M-CSF, c-Fms, and signaling in osteoclasts and their precursors

Prevention of conditions, such as osteoporosis, requires an understanding of the molecular mechanisms of bone resorption. The understanding that cells of the myeloid lineage are osteoclast precursors suggests that macrophage colony-stimulating factor (M-CSF) plays an important role in osteoclast biology. Signals generated by the binding of M-CSF to the cell-surface receptor c-Fms appear to trigger events leading to osteoclast differentiation. We have created a chimeric variant of the c-Fms receptor, which has allowed study of downstream events activated by M-CSF in a model more relevant to normal physiology than prior studies, which have relied on myeloid tissues. Our studies suggest novel regulatory signaling pathways initiated via the c-Fms receptor.

Glucocorticoids suppress bone formation via the osteoclast

The pathogenesis of glucocorticoid-induced (GC-induced) bone loss is unclear. For example, osteoblast apoptosis is enhanced by GCs in vivo, but they stimulate bone formation in vitro. This conundrum suggests that an intermediary cell transmits a component of the bone-suppressive effects of GCs to osteoblasts in the intact animal. Bone remodeling is characterized by tethering of the activities of osteoclasts and osteoblasts. Hence, the osteoclast is a potential modulator of the effect of GCs on osteoblasts. To define the direct impact of GCs on bone-resorptive cells, we compared the effects of dexamethasone (DEX) on WT osteoclasts with those derived from mice with disruption of the GC receptor in osteoclast lineage cells (GRoc-/- mice). While the steroid prolonged longevity of osteoclasts, their bone-degrading capacity was suppressed. The inhibitory effect of DEX on bone resorption reflects failure of osteoclasts to organize their cytoskeleton in response to M-CSF. DEX specifically arrested M-CSF activation of RhoA, Rac, and Vav3, each of which regulate the osteoclast cytoskeleton. In all circumstances GRoc-/- mice were spared the impact of DEX on osteoclasts and their precursors. Consistent with osteoclasts modulating the osteoblast-suppressive effect of DEX, GRoc-/- mice are protected from the steroid's inhibition of bone formation.

Nothing but skin and bone

Skin and bone - what comes to mind at hearing this phrase? While certainly a metaphor for disease, it also defines two very different tissues, one a flexible and contiguous outer covering, the other a morphologically diverse hard tissue distributed at over 200 sites in the body. As the accompanying series of Reviews highlights, these tissues are indeed diverse, but there are also surprising similarities. Skin is the interface between the internal organs and the environment, and as such plays a crucial role in the body's defense mechanism. The skin and its many appendages are responsible for functions as diverse as epidermal barrier and defense, immune surveillance, UV protection, thermoregulation, sweating, lubrication, pigmentation, the sensations of pain and touch, and, importantly, the protection of various stem cell niches in the skin. Bone serves a number of purposes: it provides protection for vital organs, a lever for locomotion, a reservoir for calcium, and the site of adult hematopoiesis. The tissue is composed of osteoblasts, osteoclasts, and their individual precursors plus a complex mixture of mesenchymal, myeloid, and lymphoid cells in the marrow space. Finally, the endothelial microenvironment provides nutrition and is a conduit for the influx and emigration of cells that impact bone biology in several important ways. This Review series guides the reader through these various facets of 2 diverse, yet interdependent, tissues.

