Altered vascular permeability and early onset of experimental autoimmune encephalomyelitis in PECAM-1-deficient mice

CD31 signals confer immune privilege to the vascular endothelium

Constitutive resistance to cell death induced by inflammatory stimuli activating the extrinsic pathway of apoptosis is a key feature of vascular endothelial cells (ECs). Although this property is central to the maintenance of the endothelial barrier during inflammation, the molecular mechanisms of EC protection from cell-extrinsic, proapoptotic stimuli have not been investigated. We show that the Ig-family member CD31, which is expressed by endothelial but not epithelial cells, is necessary to prevent EC death induced by TNF-α and cytotoxic T lymphocytes in vitro. Combined quantitative RT-PCR array and biochemical analysis show that, upon the engagement of the TNF receptor with TNF-α on ECs, CD31 becomes activated and, in turn, counteracts the proapoptotic transcriptional program induced by TNF-α via activation of the Erk/Akt pathway. Specifically, Akt activation by CD31 signals prevents the localization of the forkhead transcription factor FoxO3 to the nucleus, thus inhibiting transcription of the proapoptotic genes CD95/Fas and caspase 7 and de-repressing the expression of the antiapoptotic gene cFlar. Both CD31 intracellular immunoreceptor tyrosine-based inhibition motifs are required for its prosurvival function. In vivo, CD31 gene transfer is sufficient to recapitulate the cytoprotective mechanisms in CD31(-) pancreatic β cells, which become resistant to immune-mediated rejection when grafted in fully allogeneic recipients.

Brain barriers: Crosstalk between complex tight junctions and adherens junctions

Unique intercellular junctional complexes between the central nervous system (CNS) microvascular endothelial cells and the choroid plexus epithelial cells form the endothelial blood–brain barrier (BBB) and the epithelial blood–cerebrospinal fluid barrier (BCSFB), respectively. These barriers inhibit paracellular diffusion, thereby protecting the CNS from fluctuations in the blood. Studies of brain barrier integrity during development, normal physiology, and disease have focused on BBB and BCSFB tight junctions but not the corresponding endothelial and epithelial adherens junctions. The crosstalk between adherens junctions and tight junctions in maintaining barrier integrity is an understudied area that may represent a promising target for influencing brain barrier function.

PECAM-1 isoforms, eNOS and endoglin axis in regulation of angiogenesis

Vascular development and maintenance of proper vascular function through various regulatory mechanisms are critical to our wellbeing. Delineation of the regulatory processes involved in development of the vascular system and its function is one of the most important topics in human physiology and pathophysiology. Platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31), a cell adhesion molecule with proangiogenic and proinflammatory activity, has been the subject of numerous studies. In the present review, we look at the important roles that PECAM-1 and its isoforms play during angiogenesis, and its molecular mechanisms of action in the endothelium. In the endothelium, PECAM-1 not only plays a role as an adhesion molecule but also participates in intracellular signalling pathways which have an impact on various cell adhesive mechanisms and endothelial nitric oxide synthase (eNOS) expression and activity. In addition, recent studies from our laboratory have revealed an important relationship between PECAM-1 and endoglin expression. Endoglin is an essential molecule during angiogenesis, vascular development and integrity, and its expression and activity are compromised in the absence of PECAM-1. In the present review we discuss the roles that PECAM-1 isoforms may play in modulation of endothelial cell adhesive mechanisms, eNOS and endoglin expression and activity, and angiogenesis.

Role of PECAM-1 in radiation-induced liver inflammation

Platelet endothelial cell adhesion molecule-1 (PECAM-1, CD31) is known to play an important role in hepatic inflammation. Therefore, we investigated the role of PECAM-1 in wild-type (WT) and knock-out (KO)-mice after single-dose liver irradiation (25 Gy). Both, at mRNA and protein level, a time-dependent decrease in hepatic PECAM-1, corresponding to an increase in intercellular cell adhesion molecule-1 (ICAM-1) (6 hrs) was detected in WT-mice after irradiation. Immunohistologically, an increased number of neutrophil granulocytes (NG) (but not of mononuclear phagocytes) was observed in the liver of WT and PECAM-1-KO mice at 6 hrs after irradiation. The number of recruited NG was higher and prolonged until 24 hrs in KO compared to WT-mice. Correspondingly, a significant induction of hepatic tumour necrosis factor (TNF)-α and CXC-chemokines (KC/CXCL1 interleukin-8/CXCL8) was detected together with an elevation of serum liver transaminases (6–24 hrs) in WT and KO-mice. Likewise, phosphorylation of signal transducer and activator of transcription-3 (STAT-3) was observed in both animal groups after irradiation. The level of all investigated proteins as well as of the liver transaminases was significantly higher in KO than WT-mice. In the cell-line U937, irradiation led to a reduction in PECAM-1 in parallel to an increased ICAM-1 expression. TNF-α-blockage by anti-TNF-α prevented this change in both proteins in cell culture. Radiation-induced stress conditions induce a transient accumulation of granulocytes within the liver by down-regulation/absence of PECAM-1. It suggests that reduction/lack in PECAM-1 may lead to greater and prolonged inflammation which can be prevented by anti-TNFα.

CXCR7 expression disrupts endothelial cell homeostasis and causes ligand-dependent invasion

The homeostatic function of endothelial cells (EC) is critical for a number of physiological processes including vascular integrity, immunity, and wound healing. Indeed, vascular abnormalities resulting from EC dysfunction contribute to the development and spread of malignancies. The alternative SDF-1/CXCL12 receptor CXCR7 is frequently and specifically highly expressed in tumor-associated vessels. In this study, we investigate whether CXCR7 contributes to vascular dysfunction by specifically examining the effect of CXCR7 expression on EC barrier function and motility. We demonstrate that CXCR7 expression in EC results in redistribution of CD31/PECAM-1 and loss of contact inhibition. Moreover, CXCR7+ EC are deficient in barrier formation. We show that CXCR7-mediated motility has no influence on angiogenesis but contributes to another motile process, the invasion of CXCR7+ EC into ligand-rich niches. These results identify CXCR7 as a novel manipulator of EC barrier function via alteration of PECAM-1 homophilic junctions. As such, aberrant expression of CXCR7 in the vasculature has the potential to disrupt vascular homeostasis and could contribute to vascular dysfunction in cancer systems.

Control of vascular permeability by adhesion molecules

Vascular permeability is a vital function of the circulatory system that is regulated in large part by the limited flux of solutes, water, and cells through the endothelial cell layer. One major pathway through this barrier is via the inter-endothelial junction, which is driven by the regulation of cadherin-based adhesions. The endothelium also forms attachments with surrounding proteins and cells via 2 classes of adhesion molecules, the integrins and IgCAMs. Integrins and IgCAMs propagate activation of multiple downstream signals that potentially impact cadherin adhesion. Here we discuss the known contributions of integrin and IgCAM signaling to the regulation of cadherin adhesion stability, endothelial barrier function, and vascular permeability. Emphasis is placed on known and prospective crosstalk signaling mechanisms between integrins, the IgCAMs- ICAM-1 and PECAM-1, and inter-endothelial cadherin adhesions, as potential strategic signaling nodes for multipartite regulation of cadherin adhesion.

Astrocytic TYMP and VEGFA drive blood-brain barrier opening in inflammatory central nervous system lesions

In inflammatory central nervous system conditions such as multiple sclerosis, breakdown of the blood-brain barrier is a key event in lesion pathogenesis, predisposing to oedema, excitotoxicity, and ingress of plasma proteins and inflammatory cells. Recently, we showed that reactive astrocytes drive blood-brain barrier opening, via production of vascular endothelial growth factor A (VEGFA). Here, we now identify thymidine phosphorylase (TYMP; previously known as endothelial cell growth factor 1, ECGF1) as a second key astrocyte-derived permeability factor, which interacts with VEGFA to induce blood-brain barrier disruption. The two are co-induced NFκB1-dependently in human astrocytes by the cytokine interleukin 1 beta (IL1B), and inactivation of Vegfa in vivo potentiates TYMP induction. In human central nervous system microvascular endothelial cells, VEGFA and the TYMP product 2-deoxy-d-ribose cooperatively repress tight junction proteins, driving permeability. Notably, this response represents part of a wider pattern of endothelial plasticity: 2-deoxy-d-ribose and VEGFA produce transcriptional programs encompassing angiogenic and permeability genes, and together regulate a third unique cohort. Functionally, each promotes proliferation and viability, and they cooperatively drive motility and angiogenesis. Importantly, introduction of either into mouse cortex promotes blood-brain barrier breakdown, and together they induce severe barrier disruption. In the multiple sclerosis model experimental autoimmune encephalitis, TYMP and VEGFA co-localize to reactive astrocytes, and correlate with blood-brain barrier permeability. Critically, blockade of either reduces neurologic deficit, blood-brain barrier disruption and pathology, and inhibiting both in combination enhances tissue preservation. Suggesting importance in human disease, TYMP and VEGFA both localize to reactive astrocytes in multiple sclerosis lesion samples. Collectively, these data identify TYMP as an astrocyte-derived permeability factor, and suggest TYMP and VEGFA together promote blood-brain barrier breakdown.

