Enzymes in mineralizing systems: state of the art

A novel in vitro assay to study chondrocyte-to-osteoblast transdifferentiation

PURPOSE

Endochondral ossification, which involves transdifferentiation of chondrocytes into osteoblasts, is an important process involved in the development and postnatal growth of most vertebrate bones as well as in bone fracture healing. To study the basic molecular mechanisms of this process, a robust and easy-to-use in vitro model is desirable. Therefore, we aimed to develop a standardized in vitro assay for the transdifferentiation of chondrogenic cells towards the osteogenic lineage.

METHODS

Murine chondrogenic ATDC5 cells were differentiated into the chondrogenic lineage for seven days and subsequently differentiated towards the osteogenic direction. Gene expression analysis of pluripotency, as well as chondrogenic and osteogenic markers, cell-matrix staining, and immunofluorescent staining, were performed to assess the differentiation. In addition, the effects of Wnt3a and lipopolysaccharides (LPS) on the transdifferentiation were tested by their addition to the osteogenic differentiation medium.

RESULTS

Following osteogenic differentiation, chondrogenically pe-differentiated cells displayed the expression of pluripotency and osteogenic marker genes as well as alkaline phosphatase activity and a mineralized matrix. Co-expression of Col2a1 and Col1a1 after one day of osteogenic differentiation indicated that osteogenic cells had differentiated from chondrogenic cells. Wnt3a increased and LPS decreased transdifferentiation towards the osteogenic lineage.

CONCLUSION

We successfully established a rapid, standardized in vitro assay for the transdifferentiation of chondrogenic cells into osteogenic cells, which is suitable for testing the effects of different compounds on this cellular process.

Enzymatic Approach in Calcium Phosphate Biomineralization: A Contribution to Reconcile the Physicochemical with the Physiological View

Biomineralization is the process by which organisms produce hard inorganic matter from soft tissues with outstanding control of mineral deposition in time and space. For this purpose, organisms deploy a sophisticated "toolkit" that has resulted in significant evolutionary innovations, for which calcium phosphate (CaP) is the biomineral selected for the skeleton of vertebrates. While CaP mineral formation in aqueous media can be investigated by studying thermodynamics and kinetics of phase transitions in supersaturated solutions, biogenic mineralization requires coping with the inherent complexity of biological systems. This mainly includes compartmentalization and homeostatic processes used by organisms to regulate key physiological factors, including temperature, pH and ion concentration. A detailed analysis of the literature shows the emergence of two main views describing the mechanism of CaP biomineralization. The first one, more dedicated to the study of in vivo systems and supported by researchers in physiology, often involves matrix vesicles (MVs). The second one, more investigated by the physicochemistry community, involves collagen intrafibrillar mineralization particularly through in vitro acellular models. Herein, we show that there is an obvious need in the biological systems to control both where and when the mineral forms through an in-depth survey of the mechanism of CaP mineralization. This necessity could gather both communities of physiologists and physicochemists under a common interest for an enzymatic approach to better describe CaP biomineralization. Both homogeneous and heterogeneous enzymatic catalyses are conceivable for these systems, and a few preliminary promising results on CaP mineralization for both types of enzymatic catalysis are reported in this work. Through them, we aim to describe the relevance of our point of view and the likely findings that could be obtained when adding an enzymatic approach to the already rich and creative research field dealing with CaP mineralization. This complementary approach could lead to a better understanding of the biomineralization mechanism and inspire the biomimetic design of new materials.

Evaluation of the viability and osteogenic differentiation of cryopreserved human adipose-derived stem cells

Human adipose-derived stem cells (ASCs) have the ability to differentiate into osteoblasts and thus the potential therapeutic use to tissue-engineer bone, so a reliable method for cell storage is necessary. The aim of this study was to determine whether a simple method of cryopreservation with 10% Me(2)SO as a protectant had an effect on proliferation potential and osteogenic differentiation of ASCs isolated from fresh human adipose tissue. ASCs were harvested from 6 human lipoaspirates and each was halved for either cryopreservation in liquid nitrogen for 2 weeks or for control culture. Cells from the second-passage were plated at a density of 5000cells/well in 24-well plates and cultured with or without osteogenic media for 14 days. Cell surface antigens were used to identify the cryopreserved ASCs by flow cytometry. The proliferation rate of both populations was evaluated using a cell DNA assay. To detect osteogenic differentiation of both the cryopreserved and non-cryopreserved populations, determination of osteoblastic protein production (alkaline phosphatase and osteocalcin) and excellular matrix calcification (calcium content) was applied. The expression of osteoblastic-associated genes was also analyzed using reverse-transcription polymerase chain reaction. These results demonstrate that cryopreservation has no effect on the phenotype, proliferation or osteogenic differentiation of human ASCs, showing cryopreserved human ASCs might be applied for bone tissue engineering.

