Abstract The human skeleton is a miracle of engineering, combining both toughness and light weight. It does so because bones possess cellular mechanisms wherein external mechanical loads are sensed. ...These mechanical loads are transformed into biological signals, which ultimately direct bone formation and/or bone resorption. Osteocytes, since they are ubiquitous in the mineralized matrix, are the cells that sense mechanical loads and transduce the mechanical signals into a chemical response. The osteocytes then release signaling molecules, which orchestrate the recruitment and activity of osteoblasts or osteoclasts, resulting in the adaptation of bone mass and structure. In this review, we highlight current insights in bone adaptation to external mechanical loading, with an emphasis on how a mechanical load placed on whole bones is translated and amplified into a mechanical signal that is subsequently sensed by the osteocytes. This article is part of a Special Issue entitled "The Osteocyte".
Mechanical instability of bone implants stimulate osteoclast differentiation and peri‐implant bone loss, leading to prosthetic loosening. It is unclear which cells at the periprosthetic interface ...transduce mechanical signals into a biochemical response, and subsequently facilitate bone loss. We hypothesized that mechanical overloading of hematopoietic bone marrow progenitor cells, which are located near to the inserted bone implants, stimulates the release of osteoclast‐inducing soluble factors. Using a novel in vitro model to apply mechanical overloading, we found that hematopoietic progenitor cells released adenosine triphosphate (ATP) after only 2 min of mechanical loading. The released ATP interacts with its specific receptor P2X7 to stimulate the release of unknown soluble factors that inhibit (physiological loading) or promote (supraphysiological loading) the differentiation of multinucleated osteoclasts derived from bone marrow cultures. Inhibition of ATP‐receptor P2X7 by Brilliant Blue G completely abolished the overloading‐induced stimulation of osteoclast formation. Likewise, stimulation of P2X7 receptor on hematopoietic cells by BzATP enhanced the release of osteoclastogenesis‐stimulating signaling molecules to a similar extent as supraphysiological loading. Supraphysiological loading affected neither gene expression of inflammatory markers involved in aseptic implant loosening (e.g., interleukin‐1β (IL‐1β), IL‐6, tumor necrosis factor‐α, and PTGES2) nor expression of the osteoclast modulators receptor activator of nuclear factor κ‐Β ligand and osteoprotegerin. Our findings suggest that murine hematopoietic progenitor cells are a potential key player in local mechanical loading‐induced bone implant loosening via the ATP/P2X7‐axis. Our approach identifies potential therapeutic targets to prevent prosthetic loosening.
Hematopoietic progenitor cells responded to 2 min of supraphysiological loading with the release of soluble factors that increased osteoclast formation. In contrast, osteoclast formation upon physiological loading was reduced. Both responses were dependent on the ATP/P2X7 axis, as inhibition of P2X7 by Brilliant Blue G before mechanical stimulation of murine hematopoietic progenitor cells completely abolished these effects of loading.
Aging, Osteocytes, and Mechanotransduction Hemmatian, Haniyeh; Bakker, Astrid D.; Klein-Nulend, Jenneke ...
Current osteoporosis reports,
10/2017, Letnik:
15, Številka:
5
Journal Article
Recenzirano
Odprti dostop
Purpose of Review
The bone is able to adapt its structure to mechanical signals via the bone remodeling process governed by mechanosensitive osteocytes. With aging, an imbalance in bone remodeling ...results in osteoporosis. In this review, we hypothesized that changes in lacunar morphology underlie the decreased bone mechanoresponsiveness to mechanical loading with aging.
Recent Findings
Several studies have reported considerable variations in the shape of osteocytes and their lacunae with aging. Since osteocytes can sense matrix strain directly via their cell bodies, the variations in osteocyte morphology may cause changes in osteocyte mechanosensitivity. As a consequence, the load-adaptive response of osteocytes may change with aging, even when mechanical loading would remain unchanged.
Summary
Though extensive quantitative data is lacking, evidence exists that the osteocyte lacunae are becoming smaller and more spherical with aging. Future dedicated studies might reveal whether these changes would affect osteocyte mechanosensation and the subsequent biological response, and whether this is (one of) the pathways involved in age-related bone loss.
