The Wnt co-receptor LRP5 is essential for skeletal mechanotransduction but not for the anabolic bone response to parathyroid hormone treatment

Deletion of the auxiliary α2δ1 voltage sensitive calcium channel subunit in osteocytes and late-stage osteoblasts impairs femur strength and load-induced bone formation in male mice

Osteocytes sense and respond to mechanical force by controlling the activity of other bone cells. However, the mechanisms by which osteocytes sense mechanical input and transmit biological signals remain unclear. Voltage-sensitive calcium channels (VSCCs) regulate calcium (Ca2+) influx in response to external stimuli. Inhibition or deletion of VSCCs impairs osteogenesis and skeletal responses to mechanical loading. VSCC activity is influenced by its auxiliary subunits, which bind the channel's α1 pore-forming subunit to alter intracellular Ca2+ concentrations. The α2δ1 auxiliary subunit associates with the pore-forming subunit via a glycosylphosphatidylinositol anchor and regulates the channel's calcium-gating kinetics. Knockdown of α2δ1 in osteocytes impairs responses to membrane stretch, and global deletion of α2δ1 in mice results in osteopenia and impaired skeletal responses to loading in vivo. Therefore, we hypothesized that the α2δ1 subunit functions as a mechanotransducer, and its deletion in osteocytes would impair skeletal development and load-induced bone formation. Mice (C57BL/6) with LoxP sequences flanking Cacna2d1, the gene encoding α2δ1, were crossed with mice expressing Cre under the control of the Dmp1 promoter (10 kb). Deletion of α2δ1 in osteocytes and late-stage osteoblasts decreased femoral bone quantity (P < .05) by DXA, reduced relative osteoid surface (P < .05), and altered osteoblast and osteocyte regulatory gene expression (P < .01). Cacna2d1f/f, Cre + male mice displayed decreased femoral strength and lower 10-wk cancellous bone in vivo micro-computed tomography measurements at the proximal tibia (P < .01) compared to controls, whereas Cacna2d1f/f, Cre + female mice showed impaired 20-wk cancellous and cortical bone ex vivo micro-computed tomography measurements (P < .05) vs controls. Deletion of α2δ1 in osteocytes and late-stage osteoblasts suppressed load-induced calcium signaling in vivo and decreased anabolic responses to mechanical loading in male mice, demonstrating decreased mechanosensitivity. Collectively, the α2δ1 auxiliary subunit is essential for the regulation of osteoid-formation, femur strength, and load-induced bone formation in male mice.

Time of day of exercise does not affect the beneficial effect of exercise on bone structure in older female rats

Background: Circadian clock genes are expressed in bone and biomarkers of bone resorption and formation exhibit diurnal patterns in animals and humans. Disruption of the diurnal rhythms may affect the balance of bone turnover and compromise the beneficial effects of exercise on bone. Objective: This study investigated whether the time of day of exercise alters bone metabolism in a rodent model. We hypothesized that exercise during the active phase results in greater bone mass than exercise during the rest phase in older female rats. Methods: Fifty-five, female 12-month-old Sprague Dawley rats were randomly assigned to four treatment groups (n = 13-14/group). Rats were subjected to no exercise or 2 h of involuntary exercise at 9 m/min and 5 days/wk for 15 weeks using motor-driven running wheels at Zeitgeber time (ZT) 4-6 (rest phase), 12-14 (early active phase), or 22-24 (late active phase). ZT 0 is defined as light on, the start of the rest phase. A red lamp was used at minimal intensity during the active, dark phase exercise period, i.e., ZT 12-14 and 22-24. Bone structure, body composition, and bone-related cytokines in serum and gene expression in bone were measured. Data were analyzed using one-way ANOVA followed by Tukey-Kramer post hoc contrasts. Results: Exercise at different ZT did not affect body weight, fat mass, lean mass, the serum bone biomarkers, bone structural or mechanical parameters, or expression of circadian genes. Exercise pooled exercise data from different ZT were compared to the No-Exercise data (a priori contrast) increased serum IGF-1 and irisin concentrations, compared to No-Exercise. Exercise increased tibial bone volume/total volume (p = 0.01), connectivity density (p = 0.04), and decreased structural model index (p = 0.02). Exercise did not affect expression of circadian genes. Conclusion: These data indicate that exercise is beneficial to bone structure and that the time of day of exercise does not alter the beneficial effect of exercise on bone in older female rats.

LRP5, Bone Mass Polymorphisms and Skeletal Disorders

The formation and maintenance of the gross structure and microarchitecture of the human skeleton require the concerted functioning of a plethora of morphogenic signaling processes. Through recent discoveries in the field of genetics, numerous genotypic variants have been implicated in pathologic skeletal phenotypes and disorders arising from the disturbance of one or more of these processes. For example, total loss-of-function variants of LRP5 were found to be the cause of osteoporosis-pseudoglioma syndrome (OPPG). LRP5 encodes for the low-density lipoprotein receptor-related protein 5, a co-receptor in the canonical WNT-β-catenin signaling pathway and a crucial protein involved in the formation and maintenance of homeostasis of the human skeleton. Beyond OPPG, other partial loss-of-function variants of LRP5 have been found to be associated with other low bone mass phenotypes and disorders, while LRP5 gain-of-function variants have been implicated in high bone mass phenotypes. This review introduces the roles that LRP5 plays in skeletal morphogenesis and discusses some of the structural consequences that result from abnormalities in LRP5. A greater understanding of how the LRP5 receptor functions in bone and other body tissues could provide insights into a variety of pathologies and their potential treatments, from osteoporosis and a variety of skeletal abnormalities to congenital disorders that can lead to lifelong disabilities.

The Past, Present, and Future of Genetically Engineered Mouse Models for Skeletal Biology

The ability to create genetically engineered mouse models (GEMMs) has exponentially increased our understanding of many areas of biology. Musculoskeletal biology is no exception. In this review, we will first discuss the historical development of GEMMs and how these developments have influenced musculoskeletal disease research. This review will also update our 2008 review that appeared in BONEKey, a journal that is no longer readily available online. We will first review the historical development of GEMMs in general, followed by a particular emphasis on the ability to perform tissue-specific (conditional) knockouts focusing on musculoskeletal tissues. We will then discuss how the development of CRISPR/Cas-based technologies during the last decade has revolutionized the generation of GEMMs.

Regulation of the Osteocyte Secretome with Aging and Disease

As the most numerous and long-lived of all bone cells, osteocytes have essential functions in regulating skeletal health. Through the lacunar-canalicular system, secreted proteins from osteocytes can reach cells throughout the bone. Furthermore, the intimate connectivity between the lacunar-canalicular system and the bone vasculature allows for the transport of osteocyte-secreted factors into the circulation to reach the entire body. Local and endocrine osteocyte signaling regulates physiological processes such as bone remodeling, bone mechanoadaptation, and mineral homeostasis. However, these processes are disrupted by impaired osteocyte function induced by aging and disease. Dysfunctional osteocyte signaling is now associated with the pathogenesis of many disorders, including chronic kidney disease, cancer, diabetes mellitus, and periodontitis. In this review, we focus on the targeting of bone and extraskeletal tissues by the osteocyte secretome. In particular, we highlight the secreted osteocyte proteins, which are known to be dysregulated during aging and disease, and their roles during disease progression. We also discuss how therapeutic or genetic targeting of osteocyte-secreted proteins can improve both skeletal and systemic health.

Connexin 43 hemichannels and prostaglandin E2 release in anabolic function of the skeletal tissue to mechanical stimulation

Bone adapts to changes in the physical environment by modulating remodeling through bone resorption and formation to maintain optimal bone mass. As the most abundant connexin subtype in bone tissue, connexin 43 (Cx43)-forming hemichannels are highly responsive to mechanical stimulation by permitting the exchange of small molecules (<1.2 kDa) between bone cells and the extracellular environment. Upon mechanical stimulation, Cx43 hemichannels facilitate the release of prostaglandins E2 (PGE2), a vital bone anabolic factor from osteocytes. Although most bone cells are involved in mechanosensing, osteocytes are the principal mechanosensitive cells, and PGE2 biosynthesis is greatly enhanced by mechanical stimulation. Mechanical stimulation-induced PGE2 released from osteocytic Cx43 hemichannels acts as autocrine effects that promote β-catenin nuclear accumulation, Cx43 expression, gap junction function, and protects osteocytes against glucocorticoid-induced osteoporosis in cultured osteocytes. In vivo, Cx43 hemichannels with PGE2 release promote bone formation and anabolism in response to mechanical loading. This review summarizes current in vitro and in vivo understanding of Cx43 hemichannels and extracellular PGE2 release, and their roles in bone function and mechanical responses. Cx43 hemichannels could be a significant potential new therapeutic target for treating bone loss and osteoporosis.

