Heterogenous impairment of α cell function in type 2 diabetes is linked to cell maturation state

Molecular correlates of glycine receptor activity in human β cells

OBJECTIVES

Glycine acts in an autocrine positive feedback loop in human β cells through its ionotropic receptors (GlyRs). In type 2 diabetes (T2D), islet GlyR activity is impaired by unknown mechanisms. We sought to investigate if the GlyR dysfunction in T2D is replicated by hyperglycemia per se, and to further characterize its action in β cells and islets.

METHODS

GlyR-mediated currents were measured using whole-cell patch-clamp in human β cells from donors with or without T2D, or after high glucose (15 mM) culture. We also correlated glycine-induced current amplitude with transcript expression levels through patch-seq. The expression of the GlyR α1, α3, and β subunit mRNA splice variants was compared between islets from donors with and without T2D, and after high glucose culture. Insulin secretion from human islets was measured in the presence or absence of the GlyR antagonist strychnine.

RESULTS

Although gene expression of GlyRs was decreased in T2D islets, and β cell GlyR-mediated currents were smaller, we found no evidence for a shift in GlyR subunit splicing. Glycine-induced currents are also reduced after 48 h culture of islets from donors without diabetes in high glucose, where we also find the reduction of the α1 subunit expression, but an increase in the α3 subunit. We discovered that glycine-evoked currents are highly heterogeneous amongst β cells, inversely correlate with donor HbA1c, and are significantly correlated to the expression of 92 different transcripts and gene regulatory networks (GRNs) that include CREB3(+), RREB1(+) and ZNF697(+). Finally, glucose-stimulated insulin secretion is decreased in the presence of the GlyR antagonist strychnine.

CONCLUSIONS

We demonstrate that glucose can modulate GlyR expression, and that the current decrease in T2D is likely due to the receptor gene expression downregulation, and not a change in transcript splicing. Moreover, we define a previously unknown set of genes and regulons that are correlated to GlyR-mediated currents and could be involved in GlyR downregulation in T2D. Among those we validate the negative impact of EIF4EBP1 expression on GlyR activity.

TMEM55A-mediated PI5P signalling regulates alpha cell actin depolymerisation and glucagon secretion

AIMS/HYPOTHESIS

Diabetes is associated with the dysfunction of glucagon-producing pancreatic islet alpha cells, although the underlying mechanisms regulating glucagon secretion and alpha cell dysfunction remain unclear. While insulin secretion from pancreatic beta cells has long been known to be controlled partly by intracellular phospholipid signalling, very little is known about the role of phospholipids in glucagon secretion. Using patch-clamp electrophysiology and single-cell RNA sequencing, we previously found that expression of PIP4P2 (encoding TMEM55A, a lipid phosphatase that dephosphorylates phosphatidylinositol-4,5-bisphosphate [PIP2] to phosphatidylinositol-5-phosphate [PI5P]) correlates with alpha cell function. We hypothesise that TMEM55A is involved in glucagon secretion and aim to validate the role of TMEM55A and its potential signalling molecules in alpha cell function and glucagon secretion.

METHODS

Correlation analysis was generated from the data in www.humanislets.com . Human islets were isolated at the Alberta Diabetes Institute IsletCore. Electrical recordings were performed on dispersed human or mouse islets with scrambled siRNA or si-PIP4P2 (si-Pip4p2 for mouse) transfection. Glucagon secretion was measured using an islet perfusion system with intact mouse islets. TMEM55A activity was measured using an in vitro on-beads phosphatase assay and live-cell imaging. GTPase activity was measured using an active GTPase pull-down assay. Confocal microscopy was used to quantify F-actin intensity using primary alpha cells and alphaTC1-9 cell lines after chemical treatment.

RESULTS

TMEM55A regulated alpha cell exocytosis and glucagon secretion. TMEM55A knockdown in both human and mouse alpha cells reduced exocytosis at low glucose levels and this was rescued by the direct reintroduction of PI5P. PI5P, instead of PIP2 increased the glucagon secretion using intact mouse islets. This did not occur through an effect on Ca2+ channel activity but through a remodelling of cortical F-actin dependent on TMEM55A lipid phosphatase activity, which occurred in response to oxidative stress. TMEM55A- and PI5P-induced F-actin remodelling depends on the inactivation of GTPase and RhoA, instead of Ras-related C3 botulinum toxin substrate 1 or CDC42.

CONCLUSIONS/INTERPRETATION

We reveal a novel pathway by which TMEM55A regulates alpha cell exocytosis by controlling intracellular PI5P and the F-actin network.

NKX2.2 and KLF4 cooperate to regulate α-cell identity

Transcription factors (TFs) are indispensable for maintaining cell identity through regulating cell-specific gene expression. Distinct cell identities derived from a common progenitor are frequently perpetuated by shared TFs, yet the mechanisms that enable these TFs to regulate cell-specific targets are poorly characterized. We report that the TF NKX2.2 is critical for the identity of pancreatic islet α cells by directly activating α-cell genes and repressing alternate islet cell fate genes. When compared with the known role of NKX2.2 in islet β cells, we demonstrate that NKX2.2 regulates α-cell genes, facilitated in part by α-cell-specific DNA binding at gene promoters. Furthermore, we have identified the reprogramming factor KLF4 as having enriched expression in α cells, where it co-occupies NKX2.2-bound α-cell promoters, is necessary for NKX2.2 promoter occupancy in α cells, and coregulates many NKX2.2 α-cell transcriptional targets. Overexpression of Klf4 in β cells is sufficient to manipulate chromatin accessibility, increase binding of NKX2.2 at α-cell-specific promoter sites, and alter expression of NKX2.2-regulated cell-specific targets. This study identifies KLF4 as a novel α-cell factor that cooperates with NKX2.2 to regulate α-cell identity.

Hyperglucagonemia and glucagon hypersecretion in early type 2 diabetes result from multifaceted dysregulation of pancreatic mouse α-cells

Hyperglucagonemia has been implicated in the pathogenesis of type 2 diabetes (T2D). In contrast to β-cells, studies on the function of the pancreatic α-cell in T2D are scarce. Consequently, the processes underlying hyperglucagonemia and α-cell dysfunction are largely unknown, limiting the appropriate design of specific pharmacological and therapeutic strategies. In the current study, we aimed to analyze the alterations of the pancreatic α-cell and its glucagon responses in diabetic db/db mice at early stages of the disease. In this context of glucose intolerance, hyperinsulinemia, and β-cell dysfunction, hyperglucagonemia was only present at fed conditions and was associated with insulin resistance. Yet, we found that the glucagon-to-insulin ratio in db/db mice did not change with fed or fasted states, further supporting that the metabolic regulation of glucagon release was impaired. Pancreatic β-cell dysfunction in db/db mice was manifested by increased basal secretion from isolated islets along with reduced insulin content. In contrast, α-cells from diabetic animals presented upregulated secretion and islet content of glucagon compared with controls. Electrophysiological analysis of dispersed α-cells revealed that altered secretion was not the result of impaired exocytosis. Instead, we found defective regulation of Ca2+ signaling by glucose. Besides these functional alterations, we also observed augmented α-cell mass in diabetic mice, which was accompanied by disrupted islet cytoarchitecture as well as increased α-cell size and number, without pieces of evidence of upregulated proliferation. Overall, these findings indicate that hyperglucagonemia in early T2D results from multifaceted α-cell deregulation in mice.

