Phenotypic effects of biglycan deficiency are linked to collagen fibril abnormalities, are synergized by decorin deficiency, and mimic Ehlers-Danlos-like changes in bone and other connective tissues

Decorin promotes aortic smooth muscle cell calcification and colocalizes to calcified regions in human atherosclerotic lesions

OBJECTIVE

Ectopic calcification localized to the intima of atherosclerotic plaque is a risk marker for cardiovascular events and increases the risk of aortic dissection during angioplasty. A variety of extracellular matrix molecules such as collagen type 1, bone sialoprotein, and osteopontin are known to regulate the biomineralization of bone and ectopic vascular calcification. In the present study, it was investigated whether decorin, a small leucine-rich proteoglycan expressed in bone and atherosclerotic plaque, is involved in arterial calcification.

METHODS AND RESULTS

Calcification was induced in cultured bovine aortic smooth muscle cell (BASMC) by the addition of beta-glycerophosphate or inorganic phosphate. Northern and Western analysis revealed that decorin expression was strongly upregulated in mineralizing BASMC. Furthermore, overexpression of decorin using a retroviral expression vector resulted in a 3- to 4-fold elevation of calcium deposited on the BASMC monolayer. Increased calcification in response to decorin could also be mimicked by adding exogenous decorin to the cultures. In addition, human coronary atherosclerotic lesions taken from sudden-death patients showed marked colocalization of calcium deposits with decorin.

CONCLUSIONS

Decorin induces calcification of arterial smooth muscle cell cultures and colocalizes to mineral deposition in human atherosclerotic plaque, suggesting that decorin functions as promoter of intimal calcification.

WISP-1 is an osteoblastic regulator expressed during skeletal development and fracture repair

Wnt-1-induced secreted protein 1 (WISP-1) is a member of the CCN (connective tissue growth factor, Cyr61, NOV) family of growth factors. Experimental evidence suggests that CCN family members are involved in skeletogenesis and bone healing. To investigate the role of WISP-1 in osteogenic processes, we characterized its tissue and cellular expression and evaluated its activity in osteoblastic and chondrocytic cell culture models. During embryonic development, WISP-1 expression was restricted to osteoblasts and to osteoblastic progenitor cells of the perichondral mesenchyme. In vitro, we showed that WISP-1 expression in differentiating osteoblasts promotes BMP-2-induced osteoblastic differentiation. Using in situ and cell binding analysis, we demonstrated WISP-1 interaction with perichondral mesenchyme and undifferentiated chondrocytes. We evaluated the effect of WISP-1 on chondrocytes by generating stably transfected mouse chondrocytic cell lines. In these cells, WISP-1 increased proliferation and saturation density but repressed chondrocytic differentiation. Because of the similarity between skeletogenesis and bone healing, we also analyzed WISP-1 spatiotemporal expression in a fracture repair model. We found that WISP-1 expression recapitulates the pattern observed during skeletal development. Our data demonstrate that WISP-1 is an osteogenic potentiating factor promoting mesenchymal cell proliferation and osteoblastic differentiation while repressing chondrocytic differentiation. Therefore, we propose that WISP-1 plays an important regulatory role during bone development and fracture repair.

Regulation of fibrillin-1 by biglycan and decorin is important for tissue preservation in the kidney during pressure-induced injury

There is growing evidence that the two small leucine-rich proteoglycans biglycan and decorin regulate the assembly of connective tissues and alter cell behavior during development and pathological processes. In this study, we have used an experimental animal model of unilateral ureteral ligation and mice deficient in either biglycan or decorin. We discovered that pressure-induced injury to the wild-type kidneys led to overexpression of decorin, biglycan, fibrillin-1, and fibrillin-2. In contrast, in biglycan-deficient kidneys the overexpression of fibrillin-1 was markedly attenuated and this was associated with cystic dilatation of Bowman's capsule and proximal tubules. Notably, we found that in ligated kidneys from decorin-null mice, fibrillin-1 expression was initially enhanced to the same extent as in wild-type animals. However, long-term obstruction resulted in down-regulation of fibrillin-1 and concurrent cystic dilatation of Bowman's capsule in 33% of kidneys at 5 months after obstruction. In all of the genotypes, no differences in fibrillin-2 expression were observed. These in vivo data correlated with a significant induction of fibrillin-1 expression in renal fibroblasts and mesangial cells by recombinant biglycan and decorin. Our results indicate a novel role for decorin and biglycan during pressure-induced renal injury by stimulating fibrillin-1 expression.

