Chondrocyte deformation and local tissue strain in articular cartilage: a confocal microscopy study

Controlled Mechanical Property Gradients Within a Digital Light Processing Printed Hydrogel-Composite Osteochondral Scaffold

Tissue engineered scaffolds are needed to support physiological loads and emulate the micrometer-scale strain gradients within tissues that guide cell mechanobiological responses. We designed and fabricated micro-truss structures to possess spatially varying geometry and controlled stiffness gradients. Using a custom projection microstereolithography (μSLA) system, using digital light projection (DLP), and photopolymerizable poly(ethylene glycol) diacrylate (PEGDA) hydrogel monomers, three designs with feature sizes < 200 μm were formed: (1) uniform structure with 1 MPa structural modulus ( E ) designed to match equilibrium modulus of healthy articular cartilage, (2) E  = 1 MPa gradient structure designed to vary strain with depth, and (3) osteochondral bilayer with distinct cartilage ( E  = 1 MPa) and bone ( E  = 7 MPa) layers. Finite element models (FEM) guided design and predicted the local mechanical environment. Empty trusses and poly(ethylene glycol) norbornene hydrogel-infilled composite trusses were compressed during X-ray microscopy (XRM) imaging to evaluate regional stiffnesses. Our designs achieved target moduli for cartilage and bone while maintaining 68-81% porosity. Combined XRM imaging and compression of empty and hydrogel-infilled micro-truss structures revealed regional stiffnesses that were accurately predicted by FEM. In the infilling hydrogel, FEM demonstrated the stress-shielding effect of reinforcing structures while predicting strain distributions. Composite scaffolds made from stiff μSLA-printed polymers support physiological load levels and enable controlled mechanical property gradients which may improve in vivo outcomes for osteochondral defect tissue regeneration. Advanced 3D imaging and FE analysis provide insights into the local mechanical environment surrounding cells in composite scaffolds.

Chondrocyte deformation during the unloading phase of cyclic compression loading

Cell volume and shape changes play a pivotal role in cellular mechanotransduction, governing cellular responses to external loading. Understanding the dynamics of cell behavior under loading conditions is essential to elucidate cell adaptation mechanisms in physiological and pathological contexts. In this study, we investigated the effects of dynamic cyclic compression loading on cell volume and shape changes, comparing them with static conditions. Using a custom-designed platform which allowed for simultaneous loading and imaging of cartilage tissue, tissues were subjected to 100 cycles of mechanical loading while measuring cell volume and shape alterations during the unloading phase at specific time points. The findings revealed a transient decrease in cell volume (13%) during the early cycles, followed by a gradual recovery to baseline levels after approximately 20 cycles, despite the cartilage tissue not being fully recovered at the unloading phase. This observed pattern indicates a temporal cell volume response that may be associated with cellular adaptation to the mechanical stimulus through mechanisms related to active cell volume regulation. Additionally, this study demonstrated that cell volume and shape responses during dynamic loading were significantly distinct from those observed under static conditions. Such findings suggest that cells in their natural tissue environment perceive and respond differently to dynamic compared to static mechanical cues, highlighting the significance of considering dynamic loading environments in studies related to cellular mechanics. Overall, this research contributes to the broader understanding of cellular behavior under mechanical stimuli, providing valuable insights into their ability to adapt to dynamic mechanical loading.

Primary cilium-mediated mechanotransduction in cartilage chondrocytes

Osteoarthritis (OA) is one of the most prevalent joint disorders associated with the degradation of articular cartilage and an abnormal mechanical microenvironment. Mechanical stimuli, including compression, shear stress, stretching strain, osmotic challenge, and the physical properties of the matrix microenvironment, play pivotal roles in the tissue homeostasis of articular cartilage. The primary cilium, as a mechanosensory and chemosensory organelle, is important for detecting and transmitting both mechanical and biochemical signals in chondrocytes within the matrix microenvironment. Growing evidence indicates that primary cilia are critical for chondrocytes signaling transduction and the matrix homeostasis of articular cartilage. Furthermore, the ability of primary cilium to regulate cellular signaling is dynamic and dependent on the cellular matrix microenvironment. In the current review, we aim to elucidate the key mechanisms by which primary cilia mediate chondrocytes sensing and responding to the matrix mechanical microenvironment. This might have potential therapeutic applications in injuries and OA-associated degeneration of articular cartilage.

Membrane stretch as the mechanism of activation of PIEZO1 ion channels in chondrocytes

Osteoarthritis is a chronic disease that can be initiated by altered joint loading or injury of the cartilage. The mechanically sensitive PIEZO ion channels have been shown to transduce injurious levels of biomechanical strain in articular chondrocytes and mediate cell death. However, the mechanisms of channel gating in response to high cellular deformation and the strain thresholds for activating PIEZO channels remain unclear. We coupled studies of single-cell compression using atomic force microscopy (AFM) with finite element modeling (FEM) to identify the biophysical mechanisms of PIEZO-mediated calcium (Ca2+) signaling in chondrocytes. We showed that PIEZO1 and PIEZO2 are needed for initiating Ca2+ signaling at moderately high levels of cellular deformation, but at the highest strains, PIEZO1 functions independently of PIEZO2. Biophysical factors that increase apparent chondrocyte membrane tension, including hypoosmotic prestrain, high compression magnitudes, and low deformation rates, also increased PIEZO1-driven Ca2+ signaling. Combined AFM/FEM studies showed that 50% of chondrocytes exhibit Ca2+ signaling at 80 to 85% nominal cell compression, corresponding to a threshold of apparent membrane finite principal strain of E = 1.31, which represents a membrane stretch ratio (λ) of 1.9. Both intracellular and extracellular Ca2+ are necessary for the PIEZO1-mediated Ca2+ signaling response to compression. Our results suggest that PIEZO1-induced signaling drives chondrocyte mechanical injury due to high membrane tension, and this threshold can be altered by factors that influence membrane prestress, such as cartilage hypoosmolarity, secondary to proteoglycan loss. These findings suggest that modulating PIEZO1 activation or downstream signaling may offer avenues for the prevention or treatment of osteoarthritis.

Biomechanics of Chondrocytes and Chondrons in Healthy Conditions and Osteoarthritis: A Review of the Mechanical Characterisations at the Microscale

Biomechanical studies are expanding across a variety of fields, from biomedicine to biomedical engineering. From the molecular to the system level, mechanical stimuli are crucial regulators of the development of organs and tissues, their growth and related processes such as remodelling, regeneration or disease. When dealing with cell mechanics, various experimental techniques have been developed to analyse the passive response of cells; however, cell variability and the extraction process, complex experimental procedures and different models and assumptions may affect the resulting mechanical properties. For these purposes, this review was aimed at collecting the available literature focused on experimental chondrocyte and chondron biomechanics with direct connection to their biochemical functions and activities, in order to point out important information regarding the planning of an experimental test or a comparison with the available results. In particular, this review highlighted (i) the most common experimental techniques used, (ii) the results and models adopted by different authors, (iii) a critical perspective on features that could affect the results and finally (iv) the quantification of structural and mechanical changes due to a degenerative pathology such as osteoarthritis.

