Exercise Prevents Formation of Tubular Aggregates in Ageing Skeletal Muscle Fibers of Wild-Type Mice

Abstract

Tubular aggregates (TAs) are ordered arrays of sarcoplasmic reticulum (SR) tubes found in muscle biopsies from patients affected by TA myopathy (TAM). TAM is a disorder linked to mutations in STIM1 and Orai1, the two main players in store-operated Ca2+ entry (SOCE), a mechanism that allows recovery of extracellular Ca2+ during repetitive muscle activity. TAs are also found in ageing 24 month old C57Bl6 mice in about 50% of fast-twitch EDL fibers (while they are not present in adult 4 month old mice) and, using immunofluorescence, we verified that such TAs are strongly positive for both STIM1 and Orai1. Measurements of contractile force during repetitive stimulation (30 consecutive tetanic pulses of 1 sec at 60 Hz applied every 5 seconds) revealed that 24 month old EDL muscles exhibit a faster decay of contractile force than adult (relative force at the 15th tetanus: 40.7±2.1% vs. 48.3.6±2.8% respectively). In addition, experiments performed in presence or absence of 2.5 mM extracellular Ca2+ suggest that the faster force decay of ageing EDLs is likely caused by their inability to recruit extracellular Ca2+. We finally analyzed EDL muscles from 24 month old mice that exercised voluntarily in wheel cages for 15 months (average of 396±29 km; n=3): formation of TAs was greatly reduced in trained vs. untrained age-matched mice (found in only 7% of fibers), while the capability of EDL muscles to maintain contractile force during repetitive stimulation was significantly rescued (relative force after 15th tetanus: 52.4±4.7%). Our findings suggest that: a) TAs accumulate dysfunctional STIM1 and Orai1; b) exercise limits formation of TAs and improves muscle function during repetitive stimulation, possibly improving the capability of aged muscle to use external Ca2+ via SOCE.