The tyrosine kinase activity of c-Src regulates actin dynamics and organization of podosomes in osteoclasts

The podosomal-adaptor protein SH3PXD2B is essential for normal postnatal development

Podosome-type adhesions are actin-based membrane protrusions involved in cell-matrix adhesion and extracellular matrix degradation. Despite growing knowledge of many proteins associated with podosome-type adhesions, much remains unknown concerning the function of podosomal proteins at the level of the whole animal. In this study, the spontaneous mouse mutant nee was used to identify a component of podosome-type adhesions that is essential for normal postnatal growth and development. Mice homozygous for the nee allele exhibited runted growth, craniofacial and skeletal abnormalities, ocular anterior segment dysgenesis, and hearing impairment. Adults also exhibited infertility and a form of lipodystrophy. Using genetic mapping and DNA sequencing, the cause of nee phenotypes was identified as a 1-bp deletion within the Sh3pxd2b gene on mouse Chromosome 11. Whereas the wild-type Sh3pxd2b gene is predicted to encode a protein with one PX domain and four SH3 domains, the nee mutation is predicted to cause a frameshift and a protein truncation altering a portion of the third SH3 domain and deleting all of the fourth SH3 domain. The SH3PXD2B protein is believed to be an important component of podosomes likely to mediate protein-protein interactions with membrane-spanning metalloproteinases. Testing this directly, SH3PXD2B localized to podosomes in constitutively active Src-transfected fibroblasts and through its last SH3 domain associated with a transmembrane member of a disintegrin and metalloproteinase family of proteins, ADAM15. These results identify SH3PXD2B as a podosomal-adaptor protein required for postnatal growth and development, particularly within physiologic contexts involving extracellular matrix regulation.

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

c-Cbl and Cbl-b are highly conserved adaptor proteins that participate in integrin signaling, regulating cytoskeletal organization, motility, and bone resorption. Deletion of both c-Cbl and Cbl-b in mice leads to embryonic lethality, indicating that the two proteins perform essential redundant functions. To examine the redundant actions of c-Cbl and Cbl-b in osteoclasts, we depleted c-Cbl in Cbl-b(-/-) osteoclasts by using a short hairpin RNA. Depleting both Cbl proteins disrupted both the podosome belt and the microtubule network and decreased bone-resorbing activity. Stabilizing the microtubules with paclitaxel or inhibiting histone deacetylase 6 (HDAC6), which destabilizes microtubules by deacetylating beta-tubulin, protected both the microtubule network and the podosome belt. Examination of the mechanism involved demonstrated that the conserved four-helix bundle of c-Cbl's tyrosine kinase binding domain bound to beta-tubulin, and both c-Cbl and Cbl-b displaced HDAC6. In addition to the effects on microtubules and the podosome belt, depleting both Cbls significantly increased the levels of the proapoptotic protein Bim and apoptosis relative to the levels induced by eliminating either protein alone. Thus, both c-Cbl and Cbl-b promote bone resorption via the stabilization of microtubules, allowing the formation of the podosome belt in osteoclasts, and by promoting osteoclast survival.

Dasatinib inhibits the growth of prostate cancer in bone and provides additional protection from osteolysis

Background:

Dasatinib is a small molecule kinase inhibitor that has recently been shown to inhibit Src family kinases (SFK) and also has activity against CaP. Of importance to metastatic CaP, which frequently metastasises to bone, SFK are also vital to the regulation of bone remodelling. We sought to determine the ability of dasatinib to inhibit growth of CaP in bone.

Methods:

C4-2B CaP cells were injected into tibiae of SCID mice and treated with dasatinib, alone or in combination with docetaxel. Serum prostate-specific antigen levels, bone mineral density, radiographs and histology were analysed.

Results:

Treatment with dasatinib alone significantly lowered sacrifice serum prostate-specific antigen levels compared to control, 2.3±0.4 vs 9.2±2.1 (P=0.004). Combination therapy improved efficacy over dasatinib alone (P=0.010). Dasatinib increased bone mineral density in tumoured tibiae by 25% over control tumoured tibiae (P<0.001).

Conclusion:

Dasatinib inhibits growth of C4-2B cells in bone with improved efficacy when combined with docetaxel. Additionally, dasatinib inhibits osteolysis associated with CaP. These data support further study of dasatinib in clinical trials for men with CaP bone metastases.

