Na+/Ca2+ exchange in rat osteoblast-like UMR 106 cells

Two-Armed Activation of Bone Mineral Deposition by the Flavones Baicalin and Baicalein, Encapsulated in Polyphosphate Microparticles

In this study, we investigated the effect of the two flavonoids, baicalin (baicalein 7-O-[Formula: see text]- d-glucuronic acid) and its aglycone, baicalein (5,6,7-trihydroxyflavone), after encapsulation into amorphous calcium polyphosphate (Ca-polyP) microparticles on mineralization of primary human osteoblasts (phOSB). Both flavonoids, which come from root extracts of Scutellaria baicalensis Georgi, are used in Traditional Chinese Medicine, and are nontoxic in cells up to a concentration of 3[Formula: see text][Formula: see text]g/ml. The morphogenetically active, energy-rich Ca-polyP particles with a stoichiometric P:Ca ratio of 1:2 are degraded by cellular alkaline phosphatase (ALP) to ortho-phosphate used for bone hydroxyapatite formation. Here we show that the flavone-loaded Ca-polyP microparticles are readily taken up by phOSB, resulting in the accumulation of polyP around the nuclei and the formation of intracellular vesicles containing the ALP. In addition, we demonstrate that baicalin/baicalein causes a rise of the intracellular calcium [Ca[Formula: see text]]_i a level which markedly is augmented after encapsulation into Ca-polyP, through activation of the phospholipase C. Moreover, both flavones, either alone or associated with Ca-polyP microparticles, upregulate the expression of the osteoblast calcium efflux channel, the plasma membrane Ca[Formula: see text]-ATPase (PMCA), while the expression of ALP, which promotes bone mineralization, is induced by Ca-polyP and by the flavones only if present in the Ca-polyP-microparticle-associated form. As a result, the extent of bone mineralization is markedly enhanced. Based on the two-armed activating function, new applications of baicalin/baicalein as a component of nutriceuticals for osteoporosis prevention or bone implants can be envisaged.

NCX3 is a major functional isoform of the sodium-calcium exchanger in osteoblasts

The calcium phosphate-based skeleton of vertebrates serves as the major reservoir for metabolically available calcium ions. The skeleton is formed by osteoblasts which first secrete a proteinaceous matrix and then provide Ca++ for the calcification process. The two calcium efflux ports found in most cells are the plasma membrane Ca-ATPase (PMCA) and the sodium-calcium exchanger (NCX). In osteoblasts, PMCA and NCX are located on opposing sides of the cell with NCX facing the mineralizing bone surface. Two isoforms of NCX have been identified in osteoblasts NCX1, and NCX3. The purpose of this study was to determine the extent to which each of the two NCX isoforms support delivery of Ca++ into sites of calcification and to discern if one could compensate for the other. SiRNA technology was used to knockdown each isoform separately in MC3T3-E1 osteoblasts. Osteoblasts in which either NCX1 or NCX3 was impaired were tested for Ca++ efflux using the Ca++ specific fluorophore, fluo-4, in a sodium-dependent calcium uptake assay adapted for image analysis. NCX3 was found to serve as a major contributor of Ca++ translocation out of osteoblasts into calcifying bone matrix. NCX1 had little to no involvement.

