IGF-1 signaling and aging

New insights into phytochemicals via protein glycosylation focused on aging and diabetes

BACKGROUND

Protein glycosylation as a common post-translational modification that has significant impacts on protein folding, enzymatic activity, and interfering with receptor functioning. In recent years, with the rapid development of glycopeptide enrichment and analysis technology and the deepening of glycosylation research, glycosylation has gradually become a sign of disease occurrence and development. Multiple investigations suggest that protein glycosylation affect the advances of diabetes and aging.

PURPOSE AND METHODS

This review was focused on the action mechanisms of glycosylated proteins production, permanent abnormalities in extracellular matrix component function, inflammatory and reactive oxygen species production, as well as the glycosylated characterizations of diabetes and aging. Further, advances in glycosylation analysis and detection methods are presented for the first time, highlighting for needed future developments. All literatures were gathered from PubMed and Google Scholar.

RESULTS

Herein, we review how protein glycosylation impacts the progression of diabetes and aging. Specifically, we focus on various types of glycosylation, including N-linked glycosylation, O-linked glycosylation, C-glycosylation, S-glycosylation, and glycophosphatidylinositol (GPI) anchors. N-linked glycosylation and O-linked glycosylation are commonly observed glycosylation forms, wherein O-GlcNAcylation plays a significant role in diabetes, while N-glycan could serve as biomarkers for identifying inflammation and aging.

CONCLUSIONS

Protein glycosylation produces a vastly larger number of core glycan structures through utilizing at least 173 glycosyltransferases and repeated common scaffolds. Single protein may contain multiple glycosylation sites, and the structure and occupancy of glycan at each site may be different, resulting in the macro heterogeneity of protein glycosylation. This review will contribute to how protein glycosylation impacts the life progress of cells and its association with diseases.

Responses to Many Anti-Aging Interventions Are Sexually Dimorphic

There is increasing appreciation that sex differences are not limited to reproductive organs or traits related to reproduction and that sex is an important biological variable in most characteristics of a living organism. The biological process of aging and aging-related traits are no exception and exhibit numerous, often major, sex differences. This article explores one aspect of these differences, namely sex differences in the responses to anti-aging interventions. Aging can be slowed down and/or postponed by a variety of environmental ("lifestyle"), genetic or pharmacological interventions. Although many, particularly older studies utilized only one sex of experimental animals, there is considerable evidence that responses to these interventions can be very different in females and males. Calorie restriction (CR), that is reducing food intake without malnutrition can extend longevity in both sexes, but specific metabolic alterations and health benefits induced by CR are not the same in women and men. In laboratory mice, several of the genetic alterations that reduce insulin-like growth factor I (IGF-1) signaling extend longevity more effectively in females or in females only. Beneficial effects of rapamycin, an inhibitor of mTOR signaling, on mouse longevity are greater in females. In contrast, several anti-aging compounds, including a weak estrogen, 17 alpha estradiol, extend longevity of male, but not female, mice. Apparently, fundamental mechanisms of aging are not identical in females and males and it is essential to use both sexes in studies aimed at identifying novel anti-aging interventions. Recommendations for lifestyle modifications, drugs, and dietary supplements to maintain good health and functionality into advanced age and to live longer will likely need to be tailored to the sex of the user.

Bifidobacterium animalis subsp. lactis BPL1™ and Its Lipoteichoic Acid Modulate Longevity and Improve Age/Stress-Related Behaviors in Caenorhabditis elegans

Life expectancy has increased globally in recent decades, driving interest in maintaining a healthy life that includes preservation of physical and mental abilities, particularly in elderly people. The gut microbiome becomes increasingly perturbed with aging so the use of probiotics can be a strategy for maintaining a balanced gut microbiome. A previous report showed that Bifidobacterium animalis subsp. lactis BPL1™ induces through its lipoteichoic acid (LTA) fat reduction activities via the insulin/IGF-1 signaling pathway. Here, we have delved into the mechanism of action, eliminating alternative pathways as putative mechanisms. Furthermore, we have identified that BPL1™, its heat treated form (BPL1™ HT) and its LTA prolong longevity in Caenorhabditis elegans (C. elegans) in an insulin/IGF-1-dependent mechanism, and its consumption improves the oxidative stress response, gut permeability and protection against pathogenic infections. Furthermore, positive effects on C. elegans stress-related behaviors and in the Alzheimer's Disease model were found, highlighting the potential of the strain in improving the cognitive functions and proteotoxicity in the nematode. These results indicate the pivotal role of the IGF-1 pathway in the activity of the strain and pave the way for potential applications of BPL1™, BPL1™ HT and its LTA in the field of longevity and age-related markers.

Astragalus polysaccharide ameliorates insulin resistance in HepG2 cells through activating the STAT5/IGF-1 pathway

BACKGROUND

Insulin resistance (IR) is considered as a major factor initiating type 2 diabetes mellitus and can lead to a reduction in glucose uptake that mainly occurs in the liver. Astragalus polysaccharide (APC), extracted from the traditional Chinese medicine, has been recorded to suppress IR. However, the underlying mechanism remains inadequately explored.

METHODS

IR was induced in HepG2 cells which further underwent APC treatment. Cell viability was determined by cell counting kit-8 assay. Pretreatment with AG490, an inhibitor of signal transducer and activator of transcription 5 (STAT5) signaling, was performed for investigating the influence of STAT5 on APC. Glucose uptake level was reflected by 2-deoxyglucose-6-phosphate content determined through colorimetric assay. Expression levels of insulin-like growth factor 1 (IGF-1), IGF-1 receptor (IGF-1R), phosphorylated-STAT5/STAT5, and p-protein kinase B (AKT)/AKT in the cells were assessed by Western blot. Radioimmunoassay (RIA) was used to detect IGF-1 secretion in the cells.

RESULTS

APC at doses of 10 and 20 mg increased the viability of HepG2 cells with/without IR induction, and abrogated IR-induced inhibition of glucose intake. Meanwhile, APC (10 mg) offset IR-induced inhibition on the expressions of IGF-1R and IGF-1, the activation of AKT and STAT5, and the secretion of IGF-1 in HepG2 cells. More importantly, the reversal effect of APC on IR-induced alterations in HepG2 cells was counteracted by AG490.

