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Hallmarks of Ageing

What are the Hallmarks of Aging?

 

In 2013 a landscape-changing paper was published, The Hallmarks of Aging by Carlos López-Otín Maria A. Blasco, Linda Partridge,Manuel Serrano, Guido Kroemer, which conceptualized the essence of biological aging and its underlying mechanisms. This established a new paradigm for longevity science. The hallmarks of aging are the types of biochemical changes that occur in all organisms that experience biological aging and lead to a progressive loss of physiological integrity, impaired function and, eventually, death. 

 

As is stands there are officially 9 Hallmarks of Aging: Genomic Instability, Telomere Attrition, Epigenetic Alteration, Loss of Proteostasis, Deregulated Nutrient Sensing, Mitochondrial Dysfunction, Cellular Senescence, Stem Cell Exhaustion, and Altered Intercellular Communication. There is also a proposed tenth hallmark, which will be discussed at the end. 

 

How the Hallmarks are Categorized

The Hallmarks of Aging are classified into three categories: primary, antagonistic, and integrative hallmarks. The primary hallmarks are sources of direct damage, and are all wholly negative in effect. These include Genomic Instability, Telomere attrition, Epigenetic alterations, and Loss of proteostasis. The second type are the antagonistic hallmarks, these have opposite effects depending on the degree of intensity. At low levels, they can be beneficial and protective, but at high levels, they become more damaging. These include deregulated nutrient sensing, mitochondrial dysfunction, and cellular senescence. The final category are the integrative hallmarks, which directly affect tissue homeostasis. These are the result of accumulated damage from the other hallmarks, and include stem cell exhaustion and altered intercellular communication. Generally speaking, the primary hallmarks are the initial triggers/damage, the antagonistic hallmarks are the attempted mitigation of that damage, and the integrative hallmarks are the end result when all mitigation mechanisms fail. 

  • Genomic Instability

As we age, the genome gradually becomes unstable, tending toward entropy with increasing mutations. While some degree of mutation is essential for evolution and genetic diversity, genetic instability in the form of mutations and chromosome rearrangements is typically related to pathological disorders. These mutations can include changes in nucleic acid sequences, chromosomal rearrangements or aneuploidy (unequal distribution of chromosomes during cell division). Exogenous sources of these changes include overexposure to UV rays, x-rays, smoking, excessive alcohol use, among others. Our cells have intricate and sophisticated systems that can repair DNA and reduce the harmful effects of DNA damage, however, these repair processes are not always successful. Once the damage starts to accumulate, the nucleotide bases of the DNA start to mutate. If not identified and repaired, the mutations are passed on and replicated, which leads to conditions such as cancer.

  • Telomere Attrition

Telomeres, particularly telomere length, is one of the most widely known technical contributing factors to aging. A telomere is a protein “cap” at the end of a chromosome. It is a DNA sequence that is repeated at the end of each and every chromosome. This sequence serves two purposes: 1) To protect the coding regions of the chromosomes from damage, and 2) To provide a “clock” that measures the age of the cell. DNA molecules are made to bond with one another, unfortunately this characteristic makes them good at bonding with other molecules as well. This can cause severe problems–two chromosomes could bond together, or a chromosome could bond to an entirely different molecule. To prevent this from happening, our cells generate the telomere sequence on the end of each chromosome. Having a sequence whose sole function is to signal “this is the end of a chromosome” avoids improper binding.

 

Telomeres shorten with every cell division (in the normal process of aging). Due to their repetitive pattern, it is relatively easy to lengthen telomeres after they have been shortened with an enzyme called telomerase. However, in all cells except sperm, egg and stem cells, the lengthening process either does not occur or replaces less than what was lost. As a result, telomeres become and stay shorter with every cell division. Once a telomere shortens enough, its cell will no longer divide and eventually enters cellular senescence (another hallmark of aging). Many lifestyle choices associated with good health (healthy diet, exercise, meditation, etc) are associated with not only long telomeres overall, but lengthened telomeres.

  • Epigenetic Alterations 

 

The epigenome is a biochemical mechanism made of chemical compounds that modify, or “mark” the genome. The epigenome is not the DNA itself, but sits atop of the genome, acting as a switchboard that controls the expression of genes. As we age, the function of these markers undergo changes, consequently affecting gene expression in ways that can potentially change and ultimately compromise cell function. These changes are referred to as “epigenetic alterations”. Epigenetic alterations are closely linked with inflammation, which facilitates a negative feedback loop leading to ever-worsening epigenetic alterations and increasing fragility of chromosomes. The epigenome becomes increasingly more dysregulated; beneficial genes are switched off that should be turned on, and deleterious genes are turned on that should be switched off. As an example, changes in the epigenome of the immune system can impair its activation, leading to suppression of immune cells, thus leaving your body vulnerable to pathogens. 

