Bone morphogenetic protein-2 modulation of chondrogenic differentiation in vitro involves gap junction-mediated intercellular communication

Undifferentiated mesenchymal cells in the limb bud integrate a complex array of local and systemic signals during the process of cell condensation and chondrogenic differentiation. To address the relationship between bone morphogenetic protein (BMP) signaling and gap junction-mediated intercellular communication, we examined the effects of BMP-2 and a gap junction blocker 18 alpha glycyrrhetinic acid (18alpha-GCA) on mesenchymal cell condensation and chondrogenic differentiation in an in vitro chondrogenic model. We find that connexin43 protein expression significantly correlates with early mesenchymal cellular condensation and chondrogenesis in high-density limb bud cell culture. The level of connexin43 mRNA is maximally upregulated 48 h after treatment with recombinant human BMP-2 with corresponding changes in protein expression. Inhibition of gap junction-mediated intercellular communication with 2.5 microM 18alpha-GCA decreases chondrogenic differentiation by 50% at 96 h without effects on housekeeping genes. Exposure to 18alpha-GCA for only the first 24-48 h after plating does not affect condensation or later chondrogenic differentiation suggesting that gap junction-mediated intercellular communication is not critical for the initial phase of condensation but is important for the onset of differentiation. 18alpha-GCA can also block the chondrogenic effects of BMP-2 without effects on cell number or connexin43 expression. These observations demonstrate 18alpha-GCA-sensitive regulation of intercellular communication in limb mesenchymal cells undergoing chondrogenic differentiation and suggest that BMP-2 induced chondrogenic differentiation may be mediated in part through the modulation of connexin43 expression and gap junction-mediated intercellular communication.     

Accelerated maturation of limb mesenchyme by the BrachypodH mouse mutation

Mesenchyme cell populations prepared from proximal and distal halves of stage 20 mouse forelimb buds are shown to behave under in vitro micromass culture conditions like analogous cell populations obtained from chick embryo limb buds. While the distal cells are spontaneously chondrogenic, the proximal cells make aggregates which are only potentially chondrogenic after treatment with dibutyryl cyclic AMP. In addition, stage 20 mouse whole limb bud cells homozygous for the brachypodismH (bpH) mutation are shown to behave similarly to 'normal' proximal cells. Both make fewer aggregates and nodules and both have faster aggregation rates (determined as the rate of disappearance of single cells over time) in rotation cultures than 'normal' distal or whole limb bud cells. These results support the hypothesis that the bpH mutation specifically decreases the proportion of spontaneously chondrogenic mesenchyme cells (that is, distal-like cells) present at certain developmental stages in the limb bud, resulting in a prematurely high proportion of proximal-like cells.     

Accelerated maturation of limb mesenchyme by the BrachypodH mouse mutation

Abstract. Mesenchyme cell populations prepared from proximal and distal halves of stage 20 mouse forelimb buds are shown to behave under in vitro micromass culture conditions like analogous cell populations obtained from chick embryo limb buds. While the distal cells are spontaneously chondrogenic, the proximal cells make aggregates which are only potentially chondrogenic after treatment with dibutyryl cyclic AMP. In addition, stage 20 mouse whole limb bud cells homozygous for the brachypodism^H ( bp ^H ) mutation are shown to behave similarly to 'normal' proximal cells. Both make fewer aggregates and nodules and both have faster aggregation rates (determined as the rate of disappearance of single cells over time) in rotation cultures than 'normal' distal or whole limb bud cells. These results support the hypothesis that the bp ^H mutation specifically decreases the proportion of spontaneously chondrogenic mesenchyme cells (that is, distal-like cells) present at certain developmental stages in the limb bud, resulting in a prematurely high proportion of proximal-like cells.     

Lectins influence chondrogenesis and osteogenesis in limb bud mesenchymal cells

The role of cell surface glycoproteins in cell behavior can be characterized by their interactions with plant lectins. This study was designed to identify the effects of lectins on chondrogenesis and osteogenesis in limb bud mesenchymal cells in vitro. Limb bud mesenchymal cells from mouse embryos were cultured in high-density micromass culture. Wheat germ agglutinin (WGA), concanavalin A (ConA), peanut agglutinin (PNA), Dolichos biflorus agglutinin (DBA) and Ricinus communis agglutinin (RCA) were added separately to the culture media. Cells were cultured for 5 or 9 days, and cell viability was assayed by neutral red on day 5. The micromasses were stained with alcian blue, alizarin red S and Von Kossa stains, and alkaline phosphatase assays were also done. Dolichos biflorus agglutinin induced an increase in chondrogenesis, calcium precipitation and proteoglycan production. ConA and PNA did not affect chondrocyte differentiation but induced chondrocytes to produce more proteoglycan. Wheat germ agglutinin reduced chondrification and ossification but induced mesenchymal cells to store lipid droplets. Ricinus communis agglutinin 1 was toxic and significantly reduced cell survival. In conclusion, DBA was the most effective inducer of ossification and chondrification. Wheat germ agglutinin induced adipogenesis instead. These assays showed that lectins play important roles in limb bud development.     

