Why pancreatic islets should be regarded and regulated like organs

There are strong reasons to say that pancreatic islets are organs before they are isolated and that they should be considered to be organs once transplanted. Thus, taking into account how much we have learned about the structure and function of islet micro-organs, it seems highly illogical to on one hand consider autologous islets be regulated as organ transplants and alloislets to be regulated with the very restrictive rules used for cell transplantation. It is particularly problematic that this policy has led to restrictions that have made it next to impossible for transplants of alloislets to be carried out in the US, which is a very sad situation for the country that made so many of the advances that brought islet transplantation to the clinic.

Number and Volume of Islets Transplanted in Immunobarrier Devices

Immunobarrier devices may prevent immune destruction of transplanted islets, but there are concerns about survival within such devices. Islets were transplanted in diffusion chambers that employed two laminated polytetrafluoroethylene membranes held together with titanium rings. Five hundred syngeneic mouse islets placed in devices were transplanted into the epididymal fat pads of streptozotocin (STZ) diabetic mice (B6AF1). After 2 wk the devices were removed. Sections were made parallel to the membrane surface. Eight to 13 systematically selected sections of each device were analyzed by planimetry to determine the area of the device space and of the islets within that space. From these data we estimated total volume of the device, volume of islets, and number of islets in a device. The data were segregated into two groups: group I (blood glucose less than 100 mg/dL 2 wk after implantation), and group II (over 150 mg/dL). The volume (mean + SE) of devices implanted for 2 wk was 2.1 + 0.4 μL in group I and 2.2 + 0.2 μL in group II. The islet volume and number within devices were 0.30 + 0.06 and 0.17 + 0.01 μL, or 340 + 50 and 230 + 20 islets in group I and group II, respectively. The volume of fibrous tissue in devices was about 0.50 μL. About 10% of the islet tissue had central necrosis. The beta cell volume in a membrane device needed for cure is comparable to that required with islets under the kidney capsule (0.25-0.80 μL). The mass of islets contained within membrane devices needed to cure diabetes is equivalent to that of a graft in an optimal transplant site such as under the kidney capsule.

A Method for Estimating Number and Mass of Islets Transplanted within a Membrane Device

Immunobarrier devices may prevent the immune destruction of pancreatic islets transplanted into diabetic recipients, but there are concerns about the survival of islets within such devices. In this manuscript we described a method for estimating islet mass and number within a membrane device. Five hundred syngeneic mouse islets were placed in a membrane device, which was then transplanted into the epididymal space of streptozotocin diabetic mouse. After 14 days the device was removed from the recipient, fixed, and embedded in paraffin. Sections were made and then stained with hematoxylin. From a total of 58 sections, 12 sample sections were selected for analysis by computerized planimetry to determine area of the device chamber and the islet area for each of these sections. By making certain assumptions, it was possible to estimate the total volume of the device chamber, the number of islets contained within the chamber, and the mass of islet tissue. Two weeks after implantation of the chamber, into which approximately 500 microencapsulated islets were loaded, the blood glucose level of the recipient fell to 73 mg/dL. The volume of the device chamber, the total volume of islets in the device, and the total islet number within the device were 1.78 μL, 0.57 μL, and 277 islets, respectively. The detailed methodology, assumptions, and calculations for this approach are described in this manuscript. This new method makes it possible to determine islet mass within a membrane device by analyzing a relatively small number of selected sections. This approach should make it possible to carry out comprehensive studies on the fate of transplanted islets contained in such immunobarrier membrane devices.

Reanalysis of study of pancreatic effects of incretin therapy: methodological deficiencies

A recently published study by Butler et al. concluded that incretin treatment had adverse effects on the human type 2 diabetic pancreas including 'a marked expansion of the exocrine and endocrine pancreatic compartments, the former being accompanied by increased proliferation and dysplasia and the latter by α-cell hyperplasia with the potential for evolution into neuroendocrine tumours'. Incretin therapy has become widely used for type 2 diabetes, so these conclusions have instigated major concerns with regard to patient safety. We reassessed both the clinical case information and virtual microscopy images of the same 34 cases that were used in the Butler study as well as Network for Pancreatic Organ Donation (nPOD) cases that were not included. Whereas we would like to stress that it is important to investigate in depth any indication that incretin treatment may lead to inflammation or dysplasia in the pancreas, we find that the data presented in the Butler paper have serious methodological deficiencies that preclude any meaningful conclusions.

Rat neonatal beta cells lack the specialised metabolic phenotype of mature beta cells

AIMS/HYPOTHESIS

Fetal and neonatal beta cells have poor glucose-induced insulin secretion and only gain robust glucose responsiveness several weeks after birth. We hypothesise that this unresponsiveness is due to a generalised immaturity of the metabolic pathways normally found in beta cells rather than to a specific defect.

METHODS

Using laser-capture microdissection we excised beta cell-enriched cores of pancreatic islets from day 1 (P1) neonatal and young adult Sprague-Dawley rats in order to compare their gene-expression profiles using Affymetrix U34A microarrays (neonatal, n = 4; adult, n = 3).

RESULTS

Using dChip software for analysis, 217 probe sets for genes/38 expressed sequence tags (ESTs) were significantly higher and 345 probe sets for genes/33 ESTs significantly lower in beta cell-enriched cores of neonatal islets compared with those of adult islets. Among the genes lower in the neonatal beta cells were key metabolic genes including mitochondrial shuttles (malate dehydrogenase, glycerol-3-phosphate dehydrogenase and glutamate oxalacetate transaminase), pyruvate carboxylase and carnitine palmitoyl transferase 2. Differential expression of these enzyme genes was confirmed by quantitative PCR on RNA from isolated neonatal (P2 until P28) and adult islets and with immunostaining of pancreas. Even by 28 days of age some of these genes were still expressed at lower levels than in adults.

CONCLUSIONS/INTERPRETATION

The lack of glucose responsiveness in neonatal islets is likely to be due to a generalised immaturity of the metabolic specialisation of pancreatic beta cells.

Mafa expression enhances glucose-responsive insulin secretion in neonatal rat beta cells

AIM/HYPOTHESIS

Neonatal beta cells lack glucose-stimulated insulin secretion and are thus functionally immature. We hypothesised that this lack of glucose responsiveness results from a generalised low expression of genes characteristic of mature functional beta cells. Important glucose-responsive transcription factors, Mafa and Pdx1, regulate genes involved in insulin synthesis and secretion, and have been implicated in late beta cell development. The aim of this study was to assess whether Mafa and/or Pdx1 regulates the postnatal functional maturation of beta cells.

METHODS

By quantitative PCR we evaluated expression of these and other beta cell genes over the first month compared with adult. After infection with adenovirus expressing MAFA, Pdx1 or green fluorescent protein (Gfp), P2 rat islets were evaluated by RT-PCR and insulin secretion with static incubation and reverse haemolytic plaque assay (RHPA).

RESULTS

At P2 most beta cell genes were expressed at about 10% of adult, but by P7 Pdx1 and Neurod1 no longer differ from adult; by contrast, Mafa expression remained significantly lower than adult through P21. Overexpression of Pdx1 increased Mafa, Neurod1, glucokinase (Gck) mRNA and insulin content but failed to enhance glucose responsiveness. Similar overexpression of MAFA resulted in increased Neurod1, Nkx6-1, Gck and Glp1r mRNAs and no change in insulin content but, importantly, acquisition of glucose-responsive insulin secretion. Both the percentage of secreting beta cells and the amount of insulin secreted per beta cell increased, approaching that of adult beta cells.

CONCLUSIONS/INTERPRETATION

In the process of functional maturation acquiring glucose-responsive insulin secretion, neonatal beta cells undergo a coordinated gene expression programme in which Mafa plays a crucial role.

