Protective Efficacy of Novel Engineered Human ACE2-Fc Fusion Protein Against Pan-SARS-CoV-2 Infection In Vitro and in Vivo

Enduring occurrence of severe COVID-19 for unvaccinated, aged, or immunocompromised individuals remains an urgent need. Soluble human angiotensin-converting enzyme 2 (ACE2) has been used as a decoy receptor to inhibit SARS-CoV-2 infection, which is limited by moderate affinity. We describe an engineered, high-affinity ACE2 that is consistently effective in tissue cultures in neutralizing all strains tested, including Delta and Omicron. We also found that treatment of AC70 hACE2 transgenic mice with hACE2-Fc receptor decoys effectively reduced viral infection, attenuated tissue histopathology, and delayed the onset of morbidity and mortality caused by SARS-CoV-2 infection. We believe that using this ACE2-Fc protein would be less likely to promote the escape mutants of SARS-CoV-2 as frequently as did those neutralizing antibody therapies. Together, our results emphasize the suitability of our newly engineered hACE2-Fc fusion protein for further development as a potent antiviral agent against Pan-SARS-CoV-2 infection.

Applying Artificial Intelligence to Identify Common Targets for Treatment of Asthma, Eczema, and Food Allergy

INTRODUCTION

Allergic disorders are common diseases marked by the abnormal immune response toward foreign antigens that are not pathogens. Often patients with food allergy also suffer from asthma and eczema. Given the similarities of these diseases and a shortage of effective treatments, developing novel therapeutics against common targets of multiple allergies would offer an efficient and cost-effective treatment for patients.

METHODS

We employed the artificial intelligence-driven target discovery platform, PandaOmics, to identify common targets for treating asthma, eczema, and food allergy. Thirty-two case-control comparisons were generated from 15, 11, and 6 transcriptomics datasets related to asthma (558 cases, 315 controls), eczema (441 cases, 371 controls), and food allergy (208 cases, 106 controls), respectively, and allocated into three meta-analyses for target identification. Top-100 high-confidence targets and Top-100 novel targets were prioritized by PandaOmics for each allergic disease.

RESULTS

Six common high-confidence targets (i.e., IL4R, IL5, JAK1, JAK2, JAK3, and NR3C1) across all three allergic diseases have approved drugs for treating asthma and eczema. Based on the targets' dysregulated expression profiles and their mechanism of action in allergic diseases, three potential therapeutic targets were proposed. IL5 was selected as a high-confidence target due to its strong involvement in allergies. PTAFR was identified for drug repurposing, while RNF19B was selected as a novel target for therapeutic innovation. Analysis of the dysregulated pathways commonly identified across asthma, eczema, and food allergy revealed the well-characterized disease signature and novel biological processes that may underlie the pathophysiology of allergies.

CONCLUSION

Altogether, our study dissects the shared pathophysiology of allergic disorders and reveals the power of artificial intelligence in the exploration of novel therapeutic targets.

Anti-aging vaccines targeting senescent cells

The development of senomorphic drugs to attenuate the senescent phenotype and senolytics to clear pro-inflammatory senescent cells to treat aging-associated disorders is being hotly pursued. The effort is complicated by the fact that senescent cells play a constructive role in some cellular processes such as tissue repair and wound healing. However, concerns about efficacy, which senescent cells to target, and unwanted side effects have created potential roadblocks. Chimeric Antigen Receptor (CAR) T cells directed against urokinase-type plasminogen activator receptor (uPAR), which is expressed on at least a subset of senescent cells (SC) in atherosclerotic plaques and fibrotic livers, removed SC and improved glucose metabolism. A conventional vaccine targeting CD153-expressing senescent T-cells, also improved glucose metabolism in obese mice. Recent work to selectively target senescent cells associated with several pathologies has resulted in the creation of a peptide vaccine that primarily targets endothelial cells expressing high levels of GPNMB, recently identified as a biomarker of senescence. The vaccine reduces atherosclerotic plaque burden and metabolic dysfunction such as glucose intolerance in mouse models of obesity and atherosclerosis. For translation to humans the activity of the vaccine will need to be tightly controlled, as the target, GPNMB has multiple roles in normal physiology including acting to inhibit and possibly resolve inflammation. A promising alternative approach would be to use passive immunization with a monoclonal antibody directed against GPNMB.

Stem Cell Rejuvenation by Restoration of Youthful Metabolic Compartmentalization

Stem cell dysfunction is a hallmark of aging. Much recent study suggests that epigenetic changes play a critical role in the loss of stem cell function with age. However, the underlying mechanisms require elucidation. A recent report describes a process by which mild mitochondrial stress associated with aging causes lysosomal-mediated decreases in CiC, the mitochondrial citrate transporter, in bone marrow-derived mesenchymal stem cells (MSCs). This, in turn, results in a deficit of acetyl-CoA in the nucleus and hypoacetylation of histones. The altered epigenome results in skewered stem cell differentiation favoring adipogenesis and disfavoring osteogenesis, which is problematic given the role the MSCs play in maintaining the integrity of bone tissue. Restoration of nuclear acetyl-CoA by either ectopic expression of CiC or acetate supplementation of MSCs in culture rejuvenates the MSC, restoring the potential to efficiently differentiate along the osteogenic lineage. Citrate, which has recently been reported to extend lifespan in Drosophila, chemically incorporates acetyl-CoA and may prove useful to restore cytoplasmic and nuclear acetyl-CoA levels. The general applicability of the CiC defect in old cells, particularly stem cells, should be established.

Modulation of cGAS-STING Pathway by Nicotinamide Riboside in Alzheimer's Disease

Numerous studies demonstrate a global decrease in nicotinamide adenine dinucleotide (NAD+) with aging. This decline is associated with the development of several of the hallmarks of aging such as reduced mitophagy and neuroinflammation, processes thought to play a significant role in the progression of Alzheimer's disease (AD). Augmentation of NAD+ by oral administration of a precursor, nicotinamide riboside (NR), reduces senescence of affected cells, attenuates DNA damage and neuroinflammation in the transgenic APP/PS1 murine model of AD. Inflammation mediated by microglial cells plays an important role in progression of AD and other neurodegenerative diseases. The cytoplasmic DNA sensor, cyclic GMP-AMP synthase (cGAS) and downstream stimulator of interferon genes (STING), generates an interferon signature characteristic of senescence and inflammaging in the brain of AD mice. Elevated cGAS-STING observed in the AD mouse brains and human AD fibroblasts was normalized by NR. This intervention also increased mitophagy with improved cognition and behavior in the APP/PS1 mice. These studies suggest that modulation of the cGAS-STING pathway may benefit AD patients and possibly other disorders characterized by compromised mitophagy and excessive neuroinflammation.

