Neonatal β cell development in mice and humans is regulated by calcineurin/NFAT

Human Menstrual Blood-Derived Stem Cells Protect against Tacrolimus-Induced Islet Dysfunction via Cystathionine β-Synthase Mediated IL-6/STAT3 Inactivation

Diabetes imposes a huge burden worldwide. Islet transplantation is an alternative therapy for diabetes. However, tacrolimus, a kind of immunosuppressant after organ transplantation, is closely related to post-transplant diabetes mellitus. Mesenchymal stem cells (MSCs) have attracted interest for their potential to alleviate diabetes. In vivo experiments revealed that human menstrual blood-derived stem cells (MenSCs) treatment improved tacrolimus-induced blood glucose, body weight, and glucose tolerance disorders in mice. RNA sequencing was used to analyze the potential therapeutic targets of MenSCs. In this study, we illustrated that cystathionine β-synthase (CBS) contributed to tacrolimus -induced islet dysfunction. Using β-cell lines (MIN6, β-TC-6), we demonstrated that MenSCs ameliorated tacrolimus-induced islet dysfunction in vitro. Moreover, MenSC reduced the tacrolimus-induced elevation of CBS levels and significantly enhanced the viability, anti-apoptotic ability, glucose-stimulated insulin secretion (GSIS), and glycolytic flux of β-cells. We further revealed that MenSCs exerted their therapeutic effects by inhibiting CBS expression to activate the IL6/JAK2/STAT3 pathway. In conclusion, we showed that MenSCs may be a potential strategy to improve tacrolimus-induced islet dysfunction.

Pancreatic β-cell failure, clinical implications, and therapeutic strategies in type 2 diabetes

ABSTRACT

Pancreatic β-cell failure due to a reduction in function and mass has been defined as a primary contributor to the progression of type 2 diabetes (T2D). Reserving insulin-producing β-cells and hence restoring insulin production are gaining attention in translational diabetes research, and β-cell replenishment has been the main focus for diabetes treatment. Significant findings in β-cell proliferation, transdifferentiation, pluripotent stem cell differentiation, and associated small molecules have served as promising strategies to regenerate β-cells. In this review, we summarize current knowledge on the mechanisms implicated in β-cell dynamic processes under physiological and diabetic conditions, in which genetic factors, age-related alterations, metabolic stresses, and compromised identity are critical factors contributing to β-cell failure in T2D. The article also focuses on recent advances in therapeutic strategies for diabetes treatment by promoting β-cell proliferation, inducing non-β-cell transdifferentiation, and reprograming stem cell differentiation. Although a significant challenge remains for each of these strategies, the recognition of the mechanisms responsible for β-cell development and mature endocrine cell plasticity and remarkable advances in the generation of exogenous β-cells from stem cells and single-cell studies pave the way for developing potential approaches to cure diabetes.

Adjudin improves beta cell maturation, hepatic glucose uptake and glucose homeostasis

AIMS/HYPOTHESIS

Recovering functional beta cell mass is a promising approach for future diabetes therapies. The aim of the present study is to investigate the effects of adjudin, a small molecule identified in a beta cell screen using zebrafish, on pancreatic beta cells and diabetes conditions in mice and human spheroids.

METHODS

In zebrafish, insulin expression was examined by bioluminescence and quantitative real-time PCR (qPCR), glucose levels were examined by direct measurements and distribution using a fluorescent glucose analogue, and calcium activity in beta cells was analysed by in vivo live imaging. Pancreatic islets of wild-type postnatal day 0 (P0) and 3-month-old (adult) mice, as well as adult db/db mice (i.e. BKS(D)-Leprdb/JOrlRj), were cultured in vitro and analysed by qPCR, glucose stimulated insulin secretion and whole mount staining. RNA-seq was performed for islets of P0 and db/db mice. For in vivo assessment, db/db mice were treated with adjudin and subjected to analysis of metabolic variables and islet cells. Glucose consumption was examined in primary human hepatocyte spheroids.

RESULTS

Adjudin treatment increased insulin expression and calcium response to glucose in beta cells and decreased glucose levels after beta cell ablation in zebrafish. Adjudin led to improved beta cell function, decreased beta cell proliferation and glucose responsive insulin secretion by decreasing basal insulin secretion in in vitro cultured newborn mouse islets. RNA-seq of P0 islets indicated that adjudin treatment resulted in increased glucose metabolism and mitochondrial function, as well as downstream signalling pathways involved in insulin secretion. In islets from db/db mice cultured in vitro, adjudin treatment strengthened beta cell identity and insulin secretion. RNA-seq of db/db islets indicated adjudin-upregulated genes associated with insulin secretion, membrane ion channel activity and exocytosis. Moreover, adjudin promoted glucose uptake in the liver of zebrafish in an insulin-independent manner, and similarly promoted glucose consumption in primary human hepatocyte spheroids with insulin resistance. In vivo studies using db/db mice revealed reduced nonfasting blood glucose, improved glucose tolerance and strengthened beta cell identity after adjudin treatment.

CONCLUSIONS/INTERPRETATION

Adjudin promoted functional maturation of immature islets, improved function of dysfunctional islets, stimulated glucose uptake in liver and improved glucose homeostasis in db/db mice. Thus, the multifunctional drug adjudin, previously studied in various contexts and conditions, also shows promise in the management of diabetic states.

DATA AVAILABILITY

Raw and processed RNA-seq data for this study have been deposited in the Gene Expression Omnibus under accession number GSE235398 ( https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE235398 ).

Signaling pathways and intervention for therapy of type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) represents one of the fastest growing epidemic metabolic disorders worldwide and is a strong contributor for a broad range of comorbidities, including vascular, visual, neurological, kidney, and liver diseases. Moreover, recent data suggest a mutual interplay between T2DM and Corona Virus Disease 2019 (COVID-19). T2DM is characterized by insulin resistance (IR) and pancreatic β cell dysfunction. Pioneering discoveries throughout the past few decades have established notable links between signaling pathways and T2DM pathogenesis and therapy. Importantly, a number of signaling pathways substantially control the advancement of core pathological changes in T2DM, including IR and β cell dysfunction, as well as additional pathogenic disturbances. Accordingly, an improved understanding of these signaling pathways sheds light on tractable targets and strategies for developing and repurposing critical therapies to treat T2DM and its complications. In this review, we provide a brief overview of the history of T2DM and signaling pathways, and offer a systematic update on the role and mechanism of key signaling pathways underlying the onset, development, and progression of T2DM. In this content, we also summarize current therapeutic drugs/agents associated with signaling pathways for the treatment of T2DM and its complications, and discuss some implications and directions to the future of this field.

Ca2+-Mediated Signaling Pathways: A Promising Target for the Successful Generation of Mature and Functional Stem Cell-Derived Pancreatic Beta Cells In Vitro

Diabetes mellitus is a chronic disease affecting over 500 million adults globally and is mainly categorized as type 1 diabetes mellitus (T1DM), where pancreatic beta cells are destroyed, and type 2 diabetes mellitus (T2DM), characterized by beta cell dysfunction. This review highlights the importance of the divalent cation calcium (Ca²⁺) and its associated signaling pathways in the proper functioning of beta cells and underlines the effects of Ca²⁺ dysfunction on beta cell function and its implications for the onset of diabetes. Great interest and promise are held by human pluripotent stem cell (hPSC) technology to generate functional pancreatic beta cells from diabetic patient-derived stem cells to replace the dysfunctional cells, thereby compensating for insulin deficiency and reducing the comorbidities of the disease and its associated financial and social burden on the patient and society. Beta-like cells generated by most current differentiation protocols have blunted functionality compared to their adult human counterparts. The Ca²⁺ dynamics in stem cell-derived beta-like cells and adult beta cells are summarized in this review, revealing the importance of proper Ca²⁺ homeostasis in beta-cell function. Consequently, the importance of targeting Ca²⁺ function in differentiation protocols is suggested to improve current strategies to use hPSCs to generate mature and functional beta-like cells with a comparable glucose-stimulated insulin secretion (GSIS) profile to adult beta cells.

BefA, a microbiota-secreted membrane disrupter, disseminates to the pancreas and increases β cell mass

Microbiome dysbiosis is a feature of diabetes, but how microbial products influence insulin production is poorly understood. We report the mechanism of BefA, a microbiome-derived protein that increases proliferation of insulin-producing β cells during development in gnotobiotic zebrafish and mice. BefA disseminates systemically by multiple anatomic routes to act directly on pancreatic islets. We detail BefA's atomic structure, containing a lipid-binding SYLF domain, and demonstrate that it permeabilizes synthetic liposomes and bacterial membranes. A BefA mutant impaired in membrane disruption fails to expand β cells, whereas the pore-forming host defense protein, Reg3, stimulates β cell proliferation. Our work demonstrates that membrane permeabilization by microbiome-derived and host defense proteins is necessary and sufficient for β cell expansion during pancreas development, potentially connecting microbiome composition with diabetes risk.