Rat islet cell aggregates are superior to islets for transplantation in microcapsules

AIMS/HYPOTHESIS

Islet transplantation is a promising treatment for type 1 diabetes but is hampered by a shortage of donor human tissue and early failure. Research on islet cell transplantation includes finding new sources of cells and immunoisolation to protect from immune assault and tumourigenic potential. Small islet cell aggregates were studied to determine if their survival and function were superior to intact islets within microcapsules because of reduced oxygen transport limitation and inflammatory mediators.

METHODS

Islet cell aggregates were generated by dispersing rat islets into single cells and allowing them to re-aggregate in culture. Rat islets and islet cell aggregates were encapsulated in barium alginate capsules and studied when cultured in low (0.5% or 2%) or normal (20%) oxygen, or transplanted into mice.

RESULTS

Encapsulated islet cell aggregates were able to survive and function better than intact islets in terms of oxygen-consumption rate, nuclei counts, insulin-to-DNA ratio and glucose-stimulated insulin secretion. They also had reduced expression of pro-inflammatory genes. Islet cell aggregates showed reduced tissue necrosis in an immunodeficient transplant model and a much greater proportion of diabetic xenogeneic transplant recipients receiving islet cell aggregates (tissue volume of only 85 islet equivalents) had reversal of hyperglycaemia than recipients receiving intact islets.

CONCLUSIONS/INTERPRETATION

These aggregates were superior to intact islets in terms of survival and function in low-oxygen culture and during transplantation and are likely to provide more efficient utilisation of islet tissue, a finding of importance for the future of cell therapy for diabetes.

Dimorphic histopathology of long-standing childhood-onset diabetes

AIMS/HYPOTHESIS

Childhood diabetes is thought to usually result from autoimmune beta cell destruction (type 1A) with eventual total loss of beta cells. Analysis of C-peptide in children characterised at diabetes onset for autoantibodies shows heterogeneous preservation of insulin secretion in long-standing diabetes. The aim of this study was to characterise the pancreases of childhood-onset diabetes in order to define the pathological basis of this heterogeneity.

METHODS

We evaluated 20 cadaveric organ donor pancreases of childhood-onset long-term patients for disease heterogeneity and obtained corresponding C-peptide measurements.

RESULTS

Pancreases from the majority of cadaveric donors contained only insulin-deficient islets (14 of 20). The remaining six patients (30%) had numerous insulin-positive cells within at least some islets, with two different histological patterns. Pattern A (which we would associate with type 1A diabetes) had lobular retention of areas with 'abnormal' beta cells producing the apoptosis inhibitor survivin and HLA class I. In pattern B, 100% of all islets contained normal-appearing but quantitatively reduced beta cells without survivin or HLA class I.

CONCLUSIONS/INTERPRETATION

Our data demonstrate that C-peptide secretion in long-standing diabetic patients can be explained by two different patterns of beta cell survival,possibly reflecting different subsets of type 1 diabetes.

Single pancreatic beta cells co-express multiple islet hormone genes in mice

AIMS/HYPOTHESIS

It is widely accepted that production of insulin, glucagon, somatostatin and pancreatic polypeptide in islet cells is specific to beta, alpha, delta and pancreatic polypeptide cells, respectively. We examined whether beta cells express other genes encoding islet hormones.

METHODS

Nested RT-PCR was performed on single beta cells of transgenic mice with green fluorescent protein (GFP) driven by mouse insulin I promoter (MIP-GFP).

RESULTS

Only 55% of adult beta cells expressed the insulin gene alone, while others expressed two or more islet hormone genes; 4% expressed all four hormone genes. In embryonic and neonatal cells, 60% to 80% of GFP(+) cells co-expressed pancreatic polypeptide and insulin genes in contrast to 29% in adult. To clarify cell fate, we conducted lineage tracing using rat insulin II promoter-cre mice crossed with reporter mice Gt(ROSA)26Sor-loxP-flanked STOP-cassette-GFP. All GFP(+) cells expressed insulin I and II genes, and showed similar heterogeneity of co-expression to that seen in MIP-GFP mice. Although we report expression of other hormone genes in a significant proportion of beta cells, our lineage tracing results demonstrate that after inducing InsII (also known as Ins2) expression, beta cell progenitors do not redifferentiate to non-beta cells.

CONCLUSIONS/INTERPRETATION

This study shows co-expression of multiple hormone genes in beta cells of adult mice as well as in embryos and neonates. This finding could: (1) represent residual expression from beta cell precursors; (2) result from alternative developmental pathways for beta cells; or (3) denote the differentiation potential of these cells. It may be linked to functional heterogeneity. This heterogeneity in gene expression may provide a means to characterise the functional, cellular and developmental heterogeneity seen in beta cells.

OVO homologue-like 1 (Ovol1) transcription factor: a novel target of neurogenin-3 in rodent pancreas

AIMS/HYPOTHESIS

The basic helix-loop-helix transcription factor neurogenin-3 (NGN3) commits the fates of pancreatic progenitors to endocrine cell types, but knowledge of the mechanisms regulating the choice between proliferation and differentiation of these progenitors is limited.

METHODS

Using a chromatin immunoprecipitation cloning approach, we searched for direct targets of NGN3 and identified a zinc-finger transcription factor, OVO homologue-like 1 (OVOL1). Transactivation experiments were carried out to elucidate the functional role of NGN3 in Ovol1 gene expression. Embryonic and adult rodents pancreases were immunostained for OVOL1, Ki67 and NGN3.

RESULTS

We showed that NGN3 negatively regulates transcription of Ovol1 in an E-box-dependent fashion. The presence of either NGN3 or NEUROD1, but not MYOD, reduced endogenous Ovol1 mRNA. OVOL1 was detected in pancreatic tissue around embryonic day 15.5, after which OVOL1 levels dramatically increased. In embryonic pancreas, OVOL1 protein levels were low in NGN3(+) or Ki67(+) cells, but high in quiescent differentiated cells. OVOL1 presence was maintained in adult pancreas, where it was detected in islets, pancreatic ducts and some acinar cells. Additionally OVOL1 presence was lacking in proliferating ductules in regenerating pancreas and induced in cells as they began to acquire their differentiated phenotype.

CONCLUSIONS/INTERPRETATION

The timing of OVOL1 appearance in pancreas and its increased levels in differentiated cells suggest that OVOL1 promotes the transition of cells from a proliferating, less-differentiated state to a quiescent more-differentiated state. We conclude that OVOL1, a downstream target of NGN3, may play an important role in regulating the balance between proliferation and differentiation of pancreatic cells.

Towards better understanding of the contributions of overwork and glucotoxicity to the beta-cell inadequacy of type 2 diabetes

Type 2 diabetes (T2D) is characterized by reduction of beta-cell mass and dysfunctional insulin secretion. Understanding beta-cell phenotype changes as T2D progresses should help explain these abnormalities. The normal phenotype should differ from the state of overwork when beta-cells compensate for insulin resistance to keep glucose levels normal. When only mild hyperglycaemia develops, beta-cells are subjected to glucotoxicity. As hyperglycaemia becomes more severe, so does glucotoxicity. beta-Cells in all four of these situations should have separate phenotypes. When assessing phenotype with gene expression, isolated islets have artefacts resulting from the trauma of isolation and hypoxia of islet cores. An advantage comes from laser capture microdissection (LCM), which obtains beta-cell-rich tissue from pancreatic frozen sections. Valuable data can be obtained from animal models, but the real goal is human beta-cells. Our experience with LCM and gene arrays on frozen pancreatic sections from cadaver donors with T2D and controls is described. Although valuable data was obtained, we predict that the approach of taking fresh samples at the time of surgery is an even greater opportunity to markedly advance our understanding of how beta-cell phenotype evolves as T2D develops and progresses.

