Quantitative analysis of immune cell subset infiltration of supraspinatus muscle after severe rotator cuff injury

A causal association between lipid-lowering medications and rotator cuff syndrome: a drug-targeted mendelian randomization study

Background: Previous research has suggested that dyslipidemia may be a risk factor for rotator cuff syndrome (RCS), and lipid-lowering drugs may aid in its treatment, though conclusions have not been definitive. Mendelian randomization is a statistical method that explores the causal relationships between exposure factors and diseases. It overcomes the confounding issues inherent in traditional observational studies, thereby providing more reliable causal inferences. We employed this method to investigate whether hyperlipidemia is a risk factor for rotator cuff syndrome and whether lipid-lowering drugs can effectively treat this condition. Methods: Genetic variations linked to lipid traits low-density lipoprotein cholesterol (LDL-C), triglyceride (TG), and total cholesterol (TC) were acquired from the UK Biobank and the Global Lipids Genetics Consortium (GLGC). Data on genetic variation in rotator cuff syndrome were obtained from FinnGen, including 24,061 patients and 275,212 controls. In the next step, we carried out two-sample Mendelian randomization analyses to determine whether lipid traits correlate with rotator cuff syndrome risk. Additionally, we performed drug-target Mendelian randomization (MR) analyses on 10 drug targets related to rotator cuff syndrome. For the drug targets that showed significant results, further analysis was done using Summary-data-based Mendelian Randomization (SMR) and colocalization techniques. We performed a mediation analysis to identify potential mediators between HMG-CoA reductase (HMGCR) and RCS. Results: No causative link was established between these lipid traits and rotator cuff syndrome. However, a significant association has been identified where HMGCR inhibition corresponds to a reduced risk of rotator cuff disease (OR = 0.68, [95% CI, 0.56-0.83], p = 1.510 × 10-4). Additionally, enhanced expression of HMGCR in muscle tissues is also linked to a decreased risk of rotator cuff syndrome (OR = 0.88, [95% CI, 0.76-0.99], p = 0.03). Body mass index (BMI) mediated 22.97% of the total effect of HMGCR on RCS. Conclusion: This study does not support low-density LDL-C, TG, and TC as risk factors for rotator cuff syndrome. HMGCR represents a potential pharmaceutical target for preventing and treating rotator cuff syndrome. The protective action of statins on the rotator cuff syndrome might not be associated with their lipid-lowering properties.

Mitochondrial Biogenesis as a Therapeutic Target for Rotator Cuff Tendon Tears

Rotator Cuff Injuries (RCI) are highly prevalent and characterized by shoulder pain, restricted shoulder movement, and difficulty with overhead activity, radiating pain in the deltoid muscle, and atrophy of the rotator cuff muscles. Increasing age, hand dominance, smoking, hypertension, hyperlipidemia, and obesity are common risk factors. Chronic inflammation plays a critical role in the underlying pathogenesis. RCI accounts for massive healthcare expenditure costing about $15,000 per repair, and over 4.5 million physician visits per year, however, there is still no therapeutic target to improve clinical outcomes. Mitochondrial biogenesis in response to inflammatory stimuli supports increased cellular energy requirements, cell proliferation, and differentiation. This suggests that mitochondrial biogenesis may play a role in healing RCI by serving as a protective factor against free oxygen species and promoting homeostasis within the rotator cuff. There is evidence highlighting the potential therapeutic benefits of mitochondrial biogenesis in various inflammatory diseases, but no study explored the role of mitochondrial biogenesis in rotator cuff tears. Since hypercholesterolemia is a risk factor for RCI, we investigated the effects of hypercholesterolemia on the expression of PGC-1α, a marker of mitochondrial biogenesis, in rotator cuff muscle. The findings revealed an increased gene and protein expression of inflammatory mediators and PGC-1α, suggesting enhanced inflammation and increased mitochondrial biogenesis due to hypercholesterolemia. Additional studies are warranted to further investigate the chronic effect of hyperlipidemia induced RCI to elucidate the cause of insufficient mitochondrial biogenesis unable to protect the rotator cuff and the therapeutic effect of promoting mitochondrial biogenesis.

