The transcriptional landscape of mouse beta cells compared to human beta cells reveals notable species differences in long non-coding RNA and protein-coding gene expression

The Importance of Intra-Islet Communication in the Function and Plasticity of the Islets of Langerhans during Health and Diabetes

Islets of Langerhans are anatomically dispersed within the pancreas and exhibit regulatory coordination between islets in response to nutritional and inflammatory stimuli. However, within individual islets, there is also multi-faceted coordination of function between individual beta-cells, and between beta-cells and other endocrine and vascular cell types. This is mediated partly through circulatory feedback of the major secreted hormones, insulin and glucagon, but also by autocrine and paracrine actions within the islet by a range of other secreted products, including somatostatin, urocortin 3, serotonin, glucagon-like peptide-1, acetylcholine, and ghrelin. Their availability can be modulated within the islet by pericyte-mediated regulation of microvascular blood flow. Within the islet, both endocrine progenitor cells and the ability of endocrine cells to trans-differentiate between phenotypes can alter endocrine cell mass to adapt to changed metabolic circumstances, regulated by the within-islet trophic environment. Optimal islet function is precariously balanced due to the high metabolic rate required by beta-cells to synthesize and secrete insulin, and they are susceptible to oxidative and endoplasmic reticular stress in the face of high metabolic demand. Resulting changes in paracrine dynamics within the islets can contribute to the emergence of Types 1, 2 and gestational diabetes.

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

Disrupted RNA editing in beta cells mimics early-stage type 1 diabetes

A major hypothesis for the etiology of type 1 diabetes (T1D) postulates initiation by viral infection, leading to double-stranded RNA (dsRNA)-mediated interferon response and inflammation; however, a causal virus has not been identified. Here, we use a mouse model, corroborated with human islet data, to demonstrate that endogenous dsRNA in beta cells can lead to a diabetogenic immune response, thus identifying a virus-independent mechanism for T1D initiation. We found that disruption of the RNA editing enzyme adenosine deaminases acting on RNA (ADAR) in beta cells triggers a massive interferon response, islet inflammation, and beta cell failure and destruction, with features bearing striking similarity to early-stage human T1D. Glycolysis via calcium enhances the interferon response, suggesting an actionable vicious cycle of inflammation and increased beta cell workload.

LncRNA Malat1 regulates iPSC-derived β-cell differentiation by targeting the miR-15b-5p/Ihh axis

BACKGROUND

Differentiation of induced pluripotent stem cells (iPSCs)-derived β-like cells is a novel strategy for treatment of type 1 diabetes. Elucidation of the regulatory mechanisms of long noncoding RNAs (lncRNAs) in β-like cells derived from iPSCs is important for understanding the development of the pancreas and pancreatic β-cells and may improve the quality of β-like cells for stem cell therapy.

METHODS

β-like cells were derived from iPSCs in a three-step protocol. RNA sequencing and bioinformatics analysis were carried out to screen the differentially expressed lncRNAs and identify the putative target genes separately. LncRNA Malat1 was chosen for further research. Series of loss and gain of functions experiments were performed to study the biological function of LncRNA Malat1. Quantitative real-time PCR (qRT-PCR), Western blot (WB) analysis and immunofluorescence (IF) staining were carried out to separately detect the functions of pancreatic β-cells at the mRNA and protein levels. Cytoplasmic and nuclear RNA fractionation and fluorescence in situ hybridization (FISH) were used to determine the subcellar location of lncRNA Malat1 in β-like cells. Enzyme-linked immunosorbent assays (ELISAs) were performed to examine the differentiation and insulin secretion of β-like cells after stimulation with different glucose concentrations. Structural interactions between lncRNA Malat1 and miR-15b-5p and between miR-15b-5p/Ihh were detected by dual luciferase reporter assays (LRAs).

RESULTS

We found that the expression of lncRNA Malat1 declined during differentiation, and overexpression (OE) of lncRNA Malat1 notably impaired the differentiation and maturation of β-like cells derived from iPSCs in vitro and in vivo. Most importantly, lncRNA Malat1 could function as a competing endogenous RNA (ceRNA) of miR-15b-5p to regulate the expression of Ihh according to bioinformatics prediction, mechanistic analysis and downstream experiments.

CONCLUSION

This study established an unreported regulatory network of lncRNA Malat1 and the miR-15b-5p/Ihh axis during the differentiation of iPSCs into β-like cells. In addition to acting as an oncogene promoting tumorigenesis, lncRNA Malat1 may be an effective and novel target for treatment of diabetes in the future.

CSF1R inhibition with PLX5622 affects multiple immune cell compartments and induces tissue-specific metabolic effects in lean mice

AIMS/HYPOTHESIS

Colony stimulating factor 1 (CSF1) promotes the proliferation, differentiation and survival of macrophages, which have been implicated in both beneficial and detrimental effects on glucose metabolism. However, the physiological role of CSF1 signalling in glucose homeostasis and the potential therapeutic implications of modulating this pathway are not known. We aimed to study the composition of tissue macrophages (and other immune cells) following CSF1 receptor (CSF1R) inhibition and elucidate the metabolic consequences of CSF1R inhibition.

METHODS

We assessed immune cell populations in various organs by flow cytometry, and tissue-specific metabolic effects by hyperinsulinaemic-euglycaemic clamps and insulin secretion assays in mice fed a chow diet containing PLX5622 (a CSF1R inhibitor) or a control diet.

RESULTS

CSF1R inhibition depleted macrophages in multiple tissues while simultaneously increasing eosinophils and group 2 innate lymphoid cells. These immunological changes were consistent across different organs and were sex independent and reversible after cessation of the PLX5622. CSF1R inhibition improved hepatic insulin sensitivity but concomitantly impaired insulin secretion. In healthy islets, we found a high frequency of IL-1β+ islet macrophages. Their depletion by CSF1R inhibition led to downregulation of macrophage-related pathways and mediators of cytokine activity, including Nlrp3, suggesting IL-1β as a candidate insulin secretagogue. Partial restoration of physiological insulin secretion was achieved by injecting recombinant IL-1β prior to glucose stimulation in mice lacking macrophages.

CONCLUSIONS/INTERPRETATION

Macrophages and macrophage-derived factors, such as IL-1β, play an important role in physiological insulin secretion. A better understanding of the tissue-specific effects of CSF1R inhibition on immune cells and glucose homeostasis is crucial for the development of targeted immune-modulatory treatments in metabolic disease.

DATA AVAILABILITY

The RNA-Seq dataset is available in the Gene Expression Omnibus (GEO) under the accession number GSE189434 ( http://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE189434 ).

Exploring lncRNAs associated with human pancreatic islet cell death induced by transfer of adoptive lymphocytes in a humanized mouse model

Background

Long noncoding RNA (lncRNA)-mediated posttranscriptional and epigenetic landscapes of gene regulation are associated with numerous human diseases. However, the regulatory mechanisms governing human β-cell function and survival remain unknown. Owing to technical and ethical constraints, studying the direct role of lncRNAs in β-cell function and survival in humans in vivo is difficult. Therefore, we utilized humanized mice with human islets to investigate lncRNA expression using whole transcriptome shotgun sequencing. Our study aimed to characterize lncRNAs that may be crucial for human islet cell function and survival.

Methods

Human β-cell death was induced in humanized mice engrafted with functional human islets. Using these humanized mice harboring human islets with induced β-cell death, we investigated lncRNA expression through whole transcriptome shotgun sequencing. Additionally, we systematically identified, characterized, and explored the regulatory functions of lncRNAs that are potentially important for human pancreatic islet cell function and survival.

Results

Human islet cell death was induced in humanized mice engrafted with functional human islets. RNA sequencing analysis of isolated human islets, islet grafts from humanized mice with and without induced cell death, revealed aberrant expression of a distinct set of lncRNAs that are associated with the deregulated mRNAs important for cellular processes and molecular pathways related to β-cell function and survival. A total of 10 lncRNA isoforms (SCYL1-1:22, POLG2-1:1, CTRB1-1:1, SRPK1-1:1, GTF3C5-1:1, PPY-1:1, CTRB1-1:5, CPA5-1:1, BCAR1-2:1, and CTRB1-1:4) were identified as highly enriched and specific to human islets. These lncRNAs were deregulated in human islets from donors with different BMIs and with type 2 diabetes (T2D), as well as in cultured human islets with glucose stimulation and induced cell death induced by cytokines. Aberrant expression of these lncRNAs was detected in the exosomes from the medium used to culture islets with cytokines.

