Molecular basis of joint development

Identification of the molecular response of articular cartilage to injury, by microarray screening: Wnt-16 expression and signaling after injury and in osteoarthritis

OBJECTIVE

To characterize the molecular response of adult human articular cartilage to acute mechanical injury.

METHODS

An established ex vivo model was used to compare gene expression of adult human articular cartilage explants 24 hours after mechanical injury with that of uninjured controls by microarray analysis of gene expression. Confirmation for selected genes was obtained by real-time polymerase chain reaction and immunohistochemical analysis. Expression of selected genes was also investigated in preserved and osteoarthritic (OA) cartilage.

RESULTS

Six hundred ninety genes were significantly regulated at least 2-fold following mechanical injury. They included genes previously reported to be differentially expressed in OA versus normal cartilage or having allelic variants genetically linked to OA. Significant functional clusters included genes associated with wound healing, developmental processes, and skeletal development. The transforming growth factor beta, fibroblast growth factor, and Wnt pathways were modulated. A systematic analysis of the Wnt signaling pathway revealed up-regulation of Wnt-16, down-regulation of FRZB, up-regulation of Wnt target genes, and nuclear localization of beta-catenin in injured cartilage. In addition, in OA, Wnt-16 and beta-catenin were barely detectable in preserved cartilage areas, but were dramatically up-regulated in areas of the same joint with moderate to severe OA damage.

CONCLUSION

Our findings indicate that mechanical injury to adult human articular cartilage results in the activation of a signaling response, with reactivation of morphogenetic pathways. Therapeutic targeting of such pathways may improve current protocols of joint surface defect repair and/or prevent the evolution of such lesions into posttraumatic OA.

Mesenchymal multipotency of adult human periosteal cells demonstrated by single-cell lineage analysis

OBJECTIVE

To investigate whether periosteal cells from adult humans have features of multipotent mesenchymal stem cells (MSCs) at the single-cell level.

METHODS

Cell populations were enzymatically released from the periosteum of the proximal tibia obtained from adult human donors and then expanded in monolayer. Single-cell-derived clonal populations were obtained by limiting dilution. Culture-expanded periosteal cell populations were tested for their growth potential and for expression of conventional markers of MSCs and were subjected to in vitro assays to investigate their multilineage potential. To assess their multipotency in vivo, periosteal cells were injected into a regenerating mouse tibialis anterior muscle for skeletal myogenesis or were either seeded into an osteoinductive matrix and implanted subcutaneously into nude mice for osteogenesis or implanted in a joint surface defect under a periosteal flap into goats for chondrogenesis. Cell phenotypes were analyzed by histochemistry and immunohistochemistry and by reverse transcription-polymerase chain reaction for the expression of lineage-related marker genes.

RESULTS

Regardless of donor age, periosteal cells were clonogenic and could be expanded extensively in monolayer, maintaining linear growth curves over at least 30 population doublings. They displayed long telomeres and expressed markers of MSCs. Under specific conditions, both parental and single-cell-derived clonal cell populations differentiated to the chondrocyte, osteoblast, adipocyte, and skeletal myocyte lineages in vitro and in vivo.

CONCLUSION

Our study demonstrates that, regardless of donor age, the adult human periosteum contains cells that, upon enzymatic release and culture expansion, are multipotent MSCs at the single-cell level.

Failure of in vitro-differentiated mesenchymal stem cells from the synovial membrane to form ectopic stable cartilage in vivo

OBJECTIVE

We previously reported the identification in a nude mouse assay of molecular markers predictive of the capacity of articular cartilage-derived cells (ACDCs) to form ectopic stable cartilage that is resistant to vascular invasion and endochondral ossification. In the present study, we investigated whether in vitro-differentiated mesenchymal stem cells (MSCs) from the synovial membrane (SM) express the stable-chondrocyte markers and form ectopic stable cartilage in vivo.

METHODS

Chondrogenesis was induced in micromass culture with the addition of transforming growth factor beta1 (TGFbeta1). After acquisition of the cartilage phenotype, micromasses were implanted subcutaneously into nude mice. Alternatively, cells were released enzymatically and either replated in monolayer or injected intramuscularly into nude mice. Marker analysis was performed by quantitative reverse transcription-polymerase chain reaction. Cell death was detected with TUNEL assay.