Current Research on the Influence of Statin Treatment on Rotator Cuff Healing

Rotator cuff tears are a condition characterized by damage to the muscles and tendons that connect the scapula and humerus, which are responsible for shoulder rotation and arm lifting. Metabolic factors such as diabetes, thyroid disease, high cholesterol, vitamin D deficiency, obesity, and smoking have been associated with an increased risk of rotator cuff tears. Interestingly, patients with hyperlipidemia, a condition characterized by high levels of cholesterol and other fats in the blood, have been found to have a higher incidence of rotator cuff tears and breakdown of tendon matrix. As a result, statin therapy, which is commonly used to lower cholesterol levels in hyperlipidemia, has been explored as a potential treatment to improve clinical outcomes in rotator cuff tears. However, the results of preclinical and clinical studies on the effects of statins on tendon healing in rotator cuff tears are limited and not well-defined. Moreover, since hyperlipidemia and rotator cuff tears are more prevalent in older individuals, a literature review on the efficacy and safety of statin therapy in this population is needed.

Polarized macrophages regulate fibro/adipogenic progenitor (FAP) adipogenesis through exosomes

BACKGROUND

Macrophage polarization has been observed in the process of muscle injuries including rotator cuff (RC) muscle atrophy and fatty infiltration after large tendon tears. In our previous study, we showed that fibrogenesis and white adipogenesis of muscle residential fibro/adipogenic progenitors (FAPs) cause fibrosis and fatty infiltration and that brown/beige adipogenesis of FAPs promotes rotator cuff muscle regeneration. However, how polarized macrophages and their exosomes regulate FAP differentiation remains unknown.

METHODS

We cultured FAPs with M0, M1, and M2 macrophages or 2 × 109 exosomes derived from M0, M1 and M2 with and without GW4869, an exosome inhibitor. In vivo, M0, M1, and M2 macrophages were transplanted or purified macrophage exosomes (M0, M1, M2) were injected into supraspinatus muscle (SS) after massive tendon tears in mice (n = 6). SS were harvested at six weeks after surgery to evaluate the level of muscle atrophy and fatty infiltration.

RESULTS

Our results showed that M2 rather than M0 or M1 macrophages stimulates brown/beige fat differentiation of FAPs. However, the effect of GW4869, the exosome inhibitor, diminished this effect. M2 exosomes also promoted FAP Beige differentiation in vitro. The transplantation of M2 macrophages reduced supraspinatus muscle atrophy and fatty infiltration. In vivo injections of M2 exosomes significantly reduced muscle atrophy and fatty infiltration in supraspinatus muscle.

CONCLUSION

Results from our study demonstrated that polarized macrophages directly regulated FAP differentiation through their exosomes and M2 macrophage-derived exosomes may serve as a novel treatment option for RC muscle atrophy and fatty infiltration.

Rotator Cuff Health, Pathology, and Repair in the Perspective of Hyperlipidemia

Rotator Cuff Injuries (RCI) are prevalent cause of shoulder pain affecting over 20% of the population in the USA. Surgical repair of the torn rotator cuff helps in relieving the pressure on the rotator cuff tendon and from symptoms, however tendon-to-bone healing after rotator cuff surgery still has a high failure rate. Hyperlipidemia has been strongly associated with RCI although the cellular and molecular mechanisms are largely unknown. The focus of this critical review is to further explore the role of hyperlipidemia in RCI and rotator cuff tissue repair to determine its implication as a risk factor for tears, repair, and retears. A literature review was conducted to elucidate the role of hyperlipidemia as an inflammatory mediator and catalyst for structural instability within the shoulder. The results from various studies were critically reviewed to summarize the relationship between hyperlipidemia and rotator cuff pathology. Hyperlipidemia induces LDL-particle entrapment within the dense regular collagen of rotator cuff tendons resulting in foam cell aggregation and macrophage recruitment. Subsequent inflammatory pathways including the JAK2/STAT3 pathway and NLRP3 inflammasome pathway led to persistent inflammation and Extracellular Matrix (ECM) degradation within the rotator cuff. While arthroscopic repair remains the most common treatment modality, nonsurgical treatment including statins, vitamin D, and targeting miRNA are also of therapeutic benefit. Hyperlipidemia interferes with arthroscopic repairs by inducing inflammation and stiffness within tendons and increases the risk of retears. Most notably, targeting underlying mechanisms influencing inflammation has large therapeutic value as a novel treatment strategy for the management of rotator cuff pathology.

Identification of Diagnostic Biomarkers Associated with Stromal and Immune Cell Infiltration in Fatty Infiltration After Rotator Cuff Tear by Integrating Bioinformatic Analysis and Machine-Learning

Purpose

The present study aimed to explore potential diagnostic biomarkers for fatty infiltration (FI) of the rotator cuff muscles after rotator cuff tear (RCT) and investigate the influence of stromal and immune cell infiltration on this pathology.

