Simulated microgravity inhibits L-type calcium channel currents partially by the up-regulation of miR-103 in MC3T3-E1 osteoblasts

Cellular and Molecular Effects of Microgravity on the Immune System: A Focus on Bioactive Lipids

Microgravity is one of the main stressors that astronauts are exposed to during space missions. This condition has been linked to many disorders, including those that feature dysfunctional immune homeostasis and inflammatory damage. Over the past 30 years, a significant body of work has been gathered connecting weightlessness-either authentic or simulated-to an inefficient reaction to pathogens, dysfunctional production of cytokines and impaired survival of immune cells. These processes are also orchestrated by a plethora of bioactive lipids, produced by virtually all cells involved in immune events, which control the induction, magnitude, outcome, compartmentalization and trafficking of immunocytes during the response to injury. Despite their crucial importance in inflammation and its modulation, however, data concerning the role of bioactive lipids in microgravity-induced immune dysfunctions are surprisingly scarce, both in quantity and in variety, and the vast majority of it focuses on two lipid classes, namely eicosanoids and endocannabinoids. The present review aims to outline the accumulated knowledge addressing the effects elicited by microgravity-both simulated and authentic-on the metabolism and signaling of these two prominent lipid groups in the context of immune and inflammatory homeostasis.

Simulated weightlessness induces hippocampal insulin resistance and cognitive impairment

Growing evidence highlights the potential consequences of long-term spaceflight, including gray matter volume reduction and cognitive dysfunction with subclinical manifestations of diabetes mellitus among astronauts, but the underlying mechanisms remain unknown. In this study, we found that long-term simulated weightlessness induced hippocampal insulin resistance and subsequent neuronal damage and cognitive impairment in rats. Rats subjected to 4-week tail suspension exhibited peripheral insulin resistance, evidenced by increased fasting blood glucose and abnormal glucose tolerance and insulin tolerance, alongside reduced spontaneous activity and impaired recognition memory. In addition, 4 weeks of simulated weightlessness induced neuronal apoptosis and degeneration in the hippocampus, as evidenced by increased TUNEL and Fluoro-Jade B staining-positive neurons. Mechanistically, insulin-stimulated hippocampal Akt phosphorylation was decreased, while PTEN, the negative regulator of insulin signaling, was increased in the hippocampus in tail-suspended rats. Interestingly, treatment with berberine, an insulin sensitizer, partly reversed the above-mentioned effects induced by simulated weightlessness. These data suggest that long-term simulated weightlessness induces cognitive impairment as well as neuronal apoptosis and neural degeneration, partially through hippocampal insulin resistance via PTEN up-regulation. Berberine treatment attenuates hippocampal insulin resistance and improves cognitive function.

Phylobone: a comprehensive database of bone extracellular matrix proteins in human and model organisms

The bone extracellular matrix (ECM) contains minerals deposited on highly crosslinked collagen fibrils and hundreds of non-collagenous proteins. Some of these proteins are key to the regulation of bone formation and regeneration via signaling pathways, and play important regulatory and structural roles. However, the complete list of bone extracellular matrix proteins, their roles, and the extent of individual and cross-species variations have not been fully captured in both humans and model organisms. Here, we introduce the most comprehensive resource of bone extracellular matrix (ECM) proteins that can be used in research fields such as bone regeneration, osteoporosis, and mechanobiology. The Phylobone database (available at https://phylobone.com ) includes 255 proteins potentially expressed in the bone extracellular matrix (ECM) of humans and 30 species of vertebrates. A bioinformatics pipeline was used to identify the evolutionary relationships of bone ECM proteins. The analysis facilitated the identification of potential model organisms to study the molecular mechanisms of bone regeneration. A network analysis showed high connectivity of bone ECM proteins. A total of 214 functional protein domains were identified, including collagen and the domains involved in bone formation and resorption. Information from public drug repositories was used to identify potential repurposing of existing drugs. The Phylobone database provides a platform to study bone regeneration and osteoporosis in light of (biological) evolution, and will substantially contribute to the identification of molecular mechanisms and drug targets.

Regulation by noncoding RNAs of local translation, injury responses, and pain in the peripheral nervous system

Neuropathic pain is a chronic condition arising from damage to somatosensory pathways that results in pathological hypersensitivity. Persistent pain can be viewed as a consequence of maladaptive plasticity which, like most enduring forms of cellular plasticity, requires altered expression of specific gene programs. Control of gene expression at the level of protein synthesis is broadly utilized to directly modulate changes in activity and responsiveness in nociceptive pathways and provides an effective mechanism for compartmentalized regulation of the proteome in peripheral nerves through local translation. Levels of noncoding RNAs (ncRNAs) are commonly impacted by peripheral nerve injury leading to persistent pain. NcRNAs exert spatiotemporal regulation of local proteomes and affect signaling cascades supporting altered sensory responses that contribute to hyperalgesia. This review discusses ncRNAs found in the peripheral nervous system (PNS) that are dysregulated following nerve injury and the current understanding of their roles in pathophysiological pain-related responses including neuroimmune interactions, neuronal survival and axon regeneration, Schwann cell dedifferentiation and proliferation, intercellular communication, and the generation of ectopic action potentials in primary afferents. We review progress in the field beyond cataloging, with a focus on the relevant target transcripts and mechanisms underlying pain modulation by ncRNAs.

