Microglial p38α MAPK is a key regulator of proinflammatory cytokine up-regulation induced by toll-like receptor (TLR) ligands or beta-amyloid (Aβ)

Background: Overproduction of proinflammatory cytokines from activated microglia has been implicated as an important contributor to pathophysiology progression in both acute and chronic neurodegenerative diseases. Therefore, it is critical to elucidate intracellular signaling pathways that are significant contributors to cytokine overproduction in microglia exposed to specific stressors, especially pathways amenable to drug interventions. The serine/threonine protein kinase p38 alpha MAPK is a key enzyme in the parallel and convergent intracellular signaling pathways involved in stressor-induced production of IL-1 beta and TNF alpha in peripheral tissues, and is a drug development target for peripheral inflammatory diseases. However, much less is known about the quantitative importance of microglial p38 alpha MAPK in stressor-induced cytokine overproduction, or the potential of microglial p38 alpha MAPK to be a druggable target for CNS disorders. Therefore, we examined the contribution of microglial p38 alpha MAPK to cytokine up-regulation, with a focus on the potential to suppress the cytokine increase by inhibition of the kinase with pharmacological or genetic approaches. Methods: The microglial cytokine response to TLR ligands 2/3/4/7/8/9 or to A beta(1-42) was tested in the presence of a CNS-penetrant p38 alpha MAPK inhibitor, MW01-2-069A-SRM. Primary microglia from mice genetically deficient in p38 alpha MAPK were used to further establish a linkage between microglia p38 alpha MAPK and cytokine overproduction. The in vivo significance was determined by p38 alpha MAPK inhibitor treatment in a LPS-induced model of acute neuroinflammation. Results: Increased IL-1 beta and TNF alpha production by the BV-2 microglial cell line and by primary microglia cultures was inhibited in a concentration-dependent manner by the p38 alpha MAPK-targeted inhibitor. Cellular target engagement was demonstrated by the accompanying decrease in the phosphorylation state of two p38 alpha MAPK protein substrates, MK2 and MSK1. Consistent with the pharmacological findings, microglia from p38 alpha-deficient mice showed a diminished cytokine response to LPS. Further, oral administration of the inhibitor blocked the increase of IL-1 beta in the cerebral cortex of mice stressed by intraperitoneal injection of LPS. Conclusion: The p38 alpha MAPK pathway is an important contributor to the increased microglial production of proinflammatory cytokines induced by diverse stressors. The results also indicate the feasibility of targeting p38 alpha MAPK to modulate CNS proinflammatory cytokine overproduction.