Molecular basis of nanomelia, a heritable chondrodystrophy of chicken

Aggrecan governs intervertebral discs development by providing critical mechanical cues of the extracellular matrix

Aggrecan (ACAN) is localized in the intervertebral disc (IVD) in unique compartment-specific patterns where it contributes to the tissue structure and mechanical function together with collagens. The extracellular matrix (ECM) of the IVD undergoes degenerative changes during aging, misuse or trauma, which inevitably alter the biochemical and biomechanical properties of the tissue. A deeper understanding of these processes can be achieved in genetically engineered mouse models, taking into account the multifaceted aspects of IVD development. In this study, we generated aggrecan insertion mutant mice (Acan ^iE5/iE5 ) by interrupting exon 5 coding for the G1 domain of ACAN, and analyzed the morphological and mechanical properties of the different IVD compartments during embryonic development. Western blotting using an antibody against the total core protein failed to detect ACAN in cartilage extracts, whereas immunohistochemistry by a G1-specific antibody showed weak signals in vertebral tissues of Acan ^iE5/iE5 mice. Homozygous mutant mice are perinatally lethal and characterized by short snout, cleft palate and disproportionate dwarfism. Whole-mount skeletal staining and µ-CT analysis of Acan ^iE5/iE5 mice at embryonic day 18.5 revealed compressed vertebral bodies with accelerated mineralization compared to wild type controls. In Acan ^iE5/iE5 mice, histochemical staining revealed collapsed extracellular matrix with negligible sulfated glycosaminoglycan content accompanied by a high cellular density. Collagen type II deposition was not impaired in the IVD of Acan ^iE5/iE5 mice, as shown by immunohistochemistry. Mutant mice developed a severe IVD phenotype with deformed nucleus pulposus and thinned cartilaginous endplates accompanied by a disrupted growth plate structure in the vertebral body. Atomic force microscopy (AFM) imaging demonstrated a denser collagen network with thinner fibrils in the mutant IVD zones compared to wild type. Nanoscale AFM indentation revealed bimodal stiffness distribution attributable to the softer proteoglycan moiety and harder collagenous fibrils of the wild type IVD ECM. In Acan ^iE5/iE5 mice, loss of aggrecan resulted in a marked shift of the Young's modulus to higher values in all IVD zones. In conclusion, we demonstrated that aggrecan is pivotal for the determination and maintenance of the proper stiffness of IVD and vertebral tissues, which in turn could play an essential role in providing developmental biomechanical cues.

Two Modulators of Skeletal Development: BMPs and Proteoglycans

During embryogenesis, skeletal development is tightly regulated by locally secreted growth factors that interact with proteoglycans (PGs) in the extracellular matrix (ECM). Bone morphogenetic proteins (BMPs) are multifunctional growth factors that play critical roles in cartilage maturation and bone formation. BMP signals are transduced from plasma membrane receptors to the nucleus through both canonical Smad and noncanonical p38 mitogen-activated protein kinase (MAPK) pathways. BMP signalling is modulated by a variety of endogenous and exogenous molecular mechanisms at different spatiotemporal levels and in both positive and negative manners. As an endogenous example, BMPs undergo extracellular regulation by PGs, which generally regulate the efficiency of ligand-receptor binding. BMP signalling can also be exogenously perturbed by a group of small molecule antagonists, such as dorsomorphin and its derivatives, that selectively bind to and inhibit the intracellular kinase domain of BMP type I receptors. In this review, we present a current understanding of BMPs and PGs functions in cartilage maturation and osteoblast differentiation, highlighting BMP–PG interactions. We also discuss the identification of highly selective small-molecule BMP receptor type I inhibitors. This review aims to shed light on the importance of BMP signalling and PGs in cartilage maturation and bone formation.

