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Keywords:

  • Aging;
  • C. elegans;
  • endocrinology;
  • longevity

Summary

  1. Top of page
  2. Summary
  3. The endocrinology of lifespan in worms
  4. Conservation of aging pathways between worms, flies and mammals
  5. Pharmacological interventions in lifespan
  6. Prospects for high throughput screens (HTS) for aging in C. elegans
  7. Endocrine targets for pharmacological intervention
  8. Conclusions
  9. References

Studies in the nematode Caenorhabditis elegans have been instrumental in defining genetic pathways that are involved in modulating lifespan. Multiple processes such as endocrine signaling, nutritional sensing and mitochondrial function play a role in determining lifespan in the worm and these mechanisms appear to be conserved across species. These discoveries have identified a range of novel targets for pharmacological manipulation of lifespan and it is likely that the nematode model will now prove useful in the discovery of compounds that slow aging. This review will focus on the endocrine targets for intervention in aging and the use of C. elegans as a system for high throughput screens of compounds for their effects on aging.


The endocrinology of lifespan in worms

  1. Top of page
  2. Summary
  3. The endocrinology of lifespan in worms
  4. Conservation of aging pathways between worms, flies and mammals
  5. Pharmacological interventions in lifespan
  6. Prospects for high throughput screens (HTS) for aging in C. elegans
  7. Endocrine targets for pharmacological intervention
  8. Conclusions
  9. References

Insulin signaling was first implicated in lifespan regulation with the delineation of a genetic pathway in worms, which included the genes age-1 (a PI3 kinase), daf-2 (an insulin-like receptor) and daf-16 (a forkhead transcription factor) (Kenyon, 2005) (Fig. 1). age-1 and daf-2 mutants exhibit dramatic increases in adult lifespan which are dependent on the activity of daf-16. These genes form one arm of a complex signaling cascade that integrates signals from the environment such as temperature, food availability and population density to allow the nematode to either proceed with normal programs of reproductive growth or enter an alternate larval stage called the dauer larva (Riddle, 1988). The dauer larva is nonfeeding, stress resistant and nonreproducing, and can endure harsh environmental conditions for many months before exiting under favorable growth conditions to resume reproductive development. This decision to resume reproductive growth is determined by environmental signals that are transmitted not only through the insulin-signaling pathway but also a TGF-β-signaling pathway (Riddle & Albert, 1997; Patterson & Padgett, 2000). Both these primary endocrine pathways converge on a nuclear hormone receptor daf-12, which is responsible for secondary hormone signaling via lipophilic hormones. Although both insulin and TGF-β signaling are involved in the dauer decision, only the insulin arm is involved in determination of adult lifespan.

image

Figure 1. Pathways involved in dauer formation, reproductive development and lifespan determination in Caenorhabditis elegans. Solid arrows indicate an enhancing effect and dashed arrows indicate an inhibitory effect. (A) Signal transduction through the DAF-2 (insulin receptor) pathway reduces the activity of the transcription factor DAF-16. The DAF-2 pathway also impacts on the activity of DAF-9, a cytochrome P450, which synthesizes a lipophilic hormone that acts on the nuclear receptor DAF-12. During development the DAF-12 ligand receptor complex promotes reproductive growth and prevents entry into the dauer formation pathway. Normal signaling through these pathways during adulthood limits lifespan extension. (B) When signaling through the DAF-2 pathway is reduced there is increased activity of DAF-16. There is also a reduction in DAF-9 activity which leads to a reduction in levels of the DAF-12 ligand. During development these changes promote dauer arrest rather than reproductive growth. Reduced signaling through these pathways in adulthood leads to lifespan extension.

