Regulating Rho GTPases and their regulators

FBXW7 regulates endothelial barrier function by suppression of the cholesterol synthesis pathway and prenylation of RhoB

Rho GTPases control both the actin cytoskeleton and adherens junction stability and are recognized as essential regulators of endothelial barrier function. They act as molecular switches and are primarily regulated by the exchange of GDP and GTP. However, posttranslational modifications such as phosphorylation, prenylation, and ubiquitination can additionally alter their localization, stability, and activity. F-box proteins are involved in the recognition of substrate proteins predestined for ubiquitination and subsequent degradation. Given the importance of ubiquitination, we studied the effect of the loss of 62 members of the F-box protein family on endothelial barrier function in human umbilical vein endothelial cells. Endothelial barrier function was quantified by electrical cell impedance sensing and macromolecule passage assay. Our RNA interference–based screen identified FBXW7 as a key regulator of endothelial barrier function. Mechanistically, loss of FBXW7 induced the accumulation of the RhoB GTPase in endothelial cells, resulting in their increased contractility and permeability. FBXW7 knockdown induced activation of the cholesterol biosynthesis pathway and changed the prenylation of RhoB. This effect was reversed by farnesyl transferase inhibitors and by the addition of geranylgeranyl pyrophosphate. In summary, this study identifies FBXW7 as a novel regulator of endothelial barrier function in vitro. Loss of FBXW7 indirectly modulates RhoB activity via alteration of the cholesterol biosynthesis pathway and, consequently, of the prenylation status and activity of RhoB, resulting in increased contractility and disruption of the endothelial barrier.

Emerging Themes in PDZ Domain Signaling: Structure, Function, and Inhibition

Post-synaptic density-95, disks-large and zonula occludens-1 (PDZ) domains are small globular protein-protein interaction domains widely conserved from yeast to humans. They are composed of ∼90 amino acids and form a classical two α-helical/six β-strand structure. The prototypical ligand is the C-terminus of partner proteins; however, they also bind internal peptide sequences. Recent findings indicate that PDZ domains also bind phosphatidylinositides and cholesterol. Through their ligand interactions, PDZ domain proteins are critical for cellular trafficking and the surface retention of various ion channels. In addition, PDZ proteins are essential for neuronal signaling, memory, and learning. PDZ proteins also contribute to cytoskeletal dynamics by mediating interactions critical for maintaining cell-cell junctions, cell polarity, and cell migration. Given their important biological roles, it is not surprising that their dysfunction can lead to multiple disease states. As such, PDZ domain-containing proteins have emerged as potential targets for the development of small molecular inhibitors as therapeutic agents. Recent data suggest that the critical binding function of PDZ domains in cell signaling is more than just glue, and their binding function can be regulated by phosphorylation or allosterically by other binding partners. These studies also provide a wealth of structural and biophysical data that are beginning to reveal the physical features that endow this small modular domain with a central role in cell signaling.

Engineering of Human Corneal Endothelial Cells In Vitro

Human corneal endothelial cells are responsible for controlling corneal transparency, however they are notorious for their limited proliferative capability. Thus, damage to these cells may cause irreversible blindness. Currently, the only way to cure blindness caused by corneal endothelial dysfunction is via corneal transplantation of a cadaver donor cornea with healthy corneal endothelium. Due to severe shortage of donor corneas worldwide, it has become paramount to develop human corneal endothelial grafts in vitro that can subsequently be transplanted in humans. Recently, we have reported effective expansion of human corneal endothelial cells by reprogramming the cells into progenitor status through use of p120-Kaiso siRNA knockdown. This new reprogramming approach circumvents the need of using induced pluripotent stem cells or embryonic stem cells. Successful promotion of this technology will encourage scientists to re-think how "contact inhibition" can safely be perturbed to our benefit, i.e., effective engineering of an in vivo-like tissue while successful maintaining the normal phenotype. In this review, we present current advances in reprogramming corneal endothelial cells in vitro, detail the methods to successful engineer human corneal endothelial grafts, and discuss their future clinical applications to cure corneal blindness.

NEDD4 Family of E3 Ubiquitin Ligases in Breast Cancer: Spotlight on SMURFs, WWPs and NEDD4

Massively parallel sequencing, genomic and proteomic technologies have provided near complete resolution of signaling landscape of breast cancer (BCa). NEDD4 family of E3-ubiquitin ligases comprises a large family of proteins particularly, SMURFs (SMURF1, SMURF2), WWPs and NEDD4 which are ideal candidates for targeted therapy. However, it is becoming progressively more understandable that SMURFs and NEDD4 have "split-personalities". These molecules behave dualistically in breast cancer and future studies must converge on detailed identification of context specific role of these proteins in BCa. Finally, we provide scattered clues of regulation of SMURF2 by oncogenic miRNAs, specifically considering longstanding questions related to regulation of SMURF1 and WWPs by miRNAs in BCa. SMURFS, WWPs and NEDD4 are versatile regulators and represent a fast-growing field in cancer research and better understanding of the underlying mechanisms will be helpful in transition of our knowledge from a segmented view to a more conceptual continuum.

A role for Gic1 and Gic2 in Cdc42 polarization at elevated temperature

The conserved Rho-family GTPase Cdc42 is a master regulator of polarity establishment in many cell types. Cdc42 becomes activated and concentrated in a region of the cell cortex, and recruits a variety of effector proteins to that site. In turn, many effectors participate in regulation of cytoskeletal elements in order to remodel the cytoskeleton in a polarized manner. The budding yeast Saccharomyces cerevisiae has served as a tractable model system for studies of cell polarity. In yeast cells, Cdc42 polarization involves a positive feedback loop in which effectors called p21-activated kinases (PAKs) act to recruit a Cdc42-directed guanine nucleotide exchange factor (GEF), generating more GTP-Cdc42 in areas that already have GTP-Cdc42. The GTPase-interacting components (GICs) Gic1 and Gic2 are also Cdc42 effectors, and have been implicated in regulation of the actin and septin cytoskeleton. However, we report that cells lacking GICs are primarily defective in polarizing Cdc42 itself, suggesting that they act upstream as well as downstream of Cdc42 in yeast. Our findings suggest that feedback pathways involving GTPase effectors may be more prevalent than had been appreciated.

