Spontaneous polarization without spatial cues, or symmetry breaking, is a fundamental problem of spatial organization in biological systems. This question has been extensively studied using yeast models, which revealed the central role of the small GTPase switch Cdc42. Active Cdc42‐GTP forms a coherent patch at the cell cortex, thought to result from amplification of a small initial stochastic inhomogeneity through positive feedback mechanisms, which induces cell polarization. Here, I review and discuss the mechanisms of Cdc42 activity (...) self‐amplification and dynamic turnover. A robust Cdc42 patch is formed through the combined effects of Cdc42 activity promoting its own activation and active Cdc42‐GTP displaying reduced membrane detachment and lateral diffusion compared to inactive Cdc42‐GDP. I argue the role of the actin cytoskeleton in symmetry breaking is not primarily to transport Cdc42 to the active site. Finally, negative feedback and competition mechanisms serve to control the number of polarization sites. (shrink)

The Rho GTPases—Rho, Rac and Cdc42—act as molecular switches, cycling between an active GTP‐bound state and an inactive GDP‐bound state, to regulate the actin cytoskeleton. It has recently become apparent that the Rho GTPases can be activated in subcellular zones that appear semi‐stable, yet are dynamically maintained. These Rho GTPase activity zones are associated with a variety of fundamental biological processes including symmetric and asymmetric cytokinesis and cellular wound repair. Here we review the basic features of Rho (...) class='Hi'>GTPase activity zones, suggest that these zones represent a fundamental signaling mechanism, and discuss the implications of zone properties from the perspective of both their function and how they are likely to be controlled. BioEssays 28: 983–993, 2006. © 2006 Wiley Periodicals, Inc. (shrink)

Tumor progression involves the transition from normal to malignant cells, through a series of cumulative alterations. During this process, invasive and migratory properties are acquired, enabling cells to metastasize (reach and grow in tissues far from their origin). Numerous cellular changes take place during epithelial malignancy, and disruption of E‐cadherin based cell‐cell adhesion is a major event. The small Rho GTPases (Rho, Rac and Cdc42) have been implicated in multiple steps during cellular transformation, including alterations on the adhesion status (...) of the tumor cells. This review focuses on recent in vivo evidence that implicates RhoGTPases in epithelial tumor progression. In addition, we discuss different hypotheses to explain disruption of cadherin‐mediated cell–cell adhesion, directly or indirectly, through activation of Rho GTPases. Understanding the molecular mechanism of how cadherin adhesion and RhoGTPases interplay in normal cells and how this balance is altered during cellular transformation will provide clues as to how to interfere with tumor progression. © 2003 Wiley Periodicals, Inc. BioEssays 25:452–463, 2003. © 2003 Wiley Periodicals, Inc. (shrink)

The generation of asymmetric cell shapes is a recurring theme in biology. In budding yeast, one form of cell asymmetry occurs for division and is generated by anisotropic growth of the mother cell to form a daughter cell bud. Previous genetic studies uncovered key roles for the small GTPase Cdc42 in organizing the actin cytoskeleton and vesicle delivery to the site of bud growth,1,2 but a recent paper has also raised questions about how control of Cdc42 activity (...) is integrated into a proposed hierarchical regulatory pathway that specifies a unique site of bud formation.3 BioEssays 25:833–836, 2003. © 2003 Wiley Periodicals, Inc. (shrink)

Signal transduction pathways constructed around a core module of three consecutive protein kinases, the most distal being a member of the extracellular signal‐regulated kinase (ERK) family, are ubiquitous among eukaryotes. Recent work has defined two cascades activated preferentially by the inflammatory cytokines TNF‐α and IL‐1‐β, as well as by a wide variety of cellular stresses such as UV and ionizing radiation, hyperosmolarity, heat stress, oxidative stress, etc. One pathway converges on the ERK subfamily known as the ‘stress activated’ protein kinases (...) (SAPKs, also termed Jun N‐terminal kinases, JNKs), whereas the second pathway recruits the p38 kinases. Upstream inputs are diverse, and include small GTPases (primarily Rac and Cdc42; secondarily Ras) acting through mammalian homologs of the yeast Ste20 kinase, other kinase subfamilies (e.g. GC kinase) and ceramide, a putative second messenger for certain TNF‐α actions. These two cascades signal cell cycle delay, cellular repair or apoptosis in most cells, as well as activation of immune and reticuloendothelial cells. (shrink)

Rho GTPases are critically important and are centrally positioned regulators of the actomyosin cytoskeleton. By influencing the organization and architecture of the cytoskeleton, Rho proteins play prominent roles in many cellular processes including adhesion, migration, intra‐cellular transportation, and proliferation. The most important method of Rho GTPase regulation is via the GTPase cycle; however, post‐translational modifications (PTMs) also play critical roles in Rho protein regulation. Relative to other PTMs such as lipidation or phosphorylation that have been extensively characterized, protein (...) oxidation is a regulatory PTM that has been poorly studied. Protein oxidation primarily occurs from the reaction of reactive oxygen species (ROS), such as hydrogen peroxide (H2O2), with amino acid side chain thiols on cysteine (Cys) and methionine (Met) residues. The versatile redox modifications of cysteine residues exemplify their integral role in cell signalling processes. Here we review prominent members of the Rho GTPase family and discuss how lipidation, phosphorylation, and oxidation on conserved cysteine residues affects their regulation and function. (shrink)

