Endothelial cells in endogenous and transplanted pancreatic islets: differences in the expression of angiogenic peptides and receptors

Organ-specific endothelial cell heterogenicity and its impact on regenerative medicine and biomedical engineering applications

Endothelial cells (ECs) are a key cellular component of the vascular system as they form the inner lining of the blood vessels. Recent findings highlight that ECs express extensive phenotypic heterogenicity when following the vascular tree from the major vasculature down to the organ capillaries. However, in vitro models, used for drug development and testing, or to study the role of ECs in health and disease, rarely acknowledge this EC heterogenicity. In this review, we highlight the main differences between different EC types, briefly summarize their different characteristics and focus on the use of ECs in in vitro models. We introduce different approaches on how ECs can be utilized in co-culture test systems in the field of brain, pancreas, and liver research to study the role of the endothelium in health and disease. Finally, we discuss potential improvements to current state-of-the-art in vitro models and future directions.

Heterogeneity and Dynamics of Vasculature in the Endocrine System During Aging and Disease

The endocrine system consists of several highly vascularized glands that produce and secrete hormones to maintain body homeostasis and regulate a range of bodily functions and processes, including growth, metabolism and development. The dense and highly vascularized capillary network functions as the main transport system for hormones and regulatory factors to enable efficient endocrine function. The specialized capillary types provide the microenvironments to support stem and progenitor cells, by regulating their survival, maintenance and differentiation. Moreover, the vasculature interacts with endocrine cells supporting their endocrine function. However, the structure and niche function of vasculature in endocrine tissues remain poorly understood. Aging and endocrine disorders are associated with vascular perturbations. Understanding the cellular and molecular cues driving the disease, and age-related vascular perturbations hold potential to manage or even treat endocrine disorders and comorbidities associated with aging. This review aims to describe the structure and niche functions of the vasculature in various endocrine glands and define the vascular changes in aging and endocrine disorders.

Endothelial Cell-Derived Triosephosphate Isomerase Attenuates Insulin Secretion From Pancreatic Beta Cells of Male Rats

Insulin secretion from pancreatic beta cells is tightly regulated by glucose and paracrine signals within the microenvironment of islets of Langerhans. Extracellular matrix from islet microcapillary endothelial cells (IMEC) affect beta-cell spreading and amplify insulin secretion. This study was aimed at investigating the hypothesis that contact-independent paracrine signals generated from IMEC may also modulate beta-cell insulin secretory functions. For this purpose, conditioned medium (CMp) preparations were prepared from primary cultures of rat IMEC and were used to simulate contact-independent beta cell-endothelial cell communication. Glucose-stimulated insulin secretion (GSIS) assays were then performed on freshly isolated rat islets and the INS-1E insulinoma cell line, followed by fractionation of the CMp, mass spectroscopic identification of the factor, and characterization of the mechanism of action. The IMEC-derived CMp markedly attenuated first- and second-phase GSIS in a time- and dose-dependent manner without altering cellular insulin content and cell viability. Size exclusion fractionation, chromatographic and mass-spectroscopic analyses of the CMp identified the attenuating factor as the enzyme triosephosphate isomerase (TPI). An antibody against TPI abrogated the attenuating activity of the CMp while recombinant human TPI (hTPI) attenuated GSIS from beta cells. This effect was reversed in the presence of tolbutamide in the GSIS assay. In silico docking simulation identified regions on the TPI dimer that were important for potential interactions with the extracellular epitopes of the sulfonylurea receptor in the complex. This study supports the hypothesis that an effective paracrine interaction exists between IMEC and beta cells and modulates glucose-induced insulin secretion via TPI-sulfonylurea receptor-KATP channel (SUR1-Kir6.2) complex attenuating interactions.

