Analysis of mice carrying targeted mutations of the glucocorticoid receptor gene argues against an essential role of glucocorticoid signalling for generating adrenal chromaffin cells

Molecular mechanisms underlying the generation of distinct cell phenotypes is a key issue in developmental biology. A major paradigm of determination of neural cell fate concerns the development of sympathetic neurones and neuroendocrine chromaffin cells from a common sympathoadrenal (SA) progenitor cell. Two decades of in vitro experiments have suggested an essential role of glucocorticoid receptor (GR)-mediated signalling in generating chromaffin cells. Targeted mutation of the GR should consequently abolish chromaffin cells. The present analysis of mice lacking GR gene product demonstrates that animals have normal numbers of adrenal chromaffin cells. Moreover, there are no differences in terms of apoptosis and proliferation or in expression of several markers (e.g. GAP43, acetylcholinesterase, adhesion molecule L1) of chromaffin cells in GR-deficient and wild-type mice. However, GR mutant mice lack the adrenaline-synthesizing enzyme PNMT and secretogranin II. Chromaffin cells of GR-deficient mice exhibit the typical ultrastructural features of this cell phenotype, including the large chromaffin granules that distinguish them from sympathetic neurones. Peripherin, an intermediate filament of sympathetic neurones, is undetectable in chromaffin cells of GR mutants. Finally, when stimulated with nerve growth factor in vitro, identical proportions of chromaffin cells from GR-deficient and wild-type mice extend neuritic processes. We conclude that important phenotypic features of chromaffin cells that distinguish them from sympathetic neurones develop normally in the absence of GR-mediated signalling. Most importantly, chromaffin cells in GR-deficient mice do not convert to a neuronal phenotype. These data strongly suggest that the dogma of an essential role of glucocorticoid signalling for the development of chromaffin cells must be abandoned.