Trading spaces: Rap, Rac, and Rho as architects of transendothelial migration

Investigating human monocyte adhesion, migration and transmigration and their modulation by Zika virus

Human circulating monocytes are established targets for Zika virus (ZIKV) infection. Because of their important migratory properties toward any tissues, including the central nervous system (CNS), a better understanding of the mechanisms underlying monocyte transmigration upon ZIKV infection is required. Here, we monitored adhesion, migration and transmigration properties of monocytes exposed to ZIKV. We found that ZIKV enhanced monocyte adhesion on collagen compared to mock-exposed samples, and that pharmacological inhibition of mDia and Cdc42 function induced a significant decrease of adhesion in both mock- and ZIKV-exposed monocytes. In contrast, monocyte migration through collagen was inhibited by most of the tested small molecules targeting regulators of actin polymerization, including Rac1, ROCK, Cdc42, mDia, Arp2/3, Myosin-II and LFA-1. ZIKV-exposed monocyte migration showed a very similar profile to that of their mock-exposed counterparts. Finally, assessment of monocyte transmigration through human cerebral microvascular endothelial cells (hCMEC/D3) showed dependency on Rac1, ROCK, and Cdc42, independently of their infection status. In contrast, we identified that BIRT377, an antagonist of LFA-1, significantly inhibited transmigration of ZIKV-exposed but not mock-exposed monocytes. As BIRT377 increased adhesion of ZIKV-exposed monocytes, we propose that LFA-1 might be involved in a post-adhesion step to enhance viro-induced transmigration. These data suggest that ZIKV exposure triggers specific migratory properties of monocytes that are not exploited under physiological conditions. This work provides further insights on virus-host interactions important for viral neuroinvasion and offers novel targets to specifically inhibit the infiltration of infected cells to the CNS. SUMMARY SENTENCE: Monocyte transmigration involves massive actin cytoskeleton reorganization regulated by small Rho GTPases and integrins, which can be subverted by viruses.

The activation of gastric inhibitory peptide/gastric inhibitory peptide receptor axis via sonic hedgehog signaling promotes the bridging of gapped nerves in sciatic nerve injury

Schwann cells play an essential role in peripheral nerve regeneration by generating a favorable microenvironment. Gastric inhibitory peptide/gastric inhibitory peptide receptor (GIP/GIPR) axis deficiency leads to failure of sciatic nerve repair. However, the underlying mechanism remains elusive. In this study, we surprisingly found that GIP treatment significantly enhances the migration of Schwann cells and the formation of Schwann cell cords during recovery from sciatic nerve injury in rats. We further revealed that GIP and GIPR levels in Schwann cells were low under normal conditions, and significantly increased after injury demonstrated by real-time reverse transcription-polymerase chain reaction (RT-PCR) and Western blot. Wound healing and Transwell assays showed that GIP stimulation and GIPR silencing could affect Schwann cell migration. In vitro and in vivo mechanistic studies based on interference experiment revealed that GIP/GIPR might promote mechanistic target of rapamycin complex 2 (mTORC2) activity, thus facilitating cell migration; Rap1 activation might be involved in this process. Finally, we retrieved the stimulatory factors responsible for GIPR induction after injury. The results indicate that sonic hedgehog (SHH) is a potential candidate whose expression increased upon injury. Luciferase and chromatin immunoprecipitation (ChIP) assays showed that Gli3, the target transcription factor of the SHH pathway, dramatically augmented GIPR expression. Additionally, in vivo inhibition of SHH could effectively reduce GIPR expression after sciatic nerve injury. Collectively, our study reveals the importance of GIP/GIPR signaling in Schwann cell migration, providing a therapeutic avenue toward peripheral nerve injury.

HAM/TSP Pathogenesis: The Transmigration Activity of HTLV-1-Infected T Cells into Tissues

Slowly progressive spastic paraparesis with bladder dysfunction, the main clinical feature of human T-cell leukemia virus-1 (HTLV-1)-associated myelopathy/tropical spastic paraparesis (HAM/TSP), is induced by chronic inflammation in the spinal cord, mainly the lower thoracic cord. A long-standing bystander mechanism, such as the destruction of surrounding tissues by inflammatory cytokines, etc., induced under the interaction between infiltrated HTLV-1-infected CD4⁺ T cells and HTLV-1-specific CD8⁺ cytotoxic T cells, has been considered implicated for the induction of chronic inflammation. As this bystander mechanism is triggered conceivably by the transmigration of HTLV-1-infected CD4⁺ T cells to the spinal cord, heightened transmigrating activity of HTLV-1-infected CD4⁺ T cells to the spinal cord might play a crucial role as the first responder in the development of HAM/TSP. This review evaluated the functions of HTLV-1-infected CD4⁺ T cells in HAM/TSP patients as the prerequisite for the acquisition of the activity such as adhesion molecule expression changes, small GTPases activation, and expression of mediators involved in basement membrane disruption. The findings suggest that HTLV-1-infected CD4⁺ T cells in HAM/TSP patients have enough potential to facilitate transmigration into the tissues. Future HAM/TSP research should clarify the molecular mechanisms leading to the establishment of HTLV-1-infected CD4⁺ T cells as the first responder in HAM/TSP patients. In addition, a regimen with an inhibitory activity against the transmigration of HTLV-1-infected CD4⁺ T cells into the spinal cord might be recommended as one of the therapeutic strategies against HAM/TSP patients.

Aqueous humor proteomic analysis of acute angle-closure glaucoma with visual field loss

Background

Acute angle-closure glaucoma (AACG) is an ophthalmic emergency that occurs over the course of hours or days and may cause irreversible blindness if not treated immediately. In most cases, optic nerve damage is the cause of visual field (VF) loss in AACG. There has been no reliable biomarker found to evaluate optic nerve damage to date. Aqueous humor (AH) proteome analysis might reveal the proteomic alterations in AACG and provide helpful clues in the search for an AH biomarker of optic nerve damage and VF loss.

Methods

In this study, we used the AH proteome to explore the functions of differentially expressed proteins (DEPs) during disease progression. The AH proteins from the early-stage group and late-stage group were extracted and analyzed by the data-independent acquisition (DIA) method. The DEPs functions were annotated, and parallel reaction monitoring (PRM) was used to validate the key DEPs.

Results

A total of 87 DEPs were found. Gene Ontology analysis showed that most DEPs were enriched in immunology, hemodynamics, and apoptosis. Ingenuity pathway analysis found that vascular endothelial growth factor (VEGF) signaling, the production of reactive oxygen species (ROS) in macrophages, and the nuclear factor erythroid 2-related factor 2 (NRF2)-mediated oxidative stress response were active pathways in the late stage of AACG. The mechanism of retinal ganglion cell (RGC) death was hypothesized on the basis of DEP functional analysis. A total of 20 DEPs were validated by using PRM, and prostaglandin-H2 D-isomerase was found to have the potential to evaluate optic nerve damage.

Conclusions

This study showed that AH proteomic analysis could reveal the proteomic alterations in the pathogenesis of VF loss in AACG and help to provide objective protein biomarkers to evaluate VF loss. These findings will benefit the application of the AH proteome to clinical research.

