Zinc as a paracrine effector in pancreatic islet cell death

NMDA receptors - regulatory function and pathophysiological significance for pancreatic beta cells

Due to its unique features amongst ionotropic glutamate receptors, the NMDA receptor is of special interest in the physiological context but even more as a drug target. In the pathophysiology of metabolic disorders, particularly type 2 diabetes mellitus, there is evidence that NMDA receptor activation contributes to disease progression by impairing beta cell function. Consequently, channel inhibitors are suggested for treatment, but up to now there are many unanswered questions about the signaling pathways NMDA receptors are interfering with in the islets of Langerhans. In this review we give an overview about channel structure and function with special regard to the pancreatic beta cells and the regulation of insulin secretion. We sum up which signaling pathways from brain research have already been transferred to the beta cell, and what still needs to be proven. The main focus is on the relationship between an over-stimulated NMDA receptor and the production of reactive oxygen species, the amount of which is crucial for beta cell function. Finally, pilot studies using NMDA receptor blockers to protect the islet from dysfunction are reviewed and future perspectives for the use of such compounds in the context of impaired glucose homeostasis are discussed.

ZnT8 loss-of-function accelerates functional maturation of hESC-derived β cells and resists metabolic stress in diabetes

Human embryonic stem cell-derived β cells (SC-β cells) hold great promise for treatment of diabetes, yet how to achieve functional maturation and protect them against metabolic stresses such as glucotoxicity and lipotoxicity remains elusive. Our single-cell RNA-seq analysis reveals that ZnT8 loss of function (LOF) accelerates the functional maturation of SC-β cells. As a result, ZnT8 LOF improves glucose-stimulated insulin secretion (GSIS) by releasing the negative feedback of zinc inhibition on insulin secretion. Furthermore, we demonstrate that ZnT8 LOF mutations endow SC-β cells with resistance to lipotoxicity/glucotoxicity-triggered cell death by alleviating endoplasmic reticulum (ER) stress through modulation of zinc levels. Importantly, transplantation of SC-β cells with ZnT8 LOF into mice with preexisting diabetes significantly improves glycemia restoration and glucose tolerance. These findings highlight the beneficial effect of ZnT8 LOF on the functional maturation and survival of SC-β cells that are useful as a potential source for cell replacement therapies.

Immature function and fragility hinder application of hESC-derived β cells (SC-β cell) for diabetes cell therapy. Here, the authors identify ZnT8 as a gene editing target to enhance the insulin secretion and cell survival under metabolic stress by abolishing zinc transport in SC-β cells.

Lack of ZnT8 protects pancreatic islets from hypoxia- and cytokine-induced cell death

Pancreatic β-cells depend on the well-balanced regulation of cytosolic zinc concentrations, providing sufficient zinc ions for the processing and storage of insulin, but avoiding toxic effects. The zinc transporter ZnT8, encoded by SLC30A8,is a key player regarding islet cell zinc homeostasis, and polymorphisms in this gene are associated with altered type 2 diabetes susceptibility in man. The objective of this study was to investigate the role of ZnT8 and zinc in situations of cellular stress as hypoxia or inflammation. Isolated islets of WT and global ZnT8-/- mice were exposed to hypoxia or cytokines and cell death was measured. To explore the role of changing intracellular Zn2+ concentrations, WT islets were exposed to different zinc concentrations using zinc chloride or the zinc chelator N,N,N',N'-tetrakis(2-pyridinylmethyl)-1,2-ethanediamine (TPEN). Hypoxia or cytokine (TNF-α, IFN-γ, IL1-β) treatment induced islet cell death, but to a lesser extent in islets from ZnT8-/- mice, which were shown to have a reduced zinc content. Similarly, chelation of zinc with TPEN reduced cell death in WT islets treated with hypoxia or cytokines, whereas increased zinc concentrations aggravated the effects of these stressors. This study demonstrates a reduced rate of cell death in islets from ZnT8-/- mice as compared to WT islets when exposed to two distinct cellular stressors, hypoxia or cytotoxic cytokines. This protection from cell death is, in part, mediated by a reduced zinc content in islet cells of ZnT8-/- mice. These findings may be relevant for altered diabetes burden in carriers of risk SLC30A8 alleles in man.

Zn2+-induced changes in Cav2.3 channel function: An electrophysiological and modeling study

Loosely bound Zn²⁺ ions are increasingly recognized as potential modulators of synaptic plasticity and neuronal excitability under normal and pathophysiological conditions. Ca_v2.3 voltage-gated Ca²⁺ channels are among the most sensitive targets of Zn²⁺ and are therefore likely to be involved in the neuromodulatory actions of endogenous Zn²⁺. Although histidine residues on the external side of domain I have been implicated in the effects on Ca_v2.3 channel gating, the exact mechanisms involved in channel modulation remain incompletely understood. Here, we use a combination of electrophysiological recordings, modification of histidine residues, and computational modeling to analyze Zn²⁺-induced changes in Ca_v2.3 channel function. Our most important findings are that multiple high- and low-affinity mechanisms contribute to the net Zn²⁺ action, that Zn²⁺ can either inhibit or stimulate Ca²⁺ influx through Ca_v2.3 channels depending on resting membrane potential, and that Zn²⁺ effects may persist for some time even after cessation of the Zn²⁺ signal. Computer simulations show that (1) most salient features of Ca_v2.3 channel gating in the absence of trace metals can be reproduced by an obligatory model in which activation of two voltage sensors is necessary to open the pore; and (2) most, but not all, of the effects of Zn²⁺ can be accounted for by assuming that Zn²⁺ binding to a first site is associated with an electrostatic modification and mechanical slowing of one of the voltage sensors, whereas Zn²⁺ binding to a second, lower-affinity site blocks the channel and modifies the opening and closing transitions. While still far from complete, our model provides a first quantitative framework for understanding Zn²⁺ effects on Ca_v2.3 channel function and a step toward the application of computational approaches for predicting the complex actions of Zn²⁺ on neuronal excitability.

A new shortened protocol to obtain islet-like cells from hESC-derived ductal cells

Conventional methods for obtaining pancreatic β cells are based on simulating the embryonic development phase of endocrine cells via hierarchical differentiation of pluripotent stem cells (PSCs). Accordingly, we attempted to modify the protocols for obtaining insulin-secreting cells (ISCs) by sequential differentiation of a human embryonic stem cell (hESC), using the HS181 cell line. Furthermore, we hypothesize that actual pancreatic endocrine cells may arise from trans-differentiation of mature ductal cells after the embryonic developmental stage and throughout the rest of life. According to the hypothesis, ductal cells are trans-differentiated into endocrine and exocrine cells, undergoing a partial epithelial to mesenchymal transition (EMT). To address this issue, we developed two new protocols based on hESC differentiation to obtain ductal cells and then induce EMT in cells to obtain hormone-secreting islet-like cells (HSCs). The ductal (pre-EMT exocrine) cells were then induced to undergo partial EMT by treating with Wnt3a and activin A, in hypoxia. The cell derived from the latter method significantly expressed the main endocrine cell-specific markers and also β cells, in particular. These experiments not only support our hypothetical model but also offer a promising approach to develop new methods to compensate β cell depletion in patients with type 1 diabetes mellitus (T1DM). Although this protocol of generating islet-like cells from ductal cells has a potential to treat T1DM, this strategy may be exploited to optimize the function of these cells in an animal model and future clinical applications.