Pacemaker activity and ion channels in the sinoatrial node cells: MicroRNAs and arrhythmia

The primary pacemaking activity of the heart is determined by a spontaneous action potential (AP) within sinoatrial node (SAN) cells. This unique AP generation relies on two mechanisms: membrane clocks and calcium clocks. Nonhomologous arrhythmias are caused by several functional and structural changes in the myocardium. MicroRNAs (miRNAs) are essential regulators of gene expression in cardiomyocytes. These miRNAs play a vital role in regulating the stability of cardiac conduction and in the remodeling process that leads to arrhythmias. Although it remains unclear how miRNAs regulate the expression and function of ion channels in the heart, these regulatory mechanisms may support the development of emerging therapies. This study discusses the spread and generation of AP in the SAN as well as the regulation of miRNAs and individual ion channels. Arrhythmogenicity studies on ion channels will provide a research basis for miRNA modulation as a new therapeutic target.

Protection of primary cilia is an effective countermeasure against the impairment of osteoblast function induced by simulated microgravity

The molecular mechanism for the microgravity-induced decrease in bone formation remains unclear and there is a lack of effective specific preventative therapies. We recently reported that primary cilia of osteoblasts became shorter and even disappeared when the cells were exposed to random positioning machine (RPM)-simulated microgravity and that the microgravity-induced loss of osteogenic potential of osteoblasts could be attenuated when the resorption of primary cilia was prevented by treatment with 0.1 μM cytochalasin D. In the current study, it was further found that the loss of the osteogenic capacity of rat calvarial osteoblasts (ROBs) was associated with the inhibition of the BMP-2/Smad1/5/8 signalling pathway, of which most of the signalling proteins including BMP-2, BMPRII, Smad1/5/8 and p-Smad1/5/8 were found localized to primary cilia. Accompanying the resorption of primary cilia following the cells being exposed to simulated microgravity, the expression levels of these signalling proteins were reduced significantly. Furthermore, the expression of miRNA-129-3p, a microRNA previously reported to control cilium biogenesis, was found to be reduced quickly and changed in a similar tendency with the length of primary cilia. Moreover, overexpression of miRNA-129-3p in ROBs significantly attenuated microgravity-induced inhibition of BMP-2 signalling and loss of osteogenic differentiation and mineralization. These results indicated the important role of miRNA-129-3p in microgravity-induced resorption of primary cilia of osteoblasts and the potential of replenishing the miRNA-129-3p as an effective countermeasure against microgravity-induced loss of primary cilia and impairment of osteoblast function.

Changes in interstitial fluid flow, mass transport and the bone cell response in microgravity and normogravity

In recent years, our scientific interest in spaceflight has grown exponentially and resulted in a thriving area of research, with hundreds of astronauts spending months of their time in space. A recent shift toward pursuing territories farther afield, aiming at near-Earth asteroids, the Moon, and Mars combined with the anticipated availability of commercial flights to space in the near future, warrants continued understanding of the human physiological processes and response mechanisms when in this extreme environment. Acute skeletal loss, more severe than any bone loss seen on Earth, has significant implications for deep space exploration, and it remains elusive as to why there is such a magnitude of difference between bone loss on Earth and loss in microgravity. The removal of gravity eliminates a critical primary mechano-stimulus, and when combined with exposure to both galactic and solar cosmic radiation, healthy human tissue function can be negatively affected. An additional effect found in microgravity, and one with limited insight, involves changes in dynamic fluid flow. Fluids provide the most fundamental way to transport chemical and biochemical elements within our bodies and apply an essential mechano-stimulus to cells. Furthermore, the cell cytoplasm is not a simple liquid, and fluid transport phenomena together with viscoelastic deformation of the cytoskeleton play key roles in cell function. In microgravity, flow behavior changes drastically, and the impact on cells within the porous system of bone and the influence of an expanding level of adiposity are not well understood. This review explores the role of interstitial fluid motion and solute transport in porous bone under two different conditions: normogravity and microgravity.

Fluid Shear Stress Promotes Osteoblast Proliferation and Suppresses Mitochondrial-Mediated Osteoblast Apoptosis Through the miR-214-3p-ATF4 Signaling Axis

MicroRNAs (miRNAs) play vital roles in bone metabolism and participate in the mechanically induced bone alterations. The underlying molecular mechanisms by which fluid shear stress (FSS) regulate the proliferative and apoptotic phenotypic changes of osteoblasts remain elusive. The study aimed to investigate the regulatory effects of FSS on osteoblast proliferative and apoptotic phenotypes and the roles of miR-214-3p-ATF4 (activating transcription factor 4) signaling axis in the mechanomodulation processes. FSS promoted the proliferative activity of osteoblasts and suppressed mitochondrial-mediated osteoblast apoptosis. FSS decreased miR-214-3p expression and increased ATF4 expression in MC3T3-E1 osteoblasts. MiR-214-3p inhibited osteoblast proliferative activity and promoted mitochondrial-mediated osteoblast apoptosis. Overexpression of miR-214-3p attenuated FSS-enhanced osteoblast proliferation and FSS-suppressed mitochondrial-mediated osteoblast apoptosis. We validated that ATF4 acted as a target gene of miR-214-3p. Moreover, miR-214 3p regulated osteoblast proliferation and apoptosis through targeting ATF4. Taken together, our study proved that FSS could suppress mitochondrial-mediated osteoblast apoptosis and promote osteoblast proliferation through the miR-214-3p-ATF4 signaling axis.

