Reviewing Amino Acid Restriction as an Approach to Slow Aging

The practice of calorie restriction, reducing calorie intake to as much as 40% below ad libitum intake while still maintaining optimal micronutrient levels, is well demonstrated to slow aging in a range of species. Relative extension of life span is smaller as species life span increases, however, for reasons that make sense from an evolutionary perspective. It is unclear as to how the observed, sweeping changes to metabolism conspire to produce this differing outcome, however. Evidence to date suggests that increased autophagy is the primary mechanism by which reduced calorie intake produces benefits, but a full understanding remains to be achieved despite decades of research.

The altered metabolic state produced by calorie restriction is triggered by sensors detecting the availability of specific dietary components, such as the essential amino acid methionine. It is possible to create some fraction of the benefits of calorie restriction with a low methionine diet. Similarly, experiments have demonstrated that a reduced intake of various other amino acids can also produce some degree of benefits similar to those resulting from calorie restriction. Human trials of mild degrees of calorie restriction have been conducted, and analysis of that data continues years after the trials completed. There has been little comparable work on human trials of amino acid restriction, however.

Amino acid restriction, aging, and longevity: an update

Ever since the discovery that restricting laboratory rodent food consumption relative to their ad libitum (henceforth ad lib) feeding amount reliably extended their lives, prevented or delayed a host of diseases, and generally enhanced later life health, researchers have been seeking to discover the mechanisms by which such restriction works. One way to investigate this question is to determine whether a key feature of what we call the dietary restriction (DR) effect, that is, improved health, reduced disease, and extended longevity due to diminished food consumption, is to restrict various components of the diet as contrasted with simply reducing food consumption itself. By now, experimental reduction of all dietary macronutrients has been performed many times in addition to macronutrient components, particularly essential amino acids, in multiple species. Various diets, from low calorie to low protein to low methionine, branched-chain amino acids (BCAAs), or isoleucine formulations, have shown that dietary modulation can affect later life health in laboratory species. Whether these dietary enhancements of later life health will be translatable to humans is a question begging to be answered.

So far surveys of humans on plant-based low methionine or low sulfur amino acid (methionine + cysteine) diets have been reported to be associated with several beneficial health outcomes such as lower cardiometabolic risk factors or diabetes-related mortality. Short-term (4-12 weeks) clinical trials indicate that low sulfur amino acid diets as in most animal studies lead to weight loss, lower total cholesterol and LDL cholesterol, and other salubrious changes. The cancer field has been particularly interested in low methionine diets as both cell-based and preclinical studies confirm that cancer cells hunger for methionine. Yet short-term trials of medical methionine restriction, especially when combined other cancer therapies, have been generally less than successful largely because of low palatability of the diet. Clearly if low methionine or low sulfur amino acid diets are to be sustainable, the plant-based approach is more likely to be successful.

It is time to determine in human studies whether these low these amino acid restricting diets unlike chronic DR, are sustainable over the long-term and what the long-term health consequences might be. It is also important to discover how the diets affect mood, energy, and interact with other healthy-enhancing lifestyle or pharmaceutical interventions such as exercise or geroprotective drugs. We have reached the "translation stage" of biological aging research. It will be curious to see how successful that translation will be.

Less Soluble Klotho, Greater Inflammation in Osteoarthritis

Klotho is a longevity-associated protein. Studies in mice show that upregulation lengthens life, while downregulation shortens life. In humans, levels of the soluble circulating form of α-klotho correlate with many aspects of aging. More of it is better, less of it is worse. Here, researchers show this to be the case for inflammation related to osteoarthritis. Identification of the full panoply of mechanisms by which klotho acts to improve health remains a work in progress. It is predominantly active in the kidney, and is clearly protective of kidney health and function in later life. Kidney function is important to the rest of the body, and this may be enough to explain much of the effect on health, inflammation, life span, and so forth. Circulating α-klotho appears to have effects on the brain, however, improving cognitive function even in younger animals. It may be that it has meaningful effects on other organs as well.

The systemic immune-inflammation index (SII) is an indicator of neutrophil, lymphocyte, and platelet counts that is used to evaluate inflammation, and it can more objectively reflect changes in the level of inflammation in the body. The SII can be calculated through routine blood examination, which has the advantages of speed, efficiency, simplicity, and low cost. Previous studies have confirmed that the SII has good clinical value in diagnosing chronic diseases such as tumours, osteoporosis, kidney stones, and rheumatoid arthritis.

The Klotho gene (also known as the longevity gene) is related to ageing and is believed to exert antiaging effects through various biological mechanisms. With the increase in academic research on the Klotho gene, the function of the Klotho gene has gradually been elucidated. Previous studies have shown that the Klotho gene plays a key biological role in antioxidant, anti-inflammatory, and antiapoptotic mechanisms; kidney protection; and the improvement of calcium metabolism and phosphorus metabolism.

The association between the SII and serum Klotho has not yet been revealed. To fill this gap, we used data from the National Health and Nutrition Examination Survey (NHANES) database in the United States to explore the relationship between the SII and serum Klotho concentrations in osteoarthritis (OA) patients. This study revealed a significant negative linear relationship between the SII and serum Klotho concentration in OA patients, indicating that a higher SII is associated with lower Klotho concentration. The SII can serve as a predictive indicator of serum Klotho concentrations in OA patients, and Klotho may serve as a potential anti-inflammatory drug for OA treatment. The causal relationship between the SII and serum Klotho concentration still needs further prospective cohort studies or Mendelian randomised studies for verification.

