Lrp4, a Novel Receptor for Dickkopf 1 and Sclerostin,Is Expressed by Osteoblasts and Regulates Bone Growth and Turnover In Vivo

Lrp4 is a multifunctional member of the low density lipoprotein-receptor gene family and a modulator of extracellular cell signaling pathways in development. For example, Lrp4 binds Wise, a secreted Wnt modulator and BMP antagonist. Lrp4 shares structural elements within the extracellular ligand binding domain with Lrp5 and Lrp6, two established Wnt co-receptors with important roles in osteogenesis. Sclerostin is a potent osteocyte secreted inhibitor of bone formation that directly binds Lrp5 and Lrp6 and modulates both BMP and Wnt signaling. The anti-osteogenic effect of sclerostin is thought to be mediated mainly by inhibition of Wnt signaling through Lrp5/6 within osteoblasts. Dickkopf1 (Dkk1) is another potent soluble Wnt inhibitor that binds to Lrp5 and Lrp6, can displace Lrp5-bound sclerostin and is itself regulated by BMPs. In a recent genome-wide association study of bone mineral density a significant modifier locus was detected near the SOST gene at 17q21, which encodes sclerostin. In addition, nonsynonymous SNPs in the LRP4 gene were suggestively associated with bone mineral density. Here we show that Lrp4 is expressed in bone and cultured osteoblasts and binds Dkk1 and sclerostin in vitro. MicroCT analysis of Lrp4 deficient mutant mice revealed shortened total femur length, reduced cortical femoral perimeter, and reduced total femur bone mineral content (BMC) and bone mineral density (BMD). Lumbar spine trabecular bone volume per total volume (BV/TV) was significantly reduced in the mutants and the serum and urinary bone turnover markers alkaline phosphatase, osteocalcin and desoxypyridinoline were increased. We conclude that Lrp4 is a novel osteoblast expressed Dkk1 and sclerostin receptor with a physiological role in the regulation of bone growth and turnover, which is likely mediated through its function as an integrator of Wnt and BMP signaling pathways.     

Sost and its paralog Sostdc1 coordinate digit number in a Gli3-dependent manner

WNT signaling is critical in most aspects of skeletal development and homeostasis, and antagonists of WNT signaling are emerging as key regulatory proteins with great promise as therapeutic agents for bone disorders. Here we show that Sost and its paralog Sostdc1 emerged through ancestral genome duplication and their expression patterns have diverged to delineate non-overlapping domains in most organ systems including musculoskeletal, cardiovascular, nervous, digestive, reproductive and respiratory. In the developing limb, Sost and Sostdc1 display dynamic expression patterns with Sost being restricted to the distal ectoderm and Sostdc1 to the proximal ectoderm and the mesenchyme. While Sostdc1^−/− mice lack any obvious limb or skeletal defects, Sost^−/− mice recapitulate the hand defects described for Sclerosteosis patients. However, elevated WNT signaling in Sost^−/−; Sostdc1^−/− mice causes misregulation of SHH signaling, ectopic activation of Sox9 in the digit 1 field and preaxial polydactyly in a Gli1- and Gli3-dependent manner. In addition, we show that the syndactyly documented in Sclerosteosis is present in both Sost^−/− and Sost^−/−; Sostdc1^−/− mice, and is driven by misregulation of Fgf8 in the AER, a region lacking Sost and Sostdc1 expression. This study highlights the complexity of WNT signaling in skeletal biology and disease and emphasizes how redundant mechanism and non-cell autonomous effects can synergize to unveil new intricate phenotypes caused by elevated WNT signaling.     

