Early detection of aging cartilage and osteoarthritis in mice and patient samples using atomic force microscopy

AFM assessing of nanomechanical fingerprints for cancer early diagnosis and classification: from single cell to tissue level

Cancer development and progression are closely associated with changes both in the mechano-cellular phenotype of cancer and stromal cells and in the extracellular matrix (ECM) structure, composition, and mechanics. In this paper, we review the use of atomic force microscopy (AFM) as a tool for assessing the nanomechanical fingerprints of solid tumors, so as to be potentially used as a diagnostic biomarker for more accurate identification and early cancer grading/classification. The development of such a methodology is expected to provide new insights and a novel approach for cancer diagnosis. We propose that AFM measurements could be employed to complement standard biopsy procedures, offering an objective, novel and quantitative diagnostic approach with the properties of a blind assay, allowing unbiased evaluation of the sample.

Dehydroepiandrosterone: Molecular mechanisms and therapeutic implications in osteoarthritis

Dehydroepiandrosterone (DHEA), a 19-carbon steroid hormone primarily synthesized in the adrenal gland, exerts a chondroprotective effect against osteoarthritis (OA) and has been considered an effective candidate of disease-modifying OA drugs (DMOADs) that slow disease progression. We and others previously demonstrated that DHEA exerted a beneficial effect on osteoarthritic cartilage by positively modulating the balance between anabolic and catabolic factors (e.g., MMPs/TIMP-1, ADAMTS/TIMP-3 and cysteine proteinases/cystatin C), inhibiting catabolic signaling pathways (e.g., Wnt/β-catenin), and suppressing proinflammatory cytokines-mediated low-grade synovial inflammation (e.g., IL-1β). However, the full picture of the pharmacological molecular mechanism(s) underlying the activity of DHEA against OA is still incomplete, and a comprehensive and up-to-date review article in this field is unavailable. In this review, recent findings (apart from the well documented pathogenesis of OA) regarding disease-related mechanisms involving low grade synovial inflammation, cartilage matrix stiffness, chondrocyte autophagy and the roles of a variety of catabolic cellular signaling pathways are discussed. Moreover, the possible relationship between these disease-related mechanisms and DHEA action is discussed. Emerging evidence from in vivo and in vitro studies were scrutinized and are concisely presented to demonstrate the investigational and putative mechanisms underlying the anti-OA potential of DHEA.

Articular cartilage regeneration using acellular bioactive affinity-binding alginate hydrogel: A 6-month study in a mini-pig model of osteochondral defects

Background

Despite intensive research, regeneration of articular cartilage largely remains an unresolved medical concern as the clinically available modalities still suffer from long-term inconsistent data, relatively high failure rates and high prices of more promising approaches, such as cell therapy. In the present study, we aimed to evaluate the feasibility and long-term efficacy of a bilayered injectable acellular affinity-binding alginate hydrogel in a large animal model of osteochondral defects.

Methods

The affinity-binding alginate hydrogel is designed for presentation and slow release of chondrogenic and osteogenic inducers (transforming growth factor-β1 and bone morphogenic protein 4, respectively) in two distinct and separate hydrogel layers. The hydrogel was injected into the osteochondral defects created in the femoral medial condyle in mini-pigs, and various outcomes were evaluated after 6 months.

Results

Macroscopical and histological assessment of the defects treated with growth factor affinity-bound hydrogel showed effective reconstruction of articular cartilage layer, with major features of hyaline tissue, such as a glossy surface and cellular organisation, associated with marked deposition of proteoglycans and type II collagen. Microcomputed tomography showed incomplete bone formation in both treatment groups, which was nevertheless augmented by the presence of affinity-bound growth factors. Importantly, the physical nature of the applied hydrogel ensured its shear resistance, seamless integration and topographical matching to the surroundings and opposing articulating surface.

Conclusions

The treatment with acellular injectable growth factor-loaded affinity-binding alginate hydrogel resulted in effective tissue restoration with major hallmarks of hyaline cartilage, shown in large animal model after 6-month follow-up.

The translational potential of this article

This proof-of-concept study in a clinically relevant large animal model showed promising potential of an injectable acellular growth factor-loaded affinity-binding alginate hydrogel for effective repair and regeneration of articular hyaline cartilage, representing a strong candidate for future clinical development.

Advances in stem cell therapy for cartilage regeneration in osteoarthritis

INTRODUCTION

Osteoarthritis (OA) is a progressive joint disease that compromises the structural integrity of cartilage tissue. Conventional treatments based on medication or surgery are nowadays inefficient and cell-based therapy has emerged as one of the most promising methods for cartilage regeneration. The first therapy developed for cartilage defects was autologous chondrocyte implantation, but in the last few decades stem cells (SCs) from different sources have been proposed as a possible alternative for OA.

AREAS COVERED

SC sources and available delivery procedures (scaffolds/hydrogels) are presented, along with the main issues arisen in this regard. Thereafter, preclinical and clinical trials performed in recent years are reviewed in order to take a glance toward the potential benefits that such therapies could deliver to the patients.

EXPERT OPINION

SCs have proven their potential and safety for OA treatment. Nevertheless, there are still many questions to be resolved before their widespread used in clinical practice, such as the treatment mechanism, the best cell source, the most appropriate processing method, the most effective dose and delivery procedure, and their efficacy. In this sense, long-term follow-up and larger randomized controlled trials utilizing standardized and established outcome scores are mandatory to make objective conclusions.

Evaluation of surface charge shift of collagen fibrils exposed to glutaraldehyde

Collagen fibrils are a major component of the extracellular matrix. They form nanometer-scale “cables” acting as a scaffold for cells in animal tissues and are widely used in tissue-engineering. Besides controlling their structure and mechanical properties, it is crucial to have information of their surface charge, as this affects how cells attach to the scaffold. Here, we employed Kelvin-probe Force Microscopy to determine the electrostatic surface potential at the single-fibril level and investigated how glutaraldehyde, a well-established protein cross-linking agent, shifts the surface charge to more negative values without disrupting the fibrils themselves. This shift can be interpreted as the result of the reaction between the carbonyl groups of glutaraldehyde and the amine groups of collagen. It reduces the overall density of positively charged amine groups on the collagen fibril surface and, ultimately, results in the observed negative shift of the surface potential measured. Reactions between carbonyl-containing compounds and proteins are considered the first step in glycation, the non-enzymatic reaction between sugars and proteins. It is conceivable that similar charge shifts happen in vivo caused by sugars, which could have serious implications on age-related diseases such as diabetes and which has been hypothesised for many years.

