Induction of lens differentiation by activation of a bZIP transcription factor, L-Maf

FGF19-FGFR4 signaling elaborates lens induction with the FGF8-L-Maf cascade in the chick embryo

The fibroblast growth factor (FGF) family is known to be involved in vertebrate eye development. However, distinct roles of individual FGF members during eye development remain largely elusive. Here, we show a detailed expression pattern of Fgf19 in chick lens development. Fgf19 expression initiated in the forebrain, and then became restricted to the distal portion of the optic vesicle abutting the future lens placode, where FGF receptor 4 (Fgfr4), a receptor for FGF19, was expressed. Fgf8, a positive regulator for L-Maf, was expressed in a portion of the optic vesicle. To examine the role of FGF19 signaling during early eye development, Fgf19 was misexpressed near the presumptive lens ectoderm; however, no alteration in the expression of lens marker genes was observed. Conversely, a secreted form of FGFR4 was misexpressed to inhibit an FGF19 signal, resulting in the induction of L-Maf expression. To further define the relationship between L-Maf and Fgf19, L-Maf misexpression was performed, resulting in ectopic induction of Fgf19 expression by Hamburger and Hamilton's stage 12/13. Furthermore, misexpression of Fgf8 induced Fgf19 expression in addition to L-Maf. These results suggest that FGF19-FGFR4 signaling plays a role in early lens development in collaboration with FGF8 signaling and L-Maf transcriptional system.

Human telomerase reverse transcriptase immortalizes bovine lens epithelial cells and suppresses differentiation through regulation of the ERK signaling pathway

Telomerase is a specialized reverse transcriptase that extends telomeres of eukaryotic chromosomes. The functional telomerase complex contains a telomerase reverse transcriptase catalytic subunit and a telomerase template RNA. We have previously demonstrated that human telomerase reverse transcriptase (hTERT) catalytic subunit is functionally compatible with a telomerase template RNA from rabbit. In this study, we show that hTERT is also functionally compatible with a telomerase template RNA from bovine. Introduction of hTERT into bovine lens epithelial cells (BLECs) provides the transfected cells telomerase activity. The expressed hTERT in BLECs supports normal growth of the transfected cells for 108 population doublings so far, and these cells are still extremely healthy in both morphology and growth. In contrast, the vector-transfected cells display growth crisis after 20 population doublings. These cells run into cellular senescence due to shortening of the telomeres and also commit differentiation as indicated by the accumulation of the differentiation markers, beta-crystallin and filensin. hTERT prevents the occurrence of both events. By synthesizing new telomere, hTERT prevents replicative senescence, and through regulation of MEK/ERK, protein kinase C, and protein kinase A and eventual suppression of the MEK/ERK signaling pathway, hTERT inhibits differentiation of BLECs. Our finding that hTERT can suppress RAS/RAF/MEK/ERK signaling pathway to prevent differentiation provides a novel mechanism to explain how hTERT regulates cell differentiation.

Early regeneration genes: Building a molecular profile for shared expression in cornea-lens transdifferentiation and hindlimb regeneration in Xenopus laevis

Recent studies in Xenopus laevis have begun to compare gene expression during regeneration with that of the original development of specific structures (e.g., the hindlimb and lens), while other studies have sought differences in gene expression between regeneration-competent and regeneration-incompetent stages. To determine whether there are any similarities between the regeneration of different structures, we have used a differential screen to seek shared early gene expression between hindlimb regeneration and cornea-lens transdifferentiation in the Xenopus tadpole. We have isolated 13 clones representing genes whose expression is up-regulated within the first few days of both regenerating processes and which are not demonstrably up-regulated in the context of basic wound healing. Furthermore, all of these genes also show prominent late embryonic expression. The expression patterns and putative identities of all 13 genes are presented, and a model is considered that allows us to characterize and profile important changes in gene expression, which might be shared among various regenerating and developmental systems.

Phylogenomic analysis and expression patterns of large Maf genes in Xenopus tropicalis provide new insights into the functional evolution of the gene family in osteichthyans

We have performed an exhaustive characterization of the large Maf family of basic leucine zipper transcription factors in vertebrates using the genome data available, and studied the embryonic expression patterns of the four paralogous genes thus identified in Xenopus tropicalis. Our phylogenetic analysis shows that, in osteichthyans, the large Maf family contains four orthology classes, MafA, MafB, c-Maf and Nrl, which have emerged in vertebrates prior to the split between actinopterygians and sarcopterygians. It leads to the unambiguous assignment of the Xenopus laevis XLmaf gene, previously considered a MafA orthologue, to the Nrl class, the identification of the amphibian MafA and c-Maf orthologues and the identification of the zebrafish Nrl gene. The four X. tropicalis paralogues display partially redundant but nevertheless distinct expression patterns in the somites, developing hindbrain, pronephros, ventral blood island and lens. Comparisons with the data available in the mouse, chick and zebrafish show that these large Maf expression territories are highly conserved among osteichthyans but also highlight a number of differences in the timing of large Maf gene expression, the precise extent of some labelled territories and the combinations of paralogues transcribed in some organs. In particular, the availability of robust phylogenies leads to a reinterpretation of previous expression pattern comparisons, suggesting an important part for function shuffling within the gene family in the developing lens. These data highlight the importance of exhaustive characterizations of gene families for comparative analyses of the genetic mechanisms, which control developmental processes in vertebrates.

DeltaFosB expression and cataract

DeltaFosB is a truncated form of a FosB transcription factor, which is created by alternative splicing. Previous work has shown that transgenic mice expressing DeltaFosB both in the retina and in the lens developed a posterior subcapsular cataract resulting from the misalignment of the fibres in the suture region. In the previous study, it was not clear whether DeltaFosB expression was required in both tissues to produce the cataract. Therefore, DeltaFosB expression targeted to either the lens or the retina was undertaken in order to clarify the contribution of each tissue to cataract development. For lens expression, the R2betaB1DeltaFosB construct was synthesized (R2, an enhancer; betaB1, a chicken betaB1 crystallin gene promoter fragment). For the retina, RhoDeltaFosB was prepared. As a promoter, the bovine rhodopsin upstream region was used. DeltaFosB expression in heterozygote animals was monitored by Western blotting. Cataract development in heterozygotes of R2betaB1DeltaFosB transgenics and in both heterozygotes and homozygotes of RhoDeltaFosB transgenics was followed by slitlamp examination. The transgenic mice prepared with RhoDeltaFosB expressed DeltaFosB only in the retina and showed no sign of lens opacity. One line of the R2betaB1DeltaFosB transgenic was found to have expression only in the lens and developed posterior subcapsular cataract. We concluded that retinal expression of DeltaFosB is not sufficient to cause cataract while expression exclusively in the lens produces posterior subcapsular cataract.

