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

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.

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

“…Moreover, a growing body of evidence indicates that loss- and gain-of-function mutations in both STIM1 and ORAI1 are linked to multiple human diseases in which myopathy is a prominent clinical manifestation [ 57 , 58 ]. In addition, dysfunctional or enhanced STIM1/ORAI1-mediated SOCE is also implicated in the pathogenesis of other muscle disorders including muscular dystrophy [ 59 , 60 , 61 ], malignant hyperthermia [ 62 ], and sarcopenia [ 63 , 64 ]. Together, these findings demonstrate that a tight regulation of STIM1/ORAI1-dependent SOCE is critical for optimal muscle performance and that aberrant SOCE function contributes to muscle disease.…”

“…In support of this idea, muscles from 2-year-old mice exhibit reduced SOCE and increased susceptibility to fatigue during high-frequency stimulation [ 143 , 144 ]. We recently reported that extensor digitorum longus muscles from 2-year-old mice exhibited an accelerated decline in force generation during high-frequency stimulation compared to that of muscles from 4-month-old mice [ 64 ]. Consistent with a reduced role for Ca 2+ entry in aged muscle, removal of Ca 2+ from the extracellular medium during repetitive high-frequency stimulation reduced contractility of muscles from young, but not aged, mice.…”

Store-Operated Calcium Entry in Skeletal Muscle: What Makes It Different?

“…Another interesting point is related to the formation of tubular aggregates (TAs), which are abnormal structures of the SR that strongly express STIM1 and Orai1 [ 222 ]. In humans, TAs are described in patients with a gain-of-function mutation of Orai1 or STIM1 (rev in [ 216 ]).…”

“…In mice, TAs are found in old animals almost exclusively in type IIB fibers [ 223 ], which are the fastest, most easily tired ones, but also the least used fibers (rev in [ 224 ]). Interestingly, when older mice were allowed to run in a wheel during most of their life span (between 9 and 24 months), the formation of TAs was reduced [ 222 ]. In this case, the old muscles from trained mice required extracellular Ca 2+ entry for a stronger contraction and are sensitive to SOCE inhibitors [ 222 ], which was not the case in old, untrained animals [ 190 ].…”

“…Interestingly, when older mice were allowed to run in a wheel during most of their life span (between 9 and 24 months), the formation of TAs was reduced [ 222 ]. In this case, the old muscles from trained mice required extracellular Ca 2+ entry for a stronger contraction and are sensitive to SOCE inhibitors [ 222 ], which was not the case in old, untrained animals [ 190 ]. Indeed, as mentioned previously ( Section 2.2 ), it was proposed that in old animals, SOCE is not required to maintain force, essentially because SOCE is much less functional with age [ 73 ].…”