Collagen XII mediated cellular and extracellular mechanisms regulate establishment of tendon structure and function

An engineered in vitro model of the human myotendinous junction

The myotendinous junction (MTJ) is a vulnerable region at the interface of skeletal muscle and tendon that forms an integrated mechanical unit. This study presents a technique for the spatially restrictive co-culture of human embryonic stem cell (hESC)-derived skeletal myocytes and primary tenocytes for two-dimensional modeling of the MTJ. Micropatterned lanes of extracellular matrix and a 2-well culture chamber define the initial regions of occupation. On day 1, both lines occupy less than 20 % of the initially vacant interstitial zone, referred to henceforth as the junction. Myocyte-tenocyte interdigitations are observed by day 7. Immunocytochemistry reveals enhanced organization and alignment of patterned myocyte and tenocyte features, as well as differential expression of multiple MTJ markers. On day 24, electrically stimulated junction myocytes demonstrate negative contractile strains, while positive tensile strains are exhibited by mechanically passive tenocytes at the junction. Unpatterned tenocytes distal to the junction experience significantly decreased strains in comparison to cells at the interface. Unpatterned myocytes have impaired organization and uncoordinated contractile behavior. These findings suggest that this platform is capable of inducing myocyte-tenocyte junction formation and mechanical coupling similar to the native MTJ, showing transduction of force across the cell-cell interface. STATEMENT OF SIGNIFICANCE: The myotendinous junction (MTJ) is an integrated structure that transduces force across the muscle-tendon boundary, making the region vulnerable to strain injury. Despite the clinical relevance, previous in vitro models of the MTJ lack the structure and mechanical accuracy of the native tissue and have difficulty transmitting force across the cell-cell interface. This study demonstrates an in vitro model of the MTJ, using spatially restrictive cues to inform human myocyte-tenocyte interactions and architecture. The model expressed MTJ markers and developed anisotropic myocyte-tenocyte integrations that resemble the native tissue and allow for force transduction from contracting myocytes to passive tenocyte regions. As such, this study presents a system capable of investigating development, injury, and pathology in the human MTJ.

The coordinated activities of collagen VI and XII in maintenance of tissue structure, function and repair: evidence for a physical interaction

Collagen VI and collagen XII are structurally complex collagens of the extracellular matrix (ECM). Like all collagens, type VI and XII both possess triple-helical components that facilitate participation in the ECM network, but collagen VI and XII are distinct from the more abundant fibrillar collagens in that they also possess arrays of structurally globular modules with the capacity to propagate signaling to attached cells. Cell attachment to collagen VI and XII is known to regulate protective, proliferative or developmental processes through a variety of mechanisms, but a growing body of genetic and biochemical evidence suggests that at least some of these phenomena may be potentiated through mechanisms that require coordinated interaction between the two collagens. For example, genetic studies in humans have identified forms of myopathic Ehlers-Danlos syndrome with overlapping phenotypes that result from mutations in either collagen VI or XII, and biochemical and cell-based studies have identified accessory molecules that could form bridging interactions between the two collagens. However, the demonstration of a direct or ternary structural interaction between collagen VI or XII has not yet been reported. This Hypothesis and Theory review article examines the evidence that supports the existence of a functional complex between type VI and XII collagen in the ECM and discusses potential biological implications.

Collagen VI Deficiency Impairs Tendon Fibroblasts Mechanoresponse in Ullrich Congenital Muscular Dystrophy

The pericellular matrix (PCM) is a specialized extracellular matrix that surrounds cells. Interactions with the PCM enable the cells to sense and respond to mechanical signals, triggering a proper adaptive response. Collagen VI is a component of muscle and tendon PCM. Mutations in collagen VI genes cause a distinctive group of inherited skeletal muscle diseases, and Ullrich congenital muscular dystrophy (UCMD) is the most severe form. In addition to muscle weakness, UCMD patients show structural and functional changes of the tendon PCM. In this study, we investigated whether PCM alterations due to collagen VI mutations affect the response of tendon fibroblasts to mechanical stimulation. By taking advantage of human tendon cultures obtained from unaffected donors and from UCMD patients, we analyzed the morphological and functional properties of cellular mechanosensors. We found that the length of the primary cilia of UCMD cells was longer than that of controls. Unlike controls, in UCMD cells, both cilia prevalence and length were not recovered after mechanical stimulation. Accordingly, under the same experimental conditions, the activation of the Hedgehog signaling pathway, which is related to cilia activity, was impaired in UCMD cells. Finally, UCMD tendon cells exposed to mechanical stimuli showed altered focal adhesions, as well as impaired activation of Akt, ERK1/2, p38MAPK, and mechanoresponsive genes downstream of YAP. By exploring the response to mechanical stimulation, for the first time, our findings uncover novel unreported mechanistic aspects of the physiopathology of UCMD-derived tendon fibroblasts and point at a role for collagen VI in the modulation of mechanotransduction in tendons.

Alpha-1 adrenergic signaling drives cardiac regeneration via extracellular matrix remodeling transcriptional program in zebrafish macrophages

The autonomic nervous system plays a pivotal role in cardiac repair. Here, we describe the mechanistic underpinning of adrenergic signaling in fibrotic and regenerative response of the heart to be dependent on immunomodulation. A pharmacological approach identified adrenergic receptor alpha-1 as a key regulator of macrophage phenotypic diversification following myocardial damage in zebrafish. Genetic manipulation and single-cell transcriptomics showed that the receptor signals activation of an "extracellular matrix remodeling" transcriptional program in a macrophage subset, which serves as a key regulator of matrix composition and turnover. Mechanistically, adrenergic receptor alpha-1-activated macrophages determine activation of collagen-12-expressing fibroblasts, a cellular determinant of cardiac regenerative niche, through midkine-mediated paracrine crosstalk, allowing lymphatic and blood vessel growth and cardiomyocyte proliferation at the lesion site. These findings identify the mechanism of adrenergic signaling in macrophage phenotypic and functional determination and highlight the potential of neural modulation for regulation of fibrosis and coordination of myocardial regenerative response.