Tau depletion prevents progressive blood-brain barrier damage in a mouse model of tauopathy

Introduction

The blood-brain barrier (BBB) is damaged in tauopathies, including progressive supranuclear palsy (PSP) and Alzheimer’s disease (AD), which is thought to contribute to pathogenesis later in the disease course. In AD, BBB dysfunction has been associated with amyloid beta (Aß) pathology, but the role of tau in this process is not well characterized. Since increased BBB permeability is found in tauopathies without Aß pathology, like PSP, we suspected that tau accumulation alone could not only be sufficient, but even more important than Aß for BBB damage.

Results

Longitudinal evaluation of brain tissue from the tetracycline-regulatable rTg4510 tau transgenic mouse model showed progressive IgG, T cell and red blood cell infiltration. The Evans blue (EB) dye that is excluded from the brain when the BBB is intact also permeated the brains of rTg4510 mice following peripheral administration, indicative of a bonafide BBB defect, but this was only evident later in life. Thus, despite the marked brain atrophy and inflammation that occurs earlier in this model, BBB integrity is maintained. Interestingly, BBB dysfunction emerged at the same time that perivascular tau emerged around major hippocampal blood vessels. However, when tau expression was suppressed using doxycycline, BBB integrity was preserved, suggesting that the BBB can be stabilized in a tauopathic brain by reducing tau levels.

Conclusions

For the first time, these data demonstrate that tau alone can initiate breakdown of the BBB, but the BBB is remarkably resilient, maintaining its integrity in the face of marked brain atrophy, neuroinflammation and toxic tau accumulation. Moreover, the BBB can recover integrity when tau levels are reduced. Thus, late stage interventions targeting tau may slow the vascular contributions to cognitive impairment and dementia that occur in tauopathies.

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-015-0186-2) contains supplementary material, which is available to authorized users.

Platelet endothelial cell adhesion molecule-1 mediates endothelial-cardiomyocyte communication and regulates cardiac function

Background

Dilated cardiomyopathy is characterized by impaired contractility of cardiomyocytes, ventricular chamber dilatation, and systolic dysfunction. Although mutations in genes expressed in the cardiomyocyte are the best described causes of reduced contractility, the importance of endothelial‐cardiomyocyte communication for proper cardiac function is increasingly appreciated. In the present study, we investigate the role of the endothelial adhesion molecule platelet endothelial cell adhesion molecule (PECAM‐1) in the regulation of cardiac function.

Methods and Results

Using cell culture and animal models, we show that PECAM‐1 expressed in endothelial cells (ECs) regulates cardiomyocyte contractility and cardiac function via the neuregulin‐ErbB signaling pathway. Conscious echocardiography revealed left ventricular (LV) chamber dilation and systolic dysfunction in PECAM‐1^−/− mice in the absence of histological abnormalities or defects in cardiac capillary density. Despite deficits in global cardiac function, cardiomyocytes isolated from PECAM‐1^−/− hearts displayed normal baseline and isoproterenol‐stimulated contractility. Mechanistically, absence of PECAM‐1 resulted in elevated NO/ROS signaling and NRG‐1 release from ECs, which resulted in augmented phosphorylation of its receptor ErbB2. Treatment of cardiomyocytes with conditioned media from PECAM‐1^−/− ECs resulted in enhanced ErbB2 activation, which was normalized by pre‐treatment with an NRG‐1 blocking antibody. To determine whether normalization of increased NRG‐1 levels could correct cardiac function, PECAM‐1^−/− mice were treated with the NRG‐1 blocking antibody. Echocardiography showed that treatment significantly improved cardiac function of PECAM‐1^−/− mice, as revealed by increased ejection fraction and fractional shortening.

Conclusions

We identify a novel role for PECAM‐1 in regulating cardiac function via a paracrine NRG1‐ErbB pathway. These data highlight the importance of tightly regulated cellular communication for proper cardiac function.

Seminal CD38 is a pivotal regulator for fetomaternal tolerance

A successful pregnancy depends on a complex process that establishes fetomaternal tolerance. Seminal plasma is known to induce maternal immune tolerance to paternal alloantigens, but the seminal factors that regulate maternal immunity have yet to be characterized. Here, we show that a soluble form of CD38 (sCD38) released from seminal vesicles to the seminal plasma plays a crucial role in inducing tolerogenic dendritic cells and CD4(+) forkhead box P3(+) (Foxp3(+)) regulatory T cells (Tregs), thereby enhancing maternal immune tolerance and protecting the semiallogeneic fetus from resorption. The abortion rate in BALB/c females mated with C57BL/6 Cd38(-/-) males was high compared with that in females mated with Cd38(+/+) males, and this was associated with a reduced proportion of Tregs within the CD4(+) T-cell pool. Direct intravaginal injection of sCD38 to CBA/J pregnant mice at preimplantation increased Tregs and pregnancy rates in mice under abortive sonic stress from 48 h after mating until euthanasia. Thus, sCD38 released from seminal vesicles to the seminal plasma acts as an immunoregulatory factor to protect semiallogeneic fetuses from maternal immune responses.

Infection with Porphyromonas gingivalis exacerbates endothelial injury in obese mice

BACKGROUND

A number of studies have revealed a link between chronic periodontitis and cardiovascular disease in obese patients. However, there is little information about the influence of periodontitis-associated bacteria, Porphyromonas gingivalis (Pg), on pathogenesis of atherosclerosis in obesity.

METHODS

In vivo experiment: C57BL/6J mice were fed with a high-fat diet (HFD) or normal chow diet (CD), as a control. Pg was infected from the pulp chamber. At 6 weeks post-infection, histological and immunohistochemical analysis of aortal tissues was performed. In vitro experiment: hTERT-immortalized human umbilical vein endothelial cells (HuhT1) were used to assess the effect of Pg/Pg-LPS on free fatty acid (FFA) induced endothelial cells apoptosis and regulation of cytokine gene expression.

RESULTS

Weaker staining of CD31 and increased numbers of TUNEL positive cells in aortal tissue of HFD mice indicated endothelial injury. Pg infection exacerbated the endothelial injury. Immunohistochemically, Pg was detected deep in the smooth muscle of the aorta, and the number of Pg cells in the aortal wall was higher in HFD mice than in CD mice. Moreover, in vitro, FFA treatment induced apoptosis in HuhT1 cells and exposure to Pg-LPS increased this effect. In addition, Pg and Pg-LPS both attenuated cytokine production in HuhT1 cells stimulated by palmitate.

CONCLUSIONS

Dental infection of Pg may contribute to pathogenesis of atherosclerosis by accelerating FFA-induced endothelial injury.

Mechanotransduction of shear stress occurs through changes in VE-cadherin and PECAM-1 tension: implications for cell migration

Recent work has shown that cadherins at cell-cell junctions bear tensile forces. Using novel FRET-based tension sensors, we showed first that in response to shear stress, endothelial cells rapidly reduce mechanical tension on vascular endothelial (VE)-cadherin. Second, we observed a simultaneous increase in tension on platelet endothelial cell adhesion molecule (PECAM)-1, induced by an interaction with vimentin. In this commentary, we discuss how our results fit with existing data on cadherins as important mediators of mechanotransduction, in particular, in cell migration where mechanical tension across cadherins may communicate the direction of movement. The ability of PECAM-1 to bear mechanical tension may also be important in other PECAM-1 functions, such as leukocyte transmigration through the endothelium. Additionally, our observation that vimentin expression was required for PECAM-1 tension and mechanotransduction of fluid flow suggests that intermediate filaments are capable of transmitting tension. Overall, our results argue against models where an external force is passively transferred across the cytoskeleton, and instead suggest that cells actively respond to extracellular forces by modulating tension across junctional proteins.

Adhesion molecule-mediated hippo pathway modulates hemangioendothelioma cell behavior

Hemangioendotheliomas are categorized as intermediate-grade vascular tumors that are commonly localized in the lungs and livers. The regulation of this tumor cell's proliferative and apoptotic mechanisms is ill defined. We recently documented an important role for Hippo pathway signaling via endothelial cell adhesion molecules in brain microvascular endothelial cell proliferation and apoptosis. We found that endothelial cells lacking cell adhesion molecules escaped from contact inhibition and exhibited abnormal proliferation and apoptosis. Here we report on the roles of adherens junction molecule modulation of survivin and the Hippo pathway in the proliferation and apoptosis of a murine hemangioendothelioma (EOMA) cell. We demonstrated reduced adherens junction molecule (CD31 and VE-cadherin) expression, increased survivin and Ajuba expression, and a reduction in Hippo pathway signaling resulting in increased proliferation and decreased activation of effector caspase 3 in postconfluent EOMA cell cultures. Furthermore, we confirmed that YM155, an antisurvivin drug that interferes with Sp1-survivin promoter interactions, and survivin small interference RNA (siRNA) transfection elicited induction of VE-cadherin, decreased Ajuba expression, increased Hippo pathway and caspase activation and apoptosis, and decreased cell proliferation. These findings support the importance of the Hippo pathway in hemangioendothelioma cell proliferation and survival and YM155 as a potential therapeutic agent in this category of vascular tumors.

