c-Cbl and Cbl-b act redundantly to protect osteoclasts from apoptosis and to displace HDAC6 from beta-tubulin, stabilizing microtubules and podosomes

The role of E3 ubiquitin ligases in bone homeostasis and related diseases

The ubiquitin-proteasome system (UPS) dedicates to degrade intracellular proteins to modulate demic homeostasis and functions of organisms. These enzymatic cascades mark and modifies target proteins diversly through covalently binding ubiquitin molecules. In the UPS, E3 ubiquitin ligases are the crucial constituents by the advantage of recognizing and presenting proteins to proteasomes for proteolysis. As the major regulators of protein homeostasis, E3 ligases are indispensable to proper cell manners in diverse systems, and they are well described in physiological bone growth and bone metabolism. Pathologically, classic bone-related diseases such as metabolic bone diseases, arthritis, bone neoplasms and bone metastasis of the tumor, etc., were also depicted in a UPS-dependent manner. Therefore, skeletal system is versatilely regulated by UPS and it is worthy to summarize the underlying mechanism. Furthermore, based on the current status of treatment, normal or pathological osteogenesis and tumorigenesis elaborated in this review highlight the clinical significance of UPS research. As a strategy possibly remedies the limitations of UPS treatment, emerging PROTAC was described comprehensively to illustrate its potential in clinical application. Altogether, the purpose of this review aims to provide more evidence for exploiting novel therapeutic strategies based on UPS for bone associated diseases.

Sulfated Glucuronorhamnoxylan from Capsosiphon fulvescens Ameliorates Osteoporotic Bone Resorption via Inhibition of Osteoclastic Cell Differentiation and Function In Vitro and In Vivo

Excessive osteoclast differentiation and/or bone resorptive function causes a gradual loss of bone, leading to the pathogenesis of bone diseases such as osteoporosis (OP). In this study, a sulfated glucuronorhamnoxylan polysaccharide (designated SPS-CF) of the green alga Capsosiphon fulvescens was evaluated for anti-osteoporotic activity using osteoclastic cells differentiated from RAW264.7 macrophages by receptor activator of NF-κB ligand (RANKL) treatment and ovariectomized (OVX) female mice as a postmenopausal OP model. With negligible cytotoxicity, SPS-CF (50 μg/mL) significantly suppressed tartrate-resistant acid phosphatase (TRAP) activity, actin ring formation, and expression of matrix metalloproteinase 9 (MMP-9), cathepsin K, TRAF6, p-Pyk2, c-Cbl, c-Src, gelsolin, carbonic anhydrase II (CA II), and integrin β3, indicating that SPS-CF inhibits the differentiation and bone resorptive function of osteoclasts. Removal of sulfate groups from SPS-CF abolished its anti-osteoclastogenic activities, demonstrating that sulfate groups are critical for its activity. Oral administration of SPS-CF (400 mg/kg/day) to OVX mice significantly augmented the bone mineral density (BMD) and serum osteoprotegerin (OPG)/RANKL ratio. These results demonstrated that SPS-CF exerts significant anti-osteoporotic activity by dampening osteoclastogenesis and bone resorption via downregulation of TRAF6-c-Src-Pyk2-c-Cbl-gelsolin signaling and augmentation of serum OPG/RANKL ratios in OVX mice, suggesting that SPS-CF can be a novel anti-osteoporotic compound for treating postmenopausal OP.

IFT80 negatively regulates osteoclast differentiation via association with Cbl-b to disrupt TRAF6 stabilization and activation

Osteoclasts (OCs) are the sole bone resorbing cells indispensable for bone remodeling. Hence, understanding of novel signaling modulators regulating OC formation is clinically important. Intraflagellar transport (IFT) proteins are important for cilia, cell signaling, and organ development. It remains unclear whether IFT80 plays a role in OCs. This study uncovers an intriguing role of IFT80 in OCs where the ciliary protein regulates the stability of critical OC factor TRAF6 via Cbl-b and thereby contributes to the maintenance of OC numbers. These findings provide further basis for understanding and delineating the role of IFT proteins in OCs that may provide new strategies for treatment of osteolytic diseases.

Excess bone loss due to increased osteoclastogenesis is a significant clinical problem. Intraflagellar transport (IFT) proteins have been reported to regulate cell growth and differentiation. The role of IFT80, an IFT complex B protein, in osteoclasts (OCs) is completely unknown. Here, we demonstrate that deletion of IFT80 in the myeloid lineage led to increased OC formation and activity accompanied by severe bone loss in mice. IFT80 regulated OC formation by associating with Casitas B-lineage lymphoma proto-oncogene-b (Cbl-b) to promote protein stabilization and proteasomal degradation of tumor necrosis factor (TNF) receptor–associated factor 6 (TRAF6). IFT80 knockdown resulted in increased ubiquitination of Cbl-b and higher TRAF6 levels, thereby hyperactivating the receptor activator of nuclear factor-κβ (NF-κβ) ligand (RANKL) signaling axis and increased OC formation. Ectopic overexpression of IFT80 rescued osteolysis in a calvarial model of bone loss. We have thus identified a negative function of IFT80 in OCs.

Characterization of unique functionalities in c-Src domains required for osteoclast podosome belt formation

Deletion of c-Src, a ubiquitously expressed tyrosine kinase, results in osteoclast dysfunction and osteopetrosis, in which bones harden into "stone." In contrast, deletion of the genes encoding other members of the Src family kinase (SFK) fails to produce an osteopetrotic phenotype. This suggests that c-Src performs a unique function in the osteoclast that cannot be compensated for by other SFKs. We aimed to identify the molecular basis of this unique role in osteoclasts and bone resorption. We found that c-Src, Lyn, and Fyn were the most highly expressed SFKs in WT osteoclasts, whereas Hck, Lck, Blk, and Fgr displayed low levels of expression. Formation of the podosome belt, clusters of unique actin assemblies, was disrupted in src^-/- osteoclasts; introduction of constitutively activated SFKs revealed that only c-Src and Fyn could restore this process. To identify the key structural domains responsible, we constructed chimeric Src-Hck and Src-Lyn constructs in which the unique, SH3, SH2, or catalytic domains had been swapped. We found that the Src unique, SH3, and kinase domains were each crucial to establish Src functionality. The SH2 domain could however be substituted with Lyn or Hck SH2 domains. Furthermore, we demonstrate that c-Src's functionality is, in part, derived from an SH3-proximal proline-rich domain interaction with c-Cbl, leading to phosphorylation of c-Cbl Tyr700. These data help clarify Src's unique functionality in the organization of the cytoskeleton in osteoclasts, required for efficient bone resorption and explain why c-Src cannot be replaced, in osteoclasts, by other SFKs.

Trilobatin ameliorates bone loss via suppression of osteoclast cell differentiation and bone resorptive function in vitro and in vivo

AIM

Due to on-going safety concerns or lack of efficacy of currently used medications for the treatment of osteoporosis (OP), identifying new therapeutic agents is an important part of research. In the present study, potential anti-osteoporotic activity of a natural flavonoid glycoside, trilobatin (phloretin 4-O-glucoside, Tri) was evaluated.