Conclusion

Islet-enriched and specific human lncRNAs are deregulated in human islet grafts and cultured human islets with induced cell death. These lncRNAs may be crucial for human β-cell function and survival and could have an impact on identifying biomarkers for β-cell loss and discovering novel therapeutic targets to enhance β-cell function and survival.

Global proteomics reveals insulin abundance as a marker of human islet homeostasis alterations

AIM

The variation in quality between the human islet samples represents a major problem for research, especially when used as control material. The assays assessing the quality of human islets used in research are non-standardized and limited, with many important parameters not being consistently assessed. High-throughput studies aimed at characterizing the diversity and segregation markers among apparently functionally healthy islet preps are thus a requirement. Here, we designed a pilot study to comprehensively identify the diversity of global proteome signatures and the deviation from normal homeostasis in randomly selected human-isolated islet samples.

METHODS

By using Tandem Mass Tag 16-plex proteomics, we focused on the recurrently observed disparity in the detected insulin abundance between the samples, used it as a segregating parameter, and analyzed the correlated changes in the proteome signature and homeostasis by pathway analysis.

RESULTS

In this pilot study, we showed that insulin protein abundance is a predictor of human islet homeostasis and quality. This parameter is independent of other quality predictors within their acceptable range, thus being able to further stratify islets samples of apparent good quality. Human islets with low amounts of insulin displayed changes in their metabolic and signaling profile, especially in regard to energy homeostasis and cell identity maintenance. We further showed that xenotransplantation into diabetic hosts is not expected to improve the pre-transplantation signature, as it has a negative effect on energy balance, antioxidant activity, and islet cell identity.

CONCLUSIONS

Insulin protein abundance predicts significant changes in human islet homeostasis among random samples of apparently good quality.

NOD mice have distinct metabolic and immunologic profiles when compared with genetically similar MHC-matched ICR mice

Nonobese diabetic (NOD) mice are the most commonly used rodent model to study mechanisms relevant to the autoimmunity and immunology of type 1 diabetes. Although many different strains of mice have been used as controls for studies comparing nondiabetic lines to the NOD strain, we hypothesized that the parental strain that gave rise to the NOD line might be one of the best options. Therefore, we compared female ICR and NOD mice, which are matched at key major histocompatibility complex (MHC) loci, to understand their metabolic and immunologic similarities and differences. Several novel observations emerged: 1) NOD mice have greater circulating proinsulin when compared with ICR mice. 2) NOD mice display CD3+ and IBA1+ cell infiltration into and near pancreatic islets before hyperglycemia. 3) NOD mice show increased expression of the Il1b and Cxcl11 genes in islets when compared with islets from age-matched ICR mice. 4) NOD mice have a greater abundance of STAT1 and ICAM-1 protein in islets when compared with ICR mice. These data show that ICR mice, which are genetically similar to NOD mice, do not retain the same immunologic outcomes. Thus, ICR mice are an excellent choice as a genetically similar and MHC-matched control for NOD mice in studies designed to understand mechanisms relevant to autoimmune-mediated diabetes onset as well as novel therapeutic interventions.NEW & NOTEWORTHY Nonobese diabetic (NOD) mice have more proinsulin in circulation and STAT1 protein in islets compared with the major histocompatibility complex (MHC)-matched ICR line. NOD mice also display greater expression of cytokines and chemokines in pancreatic islets consistent with immune cell infiltration before hyperglycemia when compared with age-matched ICR mice. Thus, ICR mice represent an excellent control for autoimmunity and inflammation studies using the NOD line of mice.

Metabolic regulation of glucagon secretion

Since the discovery of glucagon 100 years ago, the hormone and the pancreatic islet alpha cells that produce it have remained enigmatic relative to insulin-producing beta cells. Canonically, alpha cells have been described in the context of glucagon's role in glucose metabolism in liver, with glucose as the primary nutrient signal regulating alpha cell function. However, current data reveal a more holistic model of metabolic signalling, involving glucagon-regulated metabolism of multiple nutrients by the liver and other tissues, including amino acids and lipids, providing reciprocal feedback to regulate glucagon secretion and even alpha cell mass. Here we describe how various nutrients are sensed, transported and metabolised in alpha cells, providing an integrative model for the metabolic regulation of glucagon secretion and action. Importantly, we discuss where these nutrient-sensing pathways intersect to regulate alpha cell function and highlight key areas for future research.

Species-specific roles for the MAFA and MAFB transcription factors in regulating islet β cell identity

Type 2 diabetes (T2D) is associated with compromised identity of insulin-producing pancreatic islet β cells, characterized by inappropriate production of other islet cell-enriched hormones. Here, we examined how hormone misexpression was influenced by the MAFA and MAFB transcription factors, closely related proteins that maintain islet cell function. Mice specifically lacking MafA in β cells demonstrated broad, population-wide changes in hormone gene expression with an overall gene signature closely resembling islet gastrin+ (Gast+) cells generated under conditions of chronic hyperglycemia and obesity. A human β cell line deficient in MAFB, but not one lacking MAFA, also produced a GAST+ gene expression pattern. In addition, GAST was detected in human T2D β cells with low levels of MAFB. Moreover, evidence is provided that human MAFB can directly repress GAST gene transcription. These results support a potentially novel, species-specific role for MafA and MAFB in maintaining adult mouse and human β cell identity, respectively. Here, we discuss the possibility that induction of Gast/GAST and other non-β cell hormones, by reduction in the levels of these transcription factors, represents a dysfunctional β cell signature.

Panax notoginseng alleviates oxidative stress through miRNA regulations based on systems biology approach

BACKGROUND

Herbal medicine Sanqi (SQ), the dried root or stem of Panax notoginseng (PNS), has been reported to have anti-diabetic and anti-obesity effects and is usually administered as a decoction for Chinese medicine. Alternative to utilizing PNS pure compound for treatment, we are motivated to propose an unconventional scheme to investigate the functions of PNS mixture. However, studies providing a detailed overview of the transcriptomics-based signaling network in response to PNS are seldom available.

METHODS

To explore the reasoning of PNS in treating metabolic disorders such as insulin resistance, we implemented a systems biology-based approach with RNA sequencing (RNA-seq) and miRNA sequencing data to elucidate key pathways, genes and miRNAs involved.

RESULTS

Functional enrichment analysis revealed PNS up-regulating oxidative stress-related pathways and down-regulating insulin and fatty acid metabolism. Superoxide dismutase 1 (SOD1), peroxiredoxin 1 (PRDX1), heme oxygenase-1 (Hmox1) and glutamate cysteine ligase (GCLc) mRNA and protein levels, as well as related miRNA levels, were measured in PNS treated rat pancreatic β cells (INS-1). PNS treatment up-regulated Hmox1, SOD1 and GCLc expression while down-regulating miR-24-3p and miR-139-5p to suppress oxidative stress. Furthermore, we verified the novel interactions between miR-139-5p and miR-24-3p with GCLc and SOD1.

CONCLUSION

This work has demonstrated the mechanism of how PNS regulates cellular molecules in metabolic disorders. Therefore, combining omics data with a systems biology strategy could be a practical means to explore the potential function and molecular mechanisms of Chinese herbal medicine in the treatment of metabolic disorders.

Amino acids and the changing face of the α-cell

Glucagon has long been defined by its glucogenic action and as a result α-cells have been characterised based largely on their interaction with glucose. Recent findings have challenged this preconception, bringing to the fore the significant role glucagon plays in amino acid breakdown and underlining the importance of amino acids in glucagon secretion. The challenge that remains is defining the mechanism that underlie these effects - understanding which amino acids are most important, how they act on the α-cell and how their actions integrate with other fuels such as glucose and fatty acids. This review will describe the current relationship between amino acids and glucagon and how we can use this knowledge to redefine the α-cell.