Studies have shown that caloric restriction slows the rate of epigenetic alterations. Reprogramming the epigenome has also been shown to extend lifespan in some organisms.

  • Loss of Proteostasis

 

Protein homeostasis or Proteostasis is the process by which proteins are continuously broken down, recycled and rebuilt. While this process is very efficient, it slows down over the course of our lifetime. As the body becomes less efficient at breaking down proteins, more and more begin to accumulate, forming clumps both inside and outside of cells. These protein clumps eventually grow so big that they hinder the functioning of cells, even to the point of demise of the cell. This is called proteotoxicity. Protein accumulation plays a role in many diseases of aging, such as Alzheimer’s disease, age-related heart failure, brittle blood vessels, and neurological syndromes like deterioration of reflexes and temperature regulation commonly experienced by the elderly. As with several other hallmarks, reduced caloric intake is thought to mitigate this. One possible mechanism for this is that it encourages the breakdown of proteins for fuel, which clears out damaged proteins as a byproduct. 

  • Deregulated Nutrient Sensing

 

The ability to sense nutrient levels is essential for healthy cell function. Our bodies have a complex system of regulatory mechanisms that measure nutrient levels, identifying scarcity or abundance. This information is received from four key hormone and protein signaling pathways that regulate metabolism. The first two pathways, the insulin/IGF-1 signaling (IIS) and mTOR pathways are involved in anabolic metabolism (building up and repair of tissues, healing of wounds). The activity of these pathways increases when nutrients are abundant. Turning down these pathways seems to promote longevity. The other two pathways, Sirtuins (SIRT), a family of proteins that detect low nutrient levels based on increased levels of NAD+, and AMP-activated kinase (AMPK), proteins that sense scarce nutrient levels, such as during fasting, work to promote catabolic metabolism (breaking down tissues). Conversely, the activity of these increases when nutrients are scarce, and turning up these pathways promotes longevity. That is, breaking down nutrients and nutrient scarcity are more conducive to longevity than building up and nutrient abundance (another nod in the direction of caloric restriction).

 

Sometimes proteins stop responding to their nutrient triggers–This is called deregulated nutrient sensing, and is associated with the aging process. As we get older, these proteins’ ability to sense and respond to nutrients starts to degenerate. Some specific causes of this include oxidative stress, natural mutations, and metabolic byproducts. When we take in too large a quantity of nutrients, we can enter metabolic stress. This is a result of our cells having to perform extra chemical reactions to break down the food we’ve eaten. This can speed up deregulated nutrient sensing by adding to the damage those four proteins sustain. Deregulated nutrient sensing can cause a chain reaction of damage to our cells. Interval strength training, fasting, and consuming antioxidant-rich foods help to mitigate this. 

  • Mitochondrial Dysfunction

 

Mitochondria are ancient organelles that are unique in the way that they contain their own genetic information (DNA). In addition to making energy (adenosine triphosphate, or ATP), mitochondria have many other functions such as heat production, calcium storage, cell signaling, and most interestingly, mediating cell death. As we age, our mitochondria go through changes that harm their ability to provide us with energy while causing the release of harmful reactive oxygen species (ROS), which can cause DNA mutations leading to cancer. These reactive oxygen species can also contribute to muscle weakness, exacerbation of background inflammation (inflammaging), and the associated bone frailty, increase in senescent cells, and immune suppression associated with old age. The number of mitochondria we have decreases with age, as they are unable to replace themselves as quickly in their dysfunctional state.  Additionally, as aging progresses, NAD+ levels in human cells decrease, causing a breakdown in communication between the cell nuclei and mitochondrial DNA, leading to decreased energy production and increased production of ROS. NAD+ supplementation could slow down the accumulation of this damage.