ATP-induced osteoclast function: the formation of sealing-zone like structure and the secretion of lytic granules via microtubule-deacetylation under the control of Syk

Osteoclasts are bone-resorbing cells which play an exclusive role in bone remodeling, but the molecular mechanisms of osteolysis, how osteoclasts are activated and how the lytic granules are finally released towards the bone matrix are poorly understood. Here we show that an energy molecule ATP induces osteolysis via P2X(7)-nucleotide receptor and that deacetylation of alpha-tubulin is essential for the whole process of osteolysis under the control of a tyrosine kinase Syk. By developing a traceable and reproducible in vitro analyzing system for osteoclast function, we found that ATP-signaling gives rise to two events simultaneously (i) cytoskeletal reorganization for the formation of sealing zones, ring-like adhesion structures which delimit the contact surface, and (ii) the delivery and secretion of lytic granules towards the delimited site on the matrix. We further found that deacetylation of alpha-tubulin is a critical reaction for osteoclast function. Pharmacological inhibition of alpha-tubulin deacetylation resulted in (i) failure of the sealing-zone like structure formation and (ii) ceased secretion of lytic granules. Additionally, kinetics of deacetylation was found to be regulated by Syk. These data suggest a novel P2X(7) microtubular regulation pathway related to Syk for a therapeutic target in osteolytic diseases.

SRC family kinases are required for limb trajectory selection by spinal motor axons

Signal relay by guidance receptors at the axonal growth cone is a process essential for the assembly of a functional nervous system. We investigated the in vivo function of Src family kinases (SFKs) as growth cone guidance signaling intermediates in the context of spinal lateral motor column (LMC) motor axon projection toward the ventral or dorsal limb mesenchyme. Using in situ mRNA detection we determined that Src and Fyn are expressed in LMC motor neurons of chick and mouse embryos at the time of limb trajectory selection. Inhibition of SFK activity by C-terminal Src kinase (Csk) overexpression in chick LMC axons using in ovo electroporation resulted in LMC axons selecting the inappropriate dorsoventral trajectory within the limb mesenchyme, with medial LMC axon projecting into the dorsal and ventral limb nerve with apparently random incidence. We also detected LMC axon trajectory choice errors in Src mutant mice demonstrating a nonredundant role for Src in motor axon guidance in agreement with gain and loss of Src function in chick LMC neurons which led to the redirection of LMC axons. Finally, Csk-mediated SFK inhibition attenuated the retargeting of LMC axons caused by EphA or EphB over-expression, implying the participation of SFKs in Eph-mediated LMC motor axon guidance. In summary, our findings demonstrate that SFKs are essential for motor axon guidance and suggest that they play an important role in relaying ephrin:Eph signals that mediate the selection of motor axon trajectory in the limb.

[The endothelial podosome]

Podosomes are highly dynamic adhesion microdomains formed at the ventral membrane of some monocyte-derived cells. Structurally, their most distinguishing feature is their two-part architecture, consisting in a core of F-actin and actin-associated proteins, surrounded by a ring structure consisting of plaque proteins as well as signalling proteins. In addition to the presence of specific markers, they are distinguished from other adhesion structures by the presence of metalloproteases, endowing them with the ability to degrade the extracellular matrix. Invadopodia are related structures, of similar molecular composition but of distinct architecture, made by fibroblasts or epithelial cells transformed by the v-src oncogene or aggressive carcinoma cells. Such membrane-associated cellular devices, now named invadosomes, are thought to have a central role in mediating polarized migration in cells that cross anatomical boundaries. Podosomes have now been shown to form in endothelial cells, non monocytic and non tumoral cells, endowed with tissue invasive activities during vascular remodelling. Here, we summarize the recent advances and developments in this field, discuss how endothelial podosomes combine specificities of monocytic podosomes and invadopodia and provide our provisional outlook into the future understanding of endothelial podosomes.