Divergent acute versus prolonged pharmacological GLP-1R responses in adult β cell-specific β-arrestin 2 knockout mice

The glucagon-like peptide-1 receptor (GLP-1R) is a major type 2 diabetes therapeutic target. Stimulated GLP-1Rs are rapidly desensitized by β-arrestins, scaffolding proteins that not only terminate G protein interactions but also act as independent signaling mediators. Here, we have assessed in vivo glycemic responses to the pharmacological GLP-1R agonist exendin-4 in adult β cell-specific β-arrestin 2 knockout (KO) mice. KOs displayed a sex-dimorphic phenotype consisting of weaker acute responses that improved 6 hours after agonist injection. Similar effects were observed for semaglutide and tirzepatide but not with biased agonist exendin-phe1. Acute cyclic adenosine 5'-monophosphate increases were impaired, but desensitization reduced in KO islets. The former defect was attributed to enhanced β-arrestin 1 and phosphodiesterase 4 activities, while reduced desensitization co-occurred with impaired GLP-1R recycling and lysosomal targeting, increased trans-Golgi network signaling, and reduced GLP-1R ubiquitination. This study has unveiled fundamental aspects of GLP-1R response regulation with direct application to the rational design of GLP-1R-targeting therapeutics.

SnRNA-Seq of Pancreas Revealed the Dysfunction of Endocrine and Exocrine Cells in Transgenic Pigs with Prediabetes

Diabetes poses a significant threat to human health. Exocrine pancreatic dysfunction is related to diabetes, but the exact mechanism is not fully understood. This study aimed to describe the pathological phenotype and pathological mechanisms of the pancreas of transgenic pigs (PIGinH11) that was constructed in our laboratory and to compare it with humans. We established diabetes-susceptible transgenic pigs and subjected them to high-fat and high-sucrose dietary interventions. The damage to the pancreatic endocrine and exocrine was evaluated using histopathology and the involved molecular mechanisms were analyzed using single-nucleus RNA-sequencing (SnRNA-seq). Compared to wild-type (WT) pigs, PIGinH11 pigs showed similar pathological manifestations to type 2 diabetes patients, such as insulin deficiency, fatty deposition, inflammatory infiltration, fibrosis tissue necrosis, double positive cells, endoplasmic reticulum (ER) and mitochondria damage. SnRNA-seq analysis revealed 16 clusters and cell-type-specific gene expression characterization in the pig pancreas. Notably, clusters of Ainar-M and Endocrine-U were observed at the intermediate state between the exocrine and endocrine pancreas. Beta cells of the PIGinH11 group demonstrated the dysfunction with insulin produced and secret decreased and ER stress. Moreover, like clinic patients, acinar cells expressed fewer digestive enzymes and showed organelle damage. We hypothesize that TXNIP that is upregulated by high glucose might play an important role in the dysfunction of endocrine to exocrine cells in PIGinH11 pigs.

Chromatin accessibility differences between alpha, beta, and delta cells identifies common and cell type-specific enhancers

BACKGROUND

High throughput sequencing has enabled the interrogation of the transcriptomic landscape of glucagon-secreting alpha cells, insulin-secreting beta cells, and somatostatin-secreting delta cells. These approaches have furthered our understanding of expression patterns that define healthy or diseased islet cell types and helped explicate some of the intricacies between major islet cell crosstalk and glucose regulation. All three endocrine cell types derive from a common pancreatic progenitor, yet alpha and beta cells have partially opposing functions, and delta cells modulate and control insulin and glucagon release. While gene expression signatures that define and maintain cellular identity have been widely explored, the underlying epigenetic components are incompletely characterized and understood. However, chromatin accessibility and remodeling is a dynamic attribute that plays a critical role to determine and maintain cellular identity.

RESULTS

Here, we compare and contrast the chromatin landscape between mouse alpha, beta, and delta cells using ATAC-Seq to evaluate the significant differences in chromatin accessibility. The similarities and differences in chromatin accessibility between these related islet endocrine cells help define their fate in support of their distinct functional roles. We identify patterns that suggest that both alpha and delta cells are poised, but repressed, from becoming beta-like. We also identify patterns in differentially enriched chromatin that have transcription factor motifs preferentially associated with different regions of the genome. Finally, we not only confirm and visualize previously discovered common endocrine- and cell specific- enhancer regions across differentially enriched chromatin, but identify novel regions as well. We compiled our chromatin accessibility data in a freely accessible database of common endocrine- and cell specific-enhancer regions that can be navigated with minimal bioinformatics expertise.

CONCLUSIONS

Both alpha and delta cells appear poised, but repressed, from becoming beta cells in murine pancreatic islets. These data broadly support earlier findings on the plasticity in identity of non-beta cells under certain circumstances. Furthermore, differential chromatin accessibility shows preferentially enriched distal-intergenic regions in beta cells, when compared to either alpha or delta cells.

Nutrient regulation of the islet epigenome controls adaptive insulin secretion

Adaptation of the islet β cell insulin-secretory response to changing insulin demand is critical for blood glucose homeostasis, yet the mechanisms underlying this adaptation are unknown. Here, we have shown that nutrient-stimulated histone acetylation plays a key role in adapting insulin secretion through regulation of genes involved in β cell nutrient sensing and metabolism. Nutrient regulation of the epigenome occurred at sites occupied by the chromatin-modifying enzyme lysine-specific demethylase 1 (Lsd1) in islets. β Cell-specific deletion of Lsd1 led to insulin hypersecretion, aberrant expression of nutrient-response genes, and histone hyperacetylation. Islets from mice adapted to chronically increased insulin demand exhibited shared epigenetic and transcriptional changes. Moreover, we found that genetic variants associated with type 2 diabetes were enriched at LSD1-bound sites in human islets, suggesting that interpretation of nutrient signals is genetically determined and clinically relevant. Overall, these studies revealed that adaptive insulin secretion involves Lsd1-mediated coupling of nutrient state to regulation of the islet epigenome.

Insulin secretion deficits in a Prader-Willi syndrome β-cell model are associated with a concerted downregulation of multiple endoplasmic reticulum chaperones

Prader-Willi syndrome (PWS) is a multisystem disorder with neurobehavioral, metabolic, and hormonal phenotypes, caused by loss of expression of a paternally-expressed imprinted gene cluster. Prior evidence from a PWS mouse model identified abnormal pancreatic islet development with retention of aged insulin and deficient insulin secretion. To determine the collective roles of PWS genes in β-cell biology, we used genome-editing to generate isogenic, clonal INS-1 insulinoma lines having 3.16 Mb deletions of the silent, maternal- (control) and active, paternal-allele (PWS). PWS β-cells demonstrated a significant cell autonomous reduction in basal and glucose-stimulated insulin secretion. Further, proteomic analyses revealed reduced levels of cellular and secreted hormones, including all insulin peptides and amylin, concomitant with reduction of at least ten endoplasmic reticulum (ER) chaperones, including GRP78 and GRP94. Critically, differentially expressed genes identified by whole transcriptome studies included reductions in levels of mRNAs encoding these secreted peptides and the group of ER chaperones. In contrast to the dosage compensation previously seen for ER chaperones in Grp78 or Grp94 gene knockouts or knockdown, compensation is precluded by the stress-independent deficiency of ER chaperones in PWS β-cells. Consistent with reduced ER chaperones levels, PWS INS-1 β-cells are more sensitive to ER stress, leading to earlier activation of all three arms of the unfolded protein response. Combined, the findings suggest that a chronic shortage of ER chaperones in PWS β-cells leads to a deficiency of protein folding and/or delay in ER transit of insulin and other cargo. In summary, our results illuminate the pathophysiological basis of pancreatic β-cell hormone deficits in PWS, with evolutionary implications for the multigenic PWS-domain, and indicate that PWS-imprinted genes coordinate concerted regulation of ER chaperone biosynthesis and β-cell secretory pathway function.

CAMSAP2 localizes to the Golgi in islet β-cells and facilitates Golgi-ER trafficking

Glucose stimulation induces the remodeling of microtubules, which potentiates insulin secretion in pancreatic β-cells. CAMSAP2 binds to microtubule minus ends to stabilize microtubules in several cultured clonal cells. Here, we report that the knockdown of CAMSAP2 in primary β-cells reduces total insulin content and attenuates GSIS without affecting the releasability of insulin vesicles. Surprisingly, CAMSAP2 knockdown does not change microtubule stability. Unlike in cultured insulinoma cells, CAMSAP2 in primary β-cells predominantly localizes to the Golgi apparatus instead of microtubule minus ends. This novel localization is specific to primary β- but not α-cells and is independent of microtubule binding. Consistent with its specific localization at the Golgi, CAMSAP2 promotes efficient Golgi-ER trafficking in primary β-cells. Moreover, primary β-cells and insulinoma cells likely express different CAMSAP2 isoforms. We propose that a novel CAMSAP2 isoform in primary β-cells has a non-canonical function, which promotes Golgi-ER trafficking to support efficient production of insulin and secretion.