The origin of eukaryotes is one of the major challenges of evolutionary cell biology. Other than the endosymbiotic origin of mitochondria and chloroplasts, the steps leading to eukaryotic endomembranes and endoskeleton are poorly understood. Ras‐family small GTPases are key regulators of cytoskeleton dynamics, vesicular trafficking and nuclear function. They are specific for eukaryotes and their expansion probably traces the evolution of core eukaryote features. The phylogeny of small GTPases suggests that the first endomembranes to evolve during eukaryote evolution had secretory, (...) and not phagocytic, function. Based on the reconstruction of putative roles for ancestral small GTPases, a hypothetical scenario on the origins of the first endomembranes, the nucleus, and phagocytosis is presented. BioEssays 25:1129–1138, 2003. © 2003 Wiley Periodicals, Inc. (shrink)

Dynamin is a GTPase that regulates late events in clathrin‐coated vesicle formation. Our current working model suggests that dynamin is targeted to coated pits in its unoccupied or GDP‐bound form, where it is initially distributed uniformly throughout the clathrin lattice. GTP/GDP exchange triggers its release from these sites and its assembly into short helices that encircle the necks of invaginated coated pits like a collar. GTP hydrolysis, which is required for vesicle detachment, presumably induces a concerted conformation change, tightening (...) the collar. Unlike most of its GTPase cousins that serve as molecular switches, dynamin has a low affinity for GTP, a very high intrinsic rate of GTP hydrolysis and functions as a homo‐oligomer. A concerted conformational change resulting from coordinated GTP hydrolysis by the dynamin oligomer might be sufficient to generate force. In this case, dynamin would be the first GTPase identified that acts as a structural protein with mechano‐chemical function. (shrink)

Rho proteins belong to the small GTPases superfamily. They function as molecular switches that, in response to diverse stimuli, control key signaling and structural aspects of the cell. Although early studies proposed a role for Rho GTPases in cellular transformation, this effect was underestimated due to the fact that no genetic mutations affecting Rho‐encoding genes were found in tumors. Recently, it has become evident that Rho GTPases participate in the carcinogenic process by either overexpression of some of the members of (...) the family with oncogenic activity, downmodulation of other members with suggested tumor suppressor activity, or by alteration of upstream modulators or downstream effectors. Thus, alteration of the levels of expression of different members of the family of Rho GTPases has been detected in many types of human tumors leading to a great interest in the cellular effects elicited by these oncoproteins. This essay reviews the current evidence of dysregulation of Rho signaling by overexpression in human tumors. BioEssays 27:602–613, 2005. © 2005 Wiley Periodicals, Inc. (shrink)

Ran is one of the most abundant and best conserved of the small GTP binding and hydrolyzing proteins of eukaryotes. It is located predominantly in cell nuclei. Ran is a member of the Ras family of GTPases, which includes the Ras and Ras‐like proteins that regulate cell growth and division, the Rho and Rac proteins that regulate cytoskeletal organization and the Rab proteins that regulate vesicular sorting. Ran differs most obviously from other members of the Ras family in both its (...) nuclear localization, and its lack of sites required for post‐translational lipid modification. Ran is, however, similar to other Ras family members in requiring a specific guanine nucleotide exchange factor (GEF) and a specific GTPase activating protein (GAP) as stimulators of overall GTPase activity. In this review, the multiple cellular functions of Ran are evaluated with respect to its known biochemistry and molecular interactions. (shrink)

The Arf family proteins are best known for their roles in the vesicle biogenesis. However, they also play fundamental roles in a wide range of cellular regulation besides vesicular trafficking, such as modulation of lipid metabolic enzymes, cytoskeleton remodeling, ciliogenesis, lysosomal, and mitochondrial morphology and functions. Growing studies continue to expand the downstream effector landscape of Arf proteins, especially for the less‐studied members, revealing new biological functions, such as amino acid sensing. Experiments with cutting‐edge technologies and in vivo functional studies (...) in the last decade help to provide a more comprehensive view of Arf family functions. In this review, we summarize the cellular functions that are regulated by at least two different Arf members with an emphasis on those beyond vesicle biogenesis. (shrink)

How epithelial tissues are able to self-renew to maintain homeostasis and regenerate in response to injury remains a persistent question. The transcriptional effectors YAP and TAZ are increasingly being recognized as central mediators of epithelial stem cell biology, and a wealth of recent studies have been directed at understanding the control and activity of these factors. Recent work by Hu et al. has added to this knowledge, as they identify an Integrin-FAK-CDC42-PP1A signaling cascade that directs nuclear YAP/TAZ activity in (...) stem cell populations of the mouse incisor, and define convergence on mTORC1 signaling as an important mediator of the proliferation of these cells. Here, we review recent studies on YAP/TAZ function and regulation in epithelial tissue-specific stem cells, merging the Hu et al. study together with our current knowledge of YAP/TAZ. The Hippo pathway effectors YAP and TAZ broadly regulate adult stem cells, and have recently been implicated in dental epithelial stem cell proliferation and restricted differentiation via an Integrin-FAK-CDC42-PP1A cascade. Here, we discuss the extracellular cues which control YAP/TAZ activity and examine downstream pathways that direct stem cell fate. (shrink)