Dynamin reduces Pyk2 Y402 phosphorylation and SRC binding in osteoclasts

Signaling via the Pyk2-Src-Cbl complex downstream of integrins contributes to the assembly, organization, and dynamics of podosomes, which are the transient adhesion complexes of highly motile cells such as osteoclasts and dendritic cells. We previously demonstrated that the GTPase dynamin is associated with podosomes, regulates actin flux in podosomes, and promotes bone resorption by osteoclasts. We report here that dynamin associates with Pyk2, independent of dynamin's GTPase activity, and reduces Pyk2 Y402 phosphorylation in a GTPase-dependent manner, leading to decreased Src binding to Pyk2. Overexpressing dynamin decreased the macrophage colony-stimulating factor- and adhesion-induced phosphorylation of Pyk2 in osteoclastlike cells, suggesting that dynamin is likely to regulate Src-Pyk2 binding downstream of integrins and growth factor receptors with important cellular consequences. Furthermore, catalytically active Src promotes dynamin-Pyk2 association, and mutating specific Src-phosphorylated tyrosine residues in dynamin blunts the dynamin-induced decrease in Pyk2 phosphorylation. Thus, since Src binds to Pyk2 through its interaction with phospho-Y402, our results suggest that Src activates a negative-feedback loop downstream of integrin engagement and other stimuli by promoting both the binding of dynamin to Pyk2-containing complexes and the dynamin-dependent decrease in Pyk2 Y402 phosphorylation, ultimately leading to the dissociation of Src from Pyk2.

Bovine dentine organic matrix down-regulates osteoclast activity

Physiological root resorption is a phenomenon that normally takes place in deciduous teeth; root resorption of permanent teeth occurs only under pathological conditions. The molecular mechanisms underlying these processes are still unclear. Our previous study showed that osteoclasts cultured on deciduous dentine exhibited a higher degree of resorption and higher levels of cathepsin K and MMP-9 mRNA than osteoclasts cultured on permanent dentine. These results could be because of different susceptibilities to acid and the different organic matrices between deciduous and permanent dentine. Thus, the purpose of this study was to investigate the effect of dentine extracts from bovine deciduous and permanent dentine on osteoclast activity. Osteoclasts, obtained from mouse bone marrow cells co-cultured with an osteoblast-rich fraction in the presence of 1,25-(OH)(2)-vitamin D3 and PGE2, were incubated with or without 0.6 M HCl extracts from bovine deciduous or permanent dentine for 48 h. TRAP positive cell number, TRAP activity, the areas of resorption pits, and mRNA levels of TRAP, v-ATPase, calcitonin receptor, cathepsin K, and MMP-9 were examined. The results illustrated that TRAP activity, the resorbed area, and the mRNA levels of osteoclast marker genes seemed to be suppressed by both deciduous and permanent dentine extracts. These findings indicate that some factors that suppress osteoclast activity are contained in both deciduous and permanent dentine extracts. Although there was no significant difference in osteoclast activity between deciduous and permanent dentine extracts, osteoclasts incubated with permanent dentine extracts tend to exhibit less resorption activity than those incubated with deciduous dentine extracts. However, we could not clearly explain the causes of this.

Signaling networks that control the lineage commitment and differentiation of bone cells

Osteoblasts and osteoclasts are the two major bone cells involved in the bone remodeling process. Osteoblasts are responsible for bone formation while osteoclasts are the bone-resorbing cells. The major event that triggers osteogenesis and bone remodeling is the transition of mesenchymal stem cells into differentiating osteoblast cells and monocyte/macrophage precursors into differentiating osteoclasts. Imbalance in differentiation and function of these two cell types will result in skeletal diseases such as osteoporosis, Paget's disease, rheumatoid arthritis, osteopetrosis, periodontal disease, and bone cancer metastases. Osteoblast and osteoclast commitment and differentiation are controlled by complex activities involving signal transduction and transcriptional regulation of gene expression. Recent advances in molecular and genetic studies using gene targeting in mice enable a better understanding of the multiple factors and signaling networks that control the differentiation process at a molecular level. This review summarizes recent advances in studies of signaling transduction pathways and transcriptional regulation of osteoblast and osteoclast cell lineage commitment and differentiation. Understanding the signaling networks that control the commitment and differentiation of bone cells will not only expand our basic understanding of the molecular mechanisms of skeletal development but will also aid our ability to develop therapeutic means of intervention in skeletal diseases.

Cell biology: watching the first steps of podosome formation

Podosomes and invadopodia are actin-rich structures that have come under intense scrutiny over the past several years due to their critical roles in cell migration and invasion. Examination of the initial stages of podosome formation has revealed an important role for the phosphoinositide PI(3,4)P(2) in anchoring the scaffold protein Tks5 to the plasma membrane.