Zinc transporters and their functional integration in mammalian cells

Zinc is a ubiquitous biological metal in all living organisms. The spatiotemporal zinc dynamics in cells provide crucial cellular signaling opportunities, but also challenges for intracellular zinc homeostasis with broad disease implications. Zinc transporters play a central role in regulating cellular zinc balance and subcellular zinc distributions. The discoveries of two complementary families of mammalian zinc transporters (ZnTs and ZIPs) in the mid-1990s spurred much speculation on their metal selectivity and cellular functions. After two decades of research, we have arrived at a biochemical description of zinc transport. However, in vitro functions are fundamentally different from those in living cells, where mammalian zinc transporters are directed to specific subcellular locations, engaged in dedicated macromolecular machineries, and connected with diverse cellular processes. Hence, the molecular functions of individual zinc transporters are reshaped and deeply integrated in cells to promote the utilization of zinc chemistry to perform enzymatic reactions, tune cellular responsiveness to pathophysiologic signals, and safeguard cellular homeostasis. At present, the underlying mechanisms driving the functional integration of mammalian zinc transporters are largely unknown. This knowledge gap has motivated a shift of the research focus from in vitro studies of purified zinc transporters to in cell studies of mammalian zinc transporters in the context of their subcellular locations and protein interactions. In this review, we will outline how knowledge of zinc transporters has been accumulated from in-test-tube to in-cell studies, highlighting new insights and paradigm shifts in our understanding of the molecular and cellular basis of mammalian zinc transporter functions.

Imaging β-Cell Function Using a Zinc-Responsive MRI Contrast Agent May Identify First Responder Islets

An imaging method for detecting β-cell function in real-time in the rodent pancreas could provide new insights into the biological mechanisms involving loss of β-cell function during development of type 2 diabetes and for testing of new drugs designed to modulate insulin secretion. In this study, we used a zinc-responsive MRI contrast agent and an optimized 2D MRI method to show that glucose stimulated insulin and zinc secretion can be detected as functionally active "hot spots" in the tail of the rat pancreas. A comparison of functional images with histological markers show that insulin and zinc secretion does not occur uniformly among all pancreatic islets but rather that some islets respond rapidly to an increase in glucose while others remain silent. Zinc and insulin secretion was shown to be altered in streptozotocin and exenatide treated rats thereby verifying that this simple MRI technique is responsive to changes in β-cell function.

Cav2.3 channel function and Zn2+-induced modulation: potential mechanisms and (patho)physiological relevance

Voltage-gated calcium channels (VGCCs) are critical for Ca²⁺ influx into all types of excitable cells, but their exact function is still poorly understood. Recent reconstruction of homology models for all human VGCCs at atomic resolution provides the opportunity for a structure-based discussion of VGCC function and novel insights into the mechanisms underlying Ca²⁺ selective flux through these channels. In the present review, we use these data as a basis to examine the structure, function, and Zn²⁺-induced modulation of Ca_v2.3 VGCCs, which mediate native R-type currents and belong to the most enigmatic members of the family. Their unique sensitivity to Zn²⁺ and the existence of multiple mechanisms of Zn²⁺ action strongly argue for a role of these channels in the modulatory action of endogenous loosely bound Zn²⁺, pools of which have been detected in a number of neuronal, endocrine, and reproductive tissues. Following a description of the different mechanisms by which Zn²⁺ has been shown or is thought to alter the function of these channels, we discuss their potential (patho)physiological relevance, taking into account what is known about the magnitude and function of extracellular Zn²⁺ signals in different tissues. While still far from complete, the picture that emerges is one where Ca_v2.3 channel expression parallels the occurrence of loosely bound Zn²⁺ pools in different tissues and where these channels may serve to translate physiological Zn²⁺ signals into changes of electrical activity and/or intracellular Ca²⁺ levels.

The efficiency of insulin production and its content in insulin-expressing model β-cells correlate with their Zn2+ levels

Insulin is produced and stored inside the pancreatic β-cell secretory granules, where it is assumed to form Zn2+-stabilized oligomers. However, the actual storage forms of this hormone and the impact of zinc ions on insulin production in vivo are not known. Our initial X-ray fluorescence experiment on granules from native Langerhans islets and insulinoma-derived INS-1E cells revealed a considerable difference in the zinc content. This led our further investigation to evaluate the impact of the intra-granular Zn2+ levels on the production and storage of insulin in different model β-cells. Here, we systematically compared zinc and insulin contents in the permanent INS-1E and BRIN-BD11 β-cells and in the native rat pancreatic islets by flow cytometry, confocal microscopy, immunoblotting, specific messenger RNA (mRNA) and total insulin analysis. These studies revealed an impaired insulin production in the permanent β-cell lines with the diminished intracellular zinc content. The drop in insulin and Zn2+ levels was paralleled by a lower expression of ZnT8 zinc transporter mRNA and hampered proinsulin processing/folding in both permanent cell lines. To summarize, we showed that the disruption of zinc homeostasis in the model β-cells correlated with their impaired insulin and ZnT8 production. This indicates a need for in-depth fundamental research about the role of zinc in insulin production and storage.

The development of two fluorescent chemosensors for the selective detection of Zn2+ and Al3+ ions in a quinoline platform by tuning the substituents in the receptor part: elucidation of the structures of the metal-bound chemosensors and biological studies

Two aminoquinoline-based chemosensors (HL1 and HL2) are reported for selective detection of Zn²⁺ and Al³⁺ ions.

Imaging Insulin Secretion from Mouse Pancreas by MRI Is Improved by Use of a Zinc-Responsive MRI Sensor with Lower Affinity for Zn2+ Ions

It has been demonstrated that divalent zinc ions packaged with insulin in β-cell granules can be detected by MRI during glucose-stimulated insulin secretion using a gadolinium-based Zn²⁺-sensitive agent. This study was designed to evaluate whether a simpler agent design having single Zn²⁺-sensing moieties but with variable Zn²⁺ binding affinities might also detect insulin secretion from the pancreas. Using an implanted MR-compatible window designed to hold the pancreas in a fixed position for imaging, we now demonstrate that focally intense "hot spots" can be detected in the tail of the pancreas using these agents after administration of glucose to stimulate insulin secretion. Histological staining of the same tissue verified that the hot spots identified by imaging correspond to clusters of islets, perhaps reflecting first-responder islets that are most responsive to a sudden increase in glucose. A comparison of images obtained when using a high-affinity Zn²⁺ sensor versus a lower-affinity sensor showed that the lower-affinity sensors produced the best image contrast. An equilibrium model that considers all possible complexes formed between Zn²⁺, the GdL sensor, and HSA predicts that a GdL sensor with lower affinity for Zn²⁺ generates a lower background signal from endogenous Zn²⁺ prior to glucose-stimulated insulin secretion (GSIS) and that the weaker binding affinity agent is more responsive to a further increase in Zn²⁺ concentration near β-cells after GSIS. These model predictions are consistent with the in vivo imaging observations.

Advances in gadolinium-based MRI contrast agent designs for monitoring biological processes in vivo

The gadolinium-based contrast agents widely used in diagnostic MRI exams for 30 years are all small molecule agents that distribute into all extracellular spaces in tissues without providing any specific biological information. Although many 'responsive agent' designs have been presented over the past 20 years or so, none have found use in clinical diagnostic medicine at this point. This review summarizes some recent approaches taken to enhance the sensitivity of such gadolinium-based agents, to target them to specific tissue components, and to create new systems for monitoring specific biological processes.

Age-related islet inflammation marks the proliferative decline of pancreatic beta-cells in zebrafish

The pancreatic islet, a cellular community harboring the insulin-producing beta-cells, is known to undergo age-related alterations. However, only a handful of signals associated with aging have been identified. By comparing beta-cells from younger and older zebrafish, here we show that the aging islets exhibit signs of chronic inflammation. These include recruitment of tnfα-expressing macrophages and the activation of NF-kB signaling in beta-cells. Using a transgenic reporter, we show that NF-kB activity is undetectable in juvenile beta-cells, whereas cells from older fish exhibit heterogeneous NF-kB activity. We link this heterogeneity to differences in gene expression and proliferation. Beta-cells with high NF-kB signaling proliferate significantly less compared to their neighbors with low activity. The NF-kB signaling^hi cells also exhibit premature upregulation of socs2, an age-related gene that inhibits beta-cell proliferation. Together, our results show that NF-kB activity marks the asynchronous decline in beta-cell proliferation with advancing age.