Transcriptional regulation of mouse alphaB- and gammaF-crystallin genes in lens: opposite promoter-specific interactions between Pax6 and large Maf transcription factors

Mammalian alphaB-crystallin is highly expressed both in lens epithelium and lens fibers. In contrast, gammaF-crystallin is highly expressed in the lens fiber cells. Crystallin gene expression in lens is regulated at the level of transcription by a sparse number of specific DNA-binding transcription factors. Here, we report studies on transcriptional regulation of mouse alphaB- and gammaF-crystallin promoters by specific combinations of Pax6/Pax6(5a), large Mafs (MafA, MafB, c-Maf, and NRL), Sox1, Sox2, Six3, and RARbeta/RXRbeta. Two sets of these factors, co-expressed both in lens epithelium and in lens fibers, were tested in co-transfection assays using cultured lens and non-lens cells. Regulation of alphaB-crystallin was studied in the presence of lens epithelial-factors Pax6, MafB, and RARbeta/RXRbeta, and lens fiber-factors Pax6, MafA, c-Maf, and NRL. Pax6 proteins activated the alphaB-crystallin promoter (-162 to +45) with any combination of Mafs. Addition of RARbeta/RXRbeta further increased its promoter activity. Gel shift assays using lens nuclear extracts demonstrated interactions of Pax6, Maf, and retinoic acid nuclear receptor proteins with two lens-specific regions, the distal LSR1 (-147/-118) and proximal LSR2 (-78/-40), of the alphaB-crystallin promoter. In contrast, Pax6 proteins acted as repressors of gammaF-crystallin promoter activity elicited by a combination of large Mafs, Sox, and RARbeta/RXRbeta proteins in transiently transfected lens and non-lens cells. The results show that Pax6 conversely regulates these two lens crystallin promoters. We propose that the opposite roles of Pax6 in crystallin gene regulation are results of different promoter architectures of the alphaB- and gammaF-crystallin genes, developmentally regulated association of transcription factors with the corresponding cis-regulatory sites, and specific recruitment of transcriptional co-activators and co-repressors by Pax6.

The adaptor molecule FADD from Xenopus laevis demonstrates evolutionary conservation of its pro-apoptotic activity

FADD is an adaptor protein that transmits apoptotic signals from death receptors such as Fas to downstream initiator caspases in mammals. We have identified and characterized the Xenopus orthologue of mammalian FADD (xFADD). xFADD contains both a death effector domain (DED) and a death domain (DD) that are structurally homologous to those of mammalian FADD. We observed xFADD binding to Xenopus caspase-8 and caspase-10 as well as to human caspase-8 and Fas through interactions with their homophilic DED and DD domains. When over-expressed, xFADD was also able to induce apoptosis in wild-type mouse embryonic fibroblasts (MEF), but not in caspase-8-deficient MEF cells. In contrast, DED-deficient xFADD (xFADDdn) acted as a dominant-negative mutant and prevented Fas-mediated apoptosis in mammalian cell lines. These results indicate that xFADD transmits apoptotic signals from Fas to caspase-8. Furthermore, we found that transgenic animals expressing xFADD in the developing heart or eye under the control of tissue-specific promoters show abnormal phenotypes. Taken together, these results suggest that xFADD can substitute functionally for its mammalian homologue in death receptor-mediated apoptosis, and we suggest that xFADD functions as a pro-apoptotic adaptor molecule in frogs. Thus, the structural and functional similarities between xFADD and mammalian FADD provide evidence that the apoptotic pathways are evolutionally conserved across vertebrate species.

Combination electro-gene therapy using herpes virus thymidine kinase and interleukin-12 expression plasmids is highly efficient against murine carcinomas in vivo

We report the use of plasmid DNA-mediated combination gene therapy for tumor-bearing mice using in vivo electroporation, also called electro-gene therapy (EGT), that resulted in uncomplicated and complete cures in more than 90% of the mice. Subcutaneously inoculated CT26 tumors in syngeneic BALB/c mice were subjected to repeated EGT treatments consisting of intratumoral co-injection of naked plasmids encoding the cytokine interleukin-12 (IL-12) (p35 and p40 subunits) and the suicide gene herpes simplex virus thymidine kinase (HSV-tk), followed by in vivo electroporation. The early anti-tumor effect was always stronger, and the rate of cure, as seen in the long-term follow-up, was always greater in the groups treated with combination EGT than in those treated with IL-12 or HSV-tk EGT alone. Systemic levels of IL-12 and IFN-gamma increased in both combination and IL-12-alone EGT-treated groups. Moreover, combination EGT for established subcutaneous tumors strongly reduced hematogenous lung metastases and increased survival time when live CT26 tumor cells were injected through the tail vein. Limited experiments on C57/B16 mice with murine melanoma also showed very similar trends. These results suggest that this simple and safe method of plasmid-mediated combination EGT may provide a potentially effective gene therapy for cancer.

Comparison of maf gene expression patterns during chick embryo development

Maf proteins are basic-leucine zipper transcription factors belonging to the AP1 superfamily. Several developmental processes require Maf proteins yet, the redundancy or complementarity of their respective roles in common processes has been only partially investigated. We present for the first time a complete comparative analysis of maf gene expression patterns in vertebrates. Expression of c-maf, mafB/kreisler, mafA/L-maf, mafF, mafG and mafK was analyzed by whole-mount in situ hybridization within chick embryos and their extraembryonic tissues ranging from embryonic day (E) 1 to 7. We carefully examined the extent of overlap between distinct maf genes and report that the developing lens, kidney, pancreas and apoptotic zones of limb buds show sustained co-expression of large maf genes. Small maf genes also exhibit overlap, for example in the dermomyotome. We also describe so far unidentified sites of maf gene expression. mafA is found in the developing neural tube and dorsal root ganglia. c-maf hybridization is detected in the neuroretina, the notochord and the endothelium of extraembryonic blood vessels.

Evaluation of MafG interaction with Maf recognition element arrays by surface plasmon resonance imaging technique

Specific interactions between transcription factors and cis-acting DNA sequence motifs are primary events for the transcriptional regulation. Many regulatory elements appear to diverge from the most optimal recognition sequences. To evaluate affinities of a transcription factor to various suboptimal sequences, we have developed a new detection method based on the surface plasmon resonance (SPR) imaging technique. Transcription factor MafG and its recognition sequence MARE (Maf recognition elements) were adopted to evaluate the new method. We modified DNA immobilization procedure on to the gold chip, so that a double-stranded DNA array was successfully fabricated. We further found that a hydrophilic flexible spacer composed of the poly (ethylene glycol) moiety between DNA and alkanethiol self-assembled monolayers on the surface is effective for preventing nonspecific adsorption and facilitating specific binding of MafG. Multiple interaction profiles between MafG and six of MARE-related sequences were observed by the SPR imaging technique. The kinetic values obtained by SPR imaging showed very good correlation with those obtained from electrophoretic gel mobility shift assays, although absolute values were deviated from each other. These results demonstrate that the double-stranded DNA array fabricated with the modified multistep procedure can be applied for the comprehensive analysis of the transcription factor-DNA interaction.

