A microRNA regulatory mechanism of osteoblast differentiation. Proc Natl Acad Sci U S A. 2009;106:20794-20799. [PMID: 19933329 DOI: 10.1073/pnas.0909311106]

Connexins and microRNAs: Interlinked players in regulating islet function?

Pancreatic β-cells are connected to neighboring endocrine cells through the adherin proteins and gap junctions. Connexin 36 (Cx36) is one of the most well-studied and abundantly expressed gap-junction proteins within rodent islets, which is important in coordinated insulin secretion. The expression of connexins is regulated at various levels and by several mechanisms; one of which is via microRNAs. In past 2 decades, microRNAs (miRNAs) have emerged as key molecules in developmental, physiologic and pathological processes. However, very few studies have demonstrated miRNA-mediated regulation of connexins. Even though there are no reports yet on miRNAs and Cx36; we envisage that considering the important role of connexins and microRNAs in insulin secretion, there would be common pathways interlinking these biomolecules. Here, we discuss the current literature on connexins and miRNAs specifically with reference to islet function.

Cross-talk of MicroRNA and hydrogen sulfide: A novel therapeutic approach for bone diseases

Bone homeostasis requires a balance between the bone formation of osteoblasts and bone resorption of osteoclasts to maintain ideal bone mass and bone quality. An imbalance in bone remodeling processes results in bone metabolic disorders such as osteoporosis. Hydrogen sulfide (H₂S), a gasotransmitter, has attracted the focus of many researchers due to its multiple physiological functions. It has been implicated in anti-inflammatory, vasodilatory, angiogenic, cytoprotective, anti-oxidative and anti-apoptotic mechanisms. H₂S has also been shown to exert osteoprotective activity through its anti-inflammatory and anti-oxidative effects. However, the underlying molecular mechanisms by which H₂S mitigates bone diseases are not completely understood. Experimental evidence suggests that H₂S may regulate signaling pathways by directly influencing a gene in the cascade or interacting with some other gasotransmitter (carbon monoxide or nitric oxide) or both. MicroRNAs (miRNAs) are short non-coding RNAs which regulate gene expression by targeting, binding and suppressing mRNAs; thus controlling cell fate. Certainly, bone remodeling is also regulated by miRNAs expression and has been reported in many studies. MicroRNAs also regulate H₂S biosynthesis. The inter-regulation of microRNAs and H₂S opens a new possibility for exploring the H₂S-microRNA crosstalk in bone diseases. However, the relationship between miRNAs, bone development, and H₂S is still not well explained. This review focuses on miRNAs and their roles in regulating bone remodeling and possible mechanisms behind H₂S mediated bone loss inhibition, H₂S-miRNAs crosstalk in relation to the pathophysiology of bone remodeling, and future perspectives for miRNA-H₂S as a therapeutic agent for bone diseases.

MiR-142-5p promotes bone repair by maintaining osteoblast activity

MicroRNAs play important roles in regulating bone regeneration and remodeling. However, the pathophysiological roles of microRNAs in bone repair remain unclear. Here we identify a significant upregulation of miR-142-5p correlated with active osteoblastogenesis during the bone healing process. In vitro, miR-142-5p promoted osteoblast activity and matrix mineralization by targeting the gene encoding WW-domain-containing E3 ubiquitin protein ligase 1. We also found that the expression of miR-142-5p in the callus of aged mice was lower than that in the callus of young mice and directly correlated with the age-related delay in bone healing. Furthermore, treatment with agomir-142-5p in the fracture areas stimulated osteoblast activity which repaired the bone fractures in aged mice. Thus, our study revealed that miR-142-5p plays a crucial role in healing fractures by maintaining osteoblast activity, and provided a new molecular target therapeutic strategy for bone healing.

microRNAs and connexins in bone: interaction and mechanisms of delivery

PURPOSE OF REVIEW

To describe the current knowledge on the cross-talk between connexins and microRNAs (miRs) in bone cells.

RECENT FINDINGS

Connexins play a crucial role on bone development and maintenance, and disruptions in their abundance or localization can affect how bone perceives and responds to mechanical, hormonal, and pharmacological stimuli. Connexin expression can be modified by miRs, which modulate connexin mRNA and protein levels. Recently, different manners by which miRs and connexins can interact in bone have been identified, including mechanisms that mediate miR exchange between cells in direct contact through gap junctions, or between distant cells via extracellular vesicles (EVs).

SUMMARY

We bring to light the relationship between miRs and connexins in bone tissue, with special focus on regulatory effects of miRs and connexins on gene expression, as well as the mechanisms that mediate miR exchange between cells in direct contact through gap junctions, or between distant cells via EVs.

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.

MicroRNA-4739 regulates osteogenic and adipocytic differentiation of immortalized human bone marrow stromal cells via targeting LRP3

Understanding the regulatory networks underlying lineage differentiation and fate determination of human bone marrow stromal cells (hBMSC) is a prerequisite for their therapeutic use. The goal of the current study was to unravel the novel role of the low-density lipoprotein receptor-related protein 3 (LRP3) in regulating the osteogenic and adipogenic differentiation of immortalized hBMSCs. Gene expression profiling revealed significantly higher LRP3 levels in the highly osteogenic hBMSC clone imCL1 than in the less osteogenic clone imCL2, as well as a significant upregulation of LRP3 during the osteogenic induction of the imCL1 clone. Data from functional and gene expression assays demonstrated the role of LRP3 as a molecular switch promoting hBMSC lineage differentiation into osteoblasts and inhibiting differentiation into adipocytes. Interestingly, microRNA (miRNA) expression profiling identified miR-4739 as the most under-represented miRNA (-36.11 fold) in imCL1 compared to imCL2. The TargetScan prediction algorithm, combined with functional and biochemical assays, identified LRP3 mRNA as a novel target of miR-4739, with a single potential binding site for miR-4739 located in the LRP3 3' UTR. Regulation of LRP3 expression by miR-4739 was subsequently confirmed by qRT-PCR, western blotting, and luciferase assays. Over-expression of miR-4739 mimicked the effects of LRP3 knockdown on promoting adipogenic and suppressing osteogenic differentiation of hBMSCs. Hence, we report for the first time a novel biological role for the LRP3/hsa-miR-4739 axis in balancing osteogenic and adipocytic differentiation of hBMSCs. Our data support the potential utilization of miRNA-based therapies in regenerative medicine.

What Is New in the miRNA World Regarding Osteosarcoma and Chondrosarcoma?