Extracellular RNA as a Versatile DAMP and Alarm Signal That Influences Leukocyte Recruitment in Inflammation and Infection

Upon vascular injury, tissue damage, ischemia, or microbial infection, intracellular material such as nucleic acids and histones is liberated and comes into contact with the vessel wall and circulating blood cells. Such "Danger-associated molecular patterns" (DAMPs) may thus have an enduring influence on the inflammatory defense process that involves leukocyte recruitment and wound healing reactions. While different species of extracellular RNA (exRNA), including microRNAs and long non-coding RNAs, have been implicated to influence inflammatory processes at different levels, recent in vitro and in vivo work has demonstrated a major impact of ribosomal exRNA as a prominent DAMP on various steps of leukocyte recruitment within the innate immune response. This includes the induction of vascular hyper-permeability and vasogenic edema by exRNA via the activation of the "vascular endothelial growth factor" (VEGF) receptor-2 system, as well as the recruitment of leukocytes to the inflamed endothelium, the M1-type polarization of inflammatory macrophages, or the role of exRNA as a pro-thrombotic cofactor to promote thrombosis. Beyond sterile inflammation, exRNA also augments the docking of bacteria to host cells and the subsequent microbial invasion. Moreover, upon vessel occlusion and ischemia, the shear stress-induced release of exRNA initiates arteriogenesis (i.e., formation of natural vessel bypasses) in a multistep process that resembles leukocyte recruitment. Although exRNA can be counteracted for by natural circulating RNase1, under the conditions mentioned, only the administration of exogenous, thermostable, non-toxic RNase1 provides an effective and safe therapeutic regimen for treating the damaging activities of exRNA. It remains to be investigated whether exRNA may also influence viral infections (including COVID-19), e.g., by supporting the interaction of host cells with viral particles and their subsequent invasion. In fact, as a consequence of the viral infection cycle, massive amounts of exRNA are liberated, which can provoke further tissue damage and enhance virus dissemination. Whether the application of RNase1 in this scenario may help to limit the extent of viral infections like COVID-19 and impact on leukocyte recruitment and emigration steps in immune defense in order to limit the extent of associated cardiovascular diseases remains to be studied.

Rap2c as a Novel Biomarker for Predicting Poor Prognosis in Glioma

Objective

Rap2c is a member of the Ras superfamily that has been implicated in various types of cancers. However, its role in glioma remains elusive. This study aimed to elucidate the role of Rap2c in glioma and its specific molecular mechanism.

Methods

We determined the expression of Rap2c in glioma tissues by Western blotting and immunohistochemistry (IHC) assays. The proliferation and apoptosis of cells were explored using CCK-8 and flow cytometry assay, whereas the migration and invasion of glioma cells were determined using transwell assay. The potential mechanism of Rap2c in the migration of glioma cell lines was investigated through Western blotting analysis and transwell assay. BALB/c nude mice were used to establish tumor models to test the effect of Rap2c on cancer metastasis in vivo.

Results

Our data showed that the protein expression of Rap2c was significantly up-regulated in glioma tissues compared with normal brain tissues, and Rap2c overexpression negatively correlated with 5-year overall survival rate. However, there was no correlation between Rap2c expression and clinicopathological parameters of glioma patients. Overexpression of Rap2c promoted the migration and invasion abilities of glioma cells but had no significant effect on the proliferation of glioma cells. Western blotting analysis revealed that Rap2c overexpression increased the phosphorylation level of extracellular signal-related kinase1/2 (ERK1/2), and this effect was abolished with U0126, a selective MEK inhibitor. Furthermore, overexpression of Rap2c induced lung metastasis of glioma cells in xenograft models.

Conclusion

These findings indicate that high Rap2c expression predicts poor prognosis in glioma. Rap2c-mediated ERK1/2 phosphorylation initiates EMT cascade and promotes migration and invasion of glioma cells. Thus, targeting Rap2c and ERK signaling pathway could be a novel treatment modality for glioma.

Identification of key genes and pathways in calcific aortic valve disease by bioinformatics analysis

Background

Calcific aortic valve disease (CAVD) is the most common type of valvular heart disease in the elderly. This study is aimed to explore molecular mechanism of CAVD via bioinformatics analysis.

Methods

The gene expression profiles of GSE51472 (including 5 normal aortic valve and 5 calcified aortic valve) and GSE83453 (including 8 normal aortic valve and 19 calcified aortic valve) were downloaded from the Gene Expression Omnibus (GEO) database. Differentially expressed genes (DEGs) were screened using the MetaDE package in R software. Functional and pathway enrichment analysis were performed based on Gene ontology (GO) and KEGG pathway database. Then, STRING database, Cytoscape and MCODE were applied to construct the protein-protein interaction (PPI) network and screen hub genes. Pathway enrichment analysis was further performed for hub genes and gene clusters identified via module analysis.

Results

A total of 107 DEGs were identified in CAVD (53 up-regulated genes, and 54 down-regulated genes), and they were mainly enriched in the terms of immune response, extracellular matrix organization, leukocyte transendothelial migration, cell adhesion molecules (CAMs), and fatty acid metabolism. Five hub genes including VCAM1, MMP9, ITGB2, RAC2, and vWF were identified via PPI network, which were mainly enriched in terms of leukocyte transendothelial migration and cell adhesion. An independently down-regulated protein cluster containing ALDH2, HIBCH, ACADVL, ECHDC2, VAT1L, and MAOA was also identified via PPI network.

Conclusions

The present study identified VCAM1, MMP9, ITGB2, RAC2, vWF and ALDH2 as key genes in the progression of CAVD. Immune cells infiltration might play a key role in the progression of CAVD, while ALDH2-mediated detoxification effect might play a protective role in CAVD. Further studies are needed to elucidate the pathogenesis of CAVD.

Role of Integrin Signaling Activation on the Development of Human T Cell Leukemia Virus-1 (HTLV-1)-Associated Myelopathy/Tropical Spastic Paraparesis: Its Relationship to HTLV-1-Infected CD4+ T Cell Transmigrating Activity into the Tissues

The main clinical feature of human T cell leukemia virus-1 (HTLV-1)-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is slowly progressive spastic paraparesis with bladder dysfunction. HAM/TSP is induced by chronic inflammation in the spinal cord, mainly the lower thoracic cord. A long-standing bystander mechanism, such as the destruction of surrounding tissues by the interaction between infiltrated Th1-like, HTLV-1-infected CD4⁺ T cells and HTLV-1-specific CD8⁺ cytotoxic T cells (CTL), is probably critical for the induction of chronic inflammation. Although the HTLV-1-infected CD4⁺ T cells in HAM/TSP appear to play a crucial role in the initial pathogenesis of HAM/TSP, the exact mechanisms of how these cells acquire their function as the first responders in the pathogenesis of HAM/TSP still remain unresolved. Herein, we propose the importance of the activation of both outside-in signals from integrin signaling and inside-out signals for integrin signaling in the HTLV-1-infected CD4⁺ T cells of HAM/TSP patients.

Quantitative proteomics and integrative network analysis identified novel genes and pathways related to osteoporosis

Osteoporosis is mainly characterized by low bone mineral density (BMD), and can be attributed to excessive bone resorption by osteoclasts. Migration of circulating monocytes from blood to bone is important for subsequent osteoclast differentiation and bone resorption. Identification of those genes and pathways related to osteoclastogenesis and BMD will contribute to a better understanding of the pathophysiological mechanisms of osteoporosis. In this study, we applied the LC-nano-ESI-MS(E) (Liquid Chromatograph-nano-Electrospray Ionization-Mass Spectrometry) for quantitative proteomic profiling in 33 female Caucasians with discordant BMD levels, with 16 high vs. 17 low BMD subjects. Protein quantitation was accomplished by label-free measurement of total ion currents collected from MS(E) data. Comparison of protein expression in high vs. low BMD subjects showed that ITGA2B (p=0.0063) and GSN (p=0.019) were up-regulated in the high BMD group. Additionally, our protein-RNA integrative analysis showed that RHOA (p=0.00062) differentially expressed between high vs. low BMD groups. Network analysis based on multiple tools revealed two pathways: "regulation of actin cytoskeleton" (p=1.13E-5, FDR=3.34E-4) and "leukocyte transendothelial migration" (p=2.76E-4, FDR=4.71E-3) that are functionally relevant to osteoporosis. Consistently, ITGA2B, GSN and RHOA played crucial roles in these two pathways respectively. All together, our study strongly supported the contribution of the genes ITGA2B, GSN and RHOA and the two pathways to osteoporosis risk.

