Focus on Kir7.1: physiology and channelopathy

Abstract

Genetic studies have linked alterations in Kir7.1 channel to diverse pathologies. We summarize functional relevance of Kir7.1 channel in retinal pigment epithelium (RPE), regulation of channel function by various cytoplasmic metabolites, and mutations that cause channelopathies. At the apical membrane of RPE, K(+) channels contribute to subretinal K(+) homeostasis and support Na(+)/K(+) pump and Na(+)-K(+)-2Cl(-) cotransporter function by providing a pathway for K(+) secretion. Electrophysiological studies have established that barium- and cesium-sensitive inwardly rectifying K(+) (Kir) channels make up a major component of the RPE apical membrane K(+) conductance. Native human RPE expresses transcripts for Kir1.1, Kir2.1, Kir2.2, Kir3.1, Kir3.4, Kir4.2, and Kir6.1, albeit at levels at least 50-fold lower than Kir7.1. Kir7.1 is structurally similar to other Kir channels, consisting of 2 trans-membrane domains, a pore-forming loop that contains the selectivity filter, and 2 cytoplasmic polar tails. Within the cytoplasmic structure, clusters of amino acid sequences form regulatory domains that interact with cellular metabolites and control the opening and closing of the channel. Recent evidence indicated that intrinsic sequence motifs present in Kir7.1 control surface expression. Mutant Kir7.1 channels are associated with inherited eye pathologies such as Snowflake Vitreoretinal Degeneration (SVD) and Lebers Congenital Amaurosis (LCA16). Based on the current evidence, mutations implicated in channelopathies have the potential to be used for genetic testing to diagnose blindness due to Kir7.1.

Keywords: LCA; SVD; channelopathy; potassium channel; retinal degeneration; retinal pigment epithelium.

Figure 1.

RPE cell functions crucial for vision. In combination with Na⁺/ATPase/K⁺ pump, Kir7.1 channels regulate the direction of fluid transport across the RPE, subretinal space volume, and help to maintain K⁺ homeostasis around the photoreceptor outer-segment.

Figure 2.

(A) Normal retinal architecture in showing the photoreceptors (PR), subretinal space (SRS), the retinal pigmented epithelium (RPE), the Bruch membrane (BM), and the choroidal vascular network. (B) Representative Kir7.1 current recording showing hyperpolarizing shift in membrane potential on switching extracellular K⁺ concentration from 5 mM to 2 mM (Modified from Doring et al. 1998). Gray rectangle represents current activation around the resting membrane potential of RPE cells in response to change in extracellular K⁺. (C) Schematic of rod PR: The dark current circulates between the inner and outer segments. The cytoplasmic concentration of cGMP is high, and maintains the cGMP-gated channels in an open state and allows a steady inward current (dark current) to depolarize the photoreceptor. In dark, the K⁺ concentration of the subretinal space is approximately 5 mM. Adjacent RPE cell mechanisms active during dark are shown on the right with simplified representation of ion transport. (D) On light exposure, fewer K⁺ ions leave the photoreceptors and the K+ concentration in the subretinal space decreases from 5 to 2 mM. Kir7.1 channels in the apical membrane of RPE recycle efflux K⁺. Additionally, the movement of water and Cl^- is shown. Abbreviations are: Ap – apical, Ba – Basolateral, AQP – aquaporine, KCNQ – M-type K⁺ channel, NKCC – Na⁺/K⁺/2Cl transporter, and ATPase – Na⁺/K⁺/ATPase pump. The thickness and length of the arrows depicts the relative magnitude of the driving forces.

Figure 3.

Proposed model for targeted protein secretion from the ER-Golgi secretory pathway. Proteins containing apical sorting signals (orange) and basolateral sorting signals (black) are delivered to Golgi. Upon entering the Golgi apparatus, proteins progress to the TGN and sorting occurs. In pathway A, apical signals (glycans, GPI linkages) interact with specific cellular machinery, e.g., apical targeting receptors, lipid rafts, resulting in packaging into apical vesicles. These vesicles can then traffic directly to the apical plasma membrane (PM). Alternatively, targeted protein may first traffic to the basolateral PM (Pathway B). Basally trafficked proteins are subsequently redirected to the apical PM via transcytosis, either directly (pathway C), or via endosomes (pathway D). Proteins in the endosomal pathway are subsequently trafficked to the apical PM. Abbreviations are: ER – Endoplasmic reticulum, COP – Coat Protein, AP – Associate protein, CL – Clathrin, TGN – Trans golgi network.

Figure 4.

K_ir7.1 membrane topology. Localization of consensus sites for phophorylation by Casein Kinase II (^T321 and ^T337), PKA (^S287) and PKC (^S14, ^S169 and ^S201) shown in red filled squares, as well as various protein trafficking signals (green boxes: ER retention signal; purple diamonds: diacidic motif; blue diamonds: dileucine motif). LCA and SVD mutation locations are also shown as filled circles indicated by arrows. Localization of mutations on the topological image is based on the TOPO2 program. TM – transmembrane.