Fight Aging! publishes news and commentary relevant to the goal of ending all age-related disease, to be achieved by bringing the mechanisms of aging under the control of modern medicine. This weekly newsletter is sent to thousands of interested subscribers. To subscribe or unsubscribe from the newsletter,
Longevity Industry Consulting Services
Reason, the founder of Fight Aging! and Repair Biotechnologies, offers strategic consulting services to investors, entrepreneurs, and others interested in the longevity industry and its complexities. To find out more: https://www.fightaging.org/services/
- Epigenetic Clocks are Quirky: the Biological Age of the Heart is Consistently Younger than Chronological Age
- Eating a Plant Based Diet Correlates with Better Health and Lower Mortality
- The OneSkin Technologies DNA Methylation Clock for Assessment of Skin Aging
- Do Non-Replicating Cells Exhibit Senescence During the Aging Process?
- Mechanisms by Which Somatic Mosaicism May Contribute to Degenerative Aging
- Extracellular Matrix Remodeling Following Injury is Impaired in Aged Muscle
- Type V Collagen Regulates the Degree of Scarring Following Heart Attack
- Overexpression of Exercise-Related Gpld1 Increases Neurogenesis in Old Mice
- Small Extracellular Vesicles and the Balance of Signals Between Normal and Senescent Cells in Aging Tissues
- Reviewing the Evidence for Gum Disease to Contribute to Alzheimer’s Disease
- A Small Study Shows Improved Memory in Old People Subject to Transcranial Magnetic Stimulation
- Chronic Neurovascular Inflammation in the Aging Brain
- Old Individuals with High Cognitive Function Exhibit Lower Accumulation of Amyloid-β and Tau
- Evidence for Stochastic Mitochondrial DNA Mutation in Mice to Largely be a Result of DNA Replication
- Hyperbaric Oxygen Treatment Improves Cerebral Blood Flow in a Small Clinical Trial
Epigenetic Clocks are Quirky: the Biological Age of the Heart is Consistently Younger than Chronological Age
Measures of biological age based on epigenetic marks, protein levels, transcriptomic profiles, and similar collections of biological data are proliferating rapidly. The first epigenetic clock, a weighted combination of DNA methylation status at numerous CpG sites, is barely a decade old. The results correlate quite tightly with chronological age, but it was quickly established that people with epigenetic ages greater than chronological age tend to exhibit a greater risk of mortality and presence of age-related disease, and vice versa. More clocks followed, and the diversity of data used to generate these assessments of age increased along the way.
All of these approaches to measuring the burden of age suffer the same issue: they are disconnected from the well established causative mechanisms of aging, from cellular senescence to mitochondrial dysfunction. It is near entirely unknown as to how the specifics of the epigenome, proteome, or transcriptome used in these clocks are determined by mechanisms of aging. The clocks produce an outcome, but there is no way to predict in advance how the outcome will change in response to specific interventions, or whether such changes are in any way an accurate reflection of the impact of an intervention on aging.
For example, perhaps some clocks are largely measures of inflammatory status and downstream effects of chronic inflammation. Interventions that reduce inflammation would produce impressive results, while others would not. But inflammation is only one aspect of aging. There are other mechanisms that are just as important. Similarly, the clocks all have their quirks. The original epigenetic clock is insensitive to exercise, for example. It does not distinguish between fit and sedentary twins, which makes little sense given what we know of the power of exercise to influence the course of long-term health and aging. Further, epigenetic aging doesn’t correlate well with loss of telomere length.
The open access paper I’ll point out today is a different example of the quirky nature of epigenetic clocks. Researchers have found that heart tissue consistently produces younger epigenetic ages than assessments carried out in white blood cells, using a clock based on a much smaller number of CpG sites than the original epigenetic clock. The question in all such studies is the degree to which it reflects a real phenomenon – i.e. that the heart ages more slowly than the immune system – versus being an artifact of the clock, resulting from tissue-specific interactions between processes of aging and the epigenetic regulation of cellular metabolism.
The biological age of the heart is consistently younger than chronological age
People do not age at the same rate, and some of us age much more dramatically than others. Genetic and environmental factors can contribute to biological aging, which means that people may be affected differently, appearing younger or older than their birth date may predict. Consequently, age, when measured chronologically, may not be a reliable indicator of the rate of physiological breakdown of the body or organs. Indeed, individual organ systems, cells, organelles, and molecules within individuals may age at significantly different rates. Therefore, it can be postulated that even the heart may have a different aging profile to the body.
The advent of epigenome-wide high-throughput sequencing analyses has led to a successful identification of a large number of genomic sites highly associated with age. Age-predicting models have been developed and validated for an accurate “biological age” estimation. An “epigenetic clock” has been created, with unprecedented accuracy for DNAmAge estimation with an average error of only 3.6 years. Such models were based on DNA mainly derived from blood circulating leucocytes as they represent an easily available source. In this study, we applied a well studied prediction model developed on data from five CpG sites, to increase the practicability of these tests.
We have determined the biological age of the heart, specifically of the right atrium (RA) and left atrium (LA), and of peripheral blood leucocytes, by measuring the mitotic telomere length (TL) and the non-mitotic epigenetic age (DNAmAge). We found that DNAmAge, of both atrial tissues (RA and LA), was younger in respect to the chronological age (-12 years). Furthermore, no significant difference existed between RA and LA, suggesting that, although anatomically diverse and exposed to different physiological conditions, different areas of the heart had the same epigenetic non-mitotic age. Furthermore, the epigenetic age of both RA and LA, was even younger than that of the blood (-10 years).
