Primary cilia-mediated mechanotransduction in human mesenchymal stem cells

Distraction force promotes the osteogenic differentiation of Gli1+ cells in facial sutures via primary cilia-mediated Hedgehog signaling pathway

BACKGROUND

Trans-sutural distraction osteogenesis (TSDO) involves the application of distraction force to facial sutures to stimulate osteogenesis. Gli1+ cells in the cranial sutures play an important role in bone growth. However, whether Gli1+ cells in facial sutures differentiate into bone under distraction force is unknown.

METHODS

4-week-old Gli1ER/Td and C57BL/6 mice were used to establish a TSDO model to explore osteogenesis of zygomaticomaxillary sutures. A Gli1+ cell lineage tracing model was used to observe the distribution of Gli1+ cells and explore the role of Gli1+ cells in facial bone remodeling.

RESULTS

Distraction force promoted bone remodeling during TSDO. Fluorescence and two-photon scanning images revealed the distribution of Gli1+ cells. Under distraction force, Gli1-lineage cells proliferated significantly and co-localized with Runx2+ cells. Hedgehog signaling was upregulated in Gli1+ cells. Inhibition of Hedgehog signaling suppresses the proliferation and osteogenesis of Gli1+ cells induced by distraction force. Subsequently, the stem cell characteristics of Gli1+ cells were identified. Cell-stretching experiments verified that mechanical force promoted the osteogenic differentiation of Gli1+ cells through Hh signaling. Furthermore, immunofluorescence staining and RT-qPCR experiments demonstrated that the primary cilia in Gli1+ cells exhibit Hedgehog-independent mechanosensitivity, which was required for the osteogenic differentiation induced by mechanical force.

CONCLUSIONS

Our study indicates that the primary cilia of Gli1+ cells sense mechanical stimuli, mediate Hedgehog signaling activation, and promote the osteogenic differentiation of Gli1+ cells in zygomaticomaxillary sutures.

IFT80 promotes early bone healing of tooth sockets through the activation of TAZ/RUNX2 pathway

Intraflagellar transport (IFT) proteins have been reported to regulate cell growth and differentiation as the essential functional component of primary cilia. The effects of IFT80 on early bone healing of extraction sockets have not been well studied. To investigate whether deletion of Ift80 in alveolar bone-derived mesenchymal stem cells (aBMSCs) affected socket bone healing, we generated a mouse model of specific knockout of Ift80 in Prx1 mesenchymal lineage cells (Prx1Cre ;IFT80f/f ). Our results demonstrated that deletion of IFT80 in Prx1 lineage cells decreased the trabecular bone volume, ALP-positive osteoblastic activity, TRAP-positive osteoclastic activity, and OSX-/COL I-/OCN-positive areas in tooth extraction sockets of Prx1Cre ; IFT80f/f mice compared with IFT80f/f littermates. Furthermore, aBMSCs from Prx1Cre ; IFT80f/f mice showed significantly decreased osteogenic markers and downregulated migration and proliferation capacity. Importantly, the overexpression of TAZ recovered significantly the expressions of osteogenic markers and migration capacity of aBMSCs. Lastly, the local administration of lentivirus for TAZ enhanced the expression of RUNX2 and OSX and promoted early bone healing of extraction sockets from Prx1Cre ; IFT80f/f mice. Thus, IFT80 promotes osteogenesis and early bone healing of tooth sockets through the activation of TAZ/RUNX2 pathway.

Osteocytes and Primary Cilia

PURPOSE OF REVIEW

The purpose of this review is to provide a background on osteocytes and the primary cilium, discussing the role it plays in osteocyte mechanosensing.

RECENT FINDINGS

Osteocytes are thought to be the primary mechanosensing cells in bone tissue, regulating bone adaptation in response to exercise, with the primary cilium suggested to be a key mechanosensing mechanism in bone. More recent work has suggested that, rather than being direct mechanosensors themselves, primary cilia in bone may instead form a key chemo-signalling nexus for processing mechanoregulated signalling pathways. Recent evidence suggests that pharmacologically induced lengthening of the primary cilium in osteocytes may potentiate greater mechanotransduction, rather than greater mechanosensing. While more research is required to delineate the specific osteocyte mechanobiological molecular mechanisms governed by the primary cilium, it is clear from the literature that the primary cilium has significant potential as a therapeutic target to treat mechanoregulated bone diseases, such as osteoporosis.

Towards modular engineering of cell signalling: Topographically-textured microparticles induce osteogenesis via activation of canonical hedgehog signalling

Polymer microparticles possess great potential as functional building blocks for advanced bottom-up engineering of complex tissues. Tailoring the three-dimensional architectural features of culture substrates has been shown to induce osteogenesis in mesenchymal stem cells in vitro, but the molecular mechanisms underpinning this remain unclear. This study proposes a mechanism linking the activation of Hedgehog signalling to the osteoinductive effect of surface-engineered, topographically-textured polymeric microparticles. In this study, mesenchymal progenitor C3H10T1/2 cells were cultured on smooth and dimpled poly(D,l-lactide) microparticles to assess differences in viability, cellular morphology, proliferation, and expression of a range of Hedgehog signalling components and osteogenesis-related genes. Dimpled microparticles induced osteogenesis and activated the Hedgehog signalling pathway relative to smooth microparticles and 2D-cultured controls without the addition of exogenous biochemical factors. We observed upregulation of the osteogenesis markers Runt-related transcription factor2 (Runx2) and bone gamma-carboxyglutamate protein 2 (Bglap2), as well as the Hedgehog signalling components, glioma associated oncogene homolog 1 (Gli1), Patched1 (Ptch1), and Smoothened (Smo). Treatment with the Smo antagonist KAAD-cyclopamine confirmed the involvement of Smo in Gli1 target gene activation, with a significant reduction in the expression of Gli1, Runx2 and Bglap2 (p ≤ 0.001) following KAAD-cyclopamine treatment. Overall, our study demonstrates the role of the topographical microenvironment in the modulation of Hedgehog signalling, highlighting the potential for tailoring substrate topographical design to offer cell-instructive 3D microenvironments. Topographically-textured microparticles allow the modulation of Hedgehog signalling in vitro without adding exogenous biochemical agonists, thereby eliminating potential confounding artefacts in high-throughput drug screening applications.

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.

Primary cilia in skeletal development and disease

Primary cilia are non-motile, microtubule-based sensory organelle present in most vertebrate cells with a fundamental role in the modulation of organismal development, morphogenesis, and repair. Here we focus on the role of primary cilia in embryonic and postnatal skeletal development. We examine evidence supporting its involvement in physiochemical and developmental signaling that regulates proliferation, patterning, differentiation and homeostasis of osteoblasts, chondrocytes, and their progenitor cells in the skeleton. We discuss how signaling effectors in mechanotransduction and bone development, such as Hedgehog, Wnt, Fibroblast growth factor and second messenger pathways operate at least in part at the primary cilium. The relevance of primary cilia in bone formation and maintenance is underscored by a growing list of rare genetic skeletal ciliopathies. We collate these findings and summarize the current understanding of molecular factors and mechanisms governing primary ciliogenesis and ciliary function in skeletal development and disease.

