The small GTPase RhoB regulates TNFα signaling in endothelial cells

The effect of circulating iron on barrier integrity of primary human endothelial cells

Iron is hypothesized to be one of the contributors to cardiovascular disease and its levels in the circulation may correlate with cardiovascular risk. The aim of this study is to investigate the mechanisms that underlie the effects of iron on the barrier function of primary human endothelium. We used Human Umbilical Vein Endothelial Cells (HUVEC) to investigate the effects of Fe3+ using electric cell-substrate impedance sensing, microscopy, western blot and immunofluorescence microscopy. Exposure to Fe3+ caused EC elongation and upregulation of stress-induced proteins. Analysis of barrier function showed a dose-dependent drop in endothelial integrity, which was accompanied by Reactive Oxygen Species (ROS) production and could partly be prevented by ROS scavengers. Inhibition of contractility by the ROCK inhibitor Y27632, showed even more effective rescue of barrier integrity. Using western blot, we detected an increase in expression of the small GTPase RhoB, an inducer of EC contraction, and a small decrease in VE-cadherin, suggestive for an iron-induced stress response. Co-stimulation by TNFα and iron, used to investigate the role of low-grade inflammation, revealed an additive, negative effect on barrier integrity, concomitant with an upregulation of pro-inflammatory markers ICAM-1 and RhoB. Iron induces a response in HUVEC that leads to endothelial activation and a pro-inflammatory state measured by loss of barrier integrity which can be reversed by ROS scavengers, combined with inhibition of contractility. These data suggest that ROS-mediated damage of the vascular endothelium could contribute to the increased cardiovascular risk which is associated with elevated levels of circulating iron.

Differential role for rapid proteostasis in Rho GTPase-mediated control of quiescent endothelial integrity

Endothelial monolayer permeability is regulated by actin dynamics and vesicular traffic. Recently, ubiquitination was also implicated in the integrity of quiescent endothelium, as it differentially controls the localization and stability of adhesion- and signaling proteins. However, the more general effect of fast protein turnover on endothelial integrity is not clear. Here, we found that inhibition of E1 ubiquitin ligases induces a rapid, reversible loss of integrity in quiescent, primary human endothelial monolayers, accompanied by increased F-actin stress fibers and the formation of intercellular gaps. Concomitantly, total protein and activity of the actin-regulating GTPase RhoB, but not its close homologue RhoA, increase ∼10-fold in 5-8 h. We determined that, the depletion of RhoB, but not of RhoA, the inhibition of actin contractility and the inhibition of protein synthesis all significantly rescue the loss of cell-cell contact induced by E1 ligase inhibition. Collectively, our data suggest that in quiescent human endothelial cells, the continuous and fast turnover of short-lived proteins that negatively regulate cell-cell contact, is essential to preserve monolayer integrity.

RhoB Promotes Endometrial Stromal Cells Decidualization Via Semaphorin3A/PlexinA4 Signaling in Early Pregnancy

Endometrial decidualization refers to a series of morphological changes and functional remodeling of the uterine endometrium to accept the embryo under the effect of estrogen and progesterone secreted by ovaries after ovulation. During decidualization, endometrial stromal cells (ESCs) proliferate and differentiate into decidual stromal cells, undergoing cytoskeletal rearrangement-mediated morphological changes and expressing decidualization markers, such as insulin-like growth factor-binding protein-1 and prolactin. Ras homology (Rho) proteins, a family of small G proteins, are well known as regulators of cellular morphology and involved in multiple other cellular processes. In this study, we found ras homolog family member B (RHOB) was the most significantly upregulated gene in the Rho protein family after the in vitro decidualization of human primary ESCs. RhoB expression was induced mainly by 3',5'-cyclic adenosine 5'-monophosphate (cAMP) / protein kinase A (PKA) / cyclic adenosine monophosphate-response element binding protein signaling and partly by progesterone signaling. Knockdown of RhoB in ESCs greatly inhibited actin cytoskeletal rearrangement, cell morphological transformation, and upregulation of insulin-like growth factor-binding protein-1, suggesting an indispensable role of RhoB in decidualization. Mechanistically, the downstream target of RhoB was semaphorin3A (Sema3A), which mediated its signaling via interacting with the receptor, plexinA4. More importantly, decreased expression of RhoB, Sema3A, and plexinA4 were detected in deciduas from patients with unexplained spontaneous miscarriage. Collectively, our results indicate that RhoB/Sema3A/plexinA4 signaling plays a positive role in endometrial decidualization and relates to unexplained spontaneous miscarriage, which is worthy of further exploration so as to provide new insights into therapeutic strategies for pregnancy diseases associated with poor decidualization.