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DAF-2 appears to be the only insulin/insulin-like growth factor (IGF) receptor in worms, in contrast to mammals that have discrete insulin and IGF receptors. However, what the worm lacks in the receptor department it more than compensates for when it comes to ligands: Caenorhabditis elegans has as many as 37 potential DAF-2 ligands (Pierce et al., 2001), compared to 3 in mammals. The functions of all these ligands remain to be determined but it appears that some act as antagonists (Pierce et al., 2001) and others as agonists (Li et al., 2003; Murphy et al., 2003) of the receptor. Inactivation of ins-7 by RNA interference has been shown to extend lifespan (Murphy et al., 2003) and other ligands may influence lifespan either positively or negatively. With so many ligands acting through one receptor the question arises as to how specificity of action is maintained. There is evidence that at least one of the insulin peptides (INS-7) is involved in regulating its own levels via a positive feedback loop. In this model reduced DAF-2 signaling reduces ins-7 gene expression, which in turn reduces the level of the DAF-2 agonist to further reduce signaling (Murphy et al., 2003). In mammals, a family of IGF-binding proteins (IGFBPs) has evolved to regulate the activity of IGF-I and IGF-II at the insulin-like growth factor type 1 receptor (Firth & Baxter, 2002). In worms there is no evidence as yet of IGFBP equivalents but a number of genes encoding proteins with homology to the extracellular domain of the insulin receptor have been noted (Dlakic, 2002). Some of these proteins are differentially regulated during dauer formation and may provide a mechanism by which ligand availability is modulated (Liu et al., 2004).

The key modulator of the effects of the DAF-2-signaling pathway on lifespan is the FOXO transcription factor DAF-16, whose activity is required for lifespan extension in insulin-signaling mutants (Kenyon, 2005). Microarray and other bioinformatics approaches have indicated that DAF-16 regulates the expression of a large number of genes involved in a wide variety of processes including cellular stress response, metabolism, energy generation and development as well as aging (Lee et al., 2003; McElwee et al., 2003; Murphy et al., 2003). Consistent with DAF-16 activity being required for longevity, overexpression of daf-16 alone is sufficient to extend lifespan (Henderson & Johnson, 2001).

Genetic analysis of dauer formation has identified another endocrine pathway downstream of insulin signaling which is also involved in lifespan regulation (Hsin & Kenyon, 1999; Gerisch et al., 2001; Jia et al., 2002). Two of the players in this pathway are daf-9, a cytochrome P450 that has homology to steroid and fatty acid hydroxylases, and the nuclear hormone receptor daf-12, which has homology to the ligand-binding domain of the vertebrate thyroid receptor and the DNA-binding domain (DBD) of the pregnane X and vitamin D receptors (Antebi et al., 2000). The identities of these molecules have led to the suggestion that DAF-9 is involved in the synthesis of a lipophilic hormone that acts on DAF-12 (Gerisch et al., 2001; Jia et al., 2002). The hormone synthesized by DAF-9 and predicted to act on DAF-12 has yet to be identified although nematode extracts that possess the predicted biological activity have been described (Gill et al., 2004; Matyash et al., 2004). A number of lines of evidence support the notion that the endogenous hormone is a sterol or steroid derived from cholesterol. Under laboratory conditions C. elegans must be supplemented with exogenous cholesterol since nematodes are unable to synthesize their own sterols (Fabian & Johnson, 1994). Cholesterol deprivation in wild-type animals leads to a variety of phenotypes including developmental arrest and reduced fertility (Shim et al., 2002; Merris et al., 2003). Additionally, weak alleles of daf-9 that are thought to be compromised in their ability to synthesize the hormone often show an enhanced phenotype under conditions of cholesterol deprivation (Gerisch et al., 2001; Jia et al., 2002).

Mutations affecting daf-9 lead to an increase in adult lifespan suggesting that the hormone synthesized by DAF-9 will limit lifespan extension (Gerisch et al., 2001; Jia et al., 2002). The role of DAF-12 in the regulation of lifespan is more complex. Most daf-12 mutations do not have any effect on lifespan, with the exception of a putative null allele which is short lived, and a second mutation that affects the ligand-binding domain which exhibits a mild longevity phenotype similar to that of daf-9 (A. Fisher, personal communication). Interestingly, certain daf-12 mutants can further increase longevity in combination with specific daf-2 alleles and conversely other allelic combinations result in normalization of daf-2 lifespan (Larsen et al., 1995; Gems et al., 1998). The mechanism by which the daf-12 and daf-2 mutants interact in this way has yet to be determined. daf-9 and daf-12 are also required for the lifespan extension that is seen in animals lacking a germline (Hsin & Kenyon, 1999; Gerisch et al., 2001; Arantes-Oliveira et al., 2002). This increase in lifespan is dependent on daf-16, daf-9 and daf-12 indicating that insulin signaling and a lipophilic hormone acting through DAF-12 are required to promote lifespan extension in these animals.

