Abstract P1065: Deletion Of Arginine 14 Increases Phospholamban Oligomerization And Disrupts The Cardiac Force-Frequency Relationship

Abstract

The Bowditch effect is a physiological phenomenon where accelerated cardiac pacing leads to an increase in the contractile force of the heart. While this has largely been attributed to a frequency-dependent increase in L-type calcium channel influx, evidence suggests intracellular calcium handling proteins must also play an active role. Knock-out of phospholamban (PLB), the inhibitor of the Ca ATPase (SERCA), virtually abolishes the Bowditch effect in mice, suggesting PLB is a major determinant of the cardiac force-frequency relationship. However, no molecular mechanism has explained this frequency-dependent function of PLB. Our lab has previously shown that SERCA loses affinity for PLB when intracellular calcium is elevated. Here, FRET microscopy revealed dynamic decreases in PLB-SERCA binding in response to intracellular calcium elevations. PLB monomers freed from SERCA were rapidly incorporated into PLB pentamers. Interestingly, unbinding of PLB from pentamers was very slow and delayed the recovery of PLB-SERCA complexes by several seconds after calcium elevations. In a computational model of these regulatory binding dynamics, PLB pentamers sequestered inhibitory monomers away from SERCA at fast heart rates. By lowering PLB inhibition of SERCA with increasing cardiac pacing, PLB pentamers serve as a major contributor to the Bowditch effect. Notably, deletion of arginine 14 of PLB causes arrhythmogenic cardiomyopathy and has been thought to destabilize PLB pentamers. Surprisingly, we found R14del-PLB enhanced pentamer stability and slowed the rate of PLB exchange from pentamers to SERCA. We hypothesize that R14del mutation may disrupt frequency-dependent changes in SERCA regulation by PLB during exercise.