The role of ubiquitylation and degradation in RhoGTPase signalling

DEF6 regulates renal ischemia reperfusion injury through suppressing the WWP2 mediated ubiquitination of PARP1

BACKGROUND

Renal ischemia-reperfusion injury (RIRI) stands as an unavoidable complication arising from kidney surgery, profoundly intertwined with its prognosis. The role of differentially expressed in FDCP 6 homolog (DEF6) in RIRI remains elusive, despite its confirmation as a potential therapeutic target for diverse diseases. Here, we investigated the mechanism by which DEF6 regulated RIRI.

METHODS

RNA sequencing data and IP-MS were used to identify the expression and potential targets of DEF6 through bioinformatics analysis. To elucidate the impact of DEF6 on RIRI, both an in vivo model of RIRI in mice and an in vitro model of kidney cell hypoxia/reoxygenation were established. Biochemical and histological analyses were used to investigate the influence of DEF6 on kidney damage mediated by RIRI.

RESULTS

We confirmed that DEF6 was upregulated during RIRI and had a close correlation with RIRI-related inflammation and apoptosis. Moreover, inhibition of DEF6 could mitigate RIRI-induced kidney damage, inflammation, and apoptosis. Through our comprehensive mechanistic investigation, we revealed that DEF6 interacts with poly ADP-ribose polymerase 1 (PARP1) and suppresses the ubiquitination of PARP1. Inhibition of DEF6 resulted in reduced cleaveage of PARP1, leading to a marked suppression of PARP1-mediated apoptosis activation. The aggravation effect on inflammation and apoptosis achieved through DEF6 was nullified by the inhibition of NF-κB and Bax/Bcl2 signaling activation through PARP1 deletion.

CONCLUSIONS

The findings from our study indicate that DEF6 suppressed the WWP2 mediated ubiquitination of PARP1 and modulates the activation of NF-κB and Bax/Bcl2 pathway, thus involved in RIRI-induced inflammation and apoptosis. These results suggest that DEF6 holds promise as a potential therapeutic target for mitigating RIRI.

Unravelling molecular dynamics in living cells: Fluorescent protein biosensors for cell biology

Genetically encoded, fluorescent protein (FP)-based Förster resonance energy transfer (FRET) biosensors are microscopy imaging tools tailored for the precise monitoring and detection of molecular dynamics within subcellular microenvironments. They are characterised by their ability to provide an outstanding combination of spatial and temporal resolutions in live-cell microscopy. In this review, we begin by tracing back on the historical development of genetically encoded FP labelling for detection in live cells, which lead us to the development of early biosensors and finally to the engineering of single-chain FRET-based biosensors that have become the state-of-the-art today. Ultimately, this review delves into the fundamental principles of FRET and the design strategies underpinning FRET-based biosensors, discusses their diverse applications and addresses the distinct challenges associated with their implementation. We place particular emphasis on single-chain FRET biosensors for the Rho family of guanosine triphosphate hydrolases (GTPases), pointing to their historical role in driving our understanding of the molecular dynamics of this important class of signalling proteins and revealing the intricate relationships and regulatory mechanisms that comprise Rho GTPase biology in living cells.

Molecular mechanism of regulation of RhoA GTPase by phosphorylation of RhoGDI

Rho-specific guanine nucleotide dissociation inhibitors (RhoGDIs) play a crucial role in the regulation of Rho family GTPases. They act as negative regulators that prevent the activation of Rho GTPases by forming complexes with the inactive GDP-bound state of GTPase. Release of Rho GTPase from the RhoGDI-bound complex is necessary for Rho GTPase activation. Biochemical studies provide evidence of a "phosphorylation code," where phosphorylation of some specific residues of RhoGDI selectively releases its GTPase partner (RhoA, Rac1, Cdc42, etc.). This work attempts to understand the molecular mechanism behind this specific phosphorylation-induced reduction in binding affinity. Using several microseconds long atomistic molecular dynamics simulations of the wild-type and phosphorylated states of the RhoA-RhoGDI complex, we propose a molecular-interaction-based mechanistic model for the dissociation of the complex. Phosphorylation induces major structural changes, particularly in the positively charged polybasic region (PBR) of RhoA and the negatively charged N-terminal region of RhoGDI that contribute most to the binding affinity. Molecular mechanics Poisson-Boltzmann surface area binding energy calculations show a significant weakening of interaction on phosphorylation at the RhoA-specific site of RhoGDI. In contrast, phosphorylation at a Rac1-specific site does not affect the overall binding affinity significantly, which confirms the presence of a phosphorylation code. RhoA-specific phosphorylation leads to a reduction in the number of contacts between the PBR of RhoA and the N-terminal region of RhoGDI, which manifests a reduction of the binding affinity. Using hydrogen bond occupancy analysis and energetic perturbation network, we propose a mechanistic model for the allosteric response, i.e., long-range signal propagation from the site of phosphorylation to the PBR and buried geranylgeranyl group in the form of rearrangement and rewiring of hydrogen bonds and salt bridges. Our results highlight the crucial role of specific electrostatic interactions in manifestation of the phosphorylation code.

NEDD8-activating enzyme inhibition potentiates the anti-myeloma activity of natural killer cells

Natural Killer (NK) cells act as important regulators in the development and progression of hematological malignancies and their suppressor activity against Multiple Myeloma (MM) cells has been confirmed in many studies. Significant changes in the distribution of NK cell subsets and dysfunctions of NK cell effector activities were described in MM patients and correlated with disease staging. Thus, restoring or enhancing the functionality of these effectors for the treatment of MM represents a critical need. Neddylation is a post-translational modification that adds a ubiquitin-like molecule, NEDD8, to the substrate protein. One of the outcomes is the activation of the Cullin Ring Ligases (CRLs), a class of ubiquitin-ligases that controls the degradation of about 20% of proteasome-regulated proteins. Overactivation of CRLs has been described in cancer and can lead to tumor growth and progression. Thus, targeting neddylation represents an attractive approach for cancer treatment. Our group has recently described how pharmacologic inhibition of neddylation increases the expression of the NKG2D activating receptor ligands, MICA and MICB, in MM cells, making these cells more susceptible to NK cell degranulation and killing. Here, we extended our investigation to the direct role of neddylation on NK cell effector functions exerted against MM. We observed that inhibition of neddylation enhanced NK cell-mediated degranulation and killing against MM cells and improved Daratumumab/Elotuzumab-mediated response. Mechanistically, inhibition of neddylation increased the expression of Rac1 and RhoA GTPases in NK cells, critical mediators for an efficient degranulation at the immunological synapse of cytotoxic lymphocytes, and augmented the levels of F-actin and perforin polarization in NK cells contacting target cells. Moreover, inhibition of neddylation partially abrogated TGFβ-mediated repression of NK cell effector activity. This study describes the role of neddylation on NK cell effector functions and highlights the positive immunomodulatory effects achieved by the inhibition of this pathway in MM.

BAG6 supports stress fiber formation by preventing the ubiquitin-mediated degradation of RhoA

The Rho family of small GTPases is a key regulator of cytoskeletal actin polymerization. Although the ubiquitination of Rho proteins is reported to control their activity, the mechanisms by which the ubiquitination of Rho family proteins is controlled by ubiquitin ligases have yet to be elucidated. In this study, we identified BAG6 as the first factor needed to prevent the ubiquitination of RhoA, a critical Rho family protein in F-actin polymerization. We found that BAG6 is necessary for stress fiber formation by stabilizing endogenous RhoA. BAG6 deficiency enhanced the association between RhoA and Cullin-3-based ubiquitin ligases, thus promoting its polyubiquitination and subsequent degradation, leading to the abrogation of actin polymerization. In contrast, the restoration of RhoA expression through transient overexpression rescued the stress fiber formation defects induced by BAG6 depletion. BAG6 was also necessary for the appropriate assembly of focal adhesions as well as cell migration events. These findings reveal a novel role for BAG6 in maintaining the integrity of actin fiber polymerization and establish BAG6 as a RhoA-stabilizing holdase, which binds to and supports the function of RhoA.