Human femur morphology and histology variation with ancestry and behaviour in an ancient sample from Vietnam

BACKGROUND

There is a genetic component to the minimum effective strain (MES)-a threshold which determines when bone will adapt to function-which suggests ancestry should play a role in bone (re)modelling. Further elucidating this is difficult in living human populations because of the high global genetic admixture. We examined femora from an anthropological skeletal assemblage (Mán Bạc, Vietnam) representing distinct ancestral groups. We tested whether femur morphological and histological markers of modelling and remodelling differed between ancestries despite their similar lifestyles.

METHODS

Static histomorphometry data collected from subperiosteal cortical bone of the femoral midshaft, and gross morphometric measures of femur robusticity, were studied in 17 individuals from the Mán Bạc collection dated to 1906-1523 cal. BC. This assemblage represents agricultural migrants with affinity to East Asian groups, who integrated with the local hunter-gatherers with affinity to Australo-Papuan groups during the mid-Holocene. Femur robusticity and histology data were compared between groups of 'Migrant' (n = 8), 'Admixed' (n = 4), and 'Local' (n = 5).

RESULTS

Local individuals had more robust femoral diaphyses with greater secondary osteon densities, and relatively large secondary osteon and Haversian canal parameters than the migrants. The Migrant group showed gracile femoral shafts with the least dense bone made up of small secondary osteons and Haversian canals. The Admixed individuals fell between the Migrant and Local categories in terms of their femoral data. However, we also found that measures of how densely bone is remodelled per unit area were in a tight range across all three ancestries.

CONCLUSIONS

Bone modelling and remodelling markers varied with ancestral histories in our sample. This suggests that there is an ancestry related predisposition to bone optimising its metabolic expenditure likely in relation to the MES. Our results stress the need to incorporate population genetic history into hierarchical bone analyses. Understanding ancestry effects on bone morphology has implications for interpreting biomechanical loading history in past and modern human populations.

Unlocking the Therapeutic Potential of Irisin: Harnessing Its Function in Degenerative Disorders and Tissue Regeneration

Physical activity is well-established as an important protective factor against degenerative conditions and a promoter of tissue growth and renewal. The discovery of Fibronectin domain-containing protein 5 (FNDC5) as the precursor of Irisin in 2012 sparked significant interest in its potential as a diagnostic biomarker and a therapeutic agent for various diseases. Clinical studies have examined the correlation between plasma Irisin levels and pathological conditions using a range of assays, but the lack of reliable measurements for endogenous Irisin has led to uncertainty about its prognostic/diagnostic potential as an exercise surrogate. Animal and tissue-engineering models have shown the protective effects of Irisin treatment in reversing functional impairment and potentially permanent damage, but dosage ambiguities remain unresolved. This review provides a comprehensive examination of the clinical and basic studies of Irisin in the context of degenerative conditions and explores its potential as a therapeutic approach in the physiological processes involved in tissue repair/regeneration.

Honey: A Promising Therapeutic Supplement for the Prevention and Management of Osteoporosis and Breast Cancer

Osteoporosis and breast cancer are serious diseases that have become a significant socioeconomic burden. There are biochemical associations between the two disorders in terms of the amended function of estrogen, receptor activator of nuclear factor kappa beta ligand, oxidative stress, inflammation, and lipid accumulation. Honey as a functional food with high antioxidant and anti-inflammatory properties can contribute to the prevention of various diseases. Its health benefits are mainly related to the content of polyphenols. This review aims to summarize the current knowledge from in vitro, animal, and human studies on the use of honey as a potential therapeutic agent for osteoporosis and breast cancer. Preclinical studies have revealed a beneficial impact of honey on both bone health (microstructure, strength, oxidative stress) and breast tissue health (breast cancer cell proliferation and apoptosis, tumor growth rate, and volume). The limited number of clinical trials, especially in osteoporosis, indicates the need for further research to evaluate the potential benefits of honey in the treatment. Clinical studies related to breast cancer have revealed that honey is effective in increasing blood cell counts, interleukin-3 levels, and quality of life. In summary, honey may serve as a prospective therapeutic supplement for bone and breast tissue health.

The Double-Edged Proteins in Cancer Proteomes and the Generation of Induced Tumor-Suppressing Cells (iTSCs)

Unlike a prevalent expectation that tumor cells secrete tumor-promoting proteins and stimulate the progression of neighboring tumor cells, accumulating evidence indicates that the role of tumor-secreted proteins is double-edged and context-dependent. Some of the oncogenic proteins in the cytoplasm and cell membranes, which are considered to promote the proliferation and migration of tumor cells, may inversely act as tumor-suppressing proteins in the extracellular domain. Furthermore, the action of tumor-secreted proteins by aggressive "super-fit" tumor cells can be different from those derived from "less-fit" tumor cells. Tumor cells that are exposed to chemotherapeutic agents could alter their secretory proteomes. Super-fit tumor cells tend to secrete tumor-suppressing proteins, while less-fit or chemotherapeutic agent-treated tumor cells may secrete tumor-promotive proteomes. Interestingly, proteomes derived from nontumor cells such as mesenchymal stem cells and peripheral blood mononuclear cells mostly share common features with tumor cell-derived proteomes in response to certain signals. This review introduces the double-sided functions of tumor-secreted proteins and describes the proposed underlying mechanism, which would possibly be based on cell competition.

Got WNTS? Insight into bone health from a WNT perspective

WNT signaling, essential for many aspects of development, is among the most commonly altered pathways associated with human disease. While initially studied in cancer, dysregulation of WNT signaling has been determined to be essential for skeletal development and the maintenance of bone health throughout life. In this review, we discuss the role of Wnt signaling in bone development and disease with a particular focus on two areas. First, we discuss the roles of WNT signaling pathways in skeletal development, with an emphasis on congenital and idiopathic skeletal syndromes and diseases that are associated with genetic variations in WNT signaling components. Next, we cover a topic that has long been an interest of our laboratory, how high and low levels of WNT signaling affects the establishment and maintenance of healthy bone mass. We conclude with a discussion of the status of WNT-based therapeutics in the treatment of skeletal disease.

Protein Kinase G2 Is Essential for Skeletal Homeostasis and Adaptation to Mechanical Loading in Male but Not Female Mice

We previously showed that the NO/cGMP/protein kinase G (PKG) signaling pathway positively regulates osteoblast proliferation, differentiation, and survival in vitro, and that cGMP-elevating agents have bone-anabolic effects in mice. Here, we generated mice with an osteoblast-specific (OB) knockout (KO) of type 2 PKG (gene name Prkg2) using a Col1a1(2.3 kb)-Cre driver. Compared to wild type (WT) littermates, 8-week-old male OB Prkg2-KO mice had fewer osteoblasts, reduced bone formation rates, and lower trabecular and cortical bone volumes. Female OB Prkg2-KO littermates showed no bone abnormalities, despite the same degree of PKG2 deficiency in bone. Expression of osteoblast differentiation- and Wnt/β-catenin-related genes was lower in primary osteoblasts and bones of male KO but not female KO mice compared to WT littermates. Osteoclast parameters were unaffected in both sexes. Since PKG2 is part of a mechano-sensitive complex in osteoblast membranes, we examined its role during mechanical loading. Cyclical compression of the tibia increased cortical thickness and induced mechanosensitive and Wnt/β-catenin-related genes to a similar extent in male and female WT mice and female OB Prkg2-KO mice, but loading had a minimal effect in male KO mice. We conclude that PKG2 drives bone acquisition and adaptation to mechanical loading via the Wnt/β-catenin pathway in male mice. The striking sexual dimorphism of OB Prkg2-KO mice suggests that current U.S. Food and Drug Administration-approved cGMP-elevating agents may represent novel effective treatment options for male osteoporosis. © 2022 American Society for Bone and Mineral Research (ASBMR).

Osteoporosis pathogenesis and treatment: existing and emerging avenues

Osteoporotic fractures lead to increased disability and mortality in the elderly population. With the rapid increase in the aging population around the globe, more effective treatments for osteoporosis and osteoporotic fractures are urgently required. The underlying molecular mechanisms of osteoporosis are believed to be due to the increased activity of osteoclasts, decreased activity of osteoblasts, or both, which leads to an imbalance in the bone remodeling process with accelerated bone resorption and attenuated bone formation. Currently, the available clinical treatments for osteoporosis have mostly focused on factors influencing bone remodeling; however, they have their own limitations and side effects. Recently, cytokine immunotherapy, gene therapy, and stem cell therapy have become new approaches for the treatment of various diseases. This article reviews the latest research on bone remodeling mechanisms, as well as how this underpins current and potential novel treatments for osteoporosis.

