The receptor phosphatase HmLAR2 collaborates with focal adhesion proteins in filopodial tips to control growth cone morphology

Receptor protein tyrosine phosphatases (RPTPs) have been shown to play key roles in regulating axon guidance and synaptogenesis. HmLAR2, one of two closely related LAR-like RPTPs in the embryonic leech, is expressed in a few central neurons and in a unique segmentally-iterated peripheral cell, the comb cell (CC). Here we show that tagged HmLAR2-EGFP has a punctate pattern of expression in the growth cones of the CC, particularly at the tips of extending filopodia. Moreover, although expression of the wild-type EGFP-tagged receptor does not affect CC growth cone morphology, expression of a putative dominant-negative mutant of the receptor, CS-HmLAR2, leads to the enlargement of the growth cones, a shortening of filopodia, and errors in cellular tiling. RNAi of several candidate substrate signaling proteins, Lena (leech Ena/Vasp), β-integrin and paxillin, but not β-catenin, phenocopies particular aspects of the effects of HmLAR2 RNAi. For paxillin, which co-localizes with HmLAR2 at growth cone puncta, knock-down led to a reduction in the number of such puncta. Together, our data suggests that HmLAR2 regulates the morphology of the growth cone by controlling F-actin polymerization and focal adhesion complexes.