Superficial zone chondrocytes can get compacted under physiological loading: A multiscale finite element analysis

Recent in vivo and in vitro studies have demonstrated that superficial zone (SZ) chondrocytes within articular layers of diarthrodial joints die under normal physiologic loading conditions. In order to further explore the implications of this observation in future investigations, we first needed to understand the mechanical environment of SZ chondrocytes that might cause them to die under physiological sliding contact conditions. In this study we performed a multiscale finite element analysis of articular contact to track the temporal evolution of a SZ chondrocyte's interstitial fluid pressure, hydraulic permeability, and volume under physiologic loading conditions. The effect of the pericellular matrix modulus and permeability was parametrically investigated. Results showed that SZ chondrocytes can lose ninety percent of their intracellular fluid after several hours of intermittent or continuous contact loading, resulting in a reduction of intracellular hydraulic permeability by more than three orders of magnitude. These findings are consistent with loss of cell viability due to the impediment of cellular metabolic pathways induced by the loss of fluid. They suggest that there is a simple mechanical explanation for the vulnerability of SZ chondrocytes to sustained physiological loading conditions. Future studies will focus on validating these specific findings experimentally. STATEMENT OF SIGNIFICANCE: As with any mechanical system, normal 'wear and tear' of cartilage tissue lining joints is expected. Yet incidences of osteoarthritis are uncommon in individuals younger than 45. This counter-intuitive observation suggests there must be an intrinsic repair mechanism compensating for this wear and tear over many decades of life. Recent experimental studies have shown superficial zone chondrocytes die under physiologic loading conditions, suggesting that this repair mechanism may involve cell replenishment. To better understand the mechanical environment of these cells, we performed a multiscale computational analysis of articular contact under loading. Results indicated that normal activities like walking or standing can induce significant loss of intracellular fluid volume, potentially hindering metabolic activity and fluid transport properties, and causing cell death.

Instabilities induced by mechanical loading determine the viability of chondrocytes grown on porous scaffolds

Tissue-engineered cartilage constructs have shown promise to treat focal cartilage defects in multiple clinical studies. Notably, products in clinical use or in late-stage clinical trials often utilize porous collagen scaffolds to provide mechanical support and attachment sites for chondrocytes. Under loading, both the local mechanical responses of collagen scaffolds and the corresponding cellular outcomes are poorly understood, despite their wide use. As such, the architecture of collagen scaffolds varies significantly among tissue-engineered cartilage products, but the effects of such architectures on construct mechanics and cell viability are not well understood. This study investigated the effects of local mechanical responses of collagen scaffolds on chondrocyte viability in tissue-engineered cartilage constructs. We utilized fast confocal microscopy combined with a strain mapping technique to analyze the architecture-dependent instabilities under quasi-static loading and subsequent chondrocyte death in honeycomb and sponge scaffolds. More specifically, we compared the isotropic and the orthotropic planes for each type of collagen scaffold. Under compression, both planes exhibited elastic, buckled, and densified deformation modes. In both loading directions, cell death was minimal in regions that experienced elastic deformation mode and a trend of increase in buckled mode. More interestingly, we saw a significant increase in cell death in densified mode. Overall, this study suggests that local instabilities are directly correlated to chondrocyte death in tissue-engineered cartilage constructs, highlighting the importance of understanding the architecture-dependent local mechanical responses under loading.

Effect of microgravity on mechanical loadings in lumbar spine at various postures: a numerical study

The aim of this study was to quantitatively analyze the mechanical change of spinal segments (disc, muscle, and ligament) at various postures under microgravity using a full-body musculoskeletal modeling approach. Specifically, in the lumbar spine, the vertebra were modeled as rigid bodies, the intervertebral discs were modeled as 6-degree-of-freedom joints with linear force-deformation relationships, the disc swelling pressure was deformation dependent, the ligaments were modeled as piecewise linear elastic materials, the muscle strength was dependent on its functional cross-sectional area. The neutral posture and the "fetal tuck" posture in microgravity (short as "Neutral 0G" and "Fetal Tuck 0G", in our simulation, the G constant was set to 0 for simulating microgravity), and for comparison, the relaxed standing posture in 1G and 0G gravity (short as "Neutral 1G" and "Standing 0G") were simulated. Compared to values at Neutral 1G, the mechanical response in the lower spine changed significantly at Neutral 0G. For example, the compressive forces on lumbar discs decreased 62-70%, the muscle forces decreased 55.7-92.9%, while disc water content increased 7.0-10.2%, disc height increased 2.1-3.0%, disc volume increased 6.4-9.3%, and ligament forces increased 59.5-271.3% at Neutral 0G. The fetal tuck 0G reversed these changes at Neutral 0G back toward values at Neutral 1G, with magnitudes much larger than those at Neutral 1G. Our results suggest that microgravity has significant influences on spinal biomechanics, alteration of which may increase the risks of disc herniation and degeneration, muscle atrophy, and/or ligament failure.

Characterization of intra-tissue strain fields in articular cartilage explants during post-loading recovery using high frequency ultrasound

This study aims to demonstrate the potential of ultrasound elastography as a research tool for non-destructive imaging of intra-tissue strain fields and tissue quality assessment in cartilage explants. Osteochondral plugs from bovine patellae were loaded up to 10, 40, or 70 N using a hemi-spherical indenter. The load was kept constant for 15 min, after which samples were unloaded and ultrasound imaging of strain recovery over time was performed in the indented area for 1 h. Tissue strains were determined using speckle tracking and accumulated to LaGrangian strains in the indentation direction. For all samples, strain maps showed a heterogeneous strain field, with the highest values in the superficial cartilage under the indenter tip at the bottom of the indent and decreasing values in the deeper cartilage. Strains were higher at higher load levels and tissue recovery over time was faster after indentation at 10 N than at 40 N and 70 N. At lower compression levels most displacement occurred near the surface with little deformation in the deep layers, while at higher levels strains increased more evenly in all cartilage zones. Ultrasound elastography is a promising method for high resolution imaging of intra-tissue strain fields and evaluation of cartilage quality in tissue explants in a laboratory setting. In the future, it may become a clinical diagnostic tool used to identify the extent of cartilage damage around visible defects.

Multiscale Strain Transfer in Cartilage

The transfer of stress and strain signals between the extracellular matrix (ECM) and cells is crucial for biochemical and biomechanical cues that are required for tissue morphogenesis, differentiation, growth, and homeostasis. In cartilage tissue, the heterogeneity in spatial variation of ECM molecules leads to a depth-dependent non-uniform strain transfer and alters the magnitude of forces sensed by cells in articular and fibrocartilage, influencing chondrocyte metabolism and biochemical response. It is not fully established how these nonuniform forces ultimately influence cartilage health, maintenance, and integrity. To comprehend tissue remodelling in health and disease, it is fundamental to investigate how these forces, the ECM, and cells interrelate. However, not much is known about the relationship between applied mechanical stimulus and resulting spatial variations in magnitude and sense of mechanical stimuli within the chondrocyte’s microenvironment. Investigating multiscale strain transfer and hierarchical structure-function relationships in cartilage is key to unravelling how cells receive signals and how they are transformed into biosynthetic responses. Therefore, this article first reviews different cartilage types and chondrocyte mechanosensing. Following this, multiscale strain transfer through cartilage tissue and the involvement of individual ECM components are discussed. Finally, insights to further understand multiscale strain transfer in cartilage are outlined.

Painful temporomandibular joint overloading induces structural remodeling in the pericellular matrix of that joint's chondrocytes

Mechanical stress to the temporomandibular joint (TMJ) is an important factor in cartilage degeneration, with both clinical and preclinical studies suggesting that repeated TMJ overloading could contribute to pain, inflammation, and/or structural damage in the joint. However, the relationship between pain severity and early signs of cartilage matrix microstructural dysregulation is not understood, limiting the advancement of diagnoses and treatments for temporomandibular joint-osteoarthritis (TMJ-OA). Changes in the pericellular matrix (PCM) surrounding chondrocytes may be early indicators of OA. A rat model of TMJ pain induced by repeated jaw loading (1 h/day for 7 days) was used to compare the extent of PCM modulation for different loading magnitudes with distinct pain profiles (3.5N-persistent pain, 2N-resolving pain, or unloaded controls-no pain) and macrostructural changes previously indicated by Mankin scoring. Expression of PCM structural molecules, collagen VI and aggrecan NITEGE neo-epitope, were evaluated at Day 15 by immunohistochemistry within TMJ fibrocartilage and compared between pain conditions. Pericellular collagen VI levels increased at Day 15 in both the 2N (p = 0.003) and 3.5N (p = 0.042) conditions compared to unloaded controls. PCM width expanded to a similar extent for both loading conditions at Day 15 (2N, p < 0.001; 3.5N, p = 0.002). Neo-epitope expression increased in the 3.5N group over levels in the 2N group (p = 0.041), indicating pericellular changes that were not identified in the same groups by Mankin scoring of the pericellular region. Although remodeling occurs in both pain conditions, the presence of pericellular catabolic neo-epitopes may be involved in the macrostructural changes and behavioral sensitivity observed in persistent TMJ pain.

