Beta cell heterogeneity: an evolving concept

Single-cell RNA sequencing identifies endothelial-derived HBEGF as promoting pancreatic beta cell proliferation in mice via the EGFR-Kmt5a-H4K20me pathway

AIMS/HYPOTHESIS

Pancreatic beta cell mass is dynamically regulated in response to increased physiological and pathological demands. Understanding the mechanisms that control physiological beta cell proliferation could provide valuable insights into novel therapeutic approaches to diabetes. Here, we aimed to analyse the intracellular and extracellular signalling pathways involved in regulating the physiological proliferation of beta cells using single-cell RNA-seq (scRNA-seq) and in vitro functional assays.

METHODS

Islets isolated from nulliparous mice, mice at different time points of gestation and mice at day 4 after delivery were analysed using scRNA-seq. Bioinformatics analyses of scRNA-seq data were performed to determine the heterogeneous transcriptomic characteristics of beta cells and to identify the proliferating subpopulation. CellChat was used to analyse cell-cell communication and identify the ligand-receptor pairs between beta cell subclusters as well as between non-beta cells and proliferating beta cells. In vitro functional assays were conducted in mouse and rat beta cell lines and isolated mouse primary islets to validate the role of Kmt5a- mono-methylation of histone H4 at lysine 20 (H4K20me) signalling and endothelial-derived heparin-binding EGF-like growth factor (HBEGF) in beta cell proliferation.

RESULTS

Of 43,724 endocrine and non-endocrine cells within islets analysed by scRNA-seq, 15,569 beta cells were clustered into eight distinct populations, each exhibiting unique heterogeneity. A proliferating beta cell subcluster was identified that highly expressed the histone methyltransferase Kmt5a. Activation of Kmt5a-H4K20me signalling upregulated the expression of Cdk1 and promoted beta cell proliferation. The crosstalk between endothelial cells and the proliferating beta cell subcluster, mediated by the HBEGF-EGF receptor (EGFR) ligand-receptor interaction, increased as beta cell mass expanded. HBEGF increased the expression levels of genes involved in the cell cycle and promoted beta cell proliferation by regulating the Kmt5a-H4K20me signalling pathway.

CONCLUSIONS/INTERPRETATION

Our study demonstrates that, under physiological conditions, endothelial-derived HBEGF regulates beta cell proliferation through the Kmt5a-H4K20me signalling pathway, which may serve as a potential target to promote beta cell expansion and treat diabetes.

DATA AVAILABILITY

The scRNA-seq and RNA-seq datasets are available from the Gene Expression Omnibus (GEO) using the accession numbers GSE278860 and GSE278861, respectively.

Signal transduction pathways controlling Ins2 gene activity and beta cell state transitions

Pancreatic β cells exist in low and high insulin gene activity states that are dynamic on a scale of hours to days. Here, we used live 3D imaging, mass spectrometry proteomics, and targeted perturbations of β cell signaling to comprehensively investigate Ins2(GFP)HIGH and Ins2(GFP)LOW β cell states. We identified the two Ins2 gene activity states in intact isolated islets and showed that cells in the same state were more likely to be nearer to each other. We report the proteomes of pure β cells to a depth of 5555 proteins and show that β cells with high Ins2 gene activity had reduced β cell immaturity factors, as well as increased translation. We identified activators of cAMP signaling (GLP1, IBMX) as powerful drivers of Ins2(GFP)LOW to Ins2(GFP)HIGH transitions. Okadaic acid and cyclosporine A had the opposite effects. This study provides new insight into the proteomic profiles and regulation of β cell states.

Tissue Resident and Infiltrating Immune Cells: Their Influence on the Demise of Beta Cells in Type 1 Diabetes

Type 1 diabetes (T1D) is an organ-specific autoimmune disease that results in the selective loss of pancreatic beta cells and an eventual deficit in insulin production to maintain glucose homeostasis. It is now increasingly accepted that this dynamic disease process is multifactorial; involves a variety of immune cells which contribute to an inflamed pancreatic microenvironment; and that the condition is heterogenous, resulting in variable rates of subsequent beta cell damage. In this review, we will explore the current understanding of the cellular interactions between both resident and infiltrating immune cells within the pancreatic environment, highlighting key mechanisms which may promote the beta cell destruction and islet damage associated with T1D.

Biphasic glucose-stimulated insulin secretion over decades: a journey from measurements and modeling to mechanistic insights

Glucose-stimulated insulin release from pancreatic β-cells is critical for maintaining blood glucose homeostasis. An abrupt increase in blood glucose concentration evokes a rapid and transient rise in insulin secretion followed by a prolonged, slower phase. A diminished first phase is one of the earliest indicators of β-cell dysfunction in individuals predisposed to develop type 2 diabetes. Consequently, researchers have explored the underlying mechanisms for decades, starting with plasma insulin measurements under physiological conditions and advancing to single-vesicle exocytosis measurements in individual β-cells combined with molecular manipulations. Based on a chain of evidence gathered from genetic manipulation to in vivo mouse phenotyping, a widely accepted theory posits that distinct functional insulin vesicle pools in β-cells regulate biphasic glucose-stimulated insulin secretion (GSIS) via activation of different metabolic signal pathways. Recently, we developed a high-resolution imaging technique to visualize single vesicle exocytosis from β-cells within an intact islet. Our findings reveal that β-cells within the islet exhibit heterogeneity in their secretory capabilities, which also differs from the heterogeneous Ca2+ signals observed in islet β-cells in response to glucose stimulation. Most importantly, we demonstrate that biphasic GSIS emerges from the interactions among α-, β-, and δ-cells within the islet and is driven by a small subset of hypersecretory β-cells. Finally, we propose that a shift from reductionism to holism may be required to fully understand the etiology of complex diseases such as diabetes.

