A single nucleotide polymorphism in osteonectin 3' untranslated region regulates bone volume and is targeted by miR-433

A potential function for MicroRNA-124 in normal and pathological bone conditions

Cells produce short single-stranded non-coding RNAs (ncRNAs) called microRNAs (miRNAs), which actively regulate gene expression at the posttranscriptional level. Several miRNAs have been observed to exert significant impacts on bone health and bone-related disorders. One of these, miR-124, is observed in bone microenvironments and is conserved across species. It affects bone cell growth and differentiation by activating different transcription factors and signaling pathways. In-depth functional analyses of miR-124 have revealed several physiological and pathological roles exerted through interactions with other ncRNAs. Deciphering these RNA-mediated signaling networks and pathways is essential for understanding the potential impacts of dysregulated miRNA functions on bone biology. In this review, we aim to provide a comprehensive analysis of miR-124's involvement in bone physiology and pathology. We highlight the importance of miR-124 in controlling transcription factors and signaling pathways that promote bone growth. This review reveals therapeutic implications for the treatment of bone-related diseases.

The role of miR-433-3p in vascular calcification in type 2 diabetic patients: targeting WNT/β-Catenin and RANKL/RANK/OPG signaling pathways

BACKGROUND

Vascular calcification (VC) is a major predictor of cardiovascular diseases that represent the principal cause of mortality among type-2 diabetic patients. Accumulating data suggest the vital role of some microRNAs on vascular calcification as an epigenetic regulator. Thus, we assessed herein, the role of serum miR-433-3p in vascular calcification in type-2 diabetic patients.

METHODS

Twenty healthy subjects (control group) and forty diabetic patients (20 without VC and 20 with VC) were involved in the study. miR-433-3p gene expression was measured. Runx2, Dickkopf-1 (DKK1), β-catenin, Receptor activator of nuclear factor kappa-B ligand (RANKL), and osteoprotegerin (OPG) levels in serum were assessed by ELISA technique.

RESULTS

Diabetes patients had significantly lower levels of miR-433-3p expression in comparison to the control group, with the lowest levels being found in diabetic patients with VC. Furthermore, Runx2, β-catenin, and RANKL levels were significantly increased with concomitant lower DKK1 and OPG levels detected in the two diabetic groups especially those with VC.

CONCLUSION

Collectively, the study documented that down-regulation of miR-433-3p may contribute to the development of VC through activating WNT/β-Catenin and RANKL/RANK/OPG signaling pathways.

Osteonectin bidirectionally regulates osteoblast mineralization

OBJECTIVE

The aim of this study was to investigate whether Osteonectin/Secreted protein acidic and rich in cysteine (ON/SPARC) had a two-way dose-dependent regulatory effect on osteoblast mineralization and its molecular mechanism.

METHODS

Initially, different concentrations of ON were added in osteoblasts, and the gene of bone sialoprotein (BSP), osteocalcin (OCN), osteopontin (OPN) and alkaline phosphatase (ALP) were detected using reverse-transcription quantitative polymerase chain reaction (RT-PCR). Secondly, based on the above results, the Optima and inhibitory concentration of ON for osteoblast mineralization were determined and regrouped, the Control group was also set up, and the gene detections of Collagen 1 (Col 1), Discoidin domain receptor 2 (DDR2) and p38 mitogen‑activated protein kinase were added using RT-PCR. In the third stage of the experiment, osteoblasts were pretreated with 0.4Mm ethyl-3,4-dihydroxybenzoate (DHB) (a specific inhibitor of collagen synthesis) for 3 h before adding the optima SPARC, the gene and protein expressions of OCN, OPN, BSP, ALP, DDR2, ALP, Col 1, DDR2 and P38 were detected by RT‑qPCR and western blot analysis, and the mineralized nodules were observed by alizarin red staining.

RESULTS

The results showed that the expression of OCN, OPN, BSP, ALP, DDR2, ALP, Col 1, DDR2 and P38 genes and proteins in osteoblasts were significantly enhanced by 1 ug/ml ON, 100 ug/ml ON or 1 ug/ml ON added with 3,4 DHB significantly inhibited the expressions of DDR2, P38 and the above-mentioned mineralization indexes, and significantly reduced the formation of mineralized nodules.

CONCLUSION

This study suggested that ON had a bidirectional dose-dependent regulatory effect on osteoblast mineralization, and the activation of P38 pathway by collagen binding to DDR2 was also an important molecular mechanism.

A Novel Construct of Coral Granules-Poly-L-Lactic Acid Nanomembrane Sandwich Double Stem Cell Sheet Transplantation as Regenerative Therapy of Bone Defect Model

OBJECTIVES

We examined the use of a new approach in nanotechnology and stem cell research as regenerative therapy for bone tissue defects.

MATERIALS AND METHODS

We compared in vitro osteogenic potential of human Wharton jelly mesenchymal stem cells using coral granules and poly-L-lactic acid nanofiber according to proliferation (by cck-8 kit) and osteogenes (runt-related transcription factor 2, alkaline phosphatase, osteonectin) by quantitative reverse transcription-polymerase chain reaction, alkaline phosphatase assay, calcium measurement, and assessment of mineralization by Alizarin red and von Kossa staining. To overcome the limitations of natural coral, we made a modification by packaging the coral granules-human Wharton jelly mesenchymal stem cells by nanomembrane-human Wharton jelly mesenchymal stem cells to form sandwich double cell sheets and compared this hole with other holes (one was filled by human Wharton jelly mesenchymal stem cell suspension, and the other was filled by coral granules saturated with preinduced mesenchymal stem cells) by radiological and histopathological studies for repairing the bone gap.