  • Cellular Senescence 

 

Cellular senescence is a permanent state in which a cell can no longer divide. Being an antagonistic hallmark, it is attributed to aging but also to tumor suppression and tissue repair. Whether it will express a positive or negative effect varies according to a number of factors, one of them being age. Senescent cells constantly secrete a mixture of pro-inflammatory, immunosuppressive chemicals known collectively as the senescence-associated secretory phenotype (SASP). SASP contributes to inflammaging, and has associated negative impacts on longevity. Telomere erosion is the most widely known cause of ceased cell division. In each replication, the telomeres lose a small part of DNA because the enzymes responsible for duplicating the DNA cannot reach the end of the chromosome. Thus, the chromosomes are shortened after each replication until they reach a point at which, after having lost the telomere, they lose important genetic information. At this point, cells undergo a DNA damage response;  ceasing division and becoming senescent. In addition to telomere erosion, other types of DNA damage, most commonly double strand breaks, can also induce cellular senescence. Other causes are reactive oxygen species, changes in DNA-associated proteins, cellular stress, obesity, and metabolic dysfunction. One proposed solution to the problem of senescent cell accumulation and the resulting inflammation is therapeutic removal through an experimental new class of drugs known as senolytics

  • Stem Cell Exhaustion 

 

Stem cell exhaustion is the age-related breakdown of efficiency of stem cells. This hallmark is directly responsible for many of the physiological problems associated with aging, such as frailty and weakened immune system. While every cell in our bodies has the same genetic code, certain regions of DNA are turned off and on in each one, giving way to many unique cell types. While normal cells cannot change their epigenetic settings very easily, stem cells have greater freedom, allowing them, in some cases, to effectively turn into any cell type in the body. Stem cells perform a wide range of functions, including signaling that improves tissue function, regulation and health; and replacement of damaged or lost red and white blood cells and solid tissues. Reduction in stem cell activity, and the subsequent impairment of these important functions can lead to many diseases and health issues such as immunosuppression, muscle loss, frailty, and weakened bones, and sagginess of skin associated with old age. There are several aging-related causes of stem cell exhaustion (many tied into the other hallmarks), including senescent cells producing SASP which reduces stem cell activity, and telomere shortening, which causes direct damage. While there are numerous quality-control mechanisms in place to protect stem cells, their DNA is still susceptible to gradual mutations to the point of causing senescence or cancer. Stem cells can regenerate themselves, but they do so with lower quality and speed over time, eventually contributing to chronic diseases. 

 

Overall, stem cell research has made rapid progress in the last decade and is a well-funded area of longevity medicine. There are already multiple stem cell therapies in clinical use, and many others are currently in clinical trials. Removing senescent cells and the SASP they secrete may also potentially have a positive impact on maintaining stem cell function. The reduction of sources of inflammation would also likely reduce the overall burden of inflammaging and could prevent stem cell inhibition.

  • Altered Intercellular Communication 

 

Altered intercellular communication is the degradation of signaling between cells. Cells must be able process information from the outside, such as changes in temperature, variation in light levels, and availability of nutrients in order to thrive. As age-associated inflammation, or inflammaging, occurs in the body, cellular communication begins to break down. This causes a wide range of problems, resulting in cell damage and age-related disorders. This particular hallmark of aging is closely associated with other hallmarks, such as cellular senescence, so treating those may have an added benefit in treating this hallmark. This hallmark is also affected by others such as cellular senescence. A major approach used in lab animals to try and treat this hallmark involves decreasing energy intake through food while maintaining nutrient intake, also known as caloric restriction. Another option being explored to treat this hallmark is apheresis, a process in which blood is removed from the body, pro-aging signaling molecules are removed, and the blood reintroduced back into the body. One practical way to mitigate this hallmark is by maintaining and improving the gut microbiome.

  • Extracellular Matrix Crosslinking – A Tenth Hallmark of Aging?

 

All of the previous hallmarks have been focused on intracellular components. This proposed tenth hallmark, extracellular matrix crosslinking, deals with an extracellular component that has major implications on the aging process. While this process has been studied in the context of and linked to aging for many years, most recently an important paper Stochastic non-enzymatic modification of long-lived macromolecules – A missing hallmark of aging by Alexander Fedintsev and Alexey Moskalev suggests that these ongoing changes to the extracellular matrix could be a missing hallmark of aging. 

 

The extracellular matrix or ECM is material secreted by cells that fills spaces between the cells in a tissue, protecting them and helping to hold them together. It is composed mainly of protein and includes collagens, elastin, and glycoproteins. The consistency of the ECM can range from semifluid to rigidly solid and hard as bone. As we age, the ECM becomes more rigid and less elastic, leading to a host of problems associated with the diseases of aging. This widely irreversible increasing rigidity is known as extracellular matrix crosslinking. This happens via Advanced glycation end-products or (ironically), AGEs. These modified proteins remain unrepaired and accumulate throughout a person’s lifetime, causing changes in tissues and organs, which lead to a vicious cycle of progressively increasing damage. According to some research, it is likely that these changes can also affect the morphology of mitochondria and the synthesis of ATP. The cause of cross-links is the process of glycation, the attachment of a sugar molecule to a protein or lipid. As the number of cross-links increase, collagen fibers become denser, making the extracellular matrix less accessible to enzymes that normally play a restorative role. This contributes to the accumulation of damaged proteins in collagen, which adversely affects the strength of tissues. These crosslinks are particularly problematic in the cardiovascular system, causing arteries to stiffen, which raises blood pressure and makes heart attacks or strokes more likely. Cross-links are also implicated in complications from diabetes. 

