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

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.

Helmsman is expressed in both trachea and photoreceptor development: partial inactivation alters tracheal morphology and visually guided behavior.

We have identified helmsman (hlm), which is expressed in the fruit fly photoreceptor cells during neural network development. Hlm is also expressed in the elongating cells of the embryonic trachea. Both photoreceptor neurons and embryonic trachea cells elongate in precise, targeted growth for cell-to-cell specific recognition. Expression of antisense hlm-interfering RNA during embryogenesis arrests elongation of the developing tracheal cells and blocks maturation. Expression of hlm-interfering RNA during visual system formation results in reduced visual acuity and poor performance in optomotor response, indicative of abnormal neural network development. Hlm is a unique cell surface protein with complement-like protein interaction motifs. We have also cloned hlm from Lucilia cuprina (Australian blowfly), which is approximately 100 million years divergent from Drosophila, and find a remarkable 90% protein identity over the entire 558 amino acid protein. Analysis of the hlm sequence found in other species indicates a significant evolutionary pressure to maintain the hlm protein sequence. Our interpretation is that hlm is involved in cell maturation in both the elongating trachea and elongating photoreceptor cells. Cell adhesion and cell signaling, which are known to use immunoglobulin-like cell adhesion molecules, may use molecular systems analogous to complement to create protein complexes to regulate growth.

eat-2 and eat-18 are required for nicotinic neurotransmission in the Caenorhabditis elegans pharynx.

Mutations in eat-2 and eat-18 cause the same defect in C. elegans feeding behavior: the pharynx is unable to pump rapidly in the presence of food. EAT-2 is a nicotinic acetylcholine receptor subunit that functions in the pharyngeal muscle. It is localized to the synapse between pharyngeal muscle and the main pharyngeal excitatory motor neuron MC, and it is required for MC stimulation of pharyngeal muscle. eat-18 encodes a small protein that has no homology to previously characterized proteins. It has a single transmembrane domain and a short extracellular region. Allele-specific genetic interactions between eat-2 and eat-18 suggest that EAT-18 interacts physically with the EAT-2 receptor. While eat-2 appears to be required specifically for MC neurotransmission, eat-18 also appears to be required for the function of other nicotinic receptors in the pharynx. In eat-18 mutants, the gross localization of EAT-2 at the MC synapse is normal, suggesting that it is not required for trafficking. These data indicate that eat-18 could be a novel component of the pharyngeal nicotinic receptor.

Tripeptidyl peptidase II promotes fat formation in a conserved fashion.

Tripeptidyl peptidase II (TPPII) is a multifunctional and evolutionarily conserved protease. In the mammalian hypothalamus, TPPII has a proposed anti-satiety role affected by degradation of the satiety hormone cholecystokinin 8. Here, we show that TPPII also regulates the metabolic homoeostasis of Caenorhabditis elegans; TPPII RNA interference (RNAi) decreases worm fat stores. However, this occurs independently of feeding behaviour and seems to be a function within fat-storing tissues. In mammalian cell culture, TPPII stimulates adipogenesis and TPPII RNAi blocks adipogenesis. The pro-adipogenic action of TPPII seems to be independent of protease function, as catalytically inactive TPPII also increases adipogenesis. Mice that were homozygous for an insertion in the Tpp2 locus were embryonic lethal. However, Tpp2 heterozygous mutants were lean compared with wild-type littermates, although food intake was normal. These findings indicate that TPPII has central and peripheral roles in regulating metabolism and that TPPII actions in fat-storing tissues might be an ancient function carried out in a protease-independent manner.