Transcriptomic heterogeneity of non-beta islet cells is associated with type 2 diabetes development in mouse models

AIMS/HYPOTHESIS

The aim of this work was to understand the role of non-beta cells in pancreatic islets at early stages of type 2 diabetes pathogenesis.

METHODS

Specific clustering was employed to single-cell transcriptome data from islet cells of obese mouse strains differing in their diabetes susceptibility (diabetes-resistant B6.V.Lepob/ob [OB] and diabetes-susceptible New Zealand Obese [NZO] mice) on a diabetogenic diet.

RESULTS

Refined clustering analysis revealed several heterogeneous subpopulations for alpha cells, delta cells and macrophages, of which 133 mapped to human diabetes genes identified by genome-wide association studies. Importantly, a similar non-beta cell heterogeneity was found in a dataset of human islets from donors at different stages of type 2 diabetes. The predominant alpha cell cluster in NZO mice displayed signs of cellular stress and lower mitochondrial capacity (97 differentially expressed genes [DEGs]), whereas delta cells from these mice exhibited higher expression levels of maturation marker genes (Hhex and Sst) but lower somatostatin secretion than OB mice (184 DEGs). Furthermore, a cluster of macrophages was almost twice as abundant in islets of OB mice, and displayed extensive cell-cell communication with beta cells of OB mice. Treatment of beta cells with IL-15, predicted to be released by macrophages, activated signal transducer and activator of transcription (STAT3), which may mediate anti-apoptotic effects. Similar to mice, humans without diabetes possess a greater number of macrophages than those with prediabetes (39 mmol/mol [5.7%] < HbA1c < 46 mmol/mol [6.4%]) and diabetes.

CONCLUSIONS/INTERPRETATION

Our study indicates that the transcriptional heterogeneity of non-beta cells has an impact on intra-islet crosstalk and participates in beta cell (dys)function.

DATA AVAILABILITY

scRNA-seq data from the previous study are available in gene expression omnibus under gene accession number GSE159211 ( https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE159211 ).

TMEM55A-mediated PI5P signaling regulates α-cell actin depolymerization and glucagon secretion

Diabetes is associated with the dysfunction of glucagon-producing pancreatic islet α-cells, although the underlying mechanisms regulating glucagon secretion and α-cell dysfunction remain unclear. While insulin secretion from pancreatic β-cells has long been known to be partly controlled by intracellular phospholipid signaling, very little is known about the role of phospholipids in glucagon secretion. Here we show that TMEM55A, a lipid phosphatase that dephosphorylates phosphatidylinositol-4,5-bisphosphate (PIP2) to phosphatidylinositol-5-phosphate (PI5P), regulates α-cell exocytosis and glucagon secretion. TMEM55A knockdown in both human and mouse α-cells reduces exocytosis at low glucose, and this is rescued by the direct reintroduction of PI5P. This does not occur through an effect on Ca2+ channel activity, but through a re-modelling of cortical F-actin dependent upon TMEM55A lipid phosphatase activity which occurs in response to oxidative stress. In summary, we reveal a novel pathway by which TMEM55A regulates α-cell exocytosis by manipulating intracellular PI5P level and the F-actin network.

Non-ionotropic voltage-gated calcium channel signaling

Voltage-gated calcium channels (VGCCs) are the major conduits for calcium ions (Ca2+) within excitable cells. Recent studies have highlighted the non-ionotropic functionality of VGCCs, revealing their capacity to activate intracellular pathways independently of ion flow. This non-ionotropic signaling mode plays a pivotal role in excitation-coupling processes, including gene transcription through excitation-transcription (ET), synaptic transmission via excitation-secretion (ES), and cardiac contraction through excitation-contraction (EC). However, it is noteworthy that these excitation-coupling processes require extracellular calcium (Ca2+) and Ca2+ occupancy of the channel ion pore. Analogous to the "non-canonical" characterization of the non-ionotropic signaling exhibited by the N-methyl-D-aspartate receptor (NMDA), which requires extracellular Ca2+ without the influx of ions, VGCC activation requires depolarization-triggered conformational change(s) concomitant with Ca2+ binding to the open channel. Here, we discuss the contributions of VGCCs to ES, ET, and EC coupling as Ca2+ binding macromolecules that transduces external stimuli to intracellular input prior to elevating intracellular Ca2+. We emphasize the recognition of calcium ion occupancy within the open ion-pore and its contribution to the excitation coupling processes that precede the influx of calcium. The non-ionotropic activation of VGCCs, triggered by the upstroke of an action potential, provides a conceptual framework to elucidate the mechanistic aspects underlying the microseconds nature of synaptic transmission, cardiac contractility, and the rapid induction of first-wave genes.

Heterogeneous endocrine cell composition defines human islet functional phenotypes

Phenotyping and genotyping initiatives within the Integrated Islet Distribution Program (IIDP), the largest source of human islets for research in the U.S., provide standardized assessment of islet preparations distributed to researchers, enabling the integration of multiple data types. Data from islets of the first 299 organ donors without diabetes, analyzed using this pipeline, highlights substantial heterogeneity in islet cell composition associated with hormone secretory traits, sex, reported race and ethnicity, genetically predicted ancestry, and genetic risk for type 2 diabetes (T2D). While α and β cell composition influenced insulin and glucagon secretory traits, the abundance of δ cells showed the strongest association with insulin secretion and was also associated with the genetic risk score (GRS) for T2D. These findings have important implications for understanding mechanisms underlying diabetes heterogeneity and islet dysfunction and may provide insight into strategies for personalized medicine and β cell replacement therapy.

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.

Heterogeneous enhancer states orchestrate β cell responses to metabolic stress

Obesity-induced β cell dysfunction contributes to the onset of type 2 diabetes. Nevertheless, elucidating epigenetic mechanisms underlying islet dysfunction at single cell level remains challenging. Here we profile single-nuclei RNA along with enhancer marks H3K4me1 or H3K27ac in islets from lean or obese mice. Our study identifies distinct gene signatures and enhancer states correlating with β cell dysfunction trajectory. Intriguingly, while many metabolic stress-induced genes exhibit concordant changes in both H3K4me1 and H3K27ac at their enhancers, expression changes of specific subsets are solely attributable to either H3K4me1 or H3K27ac dynamics. Remarkably, a subset of H3K4me1+H3K27ac- primed enhancers prevalent in lean β cells and occupied by FoxA2 are largely absent after metabolic stress. Lastly, cell-cell communication analysis identified the nerve growth factor (NGF) as protective paracrine signaling for β cells through repressing ER stress. In summary, our findings define the heterogeneous enhancer responses to metabolic challenges in individual β cells.