Aggrecan in Cardiovascular Development and Disease

Aggrecan is a large proteoglycan that forms giant hydrated aggregates with hyaluronan in the extracellular matrix (ECM). The extraordinary resistance of these aggregates to compression explains their abundance in articular cartilage of joints where they ensure adequate load-bearing. In the brain, they provide mechanical buffering and contribute to formation of perineuronal nets, which regulate synaptic plasticity. Aggrecan is also present in cardiac jelly, developing heart valves, and blood vessels during cardiovascular development. Whereas aggrecan is essential for skeletal development, its function in the developing cardiovascular system remains to be fully elucidated. An excess of aggrecan was demonstrated in cardiovascular tissues in aortic aneurysms, atherosclerosis, vascular re-stenosis after injury, and varicose veins. It is a product of vascular smooth muscle and is likely to be an important component of pericellular matrix, where its levels are regulated by proteases. Aggrecan can contribute to specific biophysical and regulatory properties of cardiovascular ECM via the diverse interactions of its domains, and its accumulation is likely to have a significant role in developmental and disease pathways. Here, the established biological functions of aggrecan, its cardiovascular associations, and potential roles in cardiovascular development and disease are discussed.

Aggrecan, the Primary Weight-Bearing Cartilage Proteoglycan, Has Context-Dependent, Cell-Directive Properties in Embryonic Development and Neurogenesis: Aggrecan Glycan Side Chain Modifications Convey Interactive Biodiversity

This review examines aggrecan's roles in developmental embryonic tissues, in tissues undergoing morphogenetic transition and in mature weight-bearing tissues. Aggrecan is a remarkably versatile and capable proteoglycan (PG) with diverse tissue context-dependent functional attributes beyond its established role as a weight-bearing PG. The aggrecan core protein provides a template which can be variably decorated with a number of glycosaminoglycan (GAG) side chains including keratan sulphate (KS), human natural killer trisaccharide (HNK-1) and chondroitin sulphate (CS). These convey unique tissue-specific functional properties in water imbibition, space-filling, matrix stabilisation or embryonic cellular regulation. Aggrecan also interacts with morphogens and growth factors directing tissue morphogenesis, remodelling and metaplasia. HNK-1 aggrecan glycoforms direct neural crest cell migration in embryonic development and is neuroprotective in perineuronal nets in the brain. The ability of the aggrecan core protein to assemble CS and KS chains at high density equips cartilage aggrecan with its well-known water-imbibing and weight-bearing properties. The importance of specific arrangements of GAG chains on aggrecan in all its forms is also a primary morphogenetic functional determinant providing aggrecan with unique tissue context dependent regulatory properties. The versatility displayed by aggrecan in biodiverse contexts is a function of its GAG side chains.

Knee Osteoarthritis: A Review of Pathogenesis and State-Of-The-Art Non-Operative Therapeutic Considerations

Being the most common musculoskeletal progressive condition, osteoarthritis is an interesting target for research. It is estimated that the prevalence of knee osteoarthritis (OA) among adults 60 years of age or older is approximately 10% in men and 13% in women, making knee OA one of the leading causes of disability in elderly population. Today, we know that osteoarthritis is not a disease characterized by loss of cartilage due to mechanical loading only, but a condition that affects all of the tissues in the joint, causing detectable changes in tissue architecture, its metabolism and function. All of these changes are mediated by a complex and not yet fully researched interplay of proinflammatory and anti-inflammatory cytokines, chemokines, growth factors and adipokines, all of which can be measured in the serum, synovium and histological samples, potentially serving as biomarkers of disease stage and progression. Another key aspect of disease progression is the epigenome that regulates all the genetic expression through DNA methylation, histone modifications, and mRNA interference. A lot of work has been put into developing non-surgical treatment options to slow down the natural course of osteoarthritis to postpone, or maybe even replace extensive surgeries such as total knee arthroplasty. At the moment, biological treatments such as platelet-rich plasma, bone marrow mesenchymal stem cells and autologous microfragmented adipose tissue containing stromal vascular fraction are ordinarily used. Furthermore, the latter two mentioned cell-based treatment options seem to be the only methods so far that increase the quality of cartilage in osteoarthritis patients. Yet, in the future, gene therapy could potentially become an option for orthopedic patients. In the following review, we summarized all of the latest and most important research in basic sciences, pathogenesis, and non-operative treatment.

