Aging brain

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Aging_brain 

Aging is a major risk factor for most common neurodegenerative diseases, including mild cognitive impairment, dementias including Alzheimer's disease, cerebrovascular disease, Parkinson's disease, and Lou Gehrig's disease. While much research has focused on diseases of aging, there are few informative studies on the molecular biology of the aging brain (usually spelled ageing brain in British English) in the absence of neurodegenerative disease or the neuropsychological profile of healthy older adults. However, research suggests that the aging process is associated with several structural, chemical, and functional changes in the brain as well as a host of neurocognitive changes. Recent reports in model organisms suggest that as organisms age, there are distinct changes in the expression of genes at the single neuron level. This page is devoted to reviewing the changes associated with healthy aging.

Structural changes

Aging entails many physical, biological, chemical, and psychological changes. Therefore, it is logical to assume the brain is no exception to this phenomenon. CT scans have found that the cerebral ventricles expand as a function of age. More recent MRI studies have reported age-related regional decreases in cerebral volume. Regional volume reduction is not uniform; some brain regions shrink at a rate of up to 1% per year, whereas others remain relatively stable until the end of the life-span. The brain is very complex, and is composed of many different areas and types of tissue, or matter. The different functions of different tissues in the brain may be more or less susceptible to age-induced changes. The brain matter can be broadly classified as either grey matter, or white matter. Grey matter consists of cell bodies in the cortex and subcortical nuclei, whereas white matter consists of tightly packed myelinated axons connecting the neurons of the cerebral cortex to each other and with the periphery.

Loss of neural circuits and brain plasticity

Brain plasticity refers to the brain's ability to change structure and function. This ties into the common phrase, "if you don't use it, you lose it," which is another way of saying, if you don't use it, your brain will devote less somatotopic space for it. One proposed mechanism for the observed age-related plasticity deficits in animals is the result of age-induced alterations in calcium regulation. The changes in our abilities to handle calcium will ultimately influence neuronal firing and the ability to propagate action potentials, which in turn would affect the ability of the brain to alter its structure or function (i.e. its plastic nature). Due to the complexity of the brain, with all of its structures and functions, it is logical to assume that some areas would be more vulnerable to aging than others. Two circuits worth mentioning here are the hippocampal and neocortical circuits. It has been suggested that age-related cognitive decline is due in part not to neuronal death but to synaptic alterations. Evidence in support of this idea from animal work has also suggested that this cognitive deficit is due to functional and biochemical factors such as changes in enzymatic activity, chemical messengers, or gene expression in cortical circuits.

Thinning of the cortex

Advances in MRI technology have provided the ability to see the brain structure in great detail in an easy, non-invasive manner in vivo. Bartzokis et al., has noted that there is a decrease in grey matter volume between adulthood and old age, whereas white matter volume was found to increase from age 19–40, and decline after this age. Studies using Voxel-based morphometry have identified areas such as the insula and superior parietal gyri as being especially vulnerable to age-related losses in grey matter of older adults. Sowell et al., reported that the first 6 decades of an individual's life were correlated with the most rapid decreases in grey matter density, and this occurred over dorsal, frontal, and parietal lobes on both interhemispheric and lateral brain surfaces. It is also worth noting that areas such as the cingulate gyrus, and occipital cortex surrounding the calcarine sulcus appear exempt from this decrease in grey matter density over time. Age effects on grey matter density in the posterior temporal cortex appear more predominantly in the left versus right hemisphere, and were confined to posterior language cortices. Certain language functions such as word retrieval and production were found to be located to more anterior language cortices, and deteriorate as a function of age. Sowell et al., also reported that these anterior language cortices were found to mature and decline earlier than the more posterior language cortices. It has also been found that the width of sulcus not only increases with age, but also with cognitive decline in the elderly.

Age-related neuronal morphology

There is converging evidence from cognitive neuroscientists around the world that age-induced cognitive deficits may not be due to neuronal loss or cell death, but rather may be the result of small region-specific changes to the morphology of neurons. Studies by Duan et al., have shown that dendritic arbors and dendritic spines of cortical pyramidal neurons decrease in size and/or number in specific regions and layers of human and non-human primate cortex as a result of age (Duan et al., 2003; morph). A 46% decrease in spine number and spine density has been reported in humans older than 50 compared with younger individuals. An electron microscopy study in monkeys reported a 50% loss in spines on the apical dendritic tufts of pyramidal cells in prefrontal cortex of old animals (27–32 years old) compared with young ones (6–9 years old).

Neurofibrillary tangles

Age-related neuro-pathologies such as Alzheimer's disease, Parkinson's disease, diabetes, hypertension and arteriosclerosis make it difficult to distinguish the normal patterns of aging. One of the important differences between normal aging and pathological aging is the location of neurofibrillary tangles. Neurofibrillary tangles are composed of paired helical filaments (PHF). In normal, non-demented aging, the number of tangles in each affected cell body is relatively low and restricted to the olfactory nucleus, parahippocampal gyrus, amygdala and entorhinal cortex. As the non-demented individual ages, there is a general increase in the density of tangles, but no significant difference in where tangles are found. The other main neurodegenerative contributor commonly found in the brain of patients with AD is amyloid plaques. However, unlike tangles, plaques have not been found to be a consistent feature of normal aging.

Role of oxidative stress

Main article: Oxidative stress

Cognitive impairment has been attributed to oxidative stress, inflammatory reactions and changes in the cerebral microvasculature. The exact impact of each of these mechanisms in affecting cognitive aging is unknown. Oxidative stress is the most controllable risk factor and is the best understood. The online Merriam-Webster Medical Dictionary defines oxidative stress as, "physiological stress on the body that is caused by the cumulative damage done by free radicals inadequately neutralized by antioxidants and that is to be associated with aging." Hence oxidative stress is the damage done to the cells by free radicals that have been released from the oxidation process.

Compared to other tissues in the body, the brain is deemed unusually sensitive to oxidative damage. Increased oxidative damage has been associated with neurodegenerative diseases, mild cognitive impairment and individual differences in cognition in healthy elderly people. In 'normal aging', the brain is undergoing oxidative stress in a multitude of ways. The main contributors include protein oxidation, lipid peroxidation and oxidative modifications in nuclear and mitochondrial DNA. Oxidative stress can damage DNA replication and inhibit repair through many complex processes, including telomere shortening in DNA components. Each time a somatic cell replicates, the telomeric DNA component shortens. As telomere length is partly inheritable, there are individual differences in the age of onset of cognitive decline.

DNA damage

At least 25 studies have demonstrated that DNA damage accumulates with age in the mammalian brain. This DNA damage includes the oxidized nucleoside 8-hydroxydeoxyguanosine (8-OHdG), single- and double-strand breaks, DNA-protein crosslinks and malondialdehyde adducts (reviewed in Bernstein et al.). Increasing DNA damage with age has been reported in the brains of the mouse, rat, gerbil, rabbit, dog, and human. Young 4-day-old rats have about 3,000 single-strand breaks and 156 double-strand breaks per neuron, whereas in rats older than 2 years the level of damage increases to about 7,400 single-strand breaks and 600 double-strand breaks per neuron.

Lu et al. studied the transcriptional profiles of the human frontal cortex of individuals ranging from 26 to 106 years of age. This led to the identification of a set of genes whose expression was altered after age 40. They further found that the promoter sequences of these particular genes accumulated oxidative DNA damage, including 8-OHdG, with age. They concluded that DNA damage may reduce the expression of selectively vulnerable genes involved in learning, memory and neuronal survival, initiating a pattern of brain aging that starts early in life.

Chemical changes

In addition to the structural changes that the brain incurs with age, the aging process also entails a broad range of biochemical changes. More specifically, neurons communicate with each other via specialized chemical messengers called neurotransmitters. Several studies have identified a number of these neurotransmitters, as well as their receptors, that exhibit a marked alteration in different regions of the brain as part of the normal aging process.

