Exercise induces hypoglycemia in rats with islet transplantation

Enhancing Therapeutic Insulin Transport from Macroencapsulated Islets Using Sub-Minute Pressure at Physiological Levels

Cadaveric islet and stem cell-derived transplantations hold promise as treatments for type 1 diabetes. To tackle the issue of immunocompatibility, numerous cellular macroencapsulation techniques have been developed that utilize diffusion to transport insulin across an immunoisolating barrier. However, despite several devices progressing to human clinical trials, none have successfully managed to attain physiologic glucose control or insulin independence. Based on empirical evidence, macroencapsulation methods with multilayered, high islet surface density are incompatible with homeostatic, on-demand insulin delivery and physiologic glucose regulation, when reliant solely on diffusion. An additional driving force is essential to overcome the distance limit of diffusion. In this study, we present both theoretical proof and experimental validation that applying pressure at levels comparable to physiological diastolic blood pressure significantly enhances insulin flux across immunoisolation membranes-increasing it by nearly three orders of magnitude. This significant enhancement in transport rate allows for precise, sub-minute regulation of both bolus and basal insulin delivery. By incorporating this technique with a pump-based extravascular system, we demonstrate the ability to rapidly reduce glucose levels in diabetic rodent models, effectively replicating the timescale and therapeutic effect of subcutaneous insulin injection or infusion. This advance provides a potential path towards achieving insulin independence with islet macroencapsulation.

One Sentence Summary

Towards improved glucose control, applying sub-minute pressure at physiological levels enhances therapeutic insulin transport from macroencapsulated islets.

A reappraisal of the role of cyclic AMP in the physiological action of glucagon

Effects of the metabolic hormone glucagon can be physiological or supraphysiological, based on agonist concentration and the mediating cellular signal. The threshold concentration (TC) for activating the AC/cAMP signal pathway in liver is ≥ 100 pM. By contrast, mean plasma concentrations are around 20-45 pM, depending on the vascular bed. Accordingly, effects produced at TCs below 100 pM are physiological and mediated by cellular signal pathways other than AC/cAMP. Effects generated at concentrations above 100 pM are supraphysiological, often mediated by simultaneous activation of cAMP-independent and -dependent pathways. Physiological responses, and their established or implicated signal pathways, include stimulation of: glucose mobilization, fatty acid oxidation, and urea synthesis in liver (PLC/IP3/Ca²⁺/CaM); lipolysis in white and brown adipose tissue and oxygen consumption in brown adipose of the rat but not in humans (PLC/IP3/Ca²⁺/CaM); renal potassium and phosphate excretion in rodents and GFR in humans (signal undetermined); and glucose utilization in rat heart (PI3K/akt). Supraphysiological responses involve the AC/cAMP pathway and include: enhanced stimulation of glucose mobilization and stimulation of urea synthesis in liver; further stimulation of white and brown adipose lipolysis and thermogenesis in brown adipose tissue; stimulation of renal Cl^- transport; and increased rat heart contractility. The AC/cAMP pathway is likely recruited when plasma glucagon rises above 100 pM during periods of elevated metabolic stress and systemic glucose demand, such as in the early neonate or strenuously exercising adult. The current cAMP-centered model should therefore be reconsidered and replaced with one that places more emphasis on cAMP-independent pathways.

Unravelling innervation of pancreatic islets

The central and peripheral nervous systems play critical roles in regulating pancreatic islet function and glucose metabolism. Over the last century, in vitro and in vivo studies along with examination of human pancreas samples have revealed the structure of islet innervation, investigated the contribution of sympathetic, parasympathetic and sensory neural pathways to glucose control, and begun to determine how the structure and function of pancreatic nerves are disrupted in metabolic disease. Now, state-of-the art techniques such as 3D imaging of pancreatic innervation and targeted in vivo neuromodulation provide further insights into the anatomy and physiological roles of islet innervation. Here, we provide a summary of the published work on the anatomy of pancreatic islet innervation, its roles, and evidence for disordered islet innervation in metabolic disease. Finally, we discuss the possibilities offered by new technologies to increase our knowledge of islet innervation and its contributions to metabolic regulation.

Glucagon, cyclic AMP, and hepatic glucose mobilization: A half‐century of uncertainty

For at least 50 years, the prevailing view has been that the adenylate cyclase (AC)/cyclic AMP (cAMP)/protein kinase A pathway is the predominant signal mediating the hepatic glucose‐mobilizing actions of glucagon. A wealth of evidence, however, supports the alternative, that the operative signal most of the time is the phospholipase C (PLC)/inositol‐phosphate (IP3)/calcium/calmodulin pathway. The evidence can be summarized as follows: (1) The consensus threshold glucagon concentration for activating AC ex vivo is 100 pM, but the statistical hepatic portal plasma glucagon concentration range, measured by RIA, is between 28 and 60 pM; (2) Within that physiological concentration range, glucagon stimulates the PLC/IP3 pathway and robustly increases glucose output without affecting the AC/cAMP pathway; (3) Activation of a latent, amplified AC/cAMP pathway at concentrations below 60 pM is very unlikely; and (4) Activation of the PLC/IP3 pathway at physiological concentrations produces intracellular effects that are similar to those produced by activation of the AC/cAMP pathway at concentrations above 100 pM, including elevated intracellular calcium and altered activities and expressions of key enzymes involved in glycogenolysis, gluconeogenesis, and glycogen synthesis. Under metabolically stressful conditions, as in the early neonate or exercising adult, plasma glucagon concentrations often exceed 100 pM, recruiting the AC/cAMP pathway and enhancing the activation of PLC/IP3 pathway to boost glucose output, adaptively meeting the elevated systemic glucose demand. Whether the AC/cAMP pathway is consistently activated in starvation or diabetes is not clear. Because the importance of glucagon in the pathogenesis of diabetes is becoming increasingly evident, it is even more urgent now to resolve lingering uncertainties and definitively establish glucagon’s true mechanism of glycemia regulation in health and disease.

