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Date of Award

5-1980

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Medical Sciences

Supervisor

Alan C. Peterson

Abstract

Nature mammalian myofibres are multinucleated elongate cells in which the nuclei are regularly distributed along the entire length of the cell. The multinucleated condition arises by the fusion of mononucleated myoblasts to form myotubes during primary development. A fundamental and essentially unexplored problem in muscle cell biology is the question of the nuclear-cytoplasmic relationship in muscle fibres. Although local membrane specializations occur, most notably in the region of innervation, it is not known if each myonucleus controls the biochemical functions of a local territory comprised of the immediately adjacent muscle cytoplasm, organelles, and membrane, or if the gene products of every myonucleus are uniformly distributed in the fibre. An understanding of this aspect of the organization of muscle cells could help solve other problems in the biology of skeletal muscle; for example, what is the significance of X-chromosome mosaicism in the expression of muscle disease problems in the biology of skeletal muscle; for example, what is the significance of X-chromosome mosaicism in the expression of muscle disease in carriers of X-linked myopathies? Which myonuclei are the targets of neurotrophic influences on muscle?

In this study I have taken advantage of the fact that in mouse chimaeras two genotypically distinct types of myonuclei are often incorporated into single myofibres skeletal muscle cells in vivo. Mouse chimaeras were produced by the aggregation of embryos homozygous for different alleles for both the monomers of the cytoplasmic enzyme glucosephosphate isomerase (GPI-1) and the monomers of the nuclear-coded mitochondrial matrix protein malic enzyme (MOD-2). Therefore, mosaic myofibres contained both Gpi-1ª/Gpi-1ª, Mod-2ª/Mod-2ª myonuclei and Gpi-1ᵇ/Gpi-1ᵇ, Mod-2ᵇ/Mod-2ᵇ myonuclei.

The distribution of the electrophoretic variants of GPI-1 was measured at various points along the length of mosaic muscle fibres. Changes in the isozyme composition along the length of a cell would indicate that the enzyme remains localized in cytoplasmic territories. A uniform isozyme profile would result if the products of individual myonuclei are homogeneously distributed. The results presented here indicate that the intracellular distribution of GPI-1 isozymes does not change along the length of mosaic myofibres in both adult and young mouse chimaeras. Therefore, the products of each myonucleus are widely distributed in skeletal myofibres.

Functional GPI-I is an oligomeric enzyme composed of two monomers. Analyses of the proportions of GPI-I dimer types in mosaic myofibres reveal that the homogenous distributions of GPI-I isozymes arise by the widespread distribution of precursors of GPI-I (monomers or monomer precursors such as messenger RNA) before dimer formation; dimers are not initially assembled in local territories around each myonucleus and then widely distributed.

The investigation was extended to the question of whether individual mitochondrion in skeletal myofibres have access to nuclear-toded proteins encoded by one or by multiple myonuclei. Evidence was obtained that indicates that both types of monomers of nuclear-coded mitochondrial malic enzyme (MOD-2) are present in an individual mitochondrion of a mosaic myofibre.

The results indicate that the gene predicts of different myonuclei are widely distributed both before the assembly of cytoplasmic oligomeric proteins and before the incorporation of proteins into mitochondria.

Taken together, the results obtained in this study constitute direct evidence that mammalian skeletal myofibres are functional syneitia in which the gene products of many myonuclei are distributed throughout each cell. Problems and possibilities regarding the mechanisms of distribution and the nature of the gene products distributed are discussed.

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