Nutrient control of insulin secretion in perifused adult pig islets

Donor- and isolation-related predictive factors of in vitro secretory function of cultured human islets

Background and aims

Human islet preparations designated for research exhibit diverse insulin-secretory profiles. This study aims to assess the impact of donor- and isolation-related factors on in vitro islet secretory function.

Methods

A retrospective analysis of 46 isolations from 23 pancreata discarded for clinical transplantation was conducted. In vitro islet secretory function tests were performed on Day 1 and Day 7 of culture. Linear mixed-effects models (LMMs) were employed to investigate the relationships between various predictors characterizing the patient and donor characteristics as well as the isolation effectiveness and two functional outcomes including the islet stimulation index (SI) and area under the insulin curve (AUC). Fixed effects were introduced to represent the main effects of each predictor, and backward elimination was utilized to select the most significant fixed effects for the final model. Interaction effects between the timepoint (Day 7 vs. Day 1) and the predictors were also evaluated to assess whether predictors were associated with the temporal evolution of SI and AUC. Fold-change (Fc) values associated with each predictor were obtained by exponentiating the corresponding coefficients of the models, which were built on log-transformed outcomes.

Results

Analysis using LMMs revealed that donor body mass index (BMI) (Fc = 0.961, 95% CI = 0.927-0.996, p = 0.05), donor gender (female vs. male, Fc = 0.702, 95% CI = 0.524-0.942, p = 0.04), and donor hypertension (Fc = 0.623, 95% CI = 0.466-0.832, p= <0.01) were significantly and independently associated with SI. Moreover, donor gender (Fc = 0.512, 95% CI = 0.302-0.864, p = 0.02), donor cause of death (cerebrovascular accident vs. cardiac arrest, Fc = 2.129, 95% CI = 0.915-4.946, p = 0.09; trauma vs. cardiac arrest, Fc = 2.129, 95% CI = 1.112-7.106, p = 0.04), pancreas weight (Fc = 1.01, 95% CI = 1.001-1.019, p = 0.03), and islet equivalent (IEQ)/mg (Fc = 1.277, 95% CI = 1.088-1.510, p ≤ 0.01) were significantly and independently associated with AUC. There was no predictor significantly associated with the temporal evolution between Day 1 and Day 7 for both SI and AUC outcomes.

Conclusion

This study identified donor- and isolation-related factors influencing in vitro islet secretory function. Further investigations are essential to validate the applicability of these results in clinical practice.

In vitro assessment of pancreatic hormone secretion from isolated porcine islets

The potential use of porcine islets for transplantation in humans has triggered interest in understanding porcine islet physiology. However, the number of studies dedicated to this topic has remained limited, as most islet physiologists prefer to use the less time-consuming rodent model or the more clinically relevant human islet. An often-overlooked aspect of pig islet physiology is its alpha cell activity and regulation of its glucagon secretion. In vitro islet perifusion is a reliable method to study the dynamics of hormone secretion in response to different stimuli. We thus used this method to quantify and study glucagon secretion from pig islets. Pancreatic islets were isolated from 20 neonatal (14 to 21-day old) and 5 adult (>2 years) pigs and cultured in appropriate media. Islet perifusion experiments were performed 8 to 10 days post-isolation for neonatal islets and 1 to 2 days post-isolation for adult islets. Insulin and glucagon were quantified in perifusion effluent fractions as well as in islet extracts by RIA. Increasing glucose concentration from 1 mM to 15 mM markedly inhibited glucagon secretion independently of animal age. Interestingly, the effect of high glucose was more drastic on glucagon secretion compared to its effect on insulin secretion. In vivo, glucose injection during IVGTT initiated a quick (2-10 minutes) 3-fold decrease of plasmatic glucagon whereas the increase of plasmatic insulin took 20 minutes to become significant. These results suggest that regulation of glucagon secretion significantly contributes to glucose homeostasis in pigs and might compensate for the mild changes in insulin secretion in response to changes in glucose concentration.