Exploring pancreatic beta-cell subgroups and their connectivity

Functional pancreatic islet beta cells are essential to ensure glucose homeostasis across species from zebrafish to humans. These cells show significant heterogeneity, and emerging studies have revealed that connectivity across a hierarchical network is required for normal insulin release. Here, we discuss current thinking and areas of debate around intra-islet connectivity, cellular hierarchies and potential "controlling" beta-cell populations. We focus on methodologies, including comparisons of different cell preparations as well as in vitro and in vivo approaches to imaging and controlling the activity of human and rodent islet preparations. We also discuss the analytical approaches that can be applied to live-cell data to identify and study critical subgroups of cells with a disproportionate role in control Ca2+ dynamics and thus insulin secretion (such as "first responders", "leaders" and "hubs", as defined by Ca2+ responses to glucose stimulation). Possible mechanisms by which this hierarchy is achieved, its physiological relevance and how its loss may contribute to islet failure in diabetes mellitus are also considered. A glossary of terms and links to computational resources are provided.

Twenty years of islet-on-a-chip: microfluidic tools for dissecting islet metabolism and function

Pancreatic islets are metabolically active micron-sized tissues responsible for controlling blood glucose through the secretion of insulin and glucagon. A loss of functional islet mass results in type 1 and 2 diabetes. Islet-on-a-chip devices are powerful microfluidic tools used to trap and study living ex vivo human and murine pancreatic islets and potentially stem cell-derived islet organoids. Devices developed over the past twenty years offer the ability to treat islets with controlled and dynamic microenvironments to mimic in vivo conditions and facilitate diabetes research. In this review, we explore the various islet-on-a-chip devices used to immobilize islets, regulate the microenvironment, and dynamically detect islet metabolism and insulin secretion. We first describe and assess the various methods used to immobilize islets including chambers, dam-walls, and hydrodynamic traps. We subsequently describe the surrounding methods used to create glucose gradients, enhance the reaggregation of dispersed islets, and control the microenvironment of stem cell-derived islet organoids. We focus on the various methods used to measure insulin secretion including capillary electrophoresis, droplet microfluidics, off-chip ELISAs, and on-chip fluorescence anisotropy immunoassays. Additionally, we delve into the various multiparametric readouts (NAD(P)H, Ca2+-activity, and O2-consumption rate) achieved primarily by adopting a microscopy-compatible optical window into the devices. By critical assessment of these advancements, we aim to inspire the development of new devices by the microfluidics community and accelerate the adoption of islet-on-a-chip devices by the wider diabetes research and clinical communities.

Single cell multiomic analysis reveals diabetes-associated β-cell heterogeneity driven by HNF1A

Broad heterogeneity in pancreatic β-cell function and morphology has been widely reported. However, determining which components of this cellular heterogeneity serve a diabetes-relevant function remains challenging. Here, we integrate single-cell transcriptome, single-nuclei chromatin accessibility, and cell-type specific 3D genome profiles from human islets and identify Type II Diabetes (T2D)-associated β-cell heterogeneity at both transcriptomic and epigenomic levels. We develop a computational method to explicitly dissect the intra-donor and inter-donor heterogeneity between single β-cells, which reflect distinct mechanisms of T2D pathogenesis. Integrative transcriptomic and epigenomic analysis identifies HNF1A as a principal driver of intra-donor heterogeneity between β-cells from the same donors; HNF1A expression is also reduced in β-cells from T2D donors. Interestingly, HNF1A activity in single β-cells is significantly associated with lower Na+ currents and we nominate a HNF1A target, FXYD2, as the primary mitigator. Our study demonstrates the value of investigating disease-associated single-cell heterogeneity and provides new insights into the pathogenesis of T2D.

Restored UBE2C expression in islets promotes β-cell regeneration in mice by ubiquitinating PER1

Insulin deficiency may be due to the reduced proliferation capacity of islet β-cell, contributing to the onset of diabetes. It is therefore imperative to investigate the mechanism of the β-cell regeneration in the islets. NKX6.1, one of the critical β-cell transcription factors, is a pivotal element in β-cell proliferation. The ubiquitin-binding enzyme 2C (UBE2C) was previously reported as one of the downstream molecules of NKX6.1 though the exact function and mechanism of UBE2C in β-cell remain to be elucidated. Here, we determined a subpopulation of islet β-cells highly expressing UBE2C, which proliferate actively. We also discovered that β-cell compensatory proliferation was induced by UBE2C via the cell cycle renewal pathway in weaning and high-fat diet (HFD)-fed mice. Moreover, the reduction of β-cell proliferation led to insulin deficiency in βUbe2cKO mice and, therefore, developed type 2 diabetes. UBE2C was found to regulate PER1 degradation through the ubiquitin-proteasome pathway via its association with a ubiquitin ligase, CUL1. PER1 inhibition rescues UBE2C knockout-induced β-cell growth inhibition both in vivo and in vitro. Notably, overexpression of UBE2C via lentiviral transduction in pancreatic islets was able to relaunch β-cell proliferation in STZ-induced diabetic mice and therefore partially alleviated hyperglycaemia and glucose intolerance. This study indicates that UBE2C positively regulates β-cell proliferation by promoting ubiquitination and degradation of the biological clock suppressor PER1. The beneficial effect of UBE2C on islet β-cell regeneration suggests a promising application in treating diabetic patients with β-cell deficiency.

