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

Lymphocyte enhancer-binding factor 1 (Lef1) inhibits terminal differentiation of osteoblasts

Lef1 is a transcriptional regulator of the Wnt/beta-catenin signaling cascade. Wnts directly augment bone formation and osteoblast differentiation from mesenchymal stem cells by receptor-mediated pathways involving Lrp5 and Frizzled. We previously reported that Lef1 represses Runx2-dependent activation of the late osteoblast differentiation gene, osteocalcin. Lef1 is expressed in preosteoblasts but is undetectable in fully differentiated osteoblasts. To determine if downregulation of Lef1 is necessary for osteoblast maturation, we constitutively overexpressed Lef1 in MC3T3-E1 preosteoblasts. Lef1-overexpressing cells produced alkaline phosphatase (ALP) and osteocalcin later, and at lower levels than control cells. Moreover, the extracellular matrices of Lef1-overexpressing cell cultures never mineralized. To further examine the role of Lef1 in osteoblasts, we suppressed Lef1 expression in MC3T3-E1 cells by RNA interference. Transient expression of a Lef1 shRNA efficiently reduced murine Lef1 levels and transcriptional activity. Stable suppression of Lef1 in MC3T3 preosteoblasts did not affect proliferation or Runx2 levels; however, ALP production and matrix mineralization were accelerated by 3-4 days. Gene chip analyses identified 14 genes that are differentially regulated in Lef1-suppressed cells. These data outline a role for Lef1 in delaying osteoblast maturation and suggest that Lef1 controls the expression of multiple genes in osteoblasts.

Wnt Signaling: A Key Regulator of Bone Mass

The identification of a link between bone mass in humans and gain- [high bone mass (HBM) trait] or loss-of-function [osteoporosis pseudoglioma (OPPG) syndrome] mutations in the Wnt coreceptor lipoprotein receptor-related protein (LRP)5 or in the Wnt antagonist sclerostin (sclerosteosis, Van Buchem syndrome) has called the attention of academic and industry scientists and clinicians to the importance of this signaling pathway in skeletal biology and disease. Multiple genetic and pharmacological manipulations of Wnt signaling in mice have since then confirmed the central role of this pathway in both the establishment of peak bone mass and its maintenance throughout life. Wnt signaling appears to be located downstream of bone morphogenetic proteins (BMPs), itself induced by Hedgehog (Hh) signaling, suggesting that it is the successive recruitment of these three intracellular signaling cascades that allow the full expression of the genetic patterns that characterize the osteoblast, the cell responsible for the formation of bone.

Common genetic variation of the low-density lipoprotein receptor-related protein 5 and 6 genes determines fracture risk in elderly white men

Both LRP5 and LRP6 genes have been implicated to play a role in bone metabolism. In a large population-based study, we related common variation in both genes to bone parameters and fractures. LRP5 variation was associated to both BMD and frame size, whereas both LRP5 and 6 variations were associated with an increased fracture risk in males.

INTRODUCTION

The low-density lipoprotein receptor-related protein 5 (LRP5) gene has a clear role in rare BMD traits and also in normal variation in peak BMD. We examined whether common variation in LRP5 and its close homolog, LRP6, plays a role in BMD in old age and fractures, the main clinical endpoint of osteoporosis.

MATERIALS AND METHODS

We analyzed four variants of LRP5 and one amino acid variant of the LRP6 gene in a large prospective population-based cohort study of elderly subjects.

RESULTS AND CONCLUSIONS

In men, the LRP5 1330-valine variant was associated with decreased BMD at the lumbar spine and the femoral neck with evidence for an allele-dose effect (p = 0.001 and 0.01, respectively). The Val allele was also associated with decreased vertebral body size and femoral neck width. Haplotype analysis of studied polymorphisms did not improve the association found and suggested that the 1330 variant was driving the association. We observed a borderline significant association of the LRP6 Ile1062Val polymorphism with height and vertebral body size in males. Male carriers of the LRP5 1330-valine variant had a 60% increased risk for fragility fractures, and the LRP6 1062-valine allele also conferred a 60% higher risk. Carriers of both the risk alleles of LRP5 and 6 had a 140% (p = 0.004) higher risk compared with noncarriers of both risk alleles and accounted for 10% of the fractures in males. The fracture risks were independent of age, height, weight, and BMD. In women, all of these associations were weaker and less consistent compared with men. The polymorphisms that were found associated were both situated in potentially important domains of the receptor and show considerable evolutionary conservation, which is evidence for functional importance of these residues.

Genetic determination and correlation of body mass index and bone mineral density at the spine and hip in Chinese Han ethnicity

The purpose of the present study was to evaluate the magnitude of genetic determination of spine and hip bone mineral density (BMD) and body mass index (BMI), and to explore the genetic, environmental, and phenotypic correlations among the above phenotypes in Chinese Han ethnicity. The sample was composed of at least 217 complete nuclear families in Chinese Han ethnicity. BMD at the spine and hip was measured using a dual-energy X-ray absorptiometry scanner. The heritability (h2) of BMI and BMD at the spine and hip, the genetic correlation (rhoG) and environmental correlation (rhoE) among the three phenotypes were evaluated via variance analysis, with age, sex, and age-by-sex interaction as covariates. The phenotypic correlation (rhoP) and the bivariate heritability rhoG2 were also calculated. The heritability for BMD and BMI was approximately 0.70 and approximately 0.50, respectively (p<0.0001). The common environment shared by household members (household effect) is significant for BMI variation (p=0.0004). Significant genetic, environmental, and phenotypic correlation was observed. The rhoG2 values were 0.13 for BMI/spine BMD, 0.18 for BMI/hip BMD, and 0.58 for the spine BMD/hip BMD. While BMD at the spine and hip have significant genetic determination, BMI is more likely to be affected by environmental factors than BMD. In addition, BMD at the spine and hip shares more genetic effect (pleiotropy) than BMI and BMD do in Chinese Han ethnicity, though the effects are significant for both.

The new bone biology: Pathologic, molecular, and clinical correlates

Bone and cartilage and their disorders are addressed under the following headings: functions of bone; normal and abnormal bone remodeling; osteopetrosis and osteoporosis; epithelial-mesenchymal interaction, condensation and differentiation; osteoblasts, markers of bone formation, osteoclasts, components of bone, and pathology of bone; chondroblasts, markers of cartilage formation, secondary cartilage, components of cartilage, and pathology of cartilage; intramembranous and endochondral bone formation; RUNX genes and cleidocranial dysplasia (CCD); osterix; histone deacetylase 4 and Runx2; Ligand to receptor activator of NFkappaB (RANKL), RANK, osteoprotegerin, and osteoimmunology; WNT signaling, LRP5 mutations, and beta-catenin; the role of leptin in bone remodeling; collagens, collagenopathies, and osteogenesis imperfecta; FGFs/FGFRs, FGFR3 skeletal dysplasias, craniosynostosis, and other disorders; short limb chondrodysplasias; molecular control of the growth plate in endochondral bone formation and genetic disorders of IHH and PTHR1; ANKH, craniometaphyseal dysplasia, and chondrocalcinosis; transforming growth factor beta, Camurati-Engelmann disease (CED), and Marfan syndrome, types I and II; an ACVR1 mutation and fibrodysplasia ossificans progressiva; MSX1 and MSX2: biology, mutations, and associated disorders; G protein, activation of adenylyl cyclase, GNAS1 mutations, McCune-Albright syndrome, fibrous dysplasia, and Albright hereditary osteodystrophy; FLNA and associated disorders; and morphological development of teeth and their genetic mutations.

