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

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.; 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.; 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

mTORC1 and PKB/Akt control the muscle response to denervation by regulating autophagy and HDAC4

Loss of innervation of skeletal muscle is a determinant event in several muscle diseases. Although several effectors have been identified, the pathways controlling the integrated muscle response to denervation remain largely unknown. Here, we demonstrate that PKB/Akt and mTORC1 play important roles in regulating muscle homeostasis and maintaining neuromuscular endplates after nerve injury. To allow dynamic changes in autophagy, mTORC1 activation must be tightly balanced following denervation. Acutely activating or inhibiting mTORC1 impairs autophagy regulation and alters homeostasis in denervated muscle. Importantly, PKB/Akt inhibition, conferred by sustained mTORC1 activation, abrogates denervation-induced synaptic remodeling and causes neuromuscular endplate degeneration. We establish that PKB/Akt activation promotes the nuclear import of HDAC4 and is thereby required for epigenetic changes and synaptic gene up-regulation upon denervation. Hence, our study unveils yet-unknown functions of PKB/Akt-mTORC1 signaling in the muscle response to nerve injury, with important implications for neuromuscular integrity in various pathological conditions

Understanding the pathomechanisms leading to muscle alterations in Myotonic Dystrophy type 1: Consequences of CaMKII deregulation on the maintenance of neuromuscular junctions

Myotonic Dystrophy Type 1 (DM1) is a multisystemic autosomal dominant disorder and represents the most common form of muscular dystrophy in adults. DM1 is caused by a (CTG)n repeat expansion in the 3` UTR of the DMPK gene. Once transcribed, the aberrant (CUG)n transcripts form stable double-stranded structures, which sequester multiple RNA-binding proteins, resulting in the defective splicing of numerous genes. Since the discovery that RNA-gain-of-function constitutes a major pathological event in DM1, therapeutic strategies have mainly focused on targeting mis-splicing events to counteract the spliceopathy. Although refinement of experimental and therapeutic approaches have allowed a better understanding of DM1 pathophysiology, there is still no cure available. Over the last years, studies unveiled the contribution of different deregulated cellular processes to DM1 muscle pathology. In particular, previous results in the group showed that the key metabolic pathways AMPK and mTORC1 are perturbed in DM1 muscle. They further suggested a major deregulation of Ca2+/calmodulin-dependent protein kinase (CaMK) proteins in DM1 muscle. To get further insights into the pathomechanisms underlying DM1, I investigated whether and how deregulation of CaMKIIs contribute to DM1 muscle affliction. CaMKIIs have been shown to be essential for muscle plasticity, including remodeling of muscle synapses. To this end, CaMKIIs regulate activation and translocation of key factors governing activity-dependent gene expression. In addition, CaMKIIs actively promote the recycling of AChRs to the surface of muscle membrane after their internalization, making them pivotal players in the maintenance of post-synaptic sites. Although studies have reported changes in pre- and post-synaptic compartments of neuromuscular junctions (NMJs) in muscle from DM1 patients and mouse models, a potential contribution of NMJ alterations to DM1 muscle pathogenesis remains under debate. Here I showed that the muscle-specific isoform of CaMKIIβ (CaMKIIβM) is lost in muscle of HSALR and Mbnl1Δ3/Δ3 mice, two well-characterized DM1 mouse models. Fluorescent-based staining approaches of NMJs revealed that HSALR and Mbnl1Δ3/Δ3 muscle exhibit an increased fragmentation of the motor endplates in both, basal conditions and when challenged with nerve injury. Analysis of activity-dependent pathways pointed to a deregulation of HDAC4 and synaptic gene expression, which may involve CaMKII deficiency in HSALR and Mbnl1Δ3/Δ3 muscles. Moreover, I showed that AChR turnover is increased in muscles from HSALR and Mbnl1Δ3/Δ3 mice under basal conditions. This was accompanied by a reduction in AChR recycling at post-synaptic sites of DM1 muscle, which may also arise from CaMKII deficiency. Lastly, Mbnl1Δ3/Δ3 mice showed defective up-regulation of synaptic genes upon nerve injury, while AChR turnover was strongly increased. This abnormal response to denervation may involve yet unknown CaMKII-independent mechanisms. Overall, these findings suggest that defective maintenance of NMJs in DM1 muscle may involve CaMKII-dependent and -independent processes, and may be key events contributing to DM1 muscle pathophysiology

Additional file 1: of Alterations to mTORC1 signaling in the skeletal muscle differentially affect whole-body metabolism

Supplementary Figures S1 to S5. Figure S1. Unchanged expression of Actb. (A) Actb (encoding β-actin) expression in the muscle, liver, WAT, and BAT of control (n=5) and RAmKO mice (n=5). (B) List of antibodies used. Figure S2. Metabolism of female TSCmKO and RAmKO mice. (A) Body weight, (B) lean mass, (C) fat massand (D) plasma glucose levels of 10-week-old female TSCmKO (n=9), RAmKO (n=8) and control (Ctrl) mice (n=11). (E)–(G) Fat mass (E), lean mass (F) and plasma glucose levels (G) of male RAmKO (n=4) and control (Ctrl) mice (n=6) on a HFD. Figure S3. RAmKO mice do not show changes in other organs. (A) Glycogen amount in gastrocnemius muscle of 10-week-old TSCmKO mice (n=3). (B) Western blot analysis of liver from 10-week-old TSCmKO and control (Ctrl) mice (n=4). Mice were intraperitoneally injected with insulin (+; TSC-insulin) or not (−). Protein expression is normalized to eEF2. (C)–(D) 12-week-old RAmKO mice do not show changes in Ucp2 expression in the liver and WAT (C) or Ucp1 and Ucp2 in BAT (D) compared to control mice (n=5). (E)–(F) Liver expression of genes involved in lipid (E) and glucose (F) metabolism of RAmKO mice (n=5). Figure S4. ATP levels in 12-week-old RAmKO soleus muscle (n = 5). Figure S5. Body composition in myopathic female TSCmKO and RAmKO mice. (A) Body weight, (B) lean and (C) fat mass of 40-week-old TSCmKO (n=10), 20-week-old RAmKO (n=4) and respective control (Ctrl) littermates (n=11). (D) The kidneys of 40-week-old TSCmKO mice appear polycystic. Cysts are indicated by arrows

Additional file 2: of Alterations to mTORC1 signaling in the skeletal muscle differentially affect whole-body metabolism

Supplementary Tables S1 and S2. Table S1. List of primers used. Table S2. RAmKO blood analysis