Melt electrowritten scaffold architectures to mimic tissue mechanics and guide neo-tissue orientation

All human tissues present with unique mechanical properties critical to their function. This is achieved in part through the specific architecture of the extracellular matrix (ECM) fibres within each tissue. An example of this is seen in the walls of the vasculature where each layer presents with a unique ECM orientation critical to its functions. Current adopted vascular grafts to bypass a stenosed/damaged vessel fail to recapitulate this unique mechanical behaviour, particularly in the case of small diameter vessels (<6 mm), leading to failure. Therefore, in this study, melt-electrowriting (MEW) was adopted to produce a range of fibrous scaffolds to mimic the extracellular matrix (ECM) architecture of the tunica media of the vasculature, in an attempt to match the mechanical and biological behaviour of the native porcine tissue. Initially, the range of collagen architectures within the native vessel was determined, and subsequently replicated using MEW (winding angles (WA) 45°, 26.5°, 18.4°, 11.3°). These scaffolds recapitulated the anisotropic, non-linear mechanical behaviour of native carotid blood vessels. Moreover, these grafts facilitated human mesenchymal stem cell (hMSC) infiltration, differentiation, and ECM deposition that was independent of WA. The bioinspired MEW fibre architecture promoted cell alignment and preferential neo-tissue orientation in a manner similar to that seen in native tissue, particularly for WA 18.4° and 11.3°, which is a mandatory requirement for long-term survival of the regenerated tissue post-scaffold degradation. Lastly, the WA 18.4° was translated to a tubular graft and was shown to mirror the mechanical behaviour of small diameter vessels within physiological strain. Taken together, this study demonstrates the capacity to use MEW to fabricate bioinspired scaffolds to mimic the tunica media of vessels and recapitulate vascular mechanics which could act as a framework for small diameter graft development to guide tissue regeneration and orientation.

2P-FLIM unveils time-dependent metabolic shifts during osteogenic differentiation with a key role of lactate to fuel osteogenesis via glutaminolysis identified

BACKGROUND

Human mesenchymal stem cells (hMSCs) utilize discrete biosynthetic pathways to self-renew and differentiate into specific cell lineages, with undifferentiated hMSCs harbouring reliance on glycolysis and hMSCs differentiating towards an osteogenic phenotype relying on oxidative phosphorylation as an energy source.

METHODS

In this study, the osteogenic differentiation of hMSCs was assessed and classified over 14 days using a non-invasive live-cell imaging modality-two-photon fluorescence lifetime imaging microscopy (2P-FLIM). This technique images and measures NADH fluorescence from which cellular metabolism is inferred.

RESULTS

During osteogenesis, we observe a higher dependence on oxidative phosphorylation (OxPhos) for cellular energy, concomitant with an increased reliance on anabolic pathways. Guided by these non-invasive observations, we validated this metabolic profile using qPCR and extracellular metabolite analysis and observed a higher reliance on glutaminolysis in the earlier time points of osteogenic differentiation. Based on the results obtained, we sought to promote glutaminolysis further by using lactate, to improve the osteogenic potential of hMSCs. Higher levels of mineral deposition and osteogenic gene expression were achieved when treating hMSCs with lactate, in addition to an upregulation of lactate metabolism and transmembrane cellular lactate transporters. To further clarify the interplay between glutaminolysis and lactate metabolism in osteogenic differentiation, we blocked these pathways using BPTES and α-CHC respectively. A reduction in mineralization was found after treatment with BPTES and α-CHC, demonstrating the reliance of hMSC osteogenesis on glutaminolysis and lactate metabolism.

CONCLUSION

In summary, we demonstrate that the osteogenic differentiation of hMSCs has a temporal metabolic profile and shift that is observed as early as day 3 of cell culture using 2P-FLIM. Furthermore, extracellular lactate is shown as an essential metabolite and metabolic fuel to ensure efficient osteogenic differentiation and as a signalling molecule to promote glutaminolysis. These findings have significant impact in the use of 2P-FLIM to discover potent approaches towards bone tissue engineering in vitro and in vivo by engaging directly with metabolite-driven osteogenesis.

Nano sized gallium oxide surface features for enhanced antimicrobial and osteo-integrative responses

Gallium oxide has known beneficial osteo-integrative properties. This may have importance for improving the osteointegration of orthopedic implants. At high concentrations gallium is cytotoxic. Therefore, integration of gallium into implant devices must be carefully controlled to limit its concentration and release. A strategy based on surface doping of gallium although challenging seems an appropriate approach to limit dose amounts to minimize cytotoxicity and maximize osteointegration benefits. In this work we develop a novel form of patterned surface doping via a block copolymer-based surface chemistry that enables very low gallium content but enhanced osteointegration as proven by comprehensive bioassays. Polystyrene-b-poly 4vinyl pyridine (PS-b-P4VP) BCP (block copolymer) films were produced on surfaces. Selective infiltration of the BCP pattern with a gallium salt precursor solution and subsequent UV-ozone treatment produced a surface pattern of gallium oxide nanodots as evidenced by atomic force and scanning electron microscopy. A comprehensive study of the bioactivity was carried out, including antimicrobial and sterility testing, gallium ion release kinetics and the interaction with human marrow mesenchymal stomal cells and mononuclear cells. Comparing the data from osteogenesis media assay tests with osteoclastogenesis tests demonstrated the potential for the gallium oxide nanodot doping to improve osteointegration properties of a surface.

A microphysiological model of bone development and regeneration

Endochondral ossification (EO) is an essential biological process than underpins how human bones develop, grow, and heal in the event of a fracture. So much is unknown about this process, thus clinical manifestations of dysregulated EO cannot be adequately treated. This can be partially attributed to the absence of predictive in vitro models of musculoskeletal tissue development and healing, which are integral to the development and preclinical evaluation of novel therapeutics. Microphysiological systems, or organ-on-chip devices, are advanced in vitro models designed for improved biological relevance compared to traditional in vitro culture models. Here we develop a microphysiological model of vascular invasion into developing/regenerating bone, thereby mimicking the process of EO. This is achieved by integrating endothelial cells and organoids mimicking different stages of endochondral bone development within a microfluidic chip. This microphysiological model is able to recreate key events in EO, such as the changing angiogenic profile of a maturing cartilage analogue, and vascular induced expression of the pluripotent transcription factors SOX2 and OCT4 in the cartilage analogue. This system represents an advanced in vitro platform to further EO research, and may also serve as a modular unit to monitor drug responses on such processes as part of a multi-organ system.&#xD.

Bioengineering extracellular vesicles: smart nanomaterials for bone regeneration

In the past decade, extracellular vesicles (EVs) have emerged as key regulators of bone development, homeostasis and repair. EV-based therapies have the potential to circumnavigate key issues hindering the translation of cell-based therapies including functional tissue engraftment, uncontrolled differentiation and immunogenicity issues. Due to EVs' innate biocompatibility, low immunogenicity, and high physiochemical stability, these naturally-derived nanoparticles have garnered growing interest as potential acellular nanoscale therapeutics for a variety of diseases. Our increasing knowledge of the roles these cell-derived nanoparticles play, has made them an exciting focus in the development of novel pro-regenerative therapies for bone repair. Although these nano-sized vesicles have shown promise, their clinical translation is hindered due to several challenges in the EV supply chain, ultimately impacting therapeutic efficacy and yield. From the biochemical and biophysical stimulation of parental cells to the transition to scalable manufacture or maximising vesicles therapeutic response in vivo, a multitude of techniques have been employed to improve the clinical efficacy of EVs. This review explores state of the art bioengineering strategies to promote the therapeutic utility of vesicles beyond their native capacity, thus maximising the clinical potential of these pro-regenerative nanoscale therapeutics for bone repair.

