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Abstract

Regulation and coordination of lipid metabolism involve complex interactions between the feeding regulatory centres in the nervous system and the regulated uptake, intracellular transport, storage, and utilization of stored lipids. As energy is essential to all cellular processes, it is thought that complex networks have evolved to ensure survival by maintaining adequate energy reservoirs. However, in times of nutrient abundance and imbalance, improper regulation and coordination of these networks can lead to obesity and other metabolic diseases and syndromes. Obesity genes must be considered as molecular components of such networks which function at an organismal level to orchestrate energy intake and expenditure. Thus, the functions of obesity genes must be understood within the context of these networks in intact animals. Since the majority of genes required for lipid homeostasis are evolutionarily conserved, much information can be obtained relevant to complex organisms by studying simple eukaryotes like C. elegans. Its genetic tractability makes C. elegans a highly attractive platform for identifying lipid regulatory pathways, drugs, and their molecular targets which ultimately will help us to understand the origin of metabolic diseases such as obesity and diabetes. Here we briefly present some central aspects of lipid accumulation in C. elegans and discuss its merits as a platform for identification and development of novel bioactive compounds regulating lipid storage.

Keywords

lipid accumulation neuro-endocrine signaling transcriptional regulation drug screening

Introduction

An organism's ability to regulate the production, storage, and release of energy is vital for health and survival. A major source of energy is stored as lipids, which are essential for the life cycle of many organisms. Thus, deficiencies in lipid metabolism and accumulation can result in various pathological states such as cardiomyopathies, atherosclerosis, obesity, non-insulin dependent diabetes mellitus, and certain cancer forms. The worldwide increase in the prevalence of obesity has intensified the search to identify genes that control the development, differentiation, and function of fat-storing tissues. While key regulators of these pathways have been identified by biochemical and molecular approaches using cell culture systems and animal models, a number of invertebrate genetic model systems including nematodes and fruit flies have accelerated the discovery of new genes important to maintain lipid homeostasis. Despite its apparent simplicity, the nematode worm C. elegans has become an important model for biomedical research, particularly in the functional characterization of novel gene functions and drug targets that have been identified using proteomic and genomic technologies. The cellular complexity and the shared heritage of disease pathways of C. elegans and higher organisms, together with the simplicity and cost-effectiveness of cultivation, has placed this organism at center stage as an effective in vivo model that is amenable to whole-organism high throughput screens and large-scale target validation.

Biogenesis of Lipid Storage Granules in C. elegans

Contrary to mammals, which store lipids in droplets in dedicated adipocytes, lipids are primarily stored as triacylglycerols (TAGs) in droplets in gut granules and in hypodermal cells in C. elegans (Ashrafi, 2007; Ashrafi et al. 2003). Nevertheless, the origin, form, and function of these storage granules are regulated by highly conserved nutrient sensing and signaling pathways. A hallmark study by Ashrafi et al. (Ashrafi et al. 2003) used the fluorescent vital dye Nile red to visualize fat droplets in living worms. Combining this assay with global RNAi screening, 305 genes were identified to cause reduced or distorted fat deposits when knocked down, while RNAi of 112 genes caused increased or enlarged fat droplets. Among the genes identified in this screen were genes that are required for gut granule acidification or biogenesis. For example, knock down of the five glo-genes pgp-2 (glo-5), apt-6, apt-7, glo-3, glo-4, and vps-16 led to decreased Nile red staining of intestinal cells (Ashrafi et al. 2003). Subsequently, Schroeder et al. (Schroeder et al. 2007) showed that functional loss of the ABC transporter pgp-2 does not affect total lipid levels, but results in mislocalization of Nile red-stained lipids into the embryonic intestinal lumen, as well as mislocalization of birefringent material into the intestinal lumen and lack of acidified intestinal compartments (Ashrafi, 2007; Schroeder et al. 2007). Analysis of worms carrying defects in genes required for lysosomal trafficking showed that apt-6 and apt-7, encoding the β3 and μ3 subunits of AP-3 adapter protein complex, respectively, are required for normal development of gut granules (Hermann et al. 2005). Additionally, glo-1, was found to associate with gut granules and to be required for lysosomal acidification (Hermann et al. 2005). Thus, proper lipid storage depends on normal biogenesis and function of intestinal granules.

