Class I myosins in B-cell physiology: functions in spreading, immune synapses, motility, and vesicular traffic

Class I Myosins, molecular motors involved in cell migration and cancer

Class I Myosins are a subfamily of motor proteins with ATPase activity and a characteristic structure conserved in all myosins: A N-Terminal Motor Domain, a central Neck and a C terminal Tail domain. Humans have eight genes for these myosins. Class I Myosins have different functions: regulate membrane tension, participate in endocytosis, exocytosis, intracellular trafficking and cell migration. Cell migration is influenced by many cellular components including motor proteins, like myosins. Recently has been reported that changes in myosin expression have an impact on the migration of cancer cells, the formation of infiltrates and metastasis. We propose that class I myosins might be potential markers for future diagnostic, prognostic or even as therapeutic targets in leukemia and other cancers.Abbreviations: Myo1g: Myosin 1g; ALL: Acute Lymphoblastic Leukemia, TH1: Tail Homology 1; TH2: Tail Homology 2; TH3: Tail Homology 3.

Detection of Myosin 1g Overexpression in Pediatric Leukemia by Novel Monoclonal Antibodies

Myosin 1g (Myo1g) is a mechanoenzyme associated with actin filaments, expressed exclusively in hematopoietic cells, and involved in various cellular functions, including cell migration, adhesion, and membrane trafficking. Despite the importance of Myo1g in distinct functions, there is currently no monoclonal antibody (mAb) against Myo1g. mAbs are helpful tools for the detection of specific antigens in tumor cells and other tissues. The development of mAbs against targeted dysregulated molecules in cancer cells remains a crucial tool for aiding in the diagnosis and the treatment of patients. Using hybridoma technology, we generated a panel of hybridomas specific for Myo1g. ELISA, immunofluorescence, and Western blot assay results revealed the recognition of Myo1g by these novel monoclonal antibodies in normal and transformed T and B cells. Here, we report the development and application of new monoclonal antibodies against Myo1g for their potential use to detect its overexpression in acute lymphoblastic leukemia (ALL) patients.

Myosin 1g and 1f: A Prospective Analysis in NK Cell Functions

NK cells are contained in the ILC1 group; they are recognized for their antiviral and antitumor cytotoxic capacity; NK cells also participate in other immune response processes through cytokines secretion. However, the mechanisms that regulate these functions are poorly understood since NK cells are not as abundant as other lymphocytes, which has made them difficult to study. Using public databases, we identified that NK cells express mRNA encoding class I myosins, among which Myosin 1g and Myosin 1f are prominent. Therefore, this mini-review aims to generate a model of the probable participation of Myosin 1g and 1f in NK cells, based on information reported about the function of these myosins in other leukocytes.

High expression of Myosin 1g in pediatric acute lymphoblastic leukemia

Acute Lymphoblastic Leukemia (ALL) is the most frequent cancer in pediatric population. Although the treatment has improved and almost 85% of the children are cured about 20% suffer relapse, therefore finding molecules that participate in the pathogenesis of the disease for the identification of relapse and patients at risk is an urgent unmet need. Class I myosins are molecular motors involved in membrane tension, endocytosis, phagocytosis and cell migration and recently they have been shown important for development and aggressiveness of diverse cancer types, however Myo1g an hematopoietic specific myosin has not been studied in cancer so far. We evaluated the expression of Myo1g by qRT-PCR, Immunocytochemistry and Immunofluorescence in a cohort of 133 ALL patients and correlated the expression at diagnosis and after treatment with clinical features and treatment outcomes. We found high expression levels of Myo1g in Peripheral Blood Mononuclear Cells (PBMCs) from patients with ALL at diagnosis and those levels decreased after complete remission; furthermore, we found an increase in Myo1g expression on patients with 9:22 translocation and those who relapse. This study show that Myo1g is over expressed in ALL and that may participate in the pathogenesis of the disease specially in high-risk patients.

Myo1g is required for efficient adhesion and migration of activated B lymphocytes to inguinal lymph nodes

Cell migration is a dynamic process that involves adhesion molecules and the deformation of the moving cell that depends on cytoskeletal remodeling and actin-modulating proteins such as myosins. In this work, we analyzed the role of the class I Myosin-1 g (Myo1g) in migratory processes of LPS + IL-4 activated B lymphocytes in vivo and in vitro. In vivo, the absence of Myo1g reduced homing of activated B lymphocytes into the inguinal lymph node. Using microchannel chambers and morphology analysis, we found that the lack of Myo1g caused adhesion and chemotaxis defects. Additionally, deficiency in Myo1g causes flaws in adopting a migratory morphology. Our results highlight the importance of Myo1g during B cell migration.