The mechanosensory and mechanotransductive processes mediated by ion channels and the impact on bone metabolism: A systematic review

Mechanical environments were associated with alterations in bone metabolism. Ion channels present on bone cells are indispensable for bone metabolism and can be directly or indirectly activated by mechanical stimulation. This review aimed to discuss the literature reporting the mechanical regulatory effects of ion channels on bone cells and bone tissue. An electronic search was conducted in PubMed, Embase and Web of Science. Studies about mechanically induced alteration of bone cells and bone tissue by ion channels were included. Ion channels including TRP family channels, Ca²⁺ release-activated Ca²⁺ channels (CRACs), Piezo1/2 channels, purinergic receptors, NMDA receptors, voltage-sensitive calcium channels (VSCCs), TREK2 potassium channels, calcium- and voltage-dependent big conductance potassium (BK_Ca) channels, small conductance, calcium-activated potassium (SK_Ca) channels and epithelial sodium channels (ENaCs) present on bone cells and bone tissue participate in the mechanical regulation of bone development in addition to contributing to direct or indirect mechanotransduction such as altered membrane potential and ionic flux. Physiological (beneficial) mechanical stimulation could induce the anabolism of bone cells and bone tissue through ion channels, but abnormal (harmful) mechanical stimulation could also induce the catabolism of bone cells and bone tissue through ion channels. Functional expression of ion channels is vital for the mechanotransduction of bone cells. Mechanical activation (opening) of ion channels triggers ion influx and induces the activation of intracellular modulators that can influence bone metabolism. Therefore, mechanosensitive ion channels provide new insights into therapeutic targets for the treatment of bone-related diseases such as osteopenia and aseptic implant loosening.

Update on the effects of microgravity on the musculoskeletal system

With the reignited push for manned spaceflight and the development of companies focused on commercializing spaceflight, increased human ventures into space are inevitable. However, this venture would not be without risk. The lower gravitational force, known as microgravity, that would be experienced during spaceflight significantly disrupts many physiological systems. One of the most notably affected systems is the musculoskeletal system, where exposure to microgravity causes both bone and skeletal muscle loss, both of which have significant clinical implications. In this review, we focus on recent advancements in our understanding of how exposure to microgravity affects the musculoskeletal system. We will focus on the catabolic effects microgravity exposure has on both bone and skeletal muscle cells, as well as their respective progenitor stem cells. Additionally, we report on the mechanisms that underlie bone and muscle tissue loss resulting from exposure to microgravity and then discuss current countermeasures being evaluated. We reveal the gaps in the current knowledge and expound upon how current research is filling these gaps while also identifying new avenues of study as we continue to pursue manned spaceflight.

MiR-103-3p targets the m6 A methyltransferase METTL14 to inhibit osteoblastic bone formation

Impaired osteoblast function is involved in osteoporosis, and microRNA (miRNA) dysregulation may cause abnormal osteoblast osteogenic activity. However, the influence of miRNA on osteoblast activity and the underlying mechanisms remain elusive. In this study, miR‐103‐3p was found to be negatively correlated with bone formation in bone specimens from elderly women with fractures and ovariectomized (OVX) mice. Additionally, miR‐103‐3p directly targeted Mettl14 to inhibit osteoblast activity, and METTL14‐dependent N⁶‐methyladenosine (m⁶A) methylation inhibited miR‐103‐3p processing by the microprocessor protein DGCR8 and promoted osteoblast activity. Moreover, miR‐103‐3p inhibited bone formation in vivo, and therapeutic inhibition of miR‐103‐3p counteracted the decreased bone formation in OVX mice. Further, METTL14 was negatively correlated with miR‐103‐3p but positively correlated with bone formation in bone specimens from elderly women with fractures and OVX mice. Collectively, our results highlight the critical roles of the miR‐103‐3p/METTL14/m⁶A signaling axis in osteoblast activity, identifying this axis as a potential target for ameliorating osteoporosis.

Effect of miR-27b-5p on apoptosis of human vascular endothelial cells induced by simulated microgravity

Weightlessness-induced cardiovascular dysfunction can lead to physiological and pathological consequences. It has been shown that spaceflight or simulated microgravity can alter expression profiles of some microRNAs (miRNAs). Here, we attempt to identify the role of miRNAs in human umbilical vein endothelial cells (HUVECs) apoptosis under simulated microgravity. RNA-sequencing and quantitative real-time PCR (qRT-PCR) assays were used to identify differentially expressed miRNAs in HUVECs under simulated microgravity. Then we obtained the target genes of these miRNAs through target analysis software. Moreover, GO and KEGG enrichment analysis were performed. The effects of these miRNAs on HUVECs apoptosis were evaluated by flow cytometry, Western blot and Hoechst staining. Furthermore, we obtained the target gene of miR-27b-5p by luciferase assay, qRT-PCR and Western blot. Finally, we investigated the relationship between this target gene and miR-27b-5p in HUVECs apoptosis under normal gravity or simulated microgravity. We found 29 differentially expressed miRNAs in HUVECs under simulated microgravity. Of them, the expressions of 3 miRNAs were validated by qRT-PCR. We demonstrated that miR-27b-5p affected HUVECs apoptosis by inhibiting zinc fingers and homeoboxes 1 (ZHX1). Our results reported here demonstrate for the first time that simulated microgravity can alter the expression of some miRNAs in HUVECs and miR-27b-5p may protect HUVECs from apoptosis under simulated microgravity by targeting ZHX1.

