Induction of Lrp5 HBM-causing mutations in Cathepsin-K expressing cells alters bone metabolism

The high-bone-mass phenotype of novel transgenic mice with LRP5 A241T mutation

Gain-of-function mutations in the low-density lipoprotein receptor-related protein 5 (LRP5) can cause high-bone-mass (HBM) phenotype, with 19 identified mutations so far. The A242T mutation in LRP5 has been found in 9 families, making it one of the most prevalent mutations. However, the correlation between the A242T mutation and HBM phenotype remains unverified in animal models. This study aimed to investigate the bone properties in a new transgenic mouse model carrying the LRP5 A241T missense mutation, equivalent to A242T in humans. Heterozygous Lrp5A241T mice were generated using CRISPR/Cas9 genome editing. Body weight increased with age from 4 to 16 weeks, higher in males than females, with no difference between Lrp5A241T mice and wild-type control. Micro-CT showed slightly longer femur and notably elevated trabecular bone mass of the femur and fifth lumbar spine with higher bone mineral density, bone volume fraction, and trabecular thickness in Lrp5A241T mice compared to wild-type mice. Additionally, increased cortical bone thickness and volume of the femur shaft and skull were observed in Lrp5A241T mice. Three-point bending tests of the tibia demonstrated enhanced bone strength properties in Lrp5A241T mice. Histomorphometry confirmed that the A241T mutation increased bone formation without affecting osteoblast number and reduced resorption activities in vivo. In vitro experiments indicated that the LRP5 A241T mutation enhanced osteogenic capacity of osteoblasts with upregulation of the Wnt signaling pathway, with no significant impact on the resorptive activity of osteoclasts. In summary, mice carrying the LRP5 A241T mutation displayed high bone mass and quality due to enhanced bone formation and reduced bone resorption in vivo, potentially mediated by the augmented osteogenic potential of osteoblasts. Continued investigation into the regulatory mechanisms of its bone metabolism and homeostasis may contribute to the advancement of novel therapeutic strategies for bone disorders.

The PDE4 Inhibitors Roflumilast and Rolipram Rescue ADO2 Osteoclast Resorption Dysfunction

Autosomal Dominant Osteopetrosis type II (ADO2) is a rare bone disease of impaired osteoclastic bone resorption caused by heterozygous missense mutations in the chloride channel 7 (CLCN7). Adenylate cyclase, which catalyzes the formation of cAMP, is critical for lysosomal acidification in osteoclasts. We found reduced cAMP levels in ADO2 osteoclasts compared to wild-type (WT) osteoclasts, leading us to examine whether regulating cAMP would improve ADO2 osteoclast activity. Although forskolin, a known activator of adenylate cyclase and cAMP levels, negatively affected osteoclast number, it led to an overall increase in ADO2 and WT osteoclast resorption activity in vitro. Next, we examined cAMP hydrolysis by the phosphodiesterase 4 (PDE4) proteins in ADO2 versus WT osteoclasts. QPCR analysis revealed higher expression of the three major PDE4 subtypes (4a, 4b, 4d) in ADO2 osteoclasts compared in WT, consistent with reduced cAMP levels in ADO2 osteoclasts. In addition, we found that the PDE4 antagonists, rolipram and roflumilast, stimulated ADO2 and WT osteoclast formation in a dose-dependent manner. Importantly, roflumilast and rolipram displayed a concentration-dependent increase in osteoclast resorption activity which was greater in ADO2 than WT osteoclasts. Moreover, treatment with roflumilast rescued cAMP levels in ADO2 OCLs. The key findings from our studies demonstrate that osteoclasts from ADO2 mice exhibit reduced cAMP levels and PDE4 inhibition rescues cAMP levels and ADO2 osteoclast activity dysfunction in vitro. The mechanism of action of PDE4 inhibitors and their ability to reduce the high bone mass of ADO2 mice in vivo are currently under investigation. Importantly, these studies advance the understanding of the mechanisms underlying the ADO2 osteoclast dysfunction which is critical for the development of therapeutic approaches to treat clinically affected ADO2 patients.

