LRP6 in mesenchymal stem cells is required for bone formation during bone growth and bone remodeling

RNA-binding proteins in bone pathophysiology

Bone remodelling is a highly regulated process that maintains mineral homeostasis and preserves bone integrity. During this process, intricate communication among all bone cells is required. Indeed, adapt to changing functional situations in the bone, the resorption activity of osteoclasts is tightly balanced with the bone formation activity of osteoblasts. Recent studies have reported that RNA Binding Proteins (RBPs) are involved in bone cell activity regulation. RBPs are critical effectors of gene expression and essential regulators of cell fate decision, due to their ability to bind and regulate the activity of cellular RNAs. Thus, a better understanding of these regulation mechanisms at molecular and cellular levels could generate new knowledge on the pathophysiologic conditions of bone. In this Review, we provide an overview of the basic properties and functions of selected RBPs, focusing on their physiological and pathological roles in the bone.

Multi-Targeting DKK1 and LRP6 Prevents Bone Loss and Improves Fracture Resistance in Multiple Myeloma

An imbalance between bone resorption and bone formation underlies the devastating osteolytic lesions and subsequent fractures seen in more than 90% of multiple myeloma (MM) patients. Currently, Wnt-targeted therapeutic agents that prevent soluble antagonists of the Wnt signaling pathway, sclerostin (SOST) and dickkopf-1 (DKK1), have been shown to prevent bone loss and improve bone strength in preclinical models of MM. In this study, we show increasing Wnt signaling via a novel anti-low-density lipoprotein receptor-related protein 6 (LRP6) antibody, which potentiates Wnt1-class ligand signaling through binding the Wnt receptor LRP6, prevented the development of myeloma-induced bone loss primarily through preventing bone resorption. When combined with an agent targeting the soluble Wnt antagonist DKK1, we showed more robust improvements in bone structure than anti-LRP6 treatment alone. Micro-computed tomography (μCT) analysis demonstrated substantial increases in trabecular bone volume in naïve mice given the anti-LRP6/DKK1 combination treatment strategy compared to control agents. Mice injected with 5TGM1eGFP murine myeloma cells had significant reductions in trabecular bone volume compared to naïve controls. The anti-LRP6/DKK1 combination strategy significantly improved bone volume in 5TGM1-bearing mice by 111%, which was also superior to anti-LRP6 single treatment; with similar bone structural changes observed within L4 lumbar vertebrae. Consequently, this combination strategy significantly improved resistance to fracture in lumbar vertebrae in 5TGM1-bearing mice compared to their controls, providing greater protection against fracture compared to anti-LRP6 antibody alone. Interestingly, these improvements in bone volume were primarily due to reduced bone resorption, with significant reductions in osteoclast numbers and osteoclast surface per bone surface demonstrated in 5TGM1-bearing mice treated with the anti-LRP6/DKK1 combination strategy. Importantly, Wnt stimulation with either single or combined Wnt-targeted agents did not exacerbate tumor activity. This work provides a novel approach of targeting both membrane-bound and soluble Wnt pathway components to provide superior skeletal outcomes in patients with multiple myeloma and other bone destructive cancers. © 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).

LRP6/filamentous-actin signaling facilitates osteogenic commitment in mechanically induced periodontal ligament stem cells

BACKGROUND

Mechanotransduction mechanisms whereby periodontal ligament stem cells (PDLSCs) translate mechanical stress into biochemical signals and thereby trigger osteogenic programs necessary for alveolar bone remodeling are being deciphered. Low-density lipoprotein receptor-related protein 6 (LRP6), a Wnt transmembrane receptor, has been qualified as a key monitor for mechanical cues. However, the role of LRP6 in the mechanotransduction of mechanically induced PDLSCs remains obscure.

METHODS

The Tension System and tooth movement model were established to determine the expression profile of LRP6. The loss-of-function assay was used to investigate the role of LRP6 on force-regulated osteogenic commitment in PDLSCs. The ability of osteogenic differentiation and proliferation was estimated by alkaline phosphatase (ALP) staining, ALP activity assay, western blotting, quantitative real-time PCR (qRT-PCR), and immunofluorescence. Crystalline violet staining was used to visualize cell morphological change. Western blotting, qRT-PCR, and phalloidin staining were adopted to affirm filamentous actin (F-actin) alteration. YAP nucleoplasmic localization was assessed by immunofluorescence and western blotting. YAP transcriptional response was evaluated by qRT-PCR. Cytochalasin D was used to determine the effects of F-actin on osteogenic commitment and YAP switch behavior in mechanically induced PDLSCs.

