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Burnstock G, Arnett TR, Orriss IR. Purinergic signalling in the musculoskeletal system. Purinergic Signal 2013; 9:541-72. [PMID: 23943493 PMCID: PMC3889393 DOI: 10.1007/s11302-013-9381-4] [Citation(s) in RCA: 93] [Impact Index Per Article: 8.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/08/2013] [Accepted: 07/12/2013] [Indexed: 12/11/2022] Open
Abstract
It is now widely recognised that extracellular nucleotides, signalling via purinergic receptors, participate in numerous biological processes in most tissues. It has become evident that extracellular nucleotides have significant regulatory effects in the musculoskeletal system. In early development, ATP released from motor nerves along with acetylcholine acts as a cotransmitter in neuromuscular transmission; in mature animals, ATP functions as a neuromodulator. Purinergic receptors expressed by skeletal muscle and satellite cells play important pathophysiological roles in their development or repair. In many cell types, expression of purinergic receptors is often dependent on differentiation. For example, sequential expression of P2X5, P2Y1 and P2X2 receptors occurs during muscle regeneration in the mdx model of muscular dystrophy. In bone and cartilage cells, the functional effects of purinergic signalling appear to be largely negative. ATP stimulates the formation and activation of osteoclasts, the bone-destroying cells. Another role appears to be as a potent local inhibitor of mineralisation. In osteoblasts, the bone-forming cells, ATP acts via P2 receptors to limit bone mineralisation by inhibiting alkaline phosphatase expression and activity. Extracellular ATP additionally exerts significant effects on mineralisation via its hydrolysis product, pyrophosphate. Evidence now suggests that purinergic signalling is potentially important in several bone and joint disorders including osteoporosis, rheumatoid arthritis and cancers. Strategies for future musculoskeletal therapies might involve modulation of purinergic receptor function or of the ecto-nucleotidases responsible for ATP breakdown or ATP transport inhibitors.
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Affiliation(s)
- Geoffrey Burnstock
- Autonomic Neuroscience Centre, University College Medical School, Rowland Hill Street, London, NW3 2PF, UK,
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Prasadam I, Farnaghi S, Feng JQ, Gu W, Perry S, Crawford R, Xiao Y. Impact of extracellular matrix derived from osteoarthritis subchondral bone osteoblasts on osteocytes: role of integrinβ1 and focal adhesion kinase signaling cues. Arthritis Res Ther 2013; 15:R150. [PMID: 24289792 PMCID: PMC3978998 DOI: 10.1186/ar4333] [Citation(s) in RCA: 38] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/07/2013] [Accepted: 09/17/2013] [Indexed: 02/02/2023] Open
Abstract
INTRODUCTION Our recent study indicated that subchondral bone pathogenesis in osteoarthritis (OA) is associated with osteocyte morphology and phenotypic abnormalities. However, the mechanism underlying this abnormality needs to be identified. In this study we investigated the effect of extracellular matrix (ECM) produced from normal and OA bone on osteocytic cells function. METHODS De-cellularized matrices, resembling the bone provisional ECM secreted from primary human subchondral bone osteoblasts (SBOs) of normal and OA patients were used as a model to study the effect on osteocytic cells. Osteocytic cells (MLOY4 osteocyte cell line) cultured on normal and OA derived ECMs were analyzed by confocal microscopy, scanning electron microscopy (SEM), cell attachment assays, zymography, apoptosis assays, qRT-PCR and western blotting. The role of integrinβ1 and focal adhesion kinase (FAK) signaling pathways during these interactions were monitored using appropriate blocking antibodies. RESULTS The ECM produced by OA SBOs contained less mineral content, showed altered organization of matrix proteins and matrix structure compared with the matrices produced by normal SBOs. Culture of osteocytic cells on these defective OA ECM resulted in a decrease of integrinβ1 expression and the de-activation of FAK cell signaling pathway, which subsequently affected the initial osteocytic cell's attachment and functions including morphological abnormalities of cytoskeletal structures, focal adhesions, increased apoptosis, altered osteocyte specific gene expression and increased Matrix metalloproteinases (MMP-2) and -9 expression. CONCLUSION This study provides new insights in understanding how altered OA bone matrix can lead to the abnormal osteocyte phenotypic changes, which is typical in OA pathogenesis.
