Rod photoreceptor neurite sprouting in retinitis pigmentosa

In animal models for retinitis pigmentosa (RP), rod photoreceptors show abnormal distribution of rhodopsin prior to undergoing cell death. To elucidate the steps in degeneration of human photoreceptors, immunocytochemistry was performed on donor retinas from 15 RP patients and five normal subjects. Rhodopsin immunolabeling in the normal retinas was restricted to the rod outer segments. In the RP retinas, rhodopsin was present in shortened rod outer segments and in the surface membranes of the rod inner segments and somata. In regions of photoreceptor death, the surviving rods had sprouted rhodopsin-positive neurites that were closely associated with gliotic Müller cell processes and extended to the inner limiting membrane. Rods and cones in the RP maculas did not form neurites, but the axons of peripheral cones were abnormally elongated and branched. Double immunofluorescence labeling showed that the rod neurites bypassed the horizontal and rod bipolar cells that are normally postsynaptic to rod axons. To our knowledge, this is the first report of rod neurite sprouting in vivo. We were unable to find neurites on degenerate rods in old rds mice, an animal model for RP. The rod neurites in the human RP retinas resemble the long, branched processes formed by rods cultured on Müller cells or purified N-CAM. Neurite growth by surviving rods in the RP retinas may be a response to neurotrophic factor upregulation, loss of inhibitory factors, or changes in molecules associated with reactive Müller cells. Such changes in the retinal microenvironment may impede functional integration of transplanted photoreceptors. The contributions of the rhodopsin-positive rod neurites and abnormal cone axons to the functional abnormalities observed in RP are unknown.