Tissue-specific expression of the fibril-associated collagens XII and XIV

Macromolecular crowding in human tenocyte and skin fibroblast cultures: A comparative analysis

Although human tenocytes and dermal fibroblasts have shown promise in tendon engineering, no tissue engineered medicine has been developed due to the prolonged ex vivo time required to develop an implantable device. Considering that macromolecular crowding has the potential to substantially accelerate the development of functional tissue facsimiles, herein we compared human tenocyte and dermal fibroblast behaviour under standard and macromolecular crowding conditions to inform future studies in tendon engineering. Basic cell function analysis made apparent the innocuousness of macromolecular crowding for both cell types. Gene expression analysis of the without macromolecular crowding groups revealed expression of tendon related molecules in human dermal fibroblasts and tenocytes. Protein electrophoresis and immunocytochemistry analyses showed significantly increased and similar deposition of collagen fibres by macromolecular crowding in the two cell types. Proteomics analysis demonstrated great similarities between human tenocyte and dermal fibroblast cultures, as well as the induction of haemostatic, anti-microbial and tissue-protective proteins by macromolecular crowding in both cell populations. Collectively, these data rationalise the use of either human dermal fibroblasts or tenocytes in combination with macromolecular crowding in tendon engineering.

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.

Comprehensive Matrisome Profiling of Human Adipose Tissue for Soft Tissue Reconstruction

For effective translation of research from tissue engineering and regenerative medicine domains, the cell-instructive extracellular matrix (ECM) of specific tissues must be accurately realized. As adipose tissue is gaining traction as a biomaterial for soft tissue reconstruction, with highly variable clinical outcomes obtained, a quantitative investigation of the adipose tissue matrisome is overdue. In this study, the human adipose tissue matrisome is profiled using quantitative sequential windowed acquisition of all theoretical fragment ion spectra - mass spectrometry (SWATH-MS) proteomics across a cohort of 13 fat-grafting patients, to provide characterization of ECM proteins within the tissue, and to understand human population variation. There are considerable differences in the expression of matrisome proteins across the patient cohort, with age and lipoaspirate collection technique contributing to the greatest variation across the core matrisome. A high abundance of basement membrane proteins (collagen IV and heparan sulfate proteoglycan) is detected, as well as fibrillar collagens I and II, reflecting the hierarchical structure of the tissue. This study provides a comprehensive proteomic evaluation of the adipose tissue matrisome and contributes to an enhanced understanding of the influence of the matrisome in adipose-related pathologies by providing a healthy reference cohort and details an experimental pipeline that can be further exploited for future biomaterial development.

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.

Context Matters: Response Heterogeneity to Collagen-Targeting Approaches in Desmoplastic Cancers

Simple Summary

A common feature of tumor types such as breast cancer, prostate cancer, pancreatic cancer, and soft-tissue sarcoma is the deposition of collagen-rich tissue called desmoplasia. However, efforts to control tumor growth by disrupting desmoplasia, collectively known as “collagen-targeting approaches”, have had mixed and contradictory results, sometimes even within the same cancer type. We believe that this phenomenon may be due—at least partially—to the fact that “collagen” is not a single molecule, but rather a diverse molecular family composed of 28 unique collagen types. Therefore, in this review, we discuss the diversity of collagen molecules in normal and cancer tissue, and explore how collagen heterogeneity relates to the mixed efficacy of collagen-targeting approaches for cancer therapy.

Abstract

The deposition of collagen-rich desmoplastic tissue is a well-documented feature of the solid tumor microenvironment (TME). However, efforts to target the desmoplastic extracellular matrix (ECM) en masse, or collagen molecules more specifically, have been met with mixed and sometimes paradoxical results. In this review, we posit that these discrepancies are due—at least in part—to the incredible diversity of the collagen superfamily. Specifically, whereas studies of “collagen-targeting” approaches frequently refer to “collagen” as a single molecule or relatively homogeneous molecular family, 28 individual collagens have been identified in mammalian tissues, each with a unique structure, supramolecular assembly pattern, tissue distribution, and/or function. Moreover, some collagen species have been shown to exert both pro- and anti-neoplastic effects in the desmoplastic TME, even within the same cancer type. Therefore, herein, we describe the diversity of the collagen family in normal tissues and highlight the context-specific roles of individual collagen molecules in desmoplastic tumors. We further discuss how this heterogeneity relates to the variable efficacy of “collagen-targeting” strategies in this setting and provide guidance for future directions in the field.

The effect of neonatal, juvenile, and adult donors on rejuvenated neocartilage functional properties

Cartilage does not naturally heal, and cartilage lesions from trauma and wear-and-tear can lead to eventual osteoarthritis. To address long-term repair, tissue engineering of functional biologic implants to treat cartilage lesions is desirable, but the development of such implants is hindered by several limitations including 1) donor tissue scarcity due to the presence of diseased tissues in joints, 2) dedifferentiation of chondrocytes during expansion, and 3) differences in functional output of cells dependent on donor age. Toward overcoming these challenges, 1) costal cartilage has been explored as a donor tissue, and 2) methods have been developed to rejuvenate the chondrogenic phenotype of passaged chondrocytes for generating self-assembled neocartilage. However, it remains unclear how the rejuvenation processes are influenced by donor age, and, thus, how to develop strategies that specifically target age-related differences. Using histological, biochemical, proteomic, and mechanical assays, this study sought to determine the differences among neocartilage generated from neonatal, juvenile, and adult donors using the Yucatan minipig, a clinically relevant large animal model. Based on the literature, a relatively young adult population of animals was chosen due to a reduction in functional output of human articular chondrocytes after 40 years of age. After isolation, costal chondrocytes were expanded, rejuvenated, and self-assembled, and the neocartilages were assessed. The aggregate modulus values of neonatal constructs were at least 1.65-fold of those from the juvenile or adult constructs. Poisson's ratio also significantly differed among all groups, with neonatal constructs exhibiting values 49% higher than adult constructs. Surprisingly, other functional properties such as tensile modulus and GAG content did not significantly differ among groups. Total collagen content was slightly elevated in the adult constructs when compared to neonatal and juvenile constructs. A more nuanced view via bottom-up mass spectrometry showed that Col2a1 protein was not significantly different among groups, but content of several other collagen subtypes (i.e., Col1a1, Col9a1, Col11a2, and Col12a1) was modulated by donor age. For example, Col12a1 in adult constructs was found to be 102.9% higher than neonatal-derived constructs. Despite these differences, this study shows that different aged donors can be used to generate neocartilages of similar functional properties.

