Transport of tissue inhibitor of metalloproteinases-1 through cartilage: contributions of fluid flow and electrical migration

Insulin-like growth factor-1 in articular cartilage repair for osteoarthritis treatment

Articular cartilage repair is a critical issue in osteoarthritis (OA) treatment. The insulin-like growth factor (IGF) signaling pathway has been implicated in articular cartilage repair. IGF-1 is a member of a family of growth factors that are structurally closely related to pro-insulin and can promote chondrocyte proliferation, enhance matrix production, and inhibit chondrocyte apoptosis. Here, we reviewed the role of IGF-1 in cartilage anabolism and catabolism. Moreover, we discussed the potential role of IGF-1 in OA treatment. Of note, we summarized the recent progress on IGF delivery systems. Optimization of IGF delivery systems will facilitate treatment application in cartilage repair and improve OA treatment efficacy.

Molecular transport in articular cartilage - what have we learned from the past 50 years?

Developing therapeutic molecules that target chondrocytes and locally produced inflammatory factors within arthritic cartilage is an active area of investigation. The extensive studies that have been conducted over the past 50 years have enabled the accurate prediction and reliable optimization of the transport of a wide variety of molecules into cartilage. In this Review, the factors that can be used to tune the transport kinetics of therapeutics are summarized. Overall, the most crucial factor when designing new therapeutic molecules is solute size. The diffusivity and partition coefficient of a solute both decrease with increasing solute size as indicated by molecular mass or by hydrodynamic radius. Surprisingly, despite having an effective pore size of ~6 nm, molecules of ~16 nm radius can diffuse through the cartilage matrix. Alteration of the shape or charge of a solute and the application of physiological loading to cartilage can be used to predictably improve solute transport kinetics, and this knowledge can be used to improve the development of therapeutic agents for osteoarthritis that target the cartilage.

The Effect of Charge and Mechanical Loading on Antibody Diffusion Through the Articular Surface of Cartilage

Molecular transport of osteoarthritis (OA) therapeutics within articular cartilage is influenced by many factors, such as solute charge, that have yet to be fully understood. This study characterizes how solute charge influences local diffusion and convective transport of antibodies within the heterogeneous cartilage matrix. Three fluorescently tagged solutes of varying isoelectric point (pI) (4.7–5.9) were tested in either cyclic or passive cartilage loading conditions. In each case, local diffusivities were calculated based on local fluorescence in the cartilage sample, as observed by confocal microscopy. In agreement with past research, local solute diffusivities within the heterogeneous cartilage matrix were highest around 200–275 μm from the articular surface, but 3–4 times lower at the articular surface and in the deeper zones of the tissue. Transport of all 150 kDa solutes was significantly increased by the application of mechanical loading at 1 Hz, but local transport enhancement was not significantly affected by changes in solute isoelectric point. More positively charged solutes (higher pI) had significantly higher local diffusivities 200–275 μm from the tissue surface, but no other differences were observed. This implies that there are certain regions of cartilage that are more sensitive to changes in solute charge than others, which could be useful for future development of OA therapeutics.

Cyclic Mechanical Loading Enhances Transport of Antibodies Into Articular Cartilage

The goal of this study was to characterize antibody penetration through cartilage tissue under mechanical loading. Mechanical stimulation aids in the penetration of some proteins, but this effect has not characterized molecules such as antibodies (>100 kDa), which may hold some clinical value for treating osteoarthritis (OA). For each experiment, fresh articular cartilage plugs were obtained and exposed to fluorescently labeled antibodies while under cyclic mechanical load in unconfined compression for several hours. Penetration of these antibodies was quantified using confocal microscopy, and finite element (FE) simulations were conducted to predict fluid flow patterns within loaded samples. Transport enhancement followed a linear trend with strain amplitude (0.25-5%) and a nonlinear trend with frequency (0.25-2.60 Hz), with maximum enhancement found to be at 5% cyclic strain and 1 Hz, respectively. Regions of highest enhancement of transport within the tissue were associated with the regions of highest interstitial fluid velocity, as predicted from finite-element simulations. Overall, cyclic compression-enhanced antibody transport by twofold to threefold. To our knowledge, this is the first study to test how mechanical stimulation affects the diffusion of antibodies in cartilage and suggest further study into other important factors regarding macromolecular transport.

