Worm Breeder's Gazette 13(3): 86 (June 1, 1994)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
We have been studying mutants in which the structure or development of the epithelially-derived excretory canal cell processes is abnormal. One discrete set of mutants shows normal canal development through most of embryogenesis, followed by slow partial or complete collapse of the canals into a series of large cysts. The terminal phenotype varies greatly from one exc gene to the next; it is unlikely that simple blockage of the duct, through which the canal empties, can cause these varied effects. Defects of the extracellular matrix over which the canal grows are not involved, as mutants known to be defective in these structures form canals which do not grow out properly, but are of normal width. We believe that the defect in these mutants is in the structure and placement of filaments internal to the canal cytoplasm, possibly including filaments that guide myriad vesicles to the apical surface of this cell. Such structural members are necessary for any hypotonic cell to form a shape other than a sphere, and are critical for the maintenance of long processes such as axons or canals. Other observations support this hypothesis. During canal collapse, the tubule sometimes assumes a helical shape, consistent with the presence of filaments placed non-perpendicularly under the apical surface; a thick density is seen at this surface in electron micrographs. In addition, some mutants show other phenotypes consistent with abnormal maintenance of filaments. Mutants in exc-5 (IV)are slightly Him; mutants in exc-2 ~X)have abnormal tailspikes, and in exc-8 (X)mutants, the pharynx sometimes detaches from the buccal hypoderm. In addition, all alleles of the gene sma-1 (V) exhibit very large cysts. sma-1 mutants are especially short as larvae and have rounded heads, consistent with abnormal formation of cuticular filaments.
Eight viable exc genes have been characterized and mapped to positions on all chromosomes. Six genes have been placed over deficiencies; this causes no increase in severity of canal defects. Several let genes also exhibit cystic excretory canals. These include let-4 (X), let-51 (IV),and the let-653 (IV)gene characterized by Steven Jones and Dave Baillie. These animals die as late embryos or early larvae (a few survive until L2 )with extremely large cysts at the excretory cell body underneath the posterior pharyngeal bulb. It is not clear if the canal defect is the cause of lethality in these animals, although this seems likely. Defects in the strongest viable exc genes, exc-4 (I)and exc-2 (X),cause similar large cysts at the excretory cell body, and viability and growth rate are reduced. Construction of animals doubly mutant for genes with milder cyst phenotypes causes both lethality and large cell body cysts. A survey of other let genes that arrest at these stages may reveal a large number of additional genes that encode products required for excretory canal cell structure and function.
The exc defects appear formally analogous to the genetic polycystic kidney diseases (PKD) of both man and mouse. The exc-3 (X)gene may be similar to the most frequent human defect, autosomal dominant PKD. Alleles of exc-3 are semi-dominant, showing a strong effect as a homozygote, with less common defects when hemizygous; hemizygotes for the deficiency nDf19 also exhibit infrequent (-5% of animals) defects in one or more of the canals. Woo et al. recently reported (Nature 368, 750-753, '94) that the microtubule-stabilizing drug taxol delayed onset of PKD in mice, while DNA-replication and protein-synthesis inhibitors had no effect. This further suggests that control of filamentous structure is defective in the exc genes; we are currently testing drugs that affect both actin and tubulin filaments for effects on excretory canal structure.