Mst1 positively regulates B-cell receptor signaling via CD19 transcriptional levels.

As a key regulator of hippo signaling pathway, Mst kinases are emerging as one of the key signaling molecules that influence cell proliferation, organ size, cell migration, and cell polarity. In B lymphocytes, Mst1 deficiency causes the developmental defect of marginal zone (MZ) B cells, but how Mst1 regulates B-cell receptor (BCR) activation and differentiation remains elusive. Using genetically manipulated mouse models and total internal reflection fluorescence microscopy, we have demonstrated that Mst1 positively regulates BCR signaling via modulating CD19 transcriptional levels. Consistent with this, Mst1-deficient mice exhibited reduced BCR signaling, which is concurrent with defective BCR clustering and B-cell spreading on stimulatory lipid bilayers. The disruption of CD19-mediated Btk signaling by Mst1 deficiency leads to the severe defect in the differentiation of MZ and germinal center B cells. Mechanistic analysis showed that Mst1 upregulates the messenger RNA level of CD19 via regulating the transcriptional factor TEAD2 that directly binds to the consensus motif in the 3' untranslated region of cd19. Overall, our results reveal a new function of Mst1 in B cells and the mechanism by which Mst1 regulates the activation and differentiation of peripheral B cells.