- Email: [email protected]
6 and WNT ligands in regulating bone development and acquisition
Abstracts / Bone 48 (2011) S50–S60
S59
clinical care of such children, including those receiving glucocorticoids (GC), has taught us vertebral fractures (VF) are a potentially debilitating manifestation of osteoporosis. Using the Genant semi-quantitative method, we have shown in a pan-Canadian study that 29/186 children (16%) with newly diagnosed leukemia manifested 75 VFs, with reductions in spine areal BMD and back pain associated with an increased odds for prevalent VF. At 12 months following chemotherapy initiation, the incident (i.e. new) VF rate in children with leukemia was also 16% (25/155 children), with prevalent VFs at baseline associated with an increased odds for incident spine fragility. Among children with rheumatic disorders, 9/134 children (7%) manifested 13 VFs within 30 days of GC initiation, and back pain was a highly associated clinical feature. At 12 months post-GC initiation in children with rheumatic disorders, 7 patients (6%) had 12 incident VFs; only 2 of these children reported back pain. Patients with rheumatic disorders and incident VFs had greater decreases in spine areal BMD Z-scores in the first 6 months of GC therapy and the cumulative GC dose was almost double compared to those without spine fragility. Spine fractures for both disease groups were most frequent in the mid-thoracic region and at the thoracolumbar junction; this observation was consistent across mild, moderate and severe VFs. In contrast to prevalent VFs at baseline where back pain was a highly associated clinical factor for both disease groups, at 12 months back pain was not a strong a discriminating feature for those with incident VFs compared to those without. These prospectively acquired data provide insight into the prevalence and incidence of VFs among children with serious illnesses, and highlight the associated clinical features. This article is part of a Special Issue entitled ECTS 2011. Disclosure of interest: L. Ward Grant / Research Support from the Canadian Institutes of Health Research.
phenotype. One of the most important but still poorly understood phenotypic changes associated with ageing is frailty. As expected, frailty and its constituent criteria (slowness, sarcopenia, exhaustion, low activity and weakness) increased in ageing European men. Bone density and turnover (as determined respectively by calcaneal ultrasound and serum CTX and PIP) respectively either showed only a minor decrease or no significant agerelated change in this male cohort. Serum total testosterone (T), insulin-like growth factor I (IGF-I) and dihydroepiandrostenedionesulfate (DHEAS) (respectively −2.1%, 0.5%and 3.6% /year) all decreased in ageing European men. Estradiol (E2) and 25 hydroxyvitamin D (25 D) did not change significantly during aging whereas sex hormone binding globulin (SHBG), luteinising hormone (LH), and follicle-stimulating hormone (FSH)increased with age (+ 1.3, 1 and 0.5% /year). The age-related decrease of free T was therefore greater (−3.2% per year) than total T. Free testosterone concentrations as well as levels of other anabolic hormones such as IGF-I, DHEAS, 25 D but not E2 were independently and inversely related to frailty in European men. Frailty and its constituent sarcopenia was also more prevalent in European men with primary as well as secondary hypogonadism even after correction for body size. Non – endocrine risk factors (inactivity and smoking) were however, as expected, also strong predictors of frailty. On the other hand, E2, SHBG, 25 D, IGF-I, IGFBP-3 but not free nor total T were independently and significantly associated with bone density. Genetically determined variations in E2 rather than T as well as nonendocrine risk factors (inactivity, smoking and excess alcohol) also determined bone density in these men. In conclusion, the male ageing phenotype in EMAS is associated with both endocrine and non-endocrine factors. Different hormones however appear to determine respectively frailty and bone health in ageing European men. This article is part of a Special Issue entitled ECTS 2011. Disclosure of interest: None declared.