Mechanoregulatory role of TRPV4 in prenatal skeletal development

Biophysical cues are essential for guiding skeletal development, but the mechanisms underlying the mechanical regulation of cartilage and bone formation are unknown. TRPV4 is a mechanically sensitive ion channel involved in cartilage and bone cell mechanosensing, mutations of which lead to skeletal developmental pathologies. We tested the hypothesis that loading-driven prenatal skeletal development is dependent on TRPV4 activity. We first establish that mechanically stimulating mouse embryo hindlimbs cultured ex vivo stimulates knee cartilage growth, morphogenesis, and expression of TRPV4, which localizes to areas of high biophysical stimuli. We then demonstrate that loading-driven joint cartilage growth and shape are dependent on TRPV4 activity, mediated via control of cell proliferation and matrix biosynthesis, indicating a mechanism by which mechanical loading could direct growth and morphogenesis during joint formation. We conclude that mechanoregulatory pathways initiated by TRPV4 guide skeletal development; therefore, TRPV4 is a valuable target for the development of skeletal regenerative and repair strategies.

Surface Free Energy Dominates the Biological Interactions of Postprocessed Additively Manufactured Ti-6Al-4V

Additive manufacturing (AM) has emerged as a disruptive technique within healthcare because of its ability to provide personalized devices; however, printed metal parts still present surface and microstructural defects, which may compromise mechanical and biological interactions. This has made physical and/or chemical postprocessing techniques essential for metal AM devices, although limited fundamental knowledge is available on how alterations in physicochemical properties influence AM biological outcomes. For this purpose, herein, powder bed fusion Ti-6Al-4V samples were postprocessed with three industrially relevant techniques: polishing, passivation, and vibratory finishing. These surfaces were thoroughly characterized in terms of roughness, chemistry, wettability, surface free energy, and surface ζ-potential. A significant increase in Staphylococcus epidermidis colonization was observed on both polished and passivated samples, which was linked to high surface free energy donor γ^– values in the acid–base, γ^AB component. Early osteoblast attachment and proliferation (24 h) were not influenced by these properties, although increased mineralization was observed for both these samples. In contrast, osteoblast differentiation on stainless steel was driven by a combination of roughness and chemistry. Collectively, this study highlights that surface free energy is a key driver between AM surfaces and cell interactions. In particular, while low acid–base components resulted in a desired reduction in S. epidermidis colonization, this was followed by reduced mineralization. Thus, while surface free energy can be used as a guide to AM device development, optimization of bacterial and mammalian cell interactions should be attained through a combination of different postprocessing techniques.

Hydrostatic pressure promotes chondrogenic differentiation and microvesicle release from human embryonic and bone marrow stem cells

Mechanical stimulation plays in an important role in regulating stem cell differentiation and their release of extracellular vesicles (EVs). In this study, effects of low magnitude hydrostatic pressure (HP) on the chondrogenic differentiation and microvesicle release from human embryonic stem cells (hESCs) and human bone marrow stem cells (hBMSCs) are examined. hESCs were differentiated into chondroprogenitors and then embedded in fibrin gels and subjected to HP (270 kPa, 1 Hz, 5 days per week). hBMSC pellets were differentiated in chondrogenic media and subjected to the same regime. HP significantly enhanced ACAN expression in hESCs. It also led to a significant increase in DNA content, sGAG content and total sGAG/DNA level in hBMSCs. Furthermore, HP significantly increased microvesicle protein content released from both cell types. These results highlight the benefit of HP bioreactor in promoting chondrogenesis and EV production for cartilage tissue engineering.

An ECM-Mimetic Hydrogel to Promote the Therapeutic Efficacy of Osteoblast-Derived Extracellular Vesicles for Bone Regeneration

The use of extracellular vesicles (EVs) is emerging as a promising acellular approach for bone regeneration, overcoming translational hurdles associated with cell-based therapies. Despite their potential, EVs short half-life following systemic administration hinders their therapeutic efficacy. EVs have been reported to bind to extracellular matrix (ECM) proteins and play an essential role in matrix mineralisation. Chitosan and collagen type I are naturally-derived pro-osteogenic biomaterials, which have been demonstrated to control EV release kinetics. Therefore, this study aimed to develop an injectable ECM-mimetic hydrogel capable of controlling the release of osteoblast-derived EVs to promote bone repair. Pure chitosan hydrogels significantly enhanced compressive modulus (2.48-fold) and osteogenic differentiation (3.07-fold), whilst reducing gelation times (2.09-fold) and proliferation (2.7-fold) compared to pure collagen gels (p ≤ 0.001). EV release was strongly associated with collagen concentration (R² > 0.94), where a significantly increased EV release profile was observed from chitosan containing gels using the CD63 ELISA (p ≤ 0.001). Hydrogel-released EVs enhanced human bone marrow stromal cells (hBMSCs) proliferation (1.12-fold), migration (2.55-fold), and mineralisation (3.25-fold) compared to untreated cells (p ≤ 0.001). Importantly, EV-functionalised chitosan-collagen composites significantly promoted hBMSCs extracellular matrix mineralisation when compared to the EV-free gels in a dose-dependent manner (p ≤ 0.001). Taken together, these findings demonstrate the development of a pro-osteogenic thermosensitive chitosan-collagen hydrogel capable of enhancing the therapeutic efficacy of osteoblast-derived EVs as a novel acellular tool for bone augmentation strategy.

The Impact of the Extracellular Matrix Environment on Sost Expression by the MLO-Y4 Osteocyte Cell Line

Bone is a dynamic organ that can adapt its structure to meet the demands of its biochemical and biophysical environment. Osteocytes form a sensory network throughout the tissue and orchestrate tissue adaptation via the release of soluble factors such as a sclerostin. Osteocyte physiology has traditionally been challenging to investigate due to the uniquely mineralized extracellular matrix (ECM) of bone leading to the development of osteocyte cell lines. Importantly, the most widely researched and utilized osteocyte cell line: the MLO-Y4, is limited by its inability to express sclerostin (Sost gene) in typical in-vitro culture. We theorised that culture in an environment closer to the in vivo osteocyte environment could impact on Sost expression. Therefore, this study investigated the role of composition and dimensionality in directing Sost expression in MLO-Y4 cells using collagen-based ECM analogues. A significant outcome of this study is that MLO-Y4 cells, when cultured on a hydroxyapatite (HA)-containing two-dimensional (2D) film analogue, expressed Sost. Moreover, three-dimensional (3D) culture within HA-containing collagen scaffolds significantly enhanced Sost expression, demonstrating the impact of ECM composition and dimensionality on MLO-Y4 behaviour. Importantly, in this bone mimetic ECM environment, Sost expression was found to be comparable to physiological levels. Lastly, MLO-Y4 cells cultured in these novel conditions responded accordingly to fluid flow stimulation with a decrease in expression. This study therefore presents a novel culture system for the MLO-Y4 osteocyte cell line, ensuring the expression of an important osteocyte specific gene, Sost, overcoming a major limitation of this model.

