The RhoA effector mDia is induced during T cell activation and regulates actin polymerization and cell migration in T lymphocytes

Changes in Adhesion and the Expression of Adhesion Molecules in PBMCs after Aneurysmal Subarachnoid Hemorrhage: Relation to Cerebral Vasospasm

Aneurysmal subarachnoid hemorrhage (aSAH) is a neurovascular disease produced by extravasation of blood to the subarachnoid space after rupture of the cerebral vessels. After bleeding, the immune response is activated. The role of peripheral blood mononuclear cells (PBMCs) in this response is a current subject of research. We have analysed the changes in PBMCs of patients with aSAH and their interaction with the endothelium, focusing on their adhesion and the expression of adhesion molecules. Using an in vitro adhesion assay, we observed that the adhesion of PBMCs of patients with aSAH is increased. Flow cytometry analysis shows that monocytes increased significantly in patients, especially in those who developed vasospasm (VSP). In aSAH patients, the expression of CD162, CD49d, CD62L and CD11a in T lymphocytes and of CD62L in monocytes increased. However, the expression of CD162, CD43, and CD11a decreased in monocytes. Furthermore, monocytes from patients who developed arteriographic VSP had lower expression of CD62L. In conclusion, our results confirm that after aSAH, monocyte count and adhesion of PBMCs increase, especially in patients with VSP, and that the expression of several adhesion molecules is altered. These observations can help predict VSP and to improve the treatment of this pathology.

Actin dynamics regulation by TTC7A/PI4KIIIα limits DNA damage and cell death under confinement

BACKGROUND

The actin cytoskeleton has a crucial role in the maintenance of the immune homeostasis by controlling various cellular processes, including cell migration. Mutations in TTC7A have been described as the cause of a primary immunodeficiency associated to different degrees of gut involvement and alterations in the actin cytoskeleton dynamics.

OBJECTIVES

This study investigates the impact of TTC7A deficiency in immune homeostasis. In particular, the role of the TTC7A/phosphatidylinositol 4 kinase type III α pathway in the control of leukocyte migration and actin dynamics.

METHODS

Microfabricated devices were leveraged to study cell migration and actin dynamics of murine and patient-derived leukocytes under confinement at the single-cell level.

RESULTS

We show that TTC7A-deficient lymphocytes exhibit an altered cell migration and reduced capacity to deform through narrow gaps. Mechanistically, TTC7A-deficient phenotype resulted from impaired phosphoinositide signaling, leading to the downregulation of the phosphoinositide 3-kinase/AKT/RHOA regulatory axis and imbalanced actin cytoskeleton dynamics. TTC7A-associated phenotype resulted in impaired cell motility, accumulation of DNA damage, and increased cell death in dense 3-dimensional gels in the presence of chemokines.

CONCLUSIONS

These results highlight a novel role of TTC7A as a critical regulator of lymphocyte migration. Impairment of this cellular function is likely to contribute to the pathophysiology underlying progressive immunodeficiency in patients.

Crosstalk between CD4+ T Cells and Airway Smooth Muscle in Pediatric Obesity-related Asthma

Rationale: Pediatric obesity-related asthma is a nonatopic asthma phenotype with high disease burden and few effective therapies. RhoGTPase upregulation in peripheral blood T helper (Th) cells is associated with the phenotype, but the mechanisms that underlie this association are not known. Objectives: To investigate the mechanisms by which upregulation of CDC42 (Cell Division Cycle 42), a RhoGTPase, in Th cells is associated with airway smooth muscle (ASM) biology. Methods: Chemotaxis of obese asthma and healthy-weight asthma Th cells, and their adhesion to obese and healthy-weight nonasthmatic ASM, was investigated. Transcriptomics and proteomics were used to determine the differential effect of obese and healthy-weight asthma Th cell adhesion to obese or healthy-weight ASM biology. Measurements and Main Results: Chemotaxis of obese asthma Th cells with CDC42 upregulation was resistant to CDC42 inhibition. Obese asthma Th cells were more adherent to obese ASM compared with healthy-weight asthma Th cells to healthy-weight ASM. Compared with coculture with healthy-weight ASM, obese asthma Th cell coculture with obese ASM was positively enriched for genes and proteins involved in actin cytoskeleton organization, transmembrane receptor protein kinase signaling, and cell mitosis, and negatively enriched for extracellular matrix organization. Targeted gene evaluation revealed upregulation of IFNG, TNF (tumor necrosis factor), and Cluster of Differentiation 247 (CD247) among Th cell genes, and of Ak strain transforming (AKT), Ras homolog family member A (RHOA), and CD38, with downregulation of PRKCA (Protein kinase C-alpha), among smooth muscle genes. Conclusions: Obese asthma Th cells have uninhibited chemotaxis and are more adherent to obese ASM, which is associated with upregulation of genes and proteins associated with smooth muscle proliferation and reciprocal nonatopic Th cell activation.

RHOA Therapeutic Targeting in Hematological Cancers

Primarily identified as an important regulator of cytoskeletal dynamics, the small GTPase Ras homolog gene family member A (RHOA) has been implicated in the transduction of signals regulating a broad range of cellular functions such as cell survival, migration, adhesion and proliferation. Deregulated activity of RHOA has been linked to the growth, progression and metastasis of various cancer types. Recent cancer genome-wide sequencing studies have unveiled both RHOA gain and loss-of-function mutations in primary leukemia/lymphoma, suggesting that this GTPase may exert tumor-promoting or tumor-suppressive functions depending on the cellular context. Based on these observations, RHOA signaling represents an attractive therapeutic target for the development of selective anticancer strategies. In this review, we will summarize the molecular mechanisms underlying RHOA GTPase functions in immune regulation and in the development of hematological neoplasms and will discuss the current strategies aimed at modulating RHOA functions in these diseases.

Polymerization power: effectors of actin polymerization as regulators of T lymphocyte migration through complex environments

During their life span, T cells are tasked with patrolling the body for potential pathogens. To do so, T cells migrate through numerous distinct anatomical sites and tissue environments with different biophysical characteristics. To migrate through these different environments, T cells use various motility strategies that rely on actin network remodeling to generate shape changes and mechanical forces. In this review, we initially discuss the migratory journey of T cells and then cover the actin polymerization effectors at play in T cells, and finally, we focus on the function of these effectors of actin cytoskeleton remodeling in mediating T-cell migration through diverse tissue environments. Specifically, we will discuss the current state of the field pertaining to our understanding of the roles in T-cell migration played by members of the three main families of actin polymerization machinery: the Arp2/3 complex; formin proteins; and Ena/VASP proteins.