RESULTS

Collagen-coated poly-L-lactic acid showed higher mRNA levels for all osteogenes (P < .001), higher alkaline phosphatase and calcium content (P < .001), and greater stainability. Our in vivo experiment showed that the holes implanted with sandwich double cell sheet-poly-L-lactic acid coral were completely filled mature compact bone. The holes implanted with human Wharton jelly mesenchymal stem cells alone were filled with immature compact bone. Holes implanted with coral granules-human Wharton jelly mesenchymal stem cells were filled with condensed connective tissue.

CONCLUSIONS

Poly-L-lactic acid nanofiber has greater osteogenic differentiating effect than the coral granules. The new approach of sDCS-PLLA-coral construct proved success for bone regeneration and repairing the bone gap and this may improve the design of tissue constructs for bone tissue regenerative therapy.

Type 2 diabetes and bone fragility in children and adults

Type 2 diabetes (T2D) is a global epidemic disease. The prevalence of T2D in adolescents and young adults is increasing alarmingly. The mechanisms leading to T2D in young people are similar to those in older patients. However, the severity of onset, reduced insulin sensitivity and defective insulin secretion can be different in subjects who develop the disease at a younger age. T2D is associated with different complications, including bone fragility with consequent susceptibility to fractures. The purpose of this systematic review was to describe T2D bone fragility together with all the possible involved pathways. Numerous studies have reported that patients with T2D show preserved, or even increased, bone mineral density compared with controls. This apparent paradox can be explained by the altered bone quality with increased cortical bone porosity and compr-omised mechanical properties. Furthermore, reduced bone turnover has been described in T2D with reduced markers of bone formation and resorption. These findings prompted different researchers to highlight the mechanisms leading to bone fragility, and numerous critical altered pathways have been identified and studied. In detail, we focused our attention on the role of microvascular disease, advanced glycation end products, the senescence pathway, the Wnt/β-catenin pathway, the osteoprotegerin/receptor-activator of nuclear factor kappa B ligand, osteonectin and fibroblast growth factor 23. The understanding of type 2 myeloid bone fragility is an important issue as it could suggest possible interventions for the prevention of poor bone quality in T2D and/or how to target these pathways when bone disease is clearly evident.

MicroRNA-101a enhances trabecular bone accrual in male mice

High-throughput microRNA sequencing was performed during differentiation of MC3T3-E1 osteoblasts to develop working hypotheses for specific microRNAs that control osteogenesis. The expression data show that miR-101a, which targets the mRNAs for the epigenetic enzyme Ezh2 and many other proteins, is highly upregulated during osteoblast differentiation and robustly expressed in mouse calvaria. Transient elevation of miR-101a suppresses Ezh2 levels, reduces tri-methylation of lysine 27 in histone 3 (H3K27me3; a heterochromatic mark catalyzed by Ezh2), and accelerates mineralization of MC3T3-E1 osteoblasts. We also examined skeletal phenotypes of an inducible miR-101a transgene under direct control of doxycycline administration. Experimental controls and mir-101a over-expressing mice were exposed to doxycycline in utero and postnatally (up to 8 weeks of age) to maximize penetrance of skeletal phenotypes. Male mice that over-express miR-101a have increased total body weight and longer femora. MicroCT analysis indicate that these mice have increased trabecular bone volume fraction, trabecular number and trabecular thickness with reduced trabecular spacing as compared to controls. Histomorphometric analysis demonstrates a significant reduction in osteoid volume to bone volume and osteoid surface to bone surface. Remarkably, while female mice also exhibit a significant increase in bone length, no significant changes were noted by microCT (trabecular bone parameters) and histomorphometry (osteoid parameters). Hence, miR-101a upregulation during osteoblast maturation and the concomitant reduction in Ezh2 mediated H3K27me3 levels may contribute to the enhanced trabecular bone parameters in male mice. However, the sex-specific effect of miR-101a indicates that more intricate epigenetic mechanisms mediate physiological control of bone formation and homeostasis.

Species-specific sensitivity to TGFβ signaling and changes to the Mmp13 promoter underlie avian jaw development and evolution

Precise developmental control of jaw length is critical for survival, but underlying molecular mechanisms remain poorly understood. The jaw skeleton arises from neural crest mesenchyme (NCM), and we previously demonstrated that these progenitor cells express more bone-resorbing enzymes including Matrix metalloproteinase 13 (Mmp13) when they generate shorter jaws in quail embryos versus longer jaws in duck. Moreover, if we inhibit bone resorption or Mmp13, we can increase jaw length. In the current study, we uncover mechanisms establishing species-specific levels of Mmp13 and bone resorption. Quail show greater activation of and sensitivity to transforming growth factor beta (TGFβ) signaling than duck; where intracellular mediators like SMADs and targets like Runt-related transcription factor 2 (Runx2), which bind Mmp13, become elevated. Inhibiting TGFβ signaling decreases bone resorption, and overexpressing Mmp13 in NCM shortens the duck lower jaw. To elucidate the basis for this differential regulation, we examine the Mmp13 promoter. We discover a SMAD-binding element and single nucleotide polymorphisms (SNPs) near a RUNX2-binding element that distinguish quail from duck. Altering the SMAD site and switching the SNPs abolish TGFβ sensitivity in the quail Mmp13 promoter but make the duck promoter responsive. Thus, differential regulation of TGFβ signaling and Mmp13 promoter structure underlie avian jaw development and evolution.

