Disc and Actin Associated Protein 1 influences attachment in the intestinal parasite Giardia lamblia

The deep-branching eukaryote Giardia lamblia is an extracellular parasite that attaches to the host intestine via a microtubule-based structure called the ventral disc. Control of attachment is mediated in part by the movement of two regions of the ventral disc that either permit or exclude the passage of fluid under the disc. Several known disc-associated proteins (DAPs) contribute to disc structure and function, but no force-generating protein has been identified among them. We recently identified several Giardia actin (GlActin) interacting proteins at the ventral disc, which could potentially employ actin polymerization for force generation and disc conformational changes. One of these proteins, Disc and Actin Associated Protein 1 (DAAP1), is highly enriched at the two regions of the disc previously shown to be important for fluid flow during attachment. In this study, we investigate the role of both GlActin and DAAP1 in ventral disc morphology and function. We confirmed interaction between GlActin and DAAP1 through coimmunoprecipitation, and used immunofluorescence to localize both proteins throughout the cell cycle and during trophozoite attachment. Similar to other DAPs, the association of DAAP1 with the disc is stable, except during cell division when the disc disassembles. Depletion of GlActin by translation-blocking antisense morpholinos resulted in both impaired attachment and defects in the ventral disc, indicating that GlActin contributes to disc-mediated attachment. Depletion of DAAP1 through CRISPR interference resulted in intact discs but impaired attachment, gating, and flow under the disc. As attachment is essential for infection, elucidation of these and other molecular mediators is a promising area for development of new therapeutics against a ubiquitous parasite.     

Role of Actin Polymerization and Adhesion to Extracellular Matrix in Rac- and Rho-induced Cytoskeletal Reorganization

Most animal cells use a combination of actin-myosin–based contraction and actin polymerization– based protrusion to control their shape and motility. The small GTPase Rho triggers the formation of contractile stress fibers and focal adhesion complexes (Ridley, A.J., and A. Hall. 1992. Cell. 70:389–399) while a close relative, Rac, induces lamellipodial protrusions and focal complexes in the lamellipodium (Nobes, C.D., and A. Hall. 1995. Cell. 81:53–62; Ridley, A.J., H.F. Paterson, C.L. Johnston, D. Diekmann, and A. Hall. 1992. Cell. 70:401–410); the Rho family of small GTPases may thus play an important role in regulating cell movement. Here we explore the roles of actin polymerization and extracellular matrix in Rho- and Rac-stimulated cytoskeletal changes. To examine the underlying mechanisms through which these GTPases control F-actin assembly, fluorescently labeled monomeric actin, Cy3-actin, was introduced into serum-starved Swiss 3T3 fibroblasts. Incorporation of Cy3- actin into lamellipodial protrusions is concomitant with F-actin assembly after activation of Rac, but Cy3-actin is not incorporated into stress fibers formed immediately after Rho activation. We conclude that Rac induces rapid actin polymerization in ruffles near the plasma membrane, whereas Rho induces stress fiber assembly primarily by the bundling of actin filaments. Activation of Rho or Rac also leads to the formation of integrin adhesion complexes. Integrin clustering is not required for the Rho-induced assembly of actin-myosin filament bundles, or for vinculin association with actin bundles, but is required for stress fiber formation. Integrin-dependent focal complex assembly is not required for the Rac-induced formation of lamellipodia or membrane ruffles. It appears, therefore, that the assembly of large integrin complexes is not required for most of the actin reorganization or cell morphology changes induced by Rac or Rho activation in Swiss 3T3 fibroblasts.     

A New Look at Rho GTPases in the Cell Cycle: Their Role in Kinetochore-Microtubule Attachment

Rho GTPases including Rho, Rac and Cdc42 are involved in cell morphogenesis by inducing specific types of actin cytoskeleton and alignment and stabilization of microtubules. Previous studies suggest that they also regulate cell cycle progression; Rho, Rac and Cdc42 regulate the G1-S progression and Rho controls cytokinesis. However, a role of Rho GTPases in nuclear division has not been definitely shown. We have recently found that Cdc42 and its downstream effector mDia3 are involved in bi-orientation and stabilization of spindle microtubules attachment to kinetochores and regulate chromosome alignment and segregation. Here, we discuss how this is coordinated with other events in mitosis, particularly, with the action of Rho in cytokinesis and how attachment of microtubules to kinetochores is achieved and stabilized. We also discuss redundancy of Cdc42 and Cdc42-related GTPase(s) and potential mechanisms of chromosome instability in cancer     

