Forming and maintaining tubular structure is fundamental to organismal development. The excretory canal cell forms a simple single-cell epithelial tubular model for this study. The EXC proteins regulate maintenance of the apical (lumenal) cytoskeleton of the excretory canal. Loss of exc gene function allows formation of large fluid-filled cysts in the excretory canal.
exc-9 mutants exhibit short and cystic canals, while the wild-type canal is smooth and runs almost all the way to the tail.
exc-9mutants also exhibit tail defects in hermaphrodites (40% penetrance) and ray defects in male (again at 40% penetrance). By SNP mapping, cosmid rescue, and RNAi experiments, we proved that F20D12.5 encodes
exc-9. EXC-9 is a small protein with 85 amino acids, and contains a single LIM domain. EXC-9 shows high homology to mammalian CRIP (Cysteine-Rich Intestinal Protein). B0496.7 is a highly homologous gene close to
exc-9; both genes are also present in C. briggsae . An
exc-9 translational GFP construct is highly expressed in the tissues affected in the mutant background.
exc-9is also expressed in some other cells, including UTSE, the distal tip cells, and ALM neurons. Overexpression of our
exc-9 constructs in an N2 background sometimes causes an unextended canal phenotype, in which the narrow apical surface is maintained at its proper diameter, but the basal surface breaks down, so that the canal forms a large cell body filled with lumen, but has no canals along the length of the animal. Since canal extension is sensitive to expression levels of
exc-9, injection of our construct sometimes creates partial rescue of the
exc-9 mutant phenotype, and sometimes unextended canals, but complete rescue is rare. This unextended canal phenotype is also found in animals showing high levels of
exc-5 expression. I used the unextended canal phenotype to examine epistasis of EXC-9 function with that of other EXC proteins. EXC-9 appears to function upstream of EXC-5 to regulate cytoskeletal formation at the apical surface (possibly via CDC-42); and EXC-9 in turn may depend upon EXC-2 and EXC-4 function. B0496.7 is not normally expressed in the canal, but rescues
exc-9 mutants when expressed in the canal. Expression of the mouse homologue CRIP in the canal failed to rescue
exc-9 mutants. We are investigating which sequence differences between these two proteins affects function in C. elegans..