Insulin secretion from pancreatic islets in fibrin glue clots at different fibrinogen and thrombin concentrations

The Foundation for Engineering a Pancreatic Islet Niche

Progress in diabetes research is hindered, in part, by deficiencies in current experimental systems to accurately model human pathophysiology and/or predict clinical outcomes. Engineering human-centric platforms that more closely mimic in vivo physiology, however, requires thoughtful and informed design. Summarizing our contemporary understanding of the unique and critical features of the pancreatic islet can inform engineering design criteria. Furthermore, a broad understanding of conventional experimental practices and their current advantages and limitations ensures that new models address key gaps. Improving beyond traditional cell culture, emerging platforms are combining diabetes-relevant cells within three-dimensional niches containing dynamic matrices and controlled fluidic flow. While highly promising, islet-on-a-chip prototypes must evolve their utility, adaptability, and adoptability to ensure broad and reproducible use. Here we propose a roadmap for engineers to craft biorelevant and accessible diabetes models. Concurrently, we seek to inspire biologists to leverage such tools to ask complex and nuanced questions. The progenies of such diabetes models should ultimately enable investigators to translate ambitious research expeditions from benchtop to the clinic.

Obesity-induced changes in human islet G protein-coupled receptor expression: Implications for metabolic regulation

G protein-coupled receptors (GPCRs) are a large family of cell surface receptors that are the targets for many different classes of pharmacotherapy. The islets of Langerhans are central to appropriate glucose homeostasis through their secretion of insulin, and islet function can be modified by ligands acting at the large number of GPCRs that islets express. The human islet GPCRome is not a static entity, but one that is altered under pathophysiological conditions and, in this review, we have compared expression of GPCR mRNAs in human islets obtained from normal weight range donors, and those with a weight range classified as obese. We have also considered the likely outcomes on islet function that the altered GPCR expression status confers and the possible impact that adipokines, secreted from expanded fat depots, could have at those GPCRs showing altered expression in obesity.

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.

Oxygenation strategies for encapsulated islet and beta cell transplants

Human allogeneic islet transplantation (ITx) is emerging as a promising treatment option for qualified patients with type 1 diabetes. However, widespread clinical application of allogeneic ITx is hindered by two critical barriers: the need for systemic immunosuppression and the limited supply of human islet tissue. Biocompatible, retrievable immunoisolation devices containing glucose-responsive insulin-secreting tissue may address both critical barriers by enabling the more effective and efficient use of allogeneic islets without immunosuppression in the near-term, and ultimately the use of a cell source with a virtually unlimited supply, such as human stem cell-derived β-cells or xenogeneic (porcine) islets with minimal or no immunosuppression. However, even though encapsulation methods have been developed and immunoprotection has been successfully tested in small and large animal models and to a limited extent in proof-of-concept clinical studies, the effective use of encapsulation approaches to convincingly and consistently treat diabetes in humans has yet to be demonstrated. There is increasing consensus that inadequate oxygen supply is a major factor limiting their clinical translation and routine implementation. Poor oxygenation negatively affects cell viability and β-cell function, and the problem is exacerbated with the high-density seeding required for reasonably-sized clinical encapsulation devices. Approaches for enhanced oxygen delivery to encapsulated tissues in implantable devices are therefore being actively developed and tested. This review summarizes fundamental aspects of islet microarchitecture and β-cell physiology as well as encapsulation approaches highlighting the need for adequate oxygenation; it also evaluates existing and emerging approaches for enhanced oxygen delivery to encapsulation devices, particularly with the advent of β-cell sources from stem cells that may enable the large-scale application of this approach.

Extracellular matrix molecules and their potential contribution to the function of transplanted pancreatic islets

Extracellular matrix (ECM) molecules are responsible for structural and biochemical support, as well as for regulation of molecular signalling and tissue repair in many organ structures, including the pancreas. In pancreatic islets, collagen type IV and VI, and laminins are the most abundant molecules, but other ECM molecules are also present. The ECM interacts with specific combinations of integrin α/β heterodimers on islet cells and guides many cellular processes. More specifically, some ECM molecules are involved in beta cell survival, function and insulin production, while others can fine tune the susceptibility of islet cells to cytokines. Further, some ECM induce release of growth factors to facilitate tissue repair. During enzymatic isolation of islets for transplantation, the ECM is damaged, impacting islet function. However, restoration of the ECM in human islets (for example by adding ECM to the interior of immunoprotective capsules) has been shown to enhance islet function. Here, we provide current insight into the role of ECM molecules in islet function and discuss the clinical potential of ECM manipulation to enhance pancreatic islet function and survival.

Decellularized pancreas as a native extracellular matrix scaffold for pancreatic islet seeding and culture

Diabetes mellitus involves the loss of function and/or absolute numbers of insulin-producing β cells in pancreatic islets. Islet transplantation is currently being investigated as a potential cure, and advances in tissue engineering methods can be used to improve pancreatic islets survival and functionality. Transplanted islets experience anoikis, hypoxia, and inflammation-mediated immune response, leading to early damage and subsequent failure of the graft. Recent development in tissue engineering enables the use of decellularized organs as scaffolds for cell therapies. Decellularized pancreas could be a suitable scaffold as it can retain the native extracellular matrix and vasculature. In this study, mouse pancreata were decellularized by perfusion using 0.5% sodium dodecyl sulfate. Different characterizations revealed that the resulting matrix was free of cells and retained part of the pancreas extracellular matrix including the vasculature and its internal elastic basal lamina, the ducts with their basal membrane, and the glycosaminoglycan and collagen structures. Islets were infused into the ductal system of decellularized pancreata, and glucose-stimulated insulin secretion results confirmed their functionality after 48 hr. Also, recellularizing the decellularized pancreas with green fluorescent protein-tagged INS-1 cells and culturing the system over 120 days confirmed the biocompatibility and non-toxic nature of the scaffold. Green fluorescent protein-tagged INS-1 cells formed pseudoislets that were, over time, budding out of the decellularized pancreata. Decellularized pancreatic scaffolds seeded with endocrine pancreatic tissue could be a potential bioengineered organ for transplantation.

