LDL receptor-related protein 5 (LRP5) affects bone accrual and eye development

Low-density lipoprotein receptor-related protein 5 (LRP5) gene polymorphisms are associated with bone mass in both Chinese and whites

In this study, the associations of novel LRP5 variants with BMD variation were detected and some replicated in the two ethnic groups of Chinese and white origins, respectively. These data support the concept that LRP5 variation can contribute to minor and major variation in bone structure.

INTRODUCTION

Mutations in the low-density lipoprotein receptor-related protein 5 (LRP5) gene have been shown to cause both high and low bone mass. However, it is still controversial whether LRP5 is associated with normal BMD variation. This study explored the association of LRP5 with BMD phenotypes at three clinically important skeletal sites-the spine, hip, and ultradistal radius (UD)-in two independent populations of Chinese and white ethnicities, respectively.

MATERIALS AND METHODS

The Chinese sample consisted of 733 unrelated subjects. The white sample was made up of 1873 subjects from 405 nuclear families. High-density single nucleotide polymorphisms (SNPs) across the whole LRP5 gene were genotyped and analyzed in both samples.

RESULTS

Linkage disequilibrium (LD) analyses showed that the haplotype structures of LRP5 between Chinese and whites were in good agreement. Association tests showed that polymorphisms in block 5 spanning intron 7 to intron 19 of LRP5 significantly associated with spine BMD variation in both samples. Particularly, the significant association of SNP rs491347 in intron 7 with spine BMD in the Chinese sample (p=0.002) was replicated in whites, even after adjusting for multiple testing (p=0.005). Its strongly associated SNP rs1784235 could cause the loss of an estrogen receptor alpha (ERalpha) binding site in LRP5, which could partially explain the above replicated association. However, we did not observe any significant replication with BMD variation at the hip and UD. After accounting for multiple testing, associations with BMD variation at these two sites were mainly found in Chinese. Sex-stratified analyses further revealed that the LRP5 associations with BMD in Chinese and whites were driven by male and female subjects, respectively.

CONCLUSIONS

Our work supported LRP5 genetic variants as possible susceptibility factors for osteoporosis and fractures in humans. Especially, the SNP rs491347 and its strongly associated SNPs (e.g., rs1784235) could be important to human osteoporosis phenotypes.

Genetic variation at the low-density lipoprotein receptor-related protein 5 (LRP5) locus modulates Wnt signaling and the relationship of physical activity with bone mineral density in men

Polymorphisms in the LRP5 gene have been associated with bone mineral density (BMD) in men and/or women. However, the functional basis for this association remains obscure. We hypothesized that LRP5 alleles could modulate Wnt signaling and the relationship between physical activity and BMD. This genetic association study was performed in the population-based Framingham Study Offspring Cohort, and included a subset of 1797 unrelated individuals who provided blood samples for DNA and who had BMD measurements of the hip and spine. Ten single-nucleotide polymorphisms (SNPs) spanning the LRP5 gene were genotyped and used for association and interaction analyses with BMD by regression methods. LRP5 haplotypes were transiently co-expressed with Wnt3a, MesD and Dkk1 in HEK293 cells and their activity evaluated by the TCF-Lef reporter assay. Six out of ten SNPs in LRP5 were associated with one or more of the femur or spine BMDs in men or women after adjustment for covariates, and these associations differed between genders. In men< or =age 60 years, 3 SNPs were significantly associated with BMD: rs2306862 on Exon 10 with femoral neck BMD (p=0.01) and Ward's BMD (p=0.01); rs4988321/p. V667M with Ward's BMD (p=0.02); and intronic rs901825 with trochanter BMD (p=0.03). In women, 3 SNPs in intron 2 were significantly associated with BMD: rs4988330 for trochanter (p=0.01) and spine BMD (p=0.003); rs312778 with femoral neck BMD (p=0.05); and rs4988331 with spine BMD (p=0.04). For each additional rare allele, BMD changed by 3-5% in males and 2-4% in females. Moreover, there was a significant interaction between physical activity and rs2306862 in exon 10 (p for interaction=0.02) and rs3736228/p. A1330V in exon 18 (p for interaction=0.05) on spine BMD in men. In both cases, the TT genotype was associated with lower BMD in men with higher physical activity scores, conversely with higher BMD in men with lower physical activity scores. In vitro, TCF-Lef activity in presence of Wnt3a was significantly reduced in cells expressing LRP5 haplotypes carrying the T allele of exon 10 and 18 compared to the wild-type allele, whereas co-expression of Dkk1 completely inhibited Wnt3a response through all LRP5 haplotypes. In summary, genetic variation in exons 10 and 18 of the LRP5 gene modulates Wnt signaling and the relationship between physical activity and BMD in men. These observations suggest that Wnt-LRP5 may play a role in the adaptation of bone to mechanical load in humans, and may explain some gender-related differences in bone mass.

PTH stimulates bone formation in mice deficient in Lrp5

Lrp5 deficiency decreases bone formation and results in low bone mass. This study evaluated the bone anabolic response to intermittent PTH treatment in Lrp5-deficient mice. Our results indicate that Lrp5 is not essential for the stimulatory effect of PTH on cancellous and cortical bone formation.

INTRODUCTION

Low-density lipoprotein receptor-related protein 5 (Lrp5), a co-receptor in canonical Wnt signaling, increases osteoblast proliferation, differentiation, and function. The purpose of this study was to use Lrp5-deficient mice to evaluate the potential role of this gene in mediating the bone anabolic effects of PTH.

MATERIALS AND METHODS

Adult wildtype (WT, 23 male and 25 female) and Lrp5 knockout (KO, 27 male and 26 female) mice were treated subcutaneously with either vehicle or 80 microg/kg human PTH(1-34) on alternate days for 6 weeks. Femoral BMC and BMD were determined using DXA. Lumbar vertebrae were processed for quantitative bone histomorphometry. Bone architecture was evaluated by microCT. Data were analyzed using a multiway ANOVA.

RESULTS

Cancellous and cortical bone mass were decreased with Lrp5 deficiency. Compared with WT mice, cancellous bone volume in the distal femur and the lumbar vertebra in Lrp5 KO mice was 54% and 38% lower, respectively (p<0.0001), whereas femoral cortical thickness was 11% lower in the KO mice (p<0.0001). The decrease in cancellous bone volume in the lumbar vertebrae was associated with a 45% decrease in osteoblast surface (p<0.0001) and a comparable decrease in bone formation rate (p<0.0001). Osteoclast surface, an index of bone resorption, was 24% lower in Lrp5 KO compared with WT mice (p<0.007). Treatment of mice with PTH for 6 weeks resulted in a 59% increase in osteoblast surface (p<0.0001) and a 19% increase in osteoclast surface (p=0.053) in both genotypes, but did not augment cancellous bone volume in either genotype. Femur cortical thickness was 11% higher in PTH-treated mice in comparison with vehicle-treated mice (p<0.0001), regardless of genotype.

CONCLUSIONS

Whereas disruption of Lrp5 results in decreased bone mass because of decreased bone formation, Lrp5 does not seem to be essential for the stimulatory effects of PTH on cancellous and cortical bone formation.

