Phenotypic Drift in Human Tenocyte Culture

Xenogeneic versus allogeneic serum and macromolecular crowding in human tenocyte cultures

Allogeneic serum and tissue-specific extracellular matrix have been shown to maintain permanently differentiated cell phenotype in culture. This is of particular importance for human tenocytes, a cell population that readily loses its function during ex vivo culture. With these in mind, herein we extracted human tenocytes using either foetal bovine serum or human serum, cultured them in the absence and presence of carrageenan and Ficoll®, the most widely used macromolecular crowding agents (to induce tissue-specific extracellular matrix deposition), and assessed cellular function, via metabolic activity, viability, proliferation and immunofluorescence for collagen related molecules, non-collagenous molecules and transmembrane molecules. At day 7, longest time point assessed, neither carrageenan nor Ficoll® significantly affected metabolic activity, viability and proliferation in either serum and human serum significantly increased metabolic activity and proliferation. At day 7, in the absence of macromolecular crowding, cells in human serum deposited significantly lower collagen type VI, biglycan, versican and tenomodulin than cells in foetal bovine serum. Interestingly, at day 7, in comparison to the no macromolecular crowding group, carrageenan in foetal bovine serum induced the highest effect, as judged by the highest number of significantly increased molecules (collagen type I, collagen type IV, collagen type V, collagen type VI, transforming growth factor β1, matrix metalloproteinase 14, lumican, versican, scleraxis and integrin α2β1). These data, although contradict previous observations where human serum outperformed foetal bovine serum, at the same time, support the use of foetal bovine serum in the development of cell-based medicines.

MRGPRX2-mediated mast cell activation by substance P from overloaded human tenocytes induces inflammatory and degenerative responses in tendons

Mast cells are immune cells minimally present in normal tendon tissue. The increased abundance of mast cells in tendinopathy biopsies and at the sites of tendon injury suggests an unexplored role of this cell population in overuse tendon injuries. Mast cells are particularly present in tendon biopsies from patients with more chronic symptom duration and a history of intensive mechanical loading. This study, therefore, examined the cross talk between mast cells and human tendon cells in either static or mechanically active conditions in order to explore the potential mechanistic roles of mast cells in overuse tendon injuries. A coculture of isolated human tenocytes and mast cells (HMC-1) combined with Flexcell Tension System for cyclic stretching of tenocytes was used. Additionally, human tenocytes were exposed to agonists and antagonists of substance P (SP) receptors. Mast cell degranulation was assessed by measuring β-hexosaminidase activity. Transwell and cell adhesion assays were used to evaluate mast cell migration and binding to tendon extracellular matrix components (collagen and fibronectin), respectively. Gene expressions were analyzed using real time qRT-PCR. Our results indicate that mechanical stimulation of human tenocytes leads to release of SP which, in turn, activates mast cells through the Mas-related G-protein-coupled receptor X2 (MRGPRX2). The degranulation and migration of mast cells in response to MRGPRX2 activation subsequently cause human tenocytes to increase their expression of inflammatory factors, matrix proteins and matrix metalloproteinase enzymes. These observations may be important in understanding the mechanisms by which tendons become tendinopathic in response to repetitive mechanical stimulation.

Novel In Vitro Platform for Studying the Cell Response to Healthy and Diseased Tendon Matrices

Current in vitro models poorly represent the healthy or diseased tendon microenvironment, limiting the translation of the findings to clinics. The present work aims to establish a physiologically relevant in vitro tendon platform that mimics biophysical aspects of a healthy and tendinopathic tendon matrix using a decellularized bovine tendon and to characterize tendon cells cultured using this platform. Bovine tendons were subjected to various decellularization techniques, with the efficacy of decellularization determined histologically. The biomechanical and architectural properties of the decellularized tendons were characterized using an atomic force microscope. Tendinopathy-mimicking matrices were prepared by treating the decellularized tendons with collagenase for 3 h or collagenase-chondroitinase (CC) for 1 h. The tendon tissue collected from healthy and tendinopathic patients was characterized using an atomic force microscope and compared to that of decellularized matrices. Healthy human tendon-derived cells (hTDCs) from the hamstring tendon were cultured on the decellularized matrices for 24 or 48 h, with cell morphology characterized using f-actin staining and gene expression characterized using real-time PCR. Tendon matrices prepared by freeze-thawing and 48 h nuclease treatment were fully decellularized, and the aligned structure and tendon stiffness (1.46 MPa) were maintained. Collagenase treatment prepared matrices with a disorganized architecture and reduced stiffness (0.75 MPa), mimicking chronic tendinopathy. Treatment with CC prepared matrices with a disorganized architecture without altering stiffness, mimicking early tendinopathy (1.52 MPa). hTDCs on a healthy tendon matrix were elongated, and the scleraxis (SCX) expression was maintained. On tendinopathic matrices, hTDCs had altered morphological characteristics and lower SCX expression. The expression of genes related to actin polymerization, matrix degradation and remodeling, and immune cell invasion were higher in hTDCs on tendinopathic matrices. Overall, the present study developed a physiological in vitro system to mimic healthy tendons and early and late tendinopathy, and it can be used to better understand tendon cell characteristics in healthy and diseased states.

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.

Characterization of TGFβ1-induced tendon-like structure in the scaffold-free three-dimensional tendon cell culture system

The biological mechanisms regulating tenocyte differentiation and morphological maturation have not been well-established, partly due to the lack of reliable in vitro systems that produce highly aligned collagenous tissues. In this study, we developed a scaffold-free, three-dimensional (3D) tendon culture system using mouse tendon cells in a differentially adherent growth channel. Transforming Growth Factor-β (TGFβ) signaling is involved in various biological processes in the tendon, regulating tendon cell fate, recruitment and maintenance of tenocytes, and matrix organization. This known function of TGFβ signaling in tendon prompted us to utilize TGFβ1 to induce tendon-like structures in 3D tendon constructs. TGFβ1 treatment promoted a tendon-like structure in the peripheral layer of the constructs characterized by increased thickness with a gradual decrease in cell density and highly aligned collagen matrix. TGFβ1 also enhanced cell proliferation, matrix production, and morphological maturation of cells in the peripheral layer compared to vehicle treatment. TGFβ1 treatment also induced early tenogenic differentiation and resulted in sufficient mechanical integrity, allowing biomechanical testing. The current study suggests that this scaffold-free 3D tendon cell culture system could be an in vitro platform to investigate underlying biological mechanisms that regulate tenogenic cell differentiation and matrix organization.

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

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

Microfluidically Aligned Collagen to Maintain the Phenotype of Tenocytes In Vitro

Tendon is a highly organized tissue that transmits forces between muscle and bone. The architecture of the extracellular matrix of tendon, predominantly from collagen type I, is important for maintaining tenocyte phenotype and function. Therefore, in repair and regeneration of damaged and diseased tendon tissue, it is crucial to restore the aligned arrangement of the collagen type I fibers of the original matrix. To this end, a novel, user-friendly microfluidic piggyback platform is developed allowing the controlled patterned formation and alignment of collagen fibers simply on the bottom of culture dishes. Rat tenocytes cultured on the micropatterns of aligned fibrous collagen exhibit a more elongated morphology. The cells also show an increased expression of tenogenic markers at the gene and protein level compared to tenocytes cultured on tissue culture plastic or non-fibrillar collagen coatings. Moreover, using imprinted polystyrene replicas of aligned collagen fibers, this work shows that the fibrillar structure of collagen per se affects the tenocyte morphology, whereas the biochemical nature of collagen plays a prominent role in the expression of tenogenic markers. Beyond the controlled provision of aligned collagen, the microfluidic platform can aid in developing more physiologically relevant in vitro models of tendon and its regeneration.

Endotenon-Derived Type II Tendon Stem Cells Have Enhanced Proliferative and Tenogenic Potential

Tendon injuries caused by overuse or age-related deterioration are frequent. Incomplete knowledge of somatic tendon cell biology and their progenitors has hindered interventions for the effective repair of injured tendons. Here, we sought to compare and contrast distinct tendon-derived cell populations: type I and II tendon stem cells (TSCs) and tenocytes (TNCs). Porcine type I and II TSCs were isolated via the enzymatic digestion of distinct membranes (paratenon and endotenon, respectively), while tenocytes were isolated through an explant method. Resultant cell populations were characterized by morphology, differentiation, molecular, flow cytometry, and immunofluorescence analysis. Cells were isolated, cultured, and evaluated in two alternate oxygen concentrations (physiological (2%) and air (21%)) to determine the role of oxygen in cell biology determination within this relatively avascular tissue. The different cell populations demonstrated distinct proliferative potential, morphology, and transcript levels (both for tenogenic and stem cell markers). In contrast, all tendon-derived cell populations displayed multipotent differentiation potential and immunophenotypes (positive for CD90 and CD44). Type II TSCs emerged as the most promising tendon-derived cell population for expansion, given their enhanced proliferative potential, multipotency, and maintenance of a tenogenic profile at early and late passage. Moreover, in all cases, physoxia promoted the enhanced proliferation and maintenance of a tenogenic profile. These observations help shed light on the biological mechanisms of tendon cells, with the potential to aid in the development of novel therapeutic approaches for tendon disorders.