Nanoscale dysregulation of collagen structure-function disrupts mechano-homeostasis and mediates pulmonary fibrosis

Matrix stiffening with downstream activation of mechanosensitive pathways is strongly implicated in progressive fibrosis; however, pathologic changes in extracellular matrix (ECM) that initiate mechano-homeostasis dysregulation are not defined in human disease. By integrated multiscale biomechanical and biological analyses of idiopathic pulmonary fibrosis lung tissue, we identify that increased tissue stiffness is a function of dysregulated post-translational collagen cross-linking rather than any collagen concentration increase whilst at the nanometre-scale collagen fibrils are structurally and functionally abnormal with increased stiffness, reduced swelling ratio, and reduced diameter. In ex vivo and animal models of lung fibrosis, dual inhibition of lysyl oxidase-like (LOXL) 2 and LOXL3 was sufficient to normalise collagen fibrillogenesis, reduce tissue stiffness, and improve lung function in vivo. Thus, in human fibrosis, altered collagen architecture is a key determinant of abnormal ECM structure-function, and inhibition of pyridinoline cross-linking can maintain mechano-homeostasis to limit the self-sustaining effects of ECM on progressive fibrosis.

Idiopathic pulmonary fibrosis (IPF) is a devastating disease of the lung, which scars the tissue and gradually destroys the organ, ultimately leading to death. It is still unclear what exactly causes this scarring, but it is thought that increasing amounts of proteins in the space surrounding the cells of the lungs, the extracellular matrix, could play a role.

These proteins, including collagen, normally form a ‘scaffold’ to stabilize cells, but if they accumulate uncontrollably, they can render tissues rigid. It has been assumed that these changes are a consequence of the disease. However, recent evidence suggests that the increased stiffness itself could stimulate cells to produce even more extracellular matrix, driving the progression of the disease. A better understanding of what exactly causes the tissue to become gradually stiffer may identify new ways to block the progression of IPF.

Now, Jones et al. compared measurements of the tissue stiffness and the collagen structure taken from samples of patients with IPF. The results showed that the collagen fibres were faulty and had an abnormal shape. This suggests that these problems, rather than an increased amount of collagen, alter the flexibility of the lung tissue.

Jones et al. also found that a specific family of proteins, which helps to connect the collagen fibres, was increased in the tissue of patients with IPF. When these proteins were blocked with a newly developed drug, the collagen structure returned to normal and the stiffness of the tissue decreased. As a consequence, the lung capacity improved.

This suggests that treatment approaches that help to maintain a normal collagen structure, may in future prevent the stiffening of the lung tissue and so limit feed-forward mechanisms that drive progressive IPF. Moreover, it indicates that measurements of the structure of collagen rather than the its total concentration could serve as a more suitable indicator for the disease.

An Alternative Method to Characterize the Quasi-Static, Nonlinear Material Properties of Murine Articular Cartilage

With the onset and progression of osteoarthritis (OA), articular cartilage (AC) mechanical properties are altered. These alterations can serve as an objective measure of tissue degradation. Although the mouse is a common and useful animal model for studying OA, it is extremely challenging to measure the mechanical properties of murine AC due to its small size (thickness < 50 μm). In this study, we developed novel and direct approach to independently quantify two quasi-static mechanical properties of mouse AC: the load-dependent (nonlinear) solid matrix Young's modulus (E) and drained Poisson's ratio (ν). The technique involves confocal microscope-based multiaxial strain mapping of compressed, intact murine AC followed by inverse finite element analysis (iFEA) to determine E and ν. Importantly, this approach yields estimates of E and ν that are independent of the initial guesses used for iterative optimization. As a proof of concept, mechanical properties of AC on the medial femoral condyles of wild-type mice were obtained for both trypsin-treated and control specimens. After proteolytic tissue degradation induced through trypsin treatment, a dramatic decrease in E was observed (compared to controls) at each of the three tested loading conditions. A significant decrease in ν due to trypsin digestion was also detected. These data indicate that the method developed in this study may serve as a valuable tool for comparative studies evaluating factors involved in OA pathogenesis using experimentally induced mouse OA models.

Calibration of colloidal probes with atomic force microscopy for micromechanical assessment

Mechanical assessment of biological materials and tissue-engineered scaffolds is increasingly focusing at lower length scale levels. Amongst other techniques, atomic force microscopy (AFM) has gained popularity as an instrument to interrogate material properties, such as the indentation modulus, at the microscale via cantilever-based indentation tests equipped with colloidal probes. Current analysis approaches of the indentation modulus from such tests require the size and shape of the colloidal probe as well as the spring constant of the cantilever. To make this technique reproducible, there still exist the challenge of proper calibration and validation of such mechanical assessment. Here, we present a method to (a) fabricate and characterize cantilevers with colloidal probes and (b) provide a guide for estimating the spring constant and the sphere diameter that should be used for a given sample to achieve the highest possible measurement sensitivity. We validated our method by testing agarose samples with indentation moduli ranging over three orders of magnitude via AFM and compared these results with bulk compression tests. Our results show that quantitative measurements of indentation modulus is achieved over three orders of magnitude ranging from 1 kPa to 1000 kPa via AFM cantilever-based microindentation experiments. Therefore, our approach could be used for quantitative micromechanical measurements without the need to perform further validation via bulk compression experiments.

Laser-induced surface deformation microscope for the study of the dynamic viscoelasticity of plasma membrane in a living cell

A laser-induced surface deformation (LISD) microscope is developed and applied to measurement of the dynamic relaxation responses of the plasma membrane in a living cell. A laser beam is tightly focused on an optional area of cell surface and the focused light induces microscopic deformation on the surface via radiation pressure. The LISD microscope not only allows non-contact and destruction-free measurement but provides power spectra of the surface responses depending on the frequency of the intensity of the laser beam. An optical system for the LISD is equipped via a microscope, allowing us to measure the relaxation responses in sub-cellular-sized regions of the plasma membrane. In addition, the forced oscillation caused by the radiation pressure for surface deformation extends the upper limit of the frequency range in the obtained power spectra to 106 Hz, which enables us to measure relaxation responses in local regions within the plasma membrane. From differences in power-law exponents at higher frequencies, it is realized that a cancerous cell obeys a weaker single power-law than a normal fibroblast cell. Furthermore, the power spectrum of a keratinocyte cell obeys a power-law with two exponents, indicating that alternative mechanical models to a conventional soft glassy rheology model (where single power-laws explain cells' responses below about 103 Hz) are needed for the understanding over a wider frequency range. The LISD microscope would contribute to investigation of microscopic cell rheology, which is important for clarifying the mechanisms of cell migration and tissue construction.

Chondrogenic differentiation of human chondrocytes cultured in the absence of ascorbic acid

Bioreactor systems will likely play a key role in establishing regulatory compliant and cost-effective production systems for manufacturing engineered tissue grafts for clinical applications. However, the automation of bioreactor systems could become considerably more complex and costly due to the requirements for additional storage and liquid handling technologies if unstable supplements are added to the culture medium. Ascorbic acid (AA) is a bioactive supplement that is commonly presumed to be essential for the generation of engineered cartilage tissues. However, AA can be rapidly oxidized and degraded. In this work, we addressed whether human nasal chondrocytes can redifferentiate, undergo chondrogenesis, and generate a cartilaginous extracellular matrix when cultured in the absence of AA. We found that when chondrocytes were cultured in 3D micromass pellets either with or without AA, there were no significant differences in their chondrogenic capacity in terms of gene expression or the amount of glycosaminoglycans. Moreover, 3D pellets cultured without AA contained abundant collagen Types II and I extracellular matrix. Although the amounts of Collagens II and I were significantly lower (34% and 50% lower) than in pellets cultured with AA, collagen fibers had similar thicknesses and distributions for both groups, as shown by scanning electron microscopy imaging. Despite the reduced amounts of collagen, if engineered cartilage grafts can be generated with sufficient properties that meet defined quality criteria without the use of unstable supplements such as AA, bioreactor automation requirements can be greatly simplified, thereby facilitating the development of more compact, user-friendly, and cost-effective bioreactor-based manufacturing systems.

