Muscle stem cell activation in a mouse model of rotator cuff injury

Muscle Health & Fatty Infiltration with Advanced Rotator Cuff Pathology

PURPOSE OF REVIEW

Fatty infiltration (FI) of the rotator cuff is a critical determinant of clinical outcomes following rotator cuff injuries and repairs. This review examines the natural history, pathophysiology, imaging evaluation, and treatment strategies for FI, highlighting recent insights into its cellular mechanisms and emerging therapeutic approaches.

RECENT FINDINGS

Animal models demonstrate that FI begins shortly after tendon injury, progresses with muscle retraction and denervation, and is largely irreversible despite repair. Key cellular drivers include fibroadipogenic progenitor cells (FAPs), influenced by mechanical loading and inflammatory signaling pathways. Clinical studies show that FI is associated with advanced age, female sex, and full-thickness tears. Higher degrees of preoperative FI correlate with poorer functional outcomes and increased re-tear rates. Novel therapeutic targets, including pathways regulating FAP activity, TGF-β, and cell-based therapies, show promise in preclinical studies. Emerging strategies such as leukocyte-poor platelet-rich plasma (PRP) may mitigate FI progression in clinical settings. Fatty infiltration remains a significant barrier to successful rotator cuff repair and functional recovery. While surgical repair may slow FI progression, it is not consistently effective in reversing established muscle degeneration. Improved understanding of the molecular mechanisms driving FI has identified potential therapeutic targets, but their clinical applicability requires further validation. Future advances in regenerative medicine, including cell-based therapies and modulation of fibroadipogenic progenitors, offer hope for mitigating FI and improving long-term outcomes.

Viscoelastic HyA Hydrogel Promotes Recovery of Muscle Quality and Vascularization in a Murine Model of Delayed Rotator Cuff Repair

Rotator cuff tears are common musculotendinous injuries with a high risk of permanent functional disability. Following surgical repair, sub-optimal patient outcomes are directly correlated with poor muscle quality; namely, injury site fatty infiltration, fibrosis, and muscle atrophy. Muscle resident fibro-adipogenic progenitor cells (FAPs) have been identified as key regulators of post-injury skeletal muscle regeneration and repair by maintaining a pro-myogenic environment. In this work, human FAPs (hFAPs) were encapsulated into hyaluronic acid (HyA)-based hydrogels functionalized with bsp-RGD(15) cell adhesion peptide, heparin, and a matrix metalloproteinase (MMP)-cleavable crosslinker. Hydrogel-encapsulated hFAPs increased expression of the pro-myogenic marker UCP1 and production of the anti-inflammatory cytokine IL-10 while downregulating the expression of the fibrotic marker αSMA over time. A murine model of unilateral rotator cuff transection, denervation, and delayed repair was treated with the HyA hydrogel or PBS and compared to a contralateral, non-injured control limb. Muscle histology 6 weeks post-repair revealed that the hydrogel reduced fibrosis, FI, and muscle atrophy while supporting vascularization of the injured tissue region. Collectively, these results suggest that the HyA hydrogel alone can promote muscle regeneration in a clinically relevant delayed repair model of rotator cuff tear, which is hypothesized due to controlled FAP differentiation into pro-myogenic lineages.

Effects of Release of TSG-6 from Heparin Hydrogels on Supraspinatus Muscle Regeneration

Muscle degeneration after rotator cuff tendon tear is a significant clinical problem. In these experiments, we developed a poly(ethylene glycol)-based injectable granular hydrogel containing two heparin derivatives (fully sulfated [Hep] and fully desulfated [Hep-]) as well as a matrix metalloproteinase-sensitive peptide to promote sustained release of tumor necrosis factor-stimulated gene 6 (TSG-6) over 14+ days in vivo in a rat model of rotator cuff muscle injury. The hydrogel formulations demonstrated similar release profiles in vivo, thus facilitating comparisons between delivery from heparin derivatives on the level of tissue repair in two different areas of muscle (near the myotendious junction [MTJ] and in the muscle belly [MB]) that have been shown previously to have differing responses to rotator cuff tendon injury. We hypothesized that sustained delivery of TSG-6 would enhance the anti-inflammatory response following rotator cuff injury through macrophage polarization and that release from Hep would potentiate this effect throughout the muscle. Inflammatory/immune cells, satellite cells, and fibroadipogenic progenitor cells were analyzed by flow cytometry 3 and 7 days after injury and hydrogel injection, while metrics of muscle healing were examined via immunohistochemistry up to day 14. Results showed controlled delivery of TSG-6 from Hep caused heightened macrophage response (day 7 macrophages, 4.00 ± 1.85% single cells, M2a, 3.27 ± 1.95% single cells) and increased markers of early muscle regeneration (embryonic heavy chain staining) by day 7, particularly in the MTJ region of the muscle. This work provides a novel strategy for localized, controlled delivery of TSG-6 to enhance muscle healing after rotator cuff tear.

