Controlled differentiation of human bone marrow stromal cells using magnetic nanoparticle technology

Overcoming translational challenges - The delivery of mechanical stimuli in vivo

Despite major medical advances, non-union bone fractures and skeletal defects continue to place significant burden on the patient, the clinicians and the healthcare system as a whole. Current bone substitute approaches are still limited in effectiveness and to date no adequate bone substitute material has been developed for routine clinical application. Tissue engineering presents a novel approach to tackling this clinical burden and developing an acceptable solution for the treatment of skeletal defects. Over the past three decades the field has evolved to appreciate the key biological, material and physical parameters influencing the development of a cell-based tissue engineered therapy and to create associated technologies to exploit such parameters. In recent years a number of therapies have started progressing along the pre-clinical pipeline to build a case for regulatory approval and ultimately clinical adoption. However, little emphasis has been given to the translational challenges faced when moving from "bench-to-bedside". One particular challenge lies in the delivery of functional mechanical stimuli to implanted cell populations to activate and promote osteogenic activities. This review introduces novel bio-magnetic approaches to overcoming this challenge.

Towards nanomedicines of the future: Remote magneto-mechanical actuation of nanomedicines by alternating magnetic fields

The paper describes the concept of magneto-mechanical actuation of single-domain magnetic nanoparticles (MNPs) in super-low and low frequency alternating magnetic fields (AMFs) and its possible use for remote control of nanomedicines and drug delivery systems. The applications of this approach for remote actuation of drug release as well as effects on biomacromolecules, biomembranes, subcellular structures and cells are discussed in comparison to conventional strategies employing magnetic hyperthermia in a radio frequency (RF) AMF. Several quantitative models describing interaction of functionalized MNPs with single macromolecules, lipid membranes, and proteins (e.g. cell membrane receptors, ion channels) are presented. The optimal characteristics of the MNPs and an AMF for effective magneto-mechanical actuation of single molecule responses in biological and bio-inspired systems are discussed. Altogether, the described studies and phenomena offer opportunities for the development of novel therapeutics both alone and in combination with magnetic hyperthermia.

Advanced cell therapies: targeting, tracking and actuation of cells with magnetic particles

Regenerative medicine would greatly benefit from a new platform technology that enabled measurable, controllable and targeting of stem cells to a site of disease or injury in the body. Superparamagnetic iron-oxide nanoparticles offer attractive possibilities in biomedicine and can be incorporated into cells, affording a safe and reliable means of tagging. This review describes three current and emerging methods to enhance regenerative medicine using magnetic particles to guide therapeutic cells to a target organ; track the cells using MRI and assess their spatial localization with high precision and influence the behavior of the cell using magnetic actuation. This approach is complementary to the systemic injection of cell therapies, thus expanding the horizon of stem cell therapeutics.

Physical Stimuli-Induced Chondrogenic Differentiation of Mesenchymal Stem Cells Using Magnetic Nanoparticles

Chondrogenic commitments of mesenchymal stem cells (MSCs) require 3D cellular organization. Furthermore, recent progresses in bioreactor technology have contributed to the development of various biophysical stimulation platforms for efficient cartilage tissue formation. Here, an approach is reported to drive 3D cellular organization and enhance chondrogenic commitment of bone-marrow-derived human mesenchymal stem cells (BM-hMSCs) via magnetic nanoparticle (MNP)-mediated physical stimuli. MNPs isolated from Magnetospirillum sp. AMB-1 are endocytosed by the BM-hMSCs in a highly efficient manner. MNPs-incorporated BM-hMSCs are pelleted and then subjected to static magnetic field and/or magnet-derived shear stress. Magnetic-based stimuli enhance level of sulfated glycosaminoglycan (sGAG) and collagen synthesis, and facilitate the chondrogenic differentiation of BM-hMSCs. In addition, both static magnetic field and magnet-derived shear stress applied for the chondrogenic differentiation of BM-hMSCs do not show increament of hypertrophic differentiation. This MNP-mediated physical stimulation platform demonstrates a promising strategy for efficient cartilage tissue engineering.

