Engineering pancreatic islets

Extracellular vesicle-mediated intercellular and interorgan crosstalk of pancreatic islet in health and diabetes

Diabetes mellitus (DM) is a systemic metabolic disease with high mortality and morbidity. Extracellular vesicles (EVs) have emerged as a novel class of signaling molecules, biomarkers and therapeutic agents. EVs-mediated intercellular and interorgan crosstalk of pancreatic islets plays a crucial role in the regulation of insulin secretion of β-cells and insulin action in peripheral insulin target tissues, maintaining glucose homeostasis under physiological conditions, and it's also involved in pathological changes including autoimmune response, insulin resistance and β-cell failure associated with DM. In addition, EVs may serve as biomarkers and therapeutic agents that respectively reflect the status and improve function and viability of pancreatic islets. In this review, we provide an overview of EVs, discuss EVs-mediated intercellular and interorgan crosstalk of pancreatic islet under physiological and diabetic conditions, and summarize the emerging applications of EVs in the diagnosis and treatment of DM. A better understanding of EVs-mediated intercellular and interorgan communication of pancreatic islets will broaden and enrich our knowledge of physiological homeostasis maintenance as well as the development, diagnosis and treatment of DM.

Intercellular Communication in the Islet of Langerhans in Health and Disease

Blood glucose homeostasis requires proper function of pancreatic islets, which secrete insulin, glucagon, and somatostatin from the β-, α-, and δ-cells, respectively. Each islet cell type is equipped with intrinsic mechanisms for glucose sensing and secretory actions, but these intrinsic mechanisms alone cannot explain the observed secretory profiles from intact islets. Regulation of secretion involves interconnected mechanisms among and between islet cell types. Islet cells lose their normal functional signatures and secretory behaviors upon dispersal as compared to intact islets and in vivo. In dispersed islet cells, the glucose response of insulin secretion is attenuated from that seen from whole islets, coordinated oscillations in membrane potential and intracellular Ca2+ activity, as well as the two-phase insulin secretion profile, are missing, and glucagon secretion displays higher basal secretion profile and a reverse glucose-dependent response from that of intact islets. These observations highlight the critical roles of intercellular communication within the pancreatic islet, and how these communication pathways are crucial for proper hormonal and nonhormonal secretion and glucose homeostasis. Further, misregulated secretions of islet secretory products that arise from defective intercellular islet communication are implicated in diabetes. Intercellular communication within the islet environment comprises multiple mechanisms, including electrical synapses from gap junctional coupling, paracrine interactions among neighboring cells, and direct cell-to-cell contacts in the form of juxtacrine signaling. In this article, we describe the various mechanisms that contribute to proper islet function for each islet cell type and how intercellular islet communications are coordinated among the same and different islet cell types. © 2021 American Physiological Society. Compr Physiol 11:2191-2225, 2021.

Tissue engineering and regenerative medicine: history, progress, and challenges

The past three decades have seen the emergence of an endeavor called tissue engineering and regenerative medicine in which scientists, engineers, and physicians apply tools from a variety of fields to construct biological substitutes that can mimic tissues for diagnostic and research purposes and can replace (or help regenerate) diseased and injured tissues. A significant portion of this effort has been translated to actual therapies, especially in the areas of skin replacement and, to a lesser extent, cartilage repair. A good amount of thoughtful work has also yielded prototypes of other tissue substitutes such as nerve conduits, blood vessels, liver, and even heart. Forward movement to clinical product, however, has been slow. Another offshoot of these efforts has been the incorporation of some new exciting technologies (e.g., microfabrication, 3D printing) that may enable future breakthroughs. In this review we highlight the modest beginnings of the field and then describe three application examples that are in various stages of development, ranging from relatively mature (skin) to ongoing proof-of-concept (cartilage) to early stage (liver). We then discuss some of the major issues that limit the development of complex tissues, some of which are fundamentals-based, whereas others stem from the needs of the end users.

Mathematical models for insulin secretion in pancreatic β-cells

Insulin secretion is one of the most characteristic features of β-cell physiology. As it plays a central role in glucose regulation, a number of experimental and theoretical studies have been performed since the discovery of the pancreatic β-cell. This review article aims to give an overview of the mathematical approaches to insulin secretion. Beginning with the bursting electrical activity in pancreatic β-cells, we describe effects of the gap-junction coupling between β-cells on the dynamics of insulin secretion. Then, implications of paracrine interactions among such islet cells as α-, β-, and δ-cells are discussed. Finally, we present mathematical models which incorporate effects of glycolysis and mitochondrial glucose metabolism on the control of insulin secretion.

