p190A RhoGAP is a glycogen synthase kinase-3-beta substrate required for polarized cell migration

Integrin α6β4 Upregulates PTPRZ1 Through UCHL1-Mediated Hif-1α Nuclear Accumulation to Promote Triple-Negative Breast Cancer Cell Invasive Properties

Integrin α6β4 drives triple-negative breast cancer (TNBC) aggressiveness through the transcriptional regulation of key genes. Here, we investigated how integrin α6β4 regulates protein tyrosine phosphatase receptor type Z1 (PTPRZ1). Using stable re-expression of integrin β4 (ITGB4) in cells naturally devoid of integrin α6β4 or knockdown or knockout (KO) of ITGB4, we found that integrin α6β4 regulates PTPRZ1 expression. To gain mechanistic insight, we focused on Hif-1α due to the impact of integrin α6β4 on a hypoxia-associated signature. We found that nuclear localization of Hif-1α, but not Hif-2α, was substantially enhanced with integrin α6β4 signaling. Hif-1α knockdown by shRNA or chemical inhibition decreased PTPRZ1 expression, while chemical activation of Hif-1α increased it. Upstream of Hif-1α, integrin α6β4 upregulates UCHL1 to stabilize Hif-1α and ultimately regulate PTPRZ1. Inhibition of UCHL1 and PTPRZ1 dramatically decreases integrin α6β4-mediated cell migration and three-dimensional invasive growth. Finally, public breast cancer database analyses demonstrated that ITGB4 correlates with PTPRZ1 and that high expression of ITGB4, UCHL1, HIF1A, and PTPRZ1 associated with decreased overall survival, distant metastasis free survival, post progression survival, and relapse-free survival. In summary, these findings provide a novel function of integrin α6β4 in promoting tumor invasive phenotypes through UCHL1-Hif-1α-mediated regulation of PTPRZ1.

GSK3 and lamellipodin balance lamellipodial protrusions and focal adhesion maturation in mouse neural crest migration

Neural crest cells are multipotent cells that delaminate from the neuroepithelium, migrating throughout the embryo. Aberrant migration causes developmental defects. Animal models are improving our understanding of neural crest anomalies, but in vivo migration behaviors are poorly understood. Here, we demonstrate that murine neural crest cells display actin-based lamellipodia and filopodia in vivo. Using neural crest-specific knockouts or inhibitors, we show that the serine-threonine kinase glycogen synthase kinase-3 (GSK3) and the cytoskeletal regulator lamellipodin (Lpd) are required for lamellipodia formation while preventing focal adhesion maturation. Lpd is a substrate of GSK3, and phosphorylation of Lpd favors interactions with the Scar/WAVE complex (lamellipodia formation) at the expense of VASP and Mena interactions (adhesion maturation and filopodia formation). This improved understanding of cytoskeletal regulation in mammalian neural crest migration has general implications for neural crest anomalies and cancer.

GSK3 as a Regulator of Cytoskeleton Architecture: Consequences for Health and Disease

Glycogen synthase kinase 3 (GSK3) was initially isolated as a critical protein in energy metabolism. However, subsequent studies indicate that GSK-3 is a multi-tasking kinase that links numerous signaling pathways in a cell and plays a vital role in the regulation of many aspects of cellular physiology. As a regulator of actin and tubulin cytoskeleton, GSK3 influences processes of cell polarization, interaction with the extracellular matrix, and directional migration of cells and their organelles during the growth and development of an animal organism. In this review, the roles of GSK3–cytoskeleton interactions in brain development and pathology, migration of healthy and cancer cells, and in cellular trafficking of mitochondria will be discussed.

Manipulating oligodendrocyte intrinsic regeneration mechanism to promote remyelination

In demyelinated lesions, astrocytes, activated microglia and infiltrating macrophages secrete several factors regulating oligodendrocyte precursor cells' behaviour. What appears to be the initiation of an intrinsic mechanism of myelin repair is only leading to partial recovery and inefficient remyelination, a process worsening over the course of the disease. This failure is largely due to the concomitant accumulation of inhibitory cues in and around the lesion sites opposing to growth promoting factors. Here starts a complex game of interactions between the signalling pathways controlling oligodendrocytes migration or differentiation. Receptors of positive or negative cues are modulating Ras, PI3K or RhoGTPases pathways acting on oligodendrocyte cytoskeleton remodelling. From the description of this intricate signalling network, this review addresses the extent to which the modulation of the global response to inhibitory cues may pave the route towards novel therapeutic approaches for myelin repair.

A Screen for PKN3 Substrates Reveals an Activating Phosphorylation of ARHGAP18

Protein kinase N3 (PKN3) is a serine/threonine kinase implicated in tumor progression of multiple cancer types, however, its substrates and effector proteins still remain largely understudied. In the present work we aimed to identify novel PKN3 substrates in a phosphoproteomic screen using analog sensitive PKN3. Among the identified putative substrates we selected ARHGAP18, a protein from RhoGAP family, for validation of the screen and further study. We confirmed that PKN3 can phosphorylate ARHGAP18 in vitro and we also characterized the interaction of the two proteins, which is mediated via the N-terminal part of ARHGAP18. We present strong evidence that PKN3-ARHGAP18 interaction is increased upon ARHGAP18 phosphorylation and that the phosphorylation of ARHGAP18 by PKN3 enhances its GAP domain activity and contributes to negative regulation of active RhoA. Taken together, we identified new set of potential PKN3 substrates and revealed a new negative feedback regulatory mechanism of Rho signaling mediated by PKN3-induced ARHGAP18 activation.

