Chronology of UPR activation in skeletal muscle adaptations to chronic contractile activity

Abstract

The mitochondrial and endoplasmic reticulum unfolded protein responses (UPR(mt) and UPR(ER)) are important for cellular homeostasis during stimulus-induced increases in protein synthesis. Exercise triggers the synthesis of mitochondrial proteins, regulated in part by peroxisome proliferator activator receptor-γ coactivator 1α (PGC-1α). To investigate the role of the UPR in exercise-induced adaptations, we subjected rats to 3 h of chronic contractile activity (CCA) for 1, 2, 3, 5, or 7 days followed by 3 h of recovery. Mitochondrial biogenesis signaling, through PGC-1α mRNA, increased 14-fold after 1 day of CCA. This resulted in 10-32% increases in cytochrome c oxidase activity, indicative of mitochondrial content, between days 3 and 7, as well as increases in the autophagic degradation of p62 and microtubule-associated proteins 1A/1B light chain 3A (LC3)-II protein. Before these adaptations, the UPR(ER) transcripts activating transcription factor-4, spliced X-box-binding protein 1, and binding immunoglobulin protein were elevated (1.3- to 3.8-fold) at days 1-3, while CCAAT/enhancer-binding protein homologous protein (CHOP) and chaperones binding immunoglobulin protein and heat shock protein (HSP) 70 were elevated at mRNA and protein levels (1.5- to 3.9-fold) at days 1-7 of CCA. The mitochondrial chaperones 10-kDa chaperonin, HSP60, and 75-kDa mitochondrial HSP, the protease ATP-dependent Clp protease proteolytic subunit, and the regulatory protein sirtuin-3 of the UPR(mt) were concurrently induced 10-80% between days 1 and 7 To test the role of the UPR in CCA-induced remodeling, we treated animals with the endoplasmic reticulum stress suppressor tauroursodeoxycholic acid and subjected them to 2 or 7 days of CCA. Tauroursodeoxycholic acid attenuated CHOP and HSP70 protein induction; however, this failed to impact mitochondrial remodeling. Our data indicate that signaling to the UPR is rapidly activated following acute contractile activity, that this is attenuated with repeated bouts, and that the UPR is involved in chronic adaptations to CCA; however, this appears to be independent of CHOP signaling.

Regulation of the autophagy system during chronic contractile activity-induced muscle adaptations

“…; Memme et al. ), we observed that PGC‐1 α , a master regulator of mitochondrial biogenesis, was upregulated by CCA in skeletal muscle by day 7, which was supported by the increased whole muscle COX activity. Although COX I and COX IV protein levels were changed in a time‐dependent manner, they showed similar protein expression patterns with PGC‐1 α , suggesting the likelihood of concurrent CCA‐induced mitochondrial biogenesis.…”

“…; O'Leary and Hood ; Memme et al. ). Our findings provide evidence that the autophagy and lysosomal systems adapt prior to, or concomitant with, mitochondrial changes in skeletal muscle exposed to chronic contractile activity.…”

“…; Memme et al. ) under anesthesia with isoflurane, and small incisions were made to allow a sterile wire electrode (Medwire, Leico industries, New York, NY) to pass subcutaneously from the top of the back to the left hindlimb. The electrodes were sutured on both sides of peroneal nerve that innervates the tibialis anterior (TA) and extensor digitorum longus (EDL) muscles.…”

“…,b; Memme et al. ), whereby damaged and unnecessary cellular components are degraded and recycled, contributing to the maintenance or improvement of muscle quality following chronic exercise.…”

Function of specialized regulatory proteins and signaling pathways in exercise-induced muscle mitochondrial biogenesis

“…In order to provide a more complete documentation of the time course of UPR activation in response to acute exercise, and the adaptations to training, recent work in our laboratory has examined the activation of both the UPR ER and the UPR mt in response to repeated bouts of exercise of contractile activity in an animal model. Our data indicate that both UPRs are activated in the early stages of the contractile activity training program, preceding the adaptive responses in mitochondrial biogenesis and autophagy 88 . It should be noted that PGC-1α has been shown to be induced by, and to regulate, UPR signaling in an ATF6-dependent manner, indicating an integrated link between UPR and mitochondrial biogenesis pathways 85 .…”