The use of gating technology in bioengineering insulin-secreting cells from embryonic stem cells

Embryonic stem cells display the ability to differentiate in vitro into a variety of cell types. This process is induced by embryoid body formation, addition of several soluble growth factors to the culture medium and other strategies. However, none of the used factors is capable to drive differentiation to only one specific celltype. The use of gating technology has allowed to partially overcome this problem. The rational behind this technique is based on the transfection of stem cells with a transgene carrying expression cassettes for a cell type specific promoter, regulating expression ofa selectable marker to select one cell lineage from other cell lineages.Using this system, we have obtained insulin-secreting cells by transfecting mouse embryonic stem cells with a DNA construct providing resistance to neomycin under the control of the regulatory regions of the human insulin gene. Furthermore, gating technology has been successfully used to isolate other cell types such as cardiomyocytes and neural precursors from undifferentiated embryonic stem cells. This review focuses on the possibilities offered by this technology in embryonic stem cell bioengineering, mainly to obtain insulin-secreting cells. Advantages and considerations of this selection system will be also discussed.

Modeling mechanisms of cell secretion

Secretion is a fundamental cellular process involving the regulated release of intracellular products from cells. Physiological functions such as neurotransmission, or the release of hormones and digestive enzymes, are all governed by cell secretion. Anomalies in the processes involved in secretion contribute to the development and progression of diseases such as diabetes and other hormonal disorders. To unravel the mechanisms that govern such diseases, it is essential to understand how hormones, growth factors and neurotransmitters are synthesized and processed, and how their signals are recognized, amplified and transmitted by intracellular signaling pathways in the target cells. Here, we discuss diverse aspects of the detailed mechanisms involved in secretion based on mathematical models. The models range from stochastic ones describing the trafficking of secretory vesicles to deterministic ones investigating the regulation of cellular processes that underlie hormonal secretion. In all cases, the models are closely related to experimental results and suggest theoretical predictions for the secretion mechanisms.

Lessons from models of pancreatic beta cells for engineering glucose-sensing cells

Mathematical models of pancreatic beta cells suggest design principles that can be applied to engineering cells to sense glucose and secrete insulin. Engineering cells can potentially both contribute to future diabetes therapies and generate new insights into beta-cell function. The focus is on ion channels, Ca(2+)handling, and elements of metabolism that combine to produce the varied oscillatory patterns exhibited by beta cells.

Adhesion of pancreatic beta cells to biopolymer films

Dramatic reversal of Type 1 diabetes in patients receiving pancreatic islet transplants continues to prompt vigorous research concerning the basic mechanisms underlying patient turnaround. At the most fundamental level, transplanted islets must maintain viability and function in vitro and in vivo and should be protected from host immune rejection. Our previous reports showed enhancement of islet viability and insulin secretion per tissue mass for small islets (<125 mum) as compared with large islets (>125 mum), thus, demonstrating the effect of enhancing the mass transport of islets (i.e. increasing tissue surface area to volume ratio). Here, we report the facile dispersion of rat islets into individual cells that are layered onto the surface of a biopolymer film towards the ultimate goal of improving mass transport in islet tissue. The tightly packed structure of intact islets was disrupted by incubating in calcium-free media resulting in fragmented islets, which were further dispersed into individual or small groups of cells by using a low concentration of papain. The dispersed cells were screened for adhesion to a range of biopolymers and the nature of cell adhesion was characterized for selected groups by quantifying adherent cells, measuring the surface area coverage of the cells, and immunolabeling cells for adhesion proteins interacting with selected biopolymers. Finally, beta cells in suspension were centrifuged to form controlled numbers of cell layers on films for future work determining the mass transport limitations in the adhered tissue constructs. (c) 2009 Wiley Periodicals, Inc. Biopolymers 91: 676-685, 2009.This article was originally published online as an accepted preprint. The "Published Online" date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com.

Contributions of mathematical modeling of beta cells to the understanding of beta-cell oscillations and insulin secretion

Mathematical modeling of pancreatic beta cells has contributed significantly to the understanding of the mechanisms involved in glucose-stimulated insulin secretion (GSIS). Early models of insulin secretion built in the 1970s were phenomenological with little biological foundation for the proposed mechanisms. In the 1980s, models focused on identifying the regulation of bursting electrical activity known to be important for insulin secretion. The main result was to reject proposed mechanisms as new data emerged, but important results of the role of cell-to-cell coupling were also established. New models have been proposed that provide possible explanations for the occurrence of various patterns of bursting and calcium oscillations. In addition, modeling has played an important role in comparing competing effects of calcium on both NADH and adenosine 3'-5'-cyclic monophosphate levels. Models including modern cell biological results of the regulation of insulin containing granules and cell heterogeneity have appeared, providing updated versions of the early models proposed in the 1970s. These models, when coupled to electrophysiological- and calcium-based ones, have the prospect to aid in understanding the overall picture of GSIS. In addition, they might be useful for estimating in vivo beta-cell functioning. Beta-cell modeling will likely move closer to clinical applications, where it can be expected to play an important role, as it has and will, in understanding the complex oscillatory phenomena observed in beta cells and islets.

