Evolutionary origin of the Otx2 enhancer for its expression in visceral endoderm

A distal enhancer that directs Otx2 expression in the retinal pigment epithelium and neuroretina

BACKGROUND

Homeodomain transcription factor Otx2 is essential for embryonic development of multiple head tissues, including retinal pigment epithelium (RPE) and neuroretina. Temporospatial regulation of Otx2 expression is critical for its functions. Molecular dissection of the cis-acting enhancers will help elucidate how Otx2 expression is regulated.

RESULTS

We comprehensively characterized distal enhancer hs1150 that was previously identified in a high throughput study. We established multiple transgenic mouse lines in which human hs1150, corresponding mouse hs1150, and two highly conserved sub-fragments in the mouse hs1150 were individually fused to a minimal hsp68 promoter to drive reporter expression. We found that hs1150 enhancer directed reporter expression in the RPE, neuroretina, and brain in a developmentally regulated manner. Human hs1150-directed reporter expression largely recapitulated Otx2 expression in the RPE, in the early neuroretina, and to a lesser degree in the early brain. Mouse hs1150, although shorter than human hs1150, exhibited similar enhancer activity, indicating functional conservation of hs1150 enhancer across species. Both of the highly conserved subfragments in mouse hs1150 enhancer directed reporter expression in the early neuroretina, indicating that the hs1150 enhancer has two functional components.

CONCLUSIONS

Our findings provide insight into the molecular mechanisms underlying the regulation of Otx2 retinal expression.

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.

Gbx2 directly restricts Otx2 expression to forebrain and midbrain, competing with class III POU factors

Otx2 plays essential roles in rostral brain development, and its counteraction with Gbx2 has been suggested to determine the midbrain-hindbrain boundary (MHB) in vertebrates. We previously identified the FM enhancer that is conserved among vertebrates and drives Otx2 transcription in forebrain/midbrain from the early somite stage. In this study, we found that the POU homeodomain of class III POU factors (Brn1, Brn2, Brn4, and Oct6) associates with noncanonical target sequence TAATTA in the FM enhancer. MicroRNA-mediated knockdown of Brn2 and Oct6 diminished the FM enhancer activity in anterior neural progenitor cells (NPCs) differentiated from P19 cells. The class III POU factors associate with the FM enhancer in forebrain and midbrain but not in hindbrain. We also demonstrated that the Gbx2 homeodomain recognizes the same target TAATTA in the FM enhancer, and Gbx2 associates with the FM enhancer in hindbrain. Gbx2 misexpression in the anterior NPCs repressed the FM enhancer activity and inhibited Brn2 association with the enhancer, whereas Gbx2 knockdown caused ectopic Brn2 association in the posterior NPCs. These results suggest that class III POU factors and Gbx2 share the same target site, TAATTA, in the FM enhancer and that their region-specific binding restricts Otx2 expression at the MHB.

The hypoblast (visceral endoderm): an evo-devo perspective

When amniotes appeared during evolution, embryos freed themselves from intracellular nutrition; development slowed, the mid-blastula transition was lost and maternal components became less important for polarity. Extra-embryonic tissues emerged to provide nutrition and other innovations. One such tissue, the hypoblast (visceral endoderm in mouse), acquired a role in fixing the body plan: it controls epiblast cell movements leading to primitive streak formation, generating bilateral symmetry. It also transiently induces expression of pre-neural markers in the epiblast, which also contributes to delay streak formation. After gastrulation, the hypoblast might protect prospective forebrain cells from caudalizing signals. These functions separate mesendodermal and neuroectodermal domains by protecting cells against being caught up in the movements of gastrulation.

A lineage specific enhancer drives Otx2 expression in teleost organizer tissues

In mouse Otx2 plays essential roles in anterior-posterior axis formation and head development in anterior visceral endoderm and anterior mesendoderm. The Otx2 expression in these sites is regulated by VE and CM enhancers at the 5' proximal to the translation start site, and we proposed that these enhancers would have been established in ancestral sarcoptergians after divergence from actinopterigians for the use of Otx2 as the head organizer gene (Kurokawa et al., 2010). This would make doubtful an earlier proposal of ours that a 1.1 kb fragment located at +14.4 to +15.5 kb 3' (3'En) of fugu Otx2a gene harbors enhancers phylogenetically and functionally homologous to mouse VE and CM enhancers (Kimura-Yoshida et al., 2007). In the present study, we demonstrate that fugu Otx2a is not expressed in the dorsal margin of blastoderm, shield and early anterior mesendoderm, and that the fugu Otx2a 3'En do not exhibit activities at these sites of fugu embryos. We conclude that the fugu Otx2a 3'En does not harbor an organizer enhancer, but encodes an enhancer for the expression in later anterior mesendodermal tissues. Instead, in fugu embryos Otx2b is expressed in the dorsal margin of blastoderm at blastula stage and shield at 50% epiboly, and this expression is directed by an enhancer, 5'En, located at -1000 to -800 bp, which is uniquely conserved among teleost Otx2b orthologues.

Otx2 and Otx1 protect diencephalon and mesencephalon from caudalization into metencephalon during early brain regionalization

Otx2 is expressed in each step and site of head development. To dissect each Otx2 function we have identified a series of Otx2 enhancers. The Otx2 expression in the anterior neuroectoderm is regulated by the AN enhancer and the subsequent expression in forebrain and midbrain later than E8.5 by FM1 and FM2 enhancers; the Otx1 expression takes place at E8.0. In telencephalon later than E9.5 Otx1 continues to be expressed in the entire pallium, while the Otx2 expression is confined to the most medial pallium. To determine the Otx functions in forebrain and midbrain development we have generated mouse mutants that lack both FM1 and FM2 enhancers (DKO: Otx2(ΔFM1ΔFM2/ΔFM1ΔFM2)) and examined the TKO (Otx1(-/-)Otx2(ΔFM1ΔFM2/ΔFM1ΔFM2)) phenotype. The mutants develop normally until E8.0, but subsequently by E9.5 the diencephalon, including thalamic eminence and prethalamus, and the mesencephalon are caudalized into metencephalon consisting of isthmus and rhombomere 1; the caudalization does not extend to rhombomere 2 and more caudal rhombomeres. In rostral forebrain, neopallium, ganglionic eminences and hypothalamus in front of prethalamus develop; we propose that they become insensitive to the caudalization with the switch from the Otx2 expression under the AN enhancer to that under FM1 and FM2 enhancers. In contrast, the medial pallium requires Otx1 and Otx2 for its development later than E9.5, and the Otx2 expression in diencepalon and mesencephalon later than E9.5 is also directed by an enhancer other than FM1 and FM2 enhancers.