Purpose: To examine the effect of rhodopsin mutations on cone photoreceptors in human retinas with retinitis pigmentosa (RP).
Methods: Four RP retinas with rhodopsin mutations and four normal retinas were examined by immunofluorescence with a battery of cell-specific antibodies against cone and rod cytoplasmic and outer segment membrane proteins. Areas of the retinas were studied that showed maximal preservation of photoreceptor structure.
Results: All four RP retinas showed loss of rods, ranging from mild (T-17-M), to more severe (P-23-H), to advanced degeneration (Q-64-ter and G-106-R). The majority of cones in the T-17-M and P-23-H retinas were cytologically normal but showed loss of immunoreactivity for the cytoplasmic proteins 7G6, calbindin, and X-arrestin. The cone outer segments (OS) remained positive for cone opsins and peripherin-2 (rds/peripherin). All remaining cones in the Q-64-ter and G-106-R retinas were degenerate, with short to absent OS, but had strong reactivity for these cytoplasmic and OS membrane markers. Cones in the maculas of the RP retinas were degenerate, with short to absent OS, but retained strong labeling for the cytoplasmic and OS proteins.
Conclusions: Even before cones show cytologic changes in response to rod cell degeneration, they lose immunoreactivity for certain cytoplasmic proteins. These cones later show shortening and loss of OS, although their OS membrane proteins remain well labeled. Cones may down regulate expression of both cytoplasmic and outer segment membrane proteins in response to mutant rod cell dysfunction and/or cell death in human RP retinas. Such cytologic and immunocytochemical changes in the cones may presage death of these critical cells in the later stages of RP.