Tobias Stauber

Identification of LRRC8 Heteromers as an Essential Component of the Volume-Regulated Anion Channel VRAC

One Swell Ion Channel When mammalian cells are faced with osmotic challenges, they need to swell or shrink. The molecular characterization of the volume-regulated anion channel (VRAC) remains unknown, although many candidate proteins have been proposed. Voss et al. (p. 634, published online 10 April; see the Perspective by Mindell) used a genome-wide screen to identify a group of leucine-rich repeat–containing (LRRC) proteins necessary for forming VRAC. Suppression of LRRC8A nearly eliminated the presence of VRAC in mammalian cells. A heterooligomer of LRRC proteins appears to form VRAC. Identification of VRAC components is an essential step forward in the understanding of swelling-activated ion channels and provides opportunities for understanding both the mechanism of the channel and its role in physiology. Components of an elusive swelling-activated anion channel are identified and form a structurally new class of channel. [Also see Perspective by Mindell] Regulation of cell volume is critical for many cellular and organismal functions, yet the molecular identity of a key player, the volume-regulated anion channel VRAC, has remained unknown. A genome-wide small interfering RNA screen in mammalian cells identified LRRC8A as a VRAC component. LRRC8A formed heteromers with other LRRC8 multispan membrane proteins. Genomic disruption of LRRC8A ablated VRAC currents. Cells with disruption of all five LRRC8 genes required LRRC8A cotransfection with other LRRC8 isoforms to reconstitute VRAC currents. The isoform combination determined VRAC inactivation kinetics. Taurine flux and regulatory volume decrease also depended on LRRC8 proteins. Our work shows that VRAC defines a class of anion channels, suggests that VRAC is identical to the volume-sensitive organic osmolyte/anion channel VSOAC, and explains the heterogeneity of native VRAC currents.

Lysosomal Pathology and Osteopetrosis upon Loss of H+-Driven Lysosomal Cl– Accumulation

Chloride Balancing Act The ionic composition of the cytosol and intracellular organelles must be regulated in the face of ongoing membrane traffic into and out of the cell. Now, two papers address the consequences of a change in the transport phenotype of an intracellular Cl− transport protein from a coupled exchanger to a passive Cl− conductor (see the Perspective by Smith and Schwappach). Novarino et al. (p. 1398, published online 29 April) investigated the consequence of a knock-in of the uncoupled ClC-5 transporter into mouse. The knock-out mouse of this endosomal kidney transporter has a severe endocytic phenotype believed to be due to a defect in vesicular acidification. The current study shows a similarly impaired endocytic phenotype for the uncoupled mutant, but the acidification of endosomes was unaffected. Weinert et al. (p. 1401, published online 29 April) used a similar strategy to investigate the consequence of the equivalent mutation in the lysosomal transporter ClC-7, which is highly expressed in the resorption lacuna of osteoclasts and whose knock-out in mice produces lysosomal storage disease and severe osteopetrosis. A similar (though less severe) phenotype was observed in the knock-in mice containing the uncoupled ClC-7, indicating that coupled transport plays a critical role in lysosomes. Chloride conductance and chloride-proton exchange have distinct effects on endolysosomal physiology in mice. During lysosomal acidification, proton-pump currents are thought to be shunted by a chloride ion (Cl–) channel, tentatively identified as ClC-7. Surprisingly, recent data suggest that ClC-7 instead mediates Cl–/proton (H+) exchange. We generated mice carrying a point mutation converting ClC-7 into an uncoupled (unc) Cl– conductor. Despite maintaining lysosomal conductance and normal lysosomal pH, these Clcn7unc/unc mice showed lysosomal storage disease like mice lacking ClC-7. However, their osteopetrosis was milder, and they lacked a coat color phenotype. Thus, only some roles of ClC-7 Cl–/H+ exchange can be taken over by a Cl– conductance. This conductance was even deleterious in Clcn7+/unc mice. Clcn7–/– and Clcn7unc/unc mice accumulated less Cl– in lysosomes than did wild-type mice. Thus, lowered lysosomal chloride may underlie their common phenotypes.

Kinesin-2 is a motor for late endosomes and lysosomes.