Inhibition of p38 MAP Kinase in the Early Posttransplantation Phase Redistributes Blood Vessels from the Surrounding Stroma into the Transplanted Endocrine Tissue

Transplanted pancreatic islets attain a chronically decreased vascular density following transplantation, despite the increased concentrations of vascular endothelial growth factor (VEGF) secreted from beta-cells in response to hypoxia during culture and in the immediate posttransplantation phase. VEGF, however, exerts dual effects on endothelial cells, and in islet endothelial cells of the adult, the vascular permeability-inducing effects of VEGF seem normally more pronounced than those to induce angiogenesis. p38 MAP kinase activity has recently been shown to serve as a switch to separate these properties of VEGF; inhibition of p38 MAP kinase activity enhances VEGF-induced angiogenesis and, at the same time, abrogates VEGF-induced vascular permeability. We hypothesized that the revascularization of transplanted islets may be hampered by a predisposition of adult islet endothelial cells to react to VEGF by forming fenestrae rather than migrating and proliferating. We therefore administered the p38 MAP kinase inhibitor SB203580 by daily IP injections for the first 14 days following transplantation, and then studied the influence of this treatment on the oxygen tension, blood perfusion, and vascular density of the islet grafts 1 month posttransplantation. SB203580 treatment redistributed islet graft blood vessels from the stroma into the endocrine tissue, and this redistribution of blood vessels into the endocrine tissue was accompanied by an increased oxygenation of the islet cells. However, the total number of blood vessels in the tissue was not affected. The blood perfusion of the islet grafts was also similar in control and SB203580-treated animals. Our results suggest that effects of VEGF to preferentially induce vascular permeability may partially contribute to, but is not the main cause of, low revascularization of transplanted islets.

Intra-islet endothelial cell and β-cell crosstalk: Implication for islet cell transplantation

The intra-islet microvasculature is a critical interface between the blood and islet endocrine cells governing a number of cellular and pathophysiological processes associated with the pancreatic tissue. A growing body of evidence indicates a strong functional and physical interdependency of β-cells with endothelial cells (ECs), the building blocks of islet microvasculature. Intra-islet ECs, actively regulate vascular permeability and appear to play a role in fine-tuning blood glucose sensing and regulation. These cells also tend to behave as “guardians”, controlling the expression and movement of a number of important immune mediators, thereby strongly contributing to the physiology of islets. This review will focus on the molecular signalling and crosstalk between the intra-islet ECs and β-cells and how their relationship can be a potential target for intervention strategies in islet pathology and islet transplantation.

Superior beta cell proliferation, function and gene expression in a subpopulation of rat islets identified by high blood perfusion

AIMS/HYPOTHESIS

The blood perfusion of individual pancreatic islets is highly variable, with a subgroup of islets having high perfusion and blood vessels responsive to further blood flow increase induced by glucose. This study tested the hypothesis that there is heterogeneity between islets with regard to beta cell proliferation, function and gene expression based on differences in their blood perfusion.

METHODS

Fluorescent microspheres were injected into the ascending aorta, and then microsphere-containing and non-microsphere-containing pancreatic islets were isolated for investigation. By this procedure, the 5% of islets with the greatest blood perfusion were identified for study. Islet endothelial cells were isolated separately to investigate the role of improved vascular support in the observed differences.

RESULTS

The vascular network was found to be more dense and tortuous in microsphere-containing than other islets. The most highly blood-perfused islets also had a higher rate of beta cell proliferation, superior beta cell function and a markedly different gene expression from other islets. Cultured islets exposed to islet endothelial cell products had a similarly increased beta cell proliferation rate, yet significantly fewer changes in gene expression than observed in the most highly blood-perfused islets.

CONCLUSIONS/INTERPRETATION

A novel heterogeneity between islets was observed, with superior beta cell proliferation, function and gene expression in a subpopulation of islets identified by high blood perfusion. In contrast with a previously described population of low-oxygenated, sleeping islets, which are recruited into functionality when needed, the presently described heterogeneity is shown to remain in vitro after islet isolation.

