Abstract
Purpose :
The expression of light sensitive proteins (opsins) in surviving neurons of the degenerate retina is a promising method for vision restoration, with retinal ganglion cells (RGCs) and bipolar cells (BC) as the main targets. However, comparison of the outcomes of these two targeting strategies in pre-clinical studies has been complicated by differences in the AAV transduction efficiency of RGCs and BCs. To address this, we directly compared the visual response properties of two transgenic mouse models with widespread uniform expression of the optogenetic tool, ReaChR, in either RGCs or ON BCs.
Methods :
We bred mice with a floxed ReaChR-mCitrine transgene with Grm6 Cre or Brn3C Cre mice possessing Pde6brd1 mutation to produce a retinally degenerate mouse model with ReaChR expression restricted to either ON BCs or RGCs respectively, confirmed using immunohistochemistry. We recorded responses to a standardised visual stimulus battery from the dorsal lateral geniculate nucleus (dLGN) using in vivo electrophysiology. Statistical comparisons use Mann-Whitney U-test.
Results :
Compared to BC expression, ReaChR targeted to retinal ganglion cells (n = 277 and 341 units, respectively) produced dLGN responses with higher maximum firing rate (U = -5.81, p < 0.001) and shorter response latency to onset of light step (U = 8.08, p < 0.001). Most units had ON responses, although both genotypes possessed OFF (~5% for both) and biphasic units (14% in ReaChR Grm6, 3% in ReaChR Brn3C). ReaChR Grm6 mice had more sustained ON responses (30%) compared to ReaChR Brn3C mice (14%). Contrast sensitivity was similar between genotypes (U = -1.076, p = 0.282). ReaChR Brn3C showed a preference for higher temporal frequencies compared to ReaChR Grm6 (U = -5.61, p < 0.001). Units from both genotypes clustered together when subjected to unsupervised community detection based on stimulus-response properties, suggesting no functional output channel was absent in either group, although the distribution of units across these channels varied between ReaChR Brn3c and ReaChR Grm6 units.
Conclusions :
Expression of ReaChR in RGCs improved some aspects of the restored visual response (amplitude, response latency) compared to BC expression, but also resulted in reduced diversity of response types (fewer sustained ON and biphasic responses), suggesting that ON-bipolar cell targeting could provide more complex vision.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.