Spatio-Temporal Regulation of Rac1 Mobility by Actin Islands

Rho GTPases play important roles in many aspects of cell migration, including polarity establishment and organizing actin cytoskeleton. In particular, the Rho GTPase Rac1 has been associated with the generation of protrusions at leading edge of migrating cells. Previously we showed the mobility of Rac1 molecules is not uniform throughout a migrating cell (Hinde E et. al. PNAS 2013). Specifically, the closer a Rac1 molecule is to the leading edge, the slower the molecule diffuses. Because actin-bound Rac1 diffuses slower than unbound Rac1, we hypothesized that regions of high actin concentration, called “actin islands”, act as diffusive traps and are responsible for the non-uniform diffusion observed in vivo. Here, in silico model simulations demonstrate that equally spaced actin islands can regulate the time scale for Rac1 diffusion in a manner consistent with data from live-cell imaging experiments. Additionally, we find this mechanism is robust; different patterns of Rac1 mobility can be achieved by changing the actin islands’ positions or their affinity for Rac1.     

Rac1 function during fucoid development

Morphogenesis in fucoid algae begins with adhesive secretion and rhizoid germination, developmental events that secure the alga within the intertidal zone. The importance of the actin cytoskeleton during these processes has been well established; but in general, little is known about actin regulation within the stramenopile lineage. Based on conserved strategies for regulation of actin in other lineages, co-localization of the Arp2/3 complex with actin structures that are essential for rhizoid formation may implicate members of the Rho family of small GTPases in the signaling pathway(s) regulating actin polymerization during fucoid development. Our lab recently demonstrated Rac1 dependent regulation of endomembrane polarization, polarization of adhesive secretion, germination and tip growth in the fucoid brown alga Silvetia compressa. We also present new evidence revealing Rac1 localization during germination in S. compressa, and show that membrane localization is essential for proper Rac1 function.Key words: actin, Arp2/3 complex, manumycin A, NSC23766, Rac1, Rho GTPase, Scar/WAVE, Silvetia compressa     

Desmoglein 3 acting as an upstream regulator of Rho GTPases, Rac-1/Cdc42 in the regulation of actin organisation and dynamics

Desmoglein 3 (Dsg3), a member of the desmoglein sub-family, serves as an adhesion molecule in desmosomes. Our previous study showed that overexpression of human Dsg3 in several epithelial lines induces formation of membrane protrusions, a phenotype suggestive of Rho GTPase activation. Here we examined the interaction between Dsg3 and actin in detail and showed that endogenous Dsg3 colocalises and interacts with actin, particularly the junctional actin in a Rac1-dependent manner. Ablation of Rac1 activity by dominant negative Rac1 mutant (N17Rac1) or the Rac1 specific inhibitor (NSC23766) directly disrupts the interaction between Dsg3 and actin. Assembly of the junctional actin at the cell borders is accompanied with enhanced levels of Dsg3, while inhibition of Dsg3 by RNAi results in profound changes in the organisation of actin cytoskeleton. In accordance, overexpression of Dsg3 results in a remarkable increase of Rac1 and Cdc42 activities and to a lesser extent, RhoA. The enhancements in Rho GTPases are accompanied by the pronounced actin-based membrane structures such as lamellipodia and filopodia, enhanced rate of actin turnover and cell polarisation. Together, our results reveal an important novel function for Dsg3 in promoting actin dynamics through regulating Rac1 and Cdc42 activation in epithelial cells.     