BIOLOGICAL SIGNIFICANCE

Mass spectrometry based quantitative proteomics study integrated with network analysis identified novel genes and pathways related to osteoporosis. The results were further verified in multiple level studies including protein-RNA integrative analysis and genome wide association studies.

Rap2B GTPase: structure, functions, and regulation

Rap2B GTPase, a member of Ras-related protein superfamily, was first discovered from a platelet cDNA library in the early 1990s. Since then, it has been reported to play an important role in regulating cellular processes including cytoskeletal organization, cell growth, and proliferation. It can be stimulated and suppressed by a wide range of external and internal inducers, circulating between GTP-bound active state and GDP-bound inactive state. Increasing focus on Ras signaling pathway reveals critical effects of Rap2B on tumorigenesis. In particular, Rap2B behaves in a p53-dependent manner in regulation of apoptosis and migration. Apart from being an oncogenic activator, Rap2B has been found to participate in many other physiological events via diverse downstream effectors. In this review, we present recent studies on the structure, regulation, and multiple biological functions of Rap2B, shedding light on its potential status in treatment of cancer as well as other diseases.

The regulation of transendothelial migration: new knowledge and new questions

Leucocyte transendothelial migration (TEM) involves a co-operative series of interactions between surface molecules on the leucocyte and cognate counter-ligands on the endothelial cell. These interactions set up a cascade of signalling events inside the endothelial cell that both allow for the junctions to loosen and for membrane to be recruited from the lateral border recycling compartment (LBRC). The LBRC is thought to provide an increased surface area and unligated receptors to the leucocyte to continue the process. The relative importance of the individual adhesion/signalling molecules that promote transmigration may vary depending on the type of leucocyte, the vascular bed, the inflammatory stimulus, and the stage of the inflammatory response. However, the molecular interactions between leucocyte and endothelial cell activate signalling pathways that disengage the adherens and tight junctions and recruit the LBRC to the site of transmigration. With the exception of disengaging the junctions, similar molecules and mechanisms promote transcellular migration as paracellular migration of leucocytes. This review will discuss the molecular interactions and signalling pathways that regulate transmigration, and the common themes that emerge from studying TEM of different leucocyte subsets under different inflammatory conditions. We will also raise some unanswered questions in need of future research.

Monocyte trafficking across the vessel wall

Monocytes fundamentally contribute to immune surveillance and the inflammatory response in immunoinflammatory diseases like atherosclerosis. Recruitment of these cells to the site of injury requires their trafficking across the blood vessel wall. A series of events, including capture, rolling, slow rolling, arrest, adhesion strengthening, and lateral locomotion, precede monocyte transmigration. Recent investigations have revealed new aspects of this cascade. This article revisits some conventional paradigms and selectively highlights new findings, including novel insights into monocyte differentiation and recently identified functional mediators, signalling pathways, and new structural aspects of monocyte extravasation. The emerging roles of endothelial junctional molecules like vascular endothelial-cadherin and the junctional adhesion molecule family, adhesion molecules such as intercellular adhesion molecule-1, molecules localized to the lateral border recycling compartment like cluster of differentiation 99, platelet/endothelial cell adhesion molecule-1, and poliovirus receptor (CD155), as well as other cell surface molecules such as cluster of differentiation 146 and ephrins in transendothelial migration are discussed.

Role of cortactin homolog HS1 in transendothelial migration of natural killer cells

Natural Killer (NK) cells perform many functions that depend on actin assembly, including adhesion, chemotaxis, lytic synapse assembly and cytolysis. HS1, the hematopoietic homolog of cortactin, binds to Arp2/3 complex and promotes actin assembly by helping to form and stabilize actin filament branches. We investigated the role of HS1 in transendothelial migration (TEM) by NK cells. Depletion of HS1 led to a decrease in the efficiency of TEM by NK cells, as measured by transwell assays with endothelial cell monolayers on porous filters. Transwell assays involve chemotaxis of NK cells across the filter, so to examine TEM more specifically, we imaged live-cell preparations and antibody-stained fixed preparations, with and without the chemoattractant SDF-1α. We found small to moderate effects of HS1 depletion on TEM, including whether the NK cells migrated via the transcellular or paracellular route. Expression of HS1 mutants indicated that phosphorylation of HS1 tyrosines at positions 222, 378 and 397 was required for rescue in the transwell assay, but HS1 mutations affecting interaction with Arp2/3 complex or SH3-domain ligands had no effect. The GEF Vav1, a ligand of HS1 phosphotyrosine, influenced NK cell transendothelial migration. HS1 and Vav1 also affected the speed of NK cells migrating across the surface of the endothelium. We conclude that HS1 has a role in transendothelial migration of NK cells and that HS1 tyrosine phosphorylation may signal through Vav1.

How endothelial cells regulate transmigration of leukocytes in the inflammatory response

Leukocytes attach to vascular endothelial cells at the site of inflammation via a series of intercellular adhesive interactions. In a separate step in leukocyte extravasation, transendothelial migration is regulated by molecules that play no role in the preceding steps of tethering, rolling, adhesion, and locomotion. Transendothelial migration itself can be dissected into a series of distinct interactions regulated sequentially by molecules concentrated at the endothelial cell border; these include platelet/endothelial cell adhesion molecule, poliovirus receptor (CD155), and CD99. These molecules are components of the lateral border recycling compartment (LBRC), a perijunctional network of interconnected tubulovesicular membrane that traffics to surround the leukocyte as it passes across the endothelial cell. This targeted recycling of LBRC requires kinesin to move the membrane along microtubules, and interfering with LBRC trafficking blocks transmigration of neutrophils, monocytes, and lymphocytes. The LBRC is also recruited to mediate transcellular migration when that occurs. Movement of the LBRC is coordinated with events on the luminal surface, such as clustering of intercellular adhesion molecule 1 and vascular cell adhesion molecule 1 under the migrating leukocyte, as well as movement of vascular endothelial cadherin and its associated catenins out of the junction at the site of transendothelial migration. How these events are coordinated is not known, but their regulation shares common signaling pathways that may serve to connect these steps.

HIV-1 induces cytoskeletal alterations and Rac1 activation during monocyte-blood-brain barrier interactions: modulatory role of CCR5

Background

Most HIV strains that enter the brain are macrophage-tropic and use the CCR5 receptor to bind and infect target cells. Because the cytoskeleton is a network of protein filaments involved in cellular movement and migration, we investigated whether CCR5 and the cytoskeleton are involved in endothelial-mononuclear phagocytes interactions, adhesion, and HIV-1 infection.

Results

Using a cytoskeleton phospho-antibody microarray, we showed that after co-culture with human brain microvascular endothelial cells (HBMEC), HIV-1 infected monocytes increased expression and activation of cytoskeleton-associated proteins, including Rac1/cdc42 and cortactin, compared to non-infected monocytes co-cultured with HBMEC. Analysis of brain tissues from HIV-1-infected patients validated these findings, and showed transcriptional upregulation of Rac1 and cortactin, as well as increased activation of Rac1 in brain tissues of HIV-1-infected humans, compared to seronegative individuals and subjects with HIV-1-encephalitis. Confocal imaging showed that brain cells expressing phosphorylated Rac1 were mostly macrophages and blood vessels. CCR5 antagonists TAK-799 and maraviroc prevented HIV-induced upregulation and phosphorylation of cytoskeleton-associated proteins, prevented HIV-1 infection of macrophages, and diminished viral-induced adhesion of monocytes to HBMEC. Ingenuity pathway analysis suggests that during monocyte-endothelial interactions, HIV-1 alters protein expression and phosphorylation associated with integrin signaling, cellular morphology and cell movement, cellular assembly and organization, and post-translational modifications in monocytes. CCR5 antagonists prevented these HIV-1-induced alterations.

