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

Generation of a pancreas derived hydrogel for the culture of hiPSC derived pancreatic endocrine cells

Stem cell-derived β-cells (SC-BCs) represent a potential source for curing diabetes. To date, in vitro generated SC-BCs display an immature phenotype and lack important features in comparison to their bona-fide counterparts. Transplantation into a living animal promotes SC-BCs maturation, indicating that components of the in vivo microenvironment trigger final SC-BCs development. Here, we investigated whether cues of the pancreas specific extracellular matrix (ECM) can improve the differentiation of human induced pluripotent stem cells (hiPSCs) towards β-cells in vitro. To this aim, a pancreas specific ECM (PanMa) hydrogel was generated from decellularized porcine pancreas and its effect on the differentiation of hiPSC-derived pancreatic hormone expressing cells (HECs) was tested. The hydrogel solidified upon neutralization at 37 °C with gelation kinetics similar to Matrigel. Cytocompatibility of the PanMa hydrogel was demonstrated for a culture duration of 21 days. Encapsulation and culture of HECs in the PanMa hydrogel over 7 days resulted in a stable gene and protein expression of most β-cell markers, but did not improve β-cell identity. In conclusion, the study describes the production of a PanMa hydrogel, which provides the basis for the development of ECM hydrogels that are more adapted to the demands of SC-BCs.

Extracellular matrix stiffness mediates insulin secretion in pancreatic islets via mechanosensitive Piezo1 channel regulated Ca2+ dynamics

The pancreatic islet is surrounded by ECM that provides both biochemical and mechanical cues to the islet β-cell to regulate cell survival and insulin secretion. Changes in ECM composition and mechanical properties drive β-cell dysfunction in many pancreatic diseases. While several studies have characterized changes in islet insulin secretion with changes in substrate stiffness, little is known about the mechanotransduction signaling driving altered islet function in response to mechanical cues. We hypothesized that increasing matrix stiffness will lead to insulin secretion dysfunction by opening the mechanosensitive ion channel Piezo1 and disrupting intracellular Ca2+ dynamics in mouse and human islets. To test our hypothesis, mouse and human cadaveric islets were encapsulated in a biomimetic reverse thermal gel (RTG) scaffold with tailorable stiffness that allows formation of islet focal adhesions with the scaffold and activation of Piezo1 in 3D. Our results indicate that increased scaffold stiffness causes insulin secretion dysfunction mediated by increases in Ca2+ influx and altered Ca2+ dynamics via opening of the mechanosensitive Piezo1 channel. Additionally, inhibition of Piezo1 rescued glucose-stimulated insulin secretion (GSIS) in islets in stiff scaffolds. Overall, our results emphasize the role mechanical properties of the islet microenvironment plays in regulating function. It also supports further investigation into the modulation of Piezo1 channel activity to restore islet function in diseases like type 2 diabetes (T2D) and pancreatic cancer where fibrosis of the peri-islet ECM leads to increased tissue stiffness and islet dysfunction.

Self-organized insulin-producing β-cells differentiated from human omentum-derived stem cells and their in vivo therapeutic potential

BACKGROUND

Human omentum-derived mesenchymal stem cells (hO-MSCs) possess great potential to differentiate into multiple lineages and have self-renewal capacity, allowing them to be utilized as patient-specific cell-based therapeutics. Although the use of various stem cell-derived β-cells has been proposed as a novel approach for treating diabetes mellitus, developing an efficient method to establish highly functional β-cells remains challenging.

METHODS

We aimed to develop a novel cell culture platform that utilizes a fibroblast growth factor 2 (FGF2)-immobilized matrix to regulate the adhesion and differentiation of hO-MSCs into insulin-producing β-cells via cell-matrix/cell-cell interactions. In our study, we evaluated the in vitro differentiation potential of hO-MSCs cultured on an FGF2-immobilized matrix and a round-bottom plate (RBP). Further, the in vivo therapeutic efficacy of the β-cells transplanted into kidney capsules was evaluated using animal models with streptozotocin (STZ)-induced diabetes.

RESULTS

Our findings demonstrated that cells cultured on an FGF2-immobilized matrix could self-organize into insulin-producing β-cell progenitors, as evident from the upregulation of pancreatic β-cell-specific markers (PDX-1, Insulin, and Glut-2). Moreover, we observed significant upregulation of heparan sulfate proteoglycan, gap junction proteins (Cx36 and Cx43), and cell adhesion molecules (E-cadherin and Ncam1) in cells cultured on the FGF2-immobilized matrix. In addition, in vivo transplantation of differentiated β-cells into animal models of STZ-induced diabetes revealed their survival and engraftment as well as glucose-sensitive production of insulin within the host microenvironment, at over 4 weeks after transplantation.

CONCLUSIONS

Our findings suggest that the FGF2-immobilized matrix can support initial cell adhesion, maturation, and glucose-stimulated insulin secretion within the host microenvironment. Such a cell culture platform can offer novel strategies to obtain functional pancreatic β-cells from patient-specific cell sources, ultimately enabling better treatment for diabetes mellitus.

Immune cell infiltration in the pancreas of type 1, type 2 and type 3c diabetes

The different types of diabetes differ in disease pathogenesis but share the impairment or loss of β-cell function leading to chronic hyperglycaemia. While immune cells are present throughout the whole pancreas in normality, their number and activation is increased in diabetes. Different patterns and composition of inflammation could be observed in type 1, type 2 and type 3c diabetes. Immune cells, pancreatic stellate cells and fibrosis were present in the islet microenvironment and could add to β-cell dysfunction and therefore development and progression of diabetes. First studies investigating the use of anti-inflammatory drugs demonstrate their ability to rescue remaining β-cell function and their potential benefit in diabetes treatment. This article provides an overview of immune cell infiltrates in different types of diabetes, highlights the knowledge of their impact on β-cell function and introduces the potential of immunomodulatory strategies.