Knee Osteoarthritis: A Review of Pathogenesis and State-Of-The-Art Non-Operative Therapeutic Considerations

Being the most common musculoskeletal progressive condition, osteoarthritis is an interesting target for research. It is estimated that the prevalence of knee osteoarthritis (OA) among adults 60 years of age or older is approximately 10% in men and 13% in women, making knee OA one of the leading causes of disability in elderly population. Today, we know that osteoarthritis is not a disease characterized by loss of cartilage due to mechanical loading only, but a condition that affects all of the tissues in the joint, causing detectable changes in tissue architecture, its metabolism and function. All of these changes are mediated by a complex and not yet fully researched interplay of proinflammatory and anti-inflammatory cytokines, chemokines, growth factors and adipokines, all of which can be measured in the serum, synovium and histological samples, potentially serving as biomarkers of disease stage and progression. Another key aspect of disease progression is the epigenome that regulates all the genetic expression through DNA methylation, histone modifications, and mRNA interference. A lot of work has been put into developing non-surgical treatment options to slow down the natural course of osteoarthritis to postpone, or maybe even replace extensive surgeries such as total knee arthroplasty. At the moment, biological treatments such as platelet-rich plasma, bone marrow mesenchymal stem cells and autologous microfragmented adipose tissue containing stromal vascular fraction are ordinarily used. Furthermore, the latter two mentioned cell-based treatment options seem to be the only methods so far that increase the quality of cartilage in osteoarthritis patients. Yet, in the future, gene therapy could potentially become an option for orthopedic patients. In the following review, we summarized all of the latest and most important research in basic sciences, pathogenesis, and non-operative treatment.

RNA-seq reveals downregulated osteochondral genes potentially related to tibia bacterial chondronecrosis with osteomyelitis in broilers

Background

Bacterial chondronecrosis with osteomyelitis (BCO) develops in the growth plate (GP) of the proximal femur and tibia and is initiated by damage to the less mineralized chondrocytes followed by colonization of opportunistic bacteria. This condition affects approximately 1% of all birds housed, being considered one of the major causes of lameness in fast growing broilers. Although several studies have been previously performed aiming to understand its pathogenesis, the molecular mechanisms involved with BCO remains to be elucidated. Therefore, this study aimed to generate a profile of global differential gene expression involved with BCO in the tibia of commercial broilers, through RNA sequencing analysis to identity genes and molecular pathways involved with BCO in chickens.

Results

Our data showed 192 differentially expressed (DE) genes: 63 upregulated and 129 downregulated in the GP of the tibia proximal epiphysis of BCO-affected broilers. Using all DE genes, six Biological Processes (BP) were associated with bone development (connective tissue development, cartilage development, skeletal system development, organ morphogenesis, system development and skeletal system morphogenesis). The analyses of the upregulated genes did not indicate any significant BP (FDR < 0.05). However, with the downregulated genes, the same BP were identified when using all DE genes in the analysis, with a total of 26 coding genes explaining BCO in the tibia: ACAN, ALDH1A2, CDH7, CHAD, CHADL, COL11A1, COMP, CSGALNACT1, CYR61, FRZB, GAL3ST1, HAPLN1, IHH, KIF26B, LECT1, LPPR1, PDE6B, RBP4A, SERINC5, SFRP1, SOX8, SOX9, TENM2, THBS1, UCHL1 and WFIKKN2. In addition, seven transcription factors were also associated to BCO: NFATC2, MAFB, HIF1A-ARNT, EWSR1-FLI1, NFIC, TCF3 and NF-KAPPAB.

Conclusions

Our data show that osteochondral downregulated genes are potential molecular causes of BCO in broilers, and the bacterial process seems to be, in fact, a secondary condition. Sixteen genes responsible for bone and cartilage formation were downregulated in BCO-affected broilers being strong candidate genes to trigger this disorder.

High frequency of pathogenic ACAN variants including an intragenic deletion in selected individuals with short stature

CONTEXT

Defining the underlying etiology of idiopathic short stature (ISS) improves the overall management of an individual.

OBJECTIVE

To assess the frequency of pathogenic ACAN variants in selected individuals.

