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Invited Review Ion Transport in Chondrocytes: Membrane

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Invited Review Ion Transport in Chondrocytes: Membrane Histol Histopathol (1998) 13: 893-910 Histology and 001: 10.14670/HH-13.893 Histopathology http://www.hh.um.es From Cell Biology to Tissue Engineering Invited Review Ion transport in chondrocytes: membrane transporters involved in intracellular ion homeostasis and the regulation of cell volume, free [Ca2+] and pH A. MobasherP, R. Mobasherl2, M.J.O. Francis3, E. Trujillo4, D. Alvarez de la Rosa4 and P. Martin-Vasallo4 1 University Laboratory of Physiology, University of Oxford, and Department of Biomedical Sciences, School of Biosciences, University of Westminster, London, 2United Medical and Dental Schools of Guy's and St Thomas's Hospitals, London, 3Nuffield Department of Orthopaedic Surgery, Nuffield Orthopaedic Centre, Headington, UK and 4Laboratory of Developmental Biology, Department of Biochemistry and Molecular Biology, University of La Laguna, La Laguna, Tenerife, Spain Summary. Chondrocytes exist in an unusual and their patterns of isoform expression underscore the variable ionic and osmotic environment in the extra­ subtlety of ion homeostasis and pH regulation in normal cellular matrix of cartilage and are responsible for cartilage. Perturbations in these mechanisms may affect maintaining the delicate equilibrium between extra­ the physiological turnover of cartilage and thus increase cellular matrix synthesis and degradation. The the susceptibility to degenerative joint disease. mechanical performance of cartilage relies on the biochemical properties of the matrix. Alterations to the Key words: Chondrocyte, Cartilage, Ion transport, £H ionic and osmotic extracellular environment of chondro­ regulation, Na+, K+-ATPase, Na+/H+ exchange, Ca +­ cytes have been shown to influence the volume, ATPase intracellular pH and ionic content of the cells, which in turn modify the synthesis and degradation of extra­ cellular matrix macromolecules. Physiological ion Introduction homeostasis is fundamental to the routine functioning of cartilage and the factors that control the integrity of this Articular cartilage is a mechanically unique highly evolved and specialized tissue. Ion transport in connective tissue designed to withstand load and act as cartilage is relatively unexplored and the biochemical an elastic shock absorber and wear resistant surface to properties and molecular identity of membrane transport the articulating joints. Within articular cartilage, only mechanisms employed by chondrocytes in the control of one cell type is found: the chondrocyte. This is a highly intracellular ion concentrations and pH is not fully specialized cell solely responsible for the synthesis and defined and this review focuses on these processes. maintenance of cartilage matrix, a meshwork of collagen Chondrocytes have been shown to express voltage and II, non-collagenous proteins and aggregating proteo­ stretch activated ion channels, passive exchangers and glycans (see Fig. 1) (Muir, 1995; Buckwalter and ATP dependent ion pumps. In addition, recent studies of Mankin, 1997a). Cell density in cartilage is low transport systems in chondrocytes have demonstrated the compared to other tissues. In adult human cartilage, presence of isozyme diversity that includes Na+/H+ chondrocytes occupy less than 1 % of the total tissue exchange (NHE1, NHE3), Na+, K+-ATPase (several volume (Stockwell, 1971, 1991) (Fig. 2A). In foetal isoforms) and others each of which possess considerably human cartilage and cartilage from small animals the different kinetic properties and modes of regulation. This cell density is significantly higher. Although chondro­ multitude of isozyme diversity indicates the highly cytes vary in size, shape, morphology and metabolic specialized handling of ions and protons in order to activity, they all possess the ability and intracellular accomplish a fine regulation of their transmembrane organelles for the biosynthesis and degradation of the fluxes. The complexities of these transport systems and extracellular matrix. They surround themselves with extracellular matrix but do not form cell-to-cell contacts Offprint requests to : Dr. A. Mobasheri, Lecturer in Cellular and (Fig. 2C). The tissue is organized into three principal Molecular Biology, Department of Biomedical Sciences, School of layers or zones, each of which has distinct biochemical, Biosciences, Un iversity of Westminster, 115 New Cavendish Street, biomechanical and physiological characteristics. The London WIM 8JS United Kingdom. FAX: 44·171 -9115087. e-mail: chondrocytes in each zone also have characteristic [email protected] shapes, sizes and orientations with respect to the 894 A review of ion transport in chondrocytes articular surface. For example, in the surface