Link: https://doi.org/10.1371/journal.pone.0300674

Oxidative Stress in Intervertebral Disc Degeneration

Oxidative stress is the presence of a damaging level of oxidative molecules, more than cells can cope with without resulting in harmfully altered behavior, dysfunction, cell death, and so forth. Increased levels of oxidative molecules is a feature of aging and inflamed tissue. As researchers here note, it appears in the context of degenerative disc disease. Targeting oxidative stress with antioxidant compounds has achieved some success for some conditions of local inflammation, such as the use of mitochondrially targeted antioxidants for uveitis, but the fine details of how a specific antioxidant interacts with cellular machinery matters greatly. A range of antioxidants have been tested in animal models for the treatment of degenerative disc disease, but little of this has progressed into human trials.

Intervertebral disc degeneration (IDD) is caused by aging, long-term sitting, long-term spinal load, and other factors. At the same time smoking, diet, and other factors can also lead to IDD. Abnormal accumulation of reactive oxygen species (ROS) occurs within the intervertebral disc, causing the production-clearance homeostasis to be disrupted, and excess ROS leads to activation of pathways downstream of ROS, which in turn triggers a range of symptoms.

When IDD occurs, the disc system undergoes intense, localized oxidative stress. From a molecular perspective, superoxide dismutase activity is significantly reduced in the plasma of IDD patients or rats, and levels of various biomarkers of oxidative stress, including phospholipase A, fructosamine, malondialdehyde, peroxide potential, total hydrogen peroxide, advanced oxidation protein products and NO, induce DNA damage, lipid metabolism, and protein synthesis disorder. From the cellular perspective, oxidative stress promotes the degeneration of normal nucleus pulposus cells in the IVD microenvironment, and impedes the function of collagen-secreting cells.

From a more macroscopic point of view, the degeneration of nucleus pulposus cells results in the decrease of type II collagen content, which is replaced by type I collagen. The annulus fibrosus is impacted by external force, and its effect of dispersing and relieving stress is weakened, which makes the annulus fibrosus easier to break, and causes the nucleus pulposus to expand and compress the nerve, resulting in more clinical symptoms.

Link: https://doi.org/10.1016/j.arr.2024.102323

Why is Cancer an Age-Related Disease?

Today's open access review paper goes back to the basics on aging and cancer, a first principles consideration of whether or not the evidence shows that we should think of cancer as a distinct process from aging. It is certainly the case that while cancer incidence increases with age, it doesn't keep on increasing ad infinitum. In very late life, 90 and older, those who are not already dead from one cause or another actually have lower rates of cancer than younger cohorts. This may not be a only matter of those most prone to cancer having died already, but also reflect something fundamental about the way in which cellular biochemistry changes at that age.

The majority of cancer risk scales with age-related disability of the immune system, and with a growing burden of mutational damage that spreads through tissue. That growing mutational damage enables the catastrophic final change to produce runaway cancerous replication, while immune aging prevents this first cancerous cells from being caught and destroyed by immune cells. One of the primary goals of the immune system is to destroy potentially cancerous cells, but growing levels of chronic inflammation, tissue damage, and cell dysfunction prevent that from happening efficiently in later life. Cancers that predominantly occur in children are a strange exception, not the rule.

Why does cancer risk start to drop at a very old age? Plausibly because cell activity diminishes across the board; less activity means less chance of mutational damage and the creation of a cancerous cells. These are overly simplistic summaries of a much more complex reality, but they are starting points for thinking about cancer and aging.

Aging and cancer

Aging is the most important risk factor of malignant disease, the prevalence of which dramatically increases as adults age, reaching a peak around 85 or 90 years, when the incidence of new cancer diagnoses starts to decline and that of cardiovascular and other diseases ramps up. Aging is, to some degree, modulable, meaning that chronological age (measured in years) and biological age (measured by biological tests and clinical status) can be uncoupled from each other. A young biological age is linked to a reduced risk of malignant disease. For this reason, it may even be argued - in a polemic fashion - that aging is a modifiable risk factor of cancer. This speculation is apparently supported by epidemiological data indicating that lifestyle factors that slow the aging process - such as leanness, an equilibrated mostly plant-based diet, voluntary physical activity and the avoidance of environmental mutagens - also reduce the probability to develop malignant disease. This observation suggests - but does not prove - that aging and cancer share common causes that are influenced by lifestyle or, in a slightly different vision, that manifest aging precipitates the development of clinically detectable tumors that then develop as 'age-related diseases'.

In this review, we will examine the mechanistic connections between aging and malignant disease. We will first discuss arguments in favor of the null hypothesis, namely, that aging and cancer just coincide as we become older because both are time-dependent processes but do not necessarily share a common biological basis. This null hypothesis would be in line with the existence of childhood cancers and progeroid (i.e., aging-accelerating) syndromes that do not increase the likelihood to develop cancer. We will then examine the likely more broadly applicable hypothesis that aging and cancer have common mechanistic grounds, as supported by the idea that both these processes share molecular and cellular characteristics that have been referred to as 'meta-hallmarks' or 'agonistic hallmarks'. However, this hypothesis does not explain why very old age (older than 90 years) is accompanied by a reduction of the incidence of cancers, perhaps because certain 'antagonistic hallmarks' of aging counteract carcinogenesis.