p190A RhoGAP induces CDH1 expression and cooperates with E-cadherin to activate LATS kinases and suppress tumor cell growth

The ARHGAP35 gene encoding p190A RhoGAP (p190A) is significantly altered by both mutation and allelic deletion in human cancer, but the functional implications of such alterations are not known. Here, we demonstrate for the first time that p190A is a tumor suppressor using a xenograft mouse model with carcinoma cells harboring defined ARHGAP35 alterations. In vitro, restoration of p190A expression in carcinoma cells promotes contact inhibition of proliferation (CIP) through activation of LATS kinases and phosphorylation of the proto-oncogenic transcriptional co-activator YAP. In contrast, p190A forms harboring recurrent cancer mutations exhibit loss of function in modulating the Hippo pathway, inducing CIP, as well as attenuated suppression of tumor growth in mice. We determine that p190A promotes mesenchymal to epithelial transition (MET) and elicits expression of a cassette of epithelial adherens junction-associated genes in a cell density-dependent manner. This cassette includes CDH1 encoding E-cadherin, which amplifies p190A-mediated LATS activation and is necessary for CIP. Oppositely, we establish that p190A is obligatory for E-cadherin to activate LATS kinases and induce CIP. Collectively, this work defines a novel mechanism by which p190A and E-cadherin cooperate in modulating Hippo signaling to suppress tumor cell growth.

PKCε regulates Rho GTPases and actin cytoskeleton reorganization in non-small cell lung cancer cells

Oncogenic protein kinase C epsilon (PKCε) promotes the formation of membrane ruffles and motility in non-small cell lung cancer (NSCLC) cells. We found that PKCε is down-regulated when NSCLC cells undergo epithelial-to-mesenchymal transition (EMT) in response to TGF-β, thus becoming dispensable for migration and invasion in the mesenchymal state. PKCε silencing or inhibition leads to stress fibre formation, suggesting that this kinase negatively regulates RhoA activity. Ruffle formation induced by PKCε activation in the epithelial state is dependent on PI3K, but does not involve the PI3K-dependent Rac-GEFs Ect2, Trio, Vav2 or Tiam1, suggesting alternative Rac-GEFs as mediators of this response. In the proposed model, PKCε acts as a rheostat for Rho GTPases that differs in the epithelial and mesenchymal states.

Conditional deletion of Rcan1 predisposes to hypertension-mediated intramural hematoma and subsequent aneurysm and aortic rupture

Aortic intramural hematoma (IMH) can evolve toward reabsorption, dissection or aneurysm. Hypertension is the most common predisposing factor in IMH and aneurysm patients, and the hypertensive mediator angiotensin-II induces both in mice. We have previously shown that constitutive deletion of Rcan1 isoforms prevents Angiotensin II-induced aneurysm in mice. Here we generate mice conditionally lacking each isoform or all isoforms in vascular smooth muscle cells, endothelial cells, or ubiquitously, to determine the contribution to aneurysm development of Rcan1 isoforms in vascular cells. Surprisingly, conditional Rcan1 deletion in either vascular cell-type induces a hypercontractile phenotype and aortic medial layer disorganization, predisposing to hypertension-mediated aortic rupture, IMH, and aneurysm. These processes are blocked by ROCK inhibition. We find that Rcan1 associates with GSK-3β, whose inhibition decreases myosin activation. Our results identify potential therapeutic targets for intervention in IMH and aneurysm and call for caution when interpreting phenotypes of constitutively and inducibly deficient mice.

The neglected terminators: Rho family GAPs in neutrophils

BACKGROUND

GTPase-activating proteins (GAPs) accelerate the rate of hydrolysis of GTP bound to small GTPases, thereby limiting the prevalence and concentration of the active, GTP-bound form of these proteins. The large number of potential GAPs acting on members of the Rho family of small GTPases raises the question of specificity or redundancy.

RESULTS

In this review, we summarize experimental data obtained on the role of Rho family GAPs in neutrophils, highlight cases where more than one GAP is involved in a physiological function and show examples that GAPs can be involved not only in termination but also in initiation of cellular processes. We demonstrate that the expression-level regulation of GAPs may also occur in short-living cells such as neutrophils. Finally, we provide insight into the existence and structure of molecular complexes in which Rho family GAPs are involved.

CONCLUSION

GAPs play more complex and varied roles than being simple terminators of cellular processes.

ARHGAP42 is activated by Src-mediated tyrosine phosphorylation to promote cell motility

The tyrosine kinase Src acts as a key regulator of cell motility by phosphorylating multiple protein substrates that control cytoskeletal and adhesion dynamics. In an earlier phosphotyrosine proteomics study, we identified a novel Rho-GTPase activating protein, now known as ARHGAP42, as a likely biologically relevant Src substrate. ARHGAP42 is a member of a family of RhoGAPs distinguished by tandem BAR-PH domains lying N-terminal to the GAP domain. Like other family members, ARHGAP42 acts preferentially as a GAP for RhoA. We show that Src principally phosphorylates ARHGAP42 on tyrosine 376 (Tyr-376) in the short linker between the BAR-PH and GAP domains. The expression of ARHGAP42 variants in mammalian cells was used to elucidate its regulation. We found that the BAR domain is inhibitory toward the GAP activity of ARHGAP42, such that BAR domain deletion resulted in decreased active GTP-bound RhoA and increased cell motility. With the BAR domain intact, ARHGAP42 GAP activity could be activated by phosphorylation of Tyr-376 to promote motile cell behavior. Thus, phosphorylation of ARHGAP42 Tyr-376 is revealed as a novel regulatory event by which Src can affect actin dynamics through RhoA inhibition.

