Abstract Body (Do not enter title and authors here): Background: Monocyte-derived macrophages play key roles in both the initiation and progression of atherosclerosis, an underlying cause of the majority of cardiovascular diseases. Previous investigations reported significant contributions of various secretory matricellular proteins in the pathogenesis of atherosclerosis. In our recent study, we observed elevated levels of a matricellular protein R-spondin 2 (RSPO2) in human and murine atherosclerotic arteries. Furthermore, RSPO2 expression was detected in lesional macrophages of atherosclerotic arteries. Additional experiments revealed increased RSPO2 levels in mouse primary macrophages exposed to atherogenic oxidized LDL compared with control cells. However, the role of macrophage RSPO2 in atherosclerosis development remains unknown.
Methods and Results: We generated novel myeloid cell-specific Rspo2 knockout mice (Rspo2^F/F LysM Cre^+/-; Rspo2^ΔM) by crossing Rspo2^F/F and LysM Cre^+/- mice. Eight-week-old male Rspo2^ΔM and littermate control Rspo2^F/F mice were injected intraperitoneally with AAV8-hPCSK9 and fed a Western diet for 16 weeks to induce hypercholesterolemia and atherosclerosis. No differences in various metabolic parameters, including body weight, whole-body fat/lean mass, fasting blood glucose, and plasma total cholesterol, were observed between the two groups after Western diet feeding. Histochemical staining performed on aortic root sections (at least 4 sections/mouse spaced 70-90 µm apart) demonstrated increased neointimal lesions (H&E) and elevated lipid deposition (Oil red O) in Rspo2^ΔM mice compared with controls. However, aortic root lesional collagen content (Masson’s Trichrome) and macrophage accumulation (CD68⁺ cells) were comparable between control and myeloid cell-restricted Rspo2-deficient mice. In vitro experiments performed with peritoneal macrophages (CD11b⁺ Ly6G^- Ly6C⁺) showed increased uptake of fluorescently-labeled oxidized LDL and elevated inflammation in Rspo2^ΔM cells compared with controls, with no differences in cholesterol efflux capacities.
Conclusion: These findings indicate that myeloid cell Rspo2 deletion promotes atherosclerosis by stimulating macrophage lipid uptake and promoting cellular inflammation. Collectively, these results identify myeloid cell Rspo2 as a suppressor of atherosclerosis.