Long-term MRI cell tracking after intraventricular delivery in a patient with global cerebral ischemia and prospects for magnetic navigation of stem cells within the CSF

Bright ferritin for long-term MR imaging of human embryonic stem cells

BACKGROUND

A non-invasive imaging technology that can monitor cell viability, retention, distribution, and interaction with host tissue after transplantation is needed for optimizing and translating stem cell-based therapies. Current cell imaging approaches are limited in sensitivity or specificity, or both, for in vivo cell tracking. The objective of this study was to apply a novel ferritin-based magnetic resonance imaging (MRI) platform to longitudinal tracking of human embryonic stem cells (hESCs) in vivo.

METHODS

Human embryonic stem cells (hESCs) were genetically modified to stably overexpress ferritin using the CRISPR-Cas9 system. Cellular toxicity associated with ferritin overexpression and manganese (Mn) supplementation were assessed based on cell viability, proliferation, and metabolic activity. Ferritin-overexpressing hESCs were characterized based on stem cell pluripotency and cardiac-lineage differentiation capability. Cells were supplemented with Mn and imaged in vitro as cell pellets on a preclinical 3 T MR scanner. T1-weighted images and T1 relaxation times were analyzed to assess contrast. For in vivo study, three million cells were injected into the leg muscle of non-obese diabetic severe combined immunodeficiency (NOD SCID) mice. Mn was administrated subcutaneously. T1-weighted sequences and T1 mapping were used to image the animals for longitudinal in vivo cell tracking. Cell survival, proliferation, and teratoma formation were non-invasively monitored by MRI. Histological analysis was used to validate MRI results.

RESULTS

Ferritin-overexpressing hESCs labeled with 0.1 mM MnCl2 provided significant T1-induced bright contrast on in vitro MRI, with no adverse effect on cell viability, proliferation, pluripotency, and differentiation into cardiomyocytes. Transplanted hESCs displayed significant bright contrast on MRI 24 h after Mn administration, with contrast persisting for 5 days. Bright contrast was recalled at 4-6 weeks with early teratoma outgrowth.

CONCLUSIONS

The bright-ferritin platform provides the first demonstration of longitudinal cell tracking with signal recall, opening a window on the massive cell death that hESCs undergo in the weeks following transplantation before the surviving cell fraction proliferates to form teratomas.

Magnetic resonance imaging focused on the ferritin heavy chain 1 reporter gene detects neuronal differentiation in stem cells

The neuronal differentiation of mesenchymal stem cells offers a new strategy for the treatment of neurological disorders. Thus, there is a need to identify a noninvasive and sensitive in vivo imaging approach for real-time monitoring of transplanted stem cells. Our previous study confirmed that magnetic resonance imaging, with a focus on the ferritin heavy chain 1 reporter gene, could track the proliferation and differentiation of bone marrow mesenchymal stem cells that had been transduced with lentivirus carrying the ferritin heavy chain 1 reporter gene. However, we could not determine whether or when bone marrow mesenchymal stem cells had undergone neuronal differentiation based on changes in the magnetic resonance imaging signal. To solve this problem, we identified a neuron-specific enolase that can be differentially expressed before and after neuronal differentiation in stem cells. In this study, we successfully constructed a lentivirus carrying the neuron-specific enolase promoter and expressing the ferritin heavy chain 1 reporter gene; we used this lentivirus to transduce bone marrow mesenchymal stem cells. Cellular and animal studies showed that the neuron-specific enolase promoter effectively drove the expression of ferritin heavy chain 1 after neuronal differentiation of bone marrow mesenchymal stem cells; this led to intracellular accumulation of iron and corresponding changes in the magnetic resonance imaging signal. In summary, we established an innovative magnetic resonance imaging approach focused on the induction of reporter gene expression by a neuron-specific promoter. This imaging method can be used to noninvasively and sensitively detect neuronal differentiation in stem cells, which may be useful in stem cell-based therapies.

Human integrin α10β1-selected mesenchymal stem cells home to cartilage defects in the rabbit knee and assume a chondrocyte-like phenotype

BACKGROUND

Mesenchymal stem cells (MSCs) have shown promising results in stimulating cartilage repair and in the treatment of osteoarthritis (OA). However, the fate of the MSCs after intra-articular injection and their role in cartilage regeneration is not clear. To address these questions, this study investigated (1) homing of labeled human adipose tissue derived integrin α10β1-selected MSCs (integrin α10-MSCs) to a cartilage defect in a rabbit model and (2) the ability of the integrin α10-MSCs to differentiate to chondrocytes and to produce cartilage matrix molecules in vivo.