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.

Primary cilium-mediated mechanotransduction in cartilage chondrocytes

Osteoarthritis (OA) is one of the most prevalent joint disorders associated with the degradation of articular cartilage and an abnormal mechanical microenvironment. Mechanical stimuli, including compression, shear stress, stretching strain, osmotic challenge, and the physical properties of the matrix microenvironment, play pivotal roles in the tissue homeostasis of articular cartilage. The primary cilium, as a mechanosensory and chemosensory organelle, is important for detecting and transmitting both mechanical and biochemical signals in chondrocytes within the matrix microenvironment. Growing evidence indicates that primary cilia are critical for chondrocytes signaling transduction and the matrix homeostasis of articular cartilage. Furthermore, the ability of primary cilium to regulate cellular signaling is dynamic and dependent on the cellular matrix microenvironment. In the current review, we aim to elucidate the key mechanisms by which primary cilia mediate chondrocytes sensing and responding to the matrix mechanical microenvironment. This might have potential therapeutic applications in injuries and OA-associated degeneration of articular cartilage.

Pathophysiological mechanism of acute bone loss after fracture

BACKGROUND

Acute bone loss after fracture is associated with various effects on the complete recovery process and a risk of secondary fractures among patients. Studies have reported similarities in pathophysiological mechanisms involved in acute bone loss after fractures and osteoporosis. However, given the silence nature of bone loss and bone metabolism complexities, the actual underlying pathophysiological mechanisms have yet to be fully elucidated.

AIM OF REVIEW

To elaborate the latest findings in basic research with a focus on acute bone loss after fracture. To briefly highlight potential therapeutic targets and current representative drugs. To arouse researchers' attention and discussion on acute bone loss after fracture.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Bone loss after fracture is associated with immobilization, mechanical unloading, blood supply damage, sympathetic nerve regulation, and crosstalk between musculoskeletals among other factors. Current treatment strategies rely on regulation of osteoblasts and osteoclasts, therefore, there is a need to elucidate on the underlying mechanisms of acute bone loss after fractures to inform the development of efficacious and safe drugs. In addition, attention should be paid towards ensuring long-term skeletal health.

Integrating physicomechanical and biological strategies for BTE: biomaterials-induced osteogenic differentiation of MSCs

Large bone defects are a major global health concern. Bone tissue engineering (BTE) is the most promising alternative to avoid the drawbacks of autograft and allograft bone. Nevertheless, how to precisely control stem cell osteogenic differentiation has been a long-standing puzzle. Compared with biochemical cues, physicomechanical stimuli have been widely studied for their biosafety and stability. The mechanical properties of various biomaterials (polymers, bioceramics, metal and alloys) become the main source of physicomechanical stimuli. By altering the stiffness, viscoelasticity, and topography of materials, mechanical stimuli with different strengths transmit into precise signals that mediate osteogenic differentiation. In addition, externally mechanical forces also play a critical role in promoting osteogenesis, such as compression stress, tensile stress, fluid shear stress and vibration, etc. When exposed to mechanical forces, mesenchymal stem cells (MSCs) differentiate into osteogenic lineages by sensing mechanical stimuli through mechanical sensors, including integrin and focal adhesions (FAs), cytoskeleton, primary cilium, ions channels, gap junction, and activating osteogenic-related mechanotransduction pathways, such as yes associated proteins (YAP)/TAZ, MAPK, Rho-GTPases, Wnt/β-catenin, TGFβ superfamily, Notch signaling. This review summarizes various biomaterials that transmit mechanical signals, physicomechanical stimuli that directly regulate MSCs differentiation, and the mechanical transduction mechanisms of MSCs. This review provides a deep and broad understanding of mechanical transduction mechanisms and discusses the challenges that remained in clinical translocation as well as the outlook for the future improvements.

Breathing-induced forces influence lung cell fate

Respiration exerts a mechanical strain on the lungs, which has an unclear effect on epithelial cell fate. Now in Cell, Shiraishi et al.¹ reveal the crucial role of mechanotransduction in maintaining lung epithelial cell fate, representing a significant milestone in understanding how mechanical factors regulate differentiation.

Cranium growth, patterning and homeostasis

Craniofacial development requires precise spatiotemporal regulation of multiple signaling pathways that crosstalk to coordinate the growth and patterning of the skull with surrounding tissues. Recent insights into these signaling pathways and previously uncharacterized progenitor cell populations have refined our understanding of skull patterning, bone mineralization and tissue homeostasis. Here, we touch upon classical studies and recent advances with an emphasis on developmental and signaling mechanisms that regulate the osteoblast lineage for the calvaria, which forms the roof of the skull. We highlight studies that illustrate the roles of osteoprogenitor cells and cranial suture-derived stem cells for proper calvarial growth and homeostasis. We also discuss genes and signaling pathways that control suture patency and highlight how perturbing the molecular regulation of these pathways leads to craniosynostosis. Finally, we discuss the recently discovered tissue and signaling interactions that integrate skull and cerebrovascular development, and the potential implications for both cerebrospinal fluid hydrodynamics and brain waste clearance in craniosynostosis.

The intraflagellar transport protein IFT52 associated with short-rib thoracic dysplasia is essential for ciliary function in osteogenic differentiation in vitro and for sensory perception in Drosophila

Primary cilia are non-motile sensory cell-organelle that are essential for organismal development, differentiation, and postnatal homeostasis. Their biogenesis and function are mediated by the intraflagellar transport (IFT) system. Pathogenic variants in IFT52, a central component of the IFT-B complex is associated with short-rib thoracic dysplasia with or without polydactyly 16 (SRTD16), with major skeletal manifestations, in addition to other features. Here we sought to examine the role of IFT52 in osteoblast differentiation. Using lentiviral shRNA interference Ift52 was depleted in C3H10T1/2 mouse mesenchymal stem cells. This led to the disruption of the IFT-B anterograde trafficking machinery that impaired primary ciliogenesis and blocked osteogenic differentiation. In Ift52 silenced cells, Hedgehog (Hh) pathway upregulation during osteogenesis was attenuated and despite Smoothened Agonist (SAG) based Hh activation, osteogenic differentiation was incompletely restored. Further we investigated IFT52 activity in Drosophila, wherein the only ciliated somatic cells are the bipolar sensory neurons of the peripheral nervous system. Knockdown of IFT52 in Drosophila neuronal tissues reduced lifespan with the loss of embryonic chordotonal cilia, and produced severe locomotion, auditory and proprioceptive defects in larva and adults. Together these findings improve our knowledge of the role of IFT52 in various physiological contexts and its associated human disorder.