DESIGN

The single-center cohort study was conducted at a tertiary university children's hospital. From 51 unrelated patients with ISS, the 16 probands aged between 3 and 18 years (12 females) with advanced bone age and/or autosomal dominant inheritance pattern of short stature were selected for the study. Fifteen family members of ACAN-positive probands were included. Exome sequencing was performed in all probands, and additional copy number variation (CNV) detection was applied in selected probands with a distinct ACAN-associated phenotype.

RESULTS

Systematic phenotyping of the study cohort yielded 37.5% (6/16) ACAN-positive probands, with all novel pathogenic variants, including a 6.082 kb large intragenic deletion, detected by array comparative genomic hybridization (array CGH) and exome data analysis. All variants were co-segregated with short stature phenotype, except in one family member with the intragenic deletion who had an unexpected growth pattern within the normal range (-0.5 SDS). One patient presented with otosclerosis, a sign not previously associated with aggrecanopathy.

CONCLUSIONS

ACAN pathogenic variants presented a common cause of familial ISS. The selection criteria used in our study were suggested for a personalized approach to genetic testing of the ACAN gene in clinical practice. Our results expanded the number of pathogenic ACAN variants, including the first intragenic deletion, and suggested CNV evaluation in patients with typical clinical features of aggrecanopathy as reasonable. Intra-familial phenotypic variability in growth patterns should be considered.

Aggrecan Hypomorphism Compromises Articular Cartilage Biomechanical Properties and Is Associated with Increased Incidence of Spontaneous Osteoarthritis

The gene encoding the proteoglycan aggrecan (Agc1) is abundantly expressed in cartilage during development and adulthood, and the loss or diminished deposition of the protein results in a wide range of skeletal malformations. Furthermore, aggrecan degradation is a hallmark of cartilage degeneration occurring in osteoarthritis. In the present study, we investigated the consequences of a partial loss of aggrecan in the postnatal skeleton and in the articular cartilage of adult mice. We took advantage of the previously described Agc1^{tm(IRES-CreERT2)} mouse line, which allows for conditional and timely-regulated deletion of floxed, cartilage-expressed genes. As previously reported, the introduction of the CreER^T2 cassette in the 3’UTR causes a disruption of the normal expression of Agc1 resulting in a hypomorphic deposition of the protein. In homozygous mice, we observed a dwarf phenotype, which persisted throughout adulthood supporting the evidence that reduced aggrecan amount impairs skeletal growth. Homozygous mice exhibited reduced proteoglycan staining of the articular cartilage at 6 and 12 months of age, increased stiffening of the extracellular matrix at six months, and developed severe cartilage erosion by 12 months. The osteoarthritis in the hypomorph mice was not accompanied by increased expression of catabolic enzymes and matrix degradation neoepitopes. These findings suggest that the degeneration found in homozygous mice is likely due to the compromised mechanical properties of the cartilage tissue upon aggrecan reduction.

The Effect of Intra-articular Injection of Autologous Microfragmented Fat Tissue on Proteoglycan Synthesis in Patients with Knee Osteoarthritis

Osteoarthritis (OA) is one of the leading musculoskeletal disorders in the adult population. It is associated with cartilage damage triggered by the deterioration of the extracellular matrix tissue. The present study explores the effect of intra-articular injection of autologous microfragmented adipose tissue to host chondrocytes and cartilage proteoglycans in patients with knee OA. A prospective, non-randomized, interventional, single-center, open-label clinical trial was conducted from January 2016 to April 2017. A total of 17 patients were enrolled in the study, and 32 knees with osteoarthritis were assessed. Surgical intervention (lipoaspiration) followed by tissue processing and intra-articular injection of the final microfragmented adipose tissue product into the affected knee(s) was performed in all patients. Patients were assessed for visual analogue scale (VAS), delayed gadolinium-enhanced magnetic resonance imaging of cartilage (dGEMRIC) and immunoglobulin G (IgG) glycans at the baseline, three, six and 12 months after the treatment. Magnetic resonance sequence in dGEMRIC due to infiltration of the anionic, negatively charged contrast gadopentetate dimeglumine (Gd-DTPA²⁻) into the cartilage indicated that the contents of cartilage glycosaminoglycans significantly increased in specific areas of the treated knee joint. In addition, dGEMRIC consequently reflected subsequent changes in the mechanical axis of the lower extremities. The results of our study indicate that the use of autologous and microfragmented adipose tissue in patients with knee OA (measured by dGEMRIC MRI) increased glycosaminoglycan (GAG) content in hyaline cartilage, which is in line with observed VAS and clinical results.

