Regulating Rho GTPases and their regulators

Tyrosine phosphorylation and proteolytic cleavage of Notch are required for non-canonical Notch/Abl signaling in Drosophila axon guidance

Notch signaling is required for the development and physiology of nearly every tissue in metazoans. Much of Notch signaling is mediated by transcriptional regulation of downstream target genes, but Notch controls axon patterning in Drosophila by local modulation of Abl tyrosine kinase signaling, via direct interactions with the Abl co-factors Disabled and Trio. Here, we show that Notch-Abl axonal signaling requires both of the proteolytic cleavage events that initiate canonical Notch signaling. We further show that some Notch protein is tyrosine phosphorylated in Drosophila, that this form of the protein is selectively associated with Disabled and Trio, and that relevant tyrosines are essential for Notch-dependent axon patterning but not for canonical Notch-dependent regulation of cell fate. Based on these data, we propose a model for the molecular mechanism by which Notch controls Abl signaling in Drosophila axons.

Targeting extracellular matrix stiffness to attenuate disease: From molecular mechanisms to clinical trials

Tissues stiffen during aging and during the pathological progression of cancer, fibrosis, and cardiovascular disease. Extracellular matrix stiffness is emerging as a prominent mechanical cue that precedes disease and drives its progression by altering cellular behaviors. Targeting extracellular matrix mechanics, by preventing or reversing tissue stiffening or interrupting the cellular response, is a therapeutic approach with clinical potential. Major drivers of changes to the mechanical properties of the extracellular matrix include phenotypically converted myofibroblasts, transforming growth factor β (TGFβ), and matrix cross-linking. Potential pharmacological interventions to overcome extracellular matrix stiffening are emerging clinically. Aside from targeting stiffening directly, alternative approaches to mitigate the effects of increased matrix stiffness aim to identify and inhibit the downstream cellular response to matrix stiffness. Therapeutic interventions that target tissue stiffening are discussed in the context of their limitations, preclinical drug development efforts, and clinical trials.

Endothelial Cell Metabolism

Endothelial cells (ECs) are more than inert blood vessel lining material. Instead, they are active players in the formation of new blood vessels (angiogenesis) both in health and (life-threatening) diseases. Recently, a new concept arose by which EC metabolism drives angiogenesis in parallel to well-established angiogenic growth factors (e.g., vascular endothelial growth factor). 6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase-3-driven glycolysis generates energy to sustain competitive behavior of the ECs at the tip of a growing vessel sprout, whereas carnitine palmitoyltransferase 1a-controlled fatty acid oxidation regulates nucleotide synthesis and proliferation of ECs in the stalk of the sprout. To maintain vascular homeostasis, ECs rely on an intricate metabolic wiring characterized by intracellular compartmentalization, use metabolites for epigenetic regulation of EC subtype differentiation, crosstalk through metabolite release with other cell types, and exhibit EC subtype-specific metabolic traits. Importantly, maladaptation of EC metabolism contributes to vascular disorders, through EC dysfunction or excess angiogenesis, and presents new opportunities for anti-angiogenic strategies. Here we provide a comprehensive overview of established as well as newly uncovered aspects of EC metabolism.

The RhoGAP Stard13 controls insulin secretion through F-actin remodeling

Objective

Actin cytoskeleton remodeling is necessary for glucose-stimulated insulin secretion in pancreatic β-cells. A mechanistic understanding of actin dynamics in the islet is paramount to a better comprehension of β-cell dysfunction in diabetes. Here, we investigate the Rho GTPase regulator Stard13 and its role in F-actin cytoskeleton organization and islet function in adult mice.

Methods

We used Lifeact-EGFP transgenic animals to visualize actin cytoskeleton organization and dynamics in vivo in the mouse islets. Furthermore, we applied this model to study actin cytoskeleton and insulin secretion in mutant mice deleted for Stard13 selectively in pancreatic cells. We isolated transgenic islets for 3D-imaging and perifusion studies to measure insulin secretion dynamics. In parallel, we performed histological and morphometric analyses of the pancreas and used in vivo approaches to study glucose metabolism in the mouse.

Results

In this study, we provide the first genetic evidence that Stard13 regulates insulin secretion in response to glucose. Postnatally, Stard13 expression became restricted to the mouse pancreatic islets. We showed that Stard13 deletion results in a marked increase in actin polymerization in islet cells, which is accompanied by severe reduction of insulin secretion in perifusion experiments. Consistently, Stard13-deleted mice displayed impaired glucose tolerance and reduced glucose-stimulated insulin secretion.

Conclusions

Taken together, our results suggest a previously unappreciated role for the RhoGAP protein Stard13 in the interplay between actin cytoskeletal remodeling and insulin secretion.

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Lifeact-EGFP mice allow in vivo labeling of the actin cytoskeleton in islets.

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The RhoGAP Stard13 regulates actin cytoskeleton organization in mouse islets.

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Stard13 deficiency hampers glucose-induced insulin secretion by mouse β-cells.

Induced cortical tension restores functional junctions in adhesion-defective carcinoma cells

Normal epithelial cells are stably connected to each other via the apical junctional complex (AJC). AJCs, however, tend to be disrupted during tumor progression, and this process is implicated in cancer dissemination. Here, using colon carcinoma cells that fail to form AJCs, we investigated molecular defects behind this failure through a search for chemical compounds that could restore AJCs, and found that microtubule-polymerization inhibitors (MTIs) were effective. MTIs activated GEF-H1/RhoA signaling, causing actomyosin contraction at the apical cortex. This contraction transmitted force to the cadherin-catenin complex, resulting in a mechanosensitive recruitment of vinculin to cell junctions. This process, in turn, recruited PDZ-RhoGEF to the junctions, leading to the RhoA/ROCK/LIM kinase/cofilin-dependent stabilization of the junctions. RhoGAP depletion mimicked these MTI-mediated processes. Cells that normally organize AJCs did not show such MTI/RhoA sensitivity. Thus, advanced carcinoma cells require elevated RhoA activity for establishing robust junctions, which triggers tension-sensitive reorganization of actin/adhesion regulators.

c-Abl tyrosine kinase regulates neutrophil crawling behavior under fluid shear stress via Rac/PAK/LIMK/cofilin signaling axis

The excessive recruitment and improper activation of polymorphonuclear neutrophils (PMNs) often induces serious injury of host tissues, leading to inflammatory disorders. Therefore, to understand the molecular mechanism on neutrophil recruitment possesses essential pathological and physiological importance. In this study, we found that physiological shear stress induces c-Abl kinase activation in neutrophils, and c-Abl kinase inhibitor impaired neutrophil crawling behavior on ICAM-1. We further identified Vav1 was a downstream effector phosphorylated at Y174 and Y267. Once activated, c-Abl kinase regulated the activity of Vav1, which further affected Rac1/PAK1/LIMK1/cofilin signaling pathway. Here, we demonstrate a novel signaling function and critical role of c-Abl kinase during neutrophil crawling under physiological shear by regulating Vav1. These findings provide a promising treatment strategy for inflammation-related disease by inactivation of c-Abl kinase to restrict neutrophil recruitment.

