Angiopoietin-2 causes pericyte dropout in the normal retina: evidence for involvement in diabetic retinopathy

Pericyte loss is an early pathologic feature of diabetic retinopathy, consistently present in retinae of diabetic humans and animals. Because pericyte recruitment and endothelial cell survival are controlled, in part, by the angiopoietin/Tie2 ligand/receptor system, we studied the expression of angiopoietin-2 and -1 in relation to the evolution of pericyte loss in diabetic rat retinae, using quantitative retinal morphometry, and in retinae from mice with heterozygous angiopoietin deficiency (Ang-2 LacZ knock-in mice). Finally, recombinant angiopoietin-2 was injected into eyes of nondiabetic rats, and pericyte numbers were quantitated in retinal capillaries. Angiopoietin-1 protein was present in the normal maturing retina and was upregulated 2.5-fold in diabetic retinae over 3 months of diabetes. In contrast, angiopoietin-2 protein was consistently upregulated more than 30-fold in the retinae of diabetic rats, preceding the onset of pericyte loss. Heterozygous angiopoietin-2 deficiency completely prevented diabetes-induced pericyte loss and reduced the number of acellular capillary segments. Injection of angiopoietin-2 into the eyes of normal rats induced a dose-dependent pericyte loss. These data show that upregulation of angiopoietin-2 plays a critical role in the loss of pericytes in the diabetic retina.

Roles of ephrinB ligands and EphB receptors in cardiovascular development: demarcation of arterial/venous domains, vascular morphogenesis, and sprouting angiogenesis

Eph receptor tyrosine kinases and their cell-surface-bound ligands, the ephrins, regulate axon guidance and bundling in the developing brain, control cell migration and adhesion, and help patterning the embryo. Here we report that two ephrinB ligands and three EphB receptors are expressed in and regulate the formation of the vascular network. Mice lacking ephrinB2 and a proportion of double mutants deficient in EphB2 and EphB3 receptor signaling die in utero before embryonic day 11.5 (E11.5) because of defects in the remodeling of the embryonic vascular system. Our phenotypic analysis suggests complex interactions and multiple functions of Eph receptors and ephrins in the embryonic vasculature. Interaction between ephrinB2 on arteries and its EphB receptors on veins suggests a role in defining boundaries between arterial and venous domains. Expression of ephrinB1 by arterial and venous endothelial cells and EphB3 by veins and some arteries indicates that endothelial cell-to-cell interactions between ephrins and Eph receptors are not restricted to the border between arteries and veins. Furthermore, expression of ephrinB2 and EphB2 in mesenchyme adjacent to vessels and vascular defects in ephB2/ephB3 double mutants indicate a requirement for ephrin-Eph signaling between endothelial cells and surrounding mesenchymal cells. Finally, ephrinB ligands induce capillary sprouting in vitro with a similar efficiency as angiopoietin-1 (Ang1) and vascular endothelial growth factor (VEGF), demonstrating a stimulatory role of ephrins in the remodeling of the developing vascular system.

Tie2 receptor expression and phosphorylation in cultured cells and mouse tissues

Accumulating experimental evidence indicates that endothelial cell growth and blood vessel morphogenesis are processes that are governed by the activity of specifically expressed receptor tyrosine kinases (RTKs). We have used two new rat monoclonal antibodies (mAbs) to study the expression and phosphorylation of one such receptor, mouse Tie2 (tyrosine kinase that contains immunoglobulin-like loops and epidermal-growth-factor-similar domains 2]), in transfected cells, endothelioma cell lines and mouse tissues. The Tie2 receptor was found to be constitutively autophosphorylated when over-expressed in COS7 cells. In contrast, the endogenous Tie2 protein was not phosphorylated in endothelioma cell lines. However, in these cell lines, Tie2 could be induced to become tyrosine phosphorylated, and this activation was found to be independent of Tie1. Studying Tie2 receptor activity during angiogenesis in mouse development, the receptor was only weakly phosphorylated in the early postnatal mouse brain whereas a stronger activation could be detected in mouse embryos at day 10.5 post coitum.