Conclusions

HIV-1 activates cytoskeletal proteins during monocyte-endothelial interactions and increase transcription and activation of Rac1 in brain tissues. In addition to preventing macrophage infection, CCR5 antagonists could diminish viral-induced alteration and phosphorylation of cytoskeletal proteins, monocyte adhesion to the brain endothelium and viral entry into the central nervous system.

Neutrophil migration: moving from zebrafish models to human autoimmunity

There has been a resurgence of interest in the neutrophil's role in autoimmune disease. Classically considered an early responder that dies at the site of inflammation, new findings using live imaging of embryonic zebrafish and other modalities suggest that neutrophils can reverse migrate away from sites of inflammation. These 'inflammation-sensitized' neutrophils, as well as the neutrophil extracellular traps and other products made by neutrophils in general, may have many implications for autoimmunity. Here, we review what is known about the role of neutrophils in three different autoimmune diseases: rheumatoid arthritis, systemic lupus erythematosus, and small vessel vasculitis. We then highlight recent findings related to several cytoskeletal regulators that guide neutrophil recruitment including Lyn, Rac2, and SHIP. Finally, we discuss how our improved understanding of the molecules that control neutrophil chemotaxis may impact our knowledge of autoimmunity.

Small GTPase Rap1 regulates cell migration through regulation of small GTPase RhoA activity in response to transforming growth factor-β1

Transforming growth factor (TGF)-β1 regulates diverse cellular functions. Particularly, TGF-β1 induces monocyte migration to sites of injury or inflammation in early period, whereas TGF-β1 inhibits cell migration in late phase. In this study, we attempted to understand how TGF-β1 suppresses cell migration in late phase. We found that TGF-β1 of short exposure induces the production of chemokines, such as macrophage inflammatory protein (MIP)-1α, by Raw 264.7 cells. However, knock-down of small GTPase RhoA by sh-RhoA inhibited the production of MIP-1α and macrophage migration, suggesting that RhoA is essential for expression of this chemokine. An activator of Epac (exchange proteins directly activated by cAMP; a guanine nucleotide exchange factor of Rap1), 8CPT-2Me-cAMP which leads to Rap1 activation abrogated MIP-1α expression and macrophage migration. Indeed, GTP-RhoA and GTP-Rap1 levels were reciprocally regulated in a time-dependent manner following TGF-β1 stimulation. 8CPT-2Me-cAMP suppressed GTP-RhoA levels, whereas si-Rap1 augmented GTP-RhoA levels and cell migration. TGF-β1 produced cAMP in late period and si-RNAs of Epac1 and Epac2 reduced GTP-Rap1 levels leading to promotion of GTP-RhoA levels. Furthermore, si-RNA of ARAP3 (Rap-dependent RhoGAP) increased GTP-RhoA level and cell migration. Therefore, we propose the mechanism that prolonged TGF-β1 treatment produce cAMP, which activates sequentially Epac, Rap1 and ARAP3, resulting in suppression of RhoA, chemokine expression, and macrophage migration. Contrary to the general concept that Rap1 stimulates cell migration, we demonstrated in this study that Rap1 inhibits cell migration by suppression of RhoA activity in response to TGF-β1.

Rho/Rho-associated kinase pathway in glaucoma (Review)

The Rho/ROCK pathway plays important roles in the modulation of the cytoskeletal integrity of cells, the synthesis of extracellular matrix components in the aqueous humor outflow tissue and the permeability of Schlemm's canal endothelial cells. The activation of the Rho/ROCK pathway results in trabecular meshwork (TM) contraction, and the inhibition of this pathway would provoke relaxation of TM with subsequent increase in outflow facility and, thereby, decrease intraocular pressure (IOP). ROCK inhibitors also serve as potent anti‑scarring agents via inhibition of transdifferentiation of tenon fibroblasts into myofibroblasts. Furthermore, the RhoA/ROCK pathway is involved in optic nerve neuroprotection. Inactivation of Rho/ROCK signaling increase ocular blood flow, improve retinal ganglion cell (RGC) survival and promote RGC axon regeneration. Considering the IOP modulation, potent bleb anti-scarring effect and neuroprotective properties of ROCK inhibitors, the Rho/ROCK pathway is an attractive target for anti-glaucoma therapy, and it may be used for human therapy in the near future.

Tumor cell migration and invasion are enhanced by depletion of Rap1 GTPase-activating protein (Rap1GAP)

The functional significance of the widespread down-regulation of Rap1 GTPase-activating protein (Rap1GAP), a negative regulator of Rap activity, in human tumors is unknown. Here we show that human colon cancer cells depleted of Rap1GAP are endowed with more aggressive migratory and invasive properties. Silencing Rap1GAP enhanced the migration of confluent and single cells. In the latter, migration distance, velocity, and directionality were increased. Enhanced migration was a consequence of increased endogenous Rap activity as silencing Rap expression selectively abolished the migration of Rap1GAP-depleted cells. ROCK-mediated cell contractility was suppressed in Rap1GAP-depleted cells, which exhibited a spindle-shaped morphology and abundant membrane protrusions. Tumor cells can switch between Rho/ROCK-mediated contractility-based migration and Rac1-mediated mesenchymal motility. Strikingly, the migration of Rap1GAP-depleted, but not control cells required Rac1 activity, suggesting that loss of Rap1GAP alters migratory mechanisms. Inhibition of Rac1 activity restored membrane blebbing and increased ROCK activity in Rap1GAP-depleted cells, suggesting that Rac1 contributes to the suppression of contractility. Collectively, these findings identify Rap1GAP as a critical regulator of aggressive tumor cell behavior and suggest that the level of Rap1GAP expression influences the migratory mechanisms that are operative in tumor cells.

The small Rho GTPase Rif and actin cytoskeletal remodelling

The Rif GTPase is a recent addition to small Rho GTPase family; it shares low homology with other members in the family and evolutionarily parallels with the development of vertebrates. Rif has the conserved Rho GTPase domain structures and cycles between a GDP-bound inactive form and a GTP-bound active form. In its active form, Rif signals through multiple downstream effectors. In the present review, our aim is to summarize the current information about the Rif effectors and how Rif remodels actin cytoskeleton in many aspects.

Cannabinoid receptor type 1 (CB1) activation inhibits small GTPase RhoA activity and regulates motility of prostate carcinoma cells

The cannabinoid receptor type 1 (CB1) is a G protein-coupled receptor that is activated in an autocrine fashion by the endocannabinoids (EC), N-arachidonoylethanolamine (AEA) and 2-arachidonoylglycerol (2-AG). The CB1 and its endogenous and synthetic agonists are emerging as therapeutic targets in several cancers due to their ability to suppress carcinoma cell invasion and migration. However, the mechanisms that the CB1 regulates cell motility are not well understood. In this study, we examined the molecular mechanisms that diminish cell migration upon the CB1 activation in prostate carcinoma cells. The CB1 activation with the agonist WIN55212 significantly diminishes the small GTPase RhoA activity but modestly increases the Rac1 and Cdc42 activity. The diminished RhoA activity is accompanied by the loss of actin/myosin microfilaments, cell spreading, and cell migration. Interestingly, the CB1 inactivation with the selective CB1 antagonist AM251 significantly increases RhoA activity, enhances microfilament formation and cell spreading, and promotes cell migration. This finding suggests that endogenously produced EC activate the CB1, resulting in chronic repression of RhoA activity and cell migration. Consistent with this possibility, RhoA activity is significantly diminished by the exogenous application of AEA but not by 2-AG in PC-3 cells (cells with very low AEA hydrolysis). Pretreatment of cells with a monoacylglycerol lipase inhibitor, JZL184, which blocks 2-AG hydrolysis, decreases the RhoA activity. These results indicate the unique CB1 signaling and support the model that EC, through their autocrine activation of CB1 and subsequent repression of RhoA activity, suppress migration in prostate carcinoma cells.

