The bHLH transcription factor hand2 is essential for noradrenergic differentiation of sympathetic neurons.

The basic helix-loop-helix transcription factor Hand2, together with Ascl1, Phox2a, Phox2b and Gata2/Gata3, is induced by bone morphogenetic proteins in neural crest-derived precursor cells during sympathetic neuron generation. Hand2 overexpression experiments and the analysis of its function at the Dbh promotor implicated Hand2 in the control of noradrenergic gene expression. Using the zebrafish hand2 deletion mutant hands off, we have now investigated the physiological role of hand2 in the development of sympathetic ganglia. In hands off mutant embryos, sympathetic precursor cells aggregate to form normal sympathetic ganglion primordia characterized by the expression of phox2b, phox2a and the achaete-scute family member zash1a/ascl1. The expression of the noradrenergic marker genes th and dbh is strongly reduced, as well as the transcription factors gata2 and tfap2a (Ap-2alpha). By contrast, generic neuronal differentiation seems to be unaffected, as the expression of elavl3 (HuC) is not reduced in hands off sympathetic ganglia. These results demonstrate in vivo an essential and selective function of hand2 for the noradrenergic differentiation of sympathetic neurons, and implicates tfap2a and gata2 as downstream effectors.