Controlled Release of Epigenetically-Enhanced Extracellular Vesicles from a GelMA/Nanoclay Composite Hydrogel to Promote Bone Repair

Extracellular vesicles (EVs) have garnered growing attention as promising acellular tools for bone repair. Although EVs' potential for bone regeneration has been shown, issues associated with their therapeutic potency and short half-life in vivo hinders their clinical utility. Epigenetic reprogramming with the histone deacetylase inhibitor Trichostatin A (TSA) has been reported to promote the osteoinductive potency of osteoblast-derived EVs. Gelatin methacryloyl (GelMA) hydrogels functionalised with the synthetic nanoclay laponite (LAP) have been shown to effectively bind, stabilise, and improve the retention of bioactive factors. This study investigated the potential of utilising a GelMA-LAP hydrogel to improve local retention and control delivery of epigenetically enhanced osteoblast-derived EVs as a novel bone repair strategy. LAP was found to elicit a dose-dependent increase in GelMA compressive modulus and shear-thinning properties. Incorporation of the nanoclay was also found to enhance shape fidelity when 3D printed compared to LAP-free gels. Interestingly, GelMA hydrogels containing LAP displayed increased mineralisation capacity (1.41-fold) (p ≤ 0.01) over 14 days. EV release kinetics from these nanocomposite systems were also strongly influenced by LAP concentration with significantly more vesicles being released from GelMA constructs as detected by a CD63 ELISA (p ≤ 0.001). EVs derived from TSA-treated osteoblasts (TSA-EVs) enhanced proliferation (1.09-fold), migration (1.83-fold), histone acetylation (1.32-fold) and mineralisation (1.87-fold) of human bone marrow stromal cells (hBMSCs) when released from the GelMA-LAP hydrogel compared to the untreated EV gels (p ≤ 0.01). Importantly, the TSA-EV functionalised GelMA-LAP hydrogel significantly promoted encapsulated hBMSCs extracellular matrix collagen production (≥1.3-fold) and mineralisation (≥1.78-fold) in a dose-dependent manner compared to untreated EV constructs (p ≤ 0.001). Taken together, these findings demonstrate the potential of combining epigenetically enhanced osteoblast-derived EVs with a nanocomposite photocurable hydrogel to promote the therapeutic efficacy of acellular vesicle approaches for bone regeneration.

Development of a Bone-Mimetic 3D Printed Ti6Al4V Scaffold to Enhance Osteoblast-Derived Extracellular Vesicles' Therapeutic Efficacy for Bone Regeneration

Extracellular Vesicles (EVs) are considered promising nanoscale therapeutics for bone regeneration. To date, EVs are typically procured from cells on 2D tissue culture plastic, an artificial environment that limits cell growth and does not replicate in situ biochemical or biophysical conditions. This study investigated the potential of 3D printed titanium scaffolds coated with hydroxyapatite to promote the therapeutic efficacy of osteoblast-derived EVs. Ti6Al4V titanium scaffolds with different pore sizes (500 and 1000 µm) and shapes (square and triangle) were fabricated by selective laser melting. A bone-mimetic nano-needle hydroxyapatite (nnHA) coating was then applied. EVs were procured from scaffold-cultured osteoblasts over 2 weeks and vesicle concentration was determined using the CD63 ELISA. Osteogenic differentiation of human bone marrow stromal cells (hBMSCs) following treatment with primed EVs was evaluated by assessing alkaline phosphatase activity, collagen production and calcium deposition. Triangle pore scaffolds significantly increased osteoblast mineralisation (1.5-fold) when compared to square architectures (P ≤ 0.001). Interestingly, EV yield was also significantly enhanced on these higher permeability structures (P ≤ 0.001), in particular (2.2-fold) for the larger pore structures (1000 µm). Furthermore osteoblast-derived EVs isolated from triangular pore scaffolds significantly increased hBMSCs mineralisation when compared to EVs acquired from square pore scaffolds (1.7-fold) and 2D culture (2.2-fold) (P ≤ 0.001). Coating with nnHA significantly improved osteoblast mineralisation (>2.6-fold) and EV production (4.5-fold) when compared to uncoated scaffolds (P ≤ 0.001). Together, these findings demonstrate the potential of harnessing bone-mimetic culture platforms to enhance the production of pro-regenerative EVs as an acellular tool for bone repair.

A meta-analysis of the diagnostic accuracy of Hounsfield units on computed topography relative to dual-energy X-ray absorptiometry for the diagnosis of osteoporosis in the spine surgery population

BACKGROUND

The preoperative identification of osteoporosis in the spine surgery population is of crucial importance. Limitations associated with dual-energy x-ray absorptiometry, such as access and reliability, have prompted the search for alternative methods to diagnose osteoporosis. The Hounsfield Unit(HU), a readily available measure on computed tomography, has garnered considerable attention in recent years as a potential diagnostic tool for reduced bone mineral density. However, the optimal threshold settings for diagnosing osteoporosis have yet to be determined.

METHODS

We selected studies that included comparison of the HU(index test) with dual-energy x-ray absorptiometry evaluation(reference test). Data quality was assessed using the standardised QUADAS-2 criteria. Studies were characterised into 3 categories, based on the threshold of the index test used with the goal of obtaining a high sensitivity, high specificity or balanced sensitivity-specificity test.

RESULTS

9 studies were eligible for meta-analysis. In the high specificity group, the pooled sensitivity was 0.652 (95% CI 0.526 - 0.760), specificity 0.795 (95% CI 0.711 - 0.859) and diagnostic odds ratio was 6.652 (95% CI 4.367 - 10.133). In the high sensitivity group, the overall pooled sensitivity was 0.912 (95% CI 0.718 - 0.977), specificity was 0.67 (0.57 - 0.75) and diagnostic odds ratio was 19.424 (5.446 - 69.275). In the balanced sensitivity-specificity group, the overall pooled sensitivity was 0.625 (95% CI 0.504 - 0.732), specificity was 0.914 (0.823 - 0.960) and diagnostic odds ratio was 14.880 (7.521 - 29.440). Considerable heterogeneity existed throughout the analysis.

CONCLUSION

In conclusion, the HU is a clinically useful tool to aide in the diagnosis of osteoporosis. However, the heterogeneity seen in this study warrants caution in the interpretation of results. We have demonstrated the impact of differing HU threshold values on the diagnostic ability of this test. We would propose a threshold of 135 HU to diagnose OP. Future work would investigate the optimal HU cut-off to differentiate normal from low bone mineral density.

Biofabrication of vasculature in microphysiological models of bone

Bone contains a dense network of blood vessels that are essential to its homoeostasis, endocrine function, mineral metabolism and regenerative functions. In addition, bone vasculature is implicated in a number of prominent skeletal diseases, and bone has high affinity for metastatic cancers. Despite vasculature being an integral part of bone physiology and pathophysiology, it is often ignored or oversimplified inin vitrobone models. However, 3D physiologically relevant vasculature can now be engineeredin vitro, with microphysiological systems (MPS) increasingly being used as platforms for engineering this physiologically relevant vasculature. In recent years, vascularised models of bone in MPSs systems have been reported in the literature, representing the beginning of a possible technological step change in how bone is modelledin vitro. Vascularised bone MPSs is a subfield of bone research in its nascency, however given the impact of MPSs has had inin vitroorgan modelling, and the crucial role of vasculature to bone physiology, these systems stand to have a substantial impact on bone research. However, engineering vasculature within the specific design restraints of the bone niche is significantly challenging given the different requirements for engineering bone and vasculature. With this in mind, this paper aims to serve as technical guidance for the biofabrication of vascularised bone tissue within MPS devices. We first discuss the key engineering and biological considerations for engineering more physiologically relevant vasculaturein vitrowithin the specific design constraints of the bone niche. We next explore emerging applications of vascularised bone MPSs, and conclude with a discussion on the current status of vascularised bone MPS biofabrication and suggest directions for development of next generation vascularised bone MPSs.