The role of Meteorin-like in skeletal development and bone fracture healing

Meteorin-like protein (Metrnl), homologous to the initially identified neurotrophic factor Meteorin, is a secreted, multifunctional protein. Here we used mouse models to investigate Metrnl's role in skeletal development and bone fracture healing. During development Metrnl was expressed in the perichondrium and primary ossification center. In neonates, single cell RNA-seq of diaphyseal bone demonstrated strongest expression of Metrnl transcript by osteoblasts. In vitro, Metrnl was osteoinductive, increasing osteoblast differentiation and mineralization in tissue culture models. In vivo, loss of Metrnl expression resulted in no change in skeletal metrics in utero, at birth, or during postnatal growth. Six-week-old Metrnl-null mice displayed similar body length, body weight, tibial length, femoral length, BV/TV, trabecular number, trabecular thickness, and cortical thickness as littermate controls. In 4-month-old mice, lack of Metrnl expression did not change structural stiffness, ultimate force, or energy to fracture of femora under 3-point-bending. Last, we investigated the role of Metrnl in bone fracture healing. Metrnl expression increased in response to tibial injury, however, loss of Metrnl expression did not affect the amount of bone deposited within the healing tissue nor did it change the structural parameters of healing tissue. This work identifies Metrnl as a dispensable molecule for skeletal development. However, the osteoinductive capabilities of Metrnl may be utilized to modulate osteoblast differentiation in cell-based orthopedic therapies.

Immobilization of Wnt Fragment Peptides on Magnetic Nanoparticles or Synthetic Surfaces Regulate Wnt Signaling Kinetics

Wnt signaling plays an important role in embryogenesis and adult stem cell homeostasis. Its diminished activation is implicated in osteoporosis and degenerative neural diseases. However, systematic administration of Wnt-signaling agonists carries risk, as aberrantly activated Wnt/β-catenin signaling is linked to cancer. Therefore, technologies for local modulation and control of Wnt signaling targeted to specific sites of disease or degeneration have potential therapeutic value in the treatment of degenerative diseases. We reported a facile approach to locally activate the canonical Wnt signaling cascade using nanomagnetic actuation or ligand immobilized platforms. Using a human embryonic kidney (HEK293) Luc-TCF/LEF reporter cell line, we demonstrated that targeting the cell membrane Wnt receptor, Frizzled 2, with peptide-tagged magnetic nanoparticles (MNPs) triggered canonical Wnt signaling transduction when exposed to a high-gradient, time-varying magnetic field, and the induced TCF/LEF signal transduction was shown to be avidity-dependent. We also demonstrated that the peptide retained signaling activity after functionalization onto glass surfaces, providing a versatile platform for drug discovery or recreation of the cell niche. In conclusion, these results showed that peptide-mediated Wnt signaling kinetics depended not only on ligand concentration but also on the presentation method of the ligand, which may be further modulated by magnetic actuation. This has important implications when designing future therapeutic platforms involving Wnt mimetics.

Loading-induced bone formation is mediated by Wnt1 induction in osteoblast-lineage cells

Mechanical loading on the skeleton stimulates bone formation. Although the exact mechanism underlying this process remains unknown, a growing body of evidence indicates that the Wnt signaling pathway is necessary for the skeletal response to loading. Recently, we showed that Wnts produced by osteoblast lineage cells mediate the osteo-anabolic response to tibial loading in adult mice. Here, we report that Wnt1 specifically plays a crucial role in mediating the mechano-adaptive response to loading. Independent of loading, short-term loss of Wnt1 in the Osx-lineage resulted in a decreased cortical bone area in the tibias of 5-month-old mice. In females, strain-matched loading enhanced periosteal bone formation in Wnt1F/F controls, but not in Wnt1F/F; OsxCreERT2 knockouts. In males, strain-matched loading increased periosteal bone formation in both control and knockout mice; however, the periosteal relative bone formation rate was 65% lower in Wnt1 knockouts versus controls. Together, these findings show that Wnt1 supports adult bone homeostasis and mediates the bone anabolic response to mechanical loading.

Osteocytes regulate bone anabolic response to mechanical loading in male mice via activation of integrin α5

Physical mechanical stimulation can maintain and even increase bone mass. Here, we report an important role of osteocytic integrin α5 in regulating the anabolic response of bone to mechanical loading using an Itga5 conditional gene knockout (cKO) mouse model. Integrin α5 gene deletion increased apoptotic osteocytes and reduced cortical anabolic responses to tibial compression including decreased endosteal osteoblasts and bone formation, and increased endosteal osteoclasts and bone resorption, contributing to the decreased bone area fraction and biomechanical properties, leading to an enlarged bone marrow area in cKO mice. Similar disruption of anabolic responses to mechanical loading was also detected in cKO trabecular bone. Moreover, integrin α5 deficiency impeded load-induced Cx43 hemichannel opening, and production and release of PGE2, an anabolic factor, resulting in attenuated effects of the loading on catabolic sclerostin (SOST) reduction and anabolic β-catenin increase. Together, this study shows an indispensable role of integrin α5 in osteocytes in the anabolic action of mechanical loading on skeletal tissue through activation of hemichannels and PGE2-evoked gene expression. Integrin α5 could act as a potential new therapeutic target for bone loss, especially in the elderly population with impeded mechanical sensitivity.

Molecular Mechanisms of Isolated Polycystic Liver Diseases

Polycystic liver disease (PLD) is a rare autosomal dominant disorder including two genetically and clinically distinct forms: autosomal dominant polycystic kidney disease (ADPKD) and isolated polycystic liver disease (PCLD). The main manifestation of ADPKD is kidney cysts, while PCLD has predominantly liver presentations with mild or absent kidney cysts. Over the past decade, PRKCSH, SEC63, ALG8, and LRP5 have been candidate genes of PCLD. Recently, more candidate genes such as GANAB, SEC61B, and ALR9 were also reported in PCLD patients. This review focused on all candidate genes of PCLD, including the newly established novel candidate genes. In addition, we also discussed some other genes which might also contribute to the disease.

Effects of Mechanical Stress Stimulation on Function and Expression Mechanism of Osteoblasts

Osteoclasts and osteoblasts play a major role in bone tissue homeostasis. The homeostasis and integrity of bone tissue are maintained by ensuring a balance between osteoclastic and osteogenic activities. The remodeling of bone tissue is a continuous ongoing process. Osteoclasts mainly play a role in bone resorption, whereas osteoblasts are mainly involved in bone remodeling processes, such as bone cell formation, mineralization, and secretion. These cell types balance and restrict each other to maintain bone tissue metabolism. Bone tissue is very sensitive to mechanical stress stimulation. Unloading and loading of mechanical stress are closely related to the differentiation and formation of osteoclasts and bone resorption function as well as the differentiation and formation of osteoblasts and bone formation function. Consequently, mechanical stress exerts an important influence on the bone microenvironment and bone metabolism. This review focuses on the effects of different forms of mechanical stress stimulation (including gravity, continuously compressive pressure, tensile strain, and fluid shear stress) on osteoclast and osteoblast function and expression mechanism. This article highlights the involvement of osteoclasts and osteoblasts in activating different mechanical transduction pathways and reports changings in their differentiation, formation, and functional mechanism induced by the application of different types of mechanical stress to bone tissue. This review could provide new ideas for further microscopic studies of bone health, disease, and tissue damage reconstruction.

Connexin hemichannels with prostaglandin release in anabolic function of bone to mechanical loading

Mechanical stimulation, such as physical exercise, is essential for bone formation and health. Here, we demonstrate the critical role of osteocytic Cx43 hemichannels in anabolic function of bone in response to mechanical loading. Two transgenic mouse models, R76W and Δ130–136, expressing dominant-negative Cx43 mutants in osteocytes were adopted. Mechanical loading of tibial bone increased cortical bone mass and mechanical properties in wild-type and gap junction-impaired R76W mice through increased PGE₂, endosteal osteoblast activity, and decreased sclerostin. These anabolic responses were impeded in gap junction/hemichannel-impaired Δ130–136 mice and accompanied by increased endosteal osteoclast activity. Specific inhibition of Cx43 hemichannels by Cx43(M1) antibody suppressed PGE₂ secretion and impeded loading-induced endosteal osteoblast activity, bone formation and anabolic gene expression. PGE₂ administration rescued the osteogenic response to mechanical loading impeded by impaired hemichannels. Together, osteocytic Cx43 hemichannels could be a potential new therapeutic target for treating bone loss and osteoporosis.

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.