Quantification and Examination of Lipid Stores

The genetic tractability and transparency of C. elegans have been instrumental for identifying genes required for lipid homeostasis in C. elegans by means of fluorescence microscopy using vital dyes like Nile red- and BODIPY-labelled fatty acids as probes. (Ashrafi et al. 2003; Brock et al. 2007; Forsythe et al. 2006; Greer et al. 2008; McKay et al. 2003; Nunes et al. 2005; Schroeder et al. 2007; Srinivasan et al. 2008; Watts and Browse, 2002; Yang et al. 2006). Either probes readily partition into neutral lipids and become strongly fluorescent (Greenspan et al. 1985; Listenberger and Brown, 2007), and thus stain lipid droplets in the intestinal cells in live animals. Contrary to these dyes, Sudan Black B is used to stain lipid stores in dead animals. Notably, quantitative image analyses of Sudan Black B or Nile red stained animals correlate well with direct quantification of intracellular triacylglyceroles by means of gas chromatography-mass spectrometry (Ashrafi et al. 2003; Watts and Browse, 2002) or high performance thin layer chromatography (Srinivasan et al. 2008). By virtue of the inherent vibrational properties of single and double C-C bonds and C-H chemical bonds in acyl chains, coherent anti-Stokes Raman scattering (CARS) microscopy was recently applied to visualize and monitor lipid stores in live C. elegans (Hellerer et al. 2007). Thus, this methodology is fully independent of probes and chemical labelling and provides information of not only the lipid composition and levels but also the packing of individual lipid droplets. Accordingly, the lipid volume in mutants of insulin/IGF-1 signaling (daf-2) and TGF-β signaling (daf-4) was found to be 1.4 and 2 times larger, respectively, than in wild type animals. This increase is caused by accumulation of small lipid droplets in hypodermal cells and accompanied by a shift from gel-ordered lipids to liquid-ordered lipids (Hellerer et al. 2007).

None of the described methods unravel the chemical composition of individual triacylglycerol species contained in the lipid droplets. Entchev et al. recently described that functional loss of LET-767, a major 3-ketoacyl-CoA reductase in C. elegans, results in decreased lipid stores and developmental arrest, which can be rescued by supplementation of exogenous odd-numbered fatty acids with monomethyl branched chains (Entchev et al. 2008). Subsequently, electrospray-mass spectrometry of extracted lipids showed that triacylglycerols from let-767 (RNAi) worms contain approximately 25% less fatty acids with an odd number of carbon atoms compared with triacylglycerols from the wild type animals.

Thus, proper selection and combination of these methods will provide detailed insights in to the genetics and molecular biochemistry of lipid synthesis, storage, and turn over.

Neuro-Endocrine Regulation of Lipid Accumulation

TUB-1 and BBS-1 function in neurons to regulate intestinal lipid storage

In 1990, Coleman and Eicher identified an autosomal recessive mutation for obesity in mouse, which they termed tubby (tub) (Coleman and Eicher, 1990). The phenotype observed in homozygous tub/tub mice is increased body weight at three to six months of age depending on sex and strain background. Although tubby mice develop insulin resistance as they gain weight, they do not progress to overt diabetes.

The function of TUBBY as a regulator of fat storage appears to be conserved, since functional loss of tubby called tub-1/F10B5.5 in C. elegans leads to accumulation of lipids (Ashrafi et al. 2003; Mak et al. 2006; Mukhopadhyay et al. 2005; Mukhopadhyay et al. 2007). In C. elegans, tub-1 is exclusively expressed in neurons, including the ciliated amphid neurons in the head region and ciliated phasmid neurons in the tail. Recently, it was shown that the lipid phenotype associated with functional loss of tub-1 is independent of DAF-16, a FOXO-family transcription factor, but dependent on RBG-3, a Rab GTPase-activating protein (Rab GAP), for the regulation of fat storage (Mukhopadhyay et al. 2005). As with TUB-1, RBG-3 was found to be expressed in the amphid neurons in the head as well as the phasmid neurons. RBG was found to stimulate the intrinsic GTPase activity of RAB-7 in C. elegans, which also was found to be required for normal fat storage in wild type and tub-1 mutants (Mukhopadhyay et al. 2007). The fact that the small GTPase RAB-5 and genes that regulate Rab membrane localization and nucleotide recycling are required for normal fat storage, suggests that endocytic recycling is involved in this process.

