Association between proteoglycans and matrix vesicles in the extracellular matrix of growth plate cartilage

The Role of Matrix-Bound Extracellular Vesicles in the Regulation of Endochondral Bone Formation

Matrix vesicles are key players in the development of the growth plate during endochondral bone formation. They are involved in the turnover of the extracellular matrix and its mineralization, as well as being a vehicle for chondrocyte communication and regulation. These extracellular organelles are released by the cells and are anchored to the matrix via integrin binding to collagen. The exact function and makeup of the vesicles are dependent on the zone of the growth plate in which they are produced. Early studies defined their role as sites of initial calcium phosphate deposition based on the presence of crystals on the inner leaflet of the membrane and subsequent identification of enzymes, ion transporters, and phospholipid complexes involved in mineral formation. More recent studies have shown that they contain small RNAs, including microRNAs, that are distinct from the parent cell, raising the hypothesis that they are a distinct subset of exosomes. Matrix vesicles are produced under complex regulatory pathways, which include the action of steroid hormones. Once in the matrix, their maturation is mediated by the action of secreted hormones. How they convey information to cells, either through autocrine or paracrine actions, is now being elucidated.

Tissue-derived extracellular vesicles: Research progress from isolation to application

Extracellular vesicles (EVs) are the structures that all cells release into the environment. They are separated by a lipid bilayer and contain the cellular components that release them. To date, most studies have been performed on EVs derived from cell supernatants or different body fluids, while the number of studies on EV isolation directly from tissues is still limited. Studies of EV isolation directly from tissues may provide us with better information. This review summarizes the role of EV in the extracellular matrix, the protocol for isolation of EV in the tissue interstitium, and the application of the protocol in different tissues.

Extracellular vesicles are integral and functional components of the extracellular matrix

Extracellular vesicles (EV) are small plasma membrane-derived particles released into the extracellular space by virtually all cell types. Recently, EV have received increased interest because of their capability to carry nucleic acids, proteins, lipids and signaling molecules and to transfer their cargo into the target cells. Less attention has been paid to their role in modifying the composition of the extracellular matrix (ECM), either directly or indirectly via regulating the ability of target cells to synthesize or degrade matrix molecules. Based on recent results, EV can be considered one of the structural and functional components of the ECM that participate in matrix organization, regulation of cells within it, and in determining the physical properties of soft connective tissues, bone, cartilage and dentin. This review addresses the relevance of EV as specific modulators of the ECM, such as during the assembly and disassembly of the molecular network, signaling through the ECM and formation of niches suitable for tissue regeneration, inflammation and tumor progression. Finally, we assess the potential of these aspects of EV biology to translational medicine.

5-Aza-2'-deoxycytidine acts as a modulator of chondrocyte hypertrophy and maturation in chick caudal region chondrocytes in culture

This study was carried out to explore the effect of DNA hypomethylation on chondrocytes phenotype, in particular the effect on chondrocyte hypertrophy, maturation, and apoptosis. Chondrocytes derived from caudal region of day 17 embryonic chick sterna were pretreated with hypomethylating drug 5-aza-2'-deoxycytidine for 48 hours and then maintained in the normal culture medium for up to 14 days. Histological studies showed distinct morphological changes occurred in the pretreated cultures when compared to the control cultures. The pretreated chondrocytes after 7 days in culture became bigger in size and acquired more flattened fibroblastic phenotype as well as a loss of cartilage specific extracellular matrix. Scanning electron microscopy at day 7 showed chondrocytes to have increased in cell volume and at day 14 in culture the extracellular matrix of the pretreated cultures showed regular fibrillar structure heavily embedded with matrix vesicles, which is the characteristic feature of chondrocyte hypertrophy. Transmission electron microscopic studies indicated the terminal fate of the hypertrophic cells in culture. The pretreated chondrocytes grown for 14 days in culture showed two types of cells: dark cells which had condense chromatin in dark patches and dark cytoplasm. The other light chondrocytes appeared to be heavily loaded with endoplasmic reticulum indicative of very active protein and secretory activity; their cytoplasm had large vacuoles and disintegrating cytoplasm. The biosynthetic profile showed that the pretreated cultures were actively synthesizing and secreting type X collagen and alkaline phosphatase as a major biosynthetic product.

