Effects of rapamycin on growth hormone receptor knockout mice

Metabolaging: a new geroscience perspective linking aging pathologies and metabolic dysfunction

With age, our metabolic systems undergo significant alterations, which can lead to a cascade of adverse effects that are implicated in both metabolic disorders, such as diabetes, and in the body's ability to respond to acute stress and trauma. To elucidate the metabolic imbalances arising from aging, we introduce the concept of "metabolaging." This framework encompasses the broad spectrum of metabolic disruptions associated with the hallmarks of aging, including the functional decline of key metabolically active organs, like the adipose tissue. By examining how these organs interact with essential nutrient-sensing pathways, "metabolaging" provides a more comprehensive view of the systemic metabolic imbalances that occur with age. This concept extends to understanding how age-related metabolic disturbances can influence the response to acute stressors, like burn injuries, highlighting the interplay between metabolic dysfunction and the ability to handle severe physiological challenges. Finally, we propose potential interventions that hold promise in mitigating the effects of metabolaging and its downstream consequences.

Enhancing immunity during ageing by targeting interactions within the tissue environment

Immunity declines with age. This results in a higher risk of age-related diseases, diminished ability to respond to new infections and reduced response to vaccines. The causes of this immune dysfunction are cellular senescence, which occurs in both lymphoid and non-lymphoid tissue, and chronic, low-grade inflammation known as 'inflammageing'. In this Review article, we highlight how the processes of inflammation and senescence drive each other, leading to loss of immune function. To break this cycle, therapies are needed that target the interactions between the altered tissue environment and the immune system instead of targeting each component alone. We discuss the relative merits and drawbacks of therapies that are directed at eliminating senescent cells (senolytics) and those that inhibit inflammation (senomorphics) in the context of tissue niches. Furthermore, we discuss therapeutic strategies designed to directly boost immune cell function and improve immune surveillance in tissues.

GHR disruption in mature adult mice alters xenobiotic metabolism gene expression in the liver

BACKGROUND

Lifelong reduction of growth hormone (GH) action extends lifespan and improves healthspan in mice. Moreover, congenital inactivating mutations of GH receptor (GHR) in mice and humans impart resistance to age-associated cancer, diabetes, and cognitive decline. To investigate the consequences of GHR disruption at an adult age, we recently ablated the GHR at 6-months of age in mature adult (6mGHRKO) mice. We found that both, male and female 6mGHRKO mice have reduced oxidative damage, with males 6mGHRKO showing improved insulin sensitivity and cancer resistance. Importantly, 6mGHRKO females have an extended lifespan compared to controls.

OBJECTIVE AND METHODS

To investigate the possible mechanisms leading to health improvements, we performed RNA sequencing using livers from male and female 6mGHRKO mice and controls.

RESULTS

We found that disrupting GH action at an adult age reduced the gap in liver gene expression between males and females, making gene expression between sexes more similar. However, there was still a 6-fold increase in the number of differentially expressed genes when comparing male 6mGHRKO mice vs controls than in 6mGHRKO female vs controls, suggesting that GHR ablation affects liver gene expression more in males than in females. Finally, we found that lipid metabolism and xenobiotic metabolism pathways are activated in the liver of 6mGHRKO mice.

CONCLUSION

The present study shows for the first time the specific hepatic gene expression profile, cellular pathways, biological processes and molecular mechanisms that are driven by ablating GH action at a mature adult age in males and females. Importantly, these results and future studies on xenobiotic metabolism may help explain the lifespan extension seen in 6mGHRKO mice.

Targeting the biology of aging with mTOR inhibitors

Inhibition of the protein kinase mechanistic target of rapamycin (mTOR) with the Food and Drug Administration (FDA)-approved therapeutic rapamycin promotes health and longevity in diverse model organisms. More recently, specific inhibition of mTORC1 to treat aging-related conditions has become the goal of basic and translational scientists, clinicians and biotechnology companies. Here, we review the effects of rapamycin on the longevity and survival of both wild-type mice and mouse models of human diseases. We discuss recent clinical trials that have explored whether existing mTOR inhibitors can safely prevent, delay or treat multiple diseases of aging. Finally, we discuss how new molecules may provide routes to the safer and more selective inhibition of mTOR complex 1 (mTORC1) in the decade ahead. We conclude by discussing what work remains to be done and the questions that will need to be addressed to make mTOR inhibitors part of the standard of care for diseases of aging.

Resistance to mild cold stress is greater in both wild-type and long-lived GHR-KO female mice

Adapting to stress, including cold environmental temperature (eT), is crucial for the survival of mammals, especially small rodents. Long-lived mutant mice have enhanced stress resistance against oxidative and non-oxidative challenges. However, much less is known about the response of those long-lived mice to cold stress. Growth hormone receptor knockout (GHR-KO) mice are long-lived with reduced growth hormone signaling. We wanted to test whether GHR-KO mice have enhanced resistance to cold stress. To examine the response of GHR-KO mice to cold eT, GHR-KO mice were housed at mild cold eT (16 °C) immediately following weaning. Longevity results showed that female GHR-KO and wild-type (WT) mice retained similar lifespan, while both male GHR-KO and WT mice had shortened lifespan compared to the mice housed at 23 °C eT. Female GHR-KO and WT mice housed at 16 °C had upregulated fibroblast growth factor 21 (FGF21), enhanced energy metabolism, reduced plasma triglycerides, and increased mRNA expression of some xenobiotic enzymes compared to females housed at 23 °C and male GHR-KO and WT mice housed under the same condition. In contrast, male GHR-KO and WT mice housed at 16 °C showed deleterious effects in parameters which might be associated with their shortened longevity compared to male GHR-KO and WT mice housed at 23 °C. Together, this study suggests that in response to mild cold stress, sex plays a pivotal role in the regulation of longevity, and female GHR-KO and WT mice are more resistant to this challenge than the males.

