Identification of signalling pathways triggered by changes in the mechanical environment in rat chondrocytes

Piezoelectric effect stimulates the rearrangement of chondrogenic cells and alters ciliary orientation via atypical PKCζ

Therapeutic ultrasound was administered to patients suffering from bone fracture with FDA approval. Bone and cartilage are piezoelectric materials. To investigate the effects of piezoelectricity on the cells of chondrogenic lineage, we applied ultrasound stimulation on an AT-cut quartz coverslip to generate electric field fluctuations. The bone-marrow-derived mesenchymal stem cells (BMMSC) and primary chondrocytes were cultured on either glass or quartz coverslips for ultrasound stimulation. The cells were immunofluorescent-labeled for the assessment of cell arrangement and ciliary orientation. Ultrasound and piezoelectricity both stimulate cell migration and disrupt ciliary orientation induced by directional migration. In particular, piezoelectric effects on cell rearrangement can be abolished by the inhibitor specifically targeting atypical Protein kinase C zeta (PKCζ). Our findings shed light on the possibility of cellular modulation by using piezoelectric manipulation.

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Separating the effect of piezoelectric stimulation from ultrasound stimulation.

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Cell migration accelerates upon ultrasound and piezoelectric stimulation.

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Piezoelectric stimulation influences cell polarity of chondrogenic lineage.

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Piezoelectric stimulation induces cell rearrangement via PKCζ.

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Novel strategy for modulating cell growth, cell differentiation or tissue engineering via piezoelectric stimulation.

Molecular Indicators of Biomaterials Osteoinductivity - Cell Migration, BMP Production and Signalling Turns a Key

In this work we dissected the osteoinductive properties of selected, PLGA-based scaffolds enriched with gel-derived bioactive glasses (SBGs) of either binary SiO2-CaO or ternary SiO2-CaO-P2O5 system, differing in CaO/SiO2 ratio (i.e. high -or low-calcium SBGs). To assess the inherent ability of the scaffolds to induce osteogenesis of human bone marrow stromal cells (BMSC), the study was designed to avoid any osteogenic stimuli beyond the putative osteogenic SBG component of the studied scaffolds. The bioactivity and porosity of scaffolds were confirmed by SBF test and porosimetry. Condition media (CM) from BMSC-loaded scaffolds exhibited increased Ca and decreased P content corresponding to SBGs CaO/SiO2 ratio, whereas Si content was relatively stable and overall lower in CM from scaffolds containing binary SBGs. CM from cell-loaded scaffolds containing high-calcium, binary SBGs promoted migration of BMSC and BMP-response in reporter osteoblast cell line. BMSC culture on these scaffolds or the ones containing ternary, low-calcium SBGs resulted in the activation of BMP-related signaling and expression of several osteogenic markers. Ectopic bone formation was induced by scaffolds containing binary SBGs, but high-calcium ones produced significantly more osteoid. Scaffolds containing ternary SBGs negatively influenced the expression of osteogenic transcription factors and Cx43, involved in cell-cell interactions. High-calcium scaffolds stimulated overall higher Cx43 expression. We believe the initial cell-cell communication may be crucial to induce and maintain osteogenesis and high BMP signaling on the studied scaffolds. The presented scaffolds' biological properties may also constitute new helpful markers to predict osteoinductive potential of other bioactive implant materials.

Cartilage Tissue Engineering Approaches Need to Assess Fibrocartilage When Hydrogel Constructs Are Mechanically Loaded

Chondrocytes that are impregnated within hydrogel constructs sense applied mechanical force and can respond by expressing collagens, which are deposited into the extracellular matrix (ECM). The intention of most cartilage tissue engineering is to form hyaline cartilage, but if mechanical stimulation pushes the ratio of collagen type I (Col1) to collagen type II (Col2) in the ECM too high, then fibrocartilage can form instead. With a focus on Col1 and Col2 expression, the first part of this article reviews the latest studies on hyaline cartilage regeneration within hydrogel constructs that are subjected to compression forces (one of the major types of the forces within joints) in vitro. Since the mechanical loading conditions involving compression and other forces in joints are difficult to reproduce in vitro, implantation of hydrogel constructs in vivo is also reviewed, again with a focus on Col1 and Col2 production within the newly formed cartilage. Furthermore, mechanotransduction pathways that may be related to the expression of Col1 and Col2 within chondrocytes are reviewed and examined. Also, two recently-emerged, novel approaches of load-shielding and synchrotron radiation (SR)–based imaging techniques are discussed and highlighted for future applications to the regeneration of hyaline cartilage. Going forward, all cartilage tissue engineering experiments should assess thoroughly whether fibrocartilage or hyaline cartilage is formed.