DOTA-PESIN, a DOTA-conjugated bombesin derivative designed for the imaging and targeted radionuclide treatment of bombesin receptor-positive tumours

PURPOSE: We aimed at designing and developing a novel bombesin analogue, DOTA-PEG(4)-BN(7-14) (DOTA-PESIN), with the goal of labelling it with (67/68)Ga and (177)Lu for diagnosis and radionuclide therapy of prostate and other human cancers overexpressing bombesin receptors. METHODS: The 8-amino acid peptide bombesin (7-14) was coupled to the macrocyclic chelator DOTA via the spacer 15-amino-4,7,10,13-tetraoxapentadecanoic acid (PEG(4)). The conjugate was complexed with Ga(III) and Lu(III) salts. The GRP receptor affinity and the bombesin receptor subtype profile were determined in human tumour specimens expressing the three bombesin receptor subtypes. Internalisation and efflux studies were performed with the human GRP receptor cell line PC-3. Xenografted nude mice were used for biodistribution. RESULTS: [Ga(III)/Lu(III)]-DOTA-PESIN showed good affinity to GRP and neuromedin B receptors but no affinity to BB3. [(67)Ga/(177)Lu]-DOTA-PESIN internalised rapidly into PC-3 cells whereas the efflux from PC-3 cells was relatively slow. In vivo experiments showed a high and specific tumour uptake and good retention of [(67)Ga/(177)Lu]-DOTA-PESIN. [(67)Ga/(177)Lu]-DOTA-PESIN highly accumulated in GRP receptor-expressing mouse pancreas. The uptake specificity was demonstrated by blocking tumour uptake and pancreas uptake. Fast clearance was found from blood and all non-target organs except the kidneys. High tumour-to-normal tissue ratios were achieved, which increased with time. PET imaging with [(68)Ga]-DOTA-PESIN was successful in visualising the tumour at 1 h post injection. Planar scintigraphic imaging showed that the (177)Lu-labelled peptide remained in the tumour even 3 days post injection. CONCLUSION: The newly designed ligands have high potential with regard to PET and SPECT imaging with (68/67)Ga and targeted radionuclide therapy with (177)Lu.

Mechanisms of brain injury in bacterial meningitis: workshop summary

Morbidity and mortality associated with bacterial meningitis remain high, although antibiotic therapy has improved during recent decades. The major intracranial complications of bacterial meningitis are cerebrovascular arterial and venous involvement, brain edema, and hydrocephalus with a subsequent increase of intracranial pressure. Experiments in animal models and cell culture systems have focused on the pathogenesis and pathophysiology of bacterial meningitis in an attempt to identify the bacterial and/or host factors responsible for brain injury during the course of infection. An international workshop entitled "Bacterial Meningitis: Mechanisms of Brain Injury" was organized by the Department of Neurology at the University of Munich and was held in Eibsee, Germany, in June 1993. This conference provided a forum for the exchange of current information on bacterial meningitis, including data on the clinical spectrum of complications, the associated morphological alterations, the role of soluble inflammatory mediators (in particular cytokines) and of leukocyte-endothelial cell interactions in tissue injury, and the molecular mechanisms of neuronal injury, with potential mediators such as reactive oxygen species, reactive nitrogen species, and excitatory amino acids. It is hoped that a better understanding of the pathophysiological events that take place during bacterial meningitis will lead to the development of new therapeutic regimens.

Biosynthesis and expression of VE-cadherin is regulated by the PI3K/mTOR signaling pathway

Vascular endothelial (VE)-cadherin is an essential protein of adherens junctions of endothelial cells and plays a pivotal role in vascular homeostasis. Mammalian target of rapamycin complex 2 (mTORC2) deficient mice display defects in fetal vascular development. Blocking mTOR or the upstream kinase phosphoinositide 3-kinase (PI3K) led to a dose-dependently decrease of the VE-cadherin mRNA and protein expression. Immunofluorescent staining showed a strongly decreased expression of VE-cadherin at the interface of human umbilical endothelial cells (HUVECs) followed by intercellular gap formation. Herewith, we demonstrated that the expression of VE-cadherin is dependent on mTOR and PI3K signaling.

The role of purinergic signaling in the liver and in transplantation: effects of extracellular nucleotides on hepatic graft vascular injury, rejection and metabolism

