Hormonal regulation of Na(+)-Ca2+ exchange in osteoblast-like cells

Phylogeny and chemistry of biological mineral transport

Three physiologically mineralizing tissues - teeth, cartilage and bone - have critical common elements and important evolutionary relationships. Phylogenetically the most ancient densely mineralized tissue is teeth. In jawless fishes without skeletons, tooth formation included epithelial transport of phosphates, a process echoed later in bone physiology. Cartilage and mineralized cartilage are skeletal elements separate from bone, but with metabolic features common to bone. Cartilage mineralization is coordinated with high expression of tissue nonspecific alkaline phosphatase and PHOSPHO1 to harvest available phosphate esters and support mineralization of collagen secreted locally. Mineralization in true bone results from stochastic nucleation of hydroxyapatite crystals within the cross-linked collagen fibrils. Mineral accumulation in dense collagen is, at least in major part, mediated by amorphous aggregates - often called Posner clusters - of calcium and phosphate that are small enough to diffuse into collagen fibrils. Mineral accumulation in membrane vesicles is widely suggested, but does not correlate with a definitive stage of mineralization. Conversely mineral deposition at non-physiologic sites where calcium and phosphate are adequate has been shown to be regulated in large part by pyrophosphate. All of these elements are present in vertebrate bone metabolism. A key biological element of bone formation is an epithelial-like cellular organization which allows control of phosphate, calcium and pH during mineralization.

Calcium and bone disease

Calcium transport and calcium signaling are of basic importance in bone cells. Bone is the major store of calcium and a key regulatory organ for calcium homeostasis. Bone, in major part, responds to calcium-dependent signals from the parathyroids and via vitamin D metabolites, although bone retains direct response to extracellular calcium if parathyroid regulation is lost. Improved understanding of calcium transporters and calcium-regulated cellular processes has resulted from analysis of genetic defects, including several defects with low or high bone mass. Osteoblasts deposit calcium by mechanisms including phosphate and calcium transport with alkalinization to absorb acid created by mineral deposition; cartilage calcium mineralization occurs by passive diffusion and phosphate production. Calcium mobilization by osteoclasts is mediated by acid secretion. Both bone forming and bone resorbing cells use calcium signals as regulators of differentiation and activity. This has been studied in more detail in osteoclasts, where both osteoclast differentiation and motility are regulated by calcium.

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.

Inhibition of Na+/Ca2+ exchange with KB-R7943 or bepridil diminished mineral deposition by osteoblasts

Osteoblasts form new bone by secreting a complex extracellular matrix that has the capacity to mineralize when adequate amounts of calcium and phosphate are supplied. The studies reported here show that long-term treatment of cultured, primary osteoblasts with Na+/Ca2+ exchanger (NCX) inhibitors, bepridil and KB-R7943, impacts in a dose-dependent manner the ability of the cells to form a calcified matrix. Treatment of confluent osteoblast cultures for 14 days with low levels of bepridil (3.0 microM) or KB-R7943 (1.0 microM and 0.1 microM) resulted in a significantly diminished capacity of these cells to mineralize bone matrix, without significantly altering cell morphology, viability, or cell differentiation. The data indicate that inhibition of NCX reduces mineral accumulation in the bone matrix by blocking the efflux of Ca2+ from the osteoblast into the bone fluid. In addition, immunocytochemistry of type I collagen (COLI) and bone sialoprotein (BSP) suggests that inhibition of NCX by 1.0 microM KB-R7943 also may impair the secretion of bone matrix proteins by the osteoblasts. This study is the first to show that NCX is an important regulator of the bone fluid microenvironment and that NCX appears critical to the mineralization process.

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.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

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.

Sodium-calcium exchange: recent advances

Na-Ca exchange proteins are involved in Ca homeostasis in a wide variety of tissues. Unique Na-Ca exchangers have been identified by molecular biological approaches and it appears that these may represent a superfamily of ion transporters, similar to that identified for ion channels. Major advances in our understanding of these transporters have occurred in the past decade by combining molecular approaches with electrophysiological analyses. The regulatory and transport properties of Na-Ca exchangers are beginning to become understood in molecular detail. It also appears that the physiological roles of Na-Ca exchange may be quite complex. This brief review highlights some recent advances in Na-Ca exchange research obtained through the combination of molecular biological and electrophysiological approaches.

