Bone matrix regulates osteoclast differentiation and annexin A8 gene expression

While attachment to bone is required for optimal osteoclast function, the molecular events that underlie this fact are unclear, other than that the cell requires adhesion to mineralized matrix to assume a fully differentiated phenotype. To address this issue, we cultured murine bone marrow-derived osteoclasts on either cell culture plastic or devitalized mouse calvariae to identify the distinct genetic profile induced by interaction with bone. Among a number of genes previously unknown to be expressed in osteoclasts we found that Annexin A8 (AnxA8) mRNA was markedly up-regulated by bone. AnxA8 protein was present at high levels in osteoclasts present in human tissues recovered from sites of pathological bone loss. The presence of bone mineral was required for up-regulation of AnxA8 mRNA since osteoclasts plated on decalcified bone express AnxA8 at low levels as did osteoclasts plated on native or denatured type I collagen. Finally, AnxA8-regulated cytoskeletal reorganization in osteoclasts generated on a mineralized matrix. Thus, we used a novel approach to define a distinct bone-dependent genetic program associated with terminal osteoclast differentiation and identified Anxa8 as a gene strongly induced late in osteoclast differentiation and a protein that regulates formation of the cell's characteristic actin ring.

Disruption of the Man-6-P targeting pathway in mice impairs osteoclast secretory lysosome biogenesis

Osteoclasts are specialized cells that secrete lysosomal acid hydrolases at the site of bone resorption, a process critical for skeletal formation and remodeling. However, the cellular mechanism underlying this secretion and the organization of the endo-lysosomal system of osteoclasts have remained unclear. We report that osteoclasts differentiated in vitro from murine bone marrow macrophages contain two types of lysosomes. The major species is a secretory lysosome containing cathepsin K and tartrate-resistant acid phosphatase (TRAP), two hydrolases critical for bone resorption. These secretory lysosomes are shown to fuse with the plasma membrane, allowing the regulated release of acid hydrolases at the site of bone resorption. The other type of lysosome contains cathepsin D, but little cathepsin K or TRAP. Osteoclasts from Gnptab(-/-) (gene encoding GlcNAc-1-phosphotransferase α, β-subunits) mice, which lack a functional mannose 6-phosphate (Man-6-P) targeting pathway, show increased secretion of cathepsin K and TRAP and impaired secretory lysosome formation. However, cathepsin D targeting was intact, showing that osteoclasts have a Man-6-P-independent pathway for selected acid hydrolases.

Fyn promotes proliferation, differentiation, survival and function of osteoclast lineage cells

c-Src and Lyn are the only Src family kinases (SFKs) with established activity in osteoclasts (OCs). c-Src promotes function via cytoskeletal organization of the mature resorptive cell while Lyn is a negative regulator of osteoclastogenesis. We establish that Fyn, another SFK, also impacts the OC, but in a manner distinctly different than c-Src and Lyn. Fyn deficiency principally alters cells throughout the osteoclastogenic process, resulting in diminished numbers of resorptive polykaryons. Arrested OC formation in the face of insufficient Fyn reflects reduced proliferation of precursors, in response to M-CSF and retarded RANK ligand (RANKL)-induced differentiation, attended by suppressed activation of the osteoclastogenic signaling molecules, c-Jun, and NF-κB. The anti-apoptotic properties of RANKL are also compromised in cells deleted of Fyn, an event mediated by increased Bim expression and failed activation of Akt. The defective osteoclastogenesis of Fyn-/- OCs dampens bone resorption, in vitro. Finally, while Fyn deficiency does not regulate basal osteoclastogenesis, in vivo, it reduces that stimulated by RANKL by ~2/3. Thus, Fyn is a pro-resorptive SFK, which exerts its effects by prompting proliferation and differentiation while attenuating apoptosis of OC lineage cells.

