Marco Sandri

Signalling pathways that control protein breakdown and protein synthesis

Skeletal muscles constitute the 40-50% of total body mass and play a critical role not only in locomotion but also in whole body metabolism. Many diseases, including inherited or acquired disorders, alter muscle function leading to muscle wasting and weakness. People that face with muscle loss show an increased morbidity and mortality. Despite the different aetiology of muscle loss, the pathogenetic mechanisms that induce muscle wasting are often shared by different diseases. In fact, skeletal muscle serves as the major protein reservoir of the body from which amino acids can be mobilized for liver gluconeogenesis or new protein synthesis and energy source of different tissues. Consequently, upon food deprivation and in many systemic catabolic disease states, including AIDS, cancer, burn injury, diabetes, cardiac and renal failure, there is a generalized muscle wasting, which results primarily from increased breakdown of muscle proteins, although protein synthesis also falls in most of these conditions. In all these systemic catabolic states, the loss of muscle mass involves a common pattern of transcriptional changes, including induction of genes for protein degradation, and decreased expression of various genes for growth-related and energy-yielding processes. We have termed this group of co-ordinately regulated genes, “atrogenes”. Between these genes there are several belonging to important cellular function such as energy production, chromatin remodelling, transcription, proteolysis, protein synthesis and metabolism. In the atrophying muscles, the ubiquitin-proteasome and autophagy-lysosome systems are activated and catalyze the degradation of the bulk of muscle proteins, including myofibrillar components, and of organelles. We were the first to identify the transcription factor involved in regulation of muscle atrophy program. The key mediators of this catabolic response during atrophy are the FoxO family of transcription factors, whose activity is suppressed during growth by phosphorylation by AKT, but whose expression and dephosphorylation rises in catabolic states (Sandri et al., Cell 2004). FoxO transcription factors are crucial during muscle loss and they coordinate the activation of the two most important proteolytic system of the cell, the autophagy-lysosome and ubiquitin-proteasome (Mammucari et al., Cell Metab. 2007; Milan et al., Nature Communications 2015), because FoxOs regulate the expression of few rate-limiting enzymes of the two proteolytic systems. The new frontier of my lab is the identification of novel FoxO downstream targets that orchestrate the autophagy-lysosome network and ubiquitin-proteasome system.

Role of physical activity, mitochondrial dynamics (biogenesis and fragmentation) and energy balance in muscle mass regulation and healthy ageing

According to the Worlds Health Organization (WHO) inactivity has been identified as the fourth leading risk factor for global mortality (WHO, 2009). Strikingly, sedentary behaviour has a deleterious effect on human health that has been quantified to be similar to smoking and severe obesity. Conversely, the impact of physical activity on human health is profound and unequivocal. Indeed, physical activity and exercise counteract the onset of the most deadly chronic diseases, including cardiovascular diseases, metabolic diseases, cancer, pulmonary disease, immune dysfunction, musculoskeletal and neurological disorders. Not surprising, an active life is important for a healthy ageing and to promote longevity. Despite the indisputable evidence of the myriad physiological benefits conferred by regular exercise, the exact molecular mechanisms by which physical activity promotes human health and conversely, inactivity causes disease onset remain poorly understood.

We have shown that both autophagy and mitochondrial dynamics are physical activity sensors. In fact, inactivity reduces while exercise stimulates these systems. By using genetic approaches (e.g. generation of muscle specific and inducible muscle specific transgenic and knockout mice), we have found that mitochondrial quality control (e.g. mitophagy, mitochondrial fusion and fission) plays an important role not only in muscle mass regulation but also in distal organs function and longevity. For instance, autophagy is reduced in myofibers and muscle stem cells during ageing in humans (Garcia Prat et al. Nature 2016; Carnio et al. Cell Reports 2014), and specific inhibition of autophagy in myofibers induces a precocious ageing phenotype that ultimately reduces lifespan of mice.

