Rho: a connection between membrane receptor signalling and the cytoskeleton

Assessment of Endothelial Barrier Functions in Extra Embryonic Vasculature of Chick Embryo as an Alternative Model

Vascular permeability, a tightly regulated process, is a direct measure of angiogenic and immune responses in the endothelium altered in several acute and chronic diseases such as sepsis, high-altitude pulmonary edema (HAPE), high-altitude cerebral edema (HACE), ischemia, and coronavirus disease 2019 (COVID-19) endotheliitis. Both endogenous and exogenous factors such as cytokines, chemokines, and hormones may affect vascular permeability. The conventional tools available for the measurement of vascular permeability in vitro and in vivo based on collagen-coated Transwell and dye-based spectrophotometric methods are indirect measures of permeability. In this chapter, we present our live in ovo protocols based on dextran-Texas red and avian chorioallantoic membrane assay developed using custom-made equipment to assess leakiness of endothelial cell barrier both in vitro and in vivo. Further, we validate this assay using different stressors such as ischemia and hypoxia known to affect endothelial barrier properties by potentiating actin stress fiber disorganization and disrupting the cell-cell junctions.

Modulation of Rac1/PAK1/connexin43-mediated ATP release from astrocytes contributes to retinal ganglion cell survival in experimental glaucoma

Connexin43 (Cx43) is a major gap junction protein in glial cells. Mutations have been found in the gap-junction alpha 1 gene encoding Cx43 in glaucomatous human retinas, suggestive of the involvement of Cx43 in the pathogenesis of glaucoma. However, how Cx43 is involved in glaucoma is still unknown. We showed that increased intraocular pressure in a glaucoma mouse model of chronic ocular hypertension (COH) downregulated Cx43, which was mainly expressed in retinal astrocytes. Astrocytes in the optic nerve head where they gather and wrap the axons (optic nerve) of retinal ganglion cells (RGCs) were activated earlier than neurons in COH retinas and the alterations in astrocytes plasticity in the optic nerve caused a reduction in Cx43 expression. A time course showed that reductions of Cx43 expression were correlated with the activation of Rac1, a member of the Rho family. Co-immunoprecipitation assays showed that active Rac1, or the downstream signaling effector PAK1, negatively regulated Cx43 expression, Cx43 hemichannel opening and astrocyte activation. Pharmacological inhibition of Rac1 stimulated Cx43 hemichannel opening and ATP release, and astrocytes were identified to be one of the main sources of ATP. Furthermore, conditional knockout of Rac1 in astrocytes enhanced Cx43 expression and ATP release, and promoted RGC survival by upregulating the adenosine A3 receptor in RGCs. Our study provides new insight into the relationship between Cx43 and glaucoma, and suggests that regulating the interaction between astrocytes and RGCs via the Rac1/PAK1/Cx43/ATP pathway may be used as part of a therapeutic strategy for managing glaucoma.

Nuclear RAC1 is a modulator of the doxorubicin-induced DNA damage response

Rho GTPases like RAC1 are localized on the inner side of the outer cell membrane where they act as molecular switches that can trigger signal transduction pathways in response to various extracellular stimuli. Nuclear functions of RAC1 were identified that are related to mitosis, cell cycle arrest and apoptosis. Previously, we showed that RAC1 plays a role in the doxorubicin (Dox)-induced DNA damage response (DDR). In this context it is still unknown whether cytosolic RAC1 modulates the Dox-induced DDR or if a nuclear fraction of RAC1 is involved. Here, we silenced RAC1 in mouse embryonic fibroblasts (MEF) pharmacologically with EHT1864 or by using siRNA against Rac1. Additionally, we transfected MEF with RAC1 mutants (wild-type, dominant-negative, constitutively active) containing a nuclear localization sequence (NLS). Afterwards, we analysed the Dox-induced DDR by evaluation of fluorescent nuclear γH2AX and 53BP1 foci formation, as well as by detection of activated proteins of the DDR by western blot to elucidate the role of nuclear RAC1 in the DDR. Treatment with EHT1864 as well as Rac1 knock-down reduced the Dox-induced DSB-formation to a similar extent. Enhanced nuclear localization of dominant-negative as well as constitutively active RAC1 mimicked these effects. Expression of the RAC1 mutants altered the Dox-induced amount of pP53 and pKAP1 protein. The observed effects were independent of S1981 ATM phosphorylation. We conclude that RAC1 is required for a substantial activation of the Dox-induced DDR and balanced levels of active/inactive RAC1 inside the nucleus are a prerequisite for this response.

Into the breach: how cells cope with wounds

Repair of wounds to individual cells is crucial for organisms to survive daily physiological or environmental stresses, as well as pathogen assaults, which disrupt the plasma membrane. Sensing wounds, resealing membranes, closing wounds and remodelling plasma membrane/cortical cytoskeleton are four major steps that are essential to return cells to their pre-wounded states. This process relies on dynamic changes of the membrane/cytoskeleton that are indispensable for carrying out the repairs within tens of minutes. Studies from different cell wound repair models over the last two decades have revealed that the molecular mechanisms of single cell wound repair are very diverse and dependent on wound type, size, and/or species. Interestingly, different repair models have been shown to use similar proteins to achieve the same end result, albeit sometimes by distinctive mechanisms. Recent studies using cutting edge microscopy and molecular techniques are shedding new light on the molecular mechanisms during cellular wound repair. Here, we describe what is currently known about the mechanisms underlying this repair process. In addition, we discuss how the study of cellular wound repair—a powerful and inducible model—can contribute to our understanding of other fundamental biological processes such as cytokinesis, cell migration, cancer metastasis and human diseases.

MiR-384 inhibits human colorectal cancer metastasis by targeting KRAS and CDC42

Colorectal cancer (CRC) is the third most common cancer worldwide. Metastatic progression is a primary factor contributing to lethality of CRC patients. However, the molecular mechanisms forming early local invasion and distant metastatic colonies are still unclear and the present therapeutic approaches for CRC are unsatisfactory. Therefore, novel therapies targeting metastatic invasion that could prevent tumor spreading and recurrence are urgently needed. Our study showed that the decrease of miR-384 was found in 83.0% (83/100) CRC patients. And low-leveled expression of miR-384 was closely correlated with the invasive depth, lymph node and distant metastasis of CRC. Overexpression of miR-384 could inhibit the invasive and migrating abilities of CRC cells in vitro and the metastatic potential in vivo. Luciferase assays showed that miR-384 repressed the expression of Kirsten Ras (KRAS) and Cell division cycle 42 (CDC42) by directly targeting their 3’-untranslated regions. There is functional and mechanistic relationship between miRNA-384 and KRAS, CDC42 in the invasion and metastasis of CRC. And our findings suggest that miR-384could be a potent therapeutic target for CRC. Restoration of miR-384 expression might provide novel therapeutic approach to the reduction of CRC metastasis.

Biphasic myosin II light chain activation during clot retraction

Clot retraction is an essential step during primary haemostasis, thereby promoting thrombus stability and wound healing. Integrin αIIbβ3 plays a critical role in clot retraction, by inducing acto-myosin interactions that allow platelet cytoskeleton reorganisation. However, the signalling pathways that lead to clot retraction are still misunderstood. In this study, we report the first data on the kinetics of myosin II light chain (MLC) phosphorylation during clot retraction. We found an early phosphorylation peak followed by a second peak. By using specific inhibitors of kinases and small G proteins, we showed that MLC kinase (MLCK), RhoA/ROCK, and Rac-1 were involved in clot retraction and in the early MLC phosphorylation peak. Only Rac-1 and actin polymerisation, controlled by outside-in signalling, were crucial to the second MLC phosphorylation peak.