Regenerative MRL/MpJ tendon cells exhibit sex differences in morphology, proliferation, mechanosensitivity, and cell-ECM organization

Clinical and animal studies have reported the influence of sex on the incidence and progression of tendinopathy, which results in disparate structural and biomechanical outcomes. However, there remains a paucity in our understanding of the sex-specific biological mechanisms underlying effective tendon healing. To overcome this hurdle, our group has investigated the impact of sex on tendon regeneration using the super-healer Murphy Roths Large (MRL/MpJ) mouse strain. We have previously shown that the scarless healing capacity of MRL/MpJ patellar tendons is associated with sexually dimorphic regulation of gene expression for pathways involved in fibrosis, cell migration, adhesion, and extracellular matrix (ECM) remodeling following an acute mid-substance injury. Thus, we hypothesized that MRL/MpJ scarless tendon healing is mediated by sex-specific and temporally distinct orchestration of cell-ECM interactions. Accordingly, the present study comparatively evaluated MRL/MpJ tendon cells on two-dimensional (2D; glass) and scaffold platforms to examine cell behavior under biochemical and topographical cues associated with tendon homeostasis and healing. Female MRL/MpJ cells showed reduced 2D migration and spreading area accompanied by enhanced mechanosensing, ECM alignment, and fibronectin-mediated cell proliferation compared to male MRL/MpJ cells. Interestingly, female MRL/MpJ cells cultured on isotropic scaffolds showed diminished cell-ECM organization compared to male MRL/MpJ cells. Lastly, MRL/MpJ cells elicited enhanced cytoskeletal elongation and alignment, ECM deposition and organization, and connexin 43-mediated intercellular communication compared to male B6 cells, regardless of culture condition or sex. These results provide insight into the cellular features conserved within the MRL/MpJ phenotype and potential sex-specific targets for the development of more equitable therapeutics.

Elicitation of Inhibitory Effects for AGE-Induced Oxidative Stress in Rotator Cuff-Derived Cells by Apocynin

Advanced glycation end-products (AGEs) play a critical supportive role during musculoskeletal disorders via glycosylation and oxidative stress. Though apocynin, identified as a potent and selective inhibitor of NADPH oxidase, has been reported to be involved in pathogen-induced reactive oxygen species (ROS), its role in age-related rotator cuff degeneration has not been well clarified. Therefore, this study aims to evaluate the in vitro effects of apocynin on human rotator cuff-derived cells. Twelve patients with rotator cuff tears (RCTs) participated in the study. Supraspinatus tendons from patients with RCTs were collected and cultured. After the preparation of RC-derived cells, they were divided into four groups (control group, control + apocynin group, AGEs group, AGEs + apocynin group), and gene marker expression, cell viability, and intracellular ROS production were evaluated. The gene expression of NOX, IL-6, and the receptor for AGEs (RAGE) was significantly decreased by apocynin. We also examined the effect of apocynin in vitro. The results showed that ROS induction and increasing apoptotic cells after treatment of AGEs were significantly decreased, and cell viability increased considerably. These results suggest that apocynin can effectively reduce AGE-induced oxidative stress by inhibiting NOX activation. Thus, apocynin is a potential prodrug in preventing degenerative changes of the rotor cuff.

IVM Advances for Early Antral Follicle-Enclosed Oocytes Coupling Reproductive Tissue Engineering to Inductive Influences of Human Chorionic Gonadotropin and Ovarian Surface Epithelium Coculture

In vitro maturation (IVM) is not a routine assisted reproductive technology (ART) for oocytes collected from early antral (EA) follicles, a large source of potentially available gametes. Despite substantial improvements in IVM in the past decade, the outcomes remain low for EA-derived oocytes due to their reduced developmental competences. To optimize IVM for ovine EA-derived oocytes, a three-dimensional (3D) scaffold-mediated follicle-enclosed oocytes (FEO) system was compared with a validated cumulus-oocyte complex (COC) protocol. Gonadotropin stimulation (eCG and/or hCG) and/or somatic cell coculture (ovarian vs. extraovarian-cell source) were supplied to both systems. The maturation rate and parthenogenetic activation were significantly improved by combining hCG stimulation with ovarian surface epithelium (OSE) cells coculture exclusively on the FEO system. Based on the data, the paracrine factors released specifically from OSE enhanced the hCG-triggering of oocyte maturation mechanisms by acting through the mural compartment (positive effect on FEO and not on COC) by stimulating the EGFR signaling. Overall, the FEO system performed on a developed reproductive scaffold proved feasible and reliable in promoting a synergic cytoplasmatic and nuclear maturation, offering a novel cultural strategy to widen the availability of mature gametes for ART.

Writing 3D In Vitro Models of Human Tendon within a Biomimetic Fibrillar Support Platform

Tendinopathies are poorly understood diseases for which treatment remains challenging. Relevant in vitro models to study human tendon physiology and pathophysiology are therefore highly needed. Here we propose the automated 3D writing of tendon microphysiological systems (MPSs) embedded in a biomimetic fibrillar support platform based on cellulose nanocrystals (CNCs) self-assembly. Tendon decellularized extracellular matrix (dECM) was used to formulate bioinks that closely recapitulate the biochemical signature of tendon niche. A monoculture system recreating the cellular patterns and phenotype of the tendon core was first developed and characterized. This system was then incorporated with a vascular compartment to study the crosstalk between the two cell populations. The combined biophysical and biochemical cues of the printed pattern and dECM hydrogel were revealed to be effective in inducing human-adipose-derived stem cells (hASCs) differentiation toward the tenogenic lineage. In the multicellular system, chemotactic effects promoted endothelial cells migration toward the direction of the tendon core compartment, while the established cellular crosstalk boosted hASCs tenogenesis, emulating the tendon development stages. Overall, the proposed concept is a promising strategy for the automated fabrication of humanized organotypic tendon-on-chip models that will be a valuable new tool for the study of tendon physiology and pathogenesis mechanisms and for testing new tendinopathy treatments.

Synergistic effects of growth factor-based serum-free medium and tendon-like substrate topography on tenogenesis of mesenchymal stem cells

Addressing clinical challenges for tendon injuries requires a deeper understanding of the effects that biological and biophysical cues have on tenogenesis. Although prior studies have identified tenogenic growth factors (GFs) or elucidated the effects of substrate topography on tenocyte behavior, few have characterized their combined effect in the presence of a tendon-like substrate. In this study, we assessed the effect of biological (GFs) and biophysical (substrate topography) cues on tenogenic proliferation and differentiation under defined, serum-free conditions. Specifically, human bone marrow-derived mesenchymal stem cells (hMSCs) were cultured in a serum-free culture medium containing a GF cocktail comprised of fibroblast growth factor-2 (FGF-2), transforming growth factor-beta 3 (TGF-β3), and insulin-like growth factor-1 (IGF-1), either alone or in combination with tendon-like substrate topography produced by replica casting of tendon tissue sections. Our data demonstrated that the use of serum-free GF cocktail medium alone promoted hMSC proliferation, as shown via DNA staining as well as Ki67 protein levels and gene expression. In particular, gene expression of Ki67 was increased by 8.46-fold in all three donors relative to serum-free medium control. Also, serum-free GF cocktail promoted tenogenic differentiation, on the basis of expression of tendon-associated gene and protein markers, scleraxis (SCX), tenascin C (TNC), and collagen type I (COL1A1) including increased normalized collagen production by 1.4-fold in two donors relative to serum-free medium control. Interestingly, hMSCs cultured on a tendon-like substrate exhibited highly oriented cell morphology and extracellular matrix (ECM) alignment reminiscent of tendon. In particular, when this GF cocktail was combined with tendon-like topography, they showed a synergistically increased expression of tendon-related markers and anisotropic organization of ECM proteins with moderate-to-large effect sizes. Together, in addition to showing the utility of a GF cocktail for expansion and differentiation of tenocyte-like cells, our findings clearly demonstrate the synergistic relationship between GF-mediated and substrate topography-related effects on hMSC tenogenic differentiation. This information provides insights into the design of strategies that combine biological and biophysical cues for ex vivo tenocyte production and tendon tissue engineering.

