The partial amino acid sequence of bovine cartilage proteoglycan, deduced from a cDNA clone, contains numerous Ser-Gly sequences arranged in homologous repeats

The Unifying Hypothesis of Alzheimer's Disease: Heparan Sulfate Proteoglycans/Glycosaminoglycans Are Key as First Hypothesized Over 30 Years Ago

The updated "Unifying Hypothesis of Alzheimer's disease" (AD) is described that links all the observed neuropathology in AD brain (i.e., plaques, tangles, and cerebrovascular amyloid deposits), as well as inflammation, genetic factors (involving ApoE), "AD-in-a-Dish" studies, beta-amyloid protein (Aβ) as a microbial peptide; and theories that bacteria, gut microflora, gingivitis and viruses all play a role in the cause of AD. The common link is the early accumulation of heparan sulfate proteoglycans (HSPGs) and heparan sulfate glycosaminoglycans (GAGs). HS GAG accumulation and/or decreased HS GAG degradation is postulated to be the key initiating event. HS GAGs and highly sulfated macromolecules induce Aβ 1-40 (but not 1-42) to form spherical congophilic maltese-cross star-like amyloid core deposits identical to those in the AD brain. Heparin/HS also induces tau protein to form paired helical filaments (PHFs). Increased sulfation and/or decreased degradation of HSPGs and HS GAGs that occur due to brain aging leads to the formation of plaques and tangles in AD brain. Knockout of HS genes markedly reduce the accumulation of Aβ fibrils in the brain demonstrating that HS GAGs are key. Bacteria and viruses all use cell surface HS GAGs for entry into cells, including SARS-CoV-2. Bacteria and viruses cause HS GAGs to rapidly increase to cause near-immediate aggregation of Aβ fibrils. "AD-in-a-dish" studies use "Matrigel" as the underlying scaffold that spontaneously causes plaque, and then tangle formation in a dish. Matrigel mostly contains large amounts of perlecan, the same specific HSPG implicated in AD and amyloid disorders. Mucopolysaccharidoses caused by lack of specific HS GAG enzymes lead to massive accumulation of HS in lysosomal compartments in neurons and contribute to cognitive impairment in children. Neurons full of HS demonstrate marked accumulation and fibrillization of Aβ, tau, α-synuclein, and prion protein (PrP) in mucopolysaccharidosis animal models demonstrating that HS GAG accumulation is a precursor to Aβ accumulation in neurons. Brain aging leads to changes in HSPGs, including newly identified splice variants leading to increased HS GAG sulfation in the AD brain. All of these events lead to the new "Unifying Hypothesis of Alzheimer's disease" that further implicates HSPGs /HS GAGs as key (as first hypothesized by Snow and Wight in 1989).

The different roles of aggrecan interaction domains

The aggregating proteoglycans of the lectican family are important components of extracellular matrices. Aggrecan is the most well studied of these and is central to cartilage biomechanical properties and skeletal development. Key to its biological function is the fixed charge of the many glycosaminoglycan chains, that provide the basis for the viscoelastic properties necessary for load distribution over the articular surface. This review is focused on the globular domains of aggrecan and their role in anchoring the proteoglycans to other extracellular matrix components. The N-terminal G1 domain is vital in that it binds the proteoglycan to hyaluronan in ternary complex with link protein, retaining the proteoglycan in the tissue. The importance of the C-terminal G3 domain interactions has recently been emphasized by two different human hereditary disorders: autosomal recessive aggrecan-type spondyloepimetaphyseal dysplasia and autosomal dominant familial osteochondritis dissecans. In these two conditions, different missense mutations in the aggrecan C-type lectin repeat have been described. The resulting amino acid replacements affect the ligand interactions of the G3 domain, albeit with widely different phenotypic outcomes.

Single-molecule force spectroscopy of cartilage aggrecan self-adhesion

We investigated self-adhesion between highly negatively charged aggrecan macromolecules extracted from bovine cartilage extracellular matrix by performing atomic force microscopy (AFM) imaging and single-molecule force spectroscopy (SMFS) in saline solutions. By controlling the density of aggrecan molecules on both the gold substrate and the gold-coated tip surface at submonolayer densities, we were able to detect and quantify the Ca(2+)-dependent homodimeric interaction between individual aggrecan molecules at the single-molecule level. We found a typical nonlinear sawtooth profile in the AFM force-versus-distance curves with a molecular persistence length of l(p) = 0.31 ± 0.04 nm. This is attributed to the stepwise dissociation of individual glycosaminoglycan (GAG) side chains in aggrecans, which is very similar to the known force fingerprints of other cell adhesion proteoglycan systems. After studying the GAG-GAG dissociation in a dynamic, loading-rate-dependent manner (dynamic SMFS) and analyzing the data according to the stochastic Bell-Evans model for a thermally activated decay of a metastable state under an external force, we estimated for the single glycan interaction a mean lifetime of τ = 7.9 ± 4.9 s and a reaction bond length of x(β) = 0.31 ± 0.08 nm. Whereas the x(β)-value compares well with values from other cell adhesion carbohydrate recognition motifs in evolutionary distant marine sponge proteoglycans, the rather short GAG interaction lifetime reflects high intermolecular dynamics within aggrecan complexes, which may be relevant for the viscoelastic properties of cartilage tissue.