In the present study, we demonstrated that biological age of the heart did not reflect the donor’s chronological age, while blood tracked these modifications. This would suggest that while blood is more susceptible to epigenetic changes induced by the interaction of advancing age and environmental factors, the heart is affected by these factors to a lower extent. It could be also postulated that the presence of stem cells in the cardiac muscle may explain why human heart tissue tends to have a lower DNAmAge. In fact, stem cells are found in relatively large numbers within myocardial tissue and show a DNAmAge close to zero. However, further investigation is required to elucidate the role of cardiac stem cells in determining epigenetic age of cardiac tissue and to fully understand its discrepancy with chronological age.
Eating a Plant Based Diet Correlates with Better Health and Lower Mortality
In a few recent scientific publications, the authors examined the differences in incidence of age-related disease and mortality in populations with differing levels of plant versus animal dietary protein intake. The closer to a vegan diet one approaches, the lower the risk of disease and mortality. There is already plenty of evidence for this outcome in the literature, although, as in all such things, the outstanding questions revolve around which of the possible mechanisms are the important ones.
For example, it should be expected that a lesser intake of animal protein will lower inflammation throughout the body. But does this effect really matter in comparison to the physiological response to the lower intake of calories one sees in people who adopt plant-based diets? Given the strength of the effects of calorie intake on long-term health, it is a very reasonable to make the argument that the bulk of the benefits of a vegan diet arise because of a lower calorie intake. Fewer calories means less visceral fat, greater operation of stress response mechanisms such as autophagy, and so forth. This adds up over the years.
Plant-Based Diets Promote Healthful Aging
Researchers reviewed clinical trials and epidemiological studies related to aging and found that while aging increases the risk for noncommunicable chronic diseases, healthful diets can help. The authors cite studies showing that plant-based diets rich in fruits, vegetables, grains, and legumes: reduce the risk of developing metabolic syndrome and type 2 diabetes by about 50%; reduce the risk of coronary heart disease events by an estimated 40%; reduce the risk of cerebral vascular disease events by 29%; reduce the risk of developing Alzheimer’s disease by more than 50%.
Association Between Plant and Animal Protein Intake and Overall and Cause-Specific Mortality
In this analysis of a large prospective cohort of 416,104 men and women in the US with 16 years of observation, we found higher plant protein intake was associated with reduced risk of overall mortality, with men and women experiencing (respectively) 12% and 14% lower mortality per 10 g/1000 kcal intake increment (5% lower mortality per standard deviation increment). The inverse association was apparent for cardiovascular disease and stroke mortality in both sexes, was independent of several risk factors, and was evident in most other cohort subgroups.
Replacement of 3% energy from various animal protein sources with plant protein was associated with 10% decreased overall mortality in both sexes. Of note, substitution analyses suggested that replacement of egg protein and red meat protein with plant protein resulted in the most prominent protective associations for overall mortality, representing 24% and 21% lower risk for men and women, respectively, for egg protein replacement, and 13% and 15% lower risk for men and women for red meat protein replacement. The effect sizes of these risk estimates were small.
The OneSkin Technologies DNA Methylation Clock for Assessment of Skin Aging
OneSkin Technologies is one of the first generation of startup biotech companies in the longevity industry; you’ll find an overview of their programs and technology in an interview with founder Carolina Reis last year. In summary, OneSkin works on both improved models of aging skin, and topical senolytic compounds capable of selectively destroying the senescent cells thought to be responsible for a sizable fraction of skin aging in later life. Unlike other companies in the longevity industry, the OneSkin staff is focused on the cosmetics regulatory path to market. This is in some ways more limited, and in other ways much cheaper and faster than the standard investigational new drug approach with the FDA.
Today’s news is more on the modelling front of the company’s efforts, in that OneSkin has developed a DNA methylation clock for age assessment in skin. DNA methylation is a form of epigenetic mark on DNA, an adjustment as to whether or not a gene will be expressed to produce the protein that it encodes. These marks shift constantly in response to circumstances, but some changes are characteristic of aging. The first epigenetic clock to assess chronological age, and which showed acceleration of the epigenetic age in people with greater mortality risk, was developed a decade ago. Considerable effort since then has gone into producing ever more varied (and sometimes better) assessments of biological age.
Evidence to date has suggested that different organs age at different rates, or at least that the epigenetic response to the molecular damage of aging is consistently different in different tissues. This means that tissue specific epigenetic clocks are probably necessary as this technology becomes used in practical ways. The primary obstacle to that practical use is that there is all too little connection between these epigenetic marks and the known mechanisms and processes of aging. It is very unclear, in advance, as to whether any specific intervention or mechanism should be expected to change a measurement of epigenetic age, or, when changes are observed, whether those changes are meaningful. So the clocks must be calibrated for use with any specific intervention – and that is very much an ongoing process in its earliest stages at best.
OneSkin launches MolClock, the first skin-specific molecular clock to determine the biological age of human skin
OneSkin is excited to share our new application programming interface (API), MolClock, the first ever skin-specific molecular clock designed to determine the chronological age of human skin. MolClock has the potential to drastically transform how scientists measure an individual’s skin molecular age which indicates one’s overall health, and the efficacy of skin products and interventions from a molecular level. While OneSkin owns the proprietary rights of MolClock, the tool is available for free and public use in an effort to forward the study of molecular aging and longevity research for scientists everywhere.
