Polyphosphoinositides-dependent regulation of the osteoclast actin cytoskeleton and bone resorption

Glia maturation factor beta deficiency protects against diabetic osteoporosis by suppressing osteoclast hyperactivity

Excessive osteoclast activation, which depends on dramatic changes in actin dynamics, causes osteoporosis (OP). The molecular mechanism of osteoclast activation in OP related to type 1 diabetes (T1D) remains unclear. Glia maturation factor beta (GMFB) is considered a growth and differentiation factor for both glia and neurons. Here, we demonstrated that Gmfb deficiency effectively ameliorated the phenotype of T1D-OP in rats by inhibiting osteoclast hyperactivity. In vitro assays showed that GMFB participated in osteoclast activation rather than proliferation. Gmfb deficiency did not affect osteoclast sealing zone (SZ) formation but effectively decreased the SZ area by decreasing actin depolymerization. When GMFB was overexpressed in Gmfb-deficient osteoclasts, the size of the SZ area was enlarged in a dose-dependent manner. Moreover, decreased actin depolymerization led to a decrease in nuclear G-actin, which activated MKL1/SRF-dependent gene transcription. We found that pro-osteoclastogenic factors (Mmp9 and Mmp14) were downregulated, while anti-osteoclastogenic factors (Cftr and Fhl2) were upregulated in Gmfb KO osteoclasts. A GMFB inhibitor, DS-30, targeting the binding site of GMFB and Arp2/3, was obtained. Biocore analysis revealed a high affinity between DS-30 and GMFB in a dose-dependent manner. As expected, DS-30 strongly suppressed osteoclast hyperactivity in vivo and in vitro. In conclusion, our work identified a new therapeutic strategy for T1D-OP treatment. The discovery of GMFB inhibitors will contribute to translational research on T1D-OP.

The Ethanol Extracts of Osmanthus fragrans Leaves Ameliorate the Bone Loss via the Inhibition of Osteoclastogenesis in Osteoporosis

The aim of this study was to evaluate the anti-osteoporosis effects of Osmanthus fragrans leaf ethanol extract (OFLEE) in bone marrow-derived macrophages (BMM) and animals with osteoporosis. OFLEE not only suppressed tartrate-resistant acid phosphatase (TRAP)-positive cells with multiple nuclei but also decreased TRAP activity in BMM treated with macrophage colony stimulating factor (M-CSF) and receptor activator of nuclear factor-κB (NF-κB) ligand (RANKL). The formation of F-actin rings and the expression and activation of matrix metalloproteinases were decreased by OFLEE in BMM treated with M-CSF and RANKL. OFLEE suppressed M-CSF- and RANKL-induced osteoclastogenesis by inhibiting NF-κB phosphorylation, tumor necrosis factor receptor-associated factor 6, c-fos, the nuclear factor of activated T-cells, cytoplasmic 1, and cathepsin K in BMM. OFLEE downregulated reactive oxygen species, cyclooxygenase-2, inducible nitric oxide synthase, prostaglandin E₂, tumor necrosis factor α, interleukin (IL)-1β, IL-6, IL-17, and RANKL in BMM treated with M-CSF and RANKL. Oral administration of OFLEE suppressed osteoporotic bone loss without hepatotoxicity in ovariectomy-induced osteoporosis animals. Our findings suggest that OFLEE, with anti-inflammatory effects, prevents osteoporotic bone loss through the suppression of osteoclastic differentiation in BMM and animals with osteoporosis.

L-Plastin Phosphorylation: Possible Regulation by a TNFR1 Signaling Cascade in Osteoclasts