A plasma membrane-associated glycolytic metabolon is functionally coupled to KATP channels in pancreatic α and β cells from humans and mice

The ATP-sensitive K+ (KATP) channel is a key regulator of hormone secretion from pancreatic islet endocrine cells. Using direct measurements of KATP channel activity in pancreatic β cells and the lesser-studied α cells, from both humans and mice, we provide evidence that a glycolytic metabolon locally controls KATP channels on the plasma membrane. The two ATP-consuming enzymes of upper glycolysis, glucokinase and phosphofructokinase, generate ADP that activates KATP. Substrate channeling of fructose 1,6-bisphosphate through the enzymes of lower glycolysis fuels pyruvate kinase, which directly consumes the ADP made by phosphofructokinase to raise ATP/ADP and close the channel. We further show the presence of a plasma membrane-associated NAD+/NADH cycle whereby lactate dehydrogenase is functionally coupled to glyceraldehyde-3-phosphate dehydrogenase. These studies provide direct electrophysiological evidence of a KATP-controlling glycolytic signaling complex and demonstrate its relevance to islet glucose sensing and excitability.

Single-cell infrared phenomics identifies cell heterogeneity of individual pancreatic islets in mouse model

Identifying the islet heterogeneity (cell types and the proportion of each subpopulation) and their relevance to function and disease will lead to fundamental information for the prevention and therapies of diabetes. Here, we introduce a single-cell phenotypic essay on the heterogeneity within individual pancreatic islets by using the combination of synchrotron infrared microspectroscopy and quantitative calculation. In a mouse model, the cellular heterogeneities at both the whole pancreas and single intact islet level were identified. The variation of biochemical phenotypes successfully subdivided islet cells into five main groups and quantitatively determined their proportion. These findings not only demonstrate single-cell infrared phenomics as a value complementary technique and strategy for the description of cellular heterogeneity within the pancreatic islets but also provide a quick, label-free optical platform for investigating phenotypic heterogeneity at the small-organelle level with single cell resolution.

Single-cell RNA sequencing combined with single-cell proteomics identifies the metabolic adaptation of islet cell subpopulations to high-fat diet in mice

AIMS/HYPOTHESIS

Islets have complex heterogeneity and subpopulations. Cell surface markers representing alpha, beta and delta cell subpopulations are urgently needed for investigations to explore the compositional changes of each subpopulation in obesity progress and diabetes onset, and the adaptation mechanism of islet metabolism induced by a high-fat diet (HFD).

METHODS

Single-cell RNA sequencing (scRNA-seq) was applied to identify alpha, beta and delta cell subpopulation markers in an HFD-induced mouse model of glucose intolerance. Flow cytometry and immunostaining were used to sort and assess the proportion of each subpopulation. Single-cell proteomics was performed on sorted cells, and the functional status of each alpha, beta and delta cell subpopulation in glucose intolerance was deeply elucidated based on protein expression.

RESULTS

A total of 33,999 cells were analysed by scRNA-seq and clustered into eight populations, including alpha, beta and delta cells. For alpha cells, scRNA-seq revealed that the Ace2^low subpopulation had downregulated expression of genes related to alpha cell function and upregulated expression of genes associated with beta cell characteristics in comparison with the Ace2^high subpopulation. The impaired function and increased fragility of ACE2^low alpha cells exposure to HFD was further suggested by single-cell proteomics. As for beta cells, the CD81^high subpopulation may indicate an immature signature of beta cells compared with the CD81^low subpopulation, which had robust function. We also found differential expression of Slc2a2 in delta cells and a potentially stronger cellular function and metabolism in GLUT2^low delta cells than GLUT2^high delta cells. Moreover, an increased proportion of ACE2^low alpha cells and CD81^low beta cells, with a constant proportion of GLUT2^low delta cells, were observed in HFD-induced glucose intolerance.

CONCLUSIONS/INTERPRETATION

We identified ACE2, CD81 and GLUT2 as surface markers to distinguish, respectively, alpha, beta and delta cell subpopulations with heterogeneous maturation and function. The changes in the proportion and functional status of islet endocrine subpopulations reflect the metabolic adaptation of islets to high-fat stress, which weakened the function of alpha cells and enhanced the function of beta and delta cells to bring about glycaemic homeostasis. Our findings provide a fundamental resource for exploring the mechanisms maintaining each islet endocrine subpopulation's fate and function in health and disease.

DATA AVAILABILITY

The scRNA-seq analysis datasets from the current study are available in the Gene Expression Omnibus (GEO) repository under the accession number GSE203376.

Revealing the tissue-level complexity of endogenous glucagon-like peptide-1 receptor expression and signaling

The glucagon-like peptide-1 receptor (GLP1R) is a class B G protein-coupled receptor (GPCR) involved in glucose homeostasis and food intake. GLP1R agonists (GLP1RA) are widely used in the treatment of diabetes and obesity, yet visualizing the endogenous localization, organization and dynamics of a GPCR has so far remained out of reach. In the present study, we generate mice harboring an enzyme self-label genome-edited into the endogenous Glp1r locus. We also rationally design and test various fluorescent dyes, spanning cyan to far-red wavelengths, for labeling performance in tissue. By combining these technologies, we show that endogenous GLP1R can be specifically and sensitively detected in primary tissue using multiple colors. Longitudinal analysis of GLP1R dynamics reveals heterogeneous recruitment of neighboring cell subpopulations into signaling and trafficking, with differences observed between GLP1RA classes and dual agonists. At the nanoscopic level, GLP1Rs are found to possess higher organization, undergoing GLP1RA-dependent membrane diffusion. Together, these results show the utility of enzyme self-labels for visualization and interrogation of endogenous proteins, and provide insight into the biology of a class B GPCR in primary cells and tissue.