The Regulation of Collagen Processing by miRNAs in Disease and Possible Implications for Bone Turnover

This article describes several recent examples of miRNA governing the regulation of the gene expression involved in bone matrix construction. We present the impact of miRNA on the subsequent steps in the formation of collagen type I. Collagen type I is a main factor of mechanical bone stiffness because it constitutes 90-95% of the organic components of the bone. Therefore, the precise epigenetic regulation of collagen formation may have a significant influence on bone structure. We also describe miRNA involvement in the expression of genes, the protein products of which participate in collagen maturation in various tissues and cancer cells. We show how non-collagenous proteins in the extracellular matrix are epigenetically regulated by miRNA in bone and other tissues. We also delineate collagen mineralisation in bones by factors that depend on miRNA molecules. This review reveals the tissue variability of miRNA regulation at different levels of collagen maturation and mineralisation. The functionality of collagen mRNA regulation by miRNA, as proven in other tissues, has not yet been shown in osteoblasts. Several collagen-regulating miRNAs are co-expressed with collagen in bone. We suggest that collagen mRNA regulation by miRNA could also be potentially important in bone metabolism.

Mechanosensitive miR-100 coordinates TGFβ and Wnt signaling in osteocytes during fluid shear stress

Organism scale mechanical forces elicit cellular scale changes through coordinated regulation of multiple signaling pathways. The mechanisms by which cells integrate signaling to generate a unified biological response remains a major question in mechanobiology. For example, the mechanosensitive response of bone and other tissues requires coordinated signaling by the transforming growth factor beta (TGFβ) and Wnt pathways through mechanisms that are not well‐defined. Here we report a new microRNA‐dependent mechanism that mediates mechanosensitive crosstalk between TGFβ and Wnt signaling in osteocytes exposed to fluid shear stress (FSS). From 60 mechanosensitive microRNA (miRs) identified by small‐RNAseq, miR100 expression is suppressed by in vivo hindlimb loading in the murine tibia and by cellular scale FSS in OCY454 cells. Though FSS activates both TGFβ and Wnt signaling in osteocytes, only TGFβ represses miR‐100 expression. miR‐100, in turn, antagonizes Wnt signaling by targeting and inhibiting expression of Frizzled receptors (FZD5/FZD8). Accordingly, miR‐100 inhibition blunts FSS‐ and TGFβ‐inducible Wnt signaling. Therefore, our results identify FSS‐responsive miRNAs in osteocytes, including one that integrates the mechanosensitive function of two essential signaling pathways in the osteoanabolic response of bone to mechanical load.

miR-433-3p suppresses bone formation and mRNAs critical for osteoblast function in mice

MicroRNAs (miRNAs) are key posttranscriptional regulators of osteoblastic commitment and differentiation. miR-433-3p was previously shown to target Runt-related transcription factor 2 (Runx2) and to be repressed by bone morphogenetic protein (BMP) signaling. Here, we show that miR-433-3p is progressively decreased during osteoblastic differentiation of primary mouse bone marrow stromal cells in vitro, and we confirm its negative regulation of this process. Although repressors of osteoblastic differentiation often promote adipogenesis, inhibition of miR-433-3p did not affect adipocyte differentiation in vitro. Multiple pathways regulate osteogenesis. Using luciferase-3' untranslated region (UTR) reporter assays, five novel miR-433-3p targets involved in parathyroid hormone (PTH), mitogen-activated protein kinase (MAPK), Wnt, and glucocorticoid signaling pathways were validated. We show that Creb1 is a miR-433-3p target, and this transcription factor mediates key signaling downstream of PTH receptor activation. We also show that miR-433-3p targets hydroxysteroid 11-β dehydrogenase 1 (Hsd11b1), the enzyme that locally converts inactive glucocorticoids to their active form. miR-433-3p dampens glucocorticoid signaling, and targeting of Hsd11b1 could contribute to this phenomenon. Moreover, miR-433-3p targets R-spondin 3 (Rspo3), a leucine-rich repeat-containing G-protein coupled receptor (LGR) ligand that enhances Wnt signaling. Notably, Wnt canonical signaling is also blunted by miR-433-3p activity. In vivo, expression of a miR-433-3p inhibitor or tough decoy in the osteoblastic lineage increased trabecular bone volume. Mice expressing the miR-433-3p tough decoy displayed increased bone formation without alterations in osteoblast or osteoclast numbers or surface, indicating that miR-433-3p decreases osteoblast activity. Overall, we showed that miR-433-3p is a negative regulator of bone formation in vivo, targeting key bone-anabolic pathways including those involved in PTH signaling, Wnt, and endogenous glucocorticoids. Local delivery of miR-433-3p inhibitor could present a strategy for the management of bone loss disorders and bone defect repair. © 2021 American Society for Bone and Mineral Research (ASBMR).

