Short bouts of mechanical loading are as effective as dexamethasone at inducing matrix production by human bone marrow mesenchymal stem cell

β-glycerophosphate, not low magnitude fluid shear stress, increases osteocytogenesis in the osteoblast-to-osteocyte cell line IDG-SW3

AIM

As osteoblasts deposit a mineralized collagen network, a subpopulation of these cells differentiates into osteocytes. Biochemical and mechanical stimuli, particularly fluid shear stress (FSS), are thought to regulate this, but their relative influence remains unclear. Here, we assess both biochemical and mechanical stimuli on long-term bone formation and osteocytogenesis using the osteoblast-osteocyte cell line IDG-SW3.

METHODS

Due to the relative novelty and uncommon culture conditions of IDG-SW3 versus other osteoblast-lineage cell lines, effects of temperature and media formulation on matrix deposition and osteocytogenesis were initially characterized. Subsequently, the relative influence of biochemical (β-glycerophosphate (βGP) and ascorbic acid 2-phosphate (AA2P)) and mechanical stimulation on osteocytogenesis was compared, with intermittent application of low magnitude FSS generated by see-saw rocker.

RESULTS

βGP and AA2P supplementation were required for mineralization and osteocytogenesis, with 33°C cultures retaining a more osteoblastic phenotype and 37°C cultures undergoing significantly higher osteocytogenesis. βGP concentration positively correlated with calcium deposition, whilst AA2P stimulated alkaline phosphatase (ALP) activity and collagen deposition. We demonstrate that increasing βGP concentration also significantly enhances osteocytogenesis as quantified by the expression of green fluorescent protein linked to Dmp1. Intermittent FSS (~0.06 Pa) rocker had no effect on osteocytogenesis and matrix deposition.

CONCLUSIONS

This work demonstrates the suitability and ease with which IDG-SW3 can be utilized in osteocytogenesis studies. IDG-SW3 mineralization was only mediated through biochemical stimuli with no detectable effect of low magnitude FSS. Osteocytogenesis of IDG-SW3 primarily occurred in mineralized areas, further demonstrating the role mineralization of the bone extracellular matrix has in osteocyte differentiation.

A mechanobiological model of bone metastasis reveals that mechanical stimulation inhibits the pro-osteolytic effects of breast cancer cells

Bone is highly susceptible to cancer metastasis, and both tumor and bone cells enable tumor invasion through a "vicious cycle" of biochemical signaling. Tumor metastasis into bone also alters biophysical cues to both tumor and bone cells, which are highly sensitive to their mechanical environment. However, the mechanobiological feedback between these cells that perpetuate this cycle has not been studied. Here, we develop highly advanced in vitro and computational models to provide an advanced understanding of how tumor growth is regulated by the synergistic influence of tumor-bone cell signaling and mechanobiological cues. In particular, we develop a multicellular healthy and metastatic bone model that can account for physiological mechanical signals within a custom bioreactor. These models successfully recapitulated mineralization, mechanobiological responses, osteolysis, and metastatic activity. Ultimately, we demonstrate that mechanical stimulus provided protective effects against tumor-induced osteolysis, confirming the importance of mechanobiological factors in bone metastasis development.

Mechanoregulation analysis of bone formation in tissue engineered constructs requires a volumetric method using time-lapsed micro-computed tomography

Bone can adapt its microstructure to mechanical loads through mechanoregulation of the (re)modeling process. This process has been investigated in vivo using time-lapsed micro-computed tomography (micro-CT) and micro-finite element (FE) analysis using surface-based methods, which are highly influenced by surface curvature. Consequently, when trying to investigate mechanoregulation in tissue engineered bone constructs, their concave surfaces make the detection of mechanoregulation impossible when using surface-based methods. In this study, we aimed at developing and applying a volumetric method to non-invasively quantify mechanoregulation of bone formation in tissue engineered bone constructs using micro-CT images and FE analysis. We first investigated hydroxyapatite scaffolds seeded with human mesenchymal stem cells that were incubated over 8 weeks with one mechanically loaded and one control group. Higher mechanoregulation of bone formation was measured in loaded samples with an area under the curve for the receiver operating curve (AUCformation) of 0.633-0.637 compared to non-loaded controls (AUCformation: 0.592-0.604) during culture in osteogenic medium (p < 0.05). Furthermore, we applied the method to an in vivo mouse study investigating the effect of loading frequencies on bone adaptation. The volumetric method detected differences in mechanoregulation of bone formation between loading conditions (p < 0.05). Mechanoregulation in bone formation was more pronounced (AUCformation: 0.609-0.642) compared to the surface-based method (AUCformation: 0.565-0.569, p < 0.05). Our results show that mechanoregulation of formation in bone tissue engineered constructs takes place and its extent can be quantified with a volumetric mechanoregulation method using time-lapsed micro-CT and FE analysis. STATEMENT OF SIGNIFICANCE: Many efforts have been directed towards optimizing bone scaffolds for tissue growth. However, the impact of the scaffolds mechanical environment on bone growth is still poorly understood, requiring accurate assessment of its mechanoregulation. Existing surface-based methods were unable to detect mechanoregulation in tissue engineered constructs, due to predominantly concave surfaces in scaffolds. We present a volumetric approach to enable the precise and non-invasive quantification and analysis of mechanoregulation in bone tissue engineered constructs by leveraging time-lapsed micro-CT imaging, image registration, and finite element analysis. The implications of this research extend to diverse experimental setups, encompassing culture conditions, and material optimization, and investigations into bone diseases, enabling a significant stride towards comprehensive advancements in bone tissue engineering and regenerative medicine.

Effect of whole body vibration therapy in the rat model of steroid-induced osteonecrosis of the femoral head

Background: Steroid-induced Osteonecrosis of the Femoral Head (SIONFH) is a skeletal disease with a high incidence and a poor prognosis. Whole body vibration therapy (WBVT), a new type of physical training, is known to promote bone formation. However, it remains unclear whether WBVT has a therapeutic effect on SIONFH. Materials and methods: Thirty adult male and female Sprague-Dawley (SD) rats were selected and randomly assigned to three experimental groups: the control group, the model group, and the mechanical vibration group, respectively. SIONFH induction was achieved through the combined administration of lipopolysaccharides (LPS) and methylprednisolone sodium succinate for injection (MPS). The femoral head samples underwent hematoxylin and eosin (H&E) staining to visualize tissue structures. Structural parameters of the region of interest (ROI) were compared using Micro-CT analysis. Immunohistochemistry was employed to assess the expression levels of Piezo1, BMP2, RUNX2, HIF-1, VEGF, CD31, while immunofluorescence was used to examine CD31 and Emcn expression levels. Results: The H&E staining results revealed a notable improvement in the ratio of empty lacuna in various groups following WBVT intervention. Immunohistochemical analysis showed that the expression levels of Piezo1, BMP2, RUNX2, HIF-1, VEGF, and CD31 in the WBVT group exhibited significant differences when compared to the Model group (p < 0.05). Additionally, immunofluorescence analysis demonstrated statistically significant differences in CD31 and Emcn expression levels between the WBVT group and the Model group (p < 0.05). Conclusion: WBVT upregulates Piezo1 to promote osteogenic differentiation, potentially by enhancing the HIF-1α/VEGF axis and regulating H-vessel angiogenesis through the activation of the Piezo1 ion channel. This mechanism may lead to improved blood flow supply and enhanced osteogenic differentiation within the femoral head.