MATERIAL AND METHODS

Osteoclastic cells were established by treating the RAW264.7 macrophage cells with RANKL and ovariectomized (OVX) C57BL/6 female mice were used as an animal model of postmenopausal OP. Actin ring formation, expression levels of osteoclastogenic marker genes and bone resorptive proteins were measured by RT-PCR, western blot, or fluorometric assays. Bone mineral density (BMD) was determined by pDEXA densitometric measurement and serum osteoprotegerin (OPG) and RANKL were measured by ELISA.

KEY FINDING

Tri (5-20 μM) significantly inhibited osteoclast formation and actin ring formation in RANKL-induced osteoclasts. Tri attenuated expression of osteoclastogenic genes (MMP-9 and cathepsin K), bone resorptive proteins (CA II and integrin β3), and osteoclastogenic signalling proteins (TRAF6, p-Pyk2, c-Cbl, and c-Src). Oral administration of Tri to OVX mice augmented BMD and serum OPG/RANKL ratio. Interestingly, while Tri and phloretin aglycone (Phl) showed similar levels of in vitro anti-osteoclastogenic activity, Tri more potently ameliorated bone loss than Phl in OVX mice.

SIGNIFICANCE

This study demonstrated that Tri inhibits osteoclastic cell differentiation and bone resorption by down-regulating the expression of osteoclastogenic marker genes and signalling proteins, bone resorptive proteins, and by augmenting serum OPG/RANKL ratio, suggesting that Tri can be a novel anti-osteoporotic compound for treating senile and postmenopausal OP.

The osteoclast cytoskeleton - current understanding and therapeutic perspectives for osteoporosis

Osteoclasts are giant multinucleated myeloid cells specialized for bone resorption, which is essential for the preservation of bone health throughout life. The activity of osteoclasts relies on the typical organization of osteoclast cytoskeleton components into a highly complex structure comprising actin, microtubules and other cytoskeletal proteins that constitutes the backbone of the bone resorption apparatus. The development of methods to differentiate osteoclasts in culture and manipulate them genetically, as well as improvements in cell imaging technologies, has shed light onto the molecular mechanisms that control the structure and dynamics of the osteoclast cytoskeleton, and thus the mechanism of bone resorption. Although essential for normal bone physiology, abnormal osteoclast activity can cause bone defects, in particular their hyper-activation is commonly associated with many pathologies, hormonal imbalance and medical treatments. Increased bone degradation by osteoclasts provokes progressive bone loss, leading to osteoporosis, with the resulting bone frailty leading to fractures, loss of autonomy and premature death. In this context, the osteoclast cytoskeleton has recently proven to be a relevant therapeutic target for controlling pathological bone resorption levels. Here, we review the present knowledge on the regulatory mechanisms of the osteoclast cytoskeleton that control their bone resorption activity in normal and pathological conditions.

Microtubules at focal adhesions – a double-edged sword

Cell adhesion to the extracellular matrix is essential for cellular processes, such as migration and invasion. In response to cues from the microenvironment, integrin-mediated adhesions alter cellular behaviour through cytoskeletal rearrangements. The tight association of the actin cytoskeleton with adhesive structures has been extensively studied, whereas the microtubule network in this context has gathered far less attention. In recent years, however, microtubules have emerged as key regulators of cell adhesion and migration through their participation in adhesion turnover and cellular signalling. In this Review, we focus on the interactions between microtubules and integrin-mediated adhesions, in particular, focal adhesions and podosomes. Starting with the association of microtubules with these adhesive structures, we describe the classical role of microtubules in vesicular trafficking, which is involved in the turnover of cell adhesions, before discussing how microtubules can also influence the actin–focal adhesion interplay through RhoGTPase signalling, thereby orchestrating a very crucial crosstalk between the cytoskeletal networks and adhesions.

Dock5 is a new regulator of microtubule dynamic instability in osteoclasts

BACKGROUND INFORMATION

Osteoclast resorption is dependent on a podosome-rich structure called sealing zone. It tightly attaches the osteoclast to the bone creating a favourable acidic microenvironment for bone degradation. This adhesion structure needs to be stabilised by microtubules whose acetylation is maintained by down-regulation of deacetylase HDAC6 and/or of microtubule destabilising kinase GSK3β activities. We already established that Dock5 is a guanine nucleotide exchange factor for Rac1. As a consequence, Dock5 inhibition results in a decrease of the GTPase activity associated with impaired podosome assembly into sealing zones and resorbing activity in osteoclasts. More, administration of C21, a chemical compound that directly inhibits the exchange activity of Dock5, disrupts osteoclast podosome organisation and protects mice against bone degradation in models recapitulating major osteolytic diseases.

RESULTS

In this report, we show that Dock5 knockout osteoclasts also present a reduced acetylated tubulin level leading to a decreased length and duration of microtubule growth phases, whereas their growth speed remains unaffected. Dock5 does not act by direct interaction with the polymerised tubulin. Using specific Rac inhibitors, we showed that Dock5 regulates microtubule dynamic instability through Rac-dependent and -independent pathways. The latter involves GSK3β inhibitory serine 9 phosphorylation downstream of Akt activation but not HDAC6 activity.

CONCLUSION

We showed that Dock5 is a new regulator of microtubule dynamic instability in osteoclast.

SIGNIFICANCE

Dock5 dual role in the regulation of the actin cytoskeleton and microtubule, which both need to be intact for bone resorption, reinforces the fact that it is an interesting therapeutic target for osteolytic pathologies.

Unique-region phosphorylation targets LynA for rapid degradation, tuning its expression and signaling in myeloid cells

The activity of Src-family kinases (SFKs), which phosphorylate immunoreceptor tyrosine-based activation motifs (ITAMs), is a critical factor regulating myeloid-cell activation. We reported previously that the SFK LynA is uniquely susceptible to rapid ubiquitin-mediated degradation in macrophages, functioning as a rheostat regulating signaling (Freedman et al., 2015). We now report the mechanism by which LynA is preferentially targeted for degradation and how cell specificity is built into the LynA rheostat. Using genetic, biochemical, and quantitative phosphopeptide analyses, we found that the E3 ubiquitin ligase c-Cbl preferentially targets LynA via a phosphorylated tyrosine (Y32) in its unique region. This distinct mode of c-Cbl recognition depresses steady-state expression of LynA in macrophages derived from mice. Mast cells, however, express little c-Cbl and have correspondingly high LynA. Upon activation, mast-cell LynA is not rapidly degraded, and SFK-mediated signaling is amplified relative to macrophages. Cell-specific c-Cbl expression thus builds cell specificity into the LynA checkpoint.