Disruption of Growth Hormone Receptor in Adipocytes Improves Insulin Sensitivity and Lifespan in Mice

Growth hormone receptor knockout (GHRKO) mice have been used for 25 years to uncover some of the many actions of growth hormone (GH). Since they are extremely long-lived with enhanced insulin sensitivity and protected from multiple age-related diseases, they are often used to study healthy aging. To determine the effect that adipose tissue has on the GHRKO phenotype, our laboratory recently created and characterized adipocyte-specific GHRKO (AdGHRKO) mice, which have increased adiposity but appear healthy with enhanced insulin sensitivity. To test the hypothesis that removal of GH action in adipocytes might partially replicate the increased lifespan and healthspan observed in global GHRKO mice, we assessed adiposity, cytokines/adipokines, glucose homeostasis, frailty, and lifespan in aging AdGHRKO mice of both sexes. Our results show that disrupting the GH receptor gene in adipocytes improved insulin sensitivity at advanced age and increased lifespan in male AdGHRKO mice. AdGHRKO mice also exhibited increased fat mass, reduced circulating levels of insulin, c-peptide, adiponectin, resistin, and improved frailty scores with increased grip strength at advanced ages. Comparison of published mean lifespan data from GHRKO mice to that from AdGHRKO and muscle-specific GHRKO mice suggests that approximately 23% of lifespan extension in male GHRKO is due to GHR disruption in adipocytes vs approximately 19% in muscle. Females benefited less from GHR disruption in these 2 tissues with approximately 19% and approximately 0%, respectively. These data indicate that removal of GH's action, even in a single tissue, is sufficient for observable health benefits that promote long-term health, reduce frailty, and increase longevity.

Rapamycin treatment during development extends life span and health span of male mice and Daphnia magna

Development is tightly connected to aging, but whether pharmacologically targeting development can extend life remains unknown. Here, we subjected genetically diverse UMHET3 mice to rapamycin for the first 45 days of life. The mice grew slower and remained smaller than controls for their entire lives. Their reproductive age was delayed without affecting offspring numbers. The treatment was sufficient to extend the median life span by 10%, with the strongest effect in males, and helped to preserve health as measured by frailty index scores, gait speed, and glucose and insulin tolerance tests. Mechanistically, the liver transcriptome and epigenome of treated mice were younger at the completion of treatment. Analogous to mice, rapamycin exposure during development robustly extended the life span of Daphnia magna and reduced its body size. Overall, the results demonstrate that short-term rapamycin treatment during development is a novel longevity intervention that acts by slowing down development and aging, suggesting that aging may be targeted already early in life.

Activation of Rictor/mTORC2 signaling acts as a pivotal strategy to protect against sensorineural hearing loss

Significance The mechanistic target of rapamycin (mTOR) plays a central role in growth, metabolism, and aging. It is assembled into two multiprotein complexes, namely, mTORC1 and mTORC2. We previously demonstrated the efficacy of sirolimus in ARHL in mice by decreasing mTORC1. However, the aspect of mTORC2 regulation in the cochlea is poorly characterized. Herein, based on pharmacological and genetic interventions, we found that a high dose of sirolimus resulted in severe hearing loss by reducing the mTORC2/AKT signaling pathway in the cochlea. Furthermore, selective activation of mTORC2 could protect against hearing loss induced by acoustic trauma and cisplatin-induced ototoxicity. Hence, the therapeutic activation of mTORC2 in conjunction with decreasing mTORC1 might represent a promising and effective strategy in preventing hearing loss.

Exosome Biogenesis and Lysosome Function Determine Podocyte Exosome Release and Glomerular Inflammatory Response during Hyperhomocysteinemia