Alteration of cell-cell junctions in cultured human lymphatic endothelial cells with inflammatory cytokine stimulation

BACKGROUND

To maintain normal function, the lymphatic endothelium is regulated by cell-cell junctions. There have been few studies of lymphatic endothelial cell junctions using standard cell biological methods. This study had two purposes: to characterize cell junctions in cultured lymphatic endothelial cells and to investigate the effects of the inflammatory cytokine TNF-α on altered cell-cell junctions.

METHODS AND RESULTS

Cultured human dermal lymphatic endothelial cells (HDLEC) were immunostained with the tight junction marker, ZO-1, and adherens junction markers, VE-cadherin and PECAM-1. In TNF-α-treated HDLEC, we evaluated changes in endothelial cell junctions by immunostaining and through the use of transendothelial electrical resistance (TER). Immunofluorescence staining of HDLEC revealed heterogeneity among the endothelial cell junctions, which could be classified into continuous and discontinuous junctions. In these cell junctions, ZO-1 and VE-cadherin were co-localized. Double immunofluorescence staining revealed the broad distribution of VE-cadherin at the cell periphery, where VE-cadherin and PECAM-1 were co-localized. TNF-α treatment decreased TER, caused a predominance in the appearance of discontinuous junctions with a reduction in the broad distribution of VE-cadherin at the cell periphery in HDLEC.

CONCLUSIONS

The results indicate a heterogeneous distribution of cell junctions in HDLEC involving continuous and discontinuous junctions. Our data also suggest that TNF-α alters the normal distribution of cell junctions and affects the endothelial barrier of cultured lymphatic endothelial cells. The broad distribution of VE-cadherin at the cell periphery may reflect the lymphatic permeability.

Regulation of endothelial cell barrier function by antibody-driven affinity modulation of platelet endothelial cell adhesion molecule-1 (PECAM-1)

PECAM-1 is a 130-kDa member of the immunoglobulin (Ig) superfamily that is expressed on the surface of platelets and leukocytes, and at the intracellular junctions of confluent endothelial cell monolayers. Previous studies have shown that PECAM-1/PECAM-1 homophilic interactions play a key role in leukocyte transendothelial migration, in allowing PECAM-1 to serve as a mechanosensory complex in endothelial cells, in its ability to confer cytoprotection to proapoptotic stimuli, and in maintaining endothelial cell junctional integrity. To examine the adhesive properties of full-length PECAM-1 in a native lipid environment, we purified it from platelets and assembled it into phospholipid nanodiscs. PECAM-1-containing nanodiscs retained not only their ability to bind homophilically to PECAM-1-expressing cells, but exhibited regulatable adhesive interactions that could be modulated by ligands that bind membrane- proximal Ig Domain 6. This property was exploited to enhance the rate of barrier restoration in endothelial cell monolayers subjected to inflammatory challenge. The finding that the adhesive properties of PECAM-1 are regulatable suggests novel approaches for controlling endothelial cell migration and barrier function in a variety of vascular permeability disorders.

An EMMPRIN-γ-catenin-Nm23 complex drives ATP production and actomyosin contractility at endothelial junctions

Cell-cell adhesions are important sites through which cells experience and resist forces. In endothelial cells, these forces regulate junction dynamics and determine endothelial barrier strength. We identify the Ig superfamily member EMMPRIN (also known as basigin) as a coordinator of forces at endothelial junctions. EMMPRIN localization at junctions correlates with endothelial junction strength in different mouse vascular beds. Accordingly, EMMPRIN-deficient mice show altered junctions and increased junction permeability. Lack of EMMPRIN alters the localization and function of VE-cadherin (also known as cadherin-5) by decreasing both actomyosin contractility and tugging forces at endothelial cell junctions. EMMPRIN ensures proper actomyosin-driven maturation of competent endothelial junctions by forming a molecular complex with γ-catenin (also known as junction plakoglobin) and Nm23 (also known as NME1), a nucleoside diphosphate kinase, thereby locally providing ATP to fuel the actomyosin machinery. These results provide a novel mechanism for the regulation of actomyosin contractility at endothelial junctions and might have broader implications in biological contexts such as angiogenesis, collective migration and tissue morphogenesis by coupling compartmentalized energy production to junction assembly.

Nucleoside Analog-treated Chronic Hepatitis B Patients showed Reduced Expression of PECAM-1 Gene in Peripheral Blood Mononuclear Cells in Bangladesh

Background and aim

Assessment of therapeutic response is important for monitoring the prognosis and to take decision for cessation of nucleoside analogues therapy in chronic hepatitis B patients. In addition to serum alanine aminotransferase (ALT), hepatitis B virus (HBV) deoxyribonucleic acid (DNA) load and HBeAg status, identification of molecular markers associated with host immune response would be essential to assess therapeutic response. In this regard the current study was performed with the aim to detect expression of platelet endothelial cell adhesion molecule (PECAM)-I gene in peripheral blood monocytes (PBMCs) of treated chronic hepatitis B patients and also to correlate expression of this gene with serum HBV DNA load and serum ALT levels.

Materials and methods

The study analyzed 60 chronic hepatitis B (CHB) patients, including 30 untreated and 30 nucleoside analogs treated and 10 healthy controls. PECAM-1 gene expression/ transcripts were detected by conventional RT-PCR.

Results

The expression PECAM-1 mRNA in the PBMCs of CHB patients was significantly higher in untreated (3.17 ± 0.75) than the treated patients (1.64 ± 0.29) (p < 0.01). Expression of PECAM-1 was positively correlated with serum ALT levels of both untreated (r = 0.580) and treated (r = 0.566) CHB patients. Moreover, in both untreated and treated groups, these gene expressions were positively correlated to serum HBV DNA load with the correlation coefficient r = 0.545 and r = 0.591 respectively.

Conclusion

PECAM-1 may be used as a biomarker for assessment of inflammatory activity as well as therapeutic response in CHB patients.

How to cite this article: Sultana N, Tabassum S, Munshi SU, Hossain M, Imam A. Nucleoside Analog-treated Chronic Hepatitis B Patients showed Reduced Expression of PECAM-1 Gene in Peripheral Blood Mononuclear Cells in Bangladesh. Euroasian J Hepato-Gastroenterol 2014;4(2):87-91.

MMP-9 deficiency shelters endothelial PECAM-1 expression and enhances regeneration of steatotic livers after ischemia and reperfusion injury

BACKGROUND & AIMS

Organ shortage has led to the use of steatotic livers in transplantation, despite their elevated susceptibility to ischemia/reperfusion injury (IRI). Matrix metalloproteinase-9 (MMP-9), an inducible gelatinase, is emerging as a central mediator of leukocyte traffic into inflamed tissues. However, its role in steatotic hepatic IRI has yet to be demonstrated.

METHODS

We examined the function of MMP-9 in mice fed with a high-fat diet (HFD), which developed approximately 50% hepatic steatosis, predominantly macrovesicular, prior to partial hepatic IRI.

RESULTS

The inability of MMP-9(-/-) deficient steatotic mice to express MMP-9 significantly protected these mice from liver IRI. Compared to fatty controls, MMP-9(-/-) steatotic livers showed significantly reduced leukocyte infiltration, proinflammatory cytokine expression, and liver necrosis. Loss of MMP-9 activity preserved platelet endothelial cell adhesion molecule-1 (PECAM-1) expression, a modulator of vascular integrity at the endothelial cell-cell junctions in steatotic livers after IRI. Using in vitro approaches, we show that targeted inhibition of MMP-9 sheltered the extracellular portion of PECAM-1 from proteolytic processing, and disrupted leukocyte migration across this junctional molecule. Moreover, the evaluation of distinct parameters of regeneration, proliferating cell nuclear antigen (PCNA) and histone H3 phosphorylation (pH3), provided evidence that hepatocyte progression into S phase and mitosis was notably enhanced in MMP-9(-/-) steatotic livers after IRI.

CONCLUSIONS

MMP-9 activity disrupts vascular integrity at least partially through a PECAM-1 dependent mechanism and interferes with regeneration of steatotic livers after IRI. Our novel findings establish MMP-9 as an important mediator of steatotic liver IRI.

Impaired tumor angiogenesis and VEGF-induced pathway in endothelial CD146 knockout mice

CD146 is a newly identified endothelial biomarker that has been implicated in angiogenesis. Though in vitro angiogenic function of CD146 has been extensively reported, in vivo evidence is still lacking. To address this issue, we generated endothelial-specific CD146 knockout (CD146^EC-KO) mice using the Tg(Tek-cre) system. Surprisingly, these mice did not exhibit any apparent morphological defects in the development of normal retinal vasculature. To evaluate the role of CD146 in pathological angiogenesis, a xenograft tumor model was used. We found that both tumor volume and vascular density were significantly lower in CD146^EC-KO mice when compared to WT littermates. Additionally, the ability for sprouting, migration and tube formation in response to VEGF treatment was impaired in endothelial cells (ECs) of CD146^EC-KO mice. Mechanistic studies further confirmed that VEGF-induced VEGFR-2 phosphorylation and AKT/p38 MAPKs/NF-κB activation were inhibited in these CD146-null ECs, which might present the underlying cause for the observed inhibition of tumor angiogenesis in CD146^EC-KO mice. These results suggest that CD146 plays a redundant role in physiological angiogenic processes, but becomes essential during pathological angiogenesis as observed in tumorigenesis.