Supraspinatus Muscle Regeneration Following Rotator Cuff Tear: A Study of the Biomarkers Pax7, MyoD, and Myogenin

The success of rotator cuff tendon repair relies on both tendon healing and muscle recovery. The objective of this descriptive study was to investigate the regenerative potential of the supraspinatus muscle in rotator cuff tear conditions by quantifying the expression of Pax7, MyoD, and myogenin, basic factors that regulate myogenesis. Muscle biopsies were collected from thirty-three patients aged 34 to 73 years who underwent surgery for a rotator cuff tear affecting the supraspinatus muscle. Among these patients, twenty-seven percent were women, and the age of the lesions ranged from 2 to 72 months post-initial trauma. Biopsies were harvested from the supraspinatus muscle at the end closest to the tendon, and control biopsies were harvested from the ipsilateral deltoid muscle. The densities of immunohistochemically stained Pax7+, MyoD+, and myogenin+ nuclei/mm2 were used to estimate the myogenic potential of the muscle. Adjustments were made for patient age and lesion age. We found increased density of MyoD+ and myogenin+ cells in supraspinatus muscles compared to deltoid muscles (p < 0.001 and p = 0.003, respectively). Regression analyses that combined the density of positive nuclei with patient age showed a continuous increase in Pax7 with age but also a reduction of MyoD and myogenin in older patients. When combined with lesion age, there was a decline in the density of all myogenic markers after an initial rise. Pax7 density continued to be higher in supraspinatus compared to the deltoid muscle, but the density of MyoD and myogenin terminally dropped to a density lower than in the deltoid. Our findings suggest that the supraspinatus muscle in tear conditions showed signs of initial activation of muscle regeneration. When compared to the unaffected deltoid muscle, an apparent reduction in capacity to progress to full muscle fiber maturity was also demonstrated. This pattern of inhibited myogenesis seemed to increase with both patient age and lesion age. Our results on muscle regenerative capacity indicate that younger patients with rotator cuff tears have better chances of muscle recovery and may benefit from early surgical reconstruction.

Effects of Release of TSG-6 from Heparin Hydrogels on Supraspinatus Muscle Regeneration

Muscle degeneration after rotator cuff tendon tear is a significant clinical problem. In these experiments, we developed a poly(ethylene glycol)-based injectable granular hydrogel containing two heparin derivatives (fully sulfated (Hep) and fully desulfated (Hep-)) as well as a matrix metalloproteinase-sensitive peptide to promote sustained release of Tumor Necrosis Factor Stimulated Gene 6 (TSG-6) over 14+ days in vivo in a rat model of rotator cuff muscle injury. The hydrogel formulations demonstrated similar release profiles in vivo , thus facilitating comparisons between delivery from heparin derivatives on level of tissue repair in two different areas of muscle (near the myotendious junction (MTJ) and in the muscle belly (MB)) that have been shown previously to have differing responses to rotator cuff tendon injury. We hypothesized that sustained delivery of TSG-6 would enhance the anti-inflammatory response following rotator cuff injury through macrophage polarization, and that release from a fully sulfated heparin derivative (Hep) would potentiate this effect throughout the muscle. Inflammatory/immune cells, satellite cells, and fibroadipogenic progenitor cells, were analyzed by flow cytometery 3 and 7 days after injury and hydrogel injection, while metrics of muscle healing were examined via immunohistochemistry up to Day 14. Results showed controlled delivery of TSG-6 from Hep caused heightened macrophage response (Day 14 macrophages, 4.00 ± 1.85% single cells, M2a, 3.27 ± 1.95% single cells) and increased markers of early muscle regeneration (embryonic heavy chain staining) by Day 7, particularly in the MTJ region of the muscle, compared to release from desulfated heparin hydrogels. This work provides a novel strategy for localized, controlled delivery of TSG-6 to enhance muscle healing after rotator cuff tear.

