Type VI collagen expression is upregulated in the early events of chondrocyte differentiation

Mass Spectrometry Imaging of the Subchondral Bone in Osteoarthritis Reveals Tissue Remodeling of Extracellular Matrix Proteins that Precede Cartilage Loss

Osteoarthritis (OA) of the knee is a degenerative condition of the skeletal extracellular matrix (ECM) marked by the loss of articular cartilage and subchondral bone homeostasis. Treatments for OA in the knee beyond full joint replacement are lacking primarily due to gaps in molecular knowledge of the biological drivers of disease. Here, Mass Spectrometry Imaging (MSI) enabled molecular spatial mapping of the proteomic landscape of human knee tissues. Histologic sections of human tibial plateaus from OA patients and cadaveric controls were treated with collagenase III to target ECM proteins prior to imaging using a timsTOF fleX mass spectrometer (Bruker) for matrix-assisted laser desorption ionization (MALDI)-MSI of bone and cartilage proteins in human knees. Spatial MSI data of the knee, using sections of the tibial plateau from non-arthritic, cadaveric donors or from knee replacement patients with medial OA were processed and automatically segmented identifying distinct areas of joint damage. ECM peptide markers compared either OA to cadaveric tissues or OA medial to OA lateral. Not only did candidate peptides distinguish OA relative to intact cartilage, but also emphasized a significant spatial difference between OA and intact subchondral bone (AUROC >0.85). Overall, 31 peptide candidates from ECM proteins, including COL1A1, COL3A1, and unanticipated detection of collagens COL6A1 and COL6A3 in adult bone, exhibited significantly elevated abundance in diseased tissue. Highly specific hydroxyproline-containing collagens dominated OA subchondral bone directly under regions of lost cartilage revealing dramatic tissue remodeling providing molecular details on the progression of joint degeneration in OA. The identification of specific spatial markers for the progression of subchondral bone degeneration in OA advances our molecular understanding of coupled deterioration of joint tissues.

The coordinated activities of collagen VI and XII in maintenance of tissue structure, function and repair: evidence for a physical interaction

Collagen VI and collagen XII are structurally complex collagens of the extracellular matrix (ECM). Like all collagens, type VI and XII both possess triple-helical components that facilitate participation in the ECM network, but collagen VI and XII are distinct from the more abundant fibrillar collagens in that they also possess arrays of structurally globular modules with the capacity to propagate signaling to attached cells. Cell attachment to collagen VI and XII is known to regulate protective, proliferative or developmental processes through a variety of mechanisms, but a growing body of genetic and biochemical evidence suggests that at least some of these phenomena may be potentiated through mechanisms that require coordinated interaction between the two collagens. For example, genetic studies in humans have identified forms of myopathic Ehlers-Danlos syndrome with overlapping phenotypes that result from mutations in either collagen VI or XII, and biochemical and cell-based studies have identified accessory molecules that could form bridging interactions between the two collagens. However, the demonstration of a direct or ternary structural interaction between collagen VI or XII has not yet been reported. This Hypothesis and Theory review article examines the evidence that supports the existence of a functional complex between type VI and XII collagen in the ECM and discusses potential biological implications.

Cooperation of cell adhesion and autophagy in the brain: Functional roles in development and neurodegenerative disease

Cellular adhesive connections directed by the extracellular matrix (ECM) and maintenance of cellular homeostasis by autophagy are seemingly disparate functions that are molecularly intertwined, each regulating the other. This is an emerging field in the brain where the interplay between adhesion and autophagy functions at the intersection of neuroprotection and neurodegeneration. The ECM and adhesion proteins regulate autophagic responses to direct protein clearance and guide regenerative programs that go awry in brain disorders. Concomitantly, autophagic flux acts to regulate adhesion dynamics to mediate neurite outgrowth and synaptic plasticity with functional disruption contributed by neurodegenerative disease. This review highlights the cooperative exchange between cellular adhesion and autophagy in the brain during health and disease. As the mechanistic alliance between adhesion and autophagy has been leveraged therapeutically for metastatic disease, understanding overlapping molecular functions that direct the interplay between adhesion and autophagy might uncover therapeutic strategies to correct or compensate for neurodegeneration.

