Modification of plasma membrane of differentiating preosseous chondrocytes: evidence for a degradative process in the mechanism of matrix vesicle formation

Cartilage formation by fetal rat chondrocytes cultured in alginate beads: a proposed model for investigating tissue-biomaterial interactions

Chondrocytes from 21-day-old rat fetal nasal cartilage were cultured in alginate beads for up to 20 days. It was found that chondrocytes retained their spherical shape and typical chondrocytic appearance. During the culture time, chondrocytes underwent differentiation, as demonstrated by the alkaline phosphatase-specific activity and rate of proteoglycan synthesis. Morphological data confirmed chondrocyte differentiation with the appearance of hypertrophic chondrocytes scattered in the alginate gel and a dense extracellular matrix containing filamentous structures and matrix vesicles. In addition, Northern blot analysis performed on day 8 of culture showed that chondrocytes cultured in alginate beads expressed type II collagen mRNA. The alginate bead method also appeared to be suitable for testing biomaterials, and the ready dissolution of the alginate beads by chelating agents provided a simple means for the rapid recovery of encapsulated chondrocytes. Powdered glass-ceramic particles entrapped in the alginate gel were colonized by chondrocytes, which then proliferated and formed a tissue similar to a true calcified cartilaginous structure. These results indicate that the alginate system represents a relevant model for studies of chondrogenesis and endochondral ossification. Furthermore, the encapsulation method could prove useful for studies of tissue-biomaterial interactions in an in vitro environment which more closely mirrors the cartilage matrix than other culture methods.

Sensitivity of chondrocytes of growing cartilage to reactive oxygen species

Vascular invasion of calcified cartilage, during endochondral ossification, is initiated and sustained by invasive cells (endothelial cells and macrophages) which degrade the tissue by releasing lytic enzymes. Concurrently, reactive oxygen species (ROS) are also released by these cells and we hypothesize that ROS also contribute to the degradation of the tissue. As a preliminary approach to this problem, the antioxidant activities and the effect of ROS on hypertrophic cartilage and chondrocytes (HCs) were investigated. Compared to resting or articular chondrocytes, HCs exhibited higher catalase but lower SOD specific activities and lower PHGPx concentration, thus revealing a defence activity specific against H2O2. Moreover, dose-dependent depletion of ATP occurred after few minutes of exposure to ROS, and a long-term treatment (16 h incubation with ROS) promoted the release of LDH activity and a significant variation of the poly- to mono-unsaturated fatty acid ratio. Finally, the incubation of HCs with low ROS doses induced the release of sedimentable alkaline phosphatase activity (matrix vesicles). How the obtained results fit the in vivo occurring events is discussed.

Effect of 17 beta-estradiol on chondrocyte membrane fluidity and phospholipid metabolism is membrane-specific, sex-specific, and cell maturation-dependent

In this study we examined the hypothesis that 17 beta-estradiol exerts both rapid and direct, nongenomic effects of cells in the endochondral pathway. To do this, we used a cell culture model in which chondrocytes at two distinct stages of cell maturation are isolated from the costochondral cartilage of male and female rats, and examined the short-term effect of 17 alpha- and 17 beta-estradiol on [14C]arachidonic acid turnover in the cell layer and phospholipase A2 specific activity in plasma membranes and extracellular matrix vesicles isolated from similarly prepared cultures. In addition, the effect of 17 alpha- and 17 beta-estradiol on plasma membrane and matrix vesicle membrane fluidity was assessed. The effect of hormone on arachidonic acid turnover was rapid, time- and concentration-dependent, stereo-specific, and cell maturation-specific. Only resting zone cells from female rats were affected, and only 17 beta-estradiol elicited a response. Similarly, only female rat resting zone chondrocytes exhibited a change in phospholipase A2 activity after a 24 h exposure to hormone, causing an increase in enzyme activity in the matrix vesicles, but not plasma membranes. When isolated membranes were incubated directly with hormone, membrane fluidity was decreased in both plasma membranes and matrix vesicles isolated from female rat resting zone chondrocyte cultures. This nongenomic effect was dose-dependent and stereo-specific and differentially expressed in the two membrane fractions with respect to time course and magnitude of response. These results support the hypothesis that 17 beta-estradiol has a rapid action on chondrocyte membrane lipid metabolism and suggest that specific membrane components, characteristic of a particular sex and state of cell maturation, are involved in the nongenomic effects of this sex hormone on isolated matrix vesicles and plasma membranes.