Playing with bone and fat

The relationship between bone and fat formation within the bone marrow microenvironment is complex and remains an area of active investigation. Classical in vitro and in vivo studies strongly support an inverse relationship between the commitment of bone marrow-derived mesenchymal stem cells or stromal cells to the adipocyte and osteoblast lineage pathways. In this review, we focus on the recent literature exploring the mechanisms underlying these differentiation events and discuss their implications relevant to osteoporosis and regenerative medicine.

Resolving the Two "Bony" Faces of PPAR-gamma

Bone loss with aging results from attenuated and unbalanced bone turnover that has been associated with a decreased number of bone forming osteoblasts, an increased number of bone resorbing osteoclasts, and an increased number of adipocytes (fat cells) in the bone marrow. Osteoblasts and adipocytes are derived from marrow mesenchymal stroma/stem cells (MSC). The milieu of intracellular and extracellular signals that controls MSC lineage allocation is diverse. The adipocyte-specific transcription factor peroxisome proliferator-activated receptor-gamma (PPAR-gamma) acts as a critical positive regulator of marrow adipocyte formation and as a negative regulator of osteoblast development. In vivo, increased PPAR-gamma activity leads to bone loss, similar to the bone loss observed with aging, whereas decreased PPAR-gamma activity results in increased bone mass. Emerging evidence suggests that the pro-adipocytic and the anti-osteoblastic properties of PPAR-gamma are ligand-selective, suggesting the existence of multiple mechanisms by which PPAR-gamma controls bone mass and fat mass in bone.

Effects of parathyroid hormone on Wnt signaling pathway in bone

The Wnt signaling pathway has recently been demonstrated to play an important role in bone cell function. In previous studies using DNA microarray analyses, we observed a change in some of the molecular components of the canonical Wnt pathway namely, frizzled-1 (FZD-1) and axil, in response to continuous parathyroid hormone (PTH) treatment in rats. In the present study, we further explored other components of the Wnt signaling pathway in rat distal metaphyseal bone in vivo, and rat osteoblastic osteosarcoma cells (UMR 106) in culture. Several Wnt pathway components, including low-density lipoprotein-receptor-related protein 5 (LRP5), LRP6, FZD-1, Dickkopf-1 (Dkk-1), and Kremen-1 (KRM-1), were expressed in bone in vivo and in osteoblasts in vitro. Continuous exposure to PTH (1-38) both in vivo and in vitro upregulated the mRNA expression of LRP6 and FZD-1 and decreased LRP5 and Dkk-1. These effects in UMR 106 cells were associated with an increase in beta-catenin as measured by Western blots and resulted in functional activation (three to six-fold) of a downstream Wnt responsive TBE6-luciferase (TCF/LEF-binding element) reporter gene. Activation of the TBE6-luciferase reporter gene by PTH (1-38) in UMR 106 cells was inhibited by the protein kinase A (PKA) inhibitor, H89. Activation was mimicked by PTH (1-31), PTH-related protein (1-34), and forskolin, but both PTH (3-34) and (7-34) had no effect. These findings suggest that the effect of PTH on the canonical Wnt signaling pathway occurs at least in part via the cAMP-PKA pathway through the differential regulation of the receptor complex proteins (FZD-1/LRP5 or LRP6) and the antagonist (Dkk-1). Taken together, these results reveal a possible role for the Wnt signaling pathway in PTH actions in bone.

Association between bone mineral densities and serum lipid profiles of pre- and post-menopausal rural women in South Korea

The objectives of this population-based study were to investigate the potential association between bone mineral density (BMD) and serum lipid profiles and to compare the effects of serum lipids on BMD at various skeletal sites in pre- and post-menopausal women. In July and August of 2004, BMD was measured at a variety of skeletal sites [lumbar spine (L1-4), femoral neck, trochanter, Ward's triangle, shaft and proximal total hip] using the GE/Bravo Lunar DPX dual-energy X-ray absorptiometer in a South Korean population-based sample of 375 pre-menopausal and 355 post-menopausal rural women aged 19-80 years. The levels of serum total cholesterol (TC) and low-density lipoprotein cholesterol (LDL-C) were inversely associated with BMD in both pre- and post-menopausal women. In the pre-menopausal women, correlations were shown only for lumbar 1-4 (TC: r=-0.12, P<0.05; LDL-C: r=-0.12, P<0.05), whereas in the post-menopausal women, no correlation was evident for the lumbar sites. In the post-menopausal subjects, the TC levels showed significant correlations with the BMD values at the trochanter (r=-0.15, P<0.01), shaft (r=-0.16, P<0.001) and proximal total hip (r=-0.15, P<0.01) sites, while the LDL-C levels showed significant correlations with the BMD values at the neck (r=-0.13, P<0.05), trochanter (r=-0.21, P<.001), shaft (r=-0.20, P<0.001) and proximal total hip (r=-0.20, P<0.001) sites. The levels of triglyceride (TG) were shown to have a significant positive correlation with BMD values at the trochanter site (r=0.11, P=0.05) in the post-menopausal women; by contrast, subjects in a higher quartile of TG levels show lower lumbar BMD values in the pre-menopausal women. The levels of high-density lipoprotein cholesterol (HDL-C) were not associated with BMD values at any of the sites in the pre- and post-menopausal subjects. Our data indicate a relationship between BMD values and serum lipid levels and suggest differences between pre- and post-menopausal women in terms of the effects of serum lipids on BMD at various skeletal sites.

Pathogenesis of osteoporosis: concepts, conflicts, and prospects

Osteoporosis is a disorder in which loss of bone strength leads to fragility fractures. This review examines the fundamental pathogenetic mechanisms underlying this disorder, which include: (a) failure to achieve a skeleton of optimal strength during growth and development; (b) excessive bone resorption resulting in loss of bone mass and disruption of architecture; and (c) failure to replace lost bone due to defects in bone formation. Estrogen deficiency is known to play a critical role in the development of osteoporosis, while calcium and vitamin D deficiencies and secondary hyperparathyroidism also contribute. There are multiple mechanisms underlying the regulation of bone remodeling, and these involve not only the osteoblastic and osteoclastic cell lineages but also other marrow cells, in addition to the interaction of systemic hormones, local cytokines, growth factors, and transcription factors. Polymorphisms of a large number of genes have been associated with differences in bone mass and fragility. It is now possible to diagnose osteoporosis, assess fracture risk, and reduce that risk with antiresorptive or other available therapies. However, new and more effective approaches are likely to emerge from a better understanding of the regulators of bone cell function.

Gene expression profiling and multiple myeloma

Gene expression profiling is a powerful tool through which the biology of multiple myeloma can be dissected. We will describe in this chapter how early studies using this technology have provided meaningful insights into myeloma biology, have led to the identification of new therapeutic targets, and have identified powerful prognostic and pharmacogenomic markers. Specifically, we will demonstrate that gene expression profiling can be used to segregate myeloma patients into prognostic categories within which known IgH translocation signatures can be readily defined. We also show that expression signatures can identify patients with chromosome 13 deletion. Finally, we demonstrate that global gene expression signatures can be distilled to short lists of three genes or more which together impart clinical outcome information, which is significantly more powerful than any previously defined prognostic tool. Expression profiling has also led to the identification of a number of new therapeutic targets not only in myeloma cell survival but also in the pathogenesis of the osteolysis which is a hallmark of this disease.

