Age-dependent changes in collagen crosslinks reduce the mechanical toughness of human meniscus

The mechanical resilience of the knee meniscus is provided by a group of structural proteins in the extracellular matrix. Aging can alter the quantity and molecular structure of these proteins making the meniscus more susceptible to debilitating tears. In this study, we determined the effect of aging on the quantity of structural proteins and collagen crosslinks in human lateral meniscus, and examined whether the quantity of these molecules was predictive of tensile toughness (area under the stress-strain curve). Two age groups were tested: a young group under 40 and an older group over 65 years old. Using mass spectrometry, we quantified the abundance of proteins and collagen crosslinks in meniscal tissue that was adjacent to the dumbbell-shaped specimens used to measure uniaxial tensile toughness parallel or perpendicular to the circumferential fiber orientation. We found that the enzymatic collagen crosslink deoxypyridinoline had a significant positive correlation with toughness, and reductions in the quantity of this crosslink with aging were associated with a loss of toughness in the ground substance and fibers. The non-enzymatic collagen crosslink carboxymethyl-lysine increased in quantity with aging, and these increases corresponded to reductions in ground substance toughness. For the collagenous (Types I, II, IV, VI, VIII) and non-collagenous structural proteins (elastin, decorin, biglycan, prolargin) analyzed in this study, only the quantity of collagen VIII was predictive of toughness. This study provides valuable insights on the structure-function relationships of the human meniscus, and how aging causes structural adaptations that weaken the tissue's mechanical integrity.

Effects of doxorubicin on autophagy in fibroblasts

Objectives: Doxorubicin (DOX) is a highly effective chemotherapeutic used to treat many adult and pediatric cancers, such as solid tumors, leukemia, lymphomas and breast cancer. It can also cause injuries to multiple organs, including the heart, liver, and brain or kidney, although cardiotoxicity is the most prominent side effect of DOX. In this study, we examined the potential effects of DOX on autophagy activity in two different mouse fibroblasts.Methods: Mouse embryonic fibroblasts (NIH3T3) and mouse primary cardiac fibroblasts (CFs) were treated with DOX to assess changes in the expression of two commonly used autophagy protein markers, LC3II and p62. We also examined the effects of DOX the on expression of key genes that encode components of the molecular machinery and regulators modulating autophagy in response to both extracellular and intracellular signals.Results: We observed that LC3II levels increased and p62 levels decreased following the DOX treatment in NIH3T3 cells. However, similar effects were not observed in primary cardiac fibroblasts. In addition, DOX treatment induced the upregulation of a significant number of genes involved in autophagy in NIH3T3 cells, but not in primary cardiac fibroblasts.Conclusions: Taken together, these results indicate that DOX upregulates autophagy in fibroblasts in a cell-specific manner.

Structure-Property-Processing Correlations of Graphene Bioscaffolds for Proliferation and Differentiation of C2C12 Cells

Graphene - an atomically thin layer of carbon atoms arranged in a hexagonal lattice - has gained interest as a bioscaffold for tissue engineering due to its exceptional mechanical, electrical, and thermal properties. Graphene's structure and properties are tightly coupled to synthesis and processing conditions, yet their influence on biomolecular interactions at the graphene-cell interface remains unclear. In this study, C2C12 cells were grown on graphene bioscaffolds with specific structure-property- processing-performance (SP3) correlations. Bioscaffolds were prepared using three different methods - chemical vapor deposition (CVD), sublimation of silicon carbide (SiC), and printing of liquid phase exfoliated graphene. To investigate the biocompatibility of each scaffold, cellular morphology and gene expression patterns were investigated using the bipotential mouse C2C12 cell line. Using a combination of fluorescence microscopy and qRT-PCR, we demonstrate that graphene production methods determine the structural and mechanical properties of the resulting bioscaffold, which in turn determine cell morphology, gene expression patterns, and cell differentiation fate. Therefore, production methods and resultant structure and properties of graphene bioscaffolds must be chosen carefully when considering graphene as a bioscaffold for musculoskeletal tissue engineering.