Conservation of aging pathways between worms, flies and mammals

  1. Top of page
  2. Summary
  3. The endocrinology of lifespan in worms
  4. Conservation of aging pathways between worms, flies and mammals
  5. Pharmacological interventions in lifespan
  6. Prospects for high throughput screens (HTS) for aging in C. elegans
  7. Endocrine targets for pharmacological intervention
  8. Conclusions
  9. References

Like worms, flies have a single insulin-like receptor (InR) that when mutated confers lifespan extension, as does mutation of the receptor substrate chico (Tatar et al., 2003). Insulin signaling in flies is also dependent on the activity of the Drosophila homolog of DAF-16, dFOXO, as demonstrated by overexpression of dFOXO in the fat body (Giannakou et al., 2004; Hwangbo et al., 2004). Drosophila has a conservative seven insulin ligands, compared with the 37 found in worms, that are synthesized in and secreted by insulin-producing cells (IPC) in the brain (Tatar et al., 2003). Ablation of IPC removes circulating insulin and also leads to lifespan extension (Broughton et al., 2005).

In insects, lipophilic hormones were identified long before the insulin-signaling pathway was characterized and consequently much more is known about these hormones and their action (Tatar, 2004). Juvenile hormone (JH) influences larval development and is also involved in the regulation of the adult reproductive diapause. JH promotes vitellogenesis and cessation of JH production by the corpora allata results in increased adult lifespan and arrested development. Long-lived insulin receptor mutants have arrested development of the ovary similar to that seen in reproductive diapause and are deficient in JH. Treatment of these animals with a JH analog is not only sufficient to restore ovarian development but will also restore normal lifespan (Tatar et al., 2001). A second lipophilic hormone, the steroid ecdysone, also has effects on reproduction and adult lifespan. Ecdysone is synthesized principally in the ovaries and mutants defective in ecdysone synthesis or with a mutated ecdysone receptor are also long-lived (Simon et al., 2003). Thus, in Drosophila both the insulin-signaling and lipophilic hormone-signaling pathways that influence aging in the worm are conserved.

Recent work in the mouse has provided abundant evidence to suggest that the insulin/IGF signaling pathway also influences longevity in mammals (Bartke, 2005). However, the situation in mammals is complicated by the fact that rather than a single receptor that transduces both insulin and IGF signals there are specific receptors for the glucose-regulating actions of insulin and the mitogenic or other actions of IGF-I. This has led to the question of whether it is loss of insulin or IGF signaling that contributes to the longevity phenotype. A further complication is that many of the mouse models of extended lifespan exhibit multiple hormone deficiencies in addition to growth hormone (GH) and IGF deficiency (Bartke, 2005). Extended lifespan in the Little mouse (GH-releasing hormone receptor deficiency), mice with targeted deletions of the growth hormone receptor (GHRKO), and mice heterozygous for an IGF-1 receptor knockout all strongly support IGF-I as a key player in mammalian lifespan regulation. However, there is still evidence to support a similar role for insulin since many of these mouse models have alterations in insulin levels, sensitivity and glucose regulation that are secondary to alterations in IGF (Bartke, 2005). In addition, it has been shown that loss of insulin signaling in just the adipose tissue is sufficient to extend lifespan (Bluher et al., 2003).

