Protocadherin 15 modulates oligodendrocyte progenitor cell proliferation and contact-mediated self-repulsion through two distinct signalling pathways

Oligodendrocyte progenitor cells (OPCs) comprise the largest proliferating cell population in the healthy, adult brain. Dysregulation of OPCs has been associated with the formation of pediatric and adult gliomas. The purpose of this research is to understand: how non-clustered protocadherins (Pcdhs) allow OPCs to maintain their even distribution throughout the central nervous system (CNS); whether nonclustered Pcdhs regulate OPC proliferation and the generation of new oligodendrocytes (OLs) throughout life, and whether the dysregulation of nonclustered Pcdh signalling can initiate glioma formation. Pcdh15 is a member of the cadherin superfamily of transmembrane proteins that mediate calcium-dependent cell adhesion. In chapter 1, I review literature related to the development and function of OPCs in the CNS, and outline key genes that are expressed by OPCs, including protocadherin 15 (Pcdh15). In neurons, various members of the protocadherin superfamily regulate neuron survival, neurite outgrowth and distribution. Pcdh15 is particularly critical for mechanoelectrical transduction in auditory hair cells and specific genetic variants in Pcdh15 result in the development of Usher syndrome. However, microarray and RNA sequencing data show that Pcdh15 mRNA is highly expressed by OPCs in the healthy CNS and its expression within gliomas predicts tumour aggression. Therefore, I also review the functional importance of protocadherin family members, including Pcdh15 in regulating glial cell function in the CNS. In Chapter 2, I demonstrate that OPCs express Pcdh15 and investigate the role of Pcdh15 in regulating OPCs function. More specifically, I demonstrate that Pcdh15 is critical for OPC proliferation and contact-mediated signalling between adjacent OPCs that influences the distribution of these cells and has implications for glioma formation. Using in situ hybridization and immunohistochemistry we have shown that OPCs and OLs express Pcdh15 mRNA and Pcdh15 protein. Using an shRNA-mediated gene knockdown approach, we show that a decrease in Pcdh15 expression triggers a significant increase in OPC proliferation in vitro, and that this is dependent on the increased activation of extracellular signal-related kinase (ERK). Moreover, reducing Pcdh15 expression significantly alters interactions between adjacent OPCs. Specifically, Pcdh15 knockdown affects actin polymerization at OPC processes. This manifests as increased filopodial contact time and a reduced frequency of lamellipodial extrusion and retraction (increased veiling time). The number of lamellipodia-like processes produced by each OPC is also markedly reduced. Unlike OPC proliferation, these changes were not affected by ERK activation, but by changing the expression of the cdc42/Rac1 - Arp2/3 complex.