The Actin Regulators Involved in the Function and Related Diseases of Lymphocytes

Actin is an important cytoskeletal protein involved in signal transduction, cell structure and motility. Actin regulators include actin-monomer-binding proteins, Wiskott-Aldrich syndrome (WAS) family of proteins, nucleation proteins, actin filament polymerases and severing proteins. This group of proteins regulate the dynamic changes in actin assembly/disassembly, thus playing an important role in cell motility, intracellular transport, cell division and other basic cellular activities. Lymphocytes are important components of the human immune system, consisting of T-lymphocytes (T cells), B-lymphocytes (B cells) and natural killer cells (NK cells). Lymphocytes are indispensable for both innate and adaptive immunity and cannot function normally without various actin regulators. In this review, we first briefly introduce the structure and fundamental functions of a variety of well-known and newly discovered actin regulators, then we highlight the role of actin regulators in T cell, B cell and NK cell, and finally provide a landscape of various diseases associated with them. This review provides new directions in exploring actin regulators and promotes more precise and effective treatments for related diseases.

Actin Dynamics at the T Cell Synapse as Revealed by Immune-Related Actinopathies

The actin cytoskeleton is composed of dynamic filament networks that build adaptable local architectures to sustain nearly all cellular activities in response to a myriad of stimuli. Although the function of numerous players that tune actin remodeling is known, the coordinated molecular orchestration of the actin cytoskeleton to guide cellular decisions is still ill defined. T lymphocytes provide a prototypical example of how a complex program of actin cytoskeleton remodeling sustains the spatio-temporal control of key cellular activities, namely antigen scanning and sensing, as well as polarized delivery of effector molecules, via the immunological synapse. We here review the unique knowledge on actin dynamics at the T lymphocyte synapse gained through the study of primary immunodeficiences caused by mutations in genes encoding actin regulatory proteins. Beyond the specific roles of individual actin remodelers, we further develop the view that these operate in a coordinated manner and are an integral part of multiple signaling pathways in T lymphocytes.

Formin-like 1 mediates effector T cell trafficking to inflammatory sites to enable T cell-mediated autoimmunity

Lymphocyte migration is essential for the function of the adaptive immune system, and regulation of T cell entry into tissues is an effective therapy in autoimmune diseases. Little is known about the specific role of cytoskeletal effectors that mediate mechanical forces and morphological changes essential for migration in complex environments. We developed a new Formin-like-1 (FMNL1) knock-out mouse model and determined that the cytoskeletal effector FMNL1 is selectively required for effector T cell trafficking to inflamed tissues, without affecting naïve T cell entry into secondary lymphoid organs. Here, we identify a FMNL1-dependent mechanism of actin polymerization at the back of the cell that enables migration of the rigid lymphocyte nucleus through restrictive barriers. Furthermore, FMNL1-deficiency impairs the ability of self-reactive effector T cells to induce autoimmune disease. Overall, our data suggest that FMNL1 may be a potential therapeutic target to specifically modulate T cell trafficking to inflammatory sites.

Mammalian diaphanous-related formin 1 (mDia1) coordinates mast cell migration and secretion through its actin-nucleating activity

BACKGROUND

Actin remodeling is a key regulator of mast cell (MC) migration and secretion. However, the precise mechanism underlying the coordination of these processes has remained obscure.

OBJECTIVE

We sought to characterize the actin rearrangements that occur during MC secretion or chemotactic migration and identify the underlying mechanism of their coordination.

METHODS

Using high-resolution microscopy, we analyzed the dynamics of actin rearrangements in MCs triggered to migration by IL-8 or prostaglandin E2 or to FcεRI-stimulated secretion.

RESULTS

We show that a major feature of the actin skeleton in MCs stimulated to migration is the buildup of pericentral actin clusters that prevent cell flattening and converge the secretory granules (SGs) in the cell center. This migratory phenotype is replaced on encounter of an IgE cross-linking antigen that stimulates secretion through a secretory phenotype characterized by cell flattening, reduction of actin mesh density, ruffling of cortical actin, and mobilization of SGs. Furthermore, we show that knockdown of mammalian diaphanous-related formin 1 (mDia1) inhibits chemotactic migration and its typical actin rearrangements, whereas expression of an active mDia1 mutant recapitulates the migratory actin phenotype and enhances cell migration while inhibiting FcεRI-triggered secretion. However, mice deficient in mDia1 appear to have normal numbers of MCs in various organs at baseline.

CONCLUSION

Our results demonstrate a unique role of actin rearrangements in clustering the SGs and inhibiting their secretion during MC migration. We identify mDia1 as a novel regulator of MC response that coordinates MC chemotaxis and secretion through its actin-nucleating activity.

RhoA as a Key Regulator of Innate and Adaptive Immunity

RhoA is a ubiquitously expressed cytoplasmic protein that belongs to the family of small GTPases. RhoA acts as a molecular switch that is activated in response to binding of chemokines, cytokines, and growth factors, and via mDia and the ROCK signaling cascade regulates the activation of cytoskeletal proteins, and other factors. This review aims to summarize our current knowledge on the role of RhoA as a general key regulator of immune cell differentiation and function. The contribution of RhoA for the primary functions of innate immune cell types, namely neutrophils, macrophages, and conventional dendritic cells (DC) to (i) get activated by pathogen-derived and endogenous danger signals, (ii) migrate to sites of infection and inflammation, and (iii) internalize pathogens has been fairly established. In activated DC, which constitute the most potent antigen-presenting cells of the immune system, RhoA is also important for the presentation of pathogen-derived antigen and the formation of an immunological synapse between DC and antigen-specific T cells as a prerequisite to induce adaptive T cell responses. In T cells and B cells as the effector cells of the adaptive immune system Rho signaling is pivotal for activation and migration. More recently, mutations of Rho and Rho-modulating factors have been identified to predispose for autoimmune diseases and as causative for hematopoietic malignancies.

Formins, Golgi, and the Centriole

Formin homology proteins (formins) are a highly conserved family of cytoskeletal remodeling proteins that are involved in a diverse array of cellular functions. Formins are best known for their ability to regulate actin dynamics, but the same functional domains also govern stability and organization of microtubules. It is thought that this dual activity allows them to coordinate the activity of these two major cytoskeletal networks and thereby influence cellular architecture. Golgi ribbon assembly is dependent upon cooperative interactions between actin filaments and cytoplasmic microtubules originating both at the Golgi itself and from the centrosome. Similarly, centrosome assembly, centriole duplication, and centrosome positioning are also reliant on a dialogue between both cytoskeletal networks. As presented in this chapter, a growing body of evidence suggests that multiple formin proteins play essential roles in these central cellular processes.