Tumor necrosis factor-alpha (TNF-α) signaling regulates phosphorylation of L-plastin, which is involved in forming the nascent sealing zone, a precursor zone for the matured sealing ring. This study aimed to illustrate the molecular mechanisms of L-plastin phosphorylation and the subsequent formation of the nascent sealing zone in osteoclasts treated with TNF-α. Here, we report that anti-TNF-receptor 1, inhibitors of signaling proteins (Src, PI3-K, Rho, and Rho-kinase), and siRNA of TRAF-6 attenuated the phosphorylation of LPL and filamentous actin content significantly in the presence of TNF-α. An inhibitor of integrin αvβ3, PKC, or PKA did not inhibit TNF-α-induced L-plastin phosphorylation. Inhibitors of Src and PI3-K and not Rho or Rho-kinase reduced tyrosine phosphorylation of TRAF-6, suggesting that Src and PI3-K regulate TRAF-6 phosphorylation, and Rho and Rho-kinase are downstream of TRAF-6 regulation. Osteoclasts expressing constitutively active or kinase-defective Src proteins were used to determine the role of Src on L-plastin phosphorylation; similarly, the effect of Rho was confirmed by transducing TAT-fused constitutively active (V14) or dominant-negative (N19) Rho proteins into osteoclasts. Pull-down analysis with glutathione S-transferase-fused SH2 and SH3 domains of Src and PI3-K demonstrated coprecipitation of L-plastin and TRAF-6 with the SH3 and SH2 domains of the PI3-K and Src proteins. However, the actual order of the interaction of proteins requires further elucidation; a comprehensive screening should corroborate the initial findings of protein interactions via the SH2/SH3 domains. Ultimately, inhibition of the interaction of proteins with SH2/SH3 could reduce L-plastin phosphorylation and affect NSZ formation and bone resorption in conditions that display osteoclast activation and bone loss.

Bothrops moojeni Venom and Its Components Strongly Affect Osteoclasts' Maturation and Protein Patterns

Osteoclasts (OCs) are important for bone maintenance, calcium balance, and tissue regeneration regulation and are involved in different inflammatory diseases. Our study aimed to evaluate the effect of Bothrops moojeni's venom and its low and high molecular mass (HMM and LMM) fractions on human peripheral blood mononuclear cell (PBMC)-derived OCs' in vitro differentiation. Bothrops moojeni, a Brazilian lanced-head viper, presents a rich but not well-explored, venom composition. This venom is a potent inducer of inflammation, which can be used as a tool to investigate the inflammatory process. Human PBMCs were isolated and induced to OC differentiation following routine protocol. On the fourth day of differentiation, the venom was added at different concentrations (5, 0.5, and 0.05 µg/mL). We observed a significant reduction of TRAP+ (tartrate-resistant acid phosphatase) OCs at the concentration of 5 µg/mL. We evaluated the F-actin-rich OCs structure's integrity; disruption of its integrity reflects bone adsorption capacity. F-actin rings phalloidin staining demonstrated that venom provoked their disruption in treated OCs. HMM, fraction reduces TRAP+ OCs at a concentration of 5 µg/mL and LMM fraction at 1 µg/mL, respectively. Our results indicate morphological changes that the venom induced cause in OCs. We analyzed the pattern of soluble proteins found in the conditioned cell culture medium OCs treated with venom and its fractions using mass spectrometry (LC-MS/IT-Tof). The proteomic analyses indicate the possible pathways and molecular mechanisms involved in OC reduction after the treatment.

Fisetin antagonizes cell fusion, cytoskeletal organization and bone resorption in RANKL-differentiated murine macrophages

Osteoclastogenesis is comprised of several stage s including progenitor survival, differentiation to mononuclear preosteoclasts, cell fusion to multinuclear mature osteoclasts, and activation to osteoclasts with bone resorbing activity. Botanical antioxidants are now being increasingly investigated for their health-promoting effects on bone. This study investigated that fisetin, a flavonol found naturally in many fruits and vegetables, suppressed osteoclastogenesis by disturbing receptor activator of nuclear factor (NF)-κB ligand (RANKL)-mediated signaling pathway and demoting osteoclastogenic protein induction. Nontoxic fisetin at ≤10 μM inhibited the induction of RANK, tumor necrosis factor receptor associated factor 6 (TRAF6) and the activation of NF-κB in RANKL-stimulated RAW 264.7 macrophages. In RANKL-differentiated osteoclasts cell fusion protein of E-cadherin was induced, which was dampened by fisetin. The formation of tartrate-resistance acid phosphatase-positive multinucleated osteoclasts was suppressed by adding fisetin to RANKL-exposed macrophages. It was also found that fisetin reduced actin ring formation and gelsolin induction of osteclasts enhanced by RANKL through disturbing c-Src-proline-rich tyrosine kinase 2 signaling. Fisetin deterred preosteoclasts from the cell-cell fusion and the organization of the cytoskeleton to seal the resorbing area and to secret protons for bone resorption. Consistently, the 5 day-treatment of fisetin diminished RANKL-induced cellular expression of carbonic anhydrase II and integrin β3 concurrently with a reduction of osteoclast bone-resorbing activity. Therefore, fisetin was a natural therapeutic agent retarding osteoclast fusion and cytoskeletal organization such as actin rings and ruffled boarder, which is a property of mature osteoclasts and is required for osteoclasts to resorb bone.