Dopamine

An overwhelming number of studies have reported age-related changes in dopamine synthesis, binding sites, and number of receptors. Studies using positron emission tomography (PET) in living human subjects have shown a significant age-related decline in dopamine synthesis, notably in the striatum and extrastriatal regions (excluding the midbrain). Significant age-related decreases in dopamine receptors D₁, D₂, and D₃ have also been highly reported. A general decrease in D₁ and D₂ receptors has been shown, and more specifically a decrease of D₁ and D₂ receptor binding in the caudate nucleus and putamen. A general decrease in D₁ receptor density has also been shown to occur with age. Significant age-related declines in dopamine receptors, D₂ and D₃ were detected in the anterior cingulate cortex, frontal cortex, lateral temporal cortex, hippocampus, medial temporal cortex, amygdala, medial thalamus, and lateral thalamus One study also indicated a significant inverse correlation between dopamine binding in the occipital cortex and age. Postmortem studies also show that the number of D₁ and D₂ receptors decline with age in both the caudate nucleus and the putamen, although the ratio of these receptors did not show age-related changes. The loss of dopamine with age is thought to be responsible for many neurological symptoms that increase in frequency with age, such as decreased arm swing and increased rigidity. Changes in dopamine levels may also cause age-related changes in cognitive flexibility.

Serotonin

Decreasing levels of different serotonin receptors and the serotonin transporter, 5-HTT, have also been shown to occur with age. Studies conducted using PET methods on humans, in vivo, show that levels of the 5-HT₂ receptor in the caudate nucleus, putamen, and frontal cerebral cortex, decline with age. A decreased binding capacity of the 5-HT₂ receptor in the frontal cortex was also found, as well as a decreased binding capacity of the serotonin transporter, 5-HHT, in the thalamus and the midbrain. Postmortem studies on humans have indicated decreased binding capacities of serotonin and a decrease in the number of S₁ receptors in the frontal cortex and hippocampus as well as a decrease in affinity in the putamen.

Glutamate

Glutamate is another neurotransmitter that tends to decrease with age. Studies have shown older subjects to have lower glutamate concentration in the motor cortex compared to younger subjects A significant age-related decline especially in the parietal gray matter, basal ganglia, and to a lesser degree, the frontal white matter, has also been noted. Although these levels were studied in the normal human brain, the parietal and basal ganglia regions are often affected in degenerative brain diseases associated with aging and it has therefore been suggested that brain glutamate may be useful as a marker of brain diseases that are affected by aging.

Neuropsychological changes

Changes in orientation

Orientation is defined as the awareness of self in relation to one's surroundings Often orientation is examined by distinguishing whether a person has a sense of time, place, and person. Deficits in orientation are one of the most common symptoms of brain disease, hence tests of orientation are included in almost all medical and neuropsychological evaluations. While research has primarily focused on levels of orientation among clinical populations, a small number of studies have examined whether there is a normal decline in orientation among healthy aging adults. Results have been somewhat inconclusive. Some studies suggest that orientation does not decline over the lifespan. For example, in one study 92% of normal elderly adults (65–84 years) presented with perfect or near perfect orientation. However some data suggest that mild changes in orientation may be a normal part of aging. For example, Sweet and colleagues concluded that "older persons with normal, healthy memory may have mild orientation difficulties. In contrast, younger people with normal memory have virtually no orientation problems" (p. 505). So although current research suggests that normal aging is not usually associated with significant declines in orientation, mild difficulties may be a part of normal aging and not necessarily a sign of pathology.

Changes in attention

Many older adults notice a decline in their attentional abilities. Attention is a broad construct that refers to "the cognitive ability that allows us to deal with the inherent processing limitations of the human brain by selecting information for further processing" (p. 334). Since the human brain has limited resources, people use their attention to zone in on specific stimuli and block out others.

If older adults have fewer attentional resources than younger adults, we would expect that when two tasks must be carried out at the same time, older adults' performance will decline more than that of younger adults. However, a large review of studies on cognition and aging suggest that this hypothesis has not been wholly supported. While some studies have found that older adults have a more difficult time encoding and retrieving information when their attention is divided, other studies have not found meaningful differences from younger adults. Similarly, one might expect older adults to do poorly on tasks of sustained attention, which measure the ability to attend to and respond to stimuli for an extended period of time. However, studies suggest that sustained attention shows no decline with age. Results suggest that sustained attention increases in early adulthood and then remains relatively stable, at least through the seventh decade of life. More research is needed on how normal aging impacts attention after age eighty.

It is worth noting that there are factors other than true attentional abilities that might relate to difficulty paying attention. For example, it is possible that sensory deficits impact older adults' attentional abilities. In other words, impaired hearing or vision may make it more difficult for older adults to do well on tasks of visual and verbal attention.

Changes in memory

Main article: Memory and aging

Many different types of memory have been identified in humans, such as declarative memory (including episodic memory and semantic memory), working memory, spatial memory, and procedural memory. Studies done, have found that memory functions, more specifically those associated with the medial temporal lobe are especially vulnerable to age-related decline. A number of studies utilizing a variety of methods such as histological, structural imaging, functional imaging, and receptor binding have supplied converging evidence that the frontal lobes and frontal-striatal dopaminergic pathways are especially affected by age-related processes resulting in memory changes.

Changes in language

Changes in performance on verbal tasks, as well as the location, extent, and signal intensity of BOLD signal changes measured with functional MRI, vary in predictable patterns with age. For example, behavioral changes associated with age include compromised performance on tasks related to word retrieval, comprehension of sentences with high syntactic and/or working memory demands, and production of such sentences.

Genetic changes

Variation in the effects of aging among individuals can be attributed to both genetic and environmental factors. As in so many other science disciplines, the nature and nurture debate is an ongoing conflict in the field of cognitive neuroscience. The search for genetic factors has always been an important aspect in trying to understand neuro-pathological processes. Research focused on discovering the genetic component in developing AD has also contributed greatly to the understanding the genetics behind normal or "non-pathological" aging.

The human brain shows a decline in function and a change in gene expression. This modulation in gene expression may be due to oxidative DNA damage at promoter regions in the genome. Genes that are down-regulated over the age of 40 include:

• GluR1 AMPA receptor subunit
• NMDA R2A receptor subunit (involved in learning)
• Subunits of the GABA-A receptor
• Genes involved in long-term potentiation e.g. calmodulin 1 and CAM kinase II alpha.
• Calcium signaling genes
• Synaptic plasticity genes
• Synaptic vesicle release and recycling genes

Genes that are upregulated include:

• Genes associated with stress response and DNA repair
• Antioxidant defence

Epigenetic age analysis of different brain regions

The cerebellum is the youngest brain region (and probably body part) in centenarians according to an epigenetic biomarker of tissue age known as epigenetic clock: it is about 15 years younger than expected in a centenarian. By contrast, all brain regions and brain cells appear to have roughly the same epigenetic age in subjects who are younger than 80. These findings suggest that the cerebellum is protected from aging effects, which in turn could explain why the cerebellum exhibits fewer neuropathological hallmarks of age related dementias compared to other brain regions.

Delaying the effects of aging

The process of aging may be inevitable; however, one may potentially delay the effects and severity of this progression. While there is no consensus of efficacy, the following are reported as delaying cognitive decline:

• High level of education
• Physical exercise
• Staying intellectually engaged, i.e. reading and mental activities (such as crossword puzzles)
• Maintaining social and friendship networks
• Maintaining a healthy diet, including omega-3 fatty acids, and protective antioxidants.

"Super Agers"

Longitudinal research studies have recently conducted genetic analyses of centenarians and their offspring to identify biomarkers as protective factors against the negative effects of aging. In particular, the cholesteryl ester transfer protein (CETP) gene is linked to prevention of cognitive decline and Alzheimer's disease. Specifically, valine CETP homozygotes but not heterozygotes experienced a relative 51% less decline in memory compared to a reference group after adjusting for demographic factors and APOE status.

