Bone vitamin D-dependent calcium-binding protein is localized in chondrocytes of growth-plate cartilage

Eggshell decalcification and skeletal mineralization during chicken embryonic development: defining candidate genes in the chorioallantoic membrane

During chicken embryonic development, skeleton calcification mainly relies on the eggshell, whose minerals are progressively solubilized and transported to the embryo via the chorioallantoic membrane (CAM). However, the molecular components involved in this process remain undefined. We assessed eggshell demineralization and calcification of the embryo skeleton after 12 and 16 d of incubation, and analyzed the expression of several candidate genes in the CAM: carbonic anhydrases that are likely involved in secretion of protons for eggshell dissolution (CA2, CA4, CA9), ions transporters and regulators (CALB1, SLC4A1, ATP6V1B2, SGK1, SCGN, PKD2) and vitamin-D binding protein (GC).

Our results confirmed that eggshell weight, thickness, and strength decreased during incubation, with a concomitant increase in calcification of embryonic skeletal system. In the CAM, the expression of CA2 increased during incubation while CA4 and CA9 were expressed at similar levels at both stages. SCL4A1 and SCGN were expressed, but not differentially, between the two stages, while the expression of ATP6V1B2 and PKD2 genes decreased. The expression of SGK1 and TRPV6 increased over time, although the expression of the latter gene was barely detectable. In parallel, we analyzed the expression of these candidate genes in the yolk sac (YS), which mediates the transfer of yolk minerals to the embryo during the first half of incubation. In YS, CA2 expression increases during incubation, similar to the CAM, while the expression of the other candidate genes decreases. Moreover, CALB1 and GC genes were found to be expressed during incubation in the YS, in contrast to the CAM where no expression of either was detected.

This study demonstrates that the regulation of genes involved in the mobilization of egg minerals during embryonic development is different between the YS and CAM extraembryonic structures. Identification of the full suite of molecular components involved in the transfer of eggshell calcium to the embryo via the CAM should help to better understand the role of this structure in bone mineralization.

Alteration in the expression level of calbindin D28k in the periodontal ligament of the rat molar during experimental tooth movement

The present immunohistochemical study was designed to investigate changes in the distribution and expression level of calbindin D28k in the periodontal ligament during experimental tooth movement in the rat molar to clarify the physiological role of this protein in the ligament. In normal animals, calbindin D28k-like immunoreactivity appeared sparsely in spindle-shaped cells in the alveolar half of the periodontal ligament. Electron microscopic observations showed that these immunoreactive cells were characterized by well-developed rough-surfaced endoplasmic reticulum and phagosomes--which often contained collagen fibers--suggesting that these cells could be categorized as periodontal fibroblasts. Twelve hours following the onset of the experimental tooth movement, cells positive for calbindin D28k increased in number in the periodontal ligament, especially in the alveolar half of the pressured side. Immunoelectron microscopy showed that the calbindin D28k-immunopositive cells had morphological features similar to those of fibroblasts in the normal ligament, and that these cells occasionally made contact with immunonegative macrophage-like cells. Immunopositive cells gradually decreased in number, and the distribution of the cells and intensity of the immunoreactivity returned to normal levels by 14 days following the induction of the experimental tooth movement. The present results suggest that calbindin D28k plays an important role in the homeostasis and cyto-protection of fibroblasts in the periodontal ligament at the initial phase of experimental tooth movement.

Calbindin-D9k and calbindin-D28k expression in rat mineralized tissues in vivo

Following their terminal differentiation, highly specialized cells, ameloblasts, odontoblasts, and osteoblasts sequentially elaborate mineralized tissues. While the developmental expression pattern of matrix proteins has been studied extensively, less attention has been paid to the molecules involved in calcium handling, such as calcium-binding proteins. This shortcoming, as well as previous conflicting data, led us to conduct studies on calbindin-D9k and calbindin-D28k in rat mandibular bone and incisor based on several methods established on rat ameloblasts in vivo. Radioimmunoassays showed that calbindin-D28k accounts for approximately 0.1% of cytosolic proteins in the ectomesenchymal fraction and 1% in the epithelial fraction of the rat incisor and is 100-fold more concentrated than calbindin-D9k in both tissue types. Western blot analysis confirmed that the anticalbindin-D28k reactive species corresponded to the well characterized renal calbindin-D28k in the ectomesenchyme. In this tissue, calbindin-D28k was ultrastructurally immunolocalized in the odontoblasts. Quantitative immunocytochemistry showed that labeling was distributed throughout their nucleus and cytoplasm. The similar cytoplasmic distribution of both calbindin-D proteins and mRNAs suggests that their expression is regulated at the subcellular level. In particular, immunoreactive calbindin-D28k appeared to be associated with rough endoplasmic reticulum. Calbindin-D9k antisense probe showed negligible labeling in odontoblasts, in parallel with the protein quantities measured (approximately 10 ng/mg of total protein). Finally, in situ hybridization showed transcripts for both calbindins-D in ameloblasts and also in osteoblasts. In summary, the present results support the concept that an elevated expression of these vitamin D-dependent calcium-binding proteins may characterize the phenotype of cells directly involved in the elaboration of mineralized tissues, enamel, dentine, and bone.

