Versatile roles of V-ATPases accessory subunit Ac45 in osteoclast formation and function

ATP6AP2, a regulator of LRP6/β-catenin protein trafficking, promotes Wnt/β-catenin signaling and bone formation in a cell type dependent manner

Wnt/β-catenin signaling is critical for various cellular processes in multiple cell types, including osteoblast (OB) differentiation and function. Exactly how Wnt/β-catenin signaling is regulated in OBs remain elusive. ATP6AP2, an accessory subunit of V-ATPase, plays important roles in multiple cell types/organs and multiple signaling pathways. However, little is known whether and how ATP6AP2 in OBs regulates Wnt/β-catenin signaling and bone formation. Here we provide evidence for ATP6AP2 in the OB-lineage cells to promote OB-mediated bone formation and bone homeostasis selectively in the trabecular bone regions. Conditionally knocking out (CKO) ATP6AP2 in the OB-lineage cells (Atp6ap2Ocn-Cre) reduced trabecular, but not cortical, bone formation and bone mass. Proteomic and cellular biochemical studies revealed that LRP6 and N-cadherin were reduced in ATP6AP2-KO BMSCs and OBs, but not osteocytes. Additional in vitro and in vivo studies revealed impaired β-catenin signaling in ATP6AP2-KO BMSCs and OBs, but not osteocytes, under both basal and Wnt stimulated conditions, although LRP5 was decreased in ATP6AP2-KO osteocytes, but not BMSCs. Further cell biological studies uncovered that osteoblastic ATP6AP2 is not required for Wnt3a suppression of β-catenin phosphorylation, but necessary for LRP6/β-catenin and N-cadherin/β-catenin protein complex distribution at the cell membrane, thus preventing their degradation. Expression of active β-catenin diminished the OB differentiation deficit in ATP6AP2-KO BMSCs. Taken together, these results support the view for ATP6AP2 as a critical regulator of both LRP6 and N-cadherin protein trafficking and stability, and thus regulating β-catenin levels, demonstrating an un-recognized function of osteoblastic ATP6AP2 in promoting Wnt/LRP6/β-catenin signaling and trabecular bone formation.

Pteryxin suppresses osteoclastogenesis and prevents bone loss via inhibiting the MAPK/Ca2+ signaling pathways mediated by ROS

Osteoporosis, as a severe public health problem worldwide, causes systemic damage to bone mass, strength, and microstructure with an increased propensity for fragility fractures. Given the inherent adverse effects associated with long-term use of current prescription medications for osteoporosis treatment, identifying natural alternatives to existing treatment methods is imperative. Pteryxin (PTX), a natural coumarin, is isolated from the Peucedanum species belonging to the family Apiaceae. PTX has been reported to have antioxidant, anti-inflammatory and anti-obesity properties. However, its effect on osteoporosis has not been clarified. Our study confirmed that PTX could attenuate the formation of osteoclasts and bone resorption on a dose-dependent basis in vitro. Consistently, in vivo ovariectomy (OVX)-induced osteoporosis models simulating the physiological characteristics of postmenopausal women showed that PTX could partially reverse the bone loss caused by OVX. Further study of its mechanism revealed that PTX might block the MAPK and Ca2+-calcineurin-NFATc1 signaling pathways by decreasing the reactive oxygen species (ROS) level in osteoclasts to dampen the expression of critical transcriptional NFATc1 and downstream osteoclast-specific genes. Overall, PTX may present a new or alternative treatment option for osteoporosis.

Lonafarnib Inhibits Farnesyltransferase via Suppressing ERK Signaling Pathway to Prevent Osteoclastogenesis in Titanium Particle-Induced Osteolysis