Regulation of Rho GTPase activity at the leading edge of migrating cells by p190RhoGAP

Cell migration, a key feature of embryonic development, immunity, angiogenesis, and tumor metastasis, is based on the coordinated regulation of actin dynamics and integrin-mediated adhesion. Rho GTPases play a major role in this phenomenon by regulating the onset and maintenance of actin-based protruding structures at cell leading edges (i.e. lamellipodia and filopodia) and contractile structures (i.e., stress fibers) at their trailing edge. While spatio-temporal analysis demonstrated the tight regulation of Rho GTPases at the migration front during cell locomotion, little is known about how the main regulators of Rho GTPase activity, such as GAPs, GEFs and GDIs, play a role in this process. In this review, we focus on a major negative regulator of RhoA, p190RhoGAP-A and its close isoform p190RhoGAP-B, which are necessary for efficient cell migration. Recent studies, including our, demonstrated that p190RhoGAP-A localization and activity undergo a complex regulatory mechanism, accounting for the tight regulation of RhoA, but also other members of the Rho GTPase family, at the cell periphery.

Cancer-associated mutations in the protrusion-targeting region of p190RhoGAP impact tumor cell migration

p190RhoGAP (p190A) is a negative regulator of RhoA and localizes to membrane protrusions, where its GAP activity is required for directional migration. Here, Binamé et al. identify the protrusion-localization sequence in p190A and show that cancer-associated mutations in this region affect p190A localization and function as well as tumor cell migration.

Spatiotemporal regulation of RhoGTPases such as RhoA is required at the cell leading edge to achieve cell migration. p190RhoGAP (p190A) is the main negative regulator of RhoA and localizes to membrane protrusions, where its GTPase-activating protein (GAP) activity is required for directional migration. In this study, we investigated the molecular processes responsible for p190A targeting to actin protrusions. By analyzing the subcellular localization of truncated versions of p190A in hepatocellular carcinoma cells, we identified a novel functional p190A domain: the protrusion localization sequence (PLS) necessary and sufficient for p190A targeting to leading edges. Interestingly, the PLS is also required for the negative regulation of p190A RhoGAP activity. Further, we show that the F-actin binding protein cortactin binds the PLS and is required for p190A targeting to protrusions. Lastly, we demonstrate that cancer-associated mutations in PLS affect p190A localization and function, as well as tumor cell migration. Altogether, our data unveil a new mechanism of regulation of p190A in migrating tumor cells.

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.

Structural Plasticity of Dendritic Spines Requires GSK3α and GSK3β

Although memories appear to be elusive phenomena, they are stored in the network of physical connections between neurons. Dendritic spines, which are actin-rich dendritic protrusions, serve as the contact points between networked neurons. The spines’ shape contributes to the strength of signal transmission. To acquire and store information, dendritic spines must remain plastic, i.e., able to respond to signals, by changing their shape. We asked whether glycogen synthase kinase (GSK) 3α and GSK3β, which are implicated in diseases with neuropsychiatric symptoms, such as Alzheimer's disease, bipolar disease and schizophrenia, play a role in a spine structural plasticity. We used Latrunculin B, an actin polymerization inhibitor, and chemically induced Long-Term Depression to trigger fast spine shape remodeling in cultured hippocampal neurons. Spine shrinkage induced by either stimulus required GSK3α activity. GSK3β activity was only important for spine structural changes after treatment with Latrunculin B. Our results indicate that GSK3α is an essential component for short-term spine structural plasticity. This specific function should be considered in future studies of neurodegenerative diseases and neuropsychiatric conditions that originate from suboptimal levels of GSK3α/β activity.

Phosphorylation of the cytoskeletal protein CAP1 controls its association with cofilin and actin

Cell signaling can control the dynamic balance between filamentous and monomeric actin by modulating actin regulatory proteins. One family of actin regulating proteins that controls actin dynamics comprises cyclase-associated proteins 1 and 2 (CAP1 and 2, respectively). However, cell signals that regulate CAPs remained unknown. We mapped phosphorylation sites on mouse CAP1 and found S307 and S309 to be regulatory sites. We further identified glycogen synthase kinase 3 as a kinase phosphorylating S309. The phosphomimetic mutant S307D/S309D lost binding to its partner cofilin and, when expressed in cells, caused accumulation of actin stress fibers similar to that in cells with reduced CAP expression. In contrast, the non-phosphorylatable S307A/S309A mutant showed drastically increased cofilin binding and reduced binding to actin. These results suggest that the phosphorylation serves to facilitate release of cofilin for a subsequent cycle of actin filament severing. Moreover, our results suggest that S307 and S309 function in tandem; neither the alterations in binding cofilin and/or actin, nor the defects in rescuing the phenotype of the enlarged cell size in CAP1 knockdown cells was observed in point mutants of either S307 or S309. In summary, we identify a novel regulatory mechanism of CAP1 through phosphorylation.

p120RasGAP is a mediator of rho pathway activation and tumorigenicity in the DLD1 colorectal cancer cell line