Rac GTPases in Hematological Malignancies

Emerging evidence suggests that crosstalk between hematologic tumor cells and the tumor microenvironment contributes to leukemia and lymphoma cell migration, survival, and proliferation. The supportive tumor cell-microenvironment interactions and the resulting cellular processes require adaptations and modulations of the cytoskeleton. The Rac subfamily of the Rho family GTPases includes key regulators of the cytoskeleton, with essential functions in both normal and transformed leukocytes. Rac proteins function downstream of receptor tyrosine kinases, chemokine receptors, and integrins, orchestrating a multitude of signals arising from the microenvironment. As such, it is not surprising that deregulation of Rac expression and activation plays a role in the development and progression of hematological malignancies. In this review, we will give an overview of the specific contribution of the deregulation of Rac GTPases in hematologic malignancies.

The RhoGEF protein Plekhg5 regulates apical constriction of bottle cells during gastrulation

Apical constriction regulates epithelial morphogenesis during embryonic development, but how this process is controlled is not understood completely. Here, we identify a Rho guanine nucleotide exchange factor (GEF) gene plekhg5 as an essential regulator of apical constriction of bottle cells during Xenopus gastrulation. plekhg5 is expressed in the blastopore lip and its expression is sufficient to induce ectopic bottle cells in epithelia of different germ layers in a Rho-dependent manner. This activity is not shared by arhgef3, which encodes another organizer-specific RhoGEF. Plekhg5 protein is localized in the apical cell cortex via its pleckstrin homology domain, and the GEF activity enhances its apical recruitment. Plekhg5 induces apical actomyosin accumulation and cell elongation. Knockdown of plekhg5 inhibits activin-induced bottle cell formation and endogenous blastopore lip formation in gastrulating frog embryos. Apical accumulation of actomyosin, apical constriction and bottle cell formation fail to occur in these embryos. Taken together, our data indicate that transcriptional regulation of plekhg5 expression at the blastopore lip determines bottle cell morphology via local polarized activation of Rho by Plekhg5, which stimulates apical actomyosin activity to induce apical constriction.

Branching mechanisms shaping dendrite architecture

A neuron's contribution to the information flow within a neural circuit is governed by the structure of its dendritic arbor. The geometry of the dendritic arbor directly determines synaptic density and the size of the receptive field, both of which influence the firing pattern of the neuron. Importantly, the position of individual dendritic branches determines the identity of the neuron's presynaptic partner and thus the nature of the incoming sensory information. To generate the unique stereotypic architecture of a given neuronal subtype, nascent branches must emerge from the dendritic shaft at preprogramed branch points. Subsequently, a complex array of extrinsic factors regulates the degree and orientation of branch expansion to ensure maximum coverage of the receptive field whilst constraining growth within predetermined territories. In this review we focus on studies that best illustrate how environmental cues such as the Wnts and Netrins and their receptors sculpt the dendritic arbor. We emphasize the pivotal role played by the actin cytoskeleton and its upstream regulators in branch initiation, outgrowth and navigation. Finally, we discuss how protocadherin and DSCAM contact-mediated repulsion prevents inappropriate synapse formation between sister dendrites or dendrites and the axon from the same neuron. Together these studies highlight the clever ways evolution has solved the problem of constructing complex branch geometries.

Opening up about Tissue Transglutaminase: When Conformation Matters More than Enzymatic Activity

Tissue transglutaminase (tTG), also referred to as type 2 transglutaminase or Gα_h, can bind and hydrolyze GTP, as well as function as a protein crosslinking enzyme. tTG is widely expressed and can be detected both inside cells and in the extracellular space. In contrast to many enzymes, the active and inactive conformations of tTG are markedly different. The catalytically inactive form of tTG adopts a compact “closed-state” conformation, while the catalytically active form of the protein adopts an elongated “open-state” conformation. tTG has long been appreciated as an important player in numerous diseases, including celiac disease, neuronal degenerative diseases, and cancer, and its roles in these diseases often depend as much upon its conformation as its catalytic activity. While its ability to promote these diseases has been traditionally thought to be dependent on its protein crosslinking activity, more recent findings suggest that the conformational state tTG adopts is also important for mediating its effects. In particular, we and others have shown that the closed-state of tTG is important for promoting cell growth and survival, while maintaining tTG in the open-state is cytotoxic. In this review, we examine the two unique conformations of tTG and how they contribute to distinct biological processes. We will also describe how this information can be used to generate novel therapies to treat diseases, with a special focus on cancer.

Resistance protein Pit interacts with the GEF OsSPK1 to activate OsRac1 and trigger rice immunity

Resistance (R) genes encode intracellular nucleotide-binding/leucine-rich repeat-containing (NLR) family proteins that serve as critical plant immune receptors to induce effector-triggered immunity (ETI). NLR proteins possess a tripartite domain architecture consisting of an N-terminal variable region, a central nucleotide-binding domain, and a C-terminal leucine-rich repeat. N-terminal coiled-coil (CC) or Toll-interleukin 1 receptor (TIR) domains of R proteins appear to serve as platforms to trigger immune responses, because overexpression of the CC or TIR domain of some R proteins is sufficient to induce an immune response. Because direct downstream signaling molecules of R proteins remain obscure, the molecular mechanisms by which R proteins regulate downstream signaling are largely unknown. We reported previously that a rice R protein named Pit triggers ETI through a small GTPase, OsRac1, although how Pit activates OsRac1 is unclear. Here, we identified OsSPK1, a DOCK family guanine nucleotide exchange factor, as an interactor of Pit and activator for OsRac1. OsSPK1 contributes to signaling by two disease-resistance genes, Pit and Pia, against the rice blast fungus Magnaporthe oryzae and facilitates OsRac1 activation in vitro and in vivo. The CC domain of Pit is required for its binding to OsSPK1, OsRac1 activation, and the induction of cell death. Overall, we conclude that OsSPK1 is a direct and key signaling target of Pit-mediated immunity. Our results shed light on how R proteins trigger ETI through direct downstream molecules.