The Formin mDia1 Regulates Acute Lymphoblastic Leukemia Engraftment, Migration, and Progression in vivo

Leukemias typically arise in the bone marrow and then spread to the blood and into other tissues. To disseminate into tissues, leukemia cells migrate into the blood stream and then exit the circulation by migrating across vascular endothelial barriers. Formin proteins regulate cytoskeletal remodeling and cell migration of normal and malignant cells. The Formin mDia1 is highly expressed in transformed lymphocytes and regulates lymphocyte migration. However, the role of mDia1 in regulating leukemia progression in vivo is unknown. Here, we investigated how mDia1 mediates the ability of leukemia cells to migrate and disseminate in vivo. For these studies, we used a mouse model of Bcr-Abl pre-B cell acute lymphoblastic leukemia. Our data showed that mDia1-deficient leukemia cells have reduced chemotaxis and ability to complete transendothelial migration in vitro. In vivo, mDia1 deficiency reduced the ability of leukemia cells to engraft in recipient mice. Furthermore, leukemia dissemination to various tissues and leukemia progression were inhibited by mDia1 depletion. Finally, mDia1 depletion in leukemia cells resulted in prolonged survival of recipient mice in a leukemia transfer model. Overall, our data show that the Formin mDia1 mediates leukemia cell migration, and drives leukemia engraftment and progression in vivo, suggesting that targeting mDia1 could provide a new method for treatment of leukemia.

RhoC regulates the actin remodeling required for phagosome formation during FcγR-mediated phagocytosis

Phagosome formation is a complicated process that requires spatiotemporally regulated actin reorganization. We found that RhoC GTPase is a critical regulator of FcγR-mediated phagocytosis in macrophages. Our live-cell imaging revealed that RhoC, but not RhoA, is recruited to phagocytic cups engulfing IgG-opsonized erythrocytes (IgG-Es). RhoC silencing through RNAi, CRISPR/Cas-mediated RhoC knockout, and the expression of dominant-negative or constitutively active RhoC mutants suppressed the phagocytosis of IgG-Es. Moreover, RhoC-GTP pulldown experiments showed that endogenous RhoC is transiently activated during phagosome formation. Notably, actin-driven pseudopod extension, which is required for the formation of phagocytic cups, was severely impaired in cells expressing the constitutively active mutant RhoC-G14V, which induced abnormal F-actin accumulation underneath the plasma membrane. mDia1 (encoded by DIAPH1), a Rho-dependent actin nucleation factor, and RhoC were colocalized at the phagocytic cups. Similar to what was seen for RhoC, mDia1 silencing through RNAi inhibited phagosome formation. Additionally, the coexpression of mDia1 with constitutively active mutant RhoC-G14V or expression of active mutant mDia1-ΔN3 drastically inhibited the uptake of IgG-Es. These data suggest that RhoC modulates phagosome formation be modifying actin cytoskeletal remodeling via mDia1.

Force-induced transcellular tunnel formation in endothelial cells

The endothelium serves as a protective semipermeable barrier in blood vessels and lymphatic vessels. Leukocytes and pathogens can pass directly through the endothelium by opening holes in endothelial cells, known as transcellular tunnels, which are formed by contact and self-fusion of the apical and basal plasma membranes. Here we test the hypothesis that the actin cytoskeleton is the primary barrier to transcellular tunnel formation using a combination of atomic force microscopy and fluorescence microscopy of live cells. We find that localized mechanical forces are sufficient to induce the formation of transcellular tunnels in HUVECs. When HUVECs are exposed to the bacterial toxin EDIN, which can induce spontaneous transcellular tunnels, less mechanical work is required to form tunnels due to the reduced cytoskeletal stiffness and thickness of these cells, similar to the effects of a ROCK inhibitor. We also observe actin enrichment in response to mechanical indentation that is reduced in cells exposed to the bacterial toxin. Our study shows that the actin cytoskeleton of endothelial cells provides both passive and active resistance against transcellular tunnel formation, serving as a mechanical barrier that can be overcome by mechanical force as well as disruption of the cytoskeleton.

Interaction between mDia1 and ROCK in Rho-induced migration and adhesion of human dental pulp cells

AIM

To investigate the effects of mammalian homologue of Drosophila diaphanous-1(mDia1) and Rho-associated coiled-coil-containing protein kinase (ROCK) on the migration and adhesion of dental pulp cells (DPCs).

METHODOLOGY

Lysophosphatidic acid (LPA) was used to activate Rho signalling. mDia1 and ROCK were inhibited by short interfering RNA and the specific inhibitor, Y-27632, respectively. The migration of DPCs was assessed using the transwell migration assay and scratch test. Formation of cytoskeleton and focal adhesions(FAs) was observed by confocal laser scanning microscopy. Cell adhesion and spreading assays were performed. Phosphorylation of focal adhesion kinase (FAK) and paxillin was detected by Western blotting, and the bands were analysed using Adobe Photoshop CS5 software. All experiments were performed at least three times, and data were analysed with one-way anova and a post hoc test.

RESULTS

LPA-triggered activation of Rho and inhibition of ROCK significantly increased the cell migration rate. Cell migration was inhibited by silencing mDia1. mDia1 silencing and ROCK inhibition suppressed the LPA-induced formation of the cytoskeleton, FA and phosphorylation of FAK and paxillin. Inhibition of ROCK or mDia1 facilitated early cell adhesion and spreading; by contrast, the combined inhibition of ROCK and mDia1 neutralized these effects.

CONCLUSIONS

mDia1 promoted RhoA-induced migration of DPCs, but ROCK had an opposite effect. Both mDia1 and ROCK participated in cytoskeleton formation and adhesion of DPCs. The interactions between mDia1 and ROCK might influence dental pulp repair by determining the migration and adhesion of DPCs.

T Lymphocyte Migration: An Action Movie Starring the Actin and Associated Actors

The actin cytoskeleton is composed of a dynamic filament meshwork that builds the architecture of the cell to sustain its fundamental properties. This physical structure is characterized by a continuous remodeling, which allows cells to accomplish complex motility steps such as directed migration, crossing of biological barriers, and interaction with other cells. T lymphocytes excel in these motility steps to ensure their immune surveillance duties. In particular, actin cytoskeleton remodeling is a key to facilitate the journey of T lymphocytes through distinct tissue environments and to tune their stop and go behavior during the scanning of antigen-presenting cells. The molecular mechanisms controlling actin cytoskeleton remodeling during T lymphocyte motility have been only partially unraveled, since the function of many actin regulators has not yet been assessed in these cells. Our review aims to integrate the current knowledge into a comprehensive picture of how the actin cytoskeleton drives T lymphocyte migration. We will present the molecular actors that control actin cytoskeleton remodeling, as well as their role in the different T lymphocyte motile steps. We will also highlight which challenges remain to be addressed experimentally and which approaches appear promising to tackle them.

