The Basic Helix-Loop-Helix Transcription Factor NEUROG3 Is Required for Development of the Human Endocrine Pancreas

A perfect islet: reviewing recent protocol developments and proposing strategies for stem cell derived functional pancreatic islets

The search for an effective cell replacement therapy for diabetes has driven the development of "perfect" pancreatic islets from human pluripotent stem cells (hPSCs). These hPSC-derived pancreatic islet-like β cells can overcome the limitations for disease modelling, drug development and transplantation therapies in diabetes. Nevertheless, challenges remain in generating fully functional and mature β cells from hPSCs. This review underscores the significant efforts made by researchers to optimize various differentiation protocols aimed at enhancing the efficiency and quality of hPSC-derived pancreatic islets and proposes methods for their improvement. By emulating the natural developmental processes of pancreatic embryogenesis, specific growth factors, signaling molecules and culture conditions are employed to guide hPSCs towards the formation of mature β cells capable of secreting insulin in response to glucose. However, the efficiency of these protocols varies greatly among different human embryonic stem cell (hESC) and induced pluripotent stem cell (hiPSC) lines. This variability poses a particular challenge for generating patient-specific β cells. Despite recent advancements, the ultimate goal remains to develop a highly efficient directed differentiation protocol that is applicable across all genetic backgrounds of hPSCs. Although progress has been made, further research is required to optimize the protocols and characterization methods that could ensure the safety and efficacy of hPSC-derived pancreatic islets before they can be utilized in clinical settings.

Molecular Classification and Characterization of Noninsulinoma: Ready for Prime Time in Clinical Practice?

Pancreatic neuroendocrine tumors are a heterogeneous group of rare clinical tumors, which can be classified into functional pancreatic neuroendocrine tumor (insulinoma is the most common) and noninsulinoma. Insulinoma and noninsulinoma have different mutation profiles. In noninsulinoma, ATRX/DAXX mutation is associated with alternative lengthening of telomeres-positive phenotype and positively correlated with poor prognosis. Copy number variation is also a prognostic marker for a high risk of recurrence. Scholars have used epigenetics as well as a multiomics approach (combining epigenetics, metabolomics, proteomics, etc) to molecularly type noninsulinoma, and there are huge differences in molecular expression and patient prognosis between different groups. In this manuscript, we summarize the published studies that utilized genome, epigenome, transcriptome, and proteome data to classify noninsulinoma.

Spatiotemporal profiling of adhesion G protein-coupled receptors in developing mouse and human pancreas reveals a role for GPR56 in islet development

INTRODUCTION

G protein-coupled receptors (GPCRs) are cell-surface proteins that are targeted therapeutically for a range of disorders, including diabetes. Adhesion GPCRs (aGPCRs) are the second largest class of the GPCR superfamily and some members of this family have been implicated in appropriate organ development. However, the role of aGPCRs in endocrine pancreas specification is not yet known.

METHODS

Here, we systematically characterised expression of mRNAs encoding aGPCRs and their ligands in developing mouse and human pancreas using our own and publicly available single-cell RNA sequencing and spatial transcriptomics data, and we conducted qPCR analysis of aGPCR expression in human pancreas at different gestational stages. We then investigated the role of GPR56 (ADGRG1), the most abundant aGPCR in pancreatic endocrine progenitors, in islet development using Gpr56 null mice and their wildtype littermates.

RESULTS

We demonstrated that aGPCRs are dynamically expressed during mouse and human pancreas development, with specific aGPCR mRNAs expressed in distinct endocrine, endothelial, mesenchymal, acinar, ductal, and immune cell clusters. aGPCR ligand mRNAs were mostly expressed by non-endocrine cells, and the most highly expressed receptor-ligand interacting mRNA pairs were those encoding GPR56 and COL3A1. Deletion of Gpr56 in neonatal mice was associated with an altered α-/β-/δ-cell ratio and reduced β-cell proliferation.

CONCLUSION

Our data show that aGPCRs are expressed at key stages of human and mouse pancreas endocrine lineage decisions, and analysis of pancreases from Gpr56 knockout mice implicate this aGPCR in the development of a full complement of β-cells.

Pancreatic endocrine and exocrine signaling and crosstalk in physiological and pathological status

The pancreas, an organ with dual functions, regulates blood glucose levels through the endocrine system by secreting hormones such as insulin and glucagon. It also aids digestion through the exocrine system by secreting digestive enzymes. Complex interactions and signaling mechanisms between the endocrine and exocrine functions of the pancreas play a crucial role in maintaining metabolic homeostasis and overall health. Compelling evidence indicates direct and indirect crosstalk between the endocrine and exocrine parts, influencing the development of diseases affecting both. From a developmental perspective, the exocrine and endocrine parts share the same origin-the "tip-trunk" domain. In certain circumstances, pancreatic exocrine cells may transdifferentiate into endocrine-like cells, such as insulin-secreting cells. Additionally, several pancreatic diseases, including pancreatic cancer, pancreatitis, and diabetes, exhibit potential relevance to both endocrine and exocrine functions. Endocrine cells may communicate with exocrine cells directly through cytokines or indirectly by regulating the immune microenvironment. This crosstalk affects the onset and progression of these diseases. This review summarizes the history and milestones of findings related to the exocrine and endocrine pancreas, their embryonic development, phenotypic transformations, signaling roles in health and disease, the endocrine-exocrine crosstalk from the perspective of diseases, and potential therapeutic targets. Elucidating the regulatory mechanisms of pancreatic endocrine and exocrine signaling and provide novel insights for the understanding and treatment of diseases.

Hippo pathway-mediated YAP1/TAZ inhibition is essential for proper pancreatic endocrine specification and differentiation

The Hippo pathway plays a central role in tissue development and homeostasis. However, the function of Hippo in pancreatic endocrine development remains obscure. Here, we generated novel conditional genetically engineered mouse models to examine the roles of Hippo pathway-mediated YAP1/TAZ inhibition in the development stages of endocrine specification and differentiation. While YAP1 protein was localized to the nuclei in bipotent progenitor cells, Neurogenin 3 expressing endocrine progenitors completely lost YAP1 expression. Using genetically engineered mouse models, we found that inactivation of YAP1 requires both an intact Hippo pathway and Neurogenin 3 protein. Gene deletion of Lats1 and 2 kinases (Lats1&2) in endocrine progenitor cells of developing mouse pancreas using Neurog3Cre blocked endocrine progenitor cell differentiation and specification, resulting in reduced islets size and a disorganized pancreas at birth. Loss of Lats1&2 in Neurogenin 3 expressing cells activated YAP1/TAZ transcriptional activity and recruited macrophages to the developing pancreas. These defects were rescued by deletion of Yap1/Wwtr1 genes, suggesting that tight regulation of YAP1/TAZ by Hippo signaling is crucial for pancreatic endocrine specification. In contrast, deletion of Lats1&2 using β-cell-specific Ins1CreER resulted in a phenotypically normal pancreas, indicating that Lats1&2 are indispensable for differentiation of endocrine progenitors but not for that of β-cells. Our results demonstrate that loss of YAP1/TAZ expression in the pancreatic endocrine compartment is not a passive consequence of endocrine specification. Rather, Hippo pathway-mediated inhibition of YAP1/TAZ in endocrine progenitors is a prerequisite for endocrine specification and differentiation.

