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

Abstract

BackgroundOverproduction 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 p38α MAPK is a key enzyme in the parallel and convergent intracellular signaling pathways involved in stressor-induced production of IL-1β and TNFα in peripheral tissues, and is a drug development target for peripheral inflammatory diseases. However, much less is known about the quantitative importance of microglial p38α MAPK in stressor-induced cytokine overproduction, or the potential of microglial p38α MAPK to be a druggable target for CNS disorders. Therefore, we examined the contribution of microglial p38αMAPK 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.MethodsThe microglial cytokine response to TLR ligands 2/3/4/7/8/9 or to Aβ1-42 was tested in the presence of a CNS-penetrant p38α MAPK inhibitor, MW01-2-069A-SRM. Primary microglia from mice genetically deficient in p38α MAPK were used to further establish a linkage between microglia p38α MAPK and cytokine overproduction. The in vivo significance was determined by p38α MAPK inhibitor treatment in a LPS-induced model of acute neuroinflammation.ResultsIncreased IL-1β and TNFα production by the BV-2 microglial cell line and by primary microglia cultures was inhibited in a concentration-dependent manner by the p38α MAPK-targeted inhibitor. Cellular target engagement was demonstrated by the accompanying decrease in the phosphorylation state of two p38α MAPK protein substrates, MK2 and MSK1. Consistent with the pharmacological findings, microglia from p38α-deficient mice showed a diminished cytokine response to LPS. Further, oral administration of the inhibitor blocked the increase of IL-1β in the cerebral cortex of mice stressed by intraperitoneal injection of LPS.ConclusionThe p38α 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 p38α MAPK to modulate CNS proinflammatory cytokine overproduction.

Mitogen-Activated Protein Kinase–Activated Protein Kinase 2 in Neuroinflammation, Heat Shock Protein 27 Phosphorylation, and Cell Cycle: Role and Targeting

“…Further supporting the role of MK2 in microglial inflammation, Bachstetter et al (2011) demonstrated that antineuroinflammatory efficacy of the p38 MAPK inhibitor MW01-2-069A-SRM is associated with decreased phosphorylation state of MK2. Similarly, in primary neuron-glia cocultures, the dopaminergic neurons from MK2-deficient mice were significantly more resistant to LPS-induced neurotoxicity compared with cells from wild-type mice.…”

Deficiency of Neuronal p38α MAPK Attenuates Amyloid Pathology in Alzheimer Disease Mouse and Cell Models through Facilitating Lysosomal Degradation of BACE1

“…p38 MAPK, which responds to inflammatory and oxidative stress, is activated in the AD brain (14,15), and mediates A␤-induced inflammatory activation in cultured microglia (21). Studies have suggested that inhibition of p38␣ MAPK retards AD pathogenesis; for example, oral administration of p38␣ MAPK inhibitor successfully reduced neuroinflammation and synaptic dysfunction in APP-transgenic mice (22,46).…”

“…A␤ and glutamate have each been shown to activate p38 MAPK in cultured neurons by increasing reactive oxygen species (16,17), and the A␤-triggered microglial release of inflammatory mediators, especially IL-1␤, is hypothesized to activate neuronal p38 MAPK (18 -20). p38␣ MAPK mediates A␤-initiated microglial inflammatory activation (21,22), phosphorylates Tau protein in neurons (20,23,24), and mediates A␤-induced synaptic impairment in the cultured hippocampal slice (25). However, no experiments have yet elucidated whether p38 MAPK phosphorylates BACE1 and regulates A␤ generation.…”

Priming the Inflammatory Pump of the CNS after Traumatic Brain Injury

“…These cells cluster together and exhibit a deramified and hypertrophic morphology. Morphological changes following diffuse injury in mice ( Mus musculus ) are dependent on intact p38α MAPK signaling, a pathway that also prompts microglial pro-inflammatory cytokine production [39]. Diffuse brain injury results in transiently elevated IL-1β, TNFα, and CD14 in the cortex and hippocampus of mice as early as 4 hours after injury, which returns to baseline by 72 hours [40].…”