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

Construction of multilayered molecular reservoirs on a titanium alloy implant for combinational drug delivery to promote osseointegration in osteoporotic conditions

In this study, β-cyclodextrin (β-CD) molecules are used as molecular reservoirs and grafted onto chitosan molecules for calcitriol (VD3) loading, which is a hormonally active metabolite of vitamin D. The resultant molecular complex is co-assembled with an antiosteoporosis drug calcitonin (CT) to form bio-functional multilayer structure on Ti6Al7Nb substrate via layer-by-layer self-assembly, which is capable of releasing VD3 and calcitonin in a sustained manner to modulate osteoblasts, osteoclasts, and macrophages at the bone-implant interface. In vitro results show that the released VD3 and CT individually upregulated the expression of calcium-binding protein (including Calbindin D9k and Calbindin D28k) and BMP2 in osteoblasts in peri-implant regions to stimulate their Ca deposition and differentiation. RAW264.7 cells (a murine macrophage) on the biofunctional implant displayed improved M2 phenotypical differentiation and expression of BMP2 and VEGF genes, but M1 phenotypical differentiation potential and MCF and TRAP gene expression levels are evidently lower. Results from in vivo micro-CT and histological analysis also demonstrate that VD3/CT co-loaded implant can dramatically enhance the bone remodeling under osteoporotic conditions with significantly enhanced interfacial shear strength and improved osseointegration as compared to other groups. The insights in this study offer new avenues for the rational functionalization of titanium implants to effectively repair osteoporotic fractures. STATEMENT OF SIGNIFICANCE: A promising strategy to enhance the recovery rate of osteoporotic fractures is to immobilize antiosteoporotic drugs onto the surface of titanium-based implants. In this study, we grafted beta-cyclodextrin (β-CD) onto chitosan (Chi) molecules to load VD3, which was co-assembled with calcitonin (CT) onto Ti6Al7Nb implants by the layer-by-layer assembly technique. The obtained functional titanium alloy implant (Ti6Al7Nb/LBL/Chi-CD@VD3/ CT) could stably release VD3 and calcitonin agents in a sustained manner. RAW264.7 cells grown on Ti6Al7Nb/LBL/Chi-CD@VD3/CT showed superior M2 phenotypical differentiation efficiency, but lower MCF/TRAP gene expression levels. In vitro and in vivo results showed that the released VD3 and CT individually upregulated the expression of calcium binding proteins and BMP2 in osteoblasts, promoting new bone formation in the peri-implant region.

Calcium in health and disease

Evolution has exploited the chemical properties of Ca(2+), which facilitate its reversible binding to the sites of irregular geometry offered by biological macromolecules, to select it as a carrier of cellular signals. A number of proteins bind Ca(2+) to specific sites: those intrinsic to membranes play the most important role in the spatial and temporal regulation of the concentration and movements of Ca(2+) inside cells. Those which are soluble, or organized in non-membranous structures, also decode the Ca(2+) message to be then transmitted to the targets of its regulation. Since Ca(2+) controls the most important processes in the life of cells, it must be very carefully controlled within the cytoplasm, where most of the targets of its signaling function reside. Membrane channels (in the plasma membrane and in the organelles) mediate the entrance of Ca(2+) into the cytoplasm, ATPases, exchangers, and the mitochondrial Ca(2+) uptake system remove Ca(2+) from it. The concentration of Ca(2+) in the external spaces, which is controlled essentially by its dynamic exchanges in the bone system, is much higher than inside cells, and can, under conditions of pathology, generate a situation of dangerous internal Ca(2+) overload. When massive and persistent, the Ca(2+) overload culminates in the death of the cell. Subtle conditions of cellular Ca(2+) dyshomeostasis that affect individual systems that control Ca(2+), generate cell disease phenotypes that are particularly severe in tissues in which the signaling function of Ca(2+) has special importance, e.g., the nervous system.

1,25-Dihydroxyvitamin D3-induced aortic calcifications in experimental uremia: up-regulation of osteoblast markers, calcium-transporting proteins and osterix

BACKGROUND AND OBJECTIVE

Whether treatment with vitamin D receptor activators contributes to cardiovascular disease in patients with chronic kidney disease is a matter of debate. We studied mechanisms involved in vitamin D-related vascular calcifications in vivo and in vitro.

METHODS

Aortic calcifications were induced in subtotally nephrectomized (SNX) rats by treatment with a high dose (0.25 μg/kg per day) of 1,25-dihydroxyvitamin D3 (calcitriol) given for 6 weeks. Likewise, primary rat vascular smooth muscle cells (VSMCs) were incubated with calcitriol at concentrations ranging from 10 to 10 mol/l. Immunohistochemistry revealed that the aortic expression of osteopontin, osteocalcin and bone sialoprotein was significantly increased in calcitriol-treated SNX rats compared to untreated SNX controls. In addition, aortic expression of the transient receptor potential vanilloid calcium channel 6 (TRPV6) and calbindin D9k was significantly up-regulated by treatment with calcitriol. Furthermore, calcitriol significantly increased expression of the osteogenic transcription factor osterix. In-vitro studies showed similar results, confirming that these effects could be attributed to treatment with calcitriol.

