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...

The paired-like homeobox-containing gene Rx has a critical role in the eye development of several vertebrate species including Xenopus, mouse, chicken, medaka, zebrafish and human. Rx is initially expressed in the anterior neural region of developing embryos, and later in the retina and ventral hypothalamus. Abnormal regulation or function of Rx results in severe abnormalities of eye formation. Overexpression of Rx in Xenopus and zebrafish embryos leads to overproliferation of retinal cells. A targeted elimination of Rx in mice results in a lack of eye formation. Mutations in Rx genes are the cause of the mouse mutation eyeless (ey1), the medaka temperature sensitive mutation eyeless (el) and the zebrafish mutation chokh. In humans, mutations in Rx lead to anophthalmia. All of these studies indicate that Rx genes are key factors in vertebrate eye formation. Because these results cannot be easily reconciled with the most popular dogmas of the field, we offer our interpretation of eye development and evolution.

RGS (regulators of G protein signaling) proteins regulate G protein signaling by accelerating GTP hydrolysis, but little is known about regulation of GTPase-accelerating protein (GAP) activities or roles of domains and subunits outside the catalytic cores. RGS9-1 is the GAP required for rapid recovery of light responses in vertebrate photoreceptors and the only mammalian RGS protein with a defined physiological function. It belongs to an RGS subfamily whose members have multiple domains, including G ␥ -like domains that bind G ␤5 proteins. Members of this subfamily play important roles in neuronal signaling. Within the GAP complex organized around the RGS domain of RGS9-1, we have identified a functional role for the G ␥ -like-G ␤5L complex in regulation of GAP activity by an effector subunit, cGMP phosphodiesterase ␥ and in protein folding and stability of RGS9-1. The C-terminal domain of RGS9-1 also plays a major role in conferring effector stimulation. The sequence of the RGS domain determines whether the sign of the effector effect will be positive or negative. These roles were observed in vitro using fulllength proteins or fragments for RGS9-1, RGS7, G ␤5S , and G ␤5L . The dependence of RGS9-1 on G ␤5 co-expression for folding, stability, and function has been confirmed in vivo using transgenic Xenopus laevis. These results reveal how multiple domains and regulatory polypeptides work together to fine tune G t␣ inactivation.Most pathways for transducing signals from the cell surface to amplified second messenger cascades within cells of animals are organized around G proteins. Sufficient information is now available from the genomes of nematodes and humans to conclude that heptahelical transmembrane proteins of the G protein-coupled class constitute by far the largest class of receptors in these animals. The burden of communicating complex signals from this enormous variety of receptors must be borne by the relatively small number (on the order of 20) of distinct G protein ␣ subunits (G ␣ ) found in these genomes. It is hard to imagine such a scheme operating successfully unless the G proteins are helped in their task of encoding this information by additional regulatory proteins. Indeed, a family of proteins, comparable in size to the G ␣ family, have been found to be capable of exerting such regulation on activated G ␣ ; these are the RGS (regulators of G protein signaling) family of GTPaseaccelerating proteins (GAPs) 1 (1, 2). Among vertebrate RGS proteins, one whose physiological role in G protein signaling is particularly clear is the photoreceptor-specific isoform RGS9-1. Removal of RGS9-1 by immunodepletion (3) or gene inactivation (4) leads to loss of GTPase acceleration for the phototransduction G protein transducin (G t ), and without this GTPase acceleration, mouse rods have dramatically slowed photoresponses. The catalytic core of RGS9-1 is sufficient to accelerate GTP hydrolysis by G t (5-7), but there is clear evidence that RGS9-1 does not act alone in accelerating GTP hydrolysis. The PDE␥ sub...

Little is known about the expression of Pax2 in mature retina or optic nerve. Here we probed for the expression of Pax2 in late stages of embryonic development and in mature chick retina. We find two distinct Pax2 isoforms expressed by cells within the retina and optic nerve. Surprisingly, Müller glia in central regions of the retina express Pax2, and levels of expression are decreased with increasing distance from the nerve head. In Müller glia, the expression levels of Pax2 are increased by acute retinal damage or treatment with growth factors. At the optic nerve, Pax2 is expressed by peripapillary glia, at the junction of the neural retina and optic nerve head and by glia within the optic nerve. In addition, we assayed for Pax2 expression in glial cells in mammalian retinas. In mammalian retinas, unlike the case in chick retina, the Müller glia do not express Pax2. Pax2-expressing cells are found in the optic nerve and astrocytes within the mouse retina. By comparison, Pax2-positive cells are not found within the guinea pig retina; Pax2-expressing glia are confined to the optic nerve. In dog and monkey (Macaca fascicularis), Pax2 is expressed by astrocytes that are scattered across inner retinal layers and by numerous glia within the optic nerve. Interestingly, Pax2-positive glial cells are found at the peripheral edge of the dog retina, but only in older animals. We conclude that the expression of Pax2 in the vertebrate eye is restricted to retinal astrocytes, peripapillary glia, and glia within the optic nerve.