Transforming activity of the Rho family GTPase, Wrch-1, a Wnt-regulated Cdc42 homolog, is dependent on a novel carboxyl-terminal palmitoylation motif

Atypical RhoUV GTPases in development and disease

RhoU and RhoV are members of the Rho family of small GTPases that comprise their own subfamily. RhoUV GTPases are classified as atypical due to the kinetics of their GTP/GDP binding cycles. They also possess unique N- and C-termini that regulate their subcellular localization and activity. RhoU and RhoV have been linked to cytoskeletal regulation, cell adhesion, and cell migration. They each exhibit distinct expression patterns during embryonic development and diseases such as cancer metastasis, suggesting they have specialized functions. In this review, we will discuss the known functions of RhoU and RhoV, with a focus on their roles in early development, organogenesis, and disease.

Molecular basis and current insights of atypical Rho small GTPase in cancer

Atypical Rho GTPases are a subtype of the Rho GTPase family that are involved in diverse cellular processes. The typical Rho GTPases, led by RhoA, Rac1 and Cdc42, have been well studied, while relative studies on atypical Rho GTPases are relatively still limited and have great exploration potential. With the increase in studies, current evidence suggests that atypical Rho GTPases regulate multiple biological processes and play important roles in the occurrence and development of human cancers. Therefore, this review mainly discusses the molecular basis of atypical Rho GTPases and their roles in cancer. We summarize the sequence characteristics, subcellular localization and biological functions of each atypical Rho GTPase. Moreover, we review the recent advances and potential mechanisms of atypical Rho GTPases in the development of multiple cancers. A comprehensive understanding and extensive exploration of the biological functions of atypical Rho GTPases and their molecular mechanisms in tumors will provide important insights into the pathophysiology of tumors and the development of cancer therapeutic strategies.

RhoU forms homo-oligomers to regulate cellular responses

RhoU is an atypical member of the Rho family of small G-proteins, which has N- and C-terminal extensions compared to the classic Rho GTPases RhoA, Rac1 and Cdc42, and associates with membranes through C-terminal palmitoylation rather than prenylation. RhoU mRNA expression is upregulated in prostate cancer and is considered a marker for disease progression. Here, we show that RhoU overexpression in prostate cancer cells increases cell migration and invasion. To identify RhoU targets that contribute to its function, we found that RhoU homodimerizes in cells. We map the region involved in this interaction to the C-terminal extension and show that C-terminal palmitoylation is required for self-association. Expression of the isolated C-terminal extension reduces RhoU-induced activation of p21-activated kinases (PAKs), which are known downstream targets for RhoU, and induces cell morphological changes consistent with inhibiting RhoU function. Our results show for the first time that the activity of a Rho family member is stimulated by self-association, and this is important for its activity.

PI3K Functions Downstream of Cdc42 to Drive Cancer phenotypes in a Melanoma Cell Line

Rho proteins are part of the Ras superfamily, which function to modulate cytoskeletal dynamics including cell adhesion and motility. Recently, an activating mutation in Cdc42, a Rho family GTPase, was found in a patient sample of melanoma. Previously, our work had shown the PI3K was important downstream of mutationally active Cdc42. Our present study sought to determine whether PI3K was a crucial downstream partner for Cdc42 in a melanoma cells line with a BRAF mutation, which is the most common mutation in cutaneous melanoma. In this work we were able to show that Cdc42 contributes to proliferation, anchorage-independent growth, cell motility and invasion. Treatment with a pan-PI3K inhibitor was able to effectively ameliorate all these cancer phenotypes. These data suggest that PI3K may be an important target downstream of Cdc42 in melanoma.

Rho GTPases: Non-canonical regulation by cysteine oxidation

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 (H₂ O₂ ), 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.

The atypical Rho GTPase RhoU interacts with intersectin-2 to regulate endosomal recycling pathways

Rho GTPases play a key role in various membrane trafficking processes. RhoU is an atypical small Rho GTPase related to Rac/Cdc42, which possesses unique N- and C-terminal domains that regulate its function and its subcellular localization. RhoU localizes at the plasma membrane, on endosomes and in cell adhesion structures where it governs cell signaling, differentiation and migration. However, despite its endomembrane localization, RhoU function in vesicular trafficking has been unexplored. Here, we identified intersectins (ITSNs) as new binding partners for RhoU and showed that the second PxxP motif at the N terminus of RhoU mediated interactions with the SH3 domains of ITSNs. To evaluate the function of RhoU and ITSNs in vesicular trafficking, we used fluorescent transferrin as a cargo for uptake experiments. We showed that silencing of either RhoU or ITSN2, but not ITSN1, increased transferrin accumulation in early endosomes, resulting from a defect in fast vesicle recycling. Concomitantly, RhoU and ITSN2 colocalized to a subset of Rab4-positive vesicles, suggesting that a RhoU-ITSN2 interaction may occur on fast recycling endosomes to regulate the fate of vesicular cargos.

RAS and RHO family GTPase mutations in cancer: twin sons of different mothers?

The RAS and RHO family comprise two major branches of the RAS superfamily of small GTPases. These proteins function as regulated molecular switches and control cytoplasmic signaling networks that regulate a diversity of cellular processes, including cell proliferation and cell migration. In the early 1980s, mutationally activated RAS genes encoding KRAS, HRAS and NRAS were discovered in human cancer and now comprise the most frequently mutated oncogene family in cancer. Only recently, exome sequencing studies identified cancer-associated alterations in two RHO family GTPases, RAC1 and RHOA. RAS and RHO proteins share significant identity in their amino acid sequences, protein structure and biochemistry. Cancer-associated RAS mutant proteins harbor missense mutations that are found primarily at one of three mutational hotspots (G12, G13 and Q61) and have been identified as gain-of-function oncogenic alterations. Although these residues are conserved in RHO family proteins, the gain-of-function mutations found in RAC1 are found primarily at a distinct hotspot. Unexpectedly, the cancer-associated mutations found with RHOA are located at different hotspots than those found with RAS. Furthermore, since the RHOA mutations suggested a loss-of-function phenotype, it has been unclear whether RHOA functions as an oncogene or tumor suppressor in cancer development. Finally, whereas RAS mutations are found in a broad spectrum of cancer types, RHOA and RAC1 mutations occur in a highly restricted range of cancer types. In this review, we focus on RHOA missense mutations found in cancer and their role in driving tumorigenesis, with comparisons to cancer-associated mutations in RAC1 and RAS GTPases.