Bulldog dwarfism in Dexter cattle is caused by mutations in ACAN

Bulldog dwarfism in Dexter cattle is one of the earliest single-locus disorders described in animals. Affected fetuses display extreme disproportionate dwarfism, reflecting abnormal cartilage development (chondrodysplasia). Typically, they die around the seventh month of gestation, precipitating a natural abortion. Heterozygotes show a milder form of dwarfism, most noticeably having shorter legs. Homozygosity mapping in candidate regions in a small Dexter pedigree suggested aggrecan (ACAN) as the most likely candidate gene. Mutation screening revealed a 4-bp insertion in exon 11 (2266_2267insGGCA) (called BD1 for diagnostic testing) and a second, rarer transition in exon 1 (-198C>T) (called BD2) that cosegregate with the disorder. In chondrocytes from cattle heterozygous for the insertion, mutant mRNA is subject to nonsense-mediated decay, showing only 8% of normal expression. Genotyping in Dexter families throughout the world shows a one-to-one correspondence between genotype and phenotype at this locus. The heterozygous and homozygous-affected Dexter cattle could prove invaluable as a model for human disorders caused by mutations in ACAN.

Domain organization, genomic structure, evolution, and regulation of expression of the aggrecan gene family

Proteoglycans are complex macromolecules, consisting of a polypeptide backbone to which are covalently attached one or more glycosaminoglycan chains. Molecular cloning has allowed identification of the genes encoding the core proteins of various proteoglycans, leading to a better understanding of the diversity of proteoglycan structure and function, as well as to the evolution of a classification of proteoglycans on the basis of emerging gene families that encode the different core proteins. One such family includes several proteoglycans that have been grouped with aggrecan, the large aggregating chondroitin sulfate proteoglycan of cartilage, based on a high number of sequence similarities within the N- and C-terminal domains. Thus far these proteoglycans include versican, neurocan, and brevican. It is now apparent that these proteins, as a group, are truly a gene family with shared structural motifs on the protein and nucleotide (mRNA) levels, and with nearly identical genomic organizations. Clearly a common ancestral origin is indicated for the members of the aggrecan family of proteoglycans. However, differing patterns of amplification and divergence have also occurred within certain exons across species and family members, leading to the class-characteristic protein motifs in the central carbohydrate-rich region exclusively. Thus the overall domain organization strongly suggests that sequence conservation in the terminal globular domains underlies common functions, whereas differences in the central portions of the genes account for functional specialization among the members of this gene family.

Molecular cloning and ultrastructural localization of the core protein of an eggshell matrix proteoglycan, ovocleidin-116

The role of avian eggshell matrix proteins in shell formation is poorly understood. This calcitic biomaterial forms in a uterine fluid where the protein composition varies during the initial, calcification, and terminal phases of eggshell deposition. A specific antibody was raised to a 116-kDa protein, which is most abundant in uterine fluid during active eggshell calcification. This antiserum was used to expression screen a bacteriophage cDNA library prepared using mRNA extracted from pooled uterine tissue harvested at the midpoint of eggshell calcification. Plasmids containing inserts of differing 5'-lengths were isolated with a maximum cDNA sequence of 2.4 kilobases. Northern blotting and reverse transcriptase-polymerase chain reaction demonstrated that the 2. 35-kilobase message was expressed in a uterine-specific manner. The hypothetical translational product from the open reading frame corresponded to a novel 80-kDa protein, which we have named ovocleidin-116. After removal of the predicted signal peptide, its N-terminal sequence corresponded almost exactly with that determined from direct microsequencing of the 116-kDa uterine protein (this work) and with that previously determined for the core protein of a 120-kDa eggshell dermatan sulfate proteoglycan (Corrino, D. A., Rodriguez, J. P., and Caplan, A. I. (1997) Connect. Tissue Res. 36, 175-193). Ultrastructural colloidal gold immunocytochemistry of ovocleidin-116 demonstrated its presence in the organic matrix, in small vesicles found throughout the mineralized palisade layer, and the calcium reserve assembly of the mammillary layer. Ovocleidin-116 thus is a candidate molecule for the regulation of calcite growth during eggshell calcification.

Expression of cartilage extracellular matrix and potential regulatory genes in a new human chondrosarcoma cell line

A human chondrosarcoma cell line has been established from an aggressive chondrosarcoma. The cells grow in a monolayer culture (doubling time: 2 days) and form aggregates. The aggregates consist of a rim of cells surrounding a hollow core. The cell line exhibits a unique pattern of mRNA expression with several molecules characteristic of the chondrocyte phenotype. Consistent with the chondrocyte phenotype, mRNAs encoding types IX and XI collagens were present along with an abundant expression of mRNAs encoding the core protein of the cartilage proteoglycans biglycan and aggrecan. No expression of mRNAs encoding types I or II fibrillar collagens or the proteoglycan decorin was observed. Sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis of [35S]sulfate-radiolabeled material confirmed the translation of proteoglycans containing glycosaminoglycan chains. The expression of molecules that contribute to cartilage development and tumorigenesis was examined. The cell line produces abundant mRNA that encodes transforming growth factor-beta1, a member of a family of cartilage and bone inductive proteins. The expression of mRNA encoding two proteins associated specifically with chondrogenesis was detected: Cart-1, a homeobox protein involved in cartilage differentiation, and CD-RAP, a secreted molecule restricted under normal conditions to differentiating chondrocytes and cartilage. Overexpression of p53, a tumor-suppressor gene, was detected. DNA analysis revealed a loss of heterozygosity at the chromosomal locus encoding p53, with the deletion of one p53 allele and the mutation of the remaining allele in both the parent tumor and the cell line. The malignant chondrosarcoma phenotype may be related to the unique gene expression pattern that is characteristic in many ways of differentiating chondroblasts, as well as to the inactivation of the p53 function that could contribute to the proliferative capacity of the cell line. This cell line may serve as a biological model for further investigation of the etiology of human chondrosarcomas and for the synthesis and regulation of cartilage-specific genes.