Interactions between Muscle and Bone-Where Physics Meets Biology

Muscle and bone interact via physical forces and secreted osteokines and myokines. Physical forces are generated through gravity, locomotion, exercise, and external devices. Cells sense mechanical strain via adhesion molecules and translate it into biochemical responses, modulating the basic mechanisms of cellular biology such as lineage commitment, tissue formation, and maturation. This may result in the initiation of bone formation, muscle hypertrophy, and the enhanced production of extracellular matrix constituents, adhesion molecules, and cytoskeletal elements. Bone and muscle mass, resistance to strain, and the stiffness of matrix, cells, and tissues are enhanced, influencing fracture resistance and muscle power. This propagates a dynamic and continuous reciprocity of physicochemical interaction. Secreted growth and differentiation factors are important effectors of mutual interaction. The acute effects of exercise induce the secretion of exosomes with cargo molecules that are capable of mediating the endocrine effects between muscle, bone, and the organism. Long-term changes induce adaptations of the respective tissue secretome that maintain adequate homeostatic conditions. Lessons from unloading, microgravity, and disuse teach us that gratuitous tissue is removed or reorganized while immobility and inflammation trigger muscle and bone marrow fatty infiltration and propagate degenerative diseases such as sarcopenia and osteoporosis. Ongoing research will certainly find new therapeutic targets for prevention and treatment.

Spaceflight-Induced Bone Tissue Changes that Affect Bone Quality and Increase Fracture Risk

PURPOSE OF REVIEW

Bone mineral density and systemic factors are used to assess skeletal health in astronauts. Yet, even in a general population, these measures fail to accurately predict when any individual will fracture. This review considers how long-duration human spaceflight requires evaluation of additional bone structural and material quality measures that contribute to microgravity-induced skeletal fragility.

RECENT FINDINGS

In both humans and small animal models following spaceflight, bone mass is compromised via reduced bone formation and elevated resorption levels. Concurrently, bone structural quality (e.g., trabecular microarchitecture) is diminished and the quality of bone material is reduced via impaired tissue mineralization, maturation, and maintenance (e.g., mediated by osteocytes). Bone structural and material quality are both affected by microgravity and may, together, jeopardize astronaut operational readiness and lead to increased fracture risk upon return to gravitational loading. Future studies need to directly evaluate how bone quality combines with diminished bone mass to influence bone strength and toughness (e.g., resistance to fracture). Bone quality assessment promises to identify novel biomarkers and therapeutic targets.

Non-coding RNAs and Cardiac Arrhythmias

Cardiac arrhythmias represent wide and heterogenic group of disturbances in the cardiac rhythm. Pathophysiology of individual arrhythmias is highly complex and dysfunction in ion channels/currents involved in generation or spreading of action potential is usually documented. Non-coding RNAs (ncRNAs) represent highly variable group of molecules regulating the heart expression program, including regulation of the expression of individual ion channels and intercellular connection proteins, e.g. connexins.Within this chapter, we will describe basic electrophysiological properties of the myocardium. We will focus on action potential generation and spreading in pacemaker and non-pacemaker cells, including description of individual ion channels (natrium, potassium and calcium) and their ncRNA-mediated regulation. Most of the studies have so far focused on microRNAs, thus, their regulatory function will be described into greater detail. Clinical consequences of altered ncRNA regulatory function will also be described together with potential future directions of the research in the field.

MiR-291a-3p regulates the BMSCs differentiation via targeting DKK1 in dexamethasone-induced osteoporosis

Osteoporosis is a skeleton disease affecting 55% of people over age 60, and the number is still increasing due to an ageing population. One method to prevent osteoporosis is to increase the formation of new bone while preventing the resorption of older bone. Thus, osteogenic differentiation of bone marrow-derived mesenchymal stem cells (BMSCs) is of great importance in improving the treatment of osteoporosis. On the other hand, glucocorticoids (GCs) are widely used to treat the chronic inflammatory disorders, but long-term exposure to GCs can induce osteoporosis. In present study, we treated BMSCs with dexamethasone (DEX) to simulate GC-induced osteoporosis. MTT assay, ALP activity, and Alizarin Red were used to evaluate the role miRNA-291a-3p in the DEX-induced osteogenic differentiation suppression. Further, we used qPCR and western blot to investigate the mechanisms of miRNA-291a-3p affecting BMSCs differentiation. The results showed that miRNA-291a-3p could improve the cell viability, osteogenic differentiation, and ALP activity, which are suppressed by DEX in BMSCs. Furthermore, we found that the osteogenesis genes Runx2, DMP1, and ALP were upregulated whereas the lipogenic genes C/EBPα and PPARγ were downregulated when miRNA-291a-3p mimics were transfected. Additionally, we demonstrated that miRNA-291a-3p promoted BMSCs' osteogenic differentiation by directly suppressing DKK1 mRNA and protein expression and subsequently activating Wnt/β-catenin signaling pathway. Our study suggests that miR-291a-3p plays an important role in preventing osteoporosis and may serve as a potential miRNA osteoporosis biomarker.

Gene Expression in Osteoblasts and Osteoclasts Under Microgravity Conditions: A Systematic Review

Background

Microgravity (μG) negatively influences bone metabolism by affecting normal osteoblast and osteoclast function. μG effects on bone metabolism has been an extensive field of study in recent years, due to the challenges presented by space flight.

Methods

We systematically reviewed research data from genomic studies performed in real or simulat-ed μG, on osteoblast and osteoclast cells. Our search yielded 50 studies, of which 39 concerned cells of the osteoblast family and 11 osteoclast precursors.