Nucleotide-binding oligomerization domain-like receptor containing pyrin domain 3 (NLRP3) inflammasome activation in podocytes is reportedly associated with enhanced release of exosomes containing NLRP3 inflammasome products from these cells during hyperhomocysteinemia (hHcy). This study examined the possible role of increased exosome secretion during podocyte NLRP3 inflammasome activation in the glomerular inflammatory response. Whether exosome biogenesis and lysosome function are involved in the regulation of exosome release from podocytes during hHcy in mice and upon stimulation of homocysteine (Hcy) in podocytes was tested. By nanoparticle tracking analysis, treatments of mice with amitriptyline (acid sphingomyelinase inhibitor), GW4869 (exosome biogenesis inhibitor), and rapamycin (lysosome function enhancer) were found to inhibit elevated urinary exosomes during hHcy. By examining NLRP3 inflammasome activation in glomeruli during hHcy, amitriptyline (but not GW4869 and rapamycin) was shown to have an inhibitory effect. However, all treatments attenuated glomerular inflammation and injury during hHcy. In cell studies, Hcy treatment stimulated exosome release from podocytes, which was prevented by amitriptyline, GW4869, and rapamycin. Structured illumination microscopy revealed that Hcy inhibited lysosome-multivesicular body interactions in podocytes, which was prevented by amitriptyline or rapamycin but not GW4869. Thus, the data from this study shows that activation of exosome biogenesis and dysregulated lysosome function are critically implicated in the enhancement of exosome release from podocytes leading to glomerular inflammation and injury during hHcy.

Growth hormone receptor gene disruption in mature-adult mice improves male insulin sensitivity and extends female lifespan

Studies in multiple species indicate that reducing growth hormone (GH) action enhances healthy lifespan. In fact, GH receptor knockout (GHRKO) mice hold the Methuselah prize for the world's longest-lived laboratory mouse. We previously demonstrated that GHR ablation starting at puberty (1.5 months), improved insulin sensitivity and female lifespan but results in markedly reduced body size. In this study, we investigated the effects of GHR disruption in mature-adult mice at 6 months old (6mGHRKO). These mice exhibited GH resistance (reduced IGF-1 and elevated GH serum levels), increased body adiposity, reduced lean mass, and minimal effects on body length. Importantly, 6mGHRKO males have enhanced insulin sensitivity and reduced neoplasms while females exhibited increased median and maximal lifespan. Furthermore, fasting glucose and oxidative damage was reduced in females compared to males irrespective of Ghr deletion. Overall, disrupted GH action in adult mice resulted in sexual dimorphic effects suggesting that GH reduction at older ages may have gerotherapeutic effects.

Therapeutic mTOR blockade in systemic autoimmunity: Implications for antiviral immunity and extension of lifespan

The mechanistic target of rapamycin (mTOR) pathway integrates metabolic cues into cell fate decisions. A particularly fateful event during the adaptive immune response is the engagement of a T cell receptor by its cognate antigen presented by an antigen-presenting cell (APC). Here, the induction of adequate T cell activation and lineage specification is critical to mount protective immunity; at the same time, inadequate activation, which could lead to autoimmunity, must be avoided. mTOR forms highly conserved protein complexes 1 and 2 that shape lineage specification by integrating signals originating from TCR engagement, co-stimulatory or co-inhibitory receptors and cytokines and availability of nutrients. If one considers autoimmunity as the result of aberrant lineage specification in response to such signals, the importance of this pathway becomes evident; this provides the conceptual basis for mTOR inhibition in the treatment of systemic autoimmunity, such as systemic lupus erythematosus (SLE). Clinical trials in SLE patients have provided preliminary evidence that mTOR blockade by sirolimus (rapamycin) can reverse pro-inflammatory lineage skewing, including the expansion of Th17 and double-negative T cells and plasma cells and the contraction of regulatory T cells. Moreover, sirolimus has shown promising efficacy in the treatment of refractory idiopathic multicentric Castleman disease, newly characterized by systemic autoimmunity due to mTOR overactivation. Alternatively, mTOR blockade enhances responsiveness to vaccination and reduces infections by influenza virus in healthy elderly subjects. Such seemingly contradictory findings highlight the importance to further evaluate the clinical effects of mTOR manipulation, including its potential role in treatment of COVID-19 infection. mTOR blockade may extend healthy lifespan by abrogating inflammation induced by viral infections and autoimmunity.

This review provides a mechanistic assessment of mTOR pathway activation in lineage specification within the adaptive and innate immune systems and its role in health and autoimmunity. We then discuss some of the recent experimental and clinical discoveries implicating mTOR in viral pathogensis and aging.

Effects of Rapamycin on Insulin Brain Endothelial Cell Binding and Blood-Brain Barrier Transport

Rapamycin is an exogenous compound that has been shown to improve cognition in Alzheimer’s disease mouse models and can regulate pathways downstream of the insulin receptor signaling pathway. Insulin is also known to improve cognition in rodent models of Alzheimer’s disease. Central nervous system (CNS) insulin must first cross the blood–brain barrier (BBB), a specialized network of brain endothelial cells. This transport process is regulated by physiological factors, such as insulin itself, triglycerides, cytokines, and starvation. Since rapamycin treatment can alter the metabolic state of rodents, increase the circulating triglycerides, and acts as a starvation mimetic, we hypothesized rapamycin could alter the rate of insulin transport across the BBB, providing a potential mechanism for the beneficial effects of rapamycin on cognition. Using young male and female CD-1 mice, we measured the effects of rapamycin on the basal levels of serum factors, insulin receptor signaling, vascular binding, and BBB pharmacokinetics. We found chronic rapamycin treatment was able to affect basal levels of circulating serum factors and endothelial cell insulin receptor signaling. In addition, while acute rapamycin treatment did affect insulin binding at the BBB, overall transport was unaltered. Chronic rapamycin slowed insulin BBB transport non-significantly (p = 0.055). These results suggest that rapamycin may not directly impact the transport of insulin at the BBB but could be acting to alter insulin signaling within brain endothelial cells, which can affect downstream signaling.

