Defining Endogenous Mitochondrial Transfer in Muscle After Rotator Cuff Injury.

BACKGROUND: Rotator cuff muscle degeneration leads to poor clinical outcomes for patients with rotator cuff tears. Fibroadipogenic progenitors (FAPs) are resident muscle stem cells with the ability to differentiate into fibroblasts as well as white and beige adipose tissue. Induction of the beige adipose phenotype in FAPs has been shown to improve muscle quality after rotator cuff tears, but the mechanisms of how FAPs exert their beneficial effects have not been fully elucidated. PURPOSE: To study the horizontal transfer of mitochondria from FAPs to myogenic cells and examine the effects of ß-agonism on this novel process. STUDY DESIGN: Controlled laboratory study. METHODS: In mice that had undergone a massive rotator cuff tear, single-cell RNA sequencing was performed on isolated FAPs for genes associated with mitochondrial biogenesis and transfer. Murine FAPs were isolated by fluorescence-activated cell sorting and treated with a ß-agonist versus control. FAPs were stained with mitochondrial dyes and cocultured with recipient C2C12 myoblasts, and the rate of transfer was measured after 24 hours by flow cytometry. PdgfraCreERT/MitoTag mice were generated to study the effects of a rotator cuff injury on mitochondrial transfer. PdgfraCreERT/tdTomato mice were likewise generated to perform lineage tracing of PDGFRA+ cells in this injury model. Both populations of transgenic mice underwent tendon transection and denervation surgery, and MitoTag-labeled mitochondria from Pdgfra+ FAPs were visualized by fluorescent microscopy, spinning disk confocal microscopy, and 2-photon microscopy; overall mitochondrial quantity was compared between mice treated with ß-agonists and dimethyl sulfoxide. RESULTS: Single-cell RNA sequencing in mice that underwent rotator cuff tear demonstrated an association between transcriptional markers of adipogenic differentiation and genes associated with mitochondrial biogenesis. In vitro cocultures of murine FAPs with C2C12 cells revealed that treatment of cells with a ß-agonist increased mitochondrial transfer compared to control conditions (17.8% ± 9.9% to 99.6% ± 0.13% P < .0001). Rotator cuff injury in PdgfraCreERT/MitoTag mice resulted in a robust increase in MitoTag signal in adjacent myofibers compared with uninjured mice. No accumulation of tdTomato signal from PDGFRA+ cells was seen in injured fibers at 6 weeks after injury, suggesting that FAPs do not fuse with injured muscle fibers but rather contribute their mitochondria. CONCLUSION: The authors have described a novel process of endogenous mitochondrial transfer that can occur within the injured rotator cuff between FAPs and myogenic cells. This process may be leveraged therapeutically with ß-agonist treatment and represents an exciting target for improving translational therapies available for rotator cuff muscle degeneration. CLINICAL RELEVANCE: Promoting endogenous mitochondrial transfer may represent a novel translational strategy to address muscle degeneration after rotator cuff tears.

Outcomes of anatomic and reverse total shoulder arthroplasty in patients over the age of 70: a systematic review.

Background: Both anatomic total shoulder arthroplasty (ATSA) and reverse total shoulder arthroplasty (RTSA) reliably improve pain and function for a variety of indications. However, there remain concerns about these procedures among elderly patients due to their general health, the potential for lesser functional gain, and the possible need for revision at an even older age. The purpose of this review is to compare the clinical outcomes, radiographic outcomes, and complications of ATSA and RTSA among patients older than 70 years. Methods: A systematic review was performed using searches of PubMed, Embase, and Cochrane databases. The inclusion criteria were studies with patients older than 70 years who were treated with a primary ATSA or RTSA and clinical results reported at a minimum of 2 years. All indications for primary RTSA except for tumor were included. Outcomes of interest included patient-reported outcomes (PROs), range of motion, patient satisfaction, radiographic changes, complication and revision rates, and implant survival. Results: A total of 24 studies met the inclusion criteria. At a mean follow-up of 3.4 years for ATSA and 3.1 years for RTSA, there were significant improvements in pain, range of motion, and PROs for both prostheses. Patients who underwent ATSA generally had better motion and functional outcomes compared to those who underwent RTSA, though these comparisons were made across different indications for arthroplasty. The satisfaction rate was 90.9% after ATSA and 90.8% after RTSA. Furthermore, 10.2% of ATSA patients and 9.9% of RTSA patients experienced a surgical complication, whereas 2.3% of ATSA and 2.2% of RTSA patients underwent a revision. Secondary rotator cuff tear was the most common complication after ATSA, occurring in 3.7% of patients, but only 1.1% of patients required revision surgery. Both ATSA and RTSA implant survivorship was reported to range from 93.1% to 98.9% at 5- and 8-year follow-up, respectively. Patient mortality was estimated to be 19.3% with a mean time to death of 6.1 years. Conclusions: Elderly patients with primary osteoarthritis and an intact rotator cuff can have predictable pain relief, restoration of functional range of motion, and significant improvement in PROs after ATSA with low complication rates. Secondary rotator cuff failure and revision arthroplasty occur infrequently at early to mid-term follow-up. Although elderly patients who underwent ATSA generally had better functional outcomes compared to those who underwent RTSA for differing indications, patient satisfaction after both procedures were similar.

Paraspinal muscle degeneration and regenerative potential in a Murine model of Lumbar Disc Injury.

BACKGROUND CONTEXT: The association between low back pain and lumbar disc degeneration is not fully characterized. One potentially overlooked factor of this process is the lumbar multifidus, which plays a role in segmental stabilization and locomotion. Previous reports have shown multifidus degeneration to be associated with disc degeneration. The goal of this study was to develop a mouse model of advanced lumbar disc degeneration to recapitulate the pathology of the human multifidus in patients with lumbar disc degeneration and low back pain. METHODS: C57BL/6 mice underwent a left anterolateral approach to the lumbar spine and disc puncture with a micro scalpel at L5/6 and L6/S1. Mice underwent behavioral analysis and functional gait testing. A subset of mice underwent 14T T2-weighted MRI to assess disc degeneration and paraspinal muscle quality. At 6 and 15 weeks, mice were sacrificed, and the multifidus muscles were harvested and divided into proximal and distal segments relative to disc injury. Histological analysis was performed to assess muscle degeneration, fiber type, and macrophage density. Fibroadipogenic progenitors (FAPs) were isolated for gene expression analysis with qPCR. RESULTS: MRI demonstrated decreased intervertebral disc signal and paraspinal muscle atrophy at 6 weeks, with progressive degeneration and atrophy at 15 weeks. Disc injury resulted in delayed functional recovery and impaired gait. Histology demonstrated progressive multifidus fibrofatty degeneration between 6 and 15 weeks. CD68+ macrophage density was increased at 6 weeks but not 15 weeks. FAPs exhibited increased fibrotic and adipogenic gene expression at 6 weeks compared to sham with less of a difference in gene expression by 15 weeks. CONCLUSIONS: We have developed a mouse model of disc injury-mediated paraspinal muscle degeneration that recapitulates features of degenerative muscle pathology observed in patients with lumbar disc degeneration, and highlights the role of FAPs in mediating fibrofatty muscle degeneration after disc injury.