Regulatory Interactions Governing Peripheral Nervous System Specification in the Ascidian Ciona robusta
Ciliated mechanoreceptors, such as our inner ear hair cells, receive vibrational and touch input from an animal’s surroundings and direct that information into the their brain via neuronal transmission. In ascidians, the vertebrates’ closest extant relatives, ciliated mechanoreceptors called epidermal sensory neurons (ESNs) are found primarily dorsally and ventrally along the larval tails. Ascidian ESNs are ciliated neurons which project their own axon, and thus only analogous to vertebrate inner ear mechanosensory systems.
Initially governed by Nodal signaling, the dominant gene that sets up the territory from which ESNs emerge is Msxb. In conjunction with Fgf9/16/20 signaling dorsally and Admp signaling ventrally, Msxb is responsible for establishing the future dorsal and ventral larval midlines as neurogenic. Once the field is further established by expression of Achaete-scute homolog (among others), Notch signaling locks in the number and spacing of ESNs through lateral inhibition. Pou4, a gene necessary to produce an ESN, is also sufficient: Pou4 has a special capacity for converting all epidermal cells into ESNs (or ESN-like cells). During normal development, Pou4 is involved in activating other transcription factors such as NeuroD and Atonal, and, as studied herein, perhaps MyT1. Pou4, an ancient homeobox gene probably involved in neurogenesis of the metazoan last common ancestor, remains primarily neurogenic in all animals in which it has been studied.
The Introduction of this Dissertation is an extensive review of Pou4 genes throughout the animal kingdom, examining their original roles, gene copy number history, and the patterns of systems they are deployed in. Chapter 1 is previously published work detailing techniques for using CRISPR-Cas technology to insert DNA sequences of interest into the C. robusta genome. Chapter 2 examines subsequent transcriptomes from expressing the three mouse Pou4 genes in the C. robusta larval epidermis, and demonstrates the ability of those mouse Pou4 genes to rescue a Pou4 knockdown loss of cilia phenotype. Chapter 3 concludes with my principal interest throughout my graduate tenure: an examination of genetic relationships that work to specify and differentiate ESNs; I use overexpression and knockdown data to build a working model for a GRN that involves gene relationships between ESNs and adjacent non-ESN midline epidermis.