Two distinct classes of prechondrogenic cell types in the embryonic limb bud

Differences are demonstrated in the chondrogenic potential of cells derived from the distal and proximal halves of chick wing buds from as early as stage 23, prior to the appearance of overt cartilage differentiation. In high cell density cultures, cells obtained from the distal portions of stage 23 or 24 limb buds are spontaneously chondrogenic in micromass cultures. Cells obtained from the proximal portions, however, become blocked in their differentiation as protodifferentiated cartilage cels, since these cells in micromass cultures make detectable type II collagen, but fail to synthesize significant levels of cartilage proteoglycan or to accumulate an extracellular matrix that will stain for sulfated glycosaminoglycans. Such cultures of proximal limb bud cells can be stimulated to form alcian blue staining nodules by the addition of 1 mM dbcAMP or 50 micrograms/ml ascorbate, or by mixing proximal cells with small numbers of distal cells (1 distal cell to 10 proximal cells). These results demonstrate the existence of two distinct stages among prechondrogenic mesenchyme cells. The earlier stage appears to be able to provide a chondrogenic stimulus to proximal cells.     

Altered differentiation in rat and rabbit limb bud micromass cultures by glutathione modulating agents

Glutathione (GSH) is the primary source of reducing equivalents in most cells, contributes significantly to the cellular redox potential and can control differentiation, proliferation, and apoptosis. Using limb bud micromass cultures from Sprague Dawley rats and New Zealand White rabbits, GSH modulating agents, L-buthionine-S,R-sulfoximine (BSO) and diethyl maleate (DEM) altered the formation of Alcian blue positive chondrogenic foci. Limb bud micromass cultures were treated for 5 d with BSO (50 or 100 μM) or DEM (5–25 μM). GSH content was determined by HPLC analysis. In rat cultures, BSO treatment did not affect differentiation but did show GSH depletion. In rabbit cultures, BSO completely inhibited differentiation and significantly depleted GSH. Treatment of rat cultures with DEM resulted in the dose-dependent decrease of chondrogenic foci, which correlated with a dose-dependent depletion of GSH. DEM completely inhibited rabbit limb bud cell differentiation and depleted GSH by 44%. Inhibition of differentiation was confirmed in rabbit cultures by the reduction in BMP-4 content. Addition of N-acetylcysteine to rabbit micromass cultures restored chondrogenic foci differentiation seen following treatment with both DEM and BSO. These results show species differences in GSH depletion in rat vs. rabbit limb bud cells and implicate GSH and cysteine in affecting pathways involved in chondrocyte differentiation.     

Limb bud chondrogenesis in cell culture, with particular reference to serum concentration in the culture medium

When limb bud mesodermal cells of stages 23–24 chick embryos were plated at low cell density (2 × 10⁵ cells/cm²) and cultured in medium containing 10% fetal calf serum (FCS) (serum-rich medium), all cells became fibroblastic and no chondrocyte differentiation occurred in the culture. However, when cells of the same origin were cultured in a medium containing only 0.1% FCS (serum-poor medium), almost all the cells formed aggregates which developed further to form cartilage nodules. The loss of chondrogenic activity in serum-rich medium culture was irreversible: cultivation of the limb bud cells in serum-rich medium for 12 h abolished chondrogenic activity completely and these cells could not resume activity on re-cultivation in serum-poor medium. Calf, horse and chick serum at a concentration of 10% also induced the loss of chondrogenic activity in low cell density culture. Failure of chondrogenesis in serum-rich medium culture seemed to be due to the commitment of bipotential limb bud mesodermal cells to fibroblastic cells rather than to selective detachment of pre-committed chondroblasts.     