Results

Osteoblastic cells under μG show a decreased differentiation phenotype, proved by diminished expression levels of Alkaline Phosphatase (ALP) and Osteocalcin (OCN) but no apoptosis. Receptor Activator of NF-κB Ligand (RANKL)/ Osteoprotegerine (OPG) ratio is elevated in favor of RANKL in a time-dependent manner, and further RANKL production is caused by upregulation of Interleukin-6 (IL-6) and the inflammation pathway. Extracellular signals and changes in the gravitational environment are perceived by mechanosensitive proteins of the cytoskeleton and converted to intracellular signals through the Mitogen Activated Protein Kinase pathway (MAPK). This is followed by changes in the ex-pression of nuclear transcription factors of the Activator Protein-1 (AP-1) family and in turn of the NF-κB, thus affecting osteoblast differentiation, cell cycle, proliferation and maturation. Pre-osteoclastic cells show increased expression of the marker proteins such as Tryptophan Regulated Attenuation Protein (TRAP), cathepsin K, Matrix Metalloproteinase-9 (MMP-9) under μG conditions and become sensitized to RANKL.

Conclusion

Suppressing the expression of fusion genes such as syncytine-A which acts independently of RANKL, could be possible future therapeutic targets for microgravity side effects.

Regulator of G Protein Signaling Protein 12 (Rgs12) Controls Mouse Osteoblast Differentiation via Calcium Channel/Oscillation and Gαi-ERK Signaling

Bone homeostasis intimately relies on the balance between osteoblasts (OBs) and osteoclasts (OCs). Our previous studies have revealed that regulator of G protein signaling protein 12 (Rgs12), the largest protein in the Rgs super family, is essential for osteoclastogenesis from hematopoietic cells and OC precursors. However, how Rgs12 regulates OB differentiation and function is still unknown. To understand that, we generated an OB-targeted Rgs12 conditional knockout (CKO) mice model by crossing Rgs12fl/fl mice with Osterix (Osx)-Cre transgenic mice. We found that Rgs12 was highly expressed in both OB precursor cells (OPCs) and OBs of wild-type (WT) mice, and gradually increased during OB differentiation, whereas Rgs12-CKO mice (OsxCre/+ ; Rgs12fl/fl ) exhibited a dramatic decrease in both trabecular and cortical bone mass, with reduced numbers of OBs and increased apoptotic cell population. Loss of Rgs12 in OPCs in vitro significantly inhibited OB differentiation and the expression of OB marker genes, resulting in suppression of OB maturation and mineralization. Further mechanism study showed that deletion of Rgs12 in OPCs significantly inhibited guanosine triphosphatase (GTPase) activity and cyclic adenosine monophosphate (cAMP) level, and impaired Calcium (Ca2+ ) oscillations via restraints of major Ca2+ entry sources (extracellular Ca2+ influx and intracellular Ca2+ release from endoplasmic reticulum), partially contributed by the blockage of L-type Ca2+ channel mediated Ca2+ influx. Downstream mediator extracellular signal-related protein kinase (ERK) was found inactive in OBs of OsxCre/+ ; Rgs12fl/fl mice and in OPCs after Rgs12 deletion, whereas application of pertussis toxin (PTX) or overexpression of Rgs12 could rescue the defective OB differentiation via restoration of ERK phosphorylation. Our findings reveal that Rgs12 is an important regulator during osteogenesis and highlight Rgs12 as a potential therapeutic target for bone disorders. © 2018 American Society for Bone and Mineral Research.

miR-181c-5p mediates simulated microgravity-induced impaired osteoblast proliferation by promoting cell cycle arrested in the G2 phase

Impaired osteoblast proliferation plays fundamental roles in microgravity‐induced bone loss, and cell cycle imbalance may result in abnormal osteoblast proliferation. However, whether microgravity exerts an influence on the cell cycle in osteoblasts or what mechanisms may underlie such an effect remains to be fully elucidated. Herein, we confirmed that simulated microgravity inhibits osteoblast proliferation. Then, we investigated the effect of mechanical unloading on the osteoblast cell cycle and found that simulated microgravity arrested the osteoblast cell cycle in the G₂ phase. In addition, our data showed that cell cycle arrest in osteoblasts from simulated microgravity was mainly because of decreased cyclin B1 expression. Furthermore, miR‐181c‐5p directly inhibited cyclin B1 protein translation by binding to a target site in the 3′UTR. Lastly, we demonstrated that inhibition of miR‐181c‐5p partially counteracted cell cycle arrest and decreased the osteoblast proliferation induced by simulated microgravity. In conclusion, our study demonstrates that simulated microgravity inhibits cell proliferation and induces cell cycle arrest in the G₂ phase in primary mouse osteoblasts partially through the miR‐181c‐5p/cyclin B1 pathway. This work may provide a novel mechanism of microgravity‐induced detrimental effects on osteoblasts and offer a new avenue to further investigate bone loss induced by mechanical unloading.

Simulated microgravity reduces intracellular-free calcium concentration by inhibiting calcium channels in primary mouse osteoblasts

Calcium homeostasis in osteoblasts plays fundamental roles in the physiology and pathology of bone tissue. Various types of mechanical stimuli promote osteogenesis and increase bone formation elicit increases in intracellular-free calcium concentration in osteoblasts. However, whether microgravity, a condition of mechanical unloading, exerts an influence on intracellular-free calcium concentration in osteoblasts or what mechanisms may underlie such an effect are unclear. Herein, we show that simulated microgravity reduces intracellular-free calcium concentration in primary mouse osteoblasts. In addition, simulated microgravity substantially suppresses the activities of L-type voltage-sensitive calcium channels, which selectively allow calcium to cross the plasma membrane from the extracellular space. Moreover, the functional expression of ryanodine receptors and inositol 1,4,5-trisphosphate receptors, which mediate the release of calcium from intracellular storage, decreased under simulated microgravity conditions. These results suggest that simulated microgravity substantially reduces intracellular-free calcium concentration through inhibition of calcium channels in primary mouse osteoblasts. Our study may provide a novel mechanism for microgravity-induced detrimental effects in osteoblasts, offering a new avenue to further investigate bone loss induced by mechanical unloading.

