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BACKGROUND:Tumor necrosis factor-α is a broadly acting inflammatory cytokine that plays an important role in the immune inflammatory response of the body.The current study concluded that tumor necrosis factor-α has significant biological effects on a variety of bone tissue cells. OBJECTIVE:To summarize the expression and action pathways of tumor necrosis factor-α in osteoblastic and osteoclastic cells to further elucidate the regulatory role of tumor necrosis factor-α on bone tissue cells. METHODS:PubMed and CNKI were searched until March 2023,and the Chinese search terms included"tumor necrosis factor α,osteoblast,osteoclast,osteoclast,osteoprogenitor";the English search terms included"TNF-α,osteoblast,osteoclast,osteocyte,osteoprogenitor cell".The corresponding criteria were established according to the research needs,and the final literature was screened.Finally,77 articles were included for review. RESULTS AND CONCLUSION:(1)Tumor necrosis factor-α is participating in regulating the recruitment,appreciation,and differentiation of osteoprogenitor cells,but leads to osteoprogenitor cell stripping and death under specific environments.It also participates in bone resorption directly or indirectly through secreted enzymes.(2)Tumor necrosis factor-α can increase the level of inflammatory factors in the environment by activating relevant signaling pathways in osteoclast lineage cells or directly induce the generation of osteoclasts in specific environments.(3)Tumor necrosis factor-α can inhibit osteogenic differentiation by activating nuclear factor-κB signaling pathway,inhibiting the expression of transcription factors such as RUNX2 and Osterix,and inducing apoptosis and necrotizing apoptosis in osteoblasts.(4)Tumor necrosis factor-α inhibits osteogenesis and promotes osteoclastogenesis by activating the nuclear factor-κB signaling pathway in osteocytes and inducing cytokines such as RANKL,SOST,and DKK1,while enhancing apoptosis of the osteocytes,as well as bone resorption around the apoptotic bone tissue.(5)Taken together,the effect of tumor necrosis factor-α in bone tissue is mainly to inhibit osteogenesis and promote osteoclastosis.The biological effect of tumor necrosis factor-α in bone tissue cells is usually dependent on the activation of tumor necrosis factor receptor and nuclear factor-κB signaling pathways.(6)The interaction of tumor necrosis factor-α with other tissue cell types surrounding bone tissue and its role in bone immune regulation still deserve attention in future studies.
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Objective To study changes in bone microstructure of osteoporotic rats by multiscale analysis. Methods A total of 20 5-month-old female SD rats were randomly divided into two groups, i.e., ovariectomy (OVX) group (n=12) and the SHAM group (n=8), respectively. The rats in OVX group were subjected to bilateral ovariectomy and became osteoporosis models after 8 weeks, while sham operation was performed for the SHAM group. Changes in microstructure of cortical bone and cancellous bone at tissue scale, and osteocyte lacunar-canalicular network (LCN) and extracellular matrix (ECM) at cell scale were quantitatively analyzed using Micro-CT and SR-Nano-CT. Results At tissue scale, the cross-sectional area of cortical bone in OVX group was significantly higher than that in SHAM group (P<0.05), and the bone mineral density (BMD) and thickness of cortical bone were not significantly different from those in SHAM group. The trabecular BMD, bone volume fraction, trabecular thickness and trabecular number in OVX group were significantly decreased in comparison with SHAM group (P<0.01), while the trabecular separation was significantly increased (P<0.01). At cell scale, there was no significant difference in the semiaxes of lacunae between OVX group and SHAM group, but the thickness of lacunae and the diameter of canaliculi in OVX group were significantly increased in comparison with SHAM group (P<0.05). At the same time, the porosity of cortical bone in OVX group was significantly higher than that in SHAM group at cell scale (P<0.05). Conclusions The bone microstructure in OVX group varied to different extents at tissue and cell scales. At tissue scale, the cancellous bone loss was severe, while the cortical bone had fewer changes. At cell scale, porosity of the lacunar-canalicular network significantly increased, which directly affected the BMD and strength of cortical bone. Multiscale analysis on changes in bone microstructure of OP rats has potential application value for clinical diagnosis and pathological analysis of osteoporosis.
