Extracellular matrix proteins involved in pseudoislets formation

Three-dimensional cell-culture platform based on hydrogel with tunable microenvironmental properties to improve insulin-secreting function of MIN6 cells

Pancreatic β-cells have been reported to be mechanosensitive to cellular microenvironments, and subjecting the cells to more physiologically relevant microenvironments can produce better results than when subjecting them to the conventional two-dimensional (2D) cell-culture conditions. In this work, we propose a novel three-dimensional (3D) strategy for inducing multicellular spheroid formation based on hydrogels with tunable mechanical and interfacial properties. The results indicate that MIN6 cells can sense the substrates and form tightly clustered monolayers or multicellular spheroids on hydrogels with tunable physical properties. Compared to the conventional 2D cell-culture system, the glucose sensitivities of the MIN6 cells cultured in the 3D culture model is enhanced greatly and their insulin content (relative to the amount of protein) is increased 7.3-7.9 folds. Moreover, the relative gene and protein expression levels of some key factors such as Pdx1, NeuroD1, Piezo1, and Rac1 in the MIN6 cells are significantly higher on the 3D platform, compared to the 2D control group. We believe that this 3D cell-culture system developed for the generation of multicellular spheroids will be a promising platform for diabetes treatment in clinical islet transplantation.

Collagen and Endothelial Cell Coculture Improves β-Cell Functionality and Rescues Pancreatic Extracellular Matrix

The use of biomaterials and biomaterial functionalization is a promising approach to support pancreatic islet viability posttransplantation in an effort to reduce insulin dependence for patients afflicted with diabetes mellitus type 1. Extracellular matrix (ECM) proteins are known to impact numerous reparative functions in the body. Assessing how endogenously expressed pancreatic ECM proteins are affected by posttransplant-like hypoxic conditions may provide significant insights toward the development of tissue-engineered therapeutic strategies to positively influence β-cell survival, proliferation, and functionality. Here, we investigated the expression of three relevant groups of pancreatic ECM proteins in human native tissue, including basement membrane (BM) proteins (collagen type 4 [COL4], laminins [LAM]), proteoglycans (decorin [DCN], nidogen-1 [NID1]), and fibril-forming proteins (fibronectin [FN], collagen type 1 [COL1]). In an in vitro hypoxia model, we identified that ECM proteins were differently affected by hypoxic conditions, contributing to an overall loss of β-cell functionality. The use of a COL1 hydrogel as carrier material demonstrated a protective effect on β-cells mitigating the effect of hypoxia on proteoglycans as well as fibril-forming protein expression, supporting β-cell functionality in hypoxia. We further showed that providing endothelial cells (ECs) into the COL1 hydrogel improves β-cell response as well as the expression of relevant BM proteins. Our data show that β-cells benefit from a microenvironment composed of structure-providing COL1 with the incorporation of ECs to withstand the harsh conditions of hypoxia. Such hydrogels support β-cell survival and can serve as an initial source of ECM proteins to allow cell engraftment while preserving cell functionality posttransplantation. Impact statement Expression analysis identifies hypoxia-induced pathological changes in extracellular matrix (ECM) homeostasis as potential targets to support β-cell transplants by encapsulation in biomaterials for the treatment of diabetes mellitus. A collagen-1 hydrogel is shown to attenuate the effect of hypoxia on β-cells and their ECM expression. The functionalization of the hydrogel with endothelial cells increases the β-cell response to glucose and rescues essential basement membrane proteins.

The species origin of the cellular microenvironment influences markers of beta cell fate and function in EndoC-βH1 cells

Interaction between islet cell subtypes and the extracellular matrix influences beta-cell function in mammals. The tissue architecture of rodent islets is very different to that of human islets; cell-to-cell communication and interaction with the extracellular matrix may vary between species. In this work, we have compared the responses of the human EndoC-βH1 cell line to non-human and human-derived growth matrices in terms of growth morphology, gene expression and glucose-stimulated insulin secretion (GSIS). EndoC-βH1 cells demonstrated a greater tendency to form cell clusters when cultured in a human microenvironment and exhibited reduced alpha cell markers at the mRNA level; mean expression difference - 0.23 and - 0.51; p = 0.009 and 0.002 for the Aristaless-related homeobox (ARX) and Glucagon (GCG) genes respectively. No differences were noted in the protein expression of mature beta cell markers such as Pdx1 and NeuroD1 were noted in EndoC-βH1 cells grown in a human microenvironment but cells were however more sensitive to glucose (4.3-fold increase in insulin secretion following glucose challenge compared with a 1.9-fold increase in cells grown in a non-human microenvironment; p = 0.0003). Our data suggests that the tissue origin of the cellular microenvironment has effects on the function of EndoC-βH1 cells in vitro, and the use of a more human-like culture microenvironment may bring benefits in terms of increased physiological relevance.

