Anoctamins/TMEM16 Proteins: Chloride Channels Flirting with Lipids and Extracellular Vesicles

Abstract

Anoctamin/TMEM16 proteins exhibit diverse functions in cells throughout the body and are implicated in several human diseases. Although the founding members ANO1 (TMEM16A) and ANO2 (TMEM16B) are Ca2+-activated Cl− channels, most ANO paralogs are Ca2+-dependent phospholipid scramblases that serve as channels that facilitate the movement (“scrambling”) of phospholipids between leaflets of the membrane bilayer. Phospholipid scrambling significantly alters the physical properties of the membrane and its landscape and has vast downstream signaling consequences. In particular, phosphatidylserine exposed on the external leaflet of the plasma membrane functions as a ligand for receptors vital for cell-cell communication. A major consequence of Ca2+-dependent scrambling is the release of extracellular vesicles that function as intercellular messengers by delivering signaling proteins and non-coding RNAs to alter target cell function. We discuss the physiological implications of Ca2+-dependent phospholipid scrambling, the extracellular vesicles associated with this activity, and the roles of ANOs in these processes.

Phosphatidylserine Is the Signal for TAM Receptors and Their Ligands

“…Inner-outer transmembrane exchange of any phospholipid has a high energy barrier (15–50 kcal/mol) because the hydrophilic headgroup of the phospholipid must dehydrate during transit across the hydrophobic core of the bilayer, and so the spontaneous rate of such exchange is very slow [10, 11]. The enzymes that facilitate the trans-bilayer exchange of PtdSer and other phospholipids have been designated phospholipid ‘scramblases’ [4, 7, 12].…”

“…Also referred to as Anoctamins (they were originally thought to be anion channels with 8 transmembrane helices), the 10-transmembrane domain TMEM16 proteins are especially interesting with respect to the biology discussed below. Although two of these proteins – Ano1 and 2 – appear to function exclusively as Ca 2+ -activated ion channels, there is now good evidence that Ano3, 4, 5, 7, 9, and especially Ano6 (TMEM16F), function as Ca 2+ -dependent phospholipid scramblases [11, 13]. Ano6 and a fungal homolog externalize PtdSer [13, 16, 17], and so the activation of these enzymes by Ca 2+ is almost certainly key to the deployment of PtdSer as an intercellular signal.…”

Anoctamin 8 tethers endoplasmic reticulum and plasma membrane for assembly of Ca 2+ signaling complexes at the ER/PM compartment

“…The first two members of the Anoctamin family, ANO1 and ANO2, function as Ca 2+ -activated Cl À channels (Stohr et al, 2009). ANO6 functions as a lipid scramblase and Cl À channel, and other Anoctamins have also been reported to have some scramblase activity (Whitlock & Hartzell, 2017). The family consists of 10 members, with very limited information on their properties and cellular functions, except for ANO1 (Dang et al, 2017;Paulino et al, 2017), ANO2 (Huang et al, 2012), and ANO6 (Suzuki et al, 2010;Brunner et al, 2014;Alvadia et al, 2019).…”

Phosphatidylinositol 4,5-bisphosphate, cholesterol, and fatty acids modulate the calcium-activated chloride channel TMEM16A (ANO1)

“…TMEM16A and TMEM16B are homo-dimeric channels that belong to the TMEM16 (Anoctamin) membrane protein family (Sheridan et al, 2011; Pedemonte and Galietta, 2014; Whitlock and Hartzell, 2017). Intracellular Ca 2+ activates TMEM16A and TMEM16B by binding directly to the channel.…”