Differentiation of COPAS-sorted non-endocrine pancreatic cells into insulin-positive cells in the mouse

AIMS/HYPOTHESIS

The regenerative process in the pancreas is of particular interest, since insulin-producing beta cells are lost in diabetes. Differentiation of new beta cells from pancreatic non-endocrine cells has been reported in vivo and in vitro, a finding that implies the existence of pancreatic stem/progenitor cells. However, while tissue-specific stem cells are well documented in skin, intestine and testis, pancreatic stem cells have been elusive. We hypothesised that pancreatic stem/progenitor cells within the non-endocrine fraction could be a source of new islets in vitro.

METHODS

To test if there were such cells within the pancreas, we generated pancreatic cell aggregates from tissue remaining after islet isolation from mouse insulin promoter 1-green fluorescent protein (MIP-GFP) mice. To eliminate any contamination of insulin-positive cells, we deleted all GFP-positive aggregates using COPAS Select and cultured with Matrigel. Immunohistochemistry, quantitative real-time PCR and single-cell nested RT-PCR were performed to confirm formation of insulin-producing cells.

RESULTS

The GFP-negative cells were expanded as monolayers and then differentiated into three-dimensional cystic structures. After 1 week of culture, GFP-positive cells were found as clusters or single cells. By quantitative real-time PCR, no insulin mRNA was detected immediately after COPAS sorting, but after differentiation insulin mRNA of the whole preparation was 1.91 +/- 0.31% that of purified MIP-GFP beta cells. All GFP-positive cells expressed insulin 1; most expressed insulin 2, pancreas duodenum homeobox-1 and cytokeratin 19 by single cell nested RT-PCR.

CONCLUSIONS/INTERPRETATION

Our data support the concept that within the exocrine (acinar and ductal) pancreas of the adult mouse there are cells that can give rise to insulin-positive cells in vitro.

Influence of diabetes on the loss of beta cell differentiation after islet transplantation in rats

AIMS/HYPOTHESIS

Hyperglycaemia can impair beta cell function after islet transplantation. Appropriate glucose-induced insulin secretion is dependent on a unique expression pattern of genes. Here we examined the effects of diabetes on gene expression in transplanted islets.

MATERIALS AND METHODS

Streptozotocin-induced diabetic or control non-diabetic Lewis rats were transplanted under the kidney capsule with an insufficient number (2,000) of syngeneic islets to normalise blood glucose levels in diabetic rats. Eighteen days after transplantation, islet grafts were retrieved and RT-PCR used to assess expression of selected genes critical for beta cell function. Islet grafts from diabetic rats transplanted with a sufficient number of islets (3,000) to normalise hyperglycaemia were used to assess the effects of correcting blood glucose levels. Additionally, gene expression of transplanted islets from non-diabetic rats was compared with freshly isolated islets.

RESULTS

In islet grafts from diabetic rats, mRNA levels of several transcription factors important for the maintenance of beta cell differentiation were reduced (pancreatic and duodenal homeobox 1 [Pdx1], neurogenic differentiation 1 [Neurod1], NK6 transcription factor related, locus 1 [Nkx6.1], paired box gene 6 [Pax6]), as were genes implicated in beta cell function (Glut2 [also known as solute carrier family 2 [facilitated glucose transporter], member 2 [Slc2a2], glucokinase, insulin, islet amyloid polypeptide [Iapp]). Conversely, mRNA levels of lactate dehydrogenase, which is normally suppressed in beta cells, were increased. The majority of the changes in gene expression were normalised after correction of hyperglycaemia, indicating that the severe loss of beta cell differentiation correlates with continuous exposure to diabetes. Even islet grafts from non-diabetic rats showed a few alterations in beta cell gene expression in comparison with fresh islets.

CONCLUSIONS/INTERPRETATION

Chronic hyperglycaemia contributes to the deterioration of beta cell differentiation after islet transplantation.

Changes in gene expression in beta cells after islet isolation and transplantation using laser-capture microdissection

AIMS/HYPOTHESIS

The process of islet isolation can cause chemical and mechanical injury to beta cells. In addition, hyperglycaemia after islet transplantation can compromise beta cell function. The aim of this experiment was to evaluate changes in gene expression in endogenous islets using laser-capture microdissection (LCM).

MATERIALS AND METHODS

Islets from B6AF1 mice were studied in situ in the pancreas as well as those freshly isolated or cultured for 24 h. Fresh islets were transplanted under the kidney capsule of syngeneic diabetic (streptozocin-induced) and non-diabetic mice. Frozen sections from all the samples were prepared for LCM to obtain beta cell-enriched tissue; RNA was extracted and amplified using T7 polymerase. RT-PCR was used to assess expression of selected genes critical for beta cell function (Ins, Ipf1 [previously known as Pdx1], Slc2a2 [previously known as GLUT2] and Ldha) and the stress response (Hmox1 [previously known as HO-1], Gpx1, Tnfaip3 [previously known as A20] and Fas). Immunostaining was also performed.

RESULTS

In freshly isolated and cultured islets, insulin and Ipf1 mRNA levels were decreased by 40% (compared with islets in situ), while stress genes were upregulated. Comparison between in situ pancreatic islets and engrafted beta cells of cured mice showed declines in Ipf1 expression.

CONCLUSIONS/INTERPRETATION

Our experiment, the first report to investigate changes in gene expression in endogenous islets using LCM, indicate that beta cells following islet isolation and residing in a foreign graft environment have decreased expression of genes involved in insulin production and increased expression of stress genes. Our data suggest that an islet graft, even in successful transplantation, may be different from endogenous islets in gene expression.

Severe hypoglycaemia post-gastric bypass requiring partial pancreatectomy: evidence for inappropriate insulin secretion and pancreatic islet hyperplasia

AIMS/HYPOTHESIS

Postprandial hypoglycaemia following gastric bypass for obesity is considered a late manifestation of the dumping syndrome and can usually be managed with dietary modification. We investigated three patients with severe postprandial hypoglycaemia and hyperinsulinaemia unresponsive to diet, octreotide and diazoxide with the aim of elucidating the pathological mechanisms involved.

METHODS

Glucose, insulin, and C-peptide were measured in the fasting and postprandial state, and insulin secretion was assessed following selective intra-arterial calcium injection. Pancreas histopathology was assessed in all three patients.

RESULTS

All three patients had evidence of severe postprandial hyperinsulinaemia and hypoglycaemia. In one patient, reversal of gastric bypass was ineffective in reversing hypoglycaemia. All three patients ultimately required partial pancreatectomy for control of neuroglycopenia; pancreas pathology of all patients revealed diffuse islet hyperplasia and expansion of beta cell mass.

CONCLUSIONS/INTERPRETATION

These findings suggest that gastric bypass-induced weight loss may unmask an underlying beta cell defect or contribute to pathological islet hyperplasia, perhaps via glucagon-like peptide 1-mediated pathways.

Islet transplantation outcomes in mice are better with fresh islets and exendin-4 treatment

AIMS/HYPOTHESIS

Although islet transplantation in diabetes holds great promise, two or three donor pancreases are usually required to achieve normoglycaemia in human or rodent recipients. We investigated whether there were differences between fresh and cultured islets in terms of transplantation outcome. We also investigated the effects of normoglycaemia during engraftment and the effects of exendin-4, a glucagon-like peptide-1 receptor agonist, on islet transplantation.

MATERIALS AND METHODS

Seventy-five fresh islets were transplanted to the right kidney of diabetic mice and 425 fresh islets were transplanted to the left kidney. The mice were treated with exendin-4 or vehicle for 14 days, after which the large graft was removed by left nephrectomy. In a separate set of experiments, islets cultured in the presence or absence of exendin-4 for 72 h, or fresh islets, were transplanted to diabetic mice. In both sets of experiments, blood glucose levels were monitored.