Cortactin has an essential and specific role in osteoclast actin assembly

Osteoclasts are essential for bone dynamics and calcium homeostasis. The cells form a tight seal on the bone surface, onto which they secrete acid and proteases to resorb bone. The seal is associated with a ring of actin filaments. Cortactin, a c-Src substrate known to promote Arp2/3-mediated actin assembly in vitro, is expressed in osteoclasts and localizes to the sealing ring. To address the role of cortactin and actin assembly in osteoclasts, we depleted cortactin by RNA interference. Cortactin-depleted osteoclasts displayed a complete loss of bone resorption with no formation of sealing zones. On nonosteoid surfaces, osteoclasts flatten with a dynamic, actin-rich peripheral edge that contains podosomes, filopodia, and lamellipodia. Cortactin depletion led to a specific loss of podosomes, revealing a tight spatial compartmentalization of actin assembly. Podosome formation was restored in cortactin-depleted cells by expression of wild-type cortactin or a Src homology 3 point mutant of cortactin. In contrast, expression of a cortactin mutant lacking tyrosine residues phosphorylated by Src did not restore podosome formation. Cortactin was found to be an early component of the nascent podosome belt, along with dynamin, supporting a role for cortactin in actin assembly.

alphavbeta3 and macrophage colony-stimulating factor: partners in osteoclast biology

Osteoclasts, the sole bone-resorbing cells, arise by fusion and differentiation of monocyte/macrophage precursors. Matrix degradation requires adhesion of the osteoclast to bone, an integrin alphavbeta3-mediated event that also stimulates signals which polarize the cell and secrete resorptive molecules such as hydrochloric acid and acidic proteases. Two cytokines are necessary and sufficient for osteoclastogenesis, receptor activator of nuclear factor kappaB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF), both produced by mesenchymal cells in the bone marrow environment. M-CSF promotes survival and proliferation of osteoclast precursors. It also contributes to their differentiation and regulates the cytoskeletal changes that accompany bone resorption. Binding of M-CSF to c-Fms, its receptor, recruits adapter proteins and cytosolic kinases, thereby activating a variety of intracellular signals. We herein review how alphavbeta3 and M-CSF, alone and in concert, impact production, survival, and function of the osteoclast, thereby controlling skeletal mass. Signals from alphavbeta3 and/or c-Fms activate Syk and Vav3, originally defined by their function in lymphoid cells. Genetic depletion of either protein generates a strong bone phenotype, underscoring the promise of osteoimmunobiology.

Critical role of beta3 integrin in experimental postmenopausal osteoporosis

We show that mice lacking beta3 integrin are protected from OVX-induced bone loss. Using a lentiviral-based strategy to express beta3 mutants in beta3(-/-) mice, we also show that beta3(S752), but not beta3(Y747/Y759), is important for osteoclastic bone resorption in vivo.

INTRODUCTION

Mice lacking the beta3 integrin have dysfunctional osteoclasts and therefore accumulate bone mass with age. Thus, the alphavbeta3 integrin is a potential anti-osteoporosis target. Identifying components of the beta3 integrin that determine its function in vivo is essential for therapeutically exploiting the antiresorptive properties of alphavbeta3.

MATERIALS AND METHODS

We used DXA and histomorphometry to assess bone loss after ovariectomy in wildtype and beta3 integrin null mice. We used lentiviral vectors carrying various human beta3 (hbeta3) integrin constructs to transduce beta3(-/-) bone marrow and reconstituted lethally irradiated beta3(-/-) mice with the transduced marrow. The expressed constructs include the intact integrin and two mutants, namely hbeta3(Y747F/Y759F) and hbeta3(S752P), each of which induces the bleeding dyscrasia, Glanzmann's thrombasthenia, in humans. Two months after transplantation, the expression of hbeta3 was measured by flow cytometry of marrow-derived macrophages. Osteoclast differentiation and function were assessed ex vivo by TRACP and actin-ring staining, respectively. Reconstituted mice were ovariectomized, and bone loss was assessed by DXA, histomorphometry, and serum TRACP5b assay.