Vascular Calcification in Uremia: New-Age Concepts about an Old-Age Problem

A hallmark of aging, and major contributor to the increased prevalence of cardiovascular disease in patients with chronic kidney disease (CKD), is the progressive structural and functional deterioration of the arteries and concomitant accrual of mineral. Vascular calcification (VC) was long viewed as a degenerative age-related pathology that resulted from the passive deposition of mineral in the extracellular matrix; however, since the discovery of "bone-related" protein expression in calcified atherosclerotic plaques over 20 years ago, a plethora of studies have evoked the now widely accepted view that VC is a highly regulated and principally cell-mediated phenomenon that recapitulates many features of physiologic ossification. Central to this theory are changes in vascular smooth muscle cell (VSMC) phenotype and viability, thought to be driven by chronic exposure to a number of dystrophic stimuli characteristics of the uremic state. Here, dedifferentiated synthetic VSMCs are seen to spawn calcifying matrix vesicles that actively seed mineralization of the arterial matrix. This review provides an overview of the major epidemiological, histological, and molecular aspects of VC in the context of CKD, and a counterpoint to the prevailing paradigm that emphasizes the primacy of VSMC-mediated mechanisms. Particular focus is given to the import of protein and small molecule inhibitors in regulating physiologic and pathological mineralization and the emerging role of mineral nanoparticles and their interplay with proinflammatory processes.

Proteomics approaches for the studies of bone metabolism

Bone is an active tissue, in which bone formation by osteoblast is followed by bone resorption by osteoclasts, in a repeating cycle. Proteomics approaches may allow the detection of changes in cell signal transduction, and the regulatory mechanism of cell differentiation. LC-MS/MS-based quantitative methods can be used with labeling strategies, such as SILAC, iTRAQ, TMT and enzymatic labeling. When used in combination with specific protein enrichment strategies, quantitative proteomics methods can identify various signaling molecules and modulators, and their interacting proteins in bone metabolism, to elucidate biological functions for the newly identified proteins in the cellular context. In this article, we will briefly review recent major advances in the application of proteomics for bone biology, especially from the aspect of cellular signaling. [BMB Reports 2014; 47(3): 141-148]

Proteomic characterization of biogenesis and functions of matrix vesicles released from mineralizing human osteoblast-like cells

Matrix vesicles (MVs), released by budding from apical microvilli of osteoblasts during bone formation and development, are involved in the initiation of mineralization by promoting the formation of hydroxyapatite in their lumen. To gain additional insights into MV biogenesis and functions, MVs and apical microvilli were co-isolated from mineralizing osteoblast-like Saos-2 cells and their proteomes were characterized using LC-ESI-MS/MS and compared. In total, 282 MV and 451 microvillar proteins were identified. Of those, 262 were common in both preparations, confirming that MVs originate from apical microvilli. The occurrence of vesicular trafficking molecules (e.g. Rab proteins) and of the on-site protein synthetic machinery suggests that cell polarization and apical targeting are required for the incorporation of specific lipids and proteins at the site of MV formation. MV release from microvilli may be driven by actions of actin-severing proteins (gelsolin, cofilin 1) and contractile motor proteins (myosins). In addition to the already known proteins involved in MV-mediated mineralization, new MV residents were detected, such as inorganic pyrophosphatase 1, SLC4A7 sodium bicarbonate cotransporter or sphingomyelin phosphodiesterase 3, providing additional insights into MV functions.

Promotion of articular cartilage matrix vesicle mineralization by type I collagen

OBJECTIVE

Calcium pyrophosphate dihydrate (CPPD) and basic calcium phosphate (BCP) crystals occur in up to 60% of osteoarthritic joints and predict an increased severity of arthritis. Articular cartilage vesicles (ACVs) generate CPPD crystals in the presence of ATP and BCP crystals with added beta-glycerophosphate. While ACVs are present in normal articular cartilage, they mineralize primarily in cartilage from osteoarthritic joints. The aim of this study was to explore the hypothesis that ACV mineralization is regulated by components of the surrounding extracellular matrix.

METHODS

Porcine ACVs were embedded in agarose gels containing type II and/or type I collagen and/or proteoglycans. Mineralization was measured as (45)Ca accumulation stimulated by ATP or beta-glycerophosphate and reflects both nucleation and growth. Synthetic CPPD and BCP crystals were embedded in similar gels to isolate the effect of matrix components on crystal growth.