RESULTS

Compared with cultured islets, fresh islets were more effective at reversing hyperglycaemia in mice. The treatment of the recipient mice with exendin-4 did not have beneficial effects on glucose homeostasis. However, when islets are cultured, exendin-4 treatment increases the rate of reversal of hyperglycaemia, but not to the degree of fresh islets.

CONCLUSIONS/INTERPRETATION

Fresh islets are more effective than cultured islets at reversing hyperglycaemia. Exendin-4 has beneficial effects on islet transplantation.

Expression of the intermediate filament vimentin in proliferating duct cells as a marker of pancreatic precursor cells

OBJECTIVES

The expression of the intermediate filament (IF) vimentin, usually considered a marker of mesenchymal cells, has been observed in the epithelial cells during embryogenesis, carcinogenesis, and dedifferentiation, suggesting that it might be useful as a marker of proliferating precursor cells in the pancreas.

METHODS

Rat pancreata at E18 and at different time points after partial pancreatectomy (Px) and human and neonatal pig pancreatic tissue sections and monolayer cultured pancreatic duct cells were observed. All tissues were simultaneously immunostained with pancytokeratin and vimentin antibodies. In costained duct cells, PDX-1 or PCNA expression was also analyzed using confocal microscope images.

RESULTS

In the rat embryonic pancreas at E18, all epithelial cells that formed ductlike structures expressed both cytokeratin and vimentin IF, whereas no duct cells costained for IF in the adult rat or neonatal pig pancreas. Such costaining reappeared in the following order: common pancreatic duct, main ducts, foci of regeneration and then disappeared completely at 30 days after Px. In humans, costaining was found in only 1 diabetic patient's pancreatic section, which was accompanied by massive duct cell proliferation. In monolayer culture, most of the duct cells of human and neonatal pigs coexpressed both IF proteins. Only a few costained duct cells also expressed PDX-1, and most of those cells were also stained with PCNA in rat embryonic pancreas and regenerating foci after partial Px.

CONCLUSIONS

Vimentin IF expression might be a useful marker for pancreatic precursor cells and could be used to investigate the concept of the dedifferentiation of fully matured duct cells during the process of the beta-cell neogenesis.

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.

High-fructose diet preserves beta-cell mass and prevents diabetes in nonobese diabetic mice: A potential role for increased insulin receptor substrate-2 expression

We demonstrate that a high-fructose diet reduces the incidence of diabetes in nonobese diabetic (NOD) mice (31.2% v 57.1% on regular chow (RC); P =.009). In a second cohort of mice, we evaluated potential mechanisms for the protective effect of the high-fructose (HF) diet and whether the metabolic changes are strain-specific. Sixty NOD and 60 Balb/c mice were randomized at weaning into HF- and RC-fed groups (30 mice each) and followed for 28 weeks. Glucose tolerance testing demonstrated improved glucose tolerance in HF diet groups (P =.001 in Balb/c; P =.04 in NOD mice at 6 months). beta-cell mass was preserved in NOD mice on the HF diet, but remained unchanged in Balb/c mice. In NOD mice, hepatic insulin receptor substrate (IRS)-2 protein expression increased by 2-fold (P =.01 for 2 v 6 months) in HF-fed mice and was 53% +/- 15% higher (P =.01) in the HF diet versus RC groups at 6 months of age. IRS-2 expression was also increased in skeletal muscle of NOD mice and in both liver and muscle of Balb/c mice. Our data suggest that a HF diet improves glucose tolerance in both NOD and Balb/c mice. The improved glucose tolerance may be related to increased IRS-2 expression and, in NOD mice, preservation of beta-cell mass.

Noninvasive in vivo measurement of beta-cell mass in mouse model of diabetes

Pancreatic beta-cell mass (BCM) is a major determinant of the quantity of insulin that can be secreted. BCM is markedly reduced in type 1 diabetes because of selective autoimmune destruction of beta-cells. Accurate assessment of BCM in human diabetes is limited to autopsy studies, which usually suffer from inadequate clinical information; thus, the development of noninvasive means of BCM measurement could be important in intervention therapy. The goal of this study was to develop such noninvasive methods for measuring BCM featuring target-specific imaging probes and to investigate whether this technique is feasible, accurate, and predictive of BCM in normal and diabetic states. Using a beta-cell-specific monoclonal antibody IC2, modified with a radioisotope chelator for nuclear imaging, we showed that highly specific binding and accumulation to beta-cells occurs after intravenous administration of the probe, with virtually no binding to exocrine pancreas or stromal tissues. Furthermore, we observed a direct correlation between accumulation of the probe with BCM in diabetic and normal animals. Nuclear imaging of the animals that received an injection of the radioactive probe showed major difference in signal intensity between normal and diabetic pancreases. The results from this study set the route for further development of imaging probes for measuring BCM that would aid in diagnosis and treatment of diabetic patients in the clinic.

Activation of the hexosamine pathway leads to deterioration of pancreatic beta-cell function through the induction of oxidative stress

It is known well that activation of the hexosamine pathway causes insulin resistance, but how this activation influences pancreatic beta-cell function remains unclear. In this study, we found that in isolated rat islets adenovirus-mediated overexpression of glutamine:fructose-6-phosphate amidotransferase (GFAT), the first and rate-limiting enzyme of the hexosamine pathway, leads to deterioration of beta-cell function, which is similar to that found in diabetes. Overexpression of GFAT or treatment with glucosamine results in impaired glucose-stimulated insulin secretion and reduction in the expression levels of several beta-cell specific genes (insulin, GLUT2, and glucokinase). Additionally, the DNA binding activity of PDX-1, an important transcription factor for these three genes, was markedly reduced. These phenomena were not mimicked by the induction of O-linked glycosylation with an inhibitor of O-GlcNAcase, PUGNAc. It was also found that glucosamine increases hydrogen peroxide levels and that several hexosamine pathway-mediated changes were suppressed by treatment with the antioxidant N-acetyl-l-cysteine. In conclusion, activation of the hexosamine pathway leads to deterioration of beta-cell function through the induction of oxidative stress rather than O-linked glycosylation. Thus, the hexosamine pathway may contribute to the deterioration of beta-cell function found in diabetes.

Porcine neonatal pancreatic cell clusters in tissue culture: benefits of serum and immobilization in alginate hydrogel

Porcine neonatal pancreatic cell clusters (NPCCs) may be a suitable source of insulin producing tissue for transplantation in diabetic patients. The possible beneficial effect of serum on maturation of NPCCs in vitro is difficult to achieve because of cell clumping, which can be avoided by immobilization in alginate hydrogel matrix. Collagenase treated pancreata, cultured for 4 days, formed NPCCs that were embedded in alginate cross-linked with CaCl2 and cultured in modified Ham's F10 medium with 10% fetal calf serum (FCS) for 10 days. NPCCs cultured as suspension in F10+ with 0.5% bovine serum albumin or with 10% FCS were used as control. To prevent the aggregation when cultured with serum, NPCCs were kept as a very diluted suspension. At the beginning and end of the culture, samples were taken for insulin and DNA content and immunostained for beta and non-beta cells. The culture of NPCCs immobilized in alginate resulted with 3-fold increase in insulin content and 9-fold increase in insulin/DNA ratio. Histology revealed evident increase of number of insulin- and other hormone-positive cells compared with the control. Even though 2 weeks in culture resulted in impaired glucose-induced insulin release, the amount of insulin secreted by clusters cultured in the presence of serum was 4-fold higher than in serum-free conditions. After transplantation, NPCCs retrieved from alginate reversed hyperglycemia similarly to NPCCs cultured in standard conditions. In conclusion, this study shows the feasibility of in vitro immobilization of NPCCs in alginate three-dimensional matrix, allowing cell clusters to be cultured at least two times higher density compared with culture in suspension.

