Mechanically induced osteogenic differentiation--the role of RhoA, ROCKII and cytoskeletal dynamics

Advances in Shear Stress Stimulation of Stem Cells: A Review of the Last Three Decades

Stem cells are widely used in scientific research because of their ability to self-renew and differentiate into a variety of specialized cell types needed for body functions. However, the self-renewal and differentiation of stem cells are regulated by various stimuli, with mechanical stimulation being particularly notable due to its ability to mimic the physical environment in the body. This study systematically collected 2638 research papers published between 1994 and 2024, employing tools such as VOSviewer, CiteSpace, and GraphPad Prism to uncover research hotspots, publication trends, and collaboration networks. The results indicate a yearly increase in global research on the shear stress stimulation of stem cells, with significant contributions from the United States and China in terms of research investment and output. Future research directions include a deeper understanding of the mechanisms underlying mechanical stimulation's effects on stem cell differentiation, the development of new materials and scaffold designs to better replicate the natural cellular environment, and advancements in regenerative medicine. Despite considerable progress, challenges remain in translating basic research findings into clinical applications.

Arrangement into layers and mechanobiology of multi-cell co-culture models of the uterine wall

STUDY QUESTION

Can a co-culture of three cell types mimic the in vivo layers of the uterine wall?

SUMMARY ANSWER

Three protocols tested for co-culture of endometrial epithelial cells (EEC), endometrial stromal cells (ESC), and myometrial smooth muscle cells (MSMC) led to formation of the distinct layers that are characteristic of the structure of the uterine wall in vivo.

WHAT IS KNOWN ALREADY

We previously showed that a layer-by-layer co-culture of EEC and MSMC responded to peristaltic wall shear stresses (WSS) by increasing the polymerization of F-actin in both layers. Other studies showed that WSS induced significant cellular alterations in epithelial and endothelial cells.

STUDY DESIGN, SIZE, DURATION

Human EEC and ESC cell lines and primary MSMC were co-cultured on a collagen-coated synthetic membrane in custom-designed wells. The co-culture model, created by seeding a mixture of all cells at once, was exposed to steady WSS of 0.5 dyne/cm2 for 10 and 30 min.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The co-culture of the three different cells was seeded either layer-by-layer or as a mixture of all cells at once. Validation of the models was by specific immunofluorescence staining and confocal microscopy. Alterations of the cytoskeletal F-actin in response to WSS were analyzed from the 2-dimensional confocal images through the Z-stacks following a previously published algorithm.

MAIN RESULTS AND THE ROLE OF CHANCE

We generated three multi-cell in vitro models of the uterine wall with distinct layers of EEC, ESC, and MSMC that mimic the in vivo morphology. Exposure of the mixed seeding model to WSS induced increased polymerization of F-actin in all the three layers relative to the unexposed controls. Moreover, the increased polymerization of F-actin was higher (P-value < 0.05) when the length of exposure was increased from 10 to 30 min. Furthermore, the inner layers of ESC and MSMC, which are not in direct contact with the applied shearing fluid, also increased their F-actin polymerization.

LARGE SCALE DATA

N/A.

LIMITATIONS, RESONS FOR CAUTION

The mixed seeding co-culture model was exposed to steady WSS of one magnitude, whereas the uterus is a dynamic organ with intra-uterine peristaltic fluid motions that vary in vivo with different time-dependent magnitude. Further in vitro studies may explore the response to peristaltic WSS or other physical and/or hormonal perturbations that may mimic the spectrum of pathophysiological aspects.

WIDER IMPLICATIONS OF THE FINDINGS

Numerous in vitro models were developed in order to mimic the human endometrium and endometrium-myometrium interface (EMI) region. The present co-culture models seem to be the first constructed from EEC, ESC, and MSMC on a collagen-coated synthetic membrane. These multi-cell in vitro models better represent the complex in vivo anatomy of the EMI region. The mixed seeding multi-cell in vitro model may easily be implemented in controlled studies of uterine function in reproduction and the pathogenesis of diseases.

STUDY FINDING/COMPETING INTEREST(S)

This study was supported in part by Tel Aviv University funds. All authors declare no conflict of interest.

Fluid shear force and hydrostatic pressure jointly promote osteogenic differentiation of BMSCs by activating YAP1 and NFAT2

Natural bone tissue features a complex mechanical environment, with cells responding to diverse mechanical stimuli, including fluid shear stress (FSS) and hydrostatic pressure (HP). However, current in vitro experiments commonly employ a singular mechanical stimulus to simulate the mechanical environment in vivo. The understanding of the combined effects and mechanisms of multiple mechanical stimuli remains limited. Hence, this study constructed a mechanical stimulation device capable of simultaneously applying FSS and HP to cells. This study investigated the impact of FSS and HP on the osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) and examined the distinctions and interactions between the two mechanisms. The results demonstrated that both FSS and HP individually enhanced the osteogenic differentiation of BMSCs, with a more pronounced effect observed through their combined application. BMSCs responded to external FSS and HP stimulation through the integrin-cytoskeleton and Piezo1 ion channel respectively. This led to the activation of downstream biochemical signals, resulting in the dephosphorylation and nuclear translocation of the intracellular transcription factors Yes Associated Protein 1 (YAP1) and nuclear factor of activated T cells 2 (NFAT2). Activated YAP1 could bind to NFAT2 to enhance transcriptional activity, thereby promoting osteogenic differentiation of BMSCs more effectively. This study highlights the significance of composite mechanical stimulation in BMSCs' osteogenic differentiation, offering guidance for establishing a complex mechanical environment for in vitro functional bone tissue construction.

Integrated evaluation of biomechanical and biological properties of the biomimetic structural bone scaffold: Biomechanics, simulation analysis, and osteogenesis

A porous structure is essential for bone implants because it increases the bone ingrowth space and improves mechanical and biological properties. The biomimetically designed porous Voronoi scaffold can reconstruct the structure and function of cancellous bone; however, its comprehensive properties need to be investigated further. In this study, algorithms based on scaling factors were used to design the Voronoi scaffolds. Classic approaches, such as computer-aided design and the implicit surface method, have been used to design Diamond, Gyroid, and I-WP scaffolds as controls. All scaffolds were prepared by selective laser melting of titanium alloys and three-dimensional printing. Mechanical tests, finite element analysis, and in vitro and in vivo experiments were performed to investigate the biomechanical, cytologic, and osteogenic performance of the scaffolds, while computational fluid dynamics simulations were used to explore the underlying mechanisms. Diamond scaffolds have a better loading capacity, and the mechanical behaviors and fluid flow of Voronoi scaffolds are similar to those of the human trabecular bone. Cells showed more proliferation and distribution on the Diamond and Voronoi scaffolds and exhibited evident differentiation on Gyroid and Voronoi scaffolds. Bone formation was apparent on the inner part of the Gyroid, the outer part of the I-WP, and the entire Diamond and Voronoi scaffolds. The hydrodynamic properties and stimulus response of cells influenced by the porous structure account for the varied biological performance of the scaffolds. The Voronoi scaffolds with bionic mechanical behavior and an appropriate hydrodynamic response exhibit evident cell growth and osteogenesis, making them preferable for porous structural bone implants.

Biomaterials regulates BMSCs differentiation via mechanical microenvironment

Bone mesenchymal stem cells (BMSCs) are crucial for bone tissue regeneration, the mechanical microenvironment of hard tissues, including bone and teeth, significantly affects the osteogenic differentiation of BMSCs. Biomaterials may mimic the microenvironment of the extracellular matrix and provide mechanical signals to regulate BMSCs differentiation via inducing the secretion of various intracellular factors. Biomaterials direct the differentiation of BMSCs via mechanical signals, including tension, compression, shear, hydrostatic pressure, stiffness, elasticity, and viscoelasticity, which can be transmitted to cells through mechanical signalling pathways. Besides, biomaterials with piezoelectric effects regulate BMSCs differentiation via indirect mechanical signals, such as, electronic signals, which are transformed from mechanical stimuli by piezoelectric biomaterials. Mechanical stimulation facilitates achieving vectored stem cell fate regulation, while understanding the underlying mechanisms remains challenging. Herein, this review summarizes the intracellular factors, including translation factors, epigenetic modifications, and miRNA level, as well as the extracellular factor, including direct and indirect mechanical signals, which regulate the osteogenic differentiation of BMSCs. Besides, this review will also give a comprehensive summary about how mechanical stimuli regulate cellular behaviours, as well as how biomaterials promote the osteogenic differentiation of BMSCs via mechanical microenvironments. The cellular behaviours and activated signal pathways will give more implications for the design of biomaterials with superior properties for bone tissue engineering. Moreover, it will also provide inspiration for the construction of bone organoids which is a useful tool for mimicking in vivo bone tissue microenvironments.

