Binding of tissue-specific forms of alpha A-CRYBP1 to their regulatory sequence in the mouse alpha A-crystallin-encoding gene: double-label immunoblotting of UV-crosslinked complexes

Recapitulation of human betaB1-crystallin promoter activity in transgenic zebrafish

Development of the eye is morphologically similar among vertebrates, indicating that the underlying mechanism regulating the process may have been highly conserved during evolution. Herein we analyzed the promoter of the human betaB1-crytallin gene in zebrafish by transgenic experiments. To delineate the evolutionarily conserved regulatory elements, we performed serial deletion assays in the promoter region. The results demonstrated that the -90/+61-bp upstream proximal promoter region is sufficient to confer lens-tissue specificity to the human betaB1-crystallin gene in transgenic zebrafish. Through phylogenetic sequence comparisons and an electrophoretic mobility shift assay (EMSA), a highly conserved cis-element of a six-base pair sequence TG(A/C)TGA, the consensus sequence for the Maf protein binding site, within the proximal promoter region was revealed. Further, a site-mutational assay showed that this element is crucial for promoter activity. These data suggest that the fundamental transcriptional regulatory mechanism of the betaB1-crystallin gene has been well conserved between humans and zebrafish, and plausibly among all vertebrates, during evolution.

Regulation of gene expression by Pax6 in ocular cells: a case of tissue-preferred expression of crystallins in lens

Lens development is an excellent model for genetic and biochemical studies of embryonic induction, cell cycle regulation, cellular differentiation and signal transduction. Differentiation of lens is characterized by lens-preferred expression and accumulation of water-soluble proteins, crystallins. Crystallins are required for light transparency, refraction and maintenance of lens integrity. Here, we review mechanisms of lens-preferred expression of crystallin genes by employing synergism between developmentally regulated DNA-binding transcription factors: Pax6, c-Maf, MafA/L-Maf, MafB, NRL, Sox2, Sox1, RARbeta/RXRbeta, RORalpha, Prox1, Six3, gammaFBP-B and HSF2. These factors are differentially expressed in lens precursor cells, lens epithelium and primary and secondary lens fibers. They exert their function in combination with ubiquitously expressed factors (e.g. AP-1, CREB, pRb, TFIID and USF) and co-activators/chromatin remodeling proteins (e.g. ASC-2 and CBP/p300). A special function belongs to Pax6, a paired domain and homeodomain-containing protein, which is essential for lens formation. Pax6 is expressed in lens progenitor cells before the onset of crystallin expression and it serves as an important regulatory factor required for expression of c-Maf, MafA/L-Maf, Six3, Prox1 and retinoic acid signaling both in lens precursor cells and the developing lens. The roles of these factors are illustrated by promoter studies of mouse alphaA-, alphaB-, gammaF- and guinea pig zeta-crystallins. Pax6 forms functional complexes with a number of transcription factors including the retinoblastoma protein, pRb, MafA, Mitf and Sox2. We present novel data showing that pRb antagonizes Pax6-mediated activation of the alphaA-crystallin promoter likely by inhibiting binding of Pax6 to DNA.

Mint Represses Transactivation of the Type II Collagen Gene Enhancer through Interaction with αA-crystallin-binding Protein 1

Collagen type II is an extracellular matrix protein important for cartilage and bone formation, and its expression is controlled by multiple cis- and trans-acting elements, including the zinc finger transcription factor alpha A-crystallin-binding protein 1 (CRYBP1). Here we show that MSX2-interacting nuclear target protein (MINT), a conserved transcriptional repressor, associates with CRYBP1 and negatively regulates the transactivation of the collagen type II gene (Col2a1) enhancer. We identified CRYBP1 as a binding partner of MINT by screening a mouse embryonic cDNA library using the yeast two-hybrid system. We demonstrated that the C terminus of MINT interacts with the C terminus of CRYBP1 using the mammalian cell two-hybrid assay, glutathione S-transferase pull-down, and co-immunoprecipitation analyses. Furthermore, MINT and CRYBP1 form a complex on the Col2a1 enhancer, as shown by chromatin immunoprecipitation and gel shift assays. In the presence of CRYBP1, overexpression of MINT or its C-terminal fragment in cells repressed a reporter construct driven by the Col2a1 enhancer elements. This transcription repression is dependent on histone deacetylase, the main co-repressor recruited by MINT. The present study shows that MINT is involved in CRYBP1-mediated Col2a1 gene repression and may play a role in regulation of cartilage development.