Laminin-332 γ2 Monomeric Chain Promotes Adhesion and Migration of Hepatocellular Carcinoma Cells

Extracellular matrix (ECM) has a well-recognized impact on the progression of solid tumors, including hepatocellular carcinoma (HCC). Laminin 332 (Ln332) is a ECM molecule of epithelial basal lamina, composed of three polypeptide chains (α3, β3, and γ2), that is usually poorly expressed in the normal liver but is detected at high levels in HCC. This macromolecule was shown to promote the proliferation, epithelial-to-mesenchymal transition (EMT), and drug resistance of HCC cells. The monomeric γ2 chain is up-regulated and localized preferentially at the invasive edge of metastatic intrahepatic HCC nodules, suggesting its potential involvement in the acquisition of invasive properties of HCC cells. HCC cells were tested in in vitro adhesion, scattering, and transwell migration assays in response to fibronectin and the Ln332 and Ln332 γ2 chains, and the activation status of major signaling pathways involved was evaluated. Here, we show that the Ln332 γ2 chain promotes HCC the cell adhesion, migration, and scattering of HCC cells that express the Ln332 receptor α3β1 integrin, proving to be a causal factor of the EMT program achievement. Moreover, we found that efficient HCC cell adhesion and migration on γ2 require the activation of the small cytosolic GTPase Rac1 and ERKs signaling. These data suggest that the γ2 chain, independently from the full-length Ln332, can contribute to the pro-invasive potential of aggressive HCC cell subpopulations.

Cannabinoid CB1 receptor gene inactivation in oligodendrocyte precursors disrupts oligodendrogenesis and myelination in mice

Cannabinoids are known to modulate oligodendrogenesis and developmental CNS myelination. However, the cell-autonomous action of these compounds on oligodendroglial cells in vivo, and the molecular mechanisms underlying these effects have not yet been studied. Here, by using oligodendroglial precursor cell (OPC)-targeted genetic mouse models, we show that cannabinoid CB₁ receptors exert an essential role in modulating OPC differentiation at the critical periods of postnatal myelination. We found that selective genetic inactivation of CB₁ receptors in OPCs in vivo perturbs oligodendrogenesis and postnatal myelination by altering the RhoA/ROCK signaling pathway, leading to hypomyelination, and motor and cognitive alterations in young adult mice. Conversely, pharmacological CB₁ receptor activation, by inducing E3 ubiquitin ligase-dependent RhoA proteasomal degradation, promotes oligodendrocyte development and CNS myelination in OPCs, an effect that was not evident in OPC-specific CB₁ receptor-deficient mice. Moreover, pharmacological inactivation of ROCK in vivo overcomes the defects in oligodendrogenesis and CNS myelination, and behavioral alterations found in OPC-specific CB₁ receptor-deficient mice. Overall, this study supports a cell-autonomous role for CB₁ receptors in modulating oligodendrogenesis in vivo, which may have a profound impact on the scientific knowledge and therapeutic manipulation of CNS myelination by cannabinoids.

Role of Nrf2 Signaling Cascade in Breast Cancer: Strategies and Treatment

Breast cancer is the second leading cancer among all types of cancers. It accounts for 12% of the total cases of cancers. The complex and heterogeneous nature of breast cancer makes it difficult to treat in advanced stages. The expression of various enzymes and proteins is regulated by several molecular pathways. Oxidative stress plays a vital role in cellular events that are generally regulated by nuclear factor erythroid 2-related factor 2 (Nrf2). The exact mechanism of Nrf2 behind cytoprotective and antioxidative properties is still under investigation. In healthy cells, Nrf2 expression is lower, which maintains antioxidative stress; however, cancerous cells overexpress Nrf2, which is associated with various phenomena, such as the development of drug resistance, angiogenesis, development of cancer stem cells, and metastasis. Aberrant Nrf2 expression diminishes the toxicity and potency of therapeutic anticancer drugs and provides cytoprotection to cancerous cells. In this article, we have discussed the attributes associated with Nrf2 in the development of drug resistance, angiogenesis, cancer stem cell generation, and metastasis in the specific context of breast cancer. We also discussed the therapeutic strategies employed against breast cancer exploiting Nrf2 signaling cascades.

Ubiquitylation by Rab40b/Cul5 regulates Rap2 localization and activity during cell migration

Cell migration is a complex process that involves coordinated changes in membrane transport and actin cytoskeleton dynamics. Ras-like small monomeric GTPases, such as Rap2, play a key role in regulating actin cytoskeleton dynamics and cell adhesions. However, how Rap2 function, localization, and activation are regulated during cell migration is not fully understood. We previously identified the small GTPase Rab40b as a regulator of breast cancer cell migration. Rab40b contains a suppressor of cytokine signaling (SOCS) box, which facilitates binding to Cullin5, a known E3 ubiquitin ligase component responsible for protein ubiquitylation. In this study, we show that the Rab40b/Cullin5 complex ubiquitylates Rap2. Importantly, we demonstrate that ubiquitylation regulates Rap2 activation as well as recycling of Rap2 from the endolysosomal compartment to the lamellipodia of migrating breast cancer cells. Based on these data, we propose that Rab40b/Cullin5 ubiquitylates and regulates Rap2-dependent actin dynamics at the leading edge, a process that is required for breast cancer cell migration and invasion.

STK25 and MST3 Have Overlapping Roles to Regulate Rho GTPases during Cortical Development

Precise control of neuronal migration is required for the laminar organization of the neocortex and critical for brain function. We previously reported that the acute disruption of the Stk25 gene (Stk25 conditional knock-out; cKO) during mouse embryogenesis causes anomalous neuronal migration in the neocortex, but paradoxically the Stk25 cKO did not have a cortical phenotype, suggesting some forms of compensation exist. In this study, we report that MST3, another member of the GCKIII subgroup of the Ste20-like kinase family, compensates for loss of Stk25 and vice versa with sex independent manner. MST3 overexpression rescued neuronal migration deficit and abnormal axonogenesis in Stk25 cKO brains. Mechanistically, STK25 leads to Rac1 activation and reduced RhoA levels in the developing brain, both of which are required to fully restore neuronal migration in the Stk25 cKO brain. Abnormal migration phenotypes are also rescued by overexpression of Bacurd1and Cul3, which target RhoA for degradation, and activate Rac1. This study reveals that MST3 upregulation is capable of rescuing acute Stk25 deficiency and resolves details of signaling downstream STK25 required for corticogenesis both common to and distinct from MST3 signaling.SIGNIFICANCE STATEMENT Proper neuronal migration during cortical development is required for normal neuronal function. Here, we show that STK25 and MST3 kinases regulate neuronal migration and polarization in a mutually compensatory manner. Furthermore, STK25 balances Rac1 activity and RhoA level through forming complexes with α-PIX and β-PIX, GTPase regulatory enzymes, and Cullin3-Bacurd1/Kctd13, a pair of RhoA ubiquitination molecules in a kinase activity-independent manner. Our findings demonstrate the importance of overlapping and unique roles of STK25 and MST3 to regulate Rho GTPase activities in cortical development.

The interplay of Rac1 activity, ubiquitination and GDI binding and its consequences for endothelial cell spreading

Signaling by the Rho GTPase Rac1 is key to the regulation of cytoskeletal dynamics, cell spreading and adhesion. It is widely accepted that the inactive form of Rac1 is bound by Rho GDI, which prevents Rac1 activation and Rac1-effector interactions. In addition, GDI-bound Rac1 is protected from proteasomal degradation, in line with data showing that Rac1 ubiquitination occurs exclusively when Rac1 is activated. We set out to investigate how Rac1 activity, GDI binding and ubiquitination are linked. We introduced single amino acid mutations in Rac1 which differentially altered Rac1 activity, and compared whether the level of Rac1 activity relates to Rac1 ubiquitination and GDI binding. Results show that Rac1 ubiquitination and the active Rac1 morphology is proportionally increased with Rac1 activity. Similarly, we introduced lysine-to-arginine mutations in constitutively active Rac1 to inhibit site-specific ubiquitination and analyze this effect on Rac1 signaling output and ubiquitination. These data show that the K16R mutation inhibits GTP binding, and consequently Rac1 activation, signaling and-ubiquitination, while the K147R mutation does not block Rac1 signaling, but does inhibits its ubiquitination. In both sets of mutants, no direct correlation was observed between GDI binding and Rac1 activity or -ubiquitination. Taken together, our data show that a strong, positive correlation exists between Rac1 activity and its level of ubiquitination, but also that GDI dissociation does not predispose Rac1 to ubiquitination.

