Myt1 and Ngn3 form a feed-forward expression loop to promote endocrine islet cell differentiation

Protein arginine methyltransferase 1 regulates mouse enteroendocrine cell development and homeostasis

BACKGROUND

The adult intestinal epithelium is a complex, self-renewing tissue composed of specialized cell types with diverse functions. Intestinal stem cells (ISCs) located at the bottom of crypts, where they divide to either self-renew, or move to the transit amplifying zone to divide and differentiate into absorptive and secretory cells as they move along the crypt-villus axis. Enteroendocrine cells (EECs), one type of secretory cells, are the most abundant hormone-producing cells in mammals and involved in the control of energy homeostasis. However, regulation of EEC development and homeostasis is still unclear or controversial. We have previously shown that protein arginine methyltransferase (PRMT) 1, a histone methyltransferase and transcription co-activator, is important for adult intestinal epithelial homeostasis.

RESULTS

To investigate how PRMT1 affects adult intestinal epithelial homeostasis, we performed RNA-Seq on small intestinal crypts of tamoxifen-induced intestinal epithelium-specific PRMT1 knockout and PRMT1fl/fl adult mice. We found that PRMT1fl/fl and PRMT1-deficient small intestinal crypts exhibited markedly different mRNA profiles. Surprisingly, GO terms and KEGG pathway analyses showed that the topmost significantly enriched pathways among the genes upregulated in PRMT1 knockout crypts were associated with EECs. In particular, genes encoding enteroendocrine-specific hormones and transcription factors were upregulated in PRMT1-deficient small intestine. Moreover, a marked increase in the number of EECs was found in the PRMT1 knockout small intestine. Concomitantly, Neurogenin 3-positive enteroendocrine progenitor cells was also increased in the small intestinal crypts of the knockout mice, accompanied by the upregulation of the expression levels of downstream targets of Neurogenin 3, including Neuod1, Pax4, Insm1, in PRMT1-deficient crypts.

CONCLUSIONS

Our finding for the first time revealed that the epigenetic enzyme PRMT1 controls mouse enteroendocrine cell development, most likely via inhibition of Neurogenin 3-mediated commitment to EEC lineage. It further suggests a potential role of PRMT1 as a critical transcriptional cofactor in EECs specification and homeostasis to affect metabolism and metabolic diseases.

Regulation of multiple signaling pathways promotes the consistent expansion of human pancreatic progenitors in defined conditions

The unlimited expansion of human progenitor cells in vitro could unlock many prospects for regenerative medicine. However, it remains an important challenge as it requires the decoupling of the mechanisms supporting progenitor self-renewal and expansion from those mechanisms promoting their differentiation. This study focuses on the expansion of human pluripotent stem (hPS) cell-derived pancreatic progenitors (PP) to advance novel therapies for diabetes. We obtained mechanistic insights into PP expansion requirements and identified conditions for the robust and unlimited expansion of hPS cell-derived PP cells under GMP-compliant conditions through a hypothesis-driven iterative approach. We show that the combined stimulation of specific mitogenic pathways, suppression of retinoic acid signaling, and inhibition of selected branches of the TGFβ and Wnt signaling pathways are necessary for the effective decoupling of PP proliferation from differentiation. This enabled the reproducible, 2000-fold, over 10 passages and 40-45 d, expansion of PDX1+/SOX9+/NKX6-1+ PP cells. Transcriptome analyses confirmed the stabilization of PP identity and the effective suppression of differentiation. Using these conditions, PDX1+/SOX9+/NKX6-1+ PP cells, derived from different, both XY and XX, hPS cell lines, were enriched to nearly 90% homogeneity and expanded with very similar kinetics and efficiency. Furthermore, non-expanded and expanded PP cells, from different hPS cell lines, were differentiated in microwells into homogeneous islet-like clusters (SC-islets) with very similar efficiency. These clusters contained abundant β-cells of comparable functionality as assessed by glucose-stimulated insulin secretion assays. These findings established the signaling requirements to decouple PP proliferation from differentiation and allowed the consistent expansion of hPS cell-derived PP cells. They will enable the establishment of large banks of GMP-produced PP cells derived from diverse hPS cell lines. This approach will streamline SC-islet production for further development of the differentiation process, diabetes research, personalized medicine, and cell therapies.

Single-cell transcriptome and accessible chromatin dynamics during endocrine pancreas development

Delineating gene regulatory networks that orchestrate cell-type specification is a continuing challenge for developmental biologists. Single-cell analyses offer opportunities to address these challenges and accelerate discovery of rare cell lineage relationships and mechanisms underlying hierarchical lineage decisions. Here, we describe the molecular analysis of mouse pancreatic endocrine cell differentiation using single-cell transcriptomics, chromatin accessibility assays coupled to genetic labeling, and cytometry-based cell purification. We uncover transcription factor networks that delineate β-, α-, and δ-cell lineages. Through genomic footprint analysis, we identify transcription factor-regulatory DNA interactions governing pancreatic cell development at unprecedented resolution. Our analysis suggests that the transcription factor Neurog3 may act as a pioneer transcription factor to specify the pancreatic endocrine lineage. These findings could improve protocols to generate replacement endocrine cells from renewable sources, like stem cells, for diabetes therapy.

Genetic alterations in gastric amphicrine carcinomas and comparison with gastric mixed neuroendocrine-non-neuroendocrine neoplasms

Gastric amphicrine carcinoma, in which endocrine and epithelial cell features are present within the same cells, is often confused with gastric mixed neuroendocrine-non-neuroendocrine neoplasm (MiNEN). In this study, we performed high-resolution copy number (CN) profiling and whole exome sequencing (WES) of formalin-fixed and paraffin-embedded (FFPE) tissues from eight gastric amphicrine carcinomas and compared the molecular features with those of the adenocarcinoma and neuroendocrine carcinoma (NEC) components of eight gastric MiNENs. The most frequent high-level CN variant was a gain of 20q13.12-20q13.2, which was found in five gastric amphicrine carcinomas. Amplifications of MYT1, NTSR1, and ZBTB46 located in this region were demonstrated by qPCR and immunohistochemistry. The CN characteristics of gastric amphicrine carcinomas were different from those of MiNENs in hierarchical clustering analysis, suggesting that amphicrine carcinoma is a separate entity from MiNEN. Moreover, the CN level of C5 (complement C5) was higher in amphicrine carcinoma than in both the adenocarcinoma and the NEC component of MiNENs, suggesting that amphicrine carcinomas might benefit more from C5 inhibitors than MiNENs. WES showed frequent somatic mutations of TP53 (37.5%, 3/8) and APC (25.0%, 2/8) in amphicrine carcinoma. There were no specific mutation characteristics to distinguish amphicrine carcinoma from MiNEN. An integrated KEGG pathway analysis showed that the estrogen signaling pathway was enriched in amphicrine carcinomas, which might be associated with the high morbidity of male patients. In summary, our study revealed the unique CN and mutation characteristics of gastric amphicrine carcinoma and differentiated these characteristics from those of MiNENs. These data provide a foundation for further studies on the development and progression of amphicrine carcinoma.

