Fight Aging!

Chronic, unresolved inflammation in brain tissue is a feature of age-related neurodegenerative conditions, and may even be the most important mechanism in these very complex conditions. The supporting cells of the brain, primarily microglia and astrocytes, become more active and inflammatory in later life. This overlaps with a rising count of senescent cells in these populations. Senescent cells produce an outsized contribution to inflammatory signaling, belying their relatively small numbers compared to non-senescent cells. Active microglia and astrocytes are largely not senescent, however. They are reacting to inflammatory signaling or molecular patterns resulting from cell dysfunction, stress, and death.

Clearing senescent cells from the brain dampens inflammation and pathology in animal models of neurodegeneration. It seems plausible that finding ways to turn off the activation of microglia and astrocytes will be similarly beneficial. In the case of microglia, the entire population can be removed without harm, allowing new non-active microglia to emerge and repopulate the brain. Astrocytes present a harder challenge, however. Given that they make up a sizable fraction of all cell in the brain, clearance really isn't an option. Some form of adjustment or reprogramming of regulatory mechanisms is called for. Fortunately, it may be the case that astrocyte activation is a consequence of microglial activation: further studies of microglial clearance as an approach to therapy may clarify this relationship.

Roles of neuropathology-associated reactive astrocytes: a systematic review

As opposed to being monolithic in function and morphology, astrocytes differ significantly depending on both tissue and cellular localization. While these characterizations have been recognized for some time, functional distinctions have only recently been investigated. Historically, in response to damage, astrocytes have been characterized as adopting a reactive phenotype. In contrast to the typically quiescent state of mature astrocytes, reactive astrocytes can become highly proliferative, and this astrogliosis is the foundation for glial scar formation.

To best support the central nervous system (CNS) in a system that can suffer from a variety of insults, astrocytes have seemingly evolved a diverse reactive response. Reactive phenotypic polarization depends on the nature of the inducing stimuli. Rather than eliciting a single response to CNS injury or insult, astrocyte reactivity is highly heterogenous. Borrowing from the nomenclature used to describe reactive macrophages and microglia, in response to tissue damage and ischemia, astrocytes adopt a neuroprotective A2 phenotype. A2s fit the traditional reactive astrocyte profile and have proliferative functions, resulting in glial scar formation, debris clearance, and blood-brain barrier (BBB) repair. They upregulate neurotrophic factors and pro-synaptic thrombospondins, thereby promoting neuronal growth and supporting synaptic repair. In contrast, neuroinflammation, infection, and aging induces a cytotoxic A1 reactive astrocyte phenotype. Neurotoxic A1 reactive astrocytes are pro-inflammatory and associated with neurodegeneration and chronic neuropathic pain, in addition to a repression of functions related to supporting neuronal survival and synaptogenesis.

The use of A1/A2 nomenclature is not universally accepted, as such a stringent dichotomy fails to accurately represent the diversity within each subset of cells. This system of classification can also give the false impression of reactive states being either entirely "helpful" or "harmful", when in reality these reactive states likely evolved to serve various functional purposes.

The environment of the aging brain can exacerbate inflammatory effects and contribute to gradual neuronal damage. During the course of normal aging, as opposed to age-associated pathologies like Alzheimer's disease, glia cells undergo a variety of physiological and functional changes. In addition to promoting neuroprotective signaling pathways, microglia in an aging brain upregulate expression of immune system response receptors, effectively becoming more sensitive to insults, and increasing production of pro-inflammatory signals. The neuroinflammatory astrocyte response in the brain that arises in advanced age is compounded by inflammation. In the absence of activated microglia cytokine secretion, age-induced astrocyte reactivity is reduced, supporting the role of activated microglia in age-associated A1-like responses.

Targeting or blocking astrocyte polarization may prove to be an effective avenue of symptom management and treatment for a host of neurodegenerative or neuroinflammatory disorders. The selective serotonin reuptake inhibitor (SSRI) Fluoxetine was found to inhibit neurotoxic astrocyte polarization upon inflammatory stimulation both in vitro and in vivo. The increased concentration of A1-associated markers in a chronic mild stress mouse model was rescued with Fluoxetine treatment. Using pharmacological inhibitors and siRNA technology, astrocytic 5HT2BR and downstream β-arrestin2 signaling were identified as the targets of the Fluoxetine-mediated inhibition of A1-like astrocyte polarization. Recently, NLY01, a GLP-1R agonist, has been investigated as a neuroprotective agent in Parkinson's disease, and was found to directly prevent microglia from inducing astrocyte polarization. These studies suggest that both current well-established therapies and those yet to be developed could be of use as neurotoxic reactive astrocyte inhibitors applicable to a wide array of neuropathologies. Continued investigation into the near-ubiquitous pathological roles of these reactive pro-inflammatory A1-like astrocytes will have important implications for how neuropathologies are studied and ultimately treated.

Autophagy is the name given to a complex collection of processes that recycle broken and unwanted proteins and cell structures. Autophagy declines in effectiveness with age, while upregulation of autophagy is a feature of many of the approaches shown to slow aging in laboratory species. The ability of calorie restriction to slow aging appears to depend on autophagy, for example. So far, little meaningful progress has been made towards therapies that can greatly improve on the ability of exercise to improve autophagy, though mTOR inhibitors could be argued to be somewhat better than exercise on this front, given their greater effect on longevity in short-lived mammals. As mentioned, autophagy is complicated, and the paper here is an example of that complexity, diving into what is known as non-canonical autophagy, some of the less well explored interactions taking place during cell maintenance.