Current insights into the molecular genetic basis of dwarfism in livestock

Impairment of bone growth at a young age leads to dwarfism in adulthood. Dwarfism can be categorised as either proportionate, an overall size reduction without changes in body proportions, or disproportionate, a size reduction in one or more limbs, with changes in body proportions. Many forms of dwarfism are inherited and result from structural disruptions or disrupted signalling pathways. Hormonal disruptions are evident in Brooksville miniature Brahman cattle and Z-linked dwarfism in chickens, caused by mutations in GH1 and GHR. Furthermore, mutations in IHH are the underlying cause of creeper achondroplasia in chickens. Belgian blue cattle display proportionate dwarfism caused by a mutation in RNF11, while American Angus cattle dwarfism is caused by a mutation in PRKG2. Mutations in EVC2 are associated with dwarfism in Japanese brown cattle and Tyrolean grey cattle. Fleckvieh dwarfism is caused by mutations in the GON4L gene. Mutations in COL10A1 and COL2A1 cause dwarfism in pigs and Holstein cattle, both associated with structural disruptions, while several mutations in ACAN are associated with bulldog-type dwarfism in Dexter cattle and dwarfism in American miniature horses. In other equine breeds, such as Shetland ponies and Friesian horses, dwarfism is caused by mutations in SHOX and B4GALT7. In Texel sheep, chondrodysplasia is associated with a deletion in SLC13A1. This review discusses genes known to be involved in these and other forms of dwarfism in livestock.

The aggrecanopathies; an evolving phenotypic spectrum of human genetic skeletal diseases

The large chondroitin sulphated proteoglycan aggrecan (ACAN) is the most abundant non-collagenous protein in cartilage and is essential for its structure and function. Mutations in ACAN result in a broad phenotypic spectrum of non-lethal skeletal dysplasias including spondyloepimetaphyseal dysplasia, spondyloepiphyseal dysplasia, familial osteochondritis dissecans and various undefined short stature syndromes associated with accelerated bone maturation. However, very little is currently known about the disease pathways that underlie these aggrecanopathies, although they are likely to be a combination of haploinsufficiency and dominant-negative (neomorphic) mechanisms. This review discusses the known human and animal aggrecanopathies in the context of clinical presentation and potential disease mechanisms.

Multiple enhancers associated with ACAN suggest highly redundant transcriptional regulation in cartilage

The chondroitin sulfate proteoglycan core protein aggrecan is the major protein constituent of cartilage aside from collagen, and is largely responsible for its distinctive mechanical properties. Aggrecan is required both for proper cartilage formation in development and maintenance of mature cartilage. Prominent ACAN transcription is a conserved feature of vertebrate cartilage, although little is known about its specific transcriptional regulation. We examined the genomic interval containing human ACAN for transcriptional enhancers directing expression to cartilage, using a functional assay in transgenic zebrafish. We tested 24 conserved non-coding sequences, representing ~6% of the total sequence in the interval, and identified eleven independently capable of regulating reporter gene expression in cartilage. These enhancers were widely spaced, from >100kb upstream of the gene to within the first intron. While the majority displayed broad cartilage expression in zebrafish larvae, several were restricted to a subset of cartilage cells in the craniofacial skeleton. In older fish, the enhancers displayed differential activity; some maintained expression, either in all cartilage or preferentially in articular cartilage at the joints, while others were not active. This remarkable degree of overlapping regulatory control has been highly conserved; we identified clear orthologues of six enhancers at the chicken ACAN locus, arranged in the same order relative to the gene. These were also functional in directing expression to cartilage in transgenic zebrafish. Several enhancers contain potential binding sites for Sox9, consistent with its described role as an upstream regulator of ACAN expression. However, others lacked Sox9 consensus binding sites, implicating additional pathways and transcription factors as regulators of ACAN expression in cartilage, either in development or adult tissue. Our identification of these enhancer sequences is the necessary first step in detailed examination of the upstream regulators of ACAN expression.