We conclude that aging and cancer are connected by common superior causes including endogenous and lifestyle factors, as well as by a bidirectional crosstalk, that together render old age not only a risk factor of cancer but also an important parameter that must be considered for therapeutic decisions.

Chronic Inflammatory Signaling in the Development of Aortic Aneurysms

An aneurysm is a weakened section of a major blood vessel wall that expands and remodels into a dilated bulge, vulnerable to rupture and subsequent death. Given that treatment often fails, prevention is of great interest to the research community. What are the contributing factors to the development of an aneurysm? Researchers here look at the contribution of inflammatory signaling, generally agreed upon to be central to the dysregulation of blood vessel tissue that leads to the creation of an aneurysm.

Abdominal aortic aneurysm (AAA) has been recognized as a serious chronic inflammatory degenerative aortic disease in recent years, and it is characterized by the progressive pathological dilatation of the abdominal aortic wall. Most patients who develop AAA are usually asymptomatic; however, when the aneurysm expands and ruptures, its mortality is extremely high. According to reports, even if ruptured AAAs are treated in time, the cases fatality rate is still as high as 50-70%, coupled with the cases without timely surgery, the ruptured AAAs' total mortality can be as high as 90%.

Modern studies have identified aortic extracellular matrix (ECM) degradation, the apoptosis of vascular smooth muscle cells (VSMCs), and vascular chronic inflammatory response as the three basic pathological processes in the pathogenesis of AAA. Of these, vascular chronic inflammatory response is the core process. The cytokines released by inflammatory cells not only exacerbate ECM degradation but also lead to the apoptosis of VSMCs. For example, interleukin (IL)-1β, IL-6, IL-33, and other stimuli prompt macrophages or VSMCs to secrete matrix metalloproteinases (MMPs) that degrade elastin and collagen, leading to the apoptosis of VSMCs and ECM degradation, thereby disrupting the stability of the aortic wall architecture.

It has been demonstrated in animal experiments that the use of an IL-1β receptor inhibitor (anakinra) can effectively inhibit mouse AAA formation induced by porcine pancreatic elastase (PPE) perfusion. Therefore, inflammasome regulation of the secretion of cytokines like IL-1β and IL-18 may significantly influence AAA progression, which has been recognized as a chronic inflammatory disease. This article reviews some mechanism studies to investigate the role of inflammasome in AAA and then summarizes several potential drugs targeting inflammasome for the treatment of AAA, aiming to provide new ideas for the clinical prevention and treatment of AAA beyond surgical methods.

Link: https://doi.org/10.3390/ijms25095001

The Alzheimer's Genome

The progression of Alzheimer's disease varies considerably between patients. Is this a matter of random chance in the complex dysfunction of a complex system, or should researchers be looking more closely at genetic and epigenetic differences between patients as a contributing cause of this variability? Researchers here argue for this conclusion based on what is presently known of the heritability of Alzheimer's disease risk and specific genetic variants that are correlated with Alzheimer's disease risk.

Alzheimer's disease (AD) has traditionally been considered first and foremost a neurodegenerative condition. This neuron-centric view of AD is not wholly unjustified, as synapse and neuronal loss are cornerstone features of the worsening cognitive outcomes associated with disease progression.In addition, two primary histopathological hallmarks, extracellular β-amyloid deposition and intraneuronal neurofibrillary tangles of hyperphosphorylated tau protein, have informed much of the research on AD pathogenesis and are still fundamental scoring criteria of present molecular attempts to stage disease trajectory. However, we now know that the disease is more multifaceted than this, comprising different cell types, inflammatory overloads, the vasculature, and uniquely vulnerable brain regions, among others. Therefore, the limited success of AD therapies, which have focused largely on mitigating β-amyloid pathology, may stem from our inability to tackle the complexity of the disease and the heterogenicity of those suffering from it.

The genome holds the key to many of these individual differences. Genetics account for up to 58%-79% of AD risk, and about 75 susceptibility loci have been discovered to date. For comparison, the genetic component of Parkinson's disease is about 15%. In fact, the heritability of AD is so great that parental disease history has been employed to identify AD-by-proxy cases in attempts to increase the power of genetic association studies. Still, it has not been trivial to translate these genetic links into mechanistic breakthroughs and therapeutic targets, as the resulting functional outcomes and causal genes linked to each polymorphism remain mostly unresolved.

Here, we explore how genomic research has advanced the understanding of late-onset AD. This is, for us, the first meaning of the "broken" AD genome, akin to unraveling a code. But various processes centered on our DNA become dysfunctional in AD, imparting an equally significant connotation to the term; i.e., "broken" in this context alludes to the genome as a driver of disease. We primarily highlight findings originating from human datasets, as existing disease models often fail to recapitulate the full pathological spectrum of AD. We recognize the importance of these tools and, when appropriate, reference insights obtained using them. We also identify challenges for the field and discuss strategies for amassing the wealth of genomic information now available for developing therapeutics and clinical tools.

