Fluorescence lifetime FRET assay for live-cell high-throughput screening of the cardiac SERCA pump yields multiple classes of small-molecule allosteric modulators

Pathological mutations in the phospholamban cytoplasmic region affect its topology and dynamics modulating the extent of SERCA inhibition

Phospholamban (PLN) is a 52 amino acid regulin that allosterically modulates the activity of the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) in the heart muscle. In its unphosphorylated form, PLN binds SERCA within its transmembrane (TM) domains, approximately 20 Å away from the Ca2+ binding site, reducing SERCA's apparent Ca2+ affinity (pKCa) and decreasing cardiac contractility. During the enzymatic cycle, the inhibitory TM domain of PLN remains anchored to SERCA, whereas its cytoplasmic region transiently binds the ATPase's headpiece. Phosphorylation of PLN at Ser16 by protein kinase A increases the affinity of its cytoplasmic domain to SERCA, weakening the TM interactions with the ATPase, reversing its inhibitory function, and augmenting muscle contractility. How the structural changes caused by pathological mutations in the PLN cytoplasmic region are transmitted to its inhibitory TM domain is still unclear. Using solid-state NMR spectroscopy and activity assays, we analyzed the structural and functional effects of a series of mutations and their phosphorylated forms located in the PLN cytoplasmic region and linked to dilated cardiomyopathy. We found that these missense mutations affect the overall topology and dynamics of PLN and ultimately modulate its inhibitory potency. Also, the changes in the TM tilt angle and cytoplasmic dynamics of PLN caused by these mutations correlate well with the extent of SERCA inhibition. Our study unveils new molecular determinants for designing variants of PLN that outcompete endogenous PLN to regulate SERCA in a tunable manner.

Structural Dynamics of Protein Interactions Using Site-Directed Spin Labeling of Cysteines to Measure Distances and Rotational Dynamics with EPR Spectroscopy

Here we review applications of site-directed spin labeling (SDSL) with engineered cysteines in proteins, to study the structural dynamics of muscle and non-muscle proteins, using and developing the electron paramagnetic resonance (EPR) spectroscopic techniques of dipolar EPR, double electron electron resonance (DEER), saturation transfer EPR (STEPR), and orientation measured by EPR. The SDSL technology pioneered by Wayne Hubbell and collaborators has greatly expanded the use of EPR, including the measurement of distances between spin labels covalently attached to proteins and peptides. The Thomas lab and collaborators have applied these techniques to elucidate dynamic interactions in the myosin-actin complex, myosin-binding protein C, calmodulin, ryanodine receptor, phospholamban, utrophin, dystrophin, β-III-spectrin, and Aurora kinase. The ability to design and engineer cysteines in proteins for site-directed covalent labeling has enabled the use of these powerful EPR techniques to measure distances, while showing that they are complementary with optical spectroscopy measurements.

Regulation of cardiac calcium signaling by newly identified calcium pump modulators

In cardiomyocytes, the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA2a) is a central component of intracellular Ca2+ regulation. Several heart diseases, including heart failure, are associated with reduced myocardial contraction due to SERCA2a downregulation. Therefore, the need for developing new drugs that could improve SERCA2a function is high. We have recently identified SERCA2a modulators (Compounds 6 and 8) from our screening campaigns and confirmed activation of biochemical SERCA2a ATPase activity and Ca2+ uptake activity. In this study, confocal microscopy and in-cell Ca2+ imaging were used to characterize the effects of these SERCA2a activators on Ca2+ regulation in mouse ventricular myocytes and endoplasmic reticulum (ER) Ca2+ uptake in a HEK293 cell expressing human SERCA2a. Analysis of cytosolic Ca2+ dynamics in cardiomyocytes revealed that both Compounds (6 and 8) increase the action potential-induced Ca2+ transients and sarcoplasmic reticulum (SR) Ca2+ load. While Compound 6 induced a negligible effect on Ca2+ transients invoked by the L-type Ca2+ channel (LTCC) current, Compound 8 increased Ca2+ transients during LTCC activation, suggesting an off-target protein interaction of Compound 8. Analysis of ER Ca2+ transport by human SERCA2a in HEK cells showed that only Compound 6 increased both ER Ca2+ uptake and ER Ca2+ load significantly, whereas Compound 8 had no effect on SERCA2a Ca2+ transport. This study revealed that Compound 6 exhibits promising characteristics that can improve intracellular Ca2+ dynamics by selectively enhancing SERCA2a Ca2+ uptake.