Pharmaceutical inhibition of mTOR in the common marmoset: Effect of rapamycin on regulators of proteostasis in a non-human primate

Background: Inhibition of mechanistic target of rapamycin (mTOR) has emerged as a viable means to lengthen lifespan and healthspan in mice, although it is still unclear whether these benefits will extend to other mammalian species. We previously reported results from a pilot experiment wherein common marmosets (Callithrix jacchus) were treated orally with rapamycin to reduce mTOR signaling in vivo in line with previous reports in mice and humans. Further, long-term treatment did not significantly alter body weight, daily activity, blood lipid concentrations, or glucose metabolism in this cohort. Methods: In this study, we report on the molecular consequences of rapamycin treatment in marmosets on mechanisms that regulate protein homeostasis (proteostasis) in vivo. There is growing appreciation for the role of proteostasis in longevity and for the role that mTOR plays in regulating this process. Tissue samples of liver and skeletal muscle from marmosets in our pilot cohort were assessed for expression and activity of components of the ubiquitin-proteasome system, macroautophagy, and protein chaperones. Results: Rapamycin treatment was associated with increased expression of PSMB5, a core subunit of the 20S proteasome, but not PSMB8 which is involved in the formation of the immunoproteasome, in the skeletal muscle and liver. Surprisingly, proteasome activity measured in these tissues was not affected by rapamycin. Rapamycin treatment was associated with an increased expression of mitochondria-targeted protein chaperones in skeletal muscle, but not liver. Finally, autophagy was increased in skeletal muscle and adipose, but not liver, from rapamycin-treated marmosets. Conclusions: Overall, these data show tissue-specific upregulation of some, but not all, components of the proteostasis network in common marmosets treated with a pharmaceutical inhibitor of mTOR.