KBTBD11, a novel BTB-Kelch protein, is a negative regulator of osteoclastogenesis through controlling Cullin3-mediated ubiquitination of NFATc1

Kelch repeat and BTB domain-containing protein 11 (KBTBD11) is a member of the KBTBD subfamily of proteins that possess a BTB domain and Kelch repeats. Despite the presence of the Kbtbd11 gene in mammalian genomes, there are few reports about KBTBD11 at present. In this study, we identified the novel protein KBTBD11 as a negative regulator of osteoclast differentiation. We found that expression of KBTBD11 increased during osteoclastogenesis. Small-interfering-RNA-mediated knockdown of KBTBD11 enhanced osteoclast formation, and markedly increased the expression of several osteoclast marker genes compared with control cells. Conversely, KBTBD11 overexpression impaired osteoclast differentiation, and decreased the expression of osteoclast marker genes. Among six major signaling pathways regulating osteoclast differentiation, KBTBD11 predominantly influenced the nuclear factor of activated T cell cytoplasmic-1 (NFATc1) pathway. Mechanistically, KBTBD11 was found to interact with an E3 ubiquitin ligase, Cullin3. Further experiments involving immunoprecipitation and treatment with MG132, a proteasome inhibitor, showed that the KBTBD11–Cullin3 promotes ubiquitination and degradation of NFATc1 by the proteasome. Considering that NFATc1 is an essential factor for osteoclast differentiation, the KBTBD11 and Cullin3 probably regulate the levels of NFATc1 through the ubiquitin-proteasome degradation system. Thus, KBTBD11 negatively modulates osteoclast differentiation by controlling Cullin3-mediated ubiquitination of NFATc1.

Evidence for altered osteoclastogenesis in splenocyte cultures from VDR knockout mice

The indirect action of 1α,25(OH)₂-vitamin-D₃ (1,25D) on the osteoclast through stromal signalling is well established. The role of vitamin D in osteoclasts through direct 1,25D-VDR signalling is less well known. We showed previously that local 1,25D synthesis in osteoclasts modified osteoclastogenesis and osteoclastic resorptive activity. In this study, we hypothesised that osteoclasts lacking VDR expression would display an enhanced resorptive capacity due to the loss of 1,25D signalling. Splenocytes were cultured under osteoclast-differentiating conditions from mice with global deletion of the Vdr gene (VDRKO) and this was compared with age-matched wild-type littermate controls (WT). In VDRKO cultures, osteoclastogenesis was reduced, as indicated by fewer TRAP-positive multinucleated cells at all time points measured (p<0.05) compared to WT levels. However, VDRKO osteoclasts demonstrated greater resorption on a per cell basis than their WT counterparts. VDRKO cultures expressed greatly increased c-Fos mRNA compared to WT. In addition, the ratio of expression of the pro-apoptotic gene Bax to the pro-survival gene Bcl-2 was decreased in VDRKO cultures, implying that these osteoclasts may survive longer than WT osteoclasts. Our data indicate abnormal osteoclastogenesis due to the absence of Vdr expression, consistent with direct effects of vitamin D signalling being important for regulating the maturation and resorptive activities of osteoclasts.

Class II and IV HDACs function as inhibitors of osteoclast differentiation

Histone deacetylases (HDACs) are negative regulators of transcription and have been shown to regulate specific changes in gene expression. In vertebrates, eighteen HDACs have thus far been identified and subdivided into four classes (I-IV). Key roles for several HDACs in bone development and biology have been elucidated through in vitro and in vivo models. By comparison, there is a paucity of data on the roles of individual HDACs in osteoclast formation and function. In this study, we investigated the gene expression patterns and the effects of suppressing individual class II (Hdac4, 5, 6, 9, and 10) and class IV (Hdac11) HDACs during osteoclast differentiation. We demonstrated that HDAC class II and IV members are differentially expressed during osteoclast differentiation. Additionally, individual shRNA-mediated suppression of Hdac4, 5, 9, 10 and 11 expression resulted in increased multinucleated osteoclast size and demineralization activity, with little to no change in the overall number of multinucleated osteoclasts formed compared with control shRNA-treated cells. We also detected increased expression of genes highly expressed in osteoclasts, including c-Fos, Nfatc1, Dc-stamp and Cathepsin K. These observations indicate that HDACs cooperatively regulate shared targets in a non-redundant manner.

Human histone deacetylase 6 shows strong preference for tubulin dimers over assembled microtubules

Human histone deacetylase 6 (HDAC6) is the major deacetylase responsible for removing the acetyl group from Lys40 of α-tubulin (αK40), which is located lumenally in polymerized microtubules. Here, we provide a detailed kinetic analysis of tubulin deacetylation and HDAC6/microtubule interactions using individual purified components. Our data unequivocally show that free tubulin dimers represent the preferred HDAC6 substrate, with a K _M value of 0.23 µM and a deacetylation rate over 1,500-fold higher than that of assembled microtubules. We attribute the lower deacetylation rate of microtubules to both longitudinal and lateral lattice interactions within tubulin polymers. Using TIRF microscopy, we directly visualized stochastic binding of HDAC6 to assembled microtubules without any detectable preferential binding to microtubule tips. Likewise, indirect immunofluorescence microscopy revealed that microtubule deacetylation by HDAC6 is carried out stochastically along the whole microtubule length, rather than from the open extremities. Our data thus complement prior studies on tubulin acetylation and further strengthen the rationale for the correlation between tubulin acetylation and microtubule age.

Evidence for altered osteoclastogenesis in splenocyte cultures from Cyp27b1 knockout mice

The association between increased serum 25-hydroxyvitamin D (25D) and reduced osteoclastic bone resorption is well known. Previously, we have demonstrated that mechanism by which this occurs, may include the conversion of 25D to 1,25-dihydroxyvitamin D (1,25D) by osteoclasts, catalysed by the CYP27B1 enzyme. Local 1,25D synthesis in osteoclasts was shown to regulate osteoclastogenesis and moderating resorptive activity. Thus, we hypothesised that osteoclasts differentiated from mice with global deletion of the Cyp27b1 gene (Cyp27b1 KO) would display enhanced resorptive capacity due to the lack of an ameliorating effect of 1,25D. Splenocytes isolated from Cyp27b1 KO mice or their wild-type (WT) littermates between 6 and 8 weeks of age were cultured under osteoclast-forming conditions for up to 14 days. Osteoclast formation was measured by staining for the osteoclast marker tartrate resistant acid phosphatase (TRAP). Bone resorption activity was measured by plating the cells on a bone-like substrate. In Cyp27b1 KO cultures, osteoclastogenesis was reduced, as indicated by fewer TRAP-positive multinucleated cells at all time points measured (p<0.05) when compared to wild-type (WT) levels. However, Cyp27b1 KO osteoclasts demonstrated greater resorption on a per cell basis than their WT counterparts (p<0.03). In addition, the ratio of expression of the pro-apoptotic gene Bax to the pro-survival gene Bcl-2 was decreased in Cyp27b1 KO cultures, implying that these smaller osteoclasts survive longer than WT osteoclasts. Our data indicate abnormal osteoclastogenesis due to the absence of CYP27B1 expression, consistent with the notion that endogenous metabolism of 25D optimises osteoclastogenesis and ameliorates the resulting activity of mature osteoclasts.