Beneficial effects of intercellular interactions between pancreatic islet cells in blood glucose regulation

Glucose homeostasis is controlled by the islets of Langerhans which are equipped with alpha-cells increasing the blood glucose level, beta-cells decreasing it, and delta-cells the precise role of which still needs identifying. Although intercellular communications between these endocrine cells have recently been observed, their roles in glucose homeostasis have not been clearly understood. In this study, we construct a mathematical model for an islet consisting of two-state alpha-, beta-, and delta-cells, and analyze effects of known chemical interactions between them with emphasis on the combined effects of those interactions. In particular, such features as paracrine signals of neighboring cells and cell-to-cell variations in response to external glucose concentrations as well as glucose dynamics, depending on insulin and glucagon hormone, are considered explicitly. Our model predicts three possible benefits of the cell-to-cell interactions: First, the asymmetric interaction between alpha- and beta-cells contributes to the dynamic stability while the perturbed glucose level recovers to the normal level. Second, the inhibitory interactions of delta-cells for glucagon and insulin secretion prevent the wasteful co-secretion of them at the normal glucose level. Finally, the glucose dose-responses of insulin secretion is modified to become more pronounced at high glucose levels due to the inhibition by delta-cells. It is thus concluded that the intercellular communications in islets of Langerhans should contribute to the effective control of glucose homeostasis.

Cell therapy for diabetes mellitus: an opportunity for stem cells?

Diabetes is a chronic disease characterized by a deficit in beta cell mass and a failure of glucose homeostasis. Both circumstances result in a variety of severe complications and an overall shortened life expectancy. Thus, diabetes represents an attractive candidate for cell therapy. Reversal of diabetes can be achieved through pancreas and islet transplantation, but shortage of donor organs has prompted an intensive search for alternative sources of beta cells. This achievement has stimulated the search for appropriate stem cell sources. Both embryonic and adult stem cells have been used to generate surrogate beta cells or otherwise restore beta cell functioning. In this regard, several studies have reported the generation of insulin-secreting cells from embryonic and adult stem cells that normalized blood glucose values when transplanted into diabetic animal models. Due to beta cell complexity, insulin-producing cells generated from stem cells do not possess all beta cell attributes. This indicates the need for further development of methods for differentiation and selection of completely functional beta cells. While these problems are overcome, diabetic patients may benefit from therapeutic strategies based on autologous stem cell therapies addressing late diabetic complications. In this article, we discuss the recent progress in the generation of insulin-producing cells from embryonic and adult stem cells, together with the challenges for the clinical use of diabetes stem cell therapy.

Size distribution of mouse Langerhans islets

Pancreatic beta-cells are clustered in islets of Langerhans, which are typically a few hundred micrometers in a variety of mammals. In this study, we propose a theoretical model for the growth of pancreatic islets and derive the islet size distribution, based on two recent observations: First, the neogenesis of new islets becomes negligible after some developmental stage. Second, islets grow via a random process, where any cell in an islet proliferates with the same rate regardless of the present size of the islet. Our model predicts either log-normal or Weibull distributions of the islet sizes, depending on whether cells in an islet proliferate coherently or independently. To confirm this, we also measure the islet size by selectively staining islets, which are exposed from exocrine tissues in mice after enzymatic treatment. Indeed revealed are skewed distributions with the peak size of approximately 100 cells, which fit well to the theoretically derived ones. Interestingly, most islets turned out to be bigger than the expected minimal size (approximately 10 or so cells) necessary for stable synchronization of beta-cells through electrical gap-junction coupling. The collaborative behavior among cells is known to facilitate synchronized insulin secretion and tends to saturate beyond the critical (saturation) size of approximately 100 cells. We further probe how the islets change as normal mice grow from young (6 weeks) to adult (5 months) stages. It is found that islets may not grow too large to maintain appropriate ratios between cells of different types. Our results implicate that growing of mouse islets may be regulated by several physical constraints such as the minimal size required for stable cell-to-cell coupling and the upper limit to keep the ratios between cell types. Within the lower and upper limits the observed size distributions of islets can be faithfully regenerated by assuming random and uncoordinated proliferation of each beta-cell at appropriate rates.

Islet cell transplantation today

INTRODUCTION

Long-term studies strongly suggest that tight control of blood glucose can prevent the development and retard the progression of chronic complications of type 1 diabetes mellitus. In contrast to conventional insulin treatment, replacement of a patient's islets of Langerhans either by pancreas organ transplantation or by isolated islet transplantation is the only treatment to achieve a constant normoglycemic state and avoiding hypoglycemic episodes, a typical adverse event of multiple daily insulin injections. However, the cost of this benefit is still the need for immunosuppressive treatment of the recipient with all its potential risks.

