Mechanisms Regulating Neuromuscular Junction Development and Function and Causes of Muscle Wasting

The neuromuscular junction is the chemical synapse between motor neurons and skeletal muscle fibers. It is designed to reliably convert the action potential from the presynaptic motor neuron into the contraction of the postsynaptic muscle fiber. Diseases that affect the neuromuscular junction may cause failure of this conversion and result in loss of ambulation and respiration. The loss of motor input also causes muscle wasting as muscle mass is constantly adapted to contractile needs by the balancing of protein synthesis and protein degradation. Finally, neuromuscular activity and muscle mass have a major impact on metabolic properties of the organisms. This review discusses the mechanisms involved in the development and maintenance of the neuromuscular junction, the consequences of and the mechanisms involved in its dysfunction, and its role in maintaining muscle mass during aging. As life expectancy is increasing, loss of muscle mass during aging, called sarcopenia, has emerged as a field of high medical need. Interestingly, aging is also accompanied by structural changes at the neuromuscular junction, suggesting that the mechanisms involved in neuromuscular junction maintenance might be disturbed during aging. In addition, there is now evidence that behavioral paradigms and signaling pathways that are involved in longevity also affect neuromuscular junction stability and sarcopenia.

Degradation of MyoD Mediated by the SCF (MAFbx) Ubiquitin Ligase

MyoD controls myoblast identity and differentiation and is required for myogenic stem cell function in adult skeletal muscle. MyoD is degraded by the ubiquitin-proteasome pathway mediated by different E3 ubiquitin ligases not identified as yet. Here we report that MyoD interacts with Atrogin-1/MAFbx (MAFbx), a striated muscle-specific E3 ubiquitin ligase dramatically up-regulated in atrophying muscle. A core LXXLL motif sequence in MyoD is necessary for binding to MAFbx. MAFbx associates with MyoD through an inverted LXXLL motif located in a series of helical leucine-charged residue-rich domains. Mutation in the LXXLL core motif represses ubiquitination and degradation of MyoD induced by MAFbx. Overexpression of MAFbx suppresses MyoD-induced differentiation and inhibits myotube formation. Finally the purified recombinant SCF(MAFbx) complex (SCF, Skp1, Cdc53/Cullin 1, F-box protein) mediated MyoD ubiquitination in vitro in a lysine-dependent pathway. Mutation of the lysine 133 in MyoD prevented its ubiquitination by the recombinant SCF(MAFbx) complex. These observations thus demonstrated that MAFbx functions in ubiquitinating MyoD via a sequence found in transcriptional coactivators. These transcriptional coactivators mediate the binding to liganded nuclear receptors. We also identified a novel protein-protein interaction module not yet identified in F-box proteins. MAFbx may play an important role in the course of muscle differentiation by determining the abundance of MyoD.

Sustained activation of mTORC1 in skeletal muscle inhibits constitutive and starvation-induced autophagy and causes a severe, late-onset myopathy

Autophagy is a catabolic process that ensures homeostatic cell clearance and is deregulated in a growing number of myopathological conditions. Although FoxO3 was shown to promote the expression of autophagy-related genes in skeletal muscle, the mechanisms triggering autophagy are unclear. We show that TSC1-deficient mice (TSCmKO), characterized by sustained activation of mTORC1, develop a late-onset myopathy related to impaired autophagy. In young TSCmKO mice, constitutive and starvation-induced autophagy is blocked at the induction steps via mTORC1-mediated inhibition of Ulk1, despite FoxO3 activation. Rapamycin is sufficient to restore autophagy in TSCmKO mice and improves the muscle phenotype of old mutant mice. Inversely, abrogation of mTORC1 signaling by depletion of raptor induces autophagy regardless of FoxO inhibition. Thus, mTORC1 is the dominant regulator of autophagy induction in skeletal muscle and ensures a tight coordination of metabolic pathways. These findings may open interesting avenues for therapeutic strategies directed toward autophagy-related muscle diseases.