Osteoblast-Specific Wnt Secretion Is Required for Skeletal Homeostasis and Loading-Induced Bone Formation in Adult Mice

Wnt signaling is critical to many aspects of skeletal regulation, but the importance of Wnt ligands in the bone anabolic response to mechanical loading is not well established. Recent transcriptome profiling studies by our laboratory and others show that mechanical loading potently induces genes encoding Wnt ligands, including Wnt1 and Wnt7b. Based on these findings, we hypothesized that mechanical loading stimulates adult bone formation by inducing Wnt ligand expression. To test this hypothesis, we inhibited Wnt ligand secretion in adult (5 months old) mice using a systemic (drug) and a bone-targeted (conditional gene knockout) approach, and subjected them to axial tibial loading to induce lamellar bone formation. Mice treated with the Wnt secretion inhibitor WNT974 exhibited a decrease in bone formation in non-loaded bones as well as a 54% decline in the periosteal bone formation response to tibial loading. Next, osteoblast-specific Wnt secretion was inhibited by dosing 5-month-old Osx-CreERT2; WlsF/F mice with tamoxifen. Within 1 to 2 weeks of Wls deletion, skeletal homeostasis was altered with decreased bone formation and increased resorption, and the anabolic response to loading was reduced 65% compared to control (WlsF/F ). Together, these findings show that Wnt ligand secretion is required for adult bone homeostasis, and furthermore establish a role for osteoblast-derived Wnts in mediating the bone anabolic response to tibial loading. © 2021 American Society for Bone and Mineral Research (ASBMR).

The Wnt pathway: An important control mechanism in bone's response to mechanical loading

The conversion of mechanical energy into biochemical changes within living cells is process known as mechanotransduction. Bone is a quintessential tissue for studying the molecular mechanisms of mechanotransduction, as the skeleton's mechanical competence is crucial for vertebrate movement. Bone cell mechanotransduction is facilitated by a number of cell biological pathways, one of the most prominent of which is the Wnt signaling cascade. The Wnt co-receptor Lrp5 has been identified as a crucial protein for mechanical signaling in bone, and modifiers of Lrp5 activity play important roles in mediating signaling efficiency through Lrp5, including sclerostin, Dkk1, and the co-receptor Lrp4. Mechanical regulation of sclerostin is mediated by certain members of the Hdac family. Other mechanisms that influence Wnt signaling-some of which are mechanoresponsive-are coming to light, including R-spondins and their role in organizing the Rnf43/Znrf3 and Lgr4/5/6 complex that liberates Lrp5. While the identity of the key Wnt proteins involved in bone cell mechanical signaling are elusive, the likely pool of key players is narrowing. Identification of Wnt-based molecular targets that can be modulated pharmacologically to make mechanical stimulation (e.g., exercise) more beneficial is an emerging approach to improving skeletal integrity and reducing fracture risk.

The Effect of Overexpression of Lrp5 on the Temporomandibular Joint

OBJECTIVE

The temporomandibular joint (TMJ) is a unique fibrocartilaginous joint that adapts to mechanical loading through cell signaling pathways such as the Wnt pathway. Increased expression of low-density lipoprotein receptor-related protein 5 (Lrp5), a co-receptor of the Wnt pathway, is associated with a high bone mass (HBM) phenotype. The objective of this study was to analyze the effect of overexpression of Lrp5 on the subchondral bone and cartilage of the TMJ in mice exhibiting the HBM phenotype.

DESIGN

Sixteen-week-old Lrp5 knock-in transgenic mice carrying either the A214V (EXP-A) or G171V (EXP-G) missense mutations, and wildtype controls (CTRL) were included in this study. Fluorescent bone labels, calcein, alizarin complexone, and demeclocycline were injected at 3.5, 7.5, and 11.5 weeks of age, respectively. The left mandibular condyle was used to compare the subchondral bone micro-computed tomography parameters and the right TMJ was used for histological analyses. Cartilage thickness, matrix proteoglycan accumulation, and immunohistochemical localization of Lrp5 and sclerostin were compared between the groups.

RESULTS

Subchondral bone volume (BV) and percent bone volume (BV/TV) were significantly increased in both EXP-A and EXP-G compared with CTRL (P < 0.05) whereas trabecular spacing (Tb.Sp) was decreased. Cartilage thickness, extracellular matrix production, and expression of Lrp5 and Sost were all increased in the experimental groups. The separation between the fluorescent bone labels indicated increased endochondral maturation between 3.5 and 7.5 weeks.

CONCLUSIONS

These data demonstrate that Lrp5 overexpression leads to adaptation changes in the mandibular condylar cartilage of the TMJ to prevent cartilage degradation.

Identification osteogenic signaling pathways following mechanical stimulation: A systematic review

INTRODUCTION

It has been shown that mechanical forces can induce or promote osteogenic differentiation as well as remodeling of the new created bone tissues. To apply this characteristic in bone tissue engineering, it is important to know which mechanical stimuli through which signaling pathway has a more significant impact on osteogenesis.

METHODS

In this systematic study, an electronic search was conducted using PubMed and Google Scholar databases. This study has been prepared and organized according to the preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines. Included studies were first categorized according to the in vivo and in vitro studies.

RESULTS

Six types of mechanical stresses were used in these articles and the most commonly used mechanical force and cell source were tension and bone marrow-derived mesenchymal stem cells (BMMSCs), respectively. These forces were able to trigger twelve signaling pathways in which Wnt pathway was so prominent.

CONCLUSION

1) Although specific signaling pathways are induced through specific mechanical forces, Wnt signaling pathways are predominantly activated by almost all types of force/stimulation, 2) All signaling pathways regulate expression of RUNX2, which is known as a master regulator of osteogenesis, 3) In Tension force, the mode of force administration, i.e, continuous or non-continuous tension is more important than the percentage of elongation.

Anabolic actions of PTH in murine models: two decades of insights

Parathyroid hormone (PTH) is produced by the parathyroid glands in response to low serum calcium concentrations where it targets bones, kidneys, and indirectly, intestines. The N-terminus of PTH has been investigated for decades for its ability to stimulate bone formation when administered intermittently (iPTH) and is used clinically as an effective anabolic agent for the treatment of osteoporosis. Despite great interest in iPTH and its clinical use, the mechanisms of PTH action remain complicated and not fully defined. More than 70 gene targets in more than 90 murine models have been utilized to better understand PTH anabolic actions. Because murine studies utilized wild-type mice as positive controls, a variety of variables were analyzed to better understand the optimal conditions under which iPTH functions. The greatest responses to iPTH were in male mice, with treatment starting later than 12 weeks of age, a treatment duration lasting 5-6 weeks, and a PTH dose of 30-60 μg/kg/day. This comprehensive study also evaluated these genetic models relative to the bone formative actions with a primary focus on the trabecular compartment revealing trends in critical genes and gene families relevant for PTH anabolic actions. The summation of these data revealed the gene deletions with the greatest increase in trabecular bone volume in response to iPTH. These included PTH and 1-α-hydroxylase (Pth;1α(OH)ase, 62-fold), amphiregulin (Areg, 15.8-fold), and PTH related protein (Pthrp, 10.2-fold). The deletions with the greatest inhibition of the anabolic response include deletions of: proteoglycan 4 (Prg4, -9.7-fold), low-density lipoprotein receptor-related protein 6 (Lrp6, 1.3-fold), and low-density lipoprotein receptor-related protein 5 (Lrp5, -1.0-fold). Anabolic actions of iPTH were broadly affected via multiple and diverse genes. This data provides critical insight for future research and development, as well as application to human therapeutics. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

The Diminishing Returns of Mechanical Loading and Potential Mechanisms that Desensitize Osteocytes

Adaptation to mechanical loading is critical to maintaining bone mass and offers therapeutic potential to preventing age-related bone loss and osteoporosis. However, increasing the duration of loading is met with "diminishing returns" as the anabolic response quickly becomes saturated. As a result, the anabolic response to daily activities and repetitive bouts of loading is limited by the underlying mechanisms that desensitize and render bone unresponsive at the cellular level. Osteocytes are the primary cells that respond to skeletal loading and facilitate the overall anabolic response. Although many of osteocytes' signaling mechanisms activated in response to loading are considered anabolic in nature, several of them can also render osteocytes insensitive to further stimuli and thereby creating a negative feedback loop that limits osteocytes' overall response. The purpose of this review is to examine the potential mechanisms that may contribute to the loss of mechanosensitivity. In particular, we examined the inactivation/desensitization of ion channels and signaling molecules along with the potential role of endocytosis and cytoskeletal reorganization. The significance in defining the negative feedback loop is the potential to identify unique targets for enabling osteocytes to maintain their sensitivity. In doing so, we can begin to cultivate new strategies that capitalize on the anabolic nature of daily activities that repeatedly load the skeleton.