Stable integration of an optimized inducible promoter system enables spatiotemporal control of gene expression throughout avian development

Precisely altering gene expression is critical for understanding molecular processes of embryogenesis. Although some tools exist for transgene misexpression in developing chick embryos, we have refined and advanced them by simplifying and optimizing constructs for spatiotemporal control. To maintain expression over the entire course of embryonic development we use an enhanced piggyBac transposon system that efficiently integrates sequences into the host genome. We also incorporate a DNA targeting sequence to direct plasmid translocation into the nucleus and a D4Z4 insulator sequence to prevent epigenetic silencing. We designed these constructs to minimize their size and maximize cellular uptake, and to simplify usage by placing all of the integrating sequences on a single plasmid. Following electroporation of stage HH8.5 embryos, our tetracycline-inducible promoter construct produces robust transgene expression in the presence of doxycycline at any point during embryonic development in ovo or in culture. Moreover, expression levels can be modulated by titrating doxycycline concentrations and spatial control can be achieved using beads or gels. Thus, we have generated a novel, sensitive, tunable, and stable inducible-promoter system for high-resolution gene manipulation in vivo.

Osteoporosis genome-wide association study variant c.3781 C>A is regulated by a novel anti-osteogenic factor miR-345-5p

Upstream transcription factor family member 3 (USF3) c.3781C>A (rs1026364) in the 3'-untranslated region (3'-UTR) has been firmly associated with bone mineral density (BMD) in genome-wide association study (GWAS). However, the molecular mechanism by which it influences BMD and osteoporosis is unknown. Bioinformatics analyses suggested that the risk c.3781A allele creates a target site for hsa-miR-345-5p binding. Luciferase assay validated that the c.3781A allele displayed significantly lower luciferase activities than the c.3781C allele in the human osteoblast cell line hFOB1.19, osteosarcoma cell lines U-2OS and Saos-2, and embryonic kidney cell line 293T. Furthermore, hsa-miR-345-5p regulated USF3 expression on both messenger RNA and protein levels in hFOB1.19 and U937 cells with heterozygous A/C genotype. Transfection of hsa-miR-345-5p antagomiR in heterozygous hFOB1.19 cells significantly increased the expression of osteogenic marker genes RUNX2, OSTERIX, COL1A1, ALP, OPN, OCN, and alkaline phosphatase activity and matrix mineralization level. Importantly, we found that hsa-miR-345-5p also inhibits osteoblast maturation in cell lines U-2OS with hsa-miR-345-5p nonbinding C/C genotype by targeting RUNX3 and SMAD1. Our findings uncovered a novel pathogenetic mechanism of osteoporosis by GWAS variant c.3781C>A-mediated disruption of hsa-miR-345-5p binding at the 3'-UTR of USF3 and the functional role of hsa-miR-345-5p in osteogenic differentiation.

Associations of LRP5 Gene With Bone Mineral Density, Bone Turnover Markers, and Fractures in the Elderly With Osteoporosis

Objective: The study aimed to explore the associations of rs4988300 and rs634008 in the low-density lipoprotein receptor-related protein 5 (LRP5) gene with bone mineral density (BMD), bone turnover markers (BTM), and fractures in elderly patients with osteoporosis (OP). Methods: Our study included 328 unrelated OP patients with or without fractures. Genomic DNA was extracted for genotyping. BTM levels were assessed by electrochemiluminescence (ECL). Dual-energy X-ray absorptiometry (DXA) was employed to measure BMD in the lumbar spine (LS) and proximal femur. Basic features between the OP and fracture groups were analyzed using the t-test. The Chi-square test was performed to analyze the differences in allele and genotype frequencies. The associations of single-nucleotide polymorphisms (SNPs) with BMD and BTM in the subgroups were investigated by the analysis of covariance (ANCOVA) adjusted for confounding factors. Results: In both females and males, individuals with fractures exhibited higher BTM levels and lower BMD values than those with OP (P < 0.05). The allele and genotype frequencies of rs4988300 in the subgroups were significantly different (P < 0.05). In both females and males suffering from OP, participants with rs4988300 GG or rs634008 TT presented lower procollagen I N-terminal propeptide (PINP) levels (P < 0.05). Women with OP carrying rs4988300 GG exhibited lower BMD values at FN and TH (P < 0.05). In both females and males with fractures, individuals carrying rs4988300 GG genotype or rs634008 TT genotype exhibited lower PINP levels and BMD values at FN and TH than those with other genotypes (P < 0.05). Conclusions: Rs4988300 and rs634008 polymorphisms in the LRP5 gene are associated with bone phenotypes in the elderly with OP or fractures.

Associations of IDUA and PTCH1 with Bone Mineral Density, Bone Turnover Markers, and Fractures in Chinese Elderly Patients with Osteoporosis

Introduction

Osteoporosis (OP) is a common polygenic disorder in the aging population, and several single nucleotide polymorphisms (SNPs) in the alpha-L-iduronidase (IDUA) gene and patched homolog 1 (PTCH1) gene regulate bone metabolism and affect bone mass. The study aimed at investigating the relationships of rs3755955 and rs6831280 in the IDUA gene and rs28377268 in the PTCH1 gene with bone mineral density (BMD), bone turnover markers (BTMs), and fractures in the elderly Chinese subjects with OP.

