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Abstract
Microglia are immune cells that reside in the central nervous system and are essential in innate immunity, homeostasis, and development of the central nervous system. However, excessive and prolonged activation of these cells leads to an enhanced production of inflammatory cytokines, reactive oxygen species, and prostaglandins. These factors are neurotoxic and can cause significant neuronal death, and accordingly, are implicated in the pathogenesis of several neurodegenerative diseases. Regulator of G Protein Signaling 10 (RGS10) protein has been shown to negatively regulate inflammatory cytokine production from microglia and subsequent microglia-induced neurotoxicity. However, activation of microglia induces suppression of RGS10 expression in microglia, eliminating the neuroprotective effects of RGS10. In the first study, we aimed to determine the molecular mechanisms that regulate inflammation-induced suppression of RGS10 in microglia. Our data indicate that suppression of RGS10 in activated microglia is regulated by epigenetic mechanisms, mainly through histone deacetylase (HDAC)-induced repression of RGS10 gene transcription. HDAC inhibitors restored expression of RGS10 in activated microglia, indicating that it is feasible to utilize small molecules to manipulate RGS10 expression in microglia. Another aim of our study was to identify the molecular mechanisms that mediate the RGS10 anti-inflammatory function in microglia, which are currently unknown. Canonically, RGS10 negatively regulates G protein signaling by acting as a GTPase-activating protein (GAP), thereby accelerating the inactivation reaction of G proteins. However, it has been reported that some RGS proteins can act in GAP-independent mechanisms. Here, we demonstrate for the first time that RGS10 suppresses inflammatory signaling in a G protein-independent mechanism that does not require its GAP activity. To further elucidate the nature of this mechanism, we conducted unbiased co-immunoprecipitation/mass spectrometry proteomics experiments to identify novel binding partners of endogenous RGS10 in microglia. In this study, we identified several novel partners of RGS10 in microglia associated with distinct cellular functions and localization. The findings of our study will expand our knowledge about the function and regulation of RGS10 in inflammatory signaling. Further, our study reports another GAP-independent function of RGS proteins, providing additional evidence that RGS proteins are much more than just GAPs.