Maintenance of stimulus-secretion coupling and single beta-cell function in cryopreserved-thawed human islets of Langerhans

Systematic review of islet cryopreservation

Pancreatic islet transplantation is being extensively researched as an alternative treatment for type 1 diabetic patients. This treatment is currently limited by temporal mismatch, between the availability of pancreas and isolated islets from deceased organ donor, and the recipient's need for freshly isolated islets. To solve this issue, cryopreservation of islets may offer the potential to bank islets for transplant on demand. Cryopreservation, however, introduces an overwhelmingly harsh environment to the ever-so-fragile islets. After exposure to the freezing and thawing, islets are usually either apoptotic, non-functional, or non-viable. Several studies have proposed various techniques that could lead to increased cell survival and function following a deep freeze. The purpose of this article is to critically review the techniques of islet cryopreservation, with the goal of highlighting optimization parameters that can lead to the most viable and functional islet upon recovery and/or transplant.

Beta-cell hubs maintain Ca2+ oscillations in human and mouse islet simulations

Islet β-cells are responsible for secreting all circulating insulin in response to rising plasma glucose concentrations. These cells are a phenotypically diverse population that express great functional heterogeneity. In mice, certain β-cells (termed 'hubs') have been shown to be crucial for dictating the islet response to high glucose, with inhibition of these hub cells abolishing the coordinated Ca2+ oscillations necessary for driving insulin secretion. These β-cell hubs were found to be highly metabolic and susceptible to pro-inflammatory and glucolipotoxic insults. In this study, we explored the importance of hub cells in human by constructing mathematical models of Ca2+ activity in human islets. Our simulations revealed that hubs dictate the coordinated Ca2+ response in both mouse and human islets; silencing a small proportion of hubs abolished whole-islet Ca2+ activity. We also observed that if hubs are assumed to be preferentially gap junction coupled, then the simulations better adhere to the available experimental data. Our simulations of 16 size-matched mouse and human islet architectures revealed that there are species differences in the role of hubs; Ca2+ activity in human islets was more vulnerable to hub inhibition than mouse islets. These simulation results not only substantiate the existence of β-cell hubs, but also suggest that hubs may be favorably coupled in the electrical and metabolic network of the islet, and that targeted destruction of these cells would greatly impair human islet function.

The triggering pathway to insulin secretion: Functional similarities and differences between the human and the mouse β cells and their translational relevance

In β cells, stimulation by metabolic, hormonal, neuronal, and pharmacological factors is coupled to secretion of insulin through different intracellular signaling pathways. Our knowledge about the molecular machinery supporting these pathways and the patterns of signals it generates comes mostly from rodent models, especially the laboratory mouse. The increased availability of human islets for research during the last few decades has yielded new insights into the specifics in signaling pathways leading to insulin secretion in humans. In this review, we follow the most central triggering pathway to insulin secretion from its very beginning when glucose enters the β cell to the calcium oscillations it produces to trigger fusion of insulin containing granules with the plasma membrane. Along the way, we describe the crucial building blocks that contribute to the flow of information and focus on their functional role in mice and humans and on their translational implications.

Mathematical modeling of gap junction coupling and electrical activity in human β-cells

Coordinated insulin secretion is controlled by electrical coupling of pancreatic β-cells due to connexin-36 gap junctions. Gap junction coupling not only synchronizes the heterogeneous β-cell population, but can also modify the electrical behavior of the cells. These phenomena have been widely studied with mathematical models based on data from mouse β-cells. However, it is now known that human β-cell electrophysiology shows important differences to its rodent counterpart, and although human pancreatic islets express connexin-36 and show evidence of β-cell coupling, these aspects have been little investigated in human β-cells. Here we investigate theoretically, the gap junction coupling strength required for synchronizing electrical activity in a small cluster of cells simulated with a recent mathematical model of human β-cell electrophysiology. We find a lower limit for the coupling strength of approximately 20 pS (i.e., normalized to cell size, ∼2 pS pF(-1)) below which spiking electrical activity is asynchronous. To confront this theoretical lower bound with data, we use our model to estimate from an experimental patch clamp recording that the coupling strength is approximately 100-200 pS (10-20 pS pF(-1)), similar to previous estimates in mouse β-cells. We then investigate the role of gap junction coupling in synchronizing and modifying other forms of electrical activity in human β-cell clusters. We find that electrical coupling can prolong the period of rapid bursting electrical activity, and synchronize metabolically driven slow bursting, in particular when the metabolic oscillators are in phase. Our results show that realistic coupling conductances are sufficient to promote synchrony in small clusters of human β-cells as observed experimentally, and provide motivation for further detailed studies of electrical coupling in human pancreatic islets.

