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Endocrine Reviews Oct 2013Few investigators think of bone as an endocrine gland, even after the discovery that osteocytes produce circulating fibroblast growth factor 23 that targets the kidney... (Review)
Review
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.
Topics: Bone Resorption; Cell Communication; Cell Death; Cell Differentiation; Endocrine Cells; Humans; Mechanotransduction, Cellular; Muscle Cells; Osteocytes; Osteogenesis
PubMed: 23612223
DOI: 10.1210/er.2012-1026 -
Current Osteoporosis Reports Oct 2019Bone and muscle mass increase in response to mechanical loading and biochemical cues. Bone-forming osteoblasts differentiate into early osteocytes which ultimately... (Review)
Review
PURPOSE OF REVIEW
Bone and muscle mass increase in response to mechanical loading and biochemical cues. Bone-forming osteoblasts differentiate into early osteocytes which ultimately mature into late osteocytes encapsulated in stiff calcified matrix. Increased muscle mass originates from muscle stem cells (MuSCs) enclosed between their plasma membrane and basal lamina. Stem cell fate and function are strongly determined by physical and chemical properties of their microenvironment, i.e., the cell niche.
RECENT FINDINGS
The cellular niche is a three-dimensional structure consisting of extracellular matrix components, signaling molecules, and/or other cells. Via mechanical interaction with their niche, osteocytes and MuSCs are subjected to mechanical loads causing deformations of membrane, cytoskeleton, and/or nucleus, which elicit biochemical responses and secretion of signaling molecules into the niche. The latter may modulate metabolism, morphology, and mechanosensitivity of the secreting cells, or signal to neighboring cells and cells at a distance. Little is known about how mechanical loading of bone and muscle tissue affects osteocytes and MuSCs within their niches. This review provides an overview of physicochemical niche conditions of (early) osteocytes and MuSCs and how these are sensed and determine cell fate and function. Moreover, we discuss how state-of-the-art imaging techniques may enhance our understanding of these conditions and mechanisms.
Topics: Animals; Cell Differentiation; Extracellular Matrix; Humans; Mechanotransduction, Cellular; Muscle Cells; Osteocytes; Stress, Mechanical
PubMed: 31428977
DOI: 10.1007/s11914-019-00522-0 -
Bone Feb 2023Mechanical unloading causes rapid loss of bone structure and strength, which gradually recovers after resuming normal loading. However, it is not well established how...
Mechanical unloading causes rapid loss of bone structure and strength, which gradually recovers after resuming normal loading. However, it is not well established how this adaptation to unloading and reloading changes with age. Clinically, elderly patients are more prone to musculoskeletal injury and longer periods of bedrest, therefore it is important to understand how periods of disuse will affect overall skeletal health of aged subjects. Bone also undergoes an age-related decrease in osteocyte density, which may impair mechanoresponsiveness. In this study, we examined bone adaptation during unloading and subsequent reloading in mice. Specifically, we examined the differences in bone adaptation between young mice (3-month-old), old mice (18-month-old), and transgenic mice that exhibit diminished osteocyte density at a young age (3-month-old BCL-2 transgenic mice). Mice underwent 14 days of hindlimb unloading followed by up to 14 days of reloading. We analyzed trabecular and cortical bone structure in the femur, mechanical properties of the femoral cortical diaphysis, osteocyte density and cell death in cortical bone, and serum levels of inflammatory cytokines. We found that young mice lost ~10% cortical bone volume and 27-42% trabecular bone volume during unloading and early reloading, with modest recovery of metaphyseal trabecular bone and near total recovery of epiphyseal trabecular bone, but no recovery of cortical bone after 14 days of reloading. Old mice lost 12-14% cortical bone volume and 35-50% trabecular bone volume during unloading and early reloading but had diminished recovery of trabecular bone during reloading and no recovery of cortical bone. In BCL-2 transgenic mice, no cortical bone loss was observed during unloading or reloading, but 28-31% trabecular bone loss occurred during unloading and early reloading, with little to no recovery during reloading. No significant differences in circulating inflammatory cytokine levels were observed due to unloading and reloading in any of the experimental groups. These results illustrate important differences in bone adaptation in older and osteocyte deficient mice, suggesting a possible period of vulnerability in skeletal health in older subjects during and following a period of disuse that may affect skeletal health in elderly patients.