Characterization of aggrecan retained and lost from the extracellular matrix of articular cartilage. Involvement of carboxyl-terminal processing in the catabolism of aggrecan

The catabolism of aggrecan in bovine articular cartilage explants is characterized by the release into the culture medium of high molecular weight aggrecan fragments, generated by the proteolytic cleavage of the core protein between residues Glu373 and Ala374 within the interglobular domain. In this study, the position of the carboxyl-terminus of these aggrecan fragments, as well as a major proteolytically shortened aggrecan core protein present in cartilage matrix, have been deduced by characterizing the peptides generated by the reaction of aggrecan core protein peptides with cyanogen bromide. It was shown that two out of three such peptide fragments having an amino terminus starting at Ala374 have their carboxyl terminus located within the chondroitin sulfate 1 domain. The third and largest aggrecan core protein peptide, with an amino terminus starting at Ala374, has a carboxyl terminus in a region of core protein between the chondroitin sulfate 1 domain and the chondroitin sulfate 2 domain. The carboxyl terminus of this peptide appeared to be the same as that of the proteolytically degraded aggrecan core protein, which is retained within the extracellular matrix of the tissue. Another two aggrecan fragments recovered from the medium of explant cultures with amino-terminal sequences in the chondroitin sulfate 2 domain at Ala1772 and Leu1872 were shown to have their carboxyl termini within the G3 globular domain. These results suggest that the catabolism of aggrecan between residues Glu373 and Ala374 in the interglobular domain by the putative proteinase, aggrecanase, may be dependent on prior proteolytic processing within the carboxyl-terminal region of the core protein.

Complete coding sequence of bovine aggrecan: comparative structural analysis

The previously available sequence for bovine aggrecan included only the KS domain, the C-terminal portion of the CS-2 domain, and the entire CS-3 and G3 domains. We have isolated cDNA clones for previously uncharacterized portions of the bovine aggrecan sequence, and, when we combined them with previously published incomplete sequences, have obtained a complete sequence for the entire core protein. The bovine aggrecan sequence, which is a composite of new sequence data and previously published incomplete sequences, is 2327 residues in length. Although there is significant conservation of G1, G2, and G3 globular domains between species, there are differences in the length of the interglobular domain, in the number of KS domain hexapeptide repeats and CS domain repeats, and in alternative splicing within the G3 domain. The bovine aggrecan KS domain contains 24 repeats of a hexapeptide motif. The largely uncharacterized CS-1 domain of bovine aggrecan was found to contain 27 variable repeats of a 21-residue consensus sequence. A notable feature of the bovine CS-1 domain is in the distribution of single Ser-Gly dipeptides, the majority of which are separated by 7 or 8 amino acids, compared to the human, where discrete pairs of Ser-Gly dipeptides are separated by 13 amino acids. The CS-2 domain contains a total of six "homology domains" with 4 complete and 2 partial approximately 100-residue repeats. Each "homology domain" contains a "nodal" region with few sites for CS chain addition that is highly conserved between species, suggesting a possible role in aggrecan biosynthesis or catabolism.

Mechanical loading and TGF-beta regulate proteoglycan synthesis in tendon

Fibrocartilage is found in tendon at sites where the tissue is subjected to transverse compressive loading in vivo. A significant characteristic of the tissue transition from tendon to fibrocartilage in bovine deep flexor tendon is increased gene expression, synthesis, and accumulation of both a large proteoglycan, aggrecan, and a small proteoglyoan, biglycan. In order to investigate the cellular events involved in this response, segments of fetal bovine deep flexor tendon were subjected in vitro to cyclic compressive load for 72 h. Following loading, the level of aggrecan mRNA in cells from loaded tissue was increased 200-450% compared to matched nonloaded tissue segments, as determined by slot-blot analysis. The level of biglycan mRNA increased 100%, and the level of versican mRNA increased 130% in the loaded tissue. The level of decorin mRNA remained virtually unchanged, while expression of alpha 1(I) collagen increased only 40%. When tissue segments were cultured in the presence of transforming growth factor (TGF)-beta 1 (1 ng/ml), the synthesis and expression of mRNA for both aggrecan and biglycan increased, whereas decorin expression was not affected. Similarity in both the direction and the pattern of the cellular response to mechanical load and TGF-beta suggested a causal relationship. Both loading of tendon segments and TGF-beta treatment increased expression of mRNA for TGF-beta by approximately 40% compared to control tissue. In addition, the amount of newly synthesized TGF-beta immunoprecipitated from extracts of loaded tissue was several-fold greater than that from nonloaded tissue. The experiments of this study support a hypothesis suggesting that one aspect of the response of cells in fetal tendon to compressive load is increased TGF-beta synthesis which, in turn, stimulates synthesis of extracellular matrix proteoglycans and leads toward fibrocartilage formation.

