Impact of Aging and Disuse on Markers of Endoplasmic Reticulum Stress and the Unfolded Protein Response

Abstract

Aging and disuse can impair skeletal muscle responses to stimuli and are key drivers in dysregulated proteostasis. Such dysfunction can lead to an accumulation of misfolded proteins resulting in endoplasmic reticulum stress (ERS) and the unfolded protein response (UPR). While the UPR blunts global translation and enhances the production of chaperones and foldases, little is known about ERS responses across the lifespan and in disuse contexts. Therefore, the purposes of this study were to examine if: i) the expression of ERS and UPR effector proteins differ across the lifespan in rats, and ii) age-associated patterns were recapitulated by 10 days of hindlimb casting in a separate cohort of rats. We hypothesized that aging would enhance the expression of ERS related proteins, and this expression pattern would be mimicked by 10 days of limb disuse. Male Fischer 344 rats were sacrificed at 3, 6, 12, 18, and 24 months (mo) of age, and plantaris (PLT) and soleus (SOL) muscles were collected for analysis. Six mo female Wistar rats were sacrificed after either 10 days of hindlimb casting or continued ambulation (age matched control), after which whole gastrocnemius muscles were collected for analysis. The effector eukaryotic initiation factor 2 alpha (eIF2a) was different across the lifespan (phospho/pan) in PLT and SOL muscles (p<0.001) and was upregulated in the PLT at 24 mo as compared to 3 mo (P=0.006). The downstream UPR effector C/EBP homologous protein (CHOP) was differentially expressed across the lifespan in PLT and SOL (p<0.001). CHOP was upregulated ~2.79 and 3.08-fold at 18 and 24 mo respectively in the SOL (p<0.001), and 2.62 and 2.44-fold at 18 and 24 mo respectively in the PLT. Finally, principal effector Activating Transcription Factor 6 (ATF6) was differentially expressed across the lifespan (cleaved/pan) in the PLT and SOL (P≤0.024) whereby a 1.23-fold up-regulation was evident at 18 mo in the SOL (P=0.035) and 1.34-fold at 18 mo in the PLT (P=0.026). The CHOP and ATF6 proteins were similarly dysregulated in our disuse model, whereby 1.83-fold (p<0.001) and 1.30-fold (P=0.002) greater values were observed in hind limb casted versus control rats. Effectors of the UPR pathway are upregulated in skeletal muscle across the lifespan in rats, and this effect is recapitulated with disuse. These findings warrant further research for potential therapeutics related to age- and disuse-related skeletal muscle atrophy. Funding for this study was provided by discretionary laboratory funds from MDR. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.