SDF-1 inhibits the dedifferentiation of islet β cells in hyperglycaemia by up-regulating FoxO1 via binding to CXCR4

Islet β cell dedifferentiation is one of the most important mechanisms in the occurrence and development of diabetes. We studied the possible effects of chemokine stromal cell-derived factor-1 (SDF-1) in the dedifferentiation of islet β cells. It was noted that the number of dedifferentiated islet β cells and the expression of SDF-1 in pancreatic tissues significantly increased with diabetes. In islet β cell experiments, inhibition of SDF-1 expression resulted in an increase in the number of dedifferentiated cells, while overexpression of SDF-1 resulted in a decrease. This seemed to be contradicted by the effect of diabetes on the expression of SDF-1 in pancreatic tissue, but it was concluded that this may be related to the loss of SDF-1 activity. SDF-1 binds to CXCR4 to form a complex, which activates and phosphorylates AKT, subsequently increases the expression of forkhead box O1 (FOXO1), and inhibits the dedifferentiation of islet β cells. This suggests that SDF-1 may be a novel target in the treatment of diabetes.

Insights into the etiology and physiopathology of MODY5/HNF1B pancreatic phenotype with a mouse model of the human disease

Maturity-onset diabetes of the young type 5 (MODY5) is due to heterozygous mutations or deletion of HNF1B. No mouse models are currently available to recapitulate the human MODY5 disease. Here, we investigate the pancreatic phenotype of a unique MODY5 mouse model generated by heterozygous insertion of a human HNF1B splicing mutation at the intron-2 splice donor site in the mouse genome. This Hnf1bsp2/+ model generated with targeted mutation of Hnf1b mimicking the c.544+1G>T (T) mutation identified in humans, results in alternative transcripts and a 38% decrease of native Hnf1b transcript levels. As a clinical feature of MODY5 patients, the hypomorphic mouse model Hnf1bsp2/+ displays glucose intolerance. Whereas Hnf1bsp2/+ isolated islets showed no altered insulin secretion, we found a 65% decrease in pancreatic insulin content associated with a 30% decrease in total large islet volume and a 20% decrease in total β-cell volume. These defects were associated with a 30% decrease in expression of the pro-endocrine gene Neurog3 that we previously identified as a direct target of Hnf1b, showing a developmental etiology. As another clinical feature of MODY5 patients, the Hnf1bsp2/+ pancreases display exocrine dysfunction with hypoplasia. We observed chronic pancreatitis with loss of acinar cells, acinar-to-ductal metaplasia, and lipomatosis, with upregulation of signaling pathways and impaired acinar cell regeneration. This was associated with ductal cell deficiency characterized by shortened primary cilia. Importantly, the Hnf1bsp2/+ mouse model reproduces the pancreatic features of the human MODY5/HNF1B disease, providing a unique in vivo tool for molecular studies of the endocrine and exocrine defects and to advance basic and translational research. © 2021 The Authors. The Journal of Pathology published by John Wiley & Sons, Ltd. on behalf of The Pathological Society of Great Britain and Ireland.

Exosome-Mediated Differentiation of Mouse Embryonic Fibroblasts and Exocrine Cells into β-Like Cells and the Identification of Key miRNAs for Differentiation

Diabetes is a concerning health malady worldwide. Islet or pancreas transplantation is the only long-term treatment available; however, the scarcity of transplantable tissues hampers this approach. Therefore, new cell sources and differentiation approaches are required. Apart from the genetic- and small molecule-based approaches, exosomes could induce cellular differentiation by means of their cargo, including miRNA. We developed a chemical-based protocol to differentiate mouse embryonic fibroblasts (MEFs) into β-like cells and employed mouse insulinoma (MIN6)-derived exosomes in the presence or absence of specific small molecules to encourage their differentiation into β-like cells. The differentiated β-like cells were functional and expressed pancreatic genes such as Pdx1, Nkx6.1, and insulin 1 and 2. We found that the exosome plus small molecule combination differentiated the MEFs most efficiently. Using miRNA-sequencing, we identified miR-127 and miR-709, and found that individually and in combination, the miRNAs differentiated MEFs into β-like cells similar to the exosome treatment. We also confirmed that exocrine cells can be differentiated into β-like cells by exosomes and the exosome-identified miRNAs. A new differentiation approach based on the use of exosome-identified miRNAs could help people afflicted with diabetes

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.

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.

Small Molecule-Induced Pancreatic β-Like Cell Development: Mechanistic Approaches and Available Strategies

Diabetes is a metabolic disease which affects not only glucose metabolism but also lipid and protein metabolism. It encompasses two major types: type 1 and 2 diabetes. Despite the different etiologies of type 1 and 2 diabetes mellitus (T1DM and T2DM, respectively), the defining features of the two forms are insulin deficiency and resistance, respectively. Stem cell therapy is an efficient method for the treatment of diabetes, which can be achieved by differentiating pancreatic β-like cells. The consistent generation of glucose-responsive insulin releasing cells remains challenging. In this review article, we present basic concepts of pancreatic organogenesis, which intermittently provides a basis for engineering differentiation procedures, mainly based on the use of small molecules. Small molecules are more auspicious than any other growth factors, as they have unique, valuable properties like cell-permeability, as well as a nonimmunogenic nature; furthermore, they offer immense benefits in terms of generating efficient functional beta-like cells. We also summarize advances in the generation of stem cell-derived pancreatic cell lineages, especially endocrine β-like cells or islet organoids. The successful induction of stem cells depends on the quantity and quality of available stem cells and the efficient use of small molecules.

Adaptive Landscape Shaped by Core Endogenous Network Coordinates Complex Early Progenitor Fate Commitments in Embryonic Pancreas

The classical development hierarchy of pancreatic cell fate commitments describes that multipotent progenitors (MPs) first bifurcate into tip cells and trunk cells, and then these cells give rise to acinar cells and endocrine/ductal cells separately. However, lineage tracings reveal that pancreatic progenitors are highly heterogeneous in tip and trunk domains in embryonic pancreas. The progenitor fate commitments from multipotency to unipotency during early pancreas development is insufficiently characterized. In pursuing a mechanistic understanding of the complexity in progenitor fate commitments, we construct a core endogenous network for pancreatic lineage decisions based on genetic regulations and quantified its intrinsic dynamic properties using dynamic modeling. The dynamics reveal a developmental landscape with high complexity that has not been clarified. Not only well-characterized pancreatic cells are reproduced, but also previously unrecognized progenitors-tip progenitor (TiP), trunk progenitor (TrP), later endocrine progenitor (LEP), and acinar progenitors (AciP/AciP2) are predicted. Further analyses show that TrP and LEP mediate endocrine lineage maturation, while TiP, AciP, AciP2 and TrP mediate acinar and ductal lineage maturation. The predicted cell fate commitments are validated by analyzing single-cell RNA sequencing (scRNA-seq) data. Significantly, this is the first time that a redefined hierarchy with detailed early pancreatic progenitor fate commitment is obtained.

A Comprehensive Structure-Function Study of Neurogenin3 Disease-Causing Alleles during Human Pancreas and Intestinal Organoid Development

Neurogenin3 (NEUROG3) is required for endocrine lineage formation of the pancreas and intestine. Patients with NEUROG3 mutations are born with congenital malabsorptive diarrhea due to complete loss of enteroendocrine cells, whereas endocrine pancreas development varies in an allele-specific manner. These findings suggest a context-dependent requirement for NEUROG3 in pancreas versus intestine. We utilized human tissue differentiated from NEUROG3^-/- pluripotent stem cells for functional analyses. Most disease-associated alleles had hypomorphic or null phenotype in both tissues, whereas the S171fsX68 mutation had reduced activity in the pancreas but largely null in the intestine. Biochemical studies revealed NEUROG3 variants have distinct molecular defects with altered protein stability, DNA binding, and gene transcription. Moreover, NEUROG3 was highly unstable in the intestinal epithelium, explaining the enhanced sensitivity of intestinal defects relative to the pancreas. These studies emphasize that studies of human mutations in the endogenous tissue context may be required to assess structure-function relationships.