MATERIALS AND METHODS

Islet cell transplantation offers the advantage of being performed as a minimally invasive procedure in which islets can be perfused percutaneously into the liver via the portal vein. Between January 1990 and December 2004, 458 pancreatic islet transplants worldwide have been reported to the International Islet Transplant Registry (ITR) at our Third Medical Department, University of Giessen/Germany.

RESULTS

Data analysis of islet cell transplants performed in the last 5 years (1999-2004) shows at 1 year after adult islet transplantation a patient survival rate of 97%, a functioning islet graft in 82% of the cases, whereas insulin independence was meanwhile achieved in 43% of the cases. However, using a novel protocol established by the Edmonton Center/Canada, the insulin independence rates have improved significantly reaching meanwhile a 50-80% level.

CONCLUSION

Finally, the concept of islet cell or stem cell transplantation is most attractive, as it offers many perspectives: islet cell availability could become unlimited and islet or stem cells my be transplanted without life-long immunosuppressive treatment of the recipient, just to mention two of them.

Glucose induces opposite intracellular Ca2+ concentration oscillatory patterns in identified alpha- and beta-cells within intact human islets of Langerhans

Homeostasis of blood glucose is mainly regulated by the coordinated secretion of glucagon and insulin from alpha- and beta-cells within the islets of Langerhans. The release of both hormones is Ca(2+) dependent. In the current study, we used confocal microscopy and immunocytochemistry to unequivocally characterize the glucose-induced Ca(2+) signals in alpha- and beta-cells within intact human islets. Extracellular glucose stimulation induced an opposite response in these two cell types. Although the intracellular Ca(2+) concentration ([Ca(2+)](i)) in beta-cells remained stable at low glucose concentrations, alpha-cells exhibited an oscillatory [Ca(2+)](i) response. Conversely, the elevation of extracellular glucose elicited an oscillatory [Ca(2+)](i) pattern in beta-cells but inhibited low-glucose-induced [Ca(2+)](i) signals in alpha-cells. These Ca(2+) signals were synchronic among beta-cells grouped in clusters within the islet, although they were not coordinated among the whole beta-cell population. The response of alpha-cells was totally asynchronic. Therefore, both the alpha- and beta-cell populations within human islets did not work as a syncitium in response to glucose. A deeper knowledge of alpha- and beta-cell behavior within intact human islets is important to better understand the physiology of the human endocrine pancreas and may be useful to select high-quality islets for transplantation.

The bioartificial pancreas: progress and challenges

Diabetes remains a devastating disease, with tremendous cost in terms of human suffering and healthcare expenditures. A bioartificial pancreas has the potential as a promising approach to preventing or reversing complications associated with this disease. Bioartificial pancreatic constructs are based on encapsulation of islet cells with a semipermeable membrane so that cells can be protected from the host's immune system. Encapsulation of islet cells eliminates the requirement of immunosuppressive drugs, and offers a possible solution to the shortage of donors as it may allow the use of animal islets or insulin-producing cells engineered from stem cells. During the past 2 decades, several major approaches for immunoprotection of islets have been studied. The microencapsulation approach is quite promising because of its improved diffusion capacity, and technical ease of transplantation. It has the potential for providing an effective long-term treatment or cure of Type 1 diabetes.

Therapeutic applications of embryonic stem cells

Embryonic stem (ES) cells have the potential to proliferate indefinitely in culture and can differentiate into any cell type. The emergence of ES cell lines from human embryos in the past 5 years has attracted profound public and scientific interest, given the far-reaching potential applications of these cells in regenerative medicine. In the future, it is possible that human ES (hES) cells might serve as an unlimited source of cells for transplantation therapy under conditions that result from cell degeneration or malfunction, and that genetically manipulated hES cells might serve as vectors to carry and express genes in target organs following transplantation in the course of gene therapy. This chapter reviews the properties of hES cells and their potential advantages and limitations for cell-based therapies. We also describe various approaches that might be utilized with hES cells to avoid potential immune rejection after allogeneic transplantation and hence circumvent the need for systemic immune suppression. Up-to-date research in establishing committed tissue-specific progenitors from ES cells and evidence of their function after transplantation in various animal disease models is also reviewed. The chapter concludes that hES cells show great promise for regenerative medicine although significant developments are still required to exploit their potential for cell and gene therapy.

