Single-cell RNA sequencing of mouse islets exposed to proinflammatory cytokines

RIPK1 is dispensable for cell death regulation in β-cells during hyperglycemia

OBJECTIVE

Receptor-interacting protein kinase 1 (RIPK1) orchestrates the decision between cell survival and cell death in response to tumor necrosis factor (TNF) and other cytokines. Whereas the scaffolding function of RIPK1 is crucial to prevent TNF-induced apoptosis and necroptosis, its kinase activity is required for necroptosis and partially for apoptosis. Although TNF is a proinflammatory cytokine associated with β-cell loss in diabetes, the mechanism by which TNF induces β-cell demise remains unclear.

METHODS

Here, we dissected the contribution of RIPK1 scaffold versus kinase functions to β-cell death regulation using mice lacking RIPK1 specifically in β-cells (Ripk1β-KO mice) or expressing a kinase-dead version of RIPK1 (Ripk1D138N mice), respectively. These mice were challenged with streptozotocin, a model of autoimmune diabetes. Moreover, Ripk1β-KO mice were further challenged with a high-fat diet to induce hyperglycemia. For mechanistic studies, pancreatic islets were subjected to various killing and sensitising agents.

RESULTS

Inhibition of RIPK1 kinase activity (Ripk1D138N mice) did not affect the onset and progression of hyperglycemia in a type 1 diabetes model. Moreover, the absence of RIPK1 expression in β-cells did not affect normoglycemia under basal conditions or hyperglycemia under diabetic challenges. Ex vivo, primary pancreatic islets are not sensitised to TNF-induced apoptosis and necroptosis in the absence of RIPK1. Intriguingly, we found that pancreatic islets display high levels of the antiapoptotic cellular FLICE-inhibitory protein (cFLIP) and low levels of apoptosis (Caspase-8) and necroptosis (RIPK3) components. Cycloheximide treatment, which led to a reduction in cFLIP levels, rendered primary islets sensitive to TNF-induced cell death which was fully blocked by caspase inhibition.

CONCLUSIONS

Unlike in many other cell types (e.g., epithelial, and immune), RIPK1 is not required for cell death regulation in β-cells under physiological conditions or diabetic challenges. Moreover, in vivo and in vitro evidence suggest that pancreatic β-cells do not undergo necroptosis but mainly caspase-dependent death in response to TNF. Last, our results show that β-cells have a distinct mode of regulation of TNF-cytotoxicity that is independent of RIPK1 and that may be highly dependent on cFLIP.

Single Cell RNAseq Analysis of Cytokine-Treated Human Islets: Association of Cellular Stress with Impaired Cytokine Responsiveness

Pancreatic β-cells are essential for survival, being the only cell type capable of insulin secretion. While they are believed to be vulnerable to damage by inflammatory cytokines such as interleukin-1 beta (IL-1β) and interferon-gamma, we have recently identified physiological roles for cytokine signaling in rodent β-cells that include the stimulation of antiviral and antimicrobial gene expression and the inhibition of viral replication. In this study, we examine cytokine-stimulated changes in gene expression in human islets using single-cell RNA sequencing. Surprisingly, the global responses of human islets to cytokine exposure were remarkably blunted compared to our previous observations in the mouse. The small population of human islet cells that were cytokine responsive exhibited increased expression of IL-1β-stimulated antiviral guanylate-binding proteins, just like in the mouse. Most human islet cells were not responsive to cytokines, and this lack of responsiveness was associated with high expression of genes encoding ribosomal proteins. We further correlated the expression levels of RPL5 with stress response genes, and when expressed at high levels, RPL5 is predictive of failure to respond to cytokines in all endocrine cells. We postulate that donor causes of death and isolation methodologies may contribute to stress of the islet preparation. Our findings indicate that activation of stress responses in human islets limits cytokine-stimulated gene expression, and we urge caution in the evaluation of studies that have examined cytokine-stimulated gene expression in human islets without evaluation of stress-related gene expression.

