Identification of mechanosensitive genes during embryonic bone formation

Immobilization secondary to cell death of muscle precursors with a dual transcriptional signature contributes to the emu wing skeletal pattern

Limb reduction has occurred multiple times in tetrapod history. Among ratites, wing reductions range from mild vestigialization to complete loss, with emus (Dromaius novaehollandiae) serving as a model for studying the genetic mechanisms behind limb reduction. Here, we explore the developmental mechanisms underlying wing reduction in emu. Our analyses reveal that immobilization resulting from the absence of distal muscles contributes to skeletal shortening, fusion and left-right intraindividual variation. Expression analysis and single cell-RNA sequencing identify muscle progenitors displaying a dual lateral plate mesodermal and myogenic signature. These cells aggregate at the proximal region of wing buds and undergo cell death. We propose that this cell death, linked to the lack of distal muscle masses, underlines the morphological features and variability in skeletal elements due to reduced mechanical loading. Our results demonstrate that differential mobility during embryonic development may drive morphological diversification in vestigial structures.

Hippo cell signaling and HS-proteoglycans regulate tissue form and function, age-dependent maturation, extracellular matrix remodeling, and repair

This review examined how Hippo cell signaling and heparan sulfate (HS)-proteoglycans (HSPGs) regulate tissue form and function. Despite being a nonweight-bearing tissue, the brain is regulated by Hippo mechanoresponsive cell signaling pathways during embryonic development. HS-proteoglycans interact with growth factors, morphogens, and extracellular matrix components to regulate development and pathology. Pikachurin and Eyes shut (Eys) interact with dystroglycan to stabilize the photoreceptor axoneme primary cilium and ribbon synapse facilitating phototransduction and neurotransduction with bipolar retinal neuronal networks in ocular vision, the primary human sense. Another HSPG, Neurexin interacts with structural and adaptor proteins to stabilize synapses and ensure specificity of neural interactions, and aids in synaptic potentiation and plasticity in neurotransduction. HSPGs also stabilize the blood-brain barrier and motor neuron basal structures in the neuromuscular junction. Agrin and perlecan localize acetylcholinesterase and its receptors in the neuromuscular junction essential for neuromuscular control. The primary cilium is a mechanosensory hub on neurons, utilized by YES associated protein (YAP)-transcriptional coactivator with PDZ-binding motif (TAZ) Hippo, Hh, Wnt, transforming growth factor (TGF)-β/bone matrix protein (BMP) receptor tyrosine kinase cell signaling. Members of the glypican HSPG proteoglycan family interact with Smoothened and Patched G-protein coupled receptors on the cilium to regulate Hh and Wnt signaling during neuronal development. Control of glycosyl sulfotransferases and endogenous protease expression by Hippo TAZ YAP represents a mechanism whereby the fine structure of HS-proteoglycans can be potentially modulated spatiotemporally to regulate tissue morphogenesis in a similar manner to how Hippo signaling controls sialyltransferase expression and mediation of cell-cell recognition, dysfunctional sialic acid expression is a feature of many tumors.

Material properties in regenerating axolotl limbs using inverse finite element analysis

BACKGROUND

The extracellular mechanical environment plays an important role in the skeletal development process. Characterization of the material properties of regenerating tissues that recapitulate development, provides insights into the mechanical environment experienced by the cells and the maturation of the matrix. In this study, we estimated the viscoelastic material properties of regenerating forelimbs in the axolotl (Ambystoma mexicanum) at three different regeneration stages: 27 days post-amputation (mid-late bud) and 41 days post-amputation (palette stage), and fully-grown time points. A stress-relaxation indentation test followed by two-term Prony series viscoelastic inverse finite element analysis was used to obtain material parameters. Glycosaminoglycan (GAG) content was estimated using a 1,9- dimethyl methylene blue assay.

RESULTS

The instantaneous and equilibrium shear moduli significantly increased with regeneration while the short-term stress relaxation time significantly decreased with limb regeneration. The long-term stress relaxation time in the fully-grown time point was significantly lower than 27 and 41 DPA groups. The GAG content was not significantly different between 27 and 41 DPA but the GAG content of cartilage in the fully-grown group was significantly greater than in 27 and 41 DPA.

CONCLUSIONS

The mechanical environment of the proliferating cells changes drastically during limb regeneration. Understanding how the tissue's mechanical properties change during limb regeneration is critical for linking molecular-level matrix production of the cells to tissue-level behavior and mechanical signals.

Important Role of the Ihh Signaling Pathway in Initiating Early Cranial Remodeling and Morphological Specialization in Cromileptes altivelis

In this study, we identified the important contribution of frontal bone remodeling in shaping the 'sunken head and humpback' appearance in C. altivelis. Our investigation identified a developmental milestone at a total length of 5-6 cm, making the onset of its morphologic specialization in this species. A comparative analysis with closely related species reveals heightened activity in the frontal osteoblasts of the humpback grouper, potentially providing a physiological basis for its remodeling. Furthermore, our findings highlight that a significant upregulation in the expression levels of Ihhb, Ptch1, and Gli2a genes was seen in C. altivelis within the specified developmental stage, indicating an important involvement of the Ihhb-Ptch1-Gli2a signaling pathway in initiating the morphological specialization. We hypothesized that Ihh signaling could be attributed to shifts in mechanical stress, resulting from muscle traction on the frontal bone due to changes in swimming patterns during development. This study not only offers significant insights into unraveling the molecular mechanisms that govern phenotypic specialization and ecological adaptations in the humpback grouper but also serves as a valuable reference for studies on fishes with a controversial morphology and molecular phylogeny.

Stirred culture of cartilaginous microtissues promotes chondrogenic hypertrophy through exposure to intermittent shear stress

Cartilage microtissues are promising tissue modules for bottom up biofabrication of implants leading to bone defect regeneration. Hitherto, most of the protocols for the development of these cartilaginous microtissues have been carried out in static setups, however, for achieving higher scales, dynamic process needs to be investigated. In the present study, we explored the impact of suspension culture on the cartilage microtissues in a novel stirred microbioreactor system. To study the effect of the process shear stress, experiments with three different impeller velocities were carried out. Moreover, we used mathematical modeling to estimate the magnitude of shear stress on the individual microtissues during dynamic culture. Identification of appropriate mixing intensity allowed dynamic bioreactor culture of the microtissues for up to 14 days maintaining microtissue suspension. Dynamic culture did not affect microtissue viability, although lower proliferation was observed as opposed to the statically cultured ones. However, when assessing cell differentiation, gene expression values showed significant upregulation of both Indian Hedgehog (IHH) and collagen type X (COLX), well known markers of chondrogenic hypertrophy, for the dynamically cultured microtissues. Exometabolomics analysis revealed similarly distinct metabolic profiles between static and dynamic conditions. Dynamic cultured microtissues showed a higher glycolytic profile compared with the statically cultured ones while several amino acids such as proline and aspartate exhibited significant differences. Furthermore, in vivo implantations proved that microtissues cultured in dynamic conditions are functional and able to undergo endochondral ossification. Our work demonstrated a suspension differentiation process for the production of cartilaginous microtissues, revealing that shear stress resulted to an acceleration of differentiation towards hypertrophic cartilage.

Endostructural and periosteal growth of the human humerus

The growth and development of long bones are of considerable interests in the fields of comparative anatomy and palaeoanthropology, as evolutionary changes and adaptations to specific physical activity patterns are expected to be revealed during bone ontogeny. Traditionally, the cross-sectional geometry of long bones has been examined at discrete locations usually placed at set intervals or fixed percentage distances along the midline axis of the bone shaft. More recently, the technique of morphometric mapping has enabled the continuous analysis of shape variation along the shaft. Here we extend this technique to the full sequence of late fetal and postnatal development of the humeral shaft in a modern human population sample, with the aim of establishing the shape changes during growth and their relationship with the development of the arm musculature and activity patterns. A sample of modern human humeri from individuals of age ranging from 24 weeks in utero to 18 years was imaged using microtomography at multiple resolutions and custom Matlab scripts. Standard biomechanical properties, cortical thickness, surface curvature, and pseudo-landmarks were extracted along radial vectors spaced at intervals of 1° at each 0.5% longitudinal increment measured along the shaft axis. Heat maps were also generated for cortical thickness and surface curvature. The results demonstrate that a whole bone approach to analysis of cross-sectional geometry is more desirable where possible, as there is a continuous pattern of variation along the shaft. It is also possible to discriminate very young individuals and adolescents from other groups by relative cortical thickness, and also by periosteal surface curvature.

