Deception in simplicity: Hereditary phospholamban mutations in dilated cardiomyopathy

The sarcoplasmic reticulum (SR) calcium pump (SERCA) and its regulator phospholamban are required for cardiovascular function. Phospholamban alters the apparent calcium affinity of SERCA in a process that is modulated by phosphorylation via the β-adrenergic pathway. This regulatory axis allows for the dynamic control of SR calcium stores and cardiac contractility. Herein we focus on hereditary mutants of phospholamban that are associated with heart failure, such as Arg⁹-Cys, Arg⁹-Leu, Arg⁹-His, and Arg¹⁴-deletion. Each mutant has a distinct effect on PLN function and SR calcium homeostasis. Arg⁹-Cys and Arg⁹-Leu do not inhibit SERCA, Arg¹⁴-deletion is a partial inhibitor, and Arg⁹-His is comparable to wild-type. While the mutants have distinct functional effects on SERCA, they have in common that they cannot be phosphorylated by protein kinase A (PKA). Arg⁹ and Arg¹⁴ are required for PKA recognition and phosphorylation of PLN. Thus, mutations at these positions eliminate β-adrenergic control and dynamic cardiac contractility. Hydrophobic mutations of Arg⁹ cause more complex changes in function, including loss of PLN function and dominant negative interaction with SERCA in heterozygous individuals. In addition, aberrant interaction with PKA may prevent phosphorylation of wild-type PLN and sequester PKA from other local subcellular targets. Herein we consider what is known about each mutant and how the synergistic changes in SR calcium homeostasis lead to impaired cardiac contractility and dilated cardiomyopathy.