Delta 1-activated Notch inhibits muscle differentiation without affecting Myf5 and Pax3 expression in chick limb myogenesis

The myogenic basic helix-loop-helix (bHLH) transcription factors, Myf5, Myo D, myogenin and MRF4, are unique in their ability to direct a program of sp ecific gene transcription leading to skeletal muscle phenotype, The observa tion that Myf5 and MyoD can force myogenic conversion in non-muscle cells i n vitro does not imply that they are equivalent. Tn this paper, we show tha t Myf5 transcripts are detected before those of MyoD during chick limb deve lopment. The Myf5 expression domain resembles that of Pax3 and is larger th an that of MyoD, Moreover, Myf5 and Pax3 expression is correlated with myob last proliferation, while MyoD is detected in post-mitotic myoblasts, These data indicate that Myf5 and MyoD are involved in different steps during ch ick limb bud myogenesis, Myf5 acting upstream of MyoD, The progression of m yoblasts through the differentiation steps must be carefully controlled to ensure myogenesis at the right place and time during wing development, Beca use Notch signalling is known to prevent differentiation in different syste ms and species, we sought to determine whether these molecules regulate the steps occurring during chick limb myogenesis, Notch1 transcripts are assoc iated with immature myoblasts, while cells expressing the ligands Delta1 an d Serrate2 are more advanced in myogenesis, Misexpression of Delta1 using a replication-competent retrovirus activates the Notch pathway, After activa tion of this pathway, myoblasts still express Myf5 and Pax3 but have downre gulated MyoD, resulting in inhibition of terminal muscle differentiation. W e conclude that activation of Notch signalling during chick limb myogenesis prevents Myf5-expressing myoblasts from progressing to the MyoD-expressing stage.