Current hydrogel advances in physicochemical and biological response-driven biomedical application diversity

Hydrogel is a type of versatile platform with various biomedical applications after rational structure and functional design that leverages on material engineering to modulate its physicochemical properties (e.g., stiffness, pore size, viscoelasticity, microarchitecture, degradability, ligand presentation, stimulus-responsive properties, etc.) and influence cell signaling cascades and fate. In the past few decades, a plethora of pioneering studies have been implemented to explore the cell-hydrogel matrix interactions and figure out the underlying mechanisms, paving the way to the lab-to-clinic translation of hydrogel-based therapies. In this review, we first introduced the physicochemical properties of hydrogels and their fabrication approaches concisely. Subsequently, the comprehensive description and deep discussion were elucidated, wherein the influences of different hydrogels properties on cell behaviors and cellular signaling events were highlighted. These behaviors or events included integrin clustering, focal adhesion (FA) complex accumulation and activation, cytoskeleton rearrangement, protein cyto-nuclei shuttling and activation (e.g., Yes-associated protein (YAP), catenin, etc.), cellular compartment reorganization, gene expression, and further cell biology modulation (e.g., spreading, migration, proliferation, lineage commitment, etc.). Based on them, current in vitro and in vivo hydrogel applications that mainly covered diseases models, various cell delivery protocols for tissue regeneration and disease therapy, smart drug carrier, bioimaging, biosensor, and conductive wearable/implantable biodevices, etc. were further summarized and discussed. More significantly, the clinical translation potential and trials of hydrogels were presented, accompanied with which the remaining challenges and future perspectives in this field were emphasized. Collectively, the comprehensive and deep insights in this review will shed light on the design principles of new biomedical hydrogels to understand and modulate cellular processes, which are available for providing significant indications for future hydrogel design and serving for a broad range of biomedical applications.

Substrate stiffness and sequence dependent bioactive peptide hydrogels influence the chondrogenic differentiation of human mesenchymal stem cells

N-Cadherin mimetic nanofibrous biocompatible peptide hydrogels with enhanced mechanical properties for differentiation of mesenchymal stem cells into chondrocytes.

Mechanotransduction and Stiffness-Sensing: Mechanisms and Opportunities to Control Multiple Molecular Aspects of Cell Phenotype as a Design Cornerstone of Cell-Instructive Biomaterials for Articular Cartilage Repair

Since material stiffness controls many cell functions, we reviewed the currently available knowledge on stiffness sensing and elucidated what is known in the context of clinical and experimental articular cartilage (AC) repair. Remarkably, no stiffness information on the various biomaterials for clinical AC repair was accessible. Using mRNA expression profiles and morphology as surrogate markers of stiffness-related effects, we deduced that the various clinically available biomaterials control chondrocyte (CH) phenotype well, but not to equal extents, and only in non-degenerative settings. Ample evidence demonstrates that multiple molecular aspects of CH and mesenchymal stromal cell (MSC) phenotype are susceptible to material stiffness, because proliferation, migration, lineage determination, shape, cytoskeletal properties, expression profiles, cell surface receptor composition, integrin subunit expression, and nuclear shape and composition of CHs and/or MSCs are stiffness-regulated. Moreover, material stiffness modulates MSC immuno-modulatory and angiogenic properties, transforming growth factor beta 1 (TGF-β1)-induced lineage determination, and CH re-differentiation/de-differentiation, collagen type II fragment production, and TGF-β1- and interleukin 1 beta (IL-1β)-induced changes in cell stiffness and traction force. We then integrated the available molecular signaling data into a stiffness-regulated CH phenotype model. Overall, we recommend using material stiffness for controlling cell phenotype, as this would be a promising design cornerstone for novel future-oriented, cell-instructive biomaterials for clinical high-quality AC repair tissue.