Bioluminescence and second harmonic generation imaging reveal dynamic changes in the inflammatory and collagen landscape in early osteoarthritis

Osteoarthritis (OA) is a leading cause of chronic disability whose mechanism of pathogenesis is largely elusive. Local inflammation is thought to play a key role in OA progression, especially in injury-associated OA. While multiple inflammatory cytokines are detected, the timing and extent of overall inflammatory activities in early OA and the manner by which joint inflammation correlates with cartilage structural damage are still unclear. We induced OA via destabilization of the medial meniscus (DMM) in NFκB luciferase reporter mice, whose bioluminescent signal reflects the activity of NFκB, a central mediator of inflammation. Bioluminescence imaging data showed that DMM and sham control joints had a similar surge of inflammation at 1-week post-surgery, but the DMM joint exhibited a delay in resolution of inflammation in subsequent weeks. A similar trend was observed with synovitis, which we found to be mainly driven by synovial cell density and inflammatory infiltration rather than synovial lining thickness. Interestingly, an association between synovitis and collagen structural damage was observed in early OA. Using Second Harmonic Generation (SHG) imaging, we analyzed collagen fiber organization in articular cartilage. Zonal differences in collagen fiber thickness and organization were observed as soon as OA initiated after DMM surgery, and persisted over time. Even at 1-week post-surgery, the DMM joint showed a decrease in collagen fiber thickness in the deep zone and an increase in collagen fiber disorganization in the superficial zone. Since we were able detect and quantify collagen structural changes very early in OA development by SHG imaging, we concluded that SHG imaging is a highly sensitive tool to evaluate pathological changes in OA. In summary, this study uncovered a dynamic profile of inflammation and joint cartilage damage during OA initiation and development, providing novel insights into OA pathology.

Mechanical and migratory properties of normal, scar, and Dupuytren's fibroblasts

Mechanical properties of myofibroblasts play a key role in Dupuytren's disease. Here, we used atomic force microscopy to measure the viscoelastic properties of 3 different types of human primary fibroblasts derived from a same patient: normal and scar dermal fibroblasts and palmar fascial fibroblasts from Dupuytren's nodules. Different stiffness hydrogels (soft ~1 kPa and stiff ~ 50 kPa) were used as cell culture matrix to mimic the mechanical properties of the natural tissues, and atomic force microscopy step response force curves were used to discriminate between elastic and viscous properties of cells. Since transforming growth factor-β1 (TGF-β1) is known to induce expression of α-smooth muscle actin positive stress fibers in myofibroblasts, we investigated the behavior of these fibroblasts before and after applying TGF-β1. Finally, we performed an in vitro cell motility test, the wound healing or scratch assay, to evaluate the migratory properties of these fibroblasts. We found that (1) Dupuytren's fibroblasts are stiffer than normal and scar fibroblasts, the elastic modulus E ranging from 4.4, 2.1, to 1.8 kPa, for Dupuytren's, normal and scar fibroblasts, respectively; (2) TGF-β1 enhances the level of α-smooth muscle actin expression and thus cell stiffness in Dupuytren's fibroblasts (E, ~6.2 kPa); (3) matrix stiffness influences cell mechanical properties most prominently in Dupuytren's fibroblasts; and (4) Dupuytren's fibroblasts migrate slower than the other fibroblasts by a factor of 3. Taking together, our results showed that mechanical and migratory properties of fibroblasts might help to discriminate between different pathological conditions, helping to identify and recognize specific cell phenotypes.

Collagen Fibrils: Nature's Highly Tunable Nonlinear Springs

Tissue hydration is well known to influence tissue mechanics and can be tuned via osmotic pressure. Collagen fibrils are nature's nanoscale building blocks to achieve biomechanical function in a broad range of biological tissues and across many species. Intrafibrillar covalent cross-links have long been thought to play a pivotal role in collagen fibril elasticity, but predominantly at large, far from physiological, strains. Performing nanotensile experiments of collagen fibrils at varying hydration levels by adjusting osmotic pressure in situ during atomic force microscopy experiments, we show the power the intrafibrillar noncovalent interactions have for defining collagen fibril tensile elasticity at low fibril strains. Nanomechanical tensile tests reveal that osmotic pressure increases collagen fibril stiffness up to 24-fold in transverse (nanoindentation) and up to 6-fold in the longitudinal direction (tension), compared to physiological saline in a reversible fashion. We attribute the stiffening to the density and strength of weak intermolecular forces tuned by hydration and hence collagen packing density. This reversible mechanism may be employed by cells to alter their mechanical microenvironment in a reversible manner. The mechanism could also be translated to tissue engineering approaches for customizing scaffold mechanics in spatially resolved fashion, and it may help explain local mechanical changes during development of diseases and inflammation.

Microscale mapping of extracellular matrix elasticity of mouse joint cartilage: an approach to extracting bulk elasticity of soft matter with surface roughness

Cartilage is composed of cells and an extracellular matrix, the latter being a composite of a collagen mesh interpenetrated by proteoglycans responsible for tissue osmotic swelling. The matrix composition and structure vary through the tissue depth. Mapping such variability requires tissue sectioning to gain access. The resulting surface roughness, and concomitant proteoglycan loss contribute to large uncertainties in elastic modulus estimates. To extract elasticity values for the bulk matrix which are not obfuscated by the indeterminate surface layer, we developed a novel experimental and data analysis methodology. We analyzed the surface roughness to optimize the probe size, and performed high-resolution (1 μm) elasticity mapping on thin (∼12 μm), epiphyseal newborn mouse cartilage sections cut parallel to the bone longitudinal axis or normal to the articular surface. Mild fixation prevented the major proteoglycan loss observed in unfixed specimens but not the stress release that resulted in thickness changes in the sectioned matrix. Our novel data analysis method introduces a virtual contact point as a fitting parameter for the Hertz model, to minimize the effects of surface roughness and corrects for the finite section thickness. Our estimates of cartilage elasticity converge with increasing indentation depth and, unlike previous data interpretations, are consistent with linearly elastic material. A high cell density that leaves narrow matrix septa between cells may cause the underestimation of elastic moduli, whereas fixation probably causes an overestimation. The proposed methodology has broader relevance to nano- and micro-indentation of soft materials with multiple length scales of organization and whenever surface effects (including roughness, electrostatics, van der Waals forces, etc.) become significant.