Myo1e modulates the recruitment of activated B cells to inguinal lymph nodes

The inclusion of lymphocytes in high endothelial venules and their migration to the lymph nodes are critical steps in the immune response. Cell migration is regulated by the actin cytoskeleton and myosins. Myo1e is a long-tailed class I myosin and is highly expressed in B cells, which have not been studied in the context of cell migration. By using intravital microscopy in an in vivo model and performing in vitro experiments, we studied the relevance of Myo1e for the adhesion and inclusion of activated B cells in high endothelial venules. We observed reduced expression of integrins and F-actin in the membrane protrusions of B lymphocytes, which might be explained by deficiencies in vesicular trafficking. Interestingly, the lack of Myo1e reduced the phosphorylation of focal adhesion kinase (FAK; also known as PTK2), AKT (also known as AKT1) and RAC-1, disturbing the FAK-PI3K-RAC-1 signaling pathway. Taken together, our results indicate a critical role of Myo1e in the mechanism of B-cell adhesion and migration.

Class I myosins: Highly versatile proteins with specific functions in the immune system

Connections established between cytoskeleton and plasma membrane are essential in cellular processes such as cell migration, vesicular trafficking, and cytokinesis. Class I myosins are motor proteins linking the actin-cytoskeleton with membrane phospholipids. Previous studies have implicated these molecules in cell functions including endocytosis, exocytosis, release of extracellular vesicles and the regulation of cell shape and membrane elasticity. In immune cells, those proteins also are involved in the formation and maintenance of immunological synapse-related signaling. Thus, these proteins are master regulators of actin cytoskeleton dynamics in different scenarios. Although the localization of class I myosins has been described in vertebrates, their functions, regulation, and mechanical properties are not very well understood. In this review, we focused on and summarized the current understanding of class I myosins in vertebrates with particular emphasis in leukocytes.

Integrating Physical and Molecular Insights on Immune Cell Migration

The function of most immune cells depends on their ability to migrate through complex microenvironments, either randomly to patrol for the presence of antigens or directionally to reach their next site of action. The actin cytoskeleton and its partners are key conductors of immune cell migration as they control the intrinsic migratory properties of leukocytes as well as their capacity to respond to cues present in their environment. In this review we focus on the latest discoveries regarding the role of the actomyosin cytoskeleton in optimizing immune cell migration in complex environments, with a special focus on recent insights provided by physical modeling.

Myosin 1g Contributes to CD44 Adhesion Protein and Lipid Rafts Recycling and Controls CD44 Capping and Cell Migration in B Lymphocytes

Cell migration and adhesion are critical for immune system function and involve many proteins, which must be continuously transported and recycled in the cell. Recycling of adhesion molecules requires the participation of several proteins, including actin, tubulin, and GTPases, and of membrane components such as sphingolipids and cholesterol. However, roles of actin motor proteins in adhesion molecule recycling are poorly understood. In this study, we identified myosin 1g as one of the important motor proteins that drives recycling of the adhesion protein CD44 in B lymphocytes. We demonstrate that the lack of Myo1g decreases the cell-surface levels of CD44 and of the lipid raft surrogate GM1. In cells depleted of Myo1g, the recycling of CD44 was delayed, the delay seems to be caused at the level of formation of recycling complex and entry into recycling endosomes. Moreover, a defective lipid raft recycling in Myo1g-deficient cells had an impact both on the capping of CD44 and on cell migration. Both processes required the transportation of lipid rafts to the cell surface to deliver signaling components. Furthermore, the extramembrane was essential for cell expansion and remodeling of the plasma membrane topology. Therefore, Myo1g is important during the recycling of lipid rafts to the membrane and to the accompanied proteins that regulate plasma membrane plasticity. Thus, Myosin 1g contributes to cell adhesion and cell migration through CD44 recycling in B lymphocytes.

Semaphorin 4C: A Novel Component of B-Cell Polarization in Th2-Driven Immune Responses

Background

Semaphorins are important molecules in embryonic development and multiple semaphorins have been identified as having key roles in immune regulation. To date, there is little known about Semaphorin 4C (Sema4C) in immune biology. We report for the first time that Sema4C is inducible in human and murine B-cells and may be important for normal B-cell development.

Methods

Human tonsillar B-cells were studied following activation via anti-CD40 antibodies in the presence or absence of representative Th1, Th2, and regulatory cytokines. Murine B-cells from WT and Sema4C^−/− mice were similarly stimulated. B-cell phenotyping in WT and Sema4C mutant mice was performed by flow cytometry and lymphoid architecture was studied by immunohistochemistry. Sema4C expression and synapse formation were analyzed by confocal microscopy.