Building a Co-ordinated Musculoskeletal System: The Plasticity of the Developing Skeleton in Response to Muscle Contractions

The skeletal musculature and the cartilage, bone and other connective tissues of the skeleton are intimately co-ordinated. The shape, size and structure of each bone in the body is sculpted through dynamic physical stimuli generated by muscle contraction, from early development, with onset of the first embryo movements, and through repair and remodelling in later life. The importance of muscle movement during development is shown by congenital abnormalities where infants that experience reduced movement in the uterus present a sequence of skeletal issues including temporary brittle bones and joint dysplasia. A variety of animal models, utilising different immobilisation scenarios, have demonstrated the precise timing and events that are dependent on mechanical stimulation from movement. This chapter lays out the evidence for skeletal system dependence on muscle movement, gleaned largely from mouse and chick immobilised embryos, showing the many aspects of skeletal development affected. Effects are seen in joint development, ossification, the size and shape of skeletal rudiments and tendons, including compromised mechanical function. The enormous plasticity of the skeletal system in response to muscle contraction is a key factor in building a responsive, functional system. Insights from this work have implications for our understanding of morphological evolution, particularly the challenging concept of emergence of new structures. It is also providing insight for the potential of physical therapy for infants suffering the effects of reduced uterine movement and is enhancing our understanding of the cellular and molecular mechanisms involved in skeletal tissue differentiation, with potential for informing regenerative therapies.

Lowry M, Almasri R, Gorgun C, Eichholz K, Stavenschi E, Hokamp K, Roche F, O’Driscoll L, Hoey D. Mechanically activated mesenchymal-derived bone cells drive vessel formation via an extracellular vesicle mediated mechanism

Blood vessel formation is an important initial step for bone formation during development as well as during remodelling and repair in the adult skeleton. This results in a heavily vascularized tissue where endothelial cells and skeletal cells are constantly in crosstalk to facilitate homeostasis, a process that is mediated by numerous environmental signals, including mechanical loading. Breakdown in this communication can lead to disease and/or poor fracture repair. Therefore, this study aimed to determine the role of mature bone cells in regulating angiogenesis, how this is influenced by a dynamic mechanical environment, and understand the mechanism by which this could occur. Herein, we demonstrate that both osteoblasts and osteocytes coordinate endothelial cell proliferation, migration, and blood vessel formation via a mechanically dependent paracrine mechanism. Moreover, we identified that this process is mediated via the secretion of extracellular vesicles (EVs), as isolated EVs from mechanically stimulated bone cells elicited the same response as seen with the full secretome, while the EV-depleted secretome did not elicit any effect. Despite mechanically activated bone cell-derived EVs (MA-EVs) driving a similar response to VEGF treatment, MA-EVs contain minimal quantities of this angiogenic factor. Lastly, a miRNA screen identified mechanoresponsive miRNAs packaged within MA-EVs which are linked with angiogenesis. Taken together, this study has highlighted an important mechanism in osteogenic-angiogenic coupling in bone and has identified the mechanically activated bone cell-derived EVs as a therapeutic to promote angiogenesis and potentially bone repair.

Ciliary IFT88 Protects Coordinated Adolescent Growth Plate Ossification From Disruptive Physiological Mechanical Forces

Compared with our understanding of endochondral ossification, much less is known about the coordinated arrest of growth defined by the narrowing and fusion of the cartilaginous growth plate. Throughout the musculoskeletal system, appropriate cell and tissue responses to mechanical force delineate morphogenesis and ensure lifelong health. It remains unclear how mechanical cues are integrated into many biological programs, including those coordinating the ossification of the adolescent growth plate at the cessation of growth. Primary cilia are microtubule-based organelles tuning a range of cell activities, including signaling cascades activated or modulated by extracellular biophysical cues. Cilia have been proposed to directly facilitate cell mechanotransduction. To explore the influence of primary cilia in the mouse adolescent limb, we conditionally targeted the ciliary gene Intraflagellar transport protein 88 (Ift88^fl/fl ) in the juvenile and adolescent skeleton using a cartilage-specific, inducible Cre (AggrecanCreER^T2 Ift88^fl/fl ). Deletion of IFT88 in cartilage, which reduced ciliation in the growth plate, disrupted chondrocyte differentiation, cartilage resorption, and mineralization. These effects were largely restricted to peripheral tibial regions beneath the load-bearing compartments of the knee. These regions were typified by an enlarged population of hypertrophic chondrocytes. Although normal patterns of hedgehog signaling were maintained, targeting IFT88 inhibited hypertrophic chondrocyte VEGF expression and downstream vascular recruitment, osteoclastic activity, and the replacement of cartilage with bone. In control mice, increases to physiological loading also impair ossification in the peripheral growth plate, mimicking the effects of IFT88 deletion. Limb immobilization inhibited changes to VEGF expression and epiphyseal morphology in Ift88cKO mice, indicating the effects of depletion of IFT88 in the adolescent growth plate are mechano-dependent. We propose that during this pivotal phase in adolescent skeletal maturation, ciliary IFT88 protects uniform, coordinated ossification of the growth plate from an otherwise disruptive heterogeneity of physiological mechanical forces. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Mechanical Static Force Negatively Regulates Vitality and Early Skeletal Development in Zebrafish Embryos

Skeletal system development and remodelling is regulated by several different factors, including hormones, cytokines, and mechanical forces. It is known that gravity and pressure stimulate mechanosensors on bone cells which transduce mechanical signals to chemical ones. Nevertheless, few data have been provided about the role of mechanical forces on embryo osteogenesis in vivo. Since the zebrafish is an elective model for developmental studies, in particular on bone formation and tissue mineralization, we analyzed in vivo the effects of a static mechanical force generated by a water column on fertilized zebrafish eggs. The results have shown that an increase in the hydrostatic pressure (HP) of up to 5.9% was lethal for 100% of treated embryos at 48 h post fertilization (hpf). A small decrease in length (−2%) and 49% mortality were found in the +4.4% HP embryos compared with the controls. To analyze skeletal development, we evaluated the number of mineralized vertebral bodies in the trunk at five days post fertilization. The embryos grown under +2.4% HP showed a physiological intramembranous mineralization of vertebral bodies whereas the embryos which grew with +3.4% HP showed a significant decrease in mineralization rate (−54%). Morphological analysis of cartilage and bones in embryos at +3.4% HP revealed a delay of both intramembranous and chondrogenic mineralization, respectively, in axial and head bones, whereas the chondrogenesis appeared normal. These data suggested that developing osteoblasts and different mineralization programs are sensitive to mechanical pressure when applied to early embryogenesis.

Pthlha and mechanical force control early patterning of growth zones in the zebrafish craniofacial skeleton

Secreted signals in patterning systems often induce repressive signals that shape their distributions in space and time. In developing growth plates (GPs) of endochondral long bones, Parathyroid hormone-like hormone (Pthlh) inhibits Indian hedgehog (Ihh) to form a negative-feedback loop that controls GP progression and bone size. Whether similar systems operate in other bones and how they arise during embryogenesis remain unclear. We show that Pthlha expression in the zebrafish craniofacial skeleton precedes chondrocyte differentiation and restricts where cells undergo hypertrophy, thereby initiating a future GP. Loss of Pthlha leads to an expansion of cells expressing a novel early marker of the hypertrophic zone (HZ), entpd5a, and later HZ markers, such as ihha, whereas local Pthlha misexpression induces ectopic entpd5a expression. Formation of this early pre-HZ correlates with onset of muscle contraction and requires mechanical force; paralysis leads to loss of entpd5a and ihha expression in the pre-HZ, mislocalized pthlha expression and no subsequent ossification. These results suggest that local Pthlh sources combined with force determine HZ locations, establishing the negative-feedback loop that later maintains GPs.