ZAS: C2H2 zinc finger proteins involved in growth and development

A ZAS gene encodes a large protein with two separate C2H2 zinc finger pairs that independently bind to specific DNA sequences, including the kappaB motif. Three paralogous mammalian genes, ZAS1, ZAS2, and ZAS3, and a related Drosophila gene, Schnurri, have been cloned and characterized. The ZAS genes encode transcriptional proteins that activate or repress the transcription of a variety of genes involved in growth, development, and metastasis. In addition, ZAS3 associates with a TNF receptor-associated factor to inhibit NF-kappaB- and JNK/ SAPK-mediated signaling of TNF-alpha. Genetic experiments show that ZAS3 deficiency leads to proliferation of cells and tumor formation in mice. The data suggest that ZAS3 is important in controlling cell growth, apoptosis, and inflammation. The potent vasoactive hormone endothelin and transcription factor AP2 gene families also each consist of three members. The ZAS, endothelin, and transcription factor AP2 genes form several linkage groups. Knowledge of the chromosomal locations of these genes provides valuable clues to the evolution of the vertebrate genome.

Structure of the human zinc finger protein HIVEP3: molecular cloning, expression, exon-intron structure, and comparison with paralogous genes HIVEP1 and HIVEP2

Here we report the cloning and characterization of HIVEP3, the newest member in the human immunodeficiency virus type 1 enhancer-binding protein family that encodes large zinc finger proteins and regulates transcription via the kappaB enhancer motif. The largest open reading frame of HIVEP3 contains 2406 aa. and is approximately 80% identical to the mouse counterpart. The HIVEP3 gene is located in the chromosomal region 1p34 and is at least 300 kb with 10 exons. RNA studies show that multiple HIVEP3 transcripts are differentially expressed and regulated. Additionally, transcription termination occurs in the ultimate exon, exon 10, or in exon 6. Therefore, HIVEP3 may produce protein isoforms that contain or exclude the carboxyl DNA binding domain and the leucine zipper by alternative RNA splicing and differential polyadenylation. Sequence homologous to HIVEP3 exon 6 is not found in mouse nor are the paralogous genes HIVEP1 and HIVEP2. Zoo-blot analysis suggests that sequences homologous to the human exon 6 are present only in primates and cow. Therefore, a foreign DNA harboring a termination exon likely was inserted into the HIVEP3 locus relatively recently in evolution, resulting in the acquisition of novel gene regulatory mechanisms as well as the generation of structural and functional diversity.

A zinc finger transcription factor, alphaA-crystallin binding protein 1, is a negative regulator of the chondrocyte-specific enhancer of the alpha1(II) collagen gene

Transcription of the type II collagen gene (Col2a1) is regulated by multiple cis-acting sites. The enhancer element, which is located in the first intron, is necessary for high-level and cartilage-specific expression of Col2a1. A mouse limb bud cDNA expression library was screened by the Saccharomyces cerevisiae one-hybrid screening method to identify protein factors bound to the enhancer. A zinc finger protein, alphaA-crystallin binding protein 1 (CRYBP1), which had been reported to bind to the mouse alphaA-crystallin gene promoter, was isolated. We herein demonstrate that CRYBP1 is involved in the negative regulation of Col2a1 enhancer activity. CRYBP1 mRNA expression was downregulated during chondrocyte differentiation in vitro. In situ hybridization analysis of developing mouse cartilage showed that CRYBP1 mRNA was also downregulated during mesenchymal condensation and that CRYBP1 mRNA was highly expressed by hypertrophic chondrocytes, but at very low levels by resting and proliferating chondrocytes. Expression of recombinant CRYBP1 in a transfected rat chondrosarcoma cell line inhibited Col2a1 enhancer activity. Electrophoretic mobility shift assays showed that CRYBP1 bound a specific sequence within the Col2a1 enhancer and inhibited the binding of Sox9, an activator for Col2a1, to the enhancer. Cotransfection of CRYBP1 with Sox9 into BALB/c 3T3 cells inhibited activation of the Col2a1 enhancer by Sox9. These results suggest a novel mechanism that negatively regulates cartilage-specific expression of Col2a1.

