Mechanism of polarized protrusion formation on neuronal precursors migrating in the developing chicken cerebellum.

Directed cell migration results from the polarization of the cellular motile apparatus by integration of extracellular signals, which are presented in a three-dimensional, spatiotemporal manner in living organisms. To investigate the mechanism underlying the highly polarized and directional nature of migration in vivo, we have developed an imaging system for observing rhombic lip cell migration in the developing chicken cerebellum. First, we show that Cdc42 is the central regulator of the overall polarity, morphology and protrusion formation in these cells. However, perturbation of canonical polarity effectors of Cdc42, e.g. the Par6-Par3-aPKC complex, does not disrupt the cell asymmetry, whereas it affects orientation of the tip of the leading process. In contrast to Cdc42, Rac is required for the generation of protrusions but not the overall polarity. Function interference of class IA phosphoinositide 3-kinase abrogates both directional extension and maintenance of the long leading process, whereas PTEN modulates the size of the protrusion. Actomyosin contractility is important for coordinated spreading of the tip of the leading process in situ. Finally, ErbB4 functions in the generation of protrusions on the rhombic lip cells. These results suggest that polarized protrusion formation on neuronal precursors may occur by a more divergent and complex mechanism than that seen in studies of other cell types growing on planar substrates.