AL-1-INDUCED GROWTH CONE COLLAPSE OF RAT CORTICAL-NEURONS IS CORRELATED WITH REK7 EXPRESSION AND REARRANGEMENT OF THE ACTIN CYTOSKELETON

Previous experiments identified AL-1 as a glycosylphosphatidylinositol (GPI)-linked ligand for the Eph-related receptor, REK7, and showed th at a REK7-lgG fusion protein blocks axon bundling in co-cultures of co rtical neurons on astrocytes, suggesting a role for REK7 and AL-1 in a xon fasciculation. Subsequent identification of RAGS, the chick homolo gue of AL-1, as a repellent axon guidance molecule in the developing c hick visual system led to speculation that AL-1, expressed on astrocyt es, provides a repellent stimulus for cortical axons, inducing them to bundle as an avoidance mechanism. Using a growth cone collapse assay to test this hypothesis, we show that a soluble AL-1-lgG fusion protei n is a potent collapsing factor for embryonic rat cortical neurons. Th e response is strongly correlated with REK7 expression, implicating RE K7 as a receptor mediating AL-I-induced collapse. Morphological collap se is preceded by an AL-1-lgG-induced reorganization of the actin cyto skeleton that resembles the effects of cytochalasin D. This suggests a pathway whereby REK7 activation by AL-I leads to perturbation of the actin cytoskeleton, possibly by an effect on actin polymerization, fol lowed by growth cone collapse. We further show that AL-1-lgG causes co llapse of rat hippocampal neurons and rat retinal ganglion cells. Thes e data suggest a role for REK7 and AL-1 in the patterning of axonal co nnections in the developing cortex, hippocampus and Visual system.