Inhibition of Lens Fiber Cell Morphogenesis by Expression of a Mutant SV40 Large T Antigen That Binds CREB-binding Protein/p300 but Not pRb

Lens apoptosis in the Astyanax blind cavefish is not triggered by its small size or defects in morphogenesis

Blindness is a convergent trait in many cave animals of various phyla. Astyanax mexicanus cavefish is one of the best studied cave animals; however the mechanisms underlying eye degeneration in this species are not yet completely understood. The lens seems to play a central role, but only relatively late differentiation defects have been implicated in the cavefish lens apoptosis phenotype so far. Here, we used genetic crosses between Astyanax cavefish and surface fish to confirm that during development, lens size is independent of retina size. We then investigated whether the small size of the cavefish lens could directly cause cell death. Laser ablation experiments of lens placode cells in surface fish embryos showed that a small lens size is not sufficient to trigger lens apoptosis. We further examined potential lens morphogenesis defects through classical histology and live-imaging microscopy. From lens placode to lens ball, we found that lens invagination and formation of the lens epithelium and fiber cells occur normally in cavefish. We conclude that the main and deleterious defect in the Astyanax cavefish lens must concern the molecular control of lens cell function.

Mesenchymal Deletion of Histone Demethylase NO66 in Mice Promotes Bone Formation

Our previous studies indicated that the Jumonji C (JmjC)-domain-containing NO66 is a histone demethylase with specificity for methylated histone H3K4 and H3K36. NO66 binds to the transcription factor Osterix (Osx) and inhibits its transcriptional activity in promoter assays. However, the physiological role of NO66 in formation of mammalian bones is unknown. Here, using a genetically engineered mouse model, we show that during early skeletal development, Prx1-Cre-dependent mesenchymal deletion of NO66 promotes osteogenesis and formation of both endochondral as well as intramembranous skeletal elements, leading to a larger skeleton and a high bone mass phenotype in adult mice. The excess bone formation in mice where NO66 was deleted in cells of mesenchymal origin is associated with an increase in the number of preosteoblasts and osteoblasts. Further analysis revealed that in the embryonic limbs and adult calvaria of mice with deletion of NO66 in cells of mesenchymal origin, expression of several genes including bone morphogenetic protein 2 (Bmp2), insulin-like growth factor 1 (Igf1), and osteoclast inhibitor osteoprotegerin was increased, concurrent with an increase in expression of bone formation markers such as osterix (Osx), type I collagen, and bone sialoprotein (Bsp). Taken together, our results provide the first in vivo evidence that NO66 histone demethylase plays an important role in mammalian osteogenesis during early development as well as in adult bone homeostasis. We postulate that NO66 regulates bone formation, at least in part, via regulating the number of bone-forming cells and expression of multiple genes that are critical for these processes.

Mesenchyme-specific overexpression of nucleolar protein 66 in mice inhibits skeletal growth and bone formation

Previous studies showed that nucleolar protein 66 (NO66), the Jumonji C-domain-containing histone demethylase for methylated histone H3K4 and H3K36 (H3K36me), negatively regulates osteoblast differentiation in vitro by inhibiting the activity of transcription factor osterix (Osx). However, whether NO66 affects mammalian skeletogenesis in vivo is not yet known. Here, we generated transgenic (TG) mice overexpressing a flag-tagged NO66 transgene driven by the Prx1 (paired related homeobox 1) promoter. We found that NO66 overexpression in Prx1-expressing mesenchymal cells inhibited skeletal growth and bone formation. The inhibitory phenotype was associated with >50% decreases in chondrocyte/osteoblast proliferation and differentiation. Moreover, we found that in bones of NO66-TG mice, expression of Igf1, Igf1 receptor (Igf1r), runt-related transcription factor 2, and Osx was significantly down-regulated (P < 0.05). Consistent with these results, we observed >50% reduction in levels of phosphorylated protein kinase B (Akt) and H3K36me3 in bones of NO66-TG mice, suggesting an inverse correlation between NO66 histone demethylase and the activity of IGF1R/Akt signaling. This correlation was further confirmed by in vitro assays of C2C12 cells with NO66 overexpression. We propose that the decrease in the IGF1R/Akt signaling pathway in mice with mesenchymal overexpression of NO66 may contribute in part to the inhibition of skeletal growth and bone formation.