Cyclic hydrostatic compress force regulates apoptosis of meniscus fibrochondrocytes via integrin α5β1

Meniscus is a semilunar fibrocartilaginous tissue, serving important roles in load buffering, stability, lubrication, proprioception, and nutrition of the knee joint. The degeneration and damage of meniscus has been proved to be a risk factor of knee osteoarthritis. Mechanical stimulus is a critical factor of the development, maintenance and repair of the meniscus fibrochondrocytes. However, the mechanism of the mechano-transduction process remains elusive. Here we reported that cyclic hydrostatic compress force (CHCF) treatment promotes proliferation and inhibits apoptosis of the isolated primary meniscus fibrochondrocytes (PMFs), via upregulating the expression level of integrin α5β1. Consequently, increased phosphorylated-ERK1/2 and phosphorylated-PI3K, and decreased caspase-3 were detected. These effects of CHCF treatment can be abolished by integrin α5β1 inhibitor or specific siRNA transfection. These data indicate that CHCF regulates apoptosis of PMFs via integrin α5β1-FAK-PI3K/ERK pathway, which may be an important candidate approach during meniscus degeneration.

Elf3 Contributes to Cartilage Degradation in vivo in a Surgical Model of Post-Traumatic Osteoarthritis

The E-74 like factor 3 (ELF3) is a transcription factor induced by inflammatory factors in various cell types, including chondrocytes. ELF3 levels are elevated in human cartilage from patients with osteoarthritis (OA), and ELF3 contributes to the IL-1β-induced expression of genes encoding Mmp13, Nos2, and Ptgs2/Cox2 in chondrocytes in vitro. Here, we investigated the contribution of ELF3 to cartilage degradation in vivo, using a mouse model of OA. To this end, we generated mouse strains with cartilage-specific Elf3 knockout (Col2Cre:Elf3^f/f) and Comp-driven Tet-off-inducible Elf3 overexpression (TRE-Elf3:Comp-tTA). To evaluate the contribution of ELF3 to OA, we induced OA in 12-week-old Col2Cre:Elf3^f/f and 6-month-old TRE-Elf3:Comp-tTA male mice using the destabilization of the medial meniscus (DMM) model. The chondrocyte-specific deletion of Elf3 led to decreased levels of IL-1β- and DMM-induced Mmp13 and Nos2 mRNA in vitro and in vivo, respectively. Histological grading showed attenuation of cartilage loss in Elf3 knockout mice compared to wild type (WT) littermates at 8 and 12 weeks following DMM surgery that correlated with reduced collagenase activity. Accordingly, Elf3 overexpression led to increased cartilage degradation post-surgery compared to WT counterparts. Our results provide evidence that ELF3 is a central contributing factor for cartilage degradation in post-traumatic OA in vivo.

Down-Regulation of Transglutaminase 2 Stimulates Redifferentiation of Dedifferentiated Chondrocytes through Enhancing Glucose Metabolism

Expansion of chondrocytes for repair of articular cartilage can lead to dedifferentiation, making it difficult to obtain a sufficient quantity of chondrocytes. Although previous studies have suggested that culture in a three-dimensional environment induces redifferentiation of dedifferentiated chondrocytes, its underlying mechanisms are still poorly understood in terms of metabolism compared with a two-dimensional environment. In this study, we demonstrate that attenuation of transglutaminase 2 (TG2), a multifunctional enzyme, stimulates redifferentiation of dedifferentiated chondrocytes. Fibroblast-like morphological changes increased as TG2 expression increased in passage-dependent manner. When dedifferentiated chondrocytes were cultured in a pellet culture system, TG2 expression was reduced and glycolytic enzyme expression up-regulated. Previous studies demonstrated that TG2 influences energy metabolism, and impaired glycolytic metabolism causes chondrocyte dedifferentiation. Interestingly, TG2 knockdown improved chondrogenic gene expression, glycolytic enzyme expression, and lactate production in a monolayer culture system. Taken together, down-regulation of TG2 is involved in redifferentiaton of dedifferentiated chondrocytes through enhancing glucose metabolism.

