Fight Aging! Newsletter, December 27th 2021

Osteoarthritis occurs when the protective cartilage that coats the ends of the bones breaks down over time, resulting in bone-on-bone friction. The disorder, which is often painful, can affect any joint, but most commonly affects those in our knees, hips, hands, and spine. To prevent the development of arthritis and alleviate the need for invasive joint replacement surgeries, the researchers are intervening earlier in the disease. "In some patients joint degeneration starts with posttraumatic focal lesions, which are lesions in the articular (joint) cartilage ranging from 1 to 8 cm2 in diameter. Since these can be detected by imaging techniques such as MRI, this opens up the possibility of early intervention therapies that limit the progression of these lesions so we can avoid the need for total joint replacement."

That joint preservation technology is a therapeutic bio-implant, called Plurocart, composed of a scaffold membrane seeded with stem cell-derived chondrocytes - the cells responsible for producing and maintaining healthy articular cartilage tissue. Building on previous research to develop and characterize the implant, the current study involved implantation of the Plurocart membrane into a pig model of osteoarthritis. This is the first time an orthopaedic implant composed of a living cell type was able to fully integrate in the damaged articular cartilage tissue and survive in vivo for up to six months. Molecular characterization studies showed the bio-implant mimicked natural articular cartilage, with more than 95 percent of implanted cells being identified as articular chondrocytes. The cartilage tissue generated was also biomechanically functional - both strong enough to withstand compression and elastic enough to accommodate movement without breaking.

Generation of articular chondrocytes from pluripotent stem cells (PSCs) has been challenging as most chondrogenic cells during development are fated to undergo hypertrophy and endochondral ossification rather than adopt an articular chondrocyte identity. We and others have generated articular-like chondrocytes from human pluripotent stem cells; we have subsequently shown that stable articular chondrocytes produced from GFP-labelled PSCs can engraft, integrate into and repair osteochondral defects in small animal models. Moreover, these human cells produce all layers of hyaline cartilage after 4 weeks in vivo, including a PRG4+ superficial zone. However, production scaling and assessment of long-term, clinically relevant functionality has so far limited the development of these protocols.

The Yucatan minipig presents an excellent model for pre-clinical assessment of potential orthopedic therapies due to structural similarities, comparable thickness of articular cartilage, and the ability to create defects of substantial volume; in addition, their size allows for cost-efficient care and observation for extended periods of time. Here we present data demonstrating long-term functional repair of porcine full-thickness articular cartilage defects with hyaline-like cartilage by scalable production of clinical grade human embryonic stem cell-derived immature articular chondrocytes.

Sepsis is a life-threatening organ dysfunction caused by the host's unbalanced response to infection. Septic shock is a type of sepsis in which the changes in metabolism, cells, and hemodynamics significantly increase the likelihood of fatality. Relevant studies have shown that there are more than 19 million sepsis patients worldwide each year, of which 6 million patients die, and the case fatality rate is greater than 25%. About 3 millions of those who survived had cognitive impairments that severely affected their quality of life. Septic shock is also one of the common clinical manifestations of severe patients with COVID-19. People over 65 years of age, infants, immunocompromised patients, and patients with autoimmune diseases, tumors, kidney diseases, and lung diseases are the most susceptible to sepsis. At present, treatments for sepsis mainly include fluid therapy (crystal fluid and albumin), antibacterial drugs, vasoactive drugs (norepinephrine), glucocorticoids, injection immunoglobulin, etc. Due to factors such as individual difference, aging, and antimicrobial resistance, the morbidity and mortality of sepsis remain high.

It has been documented that cytokines and reactive oxygen species (ROS) play essential roles in sepsis. ROS mainly come from cell respiration, protein folding, or various by-products of metabolism. Under pathological conditions, an unbalance of the generation and elimination of ROS results in oxidative stress with excess ROS. Since H2O2 is chemically stable and able to diffuse through cells and tissues, it may accumulate locally or systematically and activate the inflammatory response.