Insulin secretion hot spots in pancreatic β cells as secreting adhesions

Pancreatic β cell secretion of insulin is crucial to the maintenance of glucose homeostasis and prevention of diseases related to glucose regulation, including diabetes. Pancreatic β cells accomplish efficient insulin secretion by clustering secretion events at the cell membrane facing the vasculature. Regions at the cell periphery characterized by clustered secretion are currently termed insulin secretion hot spots. Several proteins, many associated with the microtubule and actin cytoskeletons, are known to localize to and serve specific functions at hot spots. Among these proteins are the scaffolding protein ELKS, the membrane-associated proteins LL5β and liprins, the focal adhesion-associated protein KANK1, and other factors typically associated with the presynaptic active zone in neurons. These hot spot proteins have been shown to contribute to insulin secretion, but many questions remain regarding their organization and dynamics at hot spots. Current studies suggest microtubule- and F-actin are involved in regulation of hot spot proteins and their function in secretion. The hot spot protein association with the cytoskeleton networks also suggests a potential role for mechanical regulation of these proteins and hot spots in general. This perspective summarizes the existing knowledge of known hot spot proteins, their cytoskeletal-mediated regulation, and discuss questions remaining regarding mechanical regulation of pancreatic beta cell hot spots.

Comparative Study of Two Common In Vitro Models for the Pancreatic Islet with MIN6

BACKGROUND

Islet transplantation is currently considered the most promising method for treating insulin-dependent diabetes. The two most-studied artificial islets are alginate-encapsulated β cells or β cell spheroids. As three-dimensional (3D) models, both artificial islets have better insulin secretory functions and transplantation efficiencies than cells in two-dimensional (2D) monolayer culture. However, the effects of these two methods have not been compared yet. Therefore, in this study, cells from the mouse islet β cell line Min6 were constructed as scaffold-free spheroids or alginate-encapsulated dispersed cells.

METHODS

MIN6 cell spheroids were prepared by using Agarose-base microwell arrays. The insulin secretion level was determined by mouse insulin ELISA kit, and the gene and protein expression status of the MIN6 were performed by Quantitative polymerase chain reaction and immunoblot, respectively.

RESULTS

Both 3D cultures effectively promoted the proliferation and glucose-stimulated insulin release (GSIS) of MIN6 cells compared to 2D adherent cells. Furthermore, 1% alginate-encapsulated MIN6 cells demonstrated more significant effects than the spheroids. In general, three pancreatic genes were expressed at higher levels in response to the 3D culture than to the 2D culture, and pancreatic/duodenal homeobox-1 (PDX1) expression was higher in the cells encapsulated in 1% alginate than that in the spheroids. A western blot analysis showed that 1% alginate-encapsulated MIN6 cells activated the phosphoinositide 3-kinase (PI3K)/serine/threonine protein kinase (AKT)/forkhead transcription factor FKHR (FoxO1) pathway more than the spheroids, 0.5% alginate-, or 2% alginate-encapsulated cells did. The 3D MIN6 culture, therefore, showed improved effects compared to the 2D culture, and the 1% alginate-encapsulated MIN6 cells exhibited better effects than the spheroids. The upregulation of PDX1 expression through the activation of the PI3K/AKT/FoxO1 pathway may mediate the improved cell proliferation and GSIS in 1% alginate-encapsulated MIN6 cells.

CONCLUSION

This study may contribute to the construction of in vitro culture systems for pancreatic islets to meet clinical requirements.

Glucose-Dependent Insulin Secretion from β Cell Spheroids Is Enhanced by Embedding into Softer Alginate Hydrogels Functionalised with RGD Peptide

Type 1 diabetes results from the loss of pancreatic β cells, reduced insulin secretion and dysregulated blood glucose levels. Replacement of these lost β cells with stem cell-derived β cells, and protecting these cells within macro-device implants is a promising approach to restore glucose homeostasis. However, to achieve this goal of restoration of glucose balance requires work to optimise β cell function within implants. We know that native β cell function is enhanced by cell-cell and cell-extracellular matrix interactions within the islets of Langerhans. Reproducing these interactions in 2D, such as culture on matrix proteins, does enhance insulin secretion. However, the impact of matrix proteins on the 3D organoids that would be in implants has not been widely studied. Here, we use native β cells that are dispersed from islets and reaggregated into small spheroids. We show these β cell spheroids have enhanced glucose-dependent insulin secretion when embedded into softer alginate hydrogels conjugated with RGD peptide (a common motif in extracellular matrix proteins). Embedding into alginate-RGD causes activation of integrin responses and repositioning of liprin, a protein that controls insulin secretion. We conclude that insulin secretion from β cell spheroids can be enhanced through manipulation of the surrounding environment.

Spleen extracellular matrix provides a supportive microenvironment for β-cell function

Objectives

Type 1 diabetes mellitus is a common autoimmune and multifactorial disorder. Researchers have been interested in making a favorable islet-like tissue model for the treatment of diabetes. The main objective of this study was to determine the effects of the spleen extracellular matrix (S-ECM) on the function of the MIN6 cell line (a β-cell model).