RESULTS

LRP6 was robustly activated in mechanically induced PDLSCs and PDL tissues. LRP6 deficiency impeded force-dependent osteogenic differentiation and proliferation in PDLSCs. Intriguingly, LRP6 loss caused cell morphological aberration, F-actin dynamics disruption, YAP nucleoplasmic relocation, and subsequent YAP inactivation. Moreover, disrupted F-actin dynamics inhibited osteogenic differentiation, proliferation, YAP nuclear translocation, and YAP activation in mechanically induced PDLSCs.

CONCLUSIONS

We identified that LRP6 in PDLSCs acted as the mechanosensor regulating mechanical stress-inducible osteogenic commitment via the F-actin/YAP cascade. Targeting LRP6 for controlling alveolar bone remodeling may be a prospective therapy to attenuate relapse of orthodontic treatment.

Calcium Homeostasis and Bone Metabolism in Goats Fed a Low Protein Diet

The objective of this study was to investigate the effects of low-protein diets on blood calcium (Ca) level, bone metabolism, and the correlation between bone metabolism and blood calcium in goats. Twenty-four female Xiangdong black goats with similar body weight (19.55 ± 3.55 kg) and age (8.0 ± 0.3 months) were selected and allocated into two groups: control group (CON, 10.77% protein content) and low-protein group (LP, 5.52% protein content). Blood samples were collected on days 1, 4, 7, 16 and 36 before morning feeding to determine the concentration of calcium (Ca), parathyroid hormone (PTH), bone gla protein (BGP), C-terminal telopeptide of type 1 collagen (CTX-1), bone alkaline phosphatase (BALP), and 1, 25-dihydroxyvitamin D3 [1,25(OH)₂D3]. Liver samples were collected to determine the expression of bone metabolism-related genes. There was no difference observed between LP and CON in concentration of plasma Ca or any of bone metabolism markers (P > 0.05). In the liver, the mRNA expression of bone gamma carboxyglutamate protein (BGLAP), alkaline phosphatase (ALPL), and mothers against decapentaplegic homolog-1 (SMAD1) were increased (P < 0.05) in LP as compared with CON. The correlation analysis of Ca and bone metabolism markers showed no significant correlation between Ca and bone metabolism. These results suggest that the blood Ca concentration in mature goats may keep at a stable level through nitrogen cycling when the providing protein is not enough.

Rare phenotype: Hand preaxial polydactyly associated with LRP6-related tooth agenesis in humans

Low-density lipoprotein receptor-related protein 6 (LRP6) is a pathogenic gene of selective tooth agenesis-7 (OMIM#616724). Although the malformation of the digits and fore- and hindlimbs has been reported in Lrp6-deficient mice, it has been rarely discovered in humans with LRP6 mutations. Here, we demonstrate an unreported autosomal dominant LRP6 heterozygous mutation (c.2840 T > C;p.Met947Thr) in a tooth agenesis family with hand polydactyly, and another unreported autosomal dominant LRP6 heterozygous mutation (c.1154 G > C;p.Arg385Pro) in a non-syndromic tooth agenesis family. Bioinformatic prediction demonstrated the deleterious effects of the mutations, and LRP6 structure changes suggested the corresponding functional impairments. Analysis on the pattern of LRP6-related tooth agenesis demonstrated the maxillary lateral incisor was the most affected. Our study report that LRP6 mutation might be associated with hand preaxial polydactyly in humans, which broaden the phenotypic spectrum of LRP6-related disorders, and provide valuable information on the characteristics of LRP6-related tooth agenesis.

