Conditionally and acutely ablating Kif3a from oligodendrocyte progenitor cells impairs primary cilum assembly and cell function

The primary cilium is a cellular organelle that can regulate the behaviour of many mitotic progenitor cells, and even some post mitotic cell types. Oligodendrocyte progenitor cells (OPCs) express genes associated with primary cilia assembly, disassembly and signalling. However, whether OPCs have primary cilia assembled on their surface and are functionally influenced by signalling at this organelle is unknown. In this thesis, I show that OPCs are ciliated, both in vitro and in vivo, while mature oligodendrocytes are not. Furthermore, OPCs disassemble and reassemble their primary cilia as they progress through the cell cycle, and this organelle is a critical regulator of OPC proliferation and oligodendrogenesis in adulthood. In order to examine the importance of the primary cilium for OPC function, the kinesin family member 3a (Kif3a) gene, critical for cilium assembly, was deleted from OPCs in vitro. Kif3adeletion significantly reduced the number of OPCs with assembled primary cilia on their surface and decreased OPC proliferation (Chapter 3). As Pdgfr˜í¬±-CreER\\(^{T2}\\) transgenic mice target DNA recombination to OPCs, without significantly affecting PDGFR˜í¬±\\(^+\\) cells in other tissues and organs (including the kidney, spleen, liver, intestine, heart, gastrocnemius, sciatic nerve, pituitary gland and adrenal gland) (Chapter 4), these mice were selected to conditionally delete Kif3a from OPCs, to examine the importance of this organelle in vivo (Chapter 5). Consistent with the in vitro findings, Kif3a-deletion from OPCs reduced the number of OPCs that had assembled primary cilia on their surface in the corpus callosum (CC) and reduced proliferation by ~30% in the CC and ~50% in the motor cortex. While Kif3a-deleation had no effect on OPC density, it halved the number of new oligodendrocytes produced in both the CC and the motor cortex. As the reduced proliferation rate of Kif3adeleted OPCs does not account for the substantial decrease in new oligodendrocyte number, these data indicate that the primary cilium also promotes oligodendrocyte differentiation and / or new oligodendrocyte survival in the brain. Genetically ablating OPCs, disrupting OPC function and preventing oligodendrogenesis can have behavioural consequences ranging from increased anxiety and depressive behaviours to reduced motor performance and impaired motor learning. However, when oligodendrogenesis was reduced in the CC and motor cortex over a 6-week period, by the conditional deletion of Kif3a from OPCs in vivo, no such behavioural phenotype was observed. These data indicate that deleting Kif3a from OPCs in the adult central nervous system (CNS), which does not remove OPCs from the brain, and only moderately reduces oligodendrogenesis, is not sufficient, at least in the short-term, to have overt behavioural consequences. However, these experiments also pose interesting questions about the function of OPCs, and whether oligodendrogenesis is their only role in the mature CNS. Overall this research demonstrates that primary cilia are present on the surface of OPCs but absent from mature, myelinating oligodendrocytes and that preventing cilium assembly reduces both OPC proliferation and oligodendrogenesis. I also determined that preventing cilium assembly and reducing oligodendrogenesis did not affect normal CNS function by 7 weeks post gene-deletion.