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BACKGROUND:The importance of autophagy for maintaining cellular homeostasis and stress response has long been recognized.As a way for cells to selectively clear their damaged organelles to achieve the recycling of cellular components,autophagy has a pivotal role in bone metabolism.OBJECTIVE:To review the role and possible mechanisms of autophagy in regulating bone-related cell activity and function among bone marrow mesenchymal stem cells,osteoblasts,osteocytes,and osteoclasts.METHODS:PubMed was searched for studies related to autophagy using the keywords of "autophagy;bone marrow mesenchymal stem cells;osteoblasts;osteocytes;osteoclasts."RESULTS AND CONCLUSION:We finally included 84 papers.Autophagy plays an important role in bone metabolism.Autophagy is involved in maintaining the balance between mineralization and absorption,and then maintaining bone homeostasis.An appropriate autophagy inducer may also benefit bone remodeling.Abnormal autophagy can lead to disorders of bone balance,leading to diseases such as osteoporosis.We may prevent or treat bone-related diseases by regulating the level of autophagy as its function in maintaining the balance of mineralization and resorption in bone homeostasis.
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Osteocytes are the main mechanical sensory and transductive cells of bone tissues. They connect with each other through many synaptic structures to form a huge regulatory network of bone steady-state cells, which are connected with osteoblasts, osteoclasts and other bone matrix surface cells. Osteocytes regulate bone metabolism and maintain bone regeneration by affecting osteoblast bone formation and osteoclast bone resorption through paracrine pathway. Aiming at the effects of some signal molecules or protein factors secreted or released by osteocytes after mechanical stimulation on the growth and differentiation of osteoblasts and osteoclasts, this paper reviews recent advances in how mechanically stimulated osteocytes communicate with osteoblasts and osteoclasts, so as to provide new ideas for the study of osteocytes biomechanics.
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Objective To screen the secretory factor-related, mechanoresponsive microRNAs (miRNA) of osteocytes. Methods Cyclic mechanical tensile strain (ε=2.5,f=0.5 Hz) was applied to osteocytes and osteoblasts cultured in vitro respectively, and the differentially expressed miRNAs only in the osteocytes were screened out by using miRNA chip. Through bioinformatics technology, in these differentially expressed miRNAs, the target genes of secretory factors including insulin-like growth factor-1(IGF-1), nitric oxide synthesase (NOS), fibroblast growth factor 23 (FGF23) and sclerostin (SOST) were further screened out. Then the selected miRNAs were compared with the biochip detected, differentially expressed miRNAs in femur bone of the mice which were trained on treadmill, and four of these miRNAs were randomly selected for quantitative PCR verification. Results For the 77 differentially expressed miRNAs only in the mechanically strained osteocytes in vitro, 22 miRNAs whose target genes were the 4 secreted factors (IGF-1, NOS, FGF23 and SOST), were screened out. Moreover, a total of 11 miRNAs in the 22 miRNAs were differentially expressed in femur bone of the treadmill trained mice with the same trend as those in osteocytes in vitro, and the randomly selected miR-361-3p, miR-3082-5p, miR-6348 and miR-706 were confirmed to be differentially expressed with the same trend in femur bone and osteocytes. Conclusions These mechanoresponsive miRNAs differentially expressed only in osteocytes, such as miR-361-3p, miR-3082-5p, miR-6348 and miR-706, probably influence osteoblastic differentiation or bone metabolism through regulating the secretory factors.
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Lysophosphatidic acid (LPA) is a small phospholipid that is present in all eukaryotic tissues and blood plasma. As an extracellular signaling molecule, LPA mediates many cellular functions by binding to six known G protein-coupled receptors and activating their downstream signaling pathways. These functions indicate that LPA may play important roles in many biological processes that include organismal development, wound healing, and carcinogenesis. Recently, many studies have found that LPA has various biological effects in different kinds of bone cells. These findings suggest that LPA is a potent regulator of bone development and remodeling and holds promising application potential in bone tissue engineering. Here, we review the recent progress on the biological regulatory function of LPA in bone tissue cells.