Bone remodeling induced by mechanical forces is regulated by miRNAs

The relationship between mechanical force and alveolar bone remodeling is an important issue in orthodontics because tooth movement is dependent on the response of bone tissue to the mechanical force induced by the appliances used. Mechanical cyclical stretch (MCS), fluid shear stress (FSS), compression, and microgravity play different roles in the cell differentiation and proliferation involved in bone remodeling. However, the underlying mechanisms are unclear, particularly the molecular pathways regulated by non-coding RNAs (ncRNAs) that play essential roles in bone remodeling. Amongst the various ncRNAs, miRNAs act as post-transcriptional regulators that inhibit the expression of their target genes. miRNAs are considered key regulators of many biologic processes including bone remodeling. Here, we review the role of miRNAs in mechanical force-induced bone metabolism.

Osteoblast-targeted delivery of miR-33-5p attenuates osteopenia development induced by mechanical unloading in mice

A growing body of evidence has revealed that microRNAs (miRNAs) play crucial roles in regulating osteoblasts and bone metabolism. However, the effects of miRNAs in osteoblast mechanotransduction remain to be defined. In this study, we investigated the regulatory effect of miR-33-5p in osteoblasts and tested its anti-osteopenia effect when delivered by an osteoblast-targeting delivery system in vivo. First, we demonstrated that miR-33-5p could promote the activity and mineralization of osteoblasts without influencing their proliferation in vitro. Then our data showed that supplementing miR-33-5p in osteoblasts by a targeted delivery system partially recovered the osteopenia induced by mechanical unloading at the biochemical, microstructural, and biomechanical levels. In summary, our findings demonstrate that miR-33-5p is a key factor in the occurrence and development of the osteopenia induced by mechanical unloading. In addition, targeted delivery of the mimics of miR-33-5p is a promising new strategy for the treatment of pathological osteopenia.

IGF-1 signaling mediated cell-specific skeletal mechano-transduction

Mechanical loading preserves bone mass and stimulates bone formation, whereas skeletal unloading leads to bone loss. In addition to osteocytes, which are considered the primary sensor of mechanical load, osteoblasts, and bone specific mesenchymal stem cells also are involved. The skeletal response to mechanical signals is a complex process regulated by multiple signaling pathways including that of insulin-like growth factor-1 (IGF-1). Conditional osteocyte deletion of IGF-1 ablates the osteogenic response to mechanical loading. Similarly, osteocyte IGF-1 receptor (IGF-1R) expression is necessary for reloading-induced periosteal bone formation. Transgenic overexpression of IGF-1 in osteoblasts results in enhanced responsiveness to in vivo mechanical loading in mice, a response which is eliminated by osteoblastic conditional disruption of IGF-1 in vivo. Bone marrow derived stem cells (BMSC) from unloaded bone fail to respond to IGF-1 in vitro. IGF-1R is required for the transduction of a mechanical stimulus to downstream effectors, transduction which is lost when the IGF-1R is deleted. Although the molecular mechanisms are not yet fully elucidated, the IGF signaling pathway and its interactions with potentially interlinked signaling cascades involving integrins, the estrogen receptor, and wnt/β-catenin play an important role in regulating adaptive response of cancer bone cells to mechanical stimuli. In this review, we discuss recent advances investigating how IGF-1 and other interlinked molecules and signaling pathways regulate skeletal mechano-transduction involving different bone cells, providing an overview of the IGF-1 signaling mediated cell-specific response to mechanical stimuli. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:576-583, 2018.

miR-208a-3p Suppresses Osteoblast Differentiation and Inhibits Bone Formation by Targeting ACVR1

Emerging evidence indicates that many microRNAs (miRNAs) are indispensable regulators of osteoblast differentiation and bone formation. However, the role of miRNAs in mechanotransduction of osteoblasts remains to be elucidated. This study aimed to identify a mechanosensitive miRNA that regulates Activin A receptor type I (ACVR1)-induced osteogenic differentiation. After 4 weeks of hindlimb unloading (HLU) suspension of 6-month-old male C57BL/6J mice, femurs and tibias were harvested to extract total bone RNAs. Elevated levels of miR-208a-3p correlated with a lower degree of bone formation in whole-bone samples of HLU mice. However, in vitro overexpression of miR-208a-3p inhibited osteoblast differentiation, whereas silencing of miR-208a-3p by antagomiR-208a-3p promoted expression of osteoblast activity, bone formation marker genes, and matrix mineralization under mechanical unloading condition. Bioinformatics analysis and a luciferase assay revealed that ACVR1 is a target gene of miR-208a-3p that negatively regulates osteoblast differentiation under mechanical unloading environment. Further, this study also demonstrates that in vivo pre-treatment with antagomiR-208a-3p led to an increase in bone formation and trabecular microarchitecture and partly rescued the bone loss caused by mechanical unloading. Collectively, these results suggest that in vivo, inhibition of miRNA-208a-3p by antagomiR-208a-3p may be a potential therapeutic strategy for ameliorating bone loss.