CONCLUSION

APC ameliorates IR in HepG2 cells through activating the STAT5/IGF-1 pathway.

S-Klotho level and physiological markers of cardiometabolic risk in healthy adult men

S-Klotho is perceived as a biomarker of healthy aging that has been shown to be inversely associated with cardiometabolic risk in elderly individuals. The aim of this study was to test if s-Klotho level is associated with cardiometabolic risk markers in younger healthy men in order to verify the possible role of s-Klotho level as an early marker of cardiometabolic risk. A cross-sectional study was conducted among 186 healthy men (M_age=35.33, SD_age=3.47) from a Western urban population. Serum basal levels of s-Klotho, lipid profile, homocysteine, glycemia markers, C-reactive protein, liver transaminases and creatinine were evaluated. Also, blood pressure was measured and cardiometabolic risk score and homeostatic model assessment for insulin resistance (HOMA-IR) were calculated. Testosterone and cortisol levels, self-reported psychological stress, physical activity, smoking in the past, alcohol use and body adiposity were controlled for. We found no relationship between levels of s-Klotho and physiological markers of cardiometabolic risk in the studied population. The results were similar when controlled for adiposity, testosterone level, physical activity, alcohol use and smoking in the past. We suggest that s-Klotho level is not an early marker of cardiometabolic risk in younger middle-aged healthy men.

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.

The Essential Role of SIRT1 in Hypothalamic-Pituitary Axis

The endocrine system plays an essential role in the physiological adaptation to malnutrition. The adaptive response of various hormones directs the energy utilization toward the survival functions and away from growth and reproduction. Particularly, the hypothalamic pituitary axis plays an integral and a central role in the regulation of endocrine organs. Sirtuin 1 (SIRT1) is a nicotinamide adenine dinucleotide (NAD)-dependent histone deacetylase that is activated in response to calorie restriction (CR). SIRT1 is involved in cellular processes via the deacetylation of histone as well as various transcription factors and signal transduction molecules and thereby modulates the endocrine/metabolic functions. There is much evidence to demonstrate clearly that SIRT1 in the hypothalamus, pituitary gland, and other target organs modifies the synthesis, secretion, and activities of hormones and in turn induces the adaptive responses. In this review, we discussed the role of SIRT1 in the hypothalamic pituitary axis and its pathophysiological significance.

Hepatic transcriptome implications for palm fruit juice deterrence of type 2 diabetes mellitus in young male Nile rats

Background

The Nile rat (NR, Arvicanthis niloticus) is a model of carbohydrate-induced type 2 diabetes mellitus (T2DM) and the metabolic syndrome. A previous study found that palm fruit juice (PFJ) delayed or prevented diabetes and in some cases even reversed its early stages in young NRs. However, the molecular mechanisms by which PFJ exerts these anti-diabetic effects are unknown. In this study, the transcriptomic effects of PFJ were studied in young male NRs, using microarray gene expression analysis.

Methods

Three-week-old weanling NRs were fed either a high-carbohydrate diet (%En from carbohydrate/fat/protein = 70:10:20, 16.7 kJ/g; n = 8) or the same high-carbohydrate diet supplemented with PFJ (415 ml of 13,000-ppm gallic acid equivalent (GAE) for a final concentration of 5.4 g GAE per kg diet or 2.7 g per 2000 kcal; n = 8). Livers were obtained from these NRs for microarray gene expression analysis using Illumina MouseRef-8 Version 2 Expression BeadChips. Microarray data were analysed along with the physiological parameters of diabetes.

Results

Compared to the control group, 71 genes were up-regulated while 108 were down-regulated in the group supplemented with PFJ. Among hepatic genes up-regulated were apolipoproteins related to high-density lipoproteins (HDL) and genes involved in hepatic detoxification, while those down-regulated were related to insulin signalling and fibrosis.

Conclusion

The results obtained suggest that the anti-diabetic effects of PFJ may be due to mechanisms other than an increase in insulin secretion.

Polμ deficiency increases resistance to oxidative damage and delays liver aging

Polμ is an error-prone PolX polymerase that contributes to classical NHEJ DNA repair. Mice lacking Polμ (Polμ^−/−) show altered hematopoiesis homeostasis and DSB repair and a more pronounced nucleolytic resection of some V(D)J junctions. We previously showed that Polμ^−/− mice have increased learning capacity at old ages, suggesting delayed brain aging. Here we investigated the effect of Polμ^−/− deficiency on liver aging. We found that old Polμ^−/− mice (>20 month) have greater liver regenerative capacity compared with wt animals. Old Polμ^−/− liver showed reduced genomic instability and increased apoptosis resistance. However, Polμ^−/− mice did not show an extended life span and other organs (e.g., heart) aged normally. Our results suggest that Polμ deficiency activates transcriptional networks that reduce constitutive apoptosis, leading to enhanced liver repair at old age.

QTLs influencing IGF-1 levels in a LOU/CxFischer 344F2 rat population. Tracks towards the metabolic theory of Ageing

OBJECTIVE

Since a reduction of the insulin/IGF-1 signaling cascade extends life span in many species and IGF-1 signaling might partly mediate the effects of caloric restriction (CR), an experimental intervention for increasing longevity, the purpose of the present study was to use quantitative trait loci (QTL) analysis, an unbiased genetic approach, to identify particular regions of the genome influencing plasma IGF-1 levels in an F2 intercross between F344 and LOU/C rats; the latter being an inbred strain of Wistar origin, considered as a model of healthy aging since it resists to age (and diet)-induced obesity.

DESIGN

F1 hybrids were obtained by crossbreeding LOU/C with F344 rats, and then F1 were bred inter se to obtain the F2 population, of which 93 males and 94 females were studied. Total plasma IGF-1 levels were determined by radioimmunoassay. A genome scan of the F2 population was made with 100 microsatellite markers) selected for their polymorphism between LOU/C and F344 strains (and by covering evenly the whole genome.