Identification of Enteroendocrine Regulators by Real-Time Single-Cell Differentiation Mapping

Homeostatic regulation of the intestinal enteroendocrine lineage hierarchy is a poorly understood process. We resolved transcriptional changes during enteroendocrine differentiation in real time at single-cell level using a novel knockin allele of Neurog3, the master regulator gene briefly expressed at the onset of enteroendocrine specification. A bi-fluorescent reporter, Neurog3Chrono, measures time from the onset of enteroendocrine differentiation and enables precise positioning of single-cell transcriptomes along an absolute time axis. This approach yielded a definitive description of the enteroendocrine hierarchy and its sub-lineages, uncovered differential kinetics between sub-lineages, and revealed time-dependent hormonal plasticity in enterochromaffin and L cells. The time-resolved map of transcriptional changes predicted multiple novel molecular regulators. Nine of these were validated by conditional knockout in mice or CRISPR modification in intestinal organoids. Six novel candidate regulators (Sox4, Rfx6, Tox3, Myt1, Runx1t1, and Zcchc12) yielded specific enteroendocrine phenotypes. Our time-resolved single-cell transcriptional map presents a rich resource to unravel enteroendocrine differentiation.

Neurog3-Independent Methylation Is the Earliest Detectable Mark Distinguishing Pancreatic Progenitor Identity

In the developing pancreas, transient Neurog3-expressing progenitors give rise to four major islet cell types: α, β, δ, and γ; when and how the Neurog3+ cells choose cell fate is unknown. Using single-cell RNA-seq, trajectory analysis, and combinatorial lineage tracing, we showed here that the Neurog3+ cells co-expressing Myt1 (i.e., Myt1+Neurog3+) were biased toward β cell fate, while those not simultaneously expressing Myt1 (Myt1-Neurog3+) favored α fate. Myt1 manipulation only marginally affected α versus β cell specification, suggesting Myt1 as a marker but not determinant for islet-cell-type specification. The Myt1+Neurog3+ cells displayed higher Dnmt1 expression and enhancer methylation at Arx, an α-fate-promoting gene. Inhibiting Dnmts in pancreatic progenitors promoted α cell specification, while Dnmt1 overexpression or Arx enhancer hypermethylation favored β cell production. Moreover, the pancreatic progenitors contained distinct Arx enhancer methylation states without transcriptionally definable sub-populations, a phenotype independent of Neurog3 activity. These data suggest that Neurog3-independent methylation on fate-determining gene enhancers specifies distinct endocrine-cell programs.

Antidiabetic Effects of the Ethanolic Root Extract of Uvaria chamae P. Beauv (Annonaceae) in Alloxan-Induced Diabetic Rats: A Potential Alternative Treatment for Diabetes Mellitus

Diabetes mellitus has been a menace to mankind from time immemorial. However, a natural product such as U. chamae P. Beauv (Annonaceae) offers alternative treatment for diabetes mellitus. The study aimed at evaluating antidiabetic activity of the ethanolic root extract of U. chamae in alloxan-induced diabetic rats. Diabetes was induced in Sprague Dawley rats after overnight fast with 150 mg/kg alloxan intraperitoneally. After 72 h, those with plasma glucose levels >200 mg/dl were classified as diabetic. Five diabetic rats in each group were treated daily for 14 days orally with 100, 250, and 400 mg/kg of the extract, glibenclamide (71 µg/kg) and pioglitazone (429 µg/kg), respectively, while another group was untreated. Control received 0.5 ml of Acacia senegal. Effects of extract on glucose, other biochemical, and hematological parameters were evaluated. α-amylase and α-glucosidase inhibitory activities of extract and its fractions were also evaluated. Percentage inhibition and IC₅₀ values were determined. Diabetic control was achieved on the 7th day of the study with 100, 250, and 400 mg/kg of the extract showing glucose reduction of 72.14%, 78.75%, and 87.71%, respectively. The HDL-cholesterol levels of diabetic rats treated with extracts were significantly increased. Extract and its fractions caused α-amylase and α-glucosidase inhibition. Histologically, pancreas of diabetic rats treated with extract showed regenerated islet cells which were not seen in rats treated with glibenclamide and pioglitazone. This study showed that U. chamae has antidiabetic activity which may be through α-amylase and α-glucosidase inhibition and regeneration of pancreatic beta cells. Also, it may reduce the risk of cardiovascular disease by increasing HDL-cholesterol levels.

ROCK-nmMyoII, Notch and Neurog3 gene-dosage link epithelial morphogenesis with cell fate in the pancreatic endocrine-progenitor niche

During mouse pancreas organogenesis, endocrine cells are born from progenitors residing in an epithelial plexus niche. After a period in a lineage-primed Neurog3^LO state, progenitors become endocrine committed via upregulation of Neurog3 We find that the Neurog3^LO to Neurog3^HI transition is associated with distinct stages of an epithelial egression process: narrowing the apical surface of the cell, basalward cell movement and eventual cell-rear detachment from the apical lumen surface to allow clustering as nascent islets under the basement membrane. Apical narrowing, basalward movement and Neurog3 transcriptional upregulation still occur without Neurog3 protein, suggesting that morphogenetic cues deployed within the plexus initiate endocrine commitment upstream or independently of Neurog3. Neurog3 is required for cell-rear detachment and complete endocrine-cell birth. The ROCK-nmMyoII pathway coordinates epithelial-cell morphogenesis and the progression through Neurog3-expressing states. NmMyoII is necessary for apical narrowing, basalward cell displacement and Neurog3 upregulation, but all three are limited by ROCK activity. We propose that ROCK-nmMyoII activity, Neurog3 gene-dose and Notch signaling integrate endocrine fate allocation with epithelial plexus growth and morphogenesis, representing a feedback control circuit that coordinates morphogenesis with lineage diversification in the endocrine-birth niche.

Neurog3-dependent pancreas dysgenesis causes ectopic pancreas in Hes1 mutant mice

Mutations in Hes1, a target gene of the Notch signalling pathway, lead to ectopic pancreas by a poorly described mechanism. Here, we use genetic inactivation of Hes1 combined with lineage tracing and live imaging to reveal an endodermal requirement for Hes1, and show that ectopic pancreas tissue is derived from the dorsal pancreas primordium. RNA-seq analysis of sorted E10.5 Hes1^+/+ and Hes1^-/- Pdx1-GFP⁺ cells suggested that upregulation of endocrine lineage genes in Hes1^-/- embryos was the major defect and, accordingly, early pancreas morphogenesis was normalized, and the ectopic pancreas phenotype suppressed, in Hes1^-/-Neurog3^-/- embryos. In Mib1 mutants, we found a near total depletion of dorsal progenitors, which was replaced by an anterior Gcg⁺ extension. Together, our results demonstrate that aberrant morphogenesis is the cause of ectopic pancreas and that a part of the endocrine differentiation program is mechanistically involved in the dysgenesis. Our results suggest that the ratio of endocrine lineage to progenitor cells is important for morphogenesis and that a strong endocrinogenic phenotype without complete progenitor depletion, as seen in Hes1 mutants, provokes an extreme dysgenesis that causes ectopic pancreas.