Calpain-6, a target molecule of glucocorticoids, regulates osteoclastic bone resorption via cytoskeletal organization and microtubule acetylation

Glucocorticoids (GCs) inhibit the resorptive capacity of the osteoclast by disrupting its cytoskeleton. We find that calpain-6 (Capn6), a unique, nonproteolytic member of its family, is suppressed 12-fold by dexamethasone (DEX) in the bone-degrading cell. While Capn6 abundance parallels commitment of naive bone marrow macrophages (BMMs) to the osteoclast phenotype, its excess or deletion does not affect the cell's differentiation. On the other hand, Capn6 localizes to the sealing zone, and its overexpression promotes osteoclast spreading and large actin ring formation, eventuating in stimulated bone degradation. Conversely, Capn6 knockdown impairs cytoskeletal organization and the cell's resorptive capacity. Capn6 complexes with tubulin, and its absence inhibits microtubule acetylation and stability in the osteoclast. Knockdown of Capn6 also reduces β(3)-integrin subunit protein, another essential regulator of osteoclast cytoskeletal function. Reflecting Capn6 as a target molecule of GCs, microtubule stability and acetylation, as well as the expression of β(3)-integrin protein, are similarly suppressed in DEX-treated osteoclasts. Moreover, overexpression of Capn6 rescues GC-mediated disruption of osteoclast cytoskeleton. Thus Capn6 promotes cytoskeletal organization and microtubule stability in osteoclasts, and its inhibition may mediate the resorption-arresting properties of GCs.

Osteoclast-specific inactivation of the integrin-linked kinase (ILK) inhibits bone resorption

Bone resorption requires the adhesion of osteoclasts to extracellular matrix (ECM) components, a process mediated by the alpha(v)beta(3) integrin. Following engagement with the ECM, integrin receptors signal via multiple downstream effectors, including the integrin-linked kinase (ILK). In order to characterize the physiological role of ILK in bone resorption, we generated mice with an osteoclast-specific Ilk gene ablation by mating mice with a floxed Ilk allele with TRAP-Cre transgenic mice. The TRAP-Cre mice specifically excised floxed alleles in osteoclasts, as revealed by crossing them with the ROSA26R reporter strain. Osteoclast-specific Ilk mutant mice appeared phenotypically normal, but histomorphometric analysis of the proximal tibia revealed an increase in bone volume and trabecular thickness. Osteoclast-specific Ilk ablation was associated with an increase in osteoclastogenesis both in vitro and in vivo. However, the mutant osteoclasts displayed a decrease in resorption activity as assessed by reduced pit formation on dentin slices in vitro and decreased serum concentrations of the C-terminal telopeptide of collagen in vivo. Interestingly, compound heterozygous mice in which one allele of Ilk and one allele of the beta(3) integrin gene were inactivated (ILK(+/-); beta(3) (+/-)) also had increased trabecular thickness, confirming that beta(3) integrin and Ilk form part of the same genetic cascade. Our results show that ILK is important for the function, but not the differentiation, of osteoclasts.

Src-like adaptor protein regulates osteoclast generation and survival

Src-like adaptor protein (SLAP) is a hematopoietic adaptor containing Src homology (SH)3 and SH2 motifs and a unique carboxy terminus. Unlike c-Src, SLAP lacks a tyrosine kinase domain. We investigated the role of SLAP in osteoclast development and resorptive function. Employing SLAP-deficient mice, we find lack of the adaptor enhances in vitro proliferation of osteoclast precursors in the form of bone marrow macrophages (BMMs), without altering their survival. Furthermore, osteoclastogenic markers appear more rapidly in SLAP-/- BMMs exposed to RANK ligand (RANKL). The accelerated proliferation of M-CSF-treated, SLAP-deficient precursors is associated with enhanced ERK activation. SLAP's role as a mediator of M-CSF signaling, in osteoclastic cells, is buttressed by complexing of the adaptor protein and c-Fms in lipid rafts. Unlike c-Src, SLAP does not impact resorptive function of mature osteoclasts but induces their early apoptosis. Thus, SLAP negatively regulates differentiation of osteoclasts and proliferation of their precursors. Conversely, SLAP decreases osteoclast death by inhibiting activation of caspase 3. These counterbalancing events yield indistinguishable bones of WT and SLAP-/- mice which contain equal numbers of osteoclasts in basal and stimulated conditions.