“The algorithm behind MolClock was constructed using machine learning to detect important epigenetic alterations that occur in our skin as we age. To train and test the MolClock algorithm, we used over 500 human skin samples and over 2,000 DNA methylation (DNAm) markers, achieving a highly accurate DNAm age predictor. MolClock allows us to predict the molecular age of someone’s skin based on their methylation profiles, which correlates strongly with one’s chronological age. Exceptions occur when there are ongoing processes that influence one’s DNAm age such as diseases including cancer and psoriasis, inflammatory disorders, and environmental exposures or lifestyle influences, such as smoking and obesity, which in most cases, will promote an acceleration of aging and increase the skin molecular age. Therefore, the DNAm age predicted by our tool is a highly accurate indicator of overall skin health.”
Highly accurate skin-specific methylome analysis algorithm as a platform to screen and validate therapeutics for healthy aging
DNA methylation (DNAm) age constitutes a powerful tool to assess the molecular age and overall health status of biological samples. Recently, it has been shown that tissue-specific DNAm age predictors may present superior performance compared to the pan- or multi-tissue counterparts. The skin is the largest organ in the body and bears important roles, such as body temperature control, barrier function, and protection from external insults. As a consequence of the constant and intimate interaction between the skin and the environment, current DNAm estimators, routinely trained using internal tissues which are influenced by other stimuli, are mostly inadequate to accurately predict skin DNAm age.
In the present study, we developed a highly accurate skin-specific DNAm age predictor, using DNAm data obtained from 508 human skin samples. Based on the analysis of 2,266 CpG sites, we accurately calculated the DNAm age of cultured skin cells and human skin biopsies. Age estimation was sensitive to the biological age of the donor, cell passage, skin disease status, as well as treatment with senotherapeutic drugs.
Do Non-Replicating Cells Exhibit Senescence During the Aging Process?
Most somatic cells in the body replicate, but sizable populations do not, known as post-mitotic cells, such as varieties of neuron in the central nervous system. Cellular senescence is fundamentally a process by which cell division is halted, a reaction to DNA damage, short telomeres, or a toxic signaling environment. Cells that become senescent swell in size, as though about to divide, but instead remain large. While in that state, they secrete a potent mix of inflammatory molecules that rouse the immune system, degrade surrounding tissue structure, and alter the behavior of nearby cells. In the short term this can be useful, as a way to suppress cancer or aid in wound healing. When sustained for the long term, it is very harmful.
Senescent cells are normally quickly destroyed, but they accumulate in tissues with age, the result of a slowing of clearance processes. This is an important contributing cause of aging, and thus considerable effort is presently devoted to the development of senolytic therapies capable of selectively destroying senescent cells. In this context, one interesting question is whether or not non-replicating cells – particularly those in the brain – are capable of becoming senescent, or something like senescent. They do undergo damage, but is cell division required for cells to become harmful in this way? What does senescence look like in post-mitotic cells? Will they be destroyed by senolytic therapies? Answering this question is more complex than one might think.
Senescence-like phenotype in post-mitotic cells of mice entering middle age
There is currently no single marker with absolute specificity for senescent cells. Some markers have more universal validity while others are related to specific senescent cell types. One of the most frequently used marker of cell senescence is the activity of senescence-associated beta-galactosidase (SA-β-gal). Since 1995, the wide use of SA-β-gal to study senescence in human or mice tissues in situ has been accompanied by controversies and technical challenges. In this respect, while senescent features have been found to be activated in a range of post-mitotic cells, independent multi-marker integration and confirmation of these results is still lacking for most of them.
Here, we attempted to independently deepen this knowledge using multiple senescence markers within the same cells of wild type mice entering middle age (9 months of age). A histochemistry protocol for the pH-dependent detection of β-galactosidase activity in several tissues was used. At pH 6, routinely utilized to detect senescence-associated β-galactosidase activity, only specific cellular populations in the mouse body (including Purkinje cells and choroid plexus in the central nervous system) were detected as strongly positive for β-galactosidase activity. These post-mitotic cells were also positive for other established markers of senescence (p16, p21, and DPP4), detected by immunofluorescence, confirming a potential senescent phenotype.
Choroid plexus produces cerebrospinal fluid (CSF) and participate in brain immunosurveillance. During ageing, CSF secretion decreases as much as 50%. These modifications are concurrent with subnormal brain activity, reduced beta-amyloid clearance, and increased glycation phenomena as well as oxidative stress. The potential interplay between senescent phenotype of the choroid plexus at young/mid age and its functional decline at older age is unknown. Senescence markers have been observed in neurons in the CNS also in a pathological context, during ischemia or Alzheimer’s disease.
Future research on senescent post-mitotic cells should encompass also the crucial role of mammalian target of rapamycin (mTOR) pathway. During cell cycle arrest caused by contact inhibition cells do not undergo a fully senescent phenotype. It was demonstrated that the conversion from cell cycle arrest to senescence, a phenomenon called geroconversion, requires stimulation of mTOR and downstream effectors, such as pS6K, concomitantly to p16/p21 activation. Therefore, our study thus encourages exploring the function of post-mitotic cells positive for SA-β-gal activity and other senescence markers in healthy adult or middle age organisms, by simultaneous assessment of related phenomena, to understand whether post-mitotic senescence plays a significant role as driver of ageing phenotypes.
Mechanisms by Which Somatic Mosaicism May Contribute to Degenerative Aging
Cells are liquid bags of molecules, constantly interacting and reacting with one another. Many of those reactions are unwanted and damaging to the molecular machinery of the cell, but repair of structures and replacement of damaged molecules is also a constant and ongoing process. The most efficient repair processes are those that attend the DNA that is folded away in the cell nucleus. Despite these processes, mutational damage to nuclear DNA slips though the layered schemes of protection and repair. It has to: without that damage, evolution would not occur.
