Abstract Body: Background: β1-adrenergic receptor (β1-AR) signaling is a primary driver of cardiac contractility, including promoting calcium reuptake into the sarcoplasmic reticulum (SR) via SERCA2a in a local SR membrane nanodomain. Although this pathway has been well established, the structural organization that ensures efficient coupling between β1-AR activation and intracellular calcium handling remains incompletely defined.
Objective: To determine whether the scaffold protein synaptic associated protein 97 (SAP97) coordinates the assembly of β1-AR signaling complexes in cardiomyocytes and to define the functional consequences of this spatial organization in vivo.
Methods and Results: Using biochemical analyses, we identified a SAP97-dependent β1-AR signaling complex associated with key components, including β1-AR, SERCA2a, SAP97, AKAP150, and PKA, of the SR calcium regulatory apparatus, supporting the existence of an organized receptor-SERCA2a signaling platform. To further test its function, we generated a cardiomyocyte-specific SAP97 knockout mouse model. SAP97 deficiency disrupted this structural organization and significantly attenuated β1-AR-stimulated phosphorylation of phospholamban, a critical calcium regulatory target governing SR calcium reuptake. These findings indicate that SAP97 is required for efficient coupling of receptor activation to downstream calcium regulatory pathways. Functionally, SAP97 deficiency impaired β-AR enhancement of calcium reuptake kinetics and reduced contractile responses in mouse isolated ventricular cardiomyocytes, consistent with compromised regulation of intracellular calcium dynamics. At the whole-heart level, loss of SAP97 blunted β1-AR induced increases in both systolic and diastolic performance, demonstrating that SAP97 is essential for maintaining β-AR contractile reserve in vivo. Together, these findings link disruption of signaling architecture to defects in calcium handling and cardiac contractility.
Conclusions: Our findings identify SAP97 as a critical structural determinant that physically and functionally integrates β1-AR signaling with SERCA2a-mediated calcium regulation in the heart. Loss of this signaling architecture compromises receptor-mediated calcium handling and limits cardiac functional reserve, highlighting the importance of spatially organized signaling in β-AR responsiveness.