Tissue Engineering Strategies for Improving Beta Cell Transplantation Outcome

Purpose of Review

Beta cell replacement therapy as a form of islet transplantation is a promising alternative therapy with the possibility to make selected patients with type 1 diabetes (T1D) insulin independent. However, this technique faces challenges such as extensive activation of the host immune system post-transplantation, lifelong need for immunosuppression, and the scarcity of islet donor pancreas. Advancement in tissue engineering strategies can improve these challenges and allow for a more widespread application of this therapy. This review will discuss the recent development and clinical translation of tissue engineering strategies in beta cell replacement therapy.

Recent Findings

Tissue engineering offers innovative solutions for producing unlimited glucose responsive cells and fabrication of appropriate devices/scaffolds for transplantation applications. Generation of pancreatic organoids with supporting cells in biocompatible biomaterials is a powerful technique to improve the function of insulin-producing cell clusters. Fabrication of physical barriers such as encapsulation strategies can protect the cells from the host immune system and allow for graft retrieval, although this strategy still faces major challenges to fully restore physiological glucose regulation.

Summary

The three main components of tissue engineering strategies including the generation of stem cell-derived insulin-producing cells and organoids and the possibilities for therapeutic delivery of cell-seeded devices to extra-hepatic sites need to come together in order to provide safe and functional insulin-producing devices for clinical beta cell replacement therapy.

Sex differences in postprandial blood glucose and body surface temperature are contingent on flight in the fruit bat, Cynopterus sphinx

The postprandial blood glucose level is very high for the body size in frugivorous bats. Like other homeotherms, bats release heat during digestion of dietary macronutrients. Despite males and females of the same species exhibiting different foraging behaviour, empirical support for sex differences in blood glucose and body surface temperature in fruit bats is poor. Moreover, while flight affects postprandial metabolism, whether such effects are different in each sex of fruit bats is unclear. Here, we studied these questions in the fruit bat, Cynopterus sphinx. We first assessed whether there are sex differences in the postprandial level of blood glucose and body surface temperature over time in rested bats. We then assessed whether flight affects outcomes of sex differences in both traits. We found that the estimated marginal means of both traits were generally higher in females than males, in rested bats. Notably, the sex difference in both traits was only significant at specific sampling time of the assay. Further, the trait means significantly differed between the sexes only in the rested, but not active, bats, meaning that signals of sex difference in metabolic traits eroded when bats were active. Taken together, our findings suggest that in C. sphinx, the sex specificity in the expression of metabolic traits is significantly dependent on physical activity.

Summary: The level of sex differences in metabolic traits is affected by flight in Cynopterus sphinx, a finding that has general implications for sex-specific life-history evolution in fruit bats.

The Liver Surface Is an Attractive Transplant Site for Pancreatic Islet Transplantation

In the current clinical islet transplantation, intraportal transplantation is regarded as the gold-standard procedure. However, in this procedure, 50 to 70% of the transplanted islets are immediately damaged due to a strong innate immune response based on islet–blood contact. We investigated the transplant efficiency of a novel method of liver surface transplantation using a syngeneic keratinocyte sheet to avoid islet–blood contact. To examine the influence of the keratinocyte sheet, substantial amounts of syngeneic islets (8 IEQs/g) were transplanted on the liver surface of diabetic rats, while marginal amounts of islets (4 IEQs/g) were transplanted via intraportal transplantation to compare the transplant efficiency. Blood glucose, intraperitoneal glucose tolerance, immunohistochemistry, and in vivo imaging findings of the cell sheet were evaluated. The study showed that islet transplantation to the liver surface immediately followed by a syngeneic keratinocyte sheet covering was effective for curing diabetic rats, while no rats were cured in the group without the cell sheet. Notably, islet grafts transplanted via this approach appeared to penetrate into the liver parenchyma. However, the transplant efficiency did not reach that of intraportal transplantation. Further refinements of this approach by introducing mesothelial or fibroblast cell sheets in combination with a preferable scaffold for islet grafts may help to improve the transplant efficiency.

Function and Gene Expression of Islets Experimentally Transplanted to Muscle and Omentum

Islet transplantation to the liver is a potential curative treatment for patients with type 1 diabetes. Muscle and the greater omentum are two alternative implantation sites, which can provide excellent engraftment and hold potential as future sites for stem-cell-derived beta-cell replacement. We evaluated the functional outcome after islet transplantation to muscle and omentum and found that alloxan-diabetic animals were cured with a low number of islets (200) at both sites. The cured animals had a normal area under the curve blood glucose response to intravenous glucose, albeit animals with intramuscular islet grafts had increased 120-min blood glucose levels. They also demonstrated an exaggerated counter regulatory response to hypoglycemia. The expression of genes important for beta-cell function was, at both implantation sites, comparable to that in native pancreatic islets. The gene expression of insulin (INS1 and INS2) and glucose transporter-2 was even increased, and the expression of lactate dehydrogenase decreased, at both sites when compared to native islets. We conclude that muscle and omentum provide excellent conditions for engraftment of transplanted islets. When compared to control, 200 islets implanted to the omentum displayed a restored glucose tolerance, whereas animals with intramuscular islet grafts of similar size displayed mild glucose intolerance.