Nongenomic regulation of protein kinase C isoforms by the vitamin D metabolites 1 alpha,25-(OH)2D3 and 24R,25-(OH)2D3

Prior studies have shown that vitamin D regulation of protein kinase C activity (PKC) in the cell layer of chondrocyte cultures is cell maturation-dependent. In the present study, we examined the membrane distribution of PKC and whether 1 alpha,25-(OH)2D3 and 24R,25-(OH)2D3 can directly regulate enzyme activity in isolated plasma membranes and extracellular matrix vesicles. Matrix vesicle PKC was activated by bryostatin-1 and inhibited by a PKC-specific pseudosubstrate inhibitor peptide. Depletion of membrane PKC activity using isoform-specific anti-PKC antibodies suggested that PKC alpha is the major isoform in cell layer lysates as well as in plasma membranes isolated from both cell types; PKC zeta is the predominant form in matrix vesicles. This was confirmed in Western blots of immunoprecipitates as well as in studies using control peptides to block binding of the isoform specific antibody to the enzyme and using a PKC zeta-specific pseudosubstrate inhibitor peptide. The presence of PKC zeta in matrix vesicles was further verified by immunoelectron microscopy. Enzyme activity in the matrix vesicle was insensitive to exogenous lipid, whereas that in the plasma membrane required lipid for full activity. 1,25-(OH)2D3 and 24,25-(OH)2D3 inhibited matrix vesicle PKC, but stimulated plasma membrane PKC when added directly to the isolated membrane fractions. PKC activity in the matrix vesicle was calcium-independent, whereas that in the plasma membrane required calcium. Moreover, the vitamin D-sensitive PKC in matrix vesicles was not dependent on calcium, whereas the vitamin D-sensitive enzyme in plasma membranes was calcium-dependent. It is concluded that PKC isoforms are differentially distributed between matrix vesicles and plasma membranes and that enzyme activity is regulated in a membrane-specific manner. This suggests the existence of a nongenomic mechanism whereby the effects of 1,25-(OH)2D3 and 24,25-(OH)2D3 may be mediated via PKC. Further, PKC zeta may be important in nongenomic, autocrine signal transduction at sites distal from the cell.

Differential scanning calorimetric study of the effect of vitamin D3 on the thermotropic phase behavior of lipids model systems

Differential scanning calorimetry has been used to investigate the effects of vitamin D3 on the physical properties of model membranes including pure phosphatidylcholines (PC's) of chain length from 14 to 18, pure dipalmitoylphosphatidylethanolamine (DPPE), and various mixtures of these lipids. The results demonstrate that the interactions of vitamin D3 with PC's are dependent on acyl chain lengths. It was found that vitamin D3 reduces the transition temperatures of PC's and PE's, broadening the transition and reducing the enthalpy, eventually abolishing the transition. The interaction of vitamin D3 with PC varies with chain length; the transition of DMPC is abolished by only 15 mol% vitamin D3 whereas 45 mol% vitamin D3 is required to abolish the transition of DSPC. These variations in vitamin D3 lipid interactions are further explored in various mixtures. In the mixture studies it is shown that vitamin D3 affects the mixing properties of the lipids in the mixtures. The results suggest that the presence of vitamin D3 in lipids can affect the lateral phase distributions of lipids, and thereby have important effects on membrane function.

Calcium, sulfur, and zinc distribution in normal and arthritic articular equine cartilage: a synchrotron radiation-induced X-ray emission (SRIXE) study

Calcium, sulfur, and zinc content in normal and arthritic equine cartilage have been studied by synchrotron radiation-induced X-ray emission (SRIXE). Ranging from the superficial to the columnar zone of the normal tissue, calcium and zinc concentrations are increasingly higher, whereas sulfur is at its highest concentration in the transitional zone. In the arthritic tissue, calcium concentration is at its maximum in the transitional zone, whereas zinc and sulfur distributions are relatively homogeneous. Sulfur concentration in arthritic cartilage is reduced to about one-third with respect to that in normal tissue. The possibility that zinc concentration reflects the distribution of the zinc-containing enzyme alkaline phosphatase is presented.

Occurrence of osteoblast necroses during ossification of long bone cortices in mouse fetuses