Collagen- and hyaluronic acid-based hydrogels and their biomedical applications

Hydrogels have been widely investigated in biomedical fields due to their similar physical and biochemical properties to the extracellular matrix (ECM). Collagen and hyaluronic acid (HA) are the main components of the ECM in many tissues. As a result, hydrogels prepared from collagen and HA hold inherent advantages in mimicking the structure and function of the native ECM. Numerous studies have focused on the development of collagen and HA hydrogels and their biomedical applications. In this extensive review, we provide a summary and analysis of the sources, features, and modifications of collagen and HA. Specifically, we highlight the fabrication, properties, and potential biomedical applications as well as promising commercialization of hydrogels based on these two natural polymers.

Identification of Underlying Hub Genes Associated with Hypertrophic Cardiomyopathy by Integrated Bioinformatics Analysis

Background

Considered as one of the major reasons of sudden cardiac death, hypertrophic cardiomyopathy (HCM) is a common inherited cardiovascular disease. However, effective treatment for HCM is still lacking. Identification of hub gene may be a powerful tool for discovering potential therapeutic targets and candidate biomarkers.

Methods

We analysed three gene expression datasets for HCM from the Gene Expression Omnibus. Two of them were merged by “sva” package. The merged dataset was used for analysis while the other dataset was used for validation. Following this, a weighted gene coexpression network analysis (WGCNA) was performed, and the key module most related to HCM was identified. Based on the intramodular connectivity, we identified the potential hub genes. Then, a receiver operating characteristic curve analysis was performed to verify the diagnostic values of hub genes. Finally, we validated changes of hub genes, for genetic transcription and protein expression levels, in datasets of HCM patients and myocardium of transverse aortic constriction (TAC) mice.

Results

In the merged dataset, a total of 455 differentially expressed genes (DEGs) were identified from normal and hypertrophic myocardium. In WGCNA, the blue module was identified as the key module and the genes in this module showed a high positive correlation with HCM. Functional enrichment analysis of DEGs and key module revealed that the extracellular matrix, fibrosis, and neurohormone pathways played important roles in HCM. FRZB, COL14A1, CRISPLD1, LUM, and sFRP4 were identified as hub genes in the key module. These genes showed a good predictive value for HCM and were significantly up-regulated in HCM patients and TAC mice. We also found protein expression of LUM and sFRP4 increased in myocardium of TAC mice.

Conclusion

This study revealed that five hub genes are involved in the occurrence and development of HCM, and they are potentially to be used as therapeutic targets and biomarkers for HCM.

Collagen Biosynthesis, Processing, and Maturation in Lung Ageing

In addition to providing a macromolecular scaffold, the extracellular matrix (ECM) is a critical regulator of cell function by virtue of specific physical, biochemical, and mechanical properties. Collagen is the main ECM component and hence plays an essential role in the pathogenesis and progression of chronic lung disease. It is well-established that many chronic lung diseases, e.g., chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF) primarily manifest in the elderly, suggesting increased susceptibility of the aged lung or accumulated alterations in lung structure over time that favour disease. Here, we review the main steps of collagen biosynthesis, processing, and turnover and summarise what is currently known about alterations upon lung ageing, including changes in collagen composition, modification, and crosslinking. Recent proteomic data on mouse lung ageing indicates that, while the ER-resident machinery of collagen biosynthesis, modification and triple helix formation appears largely unchanged, there are specific changes in levels of type IV and type VI as well as the two fibril-associated collagens with interrupted triple helices (FACIT), namely type XIV and type XVI collagens. In addition, levels of the extracellular collagen crosslinking enzyme lysyl oxidase are decreased, indicating less enzymatically mediated collagen crosslinking upon ageing. The latter contrasts with the ageing-associated increase in collagen crosslinking by advanced glycation endproducts (AGEs), a result of spontaneous reactions of protein amino groups with reactive carbonyls, e.g., from monosaccharides or reactive dicarbonyls like methylglyoxal. Given the slow turnover of extracellular collagen such modifications accumulate even more in ageing tissues. In summary, the collective evidence points mainly toward age-induced alterations in collagen composition and drastic changes in the molecular nature of collagen crosslinks. Future work addressing the consequences of these changes may provide important clues for prevention of lung disease and for lung bioengineering and ultimately pave the way to novel targeted approaches in lung regenerative medicine.

The "other" 15-40%: The Role of Non-Collagenous Extracellular Matrix Proteins and Minor Collagens in Tendon

Extracellular matrix (ECM) determines the physiological function of all tissues, including musculoskeletal tissues. In tendon, ECM provides overall tissue architecture, which is tailored to match the biomechanical requirements of their physiological function, that is, force transmission from muscle to bone. Tendon ECM also constitutes the microenvironment that allows tendon-resident cells to maintain their phenotype and that transmits biomechanical forces from the macro-level to the micro-level. The structure and function of adult tendons is largely determined by the hierarchical organization of collagen type I fibrils. However, non-collagenous ECM proteins such as small leucine-rich proteoglycans (SLRPs), ADAMTS proteases, and cross-linking enzymes play critical roles in collagen fibrillogenesis and guide the hierarchical bundling of collagen fibrils into tendon fascicles. Other non-collagenous ECM proteins such as the less abundant collagens, fibrillins, or elastin, contribute to tendon formation or determine some of their biomechanical properties. The interfascicular matrix or endotenon and the outer layer of tendons, the epi- and paratenon, includes collagens and non-collagenous ECM proteins, but their function is less well understood. The ECM proteins in the epi- and paratenon may provide the appropriate microenvironment to maintain the identity of distinct tendon cell populations that are thought to play a role during repair processes after injury. The aim of this review is to provide an overview of the role of non-collagenous ECM proteins and less abundant collagens in tendon development and homeostasis. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:23-35, 2020.