1H magnetic resonance spectroscopy of nanomelic chicken cartilage: effect of aggrecan depletion on cartilage T2

OBJECTIVE

To determine the effect of proteoglycan depletion on cartilage proton magnetic resonance (MR) spectroscopy T2 using nanomelic chicken cartilage, a genetic mutant that completely lacks aggrecan.

DESIGN

Proton MR spectroscopic T2 measurements of normal embryonic and nanomelic femoral epiphyseal cartilage were obtained using a 96-echo pulse sequence with inter-echo delay times increased logarithmically over the TE period of 60 micros to 1.7 s. The relative intensity and distribution of cartilage T2 components were determined by fitting signal decay curves to a multi-exponential function. The number of T2 components in the signal decay curves was determined by the degree of freedom limited r2 of the fit.

RESULTS

For normal fetal chicken cartilage, 97.6 +/- 0.2% (mean +/- 95% confidence interval) of the total signal comprises a long T2 component (179.1 +/- 1.3 ms) with a relatively small short T2 component (0.5 +/- 0.4 ms). The T2 distribution for nanomelic cartilage is more heterogeneous with four components identified: two short T2 components (0.5 +/- 0.02 and 7.3 +/- 0.6 ms), a large intermediate component (56.4 +/- 5.6 ms), and a broadly distributed long component (137.5 +/- 16.6 ms). In nanomelic cartilage there is greater heterogeneity of cartilage T2 indicating greater variation in water proton mobility and exchange of water with the extracellular matrix.

CONCLUSION

Absence of aggrecan in the extracellular cartilage matrix produces greater heterogeneity in cartilage T2, but will not increase T2 as has been previously reported with degenerative change of the collagen matrix.

Cytogenetic analysis of California condor (Gymnogyps californianus) chromosomes: comparison with chicken (Gallus gallus) macrochromosomes

The California condor is the largest flying bird in North America and belongs to a group of New World vultures. Recovering from a near fatal population decline, and currently with only 197 extant individuals, the species remains listed as endangered. Very little genetic information exists for this species, although sexing methods employing chromosome analysis or W-chromosome specific amplification is routinely applied for the management of this monomorphic species. Keeping in mind that genetic conditions like chondrodystrophy have been identified, preliminary steps were undertaken in this study to understand the genome organization of the condor. This included an extensive cytogenetic analysis that provided (i) a chromosome number of 80 (with a likelihood of an extra pair of microchromosomes), and (ii) information on the centromeres, telomeres and nucleolus organizer regions. Further, a comparison between condor and chicken macrochromosomes was obtained by using individual chicken chromosome specific paints 1-9 and Z and W on condor metaphase spreads. Except for chromosomes 4 and Z, each of the chicken (GGA) macrochromosomes painted a single condor (GCA) macrochromosome. GGA4 paint detected complete homology with two condor chromosomes, viz., GCA4 and GCA9 providing additional proof that the latter are ancestral chromosomes in the birds. The chicken Z chromosome showed correspondence with both Z and W in the condor. The homology suggests that the condor sex chromosomes have not completely differentiated during evolution, which is unlike the majority of the non-ratites studied up till now. Overall, the study provides detailed cytogenetic and basic comparative information on condor chromosomes. These findings significantly advance the effort to study the chondrodystrophy that is responsible for over ten percent mortality in the condor.

