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

Biogerontology: the next step

After a long period of collecting empirical data describing the changes in organisms, organs, tissues, cells, and macromolecules, biogerontological research is now able to develop various possibilities for intervention. Because aging is a stochastic and nondeterministic process characterized by a progressive failure of maintenance and repair, it is reasoned that gene involved in homeodynamic repair pathways are the most likely candidate gerontogenes. A promising approach for the identification of critical gerontogenic processes is through the hormesis-like positive effects of mild stress. Stimulation of various repair pathways by mild stress has significant effects on delaying the onset of various age-associated alterations in cells, tissues, and organisms.

Extended longevity in Drosophila is consistently associated with a decrease in developmental viability

It has proven relatively easy to select normal-lived strains of Drosophila for extended longevity in the laboratory. Long-lived strains have not been observed in the wild as yet. Of the various life-history traits that have been investigated for their role in modulating the evolution of extended longevity, none have yet shown a consistent or convincing relationship. Other than developmental time, the traits usually investigated in this regard are those associated with the adult phase of the life cycle. We assayed developmental timing and viability in six pairs of normal- and long-lived strains, four pairs of which are from previously described strains and two pairs of which are new strains that have been independently and recently selected. We find that the life-history trait most obviously associated with all our long-lived strains is a significantly reduced developmental viability, with the long-lived strains' having as much as twice the developmental lethality as do any of the normal-lived strains. The long-lived strains also pupate closer to the food, a behavior known to decrease fitness. Thus the reduced fitness of the long-lived strains appears to be due to both physiological and behavioral factors and may well explain why long lived strains are not usually found in the wild. The extension of longevity involves costs as well as benefits that, in this case, are borne by different individuals.

A look at the Caenorhabditis elegans Kex2/Subtilisin-like proprotein convertase family

Significant advances have recently been made in our understanding of the mechanisms of activation of proteins that require processing. Often this involves endoproteolytic cleavage of precursor forms at basic residues, and is carried out by a group of serine endoproteinases, termed the proprotein convertases. In mammals, seven different convertases have been identified to date. These act in both the regulated secretory pathway for the processing of prohormones and proneuropeptides and in the constitutive secretory pathway, in which a variety of proproteins are activated endoproteolytically. The recently completed sequence of the nematode Caenorhabditis elegans genome affords a unique opportunity to examine the entire proprotein convertase family in a multicellular organism. Here we review the nature of the family, emphasising the structural features, characteristic of the four nematode genes, that supply all of the necessary functions unique to this group of serine endoproteinases. Studies of the C. elegans genes not only provide important information about the evaluation of this gene family but should help to illuminate the roles of these proteins in mammalian systems. BioEssays 22:545-553, 2000.

Molecular genetic mechanisms of life span manipulation in Caenorhabditis elegans

Aging and a limited life span are fundamental biological realities. Recent studies have demonstrated that longevity can be manipulated and have revealed molecular mechanisms underlying longevity control in the soil nematode Caenorhabditis elegans. Signals from both neurons and the gonad appear to negatively regulate longevity. One tissue-specific signal involves an insulin-like phosphatidylinositol 3-OH kinase pathway, dependent upon the DAF-16 forkhead transcription factor. These signals regulate mechanisms determining longevity that include the OLD-1 (formerly referred to as TKR-1) receptor tyrosine kinase. Interestingly, increased resistance to environmental stress shows a strong correlation with life extension.

Therapeutic target discovery using Caenorhabditis elegans

Use of the human genome sequence in disease therapy will require efficient identification of disease-causing and disease-associated genes with functions that are amenable to pharmacological manipulation. The validation and development of such genes as therapeutic targets requires information about both the genes' functions and the biochemical pathways in which they participate. One powerful means of obtaining such information is the study of homologous genes in model organisms amenable to laboratory manipulation. Among model organisms the nematode Caenorhabditis elegans offers several advantages, including well-established techniques for genetic and experimental manipulation and the first completed genome sequence for a multicellular organism. Molecular genetic experiments using C. elegans can contribute at several levels to drug discovery programs, from elucidation of genetic functions and pathways to the validation of candidate targets. Additionally, the ease of culture allows adaptation of the nematode for use in high-throughput chemical screens for the identification of lead compounds in drug development.

Paralogy and orthology of tyrosine kinases that can extend the life span of Caenorhabditis elegans

Modification of any one of three transmembrane protein tyrosine kinase (PTK) genes, old-1, old-2 (formerly tkr-1 and tkr-2, respectively), and daf-2 can extend the mean and maximum life span of the nematode Caenorhabditis elegans. To identify paralogs and orthologs, we delineated relationships between these three PTKs and all known transmembrane PTKs and all known mammalian nontransmembrane PTKs using molecular phylogenetics. The tree includes a number of invertebrate receptor PTKs and a novel mammalian receptor PTK (inferred from the expressed-sequence tag database) that have not previously been analyzed. old-1 and old-2 were found to be members of a surprisingly large C. elegans PTK family having 16 members. Interestingly, only four members of this transmembrane family appeared to have receptor domains (immunoglobulin-like in each case). The C-terminal domain of this family was found to have a unique sequence motif that could be important for downstream signaling. Among mammalian PTKs, the old-1/old-2 family appeared to be most closely related to the Pdgfr, Fgfr, Ret, and Tie/Tek families. However, these families appeared to have split too early from the old-1/old-2 family to be orthologs, suggesting that a mammalian ortholog could yet be discovered. An extensive search of the expressed-sequence tag database suggested no additional candidate orthologs. In contrast to old-1 and old-2, daf-2 had no C. elegans paralogs. Although daf-2 was most closely related to the mammalian insulin receptor family, a hydra insulin receptor-like sequence suggested that daf-2 might not be an ortholog of the insulin receptor family. Among PTKs, the old-1/old-2 family and daf-2 were not particularly closely related, raising the possibility that other PTK families might extend life span. On a more general note, our survey of the expressed-sequence tag database suggested that few, if any, additional mammalian PTK families are likely to be discovered. The one novel family that was discovered could represent a novel oncogene family, given the prevalence of oncogenes among PTKs. Finally, the PTK tree was consistent with nematodes and fruit flies being as divergent as nematodes and mammals, suggesting that life extension mechanisms shared by nematodes and fruit flies would be reasonable candidates for extending mammalian life spans.