Production of transgenic tilapia with Brockmann bodies secreting [desThrB30] human insulin

BACKGROUND

Tilapia are commercially important tropical fish which, like many teleosts, have anatomically discrete islet organs called Brockmann bodies. When transplanted into diabetic nude mice, tilapia islets provide long-term normoglycemia and mammalian-like glucose tolerance profiles.

METHODS

Using site-directed mutagenesis and linker ligation we have "humanized" the tilapia insulin gene so that it codes for [desThrB30] human insulin while maintaining the tilapia regulatory sequences. Following microinjection into fertilized eggs, we screened DNA isolated from whole fry shortly after hatching by PCR. Positive fish were grown to sexual maturity and mated to wild-types and positive Fl's were further characterized.

RESULTS

Human insulin was detected in both serum and in the clusters of beta cells scattered throughout the Brockmann bodies. Surrounding non-beta cells as well as other tissues were negative indicating beta cell specific expression. Purification and sequencing of both A-and B-chains verified that the insulin was properly processed and humanized.

CONCLUSIONS

After extensive characterization, transgenic tilapia could become a suitable, inexpensive source of islet tissue that can be easily mass-produced for clinical islet xenotransplantation. Because tilapia islets are exceedingly resistant to hypoxia by mammalian standards, transgenic tilapia islets should be ideal for xenotransplantation using immunoisolation techniques.

Generation of new islets from stem cells

Spain ranks number one in organ donors (35 per million per yr). Although the prevalence of diabetes is low (100,000 type 1 diabetic patients and 2 million type 2 diabetic patients), the expected number of patients receiving islet transplants should be estimated at 200 per year. Islet replacement represents a promising cure for diabetes and has been successfully applied in a limited number of type 1 diabetic patients, resulting in insulin independence for periods longer than 3 yr. However, it has been difficult to obtain sufficient numbers of islets from cadaveric donors. Interesting alternatives include acquiring renewable sources of cells using either embryonic or adult stem cells to overcome the islet scarcity problem. Stem cells are capable of extensive proliferation rates and are capable of differentiating into other cell types of the body. In particular, totipotent stem cells are capable of differentiating into all cell types in the body, whereas pluripotent stem cells are limited to the development of a certain number of differentiated cell types. Insulin-producing cells have been obtained from both embryonic and adult stem cells using several approaches. In animal models of diabetes, the therapeutic application of bioengineered insulin-secreting cells derived from stem cells has delivered promising results. This review will summarize the different approaches that have been used to obtain insulin-producing cells from embryonic and adult stem cells and highlights the key points that will allow in vitro differentiation and subsequent transplantation in the future.

Cytological and immunocytochemical characterization of the insulin secreting insulinoma cell line RINm5F

The rat insulinoma cell line RINm5F, an insulin secreting pancreatic beta cell line, has been used as an attractive model for basic studies of the mechanisms of insulin secretion and, more recently, as a model for the development of alternative methods for the treatment of diabetes. To elucidate the cytological properties and expression patterns of hormones of the gastro-entero-pancreatic system, suspensions of RINm5F cells were investigated by various methods including immunocytochemistry on serial semithin sections, quantitative immunocytochemistry, routine electron microscopy, immuno-electron microscopy, in situ hybridization, and TUNEL technique. At the ultrastructural level, several phenotypes of RIm5F cells were characterized by differences in the number, shape, size, and density of their secretory granules. The most common type contained a mixture of round granules varying in size and electron density. A second type predominantly contained relatively large, moderately dense granules. Moreover, a minority of cells was characterized by the occurrence of polymorphous electron dense granules or the complete absence of any secretory granules. The immunohistochemical data showed that, among the established islet hormones, insulin was present in more than 50% of cells, whereas glucagon and somatostatin occurred only sporadically. Though cells positive for pancreatic polypeptide (PP) were not found, PP-related peptides (NPY and PYY) however could be detected in a minority of cells. The great majority of RINm5F cells were immunoreactive for chromogranin B (CgB), followed by insulin, chromogranin A (CgA), and serotonin (5-HT). In addition to intercellular differences in the density of immunostaining, numerous colocalizations of immunoreactivities were found, suggesting that RINm5F cells represent a mixture of subtypes concerning the individual pattern of hormone expression. The present results reveal a wide range of heterogeneity with respect to the morphology and especially the hormone content between individual RINm5F cells.

Expression in murine teratocarcinoma f9 cells of transcription factors involved in pancreas development

BACKGROUND

Although it has been established that formation and functional differentiation of the pancreas from embryonic endoderm is associated with activation/inactivation of many genes controlled by specific sets of transcription factors, the role and activation sequence of individual transcription factors has not yet been fully elucidated. This study sought to differentiate a murine teratocarcinoma cell line, F9, to endodermal-like cells and, subsequently; to investigate the effects of regulated expression of transcription factors in pancreas development.