In order to identify novel pathways that confer increased lipid accumulation Mak et al. identified kat-1 to act in a synergistic manner with tub-1 (Mak et al. 2006). kat-1 encodes a β-oxidation enzyme, 3-ketoacyl-CoA thiolase, and was found to be expressed in intestine, body wall muscles, and in the pharynx. Despite the presence of a peroxisomal targeting motif in the C-terminus, KAT-1 was found only to localize to mitochondria in the intestine and muscle (Mak et al. 2006). Expression of kat-1 under the control of an intestine-specific promoter partly rescued the lipid storage phenotype of kat-1; tub-1 animals, while kat-1 expression in ciliated neurons was unable to reduce lipid storage. This suggests that kat-1 normally mediates intestinal fatty acid oxidation to compensate for defects conferred by a mutation in tub-1, but also, due to the distinct expression patterns of tub-1 and kat-1, that neuronal signals are involved in regulating intestinal lipid degradation and accumulation.

Screening for mutations that resulted in a synergistic increase of lipid accumulation in kat-1 mutants led to the identification of bbs-1, the nematode ortholog of the human bbs1 gene, which is mutated at high frequency in individuals with Bardet-Biedl syndrome (Goldstone and Beales, 2008; Tobin and Beales, 2007). The facts that bbs-1 also is expressed in ciliated neurons, and that loss of function of both bbs-1 and tub-1 do not result in further increase in lipid accumulation, imply that bbs-1 and tub-1 are functioning in pathway(s) that maintain lipid homeostasis in C. elegans.

Serotonergic control of fat metabolism

Serotonin, also known as 5-hydroxytryptamine (5-HT), is a monoamine neurotransmitter which is evolutionary conserved from C. elegans to plants and to humans. It is well known that defects in serotonin signaling in mammals are implicated in diabetes and cardiovascular diseases along with a wide variety of neuronal diseases such as Parkinson's disease, schizophrenia, and behavioural disorders including bulimia, obsessive-compulsive disorder, and depression (Gershon and Tack, 2007; Kitson, 2007). Serotonin is synthesized from the amino acid tryptophan in a pathway that involves tryptophan hydroxylase (TPH) and the aromatic L-amino acid decarboxylase (BAS-1), where TPH catalyzes the first and rate-limiting step. In mammals, TPH exists in two isoforms, TPH2, which is solely found in the brain, and TPH1 which is less tissue specific (Gershon and Tack, 2007). The only ortholog of mammalian TPH in C. elegans is TPH-1, which shares 61% amino acid identity with mammalian TPH in the 330-amino acid catalytic domain (Sze et al. 2000). Besides sensory neurons innervating the male tail, serotonin synthesis is confined to ADFs; a set of chemosensory neurons in the lateral ganglia in the head, NSMs; a set of neurosecretory motor neurons in the pharyngeal region, and HSNs; a set of hermaphrodite-specific motor neurons that innervate vulva musculature (Sze et al. 2000). Four serotonin receptors have been identified in C. elegans; SER-1, SER-4, and SER-7, and all belong to the G-protein-coupled receptor superfamily (GPCR), resembling 5-HT₂, 5-HT₁, and 5-HT₇ of the mammalian receptors, respectively. The last receptor found in C. elegans is a ligand-gated chloride channel, MOD-1, that to a certain extent resembles the 5-HT class 3 of the mammalian receptors (Carre-Pierrat et al. 2006; Dernovici et al. 2007; Hobson et al. 2003; Ranganathan et al. 2000).

