LPR-1 belongs to the lipocalin superfamily, which is a group of structurally related secreted proteins that transport lipophilic cargos and in some cases participate in intercellular signaling.
lpr-1 loss-of-function(lf) mutants have an incompletely penetrant L1 lethal phenotype due to failure to maintain a continuous lumen between the excretory duct and pore tubes. Although
lpr-1 is normally expressed in both the duct and pore, it can function cell non-autonomously in tissue-specific rescue experiments, consistent with a transport- or signaling-related function.Epistasis experiments suggest that LPR-1 may facilitate LIN-3/EGF signaling from the excretory canal cell to the excretory duct and pore. The EGF-Ras-ERK signaling pathway plays multiple roles in the development of the excretory system, including specifying excretory duct versus pore cell fate and promoting duct morphogenesis.
lpr-1 mutant lethality was significantly suppressed by overexpressing a
lin-3/EGF genomic fragment or by global hyperactivation of the downstream Ras signaling pathway, suggesting that LPR-1 and LIN-3/EGF act in a common pathway. Temporal rescue experiments with a hsp::LIN-3(EGF) transgene demonstrated that signaling acts early, around the time of initial duct and pore tubulogenesis, to rescue
lpr-1(lf) lethality. Tissue-specific rescue experiments with LET-60/Ras(gf) showed that hyperactivation of Ras was required in both the duct and pore cells.
lpr-1 mutants do not display duct-to-pore cell fate transformations or other phenotypes expected for a general defect in EGF-Ras-ERK signaling; therefore we wondered if LPR-1 facilitates signaling by a specific LIN-3/EGF isoform. LIN-3 is the only EGF-related ligand in C. elegans, but alternative splicing generates at least four LIN-3 protein isoforms that differ in their extracellular domains and in their biological activities. In order to test which isoform(s) of LIN-3 could function in the excretory system and which might require LPR-1, single copy integration lines expressing different LIN-3 isoforms were generated. All three of the LIN-3 isoforms we tested could rescue
lin-3(lf) and
lpr-1(lf) mutant lethality, although they differ in rescue efficiency. These data do not support the idea that LPR-1 is a dedicated cofactor for a specific LIN-3 isoform. Rather, the data suggest that either LPR-1 plays a more general role in facilitating LIN-3/EGF signaling in the context of the excretory system, or that signaling upregulates other targets that in some way compensate for the absence of LPR-1.