Materials and Methods

In this experimental research, Wistar rat spleens were decellularized by sodium dodecyl sulfate (SDS) and Triton X-100. S-ECM was characterized by histological assessments, scanning electron microscopy, determination of residua DNA, and examination of the mechanical tensile property. Then, MIN6 cells were seeded on S-ECM scaffold. Glucose-stimulated insulin secretion and mRNA expression of insulin-related genes were examined to confirm the function of the cells.

Results

The main components of S-ECM such as collagen and glycosaminoglycan remained after decellularization. Furthermore, very low residual DNA and appropriate mechanical behavior of S-ECM provided an ideal extracellular microenvironment for the MIN6 cells. GSIS results showed that the seeded cells in S-ECM secreted more insulin than the traditional two-dimensional (2D) culture. The expression of specific insulin-related genes such as PDX-1, insulin, Maf-A, and Glut-2 in the recellularized scaffold was more significant than in the 2D traditional cultured cells. Also, MTT assay results showed that S-ECM were no cytotoxic effects on the MIN6 cells.

Conclusion

These results collectively have evidenced that S-ECM is a suitable scaffold for stabilizing artificial pancreatic islands.

Biomaterials to enhance stem cell transplantation

The successful transplantation of stem cells has the potential to transform regenerative medicine approaches and open promising avenues to repair, replace, and regenerate diseased, damaged, or aged tissues. However, pre-/post-transplantation issues of poor cell survival, retention, cell fate regulation, and insufficient integration with host tissues constitute significant challenges. The success of stem cell transplantation depends upon the coordinated sequence of stem cell renewal, specific lineage differentiation, assembly, and maintenance of long-term function. Advances in biomaterials can improve pre-/post-transplantation outcomes by integrating biophysiochemical cues and emulating tissue microenvironments. This review highlights leading biomaterials-based approaches for enhancing stem cell transplantation.

A human pancreatic ECM hydrogel optimized for 3-D modeling of the islet microenvironment

Extracellular matrix (ECM) plays a multitude of roles, including supporting cells through structural and biochemical interactions. ECM is damaged in the process of isolating human islets for clinical transplantation and basic research. A platform in which islets can be cultured in contact with natural pancreatic ECM is desirable to better understand and support islet health, and to recapitulate the native islet environment. Our study demonstrates the derivation of a practical and durable hydrogel from decellularized human pancreas that supports human islet survival and function. Islets embedded in this hydrogel show increased glucose- and KCl-stimulated insulin secretion, and improved mitochondrial function compared to islets cultured without pancreatic matrix. In extended culture, hydrogel co-culture significantly reduced levels of apoptosis compared to suspension culture and preserved controlled glucose-responsive function. Isolated islets displayed altered endocrine and non-endocrine cell arrangement compared to in situ islets; hydrogel preserved an islet architecture more similar to that observed in situ. RNA sequencing confirmed that gene expression differences between islets cultured in suspension and hydrogel largely fell within gene ontology terms related to extracellular signaling and adhesion. Natural pancreatic ECM improves the survival and physiology of isolated human islets.

Growth of MIN-6 Cells on Salmon Fibrinogen Scaffold Improves Insulin Secretion

The incidence of type I diabetes has been increasing worldwide at an annual rate of approximately 3%. One of the strategies to treat type I diabetes is islet transplantation, in which damaged β-cells are replaced with new islets. To improve β-cells’ expansion and pseudoislet formation, studies are focusing on using extracellular-matrix-resembling substrates. We evaluated the potential of salmon fibrinogen and chitosan electrospun scaffold as cell substrate for cultivating MIN-6 cells. The morphology of cells, insulin secretion and gene expression was evaluated and compared with other substrates (nanofibrous scaffold, microporous scaffold and tissue culture polystyrene). We found that all tested 3D conditions favored the pseudoislet formation of MIN-6 cells. The insulin secretion of MIN-6 cells after stimulation with high-glucose media shows approximately a 9-fold increase compared to the control group when a fibrinogen/chitosan-based electrospun scaffold was used for cultivation. The differences in insulin secretion were corroborated by differences in gene expression. The differences in insulin secretion could probably be attributed to the differences in the mechanical and/or chemical nature of the tested substrates.

Functional thermosensitive hydrogels based on chitin as RIN-m5F cell carrier for the treatment of diabetes

Herein, the thermosensitive hydroxypropyl chitin (HPCT) hydrogel was prepared and the chemical structures, microstructures, rheological properties and degradation in vitro were investigated. The HPCT hydrogel possessed satisfactory biocompatibility in mouse fibroblast cells and Sprague Dawley rats. On the other hand, N-acetylglucosamine (NAG) and carboxymethyl chitosan (CMCS) provided favorable capacity for promoting cell proliferation, delaying cell apoptosis, and facilitating the insulin secretion of rat pancreatic beta cells (RIN-m5F) in three-dimensional culture. Most importantly, the effects of HPCT/NAG and HPCT/CMCS thermosensitive hydrogels as RIN-m5F cells carriers were evaluated via injection into different areas of diabetic rats. Our results demonstrated that HPCT/NAG and HPCT/CMCS hydrogels loaded RIN-m5F cells could keep cells survival, maintain insulin secretion and reduce blood glucose for one week. Overall, the functional thermosensitive hydrogels based on HPCT were effective cell carriers for RIN-m5F cells and might provide novel strategy for the treatment of diabetes via cell engineering.