Identification of unique cell type responses in pancreatic islets to stress

Diabetes involves the death or dysfunction of pancreatic β-cells. Analysis of bulk sequencing from human samples and studies using in vitro and in vivo models suggest that endoplasmic reticulum and inflammatory signaling play an important role in diabetes progression. To better characterize cell type-specific stress response, we perform multiplexed single-cell RNA sequencing to define the transcriptional signature of primary human islet cells exposed to endoplasmic reticulum and inflammatory stress. Through comprehensive pair-wise analysis of stress responses across pancreatic endocrine and exocrine cell types, we define changes in gene expression for each cell type under different diabetes-associated stressors. We find that β-, α-, and ductal cells have the greatest transcriptional response. We utilize stem cell-derived islets to study islet health through the candidate gene CIB1, which was upregulated under stress in primary human islets. Our findings provide insights into cell type-specific responses to diabetes-associated stress and establish a resource to identify targets for diabetes therapeutics.

Zinc-chelating BET bromodomain inhibitors equally target islet endocrine cell types

Inhibition of the bromodomain and extraterminal domain (BET) protein family is a potential strategy to prevent and treat diabetes; however, the clinical use of BET bromodomain inhibitors (BETis) is associated with adverse effects. Here, we explore a strategy for targeting BETis to β cells by exploiting the high-zinc (Zn2+) concentration in β cells relative to other cell types. We report the synthesis of a novel, Zn2+-chelating derivative of the pan-BETi (+)-JQ1, (+)-JQ1-DPA, in which (+)-JQ1 was conjugated to dipicolyl amine (DPA). As controls, we synthesized (+)-JQ1-DBA, a non-Zn2+-chelating derivative, and (-)-JQ1-DPA, an inactive enantiomer that chelates Zn2+. Molecular modeling and biophysical assays showed that (+)-JQ1-DPA and (+)-JQ1-DBA retain potent binding to BET bromodomains in vitro. Cellular assays demonstrated (+)-JQ1-DPA attenuated NF-ĸB target gene expression in β cells stimulated with the proinflammatory cytokine interleukin 1β. To assess β-cell selectivity, we isolated islets from a mouse model that expresses green fluorescent protein in insulin-positive β cells and mTomato in insulin-negative cells (non-β cells). Surprisingly, Zn2+ chelation did not confer β-cell selectivity as (+)-JQ1-DPA was equally effective in both β and α cells; however, (+)-JQ1-DPA was less effective in macrophages, a nonendocrine islet cell type. Intriguingly, the non-Zn2+-chelating derivative (+)-JQ1-DBA displayed the opposite selectivity, with greater effect in macrophages compared with (+)-JQ1-DPA, suggesting potential as a macrophage-targeting molecule. These findings suggest that Zn2+-chelating small molecules confer endocrine cell selectivity rather than β-cell selectivity in pancreatic islets and provide valuable insights and techniques to assess Zn2+ chelation as an approach to selectively target small molecules to pancreatic β cells.NEW & NOTEWORTHY Inhibition of BET bromodomains is a novel potential strategy to prevent and treat diabetes mellitus. However, BET inhibitors have negative side effects. We synthesized a BET inhibitor expected to exploit the high zinc concentration in β cells to accumulate in β cells. We show our inhibitor targeted pancreatic endocrine cells; however, it was less effective in immune cells. A control inhibitor showed the opposite effect. These findings help us understand how to target specific cells in diabetes treatment.

Single-cell transcriptome analysis reveals the regulatory functions of islet exocrine cells after short-time obesogenic diet

PURPOSE

This study aims to investigate the functions of exocrine islet cell subtypes in the early stage of obesity induced by high-fat diet (HFD), which is accompanied with deterioration of the systemic insulin response and islet subpopulation abnormalities.

METHODS

In this study, we analyzed published islet single-cell RNA sequencing (scRNA-seq) datasets from the early stage induced by HFD feeding. Bioinformatics tools such as findMarkers, Cellchat, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, and Gene Ontology (GO) terms were applied to identify the different functions of exocrine cell clusters.

