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 Ca(2+)-handling proteins and membrane structures to conduct excitation–contraction coupling apart from having different ryanodine receptor (RyR) isoforms for Ca(2+) release. The sarcoplasmic reticulum (SR) generates heat following Adenosine triphosphate (ATP) hydrolysis at the Ca(2+) pump, which is amplified by increasing RyR1 Ca(2+) leak in mammals, subsequently increasing cytoplasmic [Ca(2+)] ([Ca(2+)](cyto)). For thermogenesis to be functional, rising [Ca(2+)](cyto) must not interfere with cytoplasmic effectors of the sympathetic nervous system (SNS) that likely increase RyR1 Ca(2+) leak; nor should it compromise the muscle remaining relaxed. To achieve this, Ca(2+) activated, regenerative Ca(2+) 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 [Ca(2+)](cyto); and ii) downstream effectors of the SNS increase RyR Ca(2+) leak and subsequently, heat generation. By positioning amphibian and malignant hyperthermia-susceptible human-skinned muscle fibers perpendicularly, we induced abrupt rises in [Ca(2+)](cyto) under identical conditions optimized for activating regenerative Ca(2+) release as Ca(2+) waves passed through the junction of fibers. Only mammalian fibers showed resistance to rising [Ca(2+)](cyto), resulting in increased SR Ca(2+) load and leak. Fiber heat output was increased by cyclic adenosine monophosphate (cAMP)-induced RyR1 phosphorylation at Ser2844 and Ca(2+) leak, indicating likely SNS regulation of thermogenesis. Thermogenesis occurred despite the absence of SR Ca(2+) pump regulator sarcolipin. Thus, evolutionary isolation of RyR1 provided increased dynamic range for thermogenesis with sensitivity to cAMP, supporting endothermy.