Molecular Genetic Regulation of Slc30a8/ZnT8 Reveals a Positive Association With Glucose Tolerance

Characterizing the effects of Dechlorane Plus on β-cells: a comparative study across models and species

Epidemiological studies consistently link environmental toxicant exposure with increased Type 2 diabetes risk. Our study investigated the diabetogenic effects of a widely used flame retardant, Dechlorane Plus (DP), on pancreatic β-cells using rodent and human model systems. We first examined pancreas tissues from male mice exposed daily to oral gavage of either vehicle (corn oil) or DP (10, 100, or 1000 μg/kg per day) and fed chow or high fat diet for 28-days in vivo. DP exposure did not affect islet size or endocrine cell composition in either diet group. Next, we assessed the effect of 48-hour exposure to vehicle (DMSO) or DP (1, 10, or 100 nM) in vitro using immortalized rat β-cells (INS-1 832/3), primary mouse and human islets, and human stem-cell derived islet-like cells (SC-islets). In INS-1 832/3 cells, DP did not impact glucose-stimulated insulin secretion (GSIS) but significantly decreased intracellular insulin content. DP had no effect on GSIS in mouse islets or SC-islets but had variable effects on GSIS in human islets depending on the donor. DP alone did not affect insulin content in mouse islets, human islets, or SC-islets, but mouse islets co-exposed to DP and glucolipotoxic (GLT) stress conditions (28.7 mM glucose + 0.5 mM palmitate) had reduced insulin content compared to control conditions. Co-exposure of mouse islets to DP + GLT amplified the upregulation of Slc30a8 compared to GLT alone. Our study highlights the importance and challenges of using different in vitro models for studying chemical toxicity.

Metabolic basis of solute carrier transporters in treatment of type 2 diabetes mellitus

Solute carriers (SLCs) constitute the largest superfamily of membrane transporter proteins. These transporters, present in various SLC families, play a vital role in energy metabolism by facilitating the transport of diverse substances, including glucose, fatty acids, amino acids, nucleotides, and ions. They actively participate in the regulation of glucose metabolism at various steps, such as glucose uptake (e.g., SLC2A4/GLUT4), glucose reabsorption (e.g., SLC5A2/SGLT2), thermogenesis (e.g., SLC25A7/UCP-1), and ATP production (e.g., SLC25A4/ANT1 and SLC25A5/ANT2). The activities of these transporters contribute to the pathogenesis of type 2 diabetes mellitus (T2DM). Notably, SLC5A2 has emerged as a valid drug target for T2DM due to its role in renal glucose reabsorption, leading to groundbreaking advancements in diabetes drug discovery. Alongside SLC5A2, multiple families of SLC transporters involved in the regulation of glucose homeostasis hold potential applications for T2DM therapy. SLCs also impact drug metabolism of diabetic medicines through gene polymorphisms, such as rosiglitazone (SLCO1B1/OATP1B1) and metformin (SLC22A1-3/OCT1-3 and SLC47A1, 2/MATE1, 2). By consolidating insights into the biological activities and clinical relevance of SLC transporters in T2DM, this review offers a comprehensive update on their roles in controlling glucose metabolism as potential drug targets.

From zinc homeostasis to disease progression: Unveiling the neurodegenerative puzzle

Zinc is a crucial trace element in the human body, playing a role in various physiological processes such as oxidative stress, neurotransmission, protein synthesis, and DNA repair. The zinc transporters (ZnTs) family members are responsible for exporting intracellular zinc, while Zrt- and Irt-like proteins (ZIPs) are involved in importing extracellular zinc. These processes are essential for maintaining cellular zinc homeostasis. Imbalances in zinc metabolism have been linked to the development of neurodegenerative diseases. Disruptions in zinc levels can impact the survival and activity of neurons, thereby contributing to the progression of neurodegenerative diseases through mechanisms like cell apoptosis regulation, protein phase separation, ferroptosis, oxidative stress, and neuroinflammation. Therefore, conducting a systematic review of the regulatory network of zinc and investigating the relationship between zinc dysmetabolism and neurodegenerative diseases can enhance our understanding of the pathogenesis of these diseases. Additionally, it may offer new insights and approaches for the treatment of neurodegenerative diseases.

Relationship Between the Single Nucleotide Polymorphism rs11558471 in the SLC30A8/ZnT8 Gene and Cardiometabolic Markers in Postmenopausal Women

Postmenopausal women have more risk factors for metabolic syndrome, and genetic alterations in SLC30A8 (zinc transporter 8 [ZnT8]) are directly related to these factors. Our aim was to assess the relationship of the single nucleotide polymorphism (SNP) rs11558471 in the SLC30A8/ZnT8 gene with cardiometabolic markers in postmenopausal women. This cross-sectional study included 53 postmenopausal women divided into two groups according to the SNP genotype (AG + GG [n = 25] and AA [n = 28]). Anthropometric, dietary, and biochemical (glycemic, lipidic, hepatic, renal, and hormonal markers) variables were evaluated and compared between groups. No differences in glycemic, hepatic, renal, and hormonal markers were found between groups. However, the group with the polymorphic allele (AG + GG) had a better lipid profile than non-carriers (total cholesterol, p = 0.041; low-density lipoprotein cholesterol [LDL-c], p = 0.035; non-high-density lipoprotein cholesterol [non-HDL-c], p = 0.043). Logistic regression showed that the group with polymorphic allele had lower chances of increasing levels of LDL-c (odds ratio [OR] = 0.225, p = 0.012) and non-HDL-c (OR = 0.316, p = 0.045). After adjusting for age, body mass index, physical activity, and use of diabetes and dyslipidemia drugs, only LDL-c remained associated (OR = 0.218; p = 0.017). The variant allele of SNP rs11558471 in the SLC30A8 gene was associated with better LDL-c levels, which helps reduce the risks for cardiovascular diseases in postmenopausal women.

ZnT8 Loss of Function Mutation Increases Resistance of Human Embryonic Stem Cell-Derived Beta Cells to Apoptosis in Low Zinc Condition

The rare SLC30A8 mutation encoding a truncating p.Arg138* variant (R138X) in zinc transporter 8 (ZnT8) is associated with a 65% reduced risk for type 2 diabetes. To determine whether ZnT8 is required for beta cell development and function, we derived human pluripotent stem cells carrying the R138X mutation and differentiated them into insulin-producing cells. We found that human pluripotent stem cells with homozygous or heterozygous R138X mutation and the null (KO) mutation have normal efficiency of differentiation towards insulin-producing cells, but these cells show diffuse granules that lack crystalline zinc-containing insulin granules. Insulin secretion is not compromised in vitro by KO or R138X mutations in human embryonic stem cell-derived beta cells (sc-beta cells). Likewise, the ability of sc-beta cells to secrete insulin and maintain glucose homeostasis after transplantation into mice was comparable across different genotypes. Interestingly, sc-beta cells with the SLC30A8 KO mutation showed increased cytoplasmic zinc, and cells with either KO or R138X mutation were resistant to apoptosis when extracellular zinc was limiting. These findings are consistent with a protective role of zinc in cell death and with the protective role of zinc in T2D.

Imaging zinc trafficking in vivo by positron emission tomography with zinc-62

Non-invasive imaging techniques to dynamically map whole-body trafficking of essential metals in vivo in health and diseases are needed. Despite 62Zn having appropriate physical properties for positron emission tomography (PET) imaging (half-life, 9.3 h; positron emission, 8.2%), its complex decay via 62Cu (half-life, 10 min; positron emission, 97%) has limited its use. We aimed to develop a method to extract 62Zn from a 62Zn/62Cu generator, and to investigate its use for in vivo imaging of zinc trafficking despite its complex decay. 62Zn prepared by proton irradiation of natural copper foil was used to construct a conventional 62Zn/62Cu generator. 62Zn was eluted using trisodium citrate and used for biological experiments, compared with 64Cu in similar buffer. PET/CT imaging and ex vivo tissue radioactivity measurements were performed following intravenous injection in healthy mice. [62Zn]Zn-citrate was readily eluted from the generator with citrate buffer. PET imaging with the eluate demonstrated biodistribution similar to previous observations with the shorter-lived 63Zn (half-life 38.5 min), with significant differences compared to [64Cu]Cu-citrate, notably in pancreas (>10-fold higher at 1 h post-injection). Between 4 and 24 h, 62Zn retention in liver, pancreas, and kidney declined over time, while brain uptake increased. Like 64Cu, 62Zn showed hepatobiliary excretion from liver to intestines, unaffected by fasting. Although it offers limited reliability of scanning before 1 h post-injection, 62Zn-PET allows investigation of zinc trafficking in vivo for >24 h and hence provides a useful new tool to investigate diseases where zinc homeostasis is disrupted in preclinical models and humans.