CD31 is a key coinhibitory receptor in the development of immunogenic dendritic cells

CD31 is a transhomophilic tyrosine-based inhibitory motif receptor and is expressed by both dendritic cells (DCs) and T lymphocytes. Previous studies have established that the engagement of CD31 drives immune-inhibitory signaling in T lymphocytes, but the effect exerted by CD31 signaling in DCs remains elusive. Here, we show that CD31 is a key coinhibitory receptor on stimulated DCs, favoring the development of tolerogenic functions and finally resulting in T-cell tolerance. The disruption of CD31 signaling favored the immunogenic maturation and migration of resident DCs to the draining lymph nodes. In contrast, sustaining the CD31/SHP-1 signaling during DC maturation resulted in reduced NF-κB nuclear translocation, expression of costimulatory molecules, and production of immunogenic cytokines (e.g., IL-12, IL-6), whereas the expression of TGF-β and IL-10 were increased. More importantly, CD31-conditioned DCs purified from the draining lymph nodes of ovalbumin-immunized mice favored the generation of antigen-specific regulatory T cells (CD25(+) forkhead box P3(+)) at the expense of effector (IFN-γ(+)) cells upon coculture with naive ovalbumin-specific CD4(+) T lymphocytes ex vivo. Finally, the adoptive transfer of CD31-conditioned myelin oligodendrocyte glycoprotein-loaded DCs carried immune tolerance against the subsequent development of MOG-induced experimental autoimmune encephalomyelitis in vivo. The key coinhibitory role exerted by CD31 on DCs highlighted by the present study may have important implications both in settings where the immunogenic function of DCs is desirable, such as infection and cancer, and in settings where tolerance-driving DCs are preferred, such as autoimmune diseases and transplantation.

Soluble platelet-endothelial cell adhesion molecule-1, a biomarker of ventilator-induced lung injury

Introduction

Endothelial cell injury is an important component of acute lung injury. Platelet-endothelial cell adhesion molecule-1 (PECAM1) is a transmembrane protein that connects endothelial cells to one another and can be detected as a soluble, truncated protein (sPECAM1) in serum. We hypothesized that injurious mechanical ventilation (MV) leads to shedding of PECAM1 from lung endothelial cells resulting in increasing sPECAM1 levels in the systemic circulation.

Methods

We studied 36 Sprague–Dawley rats in two prospective, randomized, controlled studies (healthy and septic) using established animal models of ventilator-induced lung injury. Animals (n = 6 in each group) were randomized to spontaneous breathing or two MV strategies: low tidal volume (V_T) (6 ml/kg) and high-V_T (20 ml/kg) on 2 cmH₂O of positive end-expiratory pressure (PEEP). In low-V_T septic animals, 10 cmH₂O of PEEP was applied. We performed pulmonary histological and physiological evaluation and measured lung PECAM1 protein content and serum sPECAM1 levels after four hours ventilation period.

Results

High-V_T MV caused severe lung injury in healthy and septic animals, and decreased lung PECAM1 protein content (P < 0.001). Animals on high-V_T had a four- to six-fold increase of mean sPECAM1 serum levels than the unventilated counterpart (35.4 ± 10.4 versus 5.6 ± 1.7 ng/ml in healthy rats; 156.8 ± 47.6 versus 35.6 ± 12.6 ng/ml in septic rats) (P < 0.0001). Low-V_T MV prevented these changes. Levels of sPECAM1 in healthy animals on high-V_T MV paralleled the sPECAM1 levels of non-ventilated septic animals.

Conclusions

Our findings suggest that circulating sPECAM1 may represent a promising biomarker for the detection and monitoring of ventilator-induced lung injury.

PECAM-1: regulator of endothelial junctional integrity

PECAM-1 (also known as CD31) is a cellular adhesion and signaling receptor comprising six extracellular immunoglobulin (Ig)-like homology domains, a short transmembrane domain and a 118 amino acid cytoplasmic domain that becomes serine and tyrosine phosphorylated upon cellular activation. PECAM-1 expression is restricted to blood and vascular cells. In circulating platelets and leukocytes, PECAM-1 functions largely as an inhibitory receptor that, via regulated sequential phosphorylation of its cytoplasmic domain, limits cellular activation responses. PECAM-1 is also highly expressed at endothelial cell intercellular junctions, where it functions as a mechanosensor, as a regulator of leukocyte trafficking and in the maintenance of endothelial cell junctional integrity. In this review, we will describe (1) the functional domains of PECAM-1 and how they contribute to its barrier-enhancing properties, (2) how the physical properties of PECAM-1 influence its subcellular localization and its ability to influence endothelial cell barrier function, (3) various stimuli that initiate PECAM-1 signaling and/or function at the endothelial junction and (4) cross-talk of PECAM-1 with other junctional molecules, which can influence endothelial cell function.

CD44 regulation of endothelial cell proliferation and apoptosis via modulation of CD31 and VE-cadherin expression

CD44 has been implicated in a diverse array of cell behaviors and in a diverse range of signaling pathway activations under physiological and pathophysiological conditions. We have documented a role for CD44 in mediating vascular barrier integrity via regulation of PECAM-1 (CD31) expression. We now report our findings on the roles of CD44 in modulating proliferation and apoptosis of microvascular endothelial cells via its modulation of CD31 and VE-cadherin expression and the Hippo pathway. In this report, we demonstrate persistent increased proliferation and reduced activations of both effector and initiator caspases in high cell density, postconfluent CD44 knock-out (CD44KO), and CD31KO cultures. We found that reconstitution with murine CD44 or CD31 restored the proliferative and caspase activation rates to WT levels. Moreover, we have confirmed that the CD31 ecto-domain plays a key role in specific caspase cascades as well as cell adhesion-mediated cell growth and found that CD31 deficiency results in a reduction in VE-cadherin expression. Last, we have shown that both CD44KO and CD31KO endothelial cells exhibit a reduced VE-cadherin expression correlating with increased survivin expression and YAP nuclear localization, consistent with inactivation of the Hippo pathway, resulting in increased proliferation and decreased apoptosis. These findings support the concept that CD44 mediates several of its effects on endothelia through modulation of adhesion protein expression, which, in addition to its known modulation of junctional integrity, matrix metalloproteinase levels and activation, interactions with cortical membrane proteins, and selected signaling pathways, plays a key role as a critical regulator of vascular function.

Calcium supplementation during sepsis exacerbates organ failure and mortality via calcium/calmodulin-dependent protein kinase kinase signaling

BACKGROUND

Calcium plays an essential role in nearly all cellular processes. As such, cellular and systemic calcium concentrations are tightly regulated. During sepsis, derangements in such tight regulation frequently occur, and treating hypocalcemia with parenteral calcium administration remains the current practice guideline.

OBJECTIVE

We investigated whether calcium administration worsens mortality and organ dysfunction using an experimental murine model of sepsis and explored the mechanistic role of the family of calcium/calmodulin-dependent protein kinases in mediating these physiological effects. To highlight the biological relevance of these observations, we conducted a translational study of the association between calcium administration, organ dysfunction, and mortality among a cohort of critically ill septic ICU patients.

DESIGN

Prospective, randomized controlled experimental murine study and observational clinical cohort analysis.

SETTING

University research laboratory and eight ICUs at a tertiary care center.

PATIENTS

A cohort of 870 septic ICU patients.

SUBJECTS

C57Bl/6 and CaMKK mice.

INTERVENTIONS

Mice underwent cecal ligation and puncture polymicrobial sepsis and were administered with calcium chloride (0.25 or 0.25 mg/kg) or normal saline.

MEASUREMENTS AND MAIN RESULTS

Administering calcium chloride to septic C57Bl/6 mice heightened systemic inflammation and vascular leak, exacerbated hepatic and renal dysfunction, and increased mortality. These events were significantly attenuated in CaMKK mice. In a risk-adjusted analysis of septic patients, calcium administration was associated with an increased risk of death, odds ratio 1.92 (95% CI, 1.00-3.68; p = 0.049), a significant increase in the risk of renal dysfunction, odds ratio 4.74 (95% CI, 2.48-9.08; p < 0.001), and a significant reduction in ventilator-free days, mean decrease 3.29 days (0.50-6.08 days; p = 0.02).

CONCLUSIONS

Derangements in calcium homeostasis occur during sepsis that is sensitive to calcium administration. This altered calcium signaling, transduced by the calmodulin-dependent protein kinase kinase cascade, mediates heightened inflammation and vascular leak that culminates in elevated organ dysfunction and mortality. In the clinical management of septic patients, calcium supplementation provides no benefit and may impose harm.

ALS and oxidative stress: the neurovascular scenario

Oxidative stress and angiogenic factors have been placed as the prime focus of scientific investigations after an establishment of link between vascular endothelial growth factor promoter (VEGF), hypoxia, and amyotrophic lateral sclerosis (ALS) pathogenesis. Deletion of the hypoxia-response element in the vascular endothelial growth factor promoter and mutant superoxide dismutase 1 (SOD1) which are characterised by atrophy and muscle weakness resulted in phenotype resembling human ALS in mice. This results in lower motor neurodegeneration thus establishing an important link between motor neuron degeneration, vasculature, and angiogenic molecules. In this review, we have presented human, animal, and in vitro studies which suggest that molecules like VEGF have a therapeutic, diagnostic, and prognostic potential in ALS. Involvement of vascular growth factors and hypoxia response elements also highlights the converging role of oxidative stress and neurovascular network for understanding and treatment of various neurodegenerative disorders like ALS.