Force Spectroscopy on a Cell Drum: AFM Measurements on the Basolateral Side of Cells via Inverted Cell Cultures

The elasticity of a cell is one of the most critical measures of the difference between cancerous cells and healthy cells: cancer cells tend to be softer than healthy cells, and highly invasive cells tend to be more elastic than less aggressive cells. In this work, we present the novel "bottom-up" cell force spectroscopy method for the biophysical characterization of cancer cells, in which an atomic force microscopy (AFM) tip approach from the backside of a net-shaped culture substrate exposing the basolateral cell membrane drum, and compare it with the conventional "top-down" AFM measurements. We used two different human pancreatic carcinoma cell lines, PaTu8988S and PaTu8988T. Our bottom-up AFM tip approach provided a more statistically synchronized distribution of the measured elastic moduli of the cells, demonstrating its superior applicability for the clinical use of force spectroscopy, which is not attainable with the conventional top-down AFM approach.

Physiology and Engineering of the Graded Interfaces of Musculoskeletal Junctions

The connective tissues of the musculoskeletal system can be grouped into fibrous, cartilaginous, and calcified tissues. While each tissue type has a distinct composition and function, the intersections between these tissues result in the formation of complex, composite, and graded junctions. The complexity of these interfaces is a critical aspect of their healthy function, but poses a significant challenge for their repair. In this review, we describe the organization and structure of complex musculoskeletal interfaces, identify emerging technologies for engineering such structures, and outline the requirements for assessing the complex nature of these tissues in the context of recapitulating their function through tissue engineering.

Morphological, biochemical and mechanical properties of articular cartilage and subchondral bone in rat tibial plateau are age related

The purpose of this study was to investigate age-related changes in the morphological, biochemical and mechanical properties of articular cartilage (AC) and subchondral bone in the rat tibial plateau. Female Wistar rats were grouped according to age (1, 3, 5, 7, 9, 11, 13, 15, 16 and 17 months, with 10 rats in each group). The ultrastructures, surface topographies, and biochemical and mechanical properties of the AC and subchondral bone in the knee joints of the rats were determined through X-ray micro-tomography, histology, immunohistochemistry, scanning electron microscopy (SEM), atomic force microscopy and nanoindentation. We found that cartilage thickness decreased with age. This decrease was accompanied by functional condensation of the underlying subchondral bone. Increased thickness and bone mineral density and decreased porosity were observed in the subchondral plate (SP). Growth decreased collagen II expression in the tibial cartilage. The arrangement of trabeculae in the subchondral trabecular bone became disordered. The thickness and strength of the fibers decreased with age, as detected by SEM. The SP and trabeculae in the tibial plateau increased in roughness in the first phase (1-9 months of age), and then were constant in the second phase (11-17 months of age). Meanwhile, the roughness of the AC changed significantly in the first phase (1-9 months of age), but the changes were independent of age thereafter. This study gives a comprehensive insight into the growth-related structural, biochemical and mechanical changes in the AC and subchondral bone. The results presented herein may contribute to a new understanding of the pathogenesis of age-related bone diseases.

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.

Periosteum Extracellular-Matrix-Mediated Acellular Mineralization during Bone Formation

Cell-mediated mineralization is essential for bone formation and regeneration. In this study, it is proven that extracellular matrix (ECM) of decellularized periosteum can play an initiative and independent role in bone-like apatite formation. Using decellularized periosteum scaffold, it is revealed that ECM scaffold itself can promote critical bone defect regeneration and nude mouse ectopic ossification. The natural collagen matrix of decellularized periosteum can serve as a 3D structural template for Ca-P nuclei initiation, controlling the size and orientation of bone-like mineral crystals. The naturally cross-linked and highly ordered 3D fibrillar network of decellularized periosteum not only provides a model for mimicking mineralization in vitro and in vivo to elucidate the important functions of ECM in bone formation and regeneration, but also can be a promising biomaterial for bone tissue engineering and clinical application.

Early Effects of Ionizing Radiation on the Collagen Hierarchical Structure of Bladder and Rectum Visualized by Atomic Force Microscopy

Radiation therapy, widely used in the treatment of a variety of malignancies in the pelvic area, is associated with inevitable damage to the surrounding healthy tissues. We have applied atomic force microscopy (AFM) to track the early damaging effects of ionizing radiation on the collagen structures in the experimental animals' bladder and rectum. The first signs of the low-dose radiation (2 Gy) effect were detected by AFM as early as 1 week postirradiation. The observed changes were consistent with initial radiation destruction of the protein matrix. The alterations in the collagen fibers' packing 1 month postirradiation were indicative of the onset of fibrotic processes. The destructive effect of higher radiation doses was probed 1 day posttreatment. The severity of the radiation damage was proportional to the dose, from relatively minor changes in the collagen packing at 8 Gy to the growing collagen matrix destruction at higher doses and complete three-dimensional collagen network restructuring towards fibrotic-type architecture at the dose of 22 Gy. The AFM study appeared superior to the optical microscopy-based studies in its sensitivity to early radiation damage of tissues, providing valuable additional information on the onset and development of the collagen matrix destruction and remodeling.

Tissue material properties and computational modelling of the human tibiofemoral joint: a critical review

Understanding how structural and functional alterations of individual tissues impact on whole-joint function is challenging, particularly in humans where direct invasive experimentation is difficult. Finite element (FE) computational models produce quantitative predictions of the mechanical and physiological behaviour of multiple tissues simultaneously, thereby providing a means to study changes that occur through healthy ageing and disease such as osteoarthritis (OA). As a result, significant research investment has been placed in developing such models of the human knee. Previous work has highlighted that model predictions are highly sensitive to the various inputs used to build them, particularly the mathematical definition of material properties of biological tissues. The goal of this systematic review is two-fold. First, we provide a comprehensive summation and evaluation of existing linear elastic material property data for human tibiofemoral joint tissues, tabulating numerical values as a reference resource for future studies. Second, we review efforts to model tibiofemoral joint mechanical behaviour through FE modelling with particular focus on how studies have sourced tissue material properties. The last decade has seen a renaissance in material testing fuelled by development of a variety of new engineering techniques that allow the mechanical behaviour of both soft and hard tissues to be characterised at a spectrum of scales from nano- to bulk tissue level. As a result, there now exists an extremely broad range of published values for human tibiofemoral joint tissues. However, our systematic review highlights gaps and ambiguities that mean quantitative understanding of how tissue material properties alter with age and OA is limited. It is therefore currently challenging to construct FE models of the knee that are truly representative of a specific age or disease-state. Consequently, recent tibiofemoral joint FE models have been highly generic in terms of material properties even relying on non-human data from multiple species. We highlight this by critically evaluating current ability to quantitatively compare and model (1) young and old and (2) healthy and OA human tibiofemoral joints. We suggest that future research into both healthy and diseased knee function will benefit greatly from a subject- or cohort-specific approach in which FE models are constructed using material properties, medical imagery and loading data from cohorts with consistent demographics and/or disease states.