Regulation of mesangial cell Na+/Ca2+ exchanger isoforms

An isoform of the Na(+)/Ca(2+) exchanger (SDNCX1.10) was cloned from mesangial cells of Sprague-Dawley rat. Regulation of this isoform was compared to two other clones that were derived from the Dahl/Rapp salt sensitive (SNCX) and salt resistant rat (RNCX). All isoforms differ at the alternative splice site and at amino acid 218 for SNCX. PKC activates RNCX but not SNCX while SDNCX1.10 was also activated by PKC. Regulation of exchanger activities by intracellular calcium ([Ca(2+)](i)), pH, and kinases was assessed using Na-dependent (45)Ca(2+) uptake assays in OK-PTH cells expressing the vector, RNCX, SNCX, or SDNCX1.10. [Ca(2+)](i) was elevated from 50 to 125 nM (n = 4) with thapsigargin (40 nM) and reduced from 50 to 29 nM (n = 4) and 18 nM (n = 4) with 10 or 20 microM BAPTA, respectively. RNCX was active at all three [Ca(2+)](i) while SNCX and SDNCX1.10 were only active at lower [Ca(2+)](i). Varying extracellular pH (pH(e), without nigericin) or pH(e) and intracellular pH (pH(i), with 10 microM nigericin) from pH 7.4 to 6.2, 6.8, or 8.0 showed that SNCX activity was attenuated at both low and high pHs. SDNCX1.10 activity was attenuated only at pH 6.2 and 6.8 (with or without nigericin) while RNCX activity was attenuated at pH 6.2 (with or without nigericin) and pH 6.8 (with nigericin). Finally, only SDNCX1.10 activity was stimulated by 250 microM CPT-cAMP or 250 microM DB-cGMP treatment. Thus the differential regulation of [Ca(2+)](i) by these exchangers is dependent upon the pattern of cellular Na(+)/Ca(2+) exchanger isoform expression.

Regulation of the 1b isoform of the plasma membrane calcium pump by 1,25-dihydroxyvitamin D3 in rat osteoblast-like cells

The first isogene of the plasma membrane calcium pump (PMCA1) is expressed on the apical plasma membrane of osteoblasts, but its regulation by 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] has not been studied in this cell type. We studied 1,25(OH)2D3 effects on PMCA1 function, protein, messenger RNA (mRNA), and isoform expression in osteoblasts. Of seven rat and human immortalized osteoblast-like cell lines studied, PMCA1 mRNA expression was confirmed in all. Only ROS 17/2.8 cells expressed measurable PMCA1 protein by Western analysis. Immunocytochemistry indicated that PMCA1 was expressed predominantly on the plasma membrane of ROS 17/2.8 cells. The 1,25(OH)2D3 but not 24,25-dihydroxyvitamin D3 [24,25(OH)2D3] treatment of confluent ROS 17/2.8 cells resulted in an approximate 3- to 5-fold dose-dependent increase in PMCA1 expression of message and protein as assessed by Western and Northern analysis and vesicular 45Ca uptake of membrane vesicles. 1,25(OH)2D3 had no effect on PMCA1 posttranscriptional splicing. The 1b isoform of PMCA was expressed under all experimental conditions. 1,25(OH)2D3 favored increased expression of the 5.5 kilobases (kb) over the 7.5-kb PMCA1b transcript, with a 2-fold proportional increase in the smaller transcript relative to the larger transcript evident at the highest dose of 1,25(OH)2D3 studied. The resultant proportional increase in the smaller 5.5-kb transcript may increase mRNA stability and account for the increase in PMCA1b protein and function with 1,25(OH)2D3. These data provide evidence for the role of 1,25(OH)2D3 and PMCA1b in the regulation of calcium transport in bone cells.

Asymmetric distribution of functional sodium-calcium exchanger in primary osteoblasts

To understand calcium translocation in osteoblasts, we have determined the location of sodium-calcium (Na-Ca) exchanger (NCX) in relation to actin and alpha-tubulin in primary cultures of avian osteoblasts. Osteoblasts derived from the periosteal surface of tibias from growing chickens were cultured for 8 days in Dulbecco's modified Eagle's medium containing 10% fetal bovine serum. Lysates immunoblotted with antibodies raised against the canine cardiac Na-Ca antibodies revealed a 70 kDa exchanger protein. Cross-reactivity of the anti-NCX antibody was confirmed by enriching for NCX in protein samples derived from plasma membrane vesicles by affinity chromatography using the exchanger inhibitory peptide. Fractions enriched for the exchanger were eluted from the column and subjected to immunoblotting with the anti-NCX antibody, revealing an intense single band at 70 kDa. Examination of live cells loaded with Calcium Green-1 AM ester by confocal microscopy demonstrated sodium-dependent calcium uptake, confirming the presence of functional NCX in intact cells. Immunolocalization studies of osteoblasts stained with anti-NCX antibodies revealed asymmetric localization of the exchanger in cultured osteoblasts, residing almost entirely within two 0.5-microm optical sections along the substrate adherent side of the cells. Since NCX is known to be a low-affinity, high-capacity calcium translocating molecule and also appears to be asymmetrically positioned, it is likely to play a key role in bone formation.