Young porcine endocrine pancreatic islets cultured in fibrin and alginate gels show improved resistance towards human monocytes

AIM

To investigate the protective function of alginate and fibrin gels used to embed porcine endocrine pancreatic islets towards human monocytes.

METHODS

Groups of 200 islet equivalents from young pigs were embedded in either a fibrin or in an alginate gel, and as a control seeded in tissue culture polystyrene (TCPS) well plates. The islet cultures were incubated with 2×10(5) human monocytes for 24h. In addition, both islets and monocytes were separately cultured in TCPS, fibrin and alginate. Islet morphology, viability and function were investigated as well as the secretion of cytokines TNFα, IL-6, and IL-1β.

RESULTS

When freely-floating in TCPS, non-encapsulated islets were surrounded by monocytes and started to disperse after 24h. In fibrin, monocytes could be found in close proximity to embedded islets, indicating monocyte migration through the gel. In contrast, after 24h, few monocytes were found close to islets in alginate. Immunofluorescence staining and manual counting showed that integrin expression was higher in fibrin-embedded islet cultures. A TUNEL assay revealed elevated numbers of apoptotic cells for islets in TCPS wells compared to fibrin and alginate cultures. Insulin secretion was higher with islets embedded in fibrin and alginate when compared to non-encapsulated islets. TNFα, IL-6 and IL-1β were found in high concentrations in the media of co-cultures and monocyte mono-culture in fibrin.

CONCLUSION

Both alginate and fibrin provide key structural support and offer some protection for the islets towards human monocytes. Fibrin itself triggers the cytokine secretion from monocytes.

Cytotoxic T lymphocyte trafficking and survival in an augmented fibrin matrix carrier

Cell-based therapies have intriguing potential for the treatment of a variety of neurological disorders. One such example is genetically engineered cytotoxic T lymphocytes (CTLs) that are being investigated in brain tumor clinical trials. The development of methods for CTL delivery is critical to their use in the laboratory and clinical setting. In our study, we determined whether CTLs can migrate through fibrin matrices and if their migration, survival, and function could be modulated by adding chemokines to the matrix. Our results indicated that CTLs can freely migrate through fibrin matrices. As expected, the addition of the monocyte chemotactic protein-1 (MCP-1), also known as chemokine C-C motif ligand 2 (CCL2), to the surrounding media increased egress of the CTLs out of the fibrin clot. Interleukin (IL) -2 and/or IL-15 embedded in the matrix enhanced T cell survival and further promoted T cell migration. The interleukin-13 receptor alpha 2 specific (IL-13R alpha2) T cells that traveled out of the fibrin clot retained the capacity to kill U251 glioma cells. In summary, CTLs can survive and migrate robustly in fibrin matrices. These processes can be influenced by modification of matrix constituents. We conclude that fibrin matrices may be suitable T cell carriers and can be used to facilitate understanding of T cell interaction with the surrounding microenvironment.

Effect of nicotinamide on early graft failure following intraportal islet transplantation

Intraportal islet transplantation (IPIT) may potentially cure Type 1 diabetes mellitus; however, graft failure in the early post-transplantation period presents a major obstacle. In this study, we tested the ability of nicotinamide to prevent early islet destruction in a syngeneic mouse model. Mice (C57BL/6) with chemically-induced diabetes received intraportal transplants of syngeneic islet tissue in various doses. Islets were cultured for 24 h in medium with or without 10 mM nicotinamide supplementation. Following IPIT, islet function was confirmed by an intraperitoneal glucose tolerance test (IPGTT) and hepatectomy. The effects of nicotinamide were evaluated by blood glucose concentration, serum monocyte chemoattractant protein-1 (MCP-1) concentration, and immunohistology at 3 h and 24 h after IPIT. Among the various islet doses, an infusion of 300 syngeneic islets treated with nicotinamide exhibited the greatest differences in glucose tolerance between recipients of treated and untreated (i.e., control) islets. One day after 300 islet equivalent (IEQ) transplantation, islets treated with nicotinamide were better granulated than the untreated islets (P=0.01), and the recipients displayed a slight decrease in serum MCP-1 concentration, as compared to controls. After 15 days, recipients of nicotinamide-pretreated islets showed higher levels of graft function (as measured by IPGTT) than controls. The pretreatment also prolonged graft survival (>100 days) and function; these were confirmed by partial hepatectomy, which led to the recurrence of diabetes. Pretreatment of islet grafts with nicotinamide may prevent their deterioration on the early period following IPIT in a syngeneic mouse model.

A fibrin gel carrier system for islet transplantation into kidney subcapsule

Islet transplantation is a promising therapeutic option for type 1 diabetes, and actively performed in the clinic as well as in the animal experiments. For the rodent experiments, islet transplantation into kidney subcapsule is widely used to assess islet quality, however, it is often difficult to do using a polyethylene tubing and fine needle because of inherent dead volume of needle and stickiness of the tubing to islets. This problem makes it difficult to interpret the physiological response to different islet doses. Here, we developed a simple fibrin gel carrier system for islet transplantation into kidney subcapsule and utilized it to determine the marginal islet mass sufficient for correction of hyperglycemia in diabetic nude mice.