Pax6 is a developmental control gene with an essential role in development of the eye, brain and pancreas. Pax6, as many other developmental regulators, depends on a substantial number of cis-regulatory elements in addition to its promoters for correct spatiotemporal and quantitative expression. Here we report on our analysis of a set of mice transgenic for a modified yeast artificial chromosome carrying the human PAX6 locus. In this 420 kb YAC a tauGFP-IRES-Neomycin reporter cassette has been inserted into the PAX6 translational start site in exon 4. The YAC has been further engineered to insert LoxP sites flanking a 35 kb long, distant downstream regulatory region (DRR) containing previously described DNaseI hypersensitive sites, to allow direct comparison between the presence or absence of this region in the same genomic context. Five independent transgenic lines were obtained that vary in the extent of downstream PAX6 locus that has integrated. Analysis of transgenic embryos carrying full-length and truncated versions of the YAC indicates the location and putative function of several novel tissue-specific enhancers. Absence of these distal regulatory elements abolishes expression in specific tissues despite the presence of more proximal enhancers with overlapping specificity, strongly suggesting interaction between these control elements. Using plasmid-based reporter transgenic analysis we provide detailed characterization of one of these enhancers in isolation. Furthermore, we show that overexpression of a short PAX6 isoform derived from an internal promoter in a multicopy YAC transgenic line results in a microphthalmia phenotype. Finally, direct comparison of a single-copy line with the floxed DRR before and after Cre-mediated deletion demonstrates unequivocally the essential role of these long-range control elements for PAX6 expression.

Levels of expression of the transcription factor Pax6 vary throughout corticogenesis in a rostro-lateral high to caudo-medial low gradient across the cortical proliferative zone. Previous loss-of-function studies have indicated that Pax6 is required for normal cortical progenitor proliferation, neuronal differentiation, cortical lamination and cortical arealization, but whether and how its level of expression affects its function is unclear. We studied the developing cortex of PAX77 YAC transgenic mice carrying several copies of the human PAX6 locus with its full complement of regulatory regions. We found that PAX77 embryos express Pax6 in a normal spatial pattern, with levels up to three times higher than wild type. By crossing PAX77 mice with a new YAC transgenic line that reports Pax6 expression (DTy54), we showed that increased expression is limited by negative autoregulation. Increased expression reduces proliferation of late cortical progenitors specifically, and analysis of PAX77}wild-type chimeras indicates that the defect is cell autonomous. We analyzed cortical arealization in PAX77 mice and found that, whereas the loss of Pax6 shifts caudal cortical areas rostrally, Pax6 overexpression at levels predicted to shift rostral areas caudally has very little effect. These findings indicate that Pax6 levels are stabilized by autoregulation, that the proliferation of cortical progenitors is sensitive to altered Pax6 levels and that cortical arealization is not.

Background: The Pax6 transcription factor is expressed during development in the eyes and in specific CNS regions, where it is essential for normal cell proliferation and differentiation. Mice lacking one or both copies of the Pax6 gene model closely humans with loss-of-function mutations in the PAX6 locus. The sequence of the Pax6/PAX6 protein is identical in mice and humans and previous studies have shown structural conservation of the gene's regulatory regions.

Pax6 is a member of an evolutionarily conserved family of transcription factors. It is developmentally regulated and is required for the normal embryonic development of the central nervous system, eye and pancreas. Pax6 mutations in the mouse result in the Small eye (Sey) phenotype. Heterozygous mice have eye defects and homozygotes die immediately after birth lacking eyes, nasal cavities and with severe brain abnormalities, including a malformed cerebral cortex. Recent work has established that there are changes in expression of cell adhesion molecules and these may underlie at least a part of the Pax6(Sey/Sey) phenotype. Here we used cell transplants and explant cultures to investigate the role of Pax6 in cell adhesion. Pax6(Sey/Sey) embryonic cortical cells transplanted into wild-type embryonic cortex were observed to segregate from wild-type cells and form dense clusters. Cells migrating from explants of Pax6(Sey/Sey) embryonic cortex clustered to a greater extent than cells migrating from wild-type controls. These new data support the hypothesis that Pax6 exerts a cell-autonomous effect on the adhesiveness of cortical cells.

The current consensus in the literature is that ovulated oocytes that are not fertilized die by apoptosis, but the details of the proteins involved in the apoptotic pathways have not been elucidated. In this paper we confirm that caspase-3, the executioner of apoptosis, is expressed in mouse oocytes, and show that two initiators of apoptosis, caspase-8 and caspase-9, are expressed in mouse oocytes. Comparisons were made of caspase-3, -8, and -9 activities in superovulated oocytes that were freshly collected or allowed to age in vivo or in vitro. We found that caspase-3 activity significantly increased in aged oocytes compared with young oocytes (p < 0.001), and that both caspase-8 activity and caspase-9 activity decreased in aged oocytes compared with young oocytes (p < 0.001 for caspase-8 and p < 0.05 for caspase-9 activity). A comparison of superovulated with naturally ovulated oocytes showed the same amount of caspase-8 activity in each, but a significant (p < 0.001) decrease in caspase-9 activity in naturally ovulated compared with superovulated oocytes. There was no difference in caspase-3, -8, or -9 activity in oocytes compared with zygotes. Finally, we showed that culture of oocytes in staurosporine increased the activity of caspase-8 and caspase-9. In conclusion, the finding of both caspase-8 and caspase-9 activity in oocytes shows that unfertilized oocytes have the machinery to undergo apoptosis by using either the extrinsic (caspase-8 dependent) or intrinsic (caspase-9 dependent) pathways.