Crosstalk between canonical and non-canonical Wnt signaling in C. elegans neuroblast migration converges on the Rac GTPase CED-10

Canonical and non-canonical Wnt signaling can have opposing effects on cell migration, but how input from these pathways is coordinated to control a common cellular process remains poorly understood. The Caenorhabditis elegans QR neuroblast lineage provides a tractable system to study this interplay in vivo at single-cell resolution. In this lineage, non-canonical Wnt signaling promotes migration through PIX-1, a guanine nucleotide exchange factor (GEF) that activates members of the Rho-family of small GTPases. In contrast, canonical Wnt signaling terminates migration through induction of RGA-9b, a GTPase-activating domain containing protein (GAP) of the ARHGAP family that is predicted to inhibit small GTPases. The nature of these downstream regulators suggests that the crosstalk between the two Wnt pathways converges on a shared small GTPase target. Here, we identify the Rac GTPase CED-10 as this common effector. Hypomorphic mutations of ced-10 reduce QR descendant migration without disrupting anterior polarization, and genetic interactions place ced-10 downstream of both non-canonical and canonical Wnt signaling. We further show that PIX-1 and RGA-9b regulate CED-10 through opposing activities: PIX-1 promotes CED-10 activation, whereas RGA-9b inhibits it. Mutations that disrupt the predicted GEF or GAP activities of PIX-1 and RGA-9b, respectively, impair their regulation of migration, while biochemical assays show that the two proteins antagonistically regulate the level of active CED-10. Together, these findings demonstrate that crosstalk between canonical and non-canonical Wnt signaling is mediated through antagonistic regulation of CED-10/Rac, revealing a novel role of Rho-family small GTPases as integrators of Wnt signaling in cell migration.