Sci. Aging Knowl. Environ., 21 May 2003
Vol. 2003, Issue 20, p. pe12
[DOI: 10.1126/sageke.2003.20.pe12]

PERSPECTIVES

Meeting Report--44th Annual Drosophila Research Conference

Jim Cypser

The author is in the Department of Ecology and Evolutionary Biology, Brown University, Providence, RI 02912, USA. E-mail: James_Cypser{at}brown.edu

http://sageke.sciencemag.org/cgi/content/full/sageke;2003/20/pe12

Key Words: caloric restriction • Drosophila • insulin signaling • microarray analysis • endocrinology

Introduction

Drosophila melanogaster is a rising star among model organisms used to study the process of aging (see Warner Subfield History). In spite of the evolutionary distance between flies and mammals, it is becoming clear that common mechanisms influence the rate of aging in these diverse species. The fly is well suited for testing fundamental questions regarding the influence of caloric restriction (CR) (see Masoro Subfield History) and hormones on the rate of aging, and techniques for tissue- and time-specific experimental approaches are available for this organism. The 44th Annual Drosophila Research Conference (held in March 2003 in Chicago) saw the emergence of the fly as a model for studying these aspects of the biology of aging (Fig. 1). Presentations and posters at the meeting also highlighted innovative uses of the molecular genetic and genomic tools available to fly researchers.



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Fig. 1. The fly emerges as a model of aging. [Credit: C. Slayden]

 
The Insulin/Insulin-Like Growth Factor (IGF) Signaling Pathway

The first individual genes shown to mediate life span in any organism were age-1 and daf-2 in the worm Caenorhabditis elegans (see Johnson Subfield History). These genes were later shown to encode homologs of a phosphatidylinositol-3-OH kinase (PI3K) critical to the insulin response pathway and the mammalian insulin receptor, respectively. In the intervening years, approximately half of all new gerontogenes (genes that when mutated result in life span extension) have been shown to relate to the insulin/IGF pathway. This pathway mediates the individual's response to environmental cues (notably, but not exclusively, nutrition), and the study of its role in aging has been extended to yeast, flies, and mice (see "One for All"). At this meeting, findings related to the insulin-like pathway continued to dominate new results in aging-related research, both directly and indirectly.

One such finding, presented by Missirlis et al., concerned the Slimfast gene. Selectively inhibiting the expression of Slimfast (an amino acid transporter) in the fly's fat body (a lipid storage tissue in the abdomen that serves as the functional equivalent of the mammalian liver) resulted in growth restriction of the entire body; the effect of this mutation on life span has not yet been tested, but growth and life span have been shown to be inversely correlated in certain instances. More strikingly, this tissue-specific effect was dependent on elements of the insulin-signaling pathway, including PI3K and Tor (target of rapamycin), which is a protein kinase that operates downstream of PI3K. Although Tor has multiple roles, it is most notably involved in sensing amino acid availability (1). The PI3K gene encodes a homolog of the age-1 gerontogene in worms. Thus, tissue specificity continued to emerge as a theme for pathways that mediate aging.

The lab of Ernst Hafen (University of Zurich) presented a poster providing evidence that a newly identified player in the insulin signaling pathway also affects life span. Honegger, Hafen et al. reported that Imp-L2 is a negative regulator of the insulin pathway. Flies that overexpress InR (insulin receptor) in the eyes display a "bulging eye" phenotype (overgrowth of the eyes), but when the flies also overexpress Imp-L2, the eye overgrowth is suppressed. The life span of female Imp-L2 mutant flies is reduced by 31% as compared to that of wild-type females, but the life span of male mutants is normal. Imp-L2 is a secreted member of the immunoglobulin (Ig) superfamily, and its expression can be induced by 20-hydroxy-ecdysone, an insect steroid hormone that is critical for normal development but has also recently been shown to mediate adult aging (2). Interestingly, Imp-L2 is expressed in several sets of neurosecretory cells, including 14 neurons that are the major source of dilps (Drosophila insulin-like peptides), suggesting that Imp-L2 antagonizes dilp function. One or more of the dilp gene products is likely to be the agonist of the insulin receptor of the fly and hence to play a central role in aging. Imp-L2 is also expressed in the corpora cardiaca, a tissue that serves as the functional homolog of the hypothalamus and pituitary in the fly. One of the dilp proteins (Dilp-2) secreted by the neurosecretory cells is directed to the corpora cardiaca (3). Adipokinetic hormone (AKH) is synthesized in the corpora cardiaca and has also been shown to affect the fly's immune response. This expression pattern is consistent with a model in which Imp-L2 plays a role in modulating metabolism (via InR) and the immune response (via AKH). This research is important because it might provide an avenue for investigating the relation between longevity and immunity.

