Fatty acid transport in articular cartilage

Identification of biomarkers and immune infiltration characterization of lipid metabolism-associated genes in osteoarthritis based on machine learning algorithms

Osteoarthritis (OA) is a prevalent degenerative condition commonly observed in the elderly, leading to consequential disability. Despite notable advancements made in clinical strategies for OA, its pathogenesis remains uncertain. The intricate association between OA and metabolic processes has yet to receive comprehensive exploration. In our investigation, we leveraged public databases and applied machine learning algorithms, including WGCNA, LASSO, RF, immune infiltration analysis, and pathway enrichment analysis, to scrutinize the role of lipid metabolism-associated genes (LAGs) in the OA. Our findings identified three distinct biomarkers, and evaluated their expression to assess their diagnostic value in the OA patients. The exploration of immune infiltration in these patients revealed an intricate relationship between immune cells and the identified biomarkers. In addition, in vitro experiments, including qRT-PCR, Western blot, chondrocyte lipid droplets detection and mitochondrial fatty acid oxidation measurement, further verified abnormal expressions of selected LAGs in OA cartilage and confirmed the correlation between lipid metabolism and OA.

Anatomical Tissue Engineering of the Anterior Cruciate Ligament Entheses

The firm integration of anterior cruciate ligament (ACL) grafts into bones remains the most demanding challenge in ACL reconstruction, since graft loosening means graft failure. For a functional-tissue-engineered ACL substitute to be realized in future, robust bone attachment sites (entheses) have to be re-established. The latter comprise four tissue compartments (ligament, non-calcified and calcified fibrocartilage, separated by the tidemark, bone) forming a histological and biomechanical gradient at the attachment interface between the ACL and bone. The ACL enthesis is surrounded by the synovium and exposed to the intra-articular micromilieu. This review will picture and explain the peculiarities of these synovioentheseal complexes at the femoral and tibial attachment sites based on published data. Using this, emerging tissue engineering (TE) strategies addressing them will be discussed. Several material composites (e.g., polycaprolactone and silk fibroin) and manufacturing techniques (e.g., three-dimensional-/bio-printing, electrospinning, braiding and embroidering) have been applied to create zonal cell carriers (bi- or triphasic scaffolds) mimicking the ACL enthesis tissue gradients with appropriate topological parameters for zones. Functionalized or bioactive materials (e.g., collagen, tricalcium phosphate, hydroxyapatite and bioactive glass (BG)) or growth factors (e.g., bone morphogenetic proteins [BMP]-2) have been integrated to achieve the zone-dependent differentiation of precursor cells. However, the ACL entheses comprise individual (loading history) asymmetric and polar histoarchitectures. They result from the unique biomechanical microenvironment of overlapping tensile, compressive and shear forces involved in enthesis formation, maturation and maintenance. This review should provide a road map of key parameters to be considered in future in ACL interface TE approaches.

Cholesterol-induced LRP3 downregulation promotes cartilage degeneration in osteoarthritis by targeting Syndecan-4

Emerging evidence suggests that osteoarthritis is associated with high cholesterol levels in some osteoarthritis patients. However, the specific mechanism under this metabolic osteoarthritis phenotype remains unclear. We find that cholesterol metabolism-related gene, LRP3 (low-density lipoprotein receptor-related protein 3) is significantly reduced in high-cholesterol diet mouse's cartilage. By using Lrp3^-/- mice in vivo and LRP3 lentiviral-transduced chondrocytes in vitro, we identify that LRP3 positively regulate chondrocyte extracellular matrix metabolism, and its deficiency aggravate the degeneration of cartilage. Regardless of diet, LRP3 overexpression in cartilage attenuate anterior cruciate ligament transection induced osteoarthritis progression in rats and Lrp3 knockout-induced osteoarthritis progression in mice. LRP3 knockdown upregulate syndecan-4 by activating the Ras signaling pathway. We identify syndecan-4 as a downstream molecular target of LRP3 in osteoarthritis pathogenesis. These findings suggest that cholesterol-LRP3- syndecan-4 axis plays critical roles in osteoarthritis development, and LRP3 gene therapy may provide a therapeutic regimen for osteoarthritis treatment.