PKB and the mitochondria: AKTing on apoptosis

Cellular homeostasis depends upon the strict regulation of responses to external stimuli, such as signalling cascades triggered by nutrients and growth factors, and upon cellular metabolism. One of the major molecules coordinating complex signalling pathways is protein kinase B (PKB), a serine/threonine kinase also known as Akt. The number of substrates known to be phosphorylated by PKB and its interacting partners, as well as our broad understanding of how PKB is implicated in responses to growth factors, metabolic pathways, proliferation, and cell death via apoptosis is constantly increasing. Activated by the insulin/growth factor-phosphatidylinositol 3-kinase (PI3K) cascade, PKB triggers events that promote cell survival and prevent apoptosis. It is also now widely accepted that mitochondria are not just suppliers of ATP, but that they participate in regulatory and signalling events, responding to multiple physiological inputs and genetic stresses, and regulate both cell proliferation and death. Thus, mitochondria are recognized as important players in apoptotic events and it is logical to predict some form of interplay with PKB. In this review, we will summarize mechanisms by which PKB mediates its anti-apoptotic activities in cells and survey recent developments in understanding mitochondrial dynamics and their role during apoptosis.

Sequential involvement of Cdk1, mTOR and p53 in apoptosis induced by the HIV-1 envelope

Syncytia arising from the fusion of cells expressing the HIV-1-encoded Env gene with cells expressing the CD4/CXCR4 complex undergo apoptosis following the nuclear translocation of mammalian target of rapamycin (mTOR), mTOR-mediated phosphorylation of p53 on Ser15 (p53(S15)), p53-dependent upregulation of Bax and activation of the mitochondrial death pathway. p53(S15) phosphorylation is only detected in syncytia in which nuclear fusion (karyogamy) has occurred. Karyogamy is secondary to a transient upregulation of cyclin B and a mitotic prophase-like dismantling of the nuclear envelope. Inhibition of cyclin-dependent kinase-1 (Cdk1) prevents karyogamy, mTOR activation, p53(S15) phosphorylation and apoptosis. Neutralization of p53 fails to prevent karyogamy, yet suppresses apoptosis. Peripheral blood mononuclear cells from HIV-1-infected patients exhibit an increase in cyclin B and mTOR expression, correlating with p53(S15) phosphorylation and viral load. Cdk1 inhibition prevents the death of syncytia elicited by HIV-1 infection of primary CD4 lymphoblasts. Thus, HIV-1 elicits a pro-apoptotic signal transduction pathway relying on the sequential action of cyclin B-Cdk1, mTOR and p53.

Differential response of skeletal muscles to mTORC1 signaling during atrophy and hypertrophy

BACKGROUND

Skeletal muscle mass is determined by the balance between protein synthesis and degradation. Mammalian target of rapamycin complex 1 (mTORC1) is a master regulator of protein translation and has been implicated in the control of muscle mass. Inactivation of mTORC1 by skeletal muscle-specific deletion of its obligatory component raptor results in smaller muscles and a lethal dystrophy. Moreover, raptor-deficient muscles are less oxidative through changes in the expression PGC-1α, a critical determinant of mitochondrial biogenesis. These results suggest that activation of mTORC1 might be beneficial to skeletal muscle by providing resistance to muscle atrophy and increasing oxidative function. Here, we tested this hypothesis by deletion of the mTORC1 inhibitor tuberous sclerosis complex (TSC) in muscle fibers.

METHOD

Skeletal muscles of mice with an acute or a permanent deletion of raptor or TSC1 were examined using histological, biochemical and molecular biological methods. Response of the muscles to changes in mechanical load and nerve input was investigated by ablation of synergistic muscles or by denervation .

