STIMulating stress fibers in endothelial cells

Rap1A Modulates Store-Operated Calcium Entry in the Lung Endothelium: A Novel Mechanism Controlling NFAT-Mediated Vascular Inflammation and Permeability

BACKGROUND

Store-operated calcium entry mediated by STIM (stromal interaction molecule)-1-Orai1 (calcium release-activated calcium modulator 1) is essential in endothelial cell (EC) functions, affecting signaling, NFAT (nuclear factor for activated T cells)-induced transcription, and metabolic programs. While the small GTPase Rap1 (Ras-proximate-1) isoforms, including the predominant Rap1B, are known for their role in cadherin-mediated adhesion, EC deletion of Rap1A after birth uniquely disrupts lung endothelial barrier function. Here, we elucidate the specific mechanisms by which Rap1A modulates lung vascular integrity and inflammation.

METHODS

The role of EC Rap1A in lung inflammation and permeability was examined using in vitro and in vivo approaches.

RESULTS

We explored Ca2+ signaling in human ECs following siRNA-mediated knockdown of Rap1A or Rap1B. Rap1A knockdown, unlike Rap1B, significantly increased store-operated calcium entry in response to a GPCR (G-protein-coupled receptor) agonist, ATP (500 µmol/L), or thapsigargin (250 nmol/L). This enhancement was attenuated by Orai1 channel blockers 10 μmol/L BTP2 (N-[4-[3,5-bis(trifluoromethyl)-1H-pyrazol-1-yl]phenyl]-4-methyl-1,2,3-thiadiazole-5-carboxamide), 10 μmol/L GSK-7975A, and 5 μmol/L Gd3+. Whole-cell patch clamp measurements revealed enhanced Ca2+ release-activated Ca2+ current density in siRap1A ECs. Rap1A depletion in ECs led to increased NFAT1 nuclear translocation and activity and elevated levels of proinflammatory cytokines (CXCL1 [C-X-C motif chemokine ligand 1], CXCL11 [C-X-C motif chemokine 11], CCL5 [chemokine (C-C motif) ligand 5], and IL-6 [interleukin-6]). Notably, reducing Orai1 expression in siRap1A ECs normalized store-operated calcium entry, NFAT activity, and endothelial hyperpermeability in vitro. EC-specific Rap1A knockout (Rap1AiΔEC) mice displayed an inflammatory lung phenotype with increased lung permeability and inflammation markers, along with higher Orai1 expression. Delivery of siRNA against Orai1 to lung endothelium using lipid nanoparticles effectively normalized Orai1 levels in lung ECs, consequently reducing hyperpermeability and inflammation in Rap1AiΔEC mice.

CONCLUSIONS

Our findings uncover a novel role of Rap1A in regulating Orai1-mediated Ca2+ entry and expression, crucial for NFAT-mediated transcription and endothelial inflammation. This study distinguishes the unique function of Rap1A from that of the predominant Rap1B isoform and highlights the importance of normalizing Orai1 expression in maintaining lung vascular integrity and modulating endothelial functions.

Ca2+ Signaling by TRPV4 Channels in Respiratory Function and Disease

Members of the transient receptor potential (TRP) superfamily are broadly expressed in our body and contribute to multiple cellular functions. Most interestingly, the fourth member of the vanilloid family of TRP channels (TRPV4) serves different partially antagonistic functions in the respiratory system. This review highlights the role of TRPV4 channels in lung fibroblasts, the lung endothelium, as well as the alveolar and bronchial epithelium, during physiological and pathophysiological mechanisms. Data available from animal models and human tissues confirm the importance of this ion channel in cellular signal transduction complexes with Ca²⁺ ions as a second messenger. Moreover, TRPV4 is an excellent therapeutic target with numerous specific compounds regulating its activity in diseases, like asthma, lung fibrosis, edema, and infections.

Actin fence therapy with exogenous V12Rac1 protects against acute lung injury

High mortality in acute lung injury (ALI) results from sustained proinflammatory signaling by alveolar receptors, such as TNF-α receptor type 1 (TNFR1). Factors that determine the sustained signaling are not known. Unexpectedly, optical imaging of live alveoli revealed a major TNF-α–induced surge of alveolar TNFR1 due to a Ca²⁺-dependent mechanism that decreased the cortical actin fence. Mouse mortality due to inhaled LPS was associated with cofilin activation, actin loss, and the TNFR1 surge. The constitutively active form of the GTPase, Rac1 (V12Rac1), given intranasally (i.n.) as a noncovalent construct with a cell-permeable peptide, enhanced alveolar filamentous actin (F-actin) and blocked the TNFR1 surge. V12Rac1 also protected against ALI-induced mortality resulting from i.n. instillation of LPS or of Pseudomonas aeruginosa. We propose a potentially new therapeutic paradigm in which actin enhancement by exogenous Rac1 strengthens the alveolar actin fence, protecting against proinflammatory receptor hyperexpression, and therefore blocking ALI.

