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- Reactive Astrocytes in Neurodegenerative Conditions
- So We Have Hallmarks of Aging: What Now?
- Aggrephagy in Hematopoietic Stem Cell Aging
- Towards Thymus Organoids Made From Induced Pluripotent Stem Cells
- Modeling a Cellular Cascade of Alzheimer's Disease
- Reviewing the Benefits of Intermittent Fasting
- Non-Canonical Autophagy in Aging
- TDP-43 Aggregation Leads to Loss of Stathmin-2 Expression and Inability of Neurons to Regenerate Axons
- Age-Related Impairment of Angiogenesis Impacts Proficient Regeneration in Zebrafish
- The Tradeoff of Working with Short-Lived Laboratory Species
- Glucagon-like Peptide-1 Receptor Agonists as an Approach to Modestly Slow Aging
- VGLL3 as an Important Regulator of Fibrosis
- Complicating the Relationship Between Cellular Senescence and Late Life Depression
- MANF Upregulation in Macrophages Improves Muscle Regeneration in Old Mice
- Cellular Senescence in Idiopathic Pulmonary Fibrosis
Reactive Astrocytes in Neurodegenerative Conditions
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 that will be 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.
So We Have Hallmarks of Aging: What Now?
The influential hallmarks of aging paper is now nearly ten years old. It has been twenty years since the Strategies for Engineered Negligible Senescence (SENS) categorization of causative mechanisms of aging was first put forward, an effort that inspired the hallmarks. Time moves on relentlessly! Are you feeling old yet? Unlike SENS, the hallmarks of aging made no attempt to be a to-do list of research and development approaches that we should be undertaking in order to effectively treat aging. They are, as it says on the label, hallmarks, observations of old cells and tissues. Nonetheless, a to-do list is somewhat the way in which the hallmarks have been taken in much of the research community, for better or worse. It is good that more of that community is on board with the treatment of aging as a goal to be achieved, but on the other hand some of the hallmarks are clearly far downstream from the root causes of aging, and thus probably poor targets for intervention.
Given the existence and popularity of the hallmarks of aging, far more cited and discussed than SENS ever was, what next? One might hope that today's open access paper illustrates something of the shape of what is next: that researchers talk less about the hallmarks of aging, and instead talk more about the approaches that might reverse aging, producing rejuvenation. The paper is something of a grab bag of presently popular strategies, with epigenetic reprogramming and senescent cell clearance leading the way. It offers only a partial coverage of the field, paying far too little attention to clearance of protein aggregates and lipofuscin, for example. Nonetheless, if more people thought this way, we might see faster progress towards the effective treatment of aging.
Cellular rejuvenation: molecular mechanisms and potential therapeutic interventions for diseases
For decades, one of the dominant theories in ageing research has been that ageing results from the accumulation of DNA changes, mainly genetic mutations, which prevent more and more genes from functioning properly over time. These malfunctions, in turn, can cause cells to lose their properties, leading to the breakdown of tissues and organs and ultimately to ageing and diseases. However, the emerging evidence claims that epigenetic information loss over time is the major cause of mammalian ageing, and epigenetic regulation can restore youthful gene expression patterns. For epigenetic rejuvenation, developing safe and stable strategies that modulate the epigenetic landscape of aged cells to a primitive state are important for cells to exert rejuvenating effects without cancer risk. Furthermore, systematic comparisons of epigenetic dynamics during ageing and partial reprogramming will contribute to identifying key checkpoints for reversing the ageing process and inform the design of potential intervention strategies for ageing-related diseases.
Pathological accumulation of senescent cells (SCs) is also associated with ageing and a range of diseases, and SCs may be potential pharmacological targets for delaying the ageing process. In respect of targeting SCs, there are still many potential markers, like chromatin dynamics and transcriptional signaling, and pharmacological interventions deserving exploitation, to effectively regulate the secretory phenotype of SCs. SC elimination and senescence-associated secretory phenotype inhibition have shown some efficacy in clinical studies of treating functional degeneration and chronic diseases in ageing.
Targeting the cell microenvironment and systemic signals makes sense for tissue-specific cell and organism rejuvenation. Stem cells play a crucial role in maintaining tissue homeostasis, and cell microenvironments also regulate stem cell behavior, which together form a regenerative unit. External signals from the ageing microenvironment appear to dominate the intrinsic function of young stem cells. In contrast, signals from the young microenvironment may have a limited effect on the regeneration of aged stem cells. It would be interesting to identify the genes or pathways that make aged stem cells insensitive to external signals in young microenvironment.
