N-cadherin facilitates trigeminal sensory neuron outgrowth and target tissue innervation.

During embryonic development, diverse cell types coordinate to form functionally complex tissues. Exemplifying this process, the trigeminal ganglion emerges from the condensation of two distinct precursor cell populations, cranial placodes and neural crest, with neuronal differentiation of the former preceding the latter. While the dual origin of the trigeminal ganglion has been understood for decades, the molecules orchestrating formation of the trigeminal ganglion from these precursors remain relatively obscure. Initial assembly of the trigeminal ganglion is mediated by cell adhesion molecules, including neural cadherin (N-cadherin), which is required by placodal neurons to properly condense with other neurons and neural crest cells. Whether N-cadherin is required for later growth and target innervation by trigeminal ganglion neurons, however, is unknown. To this end, we depleted N-cadherin from chick trigeminal placode cells and uncovered decreases in trigeminal ganglion size, nerve growth, and target innervation in vivo at later developmental stages. Furthermore, blocking N-cadherin-mediated adhesion prevented axon extension in some placode-derived trigeminal neurons in vitro . This indicates the existence of neuronal subtypes that may have unique requirements for N-cadherin for outgrowth, and points to this subset of placodal neurons as potential pioneers that serve as templates for additional axon outgrowth. Neurite complexity was also decreased in neural crest-derived neurons in vitro in response to N-cadherin knockdown in placode cells. Collectively, these findings reveal persistent cell autonomous and non-cell autonomous functions for N-cadherin, thus highlighting the critical role of N-cadherin in mediating reciprocal interactions between neural crest and placode neuronal derivatives during trigeminal ganglion development. Significance Statement: Our findings are significant because they demonstrate how neurons derived from two distinct cell populations, neural crest and placode cells, coordinate the outgrowth of their axons in time and space to generate the trigeminal ganglion using the cell adhesion molecule N-cadherin. Notably, our results provide evidence for the existence of subpopulations of neurons within the trigeminal ganglion that differentially require N-cadherin to facilitate axon outgrowth, and hint at the possibility that trigeminal pioneer neurons are derived from placode cells while followers arise from both placode and neural crest cells. These studies provide new insight into trigeminal gangliogenesis that will likely be translatable to other cranial ganglia and vertebrate species.

Neural crest cell-placodal neuron interactions are mediated by Cadherin-7 and N-cadherin during early chick trigeminal ganglion assembly.

Background: Arising at distinct positions in the head, the cranial ganglia are crucial for integrating various sensory inputs. The largest of these ganglia is the trigeminal ganglion, which relays pain, touch and temperature information through its three primary nerve branches to the central nervous system. The trigeminal ganglion and its nerves are composed of derivatives of two critical embryonic cell types, neural crest cells and placode cells, that migrate from different anatomical locations, coalesce together, and differentiate to form trigeminal sensory neurons and supporting glia. While the dual cellular origin of the trigeminal ganglion has been known for over 60 years, molecules expressed by neural crest cells and placode cells that regulate initial ganglion assembly remain obscure. Prior studies revealed the importance of cell surface cadherin proteins during early trigeminal gangliogenesis, with Cadherin-7 and neural cadherin (N-cadherin) expressed in neural crest cells and placode cells, respectively. Although cadherins typically interact in a homophilic ( i.e., like) fashion, the presence of different cadherins expressed in neural crest cells and placode cells raises the question as to whether heterophilic cadherin interactions may also be occurring. Given this, the aim of the study was to understand whether Cadherin-7 and N-cadherin were interacting during initial trigeminal ganglion formation. Methods: To assess potential interactions between Cadherin-7 and N-cadherin, we used biochemistry and innovative imaging assays conducted in vitro and in vivo, including in the forming chick trigeminal ganglion. Results: Our data revealed a physical interaction between Cadherin-7 and N-cadherin. Conclusions: These studies identify a new molecular basis by which neural crest cells and placode cells can aggregate in vivo to build the trigeminal ganglion during embryogenesis.