Coordinated Regulation of Cardiac Na⁺/Ca²⁺ Exchanger and Na⁺-K⁺-ATPase by Phospholemman (FXYD1)

Abstract

Phospholemman (PLM) is the founding member of the FXYD family of regulators of ion transport. PLM is a 72-amino acid protein consisting of the signature PFXYD motif in the extracellular N terminus, a single transmembrane (TM) domain, and a C-terminal cytoplasmic tail containing three phosphorylation sites. In the heart, PLM co-localizes and co-immunoprecipitates with Na⁺-K⁺-ATPase, Na⁺/Ca²⁺ exchanger, and L-type Ca²⁺ channel. The TM domain of PLM interacts with TM9 of the α-subunit of Na⁺-K⁺-ATPase, while its cytoplasmic tail interacts with two small regions (spanning residues 248–252 and 300–304) of the proximal intracellular loop of Na⁺/Ca²⁺ exchanger. Under stress, catecholamine stimulation phosphorylates PLM at serine⁶⁸, resulting in relief of inhibition of Na⁺-K⁺-ATPase by decreasing K_m for Na⁺ and increasing V_max, and simultaneous inhibition of Na⁺/Ca²⁺ exchanger. Enhanced Na⁺-K⁺-ATPase activity lowers intracellular Na⁺, thereby minimizing Ca²⁺ overload and risks of arrhythmias. Inhibition of Na⁺/Ca²⁺ exchanger reduces Ca²⁺ efflux, thereby preserving contractility. Thus, the coordinated actions of PLM during stress serve to minimize arrhythmogenesis and maintain inotropy. In acute cardiac ischemia and chronic heart failure, either expression or phosphorylation of PLM or both are altered. PLM regulates important ion transporters in the heart and offers a tempting target for development of drugs to treat heart failure.