Glucose-Dependent miR-125b Is a Negative Regulator of β-Cell Function

Impaired pancreatic β-cell function and insulin secretion are hallmarks of type 2 diabetes. miRNAs are short, noncoding RNAs that silence gene expression vital for the development and function of β cells. We have previously shown that β cell-specific deletion of the important energy sensor AMP-activated protein kinase (AMPK) results in increased miR-125b-5p levels. Nevertheless, the function of this miRNA in β cells is unclear. We hypothesized that miR-125b-5p expression is regulated by glucose and that this miRNA mediates some of the deleterious effects of hyperglycemia in β cells. Here, we show that islet miR-125b-5p expression is upregulated by glucose in an AMPK-dependent manner and that short-term miR-125b-5p overexpression impairs glucose-stimulated insulin secretion (GSIS) in the mouse insulinoma MIN6 cells and in human islets. An unbiased, high-throughput screen in MIN6 cells identified multiple miR-125b-5p targets, including the transporter of lysosomal hydrolases M6pr and the mitochondrial fission regulator Mtfp1. Inactivation of miR-125b-5p in the human β-cell line EndoCβ-H1 shortened mitochondria and enhanced GSIS, whereas mice overexpressing miR-125b-5p selectively in β cells (MIR125B-Tg) were hyperglycemic and glucose intolerant. MIR125B-Tg β cells contained enlarged lysosomal structures and had reduced insulin content and secretion. Collectively, we identify miR-125b as a glucose-controlled regulator of organelle dynamics that modulates insulin secretion.

Non-invasive radionuclide imaging of trace metal trafficking in health and disease: "PET metallomics"

Several specific metallic elements must be present in the human body to maintain health and function. Maintaining the correct quantity (from trace to bulk) and location at the cell and tissue level is essential. The study of the biological role of metals has become known as metallomics. While quantities of metals in cells and tissues can be readily measured in biopsy and autopsy samples by destructive analytical techniques, their trafficking and its role in health and disease are poorly understood. Molecular imaging with radionuclides - positron emission tomography (PET) and single photon emission computed tomography (SPECT) - is emerging as a means to non-invasively study the acute trafficking of essential metals between organs, non-invasively and in real time, in health and disease. PET scanners are increasingly widely available in hospitals, and methods for producing radionuclides of some of the key essential metals are developing fast. This review summarises recent developments in radionuclide imaging technology that permit such investigations, describes the radiological and physicochemical properties of key radioisotopes of essential trace metals and useful analogues, and introduces current and potential future applications in preclinical and clinical investigations to study the biology of essential trace metals in health and disease.

Cell Networks in Endocrine/Neuroendocrine Gland Function

Reproduction, growth, stress, and metabolism are determined by endocrine/neuroendocrine systems that regulate circulating hormone concentrations. All these systems generate rhythms and changes in hormone pulsatility observed in a variety of pathophysiological states. Thus, the output of endocrine/neuroendocrine systems must be regulated within a narrow window of effective hormone concentrations but must also maintain a capacity for plasticity to respond to changing physiological demands. Remarkably most endocrinologists still have a "textbook" view of endocrine gland organization which has emanated from 20^th century histological studies on thin 2D tissue sections. However, 21^st -century technological advances, including in-depth 3D imaging of specific cell types have vastly changed our knowledge. We now know that various levels of multicellular organization can be found across different glands, that organizational motifs can vary between species and can be modified to enhance or decrease hormonal release. This article focuses on how the organization of cells regulates hormone output using three endocrine/neuroendocrine glands that present different levels of organization and complexity: the adrenal medulla, with a single neuroendocrine cell type; the anterior pituitary, with multiple intermingled cell types; and the pancreas with multiple intermingled cell types organized into distinct functional units. We give an overview of recent methodologies that allow the study of the different components within endocrine systems, particularly their temporal and spatial relationships. We believe the emerging findings about network organization, and its impact on hormone secretion, are crucial to understanding how homeostatic regulation of endocrine axes is carried out within endocrine organs themselves. © 2022 American Physiological Society. Compr Physiol 12:3371-3415, 2022.

The Oxidative Balance Orchestrates the Main Keystones of the Functional Activity of Cardiomyocytes

This review is aimed at providing an overview of the key hallmarks of cardiomyocytes in physiological and pathological conditions. The main feature of cardiac tissue is the force generation through contraction. This process requires a conspicuous energy demand and therefore an active metabolism. The cardiac tissue is rich of mitochondria, the powerhouses in cells. These organelles, producing ATP, are also the main sources of ROS whose altered handling can cause their accumulation and therefore triggers detrimental effects on mitochondria themselves and other cell components thus leading to apoptosis and cardiac diseases. This review highlights the metabolic aspects of cardiomyocytes and wanders through the main systems of these cells: (a) the unique structural organization (such as different protein complexes represented by contractile, regulatory, and structural proteins); (b) the homeostasis of intracellular Ca²⁺ that represents a crucial ion for cardiac functions and E-C coupling; and (c) the balance of Zn²⁺, an ion with a crucial impact on the cardiovascular system. Although each system seems to be independent and finely controlled, the contractile proteins, intracellular Ca²⁺ homeostasis, and intracellular Zn²⁺ signals are strongly linked to each other by the intracellular ROS management in a fascinating way to form a "functional tetrad" which ensures the proper functioning of the myocardium. Nevertheless, if ROS balance is not properly handled, one or more of these components could be altered resulting in deleterious effects leading to an unbalance of this "tetrad" and promoting cardiovascular diseases. In conclusion, this "functional tetrad" is proposed as a complex network that communicates continuously in the cardiomyocytes and can drive the switch from physiological to pathological conditions in the heart.

A Family of [Zn6] Complexes from the Carboxylate-Bridge-Supported Assembly of [Zn2] Building Units: Synthetic, Structural, Spectroscopic, and Systematic Biological Studies

The three discrete [Zn₆] complexes [Na₃Zn₆(cpdp)₃(μ-Bz)₃(CH₃OH)₆][ZnCl₄][ZnCl₃(H₂O)]·3CH₃OH·1.5H₂O (1), [Na₃Zn₆(cpdp)₃(μ-p-OBz)₃(CH₃OH)₆]·2H₂O (2), and [Na₃Zn₆(cpdp)₃(μ-p-NO₂Bz)₃(CH₃OH)₆]Cl₃·2H₂O (3), supported by the carboxylate-based multidentate ligand N,N'-bis[2-carboxybenzomethyl]-N,N'-bis[2-pyridylmethyl]-1,3-diaminopropan-2-ol (H₃cpdp), have been successfully synthesized and fully characterized (Bz = benzoate; p-OBz = dianion of p-hydroxybenzoic acid; p-NO₂Bz = p-nitrobenzoate). The complexes have been characterized by elemental analysis, FTIR, UV-vis, NMR spectroscopy, PXRD, and thermal analysis, including single-crystal X-ray crystallography of 1 and 2. The molecular architectures of 1-3 are built from the self-assembly of their corresponding [Zn₂] units, which are interconnected to the central [Na₃(CH₃OH)₆]³⁺ core by six endogenous benzoate groups, with each linking one Zn(II) and one Na(I) ion in a μ₂:η¹:η¹-syn-anti bidentate fashion. The composition of the (cpdp^3-)₃/(Zn²⁺)₆ complexes in 1-3 has been observed to be 1:2, on the basis of the UV-vis titration and NMR spectroscopic results, which is further supported by X-ray crystallography. Systematic biological studies performed with a mice model suggested possible antidiabetic efficacy as well as anticancer activities of the complexes. When complexes 1-3 were administered intraperitoneally in mice, 1 showed a lowering in the blood glucose level, overall maintenance of the pancreatic tissue mass, restriction of DNA damage in pancreatic cells, and retention of lipid droplet (LD) frequency, whereas 2 and 3 showed hepatic tissue mass consistency by inhibiting the DNA damage in hepatic cells, prior to the exposure to a potent diabetic inducer, alloxan (ALX). Similar trends of results were observed in inhibiting the generation of reactive oxygen species (ROS) in the pancreatic and hepatic cells, as examined by spectrofluorometric methods. Thus, 1 seems to be a better compound for overall diabetic management and control, whereas 2 and 3 seem to be promising compounds for designing chemopreventive drugs against hepatic carcinoma.