Discovery of CTSK+ Periosteal Stem Cells Mediating Bone Repair in Orbital Reconstruction

Purpose

The purpose of this study was to explore the role of cathepsin K positive (CTSK+) periosteal stem cells (PSCs) in orbital bone repair and to clarify the source of endogenous stem cells for orbital bone self-repair.

Methods

Periosteum samples obtained by clinical orbital bone repair surgery were analyzed, after which immunofluorescence and immunohistochemical staining were used to detect the content of bone marrow-derived cells and CTSK+ PSCs in periosteum as well as the mobilization of PSCs. CTSK+ PSCs were characterized by flow cytometry. Transcriptome sequencing was used to compare the transcriptomic characteristics of CTSK+ PSCs and bone marrow mesenchymal stem cells (BMSCs).

Results

The orbital periosteum contained CTSK+CD200+ cell lineage, including CD200+CD105- PSCs and CD200+CD105+ progenitor cells. CTSK and osteocalcin (OCN) colocalized in the inner layer of the orbital periosteum, suggesting the osteogenic differentiation potential of CTSK+ PSCs. CTSK expression was much higher in periosteum after mobilization. Immunofluorescence showed low amounts of scattered CD31+ and CD45+ cells in the orbital periosteum. The stem cell characteristics of CTSK+ PSCs were verified by multidirectional differentiation. Flow cytometry found CD200+CD105- CTSK+ PSCs and CD200variantCD105+ progenitor cells. Transcriptome sequencing of CTSK+ PSCs and BMSCs found 3613 differential genes with significant differences. Gene Ontology (GO) analysis showed the differences between the two types of stem cells, revealing that PSCs were more suitable for intramembranous osteogenesis.

Conclusions

CTSK+ PSCs may be endogenous stem cells for orbital bone repair. They are mobilized after orbital fracture and have unique features suitable for intramembranous osteogenesis, completely different from BMSCs.

Sostdc1 Suppression in the Absence of Sclerostin Potentiates Anabolic Action of Cortical Bone in Mice

The development of Wnt-based osteoanabolic agents has progressed rapidly in recent years, given the potent effects of Wnt modulation on bone homeostasis. Simultaneous pharmacologic inhibition of the Wnt antagonists sclerostin and Dkk1 can be optimized to create potentiated effects in the cancellous bone compartment. We looked for other candidates that might be co-inhibited along with sclerostin to potentiate the effects in the cortical compartment. Sostdc1 (Wise), like sclerostin and Dkk1, also binds and inhibits Lrp5/6 coreceptors to impair canonical Wnt signaling, but Sostdc1 has greater effects in the cortical bone. To test this concept, we deleted Sostdc1 and Sost from mice and measured the skeletal effects in cortical and cancellous compartments individually. Sost deletion alone produced high bone mass in all compartments, whereas Sostdc1 deletion alone had no measurable effects on either envelope. Mice with codeletion of Sostdc1 and Sost had high bone mass and increased cortical properties (bone mass, formation rates, mechanical properties), but only among males. Combined administration of sclerostin antibody and Sostdc1 antibody in wild-type female mice produced potentiation of cortical bone gain despite no effect of Sostdc1 antibody alone. In conclusion, Sostdc1 inhibition/deletion can work in concert with sclerostin deficiency to improve cortical bone properties. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Perspective: The current state of Cre driver mouse lines in skeletal research: Challenges and opportunities

