File Name: microrna control of bone formation and homeostasis .zip
Hruska; A microRNA expression signature of osteoclastogenesis. Blood ; 13 : —
- MicroRNAs as Next Generation Therapeutics in Osteoporosis
- MicroRNAs and post-transcriptional regulation of skeletal development
- MicroRNA control of bone formation and homeostasis
As an important pathological result of rheumatoid arthritis RA , bone destruction will lead to joint injury and dysfunction. The imbalance of bone metabolism caused by increased osteoclast activities and decreased osteoblast activities is the main cause of bone destruction in RA. Recent studies have shown that miRNAs play indispensable roles in the occurrence and development of bone-related diseases including RA.
MicroRNAs are involved in many cellular and molecular activities and played important roles in many biological and pathological processes, such as tissue formation, cancer development, diabetes, neurodegenerative diseases, and cardiovascular diseases. Recently, it has been reported that microRNAs can modulate the differentiation and activities of osteoblasts and osteoclasts, the key cells that are involved in bone remodeling process. Meanwhile, the results from our and other research groups showed that the expression profiles of microRNAs in the serum and bone tissues are significantly different in postmenopausal women with or without fractures compared to the control. Therefore, it can be postulated that microRNAs might play important roles in bone remodeling and that they are very likely to be involved in the pathological process of postmenopausal osteoporosis.
MicroRNAs as Next Generation Therapeutics in Osteoporosis
After their discovery as a new class of regulatory molecules they have become a focus of scientific interest 1. MiRNAs were first described in by Lee et al. The first miRNAs identified were lin - 4 RNA and let - 7 RNA, both in Caenorhabditis elegans , where they are part of a programme that determines the timing of larval development 2 , 3.
Mature miRNAs are 19 to 24 nucleotide long RNA molecules that are involved in regulatory processes intracellularly, but were detected also in the circulation and other body fluids 4.
Studies showed that miRNAs predominantly regulate gene expression negatively, at least in humans. Still, a discrepancy remains between the theoretically predicted binding sites and the actual binding of miRNAs leading to a biological response.
Therefore, it has been suggested that the majority of predicted binding sites might not be relevant for biological processes 7 , 8. For the nomenclature of miRNAs a prefix indicates the organism, e. Mature miRNAs are listed as well as their precursors. This indicates that miRNAs are important factors in the development 10 and the cell-specific expression of protein profiles The pri-miRNA has a length of a few thousand nucleotides and assembles to one or more hairpin elements.
The first maturation step is catalyzed by the nucleus located protein Drosha that cuts the stem-loop hairpin structure from the pri-miRNA yielding an approximately 70 nucleotide long hairpin structure called pre-miRNA The protein exportin five complexes with the hairpin and together with a Ran-GTP Ras-related nuclear protein-guanosine triphosphate the pre-miRNA is actively released from the nucleus with the release of Ran-GDP and phosphate. Once in the cytosol, the second enzyme of the RNase-II family, Dicer, binds to the pre-miRNA and cuts off the loop of the hairpin, leaving the stem structure and the 19 to 22 nucleotide long miRNA duplex Argonaute AGO -proteins bind to the structure and one of the two strands, the so-called passenger strand, is removed and a single stranded RNA-molecule bound to AGO-proteins remains.
An imperfect base pairing can also lead to a silencing effect. The risk to develop osteoporosis is proportional to age Women after menopause are at highest risk, as the absence of estrogen is one key factor for osteoporosis development Vice versa, also the lack of androgenic hormones causes osteoporosis in men The exact mode of action of these sex hormones on bone cells is still under discussion, but it is known that both bone-forming osteoblasts and bone-resorbing osteoclasts have receptors for both estrogens and androgens.