Alginate-hyaluronic acid-collagen composite hydrogel favorable for the culture of chondrocytes and their phenotype maintenance

Articular cartilage has limited regeneration capacity, thus significant challenge has been made to restore the functions. The development of hydrogels that can encapsulate and multiply cells, and then effectively maintain the chondrocyte phenotype is a meaningful strategy to this cartilage repair. In this study, we prepared alginate-hyaluronic acid based hydrogel with type I collagen being incorporated, namely Alg-HA-Col composite hydrogel. The incorporation of Col enhanced the chemical interaction of molecules, and the thermal stability and dynamic mechanical properties of the resultant hydrogels. The primary chondrocytes isolated from rat cartilage were cultured within the composite hydrogel and the cell viability recorded revealed active proliferation over a period of 21 days. The mRNA levels of chondrocyte phenotypes, including SOX9, collagen type II, and aggrecan, were significantly up-regulated when the cells were cultured within the Alg-HA-Col gel than those cultured within the Alg-HA. Furthermore, the secretion of sulphated glycosaminoglycan, a cartilage-specific matrix molecule, was recorded higher in the collagen-added composite hydrogel. Although more in-depth studies are required such as the in vivo functions, the currently-prepared Alg-HA-Col composite hydrogel is considered to provide favorable 3-dimensional matrix conditions for the cultivation of chondrocytes. Moreover, the cell-cultured constructs may be useful for the cartilage repair and tissue engineering.

Collagen VI enhances cartilage tissue generation by stimulating chondrocyte proliferation

Regeneration of human cartilage is inherently inefficient. Current cell-based approaches for cartilage repair, including autologous chondrocytes, are limited by the paucity of cells, associated donor site morbidity, and generation of functionally inferior fibrocartilage rather than articular cartilage. Upon investigating the role of collagen VI (Col VI), a major component of the chondrocyte pericellular matrix (PCM), we observe that soluble Col VI stimulates chondrocyte proliferation. Interestingly, both adult and osteoarthritis chondrocytes respond to soluble Col VI in a similar manner. The proliferative effect is, however, strictly due to the soluble Col VI as no proliferation is observed upon exposure of chondrocytes to immobilized Col VI. Upon short Col VI treatment in 2D monolayer culture, chondrocytes maintain high expression of characteristic chondrocyte markers like Col2a1, agc, and Sox9 whereas the expression of the fibrocartilage marker Collagen I (Col I) and of the hypertrophy marker Collagen X (Col X) is minimal. Additionally, Col VI-expanded chondrocytes show a similar potential to untreated chondrocytes in engineering cartilage in 3D biomimetic hydrogel constructs. Our study has, therefore, identified soluble Col VI as a biologic that can be useful for the expansion and utilization of scarce sources of chondrocytes, potentially for autologous chondrocyte implantation. Additionally, our results underscore the importance of further investigating the changes in chondrocyte PCM with age and disease and the subsequent effects on chondrocyte growth and function.

Type VI collagen deficiency induces osteopenia with distortion of osteoblastic cell morphology

Bone consists of type I collagen as a major protein with minor various matrix proteins. Type VI collagen is one of bone matrix proteins but its function is not known. We therefore examined the effects of type VI collagen deficiency on bone. 3D-μCT analysis revealed that type VI collagen deficiency reduced cancellous bone mass. Cortical bone mass was not affected. Type VI collagen deficiency distorted the shape of osteoblasts both in the cancellous bone and in the cambium layer of periosteal region. Furthermore, type VI collagen deficiency disorganized collagen arrangement. These data indicate that type VI collagen contributes to maintain bone mass.

Endothelium-derived Netrin-4 supports pancreatic epithelial cell adhesion and differentiation through integrins α2β1 and α3β1

BACKGROUND

Netrins have been extensively studied in the developing central nervous system as pathfinding guidance cues, and more recently in non-neural tissues where they mediate cell adhesion, migration and differentiation. Netrin-4, a distant relative of Netrins 1-3, has been proposed to affect cell fate determination in developing epithelia, though receptors mediating these functions have yet to be identified.

METHODOLOGY/PRINCIPAL FINDINGS

Using human embryonic pancreatic cells as a model of developing epithelium, here we report that Netrin-4 is abundantly expressed in vascular endothelial cells and pancreatic ductal cells, and supports epithelial cell adhesion through integrins α2β1 and α3β1. Interestingly, we find that Netrin-4 recognition by embryonic pancreatic cells through integrins α2β1 and α3β1 promotes insulin and glucagon gene expression. In addition, full genome microarray analysis revealed that fetal pancreatic cell adhesion to Netrin-4 causes a prominent down-regulation of cyclins and up-regulation of negative regulators of the cell cycle. Consistent with these results, a number of other genes whose activities have been linked to developmental decisions and/or cellular differentiation are up-regulated.

CONCLUSIONS/SIGNIFICANCE

Given the recognized function of blood vessels in epithelial tissue morphogenesis, our results provide a mechanism by which endothelial-derived Netrin-4 may function as a pro-differentiation cue for adjacent developing pancreatic cell populations expressing adhesion receptors α2β1 and α3β1 integrins.

