Tartrate-resistant acid phosphatase (Acp 5): identification in diverse human tissues and dendritic cells

Increased expression of lysosomal acid phosphatase in CLN3-defective cells and mouse brain tissue

Juvenile neuronal ceroid lipofuscinosis (Batten disease) is a neurodegenerative disorder caused by defective function of the lysosomal membrane glycoprotein CLN3. The activity of the lysosomal acid phosphatase (LAP/ACP2) was found to be significantly increased in the cerebellum and brain stem of Cln3-targeted mice during the early stages of postnatal life. Histochemical localization studies revealed an increased LAP/ACP2 staining intensity in neurons of the cerebral cortex of 48-week-old Cln3-targeted mice as compared with controls. Additionally, the expression of another lysosomal membrane protein LAMP-2 was increased in all brain areas. Knockdown of CLN3 expression in HeLa cells by RNA interference also resulted in increased LAP/ACP2 and LAMP-2 expression. Finally in fibroblasts of two juvenile neuronal ceroid lipofuscinosis patients elevated levels of LAP/ACP2 were found. Both activation of gene transcription and increased protein half-life appear to contribute to increased LAP/ACP2 protein expression in CLN3-deficient cells. The data suggest that lysosomal dysfunction and accumulation of storage material require increased biogenesis of LAP/ACP2 and LAMP-2 positive membranes which makes LAP/ACP2 suitable as biomarker of Batten disease.

Sequence and TLR9 independent increase of TRACP expression by antisense DNA and siRNA molecules

Reactive oxygen species generating activity of tartrate-resistant acid phosphatase (TRACP) has been suggested to have several functions in TRACP expressing bone resorbing osteoclasts, macrophages, and dendritic cells. This work aimed to study the TRACP knock down phenotype in osteoclasts by using antisense DNA and RNA interference methods. Unexpectedly, both TRACP specific DNA oligonucleotides and siRNA molecules extensively increased the TRACP expression in human osteoclasts and monocytes. Toll-like receptor 9 (TLR9) is an immunity sensor for CpG motifs in DNA. We cultured bone marrow-derived osteoclast precursor cells from wild-type and TLR9-/- mice with CpG and non-CpG DNA oligonucleotides, and observed that the increased TRACP expression was sequence and TLR9 independent. In contrast, cells with increased TRACP activity showed decreased activity of tartrate-sensitive acid phosphatases.

CONCLUSION

DNA oligonucleotides and RNA molecules extensively increase TRACP expression in monocyte-macrophage lineage. These results suggest a potential role of TRACP in pathogen recognition and in innate immunity.

Altered collagen in tartrate-resistant acid phosphatase (TRAP)-deficient mice: a role for TRAP in bone collagen metabolism

Tartrate-resistant acid phosphatase (TRAP) is an iron-containing protein that is highly expressed by osteoclasts, macrophages, and dendritic cells. The enzyme is secreted by osteoclasts during bone resorption, and serum TRAP activity correlates with resorptive activity in disorders of bone metabolism. TRAP is essential for normal skeletal development. In knockout mice lacking TRAP, bone shape and modeling is altered with increased mineral density. Here, we report the effect of TRAP on the biochemical and biomechanical properties of collagen, the major protein constituting the bone matrix, using these mice. Femurs from TRAP-/- and wild-type mice were used in these studies. The biomechanical properties were investigated using a three-point bending technique. Collagen synthesis was determined by measuring cross-link content using high-performance liquid chromatography and amino acid analysis. Collagen degradation was determined by measuring matrix metalloproteinase-2 (MMP-2) activity. The rates of collagen synthesis and degradation were significantly greater in bones from TRAP-/- mice compared with wild type. At 8 weeks, there was an increase in the intermediate cross-links but no significant difference in animals aged 6 months. There was a significant increase in mature cross-links at both ages. A significant increase in MMP-2 production both pro and active was observed. A significant increase in ultimate stress and Young's modulus of elasticity was needed to fracture the bones from mice deficient in TRAP. We conclude that both synthesis as well as degradation of collagen are increased when TRAP is absent in mice at 8 weeks and 6 months of age, showing that TRAP has an important role in the metabolism of collagen.