LRP5 gene polymorphisms and idiopathic osteoporosis in men

Mutations in the low-density lipoprotein receptor-related protein 5 gene (LRP5) have demonstrated the role of LRP5 in bone mass acquisition. LRP5 variants were recently reported to contribute to the population-based variance in vertebral bone mass and size in males. To investigate whether LRP5 variants are implicated in idiopathic male osteoporosis, we studied 78 men with low BMD (<2.5 T score or < -2 Z score) aged less than 70 years (mean +/- SD: 50 +/- 16 years) in whom secondary causes of osteoporosis had been excluded and 86 controls (51 +/- 10 years). Genotypes and haplotypes were based on LRP5 missense substitutions in exons 9 (c.2047G > A, p.V667M) and 18 (c.4037C > T, p.A1330V), and their association with osteoporosis evaluated after adjustment for multiple clinical and environmental variables using logistic regression. The presence of osteoporosis was significantly associated with LRP5 haplotypes (P = 0.0036) independent of age (P = 0.006), weight (P = 0.004), calcium intake (P = 0.002), alcohol (P = 0.005) and tobacco (P = 0.004) consumption. Accordingly, the odds ratio for osteoporosis was 3.78 (95% CI 1.27-11.26, P < 0.001) in male carriers of haplotype 3 (c.2047A-4037T, n = 20 cases and 12 controls) versus homozygous carriers of haplotype 1 (c.2047G-4037C, n = 42 cases and 61 controls). In conclusion, these data indicate beyond a significant role for environmental factors, an association between LRP5 variants and idiopathic osteoporosis in males, pointing to a role of LRP5 in this disease.

Lrp5-independent activation of Wnt signaling by lithium chloride increases bone formation and bone mass in mice

One of the well characterized cell biologic actions of lithium is the inhibition of glycogen synthase kinase-3beta and the consequent activation of canonical Wnt signaling. Because deficient Wnt signaling has been implicated in disorders of reduced bone mass, we tested whether lithium could improve bone mass in mice. We gavage-fed lithium chloride to 8-week-old mice from three different strains (Lrp5(-/-), SAMP6, and C57BL/6) and assessed the effect on bone metabolism after 4 weeks of therapy. Lrp5(-/-) mice lack the Wnt coreceptor low-density lipoprotein receptor-related protein 5 and have markedly reduced bone mass. Lithium, which is predicted to act downstream of this receptor, restored bone metabolism and bone mass to near wild-type levels in these mice. SAMP6 mice have accelerated osteoporosis due to inadequate osteoblast renewal. Lithium significantly improved bone mass in these mice and in wild-type C57BL/6 mice. We found that lithium activated canonical Wnt signaling in cultured calvarial osteoblasts from Lrp5(-/-) mice ex vivo and that lithium-treated mice had increased expression of Wnt-responsive genes in their bone marrow cells in vivo. These data lead us to conclude that lithium enhances bone formation and improves bone mass in mice and that it may do so via activation of the canonical Wnt pathway. Lithium has been used safely and effectively for over half a century in the treatment of bipolar illness. Prospective studies in patients receiving lithium should determine whether it also improves bone mass in humans.

Missense mutations in LRP5 are not a common cause of idiopathic osteoporosis in adult men

We studied whether the LRP5 gene contributes to the clinical phenotype of IO in men. Mutation analysis in 66 IO men revealed a range of sequence variants, of which two missense variants were shown to be of functional relevance.

INTRODUCTION

Mutations in the LDL receptor-related protein 5 (LRP5) gene have been associated with extreme bone phenotypes, which makes LRP5 a plausible candidate gene for idiopathic osteoporosis (IO).

MATERIALS AND METHODS

In 66 men with IO, all 23 exons and exon-intron boundaries of the LRP5 gene were screened for mutations, and functional analyses were performed for those that were putatively involved in the phenotype.

RESULTS

Mutation analysis in the IO probands revealed five missense mutations, of which 1067C>T (S356L), 1364C>T (S455L), and 4609G>A (A1537T) were of potential functional significance because they were located in highly conserved regions of LRP5 and not found in a control panel. Segregation analysis in the respective families could not exclude their possible causality for IO. Furthermore, functional analyses clearly showed an inhibitory effect of mutations 1067C>T and 1364C>T on Wnt signal transduction. These effects are most likely caused by impaired LRP5 synthesis in the case of 1067C>T and failure of protein trafficking to the cell surface for 1364C>T.

CONCLUSIONS

For 2 of 66 IO probands, a mutation in the LRP5 gene with proven functionality was found. The findings indicate that carrying an LRP5 mutation is a risk factor for IO, but that overall, IO in men is infrequently underlied by such a mutation.

Polymorphisms of the CLCN7 gene are associated with BMD in women

Here we show that a common polymorphism causing a valine to methionine amino acid substitution at codon 418 (V418M) in the CLCN7 gene is associated with femoral neck BMD in women. Our study adds to accumulating evidence that shows that common allelic variants in monogenic bone disease genes often contribute to BMD regulation in normal subjects.

INTRODUCTION

The CLCN7 gene is a strong candidate for regulation of BMD, because mutations in CLCN7 cause some forms of osteopetrosis, a disease characterized by impaired osteoclast function and increased BMD. In this study, we sought to determine whether common allelic variation within CLCN7 was associated with BMD in the normal population.

MATERIALS AND METHODS

We conducted mutation screening of the exons and intron-exon boundaries in CLCN7 by DNA sequencing in 50 normal subjects. We conducted an association study between common polymorphisms in CLCN7 and haplotypes defined by these polymorphisms and BMD values at the lumbar spine and femoral neck in a population-based cohort study of 1077 Scottish women 45-55 years of age.

RESULTS

We identified 24 polymorphisms, but most were rare and only 4 had allele frequencies of >5%. These were a conservative single nucleotide polymorphism (SNP) in exon 1 (rs3751884), a 50-bp tandem repeat polymorphism within intron 8, and two SNPs within exon 15 (rs12926089 and rs12926669), of which one (rs12926669) predicts an amino acid change from valine to methionine at codon 418 (V418M). The exon 15 SNPs were in strong linkage disequilibrium and were both associated with femoral neck BMD (p = 0.001-0.003). None of the other polymorphisms were associated with BMD, and long-range haplotypes showed a much weaker association with BMD than the exon 15 SNPs. The V418M polymorphism was an independent predictor of femoral neck BMD on multiple regression analysis accounting for 1% of the variance in BMD at this site.

CONCLUSIONS

Our study indicates that the V418M polymorphism of CLCN7 contributes to the genetic regulation of femoral neck BMD in women and adds to accumulating evidence that indicates that subtle polymorphic variation in genes that cause monogenic bone diseases also contribute to regulation of BMD in normal subjects.

Advances in Multiple Myeloma and Spine Disease

Progressive bone destruction is the hallmark of multiple myeloma (MM) and is responsible for principal morbidity in the disease. The spine is the most afflicted skeletal organ, and vertebral fractures have significantly contributed to its poor prognosis. The principal underlying pathologic mechanism causing bone disease in MM is a shift in the balance of bone formation and bone resorption toward bone resorption, and eventually total dissociation between the 2 processes occurs in latter stages of the disease. During the past decade bisphosphonates have become an important adjunctive treatment in the management of MM, in which they have shown the ability to reduce bony complications associated with the disease. Advances in minimally invasive surgical techniques, such as percutaneous vertebroplasty and kyphoplasty, offer these patients less-invasive options for the treatment of vertebral collapse and restoration of their normal function. This report reviews recent advances in the understanding of bone disease in MM, the role of bisphosphonates in the prevention of skeletal events, and available data regarding percutaneous vertebroplasty and kyphoplasty.

