Life extension in the dwarf mouse

Growth hormone signaling is necessary for lifespan extension by dietary methionine

Growth hormone significantly impacts lifespan in mammals. Mouse longevity is extended when growth hormone (GH) signaling is interrupted but markedly shortened with high-plasma hormone levels. Methionine metabolism is enhanced in growth hormone deficiency, for example, in the Ames dwarf, but suppressed in GH transgenic mice. Methionine intake affects also lifespan, and thus, GH mutant mice and respective wild-type littermates were fed 0.16%, 0.43%, or 1.3% methionine to evaluate the interaction between hormone status and methionine. All wild-type and GH transgenic mice lived longer when fed 0.16% methionine but not when fed higher levels. In contrast, animals without growth hormone signaling due to hormone deficiency or resistance did not respond to altered levels of methionine in terms of lifespan, body weight, or food consumption. Taken together, our results suggest that the presence of growth hormone is necessary to sense dietary methionine changes, thus strongly linking growth and lifespan to amino acid availability.

IGF-I regulates the age-dependent signaling peptide humanin

Aging is influenced by endocrine pathways including the growth hormone/insulin-like growth factor-1 (GH/IGF) axis. Mitochondrial function has also been linked to the aging process, but the relevant mitochondrial signals mediating the effects of mitochondria are poorly understood. Humanin is a novel signaling peptide that acts as a potent regulator of cellular stress responses and protects from a variety of in vitro and in vivo toxic and metabolic insults. The circulating levels of humanin decline with age in mice and humans. Here, we demonstrate a negative correlation between the activity of the GH-IGF axis and the levels of humanin, as well as a positive correlation between humanin and lifespan in mouse models with altered GH/IGF-I axis. Long-lived, GH-deficient Ames mice displayed elevated humanin levels, while short-lived GH-transgenic mice have reduced humanin levels. Furthermore, treatment with GH or IGF-I reduced circulating humanin levels in both mice and human subjects. Our results indicate that GH and IGF are potent regulators of humanin levels and that humanin levels correlate with lifespan in mice. This suggests that humanin represents a circulating mitochondrial signal that participates in modulating the aging process, adding a coordinated mitochondrial element to the endocrine regulation of aging.

Mitochondrial chaperone TRAP1 activates the mitochondrial UPR and extends healthspan in Drosophila

The molecular mechanisms influencing healthspan are unclear but mitochondrial function, resistance to oxidative stress and proteostasis are recurring themes. Tumor necrosis factor Receptor Associated Protein 1 (TRAP1), the mitochondrial analog of Hsp75, regulates levels of reactive oxygen species in vitro and is found expressed at higher levels in tumor cells where it is thought to play a pro-survival role. While TRAP1-directed compartmentalized protein folding is a promising target for cancer therapy, its role at the organismal level is unclear. Here we report that overexpression of TRAP1 in Drosophila extends healthspan by enhancing stress resistance, locomotor activity and fertility while depletion of TRAP1 has the opposite effect, with little effect on lifespan under both conditions. In addition, modulating TRAP1 expression promotes the nuclear translocation of homeobox protein Dve and increases expression of genes associated with the mitochondrial unfolded protein response (UPR(mt)), indicating an activation of this proteostasis pathway. Notably, independent genetic knockdown of components of the UPR(mt) pathway dampen the enhanced stress resistance observed in TRAP1 overexpression flies. Together these studies suggest that TRAP1 regulates healthspan, potentially through activation of the UPR(mt).

IRS2 integrates insulin/IGF1 signalling with metabolism, neurodegeneration and longevity

Understanding how metabolism and nutrient homeostasis integrates with life span and neurodegeneration is a complicated undertaking. Important inconsistencies have emerged recently regarding the role of insulin-like signalling and the progression of neurodegenerative disease. Insulin resistance and type 2 diabetes are associated with clinical Alzheimer's disease, whereas study in lower organisms shows that reduced insulin-like signalling slows the progressive neurodegeneration and increases life span. From a clinical perspective, compensatory hyperinsulinaemia to overcome systemic insulin resistance is thought to be a healthy goal, because it circumvents immediate catastrophic consequences of hyperglycaemia; however, study in flies, nematodes and mice indicate that excess insulin signalling can damage cellular function and accelerate ageing. Maintenance of the central nervous system (CNS) has particular importance for life span and metabolism. A conflict arises because reduced insulin/IGF1 signalling in the CNS is associated with longevity, but can dysregulate glucose and energy homeostasis, and promote overweight. Here, we explore how the genetic manipulation of insulin/IGF1 signalling system can influence systemic metabolism, life span and neurodegeneration.

Growth hormone abolishes beneficial effects of calorie restriction in long-lived Ames dwarf mice

Disruption of the growth hormone (GH) axis promotes longevity and delays aging. In contrast, GH over-expression may lead to accelerated aging and shorter life. Calorie restriction (CR) improves insulin sensitivity and may extend lifespan. Long-lived Ames dwarf (df/df) mice have additional extension of longevity when subjected to 30% CR. The aim of the study was to assess effects of CR or GH replacement therapy separately and as a combined (CR+GH) treatment in GH-deficient df/df and normal mice, on selected metabolic parameters (e.g., insulin, glucose, cholesterol), insulin signaling components (e.g., insulin receptor [IR] β-subunit, phosphorylated form of IR [IR pY1158], protein kinase C ζ/λ [p-PKCζ/λ] and mTOR [p-mTOR]), transcription factor p-CREB, and components of the mitogen-activated protein kinase (MAPK) signaling (p-ERK1/2, p-p38), responsible for cell proliferation, differentiation and survival. CR decreased plasma levels of insulin, glucose, cholesterol and leptin, and increased hepatic IR β-subunit and IR pY1158 levels as well as IR, IRS-1 and GLUT-2 gene expression compared to ad libitum feeding, showing a significant beneficial diet intervention effect. Moreover, hepatic protein levels of p-PKCζ/λ, p-mTOR and p-p38 decreased, and p-CREB increased in CR mice. On the contrary, GH increased levels of glucose, cholesterol and leptin in plasma, and p-mTOR or p-p38 in livers, and decreased plasma adiponectin and hepatic IR β-subunit compared to saline treatment. There were no GH effects on adiponectin in N mice. Moreover, GH replacement therapy did not affect IR, IRS-1 and GLUT-2 gene expression. GH treatment abolishes the beneficial effects of CR; it may suggest an important role of GH-IGF1 axis in mediating the CR action. Suppressed somatotrophic signaling seems to predominate over GH replacement therapy in the context of the examined parameters and signaling pathways.

The influence of nutrition on the insulin-like growth factor system and the concentrations of growth hormone, glucose, insulin, gonadotropins and progesterone in ovarian follicular fluid and plasma from adult female horses (Equus caballus)

BACKGROUND

Feed intake affects the GH-IGF system and may be a key factor in determining the ovarian follicular growth rate. In fat mares, the plasma IGF-1 concentration is high with low GH and a quick follicular growth rate, in contrast to values observed in thin mares. Nothing is known regarding the long-term effects of differential feed intake on the IGF system. The objective of this experiment was to quantify IGFs, IGFBPs, GH, glucose, insulin, gonadotropin and progesterone (P4) in blood and in preovulatory follicular fluid (FF) in relation to feeding levels in mares.

METHODS

Three years prior to the experiment, Welsh Pony mares were assigned to a restricted diet group (R, n = 10) or a well-fed group (WF, n = 9). All mares were in good health and exhibited differences in body weight and subcutaneous fat thickness. Follicular development was scanned daily and plasma was also collected daily. Preovulatory FF was collected by ultrasound-guided follicular aspiration. Hormone levels were assayed in FF and plasma with a validated RIA.