Overexpression of Lrp5 enhanced the anti-breast cancer effects of osteocytes in bone

Osteocytes are the most abundant cells in bone, which is a frequent site of breast cancer metastasis. Here, we focused on Wnt signaling and evaluated tumor-osteocyte interactions. In animal experiments, mammary tumor cells were inoculated into the mammary fat pad and tibia. The role of Lrp5-mediated Wnt signaling was examined by overexpressing and silencing Lrp5 in osteocytes and establishing a conditional knockout mouse model. The results revealed that administration of osteocytes or their conditioned medium (CM) inhibited tumor progression and osteolysis. Osteocytes overexpressing Lrp5 or β-catenin displayed strikingly elevated tumor-suppressive activity, accompanied by downregulation of tumor-promoting chemokines and upregulation of apoptosis-inducing and tumor-suppressing proteins such as p53. The antitumor effect was also observed with osteocyte-derived CM that was pretreated with a Wnt-activating compound. Notably, silencing Lrp5 in tumors inhibited tumor progression, while silencing Lrp5 in osteocytes in conditional knockout mice promoted tumor progression. Osteocytes exhibited elevated Lrp5 expression in response to tumor cells, implying that osteocytes protect bone through canonical Wnt signaling. Thus, our results suggest that the Lrp5/β-catenin axis activates tumor-promoting signaling in tumor cells but tumor-suppressive signaling in osteocytes. We envision that osteocytes with Wnt activation potentially offer a novel cell-based therapy for breast cancer and osteolytic bone metastasis.

Osteocyte Dysfunction in Joint Homeostasis and Osteoarthritis

Structural disturbances of the subchondral bone are a hallmark of osteoarthritis (OA), including sclerotic changes, cystic lesions, and osteophyte formation. Osteocytes act as mechanosensory units for the micro-cracks in response to mechanical loading. Once stimulated, osteocytes initiate the reparative process by recruiting bone-resorbing cells and bone-forming cells to maintain bone homeostasis. Osteocyte-expressed sclerostin is known as a negative regulator of bone formation through Wnt signaling and the RANKL pathway. In this review, we will summarize current understandings of osteocytes at the crossroad of allometry and mechanobiology to exploit the relationship between osteocyte morphology and function in the context of joint aging and osteoarthritis. We also aimed to summarize the osteocyte dysfunction and its link with structural and functional disturbances of the osteoarthritic subchondral bone at the molecular level. Compared with normal bones, the osteoarthritic subchondral bone is characterized by a higher bone volume fraction, a larger trabecular bone number in the load-bearing region, and an increase in thickness of pre-existing trabeculae. This may relate to the aberrant expressions of sclerostin, periostin, dentin matrix protein 1, matrix extracellular phosphoglycoprotein, insulin-like growth factor 1, and transforming growth factor-beta, among others. The number of osteocyte lacunae embedded in OA bone is also significantly higher, yet the volume of individual lacuna is relatively smaller, which could suggest abnormal metabolism in association with allometry. The remarkably lower percentage of sclerostin-positive osteocytes, together with clustering of Runx-2 positive pre-osteoblasts, may suggest altered regulation of osteoblast differentiation and osteoblast-osteocyte transformation affected by both signaling molecules and the extracellular matrix. Aberrant osteocyte morphology and function, along with anomalies in molecular signaling mechanisms, might explain in part, if not all, the pre-osteoblast clustering and the uncoupled bone remodeling in OA subchondral bone.

Mechanical tibial loading remotely suppresses brain tumors by dopamine-mediated downregulation of CCN4

Mechanical loading to the bone is known to be beneficial for bone homeostasis and for suppressing tumor-induced osteolysis in the loaded bone. However, whether loading to a weight-bearing hind limb can inhibit distant tumor growth in the brain is unknown. We examined the possibility of bone-to-brain mechanotransduction using a mouse model of a brain tumor by focusing on the response to Lrp5-mediated Wnt signaling and dopamine in tumor cells. The results revealed that loading the tibia with elevated levels of tyrosine hydroxylase, a rate-limiting enzyme in dopamine synthesis, markedly reduced the progression of the brain tumors. The simultaneous application of fluphenazine (FP), an antipsychotic dopamine modulator, enhanced tumor suppression. Dopamine and FP exerted antitumor effects through the dopamine receptors DRD1 and DRD2, respectively. Notably, dopamine downregulated Lrp5 via DRD1 in tumor cells. A cytokine array analysis revealed that the reduction in CCN4 was critical for loading-driven, dopamine-mediated tumor suppression. The silencing of Lrp5 reduced CCN4, and the administration of CCN4 elevated oncogenic genes such as MMP9, Runx2, and Snail. In summary, this study demonstrates that mechanical loading regulates dopaminergic signaling and remotely suppresses brain tumors by inhibiting the Lrp5-CCN4 axis via DRD1, indicating the possibility of developing an adjuvant bone-mediated loading therapy.

Mechanistic Insight into Orthodontic Tooth Movement Based on Animal Studies: A Critical Review

Alveolar bone remodeling in orthodontic tooth movement (OTM) is a highly regulated process that coordinates bone resorption by osteoclasts and new bone formation by osteoblasts. Mechanisms involved in OTM include mechano-sensing, sterile inflammation-mediated osteoclastogenesis on the compression side and tensile force-induced osteogenesis on the tension side. Several intracellular signaling pathways and mechanosensors including the cilia and ion channels transduce mechanical force into biochemical signals that stimulate formation of osteoclasts or osteoblasts. To date, many studies were performed in vitro or using human gingival crevicular fluid samples. Thus, the use of transgenic animals is very helpful in examining a cause and effect relationship. Key cell types that participate in mediating the response to OTM include periodontal ligament fibroblasts, mesenchymal stem cells, osteoblasts, osteocytes, and osteoclasts. Intercellular signals that stimulate cellular processes needed for orthodontic tooth movement include receptor activator of nuclear factor-κB ligand (RANKL), tumor necrosis factor-α (TNF-α), dickkopf Wnt signaling pathway inhibitor 1 (DKK1), sclerostin, transforming growth factor beta (TGF-β), and bone morphogenetic proteins (BMPs). In this review, we critically summarize the current OTM studies using transgenic animal models in order to provide mechanistic insight into the cellular events and the molecular regulation of OTM.

Cumulative Effects of Strontium Ranelate and Impact Exercise on Bone Mass in Ovariectomized Rats

OBJECTIVE

To explore the effect of physical exercise (EXE), strontium ranelate (SR), or their combination on bone status in ovariectomized (OVX) rats.

DESIGN

Sixty female Wistar rats were randomized to one of five groups: sham (Sh), OVX (O), OVX+EXE (OE), OVX+SR (OSR), and OVX+EXE+SR (OESR). Animals in EXE groups were subjected to 10 drops per day (45 cm in height); rats in SR groups received 625 mg/kg/day of SR, 5 days/week for 8 weeks. Bone mineral density (BMD) and bone mineral content (BMC, dual-energy X-ray absorptiometry (DXA)), mechanical strength of the left femur (three-point bending test), and femur microarchitecture of (micro-computed tomography imaging, microCT) analyses were performed to characterize biomechanical and trabecular/cortical structure. Bone remodeling, osteocyte apoptosis, and lipid content were evaluated by ELISA and immunofluorescence tests.

RESULTS

In OVX rats, whole-body BMD, trabecular parameters, and osteocalcin (OCN) levels decreased, while weight, lean/fat mass, osteocyte apoptosis, and lipid content all increased. EXE after ovariectomy improved BMD and BMC, trabecular parameters, cross-sectional area (CSA), moment of inertia, and OCN levels while decreasing osteocyte apoptosis and lipid content. SR treatment increased BMD and BMC, trabecular parameters, CSA, stiffness, OCN, and alkaline phosphatase (ALP) levels. Furthermore, fat mass, N-telopeptide (NTX) level, osteocyte apoptosis, and lipid content significantly decreased. The combination of both EXE and SR improved bone parameters compared with EXE or SR alone.

CONCLUSION

EXE and SR had positive and synergistic effects on bone formation and resorption.

