Search

We introduce MSdb (https://www.msdb.org.cn), a database for visualization and integrated analysis of next-generation sequencing data from human musculoskeletal system, along with manually curated patient phenotype data. Systematic categorizing, standardized processing and freely accessible knowledge enables the reuse of public data. MSdb provides various types of analysis, including sample-level browsing of metadata information, gene/miRNA expression, and single-cell RNA-seq dataset. In addition, MSdb also allows integrated analysis for cross-samples and cross-omics analysis, including customized differentially-expressed gene/microRNA analysis, microRNA-gene network, scRNA-seq cross-sample/disease integration, and gene regulatory network analysis.

Cartilage defects are one of the most common symptoms of osteoarthritis (OA), a degenerative disease that affects millions of people world-wide and places a significant socio-economic burden on society. Hydrogels, which are a class of biomaterials that are elastic, and display smooth surfaces while exhibiting high water content, are promising candidates for cartilage regeneration. In recent years, various kinds of hydrogels have been developed and applied for the repair of cartilage defects in vitro or in vivo, some of which are hopeful to enter clinical trials. In this review, recent research findings and developments of hydrogels for cartilage defects repair are summarized. We discuss the principle of cartilage regeneration, and outline the requirements that have to be fulfilled for the deployment of hydrogels for medical applications. We also highlight the development of advanced hydrogels with tailored properties for different kinds of cartilage defects to meet the requirements of cartilage tissue engineering and precision medicine., Graphical abstract Hydrogels applied in joints for the repair of cartilage defects and cartilage regeneration. Hydrogels are three-dimensional hydrophilic polymer networks that can be fabricated from synthetic polymers, polysaccharides, protein, and peptides, and can be designed as injectable or in situ forming for practical clinical application.Image 1, Highlights • The biotechnology of developing hydrogels for cartilage defects repair is promising. • The principle for cartilage regeneration using hydrogels and requirements for clinical transformation are summarized. • Advanced hydrogels with tailored properties for different kinds of cartilage defects are discussed.

Antarctic krill (Euphausia superba) is an important source of biomass and high-quality protein. However, heat treatment of Antarctic krill negatively impacts its quality and compromises its utilization in the food industry. This study aimed to investigate the mechanisms underlying changes in Antarctic krill meat characteristics and physicochemical properties treated at different temperatures and holding times. Findings indicate that at higher temperatures and holding times, hardness and cooking loss of Antarctic krill meat increased dramatically. The low-field nuclear magnetic resonance (LF-NMR) analysis revealed that the loss of immobile water increased, whereas SDS-PAGE analysis showed that the content of myosin heavy chain decreased significantly, and that protein degradation occurred. Fourier transform infrared spectroscopy (FT-IR) and intrinsic fluorescence spectra indicated that α-helix motifs were transformed into β-sheets, and that more hydrophobic groups were exposed. The scanning electron microscopy (SEM) results showed that Antarctic krill meat formed corrugated folded regions after heat treatment without forming a three-dimensional water-entrapped structure, which led to significant water loss, resulting in rapid deterioration Antarctic krill meat. These results provide the basis for a deeper understanding of processing characteristics of Antarctic krill meat.

Cartilage adheres to subchondral bone via a specific osteochondral interface tissue where forces are transferred from soft cartilage to hard bone without conferring fatigue damage over a lifetime of load cycles. However, the fine structure and mechanical properties of the osteochondral interface tissue remain unclear. Here, we identified an ultrathin ∼20-30 μm graded calcified region with two-layered micronano structures of osteochondral interface tissue in the human knee joint, which exhibited characteristic biomolecular compositions and complex nanocrystals assembly. Results from finite element simulations revealed that within this region, an exponential increase of modulus (3 orders of magnitude) was conducive to force transmission. Nanoscale heterogeneity in the hydroxyapatite, coupled with enrichment of elastic-responsive protein-titin, which is usually present in muscle, endowed the osteochondral tissue with excellent mechanical properties. Collectively, these results provide novel insights into the potential design for high-performance interface materials for osteochondral interface regeneration.