Small GTPases in the Rho family act as major nodes with functions beyond cytoskeletal rearrangements shaping the Caenorhabditis elegans embryo during development. These small GTPases are key signal transducers that integrate diverse developmental signals to produce a coordinated response in the cell. In C. elegans, the best studied members of these highly conserved Rho family small GTPases, RHO‐1/RhoA, CED‐10/Rac, and CDC‐42, are crucial in several cellular processes dealing with cytoskeletal reorganization. In this review, we update the functions described for the (...) Rho family small GTPases in spindle orientation and cell division, engulfment, and cellular movements during C. elegans embryogenesis, focusing on the Rho subfamily Rac.Please also see the video abstract here. (shrink)

RAS GTPases play essential roles in normal development and are direct drivers of human cancers. Three decades of study have failed to wholly characterize pathways stimulated by activated RAS, driven by engagement with ‘effector’ proteins that have RAS binding domains (RBDs). Bone fide effectors must bind directly to RAS GTPases in a nucleotide‐dependent manner, and this interaction must impart a clear change in effector activity. Despite this, for most proteins currently deemed effectors there is little mechanistic understanding of how binding (...) to the GTPase alters protein function. There has also been limited effort to comprehensively resolve the specificity of effector binding to the full array of RAS superfamily GTPase proteins. This review will summarize what is known about RAS‐driven activation for an array of potential effector proteins, focusing on structural and mechanistic effects and highlighting how little is still known regarding this key paradigm of cellular signal transduction. (shrink)

The highly homologous Rac1 and Rac2 GTPases are co‐expressed in cells of haematopoietic origin and are likely to show some functional redundancy. While disruption of the Rac2 gene in mice has provided insight into some of its functions, Rac1 null mice are embryonic lethal and only recently has conditional gene disruption been possible. Consequently, two articles1,2 have recently elucidated some overlapping and unique key roles of Rac1 and Rac2 in haematopoietic processes including specialized roles in innate and humoral immunity. BioEssays (...) 26:221–224, 2004. © 2004 Wiley Periodicals, Inc. (shrink)

A hallmark of positive‐sense RNA viruses is the formation of membranous shelters for safe replication in the cytoplasm. Once considered invisible to the immune system, these viral shelters are now found to be antagonized through the cooperation of autophagy proteins and anti‐microbial GTPases. This coordinated effort of autophagy proteins guiding GTPases functions against not only the shelters of viruses but also cytoplasmic vacuoles containing bacteria or protozoa, suggesting a broad immune‐defense mechanism against disparate vacuolar pathogens. Fundamental questions regarding this process (...) remain: how the host recognizes these membranous structures as a target, how the autophagy proteins bring the GTPases to the shelters, and how the recruited GTPases disrupt these shelters. In this review, these questions are discussed, the answers to which will significantly advance our understanding of the response to vacuole‐like structures of pathogens, thereby paving the way for the development of broadly effective anti‐microbial strategies for public health. (shrink)

Rho, a member of the Ras superfamily of small GTPases, has multiple biological roles: it regulates signal trasduction pathways linking extracellular growth factors to the assembly of actin stress fibres and focal adhesion complexes; it is required for G1 progression and activates the SRF transcription factor when quiescent fibroblasts are stimulated to grow; and it plays a role later in the cell cycle during cytokinesis. Two groups have recently succeeded in identifying downstream effectors of Rho that may mediate some of (...) these biological effects. One protein identified by both groups is protein kinase N (PKN), a serine/threonine kinase whose catalytic domain is closely related to that of protein kinase C(1,2). (shrink)

Metastasis is one of the clinical parameters that has a strong negative influence on the prognosis of cancer patients. In recent years, significant advances have furthered our understanding of this process at the molecular and biological levels. This paper will discuss recent discoveries relating to the earliest, intra‐tumoral stages of metastasis in cancer cells, specifically focusing on: (i) the development of metastatic traits during primary tumorigenesis; (ii) intrinsic and extrinsic cancer cell programs associated with malignant traits; (iii) the intra‐tumoral migration (...) patterns of cancer cells and the dynamic roles played by the Rho/Rac GTPases and epithelial‐mesenchymal transitions in this process; and (iv) the genetic strategies used by metastatic cancer cells to promote intra‐tumoral cell migration and their subsequent escape to peripheral tissues. Finally, the therapeutic and diagnostic relevance of this information will be discussed, as well as potential future developments. (shrink)

The vav proto‐oncogene encodes a 95 kDa protein which is expressed exclusively in hematopoietic cells. Analysis of the deduced amino acid sequence has revealed the presence of a src‐homology 2 (SH2) domain, 2 SH3 domains, a cysteine‐rich region with similarity to protein kinase C, and a region highly similar to proteins with guanine nucleotide exchange activity on ras‐like GTPases. Recent work has shown that vav is tyrosine phosphorylated in response to stimulation of surface membrane receptors in a variety of hematopoietic (...) cell lines. Vav may play a role in hematopoietic cell signaling by coupling tyrosine kinase pathways to ras‐like GTPases through the regulation of guanine nucleotide exchange. (shrink)