Nm23-h1 binds to gelsolin and inactivates its actin-severing capacity to promote tumor cell motility and metastasis

Nm23-H1 has been identified as a metastasis suppressor gene, but its protein interactions have yet to be understood with any mechanistic clarity. In this study, we evaluated the proteomic spectrum of interactions made by Nm23-H1 in 4T1 murine breast cancer cells derived from tissue culture, primary mammary tumors, and pulmonary metastases. By this approach, we identified the actin-severing protein Gelsolin as binding partner for Nm23-H1, verifying their interaction by coimmunoprecipitation in 4T1 cells as well as in human MCF7, MDA-MB-231T, and MDA-MB-435 breast cancer cells. In Gelsolin-transfected cells, coexpression of Nm23-H1 abrogated the actin-severing activity of Gelsolin. Conversely, actin severing by Gelsolin was abrogated by RNA interference-mediated silencing of endogenous Nm23-H1. Tumor cell motility was negatively affected in parallel with Gelsolin activity, suggesting that Nm23-H1 binding inactivated the actin-depolymerizing function of Gelsolin to inhibit cell motility. Using indirect immunoflourescence to monitor complexes formed by Gelsolin and Nm23-H1 in living cells, we observed their colocalization in a perinuclear cytoplasmic compartment that was associated with the presence of disrupted actin stress fibers. In vivo analyses revealed that Gelsolin overexpression increased the metastasis of orthotopically implanted 4T1 or tail vein-injected MDA-MB-231T cells (P = 0.001 and 0.04, respectively), along with the proportion of mice with diffuse liver metastases, an effect ablated by coexpression of Nm23-H1. We observed no variation in proliferation among lung metastases. Our findings suggest a new actin-based mechanism that can suppress tumor metastasis.

Nm23-h1 binds to gelsolin and inactivates its actin-severing capacity to promote tumor cell motility and metastasis

Nm23-H1 has been identified as a metastasis suppressor gene, but its protein interactions have yet to be understood with any mechanistic clarity. In this study, we evaluated the proteomic spectrum of interactions made by Nm23-H1 in 4T1 murine breast cancer cells derived from tissue culture, primary mammary tumors, and pulmonary metastases. By this approach, we identified the actin-severing protein Gelsolin as binding partner for Nm23-H1, verifying their interaction by coimmunoprecipitation in 4T1 cells as well as in human MCF7, MDA-MB-231T, and MDA-MB-435 breast cancer cells. In Gelsolin-transfected cells, coexpression of Nm23-H1 abrogated the actin-severing activity of Gelsolin. Conversely, actin severing by Gelsolin was abrogated by RNA interference-mediated silencing of endogenous Nm23-H1. Tumor cell motility was negatively affected in parallel with Gelsolin activity, suggesting that Nm23-H1 binding inactivated the actin-depolymerizing function of Gelsolin to inhibit cell motility. Using indirect immunoflourescence to monitor complexes formed by Gelsolin and Nm23-H1 in living cells, we observed their colocalization in a perinuclear cytoplasmic compartment that was associated with the presence of disrupted actin stress fibers. In vivo analyses revealed that Gelsolin overexpression increased the metastasis of orthotopically implanted 4T1 or tail vein-injected MDA-MB-231T cells (P = 0.001 and 0.04, respectively), along with the proportion of mice with diffuse liver metastases, an effect ablated by coexpression of Nm23-H1. We observed no variation in proliferation among lung metastases. Our findings suggest a new actin-based mechanism that can suppress tumor metastasis.