Computational simulation of applying mechanical vibration to mesenchymal stem cell for mechanical modulation toward bone tissue engineering

Evaluation of cell response to mechanical stimuli at in vitro conditions is known as one of the important issues for modulating cell behavior. Mechanical stimuli, including mechanical vibration and oscillatory fluid flow, act as important biophysical signals for the mechanical modulation of stem cells. In the present study, mesenchymal stem cell (MSC) consists of cytoplasm, nucleus, actin, and microtubule. Also, integrin and primary cilium were considered as mechanoreceptors. In this study, the combined effect of vibration and oscillatory fluid flow on the cell and its components were investigated using numerical modeling. The results of the FEM and FSI model showed that the cell response (stress and strain values) at the frequency of 30 H z mechanical vibration has the highest value. The achieved results on shear stress caused by the fluid flow on the cell showed that the cell experiences shear stress in the range of 0 . 1 - 10 Pa . Mechanoreceptors that bind separately to the cell surface, can be highly stimulated by hydrodynamic pressure and, therefore, can play a role in the mechanical modulation of MSCs at in vitro conditions. The results of this research can be effective in future studies to optimize the conditions of mechanical stimuli applied to the cell culture medium and to determine the mechanisms involved in mechanotransduction.

Mechanotransduction in tissue engineering: Insights into the interaction of stem cells with biomechanical cues

Stem cells in their natural microenvironment are exposed to biochemical and biophysical cues emerging from the extracellular matrix (ECM) and neighboring cells. In particular, biomechanical forces modulate stem cell behavior, biological fate, and early developmental processes by sensing, interpreting, and responding through a series of biological processes known as mechanotransduction. Local structural changes in the ECM and mechanics are driven by reciprocal activation of the cell and the ECM itself, as the initial deposition of matrix proteins sequentially affects neighboring cells. Recent studies on stem cell mechanoregulation have provided insight into the importance of biomechanical signals on proper tissue regeneration and function and have shown that precise spatiotemporal control of these signals exists in stem cell niches. Against this background, the aim of this work is to review the current understanding of the molecular basis of mechanotransduction by analyzing how biomechanical forces are converted into biological responses via cellular signaling pathways. In addition, this work provides an overview of advanced strategies using stem cells and biomaterial scaffolds that enable precise spatial and temporal control of mechanical signals and offer great potential for the fields of tissue engineering and regenerative medicine will be presented.

Harnessing mechanical cues in the cellular microenvironment for bone regeneration

At the macroscale, bones experience a variety of compressive and tensile loads, and these loads cause deformations of the cortical and trabecular microstructure. These deformations produce a variety of stimuli in the cellular microenvironment that can influence the differentiation of marrow stromal cells (MSCs) and the activity of cells of the MSC lineage, including osteoblasts, osteocytes, and chondrocytes. Mechanotransduction, or conversion of mechanical stimuli to biochemical and biological signals, is thus part of a multiscale mechanobiological process that drives bone modeling, remodeling, fracture healing, and implant osseointegration. Despite strong evidence of the influence of a variety of mechanical cues, and multiple paradigms proposed to explain the influence of these cues on tissue growth and differentiation, even a working understanding of how skeletal cells respond to the complex combinations of stimuli in their microenvironments remains elusive. This review covers the current understanding of what types of microenvironmental mechanical cues MSCs respond to and what is known about how they respond in the presence of multiple such cues. We argue that in order to realize the vast potential for harnessing the cellular microenvironment for the enhancement of bone regeneration, additional investigations of how combinations of mechanical cues influence bone regeneration are needed.

A periosteum-derived cell line to study the role of BMP/TGFβ signaling in periosteal cell behavior and function

The periosteum is a thin tissue surrounding each skeletal element that contains stem and progenitor cells involved in bone development, postnatal appositional bone growth, load-induced bone formation, and fracture repair. BMP and TGFβ signaling are important for periosteal activity and periosteal cell behavior, but thorough examination of the influence of these pathways on specific cell populations resident in the periosteum is lacking due to limitations associated with primary periosteal cell isolations and in vitro experiments. Here we describe the generation of a novel periosteum-derived clonal cell (PDC) line from postnatal day 14 mice and use it to examine periosteal cell behavior in vitro. PDCs exhibit key characteristics of periosteal cells observed during skeletal development, maintenance, and bone repair. Specifically, PDCs express established periosteal markers, can be expanded in culture, demonstrate the ability to differentiate into chondrocytes, osteoblasts, and adipocytes, and exhibit an osteogenic response to physical stimulation. PDCs also engage in BMP and/or TGFβ signaling when treated with the activating ligands BMP2 and TGFβ-1, and in response to mechanical stimulation via fluid shear. We believe that this PDC line will be useful for large-scale, long-term experiments that were not feasible when using primary periosteal cells. Anticipated future uses include advancing our understanding of the signaling interactions that occur during appositional bone growth and fracture repair and developing drug screening platforms to discover novel growth and fracture healing factors.

InDel and CNV within the AKAP13 Gene Revealing Strong Associations with Growth Traits in Goat

A-kinase-anchoring protein 13 (AKAP13) is a member of the AKAP protein family that has been found to be associated with bone formation. Thus, we investigated the AKAP13 gene as a potential candidate gene for molecular-marker-assisted selection (MAS) in breeding. Our aim was to explore genetic variations (InDel and CNV) within the AKAP13 gene of Shaanbei white cashmere (SBWC) goats and analyze their relationship with growth traits. Ultimately, we identified three InDel loci (16-bp deletion, 15-bp insertion, and 25-bp deletion) and three CNVs, and the 16-bp and 15-bp loci were significantly associated with goat body length (p < 0.05). Both the 16-bp deletion variant and the 15-bp insertion variant facilitated an increase in body length in goats. In addition to this, there was a certain superposition effect between 16-bp and 15-bp loci, although there was no linkage. Additionally, the CNV1 locus was significantly correlated with body height and body length of goats (p < 0.05), and CNV2 was significantly correlated with chest depth, chest circumference, and cannon circumference of goats (p < 0.05). Individuals with gain type showed excellent growth performance. In conclusion, the InDel and CNV loci that we have identified could possibly serve as effective molecular markers in goat breeding, which is very essential for improving efficiency and success of breeding. Moreover, our findings provide a new avenue for further research into the function of the AKAP13 gene.

Electric field promotes dermal fibroblast transdifferentiation through activation of RhoA/ROCK1 pathway

With the increased incidence of age-related and lifestyle-related diseases, chronic wounds are sweeping the world, where recent studies reveal that dysfunction of fibroblast plays an indispensable role. Endogenous electric field (EF) generated by skin wound disrupting an epithelial layer has been used as an alternative clinical treatment in chronic wound by modulating cellular behaviours, including fibroblasts transdifferentiation. Although many molecules and signaling pathways have been reported associated with fibroblasts transdifferentiation, studies investigating how the electric field affects the cellular pathways have been limited. For this purpose, a model of electric field treatment in vitro was established, where cells were randomly divided into control and electrified groups. The changes of protein expression and distribution were detected under different conditions, along with Zeiss imaging system observing the response of cells. Results showed that fibroblast transdifferentiation was accompanied by increased expression of a-SMA and extracellular matrix (COL-1 and COL-3) under the EF. Simultaneously, fibroblast transdifferentiation was also consistent with changes of cell arrangement and enhanced motility. Furthermore, we found that electric field activated RhoA signaling pathways activity. Y-27632, a RhoA inhibitor, which was used to treat fibroblasts, resulted in reduced transdifferentiation. The connection between electric field and RhoA signaling pathways is likely to be significant in modulating fibroblast transdifferentiation in acute injury and tissue remodeling, which provides an innovative idea for the molecular mechanism of EF in promoting chronic wound healing.