Zinc transporter 8 (ZnT8) autoantibody prevalence in black South African participants with type 1 diabetes

BACKGROUND

Autoantibodies to β-cell specific antigens are markers of type 1 diabetes. The most recently identified autoantibodies are targeted to the zinc transporter 8 (ZnT8) protein located in the membrane of β-cell insulin secretory granules. The prevalence of ZnT8 autoantibodies in newly diagnosed participants with type 1 diabetes has been found to range from 33 to 80 %. Due to the lack of data on the immunological aetiology of type 1 diabetes in African populations, this study aimed to determine the prevalence of ZnT8 autoantibodies in black South Africans with type 1 diabetes and whether ZnT8 autoantibody positivity was associated with age at diagnosis and disease duration.

METHODS

Participants with type 1 diabetes and controls were recruited from the greater Johannesburg area, South Africa. Positivity for ZnT8, GAD65 and IA2 autoantibodies was determined by ELISA.

RESULTS

Participants with type 1 diabetes (n = 183) and controls (n = 49) were matched for age (29.1 ± 9.53 vs. 27.3 ± 7.29, respectively; p = 0.248). The mean age at diagnosis for participants with type 1 diabetes was 20.8 ± 8.46 years. The prevalence of ZnT8 autoantibody positivity was 17.5 % (32 of 183) in participants with type 1 diabetes with a median disease duration of 7.00 [2.00; 11.0] years. ZnT8 autoantibody prevalence in newly diagnosed participants (< 1 year duration) was 27.3 % (6 of 22). Logistic regression analysis found an association between ZnT8 autoantibody positivity and shorter disease duration (OR: 0.9 (0.81-1.00); p = 0.042). In addition, ZnT8 autoantibody positivity was significantly associated with an increased chance of being GAD65 (OR: 3.37 (1.10-10.3)) and IA2 (OR: 8.63 (2.82-26.4)) autoantibody positive. Multiple regression analysis found no association between ZnT8 autoantibody positivity and age at diagnosis. However, the presence of ≥ 2 autoantibodies was associated with a younger age at diagnosis of type 1 diabetes when compared to participants with ≤ 1 autoantibody (B = -5.270; p = 0.002).

CONCLUSIONS

The presence of ZnT8 autoantibodies was not related to a younger age at diagnosis in black South African patients with type 1 diabetes. However, the greater the numbers of autoantibodies present in an individual the earlier the age at diagnosis. ZnT8 autoantibodies decline with disease duration in the black South African population.

Pancreatic β cell-selective zinc transporter 8 insufficiency accelerates diabetes associated with islet amyloidosis

GWAS have shown that the common R325W variant of SLC30A8 (ZnT8) increases the risk of type 2 diabetes (T2D). However, ZnT8 haploinsufficiency is protective against T2D in humans, counterintuitive to earlier work in humans and mouse models. Therefore, whether decreasing ZnT8 activity is beneficial or detrimental to β cell function, especially under conditions of metabolic stress, remains unknown. In order to examine whether the existence of human islet amyloid polypeptide (hIAPP), a coresident of the insulin granule, affects the role of ZnT8 in regulating β cell function, hIAPP-expressing transgenics were generated with reduced ZnT8 (ZnT8B^+/– hIAPP) or null ZnT8 (ZnT8B^–/– hIAPP) expression specifically in β cells. We showed that ZnT8B^–/– hIAPP mice on a high-fat diet had intensified amyloid deposition and further impaired glucose tolerance and insulin secretion compared with control, ZnT8B^–/–, and hIAPP mice. This can in part be attributed to impaired glucose sensing and islet cell synchronicity. Importantly, ZnT8B^+/– hIAPP mice were also glucose intolerant and had reduced insulin secretion and increased amyloid aggregation compared with controls. These data suggest that loss of or reduced ZnT8 activity in β cells heightened the toxicity induced by hIAPP, leading to impaired β cell function and glucose homeostasis associated with metabolic stress.

Loss of Znt8 function in diabetes mellitus: risk or benefit?

The zinc transporter 8 (ZnT8) plays an essential role in zinc homeostasis inside pancreatic β cells, its function is related to the stabilization of insulin hexameric form. Genome-wide association studies (GWAS) have established a positive and negative relationship of ZnT8 variants with type 2 diabetes mellitus (T2DM), exposing a dual and controversial role. The first hypotheses about its role in T2DM indicated a higher risk of developing T2DM for loss of function; nevertheless, recent GWAS of ZnT8 loss-of-function mutations in humans have shown protection against T2DM. With regard to the ZnT8 role in T2DM, most studies have focused on rodent models and common high-risk variants; however, considerable differences between human and rodent models have been found and the new approaches have included lower-frequency variants as a tool to clarify gene functions, allowing a better understanding of the disease and offering possible therapeutic targets. Therefore, this review will discuss the physiological effects of the ZnT8 variants associated with a major and lower risk of T2DM, emphasizing the low- and rare-frequency variants.

In Vivo ZIMIR Imaging of Mouse Pancreatic Islet Cells Shows Oscillatory Insulin Secretion

Appropriate insulin secretion is essential for maintaining euglycemia, and impairment or loss of insulin release represents a causal event leading to diabetes. There have been extensive efforts of studying insulin secretion and its regulation using a variety of biological preparations, yet it remains challenging to monitor the dynamics of insulin secretion at the cellular level in the intact pancreas of living animals, where islet cells are supplied with physiological blood circulation and oxygenation, nerve innervation, and tissue support of surrounding exocrine cells. Herein we presented our pilot efforts of ZIMIR imaging in pancreatic islet cells in a living mouse. The imaging tracked insulin/Zn²⁺ release of individual islet β-cells in the intact pancreas with high spatiotemporal resolution, revealing a rhythmic secretion activity that appeared to be synchronized among islet β-cells. To facilitate probe delivery to islet cells, we also developed a chemogenetic approach by expressing the HaloTag protein on the cell surface. Finally, we demonstrated the application of a fluorescent granule zinc indicator, ZIGIR, as a selective and efficient islet cell marker in living animals through systemic delivery. We expect future optimization and integration of these approaches would enable longitudinal tracking of beta cell mass and function in vivo by optical imaging.

Restoration of mRNA Expression of Solute Carrier Proteins in Liver of Diet-Induced Obese Mice by Metformin

Metformin (MET), the most common medicine for type 2 diabetes (T2DM), improves insulin sensitivity by targeting the liver, intestine and other organs. Its impact on expression of the solute carrier (Slc) transporter genes have not been reported in the mechanism of insulin sensitization. In this study, we examined Slc gene expression in the liver and colon of diet-induced obese (DIO) mice treated with MET by transcriptomic analysis. There were 939 differentially expressed genes (DEGs) in the liver of DIO mice vs lean mice, which included 34 Slc genes. MET altered 489 DEGs in the liver of DIO mice, in which 23 were Slc genes. Expression of 20 MET-responsive Slc DEGs was confirmed by qRT-PCR, in which 15 Slc genes were altered in DIO mice and their expressions were restored by MET, including Slc2a10, Slc2a13, Slc5a9, Slc6a14, Slc7a9, Slc9a2, Slc9a3, Slc13a2, Slc15a2, Slc26a3, Slc34a2, Slc37a1, Slc44a4, Slc51b and Slc52a3. While, there were only 97 DEGs in the colon of DIO mice with 5 Slc genes, whose expression was not restored by MET. The data suggest that more genes were altered in the liver over the colon by the high fat diet (HFD). There were 20 Slc genes with alteration confirmed in the liver of DIO mice and 15 of them were restored by MET, which was associated with improvement of insulin sensitivity and obesity. The restoration may improve the uptake of glucose, amino acids, mannose, fructose, 1,5-anhydro-D-glucitol and bumetanide in hepatocytes of the liver of DIO mice. The study provides new insight into the mechanism of metformin action in insulin sensitization and obesity.