An amino-terminal fragment of apolipoprotein E4 leads to behavioral deficits, increased PHF-1 immunoreactivity, and mortality in zebrafish

Although the increased risk of developing sporadic Alzheimer's disease (AD) associated with the inheritance of the apolipoprotein E4 (APOE4) allele is well characterized, the molecular underpinnings of how ApoE4 imparts risk remains unknown. Enhanced proteolysis of the ApoE4 protein with a toxic-gain of function has been suggested and a 17 kDa amino-terminal ApoE4 fragment (nApoE41-151) has been identified in post-mortem human AD frontal cortex sections. Recently, we demonstrated in vitro, exogenous treatment of nApoE41-151 in BV2 microglial cells leads to uptake, trafficking to the nucleus and increased expression of genes associated with cell toxicity and inflammation. In the present study, we extend these findings to zebrafish (Danio rerio), an in vivo model system to assess the toxicity of nApoE41-151. Exogenous treatment of nApoE41-151 to 24-hour post-fertilization for 24 hours resulted in significant mortality. In addition, developmental abnormalities were observed following treatment with nApoE41-151 including improper folding of the hindbrain, delay in ear development, deformed yolk sac, enlarged cardiac cavity, and significantly lower heart rates. A similar nApoE31-151 fragment that differs by a single amino acid change (C>R) at position 112 had no effects on these parameters under identical treatment conditions. Decreased presence of pigmentation was noted for both nApoE31-151- and nApoE41-151-treated larvae compared with controls. Behaviorally, touch-evoked responses to stimulus were negatively impacted by treatment with nApoE41-151 but did not reach statistical significance. Additionally, triple-labeling confocal microscopy not only confirmed the nuclear localization of the nApoE41-151 fragment within neuronal populations following exogenous treatment, but also identified the presence of tau pathology, one of the hallmark features of AD. Collectively, these in vivo data demonstrating toxicity as well as sublethal effects on organ and tissue development support a novel pathophysiological function of this AD associated-risk factor.

AKT2 maintains brain endothelial claudin-5 expression and selective activation of IR/AKT2/FOXO1-signaling reverses barrier dysfunction

Inflammation-induced blood-brain barrier (BBB) dysfunction and microvascular leakage are associated with a host of neurological disorders. The tight junction protein claudin-5 (CLDN5) is a crucial protein necessary for BBB integrity and maintenance. CLDN5 is negatively regulated by the transcriptional repressor FOXO1, whose activity increases during impaired insulin/AKT signaling. Owing to an incomplete understanding of the mechanisms that regulate CLDN5 expression in BBB maintenance and dysfunction, therapeutic interventions remain underdeveloped. Here, we show a novel isoform-specific function for AKT2 in maintenance of BBB integrity. We identified that AKT2 during homeostasis specifically regulates CLDN5-dependent barrier integrity in brain microvascular endothelial cells (BMVECs) and that intervention with a selective insulin-receptor (IR) agonist, demethylasterriquinone B1 (DMAQ-B1), rescued IL-1β-induced AKT2 inactivation, FOXO1 nuclear accumulation, and loss of CLDN5-dependent barrier integrity. Moreover, DMAQ-B1 attenuated preclinical CLDN5-dependent BBB dysfunction in mice subjected to experimental autoimmune encephalomyelitis. Taken together, the data suggest a regulatory role for IR/AKT2/FOXO1-signaling in CLDN5 expression and BBB integrity during neuroinflammation.

A Cold Atmospheric Pressure Plasma Discharge Device Exerts Antimicrobial Effects

A cold atmospheric pressure plasma device was developed using two parallel plates of Low Temperature Co-fired Ceramic with embedded electrodes. The 2.4 cm wide by 1 mm deep plasma discharge operates at 20 kHz with a 2-5 kV AC drive signal across a 0.25 mm gap. Mixed Argon/oxygen plasmas were directed between the plates to flow toward a bacterial biofilm sample for treatment. Results showed that at 4-5 kV the plasma etched away a bacterial biofilm on glass in 10 minutes. In addition, we showed that short plasma treatments rapidly killed biofilm resident bacteria with ED₉₀ values of <15 s.

