Cell Biology and Physiology of CLC Chloride Channels and Transporters

Abstract

Proteins of the CLC gene family assemble to homo‐ or sometimes heterodimers and either function as Cl – channels or as Cl – /H + ‐exchangers. CLC proteins are present in all phyla. Detailed structural information is available from crystal structures of bacterial and algal CLCs. Mammals express nine CLC genes, four of which encode Cl – channels and five 2Cl – /H + ‐exchangers. Two accessory β‐subunits are known: (1) barttin and (2) Ostm1. ClC‐Ka and ClC‐Kb Cl – channels need barttin, whereas Ostm1 is required for the function of the lysosomal ClC‐7 2Cl – /H + ‐exchanger. ClC‐1, ‐2, ‐Ka and ‐Kb Cl – channels reside in the plasma membrane and function in the control of electrical excitability of muscles or neurons, in extra‐ and intracellular ion homeostasis, and in transepithelial transport. The mainly endosomal/lysosomal Cl – /H + ‐exchangers ClC‐3 to ClC‐7 may facilitate vesicular acidification by shunting currents of proton pumps and increase vesicular Cl – concentration. ClC‐3 is also present on synaptic vesicles, whereas ClC‐4 and ‐5 can reach the plasma membrane to some extent. ClC‐7/Ostm1 is coinserted with the vesicular H + ‐ATPase into the acid‐secreting ruffled border membrane of osteoclasts. Mice or humans lacking ClC‐7 or Ostm1 display osteopetrosis and lysosomal storage disease. Disruption of the endosomal ClC‐5 Cl – /H + ‐exchanger leads to proteinuria and Dent's disease. Mouse models in which ClC‐5 or ClC‐7 is converted to uncoupled Cl – conductors suggest an important role of vesicular Cl – accumulation in these pathologies. The important functions of CLC Cl – channels were also revealed by human diseases and mouse models, with phenotypes including myotonia, renal loss of salt and water, deafness, blindness, leukodystrophy, and male infertility. © 2012 American Physiological Society. Compr Physiol 2:1701‐1744, 2012.

Etiology of the membrane potential of rat white fat adipocytes

“…The close agreement between C i and the measured values for [Cl Ϫ ] i further supports Cl Ϫ as the permeant ionic species for these single-channel currents. Although ␥ and P= Cl are about twice as large as those described for the CLC family and cystic fibrosis transmembrane regulator (CFTR) Cl Ϫ channels recorded under similar conditions in recombinant cell models (16,37,46,48), the observation of substates suggests that these adipocyte Cl Ϫ channels may have a multimeric molecular identity or multiple configurations. Alternatively, they may be an adipocyte homolog of the volume-regulated anion channel (VRAC), which has similar biophysical properties to the channel we describe: an inward ␥ of 10 -20 pS under similar ionic gradients, voltage-independent gating, and slow gating kinetics (32).…”

Common Gating of Both CLC Transporter Subunits Underlies Voltage-dependent Activation of the 2Cl−/1H+ Exchanger ClC-7/Ostm1

“…Time constants of activation and deactivation were obtained from monoexponential fits to the current trace from 25 to 275 ms after the voltage step to 80 mV or to Ϫ80 mV following a 2-s pulse to 80 mV, respectively. To determine relative nitrate conductance, the same oocyte was measured both in standard ND96 and in ND96 in which Cl Ϫ was replaced by NO 3 Ϫ . Current amplitudes at 80 mV were normalized to the ones in standard ND96.…”

“…The structural basis of common gating is much less understood, but the gating glutamate may also play a role in closing the permeation pathway after a common rearrangement of both subunits (24,25). Early mutagenesis studies have implicated the cystathionine-␤-synthase (CBS) 3 domain-containing C terminus of ClC-0 in its * This work was supported, in part, by grants from the Deutsche Forschungs-slow common gating (26), and more recent studies revealed C-terminal movements during common gating of ClC-0 and ClC-1 (27,28). However, mutations in other regions also affect common gating.…”

“…The CLC gene family, first identified by the cloning of the voltage-dependent Cl Ϫ channel ClC-0 from Torpedo electric organ (1), not only encodes Cl Ϫ channels, but also encodes anion/proton exchangers (2)(3)(4)(5). Both CLC Cl Ϫ channels and Cl Ϫ /H ϩ exchangers function as homodimers with one ion translocation pathway per pore, as shown by mutational structure/function analysis (6 -8) and x-ray crystallography of prokaryotic and algal CLC proteins (9,10).…”

A Single-Pore Residue Renders the Arabidopsis Root Anion Channel SLAH2 Highly Nitrate Selective

“…Here, we used Xenopus laevis oocytes to study the anion selectivity of SLAH2 and compared it to that of SLAC1. In this way, we were able to establish SLAH2, in contrast to the NO 3 2 and Cl 2 permeable SLAC1, and homologs from other kingdoms, as an anion channel that transports nitrate exclusively (Chen and Hwang, 2008;Jentsch, 2008;Picollo et al, 2009;Accardi and Picollo, 2010;Hedrich, 2012;Stauber et al, 2012). Based on homology models for SLAC1 and SLAH2, structure-guided sitedirected mutations were used to explore the molecular basis of the extraordinarily high nitrate selectivity of SLAH2.…”