Early Physiological Changes To The Vastus Lateralis After Non-invasive Anterior Cruciate Ligament Injury

Abstract

Insufficient recovery of quadriceps muscle strength is commonly reported after anterior cruciate ligament (ACL) injury. Although weakness is secondary to a complex manifestation of inhibition, the extent and time course of the morphological changes in muscle are largely unknown. Using a novel, translational animal model of ACL injury, a longitudinal study was performed to illuminate the mechanisms underlying acute muscle atrophy. PURPOSE: To investigate the role of atrophic pathways after non-invasive ACL rupture. METHODS: Male Long-Evans rats were randomly assigned to 8 groups (n=8 per group): 1 control group and 7 ACL injury groups (6, 12, 24, 48-hours, and 1, 2, 4-weeks). The right hindlimbs of ACL injury rats were exposed to a single impulse, longitudinal tibial compression, to induce a non-invasive ACL rupture, followed by normal cage activity. After which rats were euthanized as per assigned group. Right and left vastus lateralis muscles (VL) were harvested, weighed and flash frozen in liquid nitrogen. The VL were immunoreacted for dystrophin to quantify fiber cross sectional area (CSA), and RNA was isolated to measure the abundance of MuRF-1, MAFbx (markers of protein degradation) and 45s (marker of translational capacity). rRNA expression was determined using RT-PCR. One-way ANOVAs with Bonferroni post-hoc were used to determine differences between groups, and paired t-tests were used to detect VL differences between limbs (P < 0.05). RESULTS: ACL injury resulted in a decrease in muscle wet weight (p=0.0003) and a trend toward reduced CSA (p=0.06) at 1-week post injury, compared to control, 2-and 4-week time points. CSA of the ACL injured limb VL was smaller than the VL of the contralateral limb at 1-week only (p= 0.01). MAFbx abundance was significantly increased at 48-hours post-ACL injury (p=0.0001), with no differences for 45s rRNA, total RNA concentration or MuRF-1. CONCLUSIONS: Results indicate that ACL injury induces atrophy which is transient and not related to a decrease in ribosome biogenesis but likely due to increased protein degradation. Future studies should focus on a comprehensive analysis of atrophic pathways after ACL injury, to establish key therapeutic windows for targeting therapy-resistant quadriceps weakness after ACL injury. Supported by K01AR071503.