RESULTS

beta3(-/-) mice are protected from ovariectomy-induced bone loss, showing no difference in BMD compared with sham-operated controls. We successfully expressed hbeta3 integrins in beta3(-/-) hosts using lentiviral transduction of bone marrow. Two months after transplantation, 25-35% of marrow-derived macrophages expressed the hbeta3 constructs. Similar to its effect in vitro, hbeta3(WT) completely rescued the osteoclast and platelet phenotype of beta3(-/-) mice. Whereas platelet function remained deranged in beta3(-/-) mice overexpressing hbeta3(Y747F/Y759F), osteoclast function was fully restored. In contrast, beta3(-/-) mice expressing hbeta3(S752P) continued to exhibit prolonged bleeding times and dysfunctional osteoclasts in vitro and ex vivo. Most importantly, hbeta3(WT) and hbeta3(Y747F/Y759F) transplanted mice underwent equivalent ovariectomy-induced bone loss, whereas, like those bearing the control vector, hbeta3(S752P) transplanted mice were protected.

CONCLUSIONS

Functional beta3 integrin is required for ovariectomy-induced bone loss. beta3(S752), but not beta3(Y747/Y759), is critical for osteoclast function in vivo.

M-CSF mediates TNF-induced inflammatory osteolysis

TNF-alpha is the dominant cytokine in inflammatory osteolysis. Using mice whose BM stromal cells and osteoclast precursors are chimeric for the presence of TNF receptors, we found that both cell types mediated the cytokine's osteoclastogenic properties. The greater contribution was made, however, by stromal cells that express the osteoclastogenic cytokine M-CSF. TNF-alpha stimulated M-CSF gene expression, in vivo, only in the presence of TNF-responsive stromal cells. M-CSF, in turn, induced the key osteoclastogenic cytokine receptor, receptor activator of NF-kappaB (RANK), in osteoclast precursors. In keeping with the proproliferative and survival properties of M-CSF, TNF-alpha enhanced osteoclast precursor number only in the presence of stromal cells bearing TNF receptors. To determine the clinical relevance of these observations, we induced inflammatory arthritis in wild-type mice and treated them with a mAb directed against the M-CSF receptor, c-Fms. Anti-c-Fms mAb selectively and completely arrested the profound pathological osteoclastogenesis attending this condition, the significance of which is reflected by similar blunting of the in vivo bone resorption marker tartrate-resistant acid phosphatase 5b (TRACP 5b). Confirming that inhibition of the M-CSF signaling pathway targets TNF-alpha, anti-c-Fms also completely arrested osteolysis in TNF-injected mice with nominal effect on macrophage number. M-CSF and its receptor, c-Fms, therefore present as candidate therapeutic targets in states of inflammatory bone erosion.

G-CSF potently inhibits osteoblast activity and CXCL12 mRNA expression in the bone marrow

Accumulating evidence indicates that interaction of stromal cell-derived factor 1 (SDF-1/CXCL12 [CXC motif, ligand 12]) with its cognate receptor, CXCR4 (CXC motif, receptor 4), generates signals that regulate hematopoietic progenitor cell (HPC) trafficking in the bone marrow. During granulocyte colony-stimulating factor (G-CSF)-induced HPC mobilization, CXCL12 protein expression in the bone marrow decreases. Herein, we show that in a series of transgenic mice carrying targeted mutations of their G-CSF receptor and displaying markedly different G-CSF-induced HPC mobilization responses, the decrease in bone marrow CXCL12 protein expression closely correlates with the degree of HPC mobilization. G-CSF treatment induced a decrease in bone marrow CXCL12 mRNA that closely mirrored the fall in CXCL12 protein. Cell sorting experiments showed that osteoblasts and to a lesser degree endothelial cells are the major sources of CXCL12 production in the bone marrow. Interestingly, osteoblast activity, as measured by histomorphometry and osteocalcin expression, is strongly down-regulated during G-CSF treatment. However, the G-CSF receptor is not expressed on osteoblasts; accordingly, G-CSF had no direct effect on osteoblast function. Collectively, these data suggest a model in which G-CSF, through an indirect mechanism, potently inhibits osteoblast activity resulting in decreased CXCL12 expression in the bone marrow. The consequent attenuation of CXCR4 signaling ultimately leads to HPC mobilization.