Quantifying Filopodia in Cultured Astrocytes by an Algorithm

Astrocytes in vivo extend thin processes termed peripheral astrocyte processes (PAPs), in particular around synapses where they can mediate glia-neuronal communication. The relation of PAPs to synapses is not based on coincidence, but it is not clear which stimuli and mechanisms lead to their formation and are active during process extension/ retraction in response to neuronal activity. Also, the molecular basis of the extremely fine PAP morphology (often 50 to 100 nm) is not understood. These open questions can be best investigated under in vitro conditions studying glial filopodia. We have previously analyzed filopodial mechanisms (Lavialle et al. PNAS 108:12915) applying an automated method for filopodia morphometry, which is now described in greater detail. The Filopodia Specific Shape Factor (FSSF) developed integrates number and length of filopodia. It quantifies filopodia independent of overall astrocytic shape or size, which can be intricate in itself. The algorithm supplied here permits automated image processing and measurements using ImageJ. Cells have to be sampled in higher numbers to obtain significant results. We validate the FSSF, and characterize the systematic influence of thresholding and camera pixel grid on measurements. We provide exemplary results of substance-induced filopodia dynamics (glutamate, mGluR agonists, EGF), and show that filopodia formation is highly sensitive to medium pH (CO₂) and duration of cell culture. Although the FSSF was developed to study astrocyte filopodia with focus on the perisynaptic glial sheath, we expect that this parameter can also be applied to neuronal growth cones, non-neural cell types, or cell lines.

Cdc42 in actin dynamics: An ordered pathway governed by complex equilibria and directional effector handover

The small GTPase, Cdc42, is a key regulator of actin dynamics, functioning to connect multiple signals to actin polymerization through effector proteins of the Wiskott-Aldrich syndrome protein (WASP) and Transducer of Cdc42-dependent actin assembly (TOCA) families. WASP family members serve to couple Cdc42 with the actin nucleator, the Arp2/3 complex, via direct interactions. The regulation of these proteins in the context of actin dynamics has been extensively studied. Studies on the TOCA family, however, are more limited and relatively little is known about their roles and regulation. In this commentary we highlight new structural and biophysical insight into the involvement of TOCA proteins in the pathway of Cdc42-dependent actin dynamics. We discuss the biological implications of the low affinity interactions between the TOCA family and Cdc42, as well as probing the sequential binding of TOCA1 and the WASP homolog, N-WASP, to Cdc42. We place our current research in the context of the wealth of biophysical, structural and functional data from earlier studies pertaining to the Cdc42/N-WASP/Arp2/3 pathway of actin polymerization. Finally, we describe the molecular basis for a sequential G protein-effector handover from TOCA1 to N-WASP.

Investigation of the Interaction between Cdc42 and Its Effector TOCA1: HANDOVER OF Cdc42 TO THE ACTIN REGULATOR N-WASP IS FACILITATED BY DIFFERENTIAL BINDING AFFINITIES

Transducer of Cdc42-dependent actin assembly protein 1 (TOCA1) is an effector of the Rho family small G protein Cdc42. It contains a membrane-deforming F-BAR domain as well as a Src homology 3 (SH3) domain and a G protein-binding homology region 1 (HR1) domain. TOCA1 binding to Cdc42 leads to actin rearrangements, which are thought to be involved in processes such as endocytosis, filopodia formation, and cell migration. We have solved the structure of the HR1 domain of TOCA1, providing the first structural data for this protein. We have found that the TOCA1 HR1, like the closely related CIP4 HR1, has interesting structural features that are not observed in other HR1 domains. We have also investigated the binding of the TOCA HR1 domain to Cdc42 and the potential ternary complex between Cdc42 and the G protein-binding regions of TOCA1 and a member of the Wiskott-Aldrich syndrome protein family, N-WASP. TOCA1 binds Cdc42 with micromolar affinity, in contrast to the nanomolar affinity of the N-WASP G protein-binding region for Cdc42. NMR experiments show that the Cdc42-binding domain from N-WASP is able to displace TOCA1 HR1 from Cdc42, whereas the N-WASP domain but not the TOCA1 HR1 domain inhibits actin polymerization. This suggests that TOCA1 binding to Cdc42 is an early step in the Cdc42-dependent pathways that govern actin dynamics, and the differential binding affinities of the effectors facilitate a handover from TOCA1 to N-WASP, which can then drive recruitment of the actin-modifying machinery.

Rho family GTPase functions in Drosophila epithelial wound repair

Epithelial repair in the Drosophila embryo is achieved through 2 dynamic cytoskeletal machineries: a contractile actomyosin cable and actin-based cellular protrusions. Rho family small GTPases (Rho, Rac, and Cdc42) are cytoskeletal regulators that control both of these wound repair mechanisms. Cdc42 is necessary for cellular protrusions and, when absent, wounds are slow to repair and never completely close. Rac proteins accumulate at specific regions in the wound leading edge cells and Rac-deficient embryos exhibit slower repair kinetics. Mutants for both Rho1 and its effector Rok impair the ability of wounds to close by disrupting the leading-edge actin cable. Our studies highlight the importance of these proteins in wound repair and identify a downstream effector of Rho1 signaling in this process.

Rho family GTPases bring a familiar ring to cell wound repair

Repair of wounds to single cells involves dynamic membrane and cytoskeletal rearrangements necessary to seal the wound and repair the underlying cytoskeleton cortex. One group of proteins essential to the cortical remodeling is the Rho family of small GTPases. Recently we showed that the founding members of this GTPases family, Rho, Rac, and Cdc42, are all essential for normal single cell wound repair and accumulate at the wound periphery in distinct temporal/spatial patterns in the Drosophila cell wound model. In addition, these proteins communicate with one another and with the cytoskeleton to regulate their distribution in response to wounds. Unexpectedly, we found evidence for context specific Rho GTPase binding to downstream targets or “effectors” which cannot be explained solely by means of local GTPase activation. Here we discuss these observations in relation to similar studies in single cell wound repair in the Xenopus oocyte, and highlight how these cell wound models serve as powerful tools to understand both cell wound repair and Rho GTPase biology.

Biased signalling: the instinctive skill of the cell in the selection of appropriate signalling pathways

GPCRs (G-protein-coupled receptors) are members of a family of proteins which are generally regarded as the largest group of therapeutic drug targets. Ligands of GPCRs do not usually activate all cellular signalling pathways linked to a particular seven-transmembrane receptor in a uniform manner. The fundamental idea behind this concept is that each ligand has its own ability, while interacting with the receptor, to activate different signalling pathways (or a particular set of signalling pathways) and it is this concept which is known as biased signalling. The importance of biased signalling is that it may selectively activate biological responses to favour therapeutically beneficial signalling pathways and to avoid adverse effects. There are two levels of biased signalling. First, bias can arise from the ability of GPCRs to couple to a subset of the available G-protein subtypes: Gαs, Gαq/11, Gαi/o or Gα12/13. These subtypes produce the diverse effects of GPCRs by targeting different effectors. Secondly, biased GPCRs may differentially activate G-proteins or β-arrestins. β-Arrestins are ubiquitously expressed and function to terminate or inhibit classic G-protein signalling and initiate distinct β-arrestin-mediated signalling processes. The interplay of G-protein and β-arrestin signalling largely determines the cellular consequences of the administration of GPCR-targeted drugs. In the present review, we highlight the particular functionalities of biased signalling and discuss its biological effects subsequent to GPCR activation. We consider that biased signalling is potentially allowing a choice between signalling through ‘beneficial’ pathways and the avoidance of ‘harmful’ ones.

