Retinal Cyclic Nucleotide-Gated Channels: From Pathophysiology to Therapy

Abstract

The first step in vision is the absorption of photons by the photopigments in cone and rod photoreceptors. After initial amplification within the phototransduction cascade the signal is translated into an electrical signal by the action of cyclic nucleotide-gated (CNG) channels. CNG channels are ligand-gated ion channels that are activated by the binding of cyclic guanosine monophosphate (cGMP) or cyclic adenosine monophosphate (cAMP). Retinal CNG channels transduce changes in intracellular concentrations of cGMP into changes of the membrane potential and the Ca²⁺ concentration. Structurally, the CNG channels belong to the superfamily of pore-loop cation channels and share a common gross structure with hyperpolarization-activated cyclic nucleotide-gated (HCN) channels and voltage-gated potassium channels (KCN). In this review, we provide an overview on the molecular properties of CNG channels and describe their physiological role in the phototransduction pathways. We also discuss insights into the pathophysiological role of CNG channel proteins that have emerged from the analysis of CNG channel-deficient animal models and human CNG channelopathies. Finally, we summarize recent gene therapy activities and provide an outlook for future clinical application.

Keywords: CNG; Ca2+; cGMP; channelopathies; cyclic nucleotide-gated channel; gene therapy; knockout; photoreceptor; vision.

Figure 1

(A) Membrane topology of CNG channel subunits. 1–6, transmembrane segment 1–6; C, carboxy-terminus; CNBD, cyclic nucleotide binding domain; N, amino-terminus. (B) Model of the CNG channel complex embedded in the plasma membrane based on the TAX-4 structure. (C) Top and bottom views of the tetrameric TAX-4 C. elegans CNG channel complex. (D) Subunit composition of the CNG channels from rods and cones. A1, CNGA1; A3, CNGA3; B1, CNGB1; B3, CNGB3. Structures in this figure were generated with the RSCB PDB 3D View tool (www.rcsb.org/3d-view/) based on PDB 5H3O.

Figure 2

(A,B) Phototransduction in mouse rod (A) and cone (B) outer segments. The principle of the phototransduction is similar in both cell types. In the dark, the cyclic nucleotide-gated (CNG) channel (CNGA1/B1 in rods and CNGA3/B3 in cones) of the outer membrane is kept open by high concentrations of cyclic guanosine monophosphate (cGMP) produced by retinal guanylyl cyclases (GCs) 1 and 2 (GC1/2 in rods, GC1 only in cones) present in the disc membrane. The resulting influx of Na⁺ and Ca²⁺ depolarizes the plasma membrane. Light activates rhodopsin (Rh) which in turn activates transducin (G_t1 in rods, G_t2 in cones) whose alpha subunit activates a phosphodiesterase (PDE6A/B in rods, PDE6C in cones) leading to hydrolysis of cGMP. The drop in the cGMP concentration leads to the closure of the CNG channel yielding to membrane hyperpolarization. Ca²⁺ is an important regulator of phototransduction. At high concentrations Ca²⁺ binds to guanylyl cyclase-activating proteins (GCAPs; GCAP1/2 in rods, GCAP1 only in cones) leading to an inhibition of guanylyl cyclases. High levels of Ca²⁺ also lead to a slight reduction of the cGMP affinity of the CNG channel via CaM-mediated feedback inhibition. Ca²⁺ is cleared from the outer segment via a Na⁺-Ca²⁺-K⁺-exchanger (NCKX1 in rods, NCKX2/4 in cones). At low Ca²⁺ levels GCAPs switch to the Ca²⁺-free form that is an activator of GCs.