Clinical and molecular findings in osteoporosis-pseudoglioma syndrome

Mutations in the low-density lipoprotein receptor-related protein 5 gene (LRP5) cause autosomal recessive osteoporosis-pseudoglioma syndrome (OPPG). We sequenced the coding exons of LRP5 in 37 probands suspected of having OPPG on the basis of the co-occurrence of severe congenital or childhood-onset visual impairment with bone fragility or osteoporosis recognized by young adulthood. We found two putative mutant alleles in 26 probands, only one mutant allele in 4 probands, and no mutant alleles in 7 probands. Looking for digenic inheritance, we sequenced the genes encoding the functionally related receptor LRP6, an LRP5 coreceptor FZD4, and an LRP5 ligand, NDP, in the four probands with one mutant allele, and, looking for locus heterogeneity, we sequenced FZD4 and NDP in the seven probands with no mutations, but we found no additional mutations. When we compared clinical features between probands with and without LRP5 mutations, we found no difference in the severity of skeletal disease, prevalence of cognitive impairment, or family history of consanguinity. However, four of the seven probands without detectable mutations had eye pathology that differed from pathology previously described for OPPG. Since many LRP5 mutations are missense changes, to differentiate between a disease-causing mutation and a benign variant, we measured the ability of wild-type and mutant LRP5 to transduce Wnt and Norrin signal ex vivo. Each of the seven OPPG mutations tested, had reduced signal transduction compared with wild-type mutations. These results indicate that early bilateral vitreoretinal eye pathology coupled with skeletal fragility is a strong predictor of LRP5 mutation and that mutations in LRP5 cause OPPG by impairing Wnt and Norrin signal transduction.

A clinical and molecular overview of the human osteopetroses

The osteopetroses are a heterogeneous group of bone remodeling disorders characterized by an increase in bone density due to a defect in osteoclastic bone resorption. In humans, several types can be distinguished and a classification has been made based on their mode of inheritance, age of onset, severity, and associated clinical symptoms. The best-known forms of osteopetrosis are the malignant and intermediate autosomal recessive forms and the milder autosomal dominant subtypes. In addition to these forms, a restricted number of cases have been reported in which additional clinical features unrelated to the increased bone mass occur. During the last years, molecular genetic studies have resulted in the identification of several disease-causing gene mutations. Thus far, all genes associated with a human osteopetrosis encode proteins that participate in the functioning of the differentiated osteoclast. This contributed substantially to the understanding of osteoclast functioning and the pathogenesis of the human osteopetroses and will provide deeper insights into the molecular pathways involved in other bone pathologies, including osteoporosis.

Wnt proteins prevent apoptosis of both uncommitted osteoblast progenitors and differentiated osteoblasts by beta-catenin-dependent and -independent signaling cascades involving Src/ERK and phosphatidylinositol 3-kinase/AKT

Genetic studies in humans and mice have revealed an important role of the Wnt signaling pathway in the regulation of bone mass, resulting from potent effects on the control of osteoblast progenitor proliferation, commitment, differentiation, and perhaps osteoblast apoptosis. To establish the linkage between Wnts and osteoblast survival and to elucidate the molecular pathways that link the two, we have utilized three cell models: the uncommitted bipotential C2C12 cells, the pre-osteoblastic cell line MC3T3-E1, and bone marrow-derived OB-6 osteoblasts. Serum withdrawal-induced apoptosis was prevented by the canonical Wnts (Wnt3a and Wnt1) and the noncanonical Wnt5a in all cell types. Wnt3a induced LRP5-independent transient phosphorylation and nuclear accumulation of ERKs and phosphorylation of Src and Akt. The anti-apoptotic effect of Wnt3a was abrogated by inhibitors of canonical Wnt signaling, as well as by inhibitors of MEK, Src, phosphatidylinositol 3-kinase (PI3K), or Akt kinases, or by the addition of cycloheximide to the culture medium. Wnt3a-induced phosphorylation of GSK-3beta and downstream activation of beta-catenin-mediated transcription required ERK, PI3K, and Akt signaling. Wnt3a increased the expression of the anti-apoptotic protein Bcl-2 in an ERK-dependent manner. Beta-catenin-mediated transcription was permissive for the anti-apoptotic actions of Wnt1 and Wnt3a but was dispensable for the anti-apoptotic action of Wnt5a. However, Src, ERKs, PI3K, and Akt kinases were required for the anti-apoptotic effects of Wnt5a. These results demonstrate for the first time that Wnt proteins, irrespective of their ability to stimulate canonical Wnt signaling, prolong the survival of osteoblasts and uncommitted osteoblast progenitors via activation of the Src/ERK and PI3K/Akt signaling cascades.

Genetic determinants of osteoporosis

PURPOSE OF REVIEW

Osteoporosis is a common disease with a strong genetic component characterised by reduced bone mass and an increased risk of fragility fractures. Several advances have been made over recent years in understanding the genetic basis of susceptibility to osteoporosis. This paper will review recent developments in this area.

RECENT FINDINGS

Twin studies have shown that genetic factors contribute to osteoporosis by influencing bone mineral density and other determinants of fracture risk such as ultrasound properties of bone, skeletal geometry, and bone turnover. In the normal population, many different genes contribute to the regulation of these phenotypes by interacting with environmental factors such as diet and exercise. Whereas the effect size of individual genes is small, meta-analysis has been successfully used in many cases to define the role of individual polymorphisms in predisposing to osteoporosis. Linkage studies in humans and experimental animals have identified several quantitative trait loci that regulate osteoporosis-related phenotypes, and many genes that cause monogenic bone diseases have been identified by use of this approach. It has been found that subtle polymorphisms in some of these genes also contribute to regulation of bone mass in the normal population.

SUMMARY

Research has recently begun to clarify the genes and genetic variants that predispose to osteoporosis and regulation of bone mass. Clinical applications of this research include the identification of genetic markers for assessment of fracture risk and the identification of novel molecular targets for the design of drugs that can be used to treat bone disease.

Cannabinoid receptor type 2 gene is associated with human osteoporosis

Osteoporosis is one of the most common degenerative diseases. It is characterized by reduced bone mineral density (BMD) with an increased risk for bone fractures. There is a substantial genetic contribution to BMD, although the genetic factors involved in the pathogenesis of human osteoporosis are largely unknown. Mice with a targeted deletion of either the cannabinoid receptor type 1 (Cnr1) or type 2 (Cnr2) gene show an alteration of bone mass, and pharmacological modification of both receptors can regulate osteoclast activity and BMD. We therefore analyzed both genes in a systematic genetic association study in a human sample of postmenopausal osteoporosis patients and matched female controls. We found a significant association of single polymorphisms (P = 0.0014) and haplotypes (P = 0.0001) encompassing the CNR2 gene on human chromosome 1p36, whereas we found no convincing association for CNR1. These results demonstrate a role for the peripherally expressed CB2 receptor in the etiology of osteoporosis and provide an interesting novel therapeutical target for this severe and common disease.

Downregulation of Wnt signaling by increased expression of Dickkopf-1 and -2 is a prerequisite for late-stage osteoblast differentiation of KS483 cells

We examined the role of Wnt/beta-catenin signaling in successive stages of osteoblast differentiation. It has been shown that Wnt signaling in mature osteoblasts needs to be downregulated to enable the formation of a mineralized matrix. Using RNA interference, we showed that this is, at least in part, accomplished by upregulation of the Wnt antagonists Dickkopf-1 and -2.

INTRODUCTION

The role of Wnt signaling in the initiation of osteoblast differentiation has been well studied. However, the role during late-stage differentiation is less clear. We have examined the role of Wnt/beta-catenin signaling in successive stages of osteoblast differentiation.