Exploring the design of clinical research studies on the efficacy mechanisms in type 2 diabetes mellitus

This review examines the complexities of Type 2 Diabetes Mellitus (T2DM), focusing on the critical role of integrating omics technologies with traditional experimental methods. It underscores the advancements in understanding the genetic diversity of T2DM and emphasizes the evolution towards personalized treatment modalities. The paper analyzes a variety of omics approaches, including genomics, methylation, transcriptomics, proteomics, metabolomics, and intestinal microbiomics, delineating their substantial contributions to deciphering the multifaceted mechanisms underlying T2DM. Furthermore, the review highlights the indispensable role of non-omics experimental techniques in comprehending and managing T2DM, advocating for their integration in the development of tailored medicine and precision treatment strategies. By identifying existing research gaps and suggesting future research trajectories, the review underscores the necessity for a comprehensive, multidisciplinary approach. This approach synergistically combines clinical insights with cutting-edge biotechnologies, aiming to refine the management and therapeutic interventions of T2DM, and ultimately enhancing patient outcomes. This synthesis of knowledge and methodologies paves the way for innovative advancements in T2DM research, fostering a deeper understanding and more effective treatment of this complex condition.

Leucine Suppresses α-Cell cAMP and Glucagon Secretion via a Combination of Cell-Intrinsic and Islet Paracrine Signaling

Glucagon is critical for the maintenance of blood glucose, however nutrient regulation of pancreatic α-cells remains poorly understood. Here, we identified a role of leucine, a well-known β-cell fuel, in the α-cell-intrinsic regulation of glucagon release. In islet perifusion assays, physiologic concentrations of leucine strongly inhibited alanine- and arginine-stimulated glucagon secretion from human and mouse islets under hypoglycemic conditions. Mechanistically, leucine dose-dependently reduced α-cell cAMP, independently of Ca2+, ATP/ADP, or fatty acid oxidation. Leucine also reduced α-cell cAMP in islets treated with somatostatin receptor 2 antagonists or diazoxide, compounds that limit paracrine signaling from β/δ-cells. Studies in dispersed mouse islets confirmed an α-cell-intrinsic effect. The inhibitory effect of leucine on cAMP was mimicked by glucose, α-ketoisocaproate, succinate, and the glutamate dehydrogenase activator BCH and blocked by cyanide, indicating a mechanism dependent on mitochondrial metabolism. Glucose dose-dependently reduced the impact of leucine on α-cell cAMP, indicating an overlap in function; however, leucine was still effective at suppressing glucagon secretion in the presence of elevated glucose, amino acids, and the incretin GIP. Taken together, these findings show that leucine plays an intrinsic role in limiting the α-cell secretory tone across the physiologic range of glucose levels, complementing the inhibitory paracrine actions of β/δ-cells.

ARTICLE HIGHLIGHTS

HumanIslets: An integrated platform for human islet data access and analysis

Comprehensive molecular and cellular phenotyping of human islets can enable deep mechanistic insights for diabetes research. We established the Human Islet Data Analysis and Sharing (HI-DAS) consortium to advance goals in accessibility, usability, and integration of data from human islets isolated from donors with and without diabetes at the Alberta Diabetes Institute (ADI) IsletCore. Here we introduce HumanIslets.com, an open resource for the research community. This platform, which presently includes data on 547 human islet donors, allows users to access linked datasets describing molecular profiles, islet function and donor phenotypes, and to perform various statistical and functional analyses at the donor, islet and single-cell levels. As an example of the analytic capacity of this resource we show a dissociation between cell culture effects on transcript and protein expression, and an approach to correct for exocrine contamination found in hand-picked islets. Finally, we provide an example workflow and visualization that highlights links between type 2 diabetes status, SERCA3b Ca2+-ATPase levels at the transcript and protein level, insulin secretion and islet cell phenotypes. HumanIslets.com provides a growing and adaptable set of resources and tools to support the metabolism and diabetes research community.

The endoplasmic reticulum plays a key role in α-cell intracellular Ca2+ dynamics and glucose-regulated glucagon secretion in mouse islets

Glucagon is secreted by pancreatic α-cells to counteract hypoglycaemia. How glucose regulates glucagon secretion remains unclear. Here, using mouse islets, we studied the role of transmembrane and endoplasmic reticulum (ER) Ca2+ on intrinsic α-cell glucagon secretion. Blocking isradipine-sensitive L-type voltage-gated Ca2+ (Cav) channels abolished α-cell electrical activity but had little impact on its cytosolic Ca2+ oscillations or low-glucose-stimulated glucagon secretion. In contrast, depleting ER Ca2+ with cyclopiazonic acid or blocking ER Ca2+-releasing ryanodine receptors abolished α-cell glucose sensitivity and low-glucose-stimulated glucagon secretion. ER Ca2+ mobilization in α-cells is regulated by intracellular ATP and likely to be coupled to Ca2+ influx through P/Q-type Cav channels. ω-Agatoxin IVA blocked α-cell ER Ca2+ release and cell exocytosis, but had no additive effect on glucagon secretion when combined with ryanodine. We conclude that glucose regulates glucagon secretion through the control of ER Ca2+ mobilization, a mechanism that can be independent of α-cell electrical activity.

Deconvolution of polygenic risk score in single cells unravels cellular and molecular heterogeneity of complex human diseases

Polygenic risk scores (PRSs) are commonly used for predicting an individual's genetic risk of complex diseases. Yet, their implication for disease pathogenesis remains largely limited. Here, we introduce scPRS, a geometric deep learning model that constructs single-cell-resolved PRS leveraging reference single-cell chromatin accessibility profiling data to enhance biological discovery as well as disease prediction. Real-world applications across multiple complex diseases, including type 2 diabetes (T2D), hypertrophic cardiomyopathy (HCM), and Alzheimer's disease (AD), showcase the superior prediction power of scPRS compared to traditional PRS methods. Importantly, scPRS not only predicts disease risk but also uncovers disease-relevant cells, such as hormone-high alpha and beta cells for T2D, cardiomyocytes and pericytes for HCM, and astrocytes, microglia and oligodendrocyte progenitor cells for AD. Facilitated by a layered multi-omic analysis, scPRS further identifies cell-type-specific genetic underpinnings, linking disease-associated genetic variants to gene regulation within corresponding cell types. We substantiate the disease relevance of scPRS-prioritized HCM genes and demonstrate that the suppression of these genes in HCM cardiomyocytes is rescued by Mavacamten treatment. Additionally, we establish a novel microglia-specific regulatory relationship between the AD risk variant rs7922621 and its target genes ANXA11 and TSPAN14. We further illustrate the detrimental effects of suppressing these two genes on microglia phagocytosis. Our work provides a multi-tasking, interpretable framework for precise disease prediction and systematic investigation of the genetic, cellular, and molecular basis of complex diseases, laying the methodological foundation for single-cell genetics.