Involvement of the Rac1-IRSp53-Wave2-Arp2/3 Signaling Pathway in HIV-1 Gag Particle Release in CD4 T Cells

During HIV-1 assembly, the Gag viral proteins are targeted and assemble at the inner leaflet of the cell plasma membrane. This process could modulate the cortical actin cytoskeleton, located underneath the plasma membrane, since actin dynamics are able to promote localized membrane reorganization. In addition, activated small Rho GTPases are known for regulating actin dynamics and membrane remodeling. Therefore, the modulation of such Rho GTPase activity and of F-actin by the Gag protein during virus particle formation was considered. Here, we studied the implication of the main Rac1, Cdc42, and RhoA small GTPases, and some of their effectors, in this process. The effect of small interfering RNA (siRNA)-mediated Rho GTPases and silencing of their effectors on Gag localization, Gag membrane attachment, and virus-like particle production was analyzed by immunofluorescence coupled to confocal microscopy, membrane flotation assays, and immunoblot assays, respectively. In parallel, the effect of Gag expression on the Rac1 activation level was monitored by G-LISA, and the intracellular F-actin content in T cells was monitored by flow cytometry and fluorescence microscopy. Our results revealed the involvement of activated Rac1 and of the IRSp53-Wave2-Arp2/3 signaling pathway in HIV-1 Gag membrane localization and particle release in T cells as well as a role for actin branching and polymerization, and this was solely dependent on the Gag viral protein. In conclusion, our results highlight a new role for the Rac1-IRSp53-Wave2-Arp2/3 signaling pathway in the late steps of HIV-1 replication in CD4 T lymphocytes.

IMPORTANCE

During HIV-1 assembly, the Gag proteins are targeted and assembled at the inner leaflet of the host cell plasma membrane. Gag interacts with specific membrane phospholipids that can also modulate the regulation of cortical actin cytoskeleton dynamics. Actin dynamics can promote localized membrane reorganization and thus can be involved in facilitating Gag assembly and particle formation. Activated small Rho GTPases and effectors are regulators of actin dynamics and membrane remodeling. We thus studied the effects of the Rac1, Cdc42, and RhoA GTPases and their specific effectors on HIV-1 Gag membrane localization and viral particle release in T cells. Our results show that activated Rac1 and the IRSp53-Wave2-Arp2/3 signaling pathway are involved in Gag plasma membrane localization and viral particle production. This work uncovers a role for cortical actin through the activation of Rac1 and the IRSp53/Wave2 signaling pathway in HIV-1 particle formation in CD4 T lymphocytes.

Multiple roles for RhoA during T cell transendothelial migration

T cells need to cross endothelial barriers during immune surveillance and inflammation. This involves T-cell adhesion to the endothelium followed by polarization and crawling with a lamellipodium at the front and contractile uropod at the back. T cells subsequently extend lamellipodia and filopodia under the endothelium in order to transmigrate. Rho GTPases play key roles in cell migration by regulating cytoskeletal dynamics and cell adhesion. We have found that the Rho GTPase RhoA is required for efficient T-cell polarization and migration on endothelial cells as well as transendothelial migration. RhoA-depleted cells lack both lamellipodia and uropods, and instead have narrow protrusions extending from a rounded cell body. Using a RhoA activity biosensor, we have shown that RhoA is active at the leading edge in lamellipodia and filopodia of crawling and transmigrating T cells, as well as in the uropod. In lamellipodia, its activity correlates with both protrusion and retraction. We predict that RhoA signals via the formin mDIA 1 during lamellipodial protrusion whereas it induces lamellipodial retraction via the kinase ROCK and actomyosin contractility. We propose that different guanine-nucleotide exchange factors (GEFs) are responsible for coordinating RhoA activation and signaling in different regions of transmigrating T cells.

Moving towards a paradigm: common mechanisms of chemotactic signaling in Dictyostelium and mammalian leukocytes

Chemotaxis, or directed migration of cells along a chemical gradient, is a highly coordinated process that involves gradient sensing, motility, and polarity. Most of our understanding of chemotaxis comes from studies of cells undergoing amoeboid-type migration, in particular the social amoeba Dictyostelium discoideum and leukocytes. In these amoeboid cells the molecular events leading to directed migration can be conceptually divided into four interacting networks: receptor/G protein, signal transduction, cytoskeleton, and polarity. The signal transduction network occupies a central position in this scheme as it receives direct input from the receptor/G protein network, as well as feedback from the cytoskeletal and polarity networks. Multiple overlapping modules within the signal transduction network transmit the signals to the actin cytoskeleton network leading to biased pseudopod protrusion in the direction of the gradient. The overall architecture of the networks, as well as the individual signaling modules, is remarkably conserved between Dictyostelium and mammalian leukocytes, and the similarities and differences between the two systems are the subject of this review.

Formin' cellular structures: Physiological roles of Diaphanous (Dia) in actin dynamics

Members of the Diaphanous (Dia) protein family are key regulators of fundamental actin driven cellular processes, which are conserved from yeast to humans. Researchers have uncovered diverse physiological roles in cell morphology, cell motility, cell polarity, and cell division, which are involved in shaping cells into tissues and organs. The identification of numerous binding partners led to substantial progress in our understanding of the differential functions of Dia proteins. Genetic approaches and new microscopy techniques allow important new insights into their localization, activity, and molecular principles of regulation.

Centrosome polarization in T cells: a task for formins

T-cell antigen receptor (TCR) engagement triggers the rapid reorientation of the centrosome, which is associated with the secretory machinery, toward the immunological synapse (IS) for polarized protein trafficking. Recent evidence indicates that upon TCR triggering the INF2 formin, together with the formins DIA1 and FMNL1, promotes the formation of a specialized array of stable detyrosinated MTs that breaks the symmetrical organization of the T-cell microtubule (MT) cytoskeleton. The detyrosinated MT array and TCR-induced tyrosine phosphorylation should coincide for centrosome polarization. We propose that the pushing forces produced by the detyrosinated MT array, which modify the position of the centrosome, in concert with Src kinase dependent TCR signaling, which provide the reference frame with respect to which the centrosome reorients, result in the repositioning of the centrosome to the IS.