Macroautophagy requires the conjugation of members of the ATG8 family, ubiquitin-like proteins including LC3 and GABARAP, to phosphatidylethanolamine (PE). This enables double-membrane vesicles termed autophagosomes to recruit ATG8 proteins, which mediate loading and maturation of cargo. More recently, autophagy-independent functions of ATG8 proteins have been discovered. Several recent studies have highlighted these additional roles of ATG8 proteins leading to alternative fates of their cargo in degradation and secretion, together referred to as non-canonical autophagy (NCA). With age, there is a general decrease in efficiency of degradative autophagy, both canonical and NCA. Additionally, in what is likely a response to age-associated decreased degradation through the lysosome is the shift to "secretory autophagy" (SA), release of material into the extracellular space. However, owing to the overlap of the initial steps of autophagosome formation, SA also decreases with age. Understanding the mechanisms that differentially initiate and regulate NCA will help identify how defects in these pathways contribute to aging and disease.

One of the defining hallmarks of aging is altered intercellular communication, with a prominent example being "inflammaging", or the chronic inflammation that further amplifies the aging process. Growing evidence identifies inflammaging as the driver for NCA in aged microglia. SA has been shown to maintain proteostasis when autophagy is inhibited by blocking fusion with the lysosome in vitro. However, the downstream effect of this is the release of cargo into the extracellular space, and, depending on what was targeted for degradation but is now in the extracellular space, can itself induce an immune response. Hyperactivation of macrophages will lead to increased phagocytosis of the discarded cargo, bringing it back into the cell to attempt to be cleared. However, if the limitation is at the lysosome, the effort is futile and will lead to deposition of aggregated proteins both intracellularly and in the extracellular space. Thus, chronic inflammation seen with aging is a likely driver for aggregation-associated diseases, including many neurodegenerative diseases.

The role of NCA in aging and age-related diseases is still under intense investigation. It is still not clear how cargo is recruited for NCA, whether NCA and canonical autophagy coexist, if differential signals direct the decision to complete canonical versus NCA, and whether the cell has a preference for either type. Alternatively, NCA may only be initiated when canonical autophagy cannot meet cellular requirements, and thus becomes the dominant response for cargo clearance. Furthermore, the molecular pathways and vesicular trafficking in SA are not fully described, but canonical autophagy machinery is required for the initiation. So, if the same machinery is needed, but there are different outcomes, what determines if degradation occurs in the lysosome or if SA is induced? Moreover, with so many pathways to deliver cargo to the lysosomes for degradation, does everything come down to functional lysosomes? This seems to be the case, since the switch from degradation to SA does not solve the overall problem in neurodegenerative diseases.

Link: https://doi.org/10.3389/fcell.2023.1137870

The paper noted here discusses a range of studies assessing the ability of forms of intermittent fasting to improve long-term health and life expectancy. Results are generally positive, but one should expect long-lived mammals to exhibit smaller gains in longevity than are observed in short-lived mammals, following the known outcomes of calorie restriction. Intermittent fasting is not as well studied as the practice of calorie restriction, but does appear to work via a similar set of mechanisms, even when overall calorie intake is not much reduced. Time spent in a state of hunger, and the metabolic changes provoked by hunger, are perhaps the important mechanisms shared by the various forms of dietary restriction.

Intermittent fasting (IF) is an eating pattern in which individuals go extended periods with little or no energy intake after consuming regular food in intervening periods. IF includes alternate-day fasting (ADF), modified fasting (MF), time-restricted fasting (TRF), and fasting-mimicking diet (FMD). Studies showed that IF increases the average lifespan of rats by 14-45% and mice by only 4-27%. Further, dietary restriction increases fatty acid oxidation by maintaining mitochondrial network homeostasis and functional coordination with the peroxisome, thereby promoting longevity.

Clinical studies demonstrated that long-term IF improves cognitive disorders and reduces oxidative stress in middle-aged adults. It also delays the onset of age-related brain damage. Moreover, nutrient-sensing signaling pathways such as the AMPK, SIRT1, mTOR, and insulin/IGF-1 pathways are downregulated during IF, blocking cell proliferation and activating stress factors, thereby negatively regulating various aging signals. IF can also protect the heart from ischemic damage, reduce body mass index and blood lipids, improve glucose tolerance, and reduce the incidence of coronary artery disease by increasing levels of the growth hormone. This in turn increases lipolysis and insulin secretion in addition to reducing other glucose metabolism pathway markers.

Link: https://doi.org/10.1155/2023/4038546

When reading about potential mechanistic links between chronic kidney disease and Alzheimer's disease, it is worth considering klotho. Increased expression of klotho has been shown to improve kidney function and better resist the decline of kidney function with age. It also improves cognitive function, though there is some debate over how this is happening. Klotho largely acts in the kidney, and its effects on cognitive function may simply be a compelling demonstration of the point that dysfunction of the kidneys is harmful to organs throughout the body, including the brain.

