MSP‐dependent activation of CNS Ron Receptor Tyrosine Kinase signaling attenuates neuroinflammation through the physiological regulation of IL1B and NLRP3 inflammasome pathway

Abstract

Neurodegenerative disorders, such as Alzheimer's Disease and aging associated dementia, are the leading causes of disease burden globally and are biologically very well characterized by severe central‐nervous‐system(CNS) inflammation. To date, very little is known about how chronic inflammation develops in the brain, challenging neuroimmune function and in turn regulating neural disease progression. A sentinel group of immune cells, the macrophages, closely regulate inflammation by taking on different states, classical activated(M1) or alternatively activated anti‐inflammatory (M2). The source of inflammation in many neurodegenerative diseases has been linked to a decrease in CNS M2 and increase in M1 population, associated with NLR family pyrin domain containing 3 (NLRP3) inflammasome activation and interleukin (IL)‐1β accumulation in microglia. Our lab studies a transmembrane tyrosine‐kinase receptor, Receptor D'Origine Nantais (Ron), which is expressed on tissue resident M2 macrophages including CNS microglia. Our preceding data confirmed that a loss of Ron responsiveness to its ligand, macrophage stimulating protein(MSP), exacerbates disease‐mediated neuroinflammation in murine models of multiple sclerosis and diet induced inflammation. These results demonstrated that Ron plays a key protective role in CNS health and homeostasis. However, the underlying cellular mechanism as to how Ron signaling mediates neuroprotection remains to be characterized. Herein, the objective of this study was to evaluate and identify a novel cell signaling pathway underlying Ron mediated neuroprotection, with in vitro assays and in vivo modelling. In the in vitro studies, immortalized human microglial cells (CHME‐3) were pretreated or untreated with Ron agonist MSP followed by LPS stimulation. MSP‐pretreated CHME‐3 cells had attenuated inflammation with significant decreases in gene expression and protein levels of interleukin‐IL1β following the LPS challenge. Gene profiling with qRT‐pcr revealed MSP‐dependent activation of Ron attenuated NLRP3 inflammasome activation with decreased microglial expression of AIM2, NLRP3 and its adaptor ASC and NLRP3 substrates, caspase‐1 and Interleukin‐IL18. To confirm these in vitro results, we evaluated the role of Ron signaling in an in vivo disease model of chronic neuroinflammation, whereby Apolipoprotein E knockout (ApoE−/−) and Ron receptor ApoE double knockout (DKO) transgenic mice were developed and maintained on a high fat (60% fat) high‐cholesterol diet(1.25%cholesterol) (HFHCD) for 18 weeks. DKO CNS exhibited increased protein expression of cyclooxygenase‐2(COX‐2) and the NLRP3 inflammasome complex. It is known that COX‐2 regulates NLRP3 inflammasome‐derived IL‐1β accumulation, thus this observation was a pivotal confirmation of an upstream regulator in this pathway. The tissue inflammation was associated with increased gene expression of COX‐2 effector inducible‐nitric oxide synthase (iNOS) and NLRP3 substrates IL1β and IL18. These results elucidate a neuroprotective mechanism of CNS Ron‐receptor tyrosine kinase activation induced suppression of NLRP3 inflammasome pathway, leading to decreased caspase‐1 and IL‐1β accumulation in microglial cells and CNS tissue. This is the first study to illustrate a potential mechanism underlying Ron expression in cellular neuroinflammation and has supported on‐going studies into the protective role of Ron in amyloidosis and tauopathy.Support or Funding Information1. NIH‐T32 NRSA Award NIH5T32GM108563‐02 (MPI), Pre‐Doctoral Fellowship2. American Association of Immunologists (AAI) Careers in Immunology Pre‐Doctoral Fellowship3. Penn State Department of Veterinary and Biomedical SciencesThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.