Kinetic characterization of nonmuscle myosin IIb at the single molecule level

Nonmuscle myosin IIB (NMIIB) is a cytoplasmic myosin, which plays an important role in cell motility by maintaining cortical tension. It forms bipolar thick filaments with ~14 myosin molecule dimers on each side of the bare zone. Our previous studies showed that the NMIIB is a moderately high duty ratio (~20-25%) motor. The ADP release step (~0.35 s(-1)) of NMIIB is only ~3 times faster than the rate-limiting phosphate release (0.13 ± 0.01 s(-1)). The aim of this study was to relate the known in vitro kinetic parameters to the results of single molecule experiments and to compare the kinetic and mechanical properties of single- and double-headed myosin fragments and nonmuscle IIB thick filaments. Examination of the kinetics of NMIIB interaction with actin at the single molecule level was accomplished using total internal reflection fluorescence (TIRF) with fluorescence imaging with 1-nm accuracy (FIONA) and dual-beam optical trapping. At a physiological ATP concentration (1 mm), the rate of detachment of the single-headed and double-headed molecules was similar (~0.4 s(-1)). Using optical tweezers we found that the power stroke sizes of single- and double-headed heavy meromyosin (HMM) were each ~6 nm. No signs of processive stepping at the single molecule level were observed in the case of NMIIB-HMM in optical tweezers or TIRF/in vitro motility experiments. In contrast, robust motility of individual fluorescently labeled thick filaments of full-length NMIIB was observed on actin filaments. Our results are in good agreement with the previous steady-state and transient kinetic studies and show that the individual nonprocessive nonmuscle myosin IIB molecules form a highly processive unit when polymerized into filaments.

Existence of a novel clathrin-independent endocytic pathway in yeast that depends on Rho1 and formin

Much like mammalian cells, yeast contain a Rho-dependent pathway for endocytosis in addition to canonical clathrin-dependent endocytosis.

Yeast is a powerful model organism for dissecting the temporal stages and choreography of the complex protein machinery during endocytosis. The only known mechanism for endocytosis in yeast is clathrin-mediated endocytosis, even though clathrin-independent endocytic pathways have been described in other eukaryotes. Here, we provide evidence for a clathrin-independent endocytic pathway in yeast. In cells lacking the clathrin-binding adaptor proteins Ent1, Ent2, Yap1801, and Yap1802, we identify a second endocytic pathway that depends on the GTPase Rho1, the downstream formin Bni1, and the Bni1 cofactors Bud6 and Spa2. This second pathway does not require components of the better-studied endocytic pathway, including clathrin and Arp2/3 complex activators. Thus, our results reveal the existence of a second pathway for endocytosis in yeast, which suggests similarities with the RhoA-dependent endocytic pathways of mammalian cells.

α-Syntrophin is required for the hepatocyte growth factor-induced migration of cultured myoblasts

Syntrophins are adaptor proteins that link intracellular signaling molecules to the dystrophin based scaffold. In this study, we investigated the function of syntrophins in cell migration, one of the early steps in myogenic differentiation and in regeneration of adult muscle. Hepatocyte growth factor (HGF) stimulates migration and lamellipodia formation in cultured C2 myoblasts. In the migrating cells, syntrophin concentrated in the rear-lateral region of the cell, opposite of the lamellipodia, instead of being diffusely present throughout the cytoplasm of non-migrating cells. When the expression of α-syntrophin, the major syntrophin isoform of skeletal muscle, was reduced by transfection with the α-syntrophin-specific siRNA, HGF stimulation of lamellipodia formation was prevented. Likewise, migration of myoblasts from α-syntrophin knockout mice could not be stimulated by HGF. However, HGF-induced migration was restored in myoblasts isolated from a transgenic mouse expressing α-syntrophin only in muscle cells. Treatment of C2 myoblasts with inhibitors of PI3-kinase not only reduced the rate of cell migration, but also impaired the accumulation of syntrophins in the rear-lateral region of the migrating cells. Phosphorylation of Akt was reduced in the α-syntrophin siRNA-treated C2 cells. These results suggest that α-syntrophin is required for HGF-induced migration of myoblasts and for proper PI3-kinase/Akt signaling.

Formin INF2 regulates MAL-mediated transport of Lck to the plasma membrane of human T lymphocytes

Expression of the src-family kinase lymphocyte-specific protein tyrosine kinase (Lck) at the plasma membrane is essential for it to fulfill its pivotal role in signal transduction in T lymphocytes. MAL, an integral membrane protein expressed in specific types of lymphoma, has been shown to play an important role in targeting Lck to the plasma membrane. Here we report that MAL interacts with Inverted Formin2 (INF2), a formin with the atypical property of promoting not only actin polymerization but also its depolymerization. In Jurkat T cells, INF2 colocalizes with MAL at the cell periphery and pericentriolar endosomes and along microtubules. Videomicroscopic analysis revealed that the MAL+ vesicles transporting Lck to the plasma membrane move along microtubule tracks. Knockdown of INF2 greatly reduced the formation of MAL+ transport vesicles and the levels of Lck at the plasma membrane and impaired formation of a normal immunologic synapse. The actin polymerization and depolymerization activities of INF2 were both required for efficient Lck targeting. Cdc42 and Rac1, which bind to INF2, regulate Lck transport in both Jurkat and primary human T cells. Thus, INF2 collaborates with MAL in the formation of specific carriers for targeting Lck to the plasma membrane in a process regulated by Cdc42 and Rac1.

Coordinated RhoA signaling at the leading edge and uropod is required for T cell transendothelial migration

A biosensor for the RhoA GTPase illustrates its activation patterns in both the front and rear of migrating lymphocytes.

Transendothelial migration (TEM) is a tightly regulated process whereby leukocytes migrate from the vasculature into tissues. Rho guanosine triphosphatases (GTPases) are implicated in TEM, but the contributions of individual Rho family members are not known. In this study, we use an RNA interference screen to identify which Rho GTPases affect T cell TEM and demonstrate that RhoA is critical for this process. RhoA depletion leads to loss of migratory polarity; cells lack both leading edge and uropod structures and, instead, have stable narrow protrusions with delocalized protrusions and contractions. By imaging a RhoA activity biosensor in transmigrating T cells, we find that RhoA is locally and dynamically activated at the leading edge, where its activation precedes both extension and retraction events, and in the uropod, where it is associated with ROCK-mediated contraction. The Rho guanine nucleotide exchange factor (GEF) GEF-H1 contributes to uropod contraction but does not affect the leading edge. Our data indicate that RhoA activity is dynamically regulated at the front and back of T cells to coordinate TEM.

Cytoskeletal cross-talk in the control of T cell antigen receptor signaling

T cell antigen receptor signaling is triggered and controlled in specialized cellular interfaces formed between T cells and antigen-presenting cells named immunological synapses. Both microtubules and actin cytoskeleton rearrange at the immunological synapse in response to T cell receptor triggering, ensuring in turn the accuracy of intracellular signaling. Recent reports show that the cross-talk between the cortical actin cytoskeleton and microtubule networks is key for structuring the immunological synapse and for controlling T cell receptor signaling. Immunological synapse architecture and the interaction between the signaling machinery and various cytoskeletal elements are therefore crucial for the fine-tuning of T cell signaling.