The Rho GTPase signalling pathway in urothelial carcinoma

Urothelial carcinoma remains a clinical challenge: non-muscle-invasive disease has a high rate of recurrence and risk of progression, and outcomes for patients with advanced disease are poor, owing to a lack of effective systemic therapies. The Rho GTPase family of enzymes was first identified >30 years ago and contains >20 members, which are divided into eight subfamilies: Cdc42, Rac, Rho, RhoUV, RhoBTB, RhoDF, RhoH, and Rnd. Rho GTPases are molecular on-off switches, which are increasingly being understood to have a critical role in a number of cellular processes, including cell migration, cell polarity, cell adhesion, cell cycle progression, and regulation of the cytoskeleton. This switch is an evolutionarily conserved system in which GTPases alternate between GDP-bound (inactive) and GTP-bound (active) forms. The activities of these Rho GTPases are many, context-dependent, and regulated by a number of proteins that are being progressively elucidated. Aberrations of the Rho GTPase signalling pathways have been implicated in various malignancies, including urothelial carcinoma, and understanding of the role of Rho GTPases in these diseases is increasing. This signalling pathway has the potential for therapeutic targeting in urothelial carcinoma. Research in this area is nascent, and much work is necessary before current laboratory-based research can be translated into the clinic.

Catenins Steer Cell Migration via Stabilization of Front-Rear Polarity

Cell migration plays a pivotal role in morphogenetic and pathogenetic processes. To achieve directional migration, cells must establish a front-to-rear axis of polarity. Here we show that components of the cadherin-catenin complex function to stabilize this front-rear polarity. Neural crest and glioblastoma cells undergo directional migration in vivo or in vitro. During this process, αE-catenin accumulated at lamellipodial membranes and then moved toward the rear with the support of a tyrosine-phosphorylated β-catenin. This relocating αE-catenin bound to p115RhoGEF, leading to gathering of active RhoA in front of the nucleus where myosin-IIB arcs assemble. When catenins or p115RhoGEF were removed, cells lost the polarized myosin-IIB assembly, as well as the capability for directional movement. These results suggest that, apart from its well-known function in cell adhesion, the β-catenin/αE-catenin complex regulates directional cell migration by restricting active RhoA to perinuclear regions and controlling myosin-IIB dynamics at these sites.

The Tiam1 guanine nucleotide exchange factor is auto-inhibited by its pleckstrin homology coiled-coil extension domain

T-cell lymphoma invasion and metastasis 1 (Tiam1) is a Dbl-family guanine nucleotide exchange factor (GEF) that specifically activates the Rho-family GTPase Rac1 in response to upstream signals, thereby regulating cellular processes including cell adhesion and migration. Tiam1 contains multiple domains, including an N-terminal pleckstrin homology coiled-coiled extension (PHn-CC-Ex) and catalytic Dbl homology and C-terminal pleckstrin homology (DH-PHc) domain. Previous studies indicate that larger fragments of Tiam1, such as the region encompassing the N-terminal to C-terminal pleckstrin homology domains (PHn-PHc), are auto-inhibited. However, the domains in this region responsible for inhibition remain unknown. Here, we show that the PHn-CC-Ex domain inhibits Tiam1 GEF activity by directly interacting with the catalytic DH-PHc domain, preventing Rac1 binding and activation. Enzyme kinetics experiments suggested that Tiam1 is auto-inhibited through occlusion of the catalytic site rather than by allostery. Small angle X-ray scattering and ensemble modeling yielded models of the PHn-PHc fragment that indicate it is in equilibrium between "open" and "closed" conformational states. Finally, single-molecule experiments support a model in which conformational sampling between the open and closed states of Tiam1 contributes to Rac1 dissociation. Our results highlight the role of the PHn-CC-Ex domain in Tiam1 GEF regulation and suggest a combinatorial model for GEF inhibition and activation of the Rac1 signaling pathway.

cAMP-induced activation of protein kinase A and p190B RhoGAP mediates down-regulation of TC10 activity at the plasma membrane and neurite outgrowth

Cyclic AMP plays a pivotal role in neurite growth. During outgrowth, a trafficking system supplies membrane at growth cones. However, the cAMP-induced signaling leading to the regulation of membrane trafficking remains unknown. TC10 is a Rho family GTPase that is essential for specific types of vesicular trafficking. Recent studies have shown a role of TC10 in neurite growth in NGF-treated PC12 cells. Here, we investigated a mechanical linkage between cAMP and TC10 in neuritogenesis. Plasmalemmal TC10 activity decreased abruptly after cAMP addition in neuronal cells. TC10 was locally inactivated at extending neurite tips in cAMP-treated PC12 cells. TC10 depletion led to a decrease in cAMP-induced neurite outgrowth. Constitutively active TC10 could not rescue this growth reduction, supporting our model for a role of GTP hydrolysis of TC10 in neuritogenesis by accelerating vesicle fusion. The cAMP-induced TC10 inactivation was mediated by PKA. Considering cAMP-induced RhoA inactivation, we found that p190B, but not p190A, mediated inactivation of TC10 and RhoA. Upon cAMP treatment, p190B was recruited to the plasma membrane. STEF depletion and Rac1-N17 expression reduced cAMP-induced TC10 inactivation. Together, the PKA-STEF-Rac1-p190B pathway leading to inactivation of TC10 and RhoA at the plasma membrane plays an important role in cAMP-induced neurite outgrowth.

Gain-of-function mutant p53 activates small GTPase Rac1 through SUMOylation to promote tumor progression

Here, Yue et al. investigated the mechanisms underlying p53 gain-of-function (GOF) mutations and found that mutant p53 activates small GTPase Rac1 as a critical mechanism for mutant p53 GOF to promote tumor progression. Their findings provide insight into a new mechanism for Rac1 activation in tumors and show that activation of Rac1 is an unidentified and critical mechanism for mutant p53 GOF in tumorigenesis.

Tumor suppressor p53 is frequently mutated in human cancer. Mutant p53 often promotes tumor progression through gain-of-function (GOF) mechanisms. However, the mechanisms underlying mutant p53 GOF are not well understood. In this study, we found that mutant p53 activates small GTPase Rac1 as a critical mechanism for mutant p53 GOF to promote tumor progression. Mechanistically, mutant p53 interacts with Rac1 and inhibits its interaction with SUMO-specific protease 1 (SENP1), which in turn inhibits SENP1-mediated de-SUMOylation of Rac1 to activate Rac1. Targeting Rac1 signaling by RNAi, expression of the dominant-negative Rac1 (Rac1 DN), or the specific Rac1 inhibitor NSC23766 greatly inhibits mutant p53 GOF in promoting tumor growth and metastasis. Furthermore, mutant p53 expression is associated with enhanced Rac1 activity in clinical tumor samples. These results uncover a new mechanism for Rac1 activation in tumors and, most importantly, reveal that activation of Rac1 is an unidentified and critical mechanism for mutant p53 GOF in tumorigenesis, which could be targeted for therapy in tumors containing mutant p53.