Investigation of multiphasic 3D-bioplotted scaffolds for site-specific chondrogenic and osteogenic differentiation of human adipose-derived stem cells for osteochondral tissue engineering applications

Osteoarthritis is a degenerative joint disease that limits mobility of the affected joint due to the degradation of articular cartilage and subchondral bone. The limited regenerative capacity of cartilage presents significant challenges when attempting to repair or reverse the effects of cartilage degradation. Tissue engineered medical products are a promising alternative to treat osteochondral degeneration due to their potential to integrate into the patient's existing tissue. The goal of this study was to create a scaffold that would induce site-specific osteogenic and chondrogenic differentiation of human adipose-derived stem cells (hASC) to generate a full osteochondral implant. Scaffolds were fabricated using 3D-bioplotting of biodegradable polycraprolactone (PCL) with either β-tricalcium phosphate (TCP) or decellularized bovine cartilage extracellular matrix (dECM) to drive site-specific hASC osteogenesis and chondrogenesis, respectively. PCL-dECM scaffolds demonstrated elevated matrix deposition and organization in scaffolds seeded with hASC as well as a reduction in collagen I gene expression. 3D-bioplotted PCL scaffolds with 20% TCP demonstrated elevated calcium deposition, endogenous alkaline phosphatase activity, and osteopontin gene expression. Osteochondral scaffolds comprised of hASC-seeded 3D-bioplotted PCL-TCP, electrospun PCL, and 3D-bioplotted PCL-dECM phases were evaluated and demonstrated site-specific osteochondral tissue characteristics. This technique holds great promise as cartilage morbidity is minimized since autologous cartilage harvest is not required, tissue rejection is minimized via use of an abundant and accessible source of autologous stem cells, and biofabrication techniques allow for a precise, customizable methodology to rapidly produce the scaffold.

Mechanical Properties of Graphene Foam and Graphene Foam - Tissue Composites

Graphene foam (GF), a 3-dimensional derivative of graphene, has received much attention recently for applications in tissue engineering due to its unique mechanical, electrical, and thermal properties. Although GF is an appealing material for cartilage tissue engineering, the mechanical properties of GF - tissue composites under dynamic compressive loads have not yet been reported. The objective of this study was to measure the elastic and viscoelastic properties of GF and GF-tissue composites under unconfined compression when quasi-static and dynamic loads are applied at strain magnitudes below 20%. The mechanical tests demonstrate a 46% increase in the elastic modulus and a 29% increase in the equilibrium modulus after 28-days of cell culture as compared to GF soaked in tissue culture medium for 24h. There was no significant difference in the amount of stress relaxation, however, the phase shift demonstrated a significant increase between pure GF and GF that had been soaked in tissue culture medium for 24h. Furthermore, we have shown that ATDC5 chondrocyte progenitor cells are viable on graphene foam and have identified the cellular contribution to the mechanical strength and viscoelastic properties of GF - tissue composites, with important implications for cartilage tissue engineering.

DockoMatic 2.0: high throughput inverse virtual screening and homology modeling

DockoMatic is a free and open source application that unifies a suite of software programs within a user-friendly graphical user interface (GUI) to facilitate molecular docking experiments. Here we describe the release of DockoMatic 2.0; significant software advances include the ability to (1) conduct high throughput inverse virtual screening (IVS); (2) construct 3D homology models; and (3) customize the user interface. Users can now efficiently setup, start, and manage IVS experiments through the DockoMatic GUI by specifying receptor(s), ligand(s), grid parameter file(s), and docking engine (either AutoDock or AutoDock Vina). DockoMatic automatically generates the needed experiment input files and output directories and allows the user to manage and monitor job progress. Upon job completion, a summary of results is generated by Dockomatic to facilitate interpretation by the user. DockoMatic functionality has also been expanded to facilitate the construction of 3D protein homology models using the Timely Integrated Modeler (TIM) wizard. The wizard TIM provides an interface that accesses the basic local alignment search tool (BLAST) and MODELER programs and guides the user through the necessary steps to easily and efficiently create 3D homology models for biomacromolecular structures. The DockoMatic GUI can be customized by the user, and the software design makes it relatively easy to integrate additional docking engines, scoring functions, or third party programs. DockoMatic is a free comprehensive molecular docking software program for all levels of scientists in both research and education.

Osteochondritis dissecans knee histology studies have variable findings and theories of etiology

BACKGROUND

Although many etiological theories have been proposed for osteochondritis dissecans (OCD), its etiology remains unclear. Histological analysis of the articular cartilage and subchondral bone tissues of OCD lesions can provide useful information about the cellular changes and progression of OCD. Previous research is predominantly comprised of retrospective clinical studies from which limited conclusions can be drawn.

QUESTIONS/PURPOSES

The purposes of this study were threefold: (1) Is osteonecrosis a consistent finding in OCD biopsy specimens? (2) Is normal articular cartilage a consistent finding in OCD biopsy specimens? (3) Do histological studies propose an etiology for OCD based on the tissue findings?