NCX1 Na/Ca exchanger inhibition by antisense oligonucleotides in mouse distal convoluted tubule cells

BACKGROUND

Plasma membrane NCX1 Na+/Ca2+ exchangers mediate cellular Ca2+ efflux. Renal distal convoluted tubule (DCT) cells express transcripts encoding three alternatively spliced NCX1 isoforms: NACA2 (exons B, C, D), NACA3 (exons B and D), and NACA6 (exons A, C, D). We used antisense oligodeoxynucleotides (ODNs) to determine the function of these NACA isoforms on Na+/Ca2+ exchanger activity and expression in DCT cells.

METHODS

Sense and antisense ODNs targeting exchanger transcripts were introduced into DCT cells permeabilized with streptolysin O. Na+/Ca2+ exchange activity was assessed by measuring Na+-dependent changes of free intracellular Ca2+ concentration (delta[Ca2+]i), in single cells, when the electrochemical gradient for Na+ was reversed.

RESULTS

The change of [Ca2+]i in cells treated with antisense ODNs to a downstream or upstream region common to all NCX1 isoforms was 173 nM (-66%) to the downstream region located in the putative ninth transmembrane domain, and 226 nM (-39%) with ODNs to an upstream region located 5' to the variable portion of the intracellular loop. Antisense ODNs to exon B, present in both NACA2 and NACA3, decreased delta[Ca2+]i by 209 nM (-44%), while antisense ODNs specific for NACA6 (exon A) were without effect. Antisense ODNs specific for exon C, present in NACA2 and NACA6, decreased delta[Ca2+]i by 226 nM (-39%). Northern analysis of mRNA prepared from primary cultures of distal tubule cells revealed exon B- but not exon A-containing transcripts. Immunofluorescence analysis using a polyclonal antibody that recognizes NCX1 confirmed that protein expression was inhibited after treatment with the exon B antisense ODNs.

CONCLUSION

These findings show that Na+-dependent cellular Ca2+ efflux in DCT cells is primarily mediated by NACA2 and NACA3.

Molecular dissection of Ca2+ efflux in immortalized proximal tubule cells

Plasma membrane Ca(2+)-ATPase (PMCA) and the Na+/Ca2+ exchanger participate in regulating cell function by maintaining proper intracellular Ca2+ concentrations ([Ca2+]i). In renal epithelial cells these proteins have been additionally implicated in cellular calcium absorption. The purpose of the present studies was to determine the Ca2+ extrusion mechanisms in cells derived from the proximal tubule. Homology-based RT-PCR was used to amplify PMCA transcripts from RNA isolated from mouse cell lines originating from the S1, S2, and S3 proximal tubule segments. S1, S2, and S3 cells exhibited only PMCA1 and PMCA4 products. PCR product identity was confirmed by sequence analysis. Northern analysis of proximal tubule cell RNAs revealed appropriate transcripts of 7.5 and 5.5 kb for PMCA1 and 8.5 and 7.5 kb for PMCA4, but were negative for PMCA2 and PMCA3. Western analysis with a monoclonal antibody to PMCA showed that all proximal cell lines expressed a reacting plasma membrane protein of 140 kD, the reported PMCA molecular mas. Na+/Ca2+ exchanger (NCX1) mRNA expression, analyzed by RT-PCR, protein expression by Western analysis, and functional exchange activity were uniformly absent from all proximal tubule cell lines. These observations support the idea that immortalized cells derived from the proximal tubule express PMCA1 and PMCA4, which may serve as the primary mechanism of cellular Ca2+ efflux.