EphrinB1 promotes cancer cell migration and invasion through the interaction with RhoGDI1

Eph receptors and their corresponding ephrin ligands have been associated with regulating cell–cell adhesion and motility, and thus have a critical role in various biological processes including tissue morphogenesis and homeostasis, as well as pathogenesis of several diseases. Aberrant regulation of Eph/ephrin signaling pathways is implicated in tumor progression of various human cancers. Here, we show that a Rho family GTPase regulator, Rho guanine nucleotide dissociation inhibitor 1 (RhoGDI1), can interact with ephrinB1, and this interaction is enhanced upon binding the extracellular domain of the cognate EphB2 receptor. Deletion mutagenesis revealed that amino acids 327–334 of the ephrinB1 intracellular domain are critical for the interaction with RhoGDI1. Stimulation with an EphB2 extracellular domain-Fc fusion protein (EphB2-Fc) induces RhoA activation and enhances the motility as well as invasiveness of wild-type ephrinB1-expressing cells. These Eph-Fc-induced effects were markedly diminished in cells expressing the mutant ephrinB1 construct (Δ327–334) that is ineffective at interacting with RhoGDI1. Furthermore, ephrinB1 depletion by siRNA suppresses EphB2-Fc-induced RhoA activation, and reduces motility and invasiveness of the SW480 and Hs578T human cancer cell lines. Our study connects the interaction between RhoGDI1 and ephrinB1 to the promotion of cancer cell behavior associated with tumor progression. This interaction may represent a therapeutic target in cancers that express ephrinB1.

Analysis of the interaction of Plexin-B1 and Plexin-B2 with Rnd family proteins

The Rnd family of proteins, Rnd1, Rnd2 and Rnd3, are atypical Rho family GTPases, which bind to but do not hydrolyse GTP. They interact with plexins, which are receptors for semaphorins, and are hypothesised to regulate plexin signalling. We recently showed that each Rnd protein has a distinct profile of interaction with three plexins, Plexin-B1, Plexin-B2 and Plexin-B3, in mammalian cells, although it is unclear which region(s) of these plexins contribute to this specificity. Here we characterise the binary interactions of the Rnd proteins with the Rho-binding domain (RBD) of Plexin-B1 and Plexin-B2 using biophysical approaches. Isothermal titration calorimetry (ITC) experiments for each of the Rnd proteins with Plexin-B1-RBD and Plexin-B2-RBD showed similar association constants for all six interactions, although Rnd1 displayed a small preference for Plexin-B1-RBD and Rnd3 for Plexin-B2-RBD. Furthermore, mutagenic analysis of Rnd3 suggested similarities in its interaction with both Plexin-B1-RBD and Plexin-B2-RBD. These results suggest that Rnd proteins do not have a clear-cut specificity for different Plexin-B-RBDs, possibly implying the contribution of additional regions of Plexin-B proteins in conferring functional substrate selection.

ERK signalling as a regulator of cell motility

Cell motility is regulated by multiple processes, including cell protrusion, cell retraction, cell-matrix adhesion, polarized exocytosis and polarized vesicle trafficking, each of which is spatiotemporally controlled by various intracellular signalling pathways. Dysregulation of cell motility leads to pathological conditions, such as tumour invasion and metastasis. Accumulating evidence has revealed that extracellular signal-regulated kinase (ERK) signalling is one of the critical regulators of cell motility, although it is classically known as an important regulator of cell proliferation, differentiation and survival through regulation of gene expression. ERK and its downstream kinase, p90 ribosomal S6 kinase (RSK), dynamically regulate cell motility mainly through direct phosphorylation of various molecules that are not necessarily involved in the regulation of gene transcription and translation. In this review, we summarize how ERK signalling regulates cell motility by focusing on the components of the cell motility machinery that are directly regulated by ERK or RSK.

Prepatterning by RhoGEFs governs Rho GTPase spatiotemporal dynamics during wound repair

During wound repair, Rho GTPases form dynamic spatial and temporal patterns surrounding the wound and coordinate the cytoskeletal response. Nakamura et al. show that Rho GTPase arrays form in response to prepatterning by RhoGEFs, which depends on annexin B9.

Like tissues, single cells are subjected to continual stresses and damage. As such, cells have a robust wound repair mechanism comprised of dynamic membrane resealing and cortical cytoskeletal remodeling. One group of proteins, the Rho family of small guanosine triphosphatases (GTPases), is critical for this actin and myosin cytoskeletal response in which they form distinct dynamic spatial and temporal patterns/arrays surrounding the wound. A key mechanistic question, then, is how these GTPase arrays are formed. Here, we show that in the Drosophila melanogaster cell wound repair model Rho GTPase arrays form in response to prepatterning by Rho guanine nucleotide exchange factors (RhoGEFs), a family of proteins involved in the activation of small GTPases. Furthermore, we show that Annexin B9, a member of a class of proteins associated with the membrane resealing, is involved in an early, Rho family–independent, actin stabilization that is integral to the formation of one RhoGEF array. Thus, Annexin proteins may link membrane resealing to cytoskeletal remodeling processes in single cell wound repair.

FGD5 sustains vascular endothelial growth factor A (VEGFA) signaling through inhibition of proteasome-mediated VEGF receptor 2 degradation

The complete repertoire of endothelial functions elicited by FGD5, a guanine nucleotide exchange factor activating the Rho GTPase Cdc42, has yet to be elucidated. Here we explore FGD5's importance during vascular endothelial growth factor A (VEGFA) signaling via VEGF receptor 2 (VEGFR2) in human endothelial cells. In microvascular endothelial cells, FGD5 is located at the inner surface of the cell membrane as well as at the outer surface of EEA1-positive endosomes carrying VEGFR2. The latter finding prompted us to explore if FGD5 regulates VEGFR2 dynamics. We found that depletion of FGD5 in microvascular cells inhibited their migration towards a stable VEGFA gradient. Furthermore, depletion of FGD5 resulted in accelerated VEGFR2 degradation, which was reverted by lactacystin-mediated proteasomal inhibition. Our results thus suggest a mechanism whereby FGD5 sustains VEGFA signaling and endothelial cell chemotaxis via inhibition of proteasome-dependent VEGFR2 degradation.