Hypoxia regulates RhoA and Wnt/β-catenin signaling in a context-dependent way to control re-differentiation of chondrocytes

Cartilage tissue is avascular and hypoxic which regulates chondrocyte phenotype via stabilization of HIFs. Here, we investigated the role of hypoxia and HIFs in regulation of Rho and canonical Wnt signaling in chondrocytes. Our data demonstrates that hypoxia controls the expression of RhoA in chondrocytes in a context-dependent manner on the culturing conditions. Within a 3D microenvironment, hypoxia suppresses RhoA on which hypoxia-driven expression of chondrogenic markers depends. Conversely, hypoxia leads to upregulation of RhoA in chondrocytes on 2D with a failure in re-expression of chondrogenic markers. Similarly to RhoA, hypoxic regulation of Wnt/β-catenin signaling depends on the microenvironment. Hypoxia downregulates β-catenin within 3D hydrogels whereas it causes a potent increase on 2D. Hypoxia-induced suppression of canonical Wnt signaling in 3D contributes to the promotion of chondrogenic phenotype as induction of Wnt signaling abrogates the hypoxic re-differentiation of chondrocytes. Inhibiting Wnt/β-catenin signaling via stabilization of Axin2 leads to a synergistic enhancement of hypoxia-induced expression of chondrogenic markers. The effects of hypoxia on Rho and Wnt/β-catenin signaling are HIF-dependent as stabilizing HIFs under normoxia revealed similar effects on chondrocytes. The study reveals important insights on hypoxic signaling of chondrocytes and how hypoxia regulates cellular mechanisms depending on the cellular microenvironment.

Mechano-sensitive regulation of gene expression during the embryonic development

Cell movements during embryogenesis produce mechanical tensions that shape the embryo and can also regulate gene expression, thereby affecting cell differentiation. Increasing evidence indicates that mechanosensitive regulation of gene expression plays important roles during embryogenesis by coupling the processes of morphogenesis and differentiation. However, the molecular mechanisms of this phenomenon remain poorly understood. This review focuses on the molecular mechanisms that "translate" mechanical stimuli into gene expression.

Substrate Stiffness Controls Osteoblastic and Chondrocytic Differentiation of Mesenchymal Stem Cells without Exogenous Stimuli

Stem cell fate has been linked to the mechanical properties of their underlying substrate, affecting mechanoreceptors and ultimately leading to downstream biological response. Studies have used polymers to mimic the stiffness of extracellular matrix as well as of individual tissues and shown mesenchymal stem cells (MSCs) could be directed along specific lineages. In this study, we examined the role of stiffness in MSC differentiation to two closely related cell phenotypes: osteoblast and chondrocyte. We prepared four methyl acrylate/methyl methacrylate (MA/MMA) polymer surfaces with elastic moduli ranging from 0.1 MPa to 310 MPa by altering monomer concentration. MSCs were cultured in media without exogenous growth factors and their biological responses were compared to committed chondrocytes and osteoblasts. Both chondrogenic and osteogenic markers were elevated when MSCs were grown on substrates with stiffness <10 MPa. Like chondrocytes, MSCs on lower stiffness substrates showed elevated expression of ACAN, SOX9, and COL2 and proteoglycan content; COMP was elevated in MSCs but reduced in chondrocytes. Substrate stiffness altered levels of RUNX2 mRNA, alkaline phosphatase specific activity, osteocalcin, and osteoprotegerin in osteoblasts, decreasing levels on the least stiff substrate. Expression of integrin subunits α1, α2, α5, αv, β1, and β3 changed in a stiffness- and cell type-dependent manner. Silencing of integrin subunit beta 1 (ITGB1) in MSCs abolished both osteoblastic and chondrogenic differentiation in response to substrate stiffness. Our results suggest that substrate stiffness is an important mediator of osteoblastic and chondrogenic differentiation, and integrin β1 plays a pivotal role in this process.

Hydrostatic Compress Force Enhances the Viability and Decreases the Apoptosis of Condylar Chondrocytes through Integrin-FAK-ERK/PI3K Pathway

Reduced mechanical stimuli in many pathological cases, such as hemimastication and limited masticatory movements, can significantly affect the metabolic activity of mandibular condylar chondrocytes and the growth of mandibles. However, the molecular mechanisms for these phenomena remain unclear. In this study, we hypothesized that integrin-focal adhesion kinase (FAK)-ERK (extracellular signal-regulated kinase)/PI3K (phosphatidylinositol-3-kinase) signaling pathway mediated the cellular response of condylar chondrocytes to mechanical loading. Primary condylar chondrocytes were exposed to hydrostatic compressive forces (HCFs) of different magnitudes (0, 50, 100, 150, 200, and 250 kPa) for 2 h. We measured the viability, morphology, and apoptosis of the chondrocytes with different treatments as well as the gene, protein expression, and phosphorylation of mechanosensitivity-related molecules, such as integrin α2, integrin α5, integrin β1, FAK, ERK, and PI3K. HCFs could significantly increase the viability and surface area of condylar chondrocytes and decrease their apoptosis in a dose-dependent manner. HCF of 250 kPa resulted in a 1.51 ± 0.02-fold increase of cell viability and reduced the ratio of apoptotic cells from 18.10% ± 0.56% to 7.30% ± 1.43%. HCFs could significantly enhance the mRNA and protein expression of integrin α2, integrin α5, and integrin β1 in a dose-dependent manner, but not ERK1, ERK2, or PI3K. Instead, HCF could significantly increase phosphorylation levels of FAK, ERK1/2, and PI3K in a dose-dependent manner. Cilengitide, the potent integrin inhibitor, could dose-dependently block such effects of HCFs. HCFs enhances the viability and decreases the apoptosis of condylar chondrocytes through the integrin-FAK-ERK/PI3K pathway.