Despite the availability of multimodal and aggressive therapies, currently patients with skeletal sarcomas, including osteosarcoma and chondrosarcoma, often have a poor prognosis. In recent decades, advances in sequencing technology have revealed the presence of RNAs without coding potential known as non-coding RNAs (ncRNAs), which provides evidence that protein-coding genes account for only a small percentage of the entire genome. This has suggested the influence of ncRNAs during development, apoptosis and cell proliferation. The discovery of microRNAs (miRNAs) in 1993 underscored the importance of these molecules in pathological diseases such as cancer. Increasing interest in this field has allowed researchers to study the role of miRNAs in cancer progression. Regarding skeletal sarcomas, the research surrounding which miRNAs are involved in the tumourigenesis of osteosarcoma and chondrosarcoma has rapidly gained traction, including the identification of which miRNAs act as tumour suppressors and which act as oncogenes. In this review, we will summarize what is new regarding the roles of miRNAs in chondrosarcoma as well as the latest discoveries of identified miRNAs in osteosarcoma.

MicroRNA-467g inhibits new bone regeneration by targeting Ihh/Runx-2 signaling

MicroRNAs are important post transcriptional regulators of gene expression and play critical role in osteoblast differentiation. In this study we report miR-467g, an uncharacterized novel miRNA, in regulation of osteoblast functions. Over-expression of miR-467g inhibited osteoblast differentiation. Target prediction analysis tools and experimental validation by luciferase 3' UTR reporter assay identified Runx-2 as a direct target of miR-467g. Over expression of miR-467g in osteoblasts down regulated Runx-2 and Ihh signaling components. Furthermore, silencing of miR-467g was done to see its role in Ihh and Runx-2 mediated bone healing and regeneration in a drill hole injury model in BALB/c mice. Silencing of miR-467g led to significant increase in new bone regeneration and Ihh and Runx-2 localization at injury site in a day dependent manner. In conclusion, miR-467g negatively regulates osteogenesis by targeting Ihh/Runx-2 signaling. We, thus, propose that therapeutic approaches targeting miR-467g could be useful in enhancing the new bone formation.

miR-21 is involved in skeletal deficiencies of zebrafish embryos exposed to polychlorinated biphenyls

Polychlorinated biphenyl (PCB) exposure increases the incidence and severity of skeletal diseases, but little is known about the mechanisms that mediate this relationship. We exposed zebrafish embryos to PCB₁₂₅₄ and assessed the changes in bone morphology protein receptor II (BMPRII), which is involved in bone formation and embryonic development, miRNA-21, for which BMPRII is a known target, and calcium metabolism. PCB₁₂₅₄ upregulated the expression of miR-21 and suppressed BMPRII expression. The inhibition of miR-21 reversed the downregulation of BMPRII and alleviated the PCB₁₂₅₄-induced loss of calcium. These findings suggest new biomarkers of developmental defects of the skeleton caused by PCBs.

miR-203 and miR-320 Regulate Bone Morphogenetic Protein-2-Induced Osteoblast Differentiation by Targeting Distal-Less Homeobox 5 (Dlx5)

MicroRNAs (miRNAs) are a family of small, non-coding RNAs (17–24 nucleotides), which regulate gene expression either by the degradation of the target mRNAs or inhibiting the translation of genes. Recent studies have indicated that miRNA plays an important role in regulating osteoblast differentiation. In this study, we identified miR-203 and miR-320b as important miRNAs modulating osteoblast differentiation. We identified Dlx5 as potential common target by prediction algorithms and confirmed this by knock-down and over expression of the miRNAs and assessing Dlx5 at mRNA and protein levels and specificity was verified by luciferase reporter assays. We examined the effect of miR-203 and miR-320b on osteoblast differentiation by transfecting with pre- and anti-miRs. Over-expression of miR-203 and miR-320b inhibited osteoblast differentiation, whereas inhibition of miR-203 and miR-320b stimulated alkaline phosphatase activity and matrix mineralization. We show that miR-203 and miR-320b negatively regulate BMP-2-induced osteoblast differentiation by suppressing Dlx5, which in turn suppresses the downstream osteogenic master transcription factor Runx2 and Osx and together they suppress osteoblast differentiation. Taken together, we propose a role for miR-203 and miR-320b in modulating bone metabolism.

Regulation of Connexins Expression Levels by MicroRNAs, an Update

Control of cell-cell coordination and communication is regulated by several factors, including paracrine and autocrine release of biomolecules, and direct exchange of soluble factors between cells through gap junction channels. Additionally, hemichannels also participate in cell-cell coordination through the release of signaling molecules, such as ATP and glutamate. A family of transmembrane proteins named connexins forms both gap junction channels and hemichannels. Because of their importance in cell and tissue coordination, connexins are controlled both by post-translational and post-transcriptional modifications. In recent years, non-coding RNAs have garnered research interest due to their ability to exert post-transcriptional regulation of gene expression. One of the most recent, well-documented control mechanisms of protein synthesis is found through the action of small, single-stranded RNA, called micro RNAs (miRNAs or miRs). Put simply, miRNAs are negative regulators of the expression of a myriad proteins involved in many physiological and pathological processes. This mini review will briefly summarize what is currently known about the action of miRNAs over Cxs expression/function in different organs under some relevant physiological and pathological conditions.

miRNAs Related to Skeletal Diseases

miRNAs as non-coding, short, double-stranded RNA segments are important for cellular biological functions, such as proliferation, differentiation, and apoptosis. miRNAs mainly contribute to the inhibition of important protein translations through their cleavage or direct repression of target messenger RNAs expressions. In the last decade, miRNAs got in the focus of interest with new publications on miRNAs in the context of different diseases. For many types of cancer or myocardial damage, typical signatures of local or systemically circulating miRNAs have already been described. However, little is known about miRNA expressions and their molecular effect in skeletal diseases. An overview of published studies reporting miRNAs detection linked with skeletal diseases was conducted. All regulated miRNAs were summarized and their molecular interactions were illustrated. This review summarizes the involvement and interaction of miRNAs in different skeletal diseases. Thereby, 59 miRNAs were described to be deregulated in tissue, cells, or in the circulation of osteoarthritis (OA), 23 miRNAs deregulated in osteoporosis, and 107 miRNAs deregulated in osteosarcoma (OS). The molecular influences of miRNAs regarding OA, osteoporosis, and OS were illustrated. Specific miRNA signatures for skeletal diseases are described in the literature. Some overlapped, but also unique ones for each disease exist. These miRNAs may present useful targets for the development of new therapeutic approaches and are candidates for diagnostic evaluations.