RESULTS

By simple interval mapping sex-dependent QTLs were found on chromosome 17 in males and on chromosome 18 in females. By multiple interval mapping, additional QTLs were found on chromosomes 1, 4, 5, 6, 12, 15 and 19 in males and on chromosomes 3, 5, 6, 12 and 17 in females. Only the markers D1Rat196 and D12Mgh5 were found in both males and females. The majority of QTLs corresponded to metabolic syndrome (cardiac function: n = 45 (30%), obesity/diabetes: n = 22 (15%), inflammation: n = 19 (13%) and only a limited number to body weight: n = 13 (9%), proliferation (n = 10 (7%) or ossification: n = 7 (5%). Ninety-six candidate genes were located on the different QTLs. A significant proportion of these genes are connected to IGF-1 production and receptor pathways (n = 18) or metabolic syndrome (n = 11).

CONCLUSIONS

Subsequent studies are necessary to determine whether the genetic networks underscored are also involved in age-associated obesity, diabetes and inflammation as well as cardiovascular impairments.

Neuronal inputs and outputs of aging and longevity

An animal’s survival strongly depends on its ability to maintain homeostasis in response to the changing quality of its external and internal environment. This is achieved through intracellular and intercellular communication within and among different tissues. One of the organ systems that plays a major role in this communication and the maintenance of homeostasis is the nervous system. Here we highlight different aspects of the neuronal inputs and outputs of pathways that affect aging and longevity. Accordingly, we discuss how sensory inputs influence homeostasis and lifespan through the modulation of different types of neuronal signals, which reflects the complexity of the environmental cues that affect physiology. We also describe feedback, compensatory, and feed-forward mechanisms in these longevity-modulating pathways that are necessary for homeostasis. Finally, we consider the temporal requirements for these neuronal processes and the potential role of natural genetic variation in shaping the neurobiology of aging.

Role of insulin-like growth factors (IGFs), their receptors and genetic regulation in the chondrogenesis and growth of the mandibular condylar cartilage

Growth of the mandibular condylar cartilage (MCC) is reviewed as a function of genetic and epigenetic factors. The growth centers around the differential spatial concentration of the chondrocytes, influence of growth factors like TGF-β and heterogeneity in the number of IGF receptors, control the action of IGF. Besides these factors, growth of the mandibular condyle is influenced by differential response of chondrocytes as a function of their source/ageing, which in turn is regulated by TGF-β, BMPs and IGFs. While IGF-1 promotes proteoglycan synthesis and survival of the chondrocytes to maintain cartilage homeostasis, TGF-β synergistically catalysed the effect of IGF-1, while BMPs catalysed proteolysis as and when physiologically needed. To understand these processes, role of IGF-1 and its six receptors is at the center to a number of physiological processes being regulated by its mode of application for the growth and differentiation. Probing deeper, biological functions of IGFs seemed to depend on their level of free status rather than bound status to respective IGF-binding proteins (IGF-BPs), considered prerequisite to modulate their biological functions. Genetic regulation of their secretion has thrown light on their insulin-like structural homology, level and response in osteo-arthritis (OA), rheumatic arthritis (RA) and diabetes type-II. Biochemistry and spatial distribution of IGF receptors in different domains exerts control on IGF-1 activities. In ultimate analysis, IGF-axis conserved during the evolution to regulate cell growth and proliferation affect nearly every organ in the body as judged from the techniques determining skeletal maturity and decision making dependent on it for orthodontic, orthognathic/orthopedic and dental implant applications.

Associations between genetic polymorphisms of insulin-like growth factor axis genes and risk for age-related macular degeneration

PURPOSE

To investigate whether insulin-like growth factor (IGF) axis genes, together with a novel dietary risk factor, the dietary glycemic index (dGI), and body mass index (BMI) affect the risk for age-related macular degeneration (AMD).

METHODS

This case-control study involved 962 subjects originally recruited through the Age-Related Eye Disease Study (AREDS) Genetic Repository. After those with missing covariates or invalid calorie intake (n = 23), diabetes (n = 59), and non-Caucasian race (n = 16) were excluded, 864 participants were used, including 209 AREDS category 1 participants (control group), 354 category 2 or 3 participants (drusen group), and 301 category 4 participants (advanced AMD group). A total of 25 single-nucleotide polymorphisms (SNPs) selected from IGF-1 (n = 9), IGF-2 (n = 1), IGF binding protein 1 (IGFBP1; n = 3), IGFBP3 (n = 3), acid-labile subunit of IGFBP (IGFALS; n = 2), IGF1 receptor (IGF1R; n = 4), and IGF2R (n = 3) were genotyped. SNP-AMD associations were measured with genotype, allele χ(2) tests and Armitage's trend test. Odds ratios (OR), 95% confidence intervals (CIs), and SNP-exposure interactions were evaluated by multivariate logistic regression.

RESULTS

One SNP (rs2872060) in IGF1R revealed a significant association with advanced AMD (P-allele = 0.0009, P-trend = 0.0008; the significance level was set at 0.05/25 = 0.002 for multiple comparisons). The risk allele (G) in the heterozygous and homozygous states (OR, 1.67 and 2.93; 95% CI, 1.03-2.71 and 1.60-5.36, respectively) suggests susceptibility and an additive effect on AMD risk. Further stratification analysis remained significant for both neovascularization (OR, 1.49 and 2.61; 95% CI, 0.90-2.48 and 1.39-4.90, respectively) and geographic atrophy (OR, 2.57 and 4.52; 95% CI, 0.99-6.71 and 1.49-13.74, respectively). The G allele interaction analysis with BMI was significant for neovascularization (P = 0.042) but not for geographic atrophy (P = 0.47). No significant interaction was found with dGI.

CONCLUSIONS

These data suggest a role of IGF1R on the risk for advanced AMD in this group of subjects.

GHRH and wound healing

The principal action of growth hormone-releasing hormone (GHRH) is considered to be the stimulation of GH production and release in the pituitary.1 With the recent identification of wound-associated fibroblasts as a direct target of GHRH,2 this neuropeptide emerges as a rather pleiotropic hormone implicated in the regulation of various physiological and pathological processes and affecting diverse cell types.