Estrogen withdrawal alters cytoskeletal and primary ciliary dynamics resulting in increased Hedgehog and osteoclastogenic paracrine signalling in osteocytes

Estrogen deficiency during post-menopausal osteoporosis leads to osteoclastogenesis and bone loss. Increased pro-osteoclastogenic signalling (RANKL/OPG) by osteocytes occurs following estrogen withdrawal (EW) and is associated with impaired focal adhesions (FAs) and a disrupted actin cytoskeleton. RANKL production is mediated by Hedgehog signalling in osteocytes, a signalling pathway associated with the primary cilium, and the ciliary structure is tightly coupled to the cytoskeleton. Therefore, the objective of this study was to investigate the role of the cilium and associated signalling in EW-mediated osteoclastogenic signalling in osteocytes. We report that EW leads to an elongation of the cilium and increase in Hedgehog and osteoclastogenic signalling. Significant trends were identified linking cilia elongation with reductions in cell area and % FA area/cell area, indicating that cilia elongation is associated with disruption of FAs and actin contractility. To verify this, we inhibited FA assembly via α_vβ₃ antagonism and inhibited actin contractility and demonstrated an elongated cilia and increased expression of Hh markers and Rankl expression. Therefore, our results suggest that the EW conditions associated with osteoporosis lead to a disorganisation of α_vβ₃ integrins and reduced actin contractility, which were associated with an elongation of the cilium, activation of the Hh pathway and osteoclastogenic paracrine signalling.

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.

Development of a New Bone-Mimetic Surface Treatment Platform: Nanoneedle Hydroxyapatite (nnHA) Coating

The hierarchical structure of bone plays pivotal roles in driving cell behavior and tissue regeneration and must be considered when designing materials for orthopedic applications. Herein, it is aimed to recapitulate the native bone environment by using melt electrowriting to fabricate fibrous microarchitectures which are modified with plate-shaped (pHA) or novel nanoneedle-shaped (nnHA) crystals. Nuclear magnetic resonance spectroscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction demonstrate that these coatings replicate the nanostructure and composition of native bone. Human mesenchymal stem/stromal cell (MSC) mineralization is significantly increased fivefold with pHA scaffolds and 14-fold with nnHA scaffolds. Given the protein stabilizing properties of mineral, these materials are further functionalized with bone morphogenetic protein 2 (BMP2). nnHA treatment facilitates controlled release of BMP2 which further enhance MSC mineral deposition. Finally, the versatility of this nnHA treatment method, which may be used to coat different architectures/materials including fused deposition modeling (FDM) scaffolds and Ti6Al4V titanium, is demonstrated. This study thus outlines a method for fabricating scaffolds with precise fibrous microarchitectures and bone-mimetic nnHA extrafibrillar coatings which significantly enhance MSC osteogenesis and therapeutic protein delivery, and leverages these results to show how this surface treatment method may be applied to a wider field for multiple orthopedic applications.

Human bone marrow stem/stromal cell osteogenesis is regulated via mechanically activated osteocyte-derived extracellular vesicles

Bone formation or regeneration requires the recruitment, proliferation, and osteogenic differentiation of stem/stromal progenitor cells. A potent stimulus driving this process is mechanical loading. Osteocytes are mechanosensitive cells that play fundamental roles in coordinating loading-induced bone formation via the secretion of paracrine factors. However, the exact mechanisms by which osteocytes relay mechanical signals to these progenitor cells are poorly understood. Therefore, this study aimed to demonstrate the potency of the mechanically stimulated osteocyte secretome in driving human bone marrow stem/stromal cell (hMSC) recruitment and differentiation, and characterize the secretome to identify potential factors regulating stem cell behavior and bone mechanobiology. We demonstrate that osteocytes subjected to fluid shear secrete a distinct collection of factors that significantly enhance hMSC recruitment and osteogenesis and demonstrate the key role of extracellular vesicles (EVs) in driving these effects. This demonstrates the pro-osteogenic potential of osteocyte-derived mechanically activated extracellular vesicles, which have great potential as a cell-free therapy to enhance bone regeneration and repair in diseases such as osteoporosis.

Loss of Adenylyl Cyclase 6 in Leptin Receptor-Expressing Stromal Cells Attenuates Loading-Induced Endosteal Bone Formation

Bone marrow stromal/stem cells represent a quiescent cell population that replenish the osteoblast bone‐forming cell pool with age and in response to injury, maintaining bone mass and repair. A potent mediator of stromal/stem cell differentiation in vitro and bone formation in vivo is physical loading, yet it still remains unclear whether loading‐induced bone formation requires the osteogenic differentiation of these resident stromal/stem cells. Therefore, in this study, we utilized the leptin receptor (LepR) to identify and trace the contribution of bone marrow stromal cells to mechanoadaptation of bone in vivo. Twelve‐week‐old Lepr‐cre;tdTomato mice were subjected to compressive tibia loading with an 11 N peak load for 40 cycles, every other day for 2 weeks. Histological analysis revealed that Lepr‐cre;tdTomato⁺ cells arise perinatally around blood vessels and populate bone surfaces as lining cells or osteoblasts before a percentage undergo osteocytogenesis. Lepr‐cre;tdTomato⁺ stromal cells within the marrow increase in abundance with age, but not following the application of tibial compressive loading. Mechanical loading induces an increase in bone mass and bone formation parameters, yet does not evoke an increase in Lepr‐cre;tdTomato⁺ osteoblasts or osteocytes. To investigate whether adenylyl cyclase‐6 (AC6) in LepR cells contributes to this mechanoadaptive response, Lepr‐cre;tdTomato mice were further crossed with AC6^fl/fl mice to generate a LepR⁺ cell‐specific knockout of AC6. These Lepr‐cre;tdTomato;AC6^fl/fl animals have an attenuated response to compressive tibia loading, characterized by a deficient load‐induced osteogenic response on the endosteal bone surface. This, therefore, shows that Lepr‐cre;tdTomato⁺ cells contribute to short‐term bone mechanoadaptation. © 2020 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Utilizing Osteocyte Derived Factors to Enhance Cell Viability and Osteogenic Matrix Deposition within IPN Hydrogels

Many bone defects arising due to traumatic injury, disease, or surgery are unable to regenerate, requiring intervention. More than four million graft procedures are performed each year to treat these defects making bone the second most commonly transplanted tissue worldwide. However, these types of graft suffer from a limited supply, a second surgical site, donor site morbidity, and pain. Due to the unmet clinical need for new materials to promote skeletal repair, this study aimed to produce novel biomimetic materials to enhance stem/stromal cell osteogenesis and bone repair by recapitulating aspects of the biophysical and biochemical cues found within the bone microenvironment. Utilizing a collagen type I-alginate interpenetrating polymer network we fabricated a material which mirrors the mechanical and structural properties of unmineralized bone, consisting of a porous fibrous matrix with a young's modulus of 64 kPa, both of which have been shown to enhance mesenchymal stromal/stem cell (MSC) osteogenesis. Moreover, by combining this material with biochemical paracrine factors released by statically cultured and mechanically stimulated osteocytes, we further mirrored the biochemical environment of the bone niche, enhancing stromal/stem cell viability, differentiation, and matrix deposition. Therefore, this biomimetic material represents a novel approach to promote skeletal repair.