A role for lysyl oxidase regulation in the control of normal collagen deposition in differentiating osteoblast cultures

Differentiation of phenotypically normal osteoblast cultures leads to formation of a bone-like extracellular matrix in vitro. Maximum collagen synthesis occurs early in the life of these cultures, whereas insoluble collagen deposition occurs later and is accompanied by a diminished rate of collagen synthesis. The mechanisms that control collagen deposition seem likely to include regulation of extracellular collagen biosynthetic enzymes, but expression patterns of these enzymes in differentiating osteoblasts has received little attention. The present study determined the regulation of lysyl oxidase as a function of differentiation of phenotypically normal murine MC3T3-E1 cells at the level of RNA and protein expression and enzyme activity. In addition, the regulation of BMP-1/mTLD mRNA levels that encodes procollagen C-proteinases was assayed. The role of lysyl oxidase in controlling insoluble collagen accumulation was further investigated in inhibition studies utilizing beta-aminopropionitrile, a specific inhibitor of lysyl oxidase enzyme activity. Results indicate that lysyl oxidase is regulated as a function of differentiation of MC3T3-E1 cells, and that the maximum increase in lysyl oxidase activity precedes the most efficient phase of insoluble collagen accumulation. By contrast BMP-1/mTLD is more constitutively expressed. Inhibition of lysyl oxidase in these cultures increases the accumulation of abnormal collagen fibrils, as determined by solubility studies and by electron microscopy. Taken together, these data support that regulation of lysyl oxidase activity plays a key role in the control of collagen deposition by osteoblast cultures.

Transient up-regulation of biglycan during skeletal muscle regeneration: delayed fiber growth along with decorin increase in biglycan-deficient mice

The onset and progression of skeletal muscle regeneration are controlled by a complex set of interactions between muscle precursor cells and their environment. Decorin is the main proteoglycan present in the extracellular matrix (ECM) of adult muscle while biglycan expression is lower, but both are increased in mdx mice dystrophic muscle. Both of these small leucine-rich proteoglycans (SLRPs) can bind other matrix proteins and to the three TGF-beta isoforms, acting as modulators of their biological activity. We evaluated biglycan and decorin expression in skeletal muscle during barium chloride-induced skeletal muscle regeneration in mice. A transient and dramatic up-regulation of biglycan was associated with newly formed myotubes, whereas decorin presented only minor variations. Studies both in vitro and in intact developing newborn mice showed that biglycan expression is initially high and then decreases during skeletal muscle differentiation and maturation. To further evaluate the role of biglycan during the regenerative process, skeletal muscle regeneration was studied in biglycan-null mice. Skeletal muscle maintains its regenerative capacity in the absence of biglycan, but a delay in regenerated fiber growth and a decreased expression of embryonic myosin were observed despite to normal expression of MyoD and myogenin. Transient up-regulation of decorin during muscle regeneration in these mice may possibly obscure further roles of SLRPs in this process.