Impaired osteogenic differentiation associated with connexin43/microRNA-206 in steroid-induced avascular necrosis of the femoral head

Connexin(Cx)43 and microRNA(miR)-206 play an important role in osteogenesis. However, their role in steroid-induced femoral head osteonecrosis (SANFH) is still ambiguous. The present study aimed to establish a rabbit model and investigate osteogenesis in steroid-induced femoral head osteonecrosis occurring via Cx43/miR-206 and the changes of Wnt/β-catenin signal pathway-related proteins. A total of 72 adult New Zealand white rabbits were divided randomly into a model group (Group A) and a control group (Group B) of 36 rabbits each. Group A was injected intravenously with lipopolysaccharide (10μg/kg body weight, once per day). After 48h, three injections of methylprednisolone (MPS; 20mg/kg body weight) were administered intramuscularly at 24-hour intervals. Group B were fed and housed under identical conditions but received saline injections. All animals were sacrificed at two, four, and eight weeks from the first MPS injection. Typical early osteonecrosis symptoms were observed in Group A. The expression of miR-206 in Group A was significantly higher than that of Group B. The mRNA and protein levels of Cx43, β-catenin, runt-related transcription factor 2, and alkaline phosphatase gradually decreased while Dickkopf-1 (Dkk-1) gradually increased in Group A compared with Group B. These findings indicated that Cx43/miR-206 is involved in the pathogenesis of early stage SANFH and may be associate with Wnt/β-catenin signal pathway.

Orthosilicic acid, Si(OH)4, stimulates osteoblast differentiation in vitro by upregulating miR-146a to antagonize NF-κB activation

Accumulating evidence over the last 40years suggests that silicate from dietary as well as silicate-containing biomaterials is beneficial to bone formation. However, the exact biological role(s) of silicate on bone cells are still unclear and controversial. Here, we report that orthosilicic acid (Si(OH)4) stimulated human mesenchymal stem cells (hMSCs) osteoblastic differentiation in vitro. To elucidate the possible molecular mechanisms, differential microRNA microarray analysis was used to show that Si(OH)4 significantly up-regulated microRNA-146a (miR-146a) expression during hMSC osteogenic differentiation. Si(OH)4 induced miR-146a expression profiling was further validated by quantitative RT-PCR (qRT-PCR), which indicated miR-146a was up-regulated during the late stages of hMSC osteogenic differentiation. Inhibition of miR-146a function by anti-miR-146a suppressed osteogenic differentiation of MC3T3 pre-osteoblasts, whereas Si(OH)4 treatment promoted osteoblast-specific genes transcription, alkaline phosphatase (ALP) production, and mineralization. Furthermore, luciferase reporter assay, Western blotting, enzyme-linked immunosorbent assay (ELISA), and immunofluorescence showed that Si(OH)4 decreased TNFα-induced activation of NF-κB, a signal transduction pathway that inhibits osteoblastic bone formation, through the known miR-146a negative feedback loop. Our studies established a mechanism for Si(OH)4 to promote osteogenesis by antagonizing NF-κB activation via miR-146a, which might be interesting to guide the design of osteo-inductive biomaterials for treatments of bone defects in humans.

STATEMENT OF SIGNIFICANCE

Accumulating evidence over 40years suggests that silicate is beneficial to bone formation. However, the biological role(s) of silicate on bone cells are still unclear and controversial. Here, we report that Si(OH)4, the simplest form of silicate, can stimulate human mesenchymal stem cells osteoblastic differentiation. We identified that miR-146a is the expression signature in bone cells treated with Si(OH)4. Further analysis of miR-146a in bone cells reveals that Si(OH)4 upregulates miR-146a to antagonize the activation of NF-κB. Si(OH)4 was also shown to deactivate the same NF-κB pathway to suppress osteoclast formation. Our findings are important to the development of third-generation cell-and gene affecting biomaterials, and suggest silicate and miR-146a can be used as pharmaceuticals for bone fracture prevention and therapy.

Circadian Clock Regulates Bone Resorption in Mice

The circadian clock controls many behavioral and physiological processes beyond daily rhythms. Circadian dysfunction increases the risk of cancer, obesity, and cardiovascular and metabolic diseases. Although clinical studies have shown that bone resorption is controlled by circadian rhythm, as indicated by diurnal variations in bone resorption, the molecular mechanism of circadian clock-dependent bone resorption remains unknown. To clarify the role of circadian rhythm in bone resorption, aryl hydrocarbon receptor nuclear translocator-like (Bmal1), a prototype circadian gene, was knocked out specifically in osteoclasts. Osteoclast-specific Bmal1-knockout mice showed a high bone mass phenotype due to reduced osteoclast differentiation. A cell-based assay revealed that BMAL1 upregulated nuclear factor of activated T cells, cytoplasmic, calcineurin-dependent 1 (Nfatc1) transcription through its binding to an E-box element located on the Nfatc1 promoter in cooperation with circadian locomotor output cycles kaput (CLOCK), a heterodimer partner of BMAL1. Moreover, steroid receptor coactivator (SRC) family members were shown to interact with and upregulate BMAL1:CLOCK transcriptional activity. Collectively, these data suggest that bone resorption is controlled by osteoclastic BMAL1 through interactions with the SRC family and binding to the Nfatc1 promoter. © 2016 American Society for Bone and Mineral Research.

Increased miR-155-5p and reduced miR-148a-3p contribute to the suppression of osteosarcoma cell death

Osteosarcoma (OS) is the most common cancer of bone and the 5th leading cause of cancer-related death in young adults. Currently, 5-year survival rates have plateaued at ~70% for patients with localized disease. Those with disseminated disease have an ~20% 5-year survival. An improved understanding of the molecular genetics of OS may yield new approaches to improve outcomes for OS patients. To this end, we applied murine models that replicate human OS to identify and understand dysregulated microRNAs (miRNAs) in OS. miRNA expression patterns were profiled in murine primary osteoblasts, osteoblast cultures and primary OS cell cultures (from primary and paired metastatic locations) isolated from two genetically engineered murine models of OS. The differentially expressed miRNA were further assessed by a cross-species comparison with human osteoblasts and OS cultures. We identified miR-155-5p and miR-148a-3p as deregulated in OS. miR-155-5p suppression or miR-148a-3p overexpression potently reduced proliferation and induced apoptosis in OS cells, yet strikingly, did not impact normal osteoblasts. To define how these miRNAs regulated OS cell fate, we used an integrated computational approach to identify putative candidate targets and then correlated these with the cell biological impact. Although we could not resolve the mechanism through which miR-148a-3p impacts OS, we identified that miR-155-5p overexpression suppressed its target Ripk1 (receptor (TNFRSF)-interacting serine-threonine kinase 1) expression, and miR-155-5p inhibition elevated Ripk1 levels. Ripk1 is directly involved in apoptosis/necroptosis. In OS cells, small interfering RNA against Ripk1 prevented cell death induced by the sequestration of miR-155-5p. Collectively, we show that miR-148a-3p and miR-155-5p are species-conserved deregulated miRNA in OS. Modulation of these miRNAs was specifically toxic to tumor cells but not normal osteoblasts, raising the possibility that these may be tractable targets for miRNA-based therapies for OS.

