Unexpected was the finding of a progressively accelerating early activation phase of ERG photoresponses (P3
sens) with age in
Rpe65 −/− mice, such that near-normality was reached by 12 to 15 months age. Several hypotheses, alone or in concert, can be entertained to help explain this finding. First, it may be argued that with greater photoreceptor degeneration, there is less ROS volume and thus less opsin molecules available with age. Assuming that the rate of remnant chromophore production in
Rpe65 −/− remains invariant with age, there could be an effective reduction in the number of free opsin molecules. Such an effect was hypothesized to occur when
Rpe65 −/− degeneration accelerated with elimination of phosphorylation.
53 One predicted result of the reduction of free opsin concentration would be an increase in the dark current, and consequent increase in P3
max. The fact that P3
max is tracking the retinal degeneration, however, implies that the intrinsic dark current does not change substantially with age, beyond that explained by shorter ROS. A second possibility involves a greater contribution of cone photoreceptor signaling with aging within the leading edge of the ERG photoresponse. In wild-type mice, a small, cone-derived component can indeed be demonstrated.
55 However, substantial numbers of cones in
Rpe65 −/− mice are lost early in the disease
56–59 making the cone hypothesis unlikely. A third hypothesis involves an increase in regenerated opsin concentration with age. The source of the chromophore that regenerates opsin in
Rpe65 −/− rods remains unknown,
31,40 but its significance in driving relatively large ERG waveforms in
Rpe65 −/− mice is remarkable. If retinyl esters were the substrate for the unknown isomerization reaction producing this biochemically tiny amount of chromophore, then the well-established accumulation of retinyl esters with age in
Rpe65 −/− mice
11,16,32,36,60–62 may be relevant. Of note, an age-related increase in retinaldehydes in
Rpe65 −/− retinas has been reported, although the amounts were near detectability limits of the methods used.
61 A test of this hypothesis may involve the natural history of
Lrat −/− mice which, at young ages, also show loss of P3
sens and a severe chromophore deficiency but, unlike
Rpe65 −/− mice, do not accumulate retinyl esters.
63