Immune responses at brain barriers and implications for brain development and neurological function in later life

For a long time the brain has been considered an immune-privileged site due to a muted inflammatory response and the presence of protective brain barriers. It is now recognized that neuroinflammation may play an important role in almost all neurological disorders and that the brain barriers may be contributing through either normal immune signaling or disruption of their basic physiological mechanisms. The distinction between normal function and dysfunction at the barriers is difficult to dissect, partly due to a lack of understanding of normal barrier function and partly because of physiological changes that occur as part of normal development and ageing. Brain barriers consist of a number of interacting structural and physiological elements including tight junctions between adjacent barrier cells and an array of influx and efflux transporters. Despite these protective mechanisms, the capacity for immune-surveillance of the brain is maintained, and there is evidence of inflammatory signaling at the brain barriers that may be an important part of the body's response to damage or infection. This signaling system appears to change both with normal ageing, and during disease. Changes may affect diapedesis of immune cells and active molecular transfer, or cause rearrangement of the tight junctions and an increase in passive permeability across barrier interfaces. Here we review the many elements that contribute to brain barrier functions and how they respond to inflammation, particularly during development and aging. The implications of inflammation–induced barrier dysfunction for brain development and subsequent neurological function are also discussed.

Serum levels of soluble platelet endothelial cell adhesion molecule-1 and vascular cell adhesion molecule-1 are decreased in subjects with autism spectrum disorder

BACKGROUND

Adhesion molecules, such as platelet-endothelial adhesion molecule-1 (PECAM-1), platelet selectin (P-selectin), endothelial selectin (E-selectin), intracellular adhesion molecule-1 (ICAM-1), and vascular cell adhesion molecule-1 (VCAM-1), are localized on the membranes of activated platelets and leukocytes and on the vascular endothelium. Recently, we measured serum levels of soluble (s) forms of adhesion molecules in adults,18 to 26 years old, with autism spectrum disorder (ASD) and observed low levels of sPECAM-1 and sP-selectin. A subsequent study showed a similar result in children two to four years old with ASD. However, information about school age (five to seventeen years old) ASD subjects is required to determine whether adhesion molecules are also reduced in individuals with ASD in this age range.

FINDINGS

Twenty-two subjects with high-functioning ASD and 29 healthy age-matched controls were recruited. ELISA was used for sPECAM-1, and a suspension array system was used for sP-selectin, sE-selectin, sICAM-1 and sVCAM-1 measurements. We found that serum levels of sPECAM-1 (U = 91.0, P<0.0001 by Mann-Whitney U test) and sVCAM-1 (U = 168.0, P = 0.0042) were significantly lower in ASD subjects than in controls. Subsequently, we examined the correlations between serum levels of either sPECAM-1 or sVCAM-1 and clinical variables including Autism Diagnostic Interview - Revised subscores and our previous cytokine profile data from the same ASD subjects. However, we did not find any significant correlations between them.

CONCLUSIONS

The present results, taken together with previous results, suggest that sPECAM-1 may play a role in the generation and development of ASD, beginning in childhood and lasting until adulthood.

Absence of platelet endothelial cell adhesion molecule 1, PECAM-1/CD31, in vivo increases resistance to Salmonella enterica serovar Typhimurium in mice

PECAM-1/CD31 is known to regulate inflammatory responses and exhibit pro- and anti-inflammatory functions. This study was designed to determine the functional role of PECAM-1 in susceptibility to murine primary in vivo infection with Salmonella enterica serovar Typhimurium and in in vitro inflammatory responses of peritoneal macrophages. Lectin profiling showed that cellular PECAM-1 and recombinant human PECAM-1-Ig chimera contain high levels of mannose sugars and N-acetylglucosamine. Consistent with this carbohydrate pattern, both recombinant human and murine PECAM-1-Ig chimeras were shown to bind S. Typhimurium in a dose-dependent manner in vitro. Using oral and fecal-oral transmission models of S. Typhimurium SL1344 infection, PECAM-1(-/-) mice were found to be more resistant to S. Typhimurium infection than wild-type (WT) C57BL/6 mice. While fecal shedding of S. Typhimurium was comparable in wild-type and PECAM-1(-/-) mice, the PECAM-1-deficient mice had lower bacterial loads in systemic organs such as liver, spleen, and mesenteric lymph nodes than WT mice, suggesting that extraintestinal dissemination was reduced in the absence of PECAM-1. This reduced bacterial load correlated with reduced tumor necrosis factor (TNF), interleukin-6 (IL-6), and monocyte chemoattractant protein (MCP) levels in sera of PECAM-1(-/-) mice. Following in vitro stimulation of macrophages with either whole S. Typhimurium, lipopolysaccharide (LPS) (Toll-like receptor 4 [TLR4] ligand), or poly(I·C) (TLR3 ligand), production of TNF and IL-6 by PECAM-1(-/-) macrophages was reduced. Together, these results suggest that PECAM-1 may have multiple functions in resistance to infection with S. Typhimurium, including binding to host cells, extraintestinal spread to deeper tissues, and regulation of inflammatory cytokine production by infected macrophages.

Inhibition of glycogen synthase kinase 3β promotes tight junction stability in brain endothelial cells by half-life extension of occludin and claudin-5

Neuroinflammatory conditions often involve dysfunction of the Blood-Brain Barrier (BBB). Therefore, identifying molecular targets that can maintain barrier fidelity is of clinical importance. We have previously reported on the anti-inflammatory effects that glycogen synthase kinase 3β (GSK3β) inhibition has on primary human brain endothelial cells. Here we show that GSK3β inhibitors also promote barrier tightness by affecting tight junction (TJ) protein stability. Transendothelial electrical resistance (TEER) was used to evaluate barrier integrity with both pharmacological inhibitors and mutants of GSK3β. Inhibition of GSK3β produced a gradual and sustained increase in TEER (as much as 22% over baseline). Analysis of subcellular membrane fractions revealed an increase in the amount of essential tight junction proteins, occludin and claudin-5, but not claudin-3. This phenomenon was attributed to a decrease in TJ protein turnover and not transcriptional regulation. Using a novel cell-based assay, inactivation of GSK3β significantly increased the half-life of occludin and claudin-5 by 32% and 43%, respectively. A correlation was also established between the enhanced association of β-catenin with ZO-1 as a function of GSK3β inhibition. Collectively, our findings suggest the possibility of using GSK3β inhibitors as a means to extend the half-life of key tight junction proteins to promote re-sealing of the BBB during neuroinflammation.

CD44 deficiency contributes to enhanced experimental autoimmune encephalomyelitis: a role in immune cells and vascular cells of the blood-brain barrier

Adhesion molecule CD44 is expressed by multiple cell types and is implicated in various cellular and immunological processes. In this study, we examined the effect of global CD44 deficiency on myelin oligodendrocyte glycoprotein peptide (MOG)-induced experimental autoimmune encephalomyelitis (EAE), a murine model of multiple sclerosis. Compared to C57BL/6 wild-type mice, CD44-deficient mice presented with greater disease severity, increased immune cell numbers in the central nervous system, and increased anti-MOG antibody and proinflammatory cytokine production, especially those associated with T helper 17 (Th17) cells. Further, decreased numbers of peripheral CD4(+)CD25(+)FoxP3(+) regulatory T cells (Tregs) were observed in CD44-knockout mice throughout the disease course. CD44-knockout CD4 T cells exhibited reduced transforming growth factor-β receptor type I (TGF-β RI) expression that did not impart a defect in Treg polarization in vitro, but did correlate with enhanced Th17 polarization in vitro. Further, EAE in bone marrow-chimeric animals suggested CD44 expression on both circulating and noncirculating cells limited disease severity. Endothelial expression of CD44 limited T-cell adhesion to and transmigration through murine endothelial monolayers in vitro. Importantly, we also identified increased permeability of the blood-brain barrier in vivo in CD44-deficient mice before and following immunization. These data suggest that CD44 has multiple protective roles in EAE, with effects on cytokine production, T-cell differentiation, T-cell-endothelial cell interactions, and blood-brain barrier integrity.

Deficiency for endoglin in tumor vasculature weakens the endothelial barrier to metastatic dissemination

Therapy-induced resistance remains a significant hurdle to achieve long-lasting responses and cures in cancer patients. We investigated the long-term consequences of genetically impaired angiogenesis by engineering multiple tumor models deprived of endoglin, a co-receptor for TGF-β in endothelial cells actively engaged in angiogenesis. Tumors from endoglin-deficient mice adapted to the weakened angiogenic response, and refractoriness to diminished endoglin signaling was accompanied by increased metastatic capability. Mechanistic studies in multiple mouse models of cancer revealed that deficiency for endoglin resulted in a tumor vasculature that displayed hallmarks of endothelial-to-mesenchymal transition, a process of previously unknown significance in cancer biology, but shown by us to be associated with a reduced capacity of the vasculature to avert tumor cell intra- and extravasation. Nevertheless, tumors deprived of endoglin exhibited a delayed onset of resistance to anti-VEGF (vascular endothelial growth factor) agents, illustrating the therapeutic utility of combinatorial targeting of multiple angiogenic pathways for the treatment of cancer.