Developmental and osteoarthritic changes in Col6a1-knockout mice: biomechanics of type VI collagen in the cartilage pericellular matrix

OBJECTIVE

Chondrocytes, the sole cell type in articular cartilage, maintain the extracellular matrix (ECM) through a homeostatic balance of anabolic and catabolic activities that are influenced by genetic factors, soluble mediators, and biophysical factors such as mechanical stress. Chondrocytes are encapsulated by a narrow tissue region termed the "pericellular matrix" (PCM), which in normal cartilage is defined by the exclusive presence of type VI collagen. Because the PCM completely surrounds each cell, it has been hypothesized that it serves as a filter or transducer for biochemical and/or biomechanical signals from the cartilage ECM. The present study was undertaken to investigate whether lack of type VI collagen may affect the development and biomechanical function of the PCM and alter the mechanical environment of chondrocytes during joint loading.

METHODS

Col6a1(-/-) mice, which lack type VI collagen in their organs, were generated for use in these studies. At ages 1, 3, 6, and 11 months, bone mineral density (BMD) was measured, and osteoarthritic (OA) and developmental changes in the femoral head were evaluated histomorphometrically. Mechanical properties of articular cartilage from the hip joints of 1-month-old Col6a1(-/-), Col6a1(+/-), and Col6a1(+/+) mice were assessed using an electromechanical test system, and mechanical properties of the PCM were measured using the micropipette aspiration technique.

RESULTS

In Col6a1(-/-) and Col6a1(+/-) mice the PCM was structurally intact, but exhibited significantly reduced mechanical properties as compared with wild-type controls. With age, Col6a1(-/-) mice showed accelerated development of OA joint degeneration, as well as other musculoskeletal abnormalities such as delayed secondary ossification and reduced BMD.

CONCLUSION

These findings suggest that type VI collagen has an important role in regulating the physiology of the synovial joint and provide indirect evidence that alterations in the mechanical environment of chondrocytes, due to either loss of PCM properties or Col6a1(-/-)-derived joint laxity, can lead to progression of OA.

Chondrogenic differentiation of human mesenchymal stem cells in three-dimensional alginate gels

We characterized the temporal changes in chondrogenic genes and developed a staging scheme for in vitro chondrogenic differentiation of human mesenchymal stem cells (hMSCs) in three-dimensional (3D) alginate gels. A time-dependent accumulation of glycosaminoglycans, aggrecan, and type II collagen was observed in chondrogenic but not in basal constructs over 24 days. qRT-PCR demonstrated a largely characteristic temporal pattern of chondrogenic markers and provided a basis for staging the cellular phenotype into four stages. Stage I (days 0-6) was defined by collagen types I and VI, Sox 4, and BMP-2 showing peak expression levels. In stage II (days 6-12), gene expression for cartilage oligomeric matrix protein, HAPLN1, collagen type XI, and Sox 9 reached peak levels, while gene expression of matrilin 3, Ihh, Homeobox 7, chondroadherin, and WNT 11 peaked at stage III (days 12-18). Finally, cells in stage IV (days 18-24) attained peak levels of aggrecan; collagen IX, II, and X; osteocalcin; fibromodulin; PTHrP; and alkaline phosphatase. Gene profiles at stages III and IV were analogous to those in juvenile articular and adult nucleus pulposus chondrocytes. Gene ontology analyses also demonstrated a specific expression pattern of several putative novel marker genes. These data provide comprehensive insights on chondrogenesis of hMSCs in 3D gels. The derivation of this staging scheme may aid in defining maximally responsive time points for mechanobiological modulation of constructs to produce optimally engineered tissues.

Degeneration of normal articular cartilage induced by late phase osteoarthritic synovial fluid in beagle dogs

OBJECTIVE

To investigate the pathogenesis of late phase osteoarthritic (OA) synovial fluid (SF) on normal articular cartilage in vivo and provide an understanding of degenerative cartilage extending in OA joint.

METHODS

A random knee, each of 8 beagle dogs, received anterior cruciate ligament transection (ACLT) and was confirmed to have late phase OA degenerative changes at 24 weeks after operation. Thereafter, one random elbow of each canine was injected with autologous late phase OA knee SF. The contralateral elbow was injected with normal saline (NS) of the same volume as SF aspirated from ACLT knee. These two groups of elbows were labeled "SF" and "NS". 8 other beagle dogs were left intact and placed in Group Control. After aseptic arthrocentesis was performed weekly on both elbows for 24 weeks, morphological changes were observed in the cartilage of the elbows, and expressions of 7 biological etiological factors of chondrocytes of the elbows were determined in Group SF, Group NS and Group Control, respectively.

RESULTS

Morphological changes were observed in articular cartilage of the elbows in Group SF. Levels of unit area of collagen type I in the noncalcified, calcified and full zones of articular cartilage of the elbows in Group SF increased significantly. Level of unit area of collagen type III in the calcified zone of articular cartilage of the elbows in Group SF remained unchanged. Meanwhile, expressions of MMP-1 and MMP-3 of chondrocytes of the elbows in Group SF increased significantly. There was almost no difference between articular cartilage in Group NS and Group Control.

CONCLUSION

Based on these results, we conclude that OA degeneration of normal articular cartilage can be independently induced by late phase OA SF. Endogenous OA biological etiological factor may be one of the reasons causing degenerative cartilage extending in OA joint.

