Abstract A role for Low-density lipoprotein related receptor 5 (LRP5) in human bone was first established by the identification of genetic alterations that led to dramatic changes in human bone mass. ...Shortly thereafter, mutations that altered the function of the Sclerostin ( SOST ) gene were also associated with altered human bone mass. Subsequent studies of LRP5 and Sclerostin have provided important insights into the mechanisms by which these proteins regulate skeletal homeostasis. Sclerostin normally binds to LRP5 and the related LRP6 protein and prevents their activation by Wnt ligands. The interaction of Sclerostin with LRP5 or LRP6 is facilitated by the LRP4 protein. Loss of LRP5 leads to defective osteoblast function and low bone mass, while loss of SOST or mutations in LRP5 which produce a protein that can no longer be bound by SOST result in high bone mass. Insights gained from the use of genetically engineered mouse models are presented as well as a brief summary of the status of antibodies in clinical trials that block the function of SOST as a mechanism to increase bone mass.
Wnt/β-catenin signaling supports intestinal homeostasis by regulating proliferation in the crypt. Multiple Wnts are expressed in Paneth cells as well as other intestinal epithelial and stromal cells. ...Ex vivo, Wnts secreted by Paneth cells can support intestinal stem cells when Wnt signaling is enhanced with supplemental R-Spondin 1 (RSPO1). However, in vivo, the source of Wnts in the stem cell niche is less clear. Genetic ablation of Porcn, an endoplasmic reticulum resident O-acyltransferase that is essential for the secretion and activity of all vertebrate Wnts, confirmed the role of intestinal epithelial Wnts in ex vivo culture. Unexpectedly, mice lacking epithelial Wnt activity (Porcn(Del)/Villin-Cre mice) had normal intestinal proliferation and differentiation, as well as successful regeneration after radiation injury, indicating that epithelial Wnts are dispensable for these processes. Consistent with a key role for stroma in the crypt niche, intestinal stromal cells endogenously expressing Wnts and Rspo3 support the growth of Porcn(Del) organoids ex vivo without RSPO1 supplementation. Conversely, increasing pharmacologic PORCN inhibition, affecting both stroma and epithelium, reduced Lgr5 intestinal stem cells, inhibited recovery from radiation injury, and at the highest dose fully blocked intestinal proliferation. We conclude that epithelial Wnts are dispensable and that stromal production of Wnts can fully support normal murine intestinal homeostasis.
Abstract Wnt signaling plays key roles in many aspects of development. In this review, we will briefly describe the components of signaling pathways induced by Wnt ligands and then describe the ...current state of research as this applies to aspects of development and disease as it relates to skeletal muscle and bone. We will conclude with a discussion of the parallels and differences in Wnt signaling in these two contexts and how these pathways are being (or could potentially be) targeted for therapeutic treatment of musculoskeletal diseases. This article is part of a Special Issue entitled “Muscle Bone Interactions”.
The identification of the low-density lipoprotein receptor (LDLR) provided a foundation for subsequent studies in lipoprotein metabolism, receptor-mediated endocytosis, and many other fundamental ...biological functions. The importance of the LDLR led to numerous studies that identified homologous molecules and ultimately resulted in the description of the LDL-receptor superfamily, a group of proteins that contain domains also found in the LDLR. Subsequent studies have revealed that members of the LDLR-related protein family play roles in regulating many aspects of signal transduction. This review is focused on the roles of selected members of this protein family in skeletal development and disease. We present background on the identification of this subgroup of receptors, discuss the phenotypes associated with alterations in their function in human patients and mouse models, and describe the current efforts to therapeutically target these proteins to treat human skeletal disease.
Osteocalcin (OCN), the most abundant noncollagenous protein in the bone matrix, is reported to be a bone-derived endocrine hormone with wide-ranging effects on many aspects of physiology, including ...glucose metabolism and male fertility. Many of these observations were made using an OCN-deficient mouse allele (Osc-) in which the 2 OCN-encoding genes in mice, Bglap and Bglap2, were deleted in ES cells by homologous recombination. Here we describe mice with a new Bglap and Bglap2 double-knockout (dko) allele (Bglap/2p.Pro25fs17Ter) that was generated by CRISPR/Cas9-mediated gene editing. Mice homozygous for this new allele do not express full-length Bglap or Bglap2 mRNA and have no immunodetectable OCN in their serum. FTIR imaging of cortical bone in these homozygous knockout animals finds alterations in the collagen maturity and carbonate to phosphate ratio in the cortical bone, compared with wild-type littermates. However, μCT and 3-point bending tests do not find differences from wild-type littermates with respect to bone mass and strength. In contrast to the previously reported OCN-deficient mice with the Osc-allele, serum glucose levels and male fertility in the OCN-deficient mice with the Bglap/2pPro25fs17Ter allele did not have significant differences from wild-type littermates. We cannot explain the absence of endocrine effects in mice with this new knockout allele. Possible explanations include the effects of each mutated allele on the transcription of neighboring genes, or differences in genetic background and environment. So that our findings can be confirmed and extended by other interested investigators, we are donating this new Bglap and Bglap2 double-knockout strain to the Jackson Laboratories for academic distribution.
