REST is a major negative regulator of endocrine differentiation during pancreas organogenesis

Multiple transcription factors have been shown to promote pancreatic β-cell differentiation, yet much less is known about negative regulators. Earlier epigenomic studies suggested that the transcriptional repressor REST could be a suppressor of endocrinogenesis in the embryonic pancreas. However, pancreatic Rest knockout mice failed to show abnormal numbers of endocrine cells, suggesting that REST is not a major regulator of endocrine differentiation. Using a different conditional allele that enables profound REST inactivation, we observed a marked increase in pancreatic endocrine cell formation. REST inhibition also promoted endocrinogenesis in zebrafish and mouse early postnatal ducts and induced β-cell-specific genes in human adult duct-derived organoids. We also defined genomic sites that are bound and repressed by REST in the embryonic pancreas. Our findings show that REST-dependent inhibition ensures a balanced production of endocrine cells from embryonic pancreatic progenitors.

Architecture of the Pancreatic Islets and Endocrine Cell Arrangement in the Embryonic Pancreas of the Grass Snake (Natrix natrix L.). Immunocytochemical Studies and 3D Reconstructions

During the early developmental stages of grass snakes, within the differentiating pancreas, cords of endocrine cells are formed. They differentiate into agglomerates of large islets flanked throughout subsequent developmental stages by small groups of endocrine cells forming islets. The islets are located within the cephalic part of the dorsal pancreas. At the end of the embryonic period, the pancreatic islet agglomerates branch off, and as a result of their remodeling, surround the splenic “bulb”. The stage of pancreatic endocrine ring formation is the first step in formation of intrasplenic islets characteristics for the adult specimens of the grass snake. The arrangement of endocrine cells within islets changes during pancreas differentiation. Initially, the core of islets formed from B and D cells is surrounded by a cluster of A cells. Subsequently, A, B, and D endocrine cells are mixed throughout the islets. Before grass snake hatching, A and B endocrine cells are intermingled within the islets, but D cells are arranged centrally. Moreover, the pancreatic polypeptide (PP) cells are not found within the embryonic pancreas of the grass snake. Variation in the proportions of different cell types, depending on the part of the pancreas, may affect the islet function—a higher proportion of glucagon cells is beneficial for insulin secretion.

Hnf1b-CreER, a model used for fate mapping pancreatic lineages, achieves efficient Cre-mediated recombination in duct and islet δ cells

The Hnf1b-CreERT2 BAC transgenic (Tg(Hnf1b-cre/ERT2)1Jfer) has been used extensively to trace the progeny of pancreatic ducts in development, regeneration, or cancer. This model originally showed that duct-like plexus cells of the embryonic pancreas are bipotent duct-endocrine progenitors, whereas adult mouse duct cells are not a common source of δ cells in various regenerative settings. We have now examined Hnf1b-CreERT2 mice with a Rosa26-RFP reporter transgene. This showed inducible recombination of up to 96% adult duct cells, a much higher efficiency than the previously used β-galactosidase reporter. Despite this high duct-cell excision, recombination in α and β cells remained very low, similar to the previously used reporter transgene (Rosa26-βgalactosidase). However, nearly half of somatostatin-expressing δ cells showed reporter activation, which was due to Cre expression in δ cells rather than an indication of duct to δ cell conversions. The high recombination efficiency in duct cells indicates that the Hnf1b-CreERT2 model can be useful for both ductal fate mapping and genetic inactivation studies. The recombination in δ cells does not modify the interpretation of studies that failed to show duct conversions to other cell types, but needs to be considered in studies that use this model to modify the plasticity of pancreatic duct cells.

REST is a major negative regulator of endocrine differentiation during pancreas organogenesis

SUMMARYUnderstanding genomic regulatory mechanisms of pancreas differentiation is relevant to the pathophysiology of diabetes mellitus, and to the development of replacement therapies. Numerous transcription factors promote β cell differentiation, although less is known about negative regulators. Earlier epigenomic studies suggested that the transcriptional repressor REST could be a suppressor of endocrine gene programs in the embryonic pancreas. However, pancreaticRestknock-out mice failed to show increased numbers of endocrine cells, suggesting that REST is not a major regulator of endocrine differentiation. Using a different conditional allele that enables profound REST inactivation, we now observe a marked increase in the formation of pancreatic endocrine cells. REST inhibition also promoted endocrinogenesis in zebrafish and mouse early postnatal ducts, and induced β-cell specific genes in human adult duct-derived organoids. Finally, we define REST genomic programs that suppress pancreatic endocrine differentiation. These results establish a crucial role of REST as a negative regulator of pancreatic endocrine differentiation.