GLP1R and GIPR expression and signaling in pancreatic alpha cells, beta cells and delta cells

Glucagon-like peptide-1 receptor (GLP1R) and glucose-dependent insulinotropic polypeptide receptor (GIPR) are transmembrane receptors involved in insulin, glucagon and somatostatin secretion from the pancreatic islet. Therapeutic targeting of GLP1R and GIPR restores blood glucose levels in part by influencing beta cell, alpha cell and delta cell function. Despite the importance of the incretin-mimetics for diabetes therapy, our understanding of GLP1R and GIPR expression patterns and signaling within the islet remain incomplete. Here, we present the evidence for GLP1R and GIPR expression in the major islet cell types, before addressing signaling pathway(s) engaged, as well as their influence on cell survival and function. While GLP1R is largely a beta cell-specific marker within the islet, GIPR is expressed in alpha cells, beta cells, and (possibly) delta cells. GLP1R and GIPR engage Gs-coupled pathways in most settings, although the exact outcome on hormone release depends on paracrine communication and promiscuous signaling. Biased agonism away from beta-arrestin is an emerging concept for improving therapeutic efficacy, and is also relevant for GLP1R/GIPR dual agonism. Lastly, dual agonists exert multiple effects on islet function through GIPR > GLP1R imbalance, increased GLP1R surface expression and cAMP signaling, as well as beneficial alpha cell-beta cell-delta cell crosstalk.

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.

Glucagon-producing α-cell transcriptional identity and reprogramming towards insulin production

β-Cell replacement by in situ reprogramming of non-β-cells is a promising diabetes therapy. Following the observation that near-total β-cell ablation in adult mice triggers the reprogramming of pancreatic α-, δ-, and γ-cells into insulin (INS)-producing cells, recent studies are delving deep into the mechanisms controlling adult α-cell identity. Systematic analyses of the α-cell transcriptome and epigenome have started to pinpoint features that could be crucial for maintaining α-cell identity. Using different transgenic and chemical approaches, significant advances have been made in reprogramming α-cells in vivo into INS-secreting cells in mice. The recent reprogramming of human α-cells in vitro is an important step forward that must now be complemented with a comprehensive molecular dissection of the mechanisms controlling α-cell identity.

RFX6 Maintains Gene Expression and Function of Adult Human Islet α-Cells

Mutations in the gene encoding the transcription factor regulatory factor X-box binding 6 (RFX6) are associated with human diabetes. Within pancreatic islets, RFX6 expression is most abundant in islet α-cells, and α-cell RFX6 expression is altered in diabetes. However, the roles of RFX6 in regulating gene expression, glucagon output, and other crucial human adult α-cell functions are not yet understood. We developed a method for selective genetic targeting of human α-cells and assessed RFX6-dependent α-cell function. RFX6 suppression with RNA interference led to impaired α-cell exocytosis and dysregulated glucagon secretion in vitro and in vivo. By contrast, these phenotypes were not observed with RFX6 suppression across all islet cells. Transcriptomics in α-cells revealed RFX6-dependent expression of genes governing nutrient sensing, hormone processing, and secretion, with some of these exclusively expressed in human α-cells. Mapping of RFX6 DNA-binding sites in primary human islet cells identified a subset of direct RFX6 target genes. Together, these data unveil RFX6-dependent genetic targets and mechanisms crucial for regulating adult human α-cell function.

ARTICLE HIGHLIGHTS

Integrating single-cell transcriptomics with cellular phenotypes: cell morphology, Ca2+ imaging and electrophysiology

I review recent technological advancements in coupling single-cell transcriptomics with cellular phenotypes including morphology, calcium signaling, and electrophysiology. Single-cell RNA sequencing (scRNAseq) has revolutionized cell type classifications by capturing the transcriptional diversity of cells. A new wave of methods to integrate scRNAseq and biophysical measurements is facilitating the linkage of transcriptomic data to cellular function, which provides physiological insight into cellular states. I briefly discuss critical factors of these phenotypical characterizations such as timescales, information content, and analytical tools. Dedicated sections focus on the integration with cell morphology, calcium imaging, and electrophysiology (patch-seq), emphasizing their complementary roles. I discuss their application in elucidating cellular states, refining cell type classifications, and uncovering functional differences in cell subtypes. To illustrate the practical applications and benefits of these methods, I highlight their use in tissues with excitable cell-types such as the brain, pancreatic islets, and the retina. The potential of combining functional phenotyping with spatial transcriptomics for a detailed mapping of cell phenotypes in situ is explored. Finally, I discuss open questions and future perspectives, emphasizing the need for a shift towards broader accessibility through increased throughput.

A primer on modelling pancreatic islets: from models of coupled β-cells to multicellular islet models

Pancreatic islets are mini-organs composed of hundreds or thousands of ɑ, β and δ-cells, which, respectively, secrete glucagon, insulin and somatostatin, key hormones for the regulation of blood glucose. In pancreatic islets, hormone secretion is tightly regulated by both internal and external mechanisms, including electrical communication and paracrine signaling between islet cells. Given its complexity, the experimental study of pancreatic islets has been complemented with computational modeling as a tool to gain a better understanding about how all the mechanisms involved at different levels of organization interact. In this review, we describe how multicellular models of pancreatic cells have evolved from the early models of electrically coupled β-cells to models in which experimentally derived architectures and both electrical and paracrine signals have been considered.

Patch-seq: Advances and Biological Applications

Multimodal analysis of gene-expression patterns, electrophysiological properties, and morphological phenotypes at the single-cell/single-nucleus level has been arduous because of the diversity and complexity of neurons. The emergence of Patch-sequencing (Patch-seq) directly links transcriptomics, morphology, and electrophysiology, taking neuroscience research to a multimodal era. In this review, we summarized the development of Patch-seq and recent applications in the cortex, hippocampus, and other nervous systems. Through generating multimodal cell type atlases, targeting specific cell populations, and correlating transcriptomic data with phenotypic information, Patch-seq has provided new insight into outstanding questions in neuroscience. We highlight the challenges and opportunities of Patch-seq in neuroscience and hope to shed new light on future neuroscience research.

Identify gestational diabetes mellitus by deep learning model from cell-free DNA at the early gestation stage

Gestational diabetes mellitus (GDM) is a common complication of pregnancy, which has significant adverse effects on both the mother and fetus. The incidence of GDM is increasing globally, and early diagnosis is critical for timely treatment and reducing the risk of poor pregnancy outcomes. GDM is usually diagnosed and detected after 24 weeks of gestation, while complications due to GDM can occur much earlier. Copy number variations (CNVs) can be a possible biomarker for GDM diagnosis and screening in the early gestation stage. In this study, we proposed a machine-learning method to screen GDM in the early stage of gestation using cell-free DNA (cfDNA) sequencing data from maternal plasma. Five thousand and eighty-five patients from north regions of Mainland China, including 1942 GDM, were recruited. A non-overlapping sliding window method was applied for CNV coverage screening on low-coverage (~0.2×) sequencing data. The CNV coverage was fed to a convolutional neural network with attention architecture for the binary classification. The model achieved a classification accuracy of 88.14%, precision of 84.07%, recall of 93.04%, F1-score of 88.33% and AUC of 96.49%. The model identified 2190 genes associated with GDM, including DEFA1, DEFA3 and DEFB1. The enriched gene ontology (GO) terms and KEGG pathways showed that many identified genes are associated with diabetes-related pathways. Our study demonstrates the feasibility of using cfDNA sequencing data and machine-learning methods for early diagnosis of GDM, which may aid in early intervention and prevention of adverse pregnancy outcomes.

