Cbl-b negatively regulates B cell antigen receptor signaling in mature B cells through ubiquitination of the tyrosine kinase Syk

Members of the Cbl family of molecular adaptors play key roles in regulating tyrosine kinase-dependent signaling in a variety of cellular systems. Here we provide evidence that in B cells Cbl-b functions as a negative regulator of B cell antigen receptor (BCR) signaling during the normal course of a response. In B cells from Cbl-b-deficient mice cross-linking the BCRs resulted in sustained phosphorylation of Igalpha, Syk, and phospholipase C (PLC)-gamma2, leading to prolonged Ca2+ mobilization, and increases in extracellular signal-regulated kinase (ERK) and c-Jun NH2-terminal protein kinase (JNK) phosphorylation and surface expression of the activation marker, CD69. Image analysis following BCR cross-linking showed sustained polarization of the BCRs into large signaling-active caps associated with phosphorylated Syk in Cbl-b-deficient B cells in contrast to the BCRs in Cbl-b-expressing B cells that rapidly proceeded to form small, condensed, signaling inactive caps. Significantly, prolonged phosphorylation of Syk correlated with reduced ubiquitination of Syk indicating that Cbl-b negatively regulates BCR signaling by targeting Syk for ubiquitination.