Translational control is required for the unfolded protein response and in vivo glucose homeostasis

The accumulation of unfolded protein in the endoplasmic reticulum (ER) attenuates protein synthesis initiation through phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF2alpha) at Ser51. Subsequently, transcription of genes encoding adaptive functions including the glucose-regulated proteins is induced. We show that eIF2alpha phosphorylation is required for translation attenuation, transcriptional induction, and survival in response to ER stress. Mice with a homozygous mutation at the eIF2alpha phosphorylation site (Ser51Ala) died within 18 hr after birth due to hypoglycemia associated with defective gluconeogenesis. In addition, homozygous mutant embryos and neonates displayed a deficiency in pancreatic beta cells. The results demonstrate that regulation of translation through eIF2alpha phosphorylation is essential for the ER stress response and in vivo glucose homeostasis.

Gene expression of VEGF and its receptors Flk-1/KDR and Flt-1 in cultured and transplanted rat islets

BACKGROUND

Vascular endothelial growth factor (VEGF) and its two receptor tyrosine kinases, Flk-1/KDR and Flt-1, may play an important role in mediating the revascularization of transplanted pancreatic islets.

METHODS

Using semiquantitative multiplex reverse-transcribed polymerase chain reaction we determined the gene expression of VEGF and its receptors in cultured and transplanted rat islets.

RESULTS

After exposure of islet cells to hypoxia in vitro, increases were found in the gene expression of the VEGF120 and VEGF164 isoforms, with simultaneous increases in VE-cadherin, Flk-1/KDR, and Flt-1. In vivo studies consisted of analysis of islet grafts transplanted into both normal and diabetic recipients. Expression of both VEGF120 and VEGF164 in grafts was up-regulated for the first 2-3 days after transplantation, with the response being more prolonged in the diabetic rats. These increases were followed by reduced expression of VEGF on days 5, 7, and 9. Increases in the expression of VE-cadherin in islet grafts in normal and diabetic recipients tended to parallel VEGF expression, with the increases in both probably being caused by hypoxia. The early increases of VEGF expression were followed by a rise in the expression of VEGF receptors, which probably represents the early stages of angiogenesis. Graft expression of Flk-1/KDR and Flt-1 was enhanced at 3 and 5 days in the normoglycemic recipients, while in the diabetic recipients increases were found later on days 5, 7, and 14.

CONCLUSIONS

The delayed expression of VEGF receptors in the diabetic recipients could reflect impaired angiogenesis caused by the diabetic milieu; this delay could contribute to the less outcomes of grafts transplanted into a hyperglycemic environment.

Increase in beta-cell mass in transplanted porcine neonatal pancreatic cell clusters is due to proliferation of beta-cells and differentiation of duct cells

A 20-fold increase in beta-cell mass has been found after transplantation of porcine neonatal pancreatic cell clusters (NPCCs). Here the mechanisms leading to this increased beta-cell mass were studied. NPCCs (4000 islet equivalents) generated after 8 days culture of digested neonatal pig pancreas were transplanted beneath the renal capsule of streptozotocin (STZ) diabetic and normoglycemic nude mice. Grafts were removed at 10 days, 6 weeks, and 20 weeks after transplantation for immunostaining and insulin content. Proliferation of beta-cells and duct cells was assessed morphometrically using double immunostaining for Ki-67 with insulin or cytokeratin 7 (CK7). Graft maturation was assessed with double immunostaining of CK7 and insulin. Apoptosis was determined using propidium iodide staining. beta-cell proliferation in NPCCs was higher after 8 days of culture compared with that found in neonatal pig pancreas. After transplantation, beta-cell proliferation remained high at 10 days, decreased somewhat at 6 weeks, and was much lower 20 weeks after transplantation. Diabetic recipients not cured at 6 weeks after transplantation had significantly higher beta-cell proliferation compared with those cured and to normoglycemic recipients. The size of individual beta-cells, as determined by cross-sectional area, increased as the grafts matured. Graft insulin content was 20-fold increased at 20 weeks after transplantation compared with 8 days cultured NPCCS: The proliferation index of duct cells was significantly higher in neonatal pig pancreas than in 8 days cultured NPCCs and in 10-day-old grafts. The incidence of apoptosis in duct cells appeared to be low. About 20% of duct cells 10 days post transplantation showed costaining for CK7 and insulin, a marker of protodifferentiation. In conclusion, the increase in beta-cell mass after transplantation of NPCCs is due to both proliferation of differentiated beta-cells and differentiation of duct cells into beta-cells.

Adaptation of beta-cell mass to substrate oversupply: enhanced function with normal gene expression

Although type 2 diabetes mellitus is associated with insulin resistance, many individuals compensate by increasing insulin secretion. Putative mechanisms underlying this compensation were assessed in the present study by use of 4-day glucose (GLC; 35% Glc, 2 ml/h) and lipid (LIH; 10% Intralipid + 20 U/ml heparin; 2 ml/h) infusions to rats. Within 2 days of beginning the infusion of either lipid or glucose, plasma glucose profiles were normalized (relative to saline-infused control rats; SAL; 0.45% 2 ml/h). During glucose infusion, plasma glucose was maintained in the normal range by an approximately twofold increase in plasma insulin and an approximately 80% increase in beta-cell mass. During LIH infusion, glucose profiles were also maintained in the normal range. Plasma insulin responses during feeding were doubled, and beta-cell mass increased 54%. For both groups, the increase in beta-cell mass was associated with increased beta-cell proliferation (98% increase during GLC and 125% increase during LIH). At the end of the 4-day infusions, no significant changes were observed in islet-specific gene transcription (i.e., the expression of islet hormone genes, glucose metabolism genes, and insulin transcription factors were unaffected). Two days after termination of the infusions, the glucose-stimulated plasma insulin response was increased approximately 67% in glucose-infused animals. No sustained effect on insulin secretory capacity was observed in the LIH animals. The increase in plasma insulin response after glucose infusion was achieved in the absence of any change in insulin clearance. We conclude that, in rats, an increase in insulin demand after an increase in glucose appearance or free fatty acid leads to an increase in beta-cell mass, mediated in part by an increase in beta-cell proliferation, and that these compensatory changes lead to increased insulin secretion, normal plasma glucose levels, and the maintenance of normal islet gene expression.

ENHANCED MATURATION OF PORCINE NEONATAL PANCREATIC CELL CLUSTERS WITH GROWTH FACTORS FAILS TO IMPROVE TRANSPLANTATION OUTCOME1

BACKGROUND

Porcine neonatal pancreatic cell clusters (NPCC) are a potential source of islet tissue for clinical transplantation. They can normalize glycemia after transplantation, although after a relatively long (several weeks) period of time, possibly due to the immaturity of the tissue.

METHODS

One week after isolation NPCCs were immobilized in alginate hydrogel to be cultured for 2 more weeks in the presence of different growth factors, which were applied individually or in various combinations. Their effect was assessed by measuring DNA and insulin content, and expression of islet genes by reverse transcriptase-polymerase chain reaction. Enhanced maturation of NPCCs was also evaluated after transplantation in streptozotocin-diabetic mice.

RESULTS

A combination of fetal calf serum, insulin-like growth factor-I, nicotinamide and sodium butyrate in NPCCs media from day 7 to day 21 resulted in increased insulin/DNA content and higher expression of insulin, somatostatin, GLUT2 and Nkx6.1 genes. NPCCs cultured under the same conditions from day 3 to day 12 were transplanted into diabetic mice. Control mice were transplanted with NPCCs cultured in parallel in the presence of nicotinamide, but with no serum, insulin-like growth factor-I or butyrate. Normoglycemia was achieved at the same rate in both groups. Plasma porcine C-peptide (week 6) and graft insulin content (week 20) were also similar in both groups.