METHODS

We searched the PubMed, Embase, and CINAHL databases for studies that conducted histological analyses of OCD lesions of the knee and identified 1560 articles. Of these, 11 met our inclusion criteria: a study of OCD lesions about the knee, published in the English language, and performed a histological analysis of subchondral bone and articular cartilage. These 11 studies were assessed for an etiology proposed in the study based on the study findings.

RESULTS

Seven of 11 studies reported subchondral bone necrosis. Four studies reported normal articular cartilage, two studies reported degenerated or irregular articular cartilage, and five studies found a combination of normal and degenerated or irregular articular cartilage. Five studies proposed trauma or repetitive stress and two studies proposed poor blood supply as possible etiologies.

CONCLUSIONS

We found limited research on histological analysis of OCD lesions of the knee. Future studies with consistent methodology are necessary to draw major conclusions about the histology and progression of OCD lesions. Inconsistent histologic findings have resulted in a lack of consensus regarding the presence of osteonecrosis, whether the necrosis is primary or secondary, the association of cartilage degeneration, and the etiology of OCD. Such studies could use a standardized grading system to allow better comparison of findings.

Clinical significance of interleukin (IL)-6 in cancer metastasis to bone: potential of anti-IL-6 therapies

Metastatic events to the bone occur frequently in numerous cancer types such as breast, prostate, lung, and renal carcinomas, melanoma, neuroblastoma, and multiple myeloma. Accumulating evidence suggests that the inflammatory cytokine interleukin (IL)-6 is frequently upregulated and is implicated in the ability of cancer cells to metastasize to bone. IL-6 is able to activate various cell signaling cascades that include the STAT (signal transducer and activator of transcription) pathway, the PI3K (phosphatidylinositol-3 kinase) pathway, and the MAPK (mitogen-activated protein kinase) pathway. Activation of these pathways may explain the ability of IL-6 to mediate various aspects of normal and pathogenic bone remodeling, inflammation, cell survival, proliferation, and pro-tumorigenic effects. This review article will discuss the role of IL-6: 1) in bone metabolism, 2) in cancer metastasis to bone, 3) in cancer prognosis, and 4) as potential therapies for metastatic bone cancer.

Collagen 11a1 is indirectly activated by lymphocyte enhancer-binding factor 1 (Lef1) and negatively regulates osteoblast maturation

Alpha 1 (XI) collagen (Col11a1) is essential for normal skeletal development. Mutations in Col11a1 cause Marshall and Stickler syndromes, both of which are characterized by craniofacial abnormalities, nearsightedness and hearing deficiencies. Despite its link to human diseases, few studies have described factors that control Col11a1 transcription. We previously identified Col11a1 as a differentially expressed gene in Lef1-suppressed MC3T3 preosteoblasts. Here we report that Lef1 activates the Col11a1 promoter. This activation is dependent upon the DNA binding domain of Lef1, but does not require the beta-catenin interaction domain, suggesting that it is not responsive to Wnt signals. Targeted suppression of Col11a1 with an antisense morpholino accelerated osteoblastic differentiation and mineralization in C2C12 cells, similar to what was observed in Lef1-suppressed MC3T3 cells. Moreover incubation with a purified Col11a1 N-terminal fragment, V1B, prevented alkaline phosphatase expression in MC3T3 and C2C12 cells. These results suggest that Lef1 is an activator of the Col11a1 promoter and that Col11a1 suppresses terminal osteoblast differentiation.

Calpain-cleavage of alpha-synuclein: connecting proteolytic processing to disease-linked aggregation

Parkinson's disease (PD) and dementia with Lewy bodies (DLB) are both characterized pathologically by the presence of neuronal inclusions termed Lewy bodies (LBs). A common feature found in LBs are aggregates of alpha-synuclein (alpha-Syn), and although it is now recognized that alpha-Syn is the major building block for these toxic filaments, the mechanism of how this occurs remains unknown. In the present study, we demonstrate that proteolytic processing of alpha-Syn by the protease calpain I leads to the formation of aggregated high-molecular weight species and adoption of a beta-sheet structure. To determine whether calpain-cleavage of alpha-Syn occurs in PD and DLB, we designed site-directed calpain-cleavage antibodies to alpha-Syn and tested their utility in several animal model systems. Detection of calpain-cleaved alpha-Syn was evident in mouse models of cerebral ischemia and PD and in a Drosophila model of PD. In the human PD and DLB brain, calpain-cleaved alpha-Syn antibodies immunolabeled LBs and neurites in the substantia nigra. Moreover, calpain-cleaved alpha-Syn fragments identified within LBs colocalized with activated calpain in neurons of the PD and DLB brains. These findings suggest that calpain I may participate in the disease-linked aggregation of alpha-Syn in various alpha-synucleinopathies.

