Abstract
Tubular aggregates (TAs) in skeletal muscle fibers are unusual accumulation of sarcoplasmic reticulum (SR) tubes that are found in different disorders including TA myopathy (TAM). TAM is a muscular disease characterized by muscle pain, cramping, and weakness that has been recently linked to mutations in STIM1 and ORAI1. STIM1 and ORAI1 are the two main proteins mediating store-operated Ca2+ entry (SOCE), a mechanism activated by depletion of intracellular Ca2+ stores (e.g., SR) that allows recovery of Ca2+ from the extracellular space during repetitive muscle activity. We have recently shown that exercise triggers the formation of unique intracellular junctions between SR and transverse tubules named Ca2+ entry units (CEUs). CEUs promote colocalization of STIM1 with ORAI1 and improve muscle function in presence of external Ca2+. TAs virtually identical to those of TAM patients are also found in fast-twitch fibers of aging male mice. Here, we used a combination of electron and confocal microscopy, Western blotting, and ex vivo stimulation protocols (in presence or absence of external Ca2+) to evaluate the presence of TAs, STIM1-ORAI1 localization and expression and fatigue resistance of intact extensor digitorum longus (EDL) muscles in wild-type male adult (4-month-old) and aged (24-month-old) mice and in mice trained in wheel cages for 15 months (from 9 to 24 months of age). The results collected indicate that (i) aging causes STIM1 and ORAI1 to accumulate in TAs and (ii) long-term exercise significantly reduced formation of TAs. In addition, (iii) EDL muscles from aged mice exhibited a faster decay of contractile force than adult muscles, likely caused by their inability to refill intracellular Ca2+ stores, and (iv) exercise in wheel cages restored the capability of aged EDL muscles to use external Ca2+ by promoting maintenance of CEUs. In conclusion, exercise prevented improper accumulation of STIM1 and ORAI1 in TAs during aging, maintaining the capability of aged muscle to refill intracellular Ca2+ stores via SOCE.
Highlights
The sarcoplasmic reticulum (SR) is a highly organized system of membranes that functions as the main intracellular calcium (Ca2+) storage of skeletal muscle (Franzini-Armstrong, 1980; Boncompagni et al, 2020)
Fibers were double-labeled for ryanodine receptor type 1 (RYR1) and stromal-interacting molecule-1 (STIM1) (Figure 1A) and for RYR1 and ORAI1 (Figure 1B): RYR1 staining produced the typical transverse cross striation corresponding to the position of Ca2+ release unit (CRU) at the A–I band junction, on both sides of Z-lines (Figures 1A,B; red staining)
As ORAI1 is a Ca2+-permeable channel of the plasma membrane located in transverse tubule (TT) in skeletal muscle (Wei-Lapierre et al, 2013; Boncompagni et al, 2017), the lack of colocalization with RYR1 suggests that part of ORAI1 may be trapped inside the SR membranes of tubular aggregate (TA)
Summary
The sarcoplasmic reticulum (SR) is a highly organized system of membranes that functions as the main intracellular calcium (Ca2+) storage of skeletal muscle (Franzini-Armstrong, 1980; Boncompagni et al, 2020). The SR is composed of two distinct compartments in direct continuity with each other: the SR terminal cisternae or junctional SR (jSR) and the longitudinal SR (lSR) (Franzini-Armstrong, 1984). In adult mammalian skeletal muscle fibers, CRUs are placed in proximity of the A-I band transition of relaxed sarcomeres and contain the macromolecular complex that mediates excitation– contraction (EC) coupling (Schneider and Chandler, 1973; Schneider, 1994; Franzini-Armstrong and Protasi, 1997). EC coupling is the mechanism that translates the action potential carried in the fiber interior by TTs into Ca2+ release through RYRs. On the other hand, the lSR membranes are enriched in sarco/endoplasmic reticulum Ca2+ ATPases (SERCAs), which rapidly remove cytosolic Ca2+ released by RYRs during EC coupling to replenish SR lumen after each contraction (Inui and Fleischer, 1988). The lSR membranes are enriched in sarco/endoplasmic reticulum Ca2+ ATPases (SERCAs), which rapidly remove cytosolic Ca2+ released by RYRs during EC coupling to replenish SR lumen after each contraction (Inui and Fleischer, 1988). lSR extends on both sides of the terminal cisternae and may be placed either next to A or to I bands
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