The mediating mechanisms linking kidney to brain may be the harms done to the cardiovascular system with loss of kidney function, as the brain is sensitive to vascular issues: lowered blood supply; pressure damage due to hypertension; loss of capillary density; leakage of the blood-brain barrier that wraps blood vessels passing through the central nervous system; and so forth. In today's open access paper, researchers discuss how exactly kidney disease may increase the risk and severity of Alzheimer's disease, but the details, particularly those relating to the vasculature, are relevant to other neurodegenerative conditions.

Pathogenesis of Chronic Kidney Disease Is Closely Bound up with Alzheimer's Disease, Especially via the Renin-Angiotensin System

Chronic kidney disease (CKD) is a clinical syndrome secondary to the definitive change in function and structure of the kidney, which is characterized by its irreversibility and slow and progressive evolution. Alzheimer's disease (AD) is characterized by the extracellular accumulation of misfolded β-amyloid (Aβ) proteins into senile plaques and the formation of neurofibrillary tangles (NFTs) containing hyperphosphorylated tau. In the aging population, CKD and AD are growing problems. CKD patients are prone to cognitive decline and AD. However, the connection between CKD and AD is still unclear.

The available evidence suggests that CKD and AD are pathologically related through the renin-angiotensin system (RAS), uremic toxins, and erythropoietin (EPO), which contribute to the occurrence and development of CKD and may aggravate the development of AD. In CKD, excess renin is released and increases circulating angiotensin II (Ang II) levels, resulting in AT1R upregulation and enhancing systemic vascular resistance, increasing blood pressure, and promoting sodium reabsorption in the proximal tubule and (through aldosterone) the collecting duct. In AD animal models, the cerebroventricular infusion of Ang II into aged normal rats increased both tau pathology and amyloid precursor protein (APP) levels, leading to an increase in amyloid-β (Aβ) accumulation. It was also shown that Ang (1-7) expression in the brain increased with disease progression and that there was an inverse correlation between Ang (1-7) level and tau hyperphosphorylation.

In AD model mice, Ang II not only impaired blood-brain barrier (BBB) function in the cerebral microcirculation but also induced inflammatory and thrombotic phenotypes. The binding of Ang II with AT1R damaged the BBB, leading to its leakage and the entry of circulating toxins into the brain. Additionally, AT2R and MasR promoted an M2 anti-inflammatory phenotype in microglia, which is a potential mechanism for alleviating neuronal dysfunction and inflammation and ultimately, for reversing cognition impairment. Based on the current evidence, we propose that the combination of Ang II and AT1R causes BBB leakage and activates microglia to secrete inflammatory factors that lead to apoptosis, neuronal injury, and neurodegeneration, resulting in the aggravation of AD; the activation of the AT2R/MasR axis produces the opposite physiological effect.

It remains unclear whether RAS imbalance in CKD is a cause of AD and vice versa. The following open questions warrant investigation in future studies: (1) Do CKD patients with AD have more severe imbalances in the RAS than those without AD? (2) What are the most significantly altered components of the RAS in CKD patients with AD, and are these components mainly proinflammatory (ACE/AT1R) or anti-inflammatory (ACE2/AT2R/MasR)? (3) Can the use of ACEI/ARB drugs prevent or delay the occurrence of AD? Answering these questions may provide insights that can guide the development of novel treatments for both diseases.

Loss of sensory hair cells in the inner ear, or loss of the connections between these cells and the brain, drive age-related hearing loss. Researchers here focus on the contribution of mitochondrial dysfunction to this condition, alongside the decline of autophagy in older individuals, leading to poor quality control of mitochondria and consequent loss of function. Many pharmacological approaches exist or are under development to improve autophagy to a degree similar to that resulting from structured exercise programs, but compelling evidence for significantly greater improvements are so far lacking. We can reasonably debate whether or not mTOR inhibitors will represent a meaningful step beyond exercise in humans, when it comes to improved autophagy, but even there the size of the effect is not that much greater in the best case.

Hearing loss is mainly considered a sensory disorder in humans. Multiple factors contribute to the pathogenesis of sensorineural hearing loss (SNHL), such as noise exposure, ototoxic drugs, genetic mutations, aging, and chronic conditions. Histopathological changes of SNHL are characterized by mechanosensory hair cell damage, spiral ganglion neuron (SGN) loss, and stria vascularis atrophy. Emerging studies have suggested that mitochondrial DNA damage, reactive oxygen species (ROS) overproduction, and inflammatory mediators activation are associated with subsequent cochlear damage.

Mitochondria ROS could induce inflammasome activation that promotes various disease progression. Moreover, ROS could also induce cellular defense process such as autophagy, a cytoprotective mechanism that delivers damaged organelles to lysosomes for degradation. Current studies reveal autophagy exhibits an antioxidative capacity to protect against hair cell damage and possesses the potential to alleviate noise-induced hearing loss (NIHL). Autophagy not only clears up undesired proteins and damaged mitochondria (mitophagy), but also eliminate excessive ROS. Appropriate enhancement of autophagy can reduce oxidative stress, inhibit cell apoptosis, and protect auditory cells.