SNPs in the 3'-untranslated region of SLC30A8 confer risk of type 2 diabetes mellitus in a south-east Iranian population: Evidences from case-control and bioinformatics studies

BACKGROUND

Type 2 diabetes mellitus (T2DM) is a heterogenic disease with increasing incidence. The SLC30A8 gene encodes an islet zinc transporter (ZnT8), and its variants have been associated with glucose and pro-insulin levels. This study was aimed to examine the effects of a missense variant (rs13266634 C/T), and two 3'UTR variants (rs2466294 C/G and rs2466293 T/C) in SLC30A8 gene on T2DM risk in a south-east Iranian population.

METHODS

In this experiment, 450 patients diagnosed with T2DM and 453 healthy subjects from the same geographic area were enrolled. Genotypes were amplified using the ARMS-PCR method. In silico analyses were performed to determine the effects of the variants on the local structure of mRNA, splicing patterns, and potential miRNA-gene interactions as well.

RESULTS

Significant differences were noticed between cases and controls regarding the genotypic and allelic distribution of the studied variants. As regards rs2466293 and rs2466294 variants, enhanced risk of T2DM was found under allelic, dominant, recessive, and codominant models (OR > 1). Besides, different genetic models of rs13266634 were associated with decreased risk of T2DM (OR < 1). Bioinformatics analyses indicated that the rs2466293 variant might influence the binding of some miRNAs, while the G-allele of rs2466294 decreased the stability of SLC30A8-mRNA.

CONCLUSIONS

In our population, both SNPs in the 3'-untranslated region of SLC30A8 increased the risk of T2DM, while the rs13266634 variant showed a protective association against T2DM susceptibility. Investigating the effects of other variants in this gene or other ZnTs can further indicate such associations in subjects from the same ethnicity.

Functional Genomics in Pancreatic β Cells: Recent Advances in Gene Deletion and Genome Editing Technologies for Diabetes Research

The inheritance of variants that lead to coding changes in, or the mis-expression of, genes critical to pancreatic beta cell function can lead to alterations in insulin secretion and increase the risk of both type 1 and type 2 diabetes. Recently developed clustered regularly interspaced short palindromic repeats (CRISPR/Cas9) gene editing tools provide a powerful means of understanding the impact of identified variants on cell function, growth, and survival and might ultimately provide a means, most likely after the transplantation of genetically "corrected" cells, of treating the disease. Here, we review some of the disease-associated genes and variants whose roles have been probed up to now. Next, we survey recent exciting developments in CRISPR/Cas9 technology and their possible exploitation for β cell functional genomics. Finally, we will provide a perspective as to how CRISPR/Cas9 technology may find clinical application in patients with diabetes.

Solute Carrier Transporters as Potential Targets for the Treatment of Metabolic Disease

The solute carrier (SLC) superfamily comprises more than 400 transport proteins mediating the influx and efflux of substances such as ions, nucleotides, and sugars across biological membranes. Over 80 SLC transporters have been linked to human diseases, including obesity and type 2 diabetes (T2D). This observation highlights the importance of SLCs for human (patho)physiology. Yet, only a small number of SLC proteins are validated drug targets. The most recent drug class approved for the treatment of T2D targets sodium-glucose cotransporter 2, product of the SLC5A2 gene. There is great interest in identifying other SLC transporters as potential targets for the treatment of metabolic diseases. Finding better treatments will prove essential in future years, given the enormous personal and socioeconomic burden posed by more than 500 million patients with T2D by 2040 worldwide. In this review, we summarize the evidence for SLC transporters as target structures in metabolic disease. To this end, we identified SLC13A5/sodium-coupled citrate transporter, and recent proof-of-concept studies confirm its therapeutic potential in T2D and nonalcoholic fatty liver disease. Further SLC transporters were linked in multiple genome-wide association studies to T2D or related metabolic disorders. In addition to presenting better-characterized potential therapeutic targets, we discuss the likely unnoticed link between other SLC transporters and metabolic disease. Recognition of their potential may promote research on these proteins for future medical management of human metabolic diseases such as obesity, fatty liver disease, and T2D. SIGNIFICANCE STATEMENT: Given the fact that the prevalence of human metabolic diseases such as obesity and type 2 diabetes has dramatically risen, pharmacological intervention will be a key future approach to managing their burden and reducing mortality. In this review, we present the evidence for solute carrier (SLC) genes associated with human metabolic diseases and discuss the potential of SLC transporters as therapeutic target structures.

Effects of ZnT8 on epithelial-to-mesenchymal transition and tubulointerstitial fibrosis in diabetic kidney disease

Zinc transporter 8 (ZnT8) transports zinc ions for crystallization and storage of insulin in pancreatic beta-cells and ZnT8 dysfunction is involved in pathogenesis of diabetes. The current study aimed to investigate whether ZnT8 has effects in pathophysiology of diabetic kidney disease (DKD) by using animal models for diabetes, including STZ-induced diabetic, db/db, ZnT8-KO, ZnT8-KO-STZ and ZnT8-KO-db/db mice. Results demonstrated that urine albumin to creatinine ratio and epithelial-to-mesenchymal transition (EMT) were increased in kidneys of ZnT8-KO-STZ and ZnT8-KO-db/db mice compared with C57BL/6 J and ZnT8-KO mice, while serum TGF-β1, IL-6, and TNF-α levels were elevated in parallel. In kidneys of mice intercrossed between ZnT8-KO and STZ-induced diabetic or db/db mice, these three inflammatory factors, ACR and EMT were also found to be increased compared with C57BL/6J, db/db and ZnT8-KO mice. Furthermore, ZnT8 up-regulation by hZnT8-EGFP reduced the levels of high glucose (HG)-induced EMT and inflammatory factors in normal rat kidney tubular epithelial cell (NRK-52E cells). Expression of phosphorylated Smad2/Smad3 was up-regulated after HG stimulation and further enhanced by ZnT8 siRNA but down-regulated after hZnT8-EGFP gene transfection. The current study thus provides the first evidence that ZnT8 protects against EMT-tubulointerstitial fibrosis though the restrain of TGF-β1/Smads signaling activation in DKD.

The SLC transporter in nutrient and metabolic sensing, regulation, and drug development

The prevalence of metabolic diseases is growing worldwide. Accumulating evidence suggests that solute carrier (SLC) transporters contribute to the etiology of various metabolic diseases. Consistent with metabolic characteristics, the top five organs in which SLC transporters are highly expressed are the kidney, brain, liver, gut, and heart. We aim to understand the molecular mechanisms of important SLC transporter-mediated physiological processes and their potentials as drug targets. SLC transporters serve as ‘metabolic gate’ of cells and mediate the transport of a wide range of essential nutrients and metabolites such as glucose, amino acids, vitamins, neurotransmitters, and inorganic/metal ions. Gene-modified animal models have demonstrated that SLC transporters participate in many important physiological functions including nutrient supply, metabolic transformation, energy homeostasis, tissue development, oxidative stress, host defense, and neurological regulation. Furthermore, the human genomic studies have identified that SLC transporters are susceptible or causative genes in various diseases like cancer, metabolic disease, cardiovascular disease, immunological disorders, and neurological dysfunction. Importantly, a number of SLC transporters have been successfully targeted for drug developments. This review will focus on the current understanding of SLCs in regulating physiology, nutrient sensing and uptake, and risk of diseases.

The type 2 diabetes gene product STARD10 is a phosphoinositide-binding protein that controls insulin secretory granule biogenesis

OBJECTIVE

Risk alleles for type 2 diabetes at the STARD10 locus are associated with lowered STARD10 expression in the β-cell, impaired glucose-induced insulin secretion, and decreased circulating proinsulin:insulin ratios. Although likely to serve as a mediator of intracellular lipid transfer, the identity of the transported lipids and thus the pathways through which STARD10 regulates β-cell function are not understood. The aim of this study was to identify the lipids transported and affected by STARD10 in the β-cell and the role of the protein in controlling proinsulin processing and insulin granule biogenesis and maturation.

METHODS

We used isolated islets from mice deleted selectively in the β-cell for Stard10 (βStard10KO) and performed electron microscopy, pulse-chase, RNA sequencing, and lipidomic analyses. Proteomic analysis of STARD10 binding partners was executed in the INS1 (832/13) cell line. X-ray crystallography followed by molecular docking and lipid overlay assay was performed on purified STARD10 protein.

