Investigation of mechanosensing and mechanoresponse mechanisms in osteoblasts and osteocytes: in vitro experiments targeting subcellular components

To understand adaptive bone remodeling in response to external mechanical stimuli, researchers have elucidated the mechanisms of mechanosensing and mechanoresponse through in vitro experiments targeting subcellular components from molecules to organelles. Such subcellular experiments have been performed by applying mechanical stimuli to mechanosensitive components and by measuring and observing the dynamic behaviors of the mechanosensitive and mechanoresponsive components. For a better understanding of the importance of the subcellular experiments, this article reviews the recent subcellular experiments for osteoblasts and osteocytes. First, we introduce the tools used for the stimulation and measurement/observation, and we discuss how these tools have contributed to the elucidation of the mechanisms. Second, we shed light on how the findings on the behaviors of the subcellular components have enhanced our basic understanding of the underlying mechanisms. Furthermore, we present future perspectives for subcellular experiments. To do this, we discuss the utilization of microscopes with higher spatial resolution and discuss focus points for a clearer understanding of these mechanisms in osteocytes. Future experiments will reveal how osteoblasts and osteocytes sense and respond to external mechanical stimuli in their surrounding environment in bone, and how cellular behaviors finally lead to the regulation of bone resorption and formation in adaptive bone remodeling

Effect of Uniaxial Compression Frequency on Osteogenic Cell Responses in Dynamic 3D Cultures

The application of mechanical stimulation on bone tissue engineering constructs aims to mimic the native dynamic nature of bone. Although many attempts have been made to evaluate the effect of applied mechanical stimuli on osteogenic differentiation, the conditions that govern this process have not yet been fully explored. In this study, pre-osteoblastic cells were seeded on PLLA/PCL/PHBV (90/5/5 wt.%) polymeric blend scaffolds. The constructs were subjected every day to cyclic uniaxial compression for 40 min at a displacement of 400 μm, using three frequency values, 0.5, 1, and 1.5 Hz, for up to 21 days, and their osteogenic response was compared to that of static cultures. Finite element simulation was performed to validate the scaffold design and the loading direction, and to assure that cells inside the scaffolds would be subjected to significant levels of strain during stimulation. None of the applied loading conditions negatively affected the cell viability. The alkaline phosphatase activity data indicated significantly higher values at all dynamic conditions compared to the static ones at day 7, with the highest response being observed at 0.5 Hz. Collagen and calcium production were significantly increased compared to static controls. These results indicate that all of the examined frequencies substantially promoted the osteogenic capacity

Polycystin-2 is required for chondrocyte mechanotransduction and traffics to the primary cilium in response to mechanical stimulation

Primary cilia and associated intraflagellar transport are essential for skeletal development, joint homeostasis, and the response to mechanical stimuli, although the mechanisms remain unclear. Polycystin-2 (PC2) is a member of the transient receptor potential polycystic (TRPP) family of cation channels, and together with Polycystin-1 (PC1), it has been implicated in cilia-mediated mechanotransduction in epithelial cells. The current study investigates the effect of mechanical stimulation on the localization of ciliary polycystins in chondrocytes and tests the hypothesis that they are required in chondrocyte mechanosignaling. Isolated chondrocytes were subjected to mechanical stimulation in the form of uniaxial cyclic tensile strain (CTS) in order to examine the effects on PC2 ciliary localization and matrix gene expression. In the absence of strain, PC2 localizes to the chondrocyte ciliary membrane and neither PC1 nor PC2 are required for ciliogenesis. Cartilage matrix gene expression (Acan, Col2a) is increased in response to 10% CTS. This response is inhibited by siRNA-mediated loss of PC1 or PC2 expression. PC2 ciliary localization requires PC1 and is increased in response to CTS. Increased PC2 cilia trafficking is dependent on the activation of transient receptor potential cation channel subfamily V member 4 (TRPV4) activation. Together, these findings demonstrate for the first time that polycystins are required for chondrocyte mechanotransduction and highlight the mechanosensitive cilia trafficking of PC2 as an important component of cilia-mediated mechanotransduction

