Rab GTPases regulate endothelial cell protein C receptor-mediated endocytosis and trafficking of factor VIIa

Endothelial Protein C Receptor and Its Impact on Rheumatic Disease

Endothelial Protein C Receptor (EPCR) is a key regulator of the activated protein C anti-coagulation pathway due to its role in the binding and activation of this protein. EPCR also binds to other ligands such as Factor VII and X, γδ T-cells, plasmodium falciparum erythrocyte membrane protein 1, and Secretory group V Phospholipases A2, facilitating ligand-specific functions. The functions of EPCR can also be regulated by soluble (s)EPCR that competes for the binding sites of membrane-bound (m)EPCR. sEPCR is created when mEPCR is shed from the cell surface. The propensity of shedding alters depending on the genetic haplotype of the EPCR gene that an individual may possess. EPCR plays an active role in normal homeostasis, anti-coagulation pathways, inflammation, and cell stemness. Due to these properties, EPCR is considered a potential effector/mediator of inflammatory diseases. Rheumatic diseases such as rheumatoid arthritis and systemic lupus erythematosus are autoimmune/inflammatory conditions that are associated with elevated EPCR levels and disease activity, potentially driven by EPCR. This review highlights the functions of EPCR and its contribution to rheumatic diseases.

Alterations to Sphingomyelin Metabolism Affect Hemostasis and Thrombosis

BACKGROUND

Our recent studies suggest that sphingomyelin levels in the plasma membrane influence TF (tissue factor) procoagulant activity. The current study was performed to investigate how alterations to sphingomyelin metabolic pathway would affect TF procoagulant activity and thereby affect hemostatic and thrombotic processes.

METHODS

Macrophages and endothelial cells were transfected with specific siRNAs or infected with adenoviral vectors to alter sphingomyelin levels in the membrane. TF activity was measured in factor X activation assay. Saphenous vein incision-induced bleeding and the inferior vena cava ligation-induced flow restriction mouse models were used to evaluate hemostasis and thrombosis, respectively.

RESULTS

Overexpression of SMS (sphingomyelin synthase) 1 or SMS2 in human monocyte-derived macrophages suppresses ATP-stimulated TF procoagulant activity, whereas silencing SMS1 or SMS2 increases the basal cell surface TF activity to the same level as of ATP-decrypted TF activity. Consistent with the concept that sphingomyelin metabolism influences TF procoagulant activity, silencing of acid sphingomyelinase or neutral sphingomyelinase 2 or 3 attenuates ATP-induced enhanced TF procoagulant activity in macrophages and endothelial cells. Niemann-Pick disease fibroblasts with a higher concentration of sphingomyelin exhibited lower TF activity compared with wild-type fibroblasts. In vivo studies revealed that LPS+ATP-induced TF activity and thrombin generation were attenuated in ASMase^-/- mice, while their levels were increased in SMS2^-/- mice. Further studies revealed that acid sphingomyelinase deficiency leads to impaired hemostasis, whereas SMS2 deficiency increases thrombotic risk.

CONCLUSIONS

Overall, our data indicate that alterations in sphingomyelin metabolism would influence TF procoagulant activity and affect hemostatic and thrombotic processes.

Rab11 and Its Role in Neurodegenerative Diseases

Vesicles mediate the trafficking of membranes/proteins in the endocytic and secretory pathways. These pathways are regulated by small GTPases of the Rab family. Rab proteins belong to the Ras superfamily of GTPases, which are significantly involved in various intracellular trafficking and signaling processes in the nervous system. Rab11 is known to play a key role especially in recycling many proteins, including receptors important for signal transduction and preservation of functional activities of nerve cells. Rab11 activity is controlled by GEFs (guanine exchange factors) and GAPs (GTPase activating proteins), which regulate its function through modulating GTP/GDP exchange and the intrinsic GTPase activity, respectively. Rab11 is involved in the transport of several growth factor molecules important for the development and repair of neurons. Overexpression of Rab11 has been shown to significantly enhance vesicle trafficking. On the other hand, a reduced expression of Rab11 was observed in several neurodegenerative diseases. Current evidence appears to support the notion that Rab11 and its cognate proteins may be potential targets for therapeutic intervention. In this review, we briefly discuss the function of Rab11 and its related interaction partners in intracellular pathways that may be involved in neurodegenerative processes.