FHL2 inhibits the activated osteoclast in a TRAF6-dependent manner

TNF receptor-associated factor 6 (TRAF6) associates with the cytoplasmic domain of receptor activator of NF-kappaB (RANK). This event is central to normal osteoclastogenesis. We discovered that TRAF6 also interacts with FHL2 (four and a half LIM domain 2), a LIM domain--only protein that functions as a transcriptional coactivator or corepressor in a cell-type--specific manner. FHL2 mRNA and protein are undetectable in marrow macrophages and increase pari passu with osteoclast differentiation in vitro. FHL2 inhibits TRAF6-induced NF-kappaB activity in wild-type osteoclast precursors and, in keeping with its role as a suppressor of TRAF6-mediated RANK signaling, TRAF6/RANK association is enhanced in FHL2-/- osteoclasts. FHL2 overexpression delays RANK ligand-induced (RANKL-induced) osteoclast formation and cytoskeletal organization. Interestingly, osteoclast-residing FHL2 is not detectable in naive wild-type mice, in vivo, but is abundant in those treated with RANKL and following induction of inflammatory arthritis. Reflecting increased RANKL sensitivity, osteoclasts generated from FHL2-/- mice reach maturation and optimally organize their cytoskeleton earlier than their wild-type counterparts. As a consequence, FHL2-/- osteoclasts are hyperresorptive, and mice lacking the protein undergo enhanced RANKL and inflammatory arthritis-stimulated bone loss. FHL2 is, therefore, an antiosteoclastogenic molecule exerting its effect by attenuating TRAF6-mediated RANK signaling.

Rab3D regulates a novel vesicular trafficking pathway that is required for osteoclastic bone resorption

Rab3 proteins are a subfamily of GTPases, known to mediate membrane transport in eukaryotic cells and play a role in exocytosis. Our data indicate that Rab3D is the major Rab3 species expressed in osteoclasts. To investigate the role of Rab3D in osteoclast physiology we examined the skeletal architecture of Rab3D-deficient mice and found an osteosclerotic phenotype. Although basal osteoclast number in null animals is normal the total eroded surface is significantly reduced, suggesting that the resorptive defect is due to attenuated osteoclast activity. Consistent with this hypothesis, ultrastructural analysis reveals that Rab3D(-/-) osteoclasts exhibit irregular ruffled borders. Furthermore, while overexpression of wild-type, constitutively active, or prenylation-deficient Rab3D has no significant effects, overexpression of GTP-binding-deficient Rab3D impairs bone resorption in vitro. Finally, subcellular localization studies reveal that, unlike wild-type or constitutively active Rab3D, which associate with a nonendosomal/lysosomal subset of post-trans-Golgi network (TGN) vesicles, inactive Rab3D localizes to the TGN and inhibits biogenesis of Rab3D-bearing vesicles. Collectively, our data suggest that Rab3D modulates a post-TGN trafficking step that is required for osteoclastic bone resorption.

Lipid links to better bone: a hypothesis

The combined resorptive activity of osteoclasts and the bone-generating function of osteoblasts result in the constant renewal of this vital tissue. It has long been appreciated that the coupled degradation and formation of bone is coordinately regulated by a complex interplay between endocrine and paracrine effectors; a recent report by now documents the possibility that cannabinoid receptors may also impact bone mass.

IL-1 mediates TNF-induced osteoclastogenesis

TNF-induced receptor activator NF-kappaB ligand (RANKL) synthesis by bone marrow stromal cells is a fundamental component of inflammatory osteolysis. We found that this process was abolished by IL-1 receptor antagonist (IL-1Ra) or in stromal cells derived from type I IL-1 receptor-deficient (IL-1RI-deficient) mice. Reflecting sequential signaling of the cytokines TNF and IL-1, TNF induces stromal cell expression of IL-1 and IL-1RI. These data suggest that TNF regulates RANKL expression via IL-1, and, therefore, IL-1 plays a role in TNF-induced periarticular osteolysis. Consistent with this posture, TNF-stimulated osteoclastogenesis in cultures consisting of WT marrow macrophages and stromal cells exposed to IL-1Ra or in cocultures established with IL-1RI-deficient stromal cells was reduced approximately 50%. The same magnitude of osteoclast inhibition occurred in IL-1RI-deficient mice following TNF administration in vivo. Like TNF, IL-1 directly targeted osteoclast precursors and promoted the osteoclast phenotype in a TNF-independent manner in the presence of permissive levels of RANKL. IL-1 is able to induce RANKL expression by stromal cells and directly stimulate osteoclast precursor differentiation under the aegis of p38 MAPK. Thus, IL-1 mediates the osteoclastogenic effect of TNF by enhancing stromal cell expression of RANKL and directly stimulating differentiation of osteoclast precursors.