Inflammation driven metabolic regulation and adaptation in macrophages

Macrophages are a diverse population of phagocytic immune cells involved in the host defense mechanisms and regulation of homeostasis. Usually, macrophages maintain healthy functioning at the cellular level, but external perturbation in their balanced functions can lead to acute and chronic disease conditions. By sensing the cues from the tissue microenvironment, these phagocytes adopt a plethora of phenotypes, such as inflammatory or M1 to anti-inflammatory (immunosuppressive) or M2 subtypes, to fulfill their spectral range of functions. The existing evidence in the literature supports that in macrophages, regulation of metabolic switches and metabolic adaptations are associated with their functional behaviors under various physiological and pathological conditions. Since these macrophages play a crucial role in many disorders, therefore it is necessary to understand their heterogeneity and metabolic reprogramming. Consequently, these macrophages have also emerged as a promising target for diseases in which their role is crucial in driving the disease pathology and outcome (e.g., Cancers). In this review, we discuss the recent findings that link many metabolites with macrophage functions and highlight how this metabolic reprogramming can improve our understanding of cellular malfunction in the macrophages during inflammatory disorders. A systematic analysis of the interconnecting crosstalk between metabolic pathways with macrophages should inform the selection of immunomodulatory therapies for inflammatory diseases.

A genome-wide CRISPR screen identifies CALCOCO2 as a regulator of beta cell function influencing type 2 diabetes risk

Identification of the genes and processes mediating genetic association signals for complex diseases represents a major challenge. As many of the genetic signals for type 2 diabetes (T2D) exert their effects through pancreatic islet-cell dysfunction, we performed a genome-wide pooled CRISPR loss-of-function screen in a human pancreatic beta cell line. We assessed the regulation of insulin content as a disease-relevant readout of beta cell function and identified 580 genes influencing this phenotype. Integration with genetic and genomic data provided experimental support for 20 candidate T2D effector transcripts including the autophagy receptor CALCOCO2. Loss of CALCOCO2 was associated with distorted mitochondria, less proinsulin-containing immature granules and accumulation of autophagosomes upon inhibition of late-stage autophagy. Carriers of T2D-associated variants at the CALCOCO2 locus further displayed altered insulin secretion. Our study highlights how cellular screens can augment existing multi-omic efforts to support mechanistic understanding and provide evidence for causal effects at genome-wide association studies loci.

PDX1 is the cornerstone of pancreatic β-cell functions and identity

Diabetes has been a worldwide healthcare problem for many years. Current methods of treating diabetes are still largely directed at symptoms, aiming to control the manifestations of the pathology. This creates an overall need to find alternative measures that can impact on the causes of the disease, reverse diabetes, or make it more manageable. Understanding the role of key players in the pathogenesis of diabetes and the related β-cell functions is of great importance in combating diabetes. PDX1 is a master regulator in pancreas organogenesis, the maturation and identity preservation of β-cells, and of their role in normal insulin function. Mutations in the PDX1 gene are correlated with many pancreatic dysfunctions, including pancreatic agenesis (homozygous mutation) and MODY4 (heterozygous mutation), while in other types of diabetes, PDX1 expression is reduced. Therefore, alternative approaches to treat diabetes largely depend on knowledge of PDX1 regulation, its interaction with other transcription factors, and its role in obtaining β-cells through differentiation and transdifferentiation protocols. In this article, we review the basic functions of PDX1 and its regulation by genetic and epigenetic factors. Lastly, we summarize different variations of the differentiation protocols used to obtain β-cells from alternative cell sources, using PDX1 alone or in combination with various transcription factors and modified culture conditions. This review shows the unique position of PDX1 as a potential target in the genetic and cellular treatment of diabetes.

Genetic Engineering of Immune Evasive Stem Cell-Derived Islets

Genome editing has the potential to revolutionize many investigative and therapeutic strategies in biology and medicine. In the field of regenerative medicine, one of the leading applications of genome engineering technology is the generation of immune evasive pluripotent stem cell-derived somatic cells for transplantation. In particular, as more functional and therapeutically relevant human pluripotent stem cell-derived islets (SCDI) are produced in many labs and studied in clinical trials, there is keen interest in studying the immunogenicity of these cells and modulating allogeneic and autoimmune immune responses for therapeutic benefit. Significant experimental work has already suggested that elimination of Human Leukocytes Antigen (HLA) expression and overexpression of immunomodulatory genes can impact survival of a variety of pluripotent stem cell-derived somatic cell types. Limited work published to date focuses on stem cell-derived islets and work in a number of labs is ongoing. Rapid progress is occurring in the genome editing of human pluripotent stem cells and their progeny focused on evading destruction by the immune system in transplantation models, and while much research is still needed, there is no doubt the combined technologies of genome editing and stem cell therapy will profoundly impact transplantation medicine in the future.

Dynamic Ins2 Gene Activity Defines β-Cell Maturity States

Transcriptional and functional cellular specialization has been described for insulin-secreting β-cells of the endocrine pancreas. However, it is not clear whether β-cell heterogeneity is stable or reflects dynamic cellular states. We investigated the temporal kinetics of endogenous insulin gene activity using live cell imaging, with complementary experiments using FACS and single-cell RNA sequencing, in β-cells from Ins2GFP knockin mice. In vivo staining and FACS analysis of islets from Ins2GFP mice confirmed that at a given moment, ∼25% of β-cells exhibited significantly higher activity at the evolutionarily conserved insulin gene, Ins2. Live cell imaging over days captured Ins2 gene activity dynamics in single β-cells. Autocorrelation analysis revealed a subset of oscillating cells, with mean oscillation periods of 17 h. Increased glucose concentrations stimulated more cells to oscillate and resulted in higher average Ins2 gene activity per cell. Single-cell RNA sequencing showed that Ins2(GFP)HIGH β-cells were enriched for markers of β-cell maturity. Ins2(GFP)HIGH β-cells were also significantly less viable at all glucose concentrations and in the context of endoplasmic reticulum stress. Collectively, our results demonstrate that the heterogeneity of insulin production, observed in mouse and human β-cells, can be accounted for by dynamic states of insulin gene activity.

A transcriptional cross species map of pancreatic islet cells

OBJECTIVE

Pancreatic islets of Langerhans secrete hormones to regulate systemic glucose levels. Emerging evidence suggests that islet cells are functionally heterogeneous to allow a fine-tuned and efficient endocrine response to physiological changes. A precise description of the molecular basis of this heterogeneity, in particular linking animal models to human islets, is an important step towards identifying the factors critical for endocrine cell function in physiological and pathophysiological conditions.

METHODS

In this study, we used single-cell RNA sequencing to profile more than 50'000 endocrine cells isolated from healthy human, pig and mouse pancreatic islets and characterize transcriptional heterogeneity and evolutionary conservation of those cells across the three species. We systematically delineated endocrine cell types and α- and β-cell heterogeneity through prior knowledge- and data-driven gene sets shared across species, which altogether capture common and differential cellular properties, transcriptional dynamics and putative driving factors of state transitions.

RESULTS

We showed that global endocrine expression profiles correlate, and that critical identity and functional markers are shared between species, while only approximately 20% of cell type enriched expression is conserved. We resolved distinct human α- and β-cell states that form continuous transcriptional landscapes. These states differentially activate maturation and hormone secretion programs, which are related to regulatory hormone receptor expression, signaling pathways and different types of cellular stress responses. Finally, we mapped mouse and pig cells to the human reference and observed that the spectrum of human α- and β-cell heterogeneity and aspects of such functional gene expression are better recapitulated in the pig than mouse data.

CONCLUSIONS

Here, we provide a high-resolution transcriptional map of healthy human islet cells and their murine and porcine counterparts, which is easily queryable via an online interface. This comprehensive resource informs future efforts that focus on pancreatic endocrine function, failure and regeneration, and enables to assess molecular conservation in islet biology across species for translational purposes.