Silk matrices promote formation of insulin-secreting islet-like clusters

Ex vivo expansion of endocrine cells constitutes an interesting alternative to be able to match the unmet need of transplantable pancreatic islets. However, endocrine cells become fragile once removed from their extracellular matrix (ECM) and typically become senescent and loose insulin expression during conventional 2D culture. Herein we develop a protocol where 3D silk matrices functionalized with ECM-derived motifs are used for generation of insulin-secreting islet-like clusters from mouse and human primary cells. The obtained clusters were shown to attain an islet-like spheroid shape and to maintain functional insulin release upon glucose stimulation in vitro. Furthermore, in vivo imaging of transplanted murine clusters showed engraftment with increasing vessel formation during time. There was no sign of cell death and the clusters maintained or increased in size throughout the period, thus suggesting a suitable cluster size for transplantation.

Compliant 3D microenvironment improves β-cell cluster insulin expression through mechanosensing and β-catenin signaling

Type 1 diabetes is chronic disease with numerous complications and currently no cure. Tissue engineering strategies have shown promise in providing a therapeutic solution, but maintenance of islet function and survival within these therapies represents a formidable challenge. The islet microenvironment may hold the key for proper islet maintenance. To elucidate the microenvironmental conditions necessary for improved islet function and survival, three-dimensional (3D) polyacrylamide cell scaffolds were fabricated with stiffnesses of 0.1 and 10 kPa to regulate the spatial and mechanical control of biosignals. Specifically, we show a significant increase in insulin mRNA expression of 3D primary mouse islet-derived and Min6-derived β-cell clusters grown on compliant 0.1 kPa scaffolds. Moreover, these compliant 0.1 kPa scaffolds also increase glucose sensitivity in Min6-derived β-cell clusters as demonstrated by the increased glucose stimulation index. Our data suggest that stiffness-specific insulin processing is regulated through the myosin light chain kinase (MLCK) and Rho-associated protein kinase (ROCK) mechanosensing pathways. Additionally, β-catenin is required for regulation of stiffness-dependent insulin expression. Through activation or inhibition of β-catenin signaling, reversible control of insulin expression is achieved on the compliant 0.1 kPa and overly stiff 10 kPa substrates. Understanding the role of the microenvironment on islet function can enhance the therapeutic approaches necessary to treat diabetes for improving insulin sensitivity and response.

Controlled local presentation of matrix proteins in microparticle-laden cell aggregates

Multi-cellular aggregates are found in healthy and diseased tissues, and while cell-cell contact is important for regulating many cell functions, cells also interact, to varying degrees, with extra-cellular matrix (ECM) proteins. Islets of Langerhans are one such example of cell aggregates in contact with ECM, both at the periphery of the cluster and dispersed throughout. While several studies have investigated the effect of reintroducing contact with ECM proteins on islet cell survival and function, the majority of these experiments only allow contact with the exterior cells. Thus, cell-culture platforms that enable the study of ECM-cell interactions throughout multi-cellular aggregates are of interest. Here, local presentation of ECM proteins was achieved using hydrogel microwell arrays to incorporate protein-laden microparticles during formation of MIN6 β-cell aggregates. Varying the microparticle seeding density reproducibly controlled the number of microparticles incorporated within three-dimensional aggregates (i.e., total amount of protein). Further, a relatively uniform spatial distribution of laminin- and fibronectin-coated microparticles was achieved throughout the x-, y-, and z-directions. Multiple ECM proteins were presented to β-cells in concert by incorporating two distinct populations of microparticles throughout the aggregates. Finally, scaling the microwell device dimensions allowed for the formation of two different sized cell-particle aggregates, ∼80 and 160 µm in diameter. While the total number of microparticles incorporated per aggregate varied with size, the fraction of the aggregate occupied by microparticles was affected only by the microparticle seeding density, indicating that uniform local concentrations of proteins can be preserved while changing the overall aggregate dimensions.

Improved MIN6 β-cell function on self-assembled peptide amphiphile nanomatrix inscribed with extracellular matrix-derived cell adhesive ligands

Understanding the role of the pancreatic extracellular matrix (ECM) in supporting islet survival and function drives the pursuit to create biomaterials that imitate and restore the pancreatic ECM microenvironment. To create an ECM mimic holding bioinductive cues for β-cells, self-assembled peptide amphiphiles (PAs) inscribed with four selected ECM-derived cell adhesive ligands are synthesized. After 7 days, compared to control groups cultured on biologically inert substrates, MIN6 β-cells cultured on PAs functionalized with YIGSR and RGDS cell adhesive ligands exhibit elevated insulin secretion in responses to glucose and also form β-cell clusters. These findings suggest that the self-assembled PA nanomatrix may be utilized to improve pancreatic islet transplantation for treating type 1 diabetes.