RESULTS

Genetic deletion or knockdown of raptor, causing inactivation of mTORC1, was sufficient to prevent muscle growth and enhance muscle atrophy. Conversely, short-term activation of mTORC1 by knockdown of TSC induced muscle fiber hypertrophy and atrophy-resistance upon denervation, in both fast tibialis anterior (TA) and slow soleus muscles. Surprisingly, however, sustained activation of mTORC1 by genetic deletion of Tsc1 caused muscle atrophy in all but soleus muscles. In contrast, oxidative capacity was increased in all muscles examined. Consistently, TSC1-deficient soleus muscle was atrophy-resistant whereas TA underwent normal atrophy upon denervation. Moreover, upon overloading, plantaris muscle did not display enhanced hypertrophy compared to controls. Biochemical analysis indicated that the atrophy response of muscles was based on the suppressed phosphorylation of PKB/Akt via feedback inhibition by mTORC1 and subsequent increased expression of the E3 ubiquitin ligases MuRF1 and atrogin-1/MAFbx. In contrast, expression of both E3 ligases was not increased in soleus muscle suggesting the presence of compensatory mechanisms in this muscle.

CONCLUSIONS

Our study shows that the mTORC1- and the PKB/Akt-FoxO pathways are tightly interconnected and differentially regulated depending on the muscle type. These results indicate that long-term activation of the mTORC1 signaling axis is not a therapeutic option to promote muscle growth because of its strong feedback induction of the E3 ubiquitin ligases involved in protein degradation.

Stabilization of MyoD by direct binding to p57(Kip2)

Recent data have demonstrated the role of Cdk1- and Cdk2-dependent phosphorylation of MyoD(Ser200) in the regulation of MyoD activity and protein turnover. In the present study, we show that in presence of p57(Kip2), MyoD(Ala200), a MyoD mutant that cannot be phosphorylated by cyclin-Cdk complexes, displayed activity 2-5-fold higher than of MyoD(Ala200) alone in transactivation of muscle-specific genes myosin heavy chain, creatine kinase, and myosin light chain 1. Furthermore, p57(Kip2) increases the levels of MyoD(Ala200) in cotransfected cells. This result implies that p57(Kip2) may regulate MyoD through a process distinct from its function as a cyclin-dependent kinase inhibitors. We report that overexpression of p57(Kip2) increased the half-life of MyoD(Ala200). This increased half-life of MyoD involves a physical interaction between MyoD and p57(Kip2) but not with p16(Ink4a), as shown by cross-immunoprecipitation not only on overexpressed proteins from transfected cells, but also on endogenous MyoD and p57(Kip2) from C2C12 myogenic cells. Mutational and functional analyses of the two proteins show that the NH(2) domain of p57(Kip2) associates with basic region in the basic helix-loop-helix domain of MyoD. Competition/association assays and site-directed mutagenesis of the NH(2) terminus of p57(Kip2) identified the intermediate alpha-helix domain, located between the Cdk and the cyclin binding sites, as essential for MyoD interaction. These data show that the alpha-helix domain of p57(Kip2), which is conserved in the Cip/Kip proteins, is implicated in protein-protein interaction and confers a specific regulatory mechanism, outside of their Cdk-inhibitory activity, by which the p57(Kip2) family members positively act on myogenic differentiation.

The translation regulatory subunit eIF3f controls the kinase-dependent mTOR signaling required for muscle differentiation and hypertrophy in mouse

The mTORC1 pathway is required for both the terminal muscle differentiation and hypertrophy by controlling the mammalian translational machinery via phosphorylation of S6K1 and 4E-BP1. mTOR and S6K1 are connected by interacting with the eIF3 initiation complex. The regulatory subunit eIF3f plays a major role in muscle hypertrophy and is a key target that accounts for MAFbx function during atrophy. Here we present evidence that in MAFbx-induced atrophy the degradation of eIF3f suppresses S6K1 activation by mTOR, whereas an eIF3f mutant insensitive to MAFbx polyubiquitination maintained persistent phosphorylation of S6K1 and rpS6. During terminal muscle differentiation a conserved TOS motif in eIF3f connects mTOR/raptor complex, which phosphorylates S6K1 and regulates downstream effectors of mTOR and Cap-dependent translation initiation. Thus eIF3f plays a major role for proper activity of mTORC1 to regulate skeletal muscle size.

