TYROSINE PHOSPHORYLATION AND ENHANCED EXPRESSION OF PAXILLIN DURING NEURONAL DIFFERENTIATION IN-VITRO

Tyrosine phosphorylation has been implicated as a means by which neuri te outgrowth is regulated. Because paxillin is a tyrosine-phosphorylat ed protein that may play a role in regulating cell morphology, we exam ined its expression in neuronal cells and how its tyrosine phosphoryla tion is related to neurite outgrowth. Paxillin was identified in sever al neuronal cell lines with an increased level upon differentiation. I n SH-SY5Y cells, paxillin was localized along with actin filaments whe re processes extended from the cell body and in neuritic growth cones. Furthermore, paxillin was tyrosine-phosphorylated in SH-SY5Y cells up on adhesion to laminin. Paxillin tyrosine phosphorylation paralleled t hat of focal adhesion kinase and occurred as cell spreading, and neuri te formation was initiated. Colchicine blocked neurite outgrowth but h ad no effect on cell spreading or on paxillin or focal adhesion kinase tyrosine phosphorylation. In contrast, cytochalasin D eliminated neur ite outgrowth, cell spreading, and the tyrosine phosphorylation of pax illin and focal adhesion kinase. These results show that paxillin is t yrosine phosphorylated upon integrin ligand binding in neuronal cells. Our findings suggest that paxillin tyrosine phosphorylation is linked to a remodeling of the actin cytoskeleton that leads to cell spreadin g and neurite formation and thus a differentiated neuronal phenotype.