Accumulation of glucosylceramide in the absence of the beta-glucosidase GBA2 alters cytoskeletal dynamics

Glycosphingolipids are key elements of cellular membranes, thereby, controlling a variety of cellular functions. Accumulation of the simple glycosphingolipid glucosylceramide results in life-threatening lipid storage-diseases or in male infertility. How glucosylceramide regulates cellular processes is ill defined. Here, we reveal that glucosylceramide accumulation in GBA2 knockout-mice alters cytoskeletal dynamics due to a more ordered lipid organization in the plasma membrane. In dermal fibroblasts, accumulation of glucosylceramide augments actin polymerization and promotes microtubules persistence, resulting in a higher number of filopodia and lamellipodia and longer microtubules. Similar cytoskeletal defects were observed in male germ and Sertoli cells from GBA2 knockout-mice. In particular, the organization of F-actin structures in the ectoplasmic specialization and microtubules in the sperm manchette is affected. Thus, glucosylceramide regulates cytoskeletal dynamics, providing mechanistic insights into how glucosylceramide controls signaling pathways not only during sperm development, but also in other cell types.

During mammalian spermatogenesis, sperm with a head and a tail are formed from a round cell. This process is tightly regulated and involves the close interaction of somatic Sertoli cells and germ cells. Accumulation of the glycosphingolipid glucosylceramide in the absence of the beta-glucosidase GBA2 has been proposed to disturb sperm development, leading to morphological defects. However, the underlying mechanism is not known. Here, we demonstrate that accumulation of glucosylceramide in GBA2 knockout-mice controls the dynamics of the actin and microtubule cytoskeleton, which are crucial for sperm development. In particular, cytoskeletal structures at the interface between Sertoli and germ cells are disorganized, leading to malformation of the sperm head and a defect in acrosome formation. In summary, we provide mechanistic insights into how glucosylceramide controls cellular signaling and dysregulation of this essential glycosphingolipid leads to male infertility.

Side-specific mechanical properties of valve endothelial cells

Aortic valve endothelial cells (ECs) function in vastly different levels of shear stress. The biomechanical characteristics of cells on each side of valve have not been investigated. We assessed the morphology and mechanical properties of cultured or native valve ECs on intact porcine aortic valve cusps using a scanning ion conductance microscope (SICM). The autocrine influence of several endothelial-derived mediators on cell compliance and the expression of actin were also examined. Cells on the aortic side of the valve are characterized by a more elongated shape and were aligned along a single axis. Measurement of EC membrane compliance using the SICM showed that the cells on the aortic side of intact valves were significantly softer than those on the ventricular side. A similar pattern was seen in cultured cells. Addition of 10−6 M of the nitric oxide donor sodium nitroprusside caused a significant reduction in the compliance of ventricular ECs but had no effect on cells on the aortic side of the valve. Conversely, endothelin-1 (10−10-10−8 M) caused an increase in the compliance of aortic cells but had no effect on cells on the ventricular side of the valve. Aortic side EC compliance was also increased by 10−4 M of the nitric oxide synthase inhibitor NG-nitro-l-arginine methyl ester. Immunofluorescent staining of actin filaments revealed a great density of staining in ECs on the ventricular surface. The expression of actin and the relative membrane compliance of ECs on both side of the valve were not affected by ventricular and aortic patterns of flow. This study has shown side-specific differences in the biomechanics of aortic valve ECs. These differences can have important implications for valve function.

Endothelial cell-derived fibronectin extra domain A promotes colorectal cancer metastasis via inducing epithelial-mesenchymal transition

Recent evidence has been suggesting the important roles of endothelial cells (ECs) involved in the pathogenesis of several cancers, including colorectal carcinomas (CRCs), but the underlying mechanism remains elusive. We have demonstrated previously that CRC-derived fibronectin extra domain A (EDA) promotes vasculogenesis, tumorigenesis and metastasis of CRCs. At the current study, we showed that EC-secreted EDA promotes the metastatic capacity CRC cells via inducing an epithelial-mesenchymal transition. In vitro and in vivo experiments showed that EC-secreted EDA, via the interaction with integrin α9β1 on neighboring CRC cells, leads to the activation of focal adhesion kinase as well as Rac signalings, thus strengthens the polarity of cytoskeleton and promotes the invasion capacity of CRC cells. Furthermore, Erk signaling pathway was revealed to critically mediate the effect of EC-derived EDA on CRC cells. Our findings reveal a novel oncogenic role of ECs in promoting CRC malignancy through secreting EDA.

miR-330 regulates the proliferation of colorectal cancer cells by targeting Cdc42

MicroRNAs are small non-coding RNA molecules that play important roles in the multistep process of colorectal carcinoma (CRC) development. However, the miRNA-mRNA regulatory network is far from being fully understood. The objective of this study was to investigate the expression and the biological roles of miR-330 in colorectal cancer cells. Cdc42, one of the best characterized members of the Rho GTPase family, was found to be up-regulated in several types of human tumors including CRC and has been implicated in cancer initiation and progression. In the present study, we identified miR-330, as a potential regulator of Cdc42, was found to be inversely correlated with Cdc42 expression in colorectal cancer cell lines. Ectopic expression of miR-330 down-regulated Cdc42 expression at both protein and mRNA level, mimicked the effect of Cdc42 knockdown in inhibiting proliferation, inducing G1 cell cycle arrest and apoptosis of the colorectal cancer cells, whereas restoration of Cdc42 could partially attenuate the effects of miR-330. In addition, elevated expression of miR-330 could suppress the immediate downstream effectors of Cdc42 and inhibit the growth of colorectal cancer cells in vivo. To sum up, our results establish a role of miR-330 in negatively regulating Cdc42 expression and colorectal cancer cell proliferation. They suggest that manipulating the expression level of Cdc42 by miR-330 has the potential to influence colorectal cancer progression.

Chondrogenesis of human bone marrow mesenchymal cells by transforming growth factors β1 through cell shape changes on controlled biomaterials

The phenotypic responses of human bone marrow mesenchymal cells (hBMSCs) on different ratio of chitosan/polycaprolactone (PCL) blends were investigated in this study. The results showed that hBMSCs existed different morphology on chitosan/PCL blends due to the different adhesion characteristic of cell on neat PCL and neat chitosan. Interestingly, comparing to hBMSCs on neat PCL, hBMSCs aggregated to form spheroid and to express ascendant trend of transforming growth factor β1, collagen type II, collagen type X, and Sox9 mRNA on the chitosan/PCL blended substrates with the decrease of PCL content. To confirm chondrogenesis of hBMSCs with spheroid on test substrates, Alcian Blue and Safranin O staining were used to detect the cartilaginous extracellular matrix (ECM). It revealed hBMSCs with spheroid on neat chitosan and 10 wt % PCL did turn to chondrogenic differentiation and synthesize cartilaginous ECM. Therefore, these findings provided new insights into the role of chitosan/PCL blended material could mediate the endogenous gene expression of hBMSCs to alter the phenotypic behavior through mediating the cell shape.

Microtubule depolymerization induces traction force increase through two distinct pathways

Traction forces increase after microtubule depolymerization; however, the signaling mechanisms underlying this, in particular the dependence upon myosin II, remain unclear. We investigated the mechanism of traction force increase after nocodazole-induced microtubule depolymerization by applying traction force microscopy to cells cultured on micropatterned polyacrylamide hydrogels to obtain samples of homogeneous shape and size. Control cells and cells treated with a focal adhesion kinase (FAK) inhibitor showed similar increases in traction forces, indicating that the response is independent of FAK. Surprisingly, pharmacological inhibition of myosin II did not prevent the increase of residual traction forces upon nocodazole treatment. This increase was abolished upon pharmacological inhibition of FAK. These results suggest two distinct pathways for the regulation of traction forces. First, microtubule depolymerization activates a myosin-II-dependent mechanism through a FAK-independent pathway. Second, microtubule depolymerization also enhances traction forces through a myosin-II-independent, FAK-regulated pathway. Traction forces are therefore regulated by a complex network of complementary signals and force-generating mechanisms.

