The large functional spectrum of the heparin-binding cytokines MK and HB-GAM in continuously growing organs: the rodent incisor as a model

The heparin binding molecules MK and HB-GAM are involved in the regulation of growth and differentiation of many tissues and organs. Here we analyzed the expression of MK and HB-GAM in the developing mouse incisors, which are continuously growing organs with a stem cell compartment. Overlapping but distinct expression patterns for MK and HB-GAM were observed during all stages of incisor development (initiation, morphogenesis, cytodifferentiation). Both proteins were detected in the enamel knot, a transient epithelial signaling structure that is important for tooth morphogenesis, and the cervical loop where the stem cell niche is located. The functions of MK and HB-GAM were studied in dental explants and organotypic cultures in vitro. In mesenchymal explants, MK stimulated HB-GAM expression and, vice-versa, HB-GAM upregulated MK expression, thus indicating a regulatory loop between these proteins. BMP and FGF molecules also activated expression of both cytokines in mesenchyme. The proliferative effects of MK and HB-GAM varied according to the mesenchymal or epithelial origin of the tissue. Growth, cytodifferentiation and mineralization were inhibited in incisor germs cultured in the presence of MK neutralizing antibodies. These results demonstrate that MK and HB-GAM are involved in stem cells maintenance, cytodifferentiation and mineralization processes during mouse incisor development.

A biomarker-based mathematical model to predict bone-forming potency of human synovial and periosteal mesenchymal stem cells

OBJECTIVE

To develop a biomarker-based model to predict osteogenic potency of human mesenchymal stem cells (MSCs) from synovial membrane and periosteum.

METHODS

MSC populations were derived from adult synovium and periosteum. Phenotype analysis was performed by fluorescence-activated cell sorting and real-time reverse transcriptase-polymerase chain reaction (RT-PCR). Telomere lengths were determined by Southern blot analysis. In vitro osteogenesis was assessed quantitatively by measurements of alkaline phosphatase activity and calcium deposits. To investigate bone formation in vivo, MSCs were seeded onto osteoinductive scaffolds and implanted subcutaneously in nude mice. Bone was assessed by histology, and the human origin investigated by in situ hybridization for human Alu genomic repeats. Quantitation was achieved by histomorphometry and real-time RT-PCR for human osteocalcin. Analysis at the single-cell level was performed with clonal populations obtained by limiting dilution. Multiple regressions were used to explore the incremental predictive value of the markers.

RESULTS

Periosteal MSCs had significantly greater osteogenic potency than did synovial MSCs inherent to the single cell. Bone was largely of human origin in vivo. Within the same tissue type, there was variability between different donors. To identify predictors of osteogenic potency, we measured the expression levels of osteoblast lineage genes in synovial and periosteal clonal MSCs prior to osteogenic treatment. We identified biomarkers that correlated with osteogenic outcome and developed a mathematical model based on type I collagen and osteoprotegerin expression that predicts the bone-forming potency of MSC preparations, independent of donor-related variables and tissue source.

CONCLUSION

Our findings indicate that our quality-control mathematical model estimates the bone-forming potency of MSC preparations for bone repair.

Cell therapy: a challenge in modern medicine

Cell therapies are unique in that the active component consists of living cells, which are difficult to define in their pharmacologic characteristics, and which produce variable and largely unknown amounts of bioactive molecules. Thus, the definition of the composition of a cellular product, mechanisms of action, pharmacokinetics, toxicity and efficacy assessment represent challenges never previously faced by traditional pharmacology. A pressing need for a routine use of cell therapies in the clinic is the development of quality controls for efficacy on the basis of clinically relevant potency assays, with prospective validation in human clinical trials. This review will focus on cell-based protocols for joint surface repair. In particular, we will present the case of autologous chondrocyte implantation as an example of advanced tissue engineering technology in the clinic, with the assumption that many issues discussed can be extrapolated to other cell-based approaches in regenerative medicine.

Apoptosis in developmental and repair-related human tooth remodeling: a view from the inside

Apoptosis is a key phenomenon in the regulation of the life span of odontoblasts, which are responsible for dentin matrix production of the teeth. The mechanism controlling odontoblasts loss in developing, normal, and injured human teeth is largely unknown. A possible correlation between apoptosis and dental pulp volume reduction was examined. Histomorphometric analysis was performed on intact 10 to 14 year-old premolars to follow dentin deposition and evaluate the total number of odontoblasts. Apoptosis in growing healthy teeth as well as in mature irritated human teeth was determined using a modified TUNEL technique and an anti-caspase-3 antibody. In intact growing teeth, the sequential rearrangement of odontoblasts into a multi-layer structure during tooth crown formation was correlated with an apoptotic wave that leads to the massive elimination of odontoblasts. These data suggest that apoptosis, coincident with dentin deposition changes, plays a role in tooth maturation and homeostasis. Massive apoptotic events were observed after dentin irritation. In carious and injured teeth, apoptosis was detected in cells surrounding the lesion sites, as well as in mono-nucleated cells nearby the injury. These results indicate that apoptosis is a part of the mechanism that regulate human dental pulp chamber remodeling during tooth development and pathology.

