Plasma membrane calcium ATPase regulates bone mass by fine-tuning osteoclast differentiation and survival

Molecular mechanisms and genetic factors contributing to the developmental dysplasia of the hip

The most prevalent hip disease in neonates is developmental dysplasia of the hip (DDH). A timely and accurate diagnosis is required to provide the most effective treatment for pediatric patients with DDH. Heredity and gene variation have been the subject of increased attention and research worldwide as one of the factors contributing to the pathogenesis of DDH. Genome-wide association studies (GWAS), genome-wide linkage analyses (GWLA), and exome sequencing (ES) have identified variants in numerous genes and single-nucleotide polymorphisms (SNPs) as being associated with susceptibility to DDH in sporadic and DDH family patients. Furthermore, the DDH phenotype can be observed in animal models that exhibit susceptibility genes or loci, including variants in CX3CR1, KANSL1, and GDF5. The dentification of noncoding RNAs and de novo gene variants in patients with DDH-related syndrome has enhanced our understanding of the genes implicated in DDH. This article reviews the most recent molecular mechanisms and genetic factors that contribute to DDH.

Association between polymorphism and haplotype of ATP2B1 gene and skeletal fluorosis in Han population

To evaluate the association between ATP2B1 gene polymorphisms and skeletal fluorosis, a cross-sectional study was conducted. In China, 962 individuals were recruited, including 342 cases of skeletal fluorosis. Four TP2BA1 polymorphisms (rs2070759, rs12817819, rs17249754, and rs7136259) were analysed. The results suggested that rs17249754 and rs7136259 were associated with skeletal fluorosis. After controlling confounders, the protective effect of GG genotype in rs17249754 was apparent in individuals over 45 years old, female, with urine fluoride concentration below 1.6 mg/L, serum calcium above 2.25 mmol/L or serum phosphorus between 1.1 and 1.3. Heterozygote TC in rs7136259 increased the risk of skeletal fluorosis in subjects who are elderly, female, with urinary fluoride more than 1.6 mg/L, serum calcium more than 2.25 mmol/L and blood phosphorus between 1.1 and 1.3 mmol/L. Four loci were found to be tightly related by linkage disequilibrium analysis, and the frequency of distribution of haplotype GCGT was lower in the skeletal fluorosis group.

Sugar transporter Slc37a2 regulates bone metabolism in mice via a tubular lysosomal network in osteoclasts

Osteoclasts are giant bone-digesting cells that harbor specialized lysosome-related organelles termed secretory lysosomes (SLs). SLs store cathepsin K and serve as a membrane precursor to the ruffled border, the osteoclast's 'resorptive apparatus'. Yet, the molecular composition and spatiotemporal organization of SLs remains incompletely understood. Here, using organelle-resolution proteomics, we identify member a2 of the solute carrier 37 family (Slc37a2) as a SL sugar transporter. We demonstrate in mice that Slc37a2 localizes to the SL limiting membrane and that these organelles adopt a hitherto unnoticed but dynamic tubular network in living osteoclasts that is required for bone digestion. Accordingly, mice lacking Slc37a2 accrue high bone mass owing to uncoupled bone metabolism and disturbances in SL export of monosaccharide sugars, a prerequisite for SL delivery to the bone-lining osteoclast plasma membrane. Thus, Slc37a2 is a physiological component of the osteoclast's unique secretory organelle and a potential therapeutic target for metabolic bone diseases.

PMCA Ca2+ clearance in dental enamel cells depends on the magnitude of cytosolic Ca2

Enamel formation (amelogenesis) is a two-step process whereby crystals partially grow during the secretory stage followed by a significant growth expansion during the maturation stage concurrent with an increase in vectorial Ca2+ transport. This requires tight regulation of cytosolic Ca2+ (c Ca2+ ) concentration in the enamel forming ameloblasts by controlling Ca2+ influx (entry) and Ca2+ extrusion (clearance). Gene and protein expression studies suggest that the plasma membrane Ca2+ -ATPases (PMCA1-4) are likely involved in c Ca2+ extrusion in ameloblasts, yet no functional analysis of these pumps has been reported nor whether their activity changes across amelogenesis. PMCAs have high Ca2+ affinity and low Ca2+ clearance which may be a limiting factor in their contribution to enamel formation as maturation stage ameloblasts handle high Ca2+ loads. We analyzed PMCA function in rat secretory and maturation ameloblasts by blocking or potentiating these pumps. Low/moderate elevations in c Ca2+ measured using the Ca2+ probe Fura-2-AM show that secretory ameloblasts clear Ca2+ faster than maturation stage cells through PMCAs. This process was completely inhibited by an external alkaline (pH 9.0) solution or was significantly delayed by the PMCA blockers vanadate and caloxin 1b1. Eliciting higher c Ca2+ transients via the activation of the ORAI1 Ca2+ channel showed that the PMCAs of maturation ameloblasts were more efficient. Inhibiting PMCAs decreased the rate of Ca2+ influx via ORAI1 but potentiation with forskolin had no effect. Our findings suggest that PMCAs are functional Ca2+ pumps during amelogenesis regulating c Ca2+ upon low and/or moderate Ca2+ stimulus in secretory stage, thus participating in amelogenesis.

Molecular mechanisms, physiological roles, and therapeutic implications of ion fluxes in bone cells: Emphasis on the cation-Cl- cotransporters

Bone turnover diseases are exceptionally prevalent in human and come with a high burden on physical health. While these diseases are associated with a variety of risk factors and causes, they are all characterized by common denominators, that is, abnormalities in the function or number of osteoblasts, osteoclasts, and/or osteocytes. As such, much effort has been deployed in the recent years to understand the signaling mechanisms of bone cell proliferation and differentiation with the objectives of exploiting the intermediates involved as therapeutic preys. Ion transport systems at the external and in the intracellular membranes of osteoblasts and osteoclasts also play an important role in bone turnover by coordinating the movement of Ca²⁺ , PO₄ ^2- , and H⁺ ions in and out of the osseous matrix. Even if they sustain the terminal steps of osteoformation and osteoresorption, they have been the object of very little attention in the last several years. Members of the cation-Cl^- cotransporter (CCC) family are among the systems at work as they are expressed in bone cells, are known to affect the activity of Ca²⁺ -, PO₄ ^2- -, and H⁺ -dependent transport systems and have been linked to bone mass density variation in human. In this review, the roles played by the CCCs in bone remodeling will be discussed in light of recent developments and their potential relevance in the treatment of skeletal disorders.

Population Genomic Sequencing Delineates Global Landscape of Copy Number Variations that Drive Domestication and Breed Formation of in Chicken

Copy number variation (CNV) is an important genetic mechanism that drives evolution and generates new phenotypic variations. To explore the impact of CNV on chicken domestication and breed shaping, the whole-genome CNVs were detected via multiple methods. Using the whole-genome sequencing data from 51 individuals, corresponding to six domestic breeds and wild red jungle fowl (RJF), we determined 19,329 duplications and 98,736 deletions, which covered 11,123 copy number variation regions (CNVRs) and 2,636 protein-coding genes. The principal component analysis (PCA) showed that these individuals could be divided into four populations according to their domestication and selection purpose. Seventy-two highly duplicated CNVRs were detected across all individuals, revealing pivotal roles of nervous system (NRG3, NCAM2), sensory (OR), and follicle development (VTG2) in chicken genome. When contrasting the CNVs of domestic breeds to those of RJFs, 235 CNVRs harboring 255 protein-coding genes, which were predominantly involved in pathways of nervous, immunity, and reproductive system development, were discovered. In breed-specific CNVRs, some valuable genes were identified, including HOXB7 for beard trait in Beijing You chicken; EDN3, SLMO2, TUBB1, and GFPT1 for melanin deposition in Silkie chicken; and SORCS2 for aggressiveness in Luxi Game fowl. Moreover, CSMD1 and NTRK3 with high duplications found exclusively in White Leghorn chicken, and POLR3H, MCM9, DOCK3, and AKR1B1L found in Recessive White Rock chicken may contribute to high egg production and fast-growing traits, respectively. The candidate genes of breed characteristics are valuable resources for further studies on phenotypic variation and the artificial breeding of chickens.