Therapeutic Strategies Targeting Pancreatic Islet β-Cell Proliferation, Regeneration, and Replacement

Diabetes results from insufficient insulin production by pancreatic islet β-cells or a loss of β-cells themselves. Restoration of regulated insulin production is a predominant goal of translational diabetes research. Here, we provide a brief overview of recent advances in the fields of β-cell proliferation, regeneration, and replacement. The discovery of therapeutic targets and associated small molecules has been enabled by improved understanding of β-cell development and cell cycle regulation, as well as advanced high-throughput screening methodologies. Important findings in β-cell transdifferentiation, neogenesis, and stem cell differentiation have nucleated multiple promising therapeutic strategies. In particular, clinical trials are underway using in vitro-generated β-like cells from human pluripotent stem cells. Significant challenges remain for each of these strategies, but continued support for efforts in these research areas will be critical for the generation of distinct diabetes therapies.

Wnt4 is heterogeneously activated in maturing β-cells to control calcium signaling, metabolism and function

Diabetes is a multifactorial disorder characterized by loss or dysfunction of pancreatic β-cells. β-cells are heterogeneous, exhibiting different glucose sensing, insulin secretion and gene expression. They communicate with other endocrine cell types via paracrine signals and between β-cells via gap junctions. Here, we identify the importance of signaling between β-cells via the extracellular signal WNT4. We show heterogeneity in Wnt4 expression, most strikingly in the postnatal maturation period, Wnt4-positive cells, being more mature while Wnt4-negative cells are more proliferative. Knock-out in adult β-cells shows that WNT4 controls the activation of calcium signaling in response to a glucose challenge, as well as metabolic pathways converging to lower ATP/ADP ratios, thereby reducing insulin secretion. These results reveal that paracrine signaling between β-cells is important in addition to gap junctions in controling insulin secretion. Together with previous reports of WNT4 up-regulation in obesity our observations suggest an adaptive insulin response coordinating β-cells.

Microtubules in Pancreatic β Cells: Convoluted Roadways Toward Precision

Pancreatic islet β cells regulate glucose homeostasis via glucose-stimulated insulin secretion (GSIS). Cytoskeletal polymers microtubules (MTs) serve as tracks for the transport and positioning of secretory insulin granules. MT network in β cells has unique morphology with several distinct features, which support granule biogenesis (via Golgi-derived MT array), net non-directional transport (via interlocked MT mesh), and control availability of granules at secretion sites (via submembrane MT bundle). The submembrane MT array, which is parallel to the plasma membrane and serves to withdraw excessive granules from the secretion hot spots, is destabilized and fragmented downstream of high glucose stimulation, allowing for regulated secretion. The origin of such an unusual MT network, the features that define its functionality, and metabolic pathways that regulate it are still to a large extent elusive and are a matter of active investigation and debate. Besides the MT network itself, it is important to consider the interplay of molecular motors that drive and fine-tune insulin granule transport. Importantly, activity of kinesin-1, which is the major MT-dependent motor in β cells, transports insulin granules, and has a capacity to remodel MT network, is also regulated by glucose. We discuss yet unknown potential avenues toward understanding how MT network and motor proteins provide control for secretion in coordination with other GSIS-regulating mechanisms.

Maturation of beta cells: lessons from in vivo and in vitro models

The ability to maintain normoglycaemia, through glucose-sensitive insulin release, is a key aspect of postnatal beta cell function. However, terminally differentiated beta cell identity does not necessarily imply functional maturity. Beta cell maturation is therefore a continuation of beta cell development, albeit a process that occurs postnatally in mammals. Although many important features have been identified in the study of beta cell maturation, as of yet no unified mechanistic model of beta cell functional maturity exists. Here, we review recent findings about the underlying mechanisms of beta cell functional maturation. These findings include systemic hormonal and nutritional triggers that operate through energy-sensing machinery shifts within beta cells, resulting in primed metabolic states that allow for appropriate glucose trafficking and, ultimately, insulin release. We also draw attention to the expansive synergistic nature of these pathways and emphasise that beta cell maturation is dependent on overlapping regulatory and metabolic networks.

The long-term failure of RYGB surgery in improving T2DM is related to hyperinsulinism

BACKGROUND

Roux-en-Y gastric bypass (RYGB) is the gold standard method for bariatric surgery and leads to substantial improvements in Type 2 Diabetes mellitus. However, many patients experience relapses in diabetes five years after undergoing this aggressive surgical procedure. We focus on beta-cell population changes and absorptive intestinal consequences after RYGB in a healthy nonobese animal model after a long survival period.

METHODS

For our purpose, we use three groups of Wistar rats: RYGB-operated, surgical control (Sham) and fasting control. We measure alpha-, beta-cell mass; transcription (Arx, and Pdx-1) and proliferation (Ki67) factors; glucose tolerance and insulin release after oral glucose tests; histological adaptive changes in the jejunum; and intestinal glucose transporters.