Early events in cartilage repair after subchondral bone microfracture

The current study investigated healing of large full-thickness articular cartilage defects during the first 8 weeks with and without penetration of the subchondral bone using microfracture in an established equine model of cartilage healing. Chondral defects in the weightbearing portion of the medial femoral condyle were made bilaterally; one defect in each horse was microfractured whereas the contralateral leg served as the control. The expression of cartilage extracellular matrix components (Types I and II collagen and aggrecan) was evaluated using histologic techniques, reverse transcription coupled polymerase chain reaction, in situ hybridization, and immunohistochemistry. This study confirms an increase in Type II collagen mRNA expression in repair tissue as early as 6 weeks after microfracture. Although other matrix mRNA and protein levels changed in concentration and tissue location over the course of the study, no significant differences were seen in microfractured defects. Although the microfracture techniques appear to improve clinical functionality, volume of repair tissue, and augment Type II collagen content, aggrecan content is less than ideal. Therefore, methods to enhance key matrix components such as aggrecan after microfracture may additionally improve repair tissue observed after the procedure.

Cis-acting elements regulate alternative splicing of exons 6A, 6B and 8 of the alpha1(XI) collagen gene and contribute to the regional diversification of collagen XI matrices

Consecutive exons 6A, 6B, 7 and 8 that encode the variable region of the amino-terminal domain (NTD) of the col11a1 gene product undergo a complex pattern of alternative splicing that is both tissue-dependent and developmentally regulated. Expression of col11a1 is predominantly associated with cartilage where it plays a critical role in skeletal development. At least five splice-forms (6B-7-8, 6A-7-8, 7-8, 6B-7 and 7) are found in cartilage. Splice-forms containing exon 6B or 8 have distinct distributions in the long bone during development, while in non-cartilage tissues, splice-form 6A-7-8 is typically expressed. In order to study this complex and tissue-specific alternative splicing, a mini-gene that contains mouse genomic sequence from exon 5 to 11, flanking the variable region of alpha1(XI)-NTD, was constructed. The minigene was transfected into chondrocytic (RCS) and non-chondrocytic (A204) cell lines that endogenously express alpha1(XI), as well as 293 cells which do not express alpha1(XI). Alternative splicing in RCS and A204 cells reflected the appropriate cartilage and non-cartilage patterns while 293 cells produced only 6A-7-8. This suggests that 6A-7-8 is the default splicing pathway and that cell or tissue-specific trans-acting factors are required to obtain pattern of the alternative splicing of alpha1(XI) pre-mRNA observed in chondrocytes. Deletional analysis was used to identify cis-acting regions important for regulating splicing. The presence of the intact exon 7 was required to generate the full complex chondrocytic pattern of splicing. Furthermore, deletional mapping of exon 6B identified sequences required for expression of exon 6B in RCS cells and these may correspond to purine-rich (ESE) and AC-rich (ACE) exonic splicing enhancers.

Deamidation of human beta B1 alters the elongated structure of the dimer

The purpose of these experiments was to determine if truncation and deamidation alter the structure of a human lens protein, beta B1-crystallin. Recombinant wild type and a deamidated form of recombinant beta B1 were expressed in Escherichia coli. Wild type beta B1 was also enzymatically cleaved to generate a physiologically-relevant truncated beta B1. Purity and size of the expressed proteins were confirmed by SDS-PAGE and electrospray ionization mass spectrometry. Size exclusion chromatography and light scattering were used to determine aggregation states of beta B1. Protein conformations were predicted from sedimentation velocity analysis. Molecular weights of 49,000 and 54,000 Da were obtained for wild type beta B1 by sedimentation equilibrium and light scattering, respectively. A sedimentation coefficient of 2.7 S was determined for wild type beta B1. Molecular weights of 54,000 and 60,000 Da were determined for deamidated beta B1 by sedimentation equilibrium and light scattering, respectively. However, deamidated beta B1 eluted earlier than wild type beta B1 on size exclusion chromatography, with an estimated molecular weight between 78,000 and 116,000 Da. Loss of the extensions of beta B1 caused abnormal association of the protein with the stationary phase during size exclusion chromatography. Wild type beta B1 was predicted to form a dimer with an elongated structure. The earlier elution of the deamidated beta B1 dimer on size exclusion chromatography suggested the dimer was less compact. Truncation caused abnormal column interactions suggesting an altered conformation. These changes are important because truncation and deamidation occur extensively in aging human lenses and may be important for senile cataract formation.