Development, regeneration, and physiological expansion of functional β-cells: Cellular sources and regulators

Pancreatic regeneration is a complex process observed in both normal and pathological conditions. The aim of this review is to provide a comprehensive understanding of the emergence of a functionally active population of insulin-secreting β-cells in the adult pancreas. The renewal of β-cells is governed by a multifaceted interaction between cellular sources of genetic and epigenetic factors. Understanding the development and heterogeneity of β-cell populations is crucial for functional β-cell regeneration. The functional mass of pancreatic β-cells increases in situations such as pregnancy and obesity. However, the specific markers of mature β-cell populations and postnatal pancreatic progenitors capable of increasing self-reproduction in these conditions remain to be elucidated. The capacity to regenerate the β-cell population through various pathways, including the proliferation of pre-existing β-cells, β-cell neogenesis, differentiation of β-cells from a population of progenitor cells, and transdifferentiation of non-β-cells into β-cells, reveals crucial molecular mechanisms for identifying cellular sources and inducers of functional cell renewal. This provides an opportunity to identify specific cellular sources and mechanisms of regeneration, which could have clinical applications in treating various pathologies, including in vitro cell-based technologies, and deepen our understanding of regeneration in different physiological conditions.

A critical review on therapeutic approaches of CRISPR-Cas9 in diabetes mellitus

Diabetes mellitus (D.M.) is a common metabolic disorder caused mainly by combining two primary factors, which are (1) defects in insulin production by the pancreatic β-cells and (2) responsiveness of insulin-sensitive tissues towards insulin. Despite the rapid advancement in medicine to suppress elevated blood glucose levels (hyperglycemia) and insulin resistance associated with this hazard, a demand has undoubtedly emerged to find more effective and curative dimensions in therapeutic approaches against D.M. The administration of diabetes treatment that emphasizes insulin production and sensitivity may result in unfavorable side effects, reduced adherence, and potential treatment ineffectiveness. Recent progressions in genome editing technologies, for instance, in zinc-finger nucleases, transcription activator-like effector nucleases, and clustered regularly interspaced short palindromic repeat (CRISPR-Cas)-associated nucleases, have greatly influenced the gene editing technology from concepts to clinical practices. Improvements in genome editing technologies have also opened up the possibility to target and modify specific genome sequences in a cell directly. CRISPR/Cas9 has proven effective in utilizing ex vivo gene editing in embryonic stem cells and stem cells derived from patients. This application has facilitated the exploration of pancreatic beta-cell development and function. Furthermore, CRISPR/Cas9 enables the creation of innovative animal models for diabetes and assesses the effectiveness of different therapeutic strategies in treating the condition. We, therefore, present a critical review of the therapeutic approaches of the genome editing tool CRISPR-Cas9 in treating D.M., discussing the challenges and limitations of implementing this technology.

Functional interrogation of twenty type 2 diabetes-associated genes using isogenic human embryonic stem cell-derived β-like cells

Genetic studies have identified numerous loci associated with type 2 diabetes (T2D), but the functional roles of many loci remain unexplored. Here, we engineered isogenic knockout human embryonic stem cell lines for 20 genes associated with T2D risk. We examined the impacts of each knockout on β cell differentiation, functions, and survival. We generated gene expression and chromatin accessibility profiles on β cells derived from each knockout line. Analyses of T2D-association signals overlapping HNF4A-dependent ATAC peaks identified a likely causal variant at the FAIM2 T2D-association signal. Additionally, the integrative association analyses identified four genes (CP, RNASE1, PCSK1N, and GSTA2) associated with insulin production, and two genes (TAGLN3 and DHRS2) associated with β cell sensitivity to lipotoxicity. Finally, we leveraged deep ATAC-seq read coverage to assess allele-specific imbalance at variants heterozygous in the parental line and identified a single likely functional variant at each of 23 T2D-association signals.

Integrating single-cell imaging and RNA sequencing datasets links differentiation and morphogenetic dynamics of human pancreatic endocrine progenitors

Basic helix-loop-helix genes, particularly proneural genes, are well-described triggers of cell differentiation, yet information on their dynamics is limited, notably in human development. Here, we focus on Neurogenin 3 (NEUROG3), which is crucial for pancreatic endocrine lineage initiation. By monitoring both NEUROG3 gene expression and protein in single cells using a knockin dual reporter in 2D and 3D models of human pancreas development, we show an approximately 2-fold slower expression of human NEUROG3 than that of the mouse. We observe heterogeneous peak levels of NEUROG3 expression and reveal through long-term live imaging that both low and high NEUROG3 peak levels can trigger differentiation into hormone-expressing cells. Based on fluorescence intensity, we statistically integrate single-cell transcriptome with dynamic behaviors of live cells and propose a data-mapping methodology applicable to other contexts. Using this methodology, we identify a role for KLK12 in motility at the onset of NEUROG3 expression.

Functional interrogation of twenty type 2 diabetes-associated genes using isogenic hESC-derived β-like cells

Genetic studies have identified numerous loci associated with type 2 diabetes (T2D), but the functional role of many loci has remained unexplored. In this study, we engineered isogenic knockout human embryonic stem cell (hESC) lines for 20 genes associated with T2D risk. We systematically examined β-cell differentiation, insulin production and secretion, and survival. We performed RNA-seq and ATAC-seq on hESC-β cells from each knockout line. Analyses of T2D GWAS signals overlapping with HNF4A-dependent ATAC peaks identified a specific SNP as a likely causal variant. In addition, we performed integrative association analyses and identified four genes ( CP, RNASE1, PCSK1N and GSTA2 ) associated with insulin production, and two genes ( TAGLN3 and DHRS2 ) associated with sensitivity to lipotoxicity. Finally, we leveraged deep ATAC-seq read coverage to assess allele-specific imbalance at variants heterozygous in the parental hESC line, to identify a single likely functional variant at each of 23 T2D GWAS signals.

Understanding cell fate acquisition in stem-cell-derived pancreatic islets using single-cell multiome-inferred regulomes

Pancreatic islet cells derived from human pluripotent stem cells hold great promise for modeling and treating diabetes. Differences between stem-cell-derived and primary islets remain, but molecular insights to inform improvements are limited. Here, we acquire single-cell transcriptomes and accessible chromatin profiles during in vitro islet differentiation and pancreas from childhood and adult donors for comparison. We delineate major cell types, define their regulomes, and describe spatiotemporal gene regulatory relationships between transcription factors. CDX2 emerged as a regulator of enterochromaffin-like cells, which we show resemble a transient, previously unrecognized, serotonin-producing pre-β cell population in fetal pancreas, arguing against a proposed non-pancreatic origin. Furthermore, we observe insufficient activation of signal-dependent transcriptional programs during in vitro β cell maturation and identify sex hormones as drivers of β cell proliferation in childhood. Altogether, our analysis provides a comprehensive understanding of cell fate acquisition in stem-cell-derived islets and a framework for manipulating cell identities and maturity.

