Abstract
Purpose :
Retinal remodeling, triggered by photoreceptor death, could either slow down, or exacerbate vision loss. We hypothesize that in typical less aggressive forms of retinitis pigmentosa, where a fraction of rods remains light responsive, the retina initially compensates for reduced receptor input by increasing signal transmission to bipolar cells. This hypothesis was tested, using a mouse model for the most common autosomal-dominant retinitis pigmentosa-associated mutation of rhodopsin, the P23H mutation (RhoP23H/WT = P23H mouse).
Methods :
All comparisons were made between P23H mice and their age-matched wildtype control littermates (n=4-8 per group). Both sexes were represented equally in each group. Statistical significance was determined using two-way ANOVA or two-tailed t-test, with correction for multiple comparisons where needed. For behavior and electrophysiology experiments, mice were bred onto the Gnat2-/- background to eliminate cone-mediated light signals. Sensitivity of vision was assessed by optomotor response in ambient light, ranging from scotopic to mesopic levels. Light signal transmission from rods to rod bipolar cells (RBCs) was assessed by ex vivo ERG, isolating the rod component using DL-AP4 and the RBC component by subtracting rod responses from mixed rod/RBC responses. ScRNA-Seq was used to measure differences in gene expression in the rods and RBCs of P23H mouse retinas versus controls. In addition, expression of selected proteins in whole retinas from P23H and control mice were compared, using Western blotting and immunohistochemistry.
Results :
Based on scRNA-Seq analysis we found a significant upregulation of several major rod ribbon synapse genes in P23H rod photoreceptors (p<0.00001). Ca2+ sensor protein synaptotagmin-1 (Syt1) was overexpressed at P30 (p=0.02) in whole P23H retinas, as compared to controls. Scotopic contrast threshold changed only at the dimmest background intensity (5*10-4 Cd/m2) in P60 P23H/Gnat2-/- mice, from 60±6% to 76±4% (p< 0.001); and the RBC/rod response ratio increased in 1- 6 mo P23H mice (p<0.005).
Conclusions :
We discovered increased mRNA expression of rod ribbon synapse proteins in P23H mice. These results suggest that presynaptic scaling, potentially via upregulation of Syt1, mediates a compensatory mechanism that promotes night vision during the slow progression of rod photoreceptor degeneration.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.