The Role of ARX in Human Pancreatic Endocrine Specification
Blair K. GageAli AsadiRobert K. BakerTravis D. WebberRennian WangMasayuki ItohMasaharu HayashiRie MiyataTakumi AkashiTimothy J. Kieffer
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Abstract:
The in vitro differentiation of human embryonic stem cells (hESCs) offers a model system to explore human development. Humans with mutations in the transcription factor Aristaless Related Homeobox (ARX) often suffer from the syndrome X-linked lissencephaly with ambiguous genitalia (XLAG), affecting many cell types including those of the pancreas. Indeed, XLAG pancreatic islets lack glucagon and pancreatic polypeptide-positive cells but retain somatostatin, insulin, and ghrelin-positive cells. To further examine the role of ARX in human pancreatic endocrine development, we utilized genomic editing in hESCs to generate deletions in ARX. ARX knockout hESCs retained pancreatic differentiation capacity and ARX knockout endocrine cells were biased toward somatostatin-positive cells (94% of endocrine cells) with reduced pancreatic polypeptide (rarely detected), glucagon (90% reduced) and insulin-positive (65% reduced) lineages. ARX knockout somatostatin-positive cells shared expression patterns with human fetal and adult δ-cells. Differentiated ARX knockout cells upregulated PAX4, NKX2.2, ISL1, HHEX, PCSK1, PCSK2 expression while downregulating PAX6 and IRX2. Re-expression of ARX in ARX knockout pancreatic progenitors reduced HHEX and increased PAX6 and insulin expression following differentiation. Taken together these data suggest that ARX plays a key role in pancreatic endocrine fate specification of pancreatic polypeptide, somatostatin, glucagon and insulin positive cells from hESCs.Keywords:
PAX4
Enteroendocrine cell
PAX6
The transcription factor paired-box-6 (Pax6) plays a key role in the endocrine differentiation cascade of the pancreas. Mutations in Pax6 are associated with anomalies including diabetes. This study elucidates the impact of Pax6 in pancreatic a- and s-cell function and differentiation. In a-cells, we found Pax6 to be critical for glucagon biosynthesis and processing, by directly and indirectly activating the glucagon gene through the transcription factors cMaf and NeuroD1, and the processing enzyme of proglucagon in a-cells PC2 and its molecular chaperon 7B2. In regard to s-cells, the study foundsPax6 to control the cell function and differentiation through the control of genes coding for insulin-1, insulin-2, glucose-transporter-2, glucokinase, GLP-1-receptor, FFA receptor GPR40 as well as the transcription factors Pdx1, MafA and Nkx6-1. The study maps the complex network in which Pax6 regulates transcription of genes important for differentiation of the endocrine pancreas and plays a major role in the islet function.
PAX6
PDX1
PAX4
Enteroendocrine cell
Glucokinase
Proglucagon
Liver receptor homolog-1
Sulfonylurea receptor
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The primary function of islet A cells is the synthesis and secretion of glucagon, an essential hormonal regulator of glucose homeostasis. The proglucagon gene is also expressed in enteroendocrine L cells of the intestinal epithelium, which produce glucagon-like peptide 1 (GLP-1) and glucagon-like peptide 2 (GLP-2), regulators of insulin secretion and intestinal growth, respectively. We show here that Pax6, a critical determinant of islet cell development and proglucagon gene expression in islet A cells, is also essential for glucagon gene transcription in the small and large intestine. Pax6 is expressed in enteroendocrine cells, binds to the G1 and G3 elements in the proglucagon promoter, and activates proglucagon gene transcription. The dominant negative Pax6 allele, SEYNeu, represses proglucagon gene transcription in enteroendocrine cells. Mice homozygous for the SEYNeu mutation exhibit markedly reduced levels of proglucagon mRNA transcripts in the small and large intestine, and GLP-1 or GLP-2-immunopositive enteroendocrine cells were not detected in the intestinal mucosa. These findings implicate an essential role for Pax6 in the development and function of glucagon-producing cells in both pancreatic and intestinal endodermal lineages.