Applications of Optobiology in Intact Cells and Multicellular Organisms

Temporal kinetics and spatial coordination of signal transduction in cells are vital for cell fate determination. Tools that allow for precise modulation of spatiotemporal regulation of intracellular signaling in intact cells and multicellular organisms remain limited. The emerging optobiological approaches use light to control protein-protein interaction in live cells and multicellular organisms. Optobiology empowers light-mediated control of diverse cellular and organismal functions such as neuronal activity, intracellular signaling, gene expression, cell proliferation, differentiation, migration, and apoptosis. In this review, we highlight recent developments in optobiology, focusing on new features of second-generation optobiological tools. We cover applications of optobiological approaches in the study of cellular and organismal functions, discuss current challenges, and present our outlook. Taking advantage of the high spatial and temporal resolution of light control, optobiology promises to provide new insights into the coordination of signaling circuits in intact cells and multicellular organisms.

Endothelial cell-cell adhesion and signaling

Endothelial cells line blood vessels and provide a dynamic interface between the blood and tissues. They remodel to allow leukocytes, fluid and small molecules to enter tissues during inflammation and infections. Here we compare the signaling networks that contribute to endothelial permeability and leukocyte transendothelial migration, focusing particularly on signals mediated by small GTPases that regulate cell adhesion and the actin cytoskeleton. Rho and Rap GTPase signaling is important for both processes, but they differ in that signals are activated locally under leukocytes, whereas endothelial permeability is a wider event that affects the whole cell. Some molecules play a unique role in one of the two processes, and could therefore be targeted to selectively alter either endothelial permeability or leukocyte transendothelial migration.

Neutrophils to the ROScue: Mechanisms of NADPH Oxidase Activation and Bacterial Resistance

Reactive oxygen species (ROS) generated by NADPH oxidase play an important role in antimicrobial host defense and inflammation. Their deficiency in humans results in recurrent and severe bacterial infections, while their unregulated release leads to pathology from excessive inflammation. The release of high concentrations of ROS aids in clearance of invading bacteria. Localization of ROS release to phagosomes containing pathogens limits tissue damage. Host immune cells, like neutrophils, also known as PMNs, will release large amounts of ROS at the site of infection following the activation of surface receptors. The binding of ligands to G-protein-coupled receptors (GPCRs), toll-like receptors, and cytokine receptors can prime PMNs for a more robust response if additional signals are encountered. Meanwhile, activation of Fc and integrin directly induces high levels of ROS production. Additionally, GPCRs that bind to the bacterial-peptide analog fMLP, a neutrophil chemoattractant, can both prime cells and trigger low levels of ROS production. Engagement of these receptors initiates intracellular signaling pathways, resulting in activation of downstream effector proteins, assembly of the NADPH oxidase complex, and ultimately, the production of ROS by this complex. Within PMNs, ROS released by the NADPH oxidase complex can activate granular proteases and induce the formation of neutrophil extracellular traps (NETs). Additionally, ROS can cross the membranes of bacterial pathogens and damage their nucleic acids, proteins, and cell membranes. Consequently, in order to establish infections, bacterial pathogens employ various strategies to prevent restriction by PMN-derived ROS or downstream consequences of ROS production. Some pathogens are able to directly prevent the oxidative burst of phagocytes using secreted effector proteins or toxins that interfere with translocation of the NADPH oxidase complex or signaling pathways needed for its activation. Nonetheless, these pathogens often rely on repair and detoxifying proteins in addition to these secreted effectors and toxins in order to resist mammalian sources of ROS. This suggests that pathogens have both intrinsic and extrinsic mechanisms to avoid restriction by PMN-derived ROS. Here, we review mechanisms of oxidative burst in PMNs in response to bacterial infections, as well as the mechanisms by which bacterial pathogens thwart restriction by ROS to survive under conditions of oxidative stress.

Genetically Encoded Photoactuators and Photosensors for Characterization and Manipulation of Pluripotent Stem Cells

Our knowledge of pluripotent stem cell biology has advanced considerably in the past four decades, but it has yet to deliver on the great promise of regenerative medicine. The slow progress can be mainly attributed to our incomplete understanding of the complex biologic processes regulating the dynamic developmental pathways from pluripotency to fully-differentiated states of functional somatic cells. Much of the difficulty arises from our lack of specific tools to query, or manipulate, the molecular scale circuitry on both single-cell and organismal levels. Fortunately, the last two decades of progress in the field of optogenetics have produced a variety of genetically encoded, light-mediated tools that enable visualization and control of the spatiotemporal regulation of cellular function. The merging of optogenetics and pluripotent stem cell biology could thus be an important step toward realization of the clinical potential of pluripotent stem cells. In this review, we have surveyed available genetically encoded photoactuators and photosensors, a rapidly expanding toolbox, with particular attention to those with utility for studying pluripotent stem cells.

Multicellular tumor invasion and plasticity in biomimetic materials

Cancer cell invasion through the extracellular matrix is associated with metastatic spread and therapeutic resistance. In carcinomas, the detachment and dissemination of individual cells has been associated with an epithelial-mesenchymal transition, but tumors can also invade using collective, multicellular phenotypes. This malignant tumor progression is also associated with alignment and stiffening of the surrounding extracellular matrix. Historically, tumor invasion has been investigated using 2D monolayer culture, small animal models or patient histology. These assays have been complemented by the use of natural biomaterials such as reconstituted basement membrane and collagen I. More recently, engineered materials with well-defined physical, chemical and biomolecular properties have enabled more controlled microenvironments. In this review, we highlight recent developments in multicellular tumor invasion based on microfabricated structures or hydrogels. We emphasize the role of interfacial geometries, biomaterial stiffness, matrix remodeling, and co-culture models. Finally, we discuss future directions for the field, particularly integration with precision measurements of biomaterial properties and single cell heterogeneity, standardization and scale-up of these platforms, as well as integration with patient-derived samples.

Astaxanthin induces migration in human skin keratinocytes via Rac1 activation and RhoA inhibition

BACKGROUND/OBJECTIVES

Re-epithelialization has an important role in skin wound healing. Astaxanthin (ASX), a carotenoid found in crustaceans including shrimp, crab, and salmon, has been widely used for skin protection. Therefore, we investigated the effects of ASX on proliferation and migration of human skin keratinocyte cells and explored the mechanism associated with that migration.

MATERIAL/METHOD

HaCaT keratinocyte cells were exposed to 0.25-1 µg/mL of ASX. Proliferation of keratinocytes was analyzed by using MTT assays and flow cytometry. Keratinocyte migration was determined by using a scratch wound-healing assay. A mechanism for regulation of migration was explored via immunocytochemistry and western blot analysis.

RESULTS

Our results suggest that ASX produces no significant toxicity in human keratinocyte cells. Cell-cycle analysis on ASX-treated keratinocytes demonstrated a significant increase in keratinocyte cell proliferation at the S phase. In addition, ASX increased keratinocyte motility across the wound space in a time-dependent manner. The mechanism by which ASX increased keratinocyte migration was associated with induction of filopodia and formation of lamellipodia, as well as with increased Cdc42 and Rac1 activation and decreased RhoA activation.

