Stress-induced adaptive islet cell identity changes

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.

Pancreatic islet cell plasticity: Pathogenic or therapeutically exploitable?

The development of pancreatic islet endocrine cells is a tightly regulated process leading to the generation of distinct cell types harbouring different hormones in response to small changes in environmental stimuli. Cell differentiation is driven by transcription factors that are also critical for the maintenance of the mature islet cell phenotype. Alteration of the insulin-secreting β-cell transcription factor set by prolonged metabolic stress, associated with the pathogenesis of diabetes, obesity or pregnancy, results in the loss of β-cell identity through de- or transdifferentiation. Importantly, the glucose-lowering effects of approved and experimental antidiabetic agents, including glucagon-like peptide-1 mimetics, novel peptides and small molecules, have been associated with preventing or reversing β-cell dedifferentiation or promoting the transdifferentiation of non-β-cells towards an insulin-positive β-cell-like phenotype. Therefore, we review the manifestations of islet cell plasticity in various experimental settings and discuss the physiological and therapeutic sides of this phenomenon, focusing on strategies for preventing β-cell loss or generating new β-cells in diabetes. A better understanding of the molecular mechanisms underpinning islet cell plasticity is a prerequisite for more targeted therapies to help prevent β-cell decline in diabetes.

Ghrelin deletion and conditional ghrelin cell ablation increase pancreatic islet size in mice

Ghrelin exerts key effects on islet hormone secretion to regulate blood glucose levels. Here, we sought to determine whether ghrelin's effects on islets extend to the alteration of islet size and β cell mass. We demonstrate that reducing ghrelin - by ghrelin gene knockout (GKO), conditional ghrelin cell ablation, or high-fat diet (HFD) feeding - was associated with increased mean islet size (up to 62%), percentage of large islets (up to 854%), and β cell cross-sectional area (up to 51%). In GKO mice, these effects were more apparent in 10- to 12-week-old mice than in 4-week-old mice. Higher β cell numbers from decreased β cell apoptosis drove the increase in β cell cross-sectional area. Conditional ghrelin cell ablation in adult mice increased the β cell number per islet by 40% within 4 weeks. A negative correlation between islet size and plasma ghrelin in HFD-fed plus chow-fed WT mice, together with even larger islet sizes in HFD-fed GKO mice than in HFD-fed WT mice, suggests that reduced ghrelin was not solely responsible for diet-induced obesity-associated islet enlargement. Single-cell transcriptomics revealed changes in gene expression in several GKO islet cell types, including upregulation of Manf, Dnajc3, and Gnas expression in β cells, which supports decreased β cell apoptosis and/or increased β cell proliferation. These effects of ghrelin reduction on islet morphology might prove useful when designing new therapies for diabetes.

Dynamic scRNA-seq of live human pancreatic slices reveals functional endocrine cell neogenesis through an intermediate ducto-acinar stage

Human pancreatic plasticity is implied from multiple single-cell RNA sequencing (scRNA-seq) studies. However, these have been invariably based on static datasets from which fate trajectories can only be inferred using pseudotemporal estimations. Furthermore, the analysis of isolated islets has resulted in a drastic underrepresentation of other cell types, hindering our ability to interrogate exocrine-endocrine interactions. The long-term culture of human pancreatic slices (HPSs) has presented the field with an opportunity to dynamically track tissue plasticity at the single-cell level. Combining datasets from same-donor HPSs at different time points, with or without a known regenerative stimulus (BMP signaling), led to integrated single-cell datasets storing true temporal or treatment-dependent information. This integration revealed population shifts consistent with ductal progenitor activation, blurring of ductal/acinar boundaries, formation of ducto-acinar-endocrine differentiation axes, and detection of transitional insulin-producing cells. This study provides the first longitudinal scRNA-seq analysis of whole human pancreatic tissue, confirming its plasticity in a dynamic fashion.

Epigenetic dosage identifies two major and functionally distinct β cell subtypes

The mechanisms that specify and stabilize cell subtypes remain poorly understood. Here, we identify two major subtypes of pancreatic β cells based on histone mark heterogeneity (βHI and βLO). βHI cells exhibit ∼4-fold higher levels of H3K27me3, distinct chromatin organization and compaction, and a specific transcriptional pattern. βHI and βLO cells also differ in size, morphology, cytosolic and nuclear ultrastructure, epigenomes, cell surface marker expression, and function, and can be FACS separated into CD24+ and CD24- fractions. Functionally, βHI cells have increased mitochondrial mass, activity, and insulin secretion in vivo and ex vivo. Partial loss of function indicates that H3K27me3 dosage regulates βHI/βLO ratio in vivo, suggesting that control of β cell subtype identity and ratio is at least partially uncoupled. Both subtypes are conserved in humans, with βHI cells enriched in humans with type 2 diabetes. Thus, epigenetic dosage is a novel regulator of cell subtype specification and identifies two functionally distinct β cell subtypes.

