Cathepsins in Rotator Cuff Tendinopathy: Identification in Human Chronic Tears and Temporal Induction in a Rat Model

The rat as a novel model for chronic rotator cuff injuries

Chronic rotator cuff injuries (CRCIs) still present a great challenge for orthopaedics surgeons. Many new therapeutic strategies are developed to facilitate repair and improve the healing process. However, there is no reliable animal model for chronic rotator cuff injury research. To present a new valuable rat model for future chronic rotator cuff injuries (CRCIs) repair studies, and describe the changes of CRCIs on the perspectives of histology, behavior and MRI. Sixty male Wistar rats were enrolled and underwent surgery of the left shoulder joint for persistent subacromial impingement. They were randomly divided into experimental group (n = 30, a 3D printed PEEK implant shuttled into the lower surface of the acromion) and sham operation group (n = 30, insert the same implant, but remove it immediately). Analyses of histology, behavior, MRI and inflammatory pain-related genes expression profiles were performed to evaluate the changes of CRCIs. After 2-weeks running, the rats in the experimental group exhibited compensatory gait patterns to protect the injured forelimb from loading after 2-weeks running. After 8-weeks running, the rats in the experimental group showed obvious CRCIs pathological changes: (1) acromion bone hyperplasia and thickening of the cortical bone; (2) supraspinatus muscle tendon of the humeral head: the bursal-side tendon was torn and layered with disordered structure, forming obvious gaps; the humeral-side tendon is partially broken, and has a neatly arranged collagen. Partial fat infiltration is found. The coronal T2-weighted images showed that abnormal tendon-to-bone junctions of the supraspinatus tendon. The signal intensity and continuity were destroyed with contracted tendon. At the nighttime, compared with the sham operation group, the expression level of IL-1β and COX-2 increased significantly (P = 0063, 0.0005) in the experimental group. The expression of COX-2 in experimental group is up-regulated about 1.5 times than that of daytime (P = 0.0011), but the expression of IL-1β, TNF-a, and NGF are all down-regulated (P = 0.0146, 0.0232, 0.0161). This novel rat model of chronic rotator cuff injuries has the similar characteristics with that of human shoulders. And it supplies a cost-effective, reliable animal model for advanced tissue engineered strategies and future therapeutic strategies.

Histological and immunohistochemical guide to tendon tissue

Tendons are unique dense connective tissues with discrete zones having specific structure and function. They are juxtaposed with other tissues (e.g., bone, muscle, and fat) with different compositional, structural, and mechanical properties. Additionally, tendon properties change drastically with growth and development, disease, aging, and injury. Consequently, there are unique challenges to performing high quality histological assessment of this tissue. To address this need, histological assessment was one of the breakout session topics at the 2022 Orthopaedic Research Society (ORS) Tendon Conference hosted at the University of Pennsylvania. The purpose of the breakout session was to discuss needs from members of the ORS Tendon Section related to histological procedures, data presentation, knowledge dissemination, and guidelines for future work. Therefore, this review provides a brief overview of the outcomes of this discussion and provides a set of guidelines, based on the perspectives from our laboratories, for histological assessment to assist researchers in their quest to utilize these techniques to enhance the outcomes and interpretations of their studies.

Animal model for tendinopathy

Background

Tendinopathy is a common motor system disease that leads to pain and reduced function. Despite its prevalence, our mechanistic understanding is incomplete, leading to limited efficacy of treatment options. Animal models contribute significantly to our understanding of tendinopathy and some therapeutic options. However, the inadequacies of animal models are also evident, largely due to differences in anatomical structure and the complexity of human tendinopathy. Different animal models reproduce different aspects of human tendinopathy and are therefore suitable for different scenarios. This review aims to summarize the existing animal models of tendinopathy and to determine the situations in which each model is appropriate for use, including exploring disease mechanisms and evaluating therapeutic effects.

Methods

We reviewed relevant literature in the PubMed database from January 2000 to December 2022 using the specific terms ((tendinopathy) OR (tendinitis)) AND (model) AND ((mice) OR (rat) OR (rabbit) OR (lapin) OR (dog) OR (canine) OR (sheep) OR (goat) OR (horse) OR (equine) OR (pig) OR (swine) OR (primate)). This review summarized different methods for establishing animal models of tendinopathy and classified them according to the pathogenesis they simulate. We then discussed the advantages and disadvantages of each model, and based on this, identified the situations in which each model was suitable for application.

Results

For studies that aim to study the pathophysiology of tendinopathy, naturally occurring models, treadmill models, subacromial impingement models and metabolic models are ideal. They are closest to the natural process of tendinopathy in humans. For studies that aim to evaluate the efficacy of possible treatments, the selection should be made according to the pathogenesis simulated by the modeling method. Existing tendinopathy models can be classified into six types according to the pathogenesis they simulate: extracellular matrix synthesis-decomposition imbalance, inflammation, oxidative stress, metabolic disorder, traumatism and mechanical load.