KRAS is mutated in ∼40% of colorectal cancer (CRC), and there are limited effective treatments for advanced KRAS mutant CRC. Therefore, it is crucial that downstream mediators of oncogenic KRAS continue to be studied. We identified p190RhoGAP as being phosphorylated in the DLD1 CRC cell line, which expresses a heterozygous KRAS G13D allele, and not in DKO4 in which the mutant allele has been deleted by somatic recombination. We found that a ubiquitous binding partner of p190RhoGAP, p120RasGAP (RasGAP), is expressed in much lower levels in DKO4 cells compared to DLD1, and this expression is regulated by KRAS. Rescue of RasGAP expression in DKO4 rescued Rho pathway activation and partially rescued tumorigenicity in DKO4 cells, indicating that the combination of mutant KRAS and RasGAP expression is crucial to these phenotypes. We conclude that RasGAP is an important effector of mutant KRAS in CRC.

The human papillomavirus E7 proteins associate with p190RhoGAP and alter its function

Using mass spectrometry, we identified p190RhoGAP (p190) as a binding partner of human papillomavirus 16 (HPV16) E7. p190 belongs to the GTPase activating protein (GAP) family and is one of the primary GAPs for RhoA. GAPs stimulate the intrinsic GTPase activity of the Rho proteins, leading to Rho inactivation and influencing numerous biological processes. RhoA is one of the best-characterized Rho proteins and is specifically involved in formation of focal adhesions and stress fibers, thereby regulating cell migration and cell spreading. Since this is the first report that E7 associates with p190, we carried out detailed interaction studies. We show that E7 proteins from other HPV types also bind p190. Furthermore, we found that conserved region 3 (CR3) of E7 and the middle domain of p190 are important for this interaction. More specifically, we identified two residues in CR3 of E7 that are necessary for p190 binding and used mutants of E7 with mutations of these residues to determine the biological consequences of the E7-p190 interaction. Our data suggest that the interaction of E7 with p190 dysregulates this GAP and alters the actin cytoskeleton. We also found that this interaction negatively regulates cell spreading on a fibronectin substrate and therefore likely contributes to important aspects of the HPV life cycle or HPV-induced tumorigenesis.

IMPORTANCE

This study identifies p190RhoGAP as a novel cellular binding partner for the human papillomavirus (HPV) E7 protein. Our study shows that a large number of different HPV E7 proteins bind p190RhoGAP, and it identifies regions in both E7 and p190RhoGAP which are important for the interaction to occur. This study also highlights the likelihood that the E7-p190RhoGAP interaction may have important biological consequences related to actin organization in the infected cell. These changes could be an important contributor to the viral life cycle and during progression to cancer in HPV-infected cells. Importantly, this work also emphasizes the need for further study in a field which has largely been unexplored as it relates to the HPV life cycle and HPV-induced transformation.

Polarized regulation of glycogen synthase kinase-3β is important for glioma cell invasion

Glioma malignancy greatly depends on its aggressive invasion. The establishment of cell polarity is an important initial step for cell migration, which is essential for cell-directional translocation. However, our understanding of the molecular mechanisms underlying cell polarity formation in glioma cell invasion remains limited. Glycogen synthase kinase-3 (GSK-3) has a critical role in the formation of cell polarity. We therefore investigated whether localized GSK-3β, a subtype of GSK-3, is important for glioma cell invasion. We reported here that the localized phosphorylation of GSK-3β at the Ser9 (pSer9-GSK-3β) was critical for glioma cell invasion. Scratching glioma cell monolayer up-regulated pSer9-GSK-3β specifically at the wound edge. Inhibition of GSK-3 impaired the cell polarity and reduced the directional persistence of cell migration. Consistently, down-regulation of GSK-3α and 3β by specific small interfering RNAs inhibited glioma cell invasion. Over-expressing wild-type or constitutively active forms of GSK-3β also inhibited the cell invasion. These results indicated the polarized localization of GSK-3 regulation in cell migration might be also important for glioma cell migration. Further, EGF regulated both GSK-3α and 3β, but only pSer9-GSK-3β was enriched at the leading edge of scratched glioma cells. Up- or down-regulation of GSK-3β inhibited EGF-stimulated cell invasion. Moreover, EGF specifically regulated GSK-3β, but not GSK-3α, through atypical PKC pathways. Our results indicated that GSK-3 was important for glioma cell invasion and localized inhibition of GSK-3β was critical for cell polarity formation.

PDK1 regulates platelet activation and arterial thrombosis

The effects of phosphoinositide-dependent protein kinase 1 (PDK1), a master kinase in the phosphoinositide 3-kinase/Akt pathway, on platelet activation are unknown. Accordingly, platelet-specific PDK1-deficient mice were characterized to elucidate the platelet-related function(s) of PDK1. We found that PDK1 deficiency caused mild thrombocytopenia. The aggregation of PDK1(-/-) platelets was diminished in response to low levels of thrombin, U46619, and adenosine 5'-diphosphate. Further results demonstrated that PDK1 regulates thrombin-induced platelet activation by affecting αIIbβ3-mediated outside-in signaling. This result provided an explanation for the diminished spreading of PDK1(-/-) platelets on immobilized fibrinogen (Fg) and the decreased rate of clot retraction in platelet-rich plasma (PRP) containing PDK1(-/-) platelets. PDK1 deficiency diminished agonist-induced Akt Ser473 phosphorylation and thoroughly abolished Akt Thr308 and Gsk3β Ser9 phosphorylation in response to agonist treatment and platelet spreading, respectively. A Gsk3β inhibitor fully restored the aggregation of PDK1(-/-) platelets in response to low levels of thrombin, normal spreading of PDK1(-/-) platelets on Fg, and normal clot retraction in PRP containing PDK1(-/-) platelets. Those results indicated that Gsk3β is one of the major downstream effectors of PDK1 in thrombin-induced platelet activation and αIIbβ3-mediated outside-in signaling. In addition, in vivo data demonstrated that PDK1 is an important regulator in arterial thrombosis formation.