Interestingly, estrogens protect cortical bone mass, whereas androgens are necessary for the maintenance of trabecular bone mass in male mammals, but not necessary for the anabolic effect on cortical bone The lack of vitamin D is considered as a major factor for the reduction of bone mass although study outcomes are conflicting Besides genetic predisposition there are many other risk factors for the development of osteoporosis such as metabolic diseases like diabetes mellitus type 1 and 2 see review on diabetoporosity in this issue , anorexia nervosa, thyroid and renal dysfunctions or dietary as well as lifestyle habits such as low calcium intake or immobilization The most important outcomes of osteoporosis are low impact fractures, of which the distal forearm is the most common site of a first fracture in Caucasian women Whereas vertebral fractures are frequently underdiagnosed, hip fractures can cause significant complications and mortality A major problem is still the missing awareness and the late diagnosis of osteoporosis, leading into a downward spiral through consecutive fractures, loss of quality of life and even premature death of the patient Early detection and rise of awareness could prevent fragility fractures 25 and a widespread screening of patients at risk could even help to lower the costs caused by the disease However, diagnostic tools such as bone density measurements, bone turnover markers or fracture algorithms like FRAX have not been able to cover all the needs for an individual fracture risk prediction.
Therefore, the availability of easy-to-measure biomarkers for osteoporosis, such as microRNAs miRNAs are considered to be an additional tool for the assessment of primary, as well as of secondary osteoporosis risk. In , two independent research teams identified the presence of miRNAs in the bloodstream 27 , Since then, many different sequences have been found in human and animal derived plasma and serum Since miRNAs have been successfully annotated to specific biological functions and diseases, they soon became a novel class of biomarkers and even potential targets for therapies in the future 29 - Chim et al.
Almost at the same time, tumour-associated miRNAs were detected in patients with diffuse B-cell lymphoma, which suggested for the first time their potential use as biomarkers The origin of circulating miRNAs is difficult to track. On the one hand, circulating miRNAs derive from dead blood cells, especially in the case of miRNAs bound to apoptotic bodies, but they can be also actively secreted by living cells Since circulating miRNAs are stable for a certain time and easily accessible from body fluids, they are ideal candidates for their use as novel biomarkers.
Exosomes are nanosized particles that are actively produced by a large number of cells and participate in a variety of signalling processes 41 , Exosomes have been identified to transport numerous signalling molecules, such as proteins, signalling lipids and miRNAs 36 , Exosomes have been associated to diverse physiological and pathophysiological conditions 42 - Exosomes are produced from all types of bone cells and seem to play important roles in the differentiation process of osteoclasts and osteoblasts 46 - In the recent years exosomes have been identified as important signal mediators in bone metabolism 48 - Proteomic profiling from murine osteoblasts at various differentiation stages revealed a distinct proteomic signature at each time point, suggesting that distinct functions are mediated at each stage of osteoblast differentiation Some mechanistic studies were able to show an interplay of exosomal miRNAs and osteoblast or osteoclast function in bone degradation 52 , Exosomal enriched miRa-5p was produced and released in high concentrations from bone marrow stromal cells BMSC of aged rats.
It has been shown that miRa-5p expression in BMSCs increases with age and is actively transported to osteoclasts, increasing osteoclastogenesis and thereby contributes to age related bone loss. Antagonizing miRa-5p reversed the age-related phenotype and rescued bone loss Similarly, a study by Sun et al.
Inhibition of exosomes biogenesis in vitro diminished this effect. Moreover miR abundance in serum of osteoporotic patients was increased compared to healthy controls and could serve as potential biomarker Xie et al.
They found that the proteomic signatures of these serum derived exosomes favour osteoclast activation but also bone formation in osteopenia. The authors hypothesize that this might be a compensatory effect in osteopenia to counteract loss of bone mass.
However, serum derived exosomes from patients with osteoporosis clearly induce osteoclast activation, inhibit osteoblast bone matrix mineralization and promote bone degradation Bone turnover is not only regulated via paracrine exosome delivery, also exosomes from distant tissue are able to affect bone formation 55 - For instance exosomal miRp released from prostate cancer cells is able to control for osteoblast activity and induce bone metastasis Qin et al.
Since exosomes and exosome associated miRNAs play pivotal roles in different aspects of bone turnover and associated pathologies, they represent promising and versatile tools for biomarker identification.
Furthermore exosomes are found in almost all bodily fluids including blood, saliva, and urine and would allow for non-invasive liquid biopsy collection 58 - Although many studies were able to identify exosomal cargo that is involved in bone formation and that might be able to serve as biomarker for bone loss, only a few studies exist that tried to identify exosomal markers in osteoporosis 52 , It is evident that miRNAs affect bone metabolism by influencing bone formation and resorption by targeting anabolic or catabolic processes.