Senescent immune cells release grancalcin to promote skeletal aging

Skeletal aging is characterized by low bone turnover and marrow fat accumulation. However, the underlying mechanism for this imbalance is unclear. Here, we show that during aging in rats and mice proinflammatory and senescent subtypes of immune cells, including macrophages and neutrophils, accumulate in the bone marrow and secrete abundant grancalcin. The injection of recombinant grancalcin into young mice was sufficient to induce premature skeletal aging. In contrast, genetic deletion of Gca in neutrophils and macrophages delayed skeletal aging. Mechanistically, we found that grancalcin binds to the plexin-b2 receptor and partially inactivates its downstream signaling pathways, thus repressing osteogenesis and promoting adipogenesis of bone marrow mesenchymal stromal cells. Heterozygous genetic deletion of Plexnb2 in skeletal stem cells abrogated the improved bone phenotype of Gca-knockout mice. Finally, we developed a grancalcin-neutralizing antibody and showed that its treatment of older mice improved bone health. Together, our data suggest that grancalcin could be a potential target for the treatment of age-related osteoporosis.

Inhibition of Axin1 in osteoblast precursor cells leads to defects in postnatal bone growth through suppressing osteoclast formation

Axin1 is a negative regulator of β-catenin signaling and its role in osteoblast precursor cells remains undefined. In the present studies, we determined changes in postnatal bone growth by deletion of Axin1 in osteoblast precursor cells and analyzed bone growth in newborn and postnatal Axin1Osx mice and found that hypertrophic cartilage area was largely expanded in Axin1Osx KO mice. A larger number of chondrocytes and unabsorbed cartilage matrix were found in the bone marrow cavity of Axin1Osx KO mice. Osteoclast formation in metaphyseal and subchondral bone areas was significantly decreased, demonstrated by decreased TRAP-positive cell numbers, associated with reduction of MMP9- and cathepsin K-positive cell numbers in Axin1Osx KO mice. OPG expression and the ratio of Opg to Rankl were significantly increased in osteoblasts of Axin1Osx KO mice. Osteoclast formation in primary bone marrow derived microphage (BMM) cells was significantly decreased when BMM cells were cultured with conditioned media (CM) collected from osteoblasts derived from Axin1Osx mice compared with BMM cells cultured with CM derived from WT mice. Thus, the loss of Axin1 in osteoblast precursor cells caused increased OPG and the decrease in osteoclast formation, leading to delayed bone growth in postnatal Axin1Osx KO mice.

LDL receptor-related protein LRP6 senses nutrient levels and regulates Hippo signaling

Controlled cell growth and proliferation are essential for tissue homeostasis and development. Wnt and Hippo signaling are well known as positive and negative regulators of cell proliferation, respectively. The regulation of Hippo signaling by the Wnt pathway has been shown, but how and which components of Wnt signaling are involved in the activation of Hippo signaling during nutrient starvation are unknown. Here, we report that a reduction in the level of low-density lipoprotein receptor-related protein 6 (LRP6) during nutrient starvation induces phosphorylation and cytoplasmic localization of YAP, inhibiting YAP-dependent transcription. Phosphorylation of YAP via loss of LRP6 is mediated by large tumor suppressor kinases 1/2 (LATS1/2) and Merlin. We found that O-GlcNAcylation of LRP6 was reduced, and the overall amount of LRP6 was decreased via endocytosis-mediated lysosomal degradation during nutrient starvation. Merlin binds to LRP6; when LRP6 is less O-GlcNAcylated, Merlin dissociates from it and becomes capable of interacting with LATS1 to induce phosphorylation of YAP. Our data suggest that LRP6 has unexpected roles as a nutrient sensor and Hippo signaling regulator.

Conditional deletion of Adrb2 in mesenchymal stem cells attenuates osteoarthritis-like defects in temporomandibular joint

