Embryonic and postnatal tendon cells respond differently to interleukin-1β

Exogenous interleukin-1 beta stimulation regulates equine tenocyte function and gene expression in three-dimensional culture which can be rescued by pharmacological inhibition of interleukin 1 receptor, but not nuclear factor kappa B, signaling

We investigated how Interleukin 1 beta (IL-1β) impacts equine tenocyte function and global gene expression in vitro and determined if these effects could be rescued by pharmacologically inhibiting nuclear factor-κB (NF-KB) or interleukin 1 signalling. Equine superficial digital flexor tenocytes were cultured in three-dimensional (3D) collagen gels and stimulated with IL-1β for two-weeks, with gel contraction and interleukin 6 (IL6) measured throughout and transcriptomic analysis performed at day 14. The impact of three NF-KB inhibitors on gel contraction and IL6 secretion were measured in 3D culture, with NF-KB-P65 nuclear translocation by immunofluorescence and gene expression by qPCR measured in two-dimensional (2D) monolayer culture. In addition, daily 3D gel contraction and transcriptomic analysis was performed on interleukin 1 receptor antagonist-treated 3D gels at day 14. IL-1β increased NF-KB-P65 nuclear translocation in 2D culture and IL6 secretion in 3D culture, but reduced daily tenocyte 3D gel contraction and impacted > 2500 genes at day 14, with enrichment for NF-KB signaling. Administering direct pharmacological inhibitors of NF-KB did reduce NF-KB-P65 nuclear translocation, but had no effect on 3D gel contraction or IL6 secretion in the presence of IL-1β. However, IL1Ra restored 3D gel contraction and partially rescued global gene expression. Tenocyte 3D gel contraction and gene expression is adversely impacted by IL-1β which can only be rescued by blockade of interleukin 1 receptor, but not NF-KB, signalling.

Establishing in vivo and ex vivo chick embryo models to investigate fetal tendon healing

Injured adult tendons heal fibrotically and possess high re-injury rates, whereas fetal tendons appear to heal scarlessly. However, knowledge of fetal tendon wound healing is limited due in part to the need for an accessible animal model. Here, we developed and characterized an in vivo and ex vivo chick embryo tendon model to study fetal tendon healing. In both models, injury sites filled rapidly with cells and extracellular matrix during healing, with wound closure occurring faster in vivo. Tendons injured at an earlier embryonic stage improved mechanical properties to levels similar to non-injured controls, whereas tendons injured at a later embryonic stage did not. Expression levels of tendon phenotype markers, collagens, collagen crosslinking regulators, matrix metalloproteinases, and pro-inflammatory mediators exhibited embryonic stage-dependent trends during healing. Apoptosis occurred during healing, but ex vivo tendons exhibited higher levels of apoptosis than tendons in vivo. Future studies will use these in vivo and ex vivo chick embryo tendon injury models to elucidate mechanisms of stage-specific fetal tendon healing to inform the development of therapeutic approaches to regeneratively heal adult tendons.

The Composition of Adipose-Derived Regenerative Cells Isolated from Lipoaspirate Using a Point of Care System Does Not Depend on the Subject's Individual Age, Sex, Body Mass Index and Ethnicity

Uncultured, unmodified, autologous, adipose-derived regenerative cells (UA-ADRCs) are a safe and effective treatment option for various musculoskeletal pathologies. However, it is unknown whether the composition of the final cell suspension systematically varies with the subject's individual age, sex, body mass index and ethnicity. UA-ADRCs were isolated from lipoaspirate from n = 232 subjects undergoing elective lipoplasty using the Transpose RT system (InGeneron, Inc.; Houston, TX, USA). The UA-ADRCs were assessed for the number of nucleated cells, cell viability and the number of viable nucleated cells per gram of adipose tissue harvested. Cells from n = 37 subjects were further characterized using four-channel flow cytometry. The present study shows, for the first time, that key characteristics of UA-ADRCs can be independent of the subject's age, sex, BMI and ethnicity. This result has important implications for the general applicability of UA-ADRCs in regeneration of musculoskeletal tissue. Future studies must determine whether the independence of key characteristics of UA-ADRCs of the subject's individual age, sex, BMI and ethnicity only applies to the system used in the present study, or also to others of the more than 25 different experimental methods and commercially available systems used to isolate UA-ADRCs from lipoaspirate that have been described in the literature.

