Development of the zebrafish dorsal root ganglia: The role of Shh signaling, Neurogenin1 and sensory deprived in specification of DRG neurons

The dorsal root ganglia (DRG) are the sensory ganglia that comprise the main somatosensory system in vertebrate organisms. Neurons and glia of the DRG derive from trunk and tail neural crest cells that coalesce at the lateral edge of the spinal cord. The mechanisms by which neural crest cells are specified to adopt a DRG sensory fate are poorly understood. The work presented in this thesis defines several factors important for specification of zebrafish ( Danio rerio) DRG neurons. I demonstrate that the basic helix-loop-helix transcription factor, Neurogenin1 (Ngn1), is generally required for the specification of sensory neurons, including DRG neurons. ngn1 is expressed in DRG precursors at the site of the nascent ganglion. Anti-sense morpholino-mediated knockdown of zebrafish Ngn1 function results in the complete absence of DRG neurons as well as loss of cranial ganglia and Rohon-Beard primary sensory neurons. In comparison, I show that sensory deprived ( sdp) is required for the development specifically of DRG neurons. The zebrafish sdp mutation results in the complete loss of DRG neurons yet other sensory neurons are unaffected and other neural crest derivatives develop normally. Moreover, I show that sdp is required for the expression of ngn1 in DRG precursors. Through analysis of several zebrafish mutants and through use of a pharmacological inhibitor, I show that Sonic hedgehog (Shh) signaling is also necessary for the development of DRG neurons. This requirement to receive a Shh signal is cell autonomous with respect to DRG precursors and occurs prior to the onset of ngn1 expression in the nascent ganglion. DRG expression of ngn1 is abolished in the Shh signaling mutants. Finally, I provide evidence for an additional role for Shh signaling during DRG development. Although loss of Shh signaling results in absence of DRG neurons, in some segments I also observe ganglia containing up to tenfold more neurons than normal ganglia. These abnormally large ganglia form in the absence of either Ngn1 or sdp function suggesting that these neurons may derive from a separate DRG precursor population that is normally repressed by Shh signaling.