The anoctamin (TMEM16) gene family: calcium‐activated chloride channels come of age

This issue contains papers presented at the symposium entitled The anoctamin (TMEM16) gene family: calcium-activated chloride channels come of age, held on Wednesday 13 July, during the 2011 main meeting of the Physiological Society, University of Oxford, UK. The setting for the Symposium was the lecture theatre in the Le Gros Clark building, which is part of the Department of Physiology, Anatomy and Genetics. Chloride channels that are specifically regulated by changes in intracellular calcium, so-called calcium-activated chloride channels (CaCCs), perform diverse and important roles in a variety of tissues (Duran et al. 2010). These channels help to regulate olfaction and taste, neuronal and cardiac excitability, as well as fluid movement in different epithelial tissues throughout the body. Despite much information about the biophysical and functional aspects of CaCCs in many cell types, knowledge about the gene or genes that encode for these plasma membrane proteins has remained elusive. The recent discovery that the anoctamin 1 (ANO1)/TMEM16A gene encodes for the CaCC (Caputo et al. 2008; Schroeder et al. 2008; Yang et al. 2008) has heralded a huge breakthrough in our understanding of the molecular identity of this channel. This discovery has been the catalyst for a flurry of reports that have furthered our understanding of the physiological function of anoctamins in epithelial, sensory and muscle tissues. This symposium brought together leaders in the field of CaCC physiology. The symposium began with an elegant presentation by Luis Galietta (Istituto Giannina Gaslini, Italy), one of the three laboratories that first identified the ANO1 (TMEM16A) gene as encoding a CaCC, on the structural and functional relationships of ANO1 (TMEM16A) and ANO2 (TMEM16B; Scudieri et al. 2012). He described the different electrophysiological properties of the four main splice variants of ANO1 (a, b, c and d), which are derived by alternative splicing and exon skipping, as well as the fact that they are differentially expressed in various cell types. He also showed that ANO2 (TMEM16B) behaves as a CaCC, although the channel requires a higher intracellular calcium concentration and has faster activation and deactivation channel kinetics than ANO1. These structural and physiological differences of the splice variants and paralogues may eventually help to explain the differing properties reported for CaCCs in different cell types. One of the main unresolved questions of CaCC regulation is whether the protein is directly activated by calcium binding or whether it is regulated by an associated calcium-binding protein, such as calmodulin or calmodulin-dependent protein kinase II (CaMKII). Karl Kunzelmann (University of Regensburg, Germany) presented data for hANO1 expressed in HEK 293 cells, and showed that activation of the channel has an absolute requirement for ATP as well as being dependent on calmodulin, but not CaMKII (Kunzelmann et al. 2012). This is interesting, given that many tissues have CaCCs that appear to be dependent on CaMKII for activation. It may be that the splice variants of ANO1 underlie this difference. There are many questions regarding the function of the anoctamins in different tissues. Anna Menini (International School for Advanced Studies, Italy) gave a lovely presentation on the importance of CaCCs in olfaction (Pifferi et al. 2012). Odorant molecules bind to odoursensing receptors (OSRs), leading to an increase in calcium entry that activates CaCC current that amplifies the depolarization of the OSR cells. Anna presented immunohistochemical and electrophysiological evidence for ANO2 (TMEM16B) as the underlying chloride channel involved in this process. Interestingly, recent work from Jentsch’s group, using ANO2 knockout mice, supports ANO2 as the CaCC in the main olfactory epithelium; however, these mice seem to have no reduction in olfactory behavioural tasks (Billig et al. 2011). These data therefore question the