Identification of positive and negative regulatory regions controlling expression of the cartilage matrix protein gene

The matrilins--adaptor proteins in the extracellular matrix

The matrilins form a four-member family of modular, multisubunit matrix proteins, which are expressed in cartilage but also in many other forms of extracellular matrix. They participate in the formation of fibrillar or filamentous structures and are often associated with collagens. It appears that they mediate interactions between collagen-containing fibrils and other matrix constituents, such as aggrecan. This adaptor function may be modulated by physiological proteolysis that causes the loss of single subunits and thereby a decrease in binding avidity. Attempts to study matrilin function by gene inactivation in mouse have been frustrating and so far not yielded pronounced phenotypes, presumably because of the extensive redundancy within the family allowing compensation by one family member for another. However, mutations in matrilin-3 in humans cause different forms of chondrodysplasias and perhaps also hand osteoarthritis. As loss of matrilin-3 is not critical in mouse, these phenotypes are likely to be caused by dominant negative effects.

Functional analysis of the regulatory regions of the matrilin-1 gene in transgenic mice reveals modular arrangement of tissue-specific control elements

Matrilin-1 is a non-collagenous protein, which functions in the organization of the extracellular matrix by forming collagen-dependent and -independent filamentous networks. It is secreted primarily by chondrocytes in a characteristic spatial, temporal and developmental stage-specific pattern during skeletogenesis. As a first step to define the tissue- and site-specific regulatory regions of the chicken matrilin-1 gene in vivo, we generated transgenic mice harboring various promoter and intronic fragments fused to the LacZ reporter gene. Histological analysis of the transgene expression pattern during ontogenic development revealed specific X-gal staining in most primordial elements of endochondral bones of transgenic mouse lines carrying either the long promoter between -2011 and +67 or the intronic fragment with a short promoter between -338 and +1819. The cartilage-specific activity of the latter transgene, however, was accompanied with variable ectopic expression pattern in neural and other tissues depending on the site of integration. The presence of both promoter upstream and intronic elements was necessary for the high level transgene activity in all chondrogenic tissues and for the extraskeletal transgene expression pattern resembling the most to that of the chicken matrilin-1 gene, e.g. expression in the eye, and lack of expression in the diminishing notochord and nucleus pulposus. The activity of the transgenes was restricted to the columnar proliferating and pre-hypertrophic chondrocytes visualized by BrdU incorporation and distribution of phosphorylated Sox9, respectively. DNA elements between -2011 and -338 also mediated ectopic LacZ expression in cells of neural crest origin. These results suggest that an interplay of modularly arranged cartilage- and neural crest-specific DNA elements control the expression of the matrilin-1 gene. The dispersal of cartilage-specific elements in the promoter upstream and intronic regions shows similarity to the transcriptional regulation of the Col11a2 gene.

Genomic organization of the human chondromodulin-1 gene containing a promoter region that confers the expression of reporter gene in chondrogenic ATDC5 cells

Chondromodulin-1 (ChM-1) is a cartilage-specific glycoprotein that stimulates the growth of chondrocytes and inhibits the tube formation of endothelial cells. To clarify the tissue-specific expression and the role of ChM-1 in pathophysiological conditions, we analyzed the structure of the human ChM-1 gene and its promoter. On the screening of a human genomic cosmid library using the human ChM-1 complimentary DNA (cDNA) as a probe, two clones were obtained that contained ChM-1 cDNA. The restriction enzyme map and nucleotide sequence revealed the human ChM-1 gene consisting of seven exons and exon-intron boundaries. The human ChM-1 gene was assigned to chromosome 13q14-21 by fluorescence in situ hybridization (FISH) using the clone as a probe. A primer extension analysis using total RNA extracted from human cartilage revealed a major transcription start site with the sequence CGCT+1GG. The region approximately 3-kilobase (kb) nucleotides upstream of the translation start site was then sequenced and analyzed in terms of promoter activity. We found that a region 446 base pairs (bp) upstream of the start site had promoter activity in COS7, HeLa, and ATDC5 cells. In structure the promoter is a TATA-less type without a GC-rich region. The transcription factors Sox9, Og12, and Cart-1 did not affect the promoter activity. The transcription factor Ying-Yang1 suppressed the promoter activity but GABP protein did not change the promoter activity. The construct containing -446/+87 fused to the SV40 enhancer and green fluorescent protein (GFP) exhibited expression of GFP corresponding to the differentiation of ATDC5 cells to mature chondrocytes. These results suggest that the element -446/+87 confers the cartilage-specific expression of this gene by some factor(s) other than Sox9, Og12, and Cart-1.