Bta-miR-223 Targeting the RHOB Gene in Dairy Cows Attenuates LPS-Induced Inflammatory Responses in Mammary Epithelial Cells

Bovine mammary epithelial cells (bMECs) are part of the first line of defense against pathogens. In recent studies, bta-miR-223 has been reported to activate congenital and innate immunity against inflammatory damage during the pathogenesis of mastitis in dairy cows. The purpose of this study was to identify the regulatory mechanism of bta-miR-223 and its downstream target genes in inflammatory bMECs. A double luciferase reporter gene assay demonstrated that ras homolog family member B (RHOB) was the target gene of bta-miR-223. To further elucidate the role of bta-miR-223 in congenital immune responses, bta-miR-223 mimics (mimic/inhibitor) were transfected into bMECs stimulated with lipopolysaccharide (LPS), which activates the Toll-like receptor 4/nuclear factor-κB (TLR4/NF-κB) signaling pathway. Real-time quantitative PCR (qPCR) and Western blot were used to detect the expression of related genes and proteins, and enzyme-linked immunosorbent assay (ELISA) was used to detect secreted inflammatory factors. Results showed that bta-miR-223 expression during inflammation in bMECs reduced the secretion of inflammatory factors by targeting RHOB and deactivation of NF-κB gene activity. Silencing RHOB inhibited LPS-induced inflammatory response in bMECs. Overall, bta-miR-223 attenuated LPS-induced inflammatory response, and acted as a negative feedback regulator via targeting RHOB, providing a novel avenue for mastitis treatment.

Sleep's role in the development and resolution of adolescent depression

Two adolescent mental health fields - sleep and depression - have advanced largely in parallel until about four years ago. Although sleep problems have been thought to be a symptom of adolescent depression, emerging evidence suggests that sleep difficulties arise before depression does. In this Review, we describe how the combination of adolescent sleep biology and psychology uniquely predispose adolescents to develop depression. We describe multiple pathways and contributors, including a delayed circadian rhythm, restricted sleep duration and greater opportunity for repetitive negative thinking while waiting for sleep. We match each contributor with evidence-based sleep interventions, including bright light therapy, exogenous melatonin and cognitive-behaviour therapy techniques. Such treatments improve sleep and alleviate depression symptoms, highlighting the utility of sleep treatment for comorbid disorders experienced by adolescents.

Inflammatory Immune Cytokine TNF-α Modulates Ezrin Protein Activation via FAK/RhoA Signaling Pathway in PMVECs Hyperpermeability

Background: One of the important pathogenesis of acute respiratory distress syndrome (ARDS) is the dysfunction of pulmonary microvascular endothelial barrier induced by a hyperinflammatory immune response. However, the potential mechanisms of such an imbalance in pulmonary microvascular endothelial cells (PMVECs) are not yet understood. Purpose: Explore the molecular mechanism of endothelial barrier dysfunction induced by inflammatory immune cytokines in ARDS, and find a therapeutic target for this syndrome. Methods: Rat PMVECs were cultured to form a monolayer. Immunofluorescence, flow cytometry, and Western blotting were selected to detect the distribution and the expression level of phosphorylated Ezrin protein and Ezrin protein. Transendothelial electrical resistance (TER) and transendothelial fluxes of fluorescein isothiocyanate (FITC)-labeled bovine serum albumin (BSA) were utilized to measure the permeability of the cell monolayer. Ezrin short hairpin RNA (shRNA) and Ezrin 567-site threonine mutant (Ezrin^T567A) were used to examine the role of Ezrin protein and phosphorylated Ezrin protein in endothelial response induced by tumor necrosis factor-alpha (TNF-α), respectively. The function of focal adhesion kinase (FAK) and Ras homolog gene family, member A (RhoA) signaling pathways were estimated by inhibitors and RhoA/FAK shRNA in TNF-α-stimulated rat PMVECs. The activation of FAK and RhoA was assessed by Western blotting or pull-down assay plus Western blotting. Results: The TER was decreased after TNF-α treatment, while the Ezrin protein phosphorylation was increased in a time- and dose-dependent manner. The phosphorylated Ezrin protein was localized primarily at the cell periphery, resulting in filamentous actin (F-actin) rearrangement, followed by a significant decrease in TER and increase in fluxes of FITC-BSA. Moreover, FAK and RhoA signaling pathways were required in the phosphorylation of Ezrin protein, and the former positively regulated the latter. Conclusion: The phosphorylated Ezrin protein was induced by TNF-α via the FAK/RhoA signaling pathway leading to endothelial hyperpermeability in PMVECs.

