Worm Breeder's Gazette 13(1): 45 (October 1, 1993)

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.

eat-12 as a pivotal control in muscle contraction

Raymond Lee[1], Leon Avery[2], Michael Hengartner, Bob Horvitz[3]

[2]Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, TX 75235-9038
[3]HHMI, Department of Biology, MIT, 77 Massachusetts Ave., Cambridge, MA 02139.

Recent developments suggest that eat-12 plays an important role in muscle contraction.

eat-12 was originally defined by one semidominant allele, ad695 . ad695 is slightly Dpy, completely male-mating defective (possibly because the spicules are often protruded), and moderately Eat, because the relaxation of the terminal bulb of the pharynx is delayed in some pumps (Avery 1993, Genetics 133: 897). Recently we found a new mutation that causes an ad695 -1ikephenotype, n2368 ,in a screen for dominant suppressors of egl-1 ( n986 ),which causes the HSNs to undergo programmed cell death in hermaphrodites. We subsequently found that n2368 does not suppress the HSN deaths, but rather causes the vulval muscles to contract in the absence of the HSNs, leading to an HSN-independent egg-laying constitutive (Egl-c) phenotype. We suspect that n2368 is allelic to ad695 because n2368 affects all the phenotypes affected by ad695 ,but to a greater extent, and maps close to ad695 (between bli-6 and unc-24 ).To check whether ad695 worms are also Egl-c we staged the most developed embryo in each of several ad695 gravid adults by counting the number of nuclei. The oldest embryos retained by ad695 worms had 7.6±1.8 nuclei (mean±se, n=8). In contrast, each of four wild-type worms retained embryos with more than 20 nuclei. This result suggests that eggs of earlier stages are laid by ad695 mutants. Thus ad695 ,like n2368 ,causes an Egl-c phenotype. However, unlike n2368 , ad695 doesn't suppress egl-1 ,perhaps a reflection of its weaker phenotype.

The semi-dominance of ad695 and n2368 suggested they might be gain-of-function (gf) mutations. We reasoned that loss-of-function (If) mutations might be egg-laying-defective (Egl-d). Since extensive screens for Egl-d mutants have been carried out previously (e.g., Trent et al. 1983, Genetics 104: 619), it seemed possible that If mutations had already been found. egl-19 ( n582 )in particular has a map position consistent with that of ad695 ,and it is Lon and Unc (slow and floppy). We further noticed that n582 worms are slightly Eat; the terminal bulb has occasional brief, partial contractions, a phenotype opposite to that of ad695 and n2368 .Furthermore, we have mapped n582 within 0.2 mu (95% confidence interval) of ad695 .The phenotypic resemblance between ad695 and n2368 ,the opposite phenotypes of n582 from those of n2368 and ad695 ,and the close genetic linkage between ad695 and n582 together suggest that the three mutations are allelic.

If these three mutations are indeed allelic, the gene eat-12 / egl-19 may play a pivotal role in the control of muscle contraction, since opposite mutations have opposite effects on terminal bulb, vulval, and possibly body wall muscle contraction. Furthermore, since the Egl-d phenotype of n582 is insensitive to serotonin (Trent et al. 1983, Genetics 104: 619) and n2368 vulval muscles are capable of contracting without HSNs, it seems likely that these mutations affect the muscle itself, not the nervous system. We have failed to detect structural defects in the muscles of these three mutants under polarized light. We therefore suspect that the defects may be at the level of membrane excitability or excitation-contraction coupling.