Deletion of serine racemase reverses neuronal insulin signaling inhibition by amyloid-β oligomers

Dysregulation of insulin signaling in the Alzheimer's (AD) brain has been extensively reported. Serine racemase(SR) modulates insulin secretion in pancreatic islets. Similarly, we wonder whether or not SR regulates insulin synthesis and secretion in neurons, thereby modulating insulin signaling in the AD brain. Srr-knockout (Srr^-/- ) mice generated with the CRISPR/Cas9 technique were used. Using immunofluorescence and fluorescence in situ hybridization, the levels of insulin protein and insulin(ins2) mRNA significantly increased in the hippocampal but not in the hypothalamic sections of Srr^-/- mice compared with WT mice. Using real-time quantitative PCR, ins2 mRNA from primary hippocampal neuronal cultures of Srr^-/- mice significantly increased compared with the cultured neurons from WT mice. Notably, the secretion of proinsulin C-peptide increased in Srr^-/- neurons relative to WT neurons. By examining the membrane fractional proteins with immunoblotting, Srr^-/- neurons retained ATP-dependent potassium channel on plasmalemma and correspondingly contained higher levels of p-AMPK. Under treatment by Aβ42, the phosphorylation levels of insulin receptor substrate at serine 616,636 (p-IRS1^ser616,636 ) were significantly lower whereas p-AKT³⁰⁸ and p-AKT⁴⁷³ were higher in Srr^-/- neurons, compared with WT neurons, respectively. The phosphorylated form of c-Jun N-terminal kinase decreased in the cultured Srr^-/- neurons relative to the WT neurons upon Aβ42 treatment. In contrast, the phosphorylated protein kinase R remained at the same levels. Further, reactive oxygen species reduced in the cultured Srr^-/- neurons under Aβ42 treatment relative to the WT neurons. Altogether, our study indicated that Srr deletion promoted insulin synthesis and secretion of proinsulin C-peptide, thereby reversing insulin resistance by Aβ42. This study suggests that targeting the neuronal SR may be utilized to enhance insulin signaling which is inhibited at the early stage of the AD brain.

Mitofusins Mfn1 and Mfn2 Are Required to Preserve Glucose- but Not Incretin-Stimulated β-Cell Connectivity and Insulin Secretion

Mitochondrial glucose metabolism is essential for stimulated insulin release from pancreatic β-cells. Whether mitofusin gene expression, and hence, mitochondrial network integrity, is important for glucose or incretin signaling has not previously been explored. Here, we generated mice with β-cell-selective, adult-restricted deletion knock-out (dKO) of the mitofusin genes Mfn1 and Mfn2 (βMfn1/2 dKO). βMfn1/2-dKO mice displayed elevated fed and fasted glycemia and a more than fivefold decrease in plasma insulin. Mitochondrial length, glucose-induced polarization, ATP synthesis, and cytosolic and mitochondrial Ca2+ increases were all reduced in dKO islets. In contrast, oral glucose tolerance was more modestly affected in βMfn1/2-dKO mice, and glucagon-like peptide 1 or glucose-dependent insulinotropic peptide receptor agonists largely corrected defective glucose-stimulated insulin secretion through enhanced EPAC-dependent signaling. Correspondingly, cAMP increases in the cytosol, as measured with an Epac-camps-based sensor, were exaggerated in dKO mice. Mitochondrial fusion and fission cycles are thus essential in the β-cell to maintain normal glucose, but not incretin, sensing. These findings broaden our understanding of the roles of mitofusins in β-cells, the potential contributions of altered mitochondrial dynamics to diabetes development, and the impact of incretins on this process.

Opposing effects on regulated insulin secretion of acute vs chronic stimulation of AMP-activated protein kinase

AIMS/HYPOTHESIS

Although targeted in extrapancreatic tissues by several drugs used to treat type 2 diabetes, the role of AMP-activated protein kinase (AMPK) in the control of insulin secretion is still debatable. Previous studies have used pharmacological activators of limited selectivity and specificity, and none has examined in primary pancreatic beta cells the actions of the latest generation of highly potent and specific activators that act via the allosteric drug and metabolite (ADaM) site.

METHODS

AMPK was activated acutely in islets isolated from C57BL6/J mice, and in an EndoC-βH3 cell line, using three structurally distinct ADaM site activators (991, PF-06409577 and RA089), with varying selectivity for β1- vs β2-containing complexes. Mouse lines expressing a gain-of-function mutation in the γ1 AMPK subunit (D316a) were generated to examine the effects of chronic AMPK stimulation in the whole body, or selectively in the beta cell.

RESULTS

Acute (1.5 h) treatment of wild-type mouse islets with 991, PF-06409577 or RA089 robustly stimulated insulin secretion at high glucose concentrations (p<0.01, p<0.05 and p<0.001, respectively), despite a lowering of glucose-induced intracellular free Ca2+ dynamics in response to 991 (AUC, p<0.05) and to RA089 at the highest dose (25 μmol/l) at 5.59 min (p<0.05). Although abolished in the absence of AMPK, the effects of 991 were observed in the absence of the upstream kinase, liver kinase B1, further implicating 'amplifying' pathways. In marked contrast, chronic activation of AMPK, either globally or selectively in the beta cell, achieved using a gain-of-function mutant, impaired insulin release in vivo (p<0.05 at 15 min following i.p. injection of 3 mmol/l glucose) and in vitro (p<0.01 following incubation of islets with 17 mmol/l glucose), and lowered glucose tolerance (p<0.001).

CONCLUSIONS/INTERPRETATION

AMPK activation exerts complex, time-dependent effects on insulin secretion. These observations should inform the design and future clinical use of AMPK modulators.

Opposing roles of the entero-pancreatic hormone urocortin-3 in glucose metabolism in rats

AIM/HYPOTHESIS

Urocortin-3 (UCN3) is a glucoregulatory peptide produced in the gut and pancreatic islets. The aim of this study was to clarify the acute effects of UCN3 on glucose regulation following an oral glucose challenge and to investigate the mechanisms involved.

METHODS

We studied the effect of UCN3 on blood glucose, gastric emptying, glucose absorption and secretion of gut and pancreatic hormones in male rats. To supplement these physiological studies, we mapped the expression of UCN3 and the UCN3-sensitive receptor, type 2 corticotropin-releasing factor receptor (CRHR2), by means of fluorescence in situ hybridisation and by gene expression analysis.

RESULTS

In rats, s.c. administration of UCN3 strongly inhibited gastric emptying and glucose absorption after oral administration of glucose. Direct inhibition of gastrointestinal motility may be responsible because UCN3's cognate receptor, CRHR2, was detected in gastric submucosal plexus and in interstitial cells of Cajal. Despite inhibited glucose absorption, post-challenge blood glucose levels matched those of rats given vehicle in the low-dose UCN3 group, because UCN3 concomitantly inhibited insulin secretion. Higher UCN3 doses did not further inhibit gastric emptying, but the insulin inhibition progressed resulting in elevated post-challenge glucose and lipolysis. Incretin hormones and somatostatin (SST) secretion from isolated perfused rat small intestine was unaffected by UCN3 infusion; however, UCN3 infusion stimulated secretion of somatostatin from delta cells in the isolated perfused rat pancreas which, unlike alpha cells and beta cells, expressed Crhr2. Conversely, acute antagonism of CRHR2 signalling increased insulin secretion by reducing SST signalling. Consistent with these observations, acute drug-induced inhibition of CRHR2 signalling improved glucose tolerance in rats to a similar degree as administration of glucagon-like peptide-1. UCN3 also powerfully inhibited glucagon secretion from isolated perfused rat pancreas (perfused with 3.5 mmol/l glucose) in a SST-dependent manner, suggesting that UCN3 may be involved in glucose-induced inhibition of glucagon secretion.

CONCLUSIONS/INTERPRETATION

Our combined data indicate that UCN3 is an important glucoregulatory hormone that acts through regulation of gastrointestinal and pancreatic functions.

G protein-coupled receptors as regulators of pancreatic islet functionality

Glucose homeostasis is maintained by hormones secreted from different types of pancreatic islets and its dysregulation can result in diseases including diabetes mellitus. The secretion of hormones from pancreatic islets is highly complex and tightly controlled by G protein-coupled receptors (GPCRs). Moreover, GPCR signaling may play a role in enhancing islet cell replication and proliferation. Thus, targeting GPCRs offers a promising strategy for regulating the functionality of pancreatic islets. Here, available RNAseq datasets from human and mouse islets were used to identify the GPCR expression profile and the impact of GPCR signaling for normal islet functionality is discussed.