Previous investigations concerned with in vitro osteogenesis and mineralization have revealed some indication of a participation of cell necroses in the course of calcification. These observations were confirmed by in vivo investigations on desmoid ossification in fetal mouse calvariae, where abundant necrotic osteoblasts were found at the mineralization border and in the osteoid. In the present study, ossification of long bone cortices from fetal mice was investigated by use of electron microscopy. Specimens obtained from the collection of the Institute of Anatomy, Free University of Berlin (mouse fetuses, forearm; rat fetuses, forearm) were reinvestigated for control purposes. In all cases, mineralization of osteoid was accompanied by cell necroses. Cell degeneration was characterized by swelling of the endoplasmic reticulum and loss of the plasma membrane resulting in freely distributed vesicular structures. Cell debris was incorporated within the mineral. Initially, cell necroses in the perichondrium occurred in the region surrounding the hypertrophic cartilage and the matrix of which showed spots of endochondral mineralization. Necrotic osteoblasts occurred simultaneously with mineralization of the osteoid. During further ossification of the long bone cortices, the number of necrotic cells increased markedly. In addition to necrotic cells, healthy osteoblasts, osteocytes and perichondral tissue were present, indicating that an artifact can be excluded. The importance of cell necroses in the process of mineralization is as yet unclear. Possibly, the cells act as calcium and/or phosphate stores, which are liberated by cell death to increase the amount of mineral constituents at sites of mineralization.

Culture and differentiation of chondrocytes entrapped in alginate gels

We studied the response to culture conditions and the differentiative ability in suspension culture in alginate gels of resting chondrocytes from the preosseous cartilage of adult pig scapula. It was found that the maximum rate of chondrocyte duplication is reached at the fourth day in culture whereas the rate of proteoglycan synthesis and alkaline phosphatase expression do not gain a maximum value before the seventh day. During the culture time, the chondrocytes undergo differentiation as it is demonstrated by the alkaline phosphatase specific activity increase and by morphological criteria (hypertrophy, increase of the number of mitochondria per cell, increased endoplasmic reticulum, matrix vesicle production). The alginate gels can be easily dissolved to obtain cell populations in which the variation of cytosolic calcium concentration following a proliferative stimulus can be conveniently observed using the conventional procedure of Fura 2.

Regulation of matrix vesicle phospholipid metabolism is cell maturation-dependent

We have developed a chondrocyte culture model for assessing the regulation of matrix vesicles at two different stages of chondrogenic maturation. These chondrocytes, resting zone (RC) and growth zone (GC), retain their phenotypic markers in culture, including production of matrix vesicles with distinctive lipid compositions and enzyme activities. Isolated matrix vesicles incubated in vitro with 1,25-(OH)2D3 (1,25) or 24,25-(OH)2D3 (24,25) respond differentially. 1,25 stimulates phospholipase A2 (PA2) in GC vesicles, but not on those from RC. 24,25 inhibits PA2 in RC vesicles, but has no effect on GC. PA2 activity is required for fatty acid turnover and is the rate-limiting step in prostaglandin production. Plasma membrane phospholipids are more susceptible to the release of arachidonic acid by PA2 than are matrix vesicle phospholipids. Matrix vesicles are distinct from the plasma membrane in terms of lipid composition and arachidonic acid incorporation. 1,25 and 24,25 stimulate arachidonic acid turnover in their target cells, but by different mechanisms. 1,25 has no effect on arachidonic acid turnover in RC; however, 24,25 inhibits turnover in RC and GC. 1,25 and 24,25 also affect isolated matrix vesicle membrane fluidity. These results suggest that vitamin D metabolites modulate PA2 activity, change the composition of membrane phospholipids by altering fatty acid composition, and affect calcium transport. The effects are mediated by altering membrane fluidity and is dependent on the stage of cell differentiation.

A potassium channel in cultured chondrocytes

Chondrocytes, obtained from preosseous cartilage, were studied by patch clamp technique in cell-attached recording configuration, and single potassium channels were characterized at different stages of culture. After 3 days, outward currents were present, with an open probability increasing with depolarization, and the K+ channels showing a mean slope conductance of 82 pS in asymmetric and 168 pS in symmetric potassium solution. Tetraethylammonium (TEA) and quinidine blocked the channels. Cells at confluence showed similar channel activity, with conductances of 121 and 252 pS, respectively. We suggest that culture time and/or conditions may modify K+ channels or induce the expression of a new type of channels.

Serum-induced cytosolic calcium movements and mitogenesis in cultured preosseous chondrocytes

The differentiation of preosseous chondrocytes begins with the proliferation of resting cells and results in the expression of the hypertrophic phenotype. The effect of fetal calf serum on chondrocyte mitogenesis and intracellular Ca2+ concentration was studied in resting and hypertrophic cells in primary culture. Resting chondrocytes respond to the growth stimulus with immediate release of Ca2+ from intracellular stores and with opening of the plasma membrane Ca2+ channels. These events may be related to the elevated [3H]thymidine incorporation observed after serum exposure. In contrast, in hypertrophic chondrocytes the lower rate of DNA synthesis seems to be coupled with a lower activity of the Ca2+ signaling mechanism and, probably, with reduced intracellular calcium stores. It is proposed that expression of the Ca2+ signaling mechanism may be modulated during the differentiation of preosseous chondrocytes.