Prickle1 is Necessary for the Caudal Migration of Murine Facial Branchiomotor Neurons

Facial branchiomotor neurons (FBMs) of vertebrates typically develop in rhombomere 4 (r4), and in mammals and several other vertebrate taxa, migrate caudally into r6 and subsequently laterally and ventrally to the pial surface. How similar or dissimilar these migratory processes between species are at a molecular level remains unclear. In zebrafish and mouse, mutations in certain PCP genes disrupt normal caudal migration of FBMs. Zebrafish prickle1a (prickle-like 1a) and prickle1b, two orthologs of Prickle1, act non-cell-autonomously and cell-autonomously, respectively, to regulate FBM migration. Here, we show that, in Prickle1 (C251X/C251X) mice which have reduced Prickle1 expression, the caudal migration of FBMs is affected. Most FBM neurons do not migrate caudally along the floor plate. However, some neurons perform limited caudal migration such that the neurons eventually lie near the pial surface from r4 to anterior r6. FBMs in Prickle1 (C251X/C251X) mice survive until P0 and form an ectopic nucleus dorsal to the olivo-cochlear efferents of r4. Ror2, which modifies the PCP pathway in other systems, is expressed by the migrating mouse FBMs, but is not required for FBM caudal migration. Our results suggest that, in mice, Prickle1 is part of a molecular mechanism that regulates FBM caudal migration and separates the FBM and the olivo-cochlear efferents. This defective caudal migration of FBMs in Prickle1C251X mutants resembles Vangl2 mutant defects. In contrast to other developing systems that show similar defects in Prickle1, Wnt5a and Ror2, the latter two only have limited or no effect on FBM caudal migration.