Cognitive reserve

Main article: Cognitive reserve

The ability of an individual to demonstrate no cognitive signs of aging despite an aging brain is called cognitive reserve. This hypothesis suggests that two patients might have the same brain pathology, with one person experiencing noticeable clinical symptoms, while the other continues to function relatively normally. Studies of cognitive reserve explore the specific biological, genetic and environmental differences which make one person susceptible to cognitive decline, and allow another to age more gracefully.

Nun Study

Main article: Nun Study

A study funded by the National Institute of Aging followed a group of 678 Roman Catholic sisters and recorded the effects of aging. The researchers used autobiographical essays collected as the nuns joined their Sisterhood. Findings suggest that early idea density, defined by number of ideas expressed and use of complex prepositions in these essays, was a significant predictor of lower risk for developing Alzheimer's disease in old age. Lower idea density was found to be significantly associated with lower brain weight, higher brain atrophy, and more neurofibrillary tangles.

Hypothalamus inflammation and GnRH

In a recent study (published May 1, 2013), it is suggested that the inflammation of the hypothalamus may be connected to our overall aging bodies. They focused on the activation of the protein complex NF-κB in mice test subjects, which showed increased activation as mice test subjects aged in the study. This activation not only affects aging, but affects a hormone known as GnRH, which has shown new anti-aging properties when injected into mice outside the hypothalamus, while causing the opposite effect when injected into the hypothalamus. It'll be some time before this can be applied to humans in a meaningful way, as more studies on this pathway are necessary to understand the mechanics of GnRH's anti-aging properties.

Inflammation

A study found that myeloid cells are drivers of a maladaptive inflammation element of brain-ageing in mice and that this can be reversed or prevented via inhibition of their EP2 signalling.

Aging Disparities

For certain demographics, the effects of normal cognitive aging are especially pronounced. Differences in cognitive aging might be tied to the lack of or reduced access to medical care and, as a result, suffer disproportionately from negative health outcomes. As the global population grows, diversifies, and grays, there is an increasing need to understand these inequities.

Race

African Americans

In the United States, Black and African American demographics suffer disproportionately from metabolic dysfunction with age. This has many downstream effects, but the most prominent of these is the toll on cardiovascular health. Metabolite profiles of the healthy aging index - a score that assesses neurocognitive function, among other correlates of health through the years - are associated with cardiovascular disease. Healthy cardiovascular function is critical for maintaining neurocognitive efficiency into old age. Attention, verbal learning, and cognitive set ability are related to diastolic blood pressure, triglyceride levels, and HDL cholesterol levels, respectively.

Latinos

The Latino demographic is most likely to suffer from metabolic syndrome - the combination of high blood pressure, high blood sugar, elevated triglyceride levels, and abdominal obesity - which not only increases the risk of cardiac events and type II diabetes but also is associated with lower neurocognitive function during midlife. Among different Latin heritages, frequency of the dementia-predisposing apoE4 allele was highest for Caribbean Latinos (Cubans, Dominicans, and Puerto Ricans) and lowest among mainland Latinos (Mexicans, Central Americans, and South Americans). Conversely, frequency of the neuroprotective apoE2 allele was highest for Caribbean Latinos and lowest for those of mainland heritage.

Indigenous Peoples

Indigenous populations are often understudied in research. Reviews of current literature studying natives in Australia, Brazil, Canada, and the United States from participants aged 45 to 94 years old reveal varied prevalence rates for cognitive impairment not related to dementia, from 4.4% to 17.7%. These results can be interpreted in the context of culturally biased neurocognitive tests, preexisting health conditions, poor access to healthcare, lower educational attainment, and/or old age.

Sex

Women

Compared to their male counterparts, women’s scores on the Mini Mental State Exam (MMSE) tend to decline at slightly faster rates with age. Males with mild cognitive impairment tend to show more microstructural damage than females with MCI, but seem to have a greater cognitive reserve due to larger absolute brain size and neuronal density. As a result, women tend to manifest symptoms of cognitive decline at lower thresholds than men do. This effect seems to be moderated by educational attainment - higher education is associated with later diagnosis of mild cognitive impairment as neuropathological load increases.

Transgender Individuals

LGBT elders face numerous disparities as they approach end-of-life. The transgender community fears the risk of hate crime, elder abuse, homelessness, loss of identity, and loss of independence as they age. As a result, depression and suicidality are particularly high within the demographic. Intersectionality - the overlap of several minority identities - can play a major role in health outcomes, as transgender people can be discriminated against for their race, sexuality, gender identity, and age. In the oldest old, these considerations are especially important - as members of this generation have survived through systematic prejudice and discrimination in a time where their identity was outlawed and labeled by the Diagnostic and Statistical Manual of Mental Disorders as a mental illness.

Socioeconomic status

Socioeconomic status is the interaction between social and economic factors. It has been demonstrated that sociodemographic factors can be used to predict cognitive profiles within older individuals to some extent.  This may be because families of higher socioeconomic status are equipped to provide their children with resources early on to facilitate cognitive development. For children in families of low SES, relatively small changes in parental income were associated with large changes in brain surface area; these losses were seen in areas associated with language, reading, executive functions, and spatial skills. Meanwhile, for children in families of high SES, small changes in parental income were associated with small changes in surface area within these regions. With respect to global cortical thickness, low SES children showed a curvilinear decrease in thickness with age while those of high SES demonstrated a steeper linear decline, suggesting that synaptic pruning is more efficient in the latter group. This trend was especially evident in the left fusiform and left superior temporal gyri - critical language and literacy supporting areas.

See also

• Aging movement control
• Life extension
• List of life extension related topics
• Neuropsychological test
• Reliability theory of aging and longevity
• Senescence
• Sensory stimulation therapy
• Software aging
• Neuroscience of aging

 

Cellular senescence

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Cellular_senescence 

 

The Hayflick limit deliberates that the average cell will divide around 50 times before reaching a stage known as senescence. As the cell divides, the telomeres on the end of a linear chromosome get shorter. The telomeres will eventually no longer be present on the chromosome. This end stage is the concept that links the deterioration of telomeres to aging.

 

(Upper) Primary mouse embryonic fibroblast cells (MEFs) before senescence. Spindle-shaped.
(Lower) MEFs became senescent after passages. Cells grow larger, flatten shape and expressed senescence-associated β-galactosidase (SABG, blue areas), a marker of cellular senescence.

Cellular senescence is a phenomenon characterized by the cessation of cell division. In their experiments during the early 1960s, Leonard Hayflick and Paul Moorhead found that normal human fetal fibroblasts in culture reach a maximum of approximately 50 cell population doublings before becoming senescent. This process is known as "replicative senescence", or the Hayflick limit. Hayflick's discovery of mortal cells paved the path for the discovery and understanding of cellular aging molecular pathways. Cellular senescence can be initiated by a wide variety of stress inducing factors. These stress factors include both environmental and internal damaging events, abnormal cellular growth, oxidative stress, autophagy factors, among many other things.

The physiological importance for cell senescence has been attributed to prevention of carcinogenesis, and more recently, aging, development, and tissue repair. Senescent cells contribute to the aging phenotype, including frailty syndrome, sarcopenia, and aging-associated diseases. Senescent astrocytes and microglia contribute to neurodegeneration.

Cellular mechanisms

Stress response and DNA damage

Mechanistically, replicative senescence can be triggered by a DNA damage response due to the shortening of telomeres. Cells can also be induced to senesce by DNA damage in response to elevated reactive oxygen species (ROS), activation of oncogenes, and cell-cell fusion. Normally, cell senescence is reached through a combination of a variety of factors (i.e., both telomere shortening and oxidative stress). The DNA damage response (DDR) arrests cell cycle progression until DNA damage, such as double-strand breaks (DSBs), are repaired. Senescent cells display persistent DDR that appears to be resistant to endogenous DNA repair activities. The prolonged DDR activates both ATM and ATR DNA damage kinases. The phosphorylation cascade initiated by these two kinases causes the eventual arrest of the cell cycle. Depending on the severity of the DNA damage, the cells may no longer be able to undergo repair and either go through apoptosis or cell senescence. Such senescent cells in mammalian culture and tissues retain DSBs and DDR markers. It has been proposed that retained DSBs are major drivers of the aging process. Mutations in genes relating to genome maintenance has been linked with premature aging diseases, supporting the role of cell senescence in aging.

