eIF3f depletion impedes mouse embryonic development, reduces adult skeletal muscle mass and amplifies muscle loss during disuse.

KEY POINTS: In muscular cells, eukaryotic initiation factor subunit f (eIF3f) activates protein synthesis by allowing physical interaction between mechanistic target of rapamycin complex 1 (MTORC1) and ribosomal protein S6 kinase 1 (S6K1), although its physiological role in animals is unknown. A knockout approach suggests that homozygous mice carrying a null mutation of the eIF3f gene fail to develop and consequently die at early embryonic stage, whereas heterozygous mice associated with a partial depletion of eIF3f gene grow normally and are phenotypically indistinguishable from wild-type mice. Heterozygous mice express reduced eIF3f mRNA and protein levels in skeletal muscles and show diminished muscle mass associated with a decrease in the protein synthesis rate and an inhibition of the MTORC1 pathway. During hindlimb immobilization, heterozygous eIF3f mice display an exacerbated immobilization-induced muscle atrophy associated with reduced protein synthesis. These results highlight the essential role of eIF3f during embryonic development and its involvement in muscular homeostasis via protein synthesis regulation. ABSTRACT: Eukaryotic translation initiation factor 3, subunit F (eIF3f), a component of eIF3 complex, plays an important role in protein synthesis regulation, although its physiological functions are unknown. We generated and analysed mice carrying a null mutation in the eIF3f gene. We showed that homozygous eIF3f knockout fail to develop and that eIF3f-/- embryos die at an early stage of development but after the pre-implantation stage. However, disrupting one eIF3f allele does not affect growth, viability and fertility of heterozygous mice but, instead, reduces eIF3f mRNA and protein levels in all tissues examined. Although heterozygous mice are phenotypically indistinguishable from wild-type mice, they present a diminished body weight and a lean mass reduction associated with normal body size. Interestingly, skeletal muscles are mainly affected and display an altered cell size without modification of fibre number. Skeletal muscles of heterozygous mice show a deficiency in polysome content, a decrease in protein synthesis rate and an inhibition of the mechanistic target of rapamycin (MTOR) pathway. We then studied the effects of hindlimb immobilization that mimic muscle disuse on heterozygous mice aiming to further explore the involvement of eIF3f in protein synthesis. We found that eIF3f partial depletion amplifies muscle atrophy compared to wild-type mice. Mass and cross-sectional area decreases were associated with reduced MTOR pathway activation and protein synthesis rate. Taken together, our data indicate that eIF3f is essential for mice embryonic development and controls adult skeletal muscle mass via protein synthesis regulation in a MTOR-dependent manner.

eIF3f: a central regulator of the antagonism atrophy/hypertrophy in skeletal muscle.

The eukaryotic initiation factor 3 subunit f (eIF3f) is one of the 13 subunits of the translation initiation factor complex eIF3 required for several steps in the initiation of mRNA translation. In skeletal muscle, recent studies have demonstrated that eIF3f plays a central role in skeletal muscle size maintenance. Accordingly, eIF3f overexpression results in hypertrophy through modulation of protein synthesis via the mTORC1 pathway. Importantly, eIF3f was described as a target of the E3 ubiquitin ligase MAFbx/atrogin-1 for proteasome-mediated breakdown under atrophic conditions. The biological importance of the MAFbx/atrogin-1-dependent targeting of eFI3f is highlighted by the finding that expression of an eIF3f mutant insensitive to MAFbx/atrogin-1 polyubiquitination is associated with enhanced protection against starvation-induced muscle atrophy. A better understanding of the precise role of this subunit should lead to the development of new therapeutic approaches to prevent or limit muscle wasting that prevails in numerous physiological and pathological states such as immobilization, aging, denervated conditions, neuromuscular diseases, AIDS, cancer, diabetes. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

The role of AMP-activated protein kinase in the coordination of skeletal muscle turnover and energy homeostasis.

The AMP-activated protein kinase (AMPK) is a serine/threonine protein kinase that acts as a sensor of cellular energy status switch regulating several systems including glucose and lipid metabolism. Recently, AMPK has been implicated in the control of skeletal muscle mass by decreasing mTORC1 activity and increasing protein degradation through regulation of ubiquitin-proteasome and autophagy pathways. In this review, we give an overview of the central role of AMPK in the control of skeletal muscle plasticity. We detail particularly its implication in the control of the hypertrophic and atrophic signaling pathways. In the light of these cumulative and attractive results, AMPK appears as a key player in regulating muscle homeostasis and the modulation of its activity may constitute a therapeutic potential in treating muscle wasting syndromes in humans.

AMPK promotes skeletal muscle autophagy through activation of forkhead FoxO3a and interaction with Ulk1.

In skeletal muscle, protein levels are determined by relative rates of protein synthesis and breakdown. The balance between synthesis and degradation of intracellular components determines the overall muscle fiber size. AMP-activated protein kinase (AMPK), a sensor of cellular energy status, was recently shown to increase myofibrillar protein degradation through the expression of MAFbx and MuRF1. In the present study, the effect of AMPK activation by AICAR on autophagy was investigated in muscle cells. Our results show that FoxO3a transcription factor activation by AMPK induces the expression of the autophagy-related proteins LC3B-II, Gabarapl1, and Beclin1 in primary mouse skeletal muscle myotubes and in the Tibialis anterior (TA) muscle. Time course studies reveal that AMPK activation by AICAR leads to a transient nuclear relocalization of FoxO3a followed by an increase of its cytosolic level. Moreover, AMPK activation leads to the inhibition of mTORC1 and its subsequent dissociation of Ulk1, Atg13, and FIP200 complex. Interestingly, we identify Ulk1 as a new interacting partner of AMPK in muscle cells and we show that Ulk1 is associated with AMPK under normal conditions and dissociates from AMPK during autophagy process. Moreover, we find that AMPK phosphorylates FoxO3a and Ulk1. In conclusion, our data show that AMPK activation stimulates autophagy in skeletal muscle cells through its effects on the transcriptional function of FoxO3a and takes part in the initiation of autophagosome formation by interacting with Ulk1. Here, we present new evidences that AMPK plays a crucial role in the fine tuning of protein expression programs that control skeletal muscle mass.