To maintain sensitivity to the changing environment, animals adapt to continuous sensory stimuli such as those of odorants. Studies on olfactory adaptation in C. elegans have so far revealed several molecules involved. For example, EGL-4(cGMP dependent protein kinase), GOA-1/EGL-30(Goa/Gqa), ARR-1(b-arrestin), and GPC-1(Gg) were all reported to regulate olfactory adaptation to benzaldehyde in the sensory neurons AWC, which sense the odorant. In contrast the Ras-MAPK pathway was reported to regulate olfactory adaptation by acting in interneurons. Therefore, neural circuit-dependent mechanisms are also important for regulation of olfactory adaptation. Here, we report a novel component of olfactory adaptation,
nep-2, which was identified through our genetic screen for adaptation-defective mutants.
nep-2 encodes a homolog of mammalian neprilysin, which is a type II integral membrane protein with an extracellular domain that acts as a metallopeptidase. Neprilysin catalyzes peptide hydrolysis at the extracellular face of the plasma membrane. NEP-2 has a conserved HExxH motif, which is indispensable for peptidase activity in neprilysin family proteases, and this motif was essential for olfactory adaptation. Therefore, NEP-2 was also suggested to act as a peptidase.
nep-2 was expressed in muscle cells and several neurons, and the Venus-fused NEP-2 localized at the surface of these cells. However, the cell-specific rescue experiments revealed that the expression of
nep-2 is not required in a specific cell. These results suggest that the substrate of NEP-2 is a broadly diffusing paracrine or endocrine peptide. To further understand the function of NEP-2 in olfactory adaptation, we screened for
nep-2 suppressors, which resulted in identification of a suppressor mutation in the gene
snet-1.
snet-1 encodes a protein of 101 amino acids similar to the precursor of Aplysia L11 peptide. SNET-1 carries sequences for a signal peptide and dibasic processing sites suggesting it is also a peptide precursor. Indeed, the results of rescue experiments with truncated
snet-1 cDNA suggested that the putative peptide-coding region of the protein is important for the function.
snet-1 is expressed in several neurons including ASK. The expression of
snet-1 in any of these neurons rescued the suppressed phenotype in the
nep-2;
snet-1 double mutant suggesting that SNET-1 acts as an endocrine or paracrine peptide signal. These results suggest a model in which SNET-1 peptide is a negative regulator of olfactory adaptation and NEP-2 restricts the function by degrading excess SNET-1 peptide.