Transient receptor potential (TRP) channels as molecular targets in lung toxicology and associated diseases

The lungs as the gateways of our body to the external environment are essential for gas exchange. They are also exposed to toxicants from two sides, the airways and the vasculature. Apart from naturally produced toxic agents, millions of human made chemicals were produced since the beginning of the industrial revolution whose toxicity still needs to be determined. While the knowledge about toxic substances is increasing only slowly, a paradigm shift regarding the proposed mechanisms of toxicity at the plasma membrane emerged. According to their broad-range chemical reactivity, the mechanism of lung injury evoked by these agents has long been described as rather unspecific. Consequently, therapeutic options are still restricted to symptomatic treatment. The identification of molecular down-stream effectors in cells was a major step forward in the mechanistic understanding of the action of toxic chemicals and will pave the way for more causal and specific toxicity testing as well as therapeutic options. In this context, the involvement of Transient Receptor Potential (TRP) channels as chemosensors involved in the detection and effectors of toxicant action is an attractive concept intensively discussed in the scientific community. In this review we will summarize recent evidence for an involvement of TRP channels (TRPA1, TRPC4, TRPC6, TRPV1, TRPV4, TRPM2 and TRPM8) expressed in the lung in pathways of toxin sensing and as mediators of lung inflammation and associated diseases like asthma, COPD, lung fibrosis and edema formation. Specific modulators of these channels may offer new therapeutic options in the future and will endorse strategies for a causal, specifically tailored treatment based on the mechanistic understanding of molecular events induced by lung-toxic agents.

STIM1 Phosphorylation at Y361 Recruits Orai1 to STIM1 Puncta and Induces Ca2+ Entry

Store-operated Ca²⁺ entry (SOCE) mediates the increase in intracellular calcium (Ca²⁺) in endothelial cells (ECs) that regulates several EC functions including tissue-fluid homeostasis. Stromal-interaction molecule 1 (STIM1), upon sensing the depletion of (Ca²⁺) from the endoplasmic reticulum (ER) store, organizes as puncta that trigger store-operated Ca²⁺ entry (SOCE) via plasmalemmal Ca²⁺-selective Orai1 channels. While the STIM1 and Orai1 binding interfaces have been mapped, signaling mechanisms activating STIM1 recruitment of Orai1 and STIM1-Orai1 interaction remains enigmatic. Here, we show that ER Ca²⁺-store depletion rapidly induces STIM1 phosphorylation at Y361 via proline-rich kinase 2 (Pyk2) in ECs. Surprisingly, the phospho-defective STIM1-Y361F mutant formed puncta but failed to recruit Orai1, thereby preventing. SOCE Furthermore, studies in mouse lungs, expression of phosphodefective STIM1-Y361F mutant in ECs prevented the increase in vascular permeability induced by the thrombin receptor, protease activated receptor 1 (PAR1). Hence, Pyk2-dependent phosphorylation of STIM1 at Y361 is a critical phospho-switch enabling recruitment of Orai1 into STIM1 puncta leading to SOCE. Therefore, Y361 in STIM1 represents a novel target for limiting SOCE-associated vascular leak.

Thrombin Induces Inositol Trisphosphate-Mediated Spatially Extensive Responses in Lung Microvessels