Moreover, we have found that mitochondrial fusion and fission proteins are downregulated during unhealthy ageing and are induced by exercise both in humans and mice. By using inducible muscle specific knockout mice, we have recently shown that inhibition of mitochondrial fusion in skeletal muscle is sufficient to activate senescence programs in epithelial tissues (e.g. skin, liver and gut), to trigger a systemic inflammatory response and to cause a premature death. Mechanistically, mitochondrial dysfunction in muscles triggers the expression and the secretion of the hormone FGF21 that induces premature ageing, systemic inflammation and the precocious death (Tezze et al. Cell Metabolism 2017). Altogether these and other findings confirm that muscles by secreting hormones, metabolites and miRNAs alter distal tissue function/metabolism ultimately affecting disease onset/proregression and organism survival. The next step is to identify which are these pro-ageing factors, to develop diagnostic tools for biological age determination and to generate new therapeutic approaches (drugs, diet, exercise) to promote healthy ageing.

Finally, we have dissected how exercise ameliorate glucose homeostasis and mitochondrial activity. In fact, we have shown that exercise stimulates the calcium dependent phosphatase, calcineurin, that dephosphorylates and activates the transcription factor TFEB. By generating Inducible muscle specific TFEB transgenic and knockout mice we showed that TFEB upregulates glucose transporters for glucose uptake and simultaneously, induces mitochondrial biogenesis independently of PGC1a (Mansueto et al. Cell Metabolsim 2017).

Role of proteostasis in inherited muscle diseases and identification of biomarkers to monitor autophagy in muscles

Proteostasis means the cellular systems that control the quality of the proteins. Among them the degradative processes such as ubiquitin proteasome and autophagy-lysosome play a critical role. Autophagy is a dynamic process regulated by around 30 proteins which commit membranes to engulf portion of cytoplasm, organelles and proteins for delivery to lysosome system to degrade the sequestered components. This process is highly conserved and is important for removal of damaged organelles, toxic proteins and pathogens. Furthermore, autophagy is critical for cell survival, during nutrient deprivation, providing alternative energy sources, but it is detrimental when is either massively activated or strongly inhibited. We found that both excessive induction of autophagy (self eating) and inhibition of basal autophagy result in muscle loss even. Because the morphological alterations of the muscle specific autophagy knockout mice are reminiscent of features present in some myopathies/dystrophies we investigated the role of autophagy in inherited muscle diseases. Importantly, we found that collagen VI myopathies, i.e. Ullrich and Bethlem dystrophy, Duchenne dystrophy and the lysosomal storage disease, i.e. Pompe and Danon diseases, are characterized by an autophagy defect. Importantly, when we restore the autophagy flux via nutritional, genetic or pharmacological approaches we greatly ameliorate muscle force and structure in these inherited diseases (Grumati et al. Nat Med. 2010). This result allow us to set up the first clinical trial based on a nutritional approach in Bethlem patients that was successful. The next step is to develop specific drug for autophagy reactivation and to develop blood biomarkers that mirror the autophagy status of muscles.

Finally, we have shown that inhibition of a muscle specific ubiquitin ligase (atrogin1) resulted in a cardiomyopathy that was consequent to autophagy impairment. Proteomic analyses identified the mechanistic link between ubiquitin proteasome, autophagy system and heart failure. Indeed, atrogin1 regulates the half life of a critical protein of autophagy-lysosome fusion whose accumulation resulted in autophagy impairment and cardiomyocyte apoptotic cell death (Taglia et al. Journal Clinical Investigation 2014). The identification of the cross-talk between these two proteolytic systems as well as with the protein synthesis is a major goal of my lab.

2025 ERC Advanced research grant award

2024/ 2023/2022/2021 Highly Cited Researcher by CLARIVATE

2025/2024/2023. Biology and Biochemistry Leader Award by Research. Com,

2023. Alfredo Margreth prize for the Muscle Biology and Physiopathology by National Academy of Lincei

1. Associazione Spaziale Italiana (ASI)
2. Fondazione AIRC