doi:10.1016/j.bone.2011.03.039
doi:10.1016/j.bone.2011.03.041
IS36 Guidelines for management of male osteoporosis R. Eastell NIHR Bone Biomedical Research Unit, University of Sheffield, Sheffield, UK
IS38 LRP5 functions in bone to produce high bone mass phenotypes M.L. Warman Orthopaedic Research Laboratories, Children's Hospital Boston, and Department of Genetics, Harvard Medical School, Boston, MA, USA
The Endocrine Society has established a working group to produce guidelines (in conjunction with other professional societies such as ECTS) on male osteoporosis. Until now, approaches to diagnosis having been focused largely on women, and so there is a need for male-specific guidelines. Osteoporosis in men leads to significant morbidity and mortality. It is estimated that one in five men over the age of 50 will have a fracture as the result of osteoporosis. The working group aims to identify strategies for the diagnosis and the clinical management of osteoporosis in men. Firstly, it is important to determine which men should be referred for bone density testing using dual-energy X-ray absorptiometry (DXA). This approach will be based on the presence of clinical risk factors such as older age, low body weight, prior fracture as an adult and smoking. ‘Secondly, it is important to decide whether the bone density T-score (the number of standard deviation units from the young normal mean) should be calculated using a male or female reference interval. Thirdly, it is important to address the question of which men should be referred for further laboratory testing for secondary osteoporosis, and which tests should be recommended. The guidelines will include nutritional advice to identify for example, adequate intakes of calcium and vitamin D, and recommendations about the avoidance of adverse lifestyle habits such as smoking and excessive alcohol. Recommendations on the use of testosterone therapy will be considered in the light of recent guidance from the Endocrine Society. This states that unless contraindicated, testosterone replacement should be considered for men with organic hypogonadism and symptomatic patients who have serum testosterone levels <200 ng/dl (<7 nmol/L). The indications for pharmacologic treatment of osteoporosis in men age ≥50 will be considered in relation to prior fracture history (such as spine or hip fractures), low bone density (such as T-scores of −2.5 or below) and the presence of significant clinical risk factors. The working group will provide advice on whether to monitor treatment in men and on the most appropriate approach to monitoring treatment effects. This article is part of a Special Issue entitled ECTS 2011. Disclosure of interest: R. Eastell Grant / Research Support from Warner Chilcott, Lilly, Amgen, AstraZeneca, Consultant for Novartis, Amgen, Ono. doi:10.1016/j.bone.2011.03.040
IS37 EU male ageing study D. Vanderschueren Department of Endocrinology, Catholic University Leuven, Leuven, Belgium The European Male Ageing study (EMAS) is a prospective observational cohort study. EMAS evaluated a randomized population sample of 3369 men between ages 40 and 79 at baseline (2003–2005) and follow-up (2007–2009) using identical instruments in 8 different European centres as well as central laboratory analysis of a fasting blood sample. A major aim is to describe and define endocrine determinants of the male ageing
Mutations in low-density lipoprotein receptor-related protein 5 (LRP5) have been associated with heritable disorders of bone mass. Loss-of-function mutations in LRP5 cause the autosomal recessive Osteoporosis-Pseudoglioma Syndrome (OPPG), and several families segregating autosomal dominant high bone mass (HBM) phenotypes have heterozygous missense mutations within the LRP5 receptor's amino-terminal, six-bladed propeller domain. We postulated that LRP5 functions as a co-receptor in the canonical Wnt signaling cascade, based upon the high sequence conservation between LRP5 and the Drosophila Wnt co-receptor Arrow. Consistent with this inferred role we found that, when expressed in cultured cells, wild-type LRP5 was able to transduce Wnt signal. We also found that HBM-causing alleles also efficiently transduced Wnt signal and that OPPGcausing alleles did not. Finally, we and other investigators found that HBM-causing alleles were less susceptible to inhibition by endogenous Wnt inhibitors, such as SOST and DKK1. However, definitive in vivo evidence that LRP5 functions in the Wnt signaling pathway and that it acts locally in bone have not been reported. I will present experiments performed in