RESULTS

After establishing baseline responsiveness of ACVs to ATP and beta-glycerophosphate in agarose gels, we examined the ability of ATP and beta-glycerophosphate to stimulate mineral formation in gels containing various matrix components. Type II collagen suppressed the ability of ATP to stimulate mineralization, while a combination of type II plus type I collagen increased the effect of ATP and beta-glycerophosphate on mineralization. Type I collagen affected ACV mineralization in a dose-responsive manner. Neither type of collagen significantly affected crystal growth or levels of mineralization-regulating enzymes. Proteoglycans suppressed mineral formation by ACVs in gels containing both type I and type II collagen.

CONCLUSION

Cartilage matrix changes that occur with osteoarthritis, such as increased quantities of type I collagen and reduced proteoglycan levels, may promote ACV mineralization.

Mineral surface in calcified plaque is like that of bone: further evidence for regulated mineralization

OBJECTIVE

Cell biological studies demonstrate remarkable similarities between mineralization processes in bone and vasculature, but knowledge of the components acting to initiate mineralization in atherosclerosis is limited. The molecular level microenvironment at the organic-inorganic interface holds a record of the mechanisms controlling mineral nucleation. This study was undertaken to compare the poorly understood interface in mineralized plaque with that of bone, which is considerably better characterized.

METHODS AND RESULTS

Solid state nuclear magnetic resonance (SSNMR) spectroscopy provides powerful tools for studying the organic-inorganic interface in calcium phosphate biominerals. The rotational echo double resonance (REDOR) technique, applied to calcified human plaque, shows that this interface predominantly comprises sugars, most likely glycosaminoglycans (GAGs). In this respect, and in the pattern of secondary effects seen to protein (mainly collagen), calcified plaque strongly resembles bone.

CONCLUSIONS

The similarity between biomineral formed under highly controlled (bone) and pathological (plaque) conditions suggests that the control mechanisms are more similar than previously thought, and may be adaptive. It is strong further evidence for regulation of plaque mineralization by osteo/chondrocytic vascular smooth muscle cells.

Mineralization of annexin-5-containing lipid-calcium-phosphate complexes: modulation by varying lipid composition and incubation with cartilage collagens

Matrix vesicles (MVs) in the growth plate bind to cartilage collagens and initiate mineralization of the extracellular matrix. Native MVs have been shown to contain a nucleational core responsible for mineral formation that is comprised of Mg(2+)-containing amorphous calcium phosphate and lipid-calcium-phosphate complexes (CPLXs) and the lipid-dependent Ca(2+)-binding proteins, especially annexin-5 (Anx-5), which greatly enhances mineral formation. Incorporation of non-Ca(2+)-binding MV lipids impedes mineral formation by phosphatidylserine (PS)-CPLX. In this study, nucleators based on amorphous calcium phosphate (with or without Anx-5) were prepared with PS alone, PS + phosphatidylethanolamine (PE), or PS + PE and other MV lipids. These were incubated in synthetic cartilage lymph containing no collagen or containing type II or type X collagen. Dilution of PS with PE and other MV lipids progressively retarded nucleation. Incorporation of Anx-5 restored nucleational activity to the PS:PE CPLX; thus PS and Anx-5 proved to be critical for nucleation of mineral. Without Anx-5, induction of mineral formation was slow unless high levels of Ca(2+) were used. The presence of type II collagen in synthetic cartilage lymph improved both the rate and amount of mineral formation but did not enhance nucleation. This stimulatory effect required the presence of the nonhelical telopeptides. Although type X collagen slowed induction, it also increased the rate and amount of mineral formation. Both type II and X collagens markedly increased mineral formation by the MV-like CPLX, requiring Anx-5 to do so. Thus, Anx-5 enhances nucleation by the CPLXs and couples this to propagation of mineral formation by the cartilage collagens.

