Identification of an Enhancer Critical for the ephirn-A5 Gene Expression in the Posterior Region of the Mesencephalon

Ephrin-A5 has been implicated in the regulation of brain morphogenesis and axon pathfinding. In this study, we used bacterial homologous recombination to express a LacZ reporter in various ephrin-A5 BAC clones to identify elements that regulate ephrin-A5 gene expression during mesencephalon development. We found that there is mesencephalon-specific enhancer activity localized to a specific +25.0 kb to +30.5 kb genomic region in the first intron of ephrin-A5. Further comparative genomic analysis indicated that two evolutionary conserved regions, ECR1 and ECR2, were present within this 5.5 kb region. Deletion of ECR1 from the enhancer resulted in disrupted mesencephalon-specific enhancer activity in transgenic embryos. We also found a consensus binding site for basic helix-loop-helix (bHLH) transcription factors (TFs) in a highly conserved region at the 3'-end of ECR1. We further demonstrated that specific deletion of the bHLH TF binding site abrogated the mesencephalon-specific enhancer activity in transgenic embryos. Finally, both electrophoretic mobility shift assay and luciferase-based transactivation assay revealed that the transcription factor Ascl1 bound the bHLH consensus binding site in the mesencephalon-specific ephrin-A5 enhancer in vitro. Together, these results suggest that the bHLH TF binding site in ECR1 is involved in the positive regulation of ephrin-A5 gene expression during the development of the mesencephalon.

Proper closure of the optic fissure requires ephrin A5-EphB2-JNK signaling

The development of complex organs such as the eye requires a delicate and coordinated balance of cell division and cell death. Although apoptosis is prevalent in the proximoventral optic cup, the precise role it plays in eye development needs to be investigated further. In this study, we show that reduced apoptosis in the proximoventral optic cup prevents closure of the optic fissure. We also show that expression of ephrin A5 (Efna5) partially overlaps with Eph receptor B2 (Ephb2) expression in the proximoventral optic cup and that binding of EphB2 to ephrin A5 induces a sustained activation of JNK. This prolonged JNK signal promotes apoptosis and prevents cell proliferation. Thus, we propose that the unique cross-subclass interaction of EphB2 with ephrin A5 has evolved to function upstream of JNK signaling for the purpose of maintaining an adequate pool of progenitor cells to ensure proper closure of the optic fissure.

Identification of the 187 bp EphA7 Genomic DNA as the Dorsal Midline-Specific Enhancer of the Diencephalon and Mesencephalon

EphA7 is a key molecule in regulating the development of the dien- and mesencephalon. To get insight into the mechanism of how EphA7 gene expression is regulated during the dorsal specification of the dien- and mesencephalon, we investigated the cis-acting regulatory sequence driving EphA7 to the dorsal midline of the dien- and mesencephalon. Transgenic LacZ reporter analysis, using overlapping EphA7 BACs, was used to narrow down the dorsal midline-specific enhancer, revealing the 25.3 kb genomic region as the enhancer candidate. Strikingly, this genomic DNA was located far downstream of the EphA7 transcription start site, +302.6 kb to +327.9 kb. Further enhancer mapping, using comparative genomic analysis and transgenic methods, showed that the 187 bp genomic DNA alone, approximately 305 kb downstream of the EphA7 transcription start site, was sufficient to act as the dorsal midline-specific enhancer of EphA7. Importantly, our results indicate that the 187 bp dorsal midline-specific enhancer is critically regulated by homeobox transcription factors during the development of the dien- and mesencephalon.

In Vivo Expression of the PTB-deleted Odin Mutant Results in Hydrocephalus

Odin has been implicated in the downstream signaling pathway of receptor tyrosine kinases, such as the epidermal growth factor and Eph receptors. However, the physiologically relevant function of Odin needs to be further determined. In this study, we used Odin heterozygous mice to analyze the Odin expression pattern; the targeted allele contained a β-geo gene trap vector inserted into the 14th intron of the Odin gene. Interestingly, we found that Odin was exclusively expressed in ependymal cells along the brain ventricles. In particular, Odin was highly expressed in the subcommissural organ, a small ependymal glandular tissue. However, we did not observe any morphological abnormalities in the brain ventricles or ependymal cells of Odin null-mutant mice. We also generated BAC transgenic mice that expressed the PTB-deleted Odin (dPTB) after a floxed GFP-STOP cassette was excised by tissue-specific Cre expression. Strikingly, Odin-dPTB expression played a causative role in the development of the hydrocephalic phenotype, primarily in the midbrain. In addition, Odin-dPTB expression disrupted proper development of the subcommissural organ and interfered with ependymal cell maturation in the cerebral aqueduct. Taken together, our findings strongly suggest that Odin plays a role in the differentiation of ependymal cells during early postnatal brain development.

