Serca Located in the Junctional SR Shapes Calcium Release in Cardiac Myocytes

Abstract

Calcium-induced calcium release is in principle an all-or-nothing mechanism and self-propagation of Ca2+ waves is a pathological manifestation of this positive feedback loop. However, since local release of Ca2+ only elicits elementary Ca2+ release events (Ca2+ sparks) that do not develop into Ca2+ waves, Ca2+ release can be graded. This local control is possible because of the spatial arrangement of L-type Ca2+ channels and clusters of ryanodine receptors at the sites of Ca2+ release (dyads) (Stern 1992). We propose that the sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA) also contributes to local control by limiting diffusion of Ca2+. Six days following conditional SERCA knockout, Ca2+ sparks exhibited broadened geometry and slowed kinetics (increase in spark width, time to peak, and duration by 33%, 33%, and 51%, respectively). At this time point, SERCA protein levels are reduced by 53% and SR Ca2+ content is decreased by 25% (Stokke et al. 2010). To determine the precise localization of SERCA we employed cryo immuno-gold electron microscopy on sections from the mouse papillary muscle. Normal cardiac myocytes showed preferential SERCA expression near the Z-lines (nearly twofold higher labeling density at the Z-line than the A-band) with SERCA molecules clearly located in the junctional SR in close proximity to the dyads. The conditional SERCA knockout resulted in preferential loss of labeling at these locations. From resin-embedded cross-sections of papillary muscles, we observed more abundant SR in the I-band compared to the A-band (by factor of 1.7 and 1.6 for control and KO respectively). These results suggest that SERCA contributes to local control of Ca2+ release by limiting diffusion of Ca2+ from the dyad. Thus, the processes of Ca2+ release and re-uptake are closely linked by a population of SERCA molecules in the junctional SR.