Expression and proteolytic processing of mammalian purple acid phosphatase in CHO-K1 cells

Rat recombinant purple acid phosphatase (PAP) stably expressed in fibroblast-like CHO-K1 cells was purified and characterized with respect to post-translational modifications such as N-glycosylation and proteolytic processing in order to elucidate subcellular and molecular pathways for proteolytic activation. In these cells, proteolytically processed PAP was more abundant than the monomeric form. PAP-transfected CHO-K1 cells were expressing active cathepsin K intracellularly, which was partially co-localized with PAP. However, neither cathepsin K nor trypsin digestion of the purified monomeric PAP in vitro did result in a two-subunit form with kinetic and electrophoretic properties resembling the endogenous cellular two-subunit form. Treatment of PAP-transfected CHO-K1 cells with the cysteine proteinase inhibitor E-64 suggested that only a minor fraction of secreted PAP is processed intracellularly by cysteine proteinases. These data do not support a dominant or critical role for cathepsins or trypsin-like serine proteinases in the proteolytic activation of PAP in CHO-K1 cells, implicating yet unidentified proteinases in the proteolytic processing of both intracellular and secreted PAP in this cell line.

Osteoclast formation is strongly reduced both in vivo and in vitro in the absence of CD47/SIRPalpha-interaction

Physical interaction between the cell surface receptors CD47 and signal regulatory protein alpha (SIRPalpha) was reported to regulate cell migration, phagocytosis, cytokine production, and macrophage fusion. However, it is unclear if the CD47/SIRPalpha-interaction can also regulate macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor (NF)-kappaB ligand (RANKL)-stimulated formation of osteoclasts. Here, we show that functional blocking antibodies to either CD47 or SIRPalpha strongly reduced formation of multinucleated tartrate-resistant acid phosphatase (TRAP)+ osteoclasts in cultures of murine hematopoietic cells, stimulated in vitro by M-CSF and RANKL. In addition, the numbers of osteoclasts formed in M-CSF/RANKL-stimulated bone marrow macrophage cultures from CD47-/- mice were strongly reduced, and bones of CD47-/- mice exhibited significantly reduced osteoclast numbers, as compared with wild-type controls. We conclude that the CD47/SIRPalpha interaction is important for M-CSF/RANKL-stimulated osteoclast formation both in vivo and in vitro, and that absence of CD47 results in decreased numbers of osteoclasts in CD47-/- mice.

TRACP Influences Th1 pathways by affecting dendritic cell function

TRACP, a marker of osteoclasts, is also expressed by cells of the immune system. We identified a novel function for TRACP in the dendritic cell. DCs from TRACP knockout mice have impaired maturation and trigger reduced Th1 responses in vivo. We postulate that TRACP has an important role in the presentation of antigens to T cells.

INTRODUCTION

TRACP is highly expressed by osteoclasts, activated macrophages, and dendritic cells (DCs). Knockout mice lacking TRACP have an intrinsic defect in osteoclastic resorption and macrophages that display abnormal immunomodulatory responses and cytokine secretion profiles. Our aim in this study was to investigate the significance of TRACP in the inductive phase of the immune response by examining dendritic cells from TRACP(-/-) mice.

MATERIALS AND METHODS

Maturational state and function of leukocyte subsets in mice was assessed by flow cytometry. The ability of the immune system to respond to nonspecific activation and to specific antigen was assessed by delayed type hypersensitivity and the presence of isotype-specific serum antibody in vivo and T-cell proliferation and cytokine production in vitro.

RESULTS

The ability of lipopolysaccharide (LPS) to upregulate MHC II and CD80 in DCs from TRACP(-/-) mice was reduced compared with wildtype mice, although production of IL-10 by DCs from TRACP-deficient animals was increased. T- and B-cell responses not involving antigen presentation (anti-CD3, TNP-ficoll) were normal in TRACP(-/-) mice, but responses to T-dependent antigens were impaired. Specifically, TRACP(-/-) mice had defective delayed hypersensitivity responses to picryl chloride and reduced proliferative responses to ovalbumin compared with wildtype mice. In response to ovalbumin, but not anti-CD3, T cells from TRACP(-/-) mice produced less interferon-gamma (IFN-gamma), but there was no difference in IL-4 production: TRACP(-/-) mice also produced less ovalbumin (OVA)-specific IgG2a after immunization.

CONCLUSIONS

The finding that DCs from TRACP(-/-) mice have impaired maturation and defective Th1 responses shows that TRACP is important for polarizing responses in naïve T cells to antigen-presented dendritic cells.