Role of epimorphin in bile duct formation of rat liver epithelial stem-like cells: involvement of small G protein RhoA and C/EBPβ

Epimorphin/syntaxin 2 is a high conserved and very abundant protein involved in epithelial morphogenesis in various organs. We have shown recently that epimorphin (EPM), a protein exclusively expressed on the surface of hepatic stellate cells and myofibroblasts of the liver, induces bile duct formation of hepatic stem-like cells (WB-F344 cells) in a putative biophysical way. Therefore, the aim of this study was to present some of the molecular mechanisms by which EPM mediates bile duct formation. We established a biliary differentiation model by co-culture of EPM-overexpressed mesenchymal cells (PT67(EPM)) with WB-F344 cells. Here, we showed that EPM could promote WB-F344 cells differentiation into bile duct-like structures. Biliary differentiation markers were also elevated by EPM including Yp, Cx43, aquaporin-1, CK19, and gamma glutamyl transpeptidase (GGT). Moreover, the signaling pathway of EPM was analyzed by focal adhesion kinase (FAK), extracellular regulated kinase 1/2 (ERK1/2), and RhoA Western blot. Also, a dominant negative (DN) RhoA-WB-F344 cell line (WB(RhoA-DN)) was constructed. We found that the levels of phosphorylation (p) of FAK and ERK1/2 were up-regulated by EPM. Most importantly, we also showed that RhoA is necessary for EPM-induced activation of FAK and ERK1/2 and bile duct formation. In addition, a dual luciferase-reporter assay and CHIP assay was performed to reveal that EPM regulates GGT IV and GGT V expression differentially, possibly mediated by C/EBPβ. Taken together, these data demonstrated that EPM regulates bile duct formation of WB-F344 cells through effects on RhoA and C/EBPβ, implicating a dual aspect of this morphoregulator in bile duct epithelial morphogenesis.

Integrin α6β4 cooperates with LPA signaling to stimulate Rac through AKAP-Lbc-mediated RhoA activation

The α(6)β(4) integrin promotes carcinoma invasion through its ability to promote directed migration and polarization of carcinoma cells. In this study, we explore how the α(6)β(4) integrin cooperates with lysophosphatidic acid (LPA) to activate Rho and Rac small GTPases. Through the use of dominant negative Rho constructs, C3 exotransferase, and Rho kinase inhibitor, we find that Rho is critical for LPA-dependent chemotaxis and lamellae formation. However, utilization of specific Rho isoforms depends on integrin α(6)β(4) expression status. Integrin α(6)β(4)-negative MDA-MB-435 cells utilize only RhoC for motility, whereas integrin α(6)β(4)-expressing cells utilize RhoC but additionally activate and utilize RhoA for LPA-dependent cell motility and lamellae formation. Notably, the activation of RhoA by cooperative LPA and integrin α(6)β(4) signaling requires the Rho guanine nucleotide exchange factor AKAP-Lbc. We also determine that integrin α(6)β(4) cannot activate Rac1 directly but promotes LPA-mediated Rac1 activation that is dependent on RhoA activity and de novo β(1) integrin ligation. Finally, we find that the regulation of Rac1 and RhoA in response to LPA is differentially regulated by phosphodiesterases, PKA, and phosphatidylinositol 3-kinase, thus supporting their spatially distinct compartmentalization. In summary, signaling from integrin α(6)β(4) facilitates LPA-stimulated chemotaxis through preferential activation of RhoA, which, in turn, facilitates activation of Rac1.

Actin as target for modification by bacterial protein toxins

Various bacterial protein toxins and effectors target the actin cytoskeleton. At least three groups of toxins/effectors can be identified, which directly modify actin molecules. One group of toxins/effectors causes ADP-ribosylation of actin at arginine-177, thereby inhibiting actin polymerization. Members of this group are numerous binary actin-ADP-ribosylating exotoxins (e.g. Clostridium botulinum C2 toxin) as well as several bacterial ADP-ribosyltransferases (e.g. Salmonella enterica SpvB) which are not binary in structure. The second group includes toxins that modify actin to promote actin polymerization and the formation of actin aggregates. To this group belongs a toxin from the Photorhabdus luminescens Tc toxin complex that ADP-ribosylates actin at threonine-148. A third group of bacterial toxins/effectors (e.g. Vibrio cholerae multifunctional, autoprocessing RTX toxin) catalyses a chemical crosslinking reaction of actin thereby forming oligomers, while blocking the polymerization of actin to functional filaments. Novel findings about members of these toxin groups are discussed in detail.

Proteomic characterization of mesenchymal stem cell-like populations derived from ovine periodontal ligament, dental pulp, and bone marrow: analysis of differentially expressed proteins

Postnatal mesenchymal stem/stromal-like cells (MSCs) including periodontal ligament stem cells (PDLSCs), dental pulp stem cells (DPSCs), and bone marrow stromal cells (BMSCs) are capable of self-renewal and differentiation into multiple mesenchymal cell lineages. Despite their similar expression of MSC-associated and osteoblastic markers, MSCs retain the capacity to generate structures resembling the microenvironments from which they are derived in vivo and represent a promising therapy for the regeneration of complex tissues in the clinical setting. With this in mind, systematic approaches are required to identify the differential protein expression patterns responsible for lineage commitment and mediating the formation of these complex structures. This is the first study to compare the differential proteomic expression profiles of ex vivo-expanded ovine PDLSCs, DPSCs, and BMSCs derived from an individual donor. The two-dimensional electrophoresis was performed and regulated proteins were identified by liquid chromatography--electrospray-ionization tandem mass spectrometry (MS and MS/MS), database searching, and de novo sequencing. In total, 58 proteins were differentially expressed between at least 2 MSC populations in both sheep, 12 of which were up-regulated in one MSC population relative to the other two. In addition, the regulation of selected proteins was also conserved between equivalent human MSC populations. We anticipate that differential protein expression profiling will provide a basis for elucidating the protein expression patterns and molecular cues that are crucial in specifying the characteristic growth and developmental capacity of dental and non-dental tissue-derived MSC populations. These expression patterns can serve as important tools for the regeneration of particular tissues in future stem cell-based tissue engineering studies using animal models.

TMPRSS4 induces invasion and epithelial-mesenchymal transition through upregulation of integrin alpha5 and its signaling pathways

TMPRSS4 is a novel type II transmembrane serine protease that is highly expressed on the cell surface in pancreatic, thyroid and other cancer tissues, although its oncogenic significance and molecular mechanisms are unknown. Previously, we have shown that TMPRSS4 promotes invasion, migration and metastasis of human tumor cells by facilitating an epithelial-mesenchymal transition (EMT). In this study, we explored the molecular basis underlying TMPRSS4-mediated effects. We show that multiple downstream signaling pathways, including focal adhesion kinase (FAK), extracellular signal-regulated kinase (ERK), Akt, Src and Rac1, are activated by TMPRSS4 expression and that FAK signaling and ERK activation are required for TMPRSS4-induced invasiveness and EMT, including cadherin switch. Inhibition of PI3K or Src reduced invasiveness and actin rearrangement mediated by TMPRSS4 without restoring E-cadherin expression. Downregulation of E-cadherin was required for TMPRSS4-mediated effects but was not sufficient to induce EMT and invasion. TMPRSS4 induced integrin alpha5 expression and its signal transduction, leading to invasiveness and EMT accompanied by downregulation of E-cadherin. Functional blocking confirmed that integrin alpha5beta1 is a critical signaling molecule that is sufficient to induce TMPRSS4-mediated effects. Immunohistochemical analysis showed that TMPRSS4 expression was significantly higher in human colorectal cancer tissues from advanced stages than in that of early stage. Furthermore, upregulation of TMPRSS4 was correlated with enhanced integrin alpha5 expression. These observations implicate integrin alpha5 upregulation as a molecular mechanism by which TMPRSS4 induces invasion and contributes to cancer progression.