The Prognostic Relevance of PMCA4 Expression in Melanoma: Gender Specificity and Implications for Immune Checkpoint Inhibition

PMCA4 is a critical regulator of Ca2+ homeostasis in mammalian cells. While its biological and prognostic relevance in several cancer types has already been demonstrated, only preclinical investigations suggested a metastasis suppressor function in melanoma. Therefore, we studied the expression pattern of PMCA4 in human skin, nevus, as well as in primary and metastatic melanoma using immunohistochemistry. Furthermore, we analyzed the prognostic power of PMCA4 mRNA levels in cutaneous melanoma both at the non-metastatic stage as well as after PD-1 blockade in advanced disease. PMCA4 localizes to the plasma membrane in a differentiation dependent manner in human skin and mucosa, while nevus cells showed no plasma membrane staining. In contrast, primary cutaneous, choroidal and conjunctival melanoma cells showed specific plasma membrane localization of PMCA4 with a wide range of intensities. Analyzing the TCGA cohort, PMCA4 mRNA levels showed a gender specific prognostic impact in stage I-III melanoma. Female patients with high transcript levels had a significantly longer progression-free survival. Melanoma cell specific PMCA4 protein expression is associated with anaplasticity in melanoma lung metastasis but had no impact on survival after lung metastasectomy. Importantly, high PMCA4 transcript levels derived from RNA-seq of cutaneous melanoma are associated with significantly longer overall survival after PD-1 blockade. In summary, we demonstrated that human melanoma cells express PMCA4 and PMCA4 transcript levels carry prognostic information in a gender specific manner.

Local delivery and controlled release of miR-34a loaded in hydroxyapatite/mesoporous organosilica nanoparticles composite-coated implant wire to accelerate bone fracture healing

Immediate mechanical stability is a prerequisite for fracture healing. In addition to bringing immediate mechanical stability in fracture site, implants with bioactive coating can release active substance to accelerate bone-fracture healing. However, limited drug-loading capacity of established coatings weakens their biological functions, which urges the engineering of more effective coating biomaterials for accelerating fracture healing. Herein, mesoporous organosilica nanoparticles (MONs), as miR-34a delivers, are loaded onto hydroxyapatite (HA)-coated Kirschner wire to engineer a HA/MONs@miR-34a composite coating. The composite coating can effectively deliver miR-34a into osteoclasts, generate gene dose-dependent inhibiting effect on differentiation and resorptive activity of osteoclasts by regulating multiple downstream gene expression at the early stage of fracture healing, which additionally exhibits decent bone regeneration potentials as evidenced in rat tibial fracture model. In particular, differentially expressed genes regulated by miR-34a are identified using RNA-seq followed by bioinformatics analysis. Functional enrichment analysis reveals that genes with altered expression mainly distribute in mainly distribute in DNA replication and cell cycle, which are associated with the development of osteoclasts. This work not only demonstrates the high clinical translation potential of HA/MONs@miR-34a to accelerate fracture healing, but also reveals the underlying molecular mechanism of regulating physiological functions of osteoclasts based on analysis of singlecell RNA sequencing.

Replicative verification of susceptibility genes previously identified from families with segregating developmental dysplasia of the hip

BACKGROUND

Developmental dysplasia of the hip (DDH) is a complex hip joint deformity with effects ranging from acetabulum malformation to irreversible hip dislocation. Previous studies suggest a significant association of four variations, teneurin transmembrane protein 3 (TENM3, OMIM * 610083) (chr4:183721398), heparan sulfate proteoglycan 2 (HSPG2, OMIM * 142461) (chr1:22201470), ATPase plasma membrane Ca²⁺ transporting 4 (ATP2B4, OMIM * 108732) (chr1:203682345), and prostaglandin F receptor (PTGFR, OMIM * 600563) (chr1:79002214), with DDH susceptibility in families with segregating DDH. However, the association was not validated in sporadic cases and remains controversial. To confirm the association of the reported variations in these four genes with DDH, we conducted replicative verification in 250 sporadic samples with DDH from a Chinese Han population.

METHODS

We conducted Sanger sequencing after amplifying the variation sites. The results were compared with the reference sequence from the GRCh37 assembly in UCSC ( http://genome.ucsc.edu ).

RESULTS

Replication analysis of 250 sporadic samples by Sanger sequencing indicated that the four variations, TENM3 (OMIM * 610083, chr4:183721398), HSPG2 (OMIM * 142461, chr1:22201470), ATP2B4 (OMIM * 108732, chr1:203682345), and PTGFR (OMIM * 600563, chr1:79002214), were not associated with the susceptibility to DDH in the Chinese Han population.

CONCLUSIONS

Further studies should be performed to identify other variations of these four genes that are potentially associated with DDH by whole-exome sequencing and the results should be verified in different populations.

Effects of photobiomodulation on bone remodeling in an osteoblast-osteoclast co-culture system

The general bone anabolic effect of photobiomodulation (PBM) is largely accepted. As a result, PBM therapy is expected to be beneficial in the medical fields of dentistry and bone healing. However, most of the previous in vitro studies on PBM and bone metabolism were performed with single-cell cultures of osteoclast-lineage cells or osteoblast-lineage cells. In the present study, the bone-modulating effects of PBM were evaluated in an in vitro osteoblast/osteoclast co-culture system. Mouse bone marrow-derived macrophages (BMMs) and mouse calvarial pre-osteoblasts cells were purified and used as precursor cells for osteoclasts and osteoblasts, respectively. The PBM effects on single-cell culture of osteoclasts or osteoblasts as well as co-culture were examined by 1.2 J/cm² low-level Ga-Al-As laser (λ  = 808 ± 3 nm, 80 mW, and 80 mA; spot size, 1cm²; NDLux, Seoul, Korea) irradiation for 30 s at daily intervals throughout culture period. At the end of culture, the osteoclast differentiation and osteoblast differentiation were assessed by TRAP staining and ALP staining, respectively. The expressions of osteoclastogenic cytokines were evaluated by RT-PCR and Western blot analyses. Under the single-cell culture condition, PBM enhanced osteoblast differentiation but had minor effects on osteoclast differentiation. However, in the co-culture condition, its osteoblastogenic effect was maintained, and osteoclast differentiation was substantially reduced. Subsequent RT-PCR analyses and western blot results revealed marked reduction in receptor activator of NF-κB ligand (RANKL) expression and elevation in osteoprotegerin (OPG) expression by PBM in co-cultured cells. More importantly, these alterations in RANKL/OPG levels were not observed under the single-cell culture conditions. Our results highlight the different effects of PBM on bone cells based on culture conditions. Further, our findings suggest the indirect anti-osteoclastogenic effect of PBM, which is accompanied by a decrease in RANKL expression and an increase in OPG expression.

Sestrin2 Regulates Osteoclastogenesis via the p62-TRAF6 Interaction

The receptor activator of nuclear factor-kappa B ligand (RANKL) mediates osteoclast differentiation and functions by inducing Ca²⁺ oscillations, activating mitogen-activated protein kinases (MAPKs), and activating nuclear factor of activated T-cells type c1 (NFATc1) via the RANK and tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6) interaction. Reactive oxygen species (ROS) also plays an important role during osteoclastogenesis and Sestrin2, an antioxidant, maintains cellular homeostasis upon stress injury via regulation of ROS, autophagy, and inflammation. However, the role of Sestrin2 in osteoclastogenesis remains unknown. In this study, we investigated the role of Sestrin2 in the RANKL-RANK-TRAF6 signaling pathway during osteoclast differentiation. Deletion of Sestrin2 (Sesn2) increased bone mass and reduced the number of multinucleated osteoclasts on bone surfaces. RANKL-induced osteoclast differentiation and function decreased in Sesn2 knockout (KO) bone marrow-derived monocytes/macrophages (BMMs) due to inhibition of NFATc1 expression, but osteoblastogenesis was not affected. mRNA expression of RANKL-induced specific osteoclastogenic genes and MAPK protein expression were lower in Sesn2 KO BMMs than wild-type (WT) BMMs after RANKL treatment. However, the Sesn2 deletion did not affect ROS generation or intracellular Ca²⁺ oscillations during osteoclastogenesis. In contrast, the interaction between TRAF6 and p62 was reduced during osteoclasts differentiation in Sesn2 KO BMMs. The reduction in the TRAF6/p62 interaction and TRAP activity in osteoclastogenesis in Sesn2 KO BMMs was recovered to the WT level upon expression of Flag-Sesn2 in Sesn2 KO BMMs. These results suggest that Sestrin2 has a novel role in bone homeostasis and osteoclasts differentiation through regulation of NFATc1 and the TRAF6/p62 interaction.

Membrane Transport Proteins in Osteoclasts: The Ins and Outs

During bone resorption, the osteoclast must sustain an extraordinarily low pH environment, withstand immense ionic pressures, and coordinate nutrient and waste exchange across its membrane to sustain its unique structural and functional polarity. To achieve this, osteoclasts are equipped with an elaborate set of membrane transport proteins (pumps, transporters and channels) that serve as molecular ‘gatekeepers’ to regulate the bilateral exchange of ions, amino acids, metabolites and macromolecules across the ruffled border and basolateral domains. Whereas the importance of the vacuolar-ATPase proton pump and chloride voltage-gated channel 7 in osteoclasts has long been established, comparatively little is known about the contributions of other membrane transport proteins, including those categorized as secondary active transporters. In this Special Issue review, we provide a contemporary update on the ‘ins and outs’ of membrane transport proteins implicated in osteoclast differentiation, function and bone homeostasis and discuss their therapeutic potential for the treatment of metabolic bone diseases.

