CELLULAR INTERACTIONS ORCHESTRATING TRIGEMINAL GANGLION FORMATION

Abstract

Cranial sensory ganglia are uniquely positioned within the vertebrate head to relay sensations from the environment back to the brain. The largest of these, the trigeminal ganglion, houses all of the cell bodies for the trigeminal nerve and conveys somatosensory information from a heterogenous neuronal population derived from neural crest cells and placode cells. Although its cellular composition is well characterized, the molecular mechanisms underlying trigeminal gangliogenesis, and subsequent maturation of the trigeminal nerve, remain understudied. However, studies on cadherin proteins, a family of cell adhesion molecules, have shed light on these processes. Neural cadherin (N-cadherin) is a classical cadherin expressed in neurons and typically engages in homophilic cadherin interactions to strengthen adhesion between cells. Prior studies have revealed its importance in placode-derived neurons during their early coalescence with neural crest cells; however, additional functions for N-cadherin throughout gangliogenesis are not known. To this end, this dissertation sought to explore the role of N-cadherin throughout trigeminal neurodevelopment. Using a GFP Reconstitution Across Synaptic Partners (GRASP) assay during early ganglion assembly, we discovered that N-cadherin-expressing placode-derived neurons engage in heterophilic interactions with Cadherin-7-expressing neural crest cells to mediate initial gangliogenesis. To examine the necessity of N-cadherin across development, we used a translation-blocking morpholino to reduce N-cadherin expression in placode-derivedneurons. N-cadherin depletion produced sustained morphological defects in the ganglion and nerve branches across multiple days of development, including when neural crest cells differentiate into neurons. Building off this, we showed that N-cadherin-mediated adhesion supports axon outgrowth of both placode- and neural crest-derived trigeminal neurons, and identified a non-cell autonomous role of N-cadherin-expressing placode-derived neurons on neural crest neurite production. In our last study, we explored interactions between N-cadherin and TrkA, a receptor tyrosine kinase expressed in nociceptive neurons at later developmental stages. We characterized several post-translational modifications of TrkA, including N- and O-linked glycosylation as well as sialyation. We also discovered that N-cadherin interacts with distinct glycoforms of TrkA in a compartment-specific manner, differing between soma and axons as TrkA matures through the secretory pathway. Membrane-bound protein extractions from trigeminal ganglia further revealed multiple N-cadherin-TrkA interactions corresponding to different stages of TrkA maturation, suggesting that these interactions occur not only intracellularly but on the plasma membrane as well. Taken together, the results from this dissertation elucidate key molecular interactions underlying trigeminal gangliogenesis and provide new insights into the mechanisms orchestrating sensory neurodevelopment.