No abstract

TMEM16A (anoctamin 1, Ano1), a member of a family of 10 homologous proteins, has been shown to form an essential component of Ca2+‐activated CU channels. TMEM16A‐null mice exhibit severe defects in epithelial transport along with tracheomalacia and death within 1 mo after birth. Despite its outstanding physiological significance, the mechanisms for activation of TMEM16A remain obscure. TMEM16A is activated on increase in intracellular Ca2+, but it is unclear whether Ca2+ binds directly to the channel or whether additional components are required. We demonstrate that TMEM16A is strictly membrane localized and requires cytoskeletal interactions to be fully activated. Despite the need for cytosolic ATP for full activation, phosphorylation by protein kinases is not required. In contrast, the Ca2+ binding protein calmodulin appears indispensable and interacts physically with TMEM16A. Openers of small‐ and intermediate‐conductance Ca2+‐activated potassium channels known to interact with calmodulin, such as 1‐EBIO, DCEBIO, or riluzole, also activated TMEM16A. These results reinforce the use of these compounds for activation of electrolyte secretion in diseases such as cystic fibrosis.—Tian, Y., Kongsuphol, P., Hug, M., Ousingsawat, J., Witzgall, R., Schreiber, R., Kunzelmann, K. Calmodulin‐dependent activation of the epithelial calcium‐dependent chloride channel TMEM16A. FASEB J. 25, 1058–1068 (2011). http://www.fasebj.org

SummaryAnoctamin 1 (Ano1; TMEM16A) and anoctamin 2 (Ano2; TMEM16B) are novel Cl 2 channels transiently activated by an increase in intracellular Ca 2+. These channels are essential for epithelial Cl 2 secretion, smooth muscle peristalsis and olfactory signal transduction. They are central to inherited diseases and cancer and can act as heat sensors. Surprisingly, another member of this protein family, Ano6, operates as a Ca 2+ -activated phospholipid scramblase, and others were reported as intracellular proteins. It is therefore unclear whether anoctamins constitute a family of Ca 2+ -activated Cl 2 channels, or are proteins with heterogeneous functions. Using whole-cell patch clamping we demonstrate that Ano4-10 are all able to produce transient Ca 2+ -activated Cl 2 currents when expressed in HEK293 cells. Although some anoctamins (Ano1, 2, 4, 6, 7) were found to be well expressed in the plasma membrane, others (Ano8, 9, 10) show rather poor membrane expression and were mostly retained in the cytosol. The transient nature of the Cl 2 currents was demonstrated to be independent of intracellular Ca 2+ levels. We show that inactivation of Ano1 currents occurs in the continuous presence of elevated Ca 2+ concentrations, possibly by calmodulin-dependent kinase. The present results demonstrate that anoctamins are a family of Ca 2+ -activated Cl 2 channels, which also induce permeability for cations. They may operate as Cl 2 channels located in the plasma membrane or in intracellular compartments. These results increase our understanding of the physiological significance of anoctamins and their role in disease.

No abstract

Endogenous Ca(2+)-activated Cl(-) channels (CaCC) demonstrate biophysical and pharmacological properties that are well represented in cells overexpressing anoctamin 1 (Ano 1, TMEM16A), a protein that has been identified recently as CaCC. Proteins of the anoctamin family (anoctamin 1-10, TMEM16A-K) are widely expressed. The number of reports demonstrating their physiological and clinical relevance is quickly rising. Anoctamins gain additional interest through their potential role in cell volume regulation and malignancy. Available data suggest that Ano 1 forms stable dimers and probably liaise with accessory proteins such as calmodulin or other anoctamins. In order to understand how anoctamins produce Ca(2+)-activated Cl(-) currents, it will be necessary to obtain better insight into their molecular structure, interactions with partner proteins, and mode of activation.