Angiopoietin 2 alters pancreatic vascularization in diabetic conditions

Aims/hypothesis

Islet vascularization, by controlling beta-cell mass expansion in response to increased insulin demand, is implicated in the progression to glucose intolerance and type 2 diabetes. We investigated how hyperglycaemia impairs expansion and differentiation of the growing pancreas. We have grafted xenogenic (avian) embryonic pancreas in severe combined immuno-deficient (SCID) mouse and analyzed endocrine and endothelial development in hyperglycaemic compared to normoglycaemic conditions.

Methods

14 dpi chicken pancreases were grafted under the kidney capsule of normoglycaemic or hyperglycaemic, streptozotocin-induced, SCID mice and analyzed two weeks later. Vascularization was analyzed both quantitatively and qualitatively using either in situ hybridization with both mouse- and chick-specific RNA probes for VEGFR2 or immunohistochemistry with an antibody to nestin, a marker of endothelial cells that is specific for murine cells. To inhibit angiopoietin 2 (Ang2), SCID mice were treated with 4 mg/kg IP L1–10 twice/week.

Results

In normoglycaemic condition, chicken-derived endocrine and exocrine cells developed well and intragraft vessels were lined with mouse endothelial cells. When pancreases were grafted in hyperglycaemic mice, growth and differentiation of the graft were altered and we observed endothelial discontinuities, large blood-filled spaces. Vessel density was decreased. These major vascular anomalies were associated with strong over-expression of chick-Ang2. To explore the possibility that Ang2 over-expression could be a key step in vascular disorganization induced by hyperglycaemia, we treated mice with L1–10, an Ang-2 specific inhibitor. Inhibition of Ang2 improved vascularization and beta-cell density.

Conclusions

This work highlighted an important role of Ang2 in pancreatic vascular defects induced by hyperglycaemia.

Thrombospondin-1: an islet endothelial cell signal of importance for β-cell function

OBJECTIVE

Loss of thrombospondin (TSP)-1 in pancreatic islets has been shown to cause islet hyperplasia. This study tested the hypothesis that endothelial-derived TSP-1 is important for β-cell function.

RESEARCH DESIGN AND METHODS

Islet function was evaluated both in vivo and in vitro. Messenger RNA and protein expression were measured by real-time PCR and Western blot, respectively. The role of endothelial-derived TSP-1 for β-cell function was determined using a transplantation design in which recipient blood vessels either were allowed to grow or not into the transplanted islets.

RESULTS

TSP-1-deficient mice were glucose intolerant, despite having an increased β-cell mass. Moreover, their islets had decreased glucose-stimulated insulin release, (pro)insulin biosynthesis, and glucose oxidation rate, as well as increased expression of uncoupling protein-2 and lactate dehydrogenase-A when compared with control islets. Almost all TSP-1 in normal islets were found to be derived from the endothelium. Transplantation of free and encapsulated neonatal wild-type and TSP-1-deficient islets was performed in order to selectively reconstitute with TSP-1-positive or -negative blood vessels in the islets and supported that the β-cell defects occurring in TSP-1-deficient islets reflected postnatal loss of the glycoprotein in the islet endothelial cells. Treatment of neonatal TSP-1-deficient mice with the transforming growth factor (TGF)β-1-activating sequence of TSP-1 showed that reconstitution of TGFβ-1 activation prevented the development of decreased glucose tolerance in these mice. Thus, endothelial-derived TSP-1 activates islet TGFβ-1 of importance for β-cells.

CONCLUSIONS

Our study indicates a novel role for endothelial cells as functional paracrine support for pancreatic β-cells.

