Long-lived Snell dwarf mice display increased proteostatic mechanisms that are not dependent on decreased mTORC1 activity.

Joshua C Drake,Karyn L Hamilton,Benjamin F Miller,Danielle R Bruns,Richard A Miller,Frederick F Peelor,Laurie M Biela

doi:10.1111/acel.12329

Abstract

Maintaining proteostasis is thought to be a key factor in slowed aging. In several growth-restricted models of long-life, we have shown evidence of increased proteostatic mechanisms, suggesting that proteostasis may be a shared characteristic of slowed aging. The Snell dwarf mouse is generated through the mutation of the Pit-1 locus causing reductions in multiple hormonal growth factors and mTORC1 signaling. Snell dwarfs are one of the longest lived rodent models of slowed aging. We hypothesized that proteostatic mechanisms would be increased in Snell compared to control (Con) as in other models of slowed aging. Using D2O, we simultaneously assessed protein synthesis in multiple subcellular fractions along with DNA synthesis in skeletal muscle, heart, and liver over 2 weeks in both sexes. We also assessed mTORC1-substrate phosphorylation. Skeletal muscle protein synthesis was decreased in all protein fractions of Snell compared to Con, varied by fraction in heart, and was not different between groups in liver. DNA synthesis was lower in Snell skeletal muscle and heart but not in liver when compared to Con. The new protein to new DNA synthesis ratio was increased threefold in Snell skeletal muscle and heart compared to Con. Snell mTORC1-substrate phosphorylation was decreased only in heart and liver. No effect of sex was seen in this study. Together with our previous investigations in long-lived models, we provide evidence further supporting proteostasis as a shared characteristic of slowed aging and show that increased proteostatic mechanisms may not necessarily require a decrease in mTORC1.

Highlights

Protein turnover decreases with age, resulting in a progressive accumulation of damaged proteins and propagation of the aging phenotypeAccepted for publication 17 January 2015(Stadtman, 1988, 1992)
We demonstrate that proteostatic mechanisms, as assessed by the new protein to new DNA synthesis ratio, were increased by threefold in skeletal muscle and heart of Snell compared to normal controls, but independent of sex
The lack of consistency between inhibition of mTORC1 substrates related to protein synthesis and the increase in proteostatic mechanisms between models of slowed aging may suggest that other processes, such as autophagy and/or protein folding efficiency (Wang & Miller, 2012; Conn & Qian, 2013), which are partially regulated by mTORC1, are more pertinent to the increase in proteostatic mechanisms we have demonstrated in slowed aging models

Summary

Introduction

Protein turnover decreases with age, resulting in a progressive accumulation of damaged proteins and propagation of the aging phenotypeAccepted for publication 17 January 2015(Stadtman, 1988, 1992). As enzymatic capacity for protein repair is limited, cells must use alternative means for dealing with the age-dependent accumulation in protein damage (Mortimore & Poso, 1987; Mary et al, 2004). In tissues where apoptosis is maladaptive in dealing with accumulated damage due to a limited ability to create new cells (e.g., skeletal muscle), damaged proteins must be removed through autophagy and/or proteolysis and replaced through the synthesis of new proteins (Mortimore & Poso, 1987; Mary et al, 2004; Poppek & Grune, 2006). Adaptation to extra- and intracellular environmental stimuli requires increases in protein synthesis to maintain cellular homeostasis but does not necessarily include cell replication. We have proposed that simultaneously assessing both protein and DNA synthesis through deuterium oxide incorporation (D2O) can provide insight into what proportion of new proteins is made in new versus existing cells (Drake et al, 2014; Miller et al, 2014)

Results

Discussion

Conclusion