Rewiring of the adult midget pathway in a primate model of acute photoreceptor loss.

Abstract

Purpose : The retinal fovea is responsible for high acuity vision. Within the primate fovea, each cone photoreceptor contacts a pair of ON and OFF midget bipolar cells (MBCs), forming the first synapses of the midget pathway. Foveal cone loss in retinal degenerative diseases such as macular degeneration is a major cause of blindness. Restoring vision in the fovea using cell-based therapies requires proper synaptic integration of new photoreceptors into the surviving midget circuitry. Thus, understanding how mature foveal circuits remodel after cone loss will be helpful for strategizing future therapies. Here, we used serial block face scanning electron microscopy (SBFSEM) to determine the connectivity of surviving foveal cones adjacent to regions of acute photoreceptor loss.

Methods : A region (200 µm by 500 µm) adjacent to the foveal pit of anesthetized 4-year-old Macaca fascicularis monkey was photocoagulated using a PASCAL laser (Topcon). The animal was euthanized 6 months later, the eye enucleated and the eyecup fixed in 4% glutaraldehyde in 0.1M cacodylate buffer. An unlasered retina from a 5-year-old Macaca fascicularis monkey served as a control. Samples were prepared for SBFSEM as previously described (Della Santina et al., 2015, Current Biology 26:2070-2077), and imaged using the VolumeScope (Thermo-Fisher). The samples, located 650 µm from the foveal center, were reconstructed using TrackEM2 software (NIH). Statistical analysis was carried out using Mann-Whitney U test.

Results : A single cone (n=10 cones) contacted a pair of ON and OFF MBCs in the control retina. A cone pedicle deprived zone (PDZ) was clearly visible 6 months after photocoagulation. As in control retina, ON and OFF MBCs within and adjacent to the PDZ were identified by their axonal stratification. At the PDZ border, ON MBCs (n=8) synapsed with single cones as in the control, whereas OFF MBCs (n=10) sometimes connected with 2 cones (average 1.6 ± 0.52; p=0.005 compared to control). Connectivity of individual cones with ON MBCs (0-3, average 1.07 ± 0.62, n=14 cones, p=0.041) and OFF MBCs (1-3, average 1.73 ± 0.90, n=11 cones, p=0.00006) increased significantly compared to the control.

Conclusions : The mature midget pathway in the primate retina rewires after acute cone loss. The extent of remodeling appears to differ between the ON and OFF pathways.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.