Biophysical quantification of reorganization dynamics of human pancreatic islets during co-culture with adipose-derived stem cells

Reorganization dynamics of human islets during co-culture with adipose stem cells depends on islet size and the heterogeneity can be assessed based on biomechanical opacity of individual islets.

Current hydrogel advances in physicochemical and biological response-driven biomedical application diversity

Hydrogel is a type of versatile platform with various biomedical applications after rational structure and functional design that leverages on material engineering to modulate its physicochemical properties (e.g., stiffness, pore size, viscoelasticity, microarchitecture, degradability, ligand presentation, stimulus-responsive properties, etc.) and influence cell signaling cascades and fate. In the past few decades, a plethora of pioneering studies have been implemented to explore the cell-hydrogel matrix interactions and figure out the underlying mechanisms, paving the way to the lab-to-clinic translation of hydrogel-based therapies. In this review, we first introduced the physicochemical properties of hydrogels and their fabrication approaches concisely. Subsequently, the comprehensive description and deep discussion were elucidated, wherein the influences of different hydrogels properties on cell behaviors and cellular signaling events were highlighted. These behaviors or events included integrin clustering, focal adhesion (FA) complex accumulation and activation, cytoskeleton rearrangement, protein cyto-nuclei shuttling and activation (e.g., Yes-associated protein (YAP), catenin, etc.), cellular compartment reorganization, gene expression, and further cell biology modulation (e.g., spreading, migration, proliferation, lineage commitment, etc.). Based on them, current in vitro and in vivo hydrogel applications that mainly covered diseases models, various cell delivery protocols for tissue regeneration and disease therapy, smart drug carrier, bioimaging, biosensor, and conductive wearable/implantable biodevices, etc. were further summarized and discussed. More significantly, the clinical translation potential and trials of hydrogels were presented, accompanied with which the remaining challenges and future perspectives in this field were emphasized. Collectively, the comprehensive and deep insights in this review will shed light on the design principles of new biomedical hydrogels to understand and modulate cellular processes, which are available for providing significant indications for future hydrogel design and serving for a broad range of biomedical applications.

Recent Advances in the Generation of β-Cells from Induced Pluripotent Stem Cells as a Potential Cure for Diabetes Mellitus

Diabetes mellitus (DM) is a group of metabolic disorders characterized by high blood glucose levels due to insufficient insulin secretion, insulin action, or both. The present-day solution to diabetes mellitus includes regular administration of insulin, which brings about many medical complications in diabetic patients. Although islet transplantation from cadaveric subjects was proposed to be a permanent cure, the increased risk of infections, the need for immunosuppressive drugs, and their unavailability had restricted its use. To overcome this, the generation of renewable and transplantable β-cells derived from autologous induced pluripotent stem cells (iPSCs) has gained enormous interest as a potential therapeutic strategy to treat diabetes mellitus permanently. To date, extensive research has been undertaken to derive transplantable insulin-producing β-cells (iβ-cells) from iPSCs in vitro by recapitulating the in vivo developmental process of the pancreas. This in vivo developmental process relies on transcription factors, signaling molecules, growth factors, and culture microenvironment. This review highlights the various factors facilitating the generation of mature β-cells from iPSCs. Moreover, this review also describes the generation of pancreatic progenitors and β-cells from diabetic patient-specific iPSCs, exploring the potential of the diabetes disease model and drug discovery. In addition, the applications of genome editing strategies have also been discussed to achieve patient-specific diabetes cell therapy. Last, we have discussed the current challenges and prospects of iPSC-derived β-cells to improve the relative efficacy of the available treatment of diabetes mellitus.

Design Considerations for Macroencapsulation Devices for Stem Cell Derived Islets for the Treatment of Type 1 Diabetes

Stem cell derived insulin producing cells or islets have shown promise in reversing Type 1 Diabetes (T1D), yet successful transplantation currently necessitates long-term modulation with immunosuppressant drugs. An alternative approach to avoiding this immune response is to utilize an islet macroencapsulation device, where islets are incorporated into a selectively permeable membrane that can protect the transplanted cells from acute host response, whilst enabling delivery of insulin. These macroencapsulation systems have to meet a number of stringent and challenging design criteria in order to achieve the ultimate goal of reversing T1D. In this progress report, the design considerations and functional requirements of macroencapsulation systems are reviewed, specifically for stem-cell derived islets (SC-islets), highlighting distinct design parameters. Additionally, a perspective on the future for macroencapsulation systems is given, and how incorporating continuous sensing and closed-loop feedback can be transformative in advancing toward an autonomous biohybrid artificial pancreas.

Cellulose-based scaffolds enhance pseudoislets formation and functionality

In vitroresearch for the study of type 2 diabetes (T2D) is frequently limited by the availability of a functional model for islets of Langerhans. To overcome the limitations of obtaining pancreatic islets from different sources, such as animal models or human donors, immortalized cell lines as the insulin-producing INS1Eβ-cells have appeared as a valid alternative to model insulin-related diseases. However, immortalized cell lines are mainly used in flat surfaces or monolayer distributions, not resembling the spheroid-like architecture of the pancreatic islets. To generate islet-like structures, the use of scaffolds appeared as a valid tool to promote cell aggregations. Traditionally-used hydrogel encapsulation methods do not accomplish all the requisites for pancreatic tissue engineering, as its poor nutrient and oxygen diffusion induces cell death. Here, we use cryogelation technology to develop a more resemblance scaffold with the mechanical and physical properties needed to engineer pancreatic tissue. This study shows that carboxymethyl cellulose (CMC) cryogels prompted cells to generateβ-cell clusters in comparison to gelatin-based scaffolds, that did not induce this cell organization. Moreover, the high porosity achieved with CMC cryogels allowed us to create specific range pseudoislets. Pseudoislets formed within CMC-scaffolds showed cell viability for up to 7 d and a better response to glucose over conventional monolayer cultures. Overall, our results demonstrate that CMC-scaffolds can be used to control the organization and function of insulin-producingβ-cells, representing a suitable technique to generateβ-cell clusters to study pancreatic islet function.

