Ligand-dependent interactions between SR-B1 and S1PR1 in macrophages and atherosclerotic plaques.

HDLs carry sphingosine-1-phosphate (S1P) and stimulate signaling pathways in different cells including macrophages and endothelial cells, involved in atherosclerotic plaque development. HDL signaling via S1P relies on the HDL receptor scavenger receptor class B, type I (SR-B1) and the sphingosine-1-phosphate receptor 1 (S1PR1), which interact when both are heterologously overexpressed in the HEK293 cell line. In this study, we set out to test if SR-B1 and S1PR1 interacted in primary murine macrophages in culture and atherosclerotic plaques. We used knock-in mice that endogenously expressed S1PR1 tagged with eGFP-(S1pr1eGFP/eGFP mice), combined with proximity ligation analysis to demonstrate that HDL stimulates the physical interaction between SR-B1 and S1PR1 in primary macrophages, that this is dependent on HDL-associated S1P and can be blocked by an inhibitor of SR-B1's lipid transfer activity or an antagonist of S1PR1. We also demonstrate that a synthetic S1PR1-selective agonist, SEW2871, stimulates the interaction between SR-B1 and S1PR1 and that this was also blocked by an inhibitor of SR-B1's lipid transport activity. Furthermore, we detected abundant SR-B1/S1PR1 complexes in atherosclerotic plaques of S1pr1eGFP/eGFP mice that also lacked apolipoprotein E. Treatment of mice with the S1PR1 antagonist, Ex26, for 12 h disrupted the SR-B1-S1PR1 interaction in atherosclerotic plaques. These findings demonstrate that SR-B1 and S1PR1 form ligand-dependent complexes both in cultured primary macrophages and within atherosclerotic plaques in mice and provide mechanistic insight into how SR-B1 and S1PR1 participate in mediating HDL signaling to activate atheroprotective responses in macrophages.

SIRT1 activation rescues the mislocalization of RNA-binding proteins and cognitive defects induced by inherited cobalamin disorders.

BACKGROUND: The molecular consequences of inborn errors of vitamin B12 or cobalamin metabolism are far from being understood. Moreover, innovative therapeutic strategies are needed for the treatment of neurological outcomes that are usually resistant to conventional treatments. Our previous findings suggest a link between SIRT1, cellular stress and RNA binding proteins (RBP) mislocalization in the pathological mechanisms triggered by impaired vitamin B12 metabolism. OBJECTIVES AND METHODS: The goal of this study was to investigate the effects of the pharmacological activation of SIRT1 using SRT1720 on the molecular mechanisms triggered by impaired methionine synthase activity. Experiments were performed in vitro with fibroblasts from patients with the cblG and cblC inherited defects of vitamin B12 metabolism and in vivo with an original transgenic mouse model of methionine synthase deficiency specific to neuronal cells. Subcellular localization of the RBPs HuR, HnRNPA1, RBM10, SRSF1 and Y14 was investigated by immunostaining and confocal microscopy in patient fibroblasts. RBPs methylation and phosphorylation were studied by co-immunoprecipitation and proximity ligation assay. Cognitive performance of the transgenic mice treated with SRT1720 was measured with an aquatic maze. RESULTS: Patient fibroblasts with cblC and cblG defects of vitamin B12 metabolism presented with endoplasmic reticulum stress, altered methylation, phosphorylation and subcellular localization of HuR, HnRNPA1 and RBM10, global mRNA mislocalization and increased HnRNPA1-dependent skipping of IRF3 exons. Incubation of fibroblasts with cobalamin, S-adenosyl methionine and okadaic acid rescued the localization of the RBPs and mRNA. The SIRT1 activating compound SRT1720 inhibited ER stress and rescued RBP and mRNA mislocalization and IRF3 splicing. Treatment with this SIRT1 agonist prevented all these hallmarks in patient fibroblasts but it also improved the deficient hippocampo-dependent learning ability of methionine synthase conditional knock-out mice. CONCLUSIONS: By unraveling the molecular mechanisms triggered by inborn errors of cbl metabolism associating ER stress, RBP mislocalization and mRNA trafficking, our study opens novel therapeutic perspectives for the treatment of inborn errors of vitamin B12 metabolism.

Methionine synthase and methionine synthase reductase interact with MMACHC and with MMADHC.

An increasing number of studies indicate that each step of the intracellular processing of vitamin B12 or cobalamin (Cbl) involves protein-protein interactions. We have previously described a novel interaction between methionine synthase (MS) and MMACHC and its effect on the regulation of MMACHC activity. Our goal is to further characterize the interactions of MS with other potential partners in a so-called MS interactome. We dissected the interactions and their alterations by co-immunoprecipitation and DuoLink proximity ligation assays in fibroblasts with cblG, cblE, and cblC genetic defects affecting respectively the expression of MS, methionine synthase reductase (MSR) and MMACHC and in HepG2 cells transfected with corresponding siRNAs. We observed the known interactions of MS with MSR and with MMACHC as well as MMADHC with MMACHC, but we also observed novel interactions for MSR with MMACHC and with MMADHC and MS with MMADHC. Furthermore, we show that the absence of MS or MMACHC expression disrupts the interactions between the other interactome members, in cblC and cblG fibroblasts and in HepG2 cells transfected with siRNAs. Our data show that the processing of Cbl in cytoplasm occurs in a multiprotein complex composed of at least MS, MSR, MMACHC and MMADHC, which could contribute to shuttle safely and efficiently Cbl towards MS. Our data suggest that defective protein-protein interactions among key players of this pathway could contribute to the molecular mechanisms of the cblC, cblG and cblE genetic defects and provide novel insights into our understanding of the pathophysiology of inherited disorders of Cbl metabolism.