Cell-based interferon gene therapy using proliferation-controllable, interferon-releasing mesenchymal stem cells

An important safety concern on cell-based gene therapy is that few methods have been available to control the proliferation and functioning of therapeutic protein-expressing cells after transplantation. We previously reported that the proliferation and functioning of the cells transfected with herpes simplex virus thymidine kinase (HSVtk) gene, a suicide gene, can be controlled by administration of ganciclovir. In this study, we tried to control the amount of murine interferon-γ (IFN-γ) secreted from transplanted murine mesenchymal stem cell line C3H10T1/2 cells to achieve safe cell-based IFN-γ gene therapy for cancer. C3H10T1/2 cells were transfected with HSVtk- and murine IFN-γ-expressing plasmid vectors to obtain C3H10T1/2/HSVtk/IFN-γ cells. C3H10T1/2/HSVtk/IFN-γ cells released IFN-γ and were sensitive to ganciclovir. C3H10T1/2/HSVtk/IFN-γ cells significantly suppressed the proliferation of murine adenocarcinoma cell line colon26 cells both in vitro and in vivo. Moreover, subcutaneous administration of ganciclovir to mice transplanted with NanoLuc luciferase-expressing C3H10T1/2/HSVtk cells for three consecutive days reduced the luminescence signals from the transplanted cells. These results indicate that the cell regulation system using HSVtk gene and ganciclovir can be useful for safe and efficient cell-based IFN-γ gene therapy for cancer.

Effects of Moderate Cycling Exercise on Blood Glucose Regulation Following Successful Clinical Islet Transplantation

CONTEXT

Islet transplantation is effective in preventing hypoglycemia in patients with type 1 diabetes (T1D). However, it is unknown whether transplanted islets regulate plasma glucose concentrations appropriately during and after exercise in human islet transplant recipient (ITxs).

OBJECTIVE

To determine the effect of exercise on plasma glucose, insulin, and glucagon concentrations in ITxs compared with control subjects (CONs) without diabetes.

INTERVENTION

Participants completed two conditions in random order: 45 minutes of aerobic exercise (60% VO2peak) and 45 minutes of seated rest. Blood samples were drawn at baseline, immediately after exercise or rest, and every 15 minutes throughout a 60-minute recovery period. Postexercise (24 hours) interstitial glucose was monitored with continuous glucose monitoring (CGM).

RESULTS

Twenty-four participants (12 ITxs, 12 CONs) completed the protocol. Plasma glucose decreased more over time with exercise in ITxs compared with CONs [main effects of treatment (P = 0.019), time (P = 0.001), and group (P = 0.012)]. Plasma glucose was lower during exercise vs rest in ITxs but not CONs [treatment by group interaction (P = 0.028)]. Plasma glucose decreased more during exercise than during rest [treatment by time interaction (P = 0.001)]. One ITx and one CON experienced plasma glucose concentrations <3.5 mmol/L at the end of exercise, both of whom returned above that threshold within 15 minutes. Nocturnal CGM glucose <3.5 mmol/L was detected in two CONs but no ITxs.

CONCLUSION

Despite a greater plasma glucose decline during exercise in ITxs, hypoglycemia risk was similar during and after exercise in ITxs compared with CONs.

Activation of µ-opioid receptors in the rostral ventrolateral medulla blocks the sympathetic counterregulatory response to glucoprivation

Activation of neurons in the rostral ventrolateral medulla (RVLM) following glucoprivation initiates sympathoadrenal activation, adrenaline release, and increased glucose production. Here, we aimed to determine the role of RVLM µ-opioid receptors in the counterregulatory response to systemic glucoprivation. Experiments were performed in pentobarbital sodium anesthetized male Sprague-Dawley rats ( n = 30). Bilateral activation of RVLM µ-opioid receptors with [d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO) (8 mM, 50 nl) depressed adrenal sympathetic nerve activity for ~60 min ( n = 6; Δ49.9 ± 5.8%, P < 0.05). The counterregulatory response to glucoprivation (measured by adrenal sympathetic efferent nerve activity) induced by 2-deoxyglucose (2-DG) ( n = 6; Δ63.6 ± 16.5%, P < 0.05) was completely blocked 60 min after DAMGO microinjections ( n = 6; Δ10.2 ± 3.5%, P < 0.05). Furthermore, DAMGO pretreatment attenuated the increase in blood glucose levels after 2-DG infusion ( n = 6; 6.1 ± 0.7mmol/l vs. baseline 5.2 ± 0.3mmol/l, P > 0.05) compared with 2-DG alone ( n = 6; 7.6 ± 0.4mmol/l vs. baseline 6.0 ± 0.4mmol/l, P < 0.05). Thus, activation of RVLM µ-opioid receptors attenuated the neural efferent response to glucoprivation and reduced glucose production.

β Cell Replacement Therapy: The Next 10 Years

β cell replacement with either pancreas or islet transplantation has progressed immensely over the last decades with current 1- and 5-year insulin independence rates of approximately 85% and 50%, respectively. Recent advances are largely attributed to improvements in immunosuppressive regimen, donor selection, and surgical technique. However, both strategies are compromised by a scarce donor source. Xenotransplantation offers a potential solution by providing a theoretically unlimited supply of islets, but clinical application has been limited by concerns for a potent immune response against xenogeneic tissue. β cell clusters derived from embryonic or induced pluripotent stem cells represent another promising unlimited source of insulin producing cells, but clinical application is pending further advances in the function of the β cell like clusters. Exciting developments and rapid progress in all areas of β cell replacement prompted a lively debate by members of the young investigator committee of the International Pancreas and Islet Transplant Association at the 15th International Pancreas and Islet Transplant Association Congress in Melbourne and at the 26th international congress of The Transplant Society in Hong Kong. This international group of young investigators debated which modality of β cell replacement would predominate the landscape in 10 years, and their arguments are summarized here.

Transplantation of Macroencapsulated Insulin-Producing Cells

PURPOSE OF REVIEW

There is considerable interest in using macroencapsulation devices as a delivery strategy for transplanting insulin-producing cells. This review aims to summarize recent advances, to highlight remaining challenges, and to provide recommendations for the field.

