Deletion of Orai1 alters expression of multiple genes during osteoclast and osteoblast maturation

Toward understanding the cellular control of vertebrate mineralization: The potential role of mitochondria

This review examines the possible role of mitochondria in maintaining calcium and phosphate ion homeostasis and participating in the mineralization of bone, cartilage and other vertebrate hard tissues. The paper builds on the known structural features of mitochondria and the documented observations in these tissues that the organelles contain calcium phosphate granules. Such deposits in mitochondria putatively form to buffer excessively high cytosolic calcium ion concentrations and prevent metabolic deficits and even cell death. While mitochondria protect cytosolic enzyme systems through this buffering capacity, the accumulation of calcium ions by mitochondria promotes the activity of enzymes of the tricarboxylic acid (TCA/Krebs) cycle, increases oxidative phosphorylation and ATP synthesis, and leads to changes in intramitochondrial pH. These pH alterations influence ion solubility and possibly the transitions and composition in the mineral phase structure of the granules. Based on these considerations, mitochondria are proposed to support the mineralization process by providing a mobile store of calcium and phosphate ions, in smaller cluster or larger granule form, while maintaining critical cellular activities. The rise in the mitochondrial calcium level also increases the generation of citrate and other TCA cycle intermediates that contribute to cell function and the development of extracellular mineral. This paper suggests that another key role of the mitochondrion, along with the effects just noted, is to supply phosphate ions, derived from the breakdown of ATP, to endolysosomes and autophagic vesicles originating in the endoplasmic reticulum and Golgi and at the plasma membrane. These many separate but interdependent mitochondrial functions emphasize the critical importance of this organelle in the cellular control of vertebrate mineralization.

KCa3.1 regulates cell cycle progression by modulating Ca2+ signaling in murine preosteoblasts

Osteoblasts synthesize and deposit essential components of the extracellular bone matrix and collagen scaffolds, leading to mineralized bone formation. Therefore, the proliferation of preosteoblasts (precursors of mature osteoblasts) helps in regulating skeletal homeostasis. This study demonstrated that the functional expression of KCa3.1, an intermediate-conductance Ca2+-activated K+ channel, is markedly upregulated in murine preosteoblastic MC3T3-E1 cells in the G0/G1 phase. The enhancement of KCa3.1 is involved in the establishment of more negative membrane potentials in MC3T3-E1 cells. This hyperpolarization can promote intracellular Ca2+ signaling because store-operated Ca2+ channels are activated. Treatment with TRAM-34, a specific KCa3.1 inhibitor, attenuated the cell cycle progression from the G0/G1 phase to the S/G2/M phases. In MC3T3-E1 cells, KCa3.1 significantly promoted the transition from the G1 phase to the S phase. KCa3.1 inhibition also caused G0 phase cell accumulation. Furthermore, TRAM-34 decreased the expression of alkaline phosphatase, bone sialoprotein, and osteocalcin, osteoblast differentiation markers in MC3T3-E1 cells, and inhibited the endochondral ossification of murine metatarsals. These results reveal novel ways by which KCa3.1 activity can strongly modulate osteoblast maturation during bone formation.

Angular limb deformity associated with TSPAN18, NRG3 and NOVA2 in Rambouillet rams

Angular limb deformity (ALD) affects many species of livestock and companion animals. The mechanisms of ALD development are not well understood, but previous research suggests the involvement of genetic risk factors. A case-control genome-wide association study (GWAS) was conducted with 40 ALD-affected and 302 unaffected Rambouillet rams and 40,945 single nucleotide polymorphisms (SNPs). Forelimbs of 6 ALD-affected rams were examined and diagnosed with osteochondrosis. Genome-wide or chromosome-wide significant SNPs were positioned exonic, intronic or within the 3'UTR of genes TSPAN18, NRG3 and NOVA2, respectively. These genes have previously described roles related to angiogenesis and osteoblast, osteoclast and chondrocyte proliferation and differentiation, which suggests the possibility for their involvement in the pathogenesis of osteochondrosis. Functional consequences of SNPs were evaluated through transcription factor binding site analysis, which predicted binding sites for transcription factors of known importance to bone growth, including SOX6, SOX9 and RUNX2. The identification of genetic risk factors for ALD may help to improve animal welfare and production in Rambouillet, a breed known to be at risk for ALD development. This study proposes genes TSPAN18, NRG3 and NOVA2 as targets for further research towards understanding the etiology of ALD in Rambouillet sheep.

The calcium channel Orai1 is required for osteoblast development: Studies in a chimeric mouse with variable in vivo Runx-cre deletion of Orai-1

The calcium-selective ion channel Orai1 has a complex role in bone homeostasis, with defects in both bone production and resorption detected in Orai1 germline knock-out mice. To determine whether Orai1 has a direct, cell-intrinsic role in osteoblast differentiation and function, we bred Orai1 flox/flox (Orai1fl/fl) mice with Runx2-cre mice to eliminate its expression in osteoprogenitor cells. Interestingly, Orai1 was expressed in a mosaic pattern in Orai1fl/fl-Runx2-cre bone. Specifically, antibody labeling for Orai1 in vertebral sections was uniform in wild type animals, but patchy regions in Orai1fl/fl-Runx2-cre bone revealed Orai1 loss while in other areas expression persisted. Nevertheless, by micro-CT, bones from Orai1fl/fl-Runx2-cre mice showed reduced bone mass overall, with impaired bone formation identified by dynamic histomorphometry. Cortical surfaces of Orai1fl/fl-Runx2-cre vertebrae however exhibited patchy defects. In cell culture, Orai1-negative osteoblasts showed profound reductions in store-operated Ca2+ entry, exhibited greatly decreased alkaline phosphatase activity, and had markedly impaired substrate mineralization. We conclude that defective bone formation observed in the absence of Orai1 reflects an intrinsic role for Orai1 in differentiating osteoblasts.

