Myf5 is a novel early axonal marker in the mouse brain and is subjected topost-transcriptional regulation in neurons

Myf5 is a key basic Helix-Loop-Helix transcription factor capable of conver ting many non-muscle cells into muscle. Together with MyoD it is essential for initiating the skeletal muscle programme in the embryo, We previously i dentified unexpected restricted domains of Myf5 transcription in the embryo nic mouse brain, first revealed by Myf5-nlacZ(+/-) embryos (Tajbakhsh, S, a nd Buckingham, M. (1995) Development 121, 4077-4083). We have now further c haracterized these Myf5 expressing neurons. Retrograde labeling with diI, a nd the use of a transgenic mouse line expressing lacZ under the control of Myf5 regulatory sequences, show that Myf5 transcription provides a novel ax onal marker of the medial longitudinal fasciculus (mlf) and the mammilloteg mental tract (mtt), the earliest longitudinal tracts to be established in t he embryonic mouse brain. Tracts projecting caudally from the de developing olfactory system are also labelled. nlacZ and lacZ expression persist in t he adult brain, in a few ventral domains such as the mammillary bodies of t he hypothalamus and the interpeduncular nucleus, potentially derived from t he embryonic structures where the Myf5 gene is transcribed. To investigate the role of Myf5 in the brain, we monitored Myf5 protein acc umulation by immunofluorescence and immunoblotting in neurons transcribing the gene. Although Myf5 was detected in muscle myotomal cells, it was absen t in neurons. This would account for the lack of myogenic conversion in bra in structures and the absence of a neural phenotype in homozygous null muta nts. RT-PCR experiments show that the splicing of Myf5 primary transcripts occurs correctly in neurons, suggesting that the lack of Myf5 protein accum ulation is due to regulation at the level of mRNA translation or protein st ability. In the embryonic neuroepithelium, Myf5 is transcribed in different iated neurons after the expression of neural basic Helix-Loop-Helix transcr iption factors. The signalling molecules Wnt1 and Sonic hedgehog, implicate d in the activation of Myf5 in myogenic progenitor cells in the somite, are also produced in the viscinity of the Myf5 expression domain in the mesenc ephalon. We show that cells expressing Wnt1 can activate neuronal Myf5-nlac Z gene expression in dissected head explants isolated from E9.5 embryos. Fu rthermore, the gene encoding the basic Helix-Loop-Helix transcription facto r mSim1 is expressed in adjacent cells in both the somite and the brain, su ggesting that signalling molecules necessary for the activation of mSim1 as well as Myf5 are present at these different sites in the embryo. This phen omenon may be widespread and it remains to be seen how many other potential ly potent regulatory genes, in addition to Myf5, when activated do not accu mulate protein at inappropriate sites in the embryo.