Wear debris after total joint arthroplasty can attract the recruitment of macrophages, which release pro-inflammatory substances, triggering the activation of osteoclasts, thereby leading to periprosthetic osteolysis (PPOL) and aseptic loosening. However, the development of pharmacological strategies targeting osteoclasts to prevent periprosthetic osteolysis has not been fruitful. In this study, we worked toward researching the effects and mechanisms of a farnesyltransferase (FTase) inhibitor Lonafarnib (Lon) on receptor activator of nuclear factor κB (NF-κB) ligand (RANKL)-induced osteoclastogenesis and bone resorption, as well as the impacts of Lon on titanium particle-induced osteolysis. To investigate the impacts of Lon on bone resorption and osteoclastogenesis in vitro, bone marrow macrophages were incubated and stimulated with RANKL and macrophage colony-stimulating factor (M-CSF). The influence of Lon on osteolysis prevention in vivo was examined utilizing a titanium particle-induced mouse calvarial osteolysis model. The osteoclast-relevant genes expression was explored by real-time quantitative PCR. Immunofluorescence was used to detect intracellular localization of nuclear factor of activated T cells 1 (NFATc1). SiRNA silence assay was applied to examine the influence of FTase on osteoclasts activation. Related signaling pathways, including NFATc1 signaling, NF-κB, mitogen-activated protein kinases pathways were identified by western blot assay. Lon was illustrated to suppress bone resorptive function and osteoclastogenesis in vitro, and it also reduced the production of pro-inflammatory substances and prevented titanium particle-induced osteolysis in vivo. Lon decreased the expression of osteoclast-relevant genes and suppressed NFATc1 nuclear translocation and auto-amplification. Mechanistically, Lon dampened FTase, and inhibition of FTase reduced osteoclast formation by suppressing ERK signaling. Lon is a promising treatment option for osteoclast-related osteolysis diseases including periprosthetic osteolysis by targeted inhibition of FTase through suppressing ERK signaling.

Pantoprazole (PPZ) Inhibits RANKL-Induced Osteoclast Formation and Function In Vitro and Prevents Lipopolysaccharide- (LPS-) Induced Inflammatory Calvarial Bone Loss In Vivo

Bone remodeling is a process delicately balanced between osteoclastic bone resorption and osteoblastic bone formation. Osteoclasts (OCs) are multinucleated giant cells formed through the fusion of monocytic precursors of the hematopoietic stem cells lineage. OCs are the exclusive cells responsible for the resorption and degradation of the mineralized bone matrix. Pantoprazole (PPZ), a proton pump inhibitor (PPI), is commonly prescribed to reduce excess gastric acid production for conditions such as gastroesophageal reflux disease and peptic ulcer disease. Studies have found contradictory effects of PPI therapy on bone metabolism due to the lack of understanding of the exact underlying mechanism. In this study, we found that PPZ inhibits receptor activator of nuclear factor-κB (NF-κB) ligand- (RANKL-) induced osteoclastogenesis from bone marrow monocytic/macrophage (BMMs) precursors and the bone-resorbing activity of mature OCs. Correspondingly, the expression of OC marker genes was also attenuated. At the molecular level, PPZ treatment was associated with reduced activation of the ERK MAPK signaling pathways crucial to OC differentiation. Additionally, the in vivo administration of PPZ protected mice against lipopolysaccharide- (LPS-) induced inflammatory calvarial bone erosion, as a result of the reduced number and activity of OCs on the calvarial bone surface. Although PPI use is associated with increased risk of osteoporosis and bone fractures, our study provides evidence for the direct inhibitory effect of PPZ on OC formation and bone resorption in vitro and in vivo, suggesting a potential therapeutic use of PPZ in the treatment of osteolytic disease with localized bone destruction.

Silencing of Ac45 Simultaneously Inhibits Osteoclast-Mediated Bone Resorption and Attenuates Dendritic Cell-Mediated Inflammation through Impairing Acidification and Cathepsin K Secretion