RESULTS

A total of 26 cell clusters were obtained were identified from this dietary intervention model. Most proportions of cell subtypes were consistent between high-fat diet (HFD) and low-fat diet (LFD) groups, except for partial endocrine islet clusters and exocrine clusters. Most differentiated expression of genes in the HFD group was found in exocrine cluster. And we also found that the cell-cell interactions between ductal and endothelial cells were reduced in the HFD group, with the significant alteration in C17 (ductal) cluster. By further analyzing the co-expression regulatory network of transcription in the C17 cluster, we speculate that differentially expressed transcription factors affected the function of duct cells by affecting the expression of related genes in intercellular interaction networks, thereby promoting insulin resistance (IR) development.

CONCLUSION

Our results provide a reference for the function and regulatory mechanisms of exocrine cells in the obesity induced by HFD and probably influence the process of following insulin resistance.

Unveiling cell subpopulations in T1D mouse islets using single-cell RNA sequencing

Type 1 diabetes (T1D) is an autoimmune disease characterized by the destruction of beta cells by immune cells. The interactions among cells within the islets may be closely linked to the pathogenesis of T1D. In this study, we used single-cell RNA sequencing (scRNA-Seq) to analyze the cellular heterogeneity within the islets of a T1D mouse model. We established a T1D mouse model induced by streptozotocin and identified cell subpopulations using scRNA-Seq technology. Our results revealed 11 major cell types in the pancreatic islets of T1D mice, with heterogeneity observed in the alpha and beta cell subgroups, which may play a crucial role in the progression of T1D. Flow cytometry further confirmed a mature alpha and beta cell reduction in T1D mice. Overall, our scRNA-Seq analysis provided insights into the cellular heterogeneity of T1D islet tissue and highlighted the potential importance of alpha and beta cells in developing T1D.NEW & NOTEWORTHY In this study, we created a comprehensive single-cell atlas of pancreatic islets in a T1D mouse model using scRNA-Seq and identified 11 major cell types in the islets, highlighting the role of alpha and beta cells in T1D. This study revealed a significant reduction in the maturity alpha and beta cells in T1D mice through flow cytometry. It also demonstrated the heterogeneity of alpha and beta cells, potentially crucial for T1D progression. Overall, our scRNA-Seq analysis provided new insights for understanding and treating T1D by studying cell subtype changes and functions.

Pancreatic β-Cell Identity Change through the Lens of Single-Cell Omics Research

The main hallmark in the development of both type 1 and type 2 diabetes is a decline in functional β-cell mass. This decline is predominantly attributed to β-cell death, although recent findings suggest that the loss of β-cell identity may also contribute to β-cell dysfunction. This phenomenon is characterized by a reduced expression of key markers associated with β-cell identity. This review delves into the insights gained from single-cell omics research specifically focused on β-cell identity. It highlights how single-cell omics based studies have uncovered an unexpected level of heterogeneity among β-cells and have facilitated the identification of distinct β-cell subpopulations through the discovery of cell surface markers, transcriptional regulators, the upregulation of stress-related genes, and alterations in chromatin activity. Furthermore, specific subsets of β-cells have been identified in diabetes, such as displaying an immature, dedifferentiated gene signature, expressing significantly lower insulin mRNA levels, and expressing increased β-cell precursor markers. Additionally, single-cell omics has increased insight into the detrimental effects of diabetes-associated conditions, including endoplasmic reticulum stress, oxidative stress, and inflammation, on β-cell identity. Lastly, this review outlines the factors that may influence the identification of β-cell subpopulations when designing and performing a single-cell omics experiment.

β Cell Stress and Endocrine Function During T1D: What Is Next to Discover?

Canonically, type 1 diabetes (T1D) is a disease characterized by autoreactive T cells as perpetrators of endocrine dysfunction and β cell death in the spiral toward loss of β cell mass, hyperglycemia, and insulin dependence. β Cells have mostly been considered as bystanders in a flurry of autoimmune processes. More recently, our framework for understanding and investigating T1D has evolved. It appears increasingly likely that intracellular β cell stress is an important component of T1D etiology/pathology that perpetuates autoimmunity during the progression to T1D. Here we discuss the emerging and complex role of β cell stress in initiating, provoking, and catalyzing T1D. We outline the bridges between hyperglycemia, endoplasmic reticulum stress, oxidative stress, and autoimmunity from the viewpoint of intrinsic β cell (dys)function, and we extend this discussion to the potential role for a therapeutic β cell stress-metabolism axis in T1D. Lastly, we mention research angles that may be pursued to improve β cell endocrine function during T1D. Biology gleaned from studying T1D will certainly overlap to innovate therapeutic strategies for T2D, and also enhance the pursuit of creating optimized stem cell-derived β cells as endocrine therapy.