Glycogen synthase kinase 3 alpha/beta deletion induces precocious growth plate remodeling in mice

Glycogen synthase kinase (GSK) 3 acts to negatively regulate multiple signaling pathways, including canonical Wnt signaling. The two mammalian GSK3 proteins (alpha and beta) are at least partially redundant. While Gsk3a KO mice are viable and display a metabolic phenotype, abnormal neuronal development, and accelerated aging, Gsk3b KO animals die late in embryogenesis or at birth. Selective Gsk3b KO in bone delays development of some bones, whereas cartilage-specific Gsk3b KO mice are normal except for elevated levels of GSK3A protein. However, the collective role of these two GSK3 proteins in cartilage was not evaluated. To address this, we generated tamoxifen-inducible, cartilage-specific Gsk3a/Gsk3b KO (described as "cDKO") in juvenile mice and investigated their skeletal phenotypes. We found that cartilage-specific Gsk3a/Gsk3b deletion in young, skeletally immature mice causes precocious growth plate (GP) remodeling, culminating in shorter long bones and hence, growth retardation. These mice exhibit inefficient breathing patterns at later stages and fail to survive. The disrupted GP in cDKO mice showed progressive loss of cellular and proteoglycan components, and immunostaining for SOX9, while BGLAP (osteocalcin) and COL2A1 increased. In addition, we observed increased osteoclast recruitment and cell apoptosis. Surprisingly, changes in articular cartilage of cDKO mice were mild compared with the GP, signifying differential regulation of articular cartilage vs GP tissues. Taken together, these findings emphasize a crucial role of two GSK3 proteins in skeletal development, in particular in the maintenance and function of GP. KEY MESSAGES: • Both GSK3 genes, together, are crucial regulators of growth plate remodeling. • Cartilage-specific deletion of both GSK3 genes causes skeletal growth retardation. • Deletion of both GSK3 genes decreases Sox9 levels and promotes chondrocyte apoptosis. • Cartilage-specific GSK3 deletion in juvenile mice culminates in premature lethality. • GSK3 deletion exhibits mild effects on articular cartilage compared to growth plate.

Mechanically Driven Counter-Regulation of Cortical Bone Formation in Response to Sclerostin-Neutralizing Antibodies

Sclerostin (Scl) antibodies (Scl-Ab) potently stimulate bone formation, but these effects are transient. Whether the rapid inhibition of Scl-Ab anabolic effects is due to a loss of bone cells' capacity to form new bone or to a mechanostatic downregulation of Wnt signaling once bone strength exceeds stress remains unclear. We hypothesized that bone formation under Scl-Ab could be reactivated by increasing the dose of Scl-Ab and/or by adding mechanical stimuli, and investigated the molecular mechanisms involved in this response, in particular the role of periostin (Postn), a co-activator of the Wnt pathway in bone. For this purpose, C57Bl/6, Postn^-/- and Postn^+/+ mice were treated with vehicle or Scl-Ab (50 to 100 mg/kg/wk) for various durations and subsequently subjected to tibia axial compressive loading. In wild-type (WT) mice, Scl-Ab anabolic effects peaked between 2 and 4 weeks and declined thereafter, with no further increase in bone volume and strength between 7 and 10 weeks. Doubling the dose of Scl-Ab did not rescue the decline in bone formation. In contrast, mechanical stimulation was able to restore cortical bone formation concomitantly to Scl-Ab treatment at both doses. Several Wnt inhibitors, including Dkk1, Sost, and Twist1, were upregulated, whereas Postn was markedly downregulated by 2 to 4 weeks of Scl-Ab. Mechanical loading specifically upregulated Postn gene expression. In turn, Scl-Ab effects on cortical bone were more rapidly downregulated in Postn^-/- mice. These results indicate that bone formation is not exhausted by Scl-Ab but inhibited by a mechanically driven downregulation of Wnt signaling. Hence, increasing mechanical loads restores bone formation on cortical surfaces, in parallel with Postn upregulation. © 2020 American Society for Bone and Mineral Research (ASBMR).

Clinical Phenotype and Relevance of LRP5 and LRP6 Variants in Patients With Early-Onset Osteoporosis (EOOP)

Reduced bone mineral density (BMD; ie, Z-score ≤-2.0) occurring at a young age (ie, premenopausal women and men <50 years) in the absence of secondary osteoporosis is considered early-onset osteoporosis (EOOP). Mutations affecting the WNT signaling pathway are of special interest because of their key role in bone mass regulation. Here, we analyzed the effects of relevant LRP5 and LRP6 variants on the clinical phenotype, bone turnover, BMD, and bone microarchitecture. After exclusion of secondary osteoporosis, EOOP patients (n = 372) were genotyped by gene panel sequencing, and segregation analysis of variants in LRP5/LRP6 was performed. The clinical assessment included the evaluation of bone turnover parameters, BMD by dual-energy X-ray absorptiometry, and microarchitecture via high-resolution peripheral quantitative computed tomography (HR-pQCT). In 50 individuals (31 EOOP index patients, 19 family members), relevant variants affecting LRP5 or LRP6 were detected (42 LRP5 and 8 LRP6 variants), including 10 novel variants. Seventeen variants were classified as disease causing, 14 were variants of unknown significance, and 19 were BMD-associated single-nucleotide polymorphisms (SNPs). One patient harbored compound heterozygous LRP5 mutations causing osteoporosis-pseudoglioma syndrome. Fractures were reported in 37 of 50 individuals, consisting of vertebral (18 of 50) and peripheral (29 of 50) fractures. Low bone formation was revealed in all individuals. A Z-score ≤-2.0 was detected in 31 of 50 individuals, and values at the spine were significantly lower than those at the hip (-2.1 ± 1.3 versus -1.6 ± 0.8; p = .003). HR-pQCT analysis (n = 34) showed impaired microarchitecture in trabecular and cortical compartments. Significant differences regarding the clinical phenotype were detectable between index patients and family members but not between different variant classes. Relevant variants in LRP5 and LRP6 contribute to EOOP in a substantial number of individuals, leading to a high number of fractures, low bone formation, reduced Z-scores, and impaired microarchitecture. This detailed skeletal characterization improves the interpretation of known and novel LRP5 and LRP6 variants. © 2020 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Direct and Indirect Effect of Honey as a Functional Food Against Metabolic Syndrome and Its Skeletal Complications

Metabolic syndrome (MetS) refers to the simultaneous presence of hypertension, hyperglycemia, dyslipidemia and/or visceral obesity, which predisposes a person to cardiovascular diseases and diabetes. Evidence suggesting the presence of direct and indirect associations between MetS and osteoporosis is growing. Many studies have reported the beneficial effects of polyphenols in alleviating MetS in in vivo and in vitro models through their antioxidant and anti-inflammation actions. This review aims to summarize the effects of honey (based on unifloral and multi-floral nectar sources) on bone metabolism and each component of MetS. A literature search was performed using the PubMed and Scopus databases using specific search strings. Original studies related to components of MetS and bone, and the effects of honey on components of MetS and bone were included. Honey polyphenols could act synergistically in alleviating MetS by preventing oxidative damage and inflammation. Honey intake is shown to reduce blood glucose levels and prevent excessive weight gain. It also improves lipid metabolism by reducing total cholesterol, triglycerides and low-density lipoprotein, as well as increasing high-density lipoprotein. Honey can prevent bone loss by reducing the adverse effects of MetS on bone homeostasis, apart from its direct action on the skeletal system. In conclusion, honey supplementation could be integrated into the management of MetS and MetS-induced bone loss as a preventive and adjunct therapeutic agent.

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.

The effect of overexpression of Lrp5 on orthodontic tooth movement

OBJECTIVE

To analyse the effect of gain-of-function mutations in the low-density lipoprotein receptor-related protein 5 (Lrp5) on orthodontic tooth movement (OTM).

SETTING AND SAMPLE POPULATION

A split-mouth study design was utilized. Thirty-two male Lrp5-high bone mass (HBM) knock-in mice including A214V and G171V mutants (n = 16/group) and sixteen C57BL/6 wild-type (WT) mice were included in the study.

MATERIALS AND METHODS

A mouse model of OTM was used for mesial movement of the maxillary first molar using a closed-coil nickel titanium (NiTi) spring attached between the molar and the incisors. After 21 days, the dissected maxillae were scanned for micro-computed tomography (micro-CT) analyses and embedded in methyl methacrylate and paraffin for histological staining and imaging. Histological analyses included immunohistochemistry for sclerostin (Sost), tartrate-resistant acid phosphatase (TRAP) staining for osteoclasts and fluorescent imaging.

RESULTS

OTM in the A214V and G171V groups was significantly less than the WT group. Bone volume (BV), per cent bone volume (BV/TV) and trabecular thickness (Tb.Th) were significantly increased in both A241V and G171V animals compared to the WT animals. On the compression side, decreased osteoclast activity was seen in both A214V and G171V groups compared to the WT group. Fluorescent labelling demonstrated that the pattern of bone deposition in the A214V animals was periosteal whereas the G171V animals added bone endocortically.