The Effect of Antibody Size and Mechanical Loading on Solute Diffusion Through the Articular Surface of Cartilage

Because of the heterogeneous nature of articular cartilage tissue, penetration of potential therapeutic molecules for osteoarthritis (OA) through the articular surface (AS) is complex, with many factors that affect transport of these solutes within the tissue. Therefore, the goal of this study is to investigate how the size of antibody (Ab) variants, as well as application of cyclic mechanical loading, affects solute transport within healthy cartilage tissue. Penetration of fluorescently tagged solutes was quantified using confocal microscopy. For all the solutes tested, fluorescence curves were obtained through the articular surface. On average, diffusivities for the solutes of sizes 200 kDa, 150 kDa, 50 kDa, and 25 kDa were 3.3, 3.4, 5.1, and 6.0 μm2/s from 0 to 100 μm from the articular surface. Diffusivities went up to a maximum of 16.5, 18.5, 20.5, and 23.4 μm2/s for the 200 kDa, 150 kDa, 50 kDa, and 25 kDa molecules, respectively, from 225 to 325 μm from the surface. Overall, the effect of loading was very significant, with maximal transport enhancement for each solute ranging from 2.2 to 3.4-fold near 275 μm. Ultimately, solutes of this size do not diffuse uniformly nor are convected uniformly, through the depth of the cartilage tissue. This research potentially holds great clinical significance to discover ways of further optimizing transport into cartilage and leads to effective antibody-based treatments for OA.

Differential morphology and homogeneity of tissue-engineered cartilage in hydrodynamic cultivation with transient exposure to insulin-like growth factor-1 and transforming growth factor-β1

Successful tissue-engineering strategies for cartilage repair must maximize the efficacy of chondrocytes within their limited life span. To that end, the combination of exogenous growth factors with mechanical stimuli holds promise for development of clinically relevant cartilage tissue substitutes. The current study aimed to determine whether incorporation of transient exposure to growth factors into a hydrodynamic bioreactor system can improve the functional maturation of tissue-engineered cartilage. Chondrocyte-seeded polyglycolic acid scaffolds were cultivated within a wavy-walled bioreactor that imparts fluid flow-induced shear stress for 4 weeks. Constructs were nourished with 100 ng/mL insulin-like growth factor-1 (IGF-1) or 10 ng/mL transforming growth factor-β1 (TGF-β1) either for the first 15 days of the culture (transient) or throughout the entire cultivation (continuous). Transiently treated constructs were found to exhibit better functional properties than continuously nourished constructs. The limited development of engineered tissues continuously stimulated by IGF-1 or TGF-β1 was related to massive growth factor leftovers in the environments that downregulated the expression of the associated receptors. Treatment with TGF-β1 eliminated the formation of a fibrous capsule at the construct periphery possibly through suppression of Smad3 phosphorylation, yielding constructs with greater homogeneity. Furthermore, TGF-β1 reversely regulated Smad2 and Smad3 pathways in articular chondrocytes under hydrodynamic stimuli partially via Smad7. Collectively, transient exposure to growth factors is likely to maintain chondrocyte homeostasis, and thus promotes their anabolic activities under hydrodynamic stimuli. The present work suggests that robust hydrodynamically engineered neocartilage with a reduced fibrotic response and enhanced tissue homogeneity can be achieved through optimization of growth factor supplementation protocols and potentially through manipulation of intracellular signals such as Smad.

