In this image of the month we highlight the protein barttin, encoded by the BSND gene, which is involved in the development of Bartter Syndrome. Barttin facilitates reabsorption of chloride in the loops of Henle in the kidneys and secretion of potassium in the stria vascularis of the inner ear (Estévez et al, 2001 ). More recently barttin has also been found to be expressed in the salivary ducts (Shinmura et al, 2018).

Barrtin is a transmembrane protein that associates with the CLCNKA and CLCNKB chloride channels. The CLCNK/barttin heteromers assist with the reabsorption of chloride ions in the kidney by facilitating its basolateral efflux. In the stria vascularis, located in the inner ear, these heteromers instead drive potassium secretion via the uptake of chloride. The BSND gene that encodes the barttin protein is found on chromosome 1 and mutations in the gene have been shown to be the cause of Bartter Syndrome type IV in which patients have sensorineural deafness and kidney failure (Birkenhäger et al, 2001). Bartter Syndrome is a rare disease in which sodium and chloride cannot be reabsorbed by the kidney in the thick ascending limb of the loop of Henle.

Barttin is mainly expressed in the kidney at the RNA level but can also be found in the pituitary and salivary gland. Using scRNAseq, barttin expression is found in ionocytes, collecting duct cells, proximal tubular cells as well as in salivary duct cells. Immunostaining found that barttin was present in salivary ducts as well as in Warthin´s and oncocytoma salivary gland tumors. Barttin was also found to be a marker for chromophobe renal cell carcinoma (Shinmura et al, 2018, Shinmura et al, 2015).