Calcium released by osteoclastic resorption stimulates autocrine/paracrine activities in local osteogenic cells to promote coupled bone formation

A major cause of osteoporosis is impaired coupled bone formation. Mechanistically, both osteoclast-derived and bone-derived growth factors have been previously implicated. We hypothesize that the release of bone calcium during osteoclastic bone resorption is essential for coupled bone formation. Osteoclastic resorption increases interstitial fluid calcium locally from the normal 1.8 mM up to 5 mM. MC3T3-E1 osteoprogenitors, cultured in a 3.6 mM calcium medium, demonstrated that calcium signaling stimulated osteogenic cell proliferation, differentiation, and migration. Calcium channel knockdown studies implicated calcium channels, Cav1.2, store-operated calcium entry (SOCE), and calcium-sensing receptor (CaSR) in regulating bone cell anabolic activities. MC3T3-E1 cultured in a 3.6 mM calcium medium expressed increased gene expression of Wnt signaling and growth factors platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), and bone morphogenic protein-2 (BMP 2). Our coupling model of bone formation, the Receptor activator of nuclear factor-kappa-Β ligand (RANKL) treated mouse calvaria, confirmed the role of calcium signaling in coupled bone formation by exhibiting increased gene expression for osterix and osteocalcin. Critically, dual immunocytochemistry showed that RANKL treatment increased osterix positive cells and increased fluorescence intensity of Cav1.2 and CaSR protein expression per osterix positive cell. The data established that calcium released by osteoclasts contributed to the regulation of coupled bone formation. CRISPR/Cas-9 knockout of Cav1.2 in osteoprogenitors cultured in basal calcium medium caused a >80% decrease in the expression of downstream osteogenic genes, emphasizing the large magnitude of the effect of calcium signaling. Thus, calcium signaling is a major regulator of coupled bone formation.

Role of K+ and Ca2+-Permeable Channels in Osteoblast Functions

Bone-forming cells or osteoblasts play an important role in bone modeling and remodeling processes. Osteoblast differentiation or osteoblastogenesis is orchestrated by multiple intracellular signaling pathways (e.g., bone morphogenetic proteins (BMP) and Wnt signaling pathways) and is modulated by the extracellular environment (e.g., parathyroid hormone (PTH), vitamin D, transforming growth factor β (TGF-β), and integrins). The regulation of bone homeostasis depends on the proper differentiation and function of osteoblast lineage cells from osteogenic precursors to osteocytes. Intracellular Ca²⁺ signaling relies on the control of numerous processes in osteoblast lineage cells, including cell growth, differentiation, migration, and gene expression. In addition, hyperpolarization via the activation of K⁺ channels indirectly promotes Ca²⁺ signaling in osteoblast lineage cells. An improved understanding of the fundamental physiological and pathophysiological processes in bone homeostasis requires detailed investigations of osteoblast lineage cells. This review summarizes the current knowledge on the functional impacts of K⁺ channels and Ca²⁺-permeable channels, which critically regulate Ca²⁺ signaling in osteoblast lineage cells to maintain bone homeostasis.

The function of the calcium channel Orai1 in osteoclast development

To determine the intrinsic role of Orai1 in osteoclast development, Orai1-floxed mice were bred with LysMcre mice to delete Orai1 from the myeloid lineage. PCR, in situ labelling and Western analysis showed Orai1 deletion in myeloid-lineage cells, including osteoclasts, as expected. Surprisingly, bone resorption was maintained in vivo, despite loss of multinucleated osteoclasts; instead, a large number of mononuclear cells bearing tartrate resistant acid phosphatase were observed on cell surfaces. An in vitro resorption assay confirmed that RANKL-treated Orai1 null cells, also TRAP-positive but mononuclear, degraded matrix, albeit at a reduced rate compared to wild type osteoclasts. This shows that mononuclear osteoclasts can degrade bone, albeit less efficiently. Further unexpected findings included that Orai1^fl/fl -LysMcre vertebrae showed slightly reduced bone density in 16-week-old mice, despite Orai1 deletion only in myeloid cells; however, this mild difference resolved with age. In summary, in vitro analysis showed a severe defect in osteoclast multinucleation in Orai1 negative mononuclear cells, consistent with prior studies using less targeted strategies, but with evidence of resorption in vivo and unexpected secondary effects on bone formation leaving bone mass largely unaffected.

ORAI1 Ca2+ Channel as a Therapeutic Target in Pathological Vascular Remodelling

In the adult, vascular smooth muscle cells (VSMC) are normally physiologically quiescent, arranged circumferentially in one or more layers within blood vessel walls. Remodelling of native VSMC to a proliferative state for vascular development, adaptation or repair is driven by platelet-derived growth factor (PDGF). A key effector downstream of PDGF receptors is store-operated calcium entry (SOCE) mediated through the plasma membrane calcium ion channel, ORAI1, which is activated by the endoplasmic reticulum (ER) calcium store sensor, stromal interaction molecule-1 (STIM1). This SOCE was shown to play fundamental roles in the pathological remodelling of VSMC. Exciting transgenic lineage-tracing studies have revealed that the contribution of the phenotypically-modulated VSMC in atherosclerotic plaque formation is more significant than previously appreciated, and growing evidence supports the relevance of ORAI1 signalling in this pathologic remodelling. ORAI1 has also emerged as an attractive potential therapeutic target as it is accessible to extracellular compound inhibition. This is further supported by the progression of several ORAI1 inhibitors into clinical trials. Here we discuss the current knowledge of ORAI1-mediated signalling in pathologic vascular remodelling, particularly in the settings of atherosclerotic cardiovascular diseases (CVDs) and neointimal hyperplasia, and the recent developments in our understanding of the mechanisms by which ORAI1 coordinates VSMC phenotypic remodelling, through the activation of key transcription factor, nuclear factor of activated T-cell (NFAT). In addition, we discuss advances in therapeutic strategies aimed at the ORAI1 target.