Skin Protein Profile after Major Weight Loss and Its Role in Body Contouring Surgery

Chronic inflammation during morbid obesity significantly alters cutaneous tissue. Large weight loss achieved after bariatric surgery minimizes or halts damage caused by metabolic syndrome, but further deteriorates the clinical condition of skin. Postbariatric skin flaccidity produces major difficulties to plastic surgery. In this study, we analyzed differences in protein composition of the skin between patients with morbid obesity and those after large weight loss and established correlations between differentially expressed proteins and clinical characteristics of postbariatric skin tissue, to improve body contouring surgery techniques.

Label-Free Quantification Proteomics for the Identification of Mesenchymal Stromal Cell Matrisome Inside 3D Poly(Ethylene Glycol) Hydrogels

Cells modulate the functional properties of their environment by depositing extracellular matrix (ECM) proteins during biological processes in vivo and in vitro. Despite the ECMs central role in tissue formation, its quantification in hydrogels like Matrigel, which have a complex materials-inherent biopolymer composition is exceptionally challenging. Here, the use of protein-free, synthetic poly(ethylene glycol) hydrogels enables the analysis of deposited human bone marrow mesenchymal stromal cells ECM directly harvested from fresh 3D cell cultures by a tandem mass spectrometry (LC-MS/MS) method. In this study, it is proved that a label-free LC-MS/MS quantification method can selectively identify proteins deposited in 3D synthetic hydrogels following different growth factor (GF) treatments. Furthermore, it is shown that the sequence in which GFs are administered and the choice of stimuli significantly influences the number and abundance of ECM proteins. Therefore, this provides a versatile method to optimize GF treatments in synthetic hydrogel-based regenerative medicine and tissue engineering approaches.

Differential Effect of Cobalt and Chromium Ions as Well as CoCr Particles on the Expression of Osteogenic Markers and Osteoblast Function

The balance of bone formation and resorption is the result of a regulated crosstalk between osteoblasts, osteoclasts, and osteocytes. Inflammation, mechanical load, and external stimuli modulate this system. Exposure of bone cells to metal ions or wear particles are thought to cause osteolysis via activation of osteoclasts and inhibition of osteoblast activity. Co²⁺ ions have been shown to impair osteoblast function and the expression of the three transforming growth factor (TGF)-β isoforms. The current study was performed to analyze how Co²⁺ and Cr³⁺ influence the expression, proliferation, and migration profile of osteoblast-like cells. The influence of Co²⁺, Cr³⁺, and CoCr particles on gene expression was analyzed using an osteogenesis PCR Array. The expression of different members of the TGF-β signaling cascade were down-regulated by Co²⁺, as well as several TGF-β regulated collagens, however, Cr³⁺ had no effect. CoCr particles partially affected similar genes as the Co²⁺treatment. Total collagen production of Co²⁺ treated osteoblasts was reduced, which can be explained by the reduced expression levels of various collagens. While proliferation of MG63 cells appears unaffected by Co²⁺, the migration capacity was impaired. Our data may improve the knowledge of changes in gene expression patterns, and the proliferation and migration effects caused by artificial materials.

Roadmap of molecular, compositional, and functional markers during embryonic tendon development

Tendon is a specialized connective tissue that connects muscle to bone, thereby enabling musculoskeletal movement. Tendon injury leads to formation of tissue with aberrant functional properties. Current approaches to treat tendon injuries, including surgical repair and tissue engineering, have not achieved normal tendon. A roadmap of markers could help with identifying when mis-steps occur during aberrant tendon formation and providing instructions for normal tendon formation. We propose this roadmap should be based on the embryo-the perfect model of tissue formation. Our prior studies have shown that adult mesenchymal stem cells mimic tendon progenitor cell behavior when treated with tendon developmental cues. Although transcription factors and extracellular matrix molecules are commonly used to assess tendon development, we have shown that these markers do not reliably reflect functional property elaboration. Thus, evaluating tendon formation on the basis of a combination of these molecular, compositional, and functional markers is important. In this review, we highlight various tendon markers with focus on their temporal profiles and roles in tendon development to outline a roadmap that may be useful for informing tendon healing and tissue engineering strategies.

Hemodynamics Modify Collagen Deposition in the Early Embryonic Chicken Heart Outflow Tract

Blood flow is critical for normal cardiac development. Hemodynamic stimuli outside of normal ranges can lead to overt cardiac defects, but how early heart tissue remodels in response to altered hemodynamics is poorly understood. This study investigated changes in tissue collagen in response to hemodynamic overload in the chicken embryonic heart outflow tract (OFT) during tubular heart stages (HH18 to HH24, ~24 h). A suture tied around the OFT at HH18 was tightened to constrict the lumen for ~24 h (constriction range at HH24: 15–60%). Expression of fibril collagens I and III and fibril organizing collagens VI and XIV were quantified at the gene and protein levels via qPCR and quantitative immunofluorescence. Collagen I was slightly elevated upstream of the band and in the cushions in banded versus control OFTs. Changes in collagen III were not observed. Collagen VI deposition was elevated downstream of the band, but not overall. Collagen XIV deposition increased throughout the OFT, and strongly correlated to lumen constriction. Interestingly, organization of collagen I fibrils was observed for the tighter banded embryos in regions that also showed increase in collagen XIV deposition, suggesting a potentially key role for collagens I and XIV in the structural adaptation of embryonic heart tissue to hemodynamic overload.

Connective tissue regeneration in skeletal muscle after eccentric contraction-induced injury

Human skeletal muscle has the potential to regenerate completely after injury induced under controlled experimental conditions. The events inside the myofibers as they undergo necrosis, followed closely by satellite cell-mediated myogenesis, have been mapped in detail. Much less is known about the adaptation throughout this process of both the connective tissue structures surrounding the myofibers and the fibroblasts, the cells responsible for synthesizing this connective tissue. However, the few studies investigating muscle connective tissue remodeling demonstrate a strong response that appears to be sustained for a long time after the major myofiber responses have subsided. While the use of electrical stimulation to induce eccentric contractions vs. voluntary eccentric contractions appears to lead to a greater extent of myofiber necrosis and regenerative response, this difference is not apparent when the muscle connective tissue responses are compared, although further work is required to confirm this. Pharmacological agents (growth hormone and angiotensin II type I receptor blockers) are considered in the context of accelerating the muscle connective tissue adaptation to loading. Cautioning against this, however, is the association between muscle matrix protein remodeling and protection against reinjury, which suggests that a (so far undefined) period of vulnerability to reinjury may exist during the remodeling phases. The role of individual muscle matrix components and their spatial interaction during adaptation to eccentric contractions is an unexplored field in human skeletal muscle and may provide insight into the optimal timing of rest vs. return to activity after muscle injury.