Downregulation of RhoA and changes in T cell cytoskeleton correlate with the abrogation of allograft rejection

Proper actin cytoskeleton architecture and dynamics are indispensable for events in the immunological response such as T cell migration, redistribution of T cell receptors, and interaction with antigen presenting cells. Thus, T cell activation, downstream signaling events and effector functions are all actin-dependent. Actin cytoskeleton architecture and dynamics are regulated by proteins belonging to the superfamily of small GTP-binding proteins, such as RhoA GTPase. We previously showed that the administration of an MHC class I allochimeric molecule [alpha1h1/u]-RT1.Aa, which contains donor-type (Wistar Furth, WF; RT1u) immunogenic epitopes displayed on recipient-type (ACI, RT1a) sequences, to the ACI recipient of heterotopic WF heart resulted in the restriction of the TCR repertoire, inhibition of T cell infiltration into the heterotopic cardiac allografts, abrogation of acute and chronic rejection, and induction of indefinite survival of the allograft. Here we show that the allochimeric molecule treatment caused downregulation of RhoA GTPase in T cells. This resulted in dramatic changes in the distribution of actin and the actin-binding protein, Hip55, in these cells, which in turn, inhibited T cell infiltration into the graft. This indicates that the immunosuppressive activity of the allochimeric molecule is achieved via downregulation of the RhoA pathway and disruption of the proper organization of T cell actin cytoskeleton to inhibit T cell functions such as motility and/or TCR signaling events.

F-actin-binding protein drebrin regulates CXCR4 recruitment to the immune synapse

The adaptive immune response depends on the interaction of T cells and antigen-presenting cells at the immune synapse. Formation of the immune synapse and the subsequent T-cell activation are highly dependent on the actin cytoskeleton. In this work, we describe that T cells express drebrin, a neuronal actin-binding protein. Drebrin colocalizes with the chemokine receptor CXCR4 and F-actin at the peripheral supramolecular activation cluster in the immune synapse. Drebrin interacts with the cytoplasmic tail of CXCR4 and both proteins redistribute to the immune synapse with similar kinetics. Drebrin knockdown in T cells impairs the redistribution of CXCR4 and inhibits actin polymerization at the immune synapse as well as IL-2 production. Our data indicate that drebrin exerts an unexpected and relevant functional role in T cells during the generation of the immune response.

The actin-bundling protein L-plastin dissociates CCR7 proximal signaling from CCR7-induced motility

Chemokines promote lymphocyte motility by triggering F-actin rearrangements and inducing cellular polarization. Chemokines can also enhance cell-cell adhesion and costimulate T cells. In this study, we establish a requirement for the actin-bundling protein L-plastin (LPL) in CCR7- and sphingosine-1-phosphate-mediated T cell chemotaxis using LPL(-/-) mice. Disrupted motility of mature LPL(-/-) thymocytes manifested in vivo as diminished thymic egress. Two-photon microscopy of LPL(-/-) lymphocytes revealed reduced velocity and motility in lymph nodes. Defective migration resulted from defective cellular polarization following CCR7 ligation, as CCR7 did not polarize to the leading edge in chemokine-stimulated LPL(-/-) T cells. However, CCR7 signaling to F-actin polymerization and CCR7-mediated costimulation was intact in LPL(-/-) lymphocytes. The differential requirement for LPL in CCR7-induced cellular adhesion and CCR7-induced motility allowed assessment of the contribution of CCR7-mediated motility to positive selection of thymocytes and lineage commitment. Results suggest that normal motility is not required for CCR7 to function in positive selection and lineage commitment. We thus identify LPL as a molecule critical for CCR7-mediated motility but dispensable for early CCR7 signaling. The requirement for actin bundling by LPL for polarization reveals a novel mechanism of regulating actin dynamics during T cell motility.

The role of formins in human disease

Formins are a conserved family of proteins that play key roles in cytoskeletal remodeling. They nucleate and processively elongate non-branched actin filaments and also modulate microtubule dynamics. Despite their significant contributions to cell biology and development, few studies have directly implicated formins in disease pathogenesis. This review highlights the roles of formins in cell division, migration, immunity, and microvesicle formation in the context of human disease. In addition, we discuss the importance of controlling formin activity and protein expression to maintain cell homeostasis.

Rho kinase: an important mediator of atherosclerosis and vascular disease

Atherosclerosis is a complex inflammatory process characterized by the cross-talk between excessive inflammation and lipid accumulation. In the past few years, compelling evidence suggests that statins can decrease vascular inflammation and attenuate the development of atherosclerosis through their so-called "pleiotropic effects". These cholesterol-independent effects are predominantly due to their ability to inhibit isoprenoid synthesis. In particular, inhibition of geranylgeranylpyrophosphate synthesis leads to inhibition of Rho and its downstream target, Rho-kinase (ROCK). Thus, one of the beneficial effects of statin therapy could be due to inhibitory effects on ROCK. ROCK is involved in mediating diverse cellular functions such as smooth muscle contraction, cell migration and proliferation. While increased ROCK activity is associated with endothelial dysfunction, cerebral ischemia, coronary vasospasms and metabolic syndrome, the inhibition of ROCK by statins or selective ROCK inhibitors leads to up-regulation of endothelial nitric oxide synthase (eNOS), decreased vascular inflammation, and reduced atherosclerotic plaque formation. This review will focus on the impact of ROCK in cardiovascular disease and its contributory role to vascular inflammation and the atherosclerosis.

Src kinase Hck association with the WASp and mDia1 cytoskeletal regulators promotes chemoattractant-induced Hck membrane targeting and activation in neutrophils

The haemopoietic cell kinase (Hck) plays an important but poorly understood role in coupling chemoattractant stimuli to the actin cytoskeletal rearrangement required for neutrophil polarization and chemotaxis. Here, we show that Hck coimmunoprecipitates with the cytoskeletal regulatory Wiskott-Aldrich syndrome protein (WASp) and mammalian diaphanous-related formin 1 (mDia1) in chemoattractant-stimulated neutrophils, and that the 3 proteins inducibly colocalize with one another at the leading edge of chemotaxing cells. Hck interaction with WASp was found to be mediated by the Hck SH3 domain binding to the WASp proline-rich region, while Hck interaction with mDia1 was indirect but was required for binding to WASp. In contrast to wild-type cells, both WASp- and mDia1-deficient neutrophils showed severe impairment of chemokine-induced Hck membrane translocation and induction of Hck binding to WASp, and Hck activation and WASp tyrosine phosphorylation were impaired in mDia1-/- cells. Thus, chemotactic stimulation appears to induce an mDia1/Hck/WASp complex required for Hck membrane targeting and for induction of the Hck-mediated WASp tyrosine phosphorylation thought to be required for WASp-driven actin polymerization. These findings reveal that Hck functions in neutrophils to be realized, at least in part, via its interaction with mDia1 and WASp, and identifies the mDia1/Hck/WASp axis as a cytoskeletal signaling interface linking tyrosine phosphorylation to chemotactic and, possibly, other actin-based neutrophil responses.

