Knocking-out matrix metalloproteinase-13 exacerbates rotator cuff muscle fatty infiltration.

INTRODUCTION: Rotator cuff (RC) tears are common tendon injuries. Clinically, both muscle atrophy and fatty infiltration have generally been attributed to poor functional outcomes. Matrix metalloproteinase-13 plays a crucial role in extracellular matrix remodeling in many physiological and pathological processes. Nevertheless, its role in rotator cuff muscle atrophy and fatty infiltration remains unknown. The purpose of this study is to define the functional role of MMP-13 in rotator cuff muscle atrophy and fatty infiltration using a mouse RC tears model. MATERIALS AND METHODS: Unilateral complete supraspinatus and infraspinatus tendon transection and suprascapular nerve transection was performed on nine of MMP-13 (-/-) knockout and nine of MMP-13 (+/+) wildtype mice at 3 months old. Mice were sacrificed 6 weeks after surgery. Supraspinatus (SS) and infraspinatus (IS) muscles were harvested for histology and gene expression analysis with RT-PCR. RESULTS: Six weeks after RC surgery, no significant difference in muscle atrophy and fibrosis between MMP-13 knockout and wild type mice was observed. However, there was a significant increase in the amount of fatty infiltration in MMP-13 knockout mice compared to the wild types. Muscles from MMP-13 knockout mice have significantly higher expression of fatty infiltration related genes. DISCUSSION: Results from this study suggest that MMP-13 plays a crucial role in rotator cuff muscle fatty degeneration. This novel finding suggests a new molecular mechanism that governs RC muscle FI and MMP-13 may serve as a target for therapeutics to treat muscle FI after RC tears.

Muscle atrophy and fatty infiltration after an acute rotator cuff repair in a sheep model.

INTRODUCTION: rotator cuff tears (RCTs) are the most common tendon injury seen in orthopedic patients. Muscle atrophy and fatty infiltration of the muscle are crucial factors that dictate the outcome following rotator cuff surgery. Though less studied in humans, rotator cuff muscle fibrosis has been seen in animal models as well and may influence outcomes as well. The purpose of this study was to determine if the rotator cuff would develop muscle changes even in the setting of an acute repair in a sheep model. We hypothesized that fatty infiltration and fibrosis would be present even after an acute repair six months after initial surgery. METHODS: twelve female adult sheep underwent an acute rotator cuff tear and immediate repair on the right shoulder. The left shoulder served as a control and did not undergo a tear or a repair. Six months following acute rotator cuff repairs, sheep muscles were harvested to study atrophy, fatty infiltration, and fibrosis by histological analysis, western blotting, and reverse transcription polymerase chain reaction (RT-PCR). RESULTS: the repair group demonstrated an increase expression of muscle atrophy, fatty infiltration, and fibrosis related genes. Significantly increased adipocytes, muscle fatty infiltration, and collagen deposition was observed in rotator cuff muscles in the tendon repair group compared to the control group. CONCLUSIONS: rotator cuff muscle undergoes degradation changes including fatty infiltration and fibrosis even after the tendons are repair immediately after rupture. LEVEL OF EVIDENCE: Basic Science Study.

Bone morphogenetic protein signaling in rotator cuff muscle atrophy and fatty infiltration.

BACKGROUND: reduced mass (atrophy) and increased fat content (fatty infiltration) of rotator cuff muscles are common complications of large or massive rotator cuff (RC) tears, and are believed to be irreversible even after tendon repairs. Clinically, both muscle atrophy and fatty infiltration are important factors contributing to poor functional outcomes after tendon repairs. The molecular mechanism of RC muscle atrophy and FI remains undefined. In this study, we investigated the role of bone morphogenetic proteins (BMP) signaling in RC muscle atrophy and fatty infiltration using a rat model. METHODS: unilateral massive RC tears was induced in adult rats. RC muscles were harvested at 2 and 6 weeks after injury for BMP signaling analysis. In a separate experiment, BMP inhibitor (LDN-193189) was injected to rats through daily intraperitoneal injection. RC muscles from rats in the treated and control groups were harvested at 6 weeks after injury for biochemistry and histology analysis. RESULTS: we found significantly increased BMP-14 and BMP-7 expression in rotator cuff muscles after RCT. Inhibiting BMP signaling resulted in increased muscle atrophy and reduced fatty infiltration in rotator cuff muscle after RC tears. CONCLUSION: this result suggests that BMP signaling inhibits RC muscle atrophy but promotes fatty infiltration.

Rat rotator cuff muscle responds differently from hindlimb muscle to a combined tendon-nerve injury.