Role of insulin and growth hormone/insulin-like growth factor-I signaling in lifespan extension: rodent longevity models for studying aging and calorie restriction

Insulin/insulin-like growth factor-I (IGF-I) pathways are recognized as critical signaling pathways involved in the control of lifespans in lower organisms to mammals. Caloric restriction (CR) reduces plasma concentration of insulin, growth hormone (GH), and IGF-I. CR retards various age-dependent disorders such as nuerodegenerative diseases and extends lifespan in laboratory rodents. These beneficial effects of CR are partly mimicked in spontaneous or genetically engineered rodent models of reduced insulin and GH/IGF-I axis. Most of these long-living rodents show increased insulin sensitivity; however, recent study has revealed that some other rodents show normal or reduced insulin sensitivity. Thus, increased insulin sensitivity might be not prerequisite for lifespan extension in insulin/GH/IGF-I altered longevity rodent models. These results highlighted that, for lifespan extension, the intracellular signaling molecules of insulin/GH/IGF-I pathways might be more important than actual peripheral or systemic insulin action.

[Growth hormone therapy in adult patients: a review]

Growth hormone deficiency (GHD) can frequently be expected in hypopituitarism of adult patients. If GHD is proven by dynamic testing of the somatotrophic axis, growth hormone substitution is useful for improving quality of life, body composition, bone and lipid metabolism, and myocardial function according to the criteria of evidence-based medicine and is admitted by most national health authorities. There are no other reasonable indications for growth hormone treatment in adulthood.

Phosphoinositide 3-kinase as a novel functional target for the regulation of the insulin signaling pathway by SIRT1

The protein deacetylase SIRT1, and its activator resveratrol, exert beneficial effects on glucose metabolism. Different SIRT1 targets have been identified, including PTP1B, AMPK, FOXO, PGC-1α and IRS2. The latter may underscore a tight link between SIRT1 and insulin signaling components. However, whether SIRT1 has a direct effect on insulin resistance and whether resveratrol acts directly or indirectly in this context is still a matter of controversy and this question has not been addressed in muscle cells. Here, we show that SIRT1 protein expression is decreased in muscle biopsies and primary myotubes derived from type 2 diabetic patients, suggesting a contribution of diminished SIRT1 in the determination of muscle insulin resistance. To investigate the functional impact of SIRT1 on the insulin pathway, the activation of insulin downstream effector PKB was evaluated after SIRT1 inactivation by RNAi, SIRT1 overexpression, or resveratrol treatments. In muscle cells and HEK293 cells, downregulation of SIRT1 reduced, while overexpression increased, insulin-induced PKB activatory phosphorylation. Further molecular characterisation revealed that SIRT1 interacts in an insulin-independent manner with the PI3K adapter subunit p85. We then investigated whether resveratrol may improve insulin signaling in muscle cells via SIRT1, or alternative targets. Incubation of muscle cells with resveratrol reverted the insulin-resistant state induced by prolonged TNFα or insulin treatment. Resveratrol-dependent improvement of insulin-resistance occurred through inhibition of serine phosphorylation of IRS1/2, implicating resveratrol as a serine kinase inhibitor. Finally, a functional interaction between PI3K and SIRT1 was demonstrated in C. elegans, where constitutively active PI3K - mimicking increased IIS signaling - lead to shortened lifespan, while removal of sir-2.1 abolished PI3K-induced lifespan shortening. Our data identify SIRT1 as a positive modulator of insulin signaling in muscle cells through PI3K, and this mechanism appears to be conserved from C. elegans through humans.

Post-translational modification of cellular proteins by ubiquitin and ubiquitin-like molecules: role in cellular senescence and aging

Ubiquitination ofendogenous proteins is one of the key regulatory steps that guides protein degradation through regulation of proteasome activity. During the last years evidence has accumulated that proteasome activity is decreased during the aging process in various model systems and that these changes might be causally related to aging and age-associated diseases. Since in most instances ubiquitination is the primary event in target selection, the system ofubiquitination and deubiquitination might be of similar importance. Furthermore, ubiquitination and proteasomal degradation are not completely congruent, since ubiquitination confers also functions different from targeting proteins for degradation. Depending on mono- and polyubiquitination and on how ubiquitin chains are linked together, post-translational modifications of cellular proteins by covalent attachment of ubiquitin and ubiquitin-like proteins are involved in transcriptional regulation, receptor internalization, DNA repair, stabilization of protein complexes and autophagy. Here, we summarize the current knowledge regarding the ubiquitinome and the underlying ubiquitin ligases and deubiquitinating enzymes in replicative senescence, tissue aging as well as in segmental progeroid syndromes and discuss potential causes and consequences for aging.

Changes in gene expression associated with aging commonly originate during juvenile growth

In mammals, proliferation is rapid in many tissues during early postnatal life, causing rapid somatic growth. This robust proliferation is then suppressed as the animal approaches adult size, bringing many tissues to a quiescent state where proliferation occurs only as needed to replace dying cells. Recent evidence suggests that the mechanism responsible for this decline in proliferation involves a multi-organ genetic program. We hypothesized that this genetic program continues to progress into later adult life, eventually suppressing proliferation to levels below those needed for tissue renewal, thus contributing to aging. We therefore used expression microarray to compare the temporal changes in gene expression that occur in adult mouse organs during aging to those occurring as juvenile proliferation slows. We found that many of the changes in gene expression that occur during the aging process originate during the period of juvenile growth deceleration. Bioinformatic analyses of the genes that show persistent decline in expression throughout postnatal life indicated that cell-cycle-related genes are strongly over-represented. Thus, the findings support the hypothesis that the genetic program that slows juvenile growth to limit body size persists into adulthood and thus may eventually hamper tissue maintenance and repair, contributing to the aging process.

