Heterogeneous muscle gene expression patterns in patients with massive rotator cuff tears

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.

Transcriptional time course after rotator cuff repair in 6 month old female rabbits

Introduction: Rotator cuff tears are prevalent in the population above the age of 60. The disease progression leads to muscle atrophy, fibrosis, and fatty infiltration, which is not improved upon with surgical repair, highlighting the need to better understand the underlying biology impairing more favorable outcomes. Methods: In this study, we collected supraspinatus muscle tissue from 6 month old female rabbits who had undergone unilateral tenotomy for 8 weeks at 1, 2, 4, or 8 weeks post-repair (n = 4/group). RNA sequencing and enrichment analyses were performed to identify a transcriptional timeline of rotator cuff muscle adaptations and related morphological sequelae. Results: There were differentially expressed (DE) genes at 1 (819 up/210 down), 2 (776/120), and 4 (63/27) weeks post-repair, with none at 8 week post-repair. Of the time points with DE genes, there were 1092 unique DE genes and 442 shared genes, highlighting that there are changing processes in the muscle at each time point. Broadly, 1-week post-repair differentially expressed genes were significantly enriched in pathways of metabolism and energetic activity, binding, and regulation. Many were also significantly enriched at 2 weeks, with the addition of NIF/NF-kappaB signaling, transcription in response to hypoxia, and mRNA stability alongside many additional pathways. There was also a shift in transcriptional activity at 4 weeks post-repair with significantly enriched pathways for lipids, hormones, apoptosis, and cytokine activity, despite an overall decrease in the number of differentially expressed genes. At 8 weeks post-repair there were no DE genes when compared to control. These transcriptional profiles were correlated with the histological findings of increased fat, degeneration, and fibrosis. Specifically, correlated gene sets were enriched for fatty acid metabolism, TGF-B-related, and other pathways. Discussion: This study identifies the timeline of transcriptional changes in muscle after RC repair, which by itself, does not induce a growth/regenerative response as desired. Instead, it is predominately related to metabolism/energetics changes at 1 week post-repair, unclear or asynchronous transcriptional diversity at 2 weeks post-repair, increased adipogenesis at 4 weeks post-repair, and a low transcriptional steady state or a dysregulated stress response at 8 weeks post-repair.

Paraspinal muscle gene expression across different aetiologies in individuals undergoing surgery for lumbar spine pathology

PURPOSE

The purpose of this study was to understand potential baseline transcriptional expression differences in paraspinal skeletal muscle from patients with different underlying lumbar pathologies by comparing multifidus gene expression profiles across individuals with either disc herniation, facet arthropathy, or degenerative spondylolisthesis.

METHODS

Multifidus biopsies were obtained from patients (n = 44) undergoing lumbar surgery for either disc herniation, facet arthropathy, or degenerative spondylolisthesis. Diagnostic categories were based on magnetic resonance images, radiology reports, and intraoperative reports. Gene expression for 42 genes was analysed using qPCR. A one-way analysis of variance was performed for each gene to determine differences in expression across diagnostic groups. Corrections for multiple comparisons across genes (Benjamini-Hochberg) and for between-group post hoc comparisons (Sidak) were applied.

RESULTS

Adipogenic gene (ADIPOQ) expression was higher in the disc herniation group when compared to the facet arthropathy group (p = 0.032). Adipogenic gene (PPARD) expression was higher in the degenerative spondylolisthesis group when compared to the disc herniation group (p = 0.013), although absolute gene expression levels for all groups was low. Fibrogenic gene (COL3A1) had significantly higher expression in the disc herniation group and facet arthropathy group when compared to the degenerative spondylolisthesis group (p < 0.001 and p = 0.038, respectively). When adjusted for multiple comparisons, only COL3A1 remained significant (p = 0.012).

CONCLUSION

Individuals with disc herniation and facet arthropathy demonstrate higher COL3A1 gene expression compared to those with degenerative spondylolisthesis. Future research is required to further understand the biological relevance of these transcriptional differences.

Spatial transcriptomics tools allow for regional exploration of heterogeneous muscle pathology in the pre-clinical rabbit model of rotator cuff tear

Background

Conditions affecting skeletal muscle, such as chronic rotator cuff tears, low back pain, dystrophies, and many others, often share changes in muscle phenotype: intramuscular adipose and fibrotic tissue increase while contractile tissue is lost. The underlying changes in cell populations and cell ratios observed with these phenotypic changes complicate the interpretation of tissue-level transcriptional data. Novel single-cell transcriptomics has limited capacity to address this problem because muscle fibers are too long to be engulfed in single-cell droplets and single nuclei transcriptomics are complicated by muscle fibers’ multinucleation. Therefore, the goal of this project was to evaluate the potential and challenges of a spatial transcriptomics technology to add dimensionality to transcriptional data in an attempt to better understand regional cellular activity in heterogeneous skeletal muscle tissue.