The rat Achilles and patellar tendons have similar increases in mechanical properties but become transcriptionally divergent during postnatal development

The molecular events that drive post-natal tendon development are poorly characterized. In this study, we profiled morphological, mechanical, and transcriptional changes in the rat Achilles and patellar tendon before walking (P7), shortly after onset of walking (P14), and at motor maturity (P28). The Achilles and patellar tendons increased collagen content and mechanical strength similarly throughout post-natal development. However, at P28 the patellar tendon tended to display a higher maximal tensile load (MTL) (P = 0.0524) than the Achilles tendon, but a similar ultimate tensile strength (UTS; load relative to cross-sectional area) probably due to its increased cross-sectional area during development. The tendons started transcriptionally similar, with overlapping PCA clusters at P7 and P14, before becoming transcriptionally distinct at P28. In both tendons, there was an increase in extracellular matrix (ECM) gene expression and a concomitant decrease in cell cycle and mitochondrial gene expression. The transcriptional divergence at P28 suggested that STAT signalling was lower in the patellar tendon where MTL increased the most. Treating engineered human ligaments with the STAT inhibitor itacitinib increased collagen content and MTL. Our results suggest that during post-natal development, cellular resources are initially allocated towards cell proliferation before shifting towards extracellular matrix development following the onset of mechanical load and provide potential targets for improving tendon function. KEY POINTS: Little is known about mechanisms of post-natal tendon growth. We characterized morphological, mechanical, and transcriptional changes that occur before (P7), and early (P14) and late after (P28) rats begin to walk. From P7 to P28, the Achilles tendon increased in length, whereas the patellar tendon increased in cross-sectional area. Mechanical and material properties of the Achilles and patellar tendon increased from P7 to P28. From P7 to P28, the Achilles and patellar tendons increased expression of ECM genes and decreased mitochondrial and cell cycle gene expression. Ribosomal gene expression also significantly decreased in the Achilles and tended to decrease in the patellar tendon. At P28, STAT1 signalling tended to be lower in the patellar tendon which had grown by increasing cross-sectional area and inhibiting STAT activation in vitro improved mechanical properties in engineered human ligaments.

Equine tendon mechanical behaviour: Prospects for repair and regeneration applications

Tendons are dense connective tissues that play an important role in the biomechanical function of the musculoskeletal system. The mechanical forces have been implicated in every aspect of tendon biology. Tendon injuries are frequently occurring and their response to treatments is often unsatisfactory. A better understanding of tendon biomechanics and mechanobiology can help develop treatment options to improve clinical outcomes. Recently, tendon tissue engineering has gained more attention as an alternative treatment due to its potential to overcome the limitations of current treatments. This review first provides a summary of tendon mechanical properties, focusing on recent findings of tendon mechanobiological responses. In the next step, we highlight the biomechanical parameters of equine energy-storing and positional tendons. The final section is devoted to how mechanical loading contributes to tenogenic differentiation using bioreactor systems. This study may help develop novel strategies for tendon injury prevention or accelerate and improve tendon healing.

A defective mechanosensing pathway affects fibroblast-to-myofibroblast transition in the old male mouse heart

The cardiac fibroblast interacts with an extracellular matrix (ECM), enabling myofibroblast maturation via a process called mechanosensing. Although in the aging male heart, ECM is stiffer than in the young mouse, myofibroblast development is impaired, as demonstrated in 2-D and 3-D experiments. In old male cardiac fibroblasts, we found a decrease in actin polymerization, α-smooth muscle actin (α-SMA), and Kindlin-2 expressions, the latter an effector of the mechanosensing. When Kindlin-2 levels were manipulated via siRNA interference, young fibroblasts developed an old-like fibroblast phenotype, whereas Kindlin-2 overexpression in old fibroblasts reversed the defective phenotype. Finally, inhibition of overactivated extracellular regulated kinases 1 and 2 (ERK1/2) in the old male fibroblasts rescued actin polymerization and α-SMA expression. Pathological ERK1/2 overactivation was also attenuated by Kindlin-2 overexpression. In contrast, old female cardiac fibroblasts retained an operant mechanosensing pathway. In conclusion, we identified defective components of the Kindlin/ERK/actin/α-SMA mechanosensing axis in aged male fibroblasts.

Cell differentiation and matrix organization are differentially affected during bone formation in osteogenesis imperfecta zebrafish models with different genetic defects impacting collagen type I structure