MicroRNAs and Bone Regeneration

Bone has multiple functions, both morphologically and physiologically, and it frequently features in the pathological condition, including fracture and osteoporosis. For bone regeneration therapy, the regulation of osteoblast differentiation is important. MicroRNA (miRNA)s are short noncoding RNA which regulate gene expression at the post-transcriptional level. MiRNAs play an important role not only in a variety of other cellular processes including differentiation, proliferation, and apoptosis but also in the pathogenesis of human diseases. Recently, miRNAs have been known to participate in osteoblast differentiation by regulating several signaling pathways including transcription factors. New insight into the mechanism during osteogenes is affected by miRNAs has been gained. Moreover, therapeutic trials for bone diseases including osteoporosis, fracture and bone defects targeting miRNAs have been examined in animal models. MiRNA therapy will enable development of a bone regeneration therapy.

Osteoclast-derived exosomal miR-214-3p inhibits osteoblastic bone formation

Emerging evidence indicates that osteoclasts direct osteoblastic bone formation. MicroRNAs (miRNAs) have a crucial role in regulating osteoclast and osteoblast function. However, whether miRNAs mediate osteoclast-directed osteoblastic bone formation is mostly unknown. Here, we show that increased osteoclastic miR-214-3p associates with both elevated serum exosomal miR-214-3p and reduced bone formation in elderly women with fractures and in ovariectomized (OVX) mice. Osteoclast-specific miR-214-3p knock-in mice have elevated serum exosomal miR-214-3p and reduced bone formation that is rescued by osteoclast-targeted antagomir-214-3p treatment. We further demonstrate that osteoclast-derived exosomal miR-214-3p is transferred to osteoblasts to inhibit osteoblast activity in vitro and reduce bone formation in vivo. Moreover, osteoclast-targeted miR-214-3p inhibition promotes bone formation in ageing OVX mice. Collectively, our results suggest that osteoclast-derived exosomal miR-214-3p transfers to osteoblasts to inhibit bone formation. Inhibition of miR-214-3p in osteoclasts may be a strategy for treating skeletal disorders involving a reduction in bone formation.

In previous studies the authors discovered that miR-214 inhibits osteoblastic bone formation. Here they extend on these findings, using ovariectomized mice and samples from patients with bone fractures, to show that miR-214 is a mediator of osteoclast-osteoblast crosstalk.

The Indispensable Role of Cyclin-Dependent Kinase 1 in Skeletal Development

Skeletal development is tightly regulated through the processes of chondrocyte proliferation and differentiation. Although the involvement of transcription and growth factors on the regulation of skeletal development has been extensively studied, the role of cell cycle regulatory proteins in this process remains elusive. To date, through cell-specific loss-of-function experiments in vivo, no cell cycle regulatory proteins have yet been conclusively shown to regulate skeletal development. Here, we demonstrate that cyclin-dependent kinase 1 (Cdk1) regulates skeletal development based on chondrocyte-specific loss-of-function experiments performed in a mouse model. Cdk1 is highly expressed in columnar proliferative chondrocytes and is greatly downregulated upon differentiation into hypertrophic chondrocytes. Cdk1 is essential for proper chondrocyte proliferation and deletion of Cdk1 resulted in accelerated differentiation of chondrocytes. In vitro and ex vivo analyses revealed that Cdk1 is an essential cell cycle regulatory protein for parathyroid hormone-related peptide (PTHrP) signaling pathway, which is critical to chondrocyte proliferation and differentiation. These results demonstrate that Cdk1 functions as a molecular switch from proliferation to hypertrophic differentiation of chondrocytes and thus is indispensable for skeletal development. Given the availability of inhibitors of Cdk1 activity, our results could provide insight for the treatment of diseases involving abnormal chondrocyte proliferation, such as osteoarthritis.

MiR-221-inhibited adipose tissue-derived mesenchymal stem cells bioengineered in a nano-hydroxy apatite scaffold

The repair of skeletal defects is the main goal of bone tissue engineering. Recent literature highlighted various regulatory roles of microRNAs in stem cell fate determination. In addition, the role of porous hydroxyapatite/polycaprolacton (nHA/PCL) as a bioactive scaffold which enhances adipose tissue-derived mesenchymal stem cells (AT-MSCs) growth and osteogenic differentiation has been proved. The aim of the present study was to investigate the synergistic potential of both down-regulating miR-221 and nHA/PCL scaffold seeding in osteogenic potential of AT-MSCs. After isolation and characterization of AT-MSCs, the transfection of anti-miR-221 was performed into the cells using lipofectamine 2000 and the transfected cells were seeded into a synthesized nHA/PCL scaffold. The DAPI staining confirmed the presence of AT-MSCs on nHA/PCL scaffold. Quantitative expression of osteoblast marker genes, Runx2, and osteocalcin of the transfected cells in the scaffold were evaluated. Interestingly, significant upregulation of transcribed Runx2 and osteocalcin genes (P < 0.01) were observed in miR-221-inhibited nHA/PCL seeded cells. Also, alkaline phosphatase activity (ALP) was significantly higher (P < 0.01) in miR-221-inhibited AT-MSCs seeded on nHA/PCL than those seeded on nHA/PCL or transfected with anti-miR-221, individually. The results of this combination suggest a valuable method for enhancing osteogenesis in AT-MSCs. This method could be applicable for gene-cell therapy of bone defects.

Understanding the Osteosarcoma Pathobiology: A Comparative Oncology Approach

Osteosarcoma is an aggressive primary bone tumor in humans and is among the most common cancer afflicting dogs. Despite surgical advancements and intensification of chemo- and targeted therapies, the survival outcome for osteosarcoma patients is, as of yet, suboptimal. The presence of metastatic disease at diagnosis or its recurrence after initial therapy is a major factor for the poor outcomes. It is thought that most human and canine patients have at least microscopic metastatic lesions at diagnosis. Osteosarcoma in dogs occurs naturally with greater frequency and shares many biological and clinical similarities with osteosarcoma in humans. From a genetic perspective, osteosarcoma in both humans and dogs is characterized by complex karyotypes with highly variable structural and numerical chromosomal aberrations. Similar molecular abnormalities have been observed in human and canine osteosarcoma. For instance, loss of TP53 and RB regulated pathways are common. While there are several oncogenes that are commonly amplified in both humans and dogs, such as MYC and RAS, no commonly activated proto-oncogene has been identified that could form the basis for targeted therapies. It remains possible that recurrent aberrant gene expression changes due to gene amplification or epigenetic alterations could be uncovered and these could be used for developing new, targeted therapies. However, the remarkably high genomic complexity of osteosarcoma has precluded their definitive identification. Several advantageous murine models of osteosarcoma have been generated. These include spontaneous and genetically engineered mouse models, including a model based on forward genetics and transposon mutagenesis allowing new genes and genetic pathways to be implicated in osteosarcoma development. The proposition of this review is that careful comparative genomic studies between human, canine and mouse models of osteosarcoma may help identify commonly affected and targetable pathways for alternative therapies for osteosarcoma patients. Translational research may be found through a path that begins in mouse models, and then moves through canine patients, and then human patients.