Subject(s)
Biological Phenomena , Bone and Bones , Lysophospholipids , Receptors, Lysophosphatidic AcidABSTRACT
Bone is a dynamic organ, and the morphology, structure and function of bone can vary with the size, direction and form of mechanical stimulation. Appropriate mechanical stimulation is the key to maintain the dynamic balance of bone formation and bone resorption. However, with aging, the senescence of bone tissues causes a series of changes, including bone microenvironment, osteocyte morphology, signaling pathways in the osteocyte, etc., which weakens its mechanical response ability and then leads to osteoporosis and other diseases. Therefore, it is of great significance to study how aging affects the mechanical response of osteocyte. This review mainly discusses the influence of aging on mechanical response of the osteocyte.
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Notch signaling pathway has a wide range of effects in the field of embryonic development, nervous system, vascular system, endocrine system and tumor. In recent years, studies have shown that Notch plays an important role in the regulation of bone metabolism, especially in bone remodeling. The disorder of bone remodeling is closely related to the progress of diseases such as osteoporosis and osteoarthritis. Notch signaling pathway can affect the process of bone remodeling by regulating the function of different cells in bone tissues, but its specific participation in different cells is still unknown. This review summarizes recent advances about the role of Notch signaling in bone remodeling.
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In the past decade, mounting evidence points to the possibility of targeting bone for treating, preventing, and predicting type 2 diabetes mellitus. Osteoblast-derived osteocalcin (OCN) can stimulate insulin secretion, enhance insulin sensitivity, and favor glucose and fatty acid uptake and utilization. Lipocalin 2 is another osteokine secreted by osteoblasts and acts in appetite suppression. Neuropeptide Y may function in browning of white adipose tissue and energy expenditure. Osteocytes are proposed to have impact on the browning of white adipose tissue and energy expenditure through the secretion of bone morphogenetic protein 7 and sclerostin. Active bone resorption is also implicated in glucose homeostasis. In addition, there is evidence indicating the involvement of bone-derived receptor activator of nuclear factor κ-B ligand in the regulation of energy metabolism. We collect and summarize recent advances and the rationales for treating, preventing, and predicting diabetes by targeting skeleton. (Chin J Endocrinol Metab, 2018, 34: 543-548)
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ABSTRACT We previously revealed the involvement of extracellular regulated protein kinases 1/2 (ERK1/2) in interleukin-6 (IL-6) secretion induced by cyclic compressive force (CCF) in MLO-Y4 cells. In this study, we investigated the contributions of the p38 mitogen-activated protein kinase (MAPK) and nuclear factor-κB (NF-κB) pathways to IL-6 secretion by stimulating MLO-Y4 cells with CCF. At 80% confluence, different magnitudes (1000μstrain, 2000 μstrain and 4000 μstrain), frequencies (0.5 Hz, 1.0 Hz and 2.0 Hz) and durations (10 min, 30 min, 1 h, 3 h and 6 h) of CCF were loaded onto cells using a four-point bending system. Flow Cytometry (FCM) analysis was used to analyze cell mortality rates after CCF loading. p38 and p65 phosphorylation as well as IκBα degradation in MLO-Y4 cells were detected by Western blotting (WB). Changes in IL-6 secretion after inhibitor treatment were assessed by enzyme-linked immunosorbent assays (ELISAs). Cellular viability was over 90 percent after CCF. p38 and p65 phosphorylation increased under all conditions, whereas IκBα protein levels decreased. However, phosphorylation and degradation were not completely dependent on the loading magnitude, frequency or duration. Furthermore, p38 inhibition using the specific inhibitor SB203580 reduced both p38 phosphorylation and IL-6 secretion. Similarly, NF-κB inhibition using BAY 11-7082 decreased p65 phosphorylation and IL-6 secretion but increased the concentration of IκBα. These findings reveal significant roles for the p38 and NF-κB signaling pathways in IL-6 secretion induced by CCF in MLO-Y4 cells.
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Summary Autophagy is a survival pathway wherein non-functional proteins and organelles are degraded in lysosomes for recycling and energy production. Therefore, autophagy is fundamental for the maintenance of cell viability, acting as a quality control process that prevents the accumulation of unnecessary structures and oxidative stress. Increasing evidence has shown that autophagy dysfunction is related to several pathologies including neurodegenerative diseases and cancer. Moreover, recent studies have shown that autophagy plays an important role for the maintenance of bone homeostasis. For instance, in vitro and animal and human studies indicate that autophagy dysfunction in bone cells is associated with the onset of bone diseases such as osteoporosis. This review had the purpose of discussing the issue to confirm whether a relationship between autophagy dysfunction and osteoporosis exits.