Urocortin3: Local inducer of somatostatin release and bellwether of beta cell maturity

Urocortin 3 (UCN3) is a peptide hormone expressed in pancreatic islets of Langerhans of both human alpha and human beta cells and solely in murine beta cells. UCN3 signaling acts locally within the islet to activate its cognate receptor, corticotropin releasing hormone receptor 2 (CRHR2), which is expressed by delta cells, to potentiate somatostatin (SST) negative feedback to reduce islet cell hormone output. The functional importance of UCN3 signaling in the islet is to modulate the amount of SST tone allowing for finely tuned regulation of insulin and glucagon secretion. UCN3 signaling is a hallmark of functional beta cell maturation, increasing the beta cell glucose threshold for insulin secretion. In doing so, UCN3 plays a relevant functional role in accurately maintaining blood glucose homeostasis. Additionally, UCN3 acts as an indicator of beta cell maturation and health, as UCN3 is not expressed in immature beta cells and is downregulated in dedifferentiated and dysfunctional beta cell states. Here, we review the mechanistic underpinnings of UCN3 signaling, its net effect on islet cell hormone output, as well as its value as a marker for beta cell maturation and functional status.

Harnessing the Power of Leptin: The Biochemical Link Connecting Obesity, Diabetes, and Cognitive Decline

In this review, the current understanding of leptin’s role in energy balance, glycemic regulation, and cognitive function is examined, and its involvement in maintaining the homeostatic “harmony” of these physiologies is explored. The effects of exercise on circulating leptin levels are summarized, and the results of clinical application of leptin to metabolic disease and neurologic dysfunction are reviewed. Finally, pre-clinical evidence is presented which suggests that synthetic peptide leptin mimetics may be useful in resolving not only the leptin resistance associated with common obesity and other elements of metabolic syndrome, but also the peripheral insulin resistance characterizing type 2 diabetes mellitus, and the central insulin resistance associated with certain neurologic deficits in humans.

Acetyl-CoA-carboxylase 1 (ACC1) plays a critical role in glucagon secretion

Dysregulated glucagon secretion from pancreatic alpha-cells is a key feature of type-1 and type-2 diabetes (T1D and T2D), yet our mechanistic understanding of alpha-cell function is underdeveloped relative to insulin-secreting beta-cells. Here we show that the enzyme acetyl-CoA-carboxylase 1 (ACC1), which couples glucose metabolism to lipogenesis, plays a key role in the regulation of glucagon secretion. Pharmacological inhibition of ACC1 in mouse islets or αTC9 cells impaired glucagon secretion at low glucose (1 mmol/l). Likewise, deletion of ACC1 in alpha-cells in mice reduced glucagon secretion at low glucose in isolated islets, and in response to fasting or insulin-induced hypoglycaemia in vivo. Electrophysiological recordings identified impaired K_ATP channel activity and P/Q- and L-type calcium currents in alpha-cells lacking ACC1, explaining the loss of glucose-sensing. ACC-dependent alterations in S-acylation of the K_ATP channel subunit, Kir6.2, were identified by acyl-biotin exchange assays. Histological analysis identified that loss of ACC1 caused a reduction in alpha-cell area of the pancreas, glucagon content and individual alpha-cell size, further impairing secretory capacity. Loss of ACC1 also reduced the release of glucagon-like peptide 1 (GLP-1) in primary gastrointestinal crypts. Together, these data reveal a role for the ACC1-coupled pathway in proglucagon-expressing nutrient-responsive endocrine cell function and systemic glucose homeostasis.

Lipid Droplets' Role in the Regulation of β-Cell Function and β-Cell Demise in Type 2 Diabetes

During development of type 2 diabetes (T2D), excessive nutritional load is thought to expose pancreatic islets to toxic effects of lipids and reduce β-cell function and mass. However, lipids also play a positive role in cellular metabolism and function. Thus, proper trafficking of lipids is critical for β cells to maximize the beneficial effects of these molecules while preventing their toxic effects. Lipid droplets (LDs) are organelles that play an important role in the storage and trafficking of lipids. In this review, we summarize the discovery of LDs in pancreatic β cells, LD lifecycle, and the effect of LD catabolism on β-cell insulin secretion. We discuss factors affecting LD formation such as age, cell type, species, and nutrient availability. We then outline published studies targeting critical LD regulators, primarily in rat and human β-cell models, to understand the molecular effect of LD formation and degradation on β-cell function and health. Furthermore, based on the abnormal LD accumulation observed in human T2D islets, we discuss the possible role of LDs during the development of β-cell failure in T2D. Current knowledge indicates that proper formation and clearance of LDs are critical to normal insulin secretion, endoplasmic reticulum homeostasis, and mitochondrial integrity in β cells. However, it remains unclear whether LDs positively or negatively affect human β-cell demise in T2D. Thus, we discuss possible research directions to address the knowledge gap regarding the role of LDs in β-cell failure.

Anti-Inflammatory and Pro-Autophagy Effects of the Cannabinoid Receptor CB2R: Possibility of Modulation in Type 1 Diabetes

Type 1 diabetes mellitus (T1DM) is an autoimmune disease resulting from loss of insulin-secreting β-cells in islets of Langerhans. The loss of β-cells is initiated when self-tolerance to β-cell-derived contents breaks down, which leads to T cell-mediated β-cell damage and, ultimately, β-cell apoptosis. Many investigations have demonstrated the positive effects of antagonizing cannabinoid receptor 1 (CB1R) in metabolic diseases such as fatty liver disease, obesity, and diabetes mellitus, but the role of cannabinoid receptor 2 (CB2R) in such diseases is relatively unknown. Activation of CB2R is known for its immunosuppressive roles in multiple sclerosis, rheumatoid arthritis, Crohn’s, celiac, and lupus diseases, and since autoimmune diseases can share common environmental and genetic factors, we propose CB2R specific agonists may also serve as disease modifiers in diabetes mellitus. The CNR2 gene, which encodes CB2R protein, is the result of a gene duplication of CNR1, which encodes CB1R protein. This ortholog evolved rapidly after transitioning from invertebrates to vertebrate hundreds of million years ago. Human specific CNR2 isoforms are induced by inflammation in pancreatic islets, and a CNR2 nonsynonymous SNP (Q63R) is associated with autoimmune diseases. We collected evidence from the literature and from our own studies demonstrating that CB2R is involved in regulating the inflammasome and especially release of the cytokine interleukin 1B (IL-1β). Furthermore, CB2R activation controls intracellular autophagy and may regulate secretion of extracellular vesicles from adipocytes that participate in recycling of lipid droplets, dysregulation of which induces chronic inflammation and obesity. CB2R activation may play a similar role in islets of Langerhans. Here, we will discuss future strategies to unravel what roles, if any, CB2R modifiers potentially play in T1DM.

Large-Scale Functional Genomics Screen to Identify Modulators of Human β-Cell Insulin Secretion

Type 2 diabetes (T2D) is a chronic metabolic disorder affecting almost half a billion people worldwide. Impaired function of pancreatic β-cells is both a hallmark of T2D and an underlying factor in the pathophysiology of the disease. Understanding the cellular mechanisms regulating appropriate insulin secretion has been of long-standing interest in the scientific and clinical communities. To identify novel genes regulating insulin secretion we developed a robust arrayed siRNA screen measuring basal, glucose-stimulated, and augmented insulin secretion by EndoC-βH1 cells, a human β-cell line, in a 384-well plate format. We screened 521 candidate genes selected by text mining for relevance to T2D biology and identified 23 positive and 68 negative regulators of insulin secretion. Among these, we validated ghrelin receptor (GHSR), and two genes implicated in endoplasmic reticulum stress, ATF4 and HSPA5. Thus, we have demonstrated the feasibility of using EndoC-βH1 cells for large-scale siRNA screening to identify candidate genes regulating β-cell insulin secretion as potential novel drug targets. Furthermore, this screening format can be adapted to other disease-relevant functional endpoints to enable large-scale screening for targets regulating cellular mechanisms contributing to the progressive loss of functional β-cell mass occurring in T2D.