RESULTS

βStard10KO islets had a sharply altered dense core granule appearance, with a dramatic increase in the number of "rod-like" dense cores. Correspondingly, basal secretion of proinsulin was increased versus wild-type islets. The solution of the crystal structure of STARD10 to 2.3 Å resolution revealed a binding pocket capable of accommodating polyphosphoinositides, and STARD10 was shown to bind to inositides phosphorylated at the 3' position. Lipidomic analysis of βStard10KO islets demonstrated changes in phosphatidylinositol levels, and the inositol lipid kinase PIP4K2C was identified as a STARD10 binding partner. Also consistent with roles for STARD10 in phosphoinositide signalling, the phosphoinositide-binding proteins Pirt and Synaptotagmin 1 were amongst the differentially expressed genes in βStard10KO islets.

CONCLUSION

Our data indicate that STARD10 binds to, and may transport, phosphatidylinositides, influencing membrane lipid composition, insulin granule biosynthesis, and insulin processing.

Multiomic Profiling Identifies cis-Regulatory Networks Underlying Human Pancreatic β Cell Identity and Function

EndoC-βH1 is emerging as a critical human β cell model to study the genetic and environmental etiologies of β cell (dys)function and diabetes. Comprehensive knowledge of its molecular landscape is lacking, yet required, for effective use of this model. Here, we report chromosomal (spectral karyotyping), genetic (genotyping), epigenomic (ChIP-seq and ATAC-seq), chromatin interaction (Hi-C and Pol2 ChIA-PET), and transcriptomic (RNA-seq and miRNA-seq) maps of EndoC-βH1. Analyses of these maps define known (e.g., PDX1 and ISL1) and putative (e.g., PCSK1 and mir-375) β cell-specific transcriptional cis-regulatory networks and identify allelic effects on cis-regulatory element use. Importantly, comparison with maps generated in primary human islets and/or β cells indicates preservation of chromatin looping but also highlights chromosomal aberrations and fetal genomic signatures in EndoC-βH1. Together, these maps, and a web application we created for their exploration, provide important tools for the design of experiments to probe and manipulate the genetic programs governing β cell identity and (dys)function in diabetes.

EndoC-βH1 is becoming an important cellular model to study genes and pathways governing human β cell identity and function, but its (epi)genomic similarity to primary human islets is unknown. Lawlor et al. complete and compare extensive EndoC and primary human islet multiomic maps to identify shared and distinct genomic circuitry.

Insights into pancreatic islet cell dysfunction from type 2 diabetes mellitus genetics

Type 2 diabetes mellitus (T2DM) is an increasingly prevalent multifactorial disease that has both genetic and environmental risk factors, resulting in impaired glucose homeostasis. Genome-wide association studies (GWAS) have identified over 400 genetic signals that are associated with altered risk of T2DM. Human physiology and epigenomic data support a central role for the pancreatic islet in the pathogenesis of T2DM. This Review focuses on the promises and challenges of moving from genetic associations to molecular mechanisms and highlights efforts to identify the causal variant and effector transcripts at T2DM GWAS susceptibility loci. In addition, we examine current human models that are used to study both β-cell development and function, including EndoC-β cell lines and human induced pluripotent stem cell-derived β-like cells. We use examples of four T2DM susceptibility loci (CDKAL1, MTNR1B, SLC30A8 and PAM) to emphasize how a holistic approach involving genetics, physiology, and cellular and developmental biology can disentangle disease mechanisms at T2DM GWAS signals.

A comprehensive review on zinc(II) complexes as anti-diabetic agents: The advances, scientific gaps and prospects

Zinc has gained notable attention in the development of potent anti-diabetic agents, due to its role in insulin storage and secretion, as well as its reported insulin mimetic properties. Consequently, zinc(II) has been complexed with numerous organic ligands as an adjuvant to develop anti-diabetic agents with improved and/or broader scope of pharmacological properties. This review focuses on the research advances thus far to identify the major scientific gaps and prospects. Peer-reviewed published data on the anti-diabetic effects of zinc(II) complexes were sourced from different scientific search engines, including, but not limited to "PubMed", "Google Scholar", "Scopus" and ScienceDirect to identify potent anti-diabetic zinc(II) complexes. The complexes were subcategorized according to their precursor ligands. A critical analysis of the outcomes from published studies shows promising leads, with Zn(II) complexes having a "tri-facet" mode of exerting pharmacological effects. However, the promising leads have been flawed by some major scientific gaps. While zinc(II) complexes of synthetic ligands with little or no anti-diabetic pharmacological history remain the most studied (about 72 %), their toxicity profile was not reported, which raises safety concerns for clinical relevance. The zinc(II) complexes of plant polyphenols; natural ligands, such as maltol and hinokitiol; and supplements, such as ascorbic acid (a natural antioxidant), l-threonine and l-carnitine, showed promising insulin mimetic and glycemic control properties but remain understudied and lack clinical validation, in spite of their minimal safety concerns and health benefits. A paradigm shift toward probing (including clinical studies) supplements, plant polyphenol and natural ligands as anti-diabetic zinc(II) complex is, therefore, recommended. Also, promising anti-diabetic Zn(II) complexes of synthetic ligands should undergo critical toxicity evaluation to address possible safety concerns.

Loss of ZnT8 function protects against diabetes by enhanced insulin secretion

A rare loss-of-function allele p.Arg138* in SLC30A8 encoding the zinc transporter 8 (ZnT8), which is enriched in Western Finland, protects against type 2 diabetes (T2D). We recruited relatives of the identified carriers and showed that protection was associated with better insulin secretion due to enhanced glucose responsiveness and proinsulin conversion, particularly when compared with individuals matched for the genotype of a common T2D-risk allele in SLC30A8, p.Arg325. In genome-edited human induced pluripotent stem cell (iPSC)-derived β-like cells, we establish that the p.Arg138* allele results in reduced SLC30A8 expression due to haploinsufficiency. In human β cells, loss of SLC30A8 leads to increased glucose responsiveness and reduced KATP channel function similar to isolated islets from carriers of the T2D-protective allele p.Trp325. These data position ZnT8 as an appealing target for treatment aimed at maintaining insulin secretion capacity in T2D.

The Role of Genetic Variant rs13266634 in SLC30A8/ZnT8 in Post-Operative Hyperglycemia after Major Abdominal Surgery

CONTEXT

Following major surgery, post-operative hyperglycemia (POHG) is associated with suboptimal outcomes, among diabetics and non-diabetics. A specific genetic variant, rs13266634 (c.973C>T; p.ARG325TRP) in zinc transporter SLC30A8/ZnT8, is associated with protection against Type-2 Diabetes, suggesting it may be actionable for predicting and preventing POHG.

OBJECTIVE

To determine independent and mediated influences of a genetic variant on POHG in patients undergoing a model major operation, complex abdominal ventral hernia repair (cVHR).

PATIENTS AND METHODS

For 110 patients (mean BMI 34.9±5.8, T2D history 28%) undergoing cVHR at a tertiary referral center (January 2012 to March 2017), multivariate regression was used to correlate the rs13266634 variant to pre-operative clinical, laboratory and imaging-based indices of liver steatosis and central abdominal adiposity to POHG. Causal Mediation Analysis (CMA) was used to determine direct and mediated contributions of SLC30A8/ZnT8 status to POHG.

RESULTS

Variant rs13266634 was present in 61 patients (55.4%). In univariate models, when compared to patients with rs13266634, the homozygous wild-genotype (C/C, n=49) was associated with significantly higher risks of POHG (OR= 0.30 95%CI =0.14, 0.67, P=0.0038). Multivariate regression indicated that the association was independent (OR= 0.39 95%CI 0.15-0.97, p=0.040). In addition, CMA suggested that rs13266634 protects against POHG directly and indirectly through its influence on liver steatosis and central adiposity.

CONCLUSIONS

In medically complex patients undergoing major operations, the rs13266634 variant protects against POHG and its associated outcomes, through independent and mediated contributions. In C/C patients undergoing major operations, SLC30A8/ZnT8 may prove useful to stratify risk of POHG and potentially as a therapeutic target.

