Myosin 1c Participates in B Cell Cytoskeleton Rearrangements, Is Recruited to the Immunologic Synapse, and Contributes to Antigen Presentation

Abstract

Myosin 1c (Myo1c) is a member of the unconventional class I myosins of vertebrates, which directly link the plasma membrane with the microfilament cortical web. Although this molecular motor has been implicated in cell functions such as cytoskeleton organization, cell motility, nuclear transcription, and endocytosis, its role in hematopoietic cells is largely unknown. In this study, we show that Myo1c is abundantly expressed in murine B lymphocytes and is preferentially located at the plasma membrane, especially in peripheral processes such as microvilli. We observed that this motor concentrates at the growing membrane protrusions generated during B cell spreading and that it is actively recruited to the immune synapse. Interestingly, Myo1c was detected in lipid rafts of B cells and showed strong colocalization with MHC-II, particularly after cross-linking of these molecules. By transfection of a dominant negative form of Myo1c or specific siRNA, we also detected alterations in the spreading and Ag-presenting ability of these cells. The data suggest that Myo1c is involved in the cytoskeleton dynamics and membrane protein anchoring or sorting in B lymphocytes.

Myosin 1g regulates cytoskeleton plasticity, cell migration, exocytosis, and endocytosis in B lymphocytes

“…It has been also determined that microvilli of human T cells and those from a mouse pre-B-cell line are rich in The distribution observed for Myo1g is highly similar to that described for Myo1c, another class I myosin also expressed by B cells [18]. Interestingly, it has been reported that the divergence in sequences of the TH1 domains of these two myosins is related with changes in lipid-binding affinities [13].…”

“…This behavior was previously observed in MyoC-deficient Dictyostelium amoebas which lack of the ability to generate directed pseudopodia [41]. In B cells, our group previously reported that down-regulation of Myo1c in B cells has the same effect [18]. A possible explanation for these observations is the membrane tension.…”

“…In experiments where Myo1c, another closely related myosin expressed by B cells, has been knocked down, we observed defects in lymphocyte migration and cytoskeleton rearrangements during cell spreading [18]. Thus, it is possible that Myo1g could also participate in these processes by regulating the membrane dynamics.…”

“…These samples were then fractionated as described previously [18,35] for the separation of the MMV fraction and the PNL fraction. Both components were then analyzed by Western blot following conventional procedures.…”

Cytoskeletal control of B cell responses to antigens

“…The meshwork is approximately 100 nm thick, has a pore size of about 50 nm and attaches to the plasma membrane via ezrin, moesin and class I myosins [15][16][17] . Actin polymerization at the plasma membrane is constantly balanced by actin destruction at the cytosolic side of the cortex by actin severing proteins, cofilin and destrin 14,18,19 , and by the debranching activity of coronins 20 .…”