Integrating evolutionary and molecular genetics of aging

Aging or senescence is an age-dependent decline in physiological function, demographically manifest as decreased survival and fecundity with increasing age. Since aging is disadvantageous it should not evolve by natural selection. So why do organisms age and die? In the 1940s and 1950s evolutionary geneticists resolved this paradox by positing that aging evolves because selection is inefficient at maintaining function late in life. By the 1980s and 1990s this evolutionary theory of aging had received firm empirical support, but little was known about the mechanisms of aging. Around the same time biologists began to apply the tools of molecular genetics to aging and successfully identified mutations that affect longevity. Today, the molecular genetics of aging is a burgeoning field, but progress in evolutionary genetics of aging has largely stalled. Here we argue that some of the most exciting and unresolved questions about aging require an integration of molecular and evolutionary approaches. Is aging a universal process? Why do species age at different rates? Are the mechanisms of aging conserved or lineage-specific? Are longevity genes identified in the laboratory under selection in natural populations? What is the genetic basis of plasticity in aging in response to environmental cues and is this plasticity adaptive? What are the mechanisms underlying trade-offs between early fitness traits and life span? To answer these questions evolutionary biologists must adopt the tools of molecular biology, while molecular biologists must put their experiments into an evolutionary framework. The time is ripe for a synthesis of molecular biogerontology and the evolutionary biology of aging.

Insulin/IGF-like signalling, the central nervous system and aging

Enormous strides in understanding aging have come from the discovery that mutations in single genes can extend healthy life-span in laboratory model organisms such as the yeast Saccharomyces, the fruit fly Drosophila melanogaster, the nematode worm Caenorhabditis elegans and the mouse. IIS [insulin/IGF (insulin-like growth factor)-like signalling] stands out as an important, evolutionarily conserved pathway involved in the determination of lifespan. The pathway has diverse functions in multicellular organisms, and mutations in IIS can affect growth, development, metabolic homoeostasis, fecundity and stress resistance, as well as lifespan. The pleiotropic nature of the pathway and the often negative effects of its disruption mean that the extent, tissue and timing of IIS manipulations are determinants of a positive effect on lifespan. One tissue of particular importance for lifespan extension in diverse organisms is the CNS (central nervous system). Although lowered IIS in the CNS can extend lifespan, IIS is also widely recognized as being neuroprotective and important for growth and survival of neurons. In the present review, we discuss our current understanding of the role of the nervous system in extension of lifespan by altered IIS, and the role of IIS in determination of neuronal function during aging. The nervous system can play both endocrine and cell-autonomous roles in extension of lifespan by IIS, and the effects of IIS on lifespan and neuronal function can be uncoupled to some extent. Tissue-specific manipulation of IIS and the cellular defence mechanisms that it regulates will better define the ways in which IIS affects neuronal and whole-organism function during aging.

High tandem repeat content in the genome of the short-lived annual fish Nothobranchius furzeri: a new vertebrate model for aging research

BACKGROUND

The annual fish Nothobranchius furzeri is the vertebrate with the shortest known life span in captivity. Fish of the GRZ strain live only three to four months under optimal laboratory conditions, show explosive growth, early sexual maturation and age-dependent physiological and behavioral decline, and express aging related biomarkers. Treatment with resveratrol and low temperature significantly extends the maximum life span. These features make N. furzeri a promising new vertebrate model for age research.

RESULTS

To contribute to establishing N. furzeri as a new model organism, we provide a first insight into its genome and a comparison to medaka, stickleback, tetraodon and zebrafish. The N. furzeri genome contains 19 chromosomes (2n = 38). Its genome of between 1.6 and 1.9 Gb is the largest among the analyzed fish species and has, at 45%, the highest repeat content. Remarkably, tandem repeats comprise 21%, which is 4-12 times more than in the other four fish species. In addition, G+C-rich tandem repeats preferentially localize to centromeric regions. Phylogenetic analysis based on coding sequences identifies medaka as the closest relative. Genotyping of an initial set of 27 markers and multi-locus fingerprinting of one microsatellite provides the first molecular evidence that the GRZ strain is highly inbred.

CONCLUSIONS

Our work presents a first basis for systematic genomic and genetic analyses aimed at understanding the mechanisms of life span determination in N. furzeri.

Integrating the stress response: lessons for neurodegenerative diseases from C. elegans

All cells possess surveillance and homeostatic mechanisms to adjust protein biogenesis to the demands of growth, differentiation, ageing and environmental stress. However, under certain circumstances, these mechanisms fail to adequately respond to proteotoxic imbalances and result in the accumulation of misfolded proteins. In humans, this can lead to neurodegeneration and other protein conformational diseases. To protect itself, the cell employs highly conserved stress responses and chaperone networks to maintain protein-folding homeostasis (proteostasis). Although the regulation of stress responses, such as the heat-shock response, and of proteostasis have been widely considered to be cell autonomous, recent studies using Caenorhabditis elegans have shown that these processes are regulated by neuronal signaling and endocrine pathways and integrated into other functions of the organism. The hierarchical control of the cellular proteostasis machinery affords insight into the organization of stress regulatory networks in multicellular organisms and offers novel targets for the treatment of human protein conformational diseases.

The role of insulin-like growth factor-I and its binding proteins in glucose homeostasis and type 2 diabetes

This review addresses the possible role of the insulin-like growth factor (IGF)-axis in normal glucose homoeostasis and in the etiopathogenesis of type 2 diabetes. IGF-I, a peptide hormone, shares amino acid sequence homology with insulin and has insulin-like activity; most notably, the promotion of glucose uptake by peripheral tissues. Type 2 diabetes as well as pre-diabetic states, including impaired fasting glucose and impaired glucose tolerance, are associated cross-sectionally with altered circulating levels of IGF-I and its binding proteins (IGFBPs). Administration of recombinant human IGF-I has been reported to improve insulin sensitivity in healthy individuals as well as in patients with insulin resistance and type 2 diabetes. Further, IGF-I may have beneficial effects on systemic inflammation, a risk factor for type 2 diabetes, and on pancreatic beta-cell mass and function. There is considerable inter-individual heterogeneity in endogenous levels of IGF-I and its binding proteins; however, the relationship between these variations and the risk of developing type 2 diabetes has not been extensively investigated. Large prospective studies are required to evaluate this association.