Regulation of connexin signaling by the epigenetic machinery

Connexins and their channels are involved in the control of all aspects of the cellular life cycle, ranging from cell growth to cell death, by mediating extracellular, intercellular and intracellular communication. These multifaceted aspects of connexin-related cellular signaling obviously require strict regulation. While connexin channel activity is mainly directed by posttranslational modifications, connexin expression as such is managed by classical cis/trans mechanisms. Over the past few years, it has become clear that connexin production is equally dictated by epigenetic actions. This paper provides an overview of the role of major determinants of the epigenome, including DNA methylation, histone acetylation and microRNA species, in connexin expression.

MicroRNAs' Involvement in Osteoarthritis and the Prospects for Treatments

Osteoarthritis (OA) is a chronic disease and its etiology is complex. With increasing OA incidence, more and more people are facing heavy financial and social burdens from the disease. Genetics-related aspects of OA pathogenesis are not well understood. Recent reports have examined the molecular mechanisms and genes related to OA. It has been realized that genetic changes in articular cartilage and bone may contribute to OA's development. Osteoclasts, osteoblasts, osteocytes, and chondrocytes in joints must express appropriate genes to achieve tissue homeostasis, and errors in this can cause OA. MicroRNAs (miRNAs) are small noncoding RNAs that have been discovered to be overarching regulators of gene expression. Their ability to repress many target genes and their target-binding specificity indicate a complex network of interactions, which is still being defined. Many studies have focused on the role of miRNAs in bone and cartilage and have identified numbers of miRNAs that play important roles in regulating bone and cartilage homeostasis. Those miRNAs may also be involved in the pathology of OA, which is the focus of this review. Future studies on the role of miRNAs in OA will provide important clues leading to a better understanding of the mechanism(s) of OA and, more particularly, to the development of therapeutic targets for OA.

MicroRNA-145 regulates osteoblastic differentiation by targeting the transcription factor Cbfb

Osteoblastic differentiation is regulated by various factors, including hormones and transcription factors. Runt-related transcription factor 2 (Runx2) is an essential player in osteoblastogenesis and transactivates its molecular target by creating a protein complex with its hetero-dimeric partner core binding factor beta (Cbfb). However, the molecular regulation of Cbfb expression remains unknown. Here, we identified miR-145 as a crucial regulator of Cbfb expression. The expression of miR-145 increased during osteoblastogenesis, indicating that miR-145 works as an inhibitor of osteoblastogenesis. Stable expression of miR-145 decreased endogenous Cbfb expression and inhibited osteoblastogenesis, in cooperation with miR-34c. Furthermore, miR-145 decreased bone regeneration in vivo. Our results indicate that miR-145 physiologically regulates osteoblast differentiation and bone formation via Cbfb expression by forming a regulatory microRNA network.

miR-21 synergizes with BMP9 in osteogenic differentiation by activating the BMP9/Smad signaling pathway in murine multilineage cells

Bone morphogenetic proteins (BMPs), particularly BMP9, have been shown to promote the osteogenic differentiation of murine multilineage cells (MMCs) and to promote bone formation in bone diseases; however, the mechanisms involved remain poorly understood. MicroRNAs (miRNAs or miRs) have been proven to regulate mesenchymal stem cell (MSC) differentiation. In this study, we identified a novel mechanism that unravels the functional axis of a key miRNA (miR-21) which contributes to BMP9-induced osteogenic differentiation. We screened differentially expressed miRNAs in MMCs during BMP9-induced osteogenic differentiation and found that miR-21 was significantly upregulated by BMP9 during the osteogenesis of MMCs. Furthermore, miR-21 was confirmed to promote the osteogenic differentiation of the MMCs by suppressing Smad7, which negatively regulates the osteogenic differentiation of MMCs. The upregulation of miR-21 may promote the osteogenic differentiation of MMCs in synergy with BMP9. The findings of our study revealed a novel function of miR-21, and suggest that the overexpression of miR-21 contributes to bone formation by promoting BMP9-induced osteogenic differentiation. Our data may provide a molecular basis for the development of novel therapeutic strategies to treat bone diseases, such as osteoporosis and other inflammatory bone diseases.

MicroRNA expression in bone marrow mesenchymal stem cells from mice with steroid-induced osteonecrosis of the femoral head

The present study aimed to identify microRNAs (miRNAs) from bone marrow mesenchymal stem cells (BMSCs) in a rat model of steroid-induced osteonecrosis of the femoral head (ONFH) using Affymetrix GeneChip®. Following identification of miRNAs, the present study aimed to elucidate the molecular mechanisms underlying steroid‑induced ONFH. A total of six C57BL/6J mice were randomly divided into two groups, control and experimental groups (n=3 per group). The mice in the experimental group were subcutaneously injected with 21 mg/kg methylprednisolone for 4 weeks, while the mice in the control group were injected with the identical dose of normal saline. The femoral head was subsequently removed and sectioned. Following sectioning, hematoxylin and eosin staining, and terminal deoxynucleotidyl transferase dUTP nick end labeling were performed to confirm the establishment of the model. To replicate the animal model ex vivo, the bone marrow was isolated. Next, different miRNAs were screened for using GeneChip®, and the key miRNAs were assessed by bioinformatics analysis and their functions were confirmed. Compared with the control, 23 miRNAs in the experimental group were identified, with seven upregulated and 16 downregulated. Of these miRNAs, putative target miRNAs were predicted by bioinformatics analysis, with two being upregulated (miR‑21‑3p and miR‑652‑5p) and five downregulated (miR‑206‑3p, miR‑196a‑5p, miR‑34b‑3p, miR‑34c‑5p and miR‑148a‑3p). The results of reverse transcription‑quantitative polymerase chain reaction were consistent with the gene‑chip results. Steroid‑induced ONFH may cause miRNA changes in BMSCs. Numerous miRNAs regulate osteogenic differentiation and the decrease in miRNA‑196a‑5p may be important in steroid-induced ONFH.

Osteogenic gene transcription is regulated via gap junction-mediated cell-cell communication

An analytical study of cell-cell communications between murine osteoblast-like MLO-A5 cells and bone marrow mesenchymal stem cell (BMSC)-like C3H10T1/2 cells was performed. C3H10T1/2 cells expressing green fluorescent protein (10T-GFP cells) were generated to enable the isolation of the BMSC-like cells from co-cultures with MLO-A5 cells. The mRNA expression levels of several osteogenic transcription factors (Runx2, Osterix, Dlx5, and Msx2) did not differ between the co-cultured and mono-cultured 10T-GFP cells, but those of alkaline phosphatase (ALP) and bone sialoprotein (BSP) were 300- to 400-fold higher in the co-cultured cells. Patch clamp and biocytin transfer assays revealed gap junction-mediated communication between co-cultured 10T-GFP and MLO-A5 cells. The addition of a gap junction inhibitor suppressed the increases in the expression levels of the ALP and BSP mRNAs in co-cultured 10T-GFP cells. Furthermore, the histone acetylation levels were higher in co-cultured 10T-GFP cells than in mono-cultured 10T-GFP cells. These results suggest that osteoblasts and BMSCs associate via gap junctions, and that gap junction-mediated signaling induces histone acetylation that leads to elevated transcription of the genes encoding ALP and BSP in BMSCs.