β2-adrenergic signal transduction in mesenchymal stem cells (MSCs) induces subchondral bone loss in osteoarthritis (OA) of temporomandibular joints (TMJs). However, whether conditional deletion of β2-adrenergic receptor (Adrb2) in nestin⁺ MSCs can alleviate TMJ-OA development remains unknown. In this study, nestin-Cre mice were crossed with Adrb2 flox mice to generate mice lacking Adrb2 expression specifically in the nestin⁺ MSCs (Adrb2^-/-), and TMJ-OA development in such mice was investigated. Adrb2 flox mice (Adrb2^+/+) and Adrb2^-/- mice were subjected to unilateral anterior crossbite (UAC), while mice in the control group were subjected to sham operation. Adrb2^+/+ and Adrb2^-/- mice in the control group showed no distinguishable phenotypic changes in body weight and length, mandibular condylar size, and other histomorphological parameters of the condylar subchondral bone. A significant increase in subchondral bone loss and cartilage degradation was observed in Adrb2^+/+ UAC mice; the former was characterized by decreased bone mineral density, bone volume fraction, and trabecular plate thickness, and increased trabecular separation, osteoclast number and osteoclast surface, and pro-osteoclastic factor expression; the latter was characterized by decreased cartilage thickness, chondrocyte density, proteoglycan area, and collagen II and aggrecan expression, but increased matrix metalloproteinase and alkaline phosphatase expression and percentage area of calcified cartilage. Adrb2 deletion in nestin⁺ MSCs largely attenuated UAC-induced increase in condylar subchondral bone loss, cartilage degradation, and aberrant calcification at the osteochondral interface. Thus, Adrb2-expressing MSCs in the condylar subchondral bone play an important role in TMJ-OA progression and may serve as novel therapeutic targets for TMJ-OA.

Dissecting the mechanisms of bone loss in Gorham-Stout disease

Gorham-Stout disease (GSD) is a rare disorder characterized by progressive osteolysis and angiomatous proliferation. Since the mechanisms leading to bone loss in GSD are not completely understood, we performed histological, serum, cellular and molecular analyses of 7 patients. Increased vessels, osteoclast number and osteocyte lacunar area were revealed in patients' bone biopsies. Biochemical analysis of sera showed high levels of ICTP, Sclerostin, VEGF-A and IL-6. In vitro experiments revealed increased osteoclast differentiation and activity, and impaired mineralization ability of osteoblasts. To evaluate the involvement of systemic factors in GSD, control cells were treated with patients' sera and displayed an increase of osteoclastogenesis, bone resorption activity and a reduction of osteoblast function. Interestingly, GSD sera stimulated the vessel formation by endothelial cells EA.hy926. These results suggest that bone cell autonomous alterations with the cooperation of systemic factors are involved in massive bone loss and angiomatous proliferation observed in GSD patients.

Curcumin suppresses osteogenesis by inducing miR-126a-3p and subsequently suppressing the WNT/LRP6 pathway

Curcumin, a natural phenolic biphenyl compound derived from the plant Curcuma longa, modulates multiple steps of carcinogenesis partly by affecting the expression of miRNAs. Interestingly, cancer development shares many of the same signalling pathways with bone formation. Reduced bone mass creates favourable conditions for tumor metastasis. However, the effects and mechanism of curcumin on bone formation and osteogenesis are relatively unknown and controversial. We demonstrated that curcumin inhibited osteogenesis of human adipose-derived mesenchymal stem cells (hADSCs) in a concentration-dependent manner. In hADSCs, curcumin modulates the expression of a series of miRNAs, including miR-126a-3p, during osteogenesis. Overexpression or inhibition of miR-126a-3p is required for the effect of curcumin on osteogenesis. Further investigation indicated that miR-126a-3p directly targets and inhibits LRP6 through binding to its 3’-UTR, and then blocks WNT activation. Our findings suggest that the use of curcumin as an anti-tumor agent may lead to decreased bone mass through the suppression of osteogenesis. Knowing whether the long-term or high doses use of curcumin will cause decreased bone mass and bone density, which might increase the potential threat of tumor metastasis, also requires a neutral assessment of the role of curcumin in both regulating bone formation and bone absorption.

Bench-to-bedside strategies for osteoporotic fracture: From osteoimmunology to mechanosensation