Transport and binding of tumor necrosis factor-α in articular cartilage depend on its quaternary structure

The effect of tumor necrosis factor-α (TNFα) on cartilage matrix degradation is mediated by its transport and binding within the extracellular matrix (ECM) of the tissue, which mediates availability to cell receptors. Since the bioactive form of TNFα is a homotrimer of monomeric subunits, conversion between trimeric and monomeric forms during intratissue transport may affect binding to ECM and, thereby, bioactivity within cartilage. We studied the transport and binding of TNFα in cartilage, considering the quaternary structure of this cytokine. Competitive binding assays showed significant binding of TNFα in cartilage tissue, leading to an enhanced uptake. However, studies in which TNFα was cross-linked to remain in the trimeric form revealed that the binding of trimeric TNFα was negligible. Thus, binding of TNFα to ECM was associated with the monomeric form. Binding of TNFα was not disrupted by pre-treating cartilage tissue with trypsin, which removes proteoglycans and glycoproteins but leaves the collagen network intact. Therefore, proteoglycan loss during osteoarthritis should only alter the passive diffusion of TNFα but not its binding interaction with the remaining matrix. Our results suggest that matrix binding and trimer-monomer conversion of TNFα both play crucial roles in regulating the accessibility of bioactive TNFα within cartilage.

What have we learned from finite element model studies of lumbar intervertebral discs in the past four decades?

Finite element analysis is a powerful tool routinely used to study complex biological systems. For the last four decades, the lumbar intervertebral disc has been the focus of many such investigations. To understand the disc functional biomechanics, a precise knowledge of the disc mechanical, structural and biochemical environments at the microscopic and macroscopic levels is essential. In response to this need, finite element model studies have proven themselves as reliable and robust tools when combined with in vitro and in vivo measurements. This paper aims to review and discuss some salient findings of reported finite element simulations of lumbar intervertebral discs with special focus on their relevance and implications in disc functional biomechanics. Towards this goal, the earlier investigations are presented, discussed and summarized separately in three distinct groups of elastic, multi-phasic transient and transport model studies. The disc overall response as well as the relative role of its constituents are markedly influenced by loading rate, magnitude, combinations/preloads and posture. The nucleus fluid content and pressurizing capacity affect the disc compliance, annulus strains and failure sites/modes. Biodynamics of the disc is affected by not only the excitation characteristics but also preloads, existing mass and nucleus condition. The role of fluid pressurization and collagen fiber stiffening diminish with time during diurnal loading. The endplates permeability influences the time-dependent response of the disc in both loaded and unloaded recovery phases. The transport of solutes is substantially influenced by the disc size, tissue diffusivity and endplates permeability.

Transport of anti-IL-6 antigen binding fragments into cartilage and the effects of injury

The efficacy of biological therapeutics against cartilage degradation in osteoarthritis is restricted by the limited transport of macromolecules through the dense, avascular extracellular matrix. The availability of biologics to cell surface and matrix targets is limited by steric hindrance of the matrix, and the microstructure of matrix itself can be dramatically altered by joint injury and the subsequent inflammatory response. We studied the transport into cartilage of a 48 kDa anti-IL-6 antigen binding fragment (Fab) using an in vitro model of joint injury to quantify the transport of Fab fragments into normal and mechanically injured cartilage. The anti-IL-6 Fab was able to diffuse throughout the depth of the tissue, suggesting that Fab fragments can have the desired property of achieving local delivery to targets within cartilage, unlike full-sized antibodies which are too large to penetrate beyond the cartilage surface. Uptake of the anti-IL-6 Fab was significantly increased following mechanical injury, and an additional increase in uptake was observed in response to combined treatment with TNFα and mechanical injury, a model used to mimic the inflammatory response following joint injury. These results suggest that joint trauma leading to cartilage degradation can further alter the transport of such therapeutics and similar-sized macromolecules.

Transport and equilibrium uptake of a peptide inhibitor of PACE4 into articular cartilage is dominated by electrostatic interactions

The availability of therapeutic molecules to targets within cartilage depends on transport through the avascular matrix. We studied equilibrium partitioning and non-equilibrium transport into cartilage of Pf-pep, a 760 Da positively charged peptide inhibitor of the proprotein convertase PACE4. Competitive binding measurements revealed negligible binding of Pf-pep to sites within cartilage. Uptake of Pf-pep depended on glycosaminoglycan charge density, and was consistent with predictions of Donnan equilibrium given the known charge of Pf-pep. In separate transport experiments, the diffusivity of Pf-pep in cartilage was measured to be approximately 1 x 10(-6) cm(2)/s, close to other similarly-sized non-binding solutes. These results suggest that small positively charged therapeutics will have a higher concentration within cartilage than in the surrounding synovial fluid, a desired property for local delivery; however, such therapeutics may rapidly diffuse out of cartilage unless there is additional specific binding to intra-tissue substrates that can maintain enhanced intra-tissue concentration for local delivery.

