Cranial sensory ganglia are components of the peripheral nervous system that possess a significant somatosensory role and include neurons within the trigeminal and epibranchial nerve bundles. Although it is well established that these ganglia arise from interactions between neural crest and neurogenic placode cells, the molecular basis of ganglia assembly is still poorly understood. Members of the Annexin protein superfamily play key roles in sensory nervous system development throughout metazoans. Annexin A6 is expressed in chick trigeminal and epibranchial placode cell-derived neuroblasts and neurons, but its function in cranial ganglia formation has not been elucidated. To this end, we interrogated the role of Annexin A6 using gene perturbation studies in the chick embryo. Our data reveal that placode cell-derived neuroblasts with reduced Annexin A6 levels ingress and migrate normally to the ganglionic anlage, where neural crest cell corridors correctly form around them. Strikingly, while Annexin A6-depleted placode cell-derived neurons still express mature neuronal markers, they fail to form two long processes, which are considered morphological features of mature neurons, and no longer innervate their designated targets due to the absence of this bipolar morphology. Moreover, overexpression of Annexin A6 causes some placode cell-derived neurons to form extra protrusions alongside these bipolar processes. These data demonstrate that the molecular program associated with neuronal maturation is distinct from that orchestrating changes in neuronal morphology, and, importantly, reveal Annexin A6 to be a key membrane scaffolding protein during sensory neuron membrane biogenesis. Collectively, our results provide novel insight into mechanisms underscoring morphological changes within placode cell-derived neurons that are essential for cranial gangliogenesis.