Prostaglandin E2 signals through PTGER2 to regulate sclerostin expression

The Wnt signaling pathway is a robust regulator of skeletal homeostasis. Gain-of-function mutations promote high bone mass, whereas loss of Lrp5 or Lrp6 co-receptors decrease bone mass. Similarly, mutations in antagonists of Wnt signaling influence skeletal integrity, in an inverse relation to Lrp receptor mutations. Loss of the Wnt antagonist Sclerostin (Sost) produces the generalized skeletal hyperostotic condition of sclerosteosis, which is characterized by increased bone mass and density due to hyperactive osteoblast function. Here we demonstrate that prostaglandin E(2) (PGE(2)), a paracrine factor with pleiotropic effects on osteoblasts and osteoclasts, decreases Sclerostin expression in osteoblastic UMR106.01 cells. Decreased Sost expression correlates with increased expression of Wnt/TCF target genes Axin2 and Tcf3. We also show that the suppressive effect of PGE(2) is mediated through a cyclic AMP/PKA pathway. Furthermore, selective agonists for the PGE(2) receptor EP2 mimic the effect of PGE(2) upon Sost, and siRNA reduction in Ptger2 prevents PGE(2)-induced Sost repression. These results indicate a functional relationship between prostaglandins and the Wnt/β-catenin signaling pathway in bone.     

Sclerostin deficient mice rapidly heal bone defects by activating β-catenin and increasing intramembranous ossification

We investigated the influence of the osteocyte protein, sclerostin, on fracture healing by examining the dynamics and mechanisms of repair of single-cortex, stabilized femoral defects in sclerostin knockout (Sost^−/−; KO) and sclerostin wild-type (Sost^+/+; WT) mice. Fourteen days following generation of bone defects, Sost KO mice had significantly more bone in the healing defect than WT mice. The increase in regenerating bone was due to an increase in the thickness of trabecularized spicules, osteoblast numbers and surfaces within the defect. Enhanced healing of bone defects in Sost KO mice was associated with significantly more activated β-catenin expression than observed in WT mice. The findings were similar to those observed in Axin2^−/− mice, in which β-catenin signaling is known to be enhanced to facilitate bone regeneration. Taken together, these data indicate that enhanced β-catenin signaling is present in Sost^−/− mice that demonstrate accelerated healing of bone defects, suggesting that modulation of β-catenin signaling in bone could be used to promote fracture repair.     

The Extracellular Domain of Lrp5/6 Inhibits Noncanonical Wnt Signaling In Vivo

Lrp5/6 are crucial coreceptors for Wnt/β-catenin signaling, a pathway biochemically distinct from noncanonical Wnt signaling pathways. Here, we examined the possible participation of Lrp5/6 in noncanonical Wnt signaling. We found that Lrp6 physically interacts with Wnt5a, but that this does not lead to phosphorylation of Lrp6 or activation of the Wnt/β-catenin pathway. Overexpression of Lrp6 blocks activation of the Wnt5a downstream target Rac1, and this effect is dependent on intact Lrp6 extracellular domains. These results suggested that the extracellular domain of Lrp6 inhibits noncanonical Wnt signaling in vitro. In vivo, Lrp6−/− mice exhibited exencephaly and a heart phenotype. Surprisingly, these defects were rescued by deletion of Wnt5a, indicating that the phenotypes resulted from noncanonical Wnt gain-of-function. Similarly, Lrp5 and Lrp6 antisense morpholino-treated Xenopus embryos exhibited convergent extension and heart phenotypes that were rescued by knockdown of noncanonical XWnt5a and XWnt11. Thus, we provide evidence that the extracellular domains of Lrp5/6 behave as physiologically relevant inhibitors of noncanonical Wnt signaling during Xenopus and mouse development in vivo.     

Craniofacial malformation in R-spondin2 knockout mice

In vertebrates, craniofacial formation is accomplished by synergistic interaction of many small elements which are generated independently from distinct germ layers. Because of its complexity, the imbalance of one signaling cascade such as Wnt/β-catenin pathway easily leads to craniofacial malformation, which is the most frequent birth defect in humans. To investigate the developmental role of a newly identified activator of Wnt/β-catenin signaling, Rspo2, we generated and characterized Rspo2^−/− mice. We found CLP with mild facial skeletal defects in Rspo2^−/− mice. Additionally, Rspo2^−/− mice also exhibited distal limb loss and lung hypoplasia, and died immediately after birth with respiratory failure. We showed the apparent reduction of Wnt/β-catenin signaling activity at the branchial arch and the apical ectodermal ridge in Rspo2^−/− mice. These findings indicate that Rspo2 regulates midfacial, limb, and lung morphogenesis during development through the Wnt/β-catenin signaling.     

