GENE TARGETING THE MYF-5 LOCUS WITH NLACZ REVEALS EXPRESSION OF THIS MYOGENIC FACTOR IN MATURE SKELETAL-MUSCLE FIBERS AS WELL AS EARLY EMBRYONIC MUSCLE

We have introduced the nlacZ reporter gene into the locus of the myoge nic factor gene myf-5 by homologous recombination in embryonic stem (E S) cells. Targeted ES clones were injected into precompaction morula, and the B-galactosidase expression pattern was monitored. These mice p ermit the sensitive visualization of myf-5 expression throughout the e mbryo, and provide a standard for comparing it with that seen with dif ferent myf-5/nlacZ transgenes. Thus, in a comparison using ES cells in chimaeric embryos containing the targeted or randomly integrated myf- 5/nlacZ construct, we demonstrate that 5.5 kbp of myf-5 upstream Banki ng sequence including exon1 and most of intron1 directs some skeletal muscle expression, but this is neither qualitatively nor quantitativel y equivalent to that of the endogenous gene. Myf-5 is expressed early, before terminal myogenesis takes place in the medial half of the somi te, and subsequently it is a major myogenic factor as skeletal muscle forms. All skeletal muscle shows P-galactosidase activity, even after birth, indicating that myf-5 expression is not confined to primary myo tubes, which are derived from embryonic myoblasts, but is also present in muscles containing different adult fibre types. The presence of my f-5 transcripts from the endogenous gene in older muscle was confirmed by in situ hybridization. These results suggest that the myf-5 gene i s not activated in only a subset of muscle cells and are consistent wi th the results on the MyoD knockout mice. (C) 1996 Wiley-Liss, Inc.