There is some debate over the degree to which nuclear DNA damage contributes to the aging process. Evidently, and well proven, it is an important reason as to why cancer is an age-related disease – due to the occurrence of mutations in cancer suppression genes, for example. Nonetheless, looking beyond the matter of cancer risk, most mutational damage occurs in places where it will do little to no harm, in unused genes in the nuclear DNA of somatic cells with few divisions remaining before they self-destruct. Still, the consensus in the research community is that sufficient disarray can be caused by random mutational damage to negatively affect the operation of metabolism and tissue function.
The primary mechanism by which random mutational damage is thought to lead to metabolic disarray is known as somatic mosaicism. When a mutation occurs in a stem cell or progenitor cell, it can spread throughout a tissue over time via the daughter somatic cells generated to support that tissue. It remains to be determined as to just how much harm is caused by somatic mosaicism, in comparison to other mechanisms of aging that are known to be important. A meaningful assessment would likely require some way to remove or reduce mosaicism, meaning identification and repair of large numbers of mutations in large numbers of cells in disparate parts of the body, which is a little beyond the capabilities of medical science at the present time.
Pathogenic Mechanisms of Somatic Mutation and Genome Mosaicism in Aging
The main argument against a causal role of random somatic mutations in aging and aging-associated disease has been that the spontaneous mutation frequency, even at an old age, is too low to impair cellular function. The exception is cancer, where particular driver mutations are selected for a growth advantage. Mutation frequencies in somatic cells have been considered to be low because estimates were based on the mutation frequencies observed in the germline of various species, including humans, as deduced from heritable changes in proteins.
However, new single-cell sequencing methods found many more mutations per cell; i.e., up to several thousands of single nucleotide variants (SNVs), depending on the age of the subject and the cell type. This suggests that the somatic mutation rate is higher than the germline mutation rate. Indeed, in a direct comparison of germline and somatic mutation rates in humans and mice, the somatic mutation rate was found to be almost 2 orders of magnitude more frequent than the germline mutation rate. To some extent, this can be explained by selection against deleterious mutations in the germline. However, most random mutations have no effect. So far there is very little insight into the mechanism through which random somatic mutations could be pathogenic in aging mammals. Here we propose that there are essentially three such mechanisms.
Clonal Expansion of Mutations in Human Disease Genes
It has been known for some time that many Mendelian genetic diseases have a somatic mutational counterpart. Somewhat surprisingly, the fraction of cells in a tissue harboring the disease-causing mutation can be as low as 1% and still show disease. In many cases, the somatic mutation confers a growth advantage to mutant cells, but often the mutation is simply clonally amplified by chance. The most dramatic example of clonal amplification of a human disease gene is an individual with sporadic early-onset Alzheimer’s disease who showed somatic mosaicism for a presenilin 1 gene mutation. The degree of mosaicism in this individual at the age of presentation was 8% in peripheral lymphocytes and 14% in the cerebral cortex.
Of course, human development and aging cannot be explained by a simple series of cell divisions, like a cell line in culture, but is subject to complex and hierarchically dictated schemes, with some cells dividing much more frequently than others and others becoming subject to apoptosis. Nevertheless, accidental somatic mutation early in development could be a significant mechanism in the etiology of human disease, alone or in combination with a germline variant. In such cases, the phenotypic effects are straightforward and associated with the known role of the target gene(s) as a germline defect. However, combinations of low-frequency disease gene mosaics could occur, in which case the phenotypic effects in terms of aging phenotypes in organs and tissues are difficult to predict.
Evolution does not only occur in populations of organisms but also in populations of cells which are genetically heterogeneous because of de novo mutations. Most attention has been focused on evolution of somatic cells in relation to the well-documented, age-related increase in cancer incidence and mortality. However, there is evidence that somatic evolution also causally contributes to age-related diseases other than cancer. Somatic evolution has also been considered a potential mechanism for cardiovascular disease, which, like cancer, is a major age-related disorder. Evidence of genomic instability in atherosclerotic cells has been reported, leading to the hypothesis that expansion of mutant cells could be a major causal factor in cardiovascular disease.
In summary, in tissues of mammals, the adaptive landscape of somatic evolution during aging is similar to the adaptive landscape of evolution but from a different perspective. Indeed, in the aging tissue, selection for fitness among individual cells tends to move them away from their optimal peak of functioning, in concert with other cells in their host, to a more selfish pattern of genetic variation. This pushes the aging process toward loss of functionality and increased risk of disease, most notably loss of proliferative homeostasis; e.g., neoplasia, fibrosis, and inflammation, long recognized as a major aging-related phenomenon.
Certain acquired gene mutations that are not by themselves disease causing can confer a selective advantage to the cell, which expands and gradually erodes organ and tissue functioning because of increasingly selfish behavior. Although the magnitude of the adverse effects of these events in aging still await more extensive studies, there is a third possible mechanism by which randomly accumulating mutations eventually affect cell fitness. This does not require clonal outgrowth and depends on the penetration of such mutations in the DNA sequence components of the gene-regulatory networks (GRNs) that provide function to a mammalian organism throughout its life. Virtually all mutations would accumulate not in the about 1% protein-coding part of the genome but in the gene-regulatory regions that make up approximately 11% of all genome sequences.
Accumulated mutations in GRNs could explain the defects in cell signaling that have been observed with age. Cells respond to environmental challenges such as temperature changes, infections, and a variety of other stressors through GRNs and their networks of regulatory interactions. Although the dynamics of these complex networks in humans are far from understood, their actions are ultimately based on genes and the regulatory sequences that control their expression.
