Ectopic expression of eIF2Bepsilon in rat skeletal muscle rescues the sepsis-induced reduction in guanine nucleotide exchange activity and protein synthesis

Glucose-Induced Activation of mTORC1 is Associated with Hexokinase2 Binding to Sestrins in HEK293T Cells

BACKGROUND

Sestrins (SESN1-3) act as proximal sensors in leucine-induced activation of the protein kinase mechanistic target of rapamycin (mTOR) in complex 1 (mTORC1), a key regulator of cell growth and metabolism.

OBJECTIVE

In the present study, the hypothesis that SESNs also mediate glucose-induced activation of mTORC1 was tested.

METHODS

Rats underwent overnight fasting, and in the morning, either saline or a glucose solution (4 g⋅kg-1 BW/10 mL⋅kg-1) was administered by oral gavage; mTORC1 activation in the tibialis anterior muscle was assessed. To further assess the mechanism through which glucose promotes mTORC1 activation, wild-type (WT) HEK293T and HEK293T cells lacking either all 3 SESNs (SESNTKO) or hexokinase 2 (HK2KO) were deprived of glucose, followed by glucose addback, and mTORC1 activation was assessed. In addition, glucose-induced changes in the association of the SESNs with components of the GAP activity toward the Rags (GATOR2) complex and with hexokinase 2 (HK2) were assessed by co-immunoprecipitation. One- and two-way ANOVA with Tukey post hoc comparisons were used.

RESULTS

Glucose administration to fasted rats promoted mTORC1 activation. Similarly, glucose readdition (GluAB) to the medium of glucose-deprived WT cells also promoted mTORC1 activation. By contrast, SESNTKO cells demonstrated attenuated mTORC1 activation following GluAB compared with WT cells. Interestingly, HK2 associated with all 3 SESNs in a glucose-dependent manner, i.e., HK2 abundance in SESN immunoprecipitates was high in cells deprived of glucose and decreased in response to GluAB. Moreover, similar to SESNTKO cells, the sensitivity of mTORC1 to GluAB was attenuated in HK2KO cells compared with WT cells.

CONCLUSIONS

The results of this study demonstrate that the SESNs and HK2 play important roles in glucose-induced mTORC1 activation in HEK293T cells. However, unlike leucine-induced mTORC1 activation, the effect was independent of the changes in SESN-GATOR2 interaction, and instead, it was associated with alterations in the association of SESNs with HK2.

CORP: Gene delivery into murine skeletal muscle using in vivo electroporation

The strategy of gene delivery into skeletal muscles has provided exciting avenues in identifying new potential therapeutics towards muscular disorders and addressing basic research questions in muscle physiology through overexpression and knockdown studies. In vivo electroporation methodology offers a simple, rapidly effective technique for the delivery of plasmid DNA into post-mitotic skeletal muscle fibers and the ability to easily explore the molecular mechanisms of skeletal muscle plasticity. The purpose of this review is to describe how to robustly electroporate plasmid DNA into different hindlimb muscles of rodent models. Further, key parameters (e.g., voltage, hyaluronidase, plasmid concentration) which contribute to the successful introduction of plasmid DNA into skeletal muscle fibers will be discussed. In addition, details on processing tissue for immunohistochemistry and fiber cross-sectional area (CSA) analysis will be outlined. The overall goal of this review is to provide the basic and necessary information needed for successful implementation of in vivo electroporation of plasmid DNA and thus open new avenues of discovery research in skeletal muscle physiology.

