Testing for a Gap Junction-Mediated Bystander Effect in Retinitis Pigmentosa: Secondary Cone Death Is Not Altered by Deletion of Connexin36 from Cones

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.

Neogenin neutralization prevents photoreceptor loss in inherited retinal degeneration

“…ing between ages P12 and P14 (28). Secondary cone loss begins shortly after the initial peak of rod apoptosis (~P15), and the majority of cones in the ONL are gone by P30 (29). Rd1 retina displayed elevated neogenin levels in the inner/outer segments of photoreceptors on P12 compared with age-matched WT retina, which was similar to the effect of 8Br-cAMP injection into WT eyes ( Figure 3, A and B).…”

Connexin 36 expression is required for electrical coupling between mouse rods and cones

“…Nonetheless, our fi ndings indicate that, in the absence of Cx36, functional channels cannot be assembled and inserted between rods and cones. Moreover, they provide the necessary support for many studies that have used Cx36 knockout mice to examine the relative roles in vision of the three rod pathways (references in the Introduction) or assessed a possible involvement of rod-cone coupling in degenerative retinal diseases (Striedinger et al, 2005 ;Kranz et al, 2013 ).…”

Functional architecture of the retina: Development and disease

“…One possibility is that there is passage of ‘apoptotic’ signals via gap-junctions amongst neighboring neurons. However, to date there is no strong evidence in support of this possibility; cones still degenerate in connexin36/rhodopsin double KO mice when rods die (Kranz et al, 2013). Instead, there is some evidence supporting a role for diffusible factors released by neighboring cells.…”