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

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

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

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

Polydopamine Codoped BaTiO3-Functionalized Polyvinylidene Fluoride Coating as a Piezo-Biomaterial Platform for an Enhanced Cellular Response and Bioactivity

For a number of clinical applications, Ti6Al4V implants with bioactive coatings are used. However, the deposition of a functional polymeric coating with desired physical properties, biocompatibility, and long-term stability remains largely unexplored. Among widely investigated synthetic biomaterials, polyvinylidene fluoride (PVDF) with β-polymorph and barium titanate (BaTiO₃, BT) are considered as good examples of piezo-biopolymers and bioceramics, respectively. In this work, an adherent PVDF-based nanocomposite coating is deposited onto a Ti6Al4V substrate to explore the impact of its functional characteristics (piezoactivity) on cellular behavior and bioactivity (apatite growth and mineralized matrix formation). The precursor solution was prepared by physically grafting PVDF with polydopamine (pDOPA), forming mPVDF. Subsequently, mPVDF was reinforced with BaTiO₃ nanoparticles in dimethylformamide/acetone solution, and the resulting nanocomposite (mPVDF-BT) was then spray-coated onto a roughened Ti6Al4V substrate using an airbrush at 140 °C under a pressure of 2 bar. The reproducibility of this simple yet effective processing approach to deposit chemically stable and adherent coatings was established. Remarkably, the modification with pDOPA and reinforcement with BaTiO₃ nanoparticles resulted in an enhanced β-fraction of PVDF up to 96%. This nanocomposite encouraged cellular viability of preosteoblasts (∼158% at day 5) and characteristic spreading, in vitro. Our findings indicate that the mPVDF-BT coating facilitated faster nucleation and growth of the biomineralized apatite layer with ∼70% coverage within 3 days of incubation in the simulated body fluid. In addition, the coupling among surface polar energy (5.5 mN/m), fractional polarity (∼117%), roughness (8.7 μm), and fibrous morphology also endorsed better cellular behavior. Taken together, this coating deposition strategy will pave the pathway toward designing cell-instructive surface-modified Ti6Al4V biomaterials with tailored biomineralization and bioactivity properties for musculoskeletal reconstruction and regeneration applications.

Biophysical Stimuli as the Fourth Pillar of Bone Tissue Engineering

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

Modeling of the Human Bone Environment: Mechanical Stimuli Guide Mesenchymal Stem Cell-Extracellular Matrix Interactions

In bone tissue engineering, the design of in vitro models able to recreate both the chemical composition, the structural architecture, and the overall mechanical environment of the native tissue is still often neglected. In this study, we apply a bioreactor system where human bone-marrow hMSCs are seeded in human femoral head-derived decellularized bone scaffolds and subjected to dynamic culture, i.e., shear stress induced by continuous cell culture medium perfusion at 1.7 mL/min flow rate and compressive stress by 10% uniaxial load at 1 Hz for 1 h per day. In silico modeling revealed that continuous medium flow generates a mean shear stress of 8.5 mPa sensed by hMSCs seeded on 3D bone scaffolds. Experimentally, both dynamic conditions improved cell repopulation within the scaffold and boosted ECM production compared with static controls. Early response of hMSCs to mechanical stimuli comprises evident cell shape changes and stronger integrin-mediated adhesion to the matrix. Stress-induced Col6 and SPP1 gene expression suggests an early hMSC commitment towards osteogenic lineage independent of Runx2 signaling. This study provides a foundation for exploring the early effects of external mechanical stimuli on hMSC behavior in a biologically meaningful in vitro environment, opening new opportunities to study bone development, remodeling, and pathologies.

Pulsating fluid flow affects pre-osteoblast behavior and osteogenic differentiation through production of soluble factors

Bone mass increases after error-loading, even in the absence of osteocytes. Loaded osteoblasts may produce a combination of growth factors affecting adjacent osteoblast differentiation. We hypothesized that osteoblasts respond to a single load in the short-term (minutes) by changing F-actin stress fiber distribution, in the intermediate-term (hours) by signaling molecule production, and in the long-term (days) by differentiation. Furthermore, growth factors produced during and after mechanical loading by pulsating fluid flow (PFF) will affect osteogenic differentiation. MC3T3-E1 pre-osteoblasts were either/not stimulated by 60 min PFF (amplitude, 1.0 Pa; frequency, 1 Hz; peak shear stress rate, 6.5 Pa/s) followed by 0-6 h, or 21/28 days of post-incubation without PFF. Computational analysis revealed that PFF immediately changed distribution and magnitude of fluid dynamics over an adherent pre-osteoblast inside a parallel-plate flow chamber (immediate impact). Within 60 min, PFF increased nitric oxide production (5.3-fold), altered actin distribution, but did not affect cell pseudopodia length and cell orientation (initial downstream impact). PFF transiently stimulated Fgf2, Runx2, Ocn, Dmp1, and Col1⍺1 gene expression between 0 and 6 h after PFF cessation. PFF did not affect alkaline phosphatase nor collagen production after 21 days, but altered mineralization after 28 days. In conclusion, a single bout of PFF with indirect associated release of biochemical factors, stimulates osteoblast differentiation in the long-term, which may explain enhanced bone formation resulting from mechanical stimuli.

In Vitro and In Vivo Evaluation of Carboxymethyl Cellulose Scaffolds for Bone Tissue Engineering Applications

The present study involves the development of citric acid-cross-linked carboxymethyl cellulose (C3CA) scaffolds by a freeze-drying process. Scaffolds were fabricated at different freezing temperatures of -20, -40, or -80 °C to investigate the influence of scaffold pore size on bone regeneration. All three scaffolds were porous in structure, and the pore size was measured to be 74 ± 4, 55 ± 6, and 46 ± 5 μm for -20, -40, and -80 °C scaffolds. The pores were larger in scaffolds processed at -20 °C compared to -40 and -80 °C, indicating the reduction in pore size of the scaffolds with a decrease in freezing temperature. The cytocompatibility, cell proliferation, and differentiation in C3CA scaffolds were assessed with the Saos-2 osteoblast cell line. These scaffolds supported the proliferation and differentiation of Saos-2 cells with significant matrix mineralization in scaffolds processed at -40 °C. Subcutaneous implantation of C3CA scaffolds in the rat model was investigated for its ability of vascularization and new matrix tissue formation. The matrix formation was observed at the earliest of 14 days in the scaffolds when processed at -40 °C while it was observed only after 28 days of implantation with the scaffolds processed at -20 and -80 °C. These results suggest that the citric acid-cross-linked CMC scaffolds processed at -40 °C can be promising for bone tissue engineering application.