Integrated multiscale mathematical modeling of insulin secretion reveals the role of islet network integrity for proper oscillatory glucose-dose response

The integrated multiscale mathematical model we present in this paper is built on two of our previous ones: a model of electrical oscillation in β-cells connected to neighboring cells within a three-dimensional (3D) network, and a model of glucose-induced β-cell intracellular insulin granule trafficking and insulin secretion. In order to couple these two models, we assume that the rate at which primed and release-ready insulin granules fuse at the cell membrane increases with the intracellular calcium concentration. Moreover, by assuming that the fraction of free K_ATP-channels decreases with increasing glucose concentration, we take into account the effect of glucose dose on membrane potential and, indirectly via the effect on the potential, on intracellular calcium. Numerical analysis of our new model shows that a single step increase in glucose concentration yields the experimentally observed characteristic biphasic insulin release. We find that the biphasic response is typically oscillatory in nature for low and moderate glucose concentrations. The plateau fraction (the time that the β-cells spend in their active firing phase) increases with increasing glucose dose, as does the total insulin secretion. At high glucose concentrations, the oscillations tend to vanish due to a constantly elevated membrane potential of the β-cells. Our results also demonstrate how insulin secretion characteristics in various glucose protocols depend on the degree of β-cell loss, highlighting the potential impact from disease. In particular, both the secretory capacity (average insulin secretion rate per β-cell) and the oscillatory response diminish as the islet cell network becomes compromised.

Xenoislets: porcine pancreatic islets for the treatment of type I diabetes

PURPOSE OF REVIEW

Porcine islets are being extensively investigated as alternative sources of insulin-secreting cells for transplantation in insulin-dependent diabetic patients. The present review focuses on recent advances in porcine islet transplantation with particular emphasis on new transgenic pig models, islet encapsulation, and biosafety considerations.

RECENT FINDINGS

Genetic modifications aimed to reduce islet cell immunogenicity, to prolong their survival, and to improve their secretory function have been reported. Micro- and macroencapsulation of porcine islets should allow their use in the clinic with no or minimal immunosuppression. The risk of porcine endogenous retrovirus transmission is being re-evaluated since no evidence for infection was found in several clinical and preclinical studies.

SUMMARY

Pig islet xenotransplantation is still a serious contestant in the race for novel treatments for type I diabetes. Adequate pathogen screening, animal selection, and the establishment of microbiological, genetic, and potency release quality controls should increase safety and efficacy of future porcine islets transplantation clinical trials.

CFTR Influences Beta Cell Function and Insulin Secretion Through Non-Cell Autonomous Exocrine-Derived Factors

Although β-cell dysfunction in cystic fibrosis (CF) leads to diabetes, the mechanism by which the cystic fibrosis transmembrane conductance regulator (CFTR) channel influences islet insulin secretion remains debated. We investigated the CFTR-dependent islet-autonomous mechanisms affecting insulin secretion by using islets isolated from CFTR knockout ferrets. Total insulin content was lower in CF as compared with wild-type (WT) islets. Furthermore, glucose-stimulated insulin secretion (GSIS) was impaired in perifused neonatal CF islets, with reduced first, second, and amplifying phase secretion. Interestingly, CF islets compensated for reduced insulin content under static low-glucose conditions by secreting a larger fraction of islet insulin than WT islets, probably because of elevated SLC2A1 transcripts, increased basal inhibition of adenosine triphosphate-sensitive potassium channels (K-ATP), and elevated basal intracellular Ca2+. Interleukin (IL)-6 secretion by CF islets was higher relative to WT, and IL-6 treatment of WT ferret islets produced a CF-like phenotype with reduced islet insulin content and elevated percentage insulin secretion in low glucose. CF islets exhibited altered expression of INS, CELA3B, and several β-cell maturation and proliferation genes. Pharmacologic inhibition of CFTR reduced GSIS by WT ferret and human islets but similarly reduced insulin secretion and intracellular Ca2+ in CFTR knockout ferret islets, indicating that the mechanism of action is not through CFTR. Single-molecule fluorescent in situ hybridization, on isolated ferret and human islets and ferret pancreas, demonstrated that CFTR RNA colocalized within KRT7+ ductal cells but not endocrine cells. These results suggest that CFTR affects β-cell function via a paracrine mechanism involving proinflammatory factors secreted from islet-associated exocrine-derived cell types.

Effect of Over 10-Year Cryopreserved Encapsulated Pancreatic Islets Of Langerhans

OBJECTIVES

Immunoisolation of pancreatic islets of Langerhans performed by the encapsulation process may be a method to avoid immunosuppressive therapy after transplant. The main problem related to islet transplant is shortage of human pancreata. Resolution of this obstacle may be cryopreservation of encapsulated islets, which enables collection of sufficient numbers of isolated islets required for transplant and long-term storage. Here, we assessed the ability of encapsulated islets to function after long-term banking at low temperature.