Endodontic disease is characterized by inflammation and destruction of periapical tissues, leading to severe bone resorption and tooth loss. ATP6AP1 (Ac45) has been implicated in human immune diseases, yet the mechanism underlying how Ac45 regulates immune response and reaction in inflammatory diseases remains unknown. We generated endodontic disease mice through bacterial infection as an inflammatory disease model and used adeno-associated virus (AAV)-mediated Ac45 RNA interference knockdown to study the function of Ac45 in periapical inflammation and bone resorption. We demonstrated that the AAV small hairpin RNA targeting Ac45 (AAV-sh-Ac45) impaired cellular acidification, extracellular acidification, and bone resorption. Our results showed that local delivery of AAV-sh-Ac45 in periapical tissues in bacterium-induced inflammatory lesions largely reduced bone destruction, inhibited inflammation, and dramatically reduced mononuclear immune cells. T-cell, macrophage, and dendritic cell infiltration in the periapical lesion was dramatically reduced, and the periodontal ligament was protected from inflammation-induced destruction. Furthermore, AAV-sh-Ac45 significantly reduced osteoclast formation and the expression of proinflammatory cytokines, such as tumor necrosis factor alpha, interleukin-10 (IL-10), IL-12, IL-1α, IL-6, and IL-17. Interestingly, AAV-sh-Ac45 impaired mature cathepsin K secretion more significantly than that by AAV-sh-C1 and AAV-sh-CtsK Unbiased genome-wide transcriptome sequencing analysis of Ctsk ^-/- dendritic cells stimulated with lipopolysaccharide demonstrated that the ablation of Ctsk dramatically reduced dendritic cell-mediated inflammatory signaling. Taken together, our results indicated that AAV-sh-Ac45 simultaneously inhibits osteoclast-mediated bone resorption and attenuates dendritic cell-mediated inflammation through impairing acidification and cathepsin K secretion. Thus, Ac45 may be a novel target for therapeutic approaches to attenuate inflammation and bone erosion in endodontic disease and other inflammation-related osteolytic diseases.

Sugary Logistics Gone Wrong: Membrane Trafficking and Congenital Disorders of Glycosylation

Glycosylation is an important post-translational modification for both intracellular and secreted proteins. For glycosylation to occur, cargo must be transported after synthesis through the different compartments of the Golgi apparatus where distinct monosaccharides are sequentially bound and trimmed, resulting in increasingly complex branched glycan structures. Of utmost importance for this process is the intraorganellar environment of the Golgi. Each Golgi compartment has a distinct pH, which is maintained by the vacuolar H+-ATPase (V-ATPase). Moreover, tethering factors such as Golgins and the conserved oligomeric Golgi (COG) complex, in concert with coatomer (COPI) and soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-mediated membrane fusion, efficiently deliver glycosylation enzymes to the right Golgi compartment. Together, these factors maintain intra-Golgi trafficking of proteins involved in glycosylation and thereby enable proper glycosylation. However, pathogenic mutations in these factors can cause defective glycosylation and lead to diseases with a wide variety of symptoms such as liver dysfunction and skin and bone disorders. Collectively, this group of disorders is known as congenital disorders of glycosylation (CDG). Recent technological advances have enabled the robust identification of novel CDGs related to membrane trafficking components. In this review, we highlight differences and similarities between membrane trafficking-related CDGs.

Identification and in silico characterization of a novel p.P208PfsX1 mutation in V-ATPase a3 subunit associated with autosomal recessive osteopetrosis in a Pakistani family

Background

Osteopetrosis is a rare inherited bone disorder mainly described as an increased bone density caused by defective osteoclastic bone resorption. To date, genetic variants of eleven genes have been reported so far to be associated with different types of osteopetrosis. However, malignant infantile osteopetrosis, a lethal form of the disease, is mostly (50%) caused by mutation(s) in TCIRG1 gene. In this study, we investigated a consanguineous Pakistani family clinically and genetically to elucidate underlying molecular basis of the infantile osteopetrosis.

Methods

DNA samples from five family members were subjected to SNP-array based whole genome homozygosity mapping. Data was analyzed and potentially pathogenic mutation was identified by Sanger sequencing of two affected as well as three phenotypically healthy individuals in the family. The significance of identified pathogenic variation and its impact on protein structure and function was studied using various bioinformatics tools.

Results

DNA samples from five family members were subjected to genome-wide SNP array genotyping and homozygosity mapping which identified ~4 Mb region on chr11 harboring the TCIRG1 gene. Sanger sequencing unveiled a novel homozygous deletion c. 624delC in exon 6 of the TCIRG1 gene encodes a3 subunit of V-ATPase complex. The identified deletion resulted in a frame shift producing a truncated protein of 208 aa. In silico analysis of premature termination of the a3 subunit of V-ATPase complex revealed deleterious effects on the protein structure, predicting impaired or complete loss of V-ATPase function causing infantile osteopetrosis.