RESULTS

Cartilage-like micromasses and released cells expressed the stable-chondrocyte markers at levels comparable with those expressed by stable ACDCs. The released cells lost chondrocyte marker expression by 24 hours in monolayer and failed to form cartilage when injected intramuscularly into nude mice. Instead, myogenic differentiation was detected. When intact TGFbeta1-treated micromasses were implanted subcutaneously, they partially lost their cartilage phenotype and underwent cell death and neoangiogenesis within 1 week. At later time points (15-40 days), we retrieved neither cartilage nor bone, and human cells were not detectable.

CONCLUSION

The chondrocyte-like phenotype of human SM MSCs, induced in vitro under specific conditions, appears to be unstable and is not sufficient to obtain ectopic formation of stable cartilage in vivo. Studies in animal models of joint surface defect repair are necessary to evaluate the stability of the SM MSC chondrocyte-like phenotype within the joint environment.

Skeletal tissue engineering: opportunities and challenges

Tissue engineering is a field of biomedicine that is growing rapidly and is critically driven by scientific advances in the areas of developmental and cell biology and biomaterial sciences. Regeneration of skeletal tissues is among the most promising areas of biological tissue repair and is providing a broad spectrum of potential clinical applications, including joint resurfacing. The availability of novel tools such as pluripotent stem cells, morphogens, smart biomaterials and gene transfer technologies, makes us dream of many exciting novel therapeutic approaches. Despite these opportunities in regenerative medicine, good clinical practice requires the clinician to question the consistency, reproducibility, validation and appropriate regulation of these new biological treatments.

Caspase inhibition supports proper gene expression in ex vivo mouse limb cultures

We standardized conditions for ex vivo mouse limb culture to study cartilage maturation and joint formation. We compared 12.5 d.p.c. mouse forelimbs that were cultured either mounted or freely rotating for up to 72 h. Limb outgrowth progressed ex vivo at a variable rate as compared to its development in vivo, spanning approximately 48 h. Although cartilage maturation and joint formation developed grossly normal, aberrant expression of skeletal marker genes was seen. Interestingly, no regression of the interdigital webs took place in mounted cultures, in contrast to limited webbing under freely rotating conditions. Caspase inhibition, by addition of zVAD-fmk to the culture medium of freely rotating limbs, supported proper gene expression associated with skeletal development, and prevented interdigital web regression. Taken together, a freely rotating ex vivo culture for mouse limb outgrowth that is combined with caspase inhibition provides a good model to study cartilage maturation and joint formation.

Multipotent mesenchymal stem cells from adult human synovial membrane

OBJECTIVE

To characterize mesenchymal stem cells (MSCs) from human synovial membrane (SM).

METHODS

Cell populations were enzymatically released from the SM obtained from knee joints of adult human donors and were expanded in monolayer with serial passages at confluence. Cell clones were obtained by limiting dilution. At different passages, SM-derived cells were subjected to in vitro assays to investigate their multilineage potential. Upon treatments, phenotypes of cell cultures were analyzed by histo- and immunohistochemistry and by semiquantitative reverse transcription-polymerase chain reaction for the expression of lineage-retated marker genes.

RESULTS

SM-derived cells could be expanded extensively in monolayer, with limited senescence. Under appropriate culture conditions, SM-derived cells were induced to differentiate to the chondrocyte, osteocyte, and adipocyte lineages. Sporadic myogenesis was also observed. Five independent cell clones displayed multilineage potential. Interestingly, only 1 clone was myogenic. Donor age, cell passaging, and cryopreservation did not affect the multilineage potential of SM-derived cells. In contrast, normal dermal fibroblasts under the same culture conditions did not display this potential.

CONCLUSION

Our study demonstrates that human multipotent MSCs can be isolated from the SM of knee joints. These cells have the ability to proliferate extensively in culture, and they maintain their multilineage differentiation potential in vitro, establishing their progenitor cell nature. SM-derived MSCs may play a role in the regenerative response during arthritic diseases and are promising candidates for developing novel cell-based therapeutic approaches for postnatal skeletal tissue repair.

Molecular markers predictive of the capacity of expanded human articular chondrocytes to form stable cartilage in vivo

OBJECTIVE

To establish a model and associated molecular markers for monitoring the capacity of in vitro-expanded chondrocytes to generate stable cartilage in vivo.