Novel CTRP8-RXFP1-JAK3-STAT3 axis promotes Cdc42-dependent actin remodeling for enhanced filopodia formation and motility in human glioblastoma cells

C1q tumor necrosis factor‐related peptide 8 (CTRP8) is the least studied member of the C1Q‐TNF‐related peptide family. We identified CTRP8 as a ligand of the G protein‐coupled receptor relaxin family peptide receptor 1 (RXFP1) in glioblastoma multiforme (GBM). The CTRP8‐RXFP1 ligand–receptor system protects human GBM cells against the DNA‐alkylating damage‐inducing temozolomide (TMZ), the drug of choice for the treatment of patients with GBM. The DNA protective role of CTRP8 was dependent on a functional RXFP1‐STAT3 signaling cascade and targeted the monofunctional glycosylase N‐methylpurine DNA glycosylase (MPG) for more efficient base excision repair of TMZ‐induced DNA‐damaged sites. CTRP8 also improved the survival of GBM cells by upregulating anti‐apoptotic BCl‐2 and BCL‐XL. Here, we have identified Janus‐activated kinase 3 (JAK3) as a novel member of a novel CTRP8‐RXFP1‐JAK3‐STAT3 signaling cascade that caused an increase in cellular protein content and activity of the small Rho GTPase Cdc42. This is associated with significant F‐actin remodeling and increased GBM motility. Cdc42 was critically important for the upregulation of the actin nucleation complex N‐Wiskott–Aldrich syndrome protein/Arp3/4 and actin elongation factor profilin‐1. The activation of the RXFP1‐JAK3‐STAT3‐Cdc42 axis by both RXFP1 agonists, CTRP8 and relaxin‐2, caused extensive filopodia formation. This coincided with enhanced activity of ezrin, a key factor in tethering F‐actin to the plasma membrane, and inhibition of the actin filament severing activity of cofilin. The F‐actin remodeling and pro‐migratory activities promoted by the novel RXFP1‐JAK3‐STAT3‐Cdc42 axis were blocked by JAK3 inhibitor tofacitinib and STAT3 inhibitor STAT3 inhibitor VI. This provides a new rationale for the design of JAK3 and STAT3 inhibitors with better brain permeability for clinical treatment of the pervasive brain invasiveness of GBM.

The EphA4 Signaling is Anti-catabolic in Synoviocytes but Pro-anabolic in Articular Chondrocytes

The expression and activation of EphA4 in the various cell types in a knee joint was upregulated upon an intraarticular injury. To determine if EphA4 signaling plays a role in osteoarthritis, we determined whether deficient EphA4 expression (in EphA4 knockout mice) or upregulation of the EphA4 signaling (with the EfnA4-fc treatment) would alter cellular functions of synoviocytes and articular chondrocytes. In synoviocytes, deficient EphA4 expression enhanced, whereas activation of the EphA4 signaling reduced, expression and secretion of key inflammatory cytokines and matrix metalloproteases. Conversely, in articular chondrocytes, activation of the EphA4 signaling upregulated, while deficient EphA4 expression reduced, expression levels of chondrogenic genes (e.g., aggrecan, lubricin, type-2 collagen, and Sox9). EfnA4-fc treatment in wildtype, but not EphA4-deficient, articular chondrocytes promoted the formation and activity of acidic proteoglycan-producing colonies. Activation of the EphA4 signaling in articular chondrocytes upregulated Rac1/2 and downregulated RhoA via enhancing Vav1 and reducing Ephexin1 activation, respectively. However, activation of the EphA4 signaling in synoviocytes suppressed the Vav/Rac signaling while upregulated the Ephexin/Rho signaling. In summary, the EphA4 signaling in synoviocytes is largely of anti-catabolic nature through suppression of the expression of inflammatory cytokines and matrix proteases, but in articular chondrocytes the signaling is pro-anabolic in that it promotes the biosynthesis of articular cartilage. The contrasting action of the EphA4 signaling in synoviocytes as opposing to articular chondrocytes may in part be mediated through the opposite differential effects of the EphA4 signaling on the Vav/Rac signaling and Ephexin/Rho signaling in the two skeletal cell types.

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.

RhoJ integrates attractive and repulsive cues in directional migration of endothelial cells

During angiogenesis, VEGF acts as an attractive cue for endothelial cells (ECs), while Sema3E mediates repulsive cues. Here, we show that the small GTPase RhoJ integrates these opposing signals in directional EC migration. In the GTP-bound state, RhoJ interacts with the cytoplasmic domain of PlexinD1. Upon Sema3E stimulation, RhoJ released from PlexinD1 induces cell contraction. PlexinD1-bound RhoJ further facilitates Sema3E-induced PlexinD1-VEGFR2 association, VEGFR2 transphosphorylation at Y1214, and p38 MAPK activation, leading to reverse EC migration. Upon VEGF stimulation, RhoJ is required for the formation of the holoreceptor complex comprising VEGFR2, PlexinD1, and neuropilin-1, thereby preventing degradation of internalized VEGFR2, prolonging downstream signal transductions via PLCγ, Erk, and Akt, and promoting forward EC migration. After conversion to the GDP-bound state, RhoJ shifts from PlexinD1 to VEGFR2, which then terminates the VEGFR2 signals. RhoJ deficiency in ECs efficiently suppressed aberrant angiogenesis in ischemic retina. These findings suggest that distinct Rho GTPases may act as context-dependent integrators of chemotactic cues in directional cell migration and may serve as candidate therapeutic targets to manipulate cell motility in disease or tissue regeneration.

Molecular Characterization of Tc964, A Novel Antigenic Protein from Trypanosoma cruzi

The Tc964 protein was initially identified by its presence in the interactome associated with the LYT1 mRNAs, which code for a virulence factor of Trypanosoma cruzi. Tc964 is annotated in the T. cruzi genome as a hypothetical protein. According to phylogenetic analysis, the protein is conserved in the different genera of the Trypanosomatidae family; however, recognizable orthologues were not identified in other groups of organisms. Therefore, as a first step, an in-depth molecular characterization of the Tc946 protein was carried out. Based on structural predictions and molecular dynamics studies, the Tc964 protein would belong to a particular class of GTPases. Subcellular fractionation analysis indicated that Tc964 is a nucleocytoplasmic protein. Additionally, the protein was expressed as a recombinant protein in order to analyze its antigenicity with sera from Chagas disease (CD) patients. Tc964 was found to be antigenic, and B-cell epitopes were mapped by the use of synthetic peptides. In parallel, the Leishmania major homologue (Lm964) was also expressed as recombinant protein and used for a preliminary evaluation of antigen cross-reactivity in CD patients. Interestingly, Tc964 was recognized by sera from Chronic CD (CCD) patients at different stages of disease severity, but no reactivity against this protein was observed when sera from Colombian patients with cutaneous leishmaniasis were analyzed. Therefore, Tc964 would be adequate for CD diagnosis in areas where both infections (CD and leishmaniasis) coexist, even though additional assays using larger collections of sera are needed in order to confirm its usefulness for differential serodiagnosis.

Spatiotemporal Control of Intracellular Membrane Trafficking by Rho GTPases

As membrane-associated master regulators of cytoskeletal remodeling, Rho GTPases coordinate a wide range of biological processes such as cell adhesion, motility, and polarity. In the last years, Rho GTPases have also been recognized to control intracellular membrane sorting and trafficking steps directly; however, how Rho GTPase signaling is regulated at endomembranes is still poorly understood. In this review, we will specifically address the local Rho GTPase pools coordinating intracellular membrane trafficking with a focus on the endo- and exocytic pathways. We will further highlight the spatiotemporal molecular regulation of Rho signaling at endomembrane sites through Rho regulatory proteins, the GEFs and GAPs. Finally, we will discuss the contribution of dysregulated Rho signaling emanating from endomembranes to the development and progression of cancer.

