ThebHLH transcription factors HAND2 and HAND1 function in development of sympathetic and enteric neurons

Neurons and supporting cells of the sympathetic and enteric nervous systems derive from the neural crest, whose developmental fate is influenced by growth factors and transcription factors. The work presented in this dissertation has examined the involvement of the bHLH transcription factor, HAND2, in the development of the sympathetic and enteric neurons. In the first series of experiments, it was demonstrated that signal transduction pathways, in which HAND2 is an active component, regulate the differentiation of sympathetic CA neurons. In neural crest cultures, BMP4 promotes the differentiation of CA neurons, induces the translocation of Smad1 from the cytosol to the nucleus and upregulates the expression of transcripts encoding HAND2. The CEE-derived factor(s) promotes catecholaminergic differentiation and this function is inhibited by the blockade of the activity of protein-tyrosine kinases and MapK. Inhibition of MapK activity does not affect the expression of transcripts encoding HAND2. Stimulation of PKA signaling by CAMP promotes and inhibits the differentiation of CA neurons from neural crest-derived cells grown in medium supplemented with 2% CEE and 10% CEE, respectively. PKA signaling regulates expression of transcripts encoding Phox2a and HAND2. In the second series of experiments, we have examined the expression and regulation of transcripts encoding HAND2 and the closely related HAND1 in the developing enteric nervous system. Transcripts encoding HAND2 are constantly expressed in the enteric ganglia along the gut axis throughout embryonic development. Transcripts encoding HAND1 are only expressed in the caudal gut and the expression decreases with development. We demonstrate that HAND2 is expressed in the enteric neurons while HAND1 is expressed in the gut epithelium. We have shown that co-culture with gut explants induces the expression of transcripts encoding HAND2, but not HAND1, from neural crest-derived cells. We demonstrate that BMP4 induces the expression of transcripts encoding HAND2 from gut neural crest-derived cells. The GDNF similarly induces the expression of HAND2 in neural crest-derived cells obtained from the esophagus and the colon. BMP4 also induces the expression of HAND1 in the foregut. In conclusion, HAND2 is an important transcription factor involved in the development of the sympathetic and enteric nervous systems.