Gastrin-Releasing Peptide (GRP) Stimulates Osteoclastogenesis in Periodontitis

Periodontitis is a chronic inflammatory disease with alveolar bone resorption and subsequent tooth loss as its ultimate outcomes. Gastrin-releasing peptide (GRP) is a neuropeptide with growth-stimulatory and tumorigenic properties, and neuropeptides have previously been suggested to play a role in the complex cascade of chemical activity associated with periodontal inflammation. In this study, GRP treatment enhanced the differentiation of bone marrow-derived macrophages (BMMs) into osteoclasts, and gastrin-releasing peptide receptor (GRPR) antagonists suppressed the pro-osteoclastogenic effect of GRP. Grpr-siRNA knockdown resulted in a significantly lower number of osteoclasts formed as compared with the control. Interestingly, gene expression analysis indicated downregulation of Grp and Grpr expressions in BMMs during osteoclastogenesis. Moreover, ligature-induced periodontitis model in mice and gingival samples from patients with periodontitis displayed increased immunostaining of GRP in the oral epithelium. Subsequently, stimulation of mouse primary epithelial cells (ECs) and HaCaT cells, human epidermal keratinocytes, with lipopolysaccharides (LPS) of Porphyromonas gingivalis or live P. gingivalis upregulated Grp and Grpr expressions. Finally, coculture of P. gingivalis-stimulated ECs and BMMs using Transwell system revealed that the differentiation of BMMs was induced when subjected to paracrine activation by LPS- as well as live-P. gingivalis stimulated ECs. Taken together, our results demonstrate that the pro-osteoclastogenic properties of BMMs may be modulated by GRP produced by ECs in the periodontal microenvironment.

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.

Protective Effects of Vitamin K Compounds on the Proteomic Profile of Osteoblasts under Oxidative Stress Conditions

Oxidative stress, which accompanies the pathogenesis of many bone diseases, contributes to the reduction of osteoblast activity, resulting in the inhibition of differentiation. This study aimed to assess the effect of vitamins K1 and K2 (MK4 and MK7) on the proteomic profile of human osteoblasts cell line under oxidative conditions induced by hydrogen peroxide (H₂O₂). The analysis was performed using QExactiveHF mass spectrometer with a nanoelectrospray ionization source. The osteoblast protein exposed to oxidative stress and vitamin K was compared with the proteome of cells exposed only to oxidative stress. Our proteomic analysis identified 1234 proteins changed after 5 days, 967 after 15 days, and 1214 after 20 days of culture. We observed the most frequent changes in the expression of proteins with catalytic activity or protein/DNA binding properties (45% and 40%, respectively). Significant changes were also observed in proteins with transcription/translation regulator activity (2–6%), regulators of molecular functions (5–6%), signal transducers (1–4%), transporters (4–6%), and structural molecules (3–5%). Our results clearly show that vitamins K protect cells from H₂O₂-induced changes in protein expression, primarily through their effects on transcriptional regulators and transporter proteins. As a result, vitamins K can support the formation, remodeling, and mineralization of bone tissue.

Corticotropin-Releasing Factor Family: A Stress Hormone-Receptor System's Emerging Role in Mediating Sex-Specific Signaling

No organ in the body is impervious to the effects of stress, and a coordinated response from all organs is essential to deal with stressors. A dysregulated stress response that fails to bring systems back to homeostasis leads to compromised function and ultimately a diseased state. The components of the corticotropin-releasing factor (CRF) family, an ancient and evolutionarily conserved stress hormone-receptor system, helps both initiate stress responses and bring systems back to homeostasis once the stressors are removed. The mammalian CRF family comprises of four known agonists, CRF and urocortins (UCN1–3), and two known G protein-coupled receptors (GPCRs), CRF₁ and CRF₂. Evolutionarily, precursors of CRF- and urocortin-like peptides and their receptors were involved in osmoregulation/diuretic functions, in addition to nutrient sensing. Both CRF and UCN1 peptide hormones as well as their receptors appeared after a duplication event nearly 400 million years ago. All four agonists and both CRF receptors show sex-specific changes in expression and/or function, and single nucleotide polymorphisms are associated with a plethora of human diseases. CRF receptors harbor N-terminal cleavable peptide sequences, conferring biased ligand properties. CRF receptors have the ability to heteromerize with each other as well as with other GPCRs. Taken together, CRF receptors and their agonists due to their versatile functional adaptability mediate nuanced responses and are uniquely positioned to orchestrate sex-specific signaling and function in several tissues.

Primary Active Ca2+ Transport Systems in Health and Disease

Calcium ions (Ca²⁺) are prominent cell signaling effectors that regulate a wide variety of cellular processes. Among the different players in Ca²⁺ homeostasis, primary active Ca²⁺ transporters are responsible for keeping low basal Ca²⁺ levels in the cytosol while establishing steep Ca²⁺ gradients across intracellular membranes or the plasma membrane. This review summarizes our current knowledge on the three types of primary active Ca²⁺-ATPases: the sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) pumps, the secretory pathway Ca²⁺- ATPase (SPCA) isoforms, and the plasma membrane Ca²⁺-ATPase (PMCA) Ca²⁺-transporters. We first discuss the Ca²⁺ transport mechanism of SERCA1a, which serves as a reference to describe the Ca²⁺ transport of other Ca²⁺ pumps. We further highlight the common and unique features of each isoform and review their structure-function relationship, expression pattern, regulatory mechanisms, and specific physiological roles. Finally, we discuss the increasing genetic and in vivo evidence that links the dysfunction of specific Ca²⁺-ATPase isoforms to a broad range of human pathologies, and highlight emerging therapeutic strategies that target Ca²⁺ pumps.

Salt-inducible kinase 1 regulates bone anabolism via the CRTC1-CREB-Id1 axis

New bone anabolic agents for the effective treatment of bone metabolic diseases like osteoporosis are of high clinical demand. In the present study, we reveal the function of salt-inducible kinase 1 (SIK1) in regulating osteoblast differentiation. Gene knockdown of SIK1 but not of SIK2 or SIK3 expression in primary preosteoblasts increased osteoblast differentiation and bone matrix mineralization. SIK1 also regulated the proliferation of osteoblastic precursor cells in osteogenesis. This negative control of osteoblasts required the catalytic activity of SIK1. SIK1 phosphorylated CREB regulated transcription coactivator 1 (CRTC1), preventing CRTC1 from enhancing CREB transcriptional activity for the expression of osteogenic genes like Id1. Furthermore, SIK1 knockout (KO) mice had higher bone mass, osteoblast number, and bone formation rate versus littermate wild-type (WT) mice. Preosteoblasts from SIK1 KO mice showed more osteoblastogenic potential than did WT cells, whereas osteoclast generation among KO and WT precursors was indifferent. In addition, bone morphogenic protein 2 (BMP2) suppressed both SIK1 expression as well as SIK1 activity by protein kinase A (PKA)–dependent mechanisms to stimulate osteogenesis. Taken together, our results indicate that SIK1 is a key negative regulator of preosteoblast proliferation and osteoblast differentiation and that the repression of SIK1 is crucial for BMP2 signaling for osteogenesis. Therefore, we propose SIK1 to be a useful therapeutic target for the development of bone anabolic strategies.

S100A4 released from highly bone-metastatic breast cancer cells plays a critical role in osteolysis

Bone destruction induced by breast cancer metastasis causes severe complications, including death, in breast cancer patients. Communication between cancer cells and skeletal cells in metastatic bone microenvironments is a principal element that drives tumor progression and osteolysis. Tumor-derived factors play fundamental roles in this form of communication. To identify soluble factors released from cancer cells in bone metastasis, we established a highly bone-metastatic subline of MDA-MB-231 breast cancer cells. This subline (mtMDA) showed a markedly elevated ability to secrete S100A4 protein, which directly stimulated osteoclast formation via surface receptor RAGE. Recombinant S100A4 stimulated osteoclastogenesis in vitro and bone loss in vivo. Conditioned medium from mtMDA cells in which S100A4 was knocked down had a reduced ability to stimulate osteoclasts. Furthermore, the S100A4 knockdown cells elicited less bone destruction in mice than the control knockdown cells. In addition, administration of an anti-S100A4 monoclonal antibody (mAb) that we developed attenuated the stimulation of osteoclastogenesis and bone loss by mtMDA in mice. Taken together, our results suggest that S100A4 released from breast cancer cells is an important player in the osteolysis caused by breast cancer bone metastasis.