MATERIALS AND METHODS

Islets of Langerhans isolated from rat, pig, and human pancreata were encapsulated within alginate-poly-L-lysine-alginate microcapsules. Cryopreservation was carried out using a controlled method of freezing (Kriomedpol freezer; Kriomedpol, Warsaw, Poland), and samples were stored in liquid nitrogen. After 10 years, the samples were thawed with the rapid method (with 0.75 M of sucrose) and then cultured.

RESULTS

We observed that microcapsules containing islets maintained their shape and integrity after thawing. During culture, free islets were defragmented into single cells, whereas encapsulated islets were still round in shape and compact. After 1, 4, and 7 days of culture of encapsulated islets, the use of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide tests showed increased mitochondrial activity. After they were thawed, the insulin secretion capacity was comparable with that obtained with fresh islets.

CONCLUSIONS

Cryopreservation and storage of free and microencapsulated islets were possible for about 10 years, although only encapsulated islets retained viability and secretory properties.

Pig-to-Primate Islet Xenotransplantation: Past, Present, and Future

Islet allotransplantation results in increasing success in treating type 1 diabetes, but the shortage of deceased human donor pancreata limits progress. Islet xenotransplantation, using pigs as a source of islets, is a promising approach to overcome this limitation. The greatest obstacle is the primate immune/inflammatory response to the porcine (pig) islets, which may take the form of rapid early graft rejection (the instant blood-mediated inflammatory reaction) or T-cell-mediated rejection. These problems are being resolved by the genetic engineering of the source pigs combined with improved immunosuppressive therapy. The results of pig-to-diabetic nonhuman primate islet xenotransplantation are steadily improving, with insulin independence being achieved for periods >1 year. An alternative approach is to isolate islets within a micro- or macroencapsulation device aimed at protecting them from the human recipient's immune response. Clinical trials using this approach are currently underway. This review focuses on the major aspects of pig-to-primate islet xenotransplantation and its potential for treatment of type 1 diabetes.

Adiponectin induced AMP-activated protein kinase impairment mediates insulin resistance in Bama mini-pig fed high-fat and high-sucrose diet

OBJECTIVE

Adipose tissue is no longer considered as an inert storage organ for lipid, but instead is thought to play an active role in regulating insulin effects via secretion adipokines. However, conflicting reports have emerged regarding the effects of adipokines. In this study, we investigated the role of adipokines in glucose metabolism and insulin sensitivity in obese Bama mini-pigs.

METHODS

An obesity model was established in Bama mini-pigs, by feeding with high-fat and high-sucrose diet for 30 weeks. Plasma glucose and blood biochemistry levels were measured, and intravenous glucose tolerance test was performed. Adipokines, including adiponectin, interleukin-6 (IL-6), resistin and tumor necrosis factor alpha (TNF-α), and glucose-induced insulin secretion were also examined by radioimmunoassay. AMP-activated protein kinase (AMPK) phosphorylation in skeletal muscle, which is a useful insulin resistance marker, was examined by immunoblotting. Additionally, associations of AMPK phosphorylation with plasma adipokines and homeostasis model assessment of insulin resistance (HOMA-IR) index were assessed by Pearce's correlation analysis.

RESULTS

Obese pigs showed hyperglycemia, high triglycerides, and insulin resistance. Adiponectin levels were significantly decreased (p<0.05) and IL-6 amounts dramatically increased (p<0.05) in obese pigs both in serum and adipose tissue, corroborating data from obese mice and humans. However, circulating resistin and TNF-α showed no difference, while the values of TNF-α in adipose tissue were significantly higher in obese pigs, also in agreement with data from obese humans but not rodent models. Moreover, strong associations of skeletal muscle AMPK phosphorylation with plasma adiponectin and HOMA-IR index were obtained.

CONCLUSION

AMPK impairment induced by adiponectin decrease mediates insulin resistance in high-fat and high-sucrose diet induction. In addition, Bama mini-pig has the possibility of a conformable model for human metabolic diseases.

