Structure and mechanism of pendrin and the mutations that cause Pendred's Syndrome

Pendrin (SLC26A4) is expressed in epithelial tissues, e.g., in the inner ear, thyroid, kidney, and lung where it plays a central role in ion homeostasis and the regulation of the cell volume. Mutations in the Slc26a4 gene cause Pendred Syndrome and enlarged vestibular aqueduct syndrome (EVAS), both of which are genetic disorders characterized by childhood early hearing loss in children and account for 5-10% of hereditary hearing loss and are currently not curable. Previous studies illuminated the role of pendrin in the physiology of the cochlea, thyroid gland, kidney and proposed that it can transport iodide ions (I-), bicarbonate ions (HCO3-), chloride ions (Cl-), and hydroxide ions (OH-) across epithelial cell membranes according to an electroneutral exchange (antiport) reaction. However, our understanding of pendrin remains rudimentary due to a lack of purified protein that enables precise functional studies without the potential interference of native proteins replete in cellular/native systems and structural studies. To overcome this gap in our understanding, we have successfully expressed and purified a mammalian homolog of human pendrin and developed binding and transport assays to determine substrate selectivity and transport. Preliminary studies confirmed that purified pendrin reconstituted in lipid membranes transports I- or HCO3- in exchange with Cl- or OH- and revealed that the transport process is electrogenic, i.e., the stoichiometry of ion exchange is not 1:1 as previously postulated for electroneutral antiport. We determined I-- and HCO3--bound pendrin structures by cryo-electron microscopy, and our preliminary analysis suggests that pendrin has two anion binding sites, which may provide an explanation for the electrogenic transport process. The structure reveals novel interactions between the transmembrane domain (TMD) and the cytosolic domain, i.e., the sulphate transporter and anti-sigma factor antagonist domain (STAS) that appears to be relevant for the transport mechanism because mutations at the interface of STAS and TMD are known to cause Pendred Syndrome. Pendrin is also a promising drug target for attenuating airway hyperresponsiveness in asthma and for reducing hypertension, and many pendrin inhibitors, e.g., the non- steroidal anti-inflammatory drug niflumic acid, has been reported to target pendrin, but the mechanisms of inhibition remain unknown. Whereas these inhibitors could be repurposed to target pendrin, their action on pendrin may also cause undesired side-effects, thus highlighting the need to elucidate the mechanisms of pendrin inhibition by small molecules. We determined the structures of pendrin in complex with the anti- inflammatory drugs YS-01 and niflumic acid, and our preliminary analysis shows that the two inhibitors occupy different binding sites, providing motivation for the further determination of the mechanisms of inhibition. To this end, the overall goal of this project is to understand the mechanism and pharmacology of pendrin at the atomic level to aid in the development of efficacious drugs that specifically target pendrin in improved therapies against Pendred Syndrome and EVAS.