Although many clinical trials registered for stem cell treatments, an effective and safe stem cell therapy to slow or reverse tissue ageing has not yet been identified. Several obstacles still need to be overcome, including proper differentiation and integration of cells in tissues, maintenance of the youth of stem cells and their progeny in ageing tissues, and prevention of tumorigenesis. It will be important to determine the specific mechanisms, which have the potential to provide better treatment pathways - using stem cell transplantation or utilizing endogenous stem cell banks. Recent advances in single-cell transcriptomics and pedigree tracing techniques provide a systematic understanding of stem cell ageing mechanisms. The systematic identification of gene networks, involved in functional changes, age-dependent changes in RNA and protein and metabolite molecules, and cellular interactions, will contribute to further studies on stem cells in tissue repair and ageing-related diseases.
Despite the great progress in cellular rejuvenation, the potential limitations have led to cellular rejuvenation rarely being tested in human studies. Cellular rejuvenation for reversing ageing and age-related diseases as well as cancers has been extensively studied. While cellular rejuvenation holds great promise, key questions remain to be addressed.
(1) Cellular reprogramming strategy can reverse age-related physiological changes and promote tissue regeneration by resetting the epigenetic clock and changing cell fate, but the problems such as relatively low reprogramming efficiency and potential safety concerns, remain the obstacle in the path of its application.
(2) The pharmacological delivery system is difficult to express the pluripotency factors with high efficacy. The toxicity might be induced by drug combinations, then reducing the effectiveness of the cocktail and causing side effects in normal cells.
(3) Clearance of SCs and decreasing SASP exert a beneficial effect on organ repair and disease treatment, but poor cell selectivity of senolytics may result in the damage of normal tissue, and SASP inhibitors targeting specific secretory factors also have limited therapeutic effects on multiple factors-mediated diseases. Besides, completely senescence-specific markers are still absent.
(4) Stem cell therapy as a rejuvenative strategy holds great promise in the reversal of ageing and alleviation of diseases. Despite the advances in many clinical trials of stem cell therapy, optimizing in vitro culture environment, improving the delivery system of stem cells, and reducing immune rejection are still the major challenges to obtain high-quality stem cells and enhance that therapeutic effect.
(5) Restoring defective intercellular communications by the inhibition of inflammation can rejuvenate ageing-impaired changes, but long-term inflammation inhibition may lead to immunosuppression.
Collectively, cellular rejuvenation holds great promise for preventing and treating ageing-related diseases from different dimensions. A healthy and rejuvenated state of the organism can maintain stable characteristics and biological functions without excessive ageing-related degeneration or deterioration. The development of various cellular rejuvenation strategies now provides compelling evidences that the ageing process is not irreversible. Particularly, the effectiveness of stem cell therapy and dietary restriction has been tested in the real world, yielding to desired results. Hopefully, there is a great possibility of translation of these rejuvenation strategies to address human ageing, age-associated diseases, and cancers. Therefore, it is reasonable to expect that clinical rejuvenation approaches to treat ageing-related diseases and even to reverse ageing will be boomed within the next two or three decades.
Aggrephagy in Hematopoietic Stem Cell Aging
Autophagy is the name given to a complex, varied set of processes that tag and recycle broken or excess proteins and structures in the cell. The destination for materials to be recycled is the lysosome, a membrane-wrapped collection of enzymes capable of breaking down near all of the proteins and other molecules a cell is likely to encounter. How materials are selected and how exactly they make their way to the lysosome varies considerably. Alongside autophagy, the ubiquitin-proteasome system is another way for cells to identify problem proteins, such as those that misfold into toxic configurations, and then break them down into their component parts for reuse.
In short-lived species, improvement in autophagy or improvement in proteasomal degradation produces a slowing of aging. More effective cellular housekeeping implies a lower burden of damage inside cells, fewer downstream issues resulting from that damage, and thus better cell and tissue function. Today's open access paper is one of many examples of researchers probing the complexities of cell maintenance, asking why some stem cell populations appear to undertake far too little proteasomal activity in order to clear out broken proteins. The authors found that these cells instead rely on a form of autophagy targeting the protein aggregates that can form as a result of misfolding.
All of these housekeeping processes decline in effectiveness with advancing age, and it is possible that ways to at least modestly slow the aging process can be found by improving cellular housekeeping in the stem cell populations responsible for supporting tissues by producing a consistent supply of new somatic cells. As noted here, the details and many possible targets for intervention are likely to be quite different from cell type to cell type. This encourages more holistic approaches such as partial reprogramming rather than going target by target in search of ways to manipulate the regulation of specific aspects of autophagy and proteasomal function.
Hematopoietic stem cells preferentially traffic misfolded proteins to aggresomes and depend on aggrephagy to maintain protein homeostasis
Maintenance of protein homeostasis (proteostasis) has emerged as fundamentally and preferentially important for stem cells. Proteostasis disruption impairs stem cell self-renewal, which contributes to poor ex vivo expansion and is associated with degenerative disorders, cancer predisposition syndromes, and age-related pathologies in vivo. To maintain proteostasis, cells employ a network of pathways to balance protein synthesis, folding, trafficking, and degradation. Despite being highly conserved, the proteostasis network can be specifically configured to support stem cell fitness and longevity.