RECENT FINDINGS

A variety of new device designs have been reported to improve biocompatibility and to provide protection for islet/beta cells from immune destruction while allowing continuous secretion of insulin. Some of these new approaches are in clinical trials, but more research is needed to determine how sufficient beta-cell mass can be transplanted in a clinically applicable device size, and that insulin is secreted with kinetics that will safely provide adequate controls of glucose levels. Macroencapsulation is a potential solution to transplant beta cells without immunosuppression in diabetes patients, but new strategies must be developed to show that this approach is feasible.

Long-term viability and function of transplanted islets macroencapsulated at high density are achieved by enhanced oxygen supply

Transplantation of encapsulated islets can cure diabetes without immunosuppression, but oxygen supply limitations can cause failure. We investigated a retrievable macroencapsulation device wherein islets are encapsulated in a planar alginate slab and supplied with exogenous oxygen from a replenishable gas chamber. Translation to clinically-useful devices entails reduction of device size by increasing islet surface density, which requires increased gas chamber pO2. Here we show that islet surface density can be substantially increased safely by increasing gas chamber pO2 to a supraphysiological level that maintains all islets viable and functional. These levels were determined from measurements of pO2 profiles in islet-alginate slabs. Encapsulated islets implanted with surface density as high as 4,800 islet equivalents/cm3 in diabetic rats maintained normoglycemia for more than 7 months and provided near-normal intravenous glucose tolerance tests. Nearly 90% of the original viable tissue was recovered after device explantation. Damaged islets failed after progressively shorter times. The required values of gas chamber pO2 were predictable from a mathematical model of oxygen consumption and diffusion in the device. These results demonstrate feasibility of developing retrievable macroencapsulated devices small enough for clinical use and provide a firm basis for design of devices for testing in large animals and humans.

Effects of Composition of Alginate-Polyethylene Glycol Microcapsules and Transplant Site on Encapsulated Islet Graft Outcomes in Mice

Background

Understanding the effects of capsule composition and transplantation site on graft outcomes of encapsulated islets will aid in the development of more effective strategies for islet transplantation without immunosuppression.

Methods

Here, we evaluated the effects of transplanting alginate (ALG)-based microcapsules (Micro) in the confined and well-vascularized epididymal fat pad (EFP) site, a model of the human omentum, as opposed to free-floating in the intraperitoneal cavity (IP) in mice. We also examined the effects of reinforcing ALG with polyethylene glycol (PEG). To allow transplantation in the EFP site, we minimized capsule size to 500 ± 17 μm. Unlike ALG, PEG resists osmotic stress, hence we generated hybrid microcapsules by mixing PEG and ALG (MicroMix) or by coating ALG capsules with a 15 ± 2 μm PEG layer (Double).

Results

We found improved engraftment of fully allogeneic BALB/c islets in Micro capsules transplanted in the EFP (median reversal time [MRT], 1 day) versus the IP site (MRT, 5 days; P < 0.01) in diabetic C57BL/6 mice and of Micro encapsulated (MRT, 8 days) versus naked (MRT, 36 days; P < 0.01) baboon islets transplanted in the EFP site. Although in vitro viability and functionality of islets within MicroMix and Double capsules were comparable to Micro, addition of PEG to ALG in MicroMix capsules improved engraftment of allogeneic islets in the IP site, but resulted deleterious in the EFP site, probably due to lower biocompatibility.

Conclusions

Our results suggest that capsule composition and transplant site affect graft outcomes through their effects on nutrient availability, capsule stability, and biocompatibility.

By evaluating the effects of the encapsulated islet grafts with different capsule compositions and transplant sites, the authors suggest that the islet grafts with micro capsules and implanted in vascularized sites may increase clinical efficacy. Supplemental digital content is available in the text.

Islet Encapsulation: Physiological Possibilities and Limitations

A logical cure for type 1 diabetes (T1D) involves replacing the lost insulin-producing cells with new ones, preferably cells from a well-characterized and unlimited source of human insulin-producing cells. This straightforward and simple solution to provide a cure for T1D is immensely attractive but entails at least two inherent and thus far unresolved hurdles: 1) provision of an unlimited source of functional human insulin-producing cells and 2) prevention of rejection without the side effects of systemic immunosuppression. Generation of transplantable insulin-producing cells from human embryonic stem cells or induced pluripotent stem cells is at present close to reality, and we are currently awaiting the first clinical studies. Focus is now directed to foster development of novel means to control the immune system to enable large-scale clinical application. Encapsulation introduces a physical barrier that prevents access of immune cells to the transplanted cells but also hinders blood vessel ingrowth. Therefore, oxygen, nutrient, and hormonal passage over the encapsulation membrane is solely dependent on diffusion over the immune barrier, contributing to delays in glucose sensing and insulin secretion kinetics. This Perspective focuses on the physiological possibilities and limitations of an encapsulation strategy to establish near-normoglycemia in subjects with T1D, assuming that glucose-responsive insulin-producing cells are available for transplantation.

Does exercise pose a challenge to glucoregulation after clinical islet transplantation?

