The search for new determinants of the retinotopic map

The ordered connections made by the retinal ganglion cells to their target neurons in the mid-brain region known as the optic tectum gives rise to a topographic map of the visual field in the brain. Roger Sperry proposed a “chemoaffinity hypothesis” to explain the mechanism of establishment of such topographic connections between the retina and the tectum. According to this hypothesis interaction between two sets of chemical tags one expressed on the retinal ganglion cell (RGC) axons and the other on the target neurons in the tectum determines the position where specific RGC axons will terminate and form synapses. Several years after Sperry’ hypothesis was proposed some of the molecules that act as these “chemoaffinity tags” have been identified. One such group of topographic guidance molecules that direct the connections between the retina and the tectum are the Eph receptor tyrosine kinases and their ligands the Ephrins which are expressed in reciprocal anterior-posterior and dorsal-ventral gradients in the retina as well as the tectum. In addition to this recently there have been some reports of identification of novel topographic guidance molecules such as Wnt3a and the transcription factor Engrailed which act through uncharacterized and potentially novel mechanisms to guide the RGC axons. This leaves open the possibility that there are as yet unidentified topographic guidance molecules that guide RGC axons. To identify such molecules we have carried out a microarray-based comparison between the anterior and posterior halves as well as the dorsal and ventral halves of the developing chick optic tectum in to identify genes expressed asymmetrically along the anterior-posterior axis or the dorsal-ventral axis. Such genes could potentially encode for candidate topographic guidance molecules or their regulators. We have validated the results of the microarray-based comparison using RNA in-situ hybridization and have selected fifteen candidate genes that exhibit distinct asymmetric expression across the anterior-posterior or the dorsal-ventral axis of the tectum. These candidate topographic guidance molecules include a diverse set of genes encoding for some transcription factors, signaling molecules and several molecules with unknown function. We are at present carrying out loss of function and gain of function experiments in-vivo in the chick to establish which of these may perturb the topographic order of RGC projections to the tectum. We are also in the process of establishing in-vitro assays to directly test the ability of any of these proteins to guide affect the growth and/or direction of growth of RGCs.