TRANSPORT PROPERTIES OF CARTILAGINOUS TISSUES

Cartilaginous tissues, such as articular cartilage and intervertebral disc, are avascular tissues which rely on transport for cellular nutrition. Comprehensive knowledge of transport properties in such tissues is therefore necessary in the understanding of nutritional supply to cells. Furthermore, poor cellular nutrition in cartilaginous tissues is believed to be a primary source of tissue degeneration, which may result in osteoarthritis (OA) or disc degeneration. In this mini-review, we present an overview of the current status of the study of transport properties and behavior in cartilaginous tissues. The mechanisms of transport in these tissues, as well as experimental approaches to measuring transport properties and results obtained are discussed. The current status of bioreactors used in cartilage tissue engineering is also presented.

Solute transport in the deep and calcified zones of articular cartilage

OBJECTIVES

(1) To establish whether the tidemark and calcified cartilage are permeable to low molecular weight solutes, thereby providing a potential pathway for nutrition of cells in the deep cartilage. (2) To investigate transport from the subchondral microcirculation into calcified cartilage in an intact perfused joint and the effects on transport of static loading.

METHODS

The permeability of the tidemark and calcified cartilage was investigated in plugs of cartilage and subchondral bone which formed the membrane of a diffusion cell. Transport from the subchondral microcirculation and the effects of load were studied in an intact perfused joint. Both preparations used the metacarpophalangeal joints of mature horses and fluorescein and rhodamine (m.w. approximately 400 Da) were employed as tracers, assayed by quantitative fluorescence microscopy on histological sections.

RESULTS

Calcified cartilage was permeable to both solutes, both from the superficial and the subchondral sides. The effective diffusivity of both solutes was of the order of 9 x 10(-9) cm(2) s(-1), fivefold less than in the uncalcified cartilage. The calcified zone was heterogeneous, with high uptake of both tracers in the vicinity of the tidemark. The distribution volume of rhodamine B was higher than for fluorescein, consistent with a significant anionic charge in the calcified matrix. Static loading of the intact joint did not affect the transport of rhodamine B but caused a significant decrease in concentration of fluorescein both in the surface and deep zones of the tissue.

CONCLUSIONS

Calcified cartilage is permeable to small solutes and the subchondral circulation may make a significant contribution to the nutrition of deep cartilage in the mature horse. Static loading reduces the uptake of small anionic solutes in the intact joint.

Gene therapy for articular cartilage repair

Articular cartilage serves as the gliding surface of joints. It is susceptible to damage from trauma and from degenerative diseases. Restoration of damaged articular cartilage may be achievable through the use of cell-regulatory molecules that augment the reparative activities of the cells, inhibit the cells' degradative activities, or both. A variety of such molecules have been identified. These include insulin-like growth factor I, fibroblast growth factor 2, bone morphogenetic proteins 2, 4, and 7, and interleukin-1 receptor antagonist. It is now possible to transfer the genes encoding such molecules into articular cartilage and synovial lining cells. Although preliminary, data from in-vitro and in-vivo studies suggest that gene therapy can deliver such potentially therapeutic agents to protect existing cartilage and to build new cartilage.

A driving force for change: interstitial flow as a morphoregulator

Dynamic stresses that are present in all living tissues drive small fluid flows, called interstitial flows, through the extracellular matrix. Interstitial flow not only helps to transport nutrients throughout the tissue, but also has important roles in tissue maintenance and pathobiology that have been, until recently, largely overlooked. Here, we present evidence for the various effects of interstitial flow on cell biology, including its roles in embryonic development, tissue morphogenesis and remodeling, inflammation and lymphedema, tumor biology and immune cell trafficking. We also discuss possible mechanisms by which interstitial flow can induce morphoregulation, including direct shear stress, matrix-cell transduction (as has been proposed in the endothelial glycocalyx) and the newly emerging concept of autologous gradient formation.

