A novel gene, tendin, is strongly expressed in tendons and ligaments and shows high homology with chondromodulin-I

A Narrative Review of the Roles of Chondromodulin-I (Cnmd) in Adult Cartilage Tissue

Articular cartilage is crucial for joint function but its avascularity limits intrinsic repair, leading to conditions like osteoarthritis (OA). Chondromodulin-I (Cnmd) has emerged as a key molecule in cartilage biology, with potential implications for OA therapy. Cnmd is primarily expressed in cartilage and plays an important role in chondrocyte proliferation, cartilage homeostasis, and the blocking of angiogenesis. In vivo and in vitro studies on Cnmd, also suggest an involvement in bone repair and in delaying OA progression. Its downregulation correlates with OA severity, indicating its potential as a therapeutic target. Further research is needed to fully understand the mode of action of Cnmd and its beneficial implications for managing OA. This comprehensive review aims to elucidate the molecular characteristics of Cnmd, from its expression pattern, role in cartilage maintenance, callus formation during bone repair and association with OA.

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.

Lineage-specific differences and regulatory networks governing human chondrocyte development

To address large gaps in our understanding of the molecular regulation of articular and growth plate cartilage development in humans, we used our directed differentiation approach to generate these distinct cartilage tissues from human embryonic stem cells. The resulting transcriptomic profiles of hESC-derived articular and growth plate chondrocytes were similar to fetal epiphyseal and growth plate chondrocytes, with respect to genes both known and previously unknown to cartilage biology. With the goal to characterize the regulatory landscapes accompanying these respective transcriptomes, we mapped chromatin accessibility in hESC-derived chondrocyte lineages, and mouse embryonic chondrocytes, using ATAC-sequencing. Integration of the expression dataset with the differentially accessible genomic regions revealed lineage-specific gene regulatory networks. We validated functional interactions of two transcription factors (TFs) (RUNX2 in growth plate chondrocytes and RELA in articular chondrocytes) with their predicted genomic targets. The maps we provide thus represent a framework for probing regulatory interactions governing chondrocyte differentiation. This work constitutes a substantial step towards comprehensive and comparative molecular characterizations of distinct chondrogenic lineages and sheds new light on human cartilage development and biology.

Introduction of tenomodulin by gene transfection vectors for rat bone tissue regeneration

Introduction

Periodontal ligament is regenerated in association with hard tissue regeneration. Tenomodulin (Tnmd) expression has been confirmed in periodontal ligament and it reportedly inhibits angiogenesis or is involved in collagen fibril maturation. The introduction of Tnmd by gene transfection in bone tissue regeneration therapy might inhibit topical hard tissue formation and induce the formation of dense fibrous tissue. Therefore, the effect of Tnmd introduction by gene transfection technique in vitro and in vivo was investigated in this study.

Methods

Osteogenesis- and chondrogenesis-related gene expression levels in osteoblastic cells (MC3T3E1) and rat bone marrow derived cells were detected using qPCR three days after gene transfection with plasmid DNA (Tnmd) using non-viral gene transfection vectors: a calcium phosphate-based gene transfection vector (CaP(Tnmd)) or a cationic polymer-based reagent (JetPEI (Tnmd)). Next, an atelocollagen scaffold with or without CaP (Tnmd) or JetPEI (Tnmd) was implanted into a rat calvaria bone defect, and the remaining bone defect volume and the tissue reaction at 28 days after surgery were evaluated.

Results

Runx 2 and SP7 mRNA was reduced by JetPEI (Tnmd) in both cells, but not in CaP(Tnmd). The volume of expressed Tnmd was at 9 ng/mL in both gene transfection vector. The remaining bone defect volume of JetPEI (Tnmd) was significantly bigger than that of the other groups and CaP (EGFP), and that of CaP (Tnmd) was significantly bigger than that of CaP (EGFP).

Conclusions

Tnmd introduction treatment inhibits bone formation in artificial bone defect, however, the effect of that was dependent on non-viral gene transfection vector.

Leucine rich repeat containing 32 accelerates tenogenic differentiation of tendon-derived stem cells and promotes Achilles tendon repair in rats

Although many surgical or non-operative therapies have been developed to treat Achilles tendon injuries, the prognosis of which is often unsatisfactory. Recently, biologic approaches using multipotent stem cells like tendon-derived stem cells (TDSCs) pose a possible treatment option. To evaluate whether the Leucine rich repeat containing 32 (Lrrc32) affects the tenogenic differentiation of TDSCs and thus promotes Achilles tendon healing. TDSCs were infected with the recombinant Lrrc32-overexpressing lentivirus (LV-Lrrc32) and then locally injected into the injured site of rat. Four weeks after surgery, the Achilles tendon tissue (~0.5 cm) around the injured area was harvested for analysis. Pathological results showed that Lrrc32-overexpressing TDSCs significantly improved the morphological changes of the injured tendons. Specifically, the increased collagen-I expression and hydroxyproline content in extracellular matrix, and more orderly arrangement of the regenerated collagen fibers were observed in the Lrrc32 overexpression group. Moreover, 4 weeks after injection of Lrrc32-overexpressing TDSCs, the expression of tenocyte-related genes such as tenomodulin (Tnmd), scleraxis (Scx) and decorin (Dcn) were upregulated in the area of the healing tendon. These findings indicated that Lrrc32 promoted the tenogenic differentiation of TDSCs in vivo. Additionally, Lrrc32 overexpression also increased the expression of TGF-β1 and p-SMAD2/3, suggesting that the beneficial effects of Lrrc32 on tendon repair might be associated with the expression of TGF-β1 and p-SMAD2/3. Our findings collectively revealed that Lrrc32-overexpressed TDSCs promoted tendon healing more effectively than TDSCs alone.

Comparison of Tendon Development Versus Tendon Healing and Regeneration

Tendon is a vital connective tissue in human skeletal muscle system, and tendon injury is very common and intractable in clinic. Tendon development and repair are two closely related but still not fully understood processes. Tendon development involves multiple germ layer, as well as the regulation of diversity transcription factors (Scx et al.), proteins (Tnmd et al.) and signaling pathways (TGFβ et al.). The nature process of tendon repair is roughly divided in three stages, which are dominated by various cells and cell factors. This review will describe the whole process of tendon development and compare it with the process of tendon repair, focusing on the understanding and recent advances in the regulation of tendon development and repair. The study and comparison of tendon development and repair process can thus provide references and guidelines for treatment of tendon injuries.

Associations between the TNMD rs4828038 and ACE2 rs879922 polymorphisms and preeclampsia susceptibility: a case-control study

A case-control study was designed to investigate the association between the angiotensin converting enzyme 2 (ACE2) rs879922, glucose-6-phosphate dehydrogenase (G6PD) rs1050828, and tenomodulin (TNMD) rs4828038 single nucleotide polymorphisms (SNPs), and preeclampsia. A total of 356 Han Chinese pregnant women (170 controls and 186 cases) were recruited into the study. ACE2 rs879922, G6PD rs1050828, and TNMD rs4828038 were tested by the targeted next-generation sequencing technology and the data were analyzed using SPSS version 18. Genotyping of results revealed that patients with the CC/CT genotype in SNP rs4828038 or CC/CG genotype in SNP rs879922 had a significantly decreased susceptibility to late-onset preeclampsia (CC/CT versus TT: OR = 0.543, 95% CI = 0.378 to 0.779, p = .001; CC/CG versus GG: OR = 0.510, 95% CI = 0.038 to 0.860, p = .012). Our study found that the polymorphisms TNMD rs4828038 and ACE2 rs879922 might be associated with late-onset preeclampsia.IMPACT STATEMENTWhat is already known on this subject? Preeclampsia is associated with multiple SNPs, and ACE2 rs879922, G6PD rs1050828, and TNMD rs4828038 are related to essential hypertension and glucose and lipid metabolism disorders. Essential hypertension, diabetes, and dyslipidemia are risks for preeclampsia. The associations between those three SNPs and preeclampsia have not been reported.What do the results of this study add? The polymorphisms of TNMD rs4828038 and ACE2 rs879922 might be associated with the risk of late-onset preeclampsia. There was no relationship between SNP rs1050828 and preeclampsia.What are the implications of these findings for clinical practice and/or further research? TNMD rs4828038 and ACE2 rs879922 might be target sites for genetic diagnosis and therapy, and the levels of mRNA and protein in pregnant women with preeclampsia should be further tested.