The relative timing of exposure to phagocytosable particulates and to osteoclastogenic cytokines is critically important in the determination of myeloid cell fate

During granulomatous inflammatory reactions, myeloid cells can differentiate into activated phagocytic macrophages, wound-healing macrophages, foreign body giant cells, and bone-resorbing osteoclasts. Although it is appreciated that a variety of stimuli, including cytokines, cell-matrix interactions, and challenge with foreign materials can influence myeloid cell fate, little is known of how these signals integrate during this process. In this study, we have investigated the cross talk between receptor activator of NF-kappaB ligand (RANKL)-induced osteoclastogenesis and particle phagocytosis-induced activation of human monocytes. Understanding interconnected signals is of particular importance to disorders, such as periprosthetic osteolysis, in which granulomatous inflammation is initiated by particle phagocytosis in proximity to bone and leads to inflammatory bone loss. Using cell-based osteoclastogenesis and phagocytosis assays together with expression analysis of key regulators of osteoclastogenesis, we show in this study that phagocytosis of disease-relevant particles inhibits RANKL-mediated osteoclastogenesis of human monocytes. Mechanistically, phagocytosis mediates this effect by downregulation of RANK and c-Fms, the receptors for the essential osteoclastogenic cytokines RANKL and M-CSF. RANKL pretreatment of monocytes generates preosteoclasts that are resistant to RANK downregulation and committed to osteoclast formation, even though they retain phagocytic activity. Thus, the relative timing of exposure to phagocytosable particulates and to osteoclastogenic cytokines is critically important in the determination of myeloid cell fate.

Cdc42 regulates bone modeling and remodeling in mice by modulating RANKL/M-CSF signaling and osteoclast polarization

The modeling and remodeling of bone requires activation and polarization of osteoclasts, achieved by reorganization of the cytoskeleton. Members of the Rho subfamily of small GTPases, including Cdc42, are known regulators of cytoskeletal components, but the role of these proteins in bone physiology and pathophysiology remains unclear. Here, we examined loss-of-function mice in which Cdc42 was selectively ablated in differentiated osteoclasts and gain-of-function animals wherein Cdc42Gap, a protein that inactivates the small GTPase, was deleted globally. Cdc42 loss-of-function mice were osteopetrotic and resistant to ovariectomy-induced bone loss, while gain-of-function animals were osteoporotic. Isolated Cdc42-deficient osteoclasts displayed suppressed bone resorption, while osteoclasts with increased Cdc42 activity had enhanced resorptive capacity. We further demonstrated that Cdc42 modulated M-CSF-stimulated cyclin D expression and phosphorylation of Rb and induced caspase 3 and Bim, thus contributing to osteoclast proliferation and apoptosis rates. Furthermore, Cdc42 was required for multiple M-CSF- and RANKL-induced osteoclastogenic signals including activation and expression of the differentiation factors MITF and NFATc1 and was a component of the Par3/Par6/atypical PKC polarization complex in osteoclasts. These data suggest that Cdc42 regulates osteoclast formation and function and may represent a promising therapeutic target for prevention of pathological bone loss.

Dissection of platelet and myeloid cell defects by conditional targeting of the beta3-integrin subunit

The purpose of this work was to determine platelet and myeloid cell-specific requirements for beta3-containing integrins in hemostasis, bone resorption, and tumor growth. LoxP-flanked mice were generated to study the conditional deletion of beta3-integrin in platelets [knockout in platelets (KOP)] and myeloid cells [knockout in myeloid (KOM)]. Using the beta3KOP and beta3KOM strains of mice, we studied the role of beta3-integrin in hemostasis, bone resorption, and subcutaneous tumor growth. Tissue-specific deletion of platelet beta3-integrins in beta3KOP mice did not affect bone mass but resulted in a severe bleeding phenotype. No growth difference of tumor xenografts or in neoangiogenesis were found in beta3KOP mice, in contrast to the defects observed in germline beta3(-/-) mice. Conditional deletion of myeloid beta3-integrins in beta3KOM mice resulted in osteopetrosis but had no effect on hemostasis or mortality. Tumor growth in beta3KOM mice was increased and accompanied by decreased macrophage infiltration, without increase in blood vessel number. Platelet beta3-integrin deficiency was sufficient to disrupt hemostasis but had no effect on bone mass or tumor growth. Myeloid-specific beta3-integrin deletion was sufficient to perturb bone mass and enhance tumor growth due to reduced macrophage infiltration in the tumors. These results suggest that beta3-integrins have cell-specific roles in complex biological processes.-Morgan, E. A., Schneider, J. G., Baroni, T. E., Uluçkan, O., Heller, E., Hurchla, M. A., Deng, H., Floyd, D., Berdy, A., Prior, J. L., Piwnica-Worms, D., Teitelbaum, S. L., Ross, F. P., Weilbaecher, K. N. Dissection of platelet and myeloid cell defects by conditional targeting of the beta3-integrin subunit.