RESULTS

According to scans, the total number of follicles in group R was 53% lower than group WF. Insulin and IGF-1 concentrations were higher in WF than in R mares. GH and IGF-2 concentrations were lower in plasma from WF mares than from R mares, but the difference was not significant in FF. The IGFBP-2/IGFBP-3 ratio in FF was not affected by feeding but was dramatically increased in R mare plasma. No difference in gonadotropin concentration was found with the exception of FSH, which was higher in the plasma of R mares. On the day of puncture, P4 concentrations were not affected by feeding but were higher in preovulatory FF than in plasma.

CONCLUSIONS

The bioavailability of IGF-1 or IGF-2, represented by the IGFBP2/IGFBP3 ratio, is modified by feed intake in plasma but not in FF. These differences partially explain the variability in follicular growth observed between well-fed mares and mares on restricted diets.

Effects of insulin-like growth factor 1 on glutathione S-transferases and thioredoxin in growth hormone receptor knockout mice

Growth hormone (GH) and insulin-like growth factor 1 (IGF-1) have been shown to affect processes involved in cellular stress defense, aging, and longevity. This study was designed to identify possible mechanisms of a disrupted GH signaling pathway on stress resistance using growth hormone receptor knockout (GHRKO) mice. GHRKO mice are GH resistant due to the targeted disruption of the GH receptor/binding protein gene, thus preventing GH from binding and exerting its downstream effects. These mice have very low circulating IGF-1 levels and high GH levels, are obese yet insulin sensitive, and live longer than their wild-type controls. Wild-type or GHRKO mice were treated with saline or IGF-1 (WT saline, GHRKO saline, GHRKO IGF-1) two times daily for 7 days. Glutathione S-transferase (GST) activities, proteins, and gene expression were determined. Liver mitochondrial GSTA1, GSTA3, and GSTZ1 proteins were significantly higher in GHRKO when compared to those of WT mice. The 4-hydroxynonenal (4-HNE) GST activity was upregulated in GHRKO mice and was suppressed after IGF-1 administration. Interestingly, thioredoxin (Trx)1, Trx2, thioredoxin reductase (TrxR)1, and TrxR2 messenger RNA (mRNA) levels were significantly higher in the GHRKO as compared to WT mice, and IGF-1 treatment suppressed the expression of each. We also found that glutaredoxin (Grx)2 mRNA and cytosolic Grx activity were higher in GHRKO mice. These results suggest that the detoxification and stress response mechanisms in GHRKO mice are contributed in part by the circulating level of IGF-1.

Pathobiology of aging mice and GEM: background strains and experimental design

The use of induced and spontaneous mutant mice and genetically engineered mice (and combinations thereof) to study cancers and other aging phenotypes to advance improved functional human life spans will involve studies of aging mice. Genetic background contributes to pathology phenotypes and to causes of death as well as to longevity. Increased recognition of expected phenotypes, experimental variables that influence phenotypes and research outcomes, and experimental design options and rationales can maximize the utility of genetically engineered mice (GEM) models to translational research on aging. This review aims to provide resources to enhance the design and practice of chronic and longevity studies involving GEM. C57BL6, 129, and FVB/N strains are emphasized because of their widespread use in the generation of knockout, transgenic, and conditional mutant GEM. Resources are included also for pathology of other inbred strain families, including A, AKR, BALB/c, C3H, C57L, C58, CBA, DBA, GR, NOD.scid, SAMP, and SJL/J, and non-inbred mice, including 4WC, AB6F1, Ames dwarf, B6, 129, B6C3F1, BALB/c,129, Het3, nude, SENCAR, and several Swiss stocks. Experimental strategies for long-term cross-sectional and longitudinal studies to assess causes of or contributors to death, disease burden, spectrum of pathology phenotypes, longevity, and functional healthy life spans (health spans) are compared and discussed.

Calcium signaling alterations, oxidative stress, and autophagy in aging

SIGNIFICANCE

Aging is a multi-factorial process that may be associated with several functional and structural deficits which can evolve into degenerative diseases. In this review, we present data that may depict an expanded view of molecular aging theories, beginning with the idea that reactive oxygen species (ROS) are the major effectors in this process. In addition, we have correlated the importance of autophagy as a neuroprotective mechanism and discussed a link between age-related molecules, Ca(2+) signaling, and oxidative stress.

RECENT ADVANCES

There is evidence suggesting that alterations in Ca(2+) homeostasis, including mitochondrial Ca(2+) overload and alterations in electron transport chain (ETC) complexes, which increase cell vulnerability, are linked to oxidative stress in aging. As much as Ca(2+) signaling is altered in aged cells, excess ROS can be produced due to an ineffective coupling of mitochondrial respiration. Damaged mitochondria might not be removed by the macroautophagic system, which is hampered in aging by lipofuscin accumulation, boosting ROS generation, damaging DNA, and, ultimately, leading to apoptosis.

CRITICAL ISSUES

This process can lead to altered protein expression (such as p53, Sirt1, and IGF-1) and progress to cell death. This cycle can lead to increased cell vulnerability in aging and contribute to an increased susceptibility to degenerative processes.

FUTURE DIRECTIONS

A better understanding of Ca(2+) signaling and molecular aging alterations is important for preventing apoptosis in age-related diseases. In addition, caloric restriction, resveratrol and autophagy modulation appear to be predominantly cytoprotective, and further studies of this process are promising in age-related disease therapeutics.

Preservation of blood glucose homeostasis in slow-senescing somatotrophism-deficient mice subjected to intermittent fasting begun at middle or old age

Poor blood glucose homeostatic regulation is common, consequential, and costly for older and elderly populations, resulting in pleiotrophically adverse clinical outcomes. Somatotrophic signaling deficiency and dietary restriction have each been shown to delay the rate of senescence, resulting in salubrious phenotypes such as increased survivorship. Using two growth hormone (GH) signaling-related, slow-aging mouse mutants we tested, via longitudinal analyses, whether genetic perturbations that increase survivorship also improve blood glucose homeostatic regulation in senescing mammals. Furthermore, we institute a dietary restriction paradigm that also decelerates aging, an intermittent fasting (IF) feeding schedule, as either a short-term or a sustained intervention beginning at either middle or old age, and assess its effects on blood glucose control. We find that either of the two genetic alterations in GH signaling ameliorates fasting hyperglycemia; additionally, both longevity-inducing somatotrophic mutations improve insulin sensitivity into old age. Strikingly, we observe major and broad improvements in blood glucose homeostatic control by IF: IF improves ad libitum-fed hyperglycemia, glucose tolerance, and insulin sensitivity, and reduces hepatic gluconeogenesis, in aging mutant and normal mice. These results on correction of aging-resultant blood glucose dysregulation have potentially important clinical and public health implications for our ever-graying global population, and are consistent with the Longevity Dividend concept.

FGF21-based pharmacotherapy--potential utility for metabolic disorders

Currently available therapies for diabetes or obesity produce modest efficacy and are usually used in combination with agents targeting cardiovascular risk factors. Fibroblast growth factor 21 (FGF21) is a circulating protein with pleiotropic metabolic actions; pharmacological doses of FGF21 produce anti-diabetic, lipid-lowering, and weight-reducing effects in rodents. Several potential benefits have translated to non-human primates and obese humans with type 2 diabetes (T2D). Accumulating results point to a specific receptor complex and actions in adipose tissue, liver, and brain; several pathways lead to enhanced fatty acid oxidation, increased insulin sensitivity, and augmented energy expenditure. A range of strategies are being explored to derive potent, safe, and convenient therapies which could potentially represent novel approaches to prevent and treat a variety of metabolic disorders.