Mesenchymal stem cells in rheumatology: a regenerative approach to joint repair

The advent of biologics in rheumatology has considerably changed the evolution and prognosis of chronic inflammatory arthritis. The success of these new treatments has contributed to steering more attention to research focussed on repair and remodelling of joint tissues. Indeed, when the tissue damage is established, treatment options are very limited and the risk of progression towards joint destruction and failure remains high. Increasing evidence indicates that mesenchymal stem cells persist postnatally within joint tissues. It is postulated that they would function to safeguard joint homoeostasis and guarantee tissue remodelling and repair throughout life. Alterations in mesenchymal stem cell biology in arthritis have indeed been reported but a causal relationship has not been demonstrated, mainly because our current knowledge of mesenchymal stem cell niches and functions within the joint in health and disease is very limited. Nonetheless, mesenchymal stem cell technologies have attracted the attention of the biomedical research community as very promising tools to achieve the repair of joint tissues such as articular cartilage, subchondral bone, menisci and tendons. This review will outline stem-cell-mediated strategies for the repair of joint tissues, spanning from the use of expanded mesenchymal stem cell populations to therapeutic targeting of endogenous stem cells, resident in their native tissues, and related reparative signals in traumatic, degenerative and inflammatory joint disorders.

Stem cell niches in mammals

Stem cells safeguard tissue homeostasis and guarantee tissue repair throughout life. The decision between self-renewal and differentiation is influenced by a specialized microenvironment called stem cell niche. Physical and molecular interactions with niche cells and orientation of the cleavage plane during stem cell mitosis control the balance between symmetric and asymmetric division of stem cells. Here we highlight recent progress made on the anatomical and molecular characterization of mammalian stem cell niches, focusing particularly on bone marrow, tooth and hair follicle. The knowledge of the regulation of stem cells within their niches in health and disease will be instrumental to develop novel therapies that target stem cell niches to achieve tissue repair and re-establish tissue homeostasis.

Immunomodulatory properties of mesenchymal stem cells: a review based on an interdisciplinary meeting held at the Kennedy Institute of Rheumatology Division, London, UK, 31 October 2005

Multipotent mesenchymal stromal cells isolated from bone marrow and other sites are currently being studied to determine their potential role in the pathogenesis and/or management of autoimmune diseases. In vitro studies have shown that they exhibit a dose-dependent antiproliferative effect on T and B lymphocytes, dendritic cells, natural killer cells and various B cell tumour lines – an effect that is both cell contact and soluble factor dependent. Animal models of autoimmune disease treated with multipotent mesenchymal stromal cells have mostly exhibited a positive clinical response, as have a limited number of patients suffering from acute graft versus host disease. This review summarizes the findings of a 1-day meeting devoted to the subject with the aim of coordinating efforts.

Activation of WNT and BMP signaling in adult human articular cartilage following mechanical injury

Acute full thickness joint surface defects can undergo repair, which involves tissue patterning and endochondral bone formation. Molecular signals regulating this process may contribute to the repair outcome, chronic evolution and, eventually, the onset of osteoarthritis. We tested the hypothesis that mechanical injury modulates morphogenetic pathways in adult human articular cartilage explants. Adjacent articular cartilage explants were obtained from preserved areas of the femoral condyles of patients undergoing arthroplasty for osteoarthritis, or from a normal joint of a patient undergoing lower limb amputation. Paired explants were individually maintained in explant culture. From each pair, one explant was mechanically injured and the other left uninjured as a control. Cultures were terminated at different time points for histochemistry, immunohistochemistry and gene expression analysis by reverse transcription real time PCR. Bone morphogenetic protein 2 (BMP-2) mRNA was upregulated in the injured explants. We detected phosphorylation of SMAD-1 and SMAD-5, consistent with activation of the bone morphogenetic protein (BMP) pathway. FRZB-1 mRNA was downregulated in the injured explants, suggesting de-repression of WNT signaling. Accordingly, expression of the canonical WNT target genes Axin-2 and c-JUN was upregulated in the injured explants. Activation of the canonical WNT signaling pathway by LiCl treatment induced upregulation of COL2A1 and Aggrecan mRNA, suggesting an anabolic effect. Phosphorylation of SMAD-1/-5 and downregulation of FRZB were confirmed in vivo in a mouse model of joint surface injury. Taken together, these data show modulation of the BMP and WNT pathways following mechanical injury in vitro and in vivo, which may play a role in the reparative response of the joint surface. These pathways may, therefore, represent potential targets in protocols of biological joint surface defect repair.