The effect of pore size within fibrous scaffolds fabricated using melt electrowriting on human bone marrow stem cell osteogenesis

Limitations associated with current bone grafting materials has necessitated the development of synthetic scaffolds that mimic the native tissue for bone repair. Scaffold parameters such as pore size, pore interconnectivity, fibre diameter, and fibre stiffness are crucial parameters of fibrous bone tissue engineering (BTE) scaffolds required to replicate the native environment. Optimum values vary with material, fabrication method and cell type. Melt electrowriting (MEW) provides precise control over extracellular matrix (ECM)-like fibrous scaffold architecture. The goal of this study was to fabricate and characterise poly-ε-caprolactone (PCL) fibrous scaffolds with 100, 200, and 300 μm pore sizes using MEW and determine the influence of pore size on human bone marrow stem cell (hMSC) adhesion, morphology, proliferation, mechanosignalling and osteogenesis. Each scaffold was fabricated with a fibre diameter of 4.01 ± 0.06 μm. The findings from this study highlight the enhanced osteogenic effects of controlled micro-scale fibre deposition using MEW, where the benefits of 100 μm square pores in comparison with larger pore sizes are illustrated, a pore size traditionally reported as a lower limit for osteogenesis. This suggests a lower pore size is optimal when hMSCs are seeded in a 3D ECM-like fibrous structure, with the 100 μm pore size optimal as it demonstrates the highest global stiffness, local fibre stiffness, highest seeding efficiency, maintains a spread cellular morphology, and significantly enhances hMSC collagen and mineral deposition. Similarly, this platform represents an effective in vitro model for the study of hMSC behaviour to determine the significant osteogenic benefits of controlling ECM-like fibrous BTE scaffold pore size using MEW.

Aged Osteoporotic Bone Marrow Stromal Cells Demonstrate Defective Recruitment, Mechanosensitivity, and Matrix Deposition

Bone formation requires the replenishment of the osteoblast from a progenitor or stem cell population, which must be recruited, expanded, and differentiated to ensure continued anabolism. How this occurs and whether it is altered in the osteoporotic environment is poorly understood. Furthermore, given that emerging treatments for osteoporosis are targeting this progenitor population, it is critical to determine the regenerative capacity of this cell type in the setting of osteoporosis. Human bone marrow stromal cells (hMSCs) from a cohort of aged osteoporotic patients were compared to MSCs isolated from healthy donors in terms of the ability to undergo recruitment and proliferation, and also respond to both the biophysical and biochemical cues that drive osteogenic matrix deposition. hMSCs isolated from healthy donors demonstrate good recruitment, mechanosensitivity, proliferation, and differentiation capacity. Contrastingly, hMSCs isolated from aged osteoporotic patients had significantly diminished regenerative potential. Interestingly, we demonstrated that osteoporotic hMSCs no longer responded to chemokine-directing recruitment and became desensitised to mechanical stimulation. The osteoporotic MSCs had a reduced proliferative potential and, importantly, they demonstrated an attenuated differentiation capability with reduced mineral and lipid formation. Moreover, during osteogenesis, despite minimal differences in the quantity of deposited collagen, the distribution of collagen was dramatically altered in osteoporosis, suggesting a potential defect in matrix quality. Taken together, this study has demonstrated that hMSCs isolated from aged osteoporotic patients demonstrate defective cell behaviour on multiple fronts, resulting in a significantly reduced regenerative potential, which must be considered during the development of new anabolic therapies that target this cell population.

Integrins in Osteocyte Biology and Mechanotransduction

PURPOSE OF REVIEW

Osteocytes are the main mechanosensitive cells in bone. Integrin-based adhesions have been shown to facilitate mechanotransduction, and therefore play an important role in load-induced bone formation. This review outlines the role of integrins in osteocyte function (cell adhesion, signalling, and mechanotransduction) and possible role in disease.

RECENT FINDINGS

Both β₁ and β₃ integrins subunits have been shown to be required for osteocyte mechanotransduction. Antagonism of these integrin subunits in osteocytes resulted in impaired responses to fluid shear stress. Various disease states (osteoporosis, osteoarthritis, bone metastases) have been shown to result in altered integrin expression and function. Osteocyte integrins are required for normal cell function, with dysregulation of integrins seen in disease. Understanding the mechanism of faulty integrins in disease may aid in the creation of novel therapeutic approaches.

Direct mechanical characterization of prostate tissue-a systematic review

BACKGROUND

Direct mechanical characterization of tissue is the application of engineering techniques to biological tissue to ascertain stiffness or elasticity, which can change in response to disease states. A number of papers have been published on the application of these techniques to prostate tissue with a range of results reported. There is a marked variability in the results depending on testing techniques and disease state of the prostate tissue. We aimed to clarify the utility of direct mechanical characterization of prostate tissue in identifying disease states.

METHODS

A systematic review of the published literature regarding direct mechanical characterization of prostate tissue was undertaking according to PRISMA guidelines.

RESULTS

A variety of testing methods have been used, including compression, indentation, and tensile testing, as well as some indirect testing techniques, such as shear-wave elastography. There is strong evidence of significant stiffness differences between cancerous and non-cancerous prostate tissue, as well as correlations with prostate cancer stage. There is a correlation with increasing prostate stiffness and increasing lower urinary tract symptoms in patients with benign prostate hyperplasia. There is a wide variation in the testing methods and protocols used in the literature making direct comparison between papers difficult. Most studies utilise ex-vivo or cadaveric tissue, while none incorporate in vivo testing.

CONCLUSION

Direct mechanical assessment of prostate tissue permits a better understanding of the pathological and physiological changes that are occurring within the tissue. Further work is needed to include prospective and in vivo data to aid medical device design and investigate non-surgical methods of managing prostate disease.

Pressure-induced mesenchymal stem cell osteogenesis is dependent on intermediate filament remodeling

Macroscale loading of bone generates a complex local mechanical microenvironment that drives osteogenesis and bone mechanoadaptation. One such mechanical stimulus generated is hydrostatic pressure (HP); however, the effect of HP on mesenchymal stem cells (MSCs) and the mechanotransduction mechanisms utilized by these cells to sense this stimulus are yet to be fully elucidated. In this study, we demonstrate that cyclic HP is a potent mediator of cytoskeletal reorganization and increases in osteogenic responses in MSCs. In particular, we demonstrate that the intermediate filament (IF) network undergoes breakdown and reorganization with centripetal translocation of IF bundles toward the perinuclear region. Furthermore, we show for the first time that this IF remodeling is required for loading-induced MSC osteogenesis, revealing a novel mechanism of MSC mechanotransduction. In addition, we demonstrate that chemical disruption of IFs with withaferin A induces a similar mechanism of IF breakdown and remodeling as well as a subsequent increase in osteogenic gene expression in MSCs, exhibiting a potential mechanotherapeutic effect to enhance MSC osteogenesis. This study therefore highlights a novel mechanotransduction mechanism of pressure-induced MSC osteogenesis involving the understudied cytoskeletal structure, the IF, and demonstrates a potential new therapy to enhance bone formation in bone-loss diseases such as osteoporosis.-Stavenschi, E., Hoey, D. A. Pressure-induced mesenchymal stem cell osteogenesis is dependent on intermediate filament remodeling.

