Different levels, but not different isoforms, of the Drosophila transcription factor DMEF2 affect distinct aspects of muscle differentiation.

mef2 genes encode alternatively spliced transcription factor isoforms that function in muscle differentiation in both Drosophila and vertebrates. Drosophila mef2 (Dmef2) has been shown to be required for the differentiation of a variety of distinct muscle types. However, many possible aspects of its function in muscle remain unexplored. There has also been no analysis in vivo of the activity of different MEF2 isoforms in any species. Our investigation centred on the role of different levels of DMEF2 in the Drosophila embryo in regulating diverse events of muscle differentiation and on the functional significance of Dmef2 alternative splicing. We used the GAL4/UAS system to both misexpress and overexpress individual DMEF2 isoforms and to rescue the different aspects of the Dmef2 mutant phenotype. Ectopic ectodermal expression of DMEF2 activated muscle gene expression and inhibited epidermal differentiation. Overexpression of DMEF2 in the mesoderm disrupted differentiation of the somatic and visceral muscle and the heart. The use of different DMEF2 levels in the rescue experiments revealed an activity range compatible with differentiation of the different muscle types: the consequence of too little or too much DMEF2 activity was disrupted differentiation. These rescue experiments also revealed that distinct DMEF2 thresholds are required for different properties within a cell and also for different cells within a muscle type and for different muscle types. Finally, each isoform functioned equivalently in these experiments, including in the stringent test of rescue of the Dmef2 mutant phenotype. Copyright 1999 Academic Press.