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

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

In vivo visualization and molecular targeting of the cardiac conduction system

Accidental injury to the cardiac conduction system (CCS), a network of specialized cells embedded within the heart and indistinguishable from the surrounding heart muscle tissue, is a major complication in cardiac surgeries. Here, we addressed this unmet need by engineering targeted antibody-dye conjugates directed against the CCS, allowing for the visualization of the CCS in vivo following a single intravenous injection in mice. These optical imaging tools showed high sensitivity, specificity, and resolution, with no adverse effects on CCS function. Further, with the goal of creating a viable prototype for human use, we generated a fully human monoclonal Fab that similarly targets the CCS with high specificity. We demonstrate that, when conjugated to an alternative cargo, this Fab can also be used to modulate CCS biology in vivo, providing a proof of principle for targeted cardiac therapeutics. Finally, in performing differential gene expression analyses of the entire murine CCS at single-cell resolution, we uncovered and validated a suite of additional cell surface markers that can be used to molecularly target the distinct subcomponents of the CCS, each prone to distinct life-threatening arrhythmias. These findings lay the foundation for translational approaches targeting the CCS for visualization and therapy in cardiothoracic surgery, cardiac imaging, and arrhythmia management.

Ser/Thr phosphatases: One of the key regulators of insulin signaling

Protein phosphorylation is an important post-translational modification that regulates several cellular processes including insulin signaling. The evidences so far have already portrayed the importance of balanced actions of kinases and phosphatases in regulating the insulin signaling cascade. Therefore, elucidating the role of both kinases and phosphatases are equally important. Unfortunately, the role of phosphatases is less studied as compared to kinases. Since brain responds to insulin and insulin signaling is reported to be crucial for many neuronal processes, it is important to understand the role of neuronal insulin signaling regulators. Ser/Thr phosphatases seem to play significant roles in regulating neuronal insulin signaling. Therefore, in this review, we discussed the involvement of Ser/Thr phosphatases in regulating insulin signaling and insulin resistance in neuronal system at the backdrop of the same phosphatases in peripheral insulin sensitive tissues.

Small RNAs derived from tRNA fragmentation regulate the functional maturation of neonatal β cells

tRNA-derived fragments (tRFs) are an emerging class of small non-coding RNAs with distinct cellular functions. Here, we studied the contribution of tRFs to the regulation of postnatal β cell maturation, a critical process that may lead to diabetes susceptibility in adulthood. We identified three tRFs abundant in neonatal rat islets originating from 5' halves (tiRNA-5s) of histidine and glutamate tRNAs. Their inhibition in these islets reduced β cell proliferation and insulin secretion. Mitochondrial respiration was also perturbed, fitting with the mitochondrial enrichment of nuclear-encoded tiRNA-5^HisGTG and tiRNA-5^GluCTC. Notably, tiRNA-5 inhibition reduced Mpc1, a mitochondrial pyruvate carrier whose knock down largely phenocopied tiRNA-5 inhibition. tiRNA-5^HisGTG interactome revealed binding to Musashi-1, which was essential for the mitochondrial enrichment of tiRNA-5^HisGTG. Finally, tiRNA-5s were dysregulated in the islets of diabetic and diabetes-prone animals. Altogether, tiRNA-5s represent a class of regulators of β cell maturation, and their deregulation in neonatal islets may lead to diabetes susceptibility in adulthood.

Disrupting the DREAM complex enables proliferation of adult human pancreatic beta cells

Resistance to regeneration of insulin-producing pancreatic β cells is a fundamental challenge for type 1 and type 2 diabetes. Recently, small molecule inhibitors of the kinase DYRK1A have proven effective in inducing adult human β cells to proliferate, but their detailed mechanism of action is incompletely understood. We interrogated our human insulinoma and β cell transcriptomic databases seeking to understand why β cells in insulinomas proliferate, while normal β cells do not. This search reveals the DREAM complex as a central regulator of quiescence in human β cells. The DREAM complex consists of a module of transcriptionally repressive proteins that assemble in response to DYRK1A kinase activity, thereby inducing and maintaining cellular quiescence. In the absence of DYRK1A, DREAM subunits reassemble into the pro-proliferative MMB complex. Here, we demonstrate that small molecule DYRK1A inhibitors induce human β cells to replicate by converting the repressive DREAM complex to its pro-proliferative MMB conformation.

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

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

Regenerating islet-derived protein 3 gamma (Reg3g) ameliorates tacrolimus-induced pancreatic β-cell dysfunction in mice by restoring mitochondrial function

BACKGROUND AND PURPOSE

Tacrolimus a first-line medication used after transplantation can induce β-cell dysfunction, causing new-onset diabetes mellitus (NODM). Regenerating islet-derived protein 3 gamma (Reg3g), a member of the pancreatic regenerative gene family, has been reported to improve type 1 diabetes by promoting β-cell regeneration. We aim to investigate the role of Reg3g in reversing tacrolimus-induced β-cell dysfunction and NODM in mice.

EXPERIMENTAL APPROACH

Circulating REG3A (the human homologue of mouse Reg3g) in heart transplantation patients treated with tacrolimus was detected. The glucose-stimulated insulin secretion and mitochondrial functions, including mitochondria membrane potential (MMP), mitochondria calcium, ATP production, oxygen consumption rate and mitochondrial morphology were investigated in β-cells. Additionally, effects of Reg3g on tacrolimus-induced NODM in mice were analysed.

KEY RESULTS

Circulating REG3A level in heart transplantation patients with NODM significantly decreased compared with those without diabetes. Tacrolimus down-regulated Reg3g via inhibiting STAT3-mediated transcription activation. Moreover, Reg3g restored glucose-stimulated insulin secretion suppressed by tacrolimus in β-cells by improving mitochondrial functions, including increased MMP, mitochondria calcium uptake, ATP production, oxygen consumption rate and contributing to an intact mitochondrial morphology. Mechanistically, Reg3g increased accumulation of pSTAT3(Ser727) in mitochondria by activating ERK1/2-STAT3 signalling pathway, leading to restoration of tacrolimus-induced mitochondrial impairment. Reg3g overexpression also effectively mitigated tacrolimus-induced NODM in mice.

CONCLUSION AND IMPLICATIONS

Reg3g can significantly ameliorate tacrolimus-induced β-cell dysfunction by restoring mitochondrial function in a pSTAT3(Ser727)-dependent manner. Our observations identify a novel Reg3g-mediated mechanism that is involved in tacrolimus-induced NODM and establish the novel role of Reg3g in reversing tacrolimus-induced β-cell dysfunction.

Calcineurin/NFATc2 and PI3K/AKT signaling maintains β-cell identity and function during metabolic and inflammatory stress

Pancreatic islets respond to metabolic and inflammatory stress by producing hormones and other factors that induce adaptive cellular and systemic responses. Here we show that intracellular Ca2+ ([Ca2+]i) and ROS signals generated by high glucose and cytokine-induced ER stress activate calcineurin (CN)/NFATc2 and PI3K/AKT to maintain β-cell identity and function. This was attributed in part by direct induction of the endocrine differentiation gene RFX6 and suppression of several β-cell "disallowed" genes, including MCT1. CN/NFATc2 targeted p300 and HDAC1 to RFX6 and MCT1 promoters to induce and suppress gene transcription, respectively. In contrast, prolonged exposure to stress, hyperstimulated [Ca2+]i, or perturbation of CN/NFATc2 resulted in downregulation of RFX6 and induction of MCT1. These findings reveal that CN/NFATc2 and PI3K/AKT maintain β-cell function during acute stress, but β-cells dedifferentiate to a dysfunctional state upon loss or exhaustion of Ca2+/CN/NFATc2 signaling. They further demonstrate the utility of targeting CN/NFATc2 to restore β-cell function.

Dynamic changes in β-cell [Ca2+] regulate NFAT activation, gene transcription, and islet gap junction communication

OBJECTIVE

Diabetes occurs because of insufficient insulin secretion due to β-cell dysfunction within the islet of Langerhans. Elevated glucose levels trigger β-cell membrane depolarization, action potential generation, and slow sustained free-Ca2+ ([Ca2+]) oscillations, which trigger insulin release. Nuclear factor of activated T-cell (NFAT) is a transcription factor, which is regulated by the increases in [Ca2+] and calceineurin (CaN) activation. NFAT regulation links cell activity with gene transcription in many systems and regulates proliferation and insulin granule biogenesis within the β-cell. However, the link between the regulation of β-cell electrical activity and oscillatory [Ca2+] dynamics with NFAT activation and downstream transcription is poorly understood. Here, we tested whether dynamic changes to β-cell electrical activity and [Ca2+] regulate NFAT activation and downstream transcription.

