Migration and differentiation of osteoclast precursors under gradient fluid shear stress

Extracellular osmolarity regulates osteoblast migration through the TRPV4-Rho/ROCK signaling

For precise bone formation, osteoblasts need to accurately migrate to specific sites guided by various biochemical and mechanical cues. During this migration, fluctuations in extracellular osmolarity may arise from shifts in the surrounding fluid environment. However, as a main regulator of cell morphology and function, whether the extracellular osmolarity change may affect osteoblast migration remains unclear. Here, we provide evidence showing that changes in extracellular osmolarity significantly impact osteoblast migration, with a hypotonic environment enhancing it while a hypertonic environment inhibiting it. Further, our findings reveal that a hypotonic treatment increases intracellular pressure, activating the Transient Receptor Potential Vanilloid 4 (TRPV4) channel. This activation of TRPV4 modulates stress fibers, focal adhesions (FAs), and cell polarity through the Rho/ROCK signaling pathway, ultimately impacting osteoblast migration. Our findings provide valuable insights into the significant influence of extracellular osmolarity on osteoblast migration, which has potential implications for enhancing our understanding of bone remodeling.

Numerical Simulation of Fluid Shear Stress Distribution in Microcracks of Trabecular Bone

Bone is one of the hardest tissues in the human body, but it can undergo microcracks under long-term and periodic mechanical loads. The Newton iterative method was used to calculate the steady state, and the effects of different inlet and outlet pressures, trabecular gap width and height, and microcrack's depth and width on the fluid shear stress (FSS) were studied, and the gradient of FSS inside the microcrack was analyzed. The results show that the pressure difference and trabecular gap heigh are positively correlated with the FSS (the linear correlation coefficients R 2 were 0.9768 and 0.96542, respectively). When the trabecular gap width was 100 μm, the peak of FSS decreased by 28.57% compared with 800 and 400 μm, and the gradient of FSS inside the microcrack was 0.1-0.4 Pa/mm. This study can help people more intuitively understand the internal fluid distribution of trabecular bone and provide a reliable theoretical basis for the subsequent construction of gradient FSS devices in vitro.

Prediction of Bone Remodeling in Rat Caudal Vertebrae Based on Fluid-Solid Coupling Simulation

Some previous researches have demonstrated that appropriate mechanical stimulation can enhance bone formation. However, most studies have employed the strain energy density (SED) method for predicting bone remodeling, with only a few considering the potential impact of wall fluid shear stress (FSS) on this process. To bridge this gap, the current study compared the prediction of bone formation and resorption via SED and wall FSS by using fluid-solid coupling numerical simulation. Specifically, 8-week-old female Sprague-Dawley rats were subjected to stretching of the eighth caudal vertebra using a custom-made device. Based on micro-computed tomography images, a three-dimensional model integrating fluid-solid coupling was created to represent compact bone, cancellous bone, and bone marrow. The animals were grouped into control, 1 Hz, and 10 Hz categories, wherein a tensile displacement load of 1000 με was applied to the loading end. The results revealed that SED values tended to increase with elevated porosity, whereas wall FSS values decreased it. Notably, wall FSS demonstrated the higher predictive accuracy for cancellous bone resorption than SED. These findings support the notion that fluid flow within cancellous bone spaces can significantly impact bone resorption. Therefore, the findings of this study contribute to a more comprehensive understanding of the role of wall FSS in bone remodeling, providing a theoretical support for the dynamic evolution of bone structures under mechanical stimulation.

Fluid-solid coupling numerical simulation of the effects of different doses of verapamil on cancellous bone in type 2 diabetic rats

BACKGROUND

The purpose of this study was to investigate the effects of four different doses of verapamil on the mechanical behaviors of solid and the characteristics of fluid flow in cancellous bone of distal femur of type 2 diabetes rats under dynamic external load.

METHODS

Based on the micro-CT images, the finite element models of cancellous bones and fluids at distal femurs of rats in control group, diabetes group, treatment groups VER 4, VER 12, VER 24, and VER 48 (verapamil doses of 4, 12, 24, and 48 mg/kg/day, respectively) were constructed. A sinusoidal time-varying displacement load with an amplitude of 0.8 μm and a period of 1s was applied to the upper surface of the solid region. Then, fluid-solid coupling numerical simulation method was used to analyze the magnitudes and distributions of von Mises stress, flow velocity, and fluid shear stress of cancellous bone models in each group.

RESULTS

The results for mean values of von Mises stress, flow velocity and FSS (t = 0.25s) were as follows: their values in control group were lower than those in diabetes group; the three parameters varied with the dose of verapamil; in the four treatment groups, the values of VER 48 group were the lowest, they were the closest to control group, and they were smaller than diabetes group. Among the four treatment groups, VER 48 group had the highest proportion of the nodes with FSS = 1-3 Pa on the surface of cancellous bone, and more areas in VER 48 group were subjected to fluid shear stress of 1-3 Pa for more than half of the time.

