Molecular mechanisms of the axonal and dendritic development of the retinal ganglion cells

Understanding how the tremendous diversity of neuronal morphology and connectivity patterns is achieved and what are the cellular and molecular mechanisms underlying these fundamental processes are the intriguing problems in modern biology. Thanks to recent methodological advances in microarray technology, laser-based microdissection, and neuronal imaging techniques, it is now possible to analyze neuronal morphology in great detail and to study the molecular mechanisms involved in the regulation of neuronal morphogenesis. The identification of molecules that control dendritic and axonal formation, maturation and refinement has become the subject of much focus in the field of neuronal development. In this study, we sought to identify molecules regulating eye specific retinogeniculate segregation---the classical models for axonal developmental plasticity (chapter 1)---and investigated the role of canonical axon guidance molecules Plexin-A3 and Plexin-A4 in dendritic and axonal development of the retinal ganglion cells (chapter 2).;In chapter 1, we employed microarray technology to identify molecules differentially expressed between the ipsi and contra projecting retinal ganglion cells during retinogeniculate segregation. We found that vesicular glutamate transporter two (VGluT2) is specifically present only in contralateral retinogeniculate synapses during segregation, and this differential expression of VGluT2 is lost soon after the segregation is complete. This remarkably precise temporal regulation of its expression suggests that VGluT2 has an important rote during eye-specific axon refinement. We speculate that VGluT2 can provide molecular cues for ipsi and contra synapse discrimination. In our microarray experiment, along with VGluT2, we have identified several novel and previously known gene expression gradients across the postnatal retina and discussed their hypothetical roles in retinal development.;In chapter 2, we studied the loss-of-function effects of the two axon guidance molecules, Plexin-A3 and Plexin-A4, on the dendritic and axonal development of the retinal ganglion cells. Neither of the two Plexins was found to be important for the guidance of the retinal ganglion axons or for the retinogeniculate segregation. We also analyzed the dendritic morphology of the retinal ganglion cells in the Plexin-A3/A4 double knockout animals and found that it was not significantly altered comparing to the wild type. The simultaneous loss of both Plexin-A3 and A4, however, affected the diameter of the retinal ganglion axons, suggesting that Plexin-A3 and A4 might well be involved in basic processes of axonogenesis.;In this work was a small step towards the understanding of general mechanisms of axonal and dendritic development and the mechanisms of retinogeniculate segregation in particular. Our findings about the expression of VGluT2 in the retinal axon terminals will stimulate further investigation of the differential organization of the ipsi- and contralateral retinogeniculate synapses and how this differential organization contributes to the process of eye-specific segregation. Our conclusion that Plexin-A3 and A4 are not largely required for the normal dendritic morphogenesis of the retinal ganglion cells will eventually help to figure out the actual functions of these two Plexins in the retinal development.