DESIGN

Integrin α10-MSCs were labeled with superparamagnetic iron oxide nanoparticles (SPIONs) co-conjugated with Rhodamine B to allow visualization by both MRI and fluorescence microscopy. A cartilage defect was created in the articular cartilage of the intertrochlear groove of the femur of rabbits. Seven days post-surgery, labeled integrin α10-MSCs or vehicle were injected into the joint. Migration and distribution of the SPION-labeled integrin α10-MSCs was evaluated by high-field 9.4 T MRI up to 10 days after injection. Tissue sections from the repair tissue in the defects were examined by fluorescence microscopy.

RESULTS

In vitro characterization of the labeled integrin α10-MSCs demonstrated maintained viability, proliferation rate and trilineage differentiation capacity compared to unlabeled MSCs. In vivo MRI analysis detected the labeled integrin α10-MSCs in the cartilage defects at all time points from 12 h after injection until day 10 with a peak concentration between day 1 and 4 after injection. The labeled MSCs were also detected lining the synovial membrane at the early time points. Fluorescence analysis confirmed the presence of the labeled integrin α10-MSCs in all layers of the cartilage repair tissue and showed co-localization between the labeled cells and the specific cartilage molecules aggrecan and collagen type II indicating in vivo differentiation of the MSCs to chondrocyte-like cells. No adverse effects of the α10-MSC treatment were detected during the study period.

CONCLUSION

Our results demonstrated migration and homing of human integrin α10β1-selected MSCs to cartilage defects in the rabbit knee after intra-articular administration as well as chondrogenic differentiation of the MSCs in the regenerated cartilage tissue.

Options for imaging cellular therapeutics in vivo: a multi-stakeholder perspective

Cell-based therapies have been making great advances toward clinical reality. Despite the increase in trial activity, few therapies have successfully navigated late-phase clinical trials and received market authorization. One possible explanation for this is that additional tools and technologies to enable their development have only recently become available. To support the safety evaluation of cell therapies, the Health and Environmental Sciences Institute Cell Therapy-Tracking, Circulation and Safety Committee, a multisector collaborative committee, polled the attendees of the 2017 International Society for Cell & Gene Therapy conference in London, UK, to understand the gaps and needs that cell therapy developers have encountered regarding safety evaluations in vivo. The goal of the survey was to collect information to inform stakeholders of areas of interest that can help ensure the safe use of cellular therapeutics in the clinic. This review is a response to the cellular imaging interests of those respondents. The authors offer a brief overview of available technologies and then highlight the areas of interest from the survey by describing how imaging technologies can meet those needs. The areas of interest include imaging of cells over time, sensitivity of imaging modalities, ability to quantify cells, imaging cellular survival and differentiation and safety concerns around adding imaging agents to cellular therapy protocols. The Health and Environmental Sciences Institute Cell Therapy-Tracking, Circulation and Safety Committee believes that the ability to understand therapeutic cell fate is vital for determining and understanding cell therapy efficacy and safety and offers this review to aid in those needs. An aim of this article is to share the available imaging technologies with the cell therapy community to demonstrate how these technologies can accomplish unmet needs throughout the translational process and strengthen the understanding of cellular therapeutics.

3D in vivo Magnetic Particle Imaging of Human Stem Cell-Derived Islet Organoid Transplantation Using a Machine Learning Algorithm

