Structural and functional implications of the phospholamban hinge domain: Impaired SR Ca²⁺ uptake as a primary cause of heart failure

Objective: The role of sarcoplasmic reticulum (SR) in the onset and progression of heart failure is controversial. We tested the hypothesis that impairment of SR Ca²⁺ sequestration may be a primary cause for progressive left ventricular (LV) dysfunction and the phospholamban hinge domain may be critical in this process.

Methods: A phospholamban hinge domain mutant (PLB/N27A) was introduced in the cardiac compartment of the phospholamban null mouse. An integrative approach was used to characterize the resulting cardiac phenotype at a structural, cellular, whole organ and intact animal level.

Results: NMR analysis revealed a defined alteration in the α-helical configuration between residues Q22 to F35 in mutant phospholamban. Transgenic lines expressing similar levels of mutant compared to wild-type phospholamban exhibited super-inhibition of the SR Ca²⁺ ATPase affinity for Ca²⁺ (EC50 0.52 µM) in oxalate-supported Ca²⁺ uptake measurements, which translated into impaired relaxation and attenuated responses to β-adrenergic stimulation. Importantly, a blunted force–frequency relation was observed in mutant hearts preceding left ventricular dilation. Upon aging to 10 months, the predominantly diastolic dysfunction progressed to congestive heart failure, characterized by induction of a fetal gene program, cardiac remodeling, lung congestion, depressed systolic function and early mortality.

Conclusion: Increased inhibition of Ca²⁺ sequestration may be a causative factor in the development of left ventricular dysfunction and myocyte remodeling leading to heart failure. Furthermore, the hinge domain may play an important role in transmitting PLB’s regulatory effects on SERCA.