CONCLUSION

Gain-of-function mutations of Lrp5 decrease orthodontic tooth movement by increasing alveolar bone mass and reducing osteoclast-mediated bone resorption.

Cortical bone adaptation to a moderate level of mechanical loading in male Sost deficient mice

Loss-of-function mutations in the Sost gene lead to high bone mass phenotypes. Pharmacological inhibition of Sost/sclerostin provides a new drug strategy for treating osteoporosis. Questions remain as to how physical activity may affect bone mass under sclerostin inhibition and if that effect differs between males and females. We previously observed in female Sost knockout (KO) mice an enhanced cortical bone formation response to a moderate level of applied loading (900 με at the tibial midshaft). The purpose of the present study was to examine cortical bone adaptation to the same strain level applied to male Sost KO mice. Strain-matched in vivo compressive loading was applied to the tibiae of 10-, 26- and 52-week-old male Sost KO and littermate control (LC) mice. The effect of tibial loading on bone (re)modeling was measured by microCT, 3D time-lapse in vivo morphometry, 2D histomorphometry and gene expression analyses. As expected, Sost deficiency led to high cortical bone mass in 10- and 26-week-old male mice as a result of increased bone formation. However, the enhanced bone formation associated with Sost deficiency did not appear to diminish with skeletal maturation. An increase in bone resorption was observed with skeletal maturation in male LC and Sost KO mice. Two weeks of in vivo loading (900 με at the tibial midshaft) induced only a mild anabolic response in 10- and 26-week-old male mice, independent of Sost deficiency. A decrease in the Wnt inhibitor Dkk1 expression was observed 3 h after loading in 52-week-old Sost KO and LC mice, and an increase in Lef1 expression was observed 8 h after loading in 10-week-old Sost KO mice. The current results suggest that long-term inhibition of sclerostin in male mice does not influence the adaptive response of cortical bone to moderate levels of loading. In contrast with our previous strain-matched study in females showing enhanced bone responses with Sost ablation, these results in males indicate that the influence of Sost deficiency on the cortical bone formation response to a moderate level of loading differs between males and females. Clinical studies examining antibodies to inhibit sclerostin may need to consider that the efficacy of additional physical activity regimens may be sex dependent.

The Intersection of Mechanotransduction and Regenerative Osteogenic Materials

Mechanical signals play a central role in cell fate determination and differentiation in both physiologic and pathologic circumstances. Such signals may be delivered using materials to generate discrete microenvironments for the purposes of tissue regeneration and have garnered increasing attention in recent years. Unlike the addition of progenitor cells or growth factors, delivery of a microenvironment is particularly attractive in that it may reduce the known untoward consequences of the former two strategies, such as excessive proliferation and potential malignant transformation. Additionally, the ability to spatially modulate the fabrication of materials allows for the creation of multiple microenvironments, particularly attractive for regenerating complex tissues. While many regenerative materials have been developed and tested for augmentation of specific cellular responses, the intersection between cell biology and material interactions have been difficult to dissect due to the complexity of both physical and chemical interactions. Specifically, modulating materials to target individual signaling pathways is an avenue of interdisciplinary research that may lead to a more effective method of optimizing regenerative materials. In this work, the aim is to summarize the major mechanotransduction pathways for osteogenic differentiation and to consolidate the known materials and material properties that activate such pathways.

Formation and Developmental Specification of the Odontogenic and Osteogenic Mesenchymes

Within the mandible, the odontogenic and osteogenic mesenchymes develop in a close proximity and form at about the same time. They both originate from the cranial neural crest. These two condensing ecto-mesenchymes are soon separated from each other by a very loose interstitial mesenchyme, whose cells do not express markers suggesting a neural crest origin. The two condensations give rise to mineralized tissues while the loose interstitial mesenchyme, remains as a soft tissue. This is crucial for proper anchorage of mammalian teeth. The situation in all three regions of the mesenchyme was compared with regard to cell heterogeneity. As the development progresses, the early phenotypic differences and the complexity in cell heterogeneity increases. The differences reported here and their evolution during development progressively specifies each of the three compartments. The aim of this review was to discuss the mechanisms underlying condensation in both the odontogenic and osteogenic compartments as well as the progressive differentiation of all three mesenchymes during development. Very early, they show physical and structural differences including cell density, shape and organization as well as the secretion of three distinct matrices, two of which will mineralize. Based on these data, this review highlights the consecutive differences in cell-cell and cell-matrix interactions, which support the cohesion as well as mechanosensing and mechanotransduction. These are involved in the conversion of mechanical energy into biochemical signals, cytoskeletal rearrangements cell differentiation, or collective cell behavior.

Molecular mechanosensors in osteocytes

Osteocytes, the most abundant and long-lived cells in bone, are the master regulators of bone remodeling. In addition to their functions in endocrine regulation and calcium and phosphate metabolism, osteocytes are the major responsive cells in force adaptation due to mechanical stimulation. Mechanically induced bone formation and adaptation, disuse-induced bone loss and skeletal fragility are mediated by osteocytes, which sense local mechanical cues and respond to these cues in both direct and indirect ways. The mechanotransduction process in osteocytes is a complex but exquisite regulatory process between cells and their environment, between neighboring cells, and between different functional mechanosensors in individual cells. Over the past two decades, great efforts have focused on finding various mechanosensors in osteocytes that transmit extracellular mechanical signals into osteocytes and regulate responsive gene expression. The osteocyte cytoskeleton, dendritic processes, Integrin-based focal adhesions, connexin-based intercellular junctions, primary cilium, ion channels, and extracellular matrix are the major mechanosensors in osteocytes reported so far with evidence from both in vitro and in vitro studies. This review aims to give a systematic introduction to osteocyte mechanobiology, provide details of osteocyte mechanosensors, and discuss the roles of osteocyte mechanosensitive signaling pathways in the regulation of bone homeostasis.

Effects of Long-Term Sclerostin Deficiency on Trabecular Bone Mass and Adaption to Limb Loading Differ in Male and Female Mice

A new therapeutic option to treat osteoporosis is focused on Wnt signaling and its inhibitor sclerostin, a product of the Sost gene. In this work, we study the effect of sclerostin deficiency on trabecular bone formation and resorption in male and female mice and whether it affects mechano-responsiveness. Male and female 10- and 26-week-old Sost knockout (KO) and littermate controls (LCs) were subjected to in vivo mechanical loading of the left tibia for 2 weeks. The right tibia served as internal control. The mice were imaged using in vivo micro-computed tomography at days 0, 5, 10, and 15 and tibiae were collected for histomorphometric analyses after euthanasia. Histomorphometry and micro-CT-based 3D time-lapse morphometry revealed an anabolic and anti-catabolic effect of Sost deficiency although increased trabecular bone resorption accompanied by diminished trabecular bone formation occurred with age. Loading led to diminished resorption in adult female but not in male mice. A net gain in bone volume could be achieved with mechanical loading in Sost KO adult female mice, which occurred through a further reduction in resorbed bone volume. Our data show that sclerostin deficiency has a particularly positive effect in adult female mice. Sclerostin antibodies are approved to treat postmenopausal women with high risk of osteoporotic fractures. Further studies are required to clarify whether both sexes benefit equally from sclerostin inhibition.

The Role of Connexin Channels in the Response of Mechanical Loading and Unloading of Bone

The skeleton adapts to mechanical loading to promote bone formation and remodeling. While most bone cells are involved in mechanosensing, it is well accepted that osteocytes are the principal mechanosensory cells. The osteocyte cell body and processes are surrounded by a fluid-filled space, forming an extensive lacuno-canalicular network. The flow of interstitial fluid is a major stress-related factor that transmits mechanical stimulation to bone cells. The long dendritic processes of osteocytes form a gap junction channel network connecting not only neighboring osteocytes, but also cells on the bone surface, such as osteoblasts and osteoclasts. Mechanosensitive osteocytes also form hemichannels that mediate the communication between the cytoplasmic and extracellular microenvironment. This paper will discuss recent research progress regarding connexin (Cx)-forming gap junctions and hemichannels in osteocytes, osteoblasts, and other bone cells, including those richly expressing Cx43. We will then cover the recent progress regarding the regulation of these channels by mechanical loading and the role of integrins and signals in mediating Cx43 channels, and bone cell function and viability. Finally, we will summarize the recent studies regarding bone responses to mechanical unloading in Cx43 transgenic mouse models. The osteocyte has been perceived as the center of bone remodeling, and connexin channels enriched in osteocytes are a likely major player in meditating the function of bone. Based on numerous studies, connexin channels may present as a potential new therapeutic target in the treatment of bone loss and osteoporosis. This review will primarily focus on Cx43, with some discussion in other connexins expressed in bone cells.