While the capacity to regenerate tissues or limbs is limited in mammals, including humans, axolotls are able to regrow entire limbs and major organs after incurring a wound. The wound blastema has been extensively studied in limb regeneration. However, due to the inadequate characterization of ECM and cell subpopulations involved in the regeneration process, the discovery of the key drivers for human limb regeneration remains unknown. In this study, we applied large-scale single-cell RNA sequencing to classify cells throughout the adult axolotl limb regeneration process, uncovering a novel regeneration-specific mitochondria-related cluster supporting regeneration through energy providing and the ECM secretion (COL2+) cluster contributing to regeneration through cell-cell interactions signals. We also discovered the dedifferentiation and re-differentiation of the COL1+/COL2+ cellular subpopulation and exposed a COL2-mitochondria subcluster supporting the musculoskeletal system regeneration. On the basis of these findings, we reconstructed the dynamic single-cell transcriptome of adult axolotl limb regenerative process, and identified the novel regenerative mitochondria-related musculoskeletal populations, which yielded deeper insights into the crucial interactions between cell clusters within the regenerative microenvironment.

Epigenetic regulation is highly correlated with osteoarthritis (OA) development, whereas its role and detailed mechanisms remain elusive. In this study, we explored the expression of EZH2, an H3K27me3 transferase, in human OA cartilages and its roles in regulating OA pathogenesis. Here, we found EZH2 was highly expressed in both mice and human OA cartilage samples by using histological analysis and RNA sequencing (RNA-Seq). The medial meniscectomy (MMx) OA model results indicated the conditional knockout of Ezh2 deteriorated OA pathological conditions. Furthermore, we showed the positive role of Ezh2 in cartilage wound healing and inhibition of hypertrophy through activating TNFSF13B, a member of the tumor necrosis factor superfamily. Further, we also indicated that the effect of TNFSF13B, increased by Ezh2, might boost the healing of chondrocytes through increasing the phosphorylation of Akt. Taken together, our results uncovered an EZH2-positive subpopulation existed in OA patients, and that EZH2-TNFSF13B signaling was responsible for regulating chondrocyte healing and hypertrophy. Thus, EZH2 might act as a new potential target for OA diagnosis and treatment. © 2020 American Society for Bone and Mineral Research.

Tendon heterotopic ossification (HO) is characterized by bone formation inside tendon tissue, which severely debilitates people in their daily life. Current therapies fail to promote functional tissue repair largely due to our limited understanding of HO pathogenesis. Here, we investigate the pathological mechanism and propose a potential treatment method for HO. Immunofluorescence assays showed that the Mohawk (MKX) expression level was decreased in human tendon HO tissue, coinciding with spontaneous HO and the upregulated expression of osteochondrogenic and angiogenic genes in the tendons of Mkx−/− mice. Single-cell RNA sequencing analyses of wild-type and Mkx−/− tendons identified three cell types and revealed the excessive activation of osteochondrogenic genes during the tenogenesis of Mkx−/− tendon cells. Single-cell analysis revealed that the gene expression program of angiogenesis, which is strongly associated with bone formation, was activated in all cell types during HO. Moreover, inhibition of angiogenesis by the small-molecule inhibitor BIBF1120 attenuated bone formation and angiogenesis in the Achilles tendons of both Mkx mutant mice and a rat traumatic model of HO. These findings provide new insights into the cellular mechanisms of tendon HO and highlight the inhibition of angiogenesis with BIBF1120 as a potential treatment strategy for HO.

The quality deterioration of Antarctic krill (Euphausia superba) after thermal processing limits its industrial application. This study sought to improve the texture characteristics of Antarctic krill after heat treatment through pre-soaking using l-lysine (Lys) solution and sodium tripolyphosphate (STPP). Moreover, the effects of Lys on heat-treated Antarctic krill were explored.Lys significantly reduced the cooking loss and improved the texture characteristics of Antarctic krill during heat treatment. The low-field nuclear magnetic resonance results showed that Lys reduced the water loss of Antarctic krill during heat treatment. Additionally, the surface hydrophobicity, Fourier transform infrared spectroscopy, and endogenous fluorescence spectroscopy results showed that Lys could inhibit the structural damage of Antarctic krill protein under the thermal denaturation condition and enhance the thermal stability of the protein. The scanning electron microscopy results showed that Lys could protect the structural integrity of Antarctic krill muscle fibers during heat treatment.The cooking loss in the Lys added groups was better than the sodium tripolyphosphate added group, and 2.0% Lys solution could minimize the cooking loss of Antarctic krill. The secondary and tertiary structures of the Antarctic krill protein were actively protected by Lys during heat treatment. Overall, the study will provide insights into the application of Lys in the food industry as a natural additive and an alternative to phosphate. © 2021 Society of Chemical Industry.

