The role of aggrecan in normal and osteoarthritic cartilage.

Peter J Roughley,John S Mort

doi:10.1186/s40634-014-0008-7

Abstract

Aggrecan is a large proteoglycan bearing numerous chondroitin sulfate and keratan sulfate chains that endow articular cartilage with its ability to withstand compressive loads. It is present in the extracellular matrix in the form of proteoglycan aggregates, in which many aggrecan molecules interact with hyaluronan and a link protein stabilizes each interaction. Aggrecan structure is not constant throughout life, but changes due to both synthetic and degradative events. Changes due to synthesis alter the structure of the chondroitin sulfate and keratan sulfate chains, whereas those due to degradation cause cleavage of all components of the aggregate. These latter changes can be viewed as being detrimental to cartilage function and are enhanced in osteoarthritic cartilage, resulting in aggrecan depletion and predisposing to cartilage erosion. Matrix metalloproteinases and aggrecanases play a major role in aggrecan degradation and their production is upregulated by mediators associated with joint inflammation and overloading. The presence of increased levels of aggrecan fragments in synovial fluid has been used as a marker of ongoing cartilage destruction in osteoarthritis. During the early stages of osteoarthritis it may be possible to retard the destructive process by enhancing the production of aggrecan and inhibiting its degradation. Aggrecan production also plays a central role in cartilage repair techniques involving stem cell or chondrocyte implantation into lesions. Thus aggrecan participates in both the demise and survival of articular cartilage.

Highlights

This review describes the role of aggrecan in the function of articular cartilage and how this role is perturbed in the osteoarthritic (OA) joint
In the case of proteases known to be active in cartilage, correlation of such cleavage sites with the sequence termini determined in aggrecan fragments isolated from human articular cartilage and synovial fluid along with the detection of the cognate
Additional proteolytic cleavage sites have been identified by digestion of aggrecan in vitro, and the presence of the corresponding products in vivo has been inferred based on the molecular sizes of aggrecan fragments extracted from cartilage or isolated from synovial fluid (Struglics and Hansson, 2012)

Summary

Introduction

This review describes the role of aggrecan in the function of articular cartilage and how this role is perturbed in the osteoarthritic (OA) joint. The OA joint is characterized by catabolic processes that degrade both the HA backbone of the aggregate and the core protein of the aggrecan molecules, so impairing aggrecan function and predisposing the articular cartilage to erosion. The lectin-like domain does have the ability to interact with a variety of ECM proteins, such as tenascins and fibulins (Aspberg, 2012), and could potentially play a role in anchoring the aggrecan within the tissue.

Results

Conclusion