Invasion of MDCK epithelial cells with altered expression of Rho GTPases by Trypanosoma cruzi amastigotes and metacyclic trypomastigotes of strains from the two major phylogenetic lineages

In order to invade mammalian cells, Trypanosoma cruzi infective forms cause distinct rearrangements of membrane and host cell cytoskeletal components. Rho GTPases have been shown to regulate three separate signal transduction pathways, linking plasma membrane receptors to the assembly of distinct actin filament structures. Here, we examined the role of Rho GTPases on the interaction between different T. cruzi infective forms of strains from the two major phylogenetic lineages with nonpolarized MDCK cells transfected with different Rho GTPase constructs. We compared the infectivity of amastigotes isolated from infected cells (intracellular amastigotes) with forms generated from the axenic differentiation of trypomastigotes (extracellular amastigotes), and also with metacyclic trypomastigotes. No detectable effect of GTPase expression was observed on metacyclic trypomastigote invasion and parasites of Y and CL (T. cruzi II) strains invaded to similar degrees all MDCK transfectants, and were more infective than either G or Tulahuen (T. cruzi I) strains. Intracellular amastigotes were complement sensitive and showed very low infectivity towards the different transfectants regardless of the parasite strain. Complement-resistant T. cruzi I extracellular amastigotes, especially of the G strain, were more infective than T. cruzi II parasites, particularly for the Rac1V12 constitutively active GTPase transfectant. The fact that in Rac1N17 dominant-negative cells, the invasion of G strain extracellular amastigotes was specifically inhibited suggested an important role for Rac1 in this process.     

p190RhoGAP has cellular RacGAP activity regulated by a polybasic region

p190RhoGAP is a GTPase-activating protein (GAP) known to regulate actin cytoskeleton dynamics by decreasing RhoGTP levels through activation of the intrinsic GTPase activity of Rho. Although the GAP domain of p190RhoGAP stimulates the intrinsic' GTPase activity of several Rho family members (Rho, Rac, Cdc42) under in vitro conditions, p190RhoGAP is generally regarded as a GAP for RhoA in the cell. The cellular RacGAP activity of the protein has not been proven directly. We have previously shown that the in vitro RacGAP and RhoGAP activity of p190RhoGAP was inversely regulated through a polybasic region of the protein. Here we provide evidence that p190RhoGAP shows remarkable GAP activity toward Rac also in the cell. The cellular RacGAP activity of p190RhoGAP requires an intact polybasic region adjacent to the GAP domain whereas the RhoGAP activity is inhibited by the same domain. Our data indicate that through its alternating RacGAP and RhoGAP activity, p190RhoGAP plays a more complex role in the Rac–Rho antagonism than it was realized earlier.     

Rac1 regulates cardiovascular development and postnatal function of endothelium

Rac1 is a member of the small Rho GTPase family, which controls actin cytoskeleton and focal adhesion dynamics in cellular protrusions. While Rac1 therefore contributes to regulation of endothelial cell-cell and cell-matrix interactions, a detailed understanding of its role in endothelium function is lacking. Recently, the role of Rac1 in development and postnatal regulation of the cardiovascular system has been investigated in murine models lacking Rac1 specifically in endothelium. Homozygous endothelial deletion was lethal, primarily due to defects in angiogenesis. Rac1-deficient endothelial cells were unable to form cellular protrusions/lamellipodia, leading to impaired cell-cell and cell-matrix interactions, and resulting in dysfunctional adhesion, motility, permeability and capillary morphogenesis. Development was normal in the heterozygous model, however a hypertensive phenotype was observed as a result of reduced nitric oxide signalling. Nitric oxide synthase activity was regulated by Rac1 at multiple levels; expression, mRNA stability and uptake of the nitric oxide synthase substrate L-arginine. Therefore, Rac1 activity is essential in regulating developmental and postnatal angiogenesis and cardiovascular function, by controlling nitric oxide production, and formation of endothelial cell protrusions.Key words: Rac1, angiogenesis, endothelial, motility, lamellipodia, nitric oxide, nitric oxide synthase     

RhoGAP18B Isoforms Act on Distinct Rho-Family GTPases and Regulate Behavioral Responses to Alcohol via Cofilin

Responses to the effects of ethanol are highly conserved across organisms, with reduced responses to the sedating effects of ethanol being predictive of increased risk for human alcohol dependence. Previously, we described that regulators of actin dynamics, such as the Rho-family GTPases Rac1, Rho1, and Cdc42, alter Drosophila’s sensitivity to ethanol-induced sedation. The GTPase activating protein RhoGAP18B also affects sensitivity to ethanol. To better understand how different RhoGAP18B isoforms affect ethanol sedation, we examined them for their effects on cell shape, GTP-loading of Rho-family GTPase, activation of the actin-severing cofilin, and actin filamentation. Our results suggest that the RhoGAP18B-PA isoform acts on Cdc42, while PC and PD act via Rac1 and Rho1 to activate cofilin. In vivo, a loss-of-function mutation in the cofilin-encoding gene twinstar leads to reduced ethanol-sensitivity and acts in concert with RhoGAP18B. Different RhoGAP18B isoforms, therefore, act on distinct subsets of Rho-family GTPases to modulate cofilin activity, actin dynamics, and ethanol-induced behaviors.     