Depletion of NAD+ can lead to DNA damage and cellular senescence in vascular smooth muscle cells.

Although senescent cells can no longer replicate, they remain metabolically active and commonly adopt an immunogenic phenotype consisting of a pro-inflammatory secretome, the up-regulation of immune ligands, a pro-survival response, promiscuous gene expression (pGE), and stain positive for senescence-associated β-galactosidase activity. Two proteins, senescence-associated beta-galactosidase and p16^Ink4A, are regarded as biomarkers of cellular senescence. However, this results in a false positive for cells that naturally have these two proteins such as maturing tissue macrophages with senescence-associated beta-galactosidase and T-cells with p16^Ink4A.

Senescent cells can undergo conversion to an immunogenic phenotype that enables them to be eliminated by the immune system. This phenotype consists of a pro-inflammatory secretome, the up-regulation of immune ligands, a pro-survival response, promiscuous gene expression (pGE) and stain positive for senescence-associated β-galactosidase activity. The nucleus of senescent cells is characterized by senescence-associated heterochromatin foci (SAHF) and DNA segments with chromatin alterations reinforcing senescence (DNA-SCARS). Senescent cells affect tumour suppression, wound healing and possibly embryonic/placental development and a pathological role in age-related diseases.

Role of telomeres

Telomeres are DNA tandem repeats at the end of chromosomes that shorten during each cycle of cell division. Recently, the role of telomeres in cellular senescence has aroused general interest, especially with a view to the possible genetically adverse effects of cloning. The successive shortening of the chromosomal telomeres with each cell cycle is also believed to limit the number of divisions of the cell, contributing to aging. After sufficient shortening, proteins responsible for maintaining telomere structure, such as TRF2, are displaced, resulting in the telomere being recognized as a site of a double-strand break. This induces replicative senescence. Some cells do not age and are described as being "biologically immortal", meaning that they are capable of dividing an unlimited amount of times. Theoretically, it is possible upon the discovery of the exact mechanism of biological immortality to genetically engineer cells with the same capability. The length of the telomere strand has senescent effects; telomere shortening activates extensive alterations in alternative RNA splicing that produce senescent toxins such as progerin, which degrades tissue and makes it more prone to failure.

Role of oncogenes

BRAF^V600E and Ras are two oncogenes implicated in cellular senescence. BRAF^V600E induces senescence through synthesis and secretion of IGFBP7. Ras activates the MAPK cascade which results in increased p53 activation and p16^INK4a upregulation. The transition to a state of senescence due to oncogene mutations are irreversible and have been termed oncogene-induced senescence (OIS).

Interestingly, even after oncogenic activation of a tissue, several researchers have identified a senescent phenotype. Researchers have identified a senescent phenotype in benign lesions of the skin carrying oncogenic mutations in neurofibroma patients with a defect that specifically causes an increase in Ras. This finding has been highly reproducible in benign prostate lesions, in melanocytic lesions of UV-irradiated HGF/SF-transgenic mice, in lymphocytes and in the mammary gland from N-Ras transgenic mice, and in hyperplasias of the pituitary gland of mice with deregulated E2F activity. The key to these findings is that genetic manipulations that abrogated the senescence response led to full-blown malignancy in those carcinomas. As such, the evidence suggests senescent cells can be associated with pre-malignant stages of the tumor. Further, it has been speculated that a senescent phenotype might serve as a promising marker for staging. There are two types of senescence in vitro. The irreversible senescence which is mediated by INK4a/Rb and p53 pathways and the reversible senescent phenotype which is mediated by p53. This suggests that p53 pathway could be effectively harnessed as a therapeutic intervention to trigger senescence and ultimately mitigate tumorigenesis.

p53 has been shown to have promising therapeutic relevance in an oncological context. In the 2007 Nature paper by Xue et al., RNAi was used to regulate endogenous p53 in a liver carcinoma model. Xue et al. utilized a chimaeric liver cancer mouse model and transduced this model with the ras oncogene. They took embryonic progenitor cells, transduced those cells with oncogenic ras, along with the tetracycline transactivator (tta) protein to control p53 expression using doxycycline, a tetracycline analog and tetracycline responsive short hairpin RNA (shRNA). In the absence of Dox, p53 was actively suppressed as the microRNA levels increased, so as Dox was administered, p53 microRNA was turned off to facilitate the expression of p53. The liver cancers that expressed Ras showed signs of senescence following p53 reactivation including an increase in senescence associated B-galactosidase protein. Even if the expression of p53 was transiently activated or deactivated, senescence via SA B-gal was observed. Xue et al. show that by briefly reactivating p53 in tumors without functional p53 activity, tumor regression is observed. The induction of cellular senescence was associated with an increase in inflammatory cytokines as is expected based on the SASP. The presence of both senescence and an increase in immune activity is able to regress and limit liver carcinoma growth in this mouse model.

Signaling pathways

There are several reported signaling pathways that lead to cellular senescence including the p53 and p16^Ink4a pathways. Both of these pathways are activated in response to cellular stressors and lead to cell cycle inhibition. p53 activates p21 which deactivates cyclin-dependent kinase 2(Cdk 2). Without Cdk 2, retinoblastoma protein (pRB) remains in its active, hypophosphorylated form and binds to the transcription factor E2F1, an important cell cycle regulator. This represses the transcriptional targets of E2F1, leading to cell cycle arrest after the G1 phase.

p16^Ink4a also activates pRB, but through inactivation of cyclin-dependent kinase 4 (Cdk 4) and cyclin-dependent kinase 6 (Cdk 6). p16^Ink4a is responsible for the induction of premature, stress-induced senescence. This is not irreversible; silencing of p16^Ink4a through promotor methylation or deletion of the p16^Ink4a locus allows the cell to resume the cell cycle if senescence was initiated by p16^Ink4a activation.

Senescence-associated secretory phenotype (SASP) gene expression is induced by a number of transcription factors, including C/EBPβ, of which the most important is NF-κB. Aberrant oncogenes, DNA damage, and oxidative stress induce mitogen-activated protein kinases, which are the upstream regulators of NF-κB.

Characteristics of senescent cells

Senescent cells are especially common in skin and adipose tissue. Senescent cells are usually larger than non-senescent cells. Transformation of a dividing cell into a non-dividing senescent cell is a slow process that can take up to six weeks.

The secretome of senescent cells is very complex. The products are mainly associated with inflammation, proliferation, and changes in the extracellular matrix. A Senescence Associated Secretory Phenotype (SASP) consisting of inflammatory cytokines, growth factors, and proteases is another characteristic feature of senescent cells. There are many SASP effector mechanisms that utilize autocrine or paracrine signalling. SASP induces an unfolded protein response in the endoplasmic reticulum because of an accumulation of unfolded proteins, resulting in proteotoxic impairment of cell function. Autophagy is upregulated to promote survival.