Regulation of eosinophil trafficking by SWAP-70 and its role in allergic airway inflammation

Eosinophils are the predominant inflammatory cells recruited to allergic airways. In this article, we show that human and murine eosinophils express SWAP-70, an intracellular RAC-binding signaling protein, and examine its role in mediating eosinophil trafficking and pulmonary recruitment in a murine model of allergic airway inflammation. Compared with wild-type eosinophils, SWAP-70-deficient (Swap-70(-/-)) eosinophils revealed altered adhesive interactions within inflamed postcapillary venules under conditions of blood flow by intravital microscopy, exhibiting enhanced slow rolling but decreased firm adhesion. In static adhesion assays, Swap-70(-/-) eosinophils adhered poorly to VCAM-1 and ICAM-1 and exhibited inefficient leading edge and uropod formation. Adherent Swap-70(-/-) eosinophils failed to translocate RAC1 to leading edges and displayed aberrant cell surface localization/distribution of α4 and Mac-1. Chemokine-induced migration of Swap-70(-/-) eosinophils was significantly decreased, correlating with reduced intracellular calcium levels, defective actin polymerization/depolymerization, and altered cytoskeletal rearrangement. In vivo, recruitment of eosinophils to the lungs of allergen-challenged Swap-70(-/-) mice, compared with wild-type mice, was significantly reduced, along with considerable attenuation of airway inflammation, indicated by diminished IL-5, IL-13, and TNF-α levels; reduced mucus secretion; and improved airway function. These findings suggest that regulation of eosinophil trafficking and migration by SWAP-70 is important for the development of eosinophilic inflammation after allergen exposure.

TCR-induced activation of LFA-1 involves signaling through Tiam1

Adhesion is pivotal for most leukocyte functions, and the β(2) integrin family of adhesion molecules plays a central role. The integrins need activation to become functional, but the molecular events resulting in adhesion have remained incompletely understood. In human T cells, activation through the TCR results in specific phosphorylation of the T758 on the β(2) chain of LFA-1. We now show that this phosphorylation leads to downstream binding of 14-3-3 proteins, followed by engagement of the guanine nucleotide exchange factor protein Tiam1 and Rac1 activation. Downregulation of the signaling molecules inhibits LFA-1 activity. Activation by the chemokine stromal cell-derived factor-1α also results in T758 phosphorylation and integrin activation. Thus, TCR and chemokine activation converges on LFA-1 phosphorylation, followed by similar downstream events affecting adhesion.

Individual rac GTPases mediate aspects of prostate cancer cell and bone marrow endothelial cell interactions

The Rho GTPases organize the actin cytoskeleton and are involved in cancer metastasis. Previously, we demonstrated that RhoC GTPase was required for PC-3 prostate cancer cell invasion. Targeted down-regulation of RhoC led to sustained activation of Rac1 GTPase and morphological, molecular and phenotypic changes reminiscent of epithelial to mesenchymal transition. We also reported that Rac1 is required for PC-3 cell diapedesis across a bone marrow endothelial cell layer. In the current study, we queried whether Rac3 and RhoG GTPases also have a role in prostate tumor cell diapedesis. Using specific siRNAs we demonstrate roles for each protein in PC-3 and C4-2 cell adhesion and diapedesis. We have shown that the chemokine CCL2 induces tumor cell diapedesis via Rac1 activation. Here we find that RhoG partially contributes to CCL2-induced tumor cell diapedesis. We also find that Rac1 GTPase mediates tight binding of prostate cancer cells to bone marrow endothelial cells and promotes retraction of endothelial cells required for tumor cell diapedesis. Finally, Rac1 leads to β1 integrin activation, suggesting a mechanism that Rac1 can mediate tight binding with endothelial cells. Together, our data suggest that Rac1 GTPase is key mediator of prostate cancer cell-bone marrow endothelial cell interactions.

A mechanism of Rap1-induced stabilization of endothelial cell--cell junctions

Rap1 stabilizes cell–cell junctions by directly binding to KRIT1, displacing it from microtubules and enabling localization at the junctions.

Activation of Rap1 small GTPases stabilizes cell–cell junctions, and this activity requires Krev Interaction Trapped gene 1 (KRIT1). Loss of KRIT1 disrupts cardiovascular development and causes autosomal dominant familial cerebral cavernous malformations. Here we report that native KRIT1 protein binds the effector loop of Rap1A but not H-Ras in a GTP-dependent manner, establishing that it is an authentic Rap1-specific effector. By modeling the KRIT1–Rap1 interface we designed a well-folded KRIT1 mutant that exhibited a ∼40-fold-reduced affinity for Rap1A and maintained other KRIT1-binding functions. Direct binding of KRIT1 to Rap1 stabilized endothelial cell–cell junctions in vitro and was required for cardiovascular development in vivo. Mechanistically, Rap1 binding released KRIT1 from microtubules, enabling it to locate to cell–cell junctions, where it suppressed Rho kinase signaling and stabilized the junctions. These studies establish that the direct physical interaction of Rap1 with KRIT1 enables the translocation of microtubule-sequestered KRIT1 to junctions, thereby supporting junctional integrity and cardiovascular development.

Interplay between FAK, PKCδ, and p190RhoGAP in the regulation of endothelial barrier function

Disruption of either intercellular or extracellular junctions involved in maintaining endothelial barrier function can result in increased endothelial permeability. Increased endothelial permeability, in turn, allows for the unregulated movement of fluid and solutes out of the vasculature and into the surrounding connective tissue, contributing to a number of disease states, including stroke and pulmonary edema (Ermert et al., 1995; Lee and Slutsky, 2010; van Hinsbergh, 1997; Waller et al., 1996; Warboys et al., 2010). Thus, a better understanding of the molecular mechanisms by which endothelial cell junction integrity is controlled is necessary for development of therapies aimed at treating such conditions. In this review, we will discuss the functions of three signaling molecules known to be involved in regulation of endothelial permeability: focal adhesion kinase (FAK), protein kinase C delta (PKCδ), and p190RhoGAP (p190). We will discuss the independent functions of each protein, as well as the interplay that exists between them and the effects of such interactions on endothelial function.

SYK regulates B-cell migration by phosphorylation of the F-actin interacting protein SWAP-70

B-cell migration into and within lymphoid tissues is not only central to the humoral immune response but also for the development of malignancies and autoimmunity. We previously demonstrated that SWAP-70, an F-actin-binding, Rho GTPase-interacting protein strongly expressed in activated B cells, is necessary for normal B-cell migration in vivo. SWAP-70 regulates integrin-mediated adhesion and cell attachment. Here we show that upon B-cell activation, SWAP-70 is extensively posttranslationally modified and becomes tyrosine phosphorylated by SYK at position 517. This phosphorylation inhibits binding of SWAP-70 to F-actin. Phospho-site mutants of SWAP-70 disrupt B-cell polarization in a dominant-negative fashion in vitro and impair migration in vivo. After CXCL12 stimulation of B cells SYK becomes activated and SWAP-70 is phosphorylated in a SYK-dependent manner. Use of the highly specific SYK inhibitor BAY61-3606 showed SYK activity is necessary for normal chemotaxis and B-cell polarization in vitro and for entry of B cells into lymph nodes in vivo. These findings demonstrate a novel requirement for SYK in migration and polarization of naive recirculating B cells and show that SWAP-70 is an important target of SYK in this pathway.