CONCLUSIONS

ASX stimulates the migration of keratinocytes through Cdc42, Rac1 activation and RhoA inhibition. ASX has a positive role in the re-epithelialization of wounds. Our results may encourage further in vivo and clinical study into the development of ASX as a potential agent for wound repair.

oxLDL induces endothelial cell proliferation via Rho/ROCK/Akt/p27kip1 signaling: opposite effects of oxLDL and cholesterol loading

Oxidized modifications of LDL (oxLDL) play a key role in the development of endothelial dysfunction and atherosclerosis. However, the underlying mechanisms of oxLDL-mediated cellular behavior are not completely understood. Here, we compared the effects of two major types of oxLDL, copper-oxidized LDL (Cu²⁺-oxLDL) and lipoxygenase-oxidized LDL (LPO-oxLDL), on proliferation of human aortic endothelial cells (HAECs). Cu²⁺-oxLDL enhanced HAECs' proliferation in a dose- and degree of oxidation-dependent manner. Similarly, LPO-oxLDL also enhanced HAEC proliferation. Mechanistically, both Cu²⁺-oxLDL and LPO-oxLDL enhance HAEC proliferation via activation of Rho, Akt phosphorylation, and a decrease in the expression of cyclin-dependent kinase inhibitor 1B (p27^kip1). Both Cu²⁺-oxLDL or LPO-oxLDL significantly increased Akt phosphorylation, whereas an Akt inhibitor, MK2206, blocked oxLDL-induced increase in HAEC proliferation. Blocking Rho with C3 or its downstream target ROCK with Y27632 significantly inhibited oxLDL-induced Akt phosphorylation and proliferation mediated by both Cu²⁺- and LPO-oxLDL. Activation of RhoA was blocked by Rho-GDI-1, which also abrogated oxLDL-induced Akt phosphorylation and HAEC proliferation. In contrast, blocking Rac1 in these cells had no effect on oxLDL-induced Akt phosphorylation or cell proliferation. Moreover, oxLDL-induced Rho/Akt signaling downregulated cell cycle inhibitor p27^kip1 Preloading these cells with cholesterol, however, prevented oxLDL-induced Akt phosphorylation and HAEC proliferation. These findings provide a new understanding of the effects of oxLDL on endothelial proliferation, which is essential for developing new treatments against neovascularization and progression of atherosclerosis.

Intermolecular steric inhibition of Ephexin4 is relieved by Elmo1

Ephexin4, a guanine nucleotide-exchange factor for RhoG, promotes engulfment of apoptotic cells and cancer cell migration in a RhoG-dependent manner, which is synergistically augmented by Elmo1, an Ephexin4-interacting protein. However, the underlying molecular mechanism remains elusive. Here, we report a mechanism by which Elmo1 cooperates with Ephexin4 to activate RhoG. We found that Ephexin4 activity was increased by elimination of its SH3 domain which intermolecularly interacts with the N20 region of Ephexin4. This interaction prevented RhoG from binding to Ephexin4 and thus inhibited RhoG activation. Moreover, we also found that Elmo1 associated with the SH3 domain as well as the N20 region and competed with the SH3 domain for binding to the N20 region, interrupting the interaction of the SH3 domain with the N20 region and thereby promoting RhoG binding to Ephexin4. In addition, the activity of Ephexin4 lacking the SH3 domain was comparable to that of Ephexin4 with Elmo1. Taken together, the data suggest that Elmo1 relieves the steric hindrance of Ephexin4 generated by the intermolecular interaction of the SH3 domain and makes Ephexin4 more accessible to RhoG.

Geranylgeraniol prevents the simvastatin-induced PCSK9 expression: Role of the small G protein Rac1

Statins are known to increase the plasma levels of proprotein convertase subtilisin kexin type 9 (PCSK9) through the activation of the sterol responsive element binding protein (SREBP) pathway due to the inhibition of cholesterol biosynthesis. In the present study, we explore a possible role of the prenylated proteins on the statin-mediated PCSK9 induction in Caco-2 cells. Simvastatin (40μM) induced both PCSK9 mRNA (10.7±3.2 fold) and protein (2.2±0.3 fold), after 24h incubation. The induction of PCSK9 mRNA was partially, but significantly, prevented by the co-incubation with mevalonate (MVA), farnesol (FOH) and geranylgeraniol (GGOH), while a complete prevention was observed on secreted PCSK9, evaluated by ELISA assay. Under the same experimental conditions, MVA, GGOH, but not FOH, prevented the activation of the PCSK9 promoter by simvastatin in a SRE-dependent manner. Simvastatin reduced by -35.7±15.2% the Rac1-GTP levels, while no changes were observed on RhoA- and Cdc42-GTP. This effect was prevented by MVA and GGOH. A Rac inhibitor, and N17Rac1 dominant negative mutant, significantly induced PCSK9 levels, and a suppression of Rac1 expression by siRNA, counteract the effect of simvastatin on the induction of PCSK9 mRNA. Finally, simvastatin, and Rac inhibitor inhibited the nuclear translocation of STAT3 and its knock-down by siRNA increased significantly the susceptibility of Caco-2 to simvastatin on PCSK9 expression. Taken together, the present study reveal a direct role of Rac1 on simvastatin-mediated PCSK9 expression via the reduction of STAT3 nuclear translocation.

Proteoglycans, ion channels and cell-matrix adhesion

Cell surface proteoglycans comprise a transmembrane or membrane-associated core protein to which one or more glycosaminoglycan chains are covalently attached. They are ubiquitous receptors on nearly all animal cell surfaces. In mammals, the cell surface proteoglycans include the six glypicans, CD44, NG2 (CSPG4), neuropilin-1 and four syndecans. A single syndecan is present in invertebrates such as nematodes and insects. Uniquely, syndecans are receptors for many classes of proteins that can bind to the heparan sulphate chains present on syndecan core proteins. These range from cytokines, chemokines, growth factors and morphogens to enzymes and extracellular matrix (ECM) glycoproteins and collagens. Extracellular interactions with other receptors, such as some integrins, are mediated by the core protein. This places syndecans at the nexus of many cellular responses to extracellular cues in development, maintenance, repair and disease. The cytoplasmic domains of syndecans, while having no intrinsic kinase activity, can nevertheless signal through binding proteins. All syndecans appear to be connected to the actin cytoskeleton and can therefore contribute to cell adhesion, notably to the ECM and migration. Recent data now suggest that syndecans can regulate stretch-activated ion channels. The structure and function of the syndecans and the ion channels are reviewed here, along with an analysis of ion channel functions in cell-matrix adhesion. This area sheds new light on the syndecans, not least since evidence suggests that this is an evolutionarily conserved relationship that is also potentially important in the progression of some common diseases where syndecans are implicated.

Diverse roles of guanine nucleotide exchange factors in regulating collective cell migration

Effective collective cell migration depends on delicate tuning of cell motility forces and cell–cell communication. Zaritsky, Tseng, and coworkers identified differential roles for RhoA, RhoC, and their upstream activators in mediating long-range transmission of guidance cues.