Considering cytokines, SASP molecules IL-6 and IL-8 are likely to cause senescence without affecting healthy neighbor cells. IL-1beta, unlike IL-6 or IL-8, is able to induce senescence in normal cells with paracrine signaling. IL-1beta is also dependent on cleavage of IL-1 by caspase-1, causing a pro-inflammatory response. Growth factors, GM-CSF and VEGF also serve as SASP molecules. From the cellular perspective, cooperation of transcriptional factors NF-κB and C/EBPβ increase the level of SASP expression. Regulation of the SASP is managed through a transcription level autocrine feedback loop, but most importantly by a continuous DDR. Proteins p53, p21, p16ink4a, and Bmi-1 have been termed as major senescence signalling factors, allowing them to serve as markers. Other markers register morphology changes, reorganization of chromatin, apoptosis resistance, altered metabolism, enlarged cytoplasm or abnormal shape of the nucleus. SASPs have distinct effects depending on the cellular context, including inflammatory or anti-inflammatory and tumor or anti-tumor effects. While considered a pro-tumorogenic effect, they likely support already tumor-primed cells instead of shifting healthy cells into transformation. Likewise, they operate as anti-tumor protectors by facilitating the elimination of damaged cells by phagocytes. The SASP is associated with many age-related diseases, including type 2 diabetes and atherosclerosis. This has motivated researchers to develop senolytic drugs to kill and eliminate senescent cells to improve health in the elderly. The nucleus of senescent cells is characterized by senescence-associated heterochromatin foci (SAHF) and DNA segments with chromatin alterations reinforcing senescence (DNA-SCARS).

Senescent cells affect tumor suppression, wound healing and possibly embryonic/placental development, and play a pathological role in age-related diseases. There are two primary tumor suppressor pathways known to mediate senescence: ARF/p53 and INK4A/RB. More specifically p16INK4a-pRb tumor suppressor and p53 are known effectors of senescence. Most cancer cells have a mutated p53 and p16INK4a-pRb, which allows the cancer cells to escape a senescent fate. The p16 protein is a cyclin dependent kinase inhibitor (CDK) inhibitor and it activates Rb tumor suppressor. p16 binds to CDK 4/6 to inhibit the kinase activity and inhibit Rb tumor suppressor via phosphorylation. The Rb tumor suppressor has been shown to associate with E2F1 (a protein necessary for transcription) in its monophosphorylated form, which inhibits transcription of downstream target genes involved in the G1/S transition. As part of a feedback loop, increased phosphorylation of Rb increases p16 expression that inhibits Cdk4/6. Reduced Cdk4/6 kinase activity results in higher levels of the hypo-phosphorylated (monophosphorylated) form of Rb, which subsequently leads to reduced levels of p16 expression.

The removal of aggregated p16 INK 4A positive senescent cells can delay tissue dysfunction and ultimately extend life. In the 2011 Nature paper by Baker et al. a novel transgene, INK-ATTAC, was used to inducibly eliminate p16 INK4A-positive senescent cells by action of a small molecule-induced activation of caspase 8, resulting in apoptosis. A BubR1 H/H mouse model known to experience the clinicopathological characteristics of aging-infertility, abnormal curvature to the spine, sarcopenia, cataracts, fat loss, dermal thinning, arrhythmias, etc. was used to test the consequences of p16INK4a removal. In these mice p16 INK4a aggregates in aging tissues including the skeletal and eye muscle, and adipose tissues. Baker et al. found that if the senescent cells are removed, it is possible to delay age-associated disorders. Not only does p16 play an important role in aging, but also in auto-immune diseases like rheumatoid arthritis that progressively lead to mobility impairment in advanced disease.

In the nervous system, senescence has been described in astrocytes and microglia, but is less understood in neurons. Because senescence arrests cell division, studies of senescence in the brain were focused mainly on glial cells and less studies were focused on nondividing neurons.

Clearance of senescent cells by the immune system

Due to the heterogeneous nature of senescent cells, different immune system cells eliminate different senescent cells. Specific components of the senescence-associated secretory phenotype (SASP) factors secreted by senescent cells attract and activate different components of both the innate and adaptive immune system.

Natural killer cells (NK cells) and macrophages play a major role in clearance of senescent cells. Natural killer cells directly kill senescent cells, and produce cytokines which activate macrophages which remove senescent cells. Senescent cells can be phagocytized by neutrophils as well as by macrophages. Senolytic drugs which induce apoptosis in senescent cells rely on phagocytic immune system cells to remove the apoptosed cells.

Natural killer cells can use NKG2D killer activation receptors to detect the MICA and ULBP2 ligands which become upregulated on senescent cells. The senescent cells are killed using perforin pore-forming cytolytic protein. CD8+ cytotoxic T-lymphocytes also use NKG2D receptors to detect senescent cells, and promote killing similar to NK cells.

Aging of the immune system (immunosenescence) results in a diminished capacity of the immune system to remove senescent cells, thereby leading to an increase in senescent cells. Chronic inflammation due to SASP from senescent cells can also reduce the capacity of the immune system to remove senescent cells. T cells, B cells, and NK cells have all been reported to become senescent themselves. Senescent-like aging CD8+ cytotoxic T-lymphocytes become more innate in structure and function, resembling NK cells. Immune system cells can be recruited by SASP to senescent cells, after which the SASP from the senescent cells can induce the immune system cells to become senescent.

Chimeric antigen receptor T cells have been proposed as an alternative means to senolytic drugs for the elimination of senescent cells. Urokinase receptors have been found to be highly expressed on senescent cells, leading researchers to use chimeric antigen receptor T cells to eliminate senescent cells in mice. Chimeric antigen receptor natural killer cells have been proposed as an allogeneic means of eliminating senescent cells. 

Transient senescence

It is important to recognize that cellular senescence is not inherently a negative phenomenon. During mammalian embryogenesis, programmed cellular senescence plays a role in tissue remodeling via macrophage infiltration and subsequent clearance of senescent cells. A study on the mesonephros and endolymphatic sac in mice highlighted the importance of cellular senescence for eventual morphogenesis of the embryonic kidney and the inner ear, respectively.

They serve to direct tissue repair and regeneration. Cellular senescence limits fibrosis during wound closure by inducing cell cycle arrest in myofibroblasts once they have fulfilled their function. When these cells have accomplished these tasks, the immune system clears them away. This phenomenon is termed acute senescence.

The negative implications of cellular senescence present themselves in the transition from acute to chronic senescence. When the immune system cannot clear senescent cells at the rate at which senescent cells are being produced, possibly as a result of the decline in immune function with age, accumulation of these cells leads to a disruption in tissue homeostasis.

Cellular senescence in mammalian disease

Transplantation of only a few (1 per 10,000) senescent cells into lean middle-aged mice was shown to be sufficient to induce frailty, early onset of aging-associated diseases, and premature death.

Biomarkers of cellular senescence have been shown to accumulate in tissues of older individuals. The accumulation of senescent cells in tissues of vertebrates with age is thought to contribute to the development of ageing-related diseases, including Alzheimer's disease, Amyotrophic lateral sclerosis, type 2 diabetes, and various cancers.

Progeria is another example of a disease that may be related to cell senescence. The disease is thought to be caused by mutations in the DNA damage response, telomere shortening, or a combination of the two. Progeroid syndromes are all examples of aging diseases where cell senescence appears to be implicated.

List of progeroid syndromes

• Hutchinson–Gilford progeria syndrome
• Rothmund–Thomson syndrome
• Werner syndrome
• Bloom syndrome
• Cockayne syndrome
• Xeroderma pigmentosum
• Trichothiodystrophy
• Xeroderma pigmentosum-Cockayne syndrome
• Restrictive dermopathy
• Mandibuloacral dysplasia
• Fanconi anaemia
• Seckel syndrome
• Ataxia telangiectasia
• Dyskeratosis congenita
• Hoyeraal-Hreidarsson syndrome
• Néstor-Guillermo progeria syndrome 

Senolytic drugs

Targeting senescent cells is a promising strategy to overcome age-related disease, simultaneous alleviate multiple comorbidities, and mitigate the effects of frailty. Removing the senescent cells by inducing apoptosis is the most straightforward option, and there are several agents that have been shown to accomplish this. Some of these senolytic drugs take advantage of the senescent-cell anti-apoptotic pathways (SCAPs); knocking out expression of the proteins involved in these pathways can lead to the death of senescent cells, leaving healthy cells.