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

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

A universal multi-platform 3D printed bioreactor chamber for tendon tissue engineering

A range of bioreactors use linear actuators to apply tensile forces in vitro, but differences in their culture environments can limit a direct comparison between studies. The widespread availability of 3D printing now provides an opportunity to develop a 'universal' bioreactor chamber that, with minimal exterior editing can be coupled to a wide range of commonly used linear actuator platforms, for example, the EBERS-TC3 and CellScale MCT6, resulting in a greater comparability between results and consistent testing of potential therapeutics. We designed a bioreactor chamber with six independent wells that was 3D printed in polylactic acid using an Ultimaker 2+ and waterproofed using a commercially available coating (XTC-3D), an oxirane resin. The cell culture wells were further coated with Sylgard-184 polydimethylsiloxane (PDMS) to produce a low-adhesion well surface. With appropriate coating and washing steps, all materials were shown to be non-cytotoxic by lactate dehydrogenase assay, and the bioreactor was waterproof, sterilisable and reusable. Tissue-engineered tendons were generated from human mesenchymal stem cells in a fibrin hydrogel and responded to 5% cyclic strain (0.5 Hz, 5 h/day, 21 days) in the bioreactor by increased production of collagen-Iα1 and decreased production of collagen-IIIα1. Calcification of the extracellular matrix was observed in unstretched tendon controls indicating abnormal differentiation, while tendons cultured under cyclic strain did not calcify and exhibited a tenogenic phenotype. The ease of manufacturing this bioreactor chamber enables researchers to quickly and cheaply reproduce this culture environment for use with many existing bioreactor actuator platforms by downloading the editable CAD files from a public database and following the manufacturing steps we describe.

Mechanosensitivity of Cells and Its Role in the Regulation of Physiological Functions and the Implementation of Physiotherapeutic Effects (Review)

Regulatory signals in the body are not limited to chemical and electrical ones. There is another type of important signals for cells: those are mechanical signals (coming from the environment or arising from within the body), which have been less known in the literature. The review summarizes new information on the mechanosensitivity of various cells of connective tissue and nervous system. Participation of mechanical stimuli in the regulation of growth, development, differentiation, and functioning of tissues is described. The data focus on bone remodeling, wound healing, neurite growth, and the formation of neural networks. Mechanotransduction, cellular organelles, and mechanosensitive molecules involved in these processes are discussed as well as the role of the extracellular matrix. The importance of mechanical characteristics of cells in the pathogenesis of diseases is highlighted. Finally, the possible role of mechanosensitivity in mediating the physiotherapeutic effects is addressed.

Mechanical regulation of nucleocytoplasmic translocation in mesenchymal stem cells: characterization and methods for investigation

Mesenchymal stem cells (MSCs) have immune-modulatory and tissue-regenerative properties that make them a suitable and promising tool for cell-based therapy application. Since the bio-chemo-mechanical environment influences MSC fate and behavior, the understanding of the mechanosensors involved in the transduction of mechanical inputs into chemical signals could be pivotal. In this context, the nuclear pore complex is a molecular machinery that is believed to have a key role in force transmission and in nucleocytoplasmic shuttling regulation. To fully understand the nuclear pore complex role and the nucleocytoplasmic transport dynamics, recent advancements in fluorescence microscopy provided the possibility to study passive and facilitated nuclear transports also in mechanically stimulated cell culture conditions. Here, we review the current available methods for the investigation of nucleocytoplasmic shuttling, including photo-perturbation-based approaches, fluorescence correlation spectroscopy, and single-particle tracking techniques. For each method, we analyze the advantages, disadvantages, and technical limitations. Finally, we summarize the recent knowledge on mechanical regulation of nucleocytoplasmic translocation in MSC, the relevant progresses made so far, and the future perspectives in the field.

Load-induced osteogenic differentiation of mesenchymal stromal cells is caused by mechano-regulated autocrine signaling

Mechanical boundary conditions critically influence the bone healing process. In this context, previous in vitro studies have demonstrated that cyclic mechanical compression alters migration and triggers osteogenesis of mesenchymal stromal cells (MSC), both processes being relevant to healing. However, it remains unclear whether this mechanosensitivity is a direct consequence of cyclic compression, an indirect effect of altered supply or a specific modulation of autocrine bone morphogenetic protein (BMP) signaling. Here, we investigate the influence of cyclic mechanical compression (ε = 5% and 10%, f = 1 Hz) on human bone marrow MSC (hBMSC) migration and osteogenic differentiation in a 3D biomaterial scaffold, an in vitro system mimicking the mechanical environment of the early bone healing phase. The open-porous architecture of the scaffold ensured sufficient supply even without cyclic compression, minimizing load-associated supply alterations. Furthermore, a large culture medium volume in relation to the cell number diminished autocrine signaling. Migration of hBMSCs was significantly downregulated under cyclic compression. Surprisingly, a decrease in migration was not associated with increased osteogenic differentiation of hBMSCs, as the expression of RUNX2 and osteocalcin decreased. In contrast, BMP2 expression was significantly upregulated. Enabling autocrine stimulation by increasing the cell-to-medium ratio in the bioreactor finally resulted in a significant upregulation of RUNX2 in response to cyclic compression, which could be reversed by rhNoggin treatment. The results indicate that osteogenesis is promoted by cyclic compression when cells condition their environment with BMP. Our findings highlight the importance of mutual interactions between mechanical forces and BMP signaling in controlling osteogenic differentiation.

Dysfunctional stem and progenitor cells impair fracture healing with age

Successful fracture healing requires the simultaneous regeneration of both the bone and vasculature; mesenchymal stem cells (MSCs) are directed to replace the bone tissue, while endothelial progenitor cells (EPCs) form the new vasculature that supplies blood to the fracture site. In the elderly, the healing process is slowed, partly due to decreased regenerative function of these stem and progenitor cells. MSCs from older individuals are impaired with regard to cell number, proliferative capacity, ability to migrate, and osteochondrogenic differentiation potential. The proliferation, migration and function of EPCs are also compromised with advanced age. Although the reasons for cellular dysfunction with age are complex and multidimensional, reduced expression of growth factors, accumulation of oxidative damage from reactive oxygen species, and altered signaling of the Sirtuin-1 pathway are contributing factors to aging at the cellular level of both MSCs and EPCs. Because of these geriatric-specific issues, effective treatment for fracture repair may require new therapeutic techniques to restore cellular function. Some suggested directions for potential treatments include cellular therapies, pharmacological agents, treatments targeting age-related molecular mechanisms, and physical therapeutics. Advanced age is the primary risk factor for a fracture, due to the low bone mass and inferior bone quality associated with aging; a better understanding of the dysfunctional behavior of the aging cell will provide a foundation for new treatments to decrease healing time and reduce the development of complications during the extended recovery from fracture healing in the elderly.

Biomimetic Designer Scaffolds Made of D,L-Lactide-ɛ-Caprolactone Polymers by 2-Photon Polymerization

IMPACT STATEMENT

In tissue engineering (TE), the establishment of cell targeting materials, which mimic the conditions of the physiological extracellular matrix (ECM), seems to be a mission impossible without advanced materials and fabrication techniques. With this in mind we established a toolbox based on (D,L)-lactide-ɛ-caprolactone methacrylate (LCM) copolymers in combination with a nano-micromaskless lithography technique, the two-photon polymerization (2-PP) to mimic the hierarchical structured and complex milieu of the natural ECM. To demonstrate the versatility of this toolbox, we choose two completely different application scenarios in bone and tumor TE to show the high potential of this concept in therapeutic and diagnostic application.