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Abstract
Few investigators think of bone as an endocrine gland, even after the discovery that osteocytes produce circulating fibroblast growth factor 23 that targets the kidney and potentially other organs. In fact, until the last few years, osteocytes were perceived by many as passive, metabolically inactive cells. However, exciting recent discoveries have shown that osteocytes encased within mineralized bone matrix are actually multifunctional cells with many key regulatory roles in bone and mineral homeostasis. In addition to serving as endocrine cells and regulators of phosphate homeostasis, these cells control bone remodeling through regulation of both osteoclasts and osteoblasts, are mechanosensory cells that coordinate adaptive responses of the skeleton to mechanical loading, and also serve as a manager of the bone's reservoir of calcium. Osteocytes must survive for decades within the bone matrix, making them one of the longest lived cells in the body. Viability and survival are therefore extremely important to ensure optimal function of the osteocyte network. As we continue to search for new therapeutics, in addition to the osteoclast and the osteoblast, the osteocyte should be considered in new strategies to prevent and treat bone disease.
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Affiliation(s)
- Sarah L Dallas
- PhD, Department of Oral and Craniofacial Sciences, School of Dentistry, University of Missouri-Kansas City, 650 East 25th Street, Kansas City, Missouri 64108.
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Matrix-dependent adhesion mediates network responses to physiological stimulation of the osteocyte cell process. Proc Natl Acad Sci U S A 2013; 110:12096-101. [PMID: 23818616 DOI: 10.1073/pnas.1310003110] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Osteocytes are bone cells that form cellular networks that sense mechanical loads distributed throughout the bone tissue. Interstitial fluid flow in the lacunar canalicular system produces focal strains at localized attachment sites around the osteocyte cell process. These regions of periodic attachment between the osteocyte cell membrane and its canalicular wall are sites where pN-level fluid-flow induced forces are generated in vivo. In this study, we show that focally applied forces of this magnitude using a newly developed Stokesian fluid stimulus probe initiate rapid and transient intercellular electrical signals in vitro. Our experiments demonstrate both direct gap junction coupling and extracellular purinergic P2 receptor signaling between MLO-Y4 cells in a connected bone cell network. Intercellular signaling was initiated by pN-level forces applied at integrin attachment sites along both appositional and distal unapposed cell processes, but not initiated at their cell bodies with equivalent forces. Electrical coupling was evident in 58% of all cell pairs tested with appositional connections; coupling strength increased with the increasing number of junctional connections. Apyrase, a nucleotide-degrading enzyme, suppressed and abolished force-induced effector responses, indicating a contribution from ATP released by the stimulated cell. This work extends the understanding of how osteocytes modulate their microenvironment in response to mechanical signals and highlights mechanisms of intercellular relay of mechanoresponsive signals in the bone network.
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Ren J, Wang XH, Wang GC, Wu JH. 17β estradiol regulation of connexin 43-based gap junction and mechanosensitivity through classical estrogen receptor pathway in osteocyte-like MLO-Y4 cells. Bone 2013; 53:587-96. [PMID: 23247057 DOI: 10.1016/j.bone.2012.12.004] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 09/11/2012] [Revised: 11/07/2012] [Accepted: 12/07/2012] [Indexed: 11/20/2022]
Abstract
Connexin 43 (Cx43) plays an essential role in osteocyte mechanotransduction. Although estrogen involves in the adaptive responses of bone cells to mechanical loadings, its effects on osteocytic Cx43-based gap junction intercellular communication (GJIC) remain obscure. We found that 17β estradiol (E2) up-regulated Cx43, and enhanced GJIC in osteocyte-like MLO-Y4 cells in fluorescence recovery after photobleaching (FRAP) assay. Combination of E2 pre-treatment and oscillating fluid flow (OFF) further enhanced Cx43 expression and mitogen-activated protein kinase (MAPK) phosphorylation, comparing to E2 or OFF treatment alone. Both blocking of classical estrogen receptors (ERα/β) by fulvestrant and ERα knockdown by small interfering RNA inhibited E2-mediated Cx43 increase, while a GPR30-specific agonist G-1 failed to promote Cx43 expression. Our results suggest that the presence of E2 enhanced Cx43-based GJIC mainly via ERα/β pathway, and sensitized osteocytes to mechanical loading.