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.

Nuclear envelope mechanobiology: linking the nuclear structure and function

The nucleus, central to cellular activity, relies on both direct mechanical input as well as its molecular transducers to sense external stimuli and respond by regulating intra-nuclear chromatin organization that determines cell function and fate. In mesenchymal stem cells of musculoskeletal tissues, changes in nuclear structures are emerging as a key modulator of their differentiation and proliferation programs. In this review we will first introduce the structural elements of the nucleoskeleton and discuss the current literature on how nuclear structure and signaling are altered in relation to environmental and tissue level mechanical cues. We will focus on state-of-the-art techniques to apply mechanical force and methods to measure nuclear mechanics in conjunction with DNA, RNA, and protein visualization in living cells. Ultimately, combining real-time nuclear deformations and chromatin dynamics can be a powerful tool to study mechanisms of how forces affect the dynamics of genome function.

Biomimetic and mechanically supportive 3D printed scaffolds for cartilage and osteochondral tissue engineering using photopolymers and digital light processing

Successful 3D scaffold designs for musculoskeletal tissue engineering necessitate full consideration of the form and function of the tissues of interest. When designing structures for engineering cartilage and osteochondral tissues, one must reconcile the need to develop a mechanically robust system that maintains the health of cells embedded in the scaffold. In this work, we present an approach that decouples the mechanical and biochemical needs and allows for the independent development of the structural and cellular niches in a scaffold. Using the highly tuned capabilities of digital light processing-based stereolithography, structures with complex architectures are achieved over a range of effective porosities and moduli. The 3D printed structure is infilled with mesenchymal stem cells and soft biomimetic hydrogels, which are specifically formulated with extracellular matrix analogs and tethered growth factors to provide selected biochemical cues for the guided differentiation towards chondrogenesis and osteogenesis. We demonstrate the ability to utilize these structures to (a) infill a focal chondral defect and mitigate macroscopic and cellular level changes in the cartilage surrounding the defect, and (b) support the development of a stratified multi-tissue scaffold for osteochondral tissue engineering.

High resolution three-dimensional strain measurements in human articular cartilage

An unresolved challenge in osteoarthritis research is characterising the localised intra-tissue mechanical response of articular cartilage. The aim of this study was to explore whether laboratory micro-computed tomography (micro-CT) and digital volume correlation (DVC) permit non-destructive quantification of three-dimensional (3D) strain fields in human articular cartilage. Human articular cartilage specimens were harvested from the knee, mounted into a loading device and imaged in the unloaded and loaded states using a micro-CT scanner. Strain was measured throughout the cartilage volume using the micro-CT image data and DVC analysis. The volumetric DVC-measured strain was within 5% of the known applied strain. Variation in strain distribution between the superficial, middle and deep zones was observed, consistent with the different architecture of the material in these locations. These results indicate DVC method may be suitable for calculating strain in human articular cartilage.

Application of 3D MAPs pipeline identifies the morphological sequence chondrocytes undergo and the regulatory role of GDF5 in this process

The activity of epiphyseal growth plates, which drives long bone elongation, depends on extensive changes in chondrocyte size and shape during differentiation. Here, we develop a pipeline called 3D Morphometric Analysis for Phenotypic significance (3D MAPs), which combines light-sheet microscopy, segmentation algorithms and 3D morphometric analysis to characterize morphogenetic cellular behaviors while maintaining the spatial context of the growth plate. Using 3D MAPs, we create a 3D image database of hundreds of thousands of chondrocytes. Analysis reveals broad repertoire of morphological changes, growth strategies and cell organizations during differentiation. Moreover, identifying a reduction in Smad 1/5/9 activity together with multiple abnormalities in cell growth, shape and organization provides an explanation for the shortening of Gdf5 KO tibias. Overall, our findings provide insight into the morphological sequence that chondrocytes undergo during differentiation and highlight the ability of 3D MAPs to uncover cellular mechanisms that may regulate this process.

Biocompatible and Enzymatically Degradable Gels for 3D Cellular Encapsulation under Extreme Compressive Strain

Cell encapsulating scaffolds are necessary for the study of cellular mechanosensing of cultured cells. However, conventional scaffolds used for loading cells in bulk generally fail at low compressive strain, while hydrogels designed for high toughness and strain resistance are generally unsuitable for cell encapsulation. Here we describe an alginate/gelatin methacryloyl interpenetrating network with multiple crosslinking modes that is robust to compressive strains greater than 70%, highly biocompatible, enzymatically degradable and able to effectively transfer strain to encapsulated cells. In future studies, this gel formula may allow researchers to probe cellular mechanosensing in bulk at levels of compressive strain previously difficult to investigate.

Polycystin-2 Is Required for Chondrocyte Mechanotransduction and Traffics to the Primary Cilium in Response to Mechanical Stimulation

Primary cilia and associated intraflagellar transport are essential for skeletal development, joint homeostasis, and the response to mechanical stimuli, although the mechanisms remain unclear. Polycystin-2 (PC2) is a member of the transient receptor potential polycystic (TRPP) family of cation channels, and together with Polycystin-1 (PC1), it has been implicated in cilia-mediated mechanotransduction in epithelial cells. The current study investigates the effect of mechanical stimulation on the localization of ciliary polycystins in chondrocytes and tests the hypothesis that they are required in chondrocyte mechanosignaling. Isolated chondrocytes were subjected to mechanical stimulation in the form of uniaxial cyclic tensile strain (CTS) in order to examine the effects on PC2 ciliary localization and matrix gene expression. In the absence of strain, PC2 localizes to the chondrocyte ciliary membrane and neither PC1 nor PC2 are required for ciliogenesis. Cartilage matrix gene expression (Acan, Col2a) is increased in response to 10% CTS. This response is inhibited by siRNA-mediated loss of PC1 or PC2 expression. PC2 ciliary localization requires PC1 and is increased in response to CTS. Increased PC2 cilia trafficking is dependent on the activation of transient receptor potential cation channel subfamily V member 4 (TRPV4) activation. Together, these findings demonstrate for the first time that polycystins are required for chondrocyte mechanotransduction and highlight the mechanosensitive cilia trafficking of PC2 as an important component of cilia-mediated mechanotransduction.

Compressive mechanical stress enhances susceptibility to interleukin-1 by increasing interleukin-1 receptor expression in 3D-cultured ATDC5 cells

Background

Mechanical overload applied on the articular cartilage may play an important role in the pathogenesis of osteoarthritis. However, the mechanism of chondrocyte mechanotransduction is not fully understood. The purpose of this study was to assess the effects of compressive mechanical stress on interleukin-1 receptor (IL-1R) and matrix-degrading enzyme expression by three-dimensional (3D) cultured ATDC5 cells. In addition, the implications of transient receptor potential vanilloid 4 (TRPV4) channel regulation in promoting effects of compressive mechanical loading were elucidated.

Methods

ATDC5 cells were cultured in alginate beads with the growth medium containing insulin-transferrin-selenium and BMP-2 for 6 days. The cultured cell pellet was seeded in collagen scaffolds to produce 3D-cultured constructs. Cyclic compressive loading was applied on the 3D-cultured constructs at 0.5 Hz for 3 h. The mRNA expressions of a disintegrin and metalloproteinases with thrombospondin motifs 4 (ADAMTS4) and IL-1R were determined with or without compressive loading, and effects of TRPV4 agonist/antagonist on mRNA expressions were examined. Immunoreactivities of reactive oxygen species (ROS), TRPV4 and IL-1R were assessed in 3D-cultured ATDC5 cells.