METHODS

In cell lines, mouse islets, and human islets, including those from donors with type 2 diabetes, we applied both agonists/antagonists of ion channels together with optogenetics to modulate β-cell electrical activity. We measured the dynamics of [Ca2+] and NFAT activation as well as performed whole transcriptome and functional analyses.

RESULTS

Both glucose-induced membrane depolarization and optogenetic stimulation triggered NFAT activation as well as increased the transcription of NFAT targets and intermediate early genes (IEGs). Importantly, slow, sustained [Ca2+] oscillation conditions led to NFAT activation and downstream transcription. In contrast, in human islets from donors with type2 diabetes, NFAT activation by glucose was diminished, but rescued upon pharmacological stimulation of electrical activity. NFAT activation regulated GJD2 expression and increased Cx36 gap junction permeability upon elevated oscillatory [Ca2+] dynamics. However, it is unclear if NFAT directly binds the GJD2 gene to regulate expression.

CONCLUSIONS

This study provides an insight into the specific patterns of electrical activity that regulate NFAT activation, gene transcription, and islet function. In addition, it provides information on how these factors are disrupted in diabetes.

Regenerative approaches to preserve pancreatic β-cell mass and function in diabetes pathogenesis

In both type 1 diabetes (T1D) and type 2 diabetes (T2D), there is a substantial β-cell mass loss. Residual β-cell mass is susceptible to cellular damage because of specific pancreatic β-cell characteristics. β cells have a low proliferation rate, being in human adults almost zero and a low antioxidant system that makes β cells susceptible to oxidative stress and increases their vulnerability to cell destruction. Different strategies have been addressed to preserve pancreatic β-cell residual mass and function in patients with diabetes. However, the effect of many compounds proposed in rodent models to trigger β-cell replication has different results in human β cells. In this review, scientific evidence of β-cell of two major regenerative approaches has been gathered. Regeneration proceedings for pancreatic β cells are promising and could improve β-cell proliferation capacity and contribute to the conservation of mature β-cell phenotypic characteristics. This evidence supports the notion that regenerative medicine could be a helpful strategy to yield amelioration of T1D and T2D pathogenesis.

Mitogen Synergy: An Emerging Route to Boosting Human Beta Cell Proliferation

Decreased number and function of beta cells are a key aspect of diabetes mellitus (diabetes), a disease that remains an onerous global health problem. Means of restoring beta cell mass are urgently being sought as a potential cure for diabetes. Several strategies, such as de novo beta cell derivation via pluripotent stem cell differentiation or mature somatic cell transdifferentiation, have yielded promising results. Beta cell expansion is another promising strategy, rendered challenging by the very low proliferative capacity of beta cells. Many effective mitogens have been identified in rodents, but the vast majority do not have similar mitogenic effects in human beta cells. Extensive research has led to the identification of several human beta cell mitogens, but their efficacy and specificity remain insufficient. An approach based on the simultaneous application of several mitogens has recently emerged and can yield human beta cell proliferation rates of up to 8%. Here, we discuss recent advances in restoration of the beta cell population, focusing on mitogen synergy, and the contribution of RNA-sequencing (RNA-seq) to accelerating the elucidation of signaling pathways in proliferating beta cells and the discovery of novel mitogens. Together, these approaches have taken beta cell research up a level, bringing us closer to a cure for diabetes.

Gsα-dependent signaling is required for postnatal establishment of a functional β-cell mass

OBJECTIVE

Early postnatal life is a critical period for the establishment of the functional β-cell mass that will sustain whole-body glucose homeostasis during the lifetime. β cells are formed from progenitors during embryonic development but undergo significant expansion in quantity and attain functional maturity after birth. The signals and pathways involved in these processes are not fully elucidated. Cyclic adenosine monophosphate (cAMP) is an intracellular signaling molecule that is known to regulate insulin secretion, gene expression, proliferation, and survival of adult β cells. The heterotrimeric G protein Gs stimulates the cAMP-dependent pathway by activating adenylyl cyclase. In this study, we sought to explore the role of Gs-dependent signaling in postnatal β-cell development.

METHODS

To study Gs-dependent signaling, we generated conditional knockout mice in which the α subunit of the Gs protein (Gsα) was ablated from β-cells using the Cre deleter line Ins1Cre. Mice were characterized in terms of glucose homeostasis, including in vivo glucose tolerance, glucose-induced insulin secretion, and insulin sensitivity. β-cell mass was studied using histomorphometric analysis and optical projection tomography. β-cell proliferation was studied by ki67 and phospho-histone H3 immunostatining, and apoptosis was assessed by TUNEL assay. Gene expression was determined in isolated islets and sorted β cells by qPCR. Intracellular cAMP was studied in isolated islets using HTRF-based technology. The activation status of the cAMP and insulin-signaling pathways was determined by immunoblot analysis of the relevant components of these pathways in isolated islets. In vitro proliferation of dissociated islet cells was assessed by BrdU incorporation.

RESULTS

Elimination of Gsα in β cells led to reduced β-cell mass, deficient insulin secretion, and severe glucose intolerance. These defects were evident by weaning and were associated with decreased proliferation and inadequate expression of key β-cell identity and maturation genes in postnatal β-cells. Additionally, loss of Gsα caused a broad multilevel disruption of the insulin transduction pathway that resulted in the specific abrogation of the islet proliferative response to insulin.

CONCLUSION

We conclude that Gsα is required for β-cell growth and maturation in the early postnatal stage and propose that this is partly mediated via its crosstalk with insulin signaling. Our findings disclose a tight connection between these two pathways in postnatal β cells, which may have implications for using cAMP-raising agents to promote β-cell regeneration and maturation in diabetes.

Identification of direct transcriptional targets of NFATC2 that promote β cell proliferation

The transcription factor NFATC2 induces β cell proliferation in mouse and human islets. However, the genomic targets that mediate these effects have not been identified. We expressed active forms of Nfatc2 and Nfatc1 in human islets. By integrating changes in gene expression with genomic binding sites for NFATC2, we identified approximately 2200 transcriptional targets of NFATC2. Genes induced by NFATC2 were enriched for transcripts that regulate the cell cycle and for DNA motifs associated with the transcription factor FOXP. Islets from an endocrine-specific Foxp1, Foxp2, and Foxp4 triple-knockout mouse were less responsive to NFATC2-induced β cell proliferation, suggesting the FOXP family works to regulate β cell proliferation in concert with NFATC2. NFATC2 induced β cell proliferation in both mouse and human islets, whereas NFATC1 did so only in human islets. Exploiting this species difference, we identified approximately 250 direct transcriptional targets of NFAT in human islets. This gene set enriches for cell cycle-associated transcripts and includes Nr4a1. Deletion of Nr4a1 reduced the capacity of NFATC2 to induce β cell proliferation, suggesting that much of the effect of NFATC2 occurs through its induction of Nr4a1. Integration of noncoding RNA expression, chromatin accessibility, and NFATC2 binding sites enabled us to identify NFATC2-dependent enhancer loci that mediate β cell proliferation.

Human Beta Cell Regenerative Drug Therapy for Diabetes: Past Achievements and Future Challenges

A quantitative deficiency of normally functioning insulin-producing pancreatic beta cells is a major contributor to all common forms of diabetes. This is the underlying premise for attempts to replace beta cells in people with diabetes by pancreas transplantation, pancreatic islet transplantation, and transplantation of beta cells or pancreatic islets derived from human stem cells. While progress is rapid and impressive in the beta cell replacement field, these approaches are expensive, and for transplant approaches, limited by donor organ availability. For these reasons, beta cell replacement will not likely become available to the hundreds of millions of people around the world with diabetes. Since the large majority of people with diabetes have some residual beta cells in their pancreata, an alternate approach to reversing diabetes would be developing pharmacologic approaches to induce these residual beta cells to regenerate and expand in a way that also permits normal function. Unfortunately, despite the broad availability of multiple classes of diabetes drugs in the current diabetes armamentarium, none has the ability to induce regeneration or expansion of human beta cells. Development of such drugs would be transformative for diabetes care around the world. This picture has begun to change. Over the past half-decade, a novel class of beta cell regenerative small molecules has emerged: the DYRK1A inhibitors. Their emergence has tremendous potential, but many areas of uncertainty and challenge remain. In this review, we summarize the accomplishments in the world of beta cell regenerative drug development and summarize areas in which most experts would agree. We also outline and summarize areas of disagreement or lack of unanimity, of controversy in the field, of obstacles to beta cell regeneration, and of challenges that will need to be overcome in order to establish human beta cell regenerative drug therapeutics as a clinically viable class of diabetes drugs.