Roles of aggrecan domains in biosynthesis, modification by glycosaminoglycans and product secretion

Aggrecan is a member of the chondroitin sulphate (CS) proteoglycan family, which also includes versican/PG-M, neurocan and brevican. Members of this family exhibit structural similarity: a G1 domain at the N-terminus and a G3 domain at the C-terminus, with a central sequence for modification by CS chains. A unique feature of aggrecan is the insertion of three additional domains, an inter-globular domain (IGD), a G2 domain and a keratan sulphate (KS) domain (sequence modified by KS chains), between the G1 domain and the CS domain (sequence modified by CS chains). The G1 and G3 domains have been implicated in product secretion, but G2, although structurally similar to the tandem repeats of G1, performs an unknown function. To define the functions of each aggrecan domain in product processing, we cloned and expressed these domains in various combinations in COS-7 cells. The results indicated that the G3 domain enhanced product secretion, alone or in combination with the KS or CS domain, and promoted glycosaminoglycan (GAG) chain attachment. Constructs containing the G1 domain were not secreted. Addition of a CS domain sequence to G1 reduced this inhibition, but GAG chain attachment was still decreased. The potential GAG chain attachment site in the IGD was occupied by GAGs, and IGD product was secreted efficiently. The KS domain was modified by GAG chains and secreted. Finally, the G2 domain was expressed but not secreted, and inhibited secretion of the IGD when expressed as an IGD-G2 combination.

Matrix proteoglycans: from molecular design to cellular function

The proteoglycan superfamily now contains more than 30 full-time molecules that fulfill a variety of biological functions. Proteoglycans act as tissue organizers, influence cell growth and the maturation of specialized tissues, play a role as biological filters and modulate growth-factor activities, regulate collagen fibrillogenesis and skin tensile strength, affect tumor cell growth and invasion, and influence corneal transparency and neurite outgrowth. Additional roles, derived from studies of mutant animals, indicate that certain proteoglycans are essential to life whereas others might be redundant. The review focuses on the most recent genetic and molecular biological studies of the matrix proteoglycans, broadly defined as proteoglycans secreted into the pericellular matrix. Special emphasis is placed on the molecular organization of the protein core, the utilization of protein modules, the gene structure and transcriptional control, and the functional roles of the various proteoglycans. When possible, proteoglycans have been grouped into distinct gene families and subfamilies offering a simplified nomenclature based on their protein core design. The structure-function relationship of some paradigmatic proteoglycans is discussed in depth and novel aspects of their biology are examined.

Gene homologs on human chromosome 15q21-q26 and a chicken microchromosome identify a new conserved segment

The genes for insulin-like growth factor 1 receptor (IGF1R), aggrecan (AGC1), beta2-microglobulin (B2M), and an H6-related gene have been mapped to a single chicken microchromosome by genetic linkage analysis. In addition, a second H6-related gene was mapped to chicken macrochromosome 3. The Igf1r and Agc1 loci are syntenic on mouse Chr 7, together with Hmx3, an H6-like locus. This suggests that the H6-related locus, which maps to the chicken microchromosome in this study, is the homolog of mouse Hmx3. The IGF1R, AGC1, and B2M loci are located on human Chr 15, probably in the same order as found for this chicken microchromosome. This conserved segment, however, is not entirely conserved in the mouse and is split between Chr 7 (Igf1r-Agc) and 2 (B2m). This comparison also predicts that the HMX3 locus may map to the short arm of human Chr 15. The conserved segment defined by the IGF1R-AGC1-HMX3-B2M loci is approximately 21-35 Mb in length and probably covers the entire chicken microchromosome. These results suggest that a segment of human Chr 15 has been conserved as a chicken microchromosome. The significance of this result is discussed with reference to the evolution of the avian and mammalian genomes.

