Human Rotator Cuff Tears Have an Endogenous, Inducible Stem Cell Source Capable of Improving Muscle Quality and Function After Rotator Cuff Repair

Abstract

Background: The muscle quality of the rotator cuff (RC), measured by atrophy and fatty infiltration (FI), is a key determinant of outcomes in RC injury and repair. The ability to regenerate muscle after repair has been shown to be limited. Purpose: To determine if there is a source of resident endogenous stem cells, fibroadipogenic progenitor cells (FAPs), within RC injury patients, and if these cells are capable of adipogenic, fibrogenic, and pro-myogenic differentiation. Study Design: Controlled laboratory study. Methods: A total of 20 patients between the ages of 40 and 75 years with partial- or full-thickness RC tears of the supraspinatus and evidence of atrophy and FI Goutallier grade 1, 2, or 3 were selected from 2 surgeons at an orthopaedic center. During the surgical repair procedure, supraspinatus muscle biopsy specimens were obtained for analysis as were deltoid muscle biopsy specimens to serve as the control. FAPs and satellite cells were quantified using fluorescence-activated cell sorting. Muscle FI and fibrosis was quantified using Oil Red O and Masson trichrome staining. FAP differentiation and gene expression profiles were compared across tear sizes after culture in adipogenic, fibrogenic, and beta-3 agonist (amibegron) conditions. Analysis of variance was used for statistical comparisons between groups, with P < .05 as statistically significant. Results: Histologic analysis confirmed the presence of fat in biopsy specimens from patients with full-thickness tears. There were more FAPs in the full-thickness tear group compared with the partial-thickness tear group (9.43% ± 4.25% vs 3.84% ± 2.54%; P < .01). Full-thickness tears were divided by tear size, with patients with larger tears having significantly more FAPs than those with smaller tears. FAPs from muscles with full-thickness tendon tears had more adipogenic and fibrogenic potential than those with partial tears. Induction of a beige adipose tissue (BAT) phenotype in FAPs was possible, as demonstrated by increased expression of BAT markers and pro-myogenic genes including insulin-like growth factor 1 and follistatin. Conclusion: Endogenous FAPs are present within the RC and likely are the source of FI. These FAPs were increased in muscles with in larger tears but are capable of adopting a pro-myogenic BAT phenotype that could be utilized to improve muscle quality and patient function after RC repair.

Rotator cuff tear degeneration and the role of fibro‐adipogenic progenitors

“…111 Feeley et al demonstrated that RC FI, fibrosis, and FAP number increased with tear size and thickness in human subjects. 100 Furthermore, the FAPs from full thickness tears displayed greater fibrogenic and adipogenic differentiation capacity in vitro than those harvested from partial tears, suggestive of FAP epigenetic changes across tear states. These studies represent important steps in confirming the cellular actors involved in human RC FI and fibrosis.…”

“…44 Recently, FAPs from the human RC have been shown to adopt a beige fat phenotype in vitro after treatment with the beta-3 agonist amibegron. 100 Furthermore, these beige-like FAPs displayed markedly increased gene expression of IGF-1 and follistatin. Beta-3 agonists, such as amibegron (SR58611A), have been shown to enhance brown fat characteristics, which may point to a potential clinical use in RC injury.…”

“…99 The correlation between increased FAP number and FI with larger tear sizes suggests that different tear states can result in epigenetic changes in FAPs that alter their proliferative and differentiation behaviors. 100 Lee et al found that of the FAPs taken from multiple uninjured muscle groups in mice, those harvested from the RC were of the highest quantity and possessed the greatest proliferative and adipogenic potential. In addition, there was significantly more FI in the RC than in the tibialis anterior (12.6% ± 3.9% versus 1.5% ± 1.0%; P < 0.05) 2 weeks after each muscle received an intramuscular injection of glycerol, suggestive of different baseline characteristics of FAPs depending on anatomic location.…”

“…112,113 A recent study evaluating human supraspinatus biopsies from patients undergoing RC repair found no differences in SC number across different tear sizes, suggesting that RC atrophy is not simply due to a lack of SCs. 100 Thus, there are likely other forces influencing atrophy progression. This is consistent with studies showing that the ability of SCs to regenerate muscle is highly dependent on the characteristics of their stem cell niche, which, in turn, is influenced by the surrounding cells and matrix.…”

The Serum from Patients with Secondary Frozen Shoulder Following Rotator Cuff Repair Induces Shoulder Capsule Fibrosis and Promotes Macrophage Polarization and Fibroblast Activation

“…HE and Masson staining were conducted based on the established method. 34,35 The thickness of joint capsule was obtained by five random measurements to obtain a mean value using ImageJ software. The relative thickness of joint capsule in different groups compared to group NC was calculated.…”