Project description:Background: An anterior cruciate ligament tear (ACLT) leads to protracted quadriceps weakness and atrophy. Protein turnover largely dictates muscle size and is highly responsive to injury and loading. Regulation of the molecular protein synthetic machinery within the quadriceps following ACLT has largely been unexplored, limiting the development of targeted therapies. Purpose: To define the effect of ACLT on 1) activation of protein synthetic and catabolic signaling within quadriceps biopsies from human participants, and 2) the time course of alterations to protein synthesis and its molecular regulation in a mouse model of ACL injury. Study Design: Descriptive laboratory studyMethods: Muscle biopsy specimens were obtained from the ACL-injured and non-injured vastus lateralis of young adults following an overnight fast (n=21, mean ± SD: 19 ± 5 years). Mice had their limbs assigned to ACLT or control, and whole quadriceps were collected 6 hours, 1, 3, or 7 days post-injury with puromycin (0.04µmol/g) injected 30 minutes before tissue collection. Muscle fiber size and expression and phosphorylation of protein anabolic signaling proteins and E3 ubiquitin ligases were assessed at the protein and transcript level. Relative protein synthesis was measured by puromycin incorporation in mice.Results: Human quadriceps showed reduced phosphorylation of ribosomal protein S6 (-41%) in the ACL-injured limb (p<0.05), in addition to elevated phosphorylation of eukaryotic initiation factor 2α (+98%, p<0.05), indicative of depressed protein anabolic signaling in the injured limb. No differences in E3 ubiquitin ligase expression were noted (p>0.05). Protein synthesis was lower at 1 and 3 days post-ACLT in mice (p<0.05 vs. control limb). Conclusions: 1) Global protein synthesis and anabolic signaling deficits occur in the quadriceps in response to ACL injury, without notable changes in measured markers of muscle protein catabolism. 2) Importantly, these deficits occur prior to the onset of significant atrophy, underscoring the need for early intervention. Clinical Relevance: These findings suggest blunted protein anabolism as opposed to increased catabolism likely mediates the quadriceps atrophy that occurs following ACL injury. Thus, future interventions should aim to restore muscle protein anabolism rapidly following the initial injury.

Project description:The anterior cruciate ligament (ACL) is an essential stabilizer of the tibiofemoral articulation. ACL tears often lead to functional instability and are associated with an increased risk for osteoarthritis. The healing potential of the injured ACL is poorly understood and is considered to be limited. Transcriptome-wide expression profiles of 24 human ACL remnants recovered at the time of surgical reconstruction were analyzed utilizing the Agilent human 8x60K microarray platform. Gene ontology was performed on differentially expressed transcripts based on time-from-injury (acute, <3 months; intermediate, 3-12 months; chronic, >12 months). A subset of transcripts was validated via microfluidic digital polymerase-chain-reaction. Expression of periostin, a highly differentially expressed transcript, was tested by immunohistochemistry. Numerous transcripts covering important functional classifications were differentially expressed by time-from-injury. In acute tears, processes representing angiogenesis were repressed while those representing stem-cell differentiation were elevated. In intermediate tears, processes representing stem-cell proliferation concomitant with cellular component organization/cellular localization were elevated. In chronic tears, processes denoting myosin filament organization were elevated while those representing cellular component organization/cell localization and extracellular matrix organization were repressed. Expression levels of periostin were down-regulated in chronic tears compared to acute (42-fold) and intermediate (29-fold) tears. Immunohistochemistry confirmed a decline in periostin expression in tissues from chronic tears. These findings suggest an initial attempt of the injured ACL to repair, which declines with time-from-injury. These findings have implications for efforts to repair the ACL and may be relevant for reconstruction of the ACL. The functional role of periostin in ACL injuries, and the potential implication for surgical treatment, warrants further investigation. Total RNA obtained frominjured anterior cruciate ligament (ACL) tissues from pateints undergoing ACL surgery.

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Genetic and monoclonal antibody blockade of GDF8 enhances musculoskeletal recovery and mitigates posttraumatic osteoarthritis following ligament injury [cartilage]

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