Osteoporosis is characterized by a decrease in bone mass and strength, rendering people prone to osteoporotic fractures caused by low-energy forces. The primary treatment strategy for osteoporotic fractures is surgery; however, the compromised and comminuted bones in osteoporotic fracture sites are not conducive to optimum reduction and rigid fixation. In addition, these patients always exhibit accompanying aging-related disorders, including high inflammatory status, decreased mechanical loading and abnormal skeletal metabolism, which are disadvantages for fracture healing around sites that have undergone orthopedic procedures. Since the incidence of osteoporosis is expected to increase worldwide, orthopedic surgeons should pay more attention to comprehensive strategies for improving the poor prognosis of osteoporotic fractures. Herein, we highlight the molecular basis of osteoimmunology and bone mechanosensation in different healing phases of elderly osteoporotic fractures, guiding perioperative management to alleviate the unfavorable effects of insufficient mechanical loading, high inflammatory levels and pathogen infection. The well-informed pharmacologic and surgical intervention, including treatment with anti-inflammatory drugs and sufficient application of antibiotics, as well as bench-to-bedside strategies for bone augmentation and hardware selection, should be made according to a comprehensive understanding of bone biomechanical properties in addition to the remodeling status of osteoporotic bones, which is necessary for creating proper biological and mechanical environments for bone union and remodeling. Multidisciplinary collaboration will facilitate the improvement of overall osteoporotic care and reduction of secondary fracture incidence.

Long noncoding RNA Bmncr regulates mesenchymal stem cell fate during skeletal aging

Bone marrow mesenchymal stem cells (BMSCs) exhibit an age-related lineage switch between osteogenic and adipogenic fates, which contributes to bone loss and adiposity. Here we identified a long noncoding RNA, Bmncr, which regulated the fate of BMSCs during aging. Mice depleted of Bmncr (Bmncr-KO) showed decreased bone mass and increased bone marrow adiposity, whereas transgenic overexpression of Bmncr (Bmncr-Tg) alleviated bone loss and bone marrow fat accumulation. Bmncr regulated the osteogenic niche of BMSCs by maintaining extracellular matrix protein fibromodulin (FMOD) and activation of the BMP2 pathway. Bmncr affected local 3D chromatin structure and transcription of Fmod. The absence of Fmod modified the bone phenotype of Bmncr-Tg mice. Further analysis revealed that Bmncr would serve as a scaffold to facilitate the interaction of TAZ and ABL, and thus facilitate the assembly of the TAZ and RUNX2/PPARG transcriptional complex, promoting osteogenesis and inhibiting adipogenesis. Adeno-associated viral-mediated overexpression of Taz in osteoprogenitors alleviated bone loss and marrow fat accumulation in Bmncr-KO mice. Furthermore, restoring BMNCR levels in human BMSCs reversed the age-related switch between osteoblast and adipocyte differentiation. Our findings indicate that Bmncr is a key regulator of the age-related osteogenic niche alteration and cell fate switch of BMSCs.

Oxidized phospholipids are ligands for LRP6

Low-density lipoprotein receptor-related protein 6 (LRP6) is a co-receptor for Wnt signaling and can be recruited by multiple growth factors/hormones to their receptors facilitating intracellular signaling activation. The ligands that bind directly to LRP6 have not been identified. Here, we report that bioactive oxidized phospholipids (oxPLs) are native ligands of LRP6, but not the closely related LRP5. oxPLs are products of lipid oxidation involving in pathological conditions such as hyperlipidemia, atherosclerosis, and inflammation. We found that cell surface LRP6 in bone marrow mesenchymal stromal cells (MSCs) decreased rapidly in response to increased oxPLs in marrow microenvironment. LRP6 directly bound and mediated the uptake of oxPLs by MSCs. oxPL-LRP6 binding induced LRP6 endocytosis through a clathrin-mediated pathway, decreasing responses of MSCs to osteogenic factors and diminishing osteoblast differentiation ability. Thus, LRP6 functions as a receptor and molecular target of oxPLs for their adverse effect on MSCs, revealing a potential mechanism underlying atherosclerosis-associated bone loss.

Immunomodulatory ECM-like Microspheres for Accelerated Bone Regeneration in Diabetes Mellitus

Bone repair and regeneration process is markedly impaired in diabetes mellitus (DM) that affects hundreds of millions of people worldwide. As a chronic inflammatory disease, DM creates a proinflammatory microenvironment in defective sites. Most of the studies on DM-associated bone regeneration, however, neglect the importance of immunomodulation under the DM condition and adopt the same approaches to normal bone healing, leading to limited bone healing. In this study, we developed a unique bioinspired injectable microsphere as an osteoimmunomodulatory biomaterial that modulates macrophages to create a prohealing microenvironment under the DM condition. The microsphere was self-assembled with heparin-modified gelatin nanofibers, and interleukin 4 (IL4) was incorporated into the nanofibrous heparin-modified gelatin microsphere (NHG-MS). IL4 has binding domains with heparin, and the binding of IL4 to heparin stabilizes this cytokine, protects it from denaturation and degradation, and subsequently prolongs its sustained release to modulate macrophage polarization. The IL4-loaded NHG-MS switched the proinflammatory M1 macrophage into a prohealing M2 phenotype, recovered the M2/M1 ratio to a normal level, efficiently resolved the inflammation, and ultimately enhanced osteoblastic differentiation and bone regeneration. The development of osteoimmunomodulatory biomaterials that harness the power of macrophages for immunomodulation, therefore, is a novel and promising strategy to enhance bone regeneration under DM condition.