Cell-Extracellular Matrix Mechanobiology: Forceful Tools and Emerging Needs for Basic and Translational Research

Extracellular biophysical cues have a profound influence on a wide range of cell behaviors, including growth, motility, differentiation, apoptosis, gene expression, adhesion, and signal transduction. Cells not only respond to definitively mechanical cues from the extracellular matrix (ECM) but can also sometimes alter the mechanical properties of the matrix and hence influence subsequent matrix-based cues in both physiological and pathological processes. Interactions between cells and materials in vitro can modify cell phenotype and ECM structure, whether intentionally or inadvertently. Interactions between cell and matrix mechanics in vivo are of particular importance in a wide variety of disorders, including cancer, central nervous system injury, fibrotic diseases, and myocardial infarction. Both the in vitro and in vivo effects of this coupling between mechanics and biology hold important implications for clinical applications.

Effect of dacarbazine on CD44 in live melanoma cells as measured by atomic force microscopy-based nanoscopy

CD44 ligand-receptor interactions are known to be involved in regulating cell migration and tumor cell metastasis. High expression levels of CD44 correlate with a poor prognosis of melanoma patients. In order to understand not only the mechanistic basis for dacarbazine (DTIC)-based melanoma treatment but also the reason for the poor prognosis of melanoma patients treated with DTIC, dynamic force spectroscopy was used to structurally map single native CD44-coupled receptors on the surface of melanoma cells. The effect of DTIC treatment was quantified by the dynamic binding strength and the ligand-binding free-energy landscape. The results demonstrated no obvious effect of DTIC on the unbinding force between CD44 ligand and its receptor, even when the CD44 nanodomains were reduced significantly. However, DTIC did perturb the kinetic and thermodynamic interactions of the CD44 ligand-receptor, with a resultant greater dissociation rate, lower affinity, lower binding free energy, and a narrower energy valley for the free-energy landscape. For cells treated with 25 and 75 μg/mL DTIC for 24 hours, the dissociation constant for CD44 increased 9- and 70-fold, respectively. The CD44 ligand binding free energy decreased from 9.94 for untreated cells to 8.65 and 7.39 kcal/mol for DTIC-treated cells, which indicated that the CD44 ligand-receptor complexes on DTIC-treated melanoma cells were less stable than on untreated cells. However, affinity remained in the micromolar range, rather than the millimolar range associated with nonaffinity ligands. Hence, the CD44 receptor could still be activated, resulting in intracellular signaling that could trigger a cellular response. These results demonstrate DTIC perturbs, but not completely inhibits, the binding of CD44 ligand to membrane receptors, suggesting a basis for the poor prognosis associated with DTIC treatment of melanoma. Overall, atomic force microscopy-based nanoscopic methods offer thermodynamic and kinetic insight into the effect of DTIC on the CD44 ligand-binding process.

Murine articular cartilage morphology and compositional quantification with high resolution cationic contrast-enhanced μCT

Articular cartilage lines the load-bearing surfaces of long bones and undergoes compositional and structural degeneration during osteoarthritis progression. Contrast enhanced microcomputed tomography (μCT) is being applied to a variety of preclinical models, including the mouse, to map structural and compositional properties in 3-D. The thinness (∼30-50 μm) and high cellularity of mouse articular cartilage presents a significant imaging challenge. Our group previously showed that mouse articular cartilage and proteoglycan (PG) content can be assessed by μCT with the ioxagalate-based contrast agent Hexabrix, but the voxel size used (6 μm) was deemed to be barely adequate. The objective of the present study is to assess the utility of a novel contrast agent, CA4+, to quantify mouse articular cartilage morphology and composition with high resolution μCT imaging (3 μm voxels) and to compare the sensitivity of CA4+ and Hexabrix to detect between-group differences. While both contrast agents are iodine-based, Hexabrix is anionic and CA4+ is cationic so they interact differently with negatively charged PGs. With CA4+, a strong correlation was found between non-calcified articular cartilage thickness measurements made with histology and μCT (R²  = 0.72, p < 0.001). Cartilage degeneration-as assessed by loss in volume, thickness, and PG content-was observed in 34-week-old mice when compared to both 7- and 12-week-old mice. High measurement precision was observed with CA4+, with the coefficient of variation after repositioning and re-imaging samples equaling 2.8%, 4.5%, 7.4% and 5.9% for attenuation, thickness, volume, and PG content, respectively. Use of CA4+ allowed increased sensitivity for assessing PG content compared to Hexabrix, but had no advantage for measurement of cartilage thickness or volume. This improvement in imaging should prove useful in preclinical studies of cartilage degeneration and regeneration. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2740-2748, 2017.

Techniques to assess bone ultrastructure organization: orientation and arrangement of mineralized collagen fibrils

Bone's remarkable mechanical properties are a result of its hierarchical structure. The mineralized collagen fibrils, made up of collagen fibrils and crystal platelets, are bone's building blocks at an ultrastructural level. The organization of bone's ultrastructure with respect to the orientation and arrangement of mineralized collagen fibrils has been the matter of numerous studies based on a variety of imaging techniques in the past decades. These techniques either exploit physical principles, such as polarization, diffraction or scattering to examine bone ultrastructure orientation and arrangement, or directly image the fibrils at the sub-micrometre scale. They make use of diverse probes such as visible light, X-rays and electrons at different scales, from centimetres down to nanometres. They allow imaging of bone sections or surfaces in two dimensions or investigating bone tissue truly in three dimensions, in vivo or ex vivo, and sometimes in combination with in situ mechanical experiments. The purpose of this review is to summarize and discuss this broad range of imaging techniques and the different modalities of their use, in order to discuss their advantages and limitations for the assessment of bone ultrastructure organization with respect to the orientation and arrangement of mineralized collagen fibrils.

Individual Collagen Fibril Thickening and Stiffening of Annulus Fibrosus in Degenerative Intervertebral Disc

STUDY DESIGN

In vitro study using rat intervertebral discs (IVDs).

OBJECTIVE

To explore the alteration of annulus fibrosus collagen fibrils after loading on IVD and to investigate the degeneration pathogenesis at the nanoscale.

SUMMARY OF BACKGROUND DATA

Abnormal loading can lead to IVD degeneration, but the precise mechanism has been hitherto elusive, especially at the nanoscale.

METHODS

A rat IVD loading model was used, which combined bending of the tail by 40° with compressive loading of 1.8, 4.5, and 7.2 N of the rat tail using an external fixation device. The structure and the elastic modulus of individual collagen fibrils within IVD Co8-Co9 was examined 2 weeks after loading at the nanoscale using atomic force microscopy.

RESULTS

Significant fibril disorder and a decrease in cell number within the annulus fibrosus after loading was observed at the microscale as judged by hematoxylin/eosin staining, suggesting initiation of rupture of the structure and degradation of the IVD. The annulus fibrosus collagen fibrils underwent a change in diameter and elastic modulus from 170 ± 18 to 310 ± 24 nm (P < 0.001) and 0.86 ± 0.12 to 1.27 ± 0.30 GPa (P = 0.003), respectively when measured on the concave side after a loading of 7.2 N. Thus the loading process resulted in a thickening and stiffening of collagen fibrils with a difference between the inner and outer layers.