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.

Reparative medicine: from tissue engineering to joint surface regeneration

Biological regeneration is proving to be an increasingly attractive alternative/complement to prosthetic replacement of tissues and organs. In cell-based therapeutic approaches, cells are manipulated in vitro and administered to patients as living and dynamic biological agents. In this review, we have focused on the regeneration of the injured joint surface to discuss novel issues that these new therapeutic agents raise and are difficult to address within the paradigms of traditional pharmacology. They include: determination of the mechanism of action and dose, evaluation of potency, safety and toxicity, as well as upscale, delivery and identification of proper indications. Finally, novel potential approaches are proposed in which resident progenitor cells are targeted to regenerate the damaged tissue.

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.

Microenvironment and phenotypic stability specify tissue formation by human articular cartilage-derived cells in vivo

During in vitro expansion, adult human articular cartilage-derived cells (HACDC) lose their phenotypic stability and capacity to form cartilage in vivo after 4-6 population doublings (PD). Nevertheless, HACDC can be efficiently expanded for up to 20 PD. Here we show that HACDC can generate cartilage, fibrous tissue, skeletal muscle, bone, and adipocytes depending on the balance between phenotypic stability and environmental cues. When 5 x 10(6) cells were injected intramuscularly into nude mice, early-passage (EP)-HACDC formed cartilage; late-passage (LP)-HACDC formed mostly fibrous tissue, but a limited number of cells contributed to muscle formation. When 0.5 x 10(6) cells were injected into regenerating mouse muscle, both EP- and LP-HACDC integrated with host myofibers and expressed muscle genes, but a number of EP-HACDC maintained collagen type II expression. HACDC seeded into Collagraft and implanted subcutaneously into nude mice formed scattered bone islands displaying immunoreactivity for human osteocalcin, and expressing human bone-specific genes. Importantly, neither collagen type II transcript nor cartilage tissue was detected at 8 weeks after implantation. Myogenic, osteogenic, and adipogenic differentiation was induced in vitro using specific culture conditions. These findings provide evidence that in vivo tissue formation by HACDC is specified by a balance between environmental cues and the inherent phenotypic stability.

Skeletal muscle repair by adult human mesenchymal stem cells from synovial membrane

We have demonstrated previously that adult human synovial membrane-derived mesenchymal stem cells (hSM-MSCs) have myogenic potential in vitro (De Bari, C., F. Dell'Accio, P. Tylzanowski, and F.P. Luyten. 2001. Arthritis Rheum. 44:1928-1942). In the present study, we have characterized their myogenic differentiation in a nude mouse model of skeletal muscle regeneration and provide proof of principle of their potential use for muscle repair in the mdx mouse model of Duchenne muscular dystrophy. When implanted into regenerating nude mouse muscle, hSM-MSCs contributed to myofibers and to long term persisting functional satellite cells. No nuclear fusion hybrids were observed between donor human cells and host mouse muscle cells. Myogenic differentiation proceeded through a molecular cascade resembling embryonic muscle development. Differentiation was sensitive to environmental cues, since hSM-MSCs injected into the bloodstream engrafted in several tissues, but acquired the muscle phenotype only within skeletal muscle. When administered into dystrophic muscles of immunosuppressed mdx mice, hSM-MSCs restored sarcolemmal expression of dystrophin, reduced central nucleation, and rescued the expression of mouse mechano growth factor.

Expanded phenotypically stable chondrocytes persist in the repair tissue and contribute to cartilage matrix formation and structural integration in a goat model of autologous chondrocyte implantation

Autologous chondrocyte implantation (ACI) is an established technique to repair joint surface defects. Although there is some indirect evidence that the expanded chondrocytes are required to achieve proper healing, the role they play in the repair process is not clear yet. To monitor the persistence and the phenotype of the injected chondrocytes in the repair tissue (RT) we have optimized a fluorescent labeling protocol for articular chondrocytes, which allows cell tracking in vivo for up to 14 weeks, using the fluorescent dye PKH26. We have combined in vivo cell tracking, with the immune-detection of collagen type II protein in a goat model of ACI. Our data indicate that the implanted cells can persist for at least 14 weeks in the defects, can participate in the integration with the surrounding tissues, and become structural part of the RT, rich in collagen type II and sulfated proteoglycans. Albeit with a small number of samples, our data provide proof of principal that the implanted chondrocytes can contribute to structural cartilage repair in a goat model of ACI.