The myosin and RhoGAP MYO9B influences osteocyte dendrite growth and responses to mechanical stimuli

Introduction: Myosin IXB (MYO9B) is an unconventional myosin with RhoGAP activity and thus is a regulator of actin cytoskeletal organization. MYO9B was previously shown to be necessary for skeletal growth and health and to play a role in actin-based functions of both osteoblasts and osteoclasts. However, its role in responses to mechanical stimulation of bone cells has not yet been described. Therefore, experiments were undertaken to determine the role of MYO9B in bone cell responses to mechanical stress both in vitro and in vivo. Methods: MYO9B expression was knocked down in osteoblast and osteocyte cell lines using RNA interference and the resulting cells were subjected to mechanical stresses including cyclic tensile strain, fluid shear stress, and plating on different substrates (no substrate vs. monomeric or polymerized collagen type I). Osteocytic cells were also subjected to MYO9B regulation through Slit-Robo signaling. Further, wild-type or Myo9b -/- mice were subjected to a regimen of whole-body vibration (WBV) and changes in bone quality were assessed by micro-CT. Results: Unlike control cells, MYO9B-deficient osteoblastic cells subjected to uniaxial cyclic tensile strain were unable to orient their actin stress fibers perpendicular to the strain. Osteocytic cells in which MYO9B was knocked down exhibited elongated dendrites but were unable to respond normally to treatments that increase dendrite length such as fluid shear stress and Slit-Robo signaling. Osteocytic responses to mechanical stimuli were also found to be dependent on the polymerization state of collagen type I substrates. Wild-type mice responded to WBV with increased bone tissue mineral density values while Myo9b -/- mice responded with bone loss. Discussion: These results demonstrate that MYO9B plays a key role in mechanical stress-induced responses of bone cells in vitro and in vivo.

Anti-osteoporosis mechanism of resistance exercise in ovariectomized rats based on transcriptome analysis: a pilot study

Postmenopausal osteoporosis is the main cause of fractures in women. Resistance exercise has a positive effect on bone mineral density in postmenopausal osteoporosis patients, but its mechanism is unclear. The purpose of this study was to explore the mechanism of resistance exercise in improving ovariectomized osteoporotic rats based on the transcriptome sequencing technique. Eighteen female Sprague-Dawley rats were randomly divided into the sham-operated group, the non-exercise group, and the resistance exercise group. The rat model of postmenopausal osteoporosis was established by bilateral ovariectomy. Ten weeks after the operation, the resistance exercise group received 2 weeks of adaptive training, and 12 weeks of resistance exercise began in the 13th week. The rats were trained 5 days per week, in 4 sets of 3 repetitions per day. After the intervention, all rats were sacrificed, and the body weight, bone mineral density, trabecular bone microarchitecture, and bone biomechanics were examined. At the same time, RNA-seq and enrichment analysis of gene ontology and Kyoto Encyclopedia of Genes and Genomes were performed on the left tibias, followed by Elisa and RT-qPCR verification. It had been found that resistance exercise can effectively counteract the weight gain of ovariectomized osteoporotic rats, and has a good effect on bone mineral density and trabecular bone microarchitecture. Enrichment analysis showed that regulation of gene expression and osteoclast differentiation is the most closely related biological process and signaling pathway shared by RE/Ovx and NE/Ovx groups. Our results revealed that resistance exercise can play a role in inhibiting osteoclast activation and preventing the enhancement of osteoclast bone resorption function in ovariectomized osteoporotic rats by inhibiting Fos/Fosb-regulated TRAP activation and relieving Calcr inhibition, which has important application value in preventing bone loss caused by estrogen deficiency.

Assembling the Puzzle Pieces. Insights for in Vitro Bone Remodeling

As a highly dynamic organ, bone changes during throughout a person's life. This process is referred to as 'bone remodeling' and it involves two stages - a well-balanced osteoclastic bone resorption and an osteoblastic bone formation. Under normal physiological conditions bone remodeling is highly regulated that ensures tight coupling between bone formation and resorption, and its disruption results in a bone metabolic disorder, most commonly osteoporosis. Though osteoporosis is one of the most prevalent skeletal ailments that affect women and men aged over 40 of all races and ethnicities, currently there are few, if any safe and effective therapeutic interventions available. Developing state-of-the-art cellular systems for bone remodeling and osteoporosis can provide important insights into the cellular and molecular mechanisms involved in skeletal homeostasis and advise better therapies for patients. This review describes osteoblastogenesis and osteoclastogenesis as two vital processes for producing mature, active bone cells in the context of interactions between cells and the bone matrix. In addition, it considers current approaches in bone tissue engineering, pointing out cell sources, core factors and matrices used in scientific practice for modeling bone diseases and testing drugs. Finally, it focuses on the challenges that bone regenerative medicine is currently facing.

Mechanobiology-informed biomaterial and tissue engineering strategies for influencing skeletal stem and progenitor cell fate

Skeletal stem and progenitor cells (SSPCs) are the multi-potent, self-renewing cell lineages that form the hematopoietic environment and adventitial structures of the skeletal tissues. Skeletal tissues are responsible for a diverse range of physiological functions because of the extensive differentiation potential of SSPCs. The differentiation fates of SSPCs are shaped by the physical properties of their surrounding microenvironment and the mechanical loading forces exerted on them within the skeletal system. In this context, the present review first highlights important biomolecules involved with the mechanobiology of how SSPCs sense and transduce these physical signals. The review then shifts focus towards how the static and dynamic physical properties of microenvironments direct the biological fates of SSPCs, specifically within biomaterial and tissue engineering systems. Biomaterial constructs possess designable, quantifiable physical properties that enable the growth of cells in controlled physical environments both in-vitro and in-vivo. The utilization of biomaterials in tissue engineering systems provides a valuable platform for controllably directing the fates of SSPCs with physical signals as a tool for mechanobiology investigations and as a template for guiding skeletal tissue regeneration. It is paramount to study this mechanobiology and account for these mechanics-mediated behaviors to develop next-generation tissue engineering therapies that synergistically combine physical and chemical signals to direct cell fate. Ultimately, taking advantage of the evolved mechanobiology of SSPCs with customizable biomaterial constructs presents a powerful method to predictably guide bone and skeletal organ regeneration.

Development of a Scaffold-on-a-Chip Platform to Evaluate Cell Infiltration and Osteogenesis on the 3D-Printed Scaffold for Bone Regeneration

Developing a scaffold for efficient and functional bone regeneration remains challenging. To accomplish this goal, a "scaffold-on-a-chip" device was developed as a platform to aid with the evaluation process. The device mimics a microenvironment experienced by a transplanted bone scaffold. The device contains a circular space at the center for scaffold insert and microfluidic channel that encloses the space. Such a design allows for monitoring of cell behavior at the blood-scaffold interphase. MC3T3-E1 cells were cultured with three different types of scaffold inserts to test its capability as an evaluation platform. Cellular behaviors, including migration, morphology, and osteogenesis with each scaffold, were analyzed through fluorescence images of live/dead assay and immunocytochemistry. Cellular behaviors, such as migration, morphology, and osteogenesis, were evaluated. The results revealed that our platform could effectively evaluate the osteoconductivity and osteoinductivity of scaffolds with various properties. In conclusion, our proposed platform is expected to replace current in vivo animal models as a highly relevant in vitro platform and can contribute to the fundamental study of bone regeneration.

Fluidic Force Microscopy and Atomic Force Microscopy Unveil New Insights into the Interactions of Preosteoblasts with 3D-Printed Submicron Patterns

Physical patterns represent potential surface cues for promoting osteogenic differentiation of stem cells and improving osseointegration of orthopedic implants. Understanding the early cell-surface interactions and their effects on late cellular functions is essential for a rational design of such topographies, yet still elusive. In this work, fluidic force microscopy (FluidFM) and atomic force microscopy (AFM) combined with optical and electron microscopy are used to quantitatively investigate the interaction of preosteoblasts with 3D-printed patterns after 4 and 24 h of culture. The patterns consist of pillars with the same diameter (200 nm) and interspace (700 nm) but distinct heights (500 and 1000 nm) and osteogenic properties. FluidFM reveals a higher cell adhesion strength after 24 h of culture on the taller pillars (32 ± 7 kPa versus 21.5 ± 12.5 kPa). This is associated with attachment of cells partly on the sidewalls of these pillars, thus requiring larger normal forces for detachment. Furthermore, the higher resistance to shear forces observed for these cells indicates an enhanced anchorage and can be related to the persistence and stability of lamellipodia. The study explains the differential cell adhesion behavior induced by different pillar heights, enabling advancements in the rational design of osteogenic patterns.

