: École d’été 2021 - Contraintes de courbures et espaces métriques

Over the last fifty years, the problem of finding sharp upper bounds for area-normalized Laplacian eigenvalues on closed surfaces has attracted the attention of many geometers, due in part to connections to the study of sphere-valued harmonic maps and minimal immersions. In this talk, I'll describe a series of results which shed new light on this problem by relating it to the variational theory of the Dirichlet energy on sphere-valued maps. Recent applications include new (H^{-1}-)stability results for the maximization of the first and second Laplacian eigenvalues, and a proof that metrics maximizing the first Steklov eigenvalue on a surface of genus g and k boundary components limit to the \lambda_1-maximizing metric on the closed surface of genus g as k becomes large (in particular, the associated free boundary minimal surfaces in B^{N+1} converge as varifolds to the associated closed minimal surface in S^N). Based on joint works with Mikhail Karpukhin, Mickael Nahon and Iosif Polterovich

Voies protéolytiques du muscle : oeuvres solistes ou ensemble orchestré ?

National audienceLe muscle squelettique est un tissu plastique dont les protéines sont en constant renouvellement. La taille du compartiment protéique musculaire dépend de l’intensité respective des vitesses de synthèse et de dégradation des protéines. De nombreuses situations physiopathologiques (maladies neuro-dégénératives, cancers, vieillissement…) sont caractérisées par une atrophie musculaire survenant lorsque la protéolyse devient supérieure à la protéosynthèse. Parmi les différents systèmes impliqués dans la protéolyse musculaire, l’attention sera focalisée sur la voie ubiquitine-protéasome dépendante (UPS) et la voie lysosomale (autophagie). Cependant, la coopération d’autres systèmes semble nécessaire pour la dégradation des protéines contractiles majeures

The ubiquitin-proteasome system and skeletal muscle wasting

Abstract The ubiquitin-proteasome system (UPS) is believed to degrade the major contractile skeletal muscle proteins and plays a major role in muscle wasting. Different and multiple events in the ubiquitination, deubiquitination and proteolytic machineries are responsible for the activation of the system and subsequent muscle wasting. However, other proteolytic enzymes act upstream (possibly m-calpain, cathepsin L, and/or caspase 3) and downstream (tripeptidyl-peptidase II and aminopeptidases) of the UPS, for the complete breakdown of the myofibrillar proteins into free amino acids. Recent studies have identified a few critical proteins that seem necessary for muscle wasting {i.e. the MAFbx (muscle atrophy F-box protein, also called atrogin-1) and MuRF-1 [muscle-specific RING (really interesting new gene) finger 1] ubiquitin-protein ligases}. The characterization of their signalling pathways is leading to new pharmacological approaches that can be useful to block or partially prevent muscle wasting in human patients. 17

Magnesium Influences Membrane Fusion during Myogenesis by Modulating Oxidative Stress in C2C12 Myoblasts

International audienceMagnesium (Mg) is essential to skeletal muscle where it plays a key role in myofiber relaxation. Although the importance of Mg in the mature skeletal muscle is well established, little is known about the role of Mg in myogenesis. We studied the effects of low and high extracellular Mg in C2C12 myogenic differentiation. Non-physiological Mg concentrations induce oxidative stress in myoblasts. The increase of reactive oxygen species, which occurs during the early phase of the differentiation process, inhibits myoblast membrane fusion, thus impairing myogenesis. Therefore, correct Mg homeostasis, also maintained through a correct dietary intake, is essential to assure the regenerative capacity of skeletal muscle fibers

Glucocorticoids regulate mRNA levels for subunits of the 19 S regulatory complex of the 26 S proteasome in fast-twitch skeletal muscles.

Circulating levels of glucocorticoids are increased in many traumatic and muscle-wasting conditions that include insulin-dependent diabetes, acidosis, infection, and starvation. On the basis of indirect findings, it appeared that these catabolic hormones are required to stimulate Ub (ubiquitin)-proteasome-dependent proteolysis in skeletal muscles in such conditions. The present studies were performed to provide conclusive evidence for an activation of Ub-proteasome-dependent proteolysis after glucocorticoid treatment. In atrophying fast-twitch muscles from rats treated with dexamethasone for 6 days, compared with pair-fed controls, we found (i) increased MG132-inhibitable proteasome-dependent proteolysis, (ii) an enhanced rate of substrate ubiquitination, (iii) increased chymotrypsin-like proteasomal activity of the proteasome, and (iv) a co-ordinate increase in the mRNA expression of several ATPase (S4, S6, S7 and S8) and non-ATPase (S1, S5a and S14) subunits of the 19 S regulatory complex, which regulates the peptidase and the proteolytic activities of the 26 S proteasome. These studies provide conclusive evidence that glucocorticoids activate Ub-proteasome-dependent proteolysis and the first in vivo evidence for a hormonal regulation of the expression of subunits of the 19 S complex. The results suggest that adaptations in gene expression of regulatory subunits of the 19 S complex by glucocorticoids are crucial in the regulation of the 26 S muscle proteasome