Capturing complex 3D tissue physiology in vitro

The emergence of tissue engineering raises new possibilities for the study of complex physiological and pathophysiological processes in vitro. Many tools are now available to create 3D tissue models in vitro, but the blueprints for what to make have been slower to arrive. We discuss here some of the 'design principles' for recreating the interwoven set of biochemical and mechanical cues in the cellular microenvironment, and the methods for implementing them. We emphasize applications that involve epithelial tissues for which 3D models could explain mechanisms of disease or aid in drug development.

Mechanobiology in the third dimension

Cells are mechanically coupled to their extracellular environments, which play critical roles in both communicating the state of the mechanical environment to the cell as well as in mediating cellular response to a variety of stimuli. Along with the molecular composition and mechanical properties of the extracellular matrix (ECM), recent work has demonstrated the importance of dimensionality in cell-ECM associations for controlling the sensitive communication between cells and the ECM. Matrix forces are generally transmitted to cells differently when the cells are on two-dimensional (2D) vs. within three-dimensional (3D) matrices, and cells in 3D environments may experience mechanical signaling that is unique vis-à-vis cells in 2D environments, such as the recently described 3D-matrix adhesion assemblies. This review examines how the dimensionality of the extracellular environment can affect in vitro cell mechanobiology, focusing on collagen and fibrin systems.

Transport of neutral solute in articular cartilage: effects of loading and particle size

We investigate the influence that matrix structure, size of diffusing molecules and type and intensity of mechanical loading have on the transport of neutral solutes in articular cartilage. Although this type of investigation has been performed in the past, earlier researchers assumed a constant diffusion coefficient. By contrast, our diffusion coefficient depends on the local deformation of the matrix, and thus varies both in space and in time during an experiment.

We derive a three-dimensional formulation of the problem based on mixture theory and utilize the commercial finite-element code ABAQUS to study it numerically. We also make use of the Cohen–Turnbull–Yasuda model to correlate the decrease of the diffusion coefficient with the increase in tortuosity, owing to the presence of the matrix. Under appropriate circumstances, the equations derived here reduce to the classical convection/diffusion equation and the equations of the biphasic cartilage model. Even though we chose axisymmetric sample geometry for the present calculations, the model can easily be applied to irregular three-dimensional samples.

Our results reinforce and refine previously published studies. The neutral solute's rate of diffusion is reduced under static compression, due to the strain dependence of the diffusion coefficient; an increase in static compression leads to a decrease in the rate of transport of solutes of all sizes. Dynamic loading, on the other hand, augments solute transport due to convection, depending on particle size. The transport of small molecular size solute is moderately enhanced, but only within the surface layer; however, the rate of transport of large molecule solute is greatly increased, even in the deep layer of the cartilage.

Engineering principles of clinical cell-based tissue engineering

Tissue engineering is a rapidly evolving discipline that seeks to repair, replace, or regenerate specific tissues or organs by translating fundamental knowledge in physics, chemistry, and biology into practical and effective materials, devices, systems, and clinical strategies. Stem cells and progenitors that are capable of forming new tissue with one or more connective tissue phenotypes are available from many adult tissues and are defined as connective tissue progenitors. There are four major cell-based tissue-engineering strategies: (1) targeting local connective tissue progenitors where new tissue is desired, (2) transplanting autogenous connective tissue progenitors, (3) transplanting culture-expanded or modified connective tissue progenitors, and (4) transplanting fully formed tissue generated in vitro or in vivo. Stem cell function is controlled by changes in stem cell activation and self-renewal or by changes in the proliferation, migration, differentiation, or survival of the progeny of stem cell activation, the downstream progenitor cells. Three-dimensional porous scaffolds promote new tissue formation by providing a surface and void volume that promotes the attachment, migration, proliferation, and desired differentiation of connective tissue progenitors throughout the region where new tissue is needed. Critical variables in scaffold design and function include the bulk material or materials from which it is made, the three-dimensional architecture, the surface chemistry, the mechanical properties, the initial environment in the area of the scaffold, and the late scaffold environment, which is often determined by degradation characteristics. Local presentation or delivery of bioactive molecules can change the function of connective tissue progenitors (activation, proliferation, migration, differentiation, or survival) in a manner that results in new or enhanced local tissue formation. All cells require access to substrate molecules (oxygen, glucose, and amino acids). A balance between consumption and local delivery of these substrates is needed if cells are to survive. Transplanted cells are particularly vulnerable. Theoretical calculations can be used to explore the relationships among cell density, diffusion distance, and cell viability within a graft and to design improved strategies for transplantation of connective tissue progenitors. Rational strategies for tissue engineering seek to optimize new tissue formation through the logical selection of conditions that modulate the performance of connective tissue progenitors in a graft site to produce a desired tissue. This increasingly involves strategies that combine cells, matrices, inductive stimuli, and techniques that enhance the survival and performance of local or transplanted connective tissue progenitors.