Generation of high yield insulin-producing cells (IPCs) from various sources of stem cells

Type 1 diabetes mellitus occurs when beta cell mass is reduced to less than 20% of the normal level due to immune system destruction of beta cell resulting in an inability to secrete enough insulin. The prevalence of diabetes is expanding according to the American Diabetes Association and the World Health Organization (WHO), foretold to exceed 350 million by 2030. The current treatment does not cure many of the serious complications associated with the disease such as neuropathy, nephropathy, dyslipidemia, retinopathy and cardiovascular disease. Whole pancreas or isolated pancreatic islet transplantation as an alternative therapy can prevent or reduce some of the complications of diabetes. However, the shortage of matched organ or islets cells donor and alloimmune responses limit this therapeutic strategy. Recently, several reports have raised extremely promising results to use different sources of stem cells to differentiate insulin-producing cells and focus on the expansion of these alternative sources. Stem cells, due to their potential for multiple differentiation and self-renewal can differentiate into all cell types, including insulin-producing cells (IPCs). Generation of new beta cells can be achieved from various stem cell sources, including embryonic stem cells (ESCs), adult stem cells, such as mesenchymal stem cells (MSCs) and induced pluripotent stem cells (iPSCs). Thus, this chapter discusses on the assistance of cellular reprogramming of various stem cells as candidates for the generation of IPCs using transcription factors/miRNA, cytokines/small molecules and tissue engineering.

Building Biomimetic Potency Tests for Islet Transplantation

Diabetes is a disease of insulin insufficiency, requiring many to rely on exogenous insulin with constant monitoring to avoid a fatal outcome. Islet transplantation is a recent therapy that can provide insulin independence, but the procedure is still limited by both the availability of human islets and reliable tests to assess their function. While stem cell technologies are poised to fill the shortage of transplantable cells, better methods are still needed for predicting transplantation outcome. To ensure islet quality, we propose that the next generation of islet potency tests should be biomimetic systems that match glucose stimulation dynamics and cell microenvironmental preferences and rapidly assess conditional and continuous insulin secretion with minimal manual handing. Here, we review the current approaches for islet potency testing and outline technologies and methods that can be used to arrive at a more predictive potency test that tracks islet secretory capacity in a relevant context. With the development of potency tests that can report on islet secretion dynamics in a context relevant to their intended function, islet transplantation can expand into a more widely accessible and reliable treatment option for individuals with diabetes.

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.

Effect of alginate matrix engineered to mimic the pancreatic microenvironment on encapsulated islet function

Islet transplantation is emerging as a therapeutic option for type 1 diabetes, albeit, only a small number of patients meeting very stringent criteria are eligible for the treatment because of the side effects of the necessary immunosuppressive therapy and the relatively short time frame of normoglycemia that most patients achieve. The challenge of the immune-suppressive regimen can be overcome through microencapsulation of the islets in a perm-selective coating of alginate microbeads with poly-l-lysine or poly- l-ornithine. In addition to other issues including the nutrient supply challenge of encapsulated islets a critical requirement for these cells has emerged as the need to engineer the microenvironment of the encapsulation matrix to mimic that of the native pancreatic scaffold that houses islet cells. That microenvironment includes biological and mechanical cues that support the viability and function of the cells. In this study, the alginate hydrogel was modified to mimic the pancreatic microenvironment by incorporation of extracellular matrix (ECM). Mechanical and biological changes in the encapsulating alginate matrix were made through stiffness modulation and incorporation of decellularized ECM, respectively. Islets were then encapsulated in this new biomimetic hydrogel and their insulin production was measured after 7 days in vitro. We found that manipulation of the alginate hydrogel matrix to simulate both physical and biological cues for the encapsulated islets enhances the mechanical strength of the encapsulated islet constructs as well as their function. Our data suggest that these modifications have the potential to improve the success rate of encapsulated islet transplantation.

Insulin/Glucose-Responsive Cells Derived from Induced Pluripotent Stem Cells: Disease Modeling and Treatment of Diabetes

Type 2 diabetes, characterized by dysfunction of pancreatic β-cells and insulin resistance in peripheral organs, accounts for more than 90% of all diabetes. Despite current developments of new drugs and strategies to prevent/treat diabetes, there is no ideal therapy targeting all aspects of the disease. Restoration, however, of insulin-producing β-cells, as well as insulin-responsive cells, would be a logical strategy for the treatment of diabetes. In recent years, generation of transplantable cells derived from stem cells in vitro has emerged as an important research area. Pluripotent stem cells, either embryonic or induced, are alternative and feasible sources of insulin-secreting and glucose-responsive cells. This notwithstanding, consistent generation of robust glucose/insulin-responsive cells remains challenging. In this review, we describe basic concepts of the generation of induced pluripotent stem cells and subsequent differentiation of these into pancreatic β-like cells, myotubes, as well as adipocyte- and hepatocyte-like cells. Use of these for modeling of human disease is now feasible, while development of replacement therapies requires continued efforts.