CONCLUSION

It could be seen that among the four treatment groups, osteoblasts on the cancellous bone surface in the highest dose group (VER 48 group) were more easily activated by mechanical loading, and the treatment effect was the best. This study might help in understanding the mechanism of verapamil's effect on the bone of type 2 diabetes mellitus, and provide theoretical guidance for the selection of verapamil dose in the clinical treatment of type 2 diabetes mellitus.

The Impact of Culture Variables on a 3D Human In Vitro Bone Remodeling Model: A Design of Experiments Approach

Human in vitro bone remodeling models, using osteoclast-osteoblast cocultures, can facilitate the investigation of human bone remodeling while reducing the need for animal experiments. Although current in vitro osteoclast-osteoblast cocultures have improved the understanding of bone remodeling, it is still unknown which culture conditions support both cell types. Therefore, in vitro bone remodeling models can benefit from a thorough evaluation of the impact of culture variables on bone turnover outcomes, with the aim to reach balanced osteoclast and osteoblast activity, mimicking healthy bone remodeling. Using a resolution III fractional factorial design, the main effects of commonly used culture variables on bone turnover markers in an in vitro human bone remodeling model are identified. This model is able to capture physiological quantitative resorption-formation coupling along all conditions. Culture conditions of two runs show promising results: conditions of one run can be used as a high bone turnover system and conditions of another run as a self-regulating system as the addition of osteoclastic and osteogenic differentiation factors is not required for remodeling. The results generated with this in vitro model allow for better translation between in vitro studies and in vivo studies, toward improved preclinical bone remodeling drug development.

Knockdown of TRPV2 inhibits the migration of RAW264.7 cells toward low fluid shear stress region

Our previous studies have demonstrated that macrophages (RAW264.7) have a special ability for sensing the gradient of fluid shear stress (FSS) and migrate toward the low-FSS region. However, the molecular mechanism regulating this phenomenon is still unclear. In this study, we examined the transcriptome genes in RAW264.7 cells, MC3T3-E1 osteoblasts, mesenchymal stem cells, canine renal epithelial cells, and periodontal ligament cells. The expression levels of genes related to cell migration, force transfer, and force sensitivity in the Ca2+ signaling pathway were analyzed. We observed that the transient receptor potential cation channel type 2 (TRPV2) was highly expressed in RAW264.7 cells. Furthermore, we used lentiviral transfection to knockdown TRPV2 expression in RAW264.7 cells and studied the effect of TRPV2 on the migration of RAW264.7 cells under a gradient FSS field. The results showed that compared with normal cells, TRPV2-knockdown cells had impaired ability for sensing FSS gradient to migrate toward the low-FSS region and lower intracellular calcium response to FSS stimulation. This study may reveal the molecular mechanism of regulating the directional migration of macrophages under a gradient FSS field.

Mechanobiology of orthodontic tooth movement: An update

The purpose of this review is to provide an update on the changes at the cellular and tissue level occurring during orthodontic force application. For the understanding of this process, knowledge of the mechanobiology of the periodontal ligament and the alveolar bone are essential. The periodontal ligament and alveolar bone make up a functional unit that undergoes robust changes during orthodontic tooth movement. Complex molecular signaling is responsible for converting mechanical stresses into biochemical events with a net result of bone apposition and/or bone resorption. Despite an improved understanding of mechanical and biochemical signaling mechanisms, it is largely unknown how mechanical stresses regulate the differentiation of stem/progenitor cells into osteoblast and osteoclast lineages. To advance orthodontics, it is crucial to gain a better understanding of osteoblast differentiation from mesenchymal stem/progenitor cells and osteoclastogenesis from the hematopoietic/monocyte lineage.

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

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

Finite element analysis on mechanical state on the osteoclasts under gradient fluid shear stress

Mechanical loading, such as fluid shear stress (FSS), is regarded as the main factor that regulates the biological responses of bone cells. Our previous studies have demonstrated that the RAW264.7 osteoclast precursors migrate toward the low-FSS region under the gradient FSS field by a cone-and-plate flow chamber, in which the FSS in the outer region is larger than that in the inner region along the radial direction. Whether the FSS distribution on a cell depends on the gradient direction of FSS field should be clarified to explain this experimental observation. In this study, the finite element models of the discretely distributed or closely packed cells adherent on the bottom plate in a cone-and-plate flow chamber were constructed, and cells were regarded as compressible isotropic Hookean solid. Results showed that the average FSS of each discretely distributed cell at the quarter sector far from the center (SFC) was about 0.1% greater than that at the quarter sector near the center (SNC). In the bands with different orientations for a cell, the relative difference between the average FSS in the SFC and the SNC becomes smaller with increased band height. For the hexagonal closely packed cells, the relative value of SFC and SNC increases with increasing cell spacing. The difference between the local wall FSS in the SFC and the SNC may activate mechanosensitive ion channels and further regulate the migration of osteoclast precursors toward the low-FSS region under the gradient FSS field.