Molecular and phenotypic analysis of rodent models reveals conserved and species-specific modulators of human sarcopenia

Sarcopenia, the age-related loss of skeletal muscle mass and function, affects 5-13% of individuals aged over 60 years. While rodents are widely-used model organisms, which aspects of sarcopenia are recapitulated in different animal models is unknown. Here we generated a time series of phenotypic measurements and RNA sequencing data in mouse gastrocnemius muscle and analyzed them alongside analogous data from rats and humans. We found that rodents recapitulate mitochondrial changes observed in human sarcopenia, while inflammatory responses are conserved at pathway but not gene level. Perturbations in the extracellular matrix are shared by rats, while mice recapitulate changes in RNA processing and autophagy. We inferred transcription regulators of early and late transcriptome changes, which could be targeted therapeutically. Our study demonstrates that phenotypic measurements, such as muscle mass, are better indicators of muscle health than chronological age and should be considered when analyzing aging-related molecular data.

Cyclin E-cdk2 phosphorylation promotes late G1-phase degradation of MyoD in muscle cells

Proliferating myoblasts already express MyoD before the induction of differentiation. Overexpression of MyoD in normal and transformed cell lines was shown to block cells from entering S phase, suggesting that the MyoD growth suppressive effect must be tightly controlled in growing myoblasts. Here we show that during G1 phase, but not in G2, MyoD abundance is down-regulated by the ubiquitin-proteasome pathway through phosphorylation of serine 200. Roscovitine, a specific inhibitor of cyclin-Cdk2 complexes, prevents both phosphorylation and degradation of MyoD in G1. Inhibition of the ubiquitin-dependent proteasome pathway by MG132 results in stabilization of MyoD-wt, with little effect on a MyoD mutant where serine 200 is replaced by an alanine. Our results show that MyoD Ser200 is the substrate for phosphorylation by cyclin E-Cdk2 stimulating its degradation by the ubiquitin-proteasome system which controls MyoD levels in G1. Phosphorylation/degradation of MyoD at the end of G1 thus represents the regulatory checkpoint in growing myoblasts allowing progression into S phase in a manner similar to the recently examplified cdk2-phosphorylation/degradation of p27(Kip1).

Myostatin inactivation increases myotube size through regulation of translational initiation machinery

Myostatin deficiency leads in skeletal muscle overgrowth but the precise molecular mechanisms underlying this hypertrophy are not well understood. In this study, to gain insight into the role of endogenous myostatin in the translational regulation, we used an in vitro model of cultured satellite cells derived from myostatin knock-out mice. Our results show that myostatin knock-out myotubes are larger than control myotubes and that this phenotype is associated with an increased activation of the Akt/mTOR signaling pathway, a known regulator of muscle hypertrophy. These results demonstrate that hypertrophy due to myostatin deficiency is preserved in vitro and suggest that myostatin deletion results in an increased protein synthesis. Accordingly, the rates of global RNA content, polysome formation and protein synthesis are all increased in myostatin-deficient myotubes while they are counteracted by the addition of recombinant myostatin. We furthermore demonstrated that genetic deletion of myostatin stimulates cap-dependent translation by positively regulating assembly of the translation preinitiation complex. Together the data indicate that myostatin controls muscle hypertrophy in part by regulating protein synthesis initiation rates, that is, translational efficiency.

Dephosphorylation and subcellular compartment change of the mitotic Bloom's syndrome DNA helicase in response to ionizing radiation

Bloom's syndrome is a rare human autosomal recessive disorder that combines a marked genetic instability and an increased risk of developing all types of cancers and which results from mutations in both copies of the BLM gene encoding a RecQ 3'-5' DNA helicase. We recently showed that BLM is phosphorylated and excluded from the nuclear matrix during mitosis. We now show that the phosphorylated mitotic BLM protein is associated with a 3'-5' DNA helicase activity and interacts with topoisomerase III alpha. We demonstrate that in mitosis-arrested cells, ionizing radiation and roscovitine treatment both result in the reversion of BLM phosphorylation, suggesting that BLM could be dephosphorylated through the inhibition of cdc2 kinase. This was supported further by our data showing that cdc2 kinase activity is inhibited in gamma-irradiated mitotic cells. Finally we show that after ionizing radiation, BLM is not involved in the establishment of the mitotic DNA damage checkpoint but is subjected to a subcellular compartment change. These findings lead us to propose that BLM may be phosphorylated during mitosis, probably through the cdc2 pathway, to form a pool of rapidly available active protein. Inhibition of cdc2 kinase after ionizing radiation would lead to BLM dephosphorylation and possibly to BLM recruitment to some specific sites for repair.