The Stiffness-Sensitive Transcriptome of Human Tendon Stromal Cells

Extracellular matrix stiffness is a major regulator of cellular states. Stiffness-sensing investigations are typically performed using cells that have acquired "mechanical memory" through prolonged conditioning in rigid environments, e.g., tissue culture plastic (TCP). This potentially masks the full extent of the matrix stiffness-driven mechanosensing programs. Here, a biomaterial composed of 2D mechanovariant silicone substrates with simplified and scalable surface biofunctionalization chemistry is developed to facilitate large-scale cell culture expansion processes. Using RNA sequencing, stiffness-mediated mechano-responses of human tendon-derived stromal cells are broadly mapped. Matrix elasticity (E.) approximating tendon microscale stiffness range (E. ≈ 35 kPa) distinctly favors transcriptional programs related to chromatin remodeling and Hippo signaling; whereas compliant stiffnesses (E. ≈ 2 kPa) are enriched in processes related to cell stemness, synapse assembly, and angiogenesis. While tendon stromal cells undergo dramatic phenotypic drift on conventional TCP, mechanovariant substrates abrogate this activation with tenogenic stiffnesses inducing a transcriptional program that strongly correlates with established tendon tissue-specific expression signature. Computational inference predicts that AKT1 and ERK1/2 are major stiffness-sensing signaling hubs. Together, these findings highlight how matrix biophysical cues may dictate the transcriptional identity of tendon cells, and how matrix mechano-reciprocity regulates diverse sets of previously underappreciated mechanosensitive processes in tendon fibroblasts.

Aberrant chromatin reorganization in cells from diseased fibrous connective tissue in response to altered chemomechanical cues

Changes in the micro-environment of fibrous connective tissue can lead to alterations in the phenotypes of tissue-resident cells, yet the underlying mechanisms are poorly understood. Here, by visualizing the dynamics of histone spatial reorganization in tenocytes and mesenchymal stromal cells from fibrous tissue of human donors via super-resolution microscopy, we show that physiological and pathological chemomechanical cues can directly regulate the spatial nanoscale organization and density of chromatin in these tissue-resident cell populations. Specifically, changes in substrate stiffness, altered oxygen tension and the presence of inflammatory signals drive chromatin relocalization and compaction into the nuclear boundary, mediated by the activity of the histone methyltransferase EZH2 and an intact cytoskeleton. In healthy cells, chemomechanically triggered changes in the spatial organization and density of chromatin are reversible and can be attenuated by dynamically stiffening the substrate. In diseased human cells, however, the link between mechanical or chemical inputs and chromatin remodelling is abrogated. Our findings suggest that aberrant chromatin organization in fibrous connective tissue may be a hallmark of disease progression that could be leveraged for therapeutic intervention.

Immortalized murine tenocyte cells: a novel and innovative tool for tendon research

Primary tenocytes rapidly undergo senescence and a phenotypic drift upon in vitro monolayer culture, which limits tendon research. The Ink4a/Arf locus encodes the proteins p16^Ink4a/Arf and p14^ARF (p19^ARF in mice) that regulate cell cycle progression and senescence. We here established an immortalized cell line using tenocytes isolated from Ink4a/Arf deficient mice (Ink4a/Arf^-/-). These cells were investigated at three distinct time points, at low (2-5), intermediate (14-17) and high (35-44) passages. Wild-type cells at low passage (2-5) served as controls. Ink4a/Arf^-/- tenocytes at all stages were comparable to wild-type cells regarding morphology, expression of tenogeneic genes (collagen type 1, 3 and 5, Scleraxis, Tenomodulin and Tenascin-C), and surface markers (CD29, CD44 and CD105) and form 3D tendon-like structures. Importantly, Ink4a/Arf^-/- tenocytes maintained their phenotypic features and proliferation potential in culture for more than 40 passages and also following freeze-thaw cycles. In contrast, wild-type tenocytes underwent senescence starting in passage 6. These data define Ink4a/Arf^-/- tenocytes as novel tool for in vitro tendon research and as valuable in vitro alternative to animal experiments.

In Vitro and In Vivo Effects of IGF-1 Delivery Strategies on Tendon Healing: A Review

Tendon injuries suffer from a slow healing, often ending up in fibrovascular scar formation, leading to inferior mechanical properties and even re-rupture upon resumption of daily work or sports. Strategies including the application of growth factors have been under view for decades. Insulin-like growth factor-1 (IGF-1) is one of the used growth factors and has been applied to tenocyte in vitro cultures as well as in animal preclinical models and to human patients due to its anabolic and matrix stimulating effects. In this narrative review, we cover the current literature on IGF-1, its mechanism of action, in vitro cell cultures (tenocytes and mesenchymal stem cells), as well as in vivo experiments. We conclude from this overview that IGF-1 is a potent stimulus for improving tendon healing due to its inherent support of cell proliferation, DNA and matrix synthesis, particularly collagen I, which is the main component of tendon tissue. Nevertheless, more in vivo studies have to be performed in order to pave the way for an IGF-1 application in orthopedic clinics.

Macromolecular crowding in animal component-free, xeno-free and foetal bovine serum media for human bone marrow mesenchymal stromal cell expansion and differentiation

Background: Cell culture media containing undefined animal-derived components and prolonged in vitro culture periods in the absence of native extracellular matrix result in phenotypic drift of human bone marrow stromal cells (hBMSCs). Methods: Herein, we assessed whether animal component-free (ACF) or xeno-free (XF) media formulations maintain hBMSC phenotypic characteristics more effectively than foetal bovine serum (FBS)-based media. In addition, we assessed whether tissue-specific extracellular matrix, induced via macromolecular crowding (MMC) during expansion and/or differentiation, can more tightly control hBMSC fate. Results: Cells expanded in animal component-free media showed overall the highest phenotype maintenance, as judged by cluster of differentiation expression analysis. Contrary to FBS media, ACF and XF media increased cellularity over time in culture, as measured by total DNA concentration. While MMC with Ficoll™ increased collagen deposition of cells in FBS media, FBS media induced significantly lower collagen synthesis and/or deposition than the ACF and XF media. Cells expanded in FBS media showed higher adipogenic differentiation than ACF and XF media, which was augmented by MMC with Ficoll™ during expansion. Similarly, Ficoll™ crowding also increased chondrogenic differentiation. Of note, donor-to-donor variability was observed for collagen type I deposition and trilineage differentiation capacity of hBMSCs. Conclusion: Collectively, our data indicate that appropriate screening of donors, media and supplements, in this case MMC agent, should be conducted for the development of clinically relevant hBMSC medicines.

Biological effects of extracorporeal shockwave therapy in tendons: A systematic review

Extracorporeal shockwave therapy was initially used for kidney stone disintegration and its application was then extended to calcific tendinitis. The therapeutic field expanded and included numerous types of tendinopathies, from shoulder to plantar fascia. The clinical benefits were documented in trials and the effects and mechanisms were studied on models including animal and human tendons. The present systematic review outlines a large spectrum of biological effects. First, an optimal dose is adapted for each species and each tendon; exceeding the optimal dose may lead to structural injury. Furthermore, the biological effects may be grouped into neovascularization induction, cellularity and extracellular matrix changes, metalloprotease and cytokine modulation, as well as lubricin production. As a result, the remodeled tendon displays improved biomechanical properties to resist stress.

CD200 as a Potential New Player in Inflammation during Rotator Cuff Tendon Injury/Repair: An In Vitro Model

Rotator cuff tendon (RCT) disease results from multifactorial mechanisms, in which inflammation plays a key role. Pro-inflammatory cytokines and tendon stem cell/progenitor cells (TSPCs) have been shown to participate in the inflammatory response. However, the underlying molecular mechanism is still not clear. In this study, flow cytometry analyses of different subpopulations of RCT-derived TSPCs demonstrate that after three days of administration, TNFα alone or in combination with IFNγ significantly decreases the percentage of CD146+CD49d+ and CD146+CD49f+ but not CD146+CD109+ TSPCs populations. In parallel, the same pro-inflammatory cytokines upregulate the expression of CD200 in the CD146+ TSPCs population. Additionally, the TNFα/IFNγ combination modulates the protein expression of STAT1, STAT3, and MMP9, but not fibromodulin. At the gene level, IRF1, CAAT (CAAT/EBPbeta), and DOK2 but not NF-κb, TGRF2 (TGFBR2), and RAS-GAP are modulated. In conclusion, although our study has several important limitations, the results highlight a new potential role of CD200 in regulating inflammation during tendon injuries. In addition, the genes analyzed here might be new potential players in the inflammatory response of TSPCs.

Hyaluronic Acid Alleviates Oxidative Stress and Apoptosis in Human Tenocytes via Caspase 3 and 7

Rotator cuff tendinopathy (RCT) is the primary reason for shoulder surgery and its clinical management is still challenging. Hyaluronic acid (HA) has been shown to have anti-inflammatory effects in vitro and in vivo under RCT conditions, characterized by an exaggerated oxidative stress (OS). However, molecular mechanisms underlying HA-related effects are still partially disclosed. With these aims, a cell model of RCT was established by exposing primary human tenocytes to H₂O₂ for up to 72 h. Four different HAs by molecular weight were administered to measure nitric oxide (NO) and OS, apoptosis, and collagen 1 expression. In parallel, the well-known antioxidant ascorbic acid was administered for comparison. The present study highlights that HAs characterized by a low molecular weight are able to counteract the H₂O₂-induced OS by decreasing the percentage of apoptotic cells and reversing the activation of caspase 3 and 7. Likewise, NO intracellular levels are comparable to the ones of controls. In parallel, collagen 1 expression was ameliorated by HAs characterized by higher molecular weights compared to AA. These findings confirm that HA plays an antioxidant role comparable to AA depending on the molecular weight, and highlight the molecular mechanisms underlying the HA anti-apoptotic effects.