The role of microRNAs in bone remodeling

Bone remodeling is balanced by bone formation and bone resorption as well as by alterations in the quantities and functions of seed cells, leading to either the maintenance or deterioration of bone status. The existing evidence indicates that microRNAs (miRNAs), known as a family of short non-coding RNAs, are the key post-transcriptional repressors of gene expression, and growing numbers of novel miRNAs have been verified to play vital roles in the regulation of osteogenesis, osteoclastogenesis, and adipogenesis, revealing how they interact with signaling molecules to control these processes. This review summarizes the current knowledge of the roles of miRNAs in regulating bone remodeling as well as novel applications for miRNAs in biomaterials for therapeutic purposes.

Manipulating Cx43 expression triggers gene reprogramming events in dermal fibroblasts from oculodentodigital dysplasia patients

Oculodentodigital dysplasia (ODDD) is primarily an autosomal dominant disorder linked to over 70 GJA1 gene [connexin43 (Cx43)] mutations. For nearly a decade, our laboratory has been investigating the relationship between Cx43 and ODDD by expressing disease-linked mutants in reference cells, tissue-relevant cell lines, 3D organ cultures and by using genetically modified mouse models of human disease. Although salient features of Cx43 mutants have been revealed, these models do not necessarily reflect the complexity of the human context. To further overcome these limitations, we have acquired dermal fibroblasts from two ODDD-affected individuals harbouring D3N and V216L mutations in Cx43, along with familial controls. Using these ODDD patient dermal fibroblasts, which naturally produce less GJA1 gene product, along with RNAi and RNA activation (RNAa) approaches, we show that manipulating Cx43 expression triggers cellular gene reprogramming. Quantitative RT-PCR, Western blot and immunofluorescent analysis of ODDD patient fibroblasts show unusually high levels of extracellular matrix (ECM)-interacting proteins, including integrin α5β1, matrix metalloproteinases as well as secreted ECM proteins collagen-I and laminin. Cx43 knockdown in familial control cells produces similar effects on ECM expression, whereas Cx43 transcriptional up-regulation using RNAa decreases production of collagen-I. Interestingly, the enhanced levels of ECM-associated proteins in ODDD V216L fibroblasts is not only a consequence of increased ECM gene expression, but also due to an apparent deficit in collagen-I secretion which may further contribute to impaired collagen gel contraction in ODDD fibroblasts. These findings further illuminate the altered function of Cx43 in ODDD-affected individuals and highlight the impact of manipulating Cx43 expression in human cells.

miR-206 Mediates YAP-Induced Cardiac Hypertrophy and Survival

RATIONALE

In Drosophila, the Hippo signaling pathway negatively regulates organ size by suppressing cell proliferation and survival through the inhibition of Yorkie, a transcriptional cofactor. Yes-associated protein (YAP), the mammalian homolog of Yorkie, promotes cardiomyocyte growth and survival in postnatal hearts. However, the underlying mechanism responsible for the beneficial effect of YAP in cardiomyocytes remains unclear.

OBJECTIVES

We investigated whether miR-206, a microRNA known to promote hypertrophy in skeletal muscle, mediates the effect of YAP on promotion of survival and hypertrophy in cardiomyocytes.

METHODS AND RESULTS

Microarray analysis indicated that YAP increased miR-206 expression in cardiomyocytes. Increased miR-206 expression induced cardiac hypertrophy and inhibited cell death in cultured cardiomyocytes, similar to that of YAP. Downregulation of endogenous miR-206 in cardiomyocytes attenuated YAP-induced cardiac hypertrophy and survival, suggesting that miR-206 plays a critical role in mediating YAP function. Cardiac-specific overexpression of miR-206 in mice induced hypertrophy and protected the heart from ischemia/reperfusion injury, whereas suppression of miR-206 exacerbated ischemia/reperfusion injury and prevented pressure overload-induced cardiac hypertrophy. miR-206 negatively regulates Forkhead box protein P1 expression in cardiomyocytes and overexpression of Forkhead box protein P1 attenuated miR-206-induced cardiac hypertrophy and survival, suggesting that Forkhead box protein P1 is a functional target of miR-206.

CONCLUSIONS

YAP increases the abundance of miR-206, which in turn plays an essential role in mediating hypertrophy and survival by silencing Forkhead box protein P1 in cardiomyocytes.

Connexins and pannexins in the skeleton: gap junctions, hemichannels and more

Regulation of bone homeostasis depends on the concerted actions of bone-forming osteoblasts and bone-resorbing osteoclasts, controlled by osteocytes, cells derived from osteoblasts surrounded by bone matrix. The control of differentiation, viability and function of bone cells relies on the presence of connexins. Connexin43 regulates the expression of genes required for osteoblast and osteoclast differentiation directly or by changing the levels of osteocytic genes, and connexin45 may oppose connexin43 actions in osteoblastic cells. Connexin37 is required for osteoclast differentiation and its deletion results in increased bone mass. Less is known on the role of connexins in cartilage, ligaments and tendons. Connexin43, connexin45, connexin32, connexin46 and connexin29 are expressed in chondrocytes, while connexin43 and connexin32 are expressed in ligaments and tendons. Similarly, although the expression of pannexin1, pannexin2 and pannexin3 has been demonstrated in bone and cartilage cells, their function in these tissues is not fully understood.

MicroRNAs in Bone Balance and Osteoporosis

Preclinical Research Bone is a rigid and dynamic organ that undergoes continuous turnover. Bone homeostasis is maintained by osteoclast-mediated bone resorption and osteoblast-mediated bone formation. The interruption of this balance can cause various diseases, including osteoporosis a public health issue due to the rate of hip fracture, the most serious outcome of osteoporosis. The bone loss in osteoporosis results from an increase in bone resorption versus bone formation. Thus, regulation of osteoblast and osteoclast activity is a main focus in the treatment of osteoporosis. MicroRNAs (miRNAs) are a class of single stranded noncoding RNAs consisting of 18-22 nucleotides that have an important role in cell differentiation, cell fate, apoptosis, and pathogenesis in various disease states. The potential therapeutic and biomarker function of miRNAs in treating bone disorders is receiving more attention. The current review summarizes the role of miRNAs in bone function at a cellular level in the context of their therapeutic potential.