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Affiliation(s)
- Jian Ren
- Imaging & Characterization lab, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia
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56
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Jing D, Lu XL, Luo E, Sajda P, Leong PL, Guo XE. Spatiotemporal properties of intracellular calcium signaling in osteocytic and osteoblastic cell networks under fluid flow. Bone 2013; 53:531-40. [PMID: 23328496 PMCID: PMC3594508 DOI: 10.1016/j.bone.2013.01.008] [Citation(s) in RCA: 34] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/09/2012] [Revised: 01/04/2013] [Accepted: 01/05/2013] [Indexed: 11/30/2022]
Abstract
Mechanical stimuli can trigger intracellular calcium (Ca(2+)) responses in osteocytes and osteoblasts. Successful construction of bone cell networks necessitates more elaborate and systematic analysis for the spatiotemporal properties of Ca(2+) signaling in the networks. In the present study, an unsupervised algorithm based on independent component analysis (ICA) was employed to extract the Ca(2+) signals of bone cells in the network. We demonstrated that the ICA-based technology could yield higher signal fidelity than the manual region of interest (ROI) method. Second, the spatiotemporal properties of Ca(2+) signaling in osteocyte-like MLO-Y4 and osteoblast-like MC3T3-E1 cell networks under laminar and steady fluid flow stimulation were systematically analyzed and compared. MLO-Y4 cells exhibited much more active Ca(2+) transients than MC3T3-E1 cells, evidenced by more Ca(2+) peaks, less time to the 1st peak and less time between the 1st and 2nd peaks. With respect to temporal properties, MLO-Y4 cells demonstrated higher spike rate and Ca(2+) oscillating frequency. The spatial intercellular synchronous activities of Ca(2+) signaling in MLO-Y4 cell networks were higher than those in MC3T3-E1 cell networks and also negatively correlated with the intercellular distance, revealing faster Ca(2+) wave propagation in MLO-Y4 cell networks. Our findings show that the unsupervised ICA-based technique results in more sensitive and quantitative signal extraction than traditional ROI analysis, with the potential to be widely employed in Ca(2+) signaling extraction in the cell networks. The present study also revealed a dramatic spatiotemporal difference in Ca(2+) signaling for osteocytic and osteoblastic cell networks in processing the mechanical stimulus. The higher intracellular Ca(2+) oscillatory behaviors and intercellular coordination of MLO-Y4 cells provided further evidences that osteocytes may behave as the major mechanical sensor in bone modeling and remodeling processes.
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Affiliation(s)
- Da Jing
- Department of Biomedical Engineering, Fourth Military Medical University, Xi’an, Shaanxi 710032, China
- Bone Bioengineering Laboratory, Department of Biomedical Engineering, Columbia University, New York, NY 10027, U.S.A
| | - X. Lucas Lu
- Bone Bioengineering Laboratory, Department of Biomedical Engineering, Columbia University, New York, NY 10027, U.S.A
- Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, U.S.A
| | - Erping Luo
- Department of Biomedical Engineering, Fourth Military Medical University, Xi’an, Shaanxi 710032, China
| | - Paul Sajda
- Laboratory for Intelligent Imaging and Neural Computing, Department of Biomedical Engineering, Columbia University, New York, NY 10027, U.S.A
| | - Pui L Leong
- Bone Bioengineering Laboratory, Department of Biomedical Engineering, Columbia University, New York, NY 10027, U.S.A
| | - X. Edward Guo
- Bone Bioengineering Laboratory, Department of Biomedical Engineering, Columbia University, New York, NY 10027, U.S.A
- Corresponding Author: X. Edward Guo, Ph.D., 351 Engineering Terrace, Mail Code 8904, 1210 Amsterdam Avenue, Columbia University, New York, NY 10027, U.S.A., Telephone: 212-854-6196, Fax: 212-854-8725,
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Rumney RMH, Wang N, Agrawal A, Gartland A. Purinergic signalling in bone. Front Endocrinol (Lausanne) 2012; 3:116. [PMID: 23049524 PMCID: PMC3446723 DOI: 10.3389/fendo.2012.00116] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/29/2012] [Accepted: 09/04/2012] [Indexed: 12/11/2022] Open
Abstract
Purinergic signaling in bone was first proposed in the early 1990s with the observation that extracellular ATP could modulate events crucial to the normal functioning of bone cells. Since then the expression of nearly all the P2Y and P2X receptors by osteoblasts and osteoclasts has been reported, mediating multiple processes including cell proliferation, differentiation, function, and death. This review will highlight the most recent developments in the field of purinergic signaling in bone, with a special emphasis on recent work resulting from the European Framework 7 funded collaboration ATPBone, as well as Arthritis Research UK and Bone Research Society supported projects.