Macromolecular crowding in the development of a three-dimensional organotypic human breast cancer model

Although cell-derived matrices are at the forefront of scientific research and technological innovation for the development of in vitro tumour models, their two-dimensional structure and low extracellular matrix composition restrict their capacity to accurately predict toxicity of candidate molecules. Herein, we assessed the potential of macromolecular crowding (a biophysical phenomenon that significantly enhances and accelerates extracellular matrix deposition, resulting in three-dimensional tissue surrogates) in improving cell-derived matrices in vitro tumour models. Among the various decellularisation protocols assessed (NH4OH, DOC, SDS/EDTA, NP40), the NP40 appeared to be the most effective in removing cellular matter and the least destructive to the deposited matrix. Among the various cell types (mammary, skin, lung fibroblasts) used to produce the cell-derived matrices, the mammary fibroblast derived matrices produced under macromolecular crowding conditions and decellularised with NP40 resulted in significant increase in focal adhesion molecules, matrix metalloproteinases and proinflammatory cytokines, when seeded with MDA-MB-231 cells. Further, macromolecular crowding derived matrices significantly increased doxorubicin resistance and reduced the impact of intracellular reactive oxygen species mediated cell death. Collectively our data clearly illustrate the potential of macromolecular crowding in the development of cell-derived matrices-based in vitro tumour models that more accurately resemble the tumour microenvironment.

Tendon healing: a concise review on cellular and molecular mechanisms with a particular focus on the Achilles tendon

Tendon is a bradytrophic and hypovascular tissue, hence, healing remains a major challenge. The molecular key events involved in successful repair have to be unravelled to develop novel strategies that reduce the risk of unfavourable outcomes such as non-healing, adhesion formation, and scarring. This review will consider the diverse pathophysiological features of tendon-derived cells that lead to failed healing, including misrouted differentiation (e.g. de- or transdifferentiation) and premature cell senescence, as well as the loss of functional progenitors. Many of these features can be attributed to disturbed cell-extracellular matrix (ECM) or unbalanced soluble mediators involving not only resident tendon cells, but also the cross-talk with immigrating immune cell populations. Unrestrained post-traumatic inflammation could hinder successful healing. Pro-angiogenic mediators trigger hypervascularization and lead to persistence of an immature repair tissue, which does not provide sufficient mechano-competence. Tendon repair tissue needs to achieve an ECM composition, structure, strength, and stiffness that resembles the undamaged highly hierarchically ordered tendon ECM. Adequate mechano-sensation and -transduction by tendon cells orchestrate ECM synthesis, stabilization by cross-linking, and remodelling as a prerequisite for the adaptation to the increased mechanical challenges during healing. Lastly, this review will discuss, from the cell biological point of view, possible optimization strategies for augmenting Achilles tendon (AT) healing outcomes, including adapted mechanostimulation and novel approaches by restraining neoangiogenesis, modifying stem cell niche parameters, tissue engineering, the modulation of the inflammatory cells, and the application of stimulatory factors.Cite this article: Bone Joint Res 2022;11(8):561-574.

A decellularized flowable placental connective tissue matrix supports cellular functions of human tenocytes in vitro

Purpose

Injectable connective tissue matrices (CTMs) may promote tendon healing, given their minimally invasive properties, structural and biochemical extracellular matrix components, and capacity to fill irregular spaces. The purpose of this study is to evaluate the effects of placental CTMs on the cellular activities of human tenocytes. Decellularization, the removal of cells, cell fragments, and DNA from CTMs, has been shown to reduce the host’s inflammatory response. Therefore, the authors hypothesize that a decellularized CTM will provide a more cell-friendly matrix to support tenocyte functions.

Methods

Three human placental CTMs were selected for comparison: AmnioFill® (A-CTM), a minimally manipulated, non-viable cellular particulate, BioRenew™ (B-CTM), a liquid matrix, and Interfyl® (I-CTM), a decellularized flowable particulate. Adhesion and proliferation were evaluated using cell viability assays and tenocyte migration using a transwell migration assay. Gene expression of tenocyte markers, cytokines, growth factors, and matrix metalloprotease (MMP) in tenocytes were assessed using quantitative polymerase chain reaction.

Results

Although A-CTM supported more tenocyte adhesion, I-CTM promoted significantly more tenocyte proliferation compared with A-CTM and B-CTM. Unlike A-CTM, tenocyte migration was higher in I-CTM than the control. The presence of I-CTM also prevented the loss of tenocyte phenotype, attenuated the expression of pro-inflammatory cytokines, growth factors, and MMP, and promoted the expression of antifibrotic growth factor, TGFβ3.

Conclusion

Compared with A-CTM and B-CTM, I-CTM interacted more favorably with human tenocytes in vitro. I-CTM supported tenocyte proliferation with reduced de-differentiation and attenuation of the inflammatory response, suggesting that I-CTM may support tendon healing and regeneration in vivo.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40634-022-00509-4.

Cells Dynamically Adapt to Surface Geometry by Remodeling Their Focal Adhesions and Actin Cytoskeleton

Cells probe their environment and adapt their shape accordingly via the organization of focal adhesions and the actin cytoskeleton. In an earlier publication, we described the relationship between cell shape and physiology, for example, shape-induced differentiation, metabolism, and proliferation in mesenchymal stem cells and tenocytes. In this study, we investigated how these cells organize their adhesive machinery over time when exposed to microfabricated surfaces of different topographies and adhesive island geometries. We further examined the reciprocal interaction between stress fiber and focal adhesion formation by pharmacological perturbations. Our results confirm the current literature that spatial organization of adhesive sites determines the ability to form focal adhesions and stress fibers. Therefore, cells on roughened surfaces have smaller focal adhesion and fewer stress fibers. Our results further highlight the importance of integrin-mediated adhesion in the adaptive properties of cells and provide clear links to the development of bioactive materials.

The loop of phenotype: Dynamic reciprocity links tenocyte morphology to tendon tissue homeostasis

Cells and their surrounding extracellular matrix (ECM) are engaged in dynamic reciprocity to maintain tissue homeostasis: cells deposit ECM, which in turn presents the signals that define cell identity. This loop of phenotype is obvious for biochemical signals, such as collagens, which are produced by and presented to cells, but the role of biomechanical signals is also increasingly recognised. In addition, cell shape goes hand in hand with cell function and tissue homeostasis. Aberrant cell shape and ECM is seen in pathological conditions, and control of cell shape in micro-fabricated platforms disclose the causal relationship between cell shape and cell function, often mediated by mechanotransduction. In this manuscript, we discuss the loop of phenotype for tendon tissue homeostasis. We describe cell shape and ECM organization in normal and diseased tissue, how ECM composition influences tenocyte shape, and how that leads to the activation of signal transduction pathways and ECM deposition. We further describe the use of technologies to control cell shape to elucidate the link between cell shape and its phenotypical markers and focus on the causal role of cell shape in the loop of phenotype. STATEMENT OF SIGNIFICANCE: The dynamic reciprocity between cells and their surrounding extracellular matrix (ECM) influences biomechanical and biochemical properties of ECM as well as cell function through activation of signal transduction pathways that regulate gene and protein expression. We refer to this reciprocity as Loop of Phenotype and it has been studied and demonstrated extensively by using micro-fabricated platforms to manipulate cell shape and cell fate. In this manuscript, we discuss this concept in tendon tissue homeostasis by giving examples in healthy and pathological tenson tissue. Furthermore, we elaborate this by showing how biomaterials are used to feed this reciprocity to manipulate cell shape and function. Finally, we elucidate the link between cell shape and its phenotypical markers and focus on the activation of signal transduction pathways and ECM deposition.

Allogeneic Serum and Macromolecular Crowding Maintain Native Equine Tenocyte Function in Culture

The absence of a native extracellular matrix and the use of xenogeneic sera are often associated with rapid tenocyte function losses during in vitro culture. Herein, we assessed the influence of different sera (equine serum and foetal bovine serum) on equine tenocyte morphology, viability, metabolic activity, proliferation and protein synthesis as a function of tissue-specific extracellular matrix deposition (induced via macromolecular crowding), aging (passages 3, 6, 9) and time in culture (days 3, 5, 7). In comparison to cells at passage 3, at day 3, in foetal bovine serum and without macromolecular crowding (traditional equine tenocyte culture), the highest number of significantly decreased readouts were observed for cells in foetal bovine serum, at passage 3, at day 5 and day 7 and without macromolecular crowding. Again, in comparison to traditional equine tenocyte culture, the highest number of significantly increased readouts were observed for cells in equine serum, at passage 3 and passage 6, at day 7 and with macromolecular crowding. Our data advocate the use of an allogeneic serum and tissue-specific extracellular matrix for effective expansion of equine tenocytes.