Effect of rapamycin on aging and age-related diseases-past and future

In 2009, rapamycin was reported to increase the lifespan of mice when implemented later in life. This observation resulted in a sea-change in how researchers viewed aging. This was the first evidence that a pharmacological agent could have an impact on aging when administered later in life, i.e., an intervention that did not have to be implemented early in life before the negative impact of aging. Over the past decade, there has been an explosion in the number of reports studying the effect of rapamycin on various diseases, physiological functions, and biochemical processes in mice. In this review, we focus on those areas in which there is strong evidence for rapamycin's effect on aging and age-related diseases in mice, e.g., lifespan, cardiac disease/function, central nervous system, immune system, and cell senescence. We conclude that it is time that pre-clinical studies be focused on taking rapamycin to the clinic, e.g., as a potential treatment for Alzheimer's disease.

Extending lifespan by modulating the growth hormone/insulin-like growth factor-1 axis: coming of age

Progress made in the years of aging research have allowed the opportunity to explore potential interventions to slow aging and extend healthy lifespan. Studies performed in yeast, worms, flies and mice subjected to genetic and pharmacological interventions have given insight into the cellular and molecular mechanisms associated with longevity. Furthermore, it is now possible to effectively modulate pathways that slow aging at different stages of life (early life or at an adult age). Interestingly, interventions that extend longevity in adult mice have had sex-specific success, suggesting a potential link between particular pathways that modulate aging and sex. For example, reduction of the growth hormone (GH)/insulin-like growth factor-1 (IGF-1) axis at an adult age extends lifespan preferentially in females. Moreover, several postnatal dietary interventions tested by the 'Intervention Testing Program (ITP)' from the National Institute of Aging (NIA) have shown that while pharmacological interventions like rapamycin affect the IGF-1/insulin pathway and preferentially extend lifespan in females; dietary compounds that target other cellular pathways are effective only in male mice-indicating mutually exclusive sex-specific pathways. Therefore, a combination of interventions that target non-overlapping aging-related pathways appears to be an effective approach to further extend healthy lifespan in both sexes. Here, we review the germline and postnatal mouse lines that target the GH/IGF-1 axis as a mechanism to extend longevity as well as the dietary compounds that tested positive in the NIA program to increase lifespan. We believe that the interventions reviewed in this paper could constitute feasible combinations for an extended healthy lifespan in both male and female mice.

Growth hormone and aging

Growth hormone (GH) actions impact growth, metabolism, and body composition and have been associated with aging and longevity. Lack of GH results in slower growth, delayed maturation, and reduced body size and can lead to delayed aging, increased healthspan, and a remarkable extension of longevity. Adult body size, which is a GH-dependent trait, has a negative association with longevity in several mammalian species. Mechanistic links between GH and aging include evolutionarily conserved insulin/insulin-like growth factors and mechanistic target of rapamycin signaling pathways in accordance with long-suspected trade-offs between anabolic/growth processes and longevity. Height and the rate and regulation of GH secretion have been related to human aging, but longevity is not extended in humans with syndromes of GH deficiency or resistance. However, the risk of age-related chronic disease is reduced in individuals affected by these syndromes and various indices of increased healthspan have been reported.

Regulation and metabolic functions of mTORC1 and mTORC2

Cells metabolize nutrients for biosynthetic and bioenergetic needs to fuel growth and proliferation. The uptake of nutrients from the environment and their intracellular metabolism is a highly controlled process that involves cross talk between growth signaling and metabolic pathways. Despite constant fluctuations in nutrient availability and environmental signals, normal cells restore metabolic homeostasis to maintain cellular functions and prevent disease. A central signaling molecule that integrates growth with metabolism is the mechanistic target of rapamycin (mTOR). mTOR is a protein kinase that responds to levels of nutrients and growth signals. mTOR forms two protein complexes, mTORC1, which is sensitive to rapamycin, and mTORC2, which is not directly inhibited by this drug. Rapamycin has facilitated the discovery of the various functions of mTORC1 in metabolism. Genetic models that disrupt either mTORC1 or mTORC2 have expanded our knowledge of their cellular, tissue, as well as systemic functions in metabolism. Nevertheless, our knowledge of the regulation and functions of mTORC2, particularly in metabolism, has lagged behind. Since mTOR is an important target for cancer, aging, and other metabolism-related pathologies, understanding the distinct and overlapping regulation and functions of the two mTOR complexes is vital for the development of more effective therapeutic strategies. This review discusses the key discoveries and recent findings on the regulation and metabolic functions of the mTOR complexes. We highlight findings from cancer models but also discuss other examples of the mTOR-mediated metabolic reprogramming occurring in stem and immune cells, type 2 diabetes/obesity, neurodegenerative disorders, and aging.

DNA- and telomere-damage does not limit lifespan: evidence from rapamycin

Failure of rapamycin to extend lifespan in DNA repair mutant and telomerase-knockout mice, while extending lifespan in normal mice, indicates that neither DNA damage nor telomere shortening limits normal lifespan or causes normal aging.