Ubiquitin-based modifications in endothelial cell-cell contact and inflammation

Endothelial cell-cell contacts are essential for vascular integrity and physiology, protecting tissues and organs from edema and uncontrolled invasion of inflammatory cells. The vascular endothelial barrier is dynamic, but its integrity is preserved through a tight control at different levels. Inflammatory cytokines and G-protein-coupled receptor agonists, such as histamine, reduce endothelial integrity and increase vascular leakage. This is due to elevated myosin-based contractility, in conjunction with phosphorylation of proteins at cell-cell contacts. Conversely, reducing contractility stabilizes or even increases endothelial junctional integrity. Rho GTPases are key regulators of such cytoskeletal dynamics and endothelial cell-cell contacts. In addition to signaling-induced regulation, the expression of junctional proteins, such as occludin, claudins and vascular endothelial cadherin, also controls endothelial barrier function. There is increasing evidence that, in addition to protein phosphorylation, ubiquitylation (also known as ubiquitination) is an important and dynamic post-translational modification that regulates Rho GTPases, junctional proteins and, consequently, endothelial barrier function. In this Review, we discuss the emerging role of ubiquitylation and deubiquitylation events in endothelial integrity and inflammation. The picture that emerges is one of increasing complexity, which is both fascinating and promising given the clinical relevance of vascular integrity in the control of inflammation, and of tissue and organ damage.

Ezrin Orchestrates Signal Transduction in Airway Cells

Ezrin is a critical structural protein that organizes receptor complexes and orchestrates their signal transduction. In this study, we review the ezrin-meditated regulation of critical receptor complexes, including the epidermal growth factor receptor (EGFR), CD44, vascular cell adhesion molecule (VCAM), and the deleted in colorectal cancer (DCC) receptor. We also analyze the ezrin-meditated regulation of critical pathways associated with asthma, such as the RhoA, Rho-associated protein kinase (ROCK), and protein kinase A (cAMP/PKA) pathways. Mounting evidence suggests that ezrin plays a role in controlling airway cell function and potentially contributes to respiratory diseases. Ezrin can participate in asthma pathogenesis by affecting bronchial epithelium repair, T lymphocyte regulation, and the contraction of the airway smooth muscle cells. These studies provide new insights for the design of novel therapeutic strategies for asthma treatment.

The TLR4 adaptor TRAM controls the phagocytosis of Gram-negative bacteria by interacting with the Rab11-family interacting protein 2

Phagocytosis is a complex process that eliminates microbes and is performed by specialised cells such as macrophages. Toll-like receptor 4 (TLR4) is expressed on the surface of macrophages and recognizes Gram-negative bacteria. Moreover, TLR4 has been suggested to play a role in the phagocytosis of Gram-negative bacteria, but the mechanisms remain unclear. Here we have used primary human macrophages and engineered THP-1 monocytes to show that the TLR4 sorting adapter, TRAM, is instrumental for phagocytosis of Escherichia coli as well as Staphylococcus aureus. We find that TRAM forms a complex with Rab11 family interacting protein 2 (FIP2) that is recruited to the phagocytic cups of E. coli. This promotes activation of the actin-regulatory GTPases Rac1 and Cdc42. Our results show that FIP2 guided TRAM recruitment orchestrates actin remodelling and IRF3 activation, two events that are both required for phagocytosis of Gram-negative bacteria.

The Gram-negative bacteria E. coli is the most common cause of severe human pathological conditions like sepsis. Sepsis is a clinical syndrome defined by pathological changes due to systemic inflammation, resulting in paralysis of adaptive T-cell immunity with IFN-β as a critical factor. TLR4 is a key sensing receptor of lipopolysaccharide on Gram-negative bacteria. Inflammatory signalling by TLR4 is initiated by the use of alternative pair of TIR-adapters, MAL-MyD88 or TRAM-TRIF. MAL-MyD88 signaling occurs mainly from the plasma membrane giving pro-inflammatory cytokines like TNF, while TRAM-TRIF signaling occurs from vacuoles like endosomes and phagosomes to give type I interferons like IFN-β. It has previously been shown that TLR4 can control phagocytosis and phagosomal maturation through MAL-MyD88 in mice, however, these data have been disputed and published before the role of TRAM was defined in the induction of IFN-β. A role for TRAM or TRIF in phagocytosis has not previously been reported. Here we describe a novel mechanism where TRAM and its binding partner Rab11-FIP2 control phagocytosis of E. coli and regulate IRF3 dependent production of IFN-β. The significance of these results is that we define Rab11-FIP2 as a potential target for modulation of TLR4-dependent signalling in different pathological states.

Role of glutamine synthetase in angiogenesis beyond glutamine synthesis

Glutamine synthetase, encoded by the gene GLUL, is an enzyme that converts glutamate and ammonia to glutamine. It is expressed by endothelial cells, but surprisingly shows negligible glutamine-synthesizing activity in these cells at physiological glutamine levels. Here we show in mice that genetic deletion of Glul in endothelial cells impairs vessel sprouting during vascular development, whereas pharmacological blockade of glutamine synthetase suppresses angiogenesis in ocular and inflammatory skin disease while only minimally affecting healthy adult quiescent endothelial cells. This relies on the inhibition of endothelial cell migration but not proliferation. Mechanistically we show that in human umbilical vein endothelial cells GLUL knockdown reduces membrane localization and activation of the GTPase RHOJ while activating other Rho GTPases and Rho kinase, thereby inducing actin stress fibres and impeding endothelial cell motility. Inhibition of Rho kinase rescues the defect in endothelial cell migration that is induced by GLUL knockdown. Notably, glutamine synthetase palmitoylates itself and interacts with RHOJ to sustain RHOJ palmitoylation, membrane localization and activation. These findings reveal that, in addition to the known formation of glutamine, the enzyme glutamine synthetase shows unknown activity in endothelial cell migration during pathological angiogenesis through RHOJ palmitoylation.

Ubiquitin, SUMO, and NEDD8: Key Targets of Bacterial Pathogens

Manipulation of host protein post-translational modifications (PTMs) is used by various pathogens to interfere with host cell functions. Among these modifications, ubiquitin (UBI) and ubiquitin-like proteins (UBLs) constitute key targets because they are regulators of pathways essential for the host cell. In particular, these PTM modifiers control pathways that have been described as crucial for infection such as pathogen entry, replication, propagation, or detection by the host. Although bacterial pathogens lack eucaryotic-like UBI or UBL systems, many of them produce proteins that specifically interfere with these host PTMs during infection. In this review we discuss the different mechanisms used by bacteria to interfere with host UBI and the two UBLs, SUMO and NEDD8.

Hypoxia-induced alterations in the lung ubiquitin proteasome system during pulmonary hypertension pathogenesis

Pulmonary hypertension (PH) is a clinical disorder characterized by sustained increases in pulmonary vascular resistance and pressure that can lead to right ventricular (RV) hypertrophy and ultimately RV failure and death. The molecular pathogenesis of PH remains incompletely defined, and existing treatments are associated with suboptimal outcomes and persistent morbidity and mortality. Reports have suggested a role for the ubiquitin proteasome system (UPS) in PH, but the extent of UPS-mediated non-proteolytic protein alterations during PH pathogenesis has not been previously defined. To further examine UPS alterations, the current study employed C57BL/6J mice exposed to normoxia or hypoxia for 3 weeks. Lung protein ubiquitination was evaluated by mass spectrometry to identify differentially ubiquitinated proteins relative to normoxic controls. Hypoxia stimulated differential ubiquitination of 198 peptides within 131 proteins (p < 0.05). These proteins were screened to identify candidates within pathways involved in PH pathogenesis. Some 51.9% of the differentially ubiquitinated proteins were implicated in at least one known pathway contributing to PH pathogenesis, and 13% were involved in three or more PH pathways. Anxa2, App, Jak1, Lmna, Pdcd6ip, Prkch1, and Ywhah were identified as mediators in PH pathways that undergo differential ubiquitination during PH pathogenesis. To our knowledge, this is the first study to report global changes in protein ubiquitination in the lung during PH pathogenesis. These findings suggest signaling nodes that are dynamically regulated by the UPS during PH pathogenesis. Further exploration of these differentially ubiquitinated proteins and related pathways can provide new insights into the role of the UPS in PH pathogenesis.