Tenomodulin and Chondromodulin-1 Are Both Required to Maintain Biomechanical Function and Prevent Intervertebral Disc Degeneration

OBJECTIVE

The underlying mechanisms and molecular factors influencing intervertebral disc (IVD) homeostasis and degeneration remain clinically relevant. Tenomodulin (Tnmd) and chondromodulin (Chm1) are antiangiogenic transmembrane glycoproteins, with cleavable C-terminus, expressed by IVD cells that are implicated in the onset of degenerative processes. We evaluate the organ-level biomechanical impact of knocking out Tnmd alone, and Tnmd and Chm1, simultaneously.

DESIGN

Caudal (c5-8) and lumbar vertebrae (L1-4) of skeletally mature male and female 9-month-old wildtype (WT), Tnmd knockout (Tnmd^-/-), and Tnmd/Chm1 double knockout (Tnmd^-/-/Chm^-/-) mice were used (n = 9-13 per group). Disc height index (DHI), histomorphological changes, and axial, torsional, creep, and failure biomechanical properties were evaluated. Differences were assessed by one-way ANOVA with post hoc Bonferroni-corrected comparisons (P < 0.05).

RESULTS

Tnmd^-/-/Chm1^-/- IVDs displayed increased DHI and histomorphological scores that indicated increased IVD degeneration compared to the WT and Tnmd^-/- groups. Double knockout IVDs required significantly less torque and energy to initiate torsional failure. Creep parameters were comparable between all groups, except for the slow time constant, which indicated faster outward fluid flow. Tnmd^-/- IVDs lost fluid faster than the WT group, and this effect was amplified in the double knockout IVDs.

CONCLUSION

Knocking out Tnmd and Chm1 affects IVD fluid flow and organ-level biomechanical function and therefore may play a role in contributing to IVD degeneration. Larger effects of the Tnmd and Chm1 double knockout mice compared to the Tnmd single mutant suggest that Chm1 may play a compensatory role in the Tnmd single mutant IVDs.

Tenomodulin knockout mice exhibit worse late healing outcomes with augmented trauma-induced heterotopic ossification of Achilles tendon

Heterotopic ossification (HO) represents a common problem after tendon injury with no effective treatment yet being developed. Tenomodulin (Tnmd), the best-known mature marker for tendon lineage cells, has important effects in tendon tissue aging and function. We have reported that loss of Tnmd leads to inferior early tendon repair characterized by fibrovascular scaring and therefore hypothesized that its lack will persistently cause deficient repair during later stages. Tnmd knockout (Tnmd-/-) and wild-type (WT) animals were subjected to complete Achilles tendon surgical transection followed by end-to-end suture. Lineage tracing revealed a reduction in tendon-lineage cells marked by ScleraxisGFP, but an increase in alpha smooth muscle actin myofibroblasts in Tnmd-/- tendon scars. At the proliferative stage, more pro-inflammatory M1 macrophages and larger collagen II cartilaginous template were detected in this group. At the remodeling stage, histological scoring revealed lower repair quality in the injured Tnmd-/- tendons, which was coupled with higher HO quantified by micro-CT. Tendon biomechanical properties were compromised in both groups upon injury, however we identified an abnormal stiffening of non-injured Tnmd-/- tendons, which possessed higher static and dynamic E-moduli. Pathologically thicker and abnormally shaped collagen fibrils were observed by TEM in Tnmd-/- tendons and this, together with augmented HO, resulted in diminished running capacity of Tnmd-/- mice. These novel findings demonstrate that Tnmd plays a protecting role against trauma-induced endochondral HO and can inspire the generation of novel therapeutics to accelerate repair.

Perspective: Why and How Ubiquitously Distributed, Vascular-Associated, Pluripotent Stem Cells in the Adult Body (vaPS Cells) Are the Next Generation of Medicine

A certain cell type can be isolated from different organs in the adult body that can differentiate into ectoderm, mesoderm, and endoderm, providing significant support for the existence of a certain type of small, vascular-associated, pluripotent stem cell ubiquitously distributed in all organs in the adult body (vaPS cells). These vaPS cells fundamentally differ from embryonic stem cells and induced pluripotent stem cells in that the latter possess the necessary genetic guidance that makes them intrinsically pluripotent. In contrast, vaPS cells do not have this intrinsic genetic guidance, but are able to differentiate into somatic cells of all three lineages under guidance of the microenvironment they are located in, independent from the original tissue or organ where they had resided. These vaPS cells are of high relevance for clinical application because they are contained in unmodified, autologous, adipose-derived regenerative cells (UA-ADRCs). The latter can be obtained from and re-applied to the same patient at the point of care, without the need for further processing, manipulation, and culturing. These findings as well as various clinical examples presented in this paper demonstrate the potential of UA-ADRCs for enabling an entirely new generation of medicine for the benefit of patients and healthcare systems.

First immunohistochemical evidence of human tendon repair following stem cell injection: A case report and review of literature

BACKGROUND

Current clinical treatment options for symptomatic, partial-thickness rotator cuff tear (sPTRCT) offer only limited potential for true tissue healing and improvement of clinical results. In animal models, injections of adult stem cells isolated from adipose tissue into tendon injuries evidenced histological regeneration of tendon tissue. However, it is unclear whether such beneficial effects could also be observed in a human tendon treated with fresh, uncultured, autologous, adipose derived regenerative cells (UA-ADRCs). A specific challenge in this regard is that UA-ADRCs cannot be labeled and, thus, not unequivocally identified in the host tissue. Therefore, histological regeneration of injured human tendons after injection of UA-ADRCs must be assessed using comprehensive, immunohistochemical and microscopic analysis of biopsies taken from the treated tendon a few weeks after injection of UA-ADRCs.

CASE SUMMARY

A 66-year-old patient suffered from sPTRCT affecting the right supraspinatus and infraspinatus tendon, caused by a bicycle accident. On day 18 post injury [day 16 post magnetic resonance imaging (MRI) examination] approximately 100 g of abdominal adipose tissue was harvested by liposuction, from which approximately 75 × 10⁶ UA-ADRCs were isolated within 2 h. Then, UA-ADRCs were injected (controlled by biplanar X-ray imaging) adjacent to the injured supraspinatus tendon immediately after isolation. Despite fast clinical recovery, a follow-up MRI examination 2.5 mo post treatment indicated the need for open revision of the injured infraspinatus tendon, which had not been treated with UA-ADRCs. During this operation, a biopsy was taken from the supraspinatus tendon at the position of the injury. A comprehensive, immunohistochemical and microscopic analysis of the biopsy (comprising 13 antibodies) was indicative of newly formed tendon tissue.

CONCLUSION

Injection of UA-ADRCs can result in regeneration of injured human tendons by formation of new tendon tissue.

Spironolactone Ameliorates Senescence and Calcification by Modulating Autophagy in Rat Tendon-Derived Stem Cells via the NF-κB/MAPK Pathway

Tendinopathy is a disabling musculoskeletal disease, the pathological process of which is tightly associated with inflammation. Spironolactone (SP) has been widely used as a diuretic in clinical practice. Recently, SP has shown anti-inflammatory features in several diseases. Tendon-derived stem cells (TDSCs), a subset cell type from tendon tissue possessing clonogenic capacity, play a vital role in the pathological process of tendinopathy. In the present study, the protective effect of SP on TDSCs was demonstrated under simulated tendinopathy conditions both in vitro and in vivo. SP contributed to the maintenance of TDSC-specific genes or proteins, while suppressing the interleukin- (IL-) 1β-induced overexpression of inflammation-mediated factors. Additionally, IL-1β-induced cellular senescence in TDSCs was inhibited, while autophagy was enhanced, as determined via β-galactosidase activity, western blot (WB), and quantitative real-time polymerase chain reaction analysis. With the aid of several emerging bioinformatics tools, the mitogen-activated protein kinase (MAPK) pathway likely participated in the effect of SP, which was further validated through WB analysis and the use of MAPK agonist. Immunofluorescence analysis and an NF-κB agonist were used to confirm the inhibitory effect of SP on IL-1β-induced activation of the NF-κB pathway. X-ray, immunofluorescence, immunohistochemistry, hematoxylin and eosin staining, histological grades, and Masson staining showed that SP ameliorated tendinopathy in an Achilles tenotomy (AT) rat model in vivo. This work elucidates the protective role of SP on the pathological process of tendinopathy both in vitro and in vivo, indicating a potential therapeutic strategy for tendinopathy treatment.