Human Pancreatic α-Cell Heterogeneity and Trajectory Inference Analysis Using Integrated Single Cell- and Single Nucleus-RNA Sequencing Platforms

Prior studies have shown that pancreatic α-cells can transdifferentiate into β-cells, and that β-cells de-differentiate and are prone to acquire an α-cell phenotype in type 2 diabetes (T2D). However, the specific human α-cell and β-cell subtypes that are involved in α-to-β-cell and β-to-α-cell transitions are unknown. Here, we have integrated single cell RNA sequencing (scRNA-seq) and single nucleus RNA-seq (snRNA-seq) of isolated human islets and human islet grafts and provide additional insight into α-β cell fate switching. Using this approach, we make seven novel observations. 1) There are five different GCG -expressing human α-cell subclusters [α1, α2, α-β-transition 1 (AB-Tr1), α-β-transition 2 (AB-Tr2), and α-β (AB) cluster] with different transcriptome profiles in human islets from non-diabetic donors. 2) The AB subcluster displays multihormonal gene expression, inferred mostly from snRNA-seq data suggesting identification by pre-mRNA expression. 3) The α1, α2, AB-Tr1, and AB-Tr2 subclusters are enriched in genes specific for α-cell function while AB cells are enriched in genes related to pancreatic progenitor and β-cell pathways; 4) Trajectory inference analysis of extracted α- and β-cell clusters and RNA velocity/PAGA analysis suggests a bifurcate transition potential for AB towards both α- and β-cells. 5) Gene commonality analysis identifies ZNF385D, TRPM3, CASR, MEG3 and HDAC9 as signature for trajectories moving towards β-cells and SMOC1, PLCE1, PAPPA2, ZNF331, ALDH1A1, SLC30A8, BTG2, TM4SF4, NR4A1 and PSCK2 as signature for trajectories moving towards α-cells. 6) Remarkably, in contrast to the events in vitro , the AB subcluster is not identified in vivo in human islet grafts and trajectory inference analysis suggests only unidirectional transition from α-to-β-cells in vivo . 7) Analysis of scRNA-seq datasets from adult human T2D donor islets reveals a clear unidirectional transition from β-to-α-cells compatible with dedifferentiation or conversion into α-cells. Collectively, these studies show that snRNA-seq and scRNA-seq can be leveraged to identify transitions in the transcriptional status among human islet endocrine cell subpopulations in vitro , in vivo , in non-diabetes and in T2D. They reveal the potential gene signatures for common trajectories involved in interconversion between α- and β-cells and highlight the utility and power of studying single nuclear transcriptomes of human islets in vivo . Most importantly, they illustrate the importance of studying human islets in their natural in vivo setting.

Hyperglucagonaemia in diabetes: altered amino acid metabolism triggers mTORC1 activation, which drives glucagon production

AIM/HYPOTHESIS

Hyperglycaemia is associated with alpha cell dysfunction, leading to dysregulated glucagon secretion in type 1 and type 2 diabetes; however, the mechanisms involved are still elusive. The nutrient sensor mammalian target of rapamycin complex 1 (mTORC1) plays a major role in the maintenance of alpha cell mass and function. We studied the regulation of alpha cell mTORC1 by nutrients and its role in the development of hyperglucagonaemia in diabetes.

METHODS

Alpha cell mTORC1 activity was assessed by immunostaining for phosphorylation of its downstream target, the ribosomal protein S6, and glucagon, followed by confocal microscopy on pancreatic sections and flow cytometry on dispersed human and mouse islets and the alpha cell line, αTC1-6. Metabolomics and metabolic flux were studied by 13C glucose labelling in 2.8 or 16.7 mmol/l glucose followed by LC-MS analysis. To study the role of mTORC1 in mediating hyperglucagonaemia in diabetes, we generated an inducible alpha cell-specific Rptor knockout in the Akita mouse model of diabetes and tested the effects on glucose tolerance by IPGTT and on glucagon secretion.

RESULTS

mTORC1 activity was increased in alpha cells from diabetic Akita mice in parallel to the development of hyperglycaemia and hyperglucagonaemia (two- to eightfold increase). Acute exposure of mouse and human islets to amino acids stimulated alpha cell mTORC1 (3.5-fold increase), whereas high glucose concentrations inhibited mTORC1 (1.4-fold decrease). The mTORC1 response to glucose was abolished in human and mouse diabetic alpha cells following prolonged islet exposure to high glucose levels, resulting in sustained activation of mTORC1, along with increased glucagon secretion. Metabolomics and metabolic flux analysis showed that exposure to high glucose levels enhanced glycolysis, glucose oxidation and the synthesis of glucose-derived amino acids. In addition, chronic exposure to high glucose levels increased the expression of Slc7a2 and Slc38a4, which encode amino acid transporters, as well as the levels of branched-chain amino acids and methionine cycle metabolites (~1.3-fold increase for both). Finally, conditional Rptor knockout in alpha cells from adult diabetic mice inhibited mTORC1, thereby inhibiting glucagon secretion (~sixfold decrease) and improving diabetes, despite persistent insulin deficiency.

CONCLUSIONS/INTERPRETATION

Alpha cell exposure to hyperglycaemia enhances amino acid synthesis and transport, resulting in sustained activation of mTORC1, thereby increasing glucagon secretion. mTORC1 therefore plays a major role in mediating alpha cell dysfunction in diabetes.

DATA AVAILABILITY

All sequencing data are available from the Gene Expression Omnibus (GEO) repository (accession no. GSE154126; https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE154126 ).

Delineating mouse β-cell identity during lifetime and in diabetes with a single cell atlas

Although multiple pancreatic islet single-cell RNA-sequencing (scRNA-seq) datasets have been generated, a consensus on pancreatic cell states in development, homeostasis and diabetes as well as the value of preclinical animal models is missing. Here, we present an scRNA-seq cross-condition mouse islet atlas (MIA), a curated resource for interactive exploration and computational querying. We integrate over 300,000 cells from nine scRNA-seq datasets consisting of 56 samples, varying in age, sex and diabetes models, including an autoimmune type 1 diabetes model (NOD), a glucotoxicity/lipotoxicity type 2 diabetes model (db/db) and a chemical streptozotocin β-cell ablation model. The β-cell landscape of MIA reveals new cell states during disease progression and cross-publication differences between previously suggested marker genes. We show that β-cells in the streptozotocin model transcriptionally correlate with those in human type 2 diabetes and mouse db/db models, but are less similar to human type 1 diabetes and mouse NOD β-cells. We also report pathways that are shared between β-cells in immature, aged and diabetes models. MIA enables a comprehensive analysis of β-cell responses to different stressors, providing a roadmap for the understanding of β-cell plasticity, compensation and demise.