Topics: Mice; Animals; Osteocytes; Bone and Bones; Cortical Bone; Femur; Hindlimb Suspension; Mice, Transgenic
PubMed: 36529445
DOI: 10.1016/j.bone.2022.116646 -
Bone Jul 2011Bisphosphonates stop bone loss by inhibiting the activity of bone-resorbing osteoclasts. However, the effect of bisphosphonates on bone mass cannot completely explain... (Review)
Review
Bisphosphonates stop bone loss by inhibiting the activity of bone-resorbing osteoclasts. However, the effect of bisphosphonates on bone mass cannot completely explain the reduction in fracture incidence observed in patients treated with these agents. Recent research efforts provided an explanation to this dichotomy by demonstrating that part of the beneficial effect of bisphosphonates on the skeleton is due to prevention of osteoblast and osteocyte apoptosis. Work of our group, independently confirmed by other investigators, demonstrated that bisphosphonates are able to prevent osteoblast and osteocyte apoptosis in vitro and in vivo. This prosurvival effect is strictly dependent on the expression of connexin (Cx) 43, as demonstrated in vitro using cells lacking Cx43 or expressing dominant-negative mutants of the protein as well as in vivo using Cx43 osteoblast/osteocyte-specific conditional knock-out mice. Remarkably, this Cx43-dependent survival effect of bisphosphonates is independent of gap junctions and results from opening of Cx43 hemichannels. Hemichannel opening leads to activation of the kinases Src and extracellular signal-regulated kinases (ERKs), followed by phosphorylation of the ERK cytoplasmic target p90(RSK) kinase and its substrates BAD and C/EBPβ, resulting in inhibition of apoptosis. The antiapoptotic effect of bisphosphonates is separate from the effect of the drugs on osteoclasts, as analogs that lack antiresorptive activity are still able to inhibit osteoblast and osteocyte apoptosis in vitro. Furthermore, a bisphosphonate analog that does not inhibit osteoclast activity prevented osteoblast and osteocyte apoptosis and the loss of bone mass and strength induced by glucocorticoids in mice. Preservation of the bone-forming function of mature osteoblasts and maintenance of the osteocytic network, in combination with lack anticatabolic actions, open new therapeutic possibilities for bisphosphonates in the treatment of osteopenic conditions in which decreased bone resorption is not desired.
Topics: Animals; Bone and Bones; Cell Survival; Connexin 43; Diphosphonates; Humans; Osteoblasts; Osteocytes
PubMed: 20727997
DOI: 10.1016/j.bone.2010.08.008 -
Frontiers in Immunology 2023The molecular mechanisms of age-related bone loss are unclear and without valid drugs yet. The aims of this study were to explore the molecular changes that occur in...
PURPOSE
The molecular mechanisms of age-related bone loss are unclear and without valid drugs yet. The aims of this study were to explore the molecular changes that occur in bone tissue during age-related bone loss, to further clarify the changes in function, and to predict potential therapeutic drugs.
METHODS
We collected bone tissues from children, middle-aged individuals, and elderly people for protein sequencing and compared the three groups of proteins pairwise, and the differentially expressed proteins (DEPs) in each group were analyzed by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG). K-means cluster analysis was then used to screen out proteins that continuously increased/decreased with age. Canonical signaling pathways that were activated or inhibited in bone tissue along with increasing age were identified by Ingenuity Pathway Analysis (IPA). Prediction of potential drugs was performed using the Connectivity Map (CMap). Finally, DEPs from sequencing were verified by Western blot, and the drug treatment effect was verified by quantitative real-time PCR.
RESULTS
The GO and KEGG analyses show that the DEPs were associated with inflammation and bone formation with aging, and the IPA analysis shows that pathways such as IL-8 signaling and acute-phase response signaling were activated, while glycolysis I and EIF2 signaling were inhibited. A total of nine potential drugs were predicted, with rapamycin ranking the highest. In cellular experiments, rapamycin reduced the senescence phenotype produced by the HO-stimulated osteocyte-like cell MLO-Y4.
CONCLUSION
With age, inflammatory pathways are activated in bone tissue, and signals that promote bone formation are inhibited. This study contributes to the understanding of the molecular changes that occur in bone tissue during age-related bone loss and provides evidence that rapamycin is a drug of potential clinical value for this disease. The therapeutic effects of the drug are to be further studied in animals.