Comparison of Northern blot hybridization and a reverse transcriptase-polymerase chain reaction technique for measurement of mRNA expression of metalloproteinases and matrix components in articular cartilage and synovial membrane from horses with osteoarthritis

OBJECTIVE

To determine relative amounts of mRNA expression of aggrecan, type-II collagen, matrix metalloproteinase (MMP) 1, and MMP3 in articular cartilage and synovial membrane samples from healthy equine joints and joints with osteoarthritis (OA) and to compare results of Northern blot hybridization with results of a reverse transcriptase-polymerase chain reaction (RT-PCR) assay.

SAMPLE POPULATION

Articular cartilage samples from 8 pairs of joints (1 with OA and 1 healthy) from 6 horses and synovial membrane samples from 6 pairs of joints from 5 horses.

PROCEDURE

RNA was extracted from samples by use of a modified Trizol procedure. Northern blot hybridization and the RT-PCR assay were performed; results were quantitated by use of glyceraldehyde 3-phosphate dehydrogenase as an internal standard.

RESULTS

Articular cartilage samples from joints with mild or moderate OA yielded less total RNA than samples from joints with severe OA. Northern blot hybridization indicated that type-II collagen mRNA expression in articular cartilage samples from joints with OA was significantly greater than expression in samples from healthy joints. The RT-PCR assay identified low levels of MMP3 mRNA expression in 4 of 8 sets of articular cartilage samples and 4 of 6 sets of synovial membrane samples, whereas Northern blot hybridization identified MMP3 mRNA expression in only 1 of 6 sets of articular cartilage samples and 1 of 6 sets of synovial membrane samples.

CONCLUSIONS

A RT-PCR assay is more sensitive than Northern blot hybridization for detection of MMP3 mRNA expression in articular cartilage and synovial membrane and requires smaller samples.

Developmentally regulated alternative splicing of the alpha1(XI) collagen chain: spatial and temporal segregation of isoforms in the cartilage of fetal rat long bones

Type XI collagen is a component of the heterotypic collagen fibrils of fetal cartilage and is required to maintain the unusually thin diameter of these fibrils. The mature matrix form of the molecule retains an N-terminal variable region whose structure is modulated by alternative exon splicing that is tissue-specific and developmentally regulated. In the alpha1(XI) chain, antibodies to two of the peptides, p6b and p8, encoded by the alternatively spliced exons localized these epitopes to the surface of the collagen fibrils and were used to determine the pattern of isoform expression during the development of rat long bones (humerus). Expression of the p6b isoform was restricted to the periphery of the cartilage underlying the perichondrium of the diaphysis, a pattern that appears de novo at embryonic Day (E) 14. P8 isoforms appeared to be associated with early stages of chondrocyte differentiation and were detected throughout prechondrogenic mesenchyme and immature cartilage. After E16, p8 isoforms gradually disappeared from the diaphysis and then from the epiphysis preceding chondrocyte hypertrophy, but were highly evident at the periarticular joint surface, where ongoing chondrogenesis accompanies the formation of articular cartilage. The spatially restricted and differentiation-specific distribution of alpha1(XI) isoforms is evidence that Type XI collagen participates in skeletal development via a mechanism that may be distinct from regulation of fibrillogenesis.