Cell responses regulated by early reorganization of actin cytoskeleton

Microfilaments exist in a dynamic equilibrium between monomeric and polymerized actin and the ratio of monomers to polymeric forms is influenced by a variety of extracellular stimuli. The polymerization, depolymerization and redistribution of actin filaments are modulated by several actin-binding proteins, which are regulated by upstream signalling molecules. Actin cytoskeleton is involved in diverse cellular functions including migration, ion channels activity, secretion, apoptosis and cell survival. In this review we have outlined the role of actin dynamics in representative cell functions induced by the early response to extracellular stimuli.

Actin polymerization modulates CD44 surface expression, MMP-9 activation, and osteoclast function

CD44 and MMP-9 are implicated in cell migration. In the current study, we tested the hypothesis that actin polymerization is critical for CD44 surface expression and MMP-9 activity on the cell surface. To understand the underlying molecular mechanisms involved in CD44 surface expression and MMP-9 activity on the cell surface, osteoclasts were treated with bisphosphonate (BP) alendronate, cytochalasin D (Cyt D), and a broad-spectrum MMP inhibitor (GM6001). BP has been reported to block the mevalonate pathway, thereby preventing prenylation of small GTPase signaling required for actin cytoskeleton modulation. We show in this study that osteoclasts secrete CD44 and MMP-9 into the resorption bay during migration and bone resorption. Results indicate that actin polymerization is critical for CD44 surface expression and osteoclast function. In particular, the surface expression of CD44 and the membrane activity of MMP-9 are reduced in osteoclasts treated with alendronate and Cyt D despite the membrane levels of MMP-9 being unaffected. Although GM6001 blocked MMP-9 activity, osteoclast migration, and bone resorption, the surface levels of CD44 were unaffected. We suggest that the surface expression of CD44 requires actin polymerization. Disruption of podosome and actin ring structures by Cyt D and alendronate not only resulted in reduced localization of MMP-9 in these structures but also in osteoclast migration and bone resorption. These results suggest that inhibition of actin polymerization by alendronate and Cyt D is effective in blocking CD44/MMP-9 complex formation on the cell surface, secretion of active form of MMP-9, and osteoclast migration. CD44/MMP-9 complex formation may signify a unique motility-enhancing signal in osteoclast function.

The Macaque Sperm Actin Cytoskeleton Reorganizes in Response to Osmotic Stress and Contributes to Morphological Defects and Decreased Motility1

Sperm undergo extreme variations in temperature and osmolality during cryopreservation, resulting in cell damage that includes plasma membrane defects, changes in cell volume, decreased motility, and flagellar defects. However, the fundamental biologic mechanisms underlying these events are poorly understood. We investigated the effects of osmotic stress and cytochalasins b (CB) and d (CD), naturally occurring toxins that disrupt actin organization, on the actin cytoskeleton and motility of Rhesus macaque sperm (Macaca mulatta). Sperm were diluted in media of low, medium, or high osmolality, or medium-osmolality media containing CB or CD, were stained with phalloidin-fluorescein isothiocyanate, and were processed for microscopy. The majority of sperm incubated in medium-osmolality media exhibited postacrosomal stain, whereas the minority displayed banding patterns of F-actin stain in the head. High-osmolality media, as well as CB and CD incubation, resulted in reorganization of F-actin into bands of stain in the majority of sperm heads. Cytochalasin b treatment also resulted in curled and looped tails, a phenomenon of hyposmotic stress, and CB and CD caused significant, dose-dependent decreases in motility determined by computer-assisted sperm assessment. Rho A cell populations were determined using flow cytometry, and immunocytochemistry analysis demonstrated that Rho A localization was altered after osmotic stress. Together, our results support a mechanism in which reorganization of the actin cytoskeleton induced by osmotic stress and potentially mediated by a Rho A signaling pathway contributes to sublethal sperm flagellar and motility defects.

Geldanamycin Anisimycins Activate Rho and Stimulate Rho- and ROCK-Dependent Actin Stress Fiber Formation

Heat shock protein 90 (Hsp90) is a member of the heat shock family of molecular chaperones that regulate protein conformation and activity. Hsp90 regulates multiple cell signaling pathways by controlling the abundance and activity of several important protein kinases and cell cycle-related proteins. In this report, we show that inhibition of Hsp90 by geldanamycin or its derivative, 17-allylamino-17-desmethoxygeldamycin, leads to activation of the Rho GTPase and a dramatic increase in actin stress fiber formation in human tumor cell lines. Inactivation of Rho prevents geldanamycin-induced actin reorganization. Hsp90 inactivation does not alter the appearance of filopodia or lamellipodia and tubulin architecture is not visibly perturbed. Our observations suggest that Hsp90 has an important and specific role in regulating Rho activity and Rho-dependent actin cytoskeleton remodeling.

Essential role of protein kinase C zeta in transducing a motility signal induced by superoxide and a chemotactic peptide, fMLP

Under various pathological conditions, including infection, malignancy, and autoimmune diseases, tissues are incessantly exposed to reactive oxygen species produced by infiltrating inflammatory cells. We show augmentation of motility associated with morphological changes of human squamous carcinoma SASH1 cells, human peripheral monocytes (hPMs), and murine macrophage-like cell line J774.1 by superoxide stimulation. We also disclose that motility of hPMs and J774.1 induced by a chemotactic peptide (N-formyl-methionyl-leucyl-phenylalanine [fMLP]) was inhibited by superoxide dismutase or N-acetylcystein, indicating stimulation of motility by superoxide generated by fMLP stimulation. In these cells, protein kinase C (PKC) ζ was activated to phosphorylate RhoGDI-1, which liberated RhoGTPases, leading to their activation. These events were inhibited by dominant-negative PKCζ in SASH1 cells, myristoylated PKCζ peptides in hPMs and J774.1, or a specific inhibitor of RhoGTPase in SASH1, hPMs, and J774.1. These results suggest a new approach for manipulation of inflammation as well as tumor cell invasion by targeting this novel signaling pathway.

The ADMA/DDAH pathway is a critical regulator of endothelial cell motility

Asymmetric dimethylarginine (ADMA) is an inhibitor of nitric oxide production associated with abnormal blood vessel growth and repair, however, the mechanism of action of ADMA is not well understood. We studied the role of exogenous and endogenous ADMA in the regulation of cell motility and actin cytoskeleton in porcine pulmonary endothelial cells (PAECs) and pulmonary microvascular endothelial cells (PMECs) from knockout mice that lack one of the enzyme metabolising ADMA, dimethylarginine dimethylaminohydrolase I (DDAHI) as well as endothelial cells overexpressing DDAH in vitro. We show that ADMA induced stress fibre and focal adhesion formation and inhibited cell motility in primary pulmonary endothelial cells. The effects of ADMA depended on the activity of RhoA and Rho kinase and were reversed by overexpression of DDAH, nitric oxide donors and protein kinase G activator, 8-bromo-cGMP. ADMA also inhibited the activities of Rac1 and Cdc42 in cells but these changes had a minor effect on cell motility. Endogenous ADMA increased RhoA activity and inhibited cell motility in PMECs from DDAHI knockout mice and inhibited angiogenesis in vitro. These results are the first demonstration that metabolism of cardiovascular risk factor ADMA regulates endothelial cell motility, an important factor in angiogenesis and vascular repair.