Muscle wasting in hemodialysis and lung cancer patients is mediated through down and up-regulation of several proteins common to both diseases

Session Proteasomes, Structure & FunctionOrganizing Committee: Chairs: Didier Attaix, Lydie Combaret and Daniel TaillandierIntroduction: Muscle atrophy is frequently encountered in diseased patients. It contributes to patient’s frailty and is associated with an increased risk of death. Studies using animal models suggest the involvement of the Ubiquitin Proteasome System (UPS) in renal failure-induced muscle atrophy. However, this remains to be established in humans. Another important goal is to detect markers that may help fighting against muscle atrophy through nutritional or pharmacological strategies. Indeed, it is very difficult to counteract the increased proteolysis when it is established. Our objectives were (i) to identify the proteolytic systems activated in chronic hemodialysis (HD) or lung cancer (LC) patients, i.e. pathologies having a different etiology and (ii) to identify markers specific to the activation of muscle atrophy processes independently of the pathology per se.Methods: Muscle biopsies (n = 7 per group) were obtained upon programmed surgery. mRNA and protein levels were determined using qRT-PCR, immunoblotting and proteomic approaches. Results: We found that the UPS and autophagy were activated in both HD and LC patients. Mass spectrometry analysis identified > 1700 proteins. Main component analysis revealed 3 distinct protein expression profiles corresponding to the 3 groups studied. We identified 106 proteins that were significantly modified (decreased or increased) in both HD and LC patients compared to controls (CT). Hierarchical cluster analysis showed that expression levels of these proteins distinguished diseased (HD or LC) vs. CT patients. Orthogonal partial least square discriminant analysis confirmed these results.Conclusion: We demonstrated that the UPS and autophagy were activated during long-term disease in humans. We also found a set of proteins whose expression levels may be specific of the atrophying process. These proteins constitute potential biomarkers witnessing the activation of muscle atrophy and/or potential therapeutic targets

Muscle wasting in patients with end-stage renal disease or early-stage lung cancer: Common mechanisms at work

Loss of muscle mass aggravates many diseases such as cancer and renal failure, contributes to the frailty syndrome and is associated with an increased risk of death (1, 2). Studies conducted on animal models have revealed the preponderant role of muscle proteolysis and in particular the activation of the ubiquitin proteasome system (UPS) (3, 4). However, studies conducted in humans remain scarce, especially within renal deficiency. In addition, whether a common/similar atrophying programme exists independently of the nature of the disease remains to be established. The aim of this work was therefore to identify common modifications at the transcriptomic or the proteomic levels in atrophying skeletal muscles from cancer and renal failure patientsOur objectives were (i) to identify the proteolytic systems activated in chronic hemodialysis (HD) or lung cancer (LC) patients, i.e. pathologies having a different etiology and (ii) to identify markers specific to the activation of muscle atrophy processes independently of the pathology per se. Muscle biopsies were performed during scheduled interventions in early stage (no treatment, no detectable muscle loss) lung cancer (LC), chronic hemodialysis (HD) or healthy (CT) patients (n = 7 per group). Proteolysis was addressed by quantitation of gene expression of members of the UPS, autophagy and apoptosis systems by quantitative RT-PCR. A global analysis of the soluble muscle proteome was conducted by shotgun proteomic for investigating the common metabolic processes altered. We found an increased expression of several UPS and autophagy-related enzymes in both LC and HD groups, suggesting that proteolytic systems were activated in both early stage and long lasting diseases. Mass spectrometry identified more than 1700 proteins and principal Component Analysis (PCA) revealed 3 differential proteomes that matched to the 3 groups of patients. Orthogonal Partial Least Square Discriminant Analysis (O-PLS-DA) was used to create a model, which distinguished the muscles of diseased patients (LC or HD) from those of healthy subjects. Proteins that most contributed to the model were selected. Functional analysis revealed up to 9 metabolic processes (including proteolysis and muscle contraction) involved in and/or altered by the atrophying programme in both LC and HD patients, which was confirmed by a co-expression network analysis.In conclusion, We were able to identify highly similar modifications of several metabolic pathways in patients exhibiting diseases with highly different etiologies (early-stage lung cancer vs. long term renal failure). This strongly suggests that a common atrophying program exists independently of the disease. The mechanisms at stake are as many avenues for the development of treatments for preventing muscle wastingReference(s) (1) von Haehling, S., Anker, M.S. & Anker, S.D. Prevalence and clinical impact of cachexia in chronic illness in Europe, USA, and Japan: facts and numbers update 2016. J Cachexia Sarcopenia Muscle 2016; 7:507-509.(2) Kalantar-Zadeh, K., et al. Why cachexia kills: examining the causality of poor outcomes in wasting conditions. J Cachexia Sarcopenia Muscle 2013; 4:89-94.(3) Aniort, J., et al. Upregulation of MuRF1 and MAFbx participates to muscle wasting upon gentamicin-induced acute kidney injury. Int. J. Biochem. Cell Biol. 2016; 79:505-516.(4) Lecker, S.H., et al. Multiple types of skeletal muscle atrophy involve a common program of changes in gene expression. FASEB J. 2004; 18:39-51