Dysregulation of Rho GTPases in Human Cancers

Rho GTPases play central roles in numerous cellular processes, including cell motility, cell polarity, and cell cycle progression, by regulating actin cytoskeletal dynamics and cell adhesion. Dysregulation of Rho GTPase signaling is observed in a broad range of human cancers, and is associated with cancer development and malignant phenotypes, including metastasis and chemoresistance. Rho GTPase activity is precisely controlled by guanine nucleotide exchange factors, GTPase-activating proteins, and guanine nucleotide dissociation inhibitors. Recent evidence demonstrates that it is also regulated by post-translational modifications, such as phosphorylation, ubiquitination, and sumoylation. Here, we review the current knowledge on the role of Rho GTPases, and the precise mechanisms controlling their activity in the regulation of cancer progression. In addition, we discuss targeting strategies for the development of new drugs to improve cancer therapy.

Lipogenic signalling modulates prostate cancer cell adhesion and migration via modification of Rho GTPases

Fatty acid synthase (FASN) is commonly overexpressed in prostate cancer and associated with tumour progression. FASN is responsible for de novo synthesis of the fatty acid palmitate; the building block for protein palmitoylation. Recent work has suggested that alongside its established role in promoting cell proliferation FASN may also promote invasion. We now find depletion of FASN expression increases prostate cancer cell adhesiveness, impairs HGF-mediated cell migration and reduces 3D invasion. These changes in motility suggest that FASN can mediate actin cytoskeletal remodelling; a process known to be downstream of Rho family GTPases. Here, we demonstrate that modulation of FASN expression specifically impacts on the palmitoylation of the atypical GTPase RhoU. Impaired RhoU activity in FASN depleted cells leads to reduced adhesion turnover downstream of paxillin serine phosphorylation, which is rescued by addition of exogenous palmitate. Moreover, canonical Cdc42 expression is dependent on the palmitoylation status of RhoU. Thus we uncover a novel relationship between FASN, RhoU and Cdc42 that directly influences cell migration potential. These results provide compelling evidence that FASN activity directly promotes cell migration and supports FASN as a potential therapeutic target in metastatic prostate cancer.

Withania somnifera root extract inhibits fatty acid synthesis in prostate cancer cells

Prior research argues for a role of increased de novo fatty acid synthesis in pathogenesis of prostate adenocarcinoma, which remains a leading cause of cancer-associated mortality in American men. A safe and effective inhibitor of fatty acid synthesis is still a clinically unmet need. Herein, we investigated the effect of ethanol extract of Withania somnifera root (WRE) standardized for one of its components (withaferin A) on fatty acid synthesis using LNCaP and 22Rv1 human prostate cancer cells. Withania somnifera is a medicinal plant used in the Ayurvedic medicine practiced in India. Western blotting and confocal microscopy revealed a statistically significant decrease in protein levels of key fatty acid metabolism enzymes including ATP citrate lyase (ACLY), acetyl-CoA carboxylase 1 (ACC1), fatty acid synthase (FASN), and carnitine palmitoyltransferase 1A (CPT1A) in WRE-treated cells compared with solvent control. The mRNA levels of ACLY, ACC1, FASN, and CPT1A were also lower in WRE-treated cells in comparison with control. Consequently, WRE treatment resulted in a significant decrease in intracellular levels of acetyl-CoA, total free fatty acids, and neutral lipid droplets in both LNCaP and 22Rv1 cells. WRE exhibited greater potency for fatty acid synthesis inhibition at equimolar concentration than cerulenin and etomoxir. Exposure to WRE results in downregulation of c-Myc and p-Akt(S473) proteins in 22Rv1 cell line. However, overexpression of only c-Myc conferred protection against clonogenic cell survival and lipogenesis inhibition by WRE. In conclusion, these results indicate that WRE is a novel inhibitor of fatty acid synthesis in human prostate cancer cells.

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Withania somnifera is a medicinal plant used in alternative medicine (Ayurvedic medicine) system practiced in India.

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Withania somnifera root extract exhibits a variety of pharmacological effects including anticancer activity.

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The present study reveals that Withania somnifera root extract is a potent inhibitor of lipogenesis in prostate cancer cells.

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Inhibition of lipogenesis seems to be a useful biomarker of anticancer efficacy of Withania somnifera in prostate cancer.

Regulation and functions of RhoU and RhoV

Rho GTPases play central roles in a wide variety of cellular processes, including cytoskeletal dynamics, cell adhesion and cell polarity. RhoU and RhoV are Rho GTPases that have some atypical properties compared with classical Rho family members, such as the presence of N- and C-terminal extension regions, unusual GDP/GTP cycling and post-translational modification by palmitoylation but not prenylation. Their activity and localization is regulated by the N-terminal and C-terminal regions, and so far no GEFs or GAPs have been identified for them. Similar to Rac and Cdc42, they interact with PAK serine/threonine kinases, and in the case of PAK4, this interaction leads to RhoU protein stabilization. In cells, RhoU and RhoV alter cell shape and cell adhesion, which probably underlies some of the phenotypes reported for these proteins in vivo, for example in heart development and epithelial morphogenesis. However, the molecular basis for these functions of RhoU and RhoV remains to be characterized.

Building Blood Vessels-One Rho GTPase at a Time

Blood vessels are required for the survival of any organism larger than the oxygen diffusion limit. Blood vessel formation is a tightly regulated event and vessel growth or changes in permeability are linked to a number of diseases. Elucidating the cell biology of endothelial cells (ECs), which are the building blocks of blood vessels, is thus critical to our understanding of vascular biology and to the development of vascular-targeted disease treatments. Small GTPases of the Rho GTPase family are known to regulate several processes critical for EC growth and maintenance. In fact, many of the 21 Rho GTPases in mammals are known to regulate EC junctional remodeling, cell shape changes, and other processes. Rho GTPases are thus an attractive target for disease treatments, as they often have unique functions in specific vascular cell types. In fact, some Rho GTPases are even expressed with relative specificity in diseased vessels. Interestingly, many Rho GTPases are understudied in ECs, despite their known expression in either developing or mature vessels, suggesting an even greater wealth of knowledge yet to be gleaned from these complex signaling pathways. This review aims to provide an overview of Rho GTPase signaling contributions to EC vasculogenesis, angiogenesis, and mature vessel barrier function. A particular emphasis is placed on so-called "alternative" Rho GTPases, as they are largely understudied despite their likely important contributions to EC biology.

The atypical RhoU/Wrch1 Rho GTPase controls cell proliferation and apoptosis in the gut epithelium

BACKGROUND

The mammalian gut epithelium displays among the highest rates of self-renewal, with a turnover time of less than 5 days. Renewal involves concerted proliferation at the bottom of the crypt, migration and differentiation along the crypt-villus axis and anoïkis/shedding in the luminal epithelium. Renewal is controlled by interplay between signalling pathways, among which canonical and non-canonical Wnt signals play prominent roles. Overall 92% of colon tumours show increased canonical Wnt signalling resulting from mutations, established as major driver steps towards carcinogenesis.