Myosin light chain kinase mediates transcellular intravasation of breast cancer cells through the underlying endothelial cells: a three-dimensional FRET study

The transient and localized signaling events between invasive breast cancer cells and the underlying endothelial cells have remained poorly characterized. We report a novel approach integrating vascular engineering with three-dimensional time-lapse fluorescence resonance energy transfer (FRET) imaging to dissect how endothelial myosin light chain kinase (MLCK) is modulated during tumor intravasation. We show that tumor transendothelial migration occurs via both paracellular (i.e. through cell-cell junctions) and transcellular (i.e. through individual endothelial cells) routes. Endothelial MLCK is activated at the invasion site, leading to regional diphosphorylation of myosin-II regulatory light chain (RLC) and myosin contraction. Blocking endothelial RLC diphosphorylation blunts tumor transcellular, but not paracellular, invasion. Our results implicate an important role for endothelial myosin-II function in tumor intravasation.

The Rac activator Tiam1 controls efficient T-cell trafficking and route of transendothelial migration

Migration toward chemoattractants is a hallmark of T-cell trafficking and is essential to produce an efficient immune response. Here, we have analyzed the function of the Rac activator Tiam1 in the control of T-cell trafficking and transendothelial migration. We found that Tiam1 is required for chemokine- and S1P-induced Rac activation and subsequent cell migration. As a result, Tiam1-deficient T cells show reduced chemotaxis in vitro, and impaired homing, egress, and contact hypersensitivity in vivo. Analysis of the T-cell transendothelial migration cascade revealed that PKCzeta/Tiam1/Rac signaling is dispensable for T-cell arrest but is essential for the stabilization of polarization and efficient crawling of T cells on endothelial cells. T cells that lack Tiam1 predominantly transmigrate through individual endothelial cells (transcellular migration) rather than at endothelial junctions (paracellular migration), suggesting that T cells are able to change their route of transendothelial migration according to their polarization status and crawling capacity.

Endothelial signaling in paracellular and transcellular leukocyte transmigration

As the primary physical barrier between blood and tissue compartments within the body, blood vessel endothelial cells and integrity of the cell junctions connecting them must be carefully regulated to support leukocyte transendothelial migration only when necessary. Leukocytes utilize two independent routes across the endothelium: the paracellular route involves migration in-between adjacent endothelial cells and requires the transient disassembly of endothelial cell junctions, while the transcellular route occurs directly through an individual endothelial cell, likely requiring the formation of a channel or pore. In this review, I will first summarize the signaling events that are transduced by leukocyte engagement of endothelial cell-surface receptors like ICAM-1 and VCAM-1. Some of these signals include activation of GTPases, production of reactive oxygen species, and phosphorylation of target proteins. These signaling pathways converge to cause junctional disruption, cytoskeletal remodeling, and/or the membrane fusion events that are associated with leukocyte transendothelial migration. The review will conclude with a detailed discussion of the newly characterized transmigratory cup structure, and the recent advances made towards understanding the mechanisms of transcellular transendothelial migration.

Cell motility in chronic lymphocytic leukemia: defective Rap1 and alphaLbeta2 activation by chemokine

Chemokine-induced activation of alpha4beta1 and alphaLbeta2 integrins (by conformational change and clustering) is required for lymphocyte transendothelial migration (TEM) and entry into lymph nodes. We have previously reported that chemokine-induced TEM is defective in chronic lymphocytic leukemia (CLL) and that this defect is a result of failure of the chemokine to induce polar clustering of alphaLbeta2; engagement of alpha4beta1 and autocrine vascular endothelial growth factor (VEGF) restore clustering and TEM. The aim of the present study was to characterize the nature of this defect in alphaLbeta2 activation and determine how it is corrected. We show here that the alphaLbeta2 of CLL cells is already in variably activated conformations, which are not further altered by chemokine treatment. Importantly, such treatment usually does not cause an increase in the GTP-loading of Rap1, a GTPase central to chemokine-induced activation of integrins. Furthermore, we show that this defect in Rap1 GTP-loading is at the level of the GTPase and is corrected in CLL cells cultured in the absence of exogenous stimuli, suggesting that the defect is the result of in vivo stimulation. Finally, we show that, because Rap1-induced activation of both alpha4beta1 and alphaLbeta2 is defective, autocrine VEGF and chemokine are necessary to activate alpha4beta1 for ligand binding. Subsequently, this binding and both VEGF and chemokine stimulation are all needed for alphaLbeta2 activation for motility and TEM. The present study not only clarifies the nature of the alphaLbeta2 defect of CLL cells but is the first to implicate activation of Rap1 in the pathophysiology of CLL.

Rho GTPases in PC-3 prostate cancer cell morphology, invasion and tumor cell diapedesis

BACKGROUND

The Rho GTPases comprise one of the eight subfamilies of the Ras superfamily of monomeric GTP-binding proteins and are involved in cytoskeletal organization. Previously, using a dominant negative construct, we demonstrated a role for RhoC GTPase in conferring invasive capabilities to PC-3 human prostate cancer cells. Further, we demonstrated that inactivation of RhoC led to morphological changes commensurate with epithelial to mesenchymal transition (EMT) and was accompanied by increased random, linear motility and decreased directed migration and invasion. EMT was related positively to sustained expression and activity of Rac GTPase. In the current study we analyze the individual roles of RhoA, RhoC and Rac1 GTPases in PC-3 cell directed migration, invasion and tumor cell diapedesis across a human bone marrow endothelial cell layer in vitro.

RESULTS

Use of specific shRNA directed against RhoA, RhoC or Rac1 GTPases demonstrated a role for each protein in maintaining cell morphology. Furthermore, we demonstrate that RhoC expression and activation is required for directed migration and invasion, while Rac1 expression and activation is required for tumor cell diapedesis. Inhibition of RhoA expression produced a slight increase in invasion and tumor cell diapedesis.

CONCLUSIONS

Individual Rho GTPases are required for critical aspects of migration, invasion and tumor cell diapedesis. These data suggest that coordinated activation of individual Rho proteins is required for cells to successfully complete the extravasation process; a key step in distant metastasis.