In silico evaluation of the mechanical stimulation effect on the regenerative rehabilitation for the articular cartilage local defects

Osteoarthritis is one of the most severe diseases of the human musculoskeletal system, and therefore, for many years, special attention has been paid to the search for effective methods of its treatment. However, even the most modern methods only in a limited number of cases in the early or intermediate stages of osteoarthritis lead to positive treatment results. In the later stages of development, osteoarthritis is practically incurable and most often ends with disability or the need for joint replacement for a large number of people. One of the main reasons hindering the development of osteoarthritis treatment methods is the peculiarities of articular cartilage, in which there is practically no vascular network and tissue homeostasis is carried out mainly due to the diffusion of nutrients present in the synovial fluid. In modern medicine, for the treatment of osteoarthritis, tissue engineering strategies have been developed based on the implantation of scaffolds populated with chondrogenic cells into the area of the defect. In vitro studies have established that these cells are highly mechanosensitive and, under the influence of mechanical stimuli of a certain type and intensity, their ability to proliferate and chondrogenesis increases. This property can be used to improve the efficiency of regenerative rehabilitation technologies based on the synergistic combination of cellular technologies, tissue engineering strategies, and mechanical tissue stimulation. In this work, using a regenerative rehabilitation mathematical model of local articular cartilage defects, numerical experiments were performed, the results of which indicate that the micro-and macro environment of the restored tissue, which changes during mechanical stimulation, has a significant effect on the formation of the extracellular matrix, and, consequently, cartilage tissue generally. The results obtained can be used to plan strategies for mechanical stimulation, based on the analysis of the results of cell proliferation experimental assessment after each stimulation procedure in vivo.

Significance of mechanical loading in bone fracture healing, bone regeneration, and vascularization

In 1892, J.L. Wolff proposed that bone could respond to mechanical and biophysical stimuli as a dynamic organ. This theory presents a unique opportunity for investigations on bone and its potential to aid in tissue repair. Routine activities such as exercise or machinery application can exert mechanical loads on bone. Previous research has demonstrated that mechanical loading can affect the differentiation and development of mesenchymal tissue. However, the extent to which mechanical stimulation can help repair or generate bone tissue and the related mechanisms remain unclear. Four key cell types in bone tissue, including osteoblasts, osteoclasts, bone lining cells, and osteocytes, play critical roles in responding to mechanical stimuli, while other cell lineages such as myocytes, platelets, fibroblasts, endothelial cells, and chondrocytes also exhibit mechanosensitivity. Mechanical loading can regulate the biological functions of bone tissue through the mechanosensor of bone cells intraosseously, making it a potential target for fracture healing and bone regeneration. This review aims to clarify these issues and explain bone remodeling, structure dynamics, and mechano-transduction processes in response to mechanical loading. Loading of different magnitudes, frequencies, and types, such as dynamic versus static loads, are analyzed to determine the effects of mechanical stimulation on bone tissue structure and cellular function. Finally, the importance of vascularization in nutrient supply for bone healing and regeneration was further discussed.

A systematic review on the effects of ROCK inhibitors on proliferation and/or differentiation in human somatic stem cells: A hypothesis that ROCK inhibitors support corneal endothelial healing via acting on the limbal stem cell niche

Rho kinase inhibitors (ROCKi) have attracted growing multidisciplinary interest, particularly in Ophthalmology where the question as to how they promote corneal endothelial healing remains unresolved. Concurrently, stem cell biology has rapidly progressed in unravelling drivers of stem cell (SC) proliferation and differentiation, where mechanical niche factors and the actin cytoskeleton are increasingly recognized as key players. There is mounting evidence from the study of the peripheral corneal endothelium that supports the likelihood of an internal limbal stem cell niche. The possibility that ROCKi stimulate the endothelial SC niche has not been addressed. Furthermore, there is currently a paucity of data that directly evaluates whether ROCKi promotes corneal endothelial healing by acting on this limbal SC niche located near the transition zone. Therefore, we performed a systematic review examining the effects ROCKi on the proliferation and differentiation of human somatic SC, to provide insight into its effects on various human SC populations. An appraisal of electronic searches of four databases identified 1 in vivo and 58 in vitro studies (36 evaluated proliferation while 53 examined differentiation). Types of SC studied included mesenchymal (n = 32), epithelial (n = 11), epidermal (n = 8), hematopoietic and other (n = 8). The ROCK 1/2 selective inhibitor Y-27632 was used in almost all studies (n = 58), while several studies evaluated ≥2 ROCKi (n = 4) including fasudil, H-1152, and KD025. ROCKi significantly influenced human somatic SC proliferation in 81% of studies (29/36) and SC differentiation in 94% of studies (50/53). The present systemic review highlights that ROCKi are influential in regulating human SC proliferation and differentiation, and provides evidence to support the hypothesis that ROCKi promotes corneal endothelial division and maintenance via acting on the inner limbal SC niche.

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.

Icariin: A Potential Alternative Against Osteoporosis

Osteoporosis is a metabolic skeletal disorder characterized by increased fragility and fracture risk as s result of reduced bone mineral density and microstructural destruction and caused a heavy burden on families and society. Current medicines, on the other hand, have some limitations, with side effects and doubts regarding long-term efficacy being highlighted. Studies seeking for natural constituents as potential treatment options therefore come into focus. Icariin is a phytochemical derived from a traditional Chinese medicine, Herba epimedium, that has been used to treat orthopedic disorders in ancient China for thousands of years, including osteoporosis, osteoarthritis, and fracture. Icariin belongs to a category of prenylated flavonoids and has been shown to help reduce osteoporosis bone loss while having relatively low side effects. Icariin's anti-osteoporosis properties manifest in a variety of ways, like promoting osteogenesis, suppressing osteoclastogenesis and bone resorption, regulating migration, proliferation, and differentiation of mesenchymal stem cells, enhancing angiogenesis, anti-inflammation, and antioxidation. These procedures entail a slew of critical signaling pathways, such as PPARγ, ERα/AKT/β-catenin, and MAPK. Therefore, icariin can be an applicable alternative to improve osteoporosis although the underlying mechanisms have yet to be fully understood. In this study, we searched using the terms “icariin” and “osteoporosis,” and included 64 articles meeting the inclusion criteria and reviewed the research of icariin in anti-osteoporosis over the last 10 years, and discussed new prospects for future study. Therefore, this review may provide some references for further studies.

Injectable nanoporous microgels generate vascularized constructs and support bone regeneration in critical-sized defects

Large and aberrant bone fractures require ossification and concomitant vascularization for proper healing. Evidence indicates that osteogenesis and vessel growth are coupled in bone fractures. Although the synergistic role of endothelial cells has been recognized, vascularizing large bone grafts remains a challenge and has apprehended the clinical translation of engineered bone constructs. Here, we describe a facile method to fabricate vascularized constructs using chitosan and gelatin-based microgels that promote osteogenesis of human mesenchymal stromal cells (MSC) while supporting endothelial sprouting and network formation. The microgels are enzymatically degradable and had a high hydration rate with a volume swelling ratio of ~ 493% and a polymer density of ~ 431 mg/cm3, which is comparable to that of native skeletal tissues. AFM indentation of the surface showed an average Young's modulus of 189 kPa, falling in a range that is conducive to both osteogenesis and vasculogenesis. The osteogenic microgel containing chitosan, gelatin, and hydroxyapatite, mimicking the bone matrix, supported robust attachment, proliferation, and differentiation of MSC. On the other hand, the vasculogenic microgels containing only gelatin, enriched endothelial phenotype and enabled vascular networks formation when embedded in 3D matrices. Combining the two types of microgels created a hybrid construct that sustained the functions of both osteogenic and vasculogenic microgels and enhanced one another. Using a murine model, we also show that the osteogenic microgels regenerate bone in a critical-sized defect with > 95% defect closure by week 12. These multifunctional microgels can be administered minimally invasively and can conformally fill large bone defects. This work lays the foundation to establish principles of designing multiphasic scaffolds with tissue-specific biophysical and biochemical properties for regenerating vascularized and interfacial tissues.