A human-specific polymorphism in the coding region of the aggrecan gene. Variable number of tandem repeats produce a range of core protein sizes in the general population

Aggrecan, one of the major structural genes of cartilage, encodes a proteoglycan core protein composed of an extended central glycosaminoglycan-bearing domain, flanked by globular domains at each end. The central region consists of long stretches of repeating amino acids that serve as attachment sites for glycosaminoglycans such as chondroitin and keratan sulfate; the terminal globular domains interact with other cartilage components. The glycosaminoglycan attachment region is encoded in several species by a single large exon, within which are several different types of repeating sequences. Several species show within this exon a similar block of conserved repeats for attachment of chondroitin sulfate, but in humans this group of repeats is particularly well conserved. Examination of genomic DNA from a population of unrelated individuals by polymerase chain reaction or Southern blot assays shows this block of repeat sequences exists in multiple allelic forms, which differ by the number of repeats at this site in each allele. Thirteen different alleles have been identified, with repeat numbers ranging from 13 to 33. This is an unusual example of an expressed variable number of tandem repeat polymorphism. This polymorphism is apparently restricted to humans, of several species examined. This polymorphism results in individuals with differing length aggrecan core proteins, bearing different numbers of potential attachment sites for chondroitin sulfate. The possibility exists for a molecular understanding of biological variation in cartilage functional properties.

Interaction of cartilage matrix protein with aggrecan. Increased covalent cross-linking with tissue maturation

Cartilage matrix protein (CMP) is a trimeric protein present in many types of cartilage extracellular matrix. It has recently been purified under native conditions that allowed the proposal of a structural model (Hauser, N., and Paulsson, M. (1994) J. Biol. Chem. 269, 25747-25753). To examine the functional properties of CMP we studied its interaction with aggrecan within cartilage extracellular matrix. Aggrecan-enriched fractions were purified from bovine tracheal cartilage of different ages under nondenaturing and denaturing conditions, respectively, and characterized by a combination of biochemical methods and electron microscopy. The fractions contained a pool of CMP noncovalently associated with aggrecan as well as a pool of CMP that appears covalently cross-linked to the aggrecan core protein. Only about two thirds of the CMP subunits could be released even upon reduction under denaturing conditions. It appears that CMP is attached by a nonreducible covalent interaction of one of its subunits with the protein core. The amount of CMP strongly bound to aggrecan increases with age. Electron microscopy revealed interaction sites for CMP in the extended chondroitin-sulfate attachment domain E2. In old tissue five distinct binding sites for CMP were found while in young cartilage only three of these were occupied. The extent of decoration of E2 with CMP increases with age.

Species-specific alternative splicing of the epidermal growth factor-like domain 1 of cartilage aggrecan

Aggrecan transcripts contain two alternatively spliced exons that code for two epidermal growth factor-like domains (EGF1 and EGF2). Whereas the EGF2 sequence is expressed at a uniform level among different species, the EGF1 sequence has been detected only in human aggrecan transcripts. In this study we have used the nested primer reverse transcription-PCR (RT-PCR) method to compare the expression of the EGF1 exon in human, bovine and dog aggrecan transcripts. Our results indicate that this exon is expressed in a species-specific manner. In addition to its significant expression level in human transcripts, the EGF1 sequence can be detected in a small portion of bovine aggrecan transcripts as shown with nested primer RT-PCR. In contrast, the same module is not detectable in dog aggrecan transcripts, although an EGF1 sequence is present in the dog aggrecan gene. The expression level of the EGF1 exon in the aggrecan transcripts correlates with the strength of the polypyrimidine tract upstream of the exon. The EGF1 sequence also shows much less conservation between the species than the EGF2 sequence. The species-specific expression and high sequence variation of the EGF1 exon imply that this sequence is likely to code for an aggrecan domain having no cartilage-specific function.

The primary structure of a basic leucine-rich repeat protein, PRELP, found in connective tissues

We have determined the primary structure of a connective tissue matrix protein from the nucleotide sequence of a clone isolated from a human articular chondrocyte cDNA library. The major part of the amino acid sequence has also been determined by direct protein sequencing. The translated primary sequence corresponds to 382 amino acid residues, including a 20-residue signal peptide. The molecular mass of the mature protein is 41,646 Da. The main part of the protein consists of 10 leucine-rich repeats ranging in length from 20 to 26 residues, with asparagine at position 10 (B-type). The N-terminal part is unusual in that it is basic and rich in arginine and proline. There are four potential N-linked glycosylation sites present. In three of these sites, post-translational modifications are likely to be present since Asn was not found by direct protein sequencing. The amino- and carboxyl-terminal parts contain four and two cysteine residues, respectively, probably forming disulfide bonds by analogy with the other members of this family. The protein shows highest identity (36%) to fibromodulin and 33% to bovine lumican, two other leucine-rich repeat connective tissue proteins. Northern blot analysis showed the presence of an approximately 3.8-kilobase mRNA in different types of bovine cartilage and cultured osteoblasts, whereas RNAs isolated from bovine kidney, skin, spleen, thymus, and trabecular bone and rat calvaria were negative. Human articular chondrocyte and rat chondrosarcoma cell RNAs contained an additional mRNA of approximately 1.6 and 1.8 kilobases, respectively.

The polycystic kidney disease 1 (PKD1) gene encodes a novel protein with multiple cell recognition domains

Characterization of the polycystic kidney disease 1 (PKD1) gene has been complicated by genomic rearrangements on chromosome 16. We have used an exon linking strategy, taking RNA from a cell line containing PKD1 but not the duplicate loci, to clone a cDNA contig of the entire transcript. The transcript consists of 14,148 bp (including a correction to the previously described C terminus), distributed among 46 exons spanning 52 kb. The predicted PKD1 protein, polycystin, is a glycoprotein with multiple transmembrane domains and a cytoplasmic C-tail. The N-terminal extracellular region of over 2,500 aa contains leucine-rich repeats, a C-type lectin, 16 immunoglobulin-like repeats and four type III fibronectin-related domains. Our results indicate that polycystin is an integral membrane protein involved in cell-cell/matrix interactions.

