daf-2, an insulin receptor-like gene that regulates longevity and diapause in Caenorhabditis elegans

A PDK1 homolog is necessary and sufficient to transduce AGE-1 PI3 kinase signals that regulate diapause in Caenorhabditis elegans

An insulin receptor-like signaling pathway regulates Caenorhabditis elegans metabolism, development, and longevity. Inactivation of the insulin receptor homolog DAF-2, the AGE-1 PI3K, or the AKT-1 and AKT-2 kinases causes a developmental arrest at the dauer stage. A null mutation in the daf-16 Fork head transcription factor alleviates the requirement for signaling through this pathway. We show here that a loss-of-function mutation in pdk-1, the C. elegans homolog of the mammalian Akt/PKB kinase PDK1, results in constitutive arrest at the dauer stage and increased life span; these phenotypes are suppressed by a loss of function mutation in daf-16. An activating mutation in pdk-1 or overexpression of wild-type pdk-1 relieves the requirement for AGE-1 PI3K signaling. Therefore, pdk-1 activity is both necessary and sufficient to propagate AGE-1 PI3K signals in the DAF-2 insulin receptor-like signaling pathway. The activating mutation in pdk-1 requires akt-1 and akt-2 gene activity in order to suppress the dauer arrest phenotype of age-1. This indicates that the major function of C. elegans PDK1 is to transduce signals from AGE-1 to AKT-1 and AKT-2. The activating pdk-1 mutation is located in a conserved region of the kinase domain; the equivalent amino acid substitution in human PDK1 activates its kinase activity toward mammalian Akt/PKB.

Signals from the reproductive system regulate the lifespan of C. elegans

Understanding how the ageing process is regulated is a fascinating and fundamental problem in biology. Here we demonstrate that signals from the reproductive system influence the lifespan of the nematode Caenorhabditis elegans. If the cells that give rise to the germ line are killed with a laser microbeam, the lifespan of the animal is extended. Our findings suggest that germline signals act by modulating the activity of an insulin/IGF-1 (insulin-like growth factor) pathway that is known to regulate the ageing of this organism. Mutants with reduced activity of the insulin/IGF-1-receptor homologue DAF-2 have been shown to live twice as long as normal, and their longevity requires the activity of DAF- 16, a member of the forkhead/winged-helix family of transcriptional regulators. We find that, in order for germline ablation to extend lifespan, DAF-16 is required, as well as a putative nuclear hormone receptor, DAF-12. In addition, our findings suggest that signals from the somatic gonad also influence ageing, and that this effect requires DAF-2 activity. Together, our findings imply that the C. elegans insulin/IGF-1 system integrates multiple signals to define the animal's rate of ageing. This study demonstrates an inherent relationship between the reproductive state of this animal and its lifespan, and may have implications for the co-evolution of reproductive capability and longevity.

A cytosolic catalase is needed to extend adult lifespan in C. elegans daf-C and clk-1 mutants

The dauer larva is an alternative larval stage in Caenorhabditis elegans which allows animals to survive through periods of low food availability. Well-fed worms live for about three weeks, but dauer larvae can live for at least two months without affecting post-dauer lifespan. Mutations in daf-2 and age-1, which produce a dauer constitutive (Daf-C) phenotype, and in clk-1, which are believed to slow metabolism, markedly increase adult lifespan. Here we show that a ctl-1 mutation reduces adult lifespan in otherwise wild-type animals and eliminates the daf-c and clk-1-mediated extension of adult lifespan. ctl-1 encodes an unusual cytosolic catalase; a second gene, ctl-2, encodes a peroxisomal catalase. ctl-1 messenger RNA is increased in dauer larvae and adults with the daf-c mutations. We suggest that the ctl-1 catalase is needed during periods of starvation, as in the dauer larva, and that its misexpression in daf-c and clk-1 adults extends lifespan. Cytosolic catalase may have evolved to protect nematodes from oxidative damage produced during prolonged dormancy before reproductive maturity, or it may represent a general mechanism for permitting organisms to cope with the metabolic changes that accompany starvation.

Apparent uncoupling of energy production and consumption in long-lived Clk mutants of Caenorhabditis elegans

Clk mutants of Caenorhabditis elegans are characterised by an overall slow down of temporal processes and increase in life span. It was hypothesised that Clk mutations slow down the pace of many cellular functions and lower the rate of energy metabolism, possibly resulting in slower production of reactive oxygen species which in turn could result in slower ageing. We tested this hypothesis by measuring respiration rates, light production capacities (a measure of metabolic potential) and ATP levels in various strains harbouring mutant alleles of the Clk genes clk-1 and gro-1 and of three other genes that interact with the Clk genes. We found a mild reduction of oxygen consumption rates but little alteration of metabolic capacities in the single Clk mutants during the first 4-5 days of their adult lives, relative to the wild-type strain. This difference tended to fade away with increasing age, however, and aged Clk mutants eventually retained higher metabolic capacities than the wild-type control strain N2. These profiles are suggestive of physiological time being retarded, relative to chronological time in Clk mutants. Ageing clk-1 and gro-1 mutants also retained substantially elevated ATP levels relative to the N2 strain, and the simultaneous presence of mutations in daf-2 or age-1 - genes that affect longevity - boosted this effect. Thus, energy production and consumption appear to be uncoupled in these mutants. Mutation in the transcription factor daf-16 suppressed the Age and ATP phenotypes, but not the reduction of respiration rate imparted by mutation in clk-1.

Interorganelle signaling is a determinant of longevity in Saccharomyces cerevisiae

Replicative capacity, which is the number of times an individual cell divides, is the measure of longevity in the yeast Saccharomyces cerevisiae. In this study, a process that involves signaling from the mitochondrion to the nucleus, called retrograde regulation, is shown to determine yeast longevity, and its induction resulted in postponed senescence. Activation of retrograde regulation, by genetic and environmental means, correlated with increased replicative capacity in four different S. cerevisiae strains. Deletion of a gene required for the retrograde response, RTG2, eliminated the increased replicative capacity. RAS2, a gene previously shown to influence longevity in yeast, interacts with retrograde regulation in setting yeast longevity. The molecular mechanism of aging elucidated here parallels the results of genetic studies of aging in nematodes and fruit flies, as well as the caloric restriction paradigm in mammals, and it underscores the importance of metabolic regulation in aging, suggesting a general applicability.