In ovo electroporation of avian somites

In ovo electroporation is a well-established method of gene transfer into neural and mesenchymal tissue in chick embryos. Electroporation of somites, however, has been hampered by low efficiency due to technical difficulties. Here, we present a powerful technique to electroporate avian somites and subpopulations of somitic cells at high efficiency in ovo. We demonstrate specific targeting of distinct somitic compartments and their derivatives using single or combinations of plasmid expression vectors. This technique opens new perspectives to investigate the morphologic and genetic basis of somite development.

Roles of Maf family proteins in lens development

Lens provides a good model for studying developmental cues relevant to cellular and molecular interactions. Basic region/leucine zipper (bZIP) transcription factors have been found to play key roles during eye formation in various species, including human, mouse, rat, Xenopus, zebrafish, chick, and quail. Different ocular developmental anomalies associated with MAF mutation in human implicate its active role during eye development. Several members of the maf gene family with this bZIP motif participate directly in lens morphogenesis. One vital Maf protein, L-Maf, is expressed in developing lens cells of chick embryos. Its homolog recently has been detected in lens placode of Xenopus embryos and regulates expression of lens fiber-specific genes in this species. Ectopic expression of L-Maf can induce lens-specific genes in cultured retina cells and embryonic ectoderm. The dominant-negative form of L-Maf causes the suppression of crystallin expression and subsequently inhibits lens formation, indicating that L-Maf plays a central role in chick lens development.

Altered Expression of Genes of the Bmp/Smad and Wnt/Calcium Signaling Pathways in the Cone-only Nrl-/- Mouse Retina, Revealed by Gene Profiling Using Custom cDNA Microarrays

Many mammalian retinas are rod-dominant, and hence our knowledge of cone photoreceptor biology is relatively limited. To gain insights into the molecular differences between rods and cones, we compared the gene expression profile of the rod-dominated retina of wild type mouse with that of the cone-only retina of Nrl(-/-) (Neural retina leucine zipper knockout) mouse. Our analysis, using custom microarrays of eye-expressed genes, provided equivalent data using either direct or reference-based experimental designs, confirmed differential expression of rod- and cone-specific genes in the Nrl(-/-) retina and identified novel genes that could serve as candidates for retinopathies or for functional studies. In addition, we detected altered expression of several genes that encode cell signaling or structural proteins. Prompted by these findings, additional real-time PCR analysis revealed that genes belonging to the Bmp/Smad and Wnt/Ca(2+) signaling pathways are expressed in the mature wild type retina and that their expression is significantly altered in the Nrl(-/-) retina. Chromatin immunoprecipitation analysis of adult retina identified Bmp4 and Smad4, which are down-regulated in the Nrl(-/-) retina, as possible direct transcriptional targets of Nrl. Consistent with these studies, Bmp4 and Smad4 are expressed in the mature rod photoreceptors of mouse retina. Modulation of Bmp4 and/or Smad4 by Nrl may provide a mechanism for integrating diverse cell signaling networks in rods. We hypothesize that Bmp/Smad and Wnt/Ca(2+) pathways participate in cell-cell communication in the mature retina, and expression changes observed in the Nrl(-/-) retina reflect their biased utilization in rod versus cone homeostasis.

Developmental contribution of c-maf in the kidney: distribution and developmental study of c-maf mRNA in normal mice kidney and histological study of c-maf knockout mice kidney and liver

Maf is a family of oncogenes which encodes a nuclear bZip transcription factor protein and has been originally identified from the avian oncogenic retrovirus, AS42. Maf genes have been reported to have critical roles in embryological development and cellular differentiation. In this study, in situ hybridization with (35)S-labeled antisense riboprobes was used to investigate the distribution of c-maf mRNA in balb/c mouse kidneys from 12 (E12) through 17 days (E17) of gestation and then 1 and 4 weeks after birth. Immunocytochemistry of 4-week-old mouse kidney using anti-c-maf antisera was also performed. Kidney and liver sections from c-maf knockout mice at 4 weeks were stained with hematoxylin-eosin, and their histological features were examined. Expression of c-maf mRNA was first detected on E16 in the renal proximal tubules, and it was expressed through 4 weeks after birth. In the c-maf knockout mice at 4 weeks the cytoplasmic volume of the proximal tubule and liver cell was smaller. These findings suggest that expression of the c-maf gene may be involved in the embryological development and/or cell differentiation of kidney and liver cells.

Temporal expression of L-Maf and RaxL in developing chicken retina are arranged into mosaic pattern

Members of the maf gene family encode basic/leucine zipper transcription factors and play important roles during cell differentiation in vertebrate and Drosophila development. To investigate the possible roles of chicken lens-specific Maf (L-Maf)/MafA in retinal development, we analyzed its expression in the developing chicken retina. We first determined that L-Maf is expressed in a subset of ganglion cells, amacrine cells and rod cells during retinal development. To characterize further the L-Maf expressing cells during photoreceptor development, we compared the expression patterns of L-Maf and RaxL, a marker of cone cells. We found the L-Maf proteins are detectable from E10 between RaxL positive cone cells in outer nuclear layer (ONL). Subsequently, the expression of L-Maf is restricted to the innermost nuclear cells in the ONL during rod differentiation. In contrast, RaxL positive cone cells are distributed in the outermost layer of ONL. Furthermore, tangential retinal sections showed that L-Maf positive rods and RaxL positive cones form a mosaic pattern during photoreceptor development.

MafT, a new member of the small Maf protein family in zebrafish

Small Maf proteins play critical roles on morphogenesis and homeostasis through associating with CNC proteins. To date, three small Maf proteins, MafF, MafG, and MafK, have been reported in vertebrates, which share redundant functions. In this study, we tried to identify and characterize small Maf proteins in zebrafish to elucidate their conservation and diversity in the fish kingdom. We identified homolog genes of MafG and MafK but not MafF in zebrafish, indicating the former two are conserved among vertebrates. In addition, a novel type of small Maf protein MafT was identified. MafT protein bound MARE sequence as a homodimer or heterodimers with zebrafish Nrf2 or p45 Nfe2. Co-overexpression of MafT and Nrf2 synergistically activated MARE-mediated gene expression in zebrafish embryos. These results indicated that MafT is a new member of small Maf proteins and involved in the Nrf2-dependent gene regulation in cellular defense system.

New tools for gene manipulation in chicken embryos

Genomics has changed the pace by which genes are analyzed. Rather than looking at genes one by one, gene expression today is studied at the genome level. Unfortunately, the data we get from microarray analysis do not give us any clues about the function of these genes. Functional analyses are still refractory to large-scale, high-throughput studies, particularly in vertebrates. With the development of in ovo RNAi as a tool for specific gene silencing, the chicken embryo has become an efficient in vivo system to study gene function during development. A major advantage of in ovo RNAi is the fact that the knowledge of a cDNA fragment of the gene of interest is sufficient to get loss-of-function phenotypes. Thus, this new approach is a valuable tool for functional genomics.

