The nuclear-cytoplasmic relationship in 'mosaic' skeletal muscle fibers from mouse chimaeras.

In mouse chimaeras, individual skeletal muscle fibers typically contain populations of myonuclei derived from both cell lines. This 'mosaic' circumstance has provided an opportunity to investigate directly whether the mammalian myofiber syncytium is functionally subdivided into territories, each preferentially influenced by products encoded by the local myonucleus, or whether the multiple nuclei direct the synthesis of products that achieve a uniform distribution throughout the fiber. Chimaeras were produced in which one cell line was derived from an embryo homozygous for gpi-1a, whereas the other was homozygous for the gpi-1b; each allele specifies electrophoretically distinguishable isozymes of the cytosolic enzyme glucosephosphate isomerase (GPI-1). Microtechniques capable of measuring the proportion of each isozyme expressed within small samples of individual muscle fibers have been established, permitting the comparison of the relative quantitative distributions of the GPI-1 isozyme types along the length of individual chimaera fibers. From individual mosaic fibers, all samples yielded identical isozyme profiles, demonstrating that GPI-1 is not sequestered adjacent to the nucleus directing its synthesis; rather, it achieves a homogeneous distribution throughout the mosaic syncytium. The GPI-1 gene locus encodes only the GPI-1 monomer, whereas the functional enzyme detected in our analysis is a dimer that results from the aggregation of monomers in the cytoplasm. The quantitative distribution of dimer types within each mosaic fiber was consistent with random aggregation amongst all monomers represented in the final isozyme pattern, a result requiring that monomers or earlier precursors were mixed in the myofiber cytoplasm prior to assembly of the enzymatically active dimer. Thus, both the final distribution of enzyme dimers within fibers and the patterns of monomer aggregation suggest that there are no subdivisions related to the spatial separation of the genotypically distinct myonuclei within mosaic muscle fibers.

A technique for detection and relative quantitative analysis of glucosephosphate isomerase isozymes from nanogram tissue samples.

An improved method for detecting and measuring the enzyme glucosephosphate isomerase after starch gel electrophoresis is described. Nitrocellulose filters are used in a gel overlay procedure which increases the sensitivity of the staining reaction and provides a simple means for accurate quantitation of the isozyme pattern. This staining technique may have wider application with other gel media and also with other enzymes.

Mitochondrial malic enzyme in mosaic skeletal muscle of mouse chimeras.

The question was investigated whether mitochondria in the mammalian skeletal muscle fiber syncytium incorporate gene products encoded by one or many nuclei. Mouse chimeras were produced from strains which differ in their electrophoretic variants of the nuclear-coded mitochondrial protein, malic enzyme (MOD-2, E.C. 1.1.1.40, L-malate NADP+ oxidoreductase decarboxylating). The MOD-2 phenotypes of skeletal muscles of these chimeras were characterized in a starch gel electrophoretic system. The results indicate that individual mitochondria can contain products encoded by multiple nuclei and therefore that, for skeletal muscle mitochondria, the cell is not subdivided into nuclear territories. Possible mechanisms of gene product distribution in skeletal muscle fibers are discussed.