Neurog3 misexpression unravels mouse pancreatic ductal cell plasticity

In the context of type 1 diabetes research and the development of insulin-producing β-cell replacement strategies, whether pancreatic ductal cells retain their developmental capability to adopt an endocrine cell identity remains debated, most likely due to the diversity of models employed to induce pancreatic regeneration. In this work, rather than injuring the pancreas, we developed a mouse model allowing the inducible misexpression of the proendocrine gene Neurog3 in ductal cells in vivo. These animals developed a progressive islet hypertrophy attributed to a proportional increase in all endocrine cell populations. Lineage tracing experiments indicated a continuous neo-generation of endocrine cells exhibiting a ductal ontogeny. Interestingly, the resulting supplementary β-like cells were found to be functional. Based on these findings, we suggest that ductal cells could represent a renewable source of new β-like cells and that strategies aiming at controlling the expression of Neurog3, or of its molecular targets/co-factors, may pave new avenues for the improved treatments of diabetes.

An overview of type 2 diabetes and importance of vitamin D3-vitamin D receptor interaction in pancreatic β-cells

One significant health issue that plagues contemporary society is that of Type 2 diabetes (T2D). This disease is characterised by higher-than-average blood glucose levels as a result of a combination of insulin resistance and insufficient insulin secretions from the β-cells of pancreatic islets of Langerhans. Previous developmental research into the pancreas has identified how early precursor genes of pancreatic β-cells, such as Cpal, Ngn3, NeuroD, Ptf1a, and cMyc, play an essential role in the differentiation of these cells. Furthermore, β-cell molecular characterization has also revealed the specific role of β-cell-markers, such as Glut2, MafA, Ins1, Ins2, and Pdx1 in insulin expression. The expression of these genes appears to be suppressed in the T2D β-cells, along with the reappearance of the early endocrine marker genes. Glucose transporters transport glucose into β-cells, thereby controlling insulin release during hyperglycaemia. This stimulates glycolysis through rises in intracellular calcium (a process enhanced by vitamin D) (Norman et al., 1980), activating 2 of 4 proteinases. The rise in calcium activates half of pancreatic β-cell proinsulinases, thus releasing free insulin from granules. The synthesis of ATP from glucose by glycolysis, Krebs cycle and oxidative phosphorylation plays a role in insulin release. Some studies have found that the β-cells contain high levels of the vitamin D receptor; however, the role that this plays in maintaining the maturity of the β-cells remains unknown. Further research is required to develop a more in-depth understanding of the role VDR plays in β-cell function and the processes by which the beta cell function is preserved.

MyT1 Counteracts the Neural Progenitor Program to Promote Vertebrate Neurogenesis

The generation of neurons from neural stem cells requires large-scale changes in gene expression that are controlled to a large extent by proneural transcription factors, such as Ascl1. While recent studies have characterized the differentiation genes activated by proneural factors, less is known on the mechanisms that suppress progenitor cell identity. Here, we show that Ascl1 induces the transcription factor MyT1 while promoting neuronal differentiation. We combined functional studies of MyT1 during neurogenesis with the characterization of its transcriptional program. MyT1 binding is associated with repression of gene transcription in neural progenitor cells. It promotes neuronal differentiation by counteracting the inhibitory activity of Notch signaling at multiple levels, targeting the Notch1 receptor and many of its downstream targets. These include regulators of the neural progenitor program, such as Hes1, Sox2, Id3, and Olig1. Thus, Ascl1 suppresses Notch signaling cell-autonomously via MyT1, coupling neuronal differentiation with repression of the progenitor fate.

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MyT1 promotes neurogenesis downstream Ascl1

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MyT1 represses Notch1 receptor and many of its downstream target genes

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MyT1 represses Hes1 expression by direct DNA binding and competition with RBPJ

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Ascl1 suppresses Notch signaling cell-autonomously while promoting differentiation

The Potential Role of Krüppel-Like Zinc-Finger Protein Glis3 in Genetic Diseases and Cancers

Gli-similar 3 (Glis3) belongs to a Glis subfamily of Krüppel-like zinc-finger transcription factors characterized to regulate a set of downstream targets essential for cellular functions, including pancreatic development, β-cell maturation and maintenance, and insulin production. Examination of the DNA-binding domain of Glis3 reveals that this domain contains a repeated cysteine 2/histidine 2 (Cys2/His2) zinc-finger motif in the central region where the recognized DNA sequence binds. The loss of the production of pancreatic hormones, such as insulin 1 and 2, is linked to the down-regulation of β cells-related genes and promotes the apoptotic death of β cells found in mutant Glis3. Although accumulating studies converge on the Glis3 functioning in β cells, recently, there have been developments in the field of Glis3 using knockdown/mutant mice to better understand the role of Glis3 in diseases. The Glis3 mutant mice have been characterized for their propensity to develop congenital hypothyroidism, polycystic kidney disease, and some types of cancer. In this review, we attempt to comprehensively summarize the knowledge of Glis3, including its structure and general function in cells. We also collected and organized the academic achievements related to the possible mechanisms of Glis3-related diseases.

Transcriptome analysis of pancreatic cells across distant species highlights novel important regulator genes

BACKGROUND

Defining the transcriptome and the genetic pathways of pancreatic cells is of great interest for elucidating the molecular attributes of pancreas disorders such as diabetes and cancer. As the function of the different pancreatic cell types has been maintained during vertebrate evolution, the comparison of their transcriptomes across distant vertebrate species is a means to pinpoint genes under strong evolutionary constraints due to their crucial function, which have therefore preserved their selective expression in these pancreatic cell types.

RESULTS

In this study, RNA-sequencing was performed on pancreatic alpha, beta, and delta endocrine cells as well as the acinar and ductal exocrine cells isolated from adult zebrafish transgenic lines. Comparison of these transcriptomes identified many novel markers, including transcription factors and signaling pathway components, specific for each cell type. By performing interspecies comparisons, we identified hundreds of genes with conserved enriched expression in endocrine and exocrine cells among human, mouse, and zebrafish. This list includes many genes known as crucial for pancreatic cell formation or function, but also pinpoints many factors whose pancreatic function is still unknown. A large set of endocrine-enriched genes can already be detected at early developmental stages as revealed by the transcriptomic profiling of embryonic endocrine cells, indicating a potential role in cell differentiation. The actual involvement of conserved endocrine genes in pancreatic cell differentiation was demonstrated in zebrafish for myt1b, whose invalidation leads to a reduction of alpha cells, and for cdx4, selectively expressed in endocrine delta cells and crucial for their specification. Intriguingly, comparison of the endocrine alpha and beta cell subtypes from human, mouse, and zebrafish reveals a much lower conservation of the transcriptomic signatures for these two endocrine cell subtypes compared to the signatures of pan-endocrine and exocrine cells. These data suggest that the identity of the alpha and beta cells relies on a few key factors, corroborating numerous examples of inter-conversion between these two endocrine cell subtypes.

CONCLUSION

This study highlights both evolutionary conserved and species-specific features that will help to unveil universal and fundamental regulatory pathways as well as pathways specific to human and laboratory animal models such as mouse and zebrafish.