METHODS

Following differentiation using retinoic acid and db cAMP (RAC), resulting F9 cells (F9-RAC) were transfected with cDNAs for PDX-1, ngn3, beta 2/NeuroD (beta 2), and Nkx2.2, singly or in combination. Expression of these transcription factors was investigated using RT-PCR and immunofluorescence techniques. RT-PCR analysis was used to assess the subsequent effects of expression of these factors on endogenous genes related to pancreas development.

RESULTS

Regulated differentiation of F9 cells generated endodermal-like cell types. Following transfection, PDX-1, ngn3, beta 2, and Nkx2.2 were expressed in F9-RAC cells, with their proteins localized mainly in cellular nuclei. Expression of these factors apparently did not affect the endogenous expression of preproinsulin, PDX-1, beta 2, Isl1, Pax4, Pax6, Sonic hedgehog, and Indian hedgehog.

CONCLUSION

This study describes the successful transient expression of transcription factors related to pancreas development, following directed differentiation of F9 cells to endoderm-like cells, and shows that treatment of F9 cells with a combination of RAC causes up-regulation of genes relevant to pancreatic development. The lack of further effect of regulated transcription factor expression on these genes may suggest that parietal endoderm- like cells derived from F9 cells is not the optimal lineage from which to develop beta cells. It may be useful to include F9-derived visceral endoderm in future studies.

Organogenesis of endocrine pancreas from transplanted embryonic anlagen

The number of donor human pancreas organs that can be transplanted directly or used for islet of Langerhans isolation is limited. We and others have shown that it is possible to 'grow' new pancreatic tissue in situ by transplanting embryonic organ-specific pancreatic precursor cells. This technology takes advantage of the fact that selective development of islets takes place post transplantation of embryonic pancreas and that the developing organ can attract its blood supply from an appropriate vascular bed post transplantation, enabling the transplantation of pancreas in 'cellular' form. Whole pancreatic anlagen implanted into a host peritoneum develop into a novel organ consisting of functional islets of Langerhans surrounded by stroma or individual alpha and beta cells within omental fat. Transplantation of developing pancreas to achieve organogenesis of its endocrine components could lead to a novel treatment for diabetes mellitus.

PDX-1 and the pancreas

Many transcription factors are critical for ensuring proper embryonic development of the endocrine pancreas and normal islet function. The transcription factor pancreatic duodenal homeobox 1 (PDX-1) is uniformly expressed in early pancreatic buds of embryos as well as the beta and delta cells of the islets of Langerhans. PDX-1 has also been found in dispersed endocrine cells of the duodenum in adults and plays a key role in pancreas formation. It has been reported that null mutation of PDX-1 in mice results in a failure of the pancreatic bud to expand; thus, the mice die 2-3 days after birth from hyperglycemia and dehydration. Heterozygous PDX-1 mice developed a pancreas but were diabetic. It has been shown that PDX-1 is required for maintaining the pancreatic islet functions by activating gene transcriptions including insulin, somatostatin (SST), islet amyloid polypeptide, glucose transporter type 2, and glucokinase. PDX-1 serves a dual role in pancreatic development. It initially contributes to pancreatic formation during embryogenesis and subsequently regulates the pancreatic islet cell physiology in mature islet cells. Understanding the underlying molecular mechanisms of pancreas formation, especially the function of PDX-1, may contribute to the enhanced treatment and prevention of debilitating diseases such as diabetes, insulinomas, and pancreatic carcinomas.

Novel players in pancreatic islet signaling: from membrane receptors to nuclear channels

Glucose and other nutrients regulate many aspects of pancreatic islet physiology. This includes not only insulin release, but also insulin synthesis and storage and other aspects of beta-cell biology, including cell proliferation, apoptosis, differentiation, and gene expression. This implies that in addition to the well-described signals for insulin release, other intracellular signaling mechanisms are needed. Here we describe the role of global and local Ca(2+) signals in insulin release, the regulation of these signals by new membrane receptors, and the generation of nuclear Ca(2+) signals involved in gene expression. An integrated view of these pathways should improve the present description of the beta-cell biology and provide new targets for novel drugs.

Bio-engineering inslulin-secreting cells from embryonic stem cells: a review of progress

According to the Edmonton protocol, human islet transplantation can result in insulin independency for periods longer than 3 years. However, this therapy for type 1 diabetes is limited by the scarcity of cadaveric donors. Owing to the ability of embryonic stem cells to expand in vitro and differentiate into a variety of cell types, research has focused on ways to manipulate these cells to overcome this problem. It has been demonstrated that mouse embryonic stem cells can differentiate into insulin-containing cells, restoring normoglycaemia in diabetic mice. To this end, mouse embryonic stem cells were transfected with a DNA construct that provides resistance to neomycin under the control of the regulatory regions of the human insulin gene. However, this protocol has a very low efficiency, needing improvements for this technology to be transferred to human stem cells. Optimum protocols will be instrumental in the production of an unlimited source of cells that synthesise, store and release insulin in a physiological manner. The review focuses on the alternative source of tissue offered by embryonic stem cells for regenerative medicine in diabetes and some key points that should be considered in order for a definitive protocol for in vitro differentiation to be established.