Reduced type I collagen utilization: A pathogenic mechanism in COL5A1 haplo-insufficient Ehlers-Danlos syndrome

To examine mechanisms by which reduced type V collagen causes weakened connective tissues in the Ehlers-Danlos syndrome (EDS), we examined matrix deposition and collagen fibril morphology in long-term dermal fibroblast cultures. EDS cells with COL5A1 haplo-insufficiency deposited less than one-half of hydroxyproline as collagen compared to control fibroblasts, though total collagen synthesis rates are near-normal because type V collagen represents a small fraction of collagen synthesized. Cells from patients with osteogenesis imperfecta (OI) and haplo-insufficiency for proalpha1(I) chains of type I collagen also incorporated about one-half the collagen as controls, but this amount was proportional to their reduced rates of total collagen synthesis. Collagen fibril diameter was inversely proportional to type V/type I collagen ratios (EDS > control > OI). However, a reduction of type V collagen, in the EDS derived cells, was associated with the assembly of significantly fewer fibrils compared to control and OI cells. These data indicate that in cell culture, the quantity of collagen fibrils deposited in matrix is highly sensitive to reduction in type V collagen, far out of proportion to type V collagen's contribution to collagen mass.

A novel missense mutation in the galactosyltransferase-I (B4GALT7) gene in a family exhibiting facioskeletal anomalies and Ehlers-Danlos syndrome resembling the progeroid type

The Ehlers-Danlos syndrome (EDS) is a heterogeneous group of heritable connective tissue disorders characterized by skin hyperextensibility, joint hypermobility, and tissue fragility. Several genes have been implicated to result in EDS phenotypes. The progeroid type of EDS is characterized by wrinkled, loose skin on the face, curly fine hair, scanty eyebrows and eyelashes, in addition to the classical features of EDS. Here we describe two similarly affected individuals in two sibships of a large consanguineous family from Qatar. DNA samples from affected and unaffected members of the family were analyzed for homozygosity of polymorphic markers associated with genes that have been implicated in EDS. Among 28 markers analyzed, homozygosity was only observed for D5S469 and D5S2111, which were markers for galactosyltransferase-I (B4GALT7) located on chromosome 5q35.2, where the previously reported progeroid-like variant of EDS has been mapped. Exons harboring the coding regions and exon-intron junctions of B4GALT7 were amplified by PCR and examined for mutations. A homozygous misssense C to T substitution at nucleotide 808 in the coding region was discovered in both affected individuals. The carrier parents were heterozygous for this mutation, which was not found among 76 DNA samples from control individuals of the same ethnicity. Segregation of this novel mutation in the family further confirmed the allelic variant and its recessive mode of inheritance in this type of EDS. The C to T substitution results in an arginine to cysteine change at amino acid residue 270 that is located in the catalytically active extracellular C-terminal domain. This change could result in abnormal protein folding and/or aberrant interactions of mutated galactosyltransferase-I with other extracellular matrix proteins leading to the development of a progeroid-like phenotype in affected individuals.

Bone Morphogenetic Protein-1/Tolloid-like Proteinases Process Dentin Matrix Protein-1

Bone morphogenetic protein-1 (BMP-1)/Tolloid-like metalloproteinases play key roles in formation of mammalian extracellular matrix (ECM), through the biosynthetic conversion of precursor proteins into their mature functional forms. These proteinases probably play a further role in formation of bone through activation of transforming growth factor beta-like BMPs. Dentin matrix protein-1 (DMP1), deposited into the ECM during assembly and involved in initiating mineralization of bones and teeth, is thought to undergo proteolysis in vivo to generate functional cleavage fragments found in extracts of mineralized tissues. Here, we have generated recombinant DMP1 and demonstrate that it is cleaved, to varying extents, by all four mammalian BMP-1/Tolloid-like proteinases, to generate fragments similar in size to those previously isolated from bone. Consistent with possible roles for the BMP-1/Tolloid-like proteinases in the physiological processing of DMP1, NH2-terminal sequences of products generated by BMP-1 cleavage of DMP1 match those predicted from processing at the predicted DMP1 site that shows greatest cross-species conservation of sequences. Moreover, fibroblasts derived from mouse embryos homozygous null for genes encoding three of the four mammalian BMP-1/Tolloid-like proteinases appear to be deficient in processing of DMP1. Thus, a further role for BMP-1-Tolloid-like proteinases in formation of mineralized tissues is indicated, via proteolytic processing of DMP1.