Antioxidant effect of nicotinamide mononucleotide in tendinopathy

Background

A link between tendinopathy and oxidative stress has been recently reported. Nicotinamide mononucleotide (NMN) is a precursor of nicotinamide adenine dinucleotide, which plays an important role in cell redox homeostasis. The aim of this study was to evaluate the antioxidant effect of NMN on tendinopathy in vitro and in vivo.

Methods

Tenocytes from healthy Sprague-Dawley rats were cultured in regular glucose (RG) and high-glucose (HG) conditions with or without NMN, and were divided into four groups: RG NMN(−), RG NMN(+), HG NMN(−), and HG NMN(+). Cell viability, reactive oxygen species (ROS) accumulation, apoptotic rate, and mRNA expression of nicotinamide adenine dinucleotide phosphate oxidase (NOX)1, NOX4, interleukin (IL)6, sirtuin (SIRT)1, and SIRT6 were investigated. In addition, rats with collagenase-induced tendinopathy were treated with or without NMN. Immunostaining of NOX1 and NOX4; mRNA expression of SIRT1, SIRT6, and IL6; and superoxide dismutase (SOD) activity measurements in the Achilles tendon were performed.

Results

NMN increased the expression of SIRT1 and SIRT6 in rat tenocytes, but decreased the levels of NOX1, NOX4, IL6, ROS, and apoptosis. In Achilles tendons with collagenase-induced tendinopathy, NMN increased the mRNA expression of SIRT1 and SIRT6, as well as SOD activity; while suppressing protein expression of NOX1 and NOX4, and mRNA expression of IL6.

Conclusion

The in vitro and in vivo results of this study show that NMN exerts an antioxidant effect on tendinopathy by promoting the expression of SIRT while inhibiting that of NOX.

Changes in Physiological Tendon Substrate Stiffness Have Moderate Effects on Tendon-Derived Cell Growth and Immune Cell Activation

Tendinopathy is characterised by pathological changes in tendon matrix composition, architecture, and stiffness, alterations in tendon resident cell characteristics, and fibrosis, with inflammation also emerging as an important factor in tendinopathy progression. The sequence of pathological changes in tendinopathy and the cellular effects of the deteriorating matrix are largely unknown. This study investigated the effects of substrate stiffness on tendon-derived cells (TDCs) and THP-1 macrophages using PDMS substrates representing physiological tendon stiffness (1.88 MPa), a stiff gel (3.17 MPa) and a soft gel (0.61 MPa). Human TDCs were cultured on the different gel substrates and on tissue culture plastic. Cell growth was determined by alamarBlue™ assay, cell morphology was analysed in f-actin labelled cells, and phenotypic markers were analysed by real-time PCR. We found that in comparison to TDCs growing on gels with physiological stiffness, cell growth increased on soft gels at 48 h (23%, p = 0.003). Cell morphology was similar on all three gels. SCX expression was slightly reduced on the soft gels (1.4-fold lower, p = 0.026) and COL1A1 expression increased on the stiff gels (2.2-fold, p = 0.041). Culturing THP-1 macrophages on soft gels induced increased expression of IL1B (2-fold, p = 0.018), and IL8 expression was inhibited on the stiffer gels (1.9-fold, p = 0.012). We also found that culturing TDCs on plastic increased cell growth, altered cell morphology, and inhibited the expression of SCX, SOX9, MMP3, and COL3. We conclude that TDCs and macrophages respond to changes in matrix stiffness. The magnitude of responses measured in TDCs were minor on the range of substrate stiffness tested by the gels. Changes in THP-1 macrophages suggested a more inflammatory phenotype on substrates with non-physiological stiffness. Although cell response to subtle variations in matrix stiffness was moderate, it is possible that these alterations may contribute to the onset and progression of tendinopathy.

Mesenchymal Stromal Cells Adapt to Chronic Tendon Disease Environment with an Initial Reduction in Matrix Remodeling

Tendon lesions are common sporting injuries in humans and horses alike. The healing process of acute tendon lesions frequently results in fibrosis and chronic disease. In horses, local mesenchymal stromal cell (MSC) injection is an accepted therapeutic strategy with positive influence on acute lesions. Concerning the use of MSCs in chronic tendon disease, data are scarce but suggest less therapeutic benefit. However, it has been shown that MSCs can have a positive effect on fibrotic tissue. Therefore, we aimed to elucidate the interplay of MSCs and healthy or chronically diseased tendon matrix. Equine MSCs were cultured either as cell aggregates or on scaffolds from healthy or diseased equine tendons. Higher expression of tendon-related matrix genes and tissue inhibitors of metalloproteinases (TIMPs) was found in aggregate cultures. However, the tenogenic transcription factor scleraxis was upregulated on healthy and diseased tendon scaffolds. Matrix metalloproteinase (MMPs) expression and activity were highest in healthy scaffold cultures but showed a strong transient decrease in diseased scaffold cultures. The release of glycosaminoglycan and collagen was also higher in scaffold cultures, even more so in those with tendon disease. This study points to an early suppression of MSC matrix remodeling activity by diseased tendon matrix, while tenogenic differentiation remained unaffected.

Rotator Cuff Tendon Healing Using Human Dermal Fibroblasts: Histological and Biomechanical Analyses in a Rabbit Model of Chronic Rotator Cuff Tears

BACKGROUND

Tenocytes derived from tendons have been reported to be effective in the treatment of rotator cuff tears through the expression of extracellular matrix proteins. Human dermal fibroblasts, known to express collagen types I and III as tenocytes do, may likely be substitutes for tenocytes to enhance healing rotator cuff tears.

PURPOSE

To demonstrate the capability of human dermal fibroblasts to enhance healing of rotator cuff tears.

STUDY DESIGN

Controlled laboratory study.

METHODS

The cellular properties and expression profiles of growth factors were compared between human dermal fibroblasts and tenocytes. In both cell types, a series of extracellular matrix proteins were analyzed along with matrix metalloproteinases and tissue inhibitors of metalloproteinases involved in the collagenolytic system. A total of 35 rabbits were divided into 5 groups: normal (n = 2), saline control (n = 9), fibrin control (n = 9), low dose of human fibroblasts (HF-LD; n = 9), and high dose of human fibroblasts (HF-HD; n = 6). Cells were injected into the sutured lesions at 6 weeks after creation of bilateral rotator cuff tears, followed by histological and biomechanical analyses at 12 weeks.

RESULTS

Human dermal fibroblasts exhibited a protein expression pattern similar to that of tenocytes. More specifically, the expression levels of collagen types I and III were comparable between fibroblasts and tenocytes. The histological analysis of 30 surviving rabbits showed that collagen fibers were more continuous and better oriented with a more mature interface between the tendon and bone in the sutured lesions in the HF-LD and HF-HD groups. Most importantly, biomechanical strength, measured using the load to failure at the injection site, was 58.8 ± 8.9 N/kg in the HF-HD group, increasing by approximately 2-fold (P = .0003) over the saline control group.

CONCLUSION

Human dermal fibroblasts, showing cellular properties comparable with tenocytes, effectively enhanced healing of chronic rotator cuff tears in rabbits.

CLINICAL RELEVANCE

Human dermal fibroblasts can be used in place of tenocytes to enhance healing of rotator cuff tears.

Adult ovine connective tissue cells resemble mesenchymal stromal cells in their propensity for extensive ex vivo expansion

Purpose/Aim: Expanded, human connective tissue cells can adopt mesenchymal stromal cell (MSC) properties that are favorable for applications in regenerative medicine. Sheep are used as a large animal model for cell therapies, although for preclinical testing it is important to establish whether ovine cells resemble humans in their tendency to adopt MSC properties. The objective of this study was to investigate whether cells from five ovine connective tissues are MSC-like in their propensity for extensive expansion and immunophenotype.Materials and Methods: Monolayer cultures were established with cells from annulus fibrosus, cartilage, meniscus, tendon, and nucleus pulposus. Bone marrow MSCs were evaluated as a control. Cultures were seeded at 500 cells/cm², and subcultured every 5 days up to day 20. Flow cytometry was used to evaluate expression of cluster of differentiation (CD) molecules associated with MSCs (29, 44, 166). Colony formation was evaluated using time-lapse imaging of individual cells.Results: By day 20, cumulative population doublings ranged between 22 (chondrocytes) and 27 (MSCs). All cells uniformly expressed CD44 and 73. Expression of CD166 for MSCs was 98-99%, and ranged between 64 and 97% for the other cell types. Time-lapse imaging demonstrated that 58-94% of the cells colonized as indicated by 3 population doublings within 52 hours.Conclusions: Cells from ovine connective tissues resembled MSCs in their propensity for sustained, colony-forming growth and expression of CD molecules. These data supports the potential for preclinical testing of MSC-like connective tissue cells in sheep.