Overexpression of the ZIP1 zinc transporter induces an osteogenic phenotype in mesenchymal stem cells

Zinc is an essential trace element that is involved in diverse metabolic and signaling pathways. Zinc deficiency is associated with retardation of bone growth. Previous in vitro studies have suggested a direct effect of zinc on both the proliferation and differentiation of osteoblast-like cells. However, the mechanisms for uptake of zinc into osteoblasts have not been examined in detail. Several families of zinc transporters have previously been characterized in mammalian cells; such transporters function in the uptake, intracellular sequestration or efflux of zinc. In the current study, we examined zinc transport in osteoprogenitor cells and have attempted to define a functional role for a zinc transport mechanism in osteogenic differentiation. We identified at least two zinc transporters in both human mesenchymal stem cells (MSCs) and in osteoblastic cells--the ubiquitous zinc transporter, ZIP1, and LIV-1, which was previously characterized as a protein that is expressed in breast cancer cells. The subcellular localization of both these zinc transporters suggested distribution in both the plasma membrane and also diffusely in the cytoplasm. During the differentiation process of pluripotent MSCs into osteoblast-like cells, both zinc uptake and expression of the ZIP1 protein were increased. An adenoviral-mediated overexpression of ZIP1 in MSCs resulted in Alizarin-red-positive mineralization and also increased expression of specific osteoblast-associated markers, such as alkaline phosphatase, and of several osteoblast differentiation genes, including osteopontin, Cbfa1/Runx2, promyelocytic leukemia zinc finger and bone sialoprotein. An siRNA-mediated reduction of ZIP1 protein expression in MSCs caused decreased zinc uptake and inhibition of osteoblastic differentiation under osteogenic culture conditions. Finally, following overexpression of ZIP1 in MSCs, cDNA microarray analysis revealed differential regulation of several genes associated with the proliferation of osteoprogenitor cells and osteoblast differentiation. In conclusion, these studies provide important insights into the role of a plasma membrane zinc transporter in the initiation of an osteogenic lineage from MSCs.

Osteoblast viability and differentiation with Me2SO as cryoprotectant compared to osteoblasts from fresh human iliac cancellous bone

The aim of this study was to compare the viability of human osteoblasts cryopreserved with Me2SO to that of fresh human iliac cancellous bone using cell culture techniques. Osteoblasts were obtained by spontaneous outgrowth of human iliac cancellous bone specimens in experiment I. In experiment II, human iliac cancellous bone was frozen with 10% Me2SO at -80 degrees C for 2 weeks and osteoblasts grew spontaneously after thawing at 37 degrees C by removing Me2SO with sucrose. The cells were grown in culture flasks containing DMEM as a culture medium, supplemented with 10% fetal calf serum. They were kept at 37 degrees C in a humidified atmosphere of 95% air and 5% CO2. Cells from the second passage were plated at a density of 5 times 10(3) cells/cm2 in 24-well plates. For detection of viability and differentiation, WST-1 assay, determination of alkaline phosphatase activity, concentration of procollagen I peptide, concentration of osteocalcin, and indirect immunofluorescence for osteopontin, collagen type I, integrin beta1, and fibronectin were applied. Experiments were conducted at four stages of confluence (days 4, 7, 14, and 21 after plating the cells). Based on the results of this study, we conclude that osteoblast-like cells survived cryopreservation and synthesized a range of markers that were consistent with this cell type.

Deep zone articular chondrocytes in vitro express genes that show specific changes with mineralization

We have developed a method to form reconstituted mineralized articular cartilagenous tissue in vitro from isolated deep zone chondrocytes. The aim of this study was to characterize further these cultures prior to and during mineralization. Histologic examination of the cells up to 8 days in culture showed that the chondrocytes had formed cartilagenous tissue. Similar to the in vivo cartilage, the chondrocytes expressed aggrecan, types II, I, and X collagens, osteopontin, and alkaline phosphatase (ALP). No osteocalcin mRNA expression was detected in either the in vivo cartilage or in vitro-generated tissue. Addition of beta-glycerophosphate (beta-GP) to the medium on day 5 induced mineralization and changes in gene expression. Expression of type X collagen, type II collagen, aggrecan core protein, and ALP were inhibited significantly between 2 h and 24 h after the addition of beta-GP. At 72 h, expression of these genes were still significantly depressed. These changes correlated with a decrease in collagen and proteoglycan synthesis, and ALP activity. Osteopontin expression increased within 8 h but returned to constitutive levels by 72 h. No change in type I collagen expression was detected. The changes in gene expression were not due to a direct effect of beta-GP itself, because similar gene changes occurred in the presence of phosphoethanolamine, another agent which induces mineralization. No changes in gene expression were seen in nonmineralizing cultures. In summary, articular chondrocytes grown on filter culture show expression of similar genes to the chondrocytes in the deep zone of articular cartilage and that changes in expression of specific genes were observed during tissue mineralization, suggesting that it is a suitable model to use to study the mechanism(s) regulating the localized mineralization of articular cartilage.