Finely-Tuned Calcium Oscillations in Osteoclast Differentiation and Bone Resorption

Calcium (Ca²⁺) plays an important role in regulating the differentiation and function of osteoclasts. Calcium oscillations (Ca oscillations) are well-known phenomena in receptor activator of nuclear factor kappa B ligand (RANKL)-induced osteoclastogenesis and bone resorption via calcineurin. Many modifiers are involved in the fine-tuning of Ca oscillations in osteoclasts. In addition to macrophage colony-stimulating factors (M-CSF; CSF-1) and RANKL, costimulatory signaling by immunoreceptor tyrosine-based activation motif-harboring adaptors is important for Ca oscillation generation and osteoclast differentiation. DNAX-activating protein of 12 kD is always necessary for osteoclastogenesis. In contrast, Fc receptor gamma (FcRγ) works as a key controller of osteoclastogenesis especially in inflammatory situation. FcRγ has a cofactor in fine-tuning of Ca oscillations. Some calcium channels and transporters are also necessary for Ca oscillations. Transient receptor potential (TRP) channels are well-known environmental sensors, and TRP vanilloid channels play an important role in osteoclastogenesis. Lysosomes, mitochondria, and endoplasmic reticulum (ER) are typical organelles for intracellular Ca²⁺ storage. Ryanodine receptor, inositol trisphosphate receptor, and sarco/endoplasmic reticulum Ca²⁺ ATPase on the ER modulate Ca oscillations. Research on Ca oscillations in osteoclasts has still many problems. Surprisingly, there is no objective definition of Ca oscillations. Causality between Ca oscillations and osteoclast differentiation and/or function remains to be examined.

STIM1/ORAI1 Loss-of-Function and Gain-of-Function Mutations Inversely Impact on SOCE and Calcium Homeostasis and Cause Multi-Systemic Mirror Diseases

Store-operated Ca²⁺ entry (SOCE) is a ubiquitous and essential mechanism regulating Ca²⁺ homeostasis in all tissues, and controls a wide range of cellular functions including keratinocyte differentiation, osteoblastogenesis and osteoclastogenesis, T cell proliferation, platelet activation, and muscle contraction. The main SOCE actors are STIM1 and ORAI1. Depletion of the reticular Ca²⁺ stores induces oligomerization of the luminal Ca²⁺ sensor STIM1, and the oligomers activate the plasma membrane Ca²⁺ channel ORAI1 to trigger extracellular Ca²⁺ entry. Mutations in STIM1 and ORAI1 result in abnormal SOCE and lead to multi-systemic disorders. Recessive loss-of-function mutations are associated with CRAC (Ca²⁺ release-activated Ca²⁺) channelopathy, involving immunodeficiency and autoimmunity, muscular hypotonia, ectodermal dysplasia, and mydriasis. In contrast, dominant STIM1 and ORAI1 gain-of-function mutations give rise to tubular aggregate myopathy and Stormorken syndrome (TAM/STRMK), forming a clinical spectrum encompassing muscle weakness, thrombocytopenia, ichthyosis, hyposplenism, short stature, and miosis. Functional studies on patient-derived cells revealed that CRAC channelopathy mutations impair SOCE and extracellular Ca²⁺ influx, while TAM/STRMK mutations induce excessive Ca²⁺ entry through SOCE over-activation. In accordance with the opposite pathomechanisms underlying both disorders, CRAC channelopathy and TAM/STRMK patients show mirror phenotypes at the clinical and molecular levels, and the respective animal models recapitulate the skin, bones, immune system, platelet, and muscle anomalies. Here we review and compare the clinical presentations of CRAC channelopathy and TAM/STRMK patients and the histological and molecular findings obtained on human samples and murine models to highlight the mirror phenotypes in different tissues, and to point out potentially undiagnosed anomalies in patients, which may be relevant for disease management and prospective therapeutic approaches.

Downregulation of the Ca2+-activated K+ channel KCa3.1 in mouse preosteoblast cells treated with vitamin D receptor agonist

The maturity of osteoblasts by proliferation and differentiation in preosteoblasts is essential for maintaining bone homeostasis. The beneficial effects of vitamin D on bone homeostasis in mammals have been demonstrated experimentally and clinically. However, the direct actions of vitamin D on preosteoblasts remain to be fully elucidated. In this study, we found that the functional activity of intermediate-conductance Ca²⁺-activated K⁺ channels (K_Ca3.1) positively regulated cell proliferation in MC3T3-E1 cells derived from mouse preosteoblasts by enhancing intracellular Ca²⁺ signaling. We examined the effects of treatment with vitamin D receptor (VDR) agonist on the expression and activity of K_Ca3.1 by real-time PCR examination, Western blotting, Ca²⁺ imaging, and patch clamp analyses in mouse MC3T3-E1 cells. Following the downregulation of K_Ca3.1 transcriptional modulators such as Fra-1 and HDAC2, K_Ca3.1 activity was suppressed in MC3T3-E1 cells treated with VDR agonists. Furthermore, application of the K_Ca3.1 activator DCEBIO attenuated the VDR agonist-evoked suppression of cell proliferation rate. These findings suggest that a decrease in K_Ca3.1 activity is involved in the suppression of cell proliferation rate in VDR agonist-treated preosteoblasts. Therefore, K_Ca3.1 plays an important role in bone formation by promoting osteoblastic proliferation under physiological conditions.

Calcium Homeostasis: A Potential Vicious Cycle of Bone Metastasis in Breast Cancers

Cancers have been considered as one of the most severe health problems in the world. Efforts to elucidate the cancer progression reveal the importance of bone metastasis for tumor malignancy, one of the leading causes for high mortality rate. Multiple cancers develop bone metastasis, from which breast cancers exhibit the highest rate and have been well-recognized. Numerous cells and environmental factors have been believed to synergistically facilitate bone metastasis in breast cancers, from which breast cancer cells, osteoclasts, osteoblasts, and their produced cytokines have been well-recognized to form a vicious cycle that aggravates tumor malignancy. Except the cytokines or chemokines, calcium ions are another element largely released from bones during bone metastasis that leads to hypercalcemia, however, have not been well-characterized yet in modulation of bone metastasis. Calcium ions act as a type of unique second messenger that exhibits omnipotent functions in numerous cells, including tumor cells, osteoclasts, and osteoblasts. Calcium ions cannot be produced in the cells and are dynamically fluxed among extracellular calcium pools, intracellular calcium storages and cytosolic calcium signals, namely calcium homeostasis, raising a possibility that calcium ions released from bone during bone metastasis would further enhance bone metastasis and aggravate tumor progression via the vicious cycle due to abnormal calcium homeostasis in breast cancer cells, osteoclasts and osteoblasts. TRPs, VGCCs, SOCE, and P2Xs are four major calcium channels/routes mediating extracellular calcium entry and affect calcium homeostasis. Here we will summarize the overall functions of these four calcium channels in breast cancer cells, osteoclasts and osteoblasts, providing evidence of calcium homeostasis as a vicious cycle in modulation of bone metastasis in breast cancers.

