Ion Channels: Regulating Cell Volume

RESEARCH HIGHLIGHTS

Nature Reviews Molecular Cell Biology | AOP, published online 9 May 2014; doi:10.1038/nrm3806

ION CHANNELS Regulating cell volume

when cell swelling causes the VRAC which ICl(swell) was inactivated. They channels to open. Focusing on pro- concluded that LRRC8 heteromers, teins that are predicted to harbour a which may be hexameric because of transmembrane domain they found their homology to channel-forming that knockdown of LRRC8A pannexins, are an integral part of the sustained fluorescence on cell swell- VRAC. ing, which suggests that LRRC8A is Qiu et al. also assessed whether part of the VRAC or is required for LRRC8A is a component of the its activation. Indeed, both groups VRAC or rather its indirect regula- also showed that knockdown of tor. The substitution of LRRC8A

LRRC8A suppressed ICl(swell) activity at residue Thr44 with Cys or Arg (a current of Cl– ions that the VRAC altered the selectivity of the VRAC, predominantly activates in response increasing or decreasing, respectively, to swelling) in patch-clamp assays. its permeability to I– ions. As ion Qiu et al. renamed this poorly selectively is mostly regulated by the characterized gene SWELL1. pore of the channel or its associated Qiu et al. and Voss et al. observed subunits, this suggests that LRRC8A that LRRC8A is located in the is part of the VRAC pore. plasma membrane and that its Finally, both groups noticed Leu-rich repeats are cytoplasmic. that swelling-induced efflux of the They also assessed whether the four osmolyte taurine was abolished in homologues of LRRC8A (LRRC8B– cells in which LRRC8A was dis- LRRC8E) regulate the VRAC. Qiu rupted. Voss et al. showed that this et al. found that knockdown of each could be rescued by co-transfection homologue had little effect in their of LRRC8A and LRRC8C. That DigitalVision/PunchStock RNAi screen, which suggests that efflux of the osmolyte taurine was Cells must maintain a constant they are not essential for VRAC dependent on LRRC8 heteromers volume, despite the rapid influx of activity on their own. In agreement, indicates that the VRAC is identical water that will cause them to swell Voss et al. found that, with the excep- to the volume-stimulated organic in response to changes in osmolar- tion of LRRC8A, the disruption of osmolyte and anion channel ity. The volume-regulated anion single LRRC8 genes using clustered, (VSOAC). Two groups channel (VRAC) promotes volume regularly interspaced short palindro- In short, these studies identify now identify decrease in response to cell swelling mic repeats (CRISPR)–Cas did LRRC8A as a component of the

Leu-rich by conducting the efflux of ions, not abolish ICl(swell). However, ICl(swell) VRAC; this finding should aid the repeat- other osmolytes and, consequently, was abolished in cell lines in which all further molecular characterization of water, but its molecular identity other isoforms (LRRC8B–LRRC8E) this channel and our understanding containing has been elusive. Two groups now were disrupted. Furthermore, in of its roles in physiology and disease. protein 8A identify Leu-rich repeat-containing cells in which all five LRRC8 genes Katharine H. Wrighton (LRRC8A) as protein 8A (LRRC8A) as an essential were disrupted, ICl(swell) was restored component of the VRAC. by the co-transfection of LRRC8A an essential ORIGINAL RESEARCH PAPERS Qiu, Z. et al. To identify genes that encode the and at least one of the other LRRC8 SWELL1, a plasma membrane protein, is an component of VRAC or its regulators, Qiu et al. isoforms; co-immunoprecipitation essential component of volume-regulated anion channel. Cell 157, 447–458 (2014) | Voss, F. K. et al. the VRAC and Voss et al. carried out genome- revealed binding between Identification of LRRC8 heteromers as an essential wide RNAi screens in human cells different LRRC8 isoforms, and the component of the volume-regulated anion expressing a fluorescent protein, combination of the transfected channel VRAC. Science http://dx.doi.org/10.1126/ science.1252826 (2014) the fluorescence of which decreases isoforms determined the kinetics by

NATURE REVIEWS | MOLECULAR CELL BIOLOGY VOLUME 15 | JUNE 2014