Mechanical Stimulation via Muscle Activity Is Necessary for the Maturation of Tendon Multiscale Mechanics During Embryonic Development

During embryonic development, tendons transform into a hypocellular tissue with robust tensile load-bearing capabilities. Previous work suggests that this mechanical transformation is due to increases in collagen fibril length and is dependent on mechanical stimulation via muscle activity. However, the relationship between changes in the microscale tissue structure and changes in macroscale tendon mechanics is still unclear. Additionally, the specific effect of mechanical stimulation on the multiscale structure-function relationships of developing tendons is also unknown. Therefore, the objective of this study was to measure the changes in tendon mechanics and structure at multiple length scales during embryonic development with and without skeletal muscle paralysis. Tensile testing of tendons from chick embryos was performed to determine the macroscale tensile modulus as well as the magnitude of the fibril strains and interfibrillar sliding with applied tissue strain. Embryos were also treated with either decamethonium bromide or pancuronium bromide to produce rigid or flaccid paralysis. Histology was performed to assess changes in tendon size, spacing between tendon subunits, and collagen fiber diameter. We found that the increase in the macroscale modulus observed with development is accompanied by an increase in the fibril:tissue strain ratio, which is consistent with an increase in collagen fibril length. Additionally, we found that flaccid paralysis reduced the macroscale tendon modulus and the fibril:tissue strain ratio, whereas less pronounced effects that were not statistically significant were observed with rigid paralysis. Finally, skeletal paralysis also reduced the size of collagen fibril bundles (i.e., fibers). Together, these data suggest that more of the applied tissue strain is transmitted to the collagen fibrils at later embryonic ages, which leads to an increase in the tendon macroscale tensile mechanics. Furthermore, our data suggest that mechanical stimulation during development is necessary to induce structural and mechanical changes at multiple physical length scales. This information provides valuable insight into the multiscale structure-function relationships of developing tendons and the importance of mechanical stimulation in producing a robust tensile load-bearing soft tissue.

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.

Joint development recovery on resumption of embryonic movement following paralysis

Fetal activity in utero is a normal part of pregnancy and reduced or absent movement can lead to long-term skeletal defects, such as Fetal Akinesia Deformation Sequence, joint dysplasia and arthrogryposis. A variety of animal models with decreased or absent embryonic movements show a consistent set of developmental defects, providing insight into the aetiology of congenital skeletal abnormalities. At developing joints, defects include reduced joint interzones with frequent fusion of cartilaginous skeletal rudiments across the joint. At the spine, defects include shortening and a spectrum of curvature deformations. An important question, with relevance to possible therapeutic interventions for human conditions, is the capacity for recovery with resumption of movement following short-term immobilisation. Here, we use the well-established chick model to compare the effects of sustained immobilisation from embryonic day (E)4-10 to two different recovery scenarios: (1) natural recovery from E6 until E10 and (2) the addition of hyperactive movement stimulation during the recovery period. We demonstrate partial recovery of movement and partial recovery of joint development under both recovery conditions, but no improvement in spine defects. The joints examined (elbow, hip and knee) showed better recovery in hindlimb than forelimb, with hyperactive mobility leading to greater recovery in the knee and hip. The hip joint showed the best recovery with improved rudiment separation, tissue organisation and commencement of cavitation. This work demonstrates that movement post paralysis can partially recover specific aspects of joint development, which could inform therapeutic approaches to ameliorate the effects of human fetal immobility. This article has an associated First Person interview with the first author of the paper.

Characterization of intrauterine growth, proliferation and biomechanical properties of the murine larynx

Current research approaches employ traditional tissue engineering strategies to promote vocal fold (VF) tissue regeneration, whereas recent novel advances seek to use principles of developmental biology to guide tissue generation by mimicking native developmental cues, causing tissue or allogenic/autologous progenitor cells to undergo the regeneration process. To address the paucity of data to direct VF differentiation and subsequent new tissue formation, we characterize structure-proliferation relationships and tissue elastic moduli over embryonic development using a murine model. Growth, cell proliferation, and tissue biomechanics were taken at E13.5, E15.5, E16.5, E18.5, P0, and adult time points. Quadratic growth patterns were found in larynx length, maximum transverse diameter, outer dorsoventral diameter, and VF thickness; internal VF length was found to mature linearly. Cell proliferation measured with EdU in the coronal and transverse planes of the VFs was found to decrease with increasing age. Exploiting atomic force microscopy, we measured significant differences in tissue stiffness across all time points except between E13.5 and E15.5. Taken together, our results indicate that as the VF mature and develop quadratically, there is a concomitant tissue stiffness increase. Greater gains in biomechanical stiffness at later prenatal stages, correlated with reduced cell proliferation, suggest that extracellular matrix deposition may be responsible for VF thickening and increased biomechanical function, and that the onset of biomechanical loading (breathing) may also contribute to increased stiffness. These data provide a profile of VF biomechanical and growth properties that can guide the development of biomechanically-relevant scaffolds and progenitor cell differentiation for VF tissue regeneration.

Localization of YAP activity in developing skeletal rudiments is responsive to mechanical stimulation

BACKGROUND

Normal skeletal development, in particular ossification, joint formation and shape features of condyles, depends on appropriate mechanical input from embryonic movement but it is unknown how such physical stimuli are transduced to alter gene regulation. Hippo/Yes-Associated Protein (YAP) signalling has been shown to respond to the physical environment of the cell and here we specifically investigate the YAP effector of the pathway as a potential mechanoresponsive mediator in the developing limb skeleton.

RESULTS

We show spatial localization of YAP protein and of pathway target gene expression within developing skeletal rudiments where predicted biophysical stimuli patterns and shape are affected in immobilization models, coincident with the period of sensitivity to movement, but not coincident with the expression of the Hippo receptor Fat4. Furthermore, we show that under reduced mechanical stimulation, in immobile, muscle-less mouse embryos, this spatial localization is lost. In culture blocking YAP reduces chondrogenesis but the effect differs depending on the timing and/or level of YAP reduction.

CONCLUSIONS

These findings implicate YAP signalling, independent of Fat4, in the transduction of mechanical signals during key stages of skeletal patterning in the developing limb, in particular endochondral ossification and shape emergence, as well as patterning of tissues at the developing synovial joint.

Short-term foetal immobility temporally and progressively affects chick spinal curvature and anatomy and rib development

Congenital spine deformities may be influenced by movements in utero, but the effects of foetal immobility on spine and rib development remain unclear. The purpose of the present study was to determine (1) critical time-periods when rigid paralysis caused the most severe disruption in spine and rib development and (2) how the effects of an early, short-term immobilisation were propagated to the different features of spine and rib development. Chick embryos were immobilised once per single embryonic day (E) between E3 and E6 and harvested at E9. To assess the ontogenetic effects following single-day immobilisation, other embryos were immobilised at E4 and harvested daily between E5 and E9. Spinal curvature, vertebral shape and segmentation and rib development were analysed by optical projection tomography and histology. The results demonstrated that periods critical for movement varied for different aspects of spine and rib development. Single-day immobilisation at E3 or E4 resulted in the most pronounced spinal curvature abnormalities, multiple wedged vertebrae and segmentation defects, while single-day immobilisation at E5 led to the most severe rib abnormalities. Assessment of ontogenetic effects following single-day immobilisation at E4 revealed that vertebral segmentation defects were subsequent to earlier vertebral body shape and spinal curvature abnormalities, while rib formation (although delayed) was independent from thoracic vertebral shape or curvature changes. A day-long immobilisation in chicks severely affected spine and rib development, highlighting the importance of abnormal foetal movements at specific time-points and motivating targeted prenatal monitoring for early diagnosis of congenital scoliosis.