Role of Nutrition

Speakers who introduce new methods that generate new data are always welcome. One such innovative talk, which provided new information about the effects of CR, was delivered by Marc Tatar of Brown University. Tatar presented results that were obtained using a powerful new experimental approach to large-scale gene expression (microarray) analysis. The approach combined three experimental steps with a novel statistical approach. First, the flies were physiologically and metabolically restricted by depriving them of one component of their diet (yeast) during the third-instar larval stage and for the first 3 days of adulthood. Then an experimental group was fed yeast, removing the restriction. The release of the restriction was followed by a closely timed series of samplings of RNA from control and experimental flies. Hybridization of the RNA from the two groups to microarrays representing the fly genome revealed the cascade of gene expression that occurred in response to the yeast refeeding. Tatar pointed out that this approach has the power to reveal groups of genes involved in a particular process and also has increased sensitivity, because the inclusion of time as a parameter in the statistical analysis enables the detection of significant changes in expression as small as 10%. Because the method detects which sets of genes change expression in a time-ordered fashion, and increasing numbers of genes are seen to change expression as time progresses, it also provides suggestions regarding the form of the regulatory cascade evoked in response to yeast refeeding.

Meng-Ping Tu of Brown University also examined the effects of altering the nutritional intake of flies. There is a well-established correlation between reduced body size and increased life span for strains of yeast, flies, mice, and dogs (see "The Shrimps Shall Inherit the Earth"). This relationship naturally provokes the question of whether small body size is itself causal of life extension within a species. Tu provided evidence in her talk that, in fact, small body size is not sufficient to extend life span, at least as modulated by nutrition. By restricting specific dietary components during the third larval stage, she was able to produce adult flies that, in some respects, were phenocopies of mutants with reduced insulin signaling. The flies that experienced yeast deprivation during the third-instar larval stage were like the chico mutant (in which the gene encoding an insulin receptor substrate protein is affected) and the InR mutant, in that they were dwarf and displayed reduced fertility. However, unlike the mutants, the flies studied by Tu displayed mortality rates virtually identical to those of controls and did not exhibit life span extension. Thus, reduced body size and life span extension are separable, at least in response to nutritional deprivation in flies.

An Emerging Central Role for sir2?

The connection between CR and the sir2 gene was elaborated by P. Kapahi of the California Institute of Technology. Overexpression of SIR2 has been shown to extend life span in yeast and worms, and SIR2 is required for the life span extension that occurs when yeast undergo CR (see Kaeberlein Perspective and "High-Octane Endurance--Yeast in the Metabolic Fast Lane Live Longer"). Now, Kapahi has shown that overexpression of the Drosophila sir2 homolog protects against the life-shortening effects of overfeeding. When flies are fed a diet consisting of 5% yeast, they have shorter life spans than do those fed a normal diet (2% yeast). Overexpression of sir2 improves the survival of the overfed flies by 40%. Kapahi also presented evidence from microarray studies that indicates overlap among genes whose transcription is altered by CR and those altered by overexpression of sir2.

New Gerontogenes

Mutations in several new genes were reported to extend life span. The lab of Michael Grotewiel (Michigan State University) presented results showing that female beta-integrin mutants (myospheroid) have significantly extended life span as compared to controls. Although beta integrins are reported to regulate the binding of IGF-1 to its receptor in mammals, the mutants had normal body size, suggesting that altered growth was not the cause of the life extension. Curiously, the long-lived myospheroid mutants displayed normal rates of behavioral aging--in other words, they lose their natural inclination to crawl upward at the same rate as flies with normal life span. This is exceptional because most long-lived mutants discovered to date (in the fly as well as in other model organisms) display delayed decline of vigor as well as delayed mortality.

Models of Human Age-Related Disease

The lab of Nancy Bonini (University of Pennsylvania/Howard Hughes Medical Institute) was a powerhouse that generated many interesting results. No less than six presentations bearing Bonini's name dealt with fly models of four different human age-related diseases: familial amyotropic lateral sclerosis (FALS), Huntington's disease (HD), Machado-Joseph disease (MJD), and Parkinson's Disease. These presentations established the fly as a model of FALS, dealt with the molecular causes of neurodegeneration observed in MJD, examined the role of axonal transport in HD, and suggested a pharmacological treatment (geldanamycin) for Parkinson's disease.

Mitochondrial Function and Aging

A poster presented by Elaine Reynolds et al. of Lafayette College lent further support to the idea that mitochondrial function plays a critical role in aging. Defective mitochondrial function would be expected to accelerate aging [although down-regulation of certain genes involved in mitochondrial function in C. elegans has been shown to result in increased life span (4) (see Melov Perspective)]. Bang-sensitive mutants, which display seizure followed by paralysis when subjected to mechanical shock, were shown to be short-lived. Two such mutants, tko and ses B, are likely to be defective for mitochondrial function; these mutants were also found to exhibit neurodegeneration with increasing age. Nonetheless, it is possible that the tko and ses B mutants are just sick; that is, the mutations might cause novel pathologies unrelated to aging and thereby result in shortened life span. The mutants might be unable to obtain and utilize energy fully because of defects in mitochondrial function.