Stem cell-derived islet organoids constitute a promising treatment of type 1 diabetes. A major hurdle in the field is the lack of appropriate in vivo method to determine graft outcome. Here, we investigate the feasibility of in vivo tracking of transplanted stem cell-derived islet organoids using magnetic particle imaging (MPI) in a mouse model. Human induced pluripotent stem cells-L1 were differentiated to islet organoids and labeled with superparamagnetic iron oxide nanoparticles. The phantoms comprising of different numbers of labeled islet organoids were imaged using an MPI system. Labeled islet organoids were transplanted into NOD/scid mice under the left kidney capsule and were then scanned using 3D MPI at 1, 7, and 28 days post transplantation. Quantitative assessment of the islet organoids was performed using the K-means++ algorithm analysis of 3D MPI. The left kidney was collected and processed for immunofluorescence staining of C-peptide and dextran. Islet organoids expressed islet cell markers including insulin and glucagon. Image analysis of labeled islet organoids phantoms revealed a direct linear correlation between the iron content and the number of islet organoids. The K-means++ algorithm showed that during the course of the study the signal from labeled islet organoids under the left kidney capsule decreased. Immunofluorescence staining of the kidney sections showed the presence of islet organoid grafts as confirmed by double staining for dextran and C-peptide. This study demonstrates that MPI with machine learning algorithm analysis can monitor islet organoids grafts labeled with super-paramagnetic iron oxide nanoparticles and provide quantitative information of their presence in vivo.

In vivo Cell Tracking Using Non-invasive Imaging of Iron Oxide-Based Particles with Particular Relevance for Stem Cell-Based Treatments of Neurological and Cardiac Disease

Stem cell-based therapeutics is a rapidly developing field associated with a number of clinical challenges. One such challenge lies in the implementation of methods to track stem cells and stem cell-derived cells in experimental animal models and in the living patient. Here, we provide an overview of cell tracking in the context of cardiac and neurological disease, focusing on the use of iron oxide-based particles (IOPs) visualized in vivo using magnetic resonance imaging (MRI). We discuss the types of IOPs available for such tracking, their advantages and limitations, approaches for labeling cells with IOPs, biological interactions and effects of IOPs at the molecular and cellular levels, and MRI-based and associated approaches for in vivo and histological visualization. We conclude with reviews of the literature on IOP-based cell tracking in cardiac and neurological disease, covering both preclinical and clinical studies.

Optimizing Stem Cell Therapy after Ischemic Brain Injury

Stem cells have been used for regenerative and therapeutic purposes in a variety of diseases. In ischemic brain injury, preclinical studies have been promising, but have failed to translate results to clinical trials. We aimed to explore the application of stem cells after ischemic brain injury by focusing on topics such as delivery routes, regeneration efficacy, adverse effects, and in vivo potential optimization. PUBMED and Web of Science were searched for the latest studies examining stem cell therapy applications in ischemic brain injury, particularly after stroke or cardiac arrest, with a focus on studies addressing delivery optimization, stem cell type comparison, or translational aspects. Other studies providing further understanding or potential contributions to ischemic brain injury treatment were also included. Multiple stem cell types have been investigated in ischemic brain injury treatment, with a strong literature base in the treatment of stroke. Studies have suggested that stem cell administration after ischemic brain injury exerts paracrine effects via growth factor release, blood-brain barrier integrity protection, and allows for exosome release for ischemic injury mitigation. To date, limited studies have investigated these therapeutic mechanisms in the setting of cardiac arrest or therapeutic hypothermia. Several delivery modalities are available, each with limitations regarding invasiveness and safety outcomes. Intranasal delivery presents a potentially improved mechanism, and hypoxic conditioning offers a potential stem cell therapy optimization strategy for ischemic brain injury. The use of stem cells to treat ischemic brain injury in clinical trials is in its early phase; however, increasing preclinical evidence suggests that stem cells can contribute to the down-regulation of inflammatory phenotypes and regeneration following injury. The safety and the tolerability profile of stem cells have been confirmed, and their potent therapeutic effects make them powerful therapeutic agents for ischemic brain injury patients.

Stem cell homing, tracking and therapeutic efficiency evaluation for stroke treatment using nanoparticles: A systematic review

BACKGROUND

Stroke is the second leading cause of death worldwide. There is a real need to develop treatment strategies for reducing neurological deficits in stroke survivors, and stem cell (SC) therapeutics appear to be a promising alternative for stroke therapy that can be used in combination with approved thrombolytic or thrombectomy approaches. However, the efficacy of SC therapy depends on the SC homing ability and engraftment into the injury site over a long period of time. Nonetheless, tracking SCs from their niche to the target tissues is a complex process.

AIM

To evaluate SC migration homing, tracking and therapeutic efficacy in the treatment of stroke using nanoparticles

METHODS

A systematic literature search was performed to identify articles published prior to November 2019 that were indexed in PubMed and Scopus. The following inclusion criteria were used: (1) Studies that used in vivo models of stroke or ischemic brain lesions; (2) Studies of SCs labeled with some type of contrast agent for cell migration detection; and (3) Studies that involved in vivo cellular homing and tracking analysis.