Fluid flow and convective transport of solutes within the intervertebral disc

Previous experimental and analytical studies of solute transport in the intervertebral disc have demonstrated that for small molecules diffusive transport alone fulfils the nutritional needs of disc cells. It has been often suggested that fluid flow into and within the disc may enhance the transport of larger molecules. The goal of the study was to predict the influence of load-induced interstitial fluid flow on mass transport in the intervertebral disc. An iterative procedure was used to predict the convective transport of physiologically relevant molecules within the disc. An axisymmetric, poroelastic finite-element structural model of the disc was developed. The diurnal loading was divided into discrete time steps. At each time step, the fluid flow within the disc due to compression or swelling was calculated. A sequentially coupled diffusion/convection model was then employed to calculate solute transport, with a constant concentration of solute being provided at the vascularised endplates and outer annulus. Loading was simulated for a complete diurnal cycle, and the relative convective and diffusive transport was compared for solutes with molecular weights ranging from 400 Da to 40 kDa. Consistent with previous studies, fluid flow did not enhance the transport of low-weight solutes. During swelling, interstitial fluid flow increased the unidirectional penetration of large solutes by approximately 100%. Due to the bi-directional temporal nature of disc loading, however, the net effect of convective transport over a full diurnal cycle was more limited (30% increase). Further study is required to determine the significance of large solutes and the timing of their delivery for disc physiology.

Modeling of neutral solute transport in a dynamically loaded porous permeable gel: implications for articular cartilage biosynthesis and tissue engineering

A primary mechanism of solute transport in articular cartilage is believed to occur through passive diffusion across the articular surface, but cyclical loading has been shown experimentally to enhance the transport of large solutes. The objective of this study is to examine the effect of dynamic loading within a theoretical context, and to investigate the circumstances under which convective transport induced by dynamic loading might supplement diffusive transport. The theory of incompressible mixtures was used to model the tissue (gel) as a mixture of a gel solid matrix (extracellular matrix/scaffold), and two fluid phases (interstitial fluid solvent and neutral solute), to solve the problem of solute transport through the lateral surface of a cylindrical sample loaded dynamically in unconfined compression with frictionless impermeable platens in a bathing solution containing an excess of solute. The resulting equations are governed by nondimensional parameters, the most significant of which are the ratio of the diffusive velocity of the interstitial fluid in the gel to the solute diffusivity in the gel (Rg), the ratio of actual to ideal solute diffusive velocities inside the gel (Rd), the ratio of loading frequency to the characteristic frequency of the gel (f), and the compressive strain amplitude (epsilon 0). Results show that when Rg > 1, Rd < 1, and f > 1, dynamic loading can significantly enhance solute transport into the gel, and that this effect is enhanced as epsilon 0 increases. Based on representative material properties of cartilage and agarose gels, and diffusivities of various solutes in these gels, it is found that the ranges Rg > 1, Rd < 1, correspond to large solutes, whereas f > 1 is in the range of physiological loading frequencies. These theoretical predictions are thus in agreement with the limited experimental data available in the literature. The results of this study apply to any porous hydrated tissue or material, and it is therefore plausible to hypothesize that dynamic loading may serve to enhance solute transport in a variety of physiological processes.