The "love-hate" relationship between osteoclasts and bone matrix

Osteoclasts are unique cells that destroy the mineralized matrix of the skeleton. There is a "love-hate" relationship between the osteoclasts and the bone matrix, whereby the osteoclast is stimulated by the contact with the matrix but, at the same time, it disrupts the matrix, which, in turn, counteracts this disruption by some of its components. The balance between these concerted events brings about bone resorption to be controlled and to contribute to bone tissue integrity and skeletal health. The matrix components released by osteoclasts are also involved in the local regulation of other bone cells and in the systemic control of organismal homeostasis. Disruption of this regulatory loop causes bone diseases, which may end up with either reduced or increased bone mass, often associated with poor bone quality. Expanding the knowledge on osteoclast-to-matrix interaction could help to counteract these diseases and improve the human bone health. In this article, we will present evidence of the physical, molecular and regulatory relationships between the osteoclasts and the mineralized matrix, discussing the underlying mechanisms as well as their pathologic alterations and potential targeting.

Histone Deacetylases in Bone Development and Skeletal Disorders

Histone deacetylases (Hdacs) are conserved enzymes that remove acetyl groups from lysine side chains in histones and other proteins. Eleven of the 18 Hdacs encoded by the human and mouse genomes depend on Zn(2+) for enzymatic activity, while the other 7, the sirtuins (Sirts), require NAD2(+). Collectively, Hdacs and Sirts regulate numerous cellular and mitochondrial processes including gene transcription, DNA repair, protein stability, cytoskeletal dynamics, and signaling pathways to affect both development and aging. Of clinical relevance, Hdacs inhibitors are United States Food and Drug Administration-approved cancer therapeutics and are candidate therapies for other common diseases including arthritis, diabetes, epilepsy, heart disease, HIV infection, neurodegeneration, and numerous aging-related disorders. Hdacs and Sirts influence skeletal development, maintenance of mineral density and bone strength by affecting intramembranous and endochondral ossification, as well as bone resorption. With few exceptions, inhibition of Hdac or Sirt activity though either loss-of-function mutations or prolonged chemical inhibition has negative and/or toxic effects on skeletal development and bone mineral density. Specifically, Hdac/Sirt suppression causes abnormalities in physiological development such as craniofacial dimorphisms, short stature, and bone fragility that are associated with several human syndromes or diseases. In contrast, activation of Sirts may protect the skeleton from aging and immobilization-related bone loss. This knowledge may prolong healthspan and prevent adverse events caused by epigenetic therapies that are entering the clinical realm at an unprecedented rate. In this review, we summarize the general properties of Hdacs/Sirts and the research that has revealed their essential functions in bone forming cells (e.g., osteoblasts and chondrocytes) and bone resorbing osteoclasts. Finally, we offer predictions on future research in this area and the utility of this knowledge for orthopedic applications and bone tissue engineering.

Podosome organization drives osteoclast-mediated bone resorption

Osteoclasts are the cells responsible for physiological bone resorption. A specific organization of their most prominent cytoskeletal structures, podosomes, is crucial for the degradation of mineralized bone matrix. Each podosome is constituted of an F-actin-enriched central core surrounded by a loose F-actin network, called the podosome cloud. In addition to intrinsic actin dynamics, podosomes are defined by their adhesion to the extracellular matrix, mainly via core-linking CD44 and cloud-linking integrins. These properties allow podosomes to collectively evolve into different patterns implicated in migration and bone resorption. Indeed, to resorb bone, osteoclasts polarize, actively secrete protons, and proteases into the resorption pit where these molecules are confined by a podosome-containing sealing zone. Here, we review recent advancements on podosome structure and regulatory pathways in osteoclasts. We also discuss the distinct functions of different podosome patterns during the lifespan of a single osteoclast.

Src blockage by siRNA inhibits VEGF-induced vascular hyperpemeability and osteoclast activity - an in vitro mechanism study for preventing destructive repair of osteonecrosis

INTRODUCTION

Destructive repair is the pathological feature of ONFH characterized with the elevated vascular permeability and persistent bone resorption, which is associated with higher VEGF expression, activated c-Src, and vascular leakage. Activated c-Src also participates in mediating endothelial permeability and osteoclasts activity. However, the molecular mechanism of the VEGF and c-Src contributing to the destructive repair process remains unknown. The purpose of this study is to delineate the role of VEGF and c-Src in triggering destructive repair of osteonecrosis in vitro, as well as to elucidate if VEGF mediating vascular permeability and osteoclastic bone resorption are Src dependent.

METHODS

We employed pharmacological VEGF to induce higher endothelial permeability and osteoclasts activity for simulating related pathological features of destructive repair in vitro. Src specific pp60(c-src)siRNA was used for determining the contribution of VEGF and Src to destructive repair. The primary endothelial cells and osteoclasts were treated with 50ng/ml VEGF and/or transfected with the pp60(c-src)siRNA, while equivalent PBS and non-targeting siRNA were treated in the control groups.

RESULTS

VEGF enhanced Src bioactivity through promoting dephosphorylation of Src at Y527 and phosphorylation of Src at Y416. Meanwhile, Src specific pp60(c-src)siRNA significantly reduced Src expression in both cells. VEGF destroyed the junctional integrity of endothelial cells resulting in higher endothelial permeability. However, Src blockade significantly relieved VEGF induced actin stress and inhibited caveolae and VVOs formation, meanwhile further stabilized the complex β-catenin/VE-cadherin/Flk-1 through decreasing phosphorylation of VE-cadherin, ultimately decreasing VEGF-mediating higher vascular permeability. In addition, VEGF promoted osteoclasts formation and function without affecting the adhesion activity and cytoskeleton. We further found that Src blockade significantly impaired cytoskeleton resulting in a lower adhesion activity through down-regulation of phosphorylation of Src, Pyk2 and Cbl, and ultimately inhibited osteoclasts formation and function.

CONCLUSIONS

These findings provide a new insight into VEGF and c-Src mode of reaction in triggering destructive repair of osteonecrosis and further indicate that VEGF mediating vascular permeability and osteoclasts activity are Src-dependent. Blockade of Src may have great potential as an effective therapy targeting destructive repair in osteonecrosis.