In vitro formation of β cell pseudoislets using islet-derived endothelial cells

β cell pseudoislets (PIs) are used for the in vitro study of β-cells in a three-dimensional (3-D) configuration. Current methods of PI induction require unique culture conditions and extensive mechanical manipulations. Here we report a novel co-culture system consisting of high passage β-cells and islet-derived endothelial cells (iECs) that results in a rapid and spontaneous formation of free-floating PIs. PI structures were formed as early as 72 h following co-culture setup and were preserved for more than 14 d. These PIs, composed solely of β-cells, were similar in size to that of native islets and showed an increased percentage of proinsulin-positive cells, increased insulin gene expression in response to glucose stimulation, and restored glucose-stimulated insulin secretion when compared to β-cells cultured as monolayers. Key extracellular matrix proteins that were absent in β-cells cultured alone were deposited by iECs on PIs and were found in and around the PIs. iEC-induced PIs are a readily available tool for examining β cell function in a native 3-D configuration and can be used for examining β-cell/iEC interactions in vitro.

Effects of Arg-Gly-Asp-modified elastin-like polypeptide on pseudoislet formation via up-regulation of cell adhesion molecules and extracellular matrix proteins

Extracellular matrix (ECM) plays an important role in controlling the β-cell morphology, survival and insulin secretary functions. An RGD-modified elastin-like polypeptide (RGD-ELP), TGPG[VGRGD(VGVPG)(6)](20)WPC, has been reported previously as a bioactive matrix. In this study, to investigate whether RGD-ELP affects β-cell growth characteristics and insulin secretion, β-TC6 cells were cultured on the RGD-ELP coatings prepared via thermally induced phase transition. On RGD-ELP, β-TC6 cells clustered into an islet-like architecture with high cell viability. Throughout 7days' culture, the proliferation rate of the cells within a pseudoislet was similar to that of monolayer culture. Under high glucose (25mM), β-TC6 pseudoislets showed up-regulated insulin gene expression and exhibited glucose-stimulated insulin secretion. Importantly, the mRNA and protein abundances of cell adhesion molecules (CAM) E-cadherin and connexin-36 were much higher in pseudoislets than in monolayer cells. The siRNA-mediated inhibition of E-cadherin or connexin-36 expression severely limited pseudoislet formation. In addition, the mRNA levels of collagen types I and IV, fibronectin and laminin were significantly elevated in pseudoislets. The results suggest that RGD-ELP promotes pseudoislet formation via up-regulation of the CAM and ECM components. The functional roles of RGD-ELP are discussed in respect of its molecular composition.

Improvement of Rat Islet Viability during Transplantation: Validation of Pharmacological Approach to Induce VEGF Overexpression

Delayed and insufficient revascularization during islet transplantation deprives islets of oxygen and nutrients, resulting in graft failure. Vascular endothelial growth factor (VEGF) could play a critical role in islet revascularization. We aimed to develop pharmacological strategies for VEGF overexpression in pancreatic islets using the iron chelator deferoxamine (DFO), thus avoiding obstacles or safety risks associated with gene therapy. Rat pancreatic islets were infected in vivo using an adenovirus (ADE) encoding human VEGF gene (4.108 pfu/pancreas) or were incubated in the presence of DFO (10 μmol/L). In vitro viability, functionality, and the secretion of VEGF were evaluated in islets 1 and 3 days after treatment. Infected islets or islets incubated with DFO were transplanted into the liver of syngenic diabetic rats and the graft efficiency was estimated in vivo by measuring body weight, glycemia, C-peptide secretion, and animal survival over a period of 2 months. DFO induced transient VEGF overexpression over 3 days, whereas infection with ADE resulted in prolonged VEGF overexpression lasting 14 days; however, this was toxic and decreased islet viability and functionality. The in vivo study showed a decrease in rat deaths after the transplantation of islets treated with DFO or ADE compared with the sham and control group. ADE treatment improved body weight and C-peptide levels. Gene therapy and DFO improved metabolic control in diabetic rats after transplantation, but this effect was limited in the presence of DFO. The pharmacological approach is an interesting strategy for improving graft efficiency during transplantation, but this approach needs to be improved with drugs that are more specific.