Response to Hypoxia in Cognitive Decline

Inflammaging, the increase of proinflammatory processes with increasing age, has multiple mechanisms from increasing numbers of senescent cells secreting cytokines to changes in metabolic processes. Alterations of oxygen metabolism with aging, especially decreased levels of O₂ with age resulting from endocrine and cardiovascular dysfunction as well as desensitization of cellular response to hypoxia, may exacerbate inflammaging, which in turn creates further oxygen metabolic dysfunction. During aging, decline in levels of erythrocyte 2,3-bisphosphoglycerate (2,3-BPG), BPG mutase, and adenosine A2B receptor, a key adenosine signaling receptor that can augment 2,3-BPG expression, may fail to protect sensitive brain tissue from subtly reduced O₂ levels, in turn resulting in increased numbers of activated microglia and secretion of proinflammatory cytokines, ultimately promoting inflammaging and senescence of endothelial cells. Interventions to restore O₂ levels directly or via increasing 2,3-BPG may help promote cognitive health in old age, but significant work to quantify the degree of reduced O₂ during aging in mammals, and especially humans, needs to be pursued.

Supplementation with Brush Border Enzyme Alkaline Phosphatase Slows Aging

Diminished integrity of the intestinal epithelial barrier with advanced age is believed to contribute to aging-associated dysfunction and pathologies in animals. In mammals, diminished gut integrity contributes to inflammaging, the increase in inflammatory processes observed in old age. Recent work suggests that expression of intestinal alkaline phosphatase (IAP) plays a key role in maintaining gut integrity. IAP expression decreases with increasing age in mice and humans. Absence of IAP leads to liver inflammation and shortened life-spans in mice lacking the IAP gene. In normal mice, exogenous supplemental IAP reverses age-induced barrier dysfunction, improves aging-associated metabolic dysfunction, prevents microbiome dysbiosis (imbalance), and extends life-span. Consistent with IAP playing a conserved role in maintaining gut integrity, increased dietary IAP increases aging-diminished physical performance in flies. IAP helps maintain gut integrity in part by supporting the expression of tight junction proteins that maintain the intestinal epithelial barrier and by inactivating bacterial pro-inflammatory factors such as lipopolysaccharides (LPS) by dephosphorylation. Recombinant IAP is in late clinical trials for sepsis-associated acute kidney injury, suggesting it may soon become available as a therapeutic. Taken together, these reports support the idea that directly increasing IAP levels by supplemental recombinant IAP or by indirectly increasing IAP levels using dietary means to induce endogenous IAP may slow the development of aging-associated pathologies.

Beneficial Gut Microbiome Remodeled During Intermittent Fasting in Humans

Intermittent fasting (IF) is the practice of restricting food intake for 12-48 hours per fasting cycle over a prolonged period of time. Previous study shows beneficial health effects such as weight loss and lower risk for cardiometabolic diseases. Although reduced calorie intake may account for some of the observed benefits of IF, exact mechanisms are still unclear. Recent evidence indicates that IF may lead to remodeling and increased taxonomic diversity in the human gut microbiome. In particular, the Lachnospiraceae family of anaerobic bacteria increased during fasting. This family, in the order Clostridiales, promotes butryogenesis in the gut, a process that is associated with healthful metabolic and prolongevity effects. IF-associated alterations to the microbiome may play a key role in the metabolic and potential healthspan-enhancing benefits of IF and dietary restriction.

Exercise Partially Rejuvenates Muscle Stem Cells

Exercise has long been known to extend health and lifespan in humans and other mammals. However, typically exercise is thought to slow the loss of function that accompanies aging. Brett et al. have now shown that exercise restores functional competency to regenerate muscle stem cells (MuSCs) in mice as well as restore a significant portion of the transcriptional signature associated with young MuSCs. The mechanism involves the likely induction of plasma-borne factors that upregulate cell cycle regulator cyclin D1, which otherwise decreases with increasing age. Cyclin D1, in turn, through its noncanonical attenuation of TGF-beta/Smad3 signaling, helps maintain the regenerative capacity of MuSCs, which is lost as TGF-beta signaling increases with age. Interestingly, elevated levels of some proinflammatory regulators including NF-κB, TNF-alpha, and interleukin 6 (IL-6) are also reduced by exercise or ectopic expression of cyclin D1. Importantly, the rejuvenation is not complete, as Notch signaling, which also decreases with age, remains at old levels and the rejuvenative effect is not permanent: wearing off in ∼2 weeks after cessation of exercise. Understanding the limitations of the rejuvenative effect of exercise on MuSCs at the molecular level, including changes in the epigenome such as altered DNA methylation age, will be critical in developing more significant rejuvenative therapies including some for aged people wherein morbidities limit exercise.

Rejuvenation Through Oxygen, More or Less

Modest modulation of oxygen intake, either by inducing mild intermittent hypoxia or hyperoxia appears to induce modest rejuvenative changes in mammals, in part, by activating key regulator hypoxia-induced factor 1a (HIF-1a). Interestingly both lower oxygen and transient higher oxygen levels induce this hypoxia regulator. Hyperbaric oxygen induces HIF-1a by the hyperoxic-hypoxic paradox that results from an overinduction of protective factors under intermittent hyperoxic conditions, leading to a state somewhat similar to that induced by hypoxia. A key difference being that SIRT1 is induced by hyperoxia, whereas it is reduced during hypoxia by the activity of HIF-1a. In a recent report, a small clinical trial employing 60 sessions of intermittent hyperbaric oxygen therapy (HBOT) studying old humans resulted in increased mean telomere length of immune cells including B cells, natural killer cells, T helper, and cytotoxic T lymphocytes. Moreover, there was a reduction in CD28^null senescent T helper and cytotoxic T cells. In a parallel report, HBOT has been reported to enhance cognition in older adults, especially attention and information processing speed through increased cerebral blood flow (CBF) in brain regions where CBF tends to decline with age. The durability of these beneficial changes is yet to be determined. These preliminary results require follow-up, including more extensive characterization of changes in aging-associated biomarkers. An interesting avenue of potential work is to elucidate potential connections between hypoxia and epigenetics, especially the induction of the master pluripotent regulatory factors, which when expressed transiently have been reported to ameliorate some aging biomarkers and pathologies.