Upon the occurrence of infection, leukocytes are attracted to affected sites and release cytokines and ROS. An excessive level of ROS may damage the biological macromolecules such as DNA, proteins, and lipids, which may cause dysfunction of cells and tissues and further exacerbate the immune response. Uncontrolled production of ROS and cytokines may eventually lead to excessive inflammatory response and cytokine storm. Therefore, eliminating the excessively produced H2O2 helps to reduce the oxidative stress and to regulate the expression of pro-inflammatory cytokines, which is beneficial for the treatment of sepsis.

Organisms can effectively regulate their H2O2 levels through efficient enzymatic reactions. Catalase is the most abundant antioxidant enzyme commonly found in the liver, erythrocytes, and alveolar epithelial cells, and is the most effective catalyst for the decomposition of H2O2. Catalase has attracted much attention in maintaining normal physiological functions and relieving pathological processes. However, exogenous catalase generally exhibits poor in vivo stability and short plasma half-life, which preclude its broad use as therapeutics. Conjugation of therapeutic proteins with poly (ethylene glycol) (PEG) is the golden standard to improve their pharmacokinetics and immunogenicity. Herein, we explore the use of PEG-conjugated catalase as a therapeutic treatment for sepsis. Our results suggest that PEGylated catalase can effectively regulate cytokine production by activated leukocytes, suppress the elevated level of AST, ALT, TNF-α, and IL-6 in mice with induced sepsis, and significantly improve the survival rate of the mice.

The kidney and the brain, as high-flow end organs relying on autoregulatory mechanisms, have unique anatomic and physiological hemodynamic properties. Similarly, the two organs share a common pattern of microvascular dysfunction as a result of aging and exposure to vascular risk factors (e.g., hypertension, diabetes, and smoking) and therefore progress in parallel into a systemic condition known as small vessel disease (SVD). Many epidemiological studies have shown that even mild renal dysfunction is robustly associated with acute and chronic forms of cerebrovascular disease.

Beyond ischemic SVD, kidney impairment increases the risk of acute cerebrovascular events related to different underlying pathologies, notably large artery stroke and intracerebral hemorrhage. Other chronic cerebral manifestations of SVD are variably associated with kidney disease. Observational data have suggested the hypothesis that kidney function influences cerebrovascular disease independently and adjunctively to the effect of known vascular risk factors, which affect both renal and cerebral microvasculature. In addition to confirming this independent association, recent large-scale human genetic studies have contributed to disentangling potentially causal associations from shared genetic predisposition and resolving the uncertainty around the direction of causality between kidney and cerebrovascular disease.

Accelerated atherosclerosis, impaired cerebral autoregulation, remodeling of the cerebral vasculature, chronic inflammation, and endothelial dysfunction can be proposed to explain the additive mechanisms through which renal dysfunction leads to cerebral SVD and other cerebrovascular events. Genetic epidemiology also can help identify new pathological pathways which wire kidney dysfunction and cerebral vascular pathology together. The need to identify the additional pathological mechanisms that underlie kidney and cerebrovascular disease is attested to by the limited effect of current therapeutic options in preventing cerebrovascular disease in patients with kidney impairment.

Age-related neurofunctional decline may negatively impact the daily life for the elderly, and no effective strategies are available so far in the clinic. This present review mainly focuses on myelin degeneration, decreased myelinogenesis during aging and the possible mechanisms. Admittedly, a lot of questions remain unanswered. For instance, are there spatial or temporal differences in the degeneration process in the central nervous system (CNS)? What is the deciding point for one oligodendrocyte (OL) or one myelin segment to initiate degeneration and could we inhibit this bad process through modulating one key factor? Is the newly generated myelin more stable compared to preexisting myelin in the aged brain? If this is the case, we may find some clues about repressing myelin degeneration in the aged. The decreased myelinogenesis during aging is likely a result of arrested differentiation of oligodendroglia precursor cells (OPCs), thus it is plausible that promoting adult OPCs maturation may be a feasible and realistic approach to improve age-related neuronal function decline for the elderly. Meanwhile, rejuvenating subventricular zone (SVZ) stem cells may also help with myelinogenesis ability in the aged. More efforts are needed to further confirm those effects in human.