It is less clear what the secondary lipophilic hormone signals are in mammals. In mouse models of longevity there are alterations in thyroid hormones and sex steroids which under normal circumstances would promote growth and reproduction at the expense of antiaging programs (Tatar et al., 2003). In addition, corticosterone levels are increased in the GHRKO mouse perhaps providing a link between secondary hormones and increased stress resistance (Al Regaiey et al., 2005). However, it is yet to be demonstrated that either one of these lipophilic hormones impact on lifespan in the manner predicted by the invertebrate models.

There is emerging evidence that the insulin/IGF system as described in worms, flies and mice may also be implicated in human longevity. Polymorphisms that affect IGF levels and are correlated to longevity have been identified from studies of centenarian populations (Franceschi et al., 2005). Correlations between polymorphisms that affect GH, IGF-I and insulin action and longevity in noncentenarian populations further suggest that this endocrine pathway is indeed involved in lifespan specification in humans as well as other systems (van Heemst et al., 2005).

Perhaps the most robust method for increasing lifespan is calorie restriction (CR). Since the 1930s CR has been known to extend the lifespan of rodents and this has also subsequently been demonstrated in worms and flies. As in the mouse models of GH-IGF deficiency, reduced insulin levels and increased insulin sensitivity are observed in CR mice. It appears that although reduced GH-IGF signaling and CR share many features they may act through distinct but overlapping mechanisms (Al Regaiey et al., 2005; Bartke, 2005).

Pharmacological interventions in lifespan

  1. Top of page
  2. Summary
  3. The endocrinology of lifespan in worms
  4. Conservation of aging pathways between worms, flies and mammals
  5. Pharmacological interventions in lifespan
  6. Prospects for high throughput screens (HTS) for aging in C. elegans
  7. Endocrine targets for pharmacological intervention
  8. Conclusions
  9. References

With the identification of conserved genetic pathways that affect lifespan there has been considerable interest in identifying pharmacological agents that could impact on the aging process. A number of observations suggest that interventions that slow aging may have broader applications to age-related disease. First, many of the basic mechanisms that underpin the aging phenotype have been shown to contribute to the pathophysiology of age-related disease, e.g. oxidative stress and protein aggregation (Hadley et al., 2005). Second, caloric restriction in rodents not only increases lifespan but also appears to prevent many of the metabolic changes associated with aging and confers resistance to many age-related diseases (Guarente & Picard, 2005). Thus, it has been hypothesized that compounds that are effective in slowing aging may also combat age-related disease. Such an outcome has already been demonstrated with the use of superoxide dismutase/catalase mimetics such as Euk-134. These compounds were shown to significantly increase the lifespan of C. elegans (Melov et al., 2000), presumably by reducing oxidative damage (Sampayo et al., 2003) and have also been shown to have beneficial effects in a number of animal models of human disease in which oxidative stress has been implicated (Gonzalez et al., 1996; Malfroy et al., 1997; Baker et al., 1998; Melov et al., 2001, 2005; Peng et al., 2005).

Invertebrate model systems have long been touted as having many advantages over in vitro and cell-based assays for drug screening. Principally, there is the opportunity to examine the effects of compounds in a whole animal setting thus quickly eliminating toxic drugs or drugs that affect multiple processes. A number of groups have demonstrated that lifespan can be increased in invertebrate model organisms by antioxidants (Harrington & Harley, 1988; Brack et al., 1997; Bauer et al., 2004; Ishii et al., 2004), anticonvulsants (Evason et al., 2005) and small molecules that target histone acetylation status (Kang et al., 2002; Howitz et al., 2003; Wood et al., 2004). However, these studies have tended to be small-scale targeted screens rather than large-scale unbiased surveys for compounds that affect aging.

