Ryanodine receptor activity and store-operated Ca2+ entry: Critical regulators of Ca2+ content and function in skeletal muscle

Abstract

Store-operated Ca²⁺ entry (SOCE) is critical to cell function. In skeletal muscle, SOCE has evolved alongside excitation-contraction coupling (EC coupling); as a result, it displays unique properties compared to SOCE in other cells. The plasma membrane of skeletal muscle is mostly internalized as the tubular system, with the tubules meeting the sarcoplasmic reticulum (SR) terminal cisternae, forming junctions where the proteins that regulate EC coupling and SOCE are positioned. In this review, we describe the properties and roles of SOCE based on direct measurements of Ca²⁺ influx during SR Ca²⁺ release and leak. SOCE is activated immediately and locally as the [Ca²⁺ ] of the junctional SR terminal cisternae ([Ca²⁺ ]_jSR ) depletes. [Ca²⁺ ]_jSR changes rapidly and steeply with increasing activity of the SR ryanodine receptor isoform 1 (RyR1). The high fidelity of [Ca²⁺ ]_jSR with RyR1 activity probably depends on the SR Ca²⁺ -buffer calsequestrin that is located immediately behind RyR1 inside the SR. This arrangement provides in-phase activation and deactivation of SOCE with a large dynamic range, allowing precise grading of SOCE flux. The in-phase activation of SOCE as the SR partially depletes traps Ca²⁺ in the cytoplasm, preventing net Ca²⁺ loss. Mild presentation of RyR1 leak can occur under physiological conditions, providing fibre Ca²⁺ redistribution without changing fibre Ca²⁺ content. This condition preserves normal contractile function at the same time as increasing basal metabolic rate. However, higher RyR1 leak drives excess cytoplasmic and mitochondrial Ca²⁺ load, setting a deleterious intracellular environment that compromises the function of the skeletal muscle.

Keywords: Ca2+; SOCE; confocal; mechanically skinned fibre; ryanodine receptor; skeletal muscle; store-operated Ca2+ entry.