ARID1A loss in pancreas leads to islet developmental defect and metabolic disturbance

ARID1A is a tumor suppressor gene mutated in 7-10% of pancreatic cancer patients. However, its function in pancreas development and endocrine regulation is unclear. We generated mice that lack Arid1a expression in the pancreas. Our results showed that deletion of the Arid1a gene in mice caused a reduction in islet numbers and insulin production, both of which are associated with diabetes mellitus (DM) phenotype. RNA sequencing of isolated islets confirmed DM gene signature and decrease of developmental lineage genes. We identified neurogenin3, a transcription factor that controls endocrine fate specification, is a direct target of Aird1a. Gene set enrichment analysis indicated the enhancement of histone deacetylase (HDAC) pathway after Arid1a depletion and a clinically approved HDAC inhibitor showed therapeutic benefit by suppressing disease onset. Our data suggest that Arid1a is required for the development of pancreatic islets by regulating Ngn3+-mediated transcriptional program and is important in maintaining endocrine function.

In vitro beta-cell killing models using immune cells and human pluripotent stem cell-derived islets: Challenges and opportunities

Type 1 diabetes (T1D) is a disease of both autoimmunity and β-cells. The β-cells play an active role in their own demise by mounting defense mechanisms that are insufficient at best, and that can become even deleterious in the long term. This complex crosstalk is important to understanding the physiological defense mechanisms at play in healthy conditions, their alterations in the T1D setting, and therapeutic agents that may boost such mechanisms. Robust protocols to develop stem-cell-derived islets (SC-islets) from human pluripotent stem cells (hPSCs), and islet-reactive cytotoxic CD8+ T-cells from peripheral blood mononuclear cells offer unprecedented opportunities to study this crosstalk. Challenges to develop in vitro β-cell killing models include the cluster morphology of SC-islets, the relatively weak cytotoxicity of most autoimmune T-cells and the variable behavior of in vitro expanded CD8+ T-cells. These challenges may however be highly rewarding in light of the opportunities offered by such models. Herein, we discuss these opportunities including: the β-cell/immune crosstalk in an islet microenvironment; the features that make β-cells more sensitive to autoimmunity; therapeutic agents that may modulate β-cell vulnerability; and the possibility to perform analyses in an autologous setting, i.e., by generating T-cell effectors and SC-islets from the same donor.

Enteroendocrine Cells Protect the Stem Cell Niche by Regulating Crypt Metabolism in Response to Nutrients

BACKGROUND & AIMS

The intestinal stem cell niche is exquisitely sensitive to changes in diet, with high-fat diet, caloric restriction, and fasting resulting in altered crypt metabolism and intestinal stem cell function. Unlike cells on the villus, cells in the crypt are not immediately exposed to the dynamically changing contents of the lumen. We hypothesized that enteroendocrine cells (EECs), which sense environmental cues and in response release hormones and metabolites, are essential for relaying the luminal and nutritional status of the animal to cells deep in the crypt.

METHODS

We used the tamoxifen-inducible VillinCreERT2 mouse model to deplete EECs (Neurog3fl/fl) from adult intestinal epithelium and we generated human intestinal organoids from wild-type and NEUROGENIN 3 (NEUROG3)-null human pluripotent stem cells. We used indirect calorimetry, 1H-Nuclear Magnetic Resonance (NMR) metabolomics, mitochondrial live imaging, and the Seahorse bioanalyzer (Agilent Technologies) to assess metabolism. Intestinal stem cell activity was measured by proliferation and enteroid-forming capacity. Transcriptional changes were assessed using 10x Genomics single-cell sequencing.

RESULTS

Loss of EECs resulted in increased energy expenditure in mice, an abundance of active mitochondria, and a shift of crypt metabolism to fatty acid oxidation. Crypts from mouse and human intestinal organoids lacking EECs displayed increased intestinal stem cell activity and failed to activate phosphorylation of downstream target S6 kinase ribosomal protein, a marker for activity of the master metabolic regulator mammalian target of rapamycin (mTOR). These phenotypes were similar to those observed when control mice were deprived of nutrients.

CONCLUSIONS

EECs are essential regulators of crypt metabolism. Depletion of EECs recapitulated a fasting metabolic phenotype despite normal levels of ingested nutrients. These data suggest that EECs are required to relay nutritional information to the stem cell niche and are essential regulators of intestinal metabolism.

Aberrant Splicing of INS Impairs Beta-Cell Differentiation and Proliferation by ER Stress in the Isogenic iPSC Model of Neonatal Diabetes

One of the causes of diabetes in infants is the defect of the insulin gene (INS). Gene mutations can lead to proinsulin misfolding, an increased endoplasmic reticulum (ER) stress and possible beta-cell apoptosis. In humans, the mechanisms underlying beta-cell failure remain unclear. We generated induced pluripotent stem cells (iPSCs) from a patient diagnosed with neonatal diabetes mellitus carrying the INS mutation in the 2nd intron (c.188-31G>A) and engineered isogenic CRISPR/Cas9 mutation-corrected cell lines. Differentiation into beta-like cells demonstrated that mutation led to the emergence of an ectopic splice site within the INS and appearance of the abnormal RNA transcript. Isogenic iPSC lines differentiated into beta-like cells showed a clear difference in formation of organoids at pancreatic progenitor stage of differentiation. Moreover, MIN6 insulinoma cell line expressing mutated cDNA demonstrated significant decrease in proliferation capacity and activation of ER stress and unfolded protein response (UPR)-associated genes. These findings shed light on the mechanism underlying the pathogenesis of monogenic diabetes.

Molecular Mechanism of Pancreatic β-Cell Failure in Type 2 Diabetes Mellitus

Various important transcription factors in the pancreas are involved in the process of pancreas development, the differentiation of endocrine progenitor cells into mature insulin-producing pancreatic β-cells and the preservation of mature β-cell function. However, when β-cells are continuously exposed to a high glucose concentration for a long period of time, the expression levels of several insulin gene transcription factors are substantially suppressed, which finally leads to pancreatic β-cell failure found in type 2 diabetes mellitus. Here we show the possible underlying pathway for β-cell failure. It is likely that reduced expression levels of MafA and PDX-1 and/or incretin receptor in β-cells are closely associated with β-cell failure in type 2 diabetes mellitus. Additionally, since incretin receptor expression is reduced in the advanced stage of diabetes mellitus, incretin-based medicines show more favorable effects against β-cell failure, especially in the early stage of diabetes mellitus compared to the advanced stage. On the other hand, many subjects have recently suffered from life-threatening coronavirus infection, and coronavirus infection has brought about a new and persistent pandemic. Additionally, the spread of coronavirus infection has led to various limitations on the activities of daily life and has restricted economic development worldwide. It has been reported recently that SARS-CoV-2 directly infects β-cells through neuropilin-1, leading to apoptotic β-cell death and a reduction in insulin secretion. In this review article, we feature a possible molecular mechanism for pancreatic β-cell failure, which is often observed in type 2 diabetes mellitus. Finally, we are hopeful that coronavirus infection will decline and normal daily life will soon resume all over the world.