Large extracellular vesicles do not mitigate the harmful effect of hyperglycemia on endothelial cell mobility

Extracellular vesicles, especially the larger fraction (LEVs - large extracellular vesicles), are believed to be an important means of intercellular communication. Earlier studies on LEVs have shown their healing properties, especially in the vascular cells of diabetic patients. Uptake of LEVs by endothelial cells and internalization of their cargo have also been demonstrated. Endothelial cells change their properties under hyperglycemic conditions (HGC), which reduces their activity and is the cause of endothelial dysfunction. The aim of our study was to investigate how human umbilical vein endothelial cells (HUVECs) change their biological properties: shape, mobility, cell surface stiffness, as well as describe the activation of metabolic pathways after exposure to the harmful effects of HGC and the administration of LEVs released by endothelial cells. We obtained LEVs from HUVEC cultures in HGC and normoglycemia (NGC) using the filtration and ultracentrifugation methods. We assessed the size of LEVs and the presence of biomarkers such as phosphatidylserine, CD63, beta-actin and HSP70. We analyzed the LEVs uptake efficiency by HUVECs, HUVEC shape, actin cytoskeleton remodeling, surface stiffness and finally gene expression by mRNA analysis. Under HGC conditions, HUVECs were larger and had a stiffened surface and a strengthened actin cortex compared to cells under NGC condition. HGC also altered the activation of metabolic pathways, especially those related to intracellular transport, metabolism, and organization of cellular components. The most interesting observation in our study is that LEVs did not restore cell motility disturbed by HGC. Although, LEVs were not able to reverse this deleterious effect of HGC, they activated transcription of genes involved in protein synthesis and vesicle trafficking in HUVECs.

Overcoming the blood-brain barrier by using a multistage exosome delivery system to inhibit central nervous system lymphoma

Due to the presence of blood-brain barrier (BBB), various chemotherapy drugs against B-cell lymphoma cannot be effectively transmitted into the brain, leading to poor prognosis of primary central nervous system lymphoma (PCNSL). Exosomes can cross the BBB as a bio- and immune-compatible drug carrier. In this study, we developed a novel drug delivery system, in which the exosomes (Exo) are conjugated with anti-CD22 monoclonal antibody fragments (CD22-F(ab')₂) and encapsulate doxorubicin (DOX) to form CD22-F(ab')₂-Exo-DOX. We showed that CD22-F(ab')₂-Exo-DOX can cross BBB and deliver DOX precisely to tumor cells. The average apoptosis rate of lymphoma cells was 84.60% ± 10.69%. The tumor-bearing mice treated with CD22-F(ab')₂-Exo-DOX have significantly prolonged life expectancy and the enhanced anti-tumor activity. CD22-F(ab')₂-Exo-DOX might be ingested by brain microvascular endothelial cells through endocytosis to cross the BBB. Therefore, targeted chemotherapy mediated by CD22-F(ab')₂-Exo-DOX is a promising option for the treatment of PCNSL.