In the last few years, increasing evidence suggested miRNAs as biomarkers for osteoporosis, since they have been identified to play a crucial role in the interplay between osteoblasts and osteoclasts. Circulating monocytes in vivo , a source of osteoclast precursors 61 , showed an association of miRa with osteoporosis MiRa and miR were also suggested to be plasma biomarkers for osteoporosis Among them miRp was the most highly upregulated one with a more than 5-fold change.
MiRp was identified to discriminate between osteoporosis and osteopenia 64 , a precursor stage of osteoporosis In another cohort of Chinese patients, circulating miRb, miR and miR were upregulated and associated with postmenopausal osteoporosis Another study investigated bone tissue of healthy and osteoporotic patients and found potential predictive miRNAs, namely miR, miRb, and miRp up-regulated and miRNAp, miR and miR down-regulated A German research group showed that miRp, miRp, miRp, and miRb-5p were significantly upregulated in serum, tissue and bone cells of osteoporotic patients, independently of gender but directly correlated with BMD Among the eight miRNAs with the lowest p-values two could be confirmed, miRa and miRp both targeting genes involved in bone metabolism A distinct miRNA-pattern in serum was found in patients with idiopathic osteoporosis.
Eight miRNAs miRp, miRe-5p, miRp, miRp, miRb-3p, miRp, miRa-3p, miRa-3p showed an association to osteoporotic fractures regardless of age and sex of the study participants Osteoporosis is a disease that causes general bone loss and also the jaw and mandibles are affected by the disease. In ovariectomized mice, an established model for postmenopausal osteoporosis, mandibles and femurs were affected by the disease and a panel of miRNAs was differentially regulated in the mandible of the mice.
The researchers suggested miRp and miRa-3p as the most promising biomarker candidates Many studies show differentially expressed miRNAs in osteoporosis, however, this data suggest that not only one differentially regulated miRNA serves as biomarker. It rather may be different patterns of numerous miRNAs that can build an algorithm-based prediction. Furthermore, different research groups find quite different patterns, depending on the cohorts they compare.
The retrospective approach of most studies limits the predictive power of the results. Furthermore, meta-analyses are still missing. In case of specific profiles and stable replication of the results, there are some promising attempts to use miRNA profiles for diagnostic purposes Table 1.
Several assays are available to perform miRNA profiling. The most common ones are summarized here in a short overview 74 , Quantitative polymerase chain reaction qPCR is the most flexible technique for small scale and versatile approaches.
It is broadly used among different laboratories with established protocols and has high sensitivity and specificity. Therefore, qPCR is considered the technique for validation of other methods With accurate primer selection, specificity can be highly increased. Most solutions work by elongating the sequence for amplification.
Disadvantages of the technique are the labour-intensive workflow and the biased approach, as the miRNA has to be annotated for primer design Although often addressed, the problem is not jet solved.
Selection of classical reference genes like Act Actin , ribosomal proteins or GAPDH glyceraldehyde 3-phosphate dehydrogenase are inappropriate in most cases as these mRNAs are way longer then miRNAs and behave differently in the extraction step. The use of short RNA species for normalization can also be problematic, as they might not be transcribed by the same polymerases as miRNA precursors. Sometimes, normalization candidates can be identified for a cohort after miRNA sequencing If no reference genes are available, one solution for the normalization problem could be that no normalization is applied.
In this case, extreme attention has to be paid on equivalent treatment of all the samples in one experiment. Especially in experiments with more than 50 target genes per sample, the global mean method can be applied, where the mean miRNA content in each sample is used for normalization 74 , As such, this technique is an ideal tool for the discovery phase of an experiment.
The system can distinguish between isoforms and closely related miRNAs. Although considered unbiased, the barcoding and pre-sequencing steps could add some bias to the experiment
MicroRNAs and post-transcriptional regulation of skeletal development
Skip to search form Skip to main content You are currently offline. Some features of the site may not work correctly. DOI: Li and X. Zhou and B. Gao Published Biology, Medicine Frontiers in bioscience.
