Dual roles of mTORC1-dependent activation of the ubiquitin-proteasome system in muscle proteostasis

Abstract

Muscle size is controlled by the PI3K-PKB/Akt-mTORC1-FoxO pathway, which integrates signals from growth factors, energy and amino acids to activate protein synthesis and inhibit protein breakdown. While mTORC1 activity is necessary for PKB/Akt-induced muscle hypertrophy, its constant activation alone induces muscle atrophy. Here we show that this paradox is based on mTORC1 activity promoting protein breakdown through the ubiquitin-proteasome system (UPS) by simultaneously inducing ubiquitin E3 ligase expression via feedback inhibition of PKB/Akt and proteasome biogenesis via Nuclear Factor Erythroid 2-Like 1 (Nrf1). Muscle growth was restored by reactivation of PKB/Akt, but not by Nrf1 knockdown, implicating ubiquitination as the limiting step. However, both PKB/Akt activation and proteasome depletion by Nrf1 knockdown led to an immediate disruption of proteome integrity with rapid accumulation of damaged material. These data highlight the physiological importance of mTORC1-mediated PKB/Akt inhibition and point to juxtaposed roles of the UPS in atrophy and proteome integrity.

Muscle fiber Myc is dispensable for muscle growth and forced expression severely perturbs homeostasis

“…7E), which were all downregulated in AktTG mice and upregulated in HSA-MycTG mice. Although the expression of ribosomal protein genes was suppressed in AktTG mice, translation of these genes is potently upregulated by PKB/Akt-mTORC1 pathway activity, resulting in higher ribosomal protein abundance [37]. Hence, despite the strong overlap of gene regulatory networks associated with cell growth, as previously observed [8], the resulting effect of Myc expression on post-mitotic muscle fibers is catastrophic, whereas PKBα/Akt1 activation results in functional hypertrophy.…”

“…In these mice, the PKBα/Akt1-ERT fusion protein is constitutively expressed in skeletal muscle fibers but immediately degraded and hence not active. Injection of tamoxifen, which binds ERT, stabilizes the PKBα/Akt1-ERT fusion protein, rendering it constitutively active [38, 39] and causing robust PKBα/Akt1-mediated muscle hypertrophy [37]. Just 3 days of tamoxifen injections was sufficient to promote measurable increases in QUAD, EDL, PLA, GAS and TRI muscle mass in female mice, while 14 days of tamoxifen treatment robustly increased the mass (25-75%) of all measured muscles, except the slow-twitch SOL muscle, which displayed just a minor ∼6% increase in mass (Fig.…”

“…As MOV-induced muscle hypertrophy was heavily MuSC-dependent, we next investigated the possibility that muscle fiber Myc expression is necessary to support myofiber hypertrophy. To this end, we crossed AktTG mice expressing an active, myristoylated form of PKBα/Akt1, fused to eGFP and ERT2 in skeletal muscle [37] with HSA-MycKO mice to create AktTG and AktTG; HSA-MycKO (Akt-MycKO) mice (Fig. 5A).…”

Nfe2l1-mediated proteasome function controls muscle energy metabolism in obesity

“…However, loss of Nfe2l1 did not completely abolish proteasomal activity, which indicates that the baseline of proteasome subunit gene expression is likely mediated by a different, less adaptive mechanism, which could be mediated by e.g., FoxO1 27 . Nfe2l1 is also linked to mTORC1 signaling, as sustained genetic activation of mTORC1 promotes Nfe2l1 activation, proteasomal activity, and muscle atrophy 28 . However, this study also demonstrated juxtaposed roles of the UPS in atrophy and proteome integrity, which is in line with our findings that loss of Nfe2l1 creates a lean phenotype.…”

Overexpression of Nfe2l1 increases proteasome activity and delays vision loss in a preclinical model of human blindness

“…An alternative approach to increase the proteolytic capacity of cells and treat human diseases caused by protein misfolding might include increasing the total proteasome pool. Several studies in diverse experimental systems, including mouse fibroblasts, the brain, the retina, and muscles, reported higher levels of proteasomes under genetic activation of the mTORC1 (mechanistic target of rapamycin complex 1) pathway (22)(23)(24)(25)(26). The mechanisms driving this transcriptional program have not been fully characterized but are proposed to be triggered by sterol-regulatory element binding protein 1 (Srebp1)-mediated transcriptional upregulation of the nuclear factor erythroid-2-like 1 (Nfe2l1) transcription factor (22,27).…”