The matrilins: a novel family of oligomeric extracellular matrix proteins

The matrilin family at present has four members that all share a structure made up of von Willebrand factor A domains, epidermal growth factor-like domains and a coiled coil alpha-helical module. The first member of the family, matrilin-1 (previously called cartilage matrix protein or CMP), is expressed mainly in cartilage. Matrilin-3 has a similar tissue distribution, while matrilin-2 and -4 occur in a wide variety of extracellular matrices. Matrilin-1 is associated with cartilage proteoglycans as well as being a component of both collagen-dependent and collagen-independent fibrils and on the basis of the related structures other matrilins may play similar roles. The matrilin genes are strictly and differently regulated and their expression may serve as markers for cellular differentiation.

Sequence, structure and chromosomal localization of Crtm gene encoding mouse cartilage matrix protein and its exclusion as a candidate for murine achondroplasia

The mouse cartilage matrix protein gene (Crtm) was isolated from a cosmid library using a mouse Crtm cDNA fragment as probe. Crtm spans 12.2 kb from the start of translation to the polyadenylation signal sequence and comprises eight exons. Sequencing of the 1.9 kb 5' flanking region revealed a TATA-like box 72 bp upstream from the initiator Met codon as well as several cis-acting motifs known to bind eukaryotic transcription factors. Analysis of the exon-intron junctions demonstrated that the last intron does not follow the gt/ag rule but belongs to the minor class of pre-mRNA introns that contain "at" and "ac" at their 5'and 3' ends, respectively. Single-strand conformation polymorphism analysis was used to map Crtm to the distal part of chromosome 4 between the microsatellite markers D4Mit16 and D4Mit339. Achodroplasia (cn), a recessive skeletal disorder in mice, has already been mapped to this region. Immunostaining for CMP and sequence of Crtm in cn/cn mice failed to reveal any disease-specific mutations, suggesting that mutations in Crtm do not cause achondroplasia.

Enhancement of cartilage matrix protein synthesis in arthritic cartilage

OBJECTIVE

To investigate whether the synthesis of cartilage matrix protein (CMP) is enhanced in arthritic cartilage.

METHODS

The content of CMP in human and pig cartilage was determined by immunoblotting, and CMP-producing chondrocytes in osteoarthritic (OA) and rheumatoid arthritic (RA) joints were immunostained.

RESULTS

CMP was undetectable in the condylar cartilage and disc of pigs, whereas it was abundant in the rib and tracheal cartilage of the same animals. By immunohistochemical analysis, CMP was localized in only a few chondrocytes (5%) in normal human joints, whereas numerous chondrocytes (>60%) were immunostained in RA joints. The number of CMP-producing cells was also increased in OA cartilage (>40%). Immunoblotting analyses confirmed that the CMP content in the cartilage from OA and RA patients was much higher than that in normal cartilage.

CONCLUSION

These findings demonstrate that articular chondrocytes can synthesize CMP, although it is suppressed under physiologic conditions. The results also suggest that articular chondrocytes express CMP in response to arthritic stimuli.

Repression of platelet-derived growth factor A-chain gene transcription by an upstream silencer element. Participation by sequence-specific single-stranded DNA-binding proteins

Platelet-derived growth factor A-chain is a potent mitogen expressed in a restricted number of normal and transformed cells. Transient transfection and deletion analysis in BSC-1 (African green monkey, renal epithelial) cells revealed that the -1680 to -1374 region of the A-chain gene repressed homologous and heterologous promoter activities by 60-80%. An S1 nuclease-hypersensitive region (5'SHS) was identified within this region (-1418 to -1388) that exhibited transcriptional silencer activity in BSC-1 and a variety of human tumor cell lines (U87, HepG2, and HeLa). Electrophoretic mobility shift assays conducted with 5'SHS oligodeoxynucleotide probes revealed several binding protein complexes that displayed unique preferences for binding to sense, antisense, and double-stranded forms of the element. Southwestern blot analysis revealed that the antisense strand of 5'SHS binds to nuclear proteins of molecular mass 97, 87, 44, and 17 kDa, whereas the double-stranded form of 5'SHS is recognized by a 70-kDa factor. Mutations within 5'SHS element indicated the necessity of a central 5'-GGGGAGGGGG-3' motif for protein binding and silencer function, while nucleotides flanking both sides of the motif were also critical for repression. These results support a model in which silencer function of 5'SHS is mediated by antisense strand binding proteins, possibly by stabilizing single-stranded DNA conformations required for interaction with enhancer sequences in the proximal promoter region of the A-chain gene.