Loss of Cannabinoid CB1 Receptors Induces Cortical Migration Malformations and Increases Seizure Susceptibility

Neuronal migration is a fundamental process of brain development, and its disruption underlies devastating neurodevelopmental disorders. The transcriptional programs governing this process are relatively well characterized. However, how environmental cues instruct neuronal migration remains poorly understood. Here, we demonstrate that the cannabinoid CB1 receptor is strictly required for appropriate pyramidal neuron migration in the developing cortex. Acute silencing of the CB1 receptor alters neuronal morphology and impairs radial migration. Consequently, CB1 siRNA-electroporated mice display cortical malformations mimicking subcortical band heterotopias and increased seizure susceptibility in adulthood. Importantly, rescuing the CB1 deficiency-induced radial migration arrest by knockdown of the GTPase protein RhoA restored the hyperexcitable neuronal network and seizure susceptibility. Our findings show that CB1 receptor/RhoA signaling regulates pyramidal neuron migration, and that deficient CB1 receptor signaling may contribute to cortical development malformations leading to refractory epilepsy independently of its canonical neuromodulatory role in the adult brain.

FK506-binding protein 10 (FKBP10) regulates lung fibroblast migration via collagen VI synthesis

Background

In idiopathic pulmonary fibrosis (IPF), fibroblasts gain a more migratory phenotype and excessively secrete extracellular matrix (ECM), ultimately leading to alveolar scarring and progressive dyspnea. Here, we analyzed the effects of deficiency of FK506-binding protein 10 (FKBP10), a potential IPF drug target, on primary human lung fibroblast (phLF) adhesion and migration.

Methods

Using siRNA, FKBP10 expression was inhibited in phLF in absence or presence of 2ng/ml transforming growth factor-β1 (TGF-β1) and 0.1mM 2-phosphoascorbate. Effects on cell adhesion and migration were monitored by an immunofluorescence (IF)-based attachment assay, a conventional scratch assay, and single cell tracking by time-lapse microscopy. Effects on expression of key players in adhesion dynamics and migration were analyzed by qPCR and Western Blot. Colocalization was evaluated by IF microscopy and by proximity ligation assays.

Results

FKBP10 knockdown significantly attenuated adhesion and migration of phLF. Expression of collagen VI was decreased, while expression of key components of the focal adhesion complex was mostly upregulated. The effects on migration were 2-phosphoascorbate-dependent, suggesting collagen synthesis as the underlying mechanism. FKBP10 colocalized with collagen VI and coating culture dishes with collagen VI, and to a lesser extent with collagen I, abolished the effect of FKBP10 deficiency on migration.

Conclusions

These findings show, to our knowledge for the first time, that FKBP10 interacts with collagen VI and that deficiency of FKBP10 reduces phLF migration mainly by downregulation of collagen VI synthesis. The results strengthen FKBP10 as an important intracellular regulator of ECM remodeling and support the concept of FKBP10 as drug target in IPF.

The Cullin-3-Rbx1-KCTD10 complex controls endothelial barrier function via K63 ubiquitination of RhoB

The RhoA GTPase controls endothelial cell migration, adhesion, and barrier formation but the role of RhoB is unclear. Kovačević et al. now discover that RhoB is ubiquitinated by the CUL3–Rbx1–KCTD10 complex and that this is a prerequisite for lysosomal degradation of RhoB and the maintenance of endothelial barrier integrity. 

RhoGTPases control endothelial cell (EC) migration, adhesion, and barrier formation. Whereas the relevance of RhoA for endothelial barrier function is widely accepted, the role of the RhoA homologue RhoB is poorly defined. RhoB and RhoA are 85% identical, but RhoB’s subcellular localization and half-life are uniquely different. Here, we studied the role of ubiquitination for the function and stability of RhoB in primary human ECs. We show that the K63 polyubiquitination at lysine 162 and 181 of RhoB targets the protein to lysosomes. Moreover, we identified the RING E3 ligase complex Cullin-3–Rbx1–KCTD10 as key modulator of endothelial barrier integrity via its regulation of the ubiquitination, localization, and activity of RhoB. In conclusion, our data show that ubiquitination controls the subcellular localization and lysosomal degradation of RhoB and thereby regulates the stability of the endothelial barrier through control of RhoB-mediated EC contraction.

The phosphomimetic mutation of syndecan-4 binds and inhibits Tiam1 modulating Rac1 activity in PDZ interaction-dependent manner

The small GTPases of the Rho family comprising RhoA, Rac1 and Cdc42 function as molecular switches controlling several essential biochemical pathways in eukaryotic cells. Their activity is cycling between an active GTP-bound and an inactive GDP-bound conformation. The exchange of GDP to GTP is catalyzed by guanine nucleotide exchange factors (GEFs). Here we report a novel regulatory mechanism of Rac1 activity, which is controlled by a phosphomimetic (Ser179Glu) mutant of syndecan-4 (SDC4). SDC4 is a ubiquitously expressed transmembrane, heparan sulfate proteoglycan. In this study we show that the Ser179Glu mutant binds strongly Tiam1, a Rac1-GEF reducing Rac1-GTP by 3-fold in MCF-7 breast adenocarcinoma cells. Mutational analysis unravels the PDZ interaction between SDC4 and Tiam1 is indispensable for the suppression of the Rac1 activity. Neither of the SDC4 interactions is effective alone to block the Rac1 activity, on the contrary, lack of either of interactions can increase the activity of Rac1, therefore the Rac1 activity is the resultant of the inhibitory and stimulatory effects. In addition, SDC4 can bind and tether RhoGDI1 (GDP-dissociation inhibitor 1) to the membrane. Expression of the phosphomimetic SDC4 results in the accumulation of the Rac1–RhoGDI1 complex. Co-immunoprecipitation assays (co-IP-s) reveal that SDC4 can form complexes with RhoGDI1. Together, the regulation of the basal activity of Rac1 is fine tuned and SDC4 is implicated in multiple ways.

Ubiquitin-dependent regulation of Cdc42 by XIAP

Rho GTPases control fundamental cellular processes and Cdc42 is a well-studied member of the family that controls filopodia formation and cell migration. Although the regulation of Cdc42 activity by nucleotide binding is well documented, the mechanisms driving its proteostasis are not clear. Here, we demonstrate that the highly conserved, RING domain containing E3 ubiquitin ligase XIAP controls the protein stability of Cdc42. XIAP binds to Cdc42 and directly conjugates poly ubiquitin chains to the Lysine 166 of Cdc42 targeting it for proteasomal degradation. Depletion of XIAP led to an increased protein stability and activity of Cdc42 in normal and tumor cells. Consistently, loss of XIAP enhances filopodia formation in a Cdc42-dependent manner and this phenomenon phenocopies EGF stimulation. Further, XIAP depletion promotes lung colonization of tumor cells in mice in a Cdc42-dependent manner. These observations shed molecular insights into ubiquitin-dependent regulation of Cdc42 and that of actin cytoskeleton.

RhoA and RhoC are involved in stromal cell-derived factor-1-induced cell migration by regulating F-actin redistribution and assembly

Stromal cell-derived factor-1 (SDF-1) signaling is important to the maintenance and progression of T-cell acute lymphoblastic leukemia by inducing chemotaxis migration. To identify the mechanism of SDF-1 signaling in the migration of T-ALL, Jurkat acute lymphoblastic leukemia cells were used. Results showed that SDF-1 induces Jurkat cell migration by F-actin redistribution and assembly, which is dependent on Rho activity. SDF-1 induced RhoA and RhoC activation, as well as reactive oxygen species (ROS) production, which was inhibited by Rho inhibitor. The Rho-dependent ROS production led to subsequent cytoskeleton redistribution and assembly in the process of migration. Additionally, RhoA and RhoC were involved in SDF-1-induced Jurkat cell migration. Taken together, we found a SDF-1/CXCR4-RhoA and RhoC-ROS-cytoskeleton pathway that regulates Jurkat cell migration in response to SDF-1. This work will contribute to a clearer insight into the migration mechanism of acute lymphoblastic leukemia.

NRF2 promotes breast cancer cell proliferation and metastasis by increasing RhoA/ROCK pathway signal transduction

Nuclear factor erythroid 2-related factor (NRF2) is an important transcription factor in oxidative stress regulation. Overexpression of NRF2 is associated with human breast carcinogenesis, and increased NRF2 mRNA levels predict poor patient outcome for breast cancer. However, the mechanisms linking gain of NRF2 expression and poor prognosis in breast cancer are still unclear. Here, we provide evidence that NRF2 deletion inhibits proliferation and metastasis of breast cancer cells by down-regulating RhoA. Restoration of RhoA in MCF7 and MDA-MB-231 cells induced NRF2 knockdown-suppressed cell growth and metastasis in vitro, and NRF2 silencing suppressed stress fiber and focal adhesion formation leading to decreased cell migration and invasion. Mechanistic studies showed that NRF2 binds to the promoter region of estrogen-related receptor α (ERR1) and may function as a silencer. This may enhance RhoA protein stability and lead to RhoA overexpression in breast cancer cell. Our findings indicate that NRF2 silencing-mediated reduction of RhoA expression contributes, at least in part, to the poor outcome of breast cancer patients with high NRF2 expression.