Conclusions

Since a3 subunit of V-ATPase complex plays a crucial role in bone resorption process, structurally abnormal a3 subunit might have adversely affected bone resorption process, leading to infantile osteopetrosis in Pakistani family. Therefore, the present study not only expands the genotypic spectrum of osteopetrosis but also improve understandings of the role of V-ATPase a3 subunit in bone resorption process. Moreover, our findings should help in genetic counseling and provide further insight into the disease pathogenesis and potential targeted therapy.

Electronic supplementary material

The online version of this article (10.1186/s12881-017-0506-4) contains supplementary material, which is available to authorized users.

Kaempferide Prevents Titanium Particle Induced Osteolysis by Suppressing JNK Activation during Osteoclast Formation

Kaempferide (KF) is an O-methylated flavonol, a natural plant extract, which is often found in Kaempferia galanga. It has a variety of effects including anti-carcinogenic, anti-inflammatory, anti-oxidant, anti-bacterial and anti-viral properties. In this study, we aimed to investigate whether KF effectively inhibits titanium particle induced calvarial bone loss via down regulation of the JNK signaling pathway. In the mice with titanium particle induced calvarial osteolysis, the Low dose of KF mildly reduced the resorption pits while in the high dose group, fewer scattered pits were observed on the surface of calvarium. Histological examination showed fewer osteoclasts formation in the KF group. In mouse bone marrow macrophages (BMMs) and RAW264.7 cells, KF significantly inhibited the osteoclast formation and bone resorption at 12.5 μM. However, KF does not affect the mature osteoclast F-actin ring formation. But when being co-treated with KF and anisomycin, BMMs differentiated into mature osteoclasts. At the molecular levels, the JNK phosphorylation was inhibited and the osteoclastogenesis-related specific gene expression including V-ATPase d2, TRAP, calcitonin receptor (CTR), c-Fos and NFATc1 was markedly suppressed. In conclusion, these results indicated that KF is a promising agent in the treatment of osteoclast-related diseases.

ATP6AP1 deficiency causes an immunodeficiency with hepatopathy, cognitive impairment and abnormal protein glycosylation

The V-ATPase is the main regulator of intra-organellar acidification. Assembly of this complex has extensively been studied in yeast, while limited knowledge exists for man. We identified 11 male patients with hemizygous missense mutations in ATP6AP1, encoding accessory protein Ac45 of the V-ATPase. Homology detection at the level of sequence profiles indicated Ac45 as the long-sought human homologue of yeast V-ATPase assembly factor Voa1. Processed wild-type Ac45, but not its disease mutants, restored V-ATPase-dependent growth in Voa1 mutant yeast. Patients display an immunodeficiency phenotype associated with hypogammaglobulinemia, hepatopathy and a spectrum of neurocognitive abnormalities. Ac45 in human brain is present as the common, processed ∼40-kDa form, while liver shows a 62-kDa intact protein, and B-cells a 50-kDa isoform. Our work unmasks Ac45 as the functional ortholog of yeast V-ATPase assembly factor Voa1 and reveals a novel link of tissue-specific V-ATPase assembly with immunoglobulin production and cognitive function.

Alexidine Dihydrochloride Attenuates Osteoclast Formation and Bone Resorption and Protects Against LPS-Induced Osteolysis

Aseptic loosening and periprosthetic infection leading to inflammatory osteolysis is a major complication associated with total joint arthroplasty (TJA). The liberation of bacterial products and/or implant-derived wear particles activates immune cells that produce pro-osteoclastogenic cytokines that enhance osteoclast recruitment and activity, leading to bone destruction and osteolysis. Therefore, agents that prevent the inflammatory response and/or attenuate excessive osteoclast (OC) formation and bone resorption offer therapeutic potential by prolonging the life of TJA implants. Alexidine dihydrochloride (AD) is a bisbiguanide compound commonly used as an oral disinfectant and in contact lens solutions. It possesses antimicrobial, anti-inflammatory and anticancer properties; however, its effects on OC biology are poorly described. Here, we demonstrate that AD inhibits OC formation and bone resorption in vitro and exert prophylatic protection against LPS-induced osteolysis in vivo. Biochemical analysis demonstrated that AD suppressed receptor activator of NF-κB ligand (RANKL)-induced activation of mitogen-activated protein kinases (ERK, p38, and JNK), leading to the downregulation of NFATc1. Furthermore, AD disrupted F-actin ring formation and attenuated the ability of mature OC to resorb bone. Collectively, our findings suggest that AD may be a promising prophylactic anti-osteoclastic/resorptive agent for the treatment of osteolytic diseases caused by excessive OC formation and function.