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Affiliation(s)
| | | | | | - Alison Gartland
- Department of Human Metabolism, The Mellanby Centre for Bone Research, The University of SheffieldSheffield, UK
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Lu XL, Huo B, Park M, Guo XE. Calcium response in osteocytic networks under steady and oscillatory fluid flow. Bone 2012; 51:466-73. [PMID: 22750013 PMCID: PMC3412915 DOI: 10.1016/j.bone.2012.05.021] [Citation(s) in RCA: 78] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/01/2011] [Revised: 05/04/2012] [Accepted: 05/05/2012] [Indexed: 02/05/2023]
Abstract
The fluid flow in the lacunar-canalicular system of bone is an essential mechanical stimulation on the osteocyte networks. Due to the complexity of human physical activities, the fluid shear stress on osteocyte bodies and processes consists of both steady and oscillatory components. In this study, we investigated and compared the intracellular calcium ([Ca(2+)](i)) responses of osteocytic networks under steady and oscillatory fluid flows. An in vitro osteocytic network was built with MLO-Y4 osteocyte-like cells using micro-patterning techniques to simulate the in vivo orderly organization of osteocyte networks. Sinusoidal oscillating fluid flow or unidirectional steady flow was applied on the cell surface with 2Pa peak shear stress. It was found that the osteocytic networks were significantly more responsive to steady flow than to oscillatory flow. The osteocytes can release more calcium peaks with higher magnitudes at a faster speed under steady flow stimulation. The [Ca(2+)](i) signaling transients under the steady and oscillatory flows have significantly different spatiotemporal characters, but a similar responsive percentage of cells. Further signaling pathway studies using inhibitors showed that endoplasmic reticulum (ER) calcium store, extracellular calcium source, ATP, PGE(2) and NO related pathways play similar roles in the [Ca(2+)](i) signaling of osteocytes under either steady or oscillating flow. The spatiotemporal characteristics of [Ca(2+)](i) transients under oscillating fluid flow are affected more profoundly by pharmacological treatments than under the steady flow. Our findings support the hypothesis that the [Ca(2+)](i) responses of osteocytic networks are significantly dependent on the profiles of fluid flow.