Prospects for high throughput screens (HTS) for aging in C. elegans

  1. Top of page
  2. Summary
  3. The endocrinology of lifespan in worms
  4. Conservation of aging pathways between worms, flies and mammals
  5. Pharmacological interventions in lifespan
  6. Prospects for high throughput screens (HTS) for aging in C. elegans
  7. Endocrine targets for pharmacological intervention
  8. Conclusions
  9. References

The characteristics of C. elegans that make it a useful experimental system (Brenner, 1974) also make it attractive for the development of whole organism compound screens (Hertweck et al., 2003). C. elegans is a self-fertilizing hermaphrodite that can be readily cultured on solid or liquid media with a diet of Escherichia coli (Wood, 1988). Large isogenic populations can easily be grown and maintained to provide millions of synchronous individuals (Fabian & Johnson, 1994). Worms can be grown in microtiter plate formats (96- and 384-well) as individuals or small populations (20–50 worms). The development of flow cytometry methods for nematodes has further facilitated the sorting and dispensing of worms into microplates (Hertweck & Baumeister, 2005). Another benefit of C. elegans is the ease by which fluorescent proteins (Chalfie et al., 1994) and fluorescent dyes (Ashrafi et al., 2003; Gill et al., 2003) can be used to monitor cellular and morphological features over time.

One of the key features of C. elegans that makes it so useful in aging research is its relatively short lifespan, measured over days, rather than months or years. However, when considering HTS for compounds that affect lifespan it would be preferable to assess a compound's potential over an even shorter time frame. An attractive approach would be to use surrogate markers of aging or assays of other life history traits that can indicate the viability or health of the animal and which may map onto lifespan. One such approach is to measure stress resistance since a common feature of most genetic mutants that show increased lifespan is that they have increased resistance to a variety of acute stressors such as heat (Lithgow et al., 1995), oxidative stress (Larsen, 1993; Vanfleteren, 1993), UV radiation (Murakami & Johnson, 1996) and heavy metals (Barsyte et al., 2001). These surrogate assays for lifespan have been successfully used in genetic screens for novel aging genes (Sampayo et al., 2000; Munoz & Riddle, 2003; de Castro et al., 2004). It is therefore likely that such an approach will be useful in identifying compounds that affect lifespan. Replacing the standard technique of touch-provoked movement to assess survival with fluorescent dyes that can indicate time of death (Gill et al., 2003) is an additional refinement that makes HTS for lifespan in C. elegans a realistic proposition.

An alternative approach to scoring death as the end point is to use other markers of the aging process and in this respect there has been a great deal of interest in identifying biomarkers of aging in invertebrates (Herndon et al., 2002). The loss of touch-provoked movement that is generally used to manually score death in worms is commonly preceded by a decline in spontaneous motility and this phenotype has been used to assess the health of aging nematodes (Hosono et al., 1980; Huang et al., 2004). Wild-type and untreated worms tend to cease unprompted movements by 16–18 days of age and lose their characteristic sinusoidal posture. Therefore, worms that can be detected moving at 20–25 days of age are likely to have an altered rate of aging. A number of automated methods already exist for determining motility and thus viability could be assessed using image-analysis techniques (Hertweck & Baumeister, 2005). The accumulation of fluorescent pigment is also associated with increased age in C. elegans (Klass, 1977; Herndon et al., 2002) and correlates with lifespan in a variety of mutants (Hosokawa et al., 1994; Hsu et al., 2003; Gerstbrein et al., 2005). This raises the possibility that automated fluorescence detection could be used to screen for compounds that delay the age-related rise in auto-fluorescence signal. The utility of biomarkers as a means of accelerating survival analysis has recently been demonstrated in flies (Bauer et al., 2004). By coupling a lethal toxin to an age-dependent biomarker that is expressed in young adults, the lifespans of control strains were reduced to 20% of normal. Subsequent interventions, either genetic or pharmacological, that slowed aging delayed the expression of the toxic transgene and therefore provided an accelerated screening system.

Many of the methods outlined above lend themselves to low-to-moderate throughput screens of compounds. In order to achieve true high-throughput, the end point of the assay should be amenable to automated detection. The phenotypes that are scored in nematodes in relation to aging and life history are often subtle changes that are easy for a trained investigator to score but may present challenges for automation. In addition, consideration must be paid to the robustness of the assay in terms of within- and between-experiment variability (Zhang et al., 1999). This is an issue for all assays but the use of whole animal populations introduces a level of variability that is not present in other HTS approaches. Even within an isogenic organism such as C. elegans under fixed environmental conditions there is considerable variation in the time of death for a population, arising from external factors such as temperature and nutrition, as well as maternal effects (Rea et al., 2005). Thus, in developing HTS methodologies for nematodes, consideration must be paid to the intrinsic variability of the score, the magnitude of the response and the ease with which it can be detected by automated systems.