The Importance of Controlled Mismatch of Biomechanical Compliances of Implantable Scaffolds and Native Tissue for Articular Cartilage Regeneration

Scaffolds for articular cartilage repair have to be optimally biodegradable with simultaneous promotion of hyaline cartilage formation under rather complex biomechanical and physiological conditions. It has been generally accepted that scaffold structure and composition would be the best when it mimics the structure of native cartilage. However, a reparative construct mimicking the mature native tissue in a healing tissue site presents a biological mismatch of reparative stimuli. In this work, we studied a new recombinant human type III collagen-polylactide (rhCol-PLA) scaffolds. The rhCol-PLA scaffolds were assessed for their relative performance in simulated synovial fluids of 1 and 4 mg/mL sodium hyaluronate with application of model-free analysis with Biomaterials Enhanced Simulation Test (BEST). Pure PLA scaffold was used as a control. The BEST results were compared to the results of a prior in vivo study with rhCol-PLA. Collectively the data indicated that a successful articular cartilage repair require lower stiffness of the scaffold compared to surrounding cartilage yet matching the strain compliance both in static and dynamic conditions. This ensures an optimal combination of load transfer and effective oscillatory nutrients supply to the cells. The results encourage further development of intelligent scaffold structures for optimal articular cartilage repair rather than simply trying to imitate the respective original tissue.

A simple rocker-induced mechanical stimulus upregulates mineralization by human osteoprogenitor cells in fibrous scaffolds

Biodegradable electrospun polycaprolactone scaffolds can be used to support bone-forming cells and could fill a thin bony defect, such as in cleft palate. Oscillatory fluid flow has been shown to stimulate bone production in human progenitor cells in monolayer culture. The aim of this study was to examine whether bone matrix production by primary human mesenchymal stem cells from bone marrow or jaw periosteal tissue could be stimulated using oscillatory fluid flow supplied by a standard see-saw rocker. This was investigated for cells in two-dimensional culture and within electrospun polycaprolactone scaffolds. From day 4 of culture onwards, samples were rocked at 45 cycles/min for 1 h/day, 5 days/week (rocking group). Cell viability, calcium deposition, collagen production, alkaline phosphatase activity and vascular endothelial growth factor secretion were evaluated to assess the ability of the cells to undergo bone differentiation and induce vascularisation. Both cell types produced more mineralized tissue when subjected to rocking and supplemented with dexamethasone. Mesenchymal progenitors and primary human mesenchymal stem cells from bone marrow in three-dimensional scaffolds upregulated mineral deposition after rocking culture as assessed by micro-computed tomography and alizarin red staining. Interestingly, vascular endothelial growth factor secretion, which has previously been shown to be mechanically sensitive, was not altered by rocking in this system and was inhibited by dexamethasone. Rocker culture may be a cost effective, simple pretreatment for bone tissue engineering for small defects such as cleft palate.

Mesenchymal Stem Cells: Cell Fate Decision to Osteoblast or Adipocyte and Application in Osteoporosis Treatment

Osteoporosis is a progressive skeletal disease characterized by decreased bone mass and degraded bone microstructure, which leads to increased bone fragility and risks of bone fracture. Osteoporosis is generally age related and has become a major disease of the world. Uncovering the molecular mechanisms underlying osteoporosis and developing effective prevention and therapy methods has great significance for human health. Mesenchymal stem cells (MSCs) are multipotent cells capable of differentiating into osteoblasts, adipocytes, or chondrocytes, and have become the favorite source of cell-based therapy. Evidence shows that during osteoporosis, a shift of the cell differentiation of MSCs to adipocytes rather than osteoblasts partly contributes to osteoporosis. Thus, uncovering the molecular mechanisms of the osteoblast or adipocyte differentiation of MSCs will provide more understanding of MSCs and perhaps new methods of osteoporosis treatment. The MSCs have been applied to both preclinical and clinical studies in osteoporosis treatment. Here, we review the recent advances in understanding the molecular mechanisms regulating osteoblast differentiation and adipocyte differentiation of MSCs and highlight the therapeutic application studies of MSCs in osteoporosis treatment. This will provide researchers with new insights into the development and treatment of osteoporosis.

Human Mesenchymal Stem/Stromal Cells from Umbilical Cord Blood and Placenta Exhibit Similar Capacities to Promote Expansion of Hematopoietic Progenitor Cells In Vitro

Mesenchymal stem/stromal cells (MSCs) from bone marrow (BM) have been used in coculture systems as a feeder layer for promoting the expansion of hematopoietic progenitor cells (HPCs) for hematopoietic cell transplantation. Because BM has some drawbacks, umbilical cord blood (UCB) and placenta (PL) have been proposed as possible alternative sources of MSCs. However, MSCs from UCB and PL sources have not been compared to determine which of these cell populations has the best capacity of promoting hematopoietic expansion. In this study, MSCs from UCB and PL were cultured under the same conditions to compare their capacities to support the expansion of HPCs in vitro. MSCs were cocultured with CD34⁺CD38^-Lin^- HPCs in the presence or absence of early acting cytokines. HPC expansion was analyzed through quantification of colony-forming cells (CFCs), long-term culture-initiating cells (LTC-ICs), and CD34⁺CD38^-Lin^- cells. MSCs from UCB and PL have similar capacities to increase HPC expansion, and this capacity is similar to that presented by BM-MSCs. Here, we are the first to determine that MSCs from UCB and PL have similar capacities to promote HPC expansion; however, PL is a better alternative source because MSCs can be obtained from a higher proportion of samples.

Short bursts of cyclic mechanical compression modulate tissue formation in a 3D hybrid scaffold

Among the cues affecting cells behaviour, mechanical stimuli are known to have a key role in tissue formation and mineralization of bone cells. While soft scaffolds are better at mimicking the extracellular environment, they cannot withstand the high loads required to be efficient substitutes for bone in vivo. We propose a 3D hybrid scaffold combining the load-bearing capabilities of polycaprolactone (PCL) and the ECM-like chemistry of collagen gel to support the dynamic mechanical differentiation of human embryonic mesodermal progenitor cells (hES-MPs). In this study, hES-MPs were cultured in vitro and a BOSE Bioreactor was employed to induce cells differentiation by mechanical stimulation. From day 6, samples were compressed by applying a 5% strain ramp followed by peak-to-peak 1% strain sinewaves at 1Hz for 15min. Three different conditions were tested: unloaded (U), loaded from day 6 to day 10 (L1) and loaded as L1 and from day 16 to day 20 (L2). Cell viability, DNA content and osteocalcin expression were tested. Samples were further stained with 1% osmium tetroxide in order to investigate tissue growth and mineral deposition by micro-computed tomography (µCT). Tissue growth involved volumes either inside or outside samples at day 21 for L1, suggesting cyclic stimulation is a trigger for delayed proliferative response of cells. Cyclic load also had a role in the mineralization process preventing mineral deposition when applied at the early stage of culture. Conversely, cyclic load during the late stage of culture on pre-compressed samples induced mineral formation. This study shows that short bursts of compression applied at different stages of culture have contrasting effects on the ability of hES-MPs to induce tissue formation and mineral deposition. The results pave the way for a new approach using mechanical stimulation in the development of engineered in vitro tissue as replacement for large bone fractures.

