Interactome of P21 Activated Kinase 1 (PAK1) in oligodendrocytes

In the central nervous system (CNS), myelin formation by oligodendrocytes (OLs) relies on actin dynamics. Actin polymerization supports the ensheathment step, when the OL process contacts and surrounds a selected portion of axon. While a drastic shift to actin depolymerization is then required to enable the wrapping process and the formation of a multilayered myelin sheath. Molecular mechanisms regulating this switch to actin depolymerization are not fully elucidated. P21-activated kinase 1 (PAK1), a downstream effector of Rho GTPases Rac1 and Cdc42, highly expressed in OLs, can regulate actin dynamics through its kinase activity. We found that PAK1 loss-of-function in OLs, in vivo, stimulates the wrapping and the thickening of myelin sheaths. We showed that PAK1 is highly expressed in myelinating OLs, yet in its inactive form, the one with the ability to trigger actin disassembly. Interestingly, we found that modulations of PAK1 kinase activity control actin dynamics in OLs, thereby myelin formation. Our data demonstrate that PAK1 inactivation in OLs is required for actin depolymerization and proper myelin formation, suggesting the presence of an endogenous PAK1 inhibitor in myelinating OLs. Proteomic analysis of PAK1 binding partners enabled us to identify several proteins likely to regulate its activity.