The C. elegans excretory canals are formed from a single cell body located just behind the pharyngeal bulb. During the larval stages of the animal, this cell extends two tubular processes that eventually form two long canals, one on the left and one on the right, that stretch from the head of the animal to the anus. The result is a large H-shaped excretory organ. A series of genes have been identified that when mutated, cause the canals to form fluid-filled cysts in various positions and sizes. These genes have been termed exc genes (short for exc retory canal abnormal). One of these genes,
exc-5 , when mutant, causes the animal to have a canal that stops approximately midway in length, and ends in one or several large cysts. EXC-5's closest human homologue is FGD1. FGD1 is involved with the genetic condition FacioGenital Dysplasia, which causes multiple defects in skeletal structure.
exc-5 encodes a putative guanine nucleotide exchange factor (GEF); these proteins replace GDP for GTP in order to activate small GTPase proteins such as Rho, Rac, and Cdc42. We have previously found that EXC-5::GFP is expressed primarily in the excretory canals, where we believe it regulates the ratio of apical:basal cytoskeletal construction. Genetic evidence suggests that EXC-5 acts upstream of the Rho GTPase
mig-2 , a gene implicated in axonal guidance. The
mig-2 mutant phenotype, however, does not include any canal defects, which suggests that EXC-5 has several downstream targets, one of which is involved in cytoskeletal formation and maintenance of canal structure. We are currently performing a non-complementation screen in order to isolate a new EMS-induced temperature-sensitive allele of
exc-5 . Such an animal would allow the temporal requirements of EXC-5 to be investigated as well as allowing control over the amount of downstream activation in the EXC-5 pathway. To date there have been 15 alleles isolated from this screen, and more are continuing to be isolated. As expected, most so far have been found to be non-temperature sensitive. Once a temperature-sensitive allele has been isolated, all of the alleles will be sequenced and experiments involving the exact nature of the developmental requirements of EXC-5 utilizing the new temperature-sensitive allele will begin.