Long-lived crowded-litter mice exhibit lasting effects on insulin sensitivity and energy homeostasis

The action of nutrients on early postnatal growth can influence mammalian aging and longevity. Recent work has demonstrated that limiting nutrient availability in the first 3 wk of life [by increasing the number of pups in the crowded-litter (CL) model] leads to extension of mean and maximal lifespan in genetically normal mice. In this study, we aimed to characterize the impact of early-life nutrient intervention on glucose metabolism and energy homeostasis in CL mice. In our study, we used mice from litters supplemented to 12 or 15 pups and compared those to control litters limited to eight pups. At weaning and then throughout adult life, CL mice are significantly leaner and consume more oxygen relative to control mice. At 6 mo of age, CL mice had low fasting leptin concentrations, and low-dose leptin injections reduced body weight and food intake more in CL female mice than in controls. At 22 mo, CL female mice also have smaller adipocytes compared with controls. Glucose and insulin tolerance tests show an increase in insulin sensitivity in 6 mo old CL male mice, and females become more insulin sensitive later in life. Furthermore, β-cell mass was significantly reduced in the CL male mice and was associated with reduction in β-cell proliferation rate in these mice. Together, these data show that early-life nutrient intervention has a significant lifelong effect on metabolic characteristics that may contribute to the increased lifespan of CL mice.

Cardiac function of the naked mole-rat: ecophysiological responses to working underground

The naked mole-rat (NMR) is a strictly subterranean rodent with a low resting metabolic rate. Nevertheless, it can greatly increase its metabolic activity to meet the high energetic demands associated with digging through compacted soils in its xeric natural habitat where food is patchily distributed. We hypothesized that the NMR heart would naturally have low basal function and exhibit a large cardiac reserve, thereby mirroring the species' low basal metabolism and large metabolic scope. Echocardiography showed that young (2-4 yr old) healthy NMRs have low fractional shortening (28 ± 2%), ejection fraction (43 ± 2%), and cardiac output (6.5 ± 0.4 ml/min), indicating low basal cardiac function. Histology revealed large NMR cardiomyocyte cross-sectional area (216 ± 10 μm(2)) and cardiac collagen deposition of 2.2 ± 0.4%. Neither of these histomorphometric traits was considered pathological, since biaxial tensile testing showed no increase in passive ventricular stiffness. NMR cardiomyocyte fibers showed a low degree of rotation, contributing to the observed low NMR cardiac contractility. Interestingly, when the exercise mimetic dobutamine (3 μg/g ip) was administered, NMRs showed pronounced increases in fractional shortening, ejection fraction, cardiac output, and stroke volume, indicating an increased cardiac reserve. The relatively low basal cardiac function and enhanced cardiac reserve of NMRs are likely to be ecophysiological adaptations to life in an energetically taxing environment.

In search of antiaging modalities: evaluation of mTOR- and ROS/DNA damage-signaling by cytometry

This review presents the evidence in support of the IGF-1/mTOR/S6K1 signaling as the primary factor contributing to aging and cellular senescence. Reviewed are also specific interactions between mTOR/S6K1 and ROS-DNA damage signaling pathways. Outlined are critical sites along these pathways, including autophagy, as targets for potential antiaging (gero-suppressive) and/or chemopreventive agents. Presented are applications of flow- and laser scanning- cytometry utilizing phospho-specific Abs, to monitor activation along these pathways in response to the reported antiaging drugs rapamycin, metformin, berberine, resveratrol, vitamin D3, 2-deoxyglucose, and acetylsalicylic acid. Specifically, effectiveness of these agents to attenuate the level of constitutive mTOR signaling was tested by cytometry and confirmed by Western blotting through measuring phosphorylation of the mTOR-downstream targets including ribosomal protein S6. The ratiometric analysis of phosphorylated to total protein along the mTOR pathway offers a useful parameter reporting the effects of gero-suppressive agents. In parallel, their ability to suppress the level of constitutive DNA damage signaling induced by endogenous ROS was measured. While the primary target of each of these agents may be different the data obtained on several human cancer cell lines, WI-38 fibroblasts and normal lymphocytes suggest common downstream mechanism in which the decline in mTOR/S6K1 signaling and translation rate is coupled with a reduction of oxidative phosphorylation and ROS that leads to decreased oxidative DNA damage. The combined assessment of constitutive γH2AX expression, mitochondrial activity (ROS, ΔΨm), and mTOR signaling provides an adequate gamut of cell responses to test effectiveness of gero-suppressive agents. Described is also an in vitro model of induction of cellular senescence by persistent replication stress, its quantitative analysis by laser scanning cytometry, and application to detect the property of the studied agents to attenuate the induction of senescence. Discussed is cytometric analysis of cell size and heterogeneity of size as a potential biomarker used to asses gero-suppressive agents and longevity.

Insulin sensitivity in long-living Ames dwarf mice

Long-living Ames dwarf mice (df/df) characterized by growth hormone (GH) deficiency are widely used in aging research because of their 40-60 % lifespan extension compared to normal (N) littermates. Importantly, these mice not only live longer but are also protected from age-related diseases including insulin resistance. Several studies demonstrate that df/df mice have enhanced insulin signaling in different insulin-sensitive tissues and suggest that this is a mechanism for extended lifespan. However, it is unknown whether the enhanced insulin signaling in df/df mice translates to improved insulin action on hepatic glucose production and tissue glucose uptake. We performed hyperinsulinemic-euglycemic clamps to assess tissue-specific insulin action in vivo for the first time in these small long-living dwarfs. Our results demonstrate that the glucose infusion rate required to maintain euglycemia was ∼2-fold higher in df/df mice compared to N controls. Insulin-mediated glucose production was completely suppressed in dwarf mice, and stimulation of gastrocnemius and vastus muscle and adipose tissue glucose uptake was also enhanced in df/df mice (100, 86, and 65 %, respectively). These findings show that improved insulin signaling in df/df mice is associated with enhanced tissue-specific insulin action in vivo. This improved functionality of insulin action and glucose homeostasis may play a key role in promoting healthy aging and longer lifespan in df/df mice.

Phospholipid composition and longevity: lessons from Ames dwarf mice

Membrane fatty acid (FA) composition is correlated with longevity in mammals. The "membrane pacemaker hypothesis of ageing" proposes that animals which cellular membranes contain high amounts of polyunsaturated FAs (PUFAs) have shorter life spans because their membranes are more susceptible to peroxidation and further oxidative damage. It remains to be shown, however, that long-lived phenotypes such as the Ames dwarf mouse have membranes containing fewer PUFAs and thus being less prone to peroxidation, as would be predicted from the membrane pacemaker hypothesis of ageing. Here, we show that across four different tissues, i.e., muscle, heart, liver and brain as well as in liver mitochondria, Ames dwarf mice possess membrane phospholipids containing between 30 and 60 % PUFAs (depending on the tissue), which is similar to PUFA contents of their normal-sized, short-lived siblings. However, we found that that Ames dwarf mice membrane phospholipids were significantly poorer in n-3 PUFAs. While lack of a difference in PUFA contents is contradicting the membrane pacemaker hypothesis, the lower n-3 PUFAs content in the long-lived mice provides some support for the membrane pacemaker hypothesis of ageing, as n-3 PUFAs comprise those FAs being blamed most for causing oxidative damage. By comparing tissue composition between 1-, 2- and 6-month-old mice in both phenotypes, we found that membranes differed both in quantity of PUFAs and in the prevalence of certain PUFAs. In sum, membrane composition in the Ames dwarf mouse supports the concept that tissue FA composition is related to longevity.

Regulation of SIRT1 by oxidative stress-responsive miRNAs and a systematic approach to identify its role in the endothelium

SIGNIFICANCE

Oxidative stress is a common denominator of various risk factors contributing to endothelial dysfunction and vascular diseases. Accumulated evidence suggests that sirtuin 1 (SIRT1) expression and/or activity is impaired by supraphysiological levels of oxidative stress, which in turn disrupts endothelial homeostasis.

RECENT ADVANCES

Several microRNAs (miRNAs) are induced by oxidative stress and termed as oxidative stress-responsive miRNAs. They may play a role linking the imbalanced redox state with dysregulated SIRT1.