PDX1 directs a core developmentally and evolutionarily conserved gene program in the pancreatic islet

Pancreatic and duodenal homeobox 1 (PDX1) is crucial for pancreas organogenesis, yet the dynamic changes in PDX1 targets in mouse or human pancreas development have not been examined. We integrated the PDX1 cistrome with cell lineage-specific gene expression in both mouse and human developing pancreas. We identified a core set of developmentally and evolutionarily conserved PDX1 bound genes that reveal the broad multifaceted role of PDX1 in pancreas development. Despite the well-known, dramatic changes in PDX1 function and expression, we showed that PDX1 binding is largely stable from embryonic pancreas to adult islet. This may point towards a dual role of PDX1, activating or repressing the expression of its targets at different ages, dependent on other functionally-congruent or directly-interacting partners. Our work also suggests that PDX1 functions not only in initiating pancreas differentiation, but also as a potential keepsake of the progenitor program in the adult beta cells.

β cell-specific deletion of HMG-CoA (3-hydroxy-3-methylglutaryl-coenzyme A) reductase causes overt diabetes due to reduction of β cell mass and impaired insulin secretion

Inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR), statins, which are used to prevent cardiovascular diseases, are associated with a modest increase in the risk of new-onset diabetes mellitus. To investigate the role of HMGCR in the development of β cells and glucose homeostasis, we deleted <i>Hmgcr</i> in a β cell-specific manner by using the Cre-loxP technique. Mice lacking <i>Hmgcr</i> in β cells (β-KO) exhibited hypoinsulinemic hyperglycemia as early as postnatal day 9 (P9) due to decreases in both β cell mass and insulin secretion. Ki67 positive cells were reduced in β-KO mice at P9, thus β cell mass reduction was caused by proliferation disorder immediately after birth. The mRNA expression of <i>neurogenin3 (Ngn3)</i>, which is transiently expressed in endocrine progenitors of the embryonic pancreas, was maintained despite a striking reduction in the expression of β cell-associated genes, such as <i>insulin</i>, <i>Pancreatic and duodenal homeobox 1</i> <i>(Pdx1)</i> and <i>MAF BZIP transcription factor A (</i><i>Mafa)</i> in the islets from β-KO mice. Histological analyses revealed dysmorphic islets with markedly reduced numbers of β cells, some of which were also positive for glucagon. In conclusion, HMGCR plays critical roles not only in insulin secretion but also in the development of β cells in mice.

β cell-specific deletion of HMG-CoA (3-hydroxy-3-methylglutaryl-coenzyme A) reductase causes overt diabetes due to reduction of β cell mass and impaired insulin secretion

Inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR), statins, which are used to prevent cardiovascular diseases, are associated with a modest increase in the risk of new-onset diabetes mellitus. To investigate the role of HMGCR in the development of β cells and glucose homeostasis, we deleted <i>Hmgcr</i> in a β cell-specific manner by using the Cre-loxP technique. Mice lacking <i>Hmgcr</i> in β cells (β-KO) exhibited hypoinsulinemic hyperglycemia as early as postnatal day 9 (P9) due to decreases in both β cell mass and insulin secretion. Ki67 positive cells were reduced in β-KO mice at P9, thus β cell mass reduction was caused by proliferation disorder immediately after birth. The mRNA expression of <i>neurogenin3 (Ngn3)</i>, which is transiently expressed in endocrine progenitors of the embryonic pancreas, was maintained despite a striking reduction in the expression of β cell-associated genes, such as <i>insulin</i>, <i>Pancreatic and duodenal homeobox 1</i> <i>(Pdx1)</i> and <i>MAF BZIP transcription factor A (</i><i>Mafa)</i> in the islets from β-KO mice. Histological analyses revealed dysmorphic islets with markedly reduced numbers of β cells, some of which were also positive for glucagon. In conclusion, HMGCR plays critical roles not only in insulin secretion but also in the development of β cells in mice.