Mechanosensitive miRNAs and Bone Formation

Mechanical stimuli are required for the maintenance of skeletal integrity and bone mass. An increasing amount of evidence indicates that multiple regulators (e.g., hormone, cytoskeleton proteins and signaling pathways) are involved in the mechanical stimuli modulating the activities of osteogenic cells and the process of bone formation. Significantly, recent studies have showed that several microRNAs (miRNAs) were sensitive to various mechanical stimuli and played a crucial role in osteogenic differentiation and bone formation. However, the functional roles and further mechanisms of mechanosensitive miRNAs in bone formation are not yet completely understood. This review highlights the roles of mechanosensitive miRNAs in osteogenic differentiation and bone formation and underlines their potential therapeutic application for bone loss induced by the altering of mechanical stimuli.

Heterogeneous nuclear ribonucleoprotein A1 regulates rhythmic synthesis of mouse Nfil3 protein via IRES-mediated translation

Nuclear factor, interleukin 3, regulated (Nfil3, also known as E4 Promoter-Binding Protein 4 (E4BP4)) protein is a transcription factor that binds to DNA and generally represses target gene expression. In the circadian clock system, Nfil3 binds to a D-box element residing in the promoter of clock genes and contributes to their robust oscillation. Here, we show that the 5'-untranslated region (5'-UTR) of Nfil3 mRNA contains an internal ribosome entry site (IRES) and that IRES-mediated translation occurs in a phase-dependent manner. We demonstrate that heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1) binds to a specific region of Nfil3 mRNA and regulates IRES-mediated translation. Knockdown of hnRNP A1 almost completely abolishes protein oscillation without affecting mRNA oscillation. Moreover, we observe that intracellular calcium levels, which are closely related to bone formation, depend on Nfil3 levels in osteoblast cell lines. We suggest that the 5'-UTR mediated cap-independent translation of Nfil3 mRNA contributes to the rhythmic expression of Nfil3 by interacting with the RNA binding protein hnRNP A1. These data provide new evidence that the posttranscriptional regulation of clock gene expression is important during bone metabolism.

The impact of microgravity on bone in humans

Experiencing real weightlessness in space is a dream for many of us who are interested in space research. Although space traveling fascinates us, it can cause both short-term and long-term health problems. Microgravity is the most important influence on the human organism in space. The human body undergoes dramatic changes during a long-term spaceflight. In this review, we will mainly focus on changes in calcium, sodium and bone metabolism of space travelers. Moreover, we report on the current knowledge on the mechanisms of bone loss in space, available models to simulate the effects of microgravity on bone on Earth as well as the combined effects of microgravity and cosmic radiation on bone. The available countermeasures applied in space will also be evaluated.

A systems biology pipeline identifies new immune and disease related molecular signatures and networks in human cells during microgravity exposure

Microgravity is a prominent health hazard for astronauts, yet we understand little about its effect at the molecular systems level. In this study, we have integrated a set of systems-biology tools and databases and have analysed more than 8000 molecular pathways on published global gene expression datasets of human cells in microgravity. Hundreds of new pathways have been identified with statistical confidence for each dataset and despite the difference in cell types and experiments, around 100 of the new pathways are appeared common across the datasets. They are related to reduced inflammation, autoimmunity, diabetes and asthma. We have identified downregulation of NfκB pathway via Notch1 signalling as new pathway for reduced immunity in microgravity. Induction of few cancer types including liver cancer and leukaemia and increased drug response to cancer in microgravity are also found. Increase in olfactory signal transduction is also identified. Genes, based on their expression pattern, are clustered and mathematically stable clusters are identified. The network mapping of genes within a cluster indicates the plausible functional connections in microgravity. This pipeline gives a new systems level picture of human cells under microgravity, generates testable hypothesis and may help estimating risk and developing medicine for space missions.

miR-33-5p, a novel mechano-sensitive microRNA promotes osteoblast differentiation by targeting Hmga2

MicroRNAs (miRNAs) interfere with the translation of specific target mRNAs and are thought to thereby regulate many cellular processes. However, the role of miRNAs in osteoblast mechanotransduction remains to be defined. In this study, we investigated the ability of a miRNA to respond to different mechanical environments and regulate mechano-induced osteoblast differentiation. First, we demonstrated that miR-33-5p expressed by osteoblasts is sensitive to multiple mechanical environments, microgravity and fluid shear stress. We then confirmed the ability of miR-33-5p to promote osteoblast differentiation. Microgravity or fluid shear stress influences osteoblast differentiation partially via miR-33-5p. Through bioinformatics analysis and a luciferase assay, we subsequently confirmed that Hmga2 is a target gene of miR-33-5p that negatively regulates osteoblast differentiation. Moreover, miR-33-5p regulates osteoblast differentiation partially via Hmga2. In summary, our findings demonstrate that miR-33-5p is a novel mechano-sensitive miRNA that can promote osteoblast differentiation and participate in the regulation of differentiation induced by changes in the mechanical environment, suggesting this miRNA as a potential target for the treatment of pathological bone loss.