Identification and Distinction of Tenocytes and Tendon-Derived Stem Cells

Restoring the normal structure and function of injured tendons is one of the biggest challenges in orthopedics and sports medicine department. The discovery of tendon-derived stem cells (TDSCs) provides a novel perspective to treat tendon injuries, which is expected to be an ideal seed cell to promote tendon repair and regeneration. Because of the lack of specific markers, the identification of tenocytes and TDSCs has not been conclusive in the in vitro study of tendons. In addition, the morphology of tendon derived cells is similar, and the comparison and identification of tenocytes and TDSCs are insufficient, which causes some obstacles to the in vitro study of tendon. In this review, the characteristics of tenocytes and TDSCs are summarized and compared based on some existing research results (mainly in terms of biomarkers), and a potential marker selection for identification is suggested. It is of profound significance to further explore the mechanism of biomarkers in vivo and to find more specific markers.

The Story of the Hand

This review describes the Story of the Human Hand. It traces the functional needs that led to evolution of the human hand as well as its embryological development. The various in utero stages of formation of the human hand are covered along with a description of the various molecular and genetic factors that control this process.

In Vitro Innovation of Tendon Tissue Engineering Strategies

Tendinopathy is the term used to refer to tendon disorders. Spontaneous adult tendon healing results in scar tissue formation and fibrosis with suboptimal biomechanical properties, often resulting in poor and painful mobility. The biomechanical properties of the tissue are negatively affected. Adult tendons have a limited natural healing capacity, and often respond poorly to current treatments that frequently are focused on exercise, drug delivery, and surgical procedures. Therefore, it is of great importance to identify key molecular and cellular processes involved in the progression of tendinopathies to develop effective therapeutic strategies and drive the tissue toward regeneration. To treat tendon diseases and support tendon regeneration, cell-based therapy as well as tissue engineering approaches are considered options, though none can yet be considered conclusive in their reproduction of a safe and successful long-term solution for full microarchitecture and biomechanical tissue recovery. In vitro differentiation techniques are not yet fully validated. This review aims to compare different available tendon in vitro differentiation strategies to clarify the state of art regarding the differentiation process.

Rebuilding Tendons: A Concise Review on the Potential of Dermal Fibroblasts

Tendons are vital to joint movement by connecting muscles to bones. Along with an increasing incidence of tendon injuries, tendon disorders can burden the quality of life of patients or the career of athletes. Current treatments involve surgical reconstruction and conservative therapy. Especially in the elderly population, tendon recovery requires lengthy periods and it may result in unsatisfactory outcome. Cell-mediated tendon engineering is a rapidly progressing experimental and pre-clinical field, which holds great potential for an alternative approach to established medical treatments. The selection of an appropriate cell source is critical and remains under investigation. Dermal fibroblasts exhibit multiple similarities to tendon cells, suggesting they may be a promising cell source for tendon engineering. Hence, the purpose of this review article was in brief, to compare tendon to dermis tissues, and summarize in vitro studies on tenogenic differentiation of dermal fibroblasts. Furthermore, analysis of an open source Gene Expression Omnibus (GEO) data repository was carried out, revealing great overlap in the molecular profiles of both cell types. Lastly, a summary of in vivo studies employing dermal fibroblasts in tendon repair as well as pilot clinical studies in this area is included. Altogether, dermal fibroblasts hold therapeutic potential and are attractive cells for rebuilding injured tendons.

Loss of tenomodulin expression is a risk factor for age-related intervertebral disc degeneration

The intervertebral disc (IVD) degeneration is thought to be closely related to ingrowth of new blood vessels. However, the impact of anti-angiogenic factors in the maintenance of IVD avascularity remains unknown. Tenomodulin (Tnmd) is a tendon/ligament-specific marker and anti-angiogenic factor with abundant expression in the IVD. It is still unclear whether Tnmd contributes to the maintenance of IVD homeostasis, acting to inhibit vascular ingrowth into this normally avascular tissue. Herein, we investigated whether IVD degeneration could be induced spontaneously by the absence of Tnmd. Our results showed that Tnmd was expressed in an age-dependent manner primarily in the outer annulus fibrous (OAF) and it was downregulated at 6 months of age corresponding to the early IVD degeneration stage in mice. Tnmd knockout (Tnmd-/- ) mice exhibited more rapid progression of age-related IVD degeneration. These signs include smaller collagen fibril diameter, markedly lower compressive stiffness, reduced multiple IVD- and tendon/ligament-related gene expression, induced angiogenesis, and macrophage infiltration in OAF, as well as more hypertrophic-like chondrocytes in the nucleus pulposus. In addition, Tnmd and chondromodulin I (Chm1, the only homologous gene to Tnmd) double knockout (Tnmd-/- Chm1-/- ) mice displayed not only accelerated IVD degeneration, but also ectopic bone formation of IVD. Lastly, the absence of Tnmd in OAF-derived cells promoted p65 and matrix metalloproteinases upregulation, and increased migratory capacity of human umbilical vein endothelial cells. In sum, our data provide clear evidences that Tnmd acts as an angiogenic inhibitor in the IVD homeostasis and protects against age-related IVD degeneration. Targeting Tnmd may represent a novel therapeutic strategy for attenuating age-related IVD degeneration.

The synergistic effect of low oxygen tension and macromolecular crowding in the development of extracellular matrix-rich tendon equivalents

Cellular therapies play an important role in tendon tissue engineering, with tenocytes being the most prominent and potent cell population available. However, for the development of a rich extracellular matrix tenocyte-assembled tendon equivalent, prolonged in vitro culture is required, which is associated with phenotypic drift. Recapitulation of tendon tissue microenvironment in vitro with cues that enhance and accelerate extracellular matrix synthesis and deposition, whilst maintaining tenocyte phenotype, may lead to functional cell therapies. Herein, we assessed the synergistic effect of low oxygen tension (enhances extracellular matrix synthesis) and macromolecular crowding (enhances extracellular matrix deposition) in human tenocyte culture. Protein analysis demonstrated that human tenocytes at 2% oxygen tension and with 50 μg ml^-1 carrageenan (macromolecular crowder used) significantly increased synthesis and deposition of collagen types I, III, V and VI. Gene analysis at day 7 illustrated that human tenocytes at 2% oxygen tension and with 50 μg ml^-1 carrageenan significantly increased the expression of prolyl 4-hydroxylase subunit alpha 1, procollagen-lysine 2- oxoglutarate 5-dioxygenase 2, scleraxis, tenomodulin and elastin, whilst chondrogenic (e.g. runt-related transcription factor 2, cartilage oligomeric matrix protein, aggrecan) and osteogenic (e.g. secreted phosphoprotein 1, bone gamma-carboxyglutamate protein) trans-differentiation markers were significantly down-regulated or remained unchanged. Collectively, our data clearly illustrates the beneficial synergistic effect of low oxygen tension and macromolecular crowding in the accelerated development of tissue equivalents.

Cellular Heterogeneity and Lineage Restriction during Mouse Digit Tip Regeneration at Single-Cell Resolution

Innate regeneration following digit tip amputation is one of the few examples of epimorphic regeneration in mammals. Digit tip regeneration is mediated by the blastema, the same structure invoked during limb regeneration in some lower vertebrates. By genetic lineage analyses, the digit tip blastema has been defined as a population of heterogeneous, lineage-restricted progenitor cells. These previous studies, however, do not comprehensively evaluate blastema heterogeneity or address lineage restriction of closely related cell types. In this report, we present single-cell RNA sequencing of over 38,000 cells from mouse digit tip blastemas and unamputated control digit tips and generate an atlas of the cell types participating in digit tip regeneration. We computationally define differentiation trajectories of vascular, monocytic, and fibroblastic lineages over regeneration, and while our data confirm broad lineage restriction of progenitors, our analysis reveals 67 genes enriched in blastema fibroblasts including a novel regeneration-specific gene, Mest.