Prevalence of hereditary hearing impairment in adults

This contribution, part of an EU-Concerted Action on the genetics of hearing impairment (H.E.A.R.), describes the preliminary estimated prevalence of hereditary hearing impairment based on retrospective data from a clinical series. Of 27,692 subjects examined in the period 1987-91, we sampled 1265 suffering from unilateral or bilateral hereditary hearing impairment, which is roughly 5% of those examined (n = 384 (31%) male; n = 881 (69%) female). Median age of the subjects is 70 years (range 22-98). Subdividing them into 10-year birth cohorts and applying the local annual population statistics, the prevalence of an overall age-related hereditary hearing impairment was roughly estimated to be 3.2/1000, reflecting prevalences as a function of age from 0.8 to 9.4/1000--prevalence in females being significantly more than in males (4.1/1000 and 2.1/1000, respectively). Overall, a moderate hearing impairment of median 51 dB in the better hearing ear was found, averaged across 0.5-4 kHz, this being fairly constant up to the age of 60, when a significant reduction in hearing sensitivity developed. No significant differences are present as a function of gender, except for the birth cohorts 1910-19 and 1920-29. The most frequent type of hereditary hearing impairment in this sample is otosclerosis, comprising 2% of the total clinical series with a rough population prevalence estimate of 1.4/1000. It is concluded that the established database may be of importance in the aggregation of very rare diseases, and for providing the inspiration for future prospective population studies, resulting in knowledge on the epidemiology of hereditary hearing impairment in adults.

Direct control of the Forkhead transcription factor AFX by protein kinase B

The phosphatidylinositol-3-OH-kinase (PI(3)K) effector protein kinase B regulates certain insulin-responsive genes, but the transcription factors regulated by protein kinase B have yet to be identified. Genetic analysis in Caenorhabditis elegans has shown that the Forkhead transcription factor daf-16 is regulated by a pathway consisting of insulin-receptor-like daf-2 and PI(3)K-like age-1. Here we show that protein kinase B phosphorylates AFX, a human orthologue of daf-16, both in vitro and in vivo. Inhibition of endogenous PI(3)K and protein kinase B activity prevents protein kinase B-dependent phosphorylation of AFX and reveals residual protein kinase B-independent phosphorylation that requires Ras signalling towards the Ral GTPase. In addition, phosphorylation of AFX by protein kinase B inhibits its transcriptional activity. Together, these results delineate a pathway for PI(3)K-dependent signalling to the nucleus.

The Caenorhabditis elegans lim-6 LIM homeobox gene regulates neurite outgrowth and function of particular GABAergic neurons

We describe here the functional analysis of the C. elegans LIM homeobox gene lim-6, the ortholog of the mammalian Lmx-1a and b genes that regulate limb, CNS, kidney and eye development. lim-6 is expressed in a small number of sensory-, inter- and motorneurons, in epithelial cells of the uterus and in the excretory system. Loss of lim-6 function affects late events in the differentiation of two classes of GABAergic motorneurons which control rhythmic enteric muscle contraction. lim-6 is required to specify the correct axon morphology of these neurons and also regulates expression of glutamic acid decarboxylase, the rate limiting enzyme of GABA synthesis in these neurons. Moreover, lim-6 gene activity and GABA signaling regulate neuroendocrine outputs of the nervous system. In the chemosensory system lim-6 regulates the asymmetric expression of a probable chemosensory receptor. lim-6 is also required in epithelial cells for uterine morphogenesis. We compare the function of lim-6 to those of other LIM homeobox genes in C. elegans and suggest that LIM homeobox genes share the common theme of controlling terminal neural differentiation steps that when disrupted lead to specific neuroanatomical and neural function defects.

Elimination of replication block protein Fob1 extends the life span of yeast mother cells

A cause of aging in yeast is the accumulation of circular species of ribosomal DNA (rDNA) arising from the 100-200 tandemly repeated copies in the genome. We show here that mutation of the FOB1 gene slows the generation of these circles and thus extends life span. Fob1p is known to create a unidirectional block to replication forks in the rDNA. We show that Fob1p is a nucleolar protein, suggesting a direct involvement in the replication fork block. We propose that this block can trigger aging by causing chromosomal breaks, the repair of which results in the generation of rDNA circles. These findings may provide a novel link between metabolic rate and aging in yeast and, perhaps, higher organisms.

Regulation of dauer larva development in Caenorhabditis elegans by daf-18, a homologue of the tumour suppressor PTEN

The tumour suppressor gene PTEN (also called MMAC1 or TEP1) is somatically mutated in a variety of cancer types [1] [2] [3] [4]. In addition, germline mutation of PTEN is responsible for two dominantly inherited, related cancer syndromes called Cowden disease and Bannayan-Ruvalcaba-Riley syndrome [4]. PTEN encodes a dual-specificity phosphatase that inhibits cell spreading and migration partly by inhibiting integrin-mediated signalling [5] [6] [7]. Furthermore, PTEN regulates the levels of phosphatidylinositol 3,4,5-trisphosphate (PIP3) by specifically dephosphorylating position 3 on the inositol ring [8]. We report here that the dauer formation gene daf-18 is the Caenorhabditis elegans homologue of PTEN. DAF-18 is a component of the insulin-like signalling pathway controlling entry into diapause and adult longevity that is regulated by the DAF-2 receptor tyrosine kinase and the AGE-1 PI 3-kinase [9]. Others have shown that mutation of daf-18 suppresses the life extension and constitutive dauer formation associated with daf-2 or age-1 mutants. Similarly, we show that inactivation of daf-18 by RNA-mediated interference mimics this suppression, and that a wild-type daf-18 transgene rescues the dauer defect. These results indicate that PTEN/daf-18 antagonizes the DAF-2-AGE-1 pathway, perhaps by catalyzing dephosphorylation of the PIP3 generated by AGE-1. These data further support the notion that mutations of PTEN contribute to the development of human neoplasia through an aberrant activation of the PI 3-kinase signalling cascade.

