Actin-binding protein 1 and Dynamin-2 modulate attenuation of B-cell signaling and regulate B-cell function

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B-cells mediate humoral immune responses with production of an amazingly diverse repertoire of antigen specific antibodies. Antigen binding to the B-cell receptor (BCR) induces coordinated BCR signaling and BCR-mediated antigen internalization, processing, and presentation to helper T-cells, necessary for generation of humoral memory. Prolonged or uncontrolled BCR signaling has been linked to development of autoimmunity; therefore controlled attenuation of B-cell signaling is required for maintenance of B-cell tolerance. This study investigates the role of an actin adaptor protein, actin-binding protein 1 (Abp1/HIP-55/SH3P7), in BCR-mediated signal transduction and subsequent B-cell function. I found that in both Abp1-/- and Abp1-/--bone marrow chimeric mice, in which only B-cells lack Abp1 expression, the number of spontaneous germinal center and marginal zone B-cells, and levels of autoantibody are significantly increased. Serum levels of T-independent antibody and total IgM and IgG antibodies are elevated in Abp1-/- mice, whereas T-dependent IgG responses are reduced and fail to undergo affinity maturation. Upon binding membrane-associated antigen, surface BCR clustering is enhanced and B-cell contraction delayed in Abp1-/- B cells, concurrent with slow but persistent increases in F-actin at BCR signalosomes. Furthermore, BCR signaling is enhanced in Abp1-/- B cells, including Ca2+ flux and phosphorylation of B-cell linker protein (BLNK), mitogen-activated protein kinase kinase MEK1/2, and extracellular signal-regulated kinase (ERK), coinciding with reductions in recruitment of the inhibitory signaling molecules, hematopoietic progenitor kinase 1 (HPK1) and SH2-containing inositol 5-phosphatase (SHIP-1) to BCR signalosomes. Our previous studies demonstrated a role for Abp1 in BCR internalization by linking the actin cytoskeleton to the endocytic machinery protein dynamin2. This study shows that dyanmin2 is recruited to the BCR upon antigen binding, and its recruitment is dependent on its proline rich domain (PRD). BCR internalization and trafficking to late endosomes requires the PRD, GTPase function, and tyrosine phosphorylation sites of dynamin2. B-cells expressing a GTPase dead mutant of dynamin2 exhibit enhanced BCR clustering and signaling. These results indicate Abp1 negatively regulates BCR signaling by coupling actin remodeling to B-cell contraction, activation of inhibitory signaling molecules, and the endocytic machinery protein dynamin2, revealing a novel regulatory mechanism for peripheral B-cell development and antibody response.