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Affiliation(s)
- X. Lucas Lu
- Bone Bioengineering Laboratory, Department of Biomedical Engineering, Columbia University, New York, NY 10027
- Department of Mechanical Engineering, University of Delaware, Newark, DE 19716
| | - Bo Huo
- Bone Bioengineering Laboratory, Department of Biomedical Engineering, Columbia University, New York, NY 10027
- Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, P. R. China
| | - Miri Park
- Bone Bioengineering Laboratory, Department of Biomedical Engineering, Columbia University, New York, NY 10027
- Department of Mechanical Engineering, University of Delaware, Newark, DE 19716
| | - X. Edward Guo
- Bone Bioengineering Laboratory, Department of Biomedical Engineering, Columbia University, New York, NY 10027
- Address correspondence to: Professor X. Edward Guo, Department of Biomedical Engineering, Columbia University, 351 Engineering Terrace, 1210 Amsterdam Avenue, New York, NY 10027, Telephone: (212) 854-6196, Fax: (212) 854-8725,
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Orriss IR, Key ML, Brandao-Burch A, Patel JJ, Burnstock G, Arnett TR. The regulation of osteoblast function and bone mineralisation by extracellular nucleotides: The role of p2x receptors. Bone 2012; 51:389-400. [PMID: 22749889 DOI: 10.1016/j.bone.2012.06.013] [Citation(s) in RCA: 82] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/04/2012] [Revised: 06/06/2012] [Accepted: 06/09/2012] [Indexed: 10/28/2022]
Abstract
Extracellular nucleotides, signalling through P2 receptors, regulate the function of both osteoblasts and osteoclasts. Osteoblasts are known to express multiple P2 receptor subtypes (P2X2,5,7 and P2Y(1),(2,4,6)), levels of which change during differentiation. ATP and UTP potently inhibit bone mineralisation in vitro, an effect mediated, at least in part, via the P2Y(2) receptor. We report here that primary rat osteoblasts express additional, functional P2 receptors (P2X1, P2X3, P2X4, P2X6, P2Y(12), P2Y(13) and P2Y(14)). Receptor expression changed with cellular differentiation: e.g., P2X4 receptor mRNA levels were 5-fold higher in mature, bone-forming osteoblasts, relative to immature, proliferating cells. The rank order of expression of P2 receptor mRNAs in mature osteoblasts was P2X4>>P2Y(1)>P2X2>P2Y(6)>P2X1>P2Y(2)>P2Y(4)>P2X6>P2X5>P2X7>P2X3>P2Y(14)>P2Y(13)>P2Y(12). Increased intracellular Ca(2+) levels following stimulation with P2X-selective agonists indicated the presence of functional receptors. To investigate whether P2X receptors might also regulate bone formation, osteoblasts were cultured for 14days with P2X receptor agonists. The P2X1 and P2X3 receptor agonists, α,β-meATP and β,γ-meATP inhibited bone mineralisation by 70% and 90%, respectively at 1μM, with complete abolition at ≥25μM; collagen production was unaffected. Bz-ATP, a P2X7 receptor agonist, reduced bone mineralisation by 70% and 99% at 10μM and 100μM, respectively. Osteoblast alkaline phosphatase activity was similarly inhibited by these agonists, whilst ecto-nucleotide pyrophosphatase/phosphodiesterase activity was increased. The effects of α,β-meATP and Bz-ATP were attenuated by antagonists selective for the P2X1 and P2X7 receptors, respectively. Our results show that normal osteoblasts express functional P2X receptors and that the P2X1 and P2X7 receptors negatively regulate bone mineralisation.
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Affiliation(s)
- Isabel R Orriss
- Department of Cell and Developmental Biology, University College London, London, UK.
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Links Between Hypertension and Osteoporosis: Benidipine Ameliorates Osteoporosis in Ovariectomized Hypertensive Rats Through Promotion of Osteoblast Proliferation and Inhibition of Osteoclast Differentiation. CURRENT CARDIOVASCULAR RISK REPORTS 2012. [DOI: 10.1007/s12170-012-0248-y] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/28/2022]
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Thompson WR, Rubin CT, Rubin J. Mechanical regulation of signaling pathways in bone. Gene 2012; 503:179-93. [PMID: 22575727 DOI: 10.1016/j.gene.2012.04.076] [Citation(s) in RCA: 268] [Impact Index Per Article: 22.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/17/2012] [Revised: 03/20/2012] [Accepted: 04/22/2012] [Indexed: 12/21/2022]
Abstract
A wide range of cell types depend on mechanically induced signals to enable appropriate physiological responses. The skeleton is particularly dependent on mechanical information to guide the resident cell population towards adaptation, maintenance and repair. Research at the organ, tissue, cell and molecular levels has improved our understanding of how the skeleton can recognize the functional environment, and how these challenges are translated into cellular information that can site-specifically alter phenotype. This review first considers those cells within the skeleton that are responsive to mechanical signals, including osteoblasts, osteoclasts, osteocytes and osteoprogenitors. This is discussed in light of a range of experimental approaches that can vary parameters such as strain, fluid shear stress, and pressure. The identity of mechanoreceptor candidates is approached, with consideration of integrins, pericellular tethers, focal adhesions, ion channels, cadherins, connexins, and the plasma membrane including caveolar and non-caveolar lipid rafts and their influence on integral signaling protein interactions. Several mechanically regulated intracellular signaling cascades are detailed including activation of kinases (Akt, MAPK, FAK), β-catenin, GTPases, and calcium signaling events. While the interaction of bone cells with their mechanical environment is complex, an understanding of mechanical regulation of bone signaling is crucial to understanding bone physiology, the etiology of diseases such as osteoporosis, and to the development of interventions to improve bone strength.