Metabolic Reprogramming Rejuvenates Aged Myeloid Cells Restoring Cognition

Inflammaging is associated with aging-associated cognitive loss and neurodegeneration. Chronic nonsteroidal anti-inflammatory drug (NSAID) use has been reported to reduce the incidence of Alzheimer's disease (AD), presumably by inhibiting inflammation, although NSAIDs appear to not be good candidates for anti-AD therapeutics given disappointing clinical trial results. Prostaglandin E2 (PGE2) acts downstream of NSAID target COX-2, a cyclooxygenase, to activate several G-protein coupled receptors (GPCRs) including EP2, which is now reported to reduce glycolysis and oxidative phosphorylation during aging by increasing glycogen synthesis and polarizing myeloid cells toward the M1 proinflammatory phenotype. Inhibiting EP2 using small molecule drugs polarizes macrophages toward the anti-inflammatory phenotype, restores youthful metabolism and mitochondrial morphology as well as youthful hippocampus-based memory capability. EP2 may be a better target than COXs for the development of drugs that improve age-associated mild cognitive impairment and possibly even for the development of drugs to treat dementias.

SUMO Wrestles with Mitophagy to Extend Lifespan

SUMOylation, a conserved protein post-translational modification that performs multiple functions including regulation of nuclear transport and transcription, is implicated in numerous biological processes including aging. RNAi knockdown of the sole Small Ubiquitin-like MOdifier (SUMO) gene, smo-1, in Caenorhabditis elegans shortened lifespan, whereas overexpression in the intestine modestly increased lifespan. Smo-1 is required for mitochondrial fission in a tissue-specific manner. Fission, in turn, is needed for mitophagy to maintain mitochondrial homeostasis during aging. SUMOlyation affects DAuer Formation (DAF)-16, which can be directly SUMOylated, and SKN-1, the homolog of mammalian Nrf2. These regulators play key roles in maintaining mitochondrial homeostasis. However, given the modest effect of overexpressing smo-1 on lifespan enhancement and potential interference with other genes that can promote increased lifespan, caution is advised in the translation of this study based on C. elegans. Although inhibitors of SUMOlyation have been developed for cancer and activators also have been identified, broad-acting biochemical pathway modifiers such as SUMO are often suboptimal drug targets and may not be as promising for antiaging applications as they first appear.

Contribution of Ferroptosis to Aging and Frailty

Ferroptosis is a recently characterized cell death phenotype resulting from iron-catalyzed peroxidation of polyunsaturated fatty acid phospholipids. Increased dysfunctional iron metabolism is thought to lead to increased levels of iron and ferroptosis, which in turn leads to cell and organismal death at least in the nematode Caenorhabditis elegans. Drugs that block lipid peroxidation or scavenge intracellular iron extend healthspan and lifespan in C. elegans independently of other mechanisms such as the daf-1/daf-16 (insulin/insulin-like growth factor 1 [IGF-1]) pathway, but unlike many aging mechanisms do not alter temporal scaling across the life cycle of C. elegans, but rather act at specific late points in the organism's life history, temporarily blocking execution of critical dysfunction that results in listless worms. As such, inhibition of ferroptosis may be a means to extend healthspan and treat frailty and possibly neurodegenerative diseases that have a reported role for iron dyshomeostasis. However, a significant effort to understand ferroptosis in the context of mammalian and human biology is necessary. For example, some tumors block ferroptosis to survive. The constraints of balancing iron metabolism are significant and will require careful consideration in any drug development program.

Eosinophils and White Fat: Protection from Worms and Inflammaging

Proinflammatory alterations of white adipose tissue (WAT) with increasing age play an important role in mammalian aging. WAT produced eotaxin-1 (CCL11-C-C motif chemokine ligand 11) and monocyte chemoattractant protein 1 (MCP-1) (CCL2) are elevated in old mammals. Obese and old adipose tissues produce excessive proinflammatory cytokines such as interleukin (IL)-6, CCL2, and IL-1-beta that contribute to inflammaging. WAT-based inflammaging involves an altered homeostatic equilibrium between proinflammatory cells such as activated type 1 macrophages, B cells (high IgJ) and T cells, and anti-inflammatory eosinophils and Tregs. Specifically, young and lean individuals exhibit a high eosinophil-to-macrophage ratio with an enrichment of alternative activated tissue macrophages that is reduced in the WAT of aging mice. Eosinophils from young animals adoptively transferred to old mice, home to WAT and reverse many of the immunoinflammatory signatures associated with aging. Whether eosinophil-based therapies for inflammaging could be created remains an open question.

Epigenetic Age Reversal by Cell-Extrinsic and Cell-Intrinsic Means

Reversal of aging by factors or drugs that reprogram adult cells to induced pluripotent stem cells suggests that at least at the cellular level aging may be reversible by resetting somatic cell state to a "ground state." An open question has been whether such rejuvenation is possible in whole organisms, especially in mammals. A related key question is whether rejuvenation can be dissociated from dedifferentiation. Several recent reports suggest that one prominent biomarker of mammalian aging, age-associated DNA methylation (DNAm) state that has been used to create DNAm age (DNAma) clocks, can be partially reversed by intrinsic treatment of cells with sets of reprogramming factors without affecting cell fate. Partial reprogramming using a superset of reprogramming factors applied transiently or subset of Yamanaka factors applied continually can increase regenerative potential, and reverse DNAma, while maintaining cell identity. Alternatively, a cell-extrinsic manipulation can accomplish something similar. A small preliminary clinical trial in humans suggests that systemic treatment with a cocktail of growth hormone, dehydroepiandrosterone, and metformin could also partially reverse DNAma and at the same time regenerate the thymus, which shrinks with age. Important questions are raised: How completely does reversing DNAma clocks embody a reversal of other age-related phenotypes, such as functional decline in strength, cognition, or immunity? How universal are these epigenetic changes at the tissue and cell levels? For example, do populations of younger stem cells exist that respond to these manipulations and then only confer the appearance of decreasing DNAma as they proliferate and differentiate? Together, these studies have profound implications for the development of antiaging and healthspan-enhancing therapies. A combination of both intrinsic and extrinsic modalities will most likely provide an optimal benefit.