JMJD3 as an epigenetic regulator in development and disease

Gene expression is epigenetically regulated through DNA methylation and covalent chromatin modifications, such as acetylation, phosphorylation, ubiquitination, sumoylation, and methylation of histones. Histone methylation state is dynamically regulated by different groups of histone methyltransferases and demethylases. The trimethylation of histone 3 (H3K4) at lysine 4 is usually associated with the activation of gene expression, whereas trimethylation of histone 3 at lysine 27 (H3K27) is associated with the repression of gene expression. The polycomb repressive complex contains the H3K27 methyltransferase Ezh2 and controls dimethylation and trimethylation of H3K27 (H3K27me2/3). The Jumonji domain containing-3 (Jmjd3, KDM6B) and ubiquitously transcribed X-chromosome tetratricopeptide repeat protein (UTX, KDM6A) have been identified as H3K27 demethylases that catalyze the demethylation of H3K27me2/3. The role and mechanisms of both JMJD3 and UTX have been extensively studied for their involvement in development, cell plasticity, immune system, neurodegenerative disease, and cancer. In this review, we will focus on recent progresses made on understanding JMJD3 in the regulation of gene expression in development and diseases. This article is part of a Directed Issue entitled: Epigenetics dynamics in development and disease.

MicroRNA-153 suppresses the osteogenic differentiation of human mesenchymal stem cells by targeting bone morphogenetic protein receptor type II

Elucidation of the molecular mechanisms governing the osteogenic differentiation of human mesenchymal stem cells (hMSCs) is of great importance for improving the treatment of bone-related diseases. MicroRNAs (miRNAs or miRs), a class of small non-coding RNAs, are critical in a number of biological processes, including the proliferation, differentiation and survival of cells and organisms. Emerging evidence indicates that miRNAs are essential in regulating osteoblastogenesis and bone formation. However, the role of miRNAs in osteoblast mechanotransduction remains to be defined. The present study aimed to examine the role of miR-153 in the osteogenesis of hMSCs and to investigate the impact of miR-153 on bone morphogenetic protein receptor type II (BMPR2) expression. The overexpression of miR-153 inhibited the osteogenic differentiation of hMSCs, whereas downregulation of miR-153 enhanced the process. Furthermore, bioinformatic analysis predicted that miR-153 is a potential regulator of BMPR2. The direct binding of miR-153 to the BMPR2 3'-untranslated region (3'-UTR) was demonstrated by a luciferase reporter assay using a construct containing the BMPR2 3'-UTR. In addition, knockdown of BMPR2 by RNA interference inhibited the osteogenic differentiation of hMSCs, with a similar effect to the upregulation of miR-153. In conclusion, the results suggest that miR-153 is a mechano-sensitive miRNA that regulates osteoblast differentiation by directly targeting BMPR2, and that therapeutic inhibition of miR-153 may be an efficient anabolic strategy for skeletal disorders caused by pathological mechanical loading.

miR-23a impairs bone differentiation in osteosarcoma via down-regulation of GJA1

Osteosarcoma is the most common type of bone cancer in children and adolescents. Impaired differentiation of osteoblast cells is a distinguishing feature of this aggressive disease. As improvements in survival outcomes have largely plateaued, better understanding of the bone differentiation program may provide new treatment approaches. The miRNA cluster miR-23a~27a~24-2, particularly miR-23a, has been shown to interact with genes important for bone development. However, global changes in gene expression associated with functional gain of this cluster have not been fully explored. To better understand the relationship between miR-23a expression and bone cell differentiation, we carried out a large-scale gene expression analysis in HOS cells. Experimental results demonstrate that over-expression of miR-23a delays differentiation in this system. Downstream bioinformatic analysis identified miR-23a target gene connexin-43 (Cx43/GJA1), a mediator of intercellular signaling critical to osteoblast development, as acutely affected by miR-23a levels. Connexin-43 is up-regulated in the course of HOS cell differentiation and is down-regulated in cells transfected with miR-23a. Analysis of gene expression data, housed at Gene Expression Omnibus, reveals that Cx43 is consistently up-regulated during osteoblast differentiation. Suppression of Cx43 mRNA by miR-23a was confirmed in vitro using a luciferase reporter assay. This work demonstrates novel interactions between microRNA expression, intercellular signaling and bone differentiation in osteosarcoma.

microRNA regulation of Wnt signaling pathways in development and disease

Wnt signaling pathways and microRNAs (miRNAs) are critical regulators of development. Aberrant Wnt signaling pathways and miRNA levels lead to developmental defects and diverse human pathologies including but not limited to cancer. Wnt signaling pathways regulate a plethora of cellular processes during embryonic development and maintain homeostasis of adult tissues. A majority of Wnt signaling components are regulated by miRNAs which are small noncoding RNAs that are expressed in both animals and plants. In animal cells, miRNAs fine tune gene expression by pairing primarily to the 3'untranslated region of protein coding mRNAs to repress target mRNA translation and/or induce target degradation. miRNA-mediated regulation of signaling transduction pathways is important in modulating dose-sensitive response of cells to signaling molecules. This review discusses components of the Wnt signaling pathways that are regulated by miRNAs in the context of development and diseases. A fundamental understanding of miRNA functions in Wnt signaling transduction pathways may yield new insight into crosstalks of regulatory mechanisms essential for development and disease pathophysiology leading to novel therapeutics.

HIF-1α-induced microRNA-210 reduces hypoxia-induced osteoblast MG-63 cell apoptosis

To better understand the ischemic-hypoxia-induced fracture healing impairment, we determined in this study the microRNA-210 expression in broken bone specimens and in osteoblasts under hypoxia and then determined the influence of microRNA-210 overexpression on the osteoblast cell proliferation and apoptosis. Results demonstrated that microRNA-210 expression was upregulated with an association with HIF-1α overexpression in clinical human catagmatic tissues and was upregulated HIF-1α-dependently in response to hypoxia in osteoblast MG-63 cells. CCK-8 assay indicated that microRNA-210 upregulation by microRNA-210 mimics reduced the chemotherapeutic 5-FU-induced osteoblast cell death, and colony formation assay demonstrated that microRNA-210 mimics promoted osteoblast cells growth. Moreover, the microRNA-210 mimics transfection inhibited the hypoxia-induced MG-63 cell apoptosis via inhibiting the activation of caspase 3 and caspase 9. Therefore, our research indicated a protective role of microRNA-210 in response to hypoxia. And microRNA-210 might serve as a protective role in bone fracture healing.