Type I phosphotidylinosotol 4-phosphate 5-kinase γ regulates osteoclasts in a bifunctional manner

Type 1 phosphotidylinosotol-4 phosphate 5 kinase γ (PIP5KIγ) is central to generation of phosphotidylinosotol (4,5)P(2) (PI(4,5)P(2)). PIP5KIγ also participates in cytoskeletal organization by delivering talin to integrins, thereby enhancing their ligand binding capacity. As the cytoskeleton is pivotal to osteoclast function, we hypothesized that absence of PIP5KIγ would compromise their resorptive capacity. Absence of the kinase diminishes PI(4,5) abundance and desensitizes precursors to RANK ligand-stimulated differentiation. Thus, PIP5KIγ(-/-) osteoclasts are reduced in number in vitro and confirm physiological relevance in vivo. Despite reduced numbers, PIP5KIγ(-/-) osteoclasts surprisingly have normal cytoskeletons and effectively resorb bone. PIP5KIγ overexpression, which increases PI(4,5)P(2), also delays osteoclast differentiation and reduces cell number but in contrast to cells lacking the kinase, its excess disrupts the cytoskeleton. The cytoskeleton-disruptive effects of excess PIP5KIγ reflect its kinase activity and are independent of talin recognition. The combined arrested differentiation and disorganized cytoskeleton of PIP5KIγ-transduced osteoclasts compromises bone resorption. Thus, optimal PIP5KIγ and PI(4,5)P(2) expression, by osteoclasts, are essential for skeletal homeostasis.

RNA therapeutics targeting osteoclast-mediated excessive bone resorption

RNA interference (RNAi) is a sequence-specific post-transcriptional gene silencing technique developed with dramatically increasing utility for both scientific and therapeutic purposes. Short interfering RNA (siRNA) is currently exploited to regulate protein expression relevant to many therapeutic applications, and commonly used as a tool for elucidating disease-associated genes. Osteoporosis and their associated osteoporotic fragility fractures in both men and women are rapidly becoming a global healthcare crisis as average life expectancy increases worldwide. New therapeutics are needed for this increasing patient population. This review describes the diversity of molecular targets suitable for RNAi-based gene knock down in osteoclasts to control osteoclast-mediated excessive bone resorption. We identify strategies for developing targeted siRNA delivery and efficient gene silencing, and describe opportunities and challenges of introducing siRNA as a therapeutic approach to hard and connective tissue disorders.

Elevated cytokine production restores bone resorption by human Btk-deficient osteoclasts

Mutations in Bruton's tyrosine kinase (Btk) cause the B-cell disorder X-linked agammaglobulinaemia (XLA) in humans, but the effect of Btk deficiency in human bone health has not been investigated previously. In this study, we show that human Btk-deficient osteoclasts are defective at resorption activity in vitro owing to a dysregulation of the actin cytoskeletal function. Contrary to expectation, XLA patients did not exhibit increased bone density or alterations in serum markers of bone turnover, indicating that a potential compensation mechanism normalizes bone homeostasis. In contrast to the bone turnover markers, the levels of inflammatory cytokines interleukin 6 (IL-6), IL-1β, and tumor necrosis factor α (TNF-α) were significantly elevated in XLA patients' serum compared with control individuals. Supplementation of osteoclast cultures from normal and XLA subjects with serum from XLA patients or recombinant inflammatory cytokines IL-6, IL-1β, and TNF-α resulted in a stimulation of osteoclast activity in vitro, whereas the addition of cytokine-neutralizing antibodies inhibited this stimulatory effect, confirming that elevated inflammatory cytokines in XLA serum heightened osteoclast activity in vitro. This study provides novel evidence that Btk signaling is crucial for optimal actin cytoskeletal organization and lacunar resorption in isolated osteoclasts. In XLA patients, however, these inherent osteoclast defects are corrected by increased inflammatory cytokine levels, restoring osteoclast activity and leading to the normalization of bone density.

Phosphatidylinositol 4,5-bisphosphate and PIP5-kinase Ialpha are required for invadopodia formation in human breast cancer cells

Invadopodia are ventral cell protrusions formed in invasive cancer cells. Because invadopodia have extracellular matrix (ECM) degradation activity, they are thought to function in cancer invasion. In this study, we examined the roles of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)] and PI(4,5)P(2)-producing enzymes in invadopodia formation in MDA-MB-231 human breast cancer cells. Immunofluorescence analysis showed that PI(4,5)P(2) accumulates at invadopodia on the ventral cell surface. Injection of an anti-PI(4,5)P(2) antibody inhibited invadopodia formation along with gelatin degradation activity. Sequestering of PI(4,5)P(2) by overexpression of the phospholipase C (PLC) delta1-pleckstrin homology (PH) domain, a specific probe for PI(4,5)P(2), also blocked invadopodia formation, while a mutated PLCdelta1-PH domain that lacks PI(4,5)P(2)-binding activity had no effect. Cellular PI(4,5)P(2) production is mainly mediated by type-I phosphatidylinositol 4-phosphate 5-kinase (PIP5KI) family proteins, which include PIP5KIalpha, Ibeta, and Igamma. Real-time quantitative PCR analysis showed that PIP5KIalpha is a dominant isoform expressed in MDA-MB-231 cells. Knockdown of PIP5KIalpha by small-interfering RNA (siRNA) inhibited invadopodia formation and gelatin degradation. Immunofluorescence analysis revealed that endogenous PIP5KIalpha protein localizes at invadopodia, which is corroborated by the observation that exogenously expressed green fluorescent protein (GFP)-fused PIP5KIalpha protein also accumulates at gelatin degradation sites. These results indicate that localized production of PI(4,5)P(2) by PIP5KIalpha is required for invadopodia formation and ECM degradation by human breast cancer cells.