Roads to the Fountain of Youth? Rejuvenating Intestinal Stem Cells

The intestinal stem cells (ISCs) of old mice and humans exhibit a reduced capacity for regeneration and repair. Compromised intestinal function may play a key role in systemic aging-related changes: not only in the affected gut, but also in the nervous and cardiovascular systems. For example, progression of age-related neurodegenerative diseases such as Alzheimer's and Parkinson's has been linked to increased inflammation from gut microbiota in old mammals, which, in turn, may be linked bidirectionally with reduced ISC function. Intestinal organoid formation has been used to dissect the mechanisms of decline of ISC function. Alterations of the Wnt pathway, including downregulation of Wnt ligands in ISCs and upregulation of Wnt ligand inhibitor Notum in Paneth cells, and dysregulation of mTORC1 contribute to the observed age-related decline. Short-term fasting, caloric restriction, and peroxisome proliferator-activated receptor delta agonists have been reported to increase ISC function in adult mice. Moreover, the mTOR inhibitor rapamycin, NAD+ precursor nicotinamide riboside, and ABC99, a small molecule Notum inhibitor, have all been reported to rejuvenate ISC function in old mice and thus may have promise in humans. However, there is some controversy over the key mechanisms involved in loss of function of ISCs, which likely results, in part, from differences in how the in vitro organoid assays are performed. Moreover, how the microbiome modulates the function of ISCs and vice versa remains to be elucidated.

The Danger of Being Too Sympathetic: Norepinephrine in Alzheimer's Disease and Graying of Hair

Although alterations in the sympathetic nervous system (SNS) with age have been reported, and serious degenerative diseases of the autonomic nervous system such as multiple system atrophy are more likely to strike older people, connections between dysregulated adrenergic receptors and age-associated diseases and phenotypes have not been well studied. Two recent reports suggest that SNS may be more closely connected than previously appreciated. First, low nanomolar concentrations of Alzheimer's disease (AD)-associated Aβ42-amyloid oligomers alter signaling by SNS neurotransmitter norepinephrine (NE) to sufficiently activate kinase GSK3β to hyperphosphorylate tau, a key mediator of neurotoxicity in AD. Connecting beta-amyloid to tau in AD has been a key quest in understanding AD and developing therapeutics. The α₂ adrenergic receptor inhibitory drug idazoxan reduces GSK3β activity and tau phosphorylation in AD mice with improved cognitive function, even in the presence of beta-amyloid deposits. In this study, SNS activation in the brain coupled with problematic Aβ42-amyloid oligomers result in serious consequences that can be ameliorated by reducing SNS signaling. A second example of the detrimental effects of increased SNS signaling is the premature graying of hair in response to stress. Secretion of NE resulting from stress causes differentiation of most hair pigment melanocyte stem cells (MeSCs) into melanocytes, rapidly depleting the hair follicle of pigment-producing cells as mature melanocytes undergo apoptosis and MeSCs are eventually eliminated. Blockade of NE SNS signaling preserves hair coloration in stressed animals. Increased SNS activation has serious apparently irreversible effects on homeostasis in both situations. Although neither report directly addresses aging, given that AD and the loss of hair pigmentation have strong age associations, it is of interest to better understand the role that SNS has in promoting age-associated phenotypes generally and determine if tuning the SNS through drug-mediated attenuation of SNS signaling may be of medical benefit.

Interacting NAD+ and Cell Senescence Pathways Complicate Antiaging Therapies

During human aging, decrease of NAD⁺ levels is associated with potentially reversible dysfunction in the liver, kidney, skeletal and cardiac muscle, endothelial cells, and neurons. At the same time, the number of senescent cells, associated with damage or stress that secretes proinflammatory factors (SASP or senescence-associated secretory phenotype), increases with age in many key tissues, including the kidneys, lungs, blood vessels, and brain. Senescent cells are believed to contribute to numerous age-associated pathologies and their elimination by senolytic regimens appears to help in numerous preclinical aging-associated disease models, including those for atherosclerosis, idiopathic pulmonary fibrosis, diabetes, and osteoarthritis. A recent report links these processes, such that decreased NAD⁺ levels associated with aging may attenuate the SASP potentially reducing its pathological effect. Conversely, increasing NAD⁺ levels by supplementation or genetic manipulation, which may benefit tissue homeostasis, also may worsen SASP and encourage tumorigenesis at least in mouse models of cancer. Taken together, these findings suggest a fundamental trade-off in treating aging-related diseases with drugs or supplements that increase NAD⁺. Even more interesting is a report that senescent cells can induce CD38 on macrophages and endothelial cells. In turn, increased CD38 expression is believed to be the key modulator of lowered NAD⁺ levels with aging in mammals. So, accumulation of senescent cells may itself be a root cause of decreased NAD⁺, which in turn could promote dysfunction. On the contrary, the lower NAD⁺ levels may attenuate SASP, decreasing the pathological influence of senescence. The elimination of most senescent cells by senolysis before initiating NAD⁺ therapies may be beneficial and increase safety, and in the best-case scenario reduce the need for NAD⁺ supplementation.

Increased REST to Optimize Life Span?

Reduced levels of neural activity are associated with a longer life span in the nematode Caenorhabditis elegans and in mice. Augmented neural activity is associated with a shorter life span. Recent studies show that levels of repressor element 1-silencing transcription factor (REST) increase with normal aging in mice and humans, and reduce neuronal excitation. In C. elegans, increased expression of spr-4, a functional REST homologue, increased the worm life span and is required for classical life span increase mediated by reduced DAF-2/insulin-IGF-1 and increased DAF-16. Preliminary evidence shows that REST and FOXO1, a DAF-16, homologue increase during mammalian aging, and that REST activity is needed for the age-related FOXO1 increase. On the contrary, REST is activated in epilepsy and plays a role in the pathogenesis of Huntington's disease. A simple unifying hypothesis suggests that REST is a "goldilocks-effect factor": too little REST promotes excitotoxic activity, which in turn leads to neurodegenerative diseases such as Alzheimer's. Appropriate increased levels of REST maintain the excitation/inhibition (E-I) balance by reducing potential excitotoxic activity. Increased levels of REST beyond this are toxic as neurons become dysfunctional due to loss of a neuronal phenotype.