Angiostatic factors normally restrict islet endothelial cell proliferation and migration: implications for islet transplantation

New blood vessel formation in transplanted islets occurs within 7-14 days post-transplantation through both the expansion of donor islet endothelium and ingrowth of blood vessels from the implantation organ. However, several studies indicate that although the islets attract recipient blood vessels, the formed intra-islet vascular network is insufficient, which affects islet post-transplant function. This study aimed to develop an in vitro model to investigate the migration and proliferation properties of isolated liver and islet endothelium.Rat islet or liver endothelium was purified using Bandeiraea simplicifolia(BS-1)-coated Dynabeads. The liver endothelium displayed an increased migration and proliferation to islet-conditioned medium. These effects were fully prevented by adding a neutralizing vascular endothelial growth factor (VEGF)-antibody. In contrast, islet-produced VEGF failed to induce islet endothelial cell migration and only had marginal effects on islet endothelial cell proliferation.These properties could, however, be activated through blocking the effects of either endostatin, thrombospondin-1 or alpha(1)-antitrypsin. In conclusion, VEGF may attract recipient blood vessels towards intrahepatically transplanted islets,but intra-islet vascular expansion is hampered by angiostatic factors present within the islets and the islet endothelium. Inhibition of angiostatic factors early after transplantation may provide a strategy to restore the islet vascular network and improve islet graft function.

Endothelial cell signalling supports pancreatic beta cell function in the rat

AIMS/HYPOTHESIS

The proximity of endothelial cells and beta cells in islets by necessity means that they are exposed to each other's products. Whereas islet endothelial cells require signals from beta cells to function properly, endothelin-1, thrombospondin-1 and laminins, among others, have been identified as endothelial-derived molecules, although their full effects on beta cells have not been explored. We tested the hypothesis that islet endothelial-derived products affect beta cell function.

METHODS

Endothelial cells from rat islets were proliferated and purified. Endothelium-conditioned culture medium (ECCM) was obtained by maintaining the endothelial cells in culture medium. Islet function was evaluated following exposure of cultured islets to standard culture medium or ECCM. Changes in mRNA levels for key beta cell metabolic enzymes were also measured in islets after ECCM exposure.

RESULTS

Glucose-stimulated insulin release and islet insulin content were markedly enhanced by exposure to ECCM. This was at least partly explained by improved mitochondrial function, as assessed by glucose oxidation and an upregulation of the mitochondrial gene for glycerol-3-phosphate dehydrogenase (mGpdh [also known as Gpd2]), combined with upregulation of the rate-limiting enzyme in the glycolysis, glucokinase, in the islets. The intracellular degradation of insulin was also decreased in the islets. Islet endothelial cells produced laminins, and the positive effects of islet endothelial cells were prevented by addition of a neutralising antibody to the beta1-chain of laminin. Addition of exogenous laminin stimulated islet function.

CONCLUSIONS/INTERPRETATION

This study provides proof of principle that endothelial cells can affect the function of beta cells in their vicinity and that this is at least partially mediated by laminins.

Vascular niche of pancreatic islets

Pancreatic islets are highly vascularized micro-organs. Approximately 10% of an islet consists of blood vessels. The induction and maintenance of the islet vascular system depend on VEGF secreted from β-cells. VEGF is also critical for the phenotype of the islet vasculature by induction of a vast number of fenestrae. The islet vasculature serves the role of supplying the endocrine cells with oxygen and nutrients, but may also be important for proper glucose sensing of the cells, for paracrine support of endocrine function and growth, and for drainage of metabolites and secreted islet hormones into the systemic circulation. Emerging evidence suggests an important role of islet endothelial cells to maintain β-cell function and growth by secretion of molecules such as hepatocyte growth factor, thrombospondin-1 and laminins, thereby forming a vascular niche for the endocrine cells.