Clinical implications of cell-of-origin epigenetic characteristics in non-functional pancreatic neuroendocrine tumors

Primary non-functional pancreatic neuroendocrine tumors (NF-PanNETs) are a heterogeneous group of neuroendocrine neoplasms that display highly variable clinical behavior. Therefore, NF-PanNETs often present clinical teams with a dilemma: the uncertain metastatic potential of the tumor has to be weighed against the morbidity associated with surgical resection. Thus, rather than utilizing current radiologic thresholds, there is an urgent need for improved prognostic biomarkers. Recent studies aimed at understanding the epigenetic underpinnings of NF-PanNETs have led to the identification of tumor subgroups based on histone modification and DNA methylation patterns. These molecular profiles tend to resemble the cellular origins of PanNETs. Subsequent retrospective analyses have demonstrated that these molecular signatures are of prognostic value and, importantly, may be useful in the preoperative setting. These studies have highlighted that sporadic NF-PanNETs displaying biomarkers associated with disease progression and poor prognosis, such as alternative lengthening of telomeres, inactivating alpha thalassemia/mental retardation X-linked (ATRX) or death domain-associated protein (DAXX) gene mutations, or copy number variations, more often display alpha cell characteristics. Conversely, NF-PanNETs with beta cell characteristics often lack these unfavorable biomarkers. Alternative lengthening of telomeres, transcription factor protein expression, and possibly DNA methylation can be assessed in endoscopic ultrasound-guided tumor biopsies. Prospective studies focusing on cell-of-origin and epigenetic profile-driven decision making prior to surgery are likely to be routinely implemented into clinical practice in the near future. © 2021 The Authors. The Journal of Pathology published by John Wiley & Sons, Ltd. on behalf of The Pathological Society of Great Britain and Ireland.

Stem Cell-Derived β Cells: A Versatile Research Platform to Interrogate the Genetic Basis of β Cell Dysfunction

Pancreatic β cell dysfunction is a central component of diabetes progression. During the last decades, the genetic basis of several monogenic forms of diabetes has been recognized. Genome-wide association studies (GWAS) have also facilitated the identification of common genetic variants associated with an increased risk of diabetes. These studies highlight the importance of impaired β cell function in all forms of diabetes. However, how most of these risk variants confer disease risk, remains unanswered. Understanding the specific contribution of genetic variants and the precise role of their molecular effectors is the next step toward developing treatments that target β cell dysfunction in the era of personalized medicine. Protocols that allow derivation of β cells from pluripotent stem cells, represent a powerful research tool that allows modeling of human development and versatile experimental designs that can be used to shed some light on diabetes pathophysiology. This article reviews different models to study the genetic basis of β cell dysfunction, focusing on the recent advances made possible by stem cell applications in the field of diabetes research.

OUP accepted manuscript

Although genome profiling provides important genetic and phenotypic details for applying precision medicine to diabetes, it is imperative to integrate in vitro human cell models, accurately recapitulating the genetic alterations associated with diabetes. The absence of the appropriate preclinical human models and the unavailability of genetically relevant cells substantially limit the progress in developing personalized treatment for diabetes. Human pluripotent stem cells (hPSCs) provide a scalable source for generating diabetes-relevant cells carrying the genetic signatures of the patients. Remarkably, allogenic hPSC-derived pancreatic progenitors and β cells are being used in clinical trials with promising preliminary results. Autologous hiPSC therapy options exist for those with monogenic and type 2 diabetes; however, encapsulation or immunosuppression must be accompanied with in the case of type 1 diabetes. Furthermore, genome-wide association studies-identified candidate variants can be introduced in hPSCs for deciphering the associated molecular defects. The hPSC-based disease models serve as excellent resources for drug development facilitating personalized treatment. Indeed, hPSC-based diabetes models have successfully provided valuable knowledge by modeling different types of diabetes, which are discussed in this review. Herein, we also evaluate their strengths and shortcomings in dissecting the underlying pathogenic molecular mechanisms and discuss strategies for improving hPSC-based disease modeling investigations.

MicroRNA-7a inhibits Isl1 expression to regulate insulin secretion by targeting Raf1 and Mapkap1 in NIT-1 cells

Both microRNA-7a (miR-7a) and LIM-homeodomain transcription factor ISL1 are important factors regulating insulin transcription and secretion, but the functional relationship and the interacting mechanisms between miR-7a and ISL1 in pancreatic islet β-cells remain unknown. The aims of this study were thus to identify the potential interactions and signaling communication between miR-7a and ISL1 in regulating insulin transcription and secretion in the cultured NIT-1 cells. The results show that miR-7a inhibitor upregulates Isl-1 and insulin gene expressions, and the insulin secretion. Whereas miR-7a mimics inhibit ISL1 and insulin gene expressions, and decreases the insulin secretion. Furthermore, we identified the target gene of miR-7a using dual-luciferase reporter assay, and the results demonstrate that Raf1 and Mapkap1 is a direct target gene of miR-7a, modeling RAF1/MEK/ERK1/2 and mTORC2/AKT signaling pathway to regulate Isl1 expression, and thus influencing insulin expression and secretion. Our results indicate that therapeutic inhibition of miR-7a function could be of relevance for preserving the function of pancreatic β-cells during the course of diabetes development, implicating miR-7, ISL1, and/or the connecting molecules may act as novel targets for pharmacological or gene therapy in diabetes and related metabolic disease, although much detailed studies are required in the further study.

Extensive NEUROG3 occupancy in the human pancreatic endocrine gene regulatory network

Objective

Mice lacking the bHLH transcription factor (TF) Neurog3 do not form pancreatic islet cells, including insulin-secreting beta cells, the absence of which leads to diabetes. In humans, homozygous mutations of NEUROG3 manifest with neonatal or childhood diabetes. Despite this critical role in islet cell development, the precise function of and downstream genetic programs regulated directly by NEUROG3 remain elusive. Therefore, we mapped genome-wide NEUROG3 occupancy in human induced pluripotent stem cell (hiPSC)–derived endocrine progenitors and determined NEUROG3 dependency of associated genes to uncover direct targets.

Methods

We generated a novel hiPSC line (NEUROG3-HA-P2A-Venus) where NEUROG3 is HA-tagged and fused to a self-cleaving fluorescent VENUS reporter. We used the CUT&RUN technique to map NEUROG3 occupancy and epigenetic marks in pancreatic endocrine progenitors (PEP) that were differentiated from this hiPSC line. We integrated NEUROG3 occupancy data with chromatin status and gene expression in PEPs as well as their NEUROG3-dependence. In addition, we investigated whether NEUROG3 binds type 2 diabetes mellitus (T2DM)–associated variants at the PEP stage.

Results

CUT&RUN revealed a total of 863 NEUROG3 binding sites assigned to 1263 unique genes. NEUROG3 occupancy was found at promoters as well as at distant cis-regulatory elements that frequently overlapped within PEP active enhancers. De novo motif analyses defined a NEUROG3 consensus binding motif and suggested potential co-regulation of NEUROG3 target genes by FOXA or RFX transcription factors. We found that 22% of the genes downregulated in NEUROG3^−/− PEPs, and 10% of genes enriched in NEUROG3-Venus positive endocrine cells were bound by NEUROG3 and thus likely to be directly regulated. NEUROG3 binds to 138 transcription factor genes, some with important roles in islet cell development or function, such as NEUROD1, PAX4, NKX2-2, SOX4, MLXIPL, LMX1B, RFX3, and NEUROG3 itself, and many others with unknown islet function. Unexpectedly, we uncovered that NEUROG3 targets genes critical for insulin secretion in beta cells (e.g., GCK, ABCC8/KCNJ11, CACNA1A, CHGA, SCG2, SLC30A8, and PCSK1). Thus, analysis of NEUROG3 occupancy suggests that the transient expression of NEUROG3 not only promotes islet destiny in uncommitted pancreatic progenitors, but could also initiate endocrine programs essential for beta cell function. Lastly, we identified eight T2DM risk SNPs within NEUROG3-bound regions.

Conclusion

Mapping NEUROG3 genome occupancy in PEPs uncovered unexpectedly broad, direct control of the endocrine genes, raising novel hypotheses on how this master regulator controls islet and beta cell differentiation.