Lipid Metabolism in Cartilage Development, Degeneration, and Regeneration

Lipids affect cartilage growth, injury, and regeneration in diverse ways. Diet and metabolism have become increasingly important as the prevalence of obesity has risen. Proper lipid supplementation in the diet contributes to the preservation of cartilage function, whereas excessive lipid buildup is detrimental to cartilage. Lipid metabolic pathways can generate proinflammatory substances that are crucial to the development and management of osteoarthritis (OA). Lipid metabolism is a complicated metabolic process involving several regulatory systems, and lipid metabolites influence different features of cartilage. In this review, we examine the current knowledge about cartilage growth, degeneration, and regeneration processes, as well as the most recent research on the significance of lipids and their metabolism in cartilage, including the extracellular matrix and chondrocytes. An in-depth examination of the involvement of lipid metabolism in cartilage metabolism will provide insight into cartilage metabolism and lead to the development of new treatment techniques for metabolic cartilage damage.

Cholesterol metabolism related genes in osteoarthritis

Cholesterol homeostasis plays a significant role in skeletal development and the dysregulation of cholesterol-related mechanism has been shown to be involved in the development of cartilage diseases including osteoarthritis (OA). Epidemiological studies have shown an association between elevated serum cholesterol levels and OA. Furthermore, abnormal lipid accumulation in chondrocytes as a result of abnormal regulation of cholesterol homeostasis has been demonstrated to be involved in the development of OA. Although, many in vivo and in vitro studies support the connection between cholesterol and cartilage degradation, the mechanisms underlying the complex interactions between lipid metabolism, especially HDL cholesterol metabolism, and OA remain unclear. The current review aims to address this problem and focuses on key molecular players of the HDL metabolism pathway and their role in ΟΑ pathogenesis. Understanding the complexity of biological processes implicated in OA pathogenesis, such as cholesterol metabolism, may lead to new targets for drug therapy of OA patients.

Lipid metabolism in cartilage and its diseases: a concise review of the research progress

The homeostasis of the vertebrate body depends on anabolic and catabolic activities that are closely linked the inside and outside of the cell. Lipid metabolism plays an essential role in these metabolic activities. Although a large amount of evidence shows that normal lipid metabolism guarantees the conventional physiological activities of organs in the vertebrate body and that abnormal lipid metabolism plays an important role in the occurrence and deterioration of cardiovascular-related diseases, such as obesity, atherosclerosis, and type II diabetes, little is known about the role of lipid metabolism in cartilage and its diseases. This review aims to summarize the latest advances about the function of lipid metabolism in cartilage and its diseases including osteoarthritis, rheumatoid arthritis, and cartilage tumors. With the gradual in-depth understanding of lipid metabolism in cartilage, treatment methods could be explored to focus on this metabolic process in various cartilage diseases.

Mitochondria: Potential Targets for Osteoarthritis

Osteoarthritis (OA) is a common and disabling joint disorder that is mainly characterized by cartilage degeneration and narrow joint spaces. The role of mitochondrial dysfunction in promoting the development of OA has gained much attention. Targeting endogenous molecules to improve mitochondrial function is a potential treatment for OA. Moreover, research on exogenous drugs to improve mitochondrial function in OA based on endogenous molecular targets has been accomplished. In addition, stem cells and exosomes have been deeply researched in the context of cartilage regeneration, and these factors both reverse mitochondrial dysfunctions. Thus, we hypothesize that biomedical approaches will be applied to the treatment of OA. Furthermore, we have summarized the global status of mitochondria and osteoarthritis research in the past two decades, which will contribute to the research field and the development of novel treatment strategies for OA.

Osteoarthritis: New Strategies for Transport and Drug Delivery Across Length Scales

Osteoarthritis (OA) is the fourth leading cause of disability in adults. Yet, few viable pharmaceutical options exist for pain abatement and joint restoration, aside from joint replacement at late and irreversible stages of the disease. From the first onset of OA, as joint pain increases, individuals with arthritis increasingly reach for drug delivery solutions, from taking oral glycosaminoglycans (GAGs) bought over the counter from retail stores (e.g., Costco) to getting injections of viscous, GAG-containing synovial fluid supplement in the doctor's office. Little is known regarding the efficacy of delivery mode and/or treatment by such disease-modulating agents. This Review addresses the interplay of mechanics and biology on drug delivery to affected joints, which has profound implications for molecular transport in joint health and (patho)physiology. Multiscale systems biology approaches lend themselves to understand the relationship between the cell and joint health in OA and other joint (patho)physiologies. This Review first describes OA-related structural and functional changes in the context of the multilength scale anatomy of articular joints. It then summarizes and categorizes, by size and charge, published molecular transport studies, considering changes in permeability induced through inflammatory pathways. Finally, pharmacological interventions for OA are outlined in the context of molecular weights and modes of drug delivery. Taken together, the current state-of-the-art points to a need for new drug delivery strategies that harness systems-based interactions underpinning molecular transport and maintenance of joint structure and function at multiple length scales from molecular agents to cells, tissues, and tissue compartments which together make up articular joints. Cutting edge and cross-length and -time scale imaging represents a key discovery enabling technology in this process.