Transgenic Expression of Glucagon-Like Peptide-1 (GLP-1) and Activated Muscarinic Receptor (M3R) Significantly Improves Pig Islet Secretory Function

Porcine islets show notoriously low insulin secretion levels in response to glucose stimulation. While this is somehow expected in the case of immature islets isolated from fetal and neonatal pigs, disappointingly low secretory responses are frequently reported in studies using in vitro-maturated fetal and neonatal islets and even fully differentiated adult islets. Herein we show that β-cell-specific expression of a modified glucagon-like peptide-1 (GLP-1) and of a constitutively activated type 3 muscarinic receptor (M3R) efficiently amplifies glucose-stimulated insulin secretion (GSIS). Both adult and neonatal isolated pig islets were treated with adenoviral expression vectors carrying sequences encoding for GLP-1 and/or M3R. GSIS from transduced and control islets was evaluated during static incubation and dynamic perifusion assays. While expression of GLP-1 did not affect basal or stimulated insulin secretion, activated M3R produced a twofold increase in both first and second phases of GSIS. Coexpression of GLP-1 and M3R caused an even greater increase in the secretory response, which was amplified fourfold compared to controls. In conclusion, our work highlights pig islet insulin secretion deficiencies and proposes concomitant activation of cAMP-dependent and cholinergic pathways as a solution to ameliorate GSIS from pig islets used for transplantation.

Progress in Clinical Encapsulated Islet Xenotransplantation

At the 2015 combined congress of the Cell Transplant Society, International Pancreas and Islet Transplant Association, and International Xenotransplantation Association, a symposium was held to discuss recent progress in pig islet xenotransplantation. The presentations focused on 5 major topics - (1) the results of 2 recent clinical trials of encapsulated pig islet transplantation, (2) the inflammatory response to encapsulated pig islets, (3) methods to improve the secretion of insulin by pig islets, (4) genetic modifications to the islet-source pigs aimed to protect the islets from the primate immune and/or inflammatory responses, and (5) regulatory aspects of clinical pig islet xenotransplantation. Trials of microencapsulated porcine islet transplantation to treat unstable type 1 diabetic patients have been associated with encouraging preliminary results. Further advances to improve efficacy may include (1) transplantation into a site other than the peritoneal cavity, which might result in better access to blood, oxygen, and nutrients; (2) the development of a more biocompatible capsule and/or the minimization of a foreign body reaction; (3) pig genetic modification to induce a greater secretion of insulin by the islets, and/or to reduce the immune response to islets released from damaged capsules; and (4) reduction of the inflammatory response to the capsules/islets by improvements in the structure of the capsules and/or in genetic engineering of the pigs and/or in some form of drug therapy. Ethical and regulatory frameworks for islet xenotransplantation are already available in several countries, and there is now a wider international perception of the importance of developing an internationally harmonized ethical and regulatory framework.

Insulin: pancreatic secretion and adipocyte regulation

Insulin is the primary acute anabolic coordinator of nutrient partitioning. Hyperglycemia is the main stimulant of insulin secretion, but other nutrients such as specific amino acids, fatty acids, and ketoacids can potentiate pancreatic insulin release. Incretins are intestinal hormones with insulinotropic activity and are secreted in response to food ingestion, thus integrating diet chemical composition with the regulation of insulin release. In addition, prolactin is required for proper islet development, and it stimulates β-cell proliferation. Counterintuitively, bacterial components appear to signal insulin secretion. In vivo lipopolysaccharide infusion acutely increases circulating insulin, which is paradoxical as endotoxemia is a potent catabolic condition. Insulin is a potent anabolic orchestrator of nutrient partitioning, and this is particularly true in adipocytes. Insulin dictates lipid accretion in a dose-dependent manner during preadipocyte development in adipose tissue-derived stromal vascular cell culture. However, in vivo studies focused on insulin's role in regulating adipose tissue metabolism from growing, and market weight pigs are sometimes inconsistent, and this variability appears to be animal, age and depot dependent. Additionally, porcine adipose tissue synthesizes and secretes a number of adipokines (leptin, adiponectin, and so forth) that directly or indirectly influence insulin action. Therefore, because insulin has an enormous impact on agriculturally important phenotypes, it is critical to have a better understanding of how insulin homeostasis is governed.