Mesenchymal stem cell mechanotransduction is cAMP dependent and regulated by adenylyl cyclase 6 and the primary cilium

Mechanical loading is a potent stimulus of bone adaptation, requiring the replenishment of the osteoblast from a progenitor population. One such progenitor is the mesenchymal stem cell (MSC), which undergoes osteogenic differentiation in response to oscillatory fluid shear. Yet, the mechanism mediating stem cell mechanotransduction, and thus the potential to target this therapeutically, is poorly understood. In this study, we demonstrate that MSCs utilise cAMP as a second messenger in mechanotransduction, which is required for flow-mediated increases in osteogenic gene expression. Furthermore, we demonstrate that this mechanosignalling is dependent on the primary cilium and the ciliary localised adenylyl cyclase 6. Finally, we also demonstrate that this mechanotransduction mechanism can be targeted therapeutically to enhance cAMP signalling and early osteogenic signalling, mimicking the beneficial effect of physical loading. Our findings therefore demonstrate a novel mechanism of MSC mechanotransduction that can be targeted therapeutically, demonstrating a potential mechanotherapeutic for bone-loss diseases such as osteoporosis.This article has an associated First Person interview with the first author of the paper.

Physiological cyclic hydrostatic pressure induces osteogenic lineage commitment of human bone marrow stem cells: a systematic study

Background

Physical loading is necessary to maintain bone tissue integrity. Loading-induced fluid shear is recognised as one of the most potent bone micromechanical cues and has been shown to direct stem cell osteogenesis. However, the effect of pressure transients, which drive fluid flow, on human bone marrow stem cell (hBMSC) osteogenesis is undetermined. Therefore, the objective of the study is to employ a systematic analysis of cyclic hydrostatic pressure (CHP) parameters predicted to occur in vivo on early hBMSC osteogenic responses and late-stage osteogenic lineage commitment.

Methods

hBMSC were exposed to CHP of 10 kPa, 100 kPa and 300 kPa magnitudes at frequencies of 0.5 Hz, 1 Hz and 2 Hz for 1 h, 2 h and 4 h of stimulation, and the effect on early osteogenic gene expression of COX2, RUNX2 and OPN was determined. Moreover, to decipher whether CHP can induce stem cell lineage commitment, hBMSCs were stimulated for 4 days for 2 h/day using 10 kPa, 100 kPa and 300 kPa pressures at 2 Hz frequency and cultured statically for an additional 1–2 weeks. Pressure-induced osteogenesis was quantified based on ATP release, collagen synthesis and mineral deposition.

Results

CHP elicited a positive, but variable, early osteogenic response in hBMSCs in a magnitude- and frequency-dependent manner, that is gene specific. COX2 expression elicited magnitude-dependent effects which were not present for RUNX2 or OPN mRNA expression. However, the most robust pro-osteogenic response was found at the highest magnitude (300 kPa) and frequency regimes (2 Hz). Interestingly, long-term mechanical stimulation utilising 2 Hz frequency elicited a magnitude-dependent release of ATP; however, all magnitudes promoted similar levels of collagen synthesis and significant mineral deposition, demonstrating that lineage commitment is magnitude independent. This therefore demonstrates that physiological levels of pressures, as low as 10 kPa, within the bone can drive hBMSC osteogenic lineage commitment.

Conclusion

Overall, these findings demonstrate an important role for cyclic hydrostatic pressure in hBMSCs and bone mechanobiology, which should be considered when studying pressure-driven fluid shear effects in hBMSCs mechanobiology. Moreover, these findings may have clinical implication in terms of bioreactor-based bone tissue engineering strategies.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-1025-8) contains supplementary material, which is available to authorized users.

Mediating human stem cell behaviour via defined fibrous architectures by melt electrospinning writing

The architecture within which cells reside is key to mediating their specific functions within the body. In this study, we use melt electrospinning writing (MEW) to fabricate cell micro-environments with various fibrous architectures to study their effect on human stem cell behaviour. We designed, built and optimised a MEW apparatus and used it to fabricate four different platform designs of 10.4 ± 2 μm fibre diameter, with angles between fibres on adjacent layers of 90°, 45°, 10° and R (random). Mechanical characterisation was conducted via tensile testing, and human skeletal stem cells (hSSCs) were seeded to scaffolds to study the effect of architecture on cell morphology and mechanosensing (nuclear YAP). Cell morphology was significantly altered between groups, with cells on 90° scaffolds having a lower aspect ratio, greater spreading, greater cytoskeletal tension and nuclear YAP expression. Long term cell culture studies were then conducted to determine the differentiation potential of scaffolds in terms of alkaline phosphatase activity, collagen and mineral production. Across these studies, an increased cell spreading in 3-dimensions is seen with decreasing alignment of architecture correlated with enhanced osteogenesis. This study therefore highlights the critical role of fibrous architecture in regulating stem cell behaviour with implications for tissue engineering and disease progression.

STATEMENT OF SIGNIFICANCE

This is the first study which has investigated the effect of controlled fibrous architectures fabricated via melt electrospinning writing on stem cell behaviour and differentiation. After optimising the fabrication process and characterising scaffolds via SEM and mechanical testing, skeletal stem cells were seeded onto fibrous scaffolds with various micro-architectures. These architectures drove cell shape changes resulting in architecture dependent nuclear YAP localisation, suggesting altered mechanosensing at early time points. In agreement with these early markers, long term cell culture studies revealed for the first time that a 90° fibrous architecture is optimal for the osteogenic differentiation of skeletal stem cells.

Genomic identification, expression profiling, and functional characterization of CatSper channels in the bovine

Cation channels of sperm (CatSper) are sperm-specific calcium channels with identified roles in the regulation of sperm function in humans, mice, and horses. We sought to employ a comparative genomics approach to identify conserved CATSPER genes in the bovine genome, and profile their expression in reproductive tissue. We hypothesized that CATSPER proteins expressed in bull testicular tissue mediates sperm hyperactivation and their rheotactic response in the reproductive tract of the cow. Bioinformatic analysis identified all four known CATSPER genes (CATSPER 1-4) in the bovine genome, and profiling by quantitative real-time polymerase chain reaction identified site-specific variation in messenger ribonucleic acid (mRNA) expression for all four genes along the reproductive tract of the bull. Using a novel antibody against CATSPER 1, protein expression was confirmed and localized to the principal piece of bull sperm, in agreement with what has been reported in other species. Subsequent treatment of bull sperm with either the calcium chelator ethylene glycol tetraacetic acid; mibefradil, a specific blocker of CatSper channels in human sperm; or CATSPER1 antibody all significantly inhibited caffeine-induced hyperactivation and the rheotactic response, supporting the concept that the calcium influx occurs via CatSper channels. Taken together, the work here provides novel insights into expression and function of CatSper channels in bull testicular tissue and in the function of ejaculated sperm.