IMPACT STATEMENT

Rotator cuff tear is a significant problem that can cause muscle degeneration. In this study, a hydrogel particle system was developed for sustained release of an anti-inflammatory protein, Tumor Necrosis Factor Stimulated Gene 6 (TSG-6), to injured muscle. Release of the protein from a fully sulfated heparin hydrogel-based carrier demonstrated greater changes in amount inflammatory cells and more early regenerative effects than a less-sulfated carrier. Thus, this work provides a novel strategy for localized, controlled delivery of an anti-inflammatory protein to enhance muscle healing after rotator cuff tear.

The effect of tenotomy, neurotomy, and dual injury on mouse rotator cuff muscles: Consequences for the mouse as a preclinical model

A common animal model of muscle pathology following rotator cuff tear (RCT) is a tenotomy of the supraspinatus and infraspinatus, often combined with neurotomy of the suprascapular nerve, which induces a more robust atrophy response than tenotomy alone. However, the utility of this model depends on its similarity to human muscle pathology post-RCT, both in terms of the disease phenotype and mechanisms of muscle atrophy and fatty infiltration. Given the clinical prevalence of nerve injury is low and the muscular response to denervation is distinct from mechanical unloading in other models, an understanding of the biological influence of the nerve injury is critical for interpreting data from this RCT model. We evaluated the individual and combined effect of tenotomy and neurotomy across multiple biological scales, in a robust time-series in the mouse supraspinatus. Muscle composition, histological, and gene expression data related to muscle atrophy, degeneration-regeneration, fatty infiltration, and fibrosis were evaluated. Broadly, we found tenotomy alone caused small, transient changes in these pathological features, which resolved over the course of the study, while neurotomy alone caused a significant fatty atrophy phenotype. The dual injury group had a similar fatty atrophy phenotype to the neurotomy group, though the addition of tenotomy did marginally enhance the fat and connective tissue. Overall, these results suggest the most clinically relevant injury model, tenotomy alone, does not produce a clinically relevant phenotype. The dual injury model partially recapitulates the human condition, but it does so through a nerve injury, which is not well justified clinically.

Lineage tracing reveals a novel PDGFRβ+ satellite cell subset that contributes to myo-regeneration of chronically injured rotator cuff muscle

Massive rotator cuff (RC) tendon tears are associated with progressive fibro-adipogenesis and muscle atrophy that altogether cause shoulder muscle wasting. Platelet derived growth factor β (PDGFRβ) lineage cells, that co-express PDGFRα have previously been shown to directly contribute to scar formation and fat accumulation in a mouse model of irreversible tendon and nerve transection (TTDN). Conversely, PDGFRβ+ lineage cells have also been  shown to be myogenic in cultures and in other models of skeletal muscle injury. We therefore hypothesized that PDGFRβ demarcates two distinct RC residing subpopulations, fibro-adipogenic and myogenic, and aimed to elucidate the identity of the PDGFRβ myogenic precursors and evaluate their contribution, if any, to RC myo-regeneration. Lineage tracing revealed increasing contribution of PDGFRβ+ myo-progenitors to the formation of GFP+ myofibers, which were the most abundant myofiber type in regenerated muscle at 2 weeks post-TTDN. Muscle regeneration preceded muscle atrophy and both advanced from the lateral site of tendon transection to the farthest medial region. GFP+/PDGFRβ+Sca-1-lin-CXCR4+Integrin-β1+ marked a novel subset of satellite cells with confirmed myogenic properties. Further studies are warranted to identify the existence of PDGFRβ+ satellite cells in human and other mouse muscles and to define their myo-regenerative potential following acute and chronic muscle injury.