Abstract Bone has long been known to be responsive to mechanical loading. For at least 25 years it has been known that osteocytes sense mechanical load, and because of their response to mechanical ...loading, osteocytes are believed to be the mechanosensory cell. The Wnt/β-catenin signaling pathway has been shown to be crucial in bone development. Mutations in LRP5 and SOST , which cause high bone mass, have increased interest in the Wnt pathway as a potential target for osteoporosis therapy and have helped link Wnt/β-catenin signaling to bone's response to mechanical loading. Because of its specificity to osteocytes, the Wnt inhibitor sclerostin is a target for anabolic bone therapies. The response of bone to mechanical loading is critically regulated by osteocytes secreting sclerostin, which binds to Lrp5. This article is part of a Special Issue entitled "The Osteocyte".
Liver‐specific β‐catenin knockout (β‐Catenin‐LKO) mice have revealed an essential role of β‐catenin in metabolic zonation where it regulates pericentral gene expression and in initiating liver ...regeneration (LR) after partial hepatectomy (PH), by regulating expression of Cyclin‐D1. However, what regulates β‐catenin activity in these events remains an enigma. Here we investigate to what extent β‐catenin activation is Wnt‐signaling‐dependent and the potential cell source of Wnts. We studied liver‐specific Lrp5/6 KO (Lrp‐LKO) mice where Wnt‐signaling was abolished in hepatocytes while the β‐catenin gene remained intact. Intriguingly, like β‐catenin‐LKO mice, Lrp‐LKO exhibited a defect in metabolic zonation observed as a lack of glutamine synthetase (GS), Cyp1a2, and Cyp2e1. Lrp‐LKO also displayed a significant delay in initiation of LR due to the absence of β‐catenin‐TCF4 association and lack of Cyclin‐D1. To address the source of Wnt proteins in liver, we investigated conditional Wntless (Wls) KO mice, which lacked the ability to secrete Wnts from either liver epithelial cells (Wls‐LKO), or macrophages including Kupffer cells (Wls‐MKO), or endothelial cells (Wls‐EKO). While Wls‐EKO was embryonic lethal precluding further analysis in adult hepatic homeostasis and growth, Wls‐LKO and Wls‐MKO were viable but did not show any defect in hepatic zonation. Wls‐LKO showed normal initiation of LR; however, Wls‐MKO showed a significant but temporal deficit in LR that was associated with decreased β‐catenin‐TCF4 association and diminished Cyclin‐D1 expression. Conclusion: Wnt‐signaling is the major upstream effector of β‐catenin activity in pericentral hepatocytes and during LR. Hepatocytes, cholangiocytes, or macrophages are not the source of Wnts in regulating hepatic zonation. However, Kupffer cells are a major contributing source of Wnt secretion necessary for β‐catenin activation during LR. (Hepatology 2014;60:964–976)
Wnt signaling has emerged as a central regulator of skeletal modeling and remodeling. Loss‐ or gain‐of‐function mutations in two Wnt co‐receptors, Lrp5 and (more recently) Lrp6, have drawn attention ...to the importance of the Wnt pathway in bone biology. This review summarizes our current understanding of how the Wnt pathway operates on bone and the implications this has for skeletal physiology and drug discovery. Over the past 9 yr, rapid advances have been made in our understanding of the cellular targets for Wnt signaling and of the important regulatory molecules in this metabolic pathway. Both canonical and noncanonical signaling pathways seem to be important for mediating the effects of Wnt in bone. A rapidly expanding catalog of genetically engineered mice has been used to establish the importance of downstream effector molecules (such as β‐catenin) in the Wnt pathway, as well as the critical role of endogenous inhibitors of Wnt signaling (such as Dkk1 and sclerostin) in bone metabolism. Indeed, regulation of sclerostin in osteocytes is emerging as an important final pathway for regulating bone anabolism in response to diverse trophic stimuli, from mechnotransduction to the anabolic actions of PTH. From the outset, it had been assumed that the effects of Wnt signaling in bone were caused by direct actions in osteoblast precursors, osteoblasts, and osteocytes. However, startling recent findings have challenged this view and suggest that a key target, at least in mice, is the duodenal enterochromaffin cell. There, Wnt signaling transduced by Lrp5 regulates serotonin synthesis, which acts in an endocrine fashion to regulate bone cell metabolism. It will take time to reconcile this new information with the considerable body of information we already have regarding the actions of Wnt in bone. The Wnt pathway has rapidly emerged as a therapeutic target for drug discovery. Neutralizing antibodies and small‐molecule inhibitors of endogenous Wnt inhibitors have shown early promise as bone anabolic agents. However, given the central role of the Wnt pathway in regulating growth and development in extraskeletal tissues, as well as our still rudimentary understanding of how this signaling cascade actually affects bone metabolism, considerable work will be needed to ensure the safety of these new therapies.
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