CONCLUSIONS

High-dose calcitriol treatment induces an osteoblastic phenotype in VSMC both in SNX rats and in vitro, associated with up-regulation of proteins regulating mineralization and calcium transport, and of the osteogenic transcription factor osterix.

Distribution of plasma-membrane Ca2+ pump in mandibular condyles from growing and adult rabbits

Chondrocytes may control the mineralization of the extracellular matrix of condylar cartilage by several mechanisms including the release of microvesicles involved in the initial nucleation, the creation or modification of the local matrix to help propagate or restrict mineralization, and the regulation of the ionic environment at the calcifying foci within the matrix. The plasma membrane Ca2+-Mg2+ ATPase (Ca2+ pump) is known to play a part in the vectorial efflux of calcium in a variety of cells including chondrocytes. The purpose here was to study the distribution of Ca2+-pump protein in mandibular condyles from growing and adult rabbits, and compare the expression of that protein in progressively differentiating chondrocytes whose final stage is associated with a mineralized extracellular matrix. Ca2+-pump antigen was identified immunohistochemically in six growing and six adult rabbit mandibular condyles with a Ca2+ pump-specific monoclonal antibody. The presence of Ca2+-pump antigen was established in hypertrophic chondrocytes, and in osteoblasts and osteoclasts of subchondral bone. Slot-blot analysis of nitrocellulose-immobilized chondrocyte homogenates showed that the amount of Ca2+ pump in growing cartilage was more than twice that in adult cartilage (p < 0.05). The demonstration of Ca2+-pump antigen in the hypertrophic chondrocytes of growing rabbit condyles is consistent with a role for the plasma-membrane Ca2+ pump in the calcification of mandibular condylar cartilage.

Synthesis of calbindin-D28K during mineralization in human bone marrow stromal cells

1alpha,25-Dihydroxyvitamin D3 [1,25(OH)2D3] is known to modulate Ca2+ metabolism in several cell types. Vitamin-D-dependent calcium binding proteins such as calbindin-D28K (28 kDa calcium binding proteins) have been shown to be regulated by 1,25(OH)2D3 but the mechanisms controlling calbindin synthesis are still poorly understood in human osteoblast cell culture models. The human bone marrow stromal cells (HBMSC) described in this paper developed a calcified matrix, expressed osteocalcin (OC), osteopontin (OP) and responded to 1,25(OH)2D3. The expression of vitamin D receptor mRNA was demonstrated by reverse transcription-PCR. Calbindin-D28K protein was identified only in cells arising from the sixth subculture, which exhibited a calcified matrix and all of the osteoblastic markers, e.g. OC and OP. It was demonstrated by dot-immunodetection using immunological probes, and by in situ hybridization using labelled cDNA probes. Moreover, vitamin D3 enhanced calbindin-D28K synthesis as well as OC synthesis and alkaline phosphatase activity. Uptake of 45Ca induced into the matrix by 1,25(OH)2D3 supports the hypothesis that the calcium-enriched matrix could trap calbindin-D proteins. In conclusion, the studies in vitro described in the present paper indicate, for the first time, a possible role of calbindin-D28K in mineralized matrix formation in HBMSC.

Ultrastructural localization of alpha-parvalbumin in the epiphyseal plate cartilage and bone of growing rats

The distribution of the calcium-binding protein, alpha-parvalbumin, in the epiphyseal plate cartilage and bone of growing rats was examined by electron microscope immunocytochemistry of undecalcified samples. Parvalbumin immunoreactivity, as revealed by gold particles, increased with maturation of chondrocytes and was maximal in the zone of calcification. It was found in the cytoplasm of chondrocytes, osteoblasts and osteocytes, corroborating light microscope observations. The immunolabeling was associated with amorphous electron-dense material in the cytoplasm and not bound to membranes. There was moderate parvalbumin immunolabeling over the dense chromatin in the nuclei of chondrocytes and bone cells, but none in the cell processes of mature and hypertrophic chondrocytes, in the matrix vesicles themselves, or in the cell processes of osteoblasts. However, there was parvalbumin immunoreactivity in the cell processes of the osteocytes of compact cortical bone. The uncalcified and calcified matrix of the epiphyseal cartilage, the osteoid, and the fully mineralized cortical bone were devoid of parvalbumin immunoreactivity. Thus, immunoreactive parvalbumin is confined to the cell bodies of chondrocytes and osteoblasts, and is unlikely to be directly involved in mineral deposition. The maximal parvalbumin immunoreactivity in the last terminal chondrocytes of the zone of calcification suggests that the protein is involved in buffering intracellular Ca2+, preventing the stimulation of degenerative processes by high intracellular calcium. The parvalbumin immunoreactivity in the cell processes of osteocytes of compact cortical bone seems to indicate that this calcium-binding protein may be involved in the regulation of Ca2+ fluxes and hence in calcium homeostasis in bone.