The Cre/Lox system has revolutionized the ability of biomedical researchers to ask very specific questions about the function of individual genes in specific cell types at specific times during development and/or disease progression in a variety of animal models. This is true in the skeletal biology field, and numerous Cre driver lines have been created to foster conditional gene manipulation in specific subpopulations of bone cells. However, as our ability to scrutinize these models increases, an increasing number of issues have been identified with most driver lines. All existing skeletal Cre mouse models exhibit problems in one or more of the following three areas: (1) cell type specificity-avoiding Cre expression in unintended cell types; (2) Cre inducibility-improving the dynamic range for Cre in inducible models (negligible Cre activity before induction and high Cre activity after induction); and (3) Cre toxicity-reducing the unwanted biological effects of Cre (beyond loxP recombination) on cellular processes and tissue health. These issues are hampering progress in understanding the biology of skeletal disease and aging, and consequently, identification of reliable therapeutic opportunities. Skeletal Cre models have not advanced technologically in decades despite the availability of improved tools, including multi-promoter-driven expression of permissive or fragmented recombinases, new dimerization systems, and alternative forms of recombinases and DNA sequence targets. We review the current state of skeletal Cre driver lines, and highlight some of the successes, failures, and opportunities to improve fidelity in the skeleton, based on successes pioneered in other areas of biomedical science.

Strategies of Macrophages to Maintain Bone Homeostasis and Promote Bone Repair: A Narrative Review

Bone homeostasis (a healthy bone mass) is regulated by maintaining a delicate balance between bone resorption and bone formation. The regulation of physiological bone remodeling by a complex system that involves multiple cells in the skeleton is closely related to bone homeostasis. Loss of bone mass or repair of bone is always accompanied by changes in bone homeostasis. However, due to the complexity of bone homeostasis, we are currently unable to identify all the mechanisms that affect bone homeostasis. To date, bone macrophages have been considered a third cellular component in addition to osteogenic spectrum cells and osteoclasts. As confirmed by co-culture models or in vivo experiments, polarized or unpolarized macrophages interact with multiple components within the bone to ensure bone homeostasis. Different macrophage phenotypes are prone to resorption and formation of bone differently. This review comprehensively summarizes the mechanisms by which macrophages regulate bone homeostasis and concludes that macrophages can control bone homeostasis from osteoclasts, mesenchymal cells, osteoblasts, osteocytes, and the blood/vasculature system. The elaboration of these mechanisms in this narrative review facilitates the development of macrophage-based strategies for the treatment of bone metabolic diseases and bone defects.

Sex-specific differences in direct osteoclastic versus indirect osteoblastic effects underlay the low bone mass of Pannexin1 deletion in TRAP-expressing cells in mice

Pannexin1 (Panx1) is a hemichannel-forming protein that participates in the communication of cells with the extracellular space. To characterize the role of osteoclastic Panx1 on bone, Panx1fl/fl;TRAP-Cre (Panx1ΔOc) mice were generated, and compared to Panx1fl/fl littermates at 6 weeks of age. Total and femoral BMD was ~20% lower in females and males whereas spinal BMD was lower only in female Panx1ΔOc mice. μCT analyses showed that cortical bone of the femoral mid-diaphysis was not altered in Panx1ΔOc mice. In contrast, cancellous bone in the distal femur and lumbar vertebra was significantly decreased in both female and male Panx1ΔOc mice compared to Panx1fl/fl controls and was associated with higher osteoclast activity in female Panx1ΔOc mice, with no changes in the males. On the other hand, vertebral bone formation was decreased for both sexes, resulting from lower mineral apposition rate in the females and lower mineralizing surface in the males. Consistent with an osteoclastic effect in female Panx1ΔOc mice, osteoclast differentiation with RANKL/M-CSF and osteoclast bone resorbing activity in vitro were higher in female, but not male, Panx1ΔOc mice, compared to Panx1fl/fl littermates. Surprisingly, although Panx1 expression was normal in bone marrow stromal-derived osteoblasts from male and female Panx1ΔOc mice, mineral deposition by male (but not female) Panx1ΔOc osteoblasts was lower than controls, and it was reduced in male Panx1fl/fl osteoblasts when conditioned media prepared from male Panx1ΔOc osteoclast cultures was added to the cell culture media. Thus, deletion of Panx1 in TRAP-expressing cells in female mice leads to low bone mass primarily through a cell autonomous effect in osteoclast activity. In contrast, our evidence suggests that changes in the osteoclast secretome drive reduced osteoblast function in male Panx1ΔOc mice, resulting in low bone mass.

