Worm Breeder's Gazette 3(2): 30
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.
As John Sulston described at the last worm meeting (see Meeting Newsletter), sensitivity to light touch, i.e. stroking with an eyebrow hair, appears to be mediated by 6 receptor neurons: two lateral pairs of cells and two cells (derived by the Q cell lineages) which send processes into the ventral cord. Cell processes of all 6 cells are easily recognized in electron micrographs because they contain large numbers of darkly staining microtubules. (Similar microtubule containing cells in other nematodes have been seen by J. M. Smith and N. Croll. Int. J. Parasitol. 5: 289, 1975). Interestingly, the number of microtubules seen in serial sections of all 6 cells fluctuates along the length of the neuronal process. This is seen most convincingly in the process of cell Q2.paa where the number of tubules (in one series) went from 18 to 4 and then up to 20 over a distance of approximately 20 m. Although neuronal microtubules have been thought to be continuous, these cells appear to show that continuity is not important for normal cell activity. 28 mec mutations in 11 complementation groups are currently being examined. (Mec - mechanosensory abnormality - replaces the previous tin designation). These mutations do not respond to the eyebrow hair stimulus but move when prodded with a platinum wire. In addition, these mutants are 'lethargic' i.e. they are relatively inactive when compared to N2 on agar plates with ample bacterial lawns. Mec worms appear as active as N2 but are still touch insensitive on clearing plates. Larval mec males are lethargic, but adult males are not (they are still mec however). Thus, hunger and sex seem to keep the worms active. These phenotypes are also found in two unc mutations and in one che mutation. The mutations are listed below. [See Figure 1] [See Figure 2] The mec mutations can be phenocopied by growing N2 in the presence of 0.5 - 1.0mM colchicine, an antimitotic drug which binds to tubulin. Colchicine-treated animals are touch insensitive and lethargic (only in the presence of ample E. coli). This concentration of colchicine appears too low to block cell division since animals move normally and are fertile (10-20mM colchicine is lethal to C. elegans). Colchicine is effective only if the animals are treated as L2s or younger. This is consistent with the finding that young animals have significantly fewer microtubules (even accounting for the fluctuations) than adults and supports the idea that colchicine is preventing the assembly of microtubules in these cells. The colchicine effect is irreversible. ( S. Ward and Y. Argon have found that worms treated with nocodazole, another antimitotic drug, become mec and unc.) 35S-labeled tubulin can be isolated from C. elegans by successive rounds of polymerization and depolymerization with excess cold pig brain tubulin. Alpha- and -tubulin isolated in this way can be visualized on SDS polyacrylamide gels. Two other components apparently copurify with tubulin. Both have lower molecular weights; one has been tentatively identified with actin. There are no components with higher molecular weights than tubulin which copurify in this system. No difference in the gel pattern has been seen between E1343*1 (which lacks the microtubules), E1416 (which lacks the touch cells), and N2. This is not surprising since the tubulin in the touch cells is likely to be a small fraction of the total worm tubulin.