CONCLUSION

The results of the present study indicated that abnormal loading was not only associated with disorder at the microscale, but also alteration of the collagen fibrils at the nanoscale, possibly leading to changes in the mechanical and physiological environment around the cells of the annulus fibrosus.

LEVEL OF EVIDENCE

N/A.

Minimally Invasive Micro-Indentation: mapping tissue mechanics at the tip of an 18G needle

Experiments regarding the mechanical properties of soft tissues mostly rely on data collected on specimens that are extracted from their native environment. During the extraction and in the time period between the extraction and the completion of the measurements, however, the specimen may undergo structural changes which could generate unwanted artifacts. To further investigate the role of mechanics in physiology and possibly use it in clinical practices, it is thus of paramount importance to develop instruments that could measure the viscoelastic response of a tissue without necessarily excising it. Tantalized by this opportunity, we have designed a minimally invasive micro-indenter that is able to probe the mechanical response of soft tissues, in situ, via an 18G needle. Here, we discuss its working principle and validate its usability by mapping the viscoelastic properties of a complex, confined sample, namely, the nucleus pulposus of the intervertebral disc. Our findings show that the mechanical properties of a biological tissue in its local environment may be indeed different than those that one would measure after excision, and thus confirm that, to better understand the role of mechanics in life sciences, one should always perform minimally invasive measurements like those that we have here introduced.

AFM-Nanomechanical Test: An Interdisciplinary Tool That Links the Understanding of Cartilage and Meniscus Biomechanics, Osteoarthritis Degeneration, and Tissue Engineering

Our objective is to provide an in-depth review of the recent technical advances of atomic force microscopy (AFM)-based nanomechanical tests and their contribution to a better understanding and diagnosis of osteoarthritis (OA), as well as the repair of tissues undergoing degeneration during OA progression. We first summarize a range of technical approaches for AFM-based nanoindentation, including considerations in both experimental design and data analysis. We then provide a more detailed description of two recently developed modes of AFM-nanoindentation, a high-bandwidth nanorheometer system for studying poroviscoelasticity and an immunofluorescence-guided nanomechanical mapping technique for delineating the pericellular matrix (PCM) and territorial/interterritorial matrix (T/IT-ECM) of surrounding cells in connective tissues. Next, we summarize recent applications of these approaches to three aspects of joint-related healthcare and disease: cartilage aging and OA, developmental biology and OA pathogenesis in murine models, and nanomechanics of the meniscus. These studies were performed over a hierarchy of length scales, from the molecular, cellular to the whole tissue level. The advances described here have contributed greatly to advancing the fundamental knowledge base for improved understanding, detection, and treatment of OA.

Laser-induced micropore formation and modification of cartilage structure in osteoarthritis healing

Pores are vital for functioning of avascular tissues. Laser-induced pores play an important role in the process of cartilage regeneration. The aim of any treatment for osteoarthritis is to repair hyaline-type cartilage. The aims of this study are to answer two questions: (1) How do laser-assisted pores affect the cartilaginous cells to synthesize hyaline cartilage (HC)? and (2) How can the size distribution of pores arising in the course of laser radiation be controlled? We have shown that in cartilage, the pores arise predominately near chondrocytes, which promote nutrition of cells and signal molecular transfer that activates regeneration of cartilage. In vivo laser treatment of damaged cartilage of miniature pig joints provides cellular transformation and formation of HC. We propose a simple model of pore formation in biopolymers that paves the way for going beyond the trial-and-error approach when choosing an optimal laser treatment regime. Our findings support the approach toward laser healing of osteoarthritis.

Effects of stimulated aggrecanolysis on nanoscale morphological and mechanical properties of wild-type and aggrecanase-resistant mutant mice cartilages

A key event in arthritis pathogenesis is the degradation of aggrecan, the major component in articular cartilage. In this work, we investigate the effects of stimulated aggrecanolysis on the morphological and nanomechanical properties of cartilage harvested from wild-type mice and aggrecanase-resistant mutant mice named "Jaffa". The cartilages were native or were subjected to stimulated aggrecanolysis by interleukin-1[Formula: see text] (IL-1[Formula: see text]) treatment. The nanoscale morphological and mechanical properties of the sectioned cartilages were measured by using a sharp probe by atomic force microscopy (AFM). The IL-1[Formula: see text] treatment resulted in a higher nanoroughess and stiffness of the cartilage from wild-type mice. However, the same treatment did not lead to any measurable change in the nanoroughness or stiffness of the cartilage from mutant mice Jaffa. This suggests that blocking aggrecanolysis by genetic modification has created the stability in the structures and mechanical properties of the cartilage at nanoscale. The present study provides insight into the mechanism of aggrecan degradation, which can complement the examination by biochemical and histological techniques.

The structure and function of cell membranes studied by atomic force microscopy

The cell membrane, involved in almost all communications of cells and surrounding matrix, is one of the most complicated components of cells. Lack of suitable methods for the detection of cell membranes in vivo has sparked debates on the biochemical composition and structure of cell membranes over half a century. The development of single molecule techniques, such as AFM, SMFS, and TREC, provides a versatile platform for imaging and manipulating cell membranes in biological relevant environments. Here, we discuss the latest developments in AFM and the progress made in cell membrane research. In particular, we highlight novel structure models and dynamic processes, including the mechanical properties of the cell membranes.

Cross-linked matrix rigidity and soluble retinoids synergize in nuclear lamina regulation of stem cell differentiation

A nanofilm of cross-linked collagen-I is equivalent to a relatively stiff matrix, which stiffens the nucleus, correlating broadly with lamin-A (including mutant progerin), retinoic acid transcription factor level and activity, and osteoinduction. In vitro results are supported by studies of ectopic bone formation in vivo.

Synergistic cues from extracellular matrix and soluble factors are often obscure in differentiation. Here the rigidity of cross-linked collagen synergizes with retinoids in the osteogenesis of human marrow mesenchymal stem cells (MSCs). Collagen nanofilms serve as a model matrix that MSCs can easily deform unless the film is enzymatically cross-linked, which promotes the spreading of cells and the stiffening of nuclei as both actomyosin assembly and nucleoskeletal lamin-A increase. Expression of lamin-A is known to be controlled by retinoic acid receptor (RAR) transcription factors, but soft matrix prevents any response to any retinoids. Rigid matrix is needed to induce rapid nuclear accumulation of the RARG isoform and for RARG-specific antagonist to increase or maintain expression of lamin-A as well as for RARG-agonist to repress expression. A progerin allele of lamin-A is regulated in the same manner in iPSC-derived MSCs. Rigid matrices are further required for eventual expression of osteogenic markers, and RARG-antagonist strongly drives lamin-A–dependent osteogenesis on rigid substrates, with pretreated xenografts calcifying in vivo to a similar extent as native bone. Proteomics-detected targets of mechanosensitive lamin-A and retinoids underscore the convergent synergy of insoluble and soluble cues in differentiation.

Biomaterials and Tissue Biomechanics: A Match Made in Heaven?