Tools of the trade: podosomes as multipurpose organelles of monocytic cells

Podosomes are adhesion and invasion structures that are particularly prominent in cells of the monocytic lineage such as macrophages, dendritic cells, and osteoclasts. They are multifunctional organelles that combine several key abilities required for cell migration and invasion. The podosome repertoire includes well-established functions such as cell-substrate adhesion, and extracellular matrix degradation, recently discovered abilities such as rigidity and topology sensing as well as antigen sampling, and also more speculative functions such as cell protrusion stabilization and transmigration. Collectively, podosomes not only enable dynamic interactions of cells with their surroundings, they also gather information about the pericellular environment, and are actively involved in its reshaping. This review presents an overview of the current knowledge on podosome composition, architecture, and regulation. We focus in particular on the growing list of podosome functions and discuss the specific properties of podosomes in macrophages, dendritic cells, and osteoclasts. Moreover, this article highlights podosome-related intracellular transport processes, the formation of podosomes in 3D environments as well as potentially podosome-associated diseases involving monocytic cells.

Podosome-regulating kinesin KIF1C translocates to the cell periphery in a CLASP-dependent manner

The kinesin KIF1C is known to regulate podosomes, actin-rich adhesion structures that remodel the extracellular matrix during physiological processes. Here, we show that KIF1C is a player in the podosome-inducing signaling cascade. Upon induction of podosome formation by protein kinase C (PKC), KIF1C translocation to the cell periphery intensifies and KIF1C accumulates both in the proximity of peripheral microtubules that show enrichment for the plus-tip-associated proteins CLASPs and around podosomes. Importantly, without CLASPs, both KIF1C trafficking and podosome formation are suppressed. Moreover, chimeric mitochondrially targeted CLASP2 recruits KIF1C, suggesting a transient CLASP-KIF1C association. We propose that CLASPs create preferred microtubule tracks for KIF1C to promote podosome induction downstream of PKC.

Microtubule acetylation regulates dynamics of KIF1C-powered vesicles and contact of microtubule plus ends with podosomes

Microtubule dynamics are important for a variety of key cellular functions such as intracellular trafficking, adjustment of the cell surface proteome, or adhesion structure turnover. In the current study, we investigate the effects of altered microtubule acetylation levels on the subcellular distribution of kinesins and actin cytoskeletal architecture in primary human macrophages. Microtubule acetylation was altered by overexpression or siRNA-induced depletion of the acetylase MEC-17, or by blocking α-tubulin deacetylation by addition of the inhibitor tubacin. We show that microtubule acetylation influences the subcellular distribution of vesicles associated with the kinesin KIF1C, as well as their directionality, velocity and run length. Moreover, tubulin acetylation alters the targeting frequency of microtubule plus ends on podosomes and influences the number of podosomes per cell and thus the matrix-degrading capacity of macrophages. Collectively, our results point to α-tubulin acetylation as an important modification that impacts on kinesin vesicle dynamics, actin cytoskeletal architecture and cellular function of macrophages.

Reactive oxygen species induce the association of SHP-1 with c-Src and the oxidation of both to enhance osteoclast survival

Loss of ovarian function causes oxidative stress as well as bone loss. We hypothesized that reactive oxygen species (ROS) induced by the failure of ovarian function are responsible for the bone loss by increasing the number of osteoclasts (OC). We found that ROS enhanced OC survival via Src homology 2 domain-containing phosphatase-1 (SHP-1), c-Src, Akt, and ERK. ROS induced the association of SHP-1 with c-Src as well as the oxidation of c-Src and SHP-1. This resulted in inactivation of SHP-1 and activation of c-Src via phosphorylation of Tyr(416). Knockdown of c-Src or SHP-1 abolished the effect of ROS on OC survival. Moreover, downregulation of SHP-1 upregulated activation of c-Src, Akt, and ERK in the absence of any stimulus, suggesting that inactivation of SHP-1 is required for OC survival. We demonstrated that the association and oxidation of c-Src and SHP-1 by ROS are key steps in enhancing OC survival, which are responsible for increased bone loss when ovarian function ceases.

Cytoplasmic retention of a nucleocytoplasmic protein TBC1D3 by microtubule network is required for enhanced EGFR signaling

The hominoid oncogene TBC1D3 enhances epidermal growth factor receptor (EGFR) signaling and induces cell transformation. However, little is known regarding its spatio-temporal regulation and mechanism of tumorigenesis. In the current study, we identified the microtubule subunit β-tubulin as a potential interaction partner for TBC1D3 using affinity purification combined with mass spectrometry analysis. The interaction between TBC1D3 and β-tubulin was confirmed by co-immunoprecipitation. Using the same method, we also revealed that TBC1D3 co-precipitated with endogenous α-tubulin, another subunit of the microtubule. In agreement with these results, microtubule cosedimentation assays showed that TBC1D3 associated with the microtubule network. The β-tubulin-interacting site of TBC1D3 was mapped to amino acids 286∼353 near the C-terminus of the TBC domain. Deletion mutation within these amino acids was shown to abolish the interaction of TBC1D3 with β-tubulin. Interestingly, the deletion mutation caused a complete loss of TBC1D3 from the cytoplasmic filamentous and punctate structures, and TBC1D3 instead appeared in the nucleus. Consistent with this, wild-type TBC1D3 exhibited the same nucleocytoplasmic distribution in cells treated with the microtubule depolymerizing agent nocodazole, suggesting that the microtubule network associates with and retains TBC1D3 in the cytoplasm. We further found that deficiency in β-tubulin-interacting resulted in TBC1D3's inability to inhibit c-Cbl recruitment and EGFR ubiquitination, ultimately leading to dysregulation of EGFR degradation and signaling. Taken together, these studies indicate a novel model by which the microtubule network regulates EGFR stability and signaling through tubulin dimer/oligomer interaction with the nucleocytoplasmic protein TBC1D3.

Microtubule dynamic instability controls podosome patterning in osteoclasts through EB1, cortactin, and Src

In osteoclasts (OCs) podosomes are organized in a belt, a feature critical for bone resorption. Although microtubules (MTs) promote the formation and stability of the belt, the MT and/or podosome molecules that mediate the interaction of the two systems are not identified. Because the growing "plus" ends of MTs point toward the podosome belt, plus-end tracking proteins (+TIPs) might regulate podosome patterning. Among the +TIPs, EB1 increased as OCs matured and was enriched in the podosome belt, and EB1-positive MTs targeted podosomes. Suppression of MT dynamic instability, displacement of EB1 from MT ends, or EB1 depletion resulted in the loss of the podosome belt. We identified cortactin as an Src-dependent interacting partner of EB1. Cortactin-deficient OCs presented a defective MT targeting to, and patterning of, podosomes and reduced bone resorption. Suppression of MT dynamic instability or EB1 depletion increased cortactin phosphorylation, decreasing its acetylation and affecting its interaction with EB1. Thus, dynamic MTs and podosomes interact to control bone resorption.

