The pericellular hyaluronan of articular chondrocytes

Abstract

The story of hyaluronan in articular cartilage, pericellular hyaluronan in particular, essentially is also the story of aggrecan. Without properly tethered aggrecan, the load bearing function of cartilage is compromised. The anchorage of aggrecan to the cell surface only occurs due to the binding of aggrecan to hyaluronan-with hyaluronan tethered either to a hyaluronan synthase or by multivalent binding to CD44. In this review, details of hyaluronan synthesis are discussed including how HAS2 production of hyaluronan is necessary for normal chondrocyte development and matrix assembly, how an abundance or deficit of pericellular hyaluronan alters chondrocyte metabolism, and whether hyaluronan size matters or changes with aging or disease. The biomechanical role and matrix assembly function of hyaluronan in addition to the functions of hyaluronidases are discussed. The turnover of hyaluronan is considered including mechanisms by which its turnover, at least in part, is mediated by endocytosis by chondrocytes and regulated by aggrecan degradation. Differences between turnover and clearance of newly synthesized hyaluronan and aggrecan versus the half-life of hyaluronan remaining within the inter-territorial matrix of cartilage are discussed. The release of neutral pH-acting hyaluronidase activity remains one unanswered question concerning the loss of cartilage hyaluronan in osteoarthritis. Signaling events driven by changes in hyaluronan-chondrocyte interactions may involve a chaperone function of CD44 with other receptors/cofactors as well as the changes in hyaluronan production functioning as a metabolic rheostat.

Keywords: Aggrecan; Cartilage; Chondrocytes; Hyaluronan; Hyaluronidase.

Fig. 1. Exogenous aggrecan enhances the hyaluronan-dependent chondrocyte pericellular matrix

(A, D) Bovine articular chondrocytes readily assemble pericellular coats after 2 days in monolayer culture, as revealed using the particle exclusion assay [18]. (B, E) The addition of 2 mg/ml purified aggrecan (bovine articular cartilage D1 fraction) to chondrocytes with an existing coat expands the coat within 90 minutes. (G) Streptomyces hyaluronidase treatment (10 units/ml medium + FBS, 60 minutes, 37 °C) removes the endogenous coats on bovine chondrocytes. (H) No expansion by added aggrecan is observed when cells are pretreated with Streptomyces hyaluronidase. (C, F) After coats are enhanced with exogenous aggrecan, these pre-established coats were dissociated via post-treatment with Streptomyces hyaluronidase (10 units/ml medium, with gentle shaking at 30 minutes). (I) Pre-enhanced coats with exogenous aggrecan persist with gentle shaking in the absence of hyaluronidase. Bars = 50 μm in each panel.

Fig. 2. Hyaluronan turnover in cartilage differs with metabolic state and regionally within the matrix

One helpful approach to consider in the matter of hyaluronan turnover is which pool of hyaluronan (and aggrecan) is being considered as well as chondrocyte metabolism. Hyaluronan turnover within the pericellular matrix (PCM) differs, for the most part, from hyaluronan turnover within the inter-territorial extracellular matrix (ECM). Secondly, hyaluronan turnover in steady-state and hyaluronan turnover associated with altered cartilage metabolism [aging, osteoarthritis (OA), disuse, or trauma] also differ. With these considerations there are likely overlapping mechanisms as well as circumstances in which there is no overlap.

Fig. 3. Loss of aggrecan is variable in human osteoarthritic cartilage tissues

Histological images of articular cartilage stained with safranin O/fast green are shown. (A) Normal bovine articular cartilage. (B) Human osteoarthritic cartilage with fibrillated articular surface and rarefication of staining of the pericellular matrix throughout all zones. (C) Human ankle cartilage, from a 39 year old female donor with no known medical history of arthritis, with an intact articular surface but depletion of pericellular safranin O staining throughout all zones. (D) Human osteoarthritic cartilage showing the loss of safranin O straining from the inter-territorial matrix from the surface to the middle zone, but enhanced pericellular safranin O staining throughout the middle zones. (E) Human osteoarthritic cartilage with the loss of safranin O staining progressing as a wave from the articular surface but persistent pericellular safranin O staining surrounding chondrocytes at the bottom of the safranin O depleted zone.