Physiology and pathophysiology of ClC-K/barttin channels

Abstract

ClC-K channels form a subgroup of anion channels within the ClC family of anion transport proteins. They are expressed predominantly in the kidney and in the inner ear, and are necessary for NaCl resorption in the loop of Henle and for K+ secretion by the stria vascularis. Subcellular distribution as well as the function of these channels are tightly regulated by an accessory subunit, barttin. Barttin improves the stability of ClC-K channel protein, stimulates the exit from the endoplasmic reticulum and insertion into the plasma membrane and changes its function by modifying voltage-dependent gating processes. The importance of ClC-K/barttin channels is highlighted by several genetic diseases. Dysfunctions of ClC-K channels result in Bartter syndrome, an inherited human condition characterized by impaired urinary concentration. Mutations in the gene encoding barttin, BSND, affect the urinary concentration as well as the sensory function of the inner ear. Surprisingly, there is one BSND mutation that causes deafness without affecting renal function, indicating that kidney function tolerates a reduction of anion channel activity that is not sufficient to support normal signal transduction in inner hair cells. This review summarizes recent work on molecular mechanisms, physiology, and pathophysiology of ClC-K/barttin channels.

Keywords: ClC channels; barttin; epithelial transport; loop of Henle; stria vascularis.

Figure 1

Physiological functions of ClC-K channels. (A) Major expression sites of ClC-K channels in the nephron. ClC-Ka/ClC-K1 permits passive transepithelial transport of chloride in the thin ascending limb of Henle. ClC-Kb/ClC-K2 is necessary for secondary-active sodium chloride reabsorption in the outer medulla and cortical region of the kidney. (PT, proximal tubule; tDL, thin descending limb of Henle's loop; tAL, thin ascending limb of Henle's loop; TAL, thick ascending limb of Henle's loop; DCT, distal convoluted tubule; CNT, connecting tubule; CCT, cortical collecting tubule; CD, collecting duct). (B) Both ClC-Ka/ClC-K1 and ClCKb/ClC-K2 are expressed in the marginal cells of the stria vascularis of the inner ear and contribute to potassium secretion into the endolymph required for sensory transduction in inner hair cells.

Figure 2

Representative current recordings of ClC-K channels heterologously expressed in mammalian tsA201 cells. Whole-cell patch clamp recordings of macroscopic currents of rClC-K1 (A), hClC-Ka (B), and hClC-Kb (C) in the absence (upper row) and presence (lower row) of the accessory subunit barttin. Channels were studied under nearly symmetrical chloride concentrations (pipette solution: 124 mM; bathing solution: 150 mM).

Figure 3

Analysis of the functional stoichiometry of rClC-K1 channels by non-stationary noise analysis (A) and single channel recordings (B–D). (A) Non-stationary noise analysis of WT rClC-K1 currents in the absence of barttin obtained from whole-cell patch clamp recordings of heterologously expressed channels in mammalian tsA201 cells. Mean currents (upper traces) and variances (middle trace) upon a voltage step that elicits biphasic responses result in a loop-like dependence when displayed in a plot of variances versus mean current amplitudes (lower panel). (B–D) Inside–out patch clamp recordings of mutant V166E rClC-K1 under nearly symmetrical chloride concentration (pipette solution: 150 mM; bathing solution: 124 mM). (B) Single channel recordings in presence of barttin at +45 and +95 mV show fast protopore gating with two equally spaced conductance states. (C) Amplitude histogram from bursts of registration at +95 mV. (D) Probabilities of the three current levels, closed, open 1 and open 2 at +95 mV (bars). Closed circles denote the predicted values for binomially distributed states. (modified from Fischer et al., 2010).

Figure 4

Truncation mutants of barttin affect subcellular distribution and functional properties of hClC-Kb channels. (A) Localization of tested carboxy-terminal truncations that result in functional deficits of barttin. Chaperone function of barttin is assigned to the two transmembrane domains. (B) Mean isochronal current amplitudes of hClC-Kb determined 2 ms after a voltage step to −155 mV on cells co-expressing WT or various truncated versions of barttin. (C–H) Confocal images of MDCK cells co-expressing YFP-hClC-Kb (red) and various mutants of barttin-CFP (green). Scale bars: 5 μm. (modified from Scholl et al., , Proc. Natl. Acad. Sci. U. S. A 103, 11411–11416, copyright (2006) National Academy of Sciences, U.S.A).

Figure 5

Disease-causing barttin mutations result in deafness and varying severity of renal symptoms depending on the type of functional deficits of ClC-K/barttin channels.