Composition and adaptation of human myotendinous junction and neighboring muscle fibers to heavy resistance training

The myotendinous junction (MTJ) is a common site of strain injury and yet understanding of its composition and ability to adapt to loading is poor. The main aims of this study were to determine the profile of selected collagens and macrophage density in human MTJ and adjoining muscle fibers, and to investigate whether heavy exercise loading would alter this profile. Fifteen individuals scheduled for anterior cruciate ligament repair surgery were randomized into three groups: control, acute or 4 weeks heavy resistance training. MTJ samples were collected from the semitendinosus and gracilis muscles and were sectioned and stained immunohistochemically for collagen types I, III, VI, XII, XIV, XXII, Tenascin-C and CD68. Macrophage density and distribution was evaluated and the amount of each collagen type in muscle and MTJ was graded. Collagen XXII was observed solely at the MTJ, while all other collagens were abundant at the MTJ and in muscle perimysium or endomysium. The endomysial content of collagen XIV, macrophages and Tenascin-C increased following 4 weeks of training. These findings illustrate the heterogeneity of collagen type composition of human MTJ. The increase in collagen XIV following 4 weeks of training may reflect a training-induced protection against strain injuries in this region.

Collagen XII Contributes to Epicardial and Connective Tissues in the Zebrafish Heart during Ontogenesis and Regeneration

Zebrafish heart regeneration depends on cardiac cell proliferation, epicardium activation and transient reparative tissue deposition. The contribution and the regulation of specific collagen types during the regenerative process, however, remain poorly characterized. Here, we identified that the non-fibrillar type XII collagen, which serves as a matrix-bridging component, is expressed in the epicardium of the zebrafish heart, and is boosted after cryoinjury-induced ventricular damage. During heart regeneration, an intense deposition of Collagen XII covers the outer epicardial cap and the interstitial reparative tissue. Analysis of the activated epicardium and fibroblast markers revealed a heterogeneous cellular origin of Collagen XII. Interestingly, this matrix-bridging collagen co-localized with fibrillar type I collagen and several glycoproteins in the post-injury zone, suggesting its role in tissue cohesion. Using SB431542, a selective inhibitor of the TGF-β receptor, we showed that while the inhibitor treatment did not affect the expression of collagen 12 and collagen 1a2 in the epicardium, it completely suppressed the induction of both genes in the fibrotic tissue. This suggests that distinct mechanisms might regulate collagen expression in the outer heart layer and the inner injury zone. On the basis of this study, we postulate that the TGF-β signaling pathway induces and coordinates formation of a transient collagenous network that comprises fibril-forming Collagen I and fiber-associated Collagen XII, both of which contribute to the reparative matrix of the regenerating zebrafish heart.

Characteristic adaptations of the extracellular matrix in dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is a relatively common heart muscle disease characterized by the dilation and thinning of the left ventricle accompanied with left ventricular systolic dysfunction. Myocardial fibrosis is a major feature in DCM and therefore it is inevitable that corresponding extracellular matrix (ECM) changes are involved in DCM onset and progression. Increasing our understanding of how ECM adaptations are involved in DCM could be important for the development of future interventions. This review article discusses the molecular adaptations in ECM composition and structure that have been reported in both animal and human studies of DCM. Furthermore, we provide a transcriptome-based catalogue of ECM genes that are associated with DCM, generated by using NCBI Gene Expression Omnibus database sets for DCM. Based on this in silico analysis, many novel ECM components involved in DCM are identified and discussed in this review. With the information gathered, we propose putative pathways of ECM adaptations in onset and progression of DCM.

Tendon development and musculoskeletal assembly: emerging roles for the extracellular matrix

Tendons and ligaments are extracellular matrix (ECM)-rich structures that interconnect muscles and bones. Recent work has shown how tendon fibroblasts (tenocytes) interact with muscles via the ECM to establish connectivity and strengthen attachments under tension. Similarly, ECM-dependent interactions between tenocytes and cartilage/bone ensure that tendon-bone attachments form with the appropriate strength for the force required. Recent studies have also established a close lineal relationship between tenocytes and skeletal progenitors, highlighting the fact that defects in signals modulated by the ECM can alter the balance between these fates, as occurs in calcifying tendinopathies associated with aging. The dynamic fine-tuning of tendon ECM composition and assembly thus gives rise to the remarkable characteristics of this unique tissue type. Here, we provide an overview of the functions of the ECM in tendon formation and maturation that attempts to integrate findings from developmental genetics with those of matrix biology.

Collagens VI and XII form complexes mediating osteoblast interactions during osteogenesis

Bone formation is precisely regulated by cell-cell communication in osteoblasts. We have previously demonstrated that genetic deletion of Col6a1 or Col12a1 impairs osteoblast connections and/or communication in mice, resulting in bone mass reduction and bone fragility. Mutations of the genes encoding collagen VI cause Ullrich congenital muscular dystrophy (UCMD) and Bethlem myopathy (BM), which have overlapping phenotypes involving connective tissue and muscle. Recent studies have identified COL12A1 gene mutations in patients with UCMD- and BM-like disorders harboring no COL6 mutations, indicating the shared functions of these collagens in connective tissue homeostasis. The purpose of this investigation has been to test the hypothesis that collagens VI and XII have coordinate regulatory role(s) during bone formation. We analyzed the localization of collagens VI and XII relative to primary osteoblasts during osteogenesis. Immunofluorescence analysis demonstrated that collagens VI and XII colocalized in matrix bridges between adjacent cells during periods when osteoblasts were establishing cell-cell connections. Quantification of cells harboring collagen bridges demonstrated that matrix bridges were composed of collagens VI and XII but not collagen I. Interestingly, matrix bridge formation was impaired in osteoblasts deficient in either Col6a1 or Col12a1, suggesting that both collagens were indispensable for matrix bridge formation. These data demonstrate, for the first time, a functional relationship between collagens VI and XII during osteogenesis and indicate that a complex containing collagens VI and XII is essential for the formation of a communicating cellular network during bone formation.