Toxoplasma gondii is a highly successful parasite capable of infecting virtually all warm-blooded animals by actively invading nucleated host cells and forming a modified compartment where it replicates within the cytosol. The parasite-containing vacuole provides a safe haven, even in professional phagocytes such as macrophages, which normally destroy foreign microbes. In an effort to eliminate the parasite, the host up-regulates a family of immunity-related p47 GTPases (IRGs), which are recruited to the parasite-containing vacuole, resulting in membrane rupture and digestion of (...) the parasite. To avoid this fate, highly virulent strains of Toxoplasma coat the external surface of their vacuole with a secretory serine/threonine kinase, known as ROP18. At this host-pathogen interface, ROP18 phosphorylates and inactivates IRGs, thereby protecting the parasite from killing. These findings reveal a novel molecular mechanism by which the parasite disarms host innate immunity. (shrink)

Detecting and interpreting extracellular spatial signals is essential for cellular orientation within complex environments, such as during directed cell migration or growth in multicellular development. Although the molecular understanding of how cells read spatial signals like chemical gradients is still lacking, recent work has revealed that stochastic processes at different temporal and spatial scales are at the core of this gradient sensing process in a wide range of eukaryotes. Fast biochemical reactions like those underlying GTPase activity dynamics form a (...) functional module together with slower cell morphological changes driven by membrane remodelling. This biochemical‐morphological module explores the environment by stochastic local concentration sampling to determine the source of the gradient signal, enabling efficient signal detection and interpretation before polarised growth or migration towards the gradient source is initiated. Here we review recent data describing local sampling and propose a model of local fast and slow feedback counteracted by gradient‐dependent substrate limitation to be at the core of gradient sensing by local sampling. (shrink)

Rnd3/RhoE has two distinct functions, regulating the actin cytoskeleton and cell proliferation. This might explain why its expression is often altered in cancer and by multiple stimuli during development and disease. Rnd3 together with its relatives Rnd1 and Rnd2 are atypical members of the Rho GTPase family in that they do not hydrolyse GTP. Rnd3 and Rnd1 both antagonise RhoA/ROCK‐mediated actomyosin contractility, thereby regulating cell migration, smooth muscle contractility and neurite extension. In addition, Rnd3 has been shown to have (...) a separate role in inhibiting cell cycle progression by reducing translation of cell cycle regulators, including cyclin D1 and Myc. We propose that Rnd3 could act as a tumour suppressor to limit proliferation, but when mutations bypass this activity of Rnd3, it can promote cancer invasion through its effects in the actin cytoskeleton. (shrink)

The LIM domain kinases (LIMKs) are important actin cytoskeleton regulators. These proteins, LIMK1 and LIMK2, are nodes downstream of Rho GTPases and are the key enzymes that phosphorylate cofilin/actin depolymerization factors to regulate filament severing. They therefore perform an essential role in cascades that control actin depolymerization. Signaling of the LIMKs is carefully regulated by numerous inter‐ and intra‐molecular mechanisms. In this review, we discuss recent findings that improve the understanding of LIM domain kinase regulation mechanisms. We also provide an (...) up‐to‐date review of the role of the LIM domain kinases, their architectural features, how activity is impacted by other proteins, and the implications of these findings for human health and disease. (shrink)

Both RalA and RalB interact with the ubiquitous calcium sensor, calmodulin (CaM). New structural and biophysical characterisation of these interactions strongly suggests that, in the native membrane‐associated state, only RalA can be extracted from the membrane by CaM and this non‐canonical interaction could underpin the divergent signalling roles of these closely related GTPases. The isoform specificity for RalA exhibited by CaM is hypothesised to contribute to the disparate signalling roles of RalA and RalB in mitochondrial dynamics. This would lead to (...) CaM shuttling RalA to the mitochondrial membrane but leaving RalB localisation unperturbed, and in doing so triggering mitochondrial fission pathways rather than mitophagy. (shrink)

Recently, Gao et al. and Chappie et al. elucidated the crystal structures of the polytetrameric stalk domain of the dynamin‐like virus resistance protein, MxA, and of the G‐domain dimer of the large, membrane‐deforming GTPase, dynamin, respectively. Combined, they provide a hypothetical oligomeric structure for the complete dynamin protein. Here, it is discussed how the oligomers are expected to form and how they participate in dynamin mediated vesicle fission during the process of endocytosis. The proposed oligomeric structure is compared with (...) the novel mechanochemical model of dynamin function recently proposed by Bashkirov et al. and Pucadyil and Schmid. In conclusion, the new model of the dynamin oligomer has the potential to explain how short self‐limiting fissogenic dynamin assemblies are formed and how concerted GTP hydrolysis is achieved. The oligomerisation of two other dynamin superfamily proteins, the guanylate binding proteins (GBPs) and the immunity‐related GTPases (IRGs), is addressed briefly. (shrink)

The autophagy initiation complex is brought about via a highly ordered and stepwise assembly process. Two crucial signaling molecules, mTORC1 and AMPK, orchestrate this assembly by phosphorylating/dephosphorylating autophagy‐related proteins. Activation of Atg1 followed by recruitment of both Atg9 vesicles and the PI3K complex I to the PAS (phagophore assembly site) are particularly crucial steps in its formation. Ypt1, a small Rab GTPase in yeast cells, also plays an essential role in the formation of the autophagy initiation complex through multiple (...) regulatory pathways. In this review, our primary focus is to discuss how signaling molecules initiate the assembly of the autophagy initiation complex, and highlight the significant roles of Ypt1 in this process. We end by addressing issues that need future clarification. (shrink)