Mouse MafA, homologue of zebrafish somite Maf 1, contributes to the specific transcriptional activity through the insulin promoter

Large Maf transcription factors, which are members of the basic leucine zipper (b-Zip) superfamily, have been reported to be involved in embryonic development and cell differentiation. Previously, we isolated a novel zebrafish large Maf cDNA, somite Maf1 (SMaf1), which possesses transactivational activity within its N-terminus domain. To elucidate SMaf1 function in mammals, we tried to isolate the mouse homologue of zebrafish SMaf1. We isolated the mouse homologue of zebrafish SMaf1, which is the same molecule as the recently reported MafA. MafA mRNA was detected in formed somites, head neural tube, and liver cells in the embryos. In the adult mouse, MafA transcript was amplified in the brain, lung, spleen, and kidney by RT-PCR. MafA mRNA was also detectable in beta-cell line. Next, we analyzed the transcriptional activity of MafA using rat insulin promoters I and II (RIPI and II), since a part of RIP sequence was similar to the Maf recognition element (MARE) and MafA was expressed in pancreatic beta cells. MafA was able to activate transcription from RIPII, but not RIPI, in a dose dependent manner and the activity was dependent on RIPE3b/C1 sequences. In addition, the amount of MafA protein was regulated by glucose concentration. These results indicate that MafA is the homologue of zebrafish SMaf1 and acts as a transcriptional activator of the insulin gene promoter through the RIPE3b element.

The Zebrafish as a Model Organism for Eye Development

In recent years, the zebrafish has become a favourite model organism for biologists studying developmental processes in vertebrates. Its rapid embryonic development, the transparency of its embryos, the large number of offspring together with several other advantages make it ideal for discovering and understanding the genes that regulate embryonic development as well as the physiology of the adult organism. Zebrafish are very visually orientated, and their retina and lens show much the same morphology as other vertebrates including humans. For this reason, they are well suited for examining ocular development, function and disease. This review describes the advantages of the zebrafish as a model organism as well as giving an overview of eye development in this species. It has a particular focus on morphological as well as molecular aspects of the development of the lens.

Mafs, Prox1, and Pax6 can regulate chicken betaB1-crystallin gene expression

During lens fiber cell differentiation, the regulation of crystallin gene expression is coupled with dramatic morphological changes. Here we report that Mafs, Prox1, and Pax6, which are essential transcription factors for normal lens development, bind to three functionally important cis elements, PL1, PL2, and OL2, in the chicken betaB1-crystallin promoter and may cooperatively direct the transcription of this lens fiber cell preferred gene. Gel shift assays demonstrated that Mafs bind to the MARE-like sequences in the PL1 and PL2 elements, whereas Prox1, a sequence-specific DNA-binding protein like its Drosophila homolog Prospero, interacts with the OL2 element. Furthermore, Pax6, a known repressor of the chicken betaB1-crystallin promoter, binds to all three of these cis elements. In transfection assays, Mafs and Prox1 activated the chicken betaB1-crystallin promoter; however, their transactivation ability was repressed when co-transfected with Pax6. Taken together with the known spatiotemporal expression patterns of Mafs, Prox1, and Pax6 in the developing lens, we propose that Pax6 occupies and represses the chicken betaB1-crystallin promoter in lens epithelial cells, and is displaced by Prox1 and Mafs, which activate the promoter, in differentiating cortical fiber cells.

Novel metalloprotease-disintegrin, meltrin ? (ADAM35), expressed in epithelial tissues during chick embryogenesis

Members of the ADAM (a disintegrin and metalloprotease) family are involved in fertilization, morphogenesis, and pathogenesis. Their metalloprotease domains mediate limited proteolysis, including ectodomain shedding of membrane-anchored growth factors and intercellular-signaling proteins, and their disintegrin domains play regulatory roles in cell adhesion and migration. In screening for cDNAs encoding chicken ADAM proteins expressed during muscle development, we identified Meltrin epsilon as a novel member of this family. To elucidate its functions, we investigated its expression during development by using antibodies raised against its protease domain. In the somites, Meltrin epsilon protein was specifically expressed in the myotomal cells, which delaminate from the dermomyotome to form epithelial sheets. It was also found in the surface ectoderm, lens placodes, otic vesicles, and the gut epithelia. Basolateral localization of Meltrin epsilon in these epithelial cells suggests its unique roles in the organization of the epithelial tissues and development of the sensory organs and the gut.

Regulation and differential expression of the c-maf gene in differentiating cultured cells

The Maf transcription factors are involved in a variety of developmental and cellular differentiation processes, but their role in the differentiation of mesenchymal cells has not been described. Here, we have analyzed c-maf expression during the differentiation of adipocytes and muscle cells in cultured systems. The expression of c-maf mRNA was down-regulated during adipogenesis and up-regulated during myogenesis. In adipogenesis, the c-maf mRNA was down-regulated 58h after switching to the differentiation medium and just after PPARgamma2 mRNA was induced. A transient transfection analysis of a reporter gene containing the 5(')-flanking region of the c-maf gene showed that PPARgamma2 represses c-maf gene expression. We previously found that c-Maf, c-Jun, and Pax6 bind to and stimulate the c-maf gene. The PPARgamma2 repression of c-maf expression seems to be due, at least in part, to inhibition of the transactivation functions of c-Maf, c-Jun, and Pax6. The repression of c-maf was partly reversed by CBP, suggesting that these transcription factors compete for CBP or related transcription co-factors. In myogenesis, there was a differentiation-dependent stimulation of c-maf mRNA expression. The increased expression correlated with myoD expression. A transient transfection analysis showed that myoD stimulated a c-maf reporter gene through binding to two typical E-box elements located between 160 and 180 nucleotides upstream of the cap site. Binding of MyoD to the E-boxes was confirmed by a gel mobility shift assay and DNaseI footprinting analysis. Combined, these results suggest that the c-maf gene plays an important role during the differentiation of adipocyte and muscle cells from mesenchymal fibroblast cells.

The genetic and molecular basis of congenital eye defects

The mature eye is a complex organ that develops through a highly organized process during embryogenesis. Alterations in its genetic programming can lead to severe disorders that become apparent at birth or shortly afterwards; for example, one-half of the cases of blindness in children have a genetic cause. This review outlines the genetic basis of eye development, as determined by mutation analysis in patients and in model organisms. A better understanding of how this intricate organ develops at the genetic and cellular level is central to our understanding of the pathologies that afflict it.

Dmaf, a novel member of Maf transcription factor family is expressed in somatic gonadal cells during embryonic development and gametogenesis in Drosophila

Members of the maf gene family encode basic/leucine zipper transcription factors and play important roles during cell differentiation and organogenesis in vertebrate development. In this study, we show that the maf family is evolutionarily conserved and that the Drosophila maf (Dmaf) gene is expressed in somatic gonadal cells. During embryonic development, Dmaf mRNA is detected in somatic gonadal precursor cells emerging from dorsolateral mesoderm. Relatively weak expression is also observed in subset of neuronal cells in the central nervous system. In adult flies, Dmaf is expressed in somatic gonadal cells surrounding developing oocytes and spermatocytes. These results suggest a specific function for Dmaf in gonadal development, including migration and differentiation of primordial germ cells.