The bidirectional nature of late endosome/lysosome movement suggests involvement of at least two distinct motors, one minus‐end directed and one plus‐end directed. Previous work has identified dynein as the minus‐end‐directed motor for late endosome/lysosome localization and dynamics. Conventional kinesin (kinesin‐1) has been implicated in plus‐end‐directed late endosome/lysosome movement, but other kinesin family members may also be involved. Kinesin‐2 is known to drive the movement of pigment granules, a type of lysosomally derived organelle, and was recently found to be associated with purified late endosomes. To determine whether kinesin‐2 might also power endosome movement in non‐pigmented cells, we overexpressed dominant negative forms of the KIF3A motor subunit and KAP3 accessory subunit and knocked down KAP3 levels using RNAi. We found kinesin‐2 to be required for the normal steady‐state localization of late endosomes/lysosomes but not early endosomes or recycling endosomes. Despite the abnormal subcellular distribution of late endosomes/lysosomes, the uptake and trafficking of molecules through the conventional endocytic pathway appeared to be unaffected. The slow time–course of inhibition suggests that both kinesin‐2 itself and its attachment to membranes do not turn over quickly.

ClC‐7 is a slowly voltage‐gated 2Cl−/1H+‐exchanger and requires Ostm1 for transport activity

Mutations in the ClC‐7/Ostm1 ion transporter lead to osteopetrosis and lysosomal storage disease. Its lysosomal localization hitherto precluded detailed functional characterization. Using a mutated ClC‐7 that reaches the plasma membrane, we now show that both the aminoterminus and transmembrane span of the Ostm1 β‐subunit are required for ClC‐7 Cl−/H+‐exchange, whereas the Ostm1 transmembrane domain suffices for its ClC‐7‐dependent trafficking to lysosomes. ClC‐7/Ostm1 currents were strongly outwardly rectifying owing to slow gating of ion exchange, which itself displays an intrinsically almost linear voltage dependence. Reversal potentials of tail currents revealed a 2Cl−/1H+‐exchange stoichiometry. Several disease‐causing CLCN7 mutations accelerated gating. Such mutations cluster to the second cytosolic cystathionine‐β‐synthase domain and potential contact sites at the transmembrane segment. Our work suggests that gating underlies the rectification of all endosomal/lysosomal CLCs and extends the concept of voltage gating beyond channels to ion exchangers.

Chloride in Vesicular Trafficking and Function

Luminal acidification is of pivotal importance for the physiology of the secretory and endocytic pathways and its diverse trafficking events. Acidification by the proton-pumping V-ATPase requires charge compensation by counterion currents that are commonly attributed to chloride. The molecular identification of intracellular chloride transporters and the improvement of methodologies for measuring intraorganellar pH and chloride have facilitated the investigation of the physiology of vesicular chloride transport. New data question the requirement of chloride for pH regulation of various organelles and furthermore ascribe functions to chloride that are beyond merely electrically shunting the proton pump. This review surveys the currently established and proposed intracellular chloride transporters and gives an overview of membrane-trafficking steps that are affected by the perturbation of chloride transport. Finally, potential mechanisms of membrane-trafficking modulation by chloride are discussed and put into the context of organellar ion homeostasis in general.

Cell Biology and Physiology of CLC Chloride Channels and Transporters

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 Dents 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.

Subunit composition of VRAC channels determines substrate specificity and cellular resistance to Pt‐based anti‐cancer drugs

Although platinum‐based drugs are widely used chemotherapeutics for cancer treatment, the determinants of tumor cell responsiveness remain poorly understood. We show that the loss of subunits LRRC8A and LRRC8D of the heteromeric LRRC8 volume‐regulated anion channels (VRACs) increased resistance to clinically relevant cisplatin/carboplatin concentrations. Under isotonic conditions, about 50% of cisplatin uptake depended on LRRC8A and LRRC8D, but neither on LRRC8C nor on LRRC8E. Cell swelling strongly enhanced LRRC8‐dependent cisplatin uptake, bolstering the notion that cisplatin enters cells through VRAC. LRRC8A disruption also suppressed drug‐induced apoptosis independently from drug uptake, possibly by impairing VRAC‐dependent apoptotic cell volume decrease. Hence, by mediating cisplatin uptake and facilitating apoptosis, VRAC plays a dual role in the cellular drug response. Incorporation of the LRRC8D subunit into VRAC substantially increased its permeability for cisplatin and the cellular osmolyte taurine, indicating that LRRC8 proteins form the channel pore. Our work suggests that LRRC8D‐containing VRACs are crucial for cell volume regulation by an important organic osmolyte and may influence cisplatin/carboplatin responsiveness of tumors.