Evidence of a defined spatial arrangement of hyaluronate in the central filament of cartilage proteoglycan aggregates

Aggregates of proteoglycans from the Swarm rat chondrosarcoma reassembled in vitro have been studied by rotary-shadowing electron microscopy, and shown to be similar to native structures that have never been dissociated [Mörgelin, Engel, Heinegård and Paulsson (1992) J. Biol. Chem. 267, 14275-14284]. A hyaluronate with defined chain length (HAshort) has now been prepared by autoclaving high-Mr hyaluronate and fractionation to a narrow size distribution by gel filtration. Proteoglycan monomers, core protein, hyaluronate-binding region and link protein were combined with HAshort. Free chains of HAshort and reconstituted complexes with proteoglycan, link protein and aggrecan fragments were examined by electron microscopy after rotary shadowing. Length measurements showed that the hyaluronate was condensed to about half of its original length on binding intact aggrecan monomers, any aggrecan fragment or link protein alone. This strongly implies that hyaluronate adopts a defined spatial arrangement within the central filament of the aggregate, probably different from its secondary structure in solution. No differences in length were observed between link-free and link-stabilized aggregates.

Length variation in the keratan sulfate domain of mammalian aggrecan

The keratan sulfate domain of aggrecan consists of a series of tandemly repeating hexapeptides which have the consensus sequence Glu-Glu/Lys-Pro-Phe-Pro-Ser, where the serine side-chains presumably provide sites for the attachment of keratan sulfate (KS) chains. The number of hexapeptide repeats varies between species, ranging from four in rat (Doege et al., 1987) and mouse (Walcz et al., 1992) to 13 in human (Doege et al., 1991) and 23 in bovine aggrecan (Antonsson et al., 1989). Chicken aggrecan (Chandrasekaran and Tanzer, 1992) does not contain a KS domain with a recognizable hexapeptide motif. The extent of this variation among mammalian and avian species is not known, and there is currently no explanation to predict how differences in the size of the KS domain would affect aggrecan function. We used polymerase chain reaction (PCR) to amplify the portion of the human, canine and porcine aggrecan gene that codes for the KS domain. We sequenced the amplified products in each case. Human aggrecan, with 13 hexapeptide repeats (Doege et al., 1987), was used as reference and found to be essentially identical to published data. The canine and porcine KS domains consisted of six and ten hexapeptide repeats respectively. The same PCR protocol was used to amplify the KS domain from genomic DNA of eight other mammalian species. Comparison of the size of these amplified products, as determined by agarose gel electrophoresis, with those for which sequence data are available allowed us to estimate the number of repeats in the KS domain. In almost half the species examined, the KS domain consisted of 13 hexapeptide repeats.

The cartilage proteoglycan aggregate: assembly through combined protein-carbohydrate and protein-protein interactions

In vitro reassembled aggregates of cartilage proteoglycan (aggrecan) were studied by glycerol spraying/rotary shadowing electron microscopy and compared to the corresponding native (i.e. never dissociated) structures. In both cases a tightly packed central filament structure was observed consisting of the hyaluronate binding region (HABR) of the proteoglycan, link protein (LP) and hyaluronate (HA). This differs from earlier results where a discontinuous central filament structure was seen after spreading proteoglycan aggregates at a water/air interphase. Binding of isolated HABR to HA is random but upon addition of link protein a clustering of the HA-binding proteins is observed, indicating a cooperativity. In a fully saturated aggregate the HA is covered by a continuous protein shell consisting of HABR and LP. When added in amounts below saturation HABR and LP bind to the HA in clusters which are interrupted by free strands of HA. The proteoglycan aggregate is thus an example for a structure where a polysaccharide forms a template for a supramolecular assembly largely stabilized by protein-protein interactions.

Aggrecan in bovine tendon

Large proteoglycans were purified by ion-exchange chromatography, gel filtration and CsCl gradient centrifugation from the compressed and tensional regions of adult bovine deep flexor tendon. Tryptic peptide maps of proteoglycan from the compressed region were very similar to maps of aggrecan from bovine articular cartilage, with evidence for the presence of all fifteen previously identified markers from the G1, G2 and G3 domains. The presence of aggrecan in these samples was confirmed by sequencing the G1 peptide YPIHTPR. The equivalent maps for large proteoglycan from tensional tendon were also consistent with the presence of aggrecan, and this was confirmed by sequencing three marker peptides from each of the G2 and G3 domains. However, G1 marker peptides were conspicuously absent from tensional samples. Northern blots for aggrecan mRNA showed high levels in cells from compressed tendon and articular cartilage. Extended exposure revealed a lower level of hybridization to RNA from tensional tendon as well. The results confirm that aggrecan, which is similar in core protein structure to articular cartilage aggrecan, is the predominant chondroitin sulfate-bearing large proteoglycan of compressed tendon. The results also indicate that aggrecan fragments lacking the G1 domain can account for the small amounts of chondroitin sulfate-bearing large proteoglycan in tensional regions of adult tendon.

