IDENTIFICATION OF KEY MOLECULES IN PLACODE-DERIVED NEURONS THAT COORDINATE CHICK TRIGEMINAL GANGLIOGENESIS

Abstract

The trigeminal nerve is the largest of the cranial nerves, possessing three main branches (ophthalmic, maxillary, and mandibular) and relaying sensations of pain, touch, and temperature from the face and head to the brain. Cell bodies of this nerve are positioned in the trigeminal ganglion, which arises from the coalescence of neural crest cells and placode cells. These progenitor cells give rise to trigeminal sensory neurons, with placode cell differentiation occurring first. While the dual cellular origin of the trigeminal ganglion has been known for decades, the molecular mechanisms controlling trigeminal ganglion development remain obscure. To elucidate molecules involved in this process, we performed RNAsequencing on the forming chick trigeminal ganglion when only placode cells contribute neurons and identified Neurogenin2 (Neurog2), Neuronal Differentiation 1 (NeuroD1), and Elongator acetyltransferase complex subunit 1 (Elp1) for further study. While Neurog2, NeuroD1, and Elp1 have established roles in neurogenesis in other systems, their functions in placode cells during trigeminal gangliogenesis had yet to be investigated. To address this, we used the chick embryo due to experimental advantages afforded by this model for the study of trigeminal placode cells and trigeminal ganglion development. Using morpholino antisense oligonucleotides, we depleted Neurog2, NeuroD1, or Elp1 from trigeminal placode cells and demonstrated each are essential for proper trigeminal ganglion development. Knockdown of Neurog2, NeuroD1, or Elp1 reduced trigeminal ganglion size and led to aberrant innervation of the eye by the ophthalmic branch. While depletion of Neurog2 and NeuroD1 had opposite effects on the width of the ophthalmic branch, Elp1 reduction appeared to have no effect. However, Elp1 knockdown led to less compact trigeminal ganglion nerve branches, decreased axon projections, and general disorganization of neurons and neural crest cells. Taken together with prior findings, our results suggest a novel interrelationship among Neurog2, NeuroD1, and Elp1 during trigeminal gangliogenesis. Our results have potential high significance for providing new insights into the function of Neurog2, NeuroD1, and Elp1 in trigeminal ganglion development and the etiology of human and animal diseases arising from defects in neural crest cells and/or placode cells.