Tension Stimulation of Tenocytes in Aligned Hyaluronic Acid/Platelet-Rich Plasma-Polycaprolactone Core-Sheath Nanofiber Membrane Scaffold for Tendon Tissue Engineering

To recreate the in vivo niche for tendon tissue engineering in vitro, the characteristics of tendon tissue underlines the use of biochemical and biophysical cues during tenocyte culture. Herein, we prepare core-sheath nanofibers with polycaprolactone (PCL) sheath for mechanical support and hyaluronic acid (HA)/platelet-rich plasma (PRP) core for growth factor delivery. Three types of core-sheath nanofiber membrane scaffolds (CSNMS), consisting of random HA-PCL nanofibers (Random), random HA/PRP-PCL nanofibers (Random+) or aligned HA/PRP-PCL (Align⁺) nanofibers, were used to study response of rabbit tenocytes to biochemical (PRP) and biophysical (fiber alignment) stimulation. The core-sheath structures as well as other pertinent properties of CSNMS have been characterized, with Align⁺ showing the best mechanical properties. The unidirectional growth of tenocytes, as induced by aligned fiber topography, was confirmed from cell morphology and cytoskeleton expression. The combined effects of PRP and fiber alignment in Align⁺ CSNMS lead to enhanced cell proliferation rates, as well as upregulated gene expression and marker protein synthesis. Another biophysical cue on tenocytes was introduced by dynamic culture of tenocyte-seeded Align⁺ in a bioreactor with cyclic tension stimulation. Augmented by this biophysical beacon from mechanical loading, dynamic cell culture could shorten the time for tendon maturation in vitro, with improved cell proliferation rates and tenogenic phenotype maintenance, compared to static culture. Therefore, we successfully demonstrate how combined use of biochemical/topographical cues as well as mechanical stimulation could ameliorate cellular response of tenocytes in CSNMS, which can provide a functional in vitro environmental niche for tendon tissue engineering.

Minispheroids as a Tool for Ligament Tissue Engineering: Do the Self-Assembly Techniques and Spheroid Dimensions Influence the Cruciate Ligamentocyte Phenotype?

Spheroid culture might stabilize the ligamentocyte phenotype. Therefore, the phenotype of lapine cruciate ligamentocyte (L-CLs) minispheroids prepared either by hanging drop (HD) method or by using a novel spheroid plate (SP) and the option of methyl cellulose (MC) for tuning spheroid formation was tested. A total of 250 and 1000 L-CLs per spheroid were seeded as HDs or on an SP before performing cell viability assay, morphometry, gene expression (qRT-PCR) and protein immunolocalization after 7 (HD/SP) and 14 (SP) days. Stable and viable spheroids of both sizes could be produced with both methods, but more rapidly with SP. MC accelerated the formation of round spheroids (HD). Their circular areas decreased significantly during culturing. After 7 days, the diameters of HD-derived spheroids were significantly larger compared to those harvested from the SP, with a tendency of lower circularity suggesting an ellipsoid shape. Gene expression of decorin increased significantly after 7 days (HD, similar trend in SP), tenascin C tended to increase after 7 (HD/SP) and 14 days (SP), whereas collagen type 1 decreased (HD/SP) compared to the monolayer control. The cruciate ligament extracellular matrix components could be localized in all mini-spheroids, confirming their conserved expression profile and their suitability for ligament tissue engineering.

Tendon Cells Derived From The Long Head Of The Biceps And The Supraspinatus Tendons Of Patients Affected By Rotator Cuff Tears Show Different Expression Of Inflammatory Markers

AIM OF THE STUDY

Tendons are exposed to mechanical stress constantly during movements and thus they are frequently subjected to injuries. Rotator cuff tears are common musculoskeletal disorders, mainly involving the supraspinatus tendon. The characterization of the tenocytes derived from this tendon and the comparison to cells isolated from the long head of the biceps tendon obtained from donors affected by rotator cuff disease may improve the knowledge of the cellular mechanisms involved in the initiation and progression of the pathology. Thus, the aim of the present study was to characterize and compare donor-matched human tendon cells (TCs) isolated from the long head of the biceps (LHB-TCs) and the supraspinatus tendons (SSP-TCs) of patients affected by rotator cuff tears.

METHODS

donor-matched LHB-TCs and SSP-TCs were isolated and cultured up to passage 3. Phenotypic appearance, metabolic activity, DNA content, production of soluble mediators (IL-1Ra, IL-1β, IL-6, and VEGF) and gene expression of tendon markers (SCX, COL1A1, COL3A1), inflammatory (PTGS2), and catabolic enzymes (MMP-1, MMP-3) were evaluated.

RESULTS

LHB-TCs showed an elongated fibroblast-like shape, while SSP-TCs appeared irregular with jagged membrane. SSP-TCs gene expression revealed an augmented production of PTGS2, a marker of inflammation, whereas they produced a reduced amount of IL-6, in respect to LHB-TCs.

CONCLUSION

SSP-TCs showed higher cellular stress and expression of inflammatory markers with respect to donor-matched LHB-TCs, suggesting that addressing the physio-pathological state of supraspinatus tendon cells during treatment of rotator cuff tears could favor tissue healing and possibly prevent relapses.

Maintenance of Ligament Homeostasis of Spheroid-Colonized Embroidered and Functionalized Scaffolds after 3D Stretch

Anterior cruciate ligament (ACL) ruptures are usually treated with autograft implantation to prevent knee instability. Tissue engineered ACL reconstruction is becoming promising to circumvent autograft limitations. The aim was to evaluate the influence of cyclic stretch on lapine (L) ACL fibroblasts on embroidered scaffolds with respect to adhesion, DNA and sulphated glycosaminoglycan (sGAG) contents, gene expression of ligament-associated extracellular matrix genes, such as type I collagen, decorin, tenascin C, tenomodulin, gap junctional connexin 43 and the transcription factor Mohawk. Control scaffolds and those functionalized by gas phase fluorination and cross-linked collagen foam were either pre-cultured with a suspension or with spheroids of LACL cells before being subjected to cyclic stretch (4%, 0.11 Hz, 3 days). Stretch increased significantly the scaffold area colonized with cells but impaired sGAGs and decorin gene expression (functionalized scaffolds seeded with cell suspension). Stretching increased tenascin C, connexin 43 and Mohawk but decreased decorin gene expression (control scaffolds seeded with cell suspension). Pre-cultivation of functionalized scaffolds with spheroids might be the more suitable method for maintaining ligamentogenesis in 3D scaffolds compared to using a cell suspension due to a significantly higher sGAG content in response to stretching and type I collagen gene expression in functionalized scaffolds.

Impact of isolation method on cellular activation and presence of specific tendon cell subpopulations during in vitro culture

Tendon injuries are common and heal poorly, due in part to a lack of understanding of fundamental tendon cell biology. A major impediment to the study of tendon cells is the absence of robust, well-characterized in vitro models. Unlike other tissue systems, current tendon cell models do not account for how differences in isolation methodology may affect the activation state of tendon cells or the presence of various tendon cell subpopulations. The objective of this study was to characterize how common isolation methods affect the behavior, fate, and lineage composition of tendon cell cultures. Tendon cells isolated by explant exhibited reduced proliferative capacity, decreased expression of tendon marker genes, and increased expression of genes associated with fibroblast activation compared to digested cells. Consistently, explanted cells also displayed an increased propensity to differentiate to myofibroblasts compared to digested cells. Explanted cultures from multiple different tendons were substantially enriched for the presence of scleraxis-lineage (Scx-lin+) cells compared to digested cultures, while the overall percentage of S100a4-lineage (S100a4-lin+) cells was dependent on both isolation method and tendon of origin. Neither isolation methods preserved the ratios of Scx-lin+ or S100a4-lin+ to non-lineage cells seen in tendons in vivo. Combined, these data indicate that further refinement of in vitro cultures models is required in order to more accurately understand the effects of various stimuli on tendon cell behavior. Statement of clinical significance: The development of informed in vitro tendon cell models will facilitate enhanced screening of potential therapeutic candidates to improve tendon healing.