Moreover, oligodendroglial lineage cells display more behaviors than differentiation and forming new myelin sheaths. For example, OPCs may form synaptic connections with neighboring neurons, and that regulates neuronal signals in the CNS. In addition, the expression of connexin channel proteins in oligodendroglial lineage cells is an intriguing feature and the connexins could function either as hemichannels or gap junctions. The gap junction enables OLs to be connected as a glial network with astrocytes, allowing transportation of small molecules such as calcium and energy metabolites, which may be important for homeostasis of the CNS. Recent studies even showed that OPCs could exert immunomodulatory functions, which are particularly relevant in the context of neurodegeneration and demyelinating diseases. Besides, OLs are found to be heterogenetic in the mouse juvenile and adult CNS, the response of different subtypes to aging remains unknown. It is not clear whether the functions mentioned above and their correspondent molecules are altered during aging. Future works are needed to give us a more comprehensive understanding of the role oligodendroglial lineage cells played in aged brains, which could shed light on the clinical therapeutic strategies considering age-related neuronal functional diseases.

We probed the changes in the fecal microbiota and metabolomes of pre-aging male mice (C57BL/6, age: 16 months) following the oral short-term administration of nicotinamide mononucleotide (NMN). The complex interplay between age and the microbiota is well-described in several studies. The changes in the composition, diversity, and functional characters of the microbiota were observed over time. It was reported that the abundance of Proteobacteria is positively linked with aging. Proteobacteria include pathogenic representatives, such as Enterobacter spp., which may cause infection and disease. In the present study, the reduced abundance of fecal Proteobacteria in the NMN-supplemented mice suggests that NMN might have perturbed certain harmful microbes.

Surprisingly, a widely accepted probiotic strain Akkermansia (Verrucomicrobiota phylum) was lowered in the fecal microbiota of NMN-supplemented mice. In contrast, a previous study has reported that NMN administration enriches the abundance of Akkermansia muciniphila. We conjecture that the observed differences in the outcomes may be attributed to the difference in the age of the mice used in the experiments. We clearly observed that the Akkermansia abundance was negatively correlated to nicotinamide, tryptophan, and indole as well as their derivatives, which might have inhibited its growth.

Nicotinamide mononucleotide increases the abundance of butyric acid-producing Turicibacter which exhibits anti-fatigue activity, implying that NMN administration might reinforce vitality by promoting the growth of Turicibacter. Unexpectedly, in this study, oral NMN administration increased Helicobacter abundance in pre-aging mice. Some Helicobacter spp. are known as pathogenic bacteria that can cause gastric diseases, its enrichment with NMN administration should be deeply and carefully confirmed further.

In addition to these top dominant genera, the correlation analysis demonstrated that Mucispirillum was greatly associated with the altered serum metabolites. Mucispirillum was positively correlated with the metabolites relevant to purine, nicotinate, and nicotinamide metabolism, as well as arginine and proline metabolism. Mucispirillum schaedleri showed a protective effect against Salmonella enterica ser. Typhimurium colitis by interfering with the invasion gene expression. These upregulated metabolites might have beneficial effects on the inhibition of pathogenic adhesion in the gut mucus.

It is not yet fully understood how these metabolites change with the varied microbial composition in response to the NMN supplementation. Further validation of specific metabolite changes corresponding to specific microbial genera coupled with their downstream biological effects will be important to the effects of NMN supplementation on the host.

In 2019, researchers in Japan published breakthrough results detailing how NeuroD1, a protein involved in cell differentiation, could coax microglia into new neurons. Now, researchers in China have found that not only does NeuroD1 not induce microglia-to-neuron conversion, but also that the protein induces microglia death. The team set out to investigate the molecular mechanisms underpinning the original finding, since microglia and neurons descend from different cellular lineages.

The researchers applied a rigid lineage tracing protocol to follow the cellular differentiation progression in mice, as well as to monitor the effect of lentiviral vectors - an inert virus package used to carry NeuroD1 to the central nervous system - on the process. They validated their observations through live cell imaging and pharmacological approaches. "Disappointingly, our results do not support the 'microglia-to-neuron conversion. Instead, our data strongly indicate that the previously observed conversion was actually due to the experimental artifacts from viral leakage."