Extracellular Matrix Remodeling Following Injury is Impaired in Aged Muscle
The maintenance of muscle tissue declines with age, leading to both loss of muscle mass and strength, as well as impaired regeneration following injury. One of the more important aspects of this aspect of aging appears to be loss of function in muscle stem cell populations, but a broad selection of other contributing mechanisms have been identified over the years. Here, researchers dig into the biochemistry of muscle regeneration in order to identify more specific areas of dysfunction. This sort of work tends to identify changed levels of protein expression, a proximate cause of the problem at hand, but in most cases it remains a struggle to link regulatory changes in important processes with specific deeper causes of aging.
Skeletal muscle constitutes approximately 40% of the total mass of the human body and plays a central role in health and well-being. Central to the maintenance of a healthy skeletal muscle mass is its regenerative capacity, enabling muscle to completely restore function within 7-10 days after severe damage. The regeneration process can be categorized into the following three sequential but widely overlapping stages: (1) inflammation and necrosis of damaged myofibres, (2) activation, proliferation, differentiation, and fusion of satellite cells, and (3) maturation and remodeling of the regenerated muscle. Each stage is essential to drive the following subsequent stage, thereby imparting coherence to the overall regeneration process.
The extracellular matrix (ECM) is critical in maintaining normal skeletal muscle function and driving skeletal muscle regeneration. Skeletal muscle ECM is composed of a plethora of structural, adhesion, and signal-stimulating proteins that are transiently degraded and reconstituted depending on the mode and severity of tissue injury. Aged skeletal muscle does not regenerate well in response to injury, and there is evidence of impairment at each stage of the regeneration process including accumulation of collagen (i.e., fibrosis). However, it is unclear if this age-related skeletal muscle fibrosis occurs as a result of impaired degradation in the first week following tissue damage.
We investigated ECM proteins and their regulators during early regeneration timepoints. The regeneration process was compared in young (three month old) and aged (18 month old) C56BL/6J mice at 3, 5, and 7 days following cardiotoxin-induced damage to the tibialis anterior muscle. The regeneration process was impaired in aged muscle. Greater intracellular and extramyocellular PAI-1 expression was found in aged muscle. Collagen I was found to accumulate in necrotic regions, while macrophage infiltration was delayed in regenerating regions of aged muscle. Young muscle expressed higher levels of MMP-9 early in the regeneration process that primarily colocalized with macrophages, but this expression was reduced in aged muscle. Our results indicate that ECM remodeling is impaired at early time points following muscle damage, likely a result of elevated expression of the major inhibitor of ECM breakdown, PAI-1, and consequent suppression of the macrophage, MMP-9, and myogenic responses.
Type V Collagen Regulates the Degree of Scarring Following Heart Attack
The heart regenerates poorly in mammals; functional tissue is replaced by scar tissue following injury, such as a the damaged caused in a heart attack. Researchers have recently found that type V collagen is an important determinant of the extent of this scarring, which varies considerably from individual to individual. Greater scarring leads to worse heart muscle function and a poor prognosis for the patient.
Genetic engineering of animals to remove the capacity to generate this type V collagen increases scar size following the induction of a heart attack. Researchers here show that this results because differences in the mechanical properties of scar tissue lacking type V collagen cause greater efforts on the part of cells to try to reinforce and expand the scar. This discovery may or may not point the way towards strategies to minimize scar formation in heart tissue; that remains to be seen.
Following acute myocardial infarction (MI), dead cardiac muscle is replaced by scar tissue. Clinical studies demonstrate that scar size in patients with prior MI is an independent predictor of mortality and outcomes, even when normalized with respect to cardiac function. Despite the immense pathophysiologic importance of scar burden, little is known about factors that regulate scar size after ischemic cardiac injury.
To identify factors determining scar size after MI, we subjected animals to ischemic cardiac injury and performed transcriptional profiling of heart scars isolated from 3 days to 6 weeks post injury. We observed that scars rapidly attained transcriptional maturity, and there were minimal transcriptional changes in the maturing scar tissue beyond 2 weeks of injury. We thus hypothesized that genes that regulate scar size are likely to be differentially expressed early after ischemic injury. Collagens were one of the most highly differentially upregulated genes in the injured heart early after ischemic cardiac injury.
In this report, we demonstrate that collagen V (Col V), a fibrillar collagen that is minimally expressed in the uninjured heart and a minor component of scar tissue, limits scar size after ischemic cardiac injury. Animals lacking Col V in scar tissue exhibit a significant and paradoxical increase in scar size after ischemic injury. In the absence of Col V, scars exhibit altered mechanical properties that drive integrin-dependent mechanosensitive feedback on fibroblasts, augmenting fibroblast activation, extracellular matrix (ECM) secretion, and increase in scar size.
A systems genetics approach across 100 in-bred strains of mice demonstrated that collagen V is a critical driver of postinjury heart function. We show that collagen V deficiency alters the mechanical properties of scar tissue, and altered reciprocal feedback between matrix and cells induces expression of mechanosensitive integrins that drive fibroblast activation and increase scar size. Cilengitide, an inhibitor of specific integrins, rescues the phenotype of increased post-injury scarring in collagen-V-deficient mice. These observations demonstrate that collagen V regulates scar size in an integrin-dependent manner.
Overexpression of Exercise-Related Gpld1 Increases Neurogenesis in Old Mice
Studies based on the transfer of blood or plasma from young mice to old mice are resulting in a number of interesting discoveries. Researchers have found important differences in the cell signaling environment that occur with age. Whether it is possible to exploit this knowledge to produce significant gains in human health remains an open question. Early tests of plasma transfer did not produce compelling results, while efforts focused on specific proteins have yet to reach the point of clinical trials. The research noted here is illustrated of many lines of inquiry presently underway, in which a novel signal molecule is identified and shown to produce benefits in old mice. It is unusual in that it turned up a molecule that doesn’t in fact change with age, but is related to the effects of exercise on brain health.