RHOA inhibits chondrogenic differentiation of mesenchymal stem cells in adolescent idiopathic scoliosis

PURPOSE

The etiology of adolescent idiopathic scoliosis (AIS) remains unclear. The chondrogenic differentiation of mesenchymal stem cells (MSCs) is important in AIS, and the Ras homolog gene family member A (RHOA) is associated with chondrogenesis. The purpose of this study was to explore the effect of RHOA on the chondrogenic differentiation of MSCs in AIS.

METHODS

We isolated MSCs from patients with AIS (AIS MSCs) and individuals without AIS (control MSCs). The inhibitor Y27632 was used to inhibit the function of RHOA/ROCK signaling, and plasmid-based overexpression and siRNA-mediated knockdown were used to manipulate RHOA expression. CCK-8 was used to detect cell viability. The phosphorylation levels of LIMK1, MLC2 and cofilin were detected by Western blotting. The mRNA expression of aggrecan, SOX9, and COL2A1 were confirmed using RT-PCR. Immunofluorescence was used to analyze F-actin and collagen II. Alcian blue staining was performed to assess the secretion of glycosaminoglycans (GAGs).

RESULTS

We found that RHOA was significantly upregulated in AIS MSCs, and the phosphorylation levels of LIMK1, MLC2, and cofilin were increased. The mRNA expressions of aggrecan, SOX9, and COL2A1 were notably reduced in AIS MSCs. However, these effects were abolished by Y27632 treatment and RHOA knockdown in AIS MSCs. In addition, RHOA knockdown in AIS MSCs increased the content of collagen II and GAGs. RHOA overexpression in the control MSCs markedly activated the RHOA/ROCK signaling and decreased the expression of aggrecan, SOX9, and COL2A1, F-actin, and GAGs.

CONCLUSION

RHOA regulates the chondrogenic differentiation ability of MSCs in AIS via the RHOA/ROCK signaling pathway and this regulation may involve SOX9.

Low-intensity pulsed ultrasound/nanomechanical force generators enhance osteogenesis of BMSCs through microfilaments and TRPM7

BACKGROUND

Low-intensity pulsed ultrasound (LIPUS) has been reported to accelerate fracture healing, but the mechanism is unclear and its efficacy needs to be further optimized. Ultrasound in combination with functionalized microbubbles has been shown to induce local shear forces and controllable mechanical stress in cells, amplifying the mechanical effects of LIPUS. Nanoscale lipid bubbles (nanobubbles) have high stability and good biosafety. However, the effect of LIPUS combined with functionalized nanobubbles on osteogenesis has rarely been studied.

RESULTS

In this study, we report cyclic arginine-glycine-aspartic acid-modified nanobubbles (cRGD-NBs), with a particle size of ~ 500 nm, able to actively target bone marrow mesenchymal stem cells (BMSCs) via integrin receptors. cRGD-NBs can act as nanomechanical force generators on the cell membrane, and further enhance the BMSCs osteogenesis and bone formation promoted by LIPUS. The polymerization of actin microfilaments and the mechanosensitive transient receptor potential melastatin 7 (TRPM7) ion channel play important roles in BMSCs osteogenesis promoted by LIPUS/cRGD-NBs. Moreover, the mutual regulation of TRPM7 and actin microfilaments promote the effect of LIPUS/cRGD-NBs. The extracellular Ca^2 + influx, controlled partly by TRPM7, could participated in the effect of LIPUS/cRGD-NBs on BMSCs.

CONCLUSIONS

The nanomechanical force generators cRGD-NBs could promote osteogenesis of BMSCs and bone formation induced by LIPUS, through regulation TRPM7, actin cytoskeleton, and intracellular calcium oscillations. This study provides new directions for optimizing the efficacy of LIPUS for fracture healing, and a theoretical basis for the further application and development of LIPUS in clinical practice.

TiO2 Nanotubes Promote Osteogenic Differentiation Through Regulation of Yap and Piezo1

Surface modification of titanium has been a hot topic to promote bone integration between implants and bone tissue. Titanium dioxide nanotubes fabricated on the surface of titanium by anodic oxidation have been a mature scheme that has shown to promote osteogenesis in vitro. However, mechanisms behind such a phenomenon remain elusive. In this study, we verified the enhanced osteogenesis of BMSCs on nanotopographic titanium in vitro and proved its effect in vivo by constructing a bone defect model in rats. In addition, the role of the mechanosensitive molecule Yap is studied in this research by the application of the Yap inhibitor verteporfin and knockdown/overexpression of Yap in MC3T3-E1 cells. Piezo1 is a mechanosensitive ion channel discovered in recent years and found to be elemental in bone metabolism. In our study, we preliminarily figured out the regulatory relationship between Yap and Piezo1 and proved Piezo1 as a downstream effector of Yap and nanotube-stimulated osteogenesis. In conclusion, this research proved that nanotopography promoted osteogenesis by increasing nuclear localization of Yap and activating the expression of Piezo1 downstream.

Signaling network regulating osteogenesis in mesenchymal stem cells

Osteogenesis is an important developmental event that results in bone formation. Bone forming cells or osteoblasts develop from mesenchymal stem cells (MSCs) through a highly controlled process regulated by several signaling pathways. The osteogenic lineage commitment of MSCs is controlled by cell-cell interactions, paracrine factors, mechanical signals, hormones, and cytokines present in their niche, which activate a plethora of signaling molecules belonging to bone morphogenetic proteins, Wnt, Hedgehog, and Notch signaling. These signaling pathways individually as well as in coordination with other signaling molecules, regulate the osteogenic lineage commitment of MSCs by activating several osteo-lineage specific transcription factors. Here, we discuss the key signaling pathways that regulate osteogenic differentiation of MSCs and the cross-talk between them during osteogenic differentiation. We also discuss how these signaling pathways can be modified for therapy for bone repair and regeneration.

Effects of Mechanical Stress Stimulation on Function and Expression Mechanism of Osteoblasts

Osteoclasts and osteoblasts play a major role in bone tissue homeostasis. The homeostasis and integrity of bone tissue are maintained by ensuring a balance between osteoclastic and osteogenic activities. The remodeling of bone tissue is a continuous ongoing process. Osteoclasts mainly play a role in bone resorption, whereas osteoblasts are mainly involved in bone remodeling processes, such as bone cell formation, mineralization, and secretion. These cell types balance and restrict each other to maintain bone tissue metabolism. Bone tissue is very sensitive to mechanical stress stimulation. Unloading and loading of mechanical stress are closely related to the differentiation and formation of osteoclasts and bone resorption function as well as the differentiation and formation of osteoblasts and bone formation function. Consequently, mechanical stress exerts an important influence on the bone microenvironment and bone metabolism. This review focuses on the effects of different forms of mechanical stress stimulation (including gravity, continuously compressive pressure, tensile strain, and fluid shear stress) on osteoclast and osteoblast function and expression mechanism. This article highlights the involvement of osteoclasts and osteoblasts in activating different mechanical transduction pathways and reports changings in their differentiation, formation, and functional mechanism induced by the application of different types of mechanical stress to bone tissue. This review could provide new ideas for further microscopic studies of bone health, disease, and tissue damage reconstruction.

Short-Duration High Frequency MegaHertz-Order Nanomechanostimulation Drives Early and Persistent Osteogenic Differentiation in Mesenchymal Stem Cells

Stem cell fate can be directed through the application of various external physical stimuli, enabling a controlled approach to targeted differentiation. Studies involving the use of dynamic mechanical cues driven by vibrational excitation to date have, however, been limited to low frequency (Hz to kHz) forcing over extended durations (typically continuous treatment for >7 days). Contrary to previous assertions that there is little benefit in applying frequencies beyond 1 kHz, we show here that high frequency MHz-order mechanostimulation in the form of nanoscale amplitude surface reflected bulk waves are capable of triggering differentiation of human mesenchymal stem cells from various donor sources toward an osteoblast lineage, with early, short time stimuli inducing long-term osteogenic commitment. More specifically, rapid treatments (10 min daily over 5 days) of the high frequency (10 MHz) mechanostimulation are shown to trigger significant upregulation in early osteogenic markers (RUNX2, COL1A1) and sustained increase in late markers (osteocalcin, osteopontin) through a mechanistic pathway involving piezo channel activation and Rho-associated protein kinase signaling. Given the miniaturizability and low cost of the devices, the possibility for upscaling the platform toward practical bioreactors, to address a pressing need for more efficient stem cell differentiation technologies in the pursuit of translatable regenerative medicine strategies, is ensivaged.