Extracellular nucleotides (e.g. ATP, UTP, ADP) are released by activated endothelium, leukocytes and platelets within the injured vasculature and bind specific cell-surface type-2 purinergic (P2) receptors. This process drives vascular inflammation and thrombosis within grafted organs. Importantly, there are also vascular ectonucleotidases i.e. ectoenzymes that hydrolyze extracellular nucleotides in the blood to generate nucleosides (viz. adenosine). Endothelial cell NTPDase1/CD39 has been shown to critically modulate levels of circulating nucleotides. This process tends to limit the activation of platelet and leukocyte expressed P2 receptors and also generates adenosine to reverse inflammatory events. This vascular protective CD39 activity is rapidly inhibited by oxidative reactions, such as is observed with liver ischemia reperfusion injury. In this review, we chiefly address the impact of these signaling cascades following liver transplantation. Interestingly, the hepatic vasculature, hepatocytes and all non-parenchymal cell types express several components co-ordinating the purinergic signaling response. With hepatic and vascular dysfunction, we note heightened P2- expression and alterations in ectonucleotidase expression and function that may predispose to progression of disease. In addition to documented impacts upon the vasculature during engraftment, extracellular nucleotides also have direct influences upon liver function and bile flow (both under physiological and pathological states). We have recently shown that alterations in purinergic signaling mediated by altered CD39 expression have major impacts upon hepatic metabolism, repair mechanisms, regeneration and associated immune responses. Future clinical applications in transplantation might involve new therapeutic modalities using soluble recombinant forms of CD39, altering expression of this ectonucleotidase by drugs and/or using small molecules to inhibit deleterious P2-mediated signaling while augmenting beneficial adenosine-mediated effects within the transplanted liver.

Regulated catalysis of extracellular nucleotides by vascular CD39/ENTPD1 is required for liver regeneration

BACKGROUND & AIMS: Little is known about how endothelial cells respond to injury, regulate hepatocyte turnover and reconstitute the hepatic vasculature. We aimed to determine the effects of the vascular ectonucleotidase CD39 on sinusoidal endothelial cell responses following partial hepatectomy and to dissect purinergic and growth factor interactions in this model. METHODS: Parameters of liver injury and regeneration, as well as the kinetics of hepatocellular and sinusoidal endothelial cell proliferation, were assessed following partial hepatectomy in mice that do not express CD39, that do not express ATP/UTP receptor P2Y2, and in controls. The effects of extracellular ATP on vascular endothelial growth factor (VEGF), hepatocyte growth factor (HGF), and interleukin-6 responses were determined in vivo and in vitro. Phosphorylation of the endothelial VEGF receptor in response to extracellular nucleotides and growth factors was assessed in vitro. RESULTS: After partial hepatectomy, expression of the vascular ectonucleotidase CD39 increased on sinusoidal endothelial cells. Targeted disruption of CD39 impaired hepatocellular regeneration, reduced angiogenesis, and increased hepatic injury, resulting in pronounced vascular endothelial apoptosis, and decreased survival. Decreased HGF release by sinusoidal endothelial cells, despite high levels of VEGF, reduced paracrine stimulation of hepatocytes. Failure of VEGF receptor-2/KDR transactivation by extracellular nucleotides on CD39-null endothelial cells was associated with P2Y2 receptor desensitization. CONCLUSIONS: Regulated phosphohydrolysis of extracellular nucleotides by CD39 coordinates both hepatocyte and endothelial cell proliferation following partial hepatectomy. Lack of CD39 activity is associated with decreased hepatic regeneration and failure of vascular reconstitution.

Deletion of CD39 inhibits development of metastatic liver cancer

New vessel formation and tumor infiltrating lymphocytes (TIL) influence host responses to malignant tissues. Extracellular adenosine-mediated pathways promote both vascular endothelial cell proliferation and inhibit cytotoxic T cells, thereby potentiating cancer growth. CD39 is the dominant ectonucleotidase of vascular and T regulatory cells and has the potential to generate high levels of adenosine locally. We have previously shown that deletion of Cd39 results in angiogenic failure and T regulatory cell dysfunction with loss of immune suppressive functions. Aim: Investigate impact of CD39 upon development of hepatic metastases. Methods and Results: We studied the development of metastatic liver deposits following portal vein infusion of 1.5x105 melanoma B16/F10 cells, with luciferase expression, in wild type and Cd39-null C57BL/6 mice (n=24). Tumor formation in liver was directly examined and animals imaged at days 7-17 after tumor cell implantation. As predicted, the formation of hepatic malignant foci was markedly suppressed in Cd39-null mice, at all time points examined. To test whether the major impact of Cd39-deletion was upon neovasculature formation or immune responsiveness, adoptive transfer experiments were conducted. Bone marrow transplants (BMT) from Cd39-null or wild type BL/6 mice were placed in lethally irradiated control and/or null mice, in a crossover manner (total n=24 for each group, respectively). Eight weeks postadoptive transfer, melanoma cells were infused via portal vein as before and tumor growth studied. The Cd39-null mice that received wild type BMT mirrored the wild type phenotype with progressive tumor growth observed (n=8 per time point; p=0.015). In contrast, metastases were significantly inhibited in both number and size and ultimately became necrotic in the wild type mice that had received Cd39-null BMT. Conclusions: Bone marrow derived cells mediate the major inhibitory effects of CD39 deletion on tumor growth. Pharmacological inhibition of CD39 may find utility as an adjunct therapy in the management of hepatic malignancy.