Production and Characterization of a Novel, Electrospun, Tri-Layer Polycaprolactone Membrane for the Segregated Co-Culture of Bone and Soft Tissue

Composite tissue-engineered constructs combining bone and soft tissue have applications in regenerative medicine, particularly dentistry. This study generated a tri-layer, electrospun, poly-ε-caprolactone membrane, with two microfiber layers separated by a layer of nanofibers, for the spatially segregated culture of mesenchymal progenitor cells (MPCs) and fibroblasts. The two cell types were seeded on either side, and cell proliferation and spatial organization were investigated over several weeks. Calcium deposition by MPCs was detected using xylenol orange (XO) and the separation between fibroblasts and the calcified matrix was visualized by confocal laser scanning microscopy. SEM confirmed that the scaffold consisted of two layers of micron-diameter fibers with a thin layer of nano-diameter fibers in-between. Complete separation of cell types was maintained and calcified matrix was observed on only one side of the membrane. This novel tri-layer membrane is capable of supporting the formation of a bilayer of calcified and non-calcified connective tissue.

Influence of biomechanical and biochemical stimulation on the proliferation and differentiation of bone marrow stromal cells seeded on polyurethane scaffolds

The aim of the present investigation was to compare the effects of cyclic compression, perfusion, dexamethasone (DEX) and bone morphogenetic protein-7 (BMP-7) on the proliferation and differentiation of human bone marrow stromal cells (hBMSCs) in polyurethane scaffolds in a perfusion bioreactor. Polyurethane scaffolds seeded with hBMSCs were cultured under six different conditions, as follows: 10% Cyclic compression at 0.5 and 5 Hz; 10 ml/min perfusion; 100 nM DEX; 100 ng/ml BMP-7; and 1 ml/min perfusion without mechanical and biochemical stimulation (control). On days 7 and 14, samples were tested for the following data: Cell proliferation; mRNA expression of Runx2, COL1A1 and osteocalcin; osteocalcin content; calcium deposition; and the equilibrium modulus of the tissue specimen. The results indicated that BMP-7 and 10 ml/min perfusion promoted cell proliferation, which was inhibited by 5 Hz cyclic compression and DEX. On day 7, the 5 Hz cyclic compression inhibited Runx2 expression, whereas the 0.5 Hz cyclic compression and BMP-7 upregulated the COL1A1 mRNA levels on day 7 and enhanced the osteocalcin expression on day 14. The DEX-treated hBMSCs exhibited downregulated osteocalcin expression. After 14 days, the BMP-7 group exhibited the highest calcium deposition, followed by the 0.5 Hz cyclic compression and the DEX groups. The equilibrium modulus of the engineered constructs significantly increased in the BMP-7, 0.5 Hz cyclic compression and DEX groups. In conclusion, the present results suggest that BMP-7 and perfusion enhance cell proliferation, whereas high frequency cyclic compression inhibits the proliferation and osteogenic differentiation of hBMSCs. Low frequency cyclic compression is more effective than DEX, but less effective compared with BMP-7 on the osteogenic differentiation of hBMSCs seeded on polyurethane scaffolds.

Effect of the harvest procedure and tissue site on the osteogenic function of and gene expression in human mesenchymal stem cells

Evidence has indicated that mesenchymal stem cells (MSCs) harvested with the Reamer/Irrigator/Aspirator (RIA) procedure exhibited an improved osteogenic differentiation capability compared with MSCs obtained by bone marrow aspiration from the iliac crest. In the present study, we hypothesized that the harvest procedure indeed influences the osteogenic activity of human MSCs more than the tissue site itself. Concentration [by colony forming unit-fibroblast (CFU-F) assay], calcification (by von Kossa staining), collagen deposition, gene expression and the gene methylation of the bone morphogenetic protein (BMP)-2 pathway [BMP2, SMAD5 and runt-related transcription factor 2 (RUNX2)], the Wnt pathway [WNT3, dickkopf-1 (DKK1), low-density lipoprotein receptor-related protein 5 (LRP5) and β-catenin] and osteogenic genes [alkaline phosphatase (ALP), collagen, type I, alpha 1 (COL1A) and osteocalcin] were analyzed in the MSCs isolated intraoperatively from the iliac crest with a spoon (n=14), from the femur with a spoon (n=7), from the femur with the RIA procedure (n=13) and from the iliac crest by fine-needle aspiration (n=8, controls). A Bonferroni-Holm corrected p-value <0.05 indicated a statistically significant difference. The concentration of CFU-F in the MSCs was increased in the RIA debris in comparison with that in the iliac crest aspirates (trend) and the femur (spoon, significant). Calcium deposition was highest in the femur-derived MSCs (by RIA) and was significantly increased in comparison with that in the iliac crest-derived MSCs (spoon, aspirate). The gene expression of BMP2, SMAD5, RUNX2, osteocalcin, and COL1A was significantly increased in the femur-derived MSCs (spoon) and the iliac crest aspirate derived-MSCs in comparison with that in the femur-derived MSCs (by RIA). There was no significant diversity between the samples obtained using a spoon (from the femur or iliac crest). Calcium deposition and osteogenic gene expression decreased significantly with the increasing passage number in all the samples. The methylation of genes did not correlate with their respective gene expression and inconsistent differences were observed between the groups. Herein, we provide evidence that the harvest procedure is a critical factor in the osteogenesis of MSCs in vitro. The MSCs isolated from the femur and iliac crest using a spoon exhibit no significant differences. The altered gene expression and function of the femur-derived MSCs (by RIA) may be due to the harsh isolation procedure. The variable differentiation ability of the MSCs, which depends on the harvest site and the harvest technique, as well as the rapid loss of the osteogenic differentiation capacity with the increasing culture duration should be taken into consideration when using MSCs as a potential therapeutic application for bone tissue engineering.

Hematopoietic Support Capacity of Mesenchymal Stem Cells: Biology and Clinical Potential

Mesenchymal stem cells (MSCs) play an important role in the physiology and homeostasis of the hematopoietic system. Because MSCs generate most of the stromal cells present in the bone marrow (BM), form part of the hematopoietic stem cell (HSC) niche, and produce various molecules regulating hematopoiesis, their hematopoiesis-supporting capacity has been demonstrated. In the last decade, BM-MSCs have been proposed to be useful in some ex vivo protocols for HSC expansion, with the aim of expanding their numbers for transplant purposes (HSC transplant, HSCT). Furthermore, application of MSCs has been proposed as an adjuvant cellular therapy for promoting rapid hematopoietic recovery in HSCT patients. Although the MSCs used in preliminary clinical trials have come from the BM, isolation of MSCs from far more accessible sources such as neonatal tissues has now been achieved, and these cells have been found to possess similar biological characteristics to those isolated from the BM. Therefore, such tissues are now considered as a potential alternative source of MSCs for clinical applications. In this review, we discuss current knowledge regarding the biological characteristics of MSCs as related to their capacity to support the formation of hematopoietic stem and progenitor cells. We also describe MSC manipulation for ex vivo HSC expansion protocols used for transplants and their clinical relevance for hematopoietic recovery in HSCT patients.

Assessment of activated porous granules on implant fixation and early bone formation in sheep

Background/Objective

Despite recent progress in regeneration medicine, the repair of large bone defects due to trauma, inflammation and tumor surgery remains a major clinical challenge. This study was designed to produce large amounts of viable bone graft materials in a novel perfusion bioreactor to promote bone formation.

