RESUMEN
BACKGROUND: Muscle atrophy, fibrosis, and fatty infiltration are commonly seen in rotator cuff tears (RCTs), which are critical factors that directly determine the clinical outcomes for patients with this injury. Therefore, improving muscle quality after RCT is crucial in improving the clinical outcome of tendon repair. In recent years, it has been discovered that adults have functional beige/brown adipose tissue (BAT) that can secrete batokines to promote muscle growth. PRDM16, a PR-domain-containing protein, was discovered with the ability to determine the brown fat cell fate and stimulate its development. Thus, the goal of this study was to discover the role of PRDM16 in improving muscle function after massive tendon tears using a transgenic mouse model with an elevated level of PRDM16 expression. METHODS: Transgenic aP2-driven PRDM16-overexpressing mice and C57BL/6J mice underwent unilateral supraspinatus (SS) tendon transection and suprascapular nerve transection (TTDN) as described previously (n = 8 in each group). DigiGait was performed to evaluate forelimb function at 6 weeks post the TTDN injury. Bilateral SS muscles, interscapular brown fat, epididymal white fat, and inguinal beige fat were harvested for analysis. The expression of PRDM16 in adipose tissue was detected by Western blot. Masson Trichrome staining was conducted to evaluate the muscle fibrosis, and Oil Red O staining was used to determine the fat infiltration. Muscle fiber type was determined by major histocompatibility complex (MHC) expression via immunostaining. All data were presented in the form of mean ± standard deviation. t test and 2-way analysis of variance was performed to determine a statistically significant difference between groups. Significance was considered when P < .05. RESULTS: Western blot data showed an increased expression of PRDM16 protein in both white and brown fat in PRDM16-overexpressing mice compared with wild-type (WT) mice. Even though PRDM16 overexpression had no effect on increasing muscle weight, it significantly improved the forelimbs function with longer brake, stance, and stride time and larger stride length and paw area in mice after RCT. Additionally, PRDM16-overexpressing mice showed no difference in the amount of fibrosis when compared to WT mice; however, they had a significantly reduced area of fatty infiltration. These mice also exhibited abundant MHC-IIx fiber percentage in the supraspinatus muscle after TTDN. CONCLUSION: Overexpression of PRDM16 significantly improved muscle function and reduced fatty infiltration after rotator cuff tears. Promoting BAT activity is beneficial in improving rotator cuff muscle quality and shoulder function after RCT.
RESUMEN
Amibegron, a ß3-adrenergic receptor (B3AR) agonist, has recently been shown to provide therapeutic effects for chronic rotator cuff (RC) tears by inducing the expression of uncoupling protein 1 (UCP-1), a marker of brown fat, in fibroadipogenic progenitors (FAPs). However, it remains to be seen if these beneficial effects hold true with age and in older, more clinically relevant populations. This study seeks to understand the impacts of aging on the efficacy of amibegron to treat chronic RC tears. Young (4-month-old) and aged (33-month-old) C57BL/6 mice underwent a RC injury procedure with delayed repair (DR). Mice were equally randomized to receive amibegron or dimethyl sulfoxide (DMSO) treatments after repair. Functional ability was measured at baseline and 6-weeks after DR. Wet muscle weight and histology of injured and contralateral supraspinatus were also analyzed 6-weeks post-DR. For in vitro histology and real-time quantitative PCR experiments, FAPs were isolated from young and aged mice via fluorescence-activated cell sorting. Young and aged FAPs were treated with amibegron or DMSO either immediately after seeding (early exposure) or 8-days after seeding (late exposure). In vitro results showed that amibegron-mediated FAP UCP-1 expression decreases with age. In vivo data demonstrated that aged mice have a decreased responsiveness to amibegron and decreased propensity for intramuscular FAP UCP-1 expression. Further, delayed amibegron treatment with RC repair did not lead to improvements in muscle atrophy and functional outcomes. Our findings demonstrate that age and the timing of interventions play a critical role in FAP-targeted therapeutics for chronic injuries.