Matrix development in self-assembly of articular cartilage

Background

Articular cartilage is a highly functional tissue which covers the ends of long bones and serves to ensure proper joint movement. A tissue engineering approach that recapitulates the developmental characteristics of articular cartilage can be used to examine the maturation and degeneration of cartilage and produce fully functional neotissue replacements for diseased tissue.

Methodology/Principal Findings

This study examined the development of articular cartilage neotissue within a self-assembling process in two phases. In the first phase, articular cartilage constructs were examined at 1, 4, 7, 10, 14, 28, 42, and 56 days immunohistochemically, histologically, and through biochemical analysis for total collagen and glycosaminoglycan (GAG) content. Based on statistical changes in GAG and collagen levels, four time points from the first phase (7, 14, 28, and 56 days) were chosen to carry into the second phase, where the constructs were studied in terms of their mechanical characteristics, relative amounts of collagen types II and VI, and specific GAG types (chondroitin 4-sulfate, chondroitin 6-sulfate, dermatan sulfate, and hyaluronan). Collagen type VI was present in initial abundance and then localized to a pericellular distribution at 4 wks. N-cadherin activity also spiked at early stages of neotissue development, suggesting that self-assembly is mediated through a minimization of free energy. The percentage of collagen type II to total collagen significantly increased over time, while the proportion of collagen type VI to total collagen decreased between 1 and 2 wks. The chondroitin 6- to 4- sulfate ratio decreased steadily during construct maturation. In addition, the compressive properties reached a plateau and tensile characteristics peaked at 4 wks.

Conclusions/Significance

The indices of cartilage formation examined in this study suggest that tissue maturation in self-assembled articular cartilage mirrors known developmental processes for native tissue. In terms of tissue engineering, it is suggested that exogenous stimulation may be necessary after 4 wks to further augment the functionality of developing constructs.

Disc Regeneration: Why, When, and How

There is increasing acknowledgment that patients with back pain who are candidates for surgery, will benefit over the long term from less invasive procedures that facilitate dynamic stabilization, rather than fusion. Dynamic stabilization can be addressed by providing assistance using mechanical devices, or relying on biologic processes such as tissue regeneration and repair. The concept of biologic disc repair has grown in recent years because of improved understanding of the cellular and molecular events of disc aging and degeneration. This article describes approaches to cell therapy, reviews relevant studies, and discusses ways to maximize clinical efficacy. Tissue engineering approaches for disc regeneration and healing have significant clinical potential.

Light microscopic, immunohistochemical, and ultrastructural findings in congenital fibular aplasia or hypoplasia (FAH)

Congenital aplasia or hypoplasia of the fibula (FAH) is a rare malformation that is defined by a partial or complete absence of the fibular bone. Etiology and pathogenesis are unknown and the precise morphology of the tissue cord replacing the malformed fibula has not been well described. Therefore, tissue cord was examined in 8 patients with FAH. Light microscopic, immunohistochemical, and electron microscopic investigations showed a core of embryonic cartilage with collagen II and VI expressions surrounded by connective tissue. Although collagen II expression is typical for chondroid differentiation, collagen VI reactivity is normally seen in articular cartilage and tendon-like fibrocartilaginous tissue but is absent in hyaline cartilage. Further ultrastructural analyses by electron microscopy supported these findings. The histomorphologic changes correspond to the histologic findings of Papenbrock et al. (2000, Mech Dev 92:113-123) who produced a congenital malformation in transgenic mice that resembled FAH by overexpression of Hox c11.

Distribution of Type VI Collagen in the Cartilaginous Tissue of the Proximal Tibia in the Domestic Cat

To investigate the distribution of the early stage chondrocytes during the formation and closure of epiphyseal growth plate (EGP) of the domestic cat, we examined the EGP of proximal tibiae by immunohistochemistry for type VI collagen. In the epiphyseal cartilage without the secondary ossification center (SOC) and EGP in newborn cats aged 1 and 10 days, type VI collagen-positive chondrocytes were located around the cartilage canals and articular surface. In the epiphyseal cartilage with the SOC and EGP in young cats aged 1 to 3 months, type VI collagen-positive chondrocytes were located in the upper resting zone of the EGP, and then increased throughout the resting zone along with maturation. In the adult cats with the partially closed EGP, type VI collagen-positive chondrocytes were distributed throughout the remaining EGP. These findings indicate that the early stage chondrocytes characterized with type VI collagen are continuously located in the EGP during maturation. In addition, the increase of the early stage chondrocytes and the decrease of the reserve chondrocytes in the EGP along with maturation may cause the cessation of the longitudinal growth of the EGP, and finally bring about the EGP closure.