Structural organization of distinct domains within the non-collagenous N-terminal region of collagen type XI

Collagen XI is a heterotrimeric molecule found predominantly in heterotypic cartilage fibrils, where it is involved in the regulation of fibrillogenesis. This function is thought to involve the complex N-terminal domain. The goal of this current study was to examine its structural organization to further elucidate the regulatory mechanism. The amino-propeptide (alpha1-Npp) alone or with isoforms of the variable region were recombinantly expressed and purified by affinity and molecular sieve chromatography. Cys-1-Cys-4 and Cys-2-Cys-3 disulfide bonds were detected by liquid chromatography-tandem mass spectrometry. This pattern is identical to the homologous alpha2-Npp, indicating that the recombinant proteins were folded correctly. Anomalous elution on molecular sieve chromatography suggested that the variable region was extended, which was confirmed using rotary shadowing; the alpha1-Npp formed a globular "head" and the variable region an extended "tail." Circular dichroism spectra analysis determined that the alpha1-Npp comprised 33% beta-sheet, whereas the variable region largely comprised non-periodic structure. Taken together, these results imply that the alpha1-Npp cannot be accommodated within the core of the fibril and that the variable region and/or minor helix facilitates its exclusion to the fibril surface. This provides further support for regulation of fibril diameter by steric hindrance or by interactions with other matrix components that affect fibrillogenesis.

Temporal and spatial expression of alternative splice-forms of the alpha1(XI) collagen gene in fetal rat cartilage

Type XI collagen, a member of the group of fibrillar collagens, plays a regulatory role in the formation of the collagen fibril network in cartilage and consequently plays a pivotal role in the formation of the endochondral skeleton. The mechanism by which type XI collagen limits fibril growth appears to involve the large noncollagenous amino terminal domain. Complex alternative splicing occurs within this domain in two of the three constituent subunits, alpha1(XI) and alpha2(XI). In the alpha1(XI) chain, three alternatively spliced exons encoding one very basic and two very acidic peptides generate six spliceforms and protein isoforms. In order to better understand the significance of this alternative splicing, we have examined fetal rat cartilage to determine: (a) the relationship between alternative splicing and chondrogenesis in limb bud micromass culture; (b) the relative levels of expression of each of the splice-forms by ribonuclease protection; and (c) the distribution of splice-forms and protein isoforms by in situ hybridization and immunohistochemistry. The results indicate that the pattern of alternative splicing of the alpha1(XI) chain is tightly linked to chondrogenesis. The two most abundant spliceforms in fetal rib cartilage are v(o), lacking all three exons, and v1b, containing the exon encoding the basic peptide. While most of the spliceforms show a general distribution in nasal, Meckel's, and rib cartilage, v1b was restricted to the dorsal portion of the fetal rib. This distribution appears to correlate with the portion of the rib which will ultimately ossify, rather than with any of the differentiative states of chondrocytes. Together these results suggest that alternative splicing within the amino terminal domain of the alpha1(XI) chain may contribute to the function of type XI collagen and that expression of the basic v1b peptide may play a role in endochondral ossification.

Ultrastructural localization of collagen types II, IX, and XI in the growth plate of human rib and fetal bovine epiphyseal cartilage: type XI collagen is restricted to thin fibrils

The collagen fibrils of hyaline cartilage vary in diameter depending on developmental stage and location within the tissue. In general, growth plates and fetal epiphyseal cartilages contain fibrils with diameters of less than approximately 25 nm, whereas the permanent cartilage of adult tissues contains fibrils of approximately 30-200 nm. The interstitial collagen fibrils of fetal cartilage are complex, having at least three collagen types as integral components. Type XI, a member of the fibrillar collagen class, has been proposed to limit fibril diameter. To test this proposition we sought to determine if Type XI collagen was preferentially associated with fibrils of smaller diameter. We focused our study on human juvenile rib growth plate, which has thin fibrils in the hypertrophic zone, thick fibrils in the resting zone or permanent cartilage, and a mixture of thin and thick fibrils in the proliferative zone. Tissues were examined by immunoelectron microscopy with antipeptide antibodies to the carboxyl telopeptide and to the amino terminal non-triple-helical domains of alpha 1 (XI). These studies showed that (a) both epitopes of Type XI collagen were readily accessible to antibodies at the fibrillar surface, (b) Type XI collagen was associated predominantly with fibrils < 25 nm in diameter, (c) Type XI collagen was not found in thick fibrils even after disruption with chaotropic agents, and (d) collagen Types II and IX were associated with fibrils of all sizes. These studies were extended to human newborn epiphyseal cartilage and to fetal calf cartilage, with the same result.