Comparison of oral antiviral therapy with valacyclovir or acyclovir after penetrating keratoplasty for herpetic keratitis

AIMS: To compare the outcome of prophylactic oral valacyclovir (VAL) or oral acyclovir treatment (ACV) in patients having undergone penetrating keratoplasty for herpetic keratitis (HK). METHODS: All patients having received a penetrating keratoplasty for HK and being treated postoperatively with either oral VAL or oral ACV (inclusion period from 12/97 to 3/06 and 5/92 to 9/96, respectively) were retrospectively evaluated. Records were analysed for postoperative reactivation of recurrent HK, graft rejection, endothelial cell loss, central corneal thickness and visual acuity after a follow-up of up to 5 years. RESULTS: Twenty patients received VAL and were compared with 19 patients being treated with ACV. Two patients developed clinical signs of recurrent herpetic disease in the VAL group compared with three patients in the ACV group. Two patients from both groups each developed an irreversible graft failure. Best corrected visual acuity improved in both treatment groups from baseline (logMAR) -1.97 (VAL), -1.47 (ACV) to -0.85, -0.72, respectively, at the 1-year follow-up and slightly deteriorated after 5 years in the ACV group (-0.71 VAL vs -1.14 ACV). CONCLUSION: Prophylactic oral VAL treatment is at least as effective as ACV in preventing recurrence in patients who underwent corneal transplantation for HK. The tolerability of the two drugs is similar, but the dosing for VAL might be more comfortable for patients.

Inhibition of mTOR in combination with doxorubicin in an experimental model of hepatocellular carcinoma

BACKGROUND/AIMS: Hepatocellular carcinoma (HCC) is resistant to chemotherapy. We reported that sirolimus, an mTOR inhibitor, has antiangiogenic properties in HCC. Since antiangiogenic therapy may enhance chemotherapy effects, we tested the antitumorigenic properties of sirolimus combined with doxorubicin in experimental HCC. METHODS: Morris Hepatoma (MH) cells were implanted into livers of syngeneic rats. Animals were assigned to sirolimus, pegylated liposomal doxorubicin, both combined or control groups. Tumoral growth was followed by MRI. Antiangiogenic effects were assessed by CD31 immunostaining and capillary tube formation assays. Cell proliferation was monitored in vitro by thymidine incorporation. Expression of p21 and phosphorylated MAPKAP kinase-2 was quantified by immunoblotting. RESULTS: Animals treated with the combination developed smaller tumors with decreased tumor microvessel density compared to animals that received monotherapies. In vitro, inhibition of mTOR further impaired capillary formation in the presence of doxorubicin. Doxorubicin reduced endothelial cell proliferation; inhibition of mTOR accentuated this effect. Doxorubicin stimulated p21 expression and the phosphorylation of MAPKAP kinase-2 in endothelial cells. Addition of mTOR inhibitor down-regulated p21, but did not decrease MAPKAP kinase-2 phosphorylation. CONCLUSIONS: Sirolimus has additive antitumoral and antiangiogenic effects when administered with doxorubicin. These findings offer a rationale for combining mTOR inhibitors with chemotherapy in HCC treatment.

Synthesis and biological evaluation of epothilone A dimeric compounds

The preparation and biological evaluation of a novel series of dimeric epothilone A derivatives (1-6) are described. Two types of diacyl spacers were introduced to establish the various dimeric epothilone A constructs. The effect of these compounds on tubulin polymerization and their cytotoxicity against four different cancer cell lines are reported. Several of the newly synthesized compounds inhibit endothelial cell differentiation and endothelial cell migration that are key steps of the angiogenic process.