The assumed finding was likely due to NeuroD1's actual role: triggering microglial cell death. Neurons are unaffected by NeuroD1 so their numbers will stay the same, while microglia cell numbers decrease. However, due to the low purity of the microglia and the viral leakage, it could appear that while microglia cells were decreasing, non-microglia cells were increasing, leading to the conclusion in vitro that microglia were converting to neurons.

"The 'microglia-to-neuron' conversion should be verified following three principles: 1) unambiguous microglial-based lineage tracing and lack of lentiviral leakage, along with well-designed controls; 2) unambiguous live cell imaging to show how an individual microglial cell converts to a neuron; and 3) upon microglial depletion, there should be no or few microglia-converted neurons." The last point appeared to be supported in the original paper, but when researchers replicated the experiment, they found that even when 98.9% of microglia cells were killed, numerous "microglia-converted neurons" were still observed. Such a finding suggests that the converted neurons were mislabeled cells rather than the desired neurons.

Here presented for the first time, are results showing persistence over a 5+ year period, in a human who had a hormone gene therapy administered to muscle. This growth hormone releasing hormone (GHRH) therapy was administered in 2 doses, a year apart, with a mean after the second dose of 195 ng/ml (13 times normal). This level of GHRH therapy appears to be safe for the subject, although there were some adverse events. IGF-1 levels were little affected, nor were growth hormone test results, showing no indications of acromegaly for the hormone homolog used. Heart rate declined 8 to 13 bpm, persistent over 5 years. Testosterone rose by 52%. HDL/LDL ratio dropped from 3.61 to mean 2.81, triglycerides declined from 196 mg/dL to mean 94.4 mg/dL.

White blood cell counts increased, however the baseline was not strong. CD4+ and CD8+ white blood cell mean count increased 11.7% and 12.0% respectively. Ancillary observations comprise an early period of euphoria, and dramatic improvement in visual correction after the first dose, spherical correction from baseline (L/R) -2.25/-2.75 to -0.25/-0.5. Over the next 5 years correction drifted back to -1.25/-1.75. Horvath phenoage was cut 44.1% post-treatment. At completion, epigenetic age was -6 years (-9.3%), and telomere age was +7 months (+0.9%).

Blood-brain barrier (BBB) breakdown facilitates entry into the brain of neurotoxic blood-derived products and pathogens and has been linked to inflammatory and immune responses that can induce neuronal injury, synaptic dysfunction, and loss of neuronal connectivity. BBB disruption also facilitates leukocyte infiltration, which leads to glial cell death, axonal damage, lesion development, and therefore cerebrovascular dysfunction results in memory impairment, acceleration of neurovascular damage, and exacerbation of the progression of neuropathology in the central nervous system (CNS). Thus, critical demand exists for new therapies that minimize peripheral immune factors and limit the infiltration of peripheral immune cells into the brain.

The neurovascular unit, which comprises brain endothelial cells, pericytes, astrocytes, and microglia, primarily confers the low paracellular permeability of the BBB. The tight cell-to-cell contacts that these cell types establish with each other restrict the entry of red blood cells, leukocytes, and plasma components into the brain parenchyma and ensure the export of potentially neurotoxic molecules from the brain to the blood. The site of the anatomical BBB is composed of a continuous monolayer of endothelial cells that are connected by tight junctions (TJs) and adherens junctions. The interactions among the endothelial cells, pericytes, and glial cells are crucial for the formation and maintenance of the highly regulated CNS internal milieu.

Astrocytes play a dual role in limiting the entry of peripheral substances into the CNS: permeability factors secreted by reactive astrocytes open the BBB by disrupting endothelial TJs, but reactive astrocytes also perform a protective function by upregulating classical TJ proteins and using TJ proteins to corral activated T lymphocytes into distinct clusters. Microglia have also recently been shown to contribute to BBB induction, and studies have indicated that microglia also play a dual role in BBB repair. Initially, microglia maintain BBB integrity by expressing the TJ protein Claudin-5 and establishing physical contacts with endothelial cells, but during persistent inflammation, microglia engulf astrocytic endfeet and endothelial cells and impair BBB function.

