Regulatory elements in the rat betaB2-crystallin promoter

Genetic and epigenetic mechanisms of gene regulation during lens development

Recent studies demonstrated a number of links between chromatin structure, gene expression, extracellular signaling and cellular differentiation during lens development. Lens progenitor cells originate from a pool of common progenitor cells, the pre-placodal region (PPR) which is formed from a combination of extracellular signaling between the neural plate, naïve ectoderm and mesendoderm. A specific commitment to the lens program over alternate choices such as the formation of olfactory epithelium or the anterior pituitary is manifested by the formation of a thickened surface ectoderm, the lens placode. Mouse lens progenitor cells are characterized by the expression of a complement of lens lineage-specific transcription factors including Pax6, Six3 and Sox2, controlled by FGF and BMP signaling, followed later by c-Maf, Mab21like1, Prox1 and FoxE3. Proliferation of lens progenitors together with their morphogenetic movements results in the formation of the lens vesicle. This transient structure, comprised of lens precursor cells, is polarized with its anterior cells retaining their epithelial morphology and proliferative capacity, whereas the posterior lens precursor cells initiate terminal differentiation forming the primary lens fibers. Lens differentiation is marked by expression and accumulation of crystallins and other structural proteins. The transcriptional control of crystallin genes is characterized by the reiterative use of transcription factors required for the establishment of lens precursors in combination with more ubiquitously expressed factors (e.g. AP-1, AP-2alpha, CREB and USF) and recruitment of histone acetyltransferases (HATs) CBP and p300, and chromatin remodeling complexes SWI/SNF and ISWI. These studies have poised the study of lens development at the forefront of efforts to understand the connections between development, cell signaling, gene transcription and chromatin remodeling.

Large Maf Transcription Factors: Cousins of AP-1 Proteins and Important Regulators of Cellular Differentiation

A large number of mammalian transcription factors possess the evolutionary conserved basic and leucine zipper domain (bZIP). The basic domain interacts with DNA while the leucine zipper facilitates homo- and hetero-dimerization. These factors can be grouped into at least seven families: AP-1, ATF/CREB, CNC, C/EBP, Maf, PAR, and virus-encoded bZIPs. Here, we focus on a group of four large Maf proteins: MafA, MafB, c-Maf, and NRL. They act as key regulators of terminal differentiation in many tissues such as bone, brain, kidney, lens, pancreas, and retina, as well as in blood. The DNA-binding mechanism of large Mafs involves cooperation between the basic domain and an adjacent ancillary DNA-binding domain. Many genes regulated by Mafs during cellular differentiation use functional interactions between the Pax/Maf, Sox/Maf, and Ets/Maf promoter and enhancer modules. The prime examples are crystallin genes in lens and glucagon and insulin in pancreas. Novel roles for large Mafs emerged from studying generations of MafA and MafB knockouts and analysis of combined phenotypes in double or triple null mice. In addition, studies of this group of factors in invertebrates revealed the evolutionarily conserved function of these genes in the development of multicellular organisms.

Requirement for betaB1-crystallin promoter of Xenopus laevis in embryonic lens development and lens regeneration

Regulation of the lens-specific betaB1-crystallin promoter in Xenopus laevis was investigated using transgenic larvae and tadpoles. Comparison of the promoter sequence with that of chicken betaB1-crystallin gene indicates significant sequence similarity over a span of several hundred base pairs starting from the transcriptional start site. Remarkably, PL-1 and PL-2 sequences identified in the chicken promoter as essential binding sites of MAF, Pax6 and Prox1 transcription factors were conserved. Mutations of X (Xenopus) PL-1 and XPL-2 sequences eliminated the promoter activity, indicating a conserved mechanism regulating betaB1-crystallin promoter among vertebrate species. A stepwise deletion of the promoter sequence starting from 2800 bp indicated that the proximal 260 bp directly upstream of the transcription initiation site is sufficient for eliciting lens-specific expression, but the 150 bp promoter sequence is inactive despite it containing the XPL-1 and XPL-2 sequences, suggesting the presence of an additional and essential regulatory sequence located between -150 and -260 bp. Activity of the betaB1-crystallin promoter during lens regeneration from cornea was examined using transgenic tadpoles and found to have the same dependence on promoter regions as in embryonic lens development, indicating that gene regulation is largely shared by the two lens-generating processes.

Ageing and vision: structure, stability and function of lens crystallins

The alpha-, beta- and gamma-crystallins are the major protein components of the vertebrate eye lens, alpha-crystallin as a molecular chaperone as well as a structural protein, beta- and gamma-crystallins as structural proteins. For the lens to be able to retain life-long transparency in the absence of protein turnover, the crystallins must meet not only the requirement of solubility associated with high cellular concentration but that of longevity as well. For proteins, longevity is commonly assumed to be correlated with long-term retention of native structure, which in turn can be due to inherent thermodynamic stability, efficient capture and refolding of non-native protein by chaperones, or a combination of both. Understanding how the specific interactions that confer intrinsic stability of the protein fold are combined with the stabilizing effect of protein assembly, and how the non-specific interactions and associations of the assemblies enable the generation of highly concentrated solutions, is thus of importance to understand the loss of transparency of the lens with age. Post-translational modification can have a major effect on protein stability but an emerging theme of the few studies of the effect of post-translational modification of the crystallins is one of solubility and assembly. Here we review the structure, assembly, interactions, stability and post-translational modifications of the crystallins, not only in isolation but also as part of a multi-component system. The available data are discussed in the context of the establishment, the maintenance and finally, with age, the loss of transparency of the lens. Understanding the structural basis of protein stability and interactions in the healthy eye lens is the route to solve the enormous medical and economical problem of cataract.

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.

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.

CREB-binding protein/p300 co-activation of crystallin gene expression

Although some of the transcription factors that are required for expression of crystallins during lens development have been identified, the molecular interactions that contribute to enhanced crystallin expression are not yet well defined. In this study, we designed experiments to test whether the co-activators CREB-binding protein (CBP) and/or p300 interact with c-Maf, Prox-1, or Sox-1 to enhance transcription of crystallin genes. Promoter regions from the mouse alphaA-, betaB2-, and gammaF-crystallin genes were linked to a luciferase reporter. Expression of c-Maf transactivated each of these promoters. Of particular interest, co-expression of CBP or p300 with c-Maf was found to synergistically co-activate each promoter. CBP and p300 were less effective or ineffective at co-activation with Prox-1 or Sox-1. Co-immunoprecipitation and mammalian two-hybrid experiments revealed that CBP and p300 bind to c-Maf and Prox-1 but not to Sox-1. The co-activation of c-Maf by CBP/p300 requires histone acetyltransferase activity. Our results suggest that c-Maf recruits CBP and/or p300 to crystallin promoters leading to up-regulation of crystallin gene expression through localized histone acetylation and consequent chromatin re-modeling. In a promoter-specific fashion, co-activation can be modulated by Prox-1 and/or Sox-1. This modulation may help to specify the endogenous levels of crystallin gene expression.

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.