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NEUROG3 CUT&RUN analysis revealed 1263 target genes in human pancreatic endocrine progenitors (PEPs).

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NEUROG3 binding sites overlap with active chromatin regions in PEPs.

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1/5 of the genes downregulated in NEUROG3^−/− hESC-derived PEPs are bound by NEUROG3.

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NEUROG3 targets islet-specific TFs and regulators of insulin secretion.

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Several T2DM risk alleles lie within NEUROG3-bound regions.

Monogenic Diabetes Modeling: In Vitro Pancreatic Differentiation From Human Pluripotent Stem Cells Gains Momentum

The occurrence of diabetes mellitus is characterized by pancreatic β cell loss and chronic hyperglycemia. While Type 1 and Type 2 diabetes are the most common types, rarer forms involve mutations affecting a single gene. This characteristic has made monogenic diabetes an interesting disease group to model in vitro using human pluripotent stem cells (hPSCs). By altering the genotype of the original hPSCs or by deriving human induced pluripotent stem cells (hiPSCs) from patients with monogenic diabetes, changes in the outcome of the in vitro differentiation protocol can be analyzed in detail to infer the regulatory mechanisms affected by the disease-associated genes. This approach has been so far applied to a diversity of genes/diseases and uncovered new mechanisms. The focus of the present review is to discuss the latest findings obtained by modeling monogenic diabetes using hPSC-derived pancreatic cells generated in vitro. We will specifically focus on the interpretation of these studies, the advantages and limitations of the models used, and the future perspectives for improvement.

Genome engineering and disease modeling via programmable nucleases for insulin gene therapy; promises of CRISPR/Cas9 technology

Targeted genome editing is a continually evolving technology employing programmable nucleases to specifically change, insert, or remove a genomic sequence of interest. These advanced molecular tools include meganucleases, zinc finger nucleases, transcription activator-like effector nucleases and RNA-guided engineered nucleases (RGENs), which create double-strand breaks at specific target sites in the genome, and repair DNA either by homologous recombination in the presence of donor DNA or via the error-prone non-homologous end-joining mechanism. A recently discovered group of RGENs known as CRISPR/Cas9 gene-editing systems allowed precise genome manipulation revealing a causal association between disease genotype and phenotype, without the need for the reengineering of the specific enzyme when targeting different sequences. CRISPR/Cas9 has been successfully employed as an ex vivo gene-editing tool in embryonic stem cells and patient-derived stem cells to understand pancreatic beta-cell development and function. RNA-guided nucleases also open the way for the generation of novel animal models for diabetes and allow testing the efficiency of various therapeutic approaches in diabetes, as summarized and exemplified in this manuscript.

Hippo Signaling Pathway in Pancreas Development

The Hippo signaling pathway is a vital regulator of pancreatic development and homeostasis, directing cell fate decisions, morphogenesis, and adult pancreatic cellular plasticity. Through loss-of-function research, Hippo signaling has been found to play key roles in maintaining the proper balance between progenitor cell renewal, proliferation, and differentiation in pancreatic organogenesis. Other studies suggest that overactivation of YAP, a downstream effector of the pathway, promotes ductal cell development and suppresses endocrine cell fate specification via repression of Ngn3. After birth, disruptions in Hippo signaling have been found to lead to de-differentiation of acinar cells and pancreatitis-like phenotype. Further, Hippo signaling directs pancreatic morphogenesis by ensuring proper cell polarization and branching. Despite these findings, the mechanisms through which Hippo governs cell differentiation and pancreatic architecture are yet to be fully understood. Here, we review recent studies of Hippo functions in pancreatic development, including its crosstalk with NOTCH, WNT/β-catenin, and PI3K/Akt/mTOR signaling pathways.

Molecular mechanisms of β-cell dysfunction and death in monogenic forms of diabetes

Monogenetic forms of diabetes represent 1%-5% of all diabetes cases and are caused by mutations in a single gene. These mutations, that affect genes involved in pancreatic β-cell development, function and survival, or insulin regulation, may be dominant or recessive, inherited or de novo. Most patients with monogenic diabetes are very commonly misdiagnosed as having type 1 or type 2 diabetes. The severity of their symptoms depends on the nature of the mutation, the function of the affected gene and, in some cases, the influence of additional genetic or environmental factors that modulate severity and penetrance. In some patients, diabetes is accompanied by other syndromic features such as deafness, blindness, microcephaly, liver and intestinal defects, among others. The age of diabetes onset may also vary from neonatal until early adulthood manifestations. Since the different mutations result in diverse clinical presentations, patients usually need different treatments that range from just diet and exercise, to the requirement of exogenous insulin or other hypoglycemic drugs, e.g., sulfonylureas or glucagon-like peptide 1 analogs to control their glycemia. As a consequence, awareness and correct diagnosis are crucial for the proper management and treatment of monogenic diabetes patients. In this chapter, we describe mutations causing different monogenic forms of diabetes associated with inadequate pancreas development or impaired β-cell function and survival, and discuss the molecular mechanisms involved in β-cell demise.

Islet organoid as a promising model for diabetes

Studies on diabetes have long been hampered by a lack of authentic disease models that, ideally, should be unlimited and able to recapitulate the abnormalities involved in the development, structure, and function of human pancreatic islets under pathological conditions. Stem cell-based islet organoids faithfully recapitulate islet development in vitro and provide large amounts of three-dimensional functional islet biomimetic materials with a morphological structure and cellular composition similar to those of native islets. Thus, islet organoids hold great promise for modeling islet development and function, deciphering the mechanisms underlying the onset of diabetes, providing an in vitro human organ model for infection of viruses such as SARS-CoV-2, and contributing to drug screening and autologous islet transplantation. However, the currently established islet organoids are generally immature compared with native islets, and further efforts should be made to improve the heterogeneity and functionality of islet organoids, making it an authentic and informative disease model for diabetes. Here, we review the advances and challenges in the generation of islet organoids, focusing on human pluripotent stem cell-derived islet organoids, and the potential applications of islet organoids as disease models and regenerative therapies for diabetes.

Modeling different types of diabetes using human pluripotent stem cells

Diabetes mellitus (DM) is a metabolic disease characterized by chronic hyperglycemia as a result of progressive loss of pancreatic β cells, which could lead to several debilitating complications. Different paths, triggered by several genetic and environmental factors, lead to the loss of pancreatic β cells and/or function. Understanding these many paths to β cell damage or dysfunction could help in identifying therapeutic approaches specific for each path. Most of our knowledge about diabetes pathophysiology has been obtained from studies on animal models, which do not fully recapitulate human diabetes phenotypes. Currently, human pluripotent stem cell (hPSC) technology is a powerful tool for generating in vitro human models, which could provide key information about the disease pathogenesis and provide cells for personalized therapies. The recent progress in generating functional hPSC-derived β cells in combination with the rapid development in genomic and genome-editing technologies offer multiple options to understand the cellular and molecular mechanisms underlying the development of different types of diabetes. Recently, several in vitro hPSC-based strategies have been used for studying monogenic and polygenic forms of diabetes. This review summarizes the current knowledge about different hPSC-based diabetes models and how these models improved our current understanding of the pathophysiology of distinct forms of diabetes. Also, it highlights the progress in generating functional β cells in vitro, and discusses the current challenges and future perspectives related to the use of the in vitro hPSC-based strategies.