Incomplete Re-Expression of Neuroendocrine Progenitor/Stem Cell Markers is a Key Feature of β-Cell Dedifferentiation

There is increasing evidence to suggest that type 2 diabetes mellitus (T2D), a pandemic metabolic disease, may be caused by β-cell dedifferentiation (βCD). However, there is currently no universal definition of βCD, and the underlying mechanism is poorly understood. We hypothesise that a high-glucose in vitro environment mimics hyperglycaemia in vivo and that β cells grown in this milieu over a long period will undergo dedifferentiation. In the present study, we report that the pancreatic β cell line mouse insulinoma 6 (MIN6) grown under a high-glucose condition did not undergo massive cell death but exhibited a glucose-stimulated insulin-secreting profile similar to that of immature β cells. The expression of insulin and the glucose-sensing molecule glucose transporter 2 (Glut2) in late passage MIN6 cells was significantly lower than the early passage at both the RNA and protein levels. Mechanistically, these cells also expressed significantly less of the 'pancreatic and duodenal homebox1' (Pdx1) β-cell transcription factor. Finally, passaged MIN6 cells dedifferentiated to demonstrate some features of β-cell precursors, as well as neuroendocrine markers, in addition to expressing both glucagon and insulin. Thus, we concluded that high-glucose passaged MIN6 cells passaged MIN6 cells. provide a cellular model of β-cell dedifferentiation that can help researchers develop a better understanding of this process. These findings provide new insights that may enhance knowledge of the pathophysiology of T2D and facilitate the establishment of a novel strategy by which this disease can be treated.

Precommitment low-level Neurog3 expression defines a long-lived mitotic endocrine-biased progenitor pool that drives production of endocrine-committed cells

Bechard et al. show that a cell population defined as Neurog3 transcriptionally active and Sox9⁺ and often containing nonimmunodetectable Neurog3 protein has a relatively high mitotic index and prolonged epithelial residency. They propose that this endocrine-biased mitotic progenitor state is functionally separated from a pro-ductal pool and endows them with long-term capacity to make endocrine fate-directed progeny.

The current model for endocrine cell specification in the pancreas invokes high-level production of the transcription factor Neurogenin 3 (Neurog3) in Sox9⁺ bipotent epithelial cells as the trigger for endocrine commitment, cell cycle exit, and rapid delamination toward proto-islet clusters. This model posits a transient Neurog3 expression state and short epithelial residence period. We show, however, that a Neurog3^TA.LO cell population, defined as Neurog3 transcriptionally active and Sox9⁺ and often containing nonimmunodetectable Neurog3 protein, has a relatively high mitotic index and prolonged epithelial residency. We propose that this endocrine-biased mitotic progenitor state is functionally separated from a pro-ductal pool and endows them with long-term capacity to make endocrine fate-directed progeny. A novel BAC transgenic Neurog3 reporter detected two types of mitotic behavior in Sox9⁺Neurog3^TA.LO progenitors, associated with progenitor pool maintenance or derivation of endocrine-committed Neurog3^HI cells, respectively. Moreover, limiting Neurog3 expression dramatically increased the proportional representation of Sox9⁺Neurog3^TA.LO progenitors, with a doubling of its mitotic index relative to normal Neurog3 expression, suggesting that low Neurog3 expression is a defining feature of this cycling endocrine-biased state. We propose that Sox9⁺Neurog3^TA.LO endocrine-biased progenitors feed production of Neurog3^HI endocrine-committed cells during pancreas organogenesis.

Genome Editing of Lineage Determinants in Human Pluripotent Stem Cells Reveals Mechanisms of Pancreatic Development and Diabetes

Directed differentiation of human pluripotent stem cells (hPSCs) into somatic counterparts is a valuable tool for studying disease. However, examination of developmental mechanisms in hPSCs remains challenging given complex multi-factorial actions at different stages. Here, we used TALEN and CRISPR/Cas-mediated gene editing and hPSC-directed differentiation for a systematic analysis of the roles of eight pancreatic transcription factors (PDX1, RFX6, PTF1A, GLIS3, MNX1, NGN3, HES1, and ARX). Our analysis not only verified conserved gene requirements between mice and humans but also revealed a number of previously unsuspected developmental mechanisms with implications for type 2 diabetes. These include a role of RFX6 in regulating the number of pancreatic progenitors, a haploinsufficient requirement for PDX1 in pancreatic β cell differentiation, and a potentially divergent role of NGN3 in humans and mice. Our findings support use of systematic genome editing in hPSCs as a strategy for understanding mechanisms underlying congenital disorders.

Assessing Primary Neurogenesis in Xenopus Embryos Using Immunostaining

Primary neurogenesis is a dynamic and complex process during embryonic development that sets up the initial layout of the central nervous system. During this process, a portion of neural stem cells undergo differentiation and give rise to the first populations of differentiated primary neurons within the nascent central nervous system. Several vertebrate model organisms have been used to explore the mechanisms of neural cell fate specification, patterning, and differentiation. Among these is the African clawed frog, Xenopus, which provides a powerful system for investigating the molecular and cellular mechanisms responsible for primary neurogenesis due to its rapid and accessible development and ease of embryological and molecular manipulations. Here, we present a convenient and rapid method to observe the different populations of neuronal cells within Xenopus central nervous system. Using antibody staining and immunofluorescence on sections of Xenopus embryos, we are able to observe the locations of neural stem cells and differentiated primary neurons during primary neurogenesis.

Microtubules Negatively Regulate Insulin Secretion in Pancreatic β Cells

For glucose-stimulated insulin secretion (GSIS), insulin granules have to be localized close to the plasma membrane. The role of microtubule-dependent transport in granule positioning and GSIS has been debated. Here, we report that microtubules, counterintuitively, restrict granule availability for secretion. In β cells, microtubules originate at the Golgi and form a dense non-radial meshwork. Non-directional transport along these microtubules limits granule dwelling at the cell periphery, restricting granule availability for secretion. High glucose destabilizes microtubules, decreasing their density; such local microtubule depolymerization is necessary for GSIS, likely because granule withdrawal from the cell periphery becomes inefficient. Consistently, microtubule depolymerization by nocodazole blocks granule withdrawal, increases their concentration at exocytic sites, and dramatically enhances GSIS in vitro and in mice. Furthermore, glucose-driven MT destabilization is balanced by new microtubule formation, which likely prevents over-secretion. Importantly, microtubule density is greater in dysfunctional β cells of diabetic mice.

Transcription factor regulation of pancreatic organogenesis, differentiation and maturation

Lineage tracing studies have revealed that transcription factors play a cardinal role in pancreatic development, differentiation and function. Three transitions define pancreatic organogenesis, differentiation and maturation. In the primary transition, when pancreatic organogenesis is initiated, there is active proliferation of pancreatic progenitor cells. During the secondary transition, defined by differentiation, there is growth, branching, differentiation and pancreatic cell lineage allocation. The tertiary transition is characterized by differentiated pancreatic cells that undergo further remodeling, including apoptosis, replication and neogenesis thereby establishing a mature organ. Transcription factors function at multiple levels and may regulate one another and auto-regulate. The interaction between extrinsic signals from non-pancreatic tissues and intrinsic transcription factors form a complex gene regulatory network ultimately culminating in the different cell lineages and tissue types in the developing pancreas. Mutations in these transcription factors clinically manifest as subtypes of diabetes mellitus. Current treatment for diabetes is not curative and thus, developmental biologists and stem cell researchers are utilizing knowledge of normal pancreatic development to explore novel therapeutic alternatives. This review summarizes current knowledge of transcription factors involved in pancreatic development and β-cell differentiation in rodents.