Link: https://doi.org/10.1016/j.xgen.2024.100555

Clearance of Microglia as a Treatment for Macular Degeneration

Macrophages are innate immune cells of the body, microglia the analogous innate immune cells of the central nervous system. All microglia and most macrophages depend on the function of colony stimulating factor 1 receptor (CSF1R); if this protein or its function are suppressed, the cells die. Following clearance of microglia and macrophages, the populations are restored within a few weeks. If this is carried out in an old animal, the new microglia and macrophages lack some of problems exhibited by the prior population, such as excessive inflammatory signaling, a high burden of cellular senescence, and so forth. There are well established CSF1R inhibitor drugs, such as pexidartinib (PLX3397), and so one can find a number of studies in which neurodegenerative conditions associated with microglial inflammation are improved by temporary clearance followed by repopulation.

Today's open access paper is an example of this sort of work. The authors provide evidence for clearance of microglia to improve the environment of a damaged retina in a mouse model of age-related macular degeneration. One might compare this to past animal studies in which clearance of microglia improves Alzheimer's disease and reduces injury following stroke. Beyond that, a considerable weight of evidence links increased numbers of pro-inflammatory microglia, whether activated or senescent, to the onset and progression of neurodegenerative conditions. It is plausible that short-term treatment with pexidartinib or a similar CSF1R inhibitor, avoiding most of the side-effects that accompany long-term use in cancer patients, will prove to be beneficial enough to enter widespread use.

Microglial repopulation restricts ocular inflammation and choroidal neovascularization in mice

Age-related macular degeneration (AMD) is a prevalent, chronic and progressive retinal degenerative disease characterized by an inflammatory response mediated by activated microglia accumulating in the retina. While robust evidence clearly identifies the beneficial effects of microglial repopulation in degenerative neurological diseases, the contributions of repopulating microglia in the retinal degenerative diseases AMD and the potential mechanisms remain incompletely understood.

In this study, we demonstrated that ten days of the CSF1R inhibitor PLX3397 treatment to induce microglial repopulation exacerbated neovascular leakage and angiogenesis formation. We also found that the accumulation of senescent cells in laser sites and treatment with microglial repopulation increased microglial phagocytosis and led to reduced cellular senescence. In addition, new microglia produced less CXCL2 and exhibited lower levels of activation markers than resident microglia, thereby ameliorating leukocyte infiltration and attenuating the inflammatory response in choroidal neovascularization lesions. Our study provides promising insights into the potential of microglial repopulation as a novel, promising therapeutic approach for the treatment of AMD using a mouse model of laser-induced CNV.

Microglia have been implicated to accumulate in the subretinal space, subsequently switching into an activated phenotype and undergoing significant changes in their function in both AMD patients and mouse models. These activated microglia cause the excessive release of inflammatory mediators and a prolonged inflammatory response, which may result in the growth of neovascular lesions and further tissue damage. As microglial survival and function are critically dependent upon CSF1R, CSF1R inhibition can effectively deplete microglia. Withdrawal of CSF1R inhibition results in the rapid repopulation of the whole retina with naïve microglia. Now that microglial activation in CNV has been identified as a symptom of inflammatory damage which in turn exacerbates retinal degeneration, it is plausible to hypothesize that the replacement of these overactivated microglia with new microglia resembling nonreactive homeostatic microglia may relieve the inflammatory response and promote retinal tissue repair in AMD.

Arguing for Hypothalamic Neural Stem Cell Signaling to Support Function in Other Tissues

Researchers here argue for neural stem cells in the hypothalamus to support a youthful environment in many other tissues via secreted factors carried into circulation in exosomes. To the degree that this signaling falters with age, it contributes to the burden of aging and age-related dysfunction - though as ever it is challenging to assign a relative importance to this mechanism versus all of the others identified to date, or a firm place in a network of cause and consequence. I don't think that describing either the signaling or its reduction with age as a program is helpful. We might expect component parts of a complex system to evolve a dependency on the behavior of other component parts. There are any number of well-established examples of the interdependence of internal organ function in the aging body. This is just the way things work.

In contrast to the hypothesis that aging results from cell-autonomous deterioration processes, the programmed longevity theory proposes that aging arises from a partial inactivation of a "longevity program" aimed at maintaining youthfulness in organisms. Supporting this hypothesis, age-related changes in organisms can be reversed by factors circulating in young blood. Concordantly, the endocrine secretion of exosomal microRNAs (miRNAs) by hypothalamic neural stem cells (htNSCs) regulates the aging rate by enhancing physiological fitness in young animals. However, the specific molecular mechanisms through which hypothalamic-derived miRNAs exert their anti-aging effects remain unexplored.

Using experimentally validated miRNA-target gene interactions and single-cell transcriptomic data of brain cells during aging and heterochronic parabiosis, we identify the main pathways controlled by these miRNAs and the cell-type-specific gene networks that are altered due to age-related loss of htNSCs and the subsequent decline in specific miRNA levels in the cerebrospinal fluid (CSF). Our bioinformatics analysis suggests that these miRNAs modulate pathways associated with senescence and cellular stress response, targeting crucial genes such as Cdkn2a, Rps27, and Txnip. The oligodendrocyte lineage appears to be the most responsive to age-dependent loss of exosomal miRNA, leading to significant derepression of several miRNA target genes.