Phenotype stability, expansion potential, and senescence of embryonic tendon cells in vitro

Embryonic tendon cells have been studied in vitro to better understand mechanisms of tendon development. Outcomes of in vitro cell culture studies are easily affected by phenotype instability of embryonic tendon cells during expansion in vitro to achieve desired cell numbers, yet this has not been characterized. In the present study, we characterized phenotype stability, expansion potential, and onset of senescence in chick embryo tendon cells from low to high cell doublings. We focused on cells of Hamburger-Hamilton stages (HH) 40 and HH42, where HH40 is the earliest stage associated with substantial increases in extracellular matrix and mechanical properties during embryonic tendon development. HH40 and HH42 cells both downregulated expression levels of tendon phenotype markers, scleraxis and tenomodulin, and exhibited onset of senescence, based on p16 and p21 expression levels, cell surface area, and percentage of β-galactosidase positive cells, before significant decreases in proliferation rates were detected. These findings showed that embryonic tendon cells destabilize phenotype and become senescent earlier than they begin to decline in proliferation rates in vitro. Additionally, embryonic stage of isolation appears to have no effect on proliferation rates, whereas later stage HH42 cells downregulate phenotype and become susceptible to senescence sooner than earlier stage HH40 cells. Based on our data, we recommend chick embryo tendon cells be used before a maximum cumulative doubling level of 12 (passage 4 in this study) to avoid phenotype destabilization and onset of senescence.

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.

Tendon Stem/Progenitor Cell Subpopulations and Their Implications in Tendon Biology

Tendon harbors a cell population that possesses stem cell characteristics such as clonogenicity, multipotency and self-renewal capacity, commonly referred to as tendon stem/progenitor cells (TSPCs). Various techniques have been employed to study how TSPCs are implicated in tendon development, homeostasis and healing. Recent advances in single-cell analysis have enabled much progress in identifying and characterizing distinct subpopulations of TSPCs, which provides a more comprehensive view of TSPCs function in tendon biology. Understanding the mechanisms of physiological and pathological processes regulated by TSPCs, especially a particular subpopulation, would greatly benefit treatment of diseased tendons. Here, we summarize the current scientific literature on the various subpopulations of TSPCs, and discuss how TSPCs can contribute to tissue homeostasis and pathogenesis, as well as examine the key modulatory signaling pathways that determine stem/progenitor cell state. A better understanding of the roles that TSPCs play in tendon biology may facilitate the development of novel treatment strategies for tendon diseases.

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.

Tendon healing in presence of chronic low-level inflammation: a systematic review

BACKGROUND

Tendinopathy is a common musculoskeletal condition affecting subjects regardless of their activity level. Multiple inflammatory molecules found in ex vivo samples of human tendons are related to the initiation or progression of tendinopathy. Their role in tendon healing is the subject of this review.

SOURCES OF DATA

An extensive review of current literature was conducted using PubMed, Embase and Cochrane Library using the term 'tendon', as well as some common terms of tendon conditions such as 'tendon injury OR (tendon damage) OR tendonitis OR tendinopathy OR (chronic tendonitis) OR tendinosis OR (chronic tendinopathy) OR enthesitis' AND 'healing' AND '(inflammation OR immune response)' as either key words or MeSH terms.

AREAS OF AGREEMENT

An environment characterized by a low level of chronic inflammation, together with increased expression of inflammatory cytokines and growth factors, may influence the physiological tendon healing response after treatment.

AREAS OF CONTROVERSY

Most studies on this topic exhibited limited scientific translational value because of their heterogeneity. The evidence associated with preclinical studies is limited.

GROWING POINTS

The role of inflammation in tendon healing is still unclear, though it seems to affect the overall outcome. A thorough understanding of the biochemical mediators of healing and their pathway of pain could be used to target tendinopathy and possibly guide its management.

AREAS TIMELY FOR DEVELOPING RESEARCH

We require further studies with improved designs to effectively evaluate the pathogenesis and progression of tendinopathy to identify cellular and molecular targets to improve outcomes.

Models of tendon development and injury

Tendons link muscle to bone and transfer forces necessary for normal movement. Tendon injuries can be debilitating and their intrinsic healing potential is limited. These challenges have motivated the development of model systems to study the factors that regulate tendon formation and tendon injury. Recent advances in understanding of embryonic and postnatal tendon formation have inspired approaches that aimed to mimic key aspects of tendon development. Model systems have also been developed to explore factors that regulate tendon injury and healing. We highlight current model systems that explore developmentally inspired cellular, mechanical, and biochemical factors in tendon formation and tenogenic stem cell differentiation. Next, we discuss in vivo, in vitro, ex vivo, and computational models of tendon injury that examine how mechanical loading and biochemical factors contribute to tendon pathologies and healing. These tendon development and injury models show promise for identifying the factors guiding tendon formation and tendon pathologies, and will ultimately improve regenerative tissue engineering strategies and clinical outcomes.