Structure and Function of TMEM16 Proteins (Anoctamins)

Abstract

TMEM16 proteins, also known as anoctamins, are involved in a variety of functions that include ion transport, phospholipid scrambling, and regulation of other membrane proteins. The first two members of the family, TMEM16A (anoctamin-1, ANO1) and TMEM16B (anoctamin-2, ANO2), function as Ca2+-activated Cl- channels (CaCCs), a type of ion channel that plays important functions such as transepithelial ion transport, smooth muscle contraction, olfaction, phototransduction, nociception, and control of neuronal excitability. Genetic ablation of TMEM16A in mice causes impairment of epithelial Cl- secretion, tracheal abnormalities, and block of gastrointestinal peristalsis. TMEM16A is directly regulated by cytosolic Ca2+ as well as indirectly by its interaction with calmodulin. Other members of the anoctamin family, such as TMEM16C, TMEM16D, TMEM16F, TMEM16G, and TMEM16J, may work as phospholipid scramblases and/or ion channels. In particular, TMEM16F (ANO6) is a major contributor to the process of phosphatidylserine translocation from the inner to the outer leaflet of the plasma membrane. Intriguingly, TMEM16F is also associated with the appearance of anion/cation channels activated by very high Ca2+ concentrations. Furthermore, a TMEM16 protein expressed in Aspergillus fumigatus displays both ion channel and lipid scramblase activity. This finding suggests that dual function is an ancestral characteristic of TMEM16 proteins and that some members, such as TMEM16A and TMEM16B, have evolved to a pure channel function. Mutations in anoctamin genes (ANO3, ANO5, ANO6, and ANO10) cause various genetic diseases. These diseases suggest the involvement of anoctamins in a variety of cell functions whose link with ion transport and/or lipid scrambling needs to be clarified.

Role of volume-regulated and calcium-activated anion channels in cell volume homeostasis, cancer and drug resistance

“…113,114 For characterization of Anoctamin structure and function see refs. [115][116][117][118] . In the following we focus on ANO1.…”

Activation of Ca2+‐activated Cl− channel ANO1 by localized Ca2+ signals

“…Mechanisms of Cl − ‐dependent secretion in epithelia are outside the scope of this brief review (for detailed coverage see excellent recent reviews by Jang & Oh, 2014 and Pedemonte & Galietta, 2014). It is worth mentioning though that ANO1‐mediated CaCCs in epithelia are also coupled to G q/11 –GPCR–PLC–IP 3 pathways, highlighting once again a functional link between ANO1 and the ER.…”

“…Thus, the G q/11 receptor ligands ATP, substance P, acetylcholine, endothelin 1, angiotensin II and histamine have all been shown to activate CaCCs in epithelial cells of various types by inducing ER Ca 2+ release (Hartzell et al . 2005; Jang & Oh, 2014; Pedemonte & Galietta, 2014). …”

“…2009; Stohr et al . 2009) subunits identified as bona fide CaCCs (see Pedemonte & Galietta, 2014 for review). For example, ANO1 (TMEM16A), reconstituted in an expression system, reproduced key features of native CaCC currents such as higher permeability to I − over Cl − , micromolar Ca 2+ sensitivity, outwardly rectifying voltage dependence and sensitivity to the CaCC blockers 5‐nitro‐2‐(3‐phenylpropylamino)benzoic acid (NPPB), niflumic acid (NFA) and 4,4′‐diisothio‐cyanostilbene‐2,2′‐disulfonic acid (DIDS) (Hartzell et al .…”

9–Anthracene carboxylic acid is more suitable than DIDS for characterization of calcium-activated chloride current during canine ventricular action potential

“…The most likely candidate is the relatively newly identified TMEM16 protein (Yang et al 2008;Forrest et al 2012); however, the results are not absolutely conclusive yet. The discovery of TMEM16 also known as Anoctamin1, and the possibility that it is the ion channel responsible for I Cl(Ca) recently boosted the research in this field in smooth muscle (Dam et al 2014), neuronal, and other tissues (Pedemonte and Galietta 2014), highlighting the importance of I Cl(Ca) in various physiological functions.…”