Stem cells exhibit and depend on unusually low protein synthesis rates compared with restricted progenitors. Modest increases in protein synthesis disrupt stem cell proteostasis and impair self-renewal by increasing the biogenesis of misfolded proteins, but similar changes minimally impact progenitors. Similarly, activation of the unfolded protein response (UPR) has dichotomous effects in stem and progenitor cells. UPR activation safeguards the integrity of the stem cell pool by preferentially inducing apoptosis in stressed stem cells, whereas it typically promotes an adaptive response in progenitors.
In embryonic stem cells, high proteasome activity can provide the capacity for substantial proteostasis buffering by degrading and preventing the accumulation of misfolded proteins. In contrast, proteasome activity is low within some stem cells such as neural stem cells and hematopoietic stem cells (HSCs). This raises a fundamental paradox: if somatic stem cells are highly dependent on proteostasis maintenance, why do they have such limited proteasome capacity to degrade misfolded proteins?
Here, we show that in contrast to most cells that primarily utilize the proteasome to degrade misfolded proteins, HSCs preferentially traffic misfolded proteins to aggresomes in a Bag3-dependent manner and depend on aggrephagy, a selective form of autophagy, to maintain proteostasis in vivo. When autophagy is disabled, HSCs compensate by increasing proteasome activity, but proteostasis is ultimately disrupted as protein aggregates accumulate and HSC function is impaired. Bag3-deficiency blunts aggresome formation in HSCs, resulting in protein aggregate accumulation, myeloid-biased differentiation, and diminished self-renewal activity. Furthermore, HSC aging is associated with a severe loss of aggresomes and reduced autophagic flux. Protein degradation pathways are thus specifically configured in young adult HSCs to preserve proteostasis and fitness but become dysregulated during aging.
Towards Thymus Organoids Made From Induced Pluripotent Stem Cells
The adaptive immune system depends upon the thymus. Thymocyte cells are generated in the bone marrow and then migrate to the thymus, where they mature into T cells through a complex process of training and selection. The thymus is largest during development, up until the end of childhood. At that point it shrinks dramatically, and then the remainder undergoes a slow atrophy over the rest of a lifespan. In older people, the much reduced volume of active thymic tissue diminishes the supply of new T cells, leading to an adaptive immune system increasingly made up of broken, misconfigured, exhausted, and senescent cells.
Finding ways to regrow the thymus is an ongoing endeavor, a number of companies taking a variety of approaches. Some are looking for small molecules to trigger regulatory genes governing thymic activity; some intend to deliver cells that home to the thymus and encourage new growth; gene therapies have been explored, involving a search for ways to target delivery systems to the thymus; and researchers are investigating the construction of new thymic tissue for transplant. You may be familiar with the work of Lygenesis and associated scientists in building thymus organoids that can be transplanted into lymph nodes.
Today's open access paper is an example of this last sort of work, focused on being able to build thymic organoids from induced pluripotent stem cells. This leads to the possibility of universal thymic tissues, built from cell lines engineered to prevent graft rejection, that could be transplanted into any patient. Or the more costly option of patient-matched thymic tissue, grown from their own cells. Clearly there is much more work to be done in order to build an artificial thymus that matches the natural thymus in structure and function, but the progress to date is promising.
Generation of functional thymic organoids from human pluripotent stem cells
The thymus is required for the development of a functional adaptive immune system, facilitating the generation of self-tolerant T cells that can respond to foreign antigens. Thymic epithelial cells (TECs) are divided into cortical and medullary (c/m) TECs, based on their location and function. Age-related involution of the thymus results in decreased thymic function and naive T cell output and increased autoimmunity and disease risk.
Thymic organoids cultured at the air-liquid interface allow for the interrogation of thymic function and T cell development. Functional human reaggregated thymic organoid cultures (RTOCs) made with expanded 1° TECs and thymic mesenchyme (TM) combined with allogenic cord blood-derived hematopoietic stem cells (HSCs) support T cell development in vitro and in vivo. However, RTOCs depend on 1° tissue access, are allogeneic, and do not support negative selection.
Recently, we reported the directed differentiation of induced PSCs (iPSC) to functional thymic epithelial progenitors (TEPs) that support murine T cell development after transplantation in nude mice. While differentiation of TEPs from human iPSCs has been demonstrated by multiple groups, in vitro generation of functional TECs has yet to be achieved. We sought to develop a thymic organoid model with isogenic hPSC-derived cell compartments that supports patient-specific TEC and T cell development in vitro.