GABA signalling in human pancreatic islets

The pancreatic islets are essential microorgans controlling the glucose level in the blood. The islets consist of different cell types which communicate with each other by means of auto- and paracrine interactions. One of the communication molecules produced by and released within the islets is γ-aminobutyric acid (GABA), a well-known inhibitor of neuronal excitability in the mammalian nervous system. Interestingly, GABA is also present in the blood in the nanomolar concentration range. Thus, GABA can affect not only islet function per se (e.g. hormone secretion) but also interactions between immune cells and the pancreatic islet cells in physiological conditions and in pathological states (particularly in type 1 diabetes). In the last decade the interest in GABA signalling in islets has increased. The broad research scope ranges from fundamental physiological studies at the molecular and cellular level to pathological implications and clinical trials. The aim of this mini-review is to outline the current status of the islet GABA field mostly in relation to human islets, to identify the gaps in the current knowledge and what clinical implications GABA signalling may have in islets.

Hepatocyte Nuclear Factor 4-α (HNF4α) controls the insulin resistance-induced pancreatic β-cell mass expansion

BACKGROUND

Regardless of the etiology, any type of DM presents a reduction of insulin-secreting cell mass, so it is important to investigate pathways that induce the increase of this cell mass.

AIM

Based on the fact that (1) HNF4α is crucial for β-cell proliferation, (2) DEX-induced IR promotes β-cell mass expansion, and (3) the stimulation of β-cell mass expansion may be an important target for DM therapies, we aimed to investigate whether DEX-induced proliferation of β pancreatic cells is dependent on HNF4α.

METHODS

We used WildType (WT) and Knockout (KO) mice for HNF4-α, treated or not with 100 mg/Kg/day of DEX, for 5 consecutive days. One day after the last injection of DEX the IR was confirmed by ipITT and the mice were euthanized for pancreas removal.

RESULTS

In comparison to WT, KO mice presented increased glucose tolerance, lower fasting glucose and increased glucose-stimulates insulin secretion (GSIS). DEX induced IR in both KO and WT mice. In addition, DEX-induced β-cell mass expansion and an increase in the Ki67 immunostaining were observed only in WT mice, evidencing that IR-induced β-cell mass expansion is dependent on HNF4α. Also, we observed that DEX-treatment, in an HNF4α-dependent way, promoted an increase in PDX1, PAX4 and NGN3 gene expression.

CONCLUSIONS

Our results strongly suggest that DEX-induced IR promotes β-cell mass expansion through processes of proliferation and neogenesis that depend on the HNF4α activity, pointing to HNF4α as a possible therapeutic target in DM treatment.

A Method for Encapsulation and Transplantation into Diabetic Mice of Human Induced Pluripotent Stem Cells (hiPSC)-Derived Pancreatic Progenitors

Pancreatic islet endocrine cells generated from patient-derived induced pluripotent stem cells represent a great strategy for both disease modeling and regenerative medicine. Nevertheless, these cells inherently miss the effects of the intricate network of systemic signals characterizing the living organisms. Xenotransplantation of in vitro differentiating cells into murine hosts substantially compensates for this drawback.Here we describe our transplantation strategy of encapsulated differentiating pancreatic progenitors into diabetic immunosuppressed (NSG) overtly diabetic mice generated by the total ablation of insulin-producing cells following diphtheria toxin administration. We will detail the differentiation protocol employed, the alginate encapsulation procedure, and the xenotransplantation steps required for a successful and reproducible experiment.

Mechanisms of Beta-Cell Apoptosis in Type 2 Diabetes-Prone Situations and Potential Protection by GLP-1-Based Therapies

Type 2 diabetes (T2D) is characterized by chronic hyperglycemia secondary to the decline of functional beta-cells and is usually accompanied by a reduced sensitivity to insulin. Whereas altered beta-cell function plays a key role in T2D onset, a decreased beta-cell mass was also reported to contribute to the pathophysiology of this metabolic disease. The decreased beta-cell mass in T2D is, at least in part, attributed to beta-cell apoptosis that is triggered by diabetogenic situations such as amyloid deposits, lipotoxicity and glucotoxicity. In this review, we discussed the molecular mechanisms involved in pancreatic beta-cell apoptosis under such diabetes-prone situations. Finally, we considered the molecular signaling pathways recruited by glucagon-like peptide-1-based therapies to potentially protect beta-cells from death under diabetogenic situations.