Evaluation of tenogenic differentiation potential of selected subpopulations of human adipose-derived stem cells

Identification of a suitable cell source and bioactive agents guiding cell differentiation towards tenogenic phenotype represents a prerequisite for advancement of cell-based therapies for tendon repair. Human adipose-derived stem cells (hASCs) are a promising, yet intrinsically heterogenous population with diversified differentiation capacities. In this work, we investigated antigenically-defined subsets of hASCs expressing markers related to tendon phenotype or associated with pluripotency that might be more prone to tenogenic differentiation, when compared to unsorted hASCs. Subpopulations positive for tenomodulin (TNMD+ hASCs) and stage specific early antigen 4 (SSEA-4+ hASCs), as well as unsorted ASCs were cultured up to 21 days in basic medium or media supplemented with TGF-β3 (10 ng/ml), or GDF-5 (50 ng/ml). Cell response was evaluated by analysis of expression of tendon-related markers at gene level and protein level by real time RT-PCR, western blot, and immunocytochemistry. A significant upregulation of scleraxis was observed for both subpopulations and unsorted hASCs in the presence of TGF-β3. More prominent alterations in gene expression profile in response to TGF-β3 were observed for TNMD+ hASCs. Subpopulations evidenced an increased collagen III and TNC deposition in basal medium conditions in comparison with unsorted hASCs. In the particular case of TNMD+ hASCs, GDF-5 seems to influence more the deposition of TNC. Within hASCs populations, discrete subsets could be distinguished offering varied sensitivity to specific biochemical stimulation leading to differential expression of tenogenic components suggesting that cell subsets may have distinctive roles in the complex biological responses leading to tenogenic commitment to be further explored in cell based strategies for tendon tissues.

Stem Cell Therapy for Tendon Regeneration: Current Status and Future Directions

In adults the healing tendon generates fibrovascular scar tissue and recovers never histologically, mechanically, and functionally which leads to chronic and to degenerative diseases. In this review, the processes and mechanisms of tendon development and fetal regeneration in comparison to adult defect repair and degeneration are discussed in relation to regenerative therapeutic options. We focused on the application of stem cells, growth factors, transcription factors, and gene therapy in tendon injury therapies in order to intervene the scarring process and to induce functional regeneration of the lesioned tissue. Outlines for future therapeutic approaches for tendon injuries will be provided.

The cellular basis of fibrotic tendon healing: challenges and opportunities

Tendon injuries are common and can dramatically impair patient mobility and productivity, resulting in a significant socioeconomic burden and reduced quality of life. Because the tendon healing process results in the formation of a fibrotic scar, injured tendons never regain the mechanical strength of the uninjured tendon, leading to frequent reinjury. Many tendons are also prone to the development of peritendinous adhesions and excess scar formation, which further reduce tendon function and lead to chronic complications. Despite this, there are currently no treatments that adequately improve the tendon healing process due in part to a lack of information regarding the contributions of various cell types to tendon healing and how their activity may be modulated for therapeutic value. In this review, we summarize recent efforts to identify and characterize the distinct cell populations involved at each stage of tendon healing. In addition, we examine the mechanisms through which different cell populations contribute to the fibrotic response to tendon injury, and how these responses can be affected by systemic factors and comorbidities. We then discuss gaps in our current understanding of tendon fibrosis and highlight how new technologies and research areas are shedding light on this clinically important and intractable challenge. A better understanding of the complex cellular environment during tendon healing is crucial to the development of new therapies to prevent fibrosis and promote tissue regeneration.

Characterization of Tendon-Specific Markers in Various Human Tissues, Tenocytes and Mesenchymal Stem Cells

Background

Unlike bone, cartilage, or muscle, tendon-specific markers are not well established. The purpose of the study was to investigate expression pattern and level of 6 well-known tendon-specific markers, in various human musculoskeletal tissues, tenocytes, and mesenchymal stem cells (MSCs).

Methods

Musculoskeletal tissue samples of tendon, bone, cartilage, nerve, muscle, and fat were obtained from patients undergoing orthopedic surgery. Tenocytes, MSCs from bone marrow, adipose tissue, and umbilical cord were isolated from each tissue and cultured. Six tendon-specific markers, scleraxis (Scx), tenomodulin (TNMD), thrombospondin-4 (TSP-4), tenascin-C (TNC), type I collagen (Col I), and type III collagen (Col III) were investigated in tendon tissue, tenocytes, and MSCs.

Results

mRNA levels of 6 tendon-specific markers were significantly higher in tendon tissue that in other connective tissues levels of Scx, TNMD, TSP-4, and Col III immediately decreased after plating tenocytes in culture dishes whereas those of TNC and Col I did not. In comparison with tendon tissue, mRNA levels pattern of Scx, TNMD, and TSP-4 in tenocytes were significantly higher than that in MSCs, but lower than in tendon tissue whereas expression pattern of TNC, Col I and III showed different pattern with each other.

Conclusion

This study demonstrated that 6 commonly used tendon-specific markers were mainly expressed in tendon tissue, but that expression level and pattern of the tendon-specific markers with respect to kinds of tissues, culture duration of tenocytes and sources of MSCs.

Effects of X-chromosome Tenomodulin Genetic Variants on Obesity in a Children's Cohort and Implications of the Gene in Adipocyte Metabolism

Tenomodulin (TNMD) is a type II transmembrane glycoprotein that has been recently linked to obesity, and it is highly expressed in obese adipose tissue. Several sex-dependent associations have been observed between single-nucleotide polymorphisms (SNPs) of the TNMD gene, which is located in the X-chromosome, and obesity, type 2 diabetes mellitus (T2DM), and metabolic syndrome in adults. On the other hand, results are lacking for children. We aimed (i) to study the association between TNMD genetic variants and metabolic complications related to childhood obesity and (ii) to investigate the function of TNMD in human adipocytes. We conducted a case-control, multicenter study in 915 Spanish children and demonstrated significant positive associations between TNMD genetic variants and BMI z-score, waist circumference, fasting glucose, and insulin resistance in boys, highlighting the SNP rs4828038. Additionally, we showed a BMI-adjusted inverse association with waist circumference in girls. Second, in vitro experiments revealed that TNMD is involved in adipogenesis, along with glucose and lipid metabolism in differentiated adipocytes, and these effects may be mediated through AMPK activation. Hence, these results suggest that TNMD genetic variants could be potentially useful as early life risk indicators for obesity and T2DM. In addition, we support the fact that TNMD exhibits significant metabolic functions in adipocytes.

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.

Injured Achilles Tendons Treated with Adipose-Derived Stem Cells Transplantation and GDF-5

Tendon injuries represent a clinical challenge in regenerative medicine because their natural repair process is complex and inefficient. The high incidence of tendon injuries is frequently associated with sports practice, aging, tendinopathies, hypertension, diabetes mellitus, and the use of corticosteroids. The growing interest of scientists in using adipose-derived mesenchymal stem cells (ADMSC) in repair processes seems to be mostly due to their paracrine and immunomodulatory effects in stimulating specific cellular events. ADMSC activity can be influenced by GDF-5, which has been successfully used to drive tenogenic differentiation of ADMSC in vitro. Thus, we hypothesized that the application of ADMSC in isolation or in association with GDF-5 could improve Achilles tendon repair through the regulation of important remodeling genes expression. Lewis rats had tendons distributed in four groups: Transected (T), transected and treated with ADMSC (ASC) or GDF-5 (GDF5), or with both (ASC+GDF5). In the characterization of cells before application, ADMSC expressed the positive surface markers, CD90 (90%) and CD105 (95%), and the negative marker, CD45 (7%). ADMSC were also differentiated in chondrocytes, osteoblast, and adipocytes. On the 14th day after the tendon injury, GFP-ADMSC were observed in the transected region of tendons in the ASC and ASC+GDF5 groups, and exhibited and/or stimulated a similar genes expression profile when compared to the in vitro assay. ADMSC up-regulated Lox, Dcn, and Tgfb1 genes expression in comparison to T and ASC+GDF5 groups, which contributed to a lower proteoglycans arrangement, and to a higher collagen fiber organization and tendon biomechanics in the ASC group. The application of ADMSC in association with GDF-5 down-regulated Dcn, Gdf5, Lox, Tgfb1, Mmp2, and Timp2 genes expression, which contributed to a lower hydroxyproline concentration, lower collagen fiber organization, and to an improvement of the rats’ gait 24 h after the injury. In conclusion, although the literature describes the benefic effect of GDF-5 for the tendon healing process, our results show that its application, isolated or associated with ADMSC, cannot improve the repair process of partial transected tendons, indicating the higher effectiveness of the application of ADMSC in injured Achilles tendons. Our results show that the application of ADMSC in injured Achilles tendons was more effective in relation to its association with GDF-5.