Localization of the Ca(2+)-binding alpha-parvalbumin and its mRNA in epiphyseal plate cartilage and bone of growing rats

This study describes the localization of alpha-parvalbumin, in undecalcified tibial epiphyseal cartilage and bone of growing rats by immunocytochemistry in the light microscope, and of parvalbumin mRNA by in situ hybridization. They were compared to the distribution of the calbindin-D9K and its mRNA in rat epiphyseal cartilage. All the chondrocytes of the epiphyseal cartilage were parvalbumin-immunopositive, but there was no parvalbumin immunoreactivity in the uncalcified or calcified extracellular cartilage matrix. The intensity of the immunostaining increased from the resting and proliferative to the mature and hypertrophic chondrocytes, with the greatest intensity in the terminal hypertrophic chondrocytes in the calcifying zone. The parvalbumin immunostaining was located in the cytoplasm, but no immunoreactivity was detected in any chondrocyte processes. The parvalbumin mRNA distribution and levels, as revealed by in situ hybridization, exactly mirrored those of the parvalbumin protein. In contrast to parvalbumin, calbindin-D9K and its mRNA appeared in mature chondrocytes and decreased in hypertrophic up to calcifying chondrocytes. Calbindin-D9K was located in the cytoplasm and all along the cell processes. In bone, the osteoblasts and the osteocytes of trabecular and compact cortical bones were immunoreactive for parvalbumin and contained parvalbumin mRNA. Parvalbumin lay in their cytoplasm, but there was no parvalbumin immunostaining in the extracellular uncalcified or mineralized bone matrix. The long processes of osteocytes, in compact bone only, were parvalbumin immunoreactive. Osteoclasts contained cytoplasmic parvalbumin immunoreactivity. Thus, the pattern of immunoreactive parvalbumin distribution indicates that the protein is not involved in the extracellular mineralization of cartilage and bone matrix. It appears to be associated with specific calcium-related intracellular functions in chondrocytes and in osteoblasts, osteocytes, and osteoclasts. As the highest cytoplasmic concentration of parvalbumin is in the terminal hypertrophic chondrocytes, parvalbumin could act as a calcium buffer to delay the death of chondrocytes. In compact bone, parvalbumin could also have a role throughout the osteocyte processes in regulating the fluxes of calcium ions for mineral homeostatis.

Calbindin-D28K and -D9K and 1,25(OH)2 vitamin D3 receptor immunolocalization and mineralization induction in long-term primary cultures of rat epiphyseal chondrocytes

Rat epiphyseal plat chondrocytes were grown on glass slides, as nonadhering monolayer cultures for up to 6 weeks. Chondrocyte growth, differentiation and maturation, matrix formation and mineralization, and the temporospatial distribution of the vitamin D-dependent calcium-binding proteins, calbindin-D9K and -D28K, and the 1,25(OH)2D3 receptor (VDR), were all monitored. Chondrocytes became confluent in 2.5 weeks, differentiated to acquire a chondrocyte (polygonal) morphology, produced extracellular matrix, and finally formed a true monolayer mineralizing cartilaginous tissue, with all the stages of chondrocyte development within a single culture. beta-Glycerophosphate promoted initial matrix mineralization in 4 weeks and accelerated cell differentiation. High nominal calcium and ascorbic acid were needed for abundant matrix formation. VDR occurred at all differentiation stages, in the nuclei and nucleoli and in the cytoplasm. Calbindin-D28K and -D9K were not coexpressed. Calbindin-D28K was found in prechondroblasts, chondroblasts, and in newly differentiated chondrocytes. It was cytoplasmic in prechondroblasts and subsequently also in the perinuclear region and in nuclei, suggesting migration to the nuclear chromatin. Calbindin-D28K was nuclear only in newly differentiated chondrocytes in vitro and was not found in mature chondrocytes. In contrast, calbindin-D9K was present in the cytoplasm of mature and hypertrophic chondrocytes only. It was first in the cell body and eventually migrated within and to the far end of long cell processes with a decreasing cytoplasmic concentration showed by decreased immunostaining intensity, and ultimately hypertrophy of chondrocytes in culture. These in vitro patterns of calbindins-D and VDR accurately reflect their in vivo distributions. The genomic action of vitamin D, in vitro, resulted in the synthesis of nuclear VDR and calbindins-D.(ABSTRACT TRUNCATED AT 250 WORDS)