Thonzonium bromide inhibits RANKL-induced osteoclast formation and bone resorption in vitro and prevents LPS-induced bone loss in vivo

Osteoclasts (OCs) play a pivotal role in a variety of lytic bone diseases including osteoporosis, arthritis, bone tumors, Paget's disease and the aseptic loosening of orthopedic implants. The primary focus for the development of bone-protective therapies in these diseases has centered on the suppression of OC formation and function. In this study we report that thonzonium bromide (TB), a monocationic surface-active agent, inhibited RANKL-induced OC formation, the appearance of OC-specific marker genes and bone-resorbing activity in vitro. Mechanistically, TB blocked the RANKL-induced activation of NF-κB, ERK and c-Fos as well as the induction of NFATc1 which is essential for OC formation. TB disrupted F-actin ring formation resulting in disturbances in cytoskeletal structure in mature OCs during bone resorption. Furthermore, TB exhibited protective effects in an in vivo murine model of LPS-induced calvarial osteolysis. Collectively, these data suggest that TB might be a useful alternative therapy in preventing or treating osteolytic diseases.

The V-ATPase accessory protein Atp6ap1b mediates dorsal forerunner cell proliferation and left-right asymmetry in zebrafish

Asymmetric fluid flows generated by motile cilia in a transient 'organ of asymmetry' are involved in establishing the left-right (LR) body axis during embryonic development. The vacuolar-type H(+)-ATPase (V-ATPase) proton pump has been identified as an early factor in the LR pathway that functions prior to cilia, but the role(s) for V-ATPase activity are not fully understood. In the zebrafish embryo, the V-ATPase accessory protein Atp6ap1b is maternally supplied and expressed in dorsal forerunner cells (DFCs) that give rise to the ciliated organ of asymmetry called Kupffer's vesicle (KV). V-ATPase accessory proteins modulate V-ATPase activity, but little is known about their functions in development. We investigated Atp6ap1b and V-ATPase in KV development using morpholinos, mutants and pharmacological inhibitors. Depletion of both maternal and zygotic atp6ap1b expression reduced KV organ size, altered cilia length and disrupted LR patterning of the embryo. Defects in other ciliated structures-neuromasts and olfactory placodes-suggested a broad role for Atp6ap1b during development of ciliated organs. V-ATPase inhibitor treatments reduced KV size and identified a window of development in which V-ATPase activity is required for proper LR asymmetry. Interfering with Atp6ap1b or V-ATPase function reduced the rate of DFC proliferation, which resulted in fewer ciliated cells incorporating into the KV organ. Analyses of pH and subcellular V-ATPase localizations suggested Atp6ap1b functions to localize the V-ATPase to the plasma membrane where it regulates proton flux and cytoplasmic pH. These results uncover a new role for the V-ATPase accessory protein Atp6ap1b in early development to maintain the proliferation rate of precursor cells needed to construct a ciliated KV organ capable of generating LR asymmetry.

Ac45 silencing mediated by AAV-sh-Ac45-RNAi prevents both bone loss and inflammation caused by periodontitis

AIM

Periodontitis induced by oral pathogens leads to severe periodontal tissue damage and osteoclast-mediated bone resorption caused by inflammation. On the basis of the importance of Ac45 in osteoclast formation and function, we performed this study to evaluate the therapeutic potential of periodontitis by local adeno-associated virus (AAV)-mediated Ac45 gene knockdown.

MATERIAL AND METHODS

We used AAV-mediated short hairpin RNAi knockdown of Ac45 gene expression (AAV-sh-Ac45) to inhibit bone erosion and gingival inflammation simultaneously in a well-established periodontitis mouse model induced by Porphyromonas gingivalis W50. Histological studies were performed to evaluate the bone protection of AAV-sh-Ac45. Immunochemistry, ELISA and qRT-PCR were performed to reveal the role of Ac45 knockdown on inflammation, immune response and expression of cytokine.