Molecular cloning and analysis of the protein modules of aggrecans

The large aggregating chondroitin sulfate proteoglycan of cartilage, aggrecan, has served as a prototype of proteoglycan structure. Molecular cloning has elucidated its primary structure and revealed both known and unknown domains. To date the complete structures of chicken, rat and human aggrecans have been deduced, while partial sequences have been reported for bovine aggrecan. A related proteoglycan, human versican, has also been cloned and sequenced. Both aggrecan and versican have two lectin domains, one at the amino-terminus which binds hyaluronic acid and one at the carboxyl-terminus whose physiological ligand is unknown. Both lectins have homologous counterparts in other types of proteins. Within the aggrecans the keratan sulfate domain may be variably present and also has a prominent repeat in some species. The chondroitin sulfate domain has three distinct regions which vary in their prominence in different species. The complex molecular structure of aggrecans is consistent with the concept of exon shuffling and aggrecans serve as suitable prototypes for comprehending the evolution of multi-domain proteins.

Alternate exon usage is a commonly used mechanism for increasing coding diversity within genes coding for extracellular matrix proteins

Extracellular matrix proteins are a diverse family of secreted proteins and glycoproteins that are responsible for a variety of critical functions in different tissues. A large number of multiexon genes encode these proteins of the extracellular matrix. Over the last few years, it has become evident that the processing of the pre-mRNA from several of these genes involves alternative splicing. This review summarizes the known examples of alternative splicing in genes coding for the extracellular matrix and attempts to relate the increase in coding diversity generated by alternate exon usage to the function(s) of individual extracellular matrix proteins.

Pro-major basic protein has three types of sugar chains at the pro-portion

The amino-acid sequence of purified recombinant pro-major basic protein from Chinese hamster kidney cells was determined to verify the primary structure and glycosylation sites. Reduced and S-carboxamidemethylated protein was first digested with Achromobacter proteinase I. Each peptide was characterized by amino-acid analysis and amino-acid sequence analysis. We could identify all the peptides which were expected from the pro-major basic protein cDNA sequence. Sequence analysis and deglycosylation study revealed that Ser-8, Thr-9, Ser-46 and Asn-70 were glycosylated. The results indicated that proMBP has three types of sugar chains, O-glycoside, N-glycoside and glycosaminoglycan, in the pro-portion.

Molecular cloning and analysis of the protein modules of aggrecans

The large aggregating chondroitin sulfate proteoglycan of cartilage, aggrecan, has served as a prototype of proteoglycan structure. Molecular cloning has elucidated its primary structure and revealed both known and unknown domains. To date the complete structures of chicken, rat and human aggrecans have been deduced, while partial sequences have been reported for bovine aggrecan. A related proteoglycan, human versican, has also been cloned and sequenced. Both aggrecan and versican have two lectin domains, one at the amino-terminus which binds hyaluronic acid and one at the carboxyl-terminus whose physiological ligand is unknown. Both lectins have homologous counterparts in other types of proteins. Within the aggrecans the keratan sulfate domain may be variably present and also has a prominent repeat in some species. The chondroitin sulfate domain has three distinct regions which vary in their prominence in different species. The complex molecular structure of aggrecans is consistent with the concept of exon shuffling and aggrecans serve as suitable prototypes for comprehending the evolution of multi-domain proteins.

Versican gene expression in human articular cartilage and comparison of mRNA splicing variation with aggrecan

The chondrocytes in human articular cartilage from subjects of all ages express mRNAs for both of the aggregating proteoglycans aggrecan and versican, although the level of expression of versican mRNA is much lower than that of aggrecan mRNA. Aggrecan shows alternative splicing of the epidermal growth factor (EGF)-like domain within its C-terminal globular region, but there is no evidence for a major difference in situ in the relative expression of this domain with age. At all ages studied from birth to the mature adult, a greater proportion of transcripts lacked the EGF domain. The relative proportions of the two transcripts did not change upon culture and passage of isolated chondrocytes. In contrast, the neighbouring complement regulatory protein (CRP)-like domain was predominantly expressed irrespective of age, but cell culture did result in variation of the splicing of this domain. Versican possesses two EGF-like domains and one CRP-like domain, but at all ages the three domains were predominantly present in all transcripts. This situation persisted upon culture and passage of the chondrocytes. Thus, unlike aggrecan, the versican expressed by human articular cartilage does not appear to undergo alternative splicing of its C-terminal globular region, either in cartilage in situ or in chondrocytes in culture.

Period-homologous sequence polymorphisms in subterranean mammals of the Spalax ehrenbergi superspecies in Israel

Restriction fragment length polymorphism (RFLPs) of the mouse period-homologous sequence were studied in 13 populations of the four chromosomal species (2n = 52, 54, 58 and 60) of the mole rat, Spalax ehrenbergi superspecies in Israel. The period locus of Drosophila melanogaster is implicated in controlling the circadian rhythm as well as the male courtship song rhythm. Multiple DNA homologies exist in the mole rat and correspond to more than 10 loci. The level of polymorphism is very high, with a large number of alleles per locus, increasing from the northern to the southern species along a gradient of increasing aridity. Variation was also found in an isolated desert population, with a unique fragment specific to this population. Fragment variation allows distinction between chromosomal species, and confirms earlier evidence that gene flow does not occur between them. A correlation was found between some allelic fragments and the number of apparent harmonics of the courtship calls. This finding suggests an interesting testable hypothesis that the existence of a locus (homology) is responsible for the courtship call parameters.