The Intermolecular Interaction of Ephexin4 Leads to Autoinhibition by Impeding Binding of RhoG

Ephexin4 is a guanine nucleotide-exchange factor (GEF) for RhoG and is involved in various RhoG-related cellular processes such as phagocytosis of apoptotic cells and migration of cancer cells. Ephexin4 forms an oligomer via an intermolecular interaction, and its GEF activity is increased in the presence of Elmo, an Ephexin4-interacting protein. However, it is uncertain if and how Ephexin4 is autoinhibited. Here, using an Ephexin4 mutant that abrogated the intermolecular interaction, we report that this interaction impeded binding of RhoG to Ephexin4 and thus inhibited RhoG activation. Mutation of the glutamate residue at position 295, which is a highly conserved residue located in the region of Ephexin4 required for the intermolecular interaction, to alanine (Ephexin4^E295A) disrupted the intermolecular interaction and increased binding of RhoG, resulting in augmented RhoG activation. In addition, phagocytosis of apoptotic cells and formation of membrane ruffles were increased more by expression of Ephexin4^E295A than by expression of wild-type Ephexin4. Taken together, our data suggest that Ephexin4 is autoinhibited through its intermolecular interaction, which impedes binding of RhoG.

Kalirin/Trio Rho GDP/GTP exchange factors regulate proinsulin and insulin secretion

Key features for progression to pancreatic β-cell failure and disease are loss of glucose responsiveness and an increased ratio of secreted proinsulin to insulin. Proinsulin and insulin are stored in secretory granules (SGs) and the fine-tuning of hormone output requires signal mediated recruitment of select SG populations according to intracellular location and age. The GTPase Rac1 coordinates multiple signaling pathways that specify SG release and Rac1 activity is controlled in part by GDP/GTP exchange factors (GEFs). To explore the function of two large multidomain GEFs, Kalirin and Trio in β-cells, we manipulated their Rac1-specific GEF1 domain activity by using small molecule inhibitors and by genetically ablating Kalirin. We examined age related secretory granule behavior employing radiolabeling protocols. Loss of Kalirin/Trio function attenuated radioactive proinsulin release by reducing constitutive-like secretion and exocytosis of 2-hour old granules. At later chase times or at steady state, Kalirin/Trio manipulations decreased glucose stimulated insulin output. Finally, use of a Rac1 FRET biosensor with cultured β-cell lines, demonstrated that Kalirin/Trio GEF1 activity was required for normal rearrangement of Rac1 to the plasma membrane in response to glucose. Rac1 activation can be evoked by both glucose metabolism and signaling through the incretin glucagon-like peptide 1 (GLP-1) receptor. GLP-1 addition restored Rac1 localization/activity and insulin secretion in the absence of Kalirin, thereby assigning Kalirin's participation to stimulatory glucose signaling.

Aristolochic acid inhibits Slit2-induced migration and tube formation via inactivation of Robo1/Robo2-NCK1/NCK2 signaling pathway in human umbilical vein endothelial cells

Robo1/Robo2-NCK1/NCK2 signaling pathway controls endothelial cell sprouting and migration induced by Slit2 or VEGF, but whether it is involved in peritubular capillary (PTC) rarefaction of Aristolochic acid nephropathy (AAN) is unclear. In the present study, we evaluated whether AA exerts antiangiogenic effects by targeting this signaling pathways in HUVECs. HUVECs or lentivirus-mediated NCK1-overexpressing HUVECs were stimulated with AA (1, 2 or 3 μg/ml) in the absence or presence of 6 nM Slit2. Our results showed that AAІ (1-3 μg/ml) dose-dependently inhibited the migration and tube formation of HUVECs. This inhibition was in parallel with down-regulated mRNA and protein expression of Slit2/Robo1/Robo2-NCK1/NCK2 signaling pathway. Importantly, overexpression of NCK1 rescued AAІ-impaired angiogenesis, as evidenced by the increase of cell migration and tube formation of HUVECs in response to Slit2. The down-regulation of NCK2 and decreased activation of Rac1 was also restored by overexpression of NCK1. Taken together, our findings show that AA inhibits Slit2-induced migration and tube formation via inactivation of Robo1/Robo2-NCK1/NCK2 signaling pathway in HUVECs, and NCK1 might be a potential agent for vascular remodeling in AAN and diseases associated with impaired angiogenesis.

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.

Signaling by Small GTPases at Cell-Cell Junctions: Protein Interactions Building Control and Networks

A number of interesting reports highlight the intricate network of signaling proteins that coordinate formation and maintenance of cell-cell contacts. We have much yet to learn about how the in vitro binding data is translated into protein association inside the cells and whether such interaction modulates the signaling properties of the protein. What emerges from recent studies is the importance to carefully consider small GTPase activation in the context of where its activation occurs, which upstream regulators are involved in the activation/inactivation cycle and the GTPase interacting partners that determine the intracellular niche and extent of signaling. Data discussed here unravel unparalleled cooperation and coordination of functions among GTPases and their regulators in supporting strong adhesion between cells.

Compartmentalisation of RAC1 signalling

RAC1 signalling has been implicated in a variety of dynamic cell biological processes that are orchestrated through regulated localisation and activation of RAC1. As a small GTPase, RAC1 switches between active and inactive states at various subcellular locations that include the plasma membrane, nucleus and mitochondria. Once activated, RAC1 interacts with a range of effectors that then mediate various biological functions. RAC1 is regulated by a large number of proteins that can promote its recruitment, activation, deactivation, or stability. RAC1 and its regulators are subject to various post-translational modifications that further fine tune RAC1 localisation, levels and activity. Developments in technologies have enabled the accurate detection of activated RAC1 during processes such as cell migration, invasion and DNA damage. Here, we highlight recent advances in our understanding of RAC1 regulation and function at specific subcellular sites.