Mohawk promotes the tenogenesis of mesenchymal stem cells through activation of the TGFβ signaling pathway

The transcription factor Mohawk (Mkx) is expressed in developing tendons and is an important regulator of tenogenic differentiation. However, the exact roles of Mkx in tendinopathy and tendon repair remain unclear. Using gene expression Omnibus datasets and immunofluorescence assays, we found that Mkx expression level was dramatically lower in human tendinopathy tissue and it is activated at specific stages of tendon development. In mesenchymal stem cells (MSCs), ectopic Mkx expression strikingly promoted tenogenesis more efficiently than Scleraxis (Scx), a well-known master transcription factor of tendon. Significantly higher levels of tenogenic gene expression and collagen fibril growth were observed with Mkx versus Scx. Interestingly, it was observed that Mkx dramatically upregulated Scx through binding to the Tgfb2 promoter. Additionally, the transplantation of Mkx-expressing-MSC sheets promoted tendon repair in a mouse model of Achilles-tendon defect. Taken together, these data shed light on previously unrecognized roles of Mkx in tendinopathy, tenogenesis, and tendon repair as well as in regulating the TGFβ pathway.

Collagens and proteoglycans of the cornea: importance in transparency and visual disorders

The cornea represents the external part of the eye and consists of an epithelium, a stroma and an endothelium. Due to its curvature and transparency this structure makes up approximately 70% of the total refractive power of the eye. This function is partly made possible by the particular organization of the collagen extracellular matrix contained in the corneal stroma that allows a constant refractive power. The maintenance of such an organization involves other molecules such as type V collagen, FACITs (fibril-associated collagens with interrupted triple helices) and SLRPs (small leucine-rich proteoglycans). These components play crucial roles in the preservation of the correct organization and function of the cornea since their absence or modification leads to abnormalities such as corneal opacities. Thus, the aim of this review is to describe the different corneal collagens and proteoglycans by highlighting their importance in corneal transparency as well as their implication in corneal visual disorders.

Progenitor Cells of the Mandibular Condylar Cartilage

The secondary cartilage of the mandibular condyle is unique as it undergoes endochondral ossification during growth and robustly remodels in response to changes in its mechanical loading environment. This cartilage is derived from mesenchymal progenitor cells that express markers of early osteoblast differentiation, namely alkaline phosphatase (ALP) and runt-related transcription factor 2 (Runx2). Interestingly, these progenitor cells then differentiate into cartilage with appropriate mechanical loading. Our laboratory has determined that these cells can be labeled by osteoblast progenitor cell markers, including the 3.6 fragment of the rat collagen type 1. However, the role these mesenchymal progenitor cells play in adult mandibular condylar cartilage maintenance and adaptation, as well as the existence of a more potent progenitor cell population within the mandibular condylar cartilage, remain in question. Further characterization of these cells is necessary to determine their potency and regenerative capacity to elucidate their potential for regenerative therapy.

A study in familial hypercholesterolemia suggests reduced methylomic plasticity in men with coronary artery disease

Aim: To assess whether DNA methylation is associated with coronary artery disease (CAD). Materials & methods: An epigenome-wide analysis has been performed on leucocytes from familial hypercholesterolemic (FH) men with (n = 6) or without CAD (n = 6). The results were replicated in an extended sample of FH men (n = 61) and in non-FH men (n = 100) for two of the top differentially methylated loci. Results: FH men with CAD had significantly more hypomethylated and hypermethylated loci and showed less DNA methylation level variability compared with men without CAD (p < 0.001). Moreover, COL14A1 and MMP9 DNA methylation levels were associated with CAD, age of onset of CAD or CAD risk factors. Conclusion: These results suggest that epigenome-wide changes are associated with CAD occurrence in men.

Bone regeneration with osteogenically enhanced mesenchymal stem cells and their extracellular matrix proteins

Although bone has remarkable regenerative capacity, about 10% of long bone fractures and 25% to 40% of vertebral fusion procedures fail to heal. In such instances, a scaffold is employed to bridge the lesion and accommodate osteoprogenitors. Although synthetic bone scaffolds mimic some of the characteristics of bone matrix, their effectiveness can vary because of biological incompatibility. Herein, we demonstrate that a composite prepared with osteogenically enhanced mesenchymal stem cells (OEhMSCs) and their extracellular matrix (ECM) has an unprecedented capacity for the repair of critical-sized defects of murine femora. Furthermore, OEhMSCs do not cause lymphocyte activation, and ECM/OEhMSC composites retain their in vivo efficacy after cryopreservation. Finally, we show that attachment to the ECM by OEhMSCs stimulates the production of osteogenic and angiogenic factors. These data demonstrate that composites of OEhMSCs and their ECM could be utilized in the place of autologous bone graft for complex orthopedic reconstructions.

Microscopic analysis of molar--incisor malformation

OBJECTIVE

Molar-incisor malformation (MIM) is a newly discovered type of dental anomaly that involves a characteristic root malformation of the permanent first molars. The aim of this study was to reveal the microstructure of MIM teeth in order to determine their origin.

STUDY DESIGN

Four MIM teeth were extracted from a 9-year-old girl due to severe mobility. The detailed microstructure of the teeth was determined by examinations with micro-computed tomography (micro-CT), hematoxylin and eosin (H&E) staining, immunohistochemical staining, and scanning electron microscopy to reveal the detailed microstructure.

RESULTS

Micro-CT and H&E staining revealed the pulpal floor comprising three layers: upper, middle, and lower. Amorphous hard tissues and hyperactive cells were observed in the middle layer of the pulpal floor, and the cells stained positively for dentin sialoprotein and osteocalcin, but not for collagen XII.

CONCLUSION

The results of the present study imply that MIM-affected molars probably result from inappropriate differentiation of the apical pulp and dental follicle.

