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内容記述 |
Topographic maps with a defined spatial ordering of neuronal connections are a key feature of the brain's organization. The most widely used model for studies of the formation of topographic maps is the retinotectal projection. Chick brain factor-1 (CBF-1), a nasal retina-specific winged-helix transcription factor, is known to prescribe the nasal specificity which leads to the formation of the precise retinotectal map especially along the antero-posterior (A-P) axis. However, the molecular mechanisms by which CBF-1 controls the expression of topographic molecules have not been elucidated. In the present study, to gain insight into the downstream target genes of CBF-1, he employed electroporation of a retroviral vector carrying the CBF-1 gene into the optic vesicle, and examined effects of the misexpression of CBF-1 on the expression of topographic molecules and other asymmetrically distributed molecules. The in ovo electroporation of retrovirus allows the immediate and sustained expression of a transgene. Since endogenous CBF-1 begins to be topographically expressed prior to Hamburger-Hamilton (HH) stage 11 in native embryos, at which stage the polarity along the nasotemporal (N-T) axis appears to be determined, this in ovo electroporation system is suitable for the functional study of CBF-1. Here, he shows that ectopic expression of CBF-1 in the temporal retina represses expression of EphA3 and CBF-2, and induces that of SOHo-1, GH6, ephrin-A2, and ephrin-A5. A chimeric protein that consists of an even-skipped repression domain and CBF-1 DNA binding domain exerted the same effects as the wild-type CBF-1 on the expression of S0Ho-1, GH6, EphA3, CBF-2, and ephrin-AS, but not on that of ephrin-A2. On the other hand, a CBF-1 mutant deficient in DNA-binding activity exerted the same effects as the wild-type CBF-1 on the expression of S0Ho-1, GH6, EphA3, CBF-2, and ephrin-AX, but not ephrin-AS. These results suggest that CBF-1 controls ephrin-A5 by a DNA binding-dependent mechanism, ephrin-A2 by a DNA binding-independent mechanism, and CBF-2, SOHo-1, GH6, and EphA3 by dual mechanisms.<br /> Our lab recently identified a novel secreted molecule, Ventroptin, which is an antagonist of bone morphogenetic protein 4 (BMP-4) in the retina, and demonstrated that Ventroptin is implicated in the retinotectal topographic projection along both the dorsoventral (D-V) and A-P axes. Asymmetrical expression of Ventroptin along the N-T axis regulates the graded expression of ephrin-A2 along this axis but not of ephrin-A5 or EphA3 in the retina, which is associated with the retinotectal mapping along the A-P axis. Vneroptin expression is ventral-specific, complementary to the dorsal-specific BMP-4 expression at the early developmental stage of the retina. However, along with a decline of BMP-4 expression from the dorsal retina, the expression of Ventroptin forms a double-gradient pattern along the two axes from E6. This suggests that a member of the TGF-□ family other thin BMP-4 should appear in a temporal high-nasal low gradient along the N-T axis with a complementary pattern to the Ventroptin expression. In this study, he found that a TGF-□ family member, BMP-2, is expressed in a pattern complementary to that of Ventroptin, with a double-gradient along the two axes from E6 onward. Ventroptin antagonizes BMP-2 as well as BMP-4. Misexpression of BMP-2 in the developing retina repressed expression of Ventroptin and ephrin-A2. Moreover, he demonstrates that CBF-1 perturbs BMP-2 signaling through a DNA binding-independent mechanism, which resultantly causes the induction of ephrin-A2 expression. These results thus suggest that CBF-1 is located at the top of the gene cascade for the regional specification along the N-T axis in the retina and distinct BMP signals play pivotal roles in the topographic projection along both axes. |
要旨・審査要旨 / Abstract, Screening Result (252.5 kB)