Age-related responses to a bout of mechanotherapy in skeletal muscle of rats

Cyclic compressive loading (CCL) is a massage mimetic that improves muscle regrowth from atrophy in adult rats. Therefore, we tested if a single bout of CCL increases anabolic signaling and protein synthesis in muscle during normal, weight-bearing conditions in gastrocnemius muscle from adult and aged rats. Male Brown Norway/F344 rats at 10 (adult) and 30 (aged) months of age were assigned control or CCL (receiving a single bout of CCL). Twenty-four hours following a single bout of CCL there was no change in protein synthesis, Akt, or GSK3β signaling at either age, despite adult rats having higher abundance and activation of mechanosensitive pathways (integrins and integrin-linked kinase). Murf1 was elevated in response to CCL in both age groups, potentially indicating muscle remodeling. Muscle from aged rats exhibited an increase in heat shock protein (HSP) 25 and HSP70 and in the cold shock protein RNA-binding motif 3 (RBM3), demonstrating a unique stress response to CCL in aged muscle only. Finally, muscle from aged rats exhibited higher basal protein synthesis that was corroborated by elevated eIF2Bε and rpS6 signaling, without an additional effect of CCL. In summary, a single bout of CCL does not have anabolic effects on skeletal muscle during normal, weight-bearing conditions, even though it has previously been shown to improve regrowth from atrophy. These data demonstrate that interventions that may help recover from atrophy do not necessarily induce muscle hypertrophy in unperturbed conditions.NEW & NOTEWORTHY Massage has been demonstrated to be an effective mechanotherapy to improve recovery from atrophy in adult skeletal muscle; however, this study shows that a single bout of massage fails to increase protein synthesis or anabolic signaling in adult or aged skeletal muscle during normal, weight-bearing conditions. Altogether, our data suggest massage is a useful mechanotherapy for preserving skeletal muscle when combined with other interventions but is not an anabolic stimulus on its own.

Evidence for a role for Sestrin1 in mediating leucine-induced activation of mTORC1 in skeletal muscle

Previous studies established that leucine stimulates protein synthesis in skeletal muscle to the same extent as a complete mixture of amino acids, and the effect occurs through activation of the mechanistic target of rapamycin in complex 1 (mTORC1). Recent studies using cells in culture showed that the Sestrins bind leucine and are required for leucine-dependent activation of mTORC1. However, the role they play in mediating leucine-dependent activation of the kinase in vivo has been questioned because the dissociation constant of Sestrin2 for leucine is well below circulating and intramuscular levels of the amino acid. The goal of the present study was to compare expression of the Sestrins in skeletal muscle to other tissues and to assess their relative role in mediating activation of mTORC1 by leucine. The results show that the relative expression of the Sestrin proteins varies widely among tissues and that in skeletal muscle Sestrin1 expression is higher than Sestrin3, whereas Sestrin2 expression is markedly lower. Analysis of the dissociation constants of the Sestrins for leucine as assessed by leucine-induced dissociation of the Sestrin·GAP activity toward Rags 2 (GATOR2) complex revealed that Sestrin1 has the highest affinity for leucine and that Sestrin3 has the lowest affinity. In agreement with the dissociation constants calculated using cells in culture, oral leucine administration promotes disassembly of the Sestrin1·GATOR2 complex but not the Sestrin2 or Sestrin3·GATOR2 complex. Overall, the results presented herein are consistent with a model in which leucine-induced activation of mTORC1 in skeletal muscle in vivo occurs primarily through release of Sestrin1 from GATOR2.

Regulation of muscle protein synthesis and the effects of catabolic states

Protein synthesis and degradation are dynamically regulated processes that act in concert to control the accretion or loss of muscle mass. The present article focuses on the mechanisms involved in the impairment of protein synthesis that are associated with skeletal muscle atrophy. The vast majority of mechanisms known to regulate protein synthesis involve modulation of the initiation phase of mRNA translation, which comprises a series of reactions that result in the binding of initiator methionyl-tRNAi and mRNA to the 40S ribosomal subunit. The function of the proteins involved in both events has been shown to be repressed under atrophic conditions such as sepsis, cachexia, chronic kidney disease, sarcopenia, and disuse atrophy. The basis for the inhibition of protein synthesis under such conditions is likely to be multifactorial and includes insulin/insulin-like growth factor 1 resistance, pro-inflammatory cytokine expression, malnutrition, corticosteroids, and/or physical inactivity. The present article provides an overview of the existing literature regarding mechanisms and signaling pathways involved in the regulation of mRNA translation as they apply to skeletal muscle wasting, as well as the efficacy of potential clinical interventions such as nutrition and exercise in the maintenance of skeletal muscle protein synthesis under atrophic conditions. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

A self-defeating anabolic program leads to β-cell apoptosis in endoplasmic reticulum stress-induced diabetes via regulation of amino acid flux

