Phospholemman is a negative feed-forward regulator of Ca²⁺ in β-adrenergic signaling, accelerating β-adrenergic inotropy

Abstract 

Sympathetic stimulation enhances cardiac contractility by stimulating β-adrenergic signaling and protein kinase A (PKA). Recently, phospholemman (PLM) has emerged as an important PKA substrate capable of regulating cytosolic Ca²⁺ transients. However, it remains unclear how PLM contributes to β-adrenergic inotropy. Here we developed a computational model to clarify PLM's role in the β-adrenergic signaling response. Simulating Na⁺ and sarcoplasmic reticulum (SR) Ca²⁺ clamps, we identify an effect of PLM phosphorylation on SR unloading as the key mechanism by which PLM confers cytosolic Ca²⁺ adaptation to long-term β-adrenergic receptor (β-AR) stimulation. Moreover, we show that phospholamban (PLB) opposes and overtakes these actions on SR load, forming a negative feed-forward loop in the β-adrenergic signaling cascade. This network motif dominates the negative feedback conferred by β-AR desensitization and accelerates β-AR-induced inotropy. Model analysis therefore unmasks key actions of PLM phosphorylation during β-adrenergic signaling, indicating that PLM is a critical component of the fight-or-flight response.

Graphical abstract 

Abbreviations: β-AR, β-adrenergic receptor, CaMKII, Ca²⁺/Calmodulin Protein Kinase II, CICR, Ca²⁺-induced Ca²⁺-release, ECC, excitation-contraction coupling, ISO, isoproterenol, LCC, L-type Ca²⁺ channel, NCX, Na⁺/Ca²⁺-ATPase, NKA, Na⁺/K⁺-ATPase, PKA, protein kinase A, PLB, phospholamban, PLM, phospholemman, RyR, ryanodine receptor, SERCA, sarco-/endoplasmic reticulum Ca²⁺-ATPase, SR, sarcoplasmic reticulum

Keywords: Phospholemman, Na⁺, β-Adrenergic signaling, Excitation–contraction coupling, Negative feed-forward, Computational model