The Optimedin gene is a downstream target of Pax6

The Optimedin gene, also known as Olfactomedin 3, encodes an olfactomedin domain-containing protein. There are two major splice variants of the Optimedin mRNA, Optimedin A and Optimedin B, transcribed from different promoters. The expression pattern of the Optimedin A variant in the eye and brain overlaps with that for Pax6, which encodes a protein containing the paired and homeobox DNA-binding domains. The Pax6 gene plays a critical role for the development of eyes, central nervous system, and endocrine glands. The proximal promoter of the Optimedin A variant contains a putative Pax6 binding site in position -86/-70. Pax6 binds this site through the paired domain in vitro as judged by electrophoretic mobility shift assay. Mutations in this site eliminate Pax6 binding as well as stimulation of the Optimedin promoter activity by Pax6 in transfection experiments. Pax6 occupies the binding site in the proximal promoter in vivo as demonstrated by the chromatin immunoprecipitation assay. Altogether these results identify the Optimedin gene as a downstream target regulated by Pax6. Although the function of optimedin is still not clear, it is suggested to be involved in cell-cell adhesion and cell attachment to the extracellular matrix. Pax6 regulation of Optimedin in the eye and brain may directly affect multiple developmental processes, including cell migration and axon growth.     

Long-range downstream enhancers are essential for Pax6 expression

Pax6 is a developmental control gene with an essential role in development of the eye, brain and pancreas. Pax6, as many other developmental regulators, depends on a substantial number of cis-regulatory elements in addition to its promoters for correct spatiotemporal and quantitative expression. Here we report on our analysis of a set of mice transgenic for a modified yeast artificial chromosome carrying the human PAX6 locus. In this 420 kb YAC a tauGFP-IRES-Neomycin reporter cassette has been inserted into the PAX6 translational start site in exon 4. The YAC has been further engineered to insert LoxP sites flanking a 35 kb long, distant downstream regulatory region (DRR) containing previously described DNaseI hypersensitive sites, to allow direct comparison between the presence or absence of this region in the same genomic context. Five independent transgenic lines were obtained that vary in the extent of downstream PAX6 locus that has integrated. Analysis of transgenic embryos carrying full-length and truncated versions of the YAC indicates the location and putative function of several novel tissue-specific enhancers. Absence of these distal regulatory elements abolishes expression in specific tissues despite the presence of more proximal enhancers with overlapping specificity, strongly suggesting interaction between these control elements. Using plasmid-based reporter transgenic analysis we provide detailed characterization of one of these enhancers in isolation. Furthermore, we show that overexpression of a short PAX6 isoform derived from an internal promoter in a multicopy YAC transgenic line results in a microphthalmia phenotype. Finally, direct comparison of a single-copy line with the floxed DRR before and after Cre-mediated deletion demonstrates unequivocally the essential role of these long-range control elements for PAX6 expression.     

Six3 activation of Pax6 expression is essential for mammalian lens induction and specification

The homeobox gene Six3 regulates forebrain development. Here we show that Six3 is also crucial for lens formation. Conditional deletion of mouse Six3 in the presumptive lens ectoderm (PLE) disrupted lens formation. In the most severe cases, lens induction and specification were defective, and the lens placode and lens were absent. In Six3-mutant embryos, Pax6 was downregulated, and Sox2 was absent in the lens preplacodal ectoderm. Using ChIP, electrophoretic mobility shift assay, and luciferase reporter assays, we determined that Six3 activates Pax6 and Sox2 expression. Misexpression of mouse Six3 into chick embryos promoted the ectopic expansion of the ectodermal Pax6 expression domain. Our results position Six3 at the top of the regulatory pathway leading to lens formation. We conclude that Six3 directly activates Pax6 and probably also Sox2 in the PLE and regulates cell autonomously the earliest stages of mammalian lens induction.     

Pax6 is misexpressed in Sox1 null lens fiber cells

Sox1 null lens fiber cells fail to elongate and have disrupted expression of gamma-crystallin. We have evaluated the expression of Sox1 and Pax6 proteins during critical stages of lens morphogenesis, with particular focus on fiber cell differentiation. While Pax6 and Sox1 are co-expressed during early stages of fiber cell differentiation, Sox1 up-regulation coincides temporally with the down-regulation of Pax6, and these proteins therefore display a striking inverse expression pattern in the lens fiber cell compartment. Furthermore, Pax6 is inappropriately expressed in the fiber cells of Sox1 null mice and the Pax6 target, alpha5 integrin, is simultaneously misexpressed. Finally, we demonstrate a genetic interaction between Sox1 and Pax6, as Sox1 heterozygosity partially rescues the diameter of Pax6(Sey) lenses by increasing the number of cells in the fiber cell compartment.     