RESULTS

Our results showed an early increase in insulin secretion after surgery, that decrease at the end of the study. The beta-cell mass reduces twenty-four weeks after RYGB, which coincides with decrease of Pdx-1 transcription promoter factor. These was coincident with an increase in alpha-mass and a high expression of Arx in RYGB group.

CONCLUSIONS

The analysis of all data showed beta-cell mass transdifferentiation into alpha-cell mass in RYGB rats. Due to long-term exhaustion of the beta-cell population by hyperinsulinism derived from digestive tract adaptation to surgery.

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.

Bayesian metamodeling of complex biological systems across varying representations

Comprehensive modeling of a whole cell requires an integration of vast amounts of information on various aspects of the cell and its parts. To divide and conquer this task, we introduce Bayesian metamodeling, a general approach to modeling complex systems by integrating a collection of heterogeneous input models. Each input model can in principle be based on any type of data and can describe a different aspect of the modeled system using any mathematical representation, scale, and level of granularity. These input models are 1) converted to a standardized statistical representation relying on probabilistic graphical models, 2) coupled by modeling their mutual relations with the physical world, and 3) finally harmonized with respect to each other. To illustrate Bayesian metamodeling, we provide a proof-of-principle metamodel of glucose-stimulated insulin secretion by human pancreatic β-cells. The input models include a coarse-grained spatiotemporal simulation of insulin vesicle trafficking, docking, and exocytosis; a molecular network model of glucose-stimulated insulin secretion signaling; a network model of insulin metabolism; a structural model of glucagon-like peptide-1 receptor activation; a linear model of a pancreatic cell population; and ordinary differential equations for systemic postprandial insulin response. Metamodeling benefits from decentralized computing, while often producing a more accurate, precise, and complete model that contextualizes input models as well as resolves conflicting information. We anticipate Bayesian metamodeling will facilitate collaborative science by providing a framework for sharing expertise, resources, data, and models, as exemplified by the Pancreatic β-Cell Consortium.

Temporal single-cell regeneration studies: the greatest thing since sliced pancreas?

The application of single-cell analytic techniques to the study of stem/progenitor cell niches supports the emerging view that pancreatic cell lineages are in a state of flux between differentiation stages. For all their value, however, such analyses merely offer a snapshot of the cellular palette of the tissue at any given time point. Conclusions about potential developmental/regeneration paths are solely based on bioinformatics inferences. In this context, the advent of new techniques for the long-term culture and lineage tracing of human pancreatic slices offers a virtual window into the native organ and presents the field with a unique opportunity to serially resolve pancreatic regeneration dynamics at the single-cell level.

NMDA receptor inhibition increases, synchronizes, and stabilizes the collective pancreatic beta cell activity: Insights through multilayer network analysis

NMDA receptors promote repolarization in pancreatic beta cells and thereby reduce glucose-stimulated insulin secretion. Therefore, NMDA receptors are a potential therapeutic target for diabetes. While the mechanism of NMDA receptor inhibition in beta cells is rather well understood at the molecular level, its possible effects on the collective cellular activity have not been addressed to date, even though proper insulin secretion patterns result from well-synchronized beta cell behavior. The latter is enabled by strong intercellular connectivity, which governs propagating calcium waves across the islets and makes the heterogeneous beta cell population work in synchrony. Since a disrupted collective activity is an important and possibly early contributor to impaired insulin secretion and glucose intolerance, it is of utmost importance to understand possible effects of NMDA receptor inhibition on beta cell functional connectivity. To address this issue, we combined confocal functional multicellular calcium imaging in mouse tissue slices with network science approaches. Our results revealed that NMDA receptor inhibition increases, synchronizes, and stabilizes beta cell activity without affecting the velocity or size of calcium waves. To explore intercellular interactions more precisely, we made use of the multilayer network formalism by regarding each calcium wave as an individual network layer, with weighted directed connections portraying the intercellular propagation. NMDA receptor inhibition stabilized both the role of wave initiators and the course of waves. The findings obtained with the experimental antagonist of NMDA receptors, MK-801, were additionally validated with dextrorphan, the active metabolite of the approved drug dextromethorphan, as well as with experiments on NMDA receptor KO mice. In sum, our results provide additional and new evidence for a possible role of NMDA receptor inhibition in treatment of type 2 diabetes and introduce the multilayer network paradigm as a general strategy to examine effects of drugs on connectivity in multicellular systems.

Single-cell chromatin accessibility identifies pancreatic islet cell type- and state-specific regulatory programs of diabetes risk

Single-nucleus assay for transposase-accessible chromatin using sequencing (snATAC-seq) creates new opportunities to dissect cell type-specific mechanisms of complex diseases. Since pancreatic islets are central to type 2 diabetes (T2D), we profiled 15,298 islet cells by using combinatorial barcoding snATAC-seq and identified 12 clusters, including multiple alpha, beta and delta cell states. We cataloged 228,873 accessible chromatin sites and identified transcription factors underlying lineage- and state-specific regulation. We observed state-specific enrichment of fasting glucose and T2D genome-wide association studies for beta cells and enrichment for other endocrine cell types. At T2D signals localized to islet-accessible chromatin, we prioritized variants with predicted regulatory function and co-accessibility with target genes. A causal T2D variant rs231361 at the KCNQ1 locus had predicted effects on a beta cell enhancer co-accessible with INS and genome editing in embryonic stem cell-derived beta cells affected INS levels. Together our findings demonstrate the power of single-cell epigenomics for interpreting complex disease genetics.