Researchers collected plasma from either 6- or 18-month-old mice that were allowed to run on a wheel, and injected that into 18-month-old sedentary mice eight times over three weeks. The shots upped BDNF in the brain by a quarter, neurogenesis by half, and also improved the old mice’s performance in the radial arm water maze and contextual fear conditioning tests.
To find the factors responsible, the authors analyzed the plasma of exercising mice by mass spectrometry. They identified 12 proteins that were consistently elevated by exercise in both age groups. They were mostly metabolic proteins made by the liver. Among the dozen, Gpld1 and serum paraoxonase 1 stood out as key. Each is involved in numerous metabolic processes, such as cholesterol efflux, hormone response, and processing ammonium, ethanolamine, and organic hydroxy compounds.
Overexpressing serum paraoxonase 1 in old mice did them no good. In contrast, overexpression of Gpld1 elevated the mice’s hippocampal BDNF by 40 percent and nearly tripled their neurogenesis. One to two months later, the rodents did better on the radial-arm water maze, Y maze, and object-recognition tests. This was a surprise, since Gpld1 had not been linked previously to aging or cognition. In fact, its expression does not change with age in mice, the authors found. It goes up in the liver, but not other organs, after exercise. Its expression does not rise in the hippocampus after exercise.
How, then, might Gpld1 help the brain? Using a tagged construct, the authors found that very little of the enzyme gets past the blood-brain barrier, suggesting that it somehow exerts its effects from outside. How Gpld1 does this is still a mystery, but it may act by dampening peripheral inflammation, and that that may influence neuroinflammation.
Small Extracellular Vesicles and the Balance of Signals Between Normal and Senescent Cells in Aging Tissues
In this study, researchers show that small extracellular vesicles can influence the functional status of old tissues. These vesicles are membrane-bound packages of molecules that are used by cells as a form of communication, constantly secreted and taken up. Delivery of vesicles isolated from young tissues (or normal, non-senescent cells) improves function and suppresses the markers of cellular senescence in aged tissues, while delivery of vesicles isolated from old tissues (or senescent cells) degrades the function of young tissues by encouraging cellular senescence. The authors postulate a signaling environment in every tissue that slowly tips towards favoring cellular senescence and dysfunction as aging progresses. Delivering suitable vesicles in large enough numbers, and for a long enough period of time, should tip the balance back – though it is an open question as to how long the benefits would last, given the other aspects of aging still extant and still driving dysfunction.
A few decades ago, the notion of rejuvenation or amelioration of aging seemed unfeasible. However, in the last decades, the concept of parabiosis re-emerging and the rejuvenating cellular and tissue plasticity acquired by induced pluripotent stem cells have changed our views on the subject. Interestingly, we previously found that small extracellular vesicles (sEVs) isolated from senescent cells induce paracrine senescence in proliferating cells. In this study, we are describing that sEVs derived from fibroblasts isolated from young human healthy donors (sEV-Ys) ameliorate senescence in old recipient cells and old mice. Thus, there seems to be a crosstalk between both cells types via EVs; EVs inducing senescence in young cells and EVs preventing senescence in old cells. We believe this situation is what really happens in vivo.
It is known that the tissue holds a mixture of senescent and proliferating cells. We believe that the predominance of functionality between sEV-Ys and sEVs derived from senescent cells will depend on the proportion of each cell present in the tissue. When the majority of cells existing in the tissue are senescent cells, the tissue homeostasis becomes compromised as there is transmission of paracrine senescence; however, during the earlier stages of aging or during tissue damage, when there are still plenty of proliferating cells, these can “repair” tissue dysfunction by ameliorating the senescent phenotype of damaged cells through soluble factors and via sEVs as shown in this study.
Although it is tempting to speculate that according to our results sEV-Ys have rejuvenating potential to young tissues in old mice, we must be cautious to reach such conclusions as more experimental data would be needed. However, we cannot deny that sEV-Ys are helping damaged tissues to repair, which is also a very attractive tool. It would be interesting to perform longer-term experiments to determine the time period by which sEV-Ys can have rejuvenating or repairing functions.
Reviewing the Evidence for Gum Disease to Contribute to Alzheimer’s Disease
There is good mechanistic evidence for the bacteria responsible for gum disease, periodontitis, to contribute directly to age-related inflammation in the heart, brain, and other organs, and thus raise the risk of suffering cardiovascular disease, Alzheimer’s disease, and numerous other conditions that are accelerated by chronic inflammation. In the case of Alzheimer’s disease, is the effect size due to periodontitis large enough to care about in comparison to other contributing causes, however? Some research suggests that the increase in risk of Alzheimer’s is modest, but this is still a point that can be argued either way.
Alzheimer’s disease (AD) is the most common cause of dementia, and it exhibits pathological properties such as deposition of extracellular amyloid β (Aβ) and abnormally phosphorylated tau in nerve cells and a decrease of synapses. Conventionally, drugs targeting Aβ and its related molecules have been developed on the basis of the amyloid cascade hypothesis, but sufficient effects on the disease have not been obtained in past clinical trials. On the other hand, it has been pointed out that chronic inflammation and microbial infection in the brain may be involved in the pathogenesis of AD.