Serum- and Glucocorticoid-inducible Kinase 1 is Essential for Osteoclastogenesis and Promotes Breast Cancer Bone Metastasis

Bone metastasis is a severe complication associated with various carcinomas. It causes debilitating pain and pathologic fractures and dramatically impairs patients' quality of life. Drugs aimed at osteoclast formation significantly reduce the incidence of skeletal complications and are currently the standard treatment for patients with bone metastases. Here, we reported that serum- and glucocorticoid-inducible kinase 1 (SGK1) plays a pivotal role in the formation and function of osteoclasts by regulating the Ca2+ release-activated Ca2+ channel Orai1. We showed that SGK1 inhibition represses osteoclastogenesis in vitro and prevents bone loss in vivo Furthermore, we validated the effect of SGK1 on bone metastasis by using an intracardiac injection model in mice. Inhibition of SGK1 resulted in a significant reduction in bone metastasis. Subsequently, the Oncomine and the OncoLnc database were employed to verify the differential expression and the association with clinical outcome of SGK1 gene in patients with breast cancer. Our data mechanistically demonstrated the regulation of the SGK1 in the process of osteoclastogenesis and revealed SGK1 as a valuable target for curing bone metastasis diseases.

STIM1 R304W in mice causes subgingival hair growth and an increased fraction of trabecular bone

Calcium signaling plays a central role in bone development and homeostasis. Store operated calcium entry (SOCE) is an important calcium influx pathway mediated by calcium release activated calcium (CRAC) channels in the plasma membrane. Stromal interaction molecule 1 (STIM1) is an endoplasmic reticulum calcium sensing protein important for SOCE. We generated a mouse model expressing the STIM1 R304W mutation, causing Stormorken syndrome in humans. Stim1^R304W/R304W mice showed perinatal lethality, and the only three animals that survived into adulthood presented with reduced growth, low body weight, and thoracic kyphosis. Radiographs revealed a reduced number of ribs in the Stim1^R304W/R304W mice. Microcomputed tomography data revealed decreased cortical bone thickness and increased trabecular bone volume fraction in Stim1^R304W/R304W mice, which had thinner and more compact bone compared to wild type mice. The Stim1^R304W/+ mice showed an intermediate phenotype. Histological analyses showed that the Stim1^R304W/R304W mice had abnormal bone architecture, with markedly increased number of trabeculae and reduced bone marrow cavity. Homozygous mice showed STIM1 positive osteocytes and osteoblasts. These findings highlight the critical role of the gain-of-function (GoF) STIM1 R304W protein in skeletal development and homeostasis in mice. Furthermore, the novel feature of bilateral subgingival hair growth on the lower incisors in the Stim1^R304W/R304W mice and 25 % of the heterozygous mice indicate that the GoF STIM1 R304W protein also induces an abnormal epithelial cell fate.

Calcium Signaling and Tissue Calcification

Calcification is a regulated physiological process occurring in bones and teeth. However, calcification is commonly found in soft tissues in association with aging and in a variety of diseases. Over the last two decades, it has emerged that calcification occurring in diseased arteries is not simply an inevitable build-up of insoluble precipitates of calcium phosphate. In some cases, it is an active process in which transcription factors drive conversion of vascular cells to an osteoblast or chondrocyte-like phenotype, with the subsequent production of mineralizing "matrix vesicles." Early studies of bone and cartilage calcification suggested roles for cellular calcium signaling in several of the processes involved in the regulation of bone calcification. Similarly, calcium signaling has recently been highlighted as an important component in the mechanisms regulating pathological calcification. The emerging hypothesis is that ectopic/pathological calcification occurs in tissues in which there is an imbalance in the regulatory mechanisms that actively prevent calcification. This review highlights the various ways that calcium signaling regulates tissue calcification, with a particular focus on pathological vascular calcification.

The roles of Orai and Stim in bone health and disease

Orai and Stim proteins are the mediators of calcium release-activated calcium signaling and are important in the regulation of bone homeostasis and disease. This includes separate regulatory systems controlling mesenchymal stem cell differentiation to form osteoblasts, which make bone, and differentiation and regulation of osteoclasts, which resorb bone. These systems will be described separately, and their integration and relation to other systems, including Orai and Stim in teeth, will be briefly discussed at the end of this review.

Induction of osteoclast-like cell formation by leptin-induced soluble intercellular adhesion molecule secreted from cancer cells

Background:

Leptin is considered a tumorigenic adipokine, suggested to promote tumorigenesis and progression in many cancers. On the other hand, intercellular adhesion molecule-1 (ICAM-1) shows altered expression in a variety of benign and malignant diseases. Histologically, ICAM-1 expression is reported as proportional to cancer stage and considered as a potential diagnosis biomarker. The altered expressions of ICAM-1 and its soluble form in malignant diseases have gained interests in recent years.

Material and methods:

The expression of ICAM-1 and its regulatory signaling were examined by Western blot or flow cytometry. The effect of soluble ICAM-1 on osteoclast formation was investigated by tartrate-resistance acid phosphatase staining of RAW cells and tumor-induced osteolysis in vivo.

Results:

In our study, we found that leptin enhanced soluble ICAM-1 production but not surface ICAM-1 expression in lung and breast cancer cells, and this effect was regulated through leptin receptor (ObR), while silencing ObR abrogated leptin-induced soluble ICAM-1 expression. In addition, we revealed that leptin administration provoked the JAK1/2, STAT3, FAK, ERK, and GSK3αβ signaling cascade, leading to the elevation of ICAM-1 expression. Moreover, soluble ICAM-1 secreted by leptin-stimulated cancer cells synergize with the receptor activator of nuclear factor kappa-B ligand (RANKL) in inducing osteoclast formation. Soluble ICAM also enhanced tumor-induced osteolysis in vivo.