CONCLUSIONS

Increased insulin content of NPCCs was achieved in vitro by addition of fetal calf serum, insulin-like growth factor-I, nicotinamide, and sodium butyrate, but this increase did not translate into a faster achievement of normoglycemia after transplantation, which suggests that there is a time frame required for complete maturation that is difficult to alter.

beta-cell turnover: its assessment and implications

The pancreatic beta-cells are responsible for the maintenance of the body's glucose levels within a very narrow range; their population is dynamic and undergoes compensatory changes to maintain euglycemia. The structural parameters that allow mass changes (replication, neogenesis, cell volume changes, and cell death) can now be assessed and have proved to be powerful tools. Changes in one parameter can dramatically affect the beta-cell mass. Unfortunately, conclusions are often drawn on measurements that do not assess beta-cell mass but only relative volumes. Throughout the lifetime of a mammal, low levels of beta-cell replication and apoptosis are balanced and result in a slowly increasing mass. The balance allows gradual replacement of the beta-cell population; thus, beta-cells should be considered a slowly renewed tissue. Two major implications of beta-cell turnover are that 1) at any time, the beta-cells would be at different ages and 2) any limitation on replacement could have dire consequences for glucose homeostasis.

Beta-cell adaptation and decompensation during the progression of diabetes

Inadequate beta-cell function is an essential component of all forms of diabetes. The most obvious problem is a failure to maintain sufficient beta-cell mass and function to cope with whatever insulin resistance is present. The most striking functional defect is a loss of acute glucose-induced insulin secretion (GIIS). This review discusses the ways in which beta-cells successfully adapt to increased demand and then decompensate as diabetes develops. Successful adaptation is achieved through increased beta-cell mass and increased insulin secretion. The hypothesis is explored that beta-cells exposed to the diabetic milieu lose their differentiation, which leads to loss of specialized functions such as GIIS. This concept has been strengthened by the finding of dedifferentiation of beta-cells in a rat model of partial pancreatectomy that includes a reduction of insulin gene expression, which may further contribute to decreased insulin production. Another finding was increased expression of c-Myc, which probably contributes to an increase in the expression of lactate dehydrogenase and the development of beta-cell hypertrophy. Arguments are developed that the beta-cell changes found in diabetes are better correlated with increased glucose levels than with non-esterified fatty acid levels, thus supporting the importance of glucose toxicity.

PDX:PBX complexes are required for normal proliferation of pancreatic cells during development

The homeobox factor PDX-1 is a key regulator of pancreatic morphogenesis and glucose homeostasis; targeted disruption of the PDX-1 gene leads to pancreatic agenesis in pdx-1(-/-) homozygotes. Pdx-1 heterozygotes develop normally, but they display glucose intolerance in adulthood. Like certain other homeobox proteins, PDX-1 contains a consensus FPWMK motif that promotes heterodimer formation with the ubiquitous homeodomain protein PBX. To evaluate the importance of PDX-1:PBX complexes in pancreatic morphogenesis and glucose homeostasis, we expressed either wild-type or PBX interaction defective PDX-1 transgenes under control of the PDX-1 promoter. Both wild-type and mutant PDX-1 transgenes corrected glucose intolerance in pdx-1 heterozygotes. The wild-type PDX-1 transgene rescued the development of all pancreatic lineages in pdx-1(-/-) animals, and these mice survived to adulthood. In contrast, pancreata from pdx-1(-/-) mice expressing the mutant PDX-1 transgene were hypoplastic, and these mice died within 3 weeks of birth from pancreatic insufficiency. All pancreatic cell types were observed in pdx-1(-/-) mice expressing the mutant PDX-1 transgene; but the islets were smaller, and increased numbers of islet hormone-positive cells were noted within the ductal epithelium. These results indicate that PDX-1:PBX complexes are dispensable for glucose homeostasis and for differentiation of stem cells into ductal, endocrine, and acinar lineages; but they are essential for expansion of these populations during development.

Islets in alginate macrobeads reverse diabetes despite minimal acute insulin secretory responses

BACKGROUND

Encapsulation of islets has been widely investigated as a treatment for diabetes. The characteristics and dynamics of insulin secretion by encapsulated islets in response to glucose and other secretagogues are not well understood.

METHODS

In our study, macroencapsulated syngeneic islets at 3-4 wk after transplantation were studied for insulin release in response to i.v. glucose (hyperglycemic clamps at 250 or 350 mg/dl plasma glucose), arginine (i.v. bolus, 100 mg/kg), glucagon-like peptide-1 (i.v. infusion for 20 min, 2.2 pmol/kg/min), and meal challenge. Syngeneic islets (6000 islets) were encapsulated in alginate macrobeads (2-3 mm diameter) with or without poly-L-lysine coating and transplanted into the peritoneal cavity of STZ-diabetic Lewis rats. Normal (nontransplanted) and diabetic Lewis rats transplanted with "naked" islets under the kidney capsule served as controls.

RESULTS

Animals transplanted with macrobeads displayed subnormal insulin responses to glucose, arginine, and glucagon-like peptide-1 despite achieving normoglycemia faster than animals with renal subcapsular islet transplants. Plasma insulin responses to meal challenges were blunted in animals with macrobeads resulting in increased plasma glucose excursions.

CONCLUSIONS

We conclude that, after transplantation into diabetic Lewis rats, macroencapsulated islets have significantly impaired insulin secretion despite achieving normal fed glycemic levels.

Life and death of the pancreatic beta cells

Pancreatic beta cells are responsible for maintaining the body's glucose levels within a very narrow range; their population is dynamic, with compensatory changes to maintain euglycemia. Throughout the lifetime of a mammal, low levels of beta-cell replication and apoptosis are balanced and result in a slowly increasing mass of beta cells. The emphasis in this review is on recent insights on the natural history of the beta cell in a normal pancreas: sources of renewal, survival and changes in differentiation.

Improved glucose and lipid metabolism in genetically obese mice lacking aP2

Adipocyte fatty acid-binding protein, aP2, is a member of the intracellular fatty acid binding protein family. Previously, studies have shown increased insulin sensitivity in aP2-deficient mice with dietary obesity. Here, we asked whether aP2-related alterations in lipolytic response and insulin production are features of obesity-induced insulin resistance and investigated the effects of aP2-deficiency on glucose homeostasis and lipid metabolism in ob/ob mice, a model of extreme obesity. ob/ob mice homozygous for the aP2 null allele (ob/ ob-aP2-/-) became more obese than ob/ob mice as indicated by significantly increased body weight and fat pad size but unaltered body length. However, despite their extreme adiposity, ob/ob-aP2-/- animals were more insulin-sensitive compared with ob/ob controls, as demonstrated by significantly lower plasma glucose and insulin levels and better performance in both insulin and glucose tolerance tests. These animals also showed improvements in dyslipidemia and had lower plasma triglyceride and cholesterol levels. Lipolytic response to beta-adrenergic stimulation and lipolysis-associated insulin secretion was significantly reduced in ob/ob-aP2-/- mice. Interestingly, glucose-stimulated insulin secretion, while virtually abolished in ob/ob controls, was significantly improved in ob/ob-aP2-/- animals. There were no apparent morphological differences in the structure or size of the pancreatic islets between genotypes. Taken together, the data indicate that in obesity, aP2-deficiency not only improves peripheral insulin resistance but also preserves pancreatic beta cell function and has beneficial effects on lipid metabolism.