CRITICAL ISSUES

This review summarizes recent findings on oxidative stress-responsive miRNAs and their involvement in SIRT1 regulation. Because of the unique characteristics of miRNAs, research in this new area requires an integrative approach that combines bioinformatics and experimental validation. Thus, a research strategy is discussed to identify the SIRT1-regulating miRNAs under oxidative stress and their functional outcomes in relation to endothelial dysfunction. Additionally, the miRNAs implicated in vascular diseases such as atherosclerosis and abdominal aortic aneurysms are discussed along with the translational potential and challenges of using miRNAs and its analogs as therapeutic agents.

FUTURE DIRECTIONS

Although at its infancy, research on oxidative stress-responsive miRNAs and their regulation of SIRT1 may provide new insights in understanding vascular disorders. Moreover, systematic approaches integrating in silico, in vitro, and in vivo observations can be useful tools in revealing the pathways modulating endothelial biology.

GH therapy and cancer risk in hypopituitarism: what we know from human studies

It has been difficult to identify factors that affect the risk of cancer, but we know that people are at higher risk as they get older, or if they have a strong family history of cancer. The potential influence of environmental and behavioral factors remains poorly understood. Early population-based and case-control studies suggested that higher serum levels of IGF1 could be associated with increased cancer risk. Since GH therapy increases IGF1 levels, concern has been raised regarding its potential role as a cancer initiation factor. Experimental evidence and some clinical studies showed that when GH/IGF1 secretion or action was inhibited, a decreased incidence and rate of progression of cancers occurred. However, human populations comprise a garden variety of genotypes that respond differently to the same kind of exposures. Human population studies frequently reveal only very small effects to these exposures. So, are GH and cancer guilty by association? After more than 20 years, leukemia, a major safety issue initially believed associated with GH treatment in children with GH deficiency (GHD), has not been confirmed but the risk of second malignancies in patients previously treated with irradiation has been detected or confirmed through the National Cooperative Growth Study. Overall, this large study confirmed the favorable overall safety profile of GH therapy in children with GHD, and also highlighted specific populations at potential risk. The risk of secondary malignancy following radiotherapy is surely related to radiotherapy more than GH therapy that may increase growth but is less likely to start the oncogenic process. In GH-deficient adults treated with GH, observational studies (KIMS, HypoCCS) have shown that when IGF1 levels were targeted within normal age-related reference ranges, the occurrence of malignancies was not higher than in the general population.

Forkhead box(O) in control of reactive oxygen species and genomic stability to ensure healthy lifespan

SIGNIFICANCE

Transcription factors of the Forkhead box O class (FOXOs) are associated with lifespan and play a role in age-related diseases. FOXOs, therefore, serve as a paradigm for developing an understanding as to how age-related diseases, such as cancer and diabetes interconnect with lifespan. Understanding the regulatory inputs on FOXO may reveal how changes in these regulatory signaling pathways affect disease and lifespan.

RECENT ADVANCES

Numerous regulators of FOXO have now been described and a clear and evolutionary conserved role has emerged for phosphoinositide-3 kinase/protein kinase B (also known as c-Akt or AKT) signaling and c-jun N-terminal kinase signaling. Analysis of FOXO function in the context of these signaling pathways has shown the importance of FOXO-mediated transcriptional regulation on cell cycle progression and other cell fates, such as cell metabolism, stress resistance, and apoptosis in mediating disease and lifespan.

CRITICAL ISSUES

Persistent DNA damage is also tightly linked to disease and aging; yet, data on a possible link between DNA damage and FOXO have been limited. Here, we discuss possible connections between FOXO and the DNA damage response in the context of the broader role of connecting lifespan and disease.

FUTURE DIRECTIONS

Understanding the role of lifespan in diseases onset may provide unique and generic possibilities to intervene in disease processes to ensure a healthy lifespan.

Prevention of neuromusculoskeletal frailty in slow-aging ames dwarf mice: longitudinal investigation of interaction of longevity genes and caloric restriction

Ames dwarf (Prop1^df/df) mice are remarkably long-lived and exhibit many characteristics of delayed aging and extended healthspan. Caloric restriction (CR) has similar effects on healthspan and lifespan, and causes an extension of longevity in Ames dwarf mice. Our study objective was to determine whether Ames dwarfism or CR influence neuromusculoskeletal function in middle-aged (82 ± 12 weeks old) or old (128 ± 14 w.o.) mice. At the examined ages, strength was improved by dwarfism, CR, and dwarfism plus CR in male mice; balance/ motor coordination was improved by CR in old animals and in middle-aged females; and agility/ motor coordination was improved by a combination of dwarfism and CR in both genders of middle-aged mice and in old females. Therefore, extension of longevity by congenital hypopituitarism is associated with improved maintenance of the examined measures of strength, agility, and motor coordination, key elements of frailty during human aging, into advanced age. This study serves as a particularly important example of knowledge related to addressing aging-associated diseases and disorders that results from studies in long-lived mammals.

Low levels of plasma IGF-1 inhibit intracortical bone remodeling during aging

Studies linking insulin-like growth factor-1 (IGF-1) to age-related bone loss in humans have been reported but remain only correlative. In this investigation, we characterized the bone phenotype of aged WT C57BL/6J male mice in comparison to that of C57BL/6J mice with reduced serum IGF-1 levels arising from an igfals gene deletion (ALS knockout (ALSKO)). During the aging process, WT mice showed an increase in fat mass and decrease lean mass while ALSKO mice had stable lean and fat mass values. Skeletal analyses of femora from WT mice revealed an expansion of the marrow area and a significant accumulation of intracortical porosity associated with increased intracortical remodeling. In contrast, ALSKO mice showed only small age-related declines in the amount of cortical bone tissue and minimal intracortical porosity, at 2 years of age. Accordingly, mechanical tests of femora from 2-year-old WT mice revealed reduced stiffness and maximal load when compared to bones from ALSKO mice. We show here that lifelong reductions in serum IGF-1 compromise skeletal size in development leading to slender bones; they are also associated with decreased intracortical bone remodeling and preservation of bone strength during aging.

Discoidin domain receptor 2 (DDR2) regulates body size and fat metabolism in mice

Discoidin domain receptor 2 (DDR2) is a receptor tyrosine kinase that is activated by fibrillar collagens, which act as its endogenous ligand. DDR2 regulates cell proliferation, cell adhesion, migration, extracellular matrix remodeling and reproductive functions. Both DDR2 null allele mice and mice with a recessive, loss-of-function allele for Ddr2 exhibit dwarfing and a reduction in body weight. However, the detailed mechanisms by which DDR2 exerts its positive systemic regulation of whole body size, local skeletal size and fat tissue volume remain to be clarified. To investigate the systemic role of DDR2 in body size regulation, we produced transgenic mice in which the DDR2 protein is overexpressed, then screened the transgenic mice for abnormalities using systematic mouse abnormality screening. The modified-SHIPRA screen revealed that only the parameter of body size was significantly different among the genotypes. We also discovered that the body length was significantly increased, while the body weight was significantly decreased in transgenic mice compared to their littermate controls. We also found that the epididymal fat pads were significantly decreased in transgenic mice compared to normal littermate mice, which may have been the cause of the leptin decrement in the transgenic mice. The new insight that DDR2 might promote metabolism in adipocyte cells is very interesting, but more experiments will be needed to elucidate the direct relation between DDR2 and adipose-derived hormones. Taken together, our data demonstrated that DDR2 might play a systemic role in the regulation of body size thorough skeletal formation and fat metabolism.

