Presence of nerve growth factor and its receptors in an in vitro model of islet cell development: implication in normal islet morphogenesis.

In vivo, the differentiation of pancreatic islet stem cells depends on unknown soluble factors produced by the mesenchyme surrounding these cells. We have previously demonstrated that, like some neuronal cells, different beta-cell lines express functional nerve growth factor (NGF) receptors and can respond to NGF by extending neurite-like processes. NGF receptors are also expressed in vivo in mature rat islets and early during development in pancreatic ductular cells, which represent putative beta-stem cells. In this study, we have further characterized an in vitro model of islet development and studied the expression of NGF receptors and its ligand in this model. We have demonstrated the expression of Trk-A messenger RNA coding for the high affinity NGF receptor in islet cells and the localization of Trk protein in both alpha- and beta-islet cells. Moreover, the cells, from which islet cells "bud," also express Trk-A. Furthermore, NGF is produced and secreted by the nonendocrine cells surrounding the islets, suggesting a possible paracrine mode of action of NGF on the adjacent islet cells. Finally, islet morphogenesis is significantly retarded in the presence of K252a, an inhibitor of the tyrosine kinase activity of the family of Trk receptors, suggesting an implication of the neurotrophin-neurotrophin receptor axis in islet development.