Link: https://doi.org/10.3389/fcell.2023.1119773

Researchers here manipulate levels of menin, a regulator of inflammation in the hypothalamus of mice. Menin expression in the hypothalamus diminishes with age, leading to increased inflammation. Upregulation of menin expression improves health and lifespan, while also improving cognitive function. One might take this as one of many examples of the chronic inflammation that is characteristic of later life being harmful to health and disruptive to tissue function. Given the complexity of regulatory systems in cells, there are many possible approaches to suppress chronic inflammation, but the challenge is to find an approach that only reduces the unwanted, excess inflammation, and doesn't sabotage the inflammatory responses necessary in wound healing and defense against pathogens.

The hypothalamus acts as the arbiter that orchestrates systemic aging through neuroinflammatory signaling. Our recent findings revealed that Menin plays important roles in neuroinflammation and brain development. Here, we found that the hypothalamic Menin signaling diminished in aged mice, which correlates with systemic aging and cognitive deficits. Restoring Menin expression in ventromedial nucleus of hypothalamus (VMH) of aged mice extended lifespan, improved learning and memory, and ameliorated aging biomarkers, while inhibiting Menin in VMH of middle-aged mice induced premature aging and accelerated cognitive decline.

We further found that Menin epigenetically regulates neuroinflammatory and metabolic pathways, including D-serine metabolism. Aging-associated Menin reduction led to impaired D-serine release by VMH-hippocampus neural circuit, while D-serine supplement rescued cognitive decline in aged mice. Collectively, VMH Menin serves as a key regulator of systemic aging and aging-related cognitive decline.

Link: https://doi.org/10.1371/journal.pbio.3002033

The biochemistry of the central nervous system is separated from the biochemistry of the rest of the body by the blood-brain barrier, a specialized lining of cells that wrap blood vessels that pass through the brain. Only some molecules and cells are permitted to pass into and out of the brain. Like all bodily systems, the blood-brain barrier breaks down with age, leading to leakage of unwanted molecules and cells into the brain, where they can provoke inflammation and dysfunction. This is thought to provide a significant contribution to the onset and further progression of age-related neurodegenerative conditions, given that blood-brain barrier failure appears somewhat in advance of other aspects of neurodegeneration in humans and animal models.

In today's open access paper, researchers review what is known of blood-brain barrier dysfunction specifically in the context of Alzheimer's disease. Relationships are observed between blood-brain barrier leakage and mechanisms involved in the production and aggregation of amyloid-β. Despite the failures of amyloid-β clearance to produce meaningful benefits in clinical trials, the build up of amyloid-β is still considered a core process in Alzheimer's disease, a foundational pathology that sets the stage for later, more severe pathology involving inflammation and tau aggregation leading to widespread cell death.

Reconsidering the role of blood-brain barrier in Alzheimer's disease: From delivery to target

The blood-brain barrier (BBB) is a dynamic interface that regulates the cellular communication between neural tissues and the blood and its constituents. It acts as a selective semipermeable barrier that controls the transport of substances to and from the central nervous system, serving as a key player in neural homeostasis. The blood is separated from central nervous system (CNS) by brain endothelial cells separated by tight junctions, adherens junctions, and gap junctions; pericytes; the foot processes of astrocytes, and the basement membrane composed of extracellular matrix components. Two main transport pathways occur within BBB: transcellular via endothelial cell used by the vast majority of the molecules which can be active (dependent on energy) or passive; and paracellular via passive diffusion through tight junctions.

The ubiquity and importance of BBB in CNS physiology also translate to how it is also impaired in almost every neurological condition. Such is the case of the most common cause of dementia, Alzheimer's disease (AD). AD affects more than 30 million people worldwide, a number that is expected to increase dramatically in the foreseeable future. To date, intracellular hyper-phosphorylated tau protein accumulation (neurofibrillary tangles) and extracellular amyloid-β (Aβ) deposition (senile plaques) in brain parenchyma is considered the central neuropathological hallmarks of the disease. However, pathogenesis is still not fully understood, and it is unclear whether these protein abnormalities are causative or rather incidental changes in the disease. Nevertheless, it is generally accepted that both proteins play a key role in disease pathogenesis with Aβ acting upstream of tau with other hypotheses building on and extending this to explain other aspects of the disease.

Aβ deposition seems to be a critical pathological trigger in AD and disruption of BBB leads to increased vascular permeability, allowing the entrance and/or hampering the clearance of toxic molecules that can trigger inflammatory and immune responses and, ultimately, neurodegeneration. One such pathologic protein whose normal clearance is dependent on a healthy BBB is the 42 amino acid Aβ peptide (Aβ42), considered the major toxic Aβ in AD. Not surprisingly, BBB dysfunction leads to Aβ deposition by disrupting its transporters. Moreover, there is experimental evidence that a disrupted BBB promotes its production from the amyloid precursor protein (APP) through the activation of the amyloidogenic pathway where APP is cleaved in sequence by β-secretase and γ-secretase.

Several studies have demonstrated BBB breakdown and dysregulation in AD. Whether it is a cause or consequence of the disease has been a matter of debate. Available evidence points to BBB breakdown as an early event preceding AD pathology. These findings have been supporting the vascular hypothesis of AD. First published in 1993 this hypothesis postulates that neurodegeneration is the consequence of a series of pathogenic pathways originating in blood vessels. More recently, others proposed the two-hit vascular hypothesis of AD. According to this hypothesis, impairment of blood vessels leads to BBB dysfunction and initiates a cascade of events leading to neuronal dysfunction (hit one). BBB dysfunction reduces Aβ clearance and increases its production inducing accumulation of this peptide, amplifying neuronal dysfunction, and accelerating neurodegeneration (hit two).

