Satellite Cell Fusion Independent Mechanisms Are Critical For Recovery From ACL Injury Induced Muscle Atrophy

Anterior cruciate ligament (ACL) tears are debilitating sport-related injuries resulting in protracted quadriceps atrophy. Our group has previously shown lower satellite cell abundance with atrophy following ACL injury. Canonically, satellite cells fuse into muscle fibers to maintain myonuclear density and following injury, reduced satellite cell abundance compromises muscle recovery. We aimed to determine whether this loss of satellite cells is causative or a consequence of atrophy. PURPOSE: To determine the role of satellite cells in recovery from ACL injury. METHODS: Pax7CreER/CreER:Rosa26DTA/DTA mice were treated with tamoxifen (SC-) or vehicle (SC+) and randomized to quadriceps muscle tissue collection 7 (SC- n = 8, SC+ n = 9) or 28 days (SC- n = 11, SC+ n = 4) post ACL transection surgery (ACLT). Satellite cell abundance, fiber cross-sectional area (μm2), and transcriptional output via bulk tissue RNA-seq were measured. Limb differences were detected using paired t-test while limb differences between SC+ and SC- mice were detected using 2-way ANOVA. RNA-seq data was analyzed using DESeq2. RESULTS: SC- mice showed >90% satellite cell depletion. At 7 days post-ACLT, SC- mice experienced atrophy that was no different than atrophy of SC+ mice (SC-: -15.38 ± 9.68 % difference from Healthy, SC+: -13.08 ± 9.99% difference from Healthy,; p = 0.6377). At 28 days post-ACLT, SC- mice trended toward greater atrophy than SC+ mice (SC-: -10.27 ± 7.40 % difference from Healthy, SC+: -5.68 ± 1.35 % difference from Healthy; p = 0.0731). 142 differentially expressed genes (DEGs) in muscle of the ACLT limb compared to the healthy limb (FDR < 0.05) with the greatest downregulated DEGs being dominated by sarcomeric genes. Additionally, no DEGs were detected between ACLT and healthy limbs of SC+ mice. Additionally, EdU+ myonuclear abundance was nearly undetectable across all mice indicating a lack of satellite cell fusion mediating recovery from atrophy. CONCLUSION: Our results show that satellite cell presence is required for recovery of muscle size following ACL injury without concomitant changes in myonuclear density indicating a non-fusiogenic role of satellite cells in promoting recovery. SCs may represent a novel therapeutic target to mitigate muscle atrophy after ACL injury.