Collagen XII: Protecting bone and muscle integrity by organizing collagen fibrils

Collagen XII, largest member of the fibril-associated collagens with interrupted triple helix (FACIT) family, assembles from three identical α-chains encoded by the COL12A1 gene. The molecule consists of three threadlike N-terminal noncollagenous NC3 domains, joined by disulfide bonds and a short interrupted collagen triple helix toward the C-terminus. Splice variants differ considerably in size and properties: "small" collagen XIIB (220 kDa subunit) is similar to collagen XIV, whereas collagen XIIA (350 kDa) has a much larger NC3 domain carrying glycosaminoglycan chains. Collagen XII binds to collagen I-containing fibrils via its collagenous domain, whereas its large noncollagenous arms interact with other matrix proteins such as tenascin-X. In dense connective tissues and bone, collagen XII is thought to regulate organization and mechanical properties of collagen fibril bundles. Accordingly, recent findings show that collagen XII mutations cause Ehlers-Danlos/myopathy overlap syndrome associated with skeletal abnormalities and muscle weakness in mice and humans.

Recessive and dominant mutations in COL12A1 cause a novel EDS/myopathy overlap syndrome in humans and mice

Collagen VI-related myopathies are disorders of connective tissue presenting with an overlap phenotype combining clinical involvement from the muscle and from the connective tissue. Not all patients displaying related overlap phenotypes between muscle and connective tissue have mutations in collagen VI. Here, we report a homozygous recessive loss of function mutation and a de novo dominant mutation in collagen XII (COL12A1) as underlying a novel overlap syndrome involving muscle and connective tissue. Two siblings homozygous for a loss of function mutation showed widespread joint hyperlaxity combined with weakness precluding independent ambulation, while the patient with the de novo missense mutation was more mildly affected, showing improvement including the acquisition of walking. A mouse model with inactivation of the Col12a1 gene showed decreased grip strength, a delay in fiber-type transition and a deficiency in passive force generation while the muscle seems more resistant to eccentric contraction induced force drop, indicating a role for a matrix-based passive force-transducing elastic element in the generation of the weakness. This new muscle connective tissue overlap syndrome expands on the emerging importance of the muscle extracellular matrix in the pathogenesis of muscle disease.

Zebrafish collagen XIV is transiently expressed in epithelia and is required for proper function of certain basement membranes

We found that zebrafish has two differentially expressed col14a1 paralogs. col14a1a expression peaked between 18-somite stage and 24 hours postfertilization (hpf), whereas col14a1b was first expressed at 32 hpf. To uncover functions of collagen XIV (COLXIV) during early embryogenesis, we focused our study on col14a1a. We characterized the α1 (XIV-A) chain as a collagenase-sensitive 200-kDa protein that formed dimer that could be reduced at high pH. As observed for the transcript, COLXIV-A protein expression peaked between 24 and 48 hpf. Using antisense probes and polyclonal antibodies, we show that col14a1a and its protein product COLXIV-A are transiently expressed in several epithelia, including epithelia undergoing shape changes, such as the fin folds. In contrast, anti-COLXII antibodies stained only connective tissues. COLXIV-A was also detected in the basement membrane (BM), where it co-localized with COLXII. At later developmental stages, COLXIV-A was not expressed in epithelia anymore but persisted in the BM. Morpholino knockdown of COLXIV-A provoked a skin detachment phenotype. Electron microscopy analysis revealed that morpholino-injected embryos lacked a lamina densa and lamina lucida at 24 hpf, and BM defects, such as gaps in the adepidermal granules, were still detected at 48 hpf. These BM defects were accompanied by a rupture of the dermis and detachment of the epidermis. Taken together, these data suggest an unexpected role of COLXIV-A in undifferentiated epithelia and in the formation of embryonic basement membranes.

Collagen XIV is important for growth and structural integrity of the myocardium

Collagen XIV is a fibril-associated collagen with an interrupted triple helix (FACIT). Previous studies have shown that this collagen type regulates early stages of fibrillogenesis in connective tissues of high mechanical demand. Mice null for Collagen XIV are viable, however formation of the interstitial collagen network is defective in tendons and skin leading to reduced biomechanical function. The assembly of a tightly regulated collagen network is also required in the heart, not only for structural support but also for controlling cellular processes. Collagen XIV is highly expressed in the embryonic heart, notably within the cardiac interstitium of the developing myocardium, however its role has not been elucidated. To test this, we examined cardiac phenotypes in embryonic and adult mice devoid of Collagen XIV. From as early as E11.5, Col14a1(-/-) mice exhibit significant perturbations in mRNA levels of many other collagen types and remodeling enzymes (MMPs, TIMPs) within the ventricular myocardium. By post natal stages, collagen fibril organization is in disarray and the adult heart displays defects in ventricular morphogenesis. In addition to the extracellular matrix, Col14a1(-/-) mice exhibit increased cardiomyocyte proliferation at post natal, but not E11.5 stages, leading to increased cell number, yet cell size is decreased by 3 months of age. In contrast to myocytes, the number of cardiac fibroblasts is reduced after birth associated with increased apoptosis. As a result of these molecular and cellular changes during embryonic development and post natal maturation, cardiac function is diminished in Col14a1(-/-) mice from 3 months of age; associated with dilation in the absence of hypertrophy, and reduced ejection fraction. Further, Col14a1 deficiency leads to a greater increase in left ventricular wall thickening in response to pathological pressure overload compared to wild type animals. Collectively, these studies identify a new role for type XIV collagen in the formation of the cardiac interstitium during embryonic development, and highlight the importance of the collagen network for myocardial cell survival, and function of the working myocardium after birth.

Collagen XII and XIV, new partners of cartilage oligomeric matrix protein in the skin extracellular matrix suprastructure

The tensile and scaffolding properties of skin rely on the complex extracellular matrix (ECM) that surrounds cells, vasculature, nerves, and adnexus structures and supports the epidermis. In the skin, collagen I fibrils are the major structural component of the dermal ECM, decorated by proteoglycans and by fibril-associated collagens with interrupted triple helices such as collagens XII and XIV. Here we show that the cartilage oligomeric matrix protein (COMP), an abundant component of cartilage ECM, is expressed in healthy human skin. COMP expression is detected in the dermal compartment of skin and in cultured fibroblasts, whereas epidermis and HaCaT cells are negative. In addition to binding collagen I, COMP binds to collagens XII and XIV via their C-terminal collagenous domains. All three proteins codistribute in a characteristic narrow zone in the superficial papillary dermis of healthy human skin. Ultrastructural analysis by immunogold labeling confirmed colocalization and further revealed the presence of COMP along with collagens XII and XIV in anchoring plaques. On the basis of these observations, we postulate that COMP functions as an adapter protein in human skin, similar to its function in cartilage ECM, by organizing collagen I fibrils into a suprastructure, mainly in the vicinity of anchoring plaques that stabilize the cohesion between the upper dermis and the basement membrane zone.