Cullin3-TNFAIP1 E3 Ligase Controls Inflammatory Response in Hepatocellular Carcinoma Cells via Ubiquitination of RhoB

Rho family GTPase RhoB is the critical signaling component controlling the inflammatory response elicited by pro-inflammatory cytokines. However, the underlying mechanisms of RhoB degradation in inflammatory response remain unclear. In this study, for the first time, we identified that TNFAIP1, an adaptor protein of Cullin3 E3 ubiquitin ligases, coordinated with Cullin3 to mediate RhoB degradation through ubiquitin proteasome system. In addition, we demonstrated that downregulation of TNFAIP1 induced the expression of pro-inflammatory cytokines IL-6 and IL-8 in TNFα-stimulated hepatocellular carcinoma cells through the activation of p38/JNK MAPK pathway via blocking RhoB degradation. Our findings revealed a novel mechanism of RhoB degradation and provided a potential strategy for anti-inflammatory intervention of tumors by targeting TNFAIP1-RhoB axis.

RhoB is regulated by hypoxia and modulates metastasis in breast cancer

BACKGROUND

RhoB is a Rho family GTPase that is highly homologous to RhoA and RhoC. RhoA and RhoC have been shown to promote tumor progression in many cancer types; however, a distinct role for RhoB in cancer has not been delineated. Additionally, several well-characterized studies have shown that small GTPases such as RhoA, Rac1, and Cdc42 are induced in vitro under hypoxia, but whether and how hypoxia regulates RhoB in breast cancer remains elusive.

AIMS

To determine whether and how hypoxia regulates RhoB expression and to understand the role of RhoB in breast cancer metastasis.

METHODS

We investigated the effects of hypoxia on the expression and activation of RhoB using real-time quantitative polymerase chain reaction and western blotting. We also examined the significance of both decreased and increased RhoB expression in breast cancer using CRISPR depletion of RhoB or a vector overexpressing RhoB in 3D in vitro migration models and in an in vivo mouse model.

RESULTS

We found that hypoxia significantly upregulated RhoB mRNA and protein expression resulting in increased levels of activated RhoB. Both loss of RhoB and gain of RhoB expression led to reduced migration in a 3D collagen matrix and invasion within a multicellular 3D spheroid. We showed that neither the reduction nor overexpression of RhoB affected tumor growth in vivo. While the loss of RhoB had no effect on metastasis, RhoB overexpression led to decreased metastasis to the lungs, liver, and lymph nodes of mice.

CONCLUSION

Our results suggest that RhoB may have an important role in suppressing breast cancer metastasis.