Towards a Functional Cure for Diabetes Using Stem Cell-Derived Beta Cells: Are We There Yet?

Diabetes mellitus is a pandemic metabolic disorder that results from either the autoimmune destruction or the dysfunction of insulin-producing pancreatic beta cells. A promising cure is beta cell replacement through the transplantation of islets of Langerhans. However, donor shortage hinders the widespread implementation of this therapy. Human pluripotent stem cells, including embryonic stem cells and induced pluripotent stem cells, represent an attractive alternative beta cell source for transplantation. Although major advances over the past two decades have led to the generation of stem cell-derived beta-like cells that share many features with genuine beta cells, producing fully mature beta cells remains challenging. Here, we review the current status of beta cell differentiation protocols and highlight specific challenges that are associated with producing mature beta cells. We address the challenges and opportunities that are offered by monogenic forms of diabetes. Finally, we discuss the remaining hurdles for clinical application of stem cell-derived beta cells and the status of ongoing clinical trials.

Human Pluripotent Stem Cells: A Unique Tool for Toxicity Testing in Pancreatic Progenitor and Endocrine Cells

Diabetes prevalence is increasing worldwide, and epidemiological studies report an association between diabetes incidence and environmental pollutant exposure. There are >84,000 chemicals in commerce, many of which are released into the environment without a clear understanding of potential adverse health consequences. While in vivo rodent studies remain an important tool for testing chemical toxicity systemically, we urgently need high-throughput screening platforms in biologically relevant models to efficiently prioritize chemicals for in depth toxicity analysis. Given the increasing global burden of obesity and diabetes, identifying chemicals that disrupt metabolism should be a high priority. Pancreatic endocrine cells are key regulators of systemic metabolism, yet often overlooked as a target tissue in toxicology studies. Immortalized β-cell lines and primary human, porcine, and rodent islets are widely used for studying the endocrine pancreas in vitro, but each have important limitations in terms of scalability, lifespan, and/or biological relevance. Human pluripotent stem cell (hPSC) culture is a powerful tool for in vitro toxicity testing that addresses many of the limitations with other β-cell models. Current in vitro differentiation protocols can efficiently generate glucose-responsive insulin-secreting β-like cells that are not fully mature, but still valuable for high-throughput toxicity screening in vitro. Furthermore, hPSCs can be applied as a model of developing pancreatic endocrine cells to screen for chemicals that influence endocrine cell formation during critical windows of differentiation. Given their versatility, we recommend using hPSCs to identify potential β-cell toxins, which can then be prioritized as chemicals of concern for metabolic disruption.

Gene Signatures of NEUROGENIN3+ Endocrine Progenitor Cells in the Human Pancreas

NEUROGENIN3+ (NEUROG3+) cells are considered to be pancreatic endocrine progenitors. Our current knowledge on the molecular program of NEUROG3+ cells in humans is largely extrapolated from studies in mice. We hypothesized that single-cell RNA-seq enables in-depth exploration of the rare NEUROG3+ cells directly in humans. We aligned four large single-cell RNA-seq datasets from postnatal human pancreas. Our integrated analysis revealed 10 NEUROG3+ epithelial cells from a total of 11,174 pancreatic cells. Noticeably, human NEUROG3+ cells clustered with mature pancreatic cells and epsilon cells displayed the highest frequency of NEUROG3 positivity. We confirmed the co-expression of NEUROG3 with endocrine markers and the high percentage of NEUROG3+ cells among epsilon cells at the protein level based on immunostaining on pancreatic tissue sections. We further identified unique genetic signatures of the NEUROG3+ cells. Regulatory network inference revealed novel transcription factors including Prospero homeobox protein 1 (PROX1) may act jointly with NEUROG3. As NEUROG3 plays a central role in endocrine differentiation, knowledge gained from our study will accelerate the development of beta cell regeneration therapies to treat diabetes.

Human Pluripotent Stem Cells to Model Islet Defects in Diabetes

Diabetes mellitus is characterized by elevated levels of blood glucose and is ultimately caused by insufficient insulin production from pancreatic beta cells. Different research models have been utilized to unravel the molecular mechanisms leading to the onset of diabetes. The generation of pancreatic endocrine cells from human pluripotent stem cells constitutes an approach to study genetic defects leading to impaired beta cell development and function. Here, we review the recent progress in generating and characterizing functional stem cell-derived beta cells. We summarize the diabetes disease modeling possibilities that stem cells offer and the challenges that lie ahead to further improve these models.

Functional Genomics in Pancreatic β Cells: Recent Advances in Gene Deletion and Genome Editing Technologies for Diabetes Research

The inheritance of variants that lead to coding changes in, or the mis-expression of, genes critical to pancreatic beta cell function can lead to alterations in insulin secretion and increase the risk of both type 1 and type 2 diabetes. Recently developed clustered regularly interspaced short palindromic repeats (CRISPR/Cas9) gene editing tools provide a powerful means of understanding the impact of identified variants on cell function, growth, and survival and might ultimately provide a means, most likely after the transplantation of genetically "corrected" cells, of treating the disease. Here, we review some of the disease-associated genes and variants whose roles have been probed up to now. Next, we survey recent exciting developments in CRISPR/Cas9 technology and their possible exploitation for β cell functional genomics. Finally, we will provide a perspective as to how CRISPR/Cas9 technology may find clinical application in patients with diabetes.

Transcription factors that shape the mammalian pancreas

Improving our understanding of mammalian pancreas development is crucial for the development of more effective cellular therapies for diabetes. Most of what we know about mammalian pancreas development stems from mouse genetics. We have learnt that a unique set of transcription factors controls endocrine and exocrine cell differentiation. Transgenic mouse models have been instrumental in studying the function of these transcription factors. Mouse and human pancreas development are very similar in many respects, but the devil is in the detail. To unravel human pancreas development in greater detail, in vitro cellular models (including directed differentiation of stem cells, human beta cell lines and human pancreatic organoids) are used; however, in vivo validation of these results is still needed. The current best 'model' for studying human pancreas development are individuals with monogenic forms of diabetes. In this review, we discuss mammalian pancreas development, highlight some discrepancies between mouse and human, and discuss selected transcription factors that, when mutated, cause permanent neonatal diabetes. Graphical abstract.

FOXA2 Is Required for Enhancer Priming during Pancreatic Differentiation

Transcriptional regulatory mechanisms of lineage priming in embryonic development are largely uncharacterized because of the difficulty of isolating transient progenitor populations. Directed differentiation of human pluripotent stem cells (hPSCs) combined with gene editing provides a powerful system to define precise temporal gene requirements for progressive chromatin changes during cell fate transitions. Here, we map the dynamic chromatin landscape associated with sequential stages of pancreatic differentiation from hPSCs. Our analysis of chromatin accessibility dynamics led us to uncover a requirement for FOXA2, known as a pioneer factor, in human pancreas specification not previously shown from mouse knockout studies. FOXA2 knockout hPSCs formed reduced numbers of pancreatic progenitors accompanied by impaired recruitment of GATA6 to pancreatic enhancers. Furthermore, FOXA2 is required for proper chromatin remodeling and H3K4me1 deposition during enhancer priming. This work highlights the power of combining hPSC differentiation, genome editing, and computational genomics for discovering transcriptional mechanisms during development.