Transcriptional regulation of the mouse alpha A-crystallin gene: binding of USF to the -7/+5 region

Lens preferred-expression of the mouse alpha A-crystallin gene (alpha A-cry) is regulated at the transcriptional level by multiple elements located in the 5' flanking region of the gene. Here we present the first analysis of the functional role of the mouse alpha A-cry +1 region and the protein(s) which bind to it. The -7/+5 region of this promoter exhibits sequence similarity with the consensus upstream stimulating factor (USF) transcription factor binding site. A wild type oligodeoxyribonucleotide (oligo) spanning the mouse alpha A-cry -15/+15 region specifically inhibited the activity of a mouse alpha A-cry promoter-cat gene fusion (p alpha A 111aCAT) in competitive co-transfection studies in the mouse alpha TN4-1 lens cell line, as did an oligo containing the adenovirus 2 major late promoter strong USF binding site. In contrast, an alpha A-cry oligo mutated (-3/+3) within the USF-like binding site did not inhibit p alpha A111aCAT activity. Western blot analysis indicated that alpha TN4-1 cells express USF1. Co-transfection of p alpha A111aCAT and a USF1 cDNA expression vector into alpha TN4-1 cells resulted in a repression of mouse alpha A-cry promoter activity. Electrophoretic mobility shift analyses (EMSA) demonstrated that proteins in an alpha TN4-1 nuclear extract form a single major complex on synthetic oligos spanning the mouse alpha A-cry -15/+15 region. The formation of this complex was inhibited by the presence of unlabeled -15/+15 oligos or an anti-USF1 antibody. In addition, purified USF1 bound to this region, producing a complex similar in size to that observed with alpha TN4-1 nuclear extracts. Taken together, our findings show that USF can bind to the mouse alpha A-cry +1 site, and support the possibility that USF plays a role in promoter activity of this gene. Sequence similarities surrounding the +1 region of the alpha A-cry gene of the mouse, mole rat, hamster, and human, as well as the previously observed utilization of USF by different cry promoters suggest that USF contributes to the high expression of many crys in the ocular lens of diverse species.

Cellular and molecular features of lens differentiation: a review of recent advances

In this paper, the more recent literature pertaining to differentiation in the developing vertebrate lens is reviewed in relation to previous work. The literature reviewed reveals that the developing lens has been, and will continue to be, a useful model system for the examination of many fundamental processes occurring during embryonic development. Areas of lens development reviewed here include: the induction and early embryology of the lens; lens cell culture techniques; the role of growth factors and cytokines; the involvement of gap junctions in lens cell-cell communication; the role of cell adhesion molecules, integrins, and the extracellular matrix; the role of the cytoskeleton; the processes of programmed cell death (apoptosis) and lens fibre cell denucleation; the involvement of Pax and Homeobox genes; and crystallin gene regulation. Finally, some speculation is provided as to possible directions for further research in lens development.

Sequence, initial functional analysis and protein-DNA binding sites of the mouse beta B2-crystallin-encoding gene

An 800-bp fragment of genomic DNA upstream from the origin of transcription of the mouse beta B2-crystallin-encoding gene (beta B2-Cry) has been isolated and its nucleotide sequence determined. Promoter fragments 275 to +30 or -110 to +30, fused to cat reporter gene, activated transcription in transiently transfected rabbit lens epithelial cells, but not in various non-lens cells. The beta B2-Cry mouse promoter contains a typical TATA-box located approx. 25 bp upstream from the transcription start point. Binding sites (upstream from the TATA-box) for transcription factors possibly involved in the regulation of gene expression have been identified by DNaseI footprinting analysis and lens cell nuclear extracts. Most notably is the binding of the Pax-6 paired domain (PrD) which correlates with the binding of lens cell nuclear proteins at the -80 to -40 region.