Islet transplantation (ITx) is effective in preventing severe hypoglycemia by restoring glucose-dependent insulin secretion in type 1 diabetes (T1D), but may not normalize glucose regulation. Studies suggest that physical activity plays a role in maintaining β-cell mass and function in individuals with type 2 diabetes and animal models of diabetes. This could indicate that physical activity plays a role in graft survival in ITx recipients. This review's objective is to assess current knowledge related to physical activity in ITx recipients. Responses to other challenges in blood glucose control (i.e., hypoglycemia) in human ITx recipients were examined to provide in-depth background information. To identify studies involving exercise in ITx recipients, a systematic search was performed using PubMed, Medline, and Embase, which revealed 277 English language publications. Publications were excluded if they did not involve ITx recipients; did not involve physical activity or hypoglycemia; or did not report on glucose, insulin, or counterregulatory hormones. During induced hypoglycemia, studies indicate normal suppression of insulin in ITx individuals compared with healthy non-T1D controls. Studies involving exercise in ITx animals have conflicting results, with time since transplantation and transplantation site (spleen, liver, kidney, peritoneal cavity) as possible confounders. No study examining blood glucose responses to physical activity in human ITx recipients was identified. A small number of induced-hypoglycemia studies in humans, and exercise studies in animals, would suggest that glucoregulation is greatly improved yet is still imperfect in this population and that ITx does not fully restore counterregulatory responses to challenges in blood glucose homeostasis.

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.

Current status of encapsulated islet transplantation

Islet transplantation is a treatment modality for diabetes mellitus that can maintain insulin levels within a physiologically appropriate range. However, wider clinical application is limited by insufficient donor numbers and a need for lifelong immunosuppression. Despite various clinical and preclinical trials, there is no single standard immunosuppressive regimen that can suppress acute and chronic immune reactions with lower toxicity to grafted islets. One of the strategies for overcoming lifelong immunosuppression is the incorporation of encapsulation technology, which can provide a physical immune barrier by keeping out high molecular weight immune system components, while still allowing low molecular weight oxygen, insulin and nutrients to pass through. Encapsulated islet transplantation approaches that have been studied so far include macroencapsulation, microencapsulation, conformal coating and nanoencapsulation. Herein we will review the basic concepts of islet encapsulation technique, earlier works to recent progress related to clinical studies and corporate investigations on encapsulated islet transplantation.

Mesenchymal stem cells and ligand incorporation in biomimetic poly(ethylene glycol) hydrogels significantly improve insulin secretion from pancreatic islets

The main goal of this study was to investigate pancreatic islet function with mesenchymal stem cells (MSCs) in a ligand-functionalized poly(ethylene glycol) (PEG) hydrogel for the treatment of type 1 diabetes (T1D). Rat bone marrow-derived MSCs (rBM-MSCs) were encapsulated within synthetic PEG hydrogel, and cell viability and apoptosis within this 3D environment was examined in detail. ATP content and caspase-3 activity of encapsulated MSCs showed that fibronectin-derived RGDS, laminin-derived IKVAV and/or insulinotropic glucagon-like peptide (GLP-1) were required to maintain MSC survival. Incorporation of these peptides into the hydrogel environment also improved pancreatic islet viability, where combinations of peptides had altered effects on islet survival. GLP-1 alone was the leading stimulator for insulin secretion. Cell adhesion peptides RGDS and IKVAV improved insulin secretion only when they were used in combination, but could not surpass the effect of GLP-1. Further, when pancreatic islets were co-encapsulated with MSCs within synthetic PEG hydrogel, a two-fold increase in the stimulation index was measured. Synergistic effects of MSCs and peptides were observed, with a seven-fold increase in the stimulation index. The results are promising and suggest that simultaneous incorporation of MSCs and ECM-derived peptides and/or GLP-1 can improve pancreatic islet function in response to altered glucose levels in the physiological environment. Copyright © 2014 John Wiley & Sons, Ltd.

Device design and materials optimization of conformal coating for islets of Langerhans

Encapsulation of islets of Langerhans may represent a way to transplant islets in the absence of immunosuppression. Traditional methods for encapsulation lead to diffusional limitations imposed by the size of the capsules (600-1,000 μm in diameter), which results in core hypoxia and delayed insulin secretion in response to glucose. Moreover, the large volume of encapsulated cells does not allow implantation in sites that might be more favorable to islet cell engraftment. To address these issues, we have developed an encapsulation method that allows conformal coating of islets through microfluidics and minimizes capsule size and graft volume. In this method, capsule thickness, rather than capsule diameter, is constant and tightly defined by the microdevice geometry and the rheological properties of the immiscible fluids used for encapsulation within the microfluidic system. We have optimized the method both computationally and experimentally, and found that conformal coating allows for complete encapsulation of islets with a thin (a few tens of micrometers) continuous layer of hydrogel. Both in vitro and in vivo in syngeneic murine models of islet transplantation, the function of conformally coated islets was not compromised by encapsulation and was comparable to that of unencapsulated islets. We have further demonstrated that the structural support conferred by the coating materials protected islets from the loss of function experienced by uncoated islets during ex vivo culture.

Functional stimulation of graft nerves has minor effects on insulin release from transplanted rat pancreatic islets

INTRODUCTION

Morphological evidence for reinnervation of pancreatic islet grafts is plentiful. However, to what extent intra-graft nerves influence the endocrine functions of the islet transplant is largely unknown. We therefore aimed to directly stimulate nerves leading to islet grafts with electrodes and measure insulin secretion in response to this.

METHODS

We implanted syngeneic islets under the renal capsule of rats, and examined them 1 or 7-9 months later. In anesthetized rats blood samples were collected from the renal vein and femoral artery, respectively, during electrode stimulation of the nerves leading to the islet grafts.

RESULTS

As expected, nerve stimulation decreased renal blood flow. However, serum insulin concentrations in samples derived from the renal vein or femoral artery changed in concert with one another, both during normoglycemia and acute hyperglycemia.

CONCLUSION

Reinnervation which occurs after islet transplantation under the renal capsule has minor effects on graft endocrine function.

Islet transplantation in rodents. Do encapsulated islets really work?