SLP-76 couples Syk to the osteoclast cytoskeleton

The capacity of the osteoclast (OC) to resorb bone is dictated by cytoskeletal organization, which in turn emanates from signals derived from the alpha(v)beta(3) integrin and c-Fms. Syk is key to these signals and, in other cells, this tyrosine kinase exerts its effects via intermediaries including the SLP adaptors, SLP-76 and BLNK (B cell linker). Thus, we asked whether these two SLP proteins regulate OC function. We find BLNK-deficient OCs are normal, whereas cytoskeletal organization of those lacking SLP-76 is delayed, thus modestly reducing bone resorption in vitro. Cytoskeletal organization and bone resorption are more profoundly arrested in cultured OCs deficient in BLNK and SLP-76 double knockout (DKO) phenotypes. In contrast, stimulated bone resorption in vivo is inhibited approximately 40% in either SLP-76(-/-) or DKO mice. This observation, taken with the fact that DKO OCs are rescued by retroviral transduction of only SLP-76, indicates that SLP-76 is the dominant SLP family member in the resorptive process. We also find SLP-76 is phosphorylated in a Syk-dependent manner. Furthermore, in the absence of the adaptor protein, integrin-mediated phosphorylation of Vav3, the OC cytoskeleton-organizing guanine nucleotide exchange factor, is abrogated. In keeping with a central role of SLP-76/Vav3 association in osteoclastic resorption, retroviral transduction of SLP-76, in which the Vav binding site is disrupted (3YF), fails to normalize the cytoskeleton of DKO OCs and the resorptive capacity of the cells. Finally, c-Fms-activated Syk also exerts its OC cytoskeleton-organizing effect in a SLP-76/Vav3-dependent manner.

Syk tyrosine 317 negatively regulates osteoclast function via the ubiquitin-protein isopeptide ligase activity of Cbl

Cytoskeletal organization of the osteoclast (OC), which is central to the capacity of the cell to resorb bone, is induced by occupancy of the alphavbeta3 integrin or the macrophage colony-stimulating factor (M-CSF) receptor c-Fms. In both circumstances, the tyrosine kinase Syk is an essential signaling intermediary. We demonstrate that Cbl negatively regulates OC function by interacting with Syk(Y317). Expression of nonphosphorylatable Syk(Y317F) in primary Syk(-/-) OCs enhances M-CSF- and alphavbeta3-induced phosphorylation of the cytoskeleton-organizing molecules, SLP76, Vav3, and PLCgamma2, to levels greater than wild type, thereby accelerating the resorptive capacity of the cell. Syk(Y317) suppresses cytoskeletal organization and function while binding the ubiquitin-protein isopeptide ligase Cbl. Consequently, Syk(Y317F) abolishes M-CSF- and integrin-stimulated Syk ubiquitination. Thus, Cbl/Syk(Y317) association negatively regulates OC function and therefore is essential for maintenance of skeletal homeostasis.

How do bone cells secrete proteins?

The ruffled border is the most specific marker of the active osteoclast (OC) as it forms only when the cell is resorbing bone. We provide evidence that this complex cytoskeletal structure reflects insertion of the lysosomal vesicles into the bone-apposed plasma membrane under the aegis of the Ca-sensing, exocytic protein, synaptotagmin VII (SytVII). In the manner, SytVII permits transport of matrix-degrading molecules into the resorptive microenvironment. SytVII also regulates secretion of bone matrix molecules by osteoblasts. Thus, SytVII-deficient mice experience suppressed bone resorption and formation with the latter deficiency predominant thereby yielding osteoporosis characterized by attenuated remodeling.

Tumor necrosis factor receptor-associated factor 6 is an intranuclear transcriptional coactivator in osteoclasts