Cellular Senescence as the Key Intermediate in Tau-Mediated Neurodegeneration

Neuroinflammation is thought to play a key role in the progression of neurodegenerative disease such as Alzheimer's disease (AD). Given the apparent nexus of inflammatory disease with the secretory-associated senescence phenotype (SASP) of cellular senescence, two reports found that tau-mediated neurodegeneration involves induction of senescence in astrocytes, microglia, and possibly even neurons. Elimination of senescent cells by pharmacologically induced genetic ablation or by senolytic drugs blocks progression of mutant human tau-mediated neurodegeneration in mice. This work suggests a working hypothesis through which tau activation leads to senescence and then tau propagation throughout the brain is supported by, and neurotoxicity is caused by, SASP, forming a pathological positive feedback loop. Although preliminary, these data suggest that the development of senolytics for AD and other tauopathies, especially early disease, and possibly senomodulatory drugs for later stage neurodegenerative disease may prove fruitful.

Epigenetic Drift Is a Determinant of Mammalian Lifespan

The epigenome, which controls cell identity and function, is not maintained with 100% fidelity in somatic animal cells. Errors in the maintenance of the epigenome lead to epigenetic drift, an important hallmark of aging. Numerous studies have described DNA methylation clocks that correlate epigenetic drift with increasing age. The question of how significant a role epigenetic drift plays in creating the phenotypes associated with aging remains open. A recent study describes a new DNA methylation clock that can be slowed by caloric restriction (CR) in a way that correlates with the degree of lifespan and healthspan extension conferred by CR, suggesting that epigenetic drift itself is a determinant of mammalian lifespan. Genetic transplantation using genomic editing of DNA methylation homeostatic genes from long-lived to short-lived species is one way to potentially demonstrate a causative role for DNA methylation. Whether the DNA methylation clock be reset to youthful state, eliminating the effects of epigenetic drift without requiring a pluripotent cell intermediate is a critical question with profound implications for the development of aging therapeutics. Methods that transiently erase the DNA methylation pattern of somatic cells may be developed that reset this aging hallmark with potentially profound effects on lifespan, if DNA methylation-based epigenetic drift really plays a primary role in aging.

Mesenchymal Stem Cells for Frailty?

Frailty is a medical syndrome associated with advancing age characterized by reduced functional reserve, strength, endurance, and susceptibility to infection associated with high morbidity, hospitalization, and death. Nonspecific interventions to improve the healthspan of affected patients include physical therapy, exercise, improved nutrition, etc. Among the hallmarks of aging, depletion of stem cells with resultant compromise of regeneration and repair of tissues informs a rational stem cell-based replacement strategy. This hypothesis has been evaluated in a randomized, double-blind, placebo-controlled clinical trial utilizing human allogeneic mesenchymal stem cells (allo-hMSCs), a facile, scalable stem cell replacement therapy. Intravenous infusion of 100 or 200 million allo-hMSCs was deemed safe in aged frail individuals. However, modest improvement outcomes were limited to the lower dose, a finding that remains difficult to explain. Future studies are definitely warranted given the magnitude of this increasingly important medical syndrome.

Finally, a Regimen to Extend Human Life Expectancy

The United States has the most expensive healthcare system worldwide. Yet measures of health span and life expectancy are well below the major industrialized nations. With the U.S. population aged 65 years and older projected to double by mid-century, a healthcare crisis is looming. Within this context, huge interest and investment have emerged in technologies and drugs to address aging with an expected benefit to health span. The thesis being that such basic interventions will reduce morbidity caused by many chronic diseases wherein biological age itself is the major risk factor. In the light of limited progress to date, a recent study out of the Harvard School of Public Health is quite refreshing: less than half dozen lifestyle interventions can greatly increase health span. Perhaps these are familiar: cessation of smoking, ≥30 minutes of moderate daily exercise, high-quality diet (limited processed food), modest alcohol intake, and maintenance of an optimal body mass index of 18.5-24.9 kg/m². From age 50 years, women engaging in all of these behaviors versus those who do zero can expect to have a life expectancy of 43.1 additional years (an extra 14 years) with men gaining 37.6 years (an extra 12.2 years). A regimen to extend life expectancy is at hand. However, there is room for optimization by including the effects of sleep, intermittent fasting, and/or caloric restriction. Moreover, the extension of life expectancy by adherence to a healthy lifestyle revises the health span threshold for antiaging treatments under development and should provide a better set of controls for clinical trials investigating novel treatments of aging.

The NAD+/PARP1/SIRT1 Axis in Aging

NAD+ levels decline with age in diverse animals from Caenorhabditis elegans to mice. Raising NAD+ levels by dietary supplementation with NAD+ precursors, nicotinamide riboside (NR) or nicotinamide mononucleotide (NMN), improves mitochondrial function and muscle and neural and melanocyte stem cell function in mice, as well as increases murine life span. Decreased NAD+ levels with age reduce SIRT1 function and reduce the mitochondrial unfolded protein response, which can be overcome by NR supplementation. Decreased NAD+ levels cause NAD+-binding protein DBC1 to form a complex with PARP1, inhibiting poly(adenosine diphosphate-ribose) polymerase (PARP) catalytic activity. Old mice have increased amounts of DBC1-PARP1 complexes, lower PARP activity, increased DNA damage, and reduced nonhomologous end joining and homologous recombination repair. DBC1-PARP1 complexes in old mice can be broken by increasing NAD+ levels through treatment with NMN, reducing DNA damage and restoring PARP activity to youthful levels. The mechanism of declining NAD+ levels and its fundamental importance to aging are yet to be elucidated. There is a correlation of PARP activity with mammalian life span that suggests that NAD+/SIRT1/PARP1 may be more significant than the modest effects on life span observed for NR supplementation in old mice. The NAD+/PARP1/SIRT1 axis may link NAD+ levels and DNA damage with the apparent epigenomic DNA methylation clocks that have been described.

Extracellular Vesicle-Mediated Reversal of Paclitaxel Resistance in Prostate Cancer

Prostate cancer (PCa) is the most common solid tumor in males and the second leading cause of cancer-related deaths in males in the United States. The current first line therapy for metastatic PCa is androgen deprivation therapy and is initially effective against the disease. However, castrate resistant prostate cancer (CRPC) develops in many men within 18-36 months, rendering this treatment ineffective. Chemotherapy, with a class of drugs known as taxanes is the standard-of-care cytotoxic option in metastatic castrate resistant PCa (mCRPC). However, the overall survival advantage for chemotherapy in mCRPC is only 2.2 months and the cancer cells often become resistant to these drugs as well. Once patients fail chemotherapy the progression to death is inevitable. Extracellular vesicles (EVs) are involved in cell signaling and play a role in cancer progression. Previous work has demonstrated that EVs are involved in the development of drug resistance in cancer cells. We report the reversal of taxane resistance and tumorigenic phenotype in PCa cells after EVs treatment. This study suggests that EVs represent a potentially novel therapeutic treatment option for CRPC.