A functional polymorphism in the promoter of RhoB is associated with susceptibility to Vibrio anguillarum in turbot (Scophthalmus maximus)

As an isoform of Rho family GTPases, RhoB plays a pivotal role in cytoskeletal organization, cell proliferation, apoptosis and immune response. However, the regulatory mechanisms of RhoB expression in aquatic animals are still unknown. In the present study, we first construct Vibrio anguillarum infection model in S. maximus, including susceptible and resistant individuals. Then the temporal expression of RhoB was detected after V. anguillarum challenge using qRT-PCR and found that RhoB transcripts were significantly induced in the liver, gill and blood despite of differential expression levels and responsive time points. In addition, the mRNA levels of RhoB in resistant individuals were significantly higher than in susceptible ones. The length of 2083 bp sequences of RhoB promoter was cloned and characterized. Moreover, DNA methylation of the RhoB promoter was measured by bisulfite sequencing (BSP) and hypo-methylated was detected in the CpG islands. Three SNPs (-1590, -1575 and -1449) and two haplotypes in the promoter region of RhoB were identified to be associated with V. anguillarum resistance in turbot by association analysis in group 17-R and 17-S. Deletion analysis indicated that these SNPs could negatively mediate the activity of RhoB promoter. Site-directed mutagenesis and qRT-PCR of individuals with different genotypes demonstrated that -1575 T/A polymorphism affected promoter activity. Further study showed that this mutation altered the binding site of the transcription factor CREB. Co-transfection of SmCREB and RhoB promoter was performed in HEK293T cells which confirmed the -1575 allelic differences on transcriptional activity, with the susceptibility allele showing reduced activity. Taken together, our findings implicate that losing of binding of CREB to SmRhoB promoter due to -1575T/A polymorphisms enhances SmRhoB expression in resistant turbot, which provide insights into the effect of SmRhoB expression in response to V. anguillarum infection.

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.

CSN5 inhibition triggers inflammatory signaling and Rho/ROCK-dependent loss of endothelial integrity

RhoGTPases regulate cytoskeletal dynamics, migration and cell-cell adhesion in endothelial cells. Besides regulation at the level of guanine nucleotide binding, they also undergo post-translational modifications, for example ubiquitination. RhoGTPases are ubiquitinated by Cullin RING ligases which are in turn regulated by neddylation. Previously we showed that inhibition of Cullin RING ligase activity by the neddylation inhibitor MLN4924 is detrimental for endothelial barrier function, due to accumulation of RhoB and the consequent induction of contractility. Here we analyzed the effect of pharmacological activation of Cullin RING ligases on endothelial barrier integrity in vitro and in vivo. CSN5i-3 induced endothelial barrier disruption and increased macromolecule leakage in vitro and in vivo. Mechanistically, CSN5i-3 strongly induced the expression and activation of RhoB and to lesser extent of RhoA in endothelial cells, which enhanced cell contraction. Elevated expression of RhoGTPases was a consequence of activation of the NF-κB pathway. In line with this notion, CSN5i-3 treatment decreased IκBα expression and increased NF-κB-mediated ICAM-1 expression and consequent adhesion of neutrophils to endothelial cells. This study shows that sustained neddylation of Cullin RING-ligases leads to activation the NF-κB pathway in endothelial cells, elevated expression of RhoGTPases, Rho/ROCK-dependent activation of MLC and disruption of the endothelial barrier.

PSMB11 Orchestrates the Development of CD4 and CD8 Thymocytes via Regulation of Gene Expression in Cortical Thymic Epithelial Cells

T cell development depends on sequential interactions of thymocytes with cortical thymic epithelial cells (cTECs) and medullary thymic epithelial cells. PSMB11 is a catalytic proteasomal subunit present exclusively in cTECs. Because proteasomes regulate transcriptional activity, we asked whether PSMB11 might affect gene expression in cTECs. We report that PSMB11 regulates the expression of 850 cTEC genes that modulate lymphostromal interactions primarily via the WNT signaling pathway. cTECs from Psmb11 ^-/- mice 1) acquire features of medullary thymic epithelial cells and 2) retain CD8 thymocytes in the thymic cortex, thereby impairing phase 2 of positive selection, 3) perturbing CD8 T cell development, and 4) causing dramatic oxidative stress leading to apoptosis of CD8 thymocytes. Deletion of Psmb11 also causes major oxidative stress in CD4 thymocytes. However, CD4 thymocytes do not undergo apoptosis because, unlike CD8 thymocytes, they upregulate expression of chaperones and inhibitors of apoptosis. We conclude that PSMB11 has pervasive effects on both CD4 and CD8 thymocytes via regulation of gene expression in cTECs.