SCAFFOLD DESIGN PARAMETERS TO STIMULATE THE OSTEOGENIC SIGNAL EXPRESSION FOR BONE TISSUE ENGINEERING APPLICATIONS

The fundamental components of bone tissue engineering are (a) progenitor cells which subsequently express tissue matrix, (b) scaffolds which can act as temporary frameworks to support bone growth, and (c) growth factors to induce osteoblast regeneration. A variety of growth factors are involved during the differentiation cascade and these chemical and biological signals dynamically interact with cell populations to facilitate the differentiation. Therefore, enhanced expression of endogenous growth factor genes might facilitate abundant existence of growth factors in the surrounding microenvironment, stimulate the osteogenic differentiation of progenitor cell population, and finally induce bone regeneration. This work is focused on the augmentation of osteogenic signal expressions to stimulate the downstream differentiation of transplanted bone marrow stromal cells (BMSCs) population through the optimization of a variety of properties of three dimensional (3D) biodegradable poly(propylene fumarate) (PPF) scaffold. Changes in the microenvironment of cell population would affect the responses of localized cell population and the manipulated scaffold properties might be associated with induction of endogenous osteogenic signal expressions. First, the effect of cell-to-cell paracrine signaling distance, which can by modulated by initial cell seeding density, on the osteogenic signal expressions and osteoblastic differentiation of BMSCs on 2D PPF disks was investigated. Next, in order to investigate the improvement of the 3D macroporous PPF scaffold by the incorporation with nanoparticle filler materials, PPF/hydroxyapatite (HA) nanocomposite scaffolds were fabricated. The effect of HA content and initial cell seeding density on the osteogenic signal expression in 3D porous system was then determined. Finally, the incorporation of diethyl dumarate (DEF) with PPF was tested based on the photocrosslinking characteristics of PPF/DEF composite material with increased mechanical properties. The effect of two scaffold design parameters including the stiffness by modulating the DEF content as well as the pore size of porous scaffold on the signal expression and downstream osteoblastic differentiation was investigated. In addition, the feasibility of PPPF/DEF materials for stereolithographical fabrication was also tested in this work. Controlling these construction parameters to optimize engineered bone substitutes could affect various cellular functions of attachment, proliferation, signal expression, and differentiation. This research provided the insight of stimulation of the expression of target endogenous genes to induce the osteogenic differentiation and bone regeneration as well as the fabrication of improved bone substitute implant materials which is clinically applicable

Stem cell mechanobiology

Stem cells are undifferentiated cells that are capable of proliferation, self-maintenance and differentiation towards specific cell phenotypes. These processes are controlled by a variety of cues including physicochemical factors associated with the specific mechanical environment in which the cells reside. The control of stem cell biology through mechanical factors remains poorly understood and is the focus of the developing field of mechanobiology. This review provides an insight into the current knowledge of the role of mechanical forces in the induction of differentiation of stem cells. While the details associated with individual studies are complex and typically associated with the stem cell type studied and model system adopted, certain key themes emerge. First, the differentiation process affects the mechanical properties of the cells and of specific subcellular components. Secondly, that stem cells are able to detect and respond to alterations in the stiffness of their surrounding microenvironment via induction of lineage-specific differentiation. Finally, the application of external mechanical forces to stem cells, transduced through a variety of mechanisms, can initiate and drive differentiation processes. The coalescence of these three key concepts permit the introduction of a new theory for the maintenance of stem cells and alternatively their differentiation via the concept of a stem cell 'mechano-niche', defined as a specific combination of cell mechanical properties, extracellular matrix stiffness and external mechanical cues conducive to the maintenance of the stem cell population.<br/

Mechanobiological Modulation of Cytoskeleton and Calcium Influx in Osteoblastic Cells by Short-Term Focused Acoustic Radiation Force

Mechanotransduction has demonstrated potential for regulating tissue adaptation in vivo and cellular activities in vitro. It is well documented that ultrasound can produce a wide variety of biological effects in biological systems. For example, pulsed ultrasound can be used to noninvasively accelerate the rate of bone fracture healing. Although a wide range of studies has been performed, mechanism for this therapeutic effect on bone healing is currently unknown. To elucidate the mechanism of cellular response to mechanical stimuli induced by pulsed ultrasound radiation, we developed a method to apply focused acoustic radiation force (ARF) (duration, one minute) on osteoblastic MC3T3-E1 cells and observed cellular responses to ARF using a spinning disk confocal microscope. This study demonstrates that the focused ARF induced F-actin cytoskeletal rearrangement in MC3T3-E1 cells. In addition, these cells showed an increase in intracellular calcium concentration following the application of focused ARF. Furthermore, passive bending movement was noted in primary cilium that were treated with focused ARF. Cell viability was not affected. Application of pulsed ultrasound radiation generated only a minimal temperature rise of 0.1°C, and induced a streaming resulting fluid shear stress of 0.186 dyne/cm2, suggesting that hyperthermia and acoustic streaming might not be the main causes of the observed cell responses. In conclusion, these data provide more insight in the interactions between acoustic mechanical stress and osteoblastic cells. This experimental system could serve as basis for further exploration of the mechanosensing mechanism of osteoblasts triggered by ultrasound

Influence of shear stress in perfusion bioreactor cultures for the development of three-dimensional bone tissue constructs: a review.

