SIGNALING AND CELL PHYSIOLOGY ER-localized bestrophin 1 activates Ca 2+ -dependent ion channels TMEM16A and SK4 possibly by acting as a counterion channel René Barro-Soria & Fadi Aldehni & Joana Almaça & Ralph Witzgall & Rainer Schreiber & Karl Kunzelmann Received: 19 June 2009 / Revised: 29 September 2009 / Accepted: 30 September 2009 / Published online: 13 October 2009 # Springer-Verlag 2009 Abstract Bestrophins form Ca 2+ -activated Cl - channels and regulate intracellular Ca 2+ signaling. We demonstrate that bestrophin 1 is localized in the endoplasmic reticulum (ER), where it interacts with stromal interacting molecule 1, the ER-Ca 2+ sensor. Intracellular Ca 2+ transients elicited by stimulation of purinergic P2Y 2 receptors in HEK293 cells were augmented by hBest1. The p21-activated protein kinase Pak2 was found to phosphorylate hBest1, thereby enhancing Ca 2+ signaling and activation of Ca 2+ -dependent Cl - (TMEM16A) and K + (SK4) channels. Lack of bestrophin 1 expression in respiratory epithelial cells of mBest1 knockout mice caused expansion of ER cisterns and induced Ca 2+ deposits. hBest1 is, therefore, important for Ca 2+ handling of the ER store and may resemble the long-suspected counterion channel to balance transient membrane potentials occurring through inositol triphos- phate (IP 3 )-induced Ca 2+ release and store refill. Thus, bestrophin 1 regulates compartmentalized Ca 2+ signaling that plays an essential role in Best macular dystrophy, inflamma- tory diseases such as cystic fibrosis, as well as proliferation. Keywords Bestrophin . Ca 2+ -activated Cl - currents . CaCC . Ca 2+ -activated K + currents . SK4 . TMEM16A . Pak2 . Purinergic receptors . Endoplasmic reticulum . ER . Ca 2+ store Abbreviations hBest1 human bestrophin 1 CaCC Ca 2+ -activated Cl - channels SK4 small conductance calcium-activated potas- sium channel type 4 TMEM16A transmembrane protein 16A ANO1 anoctamin 1 Pak2 p21-activated protein kinase ER endoplasmic reticulum SERCA sarcoendoplasmic reticulum calcium ATPase Stim1 stromal interacting molecule 1 DIDS 4,4′-diisothio-cyanostilbene-2,2′-disulfonic acid Introduction Mutations in human bestrophin 1 (hBest1) are responsible for early onset of macular degeneration called Best vitelli- form macular dystrophy [15]. Bestrophins are also upregu- lated during inflammation and tissue repair and promote proliferation and development of cancer [1, 37]. Compel- ling evidence has been provided that bestrophins are Ca 2+ - activated and DIDS-sensitive Cl - channels (CaCC) [30, 40], but much controversy exists about the question whether these proteins indeed form apical Cl - channels in epithelial cells [23]. Although it has been shown that hBest1 enhances Ca 2+ -dependent Cl - secretion in epithelial cells and that Ca 2+ -dependent Cl - secretion is reduced in Electronic supplementary material The online version of this article (doi:10.1007/s00424-009-0745-0) contains supplementary material, which is available to authorized users. René Barro-Soria and Fadi Aldehni contributed equally to the present study. R. Barro-Soria : F. Aldehni : J. Almaça : R. Schreiber : K. Kunzelmann (*) Institut für Physiologie, Universität Regensburg, Universitätsstraße 31, 93053 Regensburg, Germany e-mail: Karl.Kunzelmann@vkl.uni-regensburg.de R. Witzgall Institut für Anatomie, Universität Regensburg, Universitätsstraße 31, 93053 Regensburg, Germany Pflugers Arch - Eur J Physiol (2010) 459:485–497 DOI 10.1007/s00424-009-0745-0