Lubricin expression in human osteoarthritic knee meniscus and synovial fluid: A morphological, immunohistochemical and biochemical study
Introduction
Knee osteoarthritis (OA) is characterized by progressive alterations in joint structures and low-grade inflammation (Berenbaum, 2013). OA is characterized as joint failure due to progressive changes in several musculoskeletal components that include, but are not limited to, articular cartilage (Berenbaum, 2013). Other joint structures, such as the menisci are altered in OA, but have not been as extensively studied as the articular cartilage (Loeser, 2011). In this study special attention was focused on the expression of lubricin in the meniscus, after partial meniscectomy, and in synovial fluid (SF) from OA patients in comparison to normal donors. Partial meniscectomy is a routine surgical procedure that is frequently carried out following damage to the meniscus. Before the introduction of arthroscopy, broken menisci were removed completely with grave consequences for joint articulation and biomechanics, facilitating subsequent progression of the arthritic knee (Musumeci et al., 2012). Today surgical procedures attempt to save as much meniscal tissue as possible, in order to preserve at least a component of its biomechanical function in the knee. Increasing evidence suggests that the meniscus may not represent a passive by-stander in the OA processes. Knee menisci are two semilunar biconcave disks (lateral and medial) that reside within the medial and lateral tibio-femoral articulations. They are divided into inner, middle and outer rim (Musumeci et al., 2013a). The inner rim is the most delicate part, because it is not vascularized. The lateral meniscus has the form of an almost complete circle and adheres to the two cruciate ligaments (Musumeci et al., 2013a). The medial meniscus has the form of a half moon and is more extensive than lateral, with its extremities adhering to anterior and posterior intercondylar areas. Between the two menisci, the medial meniscus is more subject to trauma, because it is less mobile than the lateral for the presence of the semimembranosus tendon, but also because usually we tend to have a slight valgus during gait (Musumeci et al., 2013a). Microscopically, the meniscus is composed of dense fibrocartilage, sparsely populated with cells called fibrochondrocytes because they exhibit characteristics of both fibroblasts and chondrocytes (Galanti et al., 2013). The meniscus plays an important role in the complex biomechanics of the knee joint. It is involved in load bearing, load transmission, shock absorption, joint stability, joint lubrication and joint congruity. Its loss results in higher peak stresses exerted on cartilage, and eventually in cartilage degeneration and OA (Musumeci et al., 2012). Lubricin, also known as superficial zone protein (SZP), is a chondroprotective, mucinous glycoprotein, the product of the proteoglycan 4 (PRG4) gene (Musumeci et al., 2013b). It has been found in several tissues including the synovial membranes and synovial fluid (SF) (Elsaid et al., 2005), the superficial zone of articular cartilage (Schumacher et al., 1994), tendon (Rees et al., 2002), ligament (Leonardi et al., 2012a), disk (Leonardi et al., 2012b, Leonardi et al., 2012c) and meniscus (Schumacher et al., 2005, Young et al., 2006, Zhang et al., 2011, Musumeci et al., 2013b). The essential role of lubricin is maintaining joint integrity. Lubricin was originally identified as a lubricating glycoprotein present in SF, specifically synthesized and expressed by articular chondrocytes of the superficial zone and it is recognized to play a major protective role in preventing cartilage wear and synovial cell adhesion and proliferation and reducing the coefficient of friction of the articular cartilage surface (Musumeci et al., 2011a, Musumeci et al., 2011b, Loeser, 2013). Lubricin is important for articular joint physiology, and the loss of lubricin accumulation may be involved in the pathology of OA. Previous studies demonstrated that important down-regulation of lubricin, as well as of other proteoglycans and synovial fluid biomarkers, develops in the human knee meniscus and anterior cruciate ligament (ACL) immediately after acute joint injury, underlining its involvement in the articular damage (Catterall et al., 2010, Musumeci et al., 2013b). A study in an animal model of OA demonstrated that recombinant lubricin improves the chondroprotection, which suggests that recombinant lubricin molecules may potentially be used in new approaches for the treatment of OA and associated cartilage disorders (Flannery et al., 2009). The purpose of our study was to investigate, in human patients, the expression of lubricin in menisci and synovial fluid from normal donors and patients with OA, using histology, immunohistochemistry, ELISA and Western blot analysis, to provide insights into the role of this protein in the progression of OA-related pathological processes in the meniscus.
Section snippets
Patients
Knee menisci were obtained from 40 patients, with OA, with similar height 163.5 ± 7.9 cm (range 150–175) and weight 62.2 ± 9.8 kg (range 50–96), 25 males and 15 females, who underwent isolated arthroscopic partial, medial or lateral, meniscectomy (28 and 12, respectively) performed at the Orthopaedic Surgery Unit, Fondazione GB Morgagni, Catania, Italy. Informed consent was obtained from each patient; the research was approved by the Local Medical Ethical Committee and conformed to the ethical
Histology and histochemistry
Histology (H&E staining) and histochemistry (Alcian blue pH 2.5 PAS) demonstrated the absence of structural alterations in meniscus tissue from normal donors (Fig. 1A and C), while showing structural alterations, fragmentation of tissue and clefts in the meniscus from OA patients (Fig. 1B and D). The fibrochondrocytes from normal donors did not show any sign of cellular degeneration, demonstrated by normal and intense H&E staining, while the fibrochondrocytes from OA patients showed clear signs
Discussion
OA is defined as a progressive and degenerative, rather than a chronic inflammatory disease of joints. It is characterized by gradual deterioration of joint tissues, including the loss of articular cartilage, synovial inflammation, changes in the subchondral bone, as well as osteophyte formation (Edmonds, 2009, Musumeci et al., 2011d, Liddle et al., 2013). It is one of the most common joint diseases and is associated with a negative impact on the quality of life as well as on healthcare costs (
Acknowledgement
This study was supported by grants provided by the Department of Bio-Medical Sciences, School of Medicine, University of Catania.
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