Alternative exon splicing within the amino-terminal nontriple-helical domain of the rat pro-alpha 1(XI) collagen chain generates multiple forms of the mRNA transcript which exhibit tissue-dependent variation

Type XI collagen is an integral, although minor component of cartilage collagen fibrils. We have established that alternative exon usage is a mechanism for increasing structural diversity within the amino-terminal nontriple helical domain of the pro-alpha 1(XI) collagen gene. cDNA clones spanning the amino-terminal domain were selected from a rat chondrosarcoma library, and were shown to contain two major sequence differences from the previously reported human sequence. The first difference was the replacement of sequence encoding an acidic domain of 39 amino acids in length by a sequence encoding a 51-amino acid basic domain with a predicted pI of 11.9. The second difference was the absence of a sequence that would translate into a highly acidic 85-amino acid sequence downstream from the first variation. These two changes, expressed together, result in the replacement of most of the acidic domain with one that is smaller and basic. These two sequence differences serve to identify subdomains of a variable region, designated V1 and V2, respectively. V1a is defined as the acidic 39-amino acid sequence element and V1b is defined as the 51-amino acid basic sequence. Analysis of genomic DNA revealed that both V1a and V1b are encoded by separate adjacent exons in the rat genome and V2 is also encoded in a single exon downstream. Analysis of mRNA from cartilage-derived sources revealed a complex pattern of alpha 1(XI) transcript expression due to differential exon usage. In non-cartilage sources, the pattern is less complex; the most prevalent form is the one containing the two acidic sequences, V1a and V2.

A fibrillar collagen gene, Col11a1, is essential for skeletal morphogenesis

Mice that are homozygous for the autosomal recessive chondrodysplasia (cho) mutation die at birth with abnormalities in cartilage of limbs, ribs, mandible, and trachea. Limb bones of newborn cho/cho mice are wider at the metaphyses than normal bones and only about half the normal length. By linkage analysis, the cho gene and the gene encoding the alpha 1 (XI) chain of cartilage collagen XI were mapped to the same region of chromosome 3. Deletion of a cytidine residue about 570 nt downstream of the translation initiation codon in cho alpha 1 (XI) mRNA causes a reading frame shift and introduces a premature stop codon. The data demonstrate that collagen XI is essential for normal formation of cartilage collagen fibrils and the cohesive properties of cartilage. The results also suggest that the normal differentiation and spatial organization of growth plate chondrocytes is critially dependent on the presence of type XI collagen in cartilage extracellular matrix.

Characterization of type II and type XI collagen synthesis by an immortalized rat chondrocyte cell line (IRC) having a low level of type II collagen mRNA expression

The biosynthesis of type XI and type II collagens was examined using a stable rat chondrocyte cell line established by W. E. Horton et al. (1988, Exp. Cell Res. 178, 457-468.). These cells (IRC; immortalized rat chondrocytes) were created by transformation with a murine retrovirus carrying the v-myc and v-raf oncogenes. They grow in suspension culture as multicellular aggregates and synthesize typical cartilage proteins, aggrecan and link protein. Type II collagen is absent or synthesized at severely reduced levels, as shown by Northern analysis of mRNA. Thus, this cell type represents a unique model in which to study cartilage matrix protein interactions in the absence of type II collagen. A more detailed look at the proteins secreted into the medium by metabolically labeled IRC cells revealed the presence of collagenase-sensitive bands when analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The bands were identified as the alpha 1, alpha 2, and alpha 3 chains of heterotrimeric type XI collagen by electrophoretic migration after pepsin digestion, by CNBr peptide mapping, and by immunoprecipitation with antibodies to rat alpha 1(XI). mRNA for all three chains was detected by Northern blot analysis. The data indicate that the low level of alpha 1(II) mRNA previously detected in these cells is translated into pro alpha 3(XI) polypeptide chains which are incorporated into molecules of type XI. Under normal culture conditions, homotrimers of type II collagen were not detected. The carboxyl propeptide domain of the fibrillar collagens directs chain selection and molecular assembly of the trimeric molecules. The sequence of the carboxyl propeptide domain from pro alpha 3(XI) of IRC cells was found to be identical to this domain from pro alpha 1(II) of swarm rat chondrosarcoma, supporting previous evidence that pro alpha 3(XI) and pro alpha 1(II) have the same primary structure. When cultured in the presence of 50 mM arginine, IRC cells could be induced to synthesize pro alpha 1(II) chains in excess over pro alpha 1(XI) and pro alpha 2(XI). Only under these conditions were type II collagen molecules detected, suggesting a preferential association of pro alpha 1(II) with the pro alpha 1 and/or pro alpha 2 chains of type XI collagen.