Glycosaminoglycans and proteoglycans in the skin of aneuploid fetuses with increased nuchal translucency

BACKGROUND

First trimester increased fetal nuchal translucency is associated with fetal aneuploidies. One of the mechanisms of pathophysiology could be an abnormal extracellular matrix facilitating the formation of an interstitial edema. A previous study investigating interstitial edema in first trimester fetuses found large amounts of hyaluronan in the skin of fetuses with trisomy 21. The aim of this study was to establish distribution patterns for a number of other glycosaminoglycans-dermatan, heparan and keratan sulphate, chondroitin-6-sulphate and chondroitin-4-sulphate proteoglycan-in the nuchal skin of normal and chromosomally abnormal fetuses at 11-14 weeks. We also investigated whether biglycan (BGN), which is located on chromosome X, is underexpressed in fetuses with Turner syndrome. Decorin (DCN), a similar-sized proteoglycan located on chromosome 12, was taken as a control.

METHODS

We studied the distribution and concentration of various extacellular matrix components using immunohistochemistry, a double staining technique, in-situ hybridization, Northern and Western blot analysis.

RESULTS

Chondroitin-6-sulphate and chondroitin-4-sulphate proteoglycan were increased in Turner syndrome fetuses and BGN seemed to be underexpressed compared with normal controls, while DCN was not. Dermatan, heparan and keratan sulphate showed no significant abnormal distribution in trisomies 21, 18, 13, or in Turner syndrome, compared with normal. Western and immunohistochemical analysis revealed that absence of a second X chromosome, as is the case in Turner syndrome, affects BGN protein pattern.

CONCLUSIONS

An abnormal amount of glycosaminoglycans and proteoglycans presumably contributes to increased nuchal translucency.

Biglycan deficiency interferes with ovariectomy-induced bone loss

Biglycan is a matrix proteoglycan with a possible role in bone turnover. In a 4-week study with sham-operated or OVX biglycan-deficient or wildtype mice, we show that biglycan-deficient mice are resistant to OVX-induced trabecular bone loss and that there is a gender difference in the response to biglycan deficiency.

INTRODUCTION

Biglycan (bgn) is a small extracellular matrix proteoglycan enriched in skeletal tissues, and biglycan-deficient male mice have decreased trabecular bone mass and bone strength. The purpose of this study was to investigate the bone phenotype of the biglycan-deficient female mice and to investigate the effect of estrogen depletion by ovariectomy (OVX).

MATERIALS AND METHODS

OVX or sham operations were performed on 21-week-old mice that were divided into four groups: wt sham (n = 7), wt OVX (n = 9), bgn-deficient sham (n = 10) and bgn-deficient OVX (n = 10). The mice were killed 4 weeks after surgery. Bone mass and bone turnover were analyzed by peripheral quantitative computed tomography (pQCT), biochemical markers, and histomorphometry.

RESULTS AND CONCLUSIONS

In contrast to the male mice, there were only few effects of bgn deficiency on bone metabolism in female mice, showing a clear gender difference. However, when stressed by OVX, the female bgn knockout (KO) mice were resistant to the OVX-induced trabecular bone loss. The wt mice showed a decrease in trabecular bone mineral density by pQCT measurements, a decrease in trabecular bone volume (BV/TV), and an increase in mineral apposition rate. In contrast, no significant changes were detected in bgn KO mice after OVX. In addition, analysis of the bone resorption marker deoxypyridinoline showed no significant increase in the bgn KO OVX mice compared with bgn KO sham mice. Measurements of serum osteoprotegerin (OPG) and RANKL revealed increased levels of OPG and decreased levels of RANKL in the bgn KO mice compared with wt mice. In conclusion, the bgn deficiency protects against increased trabecular bone turnover and bone loss in response to estrogen depletion, supporting the concept that bgn has dual roles in bone, where it may modulate both formation and resorption ultimately influencing the bone turnover process.