RESULTS

Here, we examined the physiological role of RhoU/Wrch1 in gut homeostasis. RhoU is an atypical Rho GTPase related to Cdc42/Rac1 and identified as a transcriptional target of non-canonical Wnt signalling. We found that RHOU expression is reduced in human colorectal tumour samples. We show that RhoU is mainly expressed in the differentiated compartment of the gut epithelium. Rhou specific invalidation in the mouse gut elicits cell hyperplasia and is associated in the colon with a highly disorganized luminal epithelium. Hyperplasia affects all cell types in the small intestine and colon and has a higher impact on goblet cells. Hyperplasia is associated with a reduction of apoptosis and an increased proliferation. RhoU knockdown in human DLD-1 colon cancer cells also elicits a higher growth index and reduces cell apoptosis. Last, loss of RhoU function in the mouse gut epithelium or in DLD-1 cells increases RhoA activity and the level of phosphorylated Myosin Light Chain-2, which may functionally link RhoU activity to apoptosis.

CONCLUSION

RhoU is mostly expressed in the differentiated compartment of the gut. It plays a role in homeostasis as its specific invalidation elicits hyperplasia of all cell types. This mainly results from a reduction of apoptosis, through actomyosin-dependent mechanisms.

SIGNIFICANCE

RhoU negatively controls cell growth in the intestinal epithelium. Since its expression is sensitive to non-canonical Wnt signals and is reduced in colorectal tumours, downregulating RhoU may thus have an instrumental role in tumour progression.

Activated Rho GTPases in Cancer-The Beginning of a New Paradigm

Involvement of Rho GTPases in cancer has been a matter of debate since the identification of the first members of this branch of the Ras superfamily of small GTPases. The Rho GTPases were ascribed important roles in the cell, although these were restricted to regulation of cytoskeletal dynamics, cell morphogenesis, and cell locomotion, with initially no clear indications of direct involvement in cancer progression. This paradigm has been challenged by numerous observations that Rho-regulated pathways are often dysregulated in cancers. More recently, identification of point mutants in the Rho GTPases Rac1, RhoA, and Cdc42 in human tumors has finally given rise to a new paradigm, and we can now state with confidence that Rho GTPases serve as oncogenes in several human cancers. This article provides an exposé of current knowledge of the roles of activated Rho GTPases in cancers.

The Atypical Rho GTPase CHW-1 Works with SAX-3/Robo To Mediate Axon Guidance in Caenorhabditis elegans

During development, neuronal cells extend an axon toward their target destination in response to a cue to form a properly functioning nervous system. Rho proteins, Ras-related small GTPases that regulate cytoskeletal organization and dynamics, cell adhesion, and motility, are known to regulate axon guidance. Despite extensive knowledge about canonical Rho proteins (RhoA/Rac1/Cdc42), little is known about the Caenorhabditis elegans (C. elegans) atypical Cdc42-like family members CHW-1 and CRP-1 in regards to axon pathfinding and neuronal migration. chw-1(Chp/Wrch) encodes a protein that resembles human Chp (Wrch-2/RhoV) and Wrch-1 (RhoU), and crp-1 encodes for a protein that resembles TC10 and TCL. Here, we show that chw-1 works redundantly with crp-1 and cdc-42 in axon guidance. Furthermore, proper levels of chw-1 expression and activity are required for proper axon guidance. When examining CHW-1 GTPase mutants, we found that the native CHW-1 protein is likely partially activated, and mutations at a conserved residue (position 12 using Ras numbering, position 18 in CHW-1) alter axon guidance and neural migration. Additionally, we showed that chw-1 genetically interacts with the guidance receptor sax-3 in PDE neurons. Finally, in VD/DD motor neurons, chw-1 works downstream of sax-3 to control axon guidance. In summary, this is the first study implicating the atypical Rho GTPases chw-1 and crp-1 in axon guidance. Furthermore, this is the first evidence of genetic interaction between chw-1 and the guidance receptor sax-3 These data suggest that chw-1 is likely acting downstream and/or in parallel to sax-3 in axon guidance.

Fast-cycling Rho GTPases

The Rho GTPases were discovered more than 30 years ago, and they were for a long time considered to follow simple cycling between GDP-bound and GTP-bound conformations, as for the Ras subfamily of small GTPases. The Rho GTPases consist of 20 members, but at least 10 of these do not follow this classical GTPase cycle. Thus, based on their kinetic properties, these Rho GTPases can instead be classified as atypical. Some of these atypical Rho GTPases do not hydrolyze GTP, and some have significantly increased intrinsic GDP/GTP exchange activity. This review focuses on this latter category of atypical Rho GTPases, the so-called 'fast-cycling' Rho GTPases. The different members of these fast-cycling atypical Rho GTPases are described in more detail here, along with their potential regulatory mechanisms. Finally, some insights are provided into the involvement of the atypical Rho GTPases in human pathologies.

Protein Lipidation: Occurrence, Mechanisms, Biological Functions, and Enabling Technologies

Protein lipidation, including cysteine prenylation, N-terminal glycine myristoylation, cysteine palmitoylation, and serine and lysine fatty acylation, occurs in many proteins in eukaryotic cells and regulates numerous biological pathways, such as membrane trafficking, protein secretion, signal transduction, and apoptosis. We provide a comprehensive review of protein lipidation, including descriptions of proteins known to be modified and the functions of the modifications, the enzymes that control them, and the tools and technologies developed to study them. We also highlight key questions about protein lipidation that remain to be answered, the challenges associated with answering such questions, and possible solutions to overcome these challenges.

Regulating Rho GTPases and their regulators

Rho GTPases regulate cytoskeletal and cell adhesion dynamics and thereby coordinate a wide range of cellular processes, including cell migration, cell polarity and cell cycle progression. Most Rho GTPases cycle between a GTP-bound active conformation and a GDP-bound inactive conformation to regulate their ability to activate effector proteins and to elicit cellular responses. However, it has become apparent that Rho GTPases are regulated by post-translational modifications and the formation of specific protein complexes, in addition to GTP-GDP cycling. The canonical regulators of Rho GTPases - guanine nucleotide exchange factors, GTPase-activating proteins and guanine nucleotide dissociation inhibitors - are regulated similarly, creating a complex network of interactions to determine the precise spatiotemporal activation of Rho GTPases.