Enteroendocrine cells couple nutrient sensing to nutrient absorption by regulating ion transport

The ability to absorb ingested nutrients is an essential function of all metazoans and utilizes a wide array of nutrient transporters found on the absorptive enterocytes of the small intestine. A unique population of patients has previously been identified with severe congenital malabsorptive diarrhea upon ingestion of any enteral nutrition. The intestines of these patients are macroscopically normal, but lack enteroendocrine cells (EECs), suggesting an essential role for this rare population of nutrient-sensing cells in regulating macronutrient absorption. Here, we use human and mouse models of EEC deficiency to identify an unappreciated role for the EEC hormone peptide YY in regulating ion-coupled absorption of glucose and dipeptides. We find that peptide YY is required in the small intestine to maintain normal electrophysiology in the presence of vasoactive intestinal polypeptide, a potent stimulator of ion secretion classically produced by enteric neurons. Administration of peptide YY to EEC-deficient mice restores normal electrophysiology, improves glucose and peptide absorption, diminishes diarrhea and rescues postnatal survival. These data suggest that peptide YY is a key regulator of macronutrient absorption in the small intestine and may be a viable therapeutic option to treat patients with electrolyte imbalance and nutrient malabsorption.

Enteroendocrine cells (EECs) are specialized gastrointestinal cells that have a role in nutrient sensing and hormone secretion. Here the authors show that peptide YY from EECs regulates nutrient absorption in intestinal organoids.

Reduced Neurog3 Gene Dosage Shifts Enteroendocrine Progenitor Towards Goblet Cell Lineage in the Mouse Intestine

BACKGROUND & AIMS

Transient expression of Neurog3 commits intestinal secretory progenitors to become enteroendocrine-biased progenitors and hence drive enteroendocrine differentiation. Loss of Neurog3 in mouse resulted in the depletion of intestinal enteroendocrine cells (EECs) and an increase in goblet cells. Earlier studies in developing mouse pancreas identified a role of Neurog3 gene dosage in endocrine and exocrine cell fate allocation. We aimed to determine whether Neurog3 gene dosage controls fate choice of enteroendocrine progenitors.

METHODS

We acquired mutant Neurog3 reporter mice carrying 2, 1, or null Neurog3 alleles to study Neurog3 gene dosage effect by lineage tracing. Cell types arising from Neurog3+ progenitors were determined by immunohistochemistry using antibodies against intestinal lineage-specific markers. RNA sequencing of sorted Neurog3^+/+, Neurog3^+/-, or bulk intestinal cells were performed and differentially expressed genes were analyzed.

RESULTS

We identified 2731 genes enriched in sorted Neurog3^+/+-derived cells in the Neurog3^+/+EYFP mouse intestine when compared with bulk duodenum epithelial cells. In the intestine of Neurog3^+/-EGFP heterozygous mouse, we observed a 63% decrease in EEC numbers. Many Neurog3-derived cells stained for goblet marker Mucin 2. RNA sequencing of sorted Neurog3^+/- cells uncovered enriched expression of genes characteristic for both goblet and enteroendocrine cells, indicating the mixed lineages arose from Neurog3+ progenitors. Consistent with this hypothesis, deletion of both Neurog3 alleles resulted in the total absence of EECs. All Neurog3⁺-derived cells stained for Mucin 2.

CONCLUSIONS

We identified that the fate of Neurog3⁺ enteroendocrine progenitors is dependent on Neurog3 gene dosage. High Neurog3 gene dosage enforces the commitment of secretory progenitors to an EE lineage, while constraining their goblet cell lineage potential. Transcriptome profiling data was deposited to Gene Ontology omnibus, accession number: GSE149203.

Enteric anendocrinosis attributable to a novel Neurogenin-3 variant

Congenital diarrhea and enteropathies (CODEs) are a group of monogenic disorders that often present with severe diarrhea in the first weeks of life. Enteric anendocrinosis (EA), an extremely rare cause of CODE, is characterized by a marked reduction of intestinal enteroendocrine cells (EC). EA is associated with recessively inherited variants in Neurogenin-3 (NEUROG3) gene. Here we investigate a case of a male infant who presented with mysterious severe malabsorptive diarrhea since birth. Thorough clinical assessments and laboratory tests were successful to exclude the majority of differential diagnosis categories. However, the patient's diagnosis was not established until the genetic test using whole-exome sequencing (WES) was performed. We identified a novel homozygous missense disease-causing variant (DCV) in NEUROG3 (c.413C>G, p.Thr138Arg). Moreover, molecular dynamic simulation analysis showed that (p.Thr138Arg) led to a global change of the NEUROG3 orientation affecting its DNA binding capacity. To the best of our knowledge, this is the first time to apply WES to reach a differential diagnosis of patients with CODEs. Our study not only expands our knowledge about NEUROG3 variants and their clinical consequences but also proves that WES is a very effective tool for the diagnosis of CODEs. This might be of value in early diagnosis of diseases and prenatal CODEs detection.

Animal Models of Pancreas Development, Developmental Disorders, and Disease

The pancreas is a glandular organ responsible for diverse homeostatic functions, including hormone production from the endocrine islet cells to regulate blood sugar levels and enzyme secretion from the exocrine acinar cells to facilitate food digestion. These pancreatic functions are essential for life; therefore, preserving pancreatic function is of utmost importance. Pancreas dysfunction can arise either from developmental disorders or adult onset disease, both of which are caused by defects in shared molecular pathways. In this chapter, we discuss what is known about the molecular mechanisms controlling pancreas development, how disruption of these mechanisms can lead to developmental defects and disease, and how essential pancreas functions can be modeled using human pluripotent stem cells. At the core of understanding of these molecular processes are animal model studies that continue to be essential for elucidating the mechanisms underlying human pancreatic functions and diseases.

Understanding generation and regeneration of pancreatic β cells from a single-cell perspective

Understanding the mechanisms that underlie the generation and regeneration of β cells is crucial for developing treatments for diabetes. However, traditional research methods, which are based on populations of cells, have limitations for defining the precise processes of β-cell differentiation and trans-differentiation, and the associated regulatory mechanisms. The recent development of single-cell technologies has enabled re-examination of these processes at a single-cell resolution to uncover intermediate cell states, cellular heterogeneity and molecular trajectories of cell fate specification. Here, we review recent advances in understanding β-cell generation and regeneration, in vivo and in vitro, from single-cell technologies, which could provide insights for optimization of diabetes therapy strategies.

Insights into pancreatic islet cell dysfunction from type 2 diabetes mellitus genetics

Type 2 diabetes mellitus (T2DM) is an increasingly prevalent multifactorial disease that has both genetic and environmental risk factors, resulting in impaired glucose homeostasis. Genome-wide association studies (GWAS) have identified over 400 genetic signals that are associated with altered risk of T2DM. Human physiology and epigenomic data support a central role for the pancreatic islet in the pathogenesis of T2DM. This Review focuses on the promises and challenges of moving from genetic associations to molecular mechanisms and highlights efforts to identify the causal variant and effector transcripts at T2DM GWAS susceptibility loci. In addition, we examine current human models that are used to study both β-cell development and function, including EndoC-β cell lines and human induced pluripotent stem cell-derived β-like cells. We use examples of four T2DM susceptibility loci (CDKAL1, MTNR1B, SLC30A8 and PAM) to emphasize how a holistic approach involving genetics, physiology, and cellular and developmental biology can disentangle disease mechanisms at T2DM GWAS signals.