Expression of the ZIP/SLC39A transporters in β-cells: a systematic review and integration of multiple datasets

BACKGROUND

Pancreatic β-cells require a constant supply of zinc to maintain normal insulin secretory function. Following co-exocytosis with insulin, zinc is replenished via the Zrt- and Irt-like (ZIP; SLC39A) family of transporters. However the ZIP paralogues of particular importance for zinc uptake, and associations with β-cell function and Type 2 Diabetes remain largely unexplored. We retrieved and statistically analysed publically available microarray and RNA-seq datasets to perform a systematic review on the expression of β-cell SLC39A paralogues. We complemented results with experimental data on expression profiling of human islets and mouse β-cell derived MIN6 cells, and compared transcriptomic and proteomic sequence conservation between human, mouse and rat.

RESULTS

The 14 ZIP paralogues have 73-98% amino sequence conservation between human and rodents. We identified 18 datasets for β-cell SLC39A analysis, which compared relative expression to non-β-cells, and expression in response to PDX-1 activity, cytokines, glucose and type 2 diabetic status. Published expression data demonstrate enrichment of transcripts for ZIP7 and ZIP9 transporters within rodent β-cells and of ZIP6, ZIP7 and ZIP14 within human β-cells, with ZIP1 most differentially expressed in response to cytokines and PDX-1 within rodent, and ZIP6 in response to diabetic status in human and glucose in rat. Our qPCR expression profiling data indicate that SLC39A6, -9, -13, and - 14 are the highest expressed paralogues in human β-cells and Slc39a6 and -7 in MIN6 cells.

CONCLUSIONS

Our systematic review, expression profiling and sequence alignment reveal similarities and potentially important differences in ZIP complements between human and rodent β-cells. We identify ZIP6, ZIP7, ZIP9, ZIP13 and ZIP14 in human and rodent and ZIP1 in rodent as potentially biologically important for β-cell zinc trafficking. We propose ZIP6 and ZIP7 are key functional orthologues in human and rodent β-cells and highlight these zinc importers as important targets for exploring associations between zinc status and normal physiology of β-cells and their decline in Type 2 Diabetes.

细胞内锌稳态调控及其在胰腺癌发生发展过程中的作用

锌参与300种以上的细胞生理过程, 包括DNA及蛋白合成、酶的活化以及细胞内信号转导过程. 同时锌也是许多酶类, 如碳酸酐酶、基质金属蛋白酶的关键组分, 而这些酶类与缺氧、血管生成、细胞增殖及肿瘤转移密切相关, 因此, 锌的获取对于恶性肿瘤的生长和进展非常重要. 细胞内锌离子的浓度变化受到ZnT1、ZIP4、金属硫蛋白及金属转录因子1等的调控, 细胞内锌稳态是多种调控机制参与下锌内流、锌外流和保留之间动态平衡的结果. 已有大量研究证实锌稳态调控失衡与胰腺癌的发生和转移有关, 因此, 锌及锌稳态的调控异常在胰腺癌的发生发展中具有重要作用.

The Role of Oxidative Stress and Hypoxia in Pancreatic Beta-Cell Dysfunction in Diabetes Mellitus

SIGNIFICANCE

Metabolic syndrome is a frequent precursor of type 2 diabetes mellitus (T2D), a disease that currently affects ∼8% of the adult population worldwide. Pancreatic beta-cell dysfunction and loss are central to the disease process, although understanding of the underlying molecular mechanisms is still fragmentary. Recent Advances: Oversupply of nutrients, including glucose and fatty acids, and the subsequent overstimulation of beta cells, are believed to be an important contributor to insulin secretory failure in T2D. Hypoxia has also recently been implicated in beta-cell damage. Accumulating evidence points to a role for oxidative stress in both processes. Although the production of reactive oxygen species (ROS) results from enhanced mitochondrial respiration during stimulation with glucose and other fuels, the expression of antioxidant defense genes is unusually low (or disallowed) in beta cells.

CRITICAL ISSUES

Not all subjects with metabolic syndrome and hyperglycemia go on to develop full-blown diabetes, implying an important role in disease risk for gene-environment interactions. Possession of common risk alleles at the SLC30A8 locus, encoding the beta-cell granule zinc transporter ZnT8, may affect cytosolic Zn2+ concentrations and thus susceptibility to hypoxia and oxidative stress.

FUTURE DIRECTIONS

Loss of normal beta-cell function, rather than total mass, is increasingly considered to be the major driver for impaired insulin secretion in diabetes. Better understanding of the role of oxidative changes, its modulation by genes involved in disease risk, and effects on beta-cell identity may facilitate the development of new therapeutic strategies to this disease. Antioxid. Redox Signal. 26, 501-518.

hZnT8 (Slc30a8) Transgenic Mice That Overexpress the R325W Polymorph Have Reduced Islet Zn2+ and Proinsulin Levels, Increased Glucose Tolerance After a High-Fat Diet, and Altered Levels of Pancreatic Zinc Binding Proteins

Zinc (Zn²⁺) is involved in both type 1 diabetes (T1DM) and type 2 diabetes (T2DM). The wild-type (WT) form of the β-cell-specific Zn²⁺ transporter, ZNT8, is linked to T2DM susceptibility. ZnT8 null mice have a mild phenotype with a slight decrease in glucose tolerance, whereas patients with the ZnT8 R325W polymorphism (rs13266634) have decreased proinsulin staining and susceptibility to T2DM. We measured Zn²⁺, insulin, and proinsulin stainings and performed intraperitoneal glucose tolerance testing in transgenic mice overexpressing hZnT8 WT or hZnT8 R325W fed a normal or high-fat diet. The hZnT8 R325W transgenic line had lower pancreatic [Zn²⁺]_i and proinsulin and higher insulin and glucose tolerance compared with control littermates after 10 weeks of a high-fat diet in male mice. The converse was true for the hZnT8 WT transgenic line, and dietary Zn²⁺ supplementation also induced glucose intolerance. Finally, pancreatic zinc binding proteins were identified by Zn²⁺-affinity chromatography and proteomics. Increasing pancreatic Zn²⁺ (hZnT8WT) induced nucleoside diphosphate kinase B, and Zn²⁺ reduction (hZnT8RW) induced carboxypeptidase A1. These data suggest that pancreatic Zn²⁺ and proinsulin levels covary but are inversely variant with insulin or glucose tolerance in the HFD model of T2DM suggesting novel therapeutic targets.

Administration of Zinc Chelators Improves Survival of Mice Infected with Aspergillus fumigatus both in Monotherapy and in Combination with Caspofungin

Aspergillus fumigatus can infect immunocompromised patients, leading to high mortality rates due to the lack of reliable treatment options. This pathogen requires uptake of zinc from host tissues in order to successfully grow and cause virulence. Reducing the availability of that micronutrient could help treat A. fumigatus infections. In this study, we examined the in vitro effects of seven chelators using a bioluminescent strain of A. fumigatus 1,10-Phenanthroline and N,N,N',N'-tetrakis(2-pyridylmethyl)ethane-1,2-diamine (TPEN) proved to be the chelators most effective at inhibiting fungal growth. Intraperitoneal administration of either phenanthroline or TPEN resulted in a significant improvement in survival and decrease of weight loss and fungal burden for immunosuppressed mice intranasally infected with A. fumigatus In vitro both chelators had an indifferent effect when employed in combination with caspofungin. The use of TPEN in combination with caspofungin also significantly increased survival compared to that when using these drugs individually. Our results suggest that zinc chelation may be a valid strategy for dealing with A. fumigatus infections and that both phenanthroline and TPEN could potentially be used either independently or in combination with caspofungin, indicating that their use in combination with other antifungal treatments might also be applicable.

Zinc-regulating Proteins, ZnT-1, and Metallothionein I/II Are Present in Different Cell Populations in the Mouse Testis

Zinc ions play an important role in testis development and spermatogenesis. Thus, nutritional zinc deficiency leads to aberrant testicular development, reduced spermatogenesis, and male sterility. The precise actions of zinc in mediating these functions and the mechanisms by which zinc is itself regulated in the testis, however, have not been adequately elucidated. We have assessed the distribution of the zinc-regulating proteins ZnT-1 and metallothionein I/II (MT I/II) in the mouse seminiferous tubule. Colabeling for ZnT-1 and MT I/II demonstrated unique patterns of distribution for these proteins, with ZnT-1 present in Sertoli cells in addition to luminal spermatozoa and MT I/II restricted to spermatocytes. These findings were confirmed by dual-label immunofluorescence for ZnT-1 and the Sertoli cell marker, vimentin, and by immunoelectron microscopy. The differential expression patterns of ZnT-1 and MTs support the hypothesis that ZnT-1 and MTs play different roles in the regulation of intracellular zinc in this organ. The specific expression of ZnT-1 in the Sertoli cells, moreover, is consistent with their role in maintaining a nurturing, closely regulated environment for spermatogenesis.