MATERIALS AND METHODS

We treated murine bone marrow and mesenchymal stem cell-like KS483 cells with either LiCl or Wnt3A during several stages of osteoblast differentiation. In addition, we generated stable KS483 cell lines silencing either the Wnt antagonist Dkk-1 or -2

RESULTS

Activation of Wnt signaling by LiCl inhibits the formation of a mineralized bone matrix in both cell types. Whereas undifferentiated KS483 cells respond to Wnt3A by inducing nuclear beta-catenin translocation, differentiated cells do not. This is at least in part accomplished by upregulated expression of Dkk-1 and -2 during osteoblast differentiation. Using RNA interference, we showed that Dkk-1 plays a crucial role in blunting the BMP-induced alkaline phosphatase (ALP) response and in the transition of an ALP+ osteoblast in a mineralizing cell. In contrast, Dkk-2 plays a role in osteoblast proliferation and the initiation of osteoblast differentiation.

CONCLUSIONS

Our data suggest that Wnt signaling in maturing osteoblasts needs to be downregulated to enable the formation of a mineralized bone matrix. Furthermore, they suggest that Dkk-1 and Dkk-2 may have distinct functions in osteoblast differentiation.

LRP5/6 in Wnt signaling and tumorigenesis

The canonical Wnt signaling pathway is frequently overactivated in several types of human cancer. Defects in different components of the Wnt signaling pathway promote tumorigenesis and tumor progression. Accordingly, the pathway has been intensely studied to understand its importance in cancer biology and as therapeutic target. Recent studies have found that the low-density lipoprotein receptor-related protein (LRP)5 and 6 are essential Wnt coreceptors and interact with several key components of the Wnt signaling pathway. Furthermore, it has been demonstrated that LRP5 and 6 are potential oncogenic proteins. Thus, blockade of LRP56 function may lead to new strategies in cancer treatment.

Wnt signalling pathway: a new target for the treatment of osteoporosis

The prevention and treatment of osteoporosis traditionally involves the use of antiresorptive agents that target osteoclast function. Antiresorptive therapy is not associated with a significant increase in bone mass and, thus, only partially reduces the risk of fractures. For that reason, the search for anabolic agents, which target osteoblast function, represents an urgent medical need. The first approved bone anabolic drug for the treatment of osteoporosis was teriparatide (human parathyroid hormone 1-34). Recently, both human genetics and animal studies have pointed out the role of the Wnt/LRP5 pathway as a major regulator of bone mass accrual. Wnts are secreted glycoproteins that bind to receptor complexes including low-density lipoprotein receptor-related protein (LRP)-5/6 and Frizzled proteins. A subsequent intracellular cascade of events stabilises beta-catenin, leading to its translocation into the nucleus where, associated with Tcf/Lef transcription factors, it triggers gene expression. The existence of many potential pharmacological targets in this pathway makes it attractive for bone anabolic drug discovery.

Cell-surface co-receptors: emerging roles in signaling and human disease

Extracellular signals are transmitted to cells through two classes of cell-surface receptors: signaling receptors that directly transduce signals and signaling co-receptors that bind ligand but that, traditionally, have not been thought to signal directly. Signaling co-receptors modulate the ligand binding and signaling of their respective signaling receptors. In recent years, roles for co-receptors have expanded to include essential functions in morphogen gradient formation, localizing signaling, signaling independently, regulating cell adhesion and orchestrating the signaling of several pathways. The importance of signaling co-receptors is demonstrated by their ubiquitous expression, their conservation during evolution, their prominent role in signaling cascades, their indispensable role during development and their frequent mutation or altered expression in human disease.

WNT7b mediates macrophage-induced programmed cell death in patterning of the vasculature

Macrophages have a critical role in inflammatory and immune responses through their ability to recognize and engulf apoptotic cells. Here we show that macrophages initiate a cell-death programme in target cells by activating the canonical WNT pathway. We show in mice that macrophage WNT7b is a short-range paracrine signal required for WNT-pathway responses and programmed cell death in the vascular endothelial cells of the temporary hyaloid vessels of the developing eye. These findings indicate that macrophages can use WNT ligands to influence cell-fate decisions--including cell death--in adjacent cells, and raise the possibility that they do so in many different cellular contexts.

Crooked tail (Cd) model of human folate-responsive neural tube defects is mutated in Wnt coreceptor lipoprotein receptor-related protein 6

A cranial neural tube defect in Crooked tail (Cd) mice is prevented with prenatal dietary folic acid Cd positional cloning reveals a missense mutation of a highly conserved amino acid in the low density lipoprotein receptor-related protein 6 (Lrp6), a coreceptor required for Wnt canonical signaling. Molecular modeling predicts that Lrp6(Cd) alters a hinge region of the second YWTD beta-propeller domain. Mutant LRP6 binds to Wnt and Dickkopf1 (Dkk1) but not Mesd1, and Dkk1 cannot antagonize Wnt in Cd/Cd cells, resulting in hyperactivity. NIH 3T3 cells transfected with a mutant Lrp6 plasmid resist Dkk1 antagonism much like Cd/+ cells, confirming the significance of the mutation. The Lrp6 mutation in Cd mice provides evidence for a functional connection between Wnt signaling and folate rescue of neural tube defects.

Cross-talk between Wnt and bone morphogenetic protein 2 (BMP-2) signaling in differentiation pathway of C2C12 myoblasts

Loss of function of the Wnt co-receptor, lipoprotein receptor-related protein 5, decreases bone formation, and a point mutation in this gene results in high bone mass, indicating the importance of this signaling pathway in bone formation. However, the exact mechanism is currently unknown. We examined a potential role for Wnt signaling and functional cross-talk of bone morphogenetic protein 2 (BMP-2) in osteoblast differentiation. To assess the contribution of Wnt, we generated C2C12 cells over-expressing Wnt3a or Wnt5a and treated these with BMP-2. We showed that expression of matrix extracellular phosphoglycoprotein was induced by BMP-2 in Wnt3a over-expressing C2C12 cells but not in Wnt5a over-expressing C2C12 cells. Over-expression of Wnt3a blocked BMP-2-induced inhibition of myotube formation in C2C12 cells when switched to low mitogen medium. In these cultures, expression of inhibitor of DNA binding/differentiation (Id) 1, a helix-loop-helix protein induced by BMP-2, decreased in stable Wnt3a- but not in Wnt5a-expressing cells. This suppression is mediated by a GC-rich region of the BMP-2-responsive element of the Id1 gene promoter, and interaction between Smad1/4 and beta-catenin is crucial for Wnt-mediated suppression of the BMP-2 response in C2C12 cells. Over-expression of the inhibitor of canonical Wnt signaling, Dickkopf, inhibits this suppression. In contrast, BMP-2 or Smad1/4 up-regulated Wnt3a or activated beta-catenin-induced lymphoid-enhancing factor 1/T cell factor-dependent transcriptional activity. These findings identify functional cross-talk of Id1 expression between Wnt and BMP signaling and demonstrate a novel mechanism for Wnt regulation of the BMP-2 response, linking Id1 expression to Wnt/beta-catenin signaling.

Genetic epidemiology of osteoporosis: past, present, and future

Research during the past several decades has unequivocally established a role of heredity in the etiology of osteoporosis. Major efforts are currently underway to identify the genes and allelic variants that confer genetic susceptibility to this common and disabling condition. Genome-wide linkage mapping in families, candidate gene association studies in unrelated individuals, and quantitative trait locus mapping in animal models are the primary strategies being used to search for the genetic contributors to osteoporosis. Genome-wide mapping efforts have identified the low-density lipoprotein receptor-related protein 5, bone morphogenetic protein 2, and 15-lipoxygenase as potential susceptibility genes for osteoporosis in the past few years, providing a rich new base for understanding bone biology. Candidate gene association analyses have also provided evidence for a modest role of allelic variants in several additional genes including collagen type Ialpha1, vitamin D receptor, and estrogen receptor-alpha. With the development of a high-density genome-wide polymorphism and haplotype map and continued improvements in high-throughput and cost-effective genotyping technologies, many more genetic contributors to osteoporosis will probably be identified in the near future. The results of this research should facilitate the development of new methods for diagnosing, preventing, and treating the growing clinical and public health problem of osteoporosis.