The real Dorian Gray mouse

Genetic variants with greatly extended lifespan are proving invaluable in uncovering signal transduction pathways that influence the rates of normal ageing. These studies have so far been confined to invertebrate models such as Caenorhabditis elegans and Drosophila, but there has been much speculation as to whether a similar approach could be applied to mammals. The recent publication of results on a mouse strain, mutant in a gene encoding the signaling molecule p66(shc), gives cause for optimism. The mutation renders the mouse resistant to the action of oxygen radical generators and appears to increase mean lifespan by 30%. This approach may provide a boost for the modeling of human age-related diseases.

The Caenorhabditis elegans gonad: a test tube for cell and developmental biology

Sexual reproduction of multicellular organisms depends critically on the coordinate development of the germ line and somatic gonad, a process known as gonadogenesis. Together these tissues ensure the formation of functional gametes and, in the female of many species, create a context for production and further development of the zygote. Since the future of the species hangs in the balance, it is not surprising that gonadogenesis is a complex process involving conserved and multi-faceted developmental mechanisms. Genetic, anatomical, cell biological, and molecular experiments have established the nematode Caenorhabditis elegans as a paradigm for studying gonadogenesis. Furthermore, these studies demonstrate the utility of C. elegans gonadogenesis for exploring broad issues in cell and developmental biology, such as cell fate specification, morphogenesis, cell signaling, cell cycle control, and programmed cell death. The synergy of molecular genetics and cell biology conducted at single-cell resolution in real time permits an extraordinary depth of analysis in this organism. In this review, we first describe the embryonic and post-embryonic development and morphology of the C. elegans gonad. Next we recount seminal experiments that established the field, highlight recent results that provide insight into conserved developmental mechanisms, and present future prospects for the field.

Genetic, behavioral and environmental determinants of male longevity in Caenorhabditis elegans

Males of the nematode Caenorhabditis elegans are shorter lived than hermaphrodites when maintained in single-sex groups. We observed that groups of young males form clumps and that solitary males live longer, indicating that male-male interactions reduce life span. By contrast, grouped or isolated hermaphrodites exhibited the same longevity. In one wild isolate of C. elegans, AB2, there was evidence of copulation between males. Nine uncoordinated (unc) mutations were used to block clumping behavior. These mutations had little effect on hermaphrodite life span in most cases, yet many increased male longevity even beyond that of solitary wild-type males. In one case, the neuronal function mutant unc-64(e246), hermaphrodite life span was also increased by up to 60%. The longevity of unc-4(e120), unc-13(e51), and unc-32(e189) males exceeded that of hermaphrodites by 70-120%. This difference appears to reflect a difference in sex-specific life span potential revealed in the absence of male behavior that is detrimental to survival. The greater longevity of males appears not to be affected by daf-2, but is influenced by daf-16. In the absence of male-male interactions, median (but not maximum) male life span was variable. This variability was reduced when dead bacteria were used as food. Maintenance on dead bacteria extended both male and hermaphrodite longevity.

Signalling through the insulin receptor

The insulin receptor substrates function at the heart of the insulin signalling network. It has recently become apparent that the intracellular localisation of these molecules is regulated in a precise manner that is critical for both the generation and the termination of the insulin signal. Some insulin receptor substrate isoforms appear to be associated with an insoluble matrix that resembles the cytoskeleton. When inappropriately dissociated from this matrix the signalling network collapses concomitant with loss of insulin sensitivity.

Ammonia, respiration, and longevity in nematodes: insights on metabolic regulation of life span from temporal rescaling

To better understand metabolic correlates of longevity, we used a graphical technique to compare the adult temporal patterns of several markers of metabolic activity - ammonia elimination, oxygen consumption rate, ATP levels, and (in freeze-permeabilized worms) the rate of NADPH-activated, lucigenin-mediated superoxide formation - as observed by us and others in normal and long-lived mutant Caenorhabditis elegans strains. All of these traits declined with time, and when their logarithms were plotted against time, appeared reasonably linear for most of the duration of the experiments. The profiles for ammonia output conformed well to parallel regression lines; those for the other metabolic parameters varied widely in slope as originally plotted by the authors, but much less so when replotted as logarithms against adjusted time, scaled by the reciprocal of strain longevity. This is consistent with coregulation of life span, respiration rate, ATP levels, lucigenin reactivity, but not ammonia excretion, by a physiological clock distinguishable from chronologic time. Plots, scaled appropriately for equalized slopes, highlighted y-axis intercept differences among strains. On rescaled plots, these constitute deviations from the expectation based on 'strain-specific clock' differences alone. With one exception, y-intercept effects were observed only for mutants in an insulin-like signaling pathway.

