Ephrins stimulate or inhibit neurite outgrowth and survival as a function of neuronal cell type

The Eph family of tyrosine kinase receptors and ligands play key roles in c ell segregation and axon targeting in the developing nervous system. Intera ctions between the ligands and receptors cause repulsion or degeneration of receptor-positive axons from several brain regions including the retina, h ippocampus, thalamus, and midbrain dopaminergic system. We extend these pre vious observations by showing that three A-ephrins also negatively regulate the growth of neurites from striatal and olfactory neurons. In addition to negative effects, however, we also report a trophic activity of the A-ephr ins: Ephrin-A2 and A5 promote survival and neurite outgrowth of sympathetic neurons. These observations provide support to the notion that ephrins may function as either negative or positive signals in the developing nervous system. (C) 2000 Wiley-Liss, Inc.