Blood-brain barrier has been emerging as a central hub for AD pathogenesis, presenting as a potential target to treat AD. Understanding its dysfunctional role in AD pathogenesis would be paramount for AD biology clarification and would probably give insights into other brain disorders. In this review, we will detail pathogenic and therapeutic links between AD and BBB offering a comprehensive and integrative view that includes the genetic landscape of AD and anticipates future research and treatment.

Chronic, unresolved, unprovoked inflammation is a feature of aging, a contributing cause of loss of tissue function and all of the common fatal age-related conditions. The biochemistry involved in the regulation of harmful age-related inflammatory signaling is complex, to say the least. There are many contributing causes, such as the signaling of senescent cells, the mislocalization of mitochondrial DNA resulting from mitochondrial dysfunction, and rising levels of other molecular debris from stressed and dying cells. How cells react in detail to inflammatory stimulation is far from fully understood. Researchers are interested in these mechanisms because it is possible that a better understanding might discover targets for intervention that can suppress only unwanted, excess inflammation.

The classic signs of acute inflammation are redness, heat, pain, swelling, and loss of function. Acute inflammation in the absence of infection can also promote wound regeneration or repair, depending on the severity of the tissue damage. In contrast, the chronic, sterile inflammation that results from repeated immune stimulation over time may be the result of the degeneration of a number of receptors that activate the innate immune system in elderly individuals.

A "generic" inflammatory pathway includes Inducers, Sensors, Mediators and Effectors. An example of this pathway in action would be the stimulation of Sensors, such as the Toll-Like Receptors (TLRs) present on macrophages or mast cells by a microbe (Inducer), leading to the production of cytokines (Mediator), which act on target tissues (Effectors) in order to promote the recruitment of pathogen-destroying cells to the affected area. These actions lead to the signs of inflammation through vasodilation, edema, and the presence of pain-promoting prostaglandins in the affected tissue.

Low-grade inflammation is often observed as part of aging. This phenomenon has been termed "inflammaging". In addition to a general decline in function during aging, the nature of the immune system also changes, in a phenomenon known as immunosenescence. This accounts for the reduced ability of the elderly to respond to antigens and correlates with increased susceptibility to infections. The co-ordination of the many processes that contribute to the effective control of the inflammatory response relating to aging is complicated, and the revelation of the mechanisms underlying this control has only recently begun.

It has been found that the production of the correct inflammatory mediator in a timely manner requires exquisite control at the transcriptional level. Importantly, all eukaryotic transcription takes place in the context of the nucleoprotein complex known as chromatin. In this review, we aim to emphasize the roles of chromatin regulation at the intersection between inflammation, aging, and metabolism to deepen our mechanistic understanding of inflammaging while we discuss the possibility of obtaining control over inflammaging and directions for further studies.

Link: https://doi.org/10.3390/ijms221910274

It is hypothesized that degeneration of neuromuscular junctions is an important contributing cause of the characteristic loss of muscle mass and strength that takes place with age, leading to sarcopenia. Researchers here use a gene therapy to upregulate expression of a gene involved in neuromuscular junction maintenance, and find that it improves muscle function in old mice. The paper includes a fairly detailed discussion of the biochemistry involved, and the researchers consider this an approach that works through similar mechanisms to those involved in the effects of exercise.

Sarcopenia is progressive loss of muscle mass and strength, occurring during normal aging with significant consequences on the quality of life for elderly. Neurotrophin 3 (NT-3) is an important autocrine factor supporting Schwann cell survival and differentiation and stimulating axon regeneration and myelination. NT-3 is involved in the maintenance of neuromuscular junction (NMJ) integrity, restoration of impaired radial growth of muscle fibers through activation of the Akt/mTOR pathway.

We tested the efficacy of NT-3 gene transfer therapy in wild type (WT)-aged C57BL/6 mice, a model for natural aging and sarcopenia. In this study, we used a triple muscle-specific creatine kinase (tMCK) promoter to restrict NT-3 expression to the skeletal muscle and self-complimentary adeno-associated virus serotype 1 (scAAV1) as vector. The treatment efficacy was assessed at 6 months post-injection using run to exhaustion and rotarod tests, in vivo muscle contractility assay, and histopathological studies of the peripheral nervous system, including NMJ connectivity and muscle.

NT-3 gene therapy in WT-aged C57BL/6 mice resulted in functional and in vivo muscle physiology improvements, supported by quantitative histology from muscle, peripheral nerves, and NMJ. Hindlimb and forelimb muscles in the untreated cohort showed the presence of a muscle- and sex-dependent remodeling and fiber size decrease with aging, which was normalized toward values obtained from 10 months old WT mice with treatment. Considering the cost and quality of life to the individual, we believe our study has important implications for management of age-related sarcopenia.