Distribution of type VI collagen in chondrocyte microenvironment: study of chondrons isolated from human normal and degenerative articular cartilage and cultured chondrocytes

The chondron is the microanatomical unit composed of a chondrocyte and its pericellular microenvironment (PCME), including the pericellular matrix and capsule. In the present study, we extracted chondrons from human articular cartilages and investigated the relationship between the distribution of the matrix molecules, including type VI collagen, and the degeneration of articular cartilage. We also investigated the effects of interleukin-1beta (IL-1beta) and transforming growth factor beta-1 (TGF-beta1) on the distribution of type VI collagen in cultured chondrocytes. Chondrons were extracted by low-speed homogenization from cartilage pieces obtained from forensic autopsies and from patients with knee osteoarthritis (OA) undergoing total knee arthroplasty. Cartilage sections were classified into three groups (normal, slight degeneration, and moderate degeneration) based on the degree of degeneration according to Mankin's score. Extracted chondrons were immunostained, and the distribution of the matrix molecules, including type VI collagen, was investigated using a confocal laser scanning microscope (CLSM). The chondrocytes isolated by enzymic treatment were subjected to three-dimensional culture in agarose gel and then treated with IL-1beta or TGF-beta1. The distribution of newly synthesized type VI collagen in agarose gel was also investigated using the CLSM. Type VI collagen was localized specifically within the PCME of chondrons. The volume ratio of PCME to chondrocyte (P/C ratio) was significantly higher in the moderate degeneration group than in the other two groups. The accumulation of type VI collagen around a chondrocyte was obviously increased by the addition of TGF-beta1. The P/C ratio significantly increased as the severity of the OA progressed, suggesting that type VI collagen distributed specifically in the PCME was playing a protective role for chondrocytes by maintaining the pericellular microenvironment in OA.

SREBP contributes to induction of collagen VI transcription by serum starvation

Collagen VI is a main extracellular matrix protein whose mutation is linked to myopathic diseases. In myoblasts and other cell types, collagen VI gene transcription peaks during cell-cycle exit that precedes differentiation, upon serum withdrawal or confluence. To get insight into this transcriptional regulation, we characterized a growth arrest responsive region (GARR) in the Col6a1 promoter responsible for this effect. In this work, we identify sterol regulatory element binding protein (SREBP) as a GARR binding protein and provide evidence that SREBP contributes to induction of Col6a1 transcription in serum free conditions. Furthermore, our data unveil a previously unexpected link between extracellular matrix production and LDL signaling.

Tissue-specific gene expression in chondrocytes grown on three-dimensional hyaluronic acid scaffolds

The re-differentiation capacities of human articular and chick embryo sternal chondrocytes were evaluated by culture on HYAFF-11 and its sulphate derivative, HYAFF-11-S, polymers derived from the benzyl esterification of hyaluronate. Initial results showed that the HYAFF-11-S material promoted the highest rate of chondrocyte proliferation. RNA isolated from human and chick embryo chondrocytes cultured in Petri dishes, HYAFF-11 or HYAFF-11-S were subjected to semi-quantitative RT-PCR analyses. Human collagen types I, II, X, human Sox9 and aggrecan, chick collagen types I, II, IX and X were analysed. Results showed that human collagen type II mRNA expression was upregulated on HYAFF-11 biomaterials. In particular, a high level of collagen type IIB expression was associated with three-dimensional culture conditions, and the HYAFF-11 material was the most supportive for human collagen type X mRNA expression. Human Sox9 mRNA levels were constantly maintained in monolayer cell culture conditions over a period of 21 days, while these were upregulated when chondrocytes were cultured on HYAFF-11 and HYAFF-11S. Furthermore, chick collagen type IIA and IIB mRNA expression was detected after only 7 days of HYAFF-11 culture. Chick collagen type IX mRNA expression decreased in scaffold cultures over time. Histochemical staining performed in engineered cartilage revealed the presence of a de novo synthesized glycosaminoglycan-rich extracellular matrix; immunohistochemistry confirmed the deposition of collagen type II. This study showed that the three-dimensional HYAFF-11 culture system is both an effective chondrocyte delivery system for the treatment of articular cartilage defects, and an excellent in vitro model for studying cartilage differentiation.

A chondrogenesis-related lipocalin cluster includes a third new gene, CALgamma

We have previously reported the modulation, during chondrogenesis and/or inflammation, of two chicken genes laying in the same genomic locus and coding for two polypeptides of the lipocalin protein family, the extracellular fatty acid binding protein (ExFABP) and the chondrogenesis associated lipocalin beta (CALbeta). A third gene, located within the same cluster and coding for a new lipocalin, CALgamma, has been identified and is here characterized. Tissue distribution analyzed by real-time quantitative reverse transcriptase-polymerase chain reaction in chicken embryos shows a ubiquitous expression with predominant levels of mRNA transcripts in the liver and the brain. In the developing tibia, a high expression of CALgamma mRNA was evidenced by in situ hybridization within the pre-hypertrophic and the hypertrophic zones of the bone-forming cartilage. In agreement, dedifferentiated chondrocytes in vitro express the transcripts to the highest level when they re-differentiate reaching hypertrophy. Such peculiar developmental pattern of expression that is analogous to those already described for Ex-FABP and CALbeta suggests that all three proteins may act synergistically in the process of endochondral bone formation. Moreover, like Ex-FABP and CALbeta, CALgamma is also highly induced in dedifferentiated chondrocytes upon stimulation with lypopolysaccharides, indicating that the whole cluster quite possibly is transcriptionally activated not only in physiological morphogenic differentiation but also in pathological acute phase response.

