Henry Wu

CHARACTERIZATION OF MISTARGETED RETINAL GANGLION CELL PROJECTIONS TO THE DORSAL LATERAL GENICULATE NUCLEUS (DLGN) OF THE THALAMUS IN THE PHR MUTANT. Henry Wu¹, A. Joseph Bloom², Susan M. Culican¹. Department of Ophthalmology and Visual Sciences, Washington University, St. Louis, MO¹; Department of Molecular Biology and Pharmacology, Washington University School of Medicine, St. Louis, MO.²

Highwire (hiw) has been shown to play a role in synaptic morphology regulation. The loss of hiw in Drosophila results in abnormally large neuromuscular synapses with lower synaptic strength per vesicle. PHR is the mouse ortholog of hiw in Drosophila. Complete loss of PHR in mouse is lethal at birth. In order to study the PHR phenotype, we have generated a conditional PHR knockout in specific parts of the mouse using the Cre/lox recombinase system. To conditionally delete PHR in the retina, we have used either the αPax or Math5 promoters, both of which are expressed specifically in the retina, to drive expression of the Cre recombinase in the floxed PHR gene background. Since retina projects to the dorsal lateral geniculate nucleus (dLGN) in the thalamus, we study the PHR synaptic phenotype by labeling ganglion cells with fluorescently labeled cholera toxin (ChB), and subsequently image their projections to the dLGN. Using this approach we saw mislocalized ipsilateral projections to the dLGN in the mature conditional PHR knockout mouse. We examined the possible processes that would lead to this phenotype including: 1.) initial mistargeting of retinal projections to the ipsilateral patch, 2.) abnormal crossing of ipsilaterally projecting axons and 3.) unbiased projection in the entire retina with abnormal maintenance of projections after segregation. To discriminate amongst these possibilities we did a time series of innervation patterns in the dLGN and performed retrograde labeling of retinal projections to the dLGN to show that the most likely mechanism to account for the mislocalized ipsilateral projection is abnormal synaptic elimination/segregation from an initially unbiased projection.