Abundant expression of parathyroid hormone-related protein in primary rat aortic smooth muscle cells accompanies serum-induced proliferation

Parathyroid hormone-related protein (PTHrP), which is responsible for producing hypercalcemia in patients with humoral hypercalcemia of malignancy, has recently been identified in several normal tissues. Because PTHrP, like parathyroid hormone (PTH), is known to exhibit vasodilatory properties, we investigated the expression and regulation of PTHrP mRNA in cultured rat aortic smooth muscle cells (SMC). We report here that PTHrP mRNA is expressed in SMC and is markedly induced by serum in a time- and concentration-dependent fashion. Addition of 10% fetal calf serum to serum-deprived, confluent cells, resulted in a marked induction of PTHrP mRNA by 2 h with a peak at 4-6 h. PTHrP was detected in SMC by immunocytochemistry and radioimmunoassay of conditioned medium, and was shown to be up-regulated within 24 h after the addition of serum. The serum induction of PTHrP mRNA was blocked by actinomycin D and by cycloheximide indicating the need for protein synthesis to evoke the serum effect on PTHrP gene transcription. In addition, treatment with dexamethasone, which has been previously shown to reduce the constitutive expression of PTHrP in human cancer cells, also blunted the serum induction of PTHrP mRNA in SMC. Treatment of quiescent cells with the serum mitogens platelet-derived growth factor or insulin-like growth factor-I had no effect on PTHrP, whereas the vasoactive peptides endothelin, norepinephrine and thrombin stimulated PTHrP expression. Exogenous addition of recombinant PTHrP-(1-141) had no significant effect on SMC DNA synthesis as measured by [3H]thymidine incorporation. In summary, the abundance of PTHrP mRNA and the characteristics of its regulation in SMC suggest a major role for PTHrP as a local modulator in vascular smooth muscle.

Subcellular co-localization and co-variations of two vitamin D-dependent proteins in rat ameloblasts

The immunocytochemical patterns of calbindin-D9k (CaBP 9k) and calbindin-D28k (CaBP 28k) were compared by light and electron microscopy throughout amelogenesis. Labelling on serial sections and co-localization of CaBPs confirmed that the two proteins were restricted to a single cell type, the ameloblasts. Their quantity increased during presecretion, was stable during secretion and alternately high and low during the cyclic modulation of ameloblasts which occurs during maturation. Ruffle-ended ameloblasts contained the highest apparent concentration. Investigations with several fixatives indicated that the CaBPs were present in the cytosol and the nucleus, although there were slight differences with various fixatives by light microscopy. Their concentrations in these compartments varied in parallel throughout amelogenesis. However, mitochondria contained only immunoreactive CaBP 9k. While the distribution of CaBP 9k in zones containing Golgi apparatus and rough endoplasmic reticulum was similar, CaBP 28k concentration has, in another paper, been shown to be higher near the rough endoplasmic reticulum.

Glucosaminidase, galactosaminidase, and glucuronidase in the growth plate

The activities of glucosaminidase, galactosaminidase, and glucuronidase were determined in fractions of bovine growth plate cartilage. Glucosaminidase and galactosaminidase activities were lowest in the area corresponding to the reserve cartilage and increased from the upper to the lower portions of the hypertrophic zones of the growth plate, reaching a maximum in the calcifying cartilage. Glucuronidase activity showed a distinct spike of activity in the calcifying cartilage. The spatial distribution of these activities suggests a role in calcification and in the dissolution of the extracellular matrix at the chondro-osseous junction of the growth plate.

Differential expression of biological effects in maturationally distinct subpopulations of growth plate chondrocytes

Countercurrent centrifugal elutriation is an accurate and reproducible technique which can separate cells on the basis of size. This technique was utilized for the separation of isolated chick growth plate chondrocytes. Mean cellular volume, alkaline phosphatase activity, and Type X collagen synthesis progressively increased in each of seven successive separated fractions. The chondrocytes from the different fractions expressed a differential response to the growth regulator, transforming growth factor beta (TGF beta). TGF beta receptor binding to these cell fractions suggests that the different biological effects may be due to differences in the number of receptors expressed per cell as well as to differences in the proportion of high and low affinity TGF beta receptors present.