Lens-specific activity of the mouse alpha A-crystallin promoter in the absence of a TATA box: functional and protein binding analysis of the mouse alpha A-crystallin PE1 region

Lens-specific expression of the mouse alpha A-crystallin gene is regulated at the level of transcription. Here, we have studied the role of the PE1 region, which contains the TATA box (-31/-26) and the immediately adjacent PE1B sequence (-25/-12), in transcriptional regulation. Deletions within either the TATA box or PE1B sequence eliminated promoter activity in transfected lens cells. Surprisingly, these deletions did not eliminate lens-specific promoter activity of the transgene of transgenic mice. Transcription of the transgene with a TATA-deleted promoter initiated at multiple sites in the lenses of the transgenic mice. Footprint analysis revealed that the entire PE1 region was protected by nuclear extracts prepared from lens cells which express the alpha A-crystallin gene and from fibroblasts which do not express the gene. The -37/+3 region formed three specific EMSA complexes using lens cell nuclear extracts, while a similar but much less intense pattern was observed when a fibroblast nuclear extract was used. Competition experiments indicated that these complexes were not due to the binding of TBP to the TATA box, but rather to the binding of other nuclear proteins to the PE1B -25/-19 region. A series of co-transfection competition studies in vivo also suggested the functional importance of proteins binding in the -25/-19 region. The PE1B protein-DNA interactions appear to be conserved in the chicken, rodent and human alpha A-crystallin gene as well as within the alpha A- and alpha B-crystallin genes in the mouse. Our findings indicate that the PE1B region is important for mouse alpha A-crystallin promoter activity; the proximity of this site to the TATA box raises the possibility for cooperativity or competition between TBP and PE1B-bound proteins.

Transcriptional regulation of the mouse alpha A-crystallin gene: activation dependent on a cyclic AMP-responsive element (DE1/CRE) and a Pax-6-binding site

Two cis-acting promoter elements (-108 to -100 and -49 to -33) of the mouse alpha A-crystallin gene, which is highly expressed in the ocular lens, were studied. Here we show that DE1 (-108 to -100; 5'TGACGGTG3'), which resembles the consensus cyclic AMP (cAMP)-responsive element sequence (CRE; 5'TGACGT[A/C][A/G]3'), behaves like a functional CRE site. Transfection experiments and electrophoretic mobility shift assays (EMSAs) using site-specific mutations correlated a loss of function with deviations from the CRE consensus sequence. Results of EMSAs in the presence of antisera against CREB, delta CREB, and CREM were consistent with the binding of CREB-like proteins to the DE1 sequence. Stimulation of alpha A-crystallin promoter activity via 8-bromo-cAMP, forskolin, or human T-cell leukemia virus type I Tax1 in transfections and reduction of activity of this site in cell-free transcription tests by competition with the somatostatin CRE supported the idea that DE1 is a functional CRE. Finally, Pax-6, a member of the paired-box family of transcription factors, activated the mouse alpha A-crystallin promoter in cotransfected COP-8 fibroblasts and bound to the -59 to -29 promoter sequence in EMSAs. These data provide evidence for a synergistic role of Pax-6 and CREB-like proteins for high expression of the mouse alpha A-crystallin gene in the lens.

Murine transcription factor alpha A-crystallin binding protein I. Complete sequence, gene structure, expression, and functional inhibition via antisense RNA

alpha A-crystallin binding protein I (alpha A-CRYBP1) is a ubiquitously expressed DNA binding protein that was previously identified by its ability to interact with a functionally important sequence in the mouse alpha A-crystallin gene promoter. Here, we have cloned a single copy gene with 10 exons spanning greater than 70 kb of genomic DNA that encodes alpha A-CRYBP1. The mouse alpha A-CRYBP1 gene specifies a 2,688-amino acid protein with 72% amino acid identity to its human homologue, PRDII-BF1. Both the human and the mouse proteins contain two sets of consensus C2H2 zinc fingers at each end as well a central nonconsensus zinc finger. The alpha A-CRYBP1 gene produces a 9.5-kb transcript in 11 different tissues as well as a testis-specific, 7.7-kb transcript. alpha A-CRYBP1 cDNA clones were isolated from adult mouse brain and testis as well as from cell lines derived from mouse lens (alpha TN4-1) and muscle (C2C12). A single clone isolated from the muscle C2C12 library contains an additional exon near the 5'-end that would prevent production of a functional protein if the normal translation start site were utilized; however, there is another potential initiation codon located downstream that is in frame with the rest of the coding region. In addition, we identified multiple cDNAs from the testis in which the final intron is still present. Finally, we used an antisense expression construct derived from an alpha A-CRYBP1 cDNA clone to provide the first functional evidence that alpha A-CRYBP1 regulates gene expression. When introduced into the alpha TN4-1 mouse lens cell line, the antisense construct significantly inhibited expression from a heterologous promoter that utilized the alpha A-CRYBP1 binding site as an enhancer.