To date, it is unclear how fluid dynamics stimulate mechanosensory cells to induce an osteoprotective or osteodestructive response. We investigated how murine hematopoietic progenitor cells respond ...to 2 minutes of dynamic fluid flow stimulation with a precisely controlled sequence of fluid shear stresses. The response was quantified by measuring extracellular adenosine triphosphate (ATP), immunocytochemistry of Piezo1, and sarcoplasmic/endoplasmic Ca2+ reticulum ATPase 2 (SERCA2), and by the ability of soluble factors produced by mechanically stimulated cells to modulate osteoclast differentiation. We rejected our initial hypothesis that peak wall shear stress rate determines the response of hematopoietic progenitor cells to dynamic fluid shear stress, as it had only a minor correlation with the abovementioned parameters. Low stimulus amplitudes corresponded to activation of Piezo1, SERCA2, low concentrations of extracellular ATP, and inhibition of osteoclastogenesis and resorption area, while high amplitudes generally corresponded to osteodestructive responses. At a given amplitude (3 Pa) and waveform (square), the duration of individual stimuli (duty cycle) showed a strong correlation with the release of ATP and osteoclast number and resorption area. Collectively, our data suggest that hematopoietic progenitor cells respond in a viscoelastic manner to loading, since a combination of high shear stress amplitude and prolonged duty cycle is needed to trigger an osteodestructive response.
Plain Language Summary
In case of painful joints or missing teeth, the current intervention is to replace them with an implant to keep a high‐quality lifestyle. When exercising or chewing, the cells in the bone around the implant experience mechanical loading. This loading generally supports bone formation to strengthen the bone and prevent breaking, but can also stimulate bone loss when the mechanical loading becomes too high around orthopedic and dental implants. We still do not fully understand how cells in the bone can distinguish between mechanical loading that strengthens or weakens the bone. We cultured cells derived from the bone marrow in the laboratory to test whether the bone loss response depends on (i) how fast a mechanical load is applied (rate), (ii) how intense the mechanical load is (amplitude), or (iii) how long each individual loading stimulus is applied (duration). We mimicked mechanical loading as it occurs in the body, by applying very precisely controlled flow of fluid over the cells. We found that a mechanosensitive receptor Piezo1 was activated by a low amplitude stimulus, which usually strengthens the bone. The potential inhibitor of Piezo1, namely SERCA2, was only activated by a low amplitude stimulus. This happened regardless of the rate of application. At a constant high amplitude, a longer duration of the stimulus enhanced the bone‐weakening response. Based on these results we deduce that a high loading amplitude tends to be bone weakening, and the longer this high amplitude persists, the worse it is for the bone.
Supraphysiological loading induced by unstable orthopedic implants initiates osteoclast formation, which results in bone degradation. We aimed to investigate which mechanosensitive cells in the ...peri-implant environment produce osteoclast-stimulating factors and how the production of these factors is stimulated by supraphysiological loading. The release of osteoclast-stimulating factors by different types of isolated bone marrow-derived hematopoietic and mesenchymal stem cells from six osteoarthritic patients was analyzed after one hour of supraphysiological loading (3.0 ± 0.2 Pa, 1 Hz) by adding their conditioned medium to osteoclast precursors. Monocytes produced factors that enhanced osteoclastogenesis by 1.6 ± 0.07-fold and mesenchymal stem cells by 1.4 ± 0.07-fold. Medium from osteoprogenitors and pre-osteoblasts enhanced osteoclastogenesis by 1.3 ± 0.09-fold and 1.4 ± 0.03-fold, respectively, where medium from four patients elicited a response and two did not. Next generation sequencing analysis of osteoprogenitors revealed that genes encoding for inflammation-related pathways and cytoskeletal rearrangements were regulated differently between responders and non-responders. Our data suggest that released osteoclast-stimulating soluble factors by progenitor cells in the bone marrow after supraphysiological loading may be related to cytoskeletal arrangement in an inflammatory environment. This connection could be relevant to better understand the aseptic loosening process of orthopedic implants.