Ectopic activation of Wnt/β-catenin signaling in lens fiber cells results in cataract formation and aberrant fiber cell differentiation

The Wnt/β-catenin signaling pathway controls many processes during development, including cell proliferation, cell differentiation and tissue homeostasis, and its aberrant regulation has been linked to various pathologies. In this study we investigated the effect of ectopic activation of Wnt/β-catenin signaling during lens fiber cell differentiation. To activate Wnt/β-catenin signaling in lens fiber cells, the transgenic mouse referred to as αA-CLEF was generated, in which the transactivation domain of β-catenin was fused to the DNA-binding protein LEF1, and expression of the transgene was controlled by αA-crystallin promoter. Constitutive activation of Wnt/β-catenin signaling in lens fiber cells of αA-CLEF mice resulted in abnormal and delayed fiber cell differentiation. Moreover, adult αA-CLEF mice developed cataract, microphthalmia and manifested downregulated levels of γ-crystallins in lenses. We provide evidence of aberrant expression of cell cycle regulators in embryonic lenses of αA-CLEF transgenic mice resulting in the delay in cell cycle exit and in the shift of fiber cell differentiation to the central fiber cell compartment. Our results indicate that precise regulation of the Wnt/β-catenin signaling activity during later stages of lens development is essential for proper lens fiber cell differentiation and lens transparency.

Histone posttranslational modifications and cell fate determination: lens induction requires the lysine acetyltransferases CBP and p300

Lens induction is a classical embryologic model to study cell fate determination. It has been proposed earlier that specific changes in core histone modifications accompany the process of cell fate specification and determination. The lysine acetyltransferases CBP and p300 function as principal enzymes that modify core histones to facilitate specific gene expression. Herein, we performed conditional inactivation of both CBP and p300 in the ectodermal cells that give rise to the lens placode. Inactivation of both CBP and p300 resulted in the dramatic discontinuation of all aspects of lens specification and organogenesis, resulting in aphakia. The CBP/p300^−/− ectodermal cells are viable and not prone to apoptosis. These cells showed reduced expression of Six3 and Sox2, while expression of Pax6 was not upregulated, indicating discontinuation of lens induction. Consequently, expression of αB- and αA-crystallins was not initiated. Mutant ectoderm exhibited markedly reduced levels of histone H3 K18 and K27 acetylation, subtly increased H3 K27me3 and unaltered overall levels of H3 K9ac and H3 K4me3. Our data demonstrate that CBP and p300 are required to establish lens cell-type identity during lens induction, and suggest that posttranslational histone modifications are integral to normal cell fate determination in the mammalian lens.

Novel and established intestinal cell line models - An indispensable tool in food science and nutrition

This review presents the applications of intestinal cell models of human and pig origin in food and nutritional sciences and highlights their potential as in vitro platforms for preclinical research. Intestinal cell models are used in studies of bioavailability, adsorption and transport in nutritional or toxicological settings, allergic effects of food components, as well as probiotics and/or host-pathogen gut interactions. In addition, this review discusses the advantages of using specialized and functional cell models over generic cancer-derived cell lines.

Retinoic acid signaling in mammalian eye development

Retinoic acid (RA) is a biologically active metabolite of vitamin A (retinol) that serves as a signaling molecule during a number of developmental and physiological processes. RA signaling plays multiple roles during embryonic eye development. RA signaling is initially required for reciprocal interactions between the optic vesicle and invaginating lens placode. RA signaling promotes normal development of the ventral retina and optic nerve through its activities in the neural crest cell-derived periocular mesenchyme. RA coordinates these processes by regulating biological activities of a family of non-steroid hormone receptors, RARalpha/beta/gamma, and RXRalpha/beta/gamma. These DNA-binding transcription factors recognize DNA as RAR/RXR heterodimers and recruit multiprotein transcriptional co-repressor complexes. RA-binding to RAR receptors induces a conformational change in the receptor, followed by the replacement of co-repressor with co-activator complexes. Inactivation of RARalpha/beta/gamma receptors in the periocular mesenchyme abrogates anterior eye segment formation. This review summarizes recent genetic studies of RA signaling and progress in understanding the molecular mechanism of transcriptional co-activators that function with RAR/RXR.