Results

In 3D-cultured ATDC5 cells, ROS was induced by cyclic compressive loading stress. The mRNA expression levels of ADAMTS4 and IL-1R were increased by cyclic compressive loading, which was mostly prevented by pyrollidine dithiocarbamate. Small amounts of IL-1β upregulated ADAMTS4 and IL-1R mRNA expressions only when combined with compressive loading. TRPV4 agonist suppressed ADAMTS4 and IL-1R mRNA levels induced by the compressive loading, whereas TRPV4 antagonist enhanced these levels. Immunoreactivities to TRPV4 and IL-1R significantly increased in constructs with cyclic compressive loading.

Conclusion

Cyclic compressive loading induced mRNA expressions of ADAMTS4 and IL-1R through reactive oxygen species. TRPV4 regulated these mRNA expressions, but excessive compressive loading may impair TRPV4 regulation. These findings suggested that TRPV4 regulates the expression level of IL-1R and subsequent IL-1 signaling induced by cyclic compressive loading and participates in cartilage homeostasis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-021-04095-x.

Homogentisic acid induces cytoskeleton and extracellular matrix alteration in alkaptonuric cartilage

Alkaptonuria (AKU) is an ultra-rare disease caused by the deficient activity of homogentisate 1,2-dioxygenase enzyme, leading the accumulation of homogentisic acid (HGA) in connective tissues implicating the formation of a black pigmentation called "ochronosis." Although AKU is a multisystemic disease, the most affected tissue is the articular cartilage, which during the pathology appears to be highly damaged. In this study, a model of alkaptonuric chondrocytes and cartilage was realized to investigate the role of HGA in the alteration of the extracellular matrix (ECM). The AKU tissues lost its architecture composed of collagen, proteoglycans, and all the proteins that characterize the ECM. The cause of this alteration in AKU cartilage is attributed to a degeneration of the cytoskeletal network in chondrocytes caused by the accumulation of HGA. The three cytoskeletal proteins, actin, vimentin, and tubulin, were analyzed and a modification in their amount and disposition in AKU chondrocytes model was identified. Cytoskeleton is involved in many fundamental cellular processes; therefore, the aberration in this complex network is involved in the manifestation of AKU disease.

Mechanical compression controls the biosynthesis of human osteoarthritic chondrocytes in vitro

BACKGROUND

Cartilage damage, which can potentially lead to osteoarthritis, is a leading cause of morbidity in the elderly population. Chondrocytes are sensitive to mechanical stimuli and their matrix-protein synthesis may be altered when chondrocytes experience a variety of in vivo loadings. Therefore, a study was conducted to evaluate the biosynthesis of isolated osteoarthritic chondrocytes which subjected to compression with varying dynamic compressive strains and loading durations.

METHODS

The proximal tibia was resected as a single osteochondral unit during total knee replacement from patients (N = 10). The osteoarthritic chondrocytes were isolated from the osteochondral units, and characterized using reverse transcriptase-polymerase chain reaction. The isolated osteoarthritic chondrocytes were cultured and embedded in agarose, and then subjected to 10% and 20% uniaxial dynamic compression up to 8-days using a bioreactor. The morphological features and changes in the osteoarthritic chondrocytes upon compression were evaluated using scanning electron microscopy. Safranin O was used to detect the presence of cartilage matrix proteoglycan expression while quantitative analysis was conducted by measuring type VI collagen using an immunohistochemistry and fluorescence intensity assay.

FINDINGS

Gene expression analysis indicated that the isolated osteoarthritic chondrocytes expressed chondrocyte-specific markers, including BGN, CD90 and HSPG-2. Moreover, the compressed osteoarthritic chondrocytes showed a more intense and broader deposition of proteoglycan and type VI collagen than control. The expression of type VI collagen was directly proportional to the duration of compression in which 8-days compression was significantly higher than 4-days compression. The 20% compression showed significantly higher intensity compared to 10% compression in 4- and 8-days.

INTERPRETATION

The biosynthetic activity of human chondrocytes from osteoarthritic joints can be enhanced using selected compression regimes.

Transient stiffening of cartilage during joint articulation: A microindentation study

As a mechanoactive tissue, articular cartilage undergoes compression and shear on a daily basis. With the advent of high resolution and sensitive mechanical testing methods, such as micro- and nanoindentation, it has become possible to assess changes in small-scale mechanical properties due to compression and shear of the tissue. However, investigations on the changes of these properties before and after joint articulation have been limited. To simulate articular loading of cartilage in the context of human gait, a previously developed bioreactor system was used. Immediately after bioreactor testing, the stiffness was measured using microindentation. Specifically, we investigated whether the mechanical response of the tissue was transient or permanent, dependent on counterface material, and an effect limited to the superficial zone of cartilage. We found that cartilage surface stiffness increases immediately after articular loading and returns to baseline values within 3 hr. Cartilage-on-cartilage stiffening was found to be higher compared to both alumina- and cobalt chromium-on-cartilage stiffening, which were not significantly different from each other. This stiffening response was found to be unique to the superficial zone, as articular loading on cartilage with the superficial zone removed showed no changes in stiffness. The findings of this study suggest that the cartilage superficial zone may adapt its stiffness as a response to articular loading. As the superficial zone is often compromised during the course of osteoarthritic disease, this finding is of clinical relevance, suggesting that the load-bearing function deteriorates over time.

Chondrocyte Deformations Under Mild Dynamic Loading Conditions

Dynamic deformation of chondrocytes are associated with cell mechanotransduction and thus may offer a new understanding of the mechanobiology of articular cartilage. Despite extensive research on chondrocyte deformations for static conditions, work for dynamic conditions remains rare. However, it is these dynamic conditions that articular cartilage in joints are exposed to everyday, and that seem to promote biological signaling in chondrocytes. Therefore, the objective of this study was to develop an experimental technique to determine the in situ deformations of chondrocytes when the cartilage is dynamically compressed. We hypothesized that dynamic deformations of chondrocytes vastly differ from those observed under steady-state static strain conditions. Real-time chondrocyte geometry was reconstructed at 10, 15, and 20% compression during ramp compressions with 20% ultimate strain, applied at a strain rate of 0.2% s^-1, followed by stress relaxation. Dynamic compressive chondrocyte deformations were non-linear as a function of nominal strain, with large deformations in the early and small deformations in the late part of compression. Early compression (up to about 10%) was associated with chondrocyte volume loss, while late compression (> ~ 10%) was associated with cell deformation but minimal volume loss. Force continued to decrease for 5 min in the stress-relaxation phase, while chondrocyte shape/volume remained unaltered after the first minute of stress-relaxation.

Viscoelasticity of hydrazone crosslinked poly(ethylene glycol) hydrogels directs chondrocyte morphology during mechanical deformation

Chondrocyte deformation influences disease progression and tissue regeneration in load-bearing joints. In this work, we found that viscoelasticity of dynamic covalent crosslinks temporally modulates the biophysical transmission of physiologically relevant compressive strains to encapsulated chondrocytes. Chondrocytes in viscoelastic alky-hydrazone hydrogels demonstrated (91.4 ± 4.5%) recovery of native rounded morphologies during mechanical deformation, whereas primarily elastic benzyl-hydrazone hydrogels significantly limited morphological recovery (21.2 ± 1.4%).