Rho GTPases: Regulation and roles in cancer cell biology

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

The E3 ubiquitin ligase Hace1 is required for early embryonic development in Xenopus laevis

BACKGROUND

HECT domain and ankyrin repeat containing E3 ubiquitin protein ligase 1 (HACE1) regulates a wide variety of cellular processes. It has been shown that one of the targets of HACE1 is the GTP-bound form of the small GTPase Rac1. However, the role of HACE1 in early development remains unknown.

RESULTS

In situ hybridization revealed that Xenopus laevis hace1 is specifically expressed in the ectoderm at the blastula and gastrula stages and in the epidermis, branchial arch, kidney, and central nervous system at the tailbud stage. Knockdown of hace1 in Xenopus laevis embryos via antisense morpholino oligonucleotides led to defects in body axis elongation, pigment formation, and eye formation at the tadpole stage. Experiments with Keller sandwich explants showed that hace1 knockdown inhibited convergent extension, a morphogenetic movement known to be crucial for body axis elongation. In addition, time lapse imaging of whole embryos during the neurula stage indicated that hace1 knockdown also delayed neural tube closure. The defects caused by hace1 knockdown were partly rescued by knockdown of rac1. Moreover, embryos expressing a constitutively active form of Rac1 displayed phenotypes similar to those of embryos with hace1 knocked down.

CONCLUSIONS

Our results suggest that Xenopus laevis hace1 plays an important role in early embryonic development, possibly via regulation of Rac1 activity.

Deregulation of Rho GTPases in cancer

In vitro and in vivo studies and evidence from human tumors have long implicated Rho GTPase signaling in the formation and dissemination of a range of cancers. Recently next generation sequencing has identified direct mutations of Rho GTPases in human cancers. Moreover, the effects of ablating genes encoding Rho GTPases and their regulators in mouse models, or through pharmacological inhibition, strongly suggests that targeting Rho GTPase signaling could constitute an effective treatment. In this review we will explore the various ways in which Rho signaling can be deregulated in human cancers.

Global Analysis of Host and Bacterial Ubiquitinome in Response to Salmonella Typhimurium Infection

Ubiquitination serves as a critical signal in the host immune response to infection. Many pathogens have evolved strategies to exploit the ubiquitin (Ub) system to promote their own survival through a complex interplay between host defense machinery and bacterial virulence factors. Here we report dynamic changes in the global ubiquitinome of host epithelial cells and invading pathogen in response to Salmonella Typhimurium infection. The most significant alterations in the host ubiquitinome concern components of the actin cytoskeleton, NF-κB and autophagy pathways, and the Ub and RHO GTPase systems. Specifically, infection-induced ubiquitination promotes CDC42 activity and linear ubiquitin chain formation, both being required for NF-κB activation. Conversely, the bacterial ubiquitinome exhibited extensive ubiquitination of various effectors and several outer membrane proteins. Moreover, we reveal that bacterial Ub-modifying enzymes modulate a unique subset of host targets, affecting different stages of Salmonella infection.

PAK4 promotes kinase-independent stabilization of RhoU to modulate cell adhesion

PAK4, via a novel kinase-independent mechanism, protects RhoU from a Rab40A/Cullin 5 ubiquitin ligase complex–driven K48 ubiquitination to regulate breast cancer cell adhesion.

P21-activated kinase 4 (PAK4) is a Cdc42 effector protein thought to regulate cell adhesion disassembly in a kinase-dependent manner. We found that PAK4 expression is significantly higher in high-grade human breast cancer patient samples, whereas depletion of PAK4 modifies cell adhesion dynamics of breast cancer cells. Surprisingly, systematic analysis of PAK4 functionality revealed that PAK4-driven adhesion turnover is neither dependent on Cdc42 binding nor kinase activity. Rather, reduced expression of PAK4 leads to a concomitant loss of RhoU expression. We report that RhoU is targeted for ubiquitination by the Rab40A–Cullin 5 complex and demonstrate that PAK4 protects RhoU from ubiquitination in a kinase-independent manner. Overexpression of RhoU rescues the PAK4 depletion phenotype, whereas loss of RhoU expression reduces cell adhesion turnover and migration. These data support a new kinase-independent mechanism for PAK4 function, where an important role of PAK4 in cellular adhesions is to stabilize RhoU protein levels. Thus, PAK4 and RhoU cooperate to drive adhesion turnover and promote cell migration.

An Impaired Respiratory Electron Chain Triggers Down-regulation of the Energy Metabolism and De-ubiquitination of Solute Carrier Amino Acid Transporters

Hundreds of genes have been associated with respiratory chain disease (RCD), the most common inborn error of metabolism so far. Elimination of the respiratory electron chain by depleting the entire mitochondrial DNA (mtDNA, ρ(0) cells) has therefore one of the most severe impacts on the energy metabolism in eukaryotic cells. In this study, proteomic data sets including the post-translational modifications (PTMs) phosphorylation and ubiquitination were integrated with metabolomic data sets and selected enzyme activities in the osteosarcoma cell line 143B.TK(-) A shotgun based SILAC LC-MS proteomics and a targeted metabolomics approach was applied to elucidate the consequences of the ρ(0) state. Pathway and protein-protein interaction (PPI) network analyses revealed a nonuniform down-regulation of the respiratory electron chain, the tricarboxylic acid (TCA) cycle, and the pyruvate metabolism in ρ(0) cells. Metabolites of the TCA cycle were dysregulated, such as a reduction of citric acid and cis-aconitic acid (six and 2.5-fold), and an increase of lactic acid, oxalacetic acid (both twofold), and succinic acid (fivefold) in ρ(0) cells. Signaling pathways such as GPCR, EGFR, G12/13 alpha, and Rho GTPases were up-regulated in ρ(0) cells, which could be indicative for the mitochondrial retrograde response, a pathway of communication from mitochondria to the nucleus. This was supported by our phosphoproteome data, which revealed two main processes, GTPase-related signal transduction and cytoskeleton organization. Furthermore, a general de-ubiquitination in ρ(0) cells was observed, for example, 80S ribosomal proteins were in average threefold and SLC amino acid transporters fivefold de-ubiquitinated. The latter might cause the observed significant increase of amino acid levels in ρ(0) cells. We conclude that an elimination of the respiratory electron chain, e.g. mtDNA depletion, not only leads to an uneven down-regulation of mitochondrial energy pathways, but also triggers the retrograde response.

A barley SKP1-like protein controls abundance of the susceptibility factor RACB and influences the interaction of barley with the barley powdery mildew fungus

In an increasing number of plant-microbe interactions, it has become evident that the abundance of immunity-related proteins is controlled by the ubiquitin-26S proteasome system. In the interaction of barley with the biotrophic barley powdery mildew fungus Blumeria graminis f.sp. hordei (Bgh), the RAC/ROP [RAT SARCOMA-related C3 botulinum toxin substrate/RAT SARCOMA HOMOLOGUE (RHO) of plants] guanosine triphosphatase (GTPase) HvRACB supports the fungus in a compatible interaction. By contrast, barley HvRBK1, a ROP-binding receptor-like cytoplasmic kinase that interacts with and can be activated by constitutively activated HvRACB, limits fungal infection success. We have identified a barley type II S-phase kinase 1-associated (SKP1)-like protein (HvSKP1-like) as a molecular interactor of HvRBK1. SKP1 proteins are subunits of the SKP1-cullin 1-F-box (SCF)-E3 ubiquitin ligase complex that acts in the specific recognition and ubiquitination of protein substrates for subsequent proteasomal degradation. Transient induced gene silencing of either HvSKP1-like or HvRBK1 increased protein abundance of constitutively activated HvRACB in barley epidermal cells, whereas abundance of dominant negative RACB only weakly increased. In addition, silencing of HvSKP1-like enhanced the susceptibility of barley to haustorium establishment by Bgh. In summary, our results suggest that HvSKP1-like, together with HvRBK1, controls the abundance of HvRACB and, at the same time, modulates the outcome of the barley-Bgh interaction. A possible feedback mechanism from RAC/ROP-activated HvRBK1 on the susceptibility factor HvRACB is discussed.