Mitochondrial gene expression in single cells shape pancreatic beta cells' sub-populations and explain variation in insulin pathway

Mitochondrial gene expression is pivotal to cell metabolism. Nevertheless, it is unknown whether it diverges within a given cell type. Here, we analysed single-cell RNA-seq experiments from human pancreatic alpha (N = 3471) and beta cells (N = 1989), as well as mouse beta cells (N = 1094). Cluster analysis revealed two distinct human beta cells populations, which diverged by mitochondrial (mtDNA) and nuclear DNA (nDNA)-encoded oxidative phosphorylation (OXPHOS) gene expression in healthy and diabetic individuals, and in newborn but not in adult mice. Insulin gene expression was elevated in beta cells with higher mtDNA gene expression in humans and in young mice. Such human beta cell populations also diverged in mitochondrial RNA mutational repertoire, and in their selective signature, thus implying the existence of two previously overlooked distinct and conserved beta cell populations. While applying our approach to human alpha cells, two sub-populations of cells were identified which diverged in mtDNA gene expression, yet these cellular populations did not consistently diverge in nDNA OXPHOS genes expression, nor did they correlate with the expression of glucagon, the hallmark of alpha cells. Thus, pancreatic beta cells within an individual are divided into distinct groups with unique metabolic-mitochondrial signature.

Integration of single-cell datasets reveals novel transcriptomic signatures of β-cells in human type 2 diabetes

Pancreatic islet β-cell failure is key to the onset and progression of type 2 diabetes (T2D). The advent of single-cell RNA sequencing (scRNA-seq) has opened the possibility to determine transcriptional signatures specifically relevant for T2D at the β-cell level. Yet, applications of this technique have been underwhelming, as three independent studies failed to show shared differentially expressed genes in T2D β-cells. We performed an integrative analysis of the available datasets from these studies to overcome confounding sources of variability and better highlight common T2D β-cell transcriptomic signatures. After removing low-quality transcriptomes, we retained 3046 single cells expressing 27 931 genes. Cells were integrated to attenuate dataset-specific biases, and clustered into cell type groups. In T2D β-cells (n = 801), we found 210 upregulated and 16 downregulated genes, identifying key pathways for T2D pathogenesis, including defective insulin secretion, SREBP signaling and oxidative stress. We also compared these results with previous data of human T2D β-cells from laser capture microdissection and diabetic rat islets, revealing shared β-cell genes. Overall, the present study encourages the pursuit of single β-cell RNA-seq analysis, preventing presently identified sources of variability, to identify transcriptomic changes associated with human T2D and underscores specific traits of dysfunctional β-cells across different models and techniques.

Cellular stressors may alter islet hormone cell proportions by moderation of alternative splicing patterns

Changes to islet cell identity in response to type 2 diabetes (T2D) have been reported in rodent models, but are less well characterized in humans. We assessed the effects of aspects of the diabetic microenvironment on hormone staining, total gene expression, splicing regulation and the alternative splicing patterns of key genes in EndoC-βH1 human beta cells. Genes encoding islet hormones [somatostatin (SST), insulin (INS), Glucagon (GCG)], differentiation markers [Forkhead box O1 (FOXO1), Paired box 6, SRY box 9, NK6 Homeobox 1, NK6 Homeobox 2] and cell stress markers (DNA damage inducible transcript 3, FOXO1) were dysregulated in stressed EndoC-βH1 cells, as were some serine arginine rich splicing factor splicing activator and heterogeneous ribonucleoprotein particle inhibitor genes. Whole transcriptome analysis of primary T2D islets and matched controls demonstrated dysregulated splicing for ~25% of splicing events, of which genes themselves involved in messenger ribonucleic acid processing and regulation of gene expression comprised the largest group. Approximately 5% of EndoC-βH1 cells exposed to these factors gained SST positivity in vitro. An increased area of SST staining was also observed ex vivo in pancreas sections recovered at autopsy from donors with type 1 diabetes (T1D) or T2D (9.3% for T1D and 3% for T2D, respectively compared with 1% in controls). Removal of the stressful stimulus or treatment with the AKT Serine/Threonine kinase inhibitor SH-6 restored splicing factor expression and reversed both hormone staining effects and patterns of gene expression. This suggests that reversible changes in hormone expression may occur during exposure to diabetomimetic cellular stressors, which may be mediated by changes in splicing regulation.

The Combination of Whole Cell Lipidomics Analysis and Single Cell Confocal Imaging of Fluidity and Micropolarity Provides Insight into Stress-Induced Lipid Turnover in Subcellular Organelles of Pancreatic Beta Cells

Modern omics techniques reveal molecular structures and cellular networks of tissues and cells in unprecedented detail. Recent advances in single cell analysis have further revolutionized all disciplines in cellular and molecular biology. These methods have also been employed in current investigations on the structure and function of insulin secreting beta cells under normal and pathological conditions that lead to an impaired glucose tolerance and type 2 diabetes. Proteomic and transcriptomic analyses have pointed to significant alterations in protein expression and function in beta cells exposed to diabetes like conditions (e.g., high glucose and/or saturated fatty acids levels). These nutritional overload stressful conditions are often defined as glucolipotoxic due to the progressive damage they cause to the cells. Our recent studies on the rat insulinoma-derived INS-1E beta cell line point to differential effects of such conditions in the phospholipid bilayers in beta cells. This review focuses on confocal microscopy-based detection of these profound alterations in the plasma membrane and membranes of insulin granules and lipid droplets in single beta cells under such nutritional load conditions.