The C. elegans Chp/Wrch Ortholog CHW-1 Contributes to LIN-18/Ryk and LIN-17/Frizzled Signaling in Cell Polarity

Wnt signaling controls various aspects of developmental and cell biology, as well as contributing to certain cancers. Expression of the human Rho family small GTPase Wrch/RhoU is regulated by Wnt signaling, and Wrch and its paralog Chp/RhoV are both implicated in oncogenic transformation and regulation of cytoskeletal dynamics. We performed developmental genetic analysis of the single Caenorhabditis elegans ortholog of Chp and Wrch, CHW-1. Using a transgenic assay of the distal tip cell migration, we found that wild-type CHW-1 is likely to be partially constitutively active and that we can alter ectopic CHW-1-dependent migration phenotypes with mutations predicted to increase or decrease intrinsic GTP hydrolysis rate. The vulval P7.p polarity decision balances multiple antagonistic Wnt signals, and also uses different types of Wnt signaling. Previously described cooperative Wnt receptors LIN-17/Frizzled and LIN-18/Ryk orient P7.p posteriorly, with LIN-17/Fz contributing approximately two-thirds of polarizing activity. CHW-1 deletion appears to equalize the contributions of these two receptors. We hypothesize that CHW-1 increases LIN-17/Fz activity at the expense of LIN-18/Ryk, thus making the contribution of these signals unequal. For P7.p to polarize correctly and form a proper vulva, LIN-17/Fz and LIN-18/Ryk antagonize other Wnt transmembrane systems VANG-1/VanGogh and CAM-1/Ror. Our genetic data suggest that LIN-17/Fz represses both VANG-1/VanGogh and CAM-1/Ror, while LIN-18/Ryk represses only VANG-1. These data expand our knowledge of a sophisticated signaling network to control P7.p polarity, and suggests that CHW-1 can alter ligand gradients or receptor priorities in the system.

Spatial and temporal control of Rho GTPase functions

Rho family GTPases control almost every aspect of cell physiology and, since their discovery, a wealth of knowledge has accumulated about their biochemical regulation and function. However, each Rho GTPase distributes between multiple cellular compartments, even within the same cell, where they are controlled by multiple regulators and signal to multiple effectors. Thus, major questions about spatial and temporal aspects of regulation remain unanswered. In particular, what are the nano-scale dynamics for their activation, membrane targeting, diffusion, effector activation and GTPase inactivation? How do these mechanisms differ in the different cellular compartments where Rho GTPases function? Addressing these complex aspects of Rho GTPase biology will significantly advance our understanding of the spatial and temporal control of cellular functions.

Dysfunction of vascular smooth muscle and vascular remodeling by simvastatin

Statins, inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, are widely prescribed for hypercholesterolemia. With the increasing use of statins, numerous reports demonstrated that statins can cause damage to skeletal muscles. However, the toxicities of statins on vascular smooth muscle, which are essential to cardiovascular homeostasis, have not been previously described. Here, we examined the effects of simvastatin on the contractile function and the integrity of vascular smooth muscle in isolated rat thoracic aortic rings, primary cultured vascular smooth muscle cells (VSMCs) in vitro and rats in vivo. In aortic rings, simvastatin suppressed the normal agonist-induced contractile responses in time- and concentration-dependent manners (0.86 g ± 0.11 at 10 μM simvastatin for 24 h compared with 1.89 g ± 0.11 at control). The suppression persisted in the endothelium-denuded aortic rings and was irreversible even after wash-out of simvastatin. Simvastatin suppressed the contraction induced by Bay K8644, an activator of voltage-operated Ca²⁺ channel (VOCC) in rat aortic rings and abolished agonist-induced intracellular Ca²⁺ increase in VSMCs. The simvastatin-induced contractile dysfunction was reversed by the supplementation of mevalonate and geranylgeranylpyrophosphate, precursors for protein isoprenylation. Consistently, activation of RhoA, a representative isoprenylated protein, was disrupted by simvastatin in VSMCs and RhoA-mediated phosphorylation of MYPT1 and CPI-17, and tonic tension were also suppressed. Notably, prolonged treatment of simvastatin up to 48 h induced apoptosis of vascular smooth muscle in aortic rings. Most importantly, simvastatin treatment in vivo significantly attenuated the agonist-induced vasoconstriction in rats ex vivo and induced a decrease in luminal area of the vascular wall. Collectively, these results demonstrate that simvastatin can impair the normal vascular contractility by disturbing Ca²⁺ influx and RhoA activity, ultimately leading to apoptosis and structural remodeling.

Recognition of the protein kinase AVRPPHB SUSCEPTIBLE1 by the disease resistance protein RESISTANCE TO PSEUDOMONAS SYRINGAE5 is dependent on s-acylation and an exposed loop in AVRPPHB SUSCEPTIBLE1

The recognition of pathogen effector proteins by plants is typically mediated by intracellular receptors belonging to the nucleotide-binding leucine-rich repeat (NLR) family. NLR proteins often detect pathogen effector proteins indirectly by detecting modification of their targets. How NLR proteins detect such modifications is poorly understood. To address these questions, we have been investigating the Arabidopsis (Arabidopsis thaliana) NLR protein RESISTANCE TO PSEUDOMONAS SYRINGAE5 (RPS5), which detects the Pseudomonas syringae effector protein Avirulence protein Pseudomonas phaseolicolaB (AvrPphB). AvrPphB is a cysteine protease that specifically targets a subfamily of receptor-like cytoplasmic kinases, including the Arabidopsis protein kinase AVRPPHB Susceptible1 (PBS1). RPS5 is activated by the cleavage of PBS1 at the apex of its activation loop. Here, we show that RPS5 activation requires that PBS1 be localized to the plasma membrane and that plasma membrane localization of PBS1 is mediated by amino-terminal S-acylation. We also describe the development of a high-throughput screen for mutations in PBS1 that block RPS5 activation, which uncovered four new pbs1 alleles, two of which blocked cleavage by AvrPphB. Lastly, we show that RPS5 distinguishes among closely related kinases by the amino acid sequence (SEMPH) within an exposed loop in the C-terminal one-third of PBS1. The SEMPH loop is located on the opposite side of PBS1 from the AvrPphB cleavage site, suggesting that RPS5 associates with the SEMPH loop while leaving the AvrPphB cleavage site exposed. These findings provide support for a model of NLR activation in which NLR proteins form a preactivation complex with effector targets and then sense a conformational change in the target induced by effector modification.

Rho GTPases in endoderm development and differentiation

The embryonic foregut of the mouse embryo is lined by a layer of endoderm cells whose architecture changes during development. The transition from a squamous to columnar epithelial morphology is accompanied by the upregulation of an atypical Rho GTPase, Rhou. Subsequently, multi-layering of the epithelium at the site of organ bud formation is associated with the downregulation of Rhou. Rho-related small GTPases are known to play multiple roles in establishing and maintaining epithelial polarity, cytoskeletal organization, morphogenesis and differentiation of epithelial tissues, but their role in the early development of the endoderm in mammals is largely unexplored. Our recent study has shown that Rhou is required for maintaining F-actin polarization, epithelial morphogenesis and differentiation of the endoderm. Rhou expression responds to canonical WNT signaling and its activity influences the cytoskeletal organization and differentiation of endodermal cells, possibly via activation of JNK-mediated pathways. In this context, Rhou provides a possible link between β-catenin dependent WNT signaling and cellular processes normally associated with WNT/PCP pathways.

