ABSTRACT
The development of the enteric nervous system is dependent upon the actions of glial cell line-derived neurotrophic factor (GDNF) on neural crest-derived precursor cells in the embryonic gut. GDNF treatment of cultured enteric precursor cells leads to an increase in the number of neurons that develop and/or survive. Here we demonstrate that, although GDNF promoted an increase in neuron number at all embryonic ages examined, there was a developmental shift from a mitogenic to a trophic response by the developing enteric neurons. The timing of this shift corresponded to developmental changes in gut expression of GFRα-1, a co-receptor in the GDNF-Ret signaling complex. GFRα-1 was broadly expressed in the gut at early developmental stages, at which times soluble GFRα-1 was released into the medium by cultured gut cells. At later times, GFRα-1 became restricted to neural crest-derived cells. GFRα-1 could participate in GDNF signaling when expressed in cis on the surface of enteric precursor cells, or as a soluble protein. The GDNF-mediated response was greater when cell surface, compared with soluble, GFRα-1 was present, with the maximal response seen the presence of both cis and trans forms of GFRα-1. In addition to contributing to GDNF signaling, cell-surface GFRα-1 modulated the specificity of interactions between GDNF and soluble GFRαs. These experiments demonstrate that complex, developmentally regulated, signaling interactions contribute to the GDNF-dependent development of enteric neurons.