Changing expression profiles of lncRNAs, mRNAs, circRNAs and miRNAs during osteoclastogenesis

Bone is a dynamic organ continuously undergoing shaping, repairing and remodeling. The homeostasis of bone is maintained by the balance between osteoblastic bone formation and osteoclastic bone resorption. Osteoclasts (OCs) are specialized multinucleated cells derived from hematopoietic stem cells (HSCs) or monocytes/macrophage progenitor cells. There are different stages during osteoclastogenesis, and one of the most important steps to form functional osteoclasts is realized by cell-cell fusion. In our study, microarray was performed to detect the expression profiles of lncRNA, mRNA, circRNA and miRNA at different stages during osteoclastogenesis of RAW264.7 cells. Often changed RNAs were selected and clustered among the four groups with Venn analysis. The results revealed that expressions of 518 lncRNAs, 207 mRNAs, 24 circRNAs and 37 miRNAs were often altered at each stage during OC differentiation. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) biological pathway analysis were performed to predict the functions of differentially expressed lncRNAs and co-expressed potential targeting genes. Co-expression networks of lncRNA-mRNA and circRNA-miRNA were constructed based on the correlation analysis between the differentially expressed RNAs. The present study provided a systematic perspective on the potential function of non-coding RNAs (ncRNAs) during osteoclastogenesis.

miRNA-132-3p inhibits osteoblast differentiation by targeting Ep300 in simulated microgravity

Recent studies have demonstrated that miRNAs can play important roles in osteoblast differentiation and bone formation. However, the function of miRNAs in bone loss induced by microgravity remains unclear. In this study, we investigated the differentially expressed miRNAs in both the femur tissues of hindlimb unloading rats and primary rat osteoblasts (prOB) exposed to simulated microgravity. Specifically, miR-132-3p was found up-regulated and negatively correlated with osteoblast differentiation. Overexpression of miR-132-3p significantly inhibited prOB differentiation, whereas inhibition of miR-132-3p function yielded an opposite effect. Furthermore, silencing of miR-132-3p expression effectively attenuated the negative effects of simulated microgravity on prOB differentiation. Further experiments confirmed that E1A binding protein p300 (Ep300), a type of histone acetyltransferase important for Runx2 activity and stability, was a direct target of miR-132-3p. Up-regulation of miR-132-3p by simulated microgravity could inhibit osteoblast differentiation in part by decreasing Ep300 protein expression, which, in turn, resulted in suppression of the activity and acetylation of Runx2, a key regulatory factor of osteoblast differentiation. Taken together, our findings are the first to demonstrate that miR-132-3p can inhibit osteoblast differentiation and participate in the regulation of bone loss induced by simulated microgravity, suggesting a potential target for counteracting decreases in bone formation.

Microgravity-driven remodeling of the proteome reveals insights into molecular mechanisms and signal networks involved in response to the space flight environment

Space is a hostile environment characterized by high vacuum, extreme temperatures, meteoroids, space debris, ionospheric plasma, microgravity and space radiation, which all represent risks for human health. A deep understanding of the biological consequences of exposure to the space environment is required to design efficient countermeasures to minimize their negative impact on human health. Recently, proteomic approaches have received a significant amount of attention in the effort to further study microgravity-induced physiological changes. In this review, we summarize the current knowledge about the effects of microgravity on microorganisms (in particular Cupriavidus metallidurans CH34, Bacillus cereus and Rhodospirillum rubrum S1H), plants (whole plants, organs, and cell cultures), mammalian cells (endothelial cells, bone cells, chondrocytes, muscle cells, thyroid cancer cells, immune system cells) and animals (invertebrates, vertebrates and mammals). Herein, we describe their proteome's response to microgravity, focusing on proteomic discoveries and their future potential applications in space research.

BIOLOGICAL SIGNIFICANCE

Space experiments and operational flight experience have identified detrimental effects on human health and performance because of exposure to weightlessness, even when currently available countermeasures are implemented. Many experimental tools and methods have been developed to study microgravity induced physiological changes. Recently, genomic and proteomic approaches have received a significant amount of attention. This review summarizes the recent research studies of the proteome response to microgravity inmicroorganisms, plants, mammalians cells and animals. Current proteomic tools allow large-scale, high-throughput analyses for the detection, identification, and functional investigation of all proteomes. Understanding gene and/or protein expression is the key to unlocking the mechanisms behind microgravity-induced problems and to finding effective countermeasures to spaceflight-induced alterations but also for the study of diseases on earth. Future perspectives are also highlighted.

Effects of angular frequency during clinorotation on mesenchymal stem cell morphology and migration

AIMS

To determine the short-term effects of simulated microgravity on mesenchymal stem cell behaviors-as a function of clinorotation speed-using time-lapse microscopy.

BACKGROUND

Ground-based microgravity simulation can reproduce the apparent effects of weightlessness in spaceflight using clinostats that continuously reorient the gravity vector on a specimen, creating a time-averaged nullification of gravity. In this work, we investigated the effects of clinorotation speed on the morphology, cytoarchitecture, and migration behavior of human mesenchymal stem cells (hMSCs).

METHODS

We compared cell responses at clinorotation speeds of 0, 30, 60, and 75 rpm over 8 h in a recently developed lab-on-chip-based clinostat system. Time-lapse light microscopy was used to visualize changes in cell morphology during and after cessation of clinorotation. Cytoarchitecture was assessed by actin and vinculin staining, and chemotaxis was examined using time-lapse light microscopy of cells in NGF (100 ng/ml) gradients.

RESULTS

Among clinorotated groups, cell area distributions indicated a greater inhibition of cell spreading with higher angular frequency (P<0.005), though average cell area at 30 rpm after 8 h became statistically similar to control (P=0.794). Cells at 75 rpm clinorotation remained viable and were able to re-spread after clinorotation. In chemotaxis chambers, clinorotation did not alter migration patterns in elongated cells, but most clinorotated cells exhibited cell retraction, which strongly compromised motility.

CONCLUSIONS

These results indicate that hMSCs respond to clinorotation by adopting more rounded, less-spread morphologies. The angular frequency-dependence suggests that a cell's ability to sense the changing gravity vector is governed by the rate of perturbation. For migration studies, cells cultured in clinorotated chemotaxis chambers were generally less motile and exhibited retraction instead of migration.