Tumor necrosis factor receptor-associated factor 6 (TRAF6) associates with the cytoplasmic domain of receptor activator of NF-kappaB (RANK) and is an essential component of the signaling complex mediating osteoclastogenesis. However, the osteoclastic activity of TRAF6 is blunted by its association with four and half LIM domain 2 (FHL2), which functions as an adaptor protein in the cytoplasm and transcriptional regulator in the nucleus. We find that TRAF6 also localizes in the nuclei of osteoclasts but not their bone marrow macrophage precursors and that osteoclast intranuclear abundance is specifically increased by RANK ligand (RANKL). TRAF6 nuclear localization requires FHL2 and is diminished in fhl2(-/-) osteoclasts. Suggesting transcriptional activity, TRAF6 interacts with the transcription factor RUNX1 in the osteoclast nucleus. FHL2 also associates with RUNX1 but does so only in the presence of TRAF6. Importantly, TRAF6 recognizes FHL2 and RUNX1 in osteoclast nuclei, and the three molecules form a DNA-binding complex that recognizes and transactivates the RUNX1 response element in the fhl2 promoter. Finally, TRAF6 and its proximal activator, RANKL, polyubiquitinate FHL2, prompting its proteasomal degradation. These observations suggest a feedback mechanism whereby TRAF6 negatively regulates osteoclast formation by intracytoplasmic sequestration of FHL2 to blunt RANK activation and as a component of a transcription complex promoting FHL2 expression.

Synaptotagmin VII regulates bone remodeling by modulating osteoclast and osteoblast secretion

Maintenance of bone mass and integrity requires a tight balance between resorption by osteoclasts and formation by osteoblasts. Exocytosis of functional proteins is a prerequisite for the activity of both cells. In the present study, we show that synaptotagmin VII, a calcium sensor protein that regulates exocytosis, is associated with lysosomes in osteoclasts and bone matrix protein-containing vesicles in osteoblasts. Absence of synaptotagmin VII inhibits cathepsin K secretion and formation of the ruffled border in osteoclasts and bone matrix protein deposition in osteoblasts, without affecting the differentiation of either cell. Reflecting these in vitro findings, synaptotagmin VII-deficient mice are osteopenic due to impaired bone resorption and formation. Therefore, synaptotagmin VII plays an important role in bone remodeling and homeostasis by modulating secretory pathways functionally important in osteoclasts and osteoblasts.

Mechanisms of osteoclastic secretion

A tight balance between bone resorption by osteoclasts and bone formation by osteoblasts is required for the maintenance of bone mass and integrity. A net increase in bone resorption over formation results in osteoporosis, a disease associated with significantly morbidity and mortality. Following attachment via the integrin alphavbeta3, osteoclasts degrade bone by generation of the ruffled border, the unique resorptive organelle of the cell. The adherent cell then secretes into the subcellular space protons and acidic proteases. We review here the concepts relating to the mechanisms of regulated secretion and provide preliminary data on the role of one protein important for secretion by osteoclasts.

Notch signaling maintains bone marrow mesenchymal progenitors by suppressing osteoblast differentiation

Postnatal bone marrow houses mesenchymal progenitor cells that are osteoblast precursors. These cells have established therapeutic potential, but they are difficult to maintain and expand in vitro, presumably because little is known about the mechanisms controlling their fate decisions. To investigate the potential role of Notch signaling in osteoblastogenesis, we used conditional alleles to genetically remove components of the Notch signaling system during skeletal development. We found that disruption of Notch signaling in the limb skeletogenic mesenchyme markedly increased trabecular bone mass in adolescent mice. Notably, mesenchymal progenitors were undetectable in the bone marrow of mice with high bone mass. As a result, these mice developed severe osteopenia as they aged. Moreover, Notch signaling seemed to inhibit osteoblast differentiation through Hes or Hey proteins, which diminished Runx2 transcriptional activity via physical interaction. These results support a model wherein Notch signaling in bone marrow normally acts to maintain a pool of mesenchymal progenitors by suppressing osteoblast differentiation. Thus, mesenchymal progenitors may be expanded in vitro by activating the Notch pathway, whereas bone formation in vivo may be enhanced by transiently suppressing this pathway.

NOTCH1 regulates osteoclastogenesis directly in osteoclast precursors and indirectly via osteoblast lineage cells