Food and metabolic signalling defects in a Caenorhabditis elegans serotonin-synthesis mutant

The functions of serotonin have been assigned through serotonin-receptor-specific drugs and mutants; however, because a constellation of receptors remains when a single receptor subtype is inhibited, the coordinate responses to modulation of serotonin levels may be missed. Here we report the analysis of behavioural and neuroendocrine defects caused by a complete lack of serotonin signalling. Analysis of the C. elegans genome sequence showed that there is a single tryptophan hydroxylase gene (tph-1)-the key enzyme for serotonin biosynthesis. Animals bearing a tph-1 deletion mutation do not synthesize serotonin but are fully viable. The tph-1 mutant shows abnormalities in behaviour and metabolism that are normally coupled with the sensation and ingestion of food: rates of feeding and egg laying are decreased; large amounts of fat are stored; reproductive lifespan is increased; and some animals arrest at the metabolically inactive dauer stage. This metabolic dysregulation is, in part, due to downregulation of transforming growth factor-beta and insulin-like neuroendocrine signals. The action of the C. elegans serotonergic system in metabolic control is similar to mammalian serotonergic input to metabolism and obesity.

21st Century: Review: Ageing and medicine

Throughout the world, populations are ageing. The response of the health services needs to be based on a knowledge of the nature of human ageing and the principles of rational health care for older people. Ageing comes about from interactions between intrinsic (genetic) and extrinsic (environment and lifestyle) factors. Health care has to be responsive to the general needs of older people, but also to recognize the heterogeneity produced by different rates and patterns of individual ageing. There are now real possibilities of improving the course of human ageing through modulation of both intrinsic and extrinsic processes. There is also a need to adapt social institutions to what is a permanent change in demography.

A common muscarinic pathway for diapause recovery in the distantly related nematode species Caenorhabditis elegans and Ancylostoma caninum

Converging TGF-beta and insulin-like neuroendocrine signaling pathways regulate whether Caenorhabditis elegans develops reproductively or arrests at the dauer larval stage. We examined whether neurotransmitters act in the dauer entry or recovery pathways. Muscarinic agonists promote recovery from dauer arrest induced by pheromone as well as by mutations in the TGF-beta pathway. Dauer recovery in these animals is inhibited by the muscarinic antagonist atropine. Muscarinic agonists do not induce dauer recovery of either daf-2 or age-1 mutant animals, which have defects in the insulin-like signaling pathway. These data suggest that a metabotropic acetylcholine signaling pathway activates an insulin-like signal during C. elegans dauer recovery. Analogous and perhaps homologous cholinergic regulation of mammalian insulin release by the autonomic nervous system has been noted. In the parasitic nematode Ancylostoma caninum, the dauer larval stage is the infective stage, and recovery to the reproductive stage normally is induced by host factors. Muscarinic agonists also induce and atropine potently inhibits in vitro recovery of A. caninum dauer arrest. We suggest that host or parasite insulin-like signals may regulate recovery of A. caninum and could be potential targets for antihelminthic drugs.

Metabolic mechanisms of longevity: Caloric restriction in mammals and longevity mutations in Caenorhabditis elegans; a common pathway??

Several recent studies in Caenorhabditis elegans have reported significant extension of the lifespan by probable loss of function mutations in various genes. When sequenced, many of these genes exhibited significant homology to genes in the mammalian insulin signaling cascade. For example, the daf-2 gene that has been shown to regulate lifespan in C elegans shares significant sequence homology with the insulin and IGF-1 receptor genes in mammals. Another longevity gene in the nematode, age-1, is homologous with the p110 subunit of phosphatidylinositol 3-kinase in mammals. This enzyme functions early in the mammalian insulin response cascade to influence many important cellular growth and metabolic processes. These findings and others have led to the suggestion that lifespan regulation in nematodes is controlled by a mechanism similar to that involved in lifespan extension by caloric restriction in mammals. Many intriguing similarities exist between these two model systems providing some support for this idea. However, at present there is insufficient data to conclude that similar genes or mechanisms regulate lifespan determination in nematodes and in mammals.

Inherited sensorineural low-frequency hearing impairment: some aspects of phenotype and epidemiology

This contribution forms part of the HEAR project. It describes some phenotypes of inherited low-frequency sensorineural hearing impairment (LFSHI) and estimates the prevalence of this inherited hearing impairment (HI) based on a clinical series. During a 10-year period (1987-1996), 418 subjects (134 males and 284 females), with a median age of 68 years (range 4-98), had been examined with LFSHI, defined as hearing loss most pronounced in the low frequencies (i.e., 250 and 500 Hz > 20 dB HL with better hearing, i.e., > or =15-dB difference at 1 and/or 2 and/or 4 kHz with an air-bone gap <15 dB for the average of 0.5, 1, and 2 kHz). The 418 subjects comprising 0.6 per cent of the total number of subjects examined (N=69,309) were subdivided into four categories: category I positive genetic subjects (N=69); category II, probably genetic (N=339); category III, uncertain genetic (N=6); and category IV, subjects with contradictory audiological findings (N=4). The phenotype in category I demonstrated a symmetrical LFSHI, with a pattern of progression showing a slow deterioration in the high frequencies (i.e., 2, 4, and 8 kHz as a function of age)--the progression comprising 40-45 dB. In the low frequencies (i.e., 250, 500, and 1,000 Hz), a deterioration of 15-25 dB could be demonstrated from the youngest to the oldest age group. In category II, a symmetrical LFSHI was found in 179 subjects, showing the same pattern of progression as in category I. However, in the age group 20-39 years, a significantly poorer hearing was found in the low frequencies compared to category I, implying that several phenotypes may be present in LFSHI. A subgroup (A) in category II exhibited normal hearing in one ear with LFSHI in the opposite ear with the same pattern of progression as in category I. Three other subgroups with LFSHI and flat/sloping audiogram in the opposite ear and asymmetrical LFSHI also showed the same type of progression in the ear with LFSHI as in category I. A prevalence of 0.18/1,000 (95 per cent CI 0.13-0.22) of LFSHI was estimated based on the background population with a fairly constant prevalence throughout life. It is concluded that inherited nonsyndromal LFSHI is a rare disease and that the many different phenotypes of LFSHI probably are associated with pronounced genetic heterogeneity.