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Affiliation(s)
- William R Thompson
- Department of Medicine, University of North Carolina, Chapel Hill, NC 27599, USA.
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Ren J, Wu JH. 17β-estradiol rapidly activates calcium release from intracellular stores via the GPR30 pathway and MAPK phosphorylation in osteocyte-like MLO-Y4 cells. Calcif Tissue Int 2012; 90:411-9. [PMID: 22392527 DOI: 10.1007/s00223-012-9581-x] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/23/2011] [Accepted: 02/11/2012] [Indexed: 10/28/2022]
Abstract
Estrogen regulates critical cellular functions, and its deficiency initiates bone turnover and the development of bone mass loss in menopausal females. Recent studies have demonstrated that 17β-estradiol (E(2)) induces rapid non-genomic responses that activate downstream signaling molecules, thus providing a new perspective to understand the relationship between estrogen and bone metabolism. In this study, we investigated rapid estrogen responses, including calcium release and MAPK phosphorylation, in osteocyte-like MLO-Y4 cells. E(2) elevated [Ca(2+)]( i ) and increased Ca(2+) oscillation frequency in a dose-dependent manner. Immunolabeling confirmed the expression of three estrogen receptors (ERα, ERβ, and G protein-coupled receptor 30 [GPR30]) in MLO-Y4 cells and localized GPR30 predominantly to the plasma membrane. E(2) mobilized calcium from intracellular stores, and the use of selective agonist(s) for each ER showed that this was mediated mainly through the GPR30 pathway. MAPK phosphorylation increased in a biphasic manner, with peaks occurring after 7 and 60 min. GPR30 and classical ERs showed different temporal effects on MAPK phosphorylation and contributed to MAPK phosphorylation sequentially. ICI182,780 inhibited E(2) activation of MAPK at 7 min, while the GPR30 agonist G-1 and antagonist G-15 failed to affect MAPK phosphorylation levels. G-1-mediated MAPK phosphorylation at 60 min was prevented by prior depletion of calcium stores. Our data suggest that E(2) induces the non-genomic responses Ca(2+) release and MAPK phosphorylation to regulate osteocyte function and indicate that multiple receptors mediate rapid E(2) responses.
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Affiliation(s)
- Jian Ren
- Imaging and Characterization Lab, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia
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Lu XL, Huo B, Chiang V, Guo XE. Osteocytic network is more responsive in calcium signaling than osteoblastic network under fluid flow. J Bone Miner Res 2012; 27:563-74. [PMID: 22113822 PMCID: PMC3343217 DOI: 10.1002/jbmr.1474] [Citation(s) in RCA: 96] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Osteocytes, regarded as the mechanical sensor in bone, respond to mechanical stimulation by activating biochemical pathways and mediating the cellular activities of other bone cells. Little is known about how osteocytic networks respond to physiological mechanical stimuli. In this study, we compared the mechanical sensitivity of osteocytic and osteoblastic networks under physiological-related fluid shear stress (0.5 to 4 Pa). The intracellular calcium ([Ca(2+)](i)) responses in micropatterned in vitro osteoblastic or osteocytic networks were recorded and analyzed. Osteocytes in the network showed highly repetitive spikelike [Ca(2+)](i) peaks under fluid flow stimulation, which are dramatically different from those in the osteoblastic network. The number of responsive osteocytes in the network remained at a constant high percentage (>95%) regardless of the magnitude of shear stress, whereas the number of responsive osteoblasts in the network significantly depends on the strength of fluid flow. All spatiotemporal parameters of calcium signaling demonstrated that osteocytic networks are more sensitive and dynamic than osteoblastic networks, especially under low-level mechanical stimulations. Furthermore, pathway studies were performed to identify the molecular mechanisms responsible for the differences in [Ca(2+)](i) signaling between osteoblastic and osteocytic networks. The results suggested that the T-type voltage-gated calcium channels (VGCC) expressed on osteocytes may play an essential role in the unique kinetics of [Ca(2+)](i) signaling in osteocytic networks, whereas the L-type VGCC is critical for both types of cells to release multiple [Ca(2+)](i) peaks. The extracellular calcium source and intracellular calcium store in ER-, ATP-, PGE₂-, NO-, and caffeine-related pathways are found to play similar roles in the [Ca(2+)](i) signaling for both osteoblasts and osteocytes. The findings in this study proved that osteocytic networks possess unique characteristics in sensing and processing mechanical signals.
