Integrin signaling via RGD peptides and anti-beta1 antibodies confers resistance to apoptosis in islets of Langerhans

The effect of extracellular matrix components on the preservation of human islet function in vitro

Human islet isolation leads to the loss of the ECM basement membrane which contributes to eventual apoptosis in vitro. The reestablishment of this environment is vital in understanding the mechanism of islet interaction with its surroundings in order to arrive at conditions favourable to islet culture in vitro. In this study, we investigated the effects of the main ECM components collagen I and IV, fibronectin, and laminin on human islet adhesion, survival, and functionality. Results have provided insight into integrin-mediated effects and behaviour. Collagen I/IV and fibronectin induced adhesion, while fibronectin was the only ECM protein capable of maintaining islet structural integrity and insulin content distribution. Furthermore, islet phenotype was eventually lost, but insulin gene expression was highest in islets cultured on collagen I and IV. However, insulin release was highest on fibronectin, along with a decrease in SUR1 expression, while glucose metabolism, along with GLUT2 and GCK expression, was highest on collagen I and IV surfaces. These findings provide a basis for the future establishment of a modified three-dimensional construct for the culture of human pancreatic islets in vitro.

beta1 integrin/FAK/ERK signalling pathway is essential for human fetal islet cell differentiation and survival

beta1 integrin and collagen matrix interactions regulate the survival of cells by associating with focal adhesion kinase (FAK) and initiating MAPK/ERK signalling, but little is known about these signalling pathways during human fetal islet ontogeny. The purpose of this study was to investigate whether beta1 integrin/FAK activation of the MAPK/ERK pathway regulates human fetal islet cell expression of endocrine cell markers and survival. Isolated human (18-21 weeks fetal age) islet-epithelial cell clusters, cultured on collagen I, were examined using beta1 integrin blocking antibody, beta1 integrin siRNA and FAK expression vector. Perturbing beta1 integrin function in the human fetal islet-epithelial cell clusters resulted in a marked decrease in cell adhesion, in parallel with a reduction in the number of cells expressing PDX-1, insulin and glucagon (p < 0.05). beta1 integrin blockade disorganized focal adhesion contacts in the PDX-1(+) cells and decreased activation of FAK and ERK1/2 signalling in parallel with an increase in expression of cleaved caspases 9 and 3 (p < 0.01). Similar results were obtained following an siRNA knock-down of beta1 integrin expression. In contrast, over-expression of FAK not only increased phospho-ERK and the expression of PDX-1, insulin and glucagon (p < 0.05) but also abrogated the decreases in phospho-ERK and PDX-1 by beta1 integrin blockade. This study demonstrates that activation of the FAK/ERK signalling cascade by beta1 integrin is involved in the differentiation and survival of human fetal pancreatic islet cells.

Extracellular matrix in pancreatic islets: relevance to scaffold design and transplantation

Intrahepatic islet transplantation provides a potentially more benign alternative to pancreatic transplantation. However, islet transplants are associated with limited engraftment potential. This inefficiency is likely at least partially attributable to the isolation process, which removes islets from their native environment. Isolation not only disrupts the internal vascularization and innervation of islets, but also fundamentally changes interactions between islet cells and macromolecules of the extracellular matrix (ECM). Signaling interactions between islet cells and ECM are known to regulate multiple aspects of islet physiology, including survival, proliferation, and insulin secretion. Although it is highly likely that disruptions to these interactions during isolation significantly affect transplant outcomes, the true implications of these conditions are not well understood. The following article reviews current understandings and uncertainties in islet-ECM interactions and explains their potential impact on posttransplant engraftment. Topics covered include matrix and receptor compositions in native islets, effects of isolation and culture on islet-ECM interactions, and potential for postisolation restoration of islet-ECM interactions. Greater understanding in these areas may help to reduce isolation and transplantation stresses and improve islet engraftment.