1,25-Dihydroxyvitamin D3 induces expression of the Wnt signaling co-regulator LRP5 via regulatory elements located significantly downstream of the gene's transcriptional start site

Canonical Wnt signaling is essential for bone formation. Activation involves binding of secreted members of the Wnt family of proteins with a membrane receptor Frizzled on osteoblasts, an interaction that is facilitated by LRP5/LRP6 co-receptors. LRP5 is known to play a particularly important role in bone formation such that the loss of this protein results in a reduction in osteoblast number, a delay in mineralization and a reduction in peak BMD. During the course of a VDR ChIP-chip analysis we found that 1,25(OH)(2)D(3) could induce binding of the VDR to sites within the Lrp5 gene locus. Importantly, this interaction between 1,25(OH)(2)D(3)-activated VDR and the Lrp5 gene led to both a modification in chromatin structure within the Lrp5 locus and the induction of LRP5 mRNA transcripts in vivo as well as in vitro. One site within Lrp5 was discovered to confer 1,25(OH)(2)D(3) response to a heterologous promoter in osteoblastic cells, permitting both the identification and characterization of the component VDRE. While the regulatory region in Lrp5 was highly conserved in the human genome, the VDRE was not. Our studies show that 1,25(OH)(2)D(3) can enhance the expression of a critical component of the Wnt signaling pathway which is known to impact osteogenesis.

Regulated proteolytic processing of LRP6 results in release of its intracellular domain

Low-density lipoprotein receptor-related protein 6 (LRP6) is a member of low-density lipoprotein receptor (LDLR) family which cooperates with Frizzled receptors to transduce the canonical Wnt signal. As a critical component of the canonical Wnt pathway, LRP6 is essential for appropriate brain development, however, the mechanism by which LRP6 facilitates Wnt canonical signaling has not been fully elucidated. Interestingly, LRP6 which lacks its extracellular domain can constitutively activate TCF/LEF and potentiate the Wnt signal. Further, the free cytosolic tail of LRP6 interacts directly with glycogen synthase kinase (GSK3) and inhibits GSK3's activity in the Wnt canonical pathway which results in increased TCF/LEF activation. However, whether these truncated forms of LRP6 are physiologically relevant is unclear. Recent studies have shown that other members of the LDLR family undergo gamma-secretase dependent regulated intramembrane proteolysis (RIP). Using independent experimental approaches, we show that LRP6 also undergoes RIP. The extracellular domain of LRP6 is shed and released into the surrounding milieu and the cytoplasmic tail is cleaved by gamma-secretase-like activity to release the intracellular domain. Furthermore, protein kinase C, Wnt 3a and Dickkopf-1 modulate this process. These findings suggest a novel mechanism for LRP6 in Wnt signaling: induction of ectodomain shedding of LRP6, followed by the gamma-secretase involved proteolytic releasing its intracellular domain (ICD) which then binds to GSK3 inhibiting its activity and thus activates the canonical Wnt signaling pathway.

Noncanonical Wnt signaling through G protein-linked PKCdelta activation promotes bone formation

Wnt signaling regulates a variety of developmental processes in animals. Although the beta-catenin-dependent (canonical) pathway is known to control cell fate, a similar role for noncanonical Wnt signaling has not been established in mammals. Moreover, the intracellular cascades for noncanonical Wnt signaling remain to be elucidated. Here, we delineate a pathway in which Wnt3a signals through the Galpha(q/11) subunits of G proteins to activate phosphatidylinositol signaling and PKCdelta in the murine ST2 cells. Galpha(q/11)-PKCdelta signaling is required for Wnt3a-induced osteoblastogenesis in these cells, and PKCdelta homozygous mutant mice exhibit a deficit in embryonic bone formation. Furthermore, Wnt7b, expressed by osteogenic cells in vivo, induces osteoblast differentiation in vitro via the PKCdelta-mediated pathway; ablation of Wnt7b in skeletal progenitors results in less bone in the mouse embryo. Together, these results reveal a Wnt-dependent osteogenic mechanism, and they provide a potential target pathway for designing therapeutics to promote bone formation.

Gene expression analysis in human osteoblasts exposed to dexamethasone identifies altered developmental pathways as putative drivers of osteoporosis

BACKGROUND

Osteoporosis, a disease of decreased bone mineral density represents a significant and growing burden in the western world. Aging population structure and therapeutic use of glucocorticoids have contributed in no small way to the increase in the incidence of this disease. Despite substantial investigative efforts over the last number of years the exact molecular mechanism underpinning the initiation and progression of osteoporosis remain to be elucidated. This has meant that no significant advances in therapeutic strategies have emerged, with joint replacement surgery being the mainstay of treatment.

METHODS

In this study we have used an integrated genomics profiling and computational biology based strategy to identify the key osteoblast genes and gene clusters whose expression is altered in response to dexamethasone exposure. Primary human osteoblasts were exposed to dexamethasone in vitro and microarray based transcriptome profiling completed.

RESULTS

These studies identified approximately 500 osteoblast genes whose expression was altered. Functional characterization of the transcriptome identified developmental networks as being reactivated with 106 development associated genes found to be differentially regulated. Pathway reconstruction revealed coordinate alteration of members of the WNT signaling pathway, including frizzled-2, frizzled-7, DKK1 and WNT5B, whose differential expression in this setting was confirmed by real time PCR.

CONCLUSION

The WNT pathway is a key regulator of skeletogenesis as well as differentiation of bone cells. Reactivation of this pathway may lead to altered osteoblast activity resulting in decreased bone mineral density, the pathological hallmark of osteoporosis. The data herein lend weight to the hypothesis that alterations in developmental pathways drive the initiation and progression of osteoporosis.

Bone anabolic effects of parathyroid hormone are blunted by deletion of the Wnt antagonist secreted frizzled-related protein-1

Secreted frizzled-related protein (sFRP)-1 is a Wnt antagonist that when deleted in mice leads to increased trabecular bone formation in adult animals after 13 weeks of age. Treatment of mice with parathyroid hormone (PTH) also increases trabecular bone formation, and some of the anabolic actions of this hormone may result from altered expression of Wnt pathway components. To test this hypothesis, we treated +/+ and -/- female sFRP-1 mice with PTH 1-34 for 30 days and measured distal femur trabecular bone parameters by peripheral quantitative computed tomography (pQCT) and high-resolution micro-computed tomography. During the course of the 32-week study, volumetric bone mineral density (vBMD) declined 41% in vehicle-treated +/+ mice, but increased 24% in vehicle-treated -/- animals. At 8 weeks of age when vBMD was not altered by deletion of sFRP-1, treatment of +/+ and -/- mice with PTH increased vBMD by 147 and 163%, respectively. In contrast, at 24 weeks of age when vBMD was 75% higher in -/- mice than in +/+ controls, treatment with PTH increased vBMD 164% in +/+ animals, but only 58% in -/- mice. Furthermore, at 36 weeks of age when vBMD was 117% higher in -/- mice than in +/+ controls, treatment with PTH increased vBMD 74% in +/+ animals, while no increase was observed in -/- mice. At each of these time points, PTH treatment increased vBMD to a similar level in +/+ and -/- mice, and this level declined with age. In addition, at 36 weeks of age, the vBMD level reached by PTH treatment of +/+ mice was the same as that achieved solely by deletion of sFRP-1. These results indicate that loss of sFRP-1 and PTH treatment increase vBMD to a similar extent. Moreover, as the effects of sFRP-1 deletion on vBMD increase, the ability of PTH to enhance vBMD declines suggesting that there are overlapping mechanisms of action.