Rejuvenation by Partial Reprogramming of the Epigenome

Epigenetic variation with age is one of the most important hallmarks of aging. Resetting or repairing the epigenome of aging cells in intact animals may rejuvenate the cells and perhaps the entire organism. In fact, differentiated adult cells, which by definition have undergone some epigenetic changes, are capable of being rejuvenated and reprogrammed to create pluripotent stem cells and viable cloned animals. Apparently, such reprogramming is capable of completely resetting the epigenome. However, attempts to fully reprogram differentiated cells in adult animals have failed in part because reprogramming leads to the formation of teratomas. A preliminary method to partially reprogram adult cells in mature Hutchinson-Gilford Progeria Syndrome (HGPS) mice by transient induction of the Yamanaka factors OSKM (Oct4/Sox2/Klf4/c-Myc) appears to ameliorate aging-like phenotypes in HGPS mice, and promote youthful regenerative capability in middle-aged wild-type individuals exposed to beta cell and muscle cell-specific toxins. However, whatever epigenetic repair is induced by transient reprogramming does not endure and may be due to the induction of key homeostatic regulators instead. Some of the effect of transient reprogramming may result from increased proliferation and enhanced function of adult stem cells. Partial reprogramming may point the way to new antiaging and proregenerative therapeutics. Redifferentiation of cells into their preexisting phenotype with simultaneous epigenomic rejuvenation is an interesting variation that also should be pursued. However, discovery of methods to more precisely repair the epigenome is the most likely avenue to the development of powerful new antiaging agents.

Pharmaceutical Rejuvenation of Age-Associated Decline in Spatial Memory

Spatial memory and cognition decline during aging. Montelukast, an FDA approved drug for the treatment of asthma, can restore spatial memory in old rats to levels similar to those of young animals. Treatment improves three hallmarks of aging in the brain: reducing microglial-mediated neuroinflammation, blood-brain barrier (BBB) permeability, and increasing neurogenesis in the hippocampus although not completely to youthful levels. Other aging-associated parameters, such as reduced synaptic density, are not affected, suggesting that anti-aging therapeutics may be further optimized. Montelukast targets leukotriene receptors GPR17 and CysLTR1 and appears to invert leukotriene signaling, converting an inflammatory signal into an anti-inflammatory signal. This acts as a dominant factor to overcome the dysfunctional effects of aging reportedly mediated, in part, by blood-borne factors such as beta-2 microglobulin that inhibit neurogenesis in the dentate gyrus of the hippocampus. The key mechanism for cognitive improvement by montelukast may be restoration of BBB integrity, which would presumably decrease the amount of deleterious blood-borne factors to enter the brain. Whether or not this hypothesis is true for montelukast, drugs that restore or maintain BBB integrity may be useful in combating age-related loss of cognitive function.

Preclinical Reversal of Atherosclerosis by FDA-Approved Compound that Transforms Cholesterol into an Anti-Inflammatory "Prodrug"

Although atherosclerosis is treatable with lipid-lowering drugs, not all patients respond. Hydroxypropyl-beta-cyclodextrin (CD) is an FDA-approved compound for solubilizing, capturing, and delivering lipophilic drugs in humans. Zimmer et al. report that CD mediates regression of atherosclerotic plaques in two mouse models by solubilizing cholesterol crystals (CCs), and promoting metabolism of CCs into water-soluble 27-hydroxycholesterol, which, in turn, activates anti-inflammatory LXR receptor target genes, promotes active and passive efflux of cholesterol from macrophages, and increases metabolic processing of cholesterol. In effect, CD inverts the role of its cargo, cholesterol, from inflammatory to anti-inflammatory by converting cholesterol into a "prodrug" that when modified to 27-hydroxycholesterol reduces atherosclerosis. This mechanism defines a new class of pharmaceuticals, "inverters": compounds that cause innate biomolecules to act opposite to their normal function. However, chronic CD treatment in animal models damages auditory cells, which must be addressed before CD can be developed as a systemic drug for atherosclerosis.

Thymus Maintenance and Regeneration by Specific Molecular Factors

Although the thymus plays a key role in T cell maturation in mammals, it begins to atrophy and involute at sexual maturity. The diminished thymic microenvironment is thought to contribute to reduced adaptive immune function during aging, leading to the increased likelihood of infectious diseases and cancer. Caloric restriction or ectopic expression of the pro-longevity growth factor fibroblast growth factor 21 has been reported to maintain the thymus in aging mice. Moreover, forced expression of the transcription factor FoxN1 has been shown to rejuvenate thymuses from old mice almost completely, restoring their youthful state. These results open the way for development of potential drugs to restore immune function in the elderly.

Reversal of Aged Muscle Stem Cell Dysfunction

The loss of muscle stem cell (MuSC) numbers and function in the elderly results in a dramatic delay or incomplete repair of muscle following injury or surgery. Prolonged immobility can exacerbate the loss of muscle mass with increased morbidity of affected patients. Stem cells and their niche cooperate to regulate the activation, self-renewal, differentiation, and return to quiescence of MuSCs. Extracellular matrix fibronectin (FN) and MuSC β1-integrin have been identified as critical factors in the dysfunction of aging muscle. Reduced amounts and/or function of β1-integrin and fibronectin are critical factors in the decline in MuSC regeneration and homeostasis with aging. Replacement of fibronectin and/or stimulation of β1-integrin may provide a novel means to augment the decline in MuSC function with age.