Due to the permanency of these crosslinks, the application of senolytics, stem cell therapies, and other “anti-aging” therapies are limited. One area of research is creating enzymes against glucosepane, one of the most abundant crosslinking products/AGEs in the human body.

 

Glossary

  • Adenosine Triphosphate (ATP) –  A molecule that carries energy within cells, the source of energy for use and storage at the cellular level
  • Advanced Glycation End-products (AGEs) – Glycated proteins or lipids formed by irreversible non-enzymatic reactions between reducing sugars, such as glucose, and amino groups in proteins, lipids, and nucleic acids. Advanced glycation end products (AGEs) are derived from complex metabolic pathways involved in the pathophysiology of various diseases, especially diabetes and diabetes-related complications. They are a bio-marker implicated in aging and the development, or worsening, of many degenerative diseases, such as diabetes, atherosclerosis, chronic kidney disease, and Alzheimer’s disease.
  • AMP-activated Kinase (AMPK) – An enzyme  that plays a role in cellular energy homeostasis, largely to activate glucose and fatty acid uptake and oxidation when cellular energy is low. 
  • Aneuploidy – The presence of an abnormal number of chromosomes in a cell
  • Antagonistic hallmarks – Hallmarks of aging which at low levels, mediate beneficial effects, but at high levels, become deleterious. In principle they are beneficial, but become progressively negative in a process that is partly promoted or accelerated by the primary hallmarks. 
  • Caloric Restriction – A reduction of total dietary energy intake while maintaining adequate levels of vitamins and minerals. In contrast to starvation, which is a pathological state, caloric restriction lowers glucose levels and raises ketone body levels within normal physiological ranges.
  • Chromosome – Any of the rod-shaped or threadlike DNA-containing structures of cellular organisms that are located in the nucleus of cells.
  • Epigenome – Chemical compounds and proteins that can attach to DNA and direct such actions as turning genes on or off, or controlling the production of proteins in particular cells. The epigenome modifies the expression and function of the genome.
  • Extracellular Matrix (ECM) – A non-cellular component present within all tissues and organs which provides essential physical scaffolding for the cellular constituents, protecting them and helping to hold them together. The extracellular matrix may be semifluid or rigidly solid and hard as in bone. It is composed mainly of protein and includes collagens, elastin, reticulin, glycoproteins, proteoglycans, fibronectin, laminins and osteopontin. 
    • Extracellular Matrix Crosslinking – A process initiated by the enzymatic action, stiffening, process of chemically joining two or more molecules by a covalent bond.
  • Genome – The entire set of genetic instructions found in a cell.
  • Inflammaging – A chronic, sterile low-grade inflammation that develops with advanced age.
  • Insulin/IGF-1 signaling (IIS) pathway – The pathway that determines an organism’s metabolism, growth, development, and longevity in relation to its nutrient status.
  • Integrative hallmarks – The end result when the accumulated damage caused by the primary and antagonistic hallmarks cannot be compensated by tissue homeostatic mechanisms. The integrative hallmarks are thought to be responsible for the functional decline associated with aging, as they directly affect tissue homeostasis and function.
  • mTOR (Mammalian target of rapamycin or mechanistic target of rapamycin) – A protein pathway that regulates cell growth & protein synthesis
  • Primary hallmarks – Initial hallmarks of aging that cause damage on a cellular level and are thought to be the main factors that start to cause your health to deteriorate. They are initiating triggers whose damaging events progressively accumulate with time. 
  • Senescence-Associated Secretory Phenotype (SASP) – A phenotype associated with senescent cells wherein those cells secrete high levels of inflammatory cytokines, immune modulators, growth factors, and proteases. The SASP reinforces the senescent cell cycle arrest, stimulates the immune-mediated clearance of potentially tumorigenic cells, limits fibrosis and promotes wound healing and tissue regeneration.
  • Senolytics – A class of small molecules under basic research to determine if they can selectively induce death of senescent cells and improve health in humans.
  • SIRT (sirtuins) – A family of signaling proteins involved in metabolic regulation, particularly signaling for nutrient scarcity and catabolism. Up-regulation of sirtuins/SIRT pathway favors healthy aging.