The translocation of tRNAs through the ribosome proceeds through numerous small steps in which tRNAs gradually shift their positions on the small and large ribosomal subunits. The most urgent questions are: (i) whether these intermediates are important; (ii) how the ribosomal translocase, the GTPase elongation factor G (EF‐G), promotes directed movement; and (iii) how the energy of GTP hydrolysis is coupled to movement. In the light of recent advances in biophysical and structural studies, we argue that intermediate states of (...) translocation are snapshots of dynamic fluctuations that guide the movement. In contrast to current models of stepwise translocation, kinetic evidence shows that the tRNAs move synchronously on the two ribosomal subunits in a rapid reaction orchestrated by EF‐G and GTP hydrolysis. EF‐G combines the energy regimes of a GTPase and a motor protein and facilitates tRNA movement by a combination of directed Brownian ratchet and power stroke mechanisms. (shrink)

Cellular mechanisms whereby quiescent stem cells sense tissue injury and transition to an activated state are largely unknown. Quiescent skeletal muscle stem cells (MuSCs, also called satellite cells) have elaborate, heterogeneous projections that rapidly retract in response to muscle injury. They may therefore act as direct sensors of their niche environment. Retraction is driven by a Rac‐to‐Rho GTPase activity switch that promotes downstream MuSC activation events. These and other observations lead to several hypotheses: (1) projections are morphologically dynamic at (...) quiescence, providing a surveillance function for muscle damage; (2) quiescent projection dynamics are regulated by the relative balance of Rac and Rho activities promoted by niche‐derived cues; (3) projections, particularly their associated filopodia, sense tissue damage via changes to the biomechanical properties of the niche and/or detection of signaling cues released by damaged myofibers; and (4) the dynamic nature of projections results in a population of MuSCs with heterogeneous functional properties. These concepts may extend to other types of quiescent stem cells, as well as prove useful in translational research settings. (shrink)

Planar cell polarity (PCP), the alignment of cells within 2D tissue planes, involves a set of core molecular regulators highly conserved between animals and cell types. These include the transmembrane proteins Frizzled (Fz) and VanGogh and the cytoplasmic regulators Dishevelled (Dsh) and Prickle. It is widely accepted that this core forms part of a ‘PCP pathway’ for signal transduction, which can affect cell morphology through activation of an evolutionary ancient regulatory module involving Rho family GTPases and Myosin II, and/or the (...) JNK kinase cascade. We have re‐examined the evidence for interactions between the proposed PCP pathway components, and question the placing of the cell morphology regulators in the same pathway as the PCP core. While Fz and Dsh are clearly involved in both PCP and Rho‐based cell morphology regulation, available evidence cannot currently discriminate whether these processes are linked mechanistically by a shared Fz/Dsh population, or pass by two distinct pathways. (shrink)

Lysophosphatidic acid (LPA) is a lipid mediator with a wide variety of biological actions, particularly as an inducer of cell proliferation, migration and survival. LPA binds to specific G‐protein‐coupled receptors and thereby activates multiple signal transduction pathways, including those initiated by the small GTPases Ras, Rho, and Rac. LPA signaling has been implicated in such diverse processes as wound healing, brain development, vascular remodeling and tumor progression. Knowledge of precisely how and where LPA is produced has long proved elusive. Excitingly, (...) it has recently been discovered that LPA is generated from precursors by ‘autotaxin’, a once enigmatic exo‐phosphodiesterase implicated in tumor cell motility. Exogenous phospholipases D can also produce LPA, which may contribute to their toxicity. Here we review recent progress in our understanding of LPA bioactivity, signaling and synthesis. BioEssays 26:870–881, 2004. © 2004 Wiley Periodicals, Inc. (shrink)

There are many morphologically distinct membrane structures with different functions at the surface of epithelial cells. Among these, adherens junctions (AJ) and tight junctions (TJ) are responsible for the mechanical linkage of epithelial cells and epithelial barrier function, respectively. In the process of new cell–cell adhesion formation between two epithelial cells, such as after wounding, AJ form first and then TJ form on the apical side of AJ. This process is very complicated because AJ formation triggers drastic changes in the (...) organization of actin cytoskeleton, the activity of Rho family of small GTPases, and the lipid composition of the plasma membrane, all of which are required for subsequent TJ formation. In this review, the authors focus on the relationship between AJ and TJ as a representative example of specialization of plasma membrane regions and introduce recent findings on how AJ formation promotes the subsequent formation of TJ. (shrink)