Cell-matrix interactions improve beta-cell survival and insulin secretion in three-dimensional culture

Controlled matrix interactions were presented to pancreatic beta-cells in three-dimensional culture within poly(ethylene glycol) hydrogels. Dispersed MIN6 beta-cells were encapsulated in gel environments containing the following entrapped extracellular matrix (ECM) proteins: collagen type I, collagen type IV, fibrinogen, fibronectin, laminin, and vitronectin. In ECM-containing gels, beta-cell survival was significantly better than in gels without ECM over 10 days. Correspondingly, apoptosis in encapsulated beta-cells was less in the presence of each matrix protein, suggesting the ability of individual matrix interactions to prevent matrix signaling-related apoptosis (anoikis). MIN6 beta-cells cultured in gels containing collagen type IV or laminin secreted more insulin in response to glucose stimulation than beta-cells in all other experimental conditions. Variations in collagen type IV or laminin concentration between 10 microg/mL and 250 microg/mL did not affect insulin secretion. Finally, beta-cell function in hydrogels presenting both collagen type IV and laminin revealed synergistic interactions. With a total protein concentration of 100 microg/mL, three gel compositions of varying ratios of collagen type IV to laminin (25:75, 50:50, and 75:25) were tested. In the presence of 25 microg/mL of collagen type IV and 75 microg/mL of laminin, beta-cell insulin secretion was greater than with laminin or collagen type IV individually. These results demonstrate that specific, rationally designed extracellular environments promote isolated beta-cell survival and function.

Mouse pancreatic islets are resistant to indoleamine 2,3 dioxygenase-induced general control nonderepressible-2 kinase stress pathway and maintain normal viability and function

Islet transplantation is a promising treatment for diabetes. However, it faces several challenges including requirement of systemic immunosuppression. Indoleamine 2,3-dioxygenase (IDO), a tryptophan degrading enzyme, is a potent immunomodulatory factor. Local expression of IDO in bystander fibroblasts suppresses islet allogeneic immune response in vitro. The aim of the present study was to investigate the impact of IDO on viability and function of mouse islets embedded within IDO-expressing fibroblast-populated collagen scaffold. Mouse islets were embedded within collagen matrix populated with IDO adenovector-transduced or control fibroblasts. Proliferation, insulin content, glucose responsiveness, and activation of general control nonderepressible-2 kinase stress-responsive pathway were then measured in IDO-exposed islets. In vivo viabilities of composite islet grafts were also tested in a syngeneic diabetic animal model. No reduction in islet cells proliferation was detected in both IDO-expressing and control composites compared to the baseline rates. Islet functional studies showed normal insulin content and secretion in both preparations. In contrast to lymphocytes, general control nonderepressible-2 kinase pathway was not activated in islets cocultured with IDO-expressing fibroblasts. When transplanted to diabetic mice, syngeneic IDO-expressing composite islet grafts were functional up to 100 days tested. These findings collectively confirm normal viability and functionality of islets cocultured with IDO-expressing cells and indicate the feasibility of development of a functional nonrejectable islet graft.

The use of an approved biodegradable polymer scaffold as a solid support system for improvement of islet engraftment

The aim of this study was to investigate whether the use of a medically approved biodegradable scaffold as a solid support system would enhance graft survival following transplantation into the omental pouch in a preclinical large animal model. Six beagle dogs underwent total pancreatectomy followed by islet autotransplantation into the omental pouch. Four dogs received islets seeded in a biodegradable polymer scaffold and two received free islets without a scaffold. All four animals that received islets in the scaffold became normoglycemic without exogeneous insulin injection. One dog, transplanted with the largest number of islets, maintained a normal metabolic state until the graft was removed at 5 months posttransplant. In two out of the three that received a marginal islet mass, insulin independence was sustained up to 2 months. In contrast, two dogs transplanted with a similar marginal mass without the scaffold never became normoglycemic. Histological examination of the grafts in the scaffold showed numerous well-granulated, insulin-containing cells as well as glucagon-positive cells. These results indicate that biodegradable scaffolds may enhance survival and function of islet grafts. Manipulation of the microenvironment of transplanted islets may constitute the basis for new approaches to enhance islet engraftment.

Unique basement membrane structure of human pancreatic islets: implications for beta-cell growth and differentiation

Basement membranes (BMs) are an important part of the physiological microenvironment of pancreatic islet cells. In mouse islets, beta-cells interact directly with BMs of capillary endothelial cells. We have shown that in the human islets, the capillaries are surrounded by a double BM both in foetal and adult tissues. The endocrine islet cells are facing a BM that is separate from the endothelia. Laminins are the functionally most important component of BMs. The only laminin isoform present in the human endocrine islet BM is laminin-511 (previously known as laminin 10). The islet cells facing this BM have a strong and polarized expression of Lutheran glycoprotein, which is a well-known receptor for the laminin alpha 5 chain. Dispersed human islet cells adhere to purified human laminin-511 and the binding is equally effectively blocked by a soluble form of Lutheran as by antibody against integrin beta1. Our results reveal unique features of the BM structure of human islets, different from rodents. This information has potentially important implications for the generation of an optimal microenvironment for beta-cell function, proliferation and differentiation.