RopGEF2 is involved in ABA-suppression of seed germination and post-germination growth of Arabidopsis

The involvement of Rho of Plants (ROP) GTPases in abscisic acid (ABA) signalling in Arabidopsis has been demonstrated in many studies. However, the roles of RopGEFs (Rop guanine nucleotide exchange factors), which modulate ROP activities in ABA signalling, are poorly understood. Here, we demonstrate that RopGEF2 may play a negative role in ABA-suppressed seed germination and post-germination growth. We show that disruption of RopGEF2 enhances sensitivity to exogenous ABA in seed germination assays and that RopGEF2pro-GUS is mainly expressed in developing embryos and germinating seeds. Interestingly, YFP-RopGEF2 is located in both the cytoplasmic region and in mitochondria. Notably, the PRONE2 (plant-specific ROP nucleotide exchanger 2) domain of RopGEF2 is detected in mitochondria, whereas the N-terminus of RopGEF2 is shown to be in the cytosol. After ABA treatment, degradation of RopGEF2 is triggered in the cytosol through the ubiquitin-26S proteasome system. The binding of RopGEF2 to ROP2, ROP6 or ROP10, which has been demonstrated to be involved in ABA signalling, not only alters the localization of RopGEF2 but also enables RopGEF2 to escape degradation in the cell. Thus, in this study, we deduce a sophisticated mechanism of ABA-mediated RopGEF2-ROP signalling, which potentially implicates the inactivation of ROPs in responsiveness to ABA.

Downregulation of the small GTPase SAR1A: a key event underlying alcohol-induced Golgi fragmentation in hepatocytes

The hepatic asialoglycoprotein receptor (ASGP-R) is posttranslationally modified in the Golgi en route to the plasma membrane, where it mediates clearance of desialylated serum glycoproteins. It is known that content of plasma membrane-associated ASGP-R is decreased after ethanol exposure, although the mechanisms remain elusive. Previously, we found that formation of compact Golgi requires dimerization of the largest Golgi matrix protein giantin. We hypothesize that ethanol-impaired giantin function may be related to altered trafficking of ASGP-R. Here we report that in HepG2 cells expressing alcohol dehydrogenase and hepatocytes of ethanol-fed rats, ethanol metabolism results in Golgi disorganization. This process is initiated by dysfunction of SAR1A GTPase followed by altered COPII vesicle formation and impaired Golgi delivery of the protein disulfide isomerase A3 (PDIA3), an enzyme that catalyzes giantin dimerization. Additionally, we show that SAR1A gene silencing in hepatocytes mimics the effect of ethanol: dedimerization of giantin, arresting PDIA3 in the endoplasmic reticulum (ER) and large-scale alterations in Golgi architecture. Ethanol-induced Golgi fission has no effect on ER-to-Golgi transportation of ASGP-R, however, it results in its deposition in cis-medial-, but not trans-Golgi. Thus, alcohol-induced deficiency in COPII vesicle formation predetermines Golgi fragmentation which, in turn, compromises the Golgi-to-plasma membrane transportation of ASGP-R.

Carbon-Ion Irradiation Suppresses Migration and Invasiveness of Human Pancreatic Carcinoma Cells MIAPaCa-2 via Rac1 and RhoA Degradation

PURPOSE

To investigate the mechanisms underlying the inhibition of cancer cell migration and invasion by carbon (C)-ion irradiation.

METHODS AND MATERIALS

Human pancreatic cancer cells MIAPaCa-2, AsPC-1, and BxPC-3 were treated by x-ray (4 Gy) or C-ion (0.5, 1, 2, or 4 Gy) irradiation, and their migration and invasion were assessed 2 days later. The levels of guanosine triphosphate (GTP)-bound Rac1 and RhoA were determined by the active GTPase pull-down assay with or without a proteasome inhibitor, and the binding of E3 ubiquitin ligase to GTP-bound Rac1 was examined by immunoprecipitation.

RESULTS

Carbon-ion irradiation reduced the levels of GTP-bound Rac1 and RhoA, 2 major regulators of cell motility, in MIAPaCa-2 cells and GTP-bound Rac1 in AsPC-1 and BxPC-3 cells. Proteasome inhibition reversed the effect, indicating that C-ion irradiation induced Rac1 and RhoA degradation via the ubiquitin (Ub)-proteasome pathway. E3 Ub ligase X-linked inhibitor of apoptosis protein (XIAP), which directly targets Rac1, was selectively induced in C-ion--irradiated MIAPaCa-2 cells and coprecipitated with GTP-bound Rac1 in C-ion--irradiated cells, which was associated with Rac1 ubiquitination. Cell migration and invasion reduced by C-ion radiation were restored by short interfering RNA--mediated XIAP knockdown, indicating that XIAP is involved in C-ion--induced inhibition of cell motility.

CONCLUSION

In contrast to x-ray irradiation, C-ion treatment inhibited the activity of Rac1 and RhoA in MIAPaCa-2 cells and Rac1 in AsPC-1 and BxPC-3 cells via Ub-mediated proteasomal degradation, thereby blocking the motility of these pancreatic cancer cells.

Loss of the E3 ubiquitin ligase HACE1 results in enhanced Rac1 signaling contributing to breast cancer progression

The transition from ductal carcinoma in situ (DCIS) to invasive breast cancer (IBC) is a crucial step in breast cancer progression. The specific alterations that govern this transition have not been elucidated. HER2/neu is frequently overexpressed in DCIS but is less common in IBC, thereby suggesting additional requirements for transformation. To identify genes capable of cooperating with HER2/neu to fully transform mammary epithelial cells, we used an insertional mutagenesis screen on cells isolated from wild-type neu expressing mice and identified the E3 ligase HACE1 as HER2 cooperative tumor suppressor gene. Loss of HACE1 expression is commonly seen in clinical breast cancer data sets. HACE1 downregulation in normal human mammary epithelial cells (HMECs) results in the accumulation of the activated GTP-bound Rac1 partially transforming these cells. Overexpression of HER2 activates Rac1, which further accumulates upon HACE1 loss resulting in Rac1 hyperactivation. Although the knockdown of HACE1 or overexpression of HER2 alone in HMECs is not sufficient for tumorigenesis, HER2 overexpression combined with HACE1 downregulation fully transforms HMECs resulting in robust tumor formation. The pharmaceutical interference of Rac function abrogates the effects of HACE1 loss both in vitro and in vivo, resulting in marked reduction in tumor burden. Our work supports a critical role for HACE1 in breast cancer progression and identifies patients that may benefit from Rac-targeted therapies.

Endothelin receptor blockade ameliorates renal injury by inhibition of RhoA/Rho-kinase signalling in deoxycorticosterone acetate-salt hypertensive rats

PURPOSE OF REVIEW

Excessive production of fibrosis is a feature of hypertension-induced renal injury. Activation of RhoA/Rho-kinase (ROCK) axis has been shown in deoxycorticosterone acetate (DOCA)-salt hypertensive rats. We assessed whether selective endothelin receptor blockers can attenuate renal fibrosis by inhibiting RhoA/ROCK axis in DOCA-salt rats.

METHODS

At 4 weeks after the start of DOCA-salt treatment and uninephrectomization, male Wistar rats were randomized into three groups for 4 weeks: vehicle, ABT-627 (endothelin-A receptor inhibitor) and A192621 (endothelin-B receptor inhibitor).

RESULTS

DOCA-salt was characterized by increased blood pressure, decreased renal function, increased proteinuria, increased glomerulosclerosis and tubulointerstitial fibrosis with myofibroblast accumulation, increased renal endothelin-1 levels and RhoA activity along with increased expression of connective tissue growth factor at both mRNA and protein levels as compared with uninephrectomized control male Wistar rats. Treatment with a selective mineralocorticoid receptor antagonist, eplerenone, ameliorated proteinuria. Impaired renal function and histological changes were overcome by treatment with ABT-627, but not with A192621. The beneficial effects of bosentan, a nonspecific endothelin receptor blocker, on proteinuria, RhoA activity, and connective tissue growth factor levels were similar to ABT-627. Furthermore, in an isolated perfuse kidney, a RhoA inhibitor, C3 exoenzyme, and two ROCK inhibitors, fasudil and Y-27632, significantly attenuated connective tissue growth factor levels.