Longitudinal determination of skin collagen glycation and glycoxidation rates predicts early death in C57BL/6NNIA mice

In 1988, the National Institute on Aging launched a 10-year program aimed at identification of biomarkers of aging. Previous results from our laboratory showed that pentosidine, an advanced glycation product, formed in skin collagen at a rate inversely related to maximum life span across several mammalian species. As part of the Biomarkers Program, we investigated the hypothesis that longitudinal determination of glycation and glycoxidation rates in skin collagen could predict longevities in ad libitum-fed (AL) and caloric restricted (CR) mice. C57BL/6NNia male mice were biopsied at age 20 months and at natural death. Glycation (furosine method) was assessed by gas chromatography/mass spectrometry (GC/MS) and the glycoxidation products carboxymethyllysine (CML) and pentosidine were determined by GC/MS and HPLC, respectively. CR vs. AL significantly (P<0.0001) increased both mean (34 vs. 27 months) and maximum (47 vs. 31 months) life spans. Skin collagen levels of furosine (pmol/micromol lysine) were approximately 2.5-fold greater than CML levels and 100-fold greater than pentosidine. Individual accumulation rates modeled as linear equations were significantly (P<0.001) inhibited by CR vs. AL for all parameters and in all cases varied inversely with longevity (P<0.1 to <0.0001). The incidence of three tissue pathologies (lymphoma, dermatitis, and seminal vesiculitis) was found to be attenuated by CR and the latter pathology correlated significantly with longevities (r=0.54, P=0. 002). The finding that markers of skin collagen glycation and glycoxidation rates can predict early deaths in AL and CR C57BL/6NNia mice strongly suggests that an age-related deterioration in glucose tolerance is a life span-determining process.

Genotype-environment interaction for quantitative trait loci affecting life span in Drosophila melanogaster

The nature of genetic variation for Drosophila longevity in a population of recombinant inbred lines was investigated by estimating quantitative genetic parameters and mapping quantitative trait loci (QTL) for adult life span in five environments: standard culture conditions, high and low temperature, and heat-shock and starvation stress. There was highly significant genetic variation for life span within each sex and environment. In the analysis of variance of life span pooled over sexes and environments, however, the significant genetic variation appeared in the genotype x sex and genotype x environment interaction terms. The genetic correlation of longevity across the sexes and environments was not significantly different from zero in these lines. We estimated map positions and effects of QTL affecting life span by linkage to highly polymorphic roo transposable element markers, using a multiple-trait composite interval mapping procedure. A minimum of 17 QTL were detected; all were sex and/or environment-specific. Ten of the QTL had sexually antagonistic or antagonistic pleiotropic effects in different environments. These data provide support for the pleiotropy theory of senescence and the hypothesis that variation for longevity might be maintained by opposing selection pressures in males and females and variable environments. Further work is necessary to assess the generality of these results, using different strains, to determine heterozygous effects and to map the life span QTL to the level of genetic loci.

Insulin and insulin-like growth factor-1 receptors in an evoluted fish, the turbot: cDNA cloning and mRNA expression

The insulin receptor (IR) and the structurally related insulin-like growth factor type 1 receptor (IGF-1R) belong to the tyrosine kinase (TK) receptor family. In this study, we have carried out the molecular characterization of both receptors from turbot (Psetta maxima), an evoluted teleost flatfish. The cDNA encoding the precursors of IGF-1R and the nearly entire IR (only the first 16 amino acids of the alpha subunit are missing when compared to the published human sequence) were cloned from an embryonic cDNA library. The deduced polypeptides contain all the topological features characterizing the insulin/IGF-1 receptor family. They are highly conserved compared to their mammalian counterparts, particularly within domains involved in the catalytic activity and in the transduction pathways. Nevertheless, some differences in the primary sequences, especially in the carboxy-terminal domain of the precursors, may affect the functions fulfilled by these receptors. As in mammals, the long IGF-1R 5'-untranslated sequence contains open reading frames and potential Sp1 binding sites. Northern blot analyses have revealed a major IR transcript of 11 kb, which is approximately the size of IGF-1R transcript (Eliès, G., Groigno, L., Wolff, J., Boeuf, G., Boujard, D., 1996. Characterization of the insulin-like growth factor type 1 receptor messenger in two teleost species. Mol. Cell. Endocrinol. 124, 131-140.). If IR and IGF-1R transcripts are detected by RT-PCR at all developmental stages and in all tissues examined, in situ hybridization studies have shown that these mRNA can be visualized as ubiquitous signals only in young larvae, whereas IGF-1R and IR expression appears weaker during later development and in adult tissues.

Regulation of lifespan by sensory perception in Caenorhabditis elegans

Caenorhabditis elegans senses environmental signals through ciliated sensory neurons located primarily in sensory organs in the head and tail. Cilia function as sensory receptors, and mutants with defective sensory cilia have impaired sensory perception. Cilia are membrane-bound microtubule-based structures and in C. elegans are only found at the dendritic endings of sensory neurons. Here we show that mutations that cause defects in sensory cilia or their support cells, or in sensory signal transduction, extend lifespan. Our findings imply that sensory perception regulates the lifespan of this animal, and suggest that in nature, its lifespan may be regulated by environmental cues.