Materials and Methods

A cohort of 328 unrelated senile osteoporosis (SOP) patients with or without osteoporotic fractures was recruited. rs3755955, rs6831280, and rs28377268 polymorphisms were identified using SNaPshot technology. BTM levels were determined by electrochemiluminescence (ECL). Bone mineral densities (BMDs) at the lumbar spine (LS) and proximal femur sites were measured by dual-energy X-ray absorptiometry (DEXA) in all subjects. The Hardy-Weinberg equilibrium (HWE) test was performed. HWE P values and comparisons of genotype frequencies were estimated using the chi-square test. Analysis of covariance (ANCOVA) adjusted for confounding factors was performed to investigate associations of SNPs with BMDs and BTMs in subgroups.

Results

The chi-square test indicated that genotype distributions in the control group conformed to HWE (P > 0.05). The distributions of allele and genotype frequencies of rs6831280 between fracture and osteoporotic participants were significantly different (P-allele = 0.002 and P-genotype = 0.012, respectively). Concerning rs6831280, ANCOVA found BMDs at LS 2-4 (L2-4) and total hip (TH) among the study subjects suffering from SOP with GA genotype were lower than in those carrying GG or AA (P-L2-4 = 0.004 and P-TH = 0.027, respectively).

Conclusions

IDUA rs6831280 is associated with BMDs at L2-4 and TH in the elderly Chinese population with SOP and may serve as a marker for the genetic susceptibility to osteoporotic fractures.

MicroRNA and Human Bone Health

The small non‐coding microRNAs (miRNAs) are post‐transcription regulators that modulate diverse cellular process in bone cells. Because optimal miRNA targeting is essential for their function, single‐nucleotide polymorphisms (SNPs) within or proximal to the loci of miRNA (miR‐SNPs) or mRNA (PolymiRTS) could potentially disrupt the miRNA‐mRNA interaction, leading to changes in bone metabolism and osteoporosis. Recent human studies of skeletal traits using miRNA profiling, genomewide association studies, and functional studies started to decipher the complex miRNA regulatory network. These studies have indicated that miRNAs may be a promising bone marker. This review focuses on human miRNA studies on bone traits and discusses how genetic variants affect bone metabolic pathways. Major ex vivo investigations using human samples supported with animal and in vitro models have shed light on the mechanistic role of miRNAs. Furthermore, studying the miRNAs’ signatures in secondary osteoporosis and osteoporotic medications such as teriparatide (TPTD) and denosumab (DMab) have provided valuable insight into clinical management of the disease. © 2018 The Authors. JBMR Plus Published by Wiley Periodicals, Inc. on behalf of the American Society for Bone and Mineral Research

Association of Adrenergic Receptor α2A (α2A-AR) Gene rs1800544 Polymorphism with Bone Mineral Density and Bone Turnover Markers in an Elderly Chinese Population

BACKGROUND Adrenergic receptor α2A (α2A-AR) is up-regulated in osteoporotic bone osteoblasts. Previous research demonstrated an association between polymorphism of a2A-AR gene and bone mineral density (BMD) as well as bone turnover markers (BTMs) in the Slovenian population. The present study aimed to investigate the association of rs1800544 polymorphism of α2A-AR gene with BMD and BTMs in the Chinese elderly population with osteoporosis (OP) or with osteoporotic fractures. MATERIAL AND METHODS A total of 346 unrelated elderly individuals were recruited in the study. Rs1800544 polymorphism was determined by Snapshot technology. BTMs were determined by electrochemiluminescence. BMDs at lumbar spine (LS) and proximal femur were measured with dual-energy X-ray absorptiometry (DEXA). Hardy-Weinberg equilibrium and distribution of genotype frequencies were verified using the chi-squared test. Analysis of co-variance (ANCOVA) adjusted for confounding factors was performed to explore the relationship of rs1800544 polymorphism with BMD and BTMs in all participants and in subgroups. RESULTS The genotype distributions in all subjects and in subgroups conformed to Hardy-Weinberg equilibrium (P>0.1). Distribution of genotype frequencies of subgroups showed no significant differences (P>0.05). Patients with GG genotype in the fracture group had significantly higher serum BTMs level compared with those carrying other genotypes (P<0.05). No significant association between rs1800544 and BTMs was detected in the elderly population with OP. Comparison of BMD at each site in all participants did not show any significant difference in subgroups with CC, CG, and GG genotypes (P>0.05). CONCLUSIONS Rs1800544 polymorphism is associated with BTMs level in Chinese elderly individuals with osteoporotic fractures, indicating the involvement of genetic variation of a2A-AR gene in bone metabolism.

Epigenetics of Skeletal Diseases

PURPOSE OF REVIEW

Epigenetic mechanisms modify gene activity in a stable manner without altering DNA sequence. They participate in the adaptation to the environment, as well as in the pathogenesis of common complex disorders. We provide an overview of the role of epigenetic mechanisms in bone biology and pathology.