Proteome analysis of matrix vesicles isolated from femurs of chicken embryo

Matrix vesicles (MVs) are extracellular organelles that initiate mineral formation, accumulating inorganic phosphate (P(i)) and calcium leading to the formation of hydroxyapatite (HA) crystals, the main mineral component of bones. MVs are produced during bone formation, as well as during the endochondral calcification of cartilage. MVs are released into the extracellular matrix from osseous cells such as osteoblasts and hypertrophic chondrocytes. In this report, using 1-D SDS-PAGE, in-gel tryptic digestion and an LC-MS-MS/MS protein identification protocol, we characterized the proteome of MVs isolated from chicken embryo (Gallus gallus) bones and cartilage. We identified 126 gene products, including proteins related to the extracellular matrix and ion transport, as well as enzymes, cytoskeletal, and regulatory proteins. Among the proteins recognized for the first time in MVs were aquaporin 1, annexin A1 (AnxA1), AnxA11, glycoprotein HT7, G(i) protein alpha2, and scavenger receptor type B. The pathways for targeting the identified proteins into MVs and their particular functions in the biomineralization process are discussed. Obtaining a knowledge of the functions and roles of these proteins during embryonic mineralization is a prerequisite for the overall understanding of the initial mineral formation mechanisms.

Subcellular targeting and function of osteoblast nucleotide pyrophosphatase phosphodiesterase 1

The ectonucleoside pyrophosphatase phosphodiesterase 1 (NPP1/PC-1) is a member of the NPP enzyme family that is critical in regulating mineralization. In certain mineralizing sites of bone and cartilage, membrane-limited vesicles [matrix vesicles (MVs)] provide a sheltered internal environment for nucleation of calcium-containing crystals, including hydroxyapatite. MV formation occurs by budding of vesicles from the plasma membrane of mineralizing cells. The MVs are enriched in proteins that promote mineralization. Paradoxically, NPP1, the type II transmembrane protein that generates the potent hydroxyapatite crystal growth inhibitor inorganic pyrophosphate (PPi), is also enriched in MVs. Although osteoblasts express NPP1, NPP2, and NPP3, only NPP1 is enriched in MVs. Therefore, this study uses mineralizing human osteoblastic SaOS-2 cells, a panel of NPP1 mutants, and NPP1 chimeras with NPP3, which does not concentrate in MVs, to investigate how NPP1 preferentially targets to MVs. We demonstrated that a cytosolic dileucine motif (amino acids 49–50) was critical in localizing NPP1 to regions of the plasma membrane that budded off into MVs. Moreover, transposition of the NPP1 cytoplasmic dileucine motif and flanking region (AAASLLAP) to NPP3 conferred to NPP3 the ability to target to the plasma membrane and, subsequently, concentrate in MVs. Functionally, the cytosolic tail dileucine motif NPP1 mutants lost the ability to support MV PPiconcentrations and to suppress calcification. The results identify a specific targeting motif in the NPP1 cytosolic tail that delivers PPi-generating NPP activity to osteoblast MVs for control of calcification.

Authentic matrix vesicles contain active metalloproteases (MMP). a role for matrix vesicle-associated MMP-13 in activation of transforming growth factor-beta

Matrix vesicles (MV) play a key role in the initiation of cartilage mineralization. Although many components in these microstructures have been identified, the specific function of each component is still poorly understood. In this study, we show that metalloproteases (MMP), MMP-2, -9, and -13 are associated with MV isolated from growth plate cartilage. In addition, we provide evidence that MV contain transforming growth factor-beta (TGF-beta) and that MV-associated MMP-13 is capable of activating latent TGF-beta. To determine whether MMPs are associated directly with MV, vesicles isolated from growth plate cartilage were sequentially treated with hyaluronidase, NaCl, and bacterial collagenase to remove matrix proteins and other components attached to their outer surface. Finally, the vesicles were incubated with detergent to rupture the MV membrane and expose components that are inside the vesicles. Each treated MV fraction was subjected to substrate zymography, immunoblotting, and substrate activity assay. Whereas active MMP-13 was lost after combined treatment with hyaluronidase and NaCl, MMP-2 and -9 activities were still retained in the pellet fraction even after detergent treatment, suggesting that the gelatinases, MMP-2 and -9, are integral components of MV. In addition, MV contain TGF-beta in the small latent complex, and MMP-13 associated with the MV surface was responsible for activation of TGF-beta. Since the amount of TGF-beta activated by hypertrophic chondrocytes increased with mineral appearance in serum-free chondrocyte cultures, a role for active MV-associated MMPs is suggested in activation of TGF-beta seen during late chondrocyte hypertrophy and mineralization of growth plate cartilage.