Gut Microbiome Prolongs an Inhibitory Effect of Korean Red Ginseng on High-Fat-Diet-Induced Mouse Obesity

Although the anti-obesity effect of Korean red ginseng (Panax ginseng Meyer) has been revealed, its underlying mechanisms are not clearly understood. Here, we demonstrate an involvement of gut microbiome in the inhibitory effect of Korean red ginseng on high-fat-diet (HFD)-induced mouse obesity, and further provides information on the effects of saponin-containing red ginseng extract (SGE) and saponin-depleted red ginseng extract (GE). Mice were fed with either SGE or GE every third day for one month, and their food intakes, fat weights, plasma glucose, and insulin and leptin levels were measured. Immunofluorescence assays were conducted to measure pancreatic islet size. Stools from the mice were subjected to metagenomic analysis. Both SGE and GE attenuated HFD-induced gain of body weight, reducing HFD-induced increase of food intakes and fat weights. They also reduced HFD-increased plasma glucose, insulin, and leptin levels, decreased both fasting and postprandial glucose concentrations, and improved both insulin resistance and glucose intolerance. Immunofluorescence assays revealed that they blocked HFD-induced increase of pancreatic islet size. Our pyrosequencing of the 16S rRNA gene V3 region from stools revealed that both SGE and GE modulated HFD-altered composition of gut microbiota. Therefore, we conclude that Korean red ginseng inhibits HFD-induced obesity and diabetes by altering gut microbiome.

GLP-1 receptor signaling increases PCSK1 and β cell features in human α cells

Glucagon-like peptide-1 (GLP-1) is an incretin hormone that potentiates glucose-stimulated insulin secretion. GLP-1 is classically produced by gut L cells; however, under certain circumstances α cells can express the prohormone convertase required for proglucagon processing to GLP-1, prohormone convertase 1/3 (PC1/3), and can produce GLP-1. However, the mechanisms through which this occurs are poorly defined. Understanding the mechanisms by which α cell PC1/3 expression can be activated may reveal new targets for diabetes treatment. Here, we demonstrate that the GLP-1 receptor (GLP-1R) agonist, liraglutide, increased α cell GLP-1 expression in a β cell GLP-1R-dependent manner. We demonstrate that this effect of liraglutide was translationally relevant in human islets through application of a new scRNA-seq technology, DART-Seq. We found that the effect of liraglutide to increase α cell PC1/3 mRNA expression occurred in a subcluster of α cells and was associated with increased expression of other β cell-like genes, which we confirmed by IHC. Finally, we found that the effect of liraglutide to increase bihormonal insulin+ glucagon+ cells was mediated by the β cell GLP-1R in mice. Together, our data validate a high-sensitivity method for scRNA-seq in human islets and identify a potentially novel GLP-1-mediated pathway regulating human α cell function.

Pancreatic β cell regeneration induced by clinical and preclinical agents

Diabetes, one of the most common chronic diseases in the modern world, has pancreatic β cell deficiency as a major part of its pathophysiological mechanism. Pancreatic regeneration is a potential therapeutic strategy for the recovery of β cell loss. However, endocrine islets have limited regenerative capacity, especially in adult humans. Almost all hypoglycemic drugs can protect β cells by inhibiting β cell apoptosis and dedifferentiation via correction of hyperglycemia and amelioration of the consequent inflammation and oxidative stress. Several agents, including glucagon-like peptide-1 and γ-aminobutyric acid, have been shown to promote β cell proliferation, which is considered the main source of the regenerated β cells in adult rodents, but with less clarity in humans. Pancreatic progenitor cells might exist and be activated under particular circumstances. Artemisinins and γ-aminobutyric acid can induce α-to-β cell conversion, although some disputes exist. Intestinal endocrine progenitors can transdeterminate into insulin-producing cells in the gut after FoxO1 deletion, and pharmacological research into FoxO1 inhibition is ongoing. Other cells, including pancreatic acinar cells, can transdifferentiate into β cells, and clinical and preclinical strategies are currently underway. In this review, we summarize the clinical and preclinical agents used in different approaches for β cell regeneration and make some suggestions regarding future perspectives for clinical application.

Chromogranin A-positive hormone-negative endocrine cells in pancreas in human pregnancy

Introduction

We sought to determine whether chromogranin A‐positive hormone‐negative (CPHN) endocrine cells are increased in the pancreas of pregnant women, offering potential evidence in support of neogenesis.

Methods

Autopsy pancreata from pregnant women (n = 14) and age‐matched non‐pregnant control women (n = 9) were obtained. Staining of pancreatic sections for chromogranin A, insulin and a cocktail of glucagon, somatostatin, pancreatic polypeptide and ghrelin was undertaken, with subsequent evaluation for CPHN cell frequency.

Results

The frequency of clustered β‐cells was increased in pregnant compared to non‐pregnant subjects (46.6 ± 5.0 vs. 31.8 ± 5.0% clustered β‐cells of total clustered endocrine cells, pregnant vs. non‐pregnant, p < .05). Frequency of endocrine cocktail cells was lower in pregnant women than non‐pregnant women (36.2 ± 4.0 vs. 57.0 ± 6.8% clustered endocrine cocktail cells of total clustered endocrine cells, pregnant vs. non‐pregnant, p < .01). No difference in frequency of CPHN cells was found in islets, nor in clustered or single cells scattered throughout the exocrine pancreas, between pregnant and non‐pregnant women. The frequency of CPHN cells in pregnancy was independent of the number of pregnancies (gravidity).