Prolactin treatment improves engraftment and function of transplanted pancreatic islets

Transplantation of pancreatic islets is clinically used to treat type 1 diabetes but requires multiple donors. Previous experimental studies demonstrated that transplanted islets have a low blood vessel density, which leads to a hypoxic microenvironment. The present study tested the hypothesis that experimental prolactin pretreatment, a substance that seems to stimulate angiogenesis in endogenous islets, would increase graft blood vessel density, thereby improving transplantation outcome. Pancreatic islets from C57BL/6 mice were incubated with prolactin (500 ng/ml) or vehicle during the last 24 h of culture before syngeneic transplantation beneath the renal capsule, or recipients were injected with prolactin or vehicle for the first 7 d after transplantation. One month after transplantation, graft vascular density, blood flow, oxygen tension, endocrine volume, and function were evaluated. Also, human islets were incubated with prolactin or vehicle before experimental transplantation and investigated for vascular engraftment. Vascular engraftment of syngeneically transplanted mouse islets was improved by both in vivo and in vitro prolactin pretreatment. Moreover, prolactin pretreatment in vitro of islets used for transplantation improved recovery from diabetes in a minimal islet mass model. Interestingly, also human islets subjected to prolactin treatment before experimental transplantation demonstrated improved revascularization, blood perfusion, and oxygen tension when evaluated 1 month after transplantation. We conclude that prolactin may improve engraftment of transplanted pancreatic islets. The protocol with pretreatment of islets ex vivo could minimize the risk of side effects when used in the clinical setting.

Improved vascular engraftment and graft function after inhibition of the angiostatic factor thrombospondin-1 in mouse pancreatic islets

OBJECTIVE

Insufficient development of a new intra-islet capillary network after transplantation may be one contributing factor to the failure of islet grafts in clinical transplantation. The present study tested the hypothesis that the angiostatic factor thrombospondin-1 (TSP-1), which is normally present in islets, restricts intra-islet vascular expansion posttransplantation.

RESEARCH DESIGN AND METHODS

Pancreatic islets of TSP-1-deficient (TSP-1(-/-)) mice or wild-type islets transfected with siRNA for TSP-1 were transplanted beneath the renal capsule of syngeneic or immunocompromised recipient mice.

RESULTS

Both genetically TSP-1(-/-) islets and TSP-1 siRNA-transfected islet cells demonstrated an increased vascular density when compared with control islets 1 month after transplantation. This was also reflected in a markedly increased blood perfusion and oxygenation of the grafts. The functional importance of the improved vascular engraftment was analyzed by comparing glucose-stimulated insulin release from islet cells transfected with either TSP-1 siRNA or scramble siRNA before implantation. These experiments showed that the increased revascularization of grafts composed of TSP-1 siRNA-transfected islet cells correlated to increments in both their first and second phase of glucose-stimulated insulin secretion.

CONCLUSIONS

Our findings demonstrate that inhibition of TSP-1 in islets intended for transplantation may be a feasible strategy to improve islet graft revascularization and function.

Effects of external ATP on Ca(2+) signalling in endothelial cells isolated from mouse islets

External ATP is believed to initiate and propagate Ca(2+) signals co-ordinating the insulin release pulses within and among the different islets in the pancreas. The possibility that islet endothelial cells participate in this process was evaluated by comparing the effects on [Ca(2+)](i) of purinoceptor activation in these cells with those in beta-cells. beta-Cell-rich pancreatic islets were isolated from ob/ob mice and dispersed into single cells/aggregates. After culture with or without endothelial cell growth supplement (ECGS), the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) was measured with ratiometric fura-2 technique. Presence of ECGS or prolongation of culture (>5 days) resulted in proliferation of endothelial cells and altered their phenotype from rounded to elongated. Endothelial cells, preliminarily identified by attachment of Dynabeads coated with the Bandeiraea simplicifolia 1 lectin (BS-1), responded in a similar way as those stained with CD31 antibodies after measurements of [Ca(2+)](i). Spontaneous transients and oscillations of [Ca(2+)](i )were seen in beta-cells, but not in endothelial cells exposed to 20 mM glucose. Addition of ATP (10 microM) resulted in pronounced and more extended rise of [Ca(2+)](i) in endothelial cells than in beta-cells. The endothelial cells differed from the beta-cells by also responding with a rise of [Ca(2+)](i) to 10 microM UTP, but not to equimolar ADP and acetylcholine. The results support the idea of mutual interactions between islet endothelium and beta-cells based on ATP-induced Ca(2+) signals. It is suggested that the endothelial cells have a tonic inhibitory action on beta-cell P2 purinoceptors resulting in impaired synchronization of the insulin release pulses.