Respiratory syncytial virus activates Rab5a to suppress IRF1-dependent IFN-λ production, subverting the antiviral defense of airway epithelial cells

The limited antiviral options and lack of an effective vaccine against human respiratory syncytial virus (RSV) highlight the need for a novel antiviral therapy. One alternative is to identify and target the host factors required for viral infection. Here, using RNA interference to knock down Rab proteins, we provide multiple lines of evidence that Rab5a is required for RSV infection: (a) Rab5a is upregulated both in RSV-A2-infected A549 cells and RSV-A2-challenged BALB/c mice's airway epithelial cells at early infection phase; (b) shRNA-mediated knockdown of Rab5a is associated with reduced lung pathology in RSV A2 challenged mice; (c) Rab5a expression is correlated with disease severity of RSV infection of infants. Knockdown of Rab5a increases IFN-λ (lambda) production by mediating IRF1 nuclear translocation. Our results highlight a new role for Rab5a in RSV infection, such that its depletion inhibits RSV infection by stimulating the endogenous respiratory epithelial antiviral immunity, which suggests that Rab5a is a potential target for novel therapeutics against RSV infection.Importance This study highlights the important role of Rab5a in RSV infection, such that its depletion inhibits RSV infection by stimulating the endogenous respiratory epithelial antiviral immunity and attenuates inflammation of the airway, which suggests that Rab5a is a powerful potential target for novel therapeutics against RSV infection.

Therapeutic doses of recombinant factor VIIa in hemophilia generates thrombin in platelet-dependent and -independent mechanisms

BACKGROUND

In hemophilia bypass therapy, a platelet-dependent mechanism is believed to be primarily responsible for recombinant factor VIIa (rFVIIa)'s hemostatic effect. rFVIIa may also possibly interact with other cells through its binding to endothelial cell protein C receptor (EPCR) or cell surface phospholipids.

OBJECTIVES

We aim to investigate the relative contribution of platelet-dependent and platelet-independent mechanisms in rFVIIa-mediated thrombin generation in hemophilic conditions at the injury site.

METHODS

Platelets were depleted in acquired and genetic hemophilia mice using anti-platelet antibodies. The mice were subjected to the saphenous vein injury, and the hemostatic effect of pharmacological concentrations of rFVIIa was evaluated by measuring thrombin generation at the injury site.

RESULTS

Administration of anti-mouse CD42 antibodies to mice depleted platelets by more than 95%. As expected, hemophilia mice, compared with wild-type mice, generated only a small fraction of thrombin at the injury site. The depletion of platelets in hemophilia mice further reduced thrombin generation. However, when pharmacological doses of rFVIIa were administered to hemophilia mice, substantial amounts of thrombin were generated even in the platelet-depleted hemophilia mice. No differences in thrombin generation were detected among FVIII-/- , EPCR-deficient FVIII-/- , and EPCR-overexpressing FVIII-/- mice depleted of platelets or not. Evaluation of platelets by flow cytometry as well as immunoblot analysis showed no detectable expression of EPCR.

CONCLUSIONS

Our data suggest that pharmacological concentrations of rFVIIa generate thrombin in hemophilia in both platelet-dependent and platelet-independent mechanisms.

Cell targeting strategy affects the intracellular trafficking of liposomes altering loaded doxorubicin release kinetics and efficacy in endothelial cells

Targeting nanocarrier drug delivery systems, that deliver drug payloads to the site of disease action, are frequently viewed as the future of nanocarrier based therapies but have struggled to breakthrough to the clinic in comparison to non-targeting counterparts. Using unilamellar liposomes as model nanocarriers, we show that cell targeting strategy (electrostatic, ligand and antigen) influences both the intracellular fate of the liposomes and the corresponding efficacy of the loaded drug, doxorubicin, in endothelial cells. We show that increased liposome uptake by cells does not translate to improved efficacy in this scenario but that liposome intracellular trafficking, particularly distribution between recycling endosomes and lysosomes, influences in vitro efficacy. Choosing targeting strategies that promote desired nanocarrier intracellular trafficking may be a viable strategy to enhance the in vivo efficacy of drug delivery systems.