C. elegans accommodates its feeding rate and overall metabolic state to food availability. Lack of food will thus lead to a metabolic shift toward accumulation of fat along with a reduction in feeding rate. This phenotype is also obtained in animals defective in serotonin biosynthesis. Conversely, increased serotonin signaling leads to reduced fat accumulation and increased feeding rate in C. elegans, as well as in mammals (Ashrafi et al. 2003; Srinivasan et al. 2008; Sze et al. 2000).

SER-1 and SER-7 are required for the increase in pharyngeal pumping and egg-laying in response to serotonin, whereas SER-4 is only involved in 5-HT-stimulated egg-laying (Hobson et al. 2006; Niacaris and Avery, 2003; Tsalik and Hobert, 2003). Interestingly, Srinivasan et al. recently found that worms carrying mutations in these receptors do not exhibit alterations in lipid accumulation but reduce their fat content in response to exogenous 5-HT, while serotonergic regulation of lipid storage involves mod-1 along with tub-1 (Srinivasan et al. 2008). Although mod-1 mutants accumulate triacylglycerol and display a partial suppression of the fat reduction associated with increased serotonin signaling, these animals enhance their feeding rate in response to 5-HT to the same extent as wild type animals. This implies that regulation of feeding rate and lipid accumulation by serotonin are molecularly separated. The lipid-reducing effect of serotonin was abolished by knock down of genes encoding intestinal and hypodermal components of peripheral fatty acid oxidation, suggesting that neuronal serotonergic signaling promotes catabolic consumption of lipids via peripheral fatty acid oxidation pathways (Srinivasan et al. 2008).

DAF-2 and DAF-7, components of two central signaling pathways regulating intestinal lipid storage

During conditions with scarce food availability, high population density, and high temperature, early larval-stage worms arrest development and enter a state of developmental diapause, called dauer, characterized by increased intestinal lipid stores. Large-scale genetic screens have identified that insulin-like-, TGF-β-related- as well as cyclic nucleotide signaling play key roles in regulating dauer development (Albert et al. 1981; Albert and Riddle, 1988; Ren et al. 1996; Riddle et al. 1981).

The C. elegans insulin/insulin-likegrowth factor 1 receptor homolog DAF-2 is one of the principal components affecting lifespan, reproduction, lipid metabolism, and entry into a state of developmental diapause, called the dauer larva (Dorman et al. 1995; Gami and Wolkow, 2006; Iser et al. 2007; Libina et al. 2003; Lin et al. 2001; Murphy et al. 2003). DAF-2 signals through a highly conserved phosphatidylinositol-3-OH kinase (encoded by the age-1 gene in worms) pathway to negatively regulate the activity of the forkhead transcription factor DAF-16 by inducing its phosphorylation and nuclear exclusion and thus affecting expression of genes involved in stress responses, longevity, antimicrobials, steroid synthesis, lipid synthesis, apolipoproteins, protein- and peptide degradation, and signaling (Baumeister et al. 2006; Kenyon and Murphy, 2006; Mukhopadhyay et al. 2006; Pinkston-Gosse and Kenyon, 2007). daf-2 and age-1 mutants are characterized by developmental arrest at the dauer larval stage and a shift in metabolism towards lipid storage (Kimura et al. 1997; Larsen et al. 1995; Wolkow et al. 2000). DAF-2–-mediated signaling is present in neurons, intestinal cells, and muscles. Ectopic expression of daf-2 and age-1 in neurons restored lifespan, reproduction, and partially lipid deposition of daf-2 and age-1 mutant animals, respectively, while intestinal-specific expression was unable to complement the metabolic defects in daf-2 and age-1 animals. Hence, despite the fact that the majority of lipid is stored in the intestinal cells, insulin-like signaling in neurons and not in intestinal cells, is considered as the key regulator lipid storage in C. elegans (Wolkow et al. 2000).