Here, we investigated the transcriptional changes that occur in microglia and astrocytes by performing RNA sequencing on samples from five time points across the BBB permeability change during aging in mice. We report that whereas microglia are characterized by marked gene-level alterations related to negative regulation of protein phosphorylation and phagocytic vesicles, astrocytes show activation of enzyme- or peptidase-inhibitor signaling after detectable changes in BBB permeability. We also identify several genes enriched in these pathways that are notably altered after BBB breakdown. Our data reveal that microglia and astrocytes play an active role in maintaining BBB stabilization and corralling infiltrating cells, and thus might potentially function in ameliorating the lesions and neurologic disabilities in CNS diseases.

Earlier this year, MyMD Pharmaceuticals merged with Akers Biosciences to form a new company focused on developing and commercialising novel immunotherapy therapies. The company's lead compound is MYMD-1, an indirect inhibitor of TNF alpha. The company CSO refers to the trial in January as a "sarcopenia/longevity" study, because "the FDA isn't accepting longevity or aging as an indication." There's also the fact that a true longevity study would require following people for many years, so it makes sense to focus on "markers like muscle loss, weakness, frailty, increased predisposition for age-related pathology, and the like."

The key proposition behind MyMD-1 is its ability to reduce chronic inflammation through the inhibition of several key cytokines, including the oft-cited TNF alpha. "TNF alpha is the star of the show. It's the first proinflammatory cytokine to go up when you get an infection or any type of inflammatory illness, and it turns on IL-6 and IL-1. It is the innate immune response, but it also gets out of control in autoimmune diseases. MyMD-1 is also selective, meaning that it doesn't take out innate immunity, but it does affect adaptive immunity, so we have selectivity there and we think that's going to make a big difference."

A mouse study conducted over the past couple of years and now submitted for publication has produced what sounds like some pretty exciting results. The study looked at the effect on lifespan on older mice (19 months old) treated with either MyMD-1, rapamycin, or a combination of rapamycin and metformin over a period of 13 months. "They saw that MyMD-1 resulted in a much more dramatic, fourfold-increased, highly statistically significant survival time. In addition, they saw absolutely no loss of muscle strength in male mice compared to those on the other compounds. Females had some loss, but nothing like they did in the other groups. What we're all looking for is a chance to slow aging and extend healthspan, and we think that this is happening both because of the anti-inflammatory effects of MyMD-1 and because it also blocks oxidative stress and things like fibrosis. We think it's the combination that really does the heavy lifting."

In 1996, a report of extended longevity in mice homozygous for a mutation producing growth hormone (GH) deficiency was quickly followed by the demonstration of extensive homology between one of the key longevity genes in a worm, Caenorhabditis elegans, and genes coding for insulin receptor and insulin-like growth factor-1 (IGF-1) receptor in mammals. Since GH is the key determinant of hepatic IGF-1 expression and circulating IGF-1 levels, and has major impact on insulin signaling, these findings led to an exciting conclusion that the insulin/insulin-like growth factor signaling (IIS) is an evolutionarily conserved mechanism which controls aging in organisms ranging from yeast and worms to insects and mammals.

Subsequent work provided much evidence in support of this exciting realization, and this has led to a focus on IIS, rather than GH signaling, in analyzing genetic control of mammalian aging. This is an important distinction. Although biosynthesis and blood plasma levels of GH and IGF-1 are closely linked, the actions of these hormones are not identical and, in some cases, opposite. For example, IGF-1 mimics some of the insulin actions and promotes insulin sensitivity, while GH is anti-insulinemic and promotes insulin resistance; IGF-1 promotes fat deposition, while GH is lipolytic. Actions of GH not shared with IGF-1 include other effects relevant to aging such as impact on reactive radicals production and anti-oxidative defenses, DNA damage and repair, macrophage reprogramming, ovarian primordial follicle reserve, bone resorption and turnover, kidney dysfunction, and cognitive functioning.