Cellular Complexity at the Interface: Challenges in Enthesis Tissue Engineering

The complex heterogeneous cellular environment found in tendon-to-bone interface makes this structure a challenge for interface tissue engineering. Orthopedic surgeons still face some problems associated with the formation of fibrotic tissue or re-tear occurring after surgical re-attachment of tendons to the bony insertion or the application of grafts. Unfortunately, an understanding of the cellular component of enthesis lags far behind of other well-known musculoskeletal interfaces, which blocks the development of new treatment options for the healing and regeneration of this multifaceted junction. In this chapter, the main characteristics of tendon and bone cell populations are introduced, followed by a brief description of the interfacial cellular niche, highlighting molecular mechanisms governing tendon-to-bone attachment and mineralization. Finally, we describe and critically assess some challenges faced concerning the use of cell-based strategies in tendon-to-bone healing and regeneration.

Investigating the mechanistic basis of biomechanical input controlling skeletal development: exploring the interplay with Wnt signalling at the joint

Embryo movement is essential to the formation of a functional skeleton. Using mouse and chick models, we previously showed that mechanical forces influence gene regulation and tissue patterning, particularly at developing limb joints. However, the molecular mechanisms that underpin the influence of mechanical signals are poorly understood. Wnt signalling is required during skeletal development and is altered under reduced mechanical stimulation. Here, to explore Wnt signalling as a mediator of mechanical input, the expression of Wnt ligand and Fzd receptor genes in the developing skeletal rudiments was profiled. Canonical Wnt activity restricted to the developing joint was shown to be reduced under immobilization, while overexpression of activated β-catenin following electroporation of chick embryo limbs led to joint expansion, supporting the proposed role for Wnt signalling in mechanoresponsive joint patterning. Two key findings advance our understanding of the interplay between Wnt signalling and mechanical stimuli: first, loss of canonical Wnt activity at the joint shows reciprocal, coordinated misregulation of BMP signalling under altered mechanical influence. Second, this occurs simultaneously with increased expression of several Wnt pathway component genes in a territory peripheral to the joint, indicating the importance of mechanical stimulation for a population of potential joint progenitor cells.This article is part of the Theo Murphy meeting issue 'Mechanics of Development'.

Changing While Staying the Same: Preservation of Structural Continuity During Limb Evolution by Developmental Integration

More than 150 years since Charles Darwin published "On the Origin of Species", gradual evolution by natural selection is still not fully reconciled with the apparent sudden appearance of complex structures, such as the bat wing, with highly derived functions. This is in part because developmental genetics has not yet identified the number and types of mutations that accumulated to drive complex morphological evolution. Here, we consider the experimental manipulations in laboratory model systems that suggest tissue interdependence and mechanical responsiveness during limb development conceptually reduce the genetic complexity required to reshape the structure as a whole. It is an exciting time in the field of evolutionary developmental biology as emerging technical approaches in a variety of non-traditional laboratory species are on the verge of filling the gaps between theory and evidence to resolve this sesquicentennial debate.

Precise spatial restriction of BMP signaling in developing joints is perturbed upon loss of embryo movement

Dynamic mechanical loading of synovial joints is necessary for normal joint development, as evidenced in certain clinical conditions, congenital disorders and animal models where dynamic muscle contractions are reduced or absent. Although the importance of mechanical forces on joint development is unequivocal, little is known about the molecular mechanisms involved. Here, using chick and mouse embryos, we observed that molecular changes in expression of multiple genes analyzed in the absence of mechanical stimulation are consistent across species. Our results suggest that abnormal joint development in immobilized embryos involves inappropriate regulation of Wnt and BMP signaling during definition of the emerging joint territories, i.e. reduced β-catenin activation and concomitant upregulation of pSMAD1/5/8 signaling. Moreover, dynamic mechanical loading of the developing knee joint activates Smurf1 expression; our data suggest that Smurf1 insulates the joint region from pSMAD1/5/8 signaling and is essential for maintenance of joint progenitor cell fate.

Wnt signalling controls the response to mechanical loading during zebrafish joint development

Joint morphogenesis requires mechanical activity during development. Loss of mechanical strain causes abnormal joint development, which can impact long-term joint health. Although cell orientation and proliferation are known to shape the joint, dynamic imaging of developing joints in vivo has not been possible in other species. Using genetic labelling techniques in zebrafish we were able, for the first time, to dynamically track cell behaviours in intact moving joints. We identify that proliferation and migration, which contribute to joint morphogenesis, are mechanically controlled and are significantly reduced in immobilised larvae. By comparison with strain maps of the developing skeleton, we identify canonical Wnt signalling as a candidate for transducing mechanical forces into joint cell behaviours. We show that, in the jaw, Wnt signalling is reduced specifically in regions of high strain in response to loss of muscle activity. By pharmacological manipulation of canonical Wnt signalling, we demonstrate that Wnt acts downstream of mechanical activity and is required for joint patterning and chondrocyte maturation. Wnt16, which is also downstream of muscle activity, controls proliferation and migration, but plays no role in chondrocyte intercalation.

Mechanical loading regulates organization of the actin cytoskeleton and column formation in postnatal growth plate

Longitudinal growth of bones occurs at the growth plates where chondrocytes align into columns that allow directional growth. Little is known about the mechanisms controlling the ability of chondrocytes to form columns. We hypothesize that mechanical load and the resulting force on chondrocytes are necessary during active growth for proper growth plate development and limb length. To test this hypothesis, we created a mouse model in which a portion of the sciatic nerve from one hind limb was transected at postnatal day 8 to cause paralysis to that limb. At 6 and 12 wk postsurgery, the hind limb had significantly less bone mineral density than contralateral controls, confirming reduced load. At 8 and 14 wk postsurgery, tibiae were significantly shorter than controls. The paralyzed growth plate showed disruptions to column organization, with fewer and shorter columns. Polarized light microscopy indicated alterations in collagen fiber organization in the growth plate. Furthermore, organization of the actin cytoskeleton in growth plate chondrocytes was disrupted. We conclude that mechanical load and force on chondrocytes within the growth plate regulate postnatal development of the long bones.

Differential effects of altered patterns of movement and strain on joint cell behaviour and skeletal morphogenesis

OBJECTIVE

There is increasing evidence that joint shape is a potent predictor of osteoarthritis (OA) risk; yet the cellular events underpinning joint morphogenesis remain unclear. We sought to develop a genetically tractable animal model to study the events controlling joint morphogenesis.

DESIGN

Zebrafish larvae were subjected to periods of flaccid paralysis, rigid paralysis or hyperactivity. Immunohistochemistry and transgenic reporters were used to monitor changes to muscle and cartilage. Finite Element Models were generated to investigate the mechanical conditions of rigid paralysis. Principal component analysis was used to test variations in skeletal morphology and metrics for shape, orientation and size were applied to describe cell behaviour.

RESULTS

We show that flaccid and rigid paralysis and hypermobility affect cartilage element and joint shape. We describe differences between flaccid and rigid paralysis in regions showing high principal strain upon muscle contraction. We identify that altered shape and high strain occur in regions of cell differentiation and we show statistically significant changes to cell maturity occur in these regions in paralysed and hypermobile zebrafish.

CONCLUSION

While flaccid and rigid paralysis and hypermobility affect skeletal morphogenesis they do so in subtly different ways. We show that some cartilage regions are unaffected in conditions such as rigid paralysis where static force is applied, whereas joint morphogenesis is perturbed by both flaccid and rigid paralysis; suggesting that joints require dynamic movement for accurate morphogenesis. A better understanding of how biomechanics impacts skeletal cell behaviour will improve our understanding of how foetal mechanics shape the developing joint.