The RhoB small GTPase in physiology and disease

RhoB is a Rho family GTPase that is highly similar to RhoA and RhoC, yet has distinct functions in cells. Its unique C-terminal region is subject to specific post-translational modifications that confer different localization and functions to RhoB. Apart from the common role with RhoA and RhoC in actin organization and cell migration, RhoB is also implicated in a variety of other cellular processes including membrane trafficking, cell proliferation, DNA-repair and apoptosis. RhoB is not an essential gene in mice, but it is implicated in several physiological and pathological processes. Its multiple roles will be discussed in this review.

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.

RhoA, RhoB and RhoC differentially regulate endothelial barrier function

RhoGTPases are known regulators of intracellular actin dynamics that are important for maintaining endothelial barrier function. RhoA is most extensively studied as a key regulator of endothelial barrier function, however the function of the 2 highly homologous family-members (> 88%) RhoB and RhoC in endothelial barrier function is still poorly understood.

This study aimed to determine whether RhoA, RhoB and RhoC have overlapping or distinct roles in barrier function and permeability in resting and activated endothelium. By using primary endothelial cells in combination with siRNA transfection to establish individual, double or triple knockdown of the RhoA/B/C RhoGTPases, we found that RhoB, but not RhoA or RhoC, is in resting endothelium a negative regulator of permeability. Loss of RhoB accounted for an accumulation of VE-cadherin at cell-cell contacts. Thrombin-induced loss of endothelial integrity is mediated primarily by RhoA and RhoB. Combined loss of RhoA/B showed decreased phosphorylation of Myosin Light Chain and increased expression of VE-cadherin at cell-cell contacts after thrombin stimulation. RhoC contributes to the Rac1-dependent restoration of endothelial barrier function.

In summary, this study shows that these highly homologous RhoGTPases differentially control the dynamics of endothelial barrier function.

Genomics of human fatty liver disease reveal mechanistically linked lipid droplet-associated gene regulations in bland steatosis and nonalcoholic steatohepatitis

Nonalcoholic fatty liver disease (NAFLD) is a common disorder hallmarked by excessive lipid deposits. Based on our recent research on lipid droplet (LD) formation in hepatocytes, we investigated LD-associated gene regulations in NAFLD of different grades, that is, steatosis vs steatohepatitis by comparing liver biopsies from healthy controls (N = 13) and NAFLD patients (N = 102). On average, more than 700 differentially expressed genes (DEGs) were identified of which 146 are mechanistically linked to LD formation. We identified 51 LD-associated DEGs frequently regulated in patient samples (range ≥5 to ≤102) with the liver-receptor homolog-1(NR5A2), that is, a key regulator of cholesterol metabolism being commonly repressed among 100 patients examined. With bland steatosis, notable regulations involved hypoxia-inducible lipid droplet-associated-protein and diacylglycerol-O-acyltransferase-2 renowned for their role in LD-growth. Conversely, nonalcoholic steatohepatitis-associated DEGs coded for epidermal growth factor receptor and TLR4 signaling with decreased expression of the GTPase Rab5 and the lipid phosphohydrolase PPAP2B thus highlighting adaptive responses to inflammation, LDL-mediated endocytosis and lipogenesis, respectively. Studies with steatotic primary human hepatocyte cultures demonstrated induction of LD-associated PLIN2, CIDEC, DNAAF1, whereas repressed expression of CPT1A, ANGPTL4, and PKLR informed on burdened mitochondrial metabolism. Equally, repressed expression of the B-lymphocyte chemoattractant CXCL13 and STAT4 as well as induced FGF21 evidenced amelioration of steatosis-related inflammation. In-vitro/in-vivo patient sample comparisons confirmed C-reactive protein, SOCS3, NR5A2, and SOD2 as commonly regulated. Lastly, STRING network analysis highlighted potential "druggable" targets with PLIN2, CIDEC, and hypoxia-inducible lipid droplet-associated-protein being confirmed by immunofluorescence microscopy. In conclusion, steatosis and steatohepatitis specific gene regulations informed on the pathogenesis of NAFLD to broaden the perspective of targeted therapies.