Mice with a heterozygous Lrp6 deletion have impaired fracture healing

Bone fracture non-unions, the failure of a fracture to heal, occur in 10%–20% of fractures and are a costly and debilitating clinical problem. The Wnt/β-catenin pathway is critical in bone development and fracture healing. Polymorphisms of linking low-density lipoprotein receptor-related protein 6 (LRP6), a Wnt-binding receptor, have been associated with decreased bone mineral density and fragility fractures, although this remains controversial. Mice with a homozygous deletion of Lrp6 have severe skeletal abnormalities and are not viable, whereas mice with a heterozygous deletion have a combinatory effect with Lrp5 to decrease bone mineral density. As fracture healing closely models embryonic skeletal development, we investigated the process of fracture healing in mice heterozygous for Lrp6 (Lrp6^+/−) and hypothesized that the heterozygous deletion of Lrp6 would impair fracture healing. Mid-diaphyseal femur fractures were induced in Lrp6^+/− mice and wild-type controls (Lrp6^+/+). Fractures were analyzed using micro-computed tomography (μCT) scans, biomechanical testing, and histological analysis. Lrp6^+/− mice had significantly decreased stiffness and strength at 28 days post fracture (PF) and significantly decreased BV/TV, total density, immature bone density, and mature area within the callus on day-14 and -21 PF; they had significantly increased empty callus area at days 14 and 21 PF. Our results demonstrate that the heterozygous deletion of Lrp6 impairs fracture healing, which suggests that Lrp6 has a role in fracture healing.

Characterization of genetically engineered mouse models carrying Col2a1-cre-induced deletions of Lrp5 and/or Lrp6

Mice carrying Collagen2a1-cre-mediated deletions of Lrp5 and/or Lrp6 were created and characterized. Mice lacking either gene alone were viable and fertile with normal knee morphology. Mice in which both Lrp5 and Lrp6 were conditionally ablated via Collagen2a1-cre-mediated deletion displayed severe defects in skeletal development during embryogenesis. In addition, adult mice carrying Collagen2a1-cre-mediated deletions of Lrp5 and/or Lrp6 displayed low bone mass suggesting that the Collagen2a1-cre transgene was active in cells that subsequently differentiated into osteoblasts. In both embryonic skeletal development and establishment of adult bone mass, Lrp5 and Lrp6 carry out redundant functions.

BMP7 and EREG Contribute to the Inductive Potential of Dental Mesenchyme

Odontogenesis is accomplished by reciprocal signaling between the epithelial and mesenchymal compartments. It is generally accepted that the inductive mesenchyme is capable of inducing the odontogenic commitment of both dental and non-dental epithelial cells. However, the duration of this signal in the developing dental mesenchyme and whether adult dental pulp tissue maintains its inductive capability remain unclear. This study investigated the contribution of growth factors to regulating the inductive potential of the dental mesenchyme. Human oral epithelial cells (OEs) were co-cultured with either human dental mesenchymal/papilla cells (FDPCs) or human dental pulp cells (ADPCs) under 2-dimensional or 3-dimensional conditions. Odontogenic-associated genes and proteins were detected by qPCR and immunofluorescence, respectively, and significant differences were observed between the two co-culture systems. The BMP7 and EREG expression levels in FDPCs were significantly higher than in ADPCs, as indicated by human growth factor PCR arrays and immunofluorescence analyses. OEs co-cultured with ADPCs supplemented with BMP7 and EREG expressed ameloblastic differentiation genes. Our study suggests that BMP7 and EREG expression in late bell-stage human dental papilla contributes to the inductive potential of dental mesenchyme. Furthermore, adult dental pulp cells supplemented with these two growth factors re-established the inductive potential of postnatal dental pulp tissue.