Alterations to mTORC1 signaling in the skeletal muscle differentially affect whole-body metabolism

Background

The mammalian target of rapamycin complex 1 (mTORC1) is a central node in a network of signaling pathways controlling cell growth and survival. This multiprotein complex integrates external signals and affects different nutrient pathways in various organs. However, it is not clear how alterations of mTORC1 signaling in skeletal muscle affect whole-body metabolism.

Results

We characterized the metabolic phenotype of young and old raptor muscle knock-out (RAmKO) and TSC1 muscle knock-out (TSCmKO) mice, where mTORC1 activity in skeletal muscle is inhibited or constitutively activated, respectively. Ten-week-old RAmKO mice are lean and insulin resistant with increased energy expenditure, and they are resistant to a high-fat diet (HFD). This correlates with an increased expression of histone deacetylases (HDACs) and a downregulation of genes involved in glucose and fatty acid metabolism. Ten-week-old TSCmKO mice are also lean, glucose intolerant with a decreased activation of protein kinase B (Akt/PKB) targets that regulate glucose transporters in the muscle. The mice are resistant to a HFD and show reduced accumulation of glycogen and lipids in the liver. Both mouse models suffer from a myopathy with age, with reduced fat and lean mass, and both RAmKO and TSCmKO mice develop insulin resistance and increased intramyocellular lipid content.

Conclusions

Our study shows that alterations of mTORC1 signaling in the skeletal muscle differentially affect whole-body metabolism. While both inhibition and constitutive activation of mTORC1 induce leanness and resistance to obesity, changes in the metabolism of muscle and peripheral organs are distinct. These results indicate that a balanced mTORC1 signaling in the muscle is required for proper metabolic homeostasis.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-016-0084-8) contains supplementary material, which is available to authorized users.

SIRT3, a mitochondrial NAD⁺-dependent deacetylase, is involved in the regulation of myoblast differentiation

Sirtuin 3 (SIRT3), one of the seven mammalian sirtuins, is a mitochondrial NAD+-dependent deacetylase known to control key metabolic pathways. SIRT3 deacetylases and activates a large number of mitochondrial enzymes involved in the respiratory chain, in ATP production, and in both the citric acid and urea cycles. We have previously shown that the regulation of myoblast differentiation is tightly linked to mitochondrial activity. Since SIRT3 modulates mitochondrial activity, we decide to address its role during myoblast differentiation. For this purpose, we first investigated the expression of endogenous SIRT3 during C2C12 myoblast differentiation. We further studied the impact of SIRT3 silencing on both the myogenic potential and the mitochondrial activity of C2C12 cells. We showed that SIRT3 protein expression peaked at the onset of myoblast differentiation. The inhibition of SIRT3 expression mediated by the stable integration of SIRT3 short inhibitory RNA (SIRT3shRNA) in C2C12 myoblasts, resulted in: 1) abrogation of terminal differentiation - as evidenced by a marked decrease in the myoblast fusion index and a significant reduction of Myogenin, MyoD, Sirtuin 1 and Troponin T protein expression - restored upon MyoD overexpression; 2) a decrease in peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) and citrate synthase protein expression reflecting an alteration of mitochondrial density; and 3) an increased production of reactive oxygen species (ROS) mirrored by the decreased activity of manganese superoxide dismutase (MnSOD). Altogether our data demonstrate that SIRT3 mainly regulates myoblast differentiation via its influence on mitochondrial activity.