Molecular cloning of chicken aggrecan. Structural analyses

The large, aggregating chondroitin sulphate proteoglycan of cartilage, aggrecan, has served as a generic model of proteoglycan structure. Molecular cloning of aggrecans has further defined their amino acid sequences and domain structures. In this study, we have obtained the complete coding sequence of chicken sternal cartilage aggrecan by a combination of cDNA and genomic DNA sequencing. The composite sequence is 6117 bp in length, encoding 1951 amino acids. Comparison of chicken aggrecan protein primary structure with rat, human and bovine aggrecans has disclosed both similarities and differences. The domains which are most highly conserved at 70-80% identity are the N-terminal domains G1 and G2 and the C-terminal domain G3. The chondroitin sulphate domain of chicken aggrecan is smaller than that of rat and human aggrecans and has very distinctive repeat sequences. It has two separate sections, one comprising 12 consecutive Ser-Gly-Glu repeats of 20 amino acids each, adjacent to the other which has 23 discontinuous Ser-Gly-Glu repeats of 10 amino acids each; this latter region, N-terminal to the former one, appears to be unique to chicken aggrecan. The two regions contain a total of 94 potential chondroitin sulphate attachment sites. Genomic comparison shows that, although chicken exons 11-14 are identical in size to the rat and human exons, chicken exon 10 is the smallest of the three species. This is also reflected in the size of its chondroitin sulphate coding region and in the total number of Ser-Gly pairs. The putative keratan sulphate domain shows 31-45% identity with the other species and lacks the repetitive sequences seen in the others. In summary, while the linear arrangement of specific domains of chicken aggrecan is identical to that in the aggrecans of other species, and while there is considerable identity of three separate domains, chicken aggrecan demonstrates unique features, notably in its chondroitin sulphate domain and its keratan sulphate domain. Thus different variants of chondroitin sulphate and keratan sulphate domains may have evolved separately to fulfil specific biochemical and physiological functions.

T-cell activation molecule 4-1BB binds to extracellular matrix proteins

The recently isolated 4-1BB cDNA clone encodes a cell surface protein expressed by activated T cells. Its extracellular domain is homologous to members of the nerve growth factor receptor super family and its cytoplasmic domain contains a sequence homologous to the binding site for the T-cell-specific tyrosine kinase p56lck found in the cytoplasmic domains of CD4 and CD8 alpha. At present the function of 4-1BB is not known. We prepared a 4-1BB-immunoglobulin fusion protein (4-1BB Rg). This protein was used in immunohistochemical studies to identify tissues that express the 4-1BB ligand. 4-1BB Rg bound to virtually all tissues examined, suggesting that extracellular components might function as its ligands. To explore this possibility, 4-1BB was expressed in COS cells and found to mediate the binding of fibronectin, vitronectin, laminin, and collagen VI but not of collagen I. The binding of extracellular matrix proteins to 4-1BB was not mediated by Arg-Gly-Asp (RGD) or CS-1 amino acid sequences. Experiments with overlapping proteolytic fragments of fibronectin showed that 4-1BB interacts with multiple regions of fibronectin. The interaction between extracellular matrix proteins and 4-1BB was completely blocked by the anionic carbohydrate polymer fucoidan and was partially blocked by the anionic carbohydrate polymer dextran sulfate and the glycosaminoglycan heparin sulfate but was unaffected by desulfated heparin. These results suggest that carbohydrates may play a role in mediating the 4-1BB-extracellular matrix protein adhesion.

N-terminal sequence of proteoglycan fragments isolated from medium of interleukin-1-treated articular-cartilage cultures. Putative site(s) of enzymic cleavage

Bovine articular cartilage was cultured both in the presence and in the absence of human recombinant interleukin-1 alpha (IL-1) (100 units/ml). Addition of this cytokine stimulated matrix degradation approx. 3-fold. This increased degradation permitted characterization of the large chondroitin sulphate proteoglycan (aggrecan) fragments accumulating in the media. When compared with controls, the proteoglycans isolated from the medium of cultures treated with IL-1 exhibited a decrease in the Kav. (control 0.25; IL-1-treated 0.37), determined by Sepharose CL-2B chromatography. This decrease in proteoglycan size was accompanied by a decreased ability of these monomers to associate with hyaluronic acid. Thus only 20% of the proteoglycans isolated from the medium of IL-1-treated cultures, compared with 39% for control cultures, had the capacity to form high-M(r) aggregates with hyaluronic acid. SDS/PAGE analysis of the proteoglycans from the media of IL-1-treated cultures demonstrated several large proteoglycan protein-core bands (M(r) 144,000-380,000). The protein-core bands with M(r) 144,000-266,000 exhibited a significantly decreased reactivity with monoclonal antibody 1-C-6 (specific for domains G1 and G2). The N-terminal amino acid sequence of four of these protein-core bands (M(r) 144,000, 173,000, 214,000 and 266,000) yielded sequences LGQRPPV-Y-PQLF(E), AGEGP(S)GILEL-GAP(S)-AP(D)M, GLG-VEL-LPGE and (A)RGSVIL-AKPDFEV-P-A. A comparison of these N-terminal amino acid sequences with the published proteoglycan sequence for bovine nasal cartilage [Oldberg, Antonsson & Heinegård (1987) Biochem. J. 243, 255-259], rat chondrosarcoma [Doege, Sasaki, Horigan, Hassell & Yamada (1987) J. Biol. Chem. 262, 17757-17769] and human articular cartilage [Doege, Sasaki, Kimura & Yamada (1991) J. Biol. Chem. 266, 894-902] permitted assignment of their relative positions on the core protein. Furthermore, on the basis of this similarity to published sequence, putative sites of enzymic cleavage were constructed. These theoretical cleavage sites revealed a glutamic acid residue in the P1 position and an uncharged polar or non-polar residue in the P1' position.