RESULTS

A total of 82 articles were identified by indexing in Scopus and PubMed. After the inclusion criteria were applied, 35 studies were selected, and the articles were assessed for eligibility; ultimately, only 25 studies were included. Most of the selected studies used SCs from human and mouse bone marrow labeled with magnetic nanoparticles alone or combined with fluorophore dyes. These cells were administered in the stroke model (to treat middle cerebral artery occlusion in 74% of studies and for photothrombotic induction in 26% of studies). Fifty-three percent of studies used xenogeneic grafts for cell therapy, and the migration homing and tracking evaluation was performed by magnetic resonance imaging as well as other techniques, such as near-infrared fluorescence imaging (12%) or bioluminescence assays (12%).

CONCLUSION

Our systematic review provided an up-to-date evaluation of SC migration homing and the efficacy of cellular therapy for stroke treatment in terms of functional and structural improvements in the late stage.

Improved therapeutics of modified mesenchymal stem cells: an update

Background

Mesenchymal stromal cells (MSCs) have attracted intense interest due to their powerful intrinsic properties of self-regeneration, immunomodulation and multi-potency, as well as being readily available and easy to isolate and culture. Notwithstanding, MSC based therapy suffers reduced efficacy due to several challenges which include unfavorable microenvironmental factors in vitro and in vivo.

Body

In the quest to circumvent these challenges, several modification techniques have been applied to the naïve MSC to improve its inherent therapeutic properties. These modification approaches can be broadly divided into two groups to include genetic modification and preconditioning modification (using drugs, growth factors and other molecules). This field has witnessed great progress and continues to gather interest and novelty. We review these innovative approaches in not only maintaining, but also enhancing the inherent biological activities and therapeutics of MSCs with respect to migration, homing to target site, adhesion, survival and reduced premature senescence. We discuss the application of the improved modified MSC in some selected human diseases. Possible ways of yet better enhancing the therapeutic outcome and overcoming challenges of MSC modification in the future are also elaborated.

Conclusion

The importance of prosurvival and promigratory abilities of MSCs in their therapeutic applications can never be overemphasized. These abilities are maintained and even further enhanced via MSC modifications against the inhospitable microenvironment during culture and transplantation. This is a turning point in MSC-based therapy with promising preclinical studies and higher future prospect.

Translational challenges in advancing regenerative therapy for treating neurological disorders using nanotechnology

The focus of regenerative therapies is to replace or enrich diseased or injured cells and tissue in an attempt to replenish the local environment and function, while slowing or halting further degeneration. Targeting neurological diseases specifically is difficult, due to the complex nature of the central nervous system, including the difficulty of bypassing the brain's natural defense systems. While cell-based regenerative therapies show promise in select tissues, preclinical and clinical studies have been largely unable to transfer these successes to the brain. Advancements in nanotechnologies have provided new methods of central nervous system access, drug and cell delivery, as well as new systems of cell maintenance and support that may bridge the gap between regenerative therapies and the brain. In this review, we discuss current regenerative therapies for neurological diseases, nanotechnology as nanocarriers, and the technical, manufacturing, and regulatory challenges that arise from inception to formulation of nanoparticle-regenerative therapies.

Labeling of human mesenchymal stem cells with different classes of vital stains: robustness and toxicity

Background

Mesenchymal stem cell (MSC) transplantation has been explored as a new clinical approach to repair injured tissues. However, in order to evaluate the therapeutic activity of MSC, cell tracking techniques are required to determine the fate of transplanted cells in both preclinical and clinical studies. In these aspects, different vital stains offer the potential for labeling and monitoring of grafted cells in vivo. It is desirable to have tracking agents which have long-term stability, are not toxic to the cells, and do not affect cell function.

Methods

Here, we selected three different labels: CellTracker™ Green CMFDA, eGFP-mRNA (genetic pre-tag), and Molday ION Rhodamine B™ (nanoparticle-based fluorescent and magnetic label) and performed extensive analysis of their influence on in vitro expansion of human bone marrow-derived mesenchymal stem cells (hBM-MSCs), as well as potential of affecting therapeutic activity and the impact on the durability of staining.