The mDial formin is required for neutrophil polarization, migration, and activation of the LARG/RhoA/ROCK signaling axis during chemotaxis

Neutrophil chemotaxis depends on actin dynamics, but the roles for specific cytoskeleton regulators in this response remain unclear. By analysis of mammalian diaphanous-related formin 1 (mDia1)-deficient mice, we have identified an essential role for this actin nucleator in neutrophil chemotaxis. Lack of mDia1 was associated with defects in chemoattractant-induced neutrophil actin polymerization, polarization, and directional migration, and also with impaired activation of RhoA, its downstream target p160-Rho-associated coil-containing protein kinase (ROCK), and the leukemia-associated RhoA guanine nucleotide exchange factor (LARG). Our data also revealed mDia1 to be associated with another cytoskeletal regulator, Wiskott-Aldrich syndrome protein (WASp), at the leading edge of chemotaxing neutrophils and revealed polarized morphology and chemotaxis to be more mildly impaired in WAS(-/-) than in mDia1(-/-) neutrophils, but essentially abrogated by combined mDia1/WASp deficiency. Thus, mDia1 roles in neutrophil chemotaxis appear to be subserved in concert with WASp and are realized at least in part by activation of the LARG/RhoA/ROCK signaling pathway.

Cytokine-induced F-actin reorganization in endothelial cells involves RhoA activation

Acute ischemic kidney injury results in marked increases in local and systemic cytokine levels. IL-1alpha, IL-6, and TNF-alpha orchestrate various inflammatory reactions influencing endothelial permeability by altering cell-to-cell and cell-to-extracellular matrix attachments. To explore the role of actin and the regulatory proteins RhoA and cofilin in this process, microvascular endothelial cells (MS1) were exposed to individual cytokines or a cytokine cocktail. Within minutes, a marked, time-dependent redistribution of the actin cytoskeleton occurred with the formation of long, dense F-actin basal stress fibers. The concentration of F-actin, normalized to nuclear staining, significantly increased compared with untreated cells (up 20%, P < or = 0.05). Western blot analysis of MS1 lysates incubated with the cytokine cocktail for 4 h showed an increase in phosphorylated/inactive cofilin (up 25 +/- 15%, P < or = 0.05) and RhoA activation (up to 227 +/- 26% increase, P < or = 0.05) compared with untreated cells. Decreasing RhoA levels using small interfering RNA blocked the effect of cytokines on stress fiber organization. Treatment with Y-27632, an inhibitor of the RhoA effector p160-ROCK, decreased levels of phosphorylated cofilin and reduced stress fiber fluorescence by 22%. In cells treated with Y-27632 followed by treatment with the cytokine cocktail, stress fiber levels were similar to control cells and cofilin phosphorylation was 55% of control levels. Taken together, these studies demonstrate cytokine stimulation of RhoA, which in turn leads to cofilin phosphorylation and formation of numerous basal actin stress fibers. These results suggest cytokines signal through the Rho-ROCK pathway, but also through another pathway to affect actin dynamics.

Clustering T-cell GM1 lipid rafts increases cellular resistance to shear on fibronectin through changes in integrin affinity and cytoskeletal dynamics

Lipid rafts are small laterally mobile microdomains that are highly enriched in lymphocyte signaling molecules. GM1 gangliosides are a common lipid raft component and have been shown to be important in many T cell functions. The aggregation of specific GM1 lipid rafts can control many T cell activation events, including their novel association with T cell integrins. We found that clustering GM1 lipid rafts can regulate β1 integrin function. This was apparent through increased resistance to shear flow dependent detachment of T cells adherent to the α4β1 and α5β1 integrin ligand fibronectin (FN). Adhesion strengthening as a result of clustering GM1 enriched lipid rafts correlated with increased cellular rigidity and morphology through the localization of cortical F-actin, the resistance to shear induced cell stretching, and an increase in the surface area and symmetry of the contact area between the cell surface and adhesive substrate. Furthermore, clustering GM1 lipid rafts could initiate integrin “inside-out” signaling mechanisms. This was seen through increased integrin-cytoskeleton associations and enhanced soluble binding of FN and VCAM-1 suggesting the induction of high affinity integrin conformations. The activation of these adhesion strengthening characteristics appear to be specific for the aggregation of GM1 lipid rafts as the aggregation of the heterogeneous raft associated molecule CD59 failed to activate these functions. These findings indicate a novel mechanism to signal to β1 integrins and to activate adhesion strengthening processes.

Chemoattract receptor signaling and its role in lymphocyte motility and trafficking

Intravital microscopy has provided extraordinary glimpses of lymphocytes crossing high endothelial venules, detailed the movements and interactions of lymphocytes within lymph organs, and recorded lymphocytes crossing the lymphatic endothelium into the efferent lymph. Helping to orchestrate these movements are signals generated by the engagement of chemoattractants with their cognate receptors. Chemokines present on high endothelial venules and within lymph organs, and the high levels of sphingosine l-phosphate in the lymph, provide signposts to help guide lymphocytes and provide intracellular signals that affect lymphocyte polarity and motility. Within lymph nodes, T and B lymphocytes migrate along networks of fibroblastic reticular cells and follicular dendritic, respectively, which provide an adhesive platform and solid phased chemokines. Illustrating the importance of chemoattractant receptors in these processes, lymphocytes that lack CXCR4, CXCR5, CCR7 or S1PR1, or which lack crucial signaling molecules activated by these receptors, exhibit defects in lymph node entrance, positioning, polarity, motility, and/or lymph node egress. This review will focus on the contributions of in vivo imaging of lymphocytes from various mouse mutants to our understanding of the roles chemoattractants play in lymphocyte entrance into and exit from lymph nodes, and in coordinating and facilitating the movements of lymphocytes within lymph nodes.