βArrestin1 Regulates the Guanine Nucleotide Exchange Factor RasGRF2 Expression and the Small GTPase Rac-mediated Formation of Membrane Protrusion and Cell Motility

βArrestin proteins shuttle between the cytosol and nucleus and have been shown to regulate G protein-coupled receptor signaling, actin remodeling, and gene expression. Here, we tested the hypothesis that βarrestin1 regulates actin remodeling and cell migration through the small GTPase Rac. Depletion of βarrestin1 promotes Rac activation, leading to the formation of multipolar protrusions and increased cell circularity, and overexpression of a dominant negative form of Rac reverses these morphological changes. Small interfering RNA library screen identifies RasGRF2 as a target of βarrestin1. RasGRF2 gene and protein expression levels are elevated following depletion of βarrestin1, and the consequent activation of Rac results in dephosphorylation of cofilin that can promote actin polymerization and formation of multipolar protrusions, thereby retarding cell migration and invasion. Together, these results suggest that βarrestin1 regulates rasgrf2 gene expression and Rac activation to affect membrane protrusion and cell migration and invasion.     

Coxiella burnetii Phagocytosis Is Regulated by GTPases of the Rho Family and the RhoA Effectors mDia1 and ROCK

The GTPases belonging to the Rho family control the actin cytoskeleton rearrangements needed for particle internalization during phagocytosis. ROCK and mDia1 are downstream effectors of RhoA, a GTPase involved in that process. Coxiella burnetii, the etiologic agent of Q fever, is internalized by the host´s cells in an actin-dependent manner. Nevertheless, the molecular mechanism involved in this process has been poorly characterized. This work analyzes the role of different GTPases of the Rho family and some downstream effectors in the internalization of C. burnetii by phagocytic and non-phagocytic cells. The internalization of C. burnetii into HeLa and RAW cells was significantly inhibited when the cells were treated with Clostridium difficile Toxin B which irreversibly inactivates members of the Rho family. In addition, the internalization was reduced in HeLa cells that overexpressed the dominant negative mutants of RhoA, Rac1 or Cdc42 or that were knocked down for the Rho GTPases. The pharmacological inhibition or the knocking down of ROCK diminished bacterium internalization. Moreover, C. burnetii was less efficiently internalized in HeLa cells overexpressing mDia1-N1, a dominant negative mutant of mDia1, while the overexpression of the constitutively active mutant mDia1-ΔN3 increased bacteria uptake. Interestingly, when HeLa and RAW cells were infected, RhoA, Rac1 and mDia1 were recruited to membrane cell fractions. Our results suggest that the GTPases of the Rho family play an important role in C. burnetii phagocytosis in both HeLa and RAW cells. Additionally, we present evidence that ROCK and mDia1, which are downstream effectors of RhoA, are involved in that process.     

The Rac1 inhibitor,NSC23766, depolarizes adhesive secretion,endomembrane cycling,and tip growth in the fucoid alga, <Emphasis Type="Italic">Silvetia compressa</Emphasis>

Proper cell morphogenesis is dependent on the establishment and expression of cellular polarity. In the fucoid zygote, cell shape is critical for establishing the developmental pattern of the adult, and is achieved by guiding insertion of new membrane and wall to the rhizoid tip. Selection and growth of the appropriate tip site are accompanied by formation of dynamic actin arrays associated with the actin-nucleating Arp2/3 complex. In eukaryotes, a major pathway for activation of the Arp2/3 complex is via the Rho family GTPase, Rac1, which stimulates the Scar/WAVE complex. To determine whether Rac1 controls actin nucleation in Silvetia compressa (J. Agardh) E. Serrao, T. O. Cho, S. M. Boo et Brawley, we tested the effects of the Rac1-specific inhibitory compound, NSC23766, on actin dependent processes and on actin arrays. We found that NSC23766 disrupted polar secretion of adhesive, polarization of endomembranes, and tip-focused growth in the rhizoid. Similarly, NSC23766 altered actin and Arp2 localization in the growing rhizoid. In contrast, NSC23766 had no effect on selection of the growth site or on cytokinesis. These data suggest that Rac1 participates in nucleation of specific actin arrays in the developing zygote.     