In contrast to the remarkable extension of longevity in female and male mice lacking GH or GH receptors, the impact of reduced IGF-1 signaling on longevity of IGF1R ± mice and mice treated with an antibody to IGF-1 receptor is modest and seen only in one sex. This difference between the effects of reduced IGF-1 and GH signaling is likely related to IGF-1 exerting both beneficial and detrimental effects on aging and age-related disease (including opposite effects on the risk of type 2 diabetes vs cardiovascular disease and cognitive decline) and GH having primarily "pro-aging" effects. Both hormones impact growth, but the metabolic effects of GH are significantly greater.

The extracellular microenvironment is critical for maintaining normal physiological functions of cells because of its role in the homeostatic regulation of various components. Various factors, such as inflammation, metabolic waste, and the blood-brain barrier, can disrupt normal brain microenvironments. Thus, the maintenance of the extracellular environment is vital for brain health. Current studies have suggested that lifestyle interventions, such as regular exercise training, a healthy diet, and sufficient sleep, protect the brain by improving the microenvironment balance under pathological conditions.

Caloric restriction effectively protects the brain microenvironment via multiple mechanisms at molecular, cellular, and tissue levels. Major benefits obtained by CR are based on recent findings in aging and neuropathological models. However, there is currently no consensus on a unified protocol for CR because the duration or starting age of CR has not been clarified in animals. Reports have shown that the initiation age and duration of CR are critical factors that influence overall efficiency. Specifically, CR started at middle-age has the most potent neuroprotective effect.

Current studies on the neuroprotective effects of CR have various weaknesses, which include the lack of a precise description of the dosage curve (i.e., the relationship between CR duration and overall efficiency of neuroprotection), the lack of systematic observations of the additive effect of CR and drugs in counteracting neurodegeneration, and the absence of a neural circuit-specific effect of dietary interventions. These factors limit the large-scale promotion of CR in aging and high-risk populations with neurodegenerative diseases. Therefore, future explorations are required to understand the neuroprotective mechanisms underlying CR to develop alternative pharmaceutical or non-drug interventions for brain aging and neurodegeneration.

With stem cells that can be activated and silenced cyclically, hair follicle experiences multiple rounds of growth phase (anagen), regression phase (catagen), and resting phase (telogen) during lifespan. Hair cycling is initiated by cyclic renewal or physiological cyclic regeneration of stem cells. However, tissues and organs undergo structural and functional declines in the aging process, with physiological and pathological changes regulated by intrinsic and extrinsic factors that dictate the cell fate. As one of the important appendages of the skin, the hair follicle is a complex mini-organ with visible signs, such as decreased regenerative ability that leads to alopecia, and hair graying due to less melanin production by melanocyte stem cells during aging.

Hair regenerative ability is gradually decreased because hair follicle stem cells enter a long quiescent state, or differentiate into other skin epithelial lineages, or escape from the hair follicle niche during aging. These features promote hair follicle to become a widely used model for studying regeneration. As over 30% of the population all over the world suffer from partial or complete hair loss, particularly most people undergo alopecia during aging, understanding the mechanism by which hair follicle changes during aging is of great interest in regenerative biology and is essential for regenerative medicine.

Although there is much debate about the age of retirement, already retirement itself is a decreasingly shared key labour market transition whereby work comes to a sudden stop at a specific age. As people work longer, the age at which they stop work varies, unretiring (ie, returning to the labour force after retirement) becomes increasingly common, people switch to part-time rather than full-time work, or engage in caring or broader social activities. As a result, older workers are characterised by increased diversity. This variety applies not only to employment, but also to health, education, and a broad range of socioeconomic factors. As a consequence, chronological age plays a declining role in defining a common set of problems among older workers; labour market policies need to focus less on age and more on worker characteristics, as is already the case for younger groups.

A focus on retirement age distracts from the importance of maintaining employment from age 50 years or older. In the UK, labour force participation is 85% for people aged 50-54 years, but falls to 58% at age 60-64 years and to 23% at age 65-69 years. Withdrawal from the labour market starts well before state pension age. If a longevity economy is to be achieved, supporting employment among people aged 50 years or older will be key.