Link: https://doi.org/10.18632/aging.204577

Senescent cells accumulate throughout the body with advancing age. Somatic cells become senescent on reaching the Hayflick limit constantly throughout life, then quickly self-destruct or are destroyed by the immune system. As the immune system ages, however, it becomes less able to remove senescent cells in a timely fashion, and their numbers grow. Senescent cells are prolific generators of inflammatory signaling, and this activity is a major contribution to the chronic, unresolved inflammation that characterizes aged tissues. This inflammation is disruptive to tissue structure and function, changing cell behavior for the worse.

Many age-related diseases are characterized by inflammatory signaling and its detrimental effects. Periodontitis, inflammatory gum disease, can occur at any age given sufficient inattention to oral hygiene, but it is more prevalent in older people. One might expect this to be the case, given the increased inflammatory signaling present in an aged body. We might also ask whether the activities of senescent cells are involved in the pathology of gum disease, and in today's open access paper, researchers provide evidence to suggest that this is in fact the case.

Cellular senescence with SASP in periodontal ligament cells triggers inflammation in aging periodontal tissue

Periodontitis is a chronic inflammatory disease characterized by periodontal tissue destruction with loss of tooth-supportive bone. It is thought to be the most common infectious disease and affects more than 40% of people aged over 30 years. Colonization of dental biofilm involving periodontopathic bacteria triggers inflammation and excessive immune responses that exacerbate breakdown of periodontal tissue. In addition to bactericidal pathogens, various environmental factors affect the pathology and progression of periodontal disease. In particular, aging has been recognized as a major risk factor that affects the onset and severity of periodontitis. Thus, understanding the biological mechanisms that regulate periodontal tissue and health by aging is an urgent issue to establish preventive protocols or specialized therapies for elderly persons in the field of periodontal medicine.

Cellular senescence is a major hallmark of senescence in organs and the whole body. Accumulated senescent cells in aged organs and tissues induce senescence of the body. A large number of studies have indicated that senescent cells secrete various proteins such as proinflammatory cytokines, chemokines, growth factors, and metalloproteinases, termed SASP (senescence-associated secretory phenotype). Therefore, understanding cellular senescence is required to develop more effective therapies and prevention protocols for age-dependent, lifestyle-related diseases. However, whether and how cell types within periodontal tissue undergo cellular senescence with SASP have not yet been clarified.

Recently, it has become evident that cellular senescence is a cause of chronic diseases through production of the SASP. In this study, we examined the pathological roles of cellular senescence in periodontitis. We found localization of senescent cells in periodontal tissue, particularly the periodontal ligament (PDL), in aged mice. Senescent human PDL (HPDL) cells showed irreversible cell cycle arrest and SASP-like phenotypes in vitro. Additionally, we observed age-dependent upregulation of miR-34a in HPDL cells. These results suggest that chronic periodontitis is mediated by senescent PDL cells that exacerbate inflammation and destruction of periodontal tissues through production of SASP proteins. Thus, miR-34a and senescent PDL cells might be promising therapeutic targets for periodontitis in elderly people.

In a recent interview, George Church offers opinions on partial reprogramming as an approach to rejuvenation. In the last few years this has moved from popular topic to becoming a sizable fraction of the longevity industry, given the large-scale funding that is now devoted to partial reprogramming groups. Short-term exposure to the Yamanaka factors can be used to reset the epigenetic patterns of a cell in old tissue to be more like those of a cell in young tissue, with corresponding gains in function. There are potentially serious issues to be worked out, such as how to eliminate the possibility of cancer due to the few cells that might fully reprogram into pluripotency in a short time, but this is nonetheless an exciting area of medical science that is now heavily funded. We should expect to see significant progress in the years ahead.

Do we understand how cellular reprogramming improves health and longevity?

There have been two major camps in aging since long ago. One says that aging happens due to damage, to proteins, lipids, RNA, and DNA, and that you have to go in there with your repair kit and fix it as a therapist. The other camp says that it's all epigenetic, and that if you convince the cell that it's young, it will get its own toolkit out and start repairing as much as it can. Some things are beyond repair. If you delete all copies of a tumor suppressor, that's not something a young cell can repair. But most things are fixable with epigenetics - at least, that's how the second hypothesis goes.

I believe in a hybrid model. I think most of the work can be done epigenetically. A surprising amount of it can be done via the bloodstream, but probably not all of it. Then, there's a residual amount that you can fix with the Yamanaka factors and another residual amount that you can fix by restoring genes. Since we do the epigenetic reprogramming by adding in genes, it's not that fundamental a difference between adding in genes that will go into the blood, adding genes that will reprogram the nucleus, and adding genes that are missing, like tumor suppressors. In a certain sense, they are all addressable by multiplex gene therapy. That's why being able to either use multiple rounds of dosing or to have bigger vectors will become increasingly important.

Given the rising popularity of partial reprogramming, what is its overall place in the longevity landscape?

I think there are subtle but important differences between anti-aging drugs and drugs that improve biomarkers in the way that statins improve cholesterol. That doesn't mean such drugs increase longevity, just that they improve this one biochemical. It could actually hurt you; for instance, it could improve cardiovascular chances for some subset of the population, but for another subset, it could hasten muscle pain. So, affecting biomarkers is one thing. Reversing diseases of aging is different. You could do it just by addressing that particular disease, or you could do it more broadly, affecting multiple diseases. You might get FDA approval for one of them, but it's actually affecting multiple ones, and maybe acting preventatively. Say, there might be a cure for muscle wasting that helps prevent a variety of diseases. Finally, you're really at the core of aging when you reprogram shared elements - with good feedback systems that already exist in the body or with feedback systems that you introduce as part of the therapy.