Developmentally-Inspired Biomimetic Culture Models to Produce Functional Islet-Like Cells From Pluripotent Precursors

Insulin-producing beta cells sourced from pluripotent stem cells hold great potential as a virtually unlimited cell source to treat diabetes. Directed pancreatic differentiation protocols aim to mimic various stimuli present during embryonic development through sequential changes of in vitro culture conditions. This is commonly accomplished by the timed addition of soluble signaling factors, in conjunction with cell-handling steps such as the formation of 3D cell aggregates. Interestingly, when stem cells at the pancreatic progenitor stage are transplanted, they form functional insulin-producing cells, suggesting that in vivo microenvironmental cues promote beta cell specification. Among these cues, biophysical stimuli have only recently emerged in the context of optimizing pancreatic differentiation protocols. This review focuses on studies of cell–microenvironment interactions and their impact on differentiating pancreatic cells when considering cell signaling, cell–cell and cell–ECM interactions. We highlight the development of in vitro cell culture models that allow systematic studies of pancreatic cell mechanobiology in response to extracellular matrix proteins, biomechanical effects, soluble factor modulation of biomechanics, substrate stiffness, fluid flow and topography. Finally, we explore how these new mechanical insights could lead to novel pancreatic differentiation protocols that improve efficiency, maturity, and throughput.

Electrospun freestanding hydrophobic fabric as a potential polymer semi-permeable membrane for islet encapsulation

One of the significant problems associated with islet encapsulation for type 1 diabetes treatment is the loss of islet functionality or cell death after transplantation because of the unfavorable environment for the cells. In this work, we propose a simple strategy to fabricate electrospun membranes that will provide a favorable environment for proper islet function and also a desirable pore size to cease cellular infiltration, protecting the encapsulated islet from immune cells. By electrospinning the wettability of three different biocompatible polymers: cellulose acetate (CA), polyethersulfone (PES), and polytetrafluoroethylene (PTFE) was greatly modified. The contact angle of electrospun CA, PES, and PTFE increased to 136°, 126°, and 155° as compared to 55°, 71°, and 128° respectively as a thin film, making the electrospun membranes hydrophobic. Commercial porous membranes of PES and PTFE show a contact angle of 30° and 118°, respectively, confirming the hydrophobicity of electrospun membranes is due to the surface morphology induced by electrospinning. In- vivo results confirm that the induced hydrophobicity and surface morphology of electrospun membranes impede cell attachment, which would help in maintaining the 3D circular morphology of islet cell. More importantly, the pore size of 0.3-0.6 μm obtained due to the densely packed structure of nanofibers, will be able to restrict immune cells but would allow free movement of molecules like insulin and glucose. Therefore, electrospun polymer fibrous membranes as fabricated in this work, with hydrophobic and porous properties, make a strong case for successful islet encapsulation.

Shaping Pancreatic β-Cell Differentiation and Functioning: The Influence of Mechanotransduction

Embryonic and pluripotent stem cells hold great promise in generating β-cells for both replacing medicine and novel therapeutic discoveries in diabetes mellitus. However, their differentiation in vitro is still inefficient, and functional studies reveal that most of these β-like cells still fail to fully mirror the adult β-cell physiology. For their proper growth and functioning, β-cells require a very specific environment, the islet niche, which provides a myriad of chemical and physical signals. While the nature and effects of chemical stimuli have been widely characterized, less is known about the mechanical signals. We here review the current status of knowledge of biophysical cues provided by the niche where β-cells normally live and differentiate, and we underline the possible machinery designated for mechanotransduction in β-cells. Although the regulatory mechanisms remain poorly understood, the analysis reveals that β-cells are equipped with all mechanosensors and signaling proteins actively involved in mechanotransduction in other cell types, and they respond to mechanical cues by changing their behavior. By engineering microenvironments mirroring the biophysical niche properties it is possible to elucidate the β-cell mechanotransductive-regulatory mechanisms and to harness them for the promotion of β-cell differentiation capacity in vitro.

Engineering the vasculature for islet transplantation

The microvasculature in the pancreatic islet is highly specialized for glucose sensing and insulin secretion. Although pancreatic islet transplantation is a potentially life-changing treatment for patients with insulin-dependent diabetes, a lack of blood perfusion reduces viability and function of newly transplanted tissues. Functional vasculature around an implant is not only necessary for the supply of oxygen and nutrients but also required for rapid insulin release kinetics and removal of metabolic waste. Inadequate vascularization is particularly a challenge in islet encapsulation. Selectively permeable membranes increase the barrier to diffusion and often elicit a foreign body reaction including a fibrotic capsule that is not well vascularized. Therefore, approaches that aid in the rapid formation of a mature and robust vasculature in close proximity to the transplanted cells are crucial for successful islet transplantation or other cellular therapies. In this paper, we review various strategies to engineer vasculature for islet transplantation. We consider properties of materials (both synthetic and naturally derived), prevascularization, local release of proangiogenic factors, and co-transplantation of vascular cells that have all been harnessed to increase vasculature. We then discuss the various other challenges in engineering mature, long-term functional and clinically viable vasculature as well as some emerging technologies developed to address them. The benefits of physiological glucose control for patients and the healthcare system demand vigorous pursuit of solutions to cell transplant challenges. STATEMENT OF SIGNIFICANCE: Insulin-dependent diabetes affects more than 1.25 million people in the United States alone. Pancreatic islets secrete insulin and other endocrine hormones that control glucose to normal levels. During preparation for transplantation, the specialized islet blood vessel supply is lost. Furthermore, in the case of cell encapsulation, cells are protected within a device, further limiting delivery of nutrients and absorption of hormones. To overcome these issues, this review considers methods to rapidly vascularize sites and implants through material properties, pre-vascularization, delivery of growth factors, or co-transplantation of vessel supporting cells. Other challenges and emerging technologies are also discussed. Proper vascular growth is a significant component of successful islet transplantation, a treatment that can provide life-changing benefits to patients.