The effect of P2X7R-mediated Ca2+ signaling in OPG-induced osteoclasts adhesive structure damage

Osteoclast adhesion is important for bone resorption. Osteoprotegerin inhibits osteoclast differentiation and bone resorption via Ca²⁺ signaling. Purinergic receptor P2X7 (P2X7R) affects osteoclastogenesis by activating transcription factor nuclear factor of activated T cells 1 (NFATc1). However, the detailed mechanism of osteoprotegerin-mediated P2X7R modulation of osteoclast adhesion is unclear. This study aimed to determine the effect of P2X7R on osteoprotegerin-induced damage to osteoclast adhesion. Osteoprotegerin reduced the expression of P2X7R, and protein tyrosine kinase 2 (PYK2) and SRC phosphorylation, and reduced calcium concentration, significantly decreasing Ca²⁺-NFATc1 signaling. 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis (acetoxymethyl ester) (BAPTA-AM)/N-(6-Aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7) partly or absolutely recovered osteoprotegerin-induced osteoclasts adhesion structure damage, including increased the PYK2 and SRC phosphorylation, changed the distribution of PYK2/SRC and integrinαvβ3, and inhibited retraction of lamellipodia and filopodia and recovered osteoclast bone resorption activity. In addition, BAPTA-AM/W-7 also increased osteoprotegerin-induced activation of Ca²⁺-NFATc1 signaling, and restored normal P2X7R levels. P2X7R knockdown significantly inhibited osteoclast differentiation, and the formation of lamellipodia and filopodia, reduced the PYK2 and SRC phosphorylation, and inhibited Ca²⁺-related protein activation. However, P2X7R knockdown aggravated osteoprotegerin-induced osteoclast adhesion damage via Ca²⁺ signaling. In conclusion, the P2X7R-Ca²⁺ NFATc1 signaling pathway has a key functional role in osteoprotegerin-induced osteoclast adhesion structure damage.

Zoledronate Enhances Osteocyte-Mediated Osteoclast Differentiation by IL-6/RANKL Axis

Bisphosphonates are one of the most widely used synthetic pyrophosphate analogues for the treatment of bone resorbing diseases such as osteoporosis, multiple myeloma, and bone metastases. Although the therapeutic usefulness of bisphosphonates mainly depends on their anti-osteoclastogenic effect, a severe side-effect of bisphosphonates called bisphosphonate-related osteonecrosis of the jaw (BRONJ) could not be explained by the anti-osteoclastogenic effect of bisphosphonates. In the present study, we have evaluated the changes in osteoclastogenesis- or osteoblastogenesis-supporting activities of osteocytes induced by bisphosphonates. Zoledronate, a nitrogen-containing bisphosphonate, markedly increased both the receptor activator of nuclear factor kB ligand (RANKL) as well as sclerostin in osteocyte-like MLO-Y4 cells, which were functionally revalidated by osteoclast/osteoblast generating activities of the conditioned medium obtained from zoledronate-treated MLO-Y4 cells. Of note, the zoledronate treatment-induced upregulation of the RANKL expression was mediated by autocrine interleukin-6 (IL-6) and subsequent activation of the signal transducer and activator of transcription 3 (STAT3) pathway. These results were evidenced by the blunted RANKL expression in the presence of a Janus activated kinase (JAK2)/STAT3 inhibitor, AG490. Also, the osteoclastogenesis-supporting activity was significantly decreased in zoledronate-treated MLO-Y4 cells in the presence of IL-6 neutralizing IgG compared to that of the control IgG. Thus, our results show previously unanticipated effects of anti-bone resorptive bisphosphonate and suggest a potential clinical importance of osteocytes in BRONJ development.

Genome-wide analysis of wheat calcium ATPases and potential role of selected ACAs and ECAs in calcium stress

BACKGROUND

P₂- type calcium ATPases (ACAs-auto inhibited calcium ATPases and ECAs-endoplasmic reticulum calcium ATPases) belong to the P- type ATPase family of active membrane transporters and are significantly involved in maintaining accurate levels of Ca²⁺, Mn²⁺ and Zn²⁺ in the cytosol as well as playing a very important role in stress signaling, stomatal opening and closing and pollen tube growth. Here we report the identification and possible role of some of these ATPases from wheat.

RESULTS

In this study, ACA and ECA sequences of six species (belonging to Poaceae) were retrieved from different databases and a phylogenetic tree was constructed. A high degree of evolutionary relatedness was observed among P₂ sequences characterized in this study. Members of the respective groups from different plant species were observed to fall under the same clade. This pattern highlights the common ancestry of P_2- type calcium ATPases. Furthermore, qRT-PCR was used to analyse the expression of selected ACAs and ECAs from Triticum aestivum (wheat) under calcium toxicity and calcium deficiency. The data indicated that expression of ECAs is enhanced under calcium stress, suggesting possible roles of these ATPases in calcium homeostasis in wheat. Similarly, the expression of ACAs was significantly different in plants grown under calcium stress as compared to plants grown under control conditions. This gives clues to the role of ACAs in signal transduction during calcium stress in wheat.

CONCLUSION

Here we concluded that wheat genome consists of nine P_2B and three P_2A -type calcium ATPases. Moreover, gene loss events in wheat ancestors lead to the loss of a particular homoeolog of a gene in wheat. To elaborate the role of these wheat ATPases, qRT-PCR was performed. The results indicated that when plants are exposed to calcium stress, both P_2A and P_2B gene expression get enhanced. This further gives clues about the possible role of these ATPases in wheat in calcium management. These findings can be useful in future for genetic manipulations as well as in wheat genome annotation process.

The serum protein renalase reduces injury in experimental pancreatitis

Acute pancreatitis is a disease associated with inflammation and tissue damage. One protein that protects against acute injury, including ischemic injury to both the kidney and heart, is renalase, which is secreted into the blood by the kidney and other tissues. However, whether renalase reduces acute injury associated with pancreatitis is unknown. Here, we used both in vitro and in vivo murine models of acute pancreatitis to study renalase's effects on this condition. In isolated pancreatic lobules, pretreatment with recombinant human renalase (rRNLS) blocked zymogen activation caused by cerulein, carbachol, and a bile acid. Renalase also blocked cerulein-induced cell injury and histological changes. In the in vivo cerulein model of pancreatitis, genetic deletion of renalase resulted in more severe disease, and administering rRNLS to cerulein-exposed WT mice after pancreatitis onset was protective. Because pathological increases in acinar cell cytosolic calcium levels are central to the initiation of acute pancreatitis, we also investigated whether rRNLS could function through its binding protein, plasma membrane calcium ATPase 4b (PMCA4b), which excretes calcium from cells. We found that PMCA4b is expressed in both murine and human acinar cells and that a PMCA4b-selective inhibitor worsens pancreatitis-induced injury and blocks the protective effects of rRNLS. These findings suggest that renalase is a protective plasma protein that reduces acinar cell injury through a plasma membrane calcium ATPase. Because exogenous rRNLS reduces the severity of acute pancreatitis, it has potential as a therapeutic agent.

Transglutaminase 2 regulates osteoclast differentiation via a Blimp1-dependent pathway

Transglutaminase 2 (TG2) performs multiple reactions, including transamidation, and also plays a role in signal transduction as a GTP-binding protein. In this study, we reveal that TG2 controls osteoclast differentiation and bone homeostasis in mice. Osteoclasts specifically expressed the TG2 isoform among eight TG family members. Suppression in TG2 expression with siRNA led to increased osteoclast formation from primary mouse precursor cells in response to receptor activator of nuclear factor kappaB ligand (RANKL). This osteoclastogenic effect of TG2 knockdown was associated with enhanced induction of c-Fos and NFATc1 by RANKL. Moreover, TG2 knockdown up-regulated B lymphocyte-induced maturation protein 1 (Blimp1), which represses anti-osteoclastogenic genes, in a manner dependent on the NF-κB signaling pathway. To the contrary, TG2 overexpression inhibited osteoclast formation and the expression of osteoclastogenic genes. Consistent with these in vitro results, TG2 knockout mice exhibited lower trabecular bone mass and increased number of osteoclasts compared with wild-type mice. Taken together, our results provide strong evidence that TG2 plays an important role in bone metabolism by suppressing excessive osteoclastogenesis via the regulation of the NF-κB-Blimp1 signaling pathway.

The Plasma Membrane Calcium ATPases and Their Role as Major New Players in Human Disease

The Ca²⁺ extrusion function of the four mammalian isoforms of the plasma membrane calcium ATPases (PMCAs) is well established. There is also ever-increasing detail known of their roles in global and local Ca²⁺ homeostasis and intracellular Ca²⁺ signaling in a wide variety of cell types and tissues. It is becoming clear that the spatiotemporal patterns of expression of the PMCAs and the fact that their abundances and relative expression levels vary from cell type to cell type both reflect and impact on their specific functions in these cells. Over recent years it has become increasingly apparent that these genes have potentially significant roles in human health and disease, with PMCAs1-4 being associated with cardiovascular diseases, deafness, autism, ataxia, adenoma, and malarial resistance. This review will bring together evidence of the variety of tissue-specific functions of PMCAs and will highlight the roles these genes play in regulating normal physiological functions and the considerable impact the genes have on human disease.