mTORC1 signalling is not essential for the maintenance of muscle mass and function in adult sedentary mice

BACKGROUND

The balance between protein synthesis and degradation (proteostasis) is a determining factor for muscle size and function. Signalling via the mammalian target of rapamycin complex 1 (mTORC1) regulates proteostasis in skeletal muscle by affecting protein synthesis and autophagosomal protein degradation. Indeed, genetic inactivation of mTORC1 in developing and growing muscle causes atrophy resulting in a lethal myopathy. However, systemic dampening of mTORC1 signalling by its allosteric inhibitor rapamycin is beneficial at the organismal level and increases lifespan. Whether the beneficial effect of rapamycin comes at the expense of muscle mass and function is yet to be established.

METHODS

We conditionally ablated the gene coding for the mTORC1-essential component raptor in muscle fibres of adult mice [inducible raptor muscle-specific knockout (iRAmKO)]. We performed detailed phenotypic and biochemical analyses of iRAmKO mice and compared them with muscle-specific raptor knockout (RAmKO) mice, which lack raptor in developing muscle fibres. We also used polysome profiling and proteomics to assess protein translation and associated signalling in skeletal muscle of iRAmKO mice.

RESULTS

Analysis at different time points reveal that, as in RAmKO mice, the proportion of oxidative fibres decreases, but slow-type fibres increase in iRAmKO mice. Nevertheless, no significant decrease in body and muscle mass or muscle fibre area was detected up to 5 months post-raptor depletion. Similarly, ex vivo muscle force was not significantly reduced in iRAmKO mice. Despite stable muscle size and function, inducible raptor depletion significantly reduced the expression of key components of the translation machinery and overall translation rates.

CONCLUSIONS

Raptor depletion and hence complete inhibition of mTORC1 signalling in fully grown muscle leads to metabolic and morphological changes without inducing muscle atrophy even after 5 months. Together, our data indicate that maintenance of muscle size does not require mTORC1 signalling, suggesting that rapamycin treatment is unlikely to negatively affect muscle mass and function.

eIF3-f function in skeletal muscles: to stand at the crossroads of atrophy and hypertrophy

The control of muscle cell size is a physiological process balanced by a fine tuning between protein synthesis and protein degradation. MAFbx/Atrogin-1 is a muscle specific E3 ubiquitin ligase upregulated during disuse, immobilization and fasting or systemic diseases such as diabetes, cancer, AIDS and renal failure. This response is necessary to induce a rapid and functional atrophy. To date, the targets of MAFbx/Atrogin-1 in skeletal muscle remain to be identified. We have recently presented evidence that eIF3-f, a regulatory subunit of the eukaryotic translation factor eIF3 is a key target that accounts for MAFbx/Atrogin-1 function in muscle atrophy. More importantly, we showed that eIF3-f acts as a "translational enhancer" that increases the efficiency of the structural muscle proteins synthesis leading to both in vitro and in vivo muscle hypertrophy. We propose that eIF3-f subunit, a mTOR/S6K1 scaffolding protein in the IGF-1/Akt/mTOR dependent control of protein translation, is a positive actor essential to the translation of specific mRNAs probably implicated in muscle hypertrophy. The central role of eIF3-f in both the atrophic and hypertrophic pathways will be discussed in the light of its promising potential in muscle wasting therapy.

Critical Role for Lysine 133 in the Nuclear Ubiquitin-mediated Degradation of MyoD

The ubiquitin-proteasome system is responsible for the regulation and turnover of the nuclear transcription factor MyoD. The degradation of MyoD can occur via an NH2 terminus-dependent pathway or a lysine-dependent pathway, suggesting that MyoD ubiquitination may be driven by different mechanisms. To understand this process, deletion analysis was used to identify the region of MyoD that is required for rapid proteolysis in the lysine-dependent pathway. Here we report that the basic helix-loop-helix domain is required for ubiquitination and lysine-dependent degradation of MyoD in the nucleus. Site-directed mutagenesis in MyoD revealed that lysine 133 is the major internal lysine of ubiquitination. The half-life of the MyoD K133R mutant protein was longer than that of wild type MyoD, substantiating the implication of lysine 133 in the turnover of MyoD in myoblasts. In addition, the MyoD K133R mutant displayed activity 2-3-fold higher than the wild type in transactivation muscle-specific gene and myogenic conversion of 10T1/2 cells. Taken together, our data demonstrate that lysine 133 is targeted for ubiquitination and rapid degradation of MyoD in the lysine-dependent pathway and plays an integral role in compromising MyoD activity in the nucleus.