Conclusions

Our findings of no increase in CPHN cell frequency in pancreas of pregnant women suggest that this potential β‐cell regenerative mechanism is not that by which the increased β‐cell mass of pregnancy is achieved. However, an increase in the percentage of clustered β‐cells was found in pregnancy, with decreased frequency of other endocrine cells in clusters, suggesting a compensatory shift from other pancreatic endocrine cell types to β‐cells as a mechanism to meet the increased insulin demands of pregnancy.

Cytohistologic analyses of β cell dedifferentiation induced by inflammation in human islets

β cell dedifferentiation is a key mechanism for β cell dysfunction in type 2 diabetes mellitus (T2DM). Although it has been indicated in previous studies that β cell dedifferentiation could be induced by inflammation, the cytohistologic analyses of inflammation-induced β cell dedifferentiation in human islets is lacking. The present study aims to cytohistologically characterize the β cell dedifferentiation of human islets treated by proinflammatory cytokines Interleukin-1β/Tuman necrosis factor-α/Interferon-γ (IL-1β/TNF-α/IFN-γ), which is a frequently-used method to mimic the islet inflammation in previous studies. The loss of cytosolic FOXO1 expression, the loss of nucleic NKX6.1 expression, and the gain of ALDH1A3 expression in β cells are proclaimed as marking events for β cell dedifferentiation. Taking advantages of islets from organ donors and the immunofluorescence staining methods, the present study visualized the β cell dedifferentiation events marked by different markers, and quantified the frequency of each event as well. We successfully captured and described the characteristics of the differentiating/differentiated β cells. We found that dedifferentiated β cells were increased in the cytokines treated islets, evidenced by the increase of β cells with FOXO1 translocated to the nucleus (INS+FOXOnuc), β cells with NKX6.1 exported from the nucleus (INS+NKX6.1cyt), and β cells loss of NKX6.1 expression (INS+NKX6.1-), and β cells with dual expression of insulin and progenitor marker ALDH1A3. Consistently, we found that proinflammatory cytokines IL-1β/TNF-α/IFN-γ treatment reduced the mRNA expression of key β cell markers, but elevated the expression of progenitor marker genes. This study gives the most direct evidence for inflammation-induced β cell dedifferentiation in human islets, and supports the concept that anti-inflammation treatments may facilitate alleviating the β cell dedifferentiation in human T2DM islets.

PRMT1 Is Required for the Maintenance of Mature β-Cell Identity

Loss of functional β-cell mass is an essential feature of type 2 diabetes, and maintaining mature β-cell identity is important for preserving a functional β-cell mass. However, it is unclear how β-cells achieve and maintain their mature identity. Here we demonstrate a novel function of protein arginine methyltransferase 1 (PRMT1) in maintaining mature β-cell identity. Prmt1 knockout in fetal and adult β-cells induced diabetes, which was aggravated by high-fat diet-induced metabolic stress. Deletion of Prmt1 in adult β-cells resulted in the immediate loss of histone H4 arginine 3 asymmetric dimethylation (H4R3me2a) and the subsequent loss of β-cell identity. The expression levels of genes involved in mature β-cell function and identity were robustly downregulated as soon as Prmt1 deletion was induced in adult β-cells. Chromatin immunoprecipitation sequencing and assay for transposase-accessible chromatin sequencing analyses revealed that PRMT1-dependent H4R3me2a increases chromatin accessibility at the binding sites for CCCTC-binding factor (CTCF) and β-cell transcription factors. In addition, PRMT1-dependent open chromatin regions may show an association with the risk of diabetes in humans. Together, our results indicate that PRMT1 plays an essential role in maintaining β-cell identity by regulating chromatin accessibility.

Development and characterisation of a peptidergic N-and C-terminally stabilised mammalian NPY1R agonist which protects against diabetes induction

BACKGROUND

PYY (1-36) peptides from phylogenetically ancient fish, such as sea lamprey, have previously been shown to function as specific neuropeptide Y1 receptor (NPYR1) agonists. Although, sea lamprey PYY (1-36) is N-terminally stable, we reveal in this study that the peptide is subject to endopeptidase mediated C-terminal dipeptide degradation. In an attempt to prevent this, (d-Arg³⁵)-sea lamprey PYY (1-36) was developed.

METHODS

In vitro bioassays assessed enzymatic stability, insulinostatic activity as well as beta-cell anti-apoptotic actions of (d-Arg³⁵)-sea lamprey PYY (1-36). Follow-up studies examined the impact of twice daily administration of sea lamprey PYY (1-36) or (d-Arg³⁵)-sea lamprey PYY (1-36) in multiple low dose STZ-induced diabetic mice.