collaboration with Dr. Alex Robling at Indiana University, which suggest that HBM-causing alleles function in bone cells to increase bone mass. We generated mice in which we “knocked” the HBM-causing mutation directly into the endogenous Lrp5 locus. This approach is different from and, in our opinion, superior to previously characterized mouse models with HBM-causing alleles. We made the HBMcausing knockin allele “conditional,” in that it requires Cre-mediated recombination to become normally expressed. This enabled us to assess the role of the HBM-causing allele in specific cell types and tissues. We found no evidence that HBM-causing alleles increase bone mass by functioning outside of the skeleton. Instead, our data suggest that LRP5 HBMcausing alleles act locally in bone, most likely via a mechanism that reduces SOST-mediated inhibition of Wnt signaling in osteocytes. This article is part of a Special Issue entitled ECTS 2011. Disclosure of interest: None declared. doi:10.1016/j.bone.2011.03.042
IS39 Evaluating the role(s) of Lrp5/6 and WNT ligands in regulating bone development and acquisition B.O. Williams Center for Skeletal Disease Research, Van Andel Research Institute, Grand Rapids, MI, USA To define the individual contributions of Lrp5 and Lrp6 in skeletal development, we used Cre-LoxP recombination to create mice lacking each co-receptor selectively in
S60
Abstracts / Bone 48 (2011) S50–S60
osteoblasts. Mice lacking Lrp5 (ΔLrp5) or Lrp6 (ΔLrp6) in mature osteoblasts were generated by crossing Lrp6flox and Lrp6flox mice with mice expressing cre-recombinase under the control of the human osteocalcin promoter (OC-Cre). DEXA indicated reduced BMD at 3 months in both ΔLrp6 and ΔLrp5 mice. Additionally, assessment of femoral bone morphology by microCT revealed significantly lower bone volume with decreased trabecular numbers in 7-week old ΔLrp6 mice when compared to controls. ΔLrp5 mice also displayed a reduction in trabecular number and increased trabecular spacing within the femur. Mice deficient for both Lrp5 and Lrp6 had severely reduced trabecular bone volume similar in magnitude to that observed previously in mice lacking β-catenin (J Biol Chem 2005 280;21162–8). Histomorphometric analysis revealed a dramatically decreased number of osteoblasts in the ΔLrp5; ΔLRP6 mice. In vitro deletion of Lrp6 impaired the differentiation of primary osteoblasts while Lrp5-deficient osteoblasts differentiated normally. Simultaneous disruption of both co-receptors in vitro altered fate-specification of calvarial cells which showed abundant alcian blue staining and increased expression of Sox9, Collagen II and Collagen X mRNA. Consistent with this, generation of mice lacking both genes via crosses to the Dermo1-cre strain resulted in a clear deficit in the ability to contribute to the osteoblast lineage in vivo. These results suggest that Lrp5 and Lrp6 can serve overlapping functions in skeletal development, but that both genes have unique roles within the osteoblast. In addition, we have also begun to assess the potential role of Wnt ligands produced by the mature osteoblast in regulating bone development and acquisition by creating mice carrying an OC-cre-mediated deletion of Wntless, a gene required for the normal secretion of Wnt ligands. Mice carrying this deletion display severely reduced bone volume similar in magnitude to mice missing both Lrp5 and Lrp6. We are carrying out more detailed characterization of these mice and assessing the cellular and molecular mechanisms underlying the observed decrease in bone mass. This article is part of a Special Issue entitled ECTS 2011. Disclosure of interest: None declared. doi:10.1016/j.bone.2011.03.043
IS40 The conundrum of Wnt/Beta-catenin signaling in bone M. Kneissel Musculoskeletal Disease Area, Novartis Institutes, Novartis Pharma AG, Basel, Switzerland Wnt/beta-catenin signaling is initiated by binding of Wnt ligands to the LRP5/6 Frizzled receptor complex. This results in inactivation of the beta-catenin destruction complex thus relieving the central signaling mediator beta-catenin from its