Recently, attention has been focused on the relationship between the periodontopathic bacterium Porphylomonas gingivalis and AD. P. gingivalis and its toxins have been detected in autopsy brain tissues from patients with AD. In addition, pathological conditions of AD are formed or exacerbated in mice infected with P. gingivalis. Compounds that target the toxins of P. gingivalis ameliorate the pathogenesis of AD triggered by P. gingivalis infection. These findings indicate that the pathological condition of AD may be regulated by controlling the bacteria in the oral cavity and the body. In the current aging society, the importance of oral and periodontal care for preventing the onset of AD will increase.
A Small Study Shows Improved Memory in Old People Subject to Transcranial Magnetic Stimulation
There is very mixed data for the ability of electromagnetic stimulation to improve cognitive function. One recent study suggests that this is because the way in which such stimulation is applied, the details of frequency, power, timing, and so forth, matters greatly. There is no one obvious way to go about this form of intervention, and most studies differ in any number of details that may or or may not turn out to be important given a better understanding of the underlying mechanisms. The small study here is an example of a case in which improved memory function is demonstrated in older people – which might be compared to other, similar studies in which no benefit was observed.
Source memory is one of the cognitive abilities that are most vulnerable to aging. Luckily, the brain plasticity could be modulated to counteract the decline. The repetitive transcranial magnetic stimulation (rTMS), a relatively non-invasive neuro-modulatory technique, could directly modulate neural excitability in the targeted cortical areas. Here, we are interested in whether the application of rTMS could enhance the source memory performance in healthy older adults. In addition, event-related potentials (ERPs) were employed to explore the specific retrieval process that rTMS could affect.
Subjects were randomly assigned to either the rTMS group or the sham group. The rTMS group received 10 sessions (20 min per session) of 10 Hz rTMS applying on the right dorsolateral prefrontal cortex (i.e., F4 site), and the sham group received 10 sessions of sham stimulation. Both groups performed source memory tests before and after the intervention while the electroencephalogram (EEG) was recorded during the retrieval process. Behavioral results showed that the source memory performance was significantly improved after rTMS compared with the sham stimulation; ERPs results showed that during the retrieval phase, the left parietal old/new effect, which reflected the process of recollection common to both young and old adults, increased in the rTMS group compared with the sham stimulation group, whereas the reversed old/new effect specific to the source retrieval of older adults showed similar attenuation after intervention in both groups.
The present results suggested that rTMS could be an effective intervention to improve source memory performance in healthy older adults and that it selectively facilitated the youth-like recollection process during retrieval.
Chronic Neurovascular Inflammation in the Aging Brain
The evidence strongly suggests that chronic inflammation in brain tissue is of great importance in the onset and progression of age-related neurodegenerative conditions. Overactivation of the immune system, resulting in chronic inflammation, is a feature of aging. It arises in part due to the accumulation of senescent cells and their inflammatory secretions, but persistent viral infection and a range of other mechanisms are also implicated.
Inflammaging represents a persistent low-grade systemic inflammation with inapparent clinical symptoms. In fact, it operates as a seesaw with a progressive pro-inflammatory “overload”. Cytokines, such as interleukins and tumor necrosis factor α (TNFα), as well as a gamut of self-debris originated from dysfunctional cells fuel the constant activated state of the immune system. With aging, accumulation of these endogenous signals is less compensated by the autophagic machinery. These stressors function as damage-associated molecular patterns (DAMPs), activating the pattern recognition receptors (PRRs) of the innate immune system.
In the brain, the neurovascular unit (NVU) establishes an intimate structural and functional connection among microvascular endothelial cells, pericytes, glial cells, neurons, and extracellular matrix components. Primary functions of the NVU are the development and maintenance of the blood-brain barrier (BBB) and neurovascular coupling. Cells of the NVU are also recognized for the role in the regulation of inflammation in the central nervous system (CNS). Inflammasome receptors appear to have a defined expression in cell types of the NVU with predominant expression of NLRP3 in endothelial cells. Experimental data strongly indicate that brain vasculature is as much affected by inflammation as neural tissue. A growing body of literature supports the idea that the NVU takes center stage in age-related neurological diseases, and of this, inflammasomes are undoubtedly crucial mediators.
Old Individuals with High Cognitive Function Exhibit Lower Accumulation of Amyloid-β and Tau
Neurodegenerative conditions are largely marked by the accumulation of a few different types of toxic protein aggregate, both amyloid-β and tau in the case of Alzheimer’s disease. These few types of protein are capable of alteration in ways that seed other molecules of the same protein to also alter in the same way, linking to form solid deposits in and around cells. These deposits are clearly toxic – but, equally, it is becoming clear that removing amyloid-β doesn’t appear to do much good in Alzheimer’s patients, for reasons that continue to be debated. Perhaps because amyloid-β aggregation is an early phase of the condition, and the real damage is done by later mechanisms triggered by amyloid, such as tau aggregation. Perhaps because amyloid-β aggregation is a side-effect of more important mechanisms such as chronic viral infection and consequent neuroinflammation.
Super-agers, or individuals whose cognitive skills are above the norm even at an advanced age, have been found to have increased resistance to tau and amyloid proteins, according to new research. An analysis of positron emission tomography (PET) scans has shown that compared to normal-agers and those with mild cognitive impairment, super-agers have a lower burden of tau and amyloid pathology associated with neurodegeneration, which probably allows them to maintain their cognitive performance.
Data from the Alzheimer’s Disease Neuroimaging Initiative was utilized to create three age- and education-matched groups of 25 super-agers, 25 normal-agers and 25 patients with mild cognitive impairment, all above 80 years old. In addition, 18 younger, cognitively normal, amyloid-negative controls were included in the comparison as a reference group. PET images obtained for all individuals and researchers compared the tau and amyloid burden between the four groups. A logistic regression was performed to identify genetic and pathophysiological factors best predicting aging processes.
