Hydroxycarboxylic acid receptor 2 (HCAR2/GPR109A) expression and signaling promotes the maintenance of an immunoinhibitory retinal environment

BackgroundExcessive oxidative stress and related chronic, sub-clinical inflammation is linked causally to the development and progression of degenerative diseases of the retina including diabetic retinopathy, age-related macular degeneration and glaucoma, leading causes of blindness worldwide. The above responses may be related directly to dysregulated retinal immunity and are potentiated by the combined actions of native retinal cells (e.g., retinal pigment epithelial (RPE) and microglial cells) and immune cells infiltrating from the periphery. Maintaining tight regulation of these cells such that effective control of pathogens is accomplished yet uncontrolled inflammation and consequent tissue damage is prevented is extremely important. However, the molecular mechanisms that control this delicate balance are poorly understood. We hypothesize that the hydroxycarboxylic acid receptor 2 (HCAR2/GPR109A) may play an important role. HCAR2/GPR109A has been shown to regulate immune cell responses that potentiate anti-inflammatory signaling upon its activation in various tissues as evidenced principally by suppressed pro-inflammatory cytokine secretion in various experimental model systems. We have demonstrated HCAR2/GPR109A expression in RPE, microglia and endothelial cells and, our in vitro studies support that the receptor elicits anti-inflammatory signaling in these cell types. However, the functional relevance of HCAR2/GPR109A expression and its activation in the retina of the living animal has not been demonstrated definitively. This is the purpose of the present study. MethodsRetinal function was evaluated temporally in wildtype (Hcar2/Gpr109a +/+, WT) and knockout (Hcar2/Gpr109 -/-, KO) mice via electroretinography (ERG). Fundoscopic imaging, spectral domain-optical coherence tomography (OCT), fluorescein angiography and post-mortem histological analyses were additionally performed to evaluate retinal health. Gene microarray, RT-qPCR studies, ingenuity analyses and proteome pathway mapping were performed to evaluate potential key differences in the molecular signatures of WT and KO mouse retinas. Leukostasis and flow cytometric assays were performed to demonstrate the in vivo impact of HCAR2/GPR109A expression and its therapeutic activation on pro-inflammatory immune cell trafficking in retina. ResultsLongitudinal studies revealed progressive anomalies in retinal morphology and function in HCAR2/GPR109A knockout mice that impacted the entire retina. Gene expression and protein interactome analyses revealed differences in gene and protein expression consistent with the increased immune reactivity and infiltration of bone-marrow derived immune cells detected in KO mouse retinas. Studies conducted in an acute model of retinal (endotoxin-induced) inflammation revealed that targeting the receptor via intraperitoneal administration of agonist, beta-hydroxybutyrate, limits immune cell activation, infiltration and related inflammation in WT retinas. ConclusionsThe present studies demonstrate a central role for HCAR2/GPR109A in regulating the complex interplay between resident retinal cells and peripheral immune cells and, the potential therapeutic utility that targeting the receptor holds with respect to preventing and treating inflammatory retinal diseases. HighlightsOxidative stress and inflammation are major causative factors in degenerative retinal diseases stemming from numerous causes (e.g., aging, diabetes, sickle cell). Thus, identifying new targets and developing strategies to counter these factors to prevent and treat retinal degeneration is important. The present in vivo study demonstrates convincingly the principal role of the hydroxycarboxylic acid receptor 2 (HCAR2/GPR109A) as a major regulator of retinal immune responses under normal conditions and therefore, as a target with extremely high potential for therapeutic modulation of these responses in retinal disease.