The orphan receptor tyrosine kinase Ror2 promotes osteoblast differentiation and enhances ex vivo bone formation

Ror2 is a receptor tyrosine kinase, the expression of which increases during differentiation of pluripotent stem cells to osteoblasts and then declines as cells progress to osteocytes. To test whether Ror2 plays a role in osteoblastogenesis, we investigated the effects of Ror2 overexpression and down-regulation on osteoblastic lineage commitment and differentiation. Expression of Ror2 in pluripotent human mesenchymal stem cells (hMSCs) by adenoviral infection caused formation of mineralized extracellular matrix, which is the ultimate phenotype of an osteogenic tissue. Concomitantly, Ror2 over-expression inhibited adipogenic differentiation of hMSCs as monitored by lipid formation. Ror2 shifted hMSC fate toward osteoblastogenesis by inducing osteogenic transcription factor osterix and suppressing adipogenic transcription factors CCAAT/enhancer-binding protein alpha and peroxisome proliferator activated receptor gamma. Infection with Ror2 virus also strongly promoted matrix mineralization in committed osteoblast-like MC3T3-E1 cells. Expression of Ror2 in a human preosteocytic cell line by stable transfection also promoted further differentiation, as judged by inhibited alkaline phosphatase activity, potentiated osteocalcin secretion, and increased cellular apoptosis. In contrast, down-regulation of Ror2 expression by short hairpin RNA essentially abrogated dexamethasone-induced mineralization of hMSCs. Furthermore, down-regulation of Ror2 expression in fully differentiated SaOS-2 osteosarcoma cells inhibited alkaline phosphatase activity. We conclude that Ror2 initiates commitment of MSCs to osteoblastic lineage and promotes differentiation at early and late stages of osteoblastogenesis. Finally, using a mouse calvariae ex vivo organ culture model, we demonstrate that these effects of Ror2 result in increased bone formation, suggesting that it may also activate mature osteoblasts.

Wnt signaling and human diseases: what are the therapeutic implications?

Wnt signaling plays an important role in regulating cell proliferation and differentiation. De-regulation of these signaling pathways has been implicated in many human diseases, ranging from cancers to skeletal disorders. Wnt proteins are a large family of secreted factors that bind to the Frizzled receptors and LRP5/6 co-receptors and initiate complex signaling cascades. Over the past two decades, our understanding of Wnt signaling has been significantly improved due to the identification of many key regulators and mediators of these pathways. Given that Wnt signaling is tightly regulated at multiple cellular levels, these pathways themselves offer ample nodal points for targeted therapeutics. Here, we focus on our current understanding of these pathways, the associations of Wnt signaling with human disorders, and the opportunities to target key components of Wnt signaling for rational drug discovery.

Differential gene expression in cultured osteoblasts and bone marrow stromal cells from patients with Paget's disease of bone

Paget's disease is a focal condition of bone. To study changes in cells within pagetic lesions, we cultured osteoblasts and stromal cells from 22 patients and compared gene expression in these cells to cells from healthy bone. We identified several differentially regulated genes, and we suggest that these changes could lead to the formation of the lesions.

INTRODUCTION

Paget's disease is a focal condition of bone of unknown cause. Although it is regarded as primarily an osteoclast disorder, the tight coupling of the activity of osteoclasts and osteoblasts suggests that the osteoblast could play a key role in its pathogenesis. The aim of the study was to identify possible changes in pagetic osteoblasts and stromal cells that might contribute to the development of pagetic lesions.

MATERIALS AND METHODS

Candidate genes were identified based on known bone cell regulators, supplemented with microarray analysis. Gene expression was determined by real-time PCR in primary cultures of osteoblasts and bone marrow stromal cells from pagetic patients and control subjects. Concentrations of secreted proteins were determined by ELISA.

RESULTS

Dickkopf1 mRNA and protein levels were increased in both pagetic osteoblast and stromal cell cultures, and interleukin (IL)-1 and IL-6 were overexpressed in pagetic osteoblasts. These changes parallel recent findings in myeloma bone disease, which shares some clinical similarities with Paget's disease. Alkaline phosphatase was overexpressed, and bone sialoprotein and osteocalcin were underexpressed in pagetic osteoblasts, consistent with their circulating levels in pagetic patients. It is hypothesized that overexpression of Dickkopf1, IL-1, and IL-6 would result in stimulation of osteoclast proliferation and inhibition of osteoblast growth, leading to the development of the characteristic lytic bone lesions. By stimulating osteoblast differentiation, Dickkopf1 and IL-6 may also promote mineralization, leading to the conversion of lytic lesions to sclerotic.

CONCLUSIONS

These findings suggest that dysregulated gene expression in pagetic osteoblasts could cause the changes in bone cell number and function characteristic of Paget's disease.

Dickkopf homolog 1 mediates endothelin-1-stimulated new bone formation

Tumor-produced endothelin-1 (ET-1) stimulates osteoblasts to form new bone and is an important mediator of osteoblastic bone metastasis. The anabolic actions of ET-1 in osteoblasts were investigated by gene microarray analyses of murine neonatal calvarial organ cultures. Targets of ET-1 action were validated by real-time RT-PCR in murine primary osteoblast cultures. IL-6, IL-11, the CCN (CYR61, CTGF, NOV) family members cysteine-rich protein 61 and connective tissue growth factor, inhibin beta-A, serum/glucocorticoid regulated kinase, receptor activator of nuclear factor kappaB ligand, snail homolog 1, tissue inhibitor of metalloproteinase 3, and TG-interacting factor transcripts were increased by ET-1. ET-1 decreased the transcript for the Wnt signaling pathway inhibitor, dickkopf homolog 1 (Dkk1). Calvarial organ cultures treated with ET-1 had lower concentrations of DKK1 protein in conditioned media than control cultures. High DKK1 concentrations in bone marrow suppress bone formation in multiple myeloma. We hypothesized that the converse occurs in osteoblastic bone metastasis, where ET-1 stimulates osteoblast activity by reducing autocrine production of DKK1. Recombinant DKK1 blocked ET-1-mediated osteoblast proliferation and new bone formation in calvarial organ cultures, whereas a DKK1-neutralizing antibody increased osteoblast numbers and new bone formation. ET-1 directed nuclear translocation of beta-catenin in osteoblasts, indicating activation of the Wnt signaling pathway. The data suggest that ET-1 increases osteoblast proliferation and new bone formation by activating the Wnt signaling pathway through suppression of the Wnt pathway inhibitor DKK1.

Dickkopf-1 is a master regulator of joint remodeling

Degenerative and inflammatory joint diseases lead to a destruction of the joint architecture. Whereas degenerative osteoarthritis results in the formation of new bone, rheumatoid arthritis leads to bone resorption. The molecular basis of these different patterns of joint disease is unknown. By inhibiting Dickkopf-1 (DKK-1), a regulatory molecule of the Wnt pathway, we were able to reverse the bone-destructive pattern of a mouse model of rheumatoid arthritis to the bone-forming pattern of osteoarthritis. In this way, no overall bone erosion resulted, although bony nodules, so-called osteophytes, did form. We identified tumor necrosis factor-alpha (TNF) as a key inducer of DKK-1 in the mouse inflammatory arthritis model and in human rheumatoid arthritis. These results suggest that the Wnt pathway is a key regulator of joint remodeling.

Function and biological roles of the Dickkopf family of Wnt modulators

Dickkopf (Dkk) genes comprise an evolutionary conserved small gene family of four members (Dkk1-4) and a unique Dkk3-related gene, Dkkl1 (soggy). They encode secreted proteins that typically antagonize Wnt/beta-catenin signaling, by inhibiting the Wnt coreceptors Lrp5 and 6. Additionally, Dkks are high affinity ligands for the transmembrane proteins Kremen1 and 2, which also modulate Wnt signaling. Dkks play an important role in vertebrate development, where they locally inhibit Wnt regulated processes such as antero-posterior axial patterning, limb development, somitogenesis and eye formation. In the adult, Dkks are implicated in bone formation and bone disease, cancer and Alzheimer's disease.