ROCK inhibition produces anxiety-related behaviors in mice

RATIONALE

The role of Rho/Rho-associated kinase (ROCK) in regulating dendritic and axonal morphology during development has gained much attention. Very little is known, however, about the role of the Rho/ROCK pathway in emotional behavior.

OBJECTIVE

To investigate the role of ROCK in emotional behaviors. We examined how the ROCK inhibitor Y27632 affects the performance of mice on three behavioral tests that measure anxiety-related behaviors.

RESULTS

In the elevated plus-maze test, Y27632 (10 nmol, intracerebroventricular) induced a significant decrease in the percentage of time spent in the open arms and in the percentage of entries into open arms. In the fear conditioning test, Y27632-treated mice froze significantly more often and longer than did saline-treated mice. In the hole-board test, Y27632 significantly suppressed head-dipping behavior in Y27632-treated mice than in saline-treated mice. On the other hand, Y27632 did not produce on spontaneous alteration performance in the Y-maze test. These results indicate that ROCK inhibition increased anxiety-related behaviors.

CONCLUSION

Our findings suggest that the ROCK pathway is involved in the expression of anxiety- and fear-related behaviors. Furthermore, we propose that if the Rho/ROCK pathway plays an important role in mediating anxiety-related behaviors in humans, it may prove to be a novel system for anxiolytics to target.

Expression and prognostic role of RhoA GTPases in hepatocellular carcinoma

The Rho sub-family of proteins is involved in regulating the organization of the cytoskeleton and in cell motility. Our aim is to clarify the clinical significance of Rho protein in hepatocellular carcinomas (HCC) and to determine the relationship between the level of expression and patient outcome following hepatectomy. The expression of RhoA protein in HCC and corresponding non-tumor tissues of 26 patients who underwent surgical resection was analyzed by immunoblotting. The expression level of each case was calculated as tumor/non-tumor (T/N) ratios. High expression (T/N> or =1) of RhoA protein in HCC compared to the paired non-tumor tissues was recognized in 18 patients (69.2%) of 26 samples. The activity of RhoA is also increased in HCC with high expression of RhoA. The high expression of RhoA protein did not correlate with various clinical parameters. However, the disease-free survival rates of the RhoA-high expression group (T/N> or =1) were significantly lower than those of the RhoA-low expression group (T/N<1) (P<0.05). The high expression of RhoA protein in HCC plays an important role in intrahepatic recurrence of patients who underwent a hepatectomy for HCC, and RhoA is a useful marker for predicting early recurrence in an early-stage HCC.

E1AF/PEA3 activates the Rho/Rho-associated kinase pathway to increase the malignancy potential of non-small-cell lung cancer cells

E1AF/PEA3, an Ets family transcription factor, is frequently overexpressed in non-small-cell lung cancers (NSCLCs). Overexpression of E1AF increases motility and invasion of VMRC-LCD and NCI-H226 NSCLC cells, which lack endogenous E1AF expression, and the effect is synergistically increased by hepatocyte growth factor (HGF). The small GTPase Rho/Rho-associated kinase (ROCK) pathway is also involved in motility and invasion. To determine the role of the Rho/ROCK pathway in malignant phenotypes induced by E1AF, we analyzed VMRC-LCD cells transfected with an E1AF expression vector (LCD-E1AF cells) or with empty vector (LCD-vector cells). LCD-E1AF cells had more GTP-bound (active) Rho than LCD-vector cells and Rho activation was synergistically increased by HGF. The Rho activation by E1AF and HGF was also shown in NCI-H226 cells. Phosphorylation of myosin light chain (MLC), a downstream effector of ROCK signaling, was higher in LCD-E1AF cells than in LCD-vector cells, especially under HGF treatment. A specific ROCK inhibitor, Y27632, strongly suppressed MLC phosphorylation, cell motility, and invasion. In nude mice implanted s.c. and intrapulmonarily, LCD-E1AF cells made more local tumors than LCD-vector cells (six of six versus one of seven mice and four of seven versus one of seven mice, respectively). Three of the four mice with lung tumors from LCD-E1AF cells had lymph node metastases whereas the mouse with LCD-vector tumors did not. LCD-E1AF tumors showed higher MLC phosphorylation than LCD-vector tumors. These results suggest that E1AF activates the Rho/ROCK pathway in an HGF-enhanced manner and its activation is important in E1AF-induced motility and invasion as well as tumorigenesis and metastasis in NSCLC cells.

Pamidronate induced anti-proliferative, apoptotic, and anti-migratory effects in hepatocellular carcinoma

BACKGROUND/AIMS

The small GTPase of Ras and Rho families are widely involved in human tumorgenesis and metastasis. It has recently been reported that pamidronate inhibits the mevalonate pathway, which is required for the prenylation of the small GTPase. We demonstrated a possible beneficial use of pamidronate in the treatment of hepatocellular carcinoma (HCC).

METHODS

The effect of pamidronate on cell proliferation was analyzed with five hepatoma cell lines using MTT assay. Apoptosis was evaluated by staining with DAPI and a histon ELISA assay. A cell migration assay was performed using the Modified Boyden Chamber. To analyze anti-proliferation effect of pamidronate in vivo, tumor volumes were monitored with the intraperitoneal injection of pamidronate after subcutaneous inoculation of PLC/PRF/5 cells into nude mice.

RESULTS

Pamidronate inhibited cell growth for all hepatoma cell lines. The amount of membrane associated Ras and phosphorylated extracellular signal-regulated kinase 2 (ERK 2) were reduced after pamidronate treatment. Pamidronate increased apoptosis and cleavage of Caspase-3, and -9. Pamidronate suppressed membrane associated RhoA and cell motility. In vivo, tumor volumes were significantly suppressed by pamidronate at three weeks (P<0.03).

CONCLUSIONS

We conclude that pamidronate has therapeutic potential in inducing anti-proliferative, apoptotic, and anti-migratory effects in HCC.

Deleted in liver cancer 2 (DLC2) suppresses cell transformation by means of inhibition of RhoA activity

The deleted in liver cancer 2 (DLC2) gene, located at chromosome 13q12.3, is a recently identified tumor suppressor gene. The gene is frequently underexpressed in human hepatocellular carcinoma, and its chromosomal region shows frequent deletion. DLC2 encodes a unique RhoGTPase-activating protein (RhoGAP) specific for small RhoGTPases, RhoA, and Cdc42. With bioinformatic analysis, we have identified four different isoforms of DLC2, which we named DLC2alpha, DLC2beta, DLC2gamma, and DLC2delta. Three of the isoforms contain the RhoGAP domain, namely, DLC2alpha, DLC2beta, and DLC2gamma. Ectopic expression of these three isoforms in mouse fibroblasts showed cytoplasmic localization. Of interest, overexpression of these isoforms suppressed the lysophosphatidic acid-induced stress fiber formation in mouse fibroblasts and changed the morphology of the transfected cells from angular and spindle to round. Furthermore, the RhoA pull-down assay demonstrated a remarkable reduction in RhoA activity in the DLC2 transiently transfected cells. In contrast, cells transfected with inactive DLC2 GAP-mutant remained unchanged in cell morphology, actin stress fiber formation, and RhoA activity. HepG2 hepatoma cells stably transfected with the DLC2gamma isoform also changed to a round morphology, as in mouse fibroblasts. Of significance, these DLC2gamma stable transfectants showed marked suppression in cell proliferation, motility, and transformation, and there was a remarkable reduction in in vivo RhoA activity in these cells. These results suggest that DLC2 exhibits its tumor suppressor functions in vivo as a GAP specific for RhoA, exerting its effects in suppression of cytoskeleton reorganization, cell growth, cell migration, and transformation.