Endoplasmic reticulum (ER) stress-induced responses are associated with the loss of insulin-producing β-cells in type 2 diabetes mellitus. β-Cell survival during ER stress is believed to depend on decreased protein synthesis rates that are mediated via phosphorylation of the translation initiation factor eIF2α. It is reported here that chronic ER stress correlated with increased islet protein synthesis and apoptosis in β-cells in vivo. Paradoxically, chronic ER stress in β-cells induced an anabolic transcription program to overcome translational repression by eIF2α phosphorylation. This program included expression of amino acid transporter and aminoacyl-tRNA synthetase genes downstream of the stress-induced ATF4-mediated transcription program. The anabolic response was associated with increased amino acid flux and charging of tRNAs for branched chain and aromatic amino acids (e.g. leucine and tryptophan), the levels of which are early serum indicators of diabetes. We conclude that regulation of amino acid transport in β-cells during ER stress involves responses leading to increased protein synthesis, which can be protective during acute stress but can lead to apoptosis during chronic stress. These studies suggest that the increased expression of amino acid transporters in islets can serve as early diagnostic biomarkers for the development of diabetes.

The mTORC1 signaling repressors REDD1/2 are rapidly induced and activation of p70S6K1 by leucine is defective in skeletal muscle of an immobilized rat hindlimb

Limb immobilization, limb suspension, and bed rest cause substantial loss of skeletal muscle mass, a phenomenon termed disuse atrophy. To acquire new knowledge that will assist in the development of therapeutic strategies for minimizing disuse atrophy, the present study was undertaken with the aim of identifying molecular mechanisms that mediate control of protein synthesis and mechanistic target of rapamycin complex 1 (mTORC1) signaling. Male Sprague-Dawley rats were subjected to unilateral hindlimb immobilization for 1, 2, 3, or 7 days or served as nonimmobilized controls. Following an overnight fast, rats received either saline or L-leucine by oral gavage as a nutrient stimulus. Hindlimb skeletal muscles were extracted 30 min postgavage and analyzed for the rate of protein synthesis, mRNA expression, phosphorylation state of key proteins in the mTORC1 signaling pathway, and mTORC1 signaling repressors. In the basal state, mTORC1 signaling and protein synthesis were repressed within 24 h in the soleus of an immobilized compared with a nonimmobilized hindlimb. These responses were accompanied by a concomitant induction in expression of the mTORC1 repressors regulated in development and DNA damage responses (REDD) 1/2. The nutrient stimulus produced an elevation of similar magnitude in mTORC1 signaling in both the immobilized and nonimmobilized muscle. In contrast, phosphorylation of 70-kDa ribosomal protein S6 kinase 1 (p70S6K1) on Thr(229) and Thr(389) in response to the nutrient stimulus was severely blunted. Phosphorylation of Thr(229) by PDK1 is a prerequisite for phosphorylation of Thr(389) by mTORC1, suggesting that signaling through PDK1 is impaired in response to immobilization. In conclusion, the results show an immobilization-induced attenuation of mTORC1 signaling mediated by induction of REDD1/2 and defective p70S6K1 phosphorylation.

The role of mTORC1 in regulating protein synthesis and skeletal muscle mass in response to various mechanical stimuli

Skeletal muscle plays a fundamental role in mobility, disease prevention, and quality of life. Skeletal muscle mass is, in part, determined by the rates of protein synthesis, and mechanical loading is a major regulator of protein synthesis and skeletal muscle mass. The mammalian/mechanistic target of rapamycin (mTOR), found in the multi-protein complex, mTORC1, is proposed to play an essential role in the regulation of protein synthesis and skeletal muscle mass. The purpose of this review is to examine the function of mTORC1 in relation to protein synthesis and cell growth, the current evidence from rodent and human studies for the activation of mTORC1 signaling by different types of mechanical stimuli, whether mTORC1 signaling is necessary for changes in protein synthesis and skeletal muscle mass that occur in response to different types of mechanical stimuli, and the proposed molecular signaling mechanisms that may be responsible for the mechanical activation of mTORC1 signaling.