Conclusions

The critical factor affecting the translational value of research results is whether the selected model is matched with the research purpose. There is no single optimal model for inducing tendinopathy, and researchers must select the model that is most appropriate for the study they are conducting.

The translational potential of this article

The critical factor affecting the translational value of research results is whether the animal model used is compatible with the research purpose. This paper provides a rationale and practical guide for the establishment and selection of animal models of tendinopathy, which is helpful to improve the clinical transformation ability of existing models and develop new models.

Microfluidic Platform for Microparticle Fabrication and Release of a Cathepsin Inhibitor

Cathepsins are a family of cysteine proteases responsible for a variety of homeostatic functions throughout the body, including extracellular matrix remodeling, and have been implicated in a variety of degenerative diseases. However, clinical trials using systemic administration of cathepsin inhibitors have been abandoned due to side effects, so local delivery of cathepsin inhibitors may be advantageous. In these experiments, a novel microfluidic device platform was developed that can synthesize uniform, hydrolytically degradable microparticles from a combination of poly(ethylene glycol) diacrylate (PEGDA) and dithiothreitol (DTT). Of the formulations examined, the 10-polymer weight percentage 10 mM DTT formulation degraded after 77 days in vitro. A modified assay using the DQ Gelatin Fluorogenic Substrate was used to demonstrate sustained release and bioactivity of a cathepsin inhibitor (E-64) released from hydrogel microparticles over 2 weeks in vitro (up to ∼13 μg/mL released with up to ∼40% original level of inhibition remaining at day 14). Altogether, the technologies developed in this study will allow a small-molecule, broad cathepsin inhibitor E-64 to be released in a sustained manner for localized inhibition of cathepsins for a wide variety of diseases.

Bioactive extracellular matrix fragments in tendon repair

Tendinopathy is a common tendon disorder that causes pain, loss of strength and function, and local inflammation mainly characterized by hypoxia, collagen degradation, and extracellular matrix (ECM) disorganization. Generally, ECM degradation and remodeling is tightly regulated; however, hyperactivation of matrix metalloproteases (MMPs) contributes to excessive collagenolysis under pathologic conditions resulting in tendon ECM degradation. This review article focuses on the production, function, and signaling of matrikines for tendon regeneration following injury with insights into the expression, tissue compliance, and cell proliferation exhibited by various matrikines. Furthermore, the regenerative properties suggest translational significance of matrikines to improve the outcomes post-injury by assisting with tendon healing.

Hypoxia-driven secretion of extracellular matrix proteins in the exosomes reflects the asymptomatic pathology of rotator cuff tendinopathies

The major hallmark of rotator cuff tendinopathies (RCT) is the disorganization of the tendon extracellular matrix (ECM), which is due to a decrease in the ratio of collagen I to collagen III. In addition, the pathology of the tendon matrisome remains asymptomatic, and hypoxia has been identified to be the priming signal to initiate the molecular pathology of RCT. Also, the secretome content of hypoxia-challenged tendon cells (tenocytes) reflects the pathological status of RCT. With this background, the present study was designed to establish the expression status and molecular crosstalk of the ECM component proteins contained in the exosomes of the hypoxia-challenged swine tenocytes. The mass spectrometry analysis revealed the upregulation of COL1A2, P4HA1, PRDX2, P3H1, COL6A1, PPIB, LCN1, and COL3A1 and the downregulation of COLA12, PDIA4, COLG, FN1, CTSK, and TNC in the exosomes of hypoxic tenocytes. These proteins interact with diverse proteins and operate multiple pathways associated with ECM homeostasis and repair as determined by NetworkAnalyst. The functional analysis of these proteins reflects the pathology of tendon ECM, which is correlated with the asymptomatic phase of RCT. Understanding the signaling mediated by these proteins would reveal the underlying molecular pathology and offers translational significance in the diagnosis and management of RCT.

Tendon response to matrix unloading is determined by the patho-physiological niche

Although the molecular mechanisms behind tendon disease remain obscure, aberrant stromal matrix turnover and tissue hypervascularity are known hallmarks of advanced tendinopathy. We harness a tendon explant model to unwind complex cross-talk between the stromal and vascular tissue compartments. We identify the hypervascular tendon niche as a state-switch that gates degenerative matrix remodeling within the tissue stroma. Here pathological conditions resembling hypervascular tendon disease provoke rapid cell-mediated tissue breakdown upon mechanical unloading, in contrast to unloaded tendons that remain functionally stable in physiological low-oxygen/-temperature niches. Analyses of the stromal tissue transcriptome and secretome reveal that a stromal niche with elevated tissue oxygenation and temperature drives a ROS mediated cellular stress response that leads to adoption of an immune-modulatory phenotype within the degrading stromal tissue. Degradomic analysis further reveals a surprisingly rich set of active matrix proteases behind the progressive loss of tissue mechanics. We conclude that the tendon stromal compartment responds to aberrant mechanical unloading in a manner that is highly dependent on the vascular niche, with ROS gating a complex proteolytic breakdown of the functional collagen backbone.