Association of polymorphisms in low-density lipoprotein receptor-related protein 5 gene with bone mineral density in postmenopausal Chinese women

AIM

To investigate the possible association of Q89R, N740N and A1330V polymorphisms in low-density lipoprotein receptor-related protein 5 (LRP5) gene with bone mineral density (BMD) in postmenopausal Chinese women.

METHODS

Q89R, N740N and A1330V genotypes were determined by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) in 647 unrelated healthy postmenopausal Han Chinese women aged 43-76 years in Shanghai. BMD at lumbar spine 1-4 and the left proximal femur including the femoral neck, trochanter and Ward's triangle were measured by dual-energy X-ray absorptionmetry in all subjects.

RESULTS

The distribution of the Q89R, N740N and A1330V genotypes in this population was as follows: QQ 80.5%, QR 18.7%, and RR 0.8%; TT 66.9%, TC 31.1%, and CC 2.0%; AA 68.0%, AV 29.7%, and VV 2.3%. The frequencies of the Q89R, N740N and A1330V genotypes and alleles did not deviate from the Hardy-Weinberg equilibrium. We found that the Q89R and A1330V polymorphisms were in linkage disequilibrium in our population (kappa2=13.50, P<0.01). Both before and after adjusting for age, years since menopause, height, and weight, the Q89R or N740N genotypes were significantly associated with BMD at the femoral neck (P<0.05). Subjects with the Q89R QQ genotype or the N740N TT genotype had a significantly higher BMD at the femoral neck, compared with those with the QR/RR or TC/CC genotypes, respectively. No significant association was found between A1330V polymorphism and BMD at any site.

CONCLUSION

Our findings suggest that the LRP5 gene is a candidate for the genetic determination of BMD in postmenopausal Chinese women.

The duality of β-catenin function: A requirement in lens morphogenesis and signaling suppression of lens fate in periocular ectoderm

In the current analysis, we have investigated both the cytoskeletal and signaling roles of beta-catenin during the early phases of lens development using conditional loss- and gain-of-function strategies. Conditional loss of beta-catenin in the presumptive lens does not perturb the normal sequential appearance of lens fate markers but results in a dramatic failure of the coordinated epithelial cell behavior that constitutes lens morphogenesis. Similarly, loss-of-function for Lrp6, the Wnt pathway coreceptor expressed in the eye primordium, does not prevent expression of lens induction markers. Surprisingly, conditional deletion of beta-catenin in periocular ectoderm results in the formation of Prox-1 and beta-crystallin-positive ectopic lentoid bodies. Combined with the observation that the Wnt pathway reporter TOPGAL is expressed in nasal periocular ectoderm, these data suggest that, in this location, the canonical Wnt signaling pathway normally suppresses lens fate in favor of other structures. Consistent with this proposal, a dominant-active form of beta-catenin causes a loss of lens fate and a complete absence of lens development when expressed in the presumptive lens ectoderm.

Atorvastatin inhibits hypercholesterolemia-induced calcification in the aortic valves via the Lrp5 receptor pathway

BACKGROUND

Calcific aortic valve disease is the most common indication for surgical valve replacement in the United States. The cellular mechanisms of valve calcification are not well understood. We have previously shown that cellular proliferation and osteoblastogenesis are important in the development of valvular heart disease. Lrp5, a known low-density receptor-related protein, plays an essential role in cellular proliferation and osteoblastogenesis via the beta-catenin signaling pathway. We hypothesize that Lrp5 also plays a role in aortic valve (AV) calcification in experimental hypercholesterolemia.

METHODS AND RESULTS

We examined the effects of cholesterol and atorvastatin in Watanabe rabbits (n=54). Group I (n=18) received a normal diet, group II (n=18) a 0.25% cholesterol diet, and group III (n=18) a 0.25% (w/w) cholesterol diet with atorvastatin for the development of calcification. The AVs were examined for cellular proliferation, Lrp5/beta-catenin, and bone matrix markers. Bone formation was assessed by micro-computed tomography, calcein injection, and osteopontin expression. Low-density lipoprotein with and without atorvastatin was also tested in AV myofibroblasts for cellular proliferation and regulation of the Lrp5/beta-catenin pathway. Our results demonstrate that the cholesterol diet induced complex bone formations in the calcified AVs with an increase in the Lrp5 receptors, osteopontin, and p42/44 expression. Atorvastatin reduced bone formation, cellular proliferation, and Lrp5/beta-catenin protein levels in the AVs. In vitro analysis confirmed the Lrp5/beta-catenin expression in myofibroblast cell proliferation.

CONCLUSIONS

Hypercholesterolemic AV calcification is attenuated by atorvastatin and is mediated in part by the Lrp5/beta-catenin pathway. This developmental pathway may be important in the signaling pathway of this disease.

Expression profiles and functional analyses of Wnt-related genes in human joint disorders

Rheumatoid arthritis (RA) and osteoarthritis (OA) are joint disorders that cause major public health problems. Previous studies of the etiology of RA and OA have implicated Wnt genes, although the exact nature of their involvement remains unclear. To further clarify the relationship between RA, OA, and the Wnt gene family, gene expression analyses were performed on articular cartilage, bone, and synovial tissues in knee joints taken from RA, OA, and nor-mal/control patients. Cytokine assays were also performed in cells transfected with Wnt-7b, a member of the gene family most closely linked to RA and OA. Of the human Wnt genes, real-time PCR analysis revealed significant up-regulation of Wnt-7b in OA cartilage and RA synovium. In situ hybridization and immunohistochemistry also revealed that Wnt-7b was present in articular cartilage, bone, and synovium of RA samples and in osteophytes, articular cartilage, bone marrow, and synovium of OA samples. The levels of the cytokines tumor necrosis factor-alpha, interleukin-1beta, and interleukin-6 were significantly increased in RA synovium and Wnt-7b-transfected normal synovial cells when compared with normal samples. These results point to the potential involvement of Wnt signaling in the pathobiology of both RA and OA.

High-content screening assay for activators of the Wnt/Fzd pathway in primary human cells

We have developed a high-content screening (HCS) assay to find activators of Wnt/Frizzled (Wnt/Fzd), a pathway known to be important in bone formation. Utilizing primary human preosteoblasts as a model, activation of the Wnt/Fzd pathway was detected by monitoring the stabilization and translocation of the transcription factor beta-catenin from cytoplasm to the nucleus. Endogenous beta-catenin was detected in preosteoblasts by immunofluorescent staining, and subcellular localization was determined by HCS using the Cellomics (Pittsburgh, PA) ArrayScan IV. Positive controls, including Wnt3A-conditioned medium and inhibitors of glycogen synthase kinase-3beta, resulted in increased nuclear beta-catenin. The assay had a Z'-factor of 0.6 and was conducive to automation for high-throughput screening/HCS. By combining standard immunofluorescence technology with automated fluorescence microscopy, we demonstrate the capability of screening cell-signaling pathways in primary human cells.