Rotator cuff tears

Rotator cuff tears are the most common upper extremity condition seen by primary care and orthopaedic surgeons, with a spectrum ranging from tendinopathy to full-thickness tears with arthritic change. Some tears are traumatic, but most rotator cuff problems are degenerative. Not all tears are symptomatic and not all progress, and many patients in whom tears become more extensive do not experience symptom worsening. Hence, a standard algorithm for managing patients is challenging. The pathophysiology of rotator cuff tears is complex and encompasses an interplay between the tendon, bone and muscle. Rotator cuff tears begin as degenerative changes within the tendon, with matrix disorganization and inflammatory changes. Subsequently, tears progress to partial-thickness and then full-thickness tears. Muscle quality, as evidenced by the overall size of the muscle and intramuscular fatty infiltration, also influences symptoms, tear progression and the outcomes of surgery. Treatment depends primarily on symptoms, with non-operative management sufficient for most patients with rotator cuff problems. Modern arthroscopic repair techniques have improved recovery, but outcomes are still limited by a lack of understanding of how to improve tendon to bone healing in many patients.

Early-stage inflammation changes in supraspinatus muscle after rotator cuff tear

BACKGROUND

Rotator cuff (RC) tendon tear leads to impaired shoulder function and pain. The supraspinatus (SS) tendon is most often affected, but the biological response of the SS muscle to SS tendon tear is largely unknown. This study aimed to investigate time-dependent muscle inflammation, degeneration, fatty infiltration, and regeneration in experimental SS tear conditions.

METHODS

Forty-five C57BL/6 mice were subjected to SS tendon tear and allowed to recover for 1, 3, 5, 7, 14, or 28 days. The extent of muscle damage was examined using histologic, flow cytometric, proteomic, and chemiluminescence analyses.

RESULTS

We found that muscle inflammation peaked around day 5 with increased monocyte infiltration and increased cytokine levels in the ipsilateral compared to the contralateral SS muscle. Bioinformatics analysis of proteomics on mice that survived 5 days after RC tendon tear revealed upregulated proteins involved in "neutrophil activation involved in immune response" and "extracellular matrix organization," whereas "skeletal muscle tissue development and contraction" and "respiratory electron transport chain" were among the most downregulated. Histologic analysis of collagen showed increased collagen accumulation and fatty infiltration of the ipsilateral SS over time. Finally, we observed time- and lesion-dependent changes in satellite cell and fibro-adipogenic progenitor populations.

CONCLUSION

Altogether, we demonstrate that the SS muscle shows severe signs of acute inflammation, early degeneration, and fatty infiltration, as well as reduced regenerative potential following SS tendon tear.

Cellular and molecular modulation of rotator cuff muscle pathophysiology

Rotator cuff (RC) tendon tears are common shoulder injuries that result in irreversible and persistent degeneration of the associated muscles, which is characterized by severe inflammation, atrophy, fibrosis, and fatty infiltration. Although RC muscle degeneration strongly dictates the overall clinical outcomes, strategies to stimulate RC muscle regeneration have largely been overlooked to date. In this review, we highlight the current understanding of the cellular processes that coordinate muscle regeneration, and the roles of muscle resident cells, including immune cells, fibroadipogenic progenitors, and muscle satellite cells in the pathophysiologic regulation of RC muscles following injury. This review also provides perspectives for potential therapies to alleviate the hallmarks of RC muscle degeneration to address current limitations in postsurgical recovery.

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

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

The high prevalence of rotator cuff tears poses challenges to individual patients and the healthcare system at large. This orthopedic injury is complicated further by high rates of retear after surgical repair. Outcomes following repair are highly dependent upon the quality of the injured rotator cuff muscles, and it is, therefore, crucial that the pathophysiology of rotator cuff degeneration continues to be explored. Fibro-adipogenic progenitors, a major population of resident muscle stem cells, have emerged as the main source of intramuscular fibrosis and fatty infiltration, both of which are key features of rotator cuff muscle degeneration. Improvements to rotator cuff repair outcomes will likely require addressing the muscle pathology produced by these cells. The aim of this review is to summarize the current rotator cuff degeneration assessment tools, the effects of poor muscle quality on patient outcomes, the role of fibro-adipogenic progenitors in mediating muscle pathology, and how these cells could be leveraged for potential therapeutics to augment current rotator cuff surgical and rehabilitative strategies.