Vav3 regulates osteoclast function and bone mass

Osteoporosis, a leading cause of morbidity in the elderly, is characterized by progressive loss of bone mass resulting from excess osteoclastic bone resorption relative to osteoblastic bone formation. Here we identify Vav3, a Rho family guanine nucleotide exchange factor, as essential for stimulated osteoclast activation and bone density in vivo. Vav3-deficient osteoclasts show defective actin cytoskeleton organization, polarization, spreading and resorptive activity resulting from impaired signaling downstream of the M-CSF receptor and alpha(v)beta3 integrin. Vav3-deficient mice have increased bone mass and are protected from bone loss induced by systemic bone resorption stimuli such as parathyroid hormone or RANKL. Moreover, we provide genetic and biochemical evidence for the role of Syk tyrosine kinase as a crucial upstream regulator of Vav3 in osteoclasts. Thus, Vav3 is a potential new target for antiosteoporosis therapy.

Mice lacking the integrin beta5 subunit have accelerated osteoclast maturation and increased activity in the estrogen-deficient state

Integrin alphavbeta5 is expressed on osteoclast precursors and is capable of recognizing the same amino acid motif as alphavbeta3. Three-month-old beta5(-/-) female OVX mice had increased osteoclastogenesis ex vivo, and microCT assessment of trabecular bone volume was 53% lower than WT-OVX animals. These preliminary data suggest alphavbeta5 integrin's presence on osteoclast precursors may inhibit of osteoclast formation.

INTRODUCTION

Osteoclasts are unique resorptive skeletal cells, capable of degrading bone on contact to the juxtaposed matrix. Integrin alphavbeta5 is expressed on osteoclast precursors, structurally similar to alphavbeta3, and capable of recognizing the same amino acid motif. Given the structural relationship and reciprocal regulation of alphavbeta3 and alphavbeta5, the purpose of this study was to evaluate how alphavbeta5 might contribute to osteoclast maturation and activity.

MATERIALS AND METHODS

Three-month-old wildtype (WT) and beta5(-/-) female mice had ovariectomy (OVX) or sham operations. The osteoclastogenic capacity of marrow-derived precursors, the kinetic, the circulating, and structural parameters of bone remodeling, was determined after 6 weeks of paired feeding.

RESULTS AND CONCLUSIONS

OVX increased osteoclastogenesis ex vivo and in vivo. Osteoclast formation and prolonged pre-osteoclast survival were substantially enhanced in cultures containing beta5(-/-) cells whether obtained from sham-operated or OVX mice. Expression of cathepsin K, beta3 integrin subunit, and calcitonin receptor were accelerated in cultured beta5(-/-)osteoclasts. beta5(-/-) osteoclasts from OVX animals showed a 3-fold enhancement of net resorptive activity, with quantitative muCT showing trabecular bone volume loss after OVX 53% greater in beta5(-/-) OVX compared with similarly treated WT OVX mice (p < 0.05). alpha5beta3 seems to be an inhibitor of osteoclast formation, in contrast to alphavbeta3. In addition, loss of alphavbeta5 seems to accelerate osteoclast formation in the OVX model. Further examination of alphavbeta5 signaling pathways may enhance our understanding of the activation of bone resorption.