Leucine suppresses glucagon secretion from pancreatic islets by directly modulating α-cell cAMP

Objective

Pancreatic islets are nutrient sensors that regulate organismal blood glucose homeostasis. Glucagon release from the pancreatic α-cell is important under fasted, fed, and hypoglycemic conditions, yet metabolic regulation of α-cells remains poorly understood. Here, we identified a previously unexplored role for physiological levels of leucine, which is classically regarded as a β-cell fuel, in the intrinsic regulation of α-cell glucagon release.

Methods

GcgCreERT:CAMPER and GcgCreERT:GCaMP6s mice were generated to perform dynamic, high-throughput functional measurements of α-cell cAMP and Ca2+ within the intact islet. Islet perifusion assays were used for simultaneous, time-resolved measurements of glucagon and insulin release from mouse and human islets. The effects of leucine were compared with glucose and the mitochondrial fuels 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid (BCH, non-metabolized leucine analog that activates glutamate dehydrogenase), α-ketoisocaproate (KIC, leucine metabolite), and methyl-succinate (complex II fuel). CYN154806 (Sstr2 antagonist), diazoxide (KATP activator, which prevents Ca2+-dependent exocytosis from α, β, and δ-cells), and dispersed α-cells were used to inhibit islet paracrine signaling and identify α-cell intrinsic effects.

Results

Mimicking the effect of glucose, leucine strongly suppressed amino acid-stimulated glucagon secretion. Mechanistically, leucine dose-dependently reduced α-cell cAMP at physiological concentrations, with an IC50 of 57, 440, and 1162 μM at 2, 6, and 10 mM glucose, without affecting α-cell Ca2+. Leucine also reduced α-cell cAMP in islets treated with Sstr2 antagonist or diazoxide, as well as dispersed α-cells, indicating an α-cell intrinsic effect. The effect of leucine was matched by KIC and the glutamate dehydrogenase activator BCH, but not methyl-succinate, indicating a dependence on mitochondrial anaplerosis. Glucose, which stimulates anaplerosis via pyruvate carboxylase, had the same suppressive effect on α-cell cAMP but with lower potency. Similarly to mouse islets, leucine suppressed glucagon secretion from human islets under hypoglycemic conditions.

Conclusions

These findings highlight an important role for physiological levels of leucine in the metabolic regulation of α-cell cAMP and glucagon secretion. Leucine functions primarily through an α-cell intrinsic effect that is dependent on glutamate dehydrogenase, in addition to the well-established α-cell regulation by β/δ-cell paracrine signaling. Our results suggest that mitochondrial anaplerosis-cataplerosis facilitates the glucagonostatic effect of both leucine and glucose, which cooperatively suppress α-cell tone by reducing cAMP.

Not the second fiddle: α cell development, identity, and function in health and diabetes

Historic and emerging studies provide evidence for the deterioration of pancreatic α cell function and identity in diabetes mellitus. Increased access to human tissue and the availability of more sophisticated molecular technologies have identified key insights into how α cell function and identity are preserved in healthy conditions and how they become dysfunctional in response to stress. These studies have revealed evidence of impaired glucagon secretion, shifts in α cell electrophysiology, changes in α cell mass, dysregulation of α cell transcription, and α-to-β cell conversion prior to and during diabetes. In this review, we outline the current state of research on α cell identity in health and disease. Evidence in model organisms and humans suggests that in addition to β cell dysfunction, diabetes is associated with a fundamental dysregulation of α cell identity. Importantly, epigenetic studies have revealed that α cells retain more poised and open chromatin at key cell-specific and diabetes-dysregulated genes, supporting the model that the inherent epigenetic plasticity of α cells makes them susceptible to the transcriptional changes that potentiate the loss of identity and function seen in diabetes. Thus, additional research into the maintenance of α cell identity and function is critical to fully understanding diabetes. Furthermore, these studies suggest α cells could represent an alternative source of new β cells for diabetes treatment.

The human α cell in health and disease

In commemoration of 100 years since the discovery of glucagon, we review current knowledge about the human α cell. Alpha cells make up 30-40% of human islet endocrine cells and play a major role in regulating whole-body glucose homeostasis, largely through the direct actions of their main secretory product - glucagon - on peripheral organs. Additionally, glucagon and other secretory products of α cells, namely acetylcholine, glutamate, and glucagon-like peptide-1, have been shown to play an indirect role in the modulation of glucose homeostasis through autocrine and paracrine interactions within the islet. Studies of glucagon's role as a counterregulatory hormone have revealed additional important functions of the α cell, including the regulation of multiple aspects of energy metabolism outside that of glucose. At the molecular level, human α cells are defined by the expression of conserved islet-enriched transcription factors and various enriched signature genes, many of which have currently unknown cellular functions. Despite these common threads, notable heterogeneity exists amongst human α cell gene expression and function. Even greater differences are noted at the inter-species level, underscoring the importance of further study of α cell physiology in the human context. Finally, studies on α cell morphology and function in type 1 and type 2 diabetes, as well as other forms of metabolic stress, reveal a key contribution of α cell dysfunction to dysregulated glucose homeostasis in disease pathogenesis, making targeting the α cell an important focus for improving treatment.

Integrating genetics with single-cell multiomic measurements across disease states identifies mechanisms of beta cell dysfunction in type 2 diabetes

Dysfunctional pancreatic islet beta cells are a hallmark of type 2 diabetes (T2D), but a comprehensive understanding of the underlying mechanisms, including gene dysregulation, is lacking. Here we integrate information from measurements of chromatin accessibility, gene expression and function in single beta cells with genetic association data to nominate disease-causal gene regulatory changes in T2D. Using machine learning on chromatin accessibility data from 34 nondiabetic, pre-T2D and T2D donors, we identify two transcriptionally and functionally distinct beta cell subtypes that undergo an abundance shift during T2D progression. Subtype-defining accessible chromatin is enriched for T2D risk variants, suggesting a causal contribution of subtype identity to T2D. Both beta cell subtypes exhibit activation of a stress-response transcriptional program and functional impairment in T2D, which is probably induced by the T2D-associated metabolic environment. Our findings demonstrate the power of multimodal single-cell measurements combined with machine learning for characterizing mechanisms of complex diseases.

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.

Metabolic Messengers: glucagon

Plasma glucose is tightly regulated via the secretion of the two glucose-regulating hormones insulin and glucagon. Situated next to the insulin-secreting β-cells, the α-cells produce and secrete glucagon-one of the body's few blood glucose-increasing hormones. Diabetes is a bihormonal disorder, resulting from both inadequate insulin secretion and dysregulation of glucagon. The year 2023 marks the 100th anniversary of the discovery of glucagon, making it particularly timely to highlight the roles of this systemic metabolic messenger in health and disease.