In vivo expression of ephrinA5-Fc in mice results in cephalic neural crest agenesis and craniofacial abnormalities

Eph receptors and their ligands ephrins have been implicated in guiding the directed migration of neural crest cells (NCCs). In this study, we found that Wnt1-Cre-mediated expression of ephrinA5-Fc along the dorsal midline of the dien- and mesencephalon resulted in severe craniofacial malformation of mouse embryo. Interestingly, expression of cephalic NCC markers decreased significantly in the frontonasal process and branchial arches 1 and 2, which are target areas for the migratory cephalic NCCs originating in the dien- and mesencephalon. In addition, these craniofacial tissues were much smaller in mutant embryos expressing ephrinA5-Fc. Importantly, EphA7-positive cephalic NCCs were absent along the dorsal dien- and mesencephalon of mutant embryos expressing ephrinA5-Fc, suggesting that the generation of cephalic NCCs is disrupted due to ephrinA5-Fc expression. NCC explant experiments suggested that ephrinA5-Fc perturbed survival of cephalic NCC precursors in the dorsal midline tissue rather than affecting their migratory capacity, which was consistent with our previous report that expression of ephrinA5-Fc in the dorsal midline is responsible for severe neuroepithelial cell apoptotic death. Taken together, our findings strongly suggest that expression of ephrinA5-Fc decreases a population of cephalic NCC precursors in the dorsal midline of the dien- and mesencephalon, thereby disrupting craniofacial development in the mouse embryos.

Expression of EphA8-Fc in transgenic mouse embryos induces apoptosis of neural epithelial cells during brain development

EphAs and ephrin-As are expressed in multiple regions of the developing brain and have been implicated in regulating brain size. Here, we report the identification of a novel mechanism in which reverse signaling through ephrin-As controls neural epithelial cell number in the developing brain. Ectopic expression of EphA8-Fc in transgenic embryos induced apoptosis of neural epithelial cells, which was accompanied by a dramatic decrease in brain size. The number of ephrin-A5-expressing cells was significantly reduced in the brain region where EphA8-Fc was ectopically expressed. Furthermore, in vitro culture of the dissociated neuroepithelial cells revealed that EphA8-Fc enhanced apoptotic cell death of the ephrinA5-expressing cells in a caspase-dependent manner. Thus, our results suggest that reverse signaling through ephrin-As is biochemically linked with caspase-dependent proapoptotic signaling during early brain development.

EphA/ephrin-A signaling is critically involved in region-specific apoptosis during early brain development

EphAs and ephrin-As have been implicated in the morphogenesis of the developing brain. We found that EphA7 and ephrin-A5 are coexpressed in the dorsal midline (DM) of the diencephalon and anterior mesencephalon. Interestingly, programmed cell death (PCD) of the neural epithelial cells normally found in this region was reduced in ephrin-A5/ephrin-A2 dual-deficient embryos. In contrast, in vivo expression of ephrin-A5-Fc or full-length ephrin-A5 strongly induced apoptosis in neural epithelial cells and was accompanied by severe brain malformation during embryonic development. Expression of ephrinA5-Fc correlated with apoptosis of EphA7-expressing cells, whereas null mutation of ephrin-A5 resulted in the converse phenotype. Importantly, null mutation of caspase-3 or endogenous ephrin-A5 attenuated the PCD induced by ectopically overexpressed ephrin-A5. Together, our results suggest that brain region-specific PCD may occur in a region where EphAs cluster with neighboring ephrin-As through cell-cell contact.

Endocytosis of EphA receptors is essential for the proper development of the retinocollicular topographic map

Endocytosis of Eph-ephrin complexes may be an important mechanism for converting cell-cell adhesion to a repulsive interaction. Here, we show that an endocytosis-defective EphA8 mutant forms a complex with EphAs and blocks their endocytosis in cultured cells. Further, we used bacterial artificial chromosome transgenic (Tg) mice to recapitulate the anterior>posterior gradient of EphA in the superior colliculus (SC). In mice expressing the endocytosis-defective EphA8 mutant, the nasal axons were aberrantly shifted to the anterior SC. In contrast, in Tg mice expressing wild-type EphA8, the nasal axons were shifted to the posterior SC, as predicted for the enhanced repellent effect of ephrinA reverse signalling. Importantly, Rac signalling was shown to be essential for EphA-ephrinA internalization and the subsequent nasal axonal repulsion in the SC. These results indicate that endocytosis of the Eph-ephrin complex is a key mechanism by which axonal repulsion is generated for proper guidance and topographic mapping.

Ectopic Expression of Ephrin-A5 Under the EphA8 Promoter at the Anterior Region of the Superior Colliculus

EphA/ephrin-A mediated signaling has emerged as a key mechanism regulating axon guidance and topographic mapping, particularly in the well-characterized visual system from the retina to the superior colliculus (SC). In this study, EphA8 bacterial artificial chromosome (BAC) was manipulated to contain a floxed eGFP and human ephrin-A5 expression cassette using homologous recombination method. In the mice containing the recombinant BAC, it was shown that GFP is expressed in an anterior>posterior gradient in the SC. Furthermore, when these mice were crossed with the transgenic mice expressing Cre under the EphA8 promoter, it was evident that a GFP expression cassette was eliminated, and that human ephrin-A5 was ectopically expressed in the anterior region of the SC. This transgenic model would be useful to analyze the role of ephrin-A5 in the SC during the retinocollicular topography formation.