Evidence-based recommendations for monitoring bone disease and the response to enzyme replacement therapy in Gaucher patients

BACKGROUND

Bone disease is a serious complication of Gaucher disease. Untreated, it can result in pain, permanent bone damage and disability. Enzyme replacement therapy reverses many of the clinical signs of Gaucher bone disease but early assessment and treatment, and regular monitoring, are essential in optimising outcomes.

SCOPE

In September 2005, a group of European experts met to review current knowledge and identify best practice and unmet needs in the monitoring of Gaucher bone disease and the response to enzyme replacement therapy.

METHODS

Medline searches of peer-reviewed literature (no date restrictions) were conducted and supplemented by additional information considered relevant by panellists to furthering discussions.

FINDINGS AND CONCLUSIONS

The group's recommendations included: currently used biochemical bone markers are not clinically practical or reliable; plain X-rays should not be the sole method of assessing bone disease; MRI is the most sensitive method for monitoring bone marrow infiltration by Gaucher cells; semi-quantitative methods for assessing bone marrow infiltration in routine clinical practice should use readily available technology, include an assessment of Gaucher cell infiltration in the lumbar spine and femur, and be validated for inter-rater reliability and in comparison to other methods; a multidisciplinary approach is required for the treatment of Gaucher patients; all Gaucher patients should receive a comprehensive initial radiologic evaluation for bone disease and ongoing radiological monitoring at least once every 2 years.

Quantitative analysis of osteoclast-specific gene markers stimulated by lipopolysaccharide

Lipopolysaccharide (LPS) in the outer layers of Gram-negative bacteria plays an important role in initiating and sustaining periapical lesions. To understand the mechanisms of osteoclastic bone resorption in periapical lesions induced by LPS, we stimulated osteoclast precursors, RAW 264.7 cells with LPS. LPS stimulated osteoclastogenesis when osteoclast precursors were primed with activator for NF-kappaB ligand (RANKL) as little as 24 h. By employing real-time PCR analysis, we have confirmed that osteoclast-like cells stimulated by LPS express high level of osteoclast-specific gene markers such as TRAP, cathepsin K, and calcitonin receptor. These results suggest that bone-resportive action by LPS is partially independent of RANKL.

Endocytosis of tartrate-resistant acid phosphatase by osteoblast-like cells is followed by inactivation of the enzyme

Tartrate-resistant acid phosphatase (TRACP) is generally used as a marker of osteoclasts. Yet, other bone-associated cells, such as osteoblasts and osteocytes, may also express activity of this enzyme. Osteoblasts containing TRACP activity are seen particularly in the vicinity of areas of bone resorption, suggesting that osteoclasts somehow induce TRACP activity in osteoblasts. In a recent study, we found that osteoblast-like cells appeared to have the capacity to endocytose TRACP released by osteoclast precursors. In the present study, we investigated the endocytosis of TRACP in more detail as well as the fate of the endocytosed enzyme. We found that incubation of osteoblast-like cells with TRACP-coated beads resulted in attachment of a high number of beads to the cells. After culturing osteoblast-like cells with medium conditioned by blood monocytes that contain TRACP, activity of the enzyme was found in the cells. Following replacement of the medium by normal medium that did not contain TRACP, a decrease in the level of TRACP activity in osteoblast-like cells occurred. Our data strongly suggest that osteoblast-like cells recognize TRACP released by osteoclast precursors and that upon endocytosis inactivation of the enzyme occurs. We propose that uptake of the enzyme is important for the control of enzyme activity, thereby preventing degradation of matrix constituents.

Secretion of tartrate-resistant acid phosphatase by osteoclasts correlates with resorptive behavior

There have been dramatic advances recently in our understanding of the regulation of osteoclastic differentiation. However, much less is known of the mechanisms responsible for the induction and modulation of resorptive behavior. We have developed a strategy whereby osteoclasts can be generated in vitro and released into suspension in a fully-functional state. We now exploit this approach to show that tartrate-resistant acid phosphatase (TRAP) is released by osteoclasts during bone resorption. TRAP release was inhibited by the secretion-inhibitor Brefeldin A, and was not accompanied by LDH release. This suggests that TRAP release is due to secretion, rather than cell death. Consistent with this, TRAP secretion was stimulated by resorbogenic cytokines, was inhibited by the resorption-inhibitor calcitonin, and correlated with excavation of the bone surface. We found that, in contrast to incubation on bone, incubation on plastic, glass, or vitronectin-coated plastic substrates did not induce secretion of TRAP. This suggests that the induction of resorptive behavior in osteoclasts depends upon stimulation by bone matrix of a putative osteoclastic "mineral receptor." Release of TRAP by osteoclasts thus represents not only a productive approach to the analysis of the mechanisms that modulate the rate of resorptive activity, but also a system whereby the mechanism through which bone substrates induce resorptive behavior can be identified.