Podocan, a novel small leucine-rich repeat protein expressed in the sclerotic glomerular lesion of experimental HIV-associated nephropathy

Growing evidence suggests that human immunodeficiency virus (HIV)-1 infection of podocytes plays a central role in the glomerular disease of HIV-associated nephropathy (HIVAN). As an approach to identify host genes involved in the pathogenesis of the sclerotic glomerular lesion in HIVAN, representational difference analysis of cDNA was used to identify differentially expressed genes in HIV-1 transgenic and nontransgenic podocytes. We isolated a novel member of the small leucine-rich repeat (SLR) protein family, podocan, that is expressed at high levels in the HIV-1 transgenic podocytes. In normal embryonic kidney, a 3.2-kb podocan transcript was detected at low levels, and expression increased dramatically within 24 h following birth. Expression of a 2.3-kb transcript became evident after birth and gradually increased to 50% of the total podocan RNA in the mature kidney. Phylogenetically, podocan represents a new class in the SLR protein gene family, an expanding protein family sharing homology with the small leucine-rich repeat proteoglycans. The 3.2-kb transcript encodes a predicted 611-amino acid secretory protein with 20 leucine-rich repeats, a unique N-terminal cysteine-rich cluster pattern and a highly acidic C-terminal domain. In situ hybridization of normal kidney revealed podocan mRNA expression in podocytes and likely vascular endothelial cells within the glomerulus. The immunohistochemical staining pattern of podocan protein in normal kidney glomeruli was consistent with that of the glomerular basement membrane, and staining was markedly increased in sclerotic glomerular lesions in the transgenic HIVAN model. Thus, podocan defines a new class within the SLR protein family and is a previously unrecognized component of the sclerotic glomerular lesion that develops in the course of experimental HIVAN.

Building bone tissue: matrices and scaffolds in physiology and biotechnology

Deposition of bone in physiology involves timed secretion, deposition and removal of a complex array of extracellular matrix proteins which appear in a defined temporal and spatial sequence. Mineralization itself plays a role in dictating and spatially orienting the deposition of matrix. Many aspects of the physiological process are recapitulated in systems of autologous or xenogeneic transplantation of osteogenic precursor cells developed for tissue engineering or modeling. For example, deposition of bone sialoprotein, a member of the small integrin-binding ligand, N-linked glycoprotein family, represents the first step of bone formation in ectopic transplantation systems in vivo. The use of mineralized scaffolds for guiding bone tissue engineering has revealed unexpected manners in which the scaffold and cells interact with each other, so that a complex interplay of integration and disintegration of the scaffold ultimately results in efficient and desirable, although unpredictable, effects. Likewise, the manner in which biomaterial scaffolds are "resorbed" by osteoclasts in vitro and in vivo highlights more complex scenarios than predicted from knowledge of physiological bone resorption per se. Investigation of novel biomaterials for bone engineering represents an essential area for the design of tissue engineering strategies.

Tenascins: regulation and putative functions during pathological stress

In this review, we discuss the structure and function of the extracellular matrix protein family of tenascins with emphasis on their involvement in human pathologies. The article is divided into the following sections:

INTRODUCTION

the tenascin family of extracellular matrix proteins; Structural roles: tenascin-X deficiency and Ehlers-Danlos syndrome; Tenascins as modulators of cell adhesion, migration, and growth; Role of tenascin-C in inflammation; Regulation of tenascins by mechanical stress: implications for wound healing and regeneration; Association of tenascin-C with cancer: antibodies as diagnostic and therapeutic tools; Conclusion and perspectives.