Combination therapy in the Col1a2G610C mouse model of Osteogenesis Imperfecta reveals an additive effect of enhancing LRP5 signaling and inhibiting TGFβ signaling on trabecular bone but not on cortical bone

Enhancing LRP5 signaling and inhibiting TGFβ signaling have each been reported to increase bone mass and improve bone strength in wild-type mice. Monotherapy targeting LRP5 signaling, or TGFβ signaling, also improved bone properties in mouse models of Osteogenesis Imperfecta (OI). We investigated whether additive or synergistic increases in bone properties would be attained if enhanced LRP5 signaling was combined with TGFβ inhibition. We crossed an Lrp5 high bone mass (HBM) allele (Lrp5^A214V) into the Col1a2^G610C/+ mouse model of OI. At 6-weeks-of-age we began treating mice with an antibody that inhibits TGFβ1, β2, and β3 (mAb 1D11), or with an isotype-matched control antibody (mAb 13C4). At 12-weeks-old, we observed that combining enhanced LRP5 signaling with inhibited TGFβ signaling produced an additive effect on femoral and vertebral trabecular bone volumes, but not on cortical bone volumes. Although enhanced LRP5 signaling increased femur strength in a 3-point bending assay in Col1a2^G610C/+ mice, femur strength did not improve further with TGFβ inhibition. Neither enhanced LRP5 signaling nor TGFβ inhibition, alone or in combination, improved femur 3-point-bending post-yield displacement in Col1a2^G610C/+ mice. These pre-clinical studies indicate combination therapies that target LRP5 and TGFβ signaling should increase trabecular bone mass in patients with OI more than targeting either signaling pathway alone. Whether additive increases in trabecular bone mass will occur in, and clinically benefit, patients with OI needs to be determined.

Osteocyte-Mediated Translation of Mechanical Stimuli to Cellular Signaling and Its Role in Bone and Non-bone-Related Clinical Complications

PURPOSE OF REVIEW

Osteocytes comprise > 95% of the cellular component in bone tissue and produce a wide range of cytokines and cellular signaling molecules in response to mechanical stimuli. In this review, we aimed to summarize the molecular mechanisms involved in the osteocyte-mediated translation of mechanical stimuli to cellular signaling, and discuss their role in skeletal (bone) diseases and extra-skeletal (non-bone) clinical complications.

RECENT FINDINGS

Two decades before, osteocytes were assumed as a dormant cells buried in bone matrix. In recent years, emerging evidences have shown that osteocytes are pivotal not only for bone homeostasis but also for vital organ functions such as muscle, kidney, and heart. Osteocyte mechanotransduction regulates osteoblast and osteoclast function and maintains bone homeostasis. Mechanical stimuli modulate the release of osteocyte-derived cytokines, signaling molecules, and extracellular cellular vesicles that regulate not only the surrounding bone cell function and bone homeostasis but also the distant organ function in a paracrine and endocrine fashion. Mechanical loading and unloading modulate the osteocytic release of NO, PGE₂, and ATPs that regulates multiple cellular signaling such as Wnt/β-catenin, RANKL/OPG, BMPs, PTH, IGF1, VEGF, sclerostin, and others. Therefore, the in-depth study of the molecular mechanism of osteocyte mechanotransduction could unravel therapeutic targets for various bone and non-bone-related clinical complications such as osteoporosis, sarcopenia, and cancer metastasis to bone.

Increased anabolic bone response in Dkk1 KO mice following tibial compressive loading

A viable Dkk1 knockout (KO) mouse strain in which embryonic lethality is rescued by developmental Wnt3 heterozygosity (Dkk1^-/-:Wnt3^+/-) exhibits increased bone formation and a high bone mass phenotype. We hypothesized that in vivo mechanical loading would further augment the bone formation response in Dkk1 KO mice, comparable to results from Sost KO mice. A cyclic loading protocol was applied to Dkk1 KO mice, wild type mice (WT; Dkk1^+/+:Wnt3^+/+), and Wnt3 heterozygote (Wnt3^+/-; Dkk1^+/+:Wnt3^+/-) controls. The left tibiae of 10-week-old female mice were dynamically loaded in vivo with 7N maximum compressive force 5 days/week for 2 weeks. Dkk1 KO bones were significantly stiffer, and so an additional group of Dkk1 KO received 12N maximum compressive force to achieve an equivalent +1200με strain at the mid-diaphysis. MicroCT and bone histomorphometry analyses were subsequently performed. All groups responded to tibial loading with increased mid-diaphyseal bone volume. The largest effect size was in the Dkk1 KO -12N group. Thus, Dkk1 KO animals had enhanced sensitivity to mechanical loading. Increases in cortical bone volume reflected increased periosteal bone formation. Bone volume and formation were not altered between WT and Wnt3^+/- controls. These data support the concept that agonists of Wnt/β-catenin signaling can act synergistically with load-bearing exercise. Notably, Sost expression decreased with loading in Dkk1 KO and WT mice, independent of genotype. These data suggest that a compensatory downregulation of Sost in Dkk1 KO mice is not likely the primary mechanism for the augmented response to mechanical load.

Sex differences in the patterning of age-related bone loss in the human hallucal metatarsal in rural and urban populations

OBJECTIVES

Age-degenerative features of the metatarsals are poorly known despite the importance of metatarsal bone properties for investigating mobility patterns. We assessed the role of habitual activity in shaping the patterning and magnitude of sexual dimorphism in age-related bone loss in the hallucal metatarsal.

MATERIALS AND METHODS

Cross-sections were extracted at midshaft from micro-computed tomography scan models of individuals from medieval rural (Abingdon Vineyard) and early industrial urban (Spitalfields) settings (n = 71). A suite of cross-sectional geometry dimensions and biomechanical properties were compared between populations.

RESULTS

The rural group display generally stronger and larger metatarsals that show a greater capacity to resist torsion and that have comparatively greater bending strength along the medio-lateral plane. Men in both groups show greater values of cortical area than women, but only in the urban group do men show lower magnitudes of age-related decline compared to females. Women in rural and urban populations show different patterns of age-related decline in bone mass, particularly old women in the urban group show a marked decline in cortical area that is absent for women in the rural group.

DISCUSSION

Lifetime exposure to hard, physical activity in an agricultural setting has contributed to the attainment of greater bone mass and stronger bones in young adults. Furthermore, over the life-course, less of this greater amount of bone is lost, such that sustained activity levels may have acted to buffer against age-related decline, and this is most pronounced for women, who are expected to experience greater bone loss later in life than men.

The Osteocyte as the New Discovery of Therapeutic Options in Rare Bone Diseases

Osteocytes are the most abundant (~95%) cells in bone with the longest half-life (~25 years) in humans. In the past osteocytes have been regarded as vestigial cells in bone, since they are buried inside the tough bone matrix. However, during the last 30 years it has become clear that osteocytes are as important as bone forming osteoblasts and bone resorbing osteoclasts in maintaining bone homeostasis. The osteocyte cell body and dendritic processes reside in bone in a complex lacuno-canalicular system, which allows the direct networking of osteocytes to their neighboring osteocytes, osteoblasts, osteoclasts, bone marrow, blood vessels, and nerves. Mechanosensing of osteocytes translates the applied mechanical force on bone to cellular signaling and regulation of bone adaptation. The osteocyte lacuno-canalicular system is highly efficient in transferring external mechanical force on bone to the osteocyte cell body and dendritic processes via displacement of fluid in the lacuno-canalicular space. Osteocyte mechanotransduction regulates the formation and function of the osteoblasts and osteoclasts to maintain bone homeostasis. Osteocytes produce a variety of proteins and signaling molecules such as sclerostin, cathepsin K, Wnts, DKK1, DMP1, IGF1, and RANKL/OPG to regulate osteoblast and osteoclast activity. Various genetic abnormality-associated rare bone diseases are related to disrupted osteocyte functions, including sclerosteosis, van Buchem disease, hypophosphatemic rickets, and WNT1 and plastin3 mutation-related disorders. Meticulous studies during the last 15 years on disrupted osteocyte function in rare bone diseases guided for the development of various novel therapeutic agents to treat bone diseases. Studies on genetic, molecular, and cellular mechanisms of sclerosteosis and van Buchem disease revealed a role for sclerostin in bone homeostasis, which led to the development of the sclerostin antibody to treat osteoporosis and other bone degenerative diseases. The mechanism of many other rare bone diseases and the role of the osteocyte in the development of such conditions still needs to be investigated. In this review, we mainly discuss the knowledge obtained during the last 30 years on the role of the osteocyte in rare bone diseases. We speculate about future research directions to develop novel therapeutic drugs targeting osteocyte functions to treat both common and rare bone diseases.