Q89R polymorphism in the LDL receptor-related protein 5 gene is associated with spinal osteoarthritis in postmenopausal Japanese women

STUDY DESIGN

An association study investigating the genetic etiology for spinal osteoarthritis.

OBJECTIVE

To determine the association of single-nucleotide polymorphism (SNP) causing an amino-acid change (Q89R) in the low-density lipoprotein receptor-related protein 5 (LRP5) coding region with spinal osteoarthritis.

SUMMARY OF BACKGROUND DATA

Wnt/beta-catenin signaling pathway regulates bone density through a Wnt coreceptor LRP5. This pathway is also involved in cartilage development and homeostasis, suggesting that genetic variation in LRP5 gene may affect the pathogenesis of cartilage-related diseases, such as osteoarthritis.

METHODS

We evaluated the presence of osteophytes, endplate sclerosis, and narrowing of disc spaces in 357 Japanese postmenopausal women. Missense coding SNP for Q89R of LRP5 gene was determined using TaqMan polymerase chain reaction (PCR) method.

RESULTS

We found that subjects without the R allele (QQ; n = 321) had a significantly lower osteophyte formation score than did subjects bearing at least one R allele (QR + RR; n = 36) (7.80 vs. 10.89, P = 0.0019 by analysis of covariance).

CONCLUSIONS

We suggest that a genetic variation at the LRP5 gene locus is associated with spinal osteoarthritis, in line with the involvement of the LRP5 gene in the bone and cartilage metabolism.

Wnt signaling antagonists are potential prognostic biomarkers for the progression of radiographic hip osteoarthritis in elderly Caucasian women

OBJECTIVE

To determine whether serum levels of 2 Wnt signaling antagonists, Frizzled-related protein (FRP) and Dkk-1, are associated with the development and progression of radiographic hip osteoarthritis (RHOA).

METHODS

Pelvic radiographs were obtained a mean of 8.3 years apart in 5,928 Caucasian women >or=65 years of age who were enrolled in the Study of Osteoporotic Fractures. Random sampling of this cohort was performed, with approximately 180 subjects per group assigned to 2 nested case-control studies on RHOA incidence and progression. Baseline serum levels of FRP and Dkk-1 were measured by capture enzyme-linked immunosorbent assay. Odds ratios (ORs) and 95% confidence intervals (95% CIs) were calculated using logistic regression analyses with adjustment for potential covariates.

RESULTS

There were no differences in serum levels of FRP and Dkk-1 between case subjects with incidence or progression of RHOA and their respective control subjects. There was a trend for higher baseline serum levels of FRP to be associated with a reduced risk of incident RHOA (age-adjusted OR 0.59 [95% CI 0.32-1.09], P = 0.09 for women in the highest quartile versus women in the lowest quartile). There was no association of serum levels of FRP with progression of RHOA. Serum levels of Dkk-1 did not correlate with incident RHOA. However, higher serum levels of Dkk-1 were associated with diminished risk of RHOA progression (age-adjusted OR 0.43 [95% CI 0.23-0.79], P = 0.007 for women in the highest quartile compared with women in the lowest quartile).

CONCLUSION

Elevated circulating levels of Dkk-1 appeared to be associated with reduced progression of RHOA in elderly women, whereas the highest quartile of serum FRP levels tended to be associated with a modest reduction in risk of incident RHOA.

Influence of an LRP5 cytoplasmic SNP on Wnt signaling and osteoblastic differentiation

The low density lipoprotein receptor-related protein 5 (LRP5) is a key determinant of bone mass, via the Wnt signaling pathway control of osteoblast function. This study examined human LRP5 signaling and the effects of an intracellular domain single nucleotide polymorphism (SNP: p.V1525A) on osteoblast differentiation and mineralization. Constitutively active LRP5 was constructed by deletion of the extracellular domain of LRP5 (LRP5DeltaN). Expression of LRP5DeltaN-V, which carries the allele p.1525V, induced higher beta-catenin/TCF-LEF activity compared to LRP5DeltaN-A, which carries the allele p.1525A. In a yeast two-hybrid assay, LRP5DeltaN-V also demonstrated a stronger interaction with AXIN than LRP5DeltaN-A. Expression of either of the alleles did not change cell proliferation. However, cells expressing LRP5DeltaN-V showed increased alkaline phosphatase activity and bone nodule formation compared to cells transfected with empty vector or LRP5DeltaN-A after osteogenic supplement (OS: beta-glycerophosphate and l-ascorbic acid) treatment. Cells expressing LRP5DeltaN-V revealed significantly increased bone sialoprotein (BSP) expression after 7 days of OS treatment and maintained elevated expression until day 21. Osteocalcin (OCN) mRNA levels were increased after 14-21 days of OS treatment in LRP5DeltaN-V expressing cells. LRP5DeltaN-V expressing cells demonstrated positive interaction with BMP-2 signaling of transcription at the SBE-luc promoter. LRP5 signaling is affected by the cytoplasmic SNP, p.V1525A. mRNA levels of Runx2 and Osterix were not affected by this SNP.

β-Catenin Signaling Pathway Is Crucial for Bone Morphogenetic Protein 2 to Induce New Bone Formation

Endochondral ossification is recapitulated during bone morphogenetic protein (BMP)-induced ectopic bone formation. Although BMP and beta-catenin have been investigated in bone development and in mesenchymal cells, how they interact in this process is not clear. We implanted recombinant BMP-2 into the muscle of mice to investigate the effect of beta-catenin signaling on BMP-induced in vivo endochondral bone formation. BMP-2 induced expression of several Wnt ligands and their receptors and also activated beta-catenin-mediated T cell factor-dependent transcriptional activity. An adenovirus expressing Dickkopf-1 (Dkk-1, an inhibitor of canonical Wnt pathway) inhibited beta-catenin signaling and endochondral bone formation. Interestingly, Dkk-1 inhibited both chondrogenesis and osteogenesis. Likewise, mice expressing conditional beta-catenin null alleles also displayed an inhibition of BMP-induced chondrogenesis and osteogenesis. This is in contrast to studies of embryonic skeletogenesis, which demonstrate that beta-catenin is required for osteogenesis but is dispensable for chondrogenesis. These findings suggest that embryonic development pathways are not always recapitulated during post-natal regenerative processes, and the biochemical pathways utilized to regulate cell differentiation may be different. During in vivo ectopic bone formation, BMP-2 induces beta-catenin-mediated signaling through Wnt ligands, and beta-catenin is required for both chondrogenesis and osteogenesis.