Metabolic consequences of pregnancy-associated plasma protein-A deficiency in mice: exploring possible relationship to the longevity phenotype

Mice born with the deletion of the gene for pregnancy-associated plasma protein-A (PAPP-A), a model of reduced local IGF activity, live approximately 30% longer than their wild-type (WT) littermates. In this study, we investigated metabolic consequences of PAPP-A gene deletion and possible relationship to lifespan extension. Specifically, we determined whether 18-month-old PAPP-A knockout (KO) mice when compared with their WT littermates have reduced energy expenditure and/or altered glucose-insulin sensitivity. Food intake, and total energy expenditure and resting energy expenditure as measured by calorimetry were not different between PAPP-A KO and WT mice when subjected to the analysis of covariance with body weight as the covariate. However, there was an increase in spontaneous physical activity in PAPP-A KO mice. Both WT and PAPP-A KO mice exhibited mild insulin resistance with age, as assessed by fasting glucose/insulin ratios. Oral glucose tolerance and insulin sensitivity were not significantly different between the two groups of mice, although there appeared to be a decrease in the average size of the pancreatic islets in PAPP-A KO mice. Thus, neither reduced 'rate of living' nor altered glucose-insulin homeostasis can be considered key determinants of the enhanced longevity of PAPP-A KO mice. These findings are discussed in the context of those from other long-lived mouse models.

Insulin-like growth factor-induced signals activate mitochondrial respiration

From experiments with lower eukaryotes it is known that the metabolic rate and also the rate of aging are tightly controlled by the insulin-like growth factor (IGF)/insulin signal transduction pathway. The mitochondrial theory of aging implies that an increased metabolic rate leads to increased mitochondrial activity; increased production of reactive oxygen species due to these alterations would speed up the aging process. To address the question if mitochondrial activity is influenced by insulin/IGF signaling, we have established an experimental system to determine the influence of IGF-I-dependent signaling on mitochondrial function. We used DU145 prostate cancer cells, known for the intact IGF signal transduction pathway, to address the influence of IGF receptor activation on mitochondrial function by high-resolution respirometry. These experiments revealed that indeed mitochondrial function is regulated by IGF signaling, and up-regulation of respiration seems to require phosphoinositide 3-kinase/AKT signaling, but is independent of IGF effects on cell cycle progression. Collectively these data establish a regulatory cross-talk between insulin/IGF signal transduction and mitochondrial function, two major pathways implicated in controlling the rate of aging.

Brain insulin growth factor-I induces diuresis increase through the inhibition of arginin-vasopressin release in aged rats

Normal aging is associated with water homeostasis impairment, arginin-vasopressin (AVP) neuron dysfunction and cerebral insulin growth factor-I (IGF-I) expression deficit. Therefore, we aimed at investigating whether a cerebral chronic treatment of IGF-I in aged rats (26-mo) could restore diuretic function comparable with that observed in adults (3-mo). By using osmotic pumps, we have shown that in aged rats, IGF-I treatment in the third ventricle for four weeks increases water intake and restores diuresis and AVP plasma release similar with that observed in adults. The decrease in AVP plasma release induced by brain IGF-I treatment was also associated with the decrease in urinary osmolality. These results indicate that the age-dependent IGF-I deficit in the brain may be involved in the age-impaired fluid homeostasis in rats.

Quantitative evidence for conserved longevity pathways between divergent eukaryotic species

Studies in invertebrate model organisms have been a driving force in aging research, leading to the identification of many genes that influence life span. Few of these genes have been examined in the context of mammalian aging, however, and it remains an open question as to whether and to what extent the pathways that modulate longevity are conserved across different eukaryotic species. Using a comparative functional genomics approach, we have performed the first quantitative analysis of the degree to which longevity genes are conserved between two highly divergent eukaryotic species, the yeast Saccharomyces cerevisiae and the nematode Caenorhabditis elegans. Here, we report the replicative life span phenotypes for single-gene deletions of the yeast orthologs of worm aging genes. We find that 15% of these yeast deletions are long-lived. In contrast, only 3.4% of a random set of deletion mutants are long-lived-a statistically significant difference. These data suggest that genes that modulate aging have been conserved not only in sequence, but also in function, over a billion years of evolution. Among the longevity determining ortholog pairs, we note a substantial enrichment for genes involved in an evolutionarily conserved pathway linking nutrient sensing and protein translation. In addition, we have identified several conserved aging genes that may represent novel longevity pathways. Together, these findings indicate that the genetic component of life span determination is significantly conserved between divergent eukaryotic species, and suggest pathways that are likely to play a similar role in mammalian aging.

Fat tissue and long life

Studies over the last several years have revealed important roles of the body fat content, caloric intake and nutrition, insulin/IGF-1 signaling systems, and pathways involved in oxidative stress and control of protein acetylation on life span. Although the discovery of longevity genes supports the concept that life span is genetically determined, adipose tissue seems to be a pivotal organ in the aging process and in the determination of life span. Leanness and caloric restriction have been shown to increase longevity in organisms ranging from yeast to mammals. Increased longevity in mice with a fat-specific disruption of the insulin receptor gene (FIRKO) suggests that reduced adiposity, even in the presence of normal or increased food intake, leads to an extended life span. Reduced fat mass has an impact on longevity in a number of other model organisms. In Drosophila, a specific reduction in the fat body through overexpression of forkhead type transcription factor (dFOXO) extends life span. Sirtuin 1 (SIRT1), the mammalian ortholog of the life-extending yeast gene silent information regulator 2 (SIR2), was proposed to be involved in the molecular mechanisms linking life span to adipose tissue. Moreover, in the control of human aging and longevity, one of the striking physiological characteristics identified in centenarians is their greatly increased insulin sensitivity even compared with younger individuals. On the other hand, overweight and obesity seem to be associated with decreased life span in humans. In addition, it was recently shown that modifiable risk factors during the later years of life, including smoking, obesity, and hypertension, are associated not only with lower life expectancy, but also with poor health and function during older age. There is growing evidence that the effect of reduced adipose tissue mass on life span could be due to the prevention of obesity-related metabolic disorders including type 2 diabetes and atherosclerosis.