Development of a Neuroprotective Erythropoietin Modified with a Novel Carrier for the Blood-Brain Barrier

Extremely high doses of erythropoietin (EPO) has been used for neuroprotection in ischemia-reperfusion brain injury to deliver sufficient amounts of EPO across the blood-brain barrier (BBB); however, harmful outcomes were observed afterward. We aimed to test the ability of HBHAc (heparin-binding haemagglutinin adhesion c), an intracellular delivery peptide for macromolecules, as an EPO carrier across the BBB. The cellular internalization and transcytosis ability of HBHAc-modified EPO (EPO-HBHAc) were evaluated in bEnd.3 cells and in the bEnd.3/CTX TNA2 co-culture BBB model, respectively. Subsequently, the NMDA-induced-toxicity model and ischemia-reperfusion rat model were used to understand the neuronal protective activity of EPO-HBHAc. The biodistribution of EPO-HBHAc was demonstrated in rats by the quantification of EPO-HBHAc in the brain, plasma, and organs by ELISA. Our results demonstrate that EPO-HBHAc exhibited significantly higher cellular internalization in dose- and time-dependent manners and better transcytosis ability than EPO. In addition, the transported EPO-HBHAc in the co-culture transwell system maintained the neuronal protective activity when primary rat cortical neurons underwent NMDA-induced toxicity. The calculated cerebral infarction area of rats treated with EPO-HBHAc was significantly reduced compared to that of rats treated with EPO (29.9 ± 7.0% vs 48.9 ± 7.9%) 24 h after occlusion in 3VO rat experiments. Moreover, the EPO amount in both CSF and damaged cortex from the EPO-HBHAc group was 4.0-fold and 3.0-fold higher than the EPO group, respectively. These results suggest that HBHAc would be a favorable tool for EPO brain delivery and would further extend the clinical applications of EPO in neuroprotection.

Tumor-Derived Extracellular Vesicles Breach the Intact Blood-Brain Barrier via Transcytosis

The restrictive nature of the blood-brain barrier (BBB) creates a major challenge for brain drug delivery with current nanomedicines lacking the ability to cross the BBB. Extracellular vesicles (EVs) have been shown to contribute to the progression of a variety of brain diseases including metastatic brain cancer and have been suggested as promising therapeutics and drug delivery vehicles. However, the ability of native tumor-derived EVs to breach the BBB and the mechanism(s) involved in this process remain unknown. Here, we demonstrate that tumor-derived EVs can breach the intact BBB in vivo, and by using state-of-the-art in vitro and in vivo models of the BBB, we have identified transcytosis as the mechanism underlying this process. Moreover, high spatiotemporal resolution microscopy demonstrated that the endothelial recycling endocytic pathway is involved in this transcellular transport. We further identify and characterize the mechanism by which tumor-derived EVs circumvent the low physiologic rate of transcytosis in the BBB by decreasing the brain endothelial expression of rab7 and increasing the efficiency of their transport. These findings identify previously unknown mechanisms by which tumor-derived EVs breach an intact BBB during the course of brain metastasis and can be leveraged to guide and inform the development of drug delivery approaches to deliver therapeutic cargoes across the BBB for treatment of a variety of brain diseases including, but not limited to, brain malignancies.