Early changes in cartilage pericellular matrix micromechanobiology portend the onset of post-traumatic osteoarthritis

The pericellular matrix (PCM) of cartilage is a structurally distinctive microdomain surrounding each chondrocyte, and is pivotal to cell homeostasis and cell-matrix interactions in healthy tissue. This study queried if the PCM is the initiation point for disease or a casualty of more widespread matrix degeneration. To address this question, we queried the mechanical properties of the PCM and chondrocyte mechanoresponsivity with the development of post-traumatic osteoarthritis (PTOA). To do so, we integrated Kawamoto's film-assisted cryo-sectioning with immunofluorescence-guided AFM nanomechanical mapping, and quantified the microscale modulus of murine cartilage PCM and further-removed extracellular matrix. Using the destabilization of the medial meniscus (DMM) murine model of PTOA, we show that decreases in PCM micromechanics are apparent as early as 3 days after injury, and that this precedes changes in the bulk ECM properties and overt indications of cartilage damage. We also show that, as a consequence of altered PCM properties, calcium mobilization by chondrocytes in response to mechanical challenge (hypo-osmotic stress) is significantly disrupted. These aberrant changes in chondrocyte micromechanobiology as a consequence of DMM could be partially blocked by early inhibition of PCM remodeling. Collectively, these results suggest that changes in PCM micromechanobiology are leading indicators of the initiation of PTOA, and that disease originates in the cartilage PCM. This insight will direct the development of early detection methods, as well as small molecule-based therapies that can stop early aberrant remodeling in this critical cartilage microdomain to slow or reverse disease progression. STATEMENT OF SIGNIFICANCE: Post-traumatic osteoarthritis (PTOA) is one prevalent musculoskeletal disease that afflicts young adults, and there are no effective strategies for early detection or intervention. This study identifies that the reduction of cartilage pericellular matrix (PCM) micromodulus is one of the earliest events in the initiation of PTOA, which, in turn, impairs the mechanosensitive activities of chondrocytes, contributing to the vicious loop of cartilage degeneration. Rescuing the integrity of PCM has the potential to restore normal chondrocyte mechanosensitive homeostasis and to prevent further degradation of cartilage. Our findings enable the development of early OA detection methods targeting changes in the PCM, and treatment strategies that can stop early aberrant remodeling in this critical microdomain to slow or reverse disease progression.

Local Strain Distribution and Increased Intracellular Ca2+ Signaling in Bovine Articular Cartilage Exposed to Compressive Strain

Articular cartilage is exposed to compressive strain of approximately 10% under physiological loads in vivo, and intracellular Ca2+ signaling is one of the earliest responses in chondrocytes under this physical stimulation. However, it remains unknown whether compressive strain itself evokes intracellular Ca2+ signaling in chondrocytes located within each layer (from surface to deep) in an equal manner with physiological levels of strain. The purpose of this study, therefore, was to determine the distribution of local strain and increased intracellular Ca2+ signaling in layer-dependent cell populations in response to 10% compressive strain loading. For this purpose, the time course of strain was measured in each layer to calculate layer-specific deformation properties. In addition, layer-specific changes in chondrocyte intracellular Ca2+ signals were recorded over time using a fluorescent Ca2+ indicator, Fluo-3, to establish ratios of cells with increased Ca2+ signaling at each depth of cartilage under static conditions or exposed to compression. The results showed that the surface layer was compressed with a larger strain compared with other layers. Few cells with Ca2+ signaling were observed under static conditions. Percentages of responsive cells within compressed cartilage were higher than those within cartilage under static conditions. However, increased intracellular Ca2+ signals were observed in a prominent number of chondrocytes within the deep layer, but not the surface layer, of compressed cartilage. Our results suggest that at a physiological compression level, Ca2+ is upregulated, but the stimulation of Ca2+ signaling in articular cartilage is not simply defined by local deformation.

A mobile MRI field study of the biochemical cartilage reaction of the knee joint during a 4,486 km transcontinental multistage ultra-marathon using T2* mapping

Nearly nothing is known about the consequences of ultra-long-distance running on knee cartilage. In this mobile MRI field study, we analysed the biochemical effects of a 4,486 km transcontinental multistage ultra-marathon on femorotibial joint (FTJ) cartilage. Serial MRI data were acquired from 22 subjects (20 male, 18 finisher) using a 1.5 T MR scanner mounted on a 38-ton trailer, travelling with the participants of the TransEurope FootRace (TEFR) day by day over 64 stages. The statistical analyses focused on intrachondral T2* behaviour during the course of the TEFR as the main outcome variable of interest. T2* mapping (sagittal FLASH T2* weighted gradient echo) is a validated and highly accurate method for quantitative compositional cartilage analysis of specific weightbearing areas of the FTJ. T2* mapping is sensitive to changes in the equilibrium of free intrachondral water, which depends on the content and orientation of collagen and the proteoglycan content in the extracellular cartilage matrix. Within the first 1,100 km, a significant running load-induced T2* increase occurred in all joint regions: 44.0% femoral-lateral, 42.9% tibial-lateral, 34.9% femoral-medial, and 25.1% tibial-medial. Osteochondral lesions showed no relevant changes or new occurrence during the TEFR. The reasons for stopping the race were not associated with knee problems. As no further T2* elevation was found in the second half of the TEFR but a decreasing T2* trend (recovery) was observed after the 3,500 km run, we assume that no further softening of the cartilage occurs with ongoing running burden over ultra-long distances extending 4,500 km. Instead, we assume the ability of the FTJ cartilage matrix to reorganize and adapt to the load.

Co-aligned chondrocytes: Zonal morphological variation and structured arrangement of cell lacunae in tessellated cartilage

In most vertebrates the embryonic cartilaginous skeleton is replaced by bone during development. During this process, cartilage cells (chondrocytes) mineralize the extracellular matrix and undergo apoptosis, giving way to bone cells (osteocytes). In contrast, sharks and rays (elasmobranchs) have cartilaginous skeletons throughout life, where only the surface mineralizes, forming a layer of tiles (tesserae). Elasmobranch chondrocytes, unlike those of other vertebrates, survive cartilage mineralization and are maintained alive in spaces (lacunae) within tesserae. However, the functions of the chondrocytes in the mineralized tissue remain unknown. Applying a custom analysis workflow to high-resolution synchrotron microCT scans of tesserae, we characterize the morphologies and arrangements of stingray chondrocyte lacunae, using lacunar morphology as a proxy for chondrocyte morphology. We show that the cell density is comparable in unmineralized and mineralized tissue and that cells maintain similar volume even when they have been incorporated into tesserae. Our findings support previous hypotheses that elasmobranch chondrocytes, unlike those of other taxa, do not proliferate, hypertrophy or undergo apoptosis during mineralization. Tessera lacunae show zonal variation in their shapes, being flatter further from and more spherical closer to the unmineralized cartilage matrix, and larger in the center of tesserae. The lacunae show pronounced organization into parallel layers and strong orientation toward neighboring tesserae. Tesserae also exhibit local variation in lacunar density, with the density considerably higher near pores passing through the tesseral layer, suggesting pores and cells interact, and that pores may contain a nutrient source. We propose that the different lacunar types reflect the stages of the tesserae formation process, while also representing local variation in tissue architecture and cell function. Lacunae are linked by small passages (canaliculi) in the matrix to form elongated series at the tesseral periphery and tight clusters in the center of tesserae, creating a rich connectivity among cells. The network arrangement and the shape variation of chondrocytes in tesserae indicate that cells may interact within and between tesserae and manage mineralization differently from chondrocytes in other vertebrates, perhaps performing analogous roles to osteocytes in bone.

Apparatus and Method for Rapid Detection of Acoustic Anisotropy in Cartilage

Purpose

Articular cartilage is known to be mechanically anisotropic. In this paper, the acoustic anisotropy of bovine articular cartilage and the effects of freeze-thaw cycling on acoustic anisotropy were investigated.

Methods

We developed apparatus and methods that use a magnetic L-shaped sample holder, which allowed minimal handling of a tissue, reduced the number of measurements compared to previous studies, and produced highly reproducible results.