Unoccupied alpha(v)beta3 integrin regulates osteoclast apoptosis by transmitting a positive death signal

Cell/matrix detachment is a general inducer of programmed cell death, an event mediated by loss of integrin/ligand association. Because alpha(v)beta3 is the major integrin expressed by the osteoclast, we asked whether its occupancy promotes survival of the resorptive cell. Thus, we generated wild-type preosteoclasts and placed them on selective matrix proteins. Consistent with the posture that alpha(v)beta3 occupancy promotes survival, preosteoclasts plated on native collagen, a matrix not recognized by the integrin, undergo apoptosis 4-fold faster than those on the alpha(v)beta3 ligand, vitronectin. To further explore the role of alpha(v)beta3 in osteoclast apoptosis, wild-type and beta3-/- preosteoclasts were suspended and apoptosis determined, with time. Beta3-/- preosteoclasts, in suspension, undergo a rate of apoptosis only 40-60% of that of their wild-type counterparts, indicating that unoccupied alpha(v)beta3 transmits a positive death signal that we find regulated by caspase-8. Attesting to specificity of the unoccupied integrin-transmitted death signal, apoptosis in the absence of alpha(v)beta3 is mediated by capsase-9. We have shown that the resorptive defect of beta3-/- osteoclasts is rescued by wild-type beta3 cDNA but not by one bearing a S752P mutation. To determine whether the same holds true regarding osteoclast apoptosis, we constructed lentivirus vectors encoding green fluorescent protein, wild-type beta3, or beta3S752P. Once again, native beta3-/- preosteoclasts were protected against apoptosis. Similar to its effect on bone resorption, transduced wild-type beta3 normalizes the apoptotic rate of beta3-/- preosteoclasts. Unexpectedly, however, beta3S752P transductants also die at a rate indistinguishable from wild type. Thus, unoccupied alpha(v)beta3 integrin regulates osteoclast apoptosis via a component of the integrin that is different than that regulating resorption.

Marrow Stromal Cells and Osteoclast Precursors Differentially Contribute to TNF-α-Induced Osteoclastogenesis In Vivo

The marrow stromal cell is the principal source of the key osteoclastogenic cytokine receptor activator of NF-kappaB (RANK) ligand (RANKL). To individualize the role of marrow stromal cells in varying states of TNF-alpha-driven osteoclast formation in vivo, we generated chimeric mice in which wild-type (WT) marrow, immunodepleted of T cells and stromal cells, is transplanted into lethally irradiated mice deleted of both the p55 and p75 TNFR. As control, similarly treated WT marrow was transplanted into WT mice. Each group was administered increasing doses of TNF-alpha. Exposure to high-dose cytokine ex vivo induces exuberant osteoclastogenesis irrespective of in vivo TNF-alpha treatment or whether the recipient animals possess TNF-alpha-responsive stromal cells. In contrast, the osteoclastogenic capacity of marrow treated with lower-dose TNF-alpha requires priming by TNFR-bearing stromal cells in vivo. Importantly, the osteoclastogenic contribution of cytokine responsive stromal cells in vivo diminishes as the dose of TNF-alpha increases. In keeping with this conclusion, mice with severe inflammatory arthritis develop profound osteoclastogenesis and bone erosion independent of stromal cell expression of TNFR. The direct induction of osteoclast recruitment by TNF-alpha is characterized by enhanced RANK expression and sensitization of precursor cells to RANKL. Thus, osteolysis attending relatively modest elevations in ambient TNF-alpha depends upon responsive stromal cells. Alternatively, in states of severe periarticular inflammation, TNF-alpha may fully exert its bone erosive effects by directly promoting the differentiation of osteoclast precursors independent of cytokine-responsive stromal cells and T lymphocytes.

