Individuals with ocular albinism (OA) lack stereoscopic vision due to a reduction of the ipsilateral component of the optic tract and have deficient melanin levels in the retinal pigment epithelium (RPE). The gene that causes the X-linked form of this disease, 0A1, has been identified and characterized. It encodes a G-protein coupled-receptor of unknown function that is localized on the membrane of melanosomes. Melanogenesis occurs in these organelles and tyrosinase is the key enzyme involved in this process. Melanosornes are present in the melanocytes, of the skin and in the RPE. The goal of this proposal is to investigate the molecular mechanisms of axon guidance that lead to the formation of abnormal synaptic connections in the brain of individuals affected with OA. The mouse albino mutant, that carries a point mutation in tyrosinase leading to decreased numbers of uncrossed retinal axons, offers a genetic model to address why the deficiency in melanin results in the abnormality at the optic chiasm seen in OA. We propose to use a genetic approach to identify the cues provided by tyrosinase, RPE cells and Oal that direct retinal axon divergence at the chiasm, and the mechanisms underlying specification of retinal ganglion cells to respond to these cues. Initially, we will genetically engineer mice having Cre-recombinase. These animals will allow us to control the timing of expression of specific genes. The Cre-mice will be crossed with transgenic albino mice expressing tyrosinase or diphteria toxin, and with transgenic mice carrying a conditional allele of 0al. The inducible restoration of melanine by tyrosinase will allow us to determine whether pigmentation has a role in axonal pathfinding. The inducible expression of diphteria toxin to ablate the RPE will indicate whether these cells influence both the differentiation of ganglion cells and their axonal pathfinding. The introduction of an """"""""on/off"""""""" switch to flip Oal coding sequences will allow us to determine whether the stages of axonal crossing can be reversed by the re-expression of the wild type gene. The information obtained with these studies will. increase our understanding of the pathology of ocular albinism and will help us . to unravel molecular mechanisms of pathfinding in the optic chiasm, a """"""""choice point"""""""" where growth cones navigate to the same or opposite side of the brain. Finding how pigmentation defects in the RPE cause abnormalities in axonal guidance and in retinal development may provide insights applicable to future therapy.