Efficient collective migration depends on a balance between contractility and cytoskeletal rearrangements, adhesion, and mechanical cell–cell communication, all controlled by GTPases of the RHO family. By comprehensive screening of guanine nucleotide exchange factors (GEFs) in human bronchial epithelial cell monolayers, we identified GEFs that are required for collective migration at large, such as SOS1 and β-PIX, and RHOA GEFs that are implicated in intercellular communication. Down-regulation of the latter GEFs differentially enhanced front-to-back propagation of guidance cues through the monolayer and was mirrored by down-regulation of RHOA expression and myosin II activity. Phenotype-based clustering of knockdown behaviors identified RHOA-ARHGEF18 and ARHGEF3-ARHGEF28-ARHGEF11 clusters, indicating that the latter may signal through other RHO-family GTPases. Indeed, knockdown of RHOC produced an intermediate between the two phenotypes. We conclude that for effective collective migration, the RHOA-GEFs → RHOA/C → actomyosin pathways must be optimally tuned to compromise between generation of motility forces and restriction of intercellular communication.

Control of astrocyte morphology by Rho GTPases

Astrocytes modulate and support neuronal and synapse function via numerous mechanisms that often rely on diffusion of signalling molecules, ions or metabolites through extracellular space. As a consequence, the spatial arrangement and the distance between astrocyte processes and neuronal structures are of functional importance. Likewise, changes of astrocyte structure will affect the ability of astrocytes to interact with neurons. In contrast to neurons, where rapid morphology changes are critically involved in many aspects of physiological brain function, a role of astrocyte restructuring in brain physiology is only beginning to emerge. In neurons, small GTPases of the Rho family are powerful initiators and modulators of structural changes. Less is known about the functional significance of these signalling molecules in astrocytes. Here, we review recent experimental evidence for the role of RhoA, Cdc42 and Rac1 in controlling dynamic astrocyte morphology as well as experimental tools and analytical approaches for studying astrocyte morphology changes.

Actomyosin Pulsing in Tissue Integrity Maintenance during Morphogenesis

The actomyosin cytoskeleton is responsible for many changes in cell and tissue shape. For a long time, the actomyosin cytoskeleton has been known to exhibit dynamic contractile behavior. Recently, discrete actomyosin assembly/disassembly cycles have also been observed in cells. These so-called actomyosin pulses have been observed in a variety of contexts, including cell polarization and division, and in epithelia, where they occur during tissue contraction, folding, and extension. In epithelia, evidence suggests that actomyosin pulsing, and more generally, actomyosin turnover, is required to maintain tissue integrity during contractile processes. This review explores possible functions for pulsing in the many instances during which pulsing has been observed, and also highlights proposed molecular mechanisms that drive pulsing.

The Salmonella effector SseJ disrupts microtubule dynamics when ectopically expressed in normal rat kidney cells

Salmonella effector protein SseJ is secreted by Salmonella into the host cell cytoplasm where it can then modify host cell processes. Whilst host cell small GTPase RhoA has previously been shown to activate the acyl-transferase activity of SseJ we show here an un-described effect of SseJ protein production upon microtubule dynamism. SseJ prevents microtubule collapse and this is independent of SseJ's acyl-transferase activity. We speculate that the effects of SseJ on microtubules would be mediated via its known interactions with the small GTPases of the Rho family.

Paxillin: a crossroad in pathological cell migration

Paxilllin is a multifunctional and multidomain focal adhesion adapter protein which serves an important scaffolding role at focal adhesions by recruiting structural and signaling molecules involved in cell movement and migration, when phosphorylated on specific Tyr and Ser residues. Upon integrin engagement with extracellular matrix, paxillin is phosphorylated at Tyr31, Tyr118, Ser188, and Ser190, activating numerous signaling cascades which promote cell migration, indicating that the regulation of adhesion dynamics is under the control of a complex display of signaling mechanisms. Among them, paxillin disassembly from focal adhesions induced by extracellular regulated kinase (ERK)-mediated phosphorylation of serines 106, 231, and 290 as well as the binding of the phosphatase PEST to paxillin have been shown to play a key role in cell migration. Paxillin also coordinates the spatiotemporal activation of signaling molecules, including Cdc42, Rac1, and RhoA GTPases, by recruiting GEFs, GAPs, and GITs to focal adhesions. As a major participant in the regulation of cell movement, paxillin plays distinct roles in specific tissues and developmental stages and is involved in immune response, epithelial morphogenesis, and embryonic development. Importantly, paxillin is also an essential player in pathological conditions including oxidative stress, inflammation, endothelial cell barrier dysfunction, and cancer development and metastasis.

A RhoG-mediated signaling pathway that modulates invadopodia dynamics in breast cancer cells

One of the hallmarks of cancer is the ability of tumor cells to invade surrounding tissues and metastasize. During metastasis, cancer cells degrade the extracellular matrix, which acts as a physical barrier, by developing specialized actin-rich membrane protrusion structures called invadopodia. The formation of invadopodia is regulated by Rho GTPases, a family of proteins that regulates the actin cytoskeleton. Here, we describe a novel role for RhoG in the regulation of invadopodia disassembly in human breast cancer cells. Our results show that RhoG and Rac1 have independent and opposite roles in the regulation of invadopodia dynamics. We also show that SGEF (also known as ARHGEF26) is the exchange factor responsible for the activation of RhoG during invadopodia disassembly. When the expression of either RhoG or SGEF is silenced, invadopodia are more stable and have a longer lifetime than in control cells. Our findings also demonstrate that RhoG and SGEF modulate the phosphorylation of paxillin, which plays a key role during invadopodia disassembly. In summary, we have identified a novel signaling pathway involving SGEF, RhoG and paxillin phosphorylation, which functions in the regulation of invadopodia disassembly in breast cancer cells.

Assessment of roles for the Rho-specific guanine nucleotide dissociation inhibitor Ly-GDI in platelet function: a spatial systems approach

On activation at sites of vascular injury, platelets undergo morphological alterations essential to hemostasis via cytoskeletal reorganizations driven by the Rho GTPases Rac1, Cdc42, and RhoA. Here we investigate roles for Rho-specific guanine nucleotide dissociation inhibitor proteins (RhoGDIs) in platelet function. We find that platelets express two RhoGDI family members, RhoGDI and Ly-GDI. Whereas RhoGDI localizes throughout platelets in a granule-like manner, Ly-GDI shows an asymmetric, polarized localization that largely overlaps with Rac1 and Cdc42 as well as microtubules and protein kinase C (PKC) in platelets adherent to fibrinogen. Antibody interference and platelet spreading experiments suggest a specific role for Ly-GDI in platelet function. Intracellular signaling studies based on interactome and pathways analyses also support a regulatory role for Ly-GDI, which is phosphorylated at PKC substrate motifs in a PKC-dependent manner in response to the platelet collagen receptor glycoprotein (GP) VI-specific agonist collagen-related peptide. Additionally, PKC inhibition diffuses the polarized organization of Ly-GDI in spread platelets relative to its colocalization with Rac1 and Cdc42. Together, our results suggest a role for Ly-GDI in the localized regulation of Rho GTPases in platelets and hypothesize a link between the PKC and Rho GTPase signaling systems in platelet function.