Mechanical stimulation promotes enthesis injury repair by mobilizing Prrx1+ cells via ciliary TGF-β signaling

Proper mechanical stimulation can improve rotator cuff enthesis injury repair. However, the underlying mechanism of mechanical stimulation promoting injury repair is still unknown. In this study, we found that Prrx1⁺ cell was essential for murine rotator cuff enthesis development identified by single-cell RNA sequence and involved in the injury repair. Proper mechanical stimulation could promote the migration of Prrx1⁺ cells to enhance enthesis injury repair. Meantime, TGF-β signaling and primary cilia played an essential role in mediating mechanical stimulation signaling transmission. Proper mechanical stimulation enhanced the release of active TGF-β1 to promote migration of Prrx1⁺ cells. Inhibition of TGF-β signaling eliminated the stimulatory effect of mechanical stimulation on Prrx1⁺ cell migration and enthesis injury repair. In addition, knockdown of Pallidin to inhibit TGF-βR2 translocation to the primary cilia or deletion of Ift88 in Prrx1⁺ cells also restrained the mechanics-induced Prrx1⁺ cells migration. These findings suggested that mechanical stimulation could increase the release of active TGF-β1 and enhance the mobilization of Prrx1⁺ cells to promote enthesis injury repair via ciliary TGF-β signaling.

Biophysical Modulation of Mesenchymal Stem Cell Differentiation in the Context of Skeletal Repair

A prominent feature of the skeleton is its ability to remodel in response to biophysical stimuli and to repair under varied biophysical conditions. This allows the skeleton considerable adaptation to meet its physiological roles of stability and movement. Skeletal cells and their mesenchymal precursors exist in a native environment rich with biophysical signals, and they sense and respond to those signals to meet organismal demands of the skeleton. While mechanical strain is the most recognized of the skeletal biophysical stimuli, signaling phenomena also include fluid flow, hydrostatic pressure, shear stress, and ion-movement-related electrokinetic phenomena including, prominently, streaming potentials. Because of the complex interactions of these electromechanical signals, it is difficult to isolate the significance of each. The application of external electrical and electromagnetic fields allows an exploration of the effects of these stimuli on cell differentiation and extra-cellular matrix formation in the absence of mechanical strain. This review takes a distinctly translational approach to mechanistic and preclinical studies of differentiation and skeletal lineage commitment of mesenchymal cells under biophysical stimulation. In vitro studies facilitate the examination of isolated cellular responses while in vivo studies permit the observation of cell differentiation and extracellular matrix synthesis.

Freestanding Magnetic Microtissues for Tissue Engineering Applications

A long-sought goal in tissue engineering (TE) is the development of tissues able to recapitulate the complex architecture of the native counterpart. Microtissues, by resembling the functional units of living structures, can be used to recreate tissues' architecture. Howbeit, microfabrication methodologies fail to reproduce cell-based tissues with uniform shape. At the macroscale, complex tissues are already produced by magnetic-TE using solely magnetized cells as building materials. The enhanced extracellular matrix (ECM) deposition guaranties the conservation of tissues' architecture, leading to a successful cellular engraftment. Following the same rational, now the combination of a versatile microfabrication-platform is proposed with magnetic-TE to generate robust micro-tissues with complex architecture for TE purposes. Small tissue units with circle, square, and fiber-like shapes are designed with high fidelity acting as building blocks for engineering complex tissues. Notably, freestanding microtissues maintain their geometry after 7 days post-culturing, overcoming the challenges of microtissues fabrication. Lastly, the ability of microtissues in invading distinct tissue models while releasing trophic factors is substantiated in methacryloyl laminarin (LAM) and platelet lysates (PLMA) hydrogels. By simply using cells as building units and such microfabrication-platform, the fabrication of complex multiscale and multifunctional tissues with clinical relevance is envisaged, including for therapies or disease models.

Mechanical regulation of bone remodeling

Bone remodeling is a lifelong process that gives rise to a mature, dynamic bone structure via a balance between bone formation by osteoblasts and resorption by osteoclasts. These opposite processes allow the accommodation of bones to dynamic mechanical forces, altering bone mass in response to changing conditions. Mechanical forces are indispensable for bone homeostasis; skeletal formation, resorption, and adaptation are dependent on mechanical signals, and loss of mechanical stimulation can therefore significantly weaken the bone structure, causing disuse osteoporosis and increasing the risk of fracture. The exact mechanisms by which the body senses and transduces mechanical forces to regulate bone remodeling have long been an active area of study among researchers and clinicians. Such research will lead to a deeper understanding of bone disorders and identify new strategies for skeletal rejuvenation. Here, we will discuss the mechanical properties, mechanosensitive cell populations, and mechanotransducive signaling pathways of the skeletal system.

Mechanical Stimulation on Mesenchymal Stem Cells and Surrounding Microenvironments in Bone Regeneration: Regulations and Applications

Treatment of bone defects remains a challenge in the clinic. Artificial bone grafts are the most promising alternative to autologous bone grafting. However, one of the limiting factors of artificial bone grafts is the limited means of regulating stem cell differentiation during bone regeneration. As a weight-bearing organ, bone is in a continuous mechanical environment. External mechanical force, a type of biophysical stimulation, plays an essential role in bone regeneration. It is generally accepted that osteocytes are mechanosensitive cells in bone. However, recent studies have shown that mesenchymal stem cells (MSCs) can also respond to mechanical signals. This article reviews the mechanotransduction mechanisms of MSCs, the regulation of mechanical stimulation on microenvironments surrounding MSCs by modulating the immune response, angiogenesis and osteogenesis, and the application of mechanical stimulation of MSCs in bone regeneration. The review provides a deep and extensive understanding of mechanical stimulation mechanisms, and prospects feasible designs of biomaterials for bone regeneration and the potential clinical applications of mechanical stimulation.

Rapid Prototypable Biomimetic Peristalsis Bioreactor Capable of Concurrent Shear and Multi-Axial Strain

Peristalsis is a nuanced mechanical stimulus comprised of multi-axial strain (radial and axial strain) and shear stress. Forces associated with peristalsis regulate diverse biological functions including digestion, reproductive function, and urine dynamics. Given the central role peristalsis plays in physiology and pathophysiology, we were motivated to design a bioreactor capable of holistically mimicking peristalsis. We engineered a novel rotating screw-drive based design combined with a peristaltic pump, in order to deliver multi-axial strain and concurrent shear stress to a biocompatible polydimethylsiloxane (PDMS) membrane "wall." Radial indentation and rotation of the screw drive against the wall demonstrated multi-axial strain evaluated via finite element modeling. Experimental measurements of strain using piezoelectric strain resistors were in close alignment with model-predicted values (15.9 ± 4.2% vs. 15.2% predicted). Modeling of shear stress on the "wall" indicated a uniform velocity profile and a moderate shear stress of 0.4 Pa. Human mesenchymal stem cells (hMSCs) seeded on the PDMS "wall" and stimulated with peristalsis demonstrated dramatic changes in actin filament alignment, proliferation, and nuclear morphology compared to static controls, perfusion, or strain, indicating that hMSCs sensed and responded to peristalsis uniquely. Lastly, significant differences were observed in gene expression patterns of calponin, caldesmon, smooth muscle actin, and transgelin, corroborating the propensity of hMSCs toward myogenic differentiation in response to peristalsis. Collectively, our data suggest that the peristalsis bioreactor is capable of generating concurrent multi-axial strain and shear stress on a "wall." hMSCs experience peristalsis differently than perfusion or strain, resulting in changes in proliferation, actin fiber organization, smooth muscle actin expression, and genetic markers of differentiation. The peristalsis bioreactor device has broad utility in the study of development and disease in several organ systems.

