Embryonic stem cell-mediated transfer and correct regulation of the chicken delta-crystallin gene in developing mouse embryos

Shedding light on developmental gene regulation through the lens

My group has long studied transcriptional gene regulation involved in cell differentiation, employing lens cell differentiation as a model. In this article, our progress over the last quarter of a century in deciphering the principles involved in developmental gene regulation is described, outlining concurrent advancement in relevant branches of developmental biology.

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.

Involvement of Sox1, 2 and 3 in the early and subsequent molecular events of lens induction

Activation of the first lens-specific gene of the chicken, delta 1-crystallin, is dependent on a group of lens nuclear factors, deltaEF2, interacting with the delta1-crystallin minimal enhancer, DC5. One of the deltaEF2 factors was previously identified as SOX2. We show that two related SOX proteins, SOX1 and SOX3, account for the remaining members of deltaEF2. Activation of the DC5 enhancer is dependent on their C-terminal domains. Expression of Sox1-3 in the eye region during lens induction was studied in comparison with Pax6 and delta1-crystallin. Pax6, known to be required for the inductive response of the ectoderm, is broadly expressed in the lateral head ectoderm from before lens induction. After tight association of the optic vesicle (around stage 10–11, 40 hours after egg incubation), expression of Sox2 and Sox3 is activated in the vesicle-facing ectoderm at stage 12 (44 hours). These cells, expressing together Pax6 and Sox2/3, subsequently give rise to the lens, beginning with formation of the lens placode and expression of delta-crystallin at stage 13 (48 hours). Sox1 then starts to be expessed in the lens-forming cells at stage 14. When the prospective retina area of the neural plate was unilaterally ablated at stage 7, expression of Sox2/3 was lost in the side of lateral head ectoderm lacking the optic cup, implying that an inductive signal from the optic cup activates Sox2/3 expression. In the mouse embryonic lens, this subfamily of Sox genes is expressed in an analogous fashion, although Sox3 transcripts have not been detected and Sox2 expression is down-regulated when Sox1 is activated. In ectodermal tissues of the chicken embryo, delta -crystallin expression occurs in a few ectopic sites. These are always characterized by overlapping expression of Sox2/3 and Pax6. Thus, an essential molecular event in lens induction is the ‘turning on’ of the transcriptional regulators SOX2/3 in the Pax6-expressing ectoderm and these SOX proteins activate crystallin gene expression. Continued activity, especially of SOX1, is then essential for further development of the lens.

Transgenic models for eye malformations

Several lines of transgenic mice developing eye malformations have been described in the literature and appear to be of increasing interest for the study of eye teratology in humans, since gene expression and regulation can be studied in the developing animal. Transgenic applications are briefly described here and an overview of existing transgenic mouse models carrying different eye abnormalities is given according to the major diagnosis (e.g., cataract, microphthalmia, anterior segment dysgenesis, retinal dysplasia). Interestingly, many transgenic models exhibit pathological findings similar to those observed in human pediatric ophthalmology. Unfortunately, detailed embryological studies in transgenic mice bearing congenital eye malformations are not available for all lines. Thus, the importance of creating further transgenic models to study the function of morphogenes and growth factors in eye development is also discussed.

A stable cellular marker for the analysis of mouse chimeras: the bacterial chloramphenicol acetyltransferase gene driven by the human elongation factor 1 alpha promoter

We have developed a method of marking of mouse cells by means of transfection of a foreign gene. The transgene chosen here was the plasmid pEF321CAT which contains the bacterial chloramphenicol acetyl transferase (CAT) gene linked to the promoter region of the human polypeptide chain elongation factor 1 alpha (hEF1 alpha) gene. Evaluation of the plasmid pEF321CAT as a cellular marker for mouse cells involved intensive examination of a transgenic mouse carrying pEF321CAT. The CAT gene was expressed in all tissues examined, demonstrating that the hEF1 alpha promoter was active in a wide range of mouse cells. The plasmid itself did not exert any harmful effect on the normal development of mice, and the CAT activity was immunohistologically detectable on sectioned tissues by the use of anti-CAT serum. When the plasmid was transferred into embryonal carcinoma (EC) cells and embryonic stem (ES) cells, the CAT gene was also found to be expressed constantly irrespective of their differentiation. These results demonstrated that the plasmid pEF321CAT can be used as a reliable and feasible cellular marker that would distinguish unequivocally the cells of each of genotype in chimeric tissues.

Regulation of the murine alpha A-crystallin promoter in transgenic mice

To identify sequences necessary for lens-specific gene expression, lines of transgenic mice were generated which contain murine alpha A-crystallin promoter sequences [-111 to +46 (alpha 111), -88 to +46 (alpha 88), and -34 to +46 (alpha 34)] fused to the bacterial chloramphenicol acetyltransferase (CAT) gene and CAT expression was analyzed. Mice carrying the alpha 111-CAT or the alpha 88-CAT fusion transgene expressed CAT exclusively in lens, except for one line containing alpha 111-CAT, which expressed low levels of CAT in several nonlenticular tissues. Transcription from these promoters in lens initiated at the same site as the endogenous alpha A-crystallin promoter. In one line of mice alpha 88-CAT transgene became active in the lens during embryonic development at approximately the same time that the alpha A-crystallin gene normally begins to be expressed. In contrast, the alpha 34-CAT fusion transgene, containing the TATA box but no sequences further upstream, was inactive in transgenic mice. Our data suggest that 134 bp of sequence (-88 to +46) in the murine alpha A-crystallin gene is sufficient to provide lens specificity, although we cannot rule out the possibility that other sequences also contribute to promoter function.