Immunoreactive calbindin-D9K localization in matrix vesicle-initiated calcification in rat epiphyseal cartilage: an immunoelectron microscope study

Calbindin-D9K immunoreactivity was localized by electron microscopy in rat calcifying epiphyseal plate cartilage. Antigen-antibody reaction sites were visualized by the presence of protein A-gold complex particles on undecalcified material embedded in Lowicryl K4M. Immunoreactive calbindin-D9K was found in the hyaloplasm of hypertrophic chondrocytes and inside and at the ends of their cell processes. It was localized outside the cells, inside matrix vesicles (MVs), often against the inner face of the delimiting membrane, and inside the trilaminar membrane. Immunoreactive calbindin-D9K appeared to be extruded from the chondrocytes into the matrix vesicles when the latter were formed during the budding of cell processes. In calcifying MVs, gold particles were detected over the needle-shaped crystallites and often over the crystallites lying against the inner leaflet of the vesicular membrane. At a later stage of matrix vesicle calcification after MV membrane disruption, the number of gold particles remained unchanged over the clusters of crystallites at the loci from which the crystallites appeared to have grown and radiated. At a yet more advanced stage of calcification, they remained in the same areas, which were limited to the lateral edges of calcified cartilage longitudinal septa. These results suggest that immunoreactive calbindin-D9K plays a role in calcium input to matrix vesicles and may be involved in matrix vesicle calcification, perhaps in the initial event of matrix vesicle crystal nucleation.

Cartilage macromolecules and the calcification of cartilage matrix

The calcification of cartilage matrix in endochondral bone formation occurs in an extracellular matrix composed of fibrils of type II collagen with which type X collagen is closely associated. Also present within this matrix are the large proteoglycans containing chondroitin sulfate which aggregate with hyaluronic acid. In addition, the matrix contains matrix vesicles containing alkaline phosphatase. There is probably a concentration of calcium as a result of its binding to the many chondroitin sulfate chains. At the time of calcification, these proteoglycans become focally concentrated in sites where mineral is deposited. This would result in an even greater focal concentration of calcium. Release of inorganic phosphate, as a result of the activity of alkaline phosphatase, can lead to the displacement of proteoglycan bound calcium and its precipitation. The C-propeptide of type II collagen becomes concentrated in the mineralizing sites, prior to which it is mainly associated with type II collagen fibrils and is present in dilated cisternae of the enlarged hypertrophic chondrocytes. The synthesis of type II collagen and the C-propeptide, together with alkaline phosphatase, are regulated by the vitamin D metabolites 24,25(OH)2 cholecalciferol and 1,25 (OH)2 cholecalciferol. At the time of calcification, type X collagen remains associated with type II collagen fibrils. It may play a role in preventing the initial calcification of these fibrils focusing mineral formation in focal interfibrillar sites. This process of calcification is clearly very complex, and involves different interacting matrix molecules and is carefully regulated at the cellular level.

A mathematical modelling study of epiphyseal cartilage calcification

Recent studies in this laboratory have suggested that proteoglycan may function as a Ca ion-exchanger in the calcification of epiphyseal growth plate cartilage. Specifically, it has been proposed that phosphate liberated from hypertrophic chondrocytes may displace calcium ions bound to the anionic groups of proteoglycans, thereby raising the Ca x PO4 activity product above the threshold for precipitation of hydroxyapatite. In order to determine whether this mechanism is quantitatively feasible, a mathematical model of the interaction between Ca, Na, proteoglycan and phosphate has now been developed. This model is based on a general binding theory, and utilizes previously-determined values for the binding constants of the Ca-proteoglycan interaction, inhibition constants for the effect of Na and phosphate on this interaction, and literature values for the concentrations of proteoglycan, Na and Ca in epiphyseal cartilage. Using this approach, it was predicted that the free Ca concentration in epiphyseal cartilage in the absence of phosphate will be 1.55 mM. At 0.7 mM phosphate, the approximate concentration in non-calcified cartilage matrix, the free Ca concentration will be 2.40 mM, corresponding to a Ca x PO4 product of 1.68 (mM)2. In order to achieve a Ca x PO4 product sufficient for spontaneous precipitation of hydroxyapatite [approximately 4.3 (mM)2], a phosphate concentration of approximately 1.40 mM is required. Therefore, calcification of epiphyseal cartilage matrix by the mechanism described above will require an approximate doubling of the phosphate concentration in the pre-calcifying zones, indicating that the release of a fraction of the intracellular phosphate could trigger the calcification process.