Generalized osteoporosis is common in patients with inflammatory diseases, possibly because of circulating inflammatory factors that affect osteoblast and osteoclast formation and activity. Serum ...levels of the inflammatory factors CXCL8 and CCL20 are elevated in rheumatoid arthritis, but whether these factors affect bone metabolism is unknown. We hypothesized that CXCL8 and CCL20 decrease osteoblast proliferation and differentiation, and enhance osteoblast-mediated osteoclast formation and activity. Human primary osteoblasts were cultured with or without CXCL8 (2-200 pg/ml) or CCL20 (5-500 pg/ml) for 14 days. Osteoblast proliferation and gene expression of matrix proteins and cytokines were analyzed. Osteoclast precursors were cultured with CXCL8 (200 pg/ml) and CCL20 (500 pg/ml), or with conditioned medium (CM) from CXCL8 and CCL20-treated osteoblasts with or without IL-6 inhibitor. After 3 weeks osteoclast formation and activity were determined. CXCL8 (200 pg/ml) and CCL20 (500 pg/ml) enhanced mRNA expression of KI67 (2.5-2.7-fold), ALP (1.6-1.7-fold), and IL-6 protein production (1.3-1.6-fold) by osteoblasts. CXCL8-CM enhanced the number of osteoclasts with 3-5 nuclei (1.7-fold), and with >5 nuclei (3-fold). CCL20-CM enhanced the number of osteoclasts with 3-5 nuclei (1.3-fold), and with >5 nuclei (2.8-fold). IL-6 inhibition reduced the stimulatory effect of CXCL8-CM and CCL20-CM on formation of osteoclasts. In conclusion, CXCL8 and CCL20 did not decrease osteoblast proliferation or gene expression of matrix proteins. CXCL8 and CCL20 did not directly affect osteoclastogenesis. However, CXCL8 and CCL20 enhanced osteoblast-mediated osteoclastogenesis, partly via IL-6 production, suggesting that CXCL8 and CCL20 may contribute to osteoporosis in rheumatoid arthritis by affecting bone cell communication.
Polypyrrole (PPy) is a conducting polymer that enables controlled drug release upon electrical stimulation. We characterized the biocompatibility of PPy with human primary osteoblasts, and the effect ...of dopants. We investigated the biocompatibility of PPy comprising various dopants, i.e. p-toluene sulfonate (PPy-pTS), chondroitin sulfate (PPy-CS), or dodecylbenzenesulfonate (PPy-DBS), with human primary osteoblasts. PPy-DBS showed the roughest appearance of all surfaces tested, and its wettability was similar to the gold-coated control. The average number of attached cells was 45% higher on PPy-DBS than on PPy-CS or PPy-pTS, although gene expression of the proliferation marker Ki-67 was similar in osteoblasts on all surfaces tested. Osteoblasts seeded on PPy-DBS or gold showed similar vinculin attachment points, vinculin area per cell area, actin filament structure, and Feret's diameter, while cells seeded on PPY-CS or PPY-pTS showed disturbed focal adhesions and were enlarged with disorganized actin filaments. Osteoblasts grown on PPy-DBS or gold showed enhanced alkaline phosphatase activity and osteocalcin gene expression, but reduced osteopontin gene expression compared to cells grown on PPy-pTS and PPy-CS. In conclusion, PPy doped with DBS showed excellent biocompatibility, which resulted in maintaining focal adhesions, cell morphology, cell number, alkaline phosphatase activity, and osteocalcin gene expression. Taken together, conducting polymers doped with DBS are well tolerated by osteoblasts. Our results could provide a basis for the development of novel orthopedic or dental implants with controlled release of antibiotics and pharmaceutics that fight infections or focally enhance bone formation in a tightly controlled manner.
Hormonal changes during lactation are associated with profound changes in bone cell biology, such as osteocytic osteolysis, resulting in larger lacunae. Larger lacuna shape theoretically enhances the ...transmission of mechanical signals to osteocytes. We aimed to provide experimental evidence supporting this theory by comparing the mechanoresponse of osteocytes in the bone of lactating mice, which have enlarged lacunae due to osteocytic osteolysis, with the response of osteocytes in bone from age-matched virgin mice. The osteocyte mechanoresponse was measured in excised fibulae that were cultured in hormone-free medium for 24 h and cyclically loaded for 10 min (sinusoidal compressive load, 3000 µε, 5 Hz) by quantifying loading-related changes in
Sost
mRNA expression (qPCR) and sclerostin and β-catenin protein expression (immunohistochemistry). Loading decreased
Sost
expression by ~ threefold in fibulae of lactating mice. The loading-induced decrease in sclerostin protein expression by osteocytes was larger in lactating mice (55% decrease ± 14 (± SD),
n
= 8) than virgin mice (33% decrease ± 15,
n
= 7). Mechanical loading upregulated β-catenin expression in osteocytes in lactating mice by 3.5-fold (± 0.2,
n
= 6) which is significantly (
p
< 0.01) higher than the 1.6-fold increase in β-catenin expression by osteocytes in fibulae from virgin mice (± 0.12,
n
= 4). These results suggest that osteocytes in fibulae from lactating mice with large lacunae may respond stronger to mechanical loading than those from virgin mice. This could indicate that osteocytes residing in larger lacuna show a stronger response to mechanical loading.