RESULTS

We found that Ac45 knockdown impaired osteoclast-mediated extracellular acidification and bone resorption in vitro and in vivo. Furthermore, local administration of AAV-sh-Ac45 protected mice from bone erosion by >85% and attenuated inflammation and decreased infiltration of T cells, dendritic cells and macrophages in the periodontal lesion. Notably, the expression of pro-inflammatory cytokines was also reduced.

CONCLUSIONS

Local AAV-sh-Ac45 gene therapy efficiently protects against periodontal tissue damage and bone erosion through both inhibition of osteoclast function and attenuating inflammation, and may represent a powerful new treatment strategy for periodontitis.

Disulfiram attenuates osteoclast differentiation in vitro: a potential antiresorptive agent

Disulfiram (DSF), a cysteine modifying compound, has long been clinically employed for the treatment of alcohol addiction. Mechanistically, DSF acts as a modulator of MAPK and NF-κB pathways signaling pathways. While these pathways are crucial for osteoclast (OC) differentiation, the potential influence of DSF on OC formation and function has not been directly assessed. Here, we explore the pharmacological effects of DSF on OC differentiation, activity and the modulation of osteoclastogenic signaling cascades. We first analyzed cytotoxicity of DSF on bone marrow monocytes isolated from C57BL/6J mice. Upon the establishment of optimal dosage, we conducted osteoclastogenesis and bone resorption assays in the presence or absence of DSF treatment. Luciferase assays in RAW264.7 cells were used to examine the effects of DSF on major transcription factors activation. Western blot, reverse transcription polymerase chain reaction, intracellular acidification and proton influx assays were employed to further dissect the underlying mechanism. DSF treatment dose-dependently inhibited both mouse and human osteoclastogenesis, especially at early stages of differentiation. This inhibition correlated with a decrease in the expression of key osteoclastic marker genes including CtsK, TRAP, DC-STAMP and Atp6v0d2 as well as a reduction in bone resorption in vitro. Suppression of OC differentiation was found to be due, at least in part, to the blockade of several key receptor activators of nuclear factor kappa-B ligand (RANKL)-signaling pathways including ERK, NF-κB and NFATc1. On the other hand, DSF failed to suppress intracellular acidification and proton influx in mouse and human osteoclasts using acridine orange quenching and microsome-based proton transport assays. Our findings indicate that DSF attenuates OC differentiation via the collective suppression of several key RANKL-mediated signaling cascades, thus making it an attractive agent for the treatment of OC-mediated disorders.

Andrographolide suppresses RANKL-induced osteoclastogenesis in vitro and prevents inflammatory bone loss in vivo

BACKGROUND AND PURPOSE

Osteoclasts play a pivotal role in diseases such as osteoporosis, rheumatoid arthritis and tumour bone metastasis. Thus, searching for natural compounds that may suppress osteoclast formation and/or function is promising for the treatment of osteoclast-related diseases. Here, we examined changes in osteoclastogenesis and LPS-induced osteolysis in response to andrographolide (AP), a diterpenoid lactone isolated from the traditional Chinese and Indian medicinal plant Andrographis paniculata.

EXPERIMENTAL APPROACH

Effects of AP on osteoclast differentiation and bone resorption were measured in vitro. Western blots and RT-PCR techniques were used to examine the underlying molecular mechanisms. The bone protective activity of AP in vivo was assessed in a mouse model of osteolysis.

KEY RESULTS

AP concentration-dependently suppressed RANKL-mediated osteoclast differentiation and bone resorption in vitro and reduced the expression of osteoclast-specific markers, including tartrate-resistant acid phosphatase, calcitonin receptors and cathepsin K. Further molecular analysis revealed that AP impaired RANKL-induced NF-κB signalling by inhibiting the phosphorylation of TGF-β-activated kinase 1, suppressing the phosphorylation and degradation of IκBα, and subsequently preventing the nuclear translocation of the NF-κB p65 subunit. AP also inhibited the ERK/MAPK signalling pathway without affecting p38 or JNK signalling.

CONCLUSIONS AND IMPLICATIONS

AP suppressed RANKL-induced osteoclastogenesis through attenuating NF-κB and ERK/MAPK signalling pathways in vitro, thus preventing bone loss in vivo. These data indicated that AP is a promising natural compound for the treatment of osteoclast-related bone diseases.