Are you bullish about longevity biotech?

I think the whole field is very healthy economically and scientifically. We have passed through multiple "valleys of death". We're now in the solid science phase, and this field is going to be very impactful, maybe more impactful than any other pharmaceuticals in history, including even antibiotics, because our very ability to fight off diseases is age-related. Almost every single form of human morbidity and mortality has an age-related component to it. If you want to have a pleiotropic effect on many different diseases, this is the way to go.

Link: https://www.lifespan.io/news/prof-george-church-on-cellular-reprogramming-and-longevity/

The end of life is not pretty. The body is a failing machine of many complex essential parts, and the failures cascade and feed into one another as it breaks down. There is pain, loss of capacity, loss of the self as the brain runs down. There is a tendency to paper over the ugly reality in public discussion, to not talk about the facts of the matter, even when we all know people who have suffered a slow and painful decline. That the slow progression towards death by aging is an ugly reality, a horrible experience in its final stages, just adds to the reasons why far more effort should go towards the development of rejuvenation therapies capable of preventing the age-related failure of our bodies and brains.

The prevalence of heart failure is increasing in aging populations. Furthermore, dementia is more prevalent in patients with heart failure. Dementia is detected 10 years earlier in patients with heart failure in Asian countries, particularly low-income countries, than in Western countries. Heart failure and dementia share similar cardiovascular risk factors, such as age, hypertension, diabetes, dyslipidemia, and increased arterial stiffness, which explains their overlap in elderly patients.

In a large database of patients with heart failure (mean age 75.3 years), 11.0% developed dementia during an average follow-up period of 4.1 years. Heart failure patients with dementia were at a 4.5-fold higher risk of all-cause mortality, 5.4-fold higher risk of cardiovascular death, and 3.8-fold higher risk of noncardiovascular death. An analysis of a longitudinal dataset indicated a causal relationship between decreased cardiac function and cognitive decline.

Heart failure patients with dementia often have sarcopenia and cachexia. Nutritional changes caused by hypoperfusion and edema in skeletal muscles, the intestines, and visceral organs may contribute to the risk of cardiovascular and noncardiovascular mortality. Autonomic nerve dysregulation has been detected in some patients with dementia, particularly those with Lewy body disease. Elderly patients with heart failure are also more susceptible to infections owing to a decreased lymphocyte count.

Heart failure patients that develop dementia enter a vicious cycle of heart failure, dementia, malnutrition, sarcopenia, and cachexia, which is associated with an increased risk of all-cause mortality. Therefore, the prevention of cognitive decline is important in elderly patients with heart failure.

Link: https://doi.org/10.1016/j.jacasi.2022.10.009

Use of senolytics to clear a sizable fraction of senescent cells in the tissues of aged mice results in a reversal of many aspects of aging, including muscle aging. Muscle tissue provides a good example of the present state of play regarding the detailed understanding of senescent cells in specific tissues, in that it remains challenging to definitively identify the senescent populations in muscle. This is particularly the case following injury, when a transient, short-lived population of senescent cells are expected to arise to aid in the regenerative process, but common markers such as senescence-associated β-galactosidase are shared with the innate immune cells that also participate in regeneration.

In older individuals, the lingering senescent cells present prior to injury, and the reduced ability of the immune system to rapidly remove senescent cells created in response to injury, once their job is done, results in impaired wound healing. Senolytic drugs can improve this situation, and it seems more than clear that there is enough evidence to run clinical trials targeting frailty, non-healing wounds, and to improve healing in the elderly. The present regulatory system makes it harder to proceed in absence of a complete mechanistic understanding of the processes taking place in response to therapy, however. Thus given the demonstrated effectiveness of senolytic therapies in animal studies, there is considerable interest in better understanding exactly what is going on. Given progress on that front, expect to see a sizable expansion of senolytic research and development programs beyond those already underway.

Senolytics improve muscle adaptation in old mice

Recently, our lab and others have utilized senolytics to examine the contribution of senescent cells to impaired muscle adaptability with age, including regeneration following injury and the anabolic response to mechanical overload, as well as any potential role in sarcopenia. There is little evidence that senescent cells are present in aged muscle causing sarcopenia, but they appear to contribute to the impaired ability of muscle to adapt to exogenous stimuli. Even so, systemic deletion of senescent cells improves physical function and has been implicated in slowing sarcopenia. This apparent disconnect between a lack of senescent cells in muscle and improvements in age-associated conditions could be due to various technical or biological reasons.

Work from our lab shows that seven days following muscle injury, there are approximately 250 times more senescence-associated β-galactosidase positive (SA β-Gal+) cells in injured muscle compared to uninjured in both young and old mice. In young mice, these numbers return close to baseline after 28 days, however, the senescent cell burden remains elevated in old mice. Treating mice with a senolytic cocktail of dasatinib and quercetin (D+Q) lowers the senescence burden in old mice, while subsequently reducing the inflammatory profile of the muscle and improving the regenerative response.