Although improving the health and education of older workers will boost their productivity, this will count for little if employers believe that older workers are not productive. Evidence about the productivity of older workers is varied and often ambiguous, and varies substantially between sectors. Put simply, age does not seem to be as important a determinant of productivity as employers assume. This corporate ageism is a problem because it makes older workers more likely to lose their jobs and less likely to be hired than their younger counterparts, which contributes to the decline in employment before retirement age. In the face of such corporate ageism, there is a growing trend of older workers moving into the contingent economy (eg, part-time, gig economy, or contract work) and entrepreneurship. In the UK, over 40% of working people aged 65 years or older are self-employed, which is much higher than for any other age group.

Supporting a longevity economy will require legislation to tackle age discrimination, but social shifts and economic incentives will also be important. For instance, social perceptions about the productivity of older workers will change in response to new cohorts of older workers who have more education and better health than in previous waves. Also, shrinking populations will result in fewer younger workers, which will persuade some employers to be increasingly interested in older workers. Similarly, an ageing consumer base and the ability of technology and robotics to sustain people's productivity will increase the appeal of older workers.

Cellular senescence is a cell state characterized by a proliferative cellular arrest, a secretory phenotype, macromolecular damage, and altered metabolism that can be triggered by several different stress mechanisms. Senescent cells produce and secrete a myriad of soluble and insoluble factors, including cytokines, chemokines, proteases, and growth factors, collectively known as the senescence-associated secretory phenotype (SASP). More recent evidence proposes that different triggers might induce distinctive SASP subsets with concrete functions. Nonetheless, the SASP has started to incite interest as a potential therapeutic target in disease. Therefore, a better understanding of the molecular machinery regulating the SASP is needed.

Pattern recognition receptors (PRRs) of the innate immune system are molecular sensors that are activated by microbial-derived pathogen-associated molecular patterns (PAMPs) or by damage-associated molecular patterns (DAMPs or alarmins) generated endogenously in cells under certain conditions of stress and damage. Emerging data indicate a close relationship between these PRRs and cellular senescence.

We have previously shown that inflammasomes are critical for the SASP. Inflammasomes are multiprotein platforms that induce the proteolytic activity of the inflammatory protease caspase-1, which activates by proteolytic cleavage the proinflammatory cytokines IL-1β and interleukin-18 (IL-18). The canonical inflammasomes are assembled by PRRs. Alternatively, the related inflammatory caspase-4 and caspase-5 (caspase-11 in mice) function as independent PRRs for cytoplasmic microbial lipopolysaccharide (LPS) activating a noncanonical inflammasome.

Because the mechanism of SASP regulation by inflammasomes remains ill-defined, we decided to define the role of these inflammatory caspases in senescence. We show here that caspase-4 activation by cytoplasmic LPS triggers a senescence phenotype. Moreover, we show here that the caspase-4 noncanonical inflammasome contributes critically to the establishment of the SASP and the reinforcement of the cell cycle arrest program during oncogene-induced senescence. In all, we describe a new and critical function for cytoplasmic sensing by the caspase-4 noncanonical inflammasome in cellular senescence.

Sirtuins may counteract organismal aging by altering the pattern of cellular stress response to generate much less disruption of tissue homeostasis. The alteration of cellular stress response pattern by sirtuins comprises (1) inhibition of apoptosis, (2) promoting DNA damage repair instead of apoptosis or induction of cellular senescence, (3) antioxidative action through activation of MnSOD, (4) preventing carcinogenesis through acting as tumor suppressor proteins, (5) inhibition of unnecessary inflammatory response/inflammaging through inactivation of NF-kB, and (6) preventing cellular senescence and senescence-associated secretory phenotype (SASP) through mitochondrial protection and promoting DNA damage repair.

All the effects listed above combined may prevent disruption of tissue homeostasis - directly responsible for organismal aging in vertebrates while being itself a distant derivative of a prolonged, inappropriate pattern of cellular response to accidental damage of the biostructure. The mechanisms discussed in this review describe how exactly sirtuin-dependent modifications of the cellular stress response can slow down aging at the tissue level. Thus, sirtuins, especially SIRT1, SIRT3, and SIRT6, can modify cellular stress response to promote maintenance of tissue homeostasis and thus slow down phenotypic aging at the organismal level.