Transcriptional mechanisms of pancreatic β-cell maturation and functional adaptation

Pancreatic β-cells secrete insulin commensurate to circulating nutrient levels to maintain normoglycemia. The ability of β-cells to couple insulin secretion to nutrient stimuli is acquired during a postnatal maturation process. In mature β-cells the insulin secretory response adapts to changes in nutrient state. Both β-cell maturation and functional adaptation rely on the interplay between extracellular cues and cell type-specific transcriptional programs. Here we review emerging evidence that developmental and homeostatic regulation of β-cell function involves collaboration between lineage-determining and signal-dependent transcription factors (LDTFs and SDTFs, respectively). A deeper understanding of β-cell SDTFs and their cognate signals would delineate mechanisms of β-cell maturation and functional adaptation, which has direct implications for diabetes therapies and for generating mature β-cells from stem cells.

Temporal Transcriptome Analysis Reveals Dynamic Gene Expression Patterns Driving β-Cell Maturation

Newly differentiated pancreatic β cells lack proper insulin secretion profiles of mature functional β cells. The global gene expression differences between paired immature and mature β cells have been studied, but the dynamics of transcriptional events, correlating with temporal development of glucose-stimulated insulin secretion (GSIS), remain to be fully defined. This aspect is important to identify which genes and pathways are necessary for β-cell development or for maturation, as defective insulin secretion is linked with diseases such as diabetes. In this study, we assayed through RNA sequencing the global gene expression across six β-cell developmental stages in mice, spanning from β-cell progenitor to mature β cells. A computational pipeline then selected genes differentially expressed with respect to progenitors and clustered them into groups with distinct temporal patterns associated with biological functions and pathways. These patterns were finally correlated with experimental GSIS, calcium influx, and insulin granule formation data. Gene expression temporal profiling revealed the timing of important biological processes across β-cell maturation, such as the deregulation of β-cell developmental pathways and the activation of molecular machineries for vesicle biosynthesis and transport, signal transduction of transmembrane receptors, and glucose-induced Ca²⁺ influx, which were established over a week before β-cell maturation completes. In particular, β cells developed robust insulin secretion at high glucose several days after birth, coincident with the establishment of glucose-induced calcium influx. Yet the neonatal β cells displayed high basal insulin secretion, which decreased to the low levels found in mature β cells only a week later. Different genes associated with calcium-mediated processes, whose alterations are linked with insulin resistance and deregulation of glucose homeostasis, showed increased expression across β-cell stages, in accordance with the temporal acquisition of proper GSIS. Our temporal gene expression pattern analysis provided a comprehensive database of the underlying molecular components and biological mechanisms driving β-cell maturation at different temporal stages, which are fundamental for better control of the in vitro production of functional β cells from human embryonic stem/induced pluripotent cell for transplantation-based type 1 diabetes therapy.

Inhibition of Syk promotes chemical reprogramming of fibroblasts via metabolic rewiring and H2 S production

Chemical compounds have recently been introduced as alternative and non-integrating inducers of pluripotent stem cell fate. However, chemical reprogramming is hampered by low efficiency and the molecular mechanisms remain poorly characterized. Here, we show that inhibition of spleen tyrosine kinase (Syk) by R406 significantly promotes mouse chemical reprogramming. Mechanistically, R406 alleviates Syk / calcineurin (Cn) / nuclear factor of activated T cells (NFAT) signaling-mediated suppression of glycine, serine, and threonine metabolic genes and dependent metabolites. Syk inhibition upregulates glycine level and downstream transsulfuration cysteine biosynthesis, promoting cysteine metabolism and cellular hydrogen sulfide (H₂ S) production. This metabolic rewiring decreased oxidative phosphorylation and ROS levels, enhancing chemical reprogramming. In sum, our study identifies Syk-Cn-NFAT signaling axis as a new barrier of chemical reprogramming and suggests metabolic rewiring and redox homeostasis as important opportunities for controlling cell fates.

Scrt1, a transcriptional regulator of β-cell proliferation identified by differential chromatin accessibility during islet maturation

Glucose-induced insulin secretion, a hallmark of mature β-cells, is achieved after birth and is preceded by a phase of intense proliferation. These events occurring in the neonatal period are decisive for establishing an appropriate functional β-cell mass that provides the required insulin throughout life. However, key regulators of gene expression involved in functional maturation of β-cells remain to be elucidated. Here, we addressed this issue by mapping open chromatin regions in newborn versus adult rat islets using the ATAC-seq assay. We obtained a genome-wide picture of chromatin accessible sites (~ 100,000) among which 20% were differentially accessible during maturation. An enrichment analysis of transcription factor binding sites identified a group of transcription factors that could explain these changes. Among them, Scrt1 was found to act as a transcriptional repressor and to control β-cell proliferation. Interestingly, Scrt1 expression was controlled by the transcriptional repressor RE-1 silencing transcription factor (REST) and was increased in an in vitro reprogramming system of pancreatic exocrine cells to β-like cells. Overall, this study led to the identification of several known and unforeseen key transcriptional events occurring during β-cell maturation. These findings will help defining new strategies to induce the functional maturation of surrogate insulin-producing cells.

DYRK1A Inhibitors as Potential Therapeutics for β-Cell Regeneration for Diabetes

According to the World Health Organization (WHO), 422 million people are suffering from diabetes worldwide. Current diabetes therapies are focused on optimizing blood glucose control to prevent long-term diabetes complications. Unfortunately, current therapies have failed to achieve glycemic targets in the majority of people with diabetes. In this context, regeneration of functional insulin-producing human β-cells in people with diabetes through the use of DYRK1A inhibitor drugs has recently received special attention. Several small molecule DYRK1A inhibitors have been identified that induce human β-cell proliferation in vitro and in vivo. Furthermore, DYRK1A inhibitors have also been shown to synergize β-cell proliferation with other classes of drugs, such as TGFβ inhibitors and GLP-1 receptor agonists. In this perspective, we review the status of DYRK1A as a therapeutic target for β-cell proliferation and provide perspectives on technical and scientific challenges for future translational development.

Single-Nucleus and In Situ RNA-Sequencing Reveal Cell Topographies in the Human Pancreas

BACKGROUND & AIMS

Molecular evidence of cellular heterogeneity in the human exocrine pancreas has not been yet established because of the local concentration and cascade of hydrolytic enzymes that can rapidly degrade cells and RNA upon pancreatic resection. We sought to better understand the heterogeneity and cellular composition of the pancreas in neonates and adults in healthy and diseased conditions using single-cell sequencing approaches.

METHODS

We innovated single-nucleus RNA-sequencing protocols and profiled more than 120,000 cells from pancreata of adult and neonatal human donors. We validated the single-nucleus findings using RNA fluorescence in situ hybridization, in situ sequencing, and computational approaches.

RESULTS

We created the first comprehensive atlas of human pancreas cells including epithelial and nonepithelial constituents, and uncovered 3 distinct acinar cell types, with possible implications for homeostatic and inflammatory processes of the pancreas. The comparison with neonatal single-nucleus sequencing data showed a different cellular composition of the endocrine tissue, highlighting the tissue dynamics occurring during development. By applying spatial cartography, involving cell proximity mapping through in situ sequencing, we found evidence of specific cell type neighborhoods, dynamic topographies in the endocrine and exocrine pancreas, and principles of morphologic organization of the organ. Furthermore, similar analyses in chronic pancreatitis biopsy samples showed the presence of acinar-REG⁺ cells, a reciprocal association between macrophages and activated stellate cells, and a new potential role of tuft cells in this disease.

CONCLUSIONS

Our human pancreas cell atlas can be interrogated to understand pancreatic cell biology and provides a crucial reference set for comparisons with diseased tissue samples to map the cellular foundations of pancreatic diseases.

Role of Wnt signaling pathways in type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) has become a major global public health issue in the twenty-first century and its incidence has increased each year. Wnt signaling pathways are a set of multi-downstream signaling pathways activated by the binding of Wnt ligands to membrane protein receptors. Wnt signaling pathways regulate protein expression and play important roles in protecting the body's normal physiological metabolism. This review describes Wnt signaling pathways, and then aims to reveal how Wnt signaling pathways participate in the occurrence and development of T2DM. We found that Wnt/c-Jun N-terminal kinase signaling was closely associated with insulin resistance, inflammatory response, and pancreatic β-cell and endothelial dysfunction. β-catenin/transcription factor 7-like 2 (TCF7L2)-mediated and calcineurin/nuclear factor of activated T cells-mediated target genes were involved in insulin synthesis and secretion, insulin degradation, pancreatic β-cell growth and regeneration, and functional application of pancreatic β-cells. In addition, polymorphisms in the TCF7L2 gene could increase risk of T2DM according to previous and the most current results, and the T allele of its variants was a more adverse factor for abnormal pancreatic β-cell function and impaired glucose tolerance in patients with T2DM. These findings indicate a strong correlation between Wnt signaling pathways and T2DM, particularly in terms of pancreatic islet dysfunction and insulin resistance, and new therapeutic targets for T2DM may be identified.