We combined hPSC-derived TEPs, hematopoietic progenitor cells (HPCs), and mesenchymal cells to generate functional isogenic stem cell-derived thymic organoids (sTOs). sTOs support TEC development as demonstrated by HLA-DR, CD205, KRT5, and autoimmune regulator (AIRE) expression after 2-4 weeks in vitro, even in the absence of HPCs. AIRE, HLA-DR, and tissue-restricted antigen (TRA) expression suggests the potential for negative selection in this system. Importantly, sTOs support T cell development, including some regulatory T cells (Tregs). For the first time, to our knowledge, we demonstrate the generation of functional hPSC-derived TECs in vitro.
Modeling a Cellular Cascade of Alzheimer's Disease
Alzheimer's disease is complex and puzzling, and massively funded, high-profile efforts to find treatments for the condition have been failing for decades. The research community has focused on clearance of amyloid-β, as this protein accumulates and misfolds in Alzheimer's patients. Yet some old individuals exhibit high levels of amyloid-β and do not suffer Alzheimer's, while clearance of extracellular amyloid-β fails to meaningfully improve the condition of patients. It may be that intracellular amyloid-β is the real target, or that amyloid-β accumulation is a side-effect of the real pathological mechanisms.
Of late, more attention is being given to overly active or senescent glial cells in the brain and their contribution to rising levels of inflammation. Chronic inflammation may well turn out to be the most important mechanism in Alzheimer's disease, and thus senolytic therapies to clear senescent cells and their inflammatory secretions may turn out to be quite effective as a treatment. We'll find out whether this is the case in the years ahead.
Today's open access paper delves into post-mortem human brain tissue in order to model the cascade of changing glial cell population characteristics. The data is supportive of a focus on glial cells and their contribution to inflammation. That activation of the immune system may be the cause of increased amounts of amyloid-β in its role as an antimicrobial peptide, a part of the innate immune response. At the end of the day, the only really compelling data in the context of Alzheimer's disease is a narrowly focused treatment that produces a reversal of pathology: that would settle the debate over which of the many possibilities is the most important pathological mechanism.
Cellular dynamics across aged human brains uncover a multicellular cascade leading to Alzheimer's disease
Alzheimer's Disease (AD) is a progressive neurodegenerative disease seen with advancing age. Recent studies have revealed diverse AD-associated cell states, yet when and how they impact the causal chain leading to AD remains unknown. To reconstruct the dynamics of the brain's cellular environment along the disease cascade and to distinguish between AD and aging effects, we built a comprehensive cell atlas of the aged prefrontal cortex from 1.64 million single-nucleus RNA-seq profiles. We associated glial, vascular, and neuronal subpopulations with AD-related traits for 424 aging individuals, and aligned them along the disease cascade using causal modeling. We found two predicted trajectories in the cellular landscape, termed (a) progression of AD (prAD) and (b) Alternative Brain Aging (ABA).
At the subpopulation level, microglial nuclei profiles were partitioned into 16 subpopulations, including proliferative (Mic.1), surveilling (Mic.2-5; expressing CX3CR1), reacting (Mic.6-8; TMEM163), enhanced-redox (Mic.9-10; FLT1), stress response (Mic.11; NLRP1, TGFBR1, upregulating genes of heat response, cellular senescence and NLRP1 inflammasome), interferon response (Mic.14, IFI6), inflammatory (Mic.15; CCL3/CCL4, NFKB1, NLRP3), SERPINE1 expressing (Mic.16) and lipid-associated (Mic.12-13; APOE) subpopulations. The lipid-associated Mic.12 and Mic.13 both expressed the AD risk genes APOE and GPNMB, with Mic.13 also expressing high levels of SPP1 and TREM2 compared to other subpopulations.
Astrocytes were partitioned into 10 subpopulations - homeostatic-like (Ast.1-2), enhanced-mitophagy (Ast.3; PINK1), reactive-like Ast.4 (GFAP, ID3) and Ast.5 (GFAP, SERPINA3, OSMR), interferon-responding (Ast.7; IFI6), and stress response (Ast.8-10): Ast.8, expressing heat stress and DNA damage, calcium, and sterol metabolism genes; Ast.9, expressing heat and oxidative stress response, tau binding and necroptosis genes; and Ast.10 (SLC38A2), expressing oxidative stress and ROS, metallothioneins and zinc ion homeostasis genes.
Oligodendrocyte lineage cells were partitioned into 13 subpopulations of mature oligodendrocytes (Oli.1-13), such as the stress response Oli.13 (SLC38A2), 3 subpopulations of oligodendrocyte precursor cells (OPC.1-3), one committed oligodendrocyte precursor (COP) subpopulation and one newly formed oligodendrocytes (NFOL) subpopulation. The newly discovered diversity of OPCs are of particular interest, and included an enhanced-mitophagy subpopulation (OPC.1; PINK1), which had higher expression of AD risk genes (e.g. APOE, CLU), and an axon projection/regeneration associated subpopulation (OPC.3; SERPINA3, OSMR).