Results

SOS was greater in the direction perpendicular to the articular surface compared to the direction parallel to the articular surface (N=17, P = 0.00001). Average SOS was 1,758 ± 107 m/s perpendicular to the surface, and 1,617 ± 55 m/s parallel to it. The average percentage difference in SOS between the perpendicular and parallel directions was 8.2% (95% CI: 5.4% to 11%). Freeze-thaw cycling did not have a significant effect on SOS (P>0.4).

Conclusion

Acoustic measurement of tissue properties is particularly attractive for work in our laboratory since it has the potential for nondestructive characterization of the properties of developing engineered cartilage. Our approach allowed us to observe acoustic anisotropy of articular cartilage rapidly and reproducibly. This property was not significantly affected by freeze-thawing of the tissue samples, making cryopreservation practical for these assays.

Evidence that reduction in volume protects in situ articular chondrocytes from mechanical impact

Chondrocytes, the resident cells in articular cartilage, carry the burden of producing and maintaining the extracellular matrix (ECM). However, as these cells have a low proliferative capacity and are not readily replaced, chondrocyte death due to extreme forces may contribute to the pathogenesis of osteoarthritis (OA) after injury or may inhibit healing after osteochondral transplantation, a restorative procedure for damaged cartilage that requires a series of mechanical impacts to insert the graft. Consequently, there is a need to understand what factors influence the vulnerability of in situ chondrocytes to mechanical trauma. To this end, the objective of this study was to investigate how altering cell volume by different means (hydrostatic pressure, uniaxial load, and osmotic challenge with and without inhibition of regulatory volume decrease) affects the vulnerability of in situ chondrocytes to extreme mechanical forces. Using a custom experimental platform enabling testing of viable and intact murine cartilage-on-bone explants, we established a strong correlation between chondrocyte volume and vulnerability to impact injury wherein reduced volume was protective. Moreover, we found that the volume-perturbing interventions did not affect cartilage ECM mechanical properties, suggesting that their effects on chondrocyte vulnerability occurred at the cellular level. The findings of this study offer new avenues for novel strategies aimed at preventing chondrocyte loss during osteochondral grafting or to halting the progression of cell death after a joint destabilizing injury.

Propagation phase-contrast micro-computed tomography allows laboratory-based three-dimensional imaging of articular cartilage down to the cellular level

OBJECTIVE

High-resolution non-invasive three-dimensional (3D) imaging of chondrocytes in articular cartilage remains elusive. The aim of this study was to explore whether laboratory micro-computed tomography (micro-CT) permits imaging cells within articular cartilage.

DESIGN

Bovine osteochondral plugs were prepared four ways: in phosphate-buffered saline (PBS) or 70% ethanol (EtOH), both with or without phosphotungstic acid (PTA) staining. Specimens were imaged with micro-CT following two protocols: 1) absorption contrast (AC) imaging 2) propagation phase-contrast (PPC) imaging. All samples were scanned in liquid. The contrast to noise ratio (C/N) of cellular features quantified scan quality and were statistically analysed. Cellular features resolved by micro-CT were validated by standard histology.

RESULTS

The highest quality images were obtained using propagation phase-contrast imaging and PTA-staining in 70% EtOH. Cellular features were also visualised when stained in PBS and unstained in EtOH. Under all conditions PPC resulted in greater contrast than AC (p < 0.0001 to p = 0.038). Simultaneous imaging of cartilage and subchondral bone did not impede image quality. Corresponding features were located in both histology and micro-CT and followed the same distribution with similar density and roundness values.

CONCLUSIONS

Three-dimensional visualisation and quantification of the chondrocyte population within articular cartilage can be achieved across a field of view of several millimetres using laboratory-based micro-CT. The ability to map chondrocytes in 3D opens possibilities for research in fields from skeletal development through to medical device design and treatment of cartilage degeneration.

Influence of Loading Conditions and Temperature on Static Friction and Contact Aging of Hydrogels with Modulated Microstructures

Biological tribosystems enable diverse functions of the human body by maintaining extremely low coefficients of friction via hydrogel-like surface layers and a water-based lubricant. Although stiction has been proposed as a precursor to damage, there is still a lack of knowledge about its origin and its relation to the hydrogel's microstructure, which impairs the design of soft matter as replacement biomaterials. In this work, the static friction of poly(acrylamide) hydrogels with modulated composition was investigated by colloidal probe lateral force microscopy as a function of load, temperature, and loading time. Temperature-dependent studies enable to build a phase diagram for hydrogel's static friction, which explains stiction via (polymer) viscoelastic and poroelastic relaxation, and a subtle transition from solid- to liquid-like interfacial behavior. At room temperature, the static friction increases with loading time, a phenomenon called contact aging, which stems from the adhesion of the polymer to the colloid and from the drainage-induced increase in contact area. Contact aging is shown to gradually vanish with increase in temperature, but this behavior strongly depends on the hydrogel's composition. This work scrutinizes the relation between the microstructure of hydrogel-like soft matter and interfacial behavior, with implications for diverse areas of inquiry, not only in biolubrication and biomedical applications but also in soft robotics and microelectromechanical devices, where the processes occurring at the migrating hydrogel interface are of relevance. The results support that modulating both the hydrogel's mesh size and the structure of the near-surface region is a means to control static friction and adhesion. This conceptual framework for static friction will foster further understanding of the wear of hydrogel-like materials.

Type III collagen is a key regulator of the collagen fibrillar structure and biomechanics of articular cartilage and meniscus

Despite the fact that type III collagen is the second most abundant collagen type in the body, its contribution to the physiologic maintenance and repair of skeletal tissues remains poorly understood. This study queried the role of type III collagen in the structure and biomechanical functions of two structurally distinctive tissues in the knee joint, type II collagen-rich articular cartilage and type I collagen-dominated meniscus. Integrating outcomes from atomic force microscopy-based nanomechanical tests, collagen fibril nanostructural analysis, collagen cross-link analysis and histology, we elucidated the impact of type III collagen haplodeficiency on the morphology, nanostructure and biomechanical properties of articular cartilage and meniscus in Col3a1^+/- mice. Reduction of type III collagen leads to increased heterogeneity and mean thickness of collagen fibril diameter, as well as reduced modulus in both tissues, and these effects became more pronounced with skeletal maturation. These data suggest a crucial role of type III collagen in mediating fibril assembly and biomechanical functions of both articular cartilage and meniscus during post-natal growth. In articular cartilage, type III collagen has a marked contribution to the micromechanics of the pericellular matrix, indicating a potential role in mediating the early stage of type II collagen fibrillogenesis and chondrocyte mechanotransduction. In both tissues, reduction of type III collagen leads to decrease in tissue modulus despite the increase in collagen cross-linking. This suggests that the disruption of matrix structure due to type III collagen deficiency outweighs the stiffening of collagen fibrils by increased cross-linking, leading to a net negative impact on tissue modulus. Collectively, this study is the first to highlight the crucial structural role of type III collagen in both articular cartilage and meniscus extracellular matrices. We expect these results to expand our understanding of type III collagen across various tissue types, and to uncover critical molecular components of the microniche for regenerative strategies targeting articular cartilage and meniscus repair.

Mechanical performance of elastomeric PGS scaffolds under dynamic conditions

In developing novel scaffolds, addressing mechanical properties is essential especially when future applications involve cyclic mechanical loading. Therefore, it is important to understand the behaviour of its physical properties with the evolution of its weight loss. Poly(glycerol sebacate) (PGS) is a promising material for tissue and biomedical engineering applications due to its biocompatibility, biodegradability and mechanical properties. To understand the impact of the hydrolytic degradation on the density, cross-linking degree and porosity; scaffolds with an average porosity of 93 ± 2% were synthetized by salt leaching technique and submitted to hydrolytic degradation. The scaffold showed a Young modulus of 17.3 ± 3.4 kPa, with a negligible energy loss during the mechanical solicitation. Moreover, a weight loss of 28 ± 2% followed by an increase in the swelling ratio of the scaffold was observed after 8 weeks of hydrolytic degradation. When submitted to cyclic mechanical loading-unloading, the PGS scaffolds present an outstanding fatigue behaviour under dry and wet conditions, with a remarkable resilience to the cyclic mechanical solicitation, and even after 1000 mechanical cycles, the construct was able to recover to its initial geometry. Overall, the PGS scaffolds demonstrate promising mechanical properties for biomedical applications, especially under dynamic conditions.