Informing β-cell regeneration strategies using studies of heterogeneity

BACKGROUND

Current therapeutic strategies for type 1 (T1DM) and type 2 diabetes mellitus (T2DM) rely on increasing or substituting endogenous insulin secretion in combination with lifestyle changes. β-cell regeneration, a process whereby new β-cells arise from progenitors, self-renewal or transdifferentiation, has the potential to become a viable route to insulin self-sufficiency. Current regeneration strategies capture many of the transcriptomic and protein features of native β-cells, generating cells capable of glucose-dependent insulin secretion in vitro and alleviation of hyperglycemia in vivo. However, whether novel β-cells display appreciable heterogeneity remains poorly understood, with potential consequences for long-term functional robustness.

SCOPE OF REVIEW

The review brings together crucial discoveries in the β-cell regeneration field with state-of-the-art knowledge regarding β-cell heterogeneity. Aspects that might aid production of longer-lasting and more plastic regenerated β-cells are highlighted and discussed.

MAJOR CONCLUSIONS

Different β-cell regeneration approaches result in a similar outcome: glucose-sensitive, insulin-positive cells that mimic the native β-cell phenotype but which lack normal plasticity. The β-cell subpopulations identified to date expand our understanding of β-cell survival, proliferation and function, signposting the direction for future regeneration strategies. Therefore, regenerated β-cells should exhibit stimulus-dependent differences in gene and protein expression, as well as establish a functional network with different β-cells, all while coexisting with other cell types on a three-dimensional platform.

Heterogeneity and Delayed Activation as Hallmarks of Self-Organization and Criticality in Excitable Tissue

Self-organized critical dynamics is assumed to be an attractive mode of functioning for several real-life systems and entails an emergent activity in which the extent of observables follows a power-law distribution. The hallmarks of criticality have recently been observed in a plethora of biological systems, including beta cell populations within pancreatic islets of Langerhans. In the present study, we systematically explored the mechanisms that drive the critical and supercritical behavior in networks of coupled beta cells under different circumstances by means of experimental and computational approaches. Experimentally, we employed high-speed functional multicellular calcium imaging of fluorescently labeled acute mouse pancreas tissue slices to record calcium signals in a large number of beta cells simultaneously, and with a high spatiotemporal resolution. Our experimental results revealed that the cellular responses to stimulation with glucose are biphasic and glucose-dependent. Under physiological as well as under supraphysiological levels of stimulation, an initial activation phase was followed by a supercritical plateau phase with a high number of global intercellular calcium waves. However, the activation phase displayed fingerprints of critical behavior under lower stimulation levels, with a progressive recruitment of cells and a power-law distribution of calcium wave sizes. On the other hand, the activation phase provoked by pathophysiologically high glucose concentrations, differed considerably and was more rapid, less continuous, and supercritical. To gain a deeper insight into the experimentally observed complex dynamical patterns, we built up a phenomenological model of coupled excitable cells and explored empirically the model’s necessities that ensured a good overlap between computational and experimental results. It turned out that such a good agreement between experimental and computational findings was attained when both heterogeneous and stimulus-dependent time lags, variability in excitability levels, as well as a heterogeneous cell-cell coupling were included into the model. Most importantly, since our phenomenological approach involved only a few parameters, it naturally lends itself not only for determining key mechanisms of self-organized criticality at the tissue level, but also points out various features for comprehensive and realistic modeling of different excitable systems in nature.

Electrophysiology of the pancreatic islet β-cell sweet taste receptor TIR3

Over recent years, the presence of the sweet taste receptor TIR3 in rodent and human insulin-producing pancreatic islet β-cells was documented. The activation of this receptor by sweet-tasting sucralose mimics several biochemical and functional effects of D-glucose in the β-cells. The present study extends this analogy to the bioelectrical response of β-cells. In this respect, sucralose was inefficient in the absence of D-glucose, but induced on occasion electrical activity in mouse β-cells exposed to low non-stimulatory concentrations of the hexose and potentiated, in a concentration-related manner, the response to stimulatory concentrations of D-glucose. These data indicate that sucralose, acting as an agonist of the TIR3 receptor, exerts an excitatory effect upon pancreatic β-cell bioelectrical activity.

Time zones of pancreatic islet metabolism

Most living beings possess an intrinsic system of circadian oscillators, allowing anticipation of the Earth's rotation around its own axis. The mammalian circadian timing system orchestrates nearly all aspects of physiology and behaviour. Together with systemic signals originating from the central clock that resides in the hypothalamic suprachiasmatic nucleus, peripheral oscillators orchestrate tissue-specific fluctuations in gene transcription and translation, and posttranslational modifications, driving overt rhythms in physiology and behaviour. There is accumulating evidence of a reciprocal connection between the circadian oscillator and most aspects of physiology and metabolism, in particular as the circadian system plays a critical role in orchestrating body glucose homeostasis. Recent reports imply that circadian clocks operative in the endocrine pancreas regulate insulin secretion, and that islet clock perturbation in rodents leads to the development of overt type 2 diabetes. While whole islet clocks have been extensively studied during the last years, the heterogeneity of islet cell oscillators and the interplay between α- and β-cellular clocks for orchestrating glucagon and insulin secretion have only recently gained attention. Here, we review recent findings on the molecular makeup of the circadian clocks operative in pancreatic islet cells in rodents and in humans, and focus on the physiologically relevant synchronizers that are resetting these time-keepers. Moreover, the implication of islet clock functional outputs in the temporal coordination of metabolism in health and disease will be highlighted.