Specifically, we suggest the following sequence of events underlying the prAD trajectory: At the early stages, selective homeostatic glial subpopulations decrease in proportion alongside an increase of, first, the lipid-associated microglia subpopulation APOE+ Mic.12 subpopulation that is itself influenced by advancing age and contributes to the accumulation of amyloid-β proteinopathy, and up-regulates immune activation pathways. Then, a distinct but related Mic.13 subpopulation of APOE+TREM2+ microglia that are influenced by APOEε4 (the strongest genetic risk factor for AD) contributes to the subsequent accumulation of tau proteinopathy.
At the next stage of the AD cascade, with the accumulation of tau proteinopathy, we observed a transient increase in the proportions of Ast.3 and OPC.1, which both upregulate genes associated with high energy demand and enhanced-mitophagy, as well as oxidative phosphorylation and glutamate secretion. OPC.1 further upregulates genes associated with response to oxidative stress aligning with reports suggesting the increased vulnerability of OPCs to oxidative stressors that are rising during this phase.
At the last stage of the AD cascade, we observed further increase in Mic.12 and Mic.13 proportions, together with a coordinated increase of Ast.10 and Oli.13, with Ast.10 playing an important role mediating the effect of tau proteinopathy on the increased rate of cognitive decline. Both Ast.10 and Oli.13 express stress response genes, with Ast.10 mainly demonstrating response to oxidative stress while Oli.13 showing response to heat stress and unfolded protein. Cognitive decline appears to be directly affected by Ast.10 that is driven by both tau and Mic.13, suggesting that the proportion of this astrocyte subpopulation may be a point of convergence for different processes leading to cognitive dysfunction.
While Mic.12 offers a good target with which to perturb the accumulation of Aβ proteinopathy to enhance therapeutic options centered on anti-amyloid-β antibodies, preventing polarization of microglia and astrocytes into Mic.13 and Ast.10 respectively, may have more immediate impact in helping to prevent cognitive impairment. The latter strategy would also be better suited for individuals who are already Aβ+ and are at risk of tauopathy.
On the other hand, these subpopulations do not appear to be relevant to the alternative brain aging (ABA) trajectory, where we found a selective decrease in homeostatic glial subpopulations and an increase in reactive microglial subpopulations. At the next stage, we observed increased proportions of reactive-like Ast.5 and OPC.3, both expressing the markers SERPINA3 and OSMR. Among participants in ABA, we found constant levels of neocortical amyloid, very limited neocortical tau and varying dynamics of cognitive decline. Thus, more work is needed to better understand this trajectory and its interesting, defining glial subpopulations.
Reviewing the Benefits of Intermittent Fasting
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.
Non-Canonical Autophagy in Aging
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.
TDP-43 Aggregation Leads to Loss of Stathmin-2 Expression and Inability of Neurons to Regenerate Axons
Researchers here delve into the mechanisms by which TDP-43 aggregation contributes to the symptoms of neurodegenerative conditions in which it is involved. It disrupts expression of another gene, stathmin-2, degrading the ability of neurons to maintain axonal connections. This is a feature of ALS, the condition most readily associated with TDP-43 aggregation. The research here points to an approach to therapy: not restoration of appropriate TDP-43 behavior, but rather finding a way to force correct expression of stathmin-2. It remains the case that TDP-43 aggregation may well cause other problems unrelated to this mechanism.
Nuclear clearance and cytoplasmic aggregation of the RNA-binding protein TDP-43 is the hallmark of neurodegenerative diseases called TDP-43 proteinopathies. This includes almost all instances of amyotrophic lateral sclerosis (ALS) and about half of frontotemporal dementia. In ALS, the motor neurons that innervate and trigger contraction of skeletal muscles degenerate, resulting in paralysis. One of the most highly abundant motor neuron mRNAs encodes stathmin-2, a protein necessary for axonal regeneration and maintenance of neuromuscular junctions (NMJs).
Recognizing that stathmin-2 is essential for axonal recovery after injury and NMJ maintenance, a central interest in TDP-43 proteinopathies is to determine the mechanism through which TDP-43 enables correct processing of STMN2 mRNAs and to develop methods to restore stathmin-2 synthesis in neurons with TDP-43 dysfunction. We found that TDP-43 binding to the first intron of the STMN2 pre-mRNA was required to suppress cryptic splicing and polyadenylation. Correct processing of this modified STMN2 pre-mRNA was restored by binding, suggesting that TDP-43 normally functions by sterically blocking access to the cryptic sites of RNA-processing factors.
Rescue of stathmin-2 expression and axonal regeneration after injury in human motor neurons depleted of TDP-43 was achieved with steric binding antisense oligonucleotides (ASOs). We identified RNA-targeted CRISPR effectors and ASOs that restored STMN2 levels despite reduced TDP-43. ASO injection into cerebral spinal fluid, an approach feasible for human therapy, rescued stathmin-2 protein levels in the central nervous system of mice with chronically misprocessed Stmn2 pre-mRNAs.