Thyroid hormone signaling and homeostasis during aging

Studies in humans and in animal models show negative correlations between thyroid hormone (TH) levels and longevity. TH signaling is implicated in maintaining and integrating metabolic homeostasis at multiple levels, notably centrally in the hypothalamus but also in peripheral tissues. The question is thus raised of how TH signaling is modulated during aging in different tissues. Classically, TH actions on mitochondria and heat production are obvious candidates to link negative effects of TH to aging. Mitochondrial effects of excess TH include reactive oxygen species and DNA damage, 2 factors often considered as aging accelerators. Inversely, caloric restriction, which can retard aging from nematodes to primates, causes a rapid reduction of circulating TH, reducing metabolism in birds and mammals. However, many other factors could link TH to aging, and it is these potentially subtler and less explored areas that are highlighted here. For example, effects of TH on membrane composition, inflammatory responses, stem cell renewal and synchronization of physiological responses to light could each contribute to TH regulation of maintenance of homeostasis during aging. We propose the hypothesis that constraints on TH signaling at certain life stages, notably during maturity, are advantageous for optimal aging.

Insulin-like growth factor-1 in CNS and cerebrovascular aging

Insulin-like growth factor-1 (IGF-1) is an important anabolic hormone that decreases with age. In the past two decades, extensive research has determined that the reduction in IGF-1 is an important component of the age-related decline in cognitive function in multiple species including humans. Deficiency in circulating IGF-1 results in impairment in processing speed and deficiencies in both spatial and working memory. Replacement of IGF-1 or factors that increase IGF-1 to old animals and humans reverses many of these cognitive deficits. Despite the overwhelming evidence for IGF-1 as an important neurotrophic agent, the specific mechanisms through which IGF-1 acts have remained elusive. Recent evidence indicates that IGF-1 is both produced by and has important actions on the cerebrovasculature as well as neurons and glia. Nevertheless, the specific regulation and actions of brain- and vascular-derived IGF-1 is poorly understood. The diverse effects of IGF-1 discovered thus far reveal a complex endocrine and paracrine system essential for integrating many of the functions necessary for brain health. Identification of the mechanisms of IGF-1 actions will undoubtedly provide critical insight into regulation of brain function in general and the causes of cognitive decline with age.

Decreased levels of proapoptotic factors and increased key regulators of mitochondrial biogenesis constitute new potential beneficial features of long-lived growth hormone receptor gene-disrupted mice

Decreased somatotrophic signaling is among the most important mechanisms associated with extended longevity. Mice homozygous for the targeted disruption of the growth hormone (GH) receptor gene (GH receptor knockout; GHRKO) are obese and dwarf, are characterized by a reduced weight and body size, undetectable levels of GH receptor, high concentration of serum GH, and greatly reduced plasma levels of insulin and insulin-like growth factor-I, and are remarkably long lived. Recent results suggest new features of GHRKO mice that may positively affect longevity-decreased levels of proapoptotic factors and increased levels of key regulators of mitochondrial biogenesis. The alterations in levels of the proapoptotic factors and key regulators of mitochondrial biogenesis were not further improved by two other potential life-extending interventions-calorie restriction and visceral fat removal. This may attribute the primary role to GH resistance in the regulation of apoptosis and mitochondrial biogenesis in GHRKO mice in terms of increased life span.

IGF1R levels in the brain negatively correlate with longevity in 16 rodent species

The insulin/insulin-like growth factor signaling (IIS) pathway is a major conserved regulator of aging. Nematode, fruit fly and mouse mutants with reduced IIS signaling exhibit extended lifespan. These mutants are often dwarfs leading to the idea that small body mass correlates with longevity within species. However, when different species are compared, larger animals are typically longer-lived. Hence, the role of IIS in the evolution of life history traits remains unresolved. Here we used comparative approach to test whether IGF1R signaling changes in response to selection on lifespan or body mass and whether specific tissues are involved. The IGF1R levels in the heart, lungs, kidneys, and brains of sixteen rodent species with highly diverse lifespans and body masses were measured via immunoblot after epitope conservation analysis. We report that IGF1R levels display strong negative correlation with maximum lifespan only in brain tissue and no significant correlations with body mass for any organ. The brain-IGF1R and lifespan correlation holds when phylogenetic non-independence of data-points is taken into account. These results suggest that modulation of IGF1R signaling in nervous tissue, but not in the peripheral tissues, is an important factor in the evolution of longevity in mammals.

Parental imprinting regulates insulin-like growth factor signaling: a Rosetta Stone for understanding the biology of pluripotent stem cells, aging and cancerogenesis

In recent years, solid evidence has accumulated that insulin-like growth factor-1 (IGF-1) and 2 (IGF-2) regulate many biological processes in normal and malignant cells. Recently, more light has been shed on the epigenetic mechanisms regulating expression of genes involved in IGF signaling (IFS) and it has become evident that these mechanisms are crucial for initiation of embryogenesis, maintaining the quiescence of pluripotent stem cells deposited in adult tissues (for example, very-small embryonic-like stem cells), the aging process, and the malignant transformation of cells. The expression of several genes involved in IFS is regulated at the epigenetic level by imprinting/methylation within differentially methylated regions (DMRs), which regulate their expression from paternal or maternal chromosomes. The most important role in the regulation of IFS gene expression is played by the Igf-2-H19 locus, which encodes the autocrine/paracrine mitogen IGF-2 and the H19 gene, which gives rise to a non-coding RNA precursor of several microRNAs that negatively affect cell proliferation. Among these, miR-675 has recently been demonstrated to downregulate expression of the IGF-1 receptor. The proper imprinting of DMRs at the Igf-2-H19 locus, with methylation of the paternal chromosome and a lack of methylation on the maternal chromosome, regulates expression of these genes so that Igf-2 is transcribed only from the paternal chromosome and H19 (including miR-675) only from the maternal chromosome. In this review, we will discuss the relevance of (i) proper somatic imprinting, (ii) erasure of imprinting and (iii) loss of imprinting within the DMRs at the Igf-2-H19 locus to the expression of genes involved in IFS, and the consequences of these alternative patterns of imprinting for stem cell biology.

Somatotropic signaling: trade-offs between growth, reproductive development, and longevity

Growth hormone (GH) is a key determinant of postnatal growth and plays an important role in the control of metabolism and body composition. Surprisingly, deficiency in GH signaling delays aging and remarkably extends longevity in laboratory mice. In GH-deficient and GH-resistant animals, the "healthspan" is also extended with delays in cognitive decline and in the onset of age-related disease. The role of hormones homologous to insulin-like growth factor (IGF, an important mediator of GH actions) in the control of aging and lifespan is evolutionarily conserved from worms to mammals with some homologies extending to unicellular yeast. The combination of reduced GH, IGF-I, and insulin signaling likely contributes to extended longevity in GH or GH receptor-deficient organisms. Diminutive body size and reduced fecundity of GH-deficient and GH-resistant mice can be viewed as trade-offs for extended longevity. Mechanisms responsible for delayed aging of GH-related mutants include enhanced stress resistance and xenobiotic metabolism, reduced inflammation, improved insulin signaling, and various metabolic adjustments. Pathological excess of GH reduces life expectancy in men as well as in mice, and GH resistance or deficiency provides protection from major age-related diseases, including diabetes and cancer, in both species. However, there is yet no evidence of increased longevity in GH-resistant or GH-deficient humans, possibly due to non-age-related deaths. Results obtained in GH-related mutant mice provide striking examples of mutations of a single gene delaying aging, reducing age-related disease, and extending lifespan in a mammal and providing novel experimental systems for the study of mechanisms of aging.

