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Ulisse Bocchero, Monica Mazzolini, Beatrice M. Tam, Colette Chiu, Vincent Torre, Orson L Moritz; Early steps of photoreceptor degeneration in a Xenoups laevis model of retinitis pigmentosa. Invest. Ophthalmol. Vis. Sci. 2018;59(9):1014. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
The rhodopsin mutation P23H is associated with retinitis pigmentosa; the exact mechanisms of toxicity are unclear. To investigate early electrophysiological changes related to expression of P23H rhodopsin and development of RP, we probed phototransduction in isolated rods from transgenic X. laevis expressing bovine P23H (bP23H) rhodopsin using electrophysiological techniques.
Tadpoles were bred and housed in continuous darkness, which prevents retinal degeneration in these animals. Isolated and intact rods of WT and bP23H X. laevis were obtained by mechanical dissociation of the retina and electrical responses to light were obtained with the suction-electrode recording method. ERG recordings were obtained using a modified Ganzfeld electroretinography unit. The amount of light required to initiate retinal degeneration was determined by exposing WT and bP23H animals to varying periods of bright light; paraformaldehyde-fixed eyes were cryosectioned and examined by confocal microscopy.
Transgenic rods, were significantly shorter compared to WT (39.7±0.7µm, n=70 and 51.5±1.3µm, n=38 respectively). Initial photoresponses determined both in vivo and in single cell recording did not show major differences, except for less efficient phototransduction machinery in bP23H rods (K1/2 is equivalent to 10-16±1 photoisomerization (Rh*) for bP23H and to 16-27±2 Rh* for WT rods). However, after repeated and saturating light stimulations, there was a significant increase in duration and decrease in amplitude of bP23H rods photoresponses (p<0.001; n=7). Exposure to bright light for at least 12 minutes was required to initiate early steps of retinal degeneration (outer segment shedding) in bP23H frogs, and did not affect WT animals.
Our study reveals two early and novel steps that occur during onset of RP, the first stage involving an impairment of the phototransduction cascade, and a second stage in which photoreceptor outer segment shedding is initiated. The initial impairment requires photoactivation of a very small fraction of the total rhodopsin, implying the existence of a mechanism that can propagate the effects associated with photoactivation events to impair phototransduction throughout the outer segment.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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