Remote activation of the Wnt/β-catenin signalling pathway using functionalised magnetic particles

Wnt signalling pathways play crucial roles in developmental biology, stem cell fate and tissue patterning and have become an attractive therapeutic target in the fields of tissue engineering and regenerative medicine. Wnt signalling has also been shown to play a role in human Mesenchymal Stem Cell (hMSC) fate, which have shown potential as a cell therapy in bone and cartilage tissue engineering. Previous work has shown that biocompatible magnetic nanoparticles (MNP) can be used to stimulate specific mechanosensitive membrane receptors and ion channels in vitro and in vivo. Using this strategy, we determined the effects of mechano-stimulation of the Wnt Frizzled receptor on Wnt pathway activation in hMSC. Frizzled receptors were tagged using anti-Frizzled functionalised MNP (Fz-MNP). A commercially available oscillating magnetic bioreactor (MICA Biosystems) was used to mechanically stimulate Frizzled receptors remotely. Our results demonstrate that Fz-MNP can activate Wnt/β-catenin signalling at key checkpoints in the signalling pathway. Immunocytochemistry indicated nuclear localisation of the Wnt intracellular messenger β-catenin after treatment with Fz-MNP. A Wnt signalling TCF/LEF responsive luciferase reporter transfected into hMSC was used to assess terminal signal activation at the nucleus. We observed an increase in reporter activity after treatment with Fz-MNP and this effect was enhanced after mechano-stimulation using the magnetic array. Western blot analysis was used to probe the mechanism of signalling activation and indicated that Fz-MNP signal through an LRP independent mechanism. Finally, the gene expression profiles of stress response genes were found to be similar when cells were treated with recombinant Wnt-3A or Fz-MNP. This study provides proof of principle that Wnt signalling and Frizzled receptors are mechanosensitive and can be remotely activated in vitro. Using magnetic nanoparticle technology it may be possible to modulate Wnt signalling pathways and thus control stem cell fate for therapeutic purposes.

Remotely Activated Mechanotransduction via Magnetic Nanoparticles Promotes Mineralization Synergistically With Bone Morphogenetic Protein 2: Applications for Injectable Cell Therapy

Bone requires dynamic mechanical stimulation to form and maintain functional tissue, yet mechanical stimuli are often lacking in many therapeutic approaches for bone regeneration. Magnetic nanoparticles provide a method for delivering these stimuli by directly targeting cell-surface mechanosensors and transducing forces from an external magnetic field, resulting in remotely controllable mechanotransduction. In this investigation, functionalized magnetic nanoparticles were attached to either the mechanically gated TREK1 K+ channel or the (integrin) RGD-binding domains of human mesenchymal stem cells. These cells were microinjected into an ex vivo chick fetal femur (embryonic day 11) that was cultured organotypically in vitro as a model for endochondral bone formation. An oscillating 25-mT magnetic field delivering a force of 4 pN per nanoparticle directly against the mechanoreceptor induced mechanotransduction in the injected mesenchymal stem cells. It was found that cells that received mechanical stimuli via the nanoparticles mineralized the epiphyseal injection site more extensively than unlabeled control cells. The nanoparticle-tagged cells were also seeded into collagen hydrogels to evaluate osteogenesis in tissue-engineered constructs: in this case, inducing mechanotransduction by targeting TREK1 resulted in a 2.4-fold increase in mineralization and significant increases in matrix density. In both models, the combination of mechanical stimulation and sustained release of bone morphogenetic protein 2 (BMP2) from polymer microspheres showed a significant additive effect on mineralization, increasing the effectiveness of BMP2 delivery and demonstrating that nanoparticle-mediated mechanotransduction can be used synergistically with pharmacological approaches for orthopedic tissue engineering to maximize bone formation.

Materials as stem cell regulators

The stem cell/material interface is a complex, dynamic microenvironment in which the cell and the material cooperatively dictate one another's fate: the cell by remodelling its surroundings, and the material through its inherent properties (such as adhesivity, stiffness, nanostructure or degradability). Stem cells in contact with materials are able to sense their properties, integrate cues via signal propagation and ultimately translate parallel signalling information into cell fate decisions. However, discovering the mechanisms by which stem cells respond to inherent material characteristics is challenging because of the highly complex, multicomponent signalling milieu present in the stem cell environment. In this Review, we discuss recent evidence that shows that inherent material properties may be engineered to dictate stem cell fate decisions, and overview a subset of the operative signal transduction mechanisms that have begun to emerge. Further developments in stem cell engineering and mechanotransduction are poised to have substantial implications for stem cell biology and regenerative medicine.