TRPV4-mediates oscillatory fluid shear mechanotransduction in mesenchymal stem cells in part via the primary cilium

Skeletal homeostasis requires the continued replenishment of the bone forming osteoblast from a mesenchymal stem cell (MSC) population, a process that has been shown to be mechanically regulated. However, the mechanisms by which a biophysical stimulus can induce a change in biochemical signaling, mechanotransduction, is poorly understood. As a precursor to loading-induced bone formation, deciphering the molecular mechanisms of MSC osteogenesis is a critical step in developing novel anabolic therapies. Therefore, in this study we characterize the expression of the mechanosensitive calcium channel Transient Receptor Potential subfamily V member 4 (TRPV4) in MSCs and demonstrate that TRPV4 localizes to areas of high strain, specifically the primary cilium. We demonstrate that TRPV4 is required for MSC mechanotransduction, mediating oscillatory fluid shear induced calcium signaling and early osteogenic gene expression. Furthermore, we demonstrate that TRPV4 can be activated pharmacologically eliciting a response that mirrors that seen with mechanical stimulation. Lastly, we show that TRPV4 localization to the primary cilium is functionally significant, with MSCs with defective primary cilia exhibiting an inhibited osteogenic response to TRPV4 activation. Collectively, this data demonstrates a novel mechanism of stem cell mechanotransduction, which can be targeted therapeutically, and further highlights the critical role of the primary cilium in MSC biology.

Centriole splitting caused by loss of the centrosomal linker protein C-NAP1 reduces centriolar satellite density and impedes centrosome amplification

Duplication of the centrosomes is a tightly regulated process. Abnormal centrosome numbers can impair cell division and cause changes in how cells migrate. Duplicated centrosomes are held together by a proteinaceous linker made up of rootletin filaments anchored to the centrioles by C-NAP1. This linker is removed in a NEK2A kinase-dependent manner as mitosis begins. To explore C-NAP1 activities in regulating centrosome activities, we used genome editing to ablate it. C-NAP1-null cells were viable and had an increased frequency of premature centriole separation, accompanied by reduced density of the centriolar satellites, with reexpression of C-NAP1 rescuing both phenotypes. We found that the primary cilium, a signaling structure that arises from the mother centriole docked to the cell membrane, was intact in the absence of C-NAP1, although components of the ciliary rootlet were aberrantly localized away from the base of the cilium. C-NAP1-deficient cells were capable of signaling through the cilium, as determined by gene expression analysis after fluid flow-induced shear stress and the relocalization of components of the Hedgehog pathway. Centrosome amplification induced by DNA damage or by PLK4 or CDK2 overexpression was markedly reduced in the absence of C-NAP1. We conclude that centriole splitting reduces the local density of key centriolar precursors to impede overduplication.

Oscillatory fluid flow induces the osteogenic lineage commitment of mesenchymal stem cells: The effect of shear stress magnitude, frequency, and duration

A potent regulator of bone anabolism is physical loading. However, it is currently unclear whether physical stimuli such as fluid shear within the marrow cavity is sufficient to directly drive the osteogenic lineage commitment of resident mesenchymal stem cells (MSC). Therefore, the objective of the study is to employ a systematic analysis of oscillatory fluid flow (OFF) parameters predicted to occur in vivo on early MSC osteogenic responses and late stage lineage commitment. MSCs were exposed to OFF of 1Pa, 2Pa and 5Pa magnitudes at frequencies of 0.5Hz, 1Hz and 2Hz for 1h, 2h and 4h of stimulation. Our findings demonstrate that OFF elicits a positive osteogenic response in MSCs in a shear stress magnitude, frequency, and duration dependent manner that is gene specific. Based on the mRNA expression of osteogenic markers Cox2, Runx2 and Opn after short-term fluid flow stimulation, we identified that a regime of 2Pa shear magnitude and 2Hz frequency induces the most robust and reliable upregulation in osteogenic gene expression. Furthermore, long-term mechanical stimulation utilising this regime, elicits a significant increase in collagen and mineral deposition when compared to static control demonstrating that mechanical stimuli predicted within the marrow is sufficient to directly drive osteogenesis.

Mechanical signals promote osteogenic fate through a primary cilia-mediated mechanism

It has long been suspected, but never directly shown, that bone formed to accommodate an increase in mechanical loading is related to the creation of osteoblasts from skeletal stem cells. Indeed, biophysical stimuli potently regulate osteogenic lineage commitmentin vitro In this study, we transplanted bone marrow cells expressing green fluorescent protein, to enable lineage tracing, and subjected mice to a biophysical stimulus, to elicit a bone-forming response. We detected cells derived from transplanted progenitors embedded within the bone matrix near active bone-forming surfaces in response to loading, demonstrating for the first time, that mechanical signals enhance the homing and attachment of bone marrow cells to bone surfaces and the commitment to an osteogenic lineage of these cellsin vivo Furthermore, we used an inducible Cre/Lox recombination system to delete kinesin family member 3A (Kif3a), a gene that is essential for primary cilia formation, at will in transplanted cells and their progeny, regardless of which tissue may have incorporated them. Disruption of the mechanosensing organelle, the primary cilium in a progenitor population, significantly decreased the amount of bone formed in response to mechanical stimulation. The collective results of our study directly demonstrate that, in a novel experimental stem cell mechanobiology model, mechanical signals enhance osteogenic lineage commitmentin vivoand that the primary cilium contributes to this process.-Chen, J. C., Hoey, D. A., Chua, M., Bellon, R., Jacobs, C. R. Mechanical signals promote osteogenic fate through a primary cilia-mediated mechanism.

Oscillatory fluid flow influences primary cilia and microtubule mechanics

Many tissues are sensitive to mechanical stimuli; however, the mechanotransduction mechanism used by cells remains unknown in many cases. The primary cilium is a solitary, immotile microtubule-based extension present on nearly every mammalian cell which extends from the basal body. The cilium is a mechanosensitive organelle and has been shown to transduce fluid flow-induced shear stress in tissues, such as the kidney and bone. The majority of microtubules assemble from the mother centriole (basal body), contributing significantly to the anchoring of the primary cilium. Several studies have attempted to quantify the number of microtubules emanating from the basal body and the results vary depending on the cell type. It has also been shown that cellular response to shear stress depends on microtubular integrity. This study hypothesizes that changing the microtubule attachment of primary cilia in response to a mechanical stimulus could change primary cilia mechanics and, possibly, mechanosensitivity. Oscillatory fluid flow was applied to two different cell types and the microtubule attachment to the ciliary base was quantified. For the first time, an increase in microtubules around primary cilia both with time and shear rate in response to oscillatory fluid flow stimulation was demonstrated. Moreover, it is presented that the primary cilium is required for this loading-induced cellular response. This study has demonstrated a new role for the cilium in regulating alterations in the cytoplasmic microtubule network in response to mechanical stimulation, and therefore provides a new insight into how cilia may regulate its mechanics and thus the cells mechanosensitivity.