Methods

Cylindrical defects were created bilaterally in the distal femurs of sheep, and titanium implants were inserted. The concentric gap around the implants was randomly filled either with allograft, granules, granules with bone marrow aspirate (BMA) or bioreactor activated granule (BAG). The viable BAG consisted of autologous bone marrow stromal cells (BMSCs) seeded upon porous scaffold granules incubated in a 3D perfusion bioreactor for 2 weeks prior to surgery. 6 weeks after, the bone formation and early implant fixation were assessed by means of micro-CT, histomorphometry, and mechanical test.

Results

Microarchitectural analysis revealed that bone volume fraction and trabecular thickness in the allograft were not statistically different than those (combination of new bone and residue of granule) in the other 3 groups. The structure of the allograft group was typically plate-like, while the other 3 groups were combination of plate and rod. Histomorphometry showed that allograft induced significantly more bone and less fibrous tissue in the concentric gap than the other 3 granule groups, while the bone ingrowth to implant porous surface was not different. No significant differences among the groups were found regarding early implant mechanical fixation.

Conclusion

In conclusion, despite nice bone formation and implant fixation in all groups, bioreactor activated graft material did not convincingly induce early implant fixation similar to allograft, and neither bioreactor nor by adding BMA credited additional benefit for bone formation in this model.

Preclinical models for in vitro mechanical loading of bone-derived cells

It is well established that bone responds to mechanical stimuli whereby physical forces are translated into chemical signals between cells, via mechanotransduction. It is difficult however to study the precise cellular and molecular responses using in vivo systems. In vitro loading models, which aim to replicate forces found within the bone microenvironment, make the underlying processes of mechanotransduction accessible to the researcher. Direct measurements in vivo and predictive modeling have been used to define these forces in normal physiological and pathological states. The types of mechanical stimuli present in the bone include vibration, fluid shear, substrate deformation and compressive loading, which can all be applied in vitro to monolayer and three-dimensional (3D) cultures. In monolayer, vibration can be readily applied to cultures via a low-magnitude, high-frequency loading rig. Fluid shear can be applied to cultures in multiwell plates via a simple rocking platform to engender gravitational fluid movement or via a pump to cells attached to a slide within a parallel-plate flow chamber, which may be micropatterned for use with osteocytes. Substrate strain can be applied via the vacuum-driven FlexCell system or via a four-point loading jig. 3D cultures better replicate the bone microenvironment and can also be subjected to the same forms of mechanical stimuli as monolayer, including vibration, fluid shear via perfusion flow, strain or compression. 3D cocultures that more closely replicate the bone microenvironment can be used to study the collective response of several cell types to loading. This technical review summarizes the methods for applying mechanical stimuli to bone cells in vitro.

Novel Perfused Compression Bioreactor System as an in vitro Model to Investigate Fracture Healing

Secondary bone fracture healing is a physiological process that leads to functional tissue regeneration via endochondral bone formation. In vivo studies have demonstrated that early mobilization and the application of mechanical loads enhances the process of fracture healing. However, the influence of specific mechanical stimuli and particular effects during specific phases of fracture healing remain to be elucidated. In this work, we have developed and provided proof-of-concept of an in vitro human organotypic model of physiological loading of a cartilage callus, based on a novel perfused compression bioreactor (PCB) system. We then used the fracture callus model to investigate the regulatory role of dynamic mechanical loading. Our findings provide a proof-of-principle that dynamic mechanical loading applied by the PCB can enhance the maturation process of mesenchymal stromal cells toward late hypertrophic chondrocytes and the mineralization of the deposited extracellular matrix. The PCB provides a promising tool to study fracture healing and for the in vitro assessment of alternative fracture treatments based on engineered tissue grafts or pharmaceutical compounds, allowing for the reduction of animal experiments.

Influence of the mechanical environment on the engineering of mineralised tissues using human dental pulp stem cells and silk fibroin scaffolds

Teeth constitute a promising source of stem cells that can be used for tissue engineering and regenerative medicine purposes. Bone loss in the craniofacial complex due to pathological conditions and severe injuries could be treated with new materials combined with human dental pulp stem cells (hDPSCs) that have the same embryonic origin as craniofacial bones. Optimising combinations of scaffolds, cells, growth factors and culture conditions still remains a great challenge. In the present study, we evaluate the mineralisation potential of hDPSCs seeded on porous silk fibroin scaffolds in a mechanically dynamic environment provided by spinner flask bioreactors. Cell-seeded scaffolds were cultured in either standard or osteogenic media in both static and dynamic conditions for 47 days. Histological analysis and micro-computed tomography of the samples showed low levels of mineralisation when samples were cultured in static conditions (0.16±0.1 BV/TV%), while their culture in a dynamic environment with osteogenic medium and weekly µCT scans (4.9±1.6 BV/TV%) significantly increased the formation of homogeneously mineralised structures, which was also confirmed by the elevated calcium levels (4.5±1.0 vs. 8.8±1.7 mg/mL). Molecular analysis of the samples showed that the expression of tooth correlated genes such as Dentin Sialophosphoprotein and Nestin were downregulated by a factor of 6.7 and 7.4, respectively, in hDPSCs when cultured in presence of osteogenic medium. This finding indicates that hDPSCs are able to adopt a non-dental identity by changing the culture conditions only. Also an increased expression of Osteocalcin (1.4x) and Collagen type I (1.7x) was found after culture under mechanically dynamic conditions in control medium. In conclusion, the combination of hDPSCs and silk scaffolds cultured under mechanical loading in spinner flask bioreactors could offer a novel and promising approach for bone tissue engineering where appropriate and rapid bone regeneration in mechanically loaded tissues is required.

Modulating the biochemical and biophysical culture environment to enhance osteogenic differentiation and maturation of human pluripotent stem cell-derived mesenchymal progenitors

Advances in the fields of stem cell biology, biomaterials, and tissue engineering over the last decades have brought the possibility of constructing tissue substitutes with a broad range of applications in regenerative medicine, disease modeling, and drug discovery. Different types of human stem cells have been used, each presenting a unique set of advantages and limitations with regard to the desired research goals. Whereas adult stem cells are at the frontier of research for tissue and organ regeneration, pluripotent stem cells represent a more challenging cell source for clinical translation. However, with their unlimited growth and wide differentiation potential, pluripotent stem cells represent an unprecedented resource for the construction of advanced human tissue models for biological studies and drug discovery. At the heart of these applications lies the challenge to reproducibly expand, differentiate, and organize stem cells into mature, stable tissue structures. In this review, we focus on the derivation of mesenchymal tissue progenitors from human pluripotent stem cells and the control of their osteogenic differentiation and maturation by modulation of the biophysical culture environment. Similarly to enhancing bone development, the described principles can be applied to the construction of other mesenchymal tissues for basic and applicative studies.

