Ring-Mesh Model of Proteoglycan Glycosaminoglycan Chains in Tendon based on Three-dimensional Reconstruction by Focused Ion Beam Scanning Electron Microscopy

Abstract

Tendons are composed of collagen fibrils and proteoglycan predominantly consisting of decorin. Decorin is located on the d-band of collagen fibrils, and its glycosaminoglycan (GAG) chains have been observed between collagen fibrils with transmission electron microscopy. GAG chains have been proposed to interact with each other or with collagen fibrils, but its threedimensional organization remains unclear. In this report, we used focused ion beam scanning electron microscopy to examine the three-dimensional organization of the GAG chain in the Achilles tendon of mature rats embedded in epoxy resin after staining with Cupromeronic blue, which specifically stains GAG chains. We used 250 serial back-scattered electron images of longitudinal sections with a 10-nm interval for reconstruction. Three-dimensional images revealed that GAG chains form a ring mesh-like structure with each ring surrounding a collagen fibril at the d-band and fusing with adjacent rings to form the planar network. This ring mesh model of GAG chains suggests that more than two GAG chains may interact with each other around collagen fibrils, which could provide new insights into the roles of GAG chains.Tendon is primarily composed of collagen fibrils and the non-collagen matrix (1). Canty et al. (2) suggested that plasma membrane protrusions called fibripositors contribute to the parallel arrangement of collagen fibrils in tendon, especially in the embryonic stage. Decorin and biglycan belong to the small leucine-rich proteoglycan family and consist of a core protein containing leucine repeats with a covalently attached chondroitin/dermatan sulfate glycosaminoglycan (GAG) 2 chain. The majority (Ͼ90%) of the tendon proteoglycan is decorin, while tendon also contains small amounts of biglycan (3). The decorin core protein binds to the surface of collagen fibrils by non-covalent binding and the attached decorin GAG chain extends from the core protein to associate closely with another decorin GAG chain on an adjacent fibril surface (4 -6).Some studies have suggested that the GAG chain forms bridges between collagen fibrils to guarantee mechanical integrity (4, 7-9). In contrast, other studies have suggested that the GAG chain is irrelevant to mechanical integrity (3, 10 -12), because results of mechanical tests using decorin-knock-out mice vary depending on the type and region of the tendon (13). Some researchers also reported that decorin might restore the alignment of the collagen fibrils deformed under load, rather than contribute to the transmission of forces (14,15). However, precise roles of decorin are not well understood, partly because its three-dimensional structure is unresolved.The structure of decorin GAG chains among collagen fibrils was observed by ultrathin sections of a sample stained with Cupromeronic blue and the critical electrolyte concentration technique, which was used to reveal the sulfated GAGs under a transmission electron microscope (TEM) (5, 16). Scott et al. (17) proposed the "shape module model" in which the decorin ...

Fibrillar architecture at three different sites of the bovine superficial digital flexor tendon

“…Collagen fibrils connect to each other by linear GAG chains which form ring mesh structure around each fibril [28]. The length of GAG chains in association with decorin that binds at D-band of the fibrils is highly relevant with the interfibrillar space [19,20,21, 27].…”

Changes in skin structure of the <i>Zip13</i>-KO mouse by Makomo (<i>Zizania latifolia</i>) feeding

“…These changes without change in the amount of collagen indicate increases in other constituents rather than collagen. One of the other constituents, decorin, which carries DS as a GAG chain, is known to bind collagen fibrils [ 18 , 19 , 22 ]. In addition, TGF-β1 is stored in the ECM through binding with decorin [ 4 , 23 ].…”

“…Since there was no change in the plasma level of TGF-β1 or ratio of GAG, it is not clear whether decorin is related to the increases in densities of collagen fibers and collagen fibrils. However, the GAG chains of decorin form a ring-mesh structure and change the length [ 13 , 22 ]. As a result of cupromeronic blue stain, it seems likely that the GAG chains can change their lengths and decrease the distance between collagen fibrils without changing the ratio of GAG in the MF group.…”

Glycosaminoglycans and Proteoglycans

“…The SLRPs exemplified by decorin have ordering functions in tissues; decorin in tendons appears to wrap round the D-band of collagen fibrils forming a ring-mesh of GAG [ 50 ]. SLRPs are also important for the structure of cornea, for which transparency requires an absolutely regular structure.…”