Polymers in cell encapsulation from an enveloped cell perspective

In the past two decades, many polymers have been proposed for producing immunoprotective capsules. Examples include the natural polymers alginate, agarose, chitosan, cellulose, collagen, and xanthan and synthetic polymers poly(ethylene glycol), polyvinyl alcohol, polyurethane, poly(ether-sulfone), polypropylene, sodium polystyrene sulfate, and polyacrylate poly(acrylonitrile-sodium methallylsulfonate). The biocompatibility of these polymers is discussed in terms of tissue responses in both the host and matrix to accommodate the functional survival of the cells. Cells should grow and function in the polymer network as adequately as in their natural environment. This is critical when therapeutic cells from scarce cadaveric donors are considered, such as pancreatic islets. Additionally, the cell mass in capsules is discussed from the perspective of emerging new insights into the release of so-called danger-associated molecular pattern molecules by clumps of necrotic therapeutic cells. We conclude that despite two decades of intensive research, drawing conclusions about which polymer is most adequate for clinical application is still difficult. This is because of the lack of documentation on critical information, such as the composition of the polymer, the presence or absence of confounding factors that induce immune responses, toxicity to enveloped cells, and the permeability of the polymer network. Only alginate has been studied extensively and currently qualifies for application. This review also discusses critical issues that are not directly related to polymers and are not discussed in the other reviews in this issue, such as the functional performance of encapsulated cells in vivo. Physiological endocrine responses may indeed not be expected because of the many barriers that the metabolites encounter when traveling from the blood stream to the enveloped cells and back to circulation. However, despite these diffusion barriers, many studies have shown optimal regulation, allowing us to conclude that encapsulated grafts do not always follow nature's course but are still a possible solution for many endocrine disorders for which the minute-to-minute regulation of metabolites is mandatory.

Receptor-targeted, magneto-mechanical stimulation of osteogenic differentiation of human bone marrow-derived mesenchymal stem cells

Mechanical cues are employed to promote stem cell differentiation and functional tissue formation in tissue engineering and regenerative medicine. We have developed a Magnetic Force Bioreactor (MFB) that delivers highly targeted local forces to cells at a pico-newton level, utilizing magnetic micro- and nano-particles to target cell surface receptors. In this study, we investigated the effects of magnetically targeting and actuating specific two mechanical-sensitive cell membrane receptors-platelet-derived growth factor receptor α (PDGFRα) and integrin ανβ3. It was found that a higher mineral-to-matrix ratio was obtained after three weeks of magneto-mechanical stimulation coupled with osteogenic medium culture by initially targeting PDGFRα compared with targeting integrin ανβ3 and non-treated controls. Moreover, different initiation sites caused a differentiated response profile when using a 2-day-lagged magneto-mechanical stimulation over culture periods of 7 and 12 days). However, both resulted in statistically higher osteogenic marker genes expression compared with immediate magneto-mechanical stimulation. These results provide insights into important parameters for designing appropriate protocols for ex vivo induced bone formation via magneto-mechanical actuation.

Nanotechnology to drive stem cell commitment

Stem cells (SCs) are undifferentiated cells responsible for the growth, homeostasis and repair of many tissues. The maintenance and survival of SCs is strongly influenced by several stimuli from the local microenvironment. The majority of signaling molecules interact with SCs at the nanoscale level. Therefore, scaffolds with surface nanostructures have potential applications for SCs and in the field of regenerative medicine. Although some strategies have already reached the field of cell biology, strategies based on modification at nanoscale level are new players in the fields of SCs and tissue regeneration. The introduction of the possibility to perform such modifications to these fields is probably due to increasing improvements in nanomaterials for biomedical applications, as well as new insights into SC biology. The aim of the present review is to exhibit the most recent applications of nanostructured materials that drive the commitment of adult SCs for potential clinical applications.