Unlike dietary restriction, rapamycin fails to extend lifespan and reduce transcription stress in progeroid DNA repair-deficient mice

Dietary restriction (DR) and rapamycin extend healthspan and life span across multiple species. We have recently shown that DR in progeroid DNA repair‐deficient mice dramatically extended healthspan and trippled life span. Here, we show that rapamycin, while significantly lowering mTOR signaling, failed to improve life span nor healthspan of DNA repair‐deficient Ercc1^∆/− mice, contrary to DR tested in parallel. Rapamycin interventions focusing on dosage, gender, and timing all were unable to alter life span. Even genetically modifying mTOR signaling failed to increase life span of DNA repair‐deficient mice. The absence of effects by rapamycin on P53 in brain and transcription stress in liver is in sharp contrast with results obtained by DR, and appoints reducing DNA damage and transcription stress as an important mode of action of DR, lacking by rapamycin. Together, this indicates that mTOR inhibition does not mediate the beneficial effects of DR in progeroid mice, revealing that DR and rapamycin strongly differ in their modes of action.

Integrative Analyses of mRNA Expression Profile Reveal SOCS2 and CISH Play Important Roles in GHR Mutation-Induced Excessive Abdominal Fat Deposition in the Sex-Linked Dwarf Chicken

Sex-linked dwarf (SLD) chicken, which is caused by a recessive mutation of the growth hormone receptor (GHR), has been widely used in the Chinese broiler industry. However, it has been found that the SLD chicken has more abdominal fat deposition than normal chicken. Excessive fat deposition not only reduced the carcass quality of the broilers but also reduced the immunity of broilers to diseases. To find out the key genes and the precise regulatory pathways that were involved in the GHR mutation-induced excessive fat deposition, we used high-fat diet (HFD) and normal diet to feed the SLD chicken and normal chicken and analyzed the differentially expressed genes (DEGs) among the four groups. Results showed that the SLD chicken had more abdominal fat deposition and larger adipocytes size than normal chicken and HFD can promote abdominal fat deposition and induce adipocyte hypertrophy. RNA sequencing results of the livers and abdominal fats from the above chickens revealed that many DEGs between the SLD and normal chickens were enriched in fat metabolic pathways, such as peroxisome proliferator-activated receptor (PPAR) signaling, extracellular matrix (ECM)-receptor pathway, and fatty acid metabolism. Importantly, by constructing and analyzing the GHR-downstream regulatory network, we found that suppressor of cytokine signaling 2 (SOCS2) and cytokine-inducible SH2-containing protein (CISH) may involve in the GHR mutation-induced abdominal fat deposition in chicken. The ectopic expression of SOCS2 and CISH in liver-related cell line leghorn strain M chicken hepatoma (LMH) cell and immortalized chicken preadipocytes (ICP) revealed that these two genes can regulate fatty acid metabolism, adipocyte differentiation, and lipid droplet accumulation. Notably, overexpression of SOCS2 and CISH can rescue the hyperactive lipid metabolism and excessive lipid droplet accumulation of primary liver cell and preadipocytes that were isolated from the SLD chicken. This study found some genes and pathways involved in abdominal fat deposition of the SLD chicken and reveals that SOCS2 and CISH are two key genes involved in the GHR mutation-induced excessive fat deposition of the SLD chicken.

Decline in biological resilience as key manifestation of aging: Potential mechanisms and role in health and longevity

Decline in biological resilience (ability to recover) is a key manifestation of aging that contributes to increase in vulnerability to death with age eventually limiting longevity even in people without major chronic diseases. Understanding the mechanisms of this decline is essential for developing efficient anti-aging and pro-longevity interventions. In this paper we discuss: a) mechanisms of the decline in resilience with age, and aging components that contribute to this decline, including depletion of body reserves, imperfect repair mechanisms, and slowdown of physiological processes and responses with age; b) anti-aging interventions that may improve resilience or attenuate its decline; c) biomarkers of resilience available in human and experimental studies; and d) genetic factors that could influence resilience. There are open questions about optimal anti-aging interventions that would oppose the decline in resilience along with extending longevity limits. However, the area develops quickly, and prospects are exciting.

PKC downregulation upon rapamycin treatment attenuates mitochondrial disease

Leigh syndrome is a fatal neurometabolic disorder caused by defects in mitochondrial function. Mechanistic target of rapamycin (mTOR) inhibition with rapamycin attenuates disease progression in a mouse model of Leigh syndrome (Ndufs4 knock-out (KO) mouse); however, the mechanism of rescue is unknown. Here we identify protein kinase C (PKC) downregulation as a key event mediating the beneficial effects of rapamycin treatment of Ndufs4 KO mice. Assessing the impact of rapamycin on the brain proteome and phosphoproteome of Ndufs4 KO mice, we find that rapamycin restores mitochondrial protein levels, inhibits signalling through both mTOR complexes and reduces the abundance and activity of multiple PKC isoforms. Administration of PKC inhibitors increases survival, delays neurological deficits, prevents hair loss and decreases inflammation in Ndufs4 KO mice. Thus, PKC may be a viable therapeutic target for treating severe mitochondrial disease.