Identification of a novel prenyl and palmitoyl modification at the CaaX motif of Cdc42 that regulates RhoGDI binding

Membrane localization of Rho GTPases is essential for their biological functions and is dictated in part by a series of posttranslational modifications at a carboxyl-terminal CaaX motif: prenylation at cysteine, proteolysis of the aaX tripeptide, and carboxymethylation. The fidelity and variability of these CaaX processing steps are uncertain. The brain-specific splice variant of Cdc42 (bCdc42) terminates in a CCIF sequence. Here we show that brain Cdc42 undergoes two different types of posttranslational modification: classical CaaX processing or novel tandem prenylation and palmitoylation at the CCaX cysteines. In the dual lipidation pathway, bCdc42 was prenylated, but it bypassed proteolysis and carboxymethylation to undergo modification with palmitate at the second cysteine. The alternative postprenylation processing fates were conserved in the GTPases RalA and RalB and the phosphatase PRL-3, proteins terminating in a CCaX motif. The differentially modified forms of bCdc42 displayed functional differences. Prenylated and palmitoylated brain Cdc42 did not interact with RhoGDIα and was enriched in the plasma membrane relative to the classically processed form. The alternative processing of prenylated CCaX motif proteins by palmitoylation or by endoproteolysis and methylation expands the diversity of signaling GTPases and enables another level of regulation through reversible modification with palmitate.

Rho GTPase regulation by miRNAs and covalent modifications

To date, most studies of Rho GTPase regulation have focused on the classic GTPase cycle - GTP binding and hydrolysis - controlled by guanine nucleotide exchange factors (GEFs), GTPase-activating proteins (GAPs) and GDP-dissociation inhibitors (GDIs). Recent investigations have unveiled important additional regulatory mechanisms: microRNA (miRNA) regulating post-transcriptional processing of Rho GTPase-encoding mRNAs; palmitoylation and nuclear targeting affecting intracellular distribution; post-translational phosphorylation, transglutamination and AMPylation impacting Rho GTPase signaling; and ubiquitination controlling Rho GTPase protein stability and turnover. These modes of regulation add to the complexity of the Rho GTPase signaling network and allow precise spatiotemporal control of individual Rho GTPases. This review discusses these 'unconventional' modes of regulation and their contribution to cellular function, focusing on post-transcriptional and post-translational events beyond the classic GTPase cycle regulatory model.

Identification of a bipartite focal adhesion localization signal in RhoU/Wrch-1, a Rho family GTPase that regulates cell adhesion and migration

BACKGROUND INFORMATION

Rho GTPases are important regulators of cytoskeleton dynamics and cell adhesion. RhoU/Wrch-1 is a Rho GTPase which shares sequence similarities with Rac1 and Cdc42 (cell division cycle 42), but has also extended N- and C-terminal domains. The N-terminal extension promotes binding to SH3 (Src homology 3)-domain-containing adaptors, whereas the C-terminal extension mediates membrane targeting through palmitoylation of its non-conventional CAAX box. RhoU/Wrch-1 possesses transforming activity, which is negatively regulated by its N-terminal extension and depends on palmitoylation.

RESULTS

In the present study, we have shown that RhoU is localized to podosomes in osteoclasts and c-Src-expressing cells, and to focal adhesions of HeLa cells and fibroblasts. The N-terminal extension and the palmitoylation site were dispensable, whereas the C-terminal extension and effector binding loop were critical for RhoU targeting to focal adhesions. Moreover, the number of focal adhesions was reduced and their distribution changed upon expression of activated RhoU. Conversely, RhoU silencing increased the number of focal adhesions. As RhoU was only transiently associated with adhesion structures, this suggests that RhoU may modify adhesion turnover and cell migration rate. Indeed, we found that migration distances were increased in cells expressing activated RhoU and decreased when RhoU was knocked-down.

CONCLUSIONS

Our data indicate that RhoU localizes to adhesion structures, regulates their number and distribution and increases cell motility. It also suggests that the RhoU effector binding and C-terminal domains are critical for these functions.

Posttranslational lipid modification of Rho family small GTPases

The Rho family comprises a major branch of the Ras superfamily of small GTPases. A majority of Rho GTPases are synthesized as inactive, cytosolic proteins. They then undergo posttranslational modification by isoprenoid or fatty acid lipids, and together with additional carboxyl-terminal sequences target Rho GTPases to specific membrane and subcellular compartments essential for function. We summarize the use of biochemical and cellular assays and pharmacologic inhibitors instrumental for the study of the role of posttranslational lipid modifications and processing in Rho GTPase biology.

Off the beaten paths: alternative and crosstalk regulation of Rho GTPases

Rho proteins are small GTPases of the Ras superfamily that regulate a wide variety of biological processes, ranging from gene expression to cell migration. Mechanistically, the major Rho GTPases function as molecular switches cycling between an inactive GDP-bound and an active GTP-bound conformation, although several Rho proteins spontaneously exchange nucleotides or are simply devoid of GTPase activity. For over a decade, RhoGEFs and RhoGAPs have been established as the mainstream regulators of Rho proteins, respectively flipping the switch on or off. However, regulation by GEFs and GAPs leaves several fundamental questions on the operation of the Rho switch unanswered, indicating that the regulation of Rho proteins does not rely exclusively on RhoGEFs and RhoGAPs. Recent evidence indeed suggests that Rho GTPases are finely tuned by multiple alternative regulatory mechanisms, including post-translational modifications and protein degradation, as well as crosstalk mechanisms between Rho proteins. Here we review these alternative mechanisms and discuss how they alter Rho protein function and signaling. We also envision how the classic binary Rho switch may indeed function more like a switchboard with multiple switches and dials that can all contribute to the regulation of Rho protein function.

Proteomic profiling of S-acylated macrophage proteins identifies a role for palmitoylation in mitochondrial targeting of phospholipid scramblase 3

S-Palmitoylation, the reversible post-translational acylation of specific cysteine residues with the fatty acid palmitate, promotes the membrane tethering and subcellular localization of proteins in several biological pathways. Although inhibiting palmitoylation holds promise as a means for manipulating protein targeting, advances in the field have been hampered by limited understanding of palmitoylation enzymology and consensus motifs. In order to define the complement of S-acylated proteins in the macrophage, we treated RAW 264.7 macrophage membranes with hydroxylamine to cleave acyl thioesters, followed by biotinylation of newly exposed sulfhydryls and streptavidin-agarose affinity chromatography. Among proteins identified by LC-MS/MS, S-acylation status was established by spectral counting to assess enrichment under hydroxylamine versus mock treatment conditions. Of 1183 proteins identified in four independent experiments, 80 proteins were significant for S-acylation at false discovery rate = 0.05, and 101 significant at false discovery rate = 0.10. Candidate S-acylproteins were identified from several functional categories, including membrane trafficking, signaling, transporters, and receptors. Among these were 29 proteins previously biochemically confirmed as palmitoylated, 45 previously reported as putative S-acylproteins in proteomic screens, 24 not previously associated with palmitoylation, and three presumed false-positives. Nearly half of the candidates were previously identified by us in macrophage detergent-resistant membranes, suggesting that palmitoylation promotes lipid raft-localization of proteins in the macrophage. Among the candidate novel S-acylproteins was phospholipid scramblase 3 (Plscr3), a protein that regulates apoptosis through remodeling the mitochondrial membrane. Palmitoylation of Plscr3 was confirmed through (3)H-palmitate labeling. Moreover, site-directed mutagenesis of a cluster of five cysteines (Cys159-161-163-164-166) abolished palmitoylation, caused Plscr3 mislocalization from mitochondrion to nucleus, and reduced macrophage apoptosis in response to etoposide, together suggesting a role for palmitoylation at this site for mitochondrial targeting and pro-apoptotic function of Plscr3. Taken together, we propose that manipulation of protein palmitoylation carries great potential for intervention in macrophage biology via reprogramming of protein localization.