RESULTS

(d-Arg³⁵)-sea lamprey PYY (1-36) was fully resistant to plasma enzymatic degradation. The peptide possessed similar significant insulinostatic, as well as positive anti-apoptotic biological actions, as the parent peptide. Sea lamprey PYY (1-36) and (d-Arg³⁵)-sea lamprey PYY (1-36) delayed diabetes progression in STZ mice. Both treatment interventions induced a significant decrease in body weight, food and fluid intake as well as glucose and glucagon concentrations. In addition, glucose tolerance, plasma and pancreatic insulin were partially normalised. (d-Arg³⁵)-sea lamprey PYY (1-36) was significantly more effective than sea lamprey PYY (1-36) in terms of enhancing glucose-stimulate insulin release. Both treatments improved pancreatic islet morphology, linked to decreased apoptosis of beta-cells.

CONCLUSION

We present (d-Arg³⁵)-sea lamprey PYY (1-36) as the first-in-class N- and C-terminally stable PYY (1-36) peptide analogue.

GENERAL SIGNIFICANCE

Enzymatically stable, long-acting PYY (1-36) peptides highlight the therapeutic benefits of sustained activation of NPYR1's in diabetes.

GABAA Receptor-Mediated Currents and Hormone mRNAs in Cells Expressing More Than One Hormone Transcript in Intact Human Pancreatic Islets

In pancreatic islets, the major cell-types are α, β and δ cells. The γ-aminobutyric acid (GABA) signalling system is expressed in human pancreatic islets. In single hormone transcript-expressing cells, we have previously characterized the functional properties of islet GABAA receptors (iGABAARs). Here, we extended these studies to islet cells expressing mRNAs for more than one hormone and sought for correlation between iGABAAR activity level and relative mRNA expression ratio. The single-cell RT-PCR in combination with the patch-clamp current recordings was used to examine functional properties of iGABAARs in the multiple hormone mRNA-expressing cells. We detected cells expressing double (α/β, α/δ, β/δ cell-types) and triple (α/β/δ cell-type) hormone transcripts. The most common mixed-identity cell-type was the α/β group where the cells could be grouped into β- and α-like subgroups. The β-like cells had low GCG/INS expression ratio (<0.6) and significantly higher frequency of iGABAAR single-channel openings than the α-like cells where the GCG/INS expression ratio was high (>1.2). The hormone expression levels and iGABAAR single-channel characteristics varied in the α/β/δ cell-type. Clearly, multiple hormone transcripts can be expressed in islet cells whereas iGABAAR single-channel functional properties appear to be α or β cell specific.

The Role of α-Cells in Islet Function and Glucose Homeostasis in Health and Type 2 Diabetes

Pancreatic α-cells are the major source of glucagon, a hormone that counteracts the hypoglycemic action of insulin and strongly contributes to the correction of acute hypoglycemia. The mechanisms by which glucose controls glucagon secretion are hotly debated, and it is still unclear to what extent this control results from a direct action of glucose on α-cells or is indirectly mediated by β- and/or δ-cells. Besides its hyperglycemic action, glucagon has many other effects, in particular on lipid and amino acid metabolism. Counterintuitively, glucagon seems also required for an optimal insulin secretion in response to glucose by acting on its cognate receptor and, even more importantly, on GLP-1 receptors. Patients with diabetes mellitus display two main alterations of glucagon secretion: a relative hyperglucagonemia that aggravates hyperglycemia, and an impaired glucagon response to hypoglycemia. Under metabolic stress states, such as diabetes, pancreatic α-cells also secrete GLP-1, a glucose-lowering hormone, whereas the gut can produce glucagon. The contribution of extrapancreatic glucagon to the abnormal glucose homeostasis is unclear. Here, I review the possible mechanisms of control of glucagon secretion and the role of α-cells on islet function in healthy state. I discuss the possible causes of the abnormal glucagonemia in diabetes, with particular emphasis on type 2 diabetes, and I briefly comment the current antidiabetic therapies affecting α-cells.

What Is the Sweetest UPR Flavor for the β-cell? That Is the Question

Unfolded protein response (UPR) is a process conserved from yeasts to mammals and, based on the generally accepted dogma, helps the secretory performance of a cell, by improving its capacity to cope with a burden in the endoplasmic reticulum (ER). The ER of β-cells, "professional secretory cells", has to manage tremendous amounts of insulin, which elicits a strong pressure on the ER intrinsic folding capacity. Thus, the constant demand for insulin production results in misfolded proinsulin, triggering a physiological upregulation of UPR to restore homeostasis. Most diabetic disorders are characterized by the loss of functional β-cells, and the pathological side of UPR plays an instrumental role. The transition from a homeostatic to a pathological UPR that ultimately leads to insulin-producing β-cell decay entails complex cellular processes and molecular mechanisms which remain poorly described so far. Here, we summarize important processes that are coupled with or driven by UPR in β-cells, such as proliferation, inflammation and dedifferentiation. We conclude that the UPR comes in different "flavors" and each of them is correlated with a specific outcome for the cell, for survival, differentiation, proliferation as well as cell death. All these greatly depend on the way UPR is triggered, however what exactly is the switch that favors the activation of one UPR as opposed to others is largely unknown. Substantial work needs to be done to progress the knowledge in this important emerging field as this will help in the development of novel and more efficient therapies for diabetes.