SOLUTE TRANSPORT IN ARTICULAR CARTILAGE UNDER ROLLING-COMPRESSION LOAD

Solute transport is one of the important aspects involved in maintaining the physiological activity of tissues. The mechanical environment drives nutrition in and waste out in articular cartilage due to its avascularity, which plays a key role in the biological activity of articular cartilage. The human knee joint motion is a complex interaction between different bones including relative rolling and/or sliding movements. Rolling-compression process is a typical physiological load in knee joint motion. To investigate solute transport behavior in articular cartilage under rolling-compression load, fluorescence tracers with molecular weights of 40kDa and 0.43kDa were used respectively to mark the transport in fresh articular cartilage of mature pigs. Solute fluorescence intensity changing with time and depth of cartilage layer was measured under rolling-compression load and static state, respectively, and the distribution of corresponding relative concentration was calculated by the fluorescence microscope imaging method. The experiment results show that the solute relative concentration in articular cartilage under rolling-compression load increases significantly, even up to 62.4%, comparing with that under static state, and the changes of concentration vary in different layers and that small molecular weight solute is easier to transport than relatively large molecular weight solute in articular cartilage. Therefore, rolling-compression load can promote the solute transport in cartilage, and the mechanical loading may have application in functional cartilage tissue engineering.

Lipid distribution, composition and uptake in bovine articular cartilage studied using Raman micro-spectrometry and confocal microscopy

The distribution and composition of endogenous lipids in articular cartilage and transport of exogenous fatty acids have been investigated on a microscopic scale in fresh bovine articular cartilage. To investigate the distribution and composition of the endogenous lipids, hyperspectral Raman maps were taken of chondrocytes and their surrounding matrix in both the deep and superficial zones. These revealed differences in both lipid distribution and composition between the two zones. Extracellular lipid was observed surrounding the cells in the superficial zone but not in the deep zone. Additionally, intracellular lipid droplets were observed that were larger and more numerous in the deep zone (P = 0.01). The extracellular lipid was primarily free saturated fatty acid, whereas the cellular lipid droplets contained triglycerides with unsaturated fatty acid chains. Fatty acid uptake and transport were investigated by incubating cartilage samples in Dulbecco's modified Eagle's medium containing fluorescently labelled palmitate for a range of times and temperatures. After incubation, the palmitate distribution was imaged using confocal microscopy. Palmitate accumulated preferentially in the territorial matrix only in the superficial zone where the concentration was up to 100‐fold greater than that in the bulk matrix (P = 0.001). Palmitate uptake by the chondrocytes in both zones showed differential temperature sensitivity (P = 0.05), which would support the idea that cells take up palmitate by both active and passive mechanisms. The study reveals large differences between chondrocytes in the superficial and deep zones in their lipid content, in their extracellular lipid environment and in their access to exogenous fatty acids.

Protective effects of mitochondria-targeted antioxidants and statins on cholesterol-induced osteoarthritis

The contribution of metabolic factors on the severity of osteoarthritis (OA) is not fully appreciated. This study aimed to define the effects of hypercholesterolemia on the progression of OA. Apolipoprotein E-deficient (ApoE^-/-) mice and rats with diet-induced hypercholesterolemia (DIHC) rats were used to explore the effects of hypercholesterolemia on the progression of OA. Both models exhibited OA-like changes, characterized primarily by a loss of proteoglycans, collagen and aggrecan degradation, osteophyte formation, changes to subchondral bone architecture, and cartilage degradation. Surgical destabilization of the knees resulted in a dramatic increase of degradative OA symptoms in animals fed a high-cholesterol diet compared with controls. Clinically relevant doses of free cholesterol resulted in mitochondrial dysfunction, overproduction of reactive oxygen species (ROS), and increased expression of degenerative and hypertrophic markers in chondrocytes and breakdown of the cartilage matrix. We showed that the severity of diet-induced OA changes could be attenuated by treatment with both atorvastatin and a mitochondrial targeting antioxidant. The protective effects of the mitochondrial targeting antioxidant were associated with suppression of oxidative damage to chondrocytes and restoration of extracellular matrix homeostasis of the articular chondrocytes. In summary, our data show that hypercholesterolemia precipitates OA progression by mitochondrial dysfunction in chondrocytes, in part by increasing ROS production and apoptosis. By addressing the mitochondrial dysfunction using antioxidants, we were able attenuate the OA progression in our animal models. This approach may form the basis for novel treatment options for this OA risk group in humans.-Farnaghi, S., Prasadam, I., Cai, G., Friis, T., Du, Z., Crawford, R., Mao, X., Xiao, Y. Protective effects of mitochondria-targeted antioxidants and statins on cholesterol-induced osteoarthritis.