In vitro cultivation of human islets from expanded ductal tissue

A major obstacle to successful islet transplantation for both type 1 and 2 diabetes is an inadequate supply of insulin-producing tissue. This need for transplantable human islets has stimulated efforts to expand existing pancreatic islets and/or grow new ones. To test the hypothesis that human adult duct tissue could be expanded and differentiated in vitro to form islet cells, digested pancreatic tissue that is normally discarded from eight human islet isolations was cultured under conditions that allowed expansion of the ductal cells as a monolayer whereupon the cells were overlaid with a thin layer of Matrigel. With this manipulation, the monolayer of epithelial cells formed three-dimensional structures of ductal cysts from which 50-to 150- micrometer diameter islet-like clusters of pancreatic endocrine cells budded. Over 3-4 weeks culture the insulin content per flask increased 10- to 15-fold as the DNA content increased up to 7-fold. The cultivated human islet buds were shown by immunofluorescence to consist of cytokeratin 19-positive duct cells and hormone-positive islet cells. Double staining of insulin and non-beta cell hormones in occasional cells indicated immature cells still in the process of differentiation. Insulin secretion studies were done over 24 h in culture. Compared with their basal secretion at 5 mM glucose, cysts/cultivated human islet buds exposed to stimulatory 20 mM glucose had a 2.3-fold increase in secreted insulin. Thus, duct tissue from human pancreas can be expanded in culture and then be directed to differentiate into glucose responsive islet tissue in vitro. This approach may provide a potential new source of pancreatic islet cells for transplantation.

Effects of diabetes and hypoxia on gene markers of angiogenesis (HGF, cMET, uPA and uPAR, TGF-alpha, TGF-beta, bFGF and Vimentin) in cultured and transplanted rat islets

AIMS/HYPOTHESIS

The vascularisation of newly transplanted islets originates from the recipients. Because islets transplanted into a diabetic do less well than those transplanted into a euglycaemic environment, we examined the hypothesis that gene expression of angiogenic factors in grafts is delayed in diabetes. These factors include hepatocyte growth factor (HGF) and its receptor c-MET, and urokinase plasminogen activator (uPA) and its receptor uPAR, basic fibroblast growth factor (bFGF), TGF-alpha and TGF beta-1.

METHODS

Isolated rat islets were studied in vitro under normoxic and hypoxic culture conditions and gene expression was determined with semi-quantitative multiplex RT-PCR. We found that HGF but not c-MET expression was induced by hypoxia in vitro. Using syngeneic Lewis rats, gene expression was also studied in grafts on days 1, 3, 5, 7 and 14 after transplantation.

RESULTS

In grafts of normoglycaemic rats, HGF expression was enhanced on day 3 and maintained whereas expression of c-MET fell and remained down until day 14. Expression of uPA was up at day 3 and remained high; expression of uPAR was also up at day 3 but then fell to control levels at day 14. Expression of bFGF, TGF-alpha and TGF beta-1 persisted throughout. Vimentin, a marker of fibroblasts, had increased expression at day 1 which was further enhanced in subsequent days. In the grafts of diabetic recipients the expression of HGF, uPA and uPAR were delayed, being clearly expressed at day 5 rather than day 3. Vimentin expression was similarly delayed.

CONCLUSION/INTERPRETATION

This apparent delay in angiogenesis provides a potential mechanism for the less favourable outcomes of islets transplanted into diabetic recipients.

Insulinotropic glucagon-like peptide 1 agonists stimulate expression of homeodomain protein IDX-1 and increase islet size in mouse pancreas

Diabetes is caused by a failure of the pancreas to produce insulin in amounts sufficient to meet the body's needs. A hallmark of diabetes is an absolute (type 1) or relative (type 2) reduction in the mass of pancreatic beta-cells that produce insulin. Mature beta-cells have a lifespan of approximately 48-56 days (rat) and are replaced by the replication of preexisting beta-cells and by the differentiation and proliferation of new beta-cells (neogenesis) derived from the pancreatic ducts. Here, we show that the insulinotropic hormone glucagon-like peptide (GLP)-1, which is produced by the intestine, enhances the pancreatic expression of the homeodomain transcription factor IDX-1 that is critical for pancreas development and the transcriptional regulation of the insulin gene. Concomitantly, GLP-1 administered to diabetic mice stimulates insulin secretion and effectively lowers their blood sugar levels. GLP-1 also enhances beta-cell neogenesis and islet size. Thus, in addition to stimulating insulin secretion, GLP-1 stimulates the expression of the transcription factor IDX-1 while stimulating beta-cell neogenesis and may thereby be an effective treatment for diabetes.

Improved vascularization of planar membrane diffusion devices following continuous infusion of vascular endothelial growth factor

Improving blood vessel formation around an immunobarrier device should improve the survival of the encapsulated tissue. In the present study we investigated the formation of new blood vessels around a planar membrane diffusion device (the Baxter Theracyte System) undergoing a continuous infusion of vascular endothelial growth factor through the membranes and into the surrounding tissue. Each device (20 microl) had both an inner immunoisolation membrane and an outer vascularizing membrane. Human recombinant vascular endothelial growth factor-165 was infused at 100 ng/day (low dose: n = 6) and 500 ng/day (high dose: n = 7) for 10 days into devices implanted s.c. in Sprague-Dawley rats; noninfused devices transplanted for an identical period were used as controls (n = 5). Two days following the termination of VEGF infusion, devices were loaded with 20 microl of Lispro insulin (1 U/kg) and the kinetics of insulin release from the lumen of the device was assessed. Devices were then explanted and the number of blood vessels (capillary and noncapillary) was quantified using morphometry. High-dose vascular endothelial growth factor infusion resulted in two- to threefold more blood vessels around the device than that obtained with the noninfused devices and devices infused with low-dose vascular endothelial growth factor. This increase in the number of blood vessels was accompanied by a modest increase in insulin diffusion from the device in the high-dose vascular endothelial growth factor infusion group. We conclude that vascular endothelial growth factor can be used to improve blood vessel formation adjacent to planar membrane diffusion devices.

Exendin-4 stimulates both beta-cell replication and neogenesis, resulting in increased beta-cell mass and improved glucose tolerance in diabetic rats

Diabetes is a disease of increasing prevalence in the general population and of unknown cause. Diabetes is manifested as hyperglycemia due to a relative deficiency of the production of insulin by the pancreatic beta-cells. One determinant in the development of diabetes is an inadequate mass of beta-cells, either absolute (type 1, juvenile diabetes) or relative (type 2, maturity-onset diabetes). Earlier, we reported that the intestinal hormone glucagon-like peptide I (GLP-I) effectively augments glucose-stimulated insulin secretion. Here we report that exendin-4, a long-acting GLP-I agonist, stimulates both the differentiation of beta-cells from ductal progenitor cells (neogenesis) and proliferation of beta-cells when administered to rats. In a partial pancreatectomy rat model of type 2 diabetes, the daily administration of exendin-4 for 10 days post-pancreatectomy attenuates the development of diabetes. We show that exendin-4 stimulates the regeneration of the pancreas and expansion of beta-cell mass by processes of both neogenesis and proliferation of beta-cells. Thus, GLP-I and analogs thereof hold promise as a novel therapy to stimulate beta-cell growth and differentiation when administered to diabetic individuals with reduced beta-cell mass.

Differentiation and expansion of beta cell mass in porcine neonatal pancreatic cell clusters transplanted into nude mice

Neonatal porcine pancreas has considerable capacity for growth and differentiation, making it an attractive potential source of islet tissue for xenotransplantation. Pancreases from 1-3-day-old newborn pigs were digested with collagenase and cultured for 8 days. The resulting cellular aggregates are called porcine neonatal pancreatic cell clusters (NPCCs). The mean yield of NPCCs from a newborn pig was 28,200 +/- 1700 islet equivalents. Cytokeratin 7 (CK7) was used as a marker for the immunostaining of pancreatic duct cells. In neonatal pancreas, 18% of the insulin-positive cells co-stained for CK7, thus being protodifferentiated. NPCCs also contained protodifferentiated cells; insulin/PP and insulin/somatostatin co-stained cells were more common than insulin/glucagon cells. Between 1 and 8 days of culture, the DNA content of the NPCCs fell to 16% and the insulin content to 33% of the starting value, mainly due to the preferential loss of exocrine cells. Transplantation of 2000 or 4000 NPCCs into diabetic nude mice typically normalized glucose values in 10-20 weeks. Mice with successful grafts had lower fasting blood glucose levels than normal mice and accelerated glucose clearance after an i.p. glucose load. The starting NPCCs consisted of 17% insulin-staining cells, but the grafts of mice with reversed diabetes consisted of 94% beta cells, with some co-stained for CK7, indicating that the grafts still contained immature cells. The mass of insulin-producing cells rose from 0.22 +/- 0.08 mg 1 week after transplantation to 4.34 +/- 0.27 mg in mice sacrificed at 27-35 weeks. In summary, NPCCs contain mostly islet precursor cells, which when transplanted into nude mice undergo striking differentiation and beta cell expansion.