Results

Our study showed that basic hBM-MSC characteristics and functions might be affected by labeling. We observed strong alterations of metabolic activity and morphology after eGFP and CellTracker™ Green CMFDA hBM-MSC staining. Molday ION Rhodamine B™ labeling revealed superior properties relatively to other vital stains. The relative expression level of most of the investigated growth factors remained stable after cell labeling, but we have observed some changes in the case of EGF, GDNF, HGF, and IGF gene expression.

Conclusions

Taken together, we suggest performing similar to ours extensive analysis prior to using any cell label to tag MSC in experiments, as it can thoroughly bias results.

Electronic supplementary material

The online version of this article (10.1186/s13287-019-1296-8) contains supplementary material, which is available to authorized users.

Biodistribution of Glial Progenitors in a Three Dimensional-Printed Model of the Piglet Cerebral Ventricular System

White matter damage persists in hypoxic-ischemic newborns even when treated with hypothermia. We have previously shown that intraventricular delivery of human glial progenitors (GPs) at the neonatal stage is capable of replacing abnormal host glia and rescuing the lifespan of dysmyelinated mice. However, such transplantation in the human brain poses significant challenges as related to high-volume ventricles and long cell migration distances. These challenges can only be studied in large animal model systems. In this study, we developed a three dimensional (3D)-printed model of the ventricular system sized to a newborn pig to investigate the parameters that can maximize a global biodistribution of injected GPs within the ventricular system, while minimizing outflow to the subarachnoid space. Bioluminescent imaging and magnetic resonance imaging were used to image the biodistribution of luciferase-transduced GPs in simple fluid containers and a custom-designed, 3D-printed model of the piglet ventricular system. Seven independent variables were investigated. The results demonstrated that a low volume (0.1 mL) of cell suspension is essential to keep cells within the ventricular system. If higher volumes (1 mL) are needed, a very slow infusion speed (0.01 mL/min) is necessary. Real-time magnetic resonance imaging demonstrated that superparamagnetic iron oxide (SPIO) labeling significantly alters the rheological properties of the GP suspension, such that, even at high speeds and high volumes, the outflow to the subarachnoid space is reduced. Several other factors, including GP species (human vs. mouse), type of catheter tip (end hole vs. side hole), catheter length (0.3 vs. 7.62 m), and cell concentration, had less effect on the overall distribution of GPs. We conclude that the use of a 3D-printed phantom model represents a robust, reproducible, and cost-saving alternative to in vivo large animal studies for determining optimal injection parameters.

Placenta-Derived Cells for Acute Brain Injury

Acute brain injury resulting from ischemic/hemorrhagic or traumatic damage is one of the leading causes of mortality and disability worldwide and is a significant burden to society. Neuroprotective options to counteract brain damage are very limited in stroke and traumatic brain injury (TBI). Given the multifaceted nature of acute brain injury and damage progression, several therapeutic targets may need to be addressed simultaneously to interfere with the evolution of the injury and improve the patient’s outcome. Stem cells are ideal candidates since they act on various mechanisms of protection and repair, improving structural and functional outcomes after experimental stroke or TBI. Stem cells isolated from placenta offer advantages due to their early embryonic origin, ease of procurement, and ethical acceptance. We analyzed the evidence for the beneficial effects of placenta-derived stem cells in acute brain injury, with the focus on experimental studies of TBI and stroke, the engineering strategies pursued to foster cell potential, and characterization of the bioactive molecules secreted by placental cells, known as their secretome, as an alternative cell-free strategy. Results from the clinical application of placenta-derived stem cells for acute brain injury and ongoing clinical trials are summarily discussed.

Promoting Brain Repair and Regeneration After Stroke: a Plea for Cell-Based Therapies

PURPOSE OF REVIEW

After decades of hype, cell-based therapies are emerging into the clinical arena for the purposes of promoting recovery after stroke. In this review, we discuss the most recent science behind the role of cell-based therapies in ischemic stroke and the efforts to translate these therapies into human clinical trials.

RECENT FINDINGS

Preclinical data support numerous beneficial effects of cell-based therapies in both small and large animal models of ischemic stroke. These benefits are driven by multifaceted mechanisms promoting brain repair through immunomodulation, trophic support, circuit reorganization, and cell replacement. Cell-based therapies offer tremendous potential for improving outcomes after stroke through multimodal support of brain repair. Based on recent clinical trials, cell-based therapies appear both feasible and safe in all phases of stroke. Ongoing translational research and clinical trials will further refine these therapies and have the potential to transform the approach to stroke recovery and rehabilitation.