Decorin modulates matrix mineralization in vitro

Decorin (DCN), a member of small leucine-rich proteoglycans, is known to modulate collagen fibrillogenesis. In order to investigate the potential roles of DCN in collagen matrix mineralization, several stable osteoblastic cell clones expressing higher (sense-DCN, S-DCN) and lower (antisense-DCN, As-DCN) levels of DCN were generated and the mineralized nodules formed by these clones were characterized. In comparison with control cells, the onset of mineralization by S-DCN clones was significantly delayed; whereas it was markedly accelerated and the number of mineralized nodules was significantly increased in As-DCN clones. The timing of mineralization was inversely correlated with the level of DCN synthesis. In these clones, the patterns of cell proliferation and differentiation appeared unaffected. These results suggest that DCN may act as an inhibitor of collagen matrix mineralization, thus modulating the timing of matrix mineralization.

Phenotype discovery by gene expression profiling: mapping of biological processes linked to BMP-2-mediated osteoblast differentiation

Understanding physiological control of osteoblast differentiation necessitates characterization of the regulatory signals that initiate the events directing a cell to lineage commitment and establishing competency for bone formation. The bone morphogenetic protein, BMP-2, a member of the TGFbeta superfamily, induces osteoblast differentiation and functions through the Smad signal transduction pathway during in vivo bone formation. However, the molecular targets of BMP-mediated gene transcription during the process of osteoblast differentiation have not been comprehensively identified. In the present study, BMP-2 responsive factors involved in the early stages of commitment and differentiation to the osteoblast phenotype were analyzed by microarray gene expression profiling in samples ranging from 1 to 24 h following BMP-2 dependent differentiation of C2C12 premyoblasts into the osteogenic lineage. A total of 1,800 genes were responsive to BMP-2 and expression was modulated from 3- to 14-fold for less than 100 genes during the time course. Approximately 50% of these 100 genes are either up- or downregulated. Major events associated with phenotypic changes towards the osteogenic lineage were identified from hierarchical and functional clustering analyses. BMP-2 immediately responsive genes (1-4 h), which exhibited either transient or sustained expression, reflect activation and repression of non-osseous BMP-2 developmental systems. This initial response was followed by waves of expression of nuclear proteins and developmental regulatory factors including inhibitors of DNA binding, Runx2, C/EBP, Zn finger binding proteins, forkhead, and numerous homeobox proteins (e.g., CDP/cut, paired, distaless, Hox) which are expressed at characterized stages during osteoblast differentiation. A sequential profile of genes mediating changes in cell morphology, cell growth, and basement membrane formation is observed as a secondary transient early response (2-8 h). Commitment to the osteogenic phenotype is recognized by 8 h, reflected by downregulation of most myogenic-related genes and induction of a spectrum of signaling proteins and enzymes facilitating synthesis and assembly of an extracellular skeletal environment. These genes included collagens Type I and VI and the small leucine rich repeat family of proteoglycans (e.g., decorin, biglycan, osteomodulin, fibromodulin, and osteoadherin/osteoglycin) that reached peak expression at 24 h. With extracellular matrix development, the bone phenotype was further established from 16 to 24 h by induction of genes for cell adhesion and communication and enzymes that organize the bone ECM. Our microarray analysis resulted in the discovery of a class of genes, initially described in relation to differentiation of astrocytes and oligodendrocytes that are functionally coupled to signals for cellular extensions. They include nexin, neuropilin, latexin, neuroglian, neuron specific gene 1, and Ulip; suggesting novel roles for these genes in the bone microenvironment. This global analysis identified a multistage molecular and cellular cascade that supports BMP-2-mediated osteoblast differentiation.