Organisms lacking senescence

Cellular senescence is not observed in some organisms, including perennial plants, sponges, corals, and lobsters. In other organisms, where cellular senescence is observed, cells eventually become post-mitotic: they can no longer replicate themselves through the process of cellular mitosis (i.e., cells experience replicative senescence). How and why cells become post-mitotic in some species has been the subject of much research and speculation, but it has been suggested that cellular senescence evolved as a way to prevent the onset and spread of cancer. Somatic cells that have divided many times will have accumulated DNA mutations and would be more susceptible to becoming cancerous if cell division continued. As such, it is becoming apparent that senescent cells undergo conversion to an immunologic phenotype that enables them to be eliminated by the immune system.

See also

• Ageing
• Senolytic
• Apoptosis
• Necrosis
• Senescence
• DNA Damage
• DNA Repair
• Cell Cycle
• Telomeres
• Progeroid syndromes
• Carcinogenesis

 

DNA damage theory of aging

From Wikipedia, the free encyclopedia

The DNA damage theory of aging proposes that aging is a consequence of unrepaired accumulation of naturally occurring DNA damage. Damage in this context is a DNA alteration that has an abnormal structure. Although both mitochondrial and nuclear DNA damage can contribute to aging, nuclear DNA is the main subject of this analysis. Nuclear DNA damage can contribute to aging either indirectly (by increasing apoptosis or cellular senescence) or directly (by increasing cell dysfunction).

Several review articles have shown that deficient DNA repair, allowing greater accumulation of DNA damage, causes premature aging; and that increased DNA repair facilitates greater longevity. Mouse models of nucleotide-excision–repair syndromes reveal a striking correlation between the degree to which specific DNA repair pathways are compromised and the severity of accelerated aging, strongly suggesting a causal relationship. Human population studies show that single-nucleotide polymorphisms in DNA repair genes, causing up-regulation of their expression, correlate with increases in longevity. Lombard et al. compiled a lengthy list of mouse mutational models with pathologic features of premature aging, all caused by different DNA repair defects. Freitas and de Magalhães presented a comprehensive review and appraisal of the DNA damage theory of aging, including a detailed analysis of many forms of evidence linking DNA damage to aging. As an example, they described a study showing that centenarians of 100 to 107 years of age had higher levels of two DNA repair enzymes, PARP1 and Ku70, than general-population old individuals of 69 to 75 years of age. Their analysis supported the hypothesis that improved DNA repair leads to longer life span. Overall, they concluded that while the complexity of responses to DNA damage remains only partly understood, the idea that DNA damage accumulation with age is the primary cause of aging remains an intuitive and powerful one.

In humans and other mammals, DNA damage occurs frequently and DNA repair processes have evolved to compensate. In estimates made for mice, DNA lesions occur on average 25 to 115 times per minute in each cell, or about 36,000 to 160,000 per cell per day. Some DNA damage may remain in any cell despite the action of repair processes. The accumulation of unrepaired DNA damage is more prevalent in certain types of cells, particularly in non-replicating or slowly replicating cells, such as cells in the brain, skeletal and cardiac muscle.

DNA damage and mutation

Further information: DNA repair, DNA damage (naturally occurring), and Mutation

 

8-Hydroxydeoxyguanosine

To understand the DNA damage theory of aging it is important to distinguish between DNA damage and mutation, the two major types of errors that occur in DNA. Damage and mutation are fundamentally different. DNA damage is any physical abnormality in the DNA, such as single and double strand breaks, 8-hydroxydeoxyguanosine residues and polycyclic aromatic hydrocarbon adducts. DNA damage can be recognized by enzymes, and thus can be correctly repaired using the complementary undamaged sequence in a homologous chromosome if it is available for copying. If a cell retains DNA damage, transcription of a gene can be prevented and thus translation into a protein will also be blocked. Replication may also be blocked and/or the cell may die. Descriptions of reduced function, characteristic of aging and associated with accumulation of DNA damage, are given later in this article.

In contrast to DNA damage, a mutation is a change in the base sequence of the DNA. A mutation cannot be recognized by enzymes once the base change is present in both DNA strands, and thus a mutation cannot be repaired. At the cellular level, mutations can cause alterations in protein function and regulation. Mutations are replicated when the cell replicates. In a population of cells, mutant cells will increase or decrease in frequency according to the effects of the mutation on the ability of the cell to survive and reproduce. Although distinctly different from each other, DNA damages and mutations are related because DNA damages often cause errors of DNA synthesis during replication or repair and these errors are a major source of mutation.

Given these properties of DNA damage and mutation, it can be seen that DNA damages are a special problem in non-dividing or slowly dividing cells, where unrepaired damages will tend to accumulate over time. On the other hand, in rapidly dividing cells, unrepaired DNA damages that do not kill the cell by blocking replication will tend to cause replication errors and thus mutation. The great majority of mutations that are not neutral in their effect are deleterious to a cell's survival. Thus, in a population of cells comprising a tissue with replicating cells, mutant cells will tend to be lost. However, infrequent mutations that provide a survival advantage will tend to clonally expand at the expense of neighboring cells in the tissue. This advantage to the cell is disadvantageous to the whole organism, because such mutant cells can give rise to cancer. Thus DNA damages in frequently dividing cells, because they give rise to mutations, are a prominent cause of cancer. In contrast, DNA damages in infrequently dividing cells are likely a prominent cause of aging.

The first person to suggest that DNA damage, as distinct from mutation, is the primary cause of aging was Alexander in 1967. By the early 1980s there was significant experimental support for this idea in the literature. By the early 1990s experimental support for this idea was substantial, and furthermore it had become increasingly evident that oxidative DNA damage, in particular, is a major cause of aging.

In a series of articles from 1970 to 1977, PV Narasimh Acharya, Phd. (1924–1993) theorized and presented evidence that cells undergo "irreparable DNA damage", whereby DNA crosslinks occur when both normal cellular repair processes fail and cellular apoptosis does not occur. Specifically, Acharya noted that double-strand breaks and a "cross-linkage joining both strands at the same point is irreparable because neither strand can then serve as a template for repair. The cell will die in the next mitosis or in some rare instances, mutate."

Age-associated accumulation of DNA damage and decline in gene expression

Further information: DNA damage (naturally occurring)

In tissues composed of non- or infrequently replicating cells, DNA damage can accumulate with age and lead either to loss of cells, or, in surviving cells, loss of gene expression. Accumulated DNA damage is usually measured directly. Numerous studies of this type have indicated that oxidative damage to DNA is particularly important. The loss of expression of specific genes can be detected at both the mRNA level and protein level.

Brain

Further information: Aging brain

The adult brain is composed in large part of terminally differentiated non-dividing neurons. Many of the conspicuous features of aging reflect a decline in neuronal function. Accumulation of DNA damage with age in the mammalian brain has been reported during the period 1971 to 2008 in at least 29 studies. This DNA damage includes the oxidized nucleoside 8-oxo-2'-deoxyguanosine (8-oxo-dG), single- and double-strand breaks, DNA-protein crosslinks and malondialdehyde adducts (reviewed in Bernstein et al.). Increasing DNA damage with age has been reported in the brains of the mouse, rat, gerbil, rabbit, dog, and human.

Rutten et al. showed that single-strand breaks accumulate in the mouse brain with age. Young 4-day-old rats have about 3,000 single-strand breaks and 156 double-strand breaks per neuron, whereas in rats older than 2 years the level of damage increases to about 7,400 single-strand breaks and 600 double-strand breaks per neuron. Sen et al. showed that DNA damages which block the polymerase chain reaction in rat brain accumulate with age. Swain and Rao observed marked increases in several types of DNA damages in aging rat brain, including single-strand breaks, double-strand breaks and modified bases (8-OHdG and uracil). Wolf et al. also showed that the oxidative DNA damage 8-OHdG accumulates in rat brain with age. Similarly, it was shown that as humans age from 48 to 97 years, 8-OHdG accumulates in the brain.