Revascularization and remodelling of pancreatic islets grafted under the kidney capsule

The revascularization and the structural changes resulting from interactions between the graft and the host were investigated in transplanted pancreatic islets under the kidney capsule. Islets were isolated from mice pancreata and transplanted in syngeneic diabetic animals. Graft-bearing kidneys were collected on different days post-transplant and processed for light microscopy, immunohistochemistry and transmission electron microscopy. A numerical analysis was performed in order to compare the percentage number of the different types of cells in native islets and at different time points after the transplant. Recipient animals reversed diabetes within 4 days. An intraperitoneal glucose tolerance test was performed to determine islet functionality under stressful conditions. During the initial few days post-transplant, the islets showed peculiar shapes and the graft tended to aggregate along the vessels. Starting at days 4-7 post-transplant, islets were revascularized from vessels connected to both the cortical and the capsular vascular network of the kidney. From day 7-14 post-transplant, the vessels progressively appeared more similar in features and size to those of in situ pancreatic islets. Both the percentage number of the different cell types and the distribution of Alpha, Beta and Delta cells inside the graft were significantly different as compared with intact islets, demonstrating quantitative and structural changes after the engraftment. No concomitant proliferation of Beta cells was detected using a bromodeoxyuridin staining method. Despite the fact that quick revascularization preserved a large mass of tissue, the remodelling process of the graft and the newly formed vascularization led to a different organization of the endocrine tissue as compared with intact in situ islets. This constitutes the morphological basis for alterations of the normal intercellular interactions and may explain the altered secretory cell function often observed in transplant.

Vascular reactivity in arterioles from normal and alloxan-diabetic mice: studies on single perfused islets

Pancreatic islets possess an autonomous mechanism of blood flow regulation, independent of that of the exocrine pancreas. To study islet vascular regulation without confounding effects of the exocrine blood vessels, we have developed a technique enabling us to isolate single pancreatic islets and then to perfuse them using their endogenous vasculature for distribution of the medium. This made it possible to directly study the vascular reactivity of islet arterioles to different substances. We confirmed that control of islet blood flow is mainly located at the precapillary level. As expected, administration of angiotensin II and l-nitro-arginine methyl ester contracted islet arterioles, whereas nitric oxide and adenosine dilated them. d-glucose, the main insulin secretagogue, had a selective dilating effect on smooth muscle in islet arterioles but not in glomerular afferent arterioles. The response to glucose was amplified in islet arterioles from diabetic animals, indicating enhanced islet blood perfusion in diabetes. This newly developed technique for perfusing isolated pancreatic islets will provide new insights into islet perfusion control and its possible contributions to the pathogenesis of type 2 diabetes.

The pancreatic islet endothelial cell: emerging roles in islet function and disease

The pancreatic islets are one of the most vascularized organs of the body. This likely reflects the requirements of the organ for a rich supply of nutrients and oxygen to the tissue, as well as the need for rapid disposal of metabolites and secreted hormones. The islet endothelium is richly fenestrated to facilitate trans-endothelial transport of secreted hormones, has a unique expression of surface markers, and produces a number of vasoactive substances and growth factors. The islet endothelial cells play a critical role in the early phase of type 1 diabetes mellitus by increasing the expression of surface leucocyte-homing receptors, thereby enabling immune cells to enter the endocrine tissue and cause beta-cell destruction. Following transplantation, pancreatic islets lack a functional capillary system and need to be properly revascularized. Insufficient revascularization may severely affect the transport properties of the islet endothelial system, resulting in a dysfunctional islet graft.