Methods

The GSE130447 and GSE103266 datasets were downloaded from the Gene Expression Omnibus database. Differentially expressed genes (DEGs) were identified, and gene set enrichment analyses were performed by R software. Two machine learning algorithms, random forest and multiple support vector machine recursive feature elimination (mSVM-RFE), were used to screen candidate biomarkers. The diagnostic value of the screened biomarkers was further validated by the area under the ROC curve (AUC) in the GSE103266 dataset. Murine microenvironment cell population counter (mMCP-counter) method was employed to estimate stromal and immune cell infiltration of FI. The correlation between biomarkers and infiltrated immune and stromal cell subsets was further analyzed.

Results

A total of 2123 DEGs were identified. The identified DEGs were predominantly linked to immune system process, extracellular matrix organization and PPAR signalling pathway. FABP5 (AUC = 0.958) and MGP (AUC = 1) were screened as diagnostic biomarkers of FI. Stromal and immune cell infiltration analysis showed that monocytes, mast cells, vessels, endothelial cells and fibroblasts may be related to the process of FI. FABP5 and MGP were positively correlated with vessels whereas negatively correlated with monocytes and mast cells.

Conclusion

FABP5 and MGP can serve as diagnostic biomarkers of FI after RCT, and stromal and immune cell infiltration may play a crucial role in this pathology.

Injection of Micronized Human Amnion/Chorion Membrane Results in Increased Early Supraspinatus Muscle Regeneration in a Chronic Model of Rotator Cuff Tear

Surgical repair of severe rotator cuff tear often results in retear due to unaddressed muscle degeneration. The objective of this study was to test the regenerative potential of micronized dehydrated Human Amnion/Chorion Membrane (dHACM), in a clinically relevant delayed reattachment model of rotator cuff repair. Micronized dHACM was injected into rat supraspinatus muscle during tendon re-attachment surgery, three weeks after original tendon injury. One week after material injection, inflammatory and mesenchymal stem cell infiltration into supraspinatus muscles was assessed via flow cytometry. Histological methods were utilized to assess structural and regenerative changes in muscle one and three weeks after material injection. Micronized dHACM injection resulted in increased M1-like macrophages (17.1 [Formula: see text] fold change over contralateral controls) and regenerating muscle fibers (4.3% vs 1.7% in saline treated muscles) one week after injection compared to saline treated muscles. Tendon reattachment itself exhibited intrinsic healing in this model, demonstrated by a general return of muscle weight and reduced fibrosis. Our results indicate that injection of micronized dHACM may initiate an inflammatory response in degenerated muscle that promotes early muscle regeneration, and that our animal model may be a suitable platform for studying treatments in muscle at early timepoints, before intrinsic healing occurs.

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.

The inflammatory response of the supraspinatus muscle in rotator cuff tear conditions

BACKGROUND

Rotator cuff (RC) disorders involve a spectrum of shoulder conditions from early tendinopathy to full-thickness tears leading to impaired shoulder function and pain. The pathology of RC disorder is, nonetheless, still largely unknown. Our hypothesis is that a supraspinatus (SS) tendon tear leads to sustained inflammatory changes of the SS muscle along with fatty infiltration and muscle degeneration, which are threshold markers for poor RC muscle function. The aim of this study was to determine the extent of this muscle inflammation in conjunction with lipid accumulation and fibrosis in RC tear conditions.

METHODS

We used proteomics, histology, electrochemiluminescence immunoassay, and quantitative polymerase chain reaction analyses to evaluate inflammatory and degenerative markers and fatty infiltration in biopsies from 22 patients undergoing surgery with repair of a full-thickness SS tendon tear.

RESULTS

Bioinformatic analysis showed that proteins involved in innate immunity, extracellular matrix organization, and lipid metabolism were among the most upregulated, whereas mitochondrial electronic transport chain along with muscle fiber function was among the most downregulated. Histologic analysis confirmed changes in muscle fiber organization and the presence of inflammation and fatty infiltration. Inflammation appeared to be driven by a high number of infiltrating macrophages, accompanied by elevated matrix metalloprotease levels and changes in transforming growth factor-β and cytokine levels in the SS compared with the deltoid muscle.

CONCLUSIONS

We demonstrated massive SS muscle inflammation after the tendon tear combined with fatty infiltration and degeneration. The regulation of tissue repair is thus extremely complex, and it may have opposite effects at different time points of healing. Inhibition or stimulation of muscle inflammation may be a potential target to enhance the outcome of the repaired torn RC.

