Testing for a gap junction-mediated bystander effect in retinitis pigmentosa: secondary cone death is not altered by deletion of connexin36 from cones.

Katharina Kranz,François Paquet-Durand,Reto Weiler,Ulrike Janssen-Bienhold,Karin Dedek,Alfred Lewin

doi:10.1371/journal.pone.0057163

Katharina Kranz, François Paquet-Durand + Show 4 more

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https://doi.org/10.1371/journal.pone.0057163

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Abstract

Retinitis pigmentosa (RP) relates to a group of hereditary neurodegenerative diseases of the retina. On the cellular level, RP results in the primary death of rod photoreceptors, caused by rod-specific mutations, followed by a secondary degeneration of genetically normal cones. Different mechanisms may influence the spread of cell death from one photoreceptor type to the other. As one of these mechanisms a gap junction-mediated bystander effect was proposed, i.e., toxic molecules generated in dying rods and propagating through gap junctions induce the death of healthy cone photoreceptors. We investigated whether disruption of rod-cone coupling can prevent secondary cone death and reduce the spread of degeneration. We tested this hypothesis in two different mouse models for retinal degeneration (rhodopsin knockout and rd1) by crossbreeding them with connexin36-deficient mice as connexin36 represents the gap junction protein on the cone side and lack thereof most likely disrupts rod-cone coupling. Using immunohistochemistry, we compared the progress of cone degeneration between connexin36-deficient mouse mutants and their connexin36-expressing littermates at different ages and assessed the accompanied morphological changes during the onset (rhodopsin knockout) and later stages of secondary cone death (rd1 mutants). Connexin36-deficient mouse mutants showed the same time course of cone degeneration and the same morphological changes in second order neurons as their connexin36-expressing littermates. Thus, our results indicate that disruption of connexin36-mediated rod-cone coupling does not stop, delay or spatially restrict secondary cone degeneration and suggest that the gap junction-mediated bystander effect does not contribute to the progression of RP.

Highlights

Retinitis pigmentosa (RP) is a group of inherited retinal degenerative diseases characterized by a progressive loss of photoreceptor cells
To examine if secondary cone degeneration in Rho2/2 and rd1 mice may potentially be influenced by the deletion of the cone connexin, we first investigated if both degeneration models exhibit a normal distribution of Cx36 in the outer plexiform layer (OPL; Fig. 1)
Consistent with previous studies [12], Cx36 immunoreactivity is stronger in the inner plexiform layer (IPL) than in the OPL where it is attributed to the dendrites of OFF bipolar cells and to cone photoreceptor endings [12]

Summary

Introduction

Retinitis pigmentosa (RP) is a group of inherited retinal degenerative diseases characterized by a progressive loss of photoreceptor cells. Genetically normal cones die, leading to loss of central vision and to blindness [1] To date it is not understood how cell death propagates from dying rods to healthy cones in those forms of RP in which mutations occur only in rods [1]. The gap junction-mediated bystander effect provides another explanation that is often considered [8,9] In this scenario, the cell death-inducing signal is not released into the extracellular space but permeates from dying rods through gap junctions directly to healthy cones, thereby carrying cell death-promoting signals from one photoreceptor type to the other [8]. Later stages of cone degeneration, when the majority of cones have died, were analyzed in the rd mouse, which represents a well-established model of fast photoreceptor degeneration [18]

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