A complex of Neuroplastin and Plasma Membrane Ca2+ ATPase controls T cell activation

The outcome of T cell activation is determined by mechanisms that balance Ca²⁺ influx and clearance. Here we report that murine CD4 T cells lacking Neuroplastin (Nptn ^-/-), an immunoglobulin superfamily protein, display elevated cytosolic Ca²⁺ and impaired post-stimulation Ca²⁺ clearance, along with increased nuclear levels of NFAT transcription factor and enhanced T cell receptor-induced cytokine production. On the molecular level, we identified plasma membrane Ca²⁺ ATPases (PMCAs) as the main interaction partners of Neuroplastin. PMCA levels were reduced by over 70% in Nptn ^-/- T cells, suggesting an explanation for altered Ca²⁺ handling. Supporting this, Ca²⁺ extrusion was impaired while Ca²⁺ levels in internal stores were increased. T cells heterozygous for PMCA1 mimicked the phenotype of Nptn ^-/- T cells. Consistent with sustained Ca²⁺ levels, differentiation of Nptn ^-/- T helper cells was biased towards the Th1 versus Th2 subset. Our study thus establishes Neuroplastin-PMCA modules as important regulators of T cell activation.

Zoledronate suppressed angiogenesis and osteogenesis by inhibiting osteoclasts formation and secretion of PDGF-BB

PURPOSE

Bisphosphonates related osteonecrosis of jaw (BRONJ) is a severe complication of systemic BPs administration, the mechanism of which is still unclarified. Recently, platelet-derived growth factor-BB (PDGF-BB) secreted by preosteoclasts was reported to promote angiogenesis and osteogenesis. This study aimed to clarify whether bisphosphonates suppressed preosteoclasts releasing PDGF-BB, and whether the suppression harmed coupling of angiogenesis and osteogenesis, which could contribute to BRONJ manifestation.

METHODS AND RESULTS

Zoledronate significantly inhibited osteoclast formation by tartrate-resistant acid phosphatase (TRAP) staining and PDGF-BB secretion tested by ELISA. In line with decreasing secretion of PDGF-BB by preosteoclasts exposed to zoledronate, conditioned medium (CM) from the cells significantly induced less migration of endothelial progenitor cells (EPCs) and mesenchymal stem cells (MSCs) compared to CM from unexposed preosteoclasts. Meanwhile, angiogenic function of EPCs and osteoblastic differentiation of MSCs also declined when culturing with CM from preosteoclasts treated by zoledronate (PZ-CM), evidenced by tube formation assay of EPCs and alkaline phosphatase activity of MSCs. Western blot assay showed that the expression of VEGF in EPCs and OCN, RUNX2 in MSCs declined when culturing with PZ-CM compared to CM from preostoeclasts without exposure of zoledronate.

CONCLUSION

Our study found that zoledronate was able to suppress preosteoclasts releasing PDGF-BB, resulting in suppression of angiogenesis and osteogenesis. Our study may partly contributed to the mechanism of BRONJ.

Suppression of Osteoclastogenesis by Melatonin: A Melatonin Receptor-Independent Action

In vertebrates, melatonin is primarily secreted from the pineal gland but it affects various biological processes including the sleep-wake cycle, vasomotor control, immune system and bone homeostasis. Melatonin has been known to promote osteoblast differentiation and bone maturation, but a direct role of melatonin on osteoclast differentiation is still elusive. The present study investigated the effect of melatonin on the differentiation of macrophages to osteoclasts. The presence of melatonin significantly reduced receptor activator of nuclear factor κB ligand (RANKL)-induced osteoclastogenesis and the siRNA-mediated knockdown of the melatonin receptor failed to overcome the anti-osteoclastogenic effect of melatonin. Although melatonin treatment did not affect the phosphorylation of extracellular signal-regulated kinase (ERK), p38 and c-Jun N-terminal kinase (JNK), it markedly inhibited the activation of NF-κB and subsequent induction of nuclear factor of activated T cell cytoplasmic 1(NFATc1). Thus, our results suggest that melatonin could suppress osteoclast differentiation through downregulation of NF-κB pathway with concomitant decrease in the NFATc1 transcription factor induction. Furthermore, melatonin seems to have an anti-osteoclastogenic effect independent of plasma membrane melatonin receptors. In addition to previously reported properties of melatonin, our study proposes another aspect of melatonin and bone homeostasis.

Multiple Calcium Export Exchangers and Pumps Are a Prominent Feature of Enamel Organ Cells

Calcium export is a key function for the enamel organ during all stages of amelogenesis. Expression of a number of ATPase calcium transporting, plasma membrane genes (ATP2B1-4/PMCA1-4), solute carrier SLC8A genes (sodium/calcium exchanger or NCX1-3), and SLC24A gene family members (sodium/potassium/calcium exchanger or NCKX1-6) have been investigated in the developing enamel organ in earlier studies. This paper reviews the calcium export pathways that have been described and adds novel insights to the spatiotemporal expression patterns of PMCA1, PMCA4, and NCKX3 during amelogenesis. New data are presented to show the mRNA expression profiles for the four Atp2b1-4 gene family members (PMCA1-4) in secretory-stage and maturation-stage rat enamel organs. These data are compared to expression profiles for all Slc8a and Slc24a gene family members. PMCA1, PMCA4, and NCKX3 immunolocalization data is also presented. Gene expression profiles quantitated by real time PCR show that: (1) PMCA1, 3, and 4, and NCKX3 are most highly expressed during secretory-stage amelogenesis; (2) NCX1 and 3, and NCKX6 are expressed during secretory and maturation stages; (3) NCKX4 is most highly expressed during maturation-stage amelogenesis; and (4) expression levels of PMCA2, NCX2, NCKX1, NCKX2, and NCKX5 are negligible throughout amelogenesis. In the enamel organ PMCA1 localizes to the basolateral membrane of both secretory and maturation ameloblasts; PMCA4 expression is seen in the basolateral membrane of secretory and maturation ameloblasts, and also cells of the stratum intermedium and papillary layer; while NCKX3 expression is limited to Tomes' processes, and the apical membrane of maturation-stage ameloblasts. These new findings are discussed in the perspective of data already present in the literature, and highlight the multiplicity of calcium export systems in the enamel organ needed to regulate biomineralization.

Peucedanum japonicum Thunb. ethanol extract suppresses RANKL-mediated osteoclastogenesis

The constituents of Peucedanum japonicum Thunb. (PJ) exhibit biological and pharmacological activities, including anti-obesity, anti-oxidant and anti-allergic activities. The aim of the present study was to examine in vitro effects of PJ in RANKL-induced signaling pathways, which determine osteoclast differentiation. PJ ethanol extract (PEE) exhibited anti-osteoporotic activity by disrupting the phospholipase C (PLC)-Ca2+-c-Fos/cAMP response element-binding protein (CREB)-nuclear factor of activated T cells, cytoplasmic 1 (NFATc1) signaling pathway during osteoclastogenesis. Murine bone marrow-derived macrophages (BMMs) were cultured and used to determine the effects of PJ in the receptor activator of nuclear factor κB ligand (RANKL)-mediated osteoclastogenesis. The effects of PEE in the RANKL-mediated signaling cascade were evaluated using a standard in vitro osteoclastogenesis system. PEE treatment of BMMs significantly reduced the number of RANKL-mediated tartrate resistant acid phosphatase (TRAP)-positive multinucleated cells (P<0.05 for 5 and 10 µg/ml PEE, P<0.01 for 25 and 50 µg/ml PEE), without cytotoxic effects. Furthermore, the expression of differentiation-related marker genes, including TRAP, Oscar, Cathepsin K, dendrocyte expressed seven transmembrane protein, ATPase H+ Transporting V0 Subunit D2 and NFATc1, were markedly suppressed. PEE induced a transient increase in free cytoplasmic Ca2+ ([Ca2+]_i) mobilization via voltage-gated Ca2+ channels and PLC-sensitive pathways. Transient [Ca2+]_i increase consequently resulted in the suppression of c-Fos, CREB and NFATc1 activities. These findings highlight the potential use of PJ in treating bone disorders caused by osteoclast overgrowth.

Exome sequencing identified rare variants in genes HSPG2 and ATP2B4 in a family segregating developmental dysplasia of the hip

Background

Developmental dysplasia of the hip (DDH) is a common pathological condition of the musculoskeletal system in infants which results in a congenital and developmental malformation of the hip joint. DDH is a spectrum of pathologies affecting the infant hip ranging from asymptomatic subtle radiographic signs through mild instability to frank dislocations with acetabular dysplasia. A Saudi family with three affected individuals with DDH was identified and genetic analysis was performed to detect the possible genetic defect(s) underlying DDH in the affected members of the family.

Methods

We performed whole genome genotyping using Illumina HumanOmni 2.5 M array and whole exome sequencing (WES) using Nextera Rapid capture kit and Illumina NextSeq500 instrument in four individuals of a family with DDH.