GC-Globulin/Vitamin D-Binding Protein Is Required for Pancreatic α-Cell Adaptation to Metabolic Stress

GC-globulin (GC), or vitamin D-binding protein, is a multifunctional protein involved in the transport of circulating vitamin 25(OH)D and fatty acids, as well as actin scavenging. In the pancreatic islets, the gene encoding GC, GC/Gc, is highly localized to glucagon-secreting α-cells. Despite this, the role of GC in α-cell function is poorly understood. We previously showed that GC is essential for α-cell morphology, electrical activity, and glucagon secretion. We now show that loss of GC exacerbates α-cell failure during metabolic stress. High-fat diet-fed GC-/- mice have basal hyperglucagonemia, which is associated with decreased α-cell size, impaired glucagon secretion and Ca2+ fluxes, and changes in glucose-dependent F-actin remodelling. Impairments in glucagon secretion can be rescued using exogenous GC to replenish α-cell GC levels, increase glucagon granule area, and restore the F-actin cytoskeleton. Lastly, GC levels decrease in α-cells of donors with type 2 diabetes, which is associated with changes in α-cell mass, morphology, and glucagon expression. Together, these data demonstrate an important role for GC in α-cell adaptation to metabolic stress.

FK506-Binding Protein 2 Participates in Proinsulin Folding

Apart from chaperoning, disulfide bond formation, and downstream processing, the molecular sequence of proinsulin folding is not completely understood. Proinsulin requires proline isomerization for correct folding. Since FK506-binding protein 2 (FKBP2) is an ER-resident proline isomerase, we hypothesized that FKBP2 contributes to proinsulin folding. We found that FKBP2 co-immunoprecipitated with proinsulin and its chaperone GRP94 and that inhibition of FKBP2 expression increased proinsulin turnover with reduced intracellular proinsulin and insulin levels. This phenotype was accompanied by an increased proinsulin secretion and the formation of proinsulin high-molecular-weight complexes, a sign of proinsulin misfolding. FKBP2 knockout in pancreatic β-cells increased apoptosis without detectable up-regulation of ER stress response genes. Interestingly, FKBP2 mRNA was overexpressed in β-cells from pancreatic islets of T2D patients. Based on molecular modeling and an in vitro enzymatic assay, we suggest that proline at position 28 of the proinsulin B-chain (P28) is the substrate of FKBP2's isomerization activity. We propose that this isomerization step catalyzed by FKBP2 is an essential sequence required for correct proinsulin folding.

Integration of single-cell multiomic measurements across disease states with genetics identifies mechanisms of beta cell dysfunction in type 2 diabetes

Altered function and gene regulation of pancreatic islet beta cells is a hallmark of type 2 diabetes (T2D), but a comprehensive understanding of mechanisms driving T2D is still missing. Here we integrate information from measurements of chromatin activity, gene expression and function in single beta cells with genetic association data to identify disease-causal gene regulatory changes in T2D. Using machine learning on chromatin accessibility data from 34 non-diabetic, pre-T2D and T2D donors, we robustly identify two transcriptionally and functionally distinct beta cell subtypes that undergo an abundance shift in T2D. Subtype-defining active chromatin is enriched for T2D risk variants, suggesting a causal contribution of subtype identity to T2D. Both subtypes exhibit activation of a stress-response transcriptional program and functional impairment in T2D, which is likely induced by the T2D-associated metabolic environment. Our findings demonstrate the power of multimodal single-cell measurements combined with machine learning for identifying mechanisms of complex diseases.

Screening of Metabolism-Disrupting Chemicals on Pancreatic α-Cells Using In Vitro Methods

Metabolism-disrupting chemicals (MDCs) are endocrine disruptors with obesogenic and/or diabetogenic action. There is mounting evidence linking exposure to MDCs to increased susceptibility to diabetes. Despite the important role of glucagon in glucose homeostasis, there is little information on the effects of MDCs on α-cells. Furthermore, there are no methods to identify and test MDCs with the potential to alter α-cell viability and function. Here, we used the mouse α-cell line αTC1-9 to evaluate the effects of MDCs on cell viability and glucagon secretion. We tested six chemicals at concentrations within human exposure (from 0.1 pM to 1 µM): bisphenol-A (BPA), tributyltin (TBT), perfluorooctanoic acid (PFOA), triphenylphosphate (TPP), triclosan (TCS), and dichlorodiphenyldichloroethylene (DDE). Using two different approaches, MTT assay and DNA-binding dyes, we observed that BPA and TBT decreased α-cell viability via a mechanism that depends on the activation of estrogen receptors and PPARγ, respectively. These two chemicals induced ROS production, but barely altered the expression of endoplasmic reticulum (ER) stress markers. Although PFOA, TPP, TCS, and DDE did not alter cell viability nor induced ROS generation or ER stress, all four compounds negatively affected glucagon secretion. Our findings suggest that αTC1-9 cells seem to be an appropriate model to test chemicals with metabolism-disrupting activity and that the improvement of the test methods proposed herein could be incorporated into protocols for the screening of diabetogenic MDCs.

Understanding islet dysfunction in type 2 diabetes through multidimensional pancreatic phenotyping: The Human Pancreas Analysis Program

In this perspective, we provide an overview of a recently established National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) initiative, the Human Pancreas Analysis Program for Type 2 Diabetes (HPAP-T2D). This program is designed to define the molecular pathogenesis of islet dysfunction by studying human pancreatic tissue samples from organ donors with T2D. HPAP-T2D generates detailed datasets of physiological, histological, transcriptomic, epigenomic, and genomic information. Importantly, all data collected, generated, and analyzed by HPAP-T2D are made immediately and freely available through a centralized database, PANC-DB, thus providing a comprehensive data resource for the diabetes research community.

Single-cell imaging of α and β cell metabolic response to glucose in living human Langerhans islets

Here we use a combination of two-photon Fluorescence Lifetime Imaging Microscopy (FLIM) of NAD(P)H free/bound ratio in living HIs with post-fixation, immunofluorescence-based, cell-type identification. FLIM allowed to measure variations in the NAD(P)H free/bound ratio induced by glucose; immunofluorescence data allowed to identify single α and β cells; finally, matching of the two datasets allowed to assign metabolic shifts to cell identity. 312 α and 654 β cells from a cohort of 4 healthy donors, 15 total islets, were measured. Both α and β cells display a wide spectrum of responses, towards either an increase or a decrease in NAD(P)H free/bound ratio. Yet, if single-cell data are averaged according to the respective donor and correlated to donor insulin secretion power, a non-random distribution of metabolic shifts emerges: robust average responses of both α and β cells towards an increase of enzyme-bound NAD(P)H belong to the donor with the lowest insulin-secretion power; by contrast, discordant responses, with α cells shifting towards an increase of free NAD(P)H and β cells towards an increase of enzyme-bound NAD(P)H, correspond to the donor with the highest insulin-secretion power. Overall, data reveal neat anti-correlation of tissue metabolic responses with respect to tissue insulin secretion power.