Resumo A autofagia é uma via de sobrevivência celular pela qual proteínas e organelas não funcionais são degradadas nos lisossomos, para reciclagem e geração de energia. Assim, a autofagia é fundamental para a manutenção da homeostase e viabilidade da célula, agindo como um controle de qualidade que evita o acúmulo de estruturas desnecessárias e o estresse oxidativo. Um número crescente de estudos tem demonstrado que disfunções na via autofágica estão relacionadas ao surgimento de diversas doenças, como as neurodegenerativas e o câncer. Estudos também têm indicado que a autofagia exerce um importante papel para a manutenção da homeostase óssea; por exemplo, estudos in vitro e em animais e humanos mostram que disfunções da autofagia nas células ósseas estão associadas ao surgimento de doenças ósseas, como a osteoporose. Nesta revisão, foram abordados esses estudos, a fim de melhor esclarecer se há uma relação entre disfunção autofágica e osteoporose.
Subject(s)
Humans , Animals , Male , Female , Rats , Osteoporosis/etiology , Osteoporosis/physiopathology , Autophagy/physiology , Oxidative Stress/physiology , Osteoblasts/pathology , Osteoclasts/pathology , Osteocytes/pathology , In Vitro Techniques , HomeostasisABSTRACT
Objective To determine the effect of ostecytic TGF-β/Smad4 signaling on osteoblastic and osteoclastic differentiation in bone marrow stromal cells (BMSCs).Methods Mice with osteocytic TGF-β/Smad4 conditional knock down (Smad4ot CKD) were generated as previously by crossing DMP1-8kb-Cre mice with Smad4lox(ex8)/lox(ex8) mice.The osteocytes were isolated from tibial and femoral diaphysis and co-cultured with wild-type BMSCs.ALP staining, Alizarin red staining and TRAP staining were performed to show osteoblastic and osteoclastic differentiation.Then, their marker genes were detected by qPCR and proteins measured by Western blot.ResultsThe expression of Runx2 and Osterix were reduced in smad4 CKDot co-cultured with BMSCs compared with controls(P<0.01).Similarly, the specific markers of osteoblastic differentiation were decreased (P<0.01).Additionally, the expression of RANKL was not significantly changed in with BMSCs.However, OPG was highly expressed incontrol group compared with smad4 CKD in co-cultured group (P<0.05).Thus, the radio of RANKL/OPG was significantly reduced (P<0.05).Furthermore, the expression of RANK was inhibited.Conclusions The terminally-differentiated osteocytes are the cells regulating bone metabolism, while down-regulation of osteocytic-TGF-β/Smad4 inhibits BMSC osteoblastic and osteoclastic differentiation.
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Objectives To observe the effect of low intensity pulsed ultrasound (LIPUS) on osteocyte injuries induced by the tricalcium phosphate(TCP) wear particles in the calvaria of mice.Methods Thirty ICR male mice of 6 to 8 weeks were randomly divided into a normal control group(n=10),a model group (n=10) and a LIPUS-treated group(n=10).A murine calvarial model of osteolysis was established in the model and LIPUS-treated groups through injecting TCP particles onto the surface of bilateral parietal bones at week 1,3,5,7 and 11.Mice in the normal group received negative ultrasound probe pressing,while those in the LIPUS-treated received LIPUS radiation.Three months later,the calvarias were obtained.The micro-CT,HE staining,flow cytometry and Western blotting were performed to estimate the calvarial osteolysis,osteocyte death,apoptosis and proteins expression of the dentin matrix protein 1 (DMP-1),sclerosis protein (SOST),glucose-regulated protein78 (GRP78),inositol-requiring enzyme(IRE 1 α),spliced X-box binding protein 1 (XBP1 s),c-Jun N-terminal kinase (JNK) and phosphorylated c-Jun N-terminal kinase(p-JNK) respectively.Results Compared with the normal control group,in the model group the viability of prosthetic osteocytes decreased significantly,and cell apoptosis was more obvious(P＜0.05);the osteocytic marker protein DMP-1 down-regulated significantly,but another marker protein SOST up-regulated significantly,which caused the decline in DMP-1/SOST(P＜0.05).Moreover,the expression levels of GRP78,IRE1,XBPls and p-JNK of the model group increased significantly(P＜0.05) in the calvaria osteocytes compared to the control group.However,in the LIPUS treatment group,osteocyte injuries and endoplasmic reticulum(ER) stress both decreased significantly,shown by a significant increase in the number and activity of osteocytes,DMP-1/SOST,and significant inhibition of the IRE1α-XBP1-JNK activation(P＜0.05).Conclusion LIPUS prevents osteocyte injuries induced by TCP wear particles in the calvaria of mice,which may be due to the inhibition of IRE1α-XBP1-JNK pathway activation through ER stress reaction.