Dissecting Human Gene Functions Regulating Islet Development With Targeted Gene Transduction

During pancreas development, endocrine precursors and their progeny differentiate, migrate, and cluster to form nascent islets. The transcription factor Neurogenin 3 (Neurog3) is required for islet development in mice, but its role in these dynamic morphogenetic steps has been inferred from fixed tissues. Moreover, little is known about the molecular genetic functions of NEUROG3 in human islet development. We developed methods for gene transduction by viral microinjection in the epithelium of cultured Neurog3-null mutant fetal pancreas, permitting genetic complementation in a developmentally relevant context. In addition, we developed methods for quantitative assessment of live-cell phenotypes in single developing islet cells. Delivery of wild-type NEUROG3 rescued islet differentiation, morphogenesis, and live cell deformation, whereas the patient-derived NEUROG3(R107S) allele partially restored indicators of islet development. NEUROG3(P39X), a previously unreported patient allele, failed to restore islet differentiation or morphogenesis and was indistinguishable from negative controls, suggesting that it is a null mutation. Our systems also permitted genetic suppression analysis and revealed that targets of NEUROG3, including NEUROD1 and RFX6, can partially restore islet development in Neurog3-null mutant mouse pancreata. Thus, advances described here permitted unprecedented assessment of gene functions in regulating crucial dynamic aspects of islet development in the fetal pancreas.

REST represses a subset of the pancreatic endocrine differentiation program

To contribute to devise successful beta-cell differentiation strategies for the cure of Type 1 diabetes we sought to uncover barriers that restrict endocrine fate acquisition by studying the role of the transcriptional repressor REST in the developing pancreas. Rest expression is prevented in neurons and in endocrine cells, which is necessary for their normal function. During development, REST represses a subset of genes in the neuronal differentiation program and Rest is down-regulated as neurons differentiate. Here, we investigate the role of REST in the differentiation of pancreatic endocrine cells, which are molecularly close to neurons. We show that Rest is widely expressed in pancreas progenitors and that it is down-regulated in differentiated endocrine cells. Sustained expression of REST in Pdx1(+) progenitors impairs the differentiation of endocrine-committed Neurog3(+) progenitors, decreases beta and alpha cell mass by E18.5, and triggers diabetes in adulthood. Conditional inactivation of Rest in Pdx1(+) progenitors is not sufficient to trigger endocrine differentiation but up-regulates a subset of differentiation genes. Our results show that the transcriptional repressor REST is active in pancreas progenitors where it gates the activation of part of the beta cell differentiation program.

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

Neurogenin3 (NEUROG3) is a basic helix-loop-helix transcription factor required for development of the endocrine pancreas in mice. In contrast, humans with NEUROG3 mutations are born with endocrine pancreas function, calling into question whether NEUROG3 is required for human endocrine pancreas development. To test this directly, we generated human embryonic stem cell (hESC) lines where both alleles of NEUROG3 were disrupted using CRISPR/Cas9-mediated gene targeting. NEUROG3(-/-) hESC lines efficiently formed pancreatic progenitors but lacked detectible NEUROG3 protein and did not form endocrine cells in vitro. Moreover, NEUROG3(-/-) hESC lines were unable to form mature pancreatic endocrine cells after engraftment of PDX1(+)/NKX6.1(+) pancreatic progenitors into mice. In contrast, a 75-90% knockdown of NEUROG3 caused a reduction, but not a loss, of pancreatic endocrine cell development. We conclude that NEUROG3 is essential for endocrine pancreas development in humans and that as little as 10% NEUROG3 is sufficient for formation of pancreatic endocrine cells.

Bicaudal C1 promotes pancreatic NEUROG3+ endocrine progenitor differentiation and ductal morphogenesis

In human, mutations in bicaudal C1 (BICC1), an RNA binding protein, have been identified in patients with kidney dysplasia. Deletion of Bicc1 in mouse leads to left-right asymmetry randomization and renal cysts. Here, we show that BICC1 is also expressed in both the pancreatic progenitor cells that line the ducts during development, and in the ducts after birth, but not in differentiated endocrine or acinar cells. Genetic inactivation of Bicc1 leads to ductal cell over-proliferation and cyst formation. Transcriptome comparison between WT and Bicc1 KO pancreata, before the phenotype onset, reveals that PKD2 functions downstream of BICC1 in preventing cyst formation in the pancreas. Moreover, the analysis highlights immune cell infiltration and stromal reaction developing early in the pancreas of Bicc1 knockout mice. In addition to these functions in duct morphogenesis, BICC1 regulates NEUROG3(+) endocrine progenitor production. Its deletion leads to a late but sustained endocrine progenitor decrease, resulting in a 50% reduction of endocrine cells. We show that BICC1 functions downstream of ONECUT1 in the pathway controlling both NEUROG3(+) endocrine cell production and ductal morphogenesis, and suggest a new candidate gene for syndromes associating kidney dysplasia with pancreatic disorders, including diabetes.

Loss of Fbw7 reprograms adult pancreatic ductal cells into α, δ, and β cells

The adult pancreas is capable of limited regeneration after injury but has no defined stem cell population. The cell types and molecular signals that govern the production of new pancreatic tissue are not well understood. Here, we show that inactivation of the SCF-type E3 ubiquitin ligase substrate recognition component Fbw7 induces pancreatic ductal cells to reprogram into α, δ, and β cells. Loss of Fbw7 stabilized the transcription factor Ngn3, a key regulator of endocrine cell differentiation. The induced β cells resemble islet β cells in morphology and histology, express genes essential for β cell function, and release insulin after glucose challenge. Thus, loss of Fbw7 appears to reawaken an endocrine developmental differentiation program in adult pancreatic ductal cells. Our study highlights the plasticity of seemingly differentiated adult cells, identifies Fbw7 as a master regulator of cell fate decisions in the pancreas, and reveals adult pancreatic duct cells as a latent multipotent cell type.

Transcriptional control of mammalian pancreas organogenesis

The field of pancreas development has markedly expanded over the last decade, significantly advancing our understanding of the molecular mechanisms that control pancreas organogenesis. This growth has been fueled, in part, by the need to generate new therapeutic approaches for the treatment of diabetes. The creation of sophisticated genetic tools in mice has been instrumental in this progress. Genetic manipulation involving activation or inactivation of genes within specific cell types has allowed the identification of many transcription factors (TFs) that play critical roles in the organogenesis of the pancreas. Interestingly, many of these TFs act at multiple stages of pancreatic development, and adult organ function or repair. Interaction with other TFs, extrinsic signals, and epigenetic regulation are among the mechanisms by which TFs may play context-dependent roles during pancreas organogenesis. Many of the pancreatic TFs directly regulate each other and their own expression. These combinatorial interactions generate very specific gene regulatory networks that can define the different cell lineages and types in the developing pancreas. Here, we review recent progress made in understanding the role of pancreatic TFs in mouse pancreas formation. We also summarize our current knowledge of human pancreas development and discuss developmental pancreatic TFs that have been associated with human pancreatic diseases.

A critical role for the neural zinc factor ST18 in pancreatic β-cell apoptosis

The tumor suppressor gene ST18 was originally characterized as the third member of the neural zinc finger transcription factor family. However, little is known about its biological functions. Herein, we demonstrate that, in the pancreas, ST18 expression is restricted to endocrine cells. The detection of ST18 expression in pancreatic β-cells prompted us to investigate its regulation and its role in β-cell mass and function. We show that ST18 expression and activity are increased by cytotoxic concentrations of fatty acids and cytokines in INS832/13 cells. Furthermore, ST18 is also increased in islets of diet-induced obese animals. Overexpression and RNA interference knockdown studies demonstrate that ST18 induces β-cell apoptosis and curtails β-cell replication. Finally, our data suggest that ST18 impairs insulin secretion. Taken together, our findings indicate that ST18 could represent a novel transcriptional mediator of lipotoxicity and cytokine-induced β-cell death. We suggest that genetic or pharmacologic manipulations of ST18 could help maintain a functional β-cell mass.