Parallels between single cell migration and barrier formation: The case of RhoB and Rac1 trafficking

The appearance of multicellularity implied the adaptation of signaling networks required for unicellular life to new functions arising in this remarkable evolutionary transition. A hallmark of multicellular organisms is the formation of cellular barriers that compartmentalize spaces and functions. Here we discuss recent findings concerning the role of RhoB in the negative control of Rac1 trafficking from endosomes to the cell border, in order to induce membrane extensions to restore endothelial barrier function after acute contraction. This role closely resembles that proposed for RhoB in controlling single cell migration through Rac1, which has also been observed in cancer cell invasion. We highlight these similarities as a signaling paradigm that shows that endothelial barrier integrity is controlled not only by the formation of cell-cell junctions, but also by a balance between ancestral mechanisms of cell spreading and contraction conserved from unicellular organisms and orchestrated by Rho GTPases.

RhoB controls endothelial barrier recovery by inhibiting Rac1 trafficking to the cell border

Endothelial barrier dysfunction underlies chronic inflammatory diseases. In searching for new proteins essential to the human endothelial inflammatory response, we have found that the endosomal GTPase RhoB is up-regulated in response to inflammatory cytokines and expressed in the endothelium of some chronically inflamed tissues. We show that although RhoB and the related RhoA and RhoC play additive and redundant roles in various aspects of endothelial barrier function, RhoB specifically inhibits barrier restoration after acute cell contraction by preventing plasma membrane extension. During barrier restoration, RhoB trafficking is induced between vesicles containing RhoB nanoclusters and plasma membrane protrusions. The Rho GTPase Rac1 controls membrane spreading and stabilizes endothelial barriers. We show that RhoB colocalizes with Rac1 in endosomes and inhibits Rac1 activity and trafficking to the cell border during barrier recovery. Inhibition of endosomal trafficking impairs barrier reformation, whereas induction of Rac1 translocation to the plasma membrane accelerates it. Therefore, RhoB-specific regulation of Rac1 trafficking controls endothelial barrier integrity during inflammation.

Spatiotemporal analysis of RhoA/B/C activation in primary human endothelial cells

Endothelial cells line the vasculature and are important for the regulation of blood pressure, vascular permeability, clotting and transendothelial migration of leukocytes and tumor cells. A group of proteins that that control the endothelial barrier function are the RhoGTPases. This study focuses on three homologous (>88%) RhoGTPases: RhoA, RhoB, RhoC of which RhoB and RhoC have been poorly characterized. Using a RhoGTPase mRNA expression analysis we identified RhoC as the highest expressed in primary human endothelial cells. Based on an existing RhoA FRET sensor we developed new RhoB/C FRET sensors to characterize their spatiotemporal activation properties. We found all these RhoGTPase sensors to respond to physiologically relevant agonists (e.g. Thrombin), reaching transient, localized FRET ratio changes up to 200%. These RhoA/B/C FRET sensors show localized GEF and GAP activity and reveal spatial activation differences between RhoA/C and RhoB. Finally, we used these sensors to monitor GEF-specific differential activation of RhoA/B/C. In summary, this study adds high-contrast RhoB/C FRET sensors to the currently available FRET sensor toolkit and uncover new insights in endothelial and RhoGTPase cell biology. This allows us to study activation and signaling by these closely related RhoGTPases with high spatiotemporal resolution in primary human cells.

RhoB regulates the function of macrophages in the hypoxia-induced inflammatory response