MYADM controls endothelial barrier function through ERM-dependent regulation of ICAM-1 expression

The endothelium maintains a barrier between blood and tissue that becomes more permeable during inflammation. Membrane rafts are ordered assemblies of cholesterol, glycolipids, and proteins that modulate proinflammatory cell signaling and barrier function. In epithelial cells, the MAL family members MAL, MAL2, and myeloid-associated differentiation marker (MYADM) regulate the function and dynamics of ordered membrane domains. We analyzed the expression of these three proteins in human endothelial cells and found that only MYADM is expressed. MYADM was confined in ordered domains at the plasma membrane, where it partially colocalized with filamentous actin and cell-cell junctions. Small interfering RNA (siRNA)-mediated MYADM knockdown increased permeability, ICAM-1 expression, and leukocyte adhesion, all of which are features of an inflammatory response. Barrier function decrease in MYADM-silenced cells was dependent on ICAM-1 expression. Membrane domains and the underlying actin cytoskeleton can regulate each other and are connected by ezrin, radixin, and moesin (ERM) proteins. In endothelial cells, MYADM knockdown induced ERM activation. Triple-ERM knockdown partially inhibited ICAM-1 increase induced by MYADM siRNA. Importantly, ERM knockdown also reduced ICAM-1 expression in response to the proinflammatory cytokine tumor necrosis factor-α. MYADM therefore regulates the connection between the plasma membrane and the cortical cytoskeleton and so can control the endothelial inflammatory response.

Short term interactions with long term consequences: modulation of chimeric vessels by neural progenitors

Vessels are a critical and necessary component of most tissues, and there has been substantial research investigating vessel formation and stabilization. Several groups have investigated coculturing endothelial cells with a second cell type to promote formation and stabilization of vessels. Some have noted that long-term vessels derived from implanted cocultures are often chimeric consisting of both host and donor cells. The questions arise as to whether the coculture cell might impact the chimeric nature of the microvessels and can modulate the density of donor cells over time. If long-term engineered microvessels are primarily of host origin, any impairment of the host's angiogenic ability has significant implications for the long-term success of the implant. If one can modulate the host versus donor response, one may be able to overcome a host's angiogenic impairment. Furthermore, if one can modulate the donor contribution, one may be able to engineer microvascular networks to deliver molecules a patient lacks systemically for long times. To investigate the impact of the cocultured cell on the host versus donor contributions of endothelial cells in engineered microvascular networks, we varied the ratio of the neural progenitors to endothelial cells in subcutaneously implanted poly(ethylene glycol)/poly-L-lysine hydrogels. We found that the coculture of neural progenitors with endothelial cells led to the formation of chimeric host-donor vessels, and the ratio of neural progenitors has a significant impact on the long term residence of donor endothelial cells in engineered microvascular networks in vivo even though the neural progenitors are only present transiently in the system. We attribute this to the short term paracrine signaling between the two cell types. This suggests that one can modulate the host versus donor contributions using short-term paracrine signaling which has broad implications for the application of engineered microvascular networks and cellular therapy more broadly.

Levels of soluble platelet endothelial cell adhesion molecule-1 and P-selectin are decreased in children with autism spectrum disorder

BACKGROUND

Although the etiopathology of autism spectrum disorder (ASD) is not clear, there is increasing evidence that dysfunction in the immune system affects many children with ASD. Findings of immune dysfunction in ASD include increases in inflammatory cytokines, chemokines, and microglial activity in brain tissue and cerebrospinal fluid, as well as abnormal peripheral immune cell function.

METHODS

Adhesion molecules, such as platelet endothelial adhesion molecule-1 (PECAM-1), intercellular adhesion molecule-1 (ICAM-1), vascular adhesion molecule-1 (VCAM-1), P-selectin, and L-selectin, function to facilitate leukocyte transendothelial migration. We assessed concentrations of soluble adhesion molecules, sPECAM-1, sICAM-1, sVCAM-1, sP-selectin, and sL-selectin in the plasma of 49 participants with ASD and 31 typically developing controls of the same age, all of whom were enrolled as part of the Autism Phenome Project. Behavioral assessment, the levels of soluble adhesion molecules, and head circumference were compared in the same subjects.

RESULTS

Levels of sPECAM-1 and sP-selectin were significantly reduced in the ASD group compared to typically developing controls (p < .02). Soluble PECAM-1 levels were negatively associated with repetitive behavior and abnormal brain growth in children with ASD (p = .03).

CONCLUSIONS

Because adhesion molecules modulate the permeability and signaling at the blood-brain barrier as well as leukocyte infiltration into the central nervous system, the current data suggest a role for these molecules in the complex pathophysiology of ASD.

CD31 exhibits multiple roles in regulating T lymphocyte trafficking in vivo

The role of CD31, an Ig-like molecule expressed by leukocytes and endothelial cells (ECs), in the regulation of T lymphocyte trafficking remains contentious. Using CD31-deficient mice, we show that CD31 regulates both constitutive and inflammation-induced T cell migration in vivo. Specifically, T cell:EC interactions mediated by CD31 molecules are required for efficient localization of naive T lymphocytes to secondary lymphoid tissue and constitutive recirculation of primed T cells to nonlymphoid tissues. In inflammatory conditions, T cell:EC CD31-mediated interactions facilitate T cell recruitment to Ag-rich sites. However, endothelial CD31 also provides a gate-keeping mechanism to limit the rate of Ag-driven T cell extravasation. This event contributes to the formation of Ag-specific effector T cell infiltrates and is induced by recognition of Ag on the endothelium. In this context, CD31 engagement is required for restoring endothelial continuity, which is temporarily lost upon MHC molecule ligation by migrating cognate T cells. We propose that integrated adhesive and signaling functions of CD31 molecules exert a complex regulation of T cell trafficking, a process that is differentially adapted depending on cell-specific expression, the presence of inflammatory conditions and the molecular mechanism facilitating T cell extravasation.

CCL2 disrupts the adherens junction: implications for neuroinflammation

Alterations to blood-brain barrier (BBB) adhesion molecules and junctional integrity during neuroinflammation can promote central nervous system (CNS) pathology. The chemokine CCL2 is elevated during CNS inflammation and is associated with endothelial dysfunction. The effects of CCL2 on endothelial adherens junctions (AJs) have not been defined. We demonstrate that CCL2 transiently induces Src-dependent disruption of human brain microvascular endothelial AJ. β-Catenin is phosphorylated and traffics from the AJ to PECAM-1 (platelet endothelial cell adhesion molecule-1), where it is sequestered at the membrane. PECAM-1 is also tyrosine-phosphorylated, an event associated with recruitment of the phosphatase SHP-2 (Src homology 2 domain-containing protein phosphatase) to PECAM-1, β-catenin release from PECAM-1, and reassociation of β-catenin with the AJ. Surface localization of PECAM-1 is increased in response to CCL2. This may enable the endothelium to sustain CCL2-induced alterations in AJ and facilitate recruitment of leukocytes into the CNS. Our novel findings provide a mechanism for CCL2-mediated disruption of endothelial junctions that may contribute to BBB dysfunction and increased leukocyte recruitment in neuroinflammatory diseases.

Astrocyte-derived VEGF-A drives blood-brain barrier disruption in CNS inflammatory disease

In inflammatory CNS conditions such as multiple sclerosis (MS), current options to treat clinical relapse are limited, and more selective agents are needed. Disruption of the blood-brain barrier (BBB) is an early feature of lesion formation that correlates with clinical exacerbation, leading to edema, excitotoxicity, and entry of serum proteins and inflammatory cells. Here, we identify astrocytic expression of VEGF-A as a key driver of BBB permeability in mice. Inactivation of astrocytic Vegfa expression reduced BBB breakdown, decreased lymphocyte infiltration and neuropathology in inflammatory and demyelinating lesions, and reduced paralysis in a mouse model of MS. Knockdown studies in CNS endothelium indicated activation of the downstream effector eNOS as the principal mechanism underlying the effects of VEGF-A on the BBB. Systemic administration of the selective eNOS inhibitor cavtratin in mice abrogated VEGF-A-induced BBB disruption and pathology and protected against neurologic deficit in the MS model system. Collectively, these data identify blockade of VEGF-A signaling as a protective strategy to treat inflammatory CNS disease.

Primed T cell responses to chemokines are regulated by the immunoglobulin-like molecule CD31

CD31, an immunoglobulin-like molecule expressed by leukocytes and endothelial cells, is thought to contribute to the physiological regulation T cell homeostasis due to the presence of two immunotyrosine-based inhibitory motifs in its cytoplasmic tail. Indeed, loss of CD31 expression leads to uncontrolled T cell-mediated inflammation in a variety of experimental models of disease and certain CD31 polymorphisms correlate with increased disease severity in human graft-versus-host disease and atherosclerosis. The molecular mechanisms underlying CD31-mediated regulation of T cell responses have not yet been clarified. We here show that CD31-mediated signals attenuate T cell chemokinesis both in vitro and in vivo. This effect selectively affects activated/memory T lymphocytes, in which CD31 is clustered on the cell membrane where it segregates to the leading edge. We provide evidence that this molecular segregation, which does not occur in naïve T lymphocytes, might lead to cis-CD31 engagement on the same membrane and subsequent interference with the chemokine-induced PI3K/Akt signalling pathway. We propose that CD31-mediated modulation of memory T cell chemokinesis is a key mechanism by which this molecule contributes to the homeostatic regulation of effector T cell immunity.

