Rac1 and Nectin3 are essential for planar cell polarity-directed axon guidance in the peripheral auditory system

Spiral ganglion neurons (SGNs) carry sound information from the cochlea to the hindbrain, and innervate either inner or outer hair cells. Type II SGNs (SGNIIs) extend peripheral afferents towards outer hair cells, which make a characteristic 90° turn towards the cochlear base and innervate multiple outer hair cells. It has been shown that the planar cell polarity (PCP) pathway acts non-autonomously in the cochlear epithelium to guide SGNII peripheral afferent turning. However, the underlying mechanisms are unknown. Here, we show that PCP signaling regulates junctional localization of the small GTPase Rac1 and the cell adhesion molecule Nectin3 in mouse cochlear supporting cells (SCs), which serve as intermediate targets of SGNII peripheral afferents. Loss of Rac1 or Nectin3 partially phenocopied SGNII peripheral afferent turning defects in Vangl2 mutants. We present evidence that Rac1 plays a non-autonomous role in part by regulating the localization of core PCP proteins Vangl2 and Dvl3 at the SC-SC junctions, while Nectin3 likely serves a cell adhesion function to control SGNII afferent turning. Together, these experiments identify Nectin3 and Rac1 as regulators of PCP-directed SGNII axon guidance in the cochlea.