Biomaterials and tissue biomechanics have been traditionally separate areas of research with relatively little overlap in terms of methodological approaches. Recent advances in both fields on the one hand and developments in fabrication techniques and design approaches on the other have prepared the ground for joint research efforts by both communities. Additive manufacturing and rational design are examples of the revolutionary fabrication techniques and design methodologies that could facilitate more intimate collaboration between biomaterial scientists and biomechanists. This editorial article highlights the various ways in which the research on tissue biomechanics and biomaterials are related to each other and could benefit from each other’s results and methodologies.

Tenomodulin is Required for Tendon Endurance Running and Collagen I Fibril Adaptation to Mechanical Load

Tendons are dense connective tissues that attach muscles to bone with an indispensable role in locomotion because of their intrinsic properties of storing and releasing muscle- generated elastic energy. Tenomodulin (Tnmd) is a well-accepted gene marker for the mature tendon/ligament lineage and its loss-of -function in mice leads to a phenotype with distinct signs of premature aging on tissue and stem/progenitor cell levels. Based on these findings, we hypothesized that Tnmd might be an important factor in the functional performance of tendons. Firstly, we revealed that Tnmd is a mechanosensitive gene and that the C-terminus of the protein co-localize with collagen I-type fibers in the extracellular matrix. Secondly, using an endurance training protocol, we compared Tnmd knockout mice with wild types and showed that Tnmd deficiency leads to significantly inferior running performance that further worsens with training. In these mice, endurance running was hindered due to abnormal response of collagen I cross-linking and proteoglycan genes leading to an inadequate collagen I fiber thickness and elasticity. In sum, our study demonstrates that Tnmd is required for proper tendon tissue adaptation to endurance running and aids in better understanding of the structural-functional relationships of tendon tissues.

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Tnmd is a mechanosensitive gene and its protein is co-localized with collagen I fibers in the ECM of tendons.

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Tnmd knockout mice fail in endurance running tests, a phenotype that worsens with training.

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Tnmd knockout tendons had significantly thicker and stiffer collagen I fibers and altered crosslinking gene expression.

We performed a multidisciplinary approach to decipher the role of tenomodulin, a gene marker for the mature tendon lineage, in tendon functional performance. Loss-of-function in mice led to significantly inferior endurance running and detailed analyses revealed that tenomodulin is involved in the regulation of collagen I fiber structural and biomechanical properties in response to exercise. Our study expands the current view on the complex structural-functional relationships of tendon tissues, and tenomodulin expression levels may indicate whether an individual is suitable for a certain sport.

Vinculin association with actin cytoskeleton is necessary for stiffness-dependent regulation of vinculin behavior

The extracellular matrix (ECM) is a major regulator of cell behavior. Recent studies have indicated the importance of the physical properties of the ECM, including its stiffness, for cell migration and differentiation. Using actomyosin-generated forces, cells pull the ECM and sense stiffness via cell-ECM adhesion structures called focal adhesions (FAs). Vinculin, an actin-binding FA protein, has emerged as a major player in FA-mediated mechanotransduction. Although vinculin is important for sensing ECM stiffness, the role of vinculin binding to actin in the ECM stiffness-mediated regulation of vinculin behavior remains unknown. Here, we show that an actin binding-deficient mutation disrupts the ECM stiffness-dependent regulation of CSB (cytoskeleton stabilization buffer) resistance and the stable localization of vinculin. These results suggest that the vinculin-actin interaction participates in FA-mediated mechanotransduction.

Unraveling the role of Calcium ions in the mechanical properties of individual collagen fibrils

Collagen, the dominating material in the extracellular matrix, provides the strength, elasticity and mechanical stability to the organisms. The mechanical property of collagen is mainly dominated by its surrounding environments. However, the variation and origin of the mechanics of collagen fibril under different concentrations of calcium ions (χ_Ca) remains unknown. By using the atomic force microscopy based nanoindentation, the mechanics and structure of individual type II collagen fibril were first investigated under different χ_Ca in this study. The results demonstrate that both of the mechanical and structural properties of the collagen fibril show a prominent dependence on χ_Ca. The mechanism of χ_Ca-dependence of the collagen fibril was attributed to the chelation between collagen molecules and the calcium ions. Given the role of calcium in the pathology of osteoarthritis, the current study may cast new light on the understanding of osteoarthritis and other soft tissue hardening related diseases in the future.

Effects of Inflammation on Multiscale Biomechanical Properties of Cartilaginous Cells and Tissues

Cells within cartilaginous tissues are mechanosensitive and thus require mechanical loading for regulation of tissue homeostasis and metabolism. Mechanical loading plays critical roles in cell differentiation, proliferation, biosynthesis, and homeostasis. Inflammation is an important event occurring during multiple processes, such as aging, injury, and disease. Inflammation has significant effects on biological processes as well as mechanical function of cells and tissues. These effects are highly dependent on cell/tissue type, timing, and magnitude. In this review, we summarize key findings pertaining to effects of inflammation on multiscale mechanical properties at subcellular, cellular, and tissue level in cartilaginous tissues, including alterations in mechanotransduction and mechanosensitivity. The emphasis is on articular cartilage and the intervertebral disc, which are impacted by inflammatory insults during degenerative conditions such as osteoarthritis, joint pain, and back pain. To recapitulate the pro-inflammatory cascades that occur in vivo, different inflammatory stimuli have been used for in vitro and in situ studies, including tumor necrosis factor (TNF), various interleukins (IL), and lipopolysaccharide (LPS). Therefore, this review will focus on the effects of these stimuli because they are the best studied pro-inflammatory cytokines in cartilaginous tissues. Understanding the current state of the field of inflammation and cell/tissue biomechanics may potentially identify future directions for novel and translational therapeutics with multiscale biomechanical considerations.

Unraveling capillary interaction and viscoelastic response in atomic force microscopy of hydrated collagen fibrils

The mechanical properties of collagen fibrils depend on the amount and the distribution of water molecules within the fibrils. Here, we use atomic force microscopy (AFM) to study the effect of hydration on the viscoelastic properties of reconstituted type I collagen fibrils in air with controlled relative humidity. With the same AFM tip, we investigate the same area of a collagen fibril with two different force spectroscopy methods: force-distance (FD) and amplitude-phase-distance (APD) measurements. This allows us to separate the contributions of the fibril's viscoelastic response and the capillary force to the tip-sample interaction. A water bridge forms between the tip apex and the surface, causing an attractive capillary force, which is the main contribution to the energy dissipated from the tip to the specimen in dynamic AFM. The force hysteresis in the FD measurements and the tip indentation of only 2 nm in the APD measurements show that the hydrated collagen fibril is a viscoelastic solid. The mechanical properties of the gap regions and the overlap regions in the fibril's D-band pattern differ only in the top 2 nm but not in the fibril's bulk. We attribute this to the reduced number of intermolecular crosslinks in the reconstituted collagen fibril. The presented methodology allows the mechanical surface properties of hydrated collagenous tissues and biomaterials to be studied with unprecedented detail on the nanometer scale.