Bone's microporosity plays important roles in bone biology and bone mechanical quality. In this study, we explored the accuracy and reproducibility of nondestructive desktop μCT for 3D visualization ...and subsequent morphometric analysis of mouse cortical bone microporosity including the vascular canal network and osteocyte lacunae. The accuracy of measurements was evaluated in five murine fibula using confocal laser scanning microscopy (CLSM) in conjunction with Fluorescein isothiocyanate (FITC) staining as the reference method. The reproducibility of μCT-derived cortical bone microstructural indices was examined in 10 fibulae of C57Bl/6J male mice at a nominal resolution of 700 nanometer. Three repeated measurements were made on different days. An excellent correlation between μCT and CLSM was observed for both mean lacuna volume (r = 0.98, p = 0.002) and for mean lacuna orientation (r = 0.93, p = 0.02). Whereas the two techniques showed no significant differences for these parameters, the mean lacuna sphericity acquired from μCT was significantly higher than CLSM (p = 0.01). Reproducibility was high, with precision errors (PE) of 1.57-4.69% for lacuna parameters, and of 1.01-9.45% for vascular canal parameters. Intraclass correlation coefficient (ICC) showed a high reliability of the measurements, ranging from 0.998-1.000 for cortical parameters, 0.973-0.999 for vascular canal parameters and 0.755-0.991 for lacuna parameters. In conclusion, desktop μCT is a valuable tool to quantify the 3D characteristics of bone vascular canals as well as lacunae which can be applied to intact murine bones with high accuracy and reproducibility. Thus, μCT might be an important tool to improve our understanding of the physiological and biomechanical significance of these cannular and lacunar structure in cortical bone.
Osteoblasts derived from mouse skulls have increased osteoclastogenic potential compared to long bone osteoblasts when stimulated with 1,25(OH)
vitamin D
(vitD
). This indicates that bone cells from ...specific sites can react differently to biochemical signals, e.g., during inflammation or as emitted by bioactive bone tissue-engineering constructs. Given the high turn-over of alveolar bone, we hypothesized that
alveolar bone-derived osteoblasts have an increased osteogenic and osteoclastogenic potential compared to the osteoblasts derived from long bone. The osteogenic and osteoclastogenic capacity of alveolar bone cells and long bone cells were assessed in the presence and absence of osteotropic agent vitD
. Both cell types were studied in osteogenesis experiments, using an osteogenic medium, and in osteoclastogenesis experiments by co-culturing osteoblasts with peripheral blood mononuclear cells (PBMCs). Both osteogenic and osteoclastic markers were measured. At day 0, long bones seem to have a more late-osteoblastic/preosteocyte-like phenotype compared to the alveolar bone cells as shown by slower proliferation, the higher expression of the matrix molecule
(
and the osteocyte-enriched cytoskeletal component
(
. This phenotype was maintained during the osteogenesis assays, where long bone-derived cells still expressed more
and
. Under co-culture conditions with PBMCs, long bone cells also had a higher
(
) expression and induced the formation of osteoclasts more than alveolar bone cells. Correspondingly, the expression of osteoclast genes
(
) and
was higher in long bone co-cultures. Together, our results indicate that long bone-derived osteoblasts are more active in bone-remodeling processes, especially in osteoclastogenesis, than alveolar bone-derived cells. This indicates that tissue-engineering solutions need to be specifically designed for the site of application, such as defects in long bones vs. the regeneration of alveolar bone after severe periodontitis.
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