Pancreatic Islet Cell Transplantation

Pancreatic islet cell transplantation as a treatment for diabetes has hitherto been confined to small patient cohorts with limited success. This article summarizes the results of islet cell transplantation before and after the advent of the new 'Edmonton protocol' of immunosuppression and management of the donor pancreas. Adopting this regimen has achieved unprecedented success and renewed interest in this potential cure for diabetes. Central to recent improvements in the technique has been the transplantation of an adequate islet mass. Improved methods to procure, isolate, and purify islets for clinical use are now being adopted as a new 'gold standard'. The use of new immunosuppressive drugs has further improved clinical results. Corticosteroid sparing-based regimens, and agents such as humanized monoclonal antibodies, are likely to form the mainstay of immunosuppressive protocols with the aim of achieving donor-specific tolerance. Alternative sources of islet cells are also required to expand the technique in an era of reduced numbers of donor pancreata. Manipulation of stem cells and xenotransplantation may yet yield sufficient islets to overcome the problem of donor shortage. Islet cell transplantation now forms the basis of a prospective multicenter trial under the aegis of the Immune Tolerance Network. The results of this are awaited, but it appears that islet cell transplantation may yet emerge as an effective treatment option for some members of the diabetic population.

NMR spectroscopy in beta cell engineering and islet transplantation

Islet transplantation is a promising method for restoring normoglycemia and alleviating the long term complications of diabetes. Widespread application of islet transplantation is hindered by the limited supply of human islets and requires a large increase in the availability of suitable insulin secreting tissue as well as robust quality assessment methodologies that can ensure safety and in vivo efficacy. We explore the application of nuclear magnetic resonance (NMR) spectroscopy in two areas relevant to beta cell engineering and islet transplantation: (1) the effect of genetic alterations on glucose metabolism, and (2) quality assessment of islet preparations prior to transplantation. Results obtained utilizing a variety of NMR techniques demonstrate the following: (1) Transfection of Rat1 cells with the c-myc oncogene (which may be involved in cell proliferation and cell cycle regulation) and overexpression of Bcl-2 (which may protect cells from stresses such as hypoxia and exposure to cytokines) introduce a wide array of alterations in cellular biochemistry, including changes in anaerobic and oxidative glucose metabolism, as assessed by 13C and 31P NMR spectroscopy. (2) Overnight incubation of islets and beta cells in the bottom of centrifuge tubes filled with medium at room temperature, as is sometimes done in islet transportation, exposes them to severe oxygen limitations that may cause cell damage. Such exposure, leading to reversible or irreversible damage, can be observed with NMR-detectable markers using conventional 13C and 31P NMR spectroscopy of extracts. In addition, markers of irreversible damage (as well as markers of hypoxia) can be detected and quantified without cell extraction using high-resolution magic angle spinning 1H NMR spectroscopy. Finally, acute ischemia in a bed of perfused beta cells leads to completely reversible changes that can be followed in real time with 31P NMR spectroscopy.

In-vitro differentiation of pancreatic beta-cells

Stem cell biology is a new field that holds promise for in-vitro mass production of pancreatic beta-cells, which are responsible for insulin synthesis, storage, and release. Lack or defect of insulin produces diabetes mellitus, a devastating disease suffered by 150 million people in the world. Transplantation of insulin-producing cells could be a cure for type 1 and some cases of type 2 diabetes, however this procedure is limited by the scarcity of material. Obtaining pancreatic beta-cells from embryonic stem cells would overcome this problem. We have derived insulin-producing cells from mouse embryonic stem cells by a 3-step in-vitro differentiation method consisting of directed differentiation, cell-lineage selection, and maturation. These insulin-producing cells normalize blood glucose when transplanted into streptozotocin-diabetic mice. Strategies to increase islet precursor cells from embryonic stem cells include the expression of relevant transcription factors (Pdx1, Ngn3, Isl-1, etc), together with the use of extracellular factors. Once a high enough proportion of islet precursors has been obtained there is a need for cell-lineage selection in order to purify the desired cell population. For this purpose, we designed a cell-trapping method based on a chimeric gene that fuses the human insulin gene regulatory region with the structural gene that confers resistance to neomycin. When incorporated into embryonic stem cells, this fusion gene will generate neomycin resistance in those cells that initiate the synthesis of insulin. Not only embryonic, but also adult stem cells are potential sources for insulin-containing cells. Duct cells from the adult pancreas are committed to differentiate into the four islet cell types; other possibilities may include nestin-positive cells from islets and adult pluripotent stem cells from other origins. Whilst the former are committed to be islet cells but have a reduced capacity to expand, the latter are more pluripotent and more expandable, but a longer pathway separates them from the insulin-producing stage. The aim of this review is to discuss the different strategies that may be followed to in-vitro differentiate pancreatic beta-cells from stem cells.