Cyclical compressive stress induces differentiation of rat primary mandibular condylar chondrocytes through phosphorylated myosin light chain II

The role of myosin light chain II (MLC‑II) in cellular differentiation of rat mandibular condylar chondrocytes (MCCs) induced by cyclical uniaxial compressive stress (CUCS) remains unclear. In the current study, a four‑point bending system was used to apply CUCS to primary cultured MCCs from rats. It was identified that CUCS stimulated features of cellular differentiation including morphological alterations, cytoskeleton rearrangement and overproduction of proteoglycans. Furthermore, CUCS promoted runt‑related transcription factor‑2 (RUNX2) expression at mRNA (P<0.01) and protein levels (P<0.05) and elevated alkaline phosphatase (ALP) activity (P<0.01), which are both markers of osteogenic differentiation. Under conditions of stress, western blotting indicated that the ratio of phosphorylated MLC‑II to total MLC‑II was increased significantly (P<0.05). Silencing MLC‑II by RNA interference reduced ALP activity (P<0.01), and eliminated RUNX2 mRNA expression (P<0.01). Addition of the MLC kinase inhibitor, ML‑7, reduced the CUCS‑associated upregulation of RUNX2 expression (P<0.01) and ALP activity (P<0.01). The data indicated that CUCS promoted cellular differentiation of rat primary MCCs, and this was suggested to be via the phosphorylation of MLC‑II.

Mechanical microenvironments and protein expression associated with formation of different skeletal tissues during bone healing

Uncovering the mechanisms of the sensitivity of bone healing to mechanical factors is critical for understanding the basic biology and mechanobiology of the skeleton, as well as for enhancing clinical treatment of bone injuries. This study refined an experimental method of measuring the strain microenvironment at the site of a bone injury during bone healing. This method used a rat model in which a well-controlled bending motion was applied to an osteotomy to induce the formation of pseudarthrosis that is composed of a range of skeletal tissues, including woven bone, cartilage, fibrocartilage, fibrous tissue, and clot tissue. The goal of this study was to identify both the features of the strain microenvironment associated with formation of these different tissues and the expression of proteins frequently implicated in sensing and transducing mechanical cues. By pairing the strain measurements with histological analyses that identified the regions in which each tissue type formed, we found that formation of the different tissue types occurs in distinct strain microenvironments and that the type of tissue formed is correlated most strongly to the local magnitudes of extensional and shear strains. Weaker correlations were found for dilatation. Immunohistochemical analyses of focal adhesion kinase and rho family proteins RhoA and CDC42 revealed differences within the cartilaginous tissues in the calluses from the pseudarthrosis model as compared to fracture calluses undergoing normal endochondral bone repair. These findings suggest the involvement of these proteins in the way by which mechanical stimuli modulate the process of cartilage formation during bone healing.

The growth plate's response to load is partially mediated by mechano-sensing via the chondrocytic primary cilium

Mechanical load plays a significant role in bone and growth-plate development. Chondrocytes sense and respond to mechanical stimulation; however, the mechanisms by which those signals exert their effects are not fully understood. The primary cilium has been identified as a mechano-sensor in several cell types, including renal epithelial cells and endothelium, and accumulating evidence connects it to mechano-transduction in chondrocytes. In the growth plate, the primary cilium is involved in several regulatory pathways, such as the non-canonical Wnt and Indian Hedgehog. Moreover, it mediates cell shape, orientation, growth, and differentiation in the growth plate. In this work, we show that mechanical load enhances ciliogenesis in the growth plate. This leads to alterations in the expression and localization of key members of the Ihh-PTHrP loop resulting in decreased proliferation and an abnormal switch from proliferation to differentiation, together with abnormal chondrocyte morphology and organization. Moreover, we use the chondrogenic cell line ATDC5, a model for growth-plate chondrocytes, to understand the mechanisms mediating the participation of the primary cilium, and in particular KIF3A, in the cell's response to mechanical stimulation. We show that this key component of the cilium mediates gene expression in response to mechanical stimulation.