The emerging field of pancreatic tissue engineering: A systematic review and evidence map of scaffold materials and scaffolding techniques for insulin-secreting cells

A bioartificial endocrine pancreas is proposed as a future alternative to current treatment options. Patients with insulin-secretion deficiency might benefit. This is the first systematic review that provides an overview of scaffold materials and techniques for insulin-secreting cells or cells to be differentiated into insulin-secreting cells. An electronic literature survey was conducted in PubMed/MEDLINE and Web of Science, limited to the past 10 years. A total of 197 articles investigating 60 different materials met the inclusion criteria. The extracted data on materials, cell types, study design, and transplantation sites were plotted into two evidence gap maps. Integral parts of the tissue engineering network such as fabrication technique, extracellular matrix, vascularization, immunoprotection, suitable transplantation sites, and the use of stem cells are highlighted. This systematic review provides an evidence-based structure for future studies. Accumulating evidence shows that scaffold-based tissue engineering can enhance the viability and function or differentiation of insulin-secreting cells both in vitro and in vivo.

Fluorescence-Based Nano-Oxygen Particles for Spatiometric Monitoring of Cell Physiological Conditions

Closed-loop artificial pancreas systems have recently been proposed as a solution for treating stage I diabetes by reproducing the function of the pancreas. However, there are many unresolved issues associated with their development, including monitoring and controlling oxygen, immune responses, and the optimization of glucose, all of which need to be monitored and controlled to produce an efficient and viable artificial organ that can become integrated in the patient and maintain homeostasis. This research focused on monitoring the oxygen concentration, specifically achieving this kinetically as the oxygen gradient in an artificial pancreas made of alginate spheres containing islet cells. Functional nanoparticles (NPs) for measuring the oxygen gradient in different hydrogel cellular environments using fluorescence-based (F) microscopy were developed and tested. By the ester bond, a linker Pluronic F127 was conjugated with a carboxylic acid-modified polystyrene NP (510 nm). A hydrophilic/hydrophobic interaction between the commercially available oxygen-sensitive fluorophore and F127 results in fluorescence-based nano-oxygen particles (FNOPs). The in-house synthesized FNOP was calibrated inside electrosprayed alginate-filled hydrogels and demonstrated a good broad dynamic range (2.73-22.23) mg/L as well as a resolution of -0.01 mg/L with an accuracy of ±4%. The calibrated FNOP was utilized for continuous measuring of the oxygen concentration gradient for cell lines RIN-m5F/HeLa for more than 5 days in alginate hydrogel spheres in vitro.

Hyaluronan content governs tissue stiffness in pancreatic islet inflammation

We have identified a novel role for hyaluronan (HA), an extracellular matrix polymer, in governing the mechanical properties of inflamed tissues. We recently reported that insulitis in type 1 diabetes of mice and humans is preceded by intraislet accumulation of HA, a highly hygroscopic polymer. Using the double transgenic DO11.10 × RIPmOVA (DORmO) mouse model of type 1 diabetes, we asked whether autoimmune insulitis was associated with changes in the stiffness of islets. To measure islet stiffness, we used atomic force microscopy (AFM) and developed a novel "bed of nails"-like approach that uses quartz glass nanopillars to anchor islets, solving a long-standing problem of keeping tissue-scale objects immobilized while performing AFM. We measured stiffness via AFM nanoindentation with a spherical indenter and found that insulitis made islets mechanically soft compared with controls. Conversely, treatment with 4-methylumbelliferone, a small-molecule inhibitor of HA synthesis, reduced HA accumulation, diminished swelling, and restored basal tissue stiffness. These results indicate that HA content governs the mechanical properties of islets. In hydrogels with variable HA content, we confirmed that increased HA leads to mechanically softer hydrogels, consistent with our model. In light of recent reports that the insulin production of islets is mechanosensitive, these findings open up an exciting new avenue of research into the fundamental mechanisms by which inflammation impacts local cellular responses.

Advances in islet encapsulation technologies

Type 1 diabetes is an autoimmune disorder in which the immune system attacks and destroys insulin-producing islet cells of the pancreas. Although islet transplantation has proved to be successful for some patients with type 1 diabetes, its widespread use is limited by islet donor shortage and the requirement for lifelong immunosuppression. An encapsulation strategy that can prevent the rejection of xenogeneic islets or of stem cell-derived allogeneic islets can potentially eliminate both of these barriers. Although encapsulation technology has met several challenges, the convergence of expertise in materials, nanotechnology, stem cell biology and immunology is allowing us to get closer to the goal of encapsulated islet cell therapy for humans.

Porcine Islet Xenografts: a Clinical Source of ß-Cell Grafts

PURPOSE OF REVIEW

Diabetes is medical and social burden affecting millions around the world. Despite intensive therapy, insulin fails to maintain adequate glucose homeostasis and often results in episodes of hypoglycemic unawareness. Islet transplantation is a propitious replacement therapy, and incremental improvements in islet isolation and immunosuppressive drugs have made this procedure a feasible option. Shortage of donors, graft loss, and toxic immunosuppressive agents are few of many hurdles against making human allogenic islet transplantation a routine procedure.