Identification of MicroRNA for Intermuscular Bone Development in Blunt Snout Bream (Megalobrama amblycephala)

Intermuscular bone (IB), which occurs only in the myosepta of the lower teleosts, is attracting more attention of researchers due to its particular development and lack of genetic information. MicroRNAs (miRNAs) are emerging as important regulators for biological processes. In the present study, miRNAs from IBs and connective tissue (CT; encircled IBs) from six-month-old Megalobrama amblycephala were characterized and compared. The results revealed the sequences and expression levels of 218 known miRNA genes (belonging to 97 families). Of these miRNAs, 44 known microRNA sequences exhibited significant expression differences between the two libraries, with 24 and 20 differentially-expressed miRNAs exhibiting higher expression in the CT and IBs libraries, respectively. The expressions of 11 miRNAs were selected to validate in nine tissues. Among the high-ranked predicted gene targets, differentiation, cell cycle, metabolism, signal transduction and transcriptional regulation were implicated. The pathway analysis of differentially-expressed miRNAs indicated that they were abundantly involved in regulating the development and differentiation of IBs and CT. This study characterized the miRNA for IBs of teleosts for the first time, which provides an opportunity for further understanding of miRNA function in the regulation of IB development.

microRNA-375 inhibits osteogenic differentiation by targeting runt-related transcription factor 2

microRNAs (miRNAs) are involved in the regulation of almost all physiological and pathological processes, including osteoblast proliferation and differentiation. miR-375 has been reported to be aberrantly expressed in various tumor types and to inhibit cell proliferation and invasion by targeting a number of key genes. However, the effects of miR-375 on osteogenic differentiation have not been investigated. Thus, the present study aimed to elucidate the function served by miR-375 in osteogenic differentiation. The expression levels of miR-375 were observed to decrease in a C2C12 cell model of osteogenic differentiation. Overexpression of miR-375 inhibited the activity of key osteoblast markers, including osteocalcin (OC), alkaline phosphatase (ALP) and collagen, type I, α 1 (COL1A1). By contrast, inhibition of miR-375 expression resulted in an increase in the osteogenic potential, as indicated by the enhanced expression levels of OC, ALP and COL1A1. In addition, a dual-luciferase reporter assay indicated that runt-related transcription factor 2 (RUNX2) was a target of miR-375. Western blot analysis revealed that the inhibition of miR-375 led to a significant increase in the protein expression levels of RUNX2. In addition, overexpression of RUNX2 was observed to attenuate the miR-375-mediated suppression of osteogenic differentiation. Therefore, the results demonstrated that miR-375 was able to inhibit osteogenic differentiation via the regulation of RUNX2 expression.

Bone marrow stroma-derived miRNAs as regulators, biomarkers and therapeutic targets of bone metastasis

MicroRNAs (miRNAs) are short, endogenous RNA molecules that have essential roles in regulating gene expression. They control numerous physiological and cellular processes, including normal bone organogenesis and homeostasis, by enhancing or inhibiting bone marrow cell growth, differentiation, functional activity and crosstalk of the multiple cell types within the bone. Hence, elucidating miRNA targets in bone marrow stromal cells has revealed novel regulations during bone development and maintenance. Moreover, recent studies have detailed the capacity for bone stromal miRNAs to influence bone metastasis from a number of primary carcinomas by interfering with bone homeostasis or by directly influencing metastatic tumor cells. Owing to the current lack of good diagnostic biomarkers of bone metastases, such changes in bone stromal miRNA expression in the presence of metastatic lesions may become useful biomarkers, and may even serve as therapeutic targets. In particular, cell-free and exosomal miRNAs shed from bone stromal cells into circulation may be developed into novel biomarkers that can be routinely measured in easily accessible samples. Taken together, these findings reveal the significant role of bone marrow stroma-derived miRNAs in the regulation of bone homeostasis and bone metastasis.

TGF-β/BMP signaling and other molecular events: regulation of osteoblastogenesis and bone formation

Transforming growth factor-beta (TGF-β)/bone morphogenetic protein (BMP) plays a fundamental role in the regulation of bone organogenesis through the activation of receptor serine/threonine kinases. Perturbations of TGF-β/BMP activity are almost invariably linked to a wide variety of clinical outcomes, i.e., skeletal, extra skeletal anomalies, autoimmune, cancer, and cardiovascular diseases. Phosphorylation of TGF-β (I/II) or BMP receptors activates intracellular downstream Smads, the transducer of TGF-β/BMP signals. This signaling is modulated by various factors and pathways, including transcription factor Runx2. The signaling network in skeletal development and bone formation is overwhelmingly complex and highly time and space specific. Additive, positive, negative, or synergistic effects are observed when TGF-β/BMP interacts with the pathways of MAPK, Wnt, Hedgehog (Hh), Notch, Akt/mTOR, and miRNA to regulate the effects of BMP-induced signaling in bone dynamics. Accumulating evidence indicates that Runx2 is the key integrator, whereas Hh is a possible modulator, miRNAs are regulators, and β-catenin is a mediator/regulator within the extensive intracellular network. This review focuses on the activation of BMP signaling and interaction with other regulatory components and pathways highlighting the molecular mechanisms regarding TGF-β/BMP function and regulation that could allow understanding the complexity of bone tissue dynamics.

MicroRNAs regulate bone development and regeneration

MicroRNAs (miRNAs) are endogenous small noncoding ~22-nt RNAs, which have been reported to play a crucial role in maintaining bone development and metabolism. Osteogenesis originates from mesenchymal stem cells (MSCs) differentiating into mature osteoblasts and each period of bone formation is inseparable from the delicate regulation of various miRNAs. Of note, apprehending the sophisticated circuit between miRNAs and osteogenic homeostasis is of great value for artificial skeletal regeneration for severe bone defects. In this review, we highlight how different miRNAs interact with diverse osteo-related genes and endeavor to sketch the contours of potential manipulations of miRNA-modulated bone repair.

The role of notch signaling in bone marrow niche

OBJECTIVE

Bone marrow (BM) niche is a three-dimensional structure composed of a series of cells and it is one of the most controversial topics in hematological malignancies, leukemia, and even metastasis. Here, we review the relationship between Notch signaling and different fates of stem cells and other BM niche cells.

METHODS

Relevant English-language literature were searched and retrieved from PubMed (2000-2013) using the terms Notch signaling, BM niche, and microRNAs (miRNAs).

DISCUSSION

Notch signaling pathway is a signaling system involved in cellular processes such as proliferation, differentiation, and apoptosis. The notch signaling pathway components are associated with interaction between leukemic, metastatic, and normal cells and their microenvironment. miRNAs play an important role in expression and regulation of signaling molecules. It is necessary to evaluate the relationship between aberrant miRNA expression and notch signaling such as miR-128 and miR-30 in glioma and angiogenesis with notch signaling, respectively.

CONCLUSIONS

Characterizing malignant cells and future studies focus on better understanding the variety of cancers and apoptosis with activated Notch signaling pathway, may remain promising this signaling system as a safe and effective therapeutic target.