Remote and local control of stimuli responsive materials for therapeutic applications

Materials offering the ability to change their characteristics in response to presented stimuli have demonstrated application in the biomedical arena, allowing control over drug delivery, protein adsorption and cell attachment to materials. Many of these smart systems are reversible, giving rise to finer control over material properties and biological interaction, useful for various therapeutic treatment strategies. Many smart materials intended for biological interaction are based around pH or thermo-responsive materials, although the use of magnetic materials, particularly in neural regeneration, has increased over the past decade. This review draws together a background of literature describing the design principles and mechanisms of smart materials. Discussion centres on recent literature regarding pH-, thermo-, magnetic and dual responsive materials, and their current applications for the treatment of neural tissue.

Magnetic nanoparticle-based approaches to locally target therapy and enhance tissue regeneration in vivo

Magnetic-based systems utilizing superparamagnetic nanoparticles and a magnetic field gradient to exert a force on these particles have been used in a wide range of biomedical applications. This review is focused on drug targeting applications that require penetration of a cellular barrier as well as strategies to improve the efficacy of targeting in these biomedical applications. Another focus of this review is regenerative applications utilizing tissue engineered scaffolds prepared with the aid of magnetic particles, the use of remote actuation for release of bioactive molecules and magneto-mechanical cell stimulation, cell seeding and cell patterning.

Magnetic poly(ε-caprolactone)/iron-doped hydroxyapatite nanocomposite substrates for advanced bone tissue engineering

In biomedicine, magnetic nanoparticles provide some attractive possibilities because they possess peculiar physical properties that permit their use in a wide range of applications. The concept of magnetic guidance basically spans from drug delivery and hyperthermia treatment of tumours, to tissue engineering, such as magneto-mechanical stimulation/activation of cell constructs and mechanosensitive ion channels, magnetic cell-seeding procedures, and controlled cell proliferation and differentiation. Accordingly, the aim of this study was to develop fully biodegradable and magnetic nanocomposite substrates for bone tissue engineering by embedding iron-doped hydroxyapatite (FeHA) nanoparticles in a poly(ε-caprolactone) (PCL) matrix. X-ray diffraction analyses enabled the demonstration that the phase composition and crystallinity of the magnetic FeHA were not affected by the process used to develop the nanocomposite substrates. The mechanical characterization performed through small punch tests has evidenced that inclusion of 10 per cent by weight of FeHA would represent an effective reinforcement. The inclusion of nanoparticles also improves the hydrophilicity of the substrates as evidenced by the lower values of water contact angle in comparison with those of neat PCL. The results from magnetic measurements confirmed the superparamagnetic character of the nanocomposite substrates, indicated by a very low coercive field, a saturation magnetization strictly proportional to the FeHA content and a strong history dependence in temperature sweeps. Regarding the biological performances, confocal laser scanning microscopy and AlamarBlue assay have provided qualitative and quantitative information on human mesenchymal stem cell adhesion and viability/proliferation, respectively, whereas the obtained ALP/DNA values have shown the ability of the nanocomposite substrates to support osteogenic differentiation.

An in vitro model of mesenchymal stem cell targeting using magnetic particle labelling

The specific targeting of cells to sites of tissue damage in vivo is a major challenge precluding the success of stem cell-based therapies. Magnetic particle-based targeting may provide a solution. Our aim was to provide a model system to study the trapping and potential targeting of human mesenchymal stem cells (MSCs) during in vitro fluid flow, which ultimately will inform cell targeting in vivo. In this system magnet arrays were used to trap superparamagnetic iron oxide particle-doped MSCs. The in vitro experiments demonstrated successful cell trapping, where the volume of cells trapped increased with magnetic particle concentration and decreased with increasing flow rate. Analysis of gene expression revealed significant increases in COL1A2 and SOX9. Using principles established in vitro, a proof-of-concept in vivo experiment demonstrated that magnetic particle-doped, luciferase-expressing MSCs were trapped by an implanted magnet in a subcutaneous wound model in nude mice. Our results demonstrate the effectiveness of using an in vitro model for testing superparamagnetic iron oxide particles to develop successful MSC targeting strategies during fluid flow, which ultimately can be translated to in vivo targeted delivery of cells via the circulation in a variety of tissue-repair models.