Abnormal compartmentalization of cartilage matrix components in mice lacking collagen X: implications for function

There are conflicting views on whether collagen X is a purely structural molecule, or regulates bone mineralization during endochondral ossification. Mutations in the human collagen alpha1 (X) gene (COL10A1) in Schmid metaphyseal chondrodysplasia (SMCD) suggest a supportive role. But mouse collagen alpha1 (X) gene (Col10a1) null mutants were previously reported to show no obvious phenotypic change. We have generated collagen X deficient mice, which shows that deficiency does have phenotypic consequences which partly resemble SMCD, such as abnormal trabecular bone architecture. In particular, the mutant mice develop coxa vara, a phenotypic change common in human SMCD. Other consequences of the mutation are reduction in thickness of growth plate resting zone and articular cartilage, altered bone content, and atypical distribution of matrix components within growth plate cartilage. We propose that collagen X plays a role in the normal distribution of matrix vesicles and proteoglycans within the growth plate matrix. Collagen X deficiency impacts on the supporting properties of the growth plate and the mineralization process, resulting in abnormal trabecular bone. This hypothesis would accommodate the previously conflicting views of the function of collagen X and of the molecular pathogenesis of SMCD.

Influence of vitamin D status on hyaluronan localization in bone

The distribution of hyaluronan was investigated in the proximal tibiotarsal bones of normal (vitamin D-treated) chicks, and chicks with rachitic lesions induced by vitamin D deficiency. Localization studies using a biotinylated hyaluronan-binding probe revealed that in vitamin D-treated chicks, a high level of hyaluronan staining was present in upper proliferative zone cartilage and upper hypertrophic zone cartilage. Hyaluronan staining was greatly reduced in the zone of provisional calcification. In the metaphyses and diaphyses of normal chicks hyaluronan was predominantly localized to the non-bone-forming surfaces of osteoblasts but was also present on the basolateral surfaces of lining cells and osteoclasts. Marked changes in hyaluronan distribution were observed in vitamin D-deficient chicks. The amount of hyaluronan present in proliferative zone growth cartilage was similar to control chicks, although with a more widespread distribution, extending into lower proliferative zone cartilage. In the zone of hypertrophy/calcification, biochemical analyses revealed that hyaluronan levels in rachitic chicks were about 3.6 times greater than in vitamin D-treated chicks; localization studies demonstrated that this increase was associated with the presence of hyaluronan-positive spindle-shaped cells in the metaphyseal vascular spaces. Intense hyaluronan staining was also associated with abundant spindle-shaped cells occupying the marrow spaces of rachitic diaphyseal bone. The distribution of hyaluronan in vitamin D-treated chick bone, and the alterations observed in rachitic tissue suggests a role for hyaluronan in endochondral bone formation.

Matrix vesicles and focal proteoglycan aggregates are the nucleation sites revealed by the lanthanum incubation method: a correlated study on the hypertrophic zone of the rat epiphyseal cartilage

Correlated studies were performed with light and electron microscopy, and backscattered electron image in conjunction with X-ray microanalysis, of lanthanum-incubated epiphyseal cartilage of the young rat. The hall-mark of this procedure is the appearance of LaP electron-dense deposits (not present in control sections) in precise sites of the hypertrophic zone. The ultrastructural study revealed a dual nature of these sites: "dense matrix vesicles" and "focal filament aggregates". The dense matrix vesicles are a specific type of matrix vesicle with the intrinsic capacity of precipitating LaP mineral, as soon as they originate from the hypertrophic chondrocytes. Furthermore, the matrix vesicles were found to be heterogeneous because lanthanum-devoid, "light matrix vesicles" were also present. The focal filament aggregates, which were not recognized in unstained sections and in controls, are apparently focal concentrations of proteoglycans with high lanthanum binding capacity, although the presence in them of other components (e.g., type X collagen, C-propeptide of type II collagen) cannot be excluded. The were in close connection with the light matrix vesicles in the upper hypertrophic zone, and were loaded with a variable quantity of LaP irregular electron-dense deposits in the lower hypertrophic zone. These irregular deposits are similar to, but distinct from, calcification nodules. The lanthanum incubation method indirectly detects the matrix Ca-binding components (which bind La ions), and the calcification initiation sites (which precipitate a LaP-mineral phase). A sequence is proposed of successive steps of LaP nucleation within the focal filament aggregates, which possibly mimics calcium phosphate deposition. Such a sequence seems to require the participation not only of dense matrix vesicles, but also of the filamentous components of the focal aggregates, possibly together with the activity of alkaline phosphatase.