Regulation of the insulin-like developmental pathway of Caenorhabditis elegans by a homolog of the PTEN tumor suppressor gene

The human PTEN tumor suppressor gene is mutated in a wide variety of sporadic tumors. To determine the function of PTEN in vivo we have studied a PTEN homolog in Caenorhabditis elegans. We have generated a strong loss-of-function allele of the PTEN homolog and shown that the deficient strain is unable to enter dauer diapause. An insulin-like phosphatidylinositol 3-OH kinase (PI3'K) signaling pathway regulates dauer-stage entry. Mutations in either the daf-2 insulin receptor-like (IRL) gene or the age-1 encoded PI3'K catalytic subunit homolog cause constitutive dauer formation and also affect the life span, brood size, and metabolism of nondauer animals. Strikingly, loss-of-function mutations in the age-1 PI3'K and daf-2 IRL genes are suppressed by loss-of-function mutations in the PTEN homolog. We establish that the PTEN homolog is encoded by daf-18, a previously uncloned gene that has been shown to interact genetically with the DAF-2 IRL AGE-1 PI3'K signaling pathway. This interaction provides clear genetic evidence that PTEN acts to antagonize PI3'K function in vivo. Given the conservation of the PI3'K signaling pathway between C. elegans and mammals, the analysis of daf-18 PTEN mutant nematodes should shed light on the role of human PTEN in the etiology of metabolic disease, aging, and cancer.

Aging in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae divides asymmetrically, giving rise to a mother cell and a smaller daughter cell. Individual mother cells produce a finite number of daughter cells before senescing, undergoing characteristic changes as they age such as a slower cell cycle and sterility. The average life span is fixed for a given strain, implying that yeast aging has a strong genetic component. Genes that determine yeast longevity have highlighted the importance of such processes as cAMP metabolism, epigenetic silencing, and genome stability. The recent finding that yeast aging is caused, in part, by the accumulation of circular rDNA molecules has unified many seemingly disparate observations.

Molecular mechanisms of bacterial virulence elucidated using a Pseudomonas aeruginosa-Caenorhabditis elegans pathogenesis model

The human opportunistic pathogen Pseudomonas aeruginosa strain PA14 kills Caenorhabditis elegans. Using systematic mutagenesis of PA14 to identify mutants that fail to kill C. elegans and a C. elegans mutant that lacks P-glycoproteins, we identified phenazines, secreted P. aeruginosa pigments, as one of the mediators of killing. Analysis of C. elegans mutants with altered responses to oxidative stress suggests that phenazines exert their toxic effects on C. elegans through the generation of reactive oxygen species. Finally, we show that phenazines and other P. aeruginosa factors required for C. elegans killing are also required for pathogenesis in plants and mice, illustrating that this model tackles the dual challenges of identifying bacterial virulence factors as well as host responses to them.

Members of the insulin receptor family contain three fibronectin type III domains

The insulin receptor is a large transmembrane dimer, comprised of several domains. Detailed 3D structural information is available for the L1-cys-rich-L2 domains in the extracellular region (ectodomain) and for the tyrosine kinase catalytic domain in the cytoplasmic portion of the receptor. In addition, previous sequence analyses have identified two fibronectin type III domains in the C-terminal half of each ectodomain monomer. In this report, evidence is provided to show that a third fibronectin type III module exists between the L2 domain and the two previously described fibronectin type III domains.

The taxonomy of developmental control in Caenorhabditis elegans

The Caenorhabditis elegans genome sequence was surveyed for transcription factor and signaling gene families that have been shown to regulate development in a variety of species. About 10 to 25 percent of the genes in most of the gene families already have been genetically analyzed in C. elegans, about half of the genes detect probable orthologs in other species, and about 10 to 25 percent of the genes are, at present, unique to C. elegans. Caenorhabditis elegans is also missing genes that are found in vertebrates and other invertebrates. Thus the genome sequence reveals universals in developmental control that are the legacy of metazoan complexity before the Cambrian explosion, as well as genes that have been more recently invented or lost in particular phylogenetic lineages.

Structure and function of phosphoinositide 3-kinases

Phosphoinositide kinases (PI3Ks) play an important role in mitogenic signaling and cell survival, cytoskeletal remodeling, metabolic control and vesicular trafficking. Here we summarize the structure-function relationships delineating the activation process of class I PI3Ks involving various domains of adapter subunits, Ras, and interacting proteins. The resulting product, PtdIns(3,4,5)P3, targets Akt/protein kinase B (PKB), Bruton's tyrosine kinase (Btk), phosphoinositide-dependent kinases (PDK), integrin-linked kinase (ILK), atypical protein kinases C (PKC), phospholipase Cgamma and more. Surface receptor-activated PI3Ks function in mammals, insects, nematodes and slime mold, but not yeast. While many members of the class II family have been identified and characterized biochemically, it is presently unknown how these C2-domain containing PI3Ks are activated, and which PI substrate they phosphorylate in vivo. PtdIns 3-P is produced by Vps34p/class III PI3Ks and operates via the PtdIns 3-P-binding proteins early endosomal antigen (EEA1), yeast Vac1p, Vps27p, Pip1p in lysosomal protein targeting. Besides the production of D3 phosphorylated lipids, PI3Ks have an intrinsic protein kinase activity. For trimeric GTP-binding protein-activated PI3Kgamma, protein kinase activity seems to be sufficient to trigger mitogen-activated protein kinase (MAPK). Recent disruption of PI3K genes in slime mold, Caenorhabditis elegans, Drosophila melanogaster and mice further underlines the importance of PI3K signaling systems and elucidates the role of PI3K signaling in multicellular organisms.