Age-Related Impairment of Angiogenesis Impacts Proficient Regeneration in Zebrafish
Zebrafish are one of the few vertebrate species capable of repeatedly regenerating major tissue loss without scarring. This ability is impacted by aging, however, as noted here. Researchers find that the age-related loss of angiogenesis capacity impairs regeneration. It is known that capillary density declines with age, hand in hand with impaired angiogenesis, and it is hypothesized that the consequent reduced delivery of oxygen and nutrients is an important contribution to many aspects of aging.
Impaired wound healing is associated with aging and has significant effects on human health on an individual level, but also the whole health care sector. Deficient angiogenesis appears to be involved in the process, but the underlying biology is still poorly understood. This is at least partially being explained by complexity and costs in using mammalian aging models.
To understand aging-related vascular biology of impaired wound healing, we have utilized zebrafish and turquoise killifish fin regeneration models. The regeneration of caudal fin after resection was significantly reduced in old individuals in both species. Age-related changes in angiogenesis, vascular density, and expression levels of angiogenesis biomarker VEGF-A were observed. Furthermore, an anti-angiogenic drug, vascular endothelial growth factor receptor blocking inhibitor SU5416, reduced regeneration indicating a key role for angiogenesis in the regeneration of aging caudal fin despite aging-related changes in vasculature.
Taken together, our data indicates that these fish fin regeneration models are suitable for studying aging-related decline in wound healing and associated alterations in aging vasculature.
The Tradeoff of Working with Short-Lived Laboratory Species
It is cheaper and faster to study aging - and potential approaches to treat aging - in short-lived species. The disadvantage is that much of what is learned and achieved will be irrelevant to aging as it occurs in longer-lived species such as our own. The response to calorie restriction, an upregulation of cellular housekeeping mechanisms that lengthens life, fortunately evolved early on in the development of life, and the biochemistry is surprisingly consistent even across widely divergent species. Thus much can be learned of it in lower animals with short life spans. Unfortunately, it turns out that this class of intervention doesn't affect life span in longer-lived species like our own to anywhere near the degree it does in short-lived species. This is the tradeoff of working with short-lived models, in a nutshell: more can be done, but all of that work may turn out to be of very limited utility.
Wouldn't it actually accelerate progress if we instead did most testing in far shorter-lived animals, like the roundworm C. elegans or the fruit fly Drosophila? On its face, that's a totally reasonable question: time is ticking for all of us, and we want to get longevity therapeutics into people's hands as quickly as possible! And certainly these short-lived animals have taught us a lot about the roles of different biological signaling pathways.
Some interventions that work in C. elegans act by altering the worms' early developmental processes, which isn't terribly helpful to those of us who "have the misfortune of already being alive." That's also becoming increasingly evident in mice. We've known for about twenty years now that mutations that block IGF-1/growth hormone signaling in mice slow down their aging and extend lifespan. But those mutations dampen down signaling through these pathways throughout the animals' entire lives. To take advantage of that discovery and develop a longevity therapeutic that would work in middle-aged and older adults, a large part of the anti-aging effect would have to be due to the hormone still being low during adulthood. Instead, studies have shown that almost all the benefit of IGF-1 signaling inhibition goes away if growth hormone production is brought back to normal during the very earliest period of life.
The preceding examples apply to studies based on trying to usurp the regulation of metabolism to slow the aging process down. SENS Research Foundation is instead grounded in the direct "damage-repair" strategy of SENS. If we're going to use an organism as a test animal for rejuvenation biotechnology, it has to accumulate similar kinds of aging damage as we do, and it must do so in similar tissues and with similar pathological results. And here C. elegans and Drosophila just aren't qualified for the job. For instance, C. elegans don't live long enough to accumulate cells overtaken by mitochondrial DNA deletions, and there is no clear link between other kinds of mitochondrial DNA damage and the rate of aging in these worms. C. elegans also have no bones, so no osteoarthritis or osteoporosis either. And they lack any of the cells dedicated to the immune system.
Glucagon-like Peptide-1 Receptor Agonists as an Approach to Modestly Slow Aging
Semaglutide is probably the best known of the glucagon-like peptide-1 receptor agonists, a class of drug deployed to treat type 2 diabetes and other consequences of obesity. This is one of a number of classes of diabetes drug where there is some suspicion that maybe these small molecules can modestly slow aging through much the same set of mechanisms that help to steer the abnormal metabolism of obesity and diabetes into a modestly less terrible state, e.g. reduced blood glucose and inflammatory signaling. Equally, the data to support that belief is far from compelling, and the effect sizes are small in comparison to, say, those produced by exercise. An equally plausible explanation for observed effects is that drugs that promote weight loss and lower calorie intake operate through the well studied mechanisms of calorie restriction and reduced impact of visceral fat.