In addition to the environmental factors that can affect lifespan there are a number of other ways in which a drug treatment in worms may lead to lifespan changes that may be unrelated to the expected target of the drug. For instance, any compound that diminishes the feeding rate of the worm is likely to extend lifespan through caloric restriction. A second mechanism that may lead to the identification of nonspecific compounds is through hormesis, whereby a sublethal stressful event can lead to improved survival when the organism is subsequently challenged by a lethal stress (Lithgow, 2001). Thus, exposure to a sublethal stress during early life may lead to increases in mean and maximum lifespan in C. elegans (Cypser & Johnson, 2002). In this way a compound that has low-level toxicity in the worm may induce a stress response, leading to increased longevity. While compounds that activate the stress response regulatory pathways directly may be of interest, it may be difficult to separate these compounds from those that induce a nonspecific toxic response. Fortunately one of the strengths of the nematode system is that a number of life history traits or genetic markers can be used in secondary screens to rule out nonspecific mechanisms. For instance a reduction in fertility or slowed development is usually a good indicator of a sublethal stress or caloric restriction. Transgenic green fluorescent protein (GFP) strains can be used to determine the mode of action of a particular compound. In this respect worms expressing GFP downstream of the small heat shock protein-16 (hsp-16) promoter have proven useful in ruling out toxic effects that induce a stress response (Link et al., 1999; Sampayo et al., 2003).

A limitation of C. elegans stems from our lack of knowledge regarding pharmacokinetics in the worm. Treatment of worms with drugs often requires doses an order of magnitude higher than in cell-based systems, for example. Compounds that are deemed noxious by the worm will be avoided if possible and may induce developmental arrest. Thus a lack of effect may simply be due to lack of uptake. Even if a compound is ingested by the worm it is difficult to know how much has been taken up, how much reaches the target cells, and how fast it is metabolized or modified thereafter. Some researchers have made attempts to directly measure compounds in vivo or indirectly by measuring a biochemical activity (Davies et al., 2003; Keaney et al., 2004; Evason et al., 2005) but this approach presents its own set of challenges. Under standard culture conditions, E. coli is used as the food source for C. elegans and it is possible that compounds will be metabolized or modified in some way by the bacteria. One way round this problem would be to use chemically defined axenic media. However, this introduces an additional set of considerations, not least of which is the dramatic alteration in life history traits in axenically cultured animals (Houthoofd et al., 2003; Szewczyk et al., 2003).

Despite the considerations outlined above, there remains a great deal of interest in the prospects of using C. elegans as part of the drug discovery process. There are many advantages to the use of whole organism model systems over cell culture systems (Carroll et al., 2003). Mutant screens and more recently RNAi and microarray have been invaluable in identifying genes that affect the aging process. Indeed the development of feeding-based RNAi strategies has facilitated comprehensive genome-wide surveys for genes that influence lifespan, identifying an additional 100 genes with this role (Hamilton et al., 2005; Hansen et al., 2005). However, not all genes involved in the aging process will be identified by these approaches particularly those that have deleterious effects during development when deleted. Furthermore, RNAi screens are limited by the fact that gene inactivation in the nervous system is not always effective (Kennedy et al., 2004), the RNAi libraries do not provide full genome coverage and many of the constructs present in the library are not optimal in their effect. A major advantage of drug screens in C. elegans is the ability to exploit the wealth of genetic information and techniques such as RNAi and microarray. Lead compounds from screens can be used in RNAi screens to identify the genes that are required for the drug effect and microarray can be used to probe the downstream effects on target genes.