Purification and cDNA cloning of a novel factor produced by a human T-cell hybridoma: sequence homology with animal lectins

We have purified a novel immunoregulatory factor (BMPG: bone-marrow proteoglycan) produced by a T-cell hybridoma, with a monoclonal antibody column. Using an oligonucleotide probe corresponding to the partial amino acid sequence of BMPG, we cloned, sequenced, and expressed a cDNA for BMPG. BMPG has 222 amino acid residues with a 16 N-terminal signal sequence, so the mature form has 206 amino acid residues. BMPG was found to have unique characteristics: it has three types of sugar chains and it shows a marked homology with animal lectins including the human asialoglycoprotein receptor, chicken hepatic lectin and the homing receptor of lymphocytes.

Extraction and isolation of mRNA from adult articular cartilage

We have developed a method to isolate RNA in high yield from adult articular cartilage. Homogenization of the articular cartilage with a freezer mill, extraction with 4 M guanidinium isothiocyanate/acid-phenol, and ultracentrifugation in cesium trifluoroacetate was found to be an effective and practical method for isolating a high yield of intact RNA from adult canine articular cartilage. The total RNA was suitable for Northern blot analysis. The mRNA that could then be isolated by oligo-dT affinity chromatography was found to be a suitable substrate for in vitro translation, for making a cDNA library, and for PCR amplification.

Mechanism of catabolism of aggrecan by articular cartilage

Characterization of aggrecan core protein peptides appearing in the medium of adult articular cartilage maintained in tissue culture showed that eight major peptides could be detected. The two largest peptides had the same N-terminal sequence as bovine aggrecan core protein and probably represent partly degraded aggrecan lost to the medium in the form of the proteoglycan aggregate. The three next smallest peptides were all shown to have another N-terminal sequence which corresponded to a sequence in the interglobular domain starting at alanine residue 393 of the human aggrecan core protein (K. Doege et al., 1991, J. Biol. Chem. 266, 894-902). Two other peptides were isolated and shown to have two different N-terminal amino sequences corresponding to sequences in the chondroitin sulfate attachment domain 2 of the core protein starting at alanine residue 1839 and leucine residue 1939 of human aggrecan. This suggests that the catabolism of aggrecan by adult articular cartilage occurs by the proteolytic cleavage of the core protein of this proteoglycan at three separate sites. Examination of the amino acid sequences around each of these cleavage sites showed a similar pattern TEGE decreases ARGS, TAQE decreases AGEG, and VSQE decreases LGQR, suggesting that a single proteinase may be involved in the catabolism of aggrecan. Analysis of synovial fluids and serum of age-matched animals revealed the presence of aggrecan core protein peptides corresponding in size to those detected in vitro, thus indicating the cleavage observed in explant culture is the same as that which occurs in vivo.

Effect of interleukin-1 and insulin like growth factor-1 on the release of proteoglycan components and hyaluronan from pig articular cartilage in explant culture

The turnover of proteoglycans was investigated in articular cartilage in explant culture by analysing the components released into the culture medium. The effect of IL-1 alpha on the release of fragments derived from different proteoglycan domains and hyaluronan (HA) was determined over 4 days in culture. The effect of IGF-1 (100 ng/ml) on matrix degradation of proteoglycan and its ability to inhibit the effects of IL-1 (10 ng/ml) was also assessed. The rate of release of G1 and G2 globular domains of proteoglycans into the culture medium was determined by radioimmunoassay. In unstimulated control cartilage there was a greater release of proteoglycan G2 domain than of G1 domain suggesting that cleavage occurred between them and that some G1 was preferentially retained bound in the matrix. Compared with control cartilage IL-1 stimulated the release of all proteoglycan components and hyaluronan. IL-1 had a greater effect on the release of G1 than on G2 domain, but also resulted in some net loss of these proteins (approximately 45% as detected in the immunoassays). In explants treated with both IL-1 and IGF-1 there was much less release of proteoglycan fragments and evidence for less extensive degradation. IGF-1 was particularly affective in preventing any increase in HA release and also preventing the apparent loss of G1 and G2 domains. It also partially inhibited the release of G1 and G2 domains and the sulphated glycosaminoglycan fragments. IGF-1 was therefore an effective antagonist of IL-1 action on cartilage. It is not known at what level it blocks the chondrocyte response to IL-1, but it clearly results in the suppression of matrix degradative activity.

Immunological detection of the EGF-like domain of the core proteins of large proteoglycans from human and baboon cartilage

Recent data from the literature have shown that cDNA clones for the carboxyterminal domain of the core protein of large proteoglycan monomers from human cartilage contain an EGF-like domain, which appears to undergo alternative splicing. In the present study we have found that articular proteoglycans from human and baboon separated on agarose flat-bed gels and blotted onto nitrocellulose react with a rabbit antiserum to mouse EGF. In addition both forms of the proteoglycans (band I and band II) seen on these gels are reactive. Reactivity is seen with proteoglycans extracted from human articular cartilage of various ages (fetal, newborn, young and aged) and with proteoglycans extracted from cartilage of thanatophoric dysplasia and homozygous achondroplasia. Reactivity is dependent on prior digestion of the nitrocellulose blot with Chase ABC, suggesting masking of epitope by chondroitin sulfate. Reactivity of the EGF antiserum with cartilage proteoglycan core protein was also demonstrated in an ELISA system with core protein as coating antigen. The reactivity appears to reside in a tryptic peptide generated from Chase/keratanase digested core protein. The immunoreactive species migrates as a 68 KDa species on gradient gels. Immunological detection and quantitative analysis of the EGF-like domain could be useful for analysis of various proteoglycan samples.