Differential Effects of Voclosporin and Tacrolimus on Insulin Secretion From Human Islets

The incidence of new onset diabetes after transplant (NODAT) has increased over the past decade, likely due to calcineurin inhibitor-based immunosuppressants, including tacrolimus (TAC) and cyclosporin. Voclosporin (VCS), a next-generation calcineurin inhibitor, is reported to cause fewer incidences of NODAT but the reason is unclear. While calcineurin signaling plays important roles in pancreatic β-cell survival, proliferation, and function, its effects on human β-cells remain understudied. In particular, we do not understand why some calcineurin inhibitors have more profound effects on the incidence of NODAT. We compared the effects of TAC and VCS on the dynamics of insulin secretory function, programmed cell death rate, and the transcriptomic profile of human islets. We studied 2 clinically relevant doses of TAC (10 ng/mL, 30 ng/mL) and VCS (20 ng/mL, 60 ng/mL), meant to approximate the clinical trough and peak concentrations. TAC, but not VCS, caused a significant impairment of 15 mM glucose-stimulated and 30 mM KCl-stimulated insulin secretion. This points to molecular defects in the distal stages of exocytosis after voltage-gated Ca2+ entry. No significant effects on islet cell survival or total insulin content were identified. RNA sequencing showed that TAC significantly decreased the expression of 17 genes, including direct and indirect regulators of exocytosis (SYT16, TBC1D30, PCK1, SMOC1, SYT5, PDK4, and CREM), whereas VCS has less broad, and milder, effects on gene expression. Clinically relevant doses of TAC, but not VCS, directly inhibit insulin secretion from human islets, likely via transcriptional control of exocytosis machinery.

Aberrant methylation underlies insulin gene expression in human insulinoma

Human insulinomas are rare, benign, slowly proliferating, insulin-producing beta cell tumors that provide a molecular "recipe" or "roadmap" for pathways that control human beta cell regeneration. An earlier study revealed abnormal methylation in the imprinted p15.5-p15.4 region of chromosome 11, known to be abnormally methylated in another disorder of expanded beta cell mass and function: the focal variant of congenital hyperinsulinism. Here, we compare deep DNA methylome sequencing on 19 human insulinomas, and five sets of normal beta cells. We find a remarkably consistent, abnormal methylation pattern in insulinomas. The findings suggest that abnormal insulin (INS) promoter methylation and altered transcription factor expression create alternative drivers of INS expression, replacing canonical PDX1-driven beta cell specification with a pathological, looping, distal enhancer-based form of transcriptional regulation. Finally, NFaT transcription factors, rather than the canonical PDX1 enhancer complex, are predicted to drive INS transactivation.

The supply chain of human pancreatic β cell lines

Patients with type 1 or type 2 diabetes have an insufficiency in their functional β cell mass. To advance diabetes treatment and to work toward a cure, a better understanding of how to protect the pancreatic β cells against autoimmune or metabolic assaults (e.g., obesity, gestation) will be required. Over the past decades, β cell protection has been extensively investigated in rodents both in vivo and in vitro using isolated islets or rodent β cell lines. Transferring these rodent data to humans has long been challenging, at least partly for technical reasons: primary human islet preparations were scarce and functional human β cell lines were lacking. In 2011, we described a robust protocol of targeted oncogenesis in human fetal pancreas and produced the first functional human β cell line, and in subsequent years additional lines with specific traits. These cell lines are currently used by more than 150 academic and industrial laboratories worldwide. In this Review, we first explain how we developed the human β cell lines and why we think we succeeded where others, despite major efforts, did not. Next, we discuss the use of such functional human β cell lines and share some perspectives on their use to advance diabetes research.

Identification of a small molecule that stimulates human β-cell proliferation and insulin secretion, and protects against cytotoxic stress in rat insulinoma cells

A key event in the development of both major forms of diabetes is the loss of functional pancreatic islet β-cell mass. Strategies aimed at enhancing β-cell regeneration have long been pursued, but methods for reliably inducing human β-cell proliferation with full retention of key functions such as glucose-stimulated insulin secretion (GSIS) are still very limited. We have previously reported that overexpression of the homeobox transcription factor NKX6.1 stimulates β-cell proliferation, while also enhancing GSIS and providing protection against β-cell cytotoxicity through induction of the VGF prohormone. We developed an NKX6.1 pathway screen by stably transfecting 832/13 rat insulinoma cells with a VGF promoter-luciferase reporter construct, using the resultant cell line to screen a 630,000 compound chemical library. We isolated three compounds with consistent effects to stimulate human islet cell proliferation, but not expression of NKX6.1 or VGF, suggesting an alternative mechanism of action. Further studies of the most potent of these compounds, GNF-9228, revealed that it selectively activates human β-cell relative to α-cell proliferation and has no effect on δ-cell replication. In addition, pre-treatment, but not short term exposure of human islets to GNF-9228 enhances GSIS. GNF-9228 also protects 832/13 insulinoma cells against ER stress- and inflammatory cytokine-induced cytotoxicity. GNF-9228 stimulates proliferation via a mechanism distinct from recently emergent DYRK1A inhibitors, as it is unaffected by DYRK1A overexpression and does not activate NFAT translocation. In conclusion, we have identified a small molecule with pleiotropic positive effects on islet biology, including stimulation of human β-cell proliferation and insulin secretion, and protection against multiple agents of cytotoxic stress.

Identifying New Substrates and Functions for an Old Enzyme: Calcineurin

Biological processes are dynamically regulated by signaling networks composed of protein kinases and phosphatases. Calcineurin, or PP3, is a conserved phosphoserine/phosphothreonine-specific protein phosphatase and member of the PPP family of phosphatases. Calcineurin is unique, however, in its activation by Ca2+ and calmodulin. This ubiquitously expressed phosphatase controls Ca2+-dependent processes in all human tissues, but is best known for driving the adaptive immune response by dephosphorylating the nuclear factor of the activated T-cells (NFAT) family of transcription factors. Therefore, calcineurin inhibitors, FK506 (tacrolimus), and cyclosporin A serve as immunosuppressants. We describe some of the adverse effects associated with calcineurin inhibitors that result from inhibition of calcineurin in nonimmune tissues, illustrating the many functions of this enzyme that have yet to be elucidated. In fact, calcineurin has essential roles beyond the immune system, from yeast to humans, but since its discovery more than 30 years ago, only a small number of direct calcineurin substrates have been shown (∼75 proteins). This is because of limitations in current methods for identification of phosphatase substrates. Here we discuss recent insights into mechanisms of calcineurin activation and substrate recognition that have been critical in the development of novel approaches for identifying its targets systematically. Rather than comprehensively reviewing known functions of calcineurin, we highlight new approaches to substrate identification for this critical regulator that may reveal molecular mechanisms underlying toxicities caused by calcineurin inhibitor-based immunosuppression.

Tacrolimus-Induced BMP/SMAD Signaling Associates With Metabolic Stress-Activated FOXO1 to Trigger β-Cell Failure

Active maintenance of β-cell identity through fine-tuned regulation of key transcription factors ensures β-cell function. Tacrolimus, a widely used immunosuppressant, accelerates onset of diabetes after organ transplantation, but underlying molecular mechanisms are unclear. Here we show that tacrolimus induces loss of human β-cell maturity and β-cell failure through activation of the BMP/SMAD signaling pathway when administered under mild metabolic stress conditions. Tacrolimus-induced phosphorylated SMAD1/5 acts in synergy with metabolic stress-activated FOXO1 through formation of a complex. This interaction is associated with reduced expression of the key β-cell transcription factor MAFA and abolished insulin secretion, both in vitro in primary human islets and in vivo in human islets transplanted into high-fat diet-fed mice. Pharmacological inhibition of BMP signaling protects human β-cells from tacrolimus-induced β-cell dysfunction in vitro. Furthermore, we confirm that BMP/SMAD signaling is activated in protocol pancreas allograft biopsies from recipients on tacrolimus. To conclude, we propose a novel mechanism underlying the diabetogenicity of tacrolimus in primary human β-cells. This insight could lead to new treatment strategies for new-onset diabetes and may have implications for other forms of diabetes.