Fluid-Solid Coupling Simulation of Wall Fluid Shear Stress on Cells under Gradient Fluid Flow

Fluid shear stress (FSS) plays a crucial role for cell migration within bone cavities filled with interstitial fluid. Whether the local wall FSS distribution on cell surface depends on the global gradient FSS of flow field should be clarified to explain our previous experimental observation. In this study, finite element models of discretely distributed or hexagonal closely packed cells adherent on the bottom plate in a modified plate flow chamber with different global FSS gradient were constructed. Fluid-solid coupling simulation of wall fluid shear stress on cells was performed, and two types of data analysis methods were used. The results showed that the profile of local FSS distribution on cell surface coincides with the angle of cell migration determined in the previous study, suggesting that RAW264.7 osteoclast precursors may sense the global FSS gradient and migrate toward the low-FSS region under a high gradient. For hexagonal closely packed cells, this profile on the surface of central cells decreased along with the increase of cell spacing, which may be caused by the higher local FSS difference along the direction of FSS gradient in the regions close to the bottom plate. This study may explain the phenomenon of the targeted migration of osteoclast precursors under gradient FSS field and further provide insights into the mechanism of mechanical stimulation-induced bone remodeling.

Bactericidal efficiency of micro- and nanostructured surfaces: a critical perspective

Micro/nanostructured surfaces (MNSS) have shown the ability to inactivate bacterial cells by physical means. An enormous amount of research has been conducted in this area over the past decade. Here, we review the various surface factors that affect the bactericidal efficiency. For example, surface hydrophobicity of the substrate has been accepted to be influential on the bactericidal effect of the surface, but a review of the literature suggests that the influence of hydrophobicity differs with the bacterial species. Also, various bacterial viability quantification methods on MNSS are critically reviewed for their suitability for the purpose, and limitations of currently used protocols are discussed. Presently used static bacterial viability assays do not represent the conditions of which those surfaces could be applied. Such application conditions do have overlaying fluid flow, and bacterial behaviours are drastically different under flow conditions compared to under static conditions. Hence, it is proposed that the bactericidal effect should be assessed under relevant fluid flow conditions with factors such as shear stress and flowrate given due significance. This review will provide a range of opportunities for future research in design and engineering of micro/nanostructured surfaces with varying experimental conditions.

Fluid-solid coupling numerical simulation of trabecular bone under cyclic loading in different directions

The structure of a bone tissue is capable of adapting to mechanical loading through the process of bone remodeling, which is regulated by osteoblasts and osteoclasts. Fluid flow within trabecular porosity under cyclic loading is one of the factors stimulating the biological response of osteoblasts and osteoclasts. However, the relation between loading directions and interstitial fluid flow was seldom studied. In the present study, a finite element model based on micro-computed tomographic reconstructions is built by using a mouse femur. Results from the fluid-solid coupling numerical simulation indicate that the loading in different directions generates a distinct distribution of von Mises stress in the bone matrix and a fluid shear stress (FSS) in the bone marrow. The loading along the physiological direction leads to a more uniform distribution of solid stress and produces an FSS level beneficial to the biological response of osteoblasts and osteoclasts compared with those along the non-physiological direction. There was a minimum threshold line of wall FSS with a specific solid stress at the bone surface, suggesting that the wall FSS is mainly induced by the solid strain. These results may offer fundamental data in understanding the mechanical environment around osteoblasts and osteoclasts and the cellular and molecular mechanisms of mechanical loading-induced bone remodeling.

Thermosensitive nanoparticle of mPEG-PTMC for oligopeptide delivery into osteoclast precursors

Transmembrane delivery of biomolecules through nanoparticles plays an important role in targeted therapy. Here, we designed a simple nanoparticle for the delivery of model peptide drug into primary osteoclast precursor cells (bone marrow macrophages) by thermosensitive and biodegradable diblock copolymer monomethoxy poly(ethylene glycol)-block-poly(trimethylene carbonate). The model peptide drug was encapsulated into the nanoparticle by dropping the drug carrier dissolved in dimethylsulfoxide solvent into water containing poly(vinyl alcohol) to achieve temperature response nanoparticles. Through size analysis, we found that the nanoparticles possessed a temperature-sensitive property between 30°C and 40°C. Moreover, flow cytometry and spectrofluorimetry analysis indicated that nanoparticle systems underwent significant cellular uptake. In addition, the evaluation of cell biology showed that nanoparticles have excellent biocompatibility. Thus, the results indicated that the temperature-sensitive nanoparticles have potential application value for targeted delivery of oligopeptide in the treatment process of osteoarthritis.