Is 14-3-3 the Combination to Unlock New Pathways to Improve Metabolic Homeostasis and β-Cell Function?

Since their discovery nearly five decades ago, molecular scaffolds belonging to the 14-3-3 protein family have been recognized as pleiotropic regulators of diverse cellular and physiological functions. With their ability to bind to proteins harboring specific serine and threonine phosphorylation motifs, 14-3-3 proteins can interact with and influence the function of docking proteins, enzymes, transcription factors, and transporters that have essential roles in metabolism and glucose homeostasis. Here, we will discuss the regulatory functions of 14-3-3 proteins that will be of great interest to the fields of metabolism, pancreatic β-cell biology, and diabetes. We first describe how 14-3-3 proteins play a central role in glucose and lipid homeostasis by modulating key pathways of glucose uptake, glycolysis, oxidative phosphorylation, and adipogenesis. This is followed by a discussion of the contributions of 14-3-3 proteins to calcium-dependent exocytosis and how this relates to insulin secretion from β-cells. As 14-3-3 proteins are major modulators of apoptosis and cell cycle progression, we will explore if 14-3-3 proteins represent a viable target for promoting β-cell regeneration and discuss the feasibility of targeting 14-3-3 proteins to treat metabolic diseases such as diabetes.

ARTICLE HIGHLIGHTS

14-3-3 proteins are ubiquitously expressed scaffolds with multiple roles in glucose homeostasis and metabolism. 14-3-3ζ regulates adipogenesis via distinct mechanisms and is required for postnatal adiposity and adipocyte function. 14-3-3ζ controls glucose-stimulated insulin secretion from pancreatic β-cells by regulating mitochondrial function and ATP synthesis as well as facilitating cross talk between β-cells and α-cells.

β-cell-selective inhibition of DNA damage response signaling by nitric oxide is associated with an attenuation in glucose uptake

Nitric oxide (NO) plays a dual role in regulating DNA damage response (DDR) signaling in pancreatic β-cells. As a genotoxic agent, NO activates two types of DDR signaling; however, when produced at micromolar levels by the inducible isoform of NO synthase, NO inhibits DDR signaling and DDR-induced apoptosis in a β-cell-selective manner. DDR signaling inhibition by NO correlates with mitochondrial oxidative metabolism inhibition and decreases in ATP and NAD+. Unlike most cell types, β-cells do not compensate for impaired mitochondrial oxidation by increasing glycolytic flux, and this metabolic inflexibility leads to a decrease in ATP and NAD+. Here, we used multiple analytical approaches to determine changes in intermediary metabolites in β-cells and non-β-cells treated with NO or complex I inhibitor rotenone. In addition to ATP and NAD+, glycolytic and tricarboxylic acid cycle intermediates as well as NADPH are significantly decreased in β-cells treated with NO or rotenone. Consistent with glucose-6-phosphate residing at the metabolic branchpoint for glycolysis and the pentose phosphate pathway (NADPH), we show that mitochondrial oxidation inhibitors limit glucose uptake in a β-cell-selective manner. Our findings indicate that the β-cell-selective inhibition of DDR signaling by NO is associated with a decrease in ATP to levels that fall significantly below the KM for ATP of glucokinase (glucose uptake) and suggest that this action places the β-cell in a state of suspended animation where it is metabolically inert until NO is removed, and metabolic function can be restored.