Furthermore, heterochronic parabiosis can reverse age-related upregulation of specific miRNA-targeted genes, predominantly in brain endothelial cells, including senescence promoting genes such as Cdkn1a and Btg2. Our findings support the presence of an anti-senescence mechanism triggered by the endocrine secretion of htNSC-derived exosomal miRNAs, which is associated with a youthful transcriptional signature.

Link: https://doi.org/10.3390/ijms25105467

A Popular Science Article on the Pace of Progress Towards Treatments for Aging

Popular science articles covering the longevity industry and research into the treatment of aging tend to be a grab-bag of different projects and people of wildly different value and characteristics, all given much the same weight in the narrative. It takes a year or two of work to come to some initial understanding of the state of the field of aging research, to be able to start to distinguish good ideas from bad ideas, and make arguments about what is likely to have larger versus smaller effects on aging. No popular science journalist has put in that time. If looking for a single point of blame, it may be that there is at present no agreed-upon way to measure efficacy of a treatment for aging in humans, coupled to the point that most approaches to slow aging via metabolic adjustment behave quite differently in long lived species (such as our own) versus short-lived species (such as laboratory mice). Absent a way to rapidly assess a treatment for aging in humans, people can continue to equate likely good and likely bad approaches without being called on it.

Longevity research is advancing - but slowly. Clinical trials are moving forward on select uses for longevity drugs, younger researchers are taking the field more seriously, and private organizations are pledging significant support to research: The Saudi-based Hevolution Foundation has promised up to $1 billion in funding annually for biotech startups and academic researchers.

But while there likely remain many promising treatment candidates that have yet to be identified, they would take decades to reach clinical trials. Even academics who are bullish on the promise of longevity research fear that, for all the fanfare, the field has become too fixated on a few drugs and lifestyle adjustments that have been under investigation for years, while neglecting the basic research that could reveal novel pathways to slow down human aging.

In the last two decades, scientists have performed hundreds of lab experiments - mostly on animals - on drugs like rapamycin, canagliflozin, acarbose, empagliflozin, metformin, and on interventions like calorie restriction in diets and removal of nondividing senescent cells. Instead of testing the effects of these treatments on specific illnesses, many of these studies test whether certain interventions slow down animals' aging processes and help them live longer.

The expansion of longevity research has unearthed some potentially useful information about which biological mechanisms control aging and how to alter them. In mice and other species, changing a single pathway has the power to extend life by significant margins, raising hopes that if humans respond similarly, certain drugs could extend human lives by years. The horizon for this future is still far off. Most researchers I spoke to didn't believe that humans were going to experience a rapid increase in life expectancy any time soon - or maybe ever. They believed progress would instead be made in healthspan, helping people stay healthier for longer and avoiding long periods of physical and cognitive decline as they get older.

Link: https://www.vox.com/the-highlight/24121932/anti-aging-longevity-science-health-drugs

FOXF1 Gene Therapy Improves Regeneration of Intervertebral Discs Following Injury

Intervertebral disc degeneration is a feature of aging, and injury can produce further challenges. A range of approaches have been assessed to enhance the regenerative capacity of disc tissue, useful not just after injury, but also to reverse some of the declines in disc structure and function produced by aging. Stem cell therapies have been attempted, and are widely available via medical tourism, but unfortunately that diverse population of patients contributes next to no publicly available data to help researchers understand whether or not this is a viable approach, and how to improve on it. First generation stem cell therapies are gradually being replaced by exosome therapies, as these are logistically easier to manage. Exosomes can be harvested and stored, it is easier to produce consistent batches from one central location for manufacture, and the benefits of stem cell therapies are in any case mediated by the signaling produced by these cells, largely carried in extracellular vesicles such as exosomes.

Thus in today's open access paper, we see an example of researchers building on the exosome therapy approach rather than the cell therapy approach. Beyond the logistics, another advantage of exosomes is that they can be readily engineered to carry additional cargo into cells. In this case, researchers are delivery a DNA plasmid to express FOXF1. The usual challenge with DNA plasmids is that they express poorly, as passage into the cell nucleus and access to transcriptional machinery that can read the plasmid only efficiently occurs during cell division. The researchers used an injury model in mice, so there will tend to be more cellular replication in this circumstance as regeneration takes place. In any case, the researchers observed improvements in the treated mice versus controls. We are likely to see a range of similar approaches based on the use of extracellular vesicles as a gene therapy vector emerge in the years ahead.

