Muscle Wasting in Aged, Sarcopenic Rats Is Associated with Enhanced Activity of the Ubiquitin Proteasome Pathway

Abstract

Among the hallmarks of aged organisms are an accumulation of misfolded proteins and a reduction in skeletal muscle mass ("sarcopenia"). We have examined the effects of aging and dietary restriction (which retards many age-related changes) on components of the ubiquitin proteasome system (UPS) in muscle. The hindlimb muscles of aged (30 months old) rats showed a marked loss of muscle mass and contained 2-3-fold higher levels of 26S proteasomes than those of adult (4 months old) controls. 26S proteasomes purified from muscles of aged and adult rats showed a similar capacity to degrade peptides, proteins, and an ubiquitylated substrate, but differed in levels of proteasome-associated proteins (e.g. the ubiquitin ligase E6AP and deubiquitylating enzyme USP14). Also, the activities of many other deubiquitylating enzymes were greatly enhanced in the aged muscles. Nevertheless, their content of polyubiquitylated proteins was higher than in adult animals. The aged muscles contained higher levels of the ubiquitin ligase CHIP, involved in eliminating misfolded proteins, and MuRF1, which ubiquitylates myofibrillar proteins. These muscles differed from ones rapidly atrophying due to disease, fasting, or disuse in that Atrogin-1/MAFbx expression was low and not inducible by glucocorticoids. Thus, the muscles of aged rats showed many adaptations indicating enhanced proteolysis by the UPS, which may enhance their capacity to eliminate misfolded proteins and seems to contribute to the sarcopenia. Accordingly, dietary restriction decreased or prevented the aging-associated increases in proteasomes and other UPS components and reduced muscle wasting.

Cellular Degradation Machineries in Age-Related Loss of Muscle Mass (Sarcopenia)

“…Since muscle wasting was reduced in these animals, these findings suggest that the buildup of proteasomes contributes to the loss of muscle mass in aged animals. In support of this notion we found that the levels of proteasome subunits in aged skeletal muscle were inversely correlated with muscle weight (subunits 1: r=-0.71, p<0.05; 5: r=-0.69; p<0.05), and no such inverse relationship was found in muscles of adult rats or aged animals maintained on dietary restriction (Altun et al, 2010). However, the underlying mechanism of this accumulation remains unclear.…”

“…A growing body of evidence suggests that loss of muscle mass in elderly occurs through mechanisms more complex than those involved in disuse and disease atrophy in younger individuals (Altun et al, 2010;Edstrom et al, 2007;Edstrom and Ulfhake, 2005). In humans and rodents alike, a number of mechanisms have been suggested to underpin sarcopenia: 1) loss of innervation 2) disuse 3) impaired maintenance and repair including decline in endo-, para-and autocrine signaling (e.g.…”

“…Because available data on the integrity of the UPS in the elderly was controversial we examined in 30-month old Sprague-Dawley rats the effects of aging on the content and activity of 26S proteasomes, proteasome-associated regulatory proteins and various other components of the UPS, including multiple DUBs (Altun et al, 2010). Muscles of these aged animals undergo marked atrophy compared to muscles of young adult animals and this age-related atrophy could be impeded if the animals were maintained on a restricted diet (Altun et al, 2007a).…”

“…4. Degradation capacity towards different substrates of proteasomes isolated from triceps surae muscle of adult rats (Ad-AL), aged (Ag-AL) and aged rats maintained on dietary restriction (Ag-DR) (original data reproduced from Altun et al, 2010). The higher content per unit muscle mass of proteasomes in aged (Ag-AL) muscles is paralleled by a corresponding increase in degradation of synthetic peptides designed for the chymotryptic and caspase sites of the proteasome (A) as well as casein (B).…”

Proteasomal activity‐based probes mark protein homeostasis in muscles

“…Decreased proteasome activity in human muscles is age‐associated,12 whereas in rats, proteasome activity in muscles increases with age 13. We reported that proteasome activity is reduced in a mouse model for muscle ageing 14.…”