Chondrogenesis in mouse limb buds in vitro: effects of dibutyryl cyclic AMP treatment

Small molecules and their controlled release that induce the osteogenic/chondrogenic commitment of stem cells

Stem cell-based tissue engineering plays a significant role in skeletal system repair and regenerative therapies. However, stem cells must be differentiated into specific mature cells prior to implantation (direct implantation may lead to tumour formation). Natural or chemically synthesised small molecules provide an efficient, accurate, reversible, and cost-effective way to differentiate stem cells compared with bioactive growth factors and gene-related methods. Thus, investigating the influences of small molecules on the differentiation of stem cells is of great significance. Here, we review a series of small molecules that can induce or/and promote the osteogenic/chondrogenic commitment of stem cells. The controlled release of these small molecules from various vehicles for stem cell-based therapies and tissue engineering applications is also discussed. The extensive studies in this field represent significant contributions to stem cell-based tissue engineering research and regenerative medicine.

Physical Stimuli-Induced Chondrogenic Differentiation of Mesenchymal Stem Cells Using Magnetic Nanoparticles

Chondrogenic commitments of mesenchymal stem cells (MSCs) require 3D cellular organization. Furthermore, recent progresses in bioreactor technology have contributed to the development of various biophysical stimulation platforms for efficient cartilage tissue formation. Here, an approach is reported to drive 3D cellular organization and enhance chondrogenic commitment of bone-marrow-derived human mesenchymal stem cells (BM-hMSCs) via magnetic nanoparticle (MNP)-mediated physical stimuli. MNPs isolated from Magnetospirillum sp. AMB-1 are endocytosed by the BM-hMSCs in a highly efficient manner. MNPs-incorporated BM-hMSCs are pelleted and then subjected to static magnetic field and/or magnet-derived shear stress. Magnetic-based stimuli enhance level of sulfated glycosaminoglycan (sGAG) and collagen synthesis, and facilitate the chondrogenic differentiation of BM-hMSCs. In addition, both static magnetic field and magnet-derived shear stress applied for the chondrogenic differentiation of BM-hMSCs do not show increament of hypertrophic differentiation. This MNP-mediated physical stimulation platform demonstrates a promising strategy for efficient cartilage tissue engineering.

REPAIR OF ARTICULAR CARTILAGE INJURY

Articular cartilage defects heal poorly and lead to consequences as osteoarthritis. Clinical experience has indicated that no existing medication would substantially promote the healing process, and the cartilage defect requires surgical replacement. Allograft decays quickly for multiple reasons including the preparation process and immune reaction, and the outcome is disappointing. The extreme shortage of sparing in articular cartilage has much discouraged the use of autograft, which requires modification. The concept that constructs a chondral or osteochondral construct for the replacement of injured native tissue introduces that of tissue engineering. Limited number of cells are expanded either in vitro or in vivo, and resided temporally on a scaffold of biomaterial, which also acts as a vehicle to transfer the cells to the recipient site. Three core elements constitute this technique: the cell, a biodegradable scaffold, and an environment suitable for cells to present their proposed activity. Modern researches have kept updating those elements for a better performance of such cultivation of living tissue.

Repair of articular cartilage defects: review and perspectives

Articular cartilage defects heal poorly and lead to catastrophic degenerative arthritis. Clinical experience has indicated that no existing medication substantially promotes the healing process and the cartilage defect requires surgical replacement, preferably with an autograft. However, there is a shortage of articular cartilage that can be donated for autografting. A review of previous unsuccessful experiences reveals the reason for the current strategy to graft cartilage defects with regenerated cartilage. Autologous cartilage regeneration is a cell-based therapy in which autogenous chondrocytes or other chondrogenic cells are cultured to constitute cartilaginous tissue according to the principles of tissue engineering. Current studies are concentrating on improving such techniques from the three elements of tissue engineering, namely the cells, biomaterial scaffolds, and culture conditions. Some models of articular cartilage regeneration have yielded good repair of cartilage defects, in animal models and clinical settings, but the overall results suggest that there is room for improvement of this technique before its routine clinical application. Autologous cartilage regeneration remains the mainstay for repairing articular cartilage defects but more studies are required to optimize the efficacy of regeneration. A more abundant supply of more stable cells, i.e. capable of maintaining the phenotype of chondrogenesis, has to be identified. Porous scaffolds of biocompatible, biodegradable materials that maintain and support the presentation of the chondrogenic cells need to be fabricated. If the cells are not implanted early to allow their in vivo constitution of cartilage, a suitable in vitro cultivation method has to be devised for a consistent yield of regenerative cartilage.