Vieillissement biologique du muscle squelettique chez l'Homme

Le thème des conférences portera sur : "La mobilité internationale pendant le doctorat"Vieillissement biologique du muscle squelettique chez l'Homme. Journée de la Recherche 2018, Ecole Doctorale Sciences, Ingénierie, Sant

Biological aging of skeletal muscle in humans

Grâce au rassemblement de la communauté de Lyon et de St Etienne, associant chercheurs, cliniciens, étudiants et entreprises, cette première conférence internationale visera à explorer comment et dans quelle mesure l’exercice peut être bénéfique pour les principales cellules de l’appareil locomoteur (muscle, tendon, os, cartilage, vaisseau). Cette journée offrira la possibilité de mettre en avant à la fois des travaux de recherche fondamentale réalisés à un niveau préclinique et des investigations conduites chez le volontaire sain ou sur des cohortes de patients. Des approches cellulaires, moléculaires et fonctionnelles seront mises en avant au cours de cette conférence.Présentation orale et posterAging is characterized by changes in body composition and particularly by a gradual loss of skeletal muscle mass, a phenomenon known as Sarcopenia. This age-related decline in muscle mass is accompanied by a loss of strength and a decline of physical performance, named Dynapenia. Both events decrease the autonomy and the quality of life of the individuals affecting about 40-50% of people over the age of 80. Nonetheless, inter-individual differences in prevalence of sarcopenia/dynapenia exist, as some remain fit and strong, whereas other become frail and weak when they get old. Until now, no study has examined the inter-individual variations of muscle tissue and its biomarkers. At the fiber level, age-related variations in skeletal muscle mass induce typological and capillarization modifications. Furthermore, the loss of muscle mass with aging could be associated with serious metabolic consequences or accumulation of intramyocellular lipid droplets. Immunohistochemical studies were performed with muscle biopsies from 30 healthy elderly men, aged 80 ±0.5 years selected from the PROgnostic indicator of cardiovascular and cerebrovascular events (PROOF) cohort, classified into three groups. On the basis of appendicular mass variation between two DEXA at mean interval of seven years, some people lose more muscle mass, named ”Lost”, others remain ”Stable”, and others ”Gain” muscle mass. The loss of skeletal muscle mass was associated with a reduction in Type-I fibers surface area (-24.6%), accompanied by a proportional loss of capillaries number around each fiber-type (CAF) and capillary-to-fiber perimeter exchange index (CFPE) (-15%, -10% respectively), compared to ”Stable” and ”Gain” groups. Also subjects from the ”Lost” group exhibited significant accumulation of intramyocellular lipid droplets in Type-I fibers compared to the ”Gain” (+23%). Lastly, this decline in muscle mass induced a remodeling of the extracellular matrix with an increase in the endomysium area (+12.2% vs Gain).If usually, it is recognized that chronological aging mainly affects Type-II motor units, our results suggest that biological aging is characterized by impairment of Type-I muscle fibers, their microvascular environment and oxidative metabolism for elderly men on their eighties