Early expression of the different isoforms of the myocyte enhancer factor-2 (MEF2) protein in myogenic as well as non-myogenic cell lineages during mouse embryogenesis.

MEF2 proteins are a family of transcription factors that have muscle-specific DNA binding activity and bind to conserved A/T rich elements in the regulatory regions of numerous muscle-specific genes. They are thought have an important role in the development and differentiation of skeletal muscle. Recent in situ hybridization studies using mouse MEF2 probes have shown that MEF2 gene transcripts are detected very early in development at high levels in the myogenic cells of the myotome and embryonic heart. However, MEF2A and MEF2D transcripts have been detected in many adult tissues where they are not translated or the corresponding proteins are rapidly degraded. Therefore, it is important to establish the temporal and spatial correlation between MEF2 RNA and protein expression. In the present study we have performed in situ immunohistochemistry of whole mount mouse embryos at different stages of development using polyclonal antibodies specific for the MEF2A, MEF2C and MEF2D isoforms. At day 8.5 of development, all three MEF2 isoforms are expressed in the heart. MEF2A and C are detected in a few cells in the rostral-most somite by day 9 of development. Their expression then proceeds in a rostro-caudal direction concurrent with somite maturation. The pattern of expression of the MEF2D isoform is similar to that of MEF2C but the amount detected is much lower. Interestingly, MEF2A is also detected as early as day 8.5 p.c. in cells of the embryonic vasculature and non-myogenic cells. These results demonstrate that MEF2 proteins are detected early in development in the somites and heart, thus supporting their importance in the early stages of the hierarchical cascade of myogenesis. Their presence in non-muscle cells further suggests they could also play a role in the determination of other mesodermal derivatives, including cells of the vasculature.