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.

Control of osteoblast arrangement by osteocyte mechanoresponse through prostaglandin E2 signaling under oscillatory fluid flow stimuli

Anisotropic collagen/apatite microstructure is a prominent determinant of bone tissue functionalization; in particular, bone matrix modulates its anisotropic microstructure depending on the surrounding mechanical condition. Although mechanotransduction in bones is governed by osteocyte function, the precise mechanisms linking mechanical stimuli and anisotropic formation of collagen/apatite microstructure are poorly understood. Here we developed a novel anisotropic mechano-coculture system which enables the understanding of the biological mechanisms regulating the oriented bone matrix formation, which is constructed by aligned osteoblasts. The developed model provides bone-mimetic coculture platform that enables simultaneous control of mechanical condition and osteoblast-osteocyte communication with an anisotropic culture scaffold. The engineered coculture device helps in understanding the relationship between osteocyte mechanoresponses and osteoblast arrangement, which is a significant contributor to anisotropic organization of bone tissue. Our study showed that osteocyte responses to oscillatory flow stimuli regulated osteoblast arrangement through soluble molecular interactions. Importantly, we found that prostaglandin E2 is a novel determinant for oriented collagen/apatite organization of bone matrix, through controlling osteoblast arrangement.

Electrospun Fiber Alignment Guides Osteogenesis and Matrix Organization Differentially in Two Different Osteogenic Cell Types

Biomimetic replication of the structural anisotropy of musculoskeletal tissues is important to restore proper tissue mechanics and function. Physical cues from the local micro-environment, such as matrix fiber orientation, may influence the differentiation and extracellular matrix (ECM) organization of osteogenic progenitor cells. This study investigates how scaffold fiber orientation affects the behavior of mature and progenitor osteogenic cells, the influence on secreted mineralized-collagenous matrix organization, and the resulting construct mechanical properties. Gelatin-coated electrospun poly(caprolactone) fibrous scaffolds were fabricated with either a low or a high degree of anisotropy and cultured with mature osteoblasts (MLO-A5s) or osteogenic mesenchymal progenitor cells (hES-MPs). For MLO-A5 cells, alkaline phosphatase (ALP) activity was highest, and more calcium-containing matrix was deposited onto aligned scaffolds. In contrast, hES-MPs, osteogenic mesenchymal progenitor cells, exhibited higher ALP activity, collagen, and calcium deposition on randomly orientated fibers compared with aligned counterparts. Deposited matrix was isotropic on random fibrous scaffolds, whereas a greater degree of anisotropy was observed in aligned fibrous constructs, as confirmed by second harmonic generation (SHG) and scanning electron microscope (SEM) imaging. This resulted in anisotropic mechanical properties on aligned constructs. This study indicates that mineralized-matrix deposition by osteoblasts can be controlled by scaffold alignment but that the early stages of osteogenesis may not benefit from culture on orientated scaffolds.

FBW7 couples structural integrity with functional output of primary cilia

Structural defects in primary cilia have robust effects in diverse tissues and systems. However, how disorders of ciliary length lead to functional outcomes are unknown. We examined the functional role of a ciliary length control mechanism of FBW7-mediated destruction of NDE1, in mesenchymal stem cell (MSC) differentiation. We show that FBW7 functions as a master regulator of both negative (NDE1) and positive (TALPID3) regulators of ciliogenesis, with an overall positive net effect on primary cilia formation, MSC differentiation to osteoblasts, and bone architecture. Deletion of Fbxw7 suppresses ciliation, Hedgehog activity, and differentiation, which are partially rescued in Fbxw7/Nde1-null cells. We also show that NDE1, despite suppressing ciliogenesis, promotes MSC differentiation by increasing the activity of the Hedgehog pathway by direct binding and enhancing GLI2 activity in a cilia-independent manner. We propose that FBW7 controls a protein-protein interaction network coupling ciliary structure and function, which is essential for stem cell differentiation.

Petsouki et al. dissect the importance of FBW7-mediated regulation of NDE1 and TALPID3 in mesenchymal stem cells (MSCs). They find that by modulating the abundance of negative (NDE1) and positive (TALPID3) cilia regulators, FBW7 contributes to both the assembly and signaling functions of primary cilia that are necessary for osteoblast differentiation.

The Primary Cilium on Cells of Developing Skeletal Rudiments; Distribution, Characteristics and Response to Mechanical Stimulation

Embryo movement is important for tissue differentiation and the formation of functional skeletal elements during embryonic development: reduced mechanical stimulation results in fused joints and misshapen skeletal rudiments with concomitant changes in the signaling environment and gene expression profiles in both mouse and chick immobile embryos. Despite the clear relationship between movement and skeletogenesis, the precise mechanisms by which mechanical stimuli influence gene regulatory processes are not clear. The primary cilium enables cells to sense mechanical stimuli in the cellular environment, playing a crucial mechanosensory role during kidney development and in articular cartilage and bone but little is known about cilia on developing skeletal tissues. Here, we examine the occurrence, length, position, and orientation of primary cilia across developing skeletal rudiments in mouse embryos during a period of pronounced mechanosensitivity and we report differences and similarities between wildtype and muscle-less mutant (Pax3^Spd/Spd) rudiments. Strikingly, joint regions tend to have cilia positioned and oriented away from the joint, while there was a less obvious, but still significant, preferred position on the posterior aspect of cells within the proliferative and hypertrophic zones. Regions of the developing rudiments have characteristic proportions of ciliated cells, with more cilia in the resting and joint zones. Comparing wildtype to muscle-less mutant embryos, cilia are shorter in the mutant with no significant difference in the proportion of ciliated cells. Cilia at the mutant joint were also oriented away from the joint line.

Biomechanical cues as master regulators of hematopoietic stem cell fate

Hematopoietic stem cells (HSCs) perceive both soluble signals and biomechanical inputs from their microenvironment and cells themselves. Emerging as critical regulators of the blood program, biomechanical cues such as extracellular matrix stiffness, fluid mechanical stress, confined adhesiveness, and cell-intrinsic forces modulate multiple capacities of HSCs through mechanotransduction. In recent years, research has furthered the scientific community's perception of mechano-based signaling networks in the regulation of several cellular processes. However, the underlying molecular details of the biomechanical regulatory paradigm in HSCs remain poorly elucidated and researchers are still lacking in the ability to produce bona fide HSCs ex vivo for clinical use. This review presents an overview of the mechanical control of both embryonic and adult HSCs, discusses some recent insights into the mechanisms of mechanosensing and mechanotransduction, and highlights the application of mechanical cues aiming at HSC expansion or differentiation.