Mechanical loading inhibits hypertrophy in chondrogenically differentiating hMSCs within a biomimetic hydrogel

Three dimensional hydrogels are a promising vehicle for delivery of adult human bone-marrow derived mesenchymal stem cells (hMSCs) for cartilage tissue engineering. One of the challenges with using this cell type is the default pathway is terminal differentiation, a hypertrophic phenotype and precursor to endochondral ossification. We hypothesized that a synthetic hydrogel consisting of extracellular matrix (ECM) analogs derived from cartilage when combined with dynamic loading provides physiochemical cues for achieving a stable chondrogenic phenotype. Hydrogels were formed from crosslinked poly(ethylyene glycol) as the base chemistry and to which (meth)acrylate functionalized ECM analogs of RGD (cell adhesion peptide) and chondroitin sulfate (ChS, a negatively charged glycosaminoglycan) were introduced. Bone-marrow derived hMSCs from three donors were encapsulated in the hydrogels and cultured under free swelling conditions or under dynamic com pressive loading with 2.5 ng/ml TGF-β3. hMSC differentiation was assessed by quantitative PCR and immunohistochemistry. Nine hydrogel formulations were initially screened containing 0, 0.1 or 1mM RGD and 0, 1 or 2wt% ChS. After 21 days, the 1% ChS and 0.1 mM RGD hydrogel had the highest collagen II gene expression, but this was accompanied by high collagen X gene expression. At the protein level, collagen II was detected in all formulations with ECM analogs, but minimally detectable in the hydrogel without ECM analogs. Collagen X protein was present in all formulations. The 0.1 mM RGD and 1% ChS formulation was selected and subjected to five loading regimes: no loading, 5% strain 0.3Hz (1.5%/s), 10% strain 0.3 Hz (3%/s), 5% strain 1 Hz (5%/s), and 10% strain 1Hz (10%/s). After 21 days, ~70-90% of cells stained positive for collagen II protein regardless of the culture condition. On the contrary, only ~20-30% of cells stained positive for collagen X protein under 3 and 5%/s loading conditions, which was accompanied by minimal staining for RunX2. The other culture conditions had more cells staining positive for collagen X (40-60%) and was accompanied by positive staining for RunX2. In summary, a cartilage-like biomimetic hydrogel supports chondrogenesis of hMSCs, but dynamic loading only under select strain rates is able to inhibit hypertrophy.

IFT88 influences chondrocyte actin organization and biomechanics

OBJECTIVES

Primary cilia are microtubule based organelles which control a variety of signalling pathways important in cartilage development, health and disease. This study examines the role of the intraflagellar transport (IFT) protein, IFT88, in regulating fundamental actin organisation and mechanics in articular chondrocytes.

METHODS

The study used an established chondrocyte cell line with and without hypomorphic mutation of IFT88 (IFT88(orpk)). Confocal microscopy was used to quantify F-actin and myosin IIB organisation. Viscoelastic cell and actin cortex mechanics were determined using micropipette aspiration with actin dynamics visualised in live cells transfected with LifeACT-GFP.

RESULTS

IFT88(orpk) cells exhibited a significant increase in acto-myosin stress fibre organisation relative to wild-type (WT) cells in monolayer and an altered response to cytochalasin D. Rounded IFT88(orpk) cells cultured in suspension exhibited reduced cortical actin expression with reduced cellular equilibrium modulus. Micropipette aspiration resulted in reduced membrane bleb formation in IFT88(orpk) cells. Following membrane blebbing, IFT88(orpk) cells exhibited slower reformation of the actin cortex. IFT88(orpk) cells showed increased actin deformability and reduced cortical tension confirming that IFT regulates actin cortex mechanics. The reduced cortical tension is also consistent with the reduced bleb formation.

CONCLUSIONS

This study demonstrates for the first time that the ciliary protein IFT88 regulates fundamental actin organisation and the stiffness of the actin cortex leading to alterations in cell deformation, mechanical properties and blebbing in an IFT88 chondrocyte cell line. This adds to the growing understanding of the role of primary cilia and IFT in regulating cartilage biology.

Strategies to engineer tendon/ligament-to-bone interface: Biomaterials, cells and growth factors

Integration between tendon/ligament and bone occurs through a specialized tissue interface called enthesis. The complex and heterogeneous structure of the enthesis is essential to ensure smooth mechanical stress transfer between bone and soft tissues. Following injury, the interface is not regenerated, resulting in high rupture recurrence rates. Tissue engineering is a promising strategy for the regeneration of a functional enthesis. However, the complex structural and cellular composition of the native interface makes enthesis tissue engineering particularly challenging. Thus, it is likely that a combination of biomaterials and cells stimulated with appropriate biochemical and mechanical cues will be needed. The objective of this review is to describe the current state-of-the-art, challenges and future directions in the field of enthesis tissue engineering focusing on four key parameters: (1) scaffold and biomaterials, (2) cells, (3) growth factors and (4) mechanical stimuli.

Patterns of activity adaptation in humeral trabecular bone in Neolithic humans and present-day people

OBJECTIVE

The annual turnover rate of trabecular bone by far exceeds that of cortical bone and, therefore, is very sensitive to its daily loading regime. Here we test the hypothesis that the study of the trabecular bone architecture of the human humerus is able to differentiate between different habitual manual activities.

MATERIALS AND METHODS

For this purpose, we compared the trabecular architecture of the humeral head in a Neolithic population to that of a sample of contemporary Europeans using micro-computed tomography (microCT). We defined in each specimen a spherical volume of interest with a diameter of 57.5 ± 2.5% of the maximal diameter of the humeral head to metrically analyze the bulk of humeral head trabecular architecture. We subsequently quantified the trabecular architectures in the VOIs, measuring seven standard 3D-morphometric parameters, and used univariate and multivariate statistical analyses for comparisons within and between populations.

RESULTS

Univariate statistical analysis showed significant differences in a combination of 3D-morphometric parameters. A principal components analysis of the 3D-morphometrics of the trabecular architectures separated the Neolithic from the contemporary samples on the basis of differences in their gross trabecular architecture, including differences in the bone volume fraction (BV/TV), the number of trabeculae per unit length (Tb N), and the distance between trabeculae (Tb Sp).

DISCUSSION

We interpret the significant differences found in the humeral trabecular bone of the Neolithic and the contemporary group as likely reflecting the distinct manual working routines. The trabecular bone configuration in the Neolithic sample shows presumably functional signatures of prehistoric subsistence techniques and activity levels.

Finite element modelling predicts changes in joint shape and cell behaviour due to loss of muscle strain in jaw development

Abnormal joint morphogenesis is linked to clinical conditions such as Developmental Dysplasia of the Hip (DDH) and to osteoarthritis (OA). Muscle activity is known to be important during the developmental process of joint morphogenesis. However, less is known about how this mechanical stimulus affects the behaviour of joint cells to generate altered morphology. Using zebrafish, in which we can image all joint musculoskeletal tissues at high resolution, we show that removal of muscle activity through anaesthetisation or genetic manipulation causes a change to the shape of the joint between the Meckel's cartilage and Palatoquadrate (the jaw joint), such that the joint develops asymmetrically leading to an overlap of the cartilage elements on the medial side which inhibits normal joint function. We identify the time during which muscle activity is critical to produce a normal joint. Using Finite Element Analysis (FEA), to model the strains exerted by muscle on the skeletal elements, we identify that minimum principal strains are located at the medial region of the joint and interzone during mouth opening. Then, by studying the cells immediately proximal to the joint, we demonstrate that biomechanical strain regulates cell orientation within the developing joint, such that when muscle-induced strain is removed, cells on the medial side of the joint notably change their orientation. Together, these data show that biomechanical forces are required to establish symmetry in the joint during development.