Topics: Animals; Osteocytes; Hydrogen Peroxide; Osteoporosis; Inflammation; Sirolimus
PubMed: 36814916
DOI: 10.3389/fimmu.2023.1114006 -
Frontiers in Endocrinology 2023For decades, functional primary human osteocyte cultures have been crucially needed for understanding their role in bone anabolic processes and in endocrine phosphate...
INTRODUCTION
For decades, functional primary human osteocyte cultures have been crucially needed for understanding their role in bone anabolic processes and in endocrine phosphate regulation via the bone-kidney axis. Mature osteocyte proteins (sclerostin, DMP1, Phex and FGF23) play a key role in various systemic diseases and are targeted by successful bone anabolic drugs (anti-sclerostin antibody and teriparatide (PTH1-34)). However, cell lines available to study osteocytes produce very little sclerostin and low levels of mature osteocyte markers. We have developed a primary human 3D organotypic culture system that replicates the formation of mature osteocytes in bone.
METHODS
Primary human osteoblasts were seeded in a fibrinogen / thrombin gel around 3D-printed hanging posts. Following contraction of the gel around the posts, cells were cultured in osteogenic media and conditioned media was collected for analysis of secreted markers of osteocyte formation.
RESULTS
The organoids were viable for at least 6 months, allowing co-culture with different cell types and testing of bone anabolic drugs. Bulk RNAseq data displayed the developing marker trajectory of ossification and human primary osteocyte formation over an initial 8- week period. Vitamin D3 supplementation increased mineralization and sclerostin secretion, while hypoxia and PTH1-34 modulated sclerostin. Our culture system also secreted FGF23, enabling the future development of a bone-kidney-parathyroid-vascular multi-organoid or organ-on-a-chip system to study disease processes and drug effects using purely human cells.
DISCUSSION
This 3D organotypic culture system provides a stable, long-lived, and regulated population of mature human primary osteocytes for a variety of research applications.
Topics: Humans; Microphysiological Systems; Osteocytes; Organoids; Osteoblasts; Biological Transport
PubMed: 37223031
DOI: 10.3389/fendo.2023.1167734 -
Bone Oct 2019Lipotoxicity is defined as cellular toxicity observed in the presence of an abnormal accumulation of fat and adipocyte-derived factors in non-fat tissues. Palmitic acid...
BACKGROUND
Lipotoxicity is defined as cellular toxicity observed in the presence of an abnormal accumulation of fat and adipocyte-derived factors in non-fat tissues. Palmitic acid (PA), an abundant fatty acid in the bone marrow and particularly in osteoporotic bones, affects osteoblastogenesis and osteoblast function, decreasing their survival through induction of apoptosis and dysfunctional autophagy. In this study, we hypothesized that PA also has a lipotoxic effect on osteocytes in vitro.
METHODS
Initially, we tested the effect of PA on osteocyte-derived factors DKK1, sclerostin and RANKL. Then, we tested whether PA affects survival and causes apoptosis in osteocytes. Subsequently, we investigated the effect of PA on autophagy by detecting the membrane component LC3-II (Western blot) and staining it and lysosomes with Lysotracker Red dye.
RESULTS
PA decreases RANKL, DKK1 and sclerostin expression in osteocytes. In addition, we found that PA induces apoptosis and reduces osteocyte survival. PA also caused autophagy failure identified by a significant increase in LC3-II and a reduced number of autophagosomes/lysosomes in the cytoplasm.
CONCLUSION
In addition to the effects of PA on RANKL, DKK1 and sclerostin expression, which could have significant deleterious impact on bone cell coupling and bone turnover, PA also induced apoptosis and reduced autophagy in osteocytes. Considering that apoptosis and cell dysfunction are two common changes occurring in the osteocytes of osteoporotic bone, our findings suggest that PA could play a role in the pathogenesis of the disease. Suppression of these effects could bring new potential targets for therapeutic interventions in the future.
Topics: Animals; Apoptosis; Autophagy; Cell Line; Cell Survival; Lysosomes; Mice; Osteocytes; Palmitates
PubMed: 31226530
DOI: 10.1016/j.bone.2019.06.016 -
Current Osteoporosis Reports Dec 2019This review summarizes recently published data on the effects of pregnancy and lactation on bone structure, mechanical properties, and mechano-responsiveness in an... (Review)
Review
PURPOSE OF REVIEW
This review summarizes recently published data on the effects of pregnancy and lactation on bone structure, mechanical properties, and mechano-responsiveness in an effort to elucidate how the balance between the structural and metabolic functions of the skeleton is achieved during these physiological processes.