Telomerase Reverse Transcriptase and Peroxisome Proliferator-Activated Receptor γ Co-Activator-1α Cooperate to Protect Cells from DNA Damage and Mitochondrial Dysfunction in Vascular Senescence

Reduced telomere length with increasing age in dividing cells has been implicated in contributing to the pathologies of human aging, which include cardiovascular and metabolic disorders, through induction of cellular senescence. Telomere shortening results from the absence of telomerase, an enzyme required to maintain telomere length. Telomerase reverse transcriptase (TERT), the protein subunit of telomerase, is expressed only transiently in a subset of adult somatic cells, which include stem cells and smooth muscle cells. A recent report from Xiong and colleagues demonstrates a pivotal role for the transcription co-factor peroxisome proliferator-activated receptor γ co-activator-1α (PGC-1α) in maintaining TERT expression and preventing vascular senescence and atherosclerosis in mice. Ablation of PGC-1α reduced TERT expression and increased DNA damage and reactive oxygen species (ROS), resulting in shortened telomeres and vascular senescence. In the ApoE(-/-) mouse model of atherosclerosis, forced expression of PGC-1α increased expression of TERT, extended telomeres, and reversed genomic DNA damage, vascular senescence, and the development of atherosclerotic plaques. Alpha lipoic acid (ALA) stimulated expression of PGC-1α and TERT and reversed DNA damage, vascular senescence, and atherosclerosis, similarly to ectopic expression of PGC-1α. ALA stimulated cyclic adenosine monophosphate (cAMP) signaling, which in turn activated the cAMP response element-binding protein (CREB), a co-factor for PGC-1α expression. The possibility that ALA might induce TERT to extend telomeres in human cells suggests that ALA may be useful in treating atherosclerosis and other aging-related diseases. However, further investigation is needed to identify whether ALA induces TERT in human cells, which cell types are susceptible, and whether such changes have clinical significance.

Stem Cell Depletion by Global Disorganization of the H3K9me3 Epigenetic Marker in Aging

Epigenomic change and stem cell exhaustion are two of the hallmarks of aging. Accumulation of molecular damage is thought to underlie aging, but the precise molecular composition of the damage remains controversial. That some aging phenotypes, especially those that result from impaired stem cell function, are reversible suggest that such "damage" is repairable. Evidence is accumulating that dysfunction in aging stem cells results from increasing, albeit, subtle disorganization of the epigenome over time. Zhang et al. (2015) report that decreasing levels of WRN, Werner's syndrome (WS) helicase, with increasing age results in loss of heterochromatin marks in mesenchymal stem cells (MSCs) and correlates with an increased rate of cellular senescence. Although WRN plays a role in DNA repair, WRN exerted its effects on aging via maintaining heterochromatin, evidenced by reduced levels of interacting chromatin regulators heterochromatin protein 1α (HP1α), suppressor of variegation 3-9 homolog 1 (SUV39H1), and lamina-associated polypeptide 2β (LAP2β) as well as modified histone H3K9me3. Reducing expression of chromatin modeling co-factors SUV39H1 or HP1α in wild-type MSCs recapitulates the phenotype of WRN deficiency, resulting in reduced H3K9me3 levels and increased senescence without induction of markers of DNA damage, suggesting that chromatin disorganization and not DNA damage is responsible for the pathology of WS during aging in animals. Ectopic expression of HP1α restored H3K9me3 levels and repressed senescence in WRN-deficient MSCs. That HP1α can also suppress senescence in Hutchinson-Gilford progeria syndrome (HGPS) and extend life span in flies when over-expressed suggests that HP1α and H3K9me3 play conserved roles in maintenance of cell state. H3K9me3 levels are dynamic and expected to be potentially responsive to manipulation by extrinsic factors. Recent reports that migration inhibitory factor (MIF) or periodic fasting rejuvenate old MSCs provide the opportunity to link intrinsic and extrinsic mechanisms of aging in novel and potentially medically important ways and may lead to anti-aging treatments that reorganize the epigenome to rejuvenate cells and tissues.

Next-Generation Sequencing for Binary Protein-Protein Interactions

The yeast two-hybrid (Y2H) system exploits host cell genetics in order to display binary protein-protein interactions (PPIs) via defined and selectable phenotypes. Numerous improvements have been made to this method, adapting the screening principle for diverse applications, including drug discovery and the scale-up for proteome wide interaction screens in human and other organisms. Here we discuss a systematic workflow and analysis scheme for screening data generated by Y2H and related assays that includes high-throughput selection procedures, readout of comprehensive results via next-generation sequencing (NGS), and the interpretation of interaction data via quantitative statistics. The novel assays and tools will serve the broader scientific community to harness the power of NGS technology to address PPI networks in health and disease. We discuss examples of how this next-generation platform can be applied to address specific questions in diverse fields of biology and medicine.

Aging Stem Cells Lose the Capability to Distribute Damaged Proteins Asymmetrically

Understanding the interplay between reversible epigenetic changes and potentially more difficult to reverse accumulation of damaged macromolecules is a central challenge in developing treatments for aging-associated dysfunction. One hypothesis is that epigenetic drift leads to subtle losses of homeostatic maintenance mechanisms, that in turn, lead to the accumulation of damaged macromolecules, which then further degrade homeostasis. A key mechanism of maintaining optimal cell function is asymmetrical division, whereby cellular damage is segregated away from cells that need to undergo further proliferation, such as stem cells. Such asymmetrical distribution of damaged macromolecules has been observed during cell division in many organisms, from yeast to human embryonic stem cells, and depends on diffusion barriers (DBs) in the membrane of the endoplasmic reticulum (ER). In a recent study, these results have been extended to neural stem cells (NSCs), in which the ability of the ER DB to promote asymmetrical distribution of damaged proteins deteriorates with age. NSC function declines with age as proliferative capacity is reduced. The loss of asymmetric protein distribution correlates with the loss of NSC proliferative capacity. Ectopic expression of progerin, an altered form of lamin A, is associated with the premature aging disorder, Hutchinson-Gilford progeria syndrome (HGPS). Progerin's expression also increases with normal aging due to mis-splicing, weakening the ER DB. Recent work suggests that many cell signaling pathway changes associated with HGPS are replicated during normal aging in cultured cells. Moreover, the detrimental changes associated with progerin expression in HGPS are partially reversible experimentally after treatment with statins, a farnesyltransferase inhibitor, a isoprenylcysteine carboxyl methyltransferase inhibitor, or sulforaphane. It will be of great interest if these compounds can also reverse the aging-associated permeability of the ER DB and restore stem cell function.