Recent update on craniofacial tissue engineering

The craniofacial region consists of several different tissue types. These tissues are quite commonly affected by traumatic/pathologic tissue loss which has so far been traditionally treated by grafting procedures. With the complications and drawbacks of grafting procedures, the emerging field of regenerative medicine has proved potential. Tissue engineering advancements and the application in the craniofacial region is quickly gaining momentum although most research is still at early in vitro/in vivo stages. We aim to provide an overview on where research stands now in tissue engineering of craniofacial tissue; namely bone, cartilage muscle, skin, periodontal ligament, and mucosa. Abstracts and full-text English articles discussing techniques used for tissue engineering/regeneration of these tissue types were summarized in this article. The future perspectives and how current technological advancements and different material applications are enhancing tissue engineering procedures are also highlighted. Clinically, patients with craniofacial defects need hybrid reconstruction techniques to overcome the complexity of these defects. Cost-effectiveness and cost-efficiency are also required in such defects. The results of the studies covered in this review confirm the potential of craniofacial tissue engineering strategies as an alternative to avoid the problems of currently employed techniques. Furthermore, 3D printing advances may allow for fabrication of patient-specific tissue engineered constructs which should improve post-operative esthetic results of reconstruction. There are on the other hand still many challenges that clearly require further research in order to catch up with engineering of other parts of the human body.

Heat stress ameliorates tenotomy-induced inflammation in muscle-specific response via regulation of macrophage subtypes

During disuse-induced muscle atrophy, macrophages play a significant role in inflammatory responses that occur with muscle degeneration and repair. Heat treatment has been shown to alleviate muscle atrophy; however, the effect of heat on inflammatory responses following tenotomy has not been evaluated. This study examined the effects of heat stress on proinflammatory (M1-like) and anti-inflammatory (M2-like) macrophage populations. Also, cytokine protein expression in oxidative soleus and glycolytic plantaris muscles following Achilles tendon transection (tenotomy) was analyzed. Male Wistar rats were assigned into control, control plus heat stress, tenotomy, and tenotomy plus heat stress groups. Tenotomy was performed for 8 (TEN8) and 14 (TEN14) days to induce muscle inflammation. Heat treatments, 30 min at 40.5–41.5°C, were given 24 h before and 1–6 consecutive days after tenotomy (TEN8 group) or every other day (TEN14 group). Tenotomy induced muscle necrosis, extensive infiltration of M1- (CD68+), and M2- (CD163+) like macrophages and increased tumor necrosis factor-α (TNFα) but not interleukin-10 (IL-10) protein expression. Heat stress caused a reduction in necrotic fibers, M1-like macrophage invasion, and TNFα protein expression in tenotomized soleus muscle. Additionally, heat stress enhanced M2-like macrophage accumulation during the 14 days following tenotomy in soleus muscle but did not affect IL-10 protein level. Our results indicate that heat stress can limit tenotomy-induced inflammatory responses through modulation of macrophage subtypes and TNFα protein expression, preferentially in oxidative muscle. These findings shed light on the ability of heat stress as a therapeutic strategy to manipulate macrophages for optimal inflammation during muscle atrophy.

NEW & NOTEWORTHY We investigated differential effects of heat stress on modulating inflammation following 8 and 14 days of tenotomy in soleus and plantaris muscles. Heat exposure could reduce necrosis, suppress pro-inflammatory macrophage infiltration, and diminish TNFα protein expression in tenotomized muscle, which preferentially occurred in soleus muscle. Additionally, heat stress enhanced anti-inflammatory macrophages in soleus muscle in the 14-day study period. Neither tenotomy nor heat stress had an impact on IL-10 protein expression in either muscle examined.

The Role of Muscle Stem Cells in Regeneration and Recovery after Denervation: A Review

BACKGROUND

Skeletal muscle denervation is a complex clinical problem that still lacks a comprehensive solution. Previous studies have suggested that prolonged periods of denervation lead to a decline in the muscle stem cell population, negatively affecting the ability of muscle to regenerate following reinnervation. Recent advances in the understanding of muscle stem cell biology, along with new techniques that increase the ability to identify and manipulate these cells, provide an opportunity to definitively address the impact of muscle stem cells in recovery from denervation and their potential role in treatment.

METHODS

A comprehensive review of the literature on the biology of muscle denervation, and the effect of denervation injury on muscle stem cell behavior, was performed.

RESULTS

In this review, the authors discuss the current understanding of muscle stem cell biology in the setting of denervation atrophy, review barriers to successful reinnervation, and review options available to patients following denervation injury. The authors also discuss potential use of muscle stem cells in future therapies.

CONCLUSIONS

Although the clinical treatment of prolonged denervation injury has improved in recent years, regeneration of native muscle remains elusive. Muscle stem cells have been demonstrated to be of central importance in muscle regeneration following injury, and may be a powerful tool that provides effective new options for future treatments. Additional work clarifying the effect of denervation injury on satellite cells is needed to determine whether they are a limiting factor in recovery and to demonstrate whether their clinical use as a cell-based therapy in denervation injury can be efficacious.