T antigen transgenic mouse models

The study of polyomavirus has benefited immensely from two scientific methodologies, cell culture and in vitro studies on one side and the use of transgenic mice as experimental models on the other. Both approaches allowed us to identify cellular products targeted by the viruses, the consequences of these interactions at the phenotypic and molecular level, and thus the potential roles of the targets within their normal cellular context. In particular, cell culture and in vitro reports suggest a model explaining partially how SV40 large T antigen contributes to oncogenic transformation. In most cases, T antigen induces cell cycle entry by inactivation of the Rb proteins (pRb, p130, and p107), thus activating E2F-dependent transcription and subsequent S-phase entry. Simultaneously, T antigen blocks p53 activity and therefore prevents the ensuing cell-cycle arrest and apoptosis. For the most part, studies of T antigen expression in transgenic mice support this model, but the use of T antigen mutants and their expression in different tissue and cell type settings have expanded our knowledge of the model system and raised important questions regarding tumorigenic mechanisms functioning in vivo.

Distinct embryonic expression and localization of CBP and p300 histone acetyltransferases at the mouse alphaA-crystallin locus in lens

Mouse alphaA-crystallin gene encodes the most abundant protein of the mammalian lens. Expression of alphaA-crystallin is regulated temporally and spatially during lens development with initial expression in the lens vesicle followed by strong upregulation in the differentiating primary lens fibers. Lens-specific expression of alphaA-crystallin is mediated by DNA-binding transcription factors Pax6, c-Maf and CREB bound to its promoter region. Its 5'-distal enhancer, DCR1, mediates regulation of alphaA-crystallin via FGF signaling, while its 3'-distal enhancer, DCR3, functions only in elongated primary lens fibers via other lens differentiation pathways. DCR1 and DCR3 establish outside borders of a lens-specific chromatin region marked by histone H3 K9 acetylation. Here, we identified CREB-binding protein (CBP) and p300 as major histone acetyltransferases (HATs) associated in vivo with the mouse alphaA-crystallin locus. Both HATs are expressed in embryonic lens. Expression of CBP in primary lens fiber cells coincides with alphaA-crystallin. In the chromatin of lens epithelial cells, chromatin immunoprecipitations (ChIPs) show that the alphaA-crystallin promoter is notably devoid of any significant presence of CBP and p300, though DCR1 and a few other regions show the presence of these HATs. In the chromatin obtained from newborn lens, CBP was localized specifically at the promoter region with about ten times higher abundance compared to the entire alphaA-crystallin locus. In contrast, p300 is distributed more evenly across the entire locus. Analysis of total histone H3 and H3 K9 acetylation revealed potential lower density of nucleosomes 2 kb upstream from the promoter region. Collectively, our data suggest that moderate level of alphaA-crystallin gene expression in lens epithelial cells does not require the presence of CBP and p300 in the promoter. However, the lens-specific chromatin domain contains both promoter localized CBP on the "background" of locus-spread presence of CBP and p300.

Apoptosis in lens development and pathology

The ocular lens is a distinct system to study cell death for the following reasons. First, during animal development, the ocular lens is crafted into its unique shape. The crafting processes include cell proliferation, cell migration, and apoptosis. Moreover, the lens epithelial cells differentiate into lens fiber cells through a process, which utilizes the same regulators as those in apoptosis at multiple signaling steps. In addition, introduction of exogenous wild-type or mutant genes or knock-out of the endogenous genes leads to apoptosis of the lens epithelial cells followed by absence of the ocular lens or formation of abnormal lens. Finally, both in vitro and in vivo studies have shown that treatment of adult lens with stress factors induces apoptosis of lens epithelial cells, which is followed by cataractogenesis. The present review summarizes the current knowledge on apoptosis in the ocular lens with emphasis on its role in lens development and pathology.