Association of a single nucleotide polymorphism in the WISP1 gene with spinal osteoarthritis in postmenopausal Japanese women

The Wnt-beta-catenin signaling pathway that regulates bone density is also involved in cartilage development and homeostasis in vivo. Here, we assumed that genetic variation in Wnt-beta-catenin signaling genes can affect the pathogenesis of cartilage related diseases, such as osteoarthritis. Wnt-1-induced secreted protein 1 (WISP1) is a target of the Wnt pathway and directly regulated by beta-catenin. In the present study, we analyzed the association of a single nucleotide polymorphism (SNP) in the WISP1 3'-UTR region with the development of radiographically observable osteoarthritis of the spine. For this purpose, we evaluated the presence of osteophytes, endplate sclerosis, and narrowing of disc spaces in 304 postmenopausal Japanese women. We compared those who carried the G allele (GG or GA, n = 184) with those who did not (AA, n = 120). We found that the subjects without the G allele (AA) were significantly over-represented in the subjects having higher endplate sclerosis score (P = 0.0069; odds ratio, 2.91; 95% confidence interval, 1.34-6.30 by logistic regression analysis). On the other hand, the occurrence of disc narrowing and osteophyte formation did not significantly differ between those with and without at least one G allele. Thus, we suggest that a genetic variation in the WISP1 gene locus is associated with spinal osteoarthritis, in line with the involvement of the Wnt-beta-catenin-regulated gene in bone and cartilage metabolism.

Wnt but not BMP signaling is involved in the inhibitory action of sclerostin on BMP-stimulated bone formation

Sclerostin is an osteocyte-derived negative regulator of bone formation. It inhibits BMP-stimulated bone formation both in vitro and in vivo but has no direct effect on BMP signaling. Instead, sclerostin inhibits Wnt signaling that is required for BMP-stimulated osteoblastic differentiation.

INTRODUCTION

Sclerostin is a member of the Dan family of glycoproteins of which many members have been reported to antagonize BMP activity. Sclerostin has been shown to inhibit BMP-stimulated bone formation, but its mechanism of action seems to be different from classical BMP antagonists. In this study, we investigated the mechanism by which sclerostin inhibits BMP-stimulated bone formation.

MATERIALS AND METHODS

DNA electroporation of calf muscle of mice using expression plasmids for BMP and sclerostin was used to study the effect of sclerostin on BMP-induced bone formation in vivo. Transcriptional profiling using microarrays of osteoblastic cells treated with BMP in the absence or presence of sclerostin was used to find specific growth factor signaling pathways affected by sclerostin. The affected pathways were further studied using growth factor-specific reporter constructs.

RESULTS

BMP-induced ectopic bone formation in calf muscle of mice was prevented by co-expression of sclerostin in vivo. Transcriptional profiling analysis of osteoblastic cultures indicated that sclerostin specifically affects BMP and Wnt signaling out of many other growth signaling pathways. Sclerostin, however, did not inhibit stimulation of direct BMP target genes. Furthermore, we did not obtain any evidence for sclerostin acting as a direct BMP antagonist using a BMP-specific reporter construct. In contrast, sclerostin shared many characteristics with the Wnt antagonist dickkopf-1 in antagonizing BMP-stimulated bone formation and BMP- and Wnt-induced Wnt reporter construct activation.

CONCLUSIONS

Sclerostin inhibits BMP-stimulated bone formation but does not affect BMP signaling. Instead, it antagonizes Wnt signaling in osteoblastic cells. High bone mass in sclerosteosis and van Buchem disease may, therefore, result from increased Wnt signaling.

Postmenopausal osteoporosis: an update on current and future therapeutic options

Recent advances in osteoporosis have dramatically changed the management and treatment of this disease. This article reviews the safety and efficacy of US FDA-approved drugs for prevention and treatment of postmenopausal osteoporosis, as well as studies on combination, sequential or intermittent use of these agents. A review of promising agents for osteoporosis therapy is provided.

Gene profiling on mixed embryonic stem cell populations reveals a biphasic role for beta-catenin in osteogenic differentiation

The differentiation of embryonic stem cells (ESCs) into osteoblasts is enhanced to 60% when exposed to vitamin D3 (VD3) but leaves a remainder of one half of the cell population unidentified. To increase differentiation outcome, the known osteoinducers retinoic acid (RA) and bone morphogenetic protein-2 (BMP-2) were evaluated. Initial studies using RA and BMP-2 during early osteogenesis in addition to VD3 increased osteogenic yield in the case of RA, but surprisingly decreased osteogenesis when BMP-2 was administered together with VD3 or RA. This paper describes a comprehensive microarray study examining the gene expression profile of differentiating osteoblasts in these mixed ESC populations. In addition to five other families of signaling molecules (insulin growth factors, prostaglandin, follistatin, TGFbeta2, and Wnt molecules), we identified an endogenous expression pattern for BMPs and RA that differed from our previous exogenous administration of these molecules. By mimicking the change in expression of the RA and BMP-2 families with exogenous supplementation at the correct time, it was then possible to increase the number of ESC-derived osteoblasts to 90%. This effect was mediated through alteration in beta-catenin (CatnB) expression levels and nuclear CatnB activity, both of which are modulated by VD3, RA, and BMP-2. Our results suggest that blockage of CatnB activity by VD3 and RA is opposed by induction of CatnB activity through BMP-2 when administered together. Hence, osteoinduction, in vitro, is an intricate process involving both temporal and quantitative changes in gene expression and CatnB activity.

Direct interactions of Runx2 and canonical Wnt signaling induce FGF18

Canonical Wnt signaling is clearly required for skeletal development and bone formation. However, the targets of Wnt signaling that convert this signal into bone are unclear. Identification of these targets will yield insight into normal bone physiology and suggest new therapeutics for treatment of bone disease. Here we show that an essential regulator of bone development, FGF18, is a direct target of canonical Wnt signaling. A single DNA binding site for the Wnt-dependent transcription factors TCF/Lef accounted for the stimulation of the fgf18 promoter in response to Wnt signaling. Additionally, targeted disruption of betacat blocked fgf18 expression in vivo. Partially overlapping the TCF/Lef binding site is a Runx2 binding site and experiments showed that Runx2 and TCF/Lef work cooperatively to induce fgf18 expression. RNA interference knockdown of Runx2 inhibited and Runx2 forced expression augmented the induction of fgf18 by canonical Wnt signaling. Significantly, Runx2 formed a complex with Lef1 or TCF4 and this complex bound the composite binding site in the fgf18 promoter. These results demonstrate that two transcription pathways that are essential for bone, physically and functionally converge at the fgf18 promoter.

Induction of osteoblast differentiation by selective activation of kinase-mediated actions of the estrogen receptor

Estrogens control gene transcription by cis or trans interactions of the estrogen receptor (ER) with target DNA or via the activation of cytoplasmic kinases. We report that selective activation of kinase-mediated actions of the ER with 4-estren-3alpha,17beta-diol (estren) or an estradiol-dendrimer conjugate, each a synthetic compound that stimulates kinase-mediated ER actions 1,000 to 10,000 times more potently than direct DNA interactions, induced osteoblastic differentiation in established cell lines of uncommitted osteoblast precursors and primary cultures of osteoblast progenitors by stimulating Wnt and BMP-2 signaling in a kinase-dependent manner. In sharp contrast, 17beta-estradiol (E(2)) suppressed BMP-2-induced osteoblast progenitor commitment and differentiation. Consistent with the in vitro findings, estren, but not E(2), stimulated Wnt/beta-catenin-mediated transcription in T-cell factor-lacZ transgenic mice. Moreover, E(2) stimulated BMP signaling in mice in which ERalpha lacks DNA binding activity and classical estrogen response element-mediated transcription (ERalpha(NERKI/-)) but not in wild-type controls. This evidence reveals for the first time the existence of a large signalosome in which inputs from the ER, kinases, bone morphogenetic proteins, and Wnt signaling converge to induce differentiation of osteoblast precursors. ER can either induce it or repress it, depending on whether the activating ligand (and presumably the resulting conformation of the receptor protein) precludes or accommodates ERE-mediated transcription.