Homologs of the yeast longevity gene LAG1 in Caenorhabditis elegans and human

LAG1 is a longevity gene, the first such gene to be identified and cloned from the yeast Saccharomyces cerevisiae. A close homolog of this gene, which we call LAC1, has been found in the yeast genome. We have cloned the human homolog of LAG1 with the ultimate goal of examining its possible function in human aging. In the process, we have also cloned a homolog from the nematode worm Caenorhabditis elegans. Both of these homologs, LAG1Hs and LAG1Ce-1, functionally complemented the lethality of a lag1delta lac1delta double deletion, despite low overall sequence similarity to the yeast proteins. The proteins shared a short sequence, the Lag1 motif, and a similar transmembrane domain profile. Another, more distant human homolog, TRAM, which lacks this motif, did not complement. LAG1Hs also restored the life span of the double deletion, demonstrating that it functions in establishing the longevity phenotype in yeast. LAG1Hs mapped to 19p12, and it was expressed in only three tissues: brain, skeletal muscle, and testis. This gene possesses a trinucleotide (CTG) repeat within exon 1. This and its expression profile raise the possibility that it may be involved in neurodegenerative disease. This possibility suggests at least one way in which LAG1Hs might be involved in human aging.

The C. elegans PTEN homolog, DAF-18, acts in the insulin receptor-like metabolic signaling pathway

An insulin-like signaling pathway, from the DAF-2 receptor, the AGE-1 phosphoinositide 3-kinase, and the AKT-1/AKT-2 serine/threonine kinases to the DAF-16 Fork head transcription factor, regulates the metabolism, development, and life span of Caenorhabditis elegans. Inhibition of daf-18 gene activity bypasses the normal requirement for AGE-1 and partially bypasses the need for DAF-2 signaling. The suppression of age-1 mutations by a daf-18 mutation depends on AKT-1/AKT-2 signaling, showing that DAF-18 acts between AGE-1 and the AKT input to DAF-16 transcriptional regulation. daf-18 encodes a homolog of the human tumor suppressor PTEN (MMAC1/TEP1), which has 3-phosphatase activity toward phosphatidylinositol 3,4,5-trisphosphate (PIP3). DAF-18 PTEN may normally limit AKT-1 and AKT-2 activation by decreasing PIP3 levels. The action of daf-18 in this metabolic control pathway suggests that mammalian PTEN may modulate insulin signaling and may be variant in diabetic pedigrees.

The pleckstrin homology and phosphotyrosine binding domains of insulin receptor substrate 1 mediate inhibition of apoptosis by insulin

Insulin and insulin-like growth factor 1 (IGF-1) evoke diverse biological effects through receptor-mediated tyrosine phosphorylation of insulin receptor substrate (IRS) proteins. We investigated the elements of IRS-1 signaling that inhibit apoptosis of interleukin 3 (IL-3)-deprived 32D myeloid progenitor cells. 32D cells have few insulin receptors and no IRS proteins; therefore, insulin failed to inhibit apoptosis during IL-3 withdrawal. Insulin stimulated mitogen-activated protein kinase in 32D cells expressing insulin receptors (32DIR) but failed to activate the phosphatidylinositol 3 (PI 3)-kinase cascade or to inhibit apoptosis. By contrast, insulin stimulated the PI 3-kinase cascade, inhibited apoptosis, and promoted replication of 32DIR cells expressing IRS-1. As expected, insulin did not stimulate PI 3-kinase in 32DIR cells, which expressed a truncated IRS-1 protein lacking the tail of tyrosine phosphorylation sites. However, this truncated IRS-1 protein, which retained the NH2-terminal pleckstrin homology (PH) and phosphotyrosine binding (PTB) domains, mediated phosphorylation of PKB/akt, inhibition of apoptosis, and replication of 32DIR cells during insulin stimulation. These results suggest that a phosphotyrosine-independent mechanism mediated by the PH and PTB domains promoted antiapoptotic and growth actions of insulin. Although PI 3-kinase was not activated, its phospholipid products were required, since LY294002 inhibited these responses. Without IRS-1, a chimeric insulin receptor containing a tail of tyrosine phosphorylation sites derived from IRS-1 activated the PI 3-kinase cascade but failed to inhibit apoptosis. Thus, phosphotyrosine-independent IRS-1-linked pathways may be critical for survival and growth of IL-3-deprived 32D cells during insulin stimulation.

Genetics of longevity

Recent studies on the genetics of aging in the yeast Saccharomyces cerevisiae, the roundworm Caenorhabditis elegans, and the fruit fly Drosophila melanogaster have converged revealing the central role of metabolic capacity and resistance to stress in determining life span. Signal transduction has emerged from these studies as an important molecular mechanism underlying longevity. In their broad features, the results obtained in these genetic models are applicable to the dietary restriction paradigm in mammals, suggesting a general significance. It will be of interest to determine whether many of the molecular details will also pertain. The examination of centenarian populations for the frequency of certain alleles of pertinent genes may provide insights into the relevance of the conclusions of studies in invertebrates to human aging. These population genetic studies can be augmented by mechanistic studies in transgenic mice.

Does growth hormone prevent or accelerate aging?

It is very well documented that plasma growth hormone (GH) levels decline with age in the human and in experimental animals, and there is considerable evidence that age-related changes in body composition may be caused by reduced function of the GH-IGF-I system. However, excessive GH levels are associated with reduced life expectancy in acromegalic patients and with symptoms of accelerated aging in GH transgenic mice. Hereditary dwarf mice deficient in GH, prolactin, and TSH live much longer than their normal siblings. Possible mechanisms of delayed aging in dwarf mice include lower core body temperature and reduced oxidative processes. It is suggested that the controversies concerning the apparent potential of GH to both prevent and accelerate aging may be reconciled by interpreting the results in light of the negative relationship between body size and life span within a species.

Extended life-span and stress resistance in the Drosophila mutant methuselah

Toward a genetic dissection of the processes involved in aging, a screen for gene mutations that extend life-span in Drosophila melanogaster was performed. The mutant line methuselah (mth) displayed approximately 35 percent increase in average life-span and enhanced resistance to various forms of stress, including starvation, high temperature, and dietary paraquat, a free-radical generator. The mth gene predicted a protein with homology to several guanosine triphosphate-binding protein-coupled seven-transmembrane domain receptors. Thus, the organism may use signal transduction pathways to modulate stress response and life-span.