The interactions between inflammation and insulin resistance: molecular mechanisms in insulin-producing and insulin-dependent tissues

In the modern world the prevalence of obesity and type 2 diabetes mellitus (T2DM) significantly increases. In this light the risks of obesity-associated complications also grow up. The crucial linkage between obesity and its metabolic and cardiovascular complications is inflammatory process. The mechanism of this linkage is similar in pancreas and insulin-dependent tissues both on cells, cell-to-cell communication and signaling pathway levels: the catalysts are different lipids (cholesterol, free fatty acids, triglycerides), which are able to activate Toll-like receptors of innate immunity and inflammation. Nextly, IKK- and JNK-dependent cascades activate the secretion of inflammatory cytokines TNFa, IL-1b, IL-6 and others, which act by paracrine and autocrine manner and support inflammation both in local and systemic levels. Thus, insulin-producing and insulin-dependent tissues, which are involved in T2DM pathogenesis, through the inflammatory process integrate in pathogenic and self-maintaining cycle, which leads to the suppression of insulin secretion, pancreatic β-cell failure and the development of insulin-dependent tissues insulin resistance.

Engineering human stem cell-derived islets to evade immune rejection and promote localized immune tolerance

Immunological protection of transplanted stem cell-derived islet (SC-islet) cells is yet to be achieved without chronic immunosuppression or encapsulation. Existing genetic engineering approaches to produce immune-evasive SC-islet cells have so far shown variable results. Here, we show that targeting human leukocyte antigens (HLAs) and PD-L1 alone does not sufficiently protect SC-islet cells from xenograft (xeno)- or allograft (allo)-rejection. As an addition to these approaches, we genetically engineer SC-islet cells to secrete the cytokines interleukin-10 (IL-10), transforming growth factor β (TGF-β), and modified IL-2 such that they promote a tolerogenic local microenvironment by recruiting regulatory T cells (T_regs) to the islet grafts. Cytokine-secreting human SC-β cells resist xeno-rejection and correct diabetes for up to 8 weeks post-transplantation in non-obese diabetic (NOD) mice. Thus, genetically engineering human embryonic SCs (hESCs) to induce a tolerogenic local microenvironment represents a promising approach to provide SC-islet cells as a cell replacement therapy for diabetes without the requirement for encapsulation or immunosuppression.

Hydrogen peroxide detoxification through the peroxiredoxin/thioredoxin antioxidant system: A look at the pancreatic β-cell oxidant defense

Reactive oxygen species (ROS), such as hydrogen peroxide, are formed when molecular oxygen obtains additional electrons, increasing its reactivity. While low concentrations of hydrogen peroxide are necessary for regulation of normal cellular signaling events, high concentrations can be toxic. To maintain this balance between beneficial and deleterious concentrations of hydrogen peroxide, cells utilize antioxidants. Our recent work supports a primary role for peroxiredoxin, thioredoxin, and thioredoxin reductase as the oxidant defense pathway used by insulin-producing pancreatic β-cells. These three players work in an antioxidant cycle based on disulfide exchange, with oxidized targets ultimately being reduced using electrons provided by NADPH. Peroxiredoxins also participate in hydrogen peroxide-based signaling through disulfide exchange with redox-regulated target proteins. This chapter will describe the catalytic mechanisms of thioredoxin, thioredoxin reductase, and peroxiredoxin and provide an in-depth look at the roles these enzymes play in antioxidant defense pathways of insulin-secreting β-cells. Finally, we will evaluate the physiological relevance of peroxiredoxin-mediated hydrogen peroxide signaling as a regulator of β-cell function.

Pancreatic Islet Cells Response to IFNγ Relies on Their Spatial Location within an Islet

