Midbrain Forebrain Research Articles

Otx2 expression in anterior neuroectoderm and forebrain/midbrain is directed by more than six enhancers

Otx2 plays essential roles in each site at each step of head development. We previously identified the AN1 enhancer at 91kb 5' upstream for the Otx2 expressions in anterior neuroectoderm (AN) at neural plate stage before E8.5, and the FM1 enhancer at 75kb 5' upstream and the FM2 enhancer at 122kb 3' downstream for the expression in forebrain/midbrain (FM) at brain vesicle stage after E8.5. The present study identified a second AN enhancer (AN2) at 88kb 5' upstream; the AN2 enhancer also recapitulates the endogenous Otx2 expression in choroid plexus, cortical hem and choroidal roof. However, the enhancer mutants indicated the presence of another AN enhancer. The study also identified a third FM enhancer (FM3) at 153kb 5' upstream. Thus, the Otx2 expressions in anterior neuroectoderm and forebrain/midbrain are regulated by more than six enhancers located far from the coding region. The enhancers identified are differentially conserved among vertebrates; none of the AN enhancers has activities in caudal forebrain and midbrain at brain vesicle stage after E8.5, nor do any of the FM enhancers in anterior neuroectoderm at neural plate stage before E8.5.

Evolutionary constraint on Otx2 neuroectoderm enhancers-deep conservation from skate to mouse and unique divergence in teleost

Otx2 is a paired type homeobox gene that plays essential roles in each step and site of head development in vertebrates. In the mouse, Otx2 expression in the anterior neuroectoderm is regulated primarily by two distinct enhancers: anterior neuroectoderm (AN) and forebrain/midbrain (FM) enhancers at 92 kb and 75 kb 5'of the Otx2 locus, respectively. The AN enhancer has activity in the entire anterior neuroectoderm at headfold and early somite stages, whereas the FM enhancer is subsequently active in the future caudal forebrain and midbrain ectoderm. In tetrapods, both AN and FM enhancers are conserved, whereas the AN region is missing in teleosts, despite overt Otx2 expression in the anterior neuroectoderm. Here, we show that zebrafish and fugu FM regions drive expression not only in the forebrain and midbrain but also in the anterior neuroectoderm at headfold stage. The analysis of coelacanth and skate genomic Otx2 orthologues suggests that the utilization of the two enhancers, AN and FM, is an ancestral condition. In contrast, the AN enhancer has been specifically lost in the teleost lineage with a compensatory establishment of AN activity within the FM enhancer. Furthermore, the AN activity in the fish FM enhancer was established by recruiting upstream factors different from those that direct the tetrapod AN enhancer, yet zebrafish FM enhancer is active in both mouse and zebrafish anterior neuroectoderm at the headfold stage.

Folic acid prevents exencephaly in Cited2 deficient mice.

Cited2 (also Mrg1/p35srj) is a member of a new conserved gene family that is expressed during mouse development and in adult tissues. In order to investigate the function of Cited2 during mouse embryogenesis, we introduced a null mutation into the Cited2 locus. Cited2(-/-) mutants died at late gestation and exhibited heart defects and exencephaly, arising from defective closure of the midbrain (MB) and hindbrain. Initiation of neural tube closure at the forebrain-midbrain (FB-MB) boundary, an essential step for closure of the cranial neural tube, was impaired in the Cited2(-/-) mutants. Gene marker analysis using in situ hybridization revealed that the patterning of the anterior neural plate and head mesenchyme was little affected or normal in the Cited2(-/-) embryos. However, Cited2 was required for the survival of neuroepithelial cells and its absence led to massive apoptosis in dorsal neuroectoderm around the FB-MB boundary and in a restricted transverse domain in the hindbrain. Treatment with folic acid significantly reduced the exencephalic phenotype in the Cited2(-/-) embryos both in vivo and in vitro. However, assessment of folate metabolism revealed no defect in the Cited2(-/-) mutants, and the elevated apoptosis observed in the neuroepithelium of the Cited2(-/-) mutants was apparently not decreased by folic acid supplementation. To our knowledge, the Cited2 mouse represents the first genetic model in which folic acid can prevent a defect in neural tube closure by a mechanism other than the neutralization of a defect in folate homeostasis.

Origins of anteroposterior patterning and Hox gene regulation during chordate evolution.

All chordates share a basic body plan and many common features of early development. Anteroposterior (AP) regions of the vertebrate neural tube are specified by a combinatorial pattern of Hox gene expression that is conserved in urochordates and cephalochordates. Another primitive feature of Hox gene regulation in all chordates is a sensitivity to retinoic acid during embryogenesis, and recent developmental genetic studies have demonstrated the essential role for retinoid signalling in vertebrates. Two AP regions develop within the chordate neural tube during gastrulation: an anterior 'forebrain-midbrain' region specified by Otx genes and a posterior 'hindbrain-spinal cord' region specified by Hox genes. A third, intermediate region corresponding to the midbrain or midbrain-hindbrain boundary develops at around the same time in vertebrates, and comparative data suggest that this was also present in the chordate ancestor. Within the anterior part of the Hox-expressing domain, however, vertebrates appear to have evolved unique roles for segmentation genes, such as Krox-20, in patterning the hindbrain. Genetic approaches in mammals and zebrafish, coupled with molecular phylogenetic studies in ascidians, amphioxus and lampreys, promise to reveal how the complex mechanisms that specify the vertebrate body plan may have arisen from a relatively simple set of ancestral developmental components.