Conclusion:

These findings suggest that soluble ICAM-1 produced under leptin treatment enhances osteoclast formation and is involved in tumor-induced osteolysis.

Leptin plays an important role in physiology in health and diseases. Leptin affects immune responses that may induce inflammation and carcinogenesis. Leptin is also considered as a tumorigenic adipokine suggested to promote tumorigenesis and progression in many cancers. On the other hand, intercellular adhesion molecule-1 (ICAM-1) shows altered expression in a variety of benign and malignant diseases. Histologically, ICAM-1 expression is reported to be proportional to cancer stage and considered as a potential diagnosis biomarker. It has been reported that soluble ICAM-1 allows tumor cells to escape from immune recognition and stimulates angiogenesis and tumor growth. The altered expressions of ICAM-1 and its soluble form in malignant diseases have gained interests in recent years. In our study, we found that leptin enhanced soluble ICAM-1 production but not surface ICAM-1 expression in lung and breast cancer cells, and this effect was regulated through leptin receptor (ObR), while silencing ObR abrogated leptin-induced soluble ICAM-1 expression. In addition, we revealed that leptin administration provoked the JAK1/2, STAT3, FAK, ERK, and GSK3αβ signaling cascade, leading to the elevation of ICAM-1 expression. Moreover, soluble ICAM-1 secreted by leptin-stimulated cancer cells synergize with receptor activator of nuclear factor-kappa B ligand in inducing osteoclast formation. Soluble ICAM also enhanced tumor-induced osteolysis in vivo. These findings suggest that soluble ICAM-1 produced under leptin treatment is possibly involved in lung and breast cancer bone metastasis.

Tmem178 negatively regulates store-operated calcium entry in myeloid cells via association with STIM1

Store-operated calcium entry (SOCE) modulates cytosolic calcium in multiple cells. Endoplasmic reticulum (ER)-localized STIM1 and plasma membrane (PM)-localized ORAI1 are two main components of SOCE. STIM1:ORAI1 association requires STIM1 oligomerization, its re-distribution to ER-PM junctions, and puncta formation. However, little is known about the negative regulation of these steps to prevent calcium overload. Here, we identified Tmem178 as a negative modulator of STIM1 puncta formation in myeloid cells. Using site-directed mutagenesis, co-immunoprecipitation assays and FRET imaging, we determined that Tmem178:STIM1 association occurs via their transmembrane motifs. Mutants that increase Tmem178:STIM1 association reduce STIM1 puncta formation, SOCE activation, impair inflammatory cytokine production in macrophages and osteoclastogenesis. Mutants that reduce Tmem178:STIM1 association reverse these effects. Furthermore, exposure to plasma from arthritic patients decreases Tmem178 expression, enhances SOCE activation and cytoplasmic calcium. In conclusion, Tmem178 modulates the rate-limiting step of STIM1 puncta formation and therefore controls SOCE in inflammatory conditions.

CRAC channels in dental enamel cells

Enamel mineralization relies on Ca2+ availability provided by Ca2+ release activated Ca2+ (CRAC) channels. CRAC channels are modulated by the endoplasmic reticulum Ca2+ sensor STIM1 which gates the pore subunit of the channel known as ORAI1, found the in plasma membrane, to enable sustained Ca2+ influx. Mutations in the STIM1 and ORAI1 genes result in CRAC channelopathy, an ensemble of diseases including immunodeficiency, muscular hypotonia, ectodermal dysplasia with defects in sweat gland function and abnormal enamel mineralization similar to amelogenesis imperfecta (AI). In some reports, the chief medical complain has been the patient's dental health, highlighting the direct and important link between CRAC channels and enamel. The reported enamel defects are apparent in both the deciduous and in permanent teeth and often require extensive dental treatment to provide the patient with a functional dentition. Among the dental phenotypes observed in the patients, discoloration, increased wear, hypoplasias (thinning of enamel) and chipping has been reported. These findings are not universal in all patients. Here we review the mutations in STIM1 and ORAI1 causing AI-like phenotype, and evaluate the enamel defects in CRAC channel deficient mice. We also provide a brief overview of the role of CRAC channels in other mineralizing systems such as dentine and bone.

TRPM3/TRPV4 regulates Ca2+-mediated RANKL/NFATc1 expression in osteoblasts

Mechanical stress plays an important role in the regulation of bone turnover. However, the mechanism underlying hypo-osmotic stress-induced cellular response in osteoblasts remains poorly understood. In this study, we investigated the effect of hypotonic stress on the expression of bone remodeling factors, including the receptor activator of nuclear factor-kappa B ligand (RANKL) and the nuclear factor of activated T cells type c1 (NFATc1) in primary mouse osteoblasts and MC3T3-E1 cells. Hypo-osmotic stress induced significant increases in RANKL mRNA expression and intracellular Ca2+ concentration ([Ca2+]i) from the extracellular space. Hypo-osmotic stress-induced effects on [Ca2+]i and RANKL and NFATc1 protein expression were decreased by antagonists of transient receptor potential melastatin 3 (TRPM3) and vanilloid 4 (TRPV4). Agonists of TRPM3 and TRPV4 activated [Ca2+]i and RANKL and NFATc1 protein expression. Furthermore, genetic suppression of Trpm3 and Trpv4 reduced hypo-osmotic stress-induced effects in mouse osteoblasts. These results suggest that hypo-osmotic stress induces increases in [Ca2+]i through TRPM3 and TRPV4 to regulate RANKL and NFATc1 expression in mouse osteoblastic cells and that mechanical stress-activated TRP channels may play a critical role in bone remodeling.

Clinical and Molecular Perspectives of Reparative Dentin Formation: Lessons Learned from Pulp-Capping Materials and the Emerging Roles of Calcium

The long-term use of calcium hydroxide and the recent increase in the use of hydraulic calcium-silicate cements as direct pulp-capping materials provide important clues in terms of how reparative dentin may be induced to form a "biological seal" to protect the underlying pulp tissues. In this review article, we discuss clinical and molecular perspectives of reparative dentin formation based on evidence learned from the use of these pulp-capping materials. We also discuss the emerging role of calcium as an odontoinductive component in these pulp-capping materials.