A chemical genomic study identifying diversity in cell migration signaling in cancer cells

The aim of this study was to analyze the diversity and consistency of regulatory signaling in cancer cell migration, using a chemical genomic approach. The effects of 34 small molecular compounds were assessed quantitatively by wound healing assay in ten types of migrating cells. Hierarchical clustering was performed on the subsequent migration inhibition profile of the compounds and cancer cell types. The result was that hierarchical clustering accurately classified the compounds according to their targets. Furthermore, the cancer cells tested in this study were classified into three clusters, and the compounds were grouped into four clusters. An inhibitor of JNK suppressed all types of cell migration; however, inhibitors of ROCK, GSK-3 and p38MAPK only inhibited the migration of a subset of cell lines. Thus, our analytical system could easily distinguish between the common and cell type-specific signals responsible for cell migration.

Cell polarity as a regulator of cancer cell behavior plasticity

Cell polarization is an evolutionarily conserved process that facilitates asymmetric distribution of organelles and proteins and that is modified dynamically during physiological processes such as cell division, migration, and morphogenesis. The plasticity with which cells change their behavior and phenotype in response to cell intrinsic and extrinsic cues is an essential feature of normal physiology. In disease states such as cancer, cells lose their ability to behave normally in response to physiological cues. A molecular understanding of mechanisms that alter the behavior of cancer cells is limited. Cell polarity proteins are a recognized class of molecules that can receive and interpret both intrinsic and extrinsic signals to modulate cell behavior. In this review, we discuss how cell polarity proteins regulate a diverse array of biological processes and how they can contribute to alterations in the behavior of cancer cells.

Rho/RacGAPs: embarras de richesse?

Regulatory proteins such as guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs) determine the activity of small GTPases. In the Rho/Rac family, the number of GEFs and GAPs largely exceeds the number of small GTPases, raising the question of specific or overlapping functions. In our recent study we investigated the first time ARHGAP25 at the protein level, determined its activity as RacGAP and showed its involvement in phagocytosis. With the discovery of ARHGAP25, the number of RacGAPs described in phagocytes is increased to six. We provide data that indicate the specific functions of selected Rho/RacGAPs and we show an example of differential regulation of a Rho/Rac family GAP by different kinases. We propose that the abundance of Rho/Rac family GAPs is an important element of the fine spatiotemporal regulation of diverse cellular functions.

A role for myosin IXb, a motor-RhoGAP chimera, in epithelial wound healing and tight junction regulation

Myo9b is a motor–RhoGAP chimera that has been implicated in inflammatory bowel disease. Findings suggest that Myo9b is essential during both collective and individual wound-induced cell migration. It is also important for maintaining tight junction barrier integrity.

Polymorphisms in the gene encoding the heavy chain of myosin IXb (Myo9b) have been linked to several forms of inflammatory bowel disease (IBD). Given that Myo9b contains a RhoGTPase-activating protein domain within its tail, it may play key roles in Rho-mediated actin cytoskeletal modifications critical to intestinal barrier function. In wounded monolayers of the intestinal epithelial cell line Caco2_BBe (BBe), Myo9b localizes to the extreme leading edge of lamellipodia of migrating cells. BBe cells exhibiting loss of Myo9b expression with RNA interference or Myo9b C-terminal dominant-negative (DN) tail-tip expression lack lamellipodia, fail to migrate into the wound, and form stress fiber–like arrays of actin at the free edges of cells facing the wound. These cells also exhibit disruption of tight junction (TJ) protein localization, including ZO-1, occludin, and claudin-1. Torsional motility and junctional permeability to dextran are greatly increased in cells expressing DN-tail-tip. Of interest, this effect is propagated to neighboring cells. Consistent with a role for Myo9b in regulating levels of active Rho, localization of both RhoGTP and myosin light chain phosphorylation corresponds to Myo9b-knockdown regions of BBe monolayers. These data reveal critical roles for Myo9b during epithelial wound healing and maintenance of TJ integrity—key functions that may be altered in patients with Myo9b-linked IBD.

Small G proteins and their regulators in cellular signalling

Small molecular weight GTPases (small G proteins) are essential in the transduction of signals from different plasma membrane receptors. Due to their endogenous GTP-hydrolyzing activity, these proteins function as time-dependent biological switches controlling diverse cellular functions including cell shape and migration, cell proliferation, gene transcription, vesicular transport and membrane-trafficking. This review focuses on endocrine diseases linked to small G proteins. We provide examples for the regulation of the activity of small G proteins by various mechanisms such as posttranslational modifications, guanine nucleotide exchange factors (GEFs), GTPase activating proteins (GAPs) or guanine nucleotide dissociation inhibitors (GDIs). Finally we summarize endocrine diseases where small G proteins or their regulatory proteins have been revealed as the cause.

