C elegans: a model for exploring the genetics of fat storage

Abstract

To gain insights into the genetic cascades that regulate fat biology, we evaluated C. elegans as an appropriate model organism. We generated worms that lack two transcription factors, SREBP and C/EBP, crucial for formation of mammalian fat. Worms deficient in either of these genes displayed a lipid-depleted phenotype-pale, skinny, larval-arrested worms that lack fat stores. On the basis of this phenotype, we used a reverse genetic screen to identify several additional genes that play a role in worm lipid storage. Two of the genes encode components of the mitochondrial respiratory chain (MRC). When the MRC was inhibited chemically in worms or in a mammalian adipocyte model, fat accumulation was markedly reduced. A third encodes lpd-3, whose homolog is also required for fat storage in a mammalian model. These data suggest that C. elegans is a genetically tractable model to study the mechanisms that underlie the biology of fat-storing tissues.

Figure 1. C. elegans Mobilize Fat Stores upon Starvation

(A) Larval and adult worms were grown on plates with food (well-fed) or on plates without food for 8 hr (starved) and then microscopically examined and photographed. Solid arrows indicate the dark gut granules present in the well-fed worms. Double arrows indicate the absence of these granules. (B) Well-fed and starved adult worms were fixed and stained with Sudan black. The solid arrow indicates fat labeled with Sudan black (Ogg et al., 1997). The double arrow indicates the absence of Sudan black staining in the starved worms.

Figure 2. SREBP and C/EBP Homologs Are Required for C. elegans Fat Storage

(A) Worm SREBP (Y47D38.7) is expressed in the intestine. Transgenic worms expressing an SREBP∷GFP translational fusion were examined by bright-field and GFP fluorescence microscopy. (B) RNAi against either Y47D38.7 (SREBP) or C48E7.3 (C/EBP) results in lipid-depleted worms. dsRNA was injected into the gonads of young adult hermaphrodites, and the progeny were examined. The RNAi worms arrested as larvae, and DIC microscopy showed that they were pale and skinny compared with wild-type worms (worms are at the same developmental stage). In wild-type worms, intestinal fat granules fluoresced brightly after soaking in Nile red, a lipid-specific dye (Greenspan et al., 1985). Worms treated with lpd-1 (SREBP) or lpd-2 (C/EBP) dsRNA displayed no Nile red fluorescence.

Figure 3. Analysis of Lipid-Depleted Worms

(A) Electron microscopy on ultrathin transverse sections of lpd-1 (SREBP) deletion mutants demonstrated that the worm intestinal cells were relatively normal, except that they lacked dark gut granules. (B) lpd-1 and lpd-2 RNAi worms eat and defecate at rates similar to wild-type worms. The number of pumps/minute and seconds/defecation cycle was confirmed twice for each worm (n = 15). To test for intestinal patency, worms were fed bacteria mixed with blue latex beads (Sigma) and then examined at 400 X. (C) The expression of several lipogenic enzymes was downregulated in lpd-1 and lpd-2 RNAi worms. Feeding RNAi was performed (Fraser et al., 2000) with an empty vector, HT115 (con), or with the indicated DNA insert, lpd-1 or lpd-2, and the worms were harvested for RT-PCR. Totals of 0.5 µg and 1 µg of cDNA template were used to ensure that the PCR was semiquantitative.

Figure 4. Inhibiting MRC Function Blocks Lipid Accumulation in Worms and 3T3-L1 Cells

(A) Worms cultured on the indicated chemical MRC inhibitor were stained with Nile red and examined by DIC and fluorescence microscopy. (B) Murine 3T3-L1 cells were induced to become adipocytes as described (MacDougald and Lane, 1995). At day 2 postinduction, 5 µM rotenone, a complex I inhibitor, or 5 mM NaN₃, a complex IV inhibitor, was added to the media. Fresh media and inhibitors were added every other day. At day 8 postinduction, cells were fixed and then stained with oil red O as described (Green and Kehinde, 1975). Induced cells (IND) accumulate lipid (red). Uninduced (UN) cells and inhibitor-treated, induced cells accumulate much less lipid. (C) 3T3-L1 cells, treated as in (B), were harvested with Trizol, treated with DNase I, and analyzed, on day 3 of induction, for expression of the indicated adipogenic markers by semiquantitative RT-PCR. The –RT samples had no reverse transcriptase included in the reaction.

Figure 5. lpd-3 Is Expressed in the Worm In testine and in Mammalian Fat

(A) lpd-3 is expressed in the intestine. Transgenic worms expressing an lpd-3∷GFP translational fusion were examined by bright-field and GFP fluorescence microscopy. (B) The human homolog of lpd-3 is expressed in fat, brain, and testis. PCR with human lpd-3 primers was performed with a panel of cDNA from 16 different human tissues as template. S9 ribosomal PCR is a loading control. (C) Mouse lpd-3 is expressed at higher levels in induced 3T3-L1 (IND) than in uninduced (UN) 3T3-L1 cells. 3T3-L1 cells were grown with (IND) or without (UN) induction media and then harvested for RT-PCR. G3PDH is a loading control. PPARγ is upregulated in induced cells, while Pref-1 is downregulated. (D) Mouse lpd-3 is expressed at higher levels in the mouse embryonic day 14.5 fat enlagen than in adult fat. PPARγ and adipsin, a marker of terminal differentiation, are expressed at higher levels in adult fat. The –RT samples had no reverse transcriptase included in the reaction.

Figure 6. lpd-3 Is Required for Mammalian Lipid Accumulation

(A) 3T3-L1 cells containing the PPARγ shRNA cassette were induced to undergo adipogenesis as described (Green and Kehinde, 1975). After 10 days of induction, lipid accumulation was assessed by visual inspection. While PPARγ shRNA stable lines 5 and 10 accumulated many lipid droplets, PPARγ shRNA stable lines 3, 4, and 6 did not accumulate lipid upon induction. (B) Cells shown in (A) were harvested and processed for RT-PCR analysis. Of note, the inability to accumulate lipid correlated with an shRNA-dependent decrease in PPARγ expression. (C) 3T3-L1 cells that contained the lpd-3 shRNA cassette were induced and evaluated for lipid accumulation. While lpd-3 shRNA stable line 1 accumulated lipid droplets, lpd-3 shRNA stable lines 4, 6, 8, and 9 did not. (D) RT-PCR of cells shown in (C) demonstrated that the lines that did not accumulate lipid after induction were depleted in lpd-3 expression. (E) RT-PCR analysis on the induced PPARγ shRNA and lpd-3 shRNA cell lines (A and C) demonstrated that cells with low levels of either lpd-3 or PPARγ did not express either C/EBPα, an adipogenic gene, or LPL and adipsin, markers of terminally differentiated fat. While the PPARγ shRNA lines lacking PPARγ expressed lpd-3, the lpd-3 shRNA cells with reduced lpd-3 levels (4, 6, 8, and 9) did not express PPARγ. HPRT is a loading control. The –RT samples had no reverse transcriptase included in the reaction.