Mathematical modeling of insulin secretion and the role of glucose-dependent mobilization, docking, priming and fusion of insulin granules

In this paper we develop a new mathematical model of glucose-induced insulin secretion from pancreatic islet β-cells, and we use this model to investigate the rate limiting factors. We assume that insulin granules reside in different pools inside each β-cell, and that all β-cells respond homogeneously to glucose with the same recruitment thresholds. Consistent with recent experimental observations, our model also accounts for the fusion of newcomer granules that are not pre-docked at the plasma membrane. In response to a single step increase in glucose concentration, our model reproduces the characteristic biphasic insulin release observed in multiple experimental systems, including perfused pancreata and isolated islets of rodent or human origin. From our model analysis we note that first-phase insulin secretion depends on rapid depletion of the primed, release-ready granule pools, while the second phase relies on granule mobilization from the reserve. Moreover, newcomers have the potential to contribute significantly to the second phase. When the glucose protocol consists of multiple changes in sequence (a so-called glucose staircase), our model predicts insulin spikes of increasing height, as has been seen experimentally. This increase stems from the glucose-dependent increase in the fusion rate of insulin granules at the plasma membrane of single β-cells. In contrast, previous mathematical models reproduced the staircase experiment by assuming heterogeneous β-cell activation. In light of experimental data indicating limited heterogeneous activation for β-cells within intact islets, our findings suggest that a graded, dose-dependent cell response to glucose may contribute to insulin secretion patterns observed in multiple experiments, and thus regulate in vivo insulin release. In addition, the strength of insulin granule mobilization, priming and fusion are critical limiting factors in determining the total amount of insulin release.

Improvement of pig islet function by in vivo pancreatic tissue remodeling: a "human-like" pig islet structure with streptozotocin treatment

Pig islets demonstrate significantly lower insulin secretion after glucose stimulation than human islets (stimulation index of ∼12 vs. 2 for glucose 1 and 15 mM, respectively) due to a major difference in β- and α-cell composition in islets (60% and 25% in humans and 90% and 8% in pigs, respectively). This leads to a lower rise in 3',5'-cyclic adenosine monophosphate (cAMP) in pig β-cells. Since glucagon is the major hormonal effector of cAMP in β-cells, we modified pig islet structure in vivo to increase the proportion of α-cells per islet and to improve insulin secretion. Selected doses (0, 30, 50, 75, and 100 mg/kg) of streptozotocin (STZ) were intravenously injected in 32 young pigs to assess pancreatic (insulin and glucagon) hormone levels, islet remodeling (histomorphometry for α- and β-cell proportions), and insulin and glucagon secretion in isolated islets. Endocrine structure and hormonal content of pig islets were compared with those of human islets. The dose of STZ was significantly correlated with reductions in pancreatic insulin content (p< 0.05, r(2) = 0.77) and the proportion of β-cells (p < 0.05, r(2) = 0.88). A maximum of 50 mg/kg STZ was required for optimal structure remodeling, with an increased proportion of α-cells per islet (26% vs. 48% α-cells per islet for STZ <50 mg/kg vs. >75 mg/kg; p < 0.05) without β-cell dysfunction. Three months after STZ treatment (30/50 mg/kg STZ), pig islets were isolated and compared with isolated control islets (0 mg/kg STZ). Isolated islets from STZ-treated (30/50 mg/kg) pigs had a higher proportion of α-cells than those from control animals (32.0% vs. 9.6%, respectively, p < 0.05). After in vitro stimulation, isolated islets from STZ-treated pigs demonstrated significantly higher glucagon content (65.4 vs. 21.0 ng/ml, p < 0.05) and insulin release (144 µU/ml) than nontreated islets (59 µU/ml, p < 0.05), respectively. Low-dose STZ (<50 mg/kg) can modify the structure of pig islets in vivo and improve insulin secretion after isolation.

