Molecular operation of the cation chloride cotransporters: ion binding and inhibitor interaction

The cation chloride cotransporters (CCCs) represent an important family of transporters that plays key roles in vectorial electrolyte movement across epithelia and in intracellular chloride homeostasis of neurons and muscle cells. The CCCs are composed of three broad groups, two of which include multiple isoforms: Na-Cl cotransporter (NCC; SLC12A3), Na-K-2Cl cotransporter (NKCC; SLC12A1-2), and K-Cl cotransporter (KCC; SLC12A4-7). The CCCs are inhibited by clinically relevant drugs, including loop diuretics that inhibit NKCC2 in the renal thick ascending limb and thiazide diuretics that inhibit NCC in the renal distal tubule. For many years, much research on this gene family has centered on understanding ion binding and inhibitor interaction which represent important features of the molecular operation of these transporters. Recently, high resolution structures of bacterial transport proteins related to the CCCs have become available, thus permitting structural context in which to evaluate previous ion and inhibitor studies of the CCCs. In this article, I review past molecular and structure-function studies that have provided key pieces of information about ion binding and inhibitor interaction primarily of NKCC for which we have the most information. I then place these findings into the structural context of recent homology models of NKCC based on the outward-facing open and occluded conformations of the related bacterial transporters. These homology models provide our first glimpse into the fine details of the molecular operation of the CCCs.