RECENT FINDINGS

Extensive evidence supports the involvement of epigenetic mechanisms (DNA methylation, post-translational modifications of histone tails, and non-coding RNAs) in the differentiation of bone cells and mechanotransduction. A variety of epigenetic abnormalities have been described in patients with osteoporosis, osteoarthritis, and skeletal cancers, but their actual pathogenetic roles are still unclear. A few drugs targeting epigenetic marks have been approved for neoplastic disorders, and many more are being actively investigated. Advances in the field of epigenetics underscore the complex interactions between genetic and environmental factors as determinants of osteoporosis and other common disorders. Likewise, they help to explain the mechanisms by which prenatal and post-natal external factors, from nutrition to psychological stress, impact our body and influence the risk of later disease.

Matricellular Proteins: Functional Insights From Non-mammalian Animal Models

The extracellular matrix (ECM) has central roles in tissue integrity and remodeling throughout the life span of animals. While collagens are the most abundant structural components of ECM in most tissues, tissue-specific molecular complexity is contributed by ECM glycoproteins. The matricellular glycoproteins are categorized primarily according to functional criteria and represented predominantly by the thrombospondin, tenascin, SPARC/osteonectin, and CCN families. These proteins do not self-assemble into ECM fibrils; nevertheless, they shape ECM properties through interactions with structural ECM proteins, growth factors, and cells. Matricellular proteins also promote cell migration or morphological changes through adhesion-modulating or counter-adhesive actions on cell-ECM adhesions, intracellular signaling, and the actin cytoskeleton. Typically, matricellular proteins are most highly expressed during embryonic development. In adult tissues, expression is more limited unless activated by cues for dynamic tissue remodeling and cell motility, such as occur during inflammatory response and wound repair. Many insights in the complex roles of matricellular proteins have been obtained from studies of gene knockout mice. However, with the exception of chordate-specific tenascins, these are highly conserved proteins that are encoded in many animal phyla. This review will consider the increasing body of research on matricellular proteins in nonmammalian animal models. These models provide better access to the very earliest stages of embryonic development and opportunities to study biological processes such as limb and organ regeneration. In aggregate, this research is expanding concepts of the functions and mechanisms of action of matricellular proteins.

Regulation of Bone Metabolism by microRNAs

PURPOSE OF REVIEW

The small non-coding microRNAs (miRNAs) have emerged as important post-transcriptional regulators of various physiological and pathological processes. The purpose of this article is to review the important recent advances on the role of miRNAs in bone remodeling and metabolic bone disorders.

RECENT FINDINGS

In a physiological context, miRNAs regulate bone formation and bone resorption, thereby contributing to the maintenance of bone homeostasis. Under pathological conditions, an aberrant miRNA signaling contributes to the onset and progression of skeletal disorders, such as osteoporosis. Furthermore, miRNAs can be secreted to circulation and have clinical potential as non-invasive biomarkers. In a therapeutic setting, miRNA delivery or antagonism has been reported to affect several diseases under pre-clinical conditions thereby emerging as novel pharmacological tools. miRNAs are key regulators of bone remodeling in health and disease. The future perspectives in the field include the role of secreted miRNAs in cell-cell communication in the bone environment. Furthermore, the clinical potential of using miRNAs as diagnostic tools and therapeutic targets to treat metabolic bone diseases provides an attractive future direction.

Genetic Polymorphism of miR-196a-2 is Associated with Bone Mineral Density (BMD)

MicroRNAs (miRNAs) are small non-coding RNA molecules that post-transcriptionally regulate the translation of messenger RNAs. Given the crucial role of miRNAs in gene expression, genetic variants within miRNA-related sequences may affect miRNA function and contribute to disease risk. Osteoporosis is characterized by reduced bone mass, and bone mineral density (BMD) is a major diagnostic proxy to assess osteoporosis risk. Here, we aimed to identify miRNAs that are involved in BMD using data from recent genome-wide association studies (GWAS) on femoral neck, lumbar spine and forearm BMD. Of 242 miRNA-variants available in the GWAS data, we found rs11614913:C > T in the precursor miR-196a-2 to be significantly associated with femoral neck-BMD (p-value = 9.9 × 10⁻⁷, β = −0.038) and lumbar spine-BMD (p-value = 3.2 × 10⁻¹¹, β = −0.061). Furthermore, our sensitivity analyses using the Rotterdam study data showed a sex-specific association of rs11614913 with BMD only in women. Subsequently, we highlighted a number of miR-196a-2 target genes, expressed in bone and associated with BMD, that may mediate the miRNA function in BMD. Collectively, our results suggest that miR-196a-2 may contribute to variations in BMD level. Further biological investigations will give more insights into the mechanisms by which miR-196a-2 control expression of BMD-related genes.

Effects of Syzygium aromaticum, Cinnamomum zeylanicum, and Salvia triloba extracts on proliferation and differentiation of dental pulp stem cells

Hypersensitivity, local irritative and cytotoxic effects are known for the chemical components of Syzygium aromaticum and Cinnamomum zeylanicum contained in dental materials. However, there is no intimate data in dentistry using the whole extracts of these plants and introducing new ones. Salvia triloba is a well-known anti-inflammatory plant that correspondingly could be used in several dental traumas.