Growth hormone receptor knockout: Relevance to xenotransplantation

Xenotransplantation research has made considerable progress in recent years, largely through the increasing availability of pigs with multiple genetic modifications, effective immunosuppressive therapy, and anti-inflammatory therapy to protect pig tissues from the primate immune and inflammatory responses and correct molecular incompatibilities. Further study is required regarding identification and investigation of physiological incompatibilities. Although the exact cause remains uncertain, we and others have observed relatively rapid growth of kidney xenografts after transplantation into nonhuman primates (NHPs). There has also been some evidence of growth, or at least ventricular hypertrophy, of the pig heart after orthotopic transplantation into NHPs. Rapid growth could be problematic, particularly with regard to the heart within the relatively restricted confines of the chest. It has been suggested that the problem of rapid growth of the pig organ after transplantation could be resolved by growth hormone receptor (GHR) gene knockout in the pig. The GHR, although most well-known for regulating growth, has many other biological functions, including regulating metabolism and controlling physiological processes. Genetically modified GHRKO pigs have recently become available. We provide data on their growth compared to comparable pigs that do not include GHRKO, and we have reviewed the literature regarding the effect of GHRKO, and its relevance to xenotransplantation.

Insulin sensitivity in long-lived growth hormone-releasing hormone knockout mice

Our previous studies showed that loss-of-function mutation of growth hormone releasing hormone (GHRH) results in increased longevity and enhanced insulin sensitivity in mice. However, the details of improved insulin action and tissue-specific insulin signaling are largely unknown in this healthy-aging mouse model. We conducted hyperinsulinemic-euglycemic clamp to investigate mechanisms underlying enhanced insulin sensitivity in growth hormone (GH) deficient mice. Further, we assessed in vivo tissue-specific insulin activity via activation of PI3K-AKT and MAPK-ERK1/2 cascades using western blot. Clamp results showed that the glucose infusion rate required for maintaining euglycemia was much higher in GHRH^-/- mice compared to WT controls. Insulin-mediated glucose production was largely suppressed, whereas glucose uptake in skeletal muscle and brown adipose tissue were significant enhanced in GHRH^-/- mice compared to WT controls. Enhanced capacity of insulin-induced activation of the PI3K-AKT and MAPK-ERK1/2 signaling were observed in a tissue-specific manner in GHRH^-/- mice. Enhanced systemic insulin sensitivity in long-lived GHRH^-/- mice is associated with differential activation of insulin signaling cascades among various organs. Improved action of insulin in the insulin sensitive tissues is likely to mediate the prolonged longevity and healthy-aging effects of GH deficiency in mice.

Lifespan of long-lived growth hormone receptor knockout mice was not normalized by housing at 30°C since weaning

Growth hormone receptor knockout (GHRKO) mice are remarkably long-lived and have improved glucose homeostasis along with altered energy metabolism which manifests through decreased respiratory quotient (RQ) and increased oxygen consumption (VO2 ). Short-term exposure of these animals to increased environmental temperature (eT) at 30°C can normalize their VO2 and RQ. We hypothesized that increased heat loss in the diminutive GHRKO mice housed at 23°C and the consequent metabolic adjustments to meet the increased energy demand for thermogenesis may promote extension of longevity, and preventing these adjustments by chronic exposure to increased eT will reduce or eliminate their longevity advantage. To test these hypotheses, GHRKO mice were housed at increased eT (30°C) since weaning. Here, we report that contrasting with the effects of short-term exposure of adult GHRKO mice to 30°C, transferring juvenile GHRKO mice to chronic housing at 30°C did not normalize the examined parameters of energy metabolism and glucose homeostasis. Moreover, despite decreased expression levels of thermogenic genes in brown adipose tissue (BAT) and elevated core body temperature, the lifespan of male GHRKO mice was not reduced, while the lifespan of female GHRKO mice was increased, along with improved glucose homeostasis. The results indicate that GHRKO mice have intrinsic features that help maintain their delayed, healthy aging, and extended longevity at both 23°C and 30°C.

Widespread sex dimorphism in aging and age-related diseases

Although aging is a conserved phenomenon across evolutionary distant species, aspects of the aging process have been found to differ between males and females of the same species. Indeed, observations across mammalian studies have revealed the existence of longevity and health disparities between sexes, including in humans (i.e. with a female or male advantage). However, the underlying mechanisms for these sex differences in health and lifespan remain poorly understood, and it is unclear which aspects of this dimorphism stem from hormonal differences (i.e. predominance of estrogens vs. androgens) or from karyotypic differences (i.e. XX vs. XY sex chromosome complement). In this review, we discuss the state of the knowledge in terms of sex dimorphism in various aspects of aging and in human age-related diseases. Where the interplay between sex differences and age-related differences has not been explored fully, we present the state of the field to highlight important future research directions. We also discuss various dietary, drug or genetic interventions that were shown to improve longevity in a sex-dimorphic fashion. Finally, emerging tools and models that can be leveraged to decipher the mechanisms underlying sex differences in aging are also briefly discussed.