Activation of peroxisome proliferator-activated receptor gamma (PPARgamma) suppresses Rho GTPases in human brain microvascular endothelial cells and inhibits adhesion and transendothelial migration of HIV-1 infected monocytes

Under inflammatory conditions (including HIV-1 encephalitis and multiple sclerosis), activated brain endothelium enhances the adhesion and transmigration of monocytes across the blood-brain barrier (BBB). Synthetic ligands that activate the peroxisome proliferator-activated receptors (PPARs) have anti-inflammatory properties, and PPAR stimulation prevents the interaction of leukocytes with cytokine stimulated-endothelium. However, the mechanism underlying these effects of PPAR ligands and their ability to intervene with leukocyte adhesion and migration across brain endothelial cells has yet to be explored. For the first time, using primary human brain endothelial cells (BMVEC), we demonstrated that monocyte adhesion and transendothelial migration across inflamed endothelium were markedly reduced by PPARgamma activation. In contrast to non-brain-derived endothelial cells, PPARalpha activation in the BMVEC had no significant effect on monocyte-endothelial interaction. Previously, our work indicated a critical role of Rho GTPases (like RhoA) in BMVEC to control migration of HIV-1 infected monocytes across BBB. In this study, we show that in the BMVEC PPARgamma stimulation prevented activation of two GTPases, Rac1 and RhoA, which correlated with decreased monocyte adhesion to and migration across brain endothelium. Relevant to HIV-1 neuropathogenesis, enhanced adhesion and migration of HIV-1 infected monocytes across the BBB were significantly reduced when BMVEC were treated with PPARgamma agonist. These findings indicate that Rac1 and RhoA inhibition by PPARgamma agonists could be a new approach for treatment of neuroinflammation by preventing monocyte migration across the BBB.

cAMP-induced Epac-Rap activation inhibits epithelial cell migration by modulating focal adhesion and leading edge dynamics

Epithelial cell migration is a complex process crucial for embryonic development, wound healing and tumor metastasis. It depends on alterations in cell-cell adhesion and integrin-extracellular matrix interactions and on actomyosin-driven, polarized leading edge protrusion. The small GTPase Rap is a known regulator of integrins and cadherins that has also been implicated in the regulation of actin and myosin, but a direct role in cell migration has not been investigated. Here, we report that activation of endogenous Rap by cAMP results in an inhibition of HGF- and TGFbeta-induced epithelial cell migration in several model systems, irrespective of the presence of E-cadherin adhesion. We show that Rap activation slows the dynamics of focal adhesions and inhibits polarized membrane protrusion. Importantly, forced integrin activation by antibodies does not mimic these effects of Rap on cell motility, even though it does mimic Rap effects in short-term cell adhesion assays. From these results, we conclude that Rap inhibits epithelial cell migration, by modulating focal adhesion dynamics and leading edge activity. This extends beyond the effect of integrin affinity modulation and argues for an additional function of Rap in controlling the migration machinery of epithelial cells.

Role of Epac1, an exchange factor for Rap GTPases, in endothelial microtubule dynamics and barrier function

Rap1 GTPase activation by its cAMP responsive nucleotide exchange factor Epac present in endothelial cells increases endothelial cell barrier function with an associated increase in cortical actin. Here, Epac1 was shown to be responsible for these actin changes and to colocalize with microtubules in human umbilical vein endothelial cells. Importantly, Epac activation with a cAMP analogue, 8-pCPT-2'O-Me-cAMP resulted in a net increase in the length of microtubules. This did not require cell-cell interactions or Rap GTPase activation, and it was attributed to microtubule growth as assessed by time-lapse microscopy of human umbilical vein endothelial cell expressing fluorophore-linked microtubule plus-end marker end-binding protein 3. An intact microtubule network was required for Epac-mediated changes in cortical actin and barrier enhancement, but it was not required for Rap activation. Finally, Epac activation reversed microtubule-dependent increases in vascular permeability induced by tumor necrosis factor-alpha and transforming growth factor-beta. Thus, Epac can directly promote microtubule growth in endothelial cells. This, together with Rap activation leads to an increase in cortical actin, which has functional significance for vascular permeability.

SWAP-70 deficiency causes high-affinity plasma cell generation despite impaired germinal center formation

Germinal centers (GCs) are lymphoid tissue structures central to the generation of long-lived, high-affinity, antibody-forming B cells. However, induction, maintenance, and regulation of GCs are not sufficiently understood. The F-actin-binding, Rac-interacting protein SWAP-70 is strongly expressed in activated B cells like those in B follicles. Recent work suggests that SWAP-70 is involved in B-cell activation, migration, and homing. Therefore, we investigated the role of SWAP-70 in the T-dependent immune response, in GC formation, and in differentiation into plasma and memory B cells. Compared with wt, sheep red blood cell (SRBC)-, or NP-KLH-immunized SWAP-70(-/-) mice have strongly reduced numbers of GCs and GC-specific B cells. However, SWAP-70(-/-) NP-specific B cells accumulate outside of the B follicles, and SWAP-70(-/-) mice show more plasma cells in the red pulp and in the bone marrow, and increased NP-specific Ig and antibody-forming B cells. Yet the memory response is impaired. Thus, SWAP-70 deficiency uncouples GC formation from T-dependent antibody and long-lived plasma cell production and causes extrafollicular generation of high-affinity plasma cells, but does not adequately support the memory response.

Getting to the site of inflammation: the leukocyte adhesion cascade updated

Neutrophil recruitment, lymphocyte recirculation and monocyte trafficking all require adhesion and transmigration through blood-vessel walls. The traditional three steps of rolling, activation and firm adhesion have recently been augmented and refined. Slow rolling, adhesion strengthening, intraluminal crawling and paracellular and transcellular migration are now recognized as separate, additional steps. In neutrophils, a second activation pathway has been discovered that does not require signalling through G-protein-coupled receptors and the signalling steps leading to integrin activation are beginning to emerge. This Review focuses on new aspects of one of the central paradigms of inflammation and immunity--the leukocyte adhesion cascade.

SmgGDS regulates cell proliferation, migration, and NF-kappaB transcriptional activity in non-small cell lung carcinoma

Non-small cell lung carcinoma (NSCLC) is promoted by the increased activities of several small GTPases, including K-Ras4B, Rap1A, Rap1B, RhoC, and Rac1. SmgGDS is an unusual guanine nucleotide exchange factor that activates many of these small GTPases, and thus may promote NSCLC development or progression. We report here that SmgGDS protein levels are elevated in NSCLC tumors, compared with normal lung tissue from the same patients or from individuals without cancer. To characterize SmgGDS functions in NSCLC, we tested the effects of silencing SmgGDS expression by transfecting cultured NSCLC cells with SmgGDS small interfering RNA (siRNA). Cells with silenced SmgGDS expression form fewer colonies in soft agar, do not proliferate in culture due to an arrest in G(1) phase, and exhibit disrupted myosin organization and reduced cell migration. The transcriptional activity of NF-kappaB in NSCLC cells is diminished by transfecting the cells with SmgGDS siRNA, and enhanced by transfecting the cells with a cDNA encoding SmgGDS. Because RhoA is a major substrate for SmgGDS, we investigated whether diminished RhoA expression mimics the effects of diminished SmgGDS expression. Silencing RhoA expression with RhoA siRNA disrupts myosin organization, but only moderately decreases cell proliferation and does not inhibit migration. Our finding that the aggressive NSCLC phenotype is more effectively suppressed by silencing SmgGDS than by silencing RhoA is consistent with the ability of SmgGDS to regulate multiple small GTPases in addition to RhoA. These results demonstrate that SmgGDS promotes the malignant NSCLC phenotype and is an intriguing therapeutic target in NSCLC.

Y-27632, a Rho-associated protein kinase inhibitor, attenuates neuronal cell death after transient retinal ischemia

PURPOSE

Transient retinal ischemia induces the death of retinal neuronal cells. Postischemic damage is associated with the infiltration of leukocytes into the neural tissue through vascular endothelia. The current study aimed to investigate whether this damage was attenuated by the inhibition of Rho/ROCK (Rho kinases) signaling, recently shown to play a critical role in the transendothelial migration of leukocytes.

METHODS

Y-27632, a selective inhibitor of ROCK, was injected intravitreally into rat eyes with transient retinal ischemia. Cell loss of the ganglion cell layer (GCL) and thinning of the inner plexiform layer (IPL) with and without the administration of Y-27632 were evaluated by histological analysis, TUNEL assay and retrograde labeling of retinal ganglion cells (RGCs). To examine the attenuation of leukocyte infiltration in postischemic retinas with the administration of Y-27632, silver nitrate staining and immunohistochemistry using an anti-LCA antibody were performed.