Type 1 diabetes (T1D) is an auto-immune disease characterized by the progressive destruction of insulin-producing pancreatic beta cells. While beta cells are the target of the immune attack, the other islet endocrine cells, namely the alpha and delta cells, can also be affected by the inflammatory milieu. Here, using a flow cytometry-based strategy, we compared the impact of IFNγ, one of the main cytokines involved in T1D, on the three endocrine cell subsets isolated from C57BL/6 mouse islets. RNA-seq analyses revealed that alpha and delta cells exposed in vitro to IFNγ display a transcriptomic profile very similar to that of beta cells, with an increased expression of inflammation key genes such as MHC class I molecules, the CXCL10 chemokine and the programmed death-ligand 1 (PD-L1), three hallmarks of IFNγ signaling. Interestingly, at low IFNγ concentration, we observed two beta cell populations (responders and non-responders) based on PD-L1 protein expression. Our data indicate that this differential sensitivity relies on the location of the cells within the islet rather than on the existence of two different beta cells subsets. The same findings were corroborated by the in vivo analysis of pancreatic islets from the non-obese diabetic mouse model of T1D, showing more intense PD-L1 staining on endocrine cells close to immune infiltrate. Collectively, our work demonstrates that alpha and delta cells are as sensitive as beta cells to IFNγ, and suggests a gradual diffusion of the cytokine into an islet. These observations provide novel insights into the in situ inflammatory processes occurring in T1D progression.

PAHSAs reduce cellular senescence and protect pancreatic beta cells from metabolic stress through regulation of Mdm2/p53

Senescence in pancreatic beta cells plays a major role in beta cell dysfunction, which leads to impaired glucose homeostasis and diabetes. Therefore, prevention of beta cell senescence could reduce the risk of diabetes. Treatment of nonobese diabetic (NOD) mice, a model of type 1 autoimmune diabetes (T1D), with palmitic acid hydroxy stearic acids (PAHSAs), a novel class of endogenous lipids with antidiabetic and antiinflammatory effects, delays the onset and reduces the incidence of T1D from 82% with vehicle treatment to 35% with PAHSAs. Here, we show that a major mechanism by which PAHSAs protect islets of the NOD mice is by directly preventing and reversing the initial steps of metabolic stress-induced senescence. In vitro PAHSAs increased Mdm2 expression, which decreases the stability of p53, a key inducer of senescence-related genes. In addition, PAHSAs enhanced expression of protective genes, such as those regulating DNA repair and glutathione metabolism and promoting autophagy. We demonstrate the translational relevance by showing that PAHSAs prevent and reverse early stages of senescence in metabolically stressed human islets by the same Mdm2 mechanism. Thus, a major mechanism for the dramatic effect of PAHSAs in reducing the incidence of type 1 diabetes in NOD mice is decreasing cellular senescence; PAHSAs may have a similar benefit in humans.

Investigating the Expression and Function of the Glucose Transporter GLUT6 in Obesity

Obesity-related insulin resistance is a highly prevalent and growing health concern, which places stress on the pancreatic islets of Langerhans by increasing insulin secretion to lower blood glucose levels. The glucose transporters GLUT1 and GLUT3 play a key role in glucose-stimulated insulin secretion in human islets, while GLUT2 is the key isoform in rodent islets. However, it is unclear whether other glucose transporters also contribute to insulin secretion by pancreatic islets. Herein, we show that SLC2A6 (GLUT6) is markedly upregulated in pancreatic islets from genetically obese leptin-mutant (ob/ob) and leptin receptor-mutant (db/db) mice, compared to lean controls. Furthermore, we observe that islet SLC2A6 expression positively correlates with body mass index in human patients with type 2 diabetes. To investigate whether GLUT6 plays a functional role in islets, we crossed GLUT6 knockout mice with C57BL/6 ob/ob mice. Pancreatic islets isolated from ob/ob mice lacking GLUT6 secreted more insulin in response to high-dose glucose, compared to ob/ob mice that were wild type for GLUT6. The loss of GLUT6 in ob/ob mice had no adverse impact on body mass, body composition, or glucose tolerance at a whole-body level. This study demonstrates that GLUT6 plays a role in pancreatic islet insulin secretion in vitro but is not a dominant glucose transporter that alters whole-body metabolic physiology in ob/ob mice.

Proinflammatory cytokines induce rapid, NO-independent apoptosis, expression of chemotactic mediators and interleukin-32 secretion in human pluripotent stem cell-derived beta cells

AIMS/HYPOTHESIS

The aim of this study was to examine the effects of proinflammatory cytokines on cells of different developmental stages during the generation of stem cell-derived beta cells (SC-beta cells) from human pluripotent stem cells (hPSCs). We wanted to find out to what extent human SC-beta cells are suitable as an experimental cellular model and, with regard to a possible therapeutic use, whether SC-beta cells have a comparable vulnerability to cytokines as bona fide beta cells.