MiR-221/222 Inhibit Insulin Production of Pancreatic β-Cells in Mice

Microribonucleic acids (miRNAs) are essential for the regulation of development, proliferation, and functions of pancreatic β-cells. The conserved miR-221/222 cluster is an important regulator in multiple cellular processes. Here we investigated the functional role of miR-221/222 in the regulation of β-cell proliferation and functions in transgenic mouse models. We generated 2 pancreatic β-cell-specific-miR-221/222 transgenic mouse models on a C57BL/6J background. The glucose metabolic phenotypes, β-cell mass, and β-cell functions were analyzed in the mouse models. Adenovirus-mediated overexpression of miR-221/222 was performed on β-cells and mouse insulinoma 6 (MIN6) cells to explore the effect and mechanisms of miR-221/222 on β-cell proliferation and functions. Luciferase reporter assay, histological analysis, and quantitative polymerase chain reaction (PCR) were carried out to study the direct target genes of miR-221/222 in β-cells. The expression of miR-221/222 was significantly upregulated in β-cells from the high-fat diet (HFD)-fed mice and db/db mice. Overexpression of miR-221/222 impaired the insulin production and secretion of β-cells and resulted in glucose intolerance in vivo. The β-cell mass and proliferation were increased by miR-221/222 expression via Cdkn1b and Cdkn1c. MiR-221/222 repressed insulin transcription activity through targeting Nfatc3 and lead to reduction of insulin in β-cells. Our findings demonstrate that miR-221/222 are important regulators of β-cell proliferation and insulin production. The expression of miR-221/222 in β-cells could regulate glucose metabolism in physiological and pathological processes.

Dual Effect of Raptor on Neonatal β-Cell Proliferation and Identity Maintenance

Immature pancreatic β-cells are highly proliferative, and the expansion of β-cells during the early neonatal period largely determines functional β-cell mass; however, the mechanisms are poorly characterized. We generated Ngn3RapKO mice (ablation of Raptor, an essential component of mechanistic target of rapamycin [mTORC1] in Ngn3⁺ endocrine progenitor cells) and found that mTORC1 was dispensable for endocrine cell lineage formation but specifically regulated both proliferation and identity maintenance of neonatal β-cells. Ablation of Raptor in neonatal β-cells led to autonomous loss of cell identity, decelerated cell cycle progression, compromised proliferation, and caused neonatal diabetes as a result of inadequate establishment of functional β-cell mass at postnatal day 14. Completely different from mature β-cells, Raptor regulated G1/S and G2/M phase cell cycle transition, thus permitting a high proliferation rate in neonatal β-cells. Moreover, Ezh2 was identified as a critical downstream target of mTORC1 in neonatal β-cells, which was responsible for G2/M phase transition and proliferation. Our discovery of the dual effect of mTORC1 in immature β-cells has revealed a potential target for replenishing functional β-cell pools by promoting both expansion and functional maturation of newly formed immature β-cells.

Reduced Compensatory β-Cell Proliferation in Nfatc3-Deficient Mice Fed on High-Fat Diet

BACKGROUND

High-fat-diet induces pancreatic β-cell compensatory proliferation, and impairments in pancreatic β-cell proliferation and function can lead to defects in insulin secretion and diabetes. NFATc3 is important for HFD-induced adipose tissue inflammation. But it is unknown whether NFATc3 is required for β cell compensatory growth in mice fed with HFD.

METHODS

NFATc3 mRNA and protein expression levels were quantified by RT-qPCR and Western blotting, respectively, in pancreatic islets of WT mice fed on HFD for 12-20 weeks. Adenoviral-mediated overexpression of NFATc3 were conducted in Min6 cells and cultured primary mouse islets. NFATc3-/- mice and WT control mice were fed with HFD and metabolic and functional parameters were measured.

RESULTS

We observed that the NFATc3 expression level was reduced in the islets of high-fat-diet (HFD)-fed mice. Adenovirus-mediated overexpression of NFATc3 enhanced glucose-stimulated insulin secretion and β-cell gene expression in cultured primary mouse islets. Nfatc3-/- mice initially developed similar glucose tolerance at 2-4 weeks after HFD feeding than HFD-fed WT mice, but Nfatc3-/- mice developed improved glucose tolerance and insulin sensitivity after 8 weeks of HFD feeding compared to Nfatc3+/+fed with HFD. Furthermore, Nfatc3-/- mice on HFD exhibited decreased β-cell mass and reduced expression of genes important for β-cell proliferation and function compared to Nfatc3+/+mice on HFD.

CONCLUSIONS

The findings suggested that NFATc3 plays a role in maintaining the pancreatic β-cell compensatory growth and gene expression in response to obesity.

Compositional analysis and biological characterization of Cornus officinalis on human 1.1B4 pancreatic β cells

Type 1 diabetes (T1D) is an autoimmune disease resulting from the loss of pancreatic β cells and subsequent insulin production. Novel interventional therapies are urgently needed that can protect existing β cells from cytokine-induced death and enhance their function before symptomatic onset. Our initial evidence is suggesting that bioactive ingredients within Cornus officinalis (CO) may be able to serve in this function. CO has been extensively used in Traditional Chinese Medicine (TCM) and reported to possess both anti-inflammatory and pro-metabolic effects. We hypothesize that CO treatment may provide a future potential candidate for interventional therapy for early stage T1D prior to significant β cell loss. Our data demonstrated that CO can inhibit cytokine-mediated β cell death, increase cell viability and oxidative capacity, and increase expression of NFATC2 (Nuclear Factor of Activated T Cells, Cytoplasmic 2). We have also profiled the bioactive components in CO from multiple sources by HPLC/MS (High Performance Liquid Chromatography/Mass Spectrometry) analysis. Altogether, CO significantly increases the energy metabolism of β cells while inducing the NFAT pathway to signal for increased proliferation and endocrine function.

Transcriptomic Profiling of the Developing Cardiac Conduction System at Single-Cell Resolution

RATIONALE

The cardiac conduction system (CCS) consists of distinct components including the sinoatrial node, atrioventricular node, His bundle, bundle branches, and Purkinje fibers. Despite an essential role for the CCS in heart development and function, the CCS has remained challenging to interrogate because of inherent obstacles including small cell numbers, large cell-type heterogeneity, complex anatomy, and difficulty in isolation. Single-cell RNA-sequencing allows for genome-wide analysis of gene expression at single-cell resolution.

OBJECTIVE

Assess the transcriptional landscape of the entire CCS at single-cell resolution by single-cell RNA-sequencing within the developing mouse heart.

METHODS AND RESULTS

Wild-type, embryonic day 16.5 mouse hearts (n=6 per zone) were harvested and 3 zones of microdissection were isolated, including: Zone I-sinoatrial node region; Zone II-atrioventricular node/His region; and Zone III-bundle branch/Purkinje fiber region. Tissue was digested into single-cell suspensions, cells isolated, mRNA reverse transcribed, and barcoded before high-throughput sequencing and bioinformatics analyses. Single-cell RNA-sequencing was performed on over 22 000 cells, and all major cell types of the murine heart were successfully captured including bona fide clusters of cells consistent with each major component of the CCS. Unsupervised weighted gene coexpression network analysis led to the discovery of a host of novel CCS genes, a subset of which were validated using fluorescent in situ hybridization as well as whole-mount immunolabeling with volume imaging (iDISCO+) in 3 dimensions on intact mouse hearts. Further, subcluster analysis unveiled isolation of distinct CCS cell subtypes, including the clinically relevant but poorly characterized transitional cells that bridge the CCS and surrounding myocardium.

CONCLUSIONS

Our study represents the first comprehensive assessment of the transcriptional profiles from the entire CCS at single-cell resolution and provides a characterization in the context of development and disease.

Cholecystokinin (CCK) Regulation of Pancreatic Acinar Cells: Physiological Actions and Signal Transduction Mechanisms

Pancreatic acinar cells synthesize and secrete about 20 digestive enzymes and ancillary proteins with the processes that match the supply of these enzymes to their need in digestion being regulated by a number of hormones (CCK, secretin and insulin), neurotransmitters (acetylcholine and VIP) and growth factors (EGF and IGF). Of these regulators, one of the most important and best studied is the gastrointestinal hormone, cholecystokinin (CCK). Furthermore, the acinar cell has become a model for seven transmembrane, heterotrimeric G protein coupled receptors to regulate multiple processes by distinct signal transduction cascades. In this review, we briefly describe the chemistry and physiology of CCK and then consider the major physiological effects of CCK on pancreatic acinar cells. The majority of the review is devoted to the physiologic signaling pathways activated by CCK receptors and heterotrimeric G proteins and the functions they affect. The pathways covered include the traditional second messenger pathways PLC-IP3-Ca²⁺ , DAG-PKC, and AC-cAMP-PKA/EPAC that primarily relate to secretion. Then there are the protein-protein interaction pathways Akt-mTOR-S6K, the three major MAPK pathways (ERK, JNK, and p38 MAPK), and Ca²⁺ -calcineurin-NFAT pathways that primarily regulate non-secretory processes including biosynthesis and growth, and several miscellaneous pathways that include the Rho family small G proteins, PKD, FAK, and Src that may regulate both secretory and nonsecretory processes but are not as well understood. © 2019 American Physiological Society. Compr Physiol 9:535-564, 2019.