RECENT FINDINGS
While pregnancy and lactation induce significant changes in bone density and structure to provide calcium for fetal/infant growth, the maternal physiology also comprises several innate compensatory mechanisms that allow for the maintenance of skeletal mechanical integrity. Both clinical and animal studies suggest that pregnancy and lactation lead to adaptations in cortical bone structure to allow for rapid calcium release from the trabecular compartment while maintaining whole bone stiffness and strength. Moreover, extents of lactation-induced bone loss and weaning-induced recovery are highly dependent on a given bone's load-bearing function, resulting in better protection of the mechanical integrity at critical load-bearing sites. The recent discovery of lactation-induced osteocytic perilacunar/canalicular remodeling (PLR) indicates a new means for osteocytes to modulate mineral homeostasis and tissue-level mechanical properties of the maternal skeleton. Furthermore, lactation-induced PLR may also play an important role in maintaining the maternal skeleton's load-bearing capacity by altering osteocyte's microenvironment and modulating the transmission of anabolic mechanical signals to osteocytes. Both clinical and animal studies show that parity and lactation have no adverse, or a positive effect on bone strength later in life. The skeletal effects during pregnancy and lactation reflect an optimized balance between the mechanical and metabolic functions of the skeleton.
Topics: Adaptation, Physiological; Animals; Biomechanical Phenomena; Bone Density; Bone Remodeling; Bone and Bones; Calcium; Cancellous Bone; Cortical Bone; Female; Humans; Lactation; Osteocytes; Pregnancy; Weaning; Weight-Bearing
PubMed: 31755029
DOI: 10.1007/s11914-019-00555-5 -
Bone Mar 2023Bone remodeling occurs through the interactions of three major cell lineages, osteoblasts, which mediate bone formation, osteocytes, which derive from osteoblasts, sense... (Review)
Review
Bone remodeling occurs through the interactions of three major cell lineages, osteoblasts, which mediate bone formation, osteocytes, which derive from osteoblasts, sense mechanical force and direct bone turnover, and osteoclasts, which mediate bone resorption. However, multiple additional cell types within the bone marrow, including macrophages, T lymphocytes and B lymphocytes influence the process. The bone marrow microenvironment, which is supported, in part, by bone cells, forms a nurturing network for B lymphopoiesis. In turn, developing B lymphocytes influence bone cells. Bone health during homeostasis depends on the normal interactions of bone cells with other lineages in the bone marrow. In disease state these interactions become pathologic and can cause abnormal function of bone cells and inadequate repair of bone after a fracture. This review summarizes what is known about the development of B lymphocytes and the interactions of B lymphocytes with bone cells in both health and disease.
Topics: Humans; Osteocytes; Osteoclasts; Osteoblasts; Bone Resorption; Bone Remodeling; B-Lymphocytes
PubMed: 34942359
DOI: 10.1016/j.bone.2021.116296 -
Biomolecules Dec 2022Osteocytes play an important role in bone metabolism. The interactions of osteocytes with the surrounding microenvironment can alter cellular and lacunar morphological...
Osteocytes play an important role in bone metabolism. The interactions of osteocytes with the surrounding microenvironment can alter cellular and lacunar morphological changes. However, objective quantification of osteocyte lacunae is challenging due to their deep location in the bone matrix. This project established a novel method for the analytical study of osteocytes/lacunae, which was then used to evaluate the osteocyte morphological changes in diabetic pig mandibular bone. Eight miniature pigs were sourced, and diabetes was randomly induced in four animals using streptozotocin (STZ) administration. The mandibular tissues were collected and processed. The jawbone density was evaluated with micro-CT. Osteocyte lacunae were effectively acquired and identified using backscattered electron scanning microscopy (BSE). A significantly decreased osteocyte lacunae size was found in the diabetic group. Using the acid etching method, it was demonstrated that the area of osteocyte and lacunae, and the pericellular areas were both significantly reduced in the diabetes group. In conclusion, a standard and relatively reliable method for analyzing osteocyte/lacunae morphological changes under compromised conditions has been successfully established. This method demonstrates that diabetes can significantly decrease osteocyte/lacunae size in a pig's mandibular cancellous bone.
Topics: Animals; Swine; Osteocytes; Cancellous Bone; Bone and Bones; Bone Matrix; X-Ray Microtomography; Diabetes Mellitus
PubMed: 36671434
DOI: 10.3390/biom13010049