The zebrafish mutants for the V-ATPase subunits d, ac45, E, H and c and their variable pigment dilution phenotype

Background

The V-ATPase is a proton pump that creates an acidic medium, necessary for lysosome function and vesicular traffic. It is also essential for several developmental processes. Many enzymes, like the V-ATPase, are assemblies of multiple subunits, in which each one performs a specific function required to achieve full activity. In the zebrafish V-ATPase 15 different subunits form this multimeric complex and mutations in any of these subunits induce hypopigmentation or pigment dilution phenotype. We have previously found variability in the pigment dilution phenotype among five of the V-ATPase zebrafish mutants. This work presents additional information about such differences and is an update from a previous report.

Findings

We describe the variable phenotype severity observed among zebrafish V-ATPase pigment dilution mutants studying mRNA expression levels from their corresponding genes. At the same time we carried out phylogenetic analysis for this genes.

Conclusions

Based in the similarities between different pigment dilution mutants we suggest that there is an essential role for V-ATPases in melanosome biogenesis and melanocyte survival. Neither variable expression levels for the different V-ATPase subunits studied here or the presence of duplicated genes seems to account for the variable phenotype severity from this group of mutants. We believe there are some similarities between the pigment dilution phenotype from zebrafish V-ATPase insertional mutants and pigment mutants obtained in a chemical screening (“Tubingen pigmentation mutants”). As for some of these “Tubingen mutants” the mutated gene has not been found we suggest that mutations in V-ATPase genes may be inducing their defects.

Sanguinarine inhibits osteoclast formation and bone resorption via suppressing RANKL-induced activation of NF-κB and ERK signaling pathways

Sanguinarine is a natural plant extract that has been supplemented in a number of gingival health products to suppress the growth of dental plaque. However, whether sanguinarine has any effect on teeth and alveolar bone health is still unclear. In this study, we demonstrated for the first time that sanguinarine could suppress osteoclastic bone resorption and osteoclast formation in a dose-dependent manner. Sanguinarine diminished the expression of osteoclast marker genes, including TRAP, cathepsin K, calcitonin receptor, DC-STAMP, V-ATPase d2, NFATc1 and c-fos. Further investigation revealed that sanguinarine attenuated RANKL-mediated IκBα phosphorylation and degradation, leading to the impairment of NF-κB signaling pathway during osteoclast differentiation. In addition, sanguinarine also affected the ERK signaling pathway by inhibiting RANKL-induced ERK phosphorylation. Collectively, this study suggested that sanguinarine has protective effects on teeth and alveolar bone health.

V-ATPase subunit ATP6AP1 (Ac45) regulates osteoclast differentiation, extracellular acidification, lysosomal trafficking, and protease exocytosis in osteoclast-mediated bone resorption

Lysosomal trafficking and protease exocytosis in osteoclasts are essential for ruffled border formation and bone resorption. Yet the mechanism underlying lysosomal trafficking and the related process of exocytosis remains largely unknown. We found ATP6ap1 (Ac45), an accessory subunit of vacuolar-type H(+)-ATPases (V-ATPases), to be highly induced by receptor activator for nuclear factor kappa B ligand (RANKL) in osteoclast differentiation. Ac45 knockdown osteoclasts formed normal actin rings, but had severely impaired extracellular acidification and bone resorption. Ac45 knockdown significantly reduced osteoclast formation. The decrease in the number of osteoclasts does not result from abnormal apoptosis; rather, it results from decreased osteoclast precursor cell proliferation and fusion, which may be partially due to the downregulation of extracellular signal-regulated kinase (ERK) phosphorylation and FBJ osteosarcoma oncogene (c-fos), nuclear factor of activated T-cells, cytoplasmic 1 (NFATc1), and "transmembrane 7 superfamily member 4" (Tm7sf4) expression. Notably, Ac45 knockdown osteoclasts exhibited impaired lysosomal trafficking and exocytosis, as indicated by the absence of lysosomal trafficking to the ruffled border and a lack of cathepsin K exocytosis into the resorption lacuna. Our data revealed that the impaired exocytosis is specifically due to Ac45 deficiency, and not the general consequence of a defective V-ATPase. Together, our results demonstrate the essential role of Ac45 in osteoclast-mediated extracellular acidification and protease exocytosis, as well as the ability of Ac45 to guide lysosomal intracellular trafficking to the ruffled border, potentially through its interaction with the small guanosine-5'-triphosphatase (GTPase) Rab7. Our work indicates that Ac45 may be a novel therapeutic target for osteolytic disease.