Adenylyl cyclase 6 mediates loading-induced bone adaptation in vivo

Primary cilia are single, nonmotile, antenna-like structures extending from the apical membrane of most mammalian cells. They may mediate mechanotransduction, the conversion of external mechanical stimuli into biochemical intracellular signals. Previously we demonstrated that adenylyl cyclase 6 (AC6), a membrane-bound enzyme enriched in primary cilia of MLO-Y4 osteocyte-like cells, may play a role in a primary cilium-dependent mechanism of osteocyte mechanotransduction in vitro. In this study, we determined whether AC6 deletion impairs loading-induced bone formation in vivo. Skeletally mature mice with a global knockout of AC6 exhibited normal bone morphology and responded to osteogenic chemical stimuli similar to wild-type mice. Following ulnar loading over 3 consecutive days, bone formation parameters were assessed using dynamic histomorphometry. Mice lacking AC6 formed significantly less bone than control animals (41% lower bone formation rate). Furthermore, there was an attenuated flow-induced increase in COX-2 mRNA expression levels in primary bone cells isolated from AC6 knockout mice compared to controls (1.3±0.1- vs. 2.6±0.2-fold increase). Collectively, these data indicate that AC6 plays a role in loading-induced bone adaptation, and these findings are consistent with our previous studies implicating primary cilia and AC6 in a novel mechanism of osteocyte mechanotransduction.

Primary cilia-mediated mechanotransduction in human mesenchymal stem cells

Physical loading is a potent stimulus required to maintain bone homeostasis, partly through the renewal and osteogenic differentiation of mesenchymal stem cells (MSCs). However, the mechanism by which MSCs sense a biophysical force and translate that into a biochemical bone forming response (mechanotransduction) remains poorly understood. The primary cilium is a single sensory cellular extension, which has recently been shown to demonstrate a role in cellular mechanotransduction and MSC lineage commitment. In this study, we present evidence that short periods of mechanical stimulation in the form of oscillatory fluid flow (OFF) is sufficient to enhance osteogenic gene expression and proliferation of human MSCs (hMSCs). Furthermore, we demonstrate that the cilium mediates fluid flow mechanotransduction in hMSCs by maintaining OFF-induced increases in osteogenic gene expression and, surprisingly, to limit OFF-induced increases in proliferation. These data therefore demonstrate a pro-osteogenic mechanosensory role for the primary cilium, establishing a novel mechanotransduction mechanism in hMSCs. Based on these findings, the application of OFF may be a beneficial component of bioreactor-based strategies to form bone-like tissues suitable for regenerative medicine and also highlights the cilium as a potential therapeutic target for efforts to mimic loading with the aim of preventing bone loss during diseases such as osteoporosis. Furthermore, this study demonstrates a role for the cilium in controlling mechanically mediated increases in the proliferation of hMSCs, which parallels proposed models of polycystic kidney disease. Unraveling the mechanisms leading to rapid proliferation of mechanically stimulated MSCs with defective cilia could provide significant insights regarding ciliopathies and cystic diseases. Stem Cells2012;30:2561–2570

An experimental and computational analysis of primary cilia deflection under fluid flow

In this study we have developed a novel model of the deflection of primary cilia experiencing fluid flow accounting for phenomena not previously considered. Specifically, we developed a large rotation formulation that accounts for rotation at the base of the cilium, the initial shape of the cilium and fluid drag at high deflection angles. We utilised this model to analyse full 3D data-sets of primary cilia deflecting under fluid flow acquired with high-speed confocal microscopy. We found a wide variety of previously unreported bending shapes and behaviours. We also analysed post-flow relaxation patterns. Results from our combined experimental and theoretical approach suggest that the average flexural rigidity of primary cilia might be higher than previously reported (Schwartz et al. 1997, Am J Physiol. 272(1 Pt 2):F132-F138). In addition our findings indicate that the mechanics of primary cilia are richly varied and mechanisms may exist to alter their mechanical behaviour.

The primary cilium as a novel extracellular sensor in bone

Mechanically induced adaptation of bone is required to maintain a healthy skeleton and defects in this process can lead to dramatic changes in bone mass, resulting in bone diseases such as osteoporosis. Therefore, understanding how this process occurs could yield novel therapeutics to treat diseases of excessive bone loss or formation. Over the past decade the primary cilium has emerged as a novel extracellular sensor in bone, being required to transduce changes in the extracellular mechanical environment into biochemical responses regulating bone adaptation. In this review, we introduce the primary cilium as a novel extracellular sensor in bone; discuss the in vitro and in vivo findings of primary cilia based sensing in bone; explore the role of the primary cilium in regulating stem cell osteogenic fate commitment and finish with future directions of research and possible development of cilia targeting therapeutics to treat bone diseases.

The mechanics of the primary cilium: an intricate structure with complex function

The primary cilium is a non-motile singular cellular structure that extends from the surface of nearly every cell in the body. The cilium has been shown to play numerous roles in maintaining tissue homeostasis, through regulating signaling pathways and sensing both biophysical and biochemical changes in the extracellular environment. The structural performance of the cilium is paramount to its function as defective cilia have been linked to numerous pathologies. In particular, the cilium has demonstrated a mechanosensory role in tissues such as the kidney, liver, endothelium and bone, where cilium deflection under mechanical loading triggers a cellular response. Understanding of how cilium structure and subsequent mechanical behavior contributes to the roles that cilium plays in regulating cellular behavior is a compelling question, yet is a relatively untouched research area. Recent advances in biophysical measurements have demonstrated the cilium to be a structurally intricate organelle containing an array of load bearing proteins. Furthermore advances in modeling of this organelle have revealed the importance of these proteins at regulating the cilium's mechanosensitivity. Remarkably, the cilium is capable of adapting its mechanical state, altering its length and possibly it's bending resistance, to regulate its mechanosensitivity demonstrating the importance of cilium mechanics in cellular responses. In this review, we introduce the cilium as a mechanosensor; discuss the advances in the mechanical modeling of cilia; explore the structural features of the cilium, which contribute to its mechanics and finish with possible mechanisms in which alteration in structure may affect ciliary mechanics, consequently affecting ciliary based mechanosensing.

A role for the primary cilium in paracrine signaling between mechanically stimulated osteocytes and mesenchymal stem cells

Bone turnover is a mechanically regulated process, coordinated in part by the network of mechanosensitive osteocytes residing within the tissue. The recruitment and bone forming activity of the mesenchymal derived osteoblast is determined by numerous factors including mechanical loading. It is therefore somewhat surprising that although mechanically regulated signaling between the coordinating osteocytes and mesenchymal stem cells (MSCs) should exist, to date it has not been directly demonstrated. In this study, conditioned media from mechanically stimulated osteocytes (MLO-Y4 cell line) was collected and added to MSCs (C3H10T1/2 cell line). The addition of mechanically stimulated osteocyte conditioned media resulted in a significant upregulation of the osteogenic genes OPN and COX-2 in MSCs compared to statically cultured conditioned media, demonstrating a novel paracrine signaling mechanism between the two cell types. The same mechanically conditioned media did not alter gene expression in osteoblasts (MC3T3 cell line), and mechanically stimulated osteoblast conditioned media did not alter gene expression in MSCs demonstrating that this signaling is unique to osteocytes and MSCs. Finally, the upregulation in osteogenic genes in MSCs was not observed if primary cilia formation was inhibited prior to mechanical stimulation of the osteocyte. In summary, the results of this study indicate that soluble factors secreted by osteocytes in response to mechanical stimulation can enhance osteogenic gene expression in MSCs demonstrating a novel, unique signaling mechanism and introduces a role for the primary cilium in flow mediated paracrine signaling in bone thereby highlighting the cilium as a potential target for therapeutics aimed at enhancing bone formation.