Transport and binding of insulin-like growth factor I through articular cartilage

This study focused on the role of insulin-like growth factor (IGF) binding proteins (IGFBPs) in cartilage on the transport and binding of IGF-I within the tissue. We have developed experimental and theoretical modeling techniques to quantify and contrast the roles of diffusion, binding, fluid convection, and electrical migration on the transport of IGF-I within cartilage tissue. Bovine articular cartilage disks were equilibrated in buffer containing 125I-IGF-I and graded levels of unlabeled IGF-I. Equilibrium binding, as measured by the uptake ratio of 125I-IGF-I in the tissue (free plus bound) to the concentration of labeled species in the buffer, was found to be consistent with a first-order reversible binding model involving one dominant family of binding sites within the matrix. Western ligand blots revealed a major IGF binding doublet around 23 kDa, which has been previously shown to coincide with IGFBP-6. Diffusive transport of 125I-IGF-I through cartilage was measured and found to be consistent with a diffusion-limited reaction theoretical model incorporating first-order reversible binding. Addition of excess amounts of unlabeled IGF-I during steady state transport of 125I-IGF-I resulted in release of bound 125I-IGF-I from the tissue, as predicted by the diffusion-reaction model. In contrast, addition of the low-affinity Des(1-3)IGF-I analog did not result in release of bound 125I-IGF-I. Application of electric current was used to augment transport of IGF-I through cartilage via electroosmosis and electrophoresis. Taken together, our results suggest that a single dominant substrate family, the high-affinity IGFBPs, is responsible for much of the observed binding of IGF-I within cartilage. The data suggest that intratissue fluid flow, such as that induced by mechanical loading of cartilage in vivo may be expected to enhance IGF transport by an order of magnitude and that this increment may help to counterbalance the restrictions encountered by the immobilization of IGFs by the binding proteins.

Effects of shear stress on nitric oxide and matrix protein gene expression in human osteoarthritic chondrocytes in vitro

Mechanical loading alters articular cartilage metabolism. However, mechanisms underlying intracellular signaling and communication between cells in response to mechanical stresses remain enigmatic. This study tested the hypothesis that shear stress-induced nitric oxide (NO) production participates in the regulation of matrix protein gene expression. The data presented here demonstrate that exposure of human osteoarthritic chondrocytes to a continuously applied shear stress (1.64 Pa) upregulated NO synthase gene expression and increased NO release by 1.8-, 2.4-, and 3.5-fold at 2, 6, and 24 h, respectively. Exposure of chondrocytes to a short duration of shear stress for 2 h resulted in the release of accumulation of NO in the culture medium. Exposure of chondrocytes to shear stress for 2, 6, and 24 h inhibited type II collagen mRNA signal levels by 27%, 18% and 20% after a constant post-shear incubation period of 24 h. Aggrecan mRNA signal levels were inhibited by 30%, 32% and 41% under identical conditions. Addition of an NO antagonist increased type II collagen mRNA signal levels by an average of 1.8-fold (137% of the un-sheared control) and reestablished the aggrecan mRNA signal levels by an average of 1.4-fold after shear stress (92% of the un-sheared control) (ANOVA p < 0.05). These data support the hypothesis that shear stress-induced NO release may influence the development of degenerative joint diseases by inhibiting matrix macromolecule synthesis.

Cartilage tissue remodeling in response to mechanical forces

Recent studies suggest that there are multiple regulatory pathways by which chondrocytes in articular cartilage sense and respond to mechanical stimuli, including upstream signaling pathways and mechanisms that may lead to direct changes at the level of transcription, translation, post-translational modifications, and cell-mediated extracellular assembly and degradation of the tissue matrix. This review focuses on the effects of mechanical loading on cartilage and the resulting chondrocyte-mediated biosynthesis, remodeling, degradation, and repair of this tissue. The effects of compression and tissue shear deformation are compared, and approaches to the study of mechanical regulation of gene expression are described. Of particular interest regarding dense connective tissues, recent experiments have shown that mechanotransduction is critically important in vivo in the cell-mediated feedback between physical stimuli, the molecular structure of newly synthesized matrix molecules, and the resulting macroscopic biomechanical properties of the tissue.