Cathepsin K-Cre causes unexpected germline deletion of genes in mice

Osteoclasts are terminally differentiated cells that attach to bone and secrete proteases to degrade the bone matrix. The primary protease responsible for the degradation of the organic component of the bone matrix is Cathepsin K, which was largely thought to be unique to osteoclasts. Given its apparent selective expression in osteoclasts, the Cathepsin K promoter has been engineered to drive the expression of Cre recombinase in mice and has been the most relevant tool for generating osteoclast-specific gene loss. In an effort to understand the role of the ARF tumor suppressor in osteoclasts, we crossed Arf ^fl/fl mice to Ctsk^Cre/+ mice, which unexpectedly resulted in the germline loss of Arf. We subsequently confirmed Cre activity in gametes by generating Ctsk^Cre/+; Rosa⁺ mice. These results raise significant concerns regarding in vivo bone phenotypes created using Ctsk^Cre/+ mice and warrant further investigation into the role of Cathepsin K in gametes as well as alternative tools for studying osteoclast-specific gene loss in vivo.

Identification of domains within the V-ATPase accessory subunit Ac45 involved in V-ATPase transport and Ca2+-dependent exocytosis

The vacuolar (H(+))-ATPase (V-ATPase) is crucial for maintenance of the acidic microenvironment in intracellular organelles, whereas its membrane-bound V(0)-sector is involved in Ca(2+)-dependent membrane fusion. In the secretory pathway, the V-ATPase is regulated by its type I transmembrane and V(0)-associated accessory subunit Ac45. To execute its function, the intact-Ac45 protein is proteolytically processed to cleaved-Ac45 thereby releasing its N-terminal domain. Here, we searched for the functional domains within Ac45 by analyzing a set of deletion mutants close to the in vivo situation, namely in transgenic Xenopus intermediate pituitary melanotrope cells. Intact-Ac45 was poorly processed and accumulated in the endoplasmic reticulum of the transgenic melanotrope cells. In contrast, cleaved-Ac45 was efficiently transported through the secretory pathway, caused an accumulation of the V-ATPase at the plasma membrane and reduced dopaminergic inhibition of Ca(2+)-dependent peptide secretion. Surprisingly, removal of the C-tail from intact-Ac45 caused cellular phenotypes also found for cleaved-Ac45, whereas C-tail removal from cleaved-Ac45 still allowed its transport to the plasma membrane, but abolished V-ATPase recruitment into the secretory pathway and left dopaminergic inhibition of the cells unaffected. We conclude that domains located in the N- and C-terminal portions of the Ac45 protein direct its trafficking, V-ATPase recruitment and Ca(2+)-dependent-regulated exocytosis.

V-ATPases in osteoclasts: structure, function and potential inhibitors of bone resorption

The vacuolar-type H(+)-ATPase (V-ATPase) proton pump is a macromolecular complex composed of at least 14 subunits organized into two functional domains, V(1) and V(0). The complex is located on the ruffled border plasma membrane of bone-resorbing osteoclasts, mediating extracellular acidification for bone demineralization during bone resorption. Genetic studies from mice to man implicate a critical role for V-ATPase subunits in osteoclast-related diseases including osteopetrosis and osteoporosis. Thus, the V-ATPase complex is a potential molecular target for the development of novel anti-resorptive agents useful for the treatment of osteolytic diseases. Here, we review the current structure and function of V-ATPase subunits, emphasizing their exquisite roles in osteoclastic function. In addition, we compare several distinct classes of V-ATPase inhibitors with specific inhibitory effects on osteoclasts. Understanding the structure-function relationship of the osteoclast V-ATPase may lead to the development of osteoclast-specific V-ATPase inhibitors that may serve as alternative therapies for the treatment of osteolytic diseases.