A Role for Kinesin-2 in COPI-Dependent Recycling between the ER and the Golgi Complex

Transport carriers operating between early compartments in the mammalian secretory pathway have to travel long distances in the cell by mostly relying on the microtubule network and its associated motor proteins. Although anterograde transport from the endoplasmic reticulum (ER) to the Golgi complex is mediated by cytoplasmic dynein, the identity of the motor(s) mediating transport in the retrograde direction is presently unclear. Some studies have suggested that the heterotrimeric kinesin-2 complex plays a role in transport between the ER and the Golgi. Here, we have examined kinesin-2 function by using an RNA-interference approach to downregulate the expression of KAP3, the nonmotor subunit of kinesin-2, in HeLa cells. KAP3 silencing results in the fragmentation of the Golgi apparatus and a change in the steady-state localization of the KDEL-receptor (KDEL-R). Using specific transport assays, we show that the rate of anterograde secretory traffic is unaffected in these cells but that KDEL-R-dependent retrograde transport is strongly abrogated. Our data strongly support a role for kinesin-2 in the KDEL-R-/COPI-dependent retrograde transport pathway from the Golgi complex to the ER.

The Late Endosomal ClC-6 Mediates Proton/Chloride Countertransport in Heterologous Plasma Membrane Expression

Members of the CLC protein family of Cl− channels and transporters display the remarkable ability to function as either chloride channels or Cl−/H+ antiporters. Due to the intracellular localization of ClC-6 and ClC-7, it has not yet been possible to study the biophysical properties of these members of the late endosomal/lysosomal CLC branch in heterologous expression. Whereas recent data suggest that ClC-7 functions as an antiporter, transport characteristics of ClC-6 have remained entirely unknown. Here, we report that fusing the green fluorescent protein (GFP) to the N terminus of ClC-6 increased its cell surface expression, allowing us to functionally characterize ClC-6. Compatible with ClC-6 mediating Cl−/H+ exchange, Xenopus oocytes expressing GFP-tagged ClC-6 alkalinized upon depolarization. This alkalinization was dependent on the presence of extracellular anions and could occur against an electrochemical proton gradient. As observed in other CLC exchangers, ClC-6-mediated H+ transport was abolished by mutations in either the “gating” or “proton” glutamate. Overexpression of GFP-tagged ClC-6 in CHO cells elicited small, outwardly rectifying currents with a Cl− > I− conductance sequence. Mutating the gating glutamate of ClC-6 yielded an ohmic anion conductance that was increased by additionally mutating the “anion-coordinating” tyrosine. Additionally changing the chloride-coordinating serine 157 to proline increased the NO3− conductance of this mutant. Taken together, these data demonstrate for the first time that ClC-6 is a Cl−/H+ antiporter.

Lysosomal degradation of endocytosed proteins depends on the chloride transport protein ClC-7.

Mutations in either ClC‐7, a late endoso‐ mal/lysosomal member of the CLC family of chloride channels and transporters, or in its β‐subunit Ostm1 cause osteopetrosis and lysosomal storage disease in mice and humans. The severe phenotype of mice globally deleted for ClC‐7 or Ostm1 and the absence of storage material in cultured cells hampered investiga‐ tions of the mechanism leading to lysosomal pathology in the absence of functional ClC‐7/Ostm1 transporters. Tissue‐specific ClC‐7‐knockout mice now reveal that accumulation of storage material occurs cell‐autono‐ mously in neurons or renal proximal tubular cells lacking ClC‐7. Almost all ClC‐7‐deficient neurons die. The activation of glia is restricted to brain regions where ClC‐7 has been inactivated. The effect of ClC‐7 disruption on lysosomal function was investigated in renal proximal tubular cells, which display high endo‐ cytotic activity. Pulse‐chase endocytosis experiments in vivo with mice carrying chimeric deletion of ClC‐7 in proximal tubules allowed a direct comparison of the handling of endocytosed protein between cells express‐ ing or lacking ClC‐7. Whereas protein was endocytosed similarly in cells of either genotype, its half‐life in‐ creased significantly in ClC‐7‐deficient cells. These ex‐ periments demonstrate that lysosomal pathology is a cell‐autonomous consequence of ClC‐7 disruption and that ClC‐7 is important for lysosomal protein degrada‐ tion.—Wartosch, L., Fuhrmann, J. C., Schweizer, M., Stauber, T., Jentsch, T. J. Lysosomal degradation of endocytosed proteins depends on the chloride trans‐ port protein ClC‐7. FASEB J. 23, 4056 – 4068 (2009). www.fasebj.org