Risk factors for islet loss during culture prior to transplantation

Culturing islets can add great flexibility to a clinical islet transplant program. However, a reduction in the islet mass has been frequently observed during culture and its degree varies. The aim of this study was to identify the risk factors associated with a significant islet loss during culture. One-hundred and four islet preparations cultured in an attempt to use for transplantation constituted this study. After culture for 20 h (median), islet yield significantly decreased from 363 309 +/- 12 647 to 313 035 +/- 10 862 islet equivalent yield (IE) (mean +/- SE), accompanied by a reduction in packed tissue volume from 3.9 +/- 0.1 to 3.0 +/- 0.1 ml and islet index (IE/islet particle count) from 1.20 +/- 0.04 to 1.05 +/- 0.04. Culture did not markedly alter islet purity or percent of trapped islet. Morphology score and viability were significantly improved after culture. Of 104 islet preparations, 37 suffered a substantial islet loss (> 20%) over culture. Factors significantly associated with risk of islet loss identified by univariate analysis were longer cold ischemia time, two-layer method (TLM) preservation, lower islet purity, and higher islet index. Multivariate analysis revealed that independent predictors of islet loss were higher islet index and the use of TLM. This study provides novel information on the link between donor- isolation factors and islet loss during culture.

Islet auto transplantation following total pancreatectomy: a long-term assessment of graft function

Total pancreatectomy is considered the final resort in the treatment of chronic pancreatitis; however, here we show that simultaneous islet autotransplantation can abrogate the onset of diabetes.

METHODS

: In Leicester, 46 patients have now undergone total pancreatectomy with immediate islet auto transplant, and they have received a median of 2246 islet equivalent (IEQ)/kg body weight (range, 405-20,385 IEQ/kg body weight).

RESULTS

: Twelve patients have shown periods of insulin independence, for a median of 16.5 months (range, 2-63 months), and 5 remain insulin independent. Over the 10 years of follow-up, there has been a notable increase in insulin requirement per kilogram per day, and percentage of glycosylated hemoglobin levels have increased significantly (r = 0.66, P = 0.01). However, 100% of patients tested were C-peptide positive at their most recent assessment, and high fasting and stimulated C-peptide values recorded at 10 years after transplantation, 1.44 (range, 1.09-1.8 ng/mL) and 2.86 ng/mL (range, 1.19-4.53 ng/mL), respectively, suggest significant graft function in the long term. In addition, median serum creatinine has increased very little after the operation (71 nmol/L [range, 49-125 nmol/L] atpreoperation vs 76.5 nmol/L [range 72-81 nmol/L] at year 10), suggesting no diabetic nephropathy.

CONCLUSIONS

: Although there is a notable decline in islet function after islet auto transplant, there is still evidence of significant long-term insulin secretion and possible protection against diabetic complications.

Hydrogel encapsulation environments functionalized with extracellular matrix interactions increase islet insulin secretion

The individual and synergistic effects of extracellular matrix interactions on isolated islet function in culture were investigated within a three-dimensional poly(ethylene glycol) (PEG) hydrogel encapsulation environment. First, we observed similar glucose-stimulated insulin secretion from unencapsulated murine islets and islets photoencapsulated in PEG gels. Then islets were encapsulated in gels containing the basement membrane proteins collagen type IV and laminin, individually and in combination, at a total protein concentration of 100 microg/ml, and islet insulin secretion in response to high glucose was measured over time. Specific laminin interactions were investigated via islet encapsulation with adhesive peptide sequences found in laminin as well as via functional blocking of cell surface receptors known to bind laminin. Over 32 days, islet interactions with collagen type IV and laminin localized within the three-dimensional extracellular environment contributed to two-fold and four-fold increases in insulin secretion, respectively, relative to islets encapsulated without matrix proteins. Hydrogel compositions containing both matrix proteins and >75% laminin further increased islet insulin secretion to approximately six-fold that of islets encapsulated in the absence of matrix proteins. Encapsulation with the peptide sequence IKVAV resulted in increased islet insulin secretion, but not to the extent observed in the presence of whole laminin. Increased insulin secretion in the presence of laminin was eliminated when islets were exposed to functionally blocking anti-alpha6 integrin antibody prior to islet encapsulation with laminin. Our results demonstrate the potential of specific matrix interactions within an islet encapsulation microenvironment to promote encapsulated islet function.