Ras signaling is essential for lens cell proliferation and lens growth during development

The vertebrate ocular lens is a simple and continuously growing tissue. Growth factor-mediated receptor tyrosine kinases (RTKs) are believed to be required for lens cell proliferation, differentiation and survival. The signaling pathways downstream of the RTKs remain to be elucidated. Here, we demonstrate the important role of Ras in lens development by expressing a dominant-negative form of Ras (dn-Ras) in the lens of transgenic mice. We show that lens in the transgenic mice was smaller and lens growth was severely inhibited as compared to the wild-type lens. However, the lens shape, polarity and transparency appeared normal in the transgenic mice. Further analysis showed that cell proliferation is inhibited in the dn-Ras lens. For example, the percentage of 5-bromo-2'-deoxyuridine (BrdU)-labeled cells in epithelial layer was about 2- to 3-fold lower in the transgenic lens than in the wild-type lens, implying that Ras activity is required for normal cell proliferation during lens development. We also found a small number of apoptotic cells in both epithelial and fiber compartment of the transgenic lens, suggesting that Ras also plays a role in cell survival. Interestingly, although there was a delay in primary fiber cell differentiation, secondary fiber cell differentiation was not significantly affected in the transgenic mice. For example, the expression of beta- and gamma-crystallins, the marker proteins for fiber differentiation, was not changed in the transgenic mice. Biochemical analysis indicated that ERK activity, but not Akt activity, was significantly reduced in the dn-Ras transgenic lenses. Overall, our data imply that the RTK-Ras-ERK signaling pathway is essential for cell proliferation and, to a lesser extent, for cell survival, but not for crystallin gene expression during fiber differentiation. Thus, some of the fiber differentiation processes are likely mediated by RTK-dependent but Ras-independent pathways.     

The mitochondrial transporter ABC-me (ABCB10), a downstream target of GATA-1, is essential for erythropoiesis in vivo

The mitochondrial transporter ATP binding cassette mitochondrial erythroid (ABC-me/ABCB10) is highly induced during erythroid differentiation by GATA-1 and its overexpression increases hemoglobin production rates in vitro. However, the role of ABC-me in erythropoiesis in vivo is unknown. Here we report for the first time that erythrocyte development in mice requires ABC-me. ABC-me-/- mice die at day 12.5 of gestation, showing nearly complete eradication of primitive erythropoiesis and lack of hemoglobinized cells at day 10.5. ABC-me-/- erythroid cells fail to differentiate because they exhibit a marked increase in apoptosis, both in vivo and ex vivo. Erythroid precursors are particularly sensitive to oxidative stress and ABC-me in the heart and its yeast ortholog multidrug resistance-like 1 have been shown to protect against oxidative stress. Thus, we hypothesized that increased apoptosis in ABC-me-/- erythroid precursors was caused by oxidative stress. Within this context, ABC-me deletion causes an increase in mitochondrial superoxide production and protein carbonylation in erythroid precursors. Furthermore, treatment of ABC-me-/- erythroid progenitors with the mitochondrial antioxidant MnTBAP (superoxide dismutase 2 mimetic) supports survival, ex vivo differentiation and increased hemoglobin production. Altogether, our findings demonstrate that ABC-me is essential for erythropoiesis in vivo.     

Function of Rx, but not Pax6, is essential for the formation of retinal progenitor cells in mice

Rx plays a critical role in eye formation. Targeted elimination of Rx results in embryos that do not develop eyes. In this study, we have investigated the expression of Otx2, Six3, and Pax6 in Rx deficient embryos. We find that these genes show normal activation in the anterior neural plate in Rx-/- embryos, but they are not upregulated in the area of the neural plate that would form the primordium of the optic vesicle. In contrast, in homozygous Small eye embryos that lack Pax6 function, Rx shows normal activation in the anterior neural plate and normal upregulation in the optic vesicle/retinal progenitor cells. This suggests that neither Rx expression nor the formation of retinal progenitor cells is dependent on a functional copy of the Pax6 gene, but that Pax6 expression and the formation of the progenitor cells of the optic cup is dependent on a functional copy of the Rx gene.