P190A RhoGAP is required for mammary gland development

P190A and p190B Rho GTPase activating proteins (GAPs) are essential genes that have distinct, but overlapping roles in the developing nervous system. Previous studies from our laboratory demonstrated that p190B is required for mammary gland morphogenesis, and we hypothesized that p190A might have a distinct role in the developing mammary gland. To test this hypothesis, we examined mammary gland development in p190A-deficient mice. P190A expression was detected by in situ hybridization in the developing E14.5day embryonic mammary bud and within the ducts, terminal end buds (TEBs), and surrounding stroma of the developing virgin mammary gland. In contrast to previous results with p190B, examination of p190A heterozygous mammary glands demonstrated that p190A deficiency disrupted TEB morphology, but did not significantly delay ductal outgrowth indicating haploinsufficiency for TEB development. To examine the effects of homozygous deletion of p190A, embryonic mammary buds were rescued by transplantation into the cleared fat pads of SCID/Beige mice. Complete loss of p190A function inhibited ductal outgrowth in comparison to wildtype transplants (51% vs. 94% fat pad filled). In addition, the transplantation take rate of p190A deficient whole gland transplants from E18.5 embryos was significantly reduced compared to wildtype transplants (31% vs. 90%, respectively). These results suggest that p190A function in both the epithelium and stroma is required for mammary gland development. Immunostaining for p63 demonstrated that the myoepithelial cell layer is disrupted in the p190A deficient glands, which may result from the defective cell adhesion between the cap and body cell layers detected in the TEBs. The number of estrogen- and progesterone receptor-positive cells, as well as the expression levels of these receptors was increased in p190A deficient outgrowths. These data suggest that p190A is required in both the epithelial and stromal compartments for ductal outgrowth and that it may play a role in mammary epithelial cell differentiation.

CdGAP is required for transforming growth factor β- and Neu/ErbB-2-induced breast cancer cell motility and invasion

RhoA, Rac1 and Cdc42, the best-characterized members of the Rho family of small GTPases, are critical regulators of many cellular activities. Cdc42 GTPase-activating protein (CdGAP) is a serine- and proline-rich RhoGAP protein showing GAP activity against both Cdc42 and Rac1 but not RhoA. CdGAP is phosphorylated downstream of the MEK-ERK (extracellular signal-regulated kinase) pathway in response to serum and is required for normal cell spreading and polarized lamellipodia formation. In this study, we found that CdGAP protein and mRNA levels are highly increased in mammary tumor explants expressing an activated Neu/ErbB-2 (Neu-NT) receptor. In response to transforming growth factor-β (TGFβ) stimulation, Neu-NT-expressing mammary tumor explants demonstrate a clear induction in cell motility and invasion. We show that downregulation of CdGAP expression by small interfering RNA abrogates the ability of TGFβ to induce cell motility and invasion of Neu-NT-expressing mammary tumor explants. However, it has no effect on TGFβ-mediated cell adhesion on type 1 collagen and fibronectin. Interestingly, protein expression of E-Cadherin is highly increased in Neu-NT-expressing mammary tumor explants depleted of CdGAP. In addition, complete loss of E-Cadherin expression is not observed in CdGAP-depleted cells during TGFβ-mediated epithelial to mesenchymal transition. Downregulation of the CdGAP expression also decreases cell proliferation of Neu-NT-expressing mammary tumor explants independently of TGFβ. Rescue analysis using re-expression of various CdGAP deletion-mutant proteins revealed that the proline-rich domain (PRD) but not the GAP domain of CdGAP is essential to mediate TGFβ-induced cell motility and invasion. Finally, we found that TGFβ induces the expression and phosphorylation of CdGAP in mammary epithelial NMuMG cells. Taken together, these studies identify CdGAP as a novel molecular target in TGFβ signaling and implicate CdGAP as an essential component in the synergistic interaction between TGFβ and Neu/ErbB-2 signaling pathways in breast cancer cells.

Neutrophil functions and autoimmune arthritis in the absence of p190RhoGAP: generation and analysis of a novel null mutation in mice

Beta(2) integrins of neutrophils play a critical role in innate immune defense, but they also participate in tissue destruction during autoimmune inflammation. p190RhoGAP (ArhGAP35), a regulator of Rho family small GTPases, is required for integrin signal transduction in fibroblasts. Prior studies have also suggested a role for p190RhoGAP in beta(2) integrin signaling in neutrophils. To directly test that possibility, we have generated a novel targeted mutation completely disrupting the p190RhoGAP-encoding gene in mice. p190RhoGAP deficiency led to perinatal lethality and defective neural development, precluding the analysis of neutrophil functions in adult p190RhoGAP(-/-) animals. This was overcome by transplantation of fetal liver cells from p190RhoGAP(-/-) fetuses into lethally irradiated wild-type recipients. Neutrophils from such p190RhoGAP(-/-) bone marrow chimeras developed normally and expressed normal levels of various cell surface receptors. Although p190RhoGAP(-/-) neutrophils showed moderate reduction of beta(2) integrin-mediated adherent activation, they showed mostly normal migration in beta(2) integrin-dependent in vitro and in vivo assays and normal beta(2) integrin-mediated killing of serum-opsonized Staphylococcus aureus and Escherichia coli. A neutrophil- and beta(2) integrin-dependent transgenic model of the effector phase of autoimmune arthritis also proceeded normally in p190RhoGAP(-/-) bone marrow chimeras. In contrast, all the above responses were completely blocked in CD18(-/-) neutrophils or CD18(-/-) bone marrow chimeras. These results suggest that p190RhoGAP likely does not play a major indispensable role in beta(2) integrin-mediated in vitro and in vivo neutrophil functions or the effector phase of experimental autoimmune arthritis.

