Differential impact of Kv8.2 loss on rod and cone signaling and degeneration

Abstract

Heteromeric Kv2.1/Kv8.2 channels are voltage-gated potassium channels localized to the photoreceptor inner segment. They carry IKx, which is largely responsible for setting the photoreceptor resting membrane potential. Mutations in Kv8.2 result in childhood-onset Cone Dystrophy with Supernormal Rod Response (CDSRR). We generated a Kv8.2 knockout (KO) mouse and examined retinal signaling and photoreceptor degeneration to gain deeper insight into the complex phenotypes of this disease. Using electroretinograms we show that there were delayed or reduced signaling from rods depending on the intensity of the light stimulus, consistent with reduced capacity for light-evoked changes in membrane potential. The delayed response was not seen ex vivo where extracellular potassium levels were controlled by the perfusion buffer, so we propose the in vivo alteration is influenced by genotype-associated ionic imbalance. We observed mild retinal degeneration. Signaling from cones was reduced but there was no loss of cone density. Loss of Kv8.2 altered responses to flickering light with responses attenuated at high frequencies and altered in shape at low frequencies. The Kv8.2 KO line on an all-cone retina background had reduced cone-driven ERG b wave amplitudes and underwent degeneration. Altogether, we provide insight into how a deficit in the dark current affects the health and function of photoreceptors.

Retinoschisin and novel Na/K-ATPase interaction partners Kv2.1 and Kv8.2 define a growing protein complex at the inner segments of mammalian photoreceptors

“…Consistent with our current findings, Kv2.1 and Kv8.2 were shown earlier to be highly expressed in photoreceptor IS, but absent or poorly expressed in other retinal layers [ 26 , 27 ]. This agrees with our immunohistochemical localization of retinoschisin and the retinal Na/K-ATPase revealing a strong enrichment at the IS of the photoreceptors [ 6 ].…”

“…Similar to retinoschisin-deficient mice [ 56 – 58 ], recent studies of Jiang and colleagues as well as of Inamdar and colleagues reported that Kv8.2 knockout mice reveal a significantly higher apoptotic cell count, a thinner retina, and increased microglia occurrence in the subretinal space [ 27 , 28 ]. Interestingly, the localization of Kv2.1 was found to be unaffected in Kv8.2 knockout mice [ 28 ].…”

Retinoschisin and novel Na/K-ATPase interaction partners Kv2.1 and Kv8.2 define a growing protein complex at the inner segments of mammalian photoreceptors

“…Consistent with our current ndings, Kv2.1 and Kv8.2 were shown earlier to be highly expressed in photoreceptor IS, but absent or poorly expressed in other retinal layers [24,31]. This agrees with our immunohistochemical localization of retinoschisin and the retinal Na/K-ATPase revealing a strong enrichment at the IS of the photoreceptors [6].…”

“…Similar to retinoschisin-de cient mice [36,46,47], recent studies of Jiang and colleagues as well as of Inamdar and colleagues showed that Kv8.2 knockout mice reveal a signi cantly higher apoptotic cell count, a thinner retina, and increased microglia occurrence in the subretinal space [31,48]. Interestingly, the localization of Kv2.1 was found to be unaffected in Kv8.2 knockout mice [48].…”

Cone-Driven Retinal Responses Are Shaped by Rod But Not Cone HCN1

“…The waveform below 2 Hz was not sinusoidal suggesting the possibility of nonlinear responses at low frequencies and strikingly the waveform and phase of the response was very different between controls and Rod-HCN1 KO. To take that into account we previously characterized the sinusoidal flicker response to low frequency waveforms as being composed of "b-wave-like" and "c-wave-like" components with a large recovery phase between the two (Inamdar et al, 2021). Applying that characterization here, the altered response waveform in the HCN1-Rod KO mice appeared consistent with these two components merging, as would be expected from a prolonged b-wave.…”