Der Einfluss von Retinsäure auf die dorsoventrale Augenachse des Haushuhns
The transcriptional activator retinoic acid (RA) has a morphogenic role in the embryonic development of vertebrates, especially in the polarization of body axes. It has been proposed to be a determining factor in setting up the dorsoventral polarity of the eye axis, thereby organizing the later developing retinotopic visual projection. The pathfinding of retinal axons along the dorsoventral axis of the visual system is controlled by the family of ephrinB ligands and EphB tyrosine kinase receptors. Their action in turn is regulated by signalling molecules with spatially restricted expression domains in the dorsal (e.g. Tbx5, BMP4) and ventral eye cup (e.g. Pax2, cVax). To test whether RA influences the distribution of this signalling system, its concentration in the early chicken eye anlage was manipulated and subsequently the expression of several signalling molecules was monitored with quantitative RT-PCR. Injections of all-trans RA were found to suppress the expression of the dorsal signals Tbx5 and BMP4, whereas the injection of citral, an inhibitor of retinoic acid synthesis, enhanced the expression of these molecules. In contrast, the distribution of the transcription factors Pax2 and cVax in the ventral eye cup were not affected by changes of retinoic acid concentration. To analyze the effect of exogenous retinoic acid on the retinotectal projection, the projection of dorsal ganglion cells in E18-embryos was studied after anterograde tracing was performed. Irrespective of the RA treatment, all labelled primary axons terminated with a normal condensation zone in the expected position within the lateral part of the contralateral tectum. Additionally the molecular analysis of the axonal guidance molecule ephrinB1 did not show significant differences in the distribution and signal strength of transcripts in RA-treated animals.In the last part of the study it was investigated, how the retinoic acid synthesizing (RALDH) and metabolizing (CYP26A1) enzymes themselves are regulated in this system. After viral transfection of retinal precursor cells with cVax and Tbx5 the distribution of retinoic acid synthesizing and metabolizing enzymes was altered in E6 tissue: In cVax infected retinae transcripts of RALDH1 and CYP26A1 were diminished, whereas RALDH3 transcripts were upregulated. Moreover in the Tbx5 infected retinae CYP26A1 was found to be upregulated. Together, the experiments of this study contribute to the clarification of the RA signalling cascade in the regulatory network of the developing visual system: RA is involved in the determination of the dorsoventral eye polarity by the regulation of dorsal signalling factors. Moreover, the influence of the RA signalling system on the eye axis is reinforced by the finding, that the master transcription factors Tbx5 and cVax themselves regulate the dynamic gradients of RA.
info:eu-repo/classification/ddc/570, Biowissenschaften, Biologie, retinotektale Projektion, Retinsäure, Augenachse
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