High dose M-CSF partially rescues the Dap12-/- osteoclast phenotype

Osteoclasts are macrophage derived cells and as such are subject to regulation by molecules impacting other members of the immune system. Dap12 is an adaptor protein expressed by NK cells and B and T lymphocytes. Dap12 also mediates maturation of myeloid cells and is expressed by osteoclasts which are dysfunctional in its absence. We find Dap12-/- osteoclast precursors fail to differentiate, in vitro, and the abnormality is partially rescued by high dose M-CSF. The relative paucity of osteoclast number, even in presence of high dose cytokine, is attended by dampened proliferation of precursor cells and their failure to normally migrate towards the osteoclast-recognized matrix protein, osteopontin. Furthermore, Dap12-/- osteoclasts generated in high dose M-CSF fail to normally organize their cytoskeleton. The incapacity of Dap12 null cells to undergo normal osteoclast differentiation is not due to blunted stimulation of major RANK ligand (RANKL) or M-CSF induced signaling pathways. On the other hand, when plated on osteopontin, Dap12-/- pre-osteoclasts do not activate the tyrosine kinase, Syk, which normally binds to the adaptor protein and transmits downstream signals. Attesting to the importance of the Dap12/Syk complex, Syk deficient macrophages do not undergo normal osteoclastogenesis. Furthermore, the same cells plated onto osteopontin, adhere poorly and fail to phosphorylate c-Src or Pyk2, two kinases central to organization of the osteoclast cytoskeleton.

The HIV protease inhibitor ritonavir blocks osteoclastogenesis and function by impairing RANKL-induced signaling

Highly active antiretroviral therapy (HAART), which includes HIV protease inhibitors (PIs), has been associated with bone demineralization. To determine if this complication reflects accelerated resorptive activity, we studied the impact of two common HIV PIs, ritonavir and indinavir, on osteoclast formation and function. Surprisingly, we find that ritonavir, but not indinavir, inhibits osteoclast differentiation in a reversible manner and also abrogates bone resorption by disrupting the osteoclast cytoskeleton, without affecting cell number. Ritonavir given in vivo completely blunts parathyroid hormone-induced osteoclastogenesis in mice, which confirms that the drug is bone sparing. In keeping with its antiresorptive properties, ritonavir impairs receptor activator of nuclear factor kappaB ligand-induced (RANKL-induced) activation of NF-kappaB and Akt signaling pathways, both critical to osteoclast formation and function. In particular, ritonavir is found to inhibit RANKL-induced Akt signaling by disrupting the recruitment of TNF receptor-associated factor 6/c-Src complex to lipid rafts. Thus, ritonavir may represent a bone-sparing PI capable of preventing development of osteopenia in patients currently on HAART.

The IkappaB function of NF-kappaB2 p100 controls stimulated osteoclastogenesis

The prototranscription factor p100 represents an intersection of the NF-kappaB and IkappaB families, potentially serving as both the precursor for the active NF-kappaB subunit p52 and as an IkappaB capable of retaining NF-kappaB in the cytoplasm. NF-kappaB-inducing kinase (NIK) controls processing of p100 to generate p52, and thus NIK-deficient mice can be used to examine the biological effects of a failure in such processing. We demonstrate that treatment of wild-type osteoclast precursors with the osteoclastogenic cytokine receptor activator of NF-kappaB ligand (RANKL) increases both expression of p100 and its conversion to p52, resulting in unchanged net levels of p100. In the absence of NIK, p100 expression is increased by RANKL, but its conversion to p52 is blocked, leading to cytosolic accumulation of p100, which, acting as an IkappaB protein, binds NF-kappaB complexes and prevents their nuclear translocation. High levels of unprocessed p100 in osteoclast precursors from NIK-/- mice or a nonprocessable form of the protein in wild-type cells impair RANKL-mediated osteoclastogenesis. Conversely, p100-deficient osteoclast precursors show enhanced sensitivity to RANKL. These data demonstrate a novel, biologically relevant means of regulating NF-kappaB signaling, with upstream control and kinetics distinct from the classical IkappaBalpha pathway.