Platelet activating factor induces cytoskeletal reorganization through Rho family pathway in THP-1 macrophages

In the process of atherosclerosis, platelet activating factor (PAF) promotes the infiltration of inflammatory cells into atherosclerotic plaque by modulating their cytoskeleton. Here, we examined whether Rho family proteins are involved in PAF-induced cytoskeletal reorganization in THP-1 macrophages. PAF stimulation rapidly induced cell elongation, accompanied by filopodia formation. The inhibition of Rho family proteins by the overexpression of Rho-GDI attenuated the PAF-mediated morphological changes. Both RhoA and Cdc42 were activated in response to PAF. Inhibition of RhoA or Cdc42 by dominant negative mutants abrogated morphological changes induced by PAF. Collectively, PAF regulates cytoarchitecture through Rho family proteins in macrophages.

p116Rip Decreases Myosin II Phosphorylation by Activating Myosin Light Chain Phosphatase and by Inactivating RhoA

p116Rip was originally found to be a RhoA-binding protein, but its function has been unknown. Here, we clarify the function of p116Rip. Two critical findings were made. First, we found that p116Rip activated the GTPase activity of RhoA in vitro and that p116Rip overexpression in cells consistently diminished the epidermal growth factor-induced increase in GTP-bound RhoA. Second, p116Rip activated the myosin light chain phosphatase (MLCP) activity of the holoenzyme. p116Rip did not activate the catalytic subunit alone, indicating that the activation is due to the binding of p116Rip to the myosin phosphatase targeting subunit MYPT1. Interestingly, the activation of phosphatase was specific to myosin as substrate, and p116Rip directly bound to myosin, thus facilitating myosin/MLCP interaction. The gene silencing of p116Rip consistently and significantly increased myosin phosphorylation as well as stress fiber formation in cells. Based upon these findings, we propose that p116Rip is an important regulatory component that controls the RhoA signaling pathway, thus regulating MLCP activity and myosin phosphorylation in cells.

Zoledronic acid up-regulates bone sialoprotein expression in osteoblastic cells through Rho GTPase inhibition

Clinical practice reveals that osteoporotic women treated with BPs (bisphosphonates) show an increased bone mass density and a reduced risk of fractures. However, the mechanisms leading to these beneficial effects of BPs are still poorly understood. We hypothesized that ZOL (zoledronic acid), a potent third-generation BP, may induce the expression of proteins associated with the bone-forming potential of osteoblastic cells such as BSP (bone sialo-protein). Expression of BSP gene is up-regulated by hormones that promote bone formation and has been associated with de novo bone mineralization. Using real-time reverse transcriptase-PCR and Western-blot analysis, we demonstrated that ZOL increased BSP expression in Saos-2 osteoblast-like cells. Nuclear run-on and mRNA decay assays showed no effect at the transcriptional level but a stabilization of BSP transcripts in ZOL-treated cells. ZOL effect on BSP expression occurred through an interference with the mevalonate pathway since it was reversed by either mevalonate pathway intermediates or a Rho GTPase activator. We showed that ZOL impaired membrane localization of RhoA in Saos-2 cells indicating reduced prenylation of this protein. By the use of small interfering RNAs directed to RhoA and Rac1, we identified both Rho GTPases as negative regulators of BSP expression in Saos-2 cells. Our study demonstrates that ZOL induces BSP expression in osteoblast-like cells through inactivation of Rho GTPases and provides a potential mechanism to explain the favourable effects of ZOL treatment on bone mass and integrity.

S1P and LPA trigger Schwann cell actin changes and migration

The processes by which a Schwann cell (SC) migrates towards, wraps around and, in some cases, myelinates an axon are incompletely understood. The complex morphological rearrangements involved in these events require fundamental changes in the actin cytoskeleton. Sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) are two modulators of the actin cytoskeleton, and receptors for these signalling lipids are expressed on SCs at the time of differentiation. Previous work has revealed a role for LPA in SC survival, morphology and differentiation, but the effects of S1P have received less attention. Here we show that S1P and LPA both cause major rearrangements to the actin cytoskeleton in primary rat SCs and the SCL4.1/F7 rat SC line. S1P and LPA caused formation of lamellipodia and a circular geodesic actin network. We also show that S1P and LPA increased cell migration. The small GTPases RhoA and Rac1 were both activated by S1P/LPA treatment, but the actin rearrangements were dependent on Rac1 and not RhoA. These effects of S1P/LPA could be mimicked by SCL4.1/F7 cell-conditioned medium, which was found to contain S1P. Reduction in cellular synthesis of S1P by adding the sphingosine kinase inhibitor dimethyl sphingosine during medium conditioning reduced the ability of conditioned medium to cause actin rearrangements. These results support a role for S1P as an autocrine signal regulating the actin cytoskeleton during Schwann cell development.

Deciphering the mechanisms of homeostatic plasticity in the hypothalamo-neurohypophyseal system—genomic and gene transfer strategies

The hypothalamo-neurohypophyseal system (HNS) is the specialised brain neurosecretory apparatus responsible for the production of a peptide hormone, vasopressin, that maintains water balance by promoting water conservation at the level of the kidney. Dehydration evokes a massive increase in the regulated release of hormone from the HNS, and this is accompanied by a plethora of changes in morphology, electrical properties and biosynthetic and secretory activity, all of which are thought to facilitate hormone production and delivery, and hence the survival of the organism. We have adopted a functional genomic strategy to understand the activity dependent plasticity of the HNS in terms of the co-ordinated action of cellular and genetic networks. Firstly, using microarray gene-profiling technologies, we are elucidating which genes are expressed in the HNS, and how the pattern of expression changes following physiological challenge. The next step is to use transgenic rats to probe the functions of these genes in the context of the physiological integrity of the whole organism.

Implication of Rho-associated kinase in the elevation of extracellular dopamine levels and its related behaviors induced by methamphetamine in rats

A growing body of evidence suggests that several protein kinases are involved in the expression of pharmacological actions induced by a psychostimulant methamphetamine. The present study was designed to investigate the role of the Rho/Rho-associated kinase (ROCK)-dependent pathway in the expression of the increase in extracellular levels of dopamine in the nucleus accumbens and its related behaviors induced by methamphetamine in rats. Methamphetamine (1 mg/kg, subcutaneously) produced a substantial increase in extracellular levels of dopamine in the nucleus accumbens, with a progressive augmentation of dopamine-related behaviors including rearing and sniffing. Methamphetamine also induced the decrease in levels of its major metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanilic acid (HVA). Both the increase in extracellular levels of dopamine and the induction of dopamine-related behaviors by methamphetamine were significantly suppressed by pretreatment with an intranucleus accumbens injection of a selective ROCK inhibitor Y-27632. In contrast, Y-27632 had no effect on the decrease in levels of DOPAC and HVA induced by methamphetamine. Under these conditions, there were no changes in protein levels of membrane-bound RhoA in the nucleus accumbens following methamphetamine treatment. It is of interest to note that the microinjection of Y-27632 into the nucleus accumbens failed to suppress the increases in extracellular levels of dopamine, DOPAC, and HVA in the nucleus accumbens induced by subcutaneous injection of a prototype of micro -opioid receptor agonist morphine (10 mg/kg). Furthermore, perfusion of a selective blocker of voltage-dependent Na+ channels, tetrodotoxin (TTx) into the rat nucleus accumbens did not affect the increase in extracellular levels of dopamine in the rat nucleus accumbens by methamphetamine, whereas the morphine-induced dopamine elevation was eliminated by this application of TTx. The extracellular level of dopamine in the nucleus accumbens was also increased by perfusion of a selective dopamine re-uptake inhibitor 1-[2-[bis(4-fluorophenyl)methoxy]-4-(3-phenylpropyl)piperazine (GBR-12909) in the nucleus accumbens. This effect was not affected by pretreatment with intranucleus accumbens injection of Y-27632. These findings provide first evidence that Rho/ROCK pathway in the nucleus accumbens may contribute to the increase in extracellular levels of dopamine in the nucleus accumbens evoked by a single subcutaneous injection of methamphetamine. In contrast, this pathway is not essential for the increased level of dopamine in this region induced by morphine, providing further evidence for the different mechanisms of dopamine release by methamphetamine and morphine in rats.