T cell activation and the cytoskeleton: you can't have one without the other

More than a quarter of a century has passed since the observation that T cells rapidly polarize their actin and microtubule cytoskeletal systems toward antigen-presenting cells during activation. Since this initial discovery, several receptors on T cells (e.g., T cell receptor [TCR], co-receptors, integrins, and chemokine receptors) have been identified to regulate these two cytoskeletal networks through complex signaling pathways, which are still being elucidated. There is now an undeniable body of biochemical, pharmacological, and genetic evidence indicating that regulators of actin and microtubule dynamics are crucial for T cell activation and effector functions. In fact, the actin cytoskeleton participates in the initial clustering of TCR-major histocompatibility complex or peptide complexes, formation and stabilization of the immune synapse, integrin-mediated adhesion, and receptor sequestration, whereas both the actin and microtubule cytoskeletons regulate the establishment of cell polarity, cell migration, and directed secretion of cytokines and cytolytic granules. Over the past several years, we have begun to more thoroughly understand the contributions of specific actin-regulatory and actin-nucleating proteins that govern these processes. Herein, we discuss our current understanding of how activating receptors on T lymphocytes regulate the actin and microtubule cytoskeletons, and how in turn, these distinct but integrated cytoskeletal networks coordinate T cell immune responses.

T Cell Responses in Mammalian Diaphanous-related Formin mDia1 Knock-out Mice

Activated T cells rapidly assemble filamentous (F-) actin networks in response to ligation of the T cell receptor or upon interaction with adhesive stimuli in order to facilitate cell migration and the formation of the immune synapse. Branched filament assembly is crucial for this process and is dependent upon activation of the Arp2/3 complex by the actin nucleation-promoting factor Wiskott-Aldrich Syndrome protein (WASp). Genetic disruption of the WAS gene has been linked to hematopoietic malignancies and various cytopenias. Although the contributions of WASp and Arp2/3 to T cell responses are fairly well characterized, the role of the mammalian Diaphanous (mDia)-related formins, which both nucleate and processively elongate non-branched F-actin, has not been demonstrated. Here, we report the effects on T cell development and function following the knock out of the murine Drf1 gene encoding the canonical formin p140mDia1. Drf1(-/-) mice develop lymphopenia characterized by diminished T cell populations in lymphoid tissues. Consistent with a role for p140mDia1 in the regulation of the actin cytoskeleton, isolated Drf1(-/-) splenic T cells adhered poorly to extracellular matrix proteins and migration in response to chemotactic stimuli was completely abrogated. Both integrin and chemokine receptor expression was unaffected by Drf1(-/-) targeting. In response to proliferative stimuli, both thymic and splenic Drf1(-/-) T cells failed to proliferate; ERK1/2 activation was also diminished in activated Drf1(-/-) T cells. These data suggest a central role for p140mDia1 in vivo in dynamic cytoskeletal remodeling events driving normal T cell responses.

Myosin IIA is involved in the endocytosis of CXCR4 induced by SDF-1alpha

Endocytosis of chemokine receptors regulates signal transduction initiated by chemokines, but the molecular mechanisms underlying this process are not fully defined. In this work, we assessed the involvement of the motor protein nonmuscle myosin heavy chain IIA (MIIA) in the endocytosis of CXCR4 induced by SDF-1alpha (also known as CXCL12) in T lymphocytes. Overexpression of the C-terminal half of MIIA inhibited the ligand-induced endocytosis of CXCR4, but not that of transferrin receptor. Targeting MIIA either by silencing its expression with small interfering RNA (siRNA) or by blebbistatin treatment also inhibited endocytosis of CXCR4. Inhibition of endocytosis of CXCR4 by targeting endogenous MIIA resulted in an increased migration of T cells induced by SDF-1alpha, and in the inhibition of the HIV-1-Env antifusogenic activity of this chemokine. Coimmunoprecipitation and protein-protein binding studies demonstrated that MIIA interacts with both the cytoplasmic tail of CXCR4 and beta-arrestin. Moreover, SDF-1alpha promotes a rapid MIIA-beta-arrestin dissociation. Our data reveal a novel role for MIIA in CXCR4 endocytosis, which involves its dynamic association with beta-arrestin and highlights the role of endogenous MIIA as a regulator of CXCR4 internalization and, therefore, the onset of SDF-1alpha signaling.

The Rho-mDia1 pathway regulates cell polarity and focal adhesion turnover in migrating cells through mobilizing Apc and c-Src

Directed cell migration requires cell polarization and adhesion turnover, in which the actin cytoskeleton and microtubules work critically. The Rho GTPases induce specific types of actin cytoskeleton and regulate microtubule dynamics. In migrating cells, Cdc42 regulates cell polarity and Rac works in membrane protrusion. However, the role of Rho in migration is little known. Rho acts on two major effectors, ROCK and mDia1, among which mDia1 produces straight actin filaments and aligns microtubules. Here we depleted mDia1 by RNA interference and found that mDia1 depletion impaired directed migration of rat C6 glioma cells by inhibiting both cell polarization and adhesion turnover. Apc and active Cdc42, which work together for cell polarization, localized in the front of migrating cells, while active c-Src, which regulates adhesion turnover, localized in focal adhesions. mDia1 depletion impaired localization of these molecules at their respective sites. Conversely, expression of active mDia1 facilitated microtubule-dependent accumulation of Apc and active Cdc42 in the polar ends of the cells and actin-dependent recruitment of c-Src in adhesions. Thus, the Rho-mDia1 pathway regulates polarization and adhesion turnover by aligning microtubules and actin filaments and delivering Apc/Cdc42 and c-Src to their respective sites of action.

Chemoattractant receptor signaling and the control of lymphocyte migration

This review focuses on mechanisms by which chemoattractant receptors activate downstream signaling pathways in lymphocytes. An emphasis is placed on heterotrimeric G protein signaling with a discussion of the specific heterotrimeric G-proteins involved in lymphocyte chemotaxis and motility and the role of regulator of G protein signaling (RGS) proteins in controlling the activation of downstream effectors. Also considered are those direct downstream effectors known to function in lymphocyte chemotaxis and/or motility. The consequences of targeting genes suspected, known, or serendipitously found to be involved in chemokine receptor signaling pathways form much of a basis for the review. When needed for clarification, reference to studies of chemoattractant signaling in model organisms and in neutrophils will be compared and contrasted to studies in lymphocytes. Finally, the emergence of tools to image lymphocyte in vitro and in vivo will be mentioned as they are increasing helpful for the analysis of lymphocyte trafficking and amendable to the study of chemokine receptor signaling.

Staying in shape with formins

Formins constitute a diverse protein family present in all eukaryotes examined. They are defined by the presence of a formin homology 2 (FH2) domain, which possesses intrinsic and conserved functions regulating cytoskeletal dynamics. Over the past few years, formins have become recognized as potent nucleators of linear actin filaments that control a large variety of cellular and morphogenetic functions. Here, we review the molecular principles of formin-induced cytoskeletal rearrangements and their consequences for a growing number of biological processes.