The genetics of caloric restriction in Caenorhabditis elegans

Low caloric intake (caloric restriction) can lengthen the life span of a wide range of animals and possibly even of humans. To understand better how caloric restriction lengthens life span, we used genetic methods and criteria to investigate its mechanism of action in the nematode Caenorhabditis elegans. Mutations in many genes (eat genes) result in partial starvation of the worm by disrupting the function of the pharynx, the feeding organ. We found that most eat mutations significantly lengthen life span (by up to 50%). In C. elegans, mutations in a number of other genes that can extend life span have been found. Two genetically distinct mechanisms of life span extension are known: a mechanism involving genes that regulate dauer formation (age-1, daf-2, daf-16, and daf-28) and a mechanism involving genes that affect the rate of development and behavior (clk-1, clk-2, clk-3, and gro-1). We find that the long life of eat-2 mutants does not require the activity of DAF-16 and that eat-2; daf-2 double mutants live even longer than extremely long-lived daf-2 mutants. These findings demonstrate that food restriction lengthens life span by a mechanism distinct from that of dauer-formation mutants. In contrast, we find that food restriction does not further increase the life span of long-lived clk-1 mutants, suggesting that clk-1 and caloric restriction affect similar processes.

Cell nonautonomy of C. elegans daf-2 function in the regulation of diapause and life span

The insulin/IGF receptor homolog DAF-2 regulates the aging in C. elegans. Decreasing daf-2 activity causes fertile adults to remain active much longer than normal and to live more than twice as long. A more severe decrease in daf-2 function causes young larvae to enter a state of diapause rather than progressing to adulthood. We have asked which cells require daf-2 gene activity in order for the animal to develop to adulthood and to age normally. We found that daf-2 functions cell nonautonomously in both processes. Our findings imply that the life span of C. elegans is determined by a signaling cascade in which the DAF-2 receptor acts in multiple cell lineages to regulate the production or activity of a secondary signal (or signals), which, in turn, controls the growth and longevity of individual tissues in the animal.

Developmental regulation of a cyclin-dependent kinase inhibitor controls postembryonic cell cycle progression in Caenorhabditis elegans

C. elegans cki-1 encodes a member of the CIP/KIP family of cyclin-dependent kinase inhibitors, and functions to link postembryonic developmental programs to cell cycle progression. The expression pattern of cki-1::GFP suggests that cki-1 is developmentally regulated in blast cells coincident with G1, and in differentiating cells. Ectopic expression of CKI-1 can prematurely arrest cells in G1, while reducing cki-1 activity by RNA-mediated interference (RNAi) causes extra larval cell divisions, suggesting a role for cki-1 in the developmental control of G1/S. cki-1 activity is required for the suspension of cell cycling that occurs in dauer larvae and starved L1 larvae in response to environmental signals. In vulva precursor cells (VPCs), a pathway of heterochronic genes acts via cki-1 to maintain VPCs in G1 during the L2 stage.

Insulin induces transcription of target genes through the hypoxia-inducible factor HIF-1alpha/ARNT

Hypoxic stress induces the expression of genes associated with increased energy flux, including the glucose transporters Glut1 and Glut3, several glycolytic enzymes, nitric oxide synthase, tyrosine hydroxylase, erythropoietin and vascular endothelial growth factor (VEGF). Induction of these genes is mediated by a common basic helix-loop-helix-PAS transcription complex, the hypoxia-inducible factor-1alpha (HIF-1alpha)/aryl hydrocarbon nuclear translocator (ARNT). Insulin also induces some of these genes; however, the underlying mechanism is unestablished. We report here that insulin shares with hypoxia the ability to induce the HIF-1alpha/ARNT transcription complex in various cell types. This induction was demonstrated by electrophoretic mobility shift of the hypoxia response element (HRE), and abolished by specific antisera to HIF-1alpha and ARNT, and by transcription activation of HRE reporter vectors. Furthermore, basal and insulin-induced expression of Glut1, Glut3, aldolase A, phosphoglycerate kinase and VEGF was reduced in cells having a defective ARNT. Similarly, the insulin-induced activation of HRE reporter vectors and VEGF was impaired in these cells and was rescued by re-introduction of ARNT. Finally, insulin-like growth factor-I (IGF-I) also induced the HIF-1alpha/ARNT transcription complex. These observations establish a novel signal transduction pathway of insulin and IGF-I and broaden considerably the scope of activity of HIF-1alpha/ARNT.

Two pleiotropic classes of daf-2 mutation affect larval arrest, adult behavior, reproduction and longevity in Caenorhabditis elegans

The nematode Caenorhabditis elegans responds to overcrowding and scarcity of food by arresting development as a dauer larva, a nonfeeding, long-lived, stress-resistant, alternative third-larval stage. Previous work has shown that mutations in the genes daf-2 (encoding a member of the insulin receptor family) and age-1 (encoding a PI 3-kinase) result in constitutive formation of dauer larvae (Daf-c), increased adult longevity (Age), and increased intrinsic thermotolerance (Itt). Some daf-2 mutants have additional developmental, behavioral, and reproductive defects. We have characterized in detail 15 temperature-sensitive and 1 nonconditional daf-2 allele to investigate the extent of daf-2 mutant defects and to examine whether specific mutant traits correlate with each other. The greatest longevity seen in daf-2 mutant adults was approximately three times that of wild type. The temperature-sensitive daf-2 mutants fell into two overlapping classes, including eight class 1 mutants, which are Daf-c, Age, and Itt, and exhibit low levels of L1 arrest at 25.5 degrees. Seven class 2 mutants also exhibit the class 1 defects as well as some or all of the following: reduced adult motility, abnormal adult body and gonad morphology, high levels of embryonic and L1 arrest, production of progeny late in life, and reduced brood size. The strengths of the Daf-c, Age, and Itt phenotypes largely correlated with each other but not with the strength of class 2-specific defects. This suggests that the DAF-2 receptor is bifunctional. Examination of the null phenotype revealed a maternally rescued egg, L1 lethal component, and a nonconditional Daf-c component. With respect to the Daf-c phenotype, the dauer-defective (Daf-d) mutation daf-12(m20) was epistatic to daf-2 class 1 alleles but not the severe class 2 alleles tested. All daf-2 mutant defects were suppressed by the daf-d mutation daf-16(m26). Our findings suggest a new model for daf-2, age-1, daf-12, and daf-16 interactions.