MafA has strong cell transforming ability but is a weak transactivator

The maf oncogene of the avian oncogenic retrovirus AS42 encodes a nuclear bZip protein, v-Maf, that recognizes sequences related to the AP-1 target site. The corresponding cellular protein, c-Maf belongs to a family of related bZip proteins together with MafA and MafB. In this paper, we compare the transactivation and cell transforming abilities of MafA and MafB along with two forms of the c-Maf protein. These proteins induce cellular transformation when expressed in chicken embryo fibroblasts. In reporter assays, MafA is a much less effective transactivator than the other Maf proteins, but unexpectedly shows the strongest activity in cell transformation. Chimeras of MafA and MafB correlate the strong cell transforming ability of MafA with its DNA-binding domain. The DNA-binding domain of MafA is also correlated with weak transactivation. Additional mutagenesis experiments show that transactivation and transformation by MafA are also controlled by phosphorylation of two conserved serine residues in the transactivation domain. Finally, we constructed MafA-estrogen receptor fusion molecules that show tightly hormone-dependent cell transforming ability. These regulatable constructs permit a kinetic characterization of target gene responses and facilitate discrimination between direct and indirect targets.

Members of the large Maf transcription family regulate insulin gene transcription in islet beta cells

The C1/RIPE3b1 (-118/-107 bp) binding factor regulates pancreatic-beta-cell-specific and glucose-regulated transcription of the insulin gene. In the present study, the C1/RIPE3b1 activator from mouse beta TC-3 cell nuclear extracts was purified by DNA affinity chromatography and two-dimensional gel electrophoresis. C1/RIPE3b1 binding activity was found in the roughly 46-kDa fraction at pH 7.0 and pH 4.5, and each contained N- and C-terminal peptides to mouse MafA as determined by peptide mass mapping and tandem spectrometry. MafA was detected in the C1/RIPE3b1 binding complex by using MafA peptide-specific antisera. In addition, MafA was shown to bind within the enhancer region (-340/-91 bp) of the endogenous insulin gene in beta TC-3 cells in the chromatin immunoprecipitation assay. These results strongly suggested that MafA was the beta-cell-enriched component of the RIPE3b1 activator. However, reverse transcription-PCR analysis demonstrated that mouse islets express not only MafA but also other members of the large Maf family, specifically c-Maf and MafB. Furthermore, immunohistochemical studies revealed that at least MafA and MafB were present within the nuclei of islet beta cells and not within pancreas acinar cells. Because MafA, MafB, and c-Maf were each capable of specifically binding to and activating insulin C1 element-mediated expression, our results suggest that all of these factors play a role in islet beta-cell function.

Gene silencing in chick embryos with a vector-based small interfering RNA system

In this paper, the use of vector-based RNA interference (RNAi) to specifically interfere with gene expression in chick embryos is reported. In ovo electroporation was carried out to transfer a small interfering RNA (siRNA) expression vector into chick embryos. En2 was chosen for the target gene because the family gene, En1, is expressed in a similar pattern. Four sets of 19-mer sequences were designed with the En2 open reading frame region connected to a sequence of short hairpin RNA (shRNA), which exerts siRNA effects after being transcribed, and inserted into pSilencer U6-1.0 vector. En2 and En1 expression were suppressed by the siRNA whose sequence completely matched En2 and En1. Suppression occurred when the siRNA sequence differed by up to two nucleotides from the target sequence. The sequence that differed by four nucleotides from the target gene did not show siRNA effects. One set that completely matched the En2 target did not show siRNA effects, which may be due to location of the siRNA in the target gene. Thus, multiple sets of shRNA must be prepared if we are to consider. This system will greatly contribute to the analysis of function of genes of interest, because the target gene can be silenced in a locally and temporally desired manner.

Crystallins, genes and cataract

Far from being a physical entity, assembled of inanimate structural proteins, the ocular lens epitomizes the biological ingenuity that sustains an essential and near-perfect physical system of immaculate optics. Crystallins (alpha, beta, and gamma) provide transparency by dint of their high concentration, but it is debatable whether proteins that provide transparency are any different, biologically or structurally, from those that are present in non-transparent structures or tissues. It is becoming increasingly clear that crystallins may have a plethora of metabolic and regulatory functions, both within the lens as well as outside of it. Alpha-crystallins are members of a small heat shock family of proteins and beta/gamma-crystallins belong to the family of epidermis-specific differentiation proteins. Crystallin gene expression has been studied from the perspective of the lens specificity of their promoters. Mutations in alpha-, beta-, and gamma-crystallins are linked with the phenotype of the loss of transparency. Understanding catalytic, non-structural properties of crystallins may be critical for understanding the malfunction in molecular cascades that lead to cataractogenesis and its eventual therapeutic amelioration.

Cooperative action between L-Maf and Sox2 on delta-crystallin gene expression during chick lens development

Lens development is regulated by a variety of transcription factors with distinct properties. The lens-specific transcription factor, L-Maf, is essential for lens formation and induces lens-specific markers, such as the crystallin genes. In this study, we analyzed the mechanism by which L-Maf regulates delta-crystallin expression. Misexpression of L-Maf in the head ectoderm of lens placode-forming embryos by in ovo electroporation induced delta-crystallin only in the region surrounding the lens. To define this restricted expression, we misexpressed L-Maf together with other transcription factors implicated in delta-crystallin expression. Sox2 plus L-Maf expanded the delta-crystallin-inducible domain to the entire head ectoderm and simultaneously increased the quantity of delta-crystallin mRNA expressed. In contrast, co-expression of L-Maf with other factors such as Pax6, Six3 and Prox1 had little or no effect on delta-crystallin. We also observed that L-Maf and Sox2 cooperatively enhanced the transactivation of a reporter gene bearing the delta-crystallin enhancer in ovo, implying that L-Maf and Sox2 can induce delta-crystallin through the same enhancer. In conclusion, we report here that L-Maf and Sox2 cooperatively regulate the expression of delta-crystallin during chick lens development.

Pulverulent cataract with variably associated microcornea and iris coloboma in a MAF mutation family

AIMS

To report the detailed clinical findings in a three generation pedigree with autosomal dominant cataract, microcornea, and coloboma resulting from mutation of the lens development gene, MAF.

METHODS

Five members of a three generation pedigree with progressive cataracts underwent detailed ophthalmic examination to characterise associated ocular phenotypic features.

RESULTS

The cataracts present in all affected individuals were cortical, and/or nuclear, pulverulent opacities. Corneal diameters of 10-10.25 mm were present in two family members. Axial lengths were in the normal range. Bilateral iris coloboma in the 6 o'clock position was present in one patient. Uveal melanoma was present in one patient, with uveal naevi in this and one other patient.

