Developmental mechanisms underlying the functional diversification of facial motor neurons

About This Grant

PROJECT SUMMARY/ABSTRACT During development, neurons that are generated at a single location can go on to innervate different targets and carry out different functions. A fundamental question in developmental neurobiology is to understand how this functional diversification occurs. The developing brainstem gives rise to facial branchiomotor neurons (FBMNs), which innervate different sets of muscles in the face and collectively control all facial movements. While the developmental programs specifying FBMNs are well understood, differences between FBMNs that enable them to wire to different upstream neurons and muscle targets to control different behaviors remain poorly understood. Agenesis, miswiring, and other developmental anomalies of FBMNs cause Congenital Cranial Dysinnervation Disorders (CCDDs), characterized by clinical presentations including facial paralysis, depending on which neurons are affected. A better understanding of FBMN development will also advance our understanding of the etiologies of CCDDs. The evolutionary conservation of FBMNs enables investigation in the transparent zebrafish embryo. To identify transcriptional heterogeneity among developing FBMNs, single-cell RNA sequencing and in situ hybridization were used, identifying two transcriptionally distinct populations in the facial motor nucleus, the brainstem structure composed of FBMN cell bodies. The positions of the two transcriptional populations correlate with developmental age: earlier-born neurons migrate to ventral positions and express different genes than later- born neurons, which migrate to dorsal positions. While previous studies have identified some topography in the facial motor nucleus, the relationship between topography, birth order, and gene expression remains unclear. These observations raise the possibility that birth order specifies distinct gene expression programs, which specify innervation targets. Aim 1 proposes to use high-resolution live imaging to uncover the organization of the facial motor nucleus and determine whether this organization is a product of developmental timing. Aim 2 proposes to combine rapid reverse genetic screening and multiplexed single-cell transcriptomic phenotyping to identify functional regulators of facial motor nucleus development, with specific insight into the etiology of the CCDD, Hereditary Congenital Facial Paresis.