Proteins and RNAs move between the nucleus and cytoplasm by translocation through nuclear pore complexes in the nuclear envelope. To do this, they require specific targeting signals, energy, and a cellular apparatus that catalyzes their transport. Several of the factors involved in nucleocytoplasmic trafficking of proteins have been identified and characterized in some detail. The emerging picture for nuclear transport proposes a central role for the small GTPase Ran and proteins with which it interacts. In particular, asymmetric distribution of (...) these proteins between nucleus and cytoplasm appears to be responsible for the vectorial nature of nucleocytoplasmic transport. Here, we summarize the role of Ran and Ran-binding proteins in nuclear trafficking of proteins with classical nuclear localisation signals. We also discuss examples of the growing number of alternative pathways that are involved in transport of proteins across the nuclear envelope. BioEssays 21:579–589, 1999. © 1999 John Wiley & Sons, Inc. (shrink)

During development one mechanism for generating different cell types is asymmetric cell division, by which a cell divides and contributes different factors to each of its daughter cells. Asymmetric cell division occurs through out the eukaryotic kingdom, from yeast to humans. Many asymmetric cell divisions occur in a defined orientation. This implies a cellular mechanism for sensing direction, which must ultimately lead to differences in gene expression between two daughter cells. In this review, we describe two classes of molecules: regulatory (...) factors that are differentially expressed upon asymmetric cell division, and components of a signal transduction pathway that may define cell polarity. The lin‐11 and mec‐3 genes of C. elegans, the Isl‐1 gene of mammals and the HO gene of yeast, encode regulatory factors that determine cell type of one daughter after asymmetric cell division. The CDC24 and CDC42 genes of yeast affect both bud positioning and orientation of mating projections, and thus may define a general cellular polarity. We speculate that molecules such as Cdc24 and Cdc42 may regulate expression of genes such as lin‐11, mec‐3, Isl‐1 and HO upon asymmetric cell division. (shrink)

Transient receptor potential vanilloid 4 (TRPV4), a member of the TRP superfamily, is a broadly expressed, cell surface‐localized cation channel that is activated by a variety of environmental stimuli. Importantly, TRPV4 has been increasingly implicated in the regulation of cellular morphology. Here we propose that TRPV4 and the cytoskeletal remodeling small GTPase RhoA together constitute an environmentally sensitive signaling complex that contributes to pathological cell cytoskeletal alterations during neurological injury and disease. Supporting this hypothesis is our recent work demonstrating (...) direct physical and bidirectional functional interactions of TRPV4 with RhoA, which can lead to activation of RhoA and reorganization of the actin cytoskeleton. Furthermore, a confluence of evidence implicates TRPV4 and/or RhoA in pathological responses triggered by a range of acute neurological insults ranging from stroke to traumatic injury. While initiated by a variety of insults, TRPV4–RhoA signaling may represent a common pathway that disrupts axonal regeneration and blood–brain barrier integrity. These insights also suggest that TRPV4 inhibition may represent a safe, feasible, and precise therapeutic strategy for limiting pathological TRPV4–RhoA activation in a range of neurological diseases. (shrink)

The mitogenic action of cytokines such as epidermal growth factor (EGF)d̊ or platelet dericed growth factor (PDGF) involves the stimulation of a signal cascade controlled by a small G protein called Ras. Mutations of Ras can cause its constitutive activation and, as a consequence, bypass the regulation of cell growth by cytokines. Both growth factor‐induced and oncogenic activation of Ras involve the conversion of Ras from the GDP‐bound (D‐Ras) to the GTP‐bound (T‐Ras) forms. T‐Ras activates a network of protein kinases (...) including c‐Mos, c‐Raf‐1 and MAP kinase. Eventually the activation of MAP kinase leads to the activation of the elongation factor 4E and several transcription factors such as c‐Jun, c‐Myc and c‐Fos. There are several modulators of Ras activity, such as the GTPase activating proteins (GAP1 and NF1), which stimulate the coversion of T‐Ras to D‐Ras. A series of small NF1 fragments, which bind T‐Ras, as well as truncated forms or derivatives of c‐Raf‐1, c‐Jun and c‐Myc, are capable of blocking the T‐Ras‐activated mitogenesis in a competitive manner. These agents offer a unique opportunity to control the proliferation of T‐Ras‐associated tumors, which represent more than 30% of total human carcinomas. (shrink)

Rho/Rac proteins constitute a subgroup of the Ras superfamily of GTP hydrolases. Although originally implicated in the control of cytoskeletal events, it is currently known that these GTPases coordinate diverse cellular functions, including cell polarity, vesicular trafficking, the cell cycle and transcriptomal dynamics. In this review, we will provide an overview on the recent advances in this field regarding the mechanism of regulation and signaling, and the roles in vivo of this important GTPase family. BioEssays 29:356–370, 2007. © 2007 (...) Wiley Periodicals, Inc. (shrink)

There is a debate regarding the function of Drp1, a GTPase involved in mitochondrial fission, during the elimination of mitochondria by autophagy. A number of experiments indicate that Drp1 is needed to eliminate mitochondria during mitophagy, either by reducing the mitochondrial size or by providing a noncanonical mitophagy function. Yet, other convincing experimental results support the conclusion that Drp1 is not necessary. Here, we review the possible functions for Drp1 in mitophagy and autophagy, depending on tissues, organisms and stresses, (...) and discuss these apparent discrepancies. In this regard, it appears that the reduction of mitochondria size is often required for mitophagy but not always in a Drp1‐dependent manner. Finally, we speculate on Drp1‐independent mitochondrial fission mechanism that may take place during mitophagy and on noncanonical roles, which Drp1 may play such as modulating organelle contact sites dynamic during the autophagosome formation. (shrink)