Results

SNP data analysis did not identify any runs of homozygosity and copy number variations. Identity-by-descent (IBD) analysis on whole genome genotyping data identified a shared haplotypes on chromosome 1 in affected individuals. An analysis of the WES data identified rare heterozygous variants in HSPG2 and ATP2B4 genes in the affected individuals. Multiple prediction software predicted that the variants identified are damaging. Moreover, in silico analysis showed that HSPG2 regulates ATP2B4 expression using a variety of transcription factors.

Conclusion

Our results indicate that there might be a functional epistatic interaction between HSPG2 and ATP2B4, and DDH in the family studied is due to a combined effect of both variants. These variants are also present in the asymptomatic mother suggesting that the variants in HSPG2 and ATP2B4 are incompletely penetrant. This study provides the first evidence of digenic inheritance of DDH in a family and extends the spectrum of genetic heterogeneity in this human disorder.

Electronic supplementary material

The online version of this article (doi:10.1186/s12881-017-0393-8) contains supplementary material, which is available to authorized users.

Decreased calcium pump expression in human erythrocytes is connected to a minor haplotype in the ATP2B4 gene

Plasma membrane Ca²⁺-ATPases are key calcium exporter proteins in most tissues, and PMCA4b is the main calcium transporter in the human red blood cells (RBCs). In order to assess the expression level of PMCA4b, we have developed a flow cytometry and specific antibody binding method to quantitatively detect this protein in the erythrocyte membrane. Interestingly, we found several healthy volunteers showing significantly reduced expression of RBC-PMCA4b. Western blot analysis of isolated RBC membranes confirmed this observation, and indicated that there are no compensatory alterations in other PMCA isoforms. In addition, reduced PMCA4b levels correlated with a lower calcium extrusion capacity in these erythrocytes. When exploring the potential genetic background of the reduced PMCA4b levels, we found no missense mutations in the ATP2B4 coding regions, while a formerly unrecognized minor haplotype in the predicted second promoter region closely correlated with lower erythrocyte PMCA4b protein levels. In recent GWA studies, SNPs in this ATP2B4 haplotype have been linked to reduced mean corpuscular hemoglobin concentrations (MCHC), and to protection against malaria infection. Our data suggest that an altered regulation of gene expression is responsible for the reduced RBC-PMCA4b levels that is probably linked to the development of human disease-related phenotypes.

SOD2 and Sirt3 Control Osteoclastogenesis by Regulating Mitochondrial ROS

Reactive oxygen species (ROS) are an indispensable element of cellular signal transduction in various cell types, including bone cells. In particular, osteoclasts (OCs), cells specialized for bone resorption, utilize ROS as second messengers during receptor activator of NF-κB ligand (RANKL)-induced differentiation and activation. In addition, because of the high energy demands of bone-resorbing activity, OCs contain large amounts of mitochondria, the source of the majority of total ROS. In this study, we focused on the regulation of ROS generated from mitochondria during osteoclastogenesis. We observed that the level of mitochondrial superoxide dismutase 2 (SOD2), an enzyme responsible for reducing superoxide radicals in mitochondria, was increased by RANKL. siRNA-mediated knockdown (KD) of SOD2 increased ROS levels and enhanced OC differentiation. Conversely, overexpression of SOD2 reduced osteoclastogenesis by decreasing ROS levels. Moreover, we found that NAD-dependent deacetylase sirtuin 3 (Sirt3), an activator of SOD2 in mitochondria, was induced by RANKL. Sirt3-targeted siRNA decreased SOD2 activity by reducing deacetylation of lysine 68 of SOD2, leading to increased osteoclastogenesis. Furthermore, in vivo KD of SOD2 or Sirt3 in ICR mouse calvariae decreased bone volume and increased OC surface, supporting the results of in vitro experiments. Taken together, our findings demonstrate for the first time to our knowledge that the regulation of mitochondrial ROS by SOD2 and Sirt3 plays an important role in fine-tuning the OC differentiation program. © 2016 American Society for Bone and Mineral Research.

Lysosomal Ca2+ Signaling is Essential for Osteoclastogenesis and Bone Remodeling

Lysosomal Ca²⁺ emerges as a critical component of receptor-evoked Ca²⁺ signaling and plays a crucial role in many lysosomal and physiological functions. Lysosomal Ca²⁺ release is mediated by the transient receptor potential (TRP) family member TRPML1, mutations that cause the lysosomal storage disease mucolipidosis type 4. Lysosomes play a key role in osteoclast function. However, nothing is known about the role of lysosomal Ca²⁺ signaling in osteoclastogenesis and bone metabolism. In this study, we addressed this knowledge gap by studying the role of lysosomal Ca²⁺ signaling in osteoclastogenesis, osteoclast and osteoblast functions, and bone homeostasis in vivo. We manipulated lysosomal Ca²⁺ signaling by acute knockdown of TRPML1, deletion of TRPML1 in mice, pharmacological inhibition of lysosomal Ca²⁺ influx, and depletion of lysosomal Ca²⁺ storage using the TRPML agonist ML-SA1. We found that knockdown and deletion of TRPML1, although it did not have an apparent effect on osteoblast differentiation and bone formation, markedly attenuated osteoclast function, RANKL-induced cytosolic Ca²⁺ oscillations, inhibited activation of NFATc1 and osteoclastogenesis-controlling genes, suppressed the formation of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells (MNCs), and markedly reduced the differentiation of bone marrow-derived macrophages into osteoclasts. Moreover, deletion of TRPML1 resulted in enlarged lysosomes, inhibition of lysosomal secretion, and attenuated the resorptive activity of mature osteoclasts. Notably, depletion of lysosomal Ca²⁺ with ML-SA1 similarly abrogated RANKL-induced Ca²⁺ oscillations and MNC formation. Deletion of TRPML1 in mice reduced the TRAP-positive bone surfaces and impaired bone remodeling, resulting in prominent osteopetrosis. These findings demonstrate the essential role of lysosomal Ca²⁺ signaling in osteoclast differentiation and mature osteoclast function, which play key roles in bone homeostasis. © 2016 American Society for Bone and Mineral Research.

Increased bone resorption by osteoclast-specific deletion of the sodium/calcium exchanger isoform 1 (NCX1)

Calcium is a key component of the bone mineral hydroxyapatite. During osteoclast-mediated bone resorption, hydroxyapatite is dissolved and significant quantities of calcium are released. Several calcium transport systems have previously been identified in osteoclasts, including members of the sodium/calcium exchanger (NCX) family. Expression pattern and physiological role of NCX isoforms in osteoclasts, however, remain largely unknown at the moment. Our data indicate that all three NCX isoforms (NCX1, NCX2, and NCX3) are present in murine osteoclasts. RANKL-induced differentiation of murine osteoclast precursors into mature osteoclasts significantly attenuated the expression of NCX1, while NCX2 and NCX3 expressions were largely unaffected. To study the role of NCX1 during osteoclast differentiation and bone resorption, we crossed mice with exon 11 of the NCX1 gene flanked by loxP sites with cathepsin K-Cre transgenic mice. Mature osteoclasts derived from transgenic mice exhibited an 80-90% reduction of NCX1 protein. In vitro studies indicate that NCX1 is dispensable for osteoclast differentiation, but NCX1-deficient osteoclasts exhibited increased resorptive activity. In line with these in vitro findings, mice with an osteoclast-targeted deletion of the NCX1 gene locus displayed an age-dependent loss of bone mass. Thus, in summary, our data reveal NCX1 as a regulator of osteoclast-mediated bone resorption.

Differences in metabolic biomarkers in the blood and gene expression profiles of peripheral blood mononuclear cells among normal weight, mildly obese and moderately obese subjects

We compared metabolic biomarkers in the blood and peripheral blood mononuclear cell (PBMC) gene expression profiles among normal weight (BMI, 18·5–23 kg/m2), mildly obese (BMI, 25–27·5 kg/m2) and moderately obese Korean adult men (BMI, 27·5–30 kg/m2). High leptin, lipids (except LDL- and HDL-cholesterol) and apoB levels and low adiponectin and HDL-cholesterol levels were present in the plasma of both mildly and moderately obese subjects. Circulating levels of inflammatory cytokines and markers of insulin resistance, oxidative stress and liver damage were altered in moderately obese subjects but not in mildly obese subjects. PBMC transcriptome data showed enrichment of pathways involved in energy metabolism, insulin resistance, bone metabolism, cancer, inflammation and fibrosis in both mildly and moderately obese subjects. Signalling pathways involved in oxidative phosphorylation, TAG synthesis, carbohydrate metabolism and insulin production; mammalian target of rapamycin, forkhead box O, ras-proximate-1, RAS and transforming growth factor-β signalling; as well as extracellular matrix–receptor interaction were enriched only in moderately obese subjects, indicating that changes in PBMC gene expression profiles, according to metabolic disturbances, were associated with the development and/or aggravation of obesity. In particular, fourteen and fifteen genes differentially expressed only in mildly obese subjects and in both mildly and moderately obese subjects, respectively, could be used as early or stable biomarkers for diagnosing and treating obesity-associated metabolic disturbance. We characterised BMI-associated metabolic and molecular biomarkers in the blood and provided clues about potential blood-based targets for preventing or treating obesity-related complications.