NOTCH signaling is a key regulator of cell fate decisions in prenatal skeletal development and is active during adult tissue renewal. In addition, its association with neoplasia suggests that it is a candidate therapeutic target. We find that attenuated NOTCH signaling enhances osteoclastogenesis and bone resorption in vitro and in vivo by a combination of molecular mechanisms. First, deletion of Notch1-3 in bone marrow macrophages directly promotes their commitment to the osteoclast phenotype. These osteoclast precursors proliferate more rapidly than the wild type in response to macrophage colony-stimulating factor and are sensitized to RANKL and macrophage colony-stimulating factor, undergoing enhanced differentiation in response to low doses of either cytokine. Conforming with a role for NOTCH in this process, presentation of the NOTCH ligand JAGGED1 blunts the capacity of wild-type bone marrow macrophages to become osteoclasts. Combined, these data establish that NOTCH suppresses osteoclastogenesis via ligand-mediated receptor activation. Although NOTCH1 and NOTCH3 collaborate in regulating osteoclast formation, NOTCH1 is the dominant paralog. In addition, NOTCH1 deficiency promotes osteoclastogenesis indirectly by enhancing the ability of osteoblast lineage cells to stimulate osteoclastogenesis. This is achieved by decreasing the osteoprotegerin/RANKL expression ratio. Thus, NOTCH1 acts as a net inhibitor of bone resorption, exerting its effect both directly in osteoclast precursors and indirectly via osteoblast lineage cells. These observations raise caution that therapeutic inhibition of NOTCH signaling may adversely accelerate bone loss in humans.

M-CSF Regulates the Cytoskeleton via Recruitment of a Multimeric Signaling Complex to c-Fms Tyr-559/697/721

M-CSF is known to induce cytoskeletal reorganization in macrophages and osteoclasts by activation of phosphatidylinositol 3-kinase (PI3K) and c-Src, but the detailed mechanisms remain unclear. We find, unexpectedly, that tyrosine (Tyr) to phenylalanine (Phe) mutation of Tyr-721, the PI3K binding site in the M-CSF receptor c-Fms, fails to suppress cytoskeletal remodeling or actin ring formation. In contrast, mutation of c-Fms Tyr-559 to Phe blocks M-CSF-induced cytoskeletal reorganization by inhibiting formation of a Src Family Kinase SFK.c-Cbl.PI3K complex and the downstream activation of Vav3 and Rac, two key mediators of actin remodeling. Using an add-back approach in which specific Tyr residues are reinserted into c-Fms inactivated by the absence of all seven functionally important Tyr residues, we find that Tyr-559 is necessary but not sufficient to transduce M-CSF-dependent cytoskeletal reorganization. Furthermore, this same add-back approach identifies important roles for Tyr-697 and Tyr-721 in collaborating with Tyr-559 to recruit a multimeric signaling complex that can transduce signals from c-Fms to the actin cytoskeleton.

Syk, c-Src, the alphavbeta3 integrin, and ITAM immunoreceptors, in concert, regulate osteoclastic bone resorption

In this study, we establish that the tyrosine kinase Syk is essential for osteoclast function in vitro and in vivo. Syk(-/-) osteoclasts fail to organize their cytoskeleton, and, as such, their bone-resorptive capacity is arrested. This defect results in increased skeletal mass in Syk(-/-) embryos and dampened basal and stimulated bone resorption in chimeric mice whose osteoclasts lack the kinase. The skeletal impact of Syk deficiency reflects diminished activity of the mature osteoclast and not impaired differentiation. Syk regulates bone resorption by its inclusion with the alpha v beta3 integrin and c-Src in a signaling complex, which is generated only when alpha v beta3 is activated. Upon integrin occupancy, c-Src phosphorylates Syk. Alpha v beta3-induced phosphorylation of Syk and the latter's capacity to associate with c-Src is mediated by the immunoreceptor tyrosine-based activation motif (ITAM) proteins Dap12 and FcRgamma. Thus, in conjunction with ITAM-bearing proteins, Syk, c-Src, and alpha v beta3 represent an essential signaling complex in the bone-resorbing osteoclast, and, therefore, each is a candidate therapeutic target.

c-Fms tyrosine 559 is a major mediator of M-CSF-induced proliferation of primary macrophages

The molecular mechanisms by which binding of monocyte/macrophage colony-stimulating factor to its receptor c-Fms promotes replication in primary macrophages are incompletely understood, as all previous studies involved overexpression of receptor mutants in transformed cells not endogenously expressing the receptor. To address this issue we retrovirally expressed, in bone marrow-derived macrophages, a chimeric receptor containing a range of tyrosine to phenylalanine mutations in the c-Fms cytoplasmic tail. We measured incorporation of bromodeoxyuridine as a marker of proliferation and phosphorylation of ERKs, Akt, and the receptor itself. Our data indicate that tyrosine 559 is the major mediator of receptor activation and cell death, intracellular signaling, and cell proliferation and that the tyrosine residues at positions 697 and 807 play lesser roles in these events. Importantly, we find that activation of the ERK and Akt pathways is necessary but not sufficient for induction of macrophage proliferation. Using specific small molecule inhibitors we find that a combination of the Src family kinase, phosphatidylinositol 3-kinase/Akt, phospholipase C, and ERK pathways mediates macrophage proliferation in response to M-CSF.