Evolution and functional significance of derived sternal ossification patterns in ornithothoracine birds

The midline pattern of sternal ossification characteristic of the Cretaceous enantiornithine birds is unique among the Ornithodira, the group containing birds, nonavian dinosaurs and pterosaurs. This has been suggested to indicate that Enantiornithes is not the sister group of Ornithuromorpha, the clade that includes living birds and their close relatives, which would imply rampant convergence in many nonsternal features between enantiornithines and ornithuromorphs. However, detailed comparisons reveal greater similarity between neornithine (i.e. crown group bird) and enantiornithine modes of sternal ossification than previously recognized. Furthermore, a new subadult enantiornithine specimen demonstrates that sternal ossification followed a more typically ornithodiran pattern in basal members of the clade. This new specimen, referable to the Pengornithidae, indicates that the unique ossification pattern observed in other juvenile enantiornithines is derived within Enantiornithes. A similar but clearly distinct pattern appears to have evolved in parallel in the ornithuromorph lineage. The atypical mode of sternal ossification in some derived enantiornithines should be regarded as an autapomorphic condition rather than an indication that enantiornithines are not close relatives of ornithuromorphs. Based on what is known about molecular mechanisms for morphogenesis and the possible selective advantages, the parallel shifts to midline ossification that took place in derived enantiornithines and living neognathous birds appear to have been related to the development of a large ventral keel, which is only present in ornithuromorphs and enantiornithines. Midline ossification can serve to medially reinforce the sternum at a relatively early ontogenetic stage, which would have been especially beneficial during the protracted development of the superprecocial Cretaceous enantiornithines.

The importance of foetal movement for co-ordinated cartilage and bone development in utero : clinical consequences and potential for therapy

Construction of a functional skeleton is accomplished through co-ordination of the developmental processes of chondrogenesis, osteogenesis, and synovial joint formation. Infants whose movement in utero is reduced or restricted and who subsequently suffer from joint dysplasia (including joint contractures) and thin hypo-mineralised bones, demonstrate that embryonic movement is crucial for appropriate skeletogenesis. This has been confirmed in mouse, chick, and zebrafish animal models, where reduced or eliminated movement consistently yields similar malformations and which provide the possibility of experimentation to uncover the precise disturbances and the mechanisms by which movement impacts molecular regulation. Molecular genetic studies have shown the important roles played by cell communication signalling pathways, namely Wnt, Hedgehog, and transforming growth factor-beta/bone morphogenetic protein. These pathways regulate cell behaviours such as proliferation and differentiation to control maturation of the skeletal elements, and are affected when movement is altered. Cell contacts to the extra-cellular matrix as well as the cytoskeleton offer a means of mechanotransduction which could integrate mechanical cues with genetic regulation. Indeed, expression of cytoskeletal genes has been shown to be affected by immobilisation. In addition to furthering our understanding of a fundamental aspect of cell control and differentiation during development, research in this area is applicable to the engineering of stable skeletal tissues from stem cells, which relies on an understanding of developmental mechanisms including genetic and physical criteria. A deeper understanding of how movement affects skeletogenesis therefore has broader implications for regenerative therapeutics for injury or disease, as well as for optimisation of physical therapy regimes for individuals affected by skeletal abnormalities.

Cite this article: Bone Joint Res 2015;4:105–116

Enthesis fibrocartilage cells originate from a population of Hedgehog-responsive cells modulated by the loading environment

Tendon attaches to bone across a specialized tissue called the enthesis. This tissue modulates the transfer of muscle forces between two materials, i.e. tendon and bone, with vastly different mechanical properties. The enthesis for many tendons consists of a mineralized graded fibrocartilage that develops postnatally, concurrent with epiphyseal mineralization. Although it is well described that the mineralization and development of functional maturity requires muscle loading, the biological factors that modulate enthesis development are poorly understood. By genetically demarcating cells expressing Gli1 in response to Hedgehog (Hh) signaling, we discovered a unique population of Hh-responsive cells in the developing murine enthesis that were distinct from tendon fibroblasts and epiphyseal chondrocytes. Lineage-tracing experiments revealed that the Gli1 lineage cells that originate in utero eventually populate the entire mature enthesis. Muscle paralysis increased the number of Hh-responsive cells in the enthesis, demonstrating that responsiveness to Hh is modulated in part by muscle loading. Ablation of the Hh-responsive cells during the first week of postnatal development resulted in a loss of mineralized fibrocartilage, with very little tissue remodeling 5 weeks after cell ablation. Conditional deletion of smoothened, a molecule necessary for responsiveness to Ihh, from the developing tendon and enthesis altered the differentiation of enthesis progenitor cells, resulting in significantly reduced fibrocartilage mineralization and decreased biomechanical function. Taken together, these results demonstrate that Hh signaling within developing enthesis fibrocartilage cells is required for enthesis formation.

Three-dimensional anatomy of the ostrich (Struthio camelus) knee joint

The three-dimensional anatomy of the ostrich (Struthio camelus) knee (femorotibial, femorofibular, and femoropatellar) joint has scarcely been studied, and could elucidate certain mechanobiological properties of sesamoid bones. The adult ostrich is unique in that it has double patellae, while another similar ratite bird, the emu, has none. Understanding why these patellae form and what purpose they may serve is dually important for future studies on ratites as well as for understanding the mechanobiological characteristics of sesamoid bone development. For this purpose, we present a three-dimensional anatomical study of the ostrich knee joint, detailing osteology, ligaments and menisci, and myology. We have identified seven muscles which connect to the two patellae and compare our findings to past descriptions. These descriptions can be used to further study the biomechanical loading and implications of the double patella in the ostrich.

Fetal jaw movement affects Ihh signaling in mandibular condylar cartilage development: the possible role of Ihh as mechanotransduction mediator

OBJECTIVE

Jaw movement is an important mechanical factor for prenatal development of the condylar cartilage of mandible. Fetal jaw movement restriction has been shown to cause deformity of the mandibular condyle. We hypothesized that this treatment affects the expression of mechanosensitive molecules, namely Indian hedgehog (Ihh) and Parathyroid hormone related protein (PTHrP) in the condyle.

EXPERIMENTAL METHODS

We restrained jaw movement by suturing the jaw of E15.5 mouse embryos and allowed them to develop until E18.5 using exo utero system, and analyzed them by immunohistochemistry and in situ hybridization methods.

RESULTS

Morphological, histomorphometric and immunohistochemical study showed that the mandibular condylar cartilage was reduced and deformed, the volume and total cell numbers in the condylar cartilage were also reduced, and number and/or distribution of 5-bromo-2'-deoxyuridine-positive cells, Ihh-positive cells in the mesenchymal and pre-hypertrophic zones were significantly and correspondingly decreased in the sutured group. Using in situ hybridization, reduced expression of Ihh, PTHrP and their related receptors were observed in condylar cartilage of the sutured embryos.

CONCLUSIONS

Our results revealed that the altered mechanical stress induced by prenatal jaw movement restriction decreased proliferating cells, the amount of cartilage, and altered expression of the Ihh and PTHrP, suggesting that Ihh act as mechanotransduction mediators in the development of mandibular condylar cartilage.

Mechanical activation of mammalian target of rapamycin pathway is required for cartilage development

Mechanical stress regulates development by modulating cell signaling and gene expression. However, the cytoplasmic components mediating mechanotransduction remain unclear. In this study, elimination of muscle contraction during chicken embryonic development resulted in a reduction in the activity of mammalian target of rapamycin (mTOR) in the cartilaginous growth plate. Inhibition of mTOR activity led to significant inhibition of chondrocyte proliferation, cartilage tissue growth, and expression of chondrogenic genes, including Indian hedgehog (Ihh), a critical mediator of mechanotransduction. Conversely, cyclic loading (1 Hz, 5% matrix deformation) of embryonic chicken growth plate chondrocytes in 3-dimensional (3D) collagen scaffolding induced sustained activation of mTOR. Mechanical activation of mTOR occurred in serum-free medium, indicating that it is independent of growth factor or nutrients. Treatment of chondrocytes with Rapa abolished mechanical activation of cell proliferation and Ihh gene expression. Cyclic loading of chondroprogenitor cells deficient in SH2-containing protein tyrosine phosphatase 2 (Shp2) further enhanced mechanical activation of mTOR, cell proliferation, and chondrogenic gene expression. This result suggests that Shp2 is an antagonist of mechanotransduction through inhibition of mTOR activity. Our data demonstrate that mechanical activation of mTOR is necessary for cell proliferation, chondrogenesis, and cartilage growth during bone development, and that mTOR is an essential mechanotransduction component modulated by Shp2 in the cytoplasm.