Serum deprivation limits loss and promotes recovery of tenogenic phenotype in tendon cell culture systems

Current knowledge gaps on tendon tissue healing can partly be ascribed to the limited availability of physiologically relevant culture models. An unnatural extracellular matrix, high serum levels and random cell morphology in vitro mimic strong vascularization and lost cell elongation in pathology, and discord with a healthy, in vivo cell microenvironment. The thereby induced phenotypic drift in tendon-derived cells (TDCs) compromises the validity of the research model. Therefore, this research quantified the extracellular matrix (ECM)-, serum-, and cell morphology-guided phenotypic changes in tendon cells of whole tendon fascicle explants with intact ECM and TDCs cultured in a controlled microenvironmental niche. Explanted murine tail tendon fascicles were cultured in serum-rich or serum-free medium and phenotype was assessed using transcriptome analysis. Next, phenotypic marker gene expression was measured in in vitro expanded murine tail TDCs upon culture in serum-rich or serum-free medium on aligned or random collagen I patterns. Freshly isolated fascicles or TDCs served as native controls. In both systems, the majority of tendon-specific genes were similarly attenuated in serum-rich culture. Strikingly, 1-week serum-deprived culture-independent of cell morphology-converged TDC gene expression toward native levels. This study reveals a dynamic serum-responsive tendon cell phenotype. Extracting fascicles or TDCs from their native environment causes large changes in cellular phenotype, which can be limited and even reversed by serum deprivation. We conclude that serum-derived factors override matrix-integrity and cell morphology cues and that serum-deprivation stimulates a more physiological microenvironment for in vitro studies.

Cellular Technologies in Traumatology: From Cells to Tissue Engineering

Injuries and degenerative changes of tendons are common damages of the musculoskeletal system. Due to its hypovascular character the tendon has a limited natural ability to recover. For typical surgical treatment, the tendon integrity is restored, but in most cases, there occurs formation of the connective tissue scar resulting in structural and mechanical functionality disruption. The insufficient effectiveness of traditional therapy methods requires the search for alternative ways to restore damaged tendon tissues. This article discusses new effective methods for improving the treatment that base on the use of cellular technologies among which one of the main directions is mesenchymal stem cell application. Due to mesenchymal stem cells, there is a shift from pro-fibrotic and pro-inflammatory reactions of cells to pro-regenerative ones. Stem cells being multipotent and having among other things tenogenic potential are considered a promising material for repairing damaged tendons. The article also describes the sources of progenitor tendon cells including the tendon bundles and pericytes the main markers of which are Scx and Mkx that are proteins of the transcription factor superfamily, and Tnmd that is transmembrane glycoprotein.

The growth factors that not only enhance the proliferative activity of mesenchymal stem cells but also promote in vitro tenogenic genes expression as well as the collagen Itype production what is necessary for tendon formation are considered. Along with growth factors, the morphogenetic protein BMP14 is presented, this protein increases themesenchymal stem cell proliferation and contributes directed tenogenic differentiation of these cells, suppressing their adipogenic and chondrogenic potentials.

In recent years, mesenchymal stem cells have been used both separately and in combination with various growth factors and different three-dimensional structures providing the interaction with all of the cell types.

The issues of the latest 3D-bioprinting technology allowing to make tissue-like structures for replacement damaged tissues and organs are discussed. 3D-bioprinting technology is known to allow acting exact spatio-temporal control of the distribution of cells, growth factors, small molecules, drugs and biologically active substances. 

Species variations in tenocytes' response to inflammation require careful selection of animal models for tendon research

For research on tendon injury, many different animal models are utilized; however, the extent to which these species simulate the clinical condition and disease pathophysiology has not yet been critically evaluated. Considering the importance of inflammation in tendon disease, this study compared the cellular and molecular features of inflammation in tenocytes of humans and four common model species (mouse, rat, sheep, and horse). While mouse and rat tenocytes most closely equalled human tenocytes’ low proliferation capacity and the negligible effect of inflammation on proliferation, the wound closure speed of humans was best approximated by rats and horses. The overall gene expression of human tenocytes was most similar to mice under healthy, to horses under transient and to sheep under constant inflammatory conditions. Humans were best matched by mice and horses in their tendon marker and collagen expression, by horses in extracellular matrix remodelling genes, and by rats in inflammatory mediators. As no single animal model perfectly replicates the clinical condition and sufficiently emulates human tenocytes, fit-for-purpose selection of the model species for each specific research question and combination of data from multiple species will be essential to optimize translational predictive validity.

In vitro and in vivo tenocyte-protective effectiveness of dehydroepiandrosterone against high glucose-induced oxidative stress

BACKGROUND

Dehydroepiandrosterone (DHEA), an adrenal steroid, has a protective role against diabetes. This study aimed to investigate the in vitro and in vivo protective effects of DHEA against high glucose-induced oxidative stress in tenocytes and tendons.

METHODS

Tenocytes from normal Sprague-Dawley rats were cultured in low-glucose (LG) or high-glucose (HG) medium with or without DHEA. The experimental groups were: control group (LG without DHEA), LG with DHEA, HG without DHEA, and HG with DHEA. Reactive oxygen species (ROS) production, apoptosis, and messenger RNA (mRNA) expression of NADPH oxidase (NOX) 1 and 4, and interleukin-6 (IL-6) were determined. Further, diabetic rats were divided into a control group and a DHEA-injected group (DHEA group). NOX1 and NOX4 protein expression and mRNA expression of NOX1, NOX4, IL-6, matrix metalloproteinase (MMP)-2, tissue inhibitors of matrix metalloproteinase (TIMP)-2, and type I and III collagens in the Achilles tendon were determined.

RESULTS

In rat tenocytes, DHEA decreased the expression of NOX1 and IL-6, ROS accumulation, and apoptotic cells. In the diabetic rat Achilles tendon, NOX1 protein expression and mRNA expression of NOX1, IL-6, MMP-2, TIMP-2, and type III collagen were significantly lower while type I collagen expression was significantly higher in the DHEA group than in the control group.

CONCLUSIONS

DHEA showed antioxidant and anti-inflammatory effects both in vitro and in vivo. Moreover, DHEA improved tendon matrix synthesis and turnover, which are affected by hyperglycemic conditions. DHEA is a potential preventive drug for diabetic tendinopathy.

Cyclically stretched ACL fibroblasts emigrating from spheroids adapt their cytoskeleton and ligament-related expression profile

Mechanical stress of ligaments varies; hence, ligament fibroblasts must adapt their expression profile to novel mechanomilieus to ensure tissue resilience. Activation of the mechanoreceptors leads to a specific signal transduction, the so-called mechanotransduction. However, with regard to their natural three-dimensional (3D) microenvironment cell reaction to mechanical stimuli during emigrating from a 3D spheroid culture is still unclear. This study aims to provide a deeper understanding of the reaction profile of anterior cruciate ligament (ACL)-derived fibroblasts exposed to cyclic uniaxial strain in two-dimensional (2D) monolayer culture and during emigration from 3D spheroids with respect to cell survival, cell and cytoskeletal orientation, distribution, and expression profile. Monolayers and spheroids were cultured in crosslinked polydimethyl siloxane (PDMS) elastomeric chambers and uniaxially stretched (14% at 0.3 Hz) for 48 h. Cell vitality, their distribution, nuclear shape, stress fiber orientation, focal adhesions, proliferation, expression of ECM components such as sulfated glycosaminoglycans, collagen type I, decorin, tenascin C and cell-cell communication-related gap junctional connexin (CXN) 43, tendon-related markers Mohawk and tenomodulin (myodulin) were analyzed. In contrast to unstretched cells, stretched fibroblasts showed elongation of stress fibers, cell and cytoskeletal alignment perpendicular to strain direction, less rounded cell nuclei, increased numbers of focal adhesions, proliferation, amplified CXN43, and main ECM component expression in both cultures. The applied cyclic stretch protocol evoked an anabolic response and enhanced tendon-related marker expression in ACL-derived fibroblasts emigrating from 3D spheroids and seems also promising to support in future tissue formation in ACL scaffolds seeded in vitro with spheroids.

Hydrogel Scaffolds to Deliver Cell Therapies for Wound Healing

Cutaneous wounds are a growing global health burden as a result of an aging population coupled with increasing incidence of diabetes, obesity, and cancer. Cell-based approaches have been used to treat wounds due to their secretory, immunomodulatory, and regenerative effects, and recent studies have highlighted that delivery of stem cells may provide the most benefits. Delivering these cells to wounds with direct injection has been associated with low viability, transient retention, and overall poor efficacy. The use of bioactive scaffolds provides a promising method to improve cell therapy delivery. Specifically, hydrogels provide a physiologic microenvironment for transplanted cells, including mechanical support and protection from native immune cells, and cell-hydrogel interactions may be tailored based on specific tissue properties. In this review, we describe the current and future directions of various cell therapies and usage of hydrogels to deliver these cells for wound healing applications.

