Signaling pathways mediated by Toll-like Receptor 2 involved in the protection of oligodendrocytes from ischaemic cell death

Abstract

Oligodendrocytes (OLs), myelinating glia of central nervous system, extend multiple branches to ensheath axons. Rapid saltatory conduction of nerve impulse depends on myelin sheath. Furthermore, recent studies have shown that axonal integrity cannot be maintained without intact myelin sheath. Therefore, devastating neurological deficits occurs when ischaemic white matter injury causes demyelination and OL loss. Treating OL loss and demyelination can be a therapeutic target for ischaemic white matter stroke. Previously, we found that Toll-like receptor 2 (TLR2) expressed in OLs provide cell-autonomous protective effects on ischaemic cell death. Here, I sought to identify the intracellular signaling pathways downstream of TLR2 that are involved in the protection of OLs from ischaemic cell death. Several protective pathways such as p38 MAP kinase, CREB, NFκB, AKT, and ERK can be activated in response of cellular stresses and participate in cytoprotective effects. Treatment of PAM3CSK4 (PAM3), a well-known TLR2 agonist, induced phosphorylation of these pathways except AKT. The same pathways were activated in response to PAM3 treatment following oxygen glucose deprivation (OGD). To identify a specific pathway causally linked to the PAM3-mediated promotion of OL survival, each pathway was inhibited pharmacologically or genetically using specific inhibitors or siRNA, respectively, and the extent of OL death was measured after using CCK or LDH assay. Among all the tested, pathways p38 MAPK inhibitor and NFκB inhibitor significantly attenuated the pro-survival effect of PAM3. siRNA knockdown experiments confirmed functional implication of the two pathways in the PAM3-mediated increase of OL survival. These results demonstrated that TLR2 provides protective effects on OLs under ischaemic stress through the p38 MAPK and NFκB pathways. Further studies will clarify effector molecules downstream of the two pathways that confer cellular protection on ischaemic OLs.