Activation of plasma membrane receptors initiates compartmentalized second messenger signaling. Whether this compartmentalization facilitates the preferential intercellular diffusion of specific second messengers is unclear. Toward this, the receptor-mediated agonist, thrombin, was instilled into microvessels in a restricted region of isolated blood-perfused mouse lungs. Subsequently, the thrombin-induced increase in endothelial F-actin was determined using confocal fluorescence microscopy. Increased F-actin was evident in microvessels directly treated with thrombin and in those located in adjoining thrombin-free regions. This increase was abrogated by inhibiting inositol trisphosphate-mediated calcium release with Xestospongin C (XeC). XeC also inhibited the thrombin-induced increase in the amplitude of endothelial cytosolic Ca²⁺ oscillations. Instillation of thrombin and XeC into adjacent restricted regions increased F-actin in microvessels in the thrombin-treated and adjacent regions but not in those in the XeC-treated region. Thus, inositol trisphosphate, and not calcium, diffused interendothelially to the spatially remote thrombin-free microvessels. Thus, activation of plasma membrane receptors increased the ambit of inflammatory responses via a second messenger different from that used by stimuli that induce cell-wide increases in second messengers. Thrombin however failed to induce the spatially extensive response in microvessels of mice lacking endothelial connexin43, suggesting a role for connexin43 gap junctions. Compartmental second messenger signaling and interendothelial communication define the specific second messenger involved in exacerbating proinflammatory responses to receptor-mediated agonists.

ROCK2 primes the endothelium for vascular hyperpermeability responses by raising baseline junctional tension

Rho kinase mediates the effects of inflammatory permeability factors by increasing actomyosin-generated traction forces on endothelial adherens junctions, resulting in disassembly of intercellular junctions and increased vascular leakage. In vitro, this is accompanied by the Rho kinase-driven formation of prominent radial F-actin fibers, but the in vivo relevance of those F-actin fibers has been debated, suggesting other Rho kinase-mediated events to occur in vascular leak. Here, we delineated the contributions of the highly homologous isoforms of Rho kinase (ROCK1 and ROCK2) to vascular hyperpermeability responses. We show that ROCK2, rather than ROCK1 is the critical Rho kinase for regulation of thrombin receptor-mediated vascular permeability. Novel traction force mapping in endothelial monolayers, however, shows that ROCK2 is not required for the thrombin-induced force enhancements. Rather, ROCK2 is pivotal to baseline junctional tension as a novel mechanism by which Rho kinase primes the endothelium for hyperpermeability responses, independent from subsequent ROCK1-mediated contractile stress-fiber formation during the late phase of the permeability response.

The Role of Thrombin and Cell Contractility in Regulating Clustering and Collective Migration of Corneal Fibroblasts in Different ECM Environments

PURPOSE

We previously reported that extracellular matrix composition (fibrin versus collagen) modulates the pattern of corneal fibroblast spreading and migration in 3-D culture. In this study, we investigate the role of thrombin and cell contractility in mediating these differences in cell behavior.

METHODS

To assess cell spreading, corneal fibroblasts were plated on top of fibrillar collagen and fibrin matrices. To assess 3-dimensional cell migration, compacted collagen matrices seeded with corneal fibroblasts were embedded inside acellular collagen or fibrin matrices. Constructs were cultured in serum-free media containing platelet-derived growth factor (PDGF), with or without thrombin, the Rho kinase inhibitor Y-27632, and/or the myosin II inhibitor blebbistatin. We used 3-dimensional and 4-dimensional imaging to assess cell mechanical behavior, connectivity and cytoskeletal organization.

RESULTS

Thrombin stimulated increased contractility of corneal fibroblasts. Thrombin also induced Rho kinase-dependent clustering of cells plated on top of compliant collagen matrices, but not on rigid substrates. In contrast, cells on fibrin matrices coalesced into clusters even when Rho kinase was inhibited. In nested matrices, cells always migrated independently through collagen, even in the presence of thrombin. In contrast, cells migrating into fibrin formed an interconnected network. Both Y-27632 and blebbistatin reduced the migration rate in fibrin, but cells continued to migrate collectively.

CONCLUSIONS

The results suggest that while thrombin-induced actomyosin contraction can induce clustering of fibroblasts plated on top of compliant collagen matrices, it does not induce collective cell migration inside 3-D collagen constructs. Furthermore, increased contractility is not required for clustering or collective migration of corneal fibroblasts interacting with fibin.

Integration of signaling and cytoskeletal remodeling by Nck in directional cell migration

Planar and apical-basal cellular polarization of epithelia and endothelia are crucial during morphogenesis. The establishment of these distinct polarity states and their transitions are regulated by signaling networks that include polarity complexes, Rho GTPases, and phosphoinositides. The spatiotemporal coordination of signaling by these molecules modulates cytoskeletal remodeling and vesicle trafficking to specify membrane domains, a prerequisite for the organization of tissues and organs. Here we present an overview of how activation of the WASp/Arp2/3 pathway of actin remodeling by Nck coordinates directional cell migration and speculate on its role as a signaling integrator in the coordination of cellular processes involved in endothelial cell polarity and vascular lumen formation.