Methods

The 3′ Visium spatial transcriptomics technology was applied to muscle tissue of a rabbit model of rotator cuff tear. Healthy control and tissue collected at 2 and 16 weeks after tenotomy was utilized and freshly snap frozen tissue was compared with tissue stored for over 6 years to evaluate whether this technology is retrospectively useful in previously acquired tissues. Transcriptional information was overlayed with standard hematoxylin and eosin (H&E) stains of the exact same histological sections.

Results

Sequencing saturation and number of genes detected was not affected by sample storage duration. Unbiased clustering matched the underlying tissue type-based on H&E assessment. Connective-tissue-rich areas presented with lower unique molecular identifier counts are compared with muscle fibers even though tissue permeabilization was standardized across the section. A qualitative analysis of resulting datasets revealed heterogeneous fiber degeneration–regeneration after tenotomy based on (neonatal) myosin heavy chain 8 detection and associated differentially expressed gene analysis.

Conclusions

This protocol can be used in skeletal muscle to explore spatial transcriptional patterns and confidently relate them to the underlying histology, even for tissues that have been stored for up to 6 years. Using this protocol, there is potential for novel transcriptional pathway discovery in longitudinal studies since the transcriptional information is unbiased by muscle composition and cell type changes.

Paraspinal Muscle Health is Related to Fibrogenic, Adipogenic, and Myogenic Gene Expression in Patients with Lumbar Spine Pathology

BACKGROUND

Lumbar spine pathology is a common feature of lower back and/or lower extremity pain and is associated with observable degenerative changes in the lumbar paraspinal muscles that are associated with poor clinical prognosis. Despite the commonly observed phenotype of muscle degeneration in this patient population, its underlying molecular mechanisms are not well understood. The aim of this study was to investigate the relationships between groups of genes within the atrophic, myogenic, fibrogenic, adipogenic, and inflammatory pathways and multifidus muscle health in individuals undergoing surgery for lumbar spine pathology.

METHODS

Multifidus muscle biopsies were obtained from patients (n = 59) undergoing surgery for lumbar spine pathology to analyze 42 genes from relevant adipogenic/metabolic, atrophic, fibrogenic, inflammatory, and myogenic gene pathways using quantitative polymerase chain reaction. Multifidus muscle morphology was examined preoperatively in these patients at the level and side of biopsy using T2-weighted magnetic resonance imaging to determine whole muscle compartment area, lean muscle area, fat cross-sectional areas, and proportion of fat within the muscle compartment. These measures were used to investigate the relationships between gene expression patterns and muscle size and quality.

RESULTS

Relationships between gene expression and imaging revealed significant associations between decreased expression of adipogenic/metabolic gene (PPARD), increased expression of fibrogenic gene (COL3A1), and lower fat fraction on MRI (r = -0.346, p = 0.018, and r = 0.386, p = 0.047 respectively). Decreased expression of myogenic gene (mTOR) was related to greater lean muscle cross-sectional area (r = 0.388, p = 0.045).

CONCLUSION

Fibrogenic and adipogenic/metabolic genes were related to pre-operative muscle quality, and myogenic genes were related to pre-operative muscle size. These findings provide insight into molecular pathways associated with muscle health in the presence of lumbar spine pathology, establishing a foundation for future research that addresses how these changes impact outcomes in this patient population.

Transcriptional Time Course After Rotator Cuff Tear

Rotator cuff (RC) tears are prevalent in the population above the age of 60. The disease progression leads to muscle atrophy, fibrosis, and fatty infiltration in the chronic state, which is not improved with intervention or surgical repair. This highlights the need to better understand the underlying dysfunction in muscle after RC tendon tear. Contemporary studies aimed at understanding muscle pathobiology after RC tear have considered transcriptional data in mice, rats and sheep models at 2–3 time points (1 to 16 weeks post injury). However, none of these studies observed a transition or resurgence of gene expression after the initial acute time points. In this study, we collected rabbit supraspinatus muscle tissue with high temporal resolution (1, 2, 4, 8, and 16 weeks) post-tenotomy (n = 6/group), to determine if unique, time-dependent transcriptional changes occur. RNA sequencing and analyses were performed to identify a transcriptional timeline of RC muscle changes and related morphological sequelae. At 1-week post-tenotomy, the greatest number of differentially expressed genes was observed (1,069 up/873 down) which decreases through 2 (170/133), 4 (86/41), and 8 weeks (16/18), followed by a resurgence and transition of expression at 16 weeks (1,421/293), a behavior which previously has not been captured or reported. Broadly, 1-week post-tenotomy is an acute time point with expected immune system responses, catabolism, and changes in energy metabolism, which continues into 2 weeks with less intensity and greater contribution from mitochondrial effects. Expression shifts at 4 weeks post-tenotomy to fatty acid oxidation, lipolysis, and general upregulation of adipogenesis related genes. The effects of previous weeks’ transcriptional dysfunction present themselves at 8 weeks post-tenotomy with enriched DNA damage binding, aggresome activity, extracellular matrix-receptor changes, and significant expression of genes known to induce apoptosis. At 16 weeks post-tenotomy, there is a range of enriched pathways including extracellular matrix constituent binding, mitophagy, neuronal activity, immune response, and more, highlighting the chaotic nature of this time point and possibility of a chronic classification. Transcriptional activity correlated significantly with histological changes and were enriched for biologically relevant pathways such as lipid metabolism. These data provide platform for understanding the biological mechanisms of chronic muscle degeneration after RC tears.