Ciliary Signalling and Mechanotransduction in the Pathophysiology of Craniosynostosis

Craniosynostosis (CS) is the second most prevalent inborn craniofacial malformation; it results from the premature fusion of cranial sutures and leads to dimorphisms of variable severity. CS is clinically heterogeneous, as it can be either a sporadic isolated defect, more frequently, or part of a syndromic phenotype with mendelian inheritance. The genetic basis of CS is also extremely heterogeneous, with nearly a hundred genes associated so far, mostly mutated in syndromic forms. Several genes can be categorised within partially overlapping pathways, including those causing defects of the primary cilium. The primary cilium is a cellular antenna serving as a signalling hub implicated in mechanotransduction, housing key molecular signals expressed on the ciliary membrane and in the cilioplasm. This mechanical property mediated by the primary cilium may also represent a cue to understand the pathophysiology of non-syndromic CS. In this review, we aimed to highlight the implication of the primary cilium components and active signalling in CS pathophysiology, dissecting their biological functions in craniofacial development and in suture biomechanics. Through an in-depth revision of the literature and computational annotation of disease-associated genes we categorised 18 ciliary genes involved in CS aetiology. Interestingly, a prevalent implication of midline sutures is observed in CS ciliopathies, possibly explained by the specific neural crest origin of the frontal bone.

Screen Key Genes Associated with Distraction-Induced Osteogenesis of Stem Cells Using Bioinformatics Methods

Background: Applying mesenchymal stem cells (MSCs), together with the distraction osteogenesis (DO) process, displayed enhanced bone quality and shorter treatment periods. The DO guides the differentiation of MSCs by providing mechanical clues. However, the underlying key genes and pathways are largely unknown. The aim of this study was to screen and identify hub genes involved in distraction-induced osteogenesis of MSCs and potential molecular mechanisms. Material and Methods: The datasets were downloaded from the ArrayExpress database. Three samples of negative control and two samples subjected to 5% cyclic sinusoidal distraction at 0.25 Hz for 6 h were selected for screening differentially expressed genes (DEGs) and then analysed via bioinformatics methods. The Gene Ontology (GO) terms and Kyoto Encyclopaedia of Genes and Genomes (KEGG) pathway enrichment were investigated. The protein–protein interaction (PPI) network was visualised through the Cytoscape software. Gene set enrichment analysis (GSEA) was conducted to verify the enrichment of a self-defined osteogenic gene sets collection and identify osteogenic hub genes. Results: Three hub genes (IL6, MMP2, and EP300) that were highly associated with distraction-induced osteogenesis of MSCs were identified via the Venn diagram. These hub genes could provide a new understanding of distraction-induced osteogenic differentiation of MSCs and serve as potential gene targets for optimising DO via targeted therapies.

Primary cilium-mediated MSC mechanotransduction is dependent on Gpr161 regulation of hedgehog signalling

The benefits of physical loading to skeletal mass are well known. The primary cilium has emerged as an important organelle in bone mechanobiology/mechanotransduction, particularly in mesenchymal stem/stromal cells, yet the molecular mechanisms of cilium mechanotransduction are poorly understood. In this study, we demonstrate that Gpr161 is a mechanoresponsive GPCR, that localises to the cilium, and is involved in fluid shear-induced cAMP signalling and downstream osteogenesis. This Gpr161-mediated mechanotransduction is dependent on IFT88/cilium and may act through adenylyl cyclase 6 (AC6) to regulate cAMP and MSC osteogenesis. Moreover, we demonstrate that Hh signalling is positively associated with osteogenesis and that Hh gene expression is mechanically regulated and required for loading-induced osteogenic differentiation through a mechanism that involves IFT88, Gpr161, AC6, and cAMP. Therefore, we have delineated a molecular mechanism of MSC mechanotransduction which likely occurs at the cilium, leading to MSC osteogenesis, highlighting novel mechanotherapeutic targets to enhance osteogenesis.

Hyaluronan Synthases' Expression and Activity Are Induced by Fluid Shear Stress in Bone Marrow-Derived Mesenchymal Stem Cells

During biomineralization, the cells generating the biominerals must be able to sense the external physical stimuli exerted by the growing mineralized tissue and change their intracellular protein composition according to these stimuli. In molluscan shell, the myosin-chitin synthases have been suggested to be the link for this communication between cells and the biomaterial. Hyaluronan synthases (HAS) belong to the same enzyme family as chitin synthases. Their product hyaluronan (HA) occurs in the bone and is supposed to have a regulatory function during bone regeneration. We hypothesize that HASes’ expression and activity are controlled by fluid-induced mechanotransduction as it is known for molluscan chitin synthases. In this study, bone marrow-derived human mesenchymal stem cells (hMSCs) were exposed to fluid shear stress of 10 Pa. The RNA transcriptome was analyzed by RNA sequencing (RNAseq). HA concentrations in the supernatants were measured by ELISA. The cellular structure of hMSCs and HAS2-overexpressing hMSCs was investigated after treatment with shear stress using confocal microscopy. Fluid shear stress upregulated the expression of genes that encode proteins belonging to the HA biosynthesis and bone mineralization pathways. The HAS activity appeared to be induced. Knowledge about the regulation mechanism governing HAS expression, trafficking, enzymatic activation and quality of the HA product in hMSCs is essential to understand the biological role of HA in the bone microenvironment.

The role of IFT140 in early bone healing of tooth extraction sockets

OBJECTIVES

Primary cilium is a key organelle of regulating bone development and maintenance. The aim of this study is to investigate whether ciliary intraflagellar transporter protein 140 (IFT140) plays a positive role in extraction socket healing by promoting bone formation.

MATERIALS AND METHODS

A left maxillary first molar extraction model was established using 6-week-old Ift140^flox/flox (Ctrl group) and Ift140^flox/flox , Osx-cre (cKO group) mice. The maxillary bone samples from 1, 2, and 3 weeks were postoperatively evaluated by micro-CT, molecular biology, and histomorphometry analysis. Alveolar bone marrow stromal cells (aBMSCs) from 4-week-old mice were cultured in vitro and tested for proliferation and osteogenic ability.

RESULTS

Ciliated cells were predominantly observed in the early socket healing stage with highly expressed ciliary protein IFT140. Compared with the Ctrl group, the healing of extraction sockets in the cKO group was significantly delayed. The proliferation and osteogenic differentiation ability of aBMSCs were reduced in the cKO group.

CONCLUSION

IFT140 has a facilitating role in the early osteogenesis of extraction socket healing and is involved in regulating the proliferation and osteogenic differentiation of aBMSCs.