Anti-aging Effects of Calorie Restriction (CR) and CR Mimetics based on the Senoinflammation Concept

Chronic inflammation, a pervasive feature of the aging process, is defined by a continuous, multifarious, low-grade inflammatory response. It is a sustained and systemic phenomenon that aggravates aging and can lead to age-related chronic diseases. In recent years, our understanding of age-related chronic inflammation has advanced through a large number of investigations on aging and calorie restriction (CR). A broader view of age-related inflammation is the concept of senoinflammation, which has an outlook beyond the traditional view, as proposed in our previous work. In this review, we discuss the effects of CR on multiple phases of proinflammatory networks and inflammatory signaling pathways to elucidate the basic mechanism underlying aging. Based on studies on senoinflammation and CR, we recognized that senescence-associated secretory phenotype (SASP), which mainly comprises cytokines and chemokines, was significantly increased during aging, whereas it was suppressed during CR. Further, we recognized that cellular metabolic pathways were also dysregulated in aging; however, CR mimetics reversed these effects. These results further support and enhance our understanding of the novel concept of senoinflammation, which is related to the metabolic changes that occur in the aging process. Furthermore, a thorough elucidation of the effect of CR on senoinflammation will reveal key insights and allow possible interventions in aging mechanisms, thus contributing to the development of new therapies focused on improving health and longevity.

Is Rapamycin a Dietary Restriction Mimetic?

Since the initial suggestion that rapamycin, an inhibitor of target of rapamycin (TOR) nutrient signaling, increased lifespan comparable to dietary restriction, investigators have viewed rapamycin as a potential dietary restriction mimetic. Both dietary restriction and rapamycin increase lifespan across a wide range of evolutionarily diverse species (including yeast, Caenorhabditis elegans, Drosophila, and mice) as well as reducing pathology and improving physiological functions that decline with age in mice. The purpose of this article is to review the research comparing the effect of dietary restriction and rapamycin in mice. The current data show that dietary restriction and rapamycin have different effects on many pathways and molecular processes. In addition, these interventions affect the lifespan of many genetically manipulated mouse models differently. In other words, while dietary restriction and rapamycin may have similar effects on some pathways and processes; overall, they affect many pathways/processes quite differently. Therefore, rapamycin is likely not a true dietary restriction mimetic. Rather dietary restriction and rapamycin appear to be increasing lifespan and retarding aging largely through different mechanisms/pathways, suggesting that a combination of dietary restriction and rapamycin will have a greater effect on lifespan than either manipulation alone.

mTORC1 restrains adipocyte lipolysis to prevent systemic hyperlipidemia

Objective

Pharmacological agents targeting the mTOR complexes are used clinically as immunosuppressants and anticancer agents and can extend the lifespan of model organisms. An undesirable side effect of these drugs is hyperlipidemia. Although multiple roles have been described for mTOR complex 1 (mTORC1) in lipid metabolism, the etiology of hyperlipidemia remains incompletely understood. The objective of this study was to determine the influence of adipocyte mTORC1 signaling in systemic lipid homeostasis in vivo.

Methods

We characterized systemic lipid metabolism in mice lacking the mTORC1 subunit Raptor (Raptor^aKO), the key lipolytic enzyme ATGL (ATGL^aKO), or both (ATGL-Raptor^aKO) in their adipocytes.

Results

Mice lacking mTORC1 activity in their adipocytes failed to completely suppress lipolysis in the fed state and displayed prominent hypertriglyceridemia and hypercholesterolemia. Blocking lipolysis in their adipose tissue restored normal levels of triglycerides and cholesterol in the fed state as well as the ability to clear triglycerides in an oral fat tolerance test.

Conclusions

Unsuppressed adipose lipolysis in the fed state interferes with triglyceride clearance and contributes to hyperlipidemia. Adipose tissue mTORC1 activity is necessary for appropriate suppression of lipolysis and for the maintenance of systemic lipid homeostasis.

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Inhibition of adipose mTORC1 causes hypertriglyceridemia prior to lipodystrophy.

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Genetically inhibiting lipolysis reverses the increase in plasma TG.

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Acute pharmacological inhibition of lipolysis reverses the increase in plasma TG caused by rapamycin treatment.

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Unrestrained lipolysis impairs LPL activity and decreases TG clearance.

Mammalian Target of Rapamycin: A Metabolic Rheostat for Regulating Adipose Tissue Function and Cardiovascular Health

The complex relationship between diet and metabolism is an important contributor to cellular metabolism and health. Over the past few decades, a central role for mammalian target of rapamycin (mTOR) in the regulation of multiple cellular processes, including the response to food intake, maintaining homeostasis, and the pathogenesis of disease, has been shown. Herein, we first review our current understanding of the biochemical functions of mTOR and its response to fluctuations in hormone levels, like insulin. Second, we highlight the role of mTOR in lipogenesis, adipogenesis, β-oxidation of lipids, and ketosis of carbohydrates, lipids, and proteins. Special attention is paid to recent advances in mTOR signaling in white versus brown adipose tissues. Finally, we review how mTOR regulates cardiovascular health and disease. Together, these insights define a clearer picture of the connection between mTOR signaling, metabolic health, and disease.