Stereotactic Body Radiotherapy and Conventional Radiotherapy Induce Cytoskeleton Extension and Enlargement of Cell Morphology in Non-Small Cell Lung Cancer

Stereotactic body radiotherapy (SBRT) is now widely used in cancer therapy. However, the biological effects of SBRT compared with conventional radiotherapy (CRT) are not clear. The cytoskeleton plays an important role in many biological processes and cellular life activities. The effects of SBRT or CRT on the morphology and cytoskeletal structure of non-small cell lung cancer (NSCLC) cells remain unknown. Based on the biologically equivalent dose (BED) formula, we designed SBRT and CRT fractionation regimens with the same BED. The morphology was captured during radiation, and rhodamine-phalloidin immunofluorescence was used to study the cytoskeleton. A lactate dehydrogenase assay kit was used to determine the cell membrane permeability, and western blot was used to detect the cytoskeleton protein expression levels. The morphology and cytoskeleton expanded after SBRT or CRT, with an increase in cell membrane permeability and stable cytoskeleton protein levels. Besides, different dose of SBRT (10,20,30 Gy) induce similar morphology and cytoskeleton enlargement. Our findings indicate that SBRT and CRT can induce cytoskeleton reorganization and the enlargement of cell morphology (at different rates) in NSCLC. The morphology and cytoskeleton enlargement after SBRT are dose independence.

Inhibition of RhoA/MRTF-A signaling alleviates nucleus pulposus fibrosis induced by mechanical stress overload

PURPOSE/AIM

: Intervertebral disc degeneration (IDD) is the leading cause of lower back pain, and clinically useful drugs for IDD are unavailable. Mechanical stress overload-induced fibrosis plays a critical role in IDD. RhoA/MRTF-A signaling is known to regulate tissue fibrosis; however, the effect of RhoA/MRTF-A on the development of IDD is unclear.

MATERIALS AND METHODS

: The expression of aggrecan, collagen I, collagen II, MMP-12, CTGF, and MRTF-A in nucleus pulposus (NP) samples from IDD patients and controls was detected by immunohistochemical staining. Primary nucleus pulposus cells (NPCs) were isolated and cultured to establish an overload strain model treated with or without CCG-1423. The protein levels of RhoA, ROCK2, MRTF-A, CTGF, and MMP-12 as well as fibrosis-associated proteins were detected by western blotting and immunofluorescence.

RESULTS

: Collagen I, MMP-12, and CTGF were significantly upregulated, and aggrecan and collagen II were significantly downregulated in the IDD samples. The cellular localization of MRTF-A was associated with intervertebral disc (IVD) degeneration. Overloaded strain enhanced the nuclear translocation of MRTF-A and changed the NPC morphology from spindle-shaped to long strips. Additional experiments showed that RhoA, ROCK2, MRTF-A, SRF, MMP-12, and CTGF were upregulated; however, aggrecan and collagen II were downregulated in NPCs under overload strain. CCG-1423, a RhoA/MRTF-A pathway inhibitor, reversed strain-induced fibrosis.

CONCLUSION

: Mechanical stress activates RhoA/MRTF-A signaling to promote extracellular matrix (ECM) degeneration in the NP, which is associated with the development of IDD. Our findings suggest that the RhoA/MRTF-A inhibitor CCG-1423 can alleviate NPC degeneration caused by overload stress and has potential as a therapeutic agent for IDD.

Insight Into Rho Kinase Isoforms in Obesity and Energy Homeostasis

Obesity and associated complications increasingly jeopardize global health and contribute to the rapidly rising prevalence of type 2 diabetes mellitus and obesity-related diseases. Developing novel methods for the prevention and treatment of excess body adipose tissue expansion can make a significant contribution to public health. Rho kinase is a Rho-associated coiled-coil-containing protein kinase (Rho kinase or ROCK). The ROCK family including ROCK1 and ROCK2 has recently emerged as a potential therapeutic target for the treatment of metabolic disorders. Up-regulated ROCK activity has been involved in the pathogenesis of all aspects of metabolic syndrome including obesity, insulin resistance, dyslipidemia and hypertension. The RhoA/ROCK-mediated actin cytoskeleton dynamics have been implicated in both white and beige adipogenesis. Studies using ROCK pan-inhibitors in animal models of obesity, diabetes, and associated complications have demonstrated beneficial outcomes. Studies via genetically modified animal models further established isoform-specific roles of ROCK in the pathogenesis of metabolic disorders including obesity. However, most reported studies have been focused on ROCK1 activity during the past decade. Due to the progress in developing ROCK2-selective inhibitors in recent years, a growing body of evidence indicates more attention should be devoted towards understanding ROCK2 isoform function in metabolism. Hence, studying individual ROCK isoforms to reveal their specific roles and principal mechanisms in white and beige adipogenesis, insulin sensitivity, energy balancing regulation, and obesity development will facilitate significant breakthroughs for systemic treatment with isoform-selective inhibitors. In this review, we give an overview of ROCK functions in the pathogenesis of obesity and insulin resistance with a particular focus on the current understanding of ROCK isoform signaling in white and beige adipogenesis, obesity and thermogenesis in adipose tissue and other major metabolic organs involved in energy homeostasis regulation.

Arhgap21 Deficiency Results in Increase of Osteoblastic Lineage Cells in the Murine Bone Marrow Microenvironment

ARHGAP21 is a member of the RhoGAP family of proteins involved in cell growth, differentiation, and adhesion. We have previously shown that the heterozygous Arhgap21 knockout mouse model (Arhgap21^+/-) presents several alterations in the hematopoietic compartment, including increased frequency of hematopoietic stem and progenitor cells (HSPC) with impaired adhesion in vitro, increased mobilization to peripheral blood, and decreased engraftment after bone marrow transplantation. Although these HSPC functions strongly depend on their interactions with the components of the bone marrow (BM) niche, the role of ARHGAP21 in the marrow microenvironment has not yet been explored. In this study, we investigated the composition and function of the BM microenvironment in Arhgap21^+/- mice. The BM of Arhgap21^+/- mice presented a significant increase in the frequency of phenotypic osteoblastic lineage cells, with no differences in the frequencies of multipotent stromal cells or endothelial cells when compared to the BM of wild type mice. Arhgap21^+/- BM cells had increased capacity of generating osteogenic colony-forming units (CFU-OB) in vitro and higher levels of osteocalcin were detected in the Arhgap21^+/- BM supernatant. Increased expression of Col1a1, Ocn and decreased expression of Trap1 were observed after osteogenic differentiation of Arhgap21^+/- BM cells. In addition, Arhgap21^+/- mice recipients of normal BM cells showed decreased leucocyte numbers during transplantation recovery. Our data suggest participation of ARHGAP21 in the balanced composition of the BM microenvironment through the regulation of osteogenic differentiation.

Role of Rho GTPases in stem cell regulation

The future of regenerative medicine relies on our understanding of stem cells which are essential for tissue/organ generation and regeneration to maintain and/or restore tissue homeostasis. Rho family GTPases are known regulators of a wide variety of cellular processes related to cytoskeletal dynamics, polarity and gene transcription. In the last decade, major new advances have been made in understanding the regulatory role and mechanism of Rho GTPases in self-renewal, differentiation, migration, and lineage specification in tissue-specific signaling mechanisms in various stem cell types to regulate embryonic development, adult tissue homeostasis, and tissue regeneration upon stress or damage. Importantly, implication of Rho GTPases and their upstream regulators or downstream effectors in the transformation, migration, invasion and tumorigenesis of diverse cancer stem cells highlights the potential of Rho GTPase targeting in cancer therapy. In this review, we discuss recent evidence of Rho GTPase signaling in the regulation of embryonic stem cells, multiple somatic stem cells, and cancer stem cells. We propose promising areas where Rho GTPase pathways may serve as useful targets for stem cell manipulation and related future therapies.