The role of Alk-1 and Alk-5 in the mechanosensing of chondrocytes

We aim to demonstrate the role of Alk receptors in the response of hydrogel expansion. Chondrocytes from rat knees were cultured onto plastic and hydrogel surfaces. Alk-1 and Alk-5 were overexpressed or silenced and the effects on cells during expansion were tested and confirmed using peptide inhibitors for TGFβ. Overexpression of Alk-5 and silencing of Alk-1 led to a loss of the chondrocyte phenotype, proving that they are key regulators of chondrocyte mechanosensing. An analysis of the gene expression profile during the expansion of these modified cartilage cells in plastic showed a better maintenance of the chondrocyte phenotype, at least during the first passages. These passages were also assayed in a mouse model of intramuscular chondrogenesis. Our findings indicate that these two receptors are important mediators in the response of chondrocytes to changes in the mechanical environment, making them suitable targets for modulating chondrogenesis. Inhibition of TGFβ could also be effective in improving chondrocyte activity in aged or expanded cells that overexpress Alk-1.

Improved Chondrogenic Capacity of Collagen Hydrogel-Expanded Chondrocytes: In Vitro and in Vivo Analyses

BACKGROUND

The use of autologous chondrocytes in cartilage repair is limited because of loss of the cartilage phenotype during expansion. The mechanosensing capacity of chondrocytes suggests evaluating the use of soft substrates for in vitro expansion. Our aim was to test the expansion of chondrocytes on collagen hydrogels to improve their capacity for chondrogenesis after a number of passages.

METHODS

Rat cartilage cells were expanded on collagen hydrogels and on plastic, and the preservation of their chondrogenic capacity was tested both in vitro and in vivo. The expression of relevant markers during expansion on each surface was measured by real-time PCR (polymerase chain reaction). Expanded cells were then implanted in focal lesions in the medial femoral condyle of healthy sheep, and the newly formed tissue was analyzed by histomorphometry.

RESULTS

Compared with cells cultured on plastic, cells cultured on hydrogels had better maintenance of the expression of the Sox9, Col2 (type-II collagen), FGFR3, and Alk-5 genes and decreased expression of Alk-1 and BMP-2. Pellets also showed increased expression of the cartilage marker genes aggrecan, Sox9, and Col2, and decreased expression of Col1 and Col10 (type-I and type-X collagen). ELISA (enzyme-linked immunosorbent assay) also showed a higher ratio of type-II to type-I collagen in pellets formed from cells expanded on hydrogels. When sheep chondrocytes were expanded and implanted in cartilage lesions in the femoral condyle of healthy sheep, hydrogel-expanded cells produced histologically better tissue compared with plastic-expanded cells.

CONCLUSIONS

The expansion of chondrocytes on collagen hydrogels yielded cells with an improved chondrogenic capacity compared with cells expanded on plastic.

CLINICAL RELEVANCE

The study results favor the use of hydrogel-expanded cells over the traditional plastic-expanded cells for autologous chondrocyte implantation.

Concise review: unraveling stem cell cocultures in regenerative medicine: which cell interactions steer cartilage regeneration and how?

Cartilage damage and osteoarthritis (OA) impose an important burden on society, leaving both young, active patients and older patients disabled and affecting quality of life. In particular, cartilage injury not only imparts acute loss of function but also predisposes to OA. The increase in knowledge of the consequences of these diseases and the exponential growth in research of regenerative medicine have given rise to different treatment types. Of these, cell-based treatments are increasingly applied because they have the potential to regenerate cartilage, treat symptoms, and ultimately prevent or delay OA. Although these approaches give promising results, they require a costly in vitro cell culture procedure. The answer may lie in single-stage procedures that, by using cell combinations, render in vitro expansion redundant. In the last two decades, cocultures of cartilage cells and a variety of (mesenchymal) stem cells have shown promising results as different studies report cartilage regeneration in vitro and in vivo. However, there is considerable debate regarding the mechanisms and cellular interactions that lead to chondrogenesis in these models. This review, which included 52 papers, provides a systematic overview of the data presented in the literature and tries to elucidate the mechanisms that lead to chondrogenesis in stem cell cocultures with cartilage cells. It could serve as a basis for research groups and clinicians aiming at designing and implementing combined cellular technologies for single-stage cartilage repair and treatment or prevention of OA.