A combination of live imaging and immunofluorescence on donor islet cells uncover an anti-correlation of enzyme-bound NAD(P)H and insulin secretion power.

The past, present, and future physiology and pharmacology of glucagon

The evolution of glucagon has seen the transition from an impurity in the preparation of insulin to the development of glucagon receptor agonists for use in type 1 diabetes. In type 2 diabetes, glucagon receptor antagonists have been explored to reduce glycemia thought to be induced by hyperglucagonemia. However, the catabolic actions of glucagon are currently being leveraged to target the rise in obesity that paralleled that of diabetes, bringing the pharmacology of glucagon full circle. During this evolution, the physiological importance of glucagon advanced beyond the control of hepatic glucose production, incorporating critical roles for glucagon to regulate both lipid and amino acid metabolism. Thus, it is unsurprising that the study of glucagon has left several paradoxes that make it difficult to distill this hormone down to a simplified action. Here, we describe the history of glucagon from the past to the present and suggest some direction to the future of this field.

Human alpha cell transcriptomic signatures of types 1 and 2 diabetes highlight disease-specific dysfunction pathways

Although glucagon secretion is perturbed in both T1D and T2D, the pathophysiological changes in individual pancreatic alpha cells are still obscure. Using recently curated single-cell RNASeq data from T1D or T2D donors and their controls, we identified alpha cell transcriptomic alterations consistent with both common and discrete pathways. Although alterations in alpha cell identity gene (ARX, MAFB) expression were conserved, cytokine-regulated genes and genes involved in glucagon biosynthesis and processing were up-regulated in T1D. Conversely, mitochondrial genes associated with ROS (COX7B, NQO2) were dysregulated in T2D. Additionally, T1D alpha cells displayed altered expression of autoimmune-induced ER stress genes (ERLEC1, HSP90), whilst those from T2D subjects showed modified glycolytic and citrate cycle gene (LDHA?, PDHB, PDK4) expression. Thus, despite conserved alterations related to loss of function, alpha cells display disease-specific gene signatures which may be secondary to the main pathogenic events in each disease, namely immune- or metabolism-mediated-stress, in T1D and T2D, respectively.

Alpha Cell Thioredoxin-interacting Protein Deletion Improves Diabetes-associated Hyperglycemia and Hyperglucagonemia

Thioredoxin-interacting protein (TXNIP) has emerged as a key factor in pancreatic beta cell biology, and its upregulation by glucose and diabetes contributes to the impairment in functional beta cell mass and glucose homeostasis. In addition, beta cell deletion of TXNIP protects against diabetes in different mouse models. However, while TXNIP is ubiquitously expressed, its role in pancreatic alpha cells has remained elusive. We generated an alpha cell TXNIP knockout (aTKO) mouse and assessed the effects on glucose homeostasis. While no significant changes were observed on regular chow, after a 30-week high-fat diet, aTKO animals showed improvement in glucose tolerance and lower blood glucose levels compared to their control littermates. Moreover, in the context of streptozotocin (STZ)-induced diabetes, aTKO mice showed significantly lower blood glucose levels compared to controls. While serum insulin levels were reduced in both control and aTKO mice, STZ-induced diabetes significantly increased glucagon levels in control mice, but this effect was blunted in aTKO mice. Moreover, glucagon secretion from aTKO islets was >2-fold lower than from control islets, while insulin secretion was unchanged in aTKO islets. At the same time, no change in alpha cell or beta cell numbers or mass was observed, and glucagon and insulin expression and content were comparable in isolated islets from aTKO and control mice. Thus together the current studies suggest that downregulation of alpha cell TXNIP is associated with reduced glucagon secretion and that this may contribute to the glucose-lowering effects observed in diabetic aTKO mice.

Glucose inhibits glucagon secretion by decreasing [Ca2+]c and by reducing the efficacy of Ca2+ on exocytosis via somatostatin-dependent and independent mechanisms

Objective

Methods

Results

Conclusions

•

Glucose modulates [Ca²⁺]_c in α-cells within islets but not in dispersed α-cells.

•

In α-cells within islets, it decreases [Ca²⁺]_c independently of their K_ATP channels.

•

It decreases α-cell [Ca²⁺]_c partly via somatostatin.

•

All glucose-induced [Ca²⁺]_c changes trigger parallel changes in glucagon release.

•

Glucose also decreases the efficacy of Ca²⁺ on exocytosis (attenuating pathway).

Pancreatic alpha cells and glucagon secretion: Novel functions and targets in glucose homeostasis

Diabetes is the result of dysregulation of both insulin and glucagon. Still, insulin has attracted much more attention than glucagon. Glucagon is released from alpha cells in the islets of Langerhans in response to low glucose and certain amino acids. Drugs with the primary aim of targeting glucagon signalling are scarce. However, glucagon is often administered to counteract severe hypoglycaemia, and commonly used diabetes medications such as GLP-1 analogues, sulfonylureas and SGLT2-inhibitors also affect alpha cells. Indeed, there are physiological and developmental similarities between the alpha cell and the insulin-secreting beta cell and new data confirm that alpha cells can be converted into insulin-secreting cells. These aspects and attributes, the need to find novel therapies targeting the alpha cell and more are considered in this review.

Alpha-cell paracrine signaling in the regulation of beta-cell insulin secretion

As an incretin hormone, glucagon-like peptide 1 (GLP-1) lowers blood glucose levels by enhancing glucose-stimulated insulin secretion from pancreatic beta-cells. Therapies targeting the GLP-1 receptor (GLP-1R) use the classical incretin model as a physiological framework in which GLP-1 secreted from enteroendocrine L-cells acts on the beta-cell GLP-1R. However, this model has come into question, as evidence demonstrating local, intra-islet GLP-1 production has advanced the competing hypothesis that the incretin activity of GLP-1 may reflect paracrine signaling of GLP-1 from alpha-cells on GLP-1Rs on beta-cells. Additionally, recent studies suggest that alpha-cell-derived glucagon can serve as an additional, albeit less potent, ligand for the beta-cell GLP-1R, thereby expanding the role of alpha-cells beyond that of a counterregulatory cell type. Efforts to understand the role of the alpha-cell in the regulation of islet function have revealed both transcriptional and functional heterogeneity within the alpha-cell population. Further analysis of this heterogeneity suggests that functionally distinct alpha-cell subpopulations display alterations in islet hormone profile. Thus, the role of the alpha-cell in glucose homeostasis has evolved in recent years, such that alpha-cell to beta-cell communication now presents a critical axis regulating the functional capacity of beta-cells. Herein, we describe and integrate recent advances in our understanding of the impact of alpha-cell paracrine signaling on insulin secretory dynamics and how this intra-islet crosstalk more broadly contributes to whole-body glucose regulation in health and under metabolic stress. Moreover, we explore how these conceptual changes in our understanding of intra-islet GLP-1 biology may impact our understanding of the mechanisms of incretin-based therapeutics.