Spatiotemporal expression pattern of Myt/NZF family zinc finger transcription factors during mouse nervous system development

BACKGROUND

Three members of the Myt/NZF family of transcription factors are involved in many processes of vertebrate development. Several studies have reported that Myt1/NZF-2 has a regulatory function in the development of cultured oligodendrocyte progenitors or in neuronal differentiation during Xenopus primary neurogenesis. However, little is known about the proper function of Myt/NZF family proteins during mammalian nervous system development. To assess the possible function of Myt/NZF transcription factors in mammalian neuronal differentiation, we determined the comparative spatial and temporal expression patterns of all three types of Myt/NZF family genes in the embryonic mouse nervous system using quantitative reverse transcriptase polymerase chain reaction and in situ hybridization.

RESULTS

All three Myt/NZF family genes were extensively expressed in developing mouse nervous tissues, and their expression was transient. NZF-1 was expressed later in post-mitotic neurons. NZF-2 was initially expressed in neuronal cells a little earlier than NZF-3. NZF-3 was initially expressed in neuronal cells, just after proliferation was complete.

CONCLUSION

These expression patterns suggest that the expression of NZF family genes is spatially and temporally regulated, and each Myt/NZF family gene may have a regulatory function in a specific phase during neuronal differentiation.

Reconstituting pancreas development from purified progenitor cells reveals genes essential for islet differentiation

Developmental biology is challenged to reveal the function of numerous candidate genes implicated by recent genome-scale studies as regulators of organ development and diseases. Recapitulating organogenesis from purified progenitor cells that can be genetically manipulated would provide powerful opportunities to dissect such gene functions. Here we describe systems for reconstructing pancreas development, including islet β-cell and α-cell differentiation, from single fetal progenitor cells. A strict requirement for native genetic regulators of in vivo pancreas development, such as Ngn3, Arx, and Pax4, revealed the authenticity of differentiation programs in vitro. Efficient genetic screens permitted by this system revealed that Prdm16 is required for pancreatic islet development in vivo. Discovering the function of genes regulating pancreas development with our system should enrich strategies for regenerating islets for treating diabetes mellitus.

Ascl1b and Neurod1, instead of Neurog3, control pancreatic endocrine cell fate in zebrafish

Background

NEUROG3 is a key regulator of pancreatic endocrine cell differentiation in mouse, essential for the generation of all mature hormone producing cells. It is repressed by Notch signaling that prevents pancreatic cell differentiation by maintaining precursors in an undifferentiated state.

Results

We show that, in zebrafish, neurog3 is not expressed in the pancreas and null neurog3 mutant embryos do not display any apparent endocrine defects. The control of endocrine cell fate is instead fulfilled by two basic helix-loop-helix factors, Ascl1b and Neurod1, that are both repressed by Notch signaling. ascl1b is transiently expressed in the mid-trunk endoderm just after gastrulation and is required for the generation of the first pancreatic endocrine precursor cells. Neurod1 is expressed afterwards in the pancreatic anlagen and pursues the endocrine cell differentiation program initiated by Ascl1b. Their complementary role in endocrine differentiation of the dorsal bud is demonstrated by the loss of all hormone-secreting cells following their simultaneous inactivation. This defect is due to a blockage of the initiation of endocrine cell differentiation.

Conclusions

This study demonstrates that NEUROG3 is not the unique pancreatic endocrine cell fate determinant in vertebrates. A general survey of endocrine cell fate determinants in the whole digestive system among vertebrates indicates that they all belong to the ARP/ASCL family but not necessarily to the Neurog3 subfamily. The identity of the ARP/ASCL factor involved depends not only on the organ but also on the species. One could, therefore, consider differentiating stem cells into insulin-producing cells without the involvement of NEUROG3 but via another ARP/ASCL factor.

Pancreas-specific Cre driver lines and considerations for their prudent use

Cre/LoxP has broad utility for studying the function, development, and oncogenic transformation of pancreatic cells in mice. Here we provide an overview of the Cre driver lines that are available for such studies. We discuss how variegated expression, transgene silencing, and recombination in undesired cell types have conspired to limit the performance of these lines, sometimes leading to serious experimental concerns. We also discuss preferred strategies for achieving high-fidelity driver lines and remind investigators of the continuing need for caution when interpreting results obtained from any Cre/LoxP-based experiment performed in mice.

Gene regulatory networks governing pancreas development

Elucidation of cellular and gene regulatory networks (GRNs) governing organ development will accelerate progress toward tissue replacement. Here, we have compiled reference GRNs underlying pancreas development from data mining that integrates multiple approaches, including mutant analysis, lineage tracing, cell purification, gene expression and enhancer analysis, and biochemical studies of gene regulation. Using established computational tools, we integrated and represented these networks in frameworks that should enhance understanding of the surging output of genomic-scale genetic and epigenetic studies of pancreas development and diseases such as diabetes and pancreatic cancer. We envision similar approaches would be useful for understanding the development of other organs.

Non-parallel recombination limits Cre-LoxP-based reporters as precise indicators of conditional genetic manipulation

Cre/LoxP-mediated recombination allows for conditional gene activation or inactivation. When combined with an independent lineage-tracing reporter allele, this technique traces the lineage of presumptive genetically modified Cre-expressing cells. Several studies have suggested that floxed alleles have differential sensitivities to Cre-mediated recombination, which raises concerns regarding utilization of Cre-reporters to monitor recombination of other floxed loci of interest. Here, we directly investigate the recombination correlation, at cellular resolution, between several floxed alleles induced by Cre-expressing mouse lines. The recombination correlation between different reporter alleles varied greatly in otherwise genetically identical cell types. The chromosomal location of floxed alleles, distance between LoxP sites, sequences flanking the LoxP sites, and the level of Cre activity per cell all likely contribute to observed variations in recombination correlation. These findings directly demonstrate that, due to non-parallel recombination events, commonly available Cre reporter mice cannot be reliably utilized, in all cases, to trace cells that have DNA recombination in independent-target floxed alleles, and that careful validation of recombination correlations are required for proper interpretation of studies designed to trace the lineage of genetically modified populations, especially in mosaic situations.

Neurogenin 3+ cells contribute to β-cell neogenesis and proliferation in injured adult mouse pancreas

We previously showed that injury by partial duct ligation (PDL) in adult mouse pancreas activates Neurogenin 3 (Ngn3)⁺ progenitor cells that can differentiate to β cells ex vivo. Here we evaluate the role of Ngn3⁺ cells in β cell expansion in situ. PDL not only induced doubling of the β cell volume but also increased the total number of islets. β cells proliferated without extended delay (the so-called ‘refractory' period), their proliferation potential was highest in small islets, and 86% of the β cell expansion was attributable to proliferation of pre-existing β cells. At sufficiently high Ngn3 expression level, upto 14% of all β cells and 40% of small islet β cells derived from non-β cells. Moreover, β cell proliferation was blunted by a selective ablation of Ngn3⁺ cells but not by conditional knockout of Ngn3 in pre-existing β cells supporting a key role for Ngn3⁺ insulin⁻ cells in β cell proliferation and expansion. We conclude that Ngn3⁺ cell-dependent proliferation of pre-existing and newly-formed β cells as well as reprogramming of non-β cells contribute to in vivo β cell expansion in the injured pancreas of adult mice.