Ovarian Tumor Microenvironment Signaling: Convergence on the Rac1 GTPase

The tumor microenvironment for epithelial ovarian cancer is complex and rich in bioactive molecules that modulate cell-cell interactions and stimulate numerous signal transduction cascades. These signals ultimately modulate all aspects of tumor behavior including progression, metastasis and therapeutic response. Many of the signaling pathways converge on the small GTPase Ras-related C3 botulinum toxin substrate (Rac)1. In addition to regulating actin cytoskeleton remodeling necessary for tumor cell adhesion, migration and invasion, Rac1 through its downstream effectors, regulates cancer cell survival, tumor angiogenesis, phenotypic plasticity, quiescence, and resistance to therapeutics. In this review we discuss evidence for Rac1 activation within the ovarian tumor microenvironment, mechanisms of Rac1 dysregulation as they apply to ovarian cancer, and the potential benefits of targeting aberrant Rac1 activity in this disease. The potential for Rac1 contribution to extraperitoneal dissemination of ovarian cancer is addressed.

Fam65b Phosphorylation Relieves Tonic RhoA Inhibition During T Cell Migration

We previously identified Fam65b as an atypical inhibitor of the small G protein RhoA. Using a conditional model of a Fam65b-deficient mouse, we first show that Fam65b restricts spontaneous RhoA activation in resting T lymphocytes and regulates intranodal T cell migration in vivo. We next aimed at understanding, at the molecular level, how the brake that Fam65b exerts on RhoA can be relieved upon signaling to allow RhoA activation. Here, we show that chemokine stimulation phosphorylates Fam65b in T lymphocytes. This post-translational modification decreases the affinity of Fam65b for RhoA and favors Fam65b shuttling from the plasma membrane to the cytosol. Functionally, we show that the degree of Fam65b phosphorylation controls some cytoskeletal alterations downstream active RhoA such as actin polymerization, as well as T cell migration in vitro. Altogether, our results show that Fam65b expression and phosphorylation can finely tune the amount of active RhoA in order to favor optimal T lymphocyte motility.

Loss of sterol metabolic homeostasis triggers inflammasomes - how and why

Proper regulation of sterol biosynthesis is critical for eukaryotic cellular homeostasis. Cholesterol and isoprenoids serve key roles in eukaryotic cells by regulating membrane fluidity and correct localization of proteins. It is becoming increasingly appreciated that dysregulated sterol metabolism engages pathways that lead to inflammation. Of particular importance are inflammasomes, which are multiplatform protein complexes that activate caspase-1 in order to process the pro-inflammatory and pyrogenic cytokines IL-1β and IL-18. In this review, we highlight recent research that links altered sterol biosynthetic pathway activity to inflammasome activation. We discuss how clues from human genetics have led to new insights into how alterations in isoprenoid biosynthesis connect to inflammation. We also discuss new mechanisms that show how macrophage cholesterol buildup can lead to inflammasome activation.

EdgeProps: A Computational Platform for Correlative Analysis of Cell Dynamics and Near-Edge Protein Activity

Developing molecular tools to visualize and control Rho GTPase signaling in living cells has been instrumental in elucidating the mechanisms of cytoskeletal reorganization and causal relationships between activation events in cell function. An indispensable part of such studies is the quantitative characterization of the spatiotemporal GTPase activity. Here we present a computational pipeline, EdgeProps, designed for comparative/correlative analysis of cell dynamics (edge velocity) and near-edge protein activity (intensity of a fluorescent signal). The tool offers a user-friendly interface with three functional modules for processing, visualization, and statistical characterization of single-cell imaging data.

A Rho-based reaction-diffusion system governs cell wall patterning in metaxylem vessels

Rho GTPases play crucial roles in cell polarity and pattern formation. ROPs, Rho of plant GTPases, are widely involved in cell wall patterning in plants, yet the molecular mechanism underlying their action remains unknown. Arabidopsis ROP11 is locally activated to form plasma membrane domains, which direct formation of cell wall pits in metaxylem vessel cells through interaction with cortical microtubules. Here, we show that the pattern formation of cell wall pits is governed by ROP activation via a reaction-diffusion mechanism. Genetic analysis and reconstructive assays revealed that ROPGEF4/7 and ROPGAP3/4, which encode ROP activators and inactivators, respectively, regulated the formation of ROP-activated domains; these in turn determined the pattern of cell wall pits. Mathematical modelling showed that ROP-activation cycle generated ROP domains by reaction-diffusion mechanism. The model predicted that a positive feedback and slow diffusion of ROP11-ROPGEF4 complex were required to generate ROP-activated domains. ROPGEF4 formed a dimer that interacted with activated ROP11 in vivo, which could provide positive feedback for ROP activation. ROPGEF4 was highly stable on the plasma membrane and inhibited ROP11 diffusion. Our study indicated that ROP-based reaction-diffusion system self-organizes ROP-activated domains, thereby determines the pit pattern of metaxylem vessels.

KIF13A-regulated RhoB plasma membrane localization governs membrane blebbing and blebby amoeboid cell migration

Membrane blebbing‐dependent (blebby) amoeboid migration can be employed by lymphoid and cancer cells to invade 3D‐environments. Here, we reveal a mechanism by which the small GTPase RhoB controls membrane blebbing and blebby amoeboid migration. Interestingly, while all three Rho isoforms (RhoA, RhoB and RhoC) regulated amoeboid migration, each controlled motility in a distinct manner. In particular, RhoB depletion blocked membrane blebbing in ALL (acute lymphoblastic leukaemia), melanoma and lung cancer cells as well as ALL cell amoeboid migration in 3D‐collagen, while RhoB overexpression enhanced blebbing and 3D‐collagen migration in a manner dependent on its plasma membrane localization and down‐stream effectors ROCK and Myosin II. RhoB localization was controlled by endosomal trafficking, being internalized via Rab5 vesicles and then trafficked either to late endosomes/lysosomes or to Rab11‐positive recycling endosomes, as regulated by KIF13A. Importantly, KIF13A depletion not only inhibited RhoB plasma membrane localization, but also cell membrane blebbing and 3D‐migration of ALL cells. In conclusion, KIF13A‐mediated endosomal trafficking modulates RhoB plasma membrane localization to control membrane blebbing and blebby amoeboid migration.