The role of the e3 ligase cbl-B in murine dendritic cells

Dendritic cells (DCs) are potent antigen-presenting cells with a promising potential in cancer immunotherapy. Cbl proteins are E3 ubiquitin ligases and have been implicated in regulating the functional activity of various immune cells. As an example, c-Cbl negatively affects DC activation. We here describe that another member of the Cbl-protein family (i.e. Cbl-b) is highly expressed in murine bone-marrow-derived DCs (BMDCs). Differentiation of cblb−/− bone marrow mononuclear cells into classical BMDCs is unaltered, except enhanced induction of DEC-205 (CD205) expression. When tested in mixed-lymphocyte reaction (MLR), cblb−/− BMDCs exhibit increased allo-stimulatory capacity in vitro. BMDCs were next in vitro stimulated by various toll like receptor (TLR)-agonists (LPS, Poly(I:C), CpG) and exposed to FITC-labeled dextran. Upon TLR-stimulation, cblb−/− BMDCs produce higher levels of proinflammatory cytokines (IL-1α, IL-6 and TNF-α) and exhibit a slightly higher level of FITC-dextran uptake. To further characterize the functional significance of cblb−/− BMDCs we tested them in antigen-specific T cell responses against ovalbumin (OVA) protein and peptides, activating either CD8⁺ OT-I or CD4⁺ OT-II transgenic T cells. However, cblb−/− BMDCs are equally effective in inducing antigen-specific T cell responses when compared to wildtype BMDCs both in vitro and in vivo. The migratory capacity into lymph nodes during inflammation was similarly not affected by the absence of Cbl-b. In line with these observations, cblb−/− peptide-pulsed BMDCs are equally effective vaccines against OVA-expressing B16 tumors in vivo when compared to wildtype BMDCs. We conclude that in contrast to c-Cbl, Cbl-b plays only a limited role in the induction of Ag-specific T cell responses by murine BMDCs in vitro and in vivo.

E3 ubiquitin ligase-mediated regulation of bone formation and tumorigenesis

The ubiquitination–proteasome and degradation system is an essential process that regulates protein homeostasis. This system is involved in the regulation of cell proliferation, differentiation and survival, and dysregulations in this system lead to pathologies including cancers. The ubiquitination system is an enzymatic cascade that mediates the marking of target proteins by an ubiquitin label and thereby directs their degradation through the proteasome pathway. The ubiquitination of proteins occurs through a three-step process involving ubiquitin activation by the E1 enzyme, allowing for the transfer to a ubiquitin-conjugated enzyme E2 and to the targeted protein via ubiquitin-protein ligases (E3), the most abundant group of enzymes involved in ubiquitination. Significant advances have been made in our understanding of the role of E3 ubiquitin ligases in the control of bone turnover and tumorigenesis. These ligases are implicated in the regulation of bone cells through the degradation of receptor tyrosine kinases, signaling molecules and transcription factors. Initial studies showed that the E3 ubiquitin ligase c-Cbl, a multi-domain scaffold protein, regulates bone resorption by interacting with several molecules in osteoclasts. Further studies showed that c-Cbl controls the ubiquitination of signaling molecules in osteoblasts and in turn regulates osteoblast proliferation, differentiation and survival. Recent data indicate that c-Cbl expression is decreased in primary bone tumors, resulting in excessive receptor tyrosine kinase signaling. Consistently, c-Cbl ectopic expression reduces bone tumorigenesis by promoting tyrosine kinase receptor degradation. Here, we review the mechanisms of action of E3 ubiquitin ligases in the regulation of normal and pathologic bone formation, and we discuss how targeting the interactions of c-Cbl with some substrates may be a potential therapeutic strategy to promote osteogenesis and to reduce tumorigenesis.

Osteoclast-specific cathepsin K deletion stimulates S1P-dependent bone formation

Cathepsin K (CTSK) is secreted by osteoclasts to degrade collagen and other matrix proteins during bone resorption. Global deletion of Ctsk in mice decreases bone resorption, leading to osteopetrosis, but also increases the bone formation rate (BFR). To understand how Ctsk deletion increases the BFR, we generated osteoclast- and osteoblast-targeted Ctsk knockout mice using floxed Ctsk alleles. Targeted ablation of Ctsk in hematopoietic cells, or specifically in osteoclasts and cells of the monocyte-osteoclast lineage, resulted in increased bone volume and BFR as well as osteoclast and osteoblast numbers. In contrast, targeted deletion of Ctsk in osteoblasts had no effect on bone resorption or BFR, demonstrating that the increased BFR is osteoclast dependent. Deletion of Ctsk in osteoclasts increased their sphingosine kinase 1 (Sphk1) expression. Conditioned media from Ctsk-deficient osteoclasts, which contained elevated levels of sphingosine-1-phosphate (S1P), increased alkaline phosphatase and mineralized nodules in osteoblast cultures. An S1P1,3 receptor antagonist inhibited these responses. Osteoblasts derived from mice with Ctsk-deficient osteoclasts had an increased RANKL/OPG ratio, providing a positive feedback loop that increased the number of osteoclasts. Our data provide genetic evidence that deletion of CTSK in osteoclasts enhances bone formation in vivo by increasing the generation of osteoclast-derived S1P.

c-Cbl shRNA-expressing adenovirus sensitizes TRAIL-induced apoptosis in prostate cancer DU-145 through increases of DR4/5

We previously demonstrated that the downregulation of Casitas B-lineage lymphoma (c-Cbl) can sensitize tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis in two different ways. One way is to block the rapid degradation of TRAIL receptors, which can sustain TRAIL-induced apoptosis for a long time. Here, we designed a replication-defective adenovirus expressing the short hairpin RNA (shRNA) against c-Cbl to test the possibility of developing a cancer gene therapy that can act as a sensitizer of TRAIL. As expected from the results of our previous study that used a stable cell line with downregulated c-Cbl, infection with the c-Cbl shRNA-expressing adenovirus led to an increase in the death receptor 4 (DR4) and DR5 levels, which is known to be a cause for the increase of TRAIL-induced apoptosis. In conclusion, we demonstrated that c-Cbl shRNA-expressing adenovirus is able to sensitize TRAIL-induced apoptosis in vivo as well as in vitro.

[Implication of the ubiquitin ligase c-Cbl in bone formation and tumorigenesis]

Cbl ubiquitin ligases are important molecules that control the process of ubiquitination and degradation of proteins by the proteasome. Because this process regulates several intracellular mechanisms, alterations in Cbl activity lead to several pathologies including cancer. In bone, the c-Cbl ubiquitin ligase is known to control osteoclast activity. Our studies indicate that c-Cbl also regulates osteoblast proliferation, differentiation and survival. We recently showed that inhibition of c-Cbl activity using a c-Cbl mutant leads to promote osteoblast differentiation in mesenchymal stromal cells as a consequence of increased receptor tyrosine kinase expression. Conversely, we found that overexpression of c-Cbl leads to inhibit osteosarcoma cell proliferation and tumorigenesis through downregulation of these receptors. Thus, the use of pharmacological agents capable of modulating c-Cbl activity may be of therapeutic interest for promoting bone formation in normal bone, or to reduce tumorigenesis in primary bone cancer.