Embryonic movement stimulates joint formation and development: Implications in arthrogryposis multiplex congenita

Arthrogryposis multiplex congenita (AMC) is a heterogeneous syndrome where multiple joints have reduced range of motion due to contracture formation prior to birth. A common cause of AMC is reduced embryonic movement in utero. This reduction in embryonic movement can perturb molecular mechanisms and signaling pathways involved in the formation of joints during development. The absence of mechanical stimuli can impair joint cavitation, resulting in joint fusion, and ultimately eliminate function. In turn, mechanical stimuli are critical for proper joint formation during development and for mitigating AMC. Studies in experimental animal models have provided a greater understanding on the molecular pathophysiology of congenital contracture formation as a consequence of embryonic immobilization. Elucidation of how the mechanical signaling environment is transduced to initiate a biological response will be necessary to gain a deeper understanding of how mechanical stimuli are intertwined in the molecular regulation of joint development.

Mechanical loading and the control of stem cell behavior

OBJECTIVE

Mechanical stimulation regulates many cell responses. The present study describes the effects of different in vitro mechanical stimulation approaches on stem cell behavior.

DESIGN

The narrative review approach was performed. The articles published in English language that addressed the effects of mechanical force on stem cells were searched on Pubmed and Scopus database. The effects of extrinsic mechanical force on stem cell response was reviewed and discussed.

RESULTS

Cells sense mechanical stimuli by the function of mechanoreceptors and further transduce force stimulation into intracellular signaling. Cell responses to mechanical stimuli depend on several factors including type, magnitude, and duration. Further, similar mechanical stimuli exhibit distinct cell responses based on numerous factors including cell type and differentiation stage. Various mechanical applications modulate stemness maintenance and cell differentiation toward specific lineages.

CONCLUSIONS

Mechanical force application modulates stemness maintenance and differentiation. Modification of force regimens could be utilized to precisely control appropriate stem cell behavior toward specific applications.

Prx1-expressing cells contributing to fracture repair require primary cilia for complete healing in mice

Bone is a dynamic organ that is continuously modified during development, load-induced adaptation, and fracture repair. Understanding the cellular and molecular mechanisms for natural fracture healing can lead to therapeutics that enhance the quality of newly formed tissue, advance the rate of healing, or replace the need for invasive surgical procedures. Prx1-expressing cells in the periosteum are thought to supply the majority of osteoblasts and chondrocytes in the fracture callus, but the exact mechanisms for this behavior are unknown. The primary cilium is a sensory organelle that is known to mediate several signaling pathways involved in fracture healing and required for Prx1-expressing cells to contribute to juvenile bone development and adult load-induced bone formation. We therefore investigated the role of Prx1-expressing cell primary cilia in fracture repair by developing a mouse model that enabled us to simultaneously track Prx1 lineage cell fate and disrupt Prx1-expressing cell primary cilia in vivo. The cilium KO mice exhibited abnormally large calluses with significantly decreased bone formation and persistent cartilage nodules. Analysis of mRNA expression in the early soft callus revealed downregulation of osteogenesis, Hh signaling, and Wnt signaling, and upregulation of chondrogenesis and angiogenesis. The mutant mice also exhibited decreased Osx and Periostin but increased αSMA and PECAM-1 protein expression in the hard callus. We further used a Gli1LacZ reporter and found that Hh signaling was significantly upregulated in the mutant callus at later stages of healing. Interestingly, altered protein expression and Hh signaling did not correlate with labeled Prx1-lineage cells, suggesting loss of cilia altered Hh signaling non-autonomously. Overall, cilium KO mice demonstrated severely delayed and incomplete fracture healing, and our findings suggest Prx1-expressing cell primary cilia are necessary to tune Hh signaling for proper fracture repair.

Response of Pluripotent Stem Cells to Environmental Stress and Its Application for Directed Differentiation

Simple Summary

Environmental changes in oxygen concentration, temperature, and mechanical stimulation lead to the activation of specific transcriptional factors and induce the expression of each downstream gene. In general, these responses are protective machinery against such environmental stresses, while these transcriptional factors also regulate cell proliferation, differentiation, and organ development in mammals. In the case of pluripotent stem cells, similar response mechanisms normally work and sometimes stimulate the differentiation cues. Up to now, differentiation protocols utilizing such environmental stresses have been reported to obtain various types of somatic cells from pluripotent stem cells. Basically, environmental stresses as hypoxia (low oxygen), hyperoxia, (high oxygen) and mechanical stress from cell culture plates are relatively safer than chemicals and gene transfers, which affect the genome irreversibly. Therefore, protocols designed with such environments in mind could be useful for the technology development of cell therapy and regenerative medicine. In this manuscript, we summarize recent findings of environmental stress-induced differentiation protocols and discuss their mechanisms.

Abstract

Pluripotent stem cells have unique characteristics compared to somatic cells. In this review, we summarize the response to environmental stresses (hypoxic, oxidative, thermal, and mechanical stresses) in embryonic stem cells (ESCs) and their applications in the differentiation methods directed to specific lineages. Those stresses lead to activation of each specific transcription factor followed by the induction of downstream genes, and one of them regulates lineage specification. In short, hypoxic stress promotes the differentiation of ESCs to mesodermal lineages via HIF-1α activation. Concerning mechanical stress, high stiffness tends to promote mesodermal differentiation, while low stiffness promotes ectodermal differentiation via the modulation of YAP1. Furthermore, each step in the same lineage differentiation favors each appropriate stiffness of culture plate; for example, definitive endoderm favors high stiffness, while pancreatic progenitor favors low stiffness during pancreatic differentiation of human ESCs. Overall, treatments utilizing those stresses have no genotoxic or carcinogenic effects except oxidative stress; therefore, the differentiated cells are safe and could be useful for cell replacement therapy. In particular, the effect of mechanical stress on differentiation is becoming attractive for the field of regenerative medicine. Therefore, the development of a stress-mediated differentiation protocol is an important matter for the future.

Stem Cell Mechanobiology and the Role of Biomaterials in Governing Mechanotransduction and Matrix Production for Tissue Regeneration

Mechanobiology has underpinned many scientific advances in understanding how biophysical and biomechanical cues regulate cell behavior by identifying mechanosensitive proteins and specific signaling pathways within the cell that govern the production of proteins necessary for cell-based tissue regeneration. It is now evident that biophysical and biomechanical stimuli are as crucial for regulating stem cell behavior as biochemical stimuli. Despite this, the influence of the biophysical and biomechanical environment presented by biomaterials is less widely accounted for in stem cell-based tissue regeneration studies. This Review focuses on key studies in the field of stem cell mechanobiology, which have uncovered how matrix properties of biomaterial substrates and 3D scaffolds regulate stem cell migration, self-renewal, proliferation and differentiation, and activation of specific biological responses. First, we provide a primer of stem cell biology and mechanobiology in isolation. This is followed by a critical review of key experimental and computational studies, which have unveiled critical information regarding the importance of the biophysical and biomechanical cues for stem cell biology. This review aims to provide an informed understanding of the intrinsic role that physical and mechanical stimulation play in regulating stem cell behavior so that researchers may design strategies that recapitulate the critical cues and develop effective regenerative medicine approaches.