Sequential, but not Concurrent, Incubation of Cathepsin K and L with Type I Collagen Results in Extended Proteolysis

Degradation of extracellular matrix (ECM) during tendinopathy is, in part, mediated by the collagenolytic cathepsin K (catK) and cathepsin L (catL), with a temporal component to their activity. The objective of this study was to determine how catK and catL act in concert or in conflict to degrade collagen and tendon ECM during tissue degeneration. To do so, type I collagen gels or ECM extracted from apolipoprotein E deficient mouse Achilles tendons were incubated with catK and catL either concurrently or sequentially, incubating catK first, then catL after a delayed time period. Sequential incubation of catK then catL caused greater degradation of substrates over concurrent incubation, and of either cathepsin alone. Zymography showed there were reduced amounts of active enzymes when co-incubated, indicating that cannibalism, or protease-on-protease degradation between catK and catL was occurring, but incubation with ECM could distract from these interactions. CatK alone was sufficient to quickly degrade tendon ECM, but catL was not, requiring the presence of catK for degradation. Together, these data identify cooperative and conflicting actions of cathepsin mediated collagen matrix degradation by considering interactive effects of multiple proteases during tissue degeneration.

Full-thickness rotator cuff tear in rat results in distinct temporal expression of multiple proteases in tendon, muscle, and cartilage

The etiology of joint tissue degeneration following rotator cuff tear remains unclear. Thus, the purpose of this study was to understand the timeline of protease activity in the soft tissues of the shoulder (tendon, muscle, and cartilage) that may lead to down-stream degeneration following rotator cuff tear. A well-established rat model involving suprascapular nerve denervation and supraspinatus/infraspinatus tendon transection was employed. Histological staining and/or micro-computed tomography (µCT) were used to observe structural damage in the supraspinatus tendon and muscle, humeral head cartilage, and subchondral bone. Multiplex gelatin zymography was utilized to assess protease activity in the supraspinatus tendon and muscle, and humeral head cartilage. Zymography analysis demonstrated that cathepsins were upregulated in the first week in all tissues, while MMP-2 maintained prolonged activity in supraspinatus tendon between 1 and 3 weeks and increased only at 3 weeks in supraspinatus muscle. In supraspinatus tendon, increased cathepsin L and MMP-2 activity in the first week was concurrent with matrix disorganization and infiltration of inflammatory cells. In contrast, significant upregulation of cathepsin L and K activity in supraspinatus muscle and humeral head cartilage did not correspond to any visible tissue damage at 1 week. However, focal defects developed in half of all animals' humeral head cartilage by 12 weeks (volume: 0.12 ± 0.09 mm³ ). This work provides a more comprehensive understanding of biochemical changes to joint tissue over time following rotator cuff tear. Overall, this provides insight into potential therapeutic targets and will better inform ideal intervention times and treatments for each tissue. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:490-502, 2019.

Supraspinatus tendon overuse results in degenerative changes to tendon insertion region and adjacent humeral cartilage in a rat model

The etiology of rotator cuff tendon overuse injuries is still not well understood. Furthermore, how this overuse injury impacts other components of the glenohumeral joint, including nearby articular cartilage, is also unclear. Therefore, this study sought to better understand the time course of tendon protease activity in a rat model of supraspinatus overuse, as well as determine effects of 10 weeks of overuse on humeral head articular cartilage. For these studies, multiplex gelatin zymography was used to characterize protease activity profiles in tendon and cartilage, while histological scoring/mechanical testing and micro-computed tomography (μCT) imaging were used to quantify structural damage in the supraspinatus tendon insertion and humeral articular cartilage, respectively. Histological scoring of supraspinatus tendon insertions revealed tendinopathic cellular and collagen fiber changes after 10 weeks of overuse when compared to controls, while mechanical testing revealed no significant differences between tensile moduli (overuse: 24.5 ± 11.5 MPa; control: 16.3 ± 8.7 MPa). EPIC-μCT imaging on humeral articular cartilage demonstrated significant cartilage thinning (overuse: 119.6 ± 6.34 μm; control: 195.4 ± 13.4μm), decreased proteoglycan content (overuse: 2.1 ± 0.18 cm^-1 ; control: 1.65 ± 0.14 cm^-1 ), and increased subchondral bone thickness (overuse: 216.2 ± 10.9 μm; control: 192 ± 17.8μm) in the overuse animals. Zymography results showed no significant upregulation of cathepsins or matrix metalloproteinases in tendon or cartilage at 2 or 10 weeks of overuse compared to controls. These results have further elucidated timing of protease activity over 10 weeks and suggest that damage occurs to other tissues in addition to the supraspinatus tendon in this overuse injury model. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1910-1918, 2017.