A magnetic switch for the control of cell death signalling in in vitro and in vivo systems

The regulation of cellular activities in a controlled manner is one of the most challenging issues in fields ranging from cell biology to biomedicine. Nanoparticles have the potential of becoming useful tools for controlling cell signalling pathways in a space and time selective fashion. Here, we have developed magnetic nanoparticles that turn on apoptosis cell signalling by using a magnetic field in a remote and non-invasive manner. The magnetic switch consists of zinc-doped iron oxide magnetic nanoparticles (Zn(0.4)Fe(2.6)O(4)), conjugated with a targeting antibody for death receptor 4 (DR4) of DLD-1 colon cancer cells. The magnetic switch, in its On mode when a magnetic field is applied to aggregate magnetic nanoparticle-bound DR4s, promotes apoptosis signalling pathways. We have also demonstrated that the magnetic switch is operable at the micrometre scale and that it can be applied in an in vivo system where apoptotic morphological changes of zebrafish are successfully induced.

Intrinsically superparamagnetic Fe-hydroxyapatite nanoparticles positively influence osteoblast-like cell behaviour

BACKGROUND

Superparamagnetic nanoparticles (MNPs) have been progressively explored for their potential in biomedical applications and in particular as a contrast agent for diagnostic imaging, for magnetic drug delivery and more recently for tissue engineering applications. Considering the importance of having safe MNPs for such applications, and the essential role of iron in bone remodelling, this study developed and analysed novel biocompatible and bioreabsorbable superparamagnetic nanoparticles, that avoid the use of poorly tolerated magnetite based nanoparticles, for bone tissue engineering applications.

RESULTS

MNPs were obtained by doping hydroxyapatite (HA) with Fe ions, by directly substituting Fe2+ and Fe3+ into the HA structure yielding superparamagnetic bioactive phase. In the current study, we have investigated the effects of increasing concentrations (2000 μg/ml; 1000 μg/ml; 500 μg/ml; 200 μg/ml) of FeHA MNPs in vitro using Saos-2 human osteoblast-like cells cultured for 1, 3 and 7 days with and without the exposure to a static magnetic field of 320 mT. Results demonstrated not only a comparable osteoblast viability and morphology, but increased in cell proliferation, when compared to a commercially available Ha nanoparticles, even with the highest dose used. Furthermore, FeHA MNPs exposure to the static magnetic field resulted in a significant increase in cell proliferation throughout the experimental period, and higher osteoblast activity.In vivo preliminary results demonstrated good biocompatibility of FeHA superparamagnetic material four weeks after implantation into a critical size lesion of the rabbit condyle.

CONCLUSIONS

The results of the current study suggest that these novel FeHA MNPs may be particularly relevant for strategies of bone tissue regeneration and open new perspectives for the application of a static magnetic field in a clinical setting of bone replacement, either for diagnostic imaging or magnetic drug delivery.

Magnetic hydroxyapatite bone substitutes to enhance tissue regeneration: evaluation in vitro using osteoblast-like cells and in vivo in a bone defect

In case of degenerative disease or lesion, bone tissue replacement and regeneration is an important clinical goal. In particular, nowadays, critical size defects rely on the engineering of scaffolds that are 3D structural supports, allowing cellular infiltration and subsequent integration with the native tissue. Several ceramic hydroxyapatite (HA) scaffolds with high porosity and good osteointegration have been developed in the past few decades but they have not solved completely the problems related to bone defects. In the present study we have developed a novel porous ceramic composite made of HA that incorporates magnetite at three different ratios: HA/Mgn 95/5, HA/Mgn 90/10 and HA/Mgn 50/50. The scaffolds, consolidated by sintering at high temperature in a controlled atmosphere, have been analysed in vitro using human osteoblast-like cells. Results indicate high biocompatibility, similar to a commercially available HA bone graft, with no negative effects arising from the presence of magnetite or by the use of a static magnetic field. HA/Mgn 90/10 was shown to enhance cell proliferation at the early stage. Moreover, it has been implanted in vivo in a critical size lesion of the rabbit condyle and a good level of histocompatibility was observed. Such results identify this scaffold as particularly relevant for bone tissue regeneration and open new perspectives for the application of a magnetic field in a clinical setting of bone replacement, either for magnetic scaffold fixation or magnetic drug delivery.