Osteogenesis imperfecta (OI) is a family of rare heritable skeletal disorders associated with dominant mutations in the collagen type I encoding genes and recessive defects in proteins involved in collagen type I synthesis and processing and in osteoblast differentiation and activity. Historically, it was believed that the OI bone phenotype was only caused by abnormal collagen type I fibrils in the extracellular matrix, but more recently it became clear that the altered bone cell homeostasis, due to mutant collagen retention, plays a relevant role in modulating disease severity in most of the OI forms and it is correlated to impaired bone cell differentiation. Despite in vitro evidence, in vivo data are missing. To better understand the physiopathology of OI, we used two zebrafish models: Chihuahua (Chi/+), carrying a dominant p.G736D substitution in the α1 chain of collagen type I, and the recessive p3h1-/-, lacking prolyl 3-hydroxylase (P3h1) enzyme. Both models share the delay of collagen type I folding, resulting in its overmodification and partial intracellular retention. The regeneration of the bony caudal fin of Chi/+ and p3h1-/- was employed to investigate the impact of abnormal collagen synthesis on bone cell differentiation. Reduced regenerative ability was evident in both models, but it was associated to impaired osteoblast differentiation and osteoblastogenesis/adipogenesis switch only in Chi/+. On the contrary, reduced osteoclast number and activity were found in both models during regeneration. The dominant OI model showed a more detrimental effect in the extracellular matrix organization. Interestingly, the chemical chaperone 4-phenylbutyrate (4-PBA), known to reduce cellular stress and increase collagen secretion, improved bone formation only in p3h1-/- by favoring caudal fin growth without affecting bone cell markers expression. Taken together, our in vivo data proved the negative impact of structurally abnormal collagen type I on bone formation but revealed a gene mutation-specific effect on bone cell differentiation and matrix organization in OI. These, together with the distinct ability to respond to the chaperone treatment, underline the need for precision medicine approaches to properly treat the disease.

Ablation of collagen XII disturbs joint extracellular matrix organization and causes patellar subluxation

Collagen XII, belonging to the fibril-associated collagens, is a homotrimeric secreted extracellular matrix (ECM) protein encoded by the COL12A1 gene. Mutations in the human COL12A1 gene cause an Ehlers-Danlos/myopathy overlap syndrome leading to skeletal abnormalities and muscle weakness. Here, we studied the role of collagen XII in joint pathophysiology by analyzing collagen XII deficient mice and human patients. We found that collagen XII is widely expressed across multiple connective tissue of the developing joint. Lack of collagen XII in mice destabilizes tendons and the femoral trochlear groove to induce patellar subluxation in the patellofemoral joint. These changes are associated with an ECM damage response in tendon and secondary quadriceps muscle degeneration. Moreover, patellar subluxation was also identified as a clinical feature of human patients with collagen XII deficiency. The results provide an explanation for joint hyperlaxity in mice and human patients with collagen XII deficiency.

Extracellular Matrix Proteomics: The mdx-4cv Mouse Diaphragm as a Surrogate for Studying Myofibrosis in Dystrophinopathy

The progressive degeneration of the skeletal musculature in Duchenne muscular dystrophy is accompanied by reactive myofibrosis, fat substitution, and chronic inflammation. Fibrotic changes and reduced tissue elasticity correlate with the loss in motor function in this X-chromosomal disorder. Thus, although dystrophinopathies are due to primary abnormalities in the DMD gene causing the almost-complete absence of the cytoskeletal Dp427-M isoform of dystrophin in voluntary muscles, the excessive accumulation of extracellular matrix proteins presents a key histopathological hallmark of muscular dystrophy. Animal model research has been instrumental in the characterization of dystrophic muscles and has contributed to a better understanding of the complex pathogenesis of dystrophinopathies, the discovery of new disease biomarkers, and the testing of novel therapeutic strategies. In this article, we review how mass-spectrometry-based proteomics can be used to study changes in key components of the endomysium, perimysium, and epimysium, such as collagens, proteoglycans, matricellular proteins, and adhesion receptors. The mdx-4cv mouse diaphragm displays severe myofibrosis, making it an ideal model system for large-scale surveys of systematic alterations in the matrisome of dystrophic fibers. Novel biomarkers of myofibrosis can now be tested for their appropriateness in the preclinical and clinical setting as diagnostic, pharmacodynamic, prognostic, and/or therapeutic monitoring indicators.

Single-cell and spatial transcriptomics reveal changes in cell heterogeneity during progression of human tendinopathy

BACKGROUND

Musculoskeletal tissue degeneration impairs the life quality and motor function of many people, especially seniors and athletes. Tendinopathy is one of the most common diseases associated with musculoskeletal tissue degeneration, representing a major global healthcare burden that affects both athletes and the general population, with the clinical presentation of long-term recurring chronic pain and decreased tolerance to activity. The cellular and molecular mechanisms at the basis of the disease process remain elusive. Here, we use a single-cell and spatial RNA sequencing approach to provide a further understanding of cellular heterogeneity and molecular mechanisms underlying tendinopathy progression.

RESULTS

To explore the changes in tendon homeostasis during the tendinopathy process, we built a cell atlas of healthy and diseased human tendons using single-cell RNA sequencing of approximately 35,000 cells and explored the variations of cell subtypes' spatial distributions using spatial RNA sequencing. We identified and localized different tenocyte subpopulations in normal and lesioned tendons, found different differentiation trajectories of tendon stem/progenitor cells in normal/diseased tendons, and revealed the spatial location relationship between stromal cells and diseased tenocytes. We deciphered the progression of tendinopathy at a single-cell level, which is characterized by inflammatory infiltration, followed by chondrogenesis and finally endochondral ossification. We found diseased tissue-specific endothelial cell subsets and macrophages as potential therapeutic targets.

CONCLUSIONS

This cell atlas provides the molecular foundation for investigating how tendon cell identities, biochemical functions, and interactions contributed to the tendinopathy process. The discoveries revealed the pathogenesis of tendinopathy at single-cell and spatial levels, which is characterized by inflammatory infiltration, followed by chondrogenesis, and finally endochondral ossification. Our results provide new insights into the control of tendinopathy and potential clues to developing novel diagnostic and therapeutic strategies.