Podosomes and Invadopodia: Related structures with Common Protein Components that May Promote Breast Cancer Cellular Invasion

A rate-limiting step in breast cancer progression is acquisition of the invasive phenotype, which can precede metastasis. Expression of cell-surface proteases at the leading edge of a migrating cell provides cells with a mechanism to cross tissue barriers. A newly appreciated mechanism that may be relevant for breast cancer cell invasion is the formation of invadopodia, well-defined structures that project from the ventral membrane and promote degradation of the extracellular matrix, allowing the cell to cross a tissue barrier. Recently, there has been some controversy and discussion as to whether invadopodia, which are associated with carcinoma cells, are related to a similar structure called podosomes, which are associated with normal cells. Invadopodia and podosomes share many common characteristics, including a similar size, shape, subcellular localization and an ability to promote invasion. These two structures also share many common protein components, which we outline herein. It has been speculated that podosomes may be precursors to invadopodia and by extension both structures may be relevant to cancer cell invasion. Here, we compare and contrast the protein components of invadopodia and podosomes and discuss a potential role for these proteins and the evidence that supports a role for invadopodia and podosomes in breast cancer invasion.

Invadopodia and matrix degradation, a new property of prostate cancer cells during migration and invasion

The present study demonstrated that invadopodia are associated with invasion by degradation of matrix in prostate cancer cells PC3. To find out the presence of invadopodia in PC3 cells, we performed a few comparative analyses with osteoclasts, which utilize podosomes for migration. Our investigations indeed demonstrated that invadopodia are comparable to podosomes in the localization of Wiskott-Aldrich syndrome protein (WASP)/matrix metalloproteinase-9 and the degradation of matrix. Invadopodia are different from podosomes in the localization of actin/vinculin, distribution during migration, and the mode of degradation of extracellular matrix. Invadopodia enable polarized invasion of PC3 cells into the gelatin matrix in a time-dependent manner. Gelatin degradation was confined within the periphery of the cell. Osteoclasts demonstrated directional migration with extensive degradation of matrix underneath and around the osteoclasts. A pathway of degradation of matrix representing a migratory track was observed due to the rearrangement of podosomes as rosettes or clusters at the leading edge. Reducing the matrix metalloproteinase-9 levels by RNA interference inhibited the degradation of matrix but not the formation of podosomes or invadopodia. Competition experiments with TAT-fused WASP peptides suggest that actin polymerization and formation of invadopodia involve the WASP-Arp2/3 complex pathway. Moreover, PC3 cells overexpressing osteopontin (OPN) displayed an increase in the number of invadopodia and gelatinolytic activity as compared with PC3 cells and PC3 cells expressing mutant OPN in integrin-binding domain and null for OPN. Thus, we conclude that OPN/integrin alphavbeta3 signaling participates in the process of migration and invasion of PC3 cells through regulating processes essential for the formation and function of invadopodia.