Mechanisms of transcriptional activation of the col6a1 gene during Schwann cell differentiation

A transgenic mouse line expressing the lacZ reporter under the control of a regulatory region of the col6a1 gene has been used to investigate differentiation of Schwann cells. The data suggest that: (1) activation of col6a1 gene transcription in the peripheral nervous system is part of the differentiation program of Schwann cells from neural crest cells stimulated by neuregulins; (2) once the Schwann cell precursors have acquired the competence of transcribing the col6a1 gene, transcriptional regulation becomes independent from neuregulins and is modulated by different mechanisms, including cell cycle; (3) activation of transgene expression after birth in sciatic nerves corresponds to the time of withdrawal of immature Schwann cells from the cell cycle and the beginning of their differentiation into myelinating Schwann cells.

Articular cartilage and changes in arthritis. An introduction: cell biology of osteoarthritis

The reaction patterns of chondrocytes in osteoarthritis can be summarized in five categories: (1) proliferation and cell death (apoptosis); changes in (2) synthetic activity and (3) degradation; (4) phenotypic modulation of the articular chondrocytes; and (5) formation of osteophytes. In osteoarthritis, the primary responses are reinitiation of synthesis of cartilage macromolecules, the initiation of synthesis of types IIA and III procollagens as markers of a more primitive phenotype, and synthesis of active proteolytic enzymes. Reversion to a fibroblast-like phenotype, known as "dedifferentiation", does not appear to be an important component. Proliferation plays a role in forming characteristic chondrocyte clusters near the surface, while apoptosis probably occurs primarily in the calcified cartilage.

Endochondral ossification of costal cartilage is arrested after chondrocytes have reached hypertrophic stage of late differentiation

Late cartilage differentiation during endochondral bone formation is a multistep process. Chondrocytes transit through a differentiation cascade under the direction of environmental signals that either stimulate or repress progression from one step to the next. In human costal cartilage, chondrocytes reach very advanced stages of late differentiation and express collagen X. However, remodeling of the tissue into bone is strongly repressed. The second hypertrophy marker, alkaline phosphatase, is not expressed before puberty. Upon sexual maturity, both alkaline phosphatase and collagen X activity levels are increased and slow ossification takes place. Thus, the expression of the two hypertrophy markers is widely separated in time in costal cartilage. Progression of endochondral ossification in this tissue beyond the stage of hypertrophic cartilage appears to be associated with the expression of alkaline phosphatase activity. Costal chondrocytes in culture are stimulated by parathyroid hormone in a PTH/PTHrP receptor-mediated manner to express the fully differentiated hypertrophic phenotype. In addition, the hormone stimulates hypertrophic development even more powerfully through its carboxyterminal domain, presumably by interaction with receptors distinct from PTH/PTHrP receptors. Therefore, PTH can support late cartilage differentiation at very advanced stages, whereas the same signal negatively controls the process at earlier stages.

Involvement of type VI collagen in interleukin-4-induced mineralization by human osteoblast-like cells in vitro

We recently showed that interleukin-4 (IL-4) enhanced collagen and osteocalcin accumulation and caused mineralization in human periosteal osteoblast-like (SaM-1) cells. At that time, the expression of alpha1(VI) collagen mRNA was induced. In the present study, the possible role of IL-4-induced type VI collagen in the in vitro mineralization in osteoblasts was investigated. Addition of IL-4 in the early stage (for the first 10 days) was essential for the mineralization. The mRNA levels of alpha1(VI) and alpha2(VI) collagen and protein level of type VI collagen were transiently increased by IL-4 treatment up to day 5, whereas the alpha1(I) procollagen mRNA level was greater at day 10 than at day 5. Addition of anti-type VI collagen antibody remarkably reduced the extracellular accumulations of calcium and hydroxyproline induced by IL-4. Furthermore, the transfection of antisense oligonucleotides of alpha1(VI) to SaM-1 cells in the presence of IL-4 partially inhibited IL-4-induced type I collagen accumulation. These results demonstrated that type VI collagen played important roles for IL-4-induced mineralization and hydroxyproline accumulation mostly type I collagen accumulation, in human periosteal osteoblast-like cells.

