Light-induced retinal degeneration suppresses developmental progression of flip-to-flop alternative splicing in GluR1

AMPA receptors are hetero-oligomers composed of subsets of four distinct subunits, termed GluR1, GluR2, GluR3, and GluR4. Using quantitative reverse transcription-PCR analysis, we have found that light-induced degeneration of rat retina dramatically suppresses developmental progression of the flip-to-flop alternative splicing switch of retinal GluR1 mRNA. When animals were raised under standard conditions of a 12 hr light/dark cycle (LD 12:12), the flop-to-flip ratio in GluR1 and GluR2 dramatically increased between postnatal day 10 (P10) and P28, and the ratios continued to increase gradually up to P84. When animals were raised in complete darkness, this increase was delayed in GluR1 between P21 and P42. In addition, the increase of the flop-to-flip ratio in GluR1 was strongly suppressed after P21 under conditions of continuous illumination from P2. This is significant because P21 is just after the eye opening and is the timing of the onset of light-induced retinal degeneration. This suppression of the increase of the flop-to-flip ratio was specific to GluR1 and was not observed in GluR2-4. Immunocytochemistry and immunoblot analysis suggested no changes in either the distribution or expression of GluR1 protein in the light-damaged retina measured at P84. When rats were raised under continuous illumination from P2 to P21 followed by LD 12:12 from P22 to P84, retinal degeneration did not progress after P22. In such animals the flop-to-flip ratio, once decreased to approximately 50% of the control (LD 12:12) at P21, was restored to the control level at P84. These findings demonstrate that developmental progression of the flip-to-flop exon switch in retinal GluR1 is affected by lighting conditions, and that light-induced retinal degeneration contributes to the mechanism of suppression of this splicing switch.