Dynamic changes in the osteoclast cytoskeleton in response to growth factors and cell attachment are controlled by beta3 integrin

The beta3 integrin cytoplasmic domain, and specifically S752, is critical for integrin localization and osteoclast (OC) function. Because growth factors such as macrophage colony-stimulating factor and hepatocyte growth factor affect integrin activation and function via inside-out signaling, a process requiring the beta integrin cytoplasmic tail, we examined the effect of these growth factors on OC precursors. To this end, we retrovirally expressed various beta3 integrins with cytoplasmic tail mutations in beta3-deficient OC precursors. We find that S752 in the beta3 cytoplasmic tail is required for growth factor-induced integrin activation, cytoskeletal reorganization, and membrane protrusion, thereby affecting OC adhesion, migration, and bone resorption. The small GTPases Rho and Rac mediate cytoskeletal reorganization, and activation of each is defective in OC precursors lacking a functional beta3 subunit. Activation of the upstream mediators c-Src and c-Cbl is also dependent on beta3. Interestingly, although the FAK-related kinase Pyk2 interacts with c-Src and c-Cbl, its activation is not disrupted in the absence of functional beta3. Instead, its activation is dependent upon intracellular calcium, and on the beta2 integrin. Thus, the beta3 cytoplasmic domain is responsible for activation of specific intracellular signals leading to cytoskeletal reorganization critical for OC function.

Genetic regulation of osteoclast development and function

Osteoclasts are the principal, if not exclusive, bone-resorbing cells, and their activity has a profound impact on skeletal health. So, disorders of skeletal insufficiency, such as osteoporosis, typically represent enhanced osteoclastic bone resorption relative to bone formation. Prevention of pathological bone loss therefore depends on an appreciation of the mechanisms by which osteoclasts differentiate from their precursors and degrade the skeleton. The past five years have witnessed important insights into osteoclast formation and function. Many of these discoveries have been made through genetic experiments that involved the rare hereditary disorder osteopetrosis.

Vitamin D metabolism in a frugivorous nocturnal mammal, the Egyptian fruit bat (Rousettus aegyptiacus)

The nocturnal, frugivorous Egyptian fruit bat (Rousettus aegyptiacus) has no obvious access to either endogenous or dietary sources of vitamin D. We hypothesized that this species under natural conditions would be vitamin D deficient and that both serum mineral concentrations and vitamin D metabolite concentrations would be low. Both wild populations and captive populations appear to have an impoverished vitamin D status, as concentrations of the principle circulating metabolite, 25-hydroxyvitamin D [25(OH)D] are undetectable (<4 ng/mL) and those of the active metabolite, 1,25-dihydroxyvitamin D [1,25(OH)(2)D] are low. Intraperitoneal administration of labelled 25(OH)D revealed enhanced 1 alpha-hydroxylase activity confirming a natural state of vitamin D deficiency. This may account for the undetectable levels of 25(OH)D; for limited amounts of the prohormone substrate are rapidly converted to the active hormone. Both vitamin D(2) and D(3) metabolites were detected in bat serum, albeit in very small amounts, inferring that in their natural habitat fruit bats may have limited access to both exogenous dietary sources and endogenous sources. Despite the low levels of vitamin D metabolites in wild-caught and captive D-unsupplemented individuals, serum mineral concentrations were well regulated and similar to those of bats receiving D-supplements, with no pathological problems associated with vitamin D deficiency evident.

Interleukin 7 and estrogen-induced bone loss

During the past decade, we have increased greatly our understanding of the pathogenesis of postmenopausal osteoporosis, a major cause of morbidity and mortality. We know now that estrogen regulates the expression of cytokines that target cell types involved in modulating bone turnover. Although early work demonstrated that tumor necrosis factor alpha plays an important role in regulating bone mass, recent studies also implicate the lymphopoietic molecule interleukin 7. This protein is unique in that it suppresses the bone-forming osteoblast, whilst stimulating formation and function of the osteoclast, the exclusive resorptive cell. Furthermore, the latest findings confirm and expand the concept that T cells are key mediators of bone loss following gonadal failure.