Calcium entry units (CEUs): perspectives in skeletal muscle function and disease

Abstract

In the last decades the term Store-operated Ca 2+ entry (SOCE) has been used in the scientific literature to describe an ubiquitous cellular mechanism that allows recovery of calcium (Ca 2+) from the extracellular space. SOCE is triggered by a reduction of Ca 2+ content (i.e. depletion) in intracellular stores, i.e. endoplasmic or sarcoplasmic reticulum (ER and SR). In skeletal muscle the mechanism is primarily mediated by a physical interaction between stromal interaction molecule-1 (STIM1), a Ca 2+ sensor located in the SR membrane, and ORAI1, a Ca 2+-permeable channel of external membranes, located in transverse tubules (TTs), the invaginations of the plasma membrane (PM) deputed to propagation of action potentials. It is generally accepted that in skeletal muscle SOCE is important to limit muscle fatigue during repetitive stimulation. We recently discovered that exercise promotes the assembly of new intracellular junctions that contains colocalized STIM1 and ORAI1, and that the presence of these new junctions increases Ca 2+ entry via ORAI1, while improving fatigue resistance during repetitive stimulation. Based on these findings we named these new junctions Ca 2+ Entry Units (CEUs). CEUs are dynamic organelles that assemble during muscle activity and disassemble during recovery thanks to the plasticity of the SR (containing STIM1) and the elongation/retraction of TTs (bearing ORAI1). Interestingly, similar structures described as SR stacks were previously reported in different mouse models carrying mutations in proteins involved in Ca 2+ handling (calsequestrin-null mice; triadin and junctin null mice, etc.) or associated to microtubules (MAP6 knockout mice). Mutations in Stim1 and Orai1 (and calsequestrin-1) genes have been associated to tubular aggregate myopathy (TAM), a muscular disease characterized by: (a) muscle pain, cramping, or weakness that begins in childhood and worsens over time, and (b) the presence of large accumulations of ordered SR tubes (tubular aggregates, TAs) that do not contain myofibrils, mitochondria, nor TTs. Interestingly, TAs are also present in fast twitch muscle fibers of ageing mice. Several important issues remain unanswered: (a) the molecular mechanisms and signals that trigger the remodeling of membranes and the functional activation of SOCE during exercise are unclear; and (b) how dysfunctional SOCE and/or mutations in Stim1, Orai1 and calsequestrin (Casq1) genes lead to the formation of tubular aggregates (TAs) in aging and disease deserve investigation.

Altered Ca2+ Handling and Oxidative Stress Underlie Mitochondrial Damage and Skeletal Muscle Dysfunction in Aging and Disease

“…Second, acute exercise drives the formation of new junctions within the I band of the sarcomere between SR-stacks and extension of T-tubule. These exercise-induced SR-T-tubule junctions are structurally distinct from triads as they lack RyR1 and DHPR proteins, but contain co-localized STIM1 and ORAI1 proteins that mediate SOCE, and thus, are referred to as “Ca 2+ entry units” (CEUs) [ 44 , 45 , 46 , 49 ]. CEUs are also present, though fewer in number, in resting (non-exercised) muscle.…”

Long-Term Exercise Reduces Formation of Tubular Aggregates and Promotes Maintenance of Ca2+ Entry Units in Aged Muscle

“…Under resting conditions in muscle, ORAI1 is located within the TT system, whereas STIM1 is mainly positioned throughout the lSR at the I band region of the sarcomere (Wei-Lapierre et al, 2013;Carrell et al, 2016;Boncompagni et al, 2017). We have recently shown that, in extensor digitorum longus (EDL) muscle fibers from adult wild-type (WT) mice, acute exercise drives the formation of Ca 2+ entry units (CEUs), new intracellular junctions between stacks of lSR membranes, and TT extensions within the I band that contain colocalized STIM1 and ORAI1 (Boncompagni et al, 2017(Boncompagni et al, , 2018Protasi et al, 2020). Interestingly, CEUs are constitutively assembled in fibers of mice lacking CASQ1, which undergoes severe Ca 2+ depletion during repetitive high-frequency stimulation (Michelucci et al, 2020).…”

Muscle: 50+ Years of Electron Microscopy

“…SR membrane stacs are not frequent in quiescent muscle, but increase in frequency under regimen of strenuous exercise and are more frequent in aging muscle; in fibers deprived of junctin/triadin (2.10.2 B); in muscles involved in repetitive contractions as in sound emission,(2.10.2 C) and in the legs of finches (2.10.2 D). Boncompagni et al, 2010Boncompagni et al, , 2017Protasi et al, 2020.…”