Cloning and developmental regulation of a novel member of the insulin-like gene family in Caenorhabditis elegans

Aging, metabolism and fat accumulation in Caenorhabditis elegans (C. elegans) are influenced by mutations in DAF-2, a putative insulin-like receptor. Ten putative insulin-like genes have been recently identified from the C. elegans genome database. However, it is unclear if these genes are orthologues of human insulin since they lack the C-peptide dibasic amino acid proteolysis sites. We have identified and measured mRNA expression during development of two novel members of the C. elegans insulin-like gene family. We also report the sequence characterization and gene structure for one of these, the insulin-like protein-1 (ILP1). We focused on ILP1 characterization because it has structural features consistent with its being a candidate insulin ligand for the DAF-2 insulin-like receptor. For example, ILP1 has a putative C-peptide flanked by dibasic amino acids, exhibits conserved cysteine residues that could provide disulfide bonds between the A and B chains, and has two introns. Northern blot analysis revealed that ILP1 mRNA is expressed at very high levels in embryos and is downregulated very early during postnatal development, suggesting that it may influence embryonic development, but not Dauer formation. We also identified a novel insulin-like growth factor-1-like protein (T28B8/IGF-I) that exhibits a very different developmental expression profile than ILP1. Our results are consistent with the hypothesis that members of the unusually large and complex C. elegans insulin-like protein family exhibit complex and perhaps redundant roles.

FRG1, a gene in the FSH muscular dystrophy region on human chromosome 4q35, is highly conserved in vertebrates and invertebrates

The human FRG1 gene maps to human chromosome 4q35 and was identified as a candidate for facioscapulohumeral muscular dystrophy. However, FRG1 is apparently not causally associated with the disease and as yet, its function remains unclear. We have cloned homologues of FRG1 from two additional vertebrates, the mouse and the Japanese puffer fish Fugu rubripes, and investigated the genomic organization of the genes in the two species. The intron/exon structure of the genes is identical throughout the protein coding region, although the Fugu gene is five times smaller than the mouse gene. We have also identified FRG1 homologues in two nematodes; Caenorhabditis elegans and Brugia malayi. The FRG1 protein is highly conserved and contains a lipocalin sequence motif, suggesting it may function as a transport protein.

Caenorhabditis elegans Akt/PKB transduces insulin receptor-like signals from AGE-1 PI3 kinase to the DAF-16 transcription factor

A neurosecretory pathway regulates a reversible developmental arrest and metabolic shift at the Caenorhabditis elegans dauer larval stage. Defects in an insulin-like signaling pathway cause arrest at the dauer stage. We show here that two C. elegans Akt/PKB homologs, akt-1 and akt-2, transduce insulin receptor-like signals that inhibit dauer arrest and that AKT-1 and AKT-2 signaling are indispensable for insulin receptor-like signaling in C. elegans. A loss-of-function mutation in the Fork head transcription factor DAF-16 relieves the requirement for Akt/PKB signaling, which indicates that AKT-1 and AKT-2 function primarily to antagonize DAF-16. This is the first evidence that the major target of Akt/PKB signaling is a transcription factor. An activating mutation in akt-1, revealed by a genetic screen, as well as increased dosage of wild-type akt-1 relieves the requirement for signaling from AGE-1 PI3K, which acts downstream of the DAF-2 insulin/IGF-1 receptor homolog. This demonstrates that Akt/PKB activity is not necessarily dependent on AGE-1 PI3K activity. akt-1 and akt-2 are expressed in overlapping patterns in the nervous system and in tissues that are remodeled during dauer formation.

Expression and function of insulin/insulin-like growth factor I hybrid receptors during differentiation of 3T3-L1 preadipocytes

During the assembly of cell surface receptors, insulin proreceptors are sometimes joined to insulin-like growth factor (IGF) receptor precursors to form covalently linked hybrid receptors. To address the biological consequences of hybrid receptor formation, we studied 3T3-L1 cells known to undergo a 50-70-fold increase in insulin binding while maintaining nearly constant levels of IGF-I binding during differentiation from preadipocytes into adipocytes. The presence of insulin/IGF receptor hybrids in 3T3-L1 adipocytes was demonstrated by the immunoprecipitation of phosphorylated receptors and a novel enzyme-linked immunoassay. Hybrid receptor levels were very low in the early stages of differentiation and increased rapidly between days 4 and 6, reaching a level about 100-fold higher in the mature adipocyte. Coincident with the hybrid assembly, the formation of archetypal (alpha2,beta2) IGF receptors decreased. In fully differentiated adipocytes, virtually all of the IGF receptors were in hybrid form. Stimulation by IGF-I of receptors isolated from mature adipocytes caused autophosphorylation of IGF receptor beta subunits in hybrid complexes, whereas autophosphorylated IGF holoreceptors were not demonstrable. Insulin and IGF-I were equipotent in stimulating glucose uptake in the differentiated adipocytes, leading to the conclusion that hybrid insulin/IGF receptors can transduce a transmembrane signal when activated by IGF-I. We conclude that hybrid formation constitutes a novel post-translational mechanism whereby increased synthesis of insulin receptors limits the cell surface expression of the homologous IGF receptor. Furthermore, biological actions in 3T3-L1 adipocytes, previously attributed to archetypal IGF receptors, are in fact mediated through hybrid receptors.