Dramatic inhibition of osteoclast sealing ring formation and bone resorption in vitro by a WASP-peptide containing pTyr294 amino acid

Wiskott Aldrich Syndrome protein (WASP) has a unique regulatory role in sealing ring formation and bone resorption in osteoclasts. Here, using the TAT-transduction method, we show the possible role of WASP domain(s) in sealing ring formation and bone resorption. Transduction of TAT-fused full-length WASP peptide induced Arp2/3 complex formation, F-actin content, sealing ring formation and bone resorption. Transduction of WASP peptides containing basic, verpolin-central, pTyr294, and proline-rich regions inhibited the processes listed above at various levels. The ability to resorb bone by WASP peptides containing basic, verpolin-central, and proline-rich regions was reduced and the resorbed area matched the size of the sealing ring. However, osteoclasts transduced with WASP peptide containing pTyr294aa demonstrated the following: a) a considerable decrease in the interaction and phosphorylation of c-Src with endogenous WASP; b) total loss of sealing ring-like structures; c) formation of actin-rich patches at the peripheral edge that contains filopodia-like projections; d) reduced capacity for bone resorption in vitro. These findings suggest that modulation of phosphorylation state of pTyr294aa assists in integrating multiple signaling molecule and pathways that partake in the assembly of sealing ring.

"Actin"g on GLUT4: membrane & cytoskeletal components of insulin action

The dissection of mechanisms that regulate glucose transport by insulin has revealed an intricate network of signaling molecules scattered from the insulin receptor to the intracellular glucose transporter GLUT4. It is also appreciated that some insulin receptor signals jaunt in different directions to regulate events essential for the efficient redistribution of GLUT4 to the plasma membrane. Moreover key assists in the process appear to be arranged by membrane lipids and cytoskeletal proteins. Following current considerations of insulin signals regulating GLUT4, this review will focus on in vitro and in vivo evidence that supports an essential role for phosphoinositides and actin filaments in the control of glucose transport. The discussion will visit recent cell culture, whole animal, and human data highlighting membrane and cytoskeletal aspects of insulin resistance.

Hela l-CaD is implicated in the migration of endothelial cells/endothelial progenitor cells in human neoplasms

Caldesmon (CaD) is a major actin-binding protein distributed in a variety of cell types. No functional differences among the isoforms in in vitro studies were found so far. In a previous study we found that the low molecular caldesmon isoform (Hela l-CaD) is expressed in endothelial cells (ECs)/endothelial progenitor cells (EPCs) in tumor vasculature of various human tumors. Activation of cell motility is necessary for the navigation of the tip ECs during angiogenesis, and migration of EPCs from the bone marrow during vasculogenesis. In the present study we searched for features of motility and the intracellular expression sites of Hela l-CaD in ECs/EPCs of various human tumors under histologically preserved microenviroment. We discovered a variety of motility-related cell protrusions like filopodia, microspikes, lamellipodia, podosomes, membrane blebs and membrane ruffles in the activated ECs/EPCs. Hela l-CaD appeared to be invariably expressed in the subregions of these cell protrusions. The findings suggest that Hela l-CaD is implicated in the migration of ECs/EPC in human neoplasms where they contribute to tumor vasculogenesis and angiogenesis.

Identification of novel RANK polymorphisms and their putative association with low BMD among postmenopausal women

INTRODUCTION

Bone mineral density (BMD) is the major factor for determining bone strength, which is closely correlated to osteoporotic fracture risk and is largely determined by multiple genetic factors. The RANK (TNFRSF11A), receptor for RANKL, is a member of the tumor necrosis factor receptor (TNFR) superfamily and plays a central role in osteoclast development.

METHODS

In order to investigate the effects of RANK polymorphism on BMD and osteoporosis, we directly sequenced the RANK gene in 24 Korean individuals and identified 25 sequence variants. Eleven of these polymorphisms were selected and genotyped in a larger-scale study of postmenopausal women (n = 560). Areal BMD (g/cm(2)) of the anterior-posterior lumbar spine and the nondominant proximal femur were measured using dual-energy X-ray absorptiometry.

RESULTS

We found that two intronic polymorphisms in the RANK gene [RANK + 34863G > A (rs12458117) and RANK + 35928insdelC (new polymorphism found in this study) in intron 6] were significantly associated with the BMD of the lumbar spine, i.e., rare alleles were significantly associated with low BMD of the lumbar spine among Korean postmenopausal women (p = 0.04 and 0.02, respectively). These polymorphisms were also associated with low BMD of proximal femur sites, including Ward's triangle, trochanter, and total femur. Our results suggest that +34863G > A and +35928insdelC polymorphisms in RANK are possible genetic factors for low BMD in postmenopausal women.