ERK2-ZEB1-miR-101-1 axis contributes to epithelial-mesenchymal transition and cell migration in cancer

Regulation of metastasis continues to remain enigmatic despite our improved understanding of cancer. Identification of microRNAs associated with metastasis in the recent past has provided a new hope. Here, we show how microRNA-101 (miR-101) regulates two independent processes of cellular metastasis by targeting pro-metastatic upstream regulatory transcription factors, ZEB1 and ZEB2, and downstream effector-actin modulators, RHOA and RAC1, providing a single target for therapeutic intervention. Further, we depict how down-regulation of miR-101 by extracellular signal-regulated kinase-2 (ERK2) is vital for MAP kinase pathway induced cellular migration and mesenchymal transition. Importantly, EKR2 induced expression of ZEB1 seems essential for down-regulation of miR-101-1 and induction of EMT. Given the role of EMT in metastasis, we also observe a significant correlation between miR-101 expression and lymph node metastasis; and identify the ERK2-ZEB1-miR-101-1 pathway active in breast cancer tissues, with an apparent clinicopathological implication.

S100A8 protein attenuates airway hyperresponsiveness by suppressing the contraction of airway smooth muscle

Airway hyperresponsiveness (AHR) is a major clinical problem in allergic asthma mainly caused by the hypercontractility of airway smooth muscles (ASM). S100A8 is an important member of the S100 calcium-binding protein family with a potential to regulate cell contractility. Here, we analyze the potential of S100A8 to regulate allergen-induced AHR and ASM contraction. Treatment with recombinant S100A8 (rS100A8) diminished airway hyperresponsiveness in OVA-sensitized rats. ASM contraction assays showed that rS100A8 reduced hypercontractility in both isolated tracheal rings and primary ASM cells treated by acetylcholine. rS100A8 markedly rescued the phosphorylation level of myosin light chain induced by acetylcholine in ASM cells. These results show that rS100A8 plays a protective role in regulating AHR in asthma by inhibiting ASM contraction. These results support S100A8 as a novel therapeutic target to control ASM contraction in asthma.

Hippo kinases maintain polarity during directional cell migration in Caenorhabditis elegans

Precise positioning of cells is crucial for metazoan development. Despite immense progress in the elucidation of the attractive cues of cell migration, the repulsive mechanisms that prevent the formation of secondary leading edges remain less investigated. Here, we demonstrate that Caenorhabditis elegans Hippo kinases promote cell migration along the anterior-posterior body axis via the inhibition of dorsal-ventral (DV) migration. Ectopic DV polarization was also demonstrated in gain-of-function mutant animals for C. elegans RhoG MIG-2. We identified serine 139 of MIG-2 as a novel conserved Hippo kinase phosphorylation site and demonstrated that purified Hippo kinases directly phosphorylate MIG-2^S139 Live imaging analysis of genome-edited animals indicates that MIG-2^S139 phosphorylation impedes actin assembly in migrating cells. Intriguingly, Hippo kinases are excluded from the leading edge in wild-type cells, while MIG-2 loss induces uniform distribution of Hippo kinases. We provide evidence that Hippo kinases inhibit RhoG activity locally and are in turn restricted to the cell body by RhoG-mediated polarization. Therefore, we propose that the Hippo-RhoG feedback regulation maintains cell polarity during directional cell motility.

Spatial Rho regulation: Molecular mechanisms controlling the GAP protein DLC3

The spatial regulation of cellular Rho signaling by GEF and GAP proteins and the molecular mechanisms controlling the Rho regulators themselves are still incompletely understood. We previously reported that the poorly characterized RhoGAP protein DLC3 localizes to cell-cell adhesions and Rab8-positive membrane tubules. However, it was unclear how DLC3 is targeted to these subcellular sites to execute its functions. In our recent work, protein partners of DLC3 were identified by mass spectrometry, identifying the basolateral polarity protein Scribble as a scaffold for DLC3 at cell-cell contacts. We found that the PDZ-mediated interaction of DLC3 and Scribble is essential for junctional DLC3 recruitment and its role as a local regulator of RhoA-ROCK signaling controlling adherens junction integrity and Scribble localization. Furthermore, DLC3 and Scribble depletion interfered with polarized lumen formation in a 3D model of cyst morphogenesis, emphasizing the relevance of both proteins in epithelial polarity. These findings reveal a new mechanism for spatial Rho regulation at adherens junctions in polarized epithelial cells and highlight the necessity to investigate DLC3 localization and function also in cellular contexts that require cell junction remodeling.

Intercellular Extensions Are Induced by the Alphavirus Structural Proteins and Mediate Virus Transmission

Alphaviruses are highly organized enveloped RNA viruses with an internal nucleocapsid surrounded by a membrane containing the E2 and E1 transmembrane proteins. Alphavirus budding takes place at the plasma membrane and requires the interaction of the cytoplasmic domain of E2 with the capsid protein. Here we used WT alphaviruses and Sindbis virus in which E2 was fused to a fluorescent protein to characterize virus exit from host cells. Our results show that alphavirus infection induced striking modifications of the host cell cytoskeleton and resulted in the formation of stable intercellular extensions that emanated exclusively from the infected cell. The intercellular extensions were long (> 10 μM), contained actin and tubulin, and formed flattened contacts with neighboring cells, but did not mediate membrane or cytoplasmic continuity between cells. Receptor down-regulation studies indicated that formation of stable extensions did not require the virus receptor, and that extensions promoted cell-to-cell virus transmission to receptor-depleted cells. Virus mutant experiments demonstrated that formation of extensions required the E2-capsid interaction but not active particle budding, while intercellular transmission of infection required the production of fusion-active virus particles. Protein expression studies showed that even in the absence of virus infection, the viral structural proteins alone induced intercellular extensions, and that these extensions were preferentially targeted to non-expressing cells. Together, our results identify a mechanism for alphavirus cell-to-cell transmission and define the key viral protein interactions that it requires.

Alphaviruses are a group of small enveloped RNA viruses that include a number of important human pathogens such as Chikungunya virus and viruses that cause fatal encephalitis. Chikungunya virus emerged recently in a number of countries worldwide including the Americas, where it has caused major outbreaks. Vaccines and anti-viral strategies for these viruses are urgently needed, and basic information on the alphavirus infection pathway will help in targeting critical steps. Here we describe the changes in the alphavirus-infected cell that allow it to transmit virus to neighboring uninfected cells. Infected cells form long extensions that contact neighboring cells and mediate cell-to-cell virus transmission. This mechanism of virus transmission may help to shield virus from neutralization by host antibodies. Surprisingly, expression of the viral structural proteins alone induces these intercellular extensions, which preferentially target non-expressing cells. We used this system to define a critical interaction of the capsid and envelope protein that is required for formation of extensions.

