Neurogenin2 expression together with NeuroM regulates GDNF family neurotrophic factor receptor α1 (GFRα1) expression in the embryonic spinal cord

In many regions of the nervous system, the combinatorial action of transcriptional factors specifies the individual fate of neuronal subtypes. Contrary to this, we report that a single transcriptional factor controls a phenotype shared by different subtypes of neurons, namely the expression of a neurotrophic factor receptor in the spinal cord. Along the dorsoventral axis of the chick embryo spinal cord, the expression pattern of a specific receptor for glial cell line derived-neurotrophic factor (GDNF family of receptors alpha 1: GFR alpha 1) was related to that of two basic helix-loop-helix (bHLH) transcriptional factors (NeuroM and Neurogenin2: Ngn2). In ovo electroporation in the chick embryo revealed that the overexpression of NeuroM alone was sufficient to induce ectopic GFR alpha 1 expression without overt neuronal differentiation, whereas the suppression of NeuroM activity resulted in the specific loss of GFR alpha 1 expression, indicating that NeuroM may act as a differentiation factor for GFR alpha 1 expression. Ngn2 overexpression was also sufficient to induce precocious GFR alpha 1 expression. However, the forced expression of both obligate suppressor and activator forms of Ngn2 also induced aberrant GFR alpha 1 expression. Thus, any deviation from an optimum level of Ngn2 expression resulted in aberrant GFR alpha 1 expression. Consistent with this, manipulation of Ngn2 expression levels by other bHLH factors also resulted in ectopic GFR alpha 1 expression. For example, the downregulation by Ascl1 and the upregulation by Ptf1a induced ectopic GFR alpha 1 expression, irrespective of endogenous expression patterns of Ascl1 and Ptf1a (Ascl1/Ptf1) in the spinal cord. The suppression of Ascl1/Ptf1a activities abolished Ngn2 and GFR alpha 1 expression, even in Ascl1/Ptf1a-negative regions. These data indicate the presence of a distinct regulatory sequence for a determinant of GFR alpha 1 expression, in which Ascl1/Ptf1a may competitively intervene to stochastically modulate default Ngn2 expression levels. Thus, Ngn2 together with NeuroM serves as readout to regulate GFR alpha 1 expression, which occurs in multiple subtypes of spinal neurons. (c) 2012 Elsevier Inc. All rights reserved.