Functionalized thermosensitive hydrogel combined with tendon stem/progenitor cells as injectable cell delivery carrier for tendon tissue engineering

Thermosensitive hydrogels have been studied for potential application as promising alternative cell carriers in cell-based regenerative therapies. In this study, a thermosensitive butane diisocyanate (BDI)-collagen hydrogel (BC hydrogel) was designed as an injectable cell delivery carrier of tendon stem/progenitor cells (TSPCs) for tendon tissue engineering. We functionalized the BDI hydrogel with the addition of 20% (v/v) collagen I gel to obtain the thermosensitive BC hydrogel, which was then seeded with TSPCs derived from human Achilles tendons. The BC hydrogel compatibility and TSPC behavior and molecular response to the 3D hydrogel were investigated. Collagen (COL) I gel served as a control group. Our findings demonstrated that the BC hydrogel was thermosensitive, and hardened above 25 °C. It supported TSPC survival, proliferation, and metabolic activity with satisfactory dimension stability and biocompatibility, as revealed by gel contraction assay, live/dead staining, DNA quantification, and resazurin metabolic assay. Phalloidin-based visualization of F-actin demonstrated that the TSPCs were stretched within COL I gel with classical spindle cell shapes; similar cell morphologies were also found in the BC hydrogel. The gene expression profile of TSPCs in the BC hydrogel was comparable with that in COL I gel. Moreover, the BC hydrogel supported capillary-like structure formation by human umbilical vein endothelial cells (HUVECs) in the hydrogel matrix. Taken together, these results suggest that the thermosensitive BC hydrogel holds great potential as an injectable cell delivery carrier of TSPCs for tendon tissue engineering.

Triggering the activation of Activin A type II receptor in human adipose stem cells towards tenogenic commitment using mechanomagnetic stimulation

Stem cell therapies hold potential to stimulate tendon regeneration and homeostasis, which is maintained in response to the native mechanical environment. Activins are members of the mechano-responsive TGF-β superfamily that participates in the regulation of several downstream biological processes. Mechanosensitive membrane receptors such as activin can be activated in different types of stem cells via magnetic nanoparticles (MNPs) through remote magnetic actuation resulting in cell differentiation. In this work, we target the Activin receptor type IIA (ActRIIA) in human adipose stem cells (hASCs), using anti-ActRIIA functionalized MNPs, externally activated through a oscillating magnetic bioreactor. Upon activation, the phosphorylation of Smad2/3 is induced allowing translocation of the complex to the nucleus, regulating tenogenic transcriptional responses. Our study demonstrates the potential remote activation of MNPs tagged hASCs to trigger the Activin receptor leading to tenogenic differentiation. These results may provide insights toward tendon regeneration therapies.

Scleraxis is a transcriptional activator that regulates the expression of Tenomodulin, a marker of mature tenocytes and ligamentocytes

Tenomodulin (Tnmd) is a type II transmembrane glycoprotein predominantly expressed in tendons and ligaments. We found that scleraxis (Scx), a member of the Twist-family of basic helix-loop-helix transcription factors, is a transcriptional activator of Tnmd expression in tenocytes. During embryonic development, Scx expression preceded that of Tnmd. Tnmd expression was nearly absent in tendons and ligaments of Scx-deficient mice generated by transcription activator-like effector nucleases-mediated gene disruption. Tnmd mRNA levels were dramatically decreased during serial passages of rat tenocytes. Scx silencing by small interfering RNA significantly suppressed endogenous Tnmd mRNA levels in tenocytes. Mouse Tnmd contains five E-box sites in the ~1-kb 5′-flanking region. A 174-base pair genomic fragment containing a TATA box drives transcription in tenocytes. Enhancer activity was increased in the upstream region (−1030 to −295) of Tnmd in tenocytes, but not in NIH3T3 and C3H10T1/2 cells. Preferential binding of both Scx and Twist1 as a heterodimer with E12 or E47 to CAGATG or CATCTG and transactivation of the 5′-flanking region were confirmed by electrophoresis mobility shift and dual luciferase assays, respectively. Scx directly transactivates Tnmd via these E-boxes to positively regulate tenocyte differentiation and maturation.

Tissue-engineered magnetic cell sheet patches for advanced strategies in tendon regeneration

Tendons are powerful 3D biomechanically structures combining a few cells in an intrincated and highly hierarchical niche environment. When tendon homeostasis is compromised, restoration of functionality upon injury is limited and requires alternatives to current augmentation or replacement strategies. Cell sheet technologies are a powerful tool for the fabrication of living extracellular-rich patches towards regeneration of tenotopic defects. Thus, we originally propose the development of magnetically responsive tenogenic patches through magnetic cell sheet (magCSs) technology that enable the remote control upon implantation of the tendon-mimicking constructs. A Tenomodulin positive (TNMD⁺) subpopulation of cells sorted from a crude population of human adipose stem cells (hASCs) previously identified as being prone to tenogenesis was selected for the magCSs patch construction. We investigated the stability, the cellular co-location of the iron oxide nanoparticles (MNPs), as well as the morphology and mechanical properties of the developed magCSs. Moreover, the expression of tendon markers and collagenous tendon-like matrix were further assessed under the actuation of an external magnetic field. Overall, this study confirms the potential to bioengineer tendon patches using a magnetic cell sheet construction with magnetic responsiveness, good mechanoelastic properties and a tenogenic prone stem cell population envisioning cell-based functional therapies towards tendon regeneration.

STATEMENT OF SIGNIFICANCE

The concept of magnetic force-based tissue engineering may assist the development of innovative solutions to treat tendon (or other tissues) disorders upon remote control of biological processes as cell migration or differentiation. Herein, we originally fabricated magnetic responsive cell sheets (magCSs) with a Tenomodulin positive subpopulation of adipose tissue derived stem cells identified to commit to the tenogenic lineage. To the best of authors knowledge, this is the first time a tendon oriented strategy resorting on magCSsis reported. Moreover, the promising role of tenogenic living constructs fabricated as magnetically responsive ECM-rich patches is highlighted, envisioning the stimulation of endogenous regenerative mechanisms. Altogether, these findings contribute to future stem cell studies and their translation toward tendon therapies.

Tenomodulin is essential for prevention of adipocyte accumulation and fibrovascular scar formation during early tendon healing

Tenomodulin (Tnmd) is the best-known mature marker for tendon and ligament lineage cells. It is important for tendon maturation, running performance and has key implications for the resident tendon stem/progenitor cells (TSPCs). However, its exact functions during the tendon repair process still remain elusive. Here, we established an Achilles tendon injury model in a Tnmd knockout (Tnmd^−/−) mouse line. Detailed analyses showed not only a very different scar organization with a clearly reduced cell proliferation and expression of certain tendon-related genes, but also increased cell apoptosis, adipocyte and blood vessel accumulation in the early phase of tendon healing compared with their wild-type (WT) littermates. In addition, Tnmd^−/− tendon scar tissue contained augmented matrix deposition of biglycan, cartilage oligomeric matrix protein (Comp) and fibronectin, altered macrophage profile and reduced numbers of CD146-positive cells. In vitro analysis revealed that Tnmd^−/− TSPCs exhibited significantly reduced migration and proliferation potential compared with that of WT TSPCs. Furthermore, Tnmd^−/− TSPCs had accelerated adipogenic differentiation accompanied with significantly increased peroxisome proliferator-activated receptor gamma (Pparγ) and lipoprotein lipase (Lpl) mRNA levels. Thus, our results demonstrate that Tnmd is required for prevention of adipocyte accumulation and fibrovascular scar formation during early tendon healing.