RECENT FINDINGS

Xenografts-especially pig islets-offer a logical alternative source for islets. Current preclinical studies have revealed problems such as optimal islet source, zoonosis, and immune rejection. These issues are slowing clinical application. Genetically modified pigs, encapsulation devices, and new immune-suppressive regimens can confer graft protection. In addition, extrahepatic transplant sites are showing promising results. Notwithstanding few approved clinical human trials, and available data from non-human primates, recent reports indicate that porcine islets are closer to be the promising solution to cure diabetes.

A Protocol to Enhance INS1E and MIN6 Functionality-The Use of Theophylline

In vitro research in the field of type I diabetes is frequently limited by the availability of a functional model for islets of Langerhans. This method shows that by the addition of theophylline to the glucose buffers, mouse insulinoma MIN6 and rat insulinoma INS1E pseudo-islets can serve as a model for islets of Langerhans for in vitro research. The effect of theophylline is dose- and cell line-dependent, resulting in a minimal stimulation index of five followed by a rapid return to baseline insulin secretion by reducing glucose concentrations after a first high glucose stimulation. This protocol solves issues concerning in vitro research for type I diabetes as donors and the availability of primary islets of Langerhans are limited. To avoid the limitations of using human donor material, cell lines represent a valid alternative. Many different β cell lines have been reported, but the lack of reproducible responsiveness to glucose stimulation remains a challenge.

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.

Tuning stiffness of cell-laden hydrogel via host–guest interactions

We report a dynamic hydrogel system with on-demand tunable matrix stiffness.

Enzymatically degradable poly(ethylene glycol) hydrogels for the 3D culture and release of human embryonic stem cell derived pancreatic precursor cell aggregates

This study aimed to develop a three dimensional culture platform for aggregates of human embryonic stem cell (hESC)-derived pancreatic progenitors that enables long-term culture, maintains aggregate size and morphology, does not adversely affect differentiation and provides a means for aggregate recovery. A platform was developed with poly(ethylene glycol) hydrogels containing collagen type I, for cell-matrix interactions, and peptide crosslinkers, for facile recovery of aggregates. The platform was first demonstrated with RIN-m5F cells, showing encapsulation and subsequent release of single cells and aggregates without adversely affecting viability. Aggregates of hESC-derived pancreatic progenitors with an effective diameter of 82 (15)μm were either encapsulated in hydrogels or cultured in suspension for 28 days. At day 14, aggregate viability was maintained in the hydrogels, but significantly reduced (88%) in suspension culture. However by day 28, viability was reduced under both culture conditions. Aggregate size was maintained in the hydrogels, but in suspension was significantly higher (∼ 2-fold) by day 28. The ability to release aggregates followed by a second enzyme treatment to achieve single cells enabled assessment by flow cytometry. Prior to encapsulation, there were 39% Pdx1(+)/Nkx6.1(+) cells, key endocrine markers required for β-cell maturation. The fraction of doubly positive cells was not affected in hydrogels but was slightly and significantly lower in suspension culture by 28 days. In conclusion, we demonstrate that a MMP-sensitive PEG hydrogel containing collagen type I is a promising platform for hESC-derived pancreatic progenitors that maintains viable aggregates, aggregate size, and progenitor state and offers facile recovery of aggregates.

Photoclick Hydrogels Prepared from Functionalized Cyclodextrin and Poly(ethylene glycol) for Drug Delivery and in Situ Cell Encapsulation

Polymers or hydrogels containing modified cyclodextrin (CD) are highly useful in drug delivery applications, as CD is a cytocompatible amphiphilic molecule that can complex with a variety of hydrophobic drugs. Here, we designed modular photoclick thiol-ene hydrogels from derivatives of βCD and poly(ethylene glycol) (PEG), including βCD-allylether (βCD-AE), βCD-thiol (βCD-SH), PEG-thiol (PEGSH), and PEG-norbornene (PEGNB). Two types of CD-PEG hybrid hydrogels were prepared using radical-mediated thiol-ene photoclick reactions. Specifically, thiol-allylether hydrogels were formed by reacting multiarm PEGSH and βCD-AE, and thiol-norbornene hydrogels were formed by cross-linking βCD-SH and multiarm PEGNB. We characterized the properties of these two types of thiol-ene hydrogels, including gelation kinetics, gel fractions, hydrolytic stability, and cytocompatibility. Compared with thiol-allylether hydrogels, thiol-norbornene photoclick reaction formed hydrogels with faster gelation kinetics at equivalent macromer contents. Using curcumin, an anti-inflammatory and anticancer hydrophobic molecule, we demonstrated that CD-cross-linked PEG-based hydrogels, when compared with pure PEG-based hydrogels, afforded higher drug loading efficiency and prolonged delivery in vitro. Cytocompatibility of these CD-cross-linked hydrogels were evaluated by in situ encapsulation of radical sensitive pancreatic MIN6 β-cells. All formulations and cross-linking conditions tested were cytocompatible for cell encapsulation. Furthermore, hydrogels cross-linked by βCD-SH showed enhanced cell proliferation and insulin secretion as compared to gels cross-linked by either dithiothreitol (DTT) or βCD-AE, suggesting the profound impact of both macromer compositions and gelation chemistry on cell fate in chemically cross-linked hydrogels.