Mechanical forces induce odontoblastic differentiation of mesenchymal stem cells on three-dimensional biomimetic scaffolds

The mechanical induction of cell differentiation is well known. However, the effect of mechanical compression on odontoblastic differentiation remains to be elucidated. Thus, we first determined the optimal conditions for the induction of human dental pulp stem cells (hDPSCs) into odontoblastic differentiation in response to mechanical compression of three-dimensional (3D) scaffolds with dentinal tubule-like pores. The odontoblastic differentiation was evaluated by gene expression and confocal laser microscopy. The optimal conditions, which were: cell density, 4.0 × 10⁵ cells/cm² ; compression magnitude, 19.6 kPa; and loading time, 9 h, significantly increased expression of the odontoblast-specific markers dentine sialophosphoprotein (DSPP) and enamelysin and enhanced the elongation of cellular processes into the pores of the membrane, a typical morphological feature of odontoblasts. In addition, upregulation of bone morphogenetic protein 7 (BMP7) and wingless-type MMTV integration site family member 10a (Wnt10a) was observed. Moreover, the phosphorylation levels of mitogen-activated protein kinases (MAPKs), extracellular signal-regulated kinase 1/2 (ERK1/2) and p38 were also enhanced by mechanical compression, indicating the involvement of the MAPK signalling pathway. It is noteworthy that human mesenchymal stem cells (MSCs) derived from bone marrow and amnion also differentiated into odontoblasts in response to the optimal mechanical compression, demonstrating the importance of the physical structure of the scaffold in odontoblastic differentiation. Thus, odontoblastic differentiation of hDPSCs is promoted by optimal mechanical compression through the MAPK signalling pathway and expression of the BMP7 and Wnt10a genes. The 3D biomimetic scaffolds with dentinal tubule-like pores were critical for the odontoblastic differentiation of MSCs induced by mechanical compression. Copyright © 2014 John Wiley & Sons, Ltd.

The effect of mechanical loading on osteogenesis of human dental pulp stromal cells in a novel in vitro model

Tooth loss often results in alveolar bone resorption because of lack of mechanical stimulation. Thus, the mechanism of mechanical loading on stem cell osteogenesis is crucial for alveolar bone regeneration. We have investigated the effect of mechanical loading on osteogenesis in human dental pulp stromal cells (hDPSCs) in a novel in vitro model. Briefly, 1 × 10(7) hDPSCs were seeded into 1 ml 3% agarose gel in a 48-well-plate. A loading tube was then placed in the middle of the gel to mimic tooth-chewing movement (1 Hz, 3 × 30 min per day, n = 3). A non-loading group was used as a control. At various time points, the distribution of live/dead cells within the gel was confirmed by fluorescence markers and confocal microscopy. The correlation and interaction between the factors (e.g. force, time, depth and distance) were statistically analysed. The samples were processed for histology and immunohistochemistry. After 1-3 weeks of culture in the in-house-designed in vitro bioreactor, fluorescence imaging confirmed that additional mechanical loading increased the viable cell numbers over time as compared with the control. Cells of various phenotypes formed different patterns away from the reaction tube. The cells in the middle part of the gel showed enhanced alkaline phosphatase staining at week 1 but reduced staining at weeks 2 and 3. Additional loading enhanced Sirius Red and type I collagen staining compared with the control. We have thus successfully developed a novel in-house-designed in vitro bioreactor mimicking the biting force to enhance hDPSC osteogenesis in an agarose scaffold and to promote bone formation and/or prevent bone resorption.

Polycaprolactone scaffold engineered for sustained release of resveratrol: therapeutic enhancement in bone tissue engineering

Biomaterials-based three-dimensional scaffolds are being extensively investigated in bone tissue engineering. A potential scaffold should be osteoconductive, osteoinductive, and osteogenic for enhanced bone formation. In this study, a three-dimensional porous polycapro-lactone (PCL) scaffold was engineered for prolonged release of resveratrol. Resveratrol-loaded albumin nanoparticles (RNP) were synthesized and entrapped into a PCL scaffold to form PCL-RNP by a solvent casting and leaching method. An X-ray diffraction study of RNP and PCL-RNP showed that resveratrol underwent amorphization, which is highly desired in drug delivery. Furthermore, Fourier transform infrared spectroscopy indicates that resveratrol was not chemically modified during the entrapment process. Release of resveratrol from PCL-RNP was sustained, with a cumulative release of 64% at the end of day 12. The scaffold was evaluated for its bone-forming potential in vitro using human bone marrow-derived mesenchymal stem cells for 16 days. Alkaline phosphatase activity assayed on days 8 and 12 showed a significant increase in activity (1.6-fold and 1.4-fold, respectively) induced by PCL-RNP compared with the PCL scaffold (the positive control). Moreover, von Kossa staining for calcium deposits on day 16 showed increased mineralization in PCL-RNP. These results suggest PCL-RNP significantly improves mineralization due to its controlled and prolonged release of resveratrol, thereby increasing the therapeutic potential in bone tissue engineering.

Gene expression responses to mechanical stimulation of mesenchymal stem cells seeded on calcium phosphate cement

INTRODUCTION

The aim of the study reported here was to investigate the molecular responses of human mesenchymal stem cells (MSC) to loading with a model that attempts to closely mimic the physiological mechanical loading of bone, using monetite calcium phosphate (CaP) scaffolds to mimic the biomechanical properties of bone and a bioreactor to induce appropriate load and strain.

METHODS

Human MSCs were seeded onto CaP scaffolds and subjected to a pulsating compressive force of 5.5±4.5 N at a frequency of 0.1 Hz. Early molecular responses to mechanical loading were assessed by microarray and quantitative reverse transcription-polymerase chain reaction and activation of signal transduction cascades was evaluated by western blotting analysis.

RESULTS

The maximum mechanical strain on cell/scaffolds was calculated at around 0.4%. After 2 h of loading, a total of 100 genes were differentially expressed. The largest cluster of genes activated with 2 h stimulation was the regulator of transcription, and it included FOSB. There were also changes in genes involved in cell cycle and regulation of protein kinase cascades. When cells were rested for 6 h after mechanical stimulation, gene expression returned to normal. Further resting for a total of 22 h induced upregulation of 63 totally distinct genes that were mainly involved in cell surface receptor signal transduction and regulation of metabolic and cell division processes. In addition, the osteogenic transcription factor RUNX-2 was upregulated. Twenty-four hours of persistent loading also markedly induced osterix expression. Mechanical loading resulted in upregulation of Erk1/2 phosphorylation and the gene expression study identified a number of possible genes (SPRY2, RIPK1, SPRED2, SERTAD1, TRIB1, and RAPGEF2) that may regulate this process.

CONCLUSION

The results suggest that mechanical loading activates a small number of immediate-early response genes that are mainly associated with transcriptional regulation, which subsequently results in activation of a wider group of genes including those associated with osteoblast proliferation and differentiation. The results provide a valuable insight into molecular events and signal transduction pathways involved in the regulation of MSC osteogenic differentiation in response to a physiological level of mechanical stimulation.

Primary cilia respond to fluid shear stress and mediate flow-induced calcium deposition in osteoblasts

Bone turnover in vivo is regulated by mechanical forces such as shear stress originating from interstitial oscillatory fluid flow (OFF), and bone cells in vitro respond to mechanical loading. However, the mechanisms by which bone cells sense mechanical forces, resulting in increased mineral deposition, are not well understood. The aim of this study was to investigate the role of the primary cilium in mechanosensing by osteoblasts. MLO-A5 murine osteoblasts were cultured in monolayer and subjected to two different OFF regimens: 5 short (2 h daily) bouts of OFF followed by morphological analysis of primary cilia; or exposure to chloral hydrate to damage or remove primary cilia and 2 short bouts (2 h on consecutive days) of OFF. Primary cilia were shorter and there were fewer cilia per cell after exposure to periods of OFF compared with static controls. Damage or removal of primary cilia inhibited OFF-induced PGE₂ release into the medium and mineral deposition, assayed by Alizarin red staining. We conclude that primary cilia are important mediators of OFF-induced mineral deposition, which has relevance for the design of bone tissue engineering strategies and may inform clinical treatments of bone disorders causes by load-deficiency.—Delaine-Smith, R. M., Sittichokechaiwut, A., Reilly, G. C. Primary cilia respond to fluid shear stress and mediate flow-induced calcium deposition in osteoblasts.