CONCLUSION

The bZIP transcription factor MAF is a key lens development gene that regulates the expression of the crystallins. Individuals with a mutation in MAF may have pulverulent cataract alone or cataract in association with microcornea or iris coloboma.

Methods for introducing morpholinos into the chicken embryo

The use of antisense morpholino oligos to inhibit the translation of a target transcript has been applied recently to studies of the chicken embryo. In contrast to other developmental systems such as in frog, sea urchin, and zebrafish that permit the direct microinjection of morpholinos into a blastomere, square pulse electroporation is used to introduce fluorescently tagged morpholinos into specific populations of chick embryo cells in ovo. This article reviews the methods that have proven successful, the types of controls that are necessary when performing knockdowns of gene expression in the chick embryo, and discusses the limitations of the current technique, as well as directions for further research.

The stability of the lens-specific Maf protein is regulated by fibroblast growth factor (FGF)/ERK signaling in lens fiber differentiation

Fibroblast growth factor (FGF) signaling is necessary for both proliferation and differentiation of lens cells. However, the molecular mechanisms by which FGFs exert their effects on the lens remain poorly understood. In this study, we show that FGF-2 repressed the expression of lens-specific genes at the proliferative phase in primary cultured lens cells. Using transfected cells, we also found that the activity of L-Maf, a lens differentiation factor, is repressed by FGF/ERK signaling. L-Maf is shown to be phosphorylated by ERK, and introduction of mutations into the ERK target sites on L-Maf promotes its stabilization. The stable L-Maf mutant protein promotes the differentiation of lens cells from neural retina cells. Taken together, these results indicate that FGF/ERK signaling negatively regulates the function of L-Maf in proliferative lens cells and that stabilization of the L-Maf protein is important for lens fiber differentiation.

Downregulation of mafB expression in T-helper cells during early differentiation in vitro

We have studied the expression of a human homologue of mafB (maf-1), a member of the family of large maf transcription factors. In support of the suggested key role that mafB expression plays in differentiating macrophages, we found mafB to be expressed at a very high level in monocytic U937 and THP-1 cell lines. However, we show here that mafB transcription is not restricted to myeloid cells but can also be detected in lymphoid cells, indicating transcriptional plasticity during haematopoiesis. In conclusion, strong proliferative signals mediated by T-cell activation and interleukins (IL-4 and IL-12) downregulate the mafB messenger RNA transcript level when resting naïve CD4+ T-helper cells enter the differentiation pathway in vitro.

The cellular and molecular bases of vertebrate lens regeneration

Lens regeneration takes place in some vertebrates through processes of cellular dedifferentiation and transdifferentiation, processes by which certain differentiated cell types can give rise to others. This review describes the principal forms of lens regeneration that occur in vivo as well as related in vitro systems of transdifferentiation. Classic experimental studies are reviewed that define the tissue interactions that trigger these events in vivo. Recent molecular analyses have begun to identify the genes associated with these processes. These latter studies generally reveal tremendous similarities between embryonic lens development and lens regeneration. Different models are proposed to describe basic molecular pathways that define the processes of lens regeneration and transdifferentiation. Finally, studies are discussed suggesting that fibroblast growth factors play key roles in supporting the process of lens regeneration. Retinoids, such as retinoic acid, may also play important roles in this process.

MafA is a glucose-regulated and pancreatic beta-cell-specific transcriptional activator for the insulin gene

The insulin gene is specifically expressed in beta-cells of the Langerhans islets of the pancreas, and its transcription is regulated by the circulating glucose level. Previous reports have shown that an unidentified beta-cell-specific nuclear factor binds to a conserved cis-regulatory element called RIPE3b and is critical for its glucose-regulated expression. Based on the sequence similarity of the RIPE3b element and the consensus binding sequence of the Maf family of basic leucine zipper transcription factors, we here identified mammalian homologue of avian MafA/L-Maf, an eye-specific member of the Maf family, as the RIPE3b-binding transcriptional activator. Reverse transcription-PCR analysis showed that mafA mRNA is detected only in the eyes and in pancreatic beta-cells and not in alpha-cells. MafA protein as well as its mRNA is up-regulated by glucose, consistent with the glucose-regulated binding of MafA to the RIPE3b element in beta-cell nuclear extracts. In transient luciferase assays, we also showed that expression of MafA greatly enhanced insulin promoter activity and that a dominant-negative form of MafA inhibited it. Therefore, MafA is a beta-cell-specific and glucose-regulated transcriptional activator for insulin gene expression and thus may be involved in the function and development of beta-cells as well as in the pathogenesis of diabetes.

Retrovirus-mediated gene expression during chick visual system development

The chick embryo is an excellent model for studying eye morphogenesis, retinal cell fate determination, and retinotectal projections due to its accessibility and the available molecular tools. Avian replication-competent retroviruses allow efficient infection of proliferating cells and stable integration of the viral genome, including up to 2.3kb of foreign cDNA, into the host chromosome. High-titer retroviruses are produced by transient transfection of avian DF-1 cells followed by centrifugation of the culture medium. Targeted infection of the optic vesicle, the lens vesicle, the retina and pigmented epithelium, the periocular mesenchyme, and the tectum can be performed at different developmental stages in ovo. In addition, retroviruses can be used to transduce genes of interest into various ocular tissue explants or cells in vitro. Virus-mediated gene expression can be detected within 12h of infection. Therefore, avian replication-competent retroviruses serve as powerful tools to misexpress wild-type and mutant gene products and to study molecular mechanisms underlying vertebrate visual system development.

Cloning MafF by recognition site screening with the NFE2 tandem repeat of HS2: analysis of its role in globin and GCSl genes regulation

The erythroid-specific enhancer within hypersensitivity site 2 (HS2) of the human beta-globin locus control region is required for high level globin gene expression. We used an oligonucleotide of the NF-E2 tandem repeat, within HS2, as recognition site probe to screen a K562 cDNA library for interacting transcription factors. A 2.3 kb full length cDNA encoding the b-zip transcription factor MafF was isolated. MafF can form both homodimers and high affinity heterodimers with Nrf1, Nrf2 and Nf-E2, three members of the CNC-bZip family. Despite obvious structural similarities with the other small Maf proteins, MafF differs in its tissue distribution and its inability to repress transcription when overexpressed as homodimer. In fact, in different cell lines and on different promoters (gamma-globin, beta-globin and glutamylcysteine synthetase genes) the MafF homodimers do not appreciably affect transcription of target promoters, whereas MafF/CNC member heterodimers act as weak transcriptional activators. Even though MafF was cloned using probes derived from the globin LCR, it is in the context of the GCSl promoter and in combination with Jun that MafF shows a rather distinct and specific regulatory role. These observations suggest that a complex network of small Maf and CNC-AP1 protein interactions might be involved in regulating transcription in diverse tissues or developmental stages.