Static compression is associated with decreased diffusivity of dextrans in cartilage explants

The chondrocytes of adult articular cartilage rely upon transport phenomena within their avascular extracellular matrix for many biological activities. Therefore, changes in matrix structure which influence cytokine transport parameters may be an important mechanism involved in the chondrocyte response to tissue compression. With this hypothesis in mind, partitioning and diffusion of 3-, 10-, and 40-kDa dextrans conjugated to tetramethylrhodamine, and 430-Da tetramethylrhodamine itself, were measured within statically compressed bovine articular cartilage explants using a novel experimental apparatus and desorption fluorescence method. Partitioning and diffusion were examined as functions of solute molecular weight and matrix proteoglycan density, and diffusion was measured versus static compression up to 35% volumetric strain. In general, partition coefficients and diffusivities were found to decrease with increasing solute molecular weight. In addition, for a given solute, diffusivities decreased significantly with increasing static compression. Results therefore suggest a possible role for transport limitations of relatively large molecular weight solutes within the extracellular matrix in mediating the biological response of chondrocytes to cartilage compression.

Effects of medium perfusion on matrix production by bovine chondrocytes in three-dimensional collagen sponges

Various culture systems have been used for examining the anabolic and catabolic functions of isolated chondrocytes as well as for tissue engineering purposes. Perfusion or frequent medium change is beneficial for three-dimensional (3D) cultures of many cell types. In this study, bovine articular chondrocytes (bACs) were grown in 3D collagen sponges with or without medium perfusion (0.33 mL/min) for up to 15 days. The influence of medium perfusion was evaluated using markers of cartilage matrix accumulation, synthesis, and gene expression. Metachromatic matrix, collagen type II, and hyaluronan accumulated around the cells within the collagen sponges. Sulfated glycosaminoglycans (S-GAGs) that accumulated in the sponge exposed to nonperfused control were 130% of that in the perfused sponge at day 7. S-GAG accumulation after 15 days in the nonperfused control was 230% more than at day 7 (p < 0.01). (35)S-sulfate incorporation during the final 18 h of culture in the sponge exposed to nonperfusion was 180% greater than that in the perfused sponge (p < 0.01). Quantitative analyses show that at day 7, aggrecan and collagen type II gene expression were 350% and 240% greater, respectively, in the nonperfused culture than in the perfused one. These results indicate that perfused conditions that are beneficial for other cell types inhibit chondrogenesis by articular chondrocytes in 3D culture.

The insulin-like growth factors (IGFs) I and II bind to articular cartilage via the IGF-binding proteins

Bovine articular cartilage discs (3 mm diameter x 400 micrometer thick) were equilibrated in buffer containing (125)I-insulin-like growth factor (IGF)-I (4 degrees C) +/- unlabeled IGF-I or IGF-II. Competition for binding to cartilage discs by each unlabeled IGF was concentration-dependent, with ED(50) values for inhibition of (125)I-IGF-I binding of 11 and 10 nM for IGF-I and -II, respectively, and saturation by 50 nM. By contrast, an analog of IGF-I with very low affinity for the insulin-like growth factor-binding proteins (IGF-BPs), des-(1-3)-IGF-I, was not competitive with (125)I-IGF-I for cartilage binding even at 100-400 nM. Binding of the (125)I-labeled IGF-II isoform to cartilage was competed for by unlabeled IGF-I or -II, with ED(50)s of 160 and 8 nM, respectively. This probably reflected the differential affinities of the endogenous IGF-BPs (IGF-BP-6 and -2) for IGF-II/IGF-I. Transport of (125)I-IGF-I was also measured in an apparatus that allows diffusion only across the discs (400 micrometer), by addition to one side and continuous monitoring of efflux on the other side. The time lag for transport of (125)I-IGF was 266 min, an order of magnitude longer than the theoretical prediction for free diffusion in the matrix. (125)I-IGF-I transport then reached a steady state rate (% efflux of total added (125)I-IGF/unit time), which was subsequently accelerated approximately 2-fold by addition of an excess of unlabeled IGF-I. Taken together, these results indicate that IGF binding to cartilage, mostly through the IGF-BPs, regulates the transport of IGFs in articular cartilage, probably contributing to the control of their paracrine activities.