Zinc-specific Autometallographic In Vivo Selenium Methods: Tracing of Zinc-enriched (ZEN) Terminals, ZEN Pathways, and Pools of Zinc Ions in a Multitude of Other ZEN Cells

In vivo-applied sodium selenide or sodium selenite causes the appearance of zinc-selenium nanocrystals in places where free or loosely bound zinc ions are present. These nanocrystals can in turn be silver enhanced by autometallographic (AMG) development. The selenium method was introduced in 1982 as a tool for zinc-ion tracing, e.g., in vesicular compartments such as synaptic vesicles of zinc-enriched (ZEN) terminals in the central nervous system, and for visualization of zinc ions in ZEN secretory vesicles of, e.g., somatotrophic cells in the pituitary, zymogene granules in pancreatic acinar cells, beta-cells of the islets of Langerhans, Paneth cells of the crypts of Lieberkühn, secretory cells of the tubuloacinar glands of prostate, epithelium of parts of ductus epididymidis, and osteoblasts. If sodium selenide/selenite is injected into brain, spinal cord, spinal nerves containing sympathetic axons, or intraperitoneally, retrograde axonal transport of zinc-selenium nanocrystals takes place in ZEN neurons, resulting in accumulation of zinc-selenium nanocrystals in lysosomes of the neuronal somata. The technique is, therefore, also a highly specific tool for tracing ZEN pathways. The present review includes an update of the 1982 paper and presents evidence that only zinc ions are traced with the AMG selenium techniques if the protocols are followed to the letter.

Imaging trace element distributions in single organelles and subcellular features

The distributions of chemical elements within cells are of prime importance in a wide range of basic and applied biochemical research. An example is the role of the subcellular Zn distribution in Zn homeostasis in insulin producing pancreatic beta cells and the development of type 2 diabetes mellitus. We combined transmission electron microscopy with micro- and nano-synchrotron X-ray fluorescence to image unequivocally for the first time, to the best of our knowledge, the natural elemental distributions, including those of trace elements, in single organelles and other subcellular features. Detected elements include Cl, K, Ca, Co, Ni, Cu, Zn and Cd (which some cells were supplemented with). Cell samples were prepared by a technique that minimally affects the natural elemental concentrations and distributions, and without using fluorescent indicators. It could likely be applied to all cell types and provide new biochemical insights at the single organelle level not available from organelle population level studies.

Amplifying the sensitivity of zinc(II) responsive MRI contrast agents by altering water exchange rates

Given the known water exchange rate limitations of a previously reported Zn(II)-sensitive MRI contrast agent, GdDOTA-diBPEN, new structural targets were rationally designed to increase the rate of water exchange to improve MRI detection sensitivity. These new sensors exhibit fine-tuned water exchange properties and, depending on the individual structure, demonstrate significantly improved longitudinal relaxivities (r1). Two sensors in particular demonstrate optimized parameters and, therefore, show exceptionally high longitudinal relaxivities of about 50 mM(-1) s(-1) upon binding to Zn(II) and human serum albumin (HSA). This value demonstrates a 3-fold increase in r1 compared to that displayed by the original sensor, GdDOTA-diBPEN. In addition, this study provides important insights into the interplay between structural modifications, water exchange rate, and kinetic stability properties of the sensors. The new high relaxivity agents were used to successfully image Zn(II) release from the mouse pancreas in vivo during glucose stimulated insulin secretion.

SLC transporters as therapeutic targets: emerging opportunities

Solute carrier (SLC) transporters - a family of more than 300 membrane-bound proteins that facilitate the transport of a wide array of substrates across biological membranes - have important roles in physiological processes ranging from the cellular uptake of nutrients to the absorption of drugs and other xenobiotics. Several classes of marketed drugs target well-known SLC transporters, such as neurotransmitter transporters, and human genetic studies have provided powerful insight into the roles of more-recently characterized SLC transporters in both rare and common diseases, indicating a wealth of new therapeutic opportunities. This Review summarizes knowledge on the roles of SLC transporters in human disease, describes strategies to target such transporters, and highlights current and investigational drugs that modulate SLC transporters, as well as promising drug targets.

The role of metallothionein-3 in streptozotocin-induced beta-islet cell death and diabetes in mice

Metallothionein-3 (Mt3), a zinc (Zn)-regulatory protein mainly expressed in the central nervous system, may contribute to oxidative cell death. In the present study, we examined the possible role of Mt3 in streptozotocin (STZ)-induced islet cell death and consequent hyperglycemia. Quantitative real-time polymerase chain reaction (RT-PCR) confirmed that islet cells expressed Mt3 mRNA. In all cases, wild-type (WT) mice injected with STZ exhibited hyperglycemia 7-21 days later. In stark contrast, all Mt3-null mice remained normoglycemic following STZ injection. STZ treatment increased free Zn levels in islet cells and induced their death in WT mice, but failed to do so in Mt3-null mice. Consistent with this, cultured Mt3-null islet cells exhibited striking resistance to STZ toxicity. Notably, PDE3a (phosphodiesterase 3A) was downregulated in islets of Mt3-null mice compared to those of WT mice, and was not induced by STZ treatment. Moreover, the PDE3 inhibitor cilostazol reduced islet cell death, likely by increasing cAMP levels, further supporting a role for PDE3 in STZ-induced islet cell death. Collectively, these results demonstrate that Mt3 may act through PDE3a to play a key role in Zn dyshomeostasis and cell death in STZ-treated islets.

Zip4 mediated zinc influx stimulates insulin secretion in pancreatic beta cells

Zinc has an important role in normal pancreatic beta cell physiology as it regulates gene transcription, insulin crystallization and secretion, and cell survival. Nevertheless, little is known about how zinc is transported through the plasma membrane of beta cells and which of the class of zinc influx transporters (Zip) is involved. Zip4 was previously shown to be expressed in human and mouse beta cells; however, its function there is still unknown. Therefore, the aim of this study was to define the zinc transport role of Zip4 in beta cells. To investigate this, Zip4 was over-expressed in MIN6 beta cells using a pCMV6-Zip4GFP plasmid. Organelle staining combined with confocal microscopy showed that Zip4 exhibits a widespread localization in MIN6 cells. Time-lapse zinc imaging experiments showed that Zip4 increases cytoplasmic zinc levels. This resulted in increased granular zinc content and glucose-stimulated insulin secretion. Interestingly, it is unlikely that the increased glucose stimulated insulin secretion was triggered by a modulation of mitochondrial function, as mitochondrial membrane potential remained unchanged. To define the role of Zip4 in-vivo, we generated a beta cell-specific knockout mouse model (Zip4BKO). Deletion of the Zip4 gene was confirmed in Zip4BKO islets by PCR, RT-PCR, and immuno-histochemistry. Zip4BKO mice showed slightly improved glucose homeostasis but no change in insulin secretion during an oral glucose tolerance test. While Zip4 was not found to be essential for proper glucose homeostasis and insulin secretion in vivo in mice, this study also found that Zip4 mediates increases in cytoplasmic and granular zinc pools and stimulates glucose dependant insulin secretion in-vitro.