Directing stem cell differentiation into the chondrogenic lineage in vitro

A major area in regenerative medicine is the application of stem cells in cartilage tissue engineering and reconstructive surgery. This requires well-defined and efficient protocols for directing the differentiation of stem cells into the chondrogenic lineage, followed by their selective purification and proliferation in vitro. The development of such protocols would reduce the likelihood of spontaneous differentiation of stem cells into divergent lineages upon transplantation, as well as reduce the risk of teratoma formation in the case of embryonic stem cells. Additionally, such protocols could provide useful in vitro models for studying chondrogenesis and cartilaginous tissue biology. The development of pharmacokinetic and cytotoxicity/genotoxicity screening tests for cartilage-related biomaterials and drugs could also utilize protocols developed for the chondrogenic differentiation of stem cells. Hence, this review critically examines the various strategies that could be used to direct the differentiation of stem cells into the chondrogenic lineage in vitro.

Protein phosphatase 2A is involved in the regulation of protein kinase A signaling pathway during in vitro chondrogenesis

We have evaluated the importance of the Ser/Thr protein phosphorylation and dephosphorylation for chondrogenesis in high-density chicken limb bud mesenchymal cell cultures (HDCs) by using H89, a cell-permeable protein kinase inhibitor, and okadaic acid (OA), a phosphoprotein phosphatase (PP)-specific inhibitor molecule. When 20 nM OA was applied to the HDCs on Days 2 and 3 of culturing, it significantly inhibited protein phosphatase 2A (PP2A), enhanced cartilage formation, and elevated the activity of cAMP-dependent protein kinase (PKA). Application of 20 microM H89 significantly decreased the activity of PKA and blocked the chondrogenesis in HDCs. Furthermore, OA enhanced cartilage formation and elevated the suppressed activity of PKA even in the H89-pretreated HDCs. cGMP-dependent protein kinase was not detected in HDCs, while protein kinase Cmu (PKCmu), which is also inhibited by nanomolar concentrations of H89, was present throughout the culturing period. Neither OA nor H89 influenced the expression of the catalytic subunit of PKA or the cAMP response element binding protein, CREB. However, a significantly elevated amount of Ser-133-phosphorylated-CREB (P-CREB) was detected following addition of OA, while H89 treatment resulted in a decrease of the amount of P-CREB. Our results demonstrate that PP2A plays a role in the regulation of the PKA signaling pathway and that the phosphorylation level of CREB is influenced by the activity of both enzymes during in vitro chondrogenesis.

Cellular interactions and signaling in cartilage development

The long bones of the developing skeleton, such as those of the limb, arise from the process of endochondral ossification, where cartilage serves as the initial anlage element and is later replaced by bone. One of the earliest events of embryonic limb development is cellular condensation, whereby pre-cartilage mesenchymal cells aggregate as a result of specific cell-cell interactions, a requisite step in the chondrogenic pathway. In this review an extensive examination of historical and recent literature pertaining to limb development and mesenchymal condensation has been undertaken. Topics reviewed include limb initiation and axial induction, mesenchymal condensation and its regulation by various adhesion molecules, and regulation of chondrocyte differentiation and limb patterning. The complexity of limb development is exemplified by the involvement of multiple growth factors and morphogens such as Wnts, transforming growth factor-beta and fibroblast growth factors, as well as condensation events mediated by both cell-cell (neural cadherin and neural cell adhesion molecule) and cell-matrix adhesion (fibronectin, proteoglycans and collagens), as well as numerous intracellular signaling pathways transduced by integrins, mitogen activated protein kinases, protein kinase C, lipid metabolites and cyclic adenosine monophosphate. Furthermore, information pertaining to limb patterning and the functional importance of Hox genes and various other signaling molecules such as radical fringe, engrailed, Sox-9, and the Hedgehog family is reviewed. The exquisite three-dimensional structure of the vertebrate limb represents the culmination of these highly orchestrated and strictly regulated events. Understanding the development of cartilage should provide insights into mechanisms underlying the biology of both normal and pathologic (e.g. osteoarthritis) adult cartilage.

Chondrogenic differentiation of clonal mouse embryonic cell line ATDC5 in vitro: differentiation-dependent gene expression of parathyroid hormone (PTH)/PTH-related peptide receptor

The regulatory role of parathyroid hormone (PTH)/PTH-related peptide (PTHrP) signaling has been implicated in embryonic skeletal development. Here, we studied chondrogenic differentiation of the mouse embryonal carcinoma-derived clonal cell line ATDC5 as a model of chondrogenesis in the early stages of endochondral bone development. ATDC5 cells retain the properties of chondroprogenitor cells, and rapidly proliferate in the presence of 5% FBS. Insulin (10 micrograms/ml) induced chondrogenic differentiation of the cells in a postconfluent phase through a cellular condensation process, resulting in the formation of cartilage nodules, as evidenced by expression of type II collagen and aggrecan genes. We found that differentiated cultures of ATDC5 cells abundantly expressed the high affinity receptor for PTH (Mr approximately 80 kD; Kd = 3.9 nM; 3.2 x 10(5) sites/cell). The receptors on differentiated cells were functionally active, as evidenced by a PTH-dependent activation of adenylate cyclase. Specific binding of PTH to cells markedly increased with the formation of cartilage nodules, while undifferentiated cells failed to show specific binding of PTH. Northern blot analysis indicated that expression of the PTH/PTHrP receptor gene became detectable at the early stage of chondrogenesis of ATDC5 cells, preceding induction of aggrecan gene expression. Expression of the PTH/PTHrP receptor gene was undetectable in undifferentiated cells. The level of PTH/PTHrP receptor mRNA was markedly elevated parallel to that of type II collagen mRNA. These lines of evidence suggest that the expression of functional PTH/PTHrP receptor is associated with the onset of chondrogenesis. In addition, activation of the receptor by exogenous PTH or PTHrP significantly interfered with cellular condensation and the subsequent formation of cartilage nodules, suggesting a novel site of PTHrP action.