Conversion of pancreatic α cells into insulin-producing cells modulated by β-cell insufficiency and supplemental insulin administration

The emergence of bihormonal (BH) cells expressing insulin and glucagon has been reported under diabetic conditions in humans and mice. Whereas lineage tracing studies demonstrated that glucagon-producing α cells can be reprogrammed into BH cells, the underlying dynamics of the conversion process remain poorly understood. In the present study, we investigated the identities of pancreatic endocrine cells by genetic lineage tracing under diabetic conditions. When β-cell ablation was induced by alloxan (ALX), a time-dependent increase in BH cells was subsequently observed. Lineage tracing experiments demonstrated that BH cells originate from α cells, but not from β cells, in ALX-induced diabetic mice. Notably, supplemental insulin administration into diabetic mice resulted in a significant increase in α-cell-derived insulin-producing cells that did not express glucagon. Furthermore, lineage tracing in Ins2^Akita diabetic mice demonstrated a significant induction of α-to-β conversion. Thus, adult α cells have plasticity, which enables them to be reprogrammed into insulin-producing cells under diabetic conditions, and this can be modulated by supplemental insulin administration.

β-Cell compensation concomitant with adaptive endoplasmic reticulum stress and β-cell neogenesis in a diet-induced type 2 diabetes model

Insulin-secreting pancreatic β-cells adapt to obesity-related insulin resistance via increases in insulin secretion and β-cell mass. Failed β-cell compensation predicts the onset of type 2 diabetes (T2D). However, the mechanisms of β-cell compensation are not fully understood. Our previous study reported changes in β-cell mass during the progression of T2D in the Nile rat (NR; Arvicanthis niloticus) fed standard chow. In the present study, we measured other β-cell adaptive responses, including glucose metabolism and β-cell insulin secretion in NRs at different ages, thus characterizing NR at 2 months as a model of β-cell compensation followed by decompensation at 6 months. We observed increased proinsulin secretion in the transition from compensation to decompensation, which is indicative of impaired insulin processing. Subsequently, we compared adaptive unfolded protein response in β-cells and demonstrated a positive role of endoplasmic reticulum (ER) chaperones in insulin secretion. In addition, the incidence of insulin-positive neogenic but not proliferative cells increased during the compensation phase, suggesting nonproliferative β-cell growth as a mechanism of β-cell mass adaptation. In contrast, decreased neogenesis and β-cell dedifferentiation were observed in β-cell dysfunction. Furthermore, the progression of T2D and pathophysiological changes of β-cells were prevented by increasing fibre content of the diet. Novelty Our study characterized a novel model for β-cell compensation with adaptive responses in cell function and mass. The temporal association of adaptive ER chaperones with blood insulin and glucose suggests upregulated chaperone capacity as an adaptive mechanism. β-Cell neogenesis but not proliferation contributes to β-cell mass adaptation.

Dimorphic autoantigenic and protective effects of Reg2 peptide in the treatment of diabetic β-cell loss

AIMS

The potential effect of regenerating (Reg) proteins in the treatment of diabetes has been indicated in the past decade, but the clinical use of Reg proteins requires more advances in translational medicine. In the present study, we produced recombinant regenerating protein 2 (rReg2), to prove its protective effect against streptozocin (STZ)-induced diabetes in BALB/c mice.

MATERIALS AND METHODS

rReg2 was administrated in STZ-induced diabetic mice. Blood glucose, body weight, serum insulin and islet β-cell loss were determined. However, Reg2 has also been reported to serve as an autoantigen that induces autoimmune attacks on islets and aggravates diabetic development in non-obese diabetic mice. To address this contradiction, complete Freund's adjuvant was injected to generate a model that was hypersensitive to Reg2. In this model, islet CD8 T-cell infiltration, serum Reg2 antibody and interleukin (IL)-4 and IL-10, and splenic CD4+/interferon (IFN)-γ+ T cells were determined.

RESULTS

Direct rReg2 pretreatment preserved islet β-cell mass against STZ and improved glycaemia, body weight and serum insulin content. The protection against cell death was further confirmed in cultured mouse islets and MIN6 cells. On the other hand, significant elevations of serum Reg2 antibody and splenic CD4+/IFN-γ+ T cells, and decreases in serum IL-4 and IL-10 were detected in rReg2-vaccinated mice, which may contribute to the accelerated diabetes. Interestingly, these mice, upon further rReg2 treatment, exhibited alleviated diabetic conditions with less islet CD8+ T-cell infiltration.

CONCLUSION

rReg2 treatment ameliorated STZ-induced diabetes in normal BALB/c mice. By contrast, rReg2 vaccination exacerbated, but further rReg2 treatment alleviated, the severity of STZ-induced diabetes. Thus, the protective effect of rReg2 is predominant over the autoantigenic β-cell destruction, supporting the potential of rReg2 in the clinical treatment of diabetes.