Lipoprotein receptor-related protein 6 is required for parathyroid hormone-induced Sost suppression

Parathyroid hormone (PTH) suppresses the expression of the bone formation inhibitor sclerostin (Sost) in osteocytes by inducing nuclear accumulation of histone deacetylases (HDACs) to inhibit the myocyte enhancer factor 2 (MEF2)-dependent Sost bone enhancer. Previous studies revealed that lipoprotein receptor-related protein 6 (LRP6) mediates the intracellular signaling activation and the anabolic bone effect of PTH. Here, we investigated whether LRP6 mediates the inhibitory effect of PTH on Sost using an osteoblast-specific Lrp6-knockout (LRP6-KO) mouse model. An increased level of Sost mRNA expression was detected in femur tissue from LRP6-KO mice, compared to wild-type littermates. The number of osteocytes expressing sclerostin protein was also increased in bone tissue of LRP6-KO littermates, indicating a negative regulatory role of LRP6 on Sost/sclerostin. In wild-type littermates, intermittent PTH treatment significantly suppressed Sost mRNA expression in bone and the number of sclerostin(+) osteocytes, while the effect of PTH was much less significant in LRP6-KO mice. Additionally, PTH-induced downregulation of MEF2C and 2D, as well as HDAC changes in osteocytes, were abrogated in LRP6-KO mice. These data indicate that LRP6 is required for PTH suppression of Sost expression.

Decreased osteogenesis of adult mesenchymal stem cells by reactive oxygen species under cyclic stretch: a possible mechanism of age related osteoporosis

Age related defect of the osteogenic differentiation of mesenchymal stem cells (MSCs) plays a key role in osteoporosis. Mechanical loading is one of the most important physical stimuli for osteoblast differentiation. Here, we compared the osteogenic potential of MSCs from young and adult rats under three rounds of 2 h of cyclic stretch of 2.5% elongation at 1 Hz on 3 consecutive days. Cyclic stretch induced a significant osteogenic differentiation of MSCs from young rats, while a compromised osteogenesis in MSCs from the adult rats. Accordingly, there were much more reactive oxygen species (ROS) production in adult MSCs under cyclic stretch compared to young MSCs. Moreover, ROS scavenger N-acetylcysteine rescued the osteogenic differentiation of adult MSCs under cyclic stretch. Gene expression analysis revealed that superoxide dismutase 1 (SOD1) was significantly downregulated in those MSCs from adult rats. In summary, our data suggest that reduced SOD1 may result in excessive ROS production in adult MSCs under cyclic stretch, and thus manipulation of the MSCs from the adult donors with antioxidant would improve their osteogenic ability.

MicroRNA-188 regulates age-related switch between osteoblast and adipocyte differentiation

Bone marrow mesenchymal stem cells (BMSCs) exhibit an age-dependent reduction in osteogenesis that is accompanied by an increased propensity toward adipocyte differentiation. This switch increases adipocyte numbers and decreases the number of osteoblasts, contributing to age-related bone loss. Here, we found that the level of microRNA-188 (miR-188) is markedly higher in BMSCs from aged compared with young mice and humans. Compared with control mice, animals lacking miR-188 showed a substantial reduction of age-associated bone loss and fat accumulation in bone marrow. Conversely, mice with transgenic overexpression of miR-188 in osterix+ osteoprogenitors had greater age-associated bone loss and fat accumulation in bone marrow relative to WT mice. Moreover, using an aptamer delivery system, we found that BMSC-specific overexpression of miR-188 in mice reduced bone formation and increased bone marrow fat accumulation. We identified histone deacetylase 9 (HDAC9) and RPTOR-independent companion of MTOR complex 2 (RICTOR) as the direct targets of miR-188. Notably, BMSC-specific inhibition of miR-188 by intra-bone marrow injection of aptamer-antagomiR-188 increased bone formation and decreased bone marrow fat accumulation in aged mice. Together, our results indicate that miR-188 is a key regulator of the age-related switch between osteogenesis and adipogenesis of BMSCs and may represent a potential therapeutic target for age-related bone loss.