Blood vessels of human islets of Langerhans are surrounded by a double basement membrane

AIMS/HYPOTHESIS

Based on mouse study findings, pancreatic islet cells are supposed to lack basement membrane (BM) and interact directly with vascular endothelial BM. Until now, the BM composition of human islets has remained elusive.

METHODS

Immunohistochemistry with specific monoclonal and polyclonal antibodies as well as electron microscopy were used to study BM organisation and composition in human adult islets. Isolated islet cells and function-blocking monoclonal antibodies and recombinant soluble Lutheran peptide were further used to study islet cell adhesion to laminin (Lm)-511. Short-term cultures of islets were used to study Lutheran and integrin distribution.

RESULTS

Immunohistochemistry revealed a unique organisation for human Lm-511/521 as a peri-islet BM, which co-invaginated into islets with vessels, forming an outer endocrine BM of the intra-islet vascular channels, and was distinct from the vascular BM that additionally contained Lm-411/421. These findings were verified by electron microscopy. Lutheran glycoprotein, a receptor for the Lm alpha5 chain, was found prominently on endocrine cells, as identified by immunohistochemistry and RT-PCR, whereas alpha(3) and beta(1) integrins were more diffusely distributed. High Lutheran content was also found on endocrine cell membranes in short-term culture of human islets. The adhesion of dispersed beta cells to Lm-511 was inhibited equally effectively by antibodies to integrin and alpha(3) and beta(1) subunits, and by soluble Lutheran peptide.

CONCLUSIONS/INTERPRETATION

The present results disclose a hitherto unrecognised BM organisation and adhesion mechanisms in human pancreatic islets as distinct from mouse islets.

Cellular islet autoimmunity associates with clinical outcome of islet cell transplantation

Background

Islet cell transplantation can cure type 1 diabetes (T1D), but only a minority of recipients remains insulin–independent in the following years. We tested the hypothesis that allograft rejection and recurrent autoimmunity contribute to this progressive loss of islet allograft function.

Methodology/Principal Findings

Twenty-one T1D patients received cultured islet cell grafts prepared from multiple donors and transplanted under anti-thymocyte globulin (ATG) induction and tacrolimus plus mycophenolate mofetil (MMF) maintenance immunosuppression. Immunity against auto- and alloantigens was measured before and during one year after transplantation. Cellular auto- and alloreactivity was assessed by lymphocyte stimulation tests against autoantigens and cytotoxic T lymphocyte precursor assays, respectively. Humoral reactivity was measured by auto- and alloantibodies. Clinical outcome parameters - including time until insulin independence, insulin independence at one year, and C-peptide levels over one year- remained blinded until their correlation with immunological parameters. All patients showed significant improvement of metabolic control and 13 out of 21 became insulin-independent. Multivariate analyses showed that presence of cellular autoimmunity before and after transplantation is associated with delayed insulin-independence (p = 0.001 and p = 0.01, respectively) and lower circulating C-peptide levels during the first year after transplantation (p = 0.002 and p = 0.02, respectively). Seven out of eight patients without pre-existent T-cell autoreactivity became insulin-independent, versus none of the four patients reactive to both islet autoantigens GAD and IA-2 before transplantation. Autoantibody levels and cellular alloreactivity had no significant association with outcome.

Conclusions/Significance

In this cohort study, cellular islet-specific autoimmunity associates with clinical outcome of islet cell transplantation under ATG-tacrolimus-MMF immunosuppression. Tailored immunotherapy targeting cellular islet autoreactivity may be required. Monitoring cellular immune reactivity can be useful to identify factors influencing graft survival and to assess efficacy of immunosuppression.