TRPM2-mediated intracellular Zn2+ release triggers pancreatic β-cell death

Reactive oxygen species (ROS) can cause pancreatic β-cell death by activating transient receptor potential (melastatin) 2 (TRPM2) channels. Cell death has been attributed to the ability of these channels to raise cytosolic Ca2+. Recent studies however revealed that TRPM2 channels can also conduct Zn2+, but the physiological relevance of this property is enigmatic. Given that Zn2+ is cytotoxic, we asked whether TRPM2 channels can permeate sufficient Zn2+ to affect cell viability. To address this, we used the insulin secreting (INS1) β-cell line, human embryonic kidney (HEK)-293 cells transfected with TRPM2 and pancreatic islets. H2O2 activation of TRPM2 channels increases the cytosolic levels of both Ca2+ and Zn2+ and causes apoptotic cell death. Interestingly, chelation of Zn2+ alone was sufficient to prevent β-cell death. The source of the cytotoxic Zn2+ is intracellular, found largely sequestered in lysosomes. Lysosomes express TRPM2 channels, providing a potential route for Zn2+ release. Zn2+ release is potentiated by extracellular Ca2+ entry, indicating that Ca2+-induced Zn2+ release leads to apoptosis. Knockout of TRPM2 channels protects mice from β-cell death and hyperglycaemia induced by multiple low-dose streptozotocin (STZ; MLDS) administration. These results argue that TRPM2-mediated, Ca2+-potentiated Zn2+ release underlies ROS-induced β-cell death and Zn2+, rather than Ca2+, plays a primary role in apoptosis.

SLC30A8 mutations in type 2 diabetes

SLC30A8 encodes the secretory granule-resident and largely endocrine pancreas-restricted zinc transporter ZnT8. Interest in this gene product was sparked amongst diabetologists in 2007 when the first genome-wide association study for type 2 diabetes identified polymorphisms in SLC30A8 as affecting disease risk. Thus, the common polymorphism rs13266634 was associated with lowered beta cell function and a 14% increase in diabetes abundance per risk (C) allele. This non-synonymous variant encodes a tryptophan-to-arginine switch at position 325 in the protein's intracellular carboxy-terminal domain, resulting in reduced zinc transport activity and, consequently, decreased intragranular zinc levels. Whereas insulin secretion from isolated islets is most often increased in mice inactivated for Slc30a8, null animals usually show impaired glucose tolerance and lowered circulating insulin. Since Slc30a8 null animals display little, if any, zinc secretion from islets, the lower plasma insulin levels could be explained by increased hepatic clearance as a result of lowered local zinc levels, or less efficient insulin action on target tissues. Despite the emerging consensus on the role of ZnT8 in glucose homeostasis, a recent genetic study in humans has unexpectedly identified loss-of-function SLC30A8 mutants that are associated with protection from diabetes. Here, we attempt to reconcile these apparently contradictory findings, implicating (1) differing degrees of inhibition of ZnT8 activity in carriers of common variants vs rare loss-of-function forms, (2) effects dependent on age or hypoxic beta cell stress. We propose that these variables conspire to affect both the size and the direction of the effect of SLC30A8 risk alleles in man.

Maternally-derived zinc transporters ZIP6 and ZIP10 drive the mammalian oocyte-to-egg transition

Rapid cellular zinc influx regulates early mammalian development during the oocyte-to-egg transition through modulation of the meiotic cell cycle. Despite the physiological necessity of this zinc influx, the molecular mechanisms that govern such accumulation are unknown. Here we show that the fully grown mammalian oocyte does not employ a transcriptionally based mechanism of zinc regulation involving metal response element-binding transcription factor-1 (MTF-1), as demonstrated by a lack of MTF-1 responsiveness to environmental zinc manipulation. Instead, the mammalian oocyte controls zinc uptake through two maternally derived and cortically distributed zinc transporters, ZIP6 and ZIP10. Targeted disruption of these transporters using several approaches during meiotic maturation perturbs the intracellular zinc quota and results in a cell cycle arrest at a telophase I-like state. This arrest phenocopies established models of zinc insufficiency during the oocyte-to-egg transition, indicating the essential function of these maternally expressed transporters. Labile zinc localizes to punctate cytoplasmic structures in the human oocyte, and ZIP6 and ZIP10 are enriched in the cortex. Altogether, we demonstrate a mechanism of metal regulation required for female gamete development that may be evolutionarily conserved.

Hypoxia lowers SLC30A8/ZnT8 expression and free cytosolic Zn2+ in pancreatic beta cells

AIMS/HYPOTHESIS

Hypoxic damage complicates islet isolation for transplantation and may contribute to beta cell failure in type 2 diabetes. Polymorphisms in the SLC30A8 gene, encoding the secretory granule zinc transporter 8 (ZnT8), influence type 2 diabetes risk, conceivably by modulating cytosolic Zn(2+) levels. We have therefore explored the role of ZnT8 and cytosolic Zn(2+) in the response to hypoxia of pancreatic islet cells.

METHODS

Human, mouse or rat islets were isolated and exposed to varying O2 tensions. Cytosolic free zinc was measured using the adenovirally expressed recombinant targeted zinc probe eCALWY4. Gene expression was measured using quantitative (q)RT-PCR, western (immuno-) blotting or immunocytochemistry. Beta cells were identified by insulin immunoreactivity.

RESULTS

Deprivation of O2 (1% vs 5% or 21%) for 24 h lowered free cytosolic Zn(2+) concentrations by ~40% (p < 0.05) and ~30% (p < 0.05) in mouse and human islet cells, respectively. Hypoxia similarly decreased SLC30A8 mRNA expression in islets, and immunoreactivity in beta cells. Implicating lowered ZnT8 levels in the hypoxia-induced fall in cytosolic Zn(2+), genetic ablation of Slc30a8 from mouse islets lowered cytosolic Zn(2+) by ~40% (p < 0.05) and decreased the induction of metallothionein (Mt1, Mt2) genes. Cell survival in the face of hypoxia was enhanced in small islets of older (>12 weeks) Slc30a8 null mice vs controls, but not younger animals.

CONCLUSIONS/INTERPRETATION

The response of pancreatic beta cells to hypoxia is characterised by decreased SLC30A8 expression and lowered cytosolic Zn(2+) concentrations. The dependence on ZnT8 of hypoxia-induced changes in cell survival may contribute to the actions of SLC30A8 variants on diabetes risk in humans.

Zinc transporters and their role in the pancreatic β-cell

Zinc is an essential nutrient with tremendous importance for human health, and zinc deficiency is a severe risk factor for increased mortality and morbidity. As abnormal zinc homeostasis causes diabetes, and because the pancreatic β-cell contains the highest zinc content of any known cell type, it is of interest to know how zinc fluxes are controlled in β-cells. The understanding of zinc homeostasis has been boosted by the discovery of multiprotein families of zinc transporters, and one of them - zinc transporter 8 (ZnT8) - is abundantly and specifically expressed in the pancreatic islets of Langerhans. In this review, we discuss the evidence for a physiological role of ZnT8 in the formation of zinc-insulin crystals, the physical form in which most insulin is stored in secretory granules. In addition, we cross-examine this information, collected in genetically modified mouse strains, to the knowledge that genetic variants of the human ZnT8 gene predispose to the onset of type 2 diabetes and that epitopes on the ZnT8 protein trigger autoimmunity in patients with type 1 diabetes. The overall conclusion is that we are still at the dawn of a complete understanding of how zinc homeostasis operates in normal β-cells and how abnormalities lead to β-cell dysfunction and diabetes. (J Diabetes Invest, doi: 10.1111/j.2040-1124.2012.00199.x, 2012).

Effects of zinc supplementation and zinc chelation on in vitro β-cell function in INS-1E cells

BACKGROUND

Zinc is essential for the activities of pancreatic β-cells, especially insulin storage and secretion. Insulin secretion leads to co-release of zinc which contributes to the paracrine communication in the pancreatic islets. Zinc-transporting proteins (zinc-regulated transporter, iron-regulated transporter-like proteins [ZIPs] and zinc transporters [ZnTs]) and metal-buffering proteins (metallothioneins, MTs) tightly regulate intracellular zinc homeostasis. The present study investigated how modulation of cellular zinc availability affects β-cell function using INS-1E cells.

