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Christian Schön, Sabrina Asteriti, Susanne Koch, Lorenzo Cangiano, Martin Biel, Stylianos Michalakis; A critical role of HCN1 in the disease progression of retinitis pigmentosa. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):5421.
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The hyperpolarization-activated cyclic nucleotide-gated channel 1 (HCN1) is expressed in photoreceptors and crucial for retinal function under mesopic conditions. So far, no clear association between the channel and any retinal disorders exists and only few studies addressed the question if HCN1 affects photoreceptor cell death in inherited retinal dystrophies. Therefore, we investigated the effects on rod photoreceptors after interfering with the expression or function of HCN1 in a mouse model of retinitis pigmentosa (RP).
Mice deficient for the Cngb1 gene, encoding the B1 subunit of the rod cyclic nucleotide-gated (CNG) channel, were used as a RP model with slow progressing rod degeneration. To interfere with HCN1 function, Cngb1 knockout (KO) mice were crossed with Hcn1 KO mice to generate Cngb1/Hcn1 double KO (DKO). In addition, Cngb1 KO mice were treated with the HCN inhibitor Zatebradine. The effects of genetic or pharmacological ablation of HCN1 on rod physiology and survival were measured ex vivo by electrophysiological measurements and histological methods and in vivo by optical coherence tomography.
We monitored the progression of rod degeneration in Cngb1 KO and Cngb1/Hcn1 DKO mice over several months and found that lack of HCN1 significantly accelerates photoreceptor degeneration in Cngb1 KO mice. In contrast, Hcn1 KO mice show no signs of retinal degeneration. Accordingly, pharmacological inhibition of HCN1 with Zatebradine led to enhanced rod degeneration in Cngb1 KO mice but not in wild type mice. The membrane potentials of Cngb1 KO and Cngb1/Hcn1 DKO rods were significantly depolarized compared to what would be expected in rods whose dark current is compromised. As a consequence, any difference in membrane potential between KO and DKO rods would be challenging to quantify, but predicted to be small. In vivo experiments and in situ calpain activity assays suggest that altered calcium homeostasis might underlie the mechanism for accelerated rod degeneration in Cngb1/Hcn1 DKO mice.
Our results implicate HCN1 as a novel and major modifier of photoreceptor degeneration in RP and suggest that pharmacological inhibition of HCN channels might accelerate disease progression in RP patients.
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