Bestrophin-3 (Vitelliform Macular Dystrophy 2–Like 3 Protein) Is Essential for the cGMP-Dependent Calcium-Activated Chloride Conductance in Vascular Smooth Muscle Cells

Although the biophysical fingerprints (ion selectivity, voltage-dependence, kinetics, etc) of Ca-activated Cl currents are well established, their molecular identity is still controversial. Several molecular candidates have been suggested; however, none of them has been fully accepted. We have recently characterized a cGMP-dependent Ca-activated Cl current with unique characteristics in smooth muscle cells. This novel current has been shown to coexist with a “classic” (cGMP-independent) Ca-activated Cl current and to have characteristics distinct from those previously known for Ca-activated Cl currents. Here, we suggest that a bestrophin, a product of the Best gene family, is responsible for the cGMP-dependent Ca-activated Cl current based on similarities between the membrane currents produced by heterologous expressions of bestrophins and the cGMP-dependent Ca-activated Cl current. This is supported by similarities in the distribution pattern of the cGMP-dependent Ca-activated Cl current and bestrophin-3 (the product of Best-3 gene) expression in different smooth muscle. Furthermore, downregulation of Best-3 gene expression with small interfering RNA both in cultured cells and in vascular smooth muscle cells in vivo was associated with a significant reduction of the cGMP-dependent Ca-activated Cl current, whereas the magnitude of the classic Ca-activated Cl current was not affected. The majority of previous suggestions that bestrophins are a new Cl channel family were based on heterologous expression in cell culture studies. Our present results demonstrate that at least 1 family member, bestrophin-3, is essential for a well-defined endogenous Ca-activated Cl current in smooth muscles in the intact vascular wall.