Factor VIIa induces anti-inflammatory signaling via EPCR and PAR1

Recent studies show that endothelial cell protein C receptor (EPCR) interacts with diverse ligands, in addition to its known ligands protein C and activated protein C (APC). We showed in earlier studies that procoagulant clotting factor VIIa (FVIIa) binds EPCR and downregulates EPCR-mediated anticoagulation and induces an endothelial barrier protective effect. Here, we investigated the effect of FVIIa's interaction with EPCR on endothelial cell inflammation and lipopolysaccharide (LPS)-induced inflammatory responses in vivo. Treatment of endothelial cells with FVIIa suppressed tumor necrosis factor α (TNF-α)- and LPS-induced expression of cellular adhesion molecules and adherence of monocytes to endothelial cells. Inhibition of EPCR or protease-activated receptor 1 (PAR1) by either specific antibodies or small interfering RNA abolished the FVIIa-induced suppression of TNF-α- and LPS-induced expression of cellular adhesion molecules and interleukin-6. β-Arrestin-1 silencing blocked the FVIIa-induced anti-inflammatory effect in endothelial cells. In vivo studies showed that FVIIa treatment markedly suppressed LPS-induced inflammatory cytokines and infiltration of innate immune cells into the lung in wild-type and EPCR-overexpressing mice, but not in EPCR-deficient mice. Mechanistic studies revealed that FVIIa treatment inhibited TNF-α-induced ERK1/2, p38 MAPK, JNK, NF-κB, and C-Jun activation indicating that FVIIa-mediated signaling blocks an upstream signaling event in TNFα-induced signaling cascade. FVIIa treatment impaired the recruitment of TNF-receptor-associated factor 2 into the TNF receptor 1 signaling complex. Overall, our present data provide convincing evidence that FVIIa binding to EPCR elicits anti-inflammatory signaling via a PAR1- and β-arrestin-1 dependent pathway. The present study suggests new therapeutic potentials for FVIIa, which is currently in clinical use for treating bleeding disorders.

The role of small GTPases and EPAC-Rap signaling in the regulation of the blood-brain and blood-retinal barriers

Maintenance and regulation of the vascular endothelial cell junctional complex is critical for proper barrier function of the blood-brain barrier (BBB) and the highly related blood-retinal barrier (BRB) that help maintain proper neuronal environment. Recent research has demonstrated that the junctional complex is actively maintained and can be dynamically regulated. Studies focusing on the mechanisms of barrier formation, maintenance, and barrier disruption have been of interest to understanding development of the BBB and BRB and identifying a means for therapeutic intervention for diseases ranging from brain tumors and dementia to blinding eye diseases. Research has increasingly revealed that small GTPases play a critical role in both barrier formation and disruption mechanisms. This review will summarize the current data on small GTPases in barrier regulation with an emphasis on the EPAC-Rap1 signaling pathway to Rho in endothelial barriers, as well as explore its potential involvement in paracellular flux and transcytosis regulation.

Kinetic analysis and binding studies of a new recombinant human factor VIIa for treatment of haemophilia

INTRODUCTION/AIM

LR769 is a new second-generation recombinant human Factor VIIa (rhFVIIa) developed for haemophilia treatment. We determined enzymatic properties of LR769 and its interaction with antithrombin, tissue factor, platelets and endothelial protein C receptor (EPCR), compared with NovoSevenRT.

METHODS

Kinetic enzyme assays and active site titration were used for enzymatic studies. Surface Plasmon Resonance (SPR) was used for determination of binding constants. Cellular binding was determined for platelets and cultured human umbilical vein endothelial cells (HUVEC).

RESULTS

The dissociation constant (K_d ) for activated platelet binding was in the 1 μm range for both products. At saturation, more LR769 than NovoSevenRT was bound to the platelets. Binding to HUVEC was 25-50% higher for LR769 than for NovoSevenRT. Protein C, soluble EPCR, and anti-EPCR antibody all reduced the binding, indicating specificity for EPCR. LR769 was similar to NovoSevenRT in all kinetic assays. Active site titration demonstrated 0.7 mole of active site/mole of protein. The k_cat /K_m values for activation of FX and FIX with purified recombinant tissue factor and phospholipids were 10.5 s^-1 /0.32 μm and 3.3 s^-1 /0.44 μm respectively. The apparent second-order rate constant for inactivation by human plasma AT was 5.9 ± 0.4 × 10³ m^-1 s^-1 . The K_d values for binding of LR769 to soluble tissue factor and full-length tissue factor were 8.1 nm and 0.9 nm, respectively, and the K_d for binding to soluble EPCR was 41 nm.

CONCLUSION

Overall, LR769 exhibited characteristics similar to NovoSevenRT, but bound EPCR on HUVEC with somewhat higher affinity than NovoSevenRT.