A single-cell transcriptional atlas reveals resident progenitor cell niche functions in TMJ disc development and injury

The biological characteristics of the temporomandibular joint disc involve complex cellular network in cell identity and extracellular matrix composition to modulate jaw function. The lack of a detailed characterization of the network severely limits the development of targeted therapies for temporomandibular joint-related diseases. Here we profiled single-cell transcriptomes of disc cells from mice at different postnatal stages, finding that the fibroblast population could be divided into chondrogenic and non-chondrogenic clusters. We also find that the resident mural cell population is the source of disc progenitors, characterized by ubiquitously active expression of the NOTCH3 and THY1 pathways. Lineage tracing reveals that Myh11⁺ mural cells coordinate angiogenesis during disc injury but lost their progenitor characteristics and ultimately become Sfrp2⁺ non-chondrogenic fibroblasts instead of Chad⁺ chondrogenic fibroblasts. Overall, we reveal multiple insights into the coordinated development of disc cells and are the first to describe the resident mural cell progenitor during disc injury.

High-Performance Polarization Microscopy Reveals Structural Remodeling in Rat Calcaneal Tendons Cultivated In Vitro

Collagenous tissues exhibit anisotropic optical properties such as birefringence and linear dichroism (LD) as a result of their structurally oriented supraorganization from the nanometer level to the collagen bundle scale. Changes in macromolecular order and in aggregational states can be evaluated in tendon collagen bundles using polarization microscopy. Because there are no reports on the status of the macromolecular organization in tendon explants, the objective of this work was to evaluate the birefringence and LD characteristics of collagen bundles in rat calcaneal tendons cultivated in vitro on substrates that differ in their mechanical stiffness (plastic vs. glass) while accompanying the expected occurrence of cell migration from these structures. Tendon explants from adult male Wistar rats were cultivated for 8 and 12 days on borosilicate glass coverslips (n = 3) and on nonpyrogenic polystyrene plastic dishes (n = 4) and were compared with tendons not cultivated in vitro (n = 3). Birefringence was investigated in unstained tendon sections using high-performance polarization microscopy and image analysis. LD was studied under polarized light in tendon sections stained with the dichroic dyes Ponceau SS and toluidine blue at pH 4.0 to evaluate the orientation of proteins and acid glycosaminoglycans (GAG) macromolecules, respectively. Structural remodeling characterized by the reduction in the macromolecular orientation, aggregation and alignment of collagen bundles, based on decreased average gray values concerned with birefringence intensity, LD and morphological changes, was detected especially in the tendon explants cultivated on the plastic substrate. These changes may have facilitated cell migration from the lateral regions of the explants to the substrates, an event that was observed earlier and more intensely upon tissue cultivation on the plastic substrate. The axial alignment of the migrating cells relative to the explant, which occurred with increased cultivation times, may be due to the mechanosensitive nature of the tenocytes. Collagen fibers possibly played a role as a signal source to cells, a hypothesis that requires further investigation, including studies on the dynamics of cell membrane receptors and cytoskeletal organization, and collagen shearing electrical properties.

Collagen XII mediated cellular and extracellular mechanisms in development, regeneration, and disease

Collagen XII, a fibril-associated collagen with interrupted triple helices (FACIT), influences fibrillogenesis in numerous tissues. In addition to this extracellular function, collagen XII also directly regulates cellular function. Collagen XII is widely expressed in connective tissues, particularly tendons, ligaments, and the periodontium and periosteum, where it is enriched in the pericellular regions. Mutations in the collagen XII gene cause myopathic Ehlers-Danlos syndrome (mEDS), an early-onset disease characterized by overlapping connective tissue abnormalities and muscle weakness. Patients with mEDS exhibit delayed motor development, muscle weakness, joint laxity, hypermobility, joint contractures, and abnormal wound healing. A mEDS mouse model was generated by deletion of the Col12a1 gene, resulting in skeletal and muscle abnormalities with disorganized tissue structures and altered mechanical properties. Extracellularly, collagen XII interacts with collagen I fibrils and regulates collagen fibril spacing and assembly during fibrillogenesis. Evidence for the binding of collagen XII to other EDS-related molecules (e.g., decorin and tenascin X) suggests that disruption of ECM molecular interactions is one of the causes of connective tissue pathology in mEDS. Collagen XII also has been shown to influence cell behavior, such as cell shape and cell-cell communication, by providing physical connection between adjacent cells during tissue development and regeneration. The focus of this review is on the functions of collagen XII in development, regeneration, and disease.

Single-cell transcriptome analysis reveals cellular heterogeneity in mouse intra- and extra articular ligaments

Ligaments are collagenous connective tissues that connect bones. Injury of knee ligaments, namely anterior cruciate ligament (ACL) and medial collateral ligament (MCL), is common in athletes. Both ligaments have important functions, but distinct regeneration capacities. The capacity for recovery after injury also diminishes with age. However, cellular heterogeneity in the ligaments remains unclear. Here, we profiled the transcriptional signatures of ACL and MCL cells in mice using single-cell RNA sequencing. These ligaments comprise three fibroblast types expressing Col22a1, Col12a1, or Col14a1, but have distinct localizations in the tissue. We found substantial heterogeneity in Col12a1- and Col14a1-positive cells between ACL and MCL. Gene Ontology analysis revealed that angiogenesis- and collagen regulation-related genes were specifically enriched in MCL cells. Furthermore, we identified age-related changes in cell composition and gene expression in the ligaments. This study delineates cellular heterogeneity in ligaments, serving as a foundation for identifying potential therapeutic targets for ligament injuries.