DAF-7, a TGF-β-related ligand, signals through a highly conserved signaling pathway involving the two R-Smads, DAF-8 and DAF-14, which transduce the DAF-7 signal to inhibit the activity of the Co-Smad DAF-3, a transcriptional regulator that is required for formation of the alternative dauer larval stage. daf-7 is expressed specifically in a specific set of sensory neurons termed ASI (Ren et al. 1996). Reduced daf-7 expression as a result of limited access to food or loss of DAF-7 signaling, even under nutritionally favourable conditions, results in constitutive-dauer formation (Ren et al. 1996; Riddle et al. 1981). As food availability increases, daf-7 is re-expressed and animals resume feeding and reproductive growth. Ashrafi and co-workers have recently shown that DAF-7 signals from the ASI neurons are conveyed to the interneurons RIM and RIC, to regulate growth, feeding, and fat storage (Greer et al. 2008). They elegantly showed that following activation of DAF-3 in RIM and RIC interneurons, tyramine and octopamine (worm catecholamines) cause feeding reduction by activating SER-2, a G-protein coupled receptor, in a subset of pharyngeal neurons, while glutamatergic signaling through MGL-1 and MGL-3 metabotropic receptors regulates lipid storage by increasing peripheral lipid synthesis.

Transcriptional Regulation of Lipid Accumulation

The mechanisms by which cells and organisms regulate uptake, biosynthesis, and oxidation of lipids to meet their nutritional and cellular needs involve a complex interplay of a large number of signaling components, metabolic enzymes, and transcription factors. In mammals, a number of transcription factors have been identified and shown to play pivotal roles in fatty acid- and lipid metabolism including peroxisome proliferator-activated receptors (PPARs), liver X receptors (LXRs), farnesoid X receptor, CCAAT/enhancer-binding proteins (C/EBPs), and sterol regulatory element-binding proteins (SREBPs) (Hansen and Connolly, 2008; Nakamura et al. 2004). Similarly, key transcription factors have been identified to orchestrate transcription of several genes, whose gene products are involved in either catabolism or anabolism of lipids in C. elegans.

In mammals, SREBP-1a and SREBP-1c proteins regulate genes involved in the de novo synthesis of monounsaturated and polyunsaturated fatty acids (PUFAs), and their subsequent incorporation into triglycerides and phospholipids, while SREBP-2 predominantly regulates genes involved in cholesterol biosynthesis (Espenshade and Hughes, 2007; Raghow et al. 2008). Since C. elegans expresses only one ortholog of SREBP called SBP-1 (McKay et al. 2003) and is auxotrophic for cholesterol, C. elegans provides an excellent model to study SREBP function in fatty acid and lipid metabolism on an organismal level. Accordingly, genes that have been shown to be direct SREBP targets in mammalian cells such as acyl-CoA carboxylase (acc-1), fatty acid synthase (fasn-1), stearoyl-CoA desaturases (fat-6/fat-7), fatty acid elongases (elo-5 and elo-6), glycerol 3-phosphate acyltransferase (G3PA), malic enzyme (ME), and ATP citrate-lyase (ACL) have all been shown to be regulated by SBP-1 in C. elegans (Kniazeva et al. 2004; McKay et al. 2003; Raghow et al. 2008; Yang et al. 2006). The shared ancestry of SREBP function is further underscored by the observation that the activation domain of SBP-1 was found to stimulate transcription in human cells and that SBP-1 interacts with human ARC105 and C. elegans MDT-15, a mediator of fatty acid metabolism (Yang et al. 2006).

Despite the presence of 284 nuclear hormone receptors in the C. elegans genome (Antebi, 2006), orthologs of mammalian PPARs have not been identified. However, despite its sequence homology to HNF4γ, NHR-49 is considered to be a functional homolog of the mammalian PPARα, as it regulates transcription of genes involved in fatty acid uptake, oxidation, desaturation, and lipid binding proteins (Van Gilst et al. 2005a; Van Gilst et al. 2005b). Besides NHR-49, knock down of NHR-8, NHR-137, NHR-178, or NHR-68 was found to negatively affect lipid levels, while loss of DAF-12, NHR-25, or NHR-140 increased lipid storage (Ashrafi et al. 2003). Based on additional studies, NHR-49 is now considered to be a central regulator of gene transcription in response to nutritional alterations, since transcriptional changes of lipid metabolic genes occurring in response to loss of nhr-49 and starvation overlap (Van Gilst et al. 2005b). Thus, NHR-49 may sense and directly respond to changes in specific intracellular lipid species in a fashion similar to mammalian PPARα (Hostetler et al. 2006; Hostetler et al. 2005; Schroeder et al. 2008).