RESULTS

Using INS-1E cells, we found that zinc supplementation and zinc chelation had significant effects on insulin content and insulin secretion. Supplemental zinc within the physiological concentration range induced insulin secretion. Insulin content was reduced by zinc chelation with N,N,N',N-tektrakis(2-pyridylmethyl)-ethylenediamine. The changes in intracellular insulin content following exposure to various concentrations of zinc were reflected by changes in the expression patterns of MT-1A, ZnT-8, ZnT-5, and ZnT-3. Furthermore, high zinc concentrations induced cell necrosis while zinc chelation induced apoptosis. Finally, cell proliferation was sensitive to changes in zinc the concentration.

CONCLUSION

These results indicate that the β-cell-like function and survival of INS-1E cells are dependent on the surrounding zinc concentrations. Our results suggest that regulation of zinc homeostasis could represent a pharmacological target.

Structural optimization of Zn(II)-activated magnetic resonance imaging probes

We report the structural optimization and mechanistic investigation of a series of bioactivated magnetic resonance imaging contrast agents that transform from low relaxivity to high relaxivity in the presence of Zn(II). The change in relaxivity results from a structural transformation of the complex that alters the coordination environment about the Gd(III) center. Here, we have performed a series of systematic modifications to determine the structure that provides the optimal change in relaxivity in response to the presence of Zn(II). Relaxivity measurements in the presence and absence of Zn(II) were used in conjunction with measurements regarding water access (namely, number of water molecules bound) to the Gd(III) center and temperature-dependent (13)C NMR spectroscopy to determine how the coordination environment about the Gd(III) center is affected by the distance between the Zn(II)-binding domain and the Gd(III) chelate, the number of functional groups on the Zn(II)-binding domain, and the presence of Zn(II). The results of this study provide valuable insight into the design principles for future bioactivated magnetic resonance probes.

The diabetes-susceptible gene SLC30A8/ZnT8 regulates hepatic insulin clearance

Recent genome-wide association studies demonstrated that common variants of solute carrier family 30 member 8 gene (SLC30A8) increase susceptibility to type 2 diabetes. SLC30A8 encodes zinc transporter-8 (ZnT8), which delivers zinc ion from the cytoplasm into insulin granules. Although it is well known that insulin granules contain high amounts of zinc, the physiological role of secreted zinc remains elusive. In this study, we generated mice with β cell-specific Slc30a8 deficiency (ZnT8KO mice) and demonstrated an unexpected functional linkage between Slc30a8 deletion and hepatic insulin clearance. The ZnT8KO mice had low peripheral blood insulin levels, despite insulin hypersecretion from pancreatic β cells. We also demonstrated that a substantial amount of the hypersecreted insulin was degraded during its first passage through the liver. Consistent with these findings, ZnT8KO mice and human individuals carrying rs13266634, a major risk allele of SLC30A8, exhibited increased insulin clearance, as assessed by c-peptide/insulin ratio. Furthermore, we demonstrated that zinc secreted in concert with insulin suppressed hepatic insulin clearance by inhibiting clathrin-dependent insulin endocytosis. Our results indicate that SLC30A8 regulates hepatic insulin clearance and that genetic dysregulation of this system may play a role in the pathogenesis of type 2 diabetes.

Zinc-dependent lysosomal enlargement in TRPML1-deficient cells involves MTF-1 transcription factor and ZnT4 (Slc30a4) transporter

Zinc is critical for a multitude of cellular processes, including gene expression, secretion and enzymatic activities. Cellular zinc is controlled by zinc-chelating proteins and by zinc transporters. The recent identification of zinc permeability of the lysosomal ion channel TRPML1 (transient receptor potential mucolipin 1), and the evidence of abnormal zinc levels in cells deficient in TRPML1, suggested a role for TRPML1 in zinc transport. In the present study we provide new evidence for such a role and identify additional cellular components responsible for it. In agreement with the previously published data, an acute siRNA (small interfering RNA)-driven TRPML1 KD (knockdown) leads to the build-up of large cytoplasmic vesicles positive for LysoTracker™ and zinc staining, when cells are exposed to high concentrations of zinc. We now show that lysosomal enlargement and zinc build-up in TRPML1-KD cells exposed to zinc are ameliorated by KD of the zinc-sensitive transcription factor MTF-1 (metal-regulatory-element-binding transcription factor-1) or the zinc transporter ZnT4. TRPML1 KD is associated with a build-up of cytoplasmic zinc and with enhanced transcriptional response of mRNA for MT2a (metallothionein 2a). TRPML1 KD did not suppress lysosomal secretion, but it did delay zinc leak from the lysosomes into the cytoplasm. These results underscore a role for TRPML1 in zinc metabolism. Furthermore, they suggest that TRPML1 works in concert with ZnT4 to regulate zinc translocation between the cytoplasm and lysosomes.

Dietary zinc reduction, pyruvate supplementation, or zinc transporter 5 knockout attenuates β-cell death in nonobese diabetic mice, islets, and insulinoma cells

Pancreatic zinc (Zn(2+)) concentrations are linked to diabetes and pancreatic dysfunction, but Zn(2+) is also required for insulin processing and packaging. Zn(2+) released with insulin increases β-cell pancreatic death after streptozotocin toxin exposure in vitro and in vivo. Triosephosphate accumulation, caused by NAD(+) loss and glycolytic enzyme dysfunction, occur in type-1 diabetics (T1DM) and animal models. We previously showed these mechanisms are also involved in Zn(2+) neurotoxicity and are attenuated by nicotinamide- or pyruvate-induced restoration of NAD(+) concentrations, Zn(2+) restriction, or inhibition of Sir2 proteins. We tested the hypothesis that similar Zn(2+)- and NAD(+)-mediated mechanisms are involved in β-cell toxicity in models of ongoing T1DM using mouse insulinoma cells, islets, and nonobese diabetic (NOD) mice. Zn(2+), streptozotocin, and cytokines caused NAD(+) loss and death in insulinoma cells and islets, which were attenuated by Zn(2+) restriction, pyruvate, nicotinamide, NAD(+), and inhibitors of Sir2 proteins. We measured diabetes incidence and mortality in NOD mice and demonstrated that pyruvate supplementation, or genetic or dietary Zn(2+) reduction, attenuated these measures. T-lymphocyte infiltration, punctate Zn(2+) staining, and β-cell loss increased with time in islets of NOD mice. Dietary Zn(2+) restriction or Zn(2+) transporter 5 knockout reduced pancreatic Zn(2+) staining and increased β-cell mass, glucose homeostasis, and survival in NOD mice, whereas Zn(2+) supplementation had the opposite effects. Pancreatic Zn(2+) reduction or NAD(+) restoration (pyruvate or nicotinamide supplementation) are suggested as novel targets for attenuating T1DM.

Imaging free zinc levels in vivo - what can be learned?

Our ever-expanding knowledge about the role of zinc in biology includes its role in redox modulation, immune response, neurotransmission, reproduction, diabetes, cancer, and Alzheimers disease is galvanizing interest in detecting and monitoring the various forms of Zn(II) in biological systems. This paper reviews reported strategies for detecting and tracking of labile or "free" unchelated Zn(II) in tissues. While different bound structural forms of Zn(II) have been identified and studied in vitro by multiple techniques, very few molecular imaging methods have successfully tracked the ion in vivo. A number of MRI and optical strategies have now been reported for detection of free Zn(II) in cells and tissues but only a few have been applied successfully in vivo. A recent report of a MRI sensor for in vivo tracking of Zn(II) released from pancreatic β-cells during insulin secretion exemplifies the promise of rational design of new Zn(II) sensors for tracking this biologically important ion in vivo. Such studies promise to provide new insights into zinc trafficking in vivo and the critical role of this ion in many human diseases.

Involvement of SIRT1 in Zn2+, Streptozotocin, Non-Obese Diabetic, and Cytokine-Mediated Toxicities of β-cells

Zn2+ toxicity is implicated in pancreatic β-cell death that occurs secondarily to: streptozotocin exposure in vitro; and both autoimmune attack or streptozotocin in vivo models of T1DM. This is demonstrated by reduced β-cell death or diabetic incidence in vitro or in NOD mice after treatment with Zn2+ preferring chelators, pyruvate, nicotinamide, a reduced zinc diet, sirtuin inhibitors, or zinc transporter knockout. These therapeutics are also demonstrated to be efficacious against Zn2+ neurotoxicity.