Control of DAF-7 TGF-(alpha) expression and neuronal process development by a receptor tyrosine kinase KIN-8 in Caenorhabditis elegans

KIN-8 in C. elegans is highly homologous to human ROR-1 and 2 receptor tyrosine kinases of unknown functions. These kinases belong to a new subfamily related to the Trk subfamily. A kin-8 promoter::gfp fusion gene was expressed in ASI and many other neurons as well as in pharyngeal and head muscles. A kin-8 deletion mutant was isolated and showed constitutive dauer larva formation (Daf-c) phenotype: about half of the F(1) progeny became dauer larvae when they were cultivated on an old lawn of E. coli as food. Among the cells expressing kin-8::gfp, only ASI sensory neurons are known to express DAF-7 TGF-(beta), a key molecule preventing dauer larva formation. In the kin-8 deletion mutant, expression of daf-7::gfp in ASI was greatly reduced, dye-filling in ASI was specifically lost and ASI sensory processes did not completely extend into the amphid pore. The Daf-c phenotype was suppressed by daf-7 cDNA expression or a daf-3 null mutation. ASI-directed expression of kin-8 cDNA under the daf-7 promoter or expression by a heat shock promoter rescued the dye-filling defect, but not the Daf-c phenotype, of the kin-8 mutant. These results show that the kin-8 mutation causes the Daf-c phenotype through reduction of the daf-7 gene expression and that KIN-8 function is cell-autonomous for the dye-filling in ASI. KIN-8 is required for the process development of ASI, and also involved in promotion of daf-7 expression through a physiological or developmental function.

Modulation of cellular apoptotic potential: contributions to oncogenesis

The importance of apoptosis as a natural means to eliminate unwanted or damaged cells has been realized over the past decade. Many components required to exercise programmed cell death have been identified and shown to pre-exist in most, if not all, cells. Such ubiquity requires that apoptosis be tightly controlled and suggests the propensity of cells to trigger the cellular death machinery can be regulated. Recently, several signaling pathways have been demonstrated to impact the apoptotic potential of cells, most notably the phosphatidylinositol 3' kinase (PI3'K) pathway. The 3' phosphorylated lipid products generated by this enzyme promote activation of a protein-serine kinase, PKB/AKT, which is necessary and sufficient to confer cell PI3'K-dependent survival signals. The relevance of this pathway to human cancer was revealed by the recent finding that the product of the PTEN tumor suppressor gene acts to antagonize PI3'K. This review focuses on the regulation and mechanisms by which PKB activation protects cells and the oncologic consequences of dysregulation of the pathway.

Regulation of nuclear translocation of forkhead transcription factor AFX by protein kinase B

The regulation of intracellular localization of AFX, a human Forkhead transcription factor, was studied. AFX was recovered as a phosphoprotein from transfected COS-7 cells growing in the presence of FBS, and the phosphorylation was eliminated by wortmannin, a potent inhibitor of phosphatidylinositol (PI) 3-kinase. AFX was phosphorylated in vitro by protein kinase B (PKB), a downstream target of PI 3-kinase, but a mutant protein in which three putative phosphorylation sites of PKB had been replaced by Ala was not recognized by PKB. In Chinese hamster ovary cells (CHO-K1) cultured with serum, the AFX protein fused with green fluorescence protein (AFX-GFP) is localized mainly in the cytoplasm, and wortmannin induced transient nuclear translocation of the fusion protein. The AFX-GFP mutant in which all three phosphorylation sites had been replaced by Ala was detected exclusively in the cell nucleus. AFX-GFP was in the nucleus when the cells were infected with an adenovirus vector encoding a dominant-negative form of either PI 3-kinase or PKB, whereas the fusion protein stayed in the cytoplasm when the cells expressed constitutively active PKB. In CHO-K1 cells expressing AFX-GFP, DNA fragmentation was induced by the stable PI 3-kinase inhibitor LY294002, and the expression of the active form of PKB suppressed this DNA fragmentation. The phosphorylation site mutant of AFX-GFP enhanced DNA fragmentation irrespective of the presence and absence of PI 3-kinase inhibitor. These results indicate that the nuclear translocation of AFX is negatively regulated through its phosphorylation by PKB.

A local, high-density, single-nucleotide polymorphism map used to clone Caenorhabditis elegans cdf-1

Ras-mediated signaling is required for induction of vulval cell fates during Caenorhabditis elegans development. By screening for suppressors of the multivulva phenotype caused by constitutively active let-60 ras, we identified the mutation n2527. To clone the gene affected by n2527, we developed a method for high-resolution mapping. We took advantage of the genomic DNA sequence of the N2 strain by using DNA sequencing to scan for single-nucleotide polymorphisms (SNPs) at defined genomic positions of the RC301 strain. An average of one polymorphism per 1.4 kb was detected in predicted intergenic regions. Because of this high frequency, DNA sequencing is an efficient method to scan for SNPs. By alternating between identifying SNPs and mapping n2527 using selected recombinants, we generated an SNP map of progressively higher density. An intensive search for SNPs resulted in a local map with an average marker spacing of approximately 4 kb. This was used to map n2527 to a 9.6-kb interval. The small size of this interval made it feasible to use DNA sequencing to identify the molecular lesion. In principle, this approach can be used for high-resolution mapping of any C. elegans mutation. Furthermore, this approach can be applied to other species as the genomic sequence becomes available. The n2527 mutation affects a previously uncharacterized gene that we named cdf-1, as it encodes a predicted protein with significant similarity to members of the cation diffusion facilitator family.

Genetic and environmental conditions that increase longevity in Caenorhabditis elegans decrease metabolic rate

Mutations that increase the longevity of the soil nematode Caenorhabditis elegans could define genes involved in a process specific for aging. Alternatively, these mutations could reduce animal metabolic rate and increase longevity as a consequence. In ectotherms, longevity is often negatively correlated with metabolic rate. Consistent with these observations, environmental conditions that reduce the metabolic rate of C. elegans also extend longevity. We found that the metabolic rate of long-lived C. elegans mutants is reduced compared with that of wild-type worms and that a genetic suppressor that restored normal longevity to long-lived mutants restored normal metabolic rate. Thus, the increased longevity of some long-lived C. elegans mutants may be a consequence of a reduction in their metabolic rate, rather than an alteration of a genetic pathway that leads to enhanced longevity while maintaining normal physiology. The actual mechanism responsible for the inverse correlation between metabolic rate and longevity remains unknown.