New mechanisms and targets in the treatment of bone fragility

Bone modelling and remodelling are cell-mediated processes responsible for the construction and reconstruction of the skeleton throughout life. These processes are chiefly mediated by locally generated cytokines and growth factors that regulate the differentiation, activation, work and life span of osteoblasts and osteoclasts, the cells that co-ordinate the volumes of bone resorbed and formed. In this way, the material composition and structural design of bone is regulated in accordance with its loading requirements. Abnormalities in this regulatory system compromise the material and structural determinants of bone strength producing bone fragility. Understanding the intercellular control processes that regulate bone modelling and remodelling is essential in planning therapeutic approaches to prevention and treatment of bone fragility. A great deal has been learnt in the last decade. Clinical trials carried out exclusively with drugs that inhibit bone resorption have identified the importance of reducing the rate of bone remodelling and so the progression of bone fragility to achieved fracture reductions of approx. 50%. These trials have also identified limitations that should be placed upon interpretation of bone mineral density changes in relation to treatment. New resorption inhibitors are being developed, based on mechanisms of action that are different from existing drugs. Some of these might offer resorption inhibition without reducing bone formation. More recent research has provided the first effective anabolic therapy for bone reconstruction. Daily injections of PTH (parathyroid hormone)-(1–34) have been shown in preclinical studies and in a large clinical trial to increase bone tissue mass and reduce the risk of fractures. The action of PTH differs from that of the resorption inhibitors, but whether it is more effective in fracture reduction is not known. Understanding the cellular and molecular mechanisms of PTH action, particularly its interactions with other pathways in determining bone formation, is likely to lead to new therapeutic developments. The recent discovery through mouse genetics that PTHrP (PTH-related protein) is a crucial bone-derived paracrine regulator of remodelling offers new and interesting therapeutic targets.

Mutational analysis of Norrin-Frizzled4 recognition

Norrin and Frizzled4 (Fz4) function as a ligand-receptor pair to control vascular development in the retina and inner ear. In mice and humans, mutations in either of the corresponding genes lead to defects in vascular development. The present work is aimed at defining the sequence determinants of binding specificity between Norrin and the Fz4 amino-terminal ligand-binding domain (the "cysteine-rich domain" (CRD)). The principal conclusions are as follows: 1) Norrin binds to the Fz4 CRD and does not detectably bind to the 14 other mammalian Frizzled and secreted Frizzled-related protein CRDs; 2) Norrin and Xenopus Wnt8 recognize largely overlapping regions of the Fz4 CRD; 3) surface determinants on the Fz4 and Fz8 CRDs that allow Norrin to distinguish between these two CRDs reside within several small regions on one face of the CRD; 4) Norrin function depends critically on three pairs of cysteines that form the highly conserved trio of disulfide bonds shared among all cystine knot proteins, but the remaining two putative disulfide bonds are less important; 5) Norrin-CRD binding depends on a largely contiguous group of amino acids in the extended beta-sheet domain of Norrin that are predicted to face away from the interface between the two monomers in the Norrin homodimer; 6) Norrin-CRD binding is strongly modulated by interactions involving charged amino acid side chains; and 7) Norrin-CRD binding is enhanced approximately 10-fold by the addition of heparin. These observations are discussed in the context of Frizzled signaling and the structure and function of other cystine knot proteins.

Wnt signalling in the mouse intestine

The Apc(Min/+) mouse has emerged as a powerful model of human intestinal tumour predisposition. As such, it has provided a platform for studying genetic and epigenetic modifiers of adenoma predisposition, and for assessing the chemotherapeutic potential of a plethora of different agents. The development of new conditional and hypomorphic Apc alleles, together with models carrying mutations in other Wnt pathway components, has greatly extended the scope of experimentation. Together these approaches are being used to identify and validate key critical targets of the Wnt pathway, such as Mash2, Tiam1 and the Eph/Ephrins. They have also established a fundamental role for Wnt in the development and maintenance of normal intestinal physiology, and in particular control of the stem cell niche. These activities are now being dissected at the level of individual Wnt components, with some surprising dependencies revealed. In terms of adenoma development, these models also support a 'just right' notion for tightly controlled beta-catenin activity both in normal physiology and neoplastic development. They also indicate a two-stage dependency for some Wnt pathway targets, with an initial requirement that is subsequently overcome to permit progression. Finally, these models establish that the Wnt pathway does not operate in isolation, and that both normal and diseased physiology develops in a dynamic interplay with other pathways such as the Notch, Hedgehog and BMP pathways. The comprehensive understanding arising from these studies should lead the identification of novel prognostic markers and therapeutic targets, and also open the possibility of tissue engineering in the intestine.

LRP5 Mutations Linked to High Bone Mass Diseases Cause Reduced LRP5 Binding and Inhibition by SOST

The low density lipoprotein (LDL) receptor-related protein 5 (LRP5) is a co-receptor for Wnt proteins and a major regulator in bone homeostasis. Human genetic studies have shown that recessive loss-of-function mutations in LRP5 are linked to osteoporosis, while on the contrary, dominant missense LRP5 mutations are associated with high bone mass (HBM) diseases. All LRP5 HBM mutations are clustered in a single region in the LRP5 extracellular domain and presumably result in elevated Wnt signaling in bone forming cells. Here we show that LRP5 HBM mutant proteins exhibit reduced binding to a secreted bone-specific LRP5 antagonist, SOST, and consequently are more refractory to inhibition by SOST. As loss-of-function mutations in the SOST gene are associated with Sclerosteosis, another disorder of excessive bone growth, our study suggests that the SOST-LRP5 antagonistic interaction plays a central role in bone mass regulation and may represent a nodal point for therapeutic intervention for osteoporosis and other bone diseases.

Lens induction in vertebrates: Variations on a conserved theme of signaling events

This review provides an overview of our current understanding of signaling mechanisms involved in lens induction, which are presented in context of the major stages of lens induction (competence, bias, inhibition and specification). Although the process of lens induction is generally well conserved, we highlight aspects of induction that vary among species. Finally, this review identifies future challenges in forming an integrated network of signaling pathways involved in lens induction.

Identification of Apolipoprotein A-I as a “STOP” Signal for Myopia

Good visual acuity requires that the axial length of the ocular globe is matched to the refractive power of the cornea and lens to focus the images of distant objects onto the retina. During the growth of the juvenile eye, this is achieved through the emmetropization process that adjusts the ocular axial length to compensate for the refractive changes that occur in the anterior segment. A failure of the emmetropization process can result in either excessive or insufficient axial growth, leading to myopia or hyperopia, respectively. Emmetropization is mainly regulated by the retina, which generates two opposite signals: "GO/GROW" signals to increase axial growth and "STOP" signals to block it. The presence of GO/GROW and STOP signals was investigated by a proteomics analysis of the retinas from chicken with experimental myopia and hyperopia. Of 18 differentially expressed proteins that were identified, five displayed an expression profile corresponding to GO/GROW signals, and two corresponded to STOP signals. Western blotting confirmed that apolipoprotein A-I (apoA-I) has the characteristics of a STOP signal both in the retina as well as in the fibrous sclera. In accordance with this, intraocular application of the peroxisome proliferator-activated receptor alpha agonist GW7647 resulted in up-regulation of apoA-I levels and in a significant reduction of experimental myopia. In conclusion, using a comprehensive functional proteomics analysis of chicken ocular growth models we identified targets for ocular growth control. The correlation of elevated apoA-I levels with reduced ocular axial growth points toward a functional relationship with the observed morphological changes of the eye.