Cdc42 and RhoA are differentially regulated during arachidonate-mediated HeLa cell adhesion

Cell adhesion to extracellular matrix requires stimulation of an eicosanoid signaling pathway through the metabolism of arachidonate by 5-lipoxygenase to leukotrienes and cyclooxygenase-1/2 to prostaglandins, as well as activation of the small GTPase signaling pathway involving Cdc42 and Rho. These signaling pathways direct remodeling of the actin cytoskeleton during the adhesion process, specifically the polymerization of actin during cell spreading and the bundling of actin filaments when cells migrate. However, few studies linking these signaling pathways have been described in the literature. We have previously shown that HeLa cell adhesion to collagen requires oxidation of arachidonic acid (AA) by lipoxygenase for actin polymerization and cell spreading, and cyclooxygenase for bundling actin filaments during cell migration. We demonstrate that small GTPase activity is required for HeLa cell spreading upon gelatin, and that Cdc42 is activated while Rho is downregulated during the spreading process. Using constitutively active and dominant negative expression studies, we show that Cdc42 is required for HeLa cell spreading and migration, while activated RhoA is antagonistic towards spreading. Constitutively active RhoA promotes cell migration and increases the degree of actin bundling in HeLa cells. Further, we demonstrate that activation of either the AA oxidation pathway or the small GTPase pathway cannot rescue inhibition of spreading when the alternate pathway is blocked. Our results suggest (1) both the eicosanoid signaling pathway and small GTPase activation are required during HeLa cell adhesion, and (2) these signaling pathways converge to properly direct remodeling of the actin cytoskeleton during HeLa cell spreading and migration upon collagen.

Functional significance of the specific sites phosphorylated in desmin at cleavage furrow: Aurora-B may phosphorylate and regulate type III intermediate filaments during cytokinesis coordinatedly with Rho-kinase

Aurora-B is a protein kinase required for chromosome segregation and the progression of cytokinesis during the cell cycle. We report here that Aurora-B phosphorylates GFAP and desmin in vitro, and this phosphorylation leads to a reduction in filament forming ability. The sites phosphorylated by Aurora-B; Thr-7/Ser-13/Ser-38 of GFAP, and Thr-16 of desmin are common with those related to Rho-associated kinase (Rho-kinase), which has been reported to phosphorylate GFAP and desmin at cleavage furrow during cytokinesis. We identified Ser-59 of desmin to be a specific site phosphorylated by Aurora-B in vitro. Use of an antibody that specifically recognized desmin phosphorylated at Ser-59 led to the finding that the site is also phosphorylated specifically at the cleavage furrow during cytokinesis in Saos-2 cells. Desmin mutants, in which in vitro phosphorylation sites by Aurora-B and/or Rho-kinase are changed to Ala or Gly, cause dramatic defects in filament separation between daughter cells in cytokinesis. The results presented here suggest the possibility that Aurora-B may regulate cleavage furrow-specific phosphorylation and segregation of type III IFs coordinatedly with Rho-kinase during cytokinesis.

Rho GTPases and spermatogenesis

Rho GTPases, such as Rho, Rac and Cdc42, are known to regulate many cellular processes including cell movement and cell adhesion. While the cellular events of germ cell movement are crucial to spermatogenesis since developing germ cells must migrate progressively from the basal to the adluminal compartment but remain attached to the seminiferous epithelium, the physiological significance of Rho GTPases in spermatogenesis remains largely unexplored. This paper reviews some recent findings on Rho GTPases in the field with emphasis on the studies in the testis, upon which future studies can be designed to delineate the role of Rho GTPases in spermatogenesis.

Substrate-dependent cellular behavior of Swiss 3T3 fibroblasts and activation of Rho family during adhesion and spreading processes

Recent biochemical studies revealed that intracellular Rho guanosine triphosphatases (Rho, Rac1, and Cdc42) are key regulatory molecules that link surface receptors to cytoskeletal organization and regulation of cell shape/morphology/motility. In this study, Swiss 3T3 fibroblasts were cultured on three representative substrates [tissue culture polystyrene dishes, nontreated polystyrene, and poly(ethylene terephthalate)] for 24 h after plating. Time-dependent changes in cell shape, morphology, cytoskeletal dynamics, and motility as well as Rho family activities were determined on each substrate. The cells on tissue culture polystyrene and on poly(ethylene terephthalate), which induced rapid and relatively rapid cell spreading, respectively, expressed Rac1 and Cdc42 activities continuously during the observation period. In contrast, such activities were suppressed in cells on polystyrene, which induced slow spreading but the highest cell motility compared with the other two substrates. Although a clear-cut relationship between cellular behavior and Rho family activation was not obtained, substrate-dependent coordinated control of cellular activities by Rho family is discussed.

Vasopressin and oxytocin reverse adenosine-induced pituicyte stellation via calcium-dependent activation of Cdc42

In view of the potential impact of pituicyte morphology on neurohypophysial hormone secretion, we have studied the mechanisms involved in the shape changes induced by vasopressin (AVP) and oxytocin (OXT) in cultured rat pituicytes. Pituicytes induced to become stellate in the presence of 10 micro m adenosine revert to their nonstellate shape approximately 20 min after application of AVP or OXT. The IC50 for this effect is 0.1 nm for AVP and 36 nm for OXT. Both agonists induce Ca2+ signals in pituicytes, comprised of a transient peak and a plateau phase that is dependent on the presence of extracellular Ca2+. The EC50 values of AVP for the transient and sustained responses are 4.5 and 0.1 nm, respectively; corresponding values for OXT are 180 and 107 nm. We determined pharmacologically that these hormone-induced Ca2+ signals are mediated by the V1a subtype of vasopressin receptors, similar to what we previously observed for hormone-induced reversal of stellation. Removal of extracellular Ca2+ or chelation of intracellular Ca2+ partially prevented AVP from reversing stellation, suggesting a role for Ca2+ in this event. We previously established that adenosine-induced stellation of pituicytes occurs via RhoA inhibition. However, pharmacological experiments and pull-down assays presented here show that AVP-induced reversal of stellation does not involve RhoA activation. Rather, AVP was found to induce a time-dependent activation of Cdc42, another small GTPase involved in cytoskeletal plasticity. Activation of Cdc42 by AVP is sensitive to intra- and extracellular Ca2+ depletion, similar to AVP-induced reversal of stellation. Furthermore, AVP-induced reversal of stellation is blocked by expression of an NWASP fragment known to inhibit endogenous Cdc42.