Divergent secretory behavior of the opposite ends of aggrecan

The proteoglycan, aggrecan has a globular domain, G1, at the N terminus and a different globular domain, G3, at the C terminus. Aggrecan produced by mutant nanomelic chickens is truncated due to a premature stop codon and consequently lacks G3 and a minor portion of its chondroitin sulfate domain (Li, H., Schwartz, N. B., and Vertel, B. M.(1993) J. Biol. Chem. 268, 23504-23511). The mutant protein is retained in the endoplasmic reticulum and fails to enter the Golgi stacks (Vertel, B. M., Walters, L. M., Grier, B., Maine, N. , and Goetinck, P. F.(1993) J. Cell Sci. 104, 939-948). The homozygous mutant is lethal because of failure of chondrogenesis and osteogenesis, while the heterozygous mutant is dwarfed. To further elucidate the pathogenetic mechanisms underlying nanomelia and to determine if G1 and G3 are themselves secreted, we expressed them in transfected host cells. Expression was performed in wild type Chinese hamster ovary (CHO) cells and in mutant CHO cells which are unable to link glycosaminoglycan (GAG) chains to core proteins. We compared: (a) secretion of expressed G1 and G3 constructs containing contiguous GAG chain consensus sites and (b) GAG chain modification of the secreted proteins. We find that: 1) G3 is 24-100 times more rapidly secreted than G1; 2) secreted G3 contains contiguous chondroitin sulfate GAG chains, while secreted G1 lacks contiguous GAG chains; 3) G3 secretion is not coupled to xylosylation of contiguous GAG chain consensus sites. These results imply that G1 and G3 intrinsically differ in passage through the cell secretory route.

Nuclear retention of COL1A1 messenger RNA identifies null alleles causing mild osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a heritable connective tissue disorder characterized by bone fragility. Most cases of severe OI result from mutations in the coding region of the COL1A1 or COL1A2 genes yielding an abnormal collagen alpha chain. In contrast, many patients with mild OI show evidence of a null allele due to a premature stop mutation in the mutant RNA transcript. We have previously described a null allele arising from a splice donor mutation where the transcript containing the included intron was sequestered in the nucleus. Here we demonstrate that transcripts from null alleles arising from premature stop mutations are also present in the nucleus and absent in the cytoplasm. Using reverse transcriptase-PCR and single-strand conformational polymorphism of COL1A1 mRNA from patients with mild OI, we describe three patients with distinct null producing mutations identified from the mutant transcript within the nuclear compartment. A fourth patient with a Gly--->Arg expressed point mutation exhibits the mutant transcript in both compartments. Defining the distribution of allelic variants of COL1A1 mRNA in the nuclear and cytoplasmic compartments gives further insight into cell biology of OI and provides a strategy for investigating potential causes of a null allele.

Age-related changes in the content of the C-terminal region of aggrecan in human articular cartilage

The content of the C-terminal region of aggrecan was investigated in samples of articular cartilage from individuals ranging in age from newborn to 65 years. This region contains the globular G3 domain which is known to be removed from aggrecan in mature cartilage, probably by proteolytic cleavage, but the age-related changes in its abundance in human cartilage have not been described previously. The analysis was performed by immunosorbant assay using an antiserum (JD5) against recombinant amino acid residues of human aggrecan, on crude extracts of cartilage without further purification of aggrecan. The results showed that the content of the C-terminal region decreased with age relative to the G1 domain content (correlation coefficient = 0.463). This represented a 92% fall in the content of this region of the molecule from newborn to 65 years of age. furthermore, when the G1 content of the cartilage extracts was corrected to only include the G1 attached to aggrecan and to exclude the G1 fragments which accumulate as a by-product of normal aggrecan turnover (free G1), the age-related decrease in the C-terminal region remained very pronounced. Analysis by composite agarose/PAGE showed that the number of subpopulations of aggrecan resolved increased from one in newborn to three in adult cartilage. All of these reacted with an antiserum to the human G1 domain, but only the slowest migrating species reacted with the C-terminal region antiserum (JD5). Similar analysis by SDS/PAGE confirmed the presence of high-molecular-mass (200 kDa) proteins reactive with JD5, but no reactive fragments of lower electrophoretic mobility were detected. In contrast, when probed with the antiserum to the human G1 domain, the immunoblots showed protein species corresponding to the free G1 and G1-G2 fragments, which were present at high concentrations in adult cartilage. The results suggest that the loss of the C-terminal region is not directly part of the process of aggrecan turnover, but it is a slow independent matrix process that occurs more extensively with aging as turnover rates become slower. Young cartilage with the fastest turnover contains least molecules lacking the C-terminal region, whereas in old tissue with slow turnover few molecules retain this region. An increase in the cleavage of this region with age may also contribute to this change. The content of the C-terminal region may thus give a measure of the abundance of newly synthesized aggrecan.