Secreted frizzled-related protein 4 is a negative regulator of peak BMD in SAMP6 mice

We segregated a QTL for peak BMD on Chr 13 by generating congenic sublines of the senescence-accelerated mouse SAMP6. Sfrp 4 within this locus was responsible for lower BMD of SAMP6.

INTRODUCTION

Our genome-wide linkage study using SAMP6 and SAMP2 showed a significant quantitative trait locus (QTL) for peak BMD on chromosome (Chr) 13. To verify the gene that regulates peak BMD, we generated a congenic strain, P6.P2-Pbd2(b), which carried a 15-cM SAMP2 interval on an osteoporotic SAMP6 background, and showed that this Pbd2 locus increased peak BMD in SAMP6.

MATERIALS AND METHODS

To narrow down this interval, we generated a new congenic subline P6.P2-13. We studied the effect of this locus on morphological and histomorphological features in vivo and on osteoblasts in vitro. The levels of expression of all genes in the segregated interval were examined, and we clarified the effect of the candidate gene, secreted frizzled-related protein (Sfrp4), on osteoblasts in vitro.

RESULTS

The new congenic strain, P6.P2-13, retained the 2.4-Mb SAMP2 interval on the SAMP6 background, and 11 genes existed in this interval. In morphometrical analysis, P6.P2-13 increased the bone area fraction (BA/TA) by 6.6% at the diaphysial cortex (p < 0.001) and increased the trabecular bone volume (BV/TV) by 54.2% at the distal metaphysis (p < 0.05) in the femora compared with those of SAMP6. The bone formation rate of P6.P2-13 was markedly increased at the periosteal surface of femoral cortex and that was caused by a higher proliferation rate of osteoblasts in P6.P2-13 compared with those in SAMP6. Quantitative RT-PCR analysis of calvaria tissue showed approximately 40-fold higher levels of expression of Sfrp4 in SAMP6 than in P6.P2-13. Taken together with the result that recombinant Sfrp4 suppressed the proliferation of osteoblasts, we hypothesized that Sfrp4 inhibited the proliferation of osteoblasts through its antagonistic effect on Wnt signaling. TCF/beta-catenin-dependent reporter activity in osteoblasts derived from SAMP6 showed lower responsiveness for the Wnt ligand, Wnt3A, than that in osteoblasts from P6.P2-13.

CONCLUSIONS

In SAMP6 mice, Sfrp4 negatively regulates bone formation and decreases BMD through the inhibition of Wnt signaling.

Bone density ligand, Sclerostin, directly interacts with LRP5 but not LRP5G171V to modulate Wnt activity

We compared and contrasted the mechanism of action for the cysteine knot protein subfamily, Wise and Sost (Sclerostin). Our data suggest that functional interactions between Sost or Wise and LRP5/LRP6 have the potential to regulate bone deposition by modulating the Wnt pathway.

INTRODUCTION

The human disease sclerosteosis exhibits an increase in bone mass thought to be caused by hyperactive osteoblasts. Sclerostin, SOST, the gene affected in this disease, has been postulated to exert its activity by functioning as a BMP antagonist. However, recent evidence indicates that SOST is highly related to Wise, which can also modulate the Wnt pathway by binding to LRP5 and LRP6.

MATERIALS AND METHODS

For this study, we used cell culture to test the BMP and Wnt activity function of both Wise and Sost. In addition, we used Xenopus in vivo Wnt assays along with Xenopus in vitro Wnt assays to support our cell culture results. Epitope tagged cell supernatants containing either Sost or soluble mutant or wildtype LRP5/LRP6 were used for immunoprecipitation. Sost immunoprecipitation results were confirmed in vivo using cell culture. Finally, to support our in vitro data, we co-localized Sost, Wise, LRP5, and LRP6 in mouse long bone sections.

RESULTS

In this study, we report in vitro and in vivo evidence to show that Sost physically interacts with Lrp5 and Lrp6 and inhibits the canonical Wnt signaling pathway. Furthermore, using in vitro and in vivo assays, we showed that a variant of LRP5 (LRP5(G171V)) known to cause the human high bone mass (HBM) trait and a homologous change in LRP6 (LRP6(G158V)) abolished protein interactions with Sost. We used variants of Sost amino acids to further identify the contact points between Sost and LRP6. In Xenopus and mammalian cell culture assays, we showed that SOST is able to attenuate Wnt signaling and that this attenuation can be rescued by the addition of alpha-Sost antibodies or by the introduction of single amino acid substitution that alter its binding to LRP6. Sost differs from Wise in that it is unable to stimulate Wnt signaling. Using immunohistochemistry, we found that Sost and Wise are co-localized to osteoblasts, along with LRP5 and LRP6.

CONCLUSIONS

Our data suggest that functional interactions between Sost or Wise and LRPs have the potential to regulate bone deposition by modulating Wnt signaling.

Antibody-based inhibition of DKK1 suppresses tumor-induced bone resorption and multiple myeloma growth in vivo

Dickkopf-1 (DKK1), a soluble inhibitor of Wnt signaling secreted by multiple myeloma (MM) cells contributes to osteolytic bone disease by inhibiting the differentiation of osteoblasts. In this study, we tested the effect of anti-DKK1 therapy on bone metabolism and tumor growth in a SCID-rab system. SCID-rab mice were engrafted with primary MM cells expressing varying levels of DKK1 from 11 patients and treated with control and DKK1-neutralizing antibodies for 4 to 6 weeks. Whereas bone mineral density (BMD) of the implanted myelomatous bone in control mice was reduced during the experimental period, the BMD in mice treated with anti-DKK1 increased from pretreatment levels (P < .001). Histologic examination revealed that myelomatous bones of anti-DKK1-treated mice had increased numbers of osteocalcin-expressing osteoblasts and reduced number of multinucleated TRAP-expressing osteoclasts. The bone anabolic effect of anti-DKK1 was associated with reduced MM burden (P < .04). Anti-DKK1 also significantly increased BMD of the implanted bone and murine femur in nonmyelomatous SCID-rab mice, suggesting that DKK1 is physiologically an important regulator of bone remodeling in adults. We conclude that DKK1 is a key player in MM bone disease and that blocking DKK1 activity in myelomatous bones reduces osteolytic bone resorption, increases bone formation, and helps control MM growth.

Craniosynostosis caused by Axin2 deficiency is mediated through distinct functions of beta-catenin in proliferation and differentiation

Targeted disruption of Axin2 in mice induces skeletal defects, a phenotype resembling craniosynostosis in humans. Premature fusion of cranial sutures, caused by deficiency in intramembranous ossification, occurs at early postnatal stages. Axin2 negatively regulates both expansion of osteoprogenitors and maturation of osteoblasts through its modulation on Wnt/beta-catenin signaling. We investigate the dual role of beta-catenin to gain further insights into the skull morphogenetic circuitry. We show that as a transcriptional co-activator, beta-catenin promotes cell division by stimulating its target cyclin D1 in osteoprogenitors. Upon differentiation of osteoprogenitors, BMP signaling is elevated to accelerate the process in a positive feedback mechanism. This Wnt-dependent BMP signal dictates cellular distribution of beta-catenin. As an adhesion molecule, beta-catenin promotes cell-cell interaction mediated by adherens junctions in mature osteoblasts. Finally, haploid deficiency of beta-catenin alleviates the Axin2-null skeletal phenotypes. These findings support a model for disparate roles of beta-catenin in osteoblast proliferation and differentiation.

