Hiroshi Kajiya

The Association of Notch2 and NF-κB Accelerates RANKL-Induced Osteoclastogenesis

ABSTRACT Notch signaling plays a key role in various cell differentiation processes including bone homeostasis. However, the specific involvement of Notch in regulating osteoclastogenesis is still controversial. In the present study, we show that RANKL induces expression of Jagged1 and Notch2 in bone marrow macrophages during osteoclast differentiation. Suppression of Notch signaling by a selective γ-secretase inhibitor or Notch2 short hairpin RNA suppresses RANKL-induced osteoclastogenesis. In contrast, induction of Notch signaling by Jagged1 or by ectopic expression of intracellular Notch2 enhances NFATc1 promoter activity and expression and promotes osteoclastogenesis. Finally, we found that Notch2 and p65 interact in the nuclei of RANKL-stimulated cells and that both proteins are recruited to the NFATc1 promoter, driving its expression. Taken together, our results show a new molecular cross talk between Notch and NF-κB pathways that is relevant in osteoclastogenesis.

Alpha9beta1: a novel osteoclast integrin that regulates osteoclast formation and function.

We identified a previously unknown integrin, α9β1, on OCLs and their precursors. Antibody to α9 inhibited OCL formation in human marrow cultures, and OCLs from α9 knockout mice had a defect in actin ring reorganization and an impaired bone resorption capacity.

α9β1: A Novel Osteoclast Integrin That Regulates Osteoclast Formation and Function

We identified a previously unknown integrin, α9β1, on OCLs and their precursors. Antibody to α9 inhibited OCL formation in human marrow cultures, and OCLs from α9 knockout mice had a defect in actin ring reorganization and an impaired bone resorption capacity.

Parathyroid-hormone-related Protein Induces Expression of Receptor Activator of NF-κB Ligand in Human Periodontal Ligament Cells via a cAMP/Protein Kinase A-independent Pathway

Periodontal ligament (PDL) cells play important roles in root resorption of human deciduous teeth by odontoclasts (osteoclast-like cells). However, it is unclear how PDL cells regulate osteoclastogenesis. We examined the effects of PTHrP, TGF-β, and EGF, which are all secreted by the tooth germ, on tartrate-resistant acid-phosphatase-positive (TRAP+) cell formation using co-cultures of human PDL cells and mouse spleen cells. Only PTHrP promoted TRAP+ cell formation in co-cultures. PTHrP induced receptor activator of NF-κB ligand (RANKL) mRNA expression and slightly reduced osteoprotegerin (OPG) expression in PDL cells. The cAMP/PKA inhibitors Rp-cAMP, H89, and PKI did not affect PTHrP-induced TRAP+ cell formation. The PKC inhibitor, Ro-32-0432, suppressed RANKL expression in PDL cells and PTHrP-induced TRAP+ cell formation. However, this inhibitor directly modulated the number of osteoclast precursors. Thus, PTHrP induces osteoclastogenesis by increasing the relative expression level of RANKL vs. OPG in PDL cells via a cAMP/PKA-independent pathway. Abbreviations: PTHrP, parathyroid-hormone-related protein; TGF-β, transforming growth factor-β; EGF, epidermal growth factor; RANKL, receptor activator of NF-κB ligand; OPG, osteoprotegerin; PDL, periodontal ligament; TRAP, tartrate-resistant acid phosphatase; PKA, protein kinase A; PKC, protein kinase C; MAP, mitogen-activated protein; ERK, extracellular signal-regulated kinase; cAMP, cyclic Adenosine 3′5′-Monophosphate.

Protein tyrosine kinase inhibitors increase cytosolic calcium and inhibit actin organization as resorbing activity in rat osteoclasts

Although there is evidence that protein tyrosine kinase inhibitors (PTKIs) suppress bone resorption activity, the mechanism of action of these compounds on osteoclastic bone resorption remains obscure. In the present study, we investigated the effect of PTKIs on cytosolic Ca2+ concentration ([Ca2+]i) and on the cytoskeleton in rat osteoclasts. The PTKIs, genistein and herbimycin A, reversibly elevated [Ca2+]i measured by fura‐2 microfluorimetry. The PTKI‐induced increase was abolished by omission of extracellular Ca2+, but was not attenuated by depletion of Ca2+ stores. The PTKI‐induced increase was inhibited by addition of La3+ and Ni2+, but not abolished by dihydropyridine (DHP) Ca2+ channel blockers. Genistin, an inactive analogue of genistein, had no effect on [Ca2+]i. In the cytoskeleton assay, genistein rapidly disrupted the actin ring formation that serves as a marker for the resorbing state of osteoclasts. Disruption of the actin ring formation was also diminished in Ca2+‐free extracellular solution. These results suggest that PTKIs in rat osteoclasts elevate [Ca2+]i via activation of a DHP‐insensitive, nonspecific Ca2+ entry pathway and disrupt the formation of actin rings, resulting in suppression of bone resorption activity. The regulation of this Ca2+‐influx by PTKIs is likely to contribute to inhibition of bone resorption by these compounds. J. Cell. Physiol. 183:83–90, 2000.