Combined Inhibition of DYRK1A, SMAD, and Trithorax Pathways Synergizes to Induce Robust Replication in Adult Human Beta Cells

Small-molecule inhibitors of dual-specificity tyrosine-regulated kinase 1A (DYRK1A) induce human beta cells to proliferate, generating a labeling index of 1.5%-3%. Here, we demonstrate that combined pharmacologic inhibition of DYRK1A and transforming growth factor beta superfamily (TGFβSF)/SMAD signaling generates remarkable further synergistic increases in human beta cell proliferation (average labeling index, 5%-8%, and as high as 15%-18%), and increases in both mouse and human beta cell numbers. This synergy reflects activation of cyclins and cdks by DYRK1A inhibition, accompanied by simultaneous reductions in key cell-cycle inhibitors (CDKN1C and CDKN1A). The latter results from interference with the basal Trithorax- and SMAD-mediated transactivation of CDKN1C and CDKN1A. Notably, combined DYRK1A and TGFβ inhibition allows preservation of beta cell differentiated function. These beneficial effects extend from normal human beta cells and stem cell-derived human beta cells to those from people with type 2 diabetes, and occur both in vitro and in vivo.

In vivo imaging of β-cell function reveals glucose-mediated heterogeneity of β-cell functional development

How pancreatic β-cells acquire function in vivo is a long-standing mystery due to the lack of technology to visualize β-cell function in living animals. Here, we applied a high-resolution two-photon light-sheet microscope for the first in vivo imaging of Ca²⁺activity of every β-cell in Tg (ins:Rcamp1.07) zebrafish. We reveal that the heterogeneity of β-cell functional development in vivo occurred as two waves propagating from the islet mantle to the core, coordinated by islet vascularization. Increasing amounts of glucose induced functional acquisition and enhancement of β-cells via activating calcineurin/nuclear factor of activated T-cells (NFAT) signaling. Conserved in mammalians, calcineurin/NFAT prompted high-glucose-stimulated insulin secretion of neonatal mouse islets cultured in vitro. However, the reduction in low-glucose-stimulated insulin secretion was dependent on optimal glucose but independent of calcineurin/NFAT. Thus, combination of optimal glucose and calcineurin activation represents a previously unexplored strategy for promoting functional maturation of stem cell-derived β-like cells in vitro.

When the amount of sugar in our body rises, specialised cells known as β-cells respond by releasing insulin, a hormone that acts on various organs to keep blood sugar levels within a healthy range. These cells cluster in small ‘islets’ inside our pancreas. If the number of working β-cells declines, diseases such as diabetes may appear and it becomes difficult to regulate the amount of sugar in our bodies. Understanding how β-cells normally develop and mature in the embryo could help us learn how to make new ones in the laboratory. In particular, researchers are interested in studying how different body signals, such as blood sugar levels, turn immature β-cells into fully productive cells. However, in mammals, the pancreas and its islets are buried deep inside the embryo and they cannot be observed easily.

Here, Zhao et al. circumvented this problem by doing experiments on zebrafish embryos, which are transparent, grow outside their mother’s body, and have pancreatic islets that are similar to the ones found in mammals. A three-dimensional microscopy technique was used to watch individual β-cells activity over long periods, which revealed that the cells start being able to produce insulin at different times. The β-cells around the edge of each islet were the first to have access to blood sugar signals: they gained their hormone-producing role earlier than the cells in the core of an islet, which only sensed the information later on.

Zhao et al. then exposed the zebrafish embryos to different amounts of sugar. This showed that there is an optimal concentration of sugar which helps β-cells develop by kick-starting a cascade of events inside the cell. Further experiments confirmed that the same pathway and optimal sugar concentration exist for mammalian islets grown in the laboratory.

These findings may help researchers find better ways of making new β-cells to treat diabetic patients. In the future, using the three-dimensional imaging technique in zebrafish embryos may lead to more discoveries on how the pancreas matures.

Isx9 Regulates Calbindin D28K Expression in Pancreatic β Cells and Promotes β Cell Survival and Function

Pancreatic β-cell dysfunction and death contribute to the onset of diabetes, and novel strategies of β-cell function and survival under diabetogenic conditions need to be explored. We previously demonstrated that Isx9, a small molecule based on the isoxazole scaffold, drives neuroendocrine phenotypes by increasing the expression of genes required for β-cell function and improves glycemia in a model of β cell regeneration. We further investigated the role of Isx9 in β-cell survival. We find that Isx9 drives the expression of Calbindin-D28K (D28K), a key regulator of calcium homeostasis, and plays a cytoprotective role through its calcium buffering capacity in β cells. Isx9 increased the activity of the calcineurin (CN)/cytoplasmic nuclear factor of the activated T-cells (NFAT) transcription factor, a key regulator of D28K, and improved the recruitment of NFATc1, cAMP response element-binding protein (CREB), and p300 to the D28K promoter. We found that nutrient stimulation increased D28K plasma membrane enrichment and modulated calcium channel activity in order to regulate glucose-induced insulin secretion. Isx9-mediated expression of D28K protected β cells against chronic stress induced by serum withdrawal or chronic inflammation by reducing caspase 3 activity. Consequently, Isx9 improved human islet function after transplantation in NOD-SCID mice in a streptozotocin-induced diabetes model. In summary, Isx9 significantly regulates expression of genes relevant to β cell survival and function, and may be an attractive therapy to treat diabetes and improve islet function post-transplantation.

Cellular signaling pathways regulating β-cell proliferation as a promising therapeutic target in the treatment of diabetes

It is established that a decrease in β-cell number and deficiency in the function of existing β-cells contribute to type 1 and type 2 diabetes mellitus. Therefore, a major focus of current research is to identify novel methods of improving the number and function of β-cells, so as to prevent and/or postpone the development of diabetes mellitus and potentially reverse diabetes mellitus. Based on prior knowledge of the above-mentioned causes, promising therapeutic approaches may include direct transplantation of islets, implantation and subsequent induced differentiation of progenitors/stem cells to β-cells, replication of pre-existing β-cells, or activation of endogenous β-cell progenitors. More recently, with regards to cell replacement and regenerative treatment for diabetes patients, the identification of cellular signaling pathways with related genes or corresponding proteins involved in diabetes has become a topic of interest. However, the majority of pathways and molecules associated with β-cells remain unresolved, and the specialized functions of known pathways remain unclear, particularly in humans. The current article has evaluated the progress of research on pivotal cellular signaling pathways involved with β-cell proliferation and survival, and their validity for therapeutic adult β-cell regeneration in diabetes. More efforts are required to elucidate the cellular events involved in human β-cell proliferation in terms of the underlying mechanisms and functions.

How does glucagon-like peptide 1 stimulate human β-cell proliferation? A lesson from islet graft experiments

The incidence of type 2 diabetes increases with age. The age‐dependent decline in functional β‐cell mass contributes to the increased risk of onset of diabetes, reflecting the central role of pancreatic β‐cells in glucose homeostasis. Indeed, the replication rate of human and rodent β‐cells is known to decline sharply with age, and such a characteristic of β‐cells might explain the increased onset of type 2 diabetes in the older population. The molecular mechanism involved in the age‐dependent decline of β‐cell proliferation has been extensively studied, mainly using rodents and in vitro culture systems, but its molecular basis is still largely unknown. A mechanism by which glucagon‐like peptide‐1 receptor activation induces human β‐cell proliferation only within a restricted time window was recently suggested in a study in which human islets were grafted into immunodeficient mice. The authors found that the mitogenic effects of exendin‐4 require calcineurin/nuclear factor of activated T‐cells signaling, and that only in juvenile islets, exendin‐4 induced the expression of nuclear factor of activated T‐cells signaling components, as well as downstream target genes that facilitate β‐cell proliferation. These findings provide a mechanistic explanation as to why glucagon‐like peptide 1 exerts mitogenic effects only in juvenile human β‐cells.

Secretagogin protects Pdx1 from proteasomal degradation to control a transcriptional program required for β cell specification

OBJECTIVE

Specification of endocrine cell lineages in the developing pancreas relies on extrinsic signals from non-pancreatic tissues, which initiate a cell-autonomous sequence of transcription factor activation and repression switches. The steps in this pathway share reliance on activity-dependent Ca²⁺ signals. However, the mechanisms by which phasic Ca²⁺ surges become converted into a dynamic, cell-state-specific and physiologically meaningful code made up by transcription factors constellations remain essentially unknown.

METHODS

We used high-resolution histochemistry to explore the coincident expression of secretagogin and transcription factors driving β cell differentiation. Secretagogin promoter activity was tested in response to genetically manipulating Pax6 and Pax4 expression. Secretagogin null mice were produced with their pancreatic islets morphologically and functionally characterized during fetal development. A proteomic approach was utilized to identify the Ca²⁺-dependent interaction of secretagogin with subunits of the 26S proteasome and verified in vitro by focusing on Pdx1 retention.