GRB2 couples RhoU to epidermal growth factor receptor signaling and cell migration

RhoU, an atypical Rho family member with unique N- and C-terminal extensions, associates with activated EGFR in a GRB2-dependent manner via its N-terminal proline-rich motifs and is integrated into EGFR-mediated signaling, leading to AP1 transcriptional activity and cell migration.

RhoU is an atypical Rho family member with high homology to CDC42 but containing unique N- and C-terminal extensions. The mechanisms regulating RhoU activation, as well as its downstream effectors, are not fully characterized. We show that after epidermal growth factor (EGF) stimulation RhoU colocalizes with EGF receptor (EGFR) on endosomes, which requires both its N- and C-terminal extension sequences. Moreover, RhoU physically associates with activated EGFR in a GRB2-dependent manner through specific proline-rich motifs within its N-terminus. Mutation of these proline-rich sequences or suppression of GRB2 by RNA interference abrogates the interaction of RhoU with activated EGFR, as well as EGF-stimulated RhoU GTP binding. In addition, RhoU is involved in EGFR-mediated signaling, leading to AP1 transcriptional activity and cell migration in pancreatic cancer cells, events that require its interaction with the Grb2–EGFR complex. Taken together, the data suggest a unique regulatory mechanism by which RhoU interaction with SH3 adaptor proteins might serve to integrate growth factor receptor signaling with RhoU activation.

Dynamic palmitoylation and the role of DHHC proteins in T cell activation and anergy

Although protein S-palmitoylation was first characterized >30 years ago, and is implicated in the function, trafficking, and localization of many proteins, little is known about the regulation and physiological implications of this posttranslational modification. Palmitoylation of various signaling proteins required for TCR-induced T cell activation is also necessary for their proper function. Linker for activation of T cells (LAT) is an essential scaffolding protein involved in T cell development and activation, and we found that its palmitoylation is selectively impaired in anergic T cells. The recent discovery of the DHHC family of palmitoyl acyl transferases and the establishment of sensitive and quantitative proteomics-based methods for global analysis of the palmitoyl proteome led to significant progress in studying the biology and underlying mechanisms of cellular protein palmitoylation. We are using these approaches to explore the palmitoyl proteome in T lymphocytes and, specifically, the mechanistic basis for the impaired palmitoylation of LAT in anergic T cells. This chapter reviews the history of protein palmitoylation and its role in T cell activation, the DHHC family and new methodologies for global analysis of the palmitoyl proteome, and summarizes our recent work in this area. The new methodologies will accelerate the pace of research and provide a greatly improved mechanistic and molecular understanding of the complex process of protein palmitoylation and its regulation, and the substrate specificity of the novel DHHC family. Reversible protein palmitoylation will likely prove to be an important posttranslational mechanism that regulates cellular responses, similar to protein phosphorylation and ubiquitination.

Activity of the RhoU/Wrch1 GTPase is critical for cranial neural crest cell migration

The neural crest (NC) is a stem cell-like population that arises at the border of neural and non-neural ectoderm. During development, NC undergoes an epithelio-mesenchymal transition (EMT), i.e. loss of epithelial junctions and acquisition of pro-migratory properties, invades the entire embryo and differentiates into a wide diversity of terminal tissues. We have studied the implication of Rho pathways in NC development and previously showed that RhoV is required for cranial neural crest (CNC) cell specification. We show here that the non-canonical Wnt response rhoU/wrch1 gene, closely related to rhoV, is also expressed in CNC cells but at later stages. Using both gain- and loss-of-function experiments, we demonstrate that the level of RhoU expression is critical for CNC cell migration and subsequent differentiation into craniofacial cartilages. In in vitro cultures, RhoU activates pathways that cooperate with PAK1 and Rac1 in epithelial adhesion, cell spreading and directional cell migration. These data support the conclusion that RhoU is an essential regulator of CNC cell migration.

Rho GTPases and exocytosis: what are the molecular links?

Delivery of proteins or lipids to the plasma membrane or into the extracellular space occurs through exocytosis, a process that requires tethering, docking, priming and fusion of vesicles, as well as F-actin rearrangements in response to specific extracellular cues. GTPases of the Rho family have been implicated as important regulators of exocytosis, but how Rho proteins control this process is an open question. In this review, we focus on molecular connections that drive Rho-dependent exocytosis in polarized and regulated exocytosis. Specifically, we present data showing that Rho proteins interaction with the exocyst complex and IQGAP mediates polarized exocytosis, whereas interaction with actin-binding proteins like N-WASP mediates regulated exocytosis.

Alkynyl-farnesol reporters for detection of protein S-prenylation in cells

Protein S-prenylation is a lipid modification that regulates membrane-protein and protein-protein interactions in cell signaling. Though sites of protein S-prenylation can be predicted based upon conserved C-terminal CaaX or CC/CXC motifs, biochemical detection of protein S-prenylation in cells is still challenging. Herein, we report an alkynyl-isoprenol chemical reporter (alk-FOH) as an efficient substrate for prenyltransferases in mammalian cells that enables sensitive detection of S-farnesylated and S-geranylgeranylated proteins using bioorthogonal ligation methods. Fluorescent detection alleviates the need to deplete cellular isoprenoids for biochemical analysis of S-prenylated proteins and enables robust characterization of S-prenylated proteins, such as effectors that are injected into host cells by bacterial pathogens. This alkynyl-prenylation reporter provides a sensitive tool for biochemical analysis and rapid profiling of prenylated proteins in cells.

Pleiotropic functions of Rho GTPase signaling: a Trojan horse or Achilles' heel for breast cancer treatment?