Zinc and its regulators in pancreas

Studies have demonstrated that susceptibility to type 2 diabetes (T2D) is influenced by common polymorphism in the zinc transporter 8 gene SLC30A8, providing novel insight into the role of zinc in diabetes. Intriguingly, zinc participates in every step of the process, including insulin synthesis, crystallization, storage, secretion and signaling. Zinc deficiency or overload is associated with various disorders, such as diabetes, cardiovascular disease and obesity. Zinc supplementation is considered as an effective means of treating or preventing T2D in people with certain SLC30A8 genotypes. Three important protein families-zinc transporters (ZnTs), zinc importers (ZiPs) and metallothionein (MT)-participate in maintaining zinc homeostasis. Here, we review research on the physiological characteristics of zinc and its role in the pancreas and homeostasis regulation mechanisms, along with the latest research on the structure and function of ZnT/ZiP and MT. In addition, we summarize the advancements in research on SLC30A8 gene polymorphism in search of a mechanism to explain the relationship between the R risk allele and zinc transporter activity.

Sensors for measuring subcellular zinc pools

Zinc homeostasis is essential for normal cellular function, and defects in this process are associated with a number of diseases including type 2 diabetes (T2D), neurological disorders and cardiovascular disease. Thus, variants in the SLC30A8 gene, encoding the vesicular/granular zinc transporter ZnT8, are associated with altered insulin release and increased T2D risk while the zinc importer ZIP12 is implicated in pulmonary hypertension. In light of these, and findings in other diseases, recent efforts have focused on the development of refined sensors for intracellular free zinc ions that can be targeted to subcellular regions including the cytosol, endoplasmic reticulum (ER), secretory granules, Golgi apparatus, nucleus and the mitochondria. Here, we discuss recent advances in Zn²⁺ probe engineering and their applications to the measurement of labile subcellular zinc pools in different cell types.

Genetic, Functional, and Immunological Study of ZnT8 in Diabetes

Zinc level in the body is finely regulated to maintain cellular function. Dysregulation of zinc metabolism may induce a variety of diseases, e.g., diabetes. Zinc participates in insulin synthesis, storage, and secretion by functioning as a "cellular second messenger" in the insulin signaling pathway and glucose homeostasis. The highest zinc concentration is in the pancreas islets. Zinc accumulation in cell granules is manipulated by ZnT8, a zinc transporter expressed predominately in pancreatic α and β cells. A common ZnT8 gene (SLC30A8) polymorphism increases the risk of type 2 diabetes mellitus (T2DM), and rare mutations may present protective effects. In type 1 diabetes mellitus (T1DM), autoantibodies show specificity for binding two variants of ZnT8 (R or W at amino acid 325) dictated by a polymorphism in SLC30A8. In this review, we summarize the structure, feature, functions, and polymorphisms of ZnT8 along with its association with diabetes and explore future study directions.

ZnT8 Arg325Trp polymorphism influences zinc transporter expression and cytokine production in PBMCs from patients with diabetes

AIMS

ZnT8 Arg325Trp polymorphism has been associated with type 2 diabetes (T2DM) susceptibility. The Arg-325 risk variant shows accelerated zinc (Zn) transport kinetic and reduced glucose-stimulated insulin secretion in pancreatic cells. However, it remains unexplored the role of Znt8 polymorphism in the regulation of Zn homeostasis and inflammatory response in peripheral blood mononuclear cells (PBMCs) from T2DM patients.

METHODS AND RESULTS

A total of 556 healthy controls and 413 T2DM patients were genotyped for ZnT8 Arg325Trp polymorphism confirming the association of Arg-325 variant with an increased T2DM risk (OR = 1.35 95% C.I: 1.10-1.66; p = 0.0044). Moreover, PBMCs from Arg/Arg T2DM subjects showed increased intracellular free Zn, higher gene expression of Metallothioneins, Znt1, Znt8, Zip2 genes, and reduced Znt4 and Znt7. Higher release of IL-1α, IL-1β, IFN-γ, IL-12p70 and TNF-α and a reduced IL-10 secretion after lipopolysaccharide (LPS) stimulation were observed in PBMCs from Arg/Arg T2DM carriers as compared to subjects with the Trp variant.

CONCLUSIONS

Our data provide evidence of a substantial different Zn homeostasis regulation between Znt8 Arg-325 and Trp-325 carriers in PBMCs from T2DM patients. Moreover, Znt8 Arg-325 risk variant shows an enhanced inflammatory response upon LPS stimulation that might aggravate insulin resistance and the progression of diabetes cardiovascular complications.

Discovering human diabetes-risk gene function with genetics and physiological assays

Developing systems to identify the cell type-specific functions regulated by genes linked to type 2 diabetes (T2D) risk could transform our understanding of the genetic basis of this disease. However, in vivo systems for efficiently discovering T2D risk gene functions relevant to human cells are currently lacking. Here we describe powerful interdisciplinary approaches combining Drosophila genetics and physiology with human islet biology to address this fundamental gap in diabetes research. We identify Drosophila orthologs of T2D-risk genes that regulate insulin output. With human islets, we perform genetic studies and identify cognate human T2D-risk genes that regulate human beta cell function. Loss of BCL11A, a transcriptional regulator, in primary human islet cells leads to enhanced insulin secretion. Gene expression profiling reveals BCL11A-dependent regulation of multiple genes involved in insulin exocytosis. Thus, genetic and physiological systems described here advance the capacity to identify cell-specific T2D risk gene functions.

Mice harboring the human SLC30A8 R138X loss-of-function mutation have increased insulin secretory capacity

SLC30A8 encodes a zinc transporter that is primarily expressed in the pancreatic islets of Langerhans. In β-cells it transports zinc into insulin-containing secretory granules. Loss-of-function (LOF) mutations in SLC30A8 protect against type 2 diabetes in humans. In this study, we generated a knockin mouse model carrying one of the most common human LOF mutations for SLC30A8, R138X. The R138X mice had normal body weight, glucose tolerance, and pancreatic β-cell mass. Interestingly, in hyperglycemic conditions induced by the insulin receptor antagonist S961, the R138X mice showed a 50% increase in insulin secretion. This effect was not associated with enhanced β-cell proliferation or mass. Our data suggest that the SLC30A8 R138X LOF mutation may exert beneficial effects on glucose metabolism by increasing the capacity of β-cells to secrete insulin under hyperglycemic conditions.

Mouse models of peripheral metabolic disease

Metabolic disease risk is driven by defects in the function of cells that regulate energy homeostasis, as well as altered communication between the different tissues or organs that these cells occupy. Thus, it is desirable to use model organisms to understand the contribution of different cells, tissues and organs to metabolism. Mice are widely used for metabolic research, since well-characterised mouse strains (in terms of their genotype and phenotype) allow comparative studies and human disease modelling. Such research involves strains containing spontaneous mutations that lead to obesity and diabetes, surgically- and chemically-induced models, those that are secondary to caloric excess, genetic mutants created by transgenesis and gene knockout technologies, and peripheral models generated by Cre-Lox or CRISPR/Cas9 approaches. Focussing on obesity and type 2 diabetes as relevant metabolic diseases, we systematically review each of these models, discussing their use, limitations, and future potential.

The beneficial effects of Zn on Akt-mediated insulin and cell survival signaling pathways in diabetes

Zinc is one of the essential trace elements and participates in numerous physiological processes. Abnormalities in zinc homeostasis often result in the pathogenesis of various chronic metabolic disorders, such as diabetes and its complications. Zinc has insulin-mimetic and anti-diabetic effects and deficiency has been shown to aggravate diabetes-induced oxidative stress and tissue injury in diabetic rodent models and human subjects with diabetes. Akt signaling pathway plays a central role in insulin-stimulated glucose metabolism and cell survival. Anti-diabetic effects of zinc are largely dependent on the activation of Akt signaling. Zn is also an inducer of metallothionein that plays important role in anti-oxidative stress and damage. However, the exact molecular mechanisms underlying zinc-induced activation of Akt signaling pathway remains to be elucidated. This review summarizes the recent advances in deciphering the possible mechanisms of zinc on Akt-mediated insulin and cell survival signaling pathways in diabetes conditions. Insights into the effects of zinc on epigenetic regulation and autophagy in diabetic nephropathy are also discussed in the latter part of this review.