Local and global Cdc42 guanine nucleotide exchange factors for fission yeast cell polarity are coordinated by microtubules and the Tea1-Tea4-Pom1 axis

The conserved Rho-family GTPase Cdc42 plays a central role in eukaryotic cell polarity. The rod-shaped fission yeast Schizosaccharomyces pombe has two Cdc42 guanine nucleotide exchange factors (GEFs), Scd1 and Gef1, but little is known about how they are coordinated in polarized growth. Although the microtubule cytoskeleton is normally not required for polarity maintenance in fission yeast, we show here that when scd1 function is compromised, disruption of microtubules or the polarity landmark proteins Tea1, Tea4 or Pom1 leads to disruption of polarized growth. Instead, cells adopt an isotropic-like pattern of growth, which we term PORTLI growth. Surprisingly, PORTLI growth is caused by spatially inappropriate activity of Gef1. Although most Cdc42 GEFs are membrane associated, we find that Gef1 is a broadly distributed cytosolic protein rather than a membrane-associated protein at cell tips like Scd1. Microtubules and the Tea1–Tea4–Pom1 axis counteract inappropriate Gef1 activity by regulating the localization of the Cdc42 GTPase-activating protein Rga4. Our results suggest a new model of fission yeast cell polarity regulation, involving coordination of ‘local’ (Scd1) and ‘global’ (Gef1) Cdc42 GEFs via microtubules and microtubule-dependent polarity landmarks.

Highlighted Article: Cell polarity in fission yeast is regulated by two different Cdc42 guanine nucleotide exchange factors, coordinated by the microtubule-dependent landmark system.

Blue-Light Receptors for Optogenetics

Sensory photoreceptors underpin light-dependent adaptations of organismal physiology, development, and behavior in nature. Adapted for optogenetics, sensory photoreceptors become genetically encoded actuators and reporters to enable the noninvasive, spatiotemporally accurate and reversible control by light of cellular processes. Rooted in a mechanistic understanding of natural photoreceptors, artificial photoreceptors with customized light-gated function have been engineered that greatly expand the scope of optogenetics beyond the original application of light-controlled ion flow. As we survey presently, UV/blue-light-sensitive photoreceptors have particularly allowed optogenetics to transcend its initial neuroscience applications by unlocking numerous additional cellular processes and parameters for optogenetic intervention, including gene expression, DNA recombination, subcellular localization, cytoskeleton dynamics, intracellular protein stability, signal transduction cascades, apoptosis, and enzyme activity. The engineering of novel photoreceptors benefits from powerful and reusable design strategies, most importantly light-dependent protein association and (un)folding reactions. Additionally, modified versions of these same sensory photoreceptors serve as fluorescent proteins and generators of singlet oxygen, thereby further enriching the optogenetic toolkit. The available and upcoming UV/blue-light-sensitive actuators and reporters enable the detailed and quantitative interrogation of cellular signal networks and processes in increasingly more precise and illuminating manners.

Regulation of the Hippo pathway in cancer biology

The Hippo tumor suppressor pathway, which is well conserved from Drosophila to humans, has emerged as the master regulator of organ size, as well as major cellular properties, such as cell proliferation, survival, stemness, and tissue homeostasis. The biological significance and deregulation of the Hippo pathway in tumorigenesis have received a surge of interest in the past decade. In the current review, we present the major discoveries that made substantial contributions to our understanding of the Hippo pathway and discuss how Hippo pathway components contribute to cellular signaling, physiology, and their potential implications in anticancer therapeutics.

Multilayered coating of titanium implants promotes coupled osteogenesis and angiogenesis in vitro and in vivo

We used surface-modified titanium (Ti) substrates with a multilayered structure composed of chitosan-catechol (Chi-C), gelatin (Gel) and hydroxyapatite (HA) nanofibers, which were previously shown to improve osteogenesis, as a platform to investigate the interaction of osteogenesis and angiogenesis during bone healing. Combined techniques of Transwell co-culture, wound healing assay, enzyme linked immunosorbent assay (ELISA), quantitative real-time polymerase chain reaction (qRT-PCR), western blotting and immunohistochemical staining were used to evaluate adhesion, morphology and migration of adipose-derived mesenchymal stem cells (Ad-MSCs) and human umbilical vein endothelial cells (HUVECs) grown on different Ti substrates. We investigated the effect of substrates on the osteogenic differentiation of Ad-MSCs and reciprocal paracrine effects of Ad-MSCs on HUVECs or vice versa. The multilayered Ti substrates directly regulated the cellular functions of Ad-MSCs and angiogenic HUVECs and mediated communication between them by enhancing paracrine effects via cell-matrix interactions in vitro. The in vivo results showed that the change of microenvironment induced by surface-modified Ti implants promoted the adhesion, recruitment and proliferation of MSCs and facilitated coupled osteogenesis and angiogenesis in bone healing. The study proved that multilayer-film-coated Ti substrates positively mediated cellular biological function in vitro and improved bone healing in vivo.

STATEMENT OF SIGNIFICANCE

Recent studies have revealed that osteogenesis and angiogenesis are coupled, and that communication between osteoblasts and endothelial cells is essential for bone healing and remodeling processes; however, these conclusions only result from in vitro studies or in vivo studies using transgenic murine models. Relatively little is known about the communication between osteoblasts and endothelial cells in peri-implants during bone healing processes. Our results revealed the cellular/molecular mechanism of how multilayered Ti substrates mediate reciprocal paracrine effects between adipose-derived mesenchymal stem cells and human umbilical vein endothelial cells; moreover, the interactions between the cell-matrix and peri-implant was proven in vivo with enhanced bone healing. This study contributes to our understanding of the fundamental mechanisms of angiogenesis and osteogenesis that affect peri-implantation, and thus, provides new insights into the design of future high-quality orthopedic implants.