ROCK-II inhibition suppresses impaired mechanobiological responses in early estrogen deficient osteoblasts

Mechanobiological responses by osteoblasts are governed by downstream Rho-ROCK signalling through actin cytoskeleton re-arrangements but whether these responses are influenced by estrogen deficiency during osteoporosis remains unknown. The objective of this study was to determine alterations in the mechanobiological responses of estrogen-deficient osteoblasts and investigate whether an inhibitor of the Rho-ROCK signalling can revert these changes. MC3T3-E1 cells were pre-treated with 10 nM 17-β estradiol for 7 days and further cultured with or without estradiol for next 2 days. These cells were treated with or without ROCK-II inhibitor, Y-27632, and oscillatory fluid flow (OFF, 1Pa, 0.5 Hz, 1 h) was applied. Here, we report that Prostaglandin E₂ release, Runt-related transcription factor 2 and Osteopontin gene expression were significantly enhanced in response to OFF in estrogen-deficient cells than in cells with estrogen (3.73 vs 1.63 pg/ng DNA; 13.5 vs 2.6 fold, 2.1 vs 0.4 fold respectively). Upon ROCK-II inhibition, these enhanced effects of estrogen deficiency were downregulated. OFF increased the fibril anisotropy in cells pre-treated with estrogen and this increase was suppressed upon ROCK-II inhibition. This study is the first to demonstrate altered mechanobiological responses by osteoblasts during early estrogen deficiency and that these responses to OFF can be suppressed upon ROCK inhibition.

The potential role of peak nasal inspiratory flow to evaluate active sinonasal inflammation and disease severity

Although the pathophysiology of nasal polyposis is incompletely understood, rhinologists have seldom studied it with rhinomanometry or peak nasal inspiratory flow (PNIF) due to technical limitations and the perception that polyp size might impair reproducibility and the usefulness of recordings. The objective of this study is to assess how measures of rhinomanometry and PNIF relate to disease activity. Nineteen patients with polyps, 15 patients with chronic sinusitis without polyps and 11 negative controls were evaluated with active anterior rhinomanometry and PNIF. Sinusitis and polyp patients were re-evaluated after medical treatment. Polyp patients had the highest median Lund-Mackay score (14) and a median Johansen score of 1. PNIF and its variation after treatment were also lowest in this group (median 90 L/min before and after treatment; median variation of 0 L/min). Nasal resistance was similar between groups, and only correlated with Johansen score (Spearman = 0.517, p = 0.048) after treatment. Our study suggests that evaluating polyp patients using rhinomanometry and PNIF may provide useful and reproducible data. Several findings considered together suggest that polyp size is not the main determinant of nasal functional changes in these patients, warranting further studies to verify whether PNIF changes reflect sinus inflammation or merely airway obstruction.

Mesenchymal Stem Cells Adaptively Respond to Environmental Cues Thereby Improving Granulation Tissue Formation and Wound Healing

Granulation tissue formation constitutes a key step during wound healing of the skin and other organs. Granulation tissue concomitantly initiates regenerative M2 macrophages polarization, fibroblast proliferation, myofibroblast differentiation with subsequent contraction of the wound, new vessel formation, and matrix deposition. Impaired granulation tissue formation either leads to delayed wound healing or excessive scar formation, conditions with high morbidity and mortality. Accumulating evidence has demonstrated that mesenchymal stem cell (MSC)-based therapy is a promising strategy to ameliorate defects in granulation tissue formation and to successfully treat non-healing chronic wounds. In this review we give an updated overview of how therapeutically administered MSCs ensure a balanced granulation tissue formation, and furthermore discuss the cellular and molecular mechanisms underlying the adaptive responses of MSCs to cue in their direct neighborhood. Improved understanding of the interplay between the exogenous MSCs and their niche in granulation tissue will foster the development of MSC-based therapies tailored for difficult-to-treat non-healing wounds.

A Novel 3D Osteoblast and Osteocyte Model Revealing Changes in Mineralization and Pro-osteoclastogenic Paracrine Signaling During Estrogen Deficiency

Recent in vitro studies have revealed that the mechanobiological responses of osteoblasts and osteocytes are fundamentally impaired during estrogen deficiency. However, these two-dimensional (2D) cell culture studies do not account for in vivo biophysical cues. Thus, the objectives of this study are to (1) develop a three-dimensional (3D) osteoblast and osteocyte model integrated into a bioreactor and (2) apply this model to investigate whether estrogen deficiency leads to changes in osteoblast to osteocyte transition, mechanosensation, mineralization, and paracrine signaling associated with bone resorption by osteoclasts. MC3T3-E1s were expanded in media supplemented with estrogen (17β-estradiol). These cells were encapsulated in gelatin-mtgase before culture in (1) continued estrogen (E) or (2) no further estrogen supplementation. Constructs were placed in gas permeable and water impermeable cell culture bags and maintained at 5% CO2 and 37°C. These bags were either mechanically stimulated in a custom hydrostatic pressure (HP) bioreactor or maintained under static conditions (control). We report that osteocyte differentiation, characterized by the presence of dendrites and staining for osteocyte marker dentin matrix acidic phosphoprotein 1 (DMP1), was significantly greater under estrogen withdrawal (EW) compared to under continuous estrogen treatment (day 21). Mineralization [bone sialoprotein (BSP), osteopontin (OPN), alkaline phosphatase (ALP), calcium] and gene expression associated with paracrine signaling for osteoclastogenesis [receptor activator of nuclear factor kappa-β ligand (RANKL)/osteoprotegerin OPG ratio] were significantly increased in estrogen deficient and mechanically stimulated cells. Interestingly, BSP and DMP-1 were also increased at day 1 and day 21, respectively, which play a role in regulation of biomineralization. Furthermore, the increase in pro-osteoclastogenic signaling may be explained by altered mechanoresponsiveness of osteoblasts or osteocytes during EW. These findings highlight the impact of estrogen deficiency on bone cell function and provide a novel in vitro model to investigate the mechanisms underpinning changes in bone cells after estrogen deficiency.

Molecular mechanosensors in osteocytes

Osteocytes, the most abundant and long-lived cells in bone, are the master regulators of bone remodeling. In addition to their functions in endocrine regulation and calcium and phosphate metabolism, osteocytes are the major responsive cells in force adaptation due to mechanical stimulation. Mechanically induced bone formation and adaptation, disuse-induced bone loss and skeletal fragility are mediated by osteocytes, which sense local mechanical cues and respond to these cues in both direct and indirect ways. The mechanotransduction process in osteocytes is a complex but exquisite regulatory process between cells and their environment, between neighboring cells, and between different functional mechanosensors in individual cells. Over the past two decades, great efforts have focused on finding various mechanosensors in osteocytes that transmit extracellular mechanical signals into osteocytes and regulate responsive gene expression. The osteocyte cytoskeleton, dendritic processes, Integrin-based focal adhesions, connexin-based intercellular junctions, primary cilium, ion channels, and extracellular matrix are the major mechanosensors in osteocytes reported so far with evidence from both in vitro and in vitro studies. This review aims to give a systematic introduction to osteocyte mechanobiology, provide details of osteocyte mechanosensors, and discuss the roles of osteocyte mechanosensitive signaling pathways in the regulation of bone homeostasis.