Mechanoadaptation of developing limbs: shaking a leg

The proportion of total limb length taken up by the individual skeletal elements (limb proportionality), varies widely between species. These diverse skeletal forms have evolved to allow for a range of limb uses and they first emerge as the embryo develops, to achieve the characteristic skeletal architecture of each species. During this time, the developing skeleton experiences mechanical loading as a result of embryonic muscle contraction. The possibility that adaptation to such mechanical input may allow embryos to coordinate the appearance of skeletal design with their expanding range of movements has so far received little attention. This is surprising, given the critical role exerted by embryo movement in normal skeletal development; stage-specific in ovo immobilisation of embryonic chicks results in joint contractures and a reduction in longitudinal bone growth in the limbs. Epigenetic mechanisms allow for selective activation of genes in response to environmental signals, resulting in the production of phenotypic complexity in morphogenesis; mechanical loading of bone during movement appears to be one such signal. It may be that 'mechanosensitive' genes under regulation of mechanical input adjust proportionality along the bone's proximo-distal axis, introducing a level of phenotypic plasticity. If this hypothesis is upheld, species with more elongated distal limb elements will have a greater dependence on mechanical input for the differences in their growth, and mechanosensitive bone growth in the embryo may have evolved as an additional source of phenotypic diversity during skeletal development.

Primary cilia disassembly down-regulates mechanosensitive hedgehog signalling: a feedback mechanism controlling ADAMTS-5 expression in chondrocytes

OBJECTIVE

Hedgehog signalling is mediated by the primary cilium and promotes cartilage degeneration in osteoarthritis. Primary cilia are influenced by pathological stimuli and cilia length and prevalence are increased in osteoarthritic cartilage. This study aims to investigate the relationship between mechanical loading, hedgehog signalling and cilia disassembly in articular chondrocytes.

METHODS

Primary bovine articular chondrocytes were subjected to cyclic tensile strain (CTS; 0.33 Hz, 10% or 20% strain). Hedgehog pathway activation (Ptch1, Gli1) and A Disintegrin And Metalloproteinase with Thrombospondin Motifs 5 (ADAMTS-5) expression were assessed by real-time PCR. A chondrocyte cell line generated from the Tg737(ORPK) mouse was used to investigate the role of the cilium in this response. Cilia length and prevalence were quantified by immunocytochemistry and confocal microscopy.

RESULTS

Mechanical strain upregulates Indian hedgehog expression and activates hedgehog signalling. Ptch1, Gli1 and ADAMTS-5 expression were increased following 10% CTS, but not 20% CTS. Pathway activation requires a functioning primary cilium and is not observed in Tg737(ORPK) cells lacking cilia. Mechanical loading significantly reduced cilium length such that cilia became progressively shorter with increasing strain magnitude. Inhibition of histone deacetylase 6 (HDAC6), a tubulin deacetylase, prevented cilia disassembly and restored mechanosensitive hedgehog signalling and ADAMTS-5 expression at 20% CTS.

CONCLUSIONS

This study demonstrates for the first time that mechanical loading activates primary cilia-mediated hedgehog signalling and ADAMTS-5 expression in adult articular chondrocytes, but that this response is lost at high strains due to HDAC6-mediated cilia disassembly. The study provides new mechanistic insight into the role of primary cilia and mechanical loading in articular cartilage.

Identification of mechanosensitive genes during skeletal development: alteration of genes associated with cytoskeletal rearrangement and cell signalling pathways

Background

Mechanical stimulation is necessary for regulating correct formation of the skeleton. Here we test the hypothesis that mechanical stimulation of the embryonic skeletal system impacts expression levels of genes implicated in developmentally important signalling pathways in a genome wide approach. We use a mutant mouse model with altered mechanical stimulation due to the absence of limb skeletal muscle (Splotch-delayed) where muscle-less embryos show specific defects in skeletal elements including delayed ossification, changes in the size and shape of cartilage rudiments and joint fusion. We used Microarray and RNA sequencing analysis tools to identify differentially expressed genes between muscle-less and control embryonic (TS23) humerus tissue.

Results

We found that 680 independent genes were down-regulated and 452 genes up-regulated in humeri from muscle-less Spd embryos compared to littermate controls (at least 2-fold; corrected p-value ≤0.05). We analysed the resulting differentially expressed gene sets using Gene Ontology annotations to identify significant enrichment of genes associated with particular biological processes, showing that removal of mechanical stimuli from muscle contractions affected genes associated with development and differentiation, cytoskeletal architecture and cell signalling. Among cell signalling pathways, the most strongly disturbed was Wnt signalling, with 34 genes including 19 pathway target genes affected. Spatial gene expression analysis showed that both a Wnt ligand encoding gene (Wnt4) and a pathway antagonist (Sfrp2) are up-regulated specifically in the developing joint line, while the expression of a Wnt target gene, Cd44, is no longer detectable in muscle-less embryos. The identification of 84 genes associated with the cytoskeleton that are down-regulated in the absence of muscle indicates a number of candidate genes that are both mechanoresponsive and potentially involved in mechanotransduction, converting a mechanical stimulus into a transcriptional response.

Conclusions

This work identifies key developmental regulatory genes impacted by altered mechanical stimulation, sheds light on the molecular mechanisms that interpret mechanical stimulation during skeletal development and provides valuable resources for further investigation of the mechanistic basis of mechanoregulation. In particular it highlights the Wnt signalling pathway as a potential point of integration of mechanical and molecular signalling and cytoskeletal components as mediators of the response.

Mechanical regulation of skeletal development

Development of the various components of a normal skeleton requires highly regulated signalling systems that co-ordinate spatial and temporal patterns of cell division, cell differentiation, and morphogenesis. Much work in recent decades has revealed cascades of molecular signalling, acting through key transcription factors to regulate, for example, organized chondrogenic and osteogenic differentiation. It is now clear that mechanical stimuli are also required for aspects of skeletogenesis but very little is known about how the mechanical signals are integrated with classic biochemical signalling. Spatially organized differentiation is vital to the production of functionally appropriate tissues contributing to precise, region specific morphologies, for example transient chondrogenesis of long bone skeletal rudiments, which prefigures osteogenic replacement of the cartilage template, compared with the production of permanent cartilage at the sites of articulation. Currently a lack of understanding of how these tissues are differentially regulated hampers efforts to specifically regenerate stable bone and cartilage. Here, we review current research revealing the influence of mechanical stimuli on specific aspects of skeletal development and refer to other developing systems to set the scene for current and future work to uncover the molecular mechanisms involved. We integrate this with a brief overview of the effects of mechanical stimulation on stem cells in culture bringing together developmental and tissue engineering aspects of mechanoregulation of cell behavior. A better understanding of the molecular mechanisms that link mechanical stimuli to transcriptional control guiding cell differentiation will lead to new ideas about how to effectively prime stem cells for tissue engineering and regenerative therapies.

The emergence of mechanoregulated endochondral ossification in evolution

The differentiation of skeletal tissue phenotypes is partly regulated by mechanical forces. This mechanoregulatory aspect of tissue differentiation has been the subject of many experimental and computational investigations. However, little is known about what factors promoted the emergence of mechanoregulated tissue differentiation in evolution, even though mechanoregulated tissue differentiation, for example during development or healing of adult bone, is crucial for vertebrate phylogeny. In this paper, we use a computational framework to test the hypothesis that the emergence of mechanosensitive genes that trigger endochondral ossification in evolution will stabilise in the population and create a variable mechanoregulated response, if the endochondral ossification process enhances fitness for survival. The model combines an evolutionary algorithm that considers genetic change with a mechanoregulated fracture healing model in which the fitness of animals in a population is determined by their ability to heal their bones. The simulations show that, with the emergence of mechanosensitive genes through evolution enabling skeletal cells to modulate their synthetic activities, novel differentiation pathways such as endochondral ossification could have emerged, which when favoured by natural selection is maintained in a population. Furthermore, the model predicts that evolutionary forces do not lead to a single optimal mechanoregulated response but that the capacity of endochondral ossification exists with variability in a population. The simulations correspond with many existing findings about the mechanosensitivity of skeletal tissues in current animal populations, therefore indicating that this kind of multi-level models could be used in future population based simulations of tissue differentiation.