constitutive proteosomal degradation. Beta-catenin subsequently accumulates in the cytoplasm and translocates into the nucleus where it associates with members of the TCF/Lef family of transcription factors to control target gene transcription. Initial interest in the pathway in the context of bone homeostasis was sparked by the discovery that loss-of-function mutations in LRP5 induce osteoporosis in humans due to decreased bone formation, a phenotype which was recapitulated in Lrp5 knockout (KO) mice. Conversely gain-of-function mutations were found to result in high bone mass (HBM) related to increased bone formation, a phenotype which was also described in mice expressing HBM mutant LRP5 in bone. Since then numerous studies, using frequently mouse genetic approaches, have implicated other pathway members in bone mass regulation related to varying impact on bone formation and resorption. For example Lrp6 hypomorphic mice had reduced bone mass due to increased bone resorption. The secreted pathway inhibitors Sfrp1, Dickkopf-1 and sclerostin primarily control bone formation. Yet gene dosage reduction or inhibition of these antagonists was found to also suppress bone resorption. Inhibition of GSK3beta, a component of the beta-catenin degradation complex, resulted in increased bone formation. Absence of Apc - another member of the complex – in osteoblasts and -cytes impacted both bone formation and resorption. Finally deletion of beta-catenin from the osteoblast or the osteocyte stage onwards resulted in increased bone resorption. As beta-catenin has additional roles beyond Wnt signaling mediation, it cannot be ruled out that also other pathways might have been affected in osteoblast / osteocyte beta-catenin KO mice. Yet consistently Tcf deletion also impacted bone resorption. These and other findings are indicative of the importance of the Wnt/beta-catenin pathway for bone homeostasis. However they also underscore the fact that we cannot easily reconcile the varying impact of different pathway components on bone formation and/or resorption. Indeed recent evidence suggested that Lrp5 does not act as a Wnt co-receptor in bone but rather controls bone formation by suppressing serotonin synthesis in the duodenum via regulation of the rate-limiting enzyme Tph1. Accordingly Tph1 loss-of-function has been described to increase bone formation and mass in mice, as did pharmacological Tph1 inhibition. However we and others cannot recapitulate these findings to date, while others have described decreased bone resorption in Tph1 KO mice, overall demonstrating the need for further elucidation of the relevance of bone- versus gutexpressed Lrp5 for bone mass regulation. An alternative explanation for the diverse bone formation and resorption phenotypes could be the implication of the pathway in osteocyte function, as osteocytes are emerging as key regulators of both osteoclastic bone resorption and osteoblastic bone formation. This article is part of a Special Issue entitled ECTS 2011. Disclosure of interest: M. Kneissel Employee of Novartis Pharma AG.
doi:10.1016/j.bone.2011.03.044
S59
clinical care of such children, including those receiving glucocorticoids (GC), has taught us vertebral fractures (VF) are a potentially debilitating manifestation of osteoporosis. Using the Genant semi-quantitative method, we have shown in a pan-Canadian study that 29/186 children (16%) with newly diagnosed leukemia manifested 75 VFs, with reductions in spine areal BMD and back pain associated with an increased odds for prevalent VF. At 12 months following chemotherapy initiation, the incident (i.e. new) VF rate in children with leukemia was also 16% (25/155 children), with prevalent VFs at baseline associated with an increased odds for incident spine fragility. Among children with rheumatic disorders, 9/134 children (7%) manifested 13 VFs within 30 days of GC initiation, and back pain was a highly associated clinical feature. At 12 months post-GC initiation in children with rheumatic disorders, 7 patients (6%) had 12 incident VFs; only 2 of these children reported back pain. Patients with rheumatic disorders and incident VFs had greater decreases in spine areal BMD Z-scores in the first 6 months of GC therapy and the cumulative GC dose was almost double compared to those without spine fragility. Spine fractures for both disease groups were most frequent in the mid-thoracic region and at the thoracolumbar junction; this observation was consistent across mild, moderate and severe VFs. In contrast to prevalent VFs at baseline where back pain was a highly associated clinical factor for both disease groups, at 12 months back pain was not a strong a discriminating feature for those with incident VFs compared to those without. These prospectively acquired data provide insight into the prevalence and incidence of VFs among children with serious illnesses, and highlight the associated clinical features. This article is part of a Special Issue entitled ECTS 2011. Disclosure of interest: L. Ward Grant / Research Support from the Canadian Institutes of Health Research.