Mechanotransduction in osteogenesis

Bone is one of the most highly adaptive tissues in the body, possessing the capability to alter its morphology and function in response to stimuli in its surrounding environment. The ability of bone to sense and convert external mechanical stimuli into a biochemical response, which ultimately alters the phenotype and function of the cell, is described as mechanotransduction. This review aims to describe the fundamental physiology and biomechanisms that occur to induce osteogenic adaptation of a cell following application of a physical stimulus. Considerable developments have been made in recent years in our understanding of how cells orchestrate this complex interplay of processes, and have become the focus of research in osteogenesis. We will discuss current areas of preclinical and clinical research exploring the harnessing of mechanotransductive properties of cells and applying them therapeutically, both in the context of fracture healing and de novo bone formation in situations such as nonunion.

Cite this article: Bone Joint Res 2019;9(1):1–14.

Exome sequencing analysis identifies frequent oligogenic involvement and FLNB variants in adolescent idiopathic scoliosis

Background

Adolescent idiopathic scoliosis (AIS) is a genetically heterogeneous disease characterised by three-dimensional deformity of the spine in the absence of a congenital spinal anomaly or neurological musculoskeletal disorder. The clinical variability and incomplete penetrance of some genes linked with AIS indicate that this disease constitutes an oligogenic trait.

Objective

We aimed to explore the oligogenic nature of this disease and identify novel AIS genes.

Methods

We analysed rare damaging variants within AIS-associated genes by using exome sequencing in 40 AIS trios and 183 sporadic patients.

Results

Multiple variants within AIS-associated genes were identified in eight AIS trios, and five individuals harboured rare damaging variants in the FLNB gene. The patients showed more frequent oligogenicity than the controls. In the gene-based burden test, the top signal resided in FLNB. In functional studies, we found that the AIS-associated FLNB variants altered the protein’s conformation and subcellular localisation and its interaction with other proteins (TTC26 and OFD1) involved in AIS. The most compelling evidence of an oligogenic basis was that the number of rare damaging variants was recognised as an independent prognostic factor for curve progression in Cox regression analysis.

Conclusion

Our data indicate that AIS is an oligogenic disease and identify FLNB as a susceptibility gene for AIS.

Optogenetic stimulation of phosphoinositides reveals a critical role of primary cilia in eye pressure regulation

Glaucoma is a group of progressive optic neuropathies that cause irreversible vision loss. Although elevated intraocular pressure (IOP) is associated with the development and progression of glaucoma, the mechanisms for its regulation are not well understood. Here, we have designed CIBN/CRY2-based optogenetic constructs to study phosphoinositide regulation within distinct subcellular compartments. We show that stimulation of CRY2-OCRL, an inositol 5-phosphatase, increases aqueous humor outflow and lowers IOP in vivo, which is caused by a calcium-dependent actin rearrangement of the trabecular meshwork cells. Phosphoinositide stimulation also rescues defective aqueous outflow and IOP in a Lowe syndrome mouse model but not in IFT88^fl/fl mice that lack functional cilia. Thus, our study is the first to use optogenetics to regulate eye pressure and demonstrate that tight regulation of phosphoinositides is critical for aqueous humor homeostasis in both normal and diseased eyes.

Lineage Commitment, Signaling Pathways, and the Cytoskeleton Systems in Mesenchymal Stem Cells

Mesenchymal stem cells (MSCs) from adult tissues are promising candidates for personalized cell therapy and tissue engineering. Significant progress was achieved in our understanding of the regulation of MSCs proliferation and differentiation by different cues during the past years. Proliferation and differentiation of MSCs are sensitive to the extracellular matrix (ECM) properties, physical cues, and chemical signaling. Sheath stress, matrix stiffness, surface adhesiveness, and micro- and nanotopography define cell shape and dictate lineage commitment of MSCs even in the absence of specific chemical signals. We discuss mechanotransduction as the major route from ECM through the cytoskeleton toward signaling pathways and gene expression. All components of the cytoskeleton from primary cilium and focal adhesions (FAs) to actin, microtubules (MTs), and intermediate filaments (IFs) are involved in the mechanotransduction. Differentiation of MSCs is regulated via the complex network of interrelated signaling pathways, including RhoA/ROCK, Akt/Erk, and YAP/TAZ effectors of Hippo pathway. These pathways could be regulated both by chemical and mechanical stimuli. Attenuation of these pathways in MSCs results in specific changes in FAs and actin cytoskeleton. Besides, differentiation of MSCs affects MTs and IFs. Recent findings highlight the role of intranuclear actin in the regulation of transcription factors in response to mechanical environmental stimuli. Alterations of cytoskeletal components reflect the MSC senescence state and their migratory capacity. In this review, we discuss the relationships between the molecular interactions in signaling pathways and morphological response of cytoskeletal components and reveal the complex interrelations between cytoskeleton systems and signaling pathways during lineage commitment of MSCs. Impact Statement This review describes the complex network of relationships between mechanical and biochemical stimuli in mesenchymal stem cells (MSC) and their balance which defines the morphological changes of cell shape due to rearrangement of cytoskeletal systems during lineage commitment of MSCs.

Prx1-Expressing Progenitor Primary Cilia Mediate Bone Formation in response to Mechanical Loading in Mice

Increases in mechanical loading can enhance the addition of new bone, altering geometry and density such that bones better withstand higher forces. Bone-forming osteoblasts have long been thought to originate from progenitors, but the exact source is yet to be identified. Previous studies indicate osteogenic precursors arise from Prx1-expressing progenitors during embryonic development and adult fracture repair. However, it is unknown whether this cell population is also a source for mechanically induced active osteoblasts. We first identified that Prx1 is expressed in skeletally mature mouse periosteum, a thin tissue covering the surface of the bone that is rich in osteoprogenitors. We then traced Prx1 progenitor lineage using a transgenic mouse model carrying both a Prx1-driven tamoxifen-inducible Cre and a ROSA-driven lacZ reporter gene. Cells that expressed Prx1 when compressive axial loading was applied were detected within the cortical bone days after stimulation, indicating osteocytes are of Prx1-expressing cell origin. In addition, we evaluated how these cells sense and respond to physical stimulation in vivo by disrupting their primary cilia, which are antenna-like sensory organelles known to enhance mechanical and chemical signaling kinetics. Although Prx1-driven primary cilium disruption did not affect osteoblast recruitment to the bone surface, the relative mineral apposition and bone formation rates were decreased by 53% and 34%, respectively. Thus, this cell population contributes to load-induced bone formation, and primary cilia are needed for a complete response. Interestingly, Prx1-expressing progenitors are easily extracted from periosteum and are perhaps an attractive alternative to marrow stem cells for bone tissue regeneration strategies.