Gene regulation and differentiation in vertebrate ocular tissues

Molecular biological techniques have contributed greatly to the study of vertebrate ocular tissues. The specification of ocular tissues has been shown to be closely related to the expression of transcription factors encoded by genes such as Pax6 and microphthalmia. Lens-specific expression of the delta 1-crystallin gene is controlled by factors, such as delta EF1, binding to its enhancer sequences. Retinal activity of the glucocorticoid hormone receptor is regulated by its binding with another transcription factor. Degeneration of photoreceptors in a retinal disease, retinitis pigmentosa, can be caused by the introduction of a mutated opsin gene into mice. In addition, the process of transdifferentiation in ocular tissues has been described at the level of gene expression.

Identification of negative-acting and protein-binding elements in the mouse alpha A-crystallin -1556/-1165 region

The mouse alpha A-crystallin-encoding gene (alpha A-cry) is expressed in a highly lens-preferred manner. To date, it has been shown that this lens-preferred expression is controlled by four proximal positive-acting transcriptional regulatory elements: DE1 (-111/-97), alpha A-CRYBP1 (-66/-57), PE1/TATA (-35/-19) and PE2 (+24/+43). The present study extends our knowledge of mouse alpha A-cry transcriptional regulatory elements to the far upstream region of that gene by demonstrating that the -1556 to -1165 region contains negative-acting sequence elements which function in transfected lens cells derived from mouse, rabbit and chicken. This is the first negative-acting regulatory region identified in mouse alpha A-cry. The -1556 to -1165 region contains sequences similar to repressor/silencer elements identified in other genes, including those highly expressed in the lens, such as the delta 1-crystallin (delta 1-cry) and vimentin (vim) genes. The -1480 to -1401 region specifically interacts with nuclear proteins isolated from the alpha TN4-1 mouse lens cell line. Contained within this protein-binding region and positioned at -1453 to -1444 is a sequence (RS1) similar to the chicken delta 1-cry intron 3 repressor, and which competes for the formation of -1480 to -1401 DNA-protein complexes. Our findings suggest that lens nuclear proteins bind to the mouse alpha A-cry RS1 region. We demonstrate that the chicken delta 1-cry intron repressor binds similar nuclear proteins in chicken embryonic lens cells and mouse alpha TN4-1 lens cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Recruitment of enzymes and stress proteins as lens crystallins

The major water-soluble proteins--or crystallins--of the eye lens are either identical to or derived from proteins with non-refractive functions in numerous tissues. In general, the recruitment of crystallins has come from metabolic enzymes (usually with detoxification functions) or stress proteins. Some crystallins have been recruited without duplication of the original gene (i.e., lactate dehydrogenase B and alpha-enolase), while others have incurred one (i.e., argininosuccinate lyase and a small heat shock protein) or several (i.e., glutathione S-transferase) gene duplications. Enzyme (or stress protein)-crystallins often maintain their non-refractive function in the lens and/or other tissues as well as their refractive role, a process we call gene sharing. alpha-Crystallin/small heat shock protein/molecular chaperone is of special interest since it is the major crystallin of humans. There are two alpha-crystallin genes (alpha A and alpha B), with alpha B retaining the full functions of a small heat shock protein. Here we describe recent evidence indicating that alpha A and alpha B have kinase activity, which would make them members of the enzyme-crystallins. We also describe various regulatory elements of the mouse alpha-crystallin genes responsible for their expression in the lens and, for alpha B, in skeletal muscle. Delineating the control elements for gene expression of these multifunctional protective proteins provides the foundations for their eventual use in gene therapy. Finally, comparison of the mouse and chicken alpha A-crystallin genes reveals similarities and differences in their functional cis-acting elements, indicative of evolution at the level of gene regulation.