Regulation of the Pancreatic Islet-Specific GeneBETA2 (neuroD) by Neurogenin 3

Abstract

The BETA2 (neuroD) gene is expressed in endocrine cells during pancreas development and is essential for proper islet morphogenesis. The objective of this study is to identify potential upstream regulators of the BETA2 gene during pancreas development. We demonstrated that the expression of neurogenin 3 (ngn3), an islet-and neuron-specific basic-helix-loop-helix transcription factor, partially overlaps that of BETA2 during early mouse development. More importantly, overexpression of ngn3 can induce the ectopic expression of BETA2 in Xenopus embryos and stimulate the endogenous RNA of BETA2 in endocrine cell lines. Furthermore, overexpression of ngn3 could cause a dose-dependent activation on the 1.0-kb BETA2 promoter in islet-derived cell lines. Deletion and mutation analyses revealed that two proximal E box sequences, E1 and E3, could bind to ngn3-E47 heterodimer and mediate ngn3 activation. Based on these results, we hypothesize that ngn3 is involved in activating the expression of BETA2 at an early stage of islet cell differentiation through the E boxes in the BETA2 promoter.The endocrine pancreas, which is organized as the islets of Langerhans, contains at least four distinct types of endocrine cells (␣, ␤, ␦, and PP). The differentiation and maturation of islet cells during development is a complex process controlled by a unique network of gene regulation. Recently, it has been demonstrated by gene targeting studies that several tissuespecific transcription factors, such as BETA2 (neuroD) (24, 25), PDX-1 (1, 27), Islet-1 (2), Nkx2.2 (42), PAX-6 (41), and PAX-4 (40), are involved in this process. These factors, alone or in concert, can activate the expression of genes encoding hormones, such as glucagon (9, 44), insulin (9, 25, 28), and somatostatin (3, 31). BETA2 (neuroD), a basic helix-loop-helix (bHLH) transcription factor, was isolated both as a transcriptional activator of the insulin gene (25) and as a differentiation factor of neurogenesis (17). BETA2 is selectively expressed in the developing endocrine pancreas, the small intestine, and the nervous system (17). It has been shown that BETA2 transactivates the insulin (25) and glucagon genes (9) by binding to the E box sequences localized in their promoters. Furthermore, the functional importance of BETA2 to pancreatic islet cell development has been demonstrated by loss-of-function studies (24). BETA2-deficient (BETA2 Ϫ/Ϫ ) mice die of severe diabetes caused by a major reduction in the number of ␤ cells and a lack of proper islet formation. These results indicate that BETA2 plays an important role in maintaining the differentiation of endocrine cells and proper islet morphogenesis. Results obtained from BETA2-deficient mice also imply that the upstream factors controlling BETA2 expression are likely to be involved in the early events which determine endocrine cell differentiation. So far, numbers of a novel family of genes, the neurogenin genes (ngn) (19, 39), have been reported to be good candidates for upstream regulators of the BETA2 gene. During n...

Neurogenin3 inhibits proliferation in endocrine progenitors by inducing Cdkn1a

“…Our data demonstrate that without normal down-regulation of Neurog3, the β-cell population does not adequately expand and diabetes ensues. Interestingly, however, a very low level of Neurog3 expression persists in adult islets (30)(31)(32)(33). Recent studies demonstrated that this sustained low-level expression of Neurog3 contributes to endocrine function (33).…”

Pancreas organogenesis: From bud to plexus to gland

“…The direct targets of Ngn3 include NeuroD1/ Beta2, Pax4, Arx, Insm1, Rfx6, Nkx2.2, Myt1, and Snail2/Slug (Huang et al, 2000;Smith et al, 2003Smith et al, , 2004Smith et al, , 2010Watada et al, 2003;Mellitzer et al, 2006;Wang et al, 2007;Rukstalis and Habener, 2007;Soyer et al, 2010) (Fig. 2B).…”

“…This class of factors also permits endocrine cells to move into the mono-hormone-producing cell state. Several TF genes that fall into this category [e.g., NeuroD1 (Huang et al, 2000), Insm-1, Rfx6, and Islet1] are downstream, primarily direct targets, of Ngn3. NeuroD mutant mice have arrested endocrine cell development during the secondary transition, with reduced numbers of all endocrine cell types.…”

Ionizing radiation induces apoptosis and inhibits neuronal differentiation in rat neural stem cells via the c-Jun NH2-terminal kinase (JNK) pathway

“…The transcriptional activity of BETA2 (NeuroD) in NSCs was determined by the BETA2 promoter-luciferase reporter plasmid kindly provided by Dr Ming-Jer Tsai (Baylor College of Medicine, Houston, TX, USA) (Huang et al, 2000). Cells were plated at a density of approximately 5 Â 10 4 cells/ml, transfected with the luciferase reporter plasmids (1 mg each) using Fugen 6 transfection Reagent (Roche Applied Science) according to the manufacturer's instruction, cultured overnight and irradiated.…”