New insights into the regulation of V-ATPase-dependent proton secretion

The vacuolar H(+)-ATPase (V-ATPase) is a key player in several aspects of cellular function, including acidification of intracellular organelles and regulation of extracellular pH. In specialized cells of the kidney, male reproductive tract and osteoclasts, proton secretion via the V-ATPase represents a major process for the regulation of systemic acid/base status, sperm maturation and bone resorption, respectively. These processes are regulated via modulation of the plasma membrane expression and activity of the V-ATPase. The present review describes selected aspects of V-ATPase regulation, including recycling of V-ATPase-containing vesicles to and from the plasma membrane, assembly/disassembly of the two domains (V(0) and V(1)) of the holoenzyme, and the coupling ratio between ATP hydrolysis and proton pumping. Modulation of the V-ATPase-rich cell phenotype and the pathophysiology of the V-ATPase in humans and experimental animals are also discussed.

Regulation of podosomes by integrin αvβ3 and Rho GTPase-facilitated phosphoinositide signaling

In osteoclasts, polyphosphoinositides such as phosphatidylinositol 4,5 bisphosphate (PI(4,5)P2) and phosphatidylinositol 3,4,5 trisphosphate (PI(3,4,5)P3) are produced in response to integrin alphavbeta3 signaling and they have a critical role in actin cytoskeleton remodeling. The levels of PI(4,5)P2 and PI(3,4,5)P3 are regulated by Rho GTPase through the activation of phosphatidylinositol 4-phosphate 5-kinase (PI4P-5 kinase) and phospatidylinositol 3-kinase (PI3 kinase), respectively. Interaction of PI(4,5)P2 with gelsolin and Wiscott-Aldrich syndrome protein (WASP) is critical for podosome assembly/disassembly and actin ring formation in osteoclasts. Interaction of PI(3,4,5)P3 with gelsolin functions in orchestrating the podosome signaling complex consisting of several key signaling molecules. Gelsolin deficiency has been shown to block podosome assembly and motility in mouse osteoclasts. However, these osteoclasts are able to form a WASP-containing actin ring and retain their resorptive function. The TAT-mediated delivery of gelsolin phosphoinositide-binding domains into osteoclasts resulted in production of podosome clusters and disruption of actin ring formation. Hence, these osteoclasts were hypomotile and less resorptive. Our observations suggest that both PI(4,5)P2 and PI(3,4,5)P3 are involved in regulating osteoclast functions through modulation of severing, capping, and nucleating functions of actin-binding proteins.

Role of acid/base transporters in the male reproductive tract and potential consequences of their malfunction

Acid/base transporters play a key role in establishing an acidic luminal environment for sperm maturation and storage in the male reproductive tract. Impairment of the acidification capacity of the epididymis, via either genetic mutations or exposure to environmental factors, may have profound consequences on male fertility.

Regulation of protein activities by phosphoinositide phosphates

Phosphoinositide phosphates (PIPs) correspond to phosphorylated derivatives of phosphatidylinositol (PI). Despite their relatively low abundance in the plasma membrane, PIPs play a crucial role as precursors of second messengers and are themselves important signaling and targeting molecules. Indeed, modulation of levels of PIPs affects, for example, cortical actin organization, membrane dynamics, and cell migration. The focus of this review is on selected interesting targets of PIPs. Those proteins that bind PIPs and are involved in regulation of actin assembly, actin membrane linkage, and actin contractility are discussed, as well as those that are involved in signaling, such as small GTPases, protein kinases, and phosphatases, or in regulation of membrane dynamics.

Interactions Between Vacuolar H+-ATPases and Microfilaments in Osteoclasts

Vacuolar H(+)-ATPases (V-ATPases) are transported from cytosolic compartments to the ruffled plasma membrane of osteoclasts as they activate to resorb bone. Transport of V-ATPases is essential for bone resorption, and is associated with binding interactions between V-ATPases and microfilaments that are mediated by an actin-binding site in subunit B. This site is contained within 44 amino acids in the amino terminal domain, and requires a sequence motif that resembles an actin-binding motif found in mammalian profilin 1. Small alterations in the profilin-like sequence disrupt the actin-binding activity of subunit B. The interaction between V-ATPases and microfilaments in osteoclasts is regulated in response to changes in phosphatidylinositol-3 kinase activity. During internalization of V-ATPases from the plasma membrane of osteoclasts after a cycle of resorption, V-ATPases bind microfilaments that are in podosomes, dynamic actin-based structures, also present in metastatic cancer cells. Studies are ongoing to establish the physiological role of the microfilament-binding activity of subunit B in osteoclasts and in other cells.