Calcium Extrusion Pump PMCA4: A New Player in Renal Calcium Handling?

Calcium (Ca²⁺) is vital for multiple processes in the body, and maintenance of the electrolyte concentration is required for everyday physiological function. In the kidney, and more specifically, in the late distal convoluted tubule and connecting tubule, the fine-tuning of Ca²⁺ reabsorption from the pro-urine takes place. Here, Ca²⁺ enters the epithelial cell via the transient receptor potential vanilloid receptor type 5 (TRPV5) channel, diffuses to the basolateral side bound to calbindin-D_28k and is extruded to the blood compartment via the Na⁺/Ca²⁺ exchanger 1 (NCX1) and the plasma membrane Ca²⁺ ATPase (PMCA). Traditionally, PMCA1 was considered to be the primary Ca²⁺ pump in this process. However, in recent studies TRPV5-expressing tubules were shown to highly express PMCA4. Therefore, PMCA4 may have a predominant role in renal Ca²⁺ handling. This study aimed to elucidate the role of PMCA4 in Ca²⁺ homeostasis by characterizing the Ca²⁺ balance, and renal and duodenal Ca²⁺-related gene expression in PMCA4 knockout mice. The daily water intake of PMCA4 knockout mice was significantly lower compared to wild type littermates. There was no significant difference in serum Ca²⁺ level or urinary Ca²⁺ excretion between groups. In addition, renal and duodenal mRNA expression levels of Ca²⁺-related genes, including TRPV5, TRPV6, calbindin-D_28k, calbindin-D_9k, NCX1 and PMCA1 were similar in wild type and knockout mice. Serum FGF23 levels were significantly increased in PMCA4 knockout mice. In conclusion, PMCA4 has no discernible role in normal renal Ca²⁺ handling as no urinary Ca²⁺ wasting was observed. Further investigation of the exact role of PMCA4 in the distal convoluted tubule and connecting tubule is required.

Multifaceted plasma membrane Ca(2+) pumps: From structure to intracellular Ca(2+) handling and cancer

Plasma membrane Ca(2+) ATPases (PMCAs) are intimately involved in the control of intracellular Ca(2+) concentration. They reduce Ca(2+) in the cytosol not only by direct ejection, but also by controlling the formation of inositol-1,4,5-trisphosphate and decreasing Ca(2+) release from the endoplasmic reticulum Ca(2+) pool. In mammals four genes (PMCA1-4) are expressed, and alternative RNA splicing generates more than twenty variants. The variants differ in their regulatory characteristics. They localize into highly specialized membrane compartments and respond to the incoming Ca(2+) with distinct temporal resolution. The expression pattern of variants depends on cell type; a change in this pattern can result in perturbed Ca(2+) homeostasis and thus altered cell function. Indeed, PMCAs undergo remarkable changes in their expression pattern during tumorigenesis that might significantly contribute to the unbalanced Ca(2+) homeostasis of cancer cells. This article is part of a Special Issue entitled: Calcium and Cell Fate. Guest Editors: Jacques Haiech, Claus Heizmann, Joachim Krebs, Thierry Capiod and Olivier Mignen.

Expression of Phospholipase D in Periodontitis and Its Role in the Inflammatory and Osteoclastic Response by Nicotine- and Lipopolysaccharide-Stimulated Human Periodontal Ligament Cells

BACKGROUND

The aim of the present study is to investigate the expression of phospholipase D (PLD) 1 and PLD2 in periodontal patients and in human periodontal ligament cells (HPDLCs) exposed to nicotine plus lipopolysaccharide (LPS) from Porphyromonas gingivalis (Toll-like receptor 2 ligand). Furthermore, the effects of PLD isoform inhibition on the inflammatory response and osteoclast differentiation and its mechanisms were determined.

METHODS

Proinflammatory mediators were examined by reverse transcription-polymerase chain reaction and enzyme-linked immunosorbent assay. To silence the gene expression of the PLD isoforms, cells were transfected with small interfering RNA (siRNA) targeting PLD1 or PLD2. Mouse bone marrow-derived macrophages (BMMs) were used as osteoclast precursor cells for in vitro osteoclastogenesis. Western blot analysis and immunofluorescence were used to assess signaling pathways.

RESULTS

Chronic smokers with periodontitis exhibited significantly higher PLD1 and PLD2 messenger RNA (mRNA) expression than non-smokers with periodontitis and healthy controls. Nicotine and LPS upregulated PLD1 and PLD2 mRNA expression in a dose-dependent manner in HPDLCs. Pharmacologic and siRNA-mediated inhibition of PLD1 and PLD2 attenuated the nicotine- and LPS-induced upregulation of inducible nitric oxide (NO) synthase and cyclooxygenase-2, production of NO, and prostaglandin E2, and mRNA expression and secretion of tumor necrosis factor-α, interleukin (IL)-1β, and IL-8. The conditioned media from HPDLCs treated with PLD isoform inhibitors or siRNA against PLD inhibited receptor activator of nuclear factor-κB (NF-κB) ligand-mediated osteoclast differentiation, as well as protein expression of nuclear factor of activated T cells c1 and c-Fos, in BMMs. In addition, PLD isoform inhibitors and siRNA inhibited the nicotine- and LPS-induced activation of phosphoinositide 3-kinase, protein kinase C, p38, extracellular signal-regulated kinase, c-Jun N-terminal protein kinase, mitogen-activated protein kinase, and NF-κB.

CONCLUSION

To the best of the authors' knowledge, this study is the first to demonstrate that PLD isoform inhibition has anti-inflammatory and antiosteoclastogenic effects and thus may be a therapeutic target for the treatment of periodontitis.

Plasma membrane calcium ATPases: From generic Ca(2+) sump pumps to versatile systems for fine-tuning cellular Ca(2.)

The plasma membrane calcium ATPases (PMCAs) are ATP-driven primary ion pumps found in all eukaryotic cells. They are the major high-affinity calcium extrusion system for expulsion of Ca(2+) ions from the cytosol and help restore the low resting levels of intracellular [Ca(2+)] following the temporary elevation of Ca(2+) generated during Ca(2+) signaling. Due to their essential role in the maintenance of cellular Ca(2+) homeostasis they were initially thought to be "sump pumps" for Ca(2+) removal needed by all cells to avoid eventual calcium overload. The discovery of multiple PMCA isoforms and alternatively spliced variants cast doubt on this simplistic assumption, and revealed instead that PMCAs are integral components of highly regulated multi-protein complexes fulfilling specific roles in calcium-dependent signaling originating at the plasma membrane. Biochemical, genetic, and physiological studies in gene-manipulated and mutant animals demonstrate the important role played by specific PMCAs in distinct diseases including those affecting the peripheral and central nervous system, cardiovascular disease, and osteoporosis. Human PMCA gene mutations and allelic variants associated with specific disorders continue to be discovered and underline the crucial role of different PMCAs in particular cells, tissues and organs.

Ultrastructural and immunohistochemical localization of plasma membrane Ca2+-ATPase 4 in Ca2+-transporting epithelia

Plasma membrane Ca(2+)-ATPases (PMCAs) participate in epithelial Ca(2+) transport and intracellular Ca(2+) signaling. The Pmca4 isoform is enriched in distal nephron isolates and decreased in mice lacking the epithelial transient receptor potential vanilloid 5 Ca(2+) channel. We therefore hypothesized that Pmca4 plays a significant role in transcellular Ca(2+) flux and investigated the localization and regulation of Pmca4 in Ca(2+)-transporting epithelia. Using antibodies directed specifically against Pmca4, we found it expressed only in the smooth muscle layer of mouse and human intestines, whereas pan-specific Pmca antibodies detected Pmca1 in lateral membranes of enterocytes. In the kidney, Pmca4 showed broad localization to the distal nephron. In the mouse, expression was most abundant in segments coexpressing the epithelial ransient receptor potential vanilloid 5 Ca(2+) channel. Significant, albeit lower, expression was also evident in the region encompassing the cortical thick ascending limbs, macula densa, and early distal tubules as well as smooth muscle layers surrounding renal vessels. In the human kidney, a similar pattern of distribution was observed, with the highest PMCA4 expression in Na(+)-Cl(-) cotransporter-positive tubules. Electron microscopy demonstrated Pmca4 localization in distal nephron cells at both the basolateral membrane and intracellular perinuclear compartments but not submembranous vesicles, suggesting rapid trafficking to the plasma membrane is unlikely to occur in vivo. Pmca4 expression was not altered by perturbations in Ca(2+) balance, pointing to a housekeeping function of the pump in Ca(2+)-transporting epithelia. In conclusion, Pmca4 shows a divergent expression pattern in Ca(2+)-transporting epithelia, inferring diverse roles for this isoform not limited to transepithelial Ca(2+) transport.