CNF1-like deamidase domains: common Lego bricks among cancer-promoting immunomodulatory bacterial virulence factors

Alterations of the cellular proteome over time due to spontaneous or toxin-mediated enzymatic deamidation of glutamine (Gln) and asparagine (Asn) residues contribute to bacterial infection and might represent a source of aging-related diseases. Here, we put into perspective what is known about the mode of action of the CNF1 toxin from pathogenic Escherichia coli, a paradigm of bacterial deamidases that activate Rho GTPases, to illustrate the importance of determining whether exposure to these factors are risk factors in the etiology age-related diseases, such as cancer. In particular, through in silico analysis of the distribution of the CNF1-like deamidase active site Gly-Cys-(Xaa)n-His sequence motif in bacterial genomes, we unveil the wide distribution of the super-family of CNF-like toxins and CNF-like deamidase domains among members of the Enterobacteriacae and in association with a large variety of toxin delivery systems. We extent our discussion with recent findings concerning cellular systems that control activated Rac1 GTPase stability and provide protection against cancer. These findings point to the urgency for developing holistic approaches toward personalized medicine that include monitoring for asymptomatic carriage of pathogenic toxin-producing bacteria and that ultimately might lead to improved public health and increased lifespans.

Local Arrangement of Fibronectin by Myofibroblasts Governs Peripheral Nuclear Positioning in Muscle Cells

Skeletal muscle cells (myofibers) are rod-shaped multinucleated cells surrounded by an extracellular matrix (ECM) basal lamina. In contrast to other cell types, nuclei in myofibers are positioned just below the plasma membrane at the cell periphery. Peripheral nuclear positioning occurs during myogenesis and is driven by myofibril crosslinking and contraction. Here we show that peripheral nuclear positioning is triggered by local accumulation of fibronectin secreted by myofibroblasts. We demonstrate that fibronectin via α5-integrin mediates peripheral nuclear positioning dependent on FAK and Src activation. Finally, we show that Cdc42, downstream of restricted fibronectin activation, is required for myofibril crosslinking but not myofibril contraction. Thus we identify that local activation of integrin by fibronectin secreted by myofibroblasts activates peripheral nuclear positioning in skeletal myofibers.

Exploratory cell dynamics: a sense of touch for cells?

Cells need to process multifaceted external cues to steer their dynamic behavior. To efficiently perform this task, cells implement several exploratory mechanisms to actively sample their environment. In particular, cells can use exploratory actin-based cell protrusions and contractions to engage and squeeze the environment and to actively probe its chemical and mechanical properties. Multiple excitable signal networks were identified that can generate local activity pulses to control these exploratory processes. Such excitable signal networks offer particularly efficient mechanisms to process chemical or mechanical signals to steer dynamic cell behavior, such as directional migration, tissue morphogenesis and cell fate decisions.

RHO GTPases in cancer: known facts, open questions, and therapeutic challenges

RHO GTPases have been traditionally associated with protumorigenic functions. While this paradigm is still valid in many cases, recent data have unexpectedly revealed that RHO proteins can also play tumor suppressor roles. RHO signaling elements can also promote both pro- and antitumorigenic effects using GTPase-independent mechanisms, thus giving an extra layer of complexity to the role of these proteins in cancer. Consistent with these variegated roles, both gain- and loss-of-function mutations in RHO pathway genes have been found in cancer patients. Collectively, these observations challenge long-held functional archetypes for RHO proteins in both normal and cancer cells. In this review, I will summarize these data and discuss new questions arising from them such as the functional and clinical relevance of the mutations found in patients, the mechanistic orchestration of those antagonistic functions in tumors, and the pros and cons that these results represent for the development of RHO-based anticancer drugs.

Targeting Rac and Cdc42 GTPases in Cancer

Rac and Cdc42 are small GTPases that have been linked to multiple human cancers and are implicated in epithelial to mesenchymal transition, cell-cycle progression, migration/invasion, tumor growth, angiogenesis, and oncogenic transformation. With the exception of the P29S driver mutation in melanoma, Rac and Cdc42 are not generally mutated in cancer, but are overexpressed (gene amplification and mRNA upregulation) or hyperactivated. Rac and Cdc42 are hyperactivated via signaling through oncogenic cell surface receptors, such as growth factor receptors, which converge on the guanine nucleotide exchange factors that regulate their GDP/GTP exchange. Hence, targeting Rac and Cdc42 represents a promising strategy for precise cancer therapy, as well as for inhibition of bypass signaling that promotes resistance to cell surface receptor-targeted therapies. Therefore, an understanding of the regulatory mechanisms of these pivotal signaling intermediates is key for the development of effective inhibitors. In this review, we focus on the role of Rac and Cdc42 in cancer and summarize the regulatory mechanisms, inhibitory efficacy, and the anticancer potential of Rac- and Cdc42-targeting agents. Cancer Res; 78(12); 3101-11. ©2018 AACR.

ARHGAP21 as a master regulator of multiple cellular processes

The cellular cytoskeleton is involved with multiple biological processes and is tightly regulated by multiple proteins and effectors. Among these, the RhoGTPases family is one of the most important players. RhoGTPAses are, in turn, regulated by many other elements. In the past decade, one of those regulators, the RhoGAP Rho GTPase Activating Protein 21 (ARHGAP21), has been overlooked, despite being implied as having an important role on many of those processes. In this paper, we aimed to review the available literature regarding ARHGAP21 to highlight its importance and the mechanisms of action that have been found so far for this still unknown protein involved with cell adhesion, migration, Golgi regulation, cell trafficking, and even insulin secretion.

Transcriptional and post-transcriptional regulation of the genes encoding the small GTPases RhoA, RhoB, and RhoC: implications for the pathogenesis of human diseases

Rho GTPases are highly conserved proteins that play critical roles in many cellular processes including actin dynamics, vesicular trafficking, gene transcription, cell-cycle progression, and cell adhesion. The main mode of regulation of Rho GTPases is through guanine nucleotide binding (cycling between an active GTP-bound form and an inactive GDP-bound form), but transcriptional, post-transcriptional, and post-translational modes of Rho regulation have also been described. In the present review, we summarize recent progress on the mechanisms that control the expression of the three members of the Rho-like subfamily (RhoA, RhoB, and RhoC) at the level of gene transcription as well as their post-transcriptional regulation by microRNAs. We also discuss the progress made in deciphering the mechanisms of cross-talk between Rho proteins and the transforming growth factor β signaling pathway and their implications for the pathogenesis of human diseases such as cancer metastasis and fibrosis.