Proglucagon
Enteroendocrine cell
PAX6
Alpha cell
Glucagon-like peptide-2
Prohormone convertase
Intestinal epithelium
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Ketogenesis
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Intestinal hormones are key regulators of digestion and energy homeostasis secreted by rare enteroendocrine cells. These cells produce over ten different hormones including GLP-1 and GIP peptides known to promote insulin secretion. To date, the molecular mechanisms controlling the specification of the various enteroendocrine subtypes from multipotent Neurog3+ endocrine progenitor cells, as well as their number, remain largely unknown. In contrast, in the embryonic pancreas, the opposite activities of Arx and Pax4 homeodomain transcription factors promote islet progenitor cells towards the different endocrine cell fates. In this study, we thus investigated the role of Arx and Pax4 in enteroendocrine subtype specification. The small intestine and colon of Arx- and Pax4-deficient mice were analyzed using histological, molecular, and lineage tracing approaches. We show that Arx is expressed in endocrine progenitors (Neurog3+) and in early differentiating (ChromograninA−) GLP-1-, GIP-, CCK-, Sct- Gastrin- and Ghrelin-producing cells. We noted a dramatic reduction or a complete loss of all these enteroendocrine cell types in Arx mutants. Serotonin- and Somatostatin-secreting cells do not express Arx and, accordingly, the differentiation of Serotonin cells was not affected in Arx mutants. However, the number of Somatostatin-expressing D-cells is increased as Arx-deficient progenitor cells are redirected to the D-cell lineage. In Pax4-deficient mice, the differentiation of Serotonin and Somatostatin cells is impaired, as well as of GIP and Gastrin cells. In contrast, the number of GLP-1 producing L-cells is increased concomitantly with an upregulation of Arx. Thus, while Arx and Pax4 are necessary for the development of L- and D-cells respectively, they conversely restrict D- and L-cells fates suggesting antagonistic functions in D/L cell allocation. In conclusion, these finding demonstrate that, downstream of Neurog3, the specification of a subset of enteroendocrine subtypes relies on both Arx and Pax4, while others depend only on Arx or Pax4.
Enteroendocrine cell
PAX4
NeuroD
PAX6
Progenitor
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Somatostatin was infused for 6 h into seven normal subjects with and without a replacement dose of glucagon. The addition of glucagon to somatostatin resulted in a 30-40% rise in plasma glucagon, whereas plasma insulin declined by 40-50% in both treatment groups. Plasma glucose and glucose production initially increased 2-fold with glucagon replacement, and subsequently declined by 2-3 h to levels comparable to those observed with somatostatin alone. After 6 h plasma glucose and glucose kinetics were no different whether or not glucagon was present. The rise in blood ketones after somatostatin was not exaggerated by glucagon replacement. We conclude that glucagon lack is not a modifying factor in the late hyperglycemic and hyperketonemic response to prolonged infusions of somatostatin.
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The glucagon gene is expressed in α ‐cells of the pancreas, L cells of the intestine and the hypothalamus. The determinants of the α ‐cell‐specific expression of the glucagon gene are not fully characterized, although Arx, Pax6 and Foxa2 are critical for α ‐cell differentiation and glucagon gene expression; in addition, the absence of the β ‐cell‐specific transcription factors Pdx1, Pax4 and Nkx6.1 may allow for the glucagon gene to be expressed. Pax6, along with cMaf and MafB, binds to the DNA control element G 1 which confers α ‐cell specificity to the promoter and to G 3 and potently activates glucagon gene transcription. In addition, to its direct role on the transcription of the glucagon gene, Pax6 controls several transcription factors involved in the activation of the glucagon gene such as cMaf, MafB and NeuroD1/Beta2 as well as different steps of glucagon biosynthesis and secretion. We conclude that Pax6 independently of Arx and Foxa2 is critical for α ‐cell function by coordinating glucagon gene expression as well as glucagon biosynthesis and secretion.
PAX6
PAX4
PDX1
Enteroendocrine cell
FOXA2
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Complex diseases, such as diabetes, are influenced by comprehensive transcriptional networks. Genome-wide association studies have revealed that variants located in regulatory elements for pancreatic transcription factors are linked to diabetes, including those functionally linked to the paired box transcription factor Pax6. Pax6 deletions in adult mice cause rapid onset of classic diabetes, but the full spectrum of pancreatic Pax6 regulators is unknown. Using a regulatory element discovery approach, we identified two novel Pax6 pancreatic cis-regulatory elements in a poorly characterized regulatory desert. Both new elements, Pax6 pancreas cis-regulatory element 3 (PE3) and PE4, are located 50 and 100 kb upstream and interact with different parts of the Pax6 promoter and nearby non-coding RNAs. They drive expression in the developing pancreas and brain and code for multiple pancreas-related transcription factor-binding sites. PE3 binds CCCTC-binding factor (CTCF) and is marked by stem cell identity markers in embryonic stem cells, whilst a common variant located in the PE4 element affects binding of Pax4, a known pancreatic regulator, altering Pax6 gene expression. To determine the ability of these elements to regulate gene expression, synthetic transcriptional activators and repressors were targeted to PE3 and PE4, modulating Pax6 gene expression, as well as influencing neighbouring genes and long non-coding RNAs, implicating the Pax6 locus in pancreas function and diabetes.
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