Pancreatic Progenitors: There and Back Again

Adult pancreatic regeneration is one of the most contentious topics in modern biology. The long-held view that the islets of Langerhans can be replenished throughout adult life through the reactivation of ductal progenitor cells has been replaced over the past decade by the now prevailing notion that regeneration does not involve progenitors and occurs only through the duplication of pre-existing mature cells. Here we dissect the limitations of lineage tracing (LT) to draw categorical conclusions about pancreatic regeneration, especially in view of emerging evidence that traditional lineages are less homogeneous and cell fates more dynamic than previously thought. This new evidence further suggests that the two competing hypotheses about regeneration are not mutually exclusive.

Inhibition of mTORC1 by ER stress impairs neonatal β-cell expansion and predisposes to diabetes in the Akita mouse

Unresolved ER stress followed by cell death is recognized as the main cause of a multitude of pathologies including neonatal diabetes. A systematic analysis of the mechanisms of β-cell loss and dysfunction in Akita mice, in which a mutation in the proinsulin gene causes a severe form of permanent neonatal diabetes, showed no increase in β-cell apoptosis throughout life. Surprisingly, we found that the main mechanism leading to β-cell dysfunction is marked impairment of β-cell growth during the early postnatal life due to transient inhibition of mTORC1, which governs postnatal β-cell growth and differentiation. Importantly, restoration of mTORC1 activity in neonate β-cells was sufficient to rescue postnatal β-cell growth, and to improve diabetes. We propose a scenario for the development of permanent neonatal diabetes, possibly also common forms of diabetes, where early-life events inducing ER stress affect β-cell mass expansion due to mTOR inhibition.

Pancreatic islet-autonomous insulin and smoothened-mediated signalling modulate identity changes of glucagon+ α-cells

The mechanisms restricting regeneration and maintaining cell identity following injury are poorly characterized in higher vertebrates. Upon β-cell loss, 1–2% of the glucagon-producing α-cells spontaneously engage in insulin production in mice. Here we explore the mechanisms inhibiting α-cell plasticity. We show that the adaptive α-cell identity changes are constrained by intra-islet Insulin- and Smoothened-mediated signaling, among others. The combination of β-cell loss, or insulin signaling inhibition, with Smoothened inactivation in α- or δ-cells, stimulates insulin production in more α-cells. These findings suggest that removing constitutive “brake signals” is crucial for neutralizing the refractoriness to adaptive cell-fate changes. It appears that cell identity maintenance is an active process mediated by repressive signals, released by neighbor cells, curbing an intrinsic trend of differentiated cells to change.

Insulin acts as a repressive factor to inhibit the ability of PAR2 to induce islet cell transdifferentiation

Recently, we showed that pancreatitis in the context of profound β-cell deficiency was sufficient to induce islet cell transdifferentiation. In some circumstances, this effect was sufficient to result in recovery from severe diabetes. More recently, we showed that the molecular mechanism by which pancreatitis induced β-cell neogenesis by transdifferentiation was activation of an atypical GPCR called Protease-Activated Receptor 2 (PAR2). However, the ability of PAR2 to induce transdifferentiation occurred only in the setting of profound β-cell deficiency, implying the existence of a repressive factor from those cells. Here we show that the repressor from β-cells is insulin. Treatment of primary islets with a PAR2 agonist (2fLI) in combination with inhibitors of insulin secretion and signaling was sufficient to induce insulin and PAX4 gene expression. Moreover, in primary human islets, this treatment also led to the induction of bihormonal islet cells coexpressing glucagon and insulin, a hallmark of islet cell transdifferentiation. Mechanistically, insulin inhibited the positive effect of a PAR2 agonist on insulin gene expression and also led to an increase in PAX4, which plays an important role in islet cell transdifferentiation. The studies presented here demonstrate that insulin represses transdifferentiation of α- to β-cells induced by activation of PAR2. This provides a mechanistic explanation for the observation that α- to β-cell transdifferentiation occurs only in the setting of severe β-cell ablation. The mechanistic understanding of islet cell transdifferentiation and the ability to modulate that process using available pharmacological reagents represents an important step along the path towards harnessing this novel mechanism of β-cell neogenesis as a therapy for diabetes.

Aetiology of type 1 diabetes: Physiological growth in children affects disease progression

The prevailing view is that type 1 diabetes (T1D) develops as a consequence of a severe decline in β-cell mass resulting from T-cell-mediated autoimmunity; however, progression from islet autoantibody seroconversion to overt diabetes and finally to total loss of C-peptide production occurs in most affected individuals only slowly over many years or even decades. This slow disease progression should be viewed in relation to the total β-cell mass of only 0.2 to 1.5 g in adults without diabetes. Focal lesions of acute pancreatitis with accumulation of leukocytes, often located around the ducts, are frequently observed in people with recent-onset T1D, and most patients display extensive periductal fibrosis, the end stage of inflammation. An injurious inflammatory adverse event, occurring within the periductal area, may have negative implications for islet neogenesis, dependent on stem cells residing within or adjacent to the ductal epithelium. This could in part prevent the 30-fold increase in β-cell mass that would normally occur during the first 20 years of life. This increase occurs in order to maintain glucose metabolism during the physiological increases in insulin production that are required to balance the 20-fold increase in body weight during childhood and increased insulin resistance during puberty. Failure to expand β-cell mass during childhood would lead to clinically overt T1D and could help to explain the apparently more aggressive form of T1D occurring in growing children when compared with that observed in affected adults.