Irregular partial-thickness cartilage defect is a common pathogenesis of osteoarthritis (OA) with no available treatment in clinical practice. Currently, cartilage tissue engineering is only suitable for a limited area of full-thickness cartilage defect. Here, we design a biomimetic joint paint for the intractable partial-thickness cartilage defect repair. The joint paint, composed of a bridging layer of chondroitin sulfate and a surface layer of gelatin methacrylate with hyaluronic acid, can quickly and tightly adhere to the cartilage defect by light activation. Being treated by the joint paint, the group of rabbit and pig models with partial-thickness cartilage defects showed a restoration of a smooth cartilage surface and the preservation of normal glycosaminoglycan content, whereas the untreated control group exhibited serious progressive OA development. This paint treatment functions by prohibiting chondrocyte apoptosis, maintaining chondrocyte phenotype, and preserving the content of glycosaminoglycan in the partial-thickness cartilage defects. These findings illustrated that the biomimetic joint paint is an effective and revolutionary therapeutics for the patients with noncurable partial-thickness cartilage defects.

Stem cell therapies are unsatisfactory due to poor cell survival and engraftment. Stem cell used for therapy must be properly “tuned” for a harsh in vivo environment. Herein, we report that transfer of exogenous mitochondria (mito) to adipose‐derived mesenchymal stem cells (ADSCs) can effectively boost their energy levels, enabling efficient cell engraftment. Importantly, the entire process of exogeneous mitochondrial endocytosis is captured by high‐content live‐cell imaging. Mitochondrial transfer leads to acutely enhanced bioenergetics, with nearly 17% of higher adenosine 5′‐triphosphate (ATP) levels in ADSCs treated with high mitochondrial dosage and further results in altered secretome profiles of ADSCs. Mitochondrial transfer also induced the expression of 334 mRNAs in ADSCs, which are mainly linked to signaling pathways associated with DNA replication and cell division. We hypothesize that increase in ATP and cyclin‐dependent kinase 1 and 2 expression might be responsible for promoting enhanced proliferation, migration, and differentiation of ADSCs in vitro. More importantly, mito‐transferred ADSCs display prolonged cell survival, engraftment and horizontal transfer of exogenous mitochondria to surrounding cells in a full‐thickness skin defect rat model with improved skin repair compared with nontreated ADSCs. These results demonstrate that intracellular mitochondrial transplantation is a promising strategy to engineer stem cells for tissue regeneration.

Adverse events related to surgical mesh implants such as mesh erosion and adhesion are inflammation-related and frequently reported in clinical use; thus, advanced real-time monitoring methods are required to evaluate the interactions between implants and host immune system in order to prevent any unfavorable events. Here, we performed single cell mass cytometry of the blood, characterizing two biomaterial-induced (silk and polypropylene) immune responses in a rodent ventral hernia model to systematically decipher the interactions between the host and implants. Compared to polypropylene, silk mesh induced less immune cell activation with lower proportions of macrophages, monocytes, and neutrophils in the innate system and lower proportions of CD4+ T cells and CD8+ T cells in the adaptive system at 3-week post-implantation. Functional markers (CD115, CD27, and CD62L) involved in the immune cell activation and migration were also downregulated in the silk group. Histological analysis of tissues confirmed that silk mesh implantation led to decreased neutrophils and macrophages recruitment, less fibrosis formation, and better tissue healing, suggesting silk mesh as a superior implant material. Importantly, in patients with abdominal hernia, the ratio of monocytes in blood in the silk group was also significantly lower when compared to the polypropylene group. Our study provides a systematical profile of the material-based foreign body response at single cell resolution, highlighting the impact of biomaterial implants on whole body immune system and introducing an innovative approach to monitor material-based immune response.