AIMS

To determine if the sirtuin pathway is involved in Zn2+-, streptozotocin-, or cytokine-mediated β-cell death in vitro, and streptozotocin-, or NOD induced T1DM in vivo.

METHODS

Sensitivity of MIN6 cells expressing empty vector, sirtuin protein-1 (SIRT1) or its siRNA, to Zn2+, streptozotocin, or cytokines, and effects on NAD+ levels were determined. Covariance of manipulating SIRT1 levels with diabetic incidence was tested in vivo.

RESULTS

1) sirtuin pathway inhibition or SIRT1 knockdown attenuated Zn2+-, STZ-, and cytokine-mediated toxicity and NAD+ loss in β-cells, 2) SIRT1 overexpression potentiated these toxicities, 3) young SIRT1 β-cell transgenic mice have improved glucose tolerance under basal conditions, but upon aging showed increased sensitivity to streptozotocin compared to SIRT1 +/- mice, and 4) SIRT1 +/- mice in an NOD background or exposed to streptozotocin trended toward reduced diabetic incidence and mortality compared to wildtype.

CONCLUSIONS

These results have implicated SIRT1-mediated NAD+ loss in Zn2+, STZ, or cytokine toxicities of MIN6, and in NOD or streptozotocin T1DM animal models. Modulation of β-cell Zn2+ and NAD+ levels, and the sirtuin pathway could be novel therapeutic targets for T1DM.

Effects of high-fat diet feeding on Znt8-null mice: differences between β-cell and global knockout of Znt8

Genomewide association studies have linked a polymorphism in the zinc transporter 8 (Znt8) gene to higher risk of developing type 2 diabetes. Znt8 is highly expressed in pancreatic β-cells where it is involved in the regulation of zinc transport into granules. However, Znt8 is also expressed in other tissues including α-cells, where its function is as yet unknown. Previous work demonstrated that mice lacking Znt8 globally were more susceptible to diet-induced obesity (Lemaire et al., Proc Natl Acad Sci USA 106: 14872-14877, 2009; Nicolson et al., Diabetes 58: 2070-2083, 2009). Therefore, the main goal of this study was to examine the physiological impact of β-cell-specific Znt8 deficiency in mice during high-fat high-calorie (HFHC) diet feeding. For these studies, we used β-cell-specific Znt8 knockout (Ins2Cre:Znt8loxP/loxP) and whole body Znt8 knockout (Cre-:Znt8(-/-)) mice placed on a HFHC diet for 16 wk. Ins2Cre:Znt8loxP/loxP mice on HFHC diet had similar body weights throughout the study but displayed impaired insulin biosynthesis and secretion and were glucose intolerant compared with littermate control Ins2Cre mice. In contrast, Cre-:Znt8(-/-) mice became remarkably obese, hyperglycemic, hyperinsulinemic, insulin resistant, and glucose intolerant compared with littermate control Cre- mice. These data show that β-cell Znt8 alone does not considerably aggravate weight gain and glucose intolerance during metabolic stress imposed by an HFHC diet. However, global loss of Znt8 is involved in exacerbating diet-induced obesity and resulting insulin resistance, and this may be due to the loss of Znt8 activity in a tissue other than the β-cell. Thus, our data suggest that Znt8 contributes to the risk of developing type 2 diabetes through β-cell- and non-β-cell-specific effects.

Noninvasive MRI of β-cell function using a Zn2+-responsive contrast agent

Elevation of postprandial glucose stimulates release of insulin from granules stored in pancreatic islet β-cells. We demonstrate here that divalent zinc ions coreleased with insulin from β-cells in response to high glucose are readily detected by MRI using the Zn(2+)-responsive T(1) agent, GdDOTA-diBPEN. Image contrast was significantly enhanced in the mouse pancreas after injection of a bolus of glucose followed by a low dose of the Zn(2+) sensor. Images of the pancreas were not enhanced by the agent in mice without addition of glucose to stimulate insulin release, nor were images enhanced in streptozotocin-treated mice with or without added glucose. These observations are consistent with MRI detection of Zn(2+) released from β-cells only during glucose-stimulated insulin secretion. Images of mice fed a high-fat (60%) diet over a 12-wk period and subjected to this same imaging protocol showed a larger volume of contrast-enhanced pancreatic tissue, consistent with the expansion of pancreatic β-cell mass during fat accumulation and progression to type 2 diabetes. This MRI sensor offers the exciting potential for deep-tissue monitoring of β-cell function in vivo during development of type 2 diabetes or after implantation of islets in type I diabetic patients.

Glucose regulates free cytosolic Zn²⁺ concentration, Slc39 (ZiP), and metallothionein gene expression in primary pancreatic islet β-cells

Zn²⁺ is an important cofactor for insulin biosynthesis and storage in pancreatic β-cells. Correspondingly, polymorphisms in the SLC30A8 gene, encoding the secretory granule Zn²⁺ transporter ZnT8, are associated with type 2 diabetes risk. Using a genetically engineered (FRET)-based sensor (eCALWY-4), we show here that elevated glucose time-dependently increases free cytosolic Zn²⁺ ([Zn²⁺](cyt)) in mouse pancreatic β-cells. These changes become highly significant (853 ± 96 pm versus 452 ± 42 pm, p < 0.001) after 24 h and are associated with increased expression of the Zn²⁺ importer family members Slc39a6, Slc39a7, and Slc39a8, and decreased expression of metallothionein 1 and 2. Arguing that altered expression of the above genes is not due to altered [Zn²⁺](cyt), elevation of extracellular (and intracellular) [Zn²⁺] failed to mimic the effects of high glucose. By contrast, increases in intracellular cAMP prompted by 3-isobutyl-1-methylxanthine and forskolin partially mimicked the effects of glucose on metallothionein, although not ZiP, gene expression. Modulation of intracellular Ca²⁺ and insulin secretion with pharmacological agents (tolbutamide and diazoxide) suggested a possible role for changes in these parameters in the regulation of Slc39a6 and Slc39a7 but not Slc39a8, nor metallothionein expression. In summary, 1) glucose induces increases in [Zn²⁺](cyt), which are then likely to facilitate the processing and/or the storage of insulin and its cocrystallization with Zn²⁺, and 2) these increases are associated with elevated expression of zinc importers. Conversely, a chronic increase in [Zn²⁺](cyt) following sustained hyperglycemia may contribute to β-cell dysfunction and death in some forms of diabetes.

A two-site mechanism for the inhibition of IAPP amyloidogenesis by zinc

Human islet amyloid polypeptide (hIAPP) is a highly amyloidogenic protein co-secreted with insulin in response to glucose levels. The formation of hIAPP amyloid plaques near islet cells has been linked to the death of insulin-secreting β-cells in humans and the progression of type II diabetes. Since both healthy individuals and those with type II diabetes produce and secrete hIAPP, it is reasonable to look for factors involved in storing hIAPP and preventing amyloidosis. We have previously shown that zinc inhibits the formation of insoluble amyloid plaques of hIAPP; however, there remains significant ambiguity in the underlying mechanisms. In this study, we show that zinc binds unaggregated hIAPP at micromolar concentrations similar to those found in the extracellular environment. By contrast, the fibrillar amyloid form of hIAPP has low affinity for zinc. The binding stoichiometry obtained from isothermal titration calorimetry experiments indicates that zinc favors the formation of hIAPP hexamers. High-resolution NMR structures of hIAPP bound to zinc reveal changes in the electron environment along residues that would be located along one face of the amphipathic hIAPP α-helix proposed as an intermediate for amyloid formation. Results from electrospray ionization mass spectroscopy investigations showed that a single zinc atom is predominantly bound to hIAPP and revealed that zinc inhibits the formation of the dimer. At higher concentrations of zinc, a second zinc atom binds to hIAPP, suggesting the presence of a low-affinity secondary binding site. Combined, these results suggest that zinc promotes the formation of oligomers while creating an energetic barrier for the formation of amyloid fibers.