Serum TRACP 5b is a useful marker for monitoring alendronate treatment: comparison with other markers of bone turnover

We studied clinical performance of serum TRACP 5b and other bone turnover markers, including S-CTX, U-DPD, S-PINP, S-BALP, and S-OC, for monitoring alendronate treatment. TRACP 5b had higher clinical sensitivity, area under the ROC curve, and signal-to-noise ratio than the other markers.

INTRODUCTION

The purpose of this study was to compare the clinical performance of serum TRACP 5b (S-TRACP5b) with that of other markers of bone turnover in the monitoring of alendronate treatment.

MATERIALS AND METHODS

This double-blinded study included 148 healthy postmenopausal women that were randomly assigned into two groups: one receiving 5 mg alendronate daily (n=75) and the other receiving placebo (n=73) for 12 months. All individuals in both groups received calcium and vitamin D daily. The bone resorption markers S-TRACP5b, serum C-terminal cross-linked telopeptides of type I collagen (S-CTX), and total urinary deoxypyridinoline (U-DPD), and the serum markers of bone formation procollagen I N-terminal propeptide (S-PINP), bone-specific alkaline phosphatase (S-BALP), and total osteocalcin (S-OC) were assessed at baseline and at 3, 6, and 12 months after initiation of treatment. Lumbar spine BMD (LBMD) was measured at baseline and 12 months.

RESULTS

Compared with the placebo group, LBMD increased, and all bone markers decreased significantly more in the alendronate group (p<0.001 for each parameter). The decrease of S-TRACP5b after first 3 months of alendronate treatment correlated significantly with the changes of all other markers except S-OC, the best correlation being with S-CTX (r=0.60, p<0.0001). The changes of LBMD at 12 months only correlated significantly with the changes of S-TRACP5b (r=-0.32, p=0.005) and S-CTX (r=-0.24, p=0.037) at 3 months. Based on clinical sensitivity, receiver operating characteristic (ROC) curves, and signal-to-noise ratio, S-TRACP5b, S-CTX, and S-PINP were the best markers for monitoring alendronate treatment. Clinical sensitivity, area under the ROC curve, and signal-to-noise ratio were higher for S-TRACP5b than for the other markers.

CONCLUSION

These results show that S-TRACP5b, S-CTX, and S-PINP are useful markers for monitoring alendronate treatment.

Proteolytic Excision of a Repressive Loop Domain in Tartrate-resistant Acid Phosphatase by Cathepsin K in Osteoclasts

Tartrate-resistant acid phosphatase (TRAP) is a metallophosphoesterase participating in osteoclast-mediated bone turnover. Activation of TRAP is associated with the redox state of the di-iron metal center as well as with limited proteolytic cleavage in an exposed loop domain. The cysteine proteinases cathepsin B, L, K, and S as well as the matrix metalloproteinase-2, -9, -13, and -14 are expressed by osteoclasts and/or other bone cells and have been implicated in the turnover of bone and cartilage. To identify proteases that could act as activators of TRAP in bone, we report here that cathepsins K and L, in contrast to the matrix metalloproteinases, efficiently cleaved and activated recombinant TRAP in vitro. Activation of TRAP by cathepsin K/L was because of increases in catalytic activity, substrate affinity, and sensitivity to reductants. Processing by cathepsin K occurred sequentially by an initial excision of the loop peptide Gly(143)-Gly(160) followed by the removal of a Val(161)-Ala(162) dipeptide at the N terminus of the C-terminal 16-kDa TRAP subunit. Cathepsin L initially released a shorter Gln(151)-Gly(160) peptide and completed processing at Ser(145) or Gly(143) at the C terminus of the N-terminal 23-kDa TRAP subunit and at Arg(163) at the N terminus of the C-terminal 16-kDa TRAP subunit. Mutation of Ser(145) to Ala partly mimicked the effect of proteolysis on catalytic activity, identifying Ser(145) as well as Asp(146) (Funhoff, E. G., Ljusberg, J., Wang, Y., Andersson, G., and Averill, B. A. (2001) Biochemistry 40, 11614-11622) as repressive amino acids of the loop region to maintain the TRAP enzyme in a catalytically latent state. The C-terminal sequence of TRAP isolated from rat bone was consistent with cathepsin K-mediated processing in vivo. Moreover, cathepsin K, but not cathepsin L, co-localized with TRAP in osteoclast-resorptive compartments, supporting a role for cathepsin K in the extracellular processing of monomeric TRAP in the resorption lacuna.