Lu et al. studied the transcriptional profiles of the human frontal cortex of individuals ranging from 26 to 106 years of age. This led to the identification of a set of genes whose expression was altered after age 40. These genes play central roles in synaptic plasticity, vesicular transport and mitochondrial function. In the brain, promoters of genes with reduced expression have markedly increased DNA damage. In cultured human neurons, these gene promoters are selectively damaged by oxidative stress. Thus Lu et al. concluded that DNA damage may reduce the expression of selectively vulnerable genes involved in learning, memory and neuronal survival, initiating a program of brain aging that starts early in adult life.

Muscle

Further information: Muscle

Muscle strength, and stamina for sustained physical effort, decline in function with age in humans and other species. Skeletal muscle is a tissue composed largely of multinucleated myofibers, elements that arise from the fusion of mononucleated myoblasts. Accumulation of DNA damage with age in mammalian muscle has been reported in at least 18 studies since 1971. Hamilton et al. reported that the oxidative DNA damage 8-OHdG accumulates in heart and skeletal muscle (as well as in brain, kidney and liver) of both mouse and rat with age. In humans, increases in 8-OHdG with age were reported for skeletal muscle. Catalase is an enzyme that removes hydrogen peroxide, a reactive oxygen species, and thus limits oxidative DNA damage. In mice, when catalase expression is increased specifically in mitochondria, oxidative DNA damage (8-OHdG) in skeletal muscle is decreased and lifespan is increased by about 20%. These findings suggest that mitochondria are a significant source of the oxidative damages contributing to aging.

Protein synthesis and protein degradation decline with age in skeletal and heart muscle, as would be expected, since DNA damage blocks gene transcription. In 2005, Piec et al. found numerous changes in protein expression in rat skeletal muscle with age, including lower levels of several proteins related to myosin and actin. Force is generated in striated muscle by the interactions between myosin thick filaments and actin thin filaments.

Liver

Further information: Liver

Liver hepatocytes do not ordinarily divide and appear to be terminally differentiated, but they retain the ability to proliferate when injured. With age, the mass of the liver decreases, blood flow is reduced, metabolism is impaired, and alterations in microcirculation occur. At least 21 studies have reported an increase in DNA damage with age in liver. For instance, Helbock et al. estimated that the steady state level of oxidative DNA base alterations increased from 24,000 per cell in the liver of young rats to 66,000 per cell in the liver of old rats.

Kidney

Further information: Kidney

In kidney, changes with age include reduction in both renal blood flow and glomerular filtration rate, and impairment in the ability to concentrate urine and to conserve sodium and water. DNA damages, particularly oxidative DNA damages, increase with age (at least 8 studies). For instance Hashimoto et al. showed that 8-OHdG accumulates in rat kidney DNA with age.

Long-lived stem cells

Further information: Stem cell and Stem cell theory of aging

Tissue-specific stem cells produce differentiated cells through a series of increasingly more committed progenitor intermediates. In hematopoiesis (blood cell formation), the process begins with long-term hematopoietic stem cells that self-renew and also produce progeny cells that upon further replication go through a series of stages leading to differentiated cells without self-renewal capacity. In mice, deficiencies in DNA repair appear to limit the capacity of hematopoietic stem cells to proliferate and self-renew with age. Sharpless and Depinho reviewed evidence that hematopoietic stem cells, as well as stem cells in other tissues, undergo intrinsic aging. They speculated that stem cells grow old, in part, as a result of DNA damage. DNA damage may trigger signalling pathways, such as apoptosis, that contribute to depletion of stem cell stocks. This has been observed in several cases of accelerated aging and may occur in normal aging too.

A key aspect of hair loss with age is the aging of the hair follicle. Ordinarily, hair follicle renewal is maintained by the stem cells associated with each follicle. Aging of the hair follicle appears to be due to the DNA damage that accumulates in renewing stem cells during aging.

Mutation theories of aging

Further information: Evolution of ageing

A popular idea, that has failed to gain significant experimental support, is the idea that mutation, as distinct from DNA damage, is the primary cause of aging. As discussed above, mutations tend to arise in frequently replicating cells as a result of errors of DNA synthesis when template DNA is damaged, and can give rise to cancer. However, in mice there is no increase in mutation in the brain with aging. Mice defective in a gene (Pms2) that ordinarily corrects base mispairs in DNA have about a 100-fold elevated mutation frequency in all tissues, but do not appear to age more rapidly. On the other hand, mice defective in one particular DNA repair pathway show clear premature aging, but do not have elevated mutation.

One variation of the idea that mutation is the basis of aging, that has received much attention, is that mutations specifically in mitochondrial DNA are the cause of aging. Several studies have shown that mutations accumulate in mitochondrial DNA in infrequently replicating cells with age. DNA polymerase gamma is the enzyme that replicates mitochondrial DNA. A mouse mutant with a defect in this DNA polymerase is only able to replicate its mitochondrial DNA inaccurately, so that it sustains a 500-fold higher mutation burden than normal mice. These mice showed no clear features of rapidly accelerated aging. Overall, the observations discussed in this section indicate that mutations are not the primary cause of aging.

Dietary restriction

Further information: Calorie restriction

In rodents, caloric restriction slows aging and extends lifespan. At least 4 studies have shown that caloric restriction reduces 8-OHdG damages in various organs of rodents. One of these studies showed that caloric restriction reduced accumulation of 8-OHdG with age in rat brain, heart and skeletal muscle, and in mouse brain, heart, kidney and liver. More recently, Wolf et al. showed that dietary restriction reduced accumulation of 8-OHdG with age in rat brain, heart, skeletal muscle, and liver. Thus reduction of oxidative DNA damage is associated with a slower rate of aging and increased lifespan.

Inherited defects that cause premature aging

Further information: DNA repair-deficiency disorder

If DNA damage is the underlying cause of aging, it would be expected that humans with inherited defects in the ability to repair DNA damages should age at a faster pace than persons without such a defect. Numerous examples of rare inherited conditions with DNA repair defects are known. Several of these show multiple striking features of premature aging, and others have fewer such features. Perhaps the most striking premature aging conditions are Werner syndrome (mean lifespan 47 years), Huchinson–Gilford progeria (mean lifespan 13 years), and Cockayne syndrome (mean lifespan 13 years).

Werner syndrome is due to an inherited defect in an enzyme (a helicase and exonuclease) that acts in base excision repair of DNA (e.g. see Harrigan et al.).

Huchinson–Gilford progeria is due to a defect in Lamin A protein which forms a scaffolding within the cell nucleus to organize chromatin and is needed for repair of double-strand breaks in DNA. A-type lamins promote genetic stability by maintaining levels of proteins that have key roles in the DNA repair processes of non-homologous end joining and homologous recombination. Mouse cells deficient for maturation of prelamin A show increased DNA damage and chromosome aberrations and are more sensitive to DNA damaging agents.

Cockayne Syndrome is due to a defect in a protein necessary for the repair process, transcription coupled nucleotide excision repair, which can remove damages, particularly oxidative DNA damages, that block transcription.

In addition to these three conditions, several other human syndromes, that also have defective DNA repair, show several features of premature aging. These include ataxia–telangiectasia, Nijmegen breakage syndrome, some subgroups of xeroderma pigmentosum, trichothiodystrophy, Fanconi anemia, Bloom syndrome and Rothmund–Thomson syndrome.

Ku bound to DNA

In addition to human inherited syndromes, experimental mouse models with genetic defects in DNA repair show features of premature aging and reduced lifespan. In particular, mutant mice defective in Ku70, or Ku80, or double mutant mice deficient in both Ku70 and Ku80 exhibit early aging. The mean lifespans of the three mutant mouse strains were similar to each other, at about 37 weeks, compared to 108 weeks for the wild-type control. Six specific signs of aging were examined, and the three mutant mice were found to display the same aging signs as the control mice, but at a much earlier age. Cancer incidence was not increased in the mutant mice. Ku70 and Ku80 form the heterodimer Ku protein essential for the non-homologous end joining (NHEJ) pathway of DNA repair, active in repairing DNA double-strand breaks. This suggests an important role of NHEJ in longevity assurance.

