Co-expression of multiple myosin heavy chain genes, in addition to a tissue-specific one, in extraocular musculature.

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Abstract
The developmental transitions of myosin H chain (MHC) gene expression were investigated in the rat extraocular musculature (EOM) at the mRNA level using S1-nuclease mapping techniques and at the protein level by polypeptide mapping and immunochemistry. A genomic clone was isolated, designated .lambda. 10B3, corresponding to an MHC gene which is expressed in the EOM fibers (recti and oblique muscles) of the adult rat but not in hind limb muscles. Using c[complementary]DNA and genomic probes for MHC genes expressed in skeletal (embryonic, neonatal, fast oxidative, fast glycolytic and slow/cardiac .beta.-MHC), cardiac (.alpha.-MHC) and EOM (.lambda. 10B3) muscles, demonstrates the concomitant expression at the mRNA level of at least 6 different MHC genes in adult EOM. Protein and immunochemical analyses confirm the presence of at least 4 different MHC types on EOM. Immunocytochemistry demonstrates that different myosin isozymes tend to segregate into individual myofibers, although some fibers seem to contain > 1 MHC type. The EOM fibers exhibit multiple patterns of MHC gene regulation. One set of fibers undergoes a sequence of isoform transitions similar to the one described for limb skeletal muscles, whereas other EOM myofiber populations arrest the MHC transition at the embryonic, neonatal/adult, or adult EOM-specific stage. The MHC gene family is not under the control of a strict development clock, but the individual genes can modify their expression by tissue-specific and/or environmental factors.