Proteinase 3 contributes to transendothelial migration of NB1-positive neutrophils

Neutrophil transmigration requires the localization of neutrophils to endothelial cell junctions, in which receptor-ligand interactions and the action of serine proteases promote leukocyte diapedesis. NB1 (CD177) is a neutrophil-expressed surface molecule that has been reported to bind proteinase 3 (PR3), a serine protease released from activated neutrophils. PR3 has demonstrated proteolytic activity on a number of substrates, including extracellular matrix proteins, although its role in neutrophil transmigration is unknown. Recently, NB1 has been shown to be a heterophilic binding partner for the endothelial cell junctional protein, PECAM-1. Disrupting the interaction between NB1 and PECAM-1 significantly inhibits neutrophil transendothelial cell migration on endothelial cell monolayers. Because NB1 interacts with endothelial cell PECAM-1 at cell junctions where transmigration occurs, we considered that NB1-PR3 interactions may play a role in aiding neutrophil diapedesis. Blocking Abs targeting the heterophilic binding domain of PECAM-1 significantly inhibited transmigration of NB1-positive neutrophils through IL-1β-stimulated endothelial cell monolayers. PR3 expression and activity were significantly increased on NB1-positive neutrophils following transmigration, whereas neutrophils lacking NB1 demonstrated no increase in PR3. Finally, using selective serine protease inhibitors, we determined that PR3 activity facilitated transmigration of NB1-positive neutrophils under both static and flow conditions. These data demonstrate that PR3 contributes in the selective recruitment of the NB1-positive neutrophil population.

Monocyte maturation, HIV susceptibility, and transmigration across the blood brain barrier are critical in HIV neuropathogenesis

HIV continues to be a global health crisis with more than 34 million people infected worldwide (UNAIDS: Report on the Global AIDS Epidemic 2010, Geneva, World Health Organization). HIV enters the CNS within 2 weeks of infection and establishes a spectrum of HAND in a large percentage of infected individuals. These neurologic deficits greatly impact the quality of life of those infected with HIV. The establishment of HAND is largely attributed to monocyte transmigration, particularly that of a mature CD14(+)CD16(+) monocyte population, which is more susceptible to HIV infection, across the BBB into the CNS parenchyma in response to chemotactic signals. To enter the CNS, junctional proteins on the monocytes must participate in homo- and heterotypic interactions with those present on BMVECs of the BBB as they transmigrate across the barrier. This transmigration is responsible for bringing virus into the brain and establishing chronic neuroinflammation. While there is baseline trafficking of monocytes into the CNS, the increased chemotactic signals present during HIV infection of the brain promote exuberant monocyte transmigration into the CNS. This review will discuss the mechanisms of monocyte differentiation/maturation, HIV infectivity, and transmigration into the CNS parenchyma that contribute to the establishment of cognitive impairment in HIV-infected individuals. It will focus on markers of monocyte subpopulations, how differentiation/maturation alters HIV infectivity, and the mechanisms that promote their increased transmigration across the BBB into the CNS.

Intravital imaging of CTLs killing islet cells in diabetic mice

Type 1 diabetes (T1D) is caused by autoimmune destruction of the insulin-producing β cells in the pancreatic islets, which are essentially mini-organs embedded in exocrine tissue. CTLs are considered to have a predominant role in the autoimmune destruction underlying T1D. Visualization of CTL-mediated killing of β cells would provide new insight into the pathogenesis of T1D, but has been technically challenging to achieve. Here, we report our use of intravital 2-photon imaging in mice to visualize the dynamic behavior of a virally expanded, diabetogenic CTL population in the pancreas at cellular resolution. Following vascular arrest and extravasation, CTLs adopted a random motility pattern throughout the compact exocrine tissue and displayed unimpeded yet nonlinear migration between anatomically nearby islets. Upon antigen encounter within islets, a confined motility pattern was acquired that allowed the CTLs to scan the target cell surface. A minority of infiltrating CTLs subsequently arrested at the β cell junction, while duration of stable CTL-target cell contact was on the order of hours. Slow-rate killing occurred in the sustained local presence of substantial numbers of effector cells. Collectively, these data portray the kinetics of CTL homing to and between antigenic target sites as a stochastic process at the sub-organ level and argue against a dominant influence of chemotactic gradients.

PECAM-1 dampens cytokine levels during LPS-induced endotoxemia by regulating leukocyte trafficking

AIMS

To investigate the mechanism by which platelet endothelial cell adhesion molecule 1 (PECAM-1/CD31), an immunoglobulin (Ig)-superfamily cell adhesion and signaling receptor, regulates pro-inflammatory cytokine levels. The purpose of the present investigation was to test the hypothesis that PECAM-1 influences circulating cytokine levels by regulating the trafficking of activated, cytokine-producing leukocytes to sites of inflammation.

MAIN METHODS

PECAM-1+/+ and PECAM-1-/- mice were subjected to lipopolysaccharide (LPS)-induced endotoxemia, and systemic cytokine levels were measured by Bioplex multiplex cytokine assays. Flow cytometry was employed to enumerate leukocytes at inflammatory sites and to measure cytokine synthesis in leukocyte sub-populations. Enzyme-linked immunosorbent assay (ELISA) was used to measure cytokine levels in tissue samples and in supernatants of in vitro-stimulated leukocytes.

KEY FINDINGS

We confirmed earlier reports that mice deficient in PECAM-1 had greater systemic levels of pro-inflammatory cytokines following intraperitoneal (IP) LPS administration. Interestingly, expression of PECAM-1, in mice, had negligible effects on the level of cytokine synthesis by leukocytes stimulated in vitro with LPS and in peritoneal macrophages isolated from LPS-injected mice. There was, however, excessive accumulation of macrophages and neutrophils in the lungs of PECAM-1-deficient, compared with wild-type, mice--an event that correlated with a prolonged increase in lung pro-inflammatory cytokine levels.

SIGNIFICANCE

Our results demonstrate that PECAM-1 normally functions to dampen systemic cytokine levels during LPS-induced endotoxemia by diminishing the accumulation of cytokine-producing leukocytes at sites of inflammation, rather than by modulating cytokine synthesis by leukocytes.

Claudin-1 induced sealing of blood-brain barrier tight junctions ameliorates chronic experimental autoimmune encephalomyelitis

In experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis (MS), loss of the blood-brain barrier (BBB) tight junction (TJ) protein claudin-3 correlates with immune cell infiltration into the CNS and BBB leakiness. Here we show that sealing BBB TJs by ectopic tetracycline-regulated expression of the TJ protein claudin-1 in Tie-2 tTA//TRE-claudin-1 double transgenic C57BL/6 mice had no influence on immune cell trafficking across the BBB during EAE and furthermore did not influence the onset and severity of the first clinical disease episode. However, expression of claudin-1 did significantly reduce BBB leakiness for both blood borne tracers and endogenous plasma proteins specifically around vessels expressing claudin-1. In addition, mice expressing claudin-1 exhibited a reduced disease burden during the chronic phase of EAE as compared to control littermates. Our study identifies BBB TJs as the critical structure regulating BBB permeability but not immune cell trafficking into CNS during EAE, and indicates BBB dysfunction is a potential key event contributing to disease burden in the chronic phase of EAE. Our observations suggest that stabilizing BBB barrier function by therapeutic targeting of TJs may be beneficial in treating MS, especially when anti-inflammatory treatments have failed.

CD31 is required on CD4+ T cells to promote T cell survival during Salmonella infection

Hematopoietic cells constitutively express CD31/PECAM1, a signaling adhesion receptor associated with controlling responses to inflammatory stimuli. Although expressed on CD4(+) T cells, its function on these cells is unclear. To address this, we have used a model of systemic Salmonella infection that induces high levels of T cell activation and depends on CD4(+) T cells for resolution. Infection of CD31-deficient (CD31KO) mice demonstrates that these mice fail to control infection effectively. During infection, CD31KO mice have diminished numbers of total CD4(+) T cells and IFN-γ-secreting Th1 cells. This is despite a higher proportion of CD31KO CD4(+) T cells exhibiting an activated phenotype and an undiminished capacity to prime normally and polarize to Th1. Reduced numbers of T cells reflected the increased propensity of naive and activated CD31KO T cells to undergo apoptosis postinfection compared with wild-type T cells. Using adoptive transfer experiments, we show that loss of CD31 on CD4(+) T cells alone is sufficient to account for the defective CD31KO T cell accumulation. These data are consistent with CD31 helping to control T cell activation, because in its absence, T cells have a greater propensity to become activated, resulting in increased susceptibility to become apoptotic. The impact of CD31 loss on T cell homeostasis becomes most pronounced during severe, inflammatory, and immunological stresses such as those caused by systemic Salmonella infection. This identifies a novel role for CD31 in regulating CD4 T cell homeostasis.