High pH-Sensitive Store-Operated Ca2+ Entry Mediated by Ca2+ Release-Activated Ca2+ Channels in Rat Odontoblasts

Odontoblasts play a crucial role in dentin formation and sensory transduction following the application of stimuli to the dentin surface. Various exogenous and endogenous stimuli elicit an increase in the intracellular free calcium concentration ([Ca²⁺]_i) in odontoblasts, which is mediated by Ca²⁺ release from intracellular Ca²⁺ stores and/or Ca²⁺ influx from the extracellular medium. In a previous study, we demonstrated that the depletion of Ca²⁺ stores in odontoblasts activated store-operated Ca²⁺ entry (SOCE), a Ca²⁺ influx pathway. However, the precise biophysical and pharmacological properties of SOCE in odontoblasts have remained unclear. In the present study, we examined the functional expression and pharmacological properties of Ca²⁺ release-activated Ca²⁺ (CRAC) channels that mediate SOCE and evaluated the alkali sensitivity of SOCE in rat odontoblasts. In the absence of extracellular Ca²⁺, treatment with thapsigargin (TG), a sarco/endoplasmic reticulum Ca²⁺-ATPase inhibitor, induced an increase in [Ca²⁺]_i. After [Ca²⁺]_i returned to near-resting levels, the subsequent application of 2.5 mM extracellular Ca²⁺ resulted in an increase in [Ca²⁺]_i which is a typical of SOCE activation. Additionally, application of 2-methylthioadenosine diphosphate trisodium salt (2-MeSADP), a P2Y₁,₁₂,₁₃ receptor agonist, or carbachol (CCh), a muscarinic cholinergic receptor agonist, in the absence of extracellular Ca²⁺, induced a transient increase in [Ca²⁺]_i. The subsequent addition of extracellular Ca²⁺ resulted in significantly higher [Ca²⁺]_i in 2-MeSADP- or CCh-treated odontoblasts than in untreated cells. SOCE, that is activated by addition of extracellular Ca²⁺ in the TG pretreated odontoblasts was then suppressed by Synta66, BTP2, or lanthanum, which are CRAC channel inhibitors. Treatment with an alkaline solution enhanced SOCE, while treatment with HC030031, a TRPA1 channel antagonist, inhibited it. The amplitude of SOCE at pH 9 in the presence of HC030031 was higher than that at pH 7.4 in the absence of HC030031. These findings indicate that CRAC channel-mediated alkali-sensitive SOCE occurs in odontoblasts. SOCE is mediated by P2Y and muscarinic-cholinergic receptors, which are activated by endogenous ligands in odontoblasts.

Deletion of Orai1 leads to bone loss aggravated with aging and impairs function of osteoblast lineage cells

Osteoblast lineage cells, a group of cells including mesenchymal progenitors, osteoblasts, and osteocytes, are tightly controlled for differentiation, proliferation and stage-specific functions in processes of skeletal development, growth and maintenance. Recently, the plasma membrane calcium channel Orai1 was highlighted for its role in skeletal development and osteoblast differentiation. Yet the roles of Orai1 in osteoblast lineage cells at various stages of maturation have not been investigated. Herein we report the severe bone loss that occurred in Orai1−/− mice, aggravated by aging, as shown by the microcomputed tomography (mCT) and bone histomorphometry analysis of 8-week and 12-week old Orai1−/− mice and sex-matched WT littermates. We also report that Orai1 deficiency affected the differentiation, proliferation, and type I collagen secretion of primary calvarial osteoblasts, mesenchymal progenitors, and osteocytes in Orai1−/− mice; specifically, our study revealed a significant decrease in the expression of osteocytic genes Fgf23, DMP1 and Phex in the cortical long bone of Orai1−/− mice; a defective cellular and nuclear morphology of Orai1−/− osteocytes; and defective osteogenic differentiation of Orai1−/− primary calvarial osteoblasts (pOBs), including a decrease in extracellular-secretion of type I collagen. An increase in the mesenchymal progenitor population of Orai1−/− bone marrow cells was indicated by a colony forming unit-fibroblasts (CFU-F) assay, and the increased proliferation of Orai1−/− pOBs was indicated by an MTT assay. Notably, Orai1 deficiency reduced the nuclear localization and transcription activity of the Nuclear Factor of Activated T-cell c1 (NFATc1), a calcium-regulated transcription factor, in pOBs. Altogether, our study demonstrated the crucial role of Orai1 in bone development and maintenance, via its diverse effects on osteoblast lineage cells from mesenchymal progenitors to osteocytes.

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Severe bone loss in adult Orai1-/- mice was aggravated by aging.

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Orai1 deficiency affected function, differentiation and proliferation of osteoblast lineage cells, from mesenchymal progenitors to and osteocytes.

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Orai1 deficiency reduced the nuclear localization and transcription activity of NFATc1, a calcium-regulated transcription factor, in primary calvarial osteoblasts.

Forms and functions of store-operated calcium entry mediators, STIM and Orai

Calcium signals arise by multiple mechanisms, including mechanisms of release of intracellular stored Ca²⁺, and the influx of Ca²⁺ through channels in the plasma membrane. One mechanism that links these two sources of Ca²⁺ is store-operated Ca²⁺ entry, the most commonly encountered version of which involves the extensively studied calcium-release-activated Ca²⁺ (CRAC) channel. The minimal and essential molecular components of the CRAC channel are the STIM proteins that function as Ca²⁺ sensors in the endoplasmic reticulum, and the Orai proteins that comprise the pore forming subunits of the CRAC channel. CRAC channels are known to play significant roles in a wide variety of physiological functions. This review discusses the multiple forms of STIM and Orai proteins encountered in mammalian cells, and discusses some specific examples of how these proteins modulate or mediate important physiological processes.