The negative effect of prolonged somatotrophic/insulin signaling on an adult bone marrow-residing population of pluripotent very small embryonic-like stem cells (VSELs)

It is well known that attenuated insulin/insulin-like growth factor signaling (IIS) has a positive effect on longevity in several animal species, including mice. Here, we demonstrate that a population of murine pluripotent very small embryonic-like stem cells (VSELs) that reside in bone marrow (BM) is protected from premature depletion during aging by intrinsic parental gene imprinting mechanisms and the level of circulating insulin-like growth factor-I (IGF-I). Accordingly, an increase in the circulating level of IGF-I, as seen in short-lived bovine growth hormone (bGH)-expressing transgenic mice, which age prematurely, as well as in wild-type animals injected for 2 months with bGH, leads to accelerated depletion of VSELs from bone marrow (BM). In contrast, long-living GHR-null or Ames dwarf mice, which have very low levels of circulating IGF-I, exhibit a significantly higher number of VSELs in BM than their littermates at the same age. However, the number of VSELs in these animals decreases after GH or IGF-I treatment. These changes in the level of plasma-circulating IGF-I corroborate with changes in the genomic imprinting status of crucial genes involved in IIS, such as Igf-2-H19, RasGRF1, and Ig2R. Thus, we propose that a chronic increase in IIS contributes to aging by premature depletion of pluripotent VSELs in adult tissues.

Murine models of atrophy, cachexia, and sarcopenia in skeletal muscle

With the extension of life span over the past several decades, the age-related loss of muscle mass and strength that characterizes sarcopenia is becoming more evident and thus, has a more significant impact on society. To determine ways to intervene and delay, or even arrest the physical frailty and dependence that accompany sarcopenia, it is necessary to identify those biochemical pathways that define this process. Animal models that mimic one or more of the physiological pathways involved with this phenomenon are very beneficial in providing an understanding of the cellular processes at work in sarcopenia. The ability to influence pathways through genetic manipulation gives insight into cellular responses and their impact on the physical expression of sarcopenia. This review evaluates several murine models that have the potential to elucidate biochemical processes integral to sarcopenia. Identifying animal models that reflect sarcopenia or its component pathways will enable researchers to better understand those pathways that contribute to age-related skeletal muscle mass loss, and in turn, develop interventions that will prevent, retard, arrest, or reverse this phenomenon. This article is part of a Special Issue entitled: Animal Models of Disease.

Integrating metabolism and longevity through insulin and IGF1 signaling

The insulin pathway coordinates growth, development, metabolic homoeostasis, fertility, and stress resistance, which influence life span. Compensatory hyperinsulinemia to overcome systemic insulin resistance circumvents the immediate consequences of hyperglycemia. Work on flies, nematodes, and mice indicate that excess insulin signaling damages cellular function and accelerates aging. Maintenance of the central nervous system (CNS) has particular importance for life span. Reduced insulin/IGF1 signaling in the CNS can dysregulate peripheral energy homeostasis and metabolism, promote obesity, and extend life span. Genetic manipulations of insulin/IGF1 signaling components are revealing neuronal circuits that might resolve the central regulation of systemic metabolism from organism longevity.

Human height genes and cancer

Body development requires the ability to control cell proliferation and metabolism, together with selective 'invasive' cell migration for organogenesis. These requirements are shared with cancer. Human height-associated loci have been recently identified by genome-wide SNP-association studies. Strikingly, most of the more than 100 genes found associated to height appear linked to neoplastic growth, and impose a higher risk for cancer. Height-associated genes drive the HH/PTCH and BMP/TGFβ pathways, with p53, c-Myc, ERα, HNF4A and SMADs as central network nodes. Genetic analysis of body-size-affecting diseases and evidence from genetically-modified animals support this model. The finding that cancer is deeply linked to normal, body-plan master genes may profoundly affect current paradigms on tumor development.

Crosstalk between Oxidative Stress and SIRT1: Impact on the Aging Process

Increased oxidative stress has been associated with the aging process. However, recent studies have revealed that a low-level oxidative stress can even extend the lifespan of organisms. Reactive oxygen species (ROS) are important signaling molecules, e.g., being required for autophagic degradation. SIRT1, a class III protein deacetylase, is a crucial cellular survival protein, which is also involved in combatting oxidative stress. For instance, SIRT1 can stimulate the expression of antioxidants via the FoxO pathways. Moreover, in contrast to ROS, SIRT1 inhibits NF-κB signaling which is a major inducer of inflammatory responses, e.g., with inflammasome pathway. Recent studies have demonstrated that an increased level of ROS can both directly and indirectly control the activity of SIRT1 enzyme. For instance, ROS can inhibit SIRT1 activity by evoking oxidative modifications on its cysteine residues. Decreased activity of SIRT1 enhances the NF-κB signaling, which supports inflammatory responses. This crosstalk between the SIRT1 and ROS signaling provokes in a context-dependent manner a decline in autophagy and a low-grade inflammatory phenotype, both being common hallmarks of ageing. We will review the major mechanisms controlling the signaling balance between the ROS production and SIRT1 activity emphasizing that this crosstalk has a crucial role in the regulation of the aging process.

Organically grown food provides health benefits to Drosophila melanogaster

The "organic food" market is the fastest growing food sector, yet it is unclear whether organically raised food is nutritionally superior to conventionally grown food and whether consuming organic food bestows health benefits. In order to evaluate potential health benefits of organic foods, we used the well-characterized fruit fly Drosophila melanogaster as a model system. Fruit flies were raised on a diets consisting of extracts of either conventionally or organically raised produce (bananas, potatoes, raisins, soy beans). Flies were then subjected to a variety of tests designed to assess overall fly health. Flies raised on diets made from organically grown produce had greater fertility and longevity. On certain food sources, greater activity and greater stress resistance was additionally observed, suggesting that organic food bestows positive effects on fly health. Our data show that Drosophila can be used as a convenient model system to experimentally test potential health effects of dietary components. Using this system, we provide evidence that organically raised food may provide animals with tangible benefits to overall health.

Sirt1 inhibits the transcription factor CREB to regulate pituitary growth hormone synthesis

Growth hormone (GH) is a major anabolic hormone and the primary regulator of organism growth. Its transcription is triggered by GH-releasing hormone (GHRH) through the transcription factor cAMP response element-binding protein (CREB) and by caloric intake. In contrast, the deacetylase Sirt1 is activated by caloric restriction. Therefore, the present study investigates how Sirt1 affects CREB function and GH synthesis. Sirt1 pharmacological activation with resveratrol (IC₅₀=87 μM) suppressed GHRH-induced GH secretion from rat anterior pituitary cells in vivo and in vitro, while vehicle controls showed no effect. Resveratrol's effects were abolished after knocking down Sirt1 with RNA interference, but not in control scrambled siRNA-transfected rat somatotrophs, confirming the Sirt1 specificity. Sirt1 activation and overexpression suppressed forskolin-induced CREB-Ser(133) phosphorylation, but no effect was seen with vehicle and empty plasmid controls. The deacetylase-dead mutant Sirt1 retained CREB-Ser(133) phosphorylation by keeping protein phosphatase protein phosphatase 1 activity low. Sirt1 activation suppressed glycogen synthase kinase 3 β acetylation, and a mutation on the GSK3β-Lys(205) residue mimicking a hypoacetylated form revealed increased activity. In summary, this is a novel mechanism through which Sirt1 intercepts the cAMP pathway by suppressing CREB transcriptional activation, resulting in decreased GH synthesis.

DNA-Reparatur und Alterung

Die letzten Jahre haben erhebliche Fortschritte im Verständnis der Biologie des Alterns erbracht. Die Bedeutung genomischer Instabilität als kausaler Faktor der funktionellen Degeneration in der Alterung wurde besonders durch Untersuchungen an progeroiden Syndromen deutlich. Humane Progerien stellen seltene Erbkrankheiten dar, welche von ausgesprochener Diversität und Komplexität gekennzeichnet sind. Die Manifestationen progeroider Syndrome reflektieren sowohl die Funktion der involvierten DNA-Reparatur- und Schadensantwortmechanismen als auch die komplexen physiologischen Reaktionen auf DNA-Läsionen. Untersuchungen an biologischen Modellsystemen progeroider Erkrankungen haben Verbindungen zwischen der DNA-Schadensantwort und genetischen Regulationsmechanismen der Alterung aufgezeigt. Durch diese Erkenntnisse hat sich einerseits das Verständnis der Ursachen von Progerie und Alterung verbessert, andererseits werden neue Strategien erkenntlich, welche zur Entwicklung präventiver Interventionen zur Behandlung progeroider Symptome und altersbedingter Erkrankungen führen könnten.