Downregulation of STARD8 in gastric cancer and its involvement in gastric cancer progression

Objective

Rho-GTPases play a pivotal role in a wide variety of signal transduction pathways and are associated with a great number of human carcinomas. STARD8, which is a Rho-GTPase-activating protein, has been proposed as a tumor suppressor gene, but its role in gastric cancer remains elusive. In this study, we investigate the expression of STARD8 in gastric cancer and its association with gastric cancer progression.

Materials and methods

One normal gastric mucosa cell line for example GES1 and six human gastric cancer cell lines such as AGS, MGC803, MKN45, SGC7901, HGC27 and BGC823 were utilized to analyze STARD8 mRNA and protein levels by reverse transcription polymerase chain reaction (RT-PCR) and Western blot. A total of 70 paired gastric tissues including corresponding nonmalignant gastric tissues and cancer tissues were utilized to analyze the protein expression of STARD8 using immunohistochemistry, and the correlation between STARD8 level and clinicopathological features was also evaluated.

Results

STARD8 was found to be downregulated in primary gastric cancer cells and tissues compared with the normal gastric mucosa cell line, GES1, and corresponding nonmalignant gastric tissues, while its decreased expression was significantly associated with TNM stage, lymph node metastasis and differentiation (p<0.05).

Conclusion

There is significantly decreased expression of STARD8 in gastric cancer cells and tissues, and its expression may contribute to gastric tumorigenesis.

PRP4 kinase induces actin rearrangement and epithelial-mesenchymal transition through modulation of the actin-binding protein cofilin

Cell actin cytoskeleton is primarily modulated by Rho family proteins. RhoA regulates several downstream targets, including Rho-associated protein kinase (ROCK), LIM-Kinase (LIMK), and cofilin. Pre-mRNA processing factor 4B (PRP4) modulates the actin cytoskeleton of cancer cells via RhoA activity inhibition. In this study, we discovered that PRP4 over-expression in HCT116 colon cancer cells induces cofilin dephosphorylation by inhibiting the Rho-ROCK-LIMK-cofilin pathway. Two-dimensional gel electrophoresis, and matrix-assisted laser desorption/ionization time-of-flight mass-spectrometry (MALDI-TOF MS) analysis indicated increased expression of protein phosphatase 1A (PP1A) in PRP4-transfected HCT116 cells. The presence of PRP4 increased the expression of PP1A both at the mRNA and protein levels, which possibly activated cofilin through dephosphorylation and subsequently modulated the cell actin cytoskeleton. Furthermore, we found that PRP4 over-expression did not induce cofilin dephosphorylation in the presence of okadaic acid, a potent phosphatase inhibitor. Moreover, we discovered that PRP4 over-expression in HCT116 cells induced dephosphorylation of migration and invasion inhibitory protein (MIIP), and down-regulation of E-cadherin protein levels, which were further restored by the presence of okadaic acid. These findings indicate a possible molecular mechanism of PRP4-induced actin cytoskeleton remodeling and epithelial-mesenchymal transition, and make PRP4 an important target in colon cancer.

RhoD localization and function is dependent on its GTP/GDP-bound state and unique N-terminal motif

The atypical Rho GTPase RhoD has previously been shown to have a major impact on the organization and function of the actin filament system. However, when first discovered, RhoD was found to regulate endosome trafficking and dynamics and we therefore sought to investigate this regulation in more detail. We found that exogenously expressed RhoD in human fibroblasts localized to vesicles and the plasma membrane and that the active GTP-bound conformation was required for the plasma membrane localization but not for vesicle localization. In contrast to the GTPase deficient atypical Rho GTPases, which have a stalled GTPase activity, RhoD has an elevated intrinsic GDP/GTP exchange activity, rendering the protein constitutively active. Importantly, RhoD can still hydrolyze GTP and we found that an intact GTPase activity was required for efficient fusion of RhoD-positive vesicles. RhoD has a unique N-terminal extension of 14 amino acid residues, which is not present in the classical Rho GTPases RhoA, Cdc42 and Rac1. Deletion of this N-terminal motif often lead to clustering of RhoD positive vesicles, which were found accumulated at the peripheral membrane border. In addition, the number of vesicles per cell was increased manifold, suggesting that the N-terminal motif has an important regulatory role in vesicle dynamics.

Small GTPase-dependent regulation of leukocyte-endothelial interactions in inflammation

Inflammation is a complex biological response that serves to protect the body's tissues following harmful stimuli such as infection, irritation or injury and initiates tissue repair. At the start of an inflammatory response, pro-inflammatory mediators induce changes in the endothelial lining of the blood vessels and in leukocytes. This results in increased vascular permeability and increased expression of adhesion proteins, and promotes adhesion of leukocytes, especially neutrophils to the endothelium. Adhesion is a prerequisite for neutrophil extravasation and chemoattractant-stimulated recruitment to inflammatory sites, where neutrophils phagocytose and kill microbes, release inflammatory mediators and cross-talk with other immune cells to co-ordinate the immune response in preparation for tissue repair. Many signalling proteins are critically involved in the complex signalling processes that underpin the inflammatory response and cross-talk between endothelium and leukocytes. As key regulators of cell-cell and cell-substratum adhesion, small GTPases (guanosine triphosphatases) act as important controls of neutrophil-endothelial cell interactions as well as neutrophil recruitment to sites of inflammation. Here, we summarise key processes that are dependent upon small GTPases in leukocytes during these early inflammatory events. We place a particular focus on the regulation of integrin-dependent events and their control by Rho and Rap family GTPases as well as their regulators during neutrophil adhesion, chemotaxis and recruitment.