Hip ontogenesis: how evolution, genes, and load history shape hip morphotype and cartilotype

BACKGROUND

Developmental hip disorders (DHDs), eg, developmental dysplasia of the hip, slipped capitis femoris epiphysis, and femoroacetabular impingement, can be considered morphology variants of the normal hip. The femoroacetabular morphology of DHD is believed to induce osteoarthritis (OA) through local cumulative mechanical overload acting on genetically controlled patterning systems and subsequent damage of joint structures. However, it is unclear why hip morphology differs between individuals with seemingly comparable load histories and why certain hips with DHD progress to symptomatic OA whereas others do not.

QUESTIONS/PURPOSES

We asked (1) which mechanical factors influence growth and development of the proximal femur; and (2) which genes or genetic mechanisms are associated with hip ontogenesis.

METHODS

We performed a systematic literature review of mechanical and genetic factors of hip ontogeny. We focused on three fields that in recent years have advanced our knowledge of adult hip morphology: imaging, evolution, and genetics. WHERE ARE WE NOW?: Mechanical factors can be understood in view of human evolutionary peculiarities and may summate to load histories conducive to DHD. Genetic factors most likely act through multiple genes, each with modest effect sizes. Single genes that explain a DHD are therefore unlikely to be found. Apparently, the interplay between genes and load history not only determines hip morphotype, but also joint cartilage robustness ("cartilotype") and resistance to symptomatic OA. WHERE DO WE NEED TO GO?: We need therapies that can improve both morphotype and cartilotype. HOW DO WE GET THERE?: Better phenotyping, improving classification systems of hip morphology, and comparative population studies can be done with existing methods. Quantifying load histories likely requires new tools, but proof of principle of modifying morphotype in treatment of DDH and of cartilotype with exercise is available.

Biophysical stimuli induced by passive movements compensate for lack of skeletal muscle during embryonic skeletogenesis

In genetically modified mice with abnormal skeletal muscle development, bones and joints are differentially affected by the lack of skeletal muscle. We hypothesise that unequal levels of biophysical stimuli in the developing humerus and femur can explain the differential effects on these rudiments when muscle is absent. We find that the expression patterns of four mechanosensitive genes important for endochondral ossification are differentially affected in muscleless limb mutants, with more extreme changes in the expression in the humerus than in the femur. Using finite element analysis, we show that the biophysical stimuli induced by muscle forces are similar in the humerus and femur, implying that the removal of muscle contractile forces should, in theory, affect the rudiments equally. However, simulations in which a displacement was applied to the end of the limb, such as could be caused in muscleless mice by movements of the mother or normal littermates, predicted higher biophysical stimuli in the femur than in the humerus. Stimuli induced by limb movement were much higher than those induced by the direct application of muscle forces, and we propose that movements of limbs caused by muscle contractions, rather than the direct application of muscle forces, provide the main mechanical stimuli for normal skeletal development. In muscleless mice, passive movement induces unequal biophysical stimuli in the humerus and femur, providing an explanation for the differential effects seen in these mice. The significance of these results is that forces originating external to the embryo may contribute to the initiation and progression of skeletal development when muscle development is abnormal.

Variability observed in mechano-regulated in vivo tissue differentiation can be explained by variation in cell mechano-sensitivity

Computational simulations of tissue differentiation have been able to capture the main aspects of tissue formation/regeneration observed in animal experiments-except for the considerable degree of variability reported. Understanding and modelling the source of this variability is crucial if computational tools are to be developed for clinical applications. The objective of this study was to test the hypothesis that differences in cell mechano-sensitivity between individuals can explain the variability of tissue differentiation patterns observed experimentally. Simulations of an experiment of tissue differentiation in a mechanically loaded bone chamber were performed. Finite element analysis was used to determine the biophysical environment, and a lattice-modelling approach was used to simulate cell activity. Differences in cell mechano-sensitivity among individuals were modelled as differences in cell activity rates, with the activation of cell activities regulated by the mechanical environment. Predictions of the tissue distribution in the chambers produced the two different classes of results found experimentally: (i) chambers with a layer of bone across the chamber covered by a layer of cartilage on top and (ii) chambers with almost no bone, mainly fibrous tissue and small islands of cartilage. This indicates that the differing cellular response to the mechanical environment (i.e., subject-specific mechano-sensitivity) could be a reason for the different outcomes found when implants (or tissue engineered constructs) are used in a population.

Mechanical influences on morphogenesis of the knee joint revealed through morphological, molecular and computational analysis of immobilised embryos

Very little is known about the regulation of morphogenesis in synovial joints. Mechanical forces generated from muscle contractions are required for normal development of several aspects of normal skeletogenesis. Here we show that biophysical stimuli generated by muscle contractions impact multiple events during chick knee joint morphogenesis influencing differential growth of the skeletal rudiment epiphyses and patterning of the emerging tissues in the joint interzone. Immobilisation of chick embryos was achieved through treatment with the neuromuscular blocking agent Decamethonium Bromide. The effects on development of the knee joint were examined using a combination of computational modelling to predict alterations in biophysical stimuli, detailed morphometric analysis of 3D digital representations, cell proliferation assays and in situ hybridisation to examine the expression of a selected panel of genes known to regulate joint development. This work revealed the precise changes to shape, particularly in the distal femur, that occur in an altered mechanical environment, corresponding to predicted changes in the spatial and dynamic patterns of mechanical stimuli and region specific changes in cell proliferation rates. In addition, we show altered patterning of the emerging tissues of the joint interzone with the loss of clearly defined and organised cell territories revealed by loss of characteristic interzone gene expression and abnormal expression of cartilage markers. This work shows that local dynamic patterns of biophysical stimuli generated from muscle contractions in the embryo act as a source of positional information guiding patterning and morphogenesis of the developing knee joint.

The role of mechanical forces in plant morphogenesis

The shape of an organism relies on a complex network of genetic regulations and on the homeostasis and distribution of growth factors. In parallel to the molecular control of growth, shape changes also involve major changes in structure, which by definition depend on the laws of mechanics. Thus, to understand morphogenesis, scientists have turned to interdisciplinary approaches associating biology and physics to investigate the contribution of mechanical forces in morphogenesis, sometimes re-examining theoretical concepts that were laid out by early physiologists. Major advances in the field have notably been possible thanks to the development of computer simulations and live quantitative imaging protocols in recent years. Here, we present the mechanical basis of shape changes in plants, focusing our discussion on undifferentiated tissues. How can growth be translated into a quantified geometrical output? What is the mechanical basis of cell and tissue growth? What is the contribution of mechanical forces in patterning?

Matrix rigidity induces osteolytic gene expression of metastatic breast cancer cells

Nearly 70% of breast cancer patients with advanced disease will develop bone metastases. Once established in bone, tumor cells produce factors that cause changes in normal bone remodeling, such as parathyroid hormone-related protein (PTHrP). While enhanced expression of PTHrP is known to stimulate osteoclasts to resorb bone, the environmental factors driving tumor cells to express PTHrP in the early stages of development of metastatic bone disease are unknown. In this study, we have shown that tumor cells known to metastasize to bone respond to 2D substrates with rigidities comparable to that of the bone microenvironment by increasing expression and production of PTHrP. The cellular response is regulated by Rho-dependent actomyosin contractility mediated by TGF-ß signaling. Inhibition of Rho-associated kinase (ROCK) using both pharmacological and genetic approaches decreased PTHrP expression. Furthermore, cells expressing a dominant negative form of the TGF-ß receptor did not respond to substrate rigidity, and inhibition of ROCK decreased PTHrP expression induced by exogenous TGF-ß. These observations suggest a role for the differential rigidity of the mineralized bone microenvironment in early stages of tumor-induced osteolysis, which is especially important in metastatic cancer since many cancers (such as those of the breast and lung) preferentially metastasize to bone.