Human islet function following 20 years of cryogenic biobanking

AIMS/HYPOTHESIS

There are potential advantages to the low-temperature (-196 °C) banking of isolated islets, including the maintenance of viable islets for future research. We therefore assessed the in vitro and in vivo function of islets cryopreserved for nearly 20 years.

METHODS

Human islets were cryopreserved from 1991 to 2001 and thawed between 2012 and 2014. These were characterised by immunostaining, patch-clamp electrophysiology, insulin secretion, transcriptome analysis and transplantation into a streptozotocin (STZ)-induced mouse model of diabetes.

RESULTS

The cryopreservation time was 17.6 ± 0.4 years (n = 43). The thawed islets stained positive with dithizone, contained insulin-positive and glucagon-positive cells, and displayed levels of apoptosis and transcriptome profiles similar to those of freshly isolated islets, although their insulin content was lower. The cryopreserved beta cells possessed ion channels and exocytotic responses identical to those of freshly isolated beta cells. Cells from a subset of five donors demonstrated similar perifusion insulin secretion profiles pre- and post-cryopreservation. The transplantation of cryopreserved islets into the diabetic mice improved their glucose tolerance but did not completely normalise their blood glucose levels. Circulating human insulin and insulin-positive grafts were detectable at 10 weeks post-transplantation.

CONCLUSIONS/INTERPRETATION

We have demonstrated the potential for long-term banking of human islets for research, which could enable the use of tissue from a large number of donors with future technologies to gain new insight into diabetes.

Multiparameter screening reveals a role for Na+ channels in cytokine-induced β-cell death

Pancreatic β-cell death plays a role in both type 1 and type 2 diabetes, but clinical treatments that specifically target β-cell survival have not yet been developed. We have recently developed live-cell imaging-based, high-throughput screening methods capable of identifying factors that modulate pancreatic β-cell death, with the hope of finding drugs that can intervene in this process. In the present study, we used a high-content screen and the Prestwick Chemical Library of small molecules to identify drugs that block cell death resulting from exposure to a cocktail of cytotoxic cytokines (25 ng/mL TNF-α, 10 ng/mL IL-1β, and 10 ng/mL IFN-γ). Data analysis with self-organizing maps revealed that 19 drugs had profiles similar to that of the no cytokine condition, indicating protection. Carbamazepine, an antiepileptic Na(+) channel inhibitor, was particularly interesting because Na(+) channels are not generally considered targets for antiapoptotic therapy in diabetes and because the function of these channels in β-cells has not been well studied. We analyzed the expression and characteristics of Na(+) currents in mature β-cells from MIP-GFP mice. We confirmed the dose-dependent protective effects of carbamazepine and another use-dependent Na(+) channel blocker in cytokine-treated mouse islet cells. Carbamazepine down-regulated the proapoptotic and endoplasmic reticulum stress signaling induced by cytokines. Together, these studies point to Na(+) channels as a novel therapeutic target in diabetes.

Mathematical modeling of heterogeneous electrophysiological responses in human β-cells

Electrical activity plays a pivotal role in glucose-stimulated insulin secretion from pancreatic β-cells. Recent findings have shown that the electrophysiological characteristics of human β-cells differ from their rodent counterparts. We show that the electrophysiological responses in human β-cells to a range of ion channels antagonists are heterogeneous. In some cells, inhibition of small-conductance potassium currents has no effect on action potential firing, while it increases the firing frequency dramatically in other cells. Sodium channel block can sometimes reduce action potential amplitude, sometimes abolish electrical activity, and in some cells even change spiking electrical activity to rapid bursting. We show that, in contrast to L-type Ca2+-channels, P/Q-type Ca2+-currents are not necessary for action potential generation, and, surprisingly, a P/Q-type Ca2+-channel antagonist even accelerates action potential firing. By including SK-channels and Ca2+ dynamics in a previous mathematical model of electrical activity in human β-cells, we investigate the heterogeneous and nonintuitive electrophysiological responses to ion channel antagonists, and use our findings to obtain insight in previously published insulin secretion measurements. Using our model we also study paracrine signals, and simulate slow oscillations by adding a glycolytic oscillatory component to the electrophysiological model. The heterogenous electrophysiological responses in human β-cells must be taken into account for a deeper understanding of the mechanisms underlying insulin secretion in health and disease, and as shown here, the interdisciplinary combination of experiments and modeling increases our understanding of human β-cell physiology.