Engineered extracellular vesicle-based gene therapy for the treatment of discogenic back pain

Painful musculoskeletal disorders such as chronic low back pain (LBP) are leading causes of disability worldwide and their prevalence and societal impact continues to rise with expansion of the aging population and growing opioid crisis. Intervertebral disc (IVD) degeneration is a major cause of LBP, often referred to as discogenic back pain (DBP), with epidemiological studies estimating that approximately 40% of cases are attributed to IVD degeneration. The IVD functions as an avascular and aneural joint, sandwiched between adjacent vertebral bodies of the spinal column. It is comprised of a gelatinous proteoglycan-rich nucleus pulposus (NP) core encapsulated by rings of collagen that form the annulus fibrosus (AF). In degeneration, mechanical imbalances, loss of critical extracellular matrix (ECM) components such as proteoglycans, increased catabolism, inflammation, and neurovascular invasion contribute to a detrimental shift in homeostasis that leads to the loss of tissue function and increased pain.

n previous studies, we have demonstrated the potential of developmental transfection factors such as Brachyury (T) and Forkhead Box F1 (FOXF1), both of which are healthy immature NP markers involved in growth during development, to drive cellular reprogramming of diseased human NP cells to a pro-anabolic phenotype in vitro. These studies also highlight the feasibility of using engineered extracellular vesicles (eEVs) to mediate the delivery of FOXF1 to diseased cells, and their potential to be used as a minimally invasive gene delivery mechanism.

Here we have developed a novel non-viral gene therapy, using eEVs to deliver FOXF1 to the degenerated IVD in an in vivo model. Injured IVDs treated with eEVs loaded with FOXF1 demonstrated robust sex-specific reductions in pain behaviors compared to control groups. Furthermore, significant restoration of IVD structure and function in animals treated with FOXF1 eEVs were observed, with significant increases in disc height, tissue hydration, proteoglycan content, and mechanical properties. This is the first study to successfully restore tissue function while modulating pain behaviors in an animal model of DBP using eEV-based non-viral delivery of transcription factor genes. Such a strategy can be readily translated to other painful musculoskeletal disorders.

Why Are Extraocular Muscles So Resilient to Aging?

Most of us put little thought into the muscles that control the movement of the eye. They just work. Researchers here ask the interesting question: why are these extraocular muscles so resilient? Why does their function decline so little with age, when other muscles throughout the body lose strength and mass, leading ultimately to sarcopenia? There is no complete answer to this question, but it is suggested here that this resilience might have something to do with the fact that the extraocular muscles are much more heavily innervated than other muscles in the body. That in turn might direct a greater focus towards the effects of aging on neuromuscular junctions and consequent loss of innervation in muscles elsewhere in the body. This loss of innervation has been suggested as a contributing cause of sarcopenia.

The extraocular muscles (EOMs) are unique in several aspects: They represent the fastest and most fatigue-resistant muscles within the human body. Extraocular muscles (EOMs) predominantly exhibit impairment in conditions such as myasthenia gravis and mitochondrial myopathies, yet, remarkably, they are spared from various muscular dystrophies, including Duchenne, Becker, limb-girdle, and congenital muscular dystrophies, as well as aging. Furthermore, EOMs demonstrate particular resistance to amyotrophic lateral sclerosis (ALS).

he complexity of the actions performed by the extraocular muscles (EOMs) is reflected in their anatomical and physiological characteristics. Morphologically and in terms of their molecular composition, they significantly differ from the muscle fibers (MFs) of other skeletal muscles. The gene expression profile of the EOMs is distinct from that of limb muscles, with differences encompassing over 330 genes involved in metabolic pathways, structural components, development markers, and regenerative processes. Unlike skeletal muscles, the EOMs predominantly utilize an aerobic pathway for carbohydrate metabolism and relies directly on the glucose influx from the blood. This metabolic strategy enables them to be among the fastest muscles in the body while also being exceptionally resistant to fatigue.

Notably, EOM fibers express a diverse array of myosin heavy-chain isoforms, retaining embryonic forms into adulthood. Moreover, their motor innervation is characterized by a high ratio of nerve fibers to muscle fibers and the presence of unique neuromuscular junctions. These features contribute to the specialized functions of EOMs, including rapid and precise eye movements. Understanding the mechanisms behind the resilience of EOMs to disease and aging may offer insights into potential therapeutic strategies for treating muscular dystrophies and myopathies affecting other skeletal muscles.

Link: https://doi.org/10.3390/ijms25094985

Calorie Restriction Slows the Aging of Stem Cells in Subcutaneous Fat

The practice of calorie restriction is well known to slow aging, though the effects on life span are much larger in short-lived species. In humans calorie restriction is demonstrated to be beneficial to long-term health, certainly on a par with the results obtained from maintenance of physical fitness. Calorie restriction has noteworthy effects on the distribution and biochemistry of fat tissue. Researchers here report that one aspect of this outcome is a slowing of age-related changes in adult stem cells associated with subcutaneous fat.

With advancing age, there is a gradual loss of subcutaneous adipose tissue volume, leading to diminished glucose and lipid uptake. This phenomenon is known as "lipid overflow hypothesis," which results in the ectopic deposition of lipids in muscles and the liver, ultimately contributing to the development of insulin resistance. Long-term calorie restriction (CR) has been found to result in reduced adipocyte size and a beneficial remodeling of body fat composition, shifting away from visceral white adipose tissue towards subcutaneous white adipose tissue. This shift is significant as subcutaneous fat tends to have positive effects on aging and obesity, whereas visceral is associated with detrimental health outcomes.