MicroRNAs in bone diseases

MicroRNAs are small, noncoding single-stranded RNAs that have emerged as important posttranscriptional regulators of gene expression, with an essential role in vertebrate development and different biological processes. This review highlights the recent advances in the function of miRNAs and their roles in bone remodeling and bone diseases. MicroRNAs (miRNAs) are a class of small (∼22 nt), noncoding single-stranded RNAs that have emerged as important posttranscriptional regulators of gene expression. They are essential for vertebrate development and play critical roles in different biological processes related to cell differentiation, activity, metabolism, and apoptosis. A rising number of experimental reports now indicate that miRNAs contribute to every step of osteogenesis and bone homeostasis, from embryonic skeletal development to maintenance of adult bone tissue, by regulating the growth, differentiation, and activity of different cell systems inside and outside the skeleton. Importantly, emerging information from animal studies suggests that targeting miRNAs might become an attractive and new therapeutic approach for osteoporosis or other skeletal diseases, even though there are still major concerns related to potential off target effects and the need of efficient delivery methods in vivo. Moreover, besides their recognized effects at the cellular level, evidence is also gathering that miRNAs are excreted and can circulate in the blood or other body fluids with potential paracrine or endocrine functions. Thus, they could represent suitable candidates for becoming sensitive disease biomarkers in different pathologic conditions, including skeletal disorders. Despite these promising perspectives more work remains to be done until miRNAs can serve as robust therapeutic targets or established diagnostic tools for precision medicine in skeletal disorders.

Associations between FGF21, osteonectin and bone turnover markers in type 2 diabetic patients with albuminuria

AIM

We measured the levels of bone turnover markers (BTMs) in patients with early diabetic nephropathy from type 2 diabetes mellitus (T2DM), and investigated the associations of BTMs with adipokines, serum fibroblast growth factor-21 (FGF21) and osteonectin.

METHODS

We included 159 males and 300 females with T2DM in this cross-sectional study. Clinical characteristics, BTMs and adipokines levels were measured.

RESULTS

One-hundred and ninety-two (41.8%) patients presented with albuminuria. Patients with albuminuria had significantly higher levels of serum osteonectin (P<0.0001) and FGF21 (P=0.0125) than those with normoalbuminuria. Serum levels of P1NP were slightly lower among patients with albuminuria (P=0.031), but the difference disappeared after adjusting for FBG, PBG, and HbA1c. Serum FGF21 levels were independently and negatively related to eGFR (overall β=-0.161, P=0.001; albuminuria group β=-0.240, P=0.001) but not related to uACR. While Osteonectin was independently and positively related to uACR (overall β=0.209, P=0.001; albuminuria group β=0.170, P=0.021). The levels of serum FGF21 were independently inversely related with P1NP (overall β=-0.192, P<0.0001; albuminuria group β=-0.195, P=0.031).

CONCLUSIONS

Our results suggest that persistent hyperglycemia may inhibit bone formation. Both osteonectin and FGF21 were associated with early nephropathy in T2DM patients, albeit with different patterns.

Heterozygous PTCH1 Mutations Impact the Bone Metabolism in Patients With Nevoid Basal Cell Carcinoma Syndrome Likely by Regulating SPARC Expression

Nevoid basal cell carcinoma syndrome (NBCCS) is an autosomal dominant disorder characterized by bone and skin abnormalities and a predisposition to various tumors. Keratocystic odontogenic tumors (KCOTs), which are common tumors of the jaw that cause extensive damage to the jawbone, are usually accompanied with NBCCS. Germline PTCH1 mutations in NBCCS tumorigenesis have been frequently studied; however, little is known regarding the pathogenesis of bone abnormalities in this disease. This study sought to investigate the mechanism underlying heterozygous PTCH1 mutation-mediated abnormal bone metabolism in patients with NBCCS. Stromal cells were isolated from the fibrous capsules of patients with NBCCS-associated or non-syndromic keratocystic odontogenic tumors and non-syndromic tumor stromal cells without PTCH1 mutations served as controls. Germline PTCH1 heterozygous mutations were confirmed in all NBCCS samples and differential protein expression was identified using tandem mass tag-labeled proteomics analysis. Our findings revealed that osteonectin/SPARC expression was significantly downregulated in syndromic stromal cells compared with non-syndromic stromal cells. SPARC expression was even lower in stromal cells carrying PTCH1 protein truncation mutations. PTCH1 siRNA transfection demonstrated that SPARC downregulation correlates with decreased PTCH1 expression. Furthermore, exogenous SPARC promoted osteogenic differentiation of syndromic stromal cells with enhanced development of calcium nodules. In addition, bone mineral density tests showed that patients with NBCCS exhibit weak bone mass compared with sex- and age-matched controls. This study indicates that germline PTCH1 heterozygous mutations play a major role in bone metabolism in patients with NBCCS, in particular in those with PTCH1 protein truncation mutations. SPARC may represent an important downstream modulator of PTCH1 mediation of bone metabolism. Thus, bone mineral density monitoring is critical for patients with NBCCS for prevention of osteoporosis. In addition, surgical procedures on syndromic-associated KCOTs should be performed with consideration of the weaker bone mass in such patients. © 2016 American Society for Bone and Mineral Research.