Targeting of p300/CREB binding protein coactivators by simian virus 40 is mediated through p53

The primary transforming functions of simian virus 40 large T antigen (SV40 LT) are conferred primarily through the binding and inactivation of p53 and the retinoblastoma family members. Normal p53 function requires an association with the CREB binding protein (CBP)/p300 coactivators, and a ternary complex containing SV40 LT, p53, and CBP/p300 has been identified previously. In this report, we have evaluated a secondary function of p53 bound to the SV40 LT complex in mediating the binding of human CBP/p300. We demonstrate that p53 associated with SV40 LT was posttranslationally modified in a manner consistent with the binding of CBP/p300. Furthermore, expression of SV40 LT induced the proportion of p53 phosphorylated on S15. An essential function for p53 in bridging the interaction between SV40 LT and CBP/p300 was identified through the reconstitution of the SV40 LT-CBP/p300 complex upon p53 reexpression in p53-null cells. In addition, the SV40 LT-CBP/p300 complex was disrupted through RNA interference-mediated depletion of endogenous p53. We also demonstrate that SV40 LT was acetylated in a p300- and p53-dependent manner, at least in part through the CH3 domain of p300. Therefore, the binding of p53 serves to modify SV40 LT by targeting CBP and p300 binding to direct the acetylation of SV40 LT.

SV40 large T antigen targets multiple cellular pathways to elicit cellular transformation

DNA tumor viruses such as simian virus 40 (SV40) express dominant acting oncoproteins that exert their effects by associating with key cellular targets and altering the signaling pathways they govern. Thus, tumor viruses have proved to be invaluable aids in identifying proteins that participate in tumorigenesis, and in understanding the molecular basis for the transformed phenotype. The roles played by the SV40-encoded 708 amino-acid large T antigen (T antigen), and 174 amino acid small T antigen (t antigen), in transformation have been examined extensively. These studies have firmly established that large T antigen's inhibition of the p53 and Rb-family of tumor suppressors and small T antigen's action on the pp2A phosphatase, are important for SV40-induced transformation. It is not yet clear if the Rb, p53 and pp2A proteins are the only targets through which SV40 transforms cells, or whether additional targets await discovery. Finally, expression of SV40 oncoproteins in transgenic mice results in effects ranging from hyperplasia to invasive carcinoma accompanied by metastasis, depending on the tissue in which they are expressed. Thus, the consequences of SV40 action on these targets depend on the cell type being studied. The identification of additional cellular targets important for transformation, and understanding the molecular basis for the cell type-specific action of the viral T antigens are two important areas through which SV40 will continue to contribute to our understanding of cancer.

Distinct capacities of individual E2Fs to induce cell cycle re-entry in postmitotic lens fiber cells of transgenic mice

PURPOSE

Inactivation of the retinoblastoma gene in human retinoblasts or mouse lens fiber cells causes inappropriate cell cycle entry, presumably as a consequence of elevated activity of the E2F transcription factors. Although E2Fs are known to be critical regulators of the cell cycle, it is still unclear whether family members E2F3a, E2F4 or E2F5 are individually capable of inducing cell cycle entry in vivo. In this study, we designed experiments to test whether lens-specific expression of these E2F family members would induce postmitotic fiber cells to re-enter the cell cycle.

METHODS

Transgenic mice were generated by microinjection of constructs that contained E2F cDNAs (E2F3a, E2F4 or E2F5) linked to the mouse lens-specific alphaA-crystallin promoter. The mice were characterized by histology, in situ hybridization, immunohistochemistry, BrdU incorporation, TUNEL assay and Western blots.

RESULTS

E2F3a expression was sufficient to induce cell cycle entry in lens fiber cells. Cell cycle re-entry was accompanied by apoptotic cell death resulting in microphthalmia. E2F4 expression stimulated a modest level of cell cycle re-entry, but the transgenic lenses remained normal in size and did not show significant apoptosis. Transgenic mice expressing E2F5 did not show lens defects. In both the E2F3a and E2F4 transgenic lenses, cyclin A2 and cyclin B1 expression were upregulated. Phosphorylated histone H3, a marker for mitosis, was detected in the E2F3a fiber cells. Western blots showed that both p53 and p73alpha were upregulated in the E2F3a lenses. However, expression of p21, a well-known p53 target gene, was not activated, suggesting that p73alpha might be responsible for inducing apoptosis and blocking unregulated proliferation in lens cells overexpressing E2F3a.

CONCLUSIONS

E2F3a and E2F4, but not E2F5, function to induce cell cycle entry, although E2F4 has more modest activity. E2F3a may induce cell death primarily through activation of p73alpha.