Involvement of Frzb-1 in mesenchymal condensation and cartilage differentiation in the chick limb bud

In developing limb bud, mesenchymal cells form cellular aggregates called "mesenchymal condensations". These condensations show the prepattern of skeletal elements of the limb prior to cartilage differentiation. Roles of various signaling molecules in chondrogenesis in the limb bud have been reported. One group of signaling factors includes the Wnt proteins, which have been shown to have an inhibitory effect on chondrogenesis in the limb bud. Therefore, regulation of Wnt activity may be important in regulating cartilage differentiation. Here we show that Frzb-1, which encodes a secreted frizzled-related protein that can bind to Wnt proteins and can antagonize the activity of some Wnts, is expressed in the developing limb bud. At early stages of limb development, Frzb-1 is expressed in the ventral core mesenchyme of the limb bud, and later Frzb-1 expression becomes restricted to the central core region where mesenchymal condensations occur. At these stages, a chondrogenic marker gene, aggrecan, is not yet expressed. As limb development proceeds, expression of Frzb-1 is detected in cartilage primordial cells, although ultimately Frzb-1 expression is down-regulated. Similar results were obtained in the recombinant limb bud, which was constructed from dissociated and re-aggregated mesenchymal cells and an ectodermal jacket with the apical ectodermal ridge. In addition, Frzb-1 expression preceded aggrecan expression in micromass cultures. These results suggest that Frzb-1 has a role in condensation formation and cartilage differentiation by regulating Wnt activity in the limb bud.     

Effects of cyclic AMP on limb bud chondrogenesis in low cell density culture

In a previous paper, it was shown that the limb bud mesodermal cells differentiated into cartilage even at low cell density by lowering the serum content in the culture medium (Hattori & Ide, Exp cell res 150 (1984) 338) [20]. The present paper describes the effects of cAMP on limb bud chondrogenesis at low cell density. cAMP promoted chondrogenesis at low cell density in cultures with various concentrations of serum. The limb bud cells differentiated into cartilage cells without forming aggregates. cAMP inhibited the loss of chondrogenic capability in serum-rich medium. The relationship between cAMP level and serum content is also discussed.     

Cell populations synthesizing cartilage proteoglycan core protein in the early chick limb bud.

Cartilage specific macromolecules are known to be synthesized in the mesenchyme of the embryonic chick limb bud, especially in areas of prechondrogenic condensations (Shinomura et al, 1984). Even though the mesenchyme seems homogeneous according to histological criteria, studies in the past have suggested the presence of different cell populations with different chondrogenic potential (Solursh et al, 1982; Swalla et al, 1984). In this study we have investigated by means of flow cytometry, the synthesis of proteoglycan core protein during early development of the chick limb bud in order to identify the different chondrocyte progenitor cells. We were able to identify by virtue of different size and density a cell population which synthesizes core protein extensively at stage 24 and stage 25 of development. This cell population synthesizes core protein predominantly at the proximal half of the limb bud at stage 24. However at stage 25 the same population synthesizes core protein predominantly at the distal half of the limb bud. These observations indicate that the distal half of stage 25 limb bud is mostly homogeneous with prechondrogenic cells and is in agreement with in vitro experiments that show high chondrogenic potential of the mesenchymal cells from this stage.     

Evaluation of the sensitive step of inhibition of chondrogenesis by retinoids in limb mesenchymal cells in vitro

The sensitive step of inhibition of chondrogenesis in vitro by retinoids was investigated in modified micromass cultures of limb bud mesenchymal cells from mouse embryos of day 11 and 12. Evaluation of chondrogenesis was performed after alcian blue staining, using a simple random hit counting of cartilage nodules. All-trans-retinoic acid, 13-cis-retinoic acid, and a newly developed arotinoid, RO 13-6298, were tested for their ability to inhibit chondrogenesis. We found that inhibition of chondrogenesis depended on the dosage and the duration of treatment with the different retinoids. Further analysis showed that chondrogenesis in limb bud mesenchymal cells from the proximal part was irreversibly inhibited after one hour of treatment, whereas distal cells showed a reduction of cartilage development only after a treatment period of 12 and more hours. In respect to the doses of the retinoids, proximal cells were about one magnitude more vulnerable than distal cells. These proximo-distal differences were obtained with 13-cis-retinoic acid at 10 micrograms/ml, with all-trans-retinoic acid at 1 microgram/ml and with arotinoid RO 13-6298 with 10 ng/ml. It is supposed that the late blastemal stage of chondrogenic differentiation before the onset of matrix synthesis is the step which is most vulnerable to retinoid treatment.     

Infection of chick limb bud presumptive chondroblasts by a temperature-sensitive mutant of Rous sarcoma virus and the reversible inhibition of their terminal differentiation in culture.

Stage 21 to 22 chicken embryo limb bud cells were infected with a temperature-sensitive mutant of Rous sarcoma virus and were grown in culture. Although control, uninfected cells yielded definitive chondroblasts (by day 4) which initiated the synthesis of the cartilage-characteristic proteoglycan, the transformed cells grown at the permissive temperature failed to do so. These effects were fully reversible after a shift to the nonpermissive temperature. In addition, infected cells at the nonpermissive temperature expressed traits of terminal chondrogenic maturation 2 to 3 days earlier than parallel, uninfected cells. Thus, Rous sarcoma virus-induced transformation reversibly blocks terminal limb bud cell chondrogenesis in culture, at the nonpermissive temperature, viral infection may also induce intracellular or extracellular conditions which favor or accelerate the process of chondrogenic cell maturation.     

