The anoctamins: Structure and function

When activated by increase in intracellular Ca²⁺, anoctamins (TMEM16 proteins) operate as phospholipid scramblases and as ion channels. Anoctamin 1 (ANO1) is the Ca²⁺-activated epithelial anion-selective channel that is coexpressed together with the abundant scramblase ANO6 and additional intracellular anoctamins. In salivary and pancreatic glands, ANO1 is tightly packed in the apical membrane and secretes Cl⁻. Epithelia of airways and gut use cystic fibrosis transmembrane conductance regulator (CFTR) as an apical Cl⁻ exit pathway while ANO1 supports Cl⁻ secretion mainly by facilitating activation of luminal CFTR and basolateral K⁺ channels. Under healthy conditions ANO1 modulates intracellular Ca²⁺ signals by tethering the endoplasmic reticulum, and except of glands its direct secretory contribution as Cl⁻ channel might be small, compared to CFTR. In the kidneys ANO1 supports proximal tubular acid secretion and protein reabsorption and probably helps to excrete HCO₃⁻in the collecting duct epithelium. However, under pathological conditions as in polycystic kidney disease, ANO1 is strongly upregulated and may cause enhanced proliferation and cyst growth. Under pathological condition, ANO1 and ANO6 are upregulated and operate as secretory channel/phospholipid scramblases, partly by supporting Ca²⁺-dependent processes. Much less is known about the role of other epithelial anoctamins whose potential functions are discussed in this review.