Rotator cuff tears (RCTs) are among the most common musculoskeletal injuries seen by orthopaedic surgeons. Clinically, massive cuff tears lead to unique pathophysiological changes in rotator cuff muscle, including atrophy, and massive fatty infiltration, which are rarely seen in other skeletal muscles. Studies in a rodent model for RCT have demonstrated that these histologic findings are accompanied by activation of the Akt/mammalian target of rapamycin (mTOR) and transforming growth factor-ß (TGF-ß) pathways following combined tendon-nerve injury. The purpose of this study was to compare the histologic and molecular features of rotator cuff muscle and gastrocnemius muscle--a major hindlimb muscle, following combined tendon-nerve injury. Six weeks after injury, the rat gastrocnemius did not exhibit notable fatty infiltration compared to the rotator cuff. Likewise, the adipogenic markers SREBP-1 and PPARγ as well as the TGF-ß canonical pathway were upregulated in the rotator cuff, but not the gastrocnemius. Our study suggests that the rat rotator cuff and hindlimb muscles differ significantly in their response to a combined tendon-nerve injury. Clinically, these findings highlight the unique response of the rotator cuff to injury, and may begin to explain the poor outcomes of massive RCTs compared to other muscle-tendon injuries.

Upregulation of transforming growth factor-ß signaling in a rat model of rotator cuff tears.

BACKGROUND: Muscle atrophy, fatty infiltration, and fibrosis of the muscle have been described as important factors governing outcome after rotator cuff injury and repair. Muscle fibrosis is also thought to have a role in determining muscle compliance at the time of surgery. The transforming growth factor-ß (TGF-ß) pathways are highly conserved pathways that exert a potent level of control over muscle gene expression and are critical regulators of fibrosis in multiple organ systems. It has been shown that TGF-ß can regulate important pathways of muscle atrophy, including the Akt/mammalian target of rapamycin pathway. The purpose of this study was to evaluate the expression of TGF-ß and its downstream effectors of fibrosis after a massive rotator cuff tear (RCT) in a previously established rat model. METHODS: To simulate a massive RCT, infraspinatus and supraspinatus tenotomy and suprascapular nerve transection were performed on Sprague-Dawley rats with use of a validated model. Two and 6 weeks after surgery, supraspinatus muscles were harvested to study alterations in TGF-ß signaling by Western blotting, quantitative polymerase chain reaction, and histologic analysis. RESULTS: There was a significant increase in fibrosis in the rotator cuff muscle after RCT in our animal model. There was a concomitant increase in TGF-ß gene and protein expression at both 2 and 6 weeks after RCT. Evaluation of the TGF-ß signaling pathway revealed an increase in SMAD2 activation but not in SMAD3. There was an increase in profibrotic markers collagen I, collagen III, and α-smooth muscle actin. CONCLUSIONS: TGF-ß signaling is significantly upregulated in rat supraspinatus muscles after RCTs.

Thapsigargin blocks Pseudomonas aeruginosa homoserine lactone-induced apoptosis in airway epithelia.

Pseudomonas aeruginosa secretes N-(3-oxododecanoyl)-homoserine lactone (C12) as a quorum-sensing molecule to regulate gene expression. Micromolar concentrations are found in the airway surface liquid of infected lungs. Exposure of the airway surface to C12 caused a loss of transepithelial resistance within 1 h that was accompanied by disassembly of tight junctions, as indicated by relocation of the tight junction protein zonula occludens 1 from the apical to the basolateral pole and into the cytosol of polarized human airway epithelial cell cultures (Calu-3 and primary tracheal epithelial cells). These effects were blocked by carbobenzoxy-valyl-alanyl-aspartyl-[O-methyl]-fluoromethylketone, a pan-caspase blocker, indicating that tight junction disassembly was an early event in C12-triggered apoptosis. Short-duration (10 min) pretreatment of airway epithelial (Calu-3 and JME) cells with 1 µM thapsigargin (Tg), an inhibitor of Ca(2+) uptake into the endoplasmic reticulum (ER), was found to be protective against the C12-induced airway epithelial barrier breakdown and also against other apoptosis-related effects, including shrinkage and fragmentation of nuclei, activation of caspase 3/7 (the executioner caspase in apoptosis), release of ER-targeted redox-sensitive green fluorescent protein into the cytosol, and depolarization of mitochondrial membrane potential. Pretreatment of Calu-3 airway cell monolayers with BAPTA-AM [to buffer cytosolic Ca(2+) concentration (Cacyto)] or Ca(2+)-free solution + BAPTA-AM reduced C12 activation of apoptotic events, suggesting that C12-triggered apoptosis may involve Ca(2+). Because C12 and Tg reduced Ca(2+) concentration in the ER and increased Cacyto, while Tg increased mitochondrial Ca(2+) concentration (Camito) and C12 reduced Camito, it is proposed that Tg may reduce C12-induced apoptosis in host cells not by raising Cacyto, but by preventing C12-induced decreases in Camito.