Cell heterogeneity in the mouse anterior cruciate ligament (ACL) and medial collateral ligament (MCL) is demonstrated using single-cell analysis with three types of fibroblasts identified, expressing Col14a1, Col12a1, or Col22a1.

Targeted conditional collagen XII deletion alters tendon function

Collagen XII is a fibril-associated collagen with interrupted triple helices (FACIT). This non-fibrillar collagen is a homotrimer composed of three α1(XII) chains assembled into a collagenous molecule with a C terminal collagenous domain and a large N terminal non-collagenous domain. During tendon development and growth, collagen XII is broadly expressed throughout the extracellular matrix and enriched pericellularly around tenocytes. Tendons in a global Col12a1 ^-/- knockout model demonstrated disrupted fibril and fiber structure and disordered tenocyte organization, highlighting the critical regulatory roles of collagen XII in determining tendon structure and function. However, muscle and bone also are affected in the collagen XII knockout model. Therefore, secondary effects on tendon due to involvement of bone and muscle may occur in the global knockout. The global knockout does not allow the definition of intrinsic mechanisms involving collagen XII in tendon versus extrinsic roles involving muscle and bone. To address this limitation, we created and characterized a conditional Col12a1-null mouse model to permit the spatial and temporal manipulation of Col12a1 expression. Collagen XII knockout was targeted to tendons by breeding conditional Col12a1 ^flox/flox mice with Scleraxis-Cre (Scx-Cre) mice to yield a tendon-specific Col12a1-null mouse line, Col12a1 ^Δten/Δten . Both mRNA and protein expression in Col12a1 ^Δten/Δten mice decreased to near baseline levels in flexor digitorum longus tendons (FDL). Collagen XII immuno-localization revealed an absence of reactivity in the tendon proper, but there was reactivity in the cells of the surrounding peritenon. This supports a targeted knockout in tenocytes while peritenon cells from a non-tendon lineage were not targeted and retained collagen XII expression. The tendon-targeted, Col12a1 ^Δten/Δten  mice had significantly reduced forelimb grip strength, altered gait and a significant decrease in biomechanical properties. While the observed decrease in tendon modulus suggests that differences in tendon material properties in the absence of Col12a1 expression underlie the functional deficiencies. Together, these findings suggest an intrinsic role for collagen XII critical for development of a functional tendon.

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.

Temporal profiling of the breast tumour microenvironment reveals collagen XII as a driver of metastasis

The tumour stroma, and in particular the extracellular matrix (ECM), is a salient feature of solid tumours that plays a crucial role in shaping their progression. Many desmoplastic tumours including breast cancer involve the significant accumulation of type I collagen. However, recently it has become clear that the precise distribution and organisation of matrix molecules such as collagen I is equally as important in the tumour as their abundance. Cancer-associated fibroblasts (CAFs) coexist within breast cancer tissues and play both pro- and anti-tumourigenic roles through remodelling the ECM. Here, using temporal proteomic profiling of decellularized tumours, we interrogate the evolving matrisome during breast cancer progression. We identify 4 key matrisomal clusters, and pinpoint collagen type XII as a critical component that regulates collagen type I organisation. Through combining our proteomics with single-cell transcriptomics, and genetic manipulation models, we show how CAF-secreted collagen XII alters collagen I organisation to create a pro-invasive microenvironment supporting metastatic dissemination. Finally, we show in patient cohorts that collagen XII may represent an indicator of breast cancer patients at high risk of metastatic relapse.

The distribution and organisation of matrix molecules in the tumour stroma help shape solid tumour progression. Here they perform temporal proteomic profiling of the matrisome during breast cancer progression and show that collagen XII secreted from CAFs provides a pro-invasive microenvironment.

A Cd9+Cd271+ stem/progenitor population and the SHP2 pathway contribute to neonatal-to-adult switching that regulates tendon maturation

Tendon maturation lays the foundation for postnatal tendon development, its proper mechanical function, and regeneration, but the critical cell populations and the entangled mechanisms remain poorly understood. Here, by integrating the structural, mechanical, and molecular properties, we show that post-natal days 7-14 are the crucial transitional stage for mouse tendon maturation. We decode the cellular and molecular regulatory networks at the single-cell level. We find that a nerve growth factor (NGF)-secreting Cd9⁺Cd271⁺ tendon stem/progenitor cell population mainly prompts conversion from neonate to adult tendon. Through single-cell gene regulatory network analysis, in vitro inhibitor identification, and in vivo tendon-specific Shp2 deletion, we find that SHP2 signaling is a regulator for tendon maturation. Our research comprehensively reveals the dynamic cell population transition during tendon maturation, implementing insights into the critical roles of the maturation-related stem cell population and SHP2 signaling pathway during tendon differentiation and regeneration.

LTBP1 promotes fibrillin incorporation into the extracellular matrix

LTBP1 is a large extracellular matrix protein and an associated ligand of fibrillin-microfibrils. Knowledge of LTBP1 functions is largely limited to its role in targeting and sequestering TGFβ growth factors within the extracellular matrix, thereby regulating their bioavailability. However, the recent description of a wide spectrum of phenotypes in multiple tissues in patients harboring LTBP1 pathogenic variants suggests a multifaceted role of the protein in the homeostasis of connective tissues. To better understand the human pathology caused by LTBP1 deficiency it is important to investigate its functional role in extracellular matrix formation. In this study, we show that LTBP1 coordinates the incorporation of fibrillin-1 and -2 into the extracellular matrix in vitro. We also demonstrate that this function is differentially exerted by the two isoforms, the short and long forms of LTBP1. Thereby our findings uncover a novel TGFβ-independent LTBP1 function potentially contributing to the development of connective tissue disorders.