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Por muchos años los osteocitos han sido las células óseas "olvidadas" y consideradas espectadores inactivos enterrados en la matriz ósea. Hoy en día se sabe que los osteocitos detectan y responden a estímulos mecánicos y hormonales para coordinar tanto la resorción como la formación ósea. Actualmente se considera que los osteocitos proveen la mayoría de las moléculas que regulan la actividad de los osteoclastos y de los osteoblastos, como RANKL y esclerostina, ya que manipulaciones genéticas y famacológicas de cualquiera de estas dos moléculas afectan marcadamente la homeostasis ósea. Este artículo resume hallazgos recientes que delinean los mecanismos por los cuales los osteocitos regulan el número y actividad de los osteoblastos afectando de esta manera la formación ósea.
For many years, osteocytes have been the forgotten bone cells and considered as inactive spectators buried in the bone matrix. We now know that osteocytes detect and respond to mechanical and hormonal stimuli to coordinate bone resorption and bone formation. Osteocytes are currently considered a major source of molecules that regulate the activity of osteoclasts and osteoblasts, such as RANKL and sclerostin; and genetic and pharmacological manipulations of either molecule markedly affect bone homeostasis. This article summarizes recent findings demonstrating the mechanisms by which osteocytes regulate the number and activity of osteoblasts and thus affect bone formation.
Durante muitos anos, os osteócitos têm sido células ósseas "esquecidas" e consideradas como espectadores inativos enterrados na matriz óssea. Hoje sabemos que os osteócitos são capazes de detectar e responder a estímulos mecânicos e hormonais para coordenar tanto a reabsorção quanto a formação óssea. Os osteócitos são considerados atualmente como aquelesque fornecema maioria das moléculas que regulam a atividade dos osteoclastos e dos osteoblastos, tais como RANKL e a esclerostina,visto que manipulações genéticas e farmacológicas de qualquer uma destas moléculas afetam consideravelmente a homeostase óssea. Este artigo resume as recentes descobertas que demarcam os mecanismos pelos quais os osteócitos regulam o número e atividade dos osteoblastos, afetando assim a formação óssea.
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Mice , Bone Remodeling , Osteoblasts , OsteocytesABSTRACT
Bisphosphonates (BPs) anti-fracture efficacy may be due in part to inhibition of osteocyte apoptosis. This effect requires opening of connexin (Cx) 43 hemichannels and phosphorylation of the extracellular signal regulated kinases (ERKs). However, unlike ERK activation by other stimuli, the Cx43/ERK pathway activated by BPs does not result in nuclear ERK accumulation. Instead, the anti-apoptotic effect of BPs depends on phosphorylation of cytoplasmic ERK targets and is abolished by forced nuclear retention of ERKs. We now report that ERKs and the scaffolding protein ß-arrestin co-immuno-precipitate with Cx43 in MLO-Y4 osteocytic cells and that the BP alendronate increases this association. Moreover, ERK2 fused to red fluorescent protein (ERK2-RFP) co-localizes with Cx43 fused to green fluorescent protein outside the nucleus in cells untreated or treated with alendronate. Alendronate does not induce ERK nuclear accumulation in cells transfected with wild type ß-arrestin (wtARR) or vector control, whereas it does in cells expressing a dominant negative ß-arrestin mutant (dnARR) consisting of the ß-arrestin-clathrin binding domain that competes with endogenous ß-arrestin for binding to clathrin. Alendronate activates ERKs in dnARRtransfected cells as effectively as in cells transfected with wtARR, demonstrating that dnARR only interferes with subcellular localization but not with activation of ERKs by BPs. Further, whereas alendronate inhibits apoptosis in cells expressing wtARR or vector control, it is ineffective in cells expressing dnARR. Thus, BPs induce the formation of a complex comprising Cx43, ß-arrestin, and clathrin, which directs ERKs outside the nucleus and is indispensable for osteocyte survival induced by BPs. (AU)
