MAFA and MAFB regulate exocytosis-related genes in human beta-cells

Aims: Reduced expression of exocytotic genes is associated with functional defects in insulin exocytosis contributing to impaired insulin secretion and type 2 diabetes (T2D) development. MAFA and MAFB transcription factors regulate beta-cell physiology, and their gene expression is reduced in T2D beta cells. We investigate if loss of MAFA and MAFB in human beta cells contributes to T2D progression by regulating genes required for insulin exocytosis. Methods: Three approaches were performed: (1) RNAseq analysis with the focus on exocytosis-related genes in MafA(-/-) mouse islets, (2) correlational analysis between MAFA, MAFB and exocytosis-related genes in human islets and (3) MAFA and MAFB silencing in human islets and EndoC-beta H1 cells followed by functional in vitro studies. Results: The expression of 30 exocytosis-related genes was significantly downregulated in MafA(-/- )mouse islets. In human islets, the expression of 29 exocytosis-related genes correlated positively with MAFA and MAFB. Eight exocytosis-related genes were downregulated in MafA(-/-) mouse islets and positively correlated with MAFA and MAFB in human islets. From this analysis, the expression of RAB3A, STXBP1, UNC13A, VAMP2, NAPA, NSF, STX1A and SYT7 was quantified after acute MAFA or MAFB silencing in EndoC-beta H1 cells and human islets. MAFA and MAFB silencing resulted in impaired insulin secretion and reduced STX1A, SYT7 and STXBP1 (EndoC-beta H1) and STX1A (human islets) mRNA expression. STX1A and STXBP1 protein expression was also impaired in islets from T2D donors which lack MAFA expression. Conclusion: Our data indicate that STXBP1 and STX1A are important MAFA/B-regulated exocytosis genes which may contribute to insulin exocytosis defects observed in MAFA-deficient human T2D beta cells.