Transcytosis of calcium from bone by osteoclast-like cells evidenced by direct visualization of calcium in cells

'Transcytosis' of calcium (Ca) from bone by osteoclasts was identified by using a newly developed method that uses fixed or living osteoclast-like cells previously differentiated in vitro, a Ca-specific cell-membrane-impermeable fluorescent dye, and confocal laser scanning microscopy. This method, called the cell-membrane-impermeable dye method, revealed that in fixed osteoclast-like cells, a large quantity of Ca was confined within vacuoles and transported toward the apical cell membrane in the cells. These Ca-confined vacuoles were co-localized with marker proteins of both ruffled border and lysosome. The vacuoles were disrupted when treated with an inhibitor of ruffled border ATPase. In living osteoclast-like cells, Ca-confined vacuoles were again preferentially located at the central region and near the apical cell membrane. These results suggest actual transcytosis of Ca from bone by osteoclasts, and are the first direct evidence of the significant role of osteoclasts in the entire process of Ca metabolism in bone.

Association of the OSCAR promoter polymorphism with BMD in postmenopausal women

In an effort to identify genetic polymorphisms in potential candidate genes for osteoporosis, 10 variants were identified in the OSCAR gene using direct DNA sequencing, and 560 postmenopausal women were genotyped at five SNP loci, using the TaqMan method. The rare allele (G allele) of OSCAR-2322A>G (SNP in the 5' flanking region) showed significant association with lower BMD at various bone sites in postmenopausal women (n = 560).

INTRODUCTION

BMD is the major factor for determining bone strength and osteoporotic fracture risk and is determined by both environmental and multiple genetic factors. The osteoclast-associated receptor (OSCAR) plays a critical role in osteoclast differentiation and thus is an important candidate gene for the modulation of BMD.

MATERIALS AND METHODS

Through direct sequencing in 24 Korean individuals, 10 sequence variants were identified: 2 in the 5' flanking region, 7 in the exons (including 6 nonsynonymous single-nucleotide polymorphisms [SNPs]), and 1 in an intron. Five of these polymorphisms were selected for larger-scale genotyping in postmenopausal women (n = 560). Areal BMD (g/cm2) of the anterior-posterior lumbar spine and the nondominant proximal femur was measured using DXA (Lunar Expert XL and Hologic QDR 4500-A). Lateral thoracolumbar radiographs were obtained in all subjects.

RESULTS

Using multiple regression analysis and controlling for age, years since menopause, height, weight, and evaluation machine as covariates, the rare allele (G allele) of OSCAR-2322A>G showed significant association with lower BMD at various bone sites in postmenopausal women.

CONCLUSION

These findings suggest that the promoter variant in OSCAR gene (OSCAR-2322A>G) might be one of genetic determinants of BMD in postmenopausal women.

Regulation of actin ring formation by rho GTPases in osteoclasts

Actin ring formation is a prerequisite for osteoclast bone resorption. Although gelsolin null osteoclasts failed to exhibit podosomes, actin ring was observed in these osteoclasts. Wiscott-Aldrich syndrome protein (WASP) was observed in the actin ring of gelsolin null osteoclast. Osteoclasts stimulated with osteopontin simulated the effects of Rho and Cdc42 in phosphatidylinositol 4,5-bisphosphate (PIP2) association with WASP as well as formation of podosomes, peripheral microfilopodia-like structures, and actin ring. To explore the potential functions of Rho and Cdc42, TAT-mediated delivery of Rho proteins into osteoclasts was performed. Although Rho and Cdc42 are required for actin ring formation, transduction of either one of the proteins alone is insufficient for this process. Addition of osteopontin to osteoclasts transduced with Cdc42Val12 or transduction of osteoclasts with both RhoVal14 and Cdc42Val12 augments the formation of WASP-Arp2/3 complex and actin ring. Neomycin, an antibiotic, blocked the effects of osteopontin or TAT-RhoVal14 on PIP2 interaction with WASP. WASP distribution was found to be cytosolic in these osteoclasts. Depletion of WASP by short interfering RNA-mediated gene silencing blocked actin polymerization as well as actin ring formation in osteoclasts. These results suggest that Rho-mediated PIP2 interaction with WASP may contribute to the activation and membrane targeting of WASP. Subsequent interaction of Cdc42 and Arp2/3 with WASP may enhance cortical actin polymerization in the process of actin ring formation in osteoclasts.