Regulation of imaginal disc cell size, cell number and organ size by Drosophila class I(A) phosphoinositide 3-kinase and its adaptor

BACKGROUND

Class I(A) phosphoinositide 3-kinases (PI 3-kinases) have been implicated in the regulation of several cellular processes including cell division, cell survival and protein synthesis. The size of Drosophila imaginal discs (epithelial structures that give rise to adult organs) is maintained by factors that can compensate for experimentally induced changes in these PI 3-kinase-regulated processes. Overexpression of the gene encoding the Drosophila class I(A) PI 3-kinase, Dp110, in imaginal discs, however, results in enlarged adult organs. These observations have led us to investigate the role of Dp100 and its adaptor, p60, in the control of imaginal disc cell size, cell number and organ size.

RESULTS

Null mutations in Dp110 and p60 were generated and used to demonstrate that they are essential genes that are autonomously required for imaginal disc cells to achieve their normal adult size. In addition, modulating Dp110 activity increases or reduces cell size in the developing imaginal disc, and does so throughout the cell cycle. The inhibition of Dp110 activity reduces the rate of increase in cell number in the imaginal discs, suggesting that Dp110 normally promotes cell division and/or cell survival. Unlike direct manipulation of cell-cycle progression, manipulation of Dp110 activity in one compartment of the disc influences the size of that compartment and the size of the disc as a whole.

CONCLUSIONS

We conclude that during imaginal disc development, Dp110 and p60 regulate cell size, cell number and organ size. Our results indicate that Dp110 and p60 signalling can affect growth in multiple ways, which has important implications for the function of signalling through class I(A) PI 3-kinases.

P13-kinase inhibition induces dauer formation, thermotolerance and longevity in C. elegans

The effects of 2-(4-Morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002), an inhibitor of mammalian phosphatidylinositol 3-OH kinase, was tested on an insulin signaling-like pathway in the nematode Caenorhabditis elegans. Populations of C. elegans were treated with LY294002 at different stages of the life cycle, and its effects on development, thermotolerance and longevity were assessed. At concentrations of 160 microM and above, LY294002 significantly induced both dauer formation and thermotolerance. Treatment of adult worms also resulted in a small, but significant, increase in life span. The results presented are consistent with the view that a neuroendocrine signaling pathway functions in adult worms to determine stress resistance and longevity.

Mechanisms of life span determination in Caenorhabditis elegans

Molecular analysis of several gerontogenes of Caenorhabditis elegans has led to the discovery of at least two life span-controlling pathways. An insulin-like signaling cascade consisting of proteins encoded by the genes daf-2, age-1, akt-1, akt-2, daf-16 and daf-18 regulates dauer diapause, reproduction, and longevity. This pathway regulates all three processes systemically. daf-12 interacts with it, affecting dauer diapause and longevity. Life span extension mediated by this pathway probably results from the activation of an enhanced life-maintenance program, which is normally operative during dauer diapause. A different mechanism is specified by the clock genes clk-1, clk-2, clk-3 and gro-1, which regulate metabolic activity and the pace of many temporal processes including longevity. There is some controversy as to whether the life span extension observed in these mutants requires the activity of daf-16. All known gerontogenes appear to confer resistance to environmental stress, usually multiple stress factors, including oxidative stress, high temperature, and exposure to ultraviolet radiation. Caloric restriction extends longevity substantially, and may act by activating the enhanced life-maintenance program.

Mutations in signal transduction proteins increase stress resistance and longevity in yeast, nematodes, fruit flies, and mammalian neuronal cells

Mutations in Ras and other signal transduction proteins increase survival and resistance to oxidative stress and starvation in stationary phase yeast, nematodes, fruit flies, and in neuronal PC12 cells. The chronological life span of yeast, based on the survival of nondividing cells in stationary phase, has allowed the identification and characterization of long-lived strains with mutations in the G-protein Ras2. This paradigm was also used to identify the in vivo sources and targets of reactive oxygen species and to examine the role of antioxidant enzymes in the longevity of yeast. I will review this model system and discuss the striking phenotypic similarities between long-lived mutants ranging from yeast to mammalian neuronal cells. Taken together, the published studies suggest that survival may be regulated by similar fundamental mechanisms in many eukaryotes and that simple model systems will contribute to our understanding of the aging process in mammals.

The spe-10 mutant has longer life and increased stress resistance

We investigated the life span of spe-10 mutant nematodes. We also tested resistance of spe-10 mutants to ultraviolet (UV) light, heat, and paraquat and examined the relationship between resistance to UV light and the fertility defect of these animals. The spe-10 mutation significantly increased mean life span. Additionally, the mutation significantly increased resistance to both UV light and to heat. Resistance to paraquat was not significantly different from that of wild-type, nor were any dauers formed at 27 degrees C. No significant correlation was found between the UV resistance and the fertility defect, nor was the UV resistance attributable to a hormetic effect. These results reinforce the importance of stress resistance in specifying increased life span and indirectly suggest that this fertility defect is not a direct cause of life span extension.

