Tgfβ signaling is required for tenocyte recruitment and functional neonatal tendon regeneration

Macrophage depletion impairs neonatal tendon regeneration

Tendons are dense connective tissues that transmit muscle forces to the skeleton. After adult injury, healing potential is generally poor and dominated by scar formation. Although the immune response is a key feature of healing, the specific immune cells and signals that drive tendon healing have not been fully defined. In particular, the immune regulators underlying tendon regeneration are almost completely unknown due to a paucity of tendon regeneration models. Using a mouse model of neonatal tendon regeneration, we screened for immune-related markers and identified upregulation of several genes associated with inflammation, macrophage chemotaxis, and TGFβ signaling after injury. Depletion of macrophages using AP20187 treatment of MaFIA mice resulted in impaired functional healing, reduced cell proliferation, reduced ScxGFP+ neo-tendon formation, and altered tendon gene expression. Collectively, these results show that inflammation is a key component of neonatal tendon regeneration and demonstrate a requirement for macrophages in effective functional healing.

Reparative and Maladaptive Inflammation in Tendon Healing

Tendon injuries are common and debilitating, with non-regenerative healing often resulting in chronic disease. While there has been considerable progress in identifying the cellular and molecular regulators of tendon healing, the role of inflammation in tendon healing is less well understood. While inflammation underlies chronic tendinopathy, it also aids debris clearance and signals tissue repair. Here, we highlight recent findings in this area, focusing on the cells and cytokines involved in reparative inflammation. We also discuss findings from other model systems when research in tendon is minimal, and explore recent studies in the treatment of human tendinopathy to glean further insights into the immunobiology of tendon healing.

Transcriptional profiling of mESC-derived tendon and fibrocartilage cell fate switch

The transcriptional regulators underlying induction and differentiation of dense connective tissues such as tendon and related fibrocartilaginous tissues (meniscus and annulus fibrosus) remain largely unknown. Using an iterative approach informed by developmental cues and single cell RNA sequencing (scRNA-seq), we establish directed differentiation models to generate tendon and fibrocartilage cells from mouse embryonic stem cells (mESCs) by activation of TGFβ and hedgehog pathways, achieving 90% induction efficiency. Transcriptional signatures of the mESC-derived cells recapitulate embryonic tendon and fibrocartilage signatures from the mouse tail. scRNA-seq further identify retinoic acid signaling as a critical regulator of cell fate switch between TGFβ-induced tendon and fibrocartilage lineages. Trajectory analysis by RNA sequencing define transcriptional modules underlying tendon and fibrocartilage fate induction and identify molecules associated with lineage-specific differentiation. Finally, we successfully generate 3-dimensional engineered tissues using these differentiation protocols and show activation of mechanotransduction markers with dynamic tensile loading. These findings provide a serum-free approach to generate tendon and fibrocartilage cells and tissues at high efficiency for modeling development and disease.

The glucagon-like peptide 1 receptor agonist Exendin-4 induces tenogenesis in human mesenchymal stem cells

Tendon injuries are common and account for up to 50% of musculoskeletal injuries in the United States. The poor healing nature of the tendon is attributed to poor vascularization and cellular composition. In the absence of FDA-approved growth factors for tendon repair, engineering strategies using bioactive factors, donor cells, and delivery matrices to promote tendon repair and regeneration are being explored. Growth factor alternatives in the form of small molecules, donor cells, and progenitors offer several advantages and enhance the tendon healing response. Small drug molecules and peptides offer stability over growth factors that are known to suffer from relatively short biological half-lives. The primary focus of this study was to assess the ability of the exendin-4 (Ex-4) peptide, a glucagon-like peptide 1 (GLP-1) receptor agonist, to induce tenocyte differentiation in bone marrow-derived human mesenchymal stem cells (hMSCs). We treated hMSCs with varied doses of Ex-4 in culture media to evaluate proliferation and tendonogenic differentiation. A 20 nM Ex-4 concentration was optimal for promoting cell proliferation and tendonogenic differentiation. Tendonogenic differentiation of hMSCs was evaluated via gene expression profile, immunofluorescence, and biochemical analyses. Collectively, the levels of tendon-related transcription factors (Mkx and Scx) and extracellular matrix (Col-I, Dcn, Bgn, and Tnc) genes and proteins were elevated compared to media without Ex-4 and other controls including insulin and IGF-1 treatments. The tendonogenic factor Ex-4 in conjunction with hMSCs appear to enhance tendon regeneration.