Calcium/calmodulin‐signaling supports TRPV4 activation in osteoclasts and regulates bone mass

Osteoclast differentiation is critically dependent on calcium (Ca2+) signaling. Transient receptor potential vanilloid 4 (TRPV4), mediates Ca2+ influx in the late stage of osteoclast differentiation and thereby regulates Ca2+ signaling. However, the system‐modifying effect of TRPV4 activity remains to be determined. To elucidate the mechanisms underlying TRPV4 activation based on osteoclast differentiation, TRPV4 gain‐of‐function mutants were generated by the amino acid substitutions R616Q and V620I in TRPV4 and were introduced into osteoclast lineage in Trpv4 null mice to generate Trpv4R616Q/V620I transgenic mice. As expected, TRPV4 activation in osteoclasts increased the number of osteoclasts and their resorption activity, thereby resulting in bone loss. During in vitro analysis, Trpv4R616Q/V620I osteoclasts showed activated Ca2+/calmodulin signaling compared with osteoclasts lacking Trpv4. In addition, studies of Trpv4R616Q/V620I mice that lacked the calmodulin‐binding domain indicated that bone loss due to TRPV4 activation was abrogated by loss of interactions between Ca2+/calmodulin signaling and TRPV4. Finally, modulators of TRPV4 interactions with the calmodulin‐binding domain were investigated by proteomic analysis. Interestingly, nonmuscle myosin IIa was identified by liquid chromatography–tandem mass spectroscopy (LC‐MS/MS) analysis, which was confirmed by immunoblotting following coimmunoprecipitation with TRPV4. Furthermore, myosin IIa gene silencing significantly reduced TRPV4 activation concomitant with impaired osteoclast maturation. These results indicate that TRPV4 activation reciprocally regulates Ca2+/calmodulin signaling, which involves an association of TRPV4 with myosin IIa, and promotes sufficient osteoclast function.

Calcium signaling in osteoclast differentiation and bone resorption.

Calcium (Ca(2+)) signaling controls multiple cellular functions and is regulated by the release of Ca(2+) from internal stores and its entry from the extracellular fluid. Ca(2+) signals in osteoclasts are essential for diverse cellular functions including differentiation, bone resorption and gene transcription. Recent studies have highlighted the importance of intracellular Ca(2+) signaling for osteoclast differentiation. Receptor activator of NF-κB ligand (RANKL) signaling induces oscillatory changes in intracellular Ca(2+) concentrations, resulting in Ca(2+)/calcineurin-dependent dephosphorylation and activation of nuclear factor of activated T cells c1 (NFATc1), which translocates to the nucleus and induces osteoclast-specific gene transcription to allow differentiation of osteoclasts. Recently, some reports indicated that RANKL-induced Ca(2+) oscillation involved not only repetitive intracellular Ca(2+) release from inositol 1, 4, 5-triphosphate channels in Ca(2+) store sites, but also via store-operated Ca(2+) entry and Ca(2+) entry via transient receptor potential V channels during osteoclast differentiation. Ca(2+)-regulatory cytokines and elevation of extracellular Ca(2+) concentrations have been shown to increase intracellular Ca(2+) concentrations ([Ca(2+)](i)) in mature osteoclasts, regulating diverse cellular functions. RANKL-induced [Ca(2+)](i) increase has been reported to inhibit cell motility and the resorption of cytoskeletal structures in mature osteoclasts, resulting in suppression of bone-resorption activity. In conclusion, Ca(2+) signaling activates differentiation in osteoclast precursors but suppresses resorption in mature osteoclasts. This chapter focuses on the roles of long-term Ca(2+) oscillations in differentiation and of short-term Ca(2+) increase in osteoclastic bone resorption activity.