Intercellular signaling by growth factors, hormones and neurotransmitters produces second messenger molecules such as cyclic adenosine monophosphate (cAMP) and diacylglycerol (DAG). Protein Kinase A and Protein Kinase C are the principal effector proteins of these prototypical second messengers in certain cell types. Recently, novel receptors for cAMP and DAG have been identified. These proteins, designated EPAC (Exchange Protein directly Activated by cAMP) or cAMP‐GEF (cAMP regulated Guanine nucleotide Exchange Factor) and CalDAG‐GEF (Calcium and Diacylglycerol regulated Guanine nucleotide Exchange Factor) or (...) RasGRP (Ras Guanine nucleotide Releasing Protein) are able to mediate some of the physiologic effects of the second messengers in a protein‐kinase‐independent fashion. These proteins are exchange factors for Ras family GTPases that operate in pathways that run parallel to the classic kinase‐dependent pathways. The rapidly emerging recognition of the functions of these “non‐kinase” effectors in diverse processes such as insulin secretion, thymocyte development, asthma and malignant transformation creates new opportunities for discovery and identifies potential new therapeutic targets. BioEssays 26:730–738, 2004. © 2004 Wiley Periodicals, Inc. (shrink)

The Hippo pathway, a cascade of protein kinases that inhibits the oncogenic transcriptional coactivators YAP and TAZ, was discovered in Drosophila as a major determinant of organ size in development. Known modes of regulation involve surface proteins that mediate cell‐cell contact or determine epithelial cell polarity which, in a tissue‐specific manner, use intracellular complexes containing FERM domain and actin‐binding proteins to modulate the kinase activities or directly sequester YAP. Unexpectedly, recent work demonstrates that GPCRs, especially those signaling through Galpha12/13 such (...) as the protease activated receptor PAR1, cause potent YAP dephosphorylation and activation. This response requires active RhoA GTPase and increased assembly of filamentous (F‐)actin. Morever, cell architectures that promote F‐actin assembly per se also activate YAP by kinase‐dependent and independent mechanisms. These findings unveil the ability of GPCRs to activate the YAP oncogene through a newly recognized signaling function of the actin cytoskeleton, likely to be especially important for normal and cancerous stem cells. (shrink)

Experiments with cell lines have unveiled the implication of the Rho/Rac family of GTPases in cytoskeletal organization, mitogenesis, and cell migration. However, there have not been adequate animal models to investigate the role of these proteins in more physiological settings. This scenario has changed recently in the case of the T‐cell lineage after the generation of animal models for Rho/Rac family members, their regulators, and effectors. These studies have revealed the implication of these GTPases on multiple regulatory layers of T‐cells, (...) including the coordination of cytoskeletal change, activation of kinase cascades, stimulation of calcium fluxes, and the induction of gene expression. These pathways affect the transition of different T‐cell maturation stages, the positive/negative selection of thymocytes, T‐cell responses to antigens, and the homeostasis of peripheral T‐lymphocytes. Moreover, these animals have revealed interesting cross‐talks between Rho/Rac pathways and other signal transduction routes that participate in lymphocyte responses. BioEssays 24:602–612, 2002. © 2002 Wiley Periodicals, Inc. (shrink)

Phagocytosis is an uptake of large particles governed by the actin-based cytoskeleton. Binding of particles to specific cell surface receptors is the first step of phagocytosis. In higher Eucaryota, the receptors able to mediate phagocytosis are expressed almost exclusively in macrophages, neutrophils, and monocytes, conferring immunodefence properties to these cells. Receptor clustering is thought to occur upon particle binding, that in turn generates a phagocytic signal. Several pathways of phagocytic signal transduction have been identified, including the activation of tyrosine kinases (...) and (or) serine/threonine kinase C in pivotal roles. Kinase activation leads to phosphorylation of the receptors and other proteins, recruited at the sites of phagocytosis. Monomeric GTPases of the Rho and ARF families are likely to be engaged downstream of activated receptors. The GTPases, in cooperation with phosphatidylinositol 4-phosphate 5-kinase and phosphatidylinositol 3-kinase lipid modifying enzymes, can modulate locally the assembly of the submembranous actin filament system leading to particle internalization. BioEssays 21:422–431, 1999. © 1999 John Wiley & Sons, Inc. (shrink)

Advances in the understanding of Ras oncoprotein function suggest novel points for anti‐tumor intervention. First, upstream‐acting guanine nucleotide exchange factors and SH2/SH3 domain‐containing adaptor proteins that link Ras with growth factor receptor tyrosine kinases have recently been characterized. Second, work on downstream‐acting Ras effector functions including the Ras GTPase‐activating protein (p120GAP) and the Raf kinase has revealed direct biochemical interactions that are functionally required for oncogenic Ras signalling. We summarize progress in these areas and discuss the potential for novel (...) applications to anti‐cancer chemotherapy. (shrink)