Decorin binds fibrinogen in a Zn2+-dependent interaction

We have previously shown that decorin, a member of the small leucine-rich proteoglycan family of extracellular matrix proteoglycans/glycoproteins is a Zn(2+) metalloprotein at physiological Zn(2+) concentrations (Yang, V. W-C., LaBrenz, S. R., Rosenberg, L. C., McQuillan, D., and Höök, M. (1999) J. Biol. Chem. 274, 12454-12460). We now report that the decorin proteoglycan binds fibrinogen in the presence of Zn(2+). The fibrinogen-binding site is located in the N-terminal domain of the decorin core protein and a 45-amino acid peptide representing this domain binds to the fibrinogen D fragment with an apparent K(D) of 1.7 x 10(-6) m, as determined from fluorescence polarization data. Furthermore, we show that Zn(2+) promotes the self-association of decorin. The N-terminal domain of the core protein also mediates this activity. The results of solid-phase binding assays and gel filtration chromatography suggest that the N-terminal domain of decorin, when present at low micromolar concentrations, forms an oligomer in a Zn(2+)-dependent manner. Thus, Zn(2+) appears to play a pivotal role in the interactions and biological function of decorin.

Bone matrix proteins: their function, regulation, and relationship to osteoporosis

Bone is a unique tissue composed of numerous cell types entombed within a mineralized matrix each with its own unique functions. While the majority of the matrix is composed of inorganic materials, study of the organic components has yielded most of the insights into the roles and regulation of cell and tissue specific functions. The goal of this review will be to describe some of the major known organic components of the bone matrix and discuss their functions as currently perceived. The potential usefulness of bone matrix protein assays for diagnosing the status of bone diseases and our current understanding of how these proteins could be related to diseases such as osteoporosis will also be reviewed.

Biglycan knockout mice: new models for musculoskeletal diseases

Biglycan is a Class I Small Leucine Rich Proteoglycans (SLRP) that is localized on human chromosome Xq28-ter. The conserved nature of its intron-exon structure and protein coding sequence compared to decorin (another Class I SLRP) indicates the two genes may have arisen from gene duplication. Biglycan contains two chondroitin sulfate glycosaminoglycan (GAG) chains attached near its NH(2) terminus making it different from decorin that has only one GAG chain. To determine the functions of biglycan in vivo, transgenic mice were developed that were deficient in the production of the protein (knockout). These mice acquire diminished bone mass progressively with age. Double tetracycline-calcein labeling revealed that the biglycan deficient mice are defective in their capacity to form bone. Based on this observation, we tested the hypothesis that the osteoporosis-like phenotype is due to defects in cells critical to the process of bone formation. Our data shows that biglycan deficient mice have diminished capacity to produce marrow stromal cells, the bone cell precursors, and that this deficiency increases with age. The cells also have reduced response to tranforming growth factor-beta (TGF-beta), reduced collagen synthesis and relatively more apoptosis than cells from normal littermates. In addition, calvaria cells isolated from biglycan deficient mice have reduced expression of late differentiation markers such as bone sialoprotein and osteocalcin and diminished ability to accumulate calcium judged by alizerin red staining. We propose that any one of these defects in osteogenic cells alone, or in combination, could contribute to the osteoporosis observed in the biglycan knockout mice. Other data suggests there is a functional relationship between biglycan and bone morphogenic protein-2/4 (BMP 2/4) action in controlling skeletal cell differentiation. In order to test the hypothesis that functional compensation can occur between SLRPs, we created mice deficient in biglycan and decorin. Decorin deficient mice have normal bone mass while the double biglycan/decorin knockout mice have more severe osteopenia than the single biglycan indicating redundancy in SLRP function in bone tissue. To further determine whether compensation could occur between different classes of SLRPs, mice were generated that are deficient in both biglycan (class I) and fibromodulin, a class II SLRP highly expressed in mineralizing tissue. These doubly deficient mice had an impaired gait, ectopic calcification of tendons and premature osteoarthritis. Transmission electron microscopy analysis showed that like the decorin and biglycan knockouts, they have severely disturbed collagen fibril structures. Biomechanical analysis of the affected tendons showed they were weaker compared to control animals leading to the conclusion that instability of the joints could be the primary cause of all the skeletal defects observed in the fibromodulin/biglycan knockout mice. These studies present important new animal models for musculoskeletal diseases and provide the opportunity to characterize the network of signals that control tissue integrity and function through SLRP activity.