CONTEXT

Diabetes mellitus type I affects around 240 million people in the world and only in the USA 7.8% of the population. It has been estimated that the costs of its complications account for 5% to 10% of the total healthcare spending around the world. According to World Health Organization, 300 million people are expected to develop diabetes mellitus by the year 2025. The pancreatic islet transplantation is expected to be less invasive than a pancreas transplant, which is currently the most commonly used approach.

OBJECTIVES

To compare the encapsulated and free islet transplantation in rodents looking at sites of islet implantation, number of injected islets, viability and immunosuppression.

METHODS

A literature search was conducted using MEDLINE/PUBMED and SCIELO with terms about islet transplantation in the rodent from 2000 to 2010. We found 2,636 articles but only 56 articles from 2000 to 2010 were selected.

RESULTS

In these 56 articles used, 34% were encapsulated and 66% were nonencapsulated islets. Analyzing both types of islets transplantation, the majority of the encapsulated islets were implanted into the peritoneal cavity and the nonencapsulated islets into the liver, through the portal vein. In addition, the great advantage of the peritoneal cavity as the site of islet transplantation is its blood supply. Both vascular endothelial cells and vascular endothelial growth factor were used to stimulate angiogenesis of the islet grafts, increasing the vascularization rapidly after implantation. It also has been proven that there is influence of the capsules, since the larger the capsule more chances there are of central necrosis. In some articles, the use of immunosuppression demonstrated to increase the life expectancy of the graft.

CONCLUSION

While significant progress has been made in the islets transplantation field, many obstacles remain to be overcome. Microencapsulation provides a means to transplant islets without immunosuppressive agents and may enable the performance of xenotransplantation. The use of alternative donor sources, fewer islets per capsule and the appropriate deployment location, such as the peritoneal cavity, may give a future perspective to the application of immunoprotective capsules and viability in clinical practice. A variety of strategies, such as genetic engineering, co-encapsulation, improvement in oxygen supply or the establishment of hypoxia resistance will also improve the islet transplantation performance. It remains to be determined which combination of strategies with encapsulation can fulfill the promise of establishing a simple and safe transplantation as a cure for diabetes.

Striated muscle as implantation site for transplanted pancreatic islets

Islet transplantation is an attractive treatment for selected patients with brittle type 1 diabetes. In the clinical setting, intraportal transplantation predominates. However, due to extensive early islet cell death, the quantity of islets needed to restore glucose homeostasis requires in general a minimum of two donors. Moreover, the deterioration of islet function over time results in few insulin-independent patients after five-year followup. Specific obstacles to the success of islet transplantation include site-specific concerns for the liver such as the instant blood mediated inflammatory reaction, islet lipotoxicity, low oxygen tension, and poor revascularization, impediments that have led to the developing interest for alternative implantation sites over recent years. Within preclinical settings, several alternative sites have now been investigated and proven favorable in various aspects. Muscle is considered a very promising site and has physiologically properties and technical advantages that could make it optimal for islet transplantation.

Islets transplanted in immunoisolation devices: a review of the progress and the challenges that remain

The concept of using an immunoisolation device to facilitate the transplantation of islets without the need for immunosuppression has been around for more than 50 yr. Significant progress has been made in developing suitable materials that satisfy the need for biocompatibility, durability, and permselectivity. However, the search is ongoing for a device that allows sufficient oxygen transfer while maintaining a barrier to immune cells and preventing rejection of the transplanted tissue. Separating the islets from the rich blood supply in the native pancreas takes its toll. The immunoisolated islets commonly suffer from hypoxia and necrosis, which in turn triggers a host immune response. Efforts have been made to improve the supply of nutrients by using proangiogenic factors to augment the development of a vascular supply in the transplant site, by using small islet cell aggregates to reduce the barrier to diffusion of oxygen, or by creating scaffolds that are in close proximity to a vascular network such as the omental blood supply. Even if these efforts are successful, the shortage of donor islet tissue available for transplantation remains a major problem. To this end, a search for a renewable source of insulin-producing cells is ongoing; whether these will come from adult or embryonic stem cells or xenogeneic sources remains to be seen. Herein we will review the above issues and chart the progress made with various immunoisolation devices in small and large animal models and the small number of clinical trials carried out to date.

Bone marrow-derived mesenchymal stem cells co-cultured with pancreatic islets display β cell plasticity

The direct co-culturing effect of rat bone-marrow-derived mesenchymal stem cells (rBM-MSCs) on the pancreatic-islets (PIs) was studied to obtain functional islet cells. MSCs were isolated from rat bone marrow and cultivated under standard conditions. Following their characterization, the rBM-MSCs were directly (with cell-islet contact) co-cultured with recovered PIs together with the single cell cultures of those cell cultures as a control. The effect of direct co-cultures of rBM-MSCs with the PIs of normal rats was investigated using immunophenotypical and functional methods. The change in the amount of insulin secretion was evaluated as an indicator for differentiation of rBM-MSCs. One approache for in vitro differentiation to achieve reprogramming for differentiation into suitable cell types by changing the microenvironment of the cells to provide signals that might activate metabolic pathways is to use co-cultures with the microenvironment of the specific cells of the desired cell type, tissue/organ extracts, extracellular matrix compounds or biologically absorbable materials. Differentiated rBM-MSCs were found to be immunopositive for the specific insulin-producing cell marker, insulin, but not in undifferentiated rBM-MSCs. The functionality tests by ELISA confirmed that insulin secretion of co-cultured MSCs with islets was higher than that of islets. These evidences indicated that PIs could be regarded as critical components of the stem cell niche, such that MSCs can be differentiated into insulin-producing cells (IPCs). Moreover, direct cell-to-cell contact might provide additional and independent support. This approach would circumvent the need for PI-stem cell co-culture and could potentially facilitate the production of functional IPCs for future clinical applications.

