Endoplasmic reticulum stress in human skeletal muscle: any contribution to sarcopenia?

Redox control of protein degradation

Intracellular proteolysis is critical to maintain timely degradation of altered proteins including oxidized proteins. This review attempts to summarize the most relevant findings about oxidant protein modification, as well as the impact of reactive oxygen species on the proteolytic systems that regulate cell response to an oxidant environment: the ubiquitin-proteasome system (UPS), autophagy and the unfolded protein response (UPR). In the presence of an oxidant environment, these systems are critical to ensure proteostasis and cell survival. An example of altered degradation of oxidized proteins in pathology is provided for neurodegenerative diseases. Future work will determine if protein oxidation is a valid target to combat proteinopathies.

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Proteins undergo reversible and irreversible redox modifications.

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Oxidized proteins are cleared mainly through the 20S proteasome and autophagy.

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The proteolytic systems exhibit a dynamic crosstalk to adapt to redox alterations.

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Protein oxidation together with impaired degradation are linked to neurodegeneration.

Fyn Activation of mTORC1 Stimulates the IRE1α-JNK Pathway, Leading to Cell Death

We previously reported that the skeletal muscle-specific overexpression of Fyn in mice resulted in a severe muscle wasting phenotype despite the activation of mTORC1 signaling. To investigate the bases for the loss of muscle fiber mass, we examined the relationship between Fyn activation of mTORC1, JNK, and endoplasmic reticulum stress. Overexpression of Fyn in skeletal muscle in vivo and in HEK293T cells in culture resulted in the activation of IRE1α and JNK, leading to increased cell death. Fyn synergized with the general endoplasmic reticulum stress inducer thapsigargin, resulting in the activation of IRE1α and further accelerated cell death. Moreover, inhibition of mTORC1 with rapamycin suppressed IRE1α activation and JNK phosphorylation, resulting in protecting cells against Fyn- and thapsigargin-induced cell death. Moreover, rapamycin treatment in vivo reduced the skeletal muscle IRE1α activation in the Fyn-overexpressing transgenic mice. Together, these data demonstrate the presence of a Fyn-induced endoplasmic reticulum stress that occurred, at least in part, through the activation of mTORC1, as well as subsequent activation of the IRE1α-JNK pathway driving cell death.

Increased endoplasmic reticulum stress in mouse osteocytes with aging alters Cox-2 response to mechanical stimuli

Aging reduces bone mass as well as the anabolic response of bone to mechanical stimuli, resulting in osteopenia. Endoplasmic reticulum (ER) stress impairs the response of myogenic cells to anabolic stimuli, and is involved in sarcopenia, but whether ER stress also contributes to osteopenia is unknown. Therefore, we tested whether ER stress exists in bones of aged mice, and whether this impairs the osteocyte response to mechanical stimulation. Primary osteocytes were obtained from long bones of adult (8 months) and old (24-26 months) mice, treated with or without the pharmacological ER stress inducer tunicamycin, and either or not subjected to mechanical loading by pulsating fluid flow (PFF). The osteocyte response to PFF was assessed by measuring cyclooxygenase-2 (Cox-2) mRNA levels and nitric oxide (NO) production. mRNA levels of ER stress markers were higher in old versus adult osteocytes (+40% for activating transcription factor-4, +120% for C/EBP homologous protein, and +120% for spliced X-box binding protein-1, p < 0.05). The Cox-2 response to PFF was fourfold decreased in cells from old bones (p < 0.001), while tunicamycin decreased PFF-induced Cox-2 expression by threefold in cells from adult bones (p < 0.01). PFF increased NO production by 50% at 60 min in osteocytes from old versus adult bones (p < 0.01). In conclusion, our data indicate that the expression of several ER stress markers was higher in osteocytes from bones of old compared to adult mice. Since ER stress altered the response of osteocytes to mechanical loading, it could be a novel factor contributing to osteopenia.

Sarcopenia and sarcopenic obesity in patients with muscular dystrophy

Aging sarcopenia and muscular dystrophy (MD) are two conditions characterized by lower skeletal muscle quantity, lower muscle strength, and lower physical performance. Aging is associated with a peculiar alteration in body composition called “sarcopenic obesity” characterized by a decrease in lean body mass and increase in fat mass. To evaluate the presence of sarcopenia and obesity in a cohort of adult patients with MD, we have used the measurement techniques considered golden standard for sarcopenia that is for muscle mass dual-energy X-ray absorptiometry (DXA), for muscle strength hand-held dynamometry (HHD), and for physical performance gait speed. The study involved 14 adult patients with different types of MD. We were able to demonstrate that all patients were sarcopenic obese. We showed, in fact, that all were sarcopenic based on appendicular lean, fat and bone free, mass index (ALMI). In addition, all resulted obese according to the percentage of body fat determined by DXA in contrast to their body mass index ranging from underweight to obese. Skeletal muscle mass determined by DXA was markedly reduced in all patients and correlated with residual muscle strength determined by HHD, and physical performances determined by gait speed and respiratory function. Finally, we showed that ALMI was the best linear explicator of muscle strength and physical function. Altogether, our study suggests the relevance of a proper evaluation of body composition in MD and we propose to use, both in research and practice, the measurement techniques that has already been demonstrated effective in aging sarcopenia.