N-glycosylation influences the latency and catalytic properties of mammalian purple acid phosphatase

Purple acid phosphatase (PAP), also known as tartrate-resistant acid phosphatase or uteroferrin, contains two potential consensus N-glycosylation sites at Asn(97) and Asn(128). In this study, endogenous rat bone PAP was found to possess similar N-glycan structures as rat recombinant PAP heterologously expressed in baculovirus-infected Sf9 insect cells. PAP from Sf9 cells was shown to contain two N-linked oligosaccharides, whereas PAP expressed by mammalian CHO-K1 cells was less extensively glycosylated. The extent of N-glycosylation affected the catalytic properties of the enzyme, as N97Q and N128Q mutants, containing a single oligosaccharide chain, exhibited a lower substrate affinity and catalytic activity compared to those of the fully glycosylated PAP in the native, monomeric state. The differences in substrate affinity and catalytic activity were abolished and partially restored, respectively, by proteolytic cleavage in the loop domain, indicating that the extent of N-glycosylation influences the interaction of the repressive loop domain with catalytically important residues.

Optimised real-time quantitative PCR assays for RANKL regulated genes

Osteoclasts are multinucleated giant cells that differentiate from precursors of the monocyte-macrophage lineage. We used receptor activator of NF-kappa B ligand (RANKL) and macrophage colony stimulating factor (M-CSF) to differentiate authentic human osteoclasts from peripheral blood mononuclear cells (PBMCs). We previously described a series of genes that are strongly regulated by RANKL. Here, we provide a set of reliable quantitative real-time PCR based assays of RANKL regulated genes as reference genes that may prove useful in the study of human osteoclasts. The SYBR-green I assays are free of primer dimer and other artefacts, and are designed to amplify in parallel, thus permitting simultaneous analysis of 12 genes. Optimised primers for 18S rRNA provide a valid housekeeping reference gene. Standard curves have been constructed for all assays, thus allowing for absolute quantification of mRNA transcript copy number. As an example, the regulation of expression of the chemokine RANTES in osteoclasts is demonstrated. These gene assays have potential utility in a variety of cell types, tissues and organs, in addition to macrophages and osteoclasts.

Tartrate-resistant acid phosphatase knockout mice

TRACP is a lysosomal enzyme found in diverse tissues, where it is expressed in dendritic cells as well as osteoclasts and macrophages. To investigate the function of TRACP in vivo, we have generated mice in which the gene-encoding TRACP has been selectively disrupted by targeted homologous recombination in murine embryonic stem cells. Homozygous TRACP "knockout" mice have progressive foreshortening and deformity of the long bones and axial skeleton suggesting a role for TRACP in endochondral ossification. There is increased mineralization reflecting a mild osteopetrosis caused by reduced osteoclast modeling activity. These knockout mice also display an impairment of macrophage function with abnormal immunomodulatory cytokine responses. Superoxide formation and nitrite production were enhanced in stimulated macrophages lacking TRACP as was the secretion of the proinflammatory cytokines TNF-alpha, interleukin (IL)-1beta, and IL-12. TRACP knockout mice showed delayed clearance of the microbial pathogen Staphylococcus aureus after sublethal intraperitoneal inoculation. The macrophages lacking TRACP showed an increase in tartrate-sensitive lysosomal acid phosphatase activity (LAP). The TRACP knockout mice were bred with mice lacking LAP. Mice lacking both TRACP and LAP had even shorter bones than the TRACP single knockouts. Osteopontin, identical to the T-cell cytokine eta-1, accumulated adjacent to actively resorbing osteoclasts suggesting that both phosphatases are important for processing this protein. We propose that TRACP may be an important regulator of osteopontin/eta-1 activity common to both the immune system and skeleton.