Defects in DNA repair cause features of premature aging

Many authors have noted an association between defects in the DNA damage response and premature aging (see e.g.). If a DNA repair protein is deficient, unrepaired DNA damages tend to accumulate. Such accumulated DNA damages appear to cause features of premature aging (segmental progeria). 

Lifespan in different mammalian species

Further information: Maximum life span

Studies comparing DNA repair capacity in different mammalian species have shown that repair capacity correlates with lifespan. The initial study of this type, by Hart and Setlow, showed that the ability of skin fibroblasts of seven mammalian species to perform DNA repair after exposure to a DNA damaging agent correlated with lifespan of the species. The species studied were shrew, mouse, rat, hamster, cow, elephant and human. This initial study stimulated many additional studies involving a wide variety of mammalian species, and the correlation between repair capacity and lifespan generally held up. In one of the more recent studies, Burkle et al. studied the level of a particular enzyme, Poly ADP ribose polymerase, which is involved in repair of single-strand breaks in DNA. They found that the lifespan of 13 mammalian species correlated with the activity of this enzyme.

The DNA repair transcriptomes of the liver of humans, naked mole-rats and mice were compared. The maximum lifespans of humans, naked mole-rat, and mouse are respectively ~120, 30 and 3 years. The longer-lived species, humans and naked mole rats expressed DNA repair genes, including core genes in several DNA repair pathways, at a higher level than did mice. In addition, several DNA repair pathways in humans and naked mole-rats were up-regulated compared with mouse. These findings suggest that increased DNA repair facilitates greater longevity.

Over the past decade, a series of papers have shown that the mitochondrial DNA (mtDNA) base composition correlates with animal species maximum life span. The mitochondrial DNA base composition is thought to reflect its nucleotide-specific (guanine, cytosine, thymidine and adenine) different mutation rates (i.e., accumulation of guanine in the mitochondrial DNA of an animal species is due to low guanine mutation rate in the mitochondria of that species).

Centenarians

Lymphoblastoid cell lines established from blood samples of humans who lived past 100 years (centenarians) have significantly higher activity of the DNA repair protein Poly (ADP-ribose) polymerase (PARP) than cell lines from younger individuals (20 to 70 years old). The lymphocytic cells of centenarians have characteristics typical of cells from young people, both in their capability of priming the mechanism of repair after H₂O₂ sublethal oxidative DNA damage and in their PARP capacity.

Menopause

As women age, they experience a decline in reproductive performance leading to menopause. This decline is tied to a decline in the number of ovarian follicles. Although 6 to 7 million oocytes are present at mid-gestation in the human ovary, only about 500 (about 0.05%) of these ovulate, and the rest are lost. The decline in ovarian reserve appears to occur at an increasing rate with age, and leads to nearly complete exhaustion of the reserve by about age 51. As ovarian reserve and fertility decline with age, there is also a parallel increase in pregnancy failure and meiotic errors resulting in chromosomally abnormal conceptions.

BRCA1 and BRCA2  are homologous recombination repair genes. The role of declining ATM-Mediated DNA double strand DNA break (DSB) repair in oocyte aging was first proposed by Kutluk Oktay, MD, PhD based on his observations that women with BRCA mutations produced fewer oocytes in response to ovarian stimulation repair. His laboratory has further studied this hypothesis and provided an explanation for the decline in ovarian reserve with age. They showed that as women age, double-strand breaks accumulate in the DNA of their primordial follicles. Primordial follicles are immature primary oocytes surrounded by a single layer of granulosa cells. An enzyme system is present in oocytes that normally accurately repairs DNA double-strand breaks. This repair system is referred to as homologous recombinational repair, and it is especially active during meiosis. Titus et al. from Oktay Laboratory also showed that expression of four key DNA repair genes that are necessary for homologous recombinational repair (BRCA1, MRE11, Rad51 and ATM) decline in oocytes with age. This age-related decline in ability to repair double-strand damages can account for the accumulation of these damages, which then likely contributes to the decline in ovarian reserve as further explained by Turan and Oktay.

Women with an inherited mutation in the DNA repair gene BRCA1 undergo menopause prematurely, suggesting that naturally occurring DNA damages in oocytes are repaired less efficiently in these women, and this inefficiency leads to early reproductive failure. Genomic data from about 70,000 women were analyzed to identify protein-coding variation associated with age at natural menopause. Pathway analyses identified a major association with DNA damage response genes, particularly those expressed during meiosis and including a common coding variant in the BRCA1 gene.

Atherosclerosis

The most important risk factor for cardiovascular problems is chronological aging. Several research groups have reviewed evidence for a key role of DNA damage in vascular aging.

Atherosclerotic plaque contains vascular smooth muscle cells, macrophages and endothelial cells and these have been found to accumulate 8-oxoG, a common type of oxidative DNA damage. DNA strand breaks also increased in atherosclerotic plaques, thus linking DNA damage to plaque formation.

Werner syndrome (WS), a premature aging condition in humans, is caused by a genetic defect in a RecQ helicase that is employed in several DNA repair processes. WS patients develop a substantial burden of atherosclerotic plaques in their coronary arteries and aorta. These findings link excessive unrepaired DNA damage to premature aging and early atherosclerotic plaque development.

DNA damage and the epigenetic clock

Endogenous, naturally occurring DNA damages are frequent, and in humans include an average of about 10,000 oxidative damages per day and 50 double-strand DNA breaks per cell cycle.

Several reviews summarize evidence that the methylation enzyme DNMT1 is recruited to sites of oxidative DNA damage. Recruitment of DNMT1 leads to DNA methylation at the promoters of genes to inhibit transcription during repair. In addition, the 2018 review describes recruitment of DNMT1 during repair of DNA double-strand breaks. DNMT1 localization results in increased DNA methylation near the site of recombinational repair, associated with altered expression of the repaired gene. In general, repair-associated hyper-methylated promoters are restored to their former methylation level after DNA repair is complete. However, these reviews also indicate that transient recruitment of epigenetic modifiers can occasionally result in subsequent stable epigenetic alterations and gene silencing after DNA repair has been completed.

In human and mouse DNA, cytosine followed by guanine (CpG) is the least frequent dinucleotide, making up less than 1% of all dinucleotides (see CG suppression). At most CpG sites cytosine is methylated to form 5-methylcytosine. As indicated in the article CpG site, in mammals, 70% to 80% of CpG cytosines are methylated. However, in vertebrates there are CpG islands, about 300 to 3,000 base pairs long, with interspersed DNA sequences that deviate significantly from the average genomic pattern by being CpG-rich. These CpG islands are predominantly nonmethylated. In humans, about 70% of promoters located near the transcription start site of a gene (proximal promoters) contain a CpG island (see CpG islands in promoters). If the initially nonmethylated CpG sites in a CpG island become largely methylated, this causes stable silencing of the associated gene.

For humans, after adulthood is reached and during subsequent aging, the majority of CpG sequences slowly lose methylation (called epigenetic drift). However, the CpG islands that control promoters tend to gain methylation with age. The gain of methylation at CpG islands in promoter regions is correlated with age, and has been used to create an epigenetic clock.

There may be some relationship between the epigenetic clock and epigenetic alterations accumulating after DNA repair. Both unrepaired DNA damage accumulated with age and accumulated methylation of CpG islands would silence genes in which they occur, interfere with protein expression, and contribute to the aging phenotype.

See also

• Aging brain
• Biological immortality
• DNA damage (naturally occurring)
• DNA repair
• Life expectancy
• Longevity
• Maximum life span
• Rejuvenation
• Senescence
• Telomere