Integration Analysis of Transcriptome and Proteome Reveal the Mechanisms of Goat Wool Bending

Zhongwei goat is a unique Chinese native goat breed for excellent lamb fur. The pattern of flower spikes of the lamb fur was significantly reduced due to the reduction of the bending of the hair strands with growth. In order to explore the molecular mechanism underlying hair bending with growth, we performed the comprehensive analysis of transcriptome and proteome of skins from 45-days, 108-days and 365-days goat based on TMT-based quantitative proteomics and RNA-seq methods. In the three comparison groups, 356, 592 and 282 differentially expressed proteins (DEPs) were screened, respectively. KEGG pathway analysis indicated that DEPs were significantly enriched in a set of signaling pathways related to wool growth and bending, such as ECM-receptor interaction, PI3K-Akt signaling pathway, PPAR signaling pathway, protein digestion and absorption, and metabolic pathways. In addition, 20 DEPs abundance of goat skin at three development stages were examined by PRM method, which validated the reliability of proteomic data. Among them, KRT and collagen alpha family may play an important role in the development of goat hair follicle and wool bending. COL6A1, COL6A2, CRNN, TNC and LOC102178129 were identified as candidate genes based on combined analysis of transcriptome and proteome data and PRM quantification. Our results identify the differential expressed proteins as well as pathways related to the wool bending of Zhongwei goats and provide a theoretical basis for further revealing the molecular mechanism underlying wool bending of goats.

The Non-pregnant and Pregnant Human Cervix: a Systematic Proteomic Analysis

Appropriate timing of cervical remodeling (CR) is key to normal term parturition. To date, mechanisms behind normal and abnormal (premature or delayed) CR remain unclear. Recent studies show regional differences exist in human cervical tissue structure. While the entire cervix contains extracellular matrix (ECM), the internal os is highly cellular containing 50-60% cervical smooth muscle (CSM). The external os contains 10-20% CSM. Previously, we reported ECM rigidity and different ECM proteins influence CSM cell function, highlighting the importance of understanding not only how cervical cells orchestrate cervical ECM remodeling in pregnancy, but also how changes in specific ECM proteins can influence resident cellular function. To understand this dynamic process, we utilized a systematic proteomic approach to understand which soluble ECM and cellular proteins exist in the different regions of the human cervix and how the proteomic profiles change from the non-pregnant (NP) to the pregnant (PG) state. We found the human cervix proteome contains at least 4548 proteins and establish the types and relative abundance of cellular and soluble matrisome proteins found in the NP and PG human cervix. Further, we report the relative abundance of proteins involved with elastic fiber formation and ECM organization/degradation were significantly increased while proteins involved in RNA polymerase I/promoter opening, DNA methylation, senescence, immune system, and compliment activation were decreased in the PG compared to NP cervix. These findings establish an initial platform from which we can further comprehend how changes in the human cervix proteome results in normal and abnormal CR.

Collagen XII Regulates Corneal Stromal Structure by Modulating Transforming Growth Factor-β Activity

Collagen XII is a regulator of corneal stroma structure and function. The current study examined the role of collagen XII in regulating corneal stromal transforming growth factor (TGF)-β activation and latency. Specifically, with the use of conventional collagen XII null mouse model, the role of collagen XII in the regulation of TGF-β latency and activity in vivo was investigated. Functional quantification of latent TGF-β in stromal matrix was performed by using transformed mink lung reporter cells that produce luciferase as a function of active TGF-β. Col12a1 knockdown with shRNA was used to test the role of collagen XII in TGF-β activation. Col12a1^-/- hypertrophic stromata were observed with keratocyte hyperplasia. Increased collagen fibril forward signal was found by second harmonic generation microscopy in the absence of collagen XII. Collagen XII regulated mRNA synthesis of Serpine1, Col1a1, and Col5a1 and deposition of collagens in the extracellular matrix. A functional plasminogen activator inhibitor luciferase assay showed that collagen XII is necessary for latent TGF-β storage in the extracellular matrix and that collagen XII down-regulates active TGF-β. Collagen XII dictates stromal structure and function by regulating TGF-β activity. A hypertrophic phenotype in Col12a1^-/- corneal tissue can be explained by abnormal up-regulation of TGF-β activation and decreased latent storage.

Animal Models of Ehlers-Danlos Syndromes: Phenotype, Pathogenesis, and Translational Potential

The Ehlers–Danlos syndromes (EDS) are a group of heritable connective tissues disorders mainly characterized by skin hyperextensibility, joint hypermobility and generalized tissue fragility. Currently, 14 EDS subtypes each with particular phenotypic features are recognized and are caused by genetic defects in 20 different genes. All of these genes are involved in the biosynthesis and/or fibrillogenesis of collagens at some level. Although great progress has been made in elucidating the molecular basis of different EDS subtypes, the pathogenic mechanisms underlying the observed phenotypes remain poorly understood, and consequentially, adequate treatment and management options for these conditions remain scarce. To date, several animal models, mainly mice and zebrafish, have been described with defects in 14 of the 20 hitherto known EDS-associated genes. These models have been instrumental in discerning the functions and roles of the corresponding proteins during development, maturation and repair and in portraying their roles during collagen biosynthesis and/or fibrillogenesis, for some even before their contribution to an EDS phenotype was elucidated. Additionally, extensive phenotypical characterization of these models has shown that they largely phenocopy their human counterparts, with recapitulation of several clinical hallmarks of the corresponding EDS subtype, including dermatological, cardiovascular, musculoskeletal and ocular features, as well as biomechanical and ultrastructural similarities in tissues. In this narrative review, we provide a comprehensive overview of animal models manifesting phenotypes that mimic EDS with a focus on engineered mouse and zebrafish models, and their relevance in past and future EDS research. Additionally, we briefly discuss domestic animals with naturally occurring EDS phenotypes. Collectively, these animal models have only started to reveal glimpses into the pathophysiological aspects associated with EDS and will undoubtably continue to play critical roles in EDS research due to their tremendous potential for pinpointing (common) signaling pathways, unveiling possible therapeutic targets and providing opportunities for preclinical therapeutic interventions.

