The activation of the B-cell receptor (BCR), which initiates B-cell activation, is triggered by antigen-induced self-aggregation and clustering of receptors at the cell surface. While antigen-induced actin reorganization is known to be involved in BCR clustering in response to membrane-associated antigen, the underlying mechanism that links actin reorganization to BCR activation remains unknown. Here we show that both the stimulatory Bruton’s tyrosine kinase (Btk) and the inhibitory SH2-containing inositol-5 phosphatase-1 (SHIP-1) are required for efficient BCR self-aggregation. In Btk-deficient B cells, the magnitude of BCR aggregation into clusters and B-cell spreading in response to antigen-tethered lipid bilayer is drastically reduced, compared to that observed in wild type B-cells. In SHIP-1−/− B-cells, while surface BCRs aggregate into microclusters, the centripetal movement and growth of BCR clusters are inhibited and B-cell spreading is increased. The persistent BCR microclusters in SHIP−/− B-cells exhibit higher levels of signaling than merged BCR clusters. Contrast to the inhibition of actin remodeling in Btk-deficient B-cells, actin polymerization, F-actin accumulation, and WASP phosphorylation are enhanced in SHIP-1−/− B-cells in a Btk-dependent manner. Thus, a balance between positive and negative signaling regulates the spatiotemporal organization of the BCR at the cell surface by controlling actin remodeling, which potentially regulates the signal transduction of the BCR. This study suggests a novel feedback loop between BCR signaling and the actin cytoskeleton.

Actin filaments and associated actin binding proteins play an essential role in governing the mechanical properties of eukaryotic cells. Even though cells have multiple actin binding proteins (ABPs) that exist simultaneously to maintain the structural and mechanical integrity of the cellular cytoskeleton, how these proteins work together to determine the properties of actin networks is not clearly understood. The ABP, palladin, is essential for the maintenance of cell morphology and the regulation of cell movement. Palladin coexists with -actinin in stress fibers and focal adhesions and binds to both actin and -actinin. To obtain insight into how mutually interacting actin crosslinking proteins modulate the properties of actin networks, we characterized the micro-structure and mechanics of actin networks crosslinked with palladin and -actinin. We first showed that palladin crosslinks actin filaments into bundled networks which are viscoelastic in nature. Our studies also showed that composite networks of -actinin/palladin/actin behave very similar to pure palladin or pure -actinin networks. However, we found evidence that palladin and -actinin synergistically modify network viscoelasticity. To our knowledge, this is the first quantitative characterization of the physical properties of actin networks crosslinked with two mutually interacting crosslinkers.

The morphology and duration of contacts between cells and adhesive surfaces play a key role in several biological processes, such as cell migration, cell differentiation, and the immune response. The interaction of receptors on the cell membrane with ligands on the adhesive surface leads to triggering of signaling pathways, which allow cytoskeletal rearrangement, and large-scale deformation of the cell membrane, which allows the cell to spread over the substrate. Despite numerous studies of cell spreading, the nanometer-scale dynamics of the membrane during formation of contacts, spreading, and initiation of signaling are not well understood. Using interference reflection microscopy, we study the kinetics of cell spreading at the micron scale, as well as the topography and fluctuations of the membrane at the nanometer scale during spreading of Jurkat T cells on antibody-coated substrates. We observed two modes of spreading, which were characterized by dramatic differences in membrane dynamics and topography. Formation of signaling clusters was closely related to the movement and morphology of the membrane in contact with the activating surface. Our results suggest that cell membrane morphology may be a critical constraint on signaling at the cell-substrate interface.

In response to antigens, the BCR forms surface signalosomes where it initiates signaling cascades and antigen internalization. This study examines the role of the actin cytoskeleton in BCR signalosome formation. Both multi-valent soluble and membrane-associated antigens induce BCR clustering and actin reorganization. Actin is actively polymerized at BCR microclusters upon their formation and surrounding BCR clusters as they are merged into central clusters. Actin regulators, WASP, cofilin and gelsolin, are activated and recruited to BCR clusters. Latrunculin or jasplakinolide treatment, which blocks BCR-induced actin reorganization by depolymerizing or stabilizing F-actin, inhibits surface BCR clustering and B cell spreading. Jasplakinolide blocks antigen-triggered tyrosine phosphorylation. In contract, latrunculin alone, in the absence of antigenic stimulation, induces BCR microclustering and tyrosine phosphorylation, however, in a much slower kinetics than those induced by antigens. Btk deficiency inhibits not only antigen-induced actin polymerization, but also antigen-induced B cell spreading and BCR clustering. These results indicate that BCR-induced, Btk-dependent actin reorganization, consisting of temporospatially regulated actin polymerization and depolymerization, promotes the formation of surface BCR signalosomes by increasing the speed and the extend of BCR engagement and clustering. Our study reveals an actin-mediated positive feedback mechanism for BCR signaling.

controls. Light and transmission electron microscopy reveals focal segmental glomerulosclerosis with thicker basement membranes and abnormal podocytes. Some of the mutant mice also have lens abnormalities consistent with early cataract formation. Our results show that even heterozygous mutations in the mouse Myh9 gene can reproduce human MYH9-related diseases. These mouse models should be useful in understanding the pathophysiology of human MYH9-related diseases and also in designing and developing therapeutic stratagies.

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