Molecular genetic studies of gene identification for osteoporosis: a 2004 update

This review summarizes comprehensively the most important and representative molecular genetics studies of gene identification for osteoporosis published up to the end of December 2004. It is intended to constitute a sequential update of our previously published review covering the available data up to the end of 2002. Evidence from candidate gene association studies and genome-wide linkage studies in humans, as well as quantitative trait locus mapping animal models are reviewed separately. Studies of transgenic and knockout mice models relevant to osteoporosis are summarized. An important extension of this update is incorporation of functional genomic studies (including DNA microarrays and proteomics) on osteogenesis and osteoporosis, in light of the rapid advances and the promising prospects of the field. Comments are made on the most notable findings and representative studies for their potential influence and implications on our present understanding of genetics of osteoporosis. The format adopted by this review should be ideal for accommodating future new advances and studies.

Dkk1-mediated inhibition of Wnt signaling in bone results in osteopenia

Mutations affecting the activity of the Wnt co-receptors LRP5 and LRP6 that cause alterations in skeletal biology confirmed the involvement of Wnt signaling in bone formation. We evaluated the potential role of Dkk1, an inhibitor of LRP5/6 activity, in bone formation by examining the normal expression pattern of Dkk1 in normal young mice and by assessing the consequences of osteoblast overexpression of Dkk1 in transgenic mice. Endogenous Dkk1 expression was detected primarily in osteoblasts and osteocytes. Transgenic over-expression of Dkk1 using two different rat collagen 1A1 promoters resulted in distinct bone phenotypes. More widespread Dkk1 expression (driven by the Col1A1 3.6 kb promoter) yielded osteopenia with forelimb deformities and hairlessness, while expression restricted to osteoblasts (driven by the Col1A1 2.3 kb promoter) induced severe osteopenia without limb defects or alopecia. The decrease in bone mass in vivo resulted from a significant 49% reduction in osteoblast numbers and was reflected in a 45% reduction in serum osteocalcin concentration; an in vitro study revealed that Dkk1 caused a dose-dependent suppression of osteoblast matrix mineralization. These data indicate that Dkk1 may directly influence bone formation and suggest that osteopenia develops in mice over-expressing Dkk1 at least in part due to diminished bone formation resulting from reduced osteoblast numbers.

Modulation of LRP6-mediated Wnt signaling by molecular chaperone Mesd

LRP6 is a Wnt coreceptor at the cell surface. Here, we report that a specialized molecular chaperone Mesd modulates LRP6-mediated Wnt signaling and how different LRP6 mutants exhibit differential effects on Wnt signaling. We found that overexpression of increasing amounts of the full-length LRP6 enhances Wnt signaling in a dose dependent manner only in the presence of a co-expression of the molecular chaperone Mesd, which promotes LRP6 folding and maturation to the cell surface. We also demonstrated that LRP6 mutant lacking the intracellular domain impedes LRP6 cell surface expression and Wnt signaling in a dominant-negative fashion by sequestering Mesd from promoting LRP6 folding. Our results present novel mechanisms by which Mesd and LRP6 modulate Wnt signaling.

Genetic regulation of bone mass and susceptibility to osteoporosis

Osteoporosis is a common disease with a strong genetic component characterized by reduced bone mass and increased risk of fragility fractures. Twin and family studies have shown that the heritability of bone mineral density (BMD) and other determinants of fracture risk-such as ultrasound properties of bone, skeletal geometry, and bone turnover-is high, although heritability of fracture is modest. Many different genetic variants of modest effect size are likely to contribute to the regulation of these phenotypes by interacting with environmental factors such as diet and exercise. Linkage studies in rare Mendelian bone diseases have identified several previously unknown genes that play key roles in regulating bone mass and bone turnover. In many instances, subtle polymorphisms in these genes have also been found to regulate BMD in the general population. Although there has been extensive progress in identifying the genetic variants that regulate susceptibility to osteoporosis, most of the genes and genetic variants that regulate bone mass and susceptibility to osteoporosis remain to be discovered.

[Pathophysiology and pathomorphology of osteoporosis]

Osteoporosis is a disease that leads to fragility fractures due to loss of bone mass and bone microstructure. This review presents an update on the fundamental pathophysiologic and pathomorphologic mechanisms of bone loss situations. Pathomorphologic characteristics such as perforations and microcallus formations are explained. The physiologic relevance of the remodeling process as well as its control by local-paracrine, systemic-endocrine and central-neural signaling pathways is discussed. Furthermore the role of hormones such as estrogen, FSH and leptin, of transcription-factors such as Runx2 and osterix and as well as that of the wnt signaling pathway for bone cell differentiation and function is presented. On the basis of current knowledge osteoporosis can be diagnosed, treated and fractures can be prevented. However, it is likely that new and even more effective diagnostic and therapeutic strategies will emerge as our understanding of the remodeling process that controls osteoblast and osteoclast function increases.

Reduced bone mineral density and hyaloid vasculature remnants in a consanguineous recessive FEVR family with a mutation in LRP5

BACKGROUND/AIMS

Familial exudative vitreoretinopathy (FEVR) is an inherited blinding condition characterised by abnormal development of the retinal vasculature. FEVR has multiple modes of inheritance, and homozygous mutations in LRP5 have recently been reported as underlying the recessive form of this disease. The aim of this study was to examine LRP5 in a consanguineous recessive FEVR family and to clarify the eye and bone phenotype associated with recessive FEVR.

METHODS

All family members were examined by slit lamp biomicroscopy and indirect ophthalmoscopy. Linkage to LRP5 was determined by genotyping microsatellite markers, constructing haplotypes and calculating lod scores. Mutation screening of LRP5 was performed by polymerase chain reaction amplification of genomic DNA followed by direct sequencing. Bone mineral density (BMD) was evaluated in all family members using dual energy x ray absorptiometry (DEXA).

RESULTS

The clinical features observed in this family were consistent with a diagnosis of recessive FEVR. A homozygous LRP5 missense mutation, G550R, was identified in all affected individuals and all unaffected family members screened were heterozygous carriers of this mutation. Reduced BMD, hyaloid vasculature remnants, and nystagmus were features of the phenotype.

CONCLUSION

Recessive mutations in LRP5 can cause FEVR with reduced BMD and hyaloid vasculature remnants. Assessment of a patient with a provisional diagnosis of FEVR should therefore include investigation of BMD, with reduced levels suggestive of an underlying LRP5 mutation.

A genetic approach to fracture epidemiology in childhood

The purpose of this report is to provide a review of both childhood fracture epidemiology and known heritable causes for fracture predisposition to the Medical Geneticist, who is frequently consulted to assess children with multiple or unexplained fractures for a physiologic etiology. A detailed knowledge of the clinical and laboratory evaluation for osteogenesis imperfecta (OI) and other single-gene disorders is obviously essential to complete a useful evaluation of such children. The experienced clinician will immediately recognize that single gene disorders represent only a small fraction of these patients. In infants, non-accidental trauma (NAT) unfortunately is the likely explanation for the fracture pattern, but in some infants, and certainly in older children with recurrent fractures, no medical explanations can be found. Recent studies in which bone mineral density (BMD) has been associated with genetic variation at a number of candidate genes are promising but these studies are too premature yet to be used clinically. Nonetheless, we do expect that in the future whole-genome approaches in conjunction with key clinical and epidemiological variables may be combined through an informatics approach to create better predictors of fracture susceptibility for these populations of patients.