Identification of a receptor for extracellular renalase

Background

An increased risk for developing essential hypertension, stroke and diabetes is associated with single nucleotide gene polymorphisms in renalase, a newly described secreted flavoprotein with oxidoreductase activity. Gene deletion causes hypertension, and aggravates acute ischemic kidney (AKI) and cardiac injury. Independent of its intrinsic enzymatic activities, extracellular renalase activates MAPK signaling and prevents acute kidney injury (AKI) in wild type (WT) mice. Therefore, we sought to identity the receptor for extracellular renalase.

Methods and Results

RP-220 is a previously identified, 20 amino acids long renalase peptide that is devoid of any intrinsic enzymatic activity, but it is equally effective as full-length recombinant renalase at protecting against toxic and ischemic injury. Using biotin transfer studies with RP-220 in the human proximal tubular cell line HK-2 and protein identification by mass spectrometry, we identified PMCA4b as a renalase binding protein. This previously characterized plasma membrane ATPase is involved in cell signaling and cardiac hypertrophy. Co-immunoprecipitation and co-immunolocalization confirmed protein-protein interaction between endogenous renalase and PMCA4b. Down-regulation of endogenous PMCA4b expression by siRNA transfection, or inhibition of its enzymatic activity by the specific peptide inhibitor caloxin1b each abrogated RP-220 dependent MAPK signaling and cytoprotection. In control studies, these maneuvers had no effect on epidermal growth factor mediated signaling, confirming specificity of the interaction between PMCA4b and renalase.

Conclusions

PMCA4b functions as a renalase receptor, and a key mediator of renalase dependent MAPK signaling.

Plasma membrane calcium ATPase isoform 4 inhibits vascular endothelial growth factor-mediated angiogenesis through interaction with calcineurin

OBJECTIVE

Vascular endothelial growth factor (VEGF) has been identified as a crucial regulator of physiological and pathological angiogenesis. Among the intracellular signaling pathways triggered by VEGF, activation of the calcineurin/nuclear factor of activated T cells (NFAT) signaling axis has emerged as a critical mediator of angiogenic processes. We and others previously reported a novel role for the plasma membrane calcium ATPase (PMCA) as an endogenous inhibitor of the calcineurin/NFAT pathway, via interaction with calcineurin, in cardiomyocytes and breast cancer cells. However, the functional significance of the PMCA/calcineurin interaction in endothelial pathophysiology has not been addressed thus far.

APPROACH AND RESULTS

Using in vitro and in vivo assays, we here demonstrate that the interaction between PMCA4 and calcineurin in VEGF-stimulated endothelial cells leads to downregulation of the calcineurin/NFAT pathway and to a significant reduction in the subsequent expression of the NFAT-dependent, VEGF-activated, proangiogenic genes RCAN1.4 and Cox-2. PMCA4-dependent inhibition of calcineurin signaling translates into a reduction in endothelial cell motility and blood vessel formation that ultimately impairs in vivo angiogenesis by VEGF.

CONCLUSIONS

Given the importance of the calcineurin/NFAT pathway in the regulation of pathological angiogenesis, targeted modulation of PMCA4 functionality might open novel therapeutic avenues to promote or attenuate new vessel formation in diseases that occur with angiogenesis.

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.

NFAT1 and NFAT3 cooperate with HDAC4 during regulation of alternative splicing of PMCA isoforms in PC12 cells

Background

The bulk of human genes undergo alternative splicing (AS) upon response to physiological stimuli. AS is a great source of protein diversity and biological processes and is associated with the development of many diseases. Pheochromocytoma is a neuroendocrine tumor, characterized by an excessive Ca²⁺-dependent secretion of catecholamines. This underlines the importance of balanced control of calcium transport via regulation of gene expression pattern, including different calcium transport systems, such as plasma membrane Ca²⁺-ATPases (PMCAs), abundantly expressed in pheochromocytoma chromaffin cells (PC12 cells). PMCAs are encoded by four genes (Atp2b1, Atp2b2, Atp2b3, Atp2b4), whose transcript products undergo alternative splicing giving almost 30 variants.

Results

In this scientific report, we propose a novel mechanism of regulation of PMCA alternative splicing in PC12 cells through cooperation of the nuclear factor of activated T-cells (NFAT) and histone deacetylases (HDACs). Luciferase assays showed increased activity of NFAT in PC12 cells, which was associated with altered expression of PMCA. RT-PCR experiments suggested that inhibition of the transcriptional activity of NFAT might result in the rearrangement of PMCA splicing variants in PC12 cells. NFAT inhibition led to dominant expression of 2x/c, 3x/a and 4x/a PMCA variants, while in untreated cells the 2w,z/b, 3z,x/b,c,e,f, and 4x/b variants were found as well. Furthermore, chromatin immunoprecipitation experiments showed that NFAT1-HDAC4 or NFAT3-HDAC4 complexes might be involved in regulation of PMCA2x splicing variant generation.

Conclusions

We suggest that the influence of NFAT/HDAC on PMCA isoform composition might be important for altered dopamine secretion by PC12 cells.

Calcineurin/NFAT signaling represses genes Vamp1 and Vamp2 via PMCA-dependent mechanism during dopamine secretion by Pheochromocytoma cells

BACKGROUND

Plasma membrane Ca(2+)-ATPases (PMCA) extrude Ca(2+) ions out of the cell and contribute to generation of calcium oscillations. Calcium signaling is crucial for transcriptional regulation of dopamine secretion by neuroendocrine PC12 cells. Low resting [Ca(2+)]c in PC12 cells is maintained mainly by two Ca(2+)-ATPases, PMCA2 and PMCA3. Recently, we found that Ca(2+) dependent phosphatase calcineurin was excessively activated under conditions of experimental downregulation of PMCA2 or PMCA3. Thus, the aim of this study was to explain if, via modulation of the Ca(2+)/calcineurin-dependent nuclear factor of activated T cells (NFAT) pathway, PMCA2 and PMCA3 affect intracellular signaling in pheochromocytoma/neuronal cells/PC12 cells. Secondly, we tested whether this might influence dopamine secretion by PC12 cells.

RESULTS

PMCA2- and PMCA3-deficient cells displayed profound decrease in dopamine secretion accompanied by a permanent increase in [Ca(2+)]c. Reduction in secretion might result from changes in NFAT signaling, following altered PMCA pattern. Consequently, activation of NFAT1 and NFAT3 transcription factors was observed in PMCA2- or PMCA3-deficient cells. Furthermore, chromatin immunoprecipitation assay indicated that NFATs could be involved in repression of Vamp genes encoding vesicle associated membrane proteins (VAMP).

CONCLUSIONS

PMCA2 and PMCA3 are crucial for dopamine secretion in PC12 cells. Reduction in PMCA2 or PMCA3 led to calcium-dependent activation of calcineurin/NFAT signaling and, in consequence, to repression of the Vamp gene and deterioration of the SNARE complex formation in PC12 cells.

Coordinated regulation of TRPV5-mediated Ca²⁺ transport in primary distal convolution cultures

Fine-tuning of renal calcium ion (Ca(2+)) reabsorption takes place in the distal convoluted and connecting tubules (distal convolution) of the kidney via transcellular Ca(2+) transport, a process controlled by the epithelial Ca(2+) channel Transient Receptor Potential Vanilloid 5 (TRPV5). Studies to delineate the molecular mechanism of transcellular Ca(2+) transport are seriously hampered by the lack of a suitable cell model. The present study describes the establishment and validation of a primary murine cell model of the distal convolution. Viable kidney tubules were isolated from mice expressing enhanced Green Fluorescent Protein (eGFP) under the control of a TRPV5 promoter (pTRPV5-eGFP), using Complex Object Parametric Analyser and Sorting (COPAS) technology. Tubules were grown into tight monolayers on semi-permeable supports. Radioactive (45)Ca(2+) assays showed apical-to-basolateral transport rates of 13.5 ± 1.2 nmol/h/cm(2), which were enhanced by the calciotropic hormones parathyroid hormone and 1,25-dihydroxy vitamin D3. Cell cultures lacking TRPV5, generated by crossbreeding pTRPV5-eGFP with TRPV5 knockout mice (TRPV5(-/-)), showed significantly reduced transepithelial Ca(2+) transport (26 % of control), for the first time directly confirming the key role of TRPV5. Most importantly, using this cell model, a novel molecular player in transepithelial Ca(2+) transport was identified: mRNA analysis revealed that ATP-dependent Ca(2+)-ATPase 4 (PMCA4) instead of PMCA1 was enriched in isolated tubules and downregulated in TRPV5(-/-) material. Immunohistochemical stainings confirmed co-localization of PMCA4 with TRPV5 in the distal convolution. In conclusion, a novel primary cell model with TRPV5-dependent Ca(2+) transport characteristics was successfully established, enabling comprehensive studies of transcellular Ca(2+) transport.