Investigating the Life Expectancy and Proteolytic Degradation of Engineered Skeletal Muscle Biological Machines

A combination of techniques from 3D printing, tissue engineering and biomaterials has yielded a new class of engineered biological robots that could be reliably controlled via applied signals. These machines are powered by a muscle strip composed of differentiated skeletal myofibers in a matrix of natural proteins, including fibrin, that provide physical support and cues to the cells as an engineered basement membrane. However, maintaining consistent results becomes challenging when sustaining a living system in vitro. Skeletal muscle must be preserved in a differentiated state and the system is subject to degradation by proteolytic enzymes that can break down its mechanical integrity. Here we examine the life expectancy, breakdown, and device failure of engineered skeletal muscle bio-bots as a result of degradation by three classes of proteases: plasmin, cathepsin L, and matrix metalloproteinases (MMP-2 and MMP-9). We also demonstrate the use of gelatin zymography to determine the effects of differentiation and inhibitor concentration on protease expression. With this knowledge, we are poised to design the next generation of complex biological machines with controllable function, specific life expectancy and greater consistency. These results could also prove useful for the study of disease-specific models, treatments of myopathies, and other tissue engineering applications.

PACMANS: A bioinformatically informed algorithm to predict, design, and disrupt protease-on-protease hydrolysis

Multiple proteases in a system hydrolyze target substrates, but recent evidence indicates that some proteases will degrade other proteases as well. Cathepsin S hydrolysis of cathepsin K is one such example. These interactions may be uni- or bi-directional and change the expected kinetics. To explore potential protease-on-protease interactions in silico, a program was developed for users to input two proteases: (1) the protease-ase that hydrolyzes (2) the substrate, protease. This program identifies putative sites on the substrate protease highly susceptible to cleavage by the protease-ase, using a sliding-window approach that scores amino acid sequences by their preference in the protease-ase active site, culled from MEROPS database. We call this PACMANS, Protease-Ase Cleavage from MEROPS ANalyzed Specificities, and test and validate this algorithm with cathepsins S and K. PACMANS cumulative likelihood scoring identified L253 and V171 as sites on cathepsin K subject to cathepsin S hydrolysis. Mutations made at these locations were tested to block hydrolysis and validate PACMANS predictions. L253A and L253V cathepsin K mutants significantly reduced cathepsin S hydrolysis, validating PACMANS unbiased identification of these sites. Interfamilial protease interactions between cathepsin S and MMP-2 or MMP-9 were tested after predictions by PACMANS, confirming its utility for these systems as well. PACMANS is unique compared to other putative site cleavage programs by allowing users to define the proteases of interest and target, and can also be employed for non-protease substrate proteins, as well as short peptide sequences.

Differential cathepsin responses to inhibitor-induced feedback: E-64 and cystatin C elevate active cathepsin S and suppress active cathepsin L in breast cancer cells

Cathepsins are powerful proteases, once referred to as the lysosomal cysteine proteases, that have been implicated in breast cancer invasion and metastasis, but pharmaceutical inhibitors have suffered failures in clinical trials due to adverse side effects. Scientific advancement from lysosomotropic to cell impermeable cathepsin inhibitors have improved efficacy in treating disease, but off-target effects have still been problematic, motivating a need to better understand cellular feedback and responses to treatment with cathepsin inhibitors. To address this need, we investigated effects of E-64 and cystatin C, two broad spectrum cathepsin inhibitors, on cathepsin levels intra- and extracellularly in MDA-MB-231 breast cancer cells. Cathepsins S and L had opposing responses to both E-64 and cystatin C inhibitor treatments with paradoxically elevated amounts of active cathepsin S, but decreased amounts of active cathepsin L, as determined by multiplex cathepsin zymography. This indicated cellular feedback to selectively sustain the amounts of active cathepsin S even in the presence of inhibitors with subnanomolar inhibitory constant values. These differences were identified in cellular locations of cathepsins L and S, trafficking for secretion, co-localization with endocytosed inhibitors, and longer protein turnover time for cathepsin S compared to cathepsin L. Together, this work demonstrates that previously underappreciated cellular compensation and compartmentalization mechanisms may sustain elevated amounts of some active cathepsins while diminishing others after inhibitor treatment. This can confound predictions based solely on inhibitor kinetics, and must be better understood to effectively deploy therapies and dosing strategies that target cathepsins to prevent cancer progression.