Lipid transport and metabolism in healthy and osteoarthritic cartilage

Cartilage is an avascular tissue and cartilage metabolism depends on molecule diffusion from synovial fluid and subchondral bone. Thus, nutrient availability is limited by matrix permeability according to the size and charge of the molecules. Matrix composition limits the access of molecules to chondrocytes, determining cell metabolism and cartilage maintenance. Lipids are important nutrients in chondrocyte metabolism and are available for these cells through de novo synthesis but also through diffusion from surrounding tissues. Cartilage status and osteoarthritis development depend on lipid availability. This paper reviews lipid transport and metabolism in cartilage. We also analyze signalling pathways directly mediated by lipids and those that involve mTOR pathways, both in normal and osteoarthritic cartilage.

Measurement of diffusion in articular cartilage using fluorescence correlation spectroscopy

BACKGROUND

Fluorescence correlation spectroscopy (FCS) provides information about translational diffusion of fluorescent molecules in tiny detection volumes at the single-molecule level. In normal states, cartilage tissue lacks vascularity, so chondrocyte metabolism depends on diffusion for molecular exchanges. The abundant extracellular matrix (ECM) of cartilage is maintained by a limited number of chondrocytes. ECM plays an important role in the regulation of chondrocyte functions. In this study, FCS was used to measure diffusion behaviors of albumin, the major protein of the intra-articular space, using normal and degenerated cartilage. Preliminary investigation of fluorescence dyes including Alexa 488, Rhodamine 6G and Rhodamine 123 was conducted to evaluate their properties in cartilage.

RESULTS

The results indicate that the diffusion behaviors of fluorescently labeled albumin can be observed using FCS in both normal and chemically degenerated cartilage.

CONCLUSIONS

This work demonstrates the capability of FCS for direct measurement of diffusion in cartilaginous ECM. When the diffusion characteristics of fluorescent probes in ECM are clarified using FCS evaluation, FCS will be applicable as a method for early diagnosis of osteoarthritis, which is accompanied by increased abnormalities of ECM and also as tool for evaluating bio-engineered artificial cartilage for autologous chondrocyte implantation.

Relative efficacies of omega-3 polyunsaturated fatty acids in reducing expression of key proteins in a model system for studying osteoarthritis

OBJECTIVE

To assess the relative efficacy of three different omega-3 (n-3) polyunsaturated fatty acids (PUFAs) in suppressing the mRNA levels for important proteins involved in the etiology of osteoarthritis (OA).

METHODS

A model cell culture system (bovine chondrocytes) was used. Inflammatory factors and enzymes involved in OA were induced by exposure of the chondrocyte cultures to interleukin-1alpha (IL-1alpha). The effect of pre-incubating cultures with various amounts of exogenous fatty acids on subsequent levels of mRNAs was assessed by reverse transcription-polymerase chain reactions (RT-PCR).

RESULTS

Exposure of cultures to IL-1alpha induced expression of the cartilage proteinases A Disintegrin And Metalloproteinase with ThromboSpondin motifs (ADAMTS)-4 and ADAMTS-5, cyclooxygenase (COX)-2, the matrix metalloproteinase (MMP)-3 and the inflammatory cytokines IL-1alpha, interleukin-1beta (IL-1beta) and tumour necrosis factor-alpha (TNF-alpha). n-3 PUFAs were able to reduce the levels of mRNA for ADAMTS-4, ADAMTS-5, MMP-3, MMP-13, COX-2 (but not COX-1), IL-1alpha, IL-1beta and TNF-alpha. Eicosapentaenoic acid (EPA) was the most effective, followed by docosahexaenoic (DHA) and then alpha-linolenic (ALA) acid. The n-6 PUFA, arachidonic acid (AA) had no effect.

CONCLUSION

These results show that omega-3 (n-3) PUFAs cause a reduction in the mRNA levels for various proteins known to be important in the pathology of OA. They provide a molecular explanation, at least in part, for beneficial effects of dietary omega-3 PUFAs for the amelioration of symptoms of the disease. The relative efficacy of EPA suggests that this omega-3 PUFA may be especially useful for dietary supplementation in patients with OA.

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.