Meis homeoproteins directly regulate Pax6 during vertebrate lens morphogenesis

Pax6 is a pivotal regulator of eye development throughout Metazoa, but the direct upstream regulators of vertebrate Pax6 expression are unknown. In vertebrates, Pax6 is required for formation of the lens placode, an ectodermal thickening that precedes lens development. Here we show that the Meis1 and Meis2 homeoproteins are direct regulators of Pax6 expression in prospective lens ectoderm. In mice, Meis1 and Meis2 are developmentally expressed in a pattern remarkably similar to Pax6 and their expression is Pax6-independent. Biochemical and transgenic experiments reveal that Meis1 and Meis2 bind a specific sequence in the Pax6 lens placode enhancer that is required for its activity. Furthermore, Pax6 and Meis2 exhibit a strong genetic interaction in lens development, and Pax6 expression is elevated in lenses of Meis2-overexpressing transgenic mice. When expressed in embryonic lens ectoderm, dominant-negative forms of Meis down-regulate endogenous Pax6. These results contrast with those in Drosophila, where the single Meis homolog, Homothorax, has been shown to negatively regulate eye formation. Therefore, despite the striking evolutionary conservation of Pax6 function, Pax6 expression in the vertebrate lens is uniquely regulated.

Distinct roles of SOX2, Pax6 and Maf transcription factors in the regulation of lens-specific delta1-crystallin enhancer

BACKGROUND

The eye lens provides a good model for the study of regulation of cell differentiation, in which lens-specific delta1-crystallin expression serves as an indicator of the differentiated state of the cells. It has been indicated that the SOX2, Pax6 and Maf proteins are the major regulators of lens cell differentiation. To clarify the individual roles of these transcription factors, we analysed their participation in regulation of the delta1-crystallin enhancer.

RESULTS

We defined the major binding sites of SOX2, Pax6 and Maf transcription factors in the delta1-crystallin enhancer and assessed the effect of mutations at these sites in the cultured lens epithelial cells and in developing lenses of transgenic mouse embryos. SOX2 (or SOX1/SOX3) is essential for activation of the enhancer under all conditions. Pax6 bound at the deltaEF3 site is required for activation of the enhancer, while Pax6 at the Pax6U site appears to be involved in the Pax6-dependent suppression of the enhancer. In contrast, Maf proteins are only required for high enhancer activity in lens fibre cells.

CONCLUSION

The distinct roles of these transcription factors in the regulation of delta1-crystallin enhancer would tend to indicate their individual functions in lens differentiation. The activity of SOX2 and the related SOX1/3 is essential at all stages of lens development as transcriptional activators. Pax6, although it is required in all steps, probably exerts complex regulatory effects, since it possesses both the potential to activate and repress. The major function of Maf proteins presumably resides in the activation of the genes in lens fibre cells.

L-Maf, a downstream target of Pax6, is essential for chick lens development

During lens development in vertebrates, the orchestration of multiple transcriptional regulators is essential for fate determination and terminal differentiation. In early development, Pax6, Sox2 and Six3 are expressed in the head ectoderm, while L-maf, Prox1 and crystallin genes are expressed at a later stage in the lens placode in a more restricted fashion. To uncover the genetic interactions among these factors during lens development, we examined the effects of dominant-negative molecules of Pax6 and L-Maf, which play decisive roles in lens formation. The two dominant-negative isoforms of Pax6 repress L-maf, Prox1 and delta-crystallin expression, resulting in failure of lens formation. These effects of dominant-negative Pax6 are fully rescued by co-expression with wild-type L-Maf. In addition, dominant-negative L-Maf inhibits the expression of Prox1 and delta-crystallin, while misexpression of L-Maf causes ectopic induction of these genes in a Sox-2-dependent fashion. Our results demonstrate that L-Maf is a downstream target of Pax6 and mediates Pax6 activity in developing lens cells.

Identification of Sox-2 regulatory region which is under the control of Oct-3/4-Sox-2 complex

Sox-2 is a transcriptional cofactor expressed in embryonic stem (ES) cells as well as in neuronal cells. It has been demonstrated that Sox-2 plays an important role in supporting gene expression in ES cells, especially by forming a complex with embryonic Octamer factor, Oct-3/4. Here, we have analyzed the regulatory regions of the Sox-2 gene and identified two enhancers which stimulate transcription in ES cells as well as in embryonal carcinoma cells. These regulatory regions, which we termed Sox regulatory regions (SRR) 1 and 2, exert their function specifically when cells are in an undifferentiated state. Interestingly, like the regulatory elements of FGF-4 and UTF1 genes, combinatorial action of Octamer and Sox-2 binding sites support the SRR2 activity. However, biochemical analyses reveal that, due to the unique sequence and/or its organization, the SRR2 bears distinct characteristics from those of FGF-4 and UTF1 regulatory elements. That is, unlike the FGF-4 gene enhancer, the SRR2 precludes the binding of the Oct-1-Sox-2 complex. The difference between the SRR2 and UTF1 regulatory element is in the ability of SRR2 to recruit the Oct-6-Sox-2 complex as well as the Oct-3/4-Sox-2 complex. Co-transfection analyses confirm that both complexes are able to stimulate transcription through the SRR2 element.

Integration and diversity of the regulatory network composed of Maf and CNC families of transcription factors

Recent progress in the analysis of transcriptional regulation has revealed the presence of an exquisite functional network comprising the Maf and Cap 'n' collar (CNC) families of regulatory proteins, many of which have been isolated. Among Maf factors, large Maf proteins are important in the regulation of embryonic development and cell differentiation, whereas small Maf proteins serve as obligatory heterodimeric partner molecules for members of the CNC family. Both Maf homodimers and CNC-small Maf heterodimers bind to the Maf recognition element (MARE). Since the MARE contains a consensus TRE sequence recognized by AP-1, Jun and Fos family members may act to compete or interfere with the function of CNC-small Maf heterodimers. Overall then, the quantitative balance of transcription factors interacting with the MARE determines its transcriptional activity. Many putative MARE-dependent target genes such as those induced by antioxidants and oxidative stress are under concerted regulation by the CNC family member Nrf2, as clearly proven by mouse germline mutagenesis. Since these genes represent a vital aspect of the cellular defense mechanism against oxidative stress, Nrf2-null mutant mice are highly sensitive to xenobiotic and oxidative insults. Deciphering the molecular basis of the regulatory network composed of Maf and CNC families of transcription factors will undoubtedly lead to a new paradigm for the cooperative function of transcription factors.

Characterization of the chicken L-Maf, MafB and c-Maf in crystallin gene regulation and lens differentiation

BACKGROUND

Members of the Maf family, including L-Maf, MafB and c-Maf, are "basic region/leucine zipper" (bZIP) transcription factors. Maf proteins contain a highly conserved acidic transactivation domain (AD), and a bZIP region that mediates DNA-binding activity. The hinge region between AD and bZIP varies considerably in length between different proteins. Recent studies reveal that L-Maf, c-Maf and MafB play key roles in vertebrate lens development.