TRACP as an osteopontin phosphatase

TRACP is synthesized as a latent proenzyme requiring proteolytic processing to attain maximal phosphatase activity. Excision of an exposed loop domain abolishes the interaction between the loop residue Asp146 and a ligand to the redox-sensitive iron of the active site, most likely Asn91, providing a mechanism for the enzyme repression. Both cathepsin K and L efficiently cleave in the loop domain and activate the latent enzyme, and we propose that cathepsin K acts as a physiological activator of TRACP in osteoclasts, whereas cathepsin L might fulfill a similar role in different types of macrophages. Considering the rather broad substrate specificity of TRACP, a tight regulation of its activity in the cell appears warranted. Besides proteolytic cleavage, the enzyme should need a specific local environment with a slightly acidic pH and reducing equivalents to keep the enzyme fully active. Cellular subcompartments where these required conditions prevail are potential subcellular site(s) of TRACP action. Of bone phosphoproteins shown to be substrates for TRACP, both osteopontin and bone sialoprotein are colocalized with TRACP in the resorption lacuna of the osteoclasts, and dephosphorylation of OPN impair its ability to promote adhesion as well as migration of osteoclasts in vitro. A role for TRACP as an osteopontin phosphatase in bone is therefore suggested. The expression of TRACP as well as OPN in other tissues with possible interactions between the two could suggest a more general function for TRACP as a regulator of OPN phosphorylation and bioactivity.

Synergistic effects of Y2 and Y4 receptors on adiposity and bone mass revealed in double knockout mice

Neuropeptide Y regulates numerous physiological processes via at least five different Y receptors, but the specific roles of each receptor are still unclear. We previously demonstrated that Y2 receptor knockout results in a lean phenotype, increased cancellous bone volume, and an increase in plasma pancreatic polypeptide (PP), a ligand for Y4 receptors. PP-overexpressing mice are also known to have a lean phenotype. Deletion of the Y4 receptor also produced a lean phenotype and increased plasma PP levels. We therefore hypothesized that part of the Y2 phenotype results from increased PP action on Y4 receptors and tested this in PP transgenic Y4(-/-) and Y2(-/-) Y4(-/-) double knockout mice. Bone mass was not altered in Y4 knockout mice. Surprisingly, despite significant hyperphagia, Y2(-/-) Y4(-/-) mice retained a markedly lean phenotype, with reduced body weight, white adipose tissue mass, leptinemia, and insulinemia. Furthermore, bone volume was also increased threefold in Y2(-/-) Y4(-/-) mice, and this was associated with enhanced osteoblastic activity. These changes were more pronounced than those observed in Y2(-/-) mice, suggesting synergy between Y2 and Y4 receptor pathways. The lack of bone changes in PP transgenic mice suggests that PP alone is not responsible for the bone mass increases but might play a major role in the lean phenotype. However, a synergistic interaction between Y2 and Y4 pathways seems to regulate bone volume and adiposity and could have important implications for possible interventions in obesity and for anabolic treatment of osteoporotic bone loss.

Acid phosphatases as markers of bone metabolism

Various biochemical markers have been used to assess bone metabolism and to monitor the effects of treatments. Tartrate resistant acid phosphatase (TRAP; EC 3.1.3.2) has often been used to assess bone absorption. Although osteoclasts contain abundant TRAP and they are responsible for bone resorption, the total TRAP activities in the serum measured by colorimetric methods little reflect the bone turnover. TRAP 5 is further separated into 5a and 5b by electrophoresis. Type 5b is considered to be derived from the osteoclast, and therefore attempts are being made to measure exclusively serum TRAP 5b by kinetic methods, immunological methods, and chromatographic methods including ion-exchange and heparin column chromatography.

Hypothalamic Y2 receptors regulate bone formation

Neuropeptide Y (NPY) is a downstream modulator of leptin action, possibly at the level of the arcuate nucleus where NPY neurons are known to express both leptin receptors and Y2 receptors. In addition to the well-described role of NPY and leptin in energy balance and obesity, intracerebroventricular administration of NPY or leptin also causes bone loss. Here we show that Y2 receptor-deficient mice have a twofold increase in trabecular bone volume as well as greater trabecular number and thickness compared with control mice. We also demonstrate that central Y2 receptors are crucial for this process, since selective deletion of hypothalamic Y2 receptors in mature conditional Y2 knockout mice results in an identical increase in trabecular bone volume within 5 weeks. This hypothalamus-specific Y2 receptor deletion stimulates osteoblast activity and increases the rate of bone mineralization and formation, with no effect on osteoblast or osteoclast surface measurements. The lack of any changes in plasma total calcium, leptinemia, or hypothalamo-pituitary-corticotropic, -thyrotropic, -somatotropic, or -gonadotropic output suggests that Y2 receptors do not modulate bone formation by humoral mechanisms, and that alteration of autonomic function through hypothalamic Y2 receptors may play a key role in a major central regulatory circuit of bone formation.