Differential expression of the glutamate transporter GLT-1 in pancreas

The glutamate uptake transporter GLT-1 is best understood for its critical role in preventing brain seizures. Increasing evidence argues that GLT-1 also modulates, and is modulated by, metabolic processes that influence glucose homeostasis. To investigate further the potential role of GLT-1 in these regards, the authors examined GLT-1 expression in pancreas and found that mature multimeric GLT-1 protein is stably expressed in the pancreas of wild-type, but not GLT-1 knockout, mice. There are three primary functional carboxyl-terminus GLT-1 splice variants, called GLT-1a, b, and c. Brain and liver express all three variants; however, the pancreas expresses GLT-1a and GLT-1b but not GLT-1c. Quantitative real time-PCR further revealed that while GLT-1a is the predominant GLT-1 splice variant in brain and liver, GLT-1b is the most abundant splice variant expressed in pancreas. Confocal microscopy and immunohistochemistry showed that GLT-1a and GLT-1b are expressed in both islet β- and α-cells. GLT-1b was also expressed in exocrine ductal domains. Finally, glutamine synthetase was coexpressed with GLT-1 in islets, which suggests that, as with liver and brain, one possible role of GLT-1 in the pancreas is to support glutamine synthesis.

Detailed protocol for evaluation of dynamic perifusion of human islets to assess β-cell function

The definitive measure of β-cell quality in an islet is the measurement of β-cell function, i.e., the ability of the islets to release insulin in a controlled manner in response to minute changes in ambient glucose levels. Continuous flow or dynamic perifusion of the solution containing glucose and secretagogues through the islets is the most accurate assessment of regulated insulin release in vitro. Here, we describe in detail a low cost, mini-perifusion system that can be adapted to any laboratory to assess islet function by examining dynamic insulin release in response to elevated glucose concentrations and addition of secretagogues. Human islets with purity > 80% and viability > 90% were perifused with low glucose (1 mM) and subsequently challenged with high glucose (16.8 mM ± KCl, 25 mM). A prototypical biphasic response to elevated glucose concentrations was observed with an average 8-fold (above basal) increase in insulin concentration at peak values. Similarly, perifusion with carbachol or exendin-4 (Byetta) with glucose (6 mM) resulted in 1.32- and 1.35-fold increase in insulin secretion above basal. Islets could be maintained in the perifusion apparatus and continued to respond to glucose for up to 3 h. At minimal financial cost and technical expertise, this apparatus can be set-up in any biological laboratory to evaluate regulated hormone release from many cell types in less than 6 h. This will allow other laboratories to measure insulin responses to their drug or modifier of interest in vitro, in a manner that better approximates islet function in vivo.

A local glucose-and oxygen concentration-based insulin secretion model for pancreatic islets

BACKGROUND

Because insulin is the main regulator of glucose homeostasis, quantitative models describing the dynamics of glucose-induced insulin secretion are of obvious interest. Here, a computational model is introduced that focuses not on organism-level concentrations, but on the quantitative modeling of local, cellular-level glucose-insulin dynamics by incorporating the detailed spatial distribution of the concentrations of interest within isolated avascular pancreatic islets.

METHODS

All nutrient consumption and hormone release rates were assumed to follow Hill-type sigmoid dependences on local concentrations. Insulin secretion rates depend on both the glucose concentration and its time-gradient, resulting in second-and first-phase responses, respectively. Since hypoxia may also be an important limiting factor in avascular islets, oxygen and cell viability considerations were also built in by incorporating and extending our previous islet cell oxygen consumption model. A finite element method (FEM) framework is used to combine reactive rates with mass transport by convection and diffusion as well as fluid-mechanics.

RESULTS

The model was calibrated using experimental results from dynamic glucose-stimulated insulin release (GSIR) perifusion studies with isolated islets. Further optimization is still needed, but calculated insulin responses to stepwise increments in the incoming glucose concentration are in good agreement with existing experimental insulin release data characterizing glucose and oxygen dependence. The model makes possible the detailed description of the intraislet spatial distributions of insulin, glucose, and oxygen levels. In agreement with recent observations, modeling also suggests that smaller islets perform better when transplanted and/or encapsulated.

CONCLUSIONS

An insulin secretion model was implemented by coupling local consumption and release rates to calculations of the spatial distributions of all species of interest. The resulting glucose-insulin control system fits in the general framework of a sigmoid proportional-integral-derivative controller, a generalized PID controller, more suitable for biological systems, which are always nonlinear due to the maximum response being limited. Because of the general framework of the implementation, simulations can be carried out for arbitrary geometries including cultured, perifused, transplanted, and encapsulated islets.

Pig islets for clinical islet xenotransplantation

PURPOSE OF REVIEW

Allogeneic islet transplantation faces difficulties because organ shortage is recurrent; several pancreas donors are often needed to treat one diabetic recipient; and the intrahepatic site of islet implantation may not be the most appropriate one. Another source of insulin-producing cells, therefore, would be of major interest, and pigs represent a possible and serious source for obtaining such cells.