Exogenous glycosaminoglycans modulate chondrogenesis, cyclic AMP level and cell growth in limb bud mesenchyme cultures

Effects of hyaluronate, heparin and chondroitin-6-sulfate were studied on micromass cultures of chick limb bud mesenchyme (Hamburger and Hamilton stages 23-24). Histochemical, electron microscopical, biochemical and radiochemical investigations of day 4 cultures revealed dose-dependent inhibitory effects of these glycosaminoglycans on chondrogenesis, cyclic AMP level and growth of cells. In addition, hyaluronate with 100 micrograms/ml dose caused a displacement of newly formed proteoglycan from cultures into the medium. It is supposed that exogenous glycosaminoglycans influence ionic equilibrium in the immediate vicinity of cells and disturb the organization of the prechondrogenic extracellular matrix resulting in alterations of cell membrane--cytoskeleton associations. These alterations may provoke a reduction in cyclic AMP level and DNA synthesis. It is suggested that a reduction in cyclic AMP level preceding the expression of cartilage phenotype results in the inhibition of chondrogenesis.

Inhibition of chondrogenesis by retinoic acid in limb mesenchymal cells in vitro: effects on PGE2 and cyclic AMP concentrations

Effects of retinoic acid (RA) on prostaglandin E2 (PGE2) and cyclic AMP (cAMP) concentrations were investigated in high density, micromass cultures of mesenchymal cells derived from chick limb buds. Exposure of cells during the initial 24 h of culture to RA concentrations between 0.05-1.0 micrograms/ml inhibited chondrogenesis in a dose-dependent manner with 1.0 micrograms/ml totally inhibiting cartilage formation. Concentrations of PGE2 and cAMP increased during the prechondrogenic period in control cells in a closely related way and remained elevated throughout the six-day period examined. Addition of RA (0.05 and 0.5 micrograms/ml) did not significantly alter cAMP concentrations at any time point, but significantly elevated PGE2 levels relative to control cells in six-day cultures in a concentration-dependent manner. Addition of dibutyryl cAMP enhanced chondrogenesis in control cells between days 3 and 4, but failed to alter the inhibitory effect of RA on chondrogenesis. The results indicate that while PGE2 and cAMP are important signals in cartilage differentiation, the inhibitory effects of RA on this process are mediated through some other mechanism.

Chondrogenesis in chick limb mesenchyme in vitro derived from distal limb bud tips: changes in cyclic AMP and in prostaglandin responsiveness

Chondrogenesis was monitored in micromass cultures of mesenchymal cells derived from the distal tip of stage-25 chick limb buds over a 6-day period. Alcian green staining and immunofluorescent localization of cartilage-specific proteoglycans revealed the appearance of cartilage matrix by day 3 of cell culture. By day 6, cultures contained a uniform and homogeneous population of fully differentiated chondrocytes throughout the cell layer, with only a narrow rim of nonchondrogenic cells around the extreme periphery of the culture. Synthesis of sulfated glycosaminoglycans also progressively increased between days 3 and 6, being 8-fold higher at day 6 than at day 1 of culture. Both adenylate cyclase (AC) activity and cAMP concentrations increased dramatically during the first 2 days of culture, reaching maximal levels by day 2, which remained elevated and stable throughout the remaining chondrogenic period (days 3-6). Responsiveness of both AC and cAMP concentrations of the cells to PGE2 was maximal by day 1 of culture and was increased over control cells by 12-fold and 8-fold respectively. Both responses, however, were dramatically reduced by day 3, at which time the initiation of cartilage formation was apparent. Responsiveness of cells during the prechondrogenic period to PGE2 was relatively specific in that no effects could be demonstrated with equivalent concentrations of PGF2 alpha or 6-keto-PGF1 alpha, although PGl2 did produce increases in cAMP concentrations of about 50% of those of PGE2. These results indicate that previously reported changes in the cAMP system in heterogeneous cell cultures derived from whole limb buds reflect changes occurring in the chondrogenic cell type and indicate further that peak responsiveness of the cAMP system of these cells to prostaglandins is restricted to prechondrogenic developmental periods.