Human adipose tissue-derived tenomodulin positive subpopulation of stem cells: A promising source of tendon progenitor cells

Cell-based therapies are of particular interest for tendon and ligament regeneration given the low regenerative potential of these tissues. Adipose tissue is an abundant source of stem cells, which may be employed for the healing of tendon lesions. However, human adult multipotent adipose-derived stem cells (hASCs) isolated from the stromal vascular fraction of adipose tissue originate highly heterogeneous cell populations that hinder their use in specific tissue-oriented applications. In this study, distinct subpopulations of hASCs were immunomagnetic separated and their tenogenic differentiation capacity evaluated in the presence of several growth factors (GFs), namely endothelial GF, basic-fibroblast GF, transforming GF-β1 and platelet-derived GF-BB, which are well-known regulators of tendon development, growth and healing. Among the screened hASCs subpopulations, tenomodulin-positive cells were shown to be more promising for tenogenic applications and therefore this subpopulation was further studied, assessing tendon-related markers (scleraxis, tenomodulin, tenascin C and decorin) both at gene and protein level. Additionally, the ability for depositing collagen type I and III forming extracellular matrix structures were weekly assessed up to 28 days. The results obtained indicated that tenomodulin-positive cells exhibit phenotypical features of tendon progenitor cells and can be biochemically induced towards tenogenic lineage, demonstrating that this subset of hASCs can provide a reliable source of progenitor cells for therapies targeting tendon regeneration.

Inhibitory effect of tenomodulin versus ranibizumab on in vitro angiogenesis

AIM

To evaluate anti-angiogenic effect of tenomodulin (TNMD) and ranibizumab on cell proliferation and capillary-like morphogenesis of vascular endothelial cells under the stimulation of vascular endothelial growth factor (VEGF) in vitro.

METHODS

The effects of TNMD and ranibizumab on VEGF-induced proliferation of human umbilical vein endothelial cells (HUVECs) were evaluated by MTT assay, and the effects of TNMD and ranibizumab on capillary-like structures formed by HUVECs under the stimulation of VEGF were examined in culture. Capillary-like morphogenesis of HUVECs was quantitatively evaluated, and total lengths of tube-like structures per field were measured in a masked way.

RESULTS

HUVECs with both ranibizumab and TNMD protein showed MTT reduction in VEGF-stimulated cell proliferation as expected, while MTT absorbance in the HUVECs with TNMD was significantly declined than that with ranibizumab (P<0.01). The capillary-like structures formed by HUVECs were markedly impaired by the presence of both TNMD and ranibizumab in the culture medium. The total length of the capillary-like structures per field was significantly shorter in the medium with TNMD than that of ranibizumab (P<0.01). The inhibitory effect of TNMD on tube formation in vitro angiogenesis was significantly stronger than that of ranibizumab.

CONCLUSION

TNMD may have stronger inhibitory effect than ranibizumab on in vitro angiogenesis.

[Effects of cryopreservation on biological characteristics of tendon-derived stem cells in rat patellar tendon]

Objective

To explore the effects of cryopreservation on the cell survival rate, cell viability, early apoptosis, migration ability, and tendon-related marker expression of tendon-derived stem cells (TDSCs) in rat patellar tendons.

Methods

The patellar tendon tissues were harvested from 12 4-month-old male Sprague Dawley rats; 12 patellar tendon tissues from 6 rats were cryopreserved (the experimental group), and the other 12 patellar tendon tissues were not treated (the control group). The patellar tendons were digested with 0.3% type I collagenase to obtain nucleated cells. The survival rate of nucleated cells was detected by trypan blue exclusion assay, and colony-forming ability by crystal violet staining. TDSCs were isolated and cultured to passage 3 (P3). The cell viability of TDSCs was detected by Alamar Blue method, the early apoptosis by Annexin V-FITC/PI assay, the cell migration ability by Transwell method, and the mRNA expressions of tendon-related markers [collagen type I (Col1α1), scleraxis (Scx), and tenomodulin (Tnmd)] by real-time quantitative PCR.

Results

The survival rate of nucleated cells was 91.00%±3.63% in the control group, and was 61.65%±4.76% in the experimental group, showing significant difference ( t=12.010, P=0.000). The formation of the primary nucleated cell clones was observed in 2 groups. At 12 days, the number of colonies forming of the experimental group [(8.41±0.33)/1 000 nucleated cells] was significantly lower than that of the control group [(15.19±0.47)/1 000 nucleated cells] ( t=28.910, P=0.000). The percentage of TDSCs in the active nucleated cells in the experimental group (1.37%±0.09%) was significantly lower than that in the control group (1.67%±0.10%) ( t=5.508, P=0.003). The growth trend of TDSCs (P3) in the 2 groups was consistent within 14 days. There was no significant difference in absorbance ( A) value between 2 groups at each time point ( P>0.05). The early apoptotic rate of TDSCs was 1.67%±0.06% in the experimental group and was 1.63%±0.06% in the control group, showing no significant difference ( t=0.707, P=0.519). Under microscope, TDSCs adhered to the lower chamber of the Transwell chamber; the number of cells was 445.00±9.70 in the experimental group and was 451.50±12.66 in the control group, showing no significant difference ( t=0.998, P=0.342). The relative mRNA expressions of Col1α1, Scx, and Tnmd were 3.498±0.065, 0.062±0.002, and (4.211±0.211)×10 ^-5 in the experimental group and were 3.499±0.113, 0.062±0.001, and (4.341±0.274)×10 ^-5 in the con-trol group, showing no significant difference ( t=0.013, P=0.991; t=0.042, P=0.969; t=0.653, P=0.549).

Conclusion

The survival rate of nucleated cells in cryopreserved rat tendon tissues is lower, but a large number of active TDSCs, and its cell viability, early apoptosis rate, migration ability in vitro, and cell tenogenic differentiation ability are remained.

Tenomodulin is Required for Tendon Endurance Running and Collagen I Fibril Adaptation to Mechanical Load

Tendons are dense connective tissues that attach muscles to bone with an indispensable role in locomotion because of their intrinsic properties of storing and releasing muscle- generated elastic energy. Tenomodulin (Tnmd) is a well-accepted gene marker for the mature tendon/ligament lineage and its loss-of -function in mice leads to a phenotype with distinct signs of premature aging on tissue and stem/progenitor cell levels. Based on these findings, we hypothesized that Tnmd might be an important factor in the functional performance of tendons. Firstly, we revealed that Tnmd is a mechanosensitive gene and that the C-terminus of the protein co-localize with collagen I-type fibers in the extracellular matrix. Secondly, using an endurance training protocol, we compared Tnmd knockout mice with wild types and showed that Tnmd deficiency leads to significantly inferior running performance that further worsens with training. In these mice, endurance running was hindered due to abnormal response of collagen I cross-linking and proteoglycan genes leading to an inadequate collagen I fiber thickness and elasticity. In sum, our study demonstrates that Tnmd is required for proper tendon tissue adaptation to endurance running and aids in better understanding of the structural-functional relationships of tendon tissues.

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Tnmd is a mechanosensitive gene and its protein is co-localized with collagen I fibers in the ECM of tendons.

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Tnmd knockout mice fail in endurance running tests, a phenotype that worsens with training.

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Tnmd knockout tendons had significantly thicker and stiffer collagen I fibers and altered crosslinking gene expression.

We performed a multidisciplinary approach to decipher the role of tenomodulin, a gene marker for the mature tendon lineage, in tendon functional performance. Loss-of-function in mice led to significantly inferior endurance running and detailed analyses revealed that tenomodulin is involved in the regulation of collagen I fiber structural and biomechanical properties in response to exercise. Our study expands the current view on the complex structural-functional relationships of tendon tissues, and tenomodulin expression levels may indicate whether an individual is suitable for a certain sport.

Scleraxis is required for maturation of tissue domains for proper integration of the musculoskeletal system

Scleraxis (Scx) is a basic helix-loop-helix transcription factor that is expressed persistently in tendons/ligaments, but transiently in entheseal cartilage. In this study, we generated a novel Scx^Cre knock-in (KI) allele, by in-frame replacement of most of Scx exon 1 with Cre recombinase (Cre), to drive Cre expression using Scx promoter and to inactivate the endogenous Scx. Reflecting the intensity and duration of endogenous expression, Cre-mediated excision occurs in tendinous and ligamentous tissues persistently expressing Scx. Expression of tenomodulin, a marker of mature tenocytes and ligamentocytes, was almost absent in tendons and ligaments of Scx^Cre/Cre KI mice lacking Scx to indicate defective maturation. In homozygotes, the transiently Scx-expressing entheseal regions such as the rib cage, patella cartilage, and calcaneus were small and defective and cartilaginous tuberosity was missing. Decreased Sox9 expression and phosphorylation of Smad1/5 and Smad3 were also observed in the developing entheseal cartilage, patella, and deltoid tuberosity of Scx^Cre/Cre KI mice. These results highlighted the functional importance of both transient and persistent expression domains of Scx for proper integration of the musculoskeletal components.