Reversal of Fatty Infiltration After Suprascapular Nerve Compression Release Is Dependent on UCP1 Expression in Mice

BACKGROUND

In large rotator cuff tears, retraction of the supraspinatus muscle creates suprascapular nerve traction and compression. However, suprascapular nerve transection, when used in previous models, is different from chronic compression of the suprascapular nerve in patients. To define the role of suprascapular nerve chronic injury in rotator cuff muscle atrophy and fatty infiltration, we developed a novel reversible suprascapular nerve compression mouse model.

QUESTIONS/PURPOSES

We asked: (1) Can suprascapular nerve injury be induced by compression but reversed after compression release? (2) Can muscle fatty infiltration be induced by suprascapular nerve compression and reversed after compression release? (3) Is white fat browning involved in fatty infiltration resorption?

METHODS

Mice in a common strain of C57BL/6J were randomly assigned to suprascapular nerve transection (n = 10), nerve compression (n = 10), nerve compression and release (n = 10), or sham control (n = 10) groups. To study the role or white fat browning on muscle fatty infiltration, additional UCP1 reporter mice (n = 4 for nerve compression and n = 4 for nerve compression release) and knockout mice (n = 4 for nerve compression and n = 4 for nerve compression release) were used. Nerve injury was testified using osmium tetroxide staining and neural muscular junction staining and then semiquantified by counting the degenerating axons and disrupted junctions. Muscle fatty infiltration was evaluated using Oil Red O staining and then semiquantified by measuring the area fraction of fat. Immunofluorescent and Oil Red O staining on UCP1 transgenic mice was conducted to testify whether white fat browning was involved in fatty infiltration resorption. Ratios of UCP1 positively stained area and fat area to muscle cross-section area were measured to semiquantify UCP1 expression and fatty infiltration in muscle by blinded reviewers. Analysis of variance with Tukey post hoc comparisons was used for statistical analysis between groups.

RESULTS

Suprascapular nerve injury was induced by compression but reversed after release. The ratios of degenerating axons were: sham control: 6% ± 3% (95% confidence interval [CI], 3%-10%); nerve compression: 58% ± 10% (95% CI, 45%-70% versus sham, p < 0.001); and nerve compression and release: 15% ± 9% (95% CI, 5%-26% versus sham, p = 0.050). The supraspinatus muscle percentage area of fatty infiltration increased after 6 weeks of nerve compression (19% ± 1%; 95% CI, 18%-20%; p < 0.001) but showed no difference after compression release for 6 weeks (5% ± 3%; 95% CI, 1%-10%; p = 0.054) compared with sham (2% ± 1%; 95% CI, 1%-3%). However, the fat area fraction in UCP1 knockout mice did not change after nerve compression release (6% ± 1%; 95% CI, 4%-8% at 2 weeks after compression and 5% ± 0.32%; 95% CI, 4%-6% after 2 weeks of release; p = 0.1095).

CONCLUSIONS

We developed a clinically relevant, reversible suprascapular nerve compression mouse model. Fatty infiltration resorption after compression release was mediated through white fat browning.

CLINICAL RELEVANCE

If the mechanism of browning of white fat in rotator cuff muscle fatty infiltration can be confirmed in humans, a UCP1 agonist may be an effective treatment for patients with suprascapular nerve injury.

Localized SDF-1α Delivery Increases Pro-Healing Bone Marrow-Derived Cells in the Supraspinatus Muscle Following Severe Rotator Cuff Injury

To examine how the chemotactic agent stromal cell-derived factor-1alpha (SDF-1α) modulates the unique cellular milieu within rotator cuff muscle following tendon injury, we developed an injectable, heparin-based microparticle platform to locally present SDF-1α within the supraspinatus muscle following severe rotator cuff injury. SDF-1α loaded, degradable, N-desulfated heparin-based microparticles were fabricated, injected into a rat model of severe rotator cuff injury, and were retained for up to 7 days at the site. The resultant inflammatory cell and mesenchymal stem cell populations were analyzed compared to uninjured contralateral controls and, after 7 days, the fold-change in anti-inflammatory, M2-like macrophages (CD11b+CD68+CD163+, 4.3X fold-change) and mesenchymal stem cells (CD29+CD44+CD90+, 3.0X, respectively) was significantly greater in muscles treated with SDF-1α loaded microparticles than unloaded microparticles or injury alone. Our results indicate that SDF-1α loaded microparticles may be a novel approach to shift the cellular composition within the supraspinatus muscle and create a more pro-regenerative milieu, which may provide a platform to improve muscle repair following rotator cuff injury in the future.