Trial Registration

Clinicaltrials.gov NCT00623610

Fibroblast growth factor receptor-1 signaling in pancreatic islet beta-cells is modulated by the extracellular matrix

Maintenance of pancreatic beta-cell mass depends on extracellular stimuli that promote survival and proliferation. In the islet, these stimuli come from the beta-cell microenvironment and include extracellular matrix deposited by associated vascular endothelial cells. Fibroblast growth factor receptor-1 (FGFR1) has recently been implicated as a signaling pathway that is important for normal beta-cell function. We would like to understand how extracellular matrix and FGFR1 signaling interact to promote beta-cell survival and proliferation. To examine beta-cell-specific receptor responses, we created lentiviral vectors with rat insulin promoter-driven expression of Venus fluorescent protein-tagged full-length (R1betav) and kinase-deficient (KDR1betav) FGFR1. Significant FGF-1-dependent activation of ERK1/2 was observed in betaTC3 cells, dispersed beta-cells, and beta-cells in intact islets. This response was enhanced by R1betav expression and reduced by KDR1betav expression. Plating-dispersed beta-cells on collagen type IV resulted in enhanced expression of endogenous FGFR1 that was associated with sustained activation of ERK1/2. Conversely, plating cells on laminin reduced expression of FGFR1, and this reduction was associated with transient activation of ERK1/2. Addition of neutralizing antibodies to inhibit beta-cell attachment to laminin via alpha(6)-integrin increased high-affinity FGF-1-binding at the plasma membrane and resulted in sustained ERK1/2 activity similar to cells plated on collagen type IV. These data show that the FGF-stimulated beta-cell response is negatively affected by alpha(6)-integrin binding to laminin and suggest regulation associated with vascular endothelial cell remodeling.

Islet encapsulation: strategies to enhance islet cell functions

Diabetes is one of the most prevalent, costly, and debilitating diseases in the world. Although traditional insulin therapy has alleviated the short-term effects, long-term complications are ubiquitous and harmful. For these reasons, alternative treatment options are being developed. This review investigates one appealing area: cell replacement using encapsulated islets. Encapsulation materials, encapsulation methods, and cell sources are presented and discussed. In addition, the major factors that currently limit cell viability and functionality are reviewed, and strategies to overcome these limitations are examined. This review is designed to introduce the reader to cell replacement therapy and cell and tissue encapsulation, especially as it applies to diabetes.

The effects of cell-matrix interactions on encapsulated beta-cell function within hydrogels functionalized with matrix-derived adhesive peptides

The influence of matrix-derived adhesive peptide sequences on encapsulated beta-cell survival and glucose-stimulated insulin release was explored by covalently incorporating synthetic peptide sequences within a model encapsulation environment. Photopolymerized poly(ethylene glycol) (PEG) hydrogels were functionalized via the addition of acrylate-PEG-peptide conjugates to the polymer precursor solution prior to beta-cell photoencapsulation. Individual MIN6 beta-cells were encapsulated in the presence of the laminin-derived recognition sequences, IKLLI, IKVAV, LRE, PDSGR, RGD, and YIGSR, and the collagen type I sequence, DGEA. In the absence of cell-cell and cell-matrix contacts, encapsulated MIN6 beta-cell survival diminishes within one week; however, in PEG hydrogel derivatives including the laminin sequences IKLLI and IKVAV, encapsulated beta-cells exhibit preserved viability, reduced apoptosis, and increased insulin secretion. Interactions with the laminin sequences LRE, PDSGR, RGD, and YIGSR contribute to improved viability, but insulin release from these samples was not statistically greater than that from controls. MIN6 beta-cells were also encapsulated with various concentrations of IKLLI and IKVAV (0.05-5.0mm), individually, and the peptide combinations IKLLI-IKVAV, IKVAV-YIGSR, and PDSGR-YIGSR to explore synergistic effects. The presented results give evidence that synthetic peptide epitopes may be useful in the design of an islet encapsulation environment that promotes cell survival and function via targeted cell-matrix interactions.

Overcoming the Challenges Now Limiting Islet Transplantation: A Sequential, Integrated Approach

Steady improvements in islet cell processing technology and immunosuppressive protocols have made pancreatic islet transplantation a clinical reality for the treatment of patients with Type 1 diabetes mellitus (T1DM). Recent trials are showing that improved glycemic metabolic control, prevention of severe hypoglycemia, and better quality of life can be reproducibly achieved after transplantation of allogeneic islets in patients with unstable T1DM. Despite these encouraging results, challenges ahead comprise obtaining adequate islet cells for transplant, enhancing islets engraftment, sustaining beta cell mass and function over time, and defining effective immune interventions, among others. In order to overcome the current hurdles to the widespread application of islet transplantation there is a need for implementation of integrated, sequential therapeutic approaches.