AIM: In the renal collecting ducts, ATP stimulates a Ca(2+) -activated chloride current. The identity of the channel responsible for the current under physiological conditions is not known and it was hypothesized that TMEM16a is a relevant candidate in the renal collecting duct.
METHODS: The cortical collecting duct cell line M-1 was used as a model of the collecting duct. The ATP induced Ca(2+) signalling was imaged in cells loaded with Ca(2+) -sensitive fluorescent probes using confocal laser-scanning fluorescence microscopy. Chloride current was determined by mounting M-1 cell layers in Ussing chamber. The expression of TMEM16a in human kidney was tested by immunohistochemistry.
RESULTS: M-1 cells displayed a transient increase in intracellular Ca(2+) concentration in response to 100 nm ATP. This response was completely blocked by addition of 100 μm suramin, indicating that ATP signals through purinergic P2 receptors. Apical addition of 100 nm ATP induced a Cl(-) current, which was blocked by suramin, DPC and the cysteine-modifying compound MTSET. M-1 cells were found to express TMEM16a at the mRNA and protein level. Functionally, it was found that knock-down of TMEM16a expression in M-1 cells inhibited the ATP induced Cl(-) -current. In human and mouse kidney sections, TMEM16a protein expression was localized to the collecting duct, and TMEM16a was found to be excreted in human urinary exosomes.
CONCLUSION: TMEM16a is a Ca(2+) -activated Cl(-) channel expressed in the collecting ducts.