Functional Interaction with Filamin a Enhances Atrial-Specific Small Conductance Ca2 Activated K+ Channel (SK2) Surface Membrane Expression

Abstract

For ion channels to function properly, a precise number of channel proteins need to be trafficked to exact locations on the cell surface membrane. Small-conductance, Ca2+-activated K+ channels (SK) are predominantly expressed in the atria and their role has been implicated in atrial fibrillation (AF).Using yeast two-hybrid screen against human heart library, we identify filamin A (FLNA) as a putative interacting protein with SK2 channel. Patch-clamp recordings and immunofuorescence studies in neonatal and adult cardiac myocytes as well as HEK293 cells suggest the interaction leads to an increase in SK2 membrane localization. SiRNA knockdown of FLNA in neonatal myocytes results in a decrease in the membrane localization of SK2 channels. Additionally, the calcium dependency of SK2 channel membrane expression was examined using Total Internal Reflection Microscopy (TIRF-M) and immunofluorescence microscopy. Importantly, intracellular Ca2+ (Ca2+i) plays a critical role in the membrane localization of SK2 channel when the channel is co-expressed with α-actinin2, another cytoskeletal protein which we have previously shown to interact with SK2 channel. In conclusion, FLNA is a regulator of SK2 channel expression. Moreover, SK2 membrane expression is critically dependent on Ca2+i. An increase in Ca2+i, for example, during AF, is predicted to result in an increase in SK2 channel expression leading to shortening of the action potentials.