Glioblastoma motility occurs in the absence of actin polymer

In fibroblasts and keratocytes, motility is actin dependent, while microtubules play a secondary role, providing directional guidance. We demonstrate here that the motility of glioblastoma cells is exceptional, in that it occurs in cells depleted of assembled actin. Cells display persistent motility in the presence of actin inhibitors at concentrations sufficient to fully disassemble actin. Such actin independent motility is characterized by the extension of cell protrusions containing abundant microtubule polymers. Strikingly, glioblastoma cells exhibit no motility in the presence of microtubule inhibitors, at concentrations that disassemble labile microtubule polymers. In accord with an unconventional mode of motility, glioblastoma cells have some unusual requirements for the Rho GTPases. While Rac1 is required for lamellipodial protrusions in fibroblasts, expression of dominant negative Rac1 does not suppress glioblastoma migration. Other GTPase mutants are largely without unique effect, except dominant positive Rac1-Q61L, and rapidly cycling Rac1-F28L, which substantially suppress glioblastoma motility. We conclude that glioblastoma cells display an unprecedented mode of intrinsic motility that can occur in the absence of actin polymer, and that appears to require polymerized microtubules.     

Rac1 regulates cardiovascular development and postnatal function of endothelium

Rac1 is a member of the small Rho GTPase family, which controls actin cytoskeleton and focal adhesion dynamics in cellular protrusions. While Rac1 therefore contributes to regulation of endothelial cell-cell and cell-matrix interactions, a detailed understanding of its role in endothelium function is lacking. Recently, the role of Rac1 in development and postnatal regulation of the cardiovascular system has been investigated in murine models lacking Rac1 specifically in endothelium. Homozygous endothelial deletion was lethal, primarily due to defects in angiogenesis. Rac1-deficient endothelial cells were unable to form cellular protrusions/lamellipodia, leading to impaired cell-cell and cell-matrix interactions, and resulting in dysfunctional adhesion, motility, permeability and capillary morphogenesis. Development was normal in the heterozygous model, however a hypertensive phenotype was observed as a result of reduced nitric oxide signalling. Nitric oxide synthase activity was regulated by Rac1 at multiple levels; expression, mRNA stability and uptake of the nitric oxide synthase substrate L-arginine. Therefore, Rac1 activity is essential in regulating developmental and postnatal angiogenesis and cardiovascular function, by controlling nitric oxide production, and formation of endothelial cell protrusions.     

Requirement for the Rac GTPase in Chlamydia trachomatis invasion of non-phagocytic cells

Chlamydiae are gram-negative obligate intracellular pathogens to which access to an intracellular environment is paramount to their survival and replication. To this end, chlamydiae have evolved extremely efficient means of invading nonphagocytic cells. To elucidate the host cell machinery utilized by Chlamydia trachomatis in invasion, we examined the roles of the Rho GTPase family members in the internalization of chlamydial elementary bodies. Upon binding of elementary bodies on the cell surface, actin is rapidly recruited to the sites of internalization. Members of the Rho GTPase family are frequently involved in localized recruitment of actin. Clostridial Toxin B, which is a known enzymatic inhibitor of Rac, Cdc42 and Rho GTPases, significantly reduced chlamydial invasion of HeLa cells. Expression of dominant negative constructs in HeLa cells revealed that chlamydial uptake was dependent on Rac, but not on Cdc42 or RhoA. Rac but not Cdc42 was found to be activated by chlamydial attachment. The effect of dominant negative Rac expression on chlamydial uptake is manifested through the inhibition of actin recruitment to the sites of chlamydial entry. Studies utilizing Green Fluorescent Protein fusion constructs of Rac, Cdc42 and RhoA, showed Rac to be the sole member of the Rho GTPase family recruited to the site of chlamydial entry.     

