Abstract
Purpose: :
An important step in the visual cycle is the clearance of all-trans retinal from the disk membranes of rod and cone outer segments following photoactivation of their visual pigment. In rods this process is facilitated by a photoreceptor-specific ABC transporter (ABCA4). In mouse rods the deletion of ABCA4 slows down significantly the removal of all-trans retinal following exposure to bright light, delays rod dark adaptation, and results in the accumulation of lipofuscin-forming A2E. In contrast to the detailed description of the function of ABCA4 in rods, very little is known about its potential role in cone photoreceptors. Here we address the role of ABCA4 in chromophore turnover in mouse cones as part of the cone-specific retina visual cycle.
Methods: :
We used suction recordings to compare the properties of light responses from individual control and Abca4-/- cones. We used trans-retinal recordings to obtain mouse cone responses from isolated control and Abca4-/- retinas with pharmacologically inhibited synaptic transmission, and compared the rates of their cone pigment regeneration through the retina visual cycle after bleaching. To facilitate cone recordings all experiments were done with mice lacking rod transducin (Gnat1-/-).
Results: :
The maximum response and light sensitivity of dark adapted control and Abca4-/- cones were comparable. In addition, the kinetics of their light responses were also similar with time-to-peak of 105 ± 17 ms (n=6) and 113 ± 21 ms (n=4) for control and Abca4-/- cones, and integration time of 190 ± 37 ms (n=6) and 203 ± 15 ms (n=4), respectively. We monitored with trans-retinal recordings the recovery of cone sensitivity in isolated control and Abca4-/- retinas following a bleach. Notably, Abca4-/- cones dark-adapted to a final level comparable to that of control cones. However, the rate of cone sensitivity recovery was significantly slower in the absence of ABCA4, with a time constant of 10.6 ± 1.1 min (n=4) for Abca4-/- cones compared with 4.3 ± 0.2 min (n=5) for control cones.
Conclusions: :
Our results demonstrate that the deletion of ABCA4 does not affect the dark-adapted properties of mouse cones or their ability to regenerate their pigment using the retina visual cycle. However, the absence of ABCA4 results in slower dark adaptation in the isolated retina demonstrating that it is required for the rapid turnover of chromophore via the retina visual cycle and thus accelerates cone pigment regeneration.
Keywords: electrophysiology: non-clinical • photoreceptors • retinoids/retinoid binding proteins