The ARC/mediator subunit ARC105/MDT-15 has emerged as a central regulator of lipid homeostasis in both mammals and C. elegans due to its ability to interact with NHR-49 as well as SBP-1 (Taubert et al. 2006; Yang et al. 2006). Loss of mdt-15 in C. elegans was found to prevent transcription of NHR-49 target genes including the two δ9-desaturases fat-5 and fat-7, as well as NHR-49-independent but SBP-1-dependent expression of e.g. another δ9-desaturase fat-6, thus leading to gross alterations in the fatty acid composition of cellular lipids (Taubert et al. 2006; Yang et al. 2006). This, along with the ability to complex with SBP-1, places MDT-15 as a central mediator of transcriptional regulation of lipid metabolic genes.

Besides SBP-1 and NHR-49, the worm homolog of mammalian C/EBP, LPD-2, has also been identified as an important regulator of lipid homeostasis. Similar to knock down of sbp-1, loss of lpd-2 was found to negatively affect lipid accumulation as well as affecting the expression of several lipogenic enzymes such as acetyl-CoA carboxylase, fatty acid synthase, acyl-CoA synthetases, and glycerol 3-phosphate acyltransferase. Yet no effect on pumping rate and defecation rate was observed (McKay et al. 2003).

Metabolic Regulation of Lipid Storage

The neuro-endocrine circuits along with the transcriptional networks must ultimately confer metabolic fluxes in order to regulate synthesis, degradation, and accumulation of lipids in the intestinal- and hypodermal storage compartments. Consistently, in the past decade, genes that encode lipid biosynthetic and utilization enzymes, lipid transporters, enzymes of sterol metabolism, cytochrome P450s, and genes that regulate the flux of energy metabolism have been identified to negatively or positively affect intestinal lipid accumulation (Ashrafi, 2007; Ashrafi et al. 2003). Notably, despite the presence of multiple ortholog genes in the C. elegans genome, it is only knock down of specific isoforms of lipid binding proteins, carnitine palmitoyl transferases, or ABC transporters which affect lipid storage; e.g. out of seven isoforms of acyl-CoA binding protein (ACBP) only ACBP-1 appears to be required to maintain wild type lipid levels (Ashrafi et al. 2003), and among the six isoforms of CPT-1, only knock down of cpt-1a affects lipid storage (Srinivasan et al. 2008). Such specificities are likely to originate from differences in tissue- and/or temporal expression.

HMG-CoA reductase, catalyzing the rate limiting step in cholesterol synthesis, was also identified to be required for normal lipid storage. As worms are cholesterol auxotrophs, it is interesting to speculate that intermediates in cholesterol synthesis, e.g. geranylgeranyl pyrophosphate and farnesyl pyrophosphate for modifications of signaling components, may be important for lipid accumulation.

Chemical Genomics in the Identification of Novel Proteins and Pathways Required for Lipid Accumulation

As described above, regulation and coordination of lipid metabolism involve a complex interplay between the feeding regulatory centres in the nervous system and the regulated uptake, intracellular transport, storage, and utilization of stored lipid(s). Since the majority of genes required for lipid homeostasis are evolutionarily conserved, and genes discovered in C. elegans point to candidate obesity or diabetes loci identified in human and rodent studies, much information can be obtained relevant to complex organisms by studying simple eukaryotes like C. elegans. Therefore, C. elegans provides a highly attractive platform for identifying drugs and their molecular targets that may cure or improve and help us understand the origin of such metabolic diseases.

