Sorting and trafficking of proteins in oligodendrocytes during myelin membrane biogenesis

During myelin formation OLGs may utilize basic mechanisms of epithelial membrane trafficking, as described and summarized in the introductory chapter (Chapter 1). However, whether specific transport pathways, unique to myelin biogenesis are involved and how such pathways might be regulated in biogenesis and maintenance of the myelin sheath, is largely unexplored. Such insight is of major relevance for devising strategies for exogenous manipulation to stimulate and/or promote de novo biogenesis of the myelin sheath and its (re)assembly at pathological demyelinating conditions, as in the case of multiple sclerosis (MS). In addition, given the special nature of OLGs, consisting of a cell body, bounded by a plasma membrane, and myelin ‘protruding’ from that cell body, yet maintaining a highly specific and quite distinct membrane composition when compared to the plasma membrane, this kind of work will also contribute to solving fundamental questions of great interest to current cell biology. Accordingly, the main purpose of the work described in this thesis was to acquire insight into the sorting and trafficking of major myelin components such as PLP and MBP, and to reveal regulatory mechanisms, instrumental in promoting myelin sheath biogenesis. It is felt that this insight is a prerequisite for a prosperous advancement of therapeutic approaches in a disease as complex as MS. The trafficking and sorting of myelin constituents was primarily studied in vitro, using isolated OLG precursor cells from newborn rats. This approach also allowed us to study myelin biogenesis as a function of cellular development. In chapter 2, molecular parameters that govern the targeting and incorporation of myelin proteins and model proteins into myelin membranes are investigated to further clarify the polarized nature of oligodendrocytes. The purpose of these studies was to reveal the nature of sorting signals in myelin-directed trafficking and whether this pathway was regulated by protein kinase activity. Chapter 3 describes the distribution and functional role of syntaxins 3 and 4 in OLGs, since both proteins usually display a specific, polarized distribution in epithelial cells. We investigated how these syntaxins regulate the trafficking of myelin specific proteins, including PLP and MBP mRNA. Apart from intracellular factors, the role of the ECM was also examined, as described in experiments presented in chapter 4. OLGs were grown on different ECM substrates and their effect on the transport of the PLP and myelin assembly was determined. In chapter 5 the role of MAL in the regulation of sorting and transport of PLP was investigated, using a GFP-MAL construct that was (over)expressed in proliferating oligodendrocyte progenitor cells. The intracellular distribution of MAL was determined and its effect on myelin protein transport was studied as a function of OLG development. Finally, in chapter 6 major observations reported in this thesis are summarized and perspectives for future studies are discussed.