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The role of the let-268 and spc-1 genes in the early stages of muscle development in Caenorhabditis elegans Norman, Kenneth Rich


Myogenesis in Caenorhabditis elegans is composed of several stages: the generation of presumptive myoblasts; the migration of these cells dorsally and ventrally from their lateral position to form the muscle quadrants; the accumulation of muscle specific proteins in these cells; the polarization of the cells; and, finally, the organization of sarcomeres and attachment structures within the cells. The goal of this thesis work has been to identify genes involved in this process. From a mutant screen designed to identify mutations that disrupt muscle development during C. elegans embryogenesis, eight mutations have been isolated and placed into five distinct classes. Two of the mutants identified in this screen have been characterized in greater detail and the affected genes have been cloned. One of the mutants, let-268 (a class IV mutant), encodes a procollagen processing enzyme, procollagen lysyl hydroxylase, required for post-translational modifications of collagen. In let-268 mutants the processing and secretion of type IV collagen is disrupted. An examination of the body wall muscle in these mutant animals reveals normal myofilament assembly prior to contraction. However, once body wall muscle contraction commences the muscle cells separate from the underlying epidermal layer (hypodermis) and the myofilaments become disorganized. These observations indicate that type IV collagen is required in the basement membrane for mechanical support and not for organogenesis of the body wall muscle. The other mutant examined in greater detail is spc-1 (class V mutant), spc-1 encodes the only α spectrin gene in the C. elegans genome. Animals lacking functional α spectrin die just after hatching and have defects in myofilament organization. More specifically, when compared to myofilaments in wild type animals, the myofilaments in the mutant animals are abnormally oriented relative to the longitudinal axis of the embryo. In cross section, the myofilaments appear to be loosely associated with the sarcolemma as compared to wild type. In addition, analysis of the basement membrane and the hypodermis of spc-1(ra409) mutants provides evidence that the body wall muscle directs where the basement membrane is established and signals the hypodermis to determine where muscle anchoring structures are assembled in this tissue.

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