Toll-like receptor activation suppresses ER stress factor CHOP and translation inhibition through activation of eIF2B

Activation of toll-like receptors (TLRs) induces the endoplasmic reticulum (ER) Unfolded Protein Response (UPR) to accommodate essential protein translation^1,2. However, despite increased p-eIF2α, a TLR-TRIF-dependent pathway assures that the cells avoid CHOP induction, apoptosis, and translational suppression of critical proteins³. Because p-eIF2α decreases the functional interaction of eIF2 with eIF2B, a guanine nucleotide exchange factor (GEF), we explored the hypothesis that TLR-TRIF signaling activates eIF2B-GEF activity to counteract the effects of p-eIF2α. We now show that TLR-TRIF signaling activates eIF2B-GEF through PP2A-mediated Ser-dephosphorylation of the eIF2B ε-subunit. PP2A itself is activated by decreased Src-family-kinase-induced Tyr-phosphorylation of its catalytic subunit. Each of these processes are required for TLR-TRIF-mediated CHOP suppression in ER-stressed cells in vitro and in vivo. Thus, in the setting of prolonged, physiologic ER stress, a unique TLR-TRIF-dependent translational control pathway enables cells to carry out essential protein synthesis and avoid CHOP-induced apoptosis while still benefitting from the protective arms of the UPR.

Recent progress toward understanding the molecular mechanisms that regulate skeletal muscle mass

The maintenance of muscle mass is critical for health and issues associated with the quality of life. Over the last decade, extensive progress has been made with regard to our understanding of the molecules that regulate skeletal muscle mass. Not surprisingly, many of these molecules are intimately involved in the regulation of protein synthesis and protein degradation [e.g. the mammalian target of rapamycin (mTOR), eukaryotic initiation factor 2B (eIF2B), eukaryotic initiation factor 3f (eIF3f) and the forkhead box O (FoxO) transcription factors]. It is also becoming apparent that molecules which sense, or control, the energetic status of the cell play a key role in the regulation of muscle mass [e.g. AMP-activated protein kinase (AMPK) and peroxisome proliferator-activated receptor gamma coactivator-1 α (PGC1α)]. In this review we will attempt to summarize the current knowledge of how these molecules regulate skeletal muscle mass.

Inhibition of glycogen synthase kinase 3[beta] activity with lithium in vitro attenuates sepsis-induced changes in muscle protein turnover

Loss of lean body mass is a characteristic feature of the septic response, and the mechanisms responsible for this decrease and means of prevention have not been fully elucidated. The present study tested the hypothesis that in vitro treatment of skeletal muscle with lithium chloride (LiCl), a glycogen synthase kinase (GSK) 3 inhibitor, would reverse both the sepsis-induced increase in muscle protein degradation and inhibition of protein synthesis. Sepsis decreased GSK-3[beta] phosphorylation and increased GSK-3[beta] activity, under basal conditions. Sepsis increased muscle protein degradation, with a concomitant increase in atrogin 1 and MuRF1 mRNA and 26S proteosome activity. Incubation of septic muscle with LiCl completely reversed the increased GSK-3[beta] activity and decreased proteolysis to basal nonseptic values, but only partially reduced proteosome activity and did not diminish atrogene expression. Lithium chloride also did not ameliorate the sepsis-induced increase in LC3-II, a marker for activated autophagy. In contrast, LiCl increased protein synthesis only in nonseptic control muscle. The inability of septic muscle to respond to LiCl was independent of its ability to reverse the sepsis-induced increase in eukaryotic initiation factor (eIF) 2B[varepsilon] phosphorylation, decreased eIF2B activity, or the reduced phosphorylation of FOXO3, but instead was more closely associated with the continued suppression of mTOR (mammalian target of rapamycin) kinase activity (e.g., reduced phosphorylation of 4E-BP1 and S6). These data suggest that in vitro lithium treatment, which inhibited GSK-3[beta] activity, (a) effectively reversed the sepsis-induced increase in proteolysis, but only in part by a reduction in the ubiquitin-proteosome pathway and not by a reduction in autophagy; and (b) was ineffective at reversing the sepsis-induced decrease in muscle protein synthesis. This lithium-resistant state seems mediated at the level of mTOR and not eIF2/eIF2B. Hence, use of GSK-3[beta] inhibitors in the treatment of sepsis may not be expected to fully correct the imbalance in muscle protein turnover.