Diapause-specific gene expression in pupae of the flesh fly Sarcophaga crassipalpis

Several cDNAs isolated from brains of diapausing pupae of the flesh fly, Sarcophaga crassipalpis, show expression patterns unique to diapause. To isolate such cDNAs a diapause pupal brain cDNA library was screened by using an elimination hybridization technique, and cDNAs that did not hybridize with cDNA probes constructed from the RNA of nondiapausing pupae were selected for further screening. The 95 clones that did not hybridize in the initial library screen were selected for further characterization. These clones were then screened against diapause and nondiapause pupal poly(A)+ Northern blots. The secondary screen identified 4 diapause-up-regulated clones, 7 diapause-down-regulated clones, 8 clones expressed equally in both diapause and nondiapause, and 75 clones without detectable expression. The diapause-up-regulated and down-regulated clones were further characterized by partial DNA sequencing and identity searches by using GenBank. Identities between our cloned cDNAs and other genes included those linked to cell cycle progression, stress responses, and DNA repair processes. The results suggest that insect diapause is not merely a shutdown of gene expression but is a unique, developmental pathway characterized by the expression of a novel set of genes.

New insulin-like proteins with atypical disulfide bond pattern characterized in Caenorhabditis elegans by comparative sequence analysis and homology modeling

We have identified three new families of insulin homologs in Caenorhabditis elegans. In two of these families, concerted mutations suggest that an additional disulfide bond links B and A domains, and that the A-domain internal disulfide bond is substituted by a hydrophobic interaction. Homology modeling remarkably confirms these predictions and shows that despite this atypical disulfide bond pattern and the absence of C-like peptide, all these proteins may adopt the same fold as the insulin. Interestingly, whereas we identified 10 insulin-like peptides, only one insulin-like-receptor (daf-2) has been found. We propose that these insulin-related peptides may correspond to different activators or inhibitors of the daf-2 insulin-regulating pathway.

daf-12 regulates developmental age and the dauer alternative in Caenorhabditis elegans

From egg through adult, C. elegans has six life stages including an option for dauer formation and diapause at larval stage L3 in adverse environments. Somatic cells throughout the organism make consistent choices and advance in unison, suggesting a mechanism of coordinate regulation at these stage transitions. Earlier studies showed that daf-12, which encodes a nuclear receptor (W. Yeh, 1991, Doctoral Thesis. University of Missouri-Columbia), regulates dauer formation; epistasis experiments placed daf-12 near the end of the dauer signaling pathway. Here we describe novel daf-12 alleles that reveal a general role in advancing L3 stage programs. In these mutants, somatic cells repeat L2-specific cellular programs of division and migration at the L3 stage; epistasis experiments place daf-12 between lin-14 and lin-28 within the heterochronic pathway. We propose daf-12 and other heterochronic genes provide cellular memories of chronological stage for selecting stage-appropriate developmental programs. Endocrine factors could coordinate these stage transitions and specify developmental alternatives.

4E-BP1, a repressor of mRNA translation, is phosphorylated and inactivated by the Akt(PKB) signaling pathway

Growth factors and hormones activate protein translation by phosphorylation and inactivation of the translational repressors, the eIF4E-binding proteins (4E-BPs), through a wortmannin- and rapamycin-sensitive signaling pathway. The mechanism by which signals emanating from extracellular signals lead to phosphorylation of 4E-BPs is not well understood. Here we demonstrate that the activity of the serine/threonine kinase Akt/PKB is required in a signaling cascade that leads to phosphorylation and inactivation of 4E-BP1. PI 3-kinase elicits the phosphorylation of 4E-BP1 in a wortmannin- and rapamycin-sensitive manner, whereas activated Akt-mediated phosphorylation of 4E-BP1 is wortmannin resistant but rapamycin sensitive. A dominant negative mutant of Akt blocks insulin-mediated phosphorylation of 4E-BP1, indicating that Akt is required for the in vivo phosphorylation of 4E-BP1. Importantly, an activated Akt induces phosphorylation of 4E-BP1 on the same sites that are phosphorylated upon serum stimulation. Similar to what has been observed with serum and growth factors, phosphorylation of 4E-BP1 by Akt inhibits the interaction between 4E-BP1 and eIF-4E. Furthermore, phosphorylation of 4E-BP1 by Akt requires the activity of FRAP/mTOR. FRAP/mTOR may lie downstream of Akt in this signaling cascade. These results demonstrate that the PI 3-kinase-Akt signaling pathway, in concert with FRAP/mTOR, induces the phosphorylation of 4E-BP1.

An insulin-like signaling pathway affects both longevity and reproduction in Caenorhabditis elegans

Mutations in daf-2 and age-1 cause a dramatic increase in longevity as well as developmental arrest at the dauer diapause stage in Caenorhabditis elegans. daf-2 and age-1 encode components of an insulin-like signaling pathway. Both daf-2 and age-1 act at a similar point in the genetic epistasis pathway for dauer arrest and longevity and regulate the activity of the daf-16 gene. Mutations in daf-16 cause a dauer-defective phenotype and are epistatic to the diapause arrest and life span extension phenotypes of daf-2 and age-1 mutants. Here we show that mutations in this pathway also affect fertility and embryonic development. Weak daf-2 alleles, and maternally rescued age-1 alleles that cause life span extension but do not arrest at the dauer stage, also reduce fertility and viability. We find that age-1(hx546) has reduced both maternal and zygotic age-1 activity. daf-16 mutations suppress all of the daf-2 and age-1 phenotypes, including dauer arrest, life span extension, reduced fertility, and viability defects. These data show that insulin signaling, mediated by DAF-2 through the AGE-1 phosphatidylinositol-3-OH kinase, regulates reproduction and embryonic development, as well as dauer diapause and life span, and that DAF-16 transduces these signals. The regulation of fertility, life span, and metabolism by an insulin-like signaling pathway is similar to the endocrine regulation of metabolism and fertility by mammalian insulin signaling.

Methuselah meets diabetes

Mutations in the daf-2 and age-1 genes cause constitutive dauer larva formation and double adult life span in C. elegans. Their effect on life span has excited considerable interest and their effect on dauer formation has facilitated rapid progress in their genetic and molecular analysis. Two recent papers report that daf-2 encodes a member of the insulin-receptor family and that age-1 encodes a PI3 kinase subunit, a second-messenger producing enzyme known to act downstream of the mammalian insulin receptor. These findings provide the first mechanistic insight into the well-established link between metabolism and aging.