Current concepts on tenogenic differentiation and clinical applications

Tendon is a tissue that transmits force from muscle to bone. Chronic or acute tendon injuries are very common, and are always accompanied by pain and a limited range of motion in patients. In clinical settings, management of tendon injuries still remains a big challenge. Cell therapies, such as the application of stem cells for tenogenic differentiation, were suggested to be an ideal strategy for clinical translation. However, there is still a lack of specific methods for tenogenic differentiation due to the limited understanding of tendon biology currently. This review focuses on the summary of current published strategies for tenogenic differentiation, such as the application of growth factors, mechanical stimulation, biomaterials, coculture, or induced pluripotent stem cells. Current clinical applications of stem cells for treatment of tendon injuries and their limitations have also been discussed in this review.

Identification of Novel Equine (Equus caballus) Tendon Markers Using RNA Sequencing

Although several tendon-selective genes exist, they are also expressed in other musculoskeletal tissues. As cell and tissue engineering is reliant on specific molecular markers to discriminate between cell types, tendon-specific genes need to be identified. In order to accomplish this, we have used RNA sequencing (RNA-seq) to compare gene expression between tendon, bone, cartilage and ligament from horses. We identified several tendon-selective gene markers, and established eyes absent homolog 2 (EYA2) and a G-protein regulated inducer of neurite outgrowth 3 (GPRIN3) as specific tendon markers using RT-qPCR. Equine tendon cells cultured as three-dimensional spheroids expressed significantly greater levels of EYA2 than GPRIN3, and stained positively for EYA2 using immunohistochemistry. EYA2 was also found in fibroblast-like cells within the tendon tissue matrix and in cells localized to the vascular endothelium. In summary, we have identified EYA2 and GPRIN3 as specific molecular markers of equine tendon as compared to bone, cartilage and ligament, and provide evidence for the use of EYA2 as an additional marker for tendon cells in vitro.

Molecular characterization and function of tenomodulin, a marker of tendons and ligaments that integrate musculoskeletal components

Tendons and ligaments are dense fibrous bands of connective tissue that integrate musculoskeletal components in vertebrates. Tendons connect skeletal muscles to the bone and function as mechanical force transmitters, whereas ligaments bind adjacent bones together to stabilize joints and restrict unwanted joint movement. Fibroblasts residing in tendons and ligaments are called tenocytes and ligamentocytes, respectively. Tenomodulin (Tnmd) is a type II transmembrane glycoprotein that is expressed at high levels in tenocytes and ligamentocytes, and is also present in periodontal ligament cells and tendon stem/progenitor cells. Tnmd is related to chondromodulin-1 (Chm1), a cartilage-derived angiogenesis inhibitor, and both Tnmd and Chm1 are expressed in the CD31⁻ avascular mesenchyme. The conserved C-terminal hydrophobic domain of these proteins, which is characterized by the eight Cys residues to form four disulfide bonds, may have an anti-angiogenic function. This review highlights the molecular characterization and function of Tnmd, a specific marker of tendons and ligaments.

Tenogenic modulating insider factor: Systematic assessment on the functions of tenomodulin gene

Tenomodulin (TNMD, Tnmd) is a gene highly expressed in tendon known to be important for tendon maturation with key implications for the residing tendon stem/progenitor cells as well as for the regulation of endothelial cell migration in chordae tendineae cordis in the heart and in experimental tumour models. This review aims at providing an encompassing overview of this gene and its protein. In addition, its known expression pattern as well as putative signalling pathways will be described. A chronological overview of the discovered functions of this gene in tendon and other tissues and cells is provided as well as its use as a tendon and ligament lineage marker is assessed in detail and discussed. Last, information about the possible connections between TNMD genomic mutations and mRNA expression to various diseases is delivered. Taken together this review offers a solid synopsis on the up-to-date information available about TNMD and aids at directing and focusing the future research to fully uncover the roles and implications of this interesting gene.

A Prospectus of Tenomodulin

Tenomodulin is a type II transmembrane glycoprotein that can regulate growth of tendon. The human tenomodulin encoding gene is mapped to Х chromosome and encodes a polypeptide consisting of 317 alpha amino acids. The protein is located on the cell surface as N-glycosylated or non-N-glycosylated polypeptide with molecular mass of 45 and 40 kDa, respectively. The molecule consists of three domains and a short cytoplasmic tail at N-terminus. Tenomodulin is predominantly expressed in dense connective hypovascularized tissues such as tendons, skeletal muscle epimysium, and ligaments. Furthermore, tenomodulin is an efficient marker of tenocyte differentiation and plays an important role in the regulation of tenocyte proliferation, tendon development, and angiogenesis inhibition. A number of tenomodulin gene polymorphisms have been recently associated with a risk of obesity, diabetes, systemic immune mediators, cholesterol levels, Alzheimer disease, and age-related macular degeneration. Tenomodulin is involved in cell adhesion, determination of cell morphology, cell aging and bone mineral density. It is expected to play an important role in tendon recovery, tendon tissue engineering, anti-tumor therapy, treatment of chordal disruption, and remodeling of extracellular matrix.

Tendon-derived stem cells undergo spontaneous tenogenic differentiation

Tendon-derived stem cell (TDSC) is a subpopulation of residing stem cells within the intact tendon tissues, with the capacities of self-renewal, clonogenicity, and three-lineage differentiation. Compared with bone marrow derived mesenchymal stem cells (BMSCs), TDSCs are superior for tendon injuries repair as they remain some tendon tissue-specific differentiation properties. In the present study, TDSC was found to undergo spontaneous tenogenic differentiation in which the expression of tenogenic markers were increased while the expression of stemness markers decreased with time in TDSCs culture (without tenogenic induction medium). The further collagen synthesis ability was correspondingly increased during this process. After a longer period of culture, the monolayer of TDSCs formed a "3D" layers with rich extracellular matrices of typical tendon tissues. In addition, the key tenogenic transcription factors, such as Scx, Mkx, Egr1 and Eya1 were all up-regulated in this process. Finally, we compared the spontaneous tenogenic differentiation with TGF-β1-induced tenogenic differentiation of TDSCs, and the results showed that the spontaneous tenogenic differentiation of TDSCs was general character of TDSCs, similar to but weaker than the effect of TDSCs under tenogenic induction. Taken together, the present study identified that TDSCs had the potential of spontaneous tenogenic differentiation, which may be a better cell source for the treatment of tendon injury.

Cytotoxic and sublethal effects of silver nanoparticles on tendon-derived stem cells - implications for tendon engineering

Tendon injuries occur commonly in sports and workplace. Tendon-derived stem cells (TDSCs) have great potential for tendon healing because they can differentiate into functional tenocytes. To grow TDSCs properly in vivo, a scaffold is needed. Silver nanoparticles (AgNPs) have been used in a range of biomedical applications for their anti-bacterial and -inflammatory effects. AgNPs are therefore expected to be a good scaffolding coating material for tendon engineering. Yet, their cytotoxicity in TDSCs remains unknown. Moreover, their sublethal effects were mysterious in TDSCs. In our study, decahedral AgNPs (43.5 nm in diameter) coated with polyvinylpyrrolidone (PVP) caused a decrease in TDSCs' viability beginning at 37.5 μg ml-1 but showed non-cytotoxic effects at concentrations below 18.8 μg ml-1. Apoptosis was observed in the TDSCs when higher doses of AgNPs (75-150 μg ml-1) were used. Mechanistically, AgNPs induced reactive oxygen species (ROS) formation and mitochondrial membrane potential (MMP) depolarization, resulting in apoptosis. Interestingly, treating TDSCs with N-acetyl-l-cysteine (NAC) antioxidant significantly antagonized the ROS formation, MMP depolarization and apoptosis indicating that ROS accumulation was a prominent mediator in the AgNP-induced cytotoxicity. On the other hand, AgNPs inhibited the tendon markers' mRNA expression (0-15 μg ml-1), proliferation and clonogenicity (0-15 μg ml-1) in TDSCs under non-cytotoxic concentrations. Taken together, we have reported here for the first time that the decahedral AgNPs are cytotoxic to rat TDSCs and their sublethal effects are also detrimental to stem cells' proliferation and tenogenic differentiation. Therefore, AgNPs are not a good scaffolding coating material for tendon engineering.