RESULTS

We investigated the transactivation activity of individual factors in culture cells to analyse their specific functions. In transient transfection assays with a reporter gene containing Maf responsive elements, MafB and c-Maf activated higher levels of the reporter gene than L-Maf. However, L-Maf transactivated the alphaA-crystallin promoter as effectively as MafB and c-Maf, and induced the expression of the endogenous delta-crystallin gene more efficiently than the other two proteins. Domain-swapping experiments reveal that the bZIP region of MafB takes part in strong transcriptional activity, while the acidic and hinge regions (AH) of c-Maf collectively serve as a strong transactivation domain. The AH region of L-Maf (but not c-Maf) conferred transactivation activity to induce delta-crystallin gene expression.

CONCLUSIONS

These results suggest that despite their similar DNA binding properties, L-Maf, MafB and c-Maf regulate different sets of target genes by complex interactions with multiple factors that recognize cis-elements in promoters. The AH region of L-Maf has a distinct role in inducing endogenous delta-crystallin gene.

Identification of beta-cell-specific insulin gene transcription factor RIPE3b1 as mammalian MafA

Of the three critical enhancer elements that mediate beta-cell-specific and glucose-responsive expression of the insulin gene, only the identity of the transcription factor binding to the RIPE3b element (RIPE3b1) has remained elusive. Using a biochemical purification approach, we have identified the RIPE3b1 factor as a mammalian homologue of avian MafA/L-Maf (mMafA). The avian MafA is a cell-type determination factor that expressed ectopically can trigger lens differentiation program, but no mammalian homologue of avian MafA has previously been identified. Here, we report cloning of the human mafA (hMafA) and demonstrate that it can specifically bind the insulin enhancer element RIPE3b and activate insulin-gene expression. In addition, mMafA has a very restrictive cellular distribution and is selectively expressed in pancreatic beta but not in alpha cells. We suggest that mMafA has an essential role in the function and differentiation of beta-cells and thus may be associated with the pathophysiological origins of diabetes.

Complex regulatory element within the gammaE- and gammaF-crystallin enhancers mediates Pax6 regulation and is required for induction by retinoic acid

The paired domain, DNA-binding domain of Pax6 and other Pax transcription factors, is composed of two subdomains (PAI and RED), each recognizing distinct half-sites of the bipartite binding site in adjacent major grooves of the DNA helix. The alternatively spliced Pax6(5a) isoform containing 14 extra amino acids within the PAI domain recognizes the 5aCON sequence consisting of four interdigitated 5' half-sites of the bipartite consensus sequence. A genome database search for similar tetrameric Pax6(A) recognition sequences led to the identification of a Pax6-binding site in the lens-specific enhancer of the mouse E- and F-crystallin genes. This binding site combines the properties of bipartite and tetrameric recognition sequences and, by mutational analysis, is shown to mediate Pax6-dependent regulation of the E- and F-crystallin promoter constructs both in primary chicken lens cells and in chicken embryo fibroblasts. The Pax6-binding site is adjacent to a previously identified retinoic acid response element and is itself required for retinoic acid induction of the F- and E-crystallin genes, suggesting that Pax proteins and retinoic acid receptors cooperate in transcriptional regulation. In summary, our protein-DNA binding and transactivation studies suggest that -crystallin genes are under the control of a multifunctional enhancer element that mediates Pax6 regulation as well as retinoic acid-mediated induction.

c-Maf, the gammaD-crystallin Maf-responsive element and growth factor regulation

The transcription factor c-Maf has been suggested to regulate the activity of gamma-crystallin promoters in lens fibre cells. We here show that the transactivation potential of c-Maf and MafB for the rat gammaD-crystallin Maf-responsive element (gammaD MARE) is dependent upon the cellular context and, using chimeric and single domain mutants, that c-Maf is most likely to be the cognate factor for the gammaD MARE in the lens. Transactivation of the gammaD MARE by c-Maf in lens cells was not enhanced by c-Fos or c-Jun and was not blocked by dominant negative c-Fos or c-Jun constructs. c-Maf can activate the gammaD MARE as a homodimer since activation of the gammaD-crystallin promoter in P19 embryonic carcinoma cells required only c-Maf, but none of a number of c-Fos and c-Jun family members tested. Transactivation by c-Maf was inhibited by activation of protein kinase A (PKA) (by signal transduction agonist forskolin) or of protein kinase C (PKC) (by signal transduction agonist tetradecanoyl phorbol acetate). Site-directed mutagenesis showed that this effect is not mediated by phosphorylation of the consensus PKA/PKC site in the extended DNA-binding domain, but likely involves activation of MAP kinase kinase, as inhibition by PD98059 increased transactivation by c-Maf.

Early eye development in vertebrates

This review provides a synthesis that combines data from classical experimentation and recent advances in our understanding of early eye development. Emphasis is placed on the events that underlie and direct neural retina formation and lens induction. Understanding these events represents a longstanding problem in developmental biology. Early interest can be attributed to the curiosity generated by the relatively frequent occurrence of disorders such as cyclopia and anophthalmia, in which dramatic changes in eye development are readily observed. However, it was the advent of experimental embryology at the turn of the century that transformed curiosity into active investigation. Pioneered by investigators such as Spemann and Adelmann, these embryological manipulations have left a profound legacy. Questions about early eye development first addressed using tissue manipulations remain topical as we try to understand the molecular basis of this process.

Structure and expression of the scallop Omega-crystallin gene. Evidence for convergent evolution of promoter sequences

Omega-crystallin of the scallop lens is an inactive aldehyde dehydrogenase (1A9). Here we have cloned the scallop Omega-crystallin gene. Except for an extra novel first exon, its 14-exon structure agrees well with that of mammalian aldehyde dehydrogenases 1, 2, and 6. The -2120/+63, -714/+63, and -156/+63 Omega-crystallin promoter fragments drive the luciferase reporter gene in transfected alphaTN4-1 lens cells and L929 fibroblasts but not in Cos7 cells. Putative binding sequences for cAMP-responsive element-binding protein (CREB)/Jun, alphaACRYBP1, AP-1, and PAX-6 in the Omega-crystallin promoter are surprisingly similar to the cis-elements used for lens promoter activity of the mouse and chicken alphaA-crystallin genes, which encode proteins homologous to small heat shock proteins. Site-specific mutations in the overlapping CREB/Jun and Pax-6 sites abolished activity of the Omega-crystallin promoter in transfected cells. Gel shift experiments utilizing extracts from the alphaTN4-1, L929, and Cos7 cells and the scallop stomach and oligonucleotides derived from the putative binding sites of the Omega-crystallin promoter showed complex formation. Gel shift experiments showed binding of recombinant Pax-6 and CREB to their respective sites. Our data suggest convergent evolutionary adaptations that underlie the preferential expression of crystallin genes in the lens of vertebrates and invertebrates.