Regulation of β-adrenergic receptor trafficking and lung microvascular endothelial cell permeability by Rab5 GTPase

Rab5 GTPase modulates the trafficking of the cell surface receptors, including G protein-coupled β-adrenergic receptors (β-ARs). Here, we have determined the role of Rab5 in regulating the internalization of β-ARs in lung microvascular endothelial cells (LMECs) and in maintaining the integrity and permeability of endothelial cell barrier. Our data demonstrate that lipopolysaccharide (LPS) treatment disrupts LMEC barrier function and reduces the cell surface expression of β-ARs. Furthermore, the activation of β-ARs, particularly β2-AR, is able to protect the LMEC permeability from LPS injury. Moreover, siRNA-mediated knockdown of Rab5 inhibits both the basal and agonist-provoked internalization of β-ARs, therefore, enhancing the cell surface expression of the receptors and receptor-mediated ERK1/2 activation. Importantly, knockdown of Rab5 not only inhibits the LPS-induced effects on β-ARs but also protects the LMEC monolayer permeability. All together, these data provide strong evidence indicating a crucial role of Rab5-mediated internalization of β-ARs in functional regulation of LMECs.

Histone-targeted Polyplexes Avoid Endosomal Escape and Enter the Nucleus During Postmitotic Redistribution of ER Membranes

Nonviral gene delivery is a promising therapeutic approach because of its safety and controllability, yet limited gene transfer efficacy is a common issue. Most nonviral strategies rely upon endosomal escape designs; however, endosomal escape is often uncorrelated with improved gene transfer and membranolytic structures are typically cytotoxic. Previously, we showed that histone-targeted polyplexes trafficked to the nucleus through an alternative route involving caveolae and the Golgi and endoplasmic reticulum (ER), using pathways similar to several pathogens. We hypothesized that the efficacy of these polyplexes was due to an increased utilization of native vesicular trafficking as well as regulation by histone effectors. Accordingly, using confocal microscopy and cellular fractionation, we determined that a key effect of histone-targeting was to route polyplexes away from clathrin-mediated recycling pathways by harnessing endomembrane transfer routes regulated by histone methyltransferases. An unprecedented finding was that polyplexes accumulated in Rab6-labeled Golgi/ER vesicles and ultimately shuttled directly into the nucleus during ER-mediated nuclear envelope reassembly. Specifically, super resolution microscopy and fluorescence correlation spectroscopy unequivocally indicated that the polyplexes remained associated with ER vesicles/membranes until mitosis, when they were redistributed into the nucleus. These novel findings highlight alternative mechanisms to subvert endolysosomal trafficking and harness the ER to enhance gene transfer.

Endothelial cell protein C receptor: a multiliganded and multifunctional receptor

Endothelial cell protein C receptor (EPCR) was first identified and isolated as a cellular receptor for protein C on endothelial cells. EPCR plays a crucial role in the protein C anticoagulant pathway by promoting protein C activation. In the last decade, EPCR has received wide attention after it was discovered to play a key role in mediating activated protein C (APC)-induced cytoprotective effects, including antiapoptotic, anti-inflammatory, and barrier stabilization. APC elicits cytoprotective signaling through activation of protease activated receptor-1 (PAR1). Understanding how EPCR-APC induces cytoprotective effects through activation of PAR1, whose activation by thrombin is known to induce a proinflammatory response, has become a major research focus in the field. Recent studies also discovered additional ligands for EPCR, which include factor VIIa, Plasmodium falciparum erythrocyte membrane protein, and a specific variant of the T-cell receptor. These observations open unsuspected new roles for EPCR in hemostasis, malaria pathogenesis, innate immunity, and cancer. Future research on these new discoveries will undoubtedly expand our understanding of the role of EPCR in normal physiology and disease, as well as provide novel insights into mechanisms for EPCR multifunctionality. Comprehensive understanding of EPCR may lead to development of novel therapeutic modalities in treating hemophilia, inflammation, cerebral malaria, and cancer.