Characterized by their slow adhering property, skeletal muscle myogenic progenitor cells (MPCs) have been widely utilized in skeletal muscle tissue engineering for muscle regeneration, but with limited efficacy. Skeletal muscle regeneration is regulated by various cell types, including a large number of rapidly adhering cells (RACs) where their functions and mechanisms are still unclear. In this study, we explored the function of RACs by co-culturing them with MPCs in a biomimetic skeletal muscle organoid system. Results showed that RACs promoted the myogenic potential of MPCs in the organoid. Single-cell RNA-Seq was also performed, classifying RACs into 7 cell subtypes, including one newly described cell subtype: teno-muscular cells (TMCs). Connectivity map of RACs and MPCs subpopulations revealed potential growth factors (VEGFA and HBEGF) and extracellular matrix (ECM) proteins involvement in the promotion of myogenesis of MPCs during muscle organoid formation. Finally, trans-well experiments and small molecular inhibitors blocking experiments confirmed the role of RACs in the promotion of myogenic differentiation of MPCs. The RACs reported here revealed complex cell diversity and connectivity with MPCs in the biomimetic skeletal muscle organoid system, which not only offers an attractive alternative for disease modeling and in vitro drug screening but also provides clues for in vivo muscle regeneration.

Adipose tissue is a highly dynamic organ with complex cellular composition. Aging induces adipose tissue function decline and relocation of peripheral adipose tissue to abdominal compartment, which often associated with inflammation and metabolic disorders. Here we performed single-cell RNA sequencing to comprehensively and unbiasedly deconvolve how subcutaneous adipose tissue (SAT) responses to aging. We collected >25,000 stromal vascular cells from abdominal and gluteofemoral SAT of young and old donors. Analyses of transcription signatures and cell networks uncovered impaired adipogenesis and extracellular matrix synthesis capacity of APC, altered metabolic phenotype of immune cells and shifted tissue-dominating cells that can be used to predict adipose tissue aging. We also reported aging-associated distinct transcriptional program between gluteofemoral SAT and abdominal SAT. Our work thus reveals unanticipated cellular, immunological, metabolic and site-specific aspects of human adipose tissues aging process, providing valuable resource for better understanding of aging-associated adipose tissue dysfunction.

Tendon maturation lays the foundation for postnatal tendon development, its proper mechanical function, and regeneration, but the critical cell populations and the entangled mechanisms remain poorly understood. Here, by integrating the structural, mechanical, and molecular properties, we show that post-natal days 7-14 are the crucial transitional stage for mouse tendon maturation. We decode the cellular and molecular regulatory networks at the single-cell level. We find that a nerve growth factor (NGF)-secreting Cd9

Stem cell especially mesenchymal stem cell (MSC) transplantation is a promising therapeutic strategy for myocardial infarction (MI) repair. However, major obstacles remain due to the poor retention and survival rate of transplanted MSCs in the harmful MI microenvironment (e.g., oxidative stress and hypoxia). Here, a novel injectable hydrogel (RCGel) with dual functions of reactive oxygen species (ROS)-scavenging and O2-generating was employed to encapsulate MSCs for MI treatment. The RCGel encapsulating exhibited anti-ROS protection through inhibiting JNK/p38 apoptosis signaling pathway to improve the viability of MSCs under oxidative stress conditions in vitro. The survival of cardiomyocytes was also improved both in the oxidative stress and hypoxia environments when being co-cultured with MSCs-encapsulated RCGel (MSC/RCGel). The RCGel encapsulating boosted the engraftment of transplanted MSCs in vivo. More viable MSCs endowed with regenerative abilities in the infarcted rat heart along with the ameliorated MI microenvironment (decreased oxidative stress and hypoxia) by the RCGel substantially inhibited cardiac apoptosis, enhanced cardiomyocyte viability, promoted angiogenesis and reduced fibrosis to improve the cardiac functions. The encapsulation of MSCs in RCGel, which exhibits beneficial effects in resisting harsh environments, provides a new way in MI repair and stem cell delivery, and has great potential in cell-based regenerative medicine.