ERK5 promotes Src-induced podosome formation by limiting Rho activation

Increased Src activity, often associated with tumorigenesis, leads to the formation of invasive adhesions termed podosomes. Podosome formation requires the function of Rho family guanosine triphosphatases and reorganization of the actin cytoskeleton. In addition, Src induces changes in gene expression required for transformation, in part by activating mitogen-activated protein kinase (MAPK) signaling pathways. We sought to determine whether MAPK signaling regulates podosome formation. Unlike extracellular signal–regulated kinase 1/2 (ERK1/2), ERK5 is constitutively activated in Src-transformed fibroblasts. ERK5-deficient cells expressing v-Src exhibited increased RhoA activation and signaling, which lead to cellular retraction and an inability to form podosomes or induce invasion. Addition of the Rho-kinase inhibitor Y27632 to ERK5-deficient cells expressing v-Src led to cellular extension and restored podosome formation. In Src-transformed cells, ERK5 induced the expression of a Rho GTPase-activating protein (RhoGAP), RhoGAP7/DLC-1, via activation of the transcription factor myocyte enhancing factor 2C, and RhoGAP7 expression restored podosome formation in ERK5-deficient cells. We conclude that ERK5 promotes Src-induced podosome formation by inducing RhoGAP7 and thereby limiting Rho activation.

Rho GTPases in osteoclasts: orchestrators of podosome arrangement

Cells from the myeloid lineage, namely macrophages, dendritic cells and osteoclasts, develop podosomes instead of stress fibers and focal adhesions to adhere and migrate. Podosomes share many components with focal adhesions but differ in their molecular organization, with a dense core of polymerized actin surrounded by scaffolding proteins, kinases and integrins. Podosomes are found either isolated both in macrophages and dendritic cells or arranged into superstructures in osteoclasts. When osteoclasts resorb bone, they form an F-actin rich sealing zone, which is a dense array of connected podosomes that firmly anchors osteoclasts to bone. It delineates a compartment in which protons and proteases are secreted to dissolve and degrade the mineralized matrix. Since Rho GTPases have been shown to control F-actin stress fibers and focal adhesions in mesenchymal cells, the question of whether they could also control podosome formation and arrangement in cells from the myeloid lineage, and particularly in osteoclasts, rapidly emerged. This article considers recent advances made in our understanding of podosome arrangements in osteoclasts and how Rho GTPases may control it.

Podosome-type adhesions and focal adhesions, so alike yet so different

Cell-matrix adhesions are essential for cell migration, tissue organization and differentiation, therefore playing central roles in embryonic development, remodeling and homeostasis of tissues and organs. Matrix adhesion-dependent signals cooperate with other pathways to regulate biological functions such as cell survival, cell proliferation, wound healing, and tumorigenesis. Cell migration and invasion are integrated processes requiring the continuous, coordinated assembly and disassembly of integrin-mediated adhesions. An understanding of how integrins regulate cell migration and invasiveness through the dynamic regulation of adhesions is fundamental to both physiological and pathological situations. A variety of cell-matrix adhesions has been identified, namely, focal complexes, focal adhesions, fibrillar adhesions, podosomes, and invadopodia (podosome-type adhesions). These adhesion sites contain integrin clusters able to develop specialized structures, which are different in their architecture and dynamics although they share almost the same proteins. Here we compare recent advances and developments in the elucidation of the organization and dynamics of focal adhesions and podosome-type adhesions, in order to understand how such subcellular sites - though closely related in their composition - can be structurally and functionally different. The underlying question is how their respective physiological or pathological roles are related to their distinct organization.

Protein tyrosine phosphatases in osteoclast differentiation, adhesion, and bone resorption

Osteoclasts are large cells derived from the monocyte-macrophage hematopoietic cell lineage. Their primary function is to degrade bone in various physiological contexts. Osteoclasts adhere to bone via podosomes, specialized adhesion structures whose structure and subcellular organization are affected by mechanical contact of the cell with bone matrix. Ample evidence indicates that reversible tyrosine phosphorylation of podosomal proteins plays a major role in determining the organization and dynamics of podosomes. Although roles of several tyrosine kinases are known in detail in this respect, little is known concerning the roles of protein tyrosine phosphatases (PTPs) in regulating osteoclast adhesion. Here we summarize available information concerning the known and hypothesized roles of the best-researched PTPs in osteoclasts - PTPRO, PTP epsilon, SHP-1, and PTP-PEST. Of these, PTPRO, PTP epsilon, and PTP-PEST appear to support osteoclast activity while SHP-1 inhibits it. Additional studies are required to provide full molecular details of the roles of these PTPs in regulating osteoclast adhesion, and to uncover additional PTPs that participate in this process.