RESULTS

Here, we show that secretagogin, a Ca²⁺ sensor protein that controls α and β cell turnover in adult, is in fact expressed in endocrine pancreas from the inception of lineage segregation in a Pax4-and Pax6-dependent fashion. By genetically and pharmacologically manipulating secretagogin expression and interactome engagement in vitro, we find secretagogin to gate excitation-driven Ca²⁺ signals for β cell differentiation and insulin production. Accordingly, secretagogin^-/- fetuses retain a non-committed pool of endocrine progenitors that co-express both insulin and glucagon. We identify the Ca²⁺-dependent interaction of secretagogin with subunits of the 26S proteasome complex to prevent Pdx1 degradation through proteasome inactivation. This coincides with retained Nkx6.1, Pax4 and insulin transcription in prospective β cells.

CONCLUSIONS

In sum, secretagogin scales the temporal availability of a Ca²⁺-dependent transcription factor network to define β cell identity.

Synaptotagmin 4 Regulates Pancreatic β Cell Maturation by Modulating the Ca2+ Sensitivity of Insulin Secretion Vesicles

Islet β cells from newborn mammals exhibit high basal insulin secretion and poor glucose-stimulated insulin secretion (GSIS). Here we show that β cells of newborns secrete more insulin than adults in response to similar intracellular Ca2+ concentrations, suggesting differences in the Ca2+ sensitivity of insulin secretion. Synaptotagmin 4 (Syt4), a non-Ca2+ binding paralog of the β cell Ca2+ sensor Syt7, increased by ∼8-fold during β cell maturation. Syt4 ablation increased basal insulin secretion and compromised GSIS. Precocious Syt4 expression repressed basal insulin secretion but also impaired islet morphogenesis and GSIS. Syt4 was localized on insulin granules and Syt4 levels inversely related to the number of readily releasable vesicles. Thus, transcriptional regulation of Syt4 affects insulin secretion; Syt4 expression is regulated in part by Myt transcription factors, which repress Syt4 transcription. Finally, human SYT4 regulated GSIS in EndoC-βH1 cells, a human β cell line. These findings reveal the role that altered Ca2+ sensing plays in regulating β cell maturation.

A Common Type 2 Diabetes Risk Variant Potentiates Activity of an Evolutionarily Conserved Islet Stretch Enhancer and Increases C2CD4A and C2CD4B Expression

Genome-wide association studies (GWASs) and functional genomics approaches implicate enhancer disruption in islet dysfunction and type 2 diabetes (T2D) risk. We applied genetic fine-mapping and functional (epi)genomic approaches to a T2D- and proinsulin-associated 15q22.2 locus to identify a most likely causal variant, determine its direction of effect, and elucidate plausible target genes. Fine-mapping and conditional analyses of proinsulin levels of 8,635 non-diabetic individuals from the METSIM study support a single association signal represented by a cluster of 16 strongly associated (p < 10^-17) variants in high linkage disequilibrium (r² > 0.8) with the GWAS index SNP rs7172432. These variants reside in an evolutionarily and functionally conserved islet and β cell stretch or super enhancer; the most strongly associated variant (rs7163757, p = 3 × 10^-19) overlaps a conserved islet open chromatin site. DNA sequence containing the rs7163757 risk allele displayed 2-fold higher enhancer activity than the non-risk allele in reporter assays (p < 0.01) and was differentially bound by β cell nuclear extract proteins. Transcription factor NFAT specifically potentiated risk-allele enhancer activity and altered patterns of nuclear protein binding to the risk allele in vitro, suggesting that it could be a factor mediating risk-allele effects. Finally, the rs7163757 proinsulin-raising and T2D risk allele (C) was associated with increased expression of C2CD4B, and possibly C2CD4A, both of which were induced by inflammatory cytokines, in human islets. Together, these data suggest that rs7163757 contributes to genetic risk of islet dysfunction and T2D by increasing NFAT-mediated islet enhancer activity and modulating C2CD4B, and possibly C2CD4A, expression in (patho)physiologic states.

SIRT6-mediated transcriptional suppression of Txnip is critical for pancreatic beta cell function and survival in mice

AIMS/HYPOTHESIS

Better understanding of how genetic and epigenetic components control beta cell differentiation and function is key to the discovery of novel therapeutic approaches to prevent beta cell dysfunction and failure in the progression of type 2 diabetes. Our goal was to elucidate the role of histone deacetylase sirtuin 6 (SIRT6) in beta cell development and homeostasis.

METHODS

Sirt6 endocrine progenitor cell conditional knockout and beta cell-specific knockout mice were generated using the Cre-loxP system. Mice were assayed for islet morphology, glucose tolerance, glucose-stimulated insulin secretion and susceptibility to streptozotocin. Transcriptional regulatory functions of SIRT6 in primary islets were evaluated by RNA-Seq analysis. Reverse transcription-quantitative (RT-q)PCR and immunoblot were used to verify and investigate the gene expression changes. Chromatin occupancies of SIRT6, H3K9Ac, H3K56Ac and active RNA polymerase II were evaluated by chromatin immunoprecipitation.

RESULTS

Deletion of Sirt6 in pancreatic endocrine progenitor cells did not affect endocrine morphology, beta cell mass or insulin production but did result in glucose intolerance and defective glucose-stimulated insulin secretion in mice. Conditional deletion of Sirt6 in adult beta cells reproduced the insulin secretion defect. Loss of Sirt6 resulted in aberrant upregulation of thioredoxin-interacting protein (TXNIP) in beta cells. SIRT6 deficiency led to increased acetylation of histone H3 lysine residue at 9 (H3K9Ac), acetylation of histone H3 lysine residue at 56 (H3K56Ac) and active RNA polymerase II at the promoter region of Txnip. SIRT6-deficient beta cells exhibited a time-dependent increase in H3K9Ac, H3K56Ac and TXNIP levels. Finally, beta cell-specific SIRT6-deficient mice showed increased sensitivity to streptozotocin.

CONCLUSIONS/INTERPRETATION

Our results reveal that SIRT6 suppresses Txnip expression in beta cells via deacetylation of histone H3 and plays a critical role in maintaining beta cell function and viability.

DATA AVAILABILITY

Sequence data have been deposited in the National Institutes of Health (NIH) Gene Expression Omnibus (GEO) with the accession code GSE104161.

Replication confers β cell immaturity

Pancreatic β cells are highly specialized to regulate systemic glucose levels by secreting insulin. In adults, increase in β-cell mass is limited due to brakes on cell replication. In contrast, proliferation is robust in neonatal β cells that are functionally immature as defined by a lower set point for glucose-stimulated insulin secretion. Here we show that β-cell proliferation and immaturity are linked by tuning expression of physiologically relevant, non-oncogenic levels of c-Myc. Adult β cells induced to replicate adopt gene expression and metabolic profiles resembling those of immature neonatal β that proliferate readily. We directly demonstrate that priming insulin-producing cells to enter the cell cycle promotes a functionally immature phenotype. We suggest that there exists a balance between mature functionality and the ability to expand, as the phenotypic state of the β cell reverts to a less functional one in response to proliferative cues.

Adult beta cells, which are highly specialised insulin-secreting cells, rarely replicate. Puri et al. demonstrate that beta cell proliferative capacity is inversely correlated with their functionality and differentiation state, by inducing proliferation of adult cells with ectopic overexpression of the cell cycle regulator c-Myc.

Age-dependent human β cell proliferation induced by glucagon-like peptide 1 and calcineurin signaling

Inadequate pancreatic β cell function underlies type 1 and type 2 diabetes mellitus. Strategies to expand functional cells have focused on discovering and controlling mechanisms that limit the proliferation of human β cells. Here, we developed an engraftment strategy to examine age-associated human islet cell replication competence and reveal mechanisms underlying age-dependent decline of β cell proliferation in human islets. We found that exendin-4 (Ex-4), an agonist of the glucagon-like peptide 1 receptor (GLP-1R), stimulates human β cell proliferation in juvenile but not adult islets. This age-dependent responsiveness does not reflect loss of GLP-1R signaling in adult islets, since Ex-4 treatment stimulated insulin secretion by both juvenile and adult human β cells. We show that the mitogenic effect of Ex-4 requires calcineurin/nuclear factor of activated T cells (NFAT) signaling. In juvenile islets, Ex-4 induced expression of calcineurin/NFAT signaling components as well as target genes for proliferation-promoting factors, including NFATC1, FOXM1, and CCNA1. By contrast, expression of these factors in adult islet β cells was not affected by Ex-4 exposure. These studies reveal age-dependent signaling mechanisms regulating human β cell proliferation, and identify elements that could be adapted for therapeutic expansion of human β cells.