METHODS

hPSCs were differentiated towards pancreatic organoids (SC-organoids) using a 3D production protocol. SC-beta cells and non-insulin-producing cells were separated by FACS and differential gene expression profiles of purified human SC-beta cells, progenitor stages and the human beta cell line EndoC-βH1, as a reference, were determined after 24 h incubation with the proinflammatory cytokines IL-1β, TNF-α and IFN-γ via a transcriptome microarray. Furthermore, we investigated apoptosis based on caspase cleavage, the generation of reactive oxygen species and activation of mitogen-activated protein-kinase (MAPK) stress-signalling pathways.

RESULTS

A 24 h exposure of SC-beta cells to proinflammatory cytokines resulted in significant activation of caspase 3/7 and apoptosis via the extrinsic and intrinsic apoptosis signalling pathways. At this time point, SC-beta cells showed a markedly higher sensitivity towards proinflammatory cytokines than non-insulin-producing cells and EndoC-βH1 cells. Furthermore, we were able to demonstrate the generation of reactive oxygen species and rule out the involvement of NO-mediated stress. A transient activation of stress-signalling pathways p38 mitogen-activated protein kinases (p38) and c-Jun N-terminal kinase (JNK) was already observed after 10 min of cytokine exposure. The transcriptome analysis revealed that the cellular response to proinflammatory cytokines increased with the degree of differentiation of the cells. Cytokines induced the expression of multiple inflammatory mediators including IL-32, CXCL9 and CXCL10 in SC-beta cells and in non-insulin-producing cells.

CONCLUSIONS/INTERPRETATION

Our results indicate that human SC-beta cells respond to proinflammatory cytokines very similarly to human islets. Due to the fast and fulminant cellular response of SC-beta cells, we conclude that SC-beta cells represent a suitable model for diabetes research. In light of the immaturity of SC-beta cells, they may be an attractive model for developmentally young beta cells as they are, for example, present in patients with early-onset type 1 diabetes. The secretion of chemotactic signals may promote communication between SC-beta cells and immune cells, and non-insulin-producing cells possibly participate in the overall immune response and are thus capable of amplifying the immune response and further stimulating inflammation. We demonstrated that cytokine-treated SC-organoids secrete IL-32, which is considered a promising candidate for type 1 diabetes onset. This underlines the need to ensure the survival of SC-beta cells in an autoimmune environment such as that found in type 1 diabetes.

Islet Inflammation and β Cell Dysfunction in Type 2 Diabetes

Pancreatic islets are the body's central rheostat that regulates glucose homeostasis through the production of different hormones, including β cell-derived insulin. During obesity-induced type 2 diabetes (T2D), islet β cells become dysfunctional and inadequate insulin secretion no longer ensures glycemic control. T2D is associated with a chronic low-grade inflammation that manifests in several metabolic organs including the pancreatic islets. Growing evidence suggests that components of the innate immune system, and especially macrophages, play a crucial role in regulating islet homeostasis. Yet, the phenotypes and functions of islet macrophages in physiology and during T2D have only started to attract attention and remain unclear. In this review, the current knowledge about islet inflammation and macrophages will be summarized in humans and rodent models. Recent findings on the cellular and molecular mechanisms involved in islet remodeling and β cell function during obesity and T2D will be discussed.

Cytokine and Nitric Oxide-Dependent Gene Regulation in Islet Endocrine and Nonendocrine Cells