RECENT FINDINGS

Pig islet grafts may appear difficult because of the species barrier, but recent studies demonstrate that pig islets may function in diabetic primates for at least 6 months.

SUMMARY

Pig islet xenotransplantation, however, must still overcome the selection of a suitable pig donor to translate preclinical findings into clinical applications. This review summarizes the actual acquired knowledge of pig islet transplantation in primates to select the 'ideal' pig donor.

Shortcomings of current models of glucose-induced insulin secretion

Glucose-induced insulin secretion by pancreatic beta-cells is generally schematized by a 'consensus model' that involves the following sequence of events: acceleration of glucose metabolism, closure of ATP-sensitive potassium channels (K(ATP) channels) in the plasma membrane, depolarization, influx of Ca(2+) through voltage-dependent calcium channels and a rise in cytosolic-free Ca(2+) concentration that induces exocytosis of insulin-containing granules. This model adequately depicts the essential triggering pathway but is incomplete. In this article, we first make a case for a model of dual regulation in which a metabolic amplifying pathway is also activated by glucose and augments the secretory response to the triggering Ca(2+) signal under physiological conditions. We next discuss experimental evidence, largely but not exclusively obtained from beta-cells lacking K(ATP) channels, which indicates that these channels are not the only possible transducers of glucose effects on the triggering Ca(2+)signal. We finally address the identity of the widely neglected background inward current (Cl(-) efflux vs. Na(+) or Ca(2+) influx through voltage-independent channels) that is necessary to cause beta-cell depolarization when glucose closes K(ATP) channels. More attention should be paid to the possibility that some components of this background current are influenced by glucose metabolism and have their place in a model of glucose-induced insulin secretion.

Regulation of insulin secretion: a matter of phase control and amplitude modulation

The consensus model of stimulus-secretion coupling in beta cells attributes glucose-induced insulin secretion to a sequence of events involving acceleration of metabolism, closure of ATP-sensitive K(+) channels, depolarisation, influx of Ca(2+) and a rise in cytosolic free Ca(2+) concentration ([Ca(2+)](c)). This triggering pathway is essential, but would not be very efficient if glucose did not also activate a metabolic amplifying pathway that does not raise [Ca(2+)](c) further but augments the action of triggering Ca(2+) on exocytosis. This review discusses how both pathways interact to achieve temporal control and amplitude modulation of biphasic insulin secretion. First-phase insulin secretion is triggered by the rise in [Ca(2+)](c) that occurs synchronously in all beta cells of every islet in response to a sudden increase in the glucose concentration. Its time course and duration are shaped by those of the Ca(2+) signal, and its amplitude is modulated by the magnitude of the [Ca(2+)](c) rise and, substantially, by amplifying mechanisms. During the second phase, synchronous [Ca(2+)](c) oscillations in all beta cells of an individual islet induce pulsatile insulin secretion, but these features of the signal and response are dampened in groups of intrinsically asynchronous islets. Glucose has hardly any influence on the amplitude of [Ca(2+)](c) oscillations and mainly controls the time course of triggering signal. Amplitude modulation of insulin secretion pulses largely depends on the amplifying pathway. There are more similarities than differences between the two phases of glucose-induced insulin secretion. Both are subject to the same dual, hierarchical control over time and amplitude by triggering and amplifying pathways, suggesting that the second phase is a sequence of iterations of the first phase.

Pig islet xenotransplantation into non-human primate model

Allogeneic islet transplantation faces difficulties because (1) organ shortage is recurrent; (2) several pancreas donors are often needed to treat one diabetic recipient; and (3) the intrahepatic site of islet implantation may not be the most appropriate site. Another source of insulin-producing cells, therefore, would be of major interest, and pigs represent a possible and serious source for obtaining such cells. Pig islet grafts may seem difficult because of the species barrier, but recent reports demonstrate that pig islets may function in primates for at least 6 months. Pig islet xenotransplantation, however, must still overcome several hurdles before becoming clinically applicable. The actual consensus is to produce more preclinical data in the pig-to-primate model as a necessary requirement to envisage any pig-to-human transplantation of islets; therefore, a summary of the actual acquired knowledge of pig islet transplantation in primates seemed useful and is summarized in this overview.