The somatostatin receptor in human pancreatic β-cells

The peptide hormone somatostatin (SST) is produced in the brain, the gut, and in δ-cells in pancreatic islets of Langerhans. SST secretion from δ-cells is stimulated by glucose, amino acids, and glucagon-like peptide-1. Exogenous SST strongly inhibits the secretion of the blood glucose-regulating hormones insulin and glucagon from pancreatic β-cells and α-cells, respectively. Endogenous SST secreted from δ-cells is a paracrine regulator of insulin and glucagon secretion, although the exact physiological significance of this regulation is unclear. Secreted SST binds to specific receptors (SSTRs), which are coupled to Gi/o proteins. In both β- and α-cells, activation of SSTRs suppresses hormone secretion by reducing cAMP levels, inhibiting electrical activity, decreasing Ca²⁺ influx through voltage-gated Ca²⁺ channels and directly reducing exocytosis in a Ca²⁺ and cAMP-independent manner. In rodents, β-cells express predominantly SSTR5, whereas α-cells express SSTR2. In human islets, SSTR2 is the dominant receptor in both β- and α-cells, but other isoforms also contribute to the SST effects. Evidence from rodent models suggests that SST secretion from δ-cells is dysregulated in diabetes mellitus, which may contribute to the metabolic disturbances in this disease. SST analogues are currently used for the treatment of hyperinsulinism and other endocrine disorders, including acromegaly and Cushing's syndrome.

The human L-type calcium channel Cav1.3 regulates insulin release and polymorphisms in CACNA1D associate with type 2 diabetes

AIMS/HYPOTHESIS

Voltage-gated calcium channels of the L-type have been shown to be essential for rodent pancreatic beta cell function, but data about their presence and regulation in humans are incomplete. We therefore sought to elucidate which L-type channel isoform is functionally important and its association with inherited diabetes-related phenotypes.

METHODS

Beta cells of human islets from cadaver donors were enriched using FACS to study the expression of the genes encoding voltage-gated calcium channel (Cav)1.2 and Cav1.3 by absolute quantitative PCR in whole human and rat islets, as well as in clonal cells. Single-cell exocytosis was monitored as increases in cell capacitance after treatment with small interfering (si)RNA against CACNA1D (which encodes Cav1.3). Three single nucleotide polymorphisms (SNPs) were genotyped in 8,987 non-diabetic and 2,830 type 2 diabetic individuals from Finland and Sweden and analysed for associations with type 2 diabetes and insulin phenotypes.

RESULTS

In FACS-enriched human beta cells, CACNA1D mRNA expression exceeded that of CACNA1C (which encodes Cav1.2) by approximately 60-fold and was decreased in islets from type 2 diabetes patients. The latter coincided with diminished secretion of insulin in vitro. CACNA1D siRNA reduced glucose-stimulated insulin release in INS-1 832/13 cells and exocytosis in human beta cells. Phenotype/genotype associations of three SNPs in the CACNA1D gene revealed an association between the C allele of the SNP rs312480 and reduced mRNA expression, as well as decreased insulin secretion in vivo, whereas both rs312486/G and rs9841978/G were associated with type 2 diabetes.

CONCLUSION/INTERPRETATION

We conclude that the L-type calcium channel Cav1.3 is important in human glucose-induced insulin secretion, and common variants in CACNA1D might contribute to type 2 diabetes.

Ion channels and regulation of insulin secretion in human β-cells: a computational systems analysis

In mammals an increase in glucose leads to block of ATP dependent potassium channels in pancreatic β cells leading to membrane depolarization. This leads to the repetitive firing of action potentials that increases calcium influx and triggers insulin granule exocytosis. Several important differences between species in this process suggest that a dedicated human-oriented approach is advantageous as extrapolating from rodent data may be misleading in several respects. We examined depolarization-induced spike activity in pancreatic human islet-attached β-cells employing whole-cell patch-clamp methods. We also reviewed the literature concerning regulation of insulin secretion by channel activity and constructed a data-based computer model of human β cell function. The model couples the Hodgkin-Huxley-type ionic equations to the equations describing intracellular Ca²⁺ homeostasis and insulin release. On the basis of this model we employed computational simulations to better understand the behavior of action potentials, calcium handling and insulin secretion in human β cells under a wide range of experimental conditions. This computational system approach provides a framework to analyze the mechanisms of human β cell insulin secretion.