Estradiol, insulin-like growth factor-I and brain aging

The decrease in some hormones with aging, such as insulin-like growth factor-I (IGF-I) and estradiol, may have a negative impact on brain function. Estradiol and IGF-I may antagonize the damaging effects of adrenal steroids and other causes of brain deterioration. The signaling of estradiol and IGF-I interact to promote neuroprotection. Estrogen receptor alpha, in an estrogen-dependent process, can physically interact with IGF-I receptor and with the downstream signaling molecules of the phosphotidylinositol 3-kinase (PI3K)/Akt/glycogen synthase kinase 3 (GSK3) pathway. Estradiol and IGF-I have a synergistic effect on the activation of Akt, which in turn decreases the activity of GSK3. This may be one of the mechanisms used by estradiol to promote neuronal survival, since the inhibition of GSK3 is associated to the activation of surviving signaling pathways in neurons. Furthermore, estradiol may control Tau phosphorylation by modulating the interactions of estrogen receptor alpha with GSK3 and beta-catenin, another molecule involved in the regulation of neuronal survival and the reorganization of the cytoskeleton. All these actions may be involved in the neuroprotective effects of the hormone. Possible aging-associated changes in the expression or activity of these signaling molecules may affect estradiol neuroprotective effects. Therefore, it is important to determine whether aging affects the signaling of estradiol and IGF-I in the brain.

Novel perspectives for progesterone in hormone replacement therapy, with special reference to the nervous system

The utility and safety of postmenopausal hormone replacement therapy has recently been put into question by large clinical trials. Their outcome has been extensively commented upon, but discussions have mainly been limited to the effects of estrogens. In fact, progestagens are generally only considered with respect to their usefulness in preventing estrogen stimulation of uterine hyperplasia and malignancy. In addition, various risks have been attributed to progestagens and their omission from hormone replacement therapy has been considered, but this may underestimate their potential benefits and therapeutic promises. A major reason for the controversial reputation of progestagens is that they are generally considered as a single class. Moreover, the term progesterone is often used as a generic one for the different types of both natural and synthetic progestagens. This is not appropriate because natural progesterone has properties very distinct from the synthetic progestins. Within the nervous system, the neuroprotective and promyelinating effects of progesterone are promising, not only for preventing but also for reversing age-dependent changes and dysfunctions. There is indeed strong evidence that the aging nervous system remains at least to some extent sensitive to these beneficial effects of progesterone. The actions of progesterone in peripheral target tissues including breast, blood vessels, and bones are less well understood, but there is evidence for the beneficial effects of progesterone. The variety of signaling mechanisms of progesterone offers exciting possibilities for the development of more selective, efficient, and safe progestagens. The recognition that progesterone is synthesized by neurons and glial cells requires a reevaluation of hormonal aging.

The role of insulin and neurotrophic factor signaling in brain aging and Alzheimer’s Disease

Although increased lifespan is associated with reduced insulin signaling, insulin signaling is essential for neuronal development and survival. Insulin resistance is central to Type II diabetes and is also implicated in the pathogenesis of Alzheimer's Disease (AD). This has prompted ongoing clinical trials in AD patients to test the efficacy of improving insulin - like signaling with dietary omega-3 fatty acids or insulin - sensitizing drugs as well as exercise regimens. Here we review the role of insulin signaling in brain aging and AD, concluding that the signaling pathways downstream to neurotrophic and insulin signaling are defective and coincident with aberrant phosphorylation and translocation of key components, notably AKT and GSK3beta, but also rac> PAK signaling. These responses are likely to contribute to defects in synaptic plasticity, learning and memory. Both oligomers of beta-amyloid (which are elevated in the AD brain) and pro-inflammatory cytokines (which are elevated in the aged or AD brain) can be used to mimic the trophic factor/insulin resistance observed in AD, but details on other factors and mechanisms contributing to this resistance remain elusive. A better understanding of the precise mechanisms underlying alterations in the insulin/neurotrophic factor signal transduction pathways should aid the search for better AD therapeutic and prevention strategies.

Aging and the ubiquitinome: traditional and non-traditional functions of ubiquitin in aging cells and tissues

Ubiquitination of endogenous proteins is one of the key regulatory steps of protein degradation followed by regulation of proteasome activity. During the last years evidence has increased that proteasome activity is decreased during the aging process in various model systems and that these changes might be causally related to aging and aging associated diseases. Since in most instances ubiquitination is the primary event in target selection, the system of ubiquitination and deubiquitination might be of similar importance. Furthermore, ubiquitination and proteasomal degradation are not completely congruent, since ubiquitination also confers functions different from giving "the kiss of death" to proteins. Depending on mono- and polyubiquitination and on how ubiquitin chains are linked together, ubiquitination is involved in transcriptional regulation, receptor internalization, DNA repair, and stabilization of protein complexes. This review is therefore the first to summarize the current knowledge regarding the ubiquitinome and the underlying ubiquitin ligases and deubiquitinating enzymes in replicative senescence, tissue aging as well as in segmental progeroid syndromes and to discuss potential causes and consequences for aging.

Molecular bases of progeroid syndromes

Progeroid syndromes (PSs) constitute a group of disorders characterized by clinical features mimicking physiological aging at an early age. In some of these syndromes, biological hallmarks of aging are also present, whereas in others, a link with physiological aging, if any, remains to be elucidated. These syndromes are clinically and genetically heterogeneous and most of them, including Werner syndrome and Hutchinson-Gilford progeria, are known as 'segmental aging syndromes', as they do not feature all aspects usually associated to physiological aging. However, all the characterized PSs enter in the field of rare monogenic disorders and several causative genes have been identified. These can be separated in subcategories corresponding to (i) genes encoding DNA repair factors, in particular, DNA helicases, and (ii) genes affecting the structure or post-translational maturation of lamin A, a major nuclear component. In addition, several animal models featuring premature aging have abnormal mitochondrial function or signal transduction between membrane receptors, nuclear regulatory proteins and mitochondria: no human pathological counterpart of these alterations has been found to date. In recent years, identification of mutations and their functional characterization have helped to unravel the cellular processes associated to segmental PSs. Recently, several studies allowed to establish a functional link between DNA repair and A-type lamins-associated syndromes, evidencing a relation between these syndromes, physiological aging and cancer. Here, we review recent data on molecular and cellular bases of PSs and discuss the mechanisms involved, with a special emphasis on lamin A-associated progeria and related disorders, for which therapeutic approaches have started to be developed.