Human mesenchymal stem cell-derived matrices for enhanced osteoregeneration

The methodology for the repair of critical-sized or non-union bone lesions has unpredictable efficacy due in part to our incomplete knowledge of bone repair and the biocompatibility of bone substitutes. Although human mesenchymal stem cells (hMSCs) differentiate into osteoblasts, which promote bone growth, their ability to repair bone in vivo has been variable. We hypothesized that given the multistage process of osteogenesis, hMSC-mediated repair might be maximal at a specific time point of healing. Using a mouse model of calvarial healing, we demonstrate that the osteo-repair capacity of hMSCs can be substantially augmented by treatment with an inhibitor of peroxisome proliferator-activated receptor γ, but efficacy is confined to the rapid osteogenic phase. Upon entry into the bone-remodeling phase, hMSC retention signals are lost, resulting in truncation of healing. To solve this limitation, we prepared a scaffold consisting of hMSC-derived extracellular matrix (ECM) containing the necessary biomolecules for extended site-specific hMSC retention. When inhibitor-treated hMSCs were coadministered with ECM, they remained at the injury, well into the remodeling phase of healing, which resulted in reproducible and complete repair of critical-sized bone defects in mice in 3 weeks. These data suggest that hMSC-derived ECM and inhibitor-treated hMSCs could be used at optimal times to substantially and reproducibly improve bone repair.

Gene expression patterns related to osteogenic differentiation of bone marrow-derived mesenchymal stem cells during ex vivo expansion

Bone marrow is commonly used as a source of adult multipotent mesenchymal stem cells (MSCs), defined for their ability to differentiate in vitro into multiple lineages. The ex vivo-expanded MSCs are currently being evaluated as a strategy for the restoration of function in damaged skeletal tissue, both in cell therapy and tissue engineering applications. The aim of this study was to define gene expression patterns underlying the differentiation of MSCs into mature osteoblasts during the expansion in vitro, and to explore a variety of cell functions that cannot be easily evaluated using morphological, cytochemical, and biochemical assays. Cell cultures were obtained from bone marrow samples of six individuals undergoing total hip replacement, and a large-scale transcriptome analysis, using Affymetrix HG-U133A Plus 2.0 array (Affymetrix((R)), Santa Clara, CA), was performed at the occurrence of specific events, including the appearance of MSC surface markers, formation of colonies, and deposition of mineral nodules. We focused our attention on 213 differentially upregulated genes, some belonging to well-known pathways and some having one or more Gene Ontology annotations related to bone cell biology, including angiogenesis, bone-related genes, cell communication, development and morphogenesis, transforming growth factor-beta signaling, and Wnt signaling. Twenty-nine genes, whose role in bone cell pathophysiology has not been described yet, were found. In conclusion, gene expression patterns that characterize the early, intermediate, and late phases of the osteogenic differentiation process of ex vivo-expanded MSCs were defined. These signatures represent a useful tool to monitor the osteogenic process, and to analyze a broad spectrum of functions of MSCs cultured on scaffolds, especially when the constructs are conceived for releasing growth factors or other signals to promote bone regeneration.

Functional tissue engineering of ligament healing

Ligaments and tendons are dense connective tissues that are important in transmitting forces and facilitate joint articulation in the musculoskeletal system. Their injury frequency is high especially for those that are functional important, like the anterior cruciate ligament (ACL) and medial collateral ligament (MCL) of the knee as well as the glenohumeral ligaments and the rotator cuff tendons of the shoulder. Because the healing responses are different in these ligaments and tendons after injury, the consequences and treatments are tissue- and site-specific. In this review, we will elaborate on the injuries of the knee ligaments as well as using functional tissue engineering (FTE) approaches to improve their healing. Specifically, the ACL of knee has limited capability to heal, and results of non-surgical management of its midsubstance rupture have been poor. Consequently, surgical reconstruction of the ACL is regularly performed to gain knee stability. However, the long-term results are not satisfactory besides the numerous complications accompanied with the surgeries. With the rapid development of FTE, there is a renewed interest in revisiting ACL healing. Approaches such as using growth factors, stem cells and scaffolds have been widely investigated. In this article, the biology of normal and healing ligaments is first reviewed, followed by a discussion on the issues related to the treatment of ACL injuries. Afterwards, current promising FTE methods are presented for the treatment of ligament injuries, including the use of growth factors, gene delivery, and cell therapy with a particular emphasis on the use of ECM bioscaffolds. The challenging areas are listed in the future direction that suggests where collection of energy could be placed in order to restore the injured ligaments and tendons structurally and functionally.

Extracellular matrix molecules: potential targets in pharmacotherapy

The extracellular matrix (ECM) consists of numerous macromolecules classified traditionally into collagens, elastin, and microfibrillar proteins, proteoglycans including hyaluronan, and noncollagenous glycoproteins. In addition to being necessary structural components, ECM molecules exhibit important functional roles in the control of key cellular events such as adhesion, migration, proliferation, differentiation, and survival. Any structural inherited or acquired defect and/or metabolic disturbance in the ECM may cause cellular and tissue alterations that can lead to the development or progression of disease. Consequently, ECM molecules are important targets for pharmacotherapy. Specific agents that prevent the excess accumulation of ECM molecules in the vascular system, liver, kidney, skin, and lung; alternatively, agents that inhibit the degradation of the ECM in degenerative diseases such as osteoarthritis would be clinically beneficial. Unfortunately, until recently, the ECM in drug discovery has been largely ignored. However, several of today's drugs that act on various primary targets affect the ECM as a byproduct of the drugs' actions, and this activity may in part be beneficial to the drugs' disease-modifying properties. In the future, agents and compounds targeting directly the ECM will significantly advance the treatment of various human diseases, even those for which efficient therapies are not yet available.