Skeletal biology: Where matrix meets mineral

The skeleton is unique from all other tissues in the body because of its ability to mineralize. The incorporation of mineral into bones and teeth is essential to give them strength and structure for body support and function. For years, researchers have wondered how mineralized tissues form and repair. A major focus in this context has been on the role of the extracellular matrix, which harbors key regulators of the mineralization process. In this introductory minireview, we will review some key concepts of matrix biology as it related to mineralized tissues. Concurrently, we will highlight the subject of this special issue covering many aspects of mineralized tissues, including bones and teeth and their associated structures cartilage and tendon. Areas of emphasis are on the generation and analysis of new animal models with permutations of matrix components as well as the development of new approaches for tissue engineering for repair of damaged hard tissue. In assembling key topics on mineralized tissues written by leaders in our field, we hope the reader will get a broad view of the topic and all of its fascinating complexities.

MicroRNA variants as genetic determinants of bone mass

Single nucleotide polymorphisms (SNPs) are the most abundant genetic variants that contribute to the heritability of bone mass. MicroRNAs (miRNAs, miRs) are key post-transcriptional regulators that modulate the differentiation and function of skeletal cells by targeting multiple genes in the same or distinct signaling pathways. SNPs in miRNA genes and miRNA binding sites can alter miRNA abundance and mRNA targeting. This review describes the potential impact of miRNA-related SNPs on skeletal phenotype. Although many associations between SNPs and bone mass have been described, this review is limited to gene variants for which a function has been experimentally validated. SNPs in miRNA genes (miR-SNPs) that impair miRNA processing and alter the abundance of mature miRNA are discussed for miR-146a, miR-125a, miR-196a, miR-149 and miR-27a. SNPs in miRNA targeting sites (miR-TS-SNPs) that alter miRNA binding are described for the bone remodeling genes bone morphogenetic protein receptor 1 (Bmpr1), fibroblast growth factor 2 (Fgf2), osteonectin (Sparc) and histone deacetylase 5 (Hdac5). The review highlights two aspects of miRNA-associated SNPs: the mechanism for altering miRNA mediated gene regulation and the potential of miR-associated SNPs to alter osteoblast, osteoclast or chondrocyte differentiation and function. Given the polygenic nature of skeletal diseases like osteoporosis and osteoarthritis, validating the function of additional miRNA-associated SNPs has the potential to enhance our understanding of the genetic determinants of bone mass and predisposition to selected skeletal diseases.

SPARC/osteonectin in mineralized tissue

Secreted protein acidic and rich in cysteine (SPARC/osteonectin/BM40) is one of the most abundant non-collagenous protein expressed in mineralized tissues. This review will focus on elucidating functional roles of SPARC in bone formation building upon results from non-mineralized cells and tissues, the phenotype of SPARC-null bones, and recent discoveries of human diseases with either dysregulated expression of SPARC or mutations in the gene encoding SPARC that give rise to bone pathologies. The capacity of SPARC to influence pathways involved in extracellular matrix assembly such as procollagen processing and collagen fibril formation as well as the capacity to influence osteoblast differentiation and osteoclast activity will be addressed. In addition, the potential for SPARC to regulate cross-linking of extracellular matrix proteins by members of the transglutaminase family of enzymes is explored. Elucidating defined biological functions of SPARC in terms of bone formation and turnover are critical. Further insight into specific cellular mechanisms involved in the formation and homeostasis of mineralized tissues will lead to a better understanding of disease progression.

Intrinsically disordered proteins and biomineralization

In vertebrates and invertebrates, biomineralization is controlled by the cell and the proteins they produce. A large number of these proteins are intrinsically disordered, gaining some secondary structure when they interact with their binding partners. These partners include the component ions of the mineral being deposited, the crystals themselves, the template on which the initial crystals form, and other intrinsically disordered proteins and peptides. This review speculates why intrinsically disordered proteins are so important for biomineralization, providing illustrations from the SIBLING (small integrin binding N-glycosylated) proteins and their peptides. It is concluded that the flexible structure, and the ability of the intrinsically disordered proteins to bind to a multitude of surfaces is crucial, but details on the precise-interactions, energetics and kinetics of binding remain to be determined.

Recessive osteogenesis imperfecta caused by missense mutations in SPARC

Secreted protein, acidic, cysteine-rich (SPARC) is a glycoprotein that binds to collagen type I and other proteins in the extracellular matrix. Using whole-exome sequencing to identify the molecular defect in two unrelated girls with severe bone fragility and a clinical diagnosis of osteogenesis imperfecta type IV, we identified two homozygous variants in SPARC (GenBank: NM_003118.3; c.497G>A [p.Arg166His] in individual 1; c.787G>A [p.Glu263Lys] in individual 2). Published modeling and site-directed mutagenesis studies had previously shown that the residues substituted by these mutations form an intramolecular salt bridge in SPARC and are essential for the binding of SPARC to collagen type I. The amount of SPARC secreted by skin fibroblasts was reduced in individual 1 but appeared normal in individual 2. The migration of collagen type I alpha chains produced by these fibroblasts was mildly delayed on SDS-PAGE gel, suggesting some overmodification of collagen during triple helical formation. Pulse-chase experiments showed that collagen type I secretion was mildly delayed in skin fibroblasts from both individuals. Analysis of an iliac bone sample from individual 2 showed that trabecular bone was hypermineralized on the material level. In conclusion, these observations show that homozygous mutations in SPARC can give rise to severe bone fragility in humans.