Fast and slow muscles in tissue culture synthesise only fast myosin

Foetal myosin light chain in human ventricle

Myosin light chain subunits from adult human atria and ventricle were characterized by two-dimensional gel electrophoresis. Atrial (ALC-1 and ALC-2) differed from ventricular (VLC-1 and VLC-2) light chains. Foetal (18--21 week) ventricle contained VLC-1 plus a foetal light chain (FLC-1) which could not be distinguished from adult ALC-1. Myosin FLC-1 was gradually lost from the ventricle and replaced by VLC-1 during foetal development and in the immediate postnatal period.

Myosin types in cultured muscle cells

Fluorescent antibodies against fast skeletal, slow skeletal, and ventricular myosins were applied to muscle cultures from embryonic pectoralis and ventricular myocadium of the chicken. A number of spindle-shaped mononucleated cells, presumably myoblasts, and all myotubes present in skeletal muscle cultures were labeled by all three antimyosin antisera. In contrast, in cultures from ventricular myocardium all muscle cells were labeled by anti-ventricular myosin, whereas only part of them were stained by anti-slow skeletal myosin and rare cells reacted with anti-fast skeletal myosin. The findings indicate that myosin(s) present in cultured embryonic skeletal muscle cells contains antigenic determinants similar to those present in adult fast skeletal, slow skeletal, and ventricular myosins.

Identification of recombinant phages containing sequences from different rat myosin heavy chain genes

The construction and identification of a recombinant plasmid containing a cDNA insert which hybridizes specifically to myosin heavy chain mRNA is described. The plasmid was used as a probe to screen a rat genomic library for recombinant phages containing myosin heavy chain sequences. Six clones with approximately 15 k bp inserts each were isolated. Digestion with several restriction enzymes and hybridization of the fractionated DNA with the plasmid probe showed that the clones contained 3 different DNA inserts. Electron microscopy of a heteroduplex made by hybridization of DNA from two clones confirmed that the inserts originated in different genes. Hybridization of size-fractionated ECOR1 digested rat spleen DNA with the cloned probe suggested the existence of at least 5 myosin heavy chain genes.

Synthesis of adult myosin light chains by embryonic muscle cultures

Myosin light chain synthesis has been analyzed in cultures of fast and slow muscles from chicken and quail embryos. Synthesis was assayed by [35S]methionine incorporation and two-dimensional electrophoresis of total cell extracts. Our results show that differentiated cultures of embryonic anterior latissimus dorsi and pectoral muscles synthesize proteins that comigrate on two-dimensional gels with the five myosin light chains of adult fast (pectoral) and slow (anterior latissimus dorsi) muscle. Partial proteolytic digestion and peptide analyses further confirm the identity of these proteins as adult light chains. Cultures of dividing myoblasts do not synthesize any of these fiber type isozymes, and synthesis of the isozymes is initiated at myoblast fusion. Also, myogenic clones drived from single myoblasts differentiate to synthesize these five myosin light chains, indicating that individual myoblasts have the potential to express the synthesis of all fiber type light chain isozymes. We conclude that the primary events in muscle differentiation include the initiation of synthesis of the entire set of adult fast and slow myosin light chain isozymes. The developmental and physiological implications of these results for the establishment of fiber type specificity are discussed.