Deptor knockdown enhances mTOR Activity and protein synthesis in myocytes and ameliorates disuse muscle atrophy

Deptor is an mTOR binding protein that affects cell metabolism. We hypothesized that knockdown (KD) of Deptor in C2C12 myocytes will increase protein synthesis via stimulating mTOR-S6K1 signaling. Deptor KD was achieved using lentiviral particles containing short hairpin (sh)RNA targeting the mouse Deptor mRNA sequence, and control cells were transfected with a scrambled control shRNA. KD reduced Deptor mRNA and protein content by 90%, which increased phosphorylation of mTOR kinase substrates, 4E-BP1 and S6K1, and concomitantly increased protein synthesis. Deptor KD myoblasts were both larger in diameter and exhibited an increased mean cell volume. Deptor KD increased the percentage of cells in the S phase, coincident with an increased phosphorylation (S807/S811) of retinoblastoma protein (pRb) that is critical for the G(1) to S phase transition. Deptor KD did not appear to alter basal apoptosis or autophagy, as evidenced by the lack of change for cleaved caspase-3 and light chain (LC)3B, respectively. Deptor KD increased proliferation rate and enhanced myotube formation. Finally, in vivo Deptor KD (~50% reduction) by electroporation into gastrocnemius of C57/BL6 mice did not alter weight or protein synthesis in control muscle. However, Deptor KD prevented atrophy produced by 3 d of hindlimb immobilization, at least in part by increasing protein synthesis. Thus, our data support the hypothesis that Deptor is an important regulator of protein metabolism in myocytes and demonstrate that decreasing Deptor expression in vivo is sufficient to ameliorate muscle atrophy.

Eukaryotic initiation factor 2B epsilon induces cap-dependent translation and skeletal muscle hypertrophy

The purpose of this study was to identify signalling components known to control mRNA translation initiation in skeletal muscle that are responsive to mechanical load and may be partly responsible for myofibre hypertrophy. To accomplish this, we first utilized a human cluster model in which skeletalmuscle samples fromsubjects with widely divergent hypertrophic responses to resistance training were used for the identification of signalling proteins associated with the degree myofibre hypertrophy. We found that of 11 translational signalling molecules examined, the response of p(T421/S424)-p70S6K phosphorylation and total eukaryotic initiation factor 2Bε (eIF2Bε) protein abundance after a single bout of unaccustomed resistance exercise was associated with myofibre hypertrophy following 16 weeks of training. Follow up studies revealed that overexpression of eIF2Bε alone was sufficient to induce an 87% increase in cap-dependent translation in L6 myoblasts in vitro and 21% hypertrophy of myofibres in mouse skeletal muscle in vivo (P<0.05).However, genetically altering p70S6K activity had no impact on eIF2Bε protein abundance in mouse skeletal muscle in vivo or multiple cell lines in vitro (P >0.05), suggesting that the two phenomena were not directly related. These are the first data that mechanistically link eIF2Bε abundance to skeletal myofibre hypertrophy, and indicate that eIF2Bε abundance may at least partially underlie the widely divergent hypertrophic phenotypes in human skeletal muscle exposed to mechanical stimuli.

Simvastatin represses protein synthesis in the muscle-derived C₂C₁₂ cell line with a concomitant reduction in eukaryotic initiation factor 2B expression

Statins are a widely prescribed class of cholesterol lowering drugs whose use is frequently associated with muscle-related ailments. A number of mechanisms have been implicated in statin-induced myotoxicity including alterations in both protein synthesis and protein degradation. The objective of the present study was to explore the mechanism(s) contributing to the statin-induced reduction in protein synthesis in the muscle-derived C₂C₁₂ cell line. Cells were treated with 10 μM simvastatin or vehicle alone for 24 h in 1% serum. Cells exposed to simvastatin exhibited reduced rates of protein synthesis, as evidenced by [(35)S]methionine and [(35)S]cysteine incorporation into protein. The reduction in protein synthesis occurred with a concomitant decrease in expression and activity of eukaryotic initiation factor 2B (eIF2B), a regulated and rate-controlling guanine nucleotide exchange factor known to affect global rates of protein synthesis. The reductions in protein synthesis and eIF2B expression were prevented by coincubation with mevalonate. Simvastatin treatment also resulted in a proteasome-sensitive reduction in the protein expression of all the subunits of the eIF2B heteropentameric complex. Finally, increased phosphorylation of the catalytic ε-subunit at Ser(535) was observed, an event consistent with an observed reduction in eIF2B activity. These results suggest that repression of eIF2B expression and activity may contribute, at least in part, to the statin-induced reduction in protein synthesis.