phenotype. One of the most important but still poorly understood phenotypic changes associated with ageing is frailty. As expected, frailty and its constituent criteria (slowness, sarcopenia, exhaustion, low activity and weakness) increased in ageing European men. Bone density and turnover (as determined respectively by calcaneal ultrasound and serum CTX and PIP) respectively either showed only a minor decrease or no significant agerelated change in this male cohort. Serum total testosterone (T), insulin-like growth factor I (IGF-I) and dihydroepiandrostenedionesulfate (DHEAS) (respectively −2.1%, 0.5%and 3.6% /year) all decreased in ageing European men. Estradiol (E2) and 25 hydroxyvitamin D (25 D) did not change significantly during aging whereas sex hormone binding globulin (SHBG), luteinising hormone (LH), and follicle-stimulating hormone (FSH)increased with age (+ 1.3, 1 and 0.5% /year). The age-related decrease of free T was therefore greater (−3.2% per year) than total T. Free testosterone concentrations as well as levels of other anabolic hormones such as IGF-I, DHEAS, 25 D but not E2 were independently and inversely related to frailty in European men. Frailty and its constituent sarcopenia was also more prevalent in European men with primary as well as secondary hypogonadism even after correction for body size. Non – endocrine risk factors (inactivity and smoking) were however, as expected, also strong predictors of frailty. On the other hand, E2, SHBG, 25 D, IGF-I, IGFBP-3 but not free nor total T were independently and significantly associated with bone density. Genetically determined variations in E2 rather than T as well as nonendocrine risk factors (inactivity, smoking and excess alcohol) also determined bone density in these men. In conclusion, the male ageing phenotype in EMAS is associated with both endocrine and non-endocrine factors. Different hormones however appear to determine respectively frailty and bone health in ageing European men. This article is part of a Special Issue entitled ECTS 2011. Disclosure of interest: None declared.
doi:10.1016/j.bone.2011.03.039
doi:10.1016/j.bone.2011.03.041
IS36 Guidelines for management of male osteoporosis R. Eastell NIHR Bone Biomedical Research Unit, University of Sheffield, Sheffield, UK
IS38 LRP5 functions in bone to produce high bone mass phenotypes M.L. Warman Orthopaedic Research Laboratories, Children's Hospital Boston, and Department of Genetics, Harvard Medical School, Boston, MA, USA
The Endocrine Society has established a working group to produce guidelines (in conjunction with other professional societies such as ECTS) on male osteoporosis. Until now, approaches to diagnosis having been focused largely on women, and so there is a need for male-specific guidelines. Osteoporosis in men leads to significant morbidity and mortality. It is estimated that one in five men over the age of 50 will have a fracture as the result of osteoporosis. The working group aims to identify strategies for the diagnosis and the clinical management of osteoporosis in men. Firstly, it is important to determine which men should be referred for bone density testing using dual-energy X-ray absorptiometry (DXA). This approach will be based on the presence of clinical risk factors such as older age, low body weight, prior fracture as an adult and smoking. ‘Secondly, it is important to decide whether the bone density T-score (the number of standard deviation units from the young normal mean) should be calculated using a male or female reference interval. Thirdly, it is important to address the question of which men should be referred for further laboratory testing for secondary osteoporosis, and which tests should be recommended. The guidelines will include nutritional advice to identify for example, adequate intakes of calcium and vitamin D, and recommendations about the avoidance of adverse lifestyle habits such as smoking and excessive alcohol. Recommendations on the use of testosterone therapy will be considered in the light of recent guidance from the Endocrine Society. This states that unless contraindicated, testosterone replacement should be considered for men with organic hypogonadism and symptomatic patients who have serum testosterone levels <200 ng/dl (<7 nmol/L). The indications for pharmacologic treatment of osteoporosis in men age ≥50 will be considered in relation to prior fracture history (such as spine or hip fractures), low bone density (such as T-scores of −2.5 or below) and the presence of significant clinical risk factors. The working group will provide advice on whether to monitor treatment in men and on the most appropriate approach to monitoring treatment effects. This article is part of a Special Issue entitled ECTS 2011. Disclosure of interest: R. Eastell Grant / Research Support from Warner Chilcott, Lilly, Amgen, AstraZeneca, Consultant for Novartis, Amgen, Ono. doi:10.1016/j.bone.2011.03.040