Mechanical loading inhibits cartilage inflammatory signalling via an HDAC6 and IFT-dependent mechanism regulating primary cilia elongation

OBJECTIVE

Physiological mechanical loading reduces inflammatory signalling in numerous cell types including articular chondrocytes however the mechanism responsible remains unclear. This study investigates the role of chondrocyte primary cilia and associated intraflagellar transport (IFT) in the mechanical regulation of interleukin-1β (IL-1β) signalling.

DESIGN

Isolated chondrocytes and cartilage explants were subjected to cyclic mechanical loading in the presence and absence of the cytokine IL-1β. Nitric oxide (NO) and prostaglandin E₂ (PGE₂) release were used to monitor IL-1β signalling whilst Sulphated glycosaminoglycan (sGAG) release provided measurement of cartilage degradation. Measurements were made of HDAC6 activity and tubulin polymerisation and acetylation. Effects on primary cilia were monitored by confocal and super resolution microscopy. Involvement of IFT was analysed using ORPK cells with hypomorphic mutation of IFT88.

RESULTS

Mechanical loading suppressed NO and PGE₂ release and prevented cartilage degradation. Loading activated HDAC6 and disrupted tubulin acetylation and cilia elongation induced by IL-1β. HDAC6 inhibition with tubacin blocked the anti-inflammatory effects of loading and restored tubulin acetylation and cilia elongation. Hypomorphic mutation of IFT88 reduced IL-1β signalling and abolished the anti-inflammatory effects of loading indicating the mechanism is IFT-dependent. Loading reduced the pool of non-polymerised tubulin which was replicated by taxol which also mimicked the anti-inflammatory effects of mechanical loading and prevented cilia elongation.

CONCLUSIONS

This study reveals that mechanical loading suppresses inflammatory signalling, partially dependent on IFT, by activation of HDAC6 and post transcriptional modulation of tubulin.

Effect of Photobiomodulation Therapy on the Increase of Viability and Proliferation of Human Mesenchymal Stem Cells

BACKGROUND AND OBJECTIVES

We have investigated how low intensity laser irradiation emitted by a multiwave-locked system (MLS M1) affects the viability and proliferation of human bone marrow mesenchymal stem cells (MSCs) depending on the parameters of the irradiation.

STUDY DESIGN/MATERIALS AND METHODS

Cells isolated surgically from the femoral bone during surgery were identified by flow cytometry and cell differentiation assays. For irradiation, two wavelengths (808 and 905 nm) with the following parameters were used: power density 195, 230, and 318 mW/cm ² , doses of energy 3, 10, and 20 J (energy density 0.93-6.27 J/cm ² ), and in continuous (CW) or pulsed emission (PE) (frequencies 1,000 and 2,000 Hz).

RESULTS

There were statistically significant increases of cell viability and proliferation after irradiation at 3 J (CW; 1,000 Hz), 10 J (1,000 Hz), and 20 J (2,000 Hz).

CONCLUSIONS

Irradiation with the MLS M1 system can be used in vitro to modulate MSCs in preparation for therapeutic applications. This will assist in designing further studies to optimize the radiation parameters and elucidate the molecular mechanisms of action of the radiation. Lasers Surg. Med. © 2019 Wiley Periodicals, Inc.

Biophysical Stimuli: A Review of Electrical and Mechanical Stimulation in Hyaline Cartilage

OBJECTIVE

Hyaline cartilage degenerative pathologies induce morphologic and biomechanical changes resulting in cartilage tissue damage. In pursuit of therapeutic options, electrical and mechanical stimulation have been proposed for improving tissue engineering approaches for cartilage repair. The purpose of this review was to highlight the effect of electrical stimulation and mechanical stimuli in chondrocyte behavior.

DESIGN

Different information sources and the MEDLINE database were systematically revised to summarize the different contributions for the past 40 years.

RESULTS

It has been shown that electric stimulation may increase cell proliferation and stimulate the synthesis of molecules associated with the extracellular matrix of the articular cartilage, such as collagen type II, aggrecan and glycosaminoglycans, while mechanical loads trigger anabolic and catabolic responses in chondrocytes.

CONCLUSION

The biophysical stimuli can increase cell proliferation and stimulate molecules associated with hyaline cartilage extracellular matrix maintenance.

Analysis of Mechanically Activated Ion Channels at the Cell-Substrate Interface: Combining Pillar Arrays and Whole-Cell Patch-Clamp

Ionic currents can be evoked by mechanical inputs applied directly at the cell-substrate interface. These ionic currents are mediated by mechanically activated ion channels, where the open probability increases with increasing mechanical input. In order to study mechanically activated ion channels directly at the interface between cells and their environment, we have developed a technique to simultaneously monitor ion channel activity whilst stimuli are applied via displacement of cell-substrate contacts. This technique utilizes whole-cell patch-clamp electrophysiology and elastomeric pillar arrays, it is quantitative and appropriate for studying channels that respond to stimuli that are propagated to an adherent cell via the physical substrate. The mammalian channels PIEZO1, PIEZO2 have been shown to be activated by substrate deflections, using this technique. In addition, TRPV4 mediated currents can be evoked by substrate deflections, in contrast to alternate stimulation methods such as membrane stretch or cellular indentation. The deflections applied at cell-substrate points mimic the magnitude of physical stimuli that impact cells in situ.

Activities of daily living influence tibial cartilage T1rho relaxation times

Quantitative T1rho magnetic resonance imaging (MRI) can potentially help identify early-stage osteoarthritis (OA) by non-invasively assessing proteoglycan concentration in articular cartilage. T1rho relaxation times are negatively correlated with proteoglycan concentration. Cartilage compresses in response to load, resulting in water exudation, a relative increase in proteoglycan concentration, and a decrease in the corresponding T1rho relaxation times. To date, there is limited information on changes in cartilage composition resulting from daily activity. Therefore, the objective of this study was to quantify changes in tibial cartilage T1rho relaxation times in healthy human subjects following activities of daily living. It was hypothesized that water exudation throughout the day would lead to decreased T1rho relaxation times. Subjects underwent MR imaging in the morning and afternoon on the same day and were free to go about their normal activities between scans. Our findings confirmed the hypothesis that tibial cartilage T1rho relaxation times significantly decreased (by 7%) over the course of the day with loading, which is indicative of a relative increase in proteoglycan concentration. Additionally, baseline T1rho values varied with position within the cartilage, supporting a need for site-specific measurements of T1rho relaxation times. Understanding how loading alters the proteoglycan concentration in healthy cartilage may hold clinical significance pertaining to cartilage homeostasis and potentially help to elucidate a mechanism for OA development. These results also indicate that future studies using T1rho relaxation times as an indicator of cartilage health should control the loading history prior to image acquisition to ensure the appropriate interpretation of the data.