Immunological characterization, developmental pattern and vitamin-D-dependency of calbindin D-28 K in rat teeth ameloblasts

It has been suggested that vitamin D is involved in the process of cell differentiation and extracellular mineralization during tooth development. One of the best-defined molecular markers of the action of vitamin D is a calcium-binding protein of Mr 28,000 called calbindin D-28 K (CaBP 28 K). Since this protein is present in growing teeth, we have examined its synthesis in teeth from vitamin D-replete and -deplete rats by Western blotting and immunocytochemistry with an antiserum to CaBP 28 K purified from rat kidney. The CaBP 28 K present in the enamel organ is a single molecular species migrating near 30 k Da, similarly to the kidney protein. The differentiation and maturation of odontogenic cells were followed during early postnatal development (2-12 days) in rat molars. At the light-microscope level, CaBP 28 K was only found in a single cell-type, the ameloblasts. The expression of this protein appeared to be developmentally controlled, since its distribution varied with the cell stage and the functional steps of amelogenesis. The protein was localized in the basal compartment of ameloblasts from the presecretory stage. During the early secretory stage, the concentration of cytoplasmic CaBP 28 K formed a gradient from the apical to the basal pole of the ameloblasts. Staining appeared homogeneous in the cytoplasm of later secretory ameloblasts. CaBP 28 K was discontinuously distributed during the maturation stage. This discontinuity might be related to cyclical changes in mature ameloblasts. In all stages, ameloblasts from vitamin-D-deficient rats appeared depleted of CaBP 28 K.

1 alpha,25(OH)2 vitamin D3: a steroid hormone capable of producing pleiotropic receptor-mediated biological responses by both genomic and nongenomic mechanisms

Vitamin D3 is now considered a pro-hormone and its biological activities are in a great majority due to its hormonally active derivative 1 alpha,25-dihydroxyvitamin D3. The actions of vitamin D3 have been extensively studied both in vivo and in vitro using different animal models as well as a diversity of cell types including intestinal epithelial cells, kidney cells, lymphocytes, macrophages and a number of malignant cell lines. Results thus obtained have considerably extended the number of target tissues for this vitamin and hormone far beyond its classical target organs, parathyroid gland, kidney, intestine and bone. Studies performed at the cellular, subcellular and molecular levels have led to the establishment of new concepts on the mode of action of vitamin D3 which is not believed to act through receptor-mediated genomic and nongenomic events leading to the expression of the full biological response.

Morphologic stages of the terminal hypertrophic chondrocyte of growth plate cartilage

Recent biochemical and morphologic evidence supports the concept that hypertrophic chondrocytes of growth plate cartilage are fully viable cells that play a major functional role in controlling endochondral ossification. However, events associated with chondrocyte death remain unknown. In this study we assess the viability of terminal hypertrophic chondrocytes in situ in an organ culture system viewed simultaneously with rectified Nomarski interference contrast optics and with vital staining under fluorescence optics. Second, we use two methods of optimal chemical fixation at the ultrastructural level to define morphologically distinct stages of the terminal hypertrophic chondrocyte, which we interpret as the stages preceding chondrocyte death. An analysis of serial sections at the light microscope level showed that terminal chondrocytes were found, with different probabilities, in three morphologically distinguishable stages. Seventy-five percent of all profiles were fully hydrated cells with an intact plasma membrane making direct contact with the pericellular matrix, a morphology identical to that of living terminal chondrocytes viewed in Nomarski optics. Approximately 1% of terminal chondrocytes, while still in a fully hydrated state, consistently made a direct asymmetrical contact of the plasma membrane with the last transverse septum. In 24% of the profiles, terminal chondrocytes were found as condensed cells that retained their attachment to the last transverse septum. The stages were not characteristic of chondrocytes positioned more proximally in the growth plate. We hypothesize that a condensed morphology eventually characterizes each hypertrophic chondrocyte, and we relate these observations to current hypotheses concerning the mechanism of death of hypertrophic chondrocytes.

An ion-exchange mechanism of cartilage calcification

The provisional calcification of epiphyseal cartilage involves deposition of hydroxyapatite (calcium phosphate) crystals in an extracellular matrix consisting principally of Type II collagen and cartilage proteoglycan. A mechanism is now proposed to explain how epiphyseal cartilage calcification is initiated. Calcium exists at high concentration in cartilage, but is mainly bound to the anionic groups of proteoglycans, and thus is unavailable for precipitation. A local increase in phosphate concentration displaces calcium ions from proteoglycan by an ion-exchange effect, raising the Ca X PO4 product above the threshold for precipitation of hydroxyapatite. Evidence for this hypothesis has been derived from studies of the effect of phosphate on the binding of calcium to cartilage proteoglycan, and on hydroxyapatite formation in the presence of chondroitin sulfate.