Although being of utmost importance for human health and mobility, stem cell identity and hierarchical organization of musculoskeletal progenitors remain largely unexplored. Here, cells from E10.5, E12.5, and E15.5 murine limbs are analyzed by high throughput single‐cell RNA sequencing to illustrate the cellular architecture during limb development. Single‐cell transcriptional profiling demonstrates the identity and differentiation architecture of musculoskeletal stem cells (MSSC), soft and hard tissue progenitors through expression pattern of musculoskeletal markers (scleraxis [Scx], Hoxd13, Sox9, and Col1a1). This is confirmed by genetic in vivo lineage tracing. Moreover, single‐cell analyses of Scx knockout mice tissues illustrates that Scx regulates MSSC self‐renewal and proliferation potential. A high‐throughput and low‐cost multi‐tissues RNA sequencing strategy further provides evidence that musculoskeletal system tissues, including muscle, bone, meniscus, and cartilage, are all abnormally developed in Scx knockout mice. These results establish the presence of an indispensable limb Scx+Hoxd13+ MSSC population and their differentiation into soft tissue progenitors (Scx+Col1a1+) and hard tissue progenitors (Scx+Sox9+). Collectively, this study paves the way for systematically decoding the complex molecular mechanisms and cellular programs of musculoskeletal tissues morphogenesis in limb development and regeneration., This study presents a comprehensive genomics resource that reveals the transcriptome dynamics of the developing limb. Single‐cell transcriptional profiling with lineage tracing data demonstrates the identity and differentiation trajectory of musculoskeletal stem cells into soft and hard tissues. These results emphasize an indispensable role of limb Scx+ musculoskeletal stem cell population and provide additional perspective on musculoskeletal biology.

Wildfire has increasingly profound and pervasive consequences for forest ecosystems via directly altering soil physicochemical properties and modulating microbial community. In this study, we examined the changes in soil properties and microbial community at different periods after highly severe wildfire events (44 plots, 113 samples) in the Chinese Great Khingan Mountains. We also separated charcoals from burnt soils to establish the relationship between soil microbes and the microbes colonized on the charcoal. Wildfire significantly altered bacterial and fungal community structures across a 29-year chronosequence. The network analysis revealed that from 17 years after fire, the complexity and connectivity of bacterial and fungal communities were significantly increased. Differential abundance analysis suggested that bacterial and fungal OTUs were enriched or depleted only during 0-4 years after fire. In addition, soil factors, including soil pH, total C and N, soil water content, and dissolved C and N, are key determinants of soil bacterial and fungal communities from 17 years after fire. The fire-derived charcoals provided a new and unusual niche for microbial colonization and charcoal microbes had a significantly different community structure from the burnt soil microbes. Our data suggest that soil bacterial and fungal communities changed dramatically during the recovery from fire events in terms of the abundance and co-occurrence networks in the boreal forest ecosystems.ImportancePervious research has reported fire altered soil microbial community composition and function during short-term succession in boreal forests. However, the long-term effect of fire and fire-derived charcoals which are regarded as fire legacy effect on soil bacterial and fungal communities composition and structure have not previously been shown. Understanding how soil microbes particularly the keystone taxa and determinative soil factors, respond to fire and its legacy matter charcoal, is critical for predicting how future fire influences soil nutrient transformations and biological processes. We accessed time chronosequence to examine the effect of fire history on soil microbial abundance and co-occurrence network. These findings suggest that soil microbes can be reshaped by fire and its legacy effect of fire-derived charcoal even in the long periods after fire and provide further insights into fire and its legacy effect.

Background In this work, low-field nuclear magnetic resonance (LF-NMR) and magnetic resonance imaging (MRI) were used to investigate the changes in protons (from water and oil) distribution of mackerel during frying process. The relationship between protons migration and some physicochemical indexes was established by partial least squares regression (PLSR). The changing mechanism of the quality characteristics and physicochemical properties of fish meat under different frying conditions was analyzed by LF-NMR combined with PLSR, which provided theoretical support for the development of canned mackerel food. Results LF-NMR results showed that three kinds of T2 protons assigned to protein-water interaction (T21 ), multilayer bound water (T22 ), oil and free water (T23 ), respectively. As the frying temperature increased, the protons from T22 peak significantly decreased, while the protons from T23 peak remarkably increased. The microstructure of fried mackerel was destroyed, the cooking loss, oil content, a* value, b* value, hardness and chewiness increased, and the protein content and L* value decreased. Furthermore, the partial least squares regression (PLSR) analysis revealed that significant correlation was observed between the cooking loss, TPA parameter (chewiness), color parameter (L*) and LF-NMR parameters. Conclusion Different frying temperature and time had a strong effect on the physicochemical properties of mackerel. Good prediction models could be established by protons migration using the LF-NMR technique and PLSR for the fried mackerel. The quality control of fried fish could be realized by monitoring the change of LF-NMR data. This article is protected by copyright. All rights reserved.