Epidermal growth factor negatively regulates chondrogenesis of mesenchymal cells by modulating the protein kinase C-alpha, Erk-1, and p38 MAPK signaling pathways

During limb development, epithelial cells in the apical ectodermal ridge keep the underlying mesenchymal cells in a proliferative state preventing differentiation by secreting signaling molecules such as epidermal growth factor (EGF). We investigated the molecular mechanism of the EGF effect on the regulation of micromass culture-induced chondrogenesis of chick limb bud mesenchymal cells as a model system. We found that expression and tyrosine phosphorylation of the EGF receptor was increased transiently during chondrogenesis. Exogenous EGF inhibited chondrogenic differentiation of mesenchymal cells, and this effect was reversed by the EGF receptor inhibitor AG1478. EGF treatment also inhibited the expression and activation of protein kinase C-alpha, whereas it activated Erk-1 and inhibited p38 mitogen-activated protein kinase, all of which appeared to be involved in the EGF-induced inhibition of chondrogenesis. Stimulation of the EGF receptor blocked precartilage condensation and altered the expression of cell adhesion molecules such as N-cadherin and integrins alpha(5) and beta(1). All these EGF effects were reversible by AG1478. The data indicate that EGF negatively regulate chondrogenesis of chick limb bud mesenchymal cells by inhibiting precartilage condensation and by modulating signaling pathways including those of protein kinase C-alpha, Erk-1, and p38 mitogen-activated protein kinase.     

Effect of 4-methylumbelliferyl-β-d-xyloside on collagen synthesis in chick limb bud mesenchymal cell cultures

Previous studies showed that cultures of chick limb bud mesenchymal cells plated at high density, to maximize chondrogenic expression, had a much reduced extracellular matrix around chondrocytes when exposed to 4-methyl-, umbelliferyl-β-d-xyloside. The majority of newly synthesized chondroitin sulfate chains were found in the culture medium presumably bound to the xyloside as opposed to their normal deposition on the core protein of proteoglycan. The question remained open as to whether the development of an abnormal matrix affected the synthesis of extracellular deposition of other cartilage-specific macromolecules. We have analyzed, both morphologically and biochemically, the synthesis and deposition of Type I and Type II collagen by β-d-xyloside-treated cultures of limb mesenchymal cells. While the rate of collagen synthesis per plate and its extracellular accumulation after 8 days in culture were reduced to some extent, the ratios of Type II to Type I collagen and the morphological distribution of these macromolecules were not affected by exposure to β-d-xyloside. We conclude that the expression of the cartilage-specific Type II collagen during chondrogenic differentiation is, although reduced, qualitatively not dependent on the amount of extracellular chondroitin sulfate chains attached to matrix-associated proteoglycan core protein. However, prolonged exposure of limb bud cells to xylosides leads to the formation of a chondroitin sulfate- and collagen-deficient matrix which, in turn, reduces the capacity of limb bud cells to synthesize Types I and II collagen.     

Cyclic AMP derivatives stimulate the chondrogenic differentiation of the mesoderm subjacent to the apical ectodermal ridge of the chick limb bud.

Recent studies indicate that one of the major functions of the apical ectodermal ridge (AER) of the embryonic chick limb bud is to maintain mesenchymal cells directly subjacent to it (i.e., cells extending 0.4-0.5 mm from the AER) in a labile, undifferentiated condition. Furthermore, when mesenchymal cells are freed from the AER's influence, either artifically or as a result of normal polarized proximal-to-distal limb outgrowth, they are freed to commence cytodifferentiation. In a preliminary attempt to investigate at a molecular level the mechanism by which the AER exerts its "negative" effect on the cytodifferentiation of subridge mesenchymal cells, we have examined the effect of a variety of agents that elevate cyclic AMP levels on the chondrogenic differentiation of the unspecialized subridge mesoderm of the limb bud in an organ culture system. Dibutyryl- and 8-hydroxy-cyclic AMP elicit a dose-dependent increase in the rate and amount of cartilage matrix formation and a corresponding dose-dependent increase in sulfated glycosaminoglycan accumulation by subridge mesoderm explants. The stimulatory effect of suboptimal concentrations of cyclic AMP derivatives is potentiated by the addition of theophylline. The stimulatory effect is limited to cyclic AMP derivatives, since dibutyryl-cyclic GMP and 5'-AMP have no effect. Thus agents that elevate intracellular cyclic AMP levels stimulate the chondrogenic differentiation of the unspecialized subridge mesoderm of the embryonic chick limb bud.