Effects of proinsulin misfolding on β-cell dynamics, differentiation and function in diabetes

ER stress due to proinsulin misfolding has an important role in the pathophysiology of rare forms of permanent neonatal diabetes (PNDM) and probably also of common type 1 (T1D) and type 2 diabetes (T2D). Accumulation of misfolded proinsulin in the ER stimulates the unfolded protein response (UPR) that may eventually lead to apoptosis through a process called the terminal UPR. However, the β-cell ER has an incredible ability to cope with accumulation of misfolded proteins; therefore, it is not clear whether in common forms of diabetes the accumulation of misfolded proinsulin exceeds the point of no return in which terminal UPR is activated. Many studies showed that the UPR is altered in both T1D and T2D; however, the observed changes in the expression of different UPR markers are inconsistent and it is not clear whether they reflect an adaptive response to stress or indeed mediate the β-cell dysfunction of diabetes. Herein, we critically review the literature on the effects of proinsulin misfolding and ER stress on β-cell dysfunction and loss in diabetes with emphasis on β-cell dynamics, and discuss the gaps in understanding the role of proinsulin misfolding in the pathophysiology of diabetes.

Advances in drug discovery for human beta cell regeneration

The numbers of insulin-secreting pancreatic beta cells are reduced in people with type 1 and type 2 diabetes. Driving beta cell regeneration in the pancreases of people with diabetes would be an attractive approach to reversing diabetes. While adult human beta cells have long been believed to be terminally differentiated and, therefore, irreversibly quiescent, it has become clear over recent years that this is not true. More specifically, both candidate and unbiased high-throughput screen approaches have revealed several classes of molecules that are clearly able to induce human beta cell proliferation. Here, we review recent approaches and accomplishments in human beta cell regenerative drug discovery. We also list the challenges that this rapidly moving field must confront to translate beta cell regenerative therapy from the laboratory to the clinic.

The role of beta cell heterogeneity in islet function and insulin release

It is becoming increasingly apparent that not all insulin-secreting beta cells are equal. Subtle differences exist at the transcriptomic and protein expression levels, with repercussions for beta cell survival/proliferation, calcium signalling and insulin release. Notably, beta cell heterogeneity displays plasticity during development, metabolic stress and type 2 diabetes mellitus (T2DM). Thus, heterogeneity or lack thereof may be an important contributor to beta cell failure during T2DM in both rodents and humans. The present review will discuss the molecular and cellular features of beta cell heterogeneity at both the single-cell and islet level, explore how this influences islet function and insulin release and look into the alterations that may occur during obesity and T2DM.

Transcription factor 7-like 2 gene links increased in vivo insulin synthesis to type 2 diabetes

Transcription factor 7-like 2 (TCF7L2) is the main susceptibility gene for type 2 diabetes, primarily through impairing the insulin secretion by pancreatic β cells. However, the exact in vivo mechanisms remain poorly understood. We performed a family study and determined if the T risk allele of the rs7903146 in the TCF7L2 gene increases the risk of type 2 diabetes based on real-time stable isotope measurements of insulin synthesis during an Oral Glucose Tolerance Test. In addition, we performed oral minimal model (OMM) analyses to assess insulin sensitivity and β cell function indices. Compared to unaffected relatives, individuals with type 2 diabetes had lower OMM indices and a higher level of insulin synthesis. We found a T allele-dosage effect on insulin synthesis and on glucose tolerance status, therefore insulin synthesis was higher among T-allele carriers with type 2 diabetes than in wild-type individuals. These results suggest that hyperinsulinemia is not only an adaptation to insulin resistance, but also a direct cause of type 2 diabetes.

The Cells of the Islets of Langerhans

Islets of Langerhans are islands of endocrine cells scattered throughout the pancreas. A number of new studies have pointed to the potential for conversion of non-β islet cells in to insulin-producing β-cells to replenish β-cell mass as a means to treat diabetes. Understanding normal islet cell mass and function is important to help advance such treatment modalities: what should be the target islet/β-cell mass, does islet architecture matter to energy homeostasis, and what may happen if we lose a particular population of islet cells in favour of β-cells? These are all questions to which we will need answers for islet replacement therapy by transdifferentiation of non-β islet cells to be a reality in humans. We know a fair amount about the biology of β-cells but not quite as much about the other islet cell types. Until recently, we have not had a good grasp of islet mass and distribution in the human pancreas. In this review, we will look at current data on islet cells, focussing more on non-β cells, and on human pancreatic islet mass and distribution.

Depletion of Beta Cell Intranuclear Rodlets in Human Type II Diabetes

Intranuclear rodlets (INRs) are rod-shaped intranuclear bodies of unknown function present in the nuclei of pancreatic beta cells. Previous studies have demonstrated a significant depletion of INRs from beta cells in mouse models of type II diabetes, suggesting that they may have pathological significance. The objective of the present study was to determine whether beta cell INRs show quantitative alterations in human type II diabetes. In sections of non-neoplastic pancreas from 23 diabetic patients and 23 controls who had undergone complete or partial pancreatectomy, we detected a significant reduction in the proportion of INRs in insulin-immunoreactive beta cells. In addition, we showed that beta cell INRs are immunoreactive for the RNA-binding protein HuR. The results of this study confirm and extend our previous study and implicate this enigmatic nuclear structure in the cellular pathophysiological mechanisms underlying the development of type II diabetes in humans.