Deafferented mouse rod bipolar cells extend their dendrites and synapse with healthy photoreceptors

Abstract

Purpose : Previous studies from our group demonstrated constructive restructuring in the rod bipolar cells of the rabbit retina in response to selectivelaser ablation of a small patch of photoreceptors. The mouse is a more convenient animal model for investigation of the molecular mechanisms responsible for this restructuring due to availability of the genetically modified lines. We investigate retinal response to photoreceptor ablation in the mouse to test if it can be used as a model for constructive retinal restructuring.

Methods : Round lesions of Barely Visible clinical grade were placed in-vivo in the mouse retina with a 532-nm laser, using 200μm spot diameter and 20ms pulse duration. Photoreceptor migration and changes in the morphology of the deafferented rod bipolar cells (RBCs) were assessed using confocal microscopy of immunostained tissue. RBCs and their synaptic contacts with photoreceptors were visualized with PKCα, CtBP2, mGluR6 and ELFN1 antibodies. Cone arrestin antibody was used to visualize cone photoreceptors.

Results : The photocoagulation procedure resulted in the death of rod but not cone photoreceptors at the lesion site. No damage to the neurons in the inner nuclear layer was observed. We find that over time (3-180 days after photocoagulation), healthy rod photoreceptors partially fill the lesioned area. During this process, RBCs change their dendritic morphology: first they lose thinner processes, leaving one or two dendrites, which then expand, on average, from 10µm to 30µm in length, and from 0.3 to 1µm in thickness. These thickened dendrites extend towards the healthy photoreceptors located around the lesion and make synapses similar to those of the healthy RBCs. The dendrites of the deafferented RBCs also approach the pedicles of the cone photoreceptors that survived the ablation in the middle of the lesion.

Conclusions : Deafferented mouse RBCs restructure their dendritic trees to form new connections with healthy photoreceptors outside the lesion, similar to our earlier observations in the rabbit retina. In the center of the lesion, they approach surviving cone photoreceptors more often than in the healthy retina. The mouse model, with the broad range of its genetic control, can be used to study molecular mechanisms behind the constructive retinal plasticity.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.