Biophysical Stimuli as the Fourth Pillar of Bone Tissue Engineering

The repair of critical bone defects remains challenging worldwide. Three canonical pillars (biomaterial scaffolds, bioactive molecules, and stem cells) of bone tissue engineering have been widely used for bone regeneration in separate or combined strategies, but the delivery of bioactive molecules has several obvious drawbacks. Biophysical stimuli have great potential to become the fourth pillar of bone tissue engineering, which can be categorized into three groups depending on their physical properties: internal structural stimuli, external mechanical stimuli, and electromagnetic stimuli. In this review, distinctive biophysical stimuli coupled with their osteoinductive windows or parameters are initially presented to induce the osteogenesis of mesenchymal stem cells (MSCs). Then, osteoinductive mechanisms of biophysical transduction (a combination of mechanotransduction and electrocoupling) are reviewed to direct the osteogenic differentiation of MSCs. These mechanisms include biophysical sensing, transmission, and regulation. Furthermore, distinctive application strategies of biophysical stimuli are presented for bone tissue engineering, including predesigned biomaterials, tissue-engineered bone grafts, and postoperative biophysical stimuli loading strategies. Finally, ongoing challenges and future perspectives are discussed.

Intrinsic Mechanical Cues and Their Impact on Stem Cells and Embryogenesis

Although understanding how soluble cues direct cellular processes revolutionised the study of cell biology in the second half of the 20th century, over the last two decades, new insights into how mechanical cues similarly impact cell fate decisions has gained momentum. During development, extrinsic cues such as fluid flow, shear stress and compressive forces are essential for normal embryogenesis to proceed. Indeed, both adult and embryonic stem cells can respond to applied forces, but they can also detect intrinsic mechanical cues from their surrounding environment, such as the stiffness of the extracellular matrix, which impacts differentiation and morphogenesis. Cells can detect changes in their mechanical environment using cell surface receptors such as integrins and focal adhesions. Moreover, dynamic rearrangements of the cytoskeleton have been identified as a key means by which forces are transmitted from the extracellular matrix to the cell and vice versa. Although we have some understanding of the downstream mechanisms whereby mechanical cues are translated into changes in cell behaviour, many of the signalling pathways remain to be defined. This review discusses the importance of intrinsic mechanical cues on adult cell fate decisions, the emerging roles of cell surface mechano-sensors and the cytoskeleton in enabling cells to sense its microenvironment, and the role of intracellular signalling in translating mechanical cues into transcriptional outputs. In addition, the contribution of mechanical cues to fundamental processes during embryogenesis such as apical constriction and convergent extension is discussed. The continued development of tools to measure the biomechanical properties of soft tissues in vivo is likely to uncover currently underestimated contributions of these cues to adult stem cell fate decisions and embryogenesis, and may inform on regenerative strategies for tissue repair.

Phactr1 negatively regulates bone mass by inhibiting osteogenesis and promoting adipogenesis of BMSCs via RhoA/ROCK2

The imbalance between osteogenic and adipogenic differentiation of Bone marrow-derived mesenchymal stem cells (BMSCs) is involved in the occurrence and development of osteoporosis (OP). Previous studies have indicated the potential of phosphatase and actin regulator 1 (Phactr1) in regulating osteogenic and adipogenic differentiation of BMSCs. The present study aims to investigate the function and mechanism of Phactr1 in regulating osteogenic and adipogenic differentiation of BMSCs. Herein, the expression of Phactr1 in bone and adipose tissue of OP rats was determined by immunohistochemical. BMSCs were subjected to osteogenic and adipogenic differentiation, and transfected with Phactr1 overexpression lentivirus, small interference RNA (siRNA) and KD025 (selective ROCK2 inhibitor). The relationship between Phactr1 and ROCK2 was detected by Co-IP experiment. The expression of Phactr1, Runx2, C/EBPα, RhoA and ROCK2 was detected by Western blot. Calcium nodule and lipid droplets were determined by alizarin red and Oil red O staining. Interestingly, Phactr1 increased in both bone and adipose tissue of OP rats. During osteogenic differentiation, Phactr1 decreased and active RhoA, ROCK2 increased, while overexpression Phactr1 inhibits the increase of Runx2. Phactr1 increased and active RhoA decreased, ROCK2 did not changed during adipogenic differentiation. While, Knockdown Phactr1 inhibits the increase of C/EBPα. Phactr1 and ROCK2 were combined in osteogenic differentiation, but not in adipogenic differentiation. By using KD025, the decrease of Phactr1 and increase of Runx2 were inhibited respectively in osteogenic differentiation. Meanwhile, when ROCK2 was inhibited, Phactr1, C/EBPα were significantly increased in adipogenic differentiation. These findings indicated that Phactr1 negatively regulates bone mass by inhibiting osteogenesis and promoting adipogenesis of BMSCs by activating RhoA/ROCK2.

A multiethnic whole genome sequencing study to identify novel loci for bone mineral density

At present, there have only been a few DNA sequencing-based studies to explore the genetic determinants of bone mineral density (BMD). We carried out the largest whole genome sequencing analysis to date for femoral neck and spine BMD (n = 4981), with one of the highest average sequencing depths implemented thus far at 22×, in a multiethnic sample (58% Caucasian and 42% African American) from the Louisiana Osteoporosis Study (LOS). The LOS samples were combined with summary statistics from the GEFOS consortium and several independent samples of various ethnicities to perform GWAS meta-analysis (n = 44 506). We identified 31 and 30 genomic risk loci for femoral neck and spine BMD, respectively. The findings substantiate many previously reported susceptibility loci (e.g. WNT16 and ESR1) and reveal several others that are either novel or have not been widely replicated in GWAS for BMD, including two for femoral neck (IGF2 and ZNF423) and one for spine (SIPA1). Although we were not able to uncover ethnicity specific differences in the genetic determinants of BMD, we did identify several loci which demonstrated sex-specific associations, including two for women (PDE4D and PIGN) and three for men (TRAF3IP2, NFIB and LYSMD4). Gene-based rare variant association testing detected MAML2, a regulator of the Notch signaling pathway, which has not previously been suggested, for association with spine BMD. The findings provide novel insights into the pathophysiological mechanisms of osteoporosis.

Regulation of the integrin αVβ3- actin filaments axis in early osteogenesis of human fibroblasts under cyclic tensile stress

Background

Integrins play a prominent role in osteogenic differentiation by transmitting both mechanical and chemical signals. Integrin expression is closely associated with tensile stress, which has a positive effect on osteogenic differentiation. We investigated the relationship between integrin αVβ3 and tensile stress.

Methods

Human fibroblasts were treated with c (RGDyk) and lentivirus transduction to inhibit function of integrin αVβ3. Y-15, cytochalasin D and verteporfin were used to inhibit phosphorylation of FAK, polymerization of microfilament and function of nuclear YAP, respectively. Fibroblasts were exposed to a cyclic tensile stress of 10% at 0.5 Hz, once a day for 2 h each application. Fibroblasts were harvested on day 4 and 7 post-treatment. The expression of ALP, RUNX2, integrin αVβ3, β-actin, talin-1, FAK, vinculin, and nuclear YAP was detected by Western blot or qRT-PCR. The expression and distribution of integrin αVβ3, vinculin, microfilament and nuclear YAP.

Results

Cyclic tensile stress was found to promote expression of ALP and RUNX2. Inhibition of integrin αVβ3 activation downregulated the rearrangement of microfilament and the expression of ALP, RUNX2 and nuclear YAP. When the polymerization of microfilament was inhibited the expression of ALP, RUNX2 and nuclear YAP were decreased. The phosphorylation of FAK induced by cyclic tensile stress reduced by the inhibition of integrin αVβ3. The expression of ALP and RUNX2 was decreased by inhibition of phosphorylation of FAK and inhibition of nuclear YAP.

Conclusions

Cyclic tensile stress promotes osteogenesis of human fibroblasts via integrin αVβ3-microfilament axis. Phosphorylation of FAK and nuclear YAP participates in this process.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02597-y.

At the nuclear envelope of bone mechanobiology

The nuclear envelope and nucleoskeleton are emerging as signaling centers that regulate how physical information from the extracellular matrix is biochemically transduced into the nucleus, affecting chromatin and controlling cell function. Bone is a mechanically driven tissue that relies on physical information to maintain its physiological function and structure. Disorder that present with musculoskeletal and cardiac symptoms, such as Emery-Dreifuss muscular dystrophies and progeria, correlate with mutations in nuclear envelope proteins including Linker of Nucleoskeleton and Cytoskeleton (LINC) complex, Lamin A/C, and emerin. However, the role of nuclear envelope mechanobiology on bone function remains underexplored. The mesenchymal stem cell (MSC) model is perhaps the most studied relationship between bone regulation and nuclear envelope function. MSCs maintain the musculoskeletal system by differentiating into multiple cell types including osteocytes and adipocytes, thus supporting the bone's ability to respond to mechanical challenge. In this review, we will focus on how MSC function is regulated by mechanical challenges both in vitro and in vivo within the context of bone function specifically focusing on integrin, β-catenin and YAP/TAZ signaling. The importance of the nuclear envelope will be explored within the context of musculoskeletal diseases related to nuclear envelope protein mutations and nuclear envelope regulation of signaling pathways relevant to bone mechanobiology in vitro and in vivo.