Insulin production by human embryonic stem cells

Type 1 diabetes generally results from autoimmune destruction of pancreatic islet beta-cells, with consequent absolute insulin deficiency and complete dependence on exogenous insulin treatment. The relative paucity of donations for pancreas or islet allograft transplantation has prompted the search for alternative sources for beta-cell replacement therapy. In the current study, we used pluripotent undifferentiated human embryonic stem (hES) cells as a model system for lineage-specific differentiation. Using hES cells in both adherent and suspension culture conditions, we observed spontaneous in vitro differentiation that included the generation of cells with characteristics of insulin-producing beta-cells. Immunohistochemical staining for insulin was observed in a surprisingly high percentage of cells. Secretion of insulin into the medium was observed in a differentiation-dependent manner and was associated with the appearance of other beta-cell markers. These findings validate the hES cell model system as a potential basis for enrichment of human beta-cells or their precursors, as a possible future source for cell replacement therapy in diabetes.

Stem cells and diabetes

Diabetes mellitus is a metabolic disorder affecting 2-5% of the population. Transplantation of isolated islets of Langerhans from donor pancreata could be a cure for diabetes; however, such an approach is limited by the scarcity of the transplantation material and the long-term side effects of immunosuppressive therapy. These problems may be overcome by using a renewable source of cells, such as islet cells derived from stem cells. Stem cells are defined as clonogenic cells capable of both self-renewal and multilineage differentiation. This mean that these cells can be expanded in vivo or in vitro and differentiated to produce the desired cell type. There exist several sources of stem cells that have been demonstrated to give rise to pluripotent cell lines: 1) embryonic stem cells; 2) embryonic germ cells; 3) embryonic carcinoma cells; and 4) adult stem cells. By using in vitro differentiation and selection protocols, embryonic stem cells can be guided into specific cell lineages and selected by applying genetic selection when a marker gene is expressed. Recently, differentiation and cell selection protocols have been used to generate embryonic stem cell-derived insulin-secreting cells that normalise blood glucose when transplanted into diabetic animals. Some recent reports suggest that functional plasticity of adult stem cells may be greater than expected. The use of adult stem cells will circumvent the ethical dilemma surrounding embryonic stem cells and will allow autotransplantation. These investigations have increased the expectations that cell therapy could be one of the solutions to diabetes.

Bioartificial pancreas: materials, devices, function, and limitations

The term "bioartificial endocrine pancreas" (BEP) was introduced by Anthony Sun in 1980. It was in 1968, however, that Thomas Chang proposed the use of microencapsulated islets as artificial beta-cells. By applying a semipermeable membrane on the top of microcapsules, a system can be produced that is impermeable to viable islet cells and large effector molecules of the immune system, thus providing a protection for transplanted islets against rejection. Since then, the term BEP has not often appeared in papers. Instead, the term "bioartificial pancreas" (BAP) has gained widespread use. In a broader sense, BAP would include an application of suitable endocrine cells and protective polymeric vehicles, but not necessarily providing a filtration barrier of precisely defined properties (e.g., cells injected into a gel of hyaluronate).

Nutrient toxicity in pancreatic beta-cell dysfunction

Nutrients, such as glucose and fatty acids, have a dual effect on pancreatic beta-cell function. Acute administration of high glucose concentrations to pancreatic beta-cells stimulates insulin secretion. In addition, short term exposure of this cell type to dietary fatty acids potentiates glucose-induced insulin release. On the other hand, long-term exposure to these nutrients causes impaired insulin secretion, characterized by elevated exocytosis at low concentrations of glucose and no response when glucose increases in the extracellular medium. In addition, other phenotypic changes are observed in these conditions. One major step in linking these phenotypic changes to the diabetic pathology has been the recognition of both glucose and fatty acids as key modulators of beta-cell gene expression. This could explain the adaptative response of the cell to sustained nutrient concentration. Once this phase is exhausted, the beta-cell becomes progressively unresponsive to glucose and this alteration is accompanied by the irreversible induction of apoptotic programs. The aim of this review is to present actual data concerning the development of the toxicity to the main nutrients glucose and fatty acids in the pancreatic beta-cell and to find a possible link to the development of type 2 diabetes.