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

Ca2+ efflux from osteoblasts is thought to be mediated by Na+/Ca2+ exchange and by a plasma membrane Ca(2+)-ATPase. The presence of plasma membrane Na+/Ca2+ exchange was determined in rat UMR 106 osteosarcoma cells by functional and molecular studies. Na+/Ca2+ exchange activity was tested by measuring changes of [Ca2+]i in single cells. After Na+ loading the cells and removing extracellular Na+, the direction of exchange was reversed and [Ca2+]i increased by 100%. Multiple isoforms of the NCX1 gene product, encoding plasma membrane Na+/Ca2+ exchangers, were cloned from UMR 106 cells and a sample of primary human osteoblasts using homology-based RT-PCR. Isoforms NACA3, NACA7, and NACA10 were found in UMR 106 cells, whereas human osteoblasts expressed NACA3 and NACA7. Transcripts for NCX2 and the Na+/Ca2+, K+ exchanger were not detected. Northern analysis of UMR 106 cells with a probe to the NCX1 gene product revealed the presence of a transcript of 7 kb, the size of the exchanger message. Western analysis of UMR 106 cell membrane preparations with a polyclonal antibody specific for the NCX1 exchanger showed the presence of reacting proteins consistent with the reported masses of the exchanger at 125 and 85 kD. These results demonstrate Na(+)-dependent Ca2+ efflux from UMR 106 cells and the presence of several NACA isoforms in UMR 106 and primary human osteoblasts.

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.

Characterization of calcium translocation across the plasma membrane of primary osteoblasts using a lipophilic calcium-sensitive fluorescent dye, calcium green C18

The synthesis of Calcium Green C18, a lipophilic fluorescent calcium-sensitive dye, and its use as a monitor of Ca2+ efflux from cells is described. This indicator consists of a Calcium Green-1 molecule conjugated to a lipophilic 18-carbon alkyl chain which will intercalate into cell membranes. The Kd of the indicator for Ca2+ in aqueous solution (pH 7.2, 22 degrees C, ionic strength 0.1 M) is 0.23 +/- 0.04 microM and in the presence of liposomes is 0.062 +/- 0.007 microM. Due to its high negativity, the calcium chelating fluorophore faces the cell exterior, when loaded under a defined set of conditions. The dye was found largely on the surface of the cells when loaded at a concentration of 5 microM for 10 min at 37 degrees C. Five minutes after introduction of EGTA, 83-95% fluorescence disappeared, indicating that most of the fluorophore was on the cell surface. Photobleaching was minimal (3-13%). A confocal laser scanning microscope was used to detect and quantify fluorescence. Internalized dye was apparent in cells loaded for longer times (30-60 min) and in membrane-impaired cells, as shown by uptake of propidium iodide. Under defined confocal laser scanning microscope settings, a transient fluorescence at the periphery of approximately 30% of the cells was observed following 10(-8) M parathyroid hormone treatment, indicating the presence of outwardly directed calcium transport across the plasma membrane. Calcium efflux usually lasted 7-10 min, peaking at around 2-3 min. Changes in cell shape were also observed. Calcium efflux was shown to be sensitive to (a) 10 microM quercetin and 10 microM vanadate, partially specific inhibitors of plasma membrane Ca(2+)-ATPase, to (b) 0.1 mM trifluoperazine, an agent which renders calmodulin ineffective, and to (c) 10 mM neomycin sulfate, which blocks release of Ca2+ from intracellular stores. Thapsigargin (5 microM), an inhibitor of Ca(2+)-ATPase of the endoplasmic reticulum, prolonged fluorescence. These observations indicate that cell surface fluorescence was due to the capture of Ca2+ by Calcium Green C18 after Ca2+ had been translocated across osteoblast plasma membranes. Involvement of the plasma membrane Ca(2+)-ATPase, known to be present in osteoblasts in substantial amounts, is implicated.