La efectividad de los bisfosfonatos (BPs) en la prevención de fracturas puede deberse en parte a la inhibición de la apoptosis de osteocitos. Este efecto depende de la apertura de hemicanales de conexina (Cx) 43 y la fosforilación de quinasas reguladas por señales extracelulares (ERKs). Sin embargo, a diferencia de la activación de ERKs debida a otros estímulos, la vía de señalización Cx43/ERK activada por BPs no conlleva la acumulación de ERKs en el núcleo. El efecto anti-apoptótico de los BPs depende de la fosforilación de blancos citoplasmáticos de ERKs y es inhibido cuando las quinasas son retenidas en el núcleo. En este estudio hemos demostrado que ERKs y la proteína "scaffolding" ß-arrestina co-inmunoprecipitan con Cx43 en células osteocíticas MLO-Y4 y que alendronato aumenta esta asociación. Más aún, ERK2 fusionada a la proteína roja fluorescente (ERK2-RFP) co-localiza con Cx43 fusionada con la proteína verde fluorescente fuera del núcleo en células tratadas con vehículo o alendronato. Alendronato no indujo la acumulación nuclear de ERK en células transfectadas con ß-arrestina nativa (wtARR) o con un vector control, pero si lo hizo en células que expresan una forma dominante negativa de ß-arrestina (dnARR), consistente en el dominio de interacción entre ß-arrestina y clatrina, y que compite con ß-arrestina endógena por la unión a clatrina. Alendronato activa ERKs con la misma eficiencia en células transfectadas con dnARR o wtARR, demostrando que dnARR sólo interfiere con la localización subcelular de ERKs, pero no con su activación inducida por los BPs. Más aún, mientras alendronato inhibe apoptosis en células que expresan wtARR o vector control, es inefectivo en células que expresan dnARR. En conclusión, los BPs inducen la formación de un complejo que incluye Cx43, ß-arrestina y clatrina, el cual retiene ERKs fuera del núcleo y es indispensable para la sobrevida de los osteocitos inducida por estas drogas. (AU)
Subject(s)
Osteocytes/cytology , Cell Nucleus/enzymology , Apoptosis/drug effects , Connexin 43/metabolism , Extracellular Signal-Regulated MAP Kinases/metabolism , Diphosphonates/pharmacology , beta-Arrestins/metabolism , Osteocytes/drug effects , Osteocytes/metabolism , Bone and Bones/cytology , Cell Survival/drug effectsABSTRACT
Objective The aim of this study was to evaluate the effects of zoledronic acid (ZA) on the cortical bone channels network (CBCN) and osteocyte organization in relation to the bone channels. Materials and methods Eighteen male Wistar rats were divided into control (CG) and test groups (TG). Twelve animals from TG received 3 ZA doses (7.5 µg/kg), and 6 animals from CG did not receive any medication. TG animals were euthanized at 14 (n = 6) and 75 (n = 6) dadys after drug injection. CBCN was analyzed in mandibles and tibias using computational routines. The osteocyte organization was qualitatively evaluated in tibias using a three-dimensional reconstruction of images from serial histological sections. Results Significant differences in CBCN of tibia were found between the treated and untreated rats, with a wider range of sizes and shapes of the channels after the use of ZA (channels area p = 0.0063, channels area SD p = 0.0276) and less bone matrix (bone volume p = 0.0388). The alterations in the channels’ morphology were more evident at 75 days after the drug injection (channels perimeter p = 0.0286). No differences were found in mandibles CBCN. The osteocyte distribution revealed more variable patterns of cell distribution in ZA groups, with non-homogeneous distribution of cells in relation to the bone channels. Conclusion Zoledronic acid induces structural changes in CBCN and modifies the osteocyte arrangement in cortical bone in the tibia; also, the variability in the morphology of bone channels became more evident after a certain time of the use of the drug.