Receptor tyrosine kinases and integrins are activated by growth factors and extracellular matrix, respectively. Their activation leads to signal transduction cascades that control many aspects of cell phenotype, including progression through the G1 phase of the cell cycle. However, the signalling cassettes driven by growth factors and matrix do not work independently of each other. Integrin triggering is essential to facilitate kinase‐ and GTPase‐mediated signals and thereby drive efficient transfer of information through the growth factor–cyclin axis. A recent study (...) indicates that an additional type of player has a key role in adhesion‐regulated control of cell cycle, namely ubiquitin ligase.(1) BioEssays 24:17–21, 2002. © 2002 John Wiley & Sons, Inc. (shrink)

Parkinson's disease (PD) is generally sporadic but a number of genetic diseases have parkinsonism as a clinical feature. Two dominant genes, α‐synuclein (SNCA) and leucine‐rich repeat kinase 2 (LRRK2), are important for understanding inherited and sporadic PD. SNCA is a major component of pathologic inclusions termed Lewy bodies found in PD. LRRK2 is found in a significant proportion of PD cases. These two proteins may be linked as most LRRK2 PD cases have SNCA‐positive Lewy bodies. Mutations in both proteins are (...) associated with toxic effects in model systems although mechanisms are unclear. LRRK2 is an intracellular signaling protein possessing both GTPase and kinase activities that may contribute to pathogenicity. A third protein, tau, is implicated as a risk factor for PD. We discuss the potential relationship between these genes and suggest a model for PD pathogenesis where LRRK2 is upstream of pathogenic effects through SNCA, tau, or both proteins. (shrink)

Adherens junctions play pivotal roles in cell and tissue organization and patterning by mediating cell adhesion and cell signaling. These junctions consist of large multiprotein complexes that join the actin cytoskeleton to the plasma membrane to form adhesive contacts between cells or between cells and extracellular matrix. The best-known adherens junction is the zonula adherens (ZA) that forms a belt surrounding the apical pole of epithelial cells. Recent studies in Drosophila have further illuminated the structure of adherens junctions. Scaffolding proteins (...) encoded by the stardust gene are novel components of the Crumbs complex, which plays a critical role in ZA assembly.1-3 The small GTPase Rap1 controls the symmetric re-assembly of the ZA after cell division.4 Finally, the asymmetric distribution of adherens junction material regulates spindle orientation during asymmetric cell division in the sensory organ lineage.5 BioEssays 24:690–695, 2002. © 2002 Wiley Periodicals, Inc. (shrink)

In mammalian cells, a complex network of signaling pathways tightly regulates a variety of cellular processes, such as proliferation and differentiation. New insights from one of the most‐important signaling cascades involved in oncogenesis, the Ras–Raf–MAPK pathway, suggest that the subcellular localisation and assembly of signaling modules of this pathway is crucial to control the biological response. This commonly requires membrane targeting events that are mediated by adaptor/scaffold proteins. Of particular interest is the translocation and complex formation of GTPase‐activating proteins (...) (GAPs), such as p120GAP, at the plasma membrane to inactivate Ras. Recent studies indicate that one member of the annexin family, annexin A6 acts as a targeting protein for p120GAP. This review discusses how annexin A6 modulates the involvement of negative regulators of the Ras–Raf–MAPK pathway contributing to Ras inactivation. BioEssays 28: 1211–1220, 2006. © 2006 Wiley Periodicals, Inc. (shrink)

The Segregation Distorter (SD) system of Drosophila melanogaster is one the best‐characterized meiotic drive complexes known. SD gains an unfair transmission advantage through heterozygous SD/SD+ males by incapacitating SD+‐bearing spermatids so that virtually all progeny inherit SD. Segregation distorter (Sd), the primary distorting locus in the SD complex, is a truncated duplication of the RanGAP gene, a major regulator of the small GTPase Ran, which has several functions including the maintenance of the nucleocytoplasmic RanGTP concentration gradient that mediates nuclear (...) transport. The truncated Sd‐RanGAP protein is enzymatically active but mislocalizes to the nucleus where it somehow causes distortion. Here I present data consistent with the idea that wild‐type RanGAP, and possibly other loci able to influence the RanGTP gradient, has been caught up in an ancient genetic conflict that predates the SD complex. The legacy of this conflict could include the unexpectedly rapid evolution of nuclear transport‐related proteins, the accumulation of chromosomal inversions, the recruitment of gene duplications, and the turnover of repetitive sequences in the centric heterochromatin. BioEssays 29:386–391, 2007. © 2007 Wiley Periodicals, Inc. (shrink)

The harmonious growth and cell‐to‐cell uniformity of steady‐state bacterial populations indicate the existence of a well‐regulated cell cycle, responding to a set of internal signals. In Escherichia coli, the key events of this cycle are the initiation of DNA replication, nucleoid segregation and the initiation of cell division. The replication initiator is the DnaA protein. In nucleoid segregation, the MukB protein, required for proper partitioning, may be a member of the myosin‐kinesin superfamily of mechanoenzymes. In cell division, the FtsZ protein (...) has a tubulin motif, is a GTPase and polymerizes in a ring around midcell during septation; the FtsA protein has an actin‐like structure. The nature of the internal signals triggering these events is not known but candidates include cell mass, the superhelical density of the chromosome and the concentration of two regulatory nucleotides, cyclic AMP and ppGpp. The involvement of cytoskeletal‐like proteins in key cycle events encourages the notion of a fundamental biological unity in cell cycle regulation in all organisms. (shrink)