Met kinase-dependent loss of the E3 ligase Cbl in gastric cancer

Strict regulation of signaling by receptor tyrosine kinases (RTKs) is essential for normal biological processes, and disruption of this regulation can lead to tumor initiation and progression. Signal duration by the Met RTK is mediated in part by the E3 ligase Cbl. Cbl is recruited to Met upon kinase activation and promotes ubiquitination, trafficking, and degradation of the receptor. The Met RTK has been demonstrated to play a role in various types of cancer. Here, we show that Met-dependent loss of Cbl protein in MET-amplified gastric cancer cell lines represents another mechanism contributing to signal dysregulation. Loss of Cbl protein is dependent on Met kinase activity and is partially rescued with a proteasome inhibitor, lactacystin. Moreover, Cbl loss not only uncouples Met from Cbl-mediated negative regulation but also releases other Cbl targets, such as the EGF receptor, from Cbl-mediated signal attenuation. Thus, Met-dependent Cbl loss may also promote cross-talk through indirect enhancement of EGF receptor signaling.

LIS1 regulates osteoclast formation and function through its interactions with dynein/dynactin and Plekhm1

Microtubule organization and lysosomal secretion are both critical for the activation and function of osteoclasts, highly specialized polykaryons that are responsible for bone resorption and skeletal homeostasis. Here, we have identified a novel interaction between microtubule regulator LIS1 and Plekhm1, a lysosome-associated protein implicated in osteoclast secretion. Decreasing LIS1 expression by shRNA dramatically attenuated osteoclast formation and function, as shown by a decreased number of mature osteoclasts differentiated from bone marrow macrophages, diminished resorption pits formation, and reduced level of CTx-I, a bone resorption marker. The ablated osteoclast formation in LIS1-depleted macrophages was associated with a significant decrease in macrophage proliferation, osteoclast survival and differentiation, which were caused by reduced activation of ERK and AKT by M-CSF, prolonged RANKL-induced JNK activation and declined expression of NFAT-c1, a master transcription factor of osteoclast differentiation. Consistent with its critical role in microtubule organization and dynein function in other cell types, we found that LIS1 binds to and colocalizes with dynein in osteoclasts. Loss of LIS1 led to disorganized microtubules and aberrant dynein function. More importantly, the depletion of LIS1 in osteoclasts inhibited the secretion of Cathepsin K, a crucial lysosomal hydrolase for bone degradation, and reduced the motility of osteoclast precursors. These results indicate that LIS1 is a previously unrecognized regulator of osteoclast formation, microtubule organization, and lysosomal secretion by virtue of its ability to modulate dynein function and Plekhm1.

The skeleton: a multi-functional complex organ: the role of key signalling pathways in osteoclast differentiation and in bone resorption

Osteoclasts are the specialised cells that resorb bone matrix and are important both for the growth and shaping of bones throughout development as well as during the process of bone remodelling that occurs throughout life to maintain a healthy skeleton. Osteoclast formation, function and survival are tightly regulated by a network of signalling pathways, many of which have been identified through the study of rare monogenic diseases, knockout mouse models and animal strains carrying naturally occurring mutations in key molecules. In this review, we describe the processes of osteoclast formation, activation and function and discuss the major transcription factors and signalling pathways (including those that control the cytoskeletal rearrangements) that are important at each stage.

Microtubule assembly affects bone mass by regulating both osteoblast and osteoclast functions: stathmin deficiency produces an osteopenic phenotype in mice

Cytoskeleton microtubules regulate various cell signaling pathways that are involved in bone cell function. We recently reported that inhibition of microtubule assembly by microtubule-targeting drugs stimulates osteoblast differentiation and bone formation. To further elucidate the role of microtubules in bone homeostasis, we characterized the skeletal phenotype of mice null for stathmin, an endogenous protein that inhibits microtubule assembly. In vivo micro-computed tomography (µCT) and histology revealed that stathmin deficiency results in a significant reduction of bone mass in adult mice concurrent with decreased osteoblast and increased osteoclast numbers in bone tissues. Phenotypic analyses of primary calvarial cells and bone marrow cells showed that stathmin deficiency inhibited osteoblast differentiation and induced osteoclast formation. In vitro overexpression studies showed that increased stathmin levels enhanced osteogenic differentiation of preosteoblast MC3T3-E1 cells and mouse bone marrow-derived cells and attenuated osteoclast formation from osteoclast precursor Raw264.7 cells and bone marrow cells. Results of immunofluorescent studies indicated that overexpression of stathmin disrupted radial microtubule filaments, whereas deficiency of stathmin stabilized the microtubule network structure in these bone cells. In addition, microtubule-targeting drugs that inhibit microtubule assembly and induce osteoblast differentiation lost these effects in the absence of stathmin. Collectively, these results suggest that stathmin, which alters microtubule dynamics, plays an essential role in maintenance of postnatal bone mass by regulating both osteoblast and osteoclast functions in bone. \

Degrading devices: invadosomes in proteolytic cell invasion

Podosomes and invadopodia, collectively known as invadosomes, are cell-matrix contacts in a variety of cell types, such as monocytic cells or cancer cells, that have to cross tissue barriers. Both structures share an actin-rich core, which distinguishes them from other matrix contacts, and are regulated by a multitude of signaling pathways including RhoGTPases, kinases, actin-associated proteins, and microtubule-dependent transport. Invadosomes recruit and secrete proteinases and are thus able to lyse extracellular matrix components. They are therefore considered to be potential key structures in proteolytic cell invasion in both physiological and pathological settings. This review provides an overview of the field, with special focus on current developments such as intracellular transport processes, ultrastructural analysis, the possible involvement of invadosomes in disease, and the tentative identification of invadosomes in 3D environments and in vivo.

Bim: guardian of tissue homeostasis and critical regulator of the immune system, tumorigenesis and bone biology

One of the most important roles of apoptosis is the maintenance of tissue homeostasis. Impairment of apoptosis leads to a number of pathological conditions. In response to apoptotic signals, various proteins are activated in a pathway and signal-specific manner. Recently, the pro-apoptotic molecule Bim has attracted increasing attention as a pivotal regulator of tissue homeostasis. The Bim expression level is strictly controlled in both transcriptional and post-transcriptional levels. This control is dependent on cell, tissue and apoptotic stimuli. The phenotype of Bim-deficient mice is a systemic lupus erythematosus-like autoimmune disease with an abnormal accumulation of hematopoietic cells. Bim is thus a critical regulator of hematopoietic cells and immune system. Further studies have revealed the critical roles of Bim in various normal and pathological conditions, including bone homeostasis and tumorigenesis. The current understanding of Bim signaling and roles in the maintenance of tissue homeostasis is reviewed in this paper, focusing on the immune system, bone biology and tumorigenesis to illustrate the diversified role of Bim.

