Differential expression of myosin heavy chain mRNA and protein isoforms in four functionally diverse rabbit skeletal muscles during pre- and postnatal development.

Myosin heavy chains (hcs) are the major determinant in the speed of contraction of skeletal muscle, and various isoforms are differentially expressed depending on the functional activity of the muscle. Using the rapid amplification of cDNA ends (3' RACE) method, we have characterised the 3' end of the embryonic, perinatal, type 1, 2a, 2x, and 2b myosin hc genes in rabbit skeletal muscle and used them as probes in RNase protection assays to quantitatively monitor their expression in different type of skeletal muscles just before and after birth. SDS PAGE was used to study the changes in the expression level of their respective protein and to determine the relative abundance of each myosin hc isoform in the muscles studied. The results show that for each anatomical muscle, the developmental changes in myosin hc gene expression at the mRNA level correlate strongly to those observed at the protein level. By studying their developmental expression in four functionally diverse skeletal muscles (semimembranosus proprius, diaphragm, tibialis anterior, and semimembranosus accessorius), it was shown that all muscles express the embryonic, perinatal, and type 1 isoform during prenatal development up to the E27 stage. In the diaphragm, low levels of the type 2a and 2x transcripts, which are adult fast isoforms, were also detected at the E27 stage. During the first week of postnatal growth the myosin hc transition leading to the expression of the adult isoforms is complex, and as many as five different myosin heavy chains are concurrently expressed in some muscles at around birth. As the animal matures, individual muscles become adapted to perform highly specialised functions, and this is reflected in the myosin hc composition within these muscles. Accordingly, the expression of the type 1 isoform, and the sequence of appearance and the expression levels of the type 2 isoforms, were exclusively dependent on the muscle type and largely reflect the functional activity of each muscle during the postnatal growth period.