Increased age is associated with frailty and diseases of varying severities, and for many, the hope of a long and healthy lifespan therefore becomes elusive. Nevertheless, overall life expectancy has increased markedly during the past decades, owing to a large extent to the introduction of medicines such as statins and anti-hypertensives. These and newer-generation drugs have resulted in a lower prevalence and severity of age-related illnesses such as cardiovascular disease (CVD). To sustain and reinforce this positive trend and help ensure a prolonged healthspan for more people across the world, novel pharmacotherapeutics and optimal use of existing options are arguably needed.
Glucagon-like peptide-1 (GLP-1) receptor agonists (RAs) are an example of a drug class with proven or potential benefits across a range of prevalent age-related conditions and complications. Originally developed to manage blood glucose levels in type 2 diabetes (T2D), GLP-1 RAs have subsequently been confirmed to have marked benefits on body weight and CVD risk. Furthermore, evidence from research and clinical use of the drug class has led to the initiation of clinical trials with GLP-1 RAs in other prominent aging-related diseases, including chronic kidney disease (CKD) and Alzheimer's disease. In sum, GLP-1 RAs are positioned as one of the pharmacotherapeutic options that can contribute to addressing the high unmet medical need characterising several prevalent aging-related diseases, potentially helping more people enjoy a prolonged healthy lifespan.
VGLL3 as an Important Regulator of Fibrosis
Fibrosis is a feature of the age-related decline of many organs and tissues, notably the heart, kidney, and liver, among others. It is a malfunction of normal tissue maintenance in which excessive extracellular matrix is created, leading to scar-like deposition that is disruptive to tissue structure and function. Chronic inflammation and the presence of senescent cells appear to be important in the development of fibrosis, but as yet the medical community lacks a proven approach to reversal of fibrosis. Much of the research continues to focus on finding regulatory genes that might be targeted in order to disrupt the formation of fibrotic structures, as in the example here, rather than looking at root causes.
When an injury is on your skin, it shows up as a visible scar, but what happens when vital organs like your heart or liver are damaged and hardens? If left unchecked, it can lead to loss of mechanics and dangerous consequences. These changes in tissues are attributed to the extracellular matrix. The extracellular matrix is a web of proteins found in every cell in the body, and acts both like wires on a circuit that allow cells to communicate with each other, and the beams in a building, giving the organs its structure. Too much extracellular matrix makes the cell, and by extension the organ, tough and inflexible, a condition known as fibrosis. In simple terms, fibrosis is a stiffening of cells and tissue. Its health implications are profound, as it can lead to poor pumping by the heart or cirrhosis in the liver.
"Myofibroblasts are a group of cells that produce collagen, a common extracellular matrix protein. In diseased organs they are seen overproducing collagen. Once myofibroblasts appear in diseased organs, fibrosis proceeds in a snowball fashion. At the same time, myofibroblasts are responsible for proper wound healing."
To understand how myofibroblasts turn pathological, researchers looked at how different physical stimuli changes the expression of genes in these cells. They found consistent changes in the expression of one gene: VGLL3. Their study showed that after a heart attack, myofibroblasts in both mouse and human hearts express more VGLL3 protein which led to the production of collagen. VGLL3 was also expressed more in fibrotic mouse liver, suggesting it contributes to fibrosis in multiple organs. Conversely, preventing VGLL3 activation in mice led to far less fibrosis in these organs. The study further showed that the relationship between matrix stiffness and VGLL3 activation becomes a pathological positive feedback loop, in that a stiffer matrix triggers more VGLL3 activation, which triggers the cell to produce more collagen.
Complicating the Relationship Between Cellular Senescence and Late Life Depression
Inflammatory signaling may be influential in major depressive disorder. For any condition in which inflammation is important, attention should be given to the possible role of cellular senescence, given the advent of senolytic therapies to clear these cells. Senescent cells grow in number throughout the body with age, and while never a large fraction of all cells, they energetically generate pro-inflammatory signals. Here, researchers discuss the sometimes there, sometimes not correlation between burden of senescent cells and incidence of major depressive disorder in later life.
Previous studies suggested the role of cellular senescence in late-life depression (LLD). However, it is unclear how this finding relates to common features of LLD, such as medical and cognitive problems. We applied factor analyses to an extensive battery of clinical variables in 426 individuals with LLD. Here we tested the relationship between these factors, age and sex, with an index of cellular senescence based on 22 senescence-associated secretory phenotype (SASP) proteins.
We found four factors: 'depression and anxiety severity', 'cognitive functioning', 'cardiovascular and cardiometabolic health' and 'blood pressure'. A higher senescence-associated secretory phenotype index was associated with poorer 'cognitive functioning' and 'cardiovascular and cardiometabolic health' but not with 'depression and anxiety severity'.