RANKL-induced TRPV2 expression regulates osteoclastogenesis via calcium oscillations

The receptor activator of NFκB ligand (RANKL) induces Ca(2+) oscillations and activates the Nuclear Factor of Activated T cells 1 (NFATc1) during osteoclast differentiation (osteoclastogenesis). Ca(2+) oscillations are an important trigger signal for osteoclastogenesis, however the molecular basis of Ca(2+) permeable influx pathways serving Ca(2+) oscillations has not yet been identified. Using a DNA microarray, we found that Transient Receptor Potential Vanilloid channels 2 (TRPV2) are expressed significantly in RANKL-treated RAW264.7 cells (preosteoclasts) compared to untreated cells. Therefore, we further investigated the expression and functional role of TRPV2 on Ca(2+) oscillations and osteoclastogenesis. We found that RANKL dominantly up-regulates TRPV2 expression in preosteoclasts, and evokes spontaneous Ca(2+) oscillations and a transient inward cation current in a time-dependent manner. TRPV inhibitor ruthenium red and tetracycline-induced TRPV2 silencing significantly decreased both the frequency of Ca(2+) oscillations and the transient inward currents in RANKL-treated preosteoclasts. Silencing of store-operated Ca(2+) entry (SOCE) proteins similarly suppressed both RANKL-induced oscillations and currents in preosteoclasts. Furthermore, suppression of TRPV2 also reduced RANKL-induced NAFTc1 expression, its nuclear translocation, and osteoclastogenesis. In summary, Ca(2+) oscillations in preosteoclasts are triggered by RANKL-dependent TRPV2 and SOCE activation and intracellular Ca(2+) release. Subsequent activation of NFATc1 promotes osteoclastogenesis.

Intracellular ClC-3 chloride channels promote bone resorption in vitro through organelle acidification in mouse osteoclasts.

ClC-7 Cl(-) channels expressed in osteoclasts are important for bone resorption since it has been shown that disruption of the ClCN7 gene in mice leads to severe osteopetrosis. We have previously reported that Cl(-) currents recorded from mouse osteoclasts resemble those of ClC-3 Cl(-) channels. The aim of the present study was to determine the expression of ClC-3 channels in mouse osteoclasts and their functional role during bone resorption. We detected transcripts for both ClC-7 and ClC-3 channels in mouse osteoclasts by RT-PCR. The expression of ClC-3 was confirmed by immunocytochemical staining. Mouse osteoclasts lacking ClC-3 Cl(-) channels (ClC-3(-/-) osteoclasts) derived from ClCN3 gene-deficient mice (ClC-3(-/-)) showed lower bone resorption activity compared with ClC-3+/+ osteoclasts derived from wild-type mice (ClC-3+/+). Treatment of ClC-3+/+ osteoclasts with small interfering RNA (siRNA) against ClC-3 also significantly reduced bone resorption activity. Electrophysiological properties of basal and hypotonicity-induced Cl(-) currents in ClC-3(-/-) osteoclasts did not differ significantly from those in ClC-3+/+ osteoclasts. Using immunocytochemistry, ClC-3 was colocalized with lysosome-associated membrane protein 2. Using pH-sensitive dyes, organelle acidification activity in ClC-3(-/-) osteoclasts was weaker than in ClC-3+/+ osteoclasts. Treatment of ClC-3+/+ osteoclasts with siRNA against ClC-3 also reduced the organelle acidification activity. In conclusion, ClC-3 Cl(-) channels are expressed in intracellular organelles of mouse osteoclasts and contribute to osteoclastic bone resorption in vitro through organelle acidification.

Ca2+ extrusion via Na+-Ca2+ exchangers in rat odontoblasts.

INTRODUCTION Intracellular Ca(2+) is essential to many signal transduction pathways, and its level is tightly regulated by the Ca(2+) extrusion system in the plasma membrane, which includes the Na(+)-Ca(2+) exchanger (NCX). Although expression of NCX1 isoforms has been demonstrated in odontoblasts, the detailed properties of NCX remain to be clarified. In this study, we investigated localization and ion-transporting/pharmacologic properties of NCX isoforms in rat odontoblasts. METHODS We characterized both the reverse and forward modes of NCX activity in odontoblasts in a dental pulp slice preparation. Ca(2+) influx by reverse NCX activity was measured by fura-2 fluorescence. Ca(2+) efflux by forward NCX activity elicited inward Na(+) current as measured by perforated-patch clamp recording. For immunohistochemical analysis, cryostat sections of incisors were incubated with antibodies against NCX. RESULTS Immunohistochemical observation revealed localization of NCX1 and NCX3 in the distal membrane of odontoblasts. Inward currents by forward NCX activity showed dependence on external Na(+). Fura-2 fluorescence measurement revealed that Ca(2+) influx by reverse NCX activity depended on extracellular Ca(2+) concentration, and that this influx was blocked by NCX inhibitor KB-R7943 in a concentration-dependent manner. However, Ca(2+) influx by NCX showed a slight sensitivity to SEA0400 (a potent NCX1 inhibitor), indicating that expression potencies in odontoblasts were NCX3 > NCX1. CONCLUSIONS These results suggest that odontoblasts express NCX1 and NCX3 at the distal membrane, and that these isoforms play an important role in the Ca(2+) extrusion system as well as in the directional Ca(2+) transport pathway from the circulation to the dentin-mineralizing front.