The matrilins: a novel family of oligomeric extracellular matrix proteins

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

Terminal differentiation of chondrocytes is arrested at distinct stages identified by their expression repertoire of marker genes

During endochondral bone formation, cells in the emerging cartilaginous model transit through a cascade of several chondrocyte differentiation stages, each characterized by a specific expression repertoire of matrix macromolecules, until, as a final step, the hypertrophic cartilage is replaced by bone. In many permanent cartilage tissues, however, late differentiation of chondrocytes does not occur, due to negative regulation by the environment of the cells. Here, addressing the reason for the difference between chondrocyte fates in the chicken embryo sternum, cells from the caudal and cranial part were cultured separately in serum-free agarose gels with complements defined earlier that either permit or prevent hypertrophic development. Total RNA was extracted using a novel protocol adapted to agarose cultures, and the temporal changes in developmental stage-specific mRNA expression were monitored by Northern hybridization and phosphor image analysis. Kinetic studies of the mRNA accumulation not only showed significant differences between the expression patterns of cranial and caudal cultures after recovery, but also revealed two checkpoints of chondrocyte differentiation in keeping with cartilage development in vivo. Terminal differentiation of caudal chondrocytes is blocked at the late proliferative stage (stage Ib), while the cranial cells can undergo hypertrophic development spontaneously. The differentiation of cranial chondrocytes is reversible, since they can re-assume an early proliferative (stage Ia) phenotype under the influence of insulin, fibroblast growth factor-2 and transforming growth factor-beta in combination. Thus, the expression pattern in the latter culture resembles that of articular chondrocytes. We also provide evidence that the capacities of caudal and sternal chondrocytes to progress from the late proliferative (stage Ib) to hypertrophic stage (stage II) correlate with their differing abilities to express the Indian hedgehog gene.

De-differentiated chondrosarcoma is not a 'de-differentiated' chondrosarcoma

AIMS

De-differentiated chondrosarcoma is characterized by the presence of two distinct chondroid and nonchondroid tumour portions. The aim of our study was to investigate the distribution of extracellular matrix components in this tumour entity and thus to shed light on its histogenetic origin.

METHODS AND RESULTS

Histochemical and immunohistochemical analyses were performed for collagen subtypes I, II, III and VI and cartilage proteoglycans in three samples of de-differentiated as well as conventional chondrosarcomas (various grades). In the chondroid tumour areas of de-differentiated chondrosarcoma, typical cartilage matrix components could be detected similar to chondroid areas of grade 1 and 2 conventional chondrosarcomas. In contrast, the tumour matrix of the nonchondroid portions of de-differentiated chondrosarcomas contained matrix molecules which are typical for fibroblastic tissue. This matrix composition was not identical with less differentiated (nonchondroid) areas of grades 2 and 3 conventional chondrosarcomas.

CONCLUSIONS

Our results confirm the chondroid nature of the differentiated portion of de-differentiated chondrosarcoma and indicate a nonchondrocytic nature of the nonchondroid portion. De-differentiated chondrosarcoma should not be considered as a 'de'-differentiated chondrosarcoma (grade 4 neoplasm), but as a tumour entity showing two types of mesenchymal differentiation.

Expression of a stable articular cartilage phenotype without evidence of hypertrophy by adult human articular chondrocytes in vitro

Chondrocytes that were isolated from adult human articular cartilage changed phenotype during monolayer tissue culture, as characterized by a fibroblastic morphology and cellular proliferation. Increased proliferation was accompanied by downregulation of the cartilage-specific extracellular matrix proteoglycan, aggrecan, by cessation of type-II collagen expression, and by upregulation of type-I collagen and versican. This phenomenon observed in monolayer was reversible after the transfer of cells to a suspension culture system. The transfer of chondrocytes to suspension culture in alginate beads resulted in the rapid upregulation of aggrecan and type-II collagen and the downregulation of expression of versican and type-I collagen. Type-X collagen and osteopontin, markers of chondrocyte hypertrophy and commitment to endochondral ossification, were not expressed by adult articular chondrocytes cultured in alginate, even after 5 months. In contrast, type-X collagen was expressed within 2 weeks in a population of cells derived from a fetal growth plate. The inability of adult articular chondrocytes to express markers of chondrocyte hypertrophy has underscored the fundamental distinction between the differentiation pathways that lead to articular cartilage or to bone. Adult articular chondrocytes expressed only hyaline articular cartilage markers without evidence of hypertrophy.