β-Gal+ cells appear to transition to senescence in muscle from old mice, developing a senescence-associated secretory phenotype (SASP) relative to cells isolated from young mice 14 days post injury. Some of these findings were recently confirmed by other researchers, who show a greater abundance of senescent cells early in the regenerative process that is reduced over time and greater in old versus young mice. They also show a reduction in senescent cell abundance in response to D+Q treatment following injury, associated with improved regeneration and improvements in muscle force production.

It is important to note that our work shows young mice treated with D+Q display an attenuated regenerative response, whereas others shows an improvement in the regenerative response of young mice as early as 7 days post injury. Considering senescence has been shown to be required for the full regenerative response in skeletal muscle, the large overlap in phenotype between β-Gal+ non-senescent macrophages and bona fide senescent cells likely contributes to the confusion and warrants further investigation.

In a model of muscle hypertrophy, old mice display a blunted hypertrophic response relative to young mice, which is accompanied by a greater senescent cell burden. Treatment with D+Q improves the hypertrophic response in old mice, in addition to lowering the abundance of senescent cells. In this model, we did not observe any change in many of the SASP genes, although genes that are crucial for extracellular matrix reorganization, along with genes that negatively regulate myostatin, were elevated. In summary, senolytics effectively lower the protracted senescent cell burden that accompanies a regenerative or hypertrophic stimulus in muscle from aged mice, resulting in increased muscle fiber size.

The lingering senescent cells characteristic of old tissues contribute to the pathology of osteoathritis via their constant disruptive inflammatory signaling, the senescence-associated secretory phenotype (SASP). Researchers recently reported that the use of antioxidant ceria nanoparticles can reduce the SASP in joint tissue, acting on important regulators that control the generation of the SASP. In this context, it is worth noting that both human and animal evidence suggests that local clearance of senescent cells or local inhibition of SASP in the joint is not sufficient to turn back osteoathritis. Senescent cells elsewhere in the body may be more distant, but there are a lot more of them, and they all generate pro-inflammatory signaling that impacts the joint environment.

Accumulation of senescent cells is the prominent risk factor for osteoarthritis (OA), accelerating the progression of OA through a senescence-associated secretory phenotype (SASP). Recent studies emphasized the existence of senescent synoviocytes in OA and the therapeutic effect of removing senescent synoviocytes. Ceria nanoparticles (CeNP) have exhibited therapeutic effects in multiple age-related diseases due to their unique capability of reactive oxygen species (ROS) scavenging. However, the role of CeNP in OA remains unknown.

Our results revealed that CeNP could inhibit the expression of senescence and SASP biomarkers in multiple passaged and hydrogen-peroxide-treated synoviocytes by removing ROS. In vivo, the concentration of ROS in the synovial tissue was remarkably suppressed after the intra-articular injection of CeNP. Likewise, CeNP reduced the expression of senescence and SASP biomarkers as determined by immunohistochemistry analysis. The mechanistic study showed that CeNP inactivated the NFκB pathway in senescent synoviocytes. Finally, safranin O-fast green staining showed milder destruction of articular cartilage in the CeNP-treated group compared with the OA group.

Overall, our study suggested that CeNP attenuated senescence and protected cartilage from degeneration via scavenging ROS and inactivating the NFκB signaling pathway. This study has potentially significant implications in the field of OA as it provides a novel strategy for OA treatment.

Link: https://doi.org/10.3390/ijms24055056

The signaling produced by transplanted mesenchymal stem cells is well known to reduce the chronic inflammation that accompanies aging. This is a temporary effect, as the transplanted cells near all die rather than engraft, but it can lead to lasting improvement should the respite allow tissues to better maintain themselves for a time. Chronic inflammation is highly disruptive to tissue function, and drives the onset and progression of many age-related conditions. It is thus an important target for interventions aiming to reduce the burden of aging. Here, researchers show that mesenchymal stem cell therapy can improve neurogenesis and cognitive function in old mice, a good example of the way in which inflammation is relevant to degenerative aging.

Age-related decline in cognitive functions is associated with reduced hippocampal neurogenesis caused by changes in the systemic inflammatory milieu. Mesenchymal stem cells (MSC) are known for their immunomodulatory properties. Accordingly, MSC are a leading candidate for cell therapy and can be applied to alleviate inflammatory diseases as well as aging frailty via systemic delivery.

Akin to immune cells, MSC can also polarize into pro-inflammatory MSC (MSC1) and anti-inflammatory MSC (MSC2) following activation of Toll-like receptor 4 (TLR4) and TLR3, respectively. In the present study, we apply pituitary adenylate cyclase-activating peptide (PACAP) to polarize bone-marrow-derived MSC towards an MSC2 phenotype. Indeed, we found that polarized anti-inflammatory MSC were able to reduce the plasma levels of aging related chemokines in aged mice (18-months old) and increased hippocampal neurogenesis following systemic administration.

Similarly, aged mice treated with polarized MSC displayed improved cognitive function in the Morris water maze and Y-maze assays compared with vehicle- and naïve-MSC-treated mice. Changes in neurogenesis and Y-maze performance were negatively and significantly correlated with sICAM, CCL2, and CCL12 serum levels. We conclude that polarized PACAP-treated MSC present anti-inflammatory properties that can mitigate age-related changes in the systemic inflammatory milieu and, as a result, ameliorate age related cognitive decline.

Link: https://doi.org/10.3390/ijms24054490