Tubular (T-) System SOCE-PMCA Balance Is Regulated by RyR Leak in Resting Skeletal Muscle

Abstract

Skeletal muscle possesses a unique, rapidly activating store-operated Ca2+ entry (SOCE) mechanism. SOCE is activated when the activity of the ryanodine receptors increase, following action potential-induced Ca2+release or even increased RyR Ca2+ leak (Koenig et al 2018, Comm Biol; Cully et al 2018, PNAS). The role of SOCE in muscle remains poorly defined, but a role in calcium content regulation is likely, which means it must work in concert with fibre Ca2+ extrusion mechanisms for balance. Kinetics of Ca2+ movements in imaging experiments performed with high spatiotemporal resolution have shown that the t-system PMCA exists in the same microdomain as RyRs and the SOCE protein components; and all components are equally distributed throughout the muscle fibre (Cully et al 2012, AJP). We studied RyR mutant mice (RyR KI; Lopez et al 2018, BJA) and calsequestrin 1 null (CSQKO) mice (Paolini et al 2007, J.Physiol) to examine the regulatory relationship of RyR Ca2+ leak with SOCE and PMCA activity using mechanically skinned fibres. We determined that RyR Ca2+ leak increased in the order WT<RyR KI HET<RyR KI HOM<CSQKO. This leak caused a steady state reduction in SR calcium content, which we quantified in absolute terms. We found PMCA activity to be increased in same order as increased RyR Ca2+ leak, where CSQKO muscle showed the most rapid rate of Ca2+ extrusion when [Ca2+]t-sys was initially 0.1 mM and [Ca2+]cyto was varied between 28-1342 nM. The increase in Ca2+ extrusion was offset by increasing SOCE so that physiological, mM [Ca2+]t-sys levels were maintained. Our data conform with Cell Boundary Theorem (Rios, 2010, J.Physiol Sci), providing a platform to explain the increased [Ca2+]cyto in resting intact fibres (Lopez et al 2018) in the presence of increased RyR Ca2+ leak.