Anoctamins are a family of Ca2+ activated Cl− channels

Abstract

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.

TMEM16F is a component of a Ca2+-activated Cl− channel but not a volume-sensitive outwardly rectifying Cl− channel

“…Szteyn et al (33) reported that endogenous TMEM16F functions as a CaCC in mouse dendritic cells on the basis of siRNA-mediated knockdown. Tian et al (34) also recently reported that Cl Ϫ current activation was induced in human TMEM16F-overexpressing HEK293 cells when the intracellular Ca 2ϩ concentration was increased to more than 10 M. These observations are in agreement with our present study. In contrast, Yang et al (36) reported that TMEM16F forms a Ca 2ϩ -activated cation channel based on the data obtained by knockout mice and heterologous expression.…”

Role of anoctamins in cancer and apoptosis

“…These results came as a surprise, since Ano6 has been characterized as a Ca 2þ -activated Cl 2 channel. Moreover, Ca 2þ -activated Cl 2 currents were also observed after overexpression of Ano4, 7 and 9 [6,55,56,61,62]. However, Cl 2 channel and scramblase activity were shown to be independent [55].…”

Anoctamin-1/TMEM16A is the major apical iodide channel of the thyrocyte

“…However, their molecular identity remained elusive until 2008, when three groups independently cloned the gene encoding the transmembrane protein called TMEM16A or anoctamin-1 (ANO1) (5,33,46). This protein was observed to function as a Ca 2ϩ -activated chloride channel (26,37) and to be widely expressed at the plasma membrane of many cells in liver, skeletal muscle, heart, pancreas, placenta, sensory neurons, kidney, airways epithelial cells, lung alveoli, and mammary and salivary glands where they concentrate at the apical membrane (8). The functional properties of ANO1 were found to be regulated by alternative splicing and some variants were observed to be expressed in the thyroid (9).…”