Bone tissue is essential for supporting and protecting our bodies. When bone fracture occurs, the loss of local conditions may cause a delay in healing, nonunion of the bone, or a persistent bone defect. Mesenchymal stem cell (MSC)-based cell therapy is a promising approach for fracture repair. MSCs are cultured, alone or on biomaterials, combined with cytokines, and supplied to the injury site. This chapter covers current understanding of MSC function in bone fracture healing, including the source of MSCs for fracture repair, internal (cytokines and chemokines) and external (physical stimulation) factors that affect MSC homing, and tissue-engineering applications of MSCs (immunogenicity, isolation/treatment approach, genetic modification, materials/growth factors, and clinical results). Here we aim to summarize clues to MSC application in fracture repair, to lay a foundation of stem cell–based therapy for future bone regeneration.

Anterior cruciate ligament (ACL) is one of the most difficult tissues to heal once injured. Ligament regeneration and tendon-bone junction healing are two major goals of ACL reconstruction. This study aimed to investigate the synergistic therapeutic effects of Stromal cell-derived factor 1 (SDF-1)-releasing collagen-silk (CSF) scaffold combined with intra-articular injection of ligament-derived stem/progenitor cells (LSPCs) for ACL regeneration and the amelioration in the long-term complication of osteoarthritis (OA). The stem cell recruitment ability of CSF scaffold and the multipotency, particularly the tendon forming ability of LSPCs from rabbits were characterized in vitro, while the synergistic effect of the CSF scaffold and LSPCs for ACL regeneration and OA amelioration were investigated in vivo at 1, 3, and 6 months with a rabbit ACL reconstruction model. The CSF scaffold was used as a substitute for the ACL, and LSPCs were injected into the joint cavity after 7 days of the ACL reconstruction. CSF scaffold displayed a controlled release pattern for the encapsulated protein for up to 7 days with an increased stiffness in the mechanical property. LSPCs, which exhibited highly I Collagen and CXCR4 expression, were attracted by SDF-1 and successfully relocated into the CSF scaffold at 1 month in vivo. At 3 and 6 months post-treatment, the CSF scaffold combined with LSPCs (CSFL group) enhanced the regeneration of ACL tissue, and promoted bone tunnel healing. Furthermore, the OA progression was impeded efficiently. Our findings here provided a new strategy that using stem cell recruiting CSF scaffold with tissue-specific stem cells, could be a promising solution for ACL regeneration. Statement of Significance In this study, we developed a silk scaffold with increased stiffness and SDF-1 controlled release capacity for ligament repair. This advanced scaffold transplantation combined with intra-articular injection of LSPCs (which was isolated from rabbit ligament for the first time in this study) promoted the regeneration of both the tendinous and bone tunnel portion of ACL. This therapeutic strategy also ameliorated cartilage degeneration and reduced the severity of arthrofibrosis. Hence, combining LSPCs injection with SDF-1-releasing silk scaffold is demonstrated as a therapeutic strategy for ACL regeneration and OA treatment in the clinic. Download : Download high-res image (146KB) Download : Download full-size image