Rho GTPase signaling is altered in human breast tumors, and elevated expression and activation of Rho GTPases correlate with tumor progression, metastasis, and poor prognosis. Here we review the evidence that Rho signaling functions as a key regulator of cell cycle, mitosis, apoptosis, and invasion during breast cancer growth and progression and discuss whether these pleiotropic actions enhance or limit the targetability of this network. We propose that depending on the stage and subtype of breast cancer, targeting Rho signaling may have chemopreventative, anti-tumor, and anti-metastatic efficacy. An understanding of how Rho signaling is perturbed in specific stages and subtypes of breast cancer and how it functions in the context of the complex in vivo environment during the stochastic process of tumor formation and progression are necessary in order to effectively target this signaling network for breast cancer treatment.

Regulation of the Rho family small GTPase Wrch-1/RhoU by C-terminal tyrosine phosphorylation requires Src

Wrch-1 is an atypical Rho family small GTPase with roles in migration, epithelial cell morphogenesis, osteoclastogenesis, and oncogenic transformation. Here, we observed rapid relocalization of Wrch-1 from the plasma membrane upon serum stimulation. Studies revealed a requirement for serum-stimulated tyrosine phosphorylation of Wrch-1 at residue Y254 within its C-terminal membrane targeting domain, mediated by the nonreceptor tyrosine kinase Src. Genetic or pharmacological loss of Src kinase activity blocked both phosphorylation and relocalization of Wrch-1. Functionally, Y254 was required for proper Wrch-1 modulation of cystogenesis in three-dimensional culture, and the phospho-deficient mutant, Y254F, was enhanced in Wrch-1-mediated anchorage-independent growth. Mechanistically, C-terminal tyrosine phosphorylation and subsequent relocalization of Wrch-1 downregulated its ability to interact with and activate its effectors by decreasing active Wrch-1-GTP, perhaps by altering proximity to a GEF or GAP. Phospho-deficient Wrch-1(Y254F) remained at the plasma membrane and GTP bound and continued to recruit and activate its effector PAK, even upon serum stimulation. In contrast, a phospho-mimetic mutant, Y254E, was constitutively endosomally localized and GDP bound and failed to recruit PAK unless mutated to be constitutively active/GAP insensitive. C-terminal tyrosine phosphorylation thus represents a new paradigm in posttranslational control of small GTPase localization, activation, and biological function.

Genetic studies on the functional relevance of the protein prenyltransferases in skin keratinocytes

The modification of proteins with farnesyl or geranylgeranyl lipids, a process called protein prenylation, facilitates interactions of proteins with membrane surfaces. Protein prenylation is carried out by a pair of cytosolic enzymes, protein farnesyltransferase (FTase) and protein geranylgeranyltransferase type I (GGTase-I). FTase and GGTase-I have attracted interest as therapeutic targets for both cancer and progeria, but very little information exists on the importance of these enzymes for homeostasis of normal tissues. One study actually suggested that FTase is entirely dispensable. To explore the importance of the protein prenyltransferases for normal tissues, we used conditional knockout alleles for Fntb and Pggt1b (which encode the beta-subunits of FTase and GGTase-I, respectively) and a keratin 14-Cre transgene to create mice lacking FTase or GGTase-I in skin keratinocytes. Keratinocyte-specific Fntb knockout mice were viable but developed severe alopecia. Although hair follicles appeared normal during development, they were morphologically abnormal after birth, and ultrastructural and immunohistochemical studies revealed many apoptotic cells. The interfollicular epidermis of Fntb-deficient mice appeared normal; however, keratinocytes from these mice could not proliferate in culture. As expected, non-farnesylated prelamin A and non-farnesylated DNAJA1 accumulated in Fntb-deficient keratinocytes. Keratinocyte-specific Pggt1b knockout mice survived development but died shortly after birth. Like Fntb-deficient keratinocytes, Pggt1b-deficient keratinocytes did not proliferate in culture. Thus, both FTase and GGTase-I are required for the homeostasis of skin keratinocytes.

The fat controller: roles of palmitoylation in intracellular protein trafficking and targeting to membrane microdomains (Review)

The attachment of palmitic acid to the amino acid cysteine via thioester linkage (S-palmitoylation) is a common post-translational modification of eukaryotic proteins. In this review, we discuss the role of palmitoylation as a versatile protein sorting signal, regulating protein trafficking between distinct intracellular compartments and the micro-localization of proteins within membranes.

Caution! Analyze transcripts from conditional knockout alleles

A common strategy for conditional knockout alleles is to "flox" (flank with loxP sites) a 5' exon within the target gene. Typically, the floxed exon does not contain a unit number of codons so that the Cre-mediated recombination event yields a frameshift and a null allele. Documenting recombination within the genomic DNA is often regarded as sufficient proof of a frameshift, and the analysis of transcripts is neglected. We evaluated a previously reported conditional knockout allele for the beta-subunit of protein farnesyltransferase. The recombination event in that allele-the excision of exon 3-was predicted to yield a frameshift. However, following the excision of exon 3, exon 4 was skipped by the mRNA splicing machinery, and the predominant transcript from the mutant allele lacked exon 3 and exon 4 sequences. The "Deltaexon 3-4 transcript" does not contain a frameshift but rather is predicted to encode a protein with a short in-frame deletion. This represents a significant concern when studying an enzyme, since an enzyme with partial function could lead to erroneous conclusions. With thousands of new conditional knockout alleles under construction within mouse mutagenesis consortiums, the protein farnesyltransferase allele holds an important lesson-to characterize knockout alleles at both the DNA and RNA levels.

The transforming Rho family GTPase Wrch-1 disrupts epithelial cell tight junctions and epithelial morphogenesis

Wrch-1, an atypical and transforming Rho GTPase, regulates cellular activities including proliferation and actin organization, but its functions and effectors remain poorly characterized. We show here that Wrch-1 distributes along the apical and basolateral membranes in MDCK cells and binds the cell polarity protein Par6 in a GTP-dependent manner. Activated Wrch-1 negatively regulates the kinetics of tight junction (TJ) assembly during epithelial cell polarization but has no detectable effect on overall cell polarity in confluent monolayers. It also causes a dramatic cytoskeletal reorganization and multilayering in cells grown in two-dimensional culture and disrupts cystogenesis of cells grown in three-dimensional (3D) culture. Similarly, short hairpin RNA-mediated knockdown of Wrch-1 perturbs cystogenesis in 3D culture, suggesting that tight regulation of Wrch-1 activity is necessary for normal epithelial morphogenesis. A weakly transforming effector domain mutant of activated Wrch-1 that inhibits Par6 binding abrogates the ability of Wrch-1 to disrupt TJ formation, actin organization, and epithelial morphogenesis. We hypothesize that Wrch-1-induced morphological and growth transformation may occur in part through Par6-mediated disruption of TJs and actin organization.