Zinc in specialized secretory tissues: roles in the pancreas, prostate, and mammary gland

Zinc (Zn) is an essential micronutrient required for over 300 different cellular processes, including DNA and protein synthesis, enzyme activity, and intracellular signaling. Cellular Zn homeostasis necessitates the compartmentalization of Zn into intracellular organelles, which is tightly regulated through the integration of Zn transporting mechanisms. The pancreas, prostate, and mammary gland are secretory tissues that have unusual Zn requirements and thus must tightly regulate Zn metabolism through integrating Zn import, sequestration, and export mechanisms. Recent findings indicate that these tissues utilize Zn for basic cellular processes but also require Zn for unique cellular needs. In addition, abundant Zn is transported into the secretory pathway and a large amount is subsequently secreted in a tightly regulated manner for unique biological processes. Expression of numerous members of the SLC30A (ZnT) and SLC39A (Zip) gene families has been documented in these tissues, yet there is limited understanding of their precise functional role in Zn metabolism or their regulation. Impairments in Zn secretion from the pancreas, prostate, and mammary gland are associated with disorders such as diabetes, infertility, and cancer, respectively. In this review, we will provide a brief summary of the specific role of Zn in each tissue and describe our current knowledge regarding how Zn metabolism is regulated. Finally, in each instance, we will reflect upon how this information shapes our current understanding of the role of Zn in these secretory tissues with respect to human health and disease.

Zinc and redox signaling: perturbations associated with cardiovascular disease and diabetes mellitus

Cellular signal transduction pathways are influenced by the zinc and redox status of the cell. Numerous chronic diseases, including cardiovascular disease (CVD) and diabetes mellitus (DM), have been associated with impaired zinc utilization and increased oxidative stress. In humans, mutations in the MT-1A and ZnT8 genes, both of which are involved in the maintenance of zinc homeostasis, have been linked with DM development. Changes in levels of intracellular free zinc may exacerbate oxidative stress in CVD and DM by impacting glutathione homeostasis, nitric oxide signaling, and nuclear factor-kappa B-dependent cellular processes. Zinc ions have been shown to influence insulin and leptin signaling via the phosphoinositide 3′-kinase/Akt pathway, potentially linking an imbalance of zinc at the cellular level to insulin resistance and dyslipidemia. The oxidative modification of cysteine residues in zinc coordination sites in proteins has been implicated in cellular signaling and regulatory pathways. Despite the many interactions between zinc and cellular stress responses, studies investigating the potential therapeutic benefit of zinc supplementation in the prevention and treatment of oxidative stress-related chronic disease in humans are few and inconsistent. Further well-designed randomized controlled trials are needed to determine the effects of zinc supplementation in populations at various stages of CVD and DM progression.

Zinc, zinc transporters and diabetes

The role of zinc in islet function has recently achieved new attention as a consequence of the identification of zinc transporter 8 (ZNT8) in islets, and the association of mutations in the gene for this zinc transporter with glucose intolerance and type 2 diabetes. ZNT8 is also an autoantigen associated with the appearance of type 1 diabetes. A number of experimental models have been employed to suggest how ZNT8 and other zinc transporters regulate beta cell insulin processing and possibly secretion. An additional role for the zinc transporters in regulating alpha cell function has been suggested. In this issue of Diabetologia, Wijesekara and colleagues, using a cell-specific Znt8 (also known as Slc30a8) knockout model, demonstrate that beta cell insulin processing and glucose tolerance is negatively affected after beta cell knock out of Znt8, whereas Znt8 knockout in alpha cells seems to have little effect on glucagon secretion or glucose tolerance. Although we are yet to see the therapeutic potential of these new findings, the area represents a field through which manipulation of islet function may eventually be possible.

The content of elements in rainwater and its relation to the frequency of hospitalization for diabetes and obesity in Opole Voivodship, Poland, during 2000-2002

Environmental factors significantly influence the incidence and course of metabolic syndrome diseases such as diabetes and obesity. The content of elements in rainwater is an indirect indicator of their presence in dust suspended in the air. In this paper we present the relationships between the content of selected elements in rainwater and hospitalization frequencies due to diabetes (E10-E13) and obesity (E66). It was assumed that the hospitalization frequency could be taken as a measure of deterioration of the metabolic process in the course of diabetes and its complications. The observations concerned the population of Opole Voivodeship, Poland (one million inhabitants), distributed in small communities of 44,000 to 151,000 inhabitants during the years 2000-2002. In cases of diabetes E10-E13 for all subjects relevant correlation indicators were found for chromium (r = 0.71), cadmium (r = 0.65), and lead (r = 0.66). Borderline relevance was seen for copper (r = 0.57) and zinc (r = 056). For diabetic men the statistically relevant correlations were chromium (r = 0.79), lead (r = 0.77), cadmium (r = 0.74), copper (r = 0.70), chloride (r = 0.69), zinc (r = 0.68), and iron (r = 0.64). For women the only relevant correlations were chromium (r = 0.62) and cadmium (r = 0.55). No significant correlations were found in obese individuals of both sexes.

Acute cytokine-mediated downregulation of the zinc transporter ZnT8 alters pancreatic beta-cell function

Genetic studies suggest that Zn transporters such as ZnT8 play a role in insulin secretion by pancreatic beta-cells; however, little is known about the dynamic roles of Zn trafficking pathways on beta-cell physiology. To test the acute effects of the inflammatory cytokines interleukin 1 beta (IL1 beta) and tumor necrosis factor alpha (TNFalpha) on Zn homeostasis, the mRNA expression profile of Zn transporters of the ZnT and ZIP families was examined. Exposure of MIN6 cells or primary murine islets to IL1 beta or TNFalpha altered the mRNA expression profile of Zn transporters; most notable was decreased ZnT8 mRNA levels. siRNA-mediated gene knockdown was used to examine the effects of decreased ZnT8 expression in primary dispersed murine islet cells from C57/BL6 mice and MIN6 cells. ZnT8 knockdown in these murine islets led to reduced glucose stimulated insulin secretion without altering the total cellular insulin content or cell viability at normal or supraphysiological Zn concentrations. The labile Zn content determined by flow cytometry after loading with the Zn-specific sensor FluoZin-3 AM was decreased in MIN6 cells following ZnT8 knockdown or IL1 beta treatment. These results suggest that an acute decrease in ZnT8 levels impairs beta-cell function and Zn homeostasis, and may contribute to inflammatory cytokine-induced alterations in beta-cell function.

Increased level of exogenous zinc induces cytotoxicity and up-regulates the expression of the ZnT-1 zinc transporter gene in pancreatic cancer cells

A balance between zinc uptake by ZIP (SLC39) and efflux of zinc from the cytoplasm into subcellular organelles and out of the cell by ZnT (SLC30) transporters is crucial for zinc homeostasis. It is not clear whether normal and cancerous pancreatic cells respond differently to increased extracellular zinc concentrations. Use of flow cytometry-based methods revealed that treatment with as little as 0.01 mM zinc induced significant cytotoxicity in two human ductal adenocarcinoma cell lines. In contrast, normal human pancreatic islet cells tolerated as high as 0.5 mM zinc. Insulinoma cell lines of mouse and rat origin also succumbed to high concentrations of zinc. Exposure to elevated zinc concentrations enhanced the numbers of carcinoma but not primary islet cells staining with the cell-permeable zinc-specific fluorescent dye, FluoZin-3, indicating increased zinc influx in transformed cells. Mitochondrial membrane depolarization, superoxide generation, decreased antioxidant thiols, intracellular acidosis and activation of intracellular caspases characterized zinc-induced carcinoma cell death. Only the antioxidant glutathione but not inhibitors of enzymes implicated in apoptosis or necrosis prevented zinc-induced cytotoxicity in insulinoma cells. Immunoblotting revealed that zinc treatment increased the ubiquitination of proteins in cancer cells. Importantly, zinc treatment up-regulated the expression of ZnT-1 gene in a rat insulinoma cell line and in two human ductal adenocarcinoma cell lines. These results indicate that the exposure of pancreatic cancer cells to elevated extracellular zinc concentrations can lead to cytotoxic cell death characterized by increased protein ubiquitination and up-regulation of the zinc transporter ZnT-1 gene expression.