CONCLUSIONS

These results indicate that DOCA-salt elevates renal endothelin-1 levels and RhoA activity via activation of mineralocorticoid receptor, resulting in renal fibrosis and proteinuria. Endothelin-A receptor blockade can attenuate DOCA-salt-induced renal fibrosis probably through the inhibition of RhoA/ROCK activity and connective tissue growth factor expression.

Estrogen-related receptor α decreases RHOA stability to induce orientated cell migration

Several physiopathological processes require orientated cellular migration. This phenomenon highly depends on members of the RHO family of GTPases. Both excessive and deficient RHO activity impair directional migration. A tight control is thus exerted on these proteins through the regulation of their activation and of their stability. Here we show that the estrogen-related receptor α (ERRα) directly activates the expression of TNFAIP1, the product of which [BTB/POZ domain-containing adapter for Cullin3-mediated RhoA degradation 2 (BACURD2)] regulates RHOA protein turnover. Inactivation of the receptor leads to enhanced RHOA stability and activation. This results in cell disorientation, increased actin network, and inability to form a lamellipodium at the migration edge. As a consequence, directional migration, but not cell motility per se, is impaired in the absence of the receptor, under pathological as well as physiological conditions. Altogether, our results show that the control exerted by ERRα on RHOA stability is required for directional migration.

The GAP activity of type III effector YopE triggers killing of Yersinia in macrophages

The mammalian immune system has the ability to discriminate between pathogens and innocuous microbes by detecting conserved molecular patterns. In addition to conserved microbial patterns, the mammalian immune system may recognize distinct pathogen-induced processes through a mechanism which is poorly understood. Previous studies have shown that a type III secretion system (T3SS) in Yersinia pseudotuberculosis leads to decreased survival of this bacterium in primary murine macrophages by unknown mechanisms. Here, we use colony forming unit assays and fluorescence microscopy to investigate how the T3SS triggers killing of Yersinia in macrophages. We present evidence that Yersinia outer protein E (YopE) delivered by the T3SS triggers intracellular killing response against Yersinia. YopE mimics eukaryotic GTPase activating proteins (GAPs) and inactivates Rho GTPases in host cells. Unlike wild-type YopE, catalytically dead YopER144A is impaired in restricting Yersinia intracellular survival, highlighting that the GAP activity of YopE is detected as a danger signal. Additionally, a second translocated effector, YopT, counteracts the YopE triggered killing effect by decreasing the translocation level of YopE and possibly by competing for the same pool of Rho GTPase targets. Moreover, inactivation of Rho GTPases by Clostridium difficile Toxin B mimics the effect of YopE and promotes increased killing of Yersinia in macrophages. Using a Rac inhibitor NSC23766 and a Rho inhibitor TAT-C3, we show that macrophages restrict Yersinia intracellular survival in response to Rac1 inhibition, but not Rho inhibition. In summary, our findings reveal that primary macrophages sense manipulation of Rho GTPases by Yersinia YopE and actively counteract pathogenic infection by restricting intracellular bacterial survival. Our results uncover a new mode of innate immune recognition in response to pathogenic infection.

The type III secretion system (T3SS) is a macromolecular protein export pathway found in gram-negative bacteria. It delivers bacterial toxins into eukaryotic cells to promote pathogenic infection. T3SSs and the bacterial toxins delivered are critical arsenals for many bacterial pathogens of clinical significance, such as Yersinia, Salmonella and Shigella. On the other hand, the mammalian immune system may recognize the T3SS as a danger signal to signify pathogenic infection, and to stimulate appropriate defense against pathogens. Here, we show that the innate immune system recognizes the activity of YopE delivered by the Yersinia T3SS. Modulation of host Rho GTPases by YopE elicits a defensive response, which results in killing of bacteria in host cells. Inhibition of host Rho GTPases by Clostridium difficile Toxin B, another bacterial toxin, mimics the YopE-triggered killing effect. Our study demonstrates that host cells sense manipulation of Rho GTPases by bacterial toxins as a surveillance mechanism, revealing new insights into innate immune recognition of pathogenic infections.

Nuclear Rac1 regulates the bFGF-induced neurite outgrowth in PC12 cells

Rac1 plays a key role in neurite outgrowth via reorganization of the actin cytoskeleton. The molecular mechanisms underlying Rac1-mediated actin dynamics in the cytosol and plasma membrane have been intensively studied, but the nuclear function of Rac1 in neurite outgrowth has not yet been addressed. Using subcellular fractionation and immunocytochemistry, we sought to explore the role of nuclear Rac1 in neurite outgrowth. bFGF, a strong agonist for neurite outgrowth in PC12 cells, stimulated the nuclear accumulation of an active form of Rac1. Rac1-PBR (Q) mutant, in which six basic residues in the polybasic region at the C-terminus were replaced by glutamine, didn’t accumulate in the nucleus. In comparison with control cells, cells expressing this mutant form of Rac1 displayed a marked defect in extending neurites that was concomitant with reduced expression of MAP2 and MEK-1. These results suggest that Rac1 translocation to the nucleus functionally correlates with bFGF-induced neurite outgrowth. [BMB Reports 2013; 46(12): 617-622]

Clinical and pathological associations of the activating RAC1 P29S mutation in primary cutaneous melanoma

Activating mutations in the GTPase RAC1 are a recurrent event in cutaneous melanoma. We investigated the clinical and pathological associations of RAC1(P29S) in a cohort of 814 primary cutaneous melanomas with known BRAF and NRAS mutation status. The RAC1(P29S) mutation had a prevalence of 3.3% and was associated with increased thickness (OR=1.6 P = 0.001), increased mitotic rate (OR=1.3 P = 0.03), ulceration (OR=2.4 P = 0.04), nodular subtype (OR=3.4 P = 0.004), and nodal disease at diagnosis (OR=3.3 P = 0.006). BRAF mutant tumors were also associated with nodal metastases (OR=1.9 P = 0.004), despite being thinner at diagnosis than BRAF WT (median 1.2 mm versus 1.6 mm, P < 0.001). Immunohistochemical analysis of 51 melanomas revealed that 47% were immunoreactive for RAC1. Melanomas were more likely to show RAC1 immunoreactivity if they were BRAF mutant (OR=5.2 P = 0.01). RAC1 may therefore be important in regulating the early migration of BRAF mutant tumors. RAC1 mutations are infrequent in primary melanomas but may accelerate disease progression.

Rho protein GTPases and their interactions with NFκB: crossroads of inflammation and matrix biology

The RhoGTPases, with RhoA, Cdc42 and Rac being major members, are a group of key ubiquitous proteins present in all eukaryotic organisms that subserve such important functions as cell migration, adhesion and differentiation. The NFκB (nuclear factor κB) is a family of constitutive and inducible transcription factors that through their diverse target genes, play a major role in processes such as cytokine expression, stress regulation, cell division and transformation. Research over the past decade has uncovered new molecular links between the RhoGTPases and the NFκB pathway, with the RhoGTPases playing a positive or negative regulatory role on NFκB activation depending on the context. The RhoA–NFκB interaction has been shown to be important in cytokine-activated NFκB processes, such as those induced by TNFα (tumour necrosis factor α). On the other hand, Rac is important for activating the NFκB response downstream of integrin activation, such as after phagocytosis. Specific residues of Rac1 are important for triggering NFκB activation, and mutations do obliterate this response. Other upstream triggers of the RhoGTPase–NFκB interactions include the suppressive p120 catenin, with implications for skin inflammation. The networks described here are not only important areas for further research, but are also significant for discovery of targets for translational medicine.

Rho GTPases, oxidation, and cell redox control

While numerous studies support regulation of Ras GTPases by reactive oxygen and nitrogen species, the Rho subfamily has received considerably less attention. Over the last few years, increasing evidence is emerging that supports the redox sensitivity of Rho GTPases. Moreover, as Rho GTPases regulate the cellular redox state by controlling enzymes that generate and convert reactive oxygen and nitrogen species, redox feedback loops likely exist. Here, we provide an overview of cellular oxidants, Rho GTPases, and their inter-dependence.