Regulated expression of muscle-specific genes introduced into mouse embryonal stem cells: inverse correlation with DNA methylation

Pluripotent embryonal stem cell lines (ES) were isolated from cultured normal mouse blastocysts. These cells retained their capacity to differentiate into a great variety of cell types in cell cultures or in tumors formed after subcutaneous injection of the cells into nude mice. A chimeric actin/globin gene containing about two-thirds of the rat skeletal muscle actin gene and 730 bp of its 5' flanking region fused to the 3' end of the human embryonic epsilon-globin gene, was inserted into a plasmid containing a neomycin resistance gene (neor) whose transcription is regulated by the SV40 early control elements. The prokaryotic vector DNA sequences of this plasmid (pAG-Neo) were deleted and the two linked genes were introduced into the ES cells by electroporation. G418-resistant clones were isolated, amplified and injected subcutaneously into nude mice. From the teratocarcinoma-like tumors which developed we isolated myogenic as well as nonmyogenic cell lines. In cell lines derived from three independent transfected ES clones, expression of the actin/globin gene was developmentally regulated in myogenic cells. In contrast, in a number of experiments in which the actin/globin gene or other muscle-specific genes were introduced into the ES cells without the removal of the pBR sequences, no expression could be detected at any stage. Moreover, in the differentiated lines derived from these clones, G418 resistance was lost, and no neor transcripts could be detected. Southern-blot analysis of MSPI- or HpaII-digested DNA revealed extensive methylation in the clones that did not express the foreign DNA, whereas no significant methylation of the inserted DNA was observed in clones which expressed the transfected genes. Examination of the DNA extracted from transgenic mice carrying the same actin/globin gene revealed an inverse correlation between methylation of the exogenous gene and its potential to be expressed in the transgenic strain. However, no tissue-specific differences in methylation, related to the tissue specificity of expression of the exogenous gene, could be detected in these experiments. These results suggest that the process of methylation reported here is causally related to constitutive inactivation of the exogenous genes.

Stage-dependent expression of the chicken delta-crystallin gene in transgenic fish embryos

To study the regulation of gene expression of vertebrate crystallin genes, the chicken delta-crystallin gene was introduced into a small freshwater fish, medaka (Oryzias latipes), which lacks this gene, and its expression was examined immunohistologically at several developmental stages before hatching. The gene expression was detected in the central fiber cells of the lens at an early stage, showing a stage-dependent expression. In non-lens tissues, the expression was barely detectable before tissue differentiation. It first became substantial mainly in mesodermal tissues and then later in a greater variety of tissues, including ectodermal and endodermal ones. Thus, the non-lens expression of delta-crystallin was also stage-dependent, with the stage being dependent on the tissue type. These results from lens and non-lens tissues are discussed in relation to tissue differentiation and two categories of delta-crystallin expression.

Developmental regulation of the chicken delta 1-crystallin gene: analysis by transgenesis and gene dissection

We previously reviewed what we had learned about the regulation of the delta 1-crystallin gene through experiments using gene transfer techniques [Kondoh et al. (1986) Cell Differ. 19, 151-160]. It was concluded then that regulatory genetic elements for the lens-specific expression are associated with the delta 1-crystallin gene, and that these chicken elements properly function in mammalian cells. In the last couple of years, we have made significant progress in the understanding of lens-specific delta-crystallin expression. This is owing to success in transgenesis of mouse with the delta 1-crystallin gene and in functional dissection of the gene which led us to the discovery of an intragenic enhancer as the major determinant for lens-specific expression. In this article, we summarize these recent advances.

Conditions permitting the homotopic expression of lens-specific crystallin genes

delta-Crystallin is a major soluble protein of the avian and reptilian lens, and its expression is highly tissue-specific in development. In order to understand regulatory mechanisms for tissue-specific expression of delta-crystallin gene, several experimental systems were established in a heterologous combination of the chicken gene and mouse cells. The expression was ectopic in various cell types differentiated in teratomas derived from mouse teratocarcinoma or embryonic stem cells which were transformed to carry the chicken delta-crystallin genes. Cells of the same transformed lines of embryonic stem cells expressed the chicken gene homotopically in chimeric embryos produced by injecting them into the blastocysts. The homotopic expression also occurred in experimental systems consisting of the heterologous introduction of the gene (1) into various mouse cells in primary cultures, and (2) into male pronuclei of mouse fertilized eggs.

Exogenous delta-crystallin gene expression as probe for differentiation of teratocarcinoma stem cells

We developed an experimental system in which differentiation of teratocarcinoma stem cell is probed by expression of stably introduced exogenous genes. We used chicken delta-crystallin gene (delta gene) and its derivative (Mo delta gene) driven by long terminal repeat (LTR) promoter of Moloney murine leukemia virus (Mo-MuLV). Neither of the genes was expressed in the undifferentiated condition. Differentiation to primitive endoderm induced by retinoic acid (RA) led to expression of delta but not Mo delta, while differentiation to more advanced endodermal cells by RA plus dibutyryl cAMP elicited Mo delta expression in addition to delta. These results are interpreted as a consequence of differential activation/suppression of gene expression through enhancer elements associated with the genes.