Spatiotemporal pattern of type X collagen gene expression and collagen deposition in embryonic chick vertebrae undergoing endochondral ossification

We examined the spatio-temporal pattern of type X collagen mRNA and its protein in the embryonic chick vertebrae undergoing ossification by in situ hybridization and immunohistochemistry. Hypertrophic chondrocytes, producing type X collagen, were developed as islands of cells in a few vertebral body segments of stage 36 embryos. These cells were increased in number at stages 37 and 38 and they expressed high levels of type X collagen mRNA and deposited its protein in the matrix. Blood vessels entered from the perichondrium at stage 37 and invaded deeply into hypertrophic cartilage at stage 38. As the vertebrae grew further at stage 40, the leading front of active hypertrophic chondrocytes with high levels of type X mRNA shifted from the midvertebral perivascular area towards intervertebral borders, while the perivascular area retained a number of inactive hypertrophic chondrocytes with low levels of type X mRNA. Type X collagen was found in large amounts throughout the matrix areas containing both active and inactive hypertrophic chondrocytes. Calcium was detected by von Kossa's technique in hypertrophic cartilage matrix in a small amount at stage 37, in parts of the matrix with type X collagen deposition in succeeding stages, and finally in almost the entire area of type X collagen deposition at stage 45. The vertebral segments of stage 45 embryos also showed a clearly reversed pattern of expression between type X collagen mRNA and types II and IX collagen mRNAs. The results demonstrate that the production of type X collagen by hypertrophic chondrocytes precedes both vascular invasion and mineralization of the matrix, suggesting that hypertrophic chondrocytes have an important role in regulating these events.

Immunoreactive calbindin-D9K in bone matrix vesicle

This electron microscope study describes the subcellular occurrence and distribution of immunoreactive calbindin-D9K in the trabecular metaphyseal and compact cortical bone of normal rats, rachitic vitamin-D-deficient rats, and rachitic rats given 1,25-(OH)2D3. Undecalcified bones were embedded in Lowicryl K4M and calbindin-D9K antigenicity was detected by the protein A-gold method. Immunoreactive calbindin-D9K was localized in the cytoplasm and cell processes of osteoblasts and osteocytes. Immunoreactive calbindin-D9K was also found within matrix vesicles and calcifying matrix vesicles, where it lay over the needle-shaped crystallites, at the apparent site of initial crystal formation, but not along the whole crystallites. In fully mineralized bone it occurred at the same site, over the crystallites. Calibindin-D9K was vitamin-D-dependent in the osteoblasts and matrix vesicles, where its presence was correlated with the reappearance of crystallites in 1,25-(OH)2D3-treated vitamin-D-deficient rats. This suggests that immunoreactive calbindin-D9K is involved in mineral deposition in bone matrix vesicles. Abnormal intracellular calcification associated with calbindin-D9K antigenicity in the osteoblasts of 1,25-(OH)2D3-treated vitamin-D-deficient rats indicates that immunoreactive calbindin-D9K may also play a part in abnormal intracellular mineral deposition.

Calbindin-D9K immunolocalization and vitamin D-dependence in the bone of growing and adult rats

This report presents evidence for the presence of the vitamin D-dependent calcium-binding protein, calbindin-D9K, in bone cells and matrix. In undecalcified frozen sections of growing and adult rat bone, calbindin-D9K was immunohistochemically localized in trabecular bone of the epiphysis and metaphysis and in cortical bone of the diaphysis. It was found within the cytoplasm of osteocytes, of osteoblasts lining the osteoid, and osteoblasts inside the osteoid seams. It was also found in the osteoblast processes and the anastomosed reticulum of the processes connecting the osteocytes with each other. Extracellularly, calbindin-D9K immunoreactivity was present in compact cortical bone in the areas of the mineralized matrix surrounding the osteocyte lacunae, and in the pericanalicular walls containing the cell processes. Calbindin-D9K immunoreactivity was low or absent from the cytoplasm of osteocytes in trabecular bone from severely vitamin D-deficient rats and restored in vitamin D-deficient rats given a single dose of 1,25(OH)2-VitD3. Thus, the synthesis of immunoreactive calbindin-D9K by osteoblasts and osteocytes in trabecular bone is vitamin D-dependent. The presence of immunoreactive calbindin-D9K in the osteocytes and their cell processes suggests that this calcium-binding protein is involved in the calcium fluxes regulating bone calcium homeostasis. Its localization in osteoblasts involved in bone formation and in their cell processes suggests that it has a role in the calcium transport from these cells towards the sites of active bone mineralization.(ABSTRACT TRUNCATED AT 250 WORDS)