Molecular genetics of life span in C. elegans: how much does it teach us?

Several loci have been identified in the nematode worm Caenorhabditis elegans that, when mutated, can increase life span. Three of these genes, age-1, daf-2 and clk-1, have now been cloned. Mutations in these three genes are highly pleiotropic and affect many aspects of worm development and behaviour, age-1 and daf-2 act in the same genetic pathway and have similar effects on the worm, age-1 encodes a homologue of the p110 subunit of phosphatidylinositol 3-kinase and daf-2 encodes an insulin receptor family member, clk-1 encodes a protein of unknown biochemical function similar to the yeast metabolic regulator Cat5p/Coq7p. The implications of these findings for our understanding of organismal ageing are discussed.

Extrachromosomal rDNA circles--a cause of aging in yeast

Although many cellular and organismal changes have been described in aging individuals, a precise, molecular cause of aging has yet to be found. A prior study of aging yeast mother cells showed a progressive enlargement and fragmentation of the nucleolus. Here we show that these nucleolar changes are likely due to the accumulation of extrachromosomal rDNA circles (ERCs) in old cells and that, in fact, ERCs cause aging. Mutants for sgs1, the yeast homolog of the Werner's syndrome gene, accumulate ERCs more rapidly, leading to premature aging and a shorter life span. We speculate on the generality of this molecular cause of aging in higher species, including mammals.

daf-16: An HNF-3/forkhead family member that can function to double the life-span of Caenorhabditis elegans

The wild-type Caenorhabditis elegans nematode ages rapidly, undergoing development, senescence, and death in less than 3 weeks. In contrast, mutants with reduced activity of the gene daf-2, a homolog of the insulin and insulin-like growth factor receptors, age more slowly than normal and live more than twice as long. These mutants are active and fully fertile and have normal metabolic rates. The life-span extension caused by daf-2 mutations requires the activity of the gene daf-16. daf-16 appears to play a unique role in life-span regulation and encodes a member of the hepatocyte nuclear factor 3 (HNF-3)/forkhead family of transcriptional regulators. In humans, insulin down-regulates the expression of certain genes by antagonizing the activity of HNF-3, raising the possibility that aspects of this regulatory system have been conserved.

The Fork head transcription factor DAF-16 transduces insulin-like metabolic and longevity signals in C. elegans

In mammals, insulin signalling regulates glucose transport together with the expression and activity of various metabolic enzymes. In the nematode Caenorhabditis elegans, a related pathway regulates metabolism, development and longevity. Wild-type animals enter the developmentally arrested dauer stage in response to high levels of a secreted pheromone, accumulating large amounts of fat in their intestines and hypodermis. Mutants in DAF-2 (a homologue of the mammalian insulin receptor) and AGE-1 (a homologue of the catalytic subunit of mammalian phosphatidylinositol 3-OH kinase) arrest development at the dauer stage. Moreover, animals bearing weak or temperature-sensitive mutations in daf-2 and age-1 can develop reproductively, but nevertheless show increased energy storage and longevity. Here we show that null mutations in daf-16 suppress the effects of mutations in daf-2 or age-1; lack of daf-16 bypasses the need for this insulin receptor-like signalling pathway. The principal role of DAF-2/AGE-1 signalling is thus to antagonize DAF-16. daf-16 is widely expressed and encodes three members of the Fork head family of transcription factors. The DAF-2 pathway acts synergistically with the pathway activated by a nematode TGF-beta-type signal, DAF-7, suggesting that DAF-16 cooperates with nematode SMAD proteins in regulating the transcription of key metabolic and developmental control genes. The probable human orthologues of DAF-16, FKHR and AFX, may also act downstream of insulin signalling and cooperate with TGF-beta effectors in mediating metabolic regulation. These genes may be dysregulated in diabetes.

Genetics of aging

The role of genetics in determining life-span is complex and paradoxical. Although the heritability of life-span is relatively minor, some genetic variants significantly modify senescence of mammals and invertebrates, with both positive and negative impacts on age-related disorders and life-spans. In certain examples, the gene variants alter metabolic pathways, which could thereby mediate interactions with nutritional and other environmental factors that influence life-span. Given the relatively minor effect and variable penetrance of genetic risk factors that appear to affect survival and health at advanced ages, life-style and other environmental influences may profoundly modify outcomes of aging.

The DAF-3 Smad protein antagonizes TGF-beta-related receptor signaling in the Caenorhabditis elegans dauer pathway

Signals from TGF-beta superfamily receptors are transduced to the nucleus by Smad proteins, which transcriptionally activate target genes. In Caenorhabditis elegans, defects in a TGF-beta-related pathway cause a reversible developmental arrest and metabolic shift at the dauer larval stage. Null mutations in daf-3 suppress mutations in genes encoding this TGF-beta signal, its receptors, and associated Smad signal transduction proteins. daf-3 encodes a Smad protein that is most closely related to mammalian DPC4, and is expressed throughout development in many of the tissues that are remodeled during dauer development. DAF-4, the type II TGF-beta receptor in this pathway, is also expressed in remodeled tissues. These data suggest that the DAF-7 signal from sensory neurons acts as a neuroendocrine signal throughout the body to directly regulate developmental and metabolic shifts in tissues that are remodeled during dauer formation. A full-length functional DAF-3/GFP fusion protein is predominantly cytoplasmic, and this localization is independent of activity of the upstream TGF-beta-related pathway. However, this fusion protein is associated with chromosomes in mitotic cells, suggesting that DAF-3 binds DNA directly or indirectly. DAF-3 transgenes also interfere with dauer formation, perhaps attributable to a dosage effect. A truncated DAF-3/GFP fusion protein that is predominantly nuclear interferes with dauer formation, implying a role for DAF-3 in the nucleus. These data suggest that DAF-7 signal transduction antagonizes or modifies DAF-3 Smad activity in the nucleus to induce reproductive development; when DAF-7 signals are disabled, unmodified DAF-3 Smad activity mediates dauer arrest and its associated metabolic shift. Therefore, daf-3 is unique in that it is antagonized, rather than activated, by a TGF-beta pathway.