Effect of leptin on hypothalamic gene expression in calorie-restricted rats

Diminished leptin signaling to the arcuate nucleus of hypothalamus (ARH) may induce calorie restriction (CR)-specific neuroendocrine and metabolic adaptation, which is potentially relevant to the effect of CR. The present study investigated whether restoration of leptin signaling to the ARH could reverse CR-induced alterations in neuropeptide gene expression in rats. Male F344 rats, fed ad libitum or a 30% CR diet from 6 weeks of age, received leptin or vehicle intracerebroventricularly for 14 days via osmotic mini-pumps implanted in the subcutis at 34 weeks of age. The messenger RNA levels were quantified by real-time reverse transcription-polymerase chain reaction using total RNA extracted from microdissected tissues containing the ARH. The results indicated that leptin administration reversed the upregulated expression of neuropeptide Y and agouti-related protein genes in CR rats, suggesting the possibility of a role for the leptin-ARH pathway in the effect of CR.

p16INK4A is a robust in vivo biomarker of cellular aging in human skin

The cell-cycle regulating gene, p16INK4A, encoding an inhibitor of cyclin-dependent kinases 4 and 6, is considered to play an important role in cellular aging and in premature senescence. Although there is an age-dependent increase of p16INK4A expression in human fibroblast senescence in vitro, no data are available regarding the age dependency of p16INK4A in vivo. To determine whether p16INK4A expression in human skin correlates with donor age, p16INK4A expression was analyzed by immunohistochemistry as well as the expression of the p16INK4A repressor BMI1. Samples from the age groups 0-20, 21-70, and 71-95 years were selected from a bank of healthy human skin. We show that the number of p16INK4A positive cells is significantly higher in elderly individuals compared to the younger age groups. The number of p16INK4A positive cells was found to be increased in both epidermis and dermis, compartments with strictly different proliferative activities. BMI1 gene expression was significantly down-regulated with increasing donor age, whereas no striking age differences were observed for Ki67. In immunofluorescence co-expression studies, Ki67-positive cells were negative for p16INK4A and BMI1-expressing cells also stained negatively for Ki67. In conclusion, we provide for the first time evidence that p16INK4A expression directly correlates with chronological aging of human skin in vivo. p16INK4A therefore is a biomarker for human aging in vivo. The data reported here suggest a model for changes in regulatory gene expression that drive aging in human skin.

Pleiotropic fitness effects of the Tre1-Gr5a region in Drosophila melanogaster

The abundance of transposable elements and DNA repeat sequences in mammalian genomes raises the question of whether such insertions represent passive evolutionary baggage or may influence the expression of complex traits. We addressed this question in Drosophila melanogaster, in which the effects of single transposable elements on complex traits can be assessed in genetically identical individuals reared in controlled environments. Here we demonstrate that single P-element insertions in the intergenic region between the gustatory receptor 5a (Gr5a, also known as Tre) and trapped in endoderm 1 (Tre1), which encodes an orphan receptor, exert complex pleiotropic effects on fitness traits, including selective nutrient intake, life span, and resistance to starvation and heat stress. Mutations in this region interact epistatically with downstream components of the insulin signaling pathway. Transposon-induced sex-specific and sex-antagonistic effects further accentuate the complex influences that intergenic transposable elements can contribute to quantitative trait phenotypes.

Hypervitaminosis D and premature aging: lessons learned from Fgf23 and Klotho mutant mice

The essential role of low levels of vitamin D during aging is well documented. However, possible effects of high levels of vitamin D on the aging process are not yet clear. Recent in vivo genetic-manipulation studies have shown increased serum level of vitamin D and altered mineral-ion homeostasis in mice that lack either fibroblast growth factor 23 (Fgf23) or klotho (Kl) genes. These mice develop identical phenotypes consistent with premature aging. Elimination or reduction of vitamin-D activity from Fgf23 and Kl mutant mice, either by dietary restriction or genetic manipulation could rescue premature aging-like features and ectopic calcifications, resulting in prolonged survival of both mutants. Such in vivo experimental studies indicated that excessive vitamin-D activity and altered mineral-ion homeostasis could accelerate the aging process.

[Growth hormone therapy in adults]

In adult patients with growth hormone deficiency due to organic hypopituitarism, substitution with growth hormone is very useful and well established. Positive influence on numerous target parameters has been investigated in randomised controlled trials (evidence level I) and could be documented. Thus, growth hormone substitution in hypopituitarism is scientifically founded and has been admitted by the authorities in the European Union. Concerning pharmacotherapy with growth hormone in healthy elderly persons without pituitary disease and also doping in sports, the situation is completely different. Use of growth hormone in these circumstances has not sufficiently been investigated concerning indications, efficacy and safety. Ethical and economical aspects have not been evaluated yet. Serious concerns about the therapeutic safety need to be considered.

Extended longevity and insulin signaling in adipose tissue

Caloric restriction and leanness have been shown to increase longevity in organisms ranging from yeast to mammals. Adipose tissue seems to be a pivotal organ in the aging process and in determination of lifespan. We have recently shown that fat-specific disruption of the insulin receptor gene is sufficient to increase lifespan in FIRKO mice, suggesting that reduced adiposity, even in the presence of normal or increased food intake, can extend lifespan. The model also suggests a special role for the insulin-signaling pathway in adipose tissue in the longevity process. Reduced fat mass has an impact on the duration of life in several other model organisms. In Drosophila, a specific reduction in the fat body through overexpression of forkhead type transcription factor (dFOXO) extends lifespan. Furthermore, sirtuin1 (SIRT1), the mammalian ortholog of the life-extending yeast gene silent information regulator 2 (SIR2), was proposed to be involved in the molecular mechanisms linking lifespan to adipose tissue. In the control of human aging and longevity, one of the striking physiological characteristics identified in centenarians is their greatly increased insulin sensitivity even compared with younger individuals. The effect of reduced adipose tissue mass on lifespan could be due to the prevention of obesity-related metabolic disorders including type 2 diabetes and atherosclerosis.