A mathematical model to quantify RYR Ca2+ leak and associated heat production in resting human skeletal muscle fibers

Evolutionary isolation of ryanodine receptor isoform 1 for muscle-based thermogenesis in mammals

Resting skeletal muscle generates heat for endothermy in mammals but not amphibians, while both use the same Ca²⁺-handling proteins and membrane structures to conduct excitation-contraction coupling apart from having different ryanodine receptor (RyR) isoforms for Ca²⁺ release. The sarcoplasmic reticulum (SR) generates heat following Adenosine triphosphate (ATP) hydrolysis at the Ca²⁺ pump, which is amplified by increasing RyR1 Ca²⁺ leak in mammals, subsequently increasing cytoplasmic [Ca²⁺] ([Ca²⁺]_cyto). For thermogenesis to be functional, rising [Ca²⁺]_cyto must not interfere with cytoplasmic effectors of the sympathetic nervous system (SNS) that likely increase RyR1 Ca²⁺ leak; nor should it compromise the muscle remaining relaxed. To achieve this, Ca²⁺ activated, regenerative Ca²⁺ release that is robust in lower vertebrates needs to be suppressed in mammals. However, it has not been clear whether: i) the RyR1 can be opened by local increases in [Ca²⁺]_cyto; and ii) downstream effectors of the SNS increase RyR Ca²⁺ leak and subsequently, heat generation. By positioning amphibian and malignant hyperthermia-susceptible human-skinned muscle fibers perpendicularly, we induced abrupt rises in [Ca²⁺]_cyto under identical conditions optimized for activating regenerative Ca²⁺ release as Ca²⁺ waves passed through the junction of fibers. Only mammalian fibers showed resistance to rising [Ca²⁺]_cyto, resulting in increased SR Ca²⁺ load and leak. Fiber heat output was increased by cyclic adenosine monophosphate (cAMP)-induced RyR1 phosphorylation at Ser2844 and Ca²⁺ leak, indicating likely SNS regulation of thermogenesis. Thermogenesis occurred despite the absence of SR Ca²⁺ pump regulator sarcolipin. Thus, evolutionary isolation of RyR1 provided increased dynamic range for thermogenesis with sensitivity to cAMP, supporting endothermy.

Elevated Ca2+ at the triad junction underlies dysregulation of Ca2+ signaling in dysferlin-null skeletal muscle

Dysferlin-null A/J myofibers generate abnormal Ca²⁺ transients that are slightly reduced in amplitude compared to controls. These are further reduced in amplitude by hypoosmotic shock and often appear as Ca²⁺ waves (Lukyanenko et al., J. Physiol., 2017). Ca²⁺ waves are typically associated with Ca²⁺-induced Ca²⁺ release, or CICR, which can be myopathic. We tested the ability of a permeable Ca²⁺ chelator, BAPTA-AM, to inhibit CICR in injured dysferlin-null fibers and found that 10–50 nM BAPTA-AM suppressed all Ca²⁺ waves. The same concentrations of BAPTA-AM increased the amplitude of the Ca²⁺ transient in A/J fibers to wild type levels and protected transients against the loss of amplitude after hypoosmotic shock, as also seen in wild type fibers. Incubation with 10 nM BAPTA-AM led to intracellular BAPTA concentrations of ∼60 nM, as estimated with its fluorescent analog, Fluo-4AM. This should be sufficient to restore intracellular Ca²⁺ to levels seen in wild type muscle. Fluo-4AM was ∼10-fold less effective than BAPTA-AM, however, consistent with its lower affinity for Ca²⁺. EGTA, which has an affinity for Ca²⁺ similar to BAPTA, but with much slower kinetics of binding, was even less potent when introduced as the -AM derivative. By contrast, a dysferlin variant with GCaMP6f_u in place of its C2A domain accumulated at triad junctions, like wild type dysferlin, and suppressed all abnormal Ca²⁺ signaling. GCaMP6f_u introduced as a Venus chimera did not accumulate at junctions and failed to suppress abnormal Ca²⁺ signaling. Our results suggest that leak of Ca²⁺ into the triad junctional cleft underlies dysregulation of Ca²⁺ signaling in dysferlin-null myofibers, and that dysferlin’s C2A domain suppresses abnormal Ca²⁺ signaling and protects muscle against injury by binding Ca²⁺ in the cleft.