Role of Zinc Homeostasis in the Pathogenesis of Diabetes and Obesity

Zinc deficiency is a risk factor for obesity and diabetes. However, until recently, the underlying molecular mechanisms remained unclear. The breakthrough discovery that the common polymorphism in zinc transporter SLC30A8/ZnT8 may increase susceptibility to type 2 diabetes provided novel insights into the role of zinc in diabetes. Our group and others showed that altered ZnT8 function may be involved in the pathogenesis of type 2 diabetes, indicating that the precise control of zinc homeostasis is crucial for maintaining health and preventing various diseases, including lifestyle-associated diseases. Recently, the role of the zinc transporter ZIP13 in the regulation of beige adipocyte biogenesis was clarified, which indicated zinc homeostasis regulation as a possible therapeutic target for obesity and metabolic syndrome. Here we review advances in the role of zinc homeostasis in the pathophysiology of diabetes, and propose that inadequate zinc distribution may affect the onset of diabetes and metabolic diseases by regulating various critical biological events.

Increased free Zn2+ correlates induction of sarco(endo)plasmic reticulum stress via altered expression levels of Zn2+ -transporters in heart failure

Zn²⁺ -homoeostasis including free Zn²⁺ ([Zn²⁺ ]_i ) is regulated through Zn²⁺ -transporters and their comprehensive understanding may be important due to their contributions to cardiac dysfunction. Herein, we aimed to examine a possible role of Zn²⁺ -transporters in the development of heart failure (HF) via induction of ER stress. We first showed localizations of ZIP8, ZIP14 and ZnT8 to both sarcolemma and S(E)R in ventricular cardiomyocytes (H9c2 cells) using confocal together with calculated Pearson's coefficients. The expressions of ZIP14 and ZnT8 were significantly increased with decreased ZIP8 level in HF. Moreover, [Zn²⁺ ]_i was significantly high in doxorubicin-treated H9c2 cells compared to their controls. We found elevated levels of ER stress markers, GRP78 and CHOP/Gadd153, confirming the existence of ER stress. Furthermore, we measured markedly increased total PKC and PKCα expression and PKCα-phosphorylation in HF. A PKC inhibition induced significant decrease in expressions of these ER stress markers compared to controls. Interestingly, direct increase in [Zn²⁺ ]_i using zinc-ionophore induced significant increase in these markers. On the other hand, when we induced ER stress directly with tunicamycin, we could not observe any effect on expression levels of these Zn²⁺ transporters. Additionally, increased [Zn²⁺ ]_i could induce marked activation of PKCα. Moreover, we observed marked decrease in [Zn²⁺ ]_i under PKC inhibition in H9c2 cells. Overall, our present data suggest possible role of Zn²⁺ transporters on an intersection pathway with increased [Zn²⁺ ]_i and PKCα activation and induction of HF, most probably via development of ER stress. Therefore, our present data provide novel information how a well-controlled [Zn²⁺ ]_i via Zn²⁺ transporters and PKCα can be important therapeutic approach in prevention/treatment of HF.

Zinc transporters and insulin resistance: therapeutic implications for type 2 diabetes and metabolic disease

BACKGROUND

Zinc is a metal ion that is essential for growth and development, immunity, and metabolism, and therefore vital for life. Recent studies have highlighted zinc's dynamic role as an insulin mimetic and a cellular second messenger that controls many processes associated with insulin signaling and other downstream pathways that are amendable to glycemic control.

MAIN BODY

Mechanisms that contribute to the decompartmentalization of zinc and dysfunctional zinc transporter mechanisms, including zinc signaling are associated with metabolic disease, including type 2 diabetes. The actions of the proteins involved in the uptake, storage, compartmentalization and distribution of zinc in cells is under intense investigation. Of these, emerging research has highlighted a role for several zinc transporters in the initiation of zinc signaling events in cells that lead to metabolic processes associated with maintaining insulin sensitivity and thus glycemic homeostasis.

CONCLUSION

This raises the possibility that zinc transporters could provide novel utility to be targeted experimentally and in a clinical setting to treat patients with insulin resistance and thus introduce a new class of drug target with utility for diabetes pharmacotherapy.

Implications of impaired zinc homeostasis in diabetic cardiomyopathy and nephropathy

Impaired zinc homeostasis is observed in diabetes mellitus (DM2) and its complications. Zinc has a specific role in pancreatic β-cells via insulin synthesis, storage, and secretion. Intracellular zinc homeostasis is tightly controlled by zinc transporters (ZnT and Zip families) and metallothioneins (MT) which modulate the uptake, storage, and distribution of zinc. Several investigations in animal models demonstrate the protective role of MT in DM2 and its cardiovascular or renal complications, while a copious literature shows that a common polymorphism (R325W) in ZnT8, which affects the protein's zinc transport activity, is associated with increased DM2 risk. Emerging studies highlight a role of other zinc transporters in β-cell function, suggesting that targeting them could make a possible contribution in managing the hyperglycemia in diabetic patients. This article summarizes the current findings concerning the role of zinc homeostasis in DM2 pathogenesis and development of diabetic cardiomyopathy and nephropathy and suggests novel therapeutic targets. © 2017 BioFactors, 43(6):770-784, 2017.

Obstacles to Translating Genotype-Phenotype Correlates in Metabolic Disease

Type 2 diabetes mellitus is a polygenic disease with a variable phenotype. Many genetic associations have been described; however, understanding their underlying pathophysiological role in Type 2 diabetes mellitus is important for development of future therapeutic targets. Here, we review the physiological mechanisms of diabetes-associated variants that affect glycemia.

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.

Local and regional control of calcium dynamics in the pancreatic islet

Ca²⁺ is the key intracellular regulator of insulin secretion, acting in the β-cell as the ultimate trigger for exocytosis. In response to high glucose, ATP-sensitive K⁺ channel closure and plasma membrane depolarization engage a sophisticated machinery to drive pulsatile cytosolic Ca²⁺ changes. Voltage-gated Ca²⁺ channels, Ca²⁺ -activated K⁺ channels and Na⁺ /Ca²⁺ exchange all play important roles. The use of targeted Ca²⁺ probes has revealed that during each cytosolic Ca²⁺ pulse, uptake of Ca²⁺ by mitochondria, endoplasmic reticulum (ER), secretory granules and lysosomes fine-tune cytosolic Ca²⁺ dynamics and control organellar function. For example, changes in the expression of the Ca²⁺ -binding protein Sorcin appear to provide a link between ER Ca²⁺ levels and ER stress, affecting β-cell function and survival. Across the islet, intercellular communication between highly interconnected "hubs," which act as pacemaker β-cells, and subservient "followers," ensures efficient insulin secretion. Loss of connectivity is seen after the deletion of genes associated with type 2 diabetes (T2D) and follows metabolic and inflammatory insults that characterize this disease. Hubs, which typically comprise ~1%-10% of total β-cells, are repurposed for their specialized role by expression of high glucokinase (Gck) but lower Pdx1 and Nkx6.1 levels. Single cell-omics are poised to provide a deeper understanding of the nature of these cells and of the networks through which they communicate. New insights into the control of both the intra- and intercellular Ca²⁺ dynamics may thus shed light on T2D pathology and provide novel opportunities for therapy.

Expression and functional assessment of candidate type 2 diabetes susceptibility genes identify four new genes contributing to human insulin secretion

OBJECTIVES

Genome-wide association studies (GWAS) have identified >100 loci independently contributing to type 2 diabetes (T2D) risk. However, translational implications for precision medicine and for the development of novel treatments have been disappointing, due to poor knowledge of how these loci impact T2D pathophysiology. Here, we aimed to measure the expression of genes located nearby T2D associated signals and to assess their effect on insulin secretion from pancreatic beta cells.

METHODS

The expression of 104 candidate T2D susceptibility genes was measured in a human multi-tissue panel, through PCR-free expression assay. The effects of the knockdown of beta-cell enriched genes were next investigated on insulin secretion from the human EndoC-βH1 beta-cell line. Finally, we performed RNA-sequencing (RNA-seq) so as to assess the pathways affected by the knockdown of the new genes impacting insulin secretion from EndoC-βH1, and we analyzed the expression of the new genes in mouse models with altered pancreatic beta-cell function.