Endocrinology

The ecdysone workshop drew special attention from the crowd interested in aging-related research, in light of the recent publication by Simon et al. (2) regarding the role of the steroid hormone ecdysone in aging (see Tatar Perspective). Ecdysone thus joins juvenile hormone (JH) as a hormone that influences insect aging. However, results regarding JH presented at the workshop captured more attention. chico and InR mutants display reduced JH levels, suggesting that this hormone mediates the rate of aging in flies. However, the identity of the receptor for JH had not been well established. Grace and Davy Jones from the University of Kentucky jointly presented data neatly establishing that the ultraspiracle gene encodes the JH receptor. utraspiracle had previously been classed as an orphan receptor. Jones and Jones were able to establish that JH binds Ultraspiracle and promotes its activation, although the receptor is constitutively active at a low level. Given the role of JH in mediating life span, a good deal of attention will likely be focused on the genes transcribed in response to Ultraspiracle activation by JH.

Techniques and Collaborations

The meeting also provided opportunities for consultation regarding improvements in techniques. Sean McGuire of Baylor University presented a useful elaboration of the GAL4/UAS system. This system employs a transcription factor from yeast (Gal4) whose expression can be driven in Drosophila by a promoter specific to the tissue of choice. Gal4 in turn binds to an upstream activating sequence (appropriately placed upstream of a gene of interest) to promote expression of that gene solely in that tissue. The system described by McGuire makes use of a construct bearing a temperature-sensitive GAL80 allele. Because Gal80 inhibits Gal4, use of the conditional GAL80 allele permits the expression of transgenes in a time- and tissue-specific manner in the fly, thus bypassing the problem of developmental confounds.

Further refinements of existing transgenic techniques were discussed informally among participants over meals or in the lounges of the hotel. Mike Stebbins (formerly at Cold Spring Harbor, currently at Nature Genetics) was able to enlighten other participants regarding some of the subtler details of a doxycyclin-inducible transgene system he improved. Any researcher trying to get a molecular biological system to work will find that there is no substitute for talking to someone who has already used and perfected it. Techniques such as the temperature-inducible GAL80 system and adaptations of the doxycycline-inducible Tet-On gene expression systems will be invaluable for removing developmental confounds from the study of aging-associated phenotypes.

Conclusions

Researchers using various model organisms have collectively identified a set of genes that influences the rate of aging. However, it is becoming increasingly clear that environmental influences, especially diet, affect the expression of these genes. In addition, some genes that influence the rate of aging also influence other life history traits such as growth rate and fertility. The emerging challenge for researchers interested in aging is to discover how environmental influences on gene expression are parceled out among life history traits--in short, how and when genes that affect aging mediate tradeoffs among life history traits. Completing this picture will require analysis of the time- and tissue-specific roles of these genes.

The Drosophila Research Conference is typically a massive affair, because the fly has been developed as a successful model for studying so many biological processes. Biologists interested in development, behavior, genomics, cell signaling, and evolution are all represented at these meetings, and specialists in each of these areas have important new results to present. Within all of these specialties there are an increasing number of researchers glad to stand on the foundation laid by others and to use the fruit fly to study aging.


May 21, 2003
  1. B. Raught, A. C. Gingras, N. Sonenberg, The target of rapamycin (TOR) proteins. Proc. Natl. Acad. Sci. U.S.A. 98, 7037-7044 (2001). [Abstract/Free Full Text]
  2. A. F. Simon, C. Shih, A. Mack, S. Benzer, Steroid control of longevity in Drosophila melanogaster. Science 299, 1407-1410 (2003). [Abstract/Free Full Text]
  3. E. J. Rulifson, S. K. Kim, R. Nusse, Ablation of insulin-producing neurons in flies: growth and diabetic phenotypes. Science 296, 1118-1120 (2002).[Abstract/Free Full Text]
  4. S. S. Lee, R. Y. Lee, A. G. Fraser, R. S. Kamath, J. Ahringer, G. Ruvkun, A systematic RNAi screen identifies a critical role for mitochondria in C. elegans longevity. Nat. Genet. 33, 40-48 (2003).[CrossRef][Medline]
Citation: J. Cypser, Meeting Report--44th Annual Drosophila Research Conference. Sci. SAGE KE 2003, pe12 (21 May 2003)
http://sageke.sciencemag.org/cgi/content/full/sageke;2003/20/pe12








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