Prolonged xenograft survival of islets infected with small doses of adenovirus expressing CTLA4Ig

BACKGROUND

Systemic administration of the inhibitor of costimulation, CTLA4Ig, has been shown to prolong islet graft survival. The purpose of this study was to compare local and systemic expression of murine CTLA4Ig in transplants of rat islets into mice.

METHODS

Murine CTLA4Ig was made by joining two polymerase chain reaction products, the extracellular portion of CTLA4 and the Fc portion of IgG2a. Recombinant adenovirus expressing CTLA4Ig (AdCTLA4Ig) was generated using the strategy of Cre-lox recombination. Isolated rat islets infected with AdCTLA4Ig at multiplicities of infection (MOIs) ranging from 0.1 to 10 were transplanted into streptozocin diabetic male B6AF1 mice. Control islets were mock infected or infected with AdLacZ or AdsIg, a recombinant adenovirus expressing only the Fc portion of IgG2a. Also, AdCTLA4Ig and control viruses were injected intramuscularly into mouse transplant recipients at the time of islet transplantation to provide CTLA4Ig systemically.

RESULTS

Control islets transplanted into diabetic mice were rejected in 13-17 days. Islets infected with AdCTLA4Ig had dose-dependent prolongation of graft survival. Prolonged survival was even found with very low MOIs of 0.1 and 0.5, with survivals of 24+/-4.2 and 25+/-2.2 days, respectively. Survival with an MOI of 10 was 39+/-8.7 days. With intramuscular injection, no prolongation was found at the lowest relative MOIs of 0.2 and 1, but there was dose-dependent prolongation of graft survival with larger doses. At the highest relative MOI of 400, survival was prolonged to 58+/-10 days.

CONCLUSIONS

Rat islets infected with AdCTLA4Ig transplanted into mice had prolonged graft survival. Prolonged survival with MOIs as low as 0.1 and 0.5 indicates that only a minority of islet cells need to express CTLA4Ig to exert an effect. Moreover, the results suggest that the improved islet graft survival is due to a local influence of CTLA4Ig.

In situ electrochemical oxygen generation with an immunoisolation device

The viability and function of transplanted tissue encapsulated in immunobarrier devices is subject to oxygen transport limitation. In this study, we have designed and used an in situ electrochemical oxygen generator which decomposes water electrolyticaly to provide oxygen to the adjacent planer immunobarrier diffusion chamber. The rate of oxygen generation, which increases linearly with electrical current, was accurately controlled. A theoretical model of oxygen diffusion was also developed and was used to calculate the oxygen profiles in some of the experimental systems. In vitro culture experiments were carried out with beta TC3 cells encapsulated in titanium ring devices. The growth and viability of cells with or without in situ oxygen generation was studied. We found that under otherwise similar culturing conditions, the thickness of the cell layer and the viability of cells was the highest in devices cultured in stirred media with oxygen generation, even though the thickness had not reached the theoretically predicted value, and lowest in those unstirred and without oxygen generation.

Chronic hyperglycemia triggers loss of pancreatic beta cell differentiation in an animal model of diabetes

Differentiated pancreatic beta cells are unique in their ability to secrete insulin in response to a rise in plasma glucose. We have proposed that the unique constellation of genes they express may be lost in diabetes due to the deleterious effect of chronic hyperglycemia. To test this hypothesis, Sprague-Dawley rats were submitted to a 85-95% pancreatectomy or sham pancreatectomy. One week later, the animals developed mild to severe chronic hyperglycemia that was stable for the next 3 weeks, without significant alteration of plasma nonesterified fatty acid levels. Expression of many genes important for glucose-induced insulin release decreased progressively with increasing hyperglycemia, in parallel with a reduction of several islet transcription factors involved in beta cell development and differentiation. In contrast, genes barely expressed in sham islets (lactate dehydrogenase A and hexokinase I) were markedly increased, in parallel with an increase in the transcription factor c-Myc, a potent stimulator of cell growth. These abnormalities were accompanied by beta cell hypertrophy. Changes in gene expression were fully developed 2 weeks after pancreatectomy. Correction of blood glucose by phlorizin for the next 2 weeks normalized islet gene expression and beta cell volume without affecting plasma nonesterified fatty acid levels, strongly suggesting that hyperglycemia triggers these abnormalities. In conclusion, chronic hyperglycemia leads to beta cell hypertrophy and loss of beta cell differentiation that is correlated with changes in c-Myc and other key transcription factors. A similar change in beta cell differentiation could contribute to the profound derangement of insulin secretion in human diabetes.

Prior streptozotocin treatment does not inhibit pancreas regeneration after 90% pancreatectomy in rats

The effects of residual beta-cell mass and glycemia on regeneration of endocrine pancreas after 90% pancreatectomy were investigated. Streptozotocin or buffer alone was injected into 4-wk-old male Lewis rats (day 0). On day 7, varying numbers of syngeneic islets were transplanted under the kidney capsule to achieve varying degrees of glucose normalization. On day 14, a 90% pancreatectomy or sham pancreatectomy was performed. On day 19, rats were killed and the pancreas was fixed for quantitative morphometric determination of beta-cell mass. Focal areas of regenerating pancreas were observed in all animals that underwent partial pancreatectomy. The percentage of remnant pancreas classified as foci was unaffected by streptozotocin treatment or by plasma glucose. Moderate to severe hyperglycemia did not promote regeneration of the pancreatic beta-cell mass; rather the total endocrine cell mass was inversely related to the plasma glucose level (r = -0.5, P < 0.01). These data suggest that the precursor population for both endocrine and exocrine tissue is not susceptible to damage by streptozotocin and that local effects of residual beta-cell mass are not important to regeneration after a 90% pancreatectomy.

Reversal of hyperglycemia in mice after subcutaneous transplantation of macroencapsulated islets

BACKGROUND

Macroencapsulated islets can reverse hyperglycemia in diabetic animals when transplanted i.p. or into the fat pad. The s.c. space is an attractive site for such transplantation because macrocapsules can be implanted with local anesthesia and be easily removed or reloaded with fresh islets.

METHODS

Immunoprotective 20 microl ported devices were transplanted under the skin of Streptozocin-diabetic nude mice. Devices were loaded with 1200 rat islets in culture medium or in alginate. Empty devices were implanted for 2 weeks and then loaded with islets. Normal mice and mice with islets transplanted under the renal capsule or under the skin were used as controls. Seven weeks after transplantation, an intraperitoneal glucose tolerance test (IPGTT) was performed, followed by implant removal.

RESULTS

Three weeks after transplantation, normal blood glucose levels were observed in all animals. Compared with those of normal controls, IPGTTs showed accelerated blood glucose clearance in mice transplanted with islets either within devices or beneath the kidney capsule. Fasted transplanted mice were hypoglycemic before glucose injection and 2 hr later. After removal of the implants, all recipient mice returned to hyperglycemia. Histological evaluation revealed viable islet cells and a network of close vascular structures outside the devices.

CONCLUSIONS

Macroencapsulated islets transplanted into the s.c. space were able to survive and regulate blood glucose levels in mice. The observed differences in glucose metabolism between normal and transplanted mice may be attributed to the site of transplantation and to the use of rat islets, which have a different set point for glucose induced insulin release.