Subject(s)
Animals , Male , Bone Density Conservation Agents/pharmacology , Diphosphonates/pharmacology , Haversian System/drug effects , Imidazoles/pharmacology , Osteocytes/drug effects , Haversian System/anatomy & histology , Imaging, Three-Dimensional , Mandible/anatomy & histology , Mandible/drug effects , Rats, Wistar , Statistics, Nonparametric , Tibia/anatomy & histology , Tibia/drug effectsABSTRACT
Osteocytes establish an extensive intracellular and extracellular communication system via gap junction-coupled cell processes and canaliculi, through which cell processes pass throughout bone, and the communication system is extended to osteoblasts on the bone surface. To examine the osteocyte function, several mouse models were established. To ablate osteocytes, osteocytes death was induced by diphtheria toxin. However, any types of osteocyte death result in necrosis, because dying osteocytes are not phagocytosed by scavengers. After the rupture of cytoplasmic membrane, immunostimulatory molecules are released from lacunae to bone surface through canaliculi, and stimulate macrophages. The stimulated macrophages produce interleukin (IL)-1, IL-6, and tumor necrosis factor-alpha (TNF-alpha), which are the most important proinflammatory cytokines triggering inflammatory bone loss. Therefore, the osteocyte ablation results in necrosis-induced severe osteoporosis. In conditional knockout mice of gap junction protein alpha-1 (GJA1), which encodes connexin 43 in Gap junction, using dentin matrix protein 1 (DMP1) Cre transgenic mice, osteocyte apoptosis and enhanced bone resorption occur, because extracellular communication is intact. Overexpression of Bcl-2 in osteoblasts using 2.3 kb collagen type I alpha1 (COL1A1) promoter causes osteocyte apoptosis due to the severe reduction in the number of osteocyte processes, resulting in the disruption of both intracellular and extracellular communication systems. This mouse model unraveled osteocyte functions. Osteocytes negatively regulate bone mass by stimulating osteoclastogenesis and inhibiting osteoblast function in physiological condition. Osteocytes are responsible for bone loss in unloaded condition, and osteocytes augment their functions by further stimulating osteoclastogenesis and further inhibiting osteoblast function, at least partly, through the upregulation of receptor activator of nuclear factor-kappa B ligand (RANKL) in osteoblasts and Sost in osteocytes in unloaded condition.
Subject(s)
Animals , Mice , Apoptosis , Bone Resorption , Cell Membrane , Collagen Type I , Connexin 43 , Connexins , Cytokines , Dentin , Diphtheria Toxin , Gap Junctions , Interleukin-6 , Interleukins , Macrophages , Mice, Knockout , Mice, Transgenic , Necrosis , Osteoblasts , Osteocytes , Osteoporosis , RANK Ligand , Rupture , Tumor Necrosis Factor-alpha , Up-RegulationABSTRACT
The osteocyte has been found to be an orchestrator of bone remodeling.The damage of bone leads to osteocyte apoptosis.Sclerostin secreted by osteocyte causes feedback inhibition of bone formation,so inhibition of sclerostin expression has become a new target of treatment for osteoporosis.It seems resonable to direct clinical practise and treatment of metabolic bone diseases through understanding the function of osteocyte in the process of bone remodeling.
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Interstitial fluid flow stress is one of the most important mechanical stimulations of bone cells under physiological conditions. Osteocytes and osteoblasts act as primary mechanosensors within bones, and in vitro are able to respond to fluid shear stress, both morphologically and functionally. However, there is little information about the response of integrin-associated molecules using both osteoblasts and osteocytes. In this study, we investigated the changes in response to 2 hours of oscillatory fluid flow stress in the MLO-Y4 osteocyte-like cell line and the MC3T3-E1 osteoblast-like cell line. MLO-Y4 cells exhibited a significant increase in the expression of integrin-associated molecules, including OPN, CD44, vinculin and integrin avp3. However, there was no or limited increase observed in MC3T3-E1 osteoblast-like cells. Cell area and fiber stress formation were also markedly promoted by fluid flow only in MLO-Y4 cells. But the numbers of processes per cell remain unaffected in both cell lines.