RESULTS

Cell loss of the GCL and thinning of the IPL were significantly attenuated when 100 nmol Y-27632 was administered within three hours of the induction of ischemia. TUNEL assay and retrograde labeling of RGCs showed a decreased number of apoptotic cells and an increased number of RGCs in Y-27632-injected retinas. Moreover, silver nitrate staining and immunohistochemical analysis using an anti-LCA antibody showed that Y-27632 injection dramatically inhibited leukocyte infiltration and endothelial disarrangement.

CONCLUSIONS

Our data suggest that inhibition of Rho/ROCK signaling offers neuroprotective therapy against postischemic neural damage, by regulating leukocyte infiltration in the neural tissue.

cAMP signaling in leukocyte transendothelial migration

The migration of leukocytes across the vascular endothelium is crucial for immunosurveillance as well as for inflammatory responses. Uncontrolled leukocyte transendothelial migration results in pathologies such as asthma, rheumatoid arthritis, and atherosclerosis. The molecular mechanisms that regulate leukocyte transendothelial migration involve signaling downstream of intracellular messengers such as cAMP, calcium, phosphoinositol lipids, or reactive oxygen species. Among these, cAMP is particularly intriguing because it is generated in both leukocytes and endothelial cells and regulates leukocyte chemotaxis as well as endothelial barrier function. In addition, physiological stimuli that induce cAMP production generate both pro- and antiinflammatory signals, underscoring the complexity of cAMP-driven signaling. This review discusses our current knowledge of the control of leukocyte transendothelial migration by two main cAMP effectors: protein kinase A and the Rap exchange factor Epac (Exchange protein directly activated by cAMP).

Resealing of endothelial junctions by focal adhesion kinase

Endothelial cell (EC) junctions determine vascular barrier properties and are subject to transient opening to allow liquid flux from blood to tissue. Although EC junctions open in the presence of permeability-enhancing factors, including oxidants, the mechanisms by which they reseal remain inadequately understood. To model opening and resealing of EC junctions in the presence of an oxidant, we quantified changes in H2O2-induced transendothelial resistance (TER) in monolayers of rat lung microvascular EC. During a 30-min exposure, H2O2(100 μM) decreased TER for an initial ∼10 min, indicating junctional opening. Subsequently, despite continuous presence of H2O2, TER recovered to baseline, indicating the activation of junctional resealing mechanisms. These bimodal TER transients matched the time course of loss and then gain of E-cadherin at EC junctions. The timing of the TER decrease matched the onset of focal adhesion formation, while F-actin increase at the cell periphery occurred with a time course that complemented the recovery of peripheral E-cadherin. In monolayers expressing a focal adhesion kinase (FAK) mutant (del-FAK) that inhibits FAK activity, the initial H2O2-induced junctional opening was present, although the subsequent junctional recovery was blocked. Expression of transfected E-cadherin was evident at the cell periphery of wild-type but not del-FAK-expressing EC. E-cadherin overexpression in del-FAK-expressing EC failed to effect major rescue of the junctional resealing response. These findings indicate that in oxidant-induced EC junction opening, FAK plays a critical role in remodeling the adherens junction to reseal the barrier.

Regulation of immune responses and hematopoiesis by the Rap1 signal

Rap1 (Ras-proximity 1), a member of the Ras family of small guanine triphosphatases (GTPases), is activated by diverse extracellular stimuli. While Rap1 has been discovered originally as a potential Ras antagonist, accumulating evidence indicates that Rap1 per se mediates unique signals and exerts biological functions distinctly different from Ras. Rap1 plays a dominant role in the control of cell-cell and cell-matrix interactions by regulating the function of integrins and other adhesion molecules in various cell types. Rap1 also regulates MAP kinase (MAPK) activity in a manner highly dependent on the context of cell types. Recent studies (including gene-targeting analysis) have uncovered that the Rap1 signal is integrated crucially and unpredictably in the diverse aspects of comprehensive biological systems. This review summarizes the role of the Rap1 signal in developments and functions of the immune and hematopoietic systems as well as in malignancy. Importantly, Rap1 activation is tightly regulated in tissue cells, and dysregulations of the Rap1 signal in specific tissues result in certain disorders, including myeloproliferative disorders and leukemia, platelet dysfunction with defective hemostasis, leukocyte adhesion-deficiency syndrome, lupus-like systemic autoimmune disease, and T cell anergy. Many of these disorders resemble human diseases, and the Rap1 signal with its regulators may provide rational molecular targets for controlling certain human diseases including malignancy.

Regulation of Rho guanine nucleotide exchange factors by G proteins

Monomeric Rho GTPases regulate cellular dynamics through remodeling of the cytoskeleton, modulation of immediate signaling pathways, and longer-term regulation of gene transcription. One family of guanine nucleotide exchange factors for Rho proteins (RhoGEFs) provides a direct pathway for regulation of RhoA by cell surface receptors coupled to heterotrimeric G proteins. Some of these RhoGEFs also contain RGS domains that can attenuate signaling by the G(12) and G(13) proteins. The regulation provided by these RhoGEFs is defined by their selective regulation by specific G proteins, phosphorylation by kinases, and potential localization with signaling partners. Evidence of their physiological importance is derived from gene knockouts in Drosophila and mice. Current understanding of the basic regulatory mechanisms of these RhoGEFs is discussed. An overview of identified interactions with other signaling proteins suggests the growing spectrum of their involvement in numerous signaling pathways.

Signaling protein SWAP-70 is required for efficient B cell homing to lymphoid organs

The migration of B cells into secondary lymphoid organs is required for the generation of an effective immune response. Here we analyzed the involvement of SWAP-70, a Rac-interacting protein involved in actin rearrangement, in B cell entry into lymph nodes. We noted reduced migration of Swap70-/- B cells into lymph nodes in vivo. Swap70-/- B cells rolled and adhered, yet accumulated in lymph node high endothelial venules. This defect was not due to impaired integrin expression or chemotaxis. Instead, Swap70-/- B cells aberrantly regulated integrin-mediated adhesion. During attachment, Swap70-/- B cells showed defective polarization and did not form uropods or stabilize lamellipodia at a defined region. Thus, SWAP-70 selectively regulates processes essential for B cell entry into lymph nodes.

Blood-brain barrier: structural components and function under physiologic and pathologic conditions

The blood-brain barrier (BBB) is the specialized system of brain microvascular endothelial cells (BMVEC) that shields the brain from toxic substances in the blood, supplies brain tissues with nutrients, and filters harmful compounds from the brain back to the bloodstream. The close interaction between BMVEC and other components of the neurovascular unit (astrocytes, pericytes, neurons, and basement membrane) ensures proper function of the central nervous system (CNS). Transport across the BBB is strictly limited through both physical (tight junctions) and metabolic barriers (enzymes, diverse transport systems). A functional polarity exists between the luminal and abluminal membrane surfaces of the BMVEC. As a result of restricted permeability, the BBB is a limiting factor for the delivery of therapeutic agents into the CNS. BBB breakdown or alterations in transport systems play an important role in the pathogenesis of many CNS diseases (HIV-1 encephalitis, Alzheimer's disease, ischemia, tumors, multiple sclerosis, and Parkinson's disease). Proinflammatory substances and specific disease-associated proteins often mediate such BBB dysfunction. Despite seemingly diverse underlying causes of BBB dysfunction, common intracellular pathways emerge for the regulation of the BBB structural and functional integrity. Better understanding of tight junction regulation and factors affecting transport systems will allow the development of therapeutics to improve the BBB function in health and disease.