Deciphering the role of B cell signal-transducing adaptor protein (STAP) 1 in the regulation of antibody production and lipid metabolism

Abstract

Introduction: Familial hypercholesterolemia is a common genetic disease mainly caused by the mutation of LDLR, APOB, and PCSK9. A fourth gene, STAP1, classically known to be expressed in immune cells, has been suggested to be one of the causative genes. However, the involvement of STAP1 in familial hypercholesterolemia is still controversial, as recent studies have also indicated otherwise. Despite the fact that STAP1 is mainly expressed in the spleen but dramatically increased in the liver of high-fat diet (HFD) fed mice, the pathophysiological function of STAP1 remains not known. In this study, I have systematically examined the role of STAP1 in B cell immune responses and its potential moonlighting function in immunometabolism by both loss of function and gain of function mouse models. Methods: STAP1 global knockout (KO) mice and a recombinant adeno-associated virus (rAAV) mediated gene delivery system specifically overexpression of STAP1 in B cells were used to study the relationship between STAP1 and immunometabolism. For the immunization assay, mice were immunized with ovalbumin as a model antigen. Antigen-specific antibody levels were assessed using ELISA. A siRNA system was employed for suppressing STAP1 in vitro. A Seahorse extracellular flux analyzer was used to measure mitochondrial respiration to determine the main function of mitochondrial-related ATP production. Flow cytometry, RT-qPCR, and Western blot analysis were performed to determine the number of mitochondria, mitochondrial function, and the expression levels of mitochondrial complexed mRNAs or proteins. Results: Deletion of STAP1 resulted in a lower antigen-specific antibody production at mucosal sites following intranasal immunization adjuvanted with facilitated with cholera toxin in mice. Consistently, knock-down STAP1 expression in hybridoma B cells by specific siRNAs decreased the secretion of the IgG1 antibody. Mechanistically, suppression or deletion of STAP1 resulted in weaker mitochondrial respiration and glycolysis by downregulating the expression of mitochondrial biogenesis genes. In contrast, enhancing mitochondrial respiration and glycolysis were observed in primary B cells from mice with overexpressing of STAP1. In addition, increased serum cholesterol and triglyceride levels after fasting under standard chow feeding were observed in STAP1 KO mice as compared with their wild-type littermates. While overexpressing STAP1 in B cell of HFD fed mice improved the lipid profiles. Conclusion: In this study, I provided compelling evidence demonstrating an important role of STAP1 in immunometabolism. STAP1 is required for optimal humoral immune responses, and hence the lipid metabolism, by maintaining mitochondrial respiratory capacity in B cells. Collectively, my work has shed initial light on the elucidation of a possible link between immunity and metabolism through the expression of STAP1 in B cells. STAP1 is a novel diagnostic and therapeutic target for B cell related immunometabolic diseases.