Immune cells, particularly macrophages, play critical roles in the hypoxia-induced inflammatory response. The small GTPase RhoB is usually rapidly induced by a variety of stimuli and has been described as an important regulator of cytoskeletal organization and vesicle and membrane receptor trafficking. However, it is unknown whether RhoB is involved in the hypoxia-induced inflammatory response. Here, we investigated the effect of hypoxia on the expression of RhoB and the mechanism and significance of RhoB expression in macrophages. We found that hypoxia significantly upregulated the expression of RhoB in RAW264.7 cells, mouse peritoneal macrophages, and the spleen of rats. Hypoxia-induced expression of RhoB was significantly blocked by a specific inhibitor of hypoxia-inducible factor-1α (HIF-1α), c-Jun N-terminal kinase (JNK), or extracellular-signal regulated protein kinase (ERK), indicating that hypoxia-activated HIF-1α, JNK, and ERK are involved in the upregulation of RhoB by hypoxia. Knockdown of RhoB expression not only significantly suppressed basal production of interleukin-1 beta (IL-1β), interleukin 6 (IL-6), and tumor necrosis factor alpha (TNF-α) in normoxia but also more markedly decreased the hypoxia-stimulated production of these cytokines. Furthermore, we showed that RhoB increased nuclear factor-kappa B (NF-κB) activity, and the inhibition of NF-κB transcriptional activity significantly decreased the RhoB-increased mRNA levels of IL-1β, IL-6, and TNF-α. Finally, we demonstrated that RhoB enhanced cell adhesion and inhibited cell migration in normoxia and hypoxia. Taken together, these results suggest that RhoB plays an important role in the hypoxia-induced activation of macrophages and the inflammatory response.Cellular & Molecular Immunology advance online publication, 21 September 2015; doi:10.1038/cmi.2015.78.

Toward understanding RhoGTPase specificity: structure, function and local activation

Cell adhesion and migration are regulated through the concerted action of cytoskeletal dynamics and adhesion proteins, the activity of which is governed by RhoGTPases. Specific RhoGTPase signaling requires spatio-temporal activation and coordination of subsequent protein-protein and protein-lipid interactions. The nature, location and duration of these interactions are dependent on polarized extracellular triggers, such as cell-cell contact, and intracellular modifying events, such as phosphorylation. RhoA, RhoB, and RhoC are highly homologous GTPases that, however, succeed in generating specific intracellular responses. Here, we discuss the key features that contribute to this specificity. These not only include the well-studied switch regions, the conformation of which is nucleotide-dependent, but also additional regions and seemingly small differences in primary sequence that also contribute to specific interactions. These differences translate into differential surface charge distribution, local exposure of amino acid side-chains and isoform-specific post-translational modifications. The available evidence supports the notion that multiple regions in RhoA/B/C cooperate to provide specificity in binding to regulators and effectors. These specific interactions are highly regulated in time and space. We therefore subsequently discuss current approaches means to visualize and analyze localized GTPase activation using biosensors that allow imaging of isoform-specific, localized regulation.

Modulation of mammalian circadian rhythms by tumor necrosis factor-α

Systemic low doses of the endotoxin lipopolysaccharide (LPS, 100 µg/kg) administered during the early night induce phase-delays of locomotor activity rhythms in mice. Our aim was to evaluate the role of tumor necrosis factor (Tnf)-alpha and its receptor 1/p55 (Tnfr1) in the modulation of LPS-induced circadian effects on the suprachiasmatic nucleus (SCN). We observed that Tnfr1-defective mice (Tnfr1 KO), although exhibiting similar circadian behavior and light response to that of control mice, did not show LPS-induced phase-delays of locomotor activity rhythms, nor LPS-induced cFos and Per2 expression in the SCN and Per1 expression in the paraventricular hypothalamic nucleus (PVN) as compared to wild-type (WT) mice. We also analyzed Tnfr1 expression in the SCN of WT mice, peaking during the early night, when LPS has a circadian effect. Peripheral inoculation of LPS induced an increase in cytokine/chemokine levels (Tnf, Il-6 and Ccl2) in the SCN and in the PVN. In conclusion, in this study, we show that LPS-induced circadian responses are mediated by Tnf. Our results also suggest that this cytokine stimulates the SCN after LPS peripheral inoculation; and the time-related effect of LPS (i.e. phase shifts elicited only at early night) might depend on the increased levels of Tnfr1 expression. We also confirmed that LPS modulates clock gene expression in the SCN and PVN in WT but not in Tnfr1 KO mice.

HIGHLIGHTS

We demonstrate a fundamental role for Tnf and its receptor in circadian modulation by immune stimuli at the level of the SCN biological clock.