The interplay between cells and materials is a fundamental topic in biomaterial-based tissue regeneration. One of the principles for biomaterial development in tendon regeneration is to stimulate tenogenic differentiation of stem cells. To this end, efforts have been made to optimize the physicochemical and bio-mechanical properties of biomaterials for tendon tissue engineering. However, recent progress indicated that innate immune cells, especially macrophages, can also respond to the material cues and undergo phenotypical changes, which will either facilitate or hinder tissue regeneration. This process has been, to some extent, neglected by traditional strategies and may partially explain the unsatisfactory outcomes of previous studies; thus, more researchers have turned their focus on developing and designing immunoregenerative biomaterials to enhance tendon regeneration. In this review, we will first summarize the effects of material cues on tenogenic differentiation and paracrine secretion of stem cells. A brief introduction will also be made on how material cues can be manipulated for the regeneration of tendon-to-bone interface. Then, we will discuss the characteristics and influences of macrophages on the repair process of tendon healing and how they respond to different materials cues. These principles may benefit the development of novel biomaterials provided with combinative bioactive cues to activate tenogenic differentiation of stem cells and pro-resolving macrophage phenotype. Statement of Significance The progress achieved with the rapid development of biomaterial-based strategies for tendon regeneration has not yielded broad benefits to clinical patients. In addition to the interplay between stem cells and biomaterials, the innate immune response to biomaterials also plays a determinant role in tissue regeneration. Here, we propose that fine-tuning of stem cell behaviors and alternative activation of macrophages through material cues may lead to effective tendon/ligament regeneration. We first review the characteristics of key material cues that have been manipulated to promote tenogenic differentiation and paracrine secretion of stem cells in tendon regeneration. Then, we discuss the potentiality of corresponding material cues in activating macrophages toward a pro-resolving phenotype to promote tissue repair.

Tendon repair is a clinical challenge because of the limited understanding on tenogenesis. The synthesis of type I collagen (Collagen I) and other extracellular matrix are essential for tendon differentiation and homeostasis. Current studies on tenogenesis focused mostly on the tenogenic transcriptional factors while the signaling controlling tenogenesis on translational level remains largely unknown. Here, we showed that mechanistic target of rapamycin (mTOR) signaling was activated by protenogenic growth factor, transforming growth factors beta1, and insulin-like growth factor-I. The expression of mTOR was upregulated during tenogenesis of mesenchymal stem cells (MSCs). Moreover, mTOR was downregulated in human tendinopathy tissues and was inactivated upon statin treatment. Both inhibition and depletion of AKT or mTOR significantly reduced type I collagen production and impaired tenogenesis of MSCs. Tendon specific-ablation of mTOR resulted in tendon defect and reduction of Collagen I. However, there is no evident downregulation of tendon associated collagens at the transcription level. Our study demonstrated that AKT-mTOR axis is a key mediator of tendon differentiation and provided a novel therapeutic target for tendinopathy and tendon injuries.

Summary Articular cartilage injury and degeneration causing pain and loss of quality-of-life has become a serious problem for increasingly aged populations. Given the poor self-renewal of adult human chondrocytes, alternative functional cell sources are needed. Direct reprogramming by small molecules potentially offers an oncogene-free and cost-effective approach to generate chondrocytes, but has yet to be investigated. Here, we directly reprogrammed mouse embryonic fibroblasts into PRG4+ chondrocytes using a 3D system with a chemical cocktail, VCRTc (valproic acid, CHIR98014, Repsox, TTNPB, and celecoxib). Using single-cell transcriptomics, we revealed the inhibition of fibroblast features and activation of chondrogenesis pathways in early reprograming, and the intermediate cellular process resembling cartilage development. The in vivo implantation of chemical-induced chondrocytes at defective articular surfaces promoted defect healing and rescued 63.4% of mechanical function loss. Our approach directly converts fibroblasts into functional cartilaginous cells, and also provides insights into potential pharmacological strategies for future cartilage regeneration., Graphical Abstract, Highlights • A chemical method to derive functional murine articular chondrocytes from fibroblasts • Chemical-induced chondrocytes promote in vivo regeneration of articular defects • In single-cell analysis, intermediate reprogramming events resemble cartilage development, In this article, Ouyang and colleagues develop a chemical-driven direct reprogramming of mouse embryonic fibroblasts into PRG4+ chondrocytes, with a screen-defined cocktail VCRTc. Their induced chondrocytes promote in vivo cartilage functional regeneration. The dynamical cellular phenotypes were analyzed by single-cell RNA sequencing, illustrating the inhibition of fibroblast features and activation of chondrogenesis pathways in early reprogramming.

Supplemental material, DS_10.1177_0363546519848678 for Single-Cell Profiles and Clinically Useful Properties of Human Mesenchymal Stem Cells of Adipose and Bone Marrow Origin by Wenyan Zhou, Junxin Lin, Kun Zhao, Kaixiu Jin, Qiulin He, Yejun Hu, Hongwei Ouyang, Gang Feng, Youzhi Cai, Chen Xia, Hua Liu, Weiliang Shen and Xueqing Hu in The American Journal of Sports Medicine