AIM:R-spondin 2 (Rspo2), one member of R-spondin family which contains four secreted proteins , plays an important role in skeletal muscle development .However, the impact of Rspo2 on vascular smooth muscle cell ( SMC) differentiation is little known . This study aims at revealing the role and mechanism of Rspo 2 on SMC differentiation from embryonic stem cells (ESCs).METHODS:A well-established model for studying SMC differentiation from ESCs were used , in which mouse embryonic stem cells ( ES-D3) were seeded on collagen IV-coated flasks and cultured in differentiation medium (DM) for 2, 4, 6 and 8 days.Smooth muscle specific markers, includingα-smooth muscle actin (α-SMA), SM22 and smooth muscle myosin heavy chain (SM-MHC), were detected to in-sure the successful model by qRT-PCR and West blot .After 3-day pre-differentiation, ESCs were treated with recombinant Rspo 2 protein, overexpression plasmid or shRNA plasmid for 96 h, and the mRNA and protein expression of smooth muscle markers was detected.To explore the role of Rspo2 on SMC differentiation in vivo, 3-day predifferentiated ESCs (106 in 50μLα-MEM) incubated with Rspo2-overexpression plasmid were mixed with 50 μL of Matrigel ( Becton Dickinson Labware ) and then subcutaneously injected into C57BL/6J mice.After 12 days, mice were sacrificed and the implants were harvested for immunofluorescence staining , qRT-PCR and West blot.Furthermore, electrophoretic mobility shift assay (EMSA), chromatin immunoprecipitation assay (ChIP) and lucif-erase reporter assay were performed to investigate the transcriptional activity of SMC differentiation related transcription factors , inclu-ding serum response factor (SRF), myocardin (MYO), myocyte-specific enhancer factor 2C (MEF-2C).Involvement of Rspo2 re-ceptor, leucine-rich repeat-containing, G-protein-coupled receptors (Lgr)4,5,6, and β-catenin pathway during Rspo2-induced MSC differentiation were also uncovered by overexpression or inhibition of the respective protein .RESULTS:Our results showed that Rspo 2 mRNA and protein expression was significantly and consistently increased during ESC differentiation towards SMCs .Recombinant Rs-po2 protein and enforced Rspo 2 expression in ESCs resulted in up-regulation of smooth muscle markers and transcription factors , while knockdown decreased the expression of these genes .Expectedly , Rspo2 overexpression also promotes SMC differentiation in vivo.
Mechanistically , our data showed that Rspo 2 could promote SRF binding to SM22 promoter region .Evidence also revealed that one of three Rspo2 receptors, LGR5, was up-regulated while the other two , LGR4 and LGR6, was down-regulated.Silencing of LGR5 inhibi-ted Rspo2-induced SMC differentiation, whereas knockdown of LGR4 had no impact.Finally, activation or inhibition of β-catenin could promote or inhibit SMC differentiation , respectively .CONCLUSION: Our findings demonstrate for the first time that Rspo 2 plays a positive role during smooth muscle cell differentiation from embryonic stem cells .We confirmed that Rspo 2 can up-regulate smooth muscle markers at transcription level .We also revealed Rspo promote smooth muscle cell differentiation through activation of LGR 5 re-ceptor and Wnt/β-catenin pathway .