Hierarchical ultrastructure: An overview of what is known about tendons and future perspective for tendon engineering

Abnormal tendons are rarely ever repaired to the natural structure and morphology of normal tendons. To better guide the repair and regeneration of injured tendons through a tissue engineering method, it is necessary to have insights into the internal morphology, organization, and composition of natural tendons. This review summarized recent researches on the structure and function of the extracellular matrix (ECM) components of tendons and highlight the application of multiple detection methodologies concerning the structure of ECMs. In addition, we look forward to the future of multi-dimensional biomaterial design methods and the potential of structural repair for tendon ECM components. In addition, focus is placed on the macro to micro detection methods for tendons, and current techniques for evaluating the extracellular matrix of tendons at the micro level are introduced in detail. Finally, emphasis is given to future extracellular matrix detection methods, as well as to how future efforts could concentrate on fabricating the biomimetic tendons.

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Summarize recent research on the structure and function of the extracellular matrix (ECM) components of tendons.

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Comments on current research methods concerning the structure of ECMs.

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Perspective on the future of multi-dimensional detection techniques and structural repair of tendon ECM components.

Collagen XII Deficiency Increases the Risk of Anterior Cruciate Ligament Injury in Mice

Anterior cruciate ligament (ACL) rupture is a common knee injury for athletes. Although surgical reconstruction is recommended for the treatment of ACL ruptures, 100% functional recovery is unlikely. Therefore, the discovery of risk factors for ACL ruptures may prevent injury. Several studies have reported an association between polymorphisms of the collagen XII gene COL12A1 and ACL rupture. Collagen XII is highly expressed in tendons and ligaments and regulates tissue structure and mechanical property. Therefore, we hypothesized that collagen XII deficiency may cause ACL injury. To elucidate the influence of collagen XII deficiency on ACL, we analyzed a mouse model deficient for Col12a1. Four- to 19-week-old male Col12a1^-/- and wild-type control mice were used for gait analysis; histological and immunofluorescent analysis of collagen XII, and real-time RT-PCR evaluation of Col12a1 mRNA expression. The Col12a1^-/- mice showed an abnormal gait with an approximately 2.7-fold increase in step angle, suggesting altered step alignment. Col12a1^-/- mice displayed 20–60% ACL discontinuities, but 0% discontinuity in the posterior cruciate ligament. No discontinuities in knee ligaments were found in wild-type mice. Collagen XII mRNA expression in the ACL tended to decrease with aging. Our study demonstrates for the first time that collagen XII deficiency increases the risk of ACL injury.

Mechanical Pressure Driving Proteoglycan Expression in Mammographic Density: a Self-perpetuating Cycle?

Regions of high mammographic density (MD) in the breast are characterised by a proteoglycan (PG)-rich fibrous stroma, where PGs mediate aligned collagen fibrils to control tissue stiffness and hence the response to mechanical forces. Literature is accumulating to support the notion that mechanical stiffness may drive PG synthesis in the breast contributing to MD. We review emerging patterns in MD and other biological settings, of a positive feedback cycle of force promoting PG synthesis, such as in articular cartilage, due to increased pressure on weight bearing joints. Furthermore, we present evidence to suggest a pro-tumorigenic effect of increased mechanical force on epithelial cells in contexts where PG-mediated, aligned collagen fibrous tissue abounds, with implications for breast cancer development attributable to high MD. Finally, we summarise means through which this positive feedback mechanism of PG synthesis may be intercepted to reduce mechanical force within tissues and thus reduce disease burden.

COL12A1 rs970547 Polymorphism Does Not Alter Susceptibility to Anterior Cruciate Ligament Rupture: A Meta-Analysis

Objective: Currently available evidence regarding the association between collagen type XII α1 (COL12A1) polymorphism and risk of anterior cruciate ligament rupture (ACLR) remains elusive. The aim of our present study was to assess the association between COL12A1 rs970547 polymorphism and ACLR risk.

Methods: Five online databases, namely, PubMed, EMBASE, ISI Web of Science, CENTRAL, and CNKI, were searched from their inception data up to December 2020 to identify relative observational studies. The methodological quality of each individual study was evaluated using the Newcastle-Ottawa Scale (NOS). The “model-free approach” was employed to estimate the magnitude of effect of COL12A1 rs970547 polymorphism on ACLR, and the association was expressed using odds ratio (OR) and its associated 95% confidence interval (95% CI). Subgroup analysis was performed by ethnicity and sex of included subjects.

Results: Eight studies involving 1,477 subjects with ACLR and 100,439 healthy controls were finally included in our study. The methodological quality of included studies was deemed moderate to high based on NOS scores. The “model-free” approach suggested no genotype differences between ACLR and healthy control for the rs970547 polymorphism, but we still used the allele model to present the combined data. Under the random-effect model, there was no significant difference in the frequency of effecting allele between ACLR and control (OR: 0.91, 95% CI 0.77, 1.08; p = 0.28). Stratified analysis by sex and ethnicity also showed no difference in allele frequency.