Skeletal development is a process that involves a complex sequence of events, which are regulated by a wide range of signaling pathways Karsenty Yet, it mainly involves only three specific types of cells: chondrocytes in cartilage and osteoblasts and osteoclasts in bone. In recent years, considerable efforts have been devoted to understanding the mechanisms that mediate the transition from mesenchymal stem cells MSCs to osteoblast and chondroblast lineages. It is well known that osteoprogenitor maturation is controlled by several extracellular signals including bone morphogenetic proteins BMPs , hedgehogs, WNTs, and fibroblast growth factors, the actions of which lead to the expression of chondroblast- or osteoblast-specific genes Karsenty Recently, numerous studies have shown that microRNAs miRNAs are important post-transcriptional regulators in virtually all biological processes Hobert The miRNA field has advanced so rapidly that it has become an integral component of the way we think gene expression is regulated in cartilage and bone development.
Skeletal development is a multistep process during which mesenchymal progenitor cells MSCs undergo proliferation and differentiation, giving rise to cartilage and bone cells. Bone is produced by two distinct processes. In the endochondral ossification process, which occurs in long bones and vertebrae, MSC-derived chondrocytes produce a cartilage template, which is subsequently replaced by a mineralized matrix, deposed by the bone-making cells, MSC-derived osteoblasts. The intramembranous ossification process, which occurs in skull bones and clavicle formation, relies instead on the direct differentiation of condensed MSCs into osteoblasts. The bone modeling occurring during development and the life-long process of remodeling are controlled by several factors, including systemic hormones, bone morphogenetic proteins BMPs , fibroblast growth factors FGFs and secreted signaling factors, including Wnt.
Bone is an active tissue that works as a tissue and an organ as well. Bone is a living structure stably undertaking continual remodeling between bone formation and bone resorption, where bone-forming cells osteoblasts and bone-resorbing cells osteoclasts exhibit a crucial role. The differentiation process of osteoblasts and osteoclasts takes place in a balanced manner under normal conditions. This intricate balance is chiefly sustained by biochemical signaling cascades, facilitating accurate bone homeostasis in the body.
MicroRNAs miRNAs repress cellular protein levels to provide a sophisticated parameter of gene regulation that coordinates a broad spectrum of biological processes. Bone organogenesis is a complex process involving the differentiation and crosstalk of multiple cell types for formation and remodeling of the skeleton. Inhibition of mRNA translation by miRNAs has emerged as an important regulator of developmental osteogenic signaling pathways, osteoblast growth and differentiation, osteoclast-mediated bone resorption activity and bone homeostasis in the adult skeleton. This Review brings into focus an emerging concept of bone-regulating miRNAs, the evidence for which has been gathered largely from in vivo mouse models and in vitro studies in human and mouse skeletal cell populations. Characterization of miRNAs that operate through tissue-specific transcription factors in osteoblast and osteoclast lineage cells, as well as intricate feedforward and reverse loops, has provided novel insights into the supervision of signaling pathways and regulatory networks controlling normal bone formation and turnover.
After their discovery as a new class of regulatory molecules they have become a focus of scientific interest 1. MiRNAs were first described in by Lee et al. The first miRNAs identified were lin - 4 RNA and let - 7 RNA, both in Caenorhabditis elegans , where they are part of a programme that determines the timing of larval development 2 , 3. Mature miRNAs are 19 to 24 nucleotide long RNA molecules that are involved in regulatory processes intracellularly, but were detected also in the circulation and other body fluids 4. Studies showed that miRNAs predominantly regulate gene expression negatively, at least in humans.
MicroRNA control of bone formation and homeostasis
Osteoporosis is caused by an unbalance between bone formation and bone resorption. Bone homeostasis is regulated by intricate mechanisms. Recently, a novel class of regulatory factors termed microRNAs miRNAs has been found to play a crucial role in cell cycle control, apoptosis and other cellular processes including metabolism and differentiation. Published data have shown that some miRNAs regulate bone homeostasis, including bone formation, resorption, remodeling, repair and bone-related disease, by regulating the expression of certain cytokines and transcription factors. This review highlights the current knowledge of miRNAs and their involvement in the regulation of bone formation, bone resorption and the pathways regulating the progression of osteoporosis. This is a preview of subscription content, access via your institution.
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The nomenclature of miRNAs
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