The Lack of a Representative Tendinopathy Model Hampers Fundamental Mesenchymal Stem Cell Research

Overuse tendon injuries are a major cause of musculoskeletal morbidity in both human and equine athletes, due to the cumulative degenerative damage. These injuries present significant challenges as the healing process often results in the formation of inferior scar tissue. The poor success with conventional therapy supports the need to search for novel treatments to restore functionality and regenerate tissue as close to native tendon as possible. Mesenchymal stem cell (MSC)-based strategies represent promising therapeutic tools for tendon repair in both human and veterinary medicine. The translation of tissue engineering strategies from basic research findings, however, into clinical use has been hampered by the limited understanding of the multifaceted MSC mechanisms of action. In vitro models serve as important biological tools to study cell behavior, bypassing the confounding factors associated with in vivo experiments. Controllable and reproducible in vitro conditions should be provided to study the MSC healing mechanisms in tendon injuries. Unfortunately, no physiologically representative tendinopathy models exist to date. A major shortcoming of most currently available in vitro tendon models is the lack of extracellular tendon matrix and vascular supply. These models often make use of synthetic biomaterials, which do not reflect the natural tendon composition. Alternatively, decellularized tendon has been applied, but it is challenging to obtain reproducible results due to its variable composition, less efficient cell seeding approaches and lack of cell encapsulation and vascularization. The current review will overview pros and cons associated with the use of different biomaterials and technologies enabling scaffold production. In addition, the characteristics of the ideal, state-of-the-art tendinopathy model will be discussed. Briefly, a representative in vitro tendinopathy model should be vascularized and mimic the hierarchical structure of the tendon matrix with elongated cells being organized in a parallel fashion and subjected to uniaxial stretching. Incorporation of mechanical stimulation, preferably uniaxial stretching may be a key element in order to obtain appropriate matrix alignment and create a pathophysiological model. Together, a thorough discussion on the current status and future directions for tendon models will enhance fundamental MSC research, accelerating translation of MSC therapies for tendon injuries from bench to bedside.

Cruciate Ligament Cell Sheets Can Be Rapidly Produced on Thermoresponsive poly(glycidyl ether) Coating and Successfully Used for Colonization of Embroidered Scaffolds

Anterior cruciate ligament (ACL) cell sheets combined with biomechanically competent scaffolds might facilitate ACL tissue engineering. Since thermoresponsive polymers allow a rapid enzyme-free detachment of cell sheets, we evaluated the applicability of a thermoresponsive poly(glycidyl ether) (PGE) coating for cruciate ligamentocyte sheet formation and its influence on ligamentocyte phenotype during sheet-mediated colonization of embroidered scaffolds. Ligamentocytes were seeded on surfaces either coated with PGE or without coating. Detached ligamentocyte sheets were cultured separately or wrapped around an embroidered scaffold made of polylactide acid (PLA) and poly(lactic-co-ε-caprolactone) (P(LA-CL)) threads functionalized by gas-phase fluorination and with collagen foam. Ligamentocyte viability, protein and gene expression were determined in sheets detached from surfaces with or without PGE coating, scaffolds seeded with sheets from PGE-coated plates and the respective monolayers. Stable and vital ligamentocyte sheets could be produced within 24 h with both surfaces, but more rapidly with PGE coating. PGE did not affect ligamentocyte phenotype. Scaffolds could be colonized with sheets associated with high cell survival, stable gene expression of ligament-related type I collagen, decorin, tenascin C and Mohawk after 14 d and extracellular matrix (ECM) deposition. PGE coating facilitates ligamentocyte sheet formation, and sheets colonizing the scaffolds displayed a ligament-related phenotype.

Effect of Muscle Cell Preservation on Viability and Differentiation of Hamstring Tendon Graft In Vitro

Muscle tissue is often removed during hamstring tendon graft preparation for anterior cruciate ligament (ACL) reconstruction. The purpose of the study was to test whether preservation of muscle remnants on a tendon graft is beneficial to the graft healing process following ACL reconstruction. Co-culturing of tendon-derived cells (TDCs) and muscle-derived cells (MDCs) was performed at various ratios, and their potential for cell viability and multilineage differentiation was compared to a single TDC cell group. Ligamentous and chondrogenic differentiation was most enhanced when a small population of MDCs was co-cultured with TDCs (6:2 co-culture group). Cell viability and osteogenic differentiation were proportionally enhanced with increasing MDC population size. MDCs co-cultured with TDCs possess both the ability to enhance cell viability and differentiate into other cell lineages.

Equine Tenocyte Seeding on Gelatin Hydrogels Improves Elongated Morphology

(1) Background: Tendinopathy is a common injury in both human and equine athletes. Representative in vitro models are mandatory to facilitate translation of fundamental research into successful clinical treatments. Natural biomaterials like gelatin provide favorable cell binding characteristics and are easily modifiable. In this study, methacrylated gelatin (gel-MA) and norbornene-functionalized gelatin (gel-NB), crosslinked with 1,4-dithiotreitol (DTT) or thiolated gelatin (gel-SH) were compared. (2) Methods: The physicochemical properties (1H-NMR spectroscopy, gel fraction, swelling ratio, and storage modulus) and equine tenocyte characteristics (proliferation, viability, and morphology) of four different hydrogels (gel-MA, gel-NB85/DTT, gel-NB55/DTT, and gel-NB85/SH75) were evaluated. Cellular functionality was analyzed using fluorescence microscopy (viability assay and focal adhesion staining). (3) Results: The thiol-ene based hydrogels showed a significantly lower gel fraction/storage modulus and a higher swelling ratio compared to gel-MA. Significantly less tenocytes were observed on gel-MA discs at 14 days compared to gel-NB85/DTT, gel-NB55/DTT and gel-NB85/SH75. At 7 and 14 days, the characteristic elongated morphology of tenocytes was significantly more pronounced on gel-NB85/DTT and gel-NB55/DTT in contrast to TCP and gel-MA. (4) Conclusions: Thiol-ene crosslinked gelatins exploiting DTT as a crosslinker are the preferred biomaterials to support the culture of tenocytes. Follow-up experiments will evaluate these biomaterials in more complex models.

Growth factor and macromolecular crowding supplementation in human tenocyte culture

Cell-assembled tissue engineering strategies hold great potential in regenerative medicine, as three-dimensional tissue-like modules can be produced, even from a patient's own cells. However, the development of such implantable devices requires prolonged in vitro culture time, which is associated with cell phenotypic drift. Considering that the cells in vivo are subjected to numerous stimuli, multifactorial approaches are continuously gaining pace towards controlling cell fate during in vitro expansion. Herein, we assessed the synergistic effect of simultaneous and serial growth factor supplementation (insulin growth factor-1, platelet-derived growth factor ββ, growth differentiation factor 5 and transforming growth factor β3) to macromolecular crowding (carrageenan) in human tenocyte function; collagen synthesis and deposition; and gene expression. TGFβ3 supplementation (without/with carrageenan) induced the highest (among all groups) DNA content. In all cases, tenocyte proliferation was significantly increased as a function of time in culture, whilst metabolic activity was not affected. Carrageenan supplementation induced significantly higher collagen deposition than groups without carrageenan (without/with any growth factor). Of all the growth factors used, TGFβ3 induced the highest collagen deposition when used together with carrageenan in both simultaneous and serial fashion. At day 13, gene expression analysis revealed that TGFβ3 in serial supplementation to carrageenan upregulated the most and downregulated the least collagen- and tendon- related genes and upregulated the least and downregulated the most osteo-, chondro-, fibrosis- and adipose- related trans-differentiation genes. Collectively, these data clearly advocate the beneficial effects of multifactorial approaches (in this case, growth factor and macromolecular crowding supplementation) in the development of functional cell-assembled tissue surrogates.

Electrospun Scaffold Micro-Architecture Induces an Activated Transcriptional Phenotype within Tendon Fibroblasts

Biomaterial augmentation of surgically repaired rotator cuff tendon tears aims to improve the high failure rates (∼40%) of traditional repairs. Biomaterials that can alter cellular phenotypes through the provision of microscale topographical cues are now under development. We aimed to systematically evaluate the effect of topographic architecture on the cellular phenotype of fibroblasts from healthy and diseased tendons. Electrospun polydioxanone scaffolds with fiber diameters ranging from 300 to 4000 nm, in either a highly aligned or random configuration, were produced. Healthy tendon fibroblasts cultured for 7 days on scaffolds with highly aligned fibers demonstrated a distinctive elongated morphology, whilst those cultured on randomly configured fibers demonstrated a flattened and spread morphology. The effect of scaffold micro-architecture on the transcriptome of both healthy and diseased tendon fibroblasts was assessed with bulk RNA-seq. Both healthy (n = 3) and diseased tendon cells (n = 3) demonstrated a similar transcriptional response to architectural variants. Gene set enrichment analysis revealed that large diameter (≥2000 nm) aligned scaffolds induced an upregulation of genes involved in cellular replication and a downregulation of genes defining inflammatory responses and cell adhesion. Similarly, PDPN and CD248, markers of inflammatory or "activated" fibroblasts, were downregulated during culture of both healthy and diseased fibroblasts on aligned scaffolds with large (≥2000 nm) fiber diameters. In conclusion scaffold architectures resembling that of disordered type III collagen, typically present during the earlier phases of wound healing, resulted in tendon fibroblast activation. Conversely, scaffolds mimicking aligned diameter collagen I fibrils, present during tissue remodelling, did not activate tendon derived fibroblasts. This has implications for the design of scaffolds used during rotator cuff repair augmentation.