While small leucine rich proteins/proteoglycans (SLRPs) are present in very low concentrations in the extracellular matrix (ECM), they have been shown to be critical determinants of the proper ECM assembly and function in connective tissues [1] including bone [2], cornea [3], and cartilage [4]. However, their direct and indirect roles in matrix biomechanics and the potential for osteoarthritis-related dysfunction of cartilage remain unclear. With the advent of new high resolution nanotechnological tools, the direct quantification of cartilage biomechanical properties using murine models can provide important insights into how secondary ECM molecules, such as SLRPs, affect the function and pathology of cartilage [5]. Previous nanoindentation studies of murine cartilage have assessed the effects of maturation and osteoarthritis-like degradation of cartilage on its biomechanical properties [6, 7]. Recently, murine models have received increased attention because of the availability of specific gene-knockout and gene alteration technologies [8]. For example, chondroadherin (CHAD) is a non-collagenous small leucine-rich proteoglycan (SLRP) with α-helix and β-sheet secondary structure, spatially localized in the territorial matrix (MW = 38 kDa) [9]. In articular cartilage, CHAD is distributed non-uniformly with depth [10], and binds to type II collagen and the α2β1 integrin and is hypothesized to function in the communication between chondrocytes and their surrounding matrix, as well as in the regulation of collagen fibril assembly [11, 12] (Fig. 1). The objective of the present study is to explore the role of CHAD and its depletion on the structure and nanomechanical properties of both superficial and middle/deep zone cartilage. The current methods thereby enabled depth-dependent analysis of cartilage nanostructure and dynamic energy-dissipative mechanisms.

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