Subject(s)
Humans , Cytoskeleton/physiology , Integrins/physiology , Mechanotransduction, Cellular/physiology , Osteoblasts/cytology , Osteocytes/physiology , Cell Line , Gene Expression Profiling , Integrins/metabolism , Osteoblasts/physiology , Osteocytes/cytology , Real-Time Polymerase Chain Reaction , Stress, MechanicalABSTRACT
La mineralizacin ortotpica comienza con la produccin de las vesculas de matriz, por brotacin polarizada de la superficie de condrocitos, osteoblastos y odontoblastos. Esta transcurre en dos etapas. La primera es la formacin de cristales de hidroxiapatita dentro de las vesculas de matriz, seguido por la propagacin de la hidroxiapatita a travs de la membrana de la vescula dentro de la matriz extracelular. En la regulacin de la mineralizacin ortotpica, aparte de las clulas tejido especficas, intervienen un gran nmero de enzimas, factores inorgnicos y peptdicos, que tienen complejas interacciones. Para que la mineralizacin normal contine se necesita un ajustado balance entre los niveles de fosfato inorgnico (Pi) y de pirofosfato inorgnico (PPi) extracelular. El PPi antagoniza la habilidad del Pi para cristalizar con el calcio y formar hidroxiapatita y por lo tanto suprime su propagacin. Se han identificado tres molculas reguladoras centrales de los niveles extracelulares de PPi: la fosfatasa alcalina tejido-no especfica (TNAP), que hidroliza el PPi, la nucletido pirofosfato fosfodiesterasa 1 (NPP1), que genera PPi de nuclesidos trifosfato y la protena transmembrana de mltiples-pasos ANK, que media la transferencia intracelular al extracelular de PPi. A su vez existen dos protenas SIBLING llamadas DMP1 y MEPE reguladoras de la mineralizacin. La expresin de DMP1 por el osteocito se induce en forma marcada en respuesta a la carga mecnica incrementando la mineralizacin sea. La protena MEPE contiene un motivo peptdico proteasa resistente llamado ASARM, que se cree es un candidato a ser un inhibidor de la mineralizacin (minhibina). La osteropontina es otro inhibidor de la mineralizacin en su forma fosforilada y su secrecin est marcadamente reducida en los ratones "knockout" para NPP1. Los datos actuales parecen sostener la hiptesis que estas molculas podran ser las transductoras del "strain" seo y participar en la regulacin de la mineralizacin del espacio osteoctico perilacunar.
Orthotopic mineralization begins with the production of matrix vesicles that are produced by polarized budding of the surface of condrocytes, osteoblasts and odontoblasts. It occurs in two steps: The first one is the formation of hydroxiapatite crystals within the matrix vesicles, followed by the propagation of the hydroxiapatite crystals through the membrane vesicle into the extra cellular matrix. In the regulation of orthotopic mineralization, apart from tissue-specific cells, a great number of enzymes, inorganic and peptide factors participate, that have complex interactions among them. Inorganic pyrophosphate (PPi) antagonizes the ability of phosphate (Pi) to crystallize with calcium and to form hydroxiapatite, thus suppressing its propagation. For the normal mineralization to continue, an adjusted balance of the extra cellular Pi and PPi levels is needed. Three molecules have been identified that have a central role in the regulation of extra cellular PPi levels: tissue non-specific alkaline phosphatase (TNAP), which hydrolyzes PPi, the nucleotide pyrophosphatase phosphodiesterase 1 (NPP1), which generates PPi from triphosphate nucleosides, and the multiple-steps transmembrane protein ANK which transfers PPi from the intracellular to the extracellular compartment. There are, in turn, two SIBLING proteins called DMP1 and MEPE that regulate mineralization. The expression of DMP1 by the osteocyte is dramatically induced in response to mechanical loading increasing bone mineralization. MEPE protein contains a protease resistant motif called ASARM, which is believed to be the candidate for the mineralization inhibitor (minhibin). Osteopontin is another mineralization inhibitor in its phosphorilated form and its secretion is markedly reduced in knockout mice for NPP1. Present data seem to support the hypothesis that these molecules could be the translators of bone strain and participate in the regulation of mineralization of the perilacunar osteocytic space.