Primary cilium-mediated regulation of axonal actin dynamics during vertebrate CNS development

Abstract

The formation of a nascent axon is of fundamental importance for the development of a functional central nervous system. This occurs in response to a plethora of extracellular signals which are transduced through the primary cilium. In the developing vertebrate spinal cord, differentiating neurons undergo dismantling and reassembly of the primary cilium, corresponding with a switch from canonical Gli transcription factor-dependent Sonic hedgehog (Shh) signal transduction to non-canonical Gli transcription factor-independent Shh signal transduction, thereby facilitating cell cycle exit and axon formation. Axon formation is characterised by dramatic remodelling of the actin cytoskeleton, and non-canonical Shh signalling has been associated with actin remodelling in vitro. However, the mechanisms through which primary cilium-mediated non-canonical Shh signalling modulates actin cytoskeleton remodelling during axon formation in the developing spinal cord in vivo remain unclear. In this thesis, using high-resolution live imaging in ex vivo chick embryo spinal cord slices, we reveal the complex actin dynamics that characterise axon initiation and extension. Perturbation of primary cilium function results in disruption of these actin dynamics, leading to errors in axon formation, thereby linking primary cilium-mediated signalling with the regulation of actin dynamics. Furthermore, we demonstrate that Smoothened, a key transducer of Shh signalling, interacts with the Rac1 guanine nucleotide exchange factor, Tiam1. We reveal evidence to suggest that ciliary accumulation of Smoothened in response to Shh leads to a de-repression of Tiam1 to facilitate the downstream actin polymerisation that drives axon initiation and extension. Overall, this work provides novel insights into the mechanisms through which primary cilium-mediated non-canonical Shh signalling facilitates the actin remodelling that drives axon extension in vivo.

Date of Award	13 May 2025
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Raman Das (Main Supervisor), Viki Allan (Co Supervisor) & Shane Herbert (Co Supervisor)