Protection of rat pancreatic islet function and viability by coculture with rat bone marrow-derived mesenchymal stem cells

The maintenance of viable and functional islets is critical in successful pancreatic islet transplantation from cadaveric sources. During the isolation procedure, islets are exposed to a number of insults including ischemia, oxidative stress and cytokine injury that cause a reduction in the recovered viable islet mass. A novel approach was designed in which streptozotocin (STZ)-damaged rat pancreatic islets (rPIs) were indirectly cocultured with rat bone marrow-derived mesenchymal stem cells (rBM-MSCs) to maintain survival of the cultured rPIs. The results indicated that islets cocultured with rBM-MSCs secreted an increased level of insulin after 14 days, whereas non-cocultured islets gradually deteriorated and cell death occurred. The cocultivation of rBM-MSCs with islets and STZ-damaged islets showed the expression of IL6 and transforming growth factor-β1 in the culture medium, besides the expression of the antiapoptotic genes (Mapkapk2, Tnip1 and Bcl3), implying the cytoprotective, anti-inflammatory and antiapoptotic effects of rBM-SCs through paracrine actions.

Transgenic expression of insulin-response element binding protein-1 in beta-cells reproduces type 2 diabetes

Recent evidence supports the idea that insulin signaling through the insulin receptor substrate/phosphatidyl-inositol 3-kinase/Akt pathway is involved in the maintenance of beta-cell mass and function. We previously identified the insulin-response element binding protein-1 (IRE-BP1) as an effector of insulin-induced Akt signaling in the liver, and showed that the 50-kDa carboxyl fragment confers the transcriptional activity of this factor. In this investigation we found that IRE-BP1 is expressed in the alpha, beta, and delta-cells of the islets of Langerhans, and is localized to the cytoplasm in beta-cells in normal rats, but is reduced and redistributed to the islet cell nuclei in obese Zucker rats. To test whether IRE-BP1 modulates beta-cell function and insulin secretion, we used the rat insulin II promoter to drive expression of the carboxyl fragment in beta-cells. Transgenic expression of IRE-BP1 in FVB mice increases nuclear IRE-BP1 expression, and produces a phenotype similar to that of type 2 diabetes, with hyperinsulinemia, hyperglycemia, and increased body weight. IRE-BP1 increased islet type I IGF receptor expression, potentially contributing to the development of islet hypertrophy. Our findings suggest that increased gene transcription mediated through IRE-BP1 may contribute to beta-cell dysfunction in insulin resistance, and allow for the hypothesis that IRE-BP1 plays a role in the pathophysiology of type 2 diabetes.

Nutritive perfusion of pancreatic endocrine tissue during hemorrhagic hypotension: how differ islets in situ from islet isografts?

Although the exocrine gland frequently has I/R-associated complications such as posttransplant pancreatitis, hypoxia-induced dysfunction of pancreatic endocrine tissue is rarely observed. However, sympathetic hypersensitivity is accused of impaired endocrine function observed in human pancreatic grafts. These tissue-confined differences in susceptibility might be attributed to a distinct islet-specific regulatory control of blood flow (BF). To investigate this hypothesis, intravital microscopy was used for visualization of islets in situ and revascularized islet isografts in Syrian golden hamsters. Blood withdrawal was performed to induce sympathetic stimulation and to evoke an appropriate stress response of the tissue under investigation. Hypotension resulted in a perfusion pressure-dependent reduction of perfusion in both islets in situ and islet isografts. This was associated with comparable microhemodynamics of the tissues in direct vicinity, that is, pancreatic exocrine and host muscle tissue. There was a progressive decrease in functional capillary density of islets in situ and islet isografts that significantly correlated with the stepwise reduction of arterial blood pressure and did not differ in the hypotension-induced perfusion pattern in the neighboring exocrine and host skeletal muscle tissue. Concomitantly, capillary BF in islets in situ and islet isografts and in pancreatic exocrine and host muscle tissue was found to be reduced due to hypotension-associated decreases in capillary diameters and BF velocity. Microvascular perfusion of pancreatic islets and islet isografts is not preserved but strongly parallels perfusion pattern of the neighboring tissue under hemorrhagic stress. This disproves the existence of an individual islet-specific regulatory control of blood flow.

Normal relationship of beta- and non-beta-cells not needed for successful islet transplantation

Islets are composed mostly of beta-cells, and therefore stem cell research has concentrated on generating purified beta-cells, neglecting the other endocrine cell types in the islet. We investigated the presence of endocrine non-beta-cells after islet transplantation. In addition, we studied whether the transplantation of pure beta-cells, in volumes similar to that used in islet transplantation, would suffice to reverse hyperglycemia in diabetic mice. Rat islets were dispersed and beta-cells were purified by fluorescence-activated cell sorting according to their endogenous fluorescence. After reaggregation, 600 islet equivalents of the purified beta-cell aggregates were implanted into diabetic SCID mice. In mice implanted with beta-cell-enriched aggregates, the hyperglycemia was reversed and good graft function over a 12-week period was observed with regard to glucose and insulin levels, glucose tolerance tests, and graft insulin content. The endocrine cell composition of the beta-cell-enriched aggregates remained constant; before and 12 weeks after transplantation, the beta-cell-enriched aggregates comprised 95% beta-cells and 5% endocrine non-beta-cells. However, islet grafts, despite originally having comprised 75% beta-cells and 25% endocrine non-beta-cells, comprised just 5% endocrine non-beta-cells after transplantation, indicating a loss of these cells. beta-Cell-enriched aggregates can effectively reverse hyperglycemia in mice, and transplanted intact islets are depleted in non-beta-cells. It is therefore likely that islet non-beta-cells are not essential for successful islet transplantation.