Characterization and reconstitution of the nucleational complex responsible for mineral formation by growth plate cartilage matrix vesicles

Previous studies revealed that matrix vesicles (MV) have an acid-labile nucleationally active core (ALNAC) essential for mineral formation; current studies were aimed at characterizing and reconstituting ALNAC. SDS-PAGE and FTIR analyses revealed the presence of lipids, proteins and amorphous calcium phosphate (ACP) in ALNAC. Extraction with chloroform-methanol reduced, but did not destroy MV calcification; treatment with chloroform-methanol-HCl destroyed all activity. This acidic solvent extracted the annexins, (phosphatidylserine (PS)-dependent Ca(2+)-binding proteins), and dissociated PS-Ca(2+)-Pi complexes present in the MV. Attempts to reconstitute ALNAC, centered on the Ca(2+)-PS-Pi complex. Various pure lipids, electrolytes and proteins were combined to form a synthetic nucleationally active complex (SNAC), analyzing the rate of Ca2+ uptake. Inclusion of phosphatidylethanolamine (PE) or sphingomyelin (SM) with PS, or Mg2+ or Zn2+ with Ca2+, strongly inhibited activity; incorporation of annexin V increased SNAC activity. Thus, approaching from either deconstruction or reconstruction, it appears that ALNAC is composed of ACP complexed with PS and the annexins. Other lipids, proteins and electrolytes modulate its activity. These findings also indicate how ALNAC must be formed in vivo.

Chondrocyte differentiation

Data obtained while investigating growth plate chondrocyte differentiation during endochondral bone formation both in vivo and in vitro indicate that initial chondrogenesis depends on positional signaling mediated by selected homeobox-containing genes and soluble mediators. Continuation of the process strongly relies on interactions of the differentiating cells with the microenvironment, that is, other cells and extracellular matrix. Production of and response to different hormones and growth factors are observed at all times and autocrine and paracrine cell stimulations are key elements of the process. Particularly relevant is the role of the TGF-beta superfamily, and more specifically of the BMP subfamily. Other factors include retinoids, FGFs, GH, and IGFs, and perhaps transferrin. The influence of local microenvironment might also offer an acceptable settlement to the debate about whether hypertrophic chondrocytes convert to bone cells and live, or remain chondrocytes and die. We suggest that the ultimate fate of hypertrophic chondrocytes may be different at different microanatomical sites.

A facilitative role for carbonic anhydrase activity in matrix vesicle mineralization

Carbonic anhydrase (CA) which catalyzes the reversible hydrolysis of carbon dioxide is known to be important in osteoclastic bone resorption, however, suggested roles in calcium phosphate mineral formation have not been previously demonstrated. Biochemical evidence is provided for the presence of CA in growth plate matrix vesicles (MV) and the level of activity determined by enzyme assay. Inhibition of CA activity with the specific inhibitor acetazolamide resulted in reduced rates of MV mineralization. Other inhibitor studies showed that MV mineralization was also impaired by 4,4-diisothiocyanatostilbene-2, 2-disulfonic acid (DIDS), a blocker of membrane bicarbonate channels. No evidence was found for the presence of any proton pumps or channels. When acetazolamide and DIDS were combined, their inhibitory effects on MV mineralization were additive. These findings suggest that MV possess a pH regulation system composed of carbonic anhydrase and a putative bicarbonate channel. This system may function in the MV by providing intraluminal buffering capacity. The control of intravesicular pH is important for the stabilization of the acid-labile nucleational core complex and in preventing the build-up of protons during calcium phosphate phase transformations.

Ca2+ binding properties of type X collagen

Type X collagen is a developmentally regulated collagen that is only synthesized by chondrocytes of the hypertrophic and calcifying zone in fetal cartilage. There is evidence in the literature that type X collagen may be involved in cartilage calcification. Here we show that type X collagen synthesis precedes calcium deposition in nodules of fetal human chondrocytes forming in cell culture and present evidence that type X collagen binds calcium in a specific and dose dependent manner. In an assay using bovine type X collagen coupled to beads and 45Ca2+ we determined a total of about 15 binding sites per alpha 1(X) chain with a dissociation of 32 microM.