Assembly of ER-PM Junctions: A Critical Determinant in the Regulation of SOCE and TRPC1

Store-operated calcium entry (SOCE), a unique plasma membrane Ca²⁺ entry mechanism, is activated when ER-[Ca²⁺] is decreased. SOCE is mediated via the primary channel, Orai1, as well as others such as TRPC1. STIM1 and STIM2 are ER-Ca²⁺ sensor proteins that regulate Orai1 and TRPC1. SOCE requires assembly of STIM proteins with the plasma membrane channels which occurs within distinct regions in the cell that have been termed as endoplasmic reticulum (ER)-plasma membrane (PM) junctions. The PM and ER are in close proximity to each other within this region, which allows STIM1 in the ER to interact with and activate either Orai1 or TRPC1 in the plasma membrane. Activation and regulation of SOCE involves dynamic assembly of various components that are involved in mediating Ca²⁺ entry as well as those that determine the formation and stabilization of the junctions. These components include proteins in the cytosol, ER and PM, as well as lipids in the PM. Recent studies have also suggested that SOCE and its components are compartmentalized within ER-PM junctions and that this process might require remodeling of the plasma membrane lipids and reorganization of structural and scaffolding proteins. Such compartmentalization leads to the generation of spatially- and temporally-controlled Ca²⁺signals that are critical for regulating many downstream cellular functions.

The functions of store-operated calcium channels

Store-operated calcium channels provide calcium signals to the cytoplasm of a wide variety of cell types. The basic components of this signaling mechanism include a mechanism for discharging Ca²⁺ stores (commonly but not exclusively phospholipase C and inositol 1,4,5-trisphosphate), a sensor in the endoplasmic reticulum that also serves as an activator of the plasma membrane channel (STIM1 and STIM2), and the store-operated channel (Orai1, 2 or 3). The advent of mice genetically altered to reduce store-operated calcium entry globally or in specific cell types has provided important tools to understand the functions of these widely encountered channels in specific and clinically important physiological systems. This review briefly discusses the history and cellular properties of store-operated calcium channels, and summarizes selected studies of their physiological functions in specific physiological or pathological contexts. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Orai1 mediates osteogenic differentiation via BMP signaling pathway in bone marrow mesenchymal stem cells

Orai1 is a pore-subunit of store-operated Ca(2+) release-activated Ca(2+) (CRAC) channel that mediates Ca(2+) influx in most non-excitable cells via store-operated Ca(2+) entry (SOCE) mechanism. We previously demonstrated that Orai1 is involved in mediating osteogenic potential of mesenchymal stem cells (MSCs), but the underlying mechanism of this function remains unknown. Here, we report that Orai1 mediates osteogenic differentiation via bone morphogenic protein (BMP) signaling pathway in bone marrow MSCs (BMSCs). In osteogenic conditions, BMSCs derived from wild-type mice underwent osteoblastic differentiation and induced mineralization as demonstrated by increased alkaline phosphatase activity and alizarin red S staining, respectively. The expression of Runx2, a master regulator of osteoblast differentiation, and osteogenic differentiation markers were markedly increased in wild-type BMSCs under osteogenic conditions. In contrast, osteogenic conditions failed to induce such effects in BMSCs derived from Orai1-deficient (Orai1(-/-)) mice, indicating that Orai1 is, in part, necessary for osteogenic differentiation of MSCs. We also found that BMP2 successfully induced phosphorylation of Smad1/5/8, the immediate effector molecules of BMP signaling, in wild-type BMSCs, but failed to do so in Orai1(-/-) BMSCs. Downstream target genes of BMP signaling pathway were consistently increased by osteogenic conditions in wild-type BMSCs, but not in Orai1(-/-) BMSCs, suggesting a novel molecular link between Orai1 and BMP signaling pathway in the osteogenic differentiation process. Further functional studies demonstrated that activation of BMP signaling rescues osteogenic differentiation capacity of Orai1(-/-) BMSCs. In conclusion, Orai1 regulates osteogenic differentiation through BMP signaling, and the Orai1-BMP signaling may be a possible therapeutic target for treating bone-related diseases.

Tmem178 acts in a novel negative feedback loop targeting NFATc1 to regulate bone mass

Phospholipase C gamma-2 (PLCγ2)-dependent calcium (Ca(2+)) oscillations are indispensable for nuclear factor of activated T-cells, cytoplasmic 1 (NFATc1) activation and downstream gene transcription driving osteoclastogenesis during skeletal remodeling and pathological bone loss. Here we describe, to our knowledge, the first known function of transmembrane protein 178 (Tmem178), a PLCγ2 downstream target gene, as a critical modulator of the NFATc1 axis. In surprising contrast to the osteopetrotic phenotype of PLCγ2(-/-) mice, Tmem178(-/-) mice are osteopenic in basal conditions and are more susceptible to inflammatory bone loss, owing to enhanced osteoclast formation. Mechanistically, Tmem178 localizes to the ER membrane and regulates RANKL-induced Ca(2+) fluxes, thus controlling NFATc1 induction. Importantly, down-regulation of Tmem178 is observed in human CD14(+) monocytes exposed to plasma from systemic juvenile idiopathic arthritis patients. Similar to the mouse model, reduced Tmem178 expression in human cells correlates with excessive osteoclastogenesis. In sum, these findings identify an essential role for Tmem178 to maintain skeletal mass and limit pathological bone loss.

The Role of ORAI1 in the Odontogenic Differentiation of Human Dental Pulp Stem Cells

Pulp capping, or placing dental materials directly onto the vital pulp tissues of affected teeth, is a dental procedure that aims to regenerate reparative dentin. Several pulp capping materials are clinically being used, and calcium ion (Ca(2+)) released from these materials is known to mediate reparative dentin formation. ORAI1 is an essential pore subunit of store-operated Ca(2+) entry (SOCE), which is a major Ca(2+) influx pathway in most nonexcitable cells. Here, we evaluated the role of ORAI1 in mediating the odontogenic differentiation and mineralization of dental pulp stem cells (DPSCs). During the odontogenic differentiation of DPSCs, the expression of ORAI1 increased in a time-dependent manner. DPSCs knocked down with ORAI1 shRNA (DPSC/ORAI1sh) or overexpressed with dominant negative mutant ORAI1(E106Q) (DPSC/E106Q) exhibited the inhibition of Ca(2+) influx and suppression of odontogenic differentiation and mineralization as demonstrated by alkaline phosphatase (ALP) activity/staining as well as alizarin red S staining when compared with DPSCs of their respective control groups (DPSC/CTLsh and DPSC/CTL). The gene expression for odontogenic differentiation markers such as osteocalcin, bone sialoprotein, and dentin matrix protein 1 (DMP1) was also suppressed. When DPSC/CTL or DPSC/E106Q cells were subcutaneously transplanted into nude mice, DPSC/CTL cells induced mineralized tissue formation with significant increases in ALP and DMP1 staining in vivo, whereas DPSC/E106Q cells did not. Collectively, our data showed that ORAI1 plays critical roles in the odontogenic differentiation and mineralization of DPSCs by regulating Ca(2+) influx and that ORAI1 may be a therapeutic target to enhance reparative dentin formation.