Growth Hormone Deficiency: Health and Longevity

The important role of GH in the control of mammalian longevity was first deduced from extended longevity of mice with genetic GH deficiency (GHD) or GH resistance. Mice with isolated GHD (IGHD) due to GHRH or GHRH receptor mutations, combined deficiency of GH, prolactin, and TSH, or global deletion of GH receptors live longer than do their normal siblings. They also exhibit multiple features of delayed and/or slower aging, accompanied by extension of healthspan. The unexpected, remarkable longevity benefit of severe endocrine defects in these animals presumably represents evolutionarily conserved trade-offs among aging, growth, maturation, fecundity, and the underlying anabolic processes. Importantly, the negative association of GH signaling with longevity extends to other mammalian species, apparently including humans. Data obtained in humans with IGHD type 1B, owing to a mutation of the GHRH receptor gene, in the Itabaianinha County, Brazil, provide a unique opportunity to study the impact of severe reduction in GH signaling on age-related characteristics, health, and functionality. Individuals with IGHD are characterized by proportional short stature, doll facies, high-pitched voices, and central obesity. They have delayed puberty but are fertile and generally healthy. Moreover, these IGHD individuals are partially protected from cancer and some of the common effects of aging and can attain extreme longevity, 103 years of age in one case. We think that low, but detectable, residual GH secretion combined with life-long reduction of circulating IGF-1 and with some tissue levels of IGF-1 and/or IGF-2 preserved may account for the normal longevity and apparent extension of healthspan in these individuals.

The Genetics of Aging: A Vertebrate Perspective

Aging negatively impacts vitality and health. Many genetic pathways that regulate aging were discovered in invertebrates. However, the genetics of aging is more complex in vertebrates because of their specialized systems. This Review discusses advances in the genetic regulation of aging in vertebrates from work in mice, humans, and organisms with exceptional lifespans. We highlight challenges for the future, including sex-dependent differences in lifespan and the interplay between genes and environment. We also discuss how the identification of reliable biomarkers of age and development of new vertebrate models can be leveraged for personalized interventions to counter aging and age-related diseases.

Plausible Links Between Metabolic Networks, Stem Cells, and Longevity

Aging is an inevitable consequence of life, and all multicellular organisms undergo a decline in tissue and organ functions as they age. Several well-known risk factors, such as obesity, diabetes, and lack of physical activity that lead to the cardiovascular system, decline and impede the function of vital organs, ultimately limit overall life span. Over recent years, aging research has experienced an unparalleled growth, particularly with the discovery and recognition of genetic pathways and biochemical processes that control to some extent the rate of aging.In this chapter, we focus on several aspects of stem cell biology and aging, beginning with major cellular hallmarks of aging, endocrine regulation of aging and its impact on stem cell compartment, and mechanisms of increased longevity. We then discuss the role of epigenetic modifications associated with aging and provide an overview on a most recent search of antiaging modalities.

Impact of Growth Hormone-Related Mutations on Mammalian Aging

Mutations of a single gene can lead to a major increase in longevity in organisms ranging from yeast and worms to insects and mammals. Discovering these mutations (sometimes referred to as “longevity genes”) led to identification of evolutionarily conserved molecular, cellular, and organismal mechanisms of aging. Studies in mice provided evidence for the important role of growth hormone (GH) signaling in mammalian aging. Mice with mutations or gene deletions leading to GH deficiency or GH resistance have reduced body size and delayed maturation, but are healthier and more resistant to stress, age slower, and live longer than their normal (wild type) siblings. Mutations of the same genes in people can provide remarkable protection from age-related disease, but have no consistent impact on lifespan. Ongoing research indicates that genetic defects in GH signaling are linked to extension of healthspan and lifespan via a variety of interlocking mechanism, including improvements in genome and stem cell maintenance, stress resistance, glucose homeostasis, and thermogenesis, along with reductions in the mechanistic target of rapamycin (mTOR) C1 complex signaling and in chronic low grade inflammation.

Growth Hormone and Aging: Updated Review

Role of growth hormone (GH) in mammalian aging is actively explored in clinical, epidemiological, and experimental studies. The age-related decline in GH levels is variously interpreted as a symptom of neuroendocrine aging, as one of causes of altered body composition and other unwelcome symptoms of aging, or as a mechanism of natural protection from cancer and other chronic diseases. Absence of GH signals due to mutations affecting anterior pituitary development, GH secretion, or GH receptors produces an impressive extension of longevity in laboratory mice. Extension of healthspan in these animals and analysis of survival curves suggest that in the absence of GH, aging is slowed down or delayed. The corresponding endocrine syndromes in the human have no consistent impact on longevity, but are associated with remarkable protection from age-related disease. Moreover, survival to extremely old age has been associated with reduced somatotropic (GH and insulin-like growth factor-1) signaling in women and men. In both humans and mice, elevation of GH levels into the supranormal (pathological) range is associated with increased disease risks and reduced life expectancy likely representing acceleration of aging. The widely advertised potential of GH as an anti-aging agent attracted much interest. However, results obtained thus far have been disappointing with few documented benefits and many troublesome side effects. Possible utility of GH in the treatment of sarcopenia and frailty remains to be explored.