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Affiliation(s)
- X Lucas Lu
- Bone Bioengineering Laboratory, Department of Biomedical Engineering, Columbia University, New York, NY 10027, USA
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Brandao-Burch A, Key ML, Patel JJ, Arnett TR, Orriss IR. The P2X7 Receptor is an Important Regulator of Extracellular ATP Levels. Front Endocrinol (Lausanne) 2012; 3:41. [PMID: 22654865 PMCID: PMC3355863 DOI: 10.3389/fendo.2012.00041] [Citation(s) in RCA: 77] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/19/2011] [Accepted: 02/29/2012] [Indexed: 11/17/2022] Open
Abstract
Controlled ATP release has been demonstrated from many neuronal and non-neuronal cell types. Once released, extracellular ATP acts on cells in a paracrine manner via purinergic receptors. Considerable evidence now suggests that extracellular nucleotides, signaling via P2 receptors, play important roles in bone homeostasis modulating both osteoblast and osteoclast function. In this study, we demonstrate that mouse osteoclasts and their precursors constitutively release ATP into their extracellular environment. Levels were highest at day 2 (precursor cells), possibly reflecting the high number of red blood cells and accessory cells present. Mature osteoclasts constitutively released ATP in the range 0.05-0.5 pmol/ml/cell. Both osteoclasts and osteoblasts express mRNA and protein for the P2X7 receptor. We found that in osteoclasts, expression levels are fourfold higher in mature cells relative to precursors, whilst in osteoblasts expression remains relatively constant during differentiation. Selective antagonists (0.1-100 μM AZ10606120, A438079, and KN-62) were used to determine whether this release was mediated via P2X7 receptors. AZ10606120, A438079, and KN-62, at 0.1-10 μM, decreased ATP release by mature osteoclasts by up to 70, 60, and 80%, respectively. No differences in cell viability were observed. ATP release also occurs via vesicular exocytosis; inhibitors of this process (1-100 μM NEM or brefeldin A) had no effect on ATP release from osteoclasts. P2X7 receptor antagonists (0.1-10 μM) also decreased ATP release from primary rat osteoblasts by up to 80%. These data show that ATP release via the P2X7 receptor contributes to extracellular ATP levels in osteoclast and osteoblast cultures, suggesting an important additional role for this receptor in autocrine/paracrine purinergic signaling in bone.
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Affiliation(s)
- Andrea Brandao-Burch
- Department of Cell and Developmental Biology, University College LondonLondon, UK
| | - Michelle L. Key
- Department of Cell and Developmental Biology, University College LondonLondon, UK
| | - Jessal J. Patel
- Department of Cell and Developmental Biology, University College LondonLondon, UK
| | - Timothy R. Arnett
- Department of Cell and Developmental Biology, University College LondonLondon, UK
| | - Isabel R. Orriss
- Department of Cell and Developmental Biology, University College LondonLondon, UK
- *Correspondence: Isabel R. Orriss, Department of Cell and Developmental Biology, University College London, London WC1E 6BT, UK. e-mail:
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