Loss of cone function without degeneration in a novel Gnat2 knock-out mouse

Abstract

Rods and cones mediate visual perception over 9 log units of light intensities, with both photoreceptor types contributing to a middle 3-log unit range that comprises most night-time conditions. Rod function in this mesopic range has been difficult to isolate and study in vivo because of the paucity of mutants that abolish cone signaling without causing photoreceptor degeneration. Here we describe a novel Gnat2 knockout mouse line (Gnat2−/−) ideal for dissecting rod and cone function. In this line, loss of Gnat2 expression abolished cone phototransduction, yet there was no loss of cones, disruption of the photoreceptor mosaic, nor change in general retinal morphology up to at least 9 months of age. Retinal microglia and Müller glia, which are highly sensitive to neuronal pathophysiology, were distributed normally with morphologies indistinguishable between Gnat2−/− and wildtype adult mice. ERG recordings demonstrated complete loss of cone-driven a-waves in Gnat2−/− mice; comparison to WT controls revealed that rods of both strains continue to function at light intensities exceeding 104 photoisomerizations rod−1 s−1. We conclude that the Gnat2−/− mouse is a preferred model for functional studies of rod pathways in the retina when degeneration could be an experimental confound.

Mutation spectrum and clinical investigation of achromatopsia patients with mutations in the GNAT2 gene

“…Interestingly, a new mouse model also carrying a CNV, namely a 7,654 bp deletion spanning exons 2–9 of the mouse Gnat2 gene, revealed in comparison with our patients no obvious cone photoreceptor loss, neither a disruption of the cone mosaic nor changes in the retinal morphology up to an age of 9 month (Ronning et al, ). However, similar to the patients, no a‐wave could be detected in ERG measurements and cone phototransduction was completely abolished (Ronning et al, ).…”

“…Interestingly, a new mouse model also carrying a CNV, namely a 7,654 bp deletion spanning exons 2–9 of the mouse Gnat2 gene, revealed in comparison with our patients no obvious cone photoreceptor loss, neither a disruption of the cone mosaic nor changes in the retinal morphology up to an age of 9 month (Ronning et al, ). However, similar to the patients, no a‐wave could be detected in ERG measurements and cone phototransduction was completely abolished (Ronning et al, ). Two other GNAT2 mouse models, the cpfl3 mouse model harboring a missense mutation (c.598 G > A;p.(Asp200Asn)) in exon 6 of Gnat2 (Chang et al, ) as well as another naturally occurring mouse carrying another missense mutation in exon 6 (c.518 A > G;p.[Asp173Gly], Jobling, Vessey, Waugh, Mills, & Fletcher, ) showed a slowly progressing degeneration of the cone photoreceptor response.…”

Mouse Models of Inherited Retinal Degeneration with Photoreceptor Cell Loss

“…Gnat2 cpfl3 mice (Table S1) show no cone cell loss for at least 14 weeks but exhibit a slow loss of rod cells [173]. In contrast to these models, a recently developed Gnat2 knockout strain abolishes GNAT2 function without PR loss or dysmorphology in the oldest mice examined at 9 months of age [174]. Although human GNAT1-variants are a rare cause of achromatopsia [175], a stationary congenital colorblindness, the clinical presentation is variable and some cases are associated with a reduction in visual acuity with age [176] that may suggest progressive cone cell loss.…”

Diminished Cone Sensitivity in cpfl3 Mice Is Caused by Defective Transducin Signaling

“…A more complete model of Gnat2 loss would be preferable in studies of this nature, to eliminate cone function fully. 1 Mutations in GNAT2 have been observed in patients with achromatopsia, 8,9 and some of these mutations map to the switch domains. 6,7 Although the cpfl3 strain is not completely lacking cone function, it could still be a useful model to understand how transducin mutations lead to reduced photoreceptor function.…”