Microglial ERK signaling is a critical regulator of pro-inflammatory immune responses in Alzheimer’s disease

Abstract

BackgroundThe mitogen-activated protein kinase (MAPK) pathway is a central regulator of gene expression, pro-survival signaling, and inflammation. However, the importance of MAPK pathway signaling in regulating microglia-mediated neuroinflammation in Alzheimer's Disease (AD) remains unclear. Here we examined the role of MAPK signaling in microglia using pre-clinical in-vitro and in-vivo models of AD pathology integrated with quantitative proteomics studies of postmortem human brains. MethodsWe performed multiplexed immunoassay analyses of MAPK phosphoproteins, particularly ERK1/2, in acutely-isolated microglia and brain tissue from wild-type and 5xFAD mice.Neuropathological studies of mouse and human brain tissues were performed to quantify total and phosphorylated ERK protein in AD. The importance of ERK signaling in unstimulated and interferon γ (IFNγ)-stimulated primary microglia cultures was investigated using NanoString transcriptomic profiling, coupled with functional assays of amyloid β (Αβ) and neuronal phagocytosis. Receptor tyrosine kinases (RTKs) likely responsible for ERK signaling in homeostatic microglia and disease-associated-microglia (DAM) states and ERK-regulated human AD risk genes were identified using gene expression data. Total and phosphorylated MAPKs in human post-mortem brain tissues were measured in quantitative proteomic datasets. ResultsPhosphorylated ERK was the most strongly up-regulated signaling protein within the MAPK pathway in microglia acutely isolated from 5xFAD brains. Neuroinflammatory transcriptomic and phagocytic profiling of mouse microglia confirmed that ERK is a critical regulator of IFNγmediated pro-inflammatory activation of microglia, although it was also important for constitutive microglial functions. Phospho-ERK was an upstream regulator of disease-associated microglia (DAM) gene expression (Trem2, Tyrobp), as well as of several human AD risk genes (Bin1, Cd33, Trem2, Cnn2). Among RTKs that signal via ERK, CSF1R and MERTK were primarily expressed by homeostatic microglia while AXL and FLT1 were likely regulators of ERK signaling in DAM. Within DAM, FLT4 and IGF1R were specifically expressed by proand anti-inflammatory DAM sub-profiles respectively. In quantitative proteomic analyses of post-mortem human brain from non-disease, asymptomatic and cognitively-impaired AD cases, 3 ERK1 and ERK2 were the only MAPK pathway signaling proteins with increased protein expression and positive associations with neuropathological grade. Moreover, in a phosphoproteomic study of post-mortem human brains from controls, asymptomatic and symptomatic AD cases, we found evidence for a progressive increased flux through the ERK signaling pathway. ConclusionsOur integrated analyses using pre-clinical models and human proteomic data strongly suggest that ERK phosphorylation in microglia is a critical regulator of pro-inflammatory immune response in AD pathogenesis and that modulation of ERK via upstream RTKs may reveal novel avenues for immunomodulation. 5 are typically receptors for gro...

Oncogenic BRAF V600E induces glial proliferation through ERK and neuronal death through JNK

“…status also predicts risk of neurodegenerative complications in these conditions. ERK activation in activated microglia has also been implicated in common age-related neurodegenerative disorders such as Parkinson's disease and Alzheimer's disease [36,37]. Our study further demonstrated in vitro a clear causal link between BRAF V600E -induced and ERK-mediated activated microglia and neuronal cell death that does not require physical proximity as in vivo.…”

Small GTPases of the Ras and Rho Families Switch on/off Signaling Pathways in Neurodegenerative Diseases

“…The Ras/MAPK pathway activity is also relevant in glia; for example, microglial phagocytosis requires its activation [ 27 ]. Nevertheless, the Ras/MAPK pathway can also be detrimental to cellular physiology, as it is involved in l -3,4 dihydroxyfenylalanine-( l -DOPA)-treated PD patient dyskinesia.…”

“…Furthermore, the implication of glial cells should not be disregarded. The description of Ras and Rho GTPases controlling microglial phagocytosis and superoxide generation in AD and PD [ 27 , 86 , 87 , 88 , 98 ] and controlling astrogliosis [ 99 ] has placed glial cells in the focus of many researchers. Therefore, methods specifically targeting glial cells could be a promising therapeutic option.…”

Presenilin 1 phosphorylation regulates amyloid-β degradation by microglia

“…Our analysis also revealed that the 14-3-3 pathway was altered in Psen1 KI/KI microglia (−log p 9.92), which represent conserved regulatory proteins that act in a variety of cellular processes [28]. Another pathway significantly affected by Psen1 KI/KI was the ERK/MAPK signaling pathway (−log p 9.61) which has been shown to be specifically activated in microglia in a pre-clinical model of AD pathology and human post-mortem AD brains [29]. Interestingly, ERK inhibition reduces the ability of microglia to phagocytose Aβ42 [29].…”

“…Another pathway significantly affected by Psen1 KI/KI was the ERK/MAPK signaling pathway (−log p 9.61) which has been shown to be specifically activated in microglia in a pre-clinical model of AD pathology and human post-mortem AD brains [29]. Interestingly, ERK inhibition reduces the ability of microglia to phagocytose Aβ42 [29]. In addition, our analysis revealed that the autophagy pathway was significantly affected by Psen1 KI/KI in microglia (−log p 2.93) ( Fig.…”