Orthopaedic applications of nanoparticle-based stem cell therapies

Stem cells have tremendous applications in the field of regenerative medicine and tissue engineering. These are pioneering fields that aim to create new treatments for disease that currently have limited therapies or cures. A particularly popular avenue of research has been the regeneration of bone and cartilage to combat various orthopaedic diseases. Magnetic nanoparticles (MNPs) have been applied to aid the development and translation of these therapies from research to the clinic. This review highlights contemporary research for the applications of iron-oxide-based MNPs for the therapeutic implementation of stem cells in orthopaedics. These MNPs comprise of an iron oxide core, coated with a choice of biological polymers that can facilitate the uptake of MNPs by cells through improving endocytic activity. The combined use of these oxides and the biological polymer coatings meet biological requirements, effectively encouraging the use of MNPs in regenerative medicine. The association of MNPs with stem cells can be achieved via the process of endocytosis resulting in the internalisation of these particles or the attachment to cell surface receptors. This allows for the investigation of migratory patterns through various tracking studies, the targeting of particle-labelled cells to desired locations via the application of an external magnetic field and, finally, for activation stem cells to initiate various cellular responses to induce the differentiation. Characterisation of cell localisation and associated tissue regeneration can therefore be enhanced, particularly for in vivo applications. MNPs have been shown to have the potential to stimulate differentiation of stem cells for orthopaedic applications, without limiting proliferation. However, careful consideration of the use of active agents associated with the MNP is suggested, for differentiation towards specific lineages. This review aims to broaden the knowledge of current applications, paving the way to translate the in vitro and in vivo work into further orthopaedic clinical studies.

Stem cell-based therapies for bone repair

This article provides an overview of the cellular and molecular events involved in bone repair and the current approaches to using stem cells as an adjunct to this process. The article emphasizes the key role of osteoprogenitor cells in the formation of bone and where the clinical applications of current research may lend themselves to large animal orthopaedics. The processes involved in osteogenic differentiation are presented and strategies for bone formation, including induction by osteogenic factors, bioscaffolds, and gene therapy, are reviewed.

The promotion of in vitro vessel-like organization of endothelial cells in magnetically responsive alginate scaffolds

One of the major challenges in engineering thick, complex tissues such as cardiac muscle, is the need to pre-vascularize the engineered tissue in vitro to enable its efficient integration with host tissue upon implantation. Herein, we explored new magnetic alginate composite scaffolds to provide means of physical stimulation to cells. Magnetite-impregnated alginate scaffolds seeded with aortic endothelial cells stimulated during the first 7 days out of a total 14 day experimental course showed significantly elevated metabolic activity during the stimulation period. Expression of proliferating cell nuclear antigen (PCNA) indicated that magnetically stimulated cells had a lower proliferation index as compared to the non-stimulated cells. This suggests that the elevated metabolic activity could instead be related to cell migration and re-organization. Immunostaining and confocal microscopy analyses supported this observation showing that on day 14 in magnetically stimulated scaffolds without supplementation of any growth factors, cellular vessel-like (loop) structures, known as indicators of vasculogenesis and angiogenesis were formed as compared to cell sheets or aggregates observed in the non-stimulated (control) scaffolds. This work is the first step in our understanding of how to accurately control cellular organization to form tissue engineered constructs, which together with additional molecular signals could lead to a creation of an efficient pre-vascularized tissue construct with potential applicability for transplantation.

The effects of mechanical loading on mesenchymal stem cell differentiation and matrix production

Mesenchymal stem cells or stromal cells (MSCs) have the potential to be used therapeutically in tissue engineering and regenerative medicine to replace or restore the function of damaged tissues. Therefore, considerable effort has been ongoing in the research community to optimize culture conditions for predifferentiation of MSCs. All mesenchymal tissues are subjected to mechanical forces in vivo and all fully differentiated mesenchymal lineage cells respond to mechanical stimulation in vivo and in vitro. Therefore, it is not surprising that MSCs are highly mechanosensitive. We present a summary of current methods of mechanical stimulation of MSCs and an overview of the outcomes of the different mechanical culture techniques tested. Tissue engineers and stem cell researchers should be able to harness this mechanosensitivity to modulate MSC differentiation and matrix production; however, more research needs to be undertaken to understand the complex interactions between the mechanosensitive and biochemically stimulated differentiation pathways.