Cartilage associated protein (CASP) is a novel developmentally regulated chick embryo protein

A subtracted cDNA library was generated to identify cDNAs specific for chondrocyte mRNAs preferentially expressed at the hypertrophic stage with respect to early differentiation stages. The characterization of a cDNA isolated from this library that hybridizes with a 1.8 kb mRNA is described here. This mRNA is expressed at extremely low levels in dedifferentiated chondrocytes cultured in adherent conditions, at very low levels in differentiating chondrocytes and at very high levels in hypertrophic chondrocytes cultured in suspension conditions. In the developing chick embryo this mRNA is detectable in RNAs extracted from several other tissues besides cartilage. The described cDNA contains a complete open reading frame coding for a polypeptide of about 33 kDa. Homology searches with known cDNA and protein sequences have revealed that the chicken protein is related to the amino-terminal half of two mammalian nuclear antigens. By immunohistochemistry with specific rabbit antisera a strong signal was detected in the cartilage extracellular matrix of selected regions of the developing skeleton. Because of this localization of the antigen we named this protein cartilage associated protein (hereafter referred to as CASP).

Differentiation of embryonic stem cells into adipocytes in vitro

Embryonic stem cells, derived from the inner cell mass of murine blastocysts, can be maintained in a totipotent state in vitro. In appropriate conditions embryonic stem cells have been shown to differentiate in vitro into various derivatives of all three primary germ layers. We describe in this paper conditions to induce differentiation of embryonic stem cells reliably and at high efficiency into adipocytes. A prerequisite is to treat early developing embryonic stem cell-derived embryoid bodies with retinoic acid for a precise period of time. Retinoic acid could not be substituted by adipogenic hormones nor by potent activators of peroxisome proliferator-activated receptors. Treatment with retinoic acid resulted in the subsequent appearance of large clusters of mature adipocytes in embryoid body outgrowths. Lipogenic and lipolytic activities as well as high level expression of adipocyte specific genes could be detected in these cultures. Analysis of expression of potential adipogenic genes, such as peroxisome proliferator-activated receptors gamma and delta and CCAAT/enhancer binding protein beta, during differentiation of retinoic acid-treated embryoid bodies has been performed. The temporal pattern of expression of genes encoding these nuclear factors resembled that found during mouse embryogenesis. The differentiation of embryonic stem cells into adipocytes will provide an invaluable model for the characterisation of the role of genes expressed during the adipocyte development programme and for the identification of new adipogenic regulatory genes.

Transcriptional activation of the alpha 1(VI) collagen gene during myoblast differentiation is mediated by multiple GA boxes

During differentiation of ClC12 myoblasts in vitro, expression of alpha 1(VI) collagen mRNA was transiently stimulated severalfold. Promoter assays on cells transfected with chloramphenicol acetyltransferase (CAT) chimeric constructs have identified a region of the alpha 1(VI) a collagen promoter that increases CAT activity about 8-fold during differentiation. The region, which overlaps with transcription initiation sites, was shown to contain three protected segments (A, B, and C) in DNase I footprinting assays. The contact points between nuclear factors and the protected segments were determined by methylation interference assay and included the sequence GGGAGGG (GA box) in all segments. Experiments in which CAT constructs were cotransfected with double-stranded oligonucleotides containing the GA box suggested that this motif was necessary for induction. Transfections with deletion constructs of the natural promoter and with minipromoters made of three copies of A, B, or C showed that the elements have inducing activity and that elements C and, to a lower extent, B are stimulatory for basal transcription, whereas the contribution of A in this process is limited. Electrophoretic mobility shift assays with nuclear extracts from C2C12 cells indicated that the three GA box-containing elements bound several transcription factors, including Sp1. Comparison of the properties of the bands shifted under different experimental conditions (presence of 10 mM EDTA, heating of the nuclear extracts, addition of different concentrations of competitor oligonucleotides) established that A, B, and C probes form nine, eight and five main retarded complexes, respectively, and indicated that nuclear factors binding to C and B are subsets of proteins binding to A. UV cross-linking assays identified several peptides (seven with probe A, six with B, And five with C) in the range of 150-32 kDa. Comparison of the gel retardation pattern obtained with nuclear extracts from proliferating and differentiating cells revealed a particular increased intensity of two retarded bands. The data establish that multiple GA boxes mediate induction of the alpha 1(VI) collagen promoter during myoblast differentiation and suggest the attractive hypothesis that the effect may be related to variations of expression of transcription factors binding to these motifs.

Chondrocyte differentiation

Data obtained while investigating growth plate chondrocyte differentiation during endochondral bone formation both in vivo and in vitro indicate that initial chondrogenesis depends on positional signaling mediated by selected homeobox-containing genes and soluble mediators. Continuation of the process strongly relies on interactions of the differentiating cells with the microenvironment, that is, other cells and extracellular matrix. Production of and response to different hormones and growth factors are observed at all times and autocrine and paracrine cell stimulations are key elements of the process. Particularly relevant is the role of the TGF-beta superfamily, and more specifically of the BMP subfamily. Other factors include retinoids, FGFs, GH, and IGFs, and perhaps transferrin. The influence of local microenvironment might also offer an acceptable settlement to the debate about whether hypertrophic chondrocytes convert to bone cells and live, or remain chondrocytes and die. We suggest that the ultimate fate of hypertrophic chondrocytes may be different at different microanatomical sites.