Two Functional Classes of Rod Bipolar Cells in the Healthy and Degenerated Optogenetically Treated Murine Retina

Giulia Schilardi,Sonja Kleinlogel

doi:10.3389/fncel.2021.809531

Abstract

Bipolar cells have become successful targets for optogenetic gene therapies that restore vision after photoreceptor degeneration. However, degeneration was shown to cause changes in neuronal connectivity and protein expression, which may impact the quality of synthetically restored signaling. Further, the expression of an optogenetic protein may alter passive membrane properties of bipolar cells affecting signal propagation. We here investigated the passive membrane properties of rod bipolar cells in three different systems, the healthy retina, the degenerated retina, and the degenerated retina expressing the optogenetic actuator Opto-mGluR6. We found that, based on the shape of their current-voltage relations, rod bipolar cells in healthy and degenerated retinas form two clear functional groups (type 1 and type 2 cells). Depolarizing the membrane potential changed recorded current-voltage curves from type 1 to type 2, confirming a single cell identity with two functional states. Expression of Opto-mGluR6 did not alter the passive properties of the rod bipolar cell. With progressing degeneration, dominant outward rectifying currents recorded in type 2 rod bipolar cells decreased significantly. We demonstrate that this is caused by a downregulation of BK channel expression in the degenerated retina. Since this BK conductance will normally recover the membrane potential after RBCs are excited by open TRPM1 channels, a loss in BK will decrease high-pass filtering at the rod bipolar cell level. A better understanding of the changes of bipolar cell physiology during retinal degeneration may pave the way to optimize future treatment strategies of blindness.

Highlights

Bipolar cells (BCs) are the first-order interneurons of the retina receiving direct input from the light-sensitive photoreceptors
In order to test if BK channels in rod BCs (RBCs) are involved in membrane potential tuning, which seems to be fundamental for the categorization of an RBC into a functional type 1 or type 2, we investigated the membrane potential oscillation amplitudes in the presence of NS1619 and paxilline (Figure 6D p ≤ 0.05∗, p ≤ 0.02∗∗, p ≤0.001∗∗∗)
Transcriptomic studies hint towards good functional preservation of BCs in the degenerated retina (Gilhooley et al, 2021), which is corroborated by multiple optogenetic studies restoring light sensitivity and basic vision in rd1 mice (Lagali et al, 2008; Cehajic-Kapetanovic et al, 2015; van Wyk et al, 2015a) and in the first human patient (Sahel et al, 2021)

Summary

Introduction

Bipolar cells (BCs) are the first-order interneurons of the retina receiving direct input from the light-sensitive photoreceptors. OBC targeted optogenetic interventions were successfully employed in rd mice, it is known that retinal degeneration causes neuronal rewiring, changes in gene expression, and protein re-localization (Strettoi et al, 2002; Marc et al, 2007; Soto and Kerschensteiner, 2015) as well as neurochemical remodeling (Chua et al, 2009). These modifications may affect the quality of synthetically restored signaling, which has never been investigated

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Results

Conclusion