Fatty degeneration and wnt10b expression in the supraspinatus muscle after surgical repair of torn rotator cuff tendon

PURPOSE

In the torn rotator cuff muscles, decreased expression of wnt10b prior to elevation of peroxisome proliferator-activated receptor γ (PPARγ) and CCAAT/enhancer-binding protein α (C/EBPα) has previously been reported. The purpose of this study is to elucidate the expression profiles of these adipogenesis-related genes after rotator cuff detachment and reattachment in a rabbit model.

METHODS

We investigated gene expression profiles of PPARγ, C/EBPα, and wnt10b in different parts of rabbit supraspinatus (SSP) muscle after tendon detachment (n = 6 for each time point). In addition, we assessed expression of the same genes after SSP reattachment with different intervals from initial detachment (n = 6). Fatty degeneration of the SSP muscle was examined by Oil red-O staining. Gene expression profiles were examined by quantitative real-time polymerase chain reaction.

RESULTS

After SSP detachment, Oil red-O-positive oil deposits increased after 3 weeks. In the SSP reattachment model, numerous Oil red-O-positive cells were present at 5-week reattachment, following 2- and 3-week detachment. PPARγ and C/EBPα messenger ribonucleic acid expression exhibited a significant increase at 2 and 3 weeks after SSP detachment and remained increased at 5-week reattachment after 2- and 3-week detachment. A decreased expression of wnt10b was observed from 1 week after SSP detachment. Expression of wnt10b was recovered not in the central area of the SSP muscle but in the periphery after reattachment. Adipogenic change was not observed when SSP tendon was reattached after 1-week detachment.

CONCLUSIONS

These results may suggest that once the adipogenic transcription factors, PPARγ and C/EBPα, were elevated, repair surgery after rotator cuff tear could not prevent the emergence of fat in the SSP muscle.

Reduced Myogenic and Increased Adipogenic Differentiation Capacity of Rotator Cuff Muscle Stem Cells

BACKGROUND

Fat accumulation commonly occurs in chronically torn rotator cuff muscles, and increased fat within the rotator cuff is correlated with poor clinical outcomes. The extent of lipid deposition is particularly pronounced in injured rotator cuff muscles compared with other commonly injured muscles such as the gastrocnemius. Satellite cells, which are a tissue-resident muscle stem-cell population, can differentiate into fat cells. We hypothesized that satellite cells from the rotator cuff have greater intrinsic adipogenic differentiation potential than do gastrocnemius satellite cells, and this difference is due to variations in epigenetic imprinting between the cells.

METHODS

Satellite cells from gastrocnemius and rotator cuff muscles of mice were cultured in adipogenic media, and the capacity to differentiate into mature muscle cells and adipogenic cells was assessed (n ≥ 9 plates per muscle group). We also performed DNA methylation analysis of gastrocnemius and rotator cuff satellite cells to determine whether epigenetic differences were present between the 2 groups (n = 5 mice per group).

RESULTS

Compared with the gastrocnemius, satellite cells from the rotator cuff had a 23% reduction in myogenic differentiation and an 87% decrease in the expression of the differentiated muscle cell marker MRF4 (myogenic regulatory factor 4). With respect to adipogenesis, rotator cuff satellite cells had a 4.3-fold increase in adipogenesis, a 12-fold increase in the adipogenic transcription factor PPARγ (peroxisome proliferator-activated receptor gamma), and a 65-fold increase in the adipogenic marker FABP4 (fatty-acid binding protein 4). Epigenetic analysis identified 355 differentially methylated regions of DNA between rotator cuff and gastrocnemius satellite cells, and pathway enrichment analysis suggested that these regions were involved with lipid metabolism and adipogenesis.

CONCLUSIONS

Satellite cells from rotator cuff muscles have reduced myogenic and increased adipogenic differentiation potential compared with gastrocnemius muscles. There appears to be a cellular and genetic basis behind the generally poor rates of rotator cuff muscle healing.

CLINICAL RELEVANCE

The reduced myogenic and increased adipogenic capacity of rotator cuff satellite cells is consistent with the increased fat content and poor muscle healing rates often observed for chronically torn rotator cuff muscles. For patients undergoing rotator cuff repair, transplantation of autologous satellite cells from other muscles less prone to fatty infiltration may improve clinical outcomes.