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

Abstract

Iodide is captured by thyrocytes through the Na ϩ /I Ϫ symporter (NIS) before being released into the follicular lumen, where it is oxidized and incorporated into thyroglobulin for the production of thyroid hormones. Several reports point to pendrin as a candidate protein for iodide export from thyroid cells into the follicular lumen. Here, we show that a recently discovered Ca 2ϩ -activated anion channel, TMEM16A or anoctamin-1 (ANO1), also exports iodide from rat thyroid cell lines and from HEK 293T cells expressing human NIS and ANO1. The Ano1 mRNA is expressed in PCCl3 and FRTL-5 rat thyroid cell lines, and this expression is stimulated by thyrotropin (TSH) in rat in vivo, leading to the accumulation of the ANO1 protein at the apical membrane of thyroid follicles. Moreover, ANO1 properties, i.e., activation by intracellular calcium (i.e., by ionomycin or by ATP), low but positive affinity for pertechnetate, and nonrequirement for chloride, better fit with the iodide release characteristics of PCCl3 and FRTL-5 rat thyroid cell lines than the dissimilar properties of pendrin. Most importantly, iodide release by PCCl3 and FRTL-5 cells is efficiently blocked by T16A inh-A01, an ANO1-specific inhibitor, and upon ANO1 knockdown by RNA interference. Finally, we show that the T16A inh-A01 inhibitor efficiently blocks ATP-induced iodide efflux from in vitro-cultured human thyrocytes. In conclusion, our data strongly suggest that ANO1 is responsible for most of the iodide efflux across the apical membrane of thyroid cells.anoctamin-1; iodide release; pendrin; thyrocyte; TMEM16A

Four basic residues critical for the ion selectivity and pore blocker sensitivity of TMEM16A calcium-activated chloride channels

“…Interestingly, a physiological role for the preference for thiocyanate may exist for these channels, where thiocyanate may be transported across the apical membrane of airway epithelial cells through CaCCs and converted to OSCN − by a combination of oxidases, such as Duox and lactoperoxidase, as an innate immunity defense mechanism against pathogens (38). On the same note, iodide ions are also strongly selected for over chloride, which may be relevant for ensuring proper thyrocyte function in humans and rats (39). In addition to identifying key residues for anion selectivity, we also identified pore blockers with potency that was affected by mutations at these basic residues.…”

Chemistry and Biology in the Biosynthesis and Action of Thyroid Hormones

“…More recently,a noctamin (ANO1, TMEM16A) has been suggested to be the dominant iodide transporter at the apical membrane of thyrocytes. [13] This protein was initially characterized as aC a 2+ -activated chloride channel composed of eight transmembrane segments,but displays specificity for iodide.…”

An extremely high dietary iodide supply forestalls severe hypothyroidism in Na+/I− symporter (NIS) knockout mice

“…However, we demonstrated that SMCT does not transport I − 17 and Slc5a8 KO mice are not hypothyroid 19 . The calcium-activated chloride channel anoctamin1 ( Ano1 ) is expressed at the apical membrane of thyroid follicular cells 20 . HEK 293 T and PCCl3 cells co-transfected with plasmids encoding NIS and Ano1 show increased I − efflux in the presence of ionomycin, a Ca + ionophore, suggesting that Ano1 may play a role in I − efflux 20 .…”

“…The calcium-activated chloride channel anoctamin1 ( Ano1 ) is expressed at the apical membrane of thyroid follicular cells 20 . HEK 293 T and PCCl3 cells co-transfected with plasmids encoding NIS and Ano1 show increased I − efflux in the presence of ionomycin, a Ca + ionophore, suggesting that Ano1 may play a role in I − efflux 20 . Ano1 KO mice die within their first month of life and displayed severe tracheomalacia, with gaps in the cartilage rings all along the trachea 21 .…”