SHIP1 negatively regulates proliferation of osteoclast precursors via Akt-dependent alterations in D-type cyclins and p27

Osteoclasts arise from macrophage progenitors in bone marrow (BMMs) as a consequence of signaling events elicited by M-CSF and receptor activator of NF-kappaB ligand, acting on their unique receptors, via c-Fms and receptor activator of NF-kappaB. Both receptors activate the PI3K and MAPK pathways, which promote cell proliferation and survival. SHIP1 is essential for normal bone homeostasis, as mice lacking the protein exhibit osteoporosis resulting from increased numbers of hyper-resorptive osteoclasts. In this study, we show that BMMs from SHIP1 null mice respond to M-CSF, but not receptor activator of NF-kappaB ligand, by increasing Akt activation. In consequence, there are up-regulation of D-type cyclins, down-regulation of the cyclin-dependent kinase inhibitor p27, and, therefore, increased phosphorylation of the retinoblastoma protein and cell proliferation. Surprisingly, cell survival of wild-type and knockout BMMs is unaltered. Finally, osteoclastogenesis and periarticular bone erosions are markedly increased in SHIP1(-/-) mice with inflammatory arthritis, a condition characterized by increased M-CSF expression. The SHIP1/Akt pathway therefore suppresses bone loss in pathological states associated with an excess of the cytokine.

The LIM Protein, LIMD1, Regulates AP-1 Activation through an Interaction with TRAF6 to Influence Osteoclast Development

Increasingly a number of proteins important in the regulation of bone osteoclast development have been shown primarily influence osteoclastogenesis under conditions of physiologic or pathologic stress. Why basal osteoclastogenesis is normal and how these proteins regulate stress osteoclastogenic responses, as opposed to basal osteoclastogenesis, is unclear. LIM proteins of the Ajuba/Zyxin family localize to cellular sites of cell adhesion where they contribute to the regulation of cell adhesion and migration, translocate into the nucleus where they can affect cell fate, but are also found in the cytoplasm where their function is largely unknown. We show that one member of this LIM protein family, Limd1, is uniquely up-regulated during osteoclast differentiation and interacts with Traf6, a critical cytosolic regulator of RANK-L-regulated osteoclast development. Limd1 positively affects the capacity of Traf6 to activate AP-1, and Limd1(-/-) osteoclast precursor cells are defective in the activation of AP-1 and thus induction of NFAT2. Limd1(-/-) mice, although having normal basal bone osteoclast numbers and bone density, are resistant to physiological and pathologic osteoclastogenic stimuli. These results implicate Limd1 as a potentially important regulator of osteoclast development under conditions of stress.

Beta3 integrin deficiency promotes cardiac hypertrophy and inflammation

Cardiac hypertrophy commonly develops in response to pressure overload and is associated with increased mortality. Mechanical stress in the heart can result in the activation of transmembrane integrin alphabeta heterodimers that are expressed in cardiomyocytes. Once activated, integrins stimulate focal adhesion kinase, Grb2, c-src, and other signaling molecules to promote cardiomyocyte growth and gene expression. Mechanical stress can also promote cardiac inflammation that may be mediated, in part, by the activation of integrins expressed in blood-borne cells. To address the role of one integrin, beta(3), in the pathogenesis of cardiac hypertrophy, beta(3)(-/-) mice were examined. beta(3)(-/-) Mice developed moderate spontaneous cardiac hypertrophy associated with systolic and diastolic dysfunction, and these abnormalities were exacerbated by transverse aortic constriction. In addition, beta(3)(-/-) mice developed mild cardiac inflammation with infiltrating macrophages at baseline that was markedly worsened by pressure overload. Bone marrow transplantation experiments showed that blood-borne cells were at least partially responsible for the cardiac hypertrophy and inflammation observed in beta(3)(-/-) mice. These results suggest that alpha(v)beta(3) expression in bone marrow has a generalized suppressive effect on cardiac inflammation.