While exposure to inflammatory cytokines is thought to contribute to pancreatic β-cell damage during diabetes, primarily because cytokine-induced nitric oxide impairs β-cell function and causes cell death with prolonged exposure, we hypothesize that there is a physiological role for cytokine signaling that protects β-cells from a number of environmental stresses. This hypothesis is derived from the knowledge that β-cells are essential for survival even though they have a limited capacity to replicate, yet they are exposed to high cytokine levels during infection as most of the pancreatic blood flow is directed to islets. Here, mouse islets were subjected to single-cell RNA sequencing following 18-h cytokine exposure. Treatment with IL-1β and IFN-γ stimulates expression of inducible nitric oxide synthase (iNOS) mRNA and antiviral and immune-associated genes as well as repression of islet identity factors in a subset of β- and non-β-endocrine cells in a nitric oxide-independent manner. Nitric oxide-dependent expression of genes encoding heat shock proteins was observed in both β- and non-β-endocrine cells. Interestingly, cells with high expression of heat shock proteins failed to increase antiviral and immune-associated gene expression, suggesting that nitric oxide may be an internal "off switch" to prevent the negative effects of prolonged cytokine signaling in islet endocrine cells. We found no evidence for pro-apoptotic gene expression following 18-h cytokine exposure. Our findings suggest that the primary functions of cytokines and nitric oxide are to protect islet endocrine cells from damage, and only when regulation of cytokine signaling is lost does irreversible damage occur.

Synchrotron fluorescence imaging of individual mouse beta-cells reveals changes in zinc, calcium, and iron in a model of low-grade inflammation

Pancreatic beta-cells synthesize and secrete insulin maintaining an organism's energy homeostasis. In humans, beta-cell dysfunction and death contribute to the pathogenesis of type 2 diabetes (T2D). Although the causes of beta-cell dysfunction are complex, obesity-induced low-grade systemic inflammation plays a role. For example, obese individuals exhibiting increased levels of proinflammatory cytokines IL-6 and IL-1beta have a higher risk of beta-cell dysfunction and T2D. Interestingly, obesity-induced inflammation changes the expression of several cellular metal regulating genes, prompting this study to examine changes in the beta-cell metallome after exposure to proinflammatory-cytokines. Primary mouse beta-cells were exposed to a combination of IL-6 and IL-1beta for 48 hours, were chemically fixed and imaged by synchrotron X-ray fluorescent microscopy. Quantitative analysis showed a surprising 2.4-fold decrease in the mean total cellular content of zinc from 158 ± 57.7 femtograms (fg) to 65.7 ± 29.7 fg; calcium decreased from 216 ± 67.4 to 154.3 ± 68.7 fg (control vs. cytokines, respectively). The mean total cellular iron content slightly increased from 30.4 ± 12.2 to 47.2 ± 36.4 fg after cytokine treatment; a sub-population of cells (38%) exhibited larger increases of iron density. Changes in the subcellular distributions of zinc and calcium were observed after cytokine exposure. Beta-cells contained numerous iron puncta that accumulated still more iron after exposure to cytokines. These findings provide evidence that exposure to low levels of cytokines is sufficient to cause changes in the total cellular content and/or subcellular distribution of several metals known to be critical for normal beta-cell function.

The Role of Thioredoxin/Peroxiredoxin in the β-Cell Defense Against Oxidative Damage

Oxidative stress is hypothesized to play a role in pancreatic β-cell damage, potentially contributing to β-cell dysfunction and death in both type 1 and type 2 diabetes. Oxidative stress arises when naturally occurring reactive oxygen species (ROS) are produced at levels that overwhelm the antioxidant capacity of the cell. ROS, including superoxide and hydrogen peroxide, are primarily produced by electron leak during mitochondrial oxidative metabolism. Additionally, peroxynitrite, an oxidant generated by the reaction of superoxide and nitric oxide, may also cause β-cell damage during autoimmune destruction of these cells. β-cells are thought to be susceptible to oxidative damage based on reports that they express low levels of antioxidant enzymes compared to other tissues. Furthermore, markers of oxidative damage are observed in islets from diabetic rodent models and human patients. However, recent studies have demonstrated high expression of various isoforms of peroxiredoxins, thioredoxin, and thioredoxin reductase in β-cells and have provided experimental evidence supporting a role for these enzymes in promoting β-cell function and survival in response to a variety of oxidative stressors. This mini-review will focus on the mechanism by which thioredoxins and peroxiredoxins detoxify ROS and on the protective roles of these enzymes in β-cells. Additionally, we speculate about the role of this antioxidant system in promoting insulin secretion.