Worm Breeder's Gazette 13(2): 59 (February 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.
The body wall muscle cells of C. elegans are anchored to the cuticle through a specialized basement membrane and hypodermal hemidesmosome structures (Francis and Waterston, JCB 1991). Assembly of muscle and muscle cell attachment structures is spatially and temporally coordinated during embryogenesis (Hresko et. al. JCB, in press). At about 290 min after the first cleavage, when muscle-specific antigens are first detected, muscle cells are aligned in two lateral rows adjacent to seam cells. As development continues, and the muscle cells migrate to contact dorsal and ventral hypodermis, a significant change in the distribution of components occurs. Myofibrillar components become localized to membranes where adjacent muscle cells contact each other and the hypodermis (that is the muscle cells become polarized), basement membrane components accumulate between muscle cells and hemidesmosome components become localized to regions of the hypodermis adjacent to muscle cells. These data suggest that important developmental signals may be passed between muscle and hypodermal cells. To test this possibility, we have ablated muscle and hypodermal cell precursors, and looked for changes in the distribution of muscle attachment and muscle proteins, respectively, using antibodies. The success of each ablation was verified using antibodies against proteins specific to the ablated cell type (anti-myosin was used to identify muscle cells and MH27 ,which recognizes hypodermal cell boundaries, was used to identify hypodermal cells). In all cases the dorsal side of the embryo was examined because the ventral side is difficult to see due to the ventral bend of the embryo. Muscle Ablations: Body wall muscle cells affect the organization of the basement membrane and hemidesmosomes. Ablation of Cap eliminates 9 posterior muscle cells from the left dorsal, and 7 from the left ventral, quadrant. In the anterior dorsal region of the embryo, the MH46 antigen (a basement membrane component synthesized by dorsal and ventral hypodermis) is localized between muscle cells, and the MH4 antigen (a hemidesmosome component) is localized to the region of the syncitial hypodermis adjacent to muscle cells. This is similar to the localization seen in wild-type embryos. However, in the posterior dorsal region of the embryo, where the progeny of Cap would have been, the MH4 and MH46 antigens are not detected. Similar results are obtained with MH46 in embryos ablated for MSap and MSpp (6 anterior muscle cells from each dorsal and 3 from each ventral quadrant), and with MH4 in embryos ablated for Cpp, the contralateral homologue to Cap. These data suggest that muscle cells trigger the recruitment of basement membrane and hemidesmosome components to regions adjacent to the muscle cells. We think it is unlikely that the muscle cells turn on the expression of the MH4 and MH46 antigens. In wild-type embryos these antigens are expressed at a time when muscle cells are adjacent to seam cells and not in contact with dorsal or ventral hypodermis. Hypodermal Ablations: Dorsal hypodermal cells affect the organization of the contractile apparatus. Ablation of Caa and Cpa results in embryos missing 10 posterior dorsal hypodermal cells (numbers 11-20 according to Sulston). The muscle cells in the anterior of both dorsal muscle quadrants migrate onto dorsal hypodermis as in wild-type embryos. The muscle cells in the posterior of the quadrants migrate off the seam cells, but do not migrate as far as those in the anterior, presumably because there is no dorsal hypodermis on which to migrate. The muscle cells in the anterior of these embryos organize myosin A into A-bands. In contrast, the myosin A in the posterior cells, which do not contact dorsal hypodermis, remains diffuse in the cytoplasm and does not organize into A-bands. Clearly these data suggest that muscle and hypodermal cells communicate during the assembly of muscle and muscle cell attachment structures. This may be a mechanism by which the muscle and hypodermal cells coordinate the position of structural elements that will form the physical links between the contractile units and the outside of the worm, linkages that are crucial for locomotion.