Dissection of the sets of genes that control the behavior of biglycan-deficient pre-osteoblasts using oligonucleotide microarrays

Biglycan (bgn) is a small leucine-rich proteoglycan (SLPR) that is enriched in the extracellular matrix of skeletal tissues. Bgn-deficient mice develop age-related osteopenia with a phenotype that resembles osteoporosis. In order to identify sets of genes that play a key role in the skeletal abnormality, we determined the global gene expression patterns in bgn-deficient (bgn-KO) pre-osteoblasts using oligonucleotide microarray technology. Calvarial cells were harvested from newborn mice and cultured in the presence or absence of BMP-4 for 7 days. The total RNA was purified, labeled and hybridized to Affymetrix chips (U74A), and analyzed with a software program called GeneSpring. Our data suggested that biglycan regulates the activity of osteoblastic progenitors through sets of genes associated with cell cycle, cell growth, and differentiation. The biological outcome from the altered expression of these genes could cause a defect in the quantity and quality of osteoblastic progenitors, which could contribute to the development of age-related osteopenia in bgn-KO mice. Moreover, the data from this approach also revealed that biglycan deficiency affected the genes that control inflammation, immune response, and growth of tumor cells. These are new and unexpected findings that lead to the formation of new paradigms for biglycan function. Based on these findings, we propose that the reduction of this small proteoglycan with aging may increase the risk of infection and autoimmune diseases, impair wound healing, and cause higher incidences of malignancy. This study provides a broad and deep foundation for understanding SLRP function at a more complex level.

Genetic disorders of the LRP5-Wnt signalling pathway affecting the skeleton

Osteoporosis is a common, increasingly prevalent and potentially debilitating condition of men and women. Genetic factors are major determinants of bone mass and the risk of fracture, but few genes have been definitively demonstrated to be involved. The identification of these factors will provide novel insights into the processes of bone formation and loss and thus the pathogenesis of osteoporosis, enabling the rational development of novel therapies. In this article, we present the extensive genetic and functional data indicating that the LRP5 gene and the Wnt signalling pathway are key players in bone formation and the risk of osteoporosis, and that LRP5 signalling is essential for normal morphology, developmental processes and bone health.

Desmoid tumor with ossification in chest wall: possible involvement of BAMBI promoter hypermethylation in metaplastic bone formation

A rare case of desmoid-type fibromatosis with focal metaplastic bone in the chest wall suggested that enhanced responsiveness to BMP signaling by decreasing BAMBI expression through promoter hypermethylation plays a crucial role in the formation of metaplastic bone.

INTRODUCTION

Desmoid-type fibromatosis, originating from mesenchymal cells with myofibroblastic features, is a locally aggressive and frequently recurring infiltrative lesion. One such sporadic case with metaplastic ossification in the chest wall is presented.

MATERIALS AND METHODS

A 43-year-old man was referred to the hospital with a gradually enlarging hard mass in the left anterolateral chest wall. A thoracotomy was carried out, and histopathological specimens were used for immunohistochemical, genetic, and methylation studies.

RESULTS

Accumulation of altered beta-catenin associated with a somatic heterozygous activating mutation in codon 41 was detected in the typical desmoid-type fibromatosis and at the ossifying focus. Among factors related to bone formation and the classical wnt-beta-catenin signaling pathway, BMP and activin membrane-bound inhibitor (BAMBI) expression was specifically downregulated at the ossifying focus. Hypermethylation of the BAMBI promoter was observed in microdissected tissue from the ossifying focus but not in that from the typical desmoid-type fibromatosis.

CONCLUSIONS

Because both BMP and classical Wnt/beta-catenin/LEF1 signaling cooperatively and mutually induce differentiation of mesenchymal cells into osteoblastic cells and promote bone formation, the epigenetic event leading to the enhanced responsiveness to BMP signaling may play a crucial role in the formation of metaplastic bone.

Complexity of the genotype-phenotype correlation in familial exudative vitreoretinopathy with mutations in theLRP5and/orFZD4genes

Familial exudative vitreoretinopathy (FEVR) is a hereditary blinding disorder that features defects in retinal vascular development. The mutations in the genes encoding the Wnt receptor pair, frizzled 4 (FZD4) and low-density-lipoprotein receptor-related protein 5 (LRP5), have been shown to cause FEVR. In this study we screened 56 unrelated patients with FEVR (31 familial and 25 simplex cases) for possible mutations in LRP5 and FZD4. Six novel mutations in either LRP5 or FZD4 were identified in six familial cases. Four novel mutations in LRP5 and one known mutation in FZD4 were detected in three simplex cases, and two of these patients carried compound heterozygous mutations in LRP5. Remarkably, c.1330C>T [p.R444C] in LRP5 was found in the family in which c.1250G>A [p.R417Q] in FZD4 had previously been identified. The phenotype of these patients suggested a synergistic effect of the two mutations in the independent FEVR-causing genes. We also demonstrated that reduced bone density is a common feature in patients with FEVR who harbor LRP5 mutations. The profile of the mutations obtained in the current study further illustrates the complexity of the disease and provides a better understanding of the spectrum, frequencies, and genotype-phenotype correlation.

Dkk2 has a role in terminal osteoblast differentiation and mineralized matrix formation

Human and mouse genetic and in vitro evidence has shown that canonical Wnt signaling promotes bone formation, but we found that mice lacking the canonical Wnt antagonist Dickkopf2 (Dkk2) were osteopenic. We reaffirmed the finding that canonical Wnt signaling stimulates osteogenesis, including the differentiation from preosteoblasts to osteoblasts, in cultured osteoblast differentiation models, but we also found that canonical Wnts upregulated the expression of Dkk2 in osteoblasts. Although exogenous overexpression of Dkk before the expression of endogenous canonical Wnt (Wnt7b) suppressed osteogenesis in cultures, its expression after peak Wnt7b expression induced a phenotype resembling terminal osteoblast differentiation leading to mineralization. In addition, osteoblasts from Dkk2-null mice were poorly mineralized upon osteogenic induction in cultures, and Dkk2 deficiency led to attenuation of the expression of osteogenic markers, which could be partially reversed by exogenous expression of Dkk2. Taken together with the finding that Dkk2-null mice have increased numbers of osteoids, these data indicate that Dkk2 has a role in late stages of osteoblast differentiation into mineralized matrices. Because expression of another Wnt antagonist, FRP3, differs from Dkk2 expression in rescuing Dkk2 deficiency and regulating osteoblast differentiation, the effects of Dkk2 on terminal osteoblast differentiation may not be entirely mediated by its Wnt signaling antagonistic activity.

Genetics of osteoporosis

Genetic factors play an important role in regulating bone mineral density and other phenotypes relevant to the pathogenesis of osteoporosis such as ultrasound properties of bone, skeletal geometry, and bone turnover. Progress has been made in identifying quantitative traits for regulation of bone mineral density by linkage studies in man and mouse, but relatively few causal genes have been identified. Dramatic progress has been made in identifying the genes responsible for monogenic bone diseases and it appears that polymorphisms in many of these genes also play a role in regulating bone mineral density in the general population. Advances in knowledge about the genetic basis of osteoporosis and other bone diseases offer the prospect of developing new markers for assessment of fracture risk and the identification of novel molecular targets for the design of new drug treatments for osteoporosis.