Adipose-derived stem cells (ASCs) are crucial for tissue regeneration, but aging diminishes their stemness and regeneration potential. Aging is associated with increased adipose tissue fibrosis but no significant change in adipocyte size was observed with age. Long term caloric restriction failed to prevent fibrotic changes but resulted in significant decrease in adipocytes size. Aged subcutaneous ASCs displayed an increased production of reactive oxygen species (ROS). Using mitochondrial membrane activity as an indicator of stem cell quiescence and senescence, we observed a significant decrease in quiescence ASCs with age exclusively in the subcutaneous adipose depot. In addition, aged subcutaneous adipose tissue accumulated more senescent ASCs having defective autophagy activity. However, long-term caloric restriction leads to a reduction in mitochondrial activity in ASCs. Furthermore, caloric restriction prevents the accumulation of senescent cells and helps retain autophagy activity in aging ASCs. These results suggest that caloric restriction and caloric restriction mimetics hold promise as a potential strategy to rejuvenate the stemness of aged ASCs.

Link: https://doi.org/10.18632/aging.205812

A Brief Tour of the Development of Senolytic Therapies to Clear Senescent Cells

Senescent cells accumulate with age as the immune system falters in its ability to clear these cells in a timely fashion. Senolytic therapies selectively destroy some fraction of senescent cells, and first generation senolytic drugs have been demonstrated to rapidly and impressively reverse age-related disease and extend life in mice. The best of these first generation drugs are repurposed cancer therapeutics such as dasatinib and navitoclax, with the jury still out on whether plant extracts like fisetin can be competitive on their own rather than in combination with the chemotherapeutics.

The second generation senolytics presently under development aim to be more selective, have fewer side-effects, require lower or more infrequent doses, or be able to target a greater range of senescent cell types. It is becoming clear that senescence is a varied collection of states, and first generation senolytics are only effective in destroying senescent cells for some of those states, and in some tissues. With this in mind, today's popular science article takes a look at some of the companies and research groups working on a broad range of second generation senolytic treatments. There are promising programs under development; we might expect a much more diverse range of options to exist for patients and self-experimenters a decade from now than is presently the case.

Researchers are using new molecules, engineered immune cells and gene therapy to kill senescent cells and treat age-related diseases

Lurking throughout your body, from your liver to your brain, are zombie-like entities known as senescent cells. They no longer divide or function as they once did, yet they resist death and spew out a noxious brew of biological signals that can slow cognition, increase frailty and weaken the immune system. Worst of all, their numbers increase as you age. For more than a decade, researchers have been trying to see whether they can selectively destroy these cells with a variety of drugs. In a pivotal study published in 2015, a team discovered that a combination of two compounds, called dasatinib and quercetin, killed senescent cells in aged mice. The treatment made the mice less frail, rejuvenated their hearts and boosted their running endurance. The finding opened the door to a new area of medicine called senolytics.

Now, fresh results from animal studies and human clinical trials have added momentum to the field. In mice and monkeys, researchers are using genetic tools to reprogram and kill senescent cells. Others are engineering senolytic immune cells. And about 20 clinical trials are ongoing. Researchers are testing new and repurposed drugs that could have senolytic properties, in the hope of combating age-related conditions, including Alzheimer's disease, pulmonary fibrosis, and chronic kidney disease.

One key strategy in senolytics involves designing drugs that stop senescent cells from resisting apoptosis. Usually, the cells survive by producing anti-death proteins. Blocking these with drugs can force the cells to succumb to death. Unity Biotechnology is at the forefront of this approach. In a recent study, researchers found that senescent cells were more abundant in the retinas of diabetic mice than in those of healthy mice. It was possible, the team predicted, that senescent cells in the blood vessels of the eye play a part in diabetes-related vision loss. The researchers designed a drug, called foselutoclax, which blocks the action of BCL-xL, a key anti-death protein that is abundant in senescent cells. When they injected the drug into the eyes of diabetic mice, it killed senescent cells in the blood vessels supplying the retina, but not healthy cells.

Rather than making senolytics from scratch, some scientists are testing drugs that already exist. In a 2019 study, researchers used dasatinib and quercetin to remove senescent brain cells in a mouse model of Alzheimer's disease. Mice treated with the senolytics had reduced brain inflammation and improved memory compared with animals that were given a placebo. Spurred on by these promising data from mice, researchers last year conducted the first safety trial of the drug combination in people with early stage Alzheimer's disease. The team gave five people dasatinib and quercetin intermittently for three months. The researchers found that the drugs were safe and that dasatinib was present in samples of cerebrospinal fluid, suggesting it could cross into the brain. Quercetin was not detected in brain fluid samples, but researchers suspect that it did reach the brain and was rapidly broken down. The team is now conducting a larger trial to track the cognition of people with and without Alzheimer's disease for nine months after they take a placebo or the drug combination. The results should be released in 2025.

When it comes to killing cells in the body, the immune system could be of help. And some researchers have latched on to the idea of using genetically engineered immune cells called chimeric antigen receptor (CAR) T cells. These can target and kill specific cells on the basis of the molecules they display on their surface. Researchers found that old mice treated with the CAR T cells selective for a marker of senescence had reduced blood-sugar levels - a sign of improved metabolic health - and that the animals ran faster and for longer. But CAR-T-cell therapies are expensive to make. Deciduous Therapeutics is also developing a more affordable approach that harnesses a different kind of immune cell called a natural killer T cell. In 2021, researchers at Deciduous Therapeutics demonstrated the senolytic role of these cells, which naturally become less effective with age. They also found that drugs that can activate the immune cells helped to eliminate senescent cells in the damaged lungs of mice, reducing lung scarring and improving survival. Safety tests will be conducted in dogs and non-human primates later this year, and clinical trials should begin in the next two years.