Gap junction coupling and islet delta-cell function in health and disease

The pancreatic islets contain beta-cells and alpha-cells, which are responsible for secreting two principal gluco-regulatory hormones; insulin and glucagon, respectively. However, they also contain delta-cells, a relatively sparse cell type that secretes somatostatin (SST). These cells have a complex morphology allowing them to establish an extensive communication network throughout the islet, despite their scarcity. Delta-cells are electrically excitable cells, and SST secretion is released in a glucose- and K_ATP-dependent manner. SST hyperpolarises the alpha-cell membrane and suppresses exocytosis. In this way, islet SST potently inhibits glucagon release. Recent studies investigating the activity of delta-cells have revealed they are electrically coupled to beta-cells via gap junctions, suggesting the delta-cell is more than just a paracrine inhibitor. In this Review, we summarize delta-cell morphology, function, and the role of SST signalling for regulating islet hormonal output. A distinguishing feature of this Review is that we attempt to use the discovery of this gap junction pathway, together with what is already known about delta-cells, to reframe the role of these cells in both health and disease. In particular, we argue that the discovery of gap junction communication between delta-cells and beta-cells provides new insights into the contribution of delta-cells to the islet hormonal defects observed in both type 1 and type 2 diabetes. This reappraisal of the delta-cell is important as it may offer novel insights into how the physiology of this cell can be utilised to restore islet function in diabetes.

Rs864745 in JAZF1, an Islet Function Associated Variant, Correlates With Plasma Lipid Levels in Both Type 1 and Type 2 Diabetes Status, but Not Healthy Subjects

OBJECTIVE

This study aims to reveal the association between JAZF1 rs864745 A>G variant and type 2 diabetes (T2D), type 1 diabetes (T1D) risk, and their correlation with clinical features, including islet function, islet autoimmunity, and plasma lipid levels.

METHODS

We included 2505 healthy controls based on oral glucose tolerance test (OGTT), 1736 unrelated T2D, and 1003 unrelated autoantibody-positive T1D individuals. Binary logistic regression was performed to evaluate the relationships between rs864745 in JAZF1 and T2D, T1D, and islet-specific autoantibody status under the additive model, while multiple linear regression was used to assess its effect on glycemic-related quantitative traits and plasma lipid levels.

RESULTS

We did not find any association between rs864745 in JAZF1 and T2D, T1D, or their subgroups (All P > 0.05). For glycemic traits, we found that the G allele of this variant was significantly associated with higher 120 min insulin level, insulinogenic index (IGI), corrected insulin response (CIR), and acute insulin response (BIGTT-AIR) (P = 0.033, 0.006, 0.009, and 0.016, respectively) in healthy individuals. Similar associations were observed in newly diagnosed T2D but not T1D individuals. Although this variant had no impact on islet autoimmunity (All P > 0.05), significant associations with plasma total cholesterol (TC) and low-density lipoprotein (LDL) level stratified by JAZF1 rs864745 variant were observed in the disease status of T2D (P = 0.002 and 0.003) and T1D (P = 0.024 and 0.009), with significant heterogeneity to healthy individuals.

CONCLUSIONS

The common JAZF1 rs864745 variant contributes to islet function and lipid metabolism, which might be put into genetic risk scores to assess the risk of related clinical features.

Potential Therapeutic Targeting Neurotransmitter Receptors in Diabetes

Neurotransmitters are signaling molecules secreted by neurons to coordinate communication and proper function among different sections in the central neural system (CNS) by binding with different receptors. Some neurotransmitters as well as their receptors are found in pancreatic islets and are involved in the regulation of glucose homeostasis. Neurotransmitters can act with their receptors in pancreatic islets to stimulate or inhibit the secretion of insulin (β cell), glucagon (α cell) or somatostatin (δ cell). Neurotransmitter receptors are either G-protein coupled receptors or ligand-gated channels, their effects on blood glucose are mainly decided by the number and location of them in islets. Dysfunction of neurotransmitters receptors in islets is involved in the development of β cell dysfunction and type 2 diabetes (T2D).Therapies targeting different transmitter systems have great potential in the prevention and treatment of T2D and other metabolic diseases.

BACH2 inhibition reverses β cell failure in type 2 diabetes models

Type 2 diabetes (T2D) is associated with defective insulin secretion and reduced β cell mass. Available treatments provide a temporary reprieve, but secondary failure rates are high, making insulin supplementation necessary. Reversibility of β cell failure is a key translational question. Here, we reverse engineered and interrogated pancreatic islet-specific regulatory networks to discover T2D-specific subpopulations characterized by metabolic inflexibility and endocrine progenitor/stem cell features. Single-cell gain- and loss-of-function and glucose-induced Ca2+ flux analyses of top candidate master regulatory (MR) proteins in islet cells validated transcription factor BACH2 and associated epigenetic effectors as key drivers of T2D cell states. BACH2 knockout in T2D islets reversed cellular features of the disease, restoring a nondiabetic phenotype. BACH2-immunoreactive islet cells increased approximately 4-fold in diabetic patients, confirming the algorithmic prediction of clinically relevant subpopulations. Treatment with a BACH inhibitor lowered glycemia and increased plasma insulin levels in diabetic mice, and restored insulin secretion in diabetic mice and human islets. The findings suggest that T2D-specific populations of failing β cells can be reversed and indicate pathways for pharmacological intervention, including via BACH2 inhibition.

Deciphering regulatory protein activity in human pancreatic islets via reverse engineering of single-cell sequencing data

The loss of functional β cell mass contributes to development and progression of type 2 diabetes (T2D). However, the molecular mechanisms differentiating islet dysfunction in T2D from nondiabetic states remain elusive. In this issue of the JCI, Son et al. applied reverse engineering to obtain the activity of gene expression regulatory proteins from single-cell RNA sequencing data of nondiabetic and T2D human islets. The authors identify unique patterns of regulatory protein activities associated with T2D. Furthermore, BACH2 emerged as a potential transcription factor that drives activation of T2D-associated regulatory proteins in human islets.

IL-1beta promotes the age-associated decline of beta cell function

Aging is the prime risk factor for the development of type 2 diabetes. We investigated the role of the interleukin-1 (IL-1) system on insulin secretion in aged mice. During aging, expression of the protective IL-1 receptor antagonist decreased in islets, whereas IL-1beta gene expression increased specifically in the CD45 + islet immune cell fraction. One-year-old mice with a whole-body knockout of IL-1beta had higher insulin secretion in vivo and in isolated islets, along with enhanced proliferation marker Ki67 and elevated size and number of islets. Myeloid cell-specific IL-1beta knockout preserved glucose-stimulated insulin secretion during aging, whereas it declined in control mice. Isolated islets from aged myeloIL-1beta ko mice secreted more insulin along with increased expression of Ins2, Kir6.2, and of the cell-cycle gene E2f1. IL-1beta treatment of isolated islets reduced E2f1, Ins2, and Kir6.2 expression in beta cells. We conclude that IL-1beta contributes the age-associated decline of beta cell function.

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Islets from aged mice have increased IL-1beta and decreased IL-1Ra expression

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Islet immune cells are the source of increased IL-1beta expression during aging

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Myeloid-cell-specific IL-1beta knockout preserves insulin secretion in aged mice

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IL-1beta targets genes regulating insulin secretion and proliferation during aging

COVID-19 and Diabetes: A Comprehensive Review of Angiotensin Converting Enzyme 2, Mutual Effects and Pharmacotherapy

The potential relationship between diabetes and COVID-19 has been evaluated. However, new knowledge is rapidly emerging. In this study, we systematically reviewed the relationship between viral cell surface receptors (ACE2, AXL, CD147, DC-SIGN, L-SIGN and DPP4) and SARS-CoV-2 infection risk, and emphasized the implications of ACE2 on SARS-CoV-2 infection and COVID-19 pathogenesis. Besides, we updated on the two-way interactions between diabetes and COVID-19, as well as the treatment options for COVID-19 comorbid patients from the perspective of ACE2. The efficacies of various clinical chemotherapeutic options, including anti-diabetic drugs, renin-angiotensin-aldosterone system inhibitors, lipid-lowering drugs, anticoagulants, and glucocorticoids for COVID-19 positive diabetic patients were discussed. Moreover, we reviewed the significance of two different forms of ACE2 (mACE2 and sACE2) and gender on COVID-19 susceptibility and severity. This review summarizes COVID-19 pathophysiology and the best strategies for clinical management of diabetes patients with COVID-19.