RESULTS

We found that the candidate T2D susceptibility genes' expression is significantly enriched in pancreatic beta cells obtained by laser capture microdissection or sorted by flow cytometry and in EndoC-βH1 cells, but not in insulin sensitive tissues. Furthermore, the knockdown of seven T2D-susceptibility genes (CDKN2A, GCK, HNF4A, KCNK16, SLC30A8, TBC1D4, and TCF19) with already known expression and/or function in beta cells changed insulin secretion, supporting our functional approach. We showed first evidence for a role in insulin secretion of four candidate T2D-susceptibility genes (PRC1, SRR, ZFAND3, and ZFAND6) with no previous knowledge of presence and function in beta cells. RNA-seq in EndoC-βH1 cells with decreased expression of PRC1, SRR, ZFAND6, or ZFAND3 identified specific gene networks related to T2D pathophysiology. Finally, a positive correlation between the expression of Ins2 and the expression of Prc1, Srr, Zfand6, and Zfand3 was found in mouse pancreatic islets with altered beta-cell function.

CONCLUSIONS

This study showed the ability of post-GWAS functional studies to identify new genes and pathways involved in human pancreatic beta-cell function and in T2D pathophysiology.

The transcription factor Pax6 is required for pancreatic β cell identity, glucose-regulated ATP synthesis, and Ca2+ dynamics in adult mice

Heterozygous mutations in the human paired box gene PAX6 lead to impaired glucose tolerance. Although embryonic deletion of the Pax6 gene in mice leads to loss of most pancreatic islet cell types, the functional consequences of Pax6 loss in adults are poorly defined. Here we developed a mouse line in which Pax6 was selectively inactivated in β cells by crossing animals with floxed Pax6 alleles to mice expressing the inducible Pdx1CreERT transgene. Pax6 deficiency, achieved by tamoxifen injection, caused progressive hyperglycemia. Although β cell mass was preserved 8 days post-injection, total insulin content and insulin:chromogranin A immunoreactivity were reduced by ∼60%, and glucose-stimulated insulin secretion was eliminated. RNA sequencing and quantitative real-time PCR analyses revealed that, although the expression of key β cell genes, including Ins2, Slc30a8, MafA, Slc2a2, G6pc2, and Glp1r, was reduced after Pax6 deletion, that of several genes that are usually selectively repressed (“disallowed”) in β cells, including Slc16a1, was increased. Assessed in intact islets, glucose-induced ATP:ADP increases were significantly reduced (p < 0.05) in βPax6KO versus control β cells, and the former displayed attenuated increases in cytosolic Ca²⁺. Unexpectedly, glucose-induced increases in intercellular connectivity were enhanced after Pax6 deletion, consistent with increases in the expression of the glucose sensor glucokinase, but decreases in that of two transcription factors usually expressed in fully differentiated β-cells, Pdx1 and Nkx6.1, were observed in islet “hub” cells. These results indicate that Pax6 is required for the functional identity of adult β cells. Furthermore, deficiencies in β cell glucose sensing are likely to contribute to defective insulin secretion in human carriers of PAX6 mutations.

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.

Exploring the Association Between Demographics, SLC30A8 Genotype, and Human Islet Content of Zinc, Cadmium, Copper, Iron, Manganese and Nickel

A widely prevalent single nucleotide polymorphism, rs13266634 in the SLC30A8 gene encoding the zinc transporter ZnT8, is associated with an increased risk for T2DM. ZnT8 is mostly expressed in pancreatic insulin-producing islets of Langerhans. The effect of this variant on the divalent metal profile in human islets is unknown. Additionally, essential and non-essential divalent metal content of human islets under normal environmental exposure conditions has not been described. We therefore examined the correlation of zinc and other divalent metals in human islets with rs13266634 genotype and demographic characteristics. We found that the diabetes risk genotype C/C at rs13266634 is associated with higher islet Zn concentration (C/C genotype: 16792 ± 1607, n = 22, C/T genotype: 11221 ± 1245, n = 18 T/T genotype: 11543 ± 6054, n = 3, all values expressed as mean nmol/g protein ± standard error of the mean, p = 0.040 by ANOVA). A positive correlation between islet cadmium content and both age (p = 0.048, R2 = 0.09) and female gender (women: 36.88 ± 4.11 vs men: 21.22 ± 3.65 nmol/g protein, p = 0.007) was observed. Our results suggest that the T2DM risk allele C is associated with higher islet zinc levels and support prior evidence of cadmium's higher bioavailability in women and its long tissue half-life.

Hepatic ZIP14-mediated Zinc Transport Contributes to Endosomal Insulin Receptor Trafficking and Glucose Metabolism

Zinc influences signaling pathways through controlled targeted zinc transport. Zinc transporter Zip14 KO mice display a phenotype that includes impaired intestinal barrier function with low grade chronic inflammation, hyperinsulinemia, and increased body fat, which are signatures of diet-induced diabetes (type 2 diabetes) and obesity in humans. Hyperglycemia in type 2 diabetes and obesity is caused by insulin resistance. Insulin resistance results in inhibition of glucose uptake by liver and other peripheral tissues, principally adipose and muscle and with concurrently higher hepatic glucose production. Therefore, modulation of hepatic glucose metabolism is an important target for antidiabetic treatment approaches. We demonstrate that during glucose uptake, cell surface abundance of zinc transporter ZIP14 and mediated zinc transport increases. Zinc is distributed to multiple sites in hepatocytes through sequential translocation of ZIP14 from plasma membrane to early and late endosomes. Endosomes from Zip14 KO mice were zinc-deficient because activities of the zinc-dependent insulin-degrading proteases insulin-degrading enzyme and cathepsin D were impaired; hence insulin receptor activity increased. Transient increases in cytosolic zinc levels are concurrent with glucose uptake and suppression of glycogen synthesis. In contrast, Zip14 KO mice exhibited greater hepatic glycogen synthesis and impaired gluconeogenesis and glycolysis related to low cytosolic zinc levels. We can conclude that ZIP14-mediated zinc transport contributes to regulation of endosomal insulin receptor activity and glucose homeostasis in hepatocytes. Therefore, modulation of ZIP14 transport activity presents a new target for management of diabetes and other glucose-related disorders.

Over-expression of Slc30a8/ZnT8 selectively in the mouse α cell impairs glucagon release and responses to hypoglycemia

BACKGROUND

The human SLC30A8 gene encodes the secretory granule-localised zinc transporter ZnT8 whose expression is chiefly restricted to the endocrine pancreas. Single nucleotide polymorphisms (SNPs) in the human SLC30A8 gene have been associated, through genome-wide studies, with altered type 2 diabetes risk. In addition to a role in the control of insulin release, recent studies involving targeted gene ablation from the pancreatic α cell (Solomou et al., J Biol Chem 290(35):21432-42) have also implicated ZnT8 in the control of glucagon release. Up to now, however, the possibility that increased levels of the transporter in these cells may impact glucagon secretion has not been explored.

METHODS

Here, we use a recently-developed reverse tetracyline transactivator promoter-regulated ZnT8 transgene to drive the over-expression of human ZnT8 selectively in the α cell in adult mice. Glucose homeostasis and glucagon secretion were subsequently assessed both in vivo during hypoglycemic clamps and from isolated islets in vitro.

RESULTS

Doxyclin-dependent human ZnT8 mRNA expression was apparent in both isolated islets and in fluorescence-activated cell sorting- (FACS) purified α cells. Examined at 12 weeks of age, intraperitoneal glucose (1 g/kg) tolerance was unchanged in transgenic mice versus wild-type littermates (n = 8-10 mice/genotype, p > 0.05) and sensitivity to intraperitoneal insulin (0.75U/kg) was similarly unaltered in transgenic animals. In contrast, under hyperinsulinemic-hypoglycemic clamp, a ~45 % (p < 0.001) reduction in glucose infusion rate was apparent, and glucagon release was significantly (~40 %, p < 0.01) impaired, in transgenic mice. Correspondingly, examined in vitro, glucagon secretion was significantly reduced (~30 %, p < 0.05) from transgenic versus control islets at low, stimulatory glucose concentrations (1 mM, p < 0.05) but not at high glucose (17 mM) glucose (p > 0.05). Over-expression of ZnT8 in glucagonoma-derived αTC1-9 cells increased granule free Zn(2+) concentrations consistent with a role for Zn(2+) in this compartment in the action of ZnT8 on glucagon secretion.

CONCLUSIONS

Increased ZnT8 expression, and a likely increase in intragranular free Zn(2+) concentration, is deleterious in pancreatic α cells for stimulated glucagon release. These data provide further evidence that type 2 diabetes-associated polymorphisms in the SLC30A8/ZnT8 gene may act in part via alterations in glucagon release and suggest that ZnT8 activation may restrict glucagon release in some settings.