Lrp6 Genotype affects Individual Susceptibility to Nonalcoholic Fatty Liver Disease and Silibinin Therapeutic Response via Wnt/β-catenin-Cyp2e1 Signaling

Background: Nonalcoholic fatty liver disease (NAFLD) is a serious threat to human health worldwide, with a high genetic susceptibility. Rs2302685, a functional germline variant of LRP6, has been recently found to associate with NAFLD risk. This study was aimed to clarify the underlying mechanism associated with rs2302685 risk and its impact on pharmacotherapy in treatment of NAFLD.Methods: Venous blood samples were collected from NAFLD and non-NAFLD patients for SNP genotyping by using mass spectrometry. The Lrp6-floxdel mouse (Lrp6^(+/-)) was generated to model the partial function associated with human rs2302685. The liver injury and therapeutic effects of silibinin were compared between Lrp6^(+/-) and Lrp6^(+/+) mice received a methionine-choline deficient (MCD) diet or normal diet. The effect of Lrp6 functional alteration on Wnt/β-catenin-Cyp2e1 signaling activities was evaluated by a series of cellular and molecular assays.Results: The T allele of LRP6 rs2302685 was confirmed to associate with a higher risk of NAFLD in human subjects. The carriers of rs2302685 had reduced level of AST and ALT as compared with the noncarriers. The Lrp6^(+/-) mice exhibited a less severe liver injury induced by MCD but a reduced response to the treatment of silibinin in comparison to the Lrp6^(+/+) mice, suggesting Lrp6 as a target of silibinin. Wnt/β-catenin-Cyp2e1 signaling together with ROS generation could be exacerbated by the overexpression of Lrp6, while decreased in response to Lrp6 siRNA or silibinin treatment under NAFLD modeling.Conclusions: The Lrp6 function affects individual susceptibility to NAFLD and the therapeutic effect of silibinin through the Wnt/β-catenin-Cyp2e1 signaling pathway. The present work has provided an underlying mechanism for human individual susceptibility to NAFLD associated with Lrp6 polymorphisms as well as a rationale for the effective use of silibinin in NAFLD patients.     

The LIM protein LIMD1 influences osteoblast differentiation and function

The balance between bone resorption and bone formation involves the coordinated activities of osteoblasts and osteoclasts. Communication between these two cell types is essential for maintenance of normal bone homeostasis; however, the mechanisms regulating this cross talk are not completely understood. Many factors that mediate differentiation and function of both osteoblasts and osteoclasts have been identified. The LIM protein Limd1 has been implicated in the regulation of stress osteoclastogenesis through an interaction with the p62/sequestosome protein. Here we show that Limd1 also influences osteoblast progenitor numbers, differentiation, and function. Limd1^−/− calvarial osteoblasts display increased mineralization and accelerated differentiation. While no significant differences in osteoblast number or function were detected in vivo, bone marrow stromal cells isolated from Limd1^−/− mice contain significantly more osteoblast progenitors compared to wild type controls when cultured ex vivo. Furthermore, we observed a significant increase in nuclear β-catenin staining in differentiating Limd1^−/− calvarial osteoblasts suggesting that Limd1 is a negative regulator of canonical Wnt signaling in osteoblasts. These results demonstrate that Limd1 influences not only stress osteoclastogenesis but also osteoblast function and osteoblast progenitor commitment. Together, these data identify Limd1 as a novel regulator of both bone osetoclast and bone osteoblast development and function.     

Mouse R-spondin2 is required for apical ectodermal ridge maintenance in the hindlimb

The R-spondin (Rspo) family of proteins consists of secreted cysteine-rich proteins that can activate β-catenin signaling via the Frizzled/LRP5/6 receptor complex. Here, we report that targeted inactivation of the mouse Rspo2 gene causes developmental limb defects, especially in the hindlimb. Although the initiation of the expression of apical ectodermal ridge (AER)-specific genes, including fibroblast growth factor 8 (FGF8) and FGF4 occurred normally, the maintenance of these marker expressions was significantly defective in the hindlimb of Rspo2(−/−) mice. Consistent with the ligand role of R-spondins in the Wnt/β-catenin signaling pathway, expression of Axin2 and Sp8, targets for β-catenin signaling, within AER was greatly reduced in Rspo2(−/−) embryos. Furthermore, sonic hedgehog (Shh) signaling within the hindlimbs of Rspo2(−/−) mice was also significantly decreased. Rspo2 is expressed in the AER of all limb buds, however the stunted phenotype is significantly more severe in the hindlimbs than the forelimbs and strongly biased to the left side. Our findings strongly suggest that Rspo2 expression in the AER is required for AER maintenance likely by regulating Wnt/β-catenin signaling.     