Mechanistic or mammalian target of rapamycin (mTOR) may determine robustness in young male mice at the cost of accelerated aging

Males, who are bigger and stronger than females, live shorter in most species from flies to mammals including humans. Cellular mass growth is driven in part by mTOR (Target of Rapamycin). When developmental growth is completed, then, instead of growth, mTOR drives aging, manifested by increased cellular functions, such as hyper-secretion by fibroblasts, thus altering homeostasis, leading to age-related diseases and death. We hypothesize that MTOR activity is elevated in male mice compared with females. Noteworthy, 6 months old males were 28 % heavier than females. Also levels of phosphorylated S6 (pS6) and phospho-AKT (p-AKT, Ser 473), markers of the mTOR activity, were higher in male organs tested. Levels of pS6 were highly variable among mice and correlated with body weight and p-AKT. With age, the difference between levels of pS6 between sexes tended to minimize, albeit males still had hyperactive mTOR. Unlike fasting, the intraperitoneal (i.p.) administration of rapamycin eliminated pS6 in all organs of all females measured by immunoblotting and immunohistochemistry without affecting p-AKT and blood insulin. Although i.p. rapamycin dramatically decreased levels of pS6 in males too, it was still detectable by immunoblotting upon longer exposure. Our study demonstrated that both tissue p-AKT and pS6 were higher in young male mice and were associated with increased body weight and insulin. These data can explain bigger body size and faster aging in males. Our data suggest higher efficacy of rapamycin compared to fasting. Higher sensitivity of females to rapamycin may explain more pronounced life extension by rapamycin observed in females compared to males in several studies.

The starvation hormone, fibroblast growth factor-21, extends lifespan in mice

Fibroblast growth factor-21 (FGF21) is a hormone secreted by the liver during fasting that elicits diverse aspects of the adaptive starvation response. Among its effects, FGF21 induces hepatic fatty acid oxidation and ketogenesis, increases insulin sensitivity, blocks somatic growth and causes bone loss. Here we show that transgenic overexpression of FGF21 markedly extends lifespan in mice without reducing food intake or affecting markers of NAD+ metabolism or AMP kinase and mTOR signaling. Transcriptomic analysis suggests that FGF21 acts primarily by blunting the growth hormone/insulin-like growth factor-1 signaling pathway in liver. These findings raise the possibility that FGF21 can be used to extend lifespan in other species.

DOI:http://dx.doi.org/10.7554/eLife.00065.001

In 1934, in a famous experiment at Cornell University, it was discovered that laboratory mice could live twice as long as expected if they were fed a low-calorie diet that included enough nutrients to avoid malnutrition. This phenomenon has since been observed in species ranging from worms to primates, but not in humans. Reducing calorie intake leads to longer lives by modifying a number of the biochemical pathways that sense nutrients, including pathways that involve insulin and various other biomolecules. Chemical and genetic methods can also increase longevity by modifying these pathways, which suggests that it might be possible to develop drugs that can increase lifespan without reducing calorie intake.

Mice, humans and other creatures respond to prolonged fasting through a number of adaptive changes that include mobilizing and burning fatty acids. The liver has an important role in this response, secreting a hormone called fibroblast growth factor-21 (FGF21) that coordinates these processes among tissues. Previous experiments on transgenic mice with high levels of this hormone have shown that it suppresses the activity of growth hormone and reduces the production of insulin-like growth factor, which prevents growth and can lead to hibernation-like behavior.

Here Zhang et al. compare groups of wild-type mice and transgenic mice with high levels of FGF21. They find that the transgenic mice have a longer median survival time than wild-type mice (38 months vs 28 months), and that the transgenic female mice on average live for 4 months longer than their male counterparts. However, unlike in other examples of increased longevity, they find that decreased food intake is not required. Instead, they find that transgenic mice eat more food than wild-type mice, yet remain profoundly insulin-sensitive. The results suggest that the longer survival times are caused by a reduction in the production of insulin-like growth factor, but they also suggest that the mechanism responsible for the increased longevity is independent of the three pathways that are usually associated with such increases. Further research is needed to understand this mechanism in greater detail and could, perhaps, pave the way for the use of FGF21-based hormone therapy to extend lifespan without the need for a low-calorie diet.

DOI:http://dx.doi.org/10.7554/eLife.00065.002

Decreased thyroid follicle size in dwarf mice may suggest the role of growth hormone signaling in thyroid growth regulation

Background

Altered somatotrophic signaling is among the most important potential mechanisms of extended longevity. Ames dwarf (df/df) mice are homozygous for mutation at the Prop-1 gene, leading to a lack of growth hormone (GH), prolactin and thyroid stimulating hormone (TSH). Mice homozygous for targeted disruption of the growth hormone receptor/growth hormone binding protein gene are known as GH receptor knockout (GHRKO) mice or “Laron dwarf”. Both, df/df and GHRKO mice, are characterized by reduced body size, low plasma insulin and insulin-like growth factor-I (IGF-I), remarkably extended longevity, and severe (in df/df mice) or mild (in GHRKO mice) thyroid hypofunction. Recently, by crossing df/df and GHRKO mice, double-mutant Ames dwarf/GHRKO (df/KO) mice were created. Interestingly, these mice are smaller than Ames dwarfs or GHRKOs, and also have reduced insulin and IGF-I levels. The aim of the study was to investigate if and to what extent certain thyroid morphological parameters, such as inner follicular surface area, inner follicular perimeter, as well as the follicular epithelium thickness are changed in the examined dwarf mice.

Methods

This quantification was performed in thyroids collected from df/df, GHRKO and df/KO female mice, at approximately 5–6 months of age. We used a computerized plotting programme that combines a live microscopic image of the slide with an operator-generated overlay.

Results

Inner follicular surface area and inner follicular perimeter were decreased in all examined kinds of dwarf mice as compared to normal animals. Furthermore, decreases in these two parameters were more pronounced in df/df and df/KO than in GHRKO mice. Concerning the follicular epithelium thickness, only a tendency towards decrease of this parameter was found in all three kinds of dwarf mice.

Conclusions

Parameters characterizing thyroid follicle size are decreased in all three examined models of dwarf mice, which may explain decreased thyroid hormone levels in both basal mutants (Ames dwarfs and GHRKOs). df/df mutation seems to predominate over GHRKO genetic intervention concerning their effects on thyroid growth. Beside TSH, also GH signaling seems to constitute a crucial element in the regulation of thyroid growth and, possibly, function.

Growth culture conditions and nutrient signaling modulating yeast chronological longevity

The manipulation of nutrient-signaling pathways in yeast has uncovered the impact of environmental growth conditions in longevity. Studies using calorie restriction show that reducing glucose concentration of the culture media is sufficient to increase replicative and chronological lifespan (CLS). Other components of the culture media and factors such as the products of fermentation have also been implicated in the regulation of CLS. Acidification of the culture media mainly due to acetic acid and other organic acids production negatively impacts CLS. Ethanol is another fermentative metabolite capable of inducing CLS reduction in aged cells by yet unknown mechanisms. Recently, ammonium was reported to induce cell death associated with shortening of CLS. This effect is correlated to the concentration of NH₄⁺ added to the culture medium and is particularly evident in cells starved for auxotrophy-complementing amino acids. Studies on the nutrient-signaling pathways regulating yeast aging had a significant impact on aging-related research, providing key insights into mechanisms that modulate aging and establishing the yeast as a powerful system to extend knowledge on longevity regulation in multicellular organisms.