IS37 EU male ageing study D. Vanderschueren Department of Endocrinology, Catholic University Leuven, Leuven, Belgium The European Male Ageing study (EMAS) is a prospective observational cohort study. EMAS evaluated a randomized population sample of 3369 men between ages 40 and 79 at baseline (2003–2005) and follow-up (2007–2009) using identical instruments in 8 different European centres as well as central laboratory analysis of a fasting blood sample. A major aim is to describe and define endocrine determinants of the male ageing
Mutations in low-density lipoprotein receptor-related protein 5 (LRP5) have been associated with heritable disorders of bone mass. Loss-of-function mutations in LRP5 cause the autosomal recessive Osteoporosis-Pseudoglioma Syndrome (OPPG), and several families segregating autosomal dominant high bone mass (HBM) phenotypes have heterozygous missense mutations within the LRP5 receptor's amino-terminal, six-bladed propeller domain. We postulated that LRP5 functions as a co-receptor in the canonical Wnt signaling cascade, based upon the high sequence conservation between LRP5 and the Drosophila Wnt co-receptor Arrow. Consistent with this inferred role we found that, when expressed in cultured cells, wild-type LRP5 was able to transduce Wnt signal. We also found that HBM-causing alleles also efficiently transduced Wnt signal and that OPPGcausing alleles did not. Finally, we and other investigators found that HBM-causing alleles were less susceptible to inhibition by endogenous Wnt inhibitors, such as SOST and DKK1. However, definitive in vivo evidence that LRP5 functions in the Wnt signaling pathway and that it acts locally in bone have not been reported. I will present experiments performed in collaboration with Dr. Alex Robling at Indiana University, which suggest that HBM-causing alleles function in bone cells to increase bone mass. We generated mice in which we “knocked” the HBM-causing mutation directly into the endogenous Lrp5 locus. This approach is different from and, in our opinion, superior to previously characterized mouse models with HBM-causing alleles. We made the HBMcausing knockin allele “conditional,” in that it requires Cre-mediated recombination to become normally expressed. This enabled us to assess the role of the HBM-causing allele in specific cell types and tissues. We found no evidence that HBM-causing alleles increase bone mass by functioning outside of the skeleton. Instead, our data suggest that LRP5 HBMcausing alleles act locally in bone, most likely via a mechanism that reduces SOST-mediated inhibition of Wnt signaling in osteocytes. This article is part of a Special Issue entitled ECTS 2011. Disclosure of interest: None declared. doi:10.1016/j.bone.2011.03.042
IS39 Evaluating the role(s) of Lrp5/6 and WNT ligands in regulating bone development and acquisition B.O. Williams Center for Skeletal Disease Research, Van Andel Research Institute, Grand Rapids, MI, USA To define the individual contributions of Lrp5 and Lrp6 in skeletal development, we used Cre-LoxP recombination to create mice lacking each co-receptor selectively in
S60
Abstracts / Bone 48 (2011) S50–S60
osteoblasts. Mice lacking Lrp5 (ΔLrp5) or Lrp6 (ΔLrp6) in mature osteoblasts were generated by crossing Lrp6flox and Lrp6flox mice with mice expressing cre-recombinase under the control of the human osteocalcin promoter (OC-Cre). DEXA indicated reduced BMD at 3 months in both ΔLrp6 and ΔLrp5 mice. Additionally, assessment of femoral bone morphology by microCT revealed significantly lower bone volume with decreased trabecular numbers in 7-week old ΔLrp6 mice when compared to controls. ΔLrp5 mice also displayed a reduction in trabecular number and increased trabecular spacing within the femur. Mice deficient for both Lrp5 and Lrp6 had severely reduced trabecular bone volume similar in magnitude to that observed previously in mice lacking β-catenin (J Biol Chem 2005 280;21162–8). Histomorphometric analysis revealed a dramatically decreased number of osteoblasts in the ΔLrp5; ΔLRP6 mice. In vitro deletion of Lrp6 impaired the differentiation of primary osteoblasts while Lrp5-deficient osteoblasts differentiated normally. Simultaneous disruption of both co-receptors in vitro altered fate-specification of calvarial cells which showed abundant alcian blue