Other teams such as Oisin Biotechnologies are using gene therapy to kill senescent cells. In this approach, researchers package a gene that encodes a lethal protein called caspase-9 into fatty capsules studded with proteins derived from a virus. In mice and monkeys, the capsules have been found to deliver the gene to cells in the lungs, heart, liver, spleen and kidneys. Healthy cells are spared, because the gene is activated only in senescent cells that have high levels of one of two proteins called p16 and p53. The researchers found that, over four months, a monthly dose of the therapy reduced frailty and cancer rates in old mice without causing harmful side effects.

Senescent T Cells Contribute to Neurodegenerative Conditions

T cell senescence is a noted feature of the aging immune system. T cells mature in the thymus, which atrophies with age. Absent a supply of new T cells, the existing populations are forced into greater cellular replication in order to (a) maintain a steady number of cells, and (b) continue to respond to infection with an expansion of the number of T cells equipped to attack the pathogen. Cellular senescence occurs when a cell reaches the Hayflick limit to replication. With each cell division, telomeres at the ends of chromosomes shorten. When telomeres become too short, a cell either self-destructs or becomes senescent. In some subsets of the immune cell population, almost half of all T cells are senescent in old people.

Senescent cells generate a pro-inflammatory, pro-growth mix of signals, the senescence-associated secretory phenotype (SASP). These cells serve a purpose when present in the short term, but when they linger the SASP becomes highly disruptive to tissue structure and function. As today's open access paper points out, even though T cells are not present in the brain in any great number, their state of senescence does matter. Inflammatory signaling moves throughout the body, and can and does link the distinct immune systems of the body and brain.

With the increasing proportion of the aging population, neurodegenerative diseases have become one of the major health issues in society. Neurodegenerative diseases (NDs), including multiple sclerosis (MS), Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS), are characterized by progressive neurodegeneration associated with aging, leading to a gradual decline in cognitive, emotional, and motor functions in patients. The process of aging is a normal physiological process in human life and is accompanied by the aging of the immune system, which is known as immunosenescence.

T-cells are an important part of the immune system, and their senescence is the main feature of immunosenescence. The appearance of senescent T-cells has been shown to potentially lead to chronic inflammation and tissue damage, with some studies indicating a direct link between T-cell senescence, inflammation, and neuronal damage. The role of these subsets with different functions in NDs is still under debate. A growing body of evidence suggests that in people with a ND, there is a prevalence of CD4+ T-cell subsets exhibiting characteristics that are linked to senescence. This underscores the significance of CD4+ T-cells in NDs. In this review, we summarize the classification and function of CD4+ T-cell subpopulations, the characteristics of CD4+ T-cell senescence, the potential roles of these cells in animal models and human studies of NDs, and therapeutic strategies targeting CD4+ T-cell senescence.

Link: https://doi.org/10.3390/cells13090749

Rare Individuals Exhibit Alzheimer's Pathology but No Symptoms

In a tissue bank of more than 5,000 donated brains, researchers found 12 in which there were signs of Alzheimer's disease pathology but for which the donors had exhibited none of the symptoms of Alzheimer's disease. Here find a report on some of the biochemical differences found in these resilient brains; it is hoped that pursuing this line of research might aid in the understanding of the condition and strategies for the development of effective therapies.

Some individuals show a discrepancy between cognition and the amount of neuropathological changes characteristic for Alzheimer's disease (AD). This phenomenon has been referred to as 'resilience'. The molecular and cellular underpinnings of resilience remain poorly understood. To obtain an unbiased understanding of the molecular changes underlying resilience, we investigated global changes in gene expression in the superior frontal gyrus of a cohort of cognitively and pathologically well-defined AD patients, resilient individuals, and age-matched controls (n = 11-12 per group).

897 genes were significantly altered between AD and control, 1121 between resilient and control and 6 between resilient and AD. Gene set enrichment analysis (GSEA) revealed that the expression of metallothionein (MT) and of genes related to mitochondrial processes was higher in the resilient donors. Weighted gene co-expression network analysis (WGCNA) identified gene modules related to the unfolded protein response, mitochondrial processes and synaptic signaling to be differentially associated with resilience or dementia.

As changes in MT, mitochondria, heat shock proteins, and the unfolded protein response (UPR) were the most pronounced changes in the GSEA and/or WGCNA, immunohistochemistry was used to further validate these processes. MT was significantly increased in astrocytes in resilient individuals. A higher proportion of the mitochondrial gene MT-CO1 was detected outside the cell body versus inside the cell body in the resilient compared to the control group and there were higher levels of heat shock protein 70 (HSP70) and X-box-binding protein 1 spliced (XBP1s), two proteins related to heat shock proteins and the UPR, in the AD donors.

Finally, we show evidence for putative sex-specific alterations in resilience, including gene expression differences related to autophagy in females compared to males. Taken together, these results show possible mechanisms involving MTs, mitochondrial processes, and the UPR by which individuals might maintain cognition despite the presence of AD pathology.

Link: https://doi.org/10.1186/s40478-024-01760-9