Lrp5 functions in bone to regulate bone mass

The human skeleton is affected by mutations in low-density lipoprotein receptor-related protein 5 (LRP5). To understand how LRP5 influences bone properties, we generated mice with osteocyte-specific expression of inducible Lrp5 mutations that cause high and low bone mass phenotypes in humans. We found that bone properties in these mice were comparable to bone properties in mice with inherited mutations. We also induced an Lrp5 mutation in cells that form the appendicular skeleton but not in cells that form the axial skeleton; we observed that bone properties were altered in the limb but not in the spine. These data indicate that Lrp5 signaling functions locally, and they suggest that increasing LRP5 signaling in mature bone cells may be a strategy for treating human disorders associated with low bone mass, such as osteoporosis.     

Wise Regulates Bone Deposition through Genetic Interactions with Lrp5

In this study using genetic approaches in mouse we demonstrate that the secreted protein Wise plays essential roles in regulating early bone formation through its ability to modulate Wnt signaling via interactions with the Lrp5 co-receptor. In Wise^−/− mutant mice we find an increase in the rate of osteoblast proliferation and a transient increase in bone mineral density. This change in proliferation is dependent upon Lrp5, as Wise;Lrp5 double mutants have normal bone mass. This suggests that Wise serves as a negative modulator of Wnt signaling in active osteoblasts. Wise and the closely related protein Sclerostin (Sost) are expressed in osteoblast cells during temporally distinct early and late phases in a manner consistent with the temporal onset of their respective increased bone density phenotypes. These data suggest that Wise and Sost may have common roles in regulating bone development through their ability to control the balance of Wnt signaling. We find that Wise is also required to potentiate proliferation in chondrocytes, serving as a potential positive modulator of Wnt activity. Our analyses demonstrate that Wise plays a key role in processes that control the number of osteoblasts and chondrocytes during bone homeostasis and provide important insight into mechanisms regulating the Wnt pathway during skeletal development.     

Mutations in NOTCH1 Cause Adams-Oliver Syndrome

Notch signaling determines and reinforces cell fate in bilaterally symmetric multicellular eukaryotes. Despite the involvement of Notch in many key developmental systems, human mutations in Notch signaling components have mainly been described in disorders with vascular and bone effects. Here, we report five heterozygous NOTCH1 variants in unrelated individuals with Adams-Oliver syndrome (AOS), a rare disease with major features of aplasia cutis of the scalp and terminal transverse limb defects. Using whole-genome sequencing in a cohort of 11 families lacking mutations in the four genes with known roles in AOS pathology (ARHGAP31, RBPJ, DOCK6, and EOGT), we found a heterozygous de novo 85 kb deletion spanning the NOTCH1 5′ region and three coding variants (c.1285T>C [p.Cys429Arg], c.4487G>A [p.Cys1496Tyr], and c.5965G>A [p.Asp1989Asn]), two of which are de novo, in four unrelated probands. In a fifth family, we identified a heterozygous canonical splice-site variant (c.743−1 G>T) in an affected father and daughter. These variants were not present in 5,077 in-house control genomes or in public databases. In keeping with the prominent developmental role described for Notch1 in mouse vasculature, we observed cardiac and multiple vascular defects in four of the five families. We propose that the limb and scalp defects might also be due to a vasculopathy in NOTCH1-related AOS. Our results suggest that mutations in NOTCH1 are the most common cause of AOS and add to a growing list of human diseases that have a vascular and/or bony component and are caused by alterations in the Notch signaling pathway.     

