Identification and characterisation of novel genetic factors which promote neuronal differentiation during cerebral cortex development

Abstract

The human cerebral cortex is highly adapted to process complex information. It plays a crucial role in the control of cognitive function, consciousness and intelligent behaviour [1-4]. These functions are dependent on the proper development of the cerebral cortex, which at its early stages involves the appropriate proliferation of progenitors and generation of postmitotic neurons [2, 5, 6]. Newborn neurons need to migrate to reach their appropriate position within the developing neocortex where they undergo terminal differentiation and form appropriate synaptic connections [7, 8]. During the course of neurodevelopment, excitatory projection neurons are generated from progenitors in the ventricular zone of the neocortex, and these migrate radially towards more superficial layers. Defects in neuronal migration lead to altered neuronal positioning and laminar patterning of the cortex, which can disrupt the assembly of functional cortical circuits [9-11]. The developmental processes of neuronal migration, dendritic arborisation and synaptic connectivity are controlled by the coordinated expressions of genes, and mediated through the activities of DNA binding transcription factors (TFs) [12-14]. However, the precise molecular mechanisms that control TF activity in neuronal development within the mammalian cerebral cortex remain poorly characterised. Recent studies have shown that the transcription activator Neurogenin2 (Ngn2) [15, 16] and transcriptional Repressor Protein 58 (Rp58) [17, 18] coordinate neuronal migration in the developing cortex by regulating the expression of downstream target genes, including Rnd2 [15-18]. Specifically, it was found that Ngn2 controls the migration of embryonic cortical neurons through activation of Rnd2 expression [15, 16], whereas Rp58 suppresses Rnd2 as neurons complete their radial migration [17, 18]. While these studies highlight the interplay between these transcriptional regulators for the control of cell migration, what remains less well understood is the signalling pathways of Rnd2 in neuronal development, and the interactions between gene regulatory pathways of Ngn2 and Rp58, both of which likely specify other aspects of neuronal development, such as the dendritic differentiation of cortical neurons. The focus of this thesis is to elucidate the molecular mechanisms that guide the radial migration and terminal differentiation of cerebral cortical neurons. In studies which address the protein signalling pathway for Rnd2 during neuronal development, I have identified Bacurd2, a member of the BTB-domain containing adaptor for Cul3-mediated RhoA degradation, as a novel interacting partner which promotes radial migration within the embryonic mouse cerebral cortex. I demonstrate that the interaction between Bacurd2 and Rnd2 is crucial for their combined roles in cell migration in vivo. My cellular analysis provides a basis for understanding the combined roles for Bacurd2 and Rnd2 in order to coordinate the multipolar-to-bipolar (MP-to-BP) transition of neurons as they migrate within the embryonic cortex. In further exploration of the biological functions for Bacurd2, and its related family member Bacurd1, I present evidence to suggest that disruptions to either of these genes impair the differentiation of neurons. Finally, I describe the generation of transgenic Rp58 mouse lines to study the development of post-migratory neurons within the cerebral cortex. Altogether, this thesis provides new insights into the development of cerebral cortical neurons and identifies Bacurds as new players in this process.