Dynamic Crosstalk between Vascular Smooth Muscle Cells and the Aged Extracellular Matrix

Vascular aging is accompanied by the fragmentation of elastic fibers and collagen deposition, leading to reduced distensibility and increased vascular stiffness. A rigid artery facilitates elastin to degradation by MMPs, exposing vascular cells to greater mechanical stress and triggering signaling mechanisms that only exacerbate aging, creating a self-sustaining inflammatory environment that also promotes vascular calcification. In this review, we highlight the role of crosstalk between smooth muscle cells and the vascular extracellular matrix (ECM) and how aging promotes smooth muscle cell phenotypes that ultimately lead to mechanical impairment of aging arteries. Understanding the underlying mechanisms and the role of associated changes in ECM during aging may contribute to new approaches to prevent or delay arterial aging and the onset of cardiovascular diseases.

Integrins in the Regulation of Mesenchymal Stem Cell Differentiation by Mechanical Signals

Mesenchymal stem cells (MSCs) can sense and convert mechanical stimuli signals into a chemical response. Integrins are involved in the mechanotransduction from inside to outside and from outside to inside, and ultimately affect the fate of MSCs responding to different mechanical signals. Different integrins participate in different signaling pathways to regulate MSCs multi-differentiation. In this review, we summarize the latest advances in the effects of mechanical signals on the differentiation of MSCs, the importance of integrins in mechanotransduction, the relationship between integrin heterodimers and different mechanical signals, and the interaction among mechanical signals. We put forward our views on the prospect and challenges of developing mechanical biology in tissue engineering and regenerative medicine.

Effect of Polymeric Matrix Stiffness on Osteogenic Differentiation of Mesenchymal Stem/Progenitor Cells: Concise Review

Mesenchymal stem/progenitor cells (MSCs) have a multi-differentiation potential into specialized cell types, with remarkable regenerative and therapeutic results. Several factors could trigger the differentiation of MSCs into specific lineages, among them the biophysical and chemical characteristics of the extracellular matrix (ECM), including its stiffness, composition, topography, and mechanical properties. MSCs can sense and assess the stiffness of extracellular substrates through the process of mechanotransduction. Through this process, the extracellular matrix can govern and direct MSCs' lineage commitment through complex intracellular pathways. Hence, various biomimetic natural and synthetic polymeric matrices of tunable stiffness were developed and further investigated to mimic the MSCs' native tissues. Customizing scaffold materials to mimic cells' natural environment is of utmost importance during the process of tissue engineering. This review aims to highlight the regulatory role of matrix stiffness in directing the osteogenic differentiation of MSCs, addressing how MSCs sense and respond to their ECM, in addition to listing different polymeric biomaterials and methods used to alter their stiffness to dictate MSCs' differentiation towards the osteogenic lineage.

The cytoskeleton and connected elements in bone cell mechano-transduction

Bone is a mechano-responsive tissue that adapts to changes in its mechanical environment. Increases in strain lead to increased bone mass acquisition, whereas decreases in strain lead to a loss of bone mass. Given that mechanical stress is a regulator of bone mass and quality, it is important to understand how bone cells sense and transduce these mechanical cues into biological changes to identify druggable targets that can be exploited to restore bone cell mechano-sensitivity or to mimic mechanical load. Many studies have identified individual cytoskeletal components - microtubules, actin, and intermediate filaments - as mechano-sensors in bone. However, given the high interconnectedness and interaction between individual cytoskeletal components, and that they can assemble into multiple discreet cellular structures, it is likely that the cytoskeleton as a whole, rather than one specific component, is necessary for proper bone cell mechano-transduction. This review will examine the role of each cytoskeletal element in bone cell mechano-transduction and will present a unified view of how these elements interact and work together to create a mechano-sensor that is necessary to control bone formation following mechanical stress.

The Paracrine Function of Mesenchymal Stem Cells in Response to Pulsed Focused Ultrasound

We studied the paracrine function of mesenchymal stem cells (MSCs) derived from various sources in response to pulsed focused ultrasound (pFUS). Human adipose tissue (AD), bone marrow (BM), and umbilical cord (UC) derived MSCs were exposed to pFUS at two intensities: 0.45 W/cm² I_SATA (310 kPa PNP) and 1.3 W/cm² I_SATA (540 kPa PNP). Following pFUS, the viability and proliferation of MSCs were assessed using a hemocytometer and confocal microscopy, and their secreted cytokine profile determined using a multiplex ELISA. Our findings showed that pFUS can stimulate the production of immunomodulatory, anti-inflammatory, and angiogenic cytokines from MSCs which was dependent on both the source of MSC being studied and the acoustic intensity employed. These important findings set the foundation for additional mechanistic and validation studies using this novel noninvasive and clinically translatable technology for modulating MSC biology.

Uniaxial Cyclic Stretching Promotes Chromatin Accessibility of Gene Loci Associated With Mesenchymal Stem Cells Morphogenesis and Osteogenesis

It has been previously demonstrated that uniaxial cyclic stretching (UCS) induces differentiation of mesenchymal stem cells (MSCs) into osteoblasts in vitro. It is also known that interactions between cells and external forces occur at various aspects including cell–matrix, cytoskeleton, nucleus membrane, and chromatin. However, changes in chromatin landscape during this process are still not clear. The present study was aimed to determine changes of chromatin accessibility under cyclic stretch. The influence of cyclic stretching on the morphology, proliferation, and differentiation of hMSCs was characterized. Changes of open chromatin sites were determined by assay for transposase accessible chromatin with high-throughput sequencing (ATAC-seq). Our results showed that UCS induced cell reorientation and actin stress fibers realignment, and in turn caused nuclear reorientation and deformation. Compared with unstrained group, the expression of osteogenic and chondrogenic marker genes were the highest in group of 1 Hz + 8% strain; this condition also led to lower cell proliferation rate. Furthermore, there were 2022 gene loci with upregulated chromatin accessibility in 1 Hz + 8% groups based on the analysis of chromatin accessibility. These genes are associated with regulation of cell morphogenesis, cell–substrate adhesion, and ossification. Signaling pathways involved in osteogenic differentiation were found in up-regulated GO biological processes. These findings demonstrated that UCS increased the openness of gene loci associated with regulation of cell morphogenesis and osteogenesis as well as the corresponding transcription activities. Moreover, the findings also connect the changes in chromatin accessibility with cell reorientation, nuclear reorientation, and deformation. Our study may provide reference for directed differentiation of stem cells induced by mechanical microenvironments.

The effects of locomotion on bone marrow mesenchymal stem cell fate: insight into mechanical regulation and bone formation

Bone marrow mesenchymal stem cells (BMSCs) refer to a heterogeneous population of cells with the capacity for self-renewal. BMSCs have multi-directional differentiation potential and can differentiate into chondrocytes, osteoblasts, and adipocytes under specific microenvironment or mechanical regulation. The activities of BMSCs are closely related to bone quality. Previous studies have shown that BMSCs and their lineage-differentiated progeny (for example, osteoblasts), and osteocytes are mechanosensitive in bone. Thus, a goal of this review is to discuss how these ubiquious signals arising from mechanical stimulation are perceived by BMSCs and then how the cells respond to them. Studies in recent years reported a significant effect of locomotion on the migration, proliferation and differentiation of BMSCs, thus, contributing to our bone mass. This regulation is realized by the various intersecting signaling pathways including RhoA/Rock, IFG, BMP and Wnt signalling. The mechanoresponse of BMSCs also provides guidance for maintaining bone health by taking appropriate exercises. This review will summarize the regulatory effects of locomotion/mechanical loading on BMSCs activities. Besides, a number of signalling pathways govern MSC fate towards osteogenic or adipocytic differentiation will be discussed. The understanding of mechanoresponse of BMSCs makes the foundation for translational medicine.