Meta-analysis of gene expression in the mouse liver reveals biomarkers associated with inflammation increased early during aging

Aging is associated with a loss of cellular homeostasis, a decline in physiological function and an increase in various pathologies. Employing a meta-analysis, hepatic gene expression profiles from four independent mouse aging studies were interrogated. There was little overlap in the number of genes or canonical pathways perturbed, suggesting that independent study-specific factors may play a significant role in determining age-dependent gene expression. However, 43 genes were consistently altered during aging in three or four of these studies, including those that (1) exhibited progressively increased expression starting from 12 months of age, (2) exhibited similar expression changes in models of progeria at young ages and dampened or no changes in old longevity mouse models, (3) were associated with inflammatory tertiary lymphoid neogenesis (TLN) associated with formation of ectopic lymphoid structures observed in chronically inflamed tissues, and (4) overlapped with genes perturbed by aging in brain, muscle, and lung. Surprisingly, around half of the genes altered by aging in wild-type mice exhibited similar expression changes in adult long-lived mice compared to wild-type controls, including those associated with intermediary metabolism and feminization of the male-dependent gene expression pattern. Genes unique to aging in wild-type mice included those linked to TLN.

GH and IGF1: roles in energy metabolism of long-living GH mutant mice

Of the multiple theories to explain exceptional longevity, the most robust of these has centered on the reduction of three anabolic protein hormones, growth hormone (GH), insulin-like growth factor, and insulin. GH mutant mice live 50% longer and exhibit significant differences in several aspects of energy metabolism as compared with wild-type mice. Mitochondrial metabolism is upregulated in the absence of GH, whereas in GH transgenic mice and dwarf mice treated with GH, multiple aspects of these pathways are suppressed. Core body temperature is markedly lower in dwarf mice, yet whole-body metabolism, as measured by indirect calorimetry, is surprisingly higher in Ames dwarf and Ghr-/- mice compared with normal controls. Elevated adiponectin, a key antiinflammatory cytokine, is also very likely to contribute to longevity in these mice. Thus, several important components related to energy metabolism are altered in GH mutant mice, and these differences are likely critical in aging processes and life-span extension.

A persistent level of Cisd2 extends healthy lifespan and delays aging in mice

The CISD2 gene, which is an evolutionarily conserved novel gene, encodes a transmembrane protein primarily associated with the mitochondrial outer membrane. Significantly, the CISD2 gene is located within the candidate region on chromosome 4q where a genetic component for human longevity has been mapped. Previously, we have shown that Cisd2 deficiency shortens lifespan resulting in premature aging in mice. Additionally, an age-dependent decrease in Cisd2 expression has been detected during normal aging. In this study, we demonstrate that a persistent level of Cisd2 achieved by transgenic expression in mice extends their median and maximum lifespan without any apparent deleterious side effects. Cisd2 also ameliorates age-associated degeneration of the skin, skeletal muscles and neurons. Moreover, Cisd2 protects mitochondria from age-associated damage and functional decline as well as attenuating the age-associated reduction in whole-body energy metabolism. These results suggest that Cisd2 is a fundamentally important regulator of lifespan and provide an experimental basis for exploring the candidacy of CISD2 in human longevity.

Diverse roles of growth hormone and insulin-like growth factor-1 in mammalian aging: progress and controversies

Because the initial reports demonstrating that circulating growth hormone and insulin-like growth factor-1 decrease with age in laboratory animals and humans, there have been numerous studies related to the importance of these hormones for healthy aging. Nevertheless, the role of these potent anabolic hormones in the genesis of the aging phenotype remains controversial. In this chapter, we review the studies demonstrating the beneficial and deleterious effects of growth hormone and insulin-like growth factor-1 deficiency and explore their effects on specific tissues and pathology as well as their potentially unique effects early during development. Based on this review, we conclude that the perceived contradictory roles of growth hormone and insulin-like growth factor-1 in the genesis of the aging phenotype should not be interpreted as a controversy on whether growth hormone or insulin-like growth factor-1 increases or decreases life span but rather as an opportunity to explore the complex roles of these hormones during specific stages of the life span.

Deletion of growth hormone receptor gene but not visceral fat removal decreases expression of apoptosis-related genes in the kidney-potential mechanism of lifespan extension

Mice homozygous for the targeted disruption of the growth hormone (GH) receptor (Ghr) gene (GH receptor knockout; GHRKO; KO) are hypoinsulinemic, highly insulin sensitive, normoglycemic, and long-lived. Visceral fat removal (VFR) is a surgical intervention which improves insulin signaling in normal (N) mice and rats and extends longevity in rats. We have previously demonstrated decreased expression level of certain pro-apoptotic genes in skeletal muscles and suggested that this may contribute to the regulation of longevity in GHRKO mice. Alterations in apoptosis-related genes expression in the kidneys also may potentially lead to lifespan extension. In this context, we decided to examine the renal expression of the following genes: caspase-3, caspase-9, caspase-8, bax, bad, bcl-2, Smac/DIABLO, Apaf-1, p53, and cytochrome c1 (cyc1) in male GHRKO and N mice subjected to VFR or sham surgery, at approximately 6 months of age. The kidneys were collected 2 months after VFR. As a result, caspase-3, caspase-9, and bax expressions were decreased in KO mice as compared to N animals. Expressions of Smac/DIABLO, caspase-8, bcl-2, bad, and p53 did not differ between KOs and N mice. VFR did not change the expression of the examined genes in KO or N mice. In conclusion, endocrine abnormalities in GHRKO mice result in decreased expression of pro-apoptotic genes and VFR did not alter the examined genes expression in N and KO mice. These data are consistent with a model in which alterations of GH signaling and/or insulin sensitivity lead to increased lifespan mediated by decreased renal expression of pro-apoptotic genes.

AMP-activated protein kinase (AMPK) controls the aging process via an integrated signaling network

Efficient control of energy metabolic homeostasis, enhanced stress resistance, and qualified cellular housekeeping are the hallmarks of improved healthspan and extended lifespan. AMPK signaling is involved in the regulation of all these characteristics via an integrated signaling network. Many studies with lower organisms have revealed that increased AMPK activity can extend the lifespan. Experiments in mammals have demonstrated that AMPK controls autophagy through mTOR and ULK1 signaling which augment the quality of cellular housekeeping. Moreover, AMPK-induced stimulation of FoxO/DAF-16, Nrf2/SKN-1, and SIRT1 signaling pathways improves cellular stress resistance. Furthermore, inhibition of NF-κB signaling by AMPK suppresses inflammatory responses. Emerging studies indicate that the responsiveness of AMPK signaling clearly declines with aging. The loss of sensitivity of AMPK activation to cellular stress impairs metabolic regulation, increases oxidative stress and reduces autophagic clearance. These age-related changes activate innate immunity defence, triggering a low-grade inflammation and metabolic disorders. We will review in detail the signaling pathways of this integrated network through which AMPK controls energy metabolism, autophagic degradation and stress resistance and ultimately the aging process.

Mitochondrial electron transport chain functions in long-lived Ames dwarf mice

The age-associated decline in tissue function has been attributed to ROS-mediated oxidative damage due to mitochondrial dysfunction. The long-lived Ames dwarf mouse exhibits resistance to oxidative stress, a physiological characteristic of longevity. It is not known, however, whether there are differences in the electron transport chain (ETC) functions in Ames tissues that are associated with their longevity. In these studies we analyzed enzyme activities of ETC complexes, CI-CV and the coupled CI-CII and CII-CIII activities of mitochondria from several tissues of young, middle aged and old Ames dwarf mice and their corresponding wild type controls to identify potential mitochondrial prolongevity functions. Our studies indicate that post-mitotic heart and skeletal muscle from Ames and wild-type mice show similar changes in ETC complex activities with aging, with the exception of complex IV. Furthermore, the kidney, a slowly proliferating tissue, shows dramatic differences in ETC functions unique to the Ames mice. Our data show that there are tissue specific mitochondrial functions that are characteristic of certain tissues of the long-lived Ames mouse. We propose that this may be a factor in the determination of extended lifespan of dwarf mice.