The Wnt Co-Receptor Lrp6 Is Required for Normal Mouse Mammary Gland Development

Canonical Wnt signals are transduced through a Frizzled receptor and either the LRP5 or LRP6 co-receptor; such signals play central roles during development and in disease. We have previously shown that Lrp5 is required for ductal stem cell activity and that loss of Lrp5 delays normal mammary development and Wnt1-induced tumorigenesis. Here we show that canonical Wnt signals through the Lrp6 co-receptor are also required for normal mouse mammary gland development. Loss of Lrp6 compromises Wnt/β-catenin signaling and interferes with mammary placode, fat pad, and branching development during embryogenesis. Heterozygosity for an inactivating mutation in Lrp6 is associated with a reduced number of terminal end buds and branches during postnatal development. While Lrp6 is expressed in both the basal and luminal mammary epithelium during embryogenesis, Lrp6 expression later becomes restricted to cells residing in the basal epithelial layer. Interestingly, these cells also express mammary stem cell markers. In humans, increased Lrp6 expression is associated with basal-like breast cancer. Taken together, our results suggest both overlapping and specific functions for Lrp5 and Lrp6 in the mammary gland.     

Fibroblast growth factor receptor 1 regulates the differentiation and activation of osteoclasts through Erk1/2 pathway

To elucidate the direct role and mechanism of FGFR1 signaling in the differentiation and activation of osteoclasts, we conditionally inactivated FGFR1 in bone marrow monocytes and mature osteoclasts of mice. Mice deficient in FGFR1 (Fgfr1^−/−) exhibited misregulated bone remodeling with reduced osteoclast number and impaired osteoclast function. In vitro assay demonstrated that the number of tartrate-resistant acid phosphatase (TRAP) positive osteoclasts derived from bone marrow monocytes of Fgfr1^−/− mice was significantly diminished. The bone resorption activity of mature osteoclasts derived from Fgfr1^−/− mice was also suppressed. Further analysis showed that the osteoclasts with FGFR1 deficiency exhibited downregulated expression of genes related to osteoclastic activity including TRAP and MMP-9. The phosphorylation of Erk1/2 mitogen-activated protein (MAP) kinase was also decreased. Our results suggest that FGFR1 is indispensable for complete differentiation and activation of osteoclasts in mice.     

Ectodomains of the LDL Receptor-Related Proteins LRP1b and LRP4 Have Anchorage Independent Functions In Vivo

Background

The low-density lipoprotein (LDL) receptor gene family is a highly conserved group of membrane receptors with diverse functions in developmental processes, lipoprotein trafficking, and cell signaling. The low-density lipoprotein (LDL) receptor-related protein 1b (LRP1B) was reported to be deleted in several types of human malignancies, including non-small cell lung cancer. Our group has previously reported that a distal extracellular truncation of murine Lrp1b that is predicted to secrete the entire intact extracellular domain (ECD) is fully viable with no apparent phenotype.

Methods and Principal Findings

Here, we have used a gene targeting approach to create two mouse lines carrying internally rearranged exons of Lrp1b that are predicted to truncate the protein closer to the N-terminus and to prevent normal trafficking through the secretary pathway. Both mutations result in early embryonic lethality, but, as expected from the restricted expression pattern of LRP1b in vivo, loss of Lrp1b does not cause cellular lethality as homozygous Lrp1b-deficient blastocysts can be propagated normally in culture. This is similar to findings for another LDL receptor family member, Lrp4. We provide in vitro evidence that Lrp4 undergoes regulated intramembraneous processing through metalloproteases and γ-secretase cleavage. We further demonstrate negative regulation of the Wnt signaling pathway by the soluble extracellular domain.

Conclusions and Significance

Our results underline a crucial role for Lrp1b in development. The expression in mice of truncated alleles of Lrp1b and Lrp4 with deletions of the transmembrane and intracellular domains leads to release of the extracellular domain into the extracellular space, which is sufficient to confer viability. In contrast, null mutations are embryonically (Lrp1b) or perinatally (Lrp4) lethal. These findings suggest that the extracellular domains of both proteins may function as a scavenger for signaling ligands or signal modulators in the extracellular space, thereby preserving signaling thresholds that are critical for embryonic development, as well as for the clear, but poorly understood role of LRP1b in cancer.