THE EXPRESSION OF THE REGULATORY MYOSIN LIGHT-CHAIN 2-GENE DURING MOUSE EMBRYOGENESIS

The fast skeletal muscle myosin light chain 2 (MLC2) gene is expressed specifically in skeletal muscles of newborn and adult mice, and has n o detectable sequence homology with any of the other MLC genes includi ng the slow cardiac MLC2 gene. The expression of the fast skeletal mus cle MLC2 gene during early mouse embryogenesis was studied by in situ hybridization. Serial sections of embryos from 8.5 to 12.5 days post c oitum (d.p.c.) were hybridized to MLC2 cRNA and to probes for the myog enic regulatory genes MyoD1 and myogenin. The results revealed differe nt temporal and spatial patterns of hybridization for different muscle groups. MLC2 transcripts were first detected 9.5 d.p.c. in the myotom al regions of rostral somites, already expressing myogenin. Surprising ly, at the same stage, a weak MLC2 signal was also detected in the car diomyocytes. The cardiac expression was transient and could not be det ected at later stages while the myotomal signal persisted and spread t o the more caudal somites, very similar to the expression of myogenin. Beginning from 10.5 d.p.c., several extramyotomal premuscle cells mas ses have been demarcated by MyoD1 expression. MLC2 transcripts were de tected in only one of these cell masses. Although, transcripts of myog enin were detected in all these cell masses, the number of expressing cells was significantly lower than that observed for MyoD1. By 11.5 d. p.c., all three hybridization signals colocalized in most extramyotoma l muscle-forming regions, with the exception of the diaphragm and the hindlimb buds, where only few cells expressed MLC2 and more cells expr essed MvoD1 than myogenin. At 12.5 d.p.c., all three studied genes dis played a similar spatial pattern of expression in most muscle-forming regions. However, in some muscles, the MyoD1 signal spread over more c ells compared to myogenin or MLC2. Our results are consistent with the suggestion that multiple myogenic programs exist for myoblasts differ entiating in the myotome and extramyotomal regions.