Conclusion: The findings of this current meta-analysis suggested that rs970547 was not associated with ACLR risk in male, female, and the overall population among Asians or Caucasians.

PRIMUS: Comprehensive proteomics of mouse intervertebral discs that inform novel biology and relevance to human disease modelling

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Proteomics of healthy mouse IVDs differentiating compartments and spine levels.

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NP cells feature vacuoles with lysosomal, transport and cell–cell communication functions.

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Collagen XII, decorin and other ECM proteins contribute to function of the AF.

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Distinct proteomics between lumbar and tail discs.

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Mouse is a relevant model for human disc biology but care is needed in its use.

Mice are commonly used to study intervertebral disc (IVD) biology and related diseases such as IVD degeneration. Discs from both the lumbar and tail regions are used. However, little is known about compartmental characteristics in the different regions, nor their relevance to the human setting, where a functional IVD unit depends on a homeostatic proteome. Here, we address these major gaps through comprehensive proteomic profiling and in-depth analyses of 8-week-old healthy murine discs, followed by comparisons with human. Leveraging on a dataset of over 2,700 proteins from 31 proteomic profiles, we identified key molecular and cellular differences between disc compartments and spine levels, but not gender. The nucleus pulposus (NP) and annulus fibrosus (AF) compartments differ the most, both in matrisome and cellularity contents. Differences in the matrisome are consistent with the fibrous nature required for tensile strength in the AF and hydration property in the NP. Novel findings for the NP cells included an enrichment in cell junction proteins for cell–cell communication (Cdh2, Dsp and Gja1) and osmoregulation (Slc12a2 and Wnk1). In NP cells, we detected heterogeneity of vacuolar organelles; where about half have potential lysosomal function (Vamp3, Copb2, Lamp1/2, Lamtor1), some contain lipid droplets and others with undefined contents. The AF is enriched in proteins for the oxidative stress responses (Sod3 and Clu). Interestingly, mitochondrial proteins are elevated in the lumbar than tail IVDs that may reflect differences in metabolic requirement. Relative to the human, cellular and structural information are conserved for the AF. Even though the NP is more divergent between mouse and human, there are similarities at the level of cell biology. Further, common cross-species markers were identified for both NP (KRT8/19, CD109) and AF (COL12A1). Overall, mouse is a relevant model to study IVD biology, and an understanding of the limitation will facilitate research planning and data interpretation, maximizing the translation of research findings to human IVDs.

COL12A1 Single Nucleotide Polymorphisms rs240736 and rs970547 Are Not Associated with Temporomandibular Joint Disc Displacement without Reduction

Temporomandibular disorders (TMDs) may affect up to 25% of the population, with almost 70% of these TMD cases developing malpositioning of the disc over time in what is known as internal derangement (ID). Despite significant efforts, the molecular mechanism underlying disease progression is not yet very well known. In this study, the role of COL12A1 rs970547 and rs240736 polymorphisms as potential genetic factors regulating ID was investigated. The study included 124 Caucasian patients of both sexes after disc displacement without reduction (DDwoR) in either one or two temporomandibular joints (TMJs), either of which meet the criteria for this condition. All patients underwent clinical examination and 3D digital imaging. The COL12A1 rs970547 and rs240736 polymorphisms were evaluated. There were no statistically significant differences in the chi-square test between the study group and healthy controls. The examined COL12A1 rs240736 and rs970547 polymorphisms do not contribute to DDwoR in Polish Caucasians.

Collagen Binding Proteins of Gram-Positive Pathogens

Collagens are the primary structural components of mammalian extracellular matrices. In addition, collagens regulate tissue development, regeneration and host defense through interaction with specific cellular receptors. Their unique triple helix structure, which requires a glycine residue every third amino acid, is the defining structural feature of collagens. There are 28 genetically distinct collagens in humans. In addition, several other unrelated human proteins contain a collagen domain. Gram-positive bacteria of the genera Staphylococcus, Streptococcus, Enterococcus, and Bacillus express cell surface proteins that bind to collagen. These proteins of Gram-positive pathogens are modular proteins that can be classified into different structural families. This review will focus on the different structural families of collagen binding proteins of Gram-positive pathogen. We will describe how these proteins interact with the triple helix in collagens and other host proteins containing a collagenous domain and discuss how these interactions can contribute to the pathogenic processes.

Tendon Extracellular Matrix Assembly, Maintenance and Dysregulation Throughout Life

In his Lissner Award medal lecture in 2000, Stephen Cowin asked the question: "How is a tissue built?" It is not a new question, but it remains as relevant today as it did when it was asked 20 years ago. In fact, research on the organization and development of tissue structure has been a primary focus of tendon and ligament research for over two centuries. The tendon extracellular matrix (ECM) is critical to overall tissue function; it gives the tissue its unique mechanical properties, exhibiting complex non-linear responses, viscoelasticity and flow mechanisms, excellent energy storage and fatigue resistance. This matrix also creates a unique microenvironment for resident cells, allowing cells to maintain their phenotype and translate mechanical and chemical signals into biological responses. Importantly, this architecture is constantly remodeled by local cell populations in response to changing biochemical (systemic and local disease or injury) and mechanical (exercise, disuse, and overuse) stimuli. Here, we review the current understanding of matrix remodeling throughout life, focusing on formation and assembly during the postnatal period, maintenance and homeostasis during adulthood, and changes to homeostasis in natural aging. We also discuss advances in model systems and novel tools for studying collagen and non-collagenous matrix remodeling throughout life, and finally conclude by identifying key questions that have yet to be answered.