Intramuscular Brown Fat Activation Decreases Muscle Atrophy and Fatty Infiltration and Improves Gait After Delayed Rotator Cuff Repair in Mice

BACKGROUND

Successful repair of large and massive rotator cuff (RC) tears remains a challenge at least partially because of secondary muscle atrophy and fatty infiltration. β3 Adrenergic agonists are a group of drugs that promote fat resorption through "white fat browning" of intramuscular stem cells.

PURPOSE

To test the role of a β3 adrenergic receptor agonist, amibegron, in improving muscle quality and forelimb function in a delayed RC repair model via promoting brown/beige adipose tissue activation.

STUDY DESIGN

Controlled laboratory study.

METHODS

Three-month-old PDGFRα-GFP reporter mice, wild type C57BL/6J mice, and uncoupling protein 1 (UCP-1) knockout mice underwent unilateral supraspinatus tendon transection with a 6-week delayed tendon repair. Animals with sham surgery served as controls. Amibegron was given either immediately after tendon transection or after repair. Gait analysis was conducted to measure forelimb function at 6 weeks after tendon repair. Animals were sacrificed at 6 weeks after repair. Supraspinatus muscles were harvested and analyzed histologically. Reverse transcription polymerase chain reaction was performed to quantify gene expression related to atrophy, fibrosis, and fatty infiltration.

RESULTS

Histology of PDGFRα reporter mice showed significantly increased UCP-1 expression, suggesting white fat browning in muscle after RC repair. As administered either immediately after tendon transection or after tendon repair, amibegron significantly reduced muscle atrophy and fatty infiltration and resumed normal upper extremity gait in wild type mice. However, the effect of amibegron was not present in UCP-1 knockout mice, suggesting that the effect of amibegron in treating RC muscle atrophy and fatty infiltration is through a UCP 1-dependent mechanism.

CONCLUSION

Amibegron reduced muscle atrophy and fatty infiltration and improved forelimb function after delayed RC repair through a UCP 1-dependent mechanism. This may be an effective clinical treatment strategy for patients to improve muscle quality after RC repair.

CLINICAL RELEVANCE

β3 Adrenergic agonists may serve as a new pharmacologic modality to treat RC muscle atrophy and fatty infiltration to improve clinical outcome of RC repair.

Increased Fibrogenic Gene Expression in Multifidus Muscles of Patients With Chronic Versus Acute Lumbar Spine Pathology

STUDY DESIGN

Prospective observational study-basic science (Level 1).

OBJECTIVE

The aim of this study was to compare expression of functional groups of genes within the atrophic, myogenic, fibrogenic, adipogenic, and inflammatory pathways between paraspinal muscle biopsies from individuals with acute and chronic lumbar spine pathology.

SUMMARY OF BACKGROUND DATA

Low back pain is a complex and multifactorial condition that affects a majority of the general population annually. Changes in muscle tissue composition (i.e., fatty and fibrotic infiltration) are a common feature in individuals with lumbar spine pathology associated with low back pain, which often results in functional loss. Understanding the molecular underpinnings of these degenerative changes in different phases of disease progression may improve disease prevention and treatment specificity.

METHODS

Intraoperative biopsies of the multifidus muscle were obtained from individuals undergoing surgery for acute (<6-month duration) or chronic (>6-month duration) lumbar spine pathology. Expression of 42 genes related to myogenesis, atrophy, adipogenesis, metabolism, inflammation, and fibrosis were measured in 33 samples (eight acute, 25 chronic) using qPCR, and tissue composition of fat, muscle, and fibrosis was quantified using histology.

RESULTS

We found that tissue composition of the biopsies was heterogeneous, resulting in a trend toward lower RNA yields in biopsies with higher proportions of fat (r <-0.39, P < 0.1). There were no significant differences in gene expression patterns for atrophy (P > 0.635), adipogenesis (P > 0.317), myogenesis (P > 0.320), or inflammatory (P > 0.413) genes after adjusting for the proportion of muscle, fat, and connective tissue. However, in the fibrogenesis pathway, we found significant upregulation of CTGF (P = 0.046), and trends for upregulation of COL1A1 (P = 0.061), and downregulation of MMP1 and MMP9 (P = 0.061) in the chronic group.

CONCLUSION

There is increased fibrogenic gene expression in individuals with chronic disease when compared to acute disease, without significant differences in atrophic, myogenic, adipogenic, or inflammatory pathways, suggesting increased efforts should be made to prevent or reverse fibrogenesis to improve patient function in this population.

LEVEL OF EVIDENCE

N/A.