Giardia spp.: Distribution of contractile proteins in the attachment organelle

Giardia spp. trophozoites isolated from rat small intestine were examined by light microscopy, electron microscopy, SDS-gel electrophoresis, and immunocytochemistry. In SDS-gels of protein extracts of isolated Giardia spp. trophozoites protein bands corresponding to myosin, α-actinin, and actin were identified by comigration with avian myofibril proteins and molecular weight standards. Actin was specifically identified in SDS-gels by immunoautoradiography. Immunostaining for actin, α-actinin, myosin, and tropomyosin in trophozoites was demonstrated in the periphery of the ventral disc in an area corresponding to the lateral crest. Electron-dense fibrillar was observed in the lateral crest of the ventral disc by electron microscopy. Immunostaining for actin and α-actinin was also observed in the area of the median body, a microtubular organelle, and in electron-dense fibrillar material associated with the intracellular axonemes of the posterior-lateral flagella. The localization of these contractile proteins in the ventral disc suggests that they may play an important role in the mechanism of trophozoite attachment.     

Diverse effects of RacV12 on cell transformation by Raf: partial inhibition of morphological transformation versus deregulation of cell cycle control

Activated Raf kinases and Rac GTPases were shown to cooperate in the oncogenic transformation of fibroblasts, which is characterised by the disassembly of the cellular actin cytoskeleton, a nearly complete loss of focal adhesion complexes and deregulated cell proliferation. This is surprising since the Rac GTPase induces actin structures and the adhesion of suspended cells to extracellular matrix proteins. NIH 3T3 cells expressing a hydroxytamoxifen-inducible oncogenic c-Raf-1-oestrogen receptor fusion protein (c-Raf-1-BxB-ER, N-BxB-ER cells) undergo morphological transformation upon stimulation of the Raf kinase. We show that treatment with the Rac, Rho and Cdc42 activating Escherichia coli toxin CNF1 or coexpression of an activated RacV12 mutant partially inhibits and reverses the disassembly of cellular actin structures and focal adhesion complexes by oncogenic Raf. Activation of the Rac GTPase restores actin structures and focal adhesion complexes at the cellular boundary, leading to spreading of the otherwise spindle-shaped Raf-transformed cells. Actin stress fibres, however, which are regulated by the function of the Rho GTPase, are disassembled by oncogenic Raf even in the presence of activated Rac and Rho. With respect to the RacV12-mediated spreading of Raf-transformed cells, we postulate an anti-oncogenic function of the activated Rac. Another feature of cell transformation is the deregulation of cell cycle control. NIH 3T3 cells expressing high levels of the c-Raf-1-BxB-ER protein undergo a cell cycle arrest upon stimulation of the oncogenic Raf kinase. Our results show that in N-BxB-ER-RacV12 cells the expression of the activated RacV12 mediates cell proliferation in the presence of high-intensity Raf signals and high levels of the Cdk inhibitor p21(Cip1). These results indicate a pro-oncogenic function of the Rac GTPase with respect to the deregulation of cell cycle control.     

In Vivo Imaging and Characterization of Actin Microridges

Actin microridges form labyrinth like patterns on superficial epithelial cells across animal species. This highly organized assembly has been implicated in mucus retention and in the mechanical structure of mucosal surfaces, however the mechanisms that regulate actin microridges remain largely unknown. Here we characterize the composition and dynamics of actin microridges on the surface of zebrafish larvae using live imaging. Microridges contain phospho-tyrosine, cortactin and VASP, but not focal adhesion kinase. Time-lapse imaging reveals dynamic changes in the length and branching of microridges in intact animals. Transient perturbation of the microridge pattern occurs before cell division with rapid re-assembly during and after cytokinesis. Microridge assembly is maintained with constitutive activation of Rho or inhibition of myosin II activity. However, expression of dominant negative RhoA or Rac alters microridge organization, with an increase in distance between microridges. Latrunculin A treatment and photoconversion experiments suggest that the F-actin filaments are actively treadmilling in microridges. Accordingly, inhibition of Arp2/3 or PI3K signaling impairs microridge structure and length. Taken together, actin microridges in zebrafish represent a tractable in vivo model to probe pattern formation and dissect Arp2/3-mediated actin dynamics in vivo.