No significant differences between super-agers and the younger control group were observed in terms of in vivo tau and amyloid burden. The normal-ager group exhibited tau burden in inferior temporal and precuneal areas and no significant differences in amyloid burden, when compared to the younger control group. Patients with mild cognitive impairment showed both high amyloid and high tau pathology burden. Differences in amyloid burden dissociated the normal-agers from those with mild cognitive impairment, whereas lower tau burden and lower polygenic risk predicted super-agers from mild cognitive impairment patients.
Evidence for Stochastic Mitochondrial DNA Mutation in Mice to Largely be a Result of DNA Replication
Mitochondria, the power plants of the cell, are the distant descendants of ancient symbiotic bacteria. They contain their own remnant mitochondrial DNA, a small genome distinct from that in the nucleus. This DNA is, unfortunately, less well protected and repaired than is the case for nuclear DNA. It can suffer forms of mutation that cause a mitochondrion to be both dysfunctional and able to resist the quality control mechanism of mitophagy that is responsible for removing damaged mitochondria. Since mitochondria reproduce by replication, this can lead to cells quickly overtaken by broken mitochondria, which in turn pollute the surrounding tissue with damaging oxidative waste products.
As is true for most low level causative mechanisms of aging, it is an open question as to the relative importance mitochondrial DNA damage in the progression of aging, when compared with other known forms of molecular damage. Mitochondrial dysfunction is implicated in many age-related conditions, and in differences in species life span. The ability to measure mutational rates in specific tissues is a necessary step on the way to a better understanding the importance of this process in age-related disease and loss of function.
Researchers used an extremely accurate DNA sequencing method to sequence the entire genome of mitochondria – organelles that are the powerhouse of the cell – in both reproductive cells and other cells in the body and showed that, depending on the cell type, ten-month-old mother mice had approximately two-to-three times more new mutations than their nearly one-month-old pups.
The study is the first to directly measure new mutations across the whole mitochondrial genome in reproductive cells. “Previous studies identified new mutations by comparing DNA sequence between parents and offspring, rather than looking directly at the reproductive cells. This could provide a biased picture of the rate and pattern of new mutations, because selection could prevent some mutations, for example those that are incompatible with life, from ever being seen.”
When the team compared the mitochondrial genome sequences of the mother mice and their pups, they found an increase in the number of mutations in the older mice for all of the tissues that they tested. This suggests that as the mice age, their mitochondrial genomes accumulate mutations, so the team wanted to know if they could identify the source of these mutations. Mutations can occur because of errors in DNA replication when a cell divides and makes copies of its genetic material for each of the resulting daughter cells. They can also be caused by environmental factors like UV light or radiation, for example, or if there are errors during DNA repair.
“When we looked at the pattern of mutations in the mitochondrial genomes it fit with what we would expect for most of them occurring through replication errors. But we also observed some differences in the mutation patterns between oocytes and body cells. This suggested that the contribution of different molecular mechanisms to mitochondrial mutations varies among these cells. Given that they undergo different numbers of cell divisions, it makes sense that the contribution of various mechanisms to the mutation process might be different between the tissues. However, because we see some evidence of replication error mutations in the mitochondrial genomes of oocytes as well, it’s possible that there is turnover of mitochondrial genomes in oocytes even though the cells are not dividing themselves.”
Hyperbaric Oxygen Treatment Improves Cerebral Blood Flow in a Small Clinical Trial
In the study noted here, researchers provide evidence for a few months of hyperbaric oxygen treatment to increase blood flow to the brain, perhaps in large part by spurring greater growth of small blood vessels in brain tissue. In older patients this produced improvements in measures of cognitive function. There is a good deal of evidence in the literature to suggests that changes in blood flow to the brain cause altered cognitive function. Consider that exercise improves memory function, for example, both immediately following exercise, and then over the long term. Further, it is the case that capillary networks decline in density throughout the body with age, and this is thought to contribute to degenerative aging by lowering the flow of blood to energy-hungry tissues such as the brain and muscles.
Besides common pathological declines such as in Alzheimer’s disease and mild cognitive impairments, normal cognitive aging is part of the normal aging process. Processing speed, conceptual reasoning, memory, and problem-solving activities are the main domains which decline gradually over time. Cerebrovascular dysfunction is an additional distinctive feature of aging that includes endothelial-dependent vasodilatation and regional decreases in cerebral blood flow (CBF). Although not associated with a specific pathology, reduced regional CBF is associated with impaired cognitive functions.
Hyperbaric oxygen therapy (HBOT) utilizes 100% oxygen in an environmental pressure higher than one absolute atmospheres (ATA) to enhance the amount of oxygen dissolved in body’s tissues. Repeated intermittent hyperoxic exposures has been shown to induce physiological effects which normally occur during hypoxia in a hyperoxic environment, including stem cells proliferation and generation of new blood vessels (angiogenesis). Angiogenesis is induced mainly in brain regions signaling ischemia or metabolic dysfunction. In turn, neovascularization can enhance cerebral blood flow and consequently improve the metabolic activity.
A randomized controlled clinical trial randomized 63 healthy adults (older than 64) either to HBOT or control arms for three months. Primary endpoint included the general cognitive function measured post intervention/control. Cerebral blood flow (CBF) was evaluated by perfusion magnetic resonance imaging. There was a significant group-by-time interaction in global cognitive function post-HBOT compared to control. The most striking improvements were in attention and information processing speed. Analysis showed significant cerebral blood flow increases in the HBOT group compared to the control group.