Signaling networks regulating leukocyte podosome dynamics and function

Podosomes are ventral adhesion structures prominent in cells of the myeloid lineage. A common aspect of these cells is that they are highly motile and must to traverse multiple tissue barriers in order to perform their functions. Recently podosomes have gathered attention from researchers as important cellular structures that can influence cell adhesion, motility and matrix remodeling. Adhesive and soluble ligands act via transmembrane receptors and propagate signals to the leukocyte cytoskeleton via small G proteins of the Rho family, tyrosine kinases and scaffold proteins and are able to induce podosome formation and rearrangements. Manipulation of the signals that regulate podosome formation and dynamics can therefore be a strategy to interfere with leukocyte functions in a multitude of pathological settings, such as infections, atherosclerosis and arthritis. Here, we review the major signaling molecules that act in the formation and regulation of podosomes.

The tale of protein lysine acetylation in the cytoplasm

Reversible posttranslational modification of internal lysines in many cellular or viral proteins is now emerging as part of critical signalling processes controlling a variety of cellular functions beyond chromatin and transcription. This paper aims at demonstrating the role of lysine acetylation in the cytoplasm driving and coordinating key events such as cytoskeleton dynamics, intracellular trafficking, vesicle fusion, metabolism, and stress response.

Hdac-mediated control of endochondral and intramembranous ossification

Histone deacetylases (Hdacs) remove acetyl groups (CH3CO-) from ε-amino groups in lysine residues within histones and other proteins. This posttranslational (de) modification alters protein stability, protein-protein interactions, and chromatin structure. Hdac activity plays important roles in the development of all organs and tissues, including the mineralized skeleton. Bone is a dynamic tissue that forms and regenerates by two processes: endochondral and intramembranous ossification. Chondrocytes and osteoblasts are responsible for producing the extracellular matrices of skeletal tissues. Several Hdacs contribute to the molecular pathways and chromatin changes that regulate tissue-specific gene expression during chondrocyte and osteoblast specification, maturation, and terminal differentiation. In this review, we summarize the roles of class I and class II Hdacs in chondrocytes and osteoblasts. The effects of small molecule Hdac inhibitors on the skeleton are also discussed.

Zfp521 is a target gene and key effector of parathyroid hormone-related peptide signaling in growth plate chondrocytes

In the growth plate, the interplay between parathyroid hormone-related peptide (PTHrP) and Indian hedgehog (Ihh) signaling tightly regulates chondrocyte proliferation and differentiation during longitudinal bone growth. We found that PTHrP increases the expression of Zfp521, a zinc finger transcriptional coregulator, in prehypertrophic chondrocytes. Mice with chondrocyte-targeted deletion of Zfp521 resembled PTHrP(-/-) and chondrocyte-specific PTHR1(-/-) mice, with decreased chondrocyte proliferation, early hypertrophic transition, and reduced growth plate thickness. Deleting Zfp521 increased expression of Runx2 and Runx2 target genes, and decreased Cyclin D1 and Bcl-2 expression while increasing Caspase-3 activation and apoptosis. Zfp521 associated with Runx2 in chondrocytes, antagonizing its activity via an HDAC4-dependent mechanism. PTHrP failed to upregulate Cyclin D1 and to antagonize Runx2, Ihh, and collagen X expression when Zfp521 was absent. Thus, Zfp521 is an important PTHrP target gene that regulates growth plate chondrocyte proliferation and differentiation.

The ins and outs of tubulin acetylation: more than just a post-translational modification?

Microtubules are highly dynamic polymers of α/β tubulin heterodimers that play key roles in cell division and in organizing cell cytoplasm. Although they have been discovered more than two decades ago, tubulin post-translational modifications recently gained a new interest as their role was increasingly highlighted in neuron differentiation and neurodegenerative disorders. Here, we specifically focus on tubulin acetylation from its discovery to recent studies that provide new insights into how it is regulated in health and disease and how it impacts microtubule functions. Even though new mechanisms involving tubulin acetylation are regularly being uncovered, the molecular links between its location inside the microtubule lumen and its regulators and effectors is still poorly understood. This review highlights the emerging roles of tubulin acetylation in multiple cellular functions, ranging from cell motility, cell cycle progression or cell differentiation to intracellular trafficking and signalling. It also points out that tubulin acetylation should no longer be seen as a passive marker of microtubule stability, but as a broad regulator of microtubule functions.

The loss of Cbl-phosphatidylinositol 3-kinase interaction perturbs RANKL-mediated signaling, inhibiting bone resorption and promoting osteoclast survival

Cbl is an adaptor protein and an E3 ligase that plays both positive and negative roles in several signaling pathways that affect various cellular functions. Tyrosine 737 is unique to Cbl and is phosphorylated by Syk and Src family kinases. Phosphorylated Cbl Tyr(737) creates a binding site for the p85 regulatory subunit of PI3K, which also plays an important role in the regulation of bone resorption by osteoclasts. To investigate the role of Cbl-PI3K interaction in bone homeostasis, we examined the knock-in mice (Cbl(YF/YF)) in which the PI3K binding site in Cbl is ablated due to the mutation in the regulatory tyrosine. We report that in Cbl(YF/YF) mice, despite increased numbers of osteoclasts, bone volume is increased due to defective osteoclast function. Additionally, in ex vivo cultures, mature Cbl(YF/YF) osteoclasts showed an increased ability to survive in the presence of RANKL due to delayed onset of apoptosis. RANKL-mediated signaling is perturbed in Cbl(YF/YF) osteoclasts, and most interestingly, AKT phosphorylation is up-regulated, suggesting that the lack of PI3K sequestration by Cbl results in increased survival and decreased bone resorption. Cumulatively, these in vivo and in vitro results show that, on one hand, binding of Cbl to PI3K negatively regulates osteoclast differentiation, survival, and signaling events (e.g. AKT phosphorylation), whereas on the other hand it positively influences osteoclast function.

c-Cbl promotes T cell receptor-induced thymocyte apoptosis by activating the phosphatidylinositol 3-kinase/Akt pathway

The ability of thymocytes to assess T cell receptor (TCR) signaling strength and initiate the appropriate downstream response is crucial for determining their fate. We have previously shown that a c-Cbl RING finger mutant knock-in mouse, in which the E3 ubiquitin ligase activity of c-Cbl is inactivated, is highly sensitive to TCR-induced death signals that cause thymic deletion. This high intensity signal involves the enhanced tyrosine phosphorylation of the mutant c-Cbl protein promoting a marked increase in the activation of Akt. Here we show that this high intensity signal in c-Cbl RING finger mutant thymocytes also promotes the enhanced induction of two mediators of TCR-directed thymocyte apoptosis, Nur77 and the pro-apoptotic Bcl-2 family member, Bim. In contrast, a knock-in mouse harboring a mutation at Tyr-737, the site in c-Cbl that activates phosphatidylinositol 3-kinase, shows reduced TCR-mediated responses including suppression of Akt activation, a reduced induction of Nur77 and Bim, and greater resistance to thymocyte death. These findings identify tyrosine-phosphorylated c-Cbl as a critical sensor of TCR signal strength that regulates the engagement of death-promoting signals.