staining and increased expression of Sox9, Collagen II and Collagen X mRNA. Consistent with this, generation of mice lacking both genes via crosses to the Dermo1-cre strain resulted in a clear deficit in the ability to contribute to the osteoblast lineage in vivo. These results suggest that Lrp5 and Lrp6 can serve overlapping functions in skeletal development, but that both genes have unique roles within the osteoblast. In addition, we have also begun to assess the potential role of Wnt ligands produced by the mature osteoblast in regulating bone development and acquisition by creating mice carrying an OC-cre-mediated deletion of Wntless, a gene required for the normal secretion of Wnt ligands. Mice carrying this deletion display severely reduced bone volume similar in magnitude to mice missing both Lrp5 and Lrp6. We are carrying out more detailed characterization of these mice and assessing the cellular and molecular mechanisms underlying the observed decrease in bone mass. This article is part of a Special Issue entitled ECTS 2011. Disclosure of interest: None declared. doi:10.1016/j.bone.2011.03.043
IS40 The conundrum of Wnt/Beta-catenin signaling in bone M. Kneissel Musculoskeletal Disease Area, Novartis Institutes, Novartis Pharma AG, Basel, Switzerland Wnt/beta-catenin signaling is initiated by binding of Wnt ligands to the LRP5/6 Frizzled receptor complex. This results in inactivation of the beta-catenin destruction complex thus relieving the central signaling mediator beta-catenin from its
constitutive proteosomal degradation. Beta-catenin subsequently accumulates in the cytoplasm and translocates into the nucleus where it associates with members of the TCF/Lef family of transcription factors to control target gene transcription. Initial interest in the pathway in the context of bone homeostasis was sparked by the discovery that loss-of-function mutations in LRP5 induce osteoporosis in humans due to decreased bone formation, a phenotype which was recapitulated in Lrp5 knockout (KO) mice. Conversely gain-of-function mutations were found to result in high bone mass (HBM) related to increased bone formation, a phenotype which was also described in mice expressing HBM mutant LRP5 in bone. Since then numerous studies, using frequently mouse genetic approaches, have implicated other pathway members in bone mass regulation related to varying impact on bone formation and resorption. For example Lrp6 hypomorphic mice had reduced bone mass due to increased bone resorption. The secreted pathway inhibitors Sfrp1, Dickkopf-1 and sclerostin primarily control bone formation. Yet gene dosage reduction or inhibition of these antagonists was found to also suppress bone resorption. Inhibition of GSK3beta, a component of the beta-catenin degradation complex, resulted in increased bone formation. Absence of Apc - another member of the complex – in osteoblasts and -cytes impacted both bone formation and resorption. Finally deletion of beta-catenin from the osteoblast or the osteocyte stage onwards resulted in increased bone resorption. As beta-catenin has additional roles beyond Wnt signaling mediation, it cannot be ruled out that also other pathways might have been affected in osteoblast / osteocyte beta-catenin KO mice. Yet consistently Tcf deletion also impacted bone resorption. These and other findings are indicative of the importance of the Wnt/beta-catenin pathway for bone homeostasis. However they also underscore the fact that we cannot easily reconcile the varying impact of different pathway components on bone formation and/or resorption. Indeed recent evidence suggested that Lrp5 does not act as a Wnt co-receptor in bone but rather controls bone formation by suppressing serotonin synthesis in the duodenum via regulation of the rate-limiting enzyme Tph1. Accordingly Tph1 loss-of-function has been described to increase bone formation and mass in mice, as did pharmacological Tph1 inhibition. However we and others cannot recapitulate these findings to date, while others have described decreased bone resorption in Tph1 KO mice, overall demonstrating the need for further elucidation of the relevance of bone- versus gutexpressed Lrp5 for bone mass regulation. An alternative explanation for the diverse bone formation and resorption phenotypes could be the implication of the pathway in osteocyte function, as osteocytes are emerging as key regulators of both osteoclastic bone resorption and osteoblastic bone formation. This article is part of a Special Issue entitled ECTS 2011. Disclosure of interest: M. Kneissel Employee of Novartis Pharma AG.
doi:10.1016/j.bone.2011.03.044