Breaking down the barrier: the effects of HIV-1 on the blood-brain barrier

Human immunodeficiency virus type 1 (HIV-1) primarily infects CD4(+) T cells and cells of the monocyte-macrophage lineage, resulting in immunodeficiency in an infected patient. Along with this immune deficiency, HIV-1 has been linked to a number of neurological symptoms in the absence of opportunistic infections or other co-morbidities, suggesting that HIV-1 is able to cross the blood-brain barrier (BBB), enter the central nervous system (CNS), and cause neurocognitive impairment. HIV-1-infected monocyte-macrophages traverse the BBB and enter the CNS throughout the course of HIV-1 disease. Once in the brain, both free virus and virus-infected cells are able to infect neighboring resident microglia and astrocytes and possibly other cell types. HIV-1-infected cells in both the periphery and the CNS give rise to elevated levels of viral proteins, including gp120, Tat, and Nef, and of host inflammatory mediators such as cytokines and chemokines. It has been shown that the viral proteins may act alone or in concert with host cytokines and chemokines, affecting the integrity of the BBB. The pathological end point of these interactions may facilitate a positive feedback loop resulting in increased penetration of HIV into the CNS. It is proposed in this review that the dysregulation of the BBB during and after neuroinvasion is a critical component of the neuropathogenic process and that dysregulation of this protective barrier is caused by a combination of viral and host factors including secreted viral proteins, components of the inflammatory process, the aging process, therapeutics, and drug or alcohol abuse.

Relative contribution of PECAM-1 adhesion and signaling to the maintenance of vascular integrity

PECAM-1 (CD31) is a cellular adhesion and signaling receptor that is highly expressed at endothelial cell-cell junctions in confluent vascular beds. Previous studies have implicated PECAM-1 in the maintenance of vascular barrier integrity; however, the mechanisms behind PECAM-1-mediated barrier protection are still poorly understood. The goal of the present study, therefore, was to examine the pertinent biological properties of PECAM-1 (i.e. adhesion and/or signaling) that allow it to support barrier integrity. We found that, compared with PECAM-1-deficient endothelial cells, PECAM-1-expressing endothelial cell monolayers exhibit increased steady-state barrier function, as well as more rapid restoration of barrier integrity following thrombin-induced perturbation of the endothelial cell monolayer. The majority of PECAM-1-mediated barrier protection was found to be due to the ability of PECAM-1 to interact homophilically and become localized to cell-cell junctions, because a homophilic binding-crippled mutant form of PECAM-1 was unable to support efficient barrier function when re-expressed in cells. By contrast, cells expressing PECAM-1 variants lacking residues known to be involved in PECAM-1-mediated signal transduction exhibited normal to near-normal barrier integrity. Taken together, these studies suggest that PECAM-1-PECAM-1 homophilic interactions are more important than its signaling function for maintaining the integrity of endothelial cell junctions.

Fluids and barriers of the CNS establish immune privilege by confining immune surveillance to a two-walled castle moat surrounding the CNS castle

Neuronal activity within the central nervous system (CNS) strictly depends on homeostasis and therefore does not tolerate uncontrolled entry of blood components. It has been generally believed that under normal conditions, the endothelial blood-brain barrier (BBB) and the epithelial blood-cerebrospinal fluid barrier (BCSFB) prevent immune cell entry into the CNS. This view has recently changed when it was realized that activated T cells are able to breach the BBB and the BCSFB to perform immune surveillance of the CNS. Here we propose that the immune privilege of the CNS is established by the specific morphological architecture of its borders resembling that of a medieval castle. The BBB and the BCSFB serve as the outer walls of the castle, which can be breached by activated immune cells serving as messengers for outside dangers. Having crossed the BBB or the BCSFB they reach the castle moat, namely the cerebrospinal fluid (CSF)-drained leptomeningeal and perivascular spaces of the CNS. Next to the CNS parenchyma, the castle moat is bordered by a second wall, the glia limitans, composed of astrocytic foot processes and a parenchymal basement membrane. Inside the castle, that is the CNS parenchyma proper, the royal family of sensitive neurons resides with their servants, the glial cells. Within the CSF-drained castle moat, macrophages serve as guards collecting all the information from within the castle, which they can present to the immune-surveying T cells. If in their communication with the castle moat macrophages, T cells recognize their specific antigen and see that the royal family is in danger, they will become activated and by opening doors in the outer wall of the castle allow the entry of additional immune cells into the castle moat. From there, immune cells may breach the inner castle wall with the aim to defend the castle inhabitants by eliminating the invading enemy. If the immune response by unknown mechanisms turns against self, that is the castle inhabitants, this may allow for continuous entry of immune cells into the castle and lead to the death of the castle inhabitants, and finally members of the royal family, the neurons. This review will summarize the molecular traffic signals known to allow immune cells to breach the outer and inner walls of the CNS castle moat and will highlight the importance of the CSF-drained castle moat in maintaining immune surveillance and in mounting immune responses in the CNS.

Platelet-endothelial cell adhesion molecule-1 regulates endothelial NO synthase activity and localization through signal transducers and activators of transcription 3-dependent NOSTRIN expression

BACKGROUND

NO produced by the endothelial NO synthase (eNOS) is an important regulator of cardiovascular physiological and pathological features. eNOS is activated by numerous stimuli, and its activity is tightly regulated. Platelet-endothelial cell adhesion molecule-1 (PECAM-1) has been implicated in regulating eNOS activity in response to shear stress. The current study was conducted to determine the role of PECAM-1 in the regulation of basal eNOS activity.

METHODS AND RESULTS

We demonstrate that PECAM-1-knockout ECs have increased basal eNOS activity and NO production. Mechanistically, increased eNOS activity is associated with a decrease in the inhibitory interaction of eNOS with caveolin-1, impaired subcellular localization of eNOS, and decreased eNOS traffic inducer (NOSTRIN) expression in the absence of PECAM-1. Furthermore, we demonstrate that activation of blunted signal transducers and activators of transcription 3 (STAT3) in the absence of PECAM-1 results in decreased NOSTRIN expression via direct binding of the signal transducers and activators of transcription 3 to the NOSTRIN promoter.

CONCLUSIONS

Our results reveal an elegant mechanism of eNOS regulation by PECAM-1 through signal transducers and activators of transcription 3-mediated transcriptional control of NOSTRIN.

Ig gene-like molecule CD31 plays a nonredundant role in the regulation of T-cell immunity and tolerance

CD31 is an Ig-like molecule expressed by leukocytes and endothelial cells with an established role in the regulation of leukocyte trafficking. Despite genetic deletion of CD31 being associated with exacerbation of T cell-mediated autoimmunity, the contribution of this molecule to T-cell responses is largely unknown. Here we report that tumor and allograft rejection are significantly enhanced in CD31-deficient mice, which are also resistant to tolerance induction. We propose that these effects are dependent on an as yet unrecognized role for CD31-mediated homophilic interactions between T cells and antigen-presenting cells (APCs) during priming. We show that loss of CD31 interactions leads to enhanced primary clonal expansion, increased killing capacity, and diminished regulatory functions by T cells. Immunomodulation by CD31 signals correlates with a partial inhibition of proximal T-cell receptor (TCR) signaling, specifically Zap-70 phosphorylation. However, CD31-deficient mice do not develop autoimmunity due to increased T-cell death following activation, and we show that CD31 triggering induces Erk-mediated prosurvival activity in T cells either in conjunction with TCR signaling or autonomously. We conclude that CD31 functions as a nonredundant comodulator of T-cell responses, which specializes in sizing the ensuing immune response by setting the threshold for T-cell activation and tolerance, while preventing memory T-cell death.

Role of PECAM-1 in arteriogenesis and specification of preexisting collaterals

RATIONALE

Hemodynamic forces caused by the altered blood flow in response to an occlusion lead to the induction of collateral remodeling and arteriogenesis. Previous work showed that platelet endothelial cell adhesion molecule (PECAM)-1 is a component of a mechanosensory complex that mediates endothelial cell responses to shear stress.

OBJECTIVE

We hypothesized that PECAM-1 plays an important role in arteriogenesis and collateral remodeling.

METHODS AND RESULTS

PECAM-1 knockout (KO) and wild-type littermates underwent femoral artery ligation. Surprisingly, tissue perfusion and collateral-dependent blood flow were significantly increased in the KO mice immediately after surgery. Histology confirmed larger caliber of preexisting collaterals in the KO mice. Additionally, KO mice showed blunted recovery of perfusion from hindlimb ischemia and reduced collateral remodeling, because of deficits in shear stress-induced signaling, including activation of the nuclear factor κB pathway and inflammatory cell accumulation. Partial recovery was associated with normal responses to circumferential wall tension in the absence of PECAM-1, as evidenced by the upregulation of ephrin B2 and monocyte chemoattractant protein-1, which are 2 stretch-induced regulators of arteriogenesis, both in vitro and in vivo.

CONCLUSIONS

Our findings suggest a novel role for PECAM-1 in arteriogenesis and collateral remodeling. Furthermore, we identify PECAM-1 as the first molecule that determines preexisting collateral diameter.