Transient FOXO1 inhibition in pancreatic endoderm promotes the generation of NGN3+ endocrine precursors from human iPSCs

In the multi-step differentiation protocol used to generate pancreatic endocrine cells from human pluripotent stem cells, the induction of NGN3+ endocrine precursors from the PDX1+/NKX6.1+ pancreatic endoderm is crucial for efficient endocrine cell production. Here, we demonstrate that transient, not prolonged FOXO1 inhibition results in enhanced NGN3+ endocrine precursors and hormone-producing cell production. FOXO1 inhibition does not directly induce NGN3 expression but stimulates PDX1+/NKX6.1+ cell proliferation. NOTCH activity, whose suppression is important for Ngn3 expression, is not suppressed but Wnt signaling is stimulated by FOXO1 inhibition. Reversely, Wnt inhibition suppresses the effects of FOXO1 inhibitor. These findings indicate that FOXO1 and Wnt are involved in regulating the proliferation of PDX1+/NKX6.1+ pancreatic endoderm that gives rise to NGN3+ endocrine precursors.

From Genetic Association to Molecular Mechanisms for Islet-cell Dysfunction in Type 2 Diabetes

Genome-wide association studies (GWAS) have identified over 400 signals robustly associated with risk for type 2 diabetes (T2D). At the vast majority of these loci, the lead single nucleotide polymorphisms (SNPs) reside in noncoding regions of the genome, which hampers biological inference and translation of genetic discoveries into disease mechanisms. The study of these T2D risk variants in normoglycemic individuals has revealed that a significant proportion are exerting their disease risk through islet-cell dysfunction. The central role of the islet is also demonstrated by numerous studies, which have shown an enrichment of these signals in islet-specific epigenomic annotations. In recent years the emergence of authentic human beta-cell lines, and advances in genome-editing technologies coupled with improved protocols differentiating human pluripotent stem cells into beta-like cells has opened up new opportunities for T2D disease modeling. Here we review the current understanding on the genetic basis of T2D focusing on approaches, which have facilitated the identification of causal variants and their effector transcripts in human islets. We will present examples of functional studies based on animal and conventional cellular systems and highlight the potential of novel stem cell-based T2D disease models.

Null mutations of NEUROG3 are associated with delayed-onset diabetes mellitus

Biallelic mutations of the gene encoding the transcription factor NEUROG3 are associated with a rare disorder that presents in neonates as generalized malabsorption - due to a complete absence of enteroendocrine cells - followed, in early childhood or beyond, by insulin-dependent diabetes mellitus (IDDM). The commonly delayed onset of IDDM suggests a differential requirement for NEUROG3 in endocrine cell generation in the human pancreas versus the intestine. However, previously identified human mutations were hypomorphic and, hence, may have had residual function in pancreas. We report 2 patients with biallelic functionally null variants of the NEUROG3 gene who nonetheless did not present with IDDM during infancy but instead developed permanent IDDM during middle childhood ages. The variants showed no evidence of function in traditional promoter-based assays of NEUROG3 function and also failed to exhibit function in a variety of potentially novel in vitro and in vivo molecular assays designed to discern residual NEUROG3 function. These findings imply that, unlike in mice, pancreatic endocrine cell generation in humans is not entirely dependent on NEUROG3 expression and, hence, suggest the presence of unidentified redundant in vivo pathways in human pancreas capable of yielding β cell mass sufficient to maintain euglycemia until early childhood.

Insights Into Human Development and Disease From Human Pluripotent Stem Cell Derived Intestinal Organoids

In recent years, advances in human pluripotent stem cell (hPSC) biology have enabled the generation of gastrointestinal (GI) organoids which recapitulate aspects of normal organ development. HPSC derived gastrointestinal organoids are comprised of epithelium and mesenchyme and have a remarkable ability to self-organize and recapitulate early stages of human intestinal development. Furthermore, hPSC derived organoids can be transplanted into immunocompromised mice which allows further maturation of both the epithelium and mesenchyme. In this review, we will briefly summarize work from model systems which has elucidated mechanisms of GI patterning and how these insights have been used to guide the differentiation of hPSCs into organoids resembling small intestine and colon. We will succinctly discuss how developmental principles have been used to promote maturation of human intestinal organoids (HIOs) in vitro as well as to introduce an enteric nervous system into HIOs. We will then concisely review how organoids have been used to study human pathogens, how new genetic and bioengineering tools are being applied to organoid research, and how this integration has allowed researchers to elucidate mechanisms of human development and disease. Finally, we will briefly discuss remaining challenges in the field and how they can be addressed. HPSC derived organoids are promising new model systems which hold the potential of unlocking unknown mechanisms of human gastrointestinal development and disease.

Pancreatic β cell regeneration: To β or not to β

Diabetes is a major worldwide health problem which results from the loss and/or dysfunction of pancreatic insulin-producing β cells in the pancreas. Therefore, there is great interest in understanding the endogenous capacity of β cells to regenerate under normal or pathological conditions, with the goal of restoring functional β cell mass in patients with diabetes. Here, we summarize the current status of β cell regeneration research, which has been broadly divided into three in vivo mechanisms: 1. proliferation of existing β cells; 2. neogenesis of β cells from adult ductal progenitors; and 3. transdifferentiation of other cell types into β cells. We discuss the evidence and controversies for each mechanism in mice and humans, as well as the prospect of using these approaches for the treatment of diabetes.

A Network of microRNAs Acts to Promote Cell Cycle Exit and Differentiation of Human Pancreatic Endocrine Cells

Pancreatic endocrine cell differentiation is orchestrated by the action of transcription factors that operate in a gene regulatory network to activate endocrine lineage genes and repress lineage-inappropriate genes. MicroRNAs (miRNAs) are important modulators of gene expression, yet their role in endocrine cell differentiation has not been systematically explored. Here we characterize miRNA-regulatory networks active in human endocrine cell differentiation by combining small RNA sequencing, miRNA over-expression, and network modeling approaches. Our analysis identified Let-7g, Let-7a, miR-200a, miR-127, and miR-375 as endocrine-enriched miRNAs that drive endocrine cell differentiation-associated gene expression changes. These miRNAs are predicted to target different transcription factors, which converge on genes involved in cell cycle regulation. When expressed in human embryonic stem cell-derived pancreatic progenitors, these miRNAs induce cell cycle exit and promote endocrine cell differentiation. Our study delineates the role of miRNAs in human endocrine cell differentiation and identifies miRNAs that could facilitate endocrine cell reprogramming.

Human stem cell models: lessons for pancreatic development and disease

A comprehensive understanding of mechanisms that underlie the development and function of human cells requires human cell models. For the pancreatic lineage, protocols have been developed to differentiate human pluripotent stem cells (hPSCs) into pancreatic endocrine and exocrine cells through intermediates resembling in vivo development. In recent years, this differentiation system has been employed to decipher mechanisms of pancreatic development, congenital defects of the pancreas, as well as genetic forms of diabetes and exocrine diseases. In this review, we summarize recent insights gained from studies of pancreatic hPSC models. We discuss how genome-scale analyses of the differentiation system have helped elucidate roles of chromatin state, transcription factors, and noncoding RNAs in pancreatic development and how the analysis of cells with disease-relevant mutations has provided insight into the molecular underpinnings of genetically determined diseases of the pancreas.