Neurogenin3 cooperates with Foxa2 to autoactivate its own expression

The transcription factor Neurogenin3 functions as a master regulator of endocrine pancreas formation, and its deficiency leads to the development of diabetes in humans and mice. In the embryonic pancreas, Neurogenin3 is transiently expressed at high levels for a narrow time window to initiate endocrine differentiation in scattered progenitor cells. The mechanisms controlling these rapid and robust changes in Neurogenin3 expression are poorly understood. In this study, we characterize a Neurogenin3 positive autoregulatory loop whereby this factor may rapidly induce its own levels. We show that Neurogenin3 binds to a conserved upstream fragment of its own gene, inducing deposition of active chromatin marks and the activation of Neurog3 transcription. Additionally, we show that the broadly expressed endodermal forkhead factors Foxa1 and Foxa2 can cooperate synergistically to amplify Neurogenin3 autoregulation in vitro. However, only Foxa2 colocalizes with Neurogenin3 in pancreatic progenitors, thus indicating a primary role for this factor in regulating Neurogenin3 expression in vivo. Furthermore, in addition to decreasing Neurog3 autoregulation, inhibition of Foxa2 by RNA interference attenuates Neurogenin3-dependent activation of the endocrine developmental program in cultured duct mPAC cells. Hence, these data uncover the potential functional cooperation between the endocrine lineage-determining factor Neurogenin3 and the widespread endoderm progenitor factor Foxa2 in the implementation of the endocrine developmental program in the pancreas.

Regulation of Neurod1 contributes to the lineage potential of Neurogenin3+ endocrine precursor cells in the pancreas

During pancreatic development, transcription factor cascades gradually commit precursor populations to the different endocrine cell fate pathways. Although mutational analyses have defined the functions of many individual pancreatic transcription factors, the integrative transcription factor networks required to regulate lineage specification, as well as their sites of action, are poorly understood. In this study, we investigated where and how the transcription factors Nkx2.2 and Neurod1 genetically interact to differentially regulate endocrine cell specification. In an Nkx2.2 null background, we conditionally deleted Neurod1 in the Pdx1+ pancreatic progenitor cells, the Neurog3+ endocrine progenitor cells, or the glucagon+ alpha cells. These studies determined that, in the absence of Nkx2.2 activity, removal of Neurod1 from the Pdx1+ or Neurog3+ progenitor populations is sufficient to reestablish the specification of the PP and epsilon cell lineages. Alternatively, in the absence of Nkx2.2, removal of Neurod1 from the Pdx1+ pancreatic progenitor population, but not the Neurog3+ endocrine progenitor cells, restores alpha cell specification. Subsequent in vitro reporter assays demonstrated that Nkx2.2 represses Neurod1 in alpha cells. Based on these findings, we conclude that, although Nkx2.2 and Neurod1 are both necessary to promote beta cell differentiation, Nkx2.2 must repress Neurod1 in a Pdx1+ pancreatic progenitor population to appropriately commit a subset of Neurog3+ endocrine progenitor cells to the alpha cell lineage. These results are consistent with the proposed idea that Neurog3+ endocrine progenitor cells represent a heterogeneous population of unipotent cells, each restricted to a particular endocrine lineage.

Diabetes mellitus is a family of metabolic diseases that can result from either destruction or dysfunction of the insulin-producing beta cells of the pancreas. Recent studies have provided hope that generating insulin-producing cells from alternative cell sources may be a possible treatment for diabetes; this includes the observation that pancreatic glucagon-expressing alpha cells can be converted into beta cells under certain physiological or genetic conditions. Our study focuses on two essential beta cell regulatory factors, Nkx2.2 and Neurod1, and demonstrates how their genetic interactions can promote the development of other hormone-expressing cell types, including alpha cells. We determined that, while Nkx2.2 is required to activate Neurod1 to promote beta cell formation, Nkx2.2 must prevent expression of Neurod1 to allow alpha cell formation. Furthermore, the inactivation of Neurod1 must occur in the earliest pancreatic progenitors, at a stage in the differentiation process earlier than previously believed. These studies contribute to our understanding of the overlapping gene regulatory networks that specify islet cell types and identify the importance of timing and cellular context for these regulatory interactions. Furthermore, our data have broad implications regarding the manipulation of alpha cells or human pluripotent stem cells to generate insulin-producing beta cells for therapeutic purposes.

Beta cell dysfunction and insulin resistance

Beta cell dysfunction and insulin resistance are inherently complex with their interrelation for triggering the pathogenesis of diabetes also somewhat undefined. Both pathogenic states induce hyperglycemia and therefore increase insulin demand. Beta cell dysfunction results from inadequate glucose sensing to stimulate insulin secretion therefore elevated glucose concentrations prevail. Persistently elevated glucose concentrations above the physiological range result in the manifestation of hyperglycemia. With systemic insulin resistance, insulin signaling within glucose recipient tissues is defective therefore hyperglycemia perseveres. Beta cell dysfunction supersedes insulin resistance in inducing diabetes. Both pathological states influence each other and presumably synergistically exacerbate diabetes. Preserving beta cell function and insulin signaling in beta cells and insulin signaling in the glucose recipient tissues will maintain glucose homeostasis.

The transcription factor Myt3 acts as a pro-survival factor in β-cells

Aims/Hypothesis

We previously identified the transcription factor Myt3 as specifically expressed in pancreatic islets. Here, we sought to determine the expression and regulation of Myt3 in islets and to determine its significance in regulating islet function and survival.

Methods

Myt3 expression was determined in embryonic pancreas and adult islets by qPCR and immunohistochemistry. ChIP-seq, ChIP-qPCR and luciferase assays were used to evaluate regulation of Myt3 expression. Suppression of Myt3 was used to evaluate gene expression, insulin secretion and apoptosis in islets.

Results

We show that Myt3 is the most abundant MYT family member in adult islets and that it is expressed in all the major endocrine cell types in the pancreas after E18.5. We demonstrate that Myt3 expression is directly regulated by Foxa2, Pdx1, and Neurod1, which are critical to normal β-cell development and function, and that Ngn3 induces Myt3 expression through alterations in the Myt3 promoter chromatin state. Further, we show that Myt3 expression is sensitive to both glucose and cytokine exposure. Of specific interest, suppressing Myt3 expression reduces insulin content and increases β-cell apoptosis, at least in part, due to reduced Pdx1, Mafa, Il-6, Bcl-xl, c-Iap2 and Igfr1 levels, while over-expression of Myt3 protects islets from cytokine induced apoptosis.

Conclusion/Interpretation

We have identified Myt3 as a novel transcriptional regulator with a critical role in β-cell survival. These data are an important step in clarifying the regulatory networks responsible for β-cell survival, and point to Myt3 as a potential therapeutic target for improving functional β-cell mass.

Pancreatic β cell dedifferentiation as a mechanism of diabetic β cell failure

Diabetes is associated with β cell failure. But it remains unclear whether the latter results from reduced β cell number or function. FoxO1 integrates β cell proliferation with adaptive β cell function. We interrogated the contribution of these two processes to β cell dysfunction, using mice lacking FoxO1 in β cells. FoxO1 ablation caused hyperglycemia with reduced β cell mass following physiologic stress, such as multiparity and aging. Surprisingly, lineage-tracing experiments demonstrated that loss of β cell mass was due to β cell dedifferentiation, not death. Dedifferentiated β cells reverted to progenitor-like cells expressing Neurogenin3, Oct4, Nanog, and L-Myc. A subset of FoxO1-deficient β cells adopted the α cell fate, resulting in hyperglucagonemia. Strikingly, we identify the same sequence of events as a feature of different models of murine diabetes. We propose that dedifferentiation trumps endocrine cell death in the natural history of β cell failure and suggest that treatment of β cell dysfunction should restore differentiation, rather than promoting β cell replication.