Kinetics of pancreatic tissue proliferation in a polymeric platform in mice

BACKGROUND/OBJECTIVES

Pancreas regenerative capacity after injury is not always sufficient to comply with the body's requirement of digestive enzymes and hormones. We present an alternative system to induce pancreas parenchyma proliferation (exocrine and endocrine components), rather than regeneration or remodeling in normoglycemic mice.

METHODS

Porous discs of polyether-polyurethane were surgically placed adjacent to the native pancreas and removed at days 15, 30 and 45 after implantation. No exogenous growth factors or extracellular matrix components were added to the platform. The synthetic matrix provided a platform that was filled with parenchymal and non-parenchymal pancreas tissue as detected by histological analysis. Immunohistochemistry analysis were performed to identify insulin positive cells in the newly formed tissue. In addition, angiogenic, inflammatory and metabolic parameters were carried out in those mice.

RESULTS

At day 15, the pores of the platform were filled with inflammatory cells, spindled-shaped like fibroblasts, extracellular matrix components, blood vessels and clusters of pancreatic parenchyma (acini, ducts and islet-like structures). At days 30 and 45 the pancreas features remained well organized; its organization resembled that of a native pancreas. Interestingly, besides islet-like structures that showed positive cells to insulin, some ductal cells were also positive for insulin immunostaining. No significant differences in serum glucose and c-peptide concentrations during the experimental period were detected.

CONCLUSIONS

The plain synthetic porous platform (without addition of exogenous molecules) placed adjacent to the native organ exhibits potential to restore and/or expand exocrine (acini, ducts) and endocrine (β-cell mass) components in pancreatic injuries and in high metabolic demand.

Evidence for Loss in Identity, De-Differentiation, and Trans-Differentiation of Islet β-Cells in Type 2 Diabetes

The two main types of diabetes mellitus have distinct etiologies, yet a similar outcome: loss of islet β-cell function that is solely responsible for the secretion of the insulin hormone to reduce elevated plasma glucose toward euglycemic levels. Type 1 diabetes (T1D) has traditionally been characterized by autoimmune-mediated β-cell death leading to insulin-dependence, whereas type 2 diabetes (T2D) has hallmarks of peripheral insulin resistance, β-cell dysfunction, and cell death. However, a growing body of evidence suggests that, especially during T2D, key components of β-cell failure involves: (1) loss of cell identity, specifically proteins associated with mature cell function (e.g., insulin and transcription factors like MAFA, PDX1, and NKX6.1), as well as (2) de-differentiation, defined by regression to a progenitor or stem cell-like state. New technologies have allowed the field to compare islet cell characteristics from normal human donors to those under pathophysiological conditions by single cell RNA-Sequencing and through epigenetic analysis. This has revealed a remarkable level of heterogeneity among histologically defined "insulin-positive" β-cells. These results not only suggest that these β-cell subsets have different responses to insulin secretagogues, but that defining their unique gene expression and epigenetic modification profiles will offer opportunities to develop cellular therapeutics to enrich/maintain certain subsets for correcting pathological glucose levels. In this review, we will summarize the recent literature describing how β-cell heterogeneity and plasticity may be influenced in T2D, and various possible avenues of therapeutic intervention.

Should I Stay or Should I Go: A Clash of α-Cell Identity

Understanding mechanisms for maintaining pancreatic islet cell fate and function is important for addressing the urgent challenge of restoring islet β- and α-cell function in T1DM. In this issue of Cell Metabolism, Chakravarthy et al. (2017) identify a genetic mechanism by which mouse β-cells are spontaneously regenerated from adult α-cells.

β Cells led astray by transcription factors and the company they keep

Pancreatic β cells have one of the highest protein secretion burdens in the body, as these cells must synthesize and secrete insulin in proportion to postprandial rises in blood glucose. Remarkably, it is now becoming clear that adult β cells retain plasticity and can dedifferentiate into embryonic fates or adopt alternate islet endocrine cell identities. This property is especially important, because changes in cell fate alter β cell function and could form the basis for defects in insulin secretion that occur early in the pathogenesis of the most prevalent form of β cell dysfunction, type 2 diabetes. In this issue, three different studies provide complementary perspectives on how the transcription factors NK2 homeobox 2 (NKX2.2), paired box 6 (PAX6), and LIM domain-binding protein 1 (LDB1) serve to maintain mature adult β cell identity, revealing clues as to how adult β cells can partially dedifferentiate or become reprogrammed into other islet endocrine cells.