Short periods of cyclic mechanical strain enhance triple-supplement directed osteogenesis and bone nodule formation by human embryonic stem cells in vitro

Human embryonic stem cells (hESCs) are uniquely endowed with a capacity for both self-renewal and multilineage differentiation. The aim of this investigation was to determine if short periods of cyclic mechanical strain enhanced dexamethasone, ascorbic acid, and β-glycerophosphate (triple-supplement)-induced osteogenesis and bone nodule formation by hESCs. Colonies were cultured for 21 days and divided into control (no stretch) and three treatment groups; these were subjected to in-plane deformation of 2% for 5 s (0.2 Hertz) every 60 s for 1 h on alternate days in BioFlex plates linked to a Flexercell strain unit over the following periods (day 7-13), (day 15-21), and (day 7-21). Numerous bone nodules were formed, which stained positively for osteocalcin and type I collagen; in addition, MTS assays for cell number as well as total collagen assays showed a significant increase in the day 7-13 group compared to controls and other treatment groups. Alizarin Red staining further showed that cyclic mechanical stretching significantly increased the nodule size and mineral density between days 7-13 compared to control cultures and the other two experimental groups. We then performed a real-time polymerase chain reaction (PCR) microarray on the day 7-13 treatment group to identify mechanoresponsive osteogenic genes. Upregulated genes included the transcription factors RUNX2 and SOX9, bone morphogenetic proteins BMP1, BMP4, BMP5, and BMP6, transforming growth factor-β family members TGFB1, TGFB2, and TGFB3, and three genes involved in mineralization-ALPL, BGLAP, and VDR. In conclusion, this investigation has demonstrated that four 1-h episodes of cyclic mechanical strain acted synergistically with triple supplement to enhance osteogenesis and bone nodule formation by cultured hESCs. This suggests the development of methods to engineer three-dimensional constructs of mineralized bone in vitro, could offer an alternative approach to osseous regeneration by producing a biomaterial capable of providing stable surfaces for osteoblasts to synthesize new bone, while at the same time able to be resorbed by an osteoclastic activity-in other words, one that can recapitulate the remodeling dynamics of a naturally occurring bone matrix.

Scaffold/Extracellular matrix hybrid constructs for bone-tissue engineering

The limited natural ability of the body to fully repair large bone defects often necessitates the implantation of a replacement material to promote healing. While the current clinical strategies to address such bone defects generally carry associated limitations, bone-tissue engineering approaches seek to minimize any adverse effects and facilitate complete regeneration of the lost tissue. Of particular interest are hybrid constructs that incorporate multiple components found within the native bone matrix to enhance the osteogenicity of biocompatible materials, which might otherwise be non-osteogenic. This Progress Report will focus on such hybrid constructs that incorporate multiple components from native bone matrix for bone-tissue engineering and will highlight the synthesis and characterization of the hybrid constructs, cellular attachment and proliferation within the constructs, in vitro osteogenicity of the constructs, and the biological response to in vivo implantation of the constructs at ectopic and orthotopic sites.

Postproduction processing of electrospun fibres for tissue engineering

Electrospinning is a commonly used and versatile method to produce scaffolds (often biodegradable) for 3D tissue engineering.(1, 2, 3) Many tissues in vivo undergo biaxial distension to varying extents such as skin, bladder, pelvic floor and even the hard palate as children grow. In producing scaffolds for these purposes there is a need to develop scaffolds of appropriate biomechanical properties (whether achieved without or with cells) and which are sterile for clinical use. The focus of this paper is not how to establish basic electrospinning parameters (as there is extensive literature on electrospinning) but on how to modify spun scaffolds post production to make them fit for tissue engineering purposes--here thickness, mechanical properties and sterilisation (required for clinical use) are considered and we also describe how cells can be cultured on scaffolds and subjected to biaxial strain to condition them for specific applications. Electrospinning tends to produce thin sheets; as the electrospinning collector becomes coated with insulating fibres it becomes a poor conductor such that fibres no longer deposit on it. Hence we describe approaches to produce thicker structures by heat or vapour annealing increasing the strength of scaffolds but not necessarily the elasticity. Sequential spinning of scaffolds of different polymers to achieve complex scaffolds is also described. Sterilisation methodologies can adversely affect strength and elasticity of scaffolds. We compare three methods for their effects on the biomechanical properties on electrospun scaffolds of poly lactic-co-glycolic acid (PLGA). Imaging of cells on scaffolds and assessment of production of extracellular matrix (ECM) proteins by cells on scaffolds is described. Culturing cells on scaffolds in vitro can improve scaffold strength and elasticity but the tissue engineering literature shows that cells often fail to produce appropriate ECM when cultured under static conditions. There are few commercial systems available that allow one to culture cells on scaffolds under dynamic conditioning regimes--one example is the Bose Electroforce 3100 which can be used to exert a conditioning programme on cells in scaffolds held using mechanical grips within a media filled chamber.(4) An approach to a budget cell culture bioreactor for controlled distortion in 2 dimensions is described. We show that cells can be induced to produce elastin under these conditions. Finally assessment of the biomechanical properties of processed scaffolds cultured with or without cells is described.

Transient 100 nM dexamethasone treatment reduces inter- and intraindividual variations in osteoblastic differentiation of bone marrow-derived human mesenchymal stem cells

The development of in vitro culturing techniques for osteoblastic differentiation of human mesenchymal stem cells (hMSC) is important for cell biology research and the development of tissue-engineering applications. Dexamethasone (Dex) is a commonly used supplement, but the optimal use of Dex treatment is still unclear. By adjusting the timing of Dex supplementation, the negative effects of long-term Dex treatment could be overcome. Transient Dex treatment could contribute toward minimizing broad donor variation, which is a major challenge. We compared the two most widely used Dex concentrations of 10 and 100 nM as transient or continuous treatment and studied inter- and intraindividual variations in osteoblastic differentiation of hMSC. Characterized bone marrow-derived hMSC from 17 female donors of different age groups were used. During osteoblastic induction, the cells were treated with 10 or 100 nM Dex either transiently for different time periods or continuously. Differentiation was evaluated by measuring alkaline phosphatase (ALP) activity and staining for ALP, von Kossa, collagen type I, and osteocalcin. Cell proliferation, cell viability, and apoptosis were also monitored. The strongest osteoblastic differentiation was observed when 100 nM Dex was present for the first week. In terms of inter- and intraindividual coefficients of variations, transient treatment with 100 nM Dex was superior to the other culture conditions and showed the lowest variations in all age groups. This study demonstrates that the temporary presence of 100 nM Dex during the first week of induction culture promotes hMSC osteoblastic differentiation and reduces inter- and intraindividual variations. With this protocol, we can reproducibly produce functional osteoblasts in vitro from the hMSC of different donor populations.