A biophysical model of electrical activity in human β-cells

Electrical activity in pancreatic β-cells plays a pivotal role in glucose-stimulated insulin secretion by coupling metabolism to calcium-triggered exocytosis. Mathematical models based on rodent data have helped in understanding the mechanisms underlying the electrophysiological patterns observed in laboratory animals. However, human β-cells differ in several aspects, and in particular in their electrophysiological characteristics, from rodent β-cells. Hence, from a clinical perspective and to obtain insight into the defects in insulin secretion relevant for diabetes mellitus, it is important to study human β-cells. This work presents the first mathematical model of electrical activity based entirely on published ion channel characteristics of human β-cells. The model reproduces satisfactorily a series of experimentally observed patterns in human β-cells, such as spiking and rapid bursting electrical activity, and their response to a range of ion channel antagonists. The possibility of Human Ether-a-Go-Go-related- and leak channels as drug targets for diabetes treatment is discussed based on model results.

The isolated pancreatic islet as a micro-organ and its transplantation to cure diabetes: celebrating the legacy of Paul Lacy

Over the past three decades the pancreatic islet of Langerhans has taken center stage as an endocrine micro-organ whose glucoregulatory function is highly explicable on the basis of the increasingly well understood activities of three highly interactive secretory cells. Islet dysfunction underlies both type 1 and type 2 diabetes mellitus (DM); its protection from immune attack and gluco-and lipo-toxicity may prevent the development of DM; and its replacement by non-surgical transplantation may be curative of DM. During a career marked by vision, focus and tenacity, Paul Lacy contributed substantially to the development of each of these concepts. In this review we focus on Lacy's contribution to the development of the concept of the islet as a micro-organ, how this foreshadowed our current detailed understanding of single cell function and cell-cell interactions and how this led to a reduced model of islet function encouraging islet transplantation. Next, we examine how clinical allotransplantation, first undertaken by Lacy, has contributed to a more complex view of the interaction of islet endocrine cells with its circulation and neighboring tissues, both "in situ" and after transplantation. Lastly, we consider recent developments in some alternative approaches to treatment of DM that Lacy could glimpse on the horizon but did not have the chance to participate in.

Calcium signaling in the islets

Easy access to rodent islets and insulinoma cells and the ease of measuring Ca(2+) by fluorescent indicators have resulted in an overflow of data that have clarified minute details of Ca(2+) signaling in the rodent islets. Our understanding of the mechanisms and the roles of Ca(2+) signaling in the human islets, under physiological conditions, has been hugely influenced by uncritical extrapolation of the rodent data obtained under suboptimal experimental conditions. More recently, electrophysiological and Ca(2+) studies have elucidated the ion channel repertoire relevant for Ca(2+) signaling in the human islets and have examined their relative importance. Many new channels belonging to the transient receptor potential (TRP) family are present in the beta-cells. Ryanodine receptors, nicotinic acid adenine dinucleotide phosphate channel, and Ca(2+)-induced Ca(2+) release add new dimension to the complexity of Ca(2+) signaling in the human beta-cells. A lot more needs to be learnt about the roles of these new channels and CICR, not because that will be easy but because that will be difficult. Much de-learning will also be needed. Human beta-cells do not have a resting state in the normal human body even under physiological fasting conditions. Their membrane potential under physiologically relevant resting conditions is approximately -50 mV. Biphasic insulin secretion is an experimental epiphenomenon unrelated to the physiological pulsatile insulin secretion into the portal vein in the human body. Human islets show a wide variety of electrical activities and patterns of [Ca(2+)](i) changes, whose roles in mediating pulsatile secretion of insulin into the portal vein remain questionable. Future studies will hopefully be directed toward a better understanding of Ca(2+) signaling in the human islets in the context of the pathogenesis and treatment of human diabetes.

Exocytotic properties of human pancreatic beta-cells

Pancreatic beta-cells secrete insulin in response to elevated blood glucose via Ca(2+)-dependent fusion of secretory granules with the plasma membrane (regulated exocytosis). While exocytosis has been extensively investigated in rodent beta-cells, studies on human beta-cells are scarce. We have characterized the exocytotic properties of human beta-cells by insulin release measurements, carbon fiber amperometry, and capacitance measurements using the patch-clamp technique. Voltage-clamp depolarizations evoked capacitance increases in single beta-cells in a time- and voltage-dependent manner. The capacitance responses as well as insulin release from intact islets were strongly amplified by elevation of intracellular cAMP levels. Exocytosis was more dependent on Ca(2+) influx through P/Q-type than L-type Ca(2+) channels, reflecting the relative contribution of these channels to the total Ca(2+) current. Exocytosis (as monitored by capacitance or amperometric measurements) decreased during repetitive stimulation as a result of inactivation of Ca(2+) channels as well as depletion of a readily releasable pool of granules. These results reveal both similarities and differences between human and rodent beta-cells.

The bioartificial pancreas: progress and challenges

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