Targeting High Dynamin-2 (DNM2) Expression by Restoring Ikaros Function in Acute Lymphoblastic Leukemia

Dynamin-2 (DNM2) is a GTPase essential for intracellular vesicle formation and trafficking, cytokinesis and receptor endocytosis. Mutations in DNM2 are common in early T-cell precursor acute lymphoblastic leukemia. However, DNM2 expression in other types of ALL are not reported. We studied DNM2 mRNA level in adults with B- and T-cell ALL. We found DNM2 is more highly expressed compared with normals in both forms of ALL. High DNM2 expression is associated with some clinical and laboratory features, inferior outcomes and with leukaemia cell proliferation. We also found Ikaros directly binds the DNM2 promoter and suppresses DNM2 expression. Consequently IKZF1 deletion is associated with high DNM2 expression. Conversely, casein kinase-2 (CK2)-inhibitor increases Ikaros function thereby inhibiting DNM2 expression. Inhibiting DNM2 suppresses proliferation of leukemia cells and synergizes with CK2 inhibition. Our data indicate high DNM2 expression is associated with Ikaros dysregulation and may be important in the development of B-ALL.

The role of Rho kinase (Rock) in re-epithelialization of adult zebrafish skin wounds

Complete re-epithelialization of full-thickness skin wounds in adult mammals takes days to complete and relies on numerous signaling cues and multiple overlapping cellular processes that take place both within the epidermis itself and in other participating tissues. We have previously shown that re-epithelialization of full-thickness skin wounds of adult zebrafish, however, is extremely rapid and largely independent of the other processes of wound healing allowing for the dissection of specific processes that occur in, or have a direct effect on, re-epithelializing keratinocytes. Recently, we have shown that, in addition to lamellipodial crawling at the leading edge, re-epithelialization of zebrafish partial- and full-thickness wounds requires long-range epithelial rearrangements including radial intercalations, flattening and directed elongation and that each of these processes involves Rho kinase (Rock) signaling. Our studies demonstrate how these coordinated signaling events allow for the rapid collective cell migration observed in adult zebrafish wound healing. Here we discuss the particular contribution of Rock to each of these processes.

Rho-Mancing to Sensitize Calcium Signaling for Contraction in the Vasculature: Role of Rho Kinase

Vascular smooth muscle contraction is an important physiological process contributing to cardiovascular homeostasis. The principal determinant of smooth muscle contraction is the intracellular free Ca²⁺ concentration, and phosphorylation of myosin light chain (MLC) by activated myosin light chain kinase (MLCK) in response to increased Ca²⁺ is the main pathway by which vasoconstrictor stimuli induce crossbridge cycling of myosin and actin filaments. A secondary pathway for vascular smooth muscle contraction that is not directly dependent on Ca²⁺ concentration, but rather mediating Ca²⁺ sensitization, is the RhoA/Rho kinase pathway. In response to contractile stimuli, the small GTPase RhoA activates its downstream effector Rho kinase which, in turn, promotes contraction via myosin light chain phosphatase (MLCP) inhibition. RhoA/Rho kinase-mediated MLCP inhibition occurs mainly by phosphorylation and inhibition of MYPT1, the regulatory subunit of MLCP, or by CPI-17-mediated inhibition of the catalytic subunit of MLCP. In this review, we describe the molecular mechanisms underlying the pivotal role exerted by Rho kinase on vascular smooth muscle contraction and discuss the main regulatory pathways for its activity.

Endothelin-1 receptor drives invadopodia: Exploiting how β-arrestin-1 guides the way

Metastatization is a complex multistep process requiring fine-tuned regulated cytoskeleton re-modeling, mediated by the cross-talk of actin with interacting partners, such as the Rho GTPases. Our expanding knowledge of invadopodia, small invasive membrane protrusions composed of a core of F-actin, actin regulators and actin-binding proteins, and hotspots for secretion of extracellular matrix (ECM) proteinases, contributes to clarify critical steps of the metastatic program. Growth factor receptors and their intermediate signaling molecules, along with matrix adhesion and rigidity, pH and hypoxia, act as drivers of cytoskeleton changes and invadopodia formation. We recently pro-posed a novel route map by which cancer cells regulates invadopodia dynamics supporting metastasis as response to the endothelin A receptor (ET_AR), among the highly druggable G-protein coupled receptors in cancer. The metastatic behavior exhibited by ovarian cancer cells overe-xpressing ET_AR is now explained by the interplay with β-arrestin1 (β-arr1), a scaffold protein acting as signal-integrating module of RhoC and cofilin signaling for specific invadopodia formation, accomplished by its interaction with a Rho guanine nucleotide exchange factor (GEF), PDZ-RhoGEF, in a G-protein independent manner. Here, we summarize this novel activation of the RhoC pathway from ET_AR/β-arr1 signaling that may be exploited therapeutically and discuss new perspectives for future directions of investigations.

Rho GTPases: Regulation and roles in cancer cell biology

Rho GTPases are well known for their roles in regulating cell migration, and also contribute to a variety of other cellular responses. They are subdivided into 2 groups: typical and atypical. The typical Rho family members, including RhoA, Rac1 and Cdc42, cycle between an active GTP-bound and inactive GDP-bound conformation, and are regulated by GEFs, GAPs and GDIs, whereas atypical Rho family members have amino acid substitutions that alter their ability to interact with GTP/GDP and hence are regulated by different mechanisms. Both typical and atypical Rho GTPases contribute to cancer progression. In a few cancers, RhoA or Rac1 are mutated, but in most cancers expression levels and/or activity of Rho GTPases is altered. Rho GTPase signaling could therefore be therapeutically targeted in cancer treatment.

Rho GTPases operating at the Golgi complex: Implications for membrane traffic and cancer biology

The Golgi complex is the central unit of the secretory pathway, modifying, processing and sorting proteins and lipids to their correct cellular localisation. Changes to proteins at the Golgi complex can have deleterious effects on the function of this organelle, impeding trafficking routes through it, potentially resulting in disease. It is emerging that several Rho GTPase proteins, namely Cdc42, RhoBTB3, RhoA and RhoD are at least in part localised to the Golgi complex, and a number of studies have shown that dysregulation of their levels or activity can be associated with cellular changes which ultimately drive cancer progression. In this mini-review we highlight some of the recent work that explores links between form and function of the Golgi complex, Rho GTPases and cancer.

Cycling Rho for tissue contraction

Cell contractility, driven by the RhoA GTPase, is a fundamental determinant of tissue morphogenesis. In this issue, Mason et al. (2016. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201603077) reveal that cyclic inactivation of RhoA, mediated by its antagonist, C-GAP, is essential for effective contractility to occur.