Mitotic down-regulation of p190RhoGAP is required for the successful completion of cytokinesis

p190RhoGAP-A (p190) is a GTPase-activating protein known to regulate actin cytoskeleton dynamics by decreasing RhoGTP levels through activation of Rho intrinsic GTPase activity. We have previously shown that p190 protein levels are cell cycle-regulated, decreasing in mitosis, and that this decrease is mediated by the ubiquitin-proteasome pathway. In addition, overexpression of p190 results in decreased RhoGTP levels at the cleavage furrow during cytokinesis, p190 and the RhoGEF Ect2 play opposing roles in cytokinesis, and sustained levels of p190 in mitosis are associated with cytokinesis failure, all findings that suggest but do not directly demonstrate that completion of cytokinesis is dependent on reduced levels of p190. Here we report, using an RNAi reconstitution approach with a degradation-resistant mutant, that decreased p190 levels are required for successful cytokinesis. We also show that the multinucleation phenotype is dependent on p190 RhoGAP activity, determine that the N-terminal GBDS1 region is necessary and sufficient for p190 mitotic ubiquitination and degradation, and identify four N-terminal residues as necessary for the degradation of p190 in mitosis. Our data indicate that in addition to activation of RhoGEF(s), reduction of RhoGAP (p190) is a critical mechanism by which increased RhoGTP levels are achieved in late mitosis, thereby ensuring proper cell division.

Cell polarity in motion: redefining mammary tissue organization through EMT and cell polarity transitions

Epithelial to mesenchymal transition (EMT) and its reversion via mesenchymal to epithelial transition (MET), represent a stepwise cycle of epithelial plasticity that allows for normal tissue remodelling and diversification during development. In particular, epithelial-mesenchymal plasticity is central to many aspects of mammary development and has been proposed to be a key process in breast cancer progression. Such epithelial-mesenchymal plasticity requires complex cellular reprogramming to orchestrate a change in cell shape to an alternate morphology more conducive to migration. During this process, epithelial characteristics, including apical-basal polarity and specialised cell-cell junctions are lost and mesenchymal properties, such as a front-rear polarity associated with weak cell-cell contacts, increased motility, resistance to apoptosis and invasiveness are gained. The ability of epithelial cells to undergo transitions through cell polarity states is a central feature of epithelial-mesenchymal plasticity. These cell polarity states comprise a set of distinct asymmetric distributions of cellular constituents that are fashioned to allow specialized cellular functions, such as the regulated homeostasis of molecules across epithelial barriers, cell migration or cell diversification via asymmetric cell divisions. Each polarity state is engineered using a molecular toolbox that is highly conserved between organisms and cell types which can direct the initiation, establishment and continued maintenance of each asymmetry. Here we discuss how EMT pathways target cell polarity mediators, and how this EMT-dependent change in polarity states impact on the various stages of breast cancer. Emerging evidence places cell polarity at the interface of proliferation and morphology control and as such the changing dynamics within polarity networks play a critical role in normal mammary gland development and breast cancer progression.

Extracellular signal-regulated kinase promotes Rho-dependent focal adhesion formation by suppressing p190A RhoGAP

Cell migration is critical for normal development and for pathological processes including cancer cell metastasis. Dynamic remodeling of focal adhesions and the actin cytoskeleton are crucial determinants of cell motility. The Rho family and the mitogen-activated protein kinase (MAPK) module consisting of MEK-extracellular signal-regulated kinase (ERK) are important regulators of these processes, but mechanisms for the integration of these signals during spreading and motility are incompletely understood. Here we show that ERK activity is required for fibronectin-stimulated Rho-GTP loading, Rho-kinase function, and the maturation of focal adhesions in spreading cells. We identify p190A RhoGAP as a major target for ERK signaling in adhesion assembly and identify roles for ERK phosphorylation of the C terminus in p190A localization and activity. These observations reveal a novel role for ERK signaling in adhesion assembly in addition to its established role in adhesion disassembly.

Glycogen synthase kinase 3 in the world of cell migration

Glycogen synthase kinase 3 (GSK3) is one of the few master switch kinases that regulate many aspects of cell functions. Recent studies on cell polarization and migration have shown that GSK3 is also essential for proper regulation of these processes. GSK3 influences cell migration as one of the regulators of the spatiotemporally controlled dynamics of the actin cytoskeleton, microtubules, and cell-to-matrix adhesions. In this mini-review, the effects of GSK3 on these three aspects of cell migration will be discussed.

PKCdelta influences p190 phosphorylation and activity: events independent of PKCdelta-mediated regulation of endothelial cell stress fiber and focal adhesion formation and barrier function

BACKGROUND

We have shown that protein kinase Cdelta (PKCdelta) inhibition results in increased endothelial cell (EC) permeability and decreased RhoA activity; which correlated with diminished stress fibers (SF) and focal adhesions (FA). We have also shown co-precipitation of p190RhoGAP (p190) with PKCdelta. Here, we investigated if PKCdelta regulates p190 and whether PKCdelta-mediated changes in SF and FA or permeability were dependent upon p190.