Atomic Force Microscopy on Biological Materials Related to Pathological Conditions

Atomic force microscopy (AFM) is an easy-to-use, powerful, high-resolution microscope that allows the user to image any surface and under any aqueous condition. AFM has been used in the investigation of the structural and mechanical properties of a wide range of biological matters including biomolecules, biomaterials, cells, and tissues. It provides the capacity to acquire high-resolution images of biosamples at the nanoscale and allows at readily carrying out mechanical characterization. The capacity of AFM to image and interact with surfaces, under physiologically relevant conditions, is of great importance for realistic and accurate medical and pharmaceutical applications. The aim of this paper is to review recent trends of the use of AFM on biological materials related to health and sickness. First, we present AFM components and its different imaging modes and we continue with combined imaging and coupled AFM systems. Then, we discuss the use of AFM to nanocharacterize collagen, the major fibrous protein of the human body, which has been correlated with many pathological conditions. In the next section, AFM nanolevel surface characterization as a tool to detect possible pathological conditions such as osteoarthritis and cancer is presented. Finally, we demonstrate the use of AFM for studying other pathological conditions, such as Alzheimer's disease and human immunodeficiency virus (HIV), through the investigation of amyloid fibrils and viruses, respectively. Consequently, AFM stands out as the ideal research instrument for exploring the detection of pathological conditions even at very early stages, making it very attractive in the area of bio- and nanomedicine.

Meso-scale topological cues influence extracellular matrix production in a large deformation, elastomeric scaffold model

Physical cues are decisive factors in extracellular matrix (ECM) formation and elaboration. Their transduction across scale lengths is an inherently symbiotic phenomenon that while influencing ECM fate is also mediated by the ECM structure itself. This study investigates the possibility of enhancing ECM elaboration by topological cues that, while not modifying the substrate macro scale mechanics, can affect the meso-scale strain range acting on cells incorporated within the scaffold. Vascular smooth muscle cell micro-integrated, electrospun scaffolds were fabricated with comparable macroscopic biaxial mechanical response, but different meso-scale topology. Seeded scaffolds were conditioned on a stretch bioreactor and exposed to large strain deformations. Samples were processed to evaluate ECM quantity and quality via: biochemical assay, qualitative and quantitative histological assessment and multi-photon analysis. Experimental evaluation was coupled to a numerical model that elucidated the relationship between the scaffold micro-architecture and the strain acting on the cells. Results showed an higher amount of ECM formation for the scaffold type characterized by lowest fiber intersection density. The numerical model simulations associated this result with the differences found for the change in cell nuclear aspect ratio and showed that given comparable macro scale mechanics, a difference in material topology created significant differences in cell-scaffold meso-scale deformations. These findings reaffirmed the role of cell shape in ECM formation and introduced a novel notion for the engineering of cardiac tissue where biomaterial structure can be designed to both mimick the organ level mechanics of a specific tissue of interest and elicit a desirable cellular response.

A compression system for studying depth-dependent mechanical properties of articular cartilage under dynamic loading conditions

The biological activities of chondrocytes are influenced by the mechanical characteristics of their environment. The overall real-time mechanical response of cartilage has been investigated earlier. However, the instantaneous local mechano-biology of cartilage has not been investigated in detail under dynamic loading conditions. In order to address this gap in the literature, we designed a compression testing device and implemented a dual photon microscopy technique with the goal of measuring local mechanical and biological responses of articular cartilage under dynamic loading conditions. The details of the compression system and results of a pilot study are presented here. A 15% ramp compression at a rate of 0.003/s with a subsequent stress relaxation phase was applied to the cartilage explant samples. The extra cellular matrix was imaged throughout the entire thickness of the cartilage sample, and local tissue strains were measured during the compression and relaxation phase. The axial compressive strains in the middle and superficial zones of cartilage were observed to increase during the relaxation phase: this was a new finding, suggesting the importance of further investigations on the real-time local behavior of cartilage. The compression system showed promising results for investigating the dynamic, real-time mechanical response of articular cartilage, and can now be used to reveal the instantaneous mechanical and biological responses of chondrocytes in response to dynamic loading conditions.

Three-dimensional micro-scale strain mapping in living biological soft tissues

Non-invasive characterization of the mechanical micro-environment surrounding cells in biological tissues at multiple length scales is important for the understanding of the role of mechanics in regulating the biosynthesis and phenotype of cells. However, there is a lack of imaging methods that allow for characterization of the cell micro-environment in three-dimensional (3D) space. The aims of this study were (i) to develop a multi-photon laser microscopy protocol capable of imprinting 3D grid lines onto living tissue at a high spatial resolution, and (ii) to develop image processing software capable of analyzing the resulting microscopic images and performing high resolution 3D strain analyses. Using articular cartilage as the biological tissue of interest, we present a novel two-photon excitation imaging technique for measuring the internal 3D kinematics in intact cartilage at sub-micrometer resolution, spanning length scales from the tissue to the cell level. Using custom image processing software, we provide accurate and robust 3D micro-strain analysis that allows for detailed qualitative and quantitative assessment of the 3D tissue kinematics. This novel technique preserves tissue structural integrity post-scanning, therefore allowing for multiple strain measurements at different time points in the same specimen. The proposed technique is versatile and opens doors for experimental and theoretical investigations on the relationship between tissue deformation and cell biosynthesis. Studies of this nature may enhance our understanding of the mechanisms underlying cell mechano-transduction, and thus, adaptation and degeneration of soft connective tissues.

STATEMENT OF SIGNIFICANCE

We presented a novel two-photon excitation imaging technique for measuring the internal 3D kinematics in intact cartilage at sub-micrometer resolution, spanning from tissue length scale to cellular length scale. Using a custom image processing software (lsmgridtrack), we provide accurate and robust micro-strain analysis that allowed for detailed qualitative and quantitative assessment of the 3D tissue kinematics. The approach presented here can also be applied to other biological tissues such as meniscus and annulus fibrosus, as well as tissue-engineered tissues for the characterization of their mechanical properties. This imaging technique opens doors for experimental and theoretical investigation on the relationship between tissue deformation and cell biosynthesis. Studies of this nature may enhance our understanding of the mechanisms underlying cell mechano-transduction, and thus, adaptation and degeneration of soft connective tissues.

Functional properties of chondrocytes and articular cartilage using optical imaging to scanning probe microscopy

Mature chondrocytes in adult articular cartilage vary in number, size, and shape, depending on their depth in the tissue, location in the joint, and source species. Chondrocytes are the primary structural, functional, and metabolic unit in articular cartilage, the loss of which will induce fatigue to the extracellular matrix (ECM), eventually leading to failure of the cartilage and impairment of the joint as a whole. This brief review focuses on the functional and biomechanical studies of chondrocytes and articular cartilage, using microscopic imaging from optical microscopies to scanning probe microscopy. Three topics are covered in this review, including the functional studies of chondrons by optical imaging (unpolarized and polarized light and infrared light, two-photon excitation microscopy), the probing of chondrocytes and cartilage directly using microscale measurement techniques, and different imaging approaches that can measure chondrocyte mechanics and chondrocyte biological signaling under in situ and in vivo environments. Technical advancement in chondrocyte research during recent years has enabled new ways to study the biomechanical and functional properties of these cells and cartilage. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:620-631, 2018.

3D Bioprinting for Cartilage and Osteochondral Tissue Engineering

Significant progress has been made in the field of cartilage and bone tissue engineering over the last two decades. As a result, there is real promise that strategies to regenerate rather than replace damaged or diseased bones and joints will one day reach the clinic however, a number of major challenges must still be addressed before this becomes a reality. These include vascularization in the context of large bone defect repair, engineering complex gradients for bone-soft tissue interface regeneration and recapitulating the stratified zonal architecture present in many adult tissues such as articular cartilage. Tissue engineered constructs typically lack such spatial complexity in cell types and tissue organization, which may explain their relatively limited success to date. This has led to increased interest in bioprinting technologies in the field of musculoskeletal tissue engineering. The additive, layer by layer nature of such biofabrication strategies makes it possible to generate zonal distributions of cells, matrix and bioactive cues in 3D. The adoption of biofabrication technology in musculoskeletal tissue engineering may therefore make it possible to produce the next generation of biological implants capable of treating a range of conditions. Here, advances in bioprinting for cartilage and osteochondral tissue engineering are reviewed.