METHODS

Adult human articular chondrocytes (AHAC) were prepared by collagenase digestion of samples obtained postmortem and were expanded in monolayer. Upon passaging, aliquots of chondrocyte suspensions were either injected intramuscularly into nude mice, cultured in agarose, or used for gene expression analysis. Cartilage formation in vivo was documented by histology, histochemistry, immunofluorescence for type II collagen, and proteoglycan analysis by 35S-sulfate incorporation and molecular sieve chromatography of the radiolabeled macromolecules. In situ hybridization for species-specific genomic repeats was used to discriminate human-derived from mouse-derived cells. Gene expression dynamics were analyzed by semiquantitative reverse transcription-polymerase chain reaction.

RESULTS

Intramuscular injection of freshly isolated AHAC into nude mice resulted in stable cartilage implants that were resistant to mineralization, vascular invasion, and replacement by bone. In vitro expansion of AHAC resulted in the loss of in vivo cartilage formation. This capacity was positively associated with the expression of fibroblast growth factor receptor 3, bone morphogenetic protein 2, and alpha1(II) collagen (COL2A1), and its loss was marked by the up-regulation of activin receptor-like kinase 1 messenger RNA. Anchorage-independent growth and the reexpression of COL2A1 in agarose culture were insufficient to predict cartilage formation in vivo.

CONCLUSION

AHAC have a finite capacity to form stable cartilage in vivo; this capacity is lost throughout passaging and can be monitored using a nude mouse model and associated molecular markers. This cartilage-forming ability in vivo may be pivotal for successful cell-based joint surface defect repair protocols.

Development of articular cartilage: what do we know about it and how may it occur?

Articular cartilage has a fundamental role in joint function. While much is known about its structure, organization and biomechanical properties, there is a very poor understanding of how articular chondrocytes develop during embryogenesis and acquire the unique ability to organize and maintain the articular tissue. Given that articular cartilage forms in close juxtaposition with the joint, here we review past studies on limb joint determination and morphogenesis and more recent studies on a number of factors thought to have roles in joint and epiphysis development. These factors include: the homeobox gene Barx-1; the bone morphogenetic protein (BMP) family member GDF-5; the growth factors HGF and PTHrP; and the transcription factor ERG. We summarize current thinking on how these factors participate in joint development and how some of these factors may influence development and behavior of epiphyseal chondrocytes. We also describe pertinent recent studies from our laboratories on ERG and the newly-identified alternatively spliced variant C-1-1, and finally propose a sequela of events that may subtend the process of determination and emergence of articular chondrocytes during limb synovial joint development.

Human periosteum-derived cells maintain phenotypic stability and chondrogenic potential throughout expansion regardless of donor age

OBJECTIVE

To assess the in vitro chondrogenic potential of adult human periosteum-derived cells (PDCs) with regard to the number of cell passages and the age of the donor.

METHODS

Cells were enzymatically released from the periosteum of the proximal tibia obtained from adult human donors and expanded in monolayer. PDCs were harvested at multiple passages for total RNA extraction and semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) gene expression analysis. For the chondrogenesis assay, cells were plated in micromass and treated with transforming growth factor beta1 (TGFbeta1) in a chemically defined medium. At different time points, micromasses were either harvested for RT-PCR analysis for cartilage and bone markers or fixed, paraffin-embedded, and stained for cartilage matrix, and immunostained for type II collagen.

RESULTS

At the first 2 passages, human PDCs from young donors formed chondrogenic nodules. This spontaneous chondrogenic activity was lost upon passaging, and it was not observed in donors older than 30 years. Using a panel of marker genes, PDCs were shown to be phenotypically stable during cell expansion. Regardless of donor age or cell passage, chondrogenesis could be induced consistently by combining micromass culture and TGFbeta1 treatment. Histochemical and immunohistochemical analyses demonstrated the hyaline-like cartilage phenotype of the tissue generated in vitro. Other TGFbeta superfamily members, such as growth differentiation factor 5/cartilage-derived morphogenetic protein 1, and bone morphogenetic proteins 2, 4, and 7, were poorly chondrogenic under the same culture conditions.

CONCLUSION

Adult human PDCs have the potential to differentiate toward the chondrocytic lineage in vitro, retaining this property even after extensive subculture. Human PDCs are easily accessible, expandable, and maintain their chondrogenic potential, and are therefore promising progenitor cells for use in the repair of joint surface defects.