Aberrations of 6q13 mapped to the COL12A1 locus in chondromyxoid fibroma

Chondromyxoid fibroma, a rare benign bone tumor, may be mistaken for chondrosarcoma. Although cytogenetic studies of chondromyxoid fibroma are few, rearrangements of the long arm of chromosome 6, frequently expressed as an inv(6)(p25q13), are prominent. In this study, conventional cytogenetic analysis of 16 chondromyxoid fibroma samples from 14 patients revealed rearrangements of chromosome 6 in 10 of 11 clonally abnormal specimens. In addition to 6q13 rearrangements, recurrent 6p25 and 6q25 anomalies were detected. Notably, an identical t(6;9)(q25;q22) translocation was identified in two cases, suggesting that it represents a distinct translocation of chondromyxoid fibroma. In an effort to further define the aberrant 6q13 breakpoint and identify the molecular consequences, a fluorescence in situ hybridization (FISH)-based positional cloning strategy on chondromyxoid fibroma abnormal metaphase and interphase cells using a series of bacterial and plasmid artificial chromosome (BAC/PAC) probe combinations spanning a 6.1 Mb region was employed. The breakpoint on 6q13 was located within the COL12A1 gene, a collagen gene purportedly involved in another benign bone tumor, subungual exostosis. The findings of this study expand our knowledge of chromosomal alterations in chondromyxoid fibroma, identify COL12A1 as the likely gene candidate within the recurrent 6q13 breakpoint, and provide an alternative approach for detecting 6q13 anomalies in nondividing cells of chondromyxoid fibroma. The latter could potentially be utilized as an adjunct in diagnostically challenging cases.

Differential gene expression in the perichondrium and cartilage of the neonatal mouse temporomandibular joint

Our goal was to discover genes differentially expressed in the perichondrium (PC) of the mandibular condylar cartilage (MCC) that might enhance regenerative medicine or orthopaedic therapies directed at the tissues of the temporomandibular joint. We used targeted gene arrays (osteogenesis, stem cell) to identify genes preferentially expressed in the PC and the cartilaginous (C) portions of the MCC in 2-day-old mice. Genes with higher expression in the PC sample related to growth factor ligand-receptor interactions [FGF-13 (6.4x), FGF-18 (4x), NCAM (2x); PGDF receptors, transforming growth factor (TGF)-beta and IGF-1], the Notch isoforms (especially Notch 3 and 4) and their ligands or structural proteins/proteoglycans [collagen XIV (21x), collagen XVIII (4x), decorin (2.5x)]. Genes with higher expression in the C sample consisted mostly of known cartilage-specific genes [aggrecan (11x), procollagens X (33x), XI (14x), IX (4.5x), Sox 9 (4.4x) and Indian hedgehog (6.7x)]. However, the functional or structural roles of several genes that were expressed at higher levels in the PC sample are unclear [myogenic factor (Myf) 9 (9x), tooth-related genes such as tuftelin (2.5x) and dentin sialophosphoprotein (1.6x), VEGF-B (2x) and its receptors (3-4x) and sclerostin (1.7x)]. FGF, Notch and TGF-beta signalling may be important regulators of MCC proliferation and differentiation; the relatively high expression of genes such as Myf6 and VEGF-B and its receptors suggests a degree of unsuspected plasticity in PC cells.

Fibroblasts-a diverse population at the center of it all

The capacity of fibroblasts to produce and organize the extracellular matrix and to communicate with other cells makes them a central component of tissue biology. Even so, fibroblasts remain a somewhat enigmatic population. Our inability to fully comprehend these cells is in large part due to the paucity of unique cellular markers and to their pervasive diversity. Much of our understanding of fibroblast diversity has evolved from studies where subpopulations of these cells have been produced without resorting to cell surface markers. In this regard, cloning and mechanical separation of tissues prior to establishing cultures has provided multiple subpopulations. Nonetheless, in isolated situations, the expression or lack of expression of Thy-1/CD90 has been used to separate fibroblast subsets. The role of fibroblasts in intercellular communication is emerging through the implementation of organotypic studies in which three-dimensional fibroblast culture are combined with other populations of cells. Such studies have revealed critical paracrine loops that are essential for organ development and for wound repair. These studies also provide a backdrop for the emerging field of tissue engineering. The participation of fibroblasts in the regulation of tissue homeostasis and their contribution to the aging process are emerging issues that require better understanding. In short, fibroblasts represent a multifaceted, complex group of cells.

Osteogenic properties of late adherent subpopulations of human bone marrow stromal cells

The nonadherent (NA) population of bone-marrow-derived mononuclear cells (MNC) has been demonstrated to be a source of osteogenic precursors in addition to the plastic-adherent mesenchymal stromal cells (MSC). In the current study, two subpopulations of late adherent (LA) osteoprogenitors were obtained by subsequent replating of NA cells, and their phenotypic, functional, and molecular properties were compared with those of early adherent (EA) MSC. Approximately 35% of MNC were LA cells, and they acquired a homogeneous expression of MSC antigens later than EA cells. In EA-MSC, the alkaline phosphatase (ALP) activity increased significantly from time of seeding to the first confluence, whereas in LA cells it raised later, after the addition of mineralization medium. All subpopulations were able to produce type I collagen and to deposit extracellular matrix with organized collagen fibrils. The proportion of large colonies with more than 50% of ALP positive cells as well as the calcium content was higher in LA than in EA cells. Molecular analysis highlighted the upregulation of bone-related genes in LA-MSC, especially after the addition of mineralization medium. Our results confirm that bone marrow contains LA osteoprogenitors which exhibit a delay in the differentiation process, despite an osteogenic potential similar to or better than EA-MSC. LA cells represent a reservoir of osteoprogenitors to be recruited to gain an adequate bone tissue repair and regeneration when a depletion of the most differentiated component occurs. Bone tissue engineering and cell therapy strategies could take advantage of LA cells, since an adequate amount of osteogenic MSCs may be obtained while avoiding bone marrow manipulation and cell culture expansion.