Implantation site-dependent dysfunction of transplanted pancreatic islets

OBJECTIVE

Clinical islet transplantations are performed through infusion of islets via the portal vein into the liver. This study aimed at characterizing the influence of the implantation microenvironment on islet graft metabolism and function.

RESEARCH DESIGN AND METHODS

Islets were transplanted into their normal environment, i.e., the pancreas, or intraportally into the liver of mice. One month posttransplantation, the transplanted islets were retrieved and investigated for changes in function and gene expression.

RESULTS

Insulin content, glucose-stimulated insulin release, (pro)insulin biosynthesis, and glucose oxidation rate were markedly decreased in islets retrieved from the liver, both when compared with islets transplanted into the pancreas and endogenous islets. Islets transplanted into the pancreas showed normal insulin content, (pro)insulin biosynthesis, and glucose oxidation rate but increased basal insulin secretion and impaired glucose stimulation index. Gene expression data for retrieved islets showed downregulation of pancreatic and duodenal homeobox gene-1, GLUT-2, glucokinase, mitochondrial glycerol-phosphate dehydrogenase, and pyruvate carboxylase, preferentially in intraportally transplanted islets.

CONCLUSIONS

Islets transplanted into their normal microenvironment, i.e., the pancreas, display gene expression changes when compared with endogenous islets but only moderate changes in metabolic functions. In contrast, site-specific properties of the liver markedly impaired the metabolic functions of intraportally transplanted islets.

Ischaemia is linked to inflammation and induction of angiogenesis in pancreatic islets

Beta-cell replacement is the only way to restore euglycaemia in patients with type-1 diabetes. Pancreatic tissue, processed for subsequent clinical islet transplantation, is exposed to ischaemia causing injury and death in a large number of islets before and after transplantation. In this review we summarize what is known on the sources of environmental stress for pancreatic islets, such as insufficient oxygen supply during pancreas procurement and in culture prior to intraportal transplantation, nutritional and oxygen deprivation during the isolation process, and the consequences of hyperglycaemia. An increasingly recognized role in the modulation of beta-cell function and these environmental stress factors plays the vascular network of the pancreatic islets. Islet revascularization by angiogenesis is relevant for the survival of the graft subsequent to transplantation. Potential strategies offered by therapeutic induction of revascularization to ameliorate the detrimental impact of these factors on the quality of islet transplants are discussed.

Islet auto-transplantation into an omental or splenic site results in a normal beta cell but abnormal alpha cell response to mild non-insulin-induced hypoglycemia

The present studies were designed to determine if totally pancreatectomized dogs that underwent islet auto-transplantation retained a functional pancreatic counterregulatory response to mild non-insulin-induced hypoglycemia. Six dogs underwent total pancreatectomy followed by islet auto-transplantation to spleen or omentum. The animals recovered and fasting plasma glucose and insulin levels were normal. Each study consisted of a 40-min control and 2-h test period. At the onset of the test period, a glycogen phosphorylase inhibitor was administered to create mild hypoglycemia. Plasma glucose in the transplanted dogs fell from 120 +/- 4 to 80 +/- 3 mg/dL, similar to the minimum in control dogs without islet auto-transplantation (108 +/- 2 to 84 +/- 5 mg/dL). The fall in plasma insulin was similar in both groups. Glucagon, however, rose in response to hypoglycemia in the control dogs (Delta24 +/- 7 pg/mL; p < 0.05), but failed to rise significantly in the transplanted dogs (Delta9 +/- 6 pg/mL). In fact, only 1 of 7 control dogs failed to increase plasma glucagon by at least 25%, whereas 4 of 6 transplanted dogs failed to do so. In conclusion, in conscious dogs with successfully auto-transplanted islets, the beta cell response to mild non-insulin-induced hypoglycemia was normal, whereas the alpha cell response was not.

Leptinomimetic effects of the AMP kinase activator AICAR in leptin-resistant rats: prevention of diabetes and ectopic lipid deposition

AIMS/HYPOTHESIS

Leptin has been shown to activate AMP-activated protein kinase (AMPK), an enzyme that regulates the activities of key enzymes of lipid synthesis and metabolism. We assess here (i) whether AMPK activity is diminished in rodents deficient in leptin or the leptin receptor, and (ii) the effects of treating the diabetes-prone, leptin-receptor-deficient Zucker Diabetic Fatty (ZDF) rat with an AMPK activator.

METHODS

AMPK activity and related parameters were measured in muscle and or liver of fa/fa and ZDF rats and ob/ob mice. We also explored the effect of treatment with the AMPK activator 5-aminoimidazole 4-carboxamide 1-beta-D ribofuranoside (AICAR) (7.4 mmol/l, on Monday, Wednesday and Friday for 15 weeks, beginning at 7 weeks of age) on the phenotype of the ZDF rat.

RESULTS

AMPK activity was diminished in muscle and/or liver of fa/fa (leptin-receptor-deficient, non-diabetic) and ZDF (leptin-receptor-deficient, diabetes-prone) rats and ob/ob mice (leptin-deficient). ZDF rats that had free access to food became hyperglycaemic (22.2 mmol/l) and hyperphagic after 2 to 5 weeks and remained so during the remainder of the study. Treatment of ZDF rats with AICAR prevented the development of diabetes, as well as increases of triglyceride content in liver, muscle and the pancreatic islets. It also attenuated the morphological abnormalities observed in the islets of untreated rats. Rats diet-matched with the AICAR-treated animals developed diabetes of intermediate severity and showed decreases in triglyceride content in the islets, but not in liver or muscle.

CONCLUSIONS/INTERPRETATION

The results indicate that a deficiency of leptin or the leptin receptor is associated with a decrease in AMPK activity in muscle and/or liver. They also suggest that treatment with an AMPK activator prevents the development of diabetes and ectopic lipid accumulation in the ZDF rat.