Concise review: unraveling stem cell cocultures in regenerative medicine: which cell interactions steer cartilage regeneration and how?

Cartilage damage and osteoarthritis (OA) impose an important burden on society, leaving both young, active patients and older patients disabled and affecting quality of life. In particular, cartilage injury not only imparts acute loss of function but also predisposes to OA. The increase in knowledge of the consequences of these diseases and the exponential growth in research of regenerative medicine have given rise to different treatment types. Of these, cell-based treatments are increasingly applied because they have the potential to regenerate cartilage, treat symptoms, and ultimately prevent or delay OA. Although these approaches give promising results, they require a costly in vitro cell culture procedure. The answer may lie in single-stage procedures that, by using cell combinations, render in vitro expansion redundant. In the last two decades, cocultures of cartilage cells and a variety of (mesenchymal) stem cells have shown promising results as different studies report cartilage regeneration in vitro and in vivo. However, there is considerable debate regarding the mechanisms and cellular interactions that lead to chondrogenesis in these models. This review, which included 52 papers, provides a systematic overview of the data presented in the literature and tries to elucidate the mechanisms that lead to chondrogenesis in stem cell cocultures with cartilage cells. It could serve as a basis for research groups and clinicians aiming at designing and implementing combined cellular technologies for single-stage cartilage repair and treatment or prevention of OA.

Effects of extracellular matrix proteins in chondrocyte-derived matrices on chondrocyte functions

Loss of cartilaginous phenotype during in vitro expansion culture of chondrocytes is a major barrier to the application of chondrocytes for tissue engineering. In previous study, we showed that dedifferentiation of chondrocytes during the passage culture was delayed by matrices formed by primary chondrocytes (P0-ECM). In this study, we investigated bovine chondrocyte functions when being cultured on isolated extracellular matrix (ECM) protein-coated substrata and P0-ECM. Low chondrocyte attachment was observed on aggrecan-coated substratum and P0-ECM. Cell proliferation on aggrecan- and type II collagen/aggrecan-coated substrata and P0-ECM was lower than that on the other ECM protein (type I collagen and type II collagen)-coated substrata. When chondrocytes were subcultured on aggrecan-coated substratum, decline of cartilaginous gene expression was delayed, which was similar to the cells subcultured on P0-ECM. These results indicate that aggrecan plays an important role in the regulation of chondrocyte functions and P0-ECM may be a good experimental control for investigating the role of each ECM protein in cartilage ECM.

Response of sheep chondrocytes to changes in substrate stiffness from 2 to 20 Pa: effect of cell passaging

AIM

The influence of culture substrate stiffness (in the kPa range) on chondrocyte behavior has been described. Here we describe the response to variations in substrate stiffness in a soft range (2-20 Pa), as it may play a role in understanding cartilage physiopathology.

METHODS

We developed a system for cell culture in substrates with different elastic moduli using collagen hydrogels and evaluated chondrocytes after 2, 4, and 7 days in monolayer and three-dimensional (3D) cultures. Experiments were performed in normoxia and hypoxia in order to describe the effect of a low oxygen environment on chondrocytes. Finally, we also evaluated if dedifferentiated cells preserve the capacity for mechanosensing.

RESULTS

Chondrocytes showed less proliferating activity when cultured in monolayer in the more compliant substrates. Expression of the cartilage markers Aggrecan (Acan), type II collagen (Col2a1), and Sox9 was upregulated in the less stiff gels (both in monolayer and in 3D culture). Stiffer gels induced an organization of the actin cytoskeleton that correlated with the loss of a chondrocyte phenotype. When cells were cultured in hypoxia, we observed changes in the cellular response that were mediated by HIF-1α. Results in 3D hypoxia cultures were opposite to those found in normoxia, but remained unchanged in monolayer hypoxic experiments. Similar results were found for dedifferentiated cells.

CONCLUSIONS

Chondrocytes respond differently according to the stiffness of the substrate. This response depends greatly on the oxygen environment and on whether the chondrocyte is embedded or grown onto the hydrogel, since mechanosensing capacity was not lost with cell expansion.