Odontogenic differentiation of vascular endothelial growth factor-transfected human dental pulp stem cells in vitro

Dental pulp stem cells (DPSCs) can be induced towards odontogenic differentiation. Previous studies have shown that vascular endothelial growth factor (VEGF) is able to induce the osteogenic differentiation of cells, but the effectiveness of VEGF in the odontogenic differentiation of DPSCs remains unclear. This study aimed to investigate the effects of lentivirus‑mediated human VEGF gene transfection on the proliferation and odontogenic differentiation of human DPSCs in vitro. DPSCs were transfected with either lentiviral pCDH‑CMV‑MCS‑EFI‑copGFP (pCDH) vector or recombinant pCDH‑VEGF vector, and the growth characteristics of the resulting DPSCs/Vector and DPSCs/VEGF were subsequently assessed. The odontogenic differentiation genes of the two groups of cells, including alkaline phosphatase, osteocalcin, dentin sialophosphoprotein and dentin matrix protein 1 (DMP1), were evaluated by quantitative polymerase chain reaction (qPCR). The specific proteins of odontogenic differentiation, including dentin sialoprotein and DMP1, were analyzed by western blotting. DPSCs/VEGF showed similar proliferation characteristics to DPSCs/Vector during the observation period. qPCR results showed that the relative VEGF gene expression was significantly higher in DPSCs/VEGF than that in DPSCs/Vector two days after transfection (P<0.01). Similarly, western blot analysis showed that the protein expression levels of VEGF were higher in DPSCs/VEGF than those in DPSCs/Vector. On the first, fourth, eighth and 16th days after lentivirus-mediated transfection, the expression of odontogenic differentiation-specific genes and proteins was higher in DPSCs/VEGF than that in DPSCs/Vector. These results indicated that lentivirus-mediated VEGF gene transfection promoted the odontogenic differentiation of human DPSCs in vitro.

Proliferation and odontogenic differentiation of BMP2 gene‑transfected stem cells from human tooth apical papilla: an in vitro study

Stem cells from the apical papilla (SCAP) have odontogenic potential, which plays a pivotal role in the root dentin development of permanent teeth. Human bone morphogenetic protein 2 (BMP2) is a well-known gene that participates in regulating the odontogenic differentiation of dental tissue-derived stem cells. However, little is known regarding the effects of the BMP2 gene on the proliferation and odontogenic differentiation of SCAP. This study aimed to evaluate the odontogenic differentiation potential of lentiviral-mediated BMP2 gene-transfected human SCAP (SCAP/BMP2) in vitro. SCAP were isolated by enzymatic dissociation of human teeth apical papillae. The multipotential of SCAP was verified by their osteogenic and adipogenic differentiation characteristics. The phenotype of SCAP was evaluated by flow cytometry (FCM). The proliferation status of the blank vector-transfected SCAP (SCAP/Vector) and SCAP/BMP2 was analyzed by a cell counting kit-8 (CCK-8). Odontogenic genes, including alkaline phosphatase (ALP), osteocalcin (OCN), dentin sialophosphoprotein (DSPP) and dentin matrix protein 1 (DMP1) of the two groups of cells were evaluated by quantitative polymerase chain reaction (qPCR). ALP staining and alizarin red (AR) staining of the cells was performed on the 16th day after transfection. In vitro results of CCK-8, qPCR, ALP and AR staining demonstrated that: i) SCAP/BMP2 had a comparable proliferation rate to SCAP/Vector; ii) SCAP/BMP2 presented significantly better potential to differentiate into odontoblasts compared to SCAP/Vector by upregulating ALP, OCN, DSPP and DMP1 genes; iii) more ALP granules and mineralized deposits were formed by SCAP/BMP2 as compared to SCAP/Vector. The results suggested that lentiviral-mediated BMP2 gene transfection enhances the odontogenic differentiation capacity of human SCAP in vitro.

Store-operated calcium entry induced by activation of Gq-coupled alpha1B adrenergic receptor in human osteoblast

Recent studies have revealed that the sympathetic nervous system is involved in bone metabolism. We previously reported that noradrenaline (NA) suppressed K(+) currents via Gi/o protein-coupled alpha1B-adrenergic receptor (α1B-AR) in human osteoblast SaM-1 cells. Additionally, it has been demonstrated that the intracellular Ca(2+) level ([Ca(2+)]i) was increased by NA via α1B-AR. In this study, we investigated the signal pathway of NA-induced [Ca(2+)]i elevation by using Ca(2+) fluorescence imaging in SaM-1 cells. NA-induced [Ca(2+)]i elevation was suppressed by pretreatment with a PLC inhibitor, U73122. This suggested that the [Ca(2+)]i elevation was mediated by Gq protein-coupled α1B-AR. On the other hand, NA-induced [Ca(2+)]i elevation was completely abolished in Ca(2+)-free solution, which suggested that Ca(2+) influx is the predominant pathway of NA-induced [Ca(2+)]i elevation. Although the inhibition of K(+) channel by NA caused membrane depolarization, the [Ca(2+)]i elevation was not affected by voltage-dependent Ca(2+) channel blockers, nifedipine and mibefradil. Meanwhile, NA-induced [Ca(2+)]i elevation was abolished following activation of store-operated Ca(2+) channel by thapsigargin. Additionally, the [Ca(2+)]i elevation was suppressed by store-operated channel inhibitors, 2-APB, flufenamate, GdCl3 and LaCl3. These results suggest that Ca(2+) influx through store-operated Ca(2+) channels plays a critical role in the signal transduction pathway of Gq protein-coupled α1B-AR in human osteoblasts.