Molecular Mechanisms of Nutrient-Mediated Regulation of MicroRNAs in Pancreatic β-cells

Pancreatic β-cells within the islets of Langerhans respond to rising blood glucose levels by secreting insulin that stimulates glucose uptake by peripheral tissues to maintain whole body energy homeostasis. To different extents, failure of β-cell function and/or β-cell loss contribute to the development of Type 1 and Type 2 diabetes. Chronically elevated glycaemia and high circulating free fatty acids, as often seen in obese diabetics, accelerate β-cell failure and the development of the disease. MiRNAs are essential for endocrine development and for mature pancreatic β-cell function and are dysregulated in diabetes. In this review, we summarize the different molecular mechanisms that control miRNA expression and function, including transcription, stability, posttranscriptional modifications, and interaction with RNA binding proteins and other non-coding RNAs. We also discuss which of these mechanisms are responsible for the nutrient-mediated regulation of the activity of β-cell miRNAs and identify some of the more important knowledge gaps in the field.

NLRP3 inflammasome and IL-1β pathway in type 2 diabetes and atherosclerosis: Friend or foe?

Type 2 diabetes and atherosclerosis have gradually garnered great attention as inflammatory diseases. Previously, the fact that Interleukin-1β (IL-1β) accelerates the development of type 2 diabetes and atherosclerosis has been proved in animal experiments and clinical trials. However, the continued studies found that the effect of IL-1β on type 2 diabetes and atherosclerosis is much more complicated than the negative impact. Nucleotide-binding oligomerization domain and leucine-rich repeat pyrin 3 domain (NLRP3) inflammasome, whose activation and assembly significantly affect the release of IL-1β, is a crucial effector activated by a variety of metabolites. The diversity of NLRP3 activation mode is one of the fundamental reasons for the intricate effects on the progression of type 2 diabetes and atherosclerosis, providing many new insights for us to intervene in metabolic diseases. This review focuses on how NLRP3 inflammasome affects the progression of type 2 diabetes and atherosclerosis and what opportunities and challenges it can bring us.

Glucose-induced [Ca2+]i oscillations in β cells are composed of trains of spikes within a subplasmalemmal microdomain

Electrical activity and oscillations of cytosolic Ca²⁺ concentrations ([Ca²⁺]_i) that trigger insulin release in response to glucose are key functions of pancreatic β cells. Although oscillatory Ca²⁺ signals have been intensively studied in β cells, their lower frequency did not match that of electrical activity. In addition, the measured peak [Ca²⁺]_i did not reach levels that are typically required by synaptotagmins to elicit the release of insulin-containing vesicles in live-cell experiments. We therefore sought to resolve the Ca²⁺ dynamics in the subplasmalemmal microdomain that is critical for triggering fast exocytosis. Applying total internal reflection fluorescence (TIRF) microscopy in insulin-producing INS-1E and primary mouse β cells, we resolved extraordinary fast trains of Ca²⁺ spiking (frequency > 3 s^-1) in response to glucose exposure. Using a low-affinity [Ca²⁺]_i indicator dye, we provide experimental evidence that Ca²⁺ spikes reach low micromolar apparent concentrations in the vicinity of the plasma membrane_. Analysis of Ca²⁺ spikes evoked by repeated depolarization for 10 ms closely matched the Ca²⁺ dynamics observed upon glucose application. To our knowledge, this is the first study that experimentally demonstrates Ca²⁺ spikes in β cells with velocities that resemble those of bursting or continuously appearing trains of action potentials (APs) in non-patched cells.

Prolyl-4-hydroxylase 3 maintains β cell glucose metabolism during fatty acid excess in mice

The α-ketoglutarate–dependent dioxygenase, prolyl-4-hydroxylase 3 (PHD3), is an HIF target that uses molecular oxygen to hydroxylate peptidyl prolyl residues. Although PHD3 has been reported to influence cancer cell metabolism and liver insulin sensitivity, relatively little is known about the effects of this highly conserved enzyme in insulin-secreting β cells in vivo. Here, we show that the deletion of PHD3 specifically in β cells (βPHD3KO) was associated with impaired glucose homeostasis in mice fed a high-fat diet. In the early stages of dietary fat excess, βPHD3KO islets energetically rewired, leading to defects in the management of pyruvate fate and a shift from glycolysis to increased fatty acid oxidation (FAO). However, under more prolonged metabolic stress, this switch to preferential FAO in βPHD3KO islets was associated with impaired glucose-stimulated ATP/ADP rises, Ca²⁺ fluxes, and insulin secretion. Thus, PHD3 might be a pivotal component of the β cell glucose metabolism machinery in mice by suppressing the use of fatty acids as a primary fuel source during the early phases of metabolic stress.

21st century excitatory amino acid research: A Q & A with Jeff Watkins and Dick Evans

In 1981 Jeff Watkins and Dick Evans wrote what was to become a seminal review on excitatory amino acids (EAAs) and their receptors (Watkins and Evans, 1981). Bringing together various lines of evidence dating back over several decades on: the distribution in the nervous system of putative amino acid neurotransmitters; enzymes involved in their production and metabolism; the uptake and release of amino acids; binding of EAAs to membranes; the pharmacological action of endogenous excitatory amino acids and their synthetic analogues, and notably the actions of antagonists for the excitations caused by both nerve stimulation and exogenous agonists, often using pharmacological tools developed by Jeff and his colleagues, they provided a compelling account for EAAs, especially l-glutamate, as a bona fide neurotransmitter in the nervous system. The rest, as they say, is history, but far from being consigned to history, EAA research is in rude health well into the 21st Century as this series of Special Issues of Neuropharmacology exemplifies. With EAAs and their receptors flourishing across a wide range of disciplines and clinical conditions, we enter into a dialogue with two of the most prominent and influential figures in the early days of EAA research: Jeff Watkins and Dick Evans.

FOXO3a acetylation regulates PINK1, mitophagy, inflammasome activation in murine palmitate-conditioned and diabetic macrophages

Nucleotide-binding oligomerization domain, leucine-rich repeat, and pyrin domain containing 3 inflammasome and mitophagy play an important role in cytokine release and diabetes progression; however, the role of saturated fatty acid that is induced under such conditions remains little explored. Therefore, the present study evaluates mechanisms regulating mitophagy and inflammasome activation in primary murine diabetic and palmitate-conditioned wild-type (WT) peritoneal macrophages. Peritoneal macrophage, from the diabetic mice and WT mice, challenged with LPS/ATP and palmitate/LPS/ATP, respectively, showed dysfunctional mitochondria as assessed by their membrane potential, mitochondrial reactive oxygen species (mtROS) production, and mitochondrial DNA (mtDNA) release. A defective mitophagy was observed in the diabetic and palmitate-conditioned macrophages stimulated with LPS/ATP as assessed by translocation of PTEN-induced kinase 1 (PINK1)/Parkin or p62 in the mitochondrial fraction. Consequently, increased apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) oligomerization, caspase-1 activation, and IL1β secretion were observed in LPS/ATP stimulated diabetic and palmitate-conditioned macrophages. LPS/ATP induced Forkhead box O3a (FOXO3a) binding to PINK1 promoter and increased PINK1 mRNA expression in WT macrophages. However, PINK1 mRNA and protein expression were significantly decreased in diabetic and palmitate-conditioned macrophages in response to LPS/ATP. Palmitate-induced acetyl CoA promoted FOXO3a acetylation, which prevented LPS/ATP-induced FOXO3a binding to the PINK1 promoter. C646 (P300 inhibitor) and SRT1720 (SIRT1 activator) prevented FOXO3a acetylation and restored FOXO3a binding to the PINK1 promoter, PINK1 mRNA expression, and mitophagy in palmitate-conditioned macrophages treated with LPS/ATP. Also, a significant decrease in the LPS/ATP-induced mtROS production, mtDNA release, ASC oligomerization, caspase-1 activation, and IL-1β release was observed in the palmitate-conditioned macrophages. Similarly, modulation of FOXO3a acetylation also prevented LPS/ATP-induced mtDNA release and inflammasome activation in diabetic macrophages. Therefore, FOXO3a acetylation regulates PINK1-dependent mitophagy and inflammasome activation in the palmitate-conditioned and diabetic macrophages.