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.
In order for an aging mechanism to classify as a Hallmark, it must meet the following criteria:
1) It should manifest or happen during normal aging.
2) Experimental acceleration or increase of the hallmark accelerates normal aging
3) Experimental reduction of the hallmark slows normal aging.
In 2023, an update to this paper by the same authors was published entitled Hallmarks of aging: An Expanding Universe, which brought the total number of hallmarks from nine to twelve. These are: genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, disabled macroautophagy, deregulated nutrient-sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, altered intercellular communication, chronic inflammation, and dysbiosis.
HOW THE HALLMARKS ARE CATEGORIZED
The Hallmarks of Aging are classified into three categories: primary, antagonistic, and integrative hallmarks. 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.
(1) The primary hallmarks are sources of direct damage, and are all wholly negative in effect. These include genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, and disabled macroautophagy.
(2) 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.
(3) 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, chronic inflammation, and dysbiosis.
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.
DISABLED MACROAUTOPHAGY
Macroautophagy, often referred to simply as autophagy, is a vital cellular recycling system. It allows cells to degrade and reuse damaged or unused components, maintaining cellular health and efficiency. This process relies on lysosomes, which break down the materials collected into usable molecules. With age, lysosomes accumulate indigestible materials, becoming less efficient and slowing the process of autophagy. This decline leads to the buildup of dysfunctional cellular components, such as damaged organelles and proteins, which can disrupt cellular function and contribute to age-related diseases like neurodegeneration, cardiovascular disease, and metabolic disorders.
Autophagy is crucial during times of stress, such as fasting, as it helps clear out cellular debris and regenerate cell components. Strategies to enhance autophagy, such as intermittent fasting, caloric restriction, or pharmacological agents like spermidine, have shown promise in slowing aging processes. Research also suggests that improving lysosomal function could be key to restoring autophagic activity, offering a potential intervention for age-related decline.
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 inflammation 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.
CHRONIC INFLAMMATION
Inflammation is an essential protective mechanism, helping the body combat infections and heal injuries. However, in aging, persistent low-level inflammation—termed "inflammaging"—becomes a damaging process. This chronic state is characterized by elevated biomarkers such as C-reactive protein (CRP), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-α), which signal widespread tissue damage and increased susceptibility to age-related diseases like cardiovascular conditions, neurodegeneration, and frailty.
Causes of chronic inflammation include cellular senescence, impaired gut microbiota, and the body’s unsuccessful attempts to clear accumulated waste. Strategies such as caloric restriction, anti-inflammatory diets, and interventions targeting cellular senescence or microbiome health may reduce its impact and improve longevity.
DYSBIOSIS
The gut microbiome, a complex community of microorganisms residing in the digestive tract, plays a central role in maintaining health by supporting digestion, regulating the immune system, and influencing brain function. Dysbiosis, or an imbalance in the gut microbiota, emerges with age and is marked by reduced microbial diversity and the loss of beneficial bacteria. This imbalance is linked to chronic inflammation (inflammaging), impaired metabolism, and increased susceptibility to diseases such as obesity, diabetes, and neurodegeneration.
Healthy aging is often associated with a more diverse and resilient gut microbiome. Centenarians, for example, exhibit a unique and varied microbiome composition compared to younger individuals with poorer health. Dietary choices have a profound impact on microbiome health. Diets rich in fiber, prebiotics (e.g., resistant starch, inulin), and fermented foods like yogurt and kimchi promote microbial diversity. Emerging therapies, such as personalized probiotics and fecal microbiota transplants, are being explored to combat dysbiosis. Regular physical activity and reducing antibiotic overuse also contribute to preserving gut microbiota health, highlighting the multifaceted approaches necessary to mitigate this hallmark of aging.
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. AGEs are implicated in aging and degenerative diseases, such as diabetes and Alzheimer’s.
Age-Related Diseases
Conditions that often show up as we age, such as heart disease, cancer, dementia, and diabetes, which are leading causes of mortality worldwide.
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 that are beneficial at low levels but become deleterious at high levels, accelerating the aging process.
Apheresis
A medical technology in which blood is passed through an apparatus that separates out one constituent and returns the rest to the circulation.
Autophagy
The body’s natural recycling system, breaking down and reusing damaged or unnecessary cellular components to maintain cellular health and function.
Biological Age
A measure of how old our bodies are based on the condition of organs, tissues, and cells, rather than chronological age.
Caloric Restriction
A reduction of total dietary energy intake while maintaining adequate levels of vitamins and minerals, promoting health without starvation.
Chromosome
DNA-containing structures in the nucleus of cells, essential for carrying genetic information.
Dysbiosis
An imbalance in the gut microbiome linked to inflammation, metabolic disorders, and age-related diseases.
Epigenome
Chemical compounds and proteins that attach to DNA to regulate gene expression and cellular function.
Extracellular Matrix (ECM)
A non-cellular component within tissues and organs, providing physical scaffolding and protection for cellular constituents.
Extracellular Matrix Crosslinking
A process involving the enzymatic or chemical joining of molecules by covalent bonds, leading to tissue stiffening.
Genome
The complete set of genetic instructions in a cell.
Geroprotectors
Substances or interventions that target the root causes of aging to slow the process and promote longer healthspan and lifespan.
Glucosepane
An irreversible protein cross-linking product and AGE derived from D-glucose, prevalent in human tissues
Healthspan
The number of years lived in good health, free from chronic diseases and disabilities, emphasizing quality of life.
Hormesis
A biological phenomenon where low doses of stress or toxins stimulate beneficial responses, promoting cellular health.
Inflammaging
Low-grade, chronic inflammation that develops with age, linked to many age-related diseases and conditions.
Insulin/IGF-1 signaling (IIS) pathway
A pathway determining metabolism, growth, and longevity based on nutrient status.
Integrative hallmarks
End results of accumulated damage from primary and antagonistic hallmarks, leading to tissue dysfunction and aging.
Lysosome
A membrane-bound organelle in animal cells that breaks down macromolecules, repairs cell membranes, and responds to foreign substances.
mTOR (Mammalian target of rapamycin)
A protein pathway that regulates cell growth and protein synthesis.
NAD (Nicotinamide Adenine Dinucleotide)
A coenzyme crucial for cellular energy production and DNA repair, with levels decreasing as we age.
Partial Reprogramming
A technology resetting age-related cellular changes without fully returning cells to a youthful state.
Primary hallmarks
Initial aging hallmarks causing cellular damage, triggering a cascade of aging-related events.
Regenerative Medicine
A field focused on healing tissues and organs to restore function lost due to aging, disease, or damage.
Senescence-Associated Secretory Phenotype (SASP)
A phenotype of senescent cells characterized by the secretion of inflammatory cytokines and growth factors.
Senolytics
A class of small molecules under research for selectively eliminating senescent cells to improve health.
SIRT (sirtuins)
A family of signaling proteins involved in metabolic regulation, promoting healthy aging by responding to nutrient scarcity.