Systemic factors mediate reversible age-associated brain dysfunction

Brain function declines in aging mammals. Recent work has identified dysregulation of key blood-borne factors whose altered expression during aging diminishes brain function in mice. Increased C-C motif chemokine 11 (CCL11) expression with aging is detrimental to brain function. On the other hand, plasma levels of the trophic factor growth/differentiation factor 11 (GDF11) decrease with aging. Restoration of youthful levels of GDF11 by injection partially restores brain function and neurogenesis by improving endothelial cell function and vasculature. Moreover, GDF11 has a rejuvenative effect on cardiac and skeletal muscle. Decreased type II interferon (IFN-II) and increased type I interferon (IFN-I) signaling during aging at the choroid plexus (CP), which constitutes the brain-cerebrospinal fluid barrier (B-CSF-B), negatively effects brain function. Blood from young mice contains factors that restore IFN-II levels. IFN-II is required for maintenance of the CP, and low IFN-II levels are associated with decreased cognitive abilities. IFN-I levels appear to drive increased CCL11 expression through the CSF. Blood from young animals does not restore IFN-I levels. However, injecting anti-interferon-α/β receptor (IFNAR) antibodies into the CSF inhibits downstream IFN-I gene and protein expression and decreases expression of CCL11, partially restoring neurogenesis and cognitive function. These results suggest that IFN-I plays a critical role in increasing CCL11 during aging of the brain. An emerging theme is that aging-associated loss of function in mammals may involve a set of defined, potentially reversible changes in many tissues and organs, including the brain, permitting development of potential rejuvenative therapies.

Epigenetic/Genetic Mismatch: Using Transdifferentiation as a Potential Cancer Therapy to Exploit the Cell Type Specificity of Cancer

Every cell type capable of proliferation can be malignantly transformed. However, there appears to be no naturally occurring universal set of genetic mutations capable of converting every cell type to a malignant state. Any specific cell type is generally resistant to transformation by the cancer mutations accumulated by cells of different lineages, presumably due to epigenetic differences. Evidence for this idea derives from experiments in which the developmental fates of cancer cells are altered to reduce malignancy. Reprogramming cancer cells to more primitive developmental states using pluripotency factors (IPS) or somatic nuclear transfer suppresses the malignant phenotype, as does subsequent directed differentiation to mature cells of lineages distinct from the originating cell. Direct transdifferentiation to an alternative cell fate also reduces tumorigenicity. In contrast, after reprogramming, cells induced to redifferentiate toward the original tumor cell type are tumorigenic. In these types of experiments an epigenetic/genetic mismatch often results in suppression of malignancy or cell death. Elucidating the specific transcription and cell signaling network incompatibilities will identify new targets for cancer therapy. Moreover, novel strategies to induce an incompatible transdifferentiated state, in which expression of thousands of genes are altered, will prove useful in controlling malignancies that otherwise easily evolve resistance to single target-based therapeutics. Engineering small molecules, genetic vectors, cytokines, growth factors, targeted extracellular vesicles, and cell fusion will help realize transdifferentiation-based therapeutics for cancer.

Partial reversal of skeletal muscle aging by restoration of normal NAD⁺ levels

That some aging-associated phenotypes may be reversible is an emerging theme in contemporary aging research. Gomes et al. report that age-associated oxidative phosphorylation (OXPHOS) defects in murine skeletal muscle are biphasic. In the first phase, OXPHOS is decreased because of reduced expression of mitochondrially encoded genes. Treatment of moderately old mice (first-phase OXPHOS defects) with nicotinamide adenine dinucleotide (NAD⁺) precursor nicotinamide mononucleotide (NMN) for 1 week restores oxidative phosphorylation activity and other markers of mitochondrial function in skeletal muscle. However, muscle strength is not restored. In very old animals (second-phase OXPHOS defects), expression of OXPHOS genes from both the nucleus and mitochondria is reduced and mitochondrial DNA integrity is diminished. Gomes et al. propose a model linking decreased NAD⁺ to loss of nuclear SIRT1 activity to stabilization of the hypoxia-associated transcription factor hypoxia-inducible factor 1-alpha (HIF-1a). HIF-1a promotes an hypoxic-like (Warburg effect) state in the cell. The HIF-1a protein interacts with c-Myc, decreasing c-Myc-regulated transcription of the key mitochondrial regulator mitochondrial transcription factor A (TFAM). Low levels of TFAM lead to first-phase OXPHOS dysfunction. The transition to irreversible phase 2 dysfunction remains to be characterized, but may be related to increased reactive oxygen species (ROS) production. This model suggests that intervention in mitochondrial aging may be possible using appropriate NAD⁺ precursors such as nicotinamide riboside. Restoring NAD⁺ levels may be beneficial throughout the organism. For example, aging-associated disturbances in circadian rhythm are linked to diminished SIRT1 activity, and loss of hematopoietic stem cell function to reduced SIRT3. Work to elucidate other biphasic aging mechanisms is strongly encouraged.

Sleep facilitates clearance of metabolites from the brain: glymphatic function in aging and neurodegenerative diseases

Decline of cognition and increasing risk of neurodegenerative diseases are major problems associated with aging in humans. Of particular importance is how the brain removes potentially toxic biomolecules that accumulate with normal neuronal function. Recently, a biomolecule clearance system using convective flow between the cerebrospinal fluid (CSF) and interstitial fluid (ISF) to remove toxic metabolites in the brain was described. Xie and colleagues now report that in mice the clearance activity of this so-called "glymphatic system" is strongly stimulated by sleep and is associated with an increase in interstitial volume, possibly by shrinkage of astroglial cells. Moreover, anesthesia and attenuation of adrenergic signaling can activate the glymphatic system to clear potentially toxic proteins known to contribute to the pathology of Alzheimer disease (AD) such as beta-amyloid (Abeta). Clearance during sleep is as much as two-fold faster than during waking hours. These results support a new hypothesis to answer the age-old question of why sleep is necessary. Glymphatic dysfunction may pay a hitherto unsuspected role in the pathogenesis of neurodegenerative diseases as well as maintenance of cognition. Furthermore, clinical studies suggest that quality and duration of sleep may be predictive of the onset of AD, and that quality sleep may significantly reduce the risk of AD for apolipoprotein E (ApoE) ɛ4 carriers, who have significantly greater chances of developing AD. Further characterization of the glymphatic system in humans may lead to new therapies and methods of prevention of neurodegenerative diseases. A public health initiative to ensure adequate sleep among middle-aged and older people may prove useful in preventing AD, especially in apolipoprotein E (ApoE) ɛ4 carriers.