Tenomodulin is necessary for tenocyte proliferation and tendon maturation

Tenomodulin (Tnmd) is a member of a new family of type II transmembrane glycoproteins. It is predominantly expressed in tendons, ligaments, and eyes, whereas the only other family member, chondromodulin I (ChM-I), is highly expressed in cartilage and at lower levels in the eye and thymus. The C-terminal extracellular domains of both proteins were shown to modulate endothelial-cell proliferation and tube formation in vitro and in vivo. We analyzed Tnmd function in vivo and provide evidence that Tnmd is processed in vivo and that the proteolytically cleaved C-terminal domain can be found in tendon extracts. Loss of Tnmd expression in gene targeted mice abated tenocyte proliferation and led to a reduced tenocyte density. The deposited amounts of extracellular matrix proteins, including collagen types I, II, III, and VI and decorin, lumican, aggrecan, and matrilin-2, were not affected, but the calibers of collagen fibrils varied significantly and exhibited increased maximal diameters. Tnmd-deficient mice did not have changes in tendon vessel density, and mice lacking both Tnmd and ChM-I had normal retinal vascularization and neovascularization after oxygen-induced retinopathy. These results suggest that Tnmd is a regulator of tenocyte proliferation and is involved in collagen fibril maturation but do not confirm an in vivo involvement of Tnmd in angiogenesis.

The blood-brain and the blood-cerebrospinal fluid barriers: function and dysfunction

The central nervous system (CNS) is tightly sealed from the changeable milieu of blood by the blood-brain barrier (BBB) and the blood-cerebrospinal fluid (CSF) barrier (BCSFB). While the BBB is considered to be localized at the level of the endothelial cells within CNS microvessels, the BCSFB is established by choroid plexus epithelial cells. The BBB inhibits the free paracellular diffusion of water-soluble molecules by an elaborate network of complex tight junctions (TJs) that interconnects the endothelial cells. Combined with the absence of fenestrae and an extremely low pinocytotic activity, which inhibit transcellular passage of molecules across the barrier, these morphological peculiarities establish the physical permeability barrier of the BBB. In addition, a functional BBB is manifested by a number of permanently active transport mechanisms, specifically expressed by brain capillary endothelial cells that ensure the transport of nutrients into the CNS and exclusion of blood-borne molecules that could be detrimental to the milieu required for neural transmission. Finally, while the endothelial cells constitute the physical and metabolic barrier per se, interactions with adjacent cellular and acellular layers are prerequisites for barrier function. The fully differentiated BBB consists of a complex system comprising the highly specialized endothelial cells and their underlying basement membrane in which a large number of pericytes are embedded, perivascular antigen-presenting cells, and an ensheathment of astrocytic endfeet and associated parenchymal basement membrane. Endothelial cell morphology, biochemistry, and function thus make these brain microvascular endothelial cells unique and distinguishable from all other endothelial cells in the body. Similar to the endothelial barrier, the morphological correlate of the BCSFB is found at the level of unique apical tight junctions between the choroid plexus epithelial cells inhibiting paracellular diffusion of water-soluble molecules across this barrier. Besides its barrier function, choroid plexus epithelial cells have a secretory function and produce the CSF. The barrier and secretory function of the choroid plexus epithelial cells are maintained by the expression of numerous transport systems allowing the directed transport of ions and nutrients into the CSF and the removal of toxic agents out of the CSF. In the event of CNS pathology, barrier characteristics of the blood-CNS barriers are altered, leading to edema formation and recruitment of inflammatory cells into the CNS. In this review we will describe current knowledge on the cellular and molecular basis of the functional and dysfunctional blood-CNS barriers with focus on CNS autoimmune inflammation.

Culture-induced changes in blood-brain barrier transcriptome: implications for amino-acid transporters in vivo

Tight homeostatic control of brain amino acids (AA) depends on transport by solute carrier family proteins expressed by the blood-brain barrier (BBB) microvascular endothelial cells (BMEC). To characterize the mouse BMEC transcriptome and probe culture-induced changes, microarray analyses of platelet endothelial cell adhesion molecule-1-positive (PECAM1(+)) endothelial cells (ppMBMECs) were compared with primary MBMECs (pMBMEC) cultured in the presence or absence of glial cells and with b.End5 endothelioma cell line. Selected cell marker and AA transporter mRNA levels were further verified by reverse transcription real-time PCR. Regardless of glial coculture, expression of a large subset of genes was strongly altered by a brief culture step. This is consistent with the known dependence of BMECs on in vivo interactions to maintain physiologic functions, for example, tight barrier formation, and their consequent dedifferentiation in culture. Seven (4F2hc, Lat1, Taut, Snat3, Snat5, Xpct, and Cat1) of nine AA transporter mRNAs highly expressed in freshly isolated ppMBMECs were strongly downregulated for all cultures and two (Snat2 and Eaat3) were variably regulated. In contrast, five AA transporter mRNAs with low expression in ppMBMECs, including y(+)Lat2, xCT, and Snat1, were upregulated by culture. We hypothesized that the AA transporters highly expressed in ppMBMECs and downregulated in culture have a major in vivo function for BBB transendothelial transport.