zone, concentrations of these cations are higher within chondrocytes are flattened and ellipsoid-shaped arranged cartilage relative to serum and synovial fluid (Table 1). in parallel to the articular surface (Fig. 2A). In the In addition, the high fixed negative charge density middle and deep zones the cells are more spheroidal in results in a high concentration of protons, leading to a shape and align themselves in columns perpendicular to lowering of cartilage pH (Gray et aI., 1988). Anions (Cl­ the articular surface and the zone of calcification. The and HC03-) are repelled by the negative charges and composition of the extracellular matrix also varies in their concentration is relatively low in cartilage (Urban each zone; in the surface layer the matrix is rich in and Hall, 1992). In general, the movement of collagen but the concentration of proteoglycans is monovalent cations in the matrix is not restricted and is relatively low whereas in the middle and deep layers the similar to that in free solution whereas divalent cations concentration of proteoglycans is significantly higher such as Ca2+ are less mobile (Maroudas, 1980). The (Stockwell and Scott, 1967). The concentration of ionic environment of chondrocytes is thus unusual proteoglycans is highest in the deep zone and the compared with most other cell types (Table 1). The chondrocytes express biochemical markers normally movement of cations into the matrix is followed by the associated with matrix mineralization and calcification movement of osmotically obliged water which may (e.g. alkaline phosphatase; Fig 2B). contribute as much as 80% of the wet weight of articular The unique load bearing and physico-chemical cartilage. The interaction between water and the ions in properties of articular cartilage depend on their high the matrix substantially influences the load bearing concentrations of complex, negatively charged proteo­ performance of cartilage. The increase in the total ionic glycans. The glycosaminoglycan side chains of proteo­ strength results in an increase in tissue osmolarity, glycans contain up to two anionic groups (carboxyl and creating a Donnan effect; the collagen network resists sulphate) per disaccharide subunit and thus provide a net the osmotic pressure exerted by the tissue water and the negative charge to the matrix of cartilage (Fig. 1). Since inflated proteoglycans. Mechanical loading itself results these ionic groups are covalently attached to the solid in changes to the ionic and osmotic environment of matrix, they are often referred to as "fixed negative chondrocytes; loading expresses interstitial fluid from charges" and thus result in a fixed charge density (FCD) the matrix of cartilage increasing the local proteoglycan (Lesperance et aI., 1992). Electrostatic interactions concentrations (Maroudas, 1979; Urban, 1994) and between the fixed charged groups provides up to 50% of hence the local ionic strength and osmotic pressure (Fig the tissue compressive stiffness and the ability to 3). The loading induced cell shrinkage will then withstand load. The ionic composition of the matrix is activate volume regulatory ion and osmolyte transport strictly dictated by the FCD; the negative charges on the mechanisms that allow the chondrocytes to return to proteoglycans draw cations (principally Na+, K+, and their original volume. This regulatory volume increase Ca2+) into the matrix to balance the negative charge (RVI) effectively raises intracellular [K+], resulting in distribution (Maroudas, 1980). Consequently, the cell swelling towards normal volume (Hoffmann and Simonsen, 1989; Hall et aI., 1996). In normal cartilage, proteoglycans help to maintain the osmotic pressure within the cartilage matrix and concentrations of electrolytes in the interstitial fluid. The influence of tissue FCD on the ion and water content of the matrix may have important biochemical, cellular and patho-physiological consequences. For example, the synthesis of cartilage matrix by chondrocytes is sensitive to extracellular Na+ concentrations (Urban and Bayliss, 1989). Furthermore, an acidic tissue pH has deleterious effects on matrix synthesis in many types of cartilage Collagen Fibril (Schwartz et aI., 1976; Wilkins and Hall, 1995). More importantly, in the early stages of osteoarthritis, proteoglycans are lost from the matrix (Stockwell, 1991; cr Mobile anion Table 1. The ionic composition of articular cartilage. Na+ (mM) K+ (mM) Ca2+ (mM) CI- (mM) pH CI Mobile amon Cartilage 240-350 7 -12 6-20 60-100 6.9-7.1 Serum/Synovial fluid 140 5 1.5 145 7.4 Glycosaminoglycan (GAG) chains (keratan and chondroit in sulphate chains) The concentrations of the principal cations and anions in cartilage matrix depends on the local concentration of proteoglycans and their fixed Fig. 1. Schematic illustration of the major constituents of cartilage matrix
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