When interpreting this finding, it is critical to note that it does not contradict our previous studies that have consistently demonstrated an increased SASP index in individuals with a major depressive disorder compared with non-depressed older adults. However, it suggests that the SASP index is more closely associated with physical health and cognitive functioning than with the severity of depression and anxiety symptoms within individuals with LLD. The question of how depressive symptoms interact with the pathophysiology of major depression is fiercely discussed.
Our findings highlight the interactive effect between LLD and physical burden. Previous research has demonstrated that depression frequently occurs in individuals with chronic illness and amplifies the disability and disablement associated with co-occurring physical illness and cognitive impairment. In addition, depression undermines adherence to co-prescribed pharmacotherapy for medical diseases and reduces healthy lifestyle choices. Therefore, evidence-based treatment of depression may also reduce mortality risk secondary to physical illness, such as cancer. On the other hand, co-occurring physical burden moderates the long-term response to antidepressant treatment and renders the individual's response more brittle.
MANF Upregulation in Macrophages Improves Muscle Regeneration in Old Mice
Regeneration following injury is an intricate, coordinated dance between stem cells, various types of somatic cell, and immune cells. Age-related changes in any of those cell populations may contribute to the declining ability to heal injuries observed in later life. Researchers here identify a specific change in the innate immune cells called macrophages that produces a significant impairment in wound healing in mice. This may prove to the basis for therapies to improve regeneration in older people, time will tell.
As our organism ages, the muscles lose the capacity to regenerate. Researchers have found a protein that regulates the function of a subset of immune cells, macrophages, by promoting their ability to clear residues in the regenerating muscle. The behavior of macrophages is altered in aged mice. Macrophages are a type of immune cells that are capable of phagocytosis, the process of ingestion and elimination of particles inside cells. During regeneration the macrophages are responsible for clearing the dead cells from the muscle after injury, which is a normal step of the process of muscle regeneration.
The researchers found that macrophages in aged mice have reduced levels of a protein, called MANF, that is crucial for this process. "In fact, this protein is so important in this process that if we decrease MANF levels in the macrophages in younger mice, their ability to regenerate muscle is also impaired. On the other hand, increasing the levels of the protein MANF in aged muscle is sufficient to recover muscle's regenerative capacity."
"A central promise of regenerative medicine is the ability to repair aged or diseased organs using stem cells. This approach will likely become an effective strategy for organ rejuvenation, holding the potential to increase human health span by delaying age-related diseases. Our study shows that immune aging is an important obstacle to the regenerative capacity of aged muscle."
Cellular Senescence in Idiopathic Pulmonary Fibrosis
This review paper goes into some detail regarding present thought on the role of senescent cells of different types in idiopathic pulmonary fibrosis. Fibrosis in general is an often age-related dysfunction of normal tissue maintenance and regeneration, in which excessive extracellular matrix is created, leading to scar-like deposits that disrupt normal tissue structure and function. In the lung, this progressively impairs breathing and is ultimately fatal. Idiopathic pulmonary fibrosis was one of the first conditions for which early senolytic drugs to clear senescent cells were tested in humans.
Idiopathic pulmonary fibrosis (IPF) is a chronic progressive interstitial lung disease of unknown origin. Histologically, IPF is characterized by massive accumulation of fibroblasts, myofibroblasts, alveolar epithelial cells (AECs), and macrophages and a significant deposition of extracellular matrix (ECM). A previous review showed that AECs, as the main source of pro-fibrogenic cytokines in IPF, express a variety of cytokines and growth factors, which can promote the migration, proliferation, and accumulation of extracellular matrix of fibroblasts; these are key events of cell dysfunction in PF.
AECs are damaged by pathogenic microorganisms, dust, drugs, chemicals, and oxygen free radicals which, when coupled with risk factors such as aging and genetics, may decrease the ability of alveolar epithelial type II (ATII) cells and lung fibroblasts (LFs) to repair damage to the lung. LFs proliferate locally, migrate to the injury site and differentiate into myofibroblasts, which produce a large amount of ECM and exhibit contractile function. These myofibroblasts typically vanish after successful repair; dysregulation of the normal repair process can lead to persistent myofibroblast activation.
PF is an aging-associated lung disease in which LF and AEC senescence play a complex role in pathogenesis. Numerous studies have revealed that ATII cell senescence and apoptosis are associated with endoplasmic reticulum stress and autophagy, telomere damage, mitochondrial dysfunction, and epigenetic changes, leading to development of pulmonary fibrosis. The activation of LF and deposition of ECM proteins are key steps in the development of IPF. Epigenetic changes and reduced activation of autophagy promote myofibroblast differentiation, ultimately leading to pulmonary fibrosis. Aging AECs promote LF activation by increasing expression of the senescence-associated secretory phenotype (SASP), thereby increasing occurrence and development of pulmonary fibrosis. In short, cellular senescence is an important mechanism of IPF pathogenesis.