Endocrine targets for pharmacological intervention

  1. Top of page
  2. Summary
  3. The endocrinology of lifespan in worms
  4. Conservation of aging pathways between worms, flies and mammals
  5. Pharmacological interventions in lifespan
  6. Prospects for high throughput screens (HTS) for aging in C. elegans
  7. Endocrine targets for pharmacological intervention
  8. Conclusions
  9. References

So what are the prospects for targeting endocrine pathways in C. elegans? There are a number of endocrine targets that could be manipulated, some of which would be more favorable than others as drug targets. The insulin-signaling pathway has long been a target for drug discovery in combating metabolic disease and some of these compounds have beneficial effects on the nematode (Anisimov et al., 2003). It has been suggested that new therapeutics for treatment of metabolic disease may now be discovered by targeting molecules that have been identified through their effects on lifespan (Curtis et al., 2005). The secondary lipophilic hormones and their nuclear receptors are particularly attractive targets for drug discovery in C. elegans. Nuclear receptors act as ligand gated transcription factors that can either activate or repress gene expression (Gronemeyer et al., 2004) and have proven excellent drug targets in humans. Selective nuclear receptor modulators (SNuRMs) which are able to elicit a specific biological response at the expense of the other activities that the natural ligand may induce (Gronemeyer et al., 2004) have been described for the estrogen receptor (Robertson, 2004) and the glucocorticoid receptor (Rosen & Miner, 2005) among others. The nematode has undergone a dramatic expansion in the NHR superfamily with 284 predicted receptors compared with 48 in humans (Sluder et al., 1999). Only a small fraction of the worm NHRs (15/284) are members of conserved subfamilies found in mammals, of which DAF-12 is one (Sluder & Maina, 2001). There is still a great deal of work to be done in defining the physiological role of the other 269 C. elegans NHRs which form part of an expanded hepatocyte nuclear receptor 4 (HNF4) family and are divergent from mammalian receptors (Robinson-Rechavi et al., 2005). With respect to aging DAF-12 remains the best characterized NHR in worms but RNAi screens have recently identified two other NHRs that when inactivated extend lifespan (Hamilton et al., 2005). In addition, a number of genes thought to be involved in lipophilic hormone transport and synthesis have also been identified (Hamilton et al., 2005; Hansen et al., 2005). Interestingly, the nematode receptor NHR-49 appears to be the functional equivalent of the peroxisome proliferator-activated receptors (PPAR) acting to control fat metabolism in the worm (Van Gilst et al., 2005). This study suggests that despite an apparent lack of sequence homology the functions of the nematode receptors may be conserved across species. These data suggest that compound screens targeting NHRs and their ligands in worms may be useful in identifying other novel regulators of the aging process. A useful starting point for such screens may be to screen existing compound libraries that contain ligands for mammalian NHRs, both natural and synthetic, as well as to identify the endogenous NHR ligands in nematodes which would then allow rational design of synthetic agonists and antagonists.

Conclusions

  1. Top of page
  2. Summary
  3. The endocrinology of lifespan in worms
  4. Conservation of aging pathways between worms, flies and mammals
  5. Pharmacological interventions in lifespan
  6. Prospects for high throughput screens (HTS) for aging in C. elegans
  7. Endocrine targets for pharmacological intervention
  8. Conclusions
  9. References

Invertebrate model systems have proved themselves over the last few years in defining the genetic landscape of aging. The door is now open for these systems, particularly C. elegans, to fulfill their potential in identifying pharmacological agents that slow the aging process. The worm has many advantages over in vitro and cell-based assays for drug screening but there remain a number of obstacles to truly high throughput screens in this organism. Once these hurdles are overcome it is likely that a range of compounds that target endocrine pathways involved in aging will be identified for use in ameliorating age-related disease.

References

  1. Top of page
  2. Summary
  3. The endocrinology of lifespan in worms
  4. Conservation of aging pathways between worms, flies and mammals
  5. Pharmacological interventions in lifespan
  6. Prospects for high throughput screens (HTS) for aging in C. elegans
  7. Endocrine targets for pharmacological intervention
  8. Conclusions
  9. References
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