R-spondin 3/LGR4 (Leucine-Rich Repeat-Containing G Protein-Coupled Receptor 4) Axis Is a Novel Inflammatory and Neurite Outgrowth Signaling System in the Ischemic Brain in Mice

Although stimulation of Wnt/β-catenin signaling is an important strategy to treat ischemic stroke, its signaling pathway has not been fully clarified yet. Recently, RSPO3 (R-spondin 3)/LGR4 (leucine-rich repeat-containing G protein-coupled receptor 4) signaling has resolved TLR4 (toll-like receptor 4)-induced inflammation in lung injury; however, whether this signal is critical in the ischemic brain remains unknown. Therefore, we investigated the role of RSPO3/LGR4 signaling in the ischemic brain.BALB/c mice were exposed to permanent distal middle cerebral artery and common carotid artery occlusion. Temporal RSPO3 and LGR4 expressions were examined, and the mice were randomly assigned to receive vehicle or recombinant RSPO3. The underlying mechanisms were investigated using microglial cell lines and primary mixed glia-endothelia-neuron and primary neuronal cultures.In the ischemic brain, RSPO3 and LGR4 were expressed in endothelial cells and microglia/macrophages and neurons, respectively. Stimulation of RSPO3/LGR4 signaling by recombinant RSPO3 recovered neurological deficits with decreased Il1β and iNOS mRNA on day 3 and increased Gap43 on day 9. In cultured cells, LGR4 was expressed in neuron and microglia, whereas RSPO3 promoted nuclear translocation of β-catenin. Neuroprotective effects with reduced expression of inflammatory cytokines were observed in lipopolysaccharide-stimulated glia-endothelium-neuron cultures but not in glutamate-, CoCl2-, H2O2-, or oxygen glucose deprivation-stimulated neuronal cultures, indicating that RSPO3/LGR4 can protect neurons by regulating inflammatory cytokines. LGR4-Fc chimera, which was used to block endogenous RSPO3/LGR4 signaling, increased LPS-induced production of inflammatory cytokines, suggesting that endogenous RSPO3 suppresses inflammation. RSPO3 decreased TLR4-related inflammatory cytokine expression by decreasing TLR4 expression without affecting the M1/M2 phenotype. RSPO3 also inhibited TLR2- and TLR9-induced inflammation but not TLR7-induced inflammation, and promoted neurite outgrowth.RSPO3/LGR4 signaling plays a critical role in regulating TLR-induced inflammation and neurite outgrowth in the ischemic brain. Enhancing this signal will be a promising approach for treating ischemic stroke.