Canonical WNT signaling promotes osteogenesis by directly stimulating Runx2 gene expression

Both activating and null mutations of proteins required for canonical WNT signaling have revealed the importance of this pathway for normal skeletal development. However, tissue-specific transcriptional mechanisms through which WNT signaling promotes the differentiation of bone-forming cells have yet to be identified. Here, we address the hypothesis that canonical WNT signaling and the bone-related transcription factor RUNX2/CBFA1/AML3 are functionally linked components of a pathway required for the onset of osteoblast differentiation. Our findings show that, in bone of the SFRP1 (secreted frizzled-related protein-1)-null mouse, which exhibits activated WNT signaling and a high bone mass phenotype, there is a significant increase in expression of T-cell factor (TCF)-1, Runx2, and the RUNX2 target gene osteocalcin. We demonstrate by mutational analysis that a functional TCF regulatory element responsive to canonical WNT signaling resides in the promoter of the Runx2 gene (-97 to -93). By chromatin immunoprecipitation, recruitment of beta-catenin and TCF1 to the endogenous Runx2 gene is shown. Coexpression of TCF1 with canonical WNT proteins resulted in a 2-5-fold activation of Runx2 promoter activity and a 7-8-fold induction of endogenous mRNA in mouse pluripotent mesenchymal and osteoprogenitor cells. This enhancement was abrogated by SFRP1. Taken together, our results provide evidence for direct regulation of Runx2 by canonical WNT signaling and suggest that Runx2 is a target of beta-catenin/TCF1 for the stimulation of bone formation. We propose that WNT/TCF1 signaling, like bone morphogenetic protein/transforming growth factor-beta signaling, activates Runx2 gene expression in mesenchymal cells for the control of osteoblast differentiation and skeletal development.

Myeloma cells suppress bone formation by secreting a soluble Wnt inhibitor, sFRP-2

Multiple myeloma (MM) develops devastating bone destruction with enhanced bone resorption and suppressed bone formation. In contrast to enhanced osteoclastogenesis, little is known about the mechanism of impaired bone formation in MM. Because a canonical Wingless-type (Wnt) signaling pathway has recently been shown to play an important role in osteoblast differentiation, we examined whether MM cells affect a canonical Wnt pathway to suppress bone formation. Conditioned media from RPMI8226 and U266 MM cell lines and primary MM cells suppressed in vitro mineralization as well as alkaline phosphatase activity in osteoblasts induced by bone morphogenetic protein 2 (BMP-2). These cell lines constitutively produced a soluble Wnt inhibitor, secreted Frizzled-related protein 2 (sFRP-2), but not other Wnt inhibitors including sFRP-1, sFRP-3, and dickkopf 1 (DKK-1) at the protein level. Most MM cells from patients with advanced bone destructive lesions also expressed sFRP-2. Furthermore, exogenous sFRP-2 suppressed osteoblast differentiation induced by BMP-2, and immunodepletion of sFRP-2 significantly restored mineralized nodule formation in vitro, suggesting a predominant role for MM cell-derived sFRP-2 in the impairment of bone formation by MM. Thus, in addition to enhanced osteolysis, MM cells also suppress bone formation at least in part through an inhibition of the canonical Wnt pathway by secreting sFRP-2.

Reduced affinity to and inhibition by DKK1 form a common mechanism by which high bone mass-associated missense mutations in LRP5 affect canonical Wnt signaling

The low-density-lipoprotein receptor-related protein 5 (LRP5), a coreceptor in the canonical Wnt signaling pathway, has been implicated in human disorders of low and high bone mass. Loss-of-function mutations cause the autosomal recessive osteoporosis-pseudoglioma syndrome, and heterozygous missense mutations in families segregating autosomal dominant high bone mass (HBM) phenotypes have been identified. We expressed seven different HBM-LRP5 missense mutations to delineate the mechanism by which they alter Wnt signaling. None of the mutations caused activation of the receptor in the absence of ligand. Each mutant receptor was able to reach the cell surface, albeit at differing amounts, and transduce exogenously supplied Wnt1 and Wnt3a signal. All HBM mutant proteins had reduced physical interaction with and reduced inhibition by DKK1. These data suggest that HBM mutant proteins can transit to the cell surface in sufficient quantity to transduce Wnt signal and that the likely mechanism for the HBM mutations' physiologic effects is via reduced affinity to and inhibition by DKK1.

Osteoporosis and atherosclerosis: biological linkages and the emergence of dual-purpose therapies

Osteoporosis and atherosclerosis are both widely prevalent in an ageing population, and induce serious morbidities and death. There is growing evidence that in addition to their relationship to ageing, osteoporosis and atherosclerosis are also linked by biological associations. This article reviews their clinical interrelations, discusses the basic biology of bone and the arterial wall, and presents five examples that illustrate their biological linkages. Current therapeutic approaches emerging from these linkages, including statins, bisphosphonates, and the thiazolidinediones, have dual effects on bone and the vasculature. Additional therapies derived from experimental studies that enhance bone density and reduce atherogenesis hold further promise to diminish the morbidity and mortality of osteoporosis and atherosclerosis, with attendant benefits to society.

TOPGAL mice show that the canonical Wnt signaling pathway is active during bone development and growth and is activated by mechanical loading in vitro

We identified cellular targets of canonical Wnt signaling within the skeleton, which included chondrocytes, osteoblasts, and osteocytes in growing bone, but only osteocytes and chondrocytes in the mature skeleton. Mechanical deformation induced Wnt signaling in osteoblasts in vitro.

INTRODUCTION

Genetic evidence in mice and humans has implicated the canonical Wnt signaling pathway in the control of skeletal development and bone mass. However, little is known of the details of Wnt signaling in the skeleton in vivo. We used Wnt indicator TOPGAL mice to identify which cells activated this pathway during bone development and in the mature skeleton.

MATERIALS AND METHODS

We examined canonical Wnt signaling during embryonic and neonatal bone development in TOPGAL mice. The TOPGAL transgene consists of a beta-galactosidase gene driven by a T cell factor (TCF)beta-catenin responsive promoter so that canonical Wnt activity can be detected by X-gal staining. Expression of Wnt signaling components was examined in primary calvarial cell cultures by RT-PCR. The effect of mechanical deformation on Wnt signaling was examined in primary calvarial cells grown on collagen I and stretched using Flexercell Tension Plus System FX-4000T. Immunohistochemistry was used to examine the localization of beta-catenin in cartilage, bone, and cultured calvarial cells exposed to physical deformation.

RESULTS AND CONCLUSIONS

Canonical Wnt signaling was active in several cell types in the fetal and neonatal skeleton, including chondrocytes, osteoblasts, and osteocytes. With age, activation of Wnt signaling became less prominent but persisted in chondrocytes and osteocytes. Although osteoblasts in culture expressed many different individual Wnt's and Wnt receptors, the TOPGAL transgene was not active in these cells at baseline. However, Wnt signaling was activated in these cells by physical deformation. Together with the activation of canonical Wnt signaling in osteocytes seen in vivo, these data suggest that Wnt signaling may be involved in the coupling of mechanical force to anabolic activity in the skeleton.

Haplotypes of the low-density lipoprotein receptor-related protein 5 (LRP5) gene: are they a risk factor in osteoarthritis?

OBJECTIVE

Several genome-wide scans have revealed an osteoarthritis (OA)-susceptibility locus on chromosome 11q in close proximity to the low-density lipoprotein receptor-related protein 5 (LRP5) gene. The regulation of bone mass is under the control of LRP5 and since increased bone mass is thought to play a role in the pathology of OA we examined LRP5 polymorphisms and haplotypes to determine if variants of this locus may predispose to OA.

METHODS

A UK control population of 187 individuals was examined for five commonly occurring polymorphisms against a cohort of 158 DNAs from patients with knee OA. An additional UK cohort was also examined to confirm the findings of the first study; this second group consisted of 110 knee OA patients. Haplotype analysis was also performed on patient and control DNAs.

RESULTS

A study of individual polymorphisms revealed no association with disease. However, haplotype analysis of the initial two populations revealed a common haplotype (C-G-C-C-A) that provided a 1.6-fold increased risk of OA (P(c)=0.021). The data obtained from the second cohort confirmed the initial findings, with a 1.6-fold increased risk observed within this cohort for the risk haplotype (P=0.012).

CONCLUSIONS

A closer investigation of LRP5 and associated Wnt signalling molecules in OA will help determine disease aetiology and the development of novel treatment strategies that specifically target the bone compartment.