Tendon tissue engineering: Cells, growth factors, scaffolds and production techniques

Tendon injuries are a global health problem that affects millions of people annually. The properties of tendons make their natural rehabilitation a very complex and long-lasting process. Thanks to the development of the fields of biomaterials, bioengineering and cell biology, a new discipline has emerged, tissue engineering. Within this discipline, diverse approaches have been proposed. The obtained results turn out to be promising, as increasingly more complex and natural tendon-like structures are obtained. In this review, the nature of the tendon and the conventional treatments that have been applied so far are underlined. Then, a comparison between the different tendon tissue engineering approaches that have been proposed to date is made, focusing on each of the elements necessary to obtain the structures that allow adequate regeneration of the tendon: growth factors, cells, scaffolds and techniques for scaffold development. The analysis of all these aspects allows understanding, in a global way, the effect that each element used in the regeneration of the tendon has and, thus, clarify the possible future approaches by making new combinations of materials, designs, cells and bioactive molecules to achieve a personalized regeneration of a functional tendon.

Bringing tendon biology to heel: Leveraging mechanisms of tendon development, healing, and regeneration to advance therapeutic strategies

Tendons are specialized matrix-rich connective tissues that transmit forces from muscle to bone and are essential for movement. As tissues that frequently transfer large mechanical loads, tendons are commonly injured in patients of all ages. Following injury, mammalian tendons heal poorly through a slow process that forms disorganized fibrotic scar tissue with inferior biomechanical function. Current treatments are limited and patients can be left with a weaker tendon that is likely to rerupture and an increased chance of developing degenerative conditions. More effective, alternative treatments are needed. However, our current understanding of tendon biology remains limited. Here, we emphasize why expanding our knowledge of tendon development, healing, and regeneration is imperative for advancing tendon regenerative medicine. We provide a comprehensive review of the current mechanisms governing tendon development and healing and further highlight recent work in regenerative tendon models including the neonatal mouse and zebrafish. Importantly, we discuss how present and future discoveries can be applied to both augment current treatments and design novel strategies to treat tendon injuries.

Akt signaling is activated by TGFβ2 and impacts tenogenic induction of mesenchymal stem cells

Background

Tissue engineered and regenerative approaches for treating tendon injuries are challenged by the limited information on the cellular signaling pathways driving tenogenic differentiation of stem cells. Members of the transforming growth factor (TGF) β family, particularly TGFβ2, play a role in tenogenesis, which may proceed via Smad-mediated signaling. However, recent evidence suggests some aspects of tenogenesis may be independent of Smad signaling, and other pathways potentially involved in tenogenesis are understudied. Here, we examined the role of Akt/mTORC1/P70S6K signaling in early TGFβ2-induced tenogenesis of mesenchymal stem cells (MSCs) and evaluated TGFβ2-induced tenogenic differentiation when Smad3 is inhibited.

Methods

Mouse MSCs were treated with TGFβ2 to induce tenogenesis, and Akt or Smad3 signaling was chemically inhibited using the Akt inhibitor, MK-2206, or the Smad3 inhibitor, SIS3. Effects of TGFβ2 alone and in combination with these inhibitors on the activation of Akt signaling and its downstream targets mTOR and P70S6K were quantified using western blot analysis, and cell morphology was assessed using confocal microscopy. Levels of the tendon marker protein, tenomodulin, were also assessed.