Calories and aging alter gene expression for gluconeogenic, glycolytic, and nitrogen-metabolizing enzymes

We characterized the effects of calorie restriction (CR) on the expression of key glycolytic, gluconeogenic, and nitrogen-metabolizing enzymes in mice. Of the gluconeogenic enzymes investigated, liver glucose-6-phosphatase mRNA increased 1.7- and 2. 3-fold in young and old CR mice. Phosphoenolpyruvate carboxykinase mRNA and activity increased 2.5- and 1.7-fold in old CR mice. Of the key glycolytic enzymes, pyruvate kinase mRNA and activity decreased approximately 60% in CR mice. Hepatic phosphofructokinase-1 and pyruvate dehydrogenase mRNA decreased 10-20% in CR mice. Of the genes that detoxify ammonia generated from protein catabolism, hepatic glutaminase, carbamyl phosphate synthase I, and tyrosine aminotransferase mRNAs increased 2.4-, 1.8-, and 1.8-fold with CR, respectively. Muscle glutamine synthetase mRNA increased 1.3- and 2. 1-fold in young and old CR mice. Hepatic glutamine synthetase mRNA and activity each decreased 38% in CR mice. These CR-induced changes are consistent with other studies suggesting that CR may decrease enzymatic capacity for glycolysis and increase the enzymatic capacity for hepatic gluconeogenesis and the disposal of byproducts of muscle protein catabolism.

TGFbeta-like signaling and spicule development in Caenorhabditis elegans

A TGFbeta-like signal is required for spicule development in Caenorhabditis elegans males. This signal appears to originate in the male-specific musculature and is required for the migrations of cells within the proctodeum. The migrations of these cells form cellular molds, the spicule traces, in which the cuticle of the spicules is secreted. Mutations in daf-4, sma-2, sma-3, and sma-4, which disrupt TGFbeta-like signaling, result in aberrant migrations and morphologically abnormal spicules. daf-4, and hence the TGFbeta-like signal, is required prior to or during cell migrations. Therefore, the TGFbeta-like signal may act to prime the migrating cells or as a guidance cue. Mutations in lin-31 result in identical cell migration and spicule morphology defects. Thus, lin-31, which encodes a "winged helix" protein (Miller et al., Genes Dev. 7, 933-947, 1993), may be a component of this TGFbeta-like signaling pathway.

FKHR binds the insulin response element in the insulin-like growth factor binding protein-1 promoter

The insulin response element (IRE) in the IGFBP-1 promoter, and in other gene promoters, contains a T(A/G)TTT motif essential for insulin inhibition of transcription. Studies presented here test whether FKHR may be the transcription factor that confers insulin inhibition through this IRE motif. Immunoblots using antiserum to the synthetic peptide FKHR413-430, RNase protection, and Northerns blots show that FKHR is expressed in HEP G2 human hepatoma cells. Southwestern blots, electromobility shift assays, and DNase I protection assays show that Escherichia coli-expressed GST-FKHR binds specifically to IREs from the IGFBP-1, PEPCK and TAT genes; however, unlike HNF3beta, another protein proposed to be the insulin regulated factor, GST-FKHR does not bind the insulin unresponsive G/C-A/C mutation of the IGFBP-1 IRE. When HEP G2 cells were cotransfected with FKHR expression vectors and with IGFBP-1 promoter plasmids containing either native or mutant IREs, FKHR expression induced a 5-fold increase in activity of the native IGFBP-1 promoter but no increase in activity of promoter constructs containing insulin unresponsive IRE mutants. These data suggest that FKHR, and/or a related family member, is the important T(G/A)TTT binding protein that confers the inhibitory effect of insulin on gene transcription.

Autonomous control of cell and organ size by CHICO, a Drosophila homolog of vertebrate IRS1-4

The control of growth is fundamental to the developing metazoan. Here, we show that CHICO, a Drosophila homolog of vertebrate IRS1-4, plays an essential role in the control of cell size and growth. Animals mutant for chico are less than half the size of wild-type flies, owing to fewer and smaller cells. In mosaic animals, chico homozygous cells grow slower than their heterozygous siblings, show an autonomous reduction in cell size, and form organs of reduced size. Although chico flies are smaller, they show an almost 2-fold increase in lipid levels. The similarities of the growth defects caused by mutations in chico and the insulin receptor gene in Drosophila and by perturbations of the insulin/IGF1 signaling pathway in vertebrates suggest that this pathway plays a conserved role in the regulation of overall growth by controling cell size, cell number, and metabolism.

The PTEN tumor suppressor homolog in Caenorhabditis elegans regulates longevity and dauer formation in an insulin receptor-like signaling pathway

Inactivation of the tumor suppressor PTEN gene is found in a variety of human cancers and in cancer predisposition syndromes. Recently, PTEN protein has been shown to possess phosphatase activity on phosphatidylinositol 3,4,5-trisphosphate, a product of phosphatidylinositol 3-kinase. We have identified a homolog of PTEN in Caenorhabditis elegans and have found that it corresponds to the daf-18 gene, which had been defined by a single, phenotypically weak allele, daf-18(e1375). By analyzing an allele, daf-18(nr2037), which bears a deletion of the catalytic portion of CePTEN/DAF-18, we have shown that mutation in daf-18 can completely suppress the dauer-constitutive phenotype caused by inactivation of daf-2 or age-1, which encode an insulin receptor-like molecule and the catalytic subunit of phosphatidylinositol 3-kinase, respectively. In addition, daf-18(nr2037) dramatically shortens lifespan, both in a wild-type background and in a daf-2 mutant background that normally prolongs lifespan. The lifespan in a daf-18(nr2037) mutant can be restored to essentially that of wild type when combined with a daf-2 mutation. Our studies provide genetic evidence that, in C. elegans, the PTEN homolog DAF-18 functions as a negative regulator of the DAF-2 and AGE-1 signaling pathway, consistent with the notion that DAF-18 acts a phosphatidylinositol 3,4,5-trisphosphate phosphatase in vivo. Furthermore, our studies have uncovered a longevity-promoting activity of the PTEN homolog in C. elegans.