Results

Gene array studies performed on human tonsillar B-cells stimulated to produce IgE revealed that Sema4C was among the top genes expressed at 24 h, and the only semaphorin to be increased under Th2 conditions. Validation studies demonstrated that human and murine B-cells expressed Sema4C under similar conditions. Sema4C^−/− mice had impaired maturation of B-cell follicles in spleens and associated decreases in follicular and marginal zone B-cells as well as impaired IgG and IgA production. In keeping with a potential role in maturation of B-cells, Sema4C was expressed predominantly on CD27⁺ human B-cells. Within 72 h of B-cell activation, Sema4C was localized to one pole in a synapse-like structure, in association with F-actin, B-cell receptor, and Plexin-B2. Cell polarization was impaired in Sema4C^−/− mice.

Conclusion

We have identified a novel immune semaphorin induced in human and murine B-cells under Th2 conditions. Sema4C appears to be a marker for human memory B-cells. It may be important for B-cell polarization and for the formation of normal splenic follicles.

Control of the initiation and termination of kinesin-1-driven transport by myosin-Ic and nonmuscle tropomyosin

Intracellular transport is largely driven by processive microtubule- and actin-based molecular motors. Nonprocessive motors have also been localized to trafficking cargos, but their roles are not well understood. Myosin-Ic (Myo1c), a nonprocessive actin motor, functions in a variety of exocytic events, although the underlying mechanisms are not yet clear. To investigate the interplay between myosin-I and the canonical long-distance transport motor kinesin-1, we attached both motor types to lipid membrane-coated bead cargo, using an attachment strategy that allows motors to actively reorganize within the membrane in response to the local cytoskeletal environment. We compared the motility of kinesin-1-driven cargos in the absence and presence of Myo1c at engineered actin/microtubule intersections. We found that Myo1c significantly increases the frequency of kinesin-1-driven microtubule-based runs that begin at actin/microtubule intersections. Myo1c also regulates the termination of processive runs. Beads with both motors bound have a significantly higher probability of pausing at actin/microtubule intersections, remaining tethered for an average of 20 s, with some pauses lasting longer than 200 s. The actin-binding protein nonmuscle tropomyosin (Tm) provides spatially specific regulation of interactions between myosin motors and actin filaments in vivo; in the crossed-filament in vitro assay, we found that Tm2-actin abolishes Myo1c-specific effects on both run initiation and run termination. Together, these observations suggest Myo1c is important for the selective initiation and termination of kinesin-1-driven runs along microtubules at specific actin filament populations within the cell.

Class I myosin Myo1e regulates TLR4-triggered macrophage spreading, chemokine release, and antigen presentation via MHC class II

TLR-mediated recognition of microbial danger induces substantial changes in macrophage migration, adherence, and phagocytosis. Recently, we described the LPS-regulated phosphorylation of many cytoskeleton-associated proteins by phosphoproteomics. The functional role of these cytoskeletal and motor proteins in innate immune cell responses is largely unexplored. Here, we first identified both long-tailed class I myosins Myo1e and Myo1f as important contributors to LPS-triggered macrophage spreading. Mouse bone marrow-derived macrophages and DCs deficient in Myo1e selectively secreted increased amounts of the chemokine CCL2. In addition, the cell surface expression of MHC class II (MHC-II) on both cell types was reduced in the absence of Myo1e. However, transcriptional changes in CCL2 and MHC-II were not observed in the absence of Myo1e, indicating that Myo1e regulates specific intracellular transport processes. The capacity of macrophages and DCs lacking Myo1e to stimulate antigen-specific CD4(+) T-cell proliferation was impaired, consistent with the reduced MHC-II surface protein levels. Surprisingly, in Myo1e-deficient DCs, the proteolytic cleavage of endocytosed antigen was also increased. Together, our results provide evidence for a non-redundant function of the motor protein Myo1e in the regulation of TLR4-controlled, cytoskeleton-associated functional properties of macrophages and DCs, and in induction of a full MHC-II-restricted adaptive immune response.

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

Myosin 1g (Myo1g) is a hematopoietic-specific myosin expressed mainly by lymphocytes. Here, we report the localization of Myo1g in B-cell membrane compartments such as lipid rafts, microvilli, and membrane extensions formed during spreading. By using Myo1g-deficient mouse B cells, we detected abnormalities in the adhesion ability and chemokine-induced directed migration of these lymphocytes. We also assessed a role for Myo1g in phagocytosis and exocytosis processes, as these were also irregular in Myo1g-deficient B cells. Taken together, our results show that Myo1g acts as a main regulator of different membrane/cytoskeleton-dependent processes in B lymphocytes.