Results

TGFβ2 alone activated Akt signaling during early tenogenic induction. Chemically inhibiting Akt prevented increases in tenomodulin and attenuated tenogenic morphology of the MSCs in response to TGFβ2. Chemically inhibiting Smad3 did not prevent tenogenesis, but appeared to accelerate it. MSCs treated with both TGFβ2 and SIS3 produced significantly higher levels of tenomodulin at 7 days and morphology appeared tenogenic, with localized cell alignment and elongation. Finally, inhibiting Smad3 did not appear to impact Akt signaling, suggesting that Akt may allow TGFβ2-induced tenogenesis to proceed during disruption of Smad3 signaling.

Conclusions

These findings show that Akt signaling plays a role in TGFβ2-induced tenogenesis and that tenogenesis of MSCs can be initiated by TGFβ2 during disruption of Smad3 signaling. These findings provide new insights into the signaling pathways that regulate tenogenic induction in stem cells.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02167-2.

ScxLin cells directly form a subset of chondrocytes in temporomandibular joint that are sharply increased in Dmp1-null mice

It has been assumed that the secondary cartilage in the temporomandibular joint (TMJ), which is the most complex and mystery joint and expands rapidly after birth, is formed by periochondrium-derived chondrocytes. The TMJ condyle has rich attachment sites of tendon, which is thought to be solely responsible for joint movement with a distinct cell lineage. Here, we used a Scx-Cre ERT2 mouse line (the tracing line for progenitor and mature tendon cells) to track the fate of tendon cells during TMJ postnatal growth. Our data showed a progressive differentiation of Scx lineage cells started at tendon and the fibrous layer, to cells at the prechondroblasts (Sox9 -/Col I +), and then to cells at the chondrocytic layer (Sox9 +/Col I -). Importantly, the Scx + chondrocytes remained as "permanent" chondrocytes to maintain cartilage mass with no further cell trandifferentiation to bone cells. This notion was substantiated in an assessment of these cells in Dmp1 -null mice (a hypophosphatemic rickets model), where there was a significant increase in the number of Scx lineage cells in response to hypophosphatemia. In addition, we showed the origin of disc, which is derived from Scx + cells. Thus, we propose Scx lineage cells play an important role in TMJ postnatal growth by forming the disc and a new subset of Scx + chondrocytes that do not undergo osteogenesis as the Scx - chondrocytes and are sensitive to the level of phosphorous.

TGF-β1-containing exosomes derived from bone marrow mesenchymal stem cells promote proliferation, migration and fibrotic activity in rotator cuff tenocytes

Objective

This study aimed to investigate effects of TGF-β1-containing exosomes derived from bone marrow mesenchymal stem cells (BMSC) on cell function of rotator cuff tenocytes and its implication to rotator cuff tear.

Methods

The primary BMSC and rotator cuff tenocytes were extracted and cultured. Identification of BMSC were performed by observing cell morphology and measurement of surface biomarkers by flow cytometry. BMSC-derived exosomes were extracted and identified by using electron microscopy, nanoparticle-tracking analysis (NTA) and western blotting. Cell proliferation and cell cycle were measured by CCK-8 assay and flow cytometry assay, respectively. Transwell assay was used for detection of tenocytes migration. The fibrotic activity of tenocytes was determined via qPCR and western blotting assays.

Results

BMSC and BMSC-derived exosomes were successfully extracted. Treatment of BMSC-derived exosomes or TGF-β1 promoted cell proliferation, migration and increased cell ratio of (S + G2/M) phases in tenocytes, as well as enhanced the expression levels of fibrotic activity associated proteins. However, inhibition of TGF-β1 by transfection of sh-TGF-β1 or treatment of TGFβR I/II inhibitor partially reversed the impact of BMSC-derived exosomes on tenocytes function.

Conclusion

Taken together, TGF-β1-containing exosomes derived from BMSC promoted proliferation, migration and fibrotic activity in rotator cuff tenocytes, providing a new direction for treatment of rotator cuff tendon healing.