Wnt Pathway Extracellular Components and Their Essential Roles in Bone Homeostasis

The Wnt pathway is involved in several processes essential for bone development and homeostasis. For proper functioning, the Wnt pathway is tightly regulated by numerous extracellular elements that act by both activating and inhibiting the pathway at different moments. This review aims to describe, summarize and update the findings regarding the extracellular modulators of the Wnt pathway, including co-receptors, ligands and inhibitors, in relation to bone homeostasis, with an emphasis on the animal models generated, the diseases associated with each gene and the bone processes in which each member is involved. The precise knowledge of all these elements will help us to identify possible targets that can be used as a therapeutic target for the treatment of bone diseases such as osteoporosis.

Zebrafish mutants reveal unexpected role of Lrp5 in osteoclast regulation

Low-density Lipoprotein Receptor-related Protein 5 (LRP5) functions as a co-receptor for Wnt ligands, controlling expression of genes involved in osteogenesis. In humans, loss-of-function mutations in LRP5 cause Osteoporosis-Pseudoglioma syndrome, a low bone mass disorder, while gain-of-function missense mutations have been observed in individuals with high bone mass. Zebrafish (Danio rerio) is a popular model for human disease research, as genetic determinants that control bone formation are generally conserved between zebrafish and mammals. We generated lrp5- knock-out zebrafish to study its role in skeletogenesis and homeostasis. Loss of lrp5 in zebrafish leads to craniofacial deformities and low bone mineral density (total body and head) at adult ages. To understand the mechanism and consequences of the observed phenotypes, we performed transcriptome analysis of the cranium of adult lrp5 mutants and siblings. Enrichment analysis revealed upregulation of genes significantly associated with hydrolase activity: mmp9, mmp13a, acp5a. acp5a encodes Tartrate-resistant acid phosphatase (TRAP) which is commonly used as an osteoclast marker, while Matrix metalloprotease 9, Mmp9, is known to be secreted by osteoclasts and stimulate bone resorption. These genes point to changes in osteoclast differentiation regulated by lrp5. To analyze these changes functionally, we assessed osteoclast dynamics in mutants and observed increased TRAP staining, significantly larger resorption areas, and developmental skeletal dysmorphologies in the mutant, suggesting higher resorptive activity in the absence of Lrp5 signaling. Our findings support a conserved role of Lrp5 in maintaining bone mineral density and revealed unexpected insights into the function of Lrp5 in bone homeostasis through moderation of osteoclast function.

Sites of Cre-recombinase activity in mouse lines targeting skeletal cells

The Cre/Lox system is a powerful tool in the biologist's toolbox, allowing loss-of-function and gain-of-function studies, as well as lineage tracing, through gene recombination in a tissue-specific and inducible manner. Evidence indicates, however, that Cre transgenic lines have a far more nuanced and broader pattern of Cre activity than initially thought, exhibiting "off-target" activity in tissues/cells other than the ones they were originally designed to target. With the goal of facilitating the comparison and selection of optimal Cre lines to be used for the study of gene function, we have summarized in a single manuscript the major sites and timing of Cre activity of the main Cre lines available to target bone mesenchymal stem cells, chondrocytes, osteoblasts, osteocytes, tenocytes, and osteoclasts, along with their reported sites of "off-target" Cre activity. We also discuss characteristics, advantages, and limitations of these Cre lines for users to avoid common risks related to overinterpretation or misinterpretation based on the assumption of strict cell-type specificity or unaccounted effect of the Cre transgene or Cre inducers. © 2021 American Society for Bone and Mineral Research (ASBMR).