Cloning, sequencing and expression analysis of mouse cartilage matrix protein cDNA

A cDNA encoding the mouse cartilage matrix protein (CMP) was cloned following the reverse-transcription polymerase chain reaction and rapid amplification of cDNA ends procedures using mRNA isolated from trachea. The open reading frame encodes a product of 500 amino acids. Large parts of the protein have been completely conserved when compared to chicken and human sequences, including all 12 cysteine residues of the mature CMP. In situ hybridization reveals an even distribution of the CMP mRNA in the developing skeleton, which is followed by a zonal distribution paralleling hypertrophy and calcification. From early cartilage differentiation and onwards, CMP transcript is absent in the forming articular surfaces and intervertebral discs. Extraskeletal expression of CMP mRNA was detected in the adult eye.

Identification of a nuclear factor-I family protein-binding site in the silencer region of the cartilage matrix protein gene

Cartilage matrix protein (CMP) is synthesized by chondrocytes in a developmentally regulated manner. Here we have dissected promoter upstream elements involved in its transcriptional regulation. We show that although the 79-base pair CMP minimal promoter is promiscuous, 1137 base pairs of 5'-flanking region are capable of directing tissue- and developmental stage-specific transcription when fused to a reporter gene. This results from two positive control regions which, in proliferating chondrocytes, relieve the repression mediated by two non-tissue-specific negative control regions. Characterization of the promoter proximal silencer by DNase I footprinting and gel shifts revealed the presence of two elements, SI and SII, which bound mesenchymal cell proteins. Methylation interference analysis indicated a gapped palindromic binding site similar to nuclear factor I (NF-I) family proteins within SI, but only a half-site within SII. Gel shift assays with specific NF-I and mutated SI competitors, binding of recombinant NF-I, as well as supershift analysis with NF-I-specific antiserum verified the binding of NF-I family proteins to the SI element. Double-stranded SI and SII oligonucleotides inserted in single copy in either orientation were found to repress both homologous and heterologous promoters upon transfection into mesenchymal cells. Transcriptional repression also occurred when a consensus NF-I site itself was fused to the CMP minimal promoter. We conclude that NF-I-related protein(s) can mediate transcriptional repression in cells of mesenchymal origin.

The zonal expression of chicken cartilage matrix protein gene in the developing skeleton of transgenic mice

Cartilage matrix protein (CMP) is a major noncollagenous glycoprotein of hyaline cartilage with a molecular mass of about 148 kDa. It has been proposed to be involved in matrix organization by its interactions with proteoglycan and type II collagen. The 54-kDa monomers form homotrimers stabilized by disulfide bonds. The gene for chicken cartilage matrix protein was isolated, and its regulation has been studied recently in transient expression experiments. To learn more about the spatial and temporal expression of the gene during ontogenic development, we created transgenic mice via microinjection of a 21.8-kb genomic fragment, encoding the chicken cartilage matrix protein. None of the founder animals exhibited any abnormal phenotype. The developmental stage-specific expression of the transgene was examined by immunostaining with a chicken CMP specific antiserum at different stages of embryonic development in cartilage from different sources: lower and upper limb, vertebrae, ribs and nasal septum. The level of transgene expression showed marked differences in various zones of cartilage. Briefly, high levels were found in the zones of proliferating chondrocytes, while little if any transgene product was detected in the very early and hypertrophic stage of chondrogenesis. The expression pattern of the transgene correlated with the endogenous mouse CMP and did not cause any morphological changes detectable by microscopic analysis of cartilage. These data indicate that the injected CMP gene with its flanking sequences contained all the information necessary for cell type-specific expression in transgenic mice.

Negative regulation of transcription in eukaryotes

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