METHODS

Protein-protein interaction and activity analyses were performed using co-precipitation assays. Analysis of p190 phosphorylation was performed using in vitro kinase assays. SF and FA were analyzed by immunofluorescence analyses. EC monolayer permeability was measured using electrical cell impedance sensor (ECIS) technique.

RESULTS

Inhibition of PKCdelta increased p190 activity, while PKCdelta overexpression diminished p190 activity. PKCdelta bound to and phosphorylated both p190FF and p190GTPase domains. p190 protein overexpression diminished SF and FA formation and RhoA activity. Disruption of SF and FA or increased permeability induced upon PKCdelta inhibition, were not attenuated in EC in which the p190 isoforms were suppressed individually or concurrently.

GENERAL SIGNIFICANCE

Our findings suggest that while PKCdelta can regulate p190 activity, possibly at the FF and/or GTPase domains, the effect of PKCdelta inhibition on SF and FA and barrier dysfunction occurs through a pathway independent of p190.

Regulation of the substrate preference of p190RhoGAP by protein kinase C-mediated phosphorylation of a phospholipid binding site

The Rho family GTPases are stringently regulated through the action of a large family of GTPase activating proteins (GAPs) that stimulate their relatively weak intrinsic GTP hydrolyzing activity. The p190RhoGAPs, which include the p190A and p190B proteins, are potent and widely expressed GAPs acting on both Rho and Rac GTPases. We have observed that several acidic phospholipids inhibit the RhoGAP activity and promote the RacGAP activity of p190 proteins. In liposome binding assays we have demonstrated that binding of p190A to phospholipids is controlled by electrostatic interactions. Using mapping techniques, we determined that a small polybasic peptide stretch within p190A is a common site for both the phospholipid binding and PKC phosphorylation. Moreover, PKC-mediated phosphorylation of two amino acids (serine-1221 and threonine-1226) within this region of p190A prevents the binding and substrate specificity regulation by phospholipids. Transfection of COS-7 cells with mutant forms of p190A either unable to bind to phospholipids or unable to become phosphorylated induced distinct morphological changes. Together, these findings reveal a novel GAP regulatory mechanism in which phosphorylation indirectly alters GTPase substrate preference by affecting the interaction with acidic phospholipids. Our observations provide a potential mechanism of Rac/Rho antagonism described in several cellular functions.

A FAK-p120RasGAP-p190RhoGAP complex regulates polarity in migrating cells

Directional motility is a complex process requiring the spatiotemporal integration of signals that regulate cytoskeletal changes, and the establishment of an anteroposterior or polarized cell axis. Focal adhesion kinase (FAK) promotes cell migration, but a molecular role for FAK in promoting cell polarity remains undefined. Here, using wound healing and Golgi-reorientation analyses, we show that fibroblast, endothelial and carcinoma polarity during cell migration requires FAK and is associated with a complex between FAK, p120RasGAP and p190RhoGAP (p190A), leading to p190A tyrosine phosphorylation. Fibronectin-integrin-mediated FAK activation and phosphorylation promote SH2-mediated binding of p120RasGAP to FAK and FAK-mediated p190A tyrosine phosphorylation. The association of p120RasGAP with FAK facilitates the formation of a FAK-p120RasGAP-p190A complex targeted to leading-edge focal adhesions by FAK. Knockdown of p120RasGAP, mutation of FAK Y397 or inhibition of FAK activity prevent the association of FAK with p190A and subsequent tyrosine phosphorylation of p190A, and result in the loss of cell polarity. Because reconstitution of FAK-null fibroblasts with FAK or a Pyk2-FAK chimera restore the normal decrease in RhoA GTP binding upon cell spreading on fibronectin, our studies support a model whereby FAK activity facilitates the recruitment and stabilization of a p120RasGAP-p190A complex at leading-edge focal adhesions connected to the transient inhibition of RhoA activity and the regulation of cell polarity.

Establishment of axon-dendrite polarity in developing neurons

Neurons are among the most highly polarized cell types in the body, and the polarization of axon and dendrites underlies the ability of neurons to integrate and transmit information in the brain. Significant progress has been made in the identification of the cellular and molecular mechanisms underlying the establishment of neuronal polarity using primarily in vitro approaches such as dissociated culture of rodent hippocampal and cortical neurons. This model has led to the predominant view suggesting that neuronal polarization is specified largely by stochastic, asymmetric activation of intracellular signaling pathways. Recent evidence shows that extracellular cues can play an instructive role during neuronal polarization in vitro and in vivo. In this review, we synthesize the recent data supporting an integrative model whereby extracellular cues orchestrate the intracellular signaling underlying the initial break of neuronal symmetry leading to axon-dendrite polarization.

Crosstalk between small GTPases and polarity proteins in cell polarization

Cell polarization is crucial for the development of multicellular organisms, and aberrant cell polarization contributes to various diseases, including cancer. How cell polarity is established and how it is maintained remain fascinating questions. Conserved proteins of the partitioning defective (PAR), Scribble and Crumbs complexes guide the establishment of cell polarity in various organisms. Moreover, GTPases that regulate actin cytoskeletal dynamics have been implicated in cell polarization. Recent findings provide insights into polarization mechanisms and show intriguing crosstalk between small GTPases and members of polarity complexes in regulating cell polarization in different cellular contexts and cell types.