Osteoarthritis: toward a comprehensive understanding of pathological mechanism

Osteoarthritis (OA) is the most common degenerative joint disease and a major cause of pain and disability in adult individuals. The etiology of OA includes joint injury, obesity, aging, and heredity. However, the detailed molecular mechanisms of OA initiation and progression remain poorly understood and, currently, there are no interventions available to restore degraded cartilage or decelerate disease progression. The diathrodial joint is a complicated organ and its function is to bear weight, perform physical activity and exhibit a joint-specific range of motion during movement. During OA development, the entire joint organ is affected, including articular cartilage, subchondral bone, synovial tissue and meniscus. A full understanding of the pathological mechanism of OA development relies on the discovery of the interplaying mechanisms among different OA symptoms, including articular cartilage degradation, osteophyte formation, subchondral sclerosis and synovial hyperplasia, and the signaling pathway(s) controlling these pathological processes.

Nanoindentation modulus of murine cartilage: a sensitive indicator of the initiation and progression of post-traumatic osteoarthritis

OBJECTIVE

This study aims to demonstrate that cartilage nanoindentation modulus is a highly sensitive indicator of the onset and spatiotemporal progression of post-traumatic osteoarthritis (PTOA) in murine models.

DESIGN

Destabilization of the medial meniscus (DMM) surgery was performed on the right knees of 12-week old male, wild-type C57BL/6 mice, with Sham control on contralateral left knees. Atomic force microscopy (AFM)-based nanoindentation was applied to quantify the nanoindentation modulus, Eind, of femoral condyle cartilage at 3 days to 12 weeks after surgery. The modulus changes were compared against the timeline of histological OA signs. Meanwhile, at 8 weeks after surgery, changes in meniscus, synovium and subchondral bone were evaluated to reveal the spatial progression of PTOA.

RESULTS

The modulus of medial condyle cartilage was significantly reduced at 1 week after DMM, preceding the histological OA signs, which only became detectable at 4-8 weeks after. This reduction is likely due to concomitantly elevated proteolytic activities, as blocking enzymatic activities in mice can attenuate this modulus reduction. In later OA, lateral condyle cartilage and medial meniscus also started to be weakened, illustrating the whole-organ nature of PTOA.

CONCLUSIONS

This study underscores the high sensitivity of nanoindentation in examining the initiation, attenuation and progression of PTOA in murine models. Meanwhile, modulus changes highlight concomitant changes in lateral cartilage and meniscus during the advancement of OA.

AFM study shows prominent physical changes in elasticity and pericellular layer in human acute leukemic cells due to inadequate cell-cell communication

Biomechanical properties of single cells in vitro or ex vivo and their pericellular interfaces have recently attracted a lot of attention as a potential biophysical (and possibly prognostic) marker of various diseases and cell abnormalities. At the same time, the influence of the cell environment on the biomechanical properties of cells is not well studied. Here we use atomic force microscopy to demonstrate that cell-cell communication can have a profound effect on both cell elasticity and its pericellular coat. A human pre-B p190^BCR/ABL acute lymphoblastic leukemia cell line (ALL3) was used in this study. Assuming that cell-cell communication is inversely proportional to the distance between cells, we study ALL3 cells in vitro growing at different cell densities. ALL3 cells demonstrate a clear density dependent behavior. These cells grow very well if started at a relatively high cell density (HD, >2 × 10⁵ cells ml^-1) and are poised to grow at low cell density (LD, <1 × 10⁴ cells ml^-1). Here we observe ∼6× increase in the elastic (Young's) modulus of the cell body and ∼3.6× decrease in the pericellular brush length of LD cells compared to HD ALL3 cells. The difference observed in the elastic modulus is much larger than typically reported for pathologically transformed cells. Thus, cell-cell communication must be taken into account when studying biomechanics of cells, in particular, correlating cell phenotype and its biophysical properties.

Influence of Donor Age and Stimulation Intensity on Osteogenic Differentiation of Rat Mesenchymal Stromal Cells in Response to Focused Low-Intensity Pulsed Ultrasound

A focused low-intensity pulsed ultrasound (FLIPUS) was used to investigate the effects of stimulation period, acoustic intensity and donor age on the osteogenic differentiation potential of rat mesenchymal stromal cells (rMSCs). rMSCs from 3- and 12-mo-old female Sprague Drawly rats were isolated from bone marrow and stimulated 20 min/d with either 11.7 or 44.5 mW/cm² (spatial average temporal average intensity) for 7 or 14 d. Osteogenic differentiation markers, i.e., Runt-related transcription factor 2 (RUNX2), osteocalcin (OCN) and degree of matrix calcification were analyzed. On day 7 of stimulation, OCN gene expression was enhanced 1.9-fold in cells from young rats when stimulated with low intensity. The low intensity also led to a 40% decrease in RUNX2 expression on day 7 in aged cells, whereas high intensity enhanced expression of RUNX2 on day 14. FLIPUS treatment with low intensity resulted in a 15% increase in extracellular matrix mineralization in young but not old rMSCs. These differences suggest the necessity of a donor-age related optimization of stimulation parameters.

Introduction to Cartilage

Cartilage is a supporting connective tissue that, together with the bone, forms the framework supporting the body as a whole. There are many distinct types of cartilage, which exhibit numerous similarities as well as differences. Among them, articular cartilage is the best known and the most studied type. Articular cartilage is the thin layer of connective tissue that covers the articulating ends of bones in synovial (diarthrodial) joints. It provides a smooth surface for joint movement and acts as a load-bearing medium that protects the bone and distributes stress. The intense interest in articular cartilage is motivated by the critical role its degradation plays in arthritis and related joint diseases, which are the number one cause of disability in humans. This chapter discusses the physical, chemical and cellular properties of cartilage that give the tissue its extraordinary load-bearing characteristics.

Supramolecular Organization of Collagen Fibrils in Healthy and Osteoarthritic Human Knee and Hip Joint Cartilage

Cartilage matrix is a composite of discrete, but interacting suprastructures, i.e. cartilage fibers with microfibrillar or network-like aggregates and penetrating extrafibrillar proteoglycan matrix. The biomechanical function of the proteoglycan matrix and the collagen fibers are to absorb compressive and tensional loads, respectively. Here, we are focusing on the suprastructural organization of collagen fibrils and the degradation process of their hierarchical organized fiber architecture studied at high resolution at the authentic location within cartilage. We present electron micrographs of the collagenous cores of such fibers obtained by an improved protocol for scanning electron microscopy (SEM). Articular cartilages are permeated by small prototypic fibrils with a homogeneous diameter of 18 ± 5 nm that can align in their D-periodic pattern and merge into larger fibers by lateral association. Interestingly, these fibers have tissue-specific organizations in cartilage. They are twisted ropes in superficial regions of knee joints or assemble into parallel aligned cable-like structures in deeper regions of knee joint- or throughout hip joints articular cartilage. These novel observations contribute to an improved understanding of collagen fiber biogenesis, function, and homeostasis in hyaline cartilage.