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

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.

Exploring the relationship between mTORC1 and the ubiquitin-proteasome system, light is shed on the paradox between mTORC1-mediated muscle hypertrophy induced by PKB/Akt and the muscle atrophy induced by mTORC1 alone.

Regulation of glioma cell invasion by 3q26 gene products PIK3CA, SOX2 and OPA1

Diffuse gliomas progress by invading neighboring brain tissue to promote postoperative relapse. Transcription factor SOX2 is highly expressed in invasive gliomas and maps to chromosome region 3q26 together with the genes for PI3K/AKT signaling activator PIK3CA and effector molecules of mitochondria fusion and cell invasion, MFN1 and OPA1. Gene copy number analysis at 3q26 from 129 glioma patient biopsies revealed mutually exclusive SOX2 amplifications (26%) and OPA1 losses (19%). Both forced SOX2 expression and OPA1 inactivation increased LN319 glioma cell invasion in vitro and promoted cell dispersion in vivo in xenotransplanted D. rerio embryos. While PI3 kinase activity sustained SOX2 expression, pharmacological PI3K/AKT pathway inhibition decreased invasion and resulted in SOX2 nucleus-to-cytoplasm translocation in an mTORC1-independent manner. Chromatin immunoprecipitation and luciferase reporter gene assays together demonstrated that SOX2 trans-activates PIK3CA and OPA1. Thus, SOX2 activates PI3K/AKT signaling in a positive feedback loop, while OPA1 deletion is interpreted to counteract OPA1 trans-activation. Remarkably, neuroimaging of human gliomas with high SOX2 or low OPA1 genomic imbalances revealed significantly larger necrotic tumor zone volumes, corresponding to higher invasive capacities of tumors, while autologous necrotic cells are capable of inducing higher invasion in SOX2 overexpressing or OPA1 knocked-down relative to parental LN319. We thus propose necrosis volume as a surrogate marker for the assessment of glioma invasive potential. Whereas glioma invasion is activated by a PI3K/AKT-SOX2 loop, it is reduced by a cryptic invasion suppressor SOX2-OPA1 pathway. Thus, PI3K/AKT-SOX2 and mitochondria fission represent connected signaling networks regulating glioma invasion.

AIMTOR, a BRET biosensor for live imaging, reveals subcellular mTOR signaling and dysfunctions

Background

mTOR signaling is an essential nutrient and energetic sensing pathway. Here we describe AIMTOR, a sensitive genetically encoded BRET (Bioluminescent Resonance Energy Transfer) biosensor to study mTOR activity in living cells.

Results

As a proof of principle, we show in both cell lines and primary cell cultures that AIMTOR BRET intensities are modified by mTOR activity changes induced by specific inhibitors and activators of mTORC1 including amino acids and insulin. We further engineered several versions of AIMTOR enabling subcellular-specific assessment of mTOR activities. We then used AIMTOR to decipher mTOR signaling in physio-pathological conditions. First, we show that mTORC1 activity increases during muscle cell differentiation and in response to leucine stimulation in different subcellular compartments such as the cytosol and at the surface of the lysosome, the nucleus, and near the mitochondria. Second, in hippocampal neurons, we found that the enhancement of neuronal activity increases mTOR signaling. AIMTOR further reveals mTOR-signaling dysfunctions in neurons from mouse models of autism spectrum disorder.

Conclusions

Altogether, our results demonstrate that AIMTOR is a sensitive and specific tool to investigate mTOR-signaling dynamics in living cells and phenotype mTORopathies.