Chemical genetics combines chemistry with genetic screens as a means of exploring the function of unknown proteins or identifying the proteins responsible for a particular phenotype induced by a small cell-permeable bioactive molecule. Classical forward genetic screens have generated an enormous amount of important detailed knowledge of C. elegans biology. In a traditional forward screen, C. elegans is randomly mutagenized followed by scoring and isolation of relevant phenotypes. Subsequently, mutated genes giving rise to a particular phenotype can readily be identified by SNP-based mapping strategies (Davis and Hammarlund, 2006; Wicks et al. 2001). In a chemical genetic screen, chemical compounds or biomolecules are used to exert an appropriate phenotypic change. Subsequently, worms are mutagenized and mutants showing either increased resistance or sensitivity towards a given compound are isolated for further analysis. Such forward genetic screens combined with chemical screens have led to the identification of Rab geranylgeranyl transferase as a proapoptotic target of farnesyl transferase inhibitors (Lackner et al. 2005), G-protein coupled receptor signaling as a target of compounds active in models of urinary incontinence (Fitzgerald et al. 2006), and several molecular targets of the antidepressant selective serotonin reuptake inhibitor Fluoxetine (Choy et al. 2006; Dempsey et al. 2005; Ranganathan et al. 2001). In this light, it is interesting that Watts and Browse have identified genes involved in synthesis of polyunsaturated fatty acids in C. elegans by chemical mutagenesis screens in combination with gas chromatography analysis of the fatty acid composition (Watts and Browse, 2002).

In mammals as well as worms, insulin signaling is the primary mechanism for controlling metabolic physiology (Ashrafi et al. 2003; Kimura et al. 1997). Accordingly, mutations in the insulin receptor can cause severe insulin resistance (Leroith and Accili 2008). Therefore, worms carrying a temperature-sensitive mutation in the worm ortholog of the insulin receptor, daf-2, have served as a model of insulin resistance (Hanover et al. 2005; Min et al. 2007). As mentioned above, the DAF-2 signaling pathway coordinates food uptake, metabolism, growth, and life span by regulating age-1, an ortholog of mammalian phosphatidylinositol-3-OH kinase (Paradis et al. 1999; Paradis and Ruvkun, 1998). By screening a tagged chemical library, Min et al. identified one compound (GAPDS) which was able to overcome the dauerforming phenotype of daf-2 mutants (Min et al. 2007). Using affinity chromatography combined with RNA interference, it was found that glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a target of GAPDS, and that GAPDH stimulates the turn-over of phosphatidylinositol (3,4,5)P3 and phosphatidylinositol (4,5)P2.

The overall goal in treating obesity is to reduce body weight and maintain the lower weight. A variety of medications are available which promote weight loss, but none of them cure obesity. This usually involves a combination of medication, diet, exercise, and other changes in lifestyle. Given that the prevalence of overweight and obesity is increasing at an alarming rate in both developing and developed countries worldwide, there is a still increasing demand for better treatments of such disorders. Thus, it is somewhat surprising, that a chemical genetic screen for the identification of compounds affecting lipid accumulation is yet to be described. Given the straightforwardness of scoring lipid accumulation in live animals by means of various fluorescent probes and fluorescence microscopy (Ashrafi et al. 2003), C. elegans should serve as an attractive model for such screens.

Footnotes

Disclosure

The authors report no conflicts of interest.

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C. elegans : A Model for Understanding Lipid Accumulation

Abstract

Keywords

Introduction

Biogenesis of Lipid Storage Granules in C. elegans

Quantification and Examination of Lipid Stores

Neuro-Endocrine Regulation of Lipid Accumulation

TUB-1 and BBS-1 function in neurons to regulate intestinal lipid storage

Serotonergic control of fat metabolism

DAF-2 and DAF-7, components of two central signaling pathways regulating intestinal lipid storage

Transcriptional Regulation of Lipid Accumulation

Metabolic Regulation of Lipid Storage

Chemical Genomics in the Identification of Novel Proteins and Pathways Required for Lipid Accumulation

Footnotes

Disclosure

References