Neurosecretory control of aging in Caenorhabditis elegans

In the nematode Caenorhabditis elegans, an insulin receptor signaling pathway regulates adult life span and developmental arrest at the dauer larval stage. Here we show that the unc-64 and unc-31 genes also function in this pathway. These two genes are involved in mediating Ca2+-regulated secretion. Mutations in unc-64 and unc-31 increase adult life span and cause constitutive dauer formation. Both phenotypes are suppressed by mutations in daf-16, which also suppresses other mutations in this pathway. We present evidence that the site of action of unc-64 is neuronal, suggesting that a neurosecretory signal regulates life span and dauer formation.

Orthologues of the Caenorhabditis elegans longevity gene clk-1 in mouse and human

The clk-1 gene was isolated from the long-lived mutant of Caenorhabditis elegans and was suggested to play a biological role in longevity (Ewbank et al., 1997, Science 275: 980-983). The primary structure of CLK-1 showed a significant homology to Saccharomyces cerevisiae Coq7p/Cat5p, which is required for the biosynthesis of ubiquinone and the derepression of gluconeogenic genes. In the present study, we isolated and characterized human and mouse orthologues of the COQ7/CLK-1 gene. Sequence analysis of both the human and the mouse COQ7 cDNAs showed an open reading frame composed of 217 amino acids with calculated molecular mass of 24,309 and 24,044 Da, respectively. Homology search revealed that human COQ7 showed 85% identity to mouse COQ7, 89% identity to rat COQ7, 53% identity to C. elegans CLK-1, and 37% identity to S. cerevisiae Coq7p/Cat5p. Zoo blot analysis implied that the COQ7 gene was well conserved among mammal, bird, and reptile genomes. Tissue blot analysis showed that human COQ7 is dominantly transcribed in heart and skeletal muscle. Genomic analyses revealed that the human COQ7 gene is composed of six exons spanning 11 kb of human genome as a single-copy gene. Radiation hybrid mapping assigned the COQ7 gene to human chromosome 16p12.3-p13.11.

Phosphorylation of serine 256 by protein kinase B disrupts transactivation by FKHR and mediates effects of insulin on insulin-like growth factor-binding protein-1 promoter activity through a conserved insulin response sequence

Insulin inhibits the expression of multiple genes in the liver containing an insulin response sequence (IRS) (CAAAA(C/T)AA), and we have reported that protein kinase B (PKB) mediates this effect of insulin. Genetic studies in Caenorhabditis elegans indicate that daf-16, a forkhead/winged-helix transcription factor, is a major target of the insulin receptor-PKB signaling pathway. FKHR, a human homologue of daf-16, contains three PKB sites and is expressed in the liver. Reporter gene studies in HepG2 hepatoma cells show that FKHR stimulates insulin-like growth factor-binding protein-1 promoter activity through an IRS, and introduction of IRSs confers this effect on a heterologous promoter. Insulin disrupts IRS-dependent transactivation by FKHR, and phosphorylation of Ser-256 by PKB is necessary and sufficient to mediate this effect. Antisense studies indicate that FKHR contributes to basal promoter function and is required to mediate effects of insulin and PKB on promoter activity via an IRS. To our knowledge, these results provide the first report that FKHR stimulates promoter activity through an IRS and that phosphorylation of FKHR by PKB mediates effects of insulin on gene expression. Signaling to FKHR-related forkhead proteins via PKB may provide an evolutionarily conserved mechanism by which insulin and related factors regulate gene expression.

Insulin stimulates phosphorylation of the forkhead transcription factor FKHR on serine 253 through a Wortmannin-sensitive pathway

In the nematode Caenorhabditis elegans, mutations of the insulin/insulin-like growth factor-1 receptor homologue Daf-2 gene cause developmental arrest at the dauer stage. The effect of Daf-2 mutations is counteracted by mutations in the Daf-16 gene, suggesting that Daf-16 is required for signaling by Daf-2. Daf-16 encodes a forkhead transcription factor. Based on sequence similarity, the FKHR genes are the likeliest mammalian Daf-16 homologues. FKHR proteins contain potential sites for phosphorylation by the serine/threonine kinase Akt. Because Akt is phosphorylated in response to insulin and has been implicated in a variety of insulin effects, we investigated whether insulin affects phosphorylation of FKHR. Insulin stimulated phosphorylation of endogenous FKHR and of a recombinant c-Myc/FKHR fusion protein transiently expressed in murine SV40-transformed hepatocytes. The effect of insulin was inhibited by wortmannin treatment, suggesting that PI 3-kinase activity is required for FKHR phosphorylation. Mutation of serine 253, located in a consensus Akt phosphorylation site at the carboxyl-terminal end of the forkhead domain, abolished the effect of insulin on FKHR phosphorylation. In contrast, mutation of two additional Akt phosphorylation sites, at amino acids threonine 24 or serine 316, did not abolish insulin-induced phosphorylation. These data indicate that FKHR may represent a distal effector of insulin action.

The evolutionary genetics of ageing and longevity

Evolutionary theories of ageing are based on the observation that the efficacy of natural selection decreases with age. This is because, even without ageing, individuals will die of environmental causes, such as predation, disease and accidents. Ageing is thought to have evolved as the result of optimising fitness early in life. A second process, namely the progressive accumulation of mutations with effects late in life, will reinforce this result. Longevity of a species is therefore determined by the amount of environmental mortality caused by the ecology of a species. The experimental data concerning the relative roles of both processes are reviewed here. Recent discoveries of the levelling of mortality curves, and of age specific mutations in mutation accumulation lines of Drosophila melanogaster, require adjustments to the original models of the evolution of ageing and species longevity. These adjustments do not invalidate the underlying rationale of evolutionary theories of ageing. With current developments in QTL mapping and genetic association studies, the unravelling of the ageing process has the potential to progress rapidly.