A kinome-wide high-content siRNA screen identifies MEK5-ERK5 signaling as critical for breast cancer cell EMT and metastasis

An epithelial to mesenchymal transition (EMT) has been correlated to malignant tumor progression and metastasis by promoting cancer cell migration and invasion and chemoresistance. Hence, finding druggable EMT effectors is critical to efficiently interfere with metastasis formation and to overcome therapy resistance. We have employed a high-content microscopy screen in combination with a kinome and phosphatome-wide siRNA library to identify signaling pathways underlying an EMT of murine mammary epithelial cells and breast cancer cells. This screen identified the MEK5-ERK5 axis as a critical player in TGFβ-mediated EMT. Suppression of MEK5-ERK5 signaling completely prevented the morphological and molecular changes occurring during a TGFβ-induced EMT and, conversely, forced highly metastatic breast cancer cells into a differentiated epithelial state. Inhibition of MEK5-ERK5 signaling also repressed breast cancer cell migration and invasion and substantially reduced lung metastasis without affecting primary tumor growth. The results suggest that the MEK5-ERK5 signaling axis via activation of MEF2B and other transcription factors plays an important role in the induction and maintenance of breast cancer cell migration and invasion and thus represents an exploitable target for the pharmacological inhibition of cancer cell metastasis.

Modulation of small GTPase activity by NME proteins

NME proteins are reported to influence signal transduction activity of small GTPases from the Ras superfamily by diverse mechanisms in addition to their generic NDP kinase activity, which replenishes the cytoplasmic pool of GTP. Comprehensive evidence shows that NME proteins modulate the activity of Ras GTPases, in particular members of the Rho family, via binding to their major activators GEFs. Direct interaction between several NMEs and Ras GTPases were also indicated in vitro and in vivo. These modes of regulation are mainly independent of the NME's kinase activity. NMEs also modulate the Ras-mediated signal transduction by interfering with the formation of a Ras signaling complex at the plasma membrane. In several examples, NMEs were proposed to perform the role of GAP proteins by promoting hydrolysis of the bound GTP, but this activity still requires additional verification. Early suggestions that NMEs can activate small GTPases by direct phosphorylation of the bound GDP, or by high-rate loading of GTP onto a closely apposed GTPase, were largely dismissed. In this review article, we survey and put into perspective published examples of identified and hypothetical mechanisms of Ras signaling modulation by NME proteins. We also point out involvement of NMEs in the transcriptional regulation of components of Ras GTPases-mediated signal transduction pathways, and reciprocal regulation of NME function by small GTPases, particularly related to NME's binding to membranes.

From Physiological Redox Signalling to Oxidant Stress

Oxidant stress is strongly associated with cardiovascular disease, including pulmonary hypertension, but antioxidant therapies have so far proven ineffective. This is partly due to a lack of understanding of the key role played by reactive oxygen species (ROS) in physiological cell signalling, and partly to the complex interrelationships between generators of ROS (e.g. mitochondria and NADPH oxidases, NOX), cellular antioxidant systems and indeed Ca²⁺ signalling. At physiological levels ROS reversibly affect the function of numerous enzymes and transcription factors, most often via oxidation of specific protein thiols. Importantly, they also affect pathways that promote ROS generation by NOX or mitochondria (ROS-induced ROS release), which has an inherent propensity for positive feedback and uncontrolled oxidant production. The reason this does not occur under normal conditions reflects in part a high level of compartmentalisation of ROS signalling within the cell, akin to that for Ca²⁺. This article considers the physiological processes which regulate NOX and mitochondrial ROS production and degradation and their interactions with each other and Ca²⁺ signalling pathways, and discusses how loss of spatiotemporal constraints and activation of positive feedback pathways may impact on their dysregulation in pulmonary hypertension.

Polyisoprenylated cysteinyl amide inhibitors disrupt actin cytoskeleton organization, induce cell rounding and block migration of non-small cell lung cancer

The malignant potential of Non-Small Cell Lung Cancer (NSCLC) is dependent on cellular processes that promote metastasis. F-actin organization is central to cell migration, invasion, adhesion and angiogenesis, processes involved in metastasis. F-actin remodeling is enhanced by the overexpression and/or hyper-activation of some members of the Rho family of small GTPases. Therefore, agents that mitigate hyperactive Rho proteins may be relevant for controlling metastasis. We previously reported the role of polyisoprenylated cysteinyl amide inhibitors (PCAIs) as potential inhibitors of cancers with hyperactive small GTPases. In this report, we investigate the potential role of PCAIs against NSCLC cells and show that as low as 0.5 μM PCAIs significantly inhibit 2D and 3D NCI-H1299 cell migration by 48% and 45%, respectively. PCAIs at 1 μM inhibited 2D and 3D NCI-H1299 cell invasion through Matrigel by 50% and 85%, respectively. Additionally, exposure to 5 μM of the PCAIs for 24 h caused at least a 66% drop in the levels of Rac1, Cdc42, and RhoA and a 38% drop in F-actin intensity at the cell membrane. This drop in F-actin was accompanied by a 73% reduction in the number of filopodia per cell. Interestingly, the polyisoprenyl group of the PCAIs is essential for these effects, as NSL-100, a non-farnesylated analog, does not elicit similar effects on F-actin assembly and organization. Our findings indicate that PCAIs disrupt F-actin assembly and organization to suppress cell motility and invasion. The PCAIs may be an effective therapy option for NSCLC metastasis and invasion control.

Lipid Cell Biology: A Focus on Lipids in Cell Division

Cells depend on hugely diverse lipidomes for many functions. The actions and structural integrity of the plasma membrane and most organelles also critically depend on membranes and their lipid components. Despite the biological importance of lipids, our understanding of lipid engagement, especially the roles of lipid hydrophobic alkyl side chains, in key cellular processes is still developing. Emerging research has begun to dissect the importance of lipids in intricate events such as cell division. This review discusses how these structurally diverse biomolecules are spatially and temporally regulated during cell division, with a focus on cytokinesis. We analyze how lipids facilitate changes in cellular morphology during division and how they participate in key signaling events. We identify which cytokinesis proteins are associated with membranes, suggesting lipid interactions. More broadly, we highlight key unaddressed questions in lipid cell biology and techniques, including mass spectrometry, advanced imaging, and chemical biology, which will help us gain insights into the functional roles of lipids.