Dynamic expression patterns of RhoV/Chp and RhoU/Wrch during chicken embryonic development

Rho GTPases play central roles in the control of cell adhesion and migration, cell cycle progression, growth, and differentiation. However, although most of our knowledge of Rho GTPase function comes from the study of the three classic Rho GTPases RhoA, Rac1, and Cdc42, recent studies have begun to explore the expression, regulation, and function of some of the lesser-known members of the Rho GTPase family. In the present study, we cloned the avian orthologues of RhoV (or Chp for Cdc42 homologous protein) and RhoU (or Wrch-1 for Wnt-regulated Cdc42 homolog-1) and examined their expression patterns by in situ hybridization analysis both during early chick embryogenesis and later on, during gastrointestinal tract development. Our data show that both GTPases are detected in the primitive streak, the somites, the neural crest cells, and the gastrointestinal tract with distinct territories and/or temporal expression windows. Although both proteins are 90% identical, our results indicate that cRhoV and cRhoU are distinctly expressed during chicken embryonic development.

Rho Family GTPase modification and dependence on CAAX motif-signaled posttranslational modification

Rho GTPases (20 human members) comprise a major branch of the Ras superfamily of small GTPases, and aberrant Rho GTPase function has been implicated in oncogenesis and other human diseases. Although many of our current concepts of Rho GTPases are based on the three classical members (RhoA, Rac1, and Cdc42), recent studies have revealed the diversity of biological functions mediated by other family members. A key basis for the functional diversity of Rho GTPases is their association with distinct subcellular compartments, which is dictated in part by three posttranslational modifications signaled by their carboxyl-terminal CAAX (where C represents cysteine, A is an aliphatic amino acid, and X is a terminal amino acid) tetrapeptide motifs. CAAX motifs are substrates for the prenyltransferase-catalyzed addition of either farnesyl or geranylgeranyl isoprenoid lipids, Rce1-catalyzed endoproteolytic cleavage of the AAX amino acids, and Icmt-catalyzed carboxyl methylation of the isoprenylcysteine. We utilized pharmacologic, biochemical, and genetic approaches to determine the sequence requirements and roles of CAAX signal modifications in dictating the subcellular locations and functions of the Rho GTPase family. Although the classical Rho GTPases are modified by geranylgeranylation, we found that a majority of the other Rho GTPases are substrates for farnesyltransferase. We found that the membrane association and/or function of Rho GTPases are differentially dependent on Rce1- and Icmt-mediated modifications. Our results further delineate the sequence requirements for prenyltransferase specificity and functional roles for protein prenylation in Rho GTPase function. We conclude that a majority of Rho GTPases are targets for pharmacologic inhibitors of farnesyltransferase, Rce1, and Icmt.

Rho GTPases in osteoclasts: orchestrators of podosome arrangement

Cells from the myeloid lineage, namely macrophages, dendritic cells and osteoclasts, develop podosomes instead of stress fibers and focal adhesions to adhere and migrate. Podosomes share many components with focal adhesions but differ in their molecular organization, with a dense core of polymerized actin surrounded by scaffolding proteins, kinases and integrins. Podosomes are found either isolated both in macrophages and dendritic cells or arranged into superstructures in osteoclasts. When osteoclasts resorb bone, they form an F-actin rich sealing zone, which is a dense array of connected podosomes that firmly anchors osteoclasts to bone. It delineates a compartment in which protons and proteases are secreted to dissolve and degrade the mineralized matrix. Since Rho GTPases have been shown to control F-actin stress fibers and focal adhesions in mesenchymal cells, the question of whether they could also control podosome formation and arrangement in cells from the myeloid lineage, and particularly in osteoclasts, rapidly emerged. This article considers recent advances made in our understanding of podosome arrangements in osteoclasts and how Rho GTPases may control it.

Paralemmin-1, a modulator of filopodia induction is required for spine maturation

Dendritic filopodia are thought to participate in neuronal contact formation and development of dendritic spines; however, molecules that regulate filopodia extension and their maturation to spines remain largely unknown. Here we identify paralemmin-1 as a regulator of filopodia induction and spine maturation. Paralemmin-1 localizes to dendritic membranes, and its ability to induce filopodia and recruit synaptic elements to contact sites requires protein acylation. Effects of paralemmin-1 on synapse maturation are modulated by alternative splicing that regulates spine formation and recruitment of AMPA-type glutamate receptors. Paralemmin-1 enrichment at the plasma membrane is subject to rapid changes in neuronal excitability, and this process controls neuronal activity-driven effects on protrusion expansion. Knockdown of paralemmin-1 in developing neurons reduces the number of filopodia and spines formed and diminishes the effects of Shank1b on the transformation of existing filopodia into spines. Our study identifies a key role for paralemmin-1 in spine maturation through modulation of filopodia induction.

The atypical Rho GTPase Wrch1 collaborates with the nonreceptor tyrosine kinases Pyk2 and Src in regulating cytoskeletal dynamics

The Cdc42-like GTPase Wnt responsive Cdc42 homolog 1 (Wrch1) has several atypical features; it has an N-terminal proline-rich extension that confers binding to SH3 domains, and it harbors an extremely high intrinsic nucleotide exchange activity, which overrides the normal GTPase activity. As a result, Wrch1 resides mainly in the active, GTP-loaded conformation under normal cellular conditions. We have previously shown that ectopic expression of Wrch1 in fibroblasts resulted in an altered cell morphology visible as a formation of filopodia, a loss of stress fibers, and a reduction in focal adhesions. Here, we show that Wrch1 binds to the nonreceptor tyrosine kinase Pyk2. The interaction required Wrch1 to be in a GTP conformation and also required an intact N-terminal proline-rich extension as well as an intact effector loop. Wrch1 requires Pyk2 in imposing the cytoskeletal effects, seen as the formation of filopodia, since treatment of cells with a Pyk2-specific small interfering RNA abrogated this response. Interestingly, we found that the presence and activity of Src were needed for the formation of a Wrch1-Pyk2 complex as well as for the Wrch1-induced formation of filopodia. We propose a model in which Pyk2 and Src function to coordinate the Wrch1-dependent effects on cytoskeletal dynamics.

Taking Rho GTPases to the next level: the cellular functions of atypical Rho GTPases

The Rho GTPases are influential regulators of signalling pathways that control vital cellular processes such as cytoskeletal dynamics, gene transcription, cell cycle progression and cell transformation. A vast majority of the studies involving Rho GTPases have been focused to the famous triad, Cdc42, Rac1 and RhoA, but this protein family actually harbours 20 members. Recently, the less known Rho GTPases have received increased attention. Many of the less studied Rho GTPases have structural, as well as, functional features which makes it pertinent to classify them as atypical Rho GTPases. This review article will focus on the critical aspects of the atypical Rho GTPases, RhoH, Wrch-1, Chp and RhoBTB. These proteins are involved in a broad spectre of biological processes, such as cytoskeletal dynamics, T-cell signalling and protein ubiquitinylation. We will also discuss the roles of atypical Rho GTPases as oncogenes or tumour suppressors, as well as their potential involvement in human diseases.