Mechanical Loading-Driven Tumor Suppression Is Mediated by Lrp5-Dependent and Independent Mechanisms

Simple Summary

Advanced breast cancer and prostate cancer metastasize to varying organs including the bone. We show here that mechanical loading to the knee suppresses tumor growth in the loaded bone and the non-loaded mammary pad. Although lipoprotein receptor-related protein 5 (Lrp5) in osteocytes is necessary to induce loading-driven bone formation, loading-driven tumor suppression is regulated by Lrp5-dependent and independent mechanisms. Lrp5 overexpression in osteocytes enhances tumor suppression, but without Lrp5 in osteocytes, mechanical loading elevates dopamine, chemerin, p53, and TNF-related apoptosis-inducing ligand (TRAIL) and reduces cholesterol and nexin. Their systemic changes contribute to inhibiting tumors without Lrp5. Osteoclast development is also inhibited by the load-driven regulation of chemerin and nexin.

Abstract

Bone is mechanosensitive and lipoprotein receptor-related protein 5 (Lrp5)-mediated Wnt signaling promotes loading-driven bone formation. While mechanical loading can suppress tumor growth, the question is whether Lrp5 mediates loading-driven tumor suppression. Herein, we examined the effect of Lrp5 using osteocyte-specific Lrp5 conditional knockout mice. All mice presented noticeable loading-driven tumor suppression in the loaded tibia and non-loaded mammary pad. The degree of suppression was more significant in wild-type than knockout mice. In all male and female mice, knee loading reduced cholesterol and elevated dopamine. It reduced tumor-promoting nexin, which was elevated by cholesterol and reduced by dopamine. By contrast, it elevated p53, TNF-related apoptosis-inducing ligand (TRAIL), and chemerin, and they were regulated reversely by dopamine and cholesterol. Notably, Lrp5 overexpression in osteocytes enhanced tumor suppression, and osteoclast development was inhibited by chemerin. Collectively, this study identified Lrp5-dependent and independent mechanisms for tumor suppression. Lrp5 in osteocytes contributed to the loaded bone, while the Lrp5-independent regulation of dopamine- and cholesterol-induced systemic suppression.

Osteoclast formation at the bone marrow/bone surface interface: Importance of structural elements, matrix, and intercellular communication

Osteoclasts, the multinucleated cells responsible for bone resorption, have an enormous destructive power which demands to be kept under tight control. Accordingly, the identification of molecular signals directing osteoclastogenesis and switching on their resorptive activity have received much attention. Mandatory factors were identified, but a very essential aspect of the control mechanism of osteoclastic resorption, i.e. its spatial control, remains poorly understood. Under physiological conditions, multinucleated osteoclasts are only detected on the bone surface, while their mono-nucleated precursors are only in the bone marrow. How are pre-osteoclasts targeted to the bone surface? How is their progressive differentiation coordinated with their approach to the bone surface sites to be resorbed, which is where they finally fuse? Here we review the information on the bone marrow distribution of differentiating pre-osteoclasts relative to the position of the mandatory factors for their differentiation as well as relative to physical entities that may affect their access to the remodelling sites. This info allows recognizing an "osteoclastogenesis route" through the bone marrow and leading to the coincident fusion/resorption site - but also points to what still remains to be clarified regarding this route and regarding the restriction of fusion at the resorption site. Finally, we discuss the mechanism responsible for the start of resorption and its spatial extension. This review underscores that fully understanding the control of bone resorption requires to consider it in both space and time - which demands taking into account the context of bone tissue.

From hepatitis C virus immunoproteomics to rheumatology via cross-reactivity in one table

PURPOSE OF REVIEW

To give an overview of molecular and immunologic data that link hepatitis C virus (HCV) infection to rheumatic diseases in the human host.

RECENT FINDINGS

A high level of peptide sharing exists between immunopositive HCV epitopes and human proteins that, when altered, associate with rheumatic manifestations.

SUMMARY

The findings suggest the involvement of HCV infection in the induction of most rheumatic diseases via a mechanism of autoimmune cross-reactivity.