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Homologues of the homeotic and segment polarity genes are involved in the postnatal development of the cat visual system Sabunciyan, Sarven Harutyun

Abstract

This thesis investigates the possible involvement of the homologues of the drosophila developmental genes in the postnatal development of the cat visual system. Initially, the cDNA's of homeotic genes PBX1, PBX2 and the homologues of the segment polarity genes BMP4, BMP6, BMP type II receptor, Wnt-1, and FrzB were partially cloned in the cat. We report that the mRNA expression of these genes is developmentally regulated in the postnatal cat visual cortex. To further substantiate our hypothesis that the homologues of the drosophila developmental genes contribute to the postnatal development of the cat visual cortex, the expression of the beta-catenin protein was characterized in the visual system of normally developing and deprived kittens. The beta-catenin protein, which is a downstream effector of the Wnt-1 signalling pathway, is capable of functioning both as a transcription factor and a cell adhesion molecule. Consistent with its characterized role as a transcription factor, the beta-catenin protein becomes nuclearized in L G N neurons at the end of the period for thalamocortical plasticity. Hence, one of the putative functions of the beta-catenin protein in postnatal visual development is proposed to be ending thalamocortical plasticity. The role of the beta-catenin protein in cellular adhesion is to anchor the cadherin cell adhesion molecules to the actin cytoskeleton. Interestingly, the beta-catenin/cadherin cell adhesion system in neurons is located at synapses. Fittingly, both the cadherin and the beta-catenin proteins are expressed in the neuropil of the geniculate and the visual cortex and this expression is prominent in layer IV of the visual cortex. In addition, the temporal expression of these proteins correlates with the critical period. Furthermore, neuropil expression of betacatenin and cadherin proteins is altered in the L G N in response to monocular deprivation. This finding suggest a role for these molecules in the competition occurring between X - and Y - cell arbours. In summary, beta-catenin appears to act as a multi-functional protein and contribute to different facets of postnatal visual development in the cat. These findings endorse our original hypothesis that the homologues of the drosophila developmental genes are involved in the development of the cat visual system.

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