REDD1 Regulates MERCS, Protein Synthesis and NMJ Stability in Fast Myofibers During Dexamethasone-Induced Muscle Wasting

Background: Glucocorticoids cause skeletal muscle atrophy preferentially affecting fast glycolytic fibers, but the mechanisms involved in this fiber selectivity is unclear. REDD1 is a glucocorticoid-induced stress protein that limits muscle protein synthesis inducing atrophy. However, it remains unknown whether REDD1 exerts myofiber type-specific effects and through which precise mechanisms it regulates protein synthesis. We investigated the role of myofiber REDD1 expression in dexamethasone (DEX)-induced muscle atrophy, with a particular focus on its involvement in mitochondria-ER contact sites (MERCS), protein synthesis, and neuromuscular junction (NMJ) integrity. Methods: We generated tamoxifen-inducible, muscle-specific REDD1 knockout mice (REDD1fl/flHSA-CreERT2) and compared them with floxed littermates (WT) in a 2x2 design (WT/KO x PBS/DEX, 7 days). We combined single-nucleus RNA sequencing, RNAscope, immunofluorescence, transmission electron microscopy, proximity ligation assay, SUnSET puromycin labelling, western blot and RT-qPCR, and AdenoFATE1-mediated MERCS disruption in C2C12 myotubes. Results: Glucocorticoid receptor and REDD1 transcripts were co-enriched in fast glycolytic fibers mostly atrophied by DEX (~20%). REDD1 deletion in myofiber drove to lower basal muscle mass and fast fiber volume but protected them from DEX-induced atrophy. DEX inhibited protein synthesis (~70%) in WT mice with no matching change in Akt/mTOR-pathway activity. In REDD1 KO mice, protein synthesis was already low and was not affected by DEX. DEX-induced REDD1 expression remodelled mitochondrial network and MERCS in a subcellular compartment-specific manner. The intermyofibrillar MERCS minimum distance shortened in both genotypes reaching pathological distances only in WT mice (WT ~28 --> ~5 nm; KO ~25 --> ~15 nm). Perinuclear MERCS and mitochondria-nuclei distances increased in WT mice only (~18 --> ~45 nm and ~130 --> ~460 nm). In WT mice only, DEX-induced alteration of the perinuclear mitochondrial network was associated with a loss of myonuclei accumulating mt-RNA and exhibiting an anabolic transcriptomic signature notably enriched in sarcomeric transcripts. These findings suggest that REDD1-dependent MERCS remodelling may regulate muscle anabolism beyond the control of mRNA translation, by shaping the myonuclear transcriptome. Finally, REDD1 localised to the NMJ and reduced endplate area during DEX treatment. Interestingly, MERCS were denser in NMJ than in myofiber body and we showed in vitro that FATE1-mediated MERCS disruption was sufficient to reduce protein synthesis and agrin-induced acetylcholine-receptor clustering demonstrating that REDD1 and MERCS are important for NMJ stabilization. Conclusions: Muscle REDD1 links the glucocorticoid response to compartment-specific mitochondrial network remodelling, protein synthesis as well as NMJ stability in fast glycolytic fibers. Our results also show that REDD1 is important for maintaining basal mitochondrial network and protein synthesis homeostasis.