Bone tissue engineering aims to generate clinically applicable bone graft substitutes in an effort to ease the demands and reduce the potential risks associated with traditional autograft and allograft bone replacement procedures. Biomechanical stimuli play an important role under physiologically relevant conditions in the normal formation, development, and homeostasis of bone tissue--predominantly, strain (predicted levels in vivo for humans \u3c2000\u3eμε) caused by physical deformation, and fluid shear stress (0.8-3 Pa), generated by interstitial fluid movement through lacunae caused by compression and tension under loading. Therefore, in vitro bone tissue cultivation strategies seek to incorporate biochemical stimuli in an effort to create more physiologically relevant constructs for grafting. This review is focused on collating information pertaining to the relationship between fluid shear stress, cellular deformation, and osteogenic differentiation, providing further insight into the optimal culture conditions for the creation of bone tissue substitutes

Effects of extracellular osteoanabolic agents on the endogenous response of osteoblastic cells

The complex multidimensional skeletal organization can adapt its structure in accordance with external contexts, demonstrating excellent self-renewal capacity. Thus, optimal extracellular environmental properties are critical for bone regeneration and inextricably linked to the mechanical and biological states of bone. It is interesting to note that the microstructure of bone depends not only on genetic determinants (which control the bone remodeling loop through autocrine and paracrine signals) but also, more importantly, on the continuous response of cells to external mechanical cues. In particular, bone cells sense mechanical signals such as shear, tensile, loading and vibration, and once activated, they react by regulating bone anabolism. Although several specific surrounding conditions needed for osteoblast cells to specifically augment bone formation have been empirically discovered, most of the underlying biomechanical cellular processes underneath remain largely unknown. Nevertheless, exogenous stimuli of endogenous osteogenesis can be applied to promote the mineral apposition rate, bone formation, bone mass and bone strength, as well as expediting fracture repair and bone regeneration. The following review summarizes the latest studies related to the proliferation and differentiation of osteoblastic cells, enhanced by mechanical forces or supplemental signaling factors (such as trace metals, nutraceuticals, vitamins and exosomes), providing a thorough overview of the exogenous osteogenic agents which can be exploited to modulate and influence the mechanically induced anabolism of bone. Furthermore, this review aims to discuss the emerging role of extracellular stimuli in skeletal metabolism as well as their potential roles and provide new perspectives for the treatment of bone disorders

Chip-Based Comparison of the Osteogenesis of Human Bone Marrow- and Adipose Tissue-Derived Mesenchymal Stem Cells under Mechanical Stimulation

Adipose tissue-derived stem cells (ASCs) are considered as an attractive stem cell source for tissue engineering and regenerative medicine. We compared human bone marrow-derived mesenchymal stem cells (hMSCs) and hASCs under dynamic hydraulic compression to evaluate and compare osteogenic abilities. A novel micro cell chip integrated with microvalves and microscale cell culture chambers separated from an air-pressure chamber was developed using microfabrication technology. The microscale chip enables the culture of two types of stem cells concurrently, where each is loaded into cell culture chambers and dynamic compressive stimulation is applied to the cells uniformly. Dynamic hydraulic compression (1 Hz, 1 psi) increased the production of osteogenic matrix components (bone sialoprotein, oateopontin, type I collagen) and integrin (CD11b and CD31) expression from both stem cell sources. Alkaline phosphatase and Alrizarin red staining were evident in the stimulated hMSCs, while the stimulated hASCs did not show significant increases in staining under the same stimulation conditions. Upon application of mechanical stimulus to the two types of stem cells, integrin (β1) and osteogenic gene markers were upregulated from both cell types. In conclusion, stimulated hMSCs and hASCs showed increased osteogenic gene expression compared to non-stimulated groups. The hMSCs were more sensitive to mechanical stimulation and more effective towards osteogenic differentiation than the hASCs under these modes of mechanical stimulation.National Institutes of Health (U.S.) (EB008392