Potassium Channels, Renal Fibrosis, and Diabetes
Progressive renal fibrosis is a common outcome of almost all forms of renal damage, including diabetic nephropathy. Fibrosis leads to chronic kidney failure and, eventually, dialysis or transplantation (1–3). Although much is known about the molecular background and mediators that prompt fibroblasts or transdifferentiated kidney cells to release collagen and matrix (4–6), the search for a unique event that initiates the process remains inconclusive. Targeting this putative key signal would enable researchers to “switch off” fibrosis and, perhaps, the progressive loss of renal function that plagues millions of individuals worldwide (1–6). The intermediate/small-conductance Ca2+-activated K+ channel (KCa3.1; KCNN4; SK4) is one intriguing candidate for such function because it promotes fibrogenesis in target tissue by altering the membrane potential of cells, thus enhancing extracellular Ca2+ entry (7,8). Subsequent Smad2/3 or mitogen-activated protein kinase (MEK)–dependent phosphorylation upregulates profibrotic genes and collagens in human and animal fibroblasts (7–9).
KCa3.1 Ca2+-activated channels regulate K+ outflow, increasing the driving force for Ca2+ entry through hyperpolarization of the plasma membrane (7,10). In KCa3.1, four identical subunits are gathered as a symmetric homotetramer. Six hydrophobic α-helical domains are inserted into the cell membrane in each subunit (Fig. 1). A five-residue loop between the fifth and sixth transmembrane domain confers K+ selectivity. K+ channels are usually tightly associated with calmodulin, the regulatory protein that accounts for the Ca2+ sensitivity of these channels, usually activated by [Ca2+] …