Expression of DMP1 in the developing mouse tongue embryo

Dentin matrix protein 1 (DMP-1) is an important factor in the mineralization of hard tissues. However, it has many other functions in addition to the regulation of mineralized tissues. We analyzed the expression and localization of DMP-1 by immunohistochemical staining and in situ hybridization in the developing mouse tongue during embryonic days 12.5 (E12.5), E14.5, E17.5, and E18.5. We also detected the mRNA abundance of tongue morphogenesis markers such as FGF6, TGF-β1, Collagen I, osteocalcin, chondromodulin 1, tenomodulin, Vascular endothelial growth factor (VEGF), caspase-3, and Aifm from embryonic stages by real-time RT-PCR. The antisense probe for DMP-1 was detected in a few mesenchymal cells surrounding blood vessels at E12.5, and faint localization was seen at E18.5 in the embryonic mouse tongue by in situ hybridization. The DMP-1 and osteocalcin abundance levels gradually increased compared with the other tongue markers from E12.5 to E18.5 (p<0.001). Cluster analyses identified the following distinct clusters for mRNA abundance in the tongue: cluster 1, E12.5; cluster 2, E14.5 and E17.5; and cluster 3, E18.5. The positive correlation between DMP-1 and osteocalcin (Pearson's r=0.685; p<0.05) and negative correlation between DMP-1 and Caspase-3 (Pearson's r=-0.632; p<0.05) were analyzed. These data suggested that DMP-1 potentially influences osteocalcin and Caspase-3 during mouse tongue development and morphogenesis. DMP-1 also affects the angiogenic marker VEGF in specific stages and areas, terminating the differentiation of the tongue from other developing tissues. We conclude that DMP-1 may be involved in regulating the temporal expression at embryonic stages in the mouse tongue.

Metabolic and cytoprotective effects of in vivo peri-patellar hyaluronic acid injections in cultured tenocytes

The purpose of this study was to investigate tenocyte mechanobiology after sudden-detraining and to examine the hypothesis that repeated peri-patellar injections of hyaluronic acid (HA) on detrained patellar tendon (PT) may reduce and limit detrained-associated damage in tenocytes. Twenty-four male Sprague-Dawley rats were divided into three groups: Untrained, Trained and Detrained. In the Detrained rats, the left tendon was untreated while the right tendon received repeated peri-patellar injections of either HA or saline (NaCl). Tenocyte morphology, metabolism and synthesis of C-terminal-propeptide of type I collagen, collagen-III, fibronectin, aggrecan, tenascin-c, interleukin-1β, matrix-metalloproteinase-1 and-3 were evaluated after 1, 3, 7 and 10 days of culture. Transmission-electronic-microscopy showed a significant increase in mitochondria and rough endoplasmic reticulum in cultured tenocytes from Detrained-HA with respect to those from Detrained-NaCl. Additionally, Detrained-HA cultures showed a significantly higher proliferation rate and viability, and increased synthesis of C-terminal-Propeptide of type I collagen, fibronectin, aggrecan, tenascin-c and matrix-metalloproteinase-3 with respect to Detrained-NaCl ones, whereas synthesis of matrix-metalloproteinase-1 and interleukin-1β was decreased. Our study demonstrates that discontinuing training activity in the short-term alters tenocyte synthetic and metabolic activity and that repeated peri-patellar infiltrations of HA during detraining allow the maintenance of tenocyte anabolic activity.

Expression of tenocyte lineage-related factors in regenerated tissue at sites of tendon defect

BACKGROUND

The healing mechanism of ruptured or injured tendons is poorly understood. To date, some lineage-specific factors, such as scleraxis and tenomodulin, have been reported as markers of tenocyte differentiation. Because few studies have focused on tenocyte lineage-related factors with respect to the repaired tissue of healing tendons, the aim of this study was to investigate their expression during the tendon healing process.

METHODS

Defects were created in the patellar tendons of rats, and the patellae and patellar tendons were harvested at 3 days and at 1, 2, 3, 6, 12, and 20 weeks after surgery. They were studied using micro-computed tomography, and paraffin-embedded sections were then prepared for histological evaluation. Reverse transcription-polymerase chain reactions were performed to analyze the expression of genes related to the tenocyte lineage, chondrogenesis, and ossification.

RESULTS

Repaired tissue became increasingly fibrous over time and contained a greater number of vessels than normal tendons, even in the later period. Safranin O staining revealed the existence of proteoglycan at 1 week and its persistence through 20 weeks. Ossification was detected in all tendons at 12 weeks. The expression of tenocyte lineage-related genes was high at 1 and 2 weeks. Chondrogenic genes were up-regulated until 6 weeks. Runt-related transcription factor 2, an osteogenic gene, was up-regulated at 20 weeks.

CONCLUSIONS

In our tendon defect model, cells participating in the tendon healing process appeared to differentiate toward tenocyte lineage only in the early phase, and chondrogenesis seemed to occur from the early phase onward. To improve tendon repair, it will be necessary to promote and maintain tenogenesis and to inhibit chondrogenesis, especially in the early phase, in order to avoid erroneous differentiation of stem cells.

Loss of tenomodulin results in reduced self-renewal and augmented senescence of tendon stem/progenitor cells

Tenomodulin (Tnmd) is a well-known gene marker for the tendon and ligament lineage, but its exact functions in these tissues still remain elusive. In this study, we investigated Tnmd loss of function in mouse tendon stem/progenitor cells (mTSPC) by implicating a previously established Tnmd knockout (KO) mouse model. mTSPC were isolated from control and Tnmd KO tail tendons and their stemness features, such as gene marker profile, multipotential, and self-renewal, were compared. Immunofluorescence and reverse transcriptase-polymerase chain reaction analyses for stem cell-, tenogenic-, osteogenic-, and chondrogenic-related genes confirmed their stemness and lineage specificity and demonstrated no profound differences between the two genotypes. Multipotential was not significantly affected since both cell types differentiated successfully into adipogenic, osteogenic, and chondrogenic lineages. In contrast, self-renewal assays validated that Tnmd KO TSPC exhibit significantly reduced proliferative potential, which was also reflected in lower Cyclin D1 levels. When analyzing possible cellular mechanisms behind the observed decreased self-renewability of Tnmd KO TSPC, we found that cellular senescence plays a major role, starting earlier and cumulating more in Tnmd KO compared with control TSPC. This was accompanied with augmented expression of the cell cycle inhibitor p53. Finally, the proliferative effect of Tnmd in TSPC was confirmed with transient transfection of Tnmd cDNA into Tnmd KO TSPC, which rescued their proliferative deficit. Taken together, we can report that loss of Tnmd affects significantly the self-renewal and senescence properties, but not the multipotential of TSPC.

Tenomodulin regulated the compartments of embryonic and early postnatal mouse masseter muscle

The masseter muscle (MM) is a complex tendinous laminar structure during development; however, the stage of the laminar structure formation is unknown. Tenomodulin (TeM) is a useful marker of tendons and has an anti-angiogenic cysteine-rich C-terminal domain. Therefore, we analyzed mRNA of TeM and angiogenesis markers (CD31 and vascular endothelial growth factor (VEGF)) and performed in situ hybridization for the TeM genes in MM from on embryonic day 12.5 (E12.5) to postnatal day 5 (P5). The TeM expression is at first detectable in the middle region of the mesenchymal connective tissue in the MM at E 12.5. The expression domains of the TeM during development typically include the middle region of the MM, particularly surrounding the vascular regions. The level of TeM mRNA in the MM increased from E12.5 to E17.5 and decreased after birth. In contrast, the levels of CD31 and VEGF mRNAs were almost constant from E12.5 to E18.5 and then low from birth onward. Therefore, the development of the laminar tendinous structure in the middle region between superficial and deeper regions of the MM first occurs during the process of tendon formation at embryonic day 12.5. In our study of MM development, the laminar structure regulating TeM also prevents vascular invasion during the formation of compartment of the MM. The tendon may relate to the components of muscle mass of MM.