Chemobrain as a Product of Growing Success in Chemotherapy - Focus on Glia as both a Victim and a Cure

Chemotherapy-induced cognitive impairment or chemobrain is a frequent consequence of cancer treatment with many psychiatric features. Ironically, the increasing efficacy of chemotherapy leaves growing number of patients alive with chemobrain. Therefore, there is an urgent need for strategies capable of returning cancer survivors back to their pre-morbid quality of life. Molecular mechanisms of chemobrain are largely unknown. Over the last decade there was a lot of emphasis in preclinical research on inflammatory consequences of chemotherapy and oxidative stress but so far none of these approaches were translated into clinical scenario. The co-administration of chemotherapy with protective agents was evaluated preclinically but it should be introduced with caution as potential interference was not yet studied and that could blunt therapeutic efficacy. Stem cell-based regenerative medicine approach has so far been exploited very sparsely in the context of chemobrain and the focus was on indirect mechanisms or neuronal replacement in the hippocampus. However, there is evidence for widespread white matter abnormalities in patients with chemobrain. This is quite logical considering life-long proliferation and turnover of glial cells, which makes them vulnerable to chemotherapeutic agents. Feasibility of glia replacement has been established in mice with global dysmyelination where profound therapeutic effect has been observed but only in case of global cell engraftment (across the entire brain). While global glia replacement has been achieved in mice translation to clinical setting might be challenging due to much larger brain size. Therefore, a lot of attention should be directed towards the route of administration to accomplish widespread cell delivery. Techniques facilitating that broad cell distribution including intra-arterial and intrathecal methods should be considered as very compelling options. Summarizing, chemobrain is a rapidly growing medical problem and global glia replacement should be considered as worthwhile therapeutic strategy.

Clinical Tracking of Cell Transfer and Cell Transplantation: Trials and Tribulations

Cell therapy has provided unprecedented opportunities for tissue repair and cancer therapy. Imaging tools for in vivo tracking of therapeutic cells have entered the clinic to evaluate therapeutic cell delivery and retention in patients. Thus far, clinical cell tracking studies have been a mere proof of principle of the feasibility of cell detection. This review centers around the main clinical queries associated with cell therapy: Have cells been delivered correctly at the targeted site of injection? Are cells still alive, and, if so, how many? Are cells being rejected by the host, and, if so, how severe is the immune response? For stem cell therapeutics, have cells differentiated into downstream cell lineages? Is there cell proliferation including tumor formation? At present, clinical cell tracking trials have only provided information on immediate cell delivery and short-term cell retention. The next big question is if these cell tracking tools can improve the clinical management of the patients and, if so, by how much, for how many, and for whom; in addition, it must be determined whether tracking therapeutic cells in every patient is needed. To become clinically relevant, it must now be demonstrated how cell tracking techniques can inform patient treatment and affect clinical outcomes.

Hydrogel-based scaffolds to support intrathecal stem cell transplantation as a gateway to the spinal cord: clinical needs, biomaterials, and imaging technologies

The prospects for cell replacement in spinal cord diseases are impeded by inefficient stem cell delivery. The deep location of the spinal cord and complex surgical access, as well as densely packed vital structures, question the feasibility of the widespread use of multiple spinal cord punctures to inject stem cells. Disorders characterized by disseminated pathology are particularly appealing for the distribution of cells globally throughout the spinal cord in a minimally invasive fashion. The intrathecal space, with access to a relatively large surface area along the spinal cord, is an attractive route for global stem cell delivery, and, indeed, is highly promising, but the success of this approach relies on the ability of cells (1) to survive in the cerebrospinal fluid (CSF), (2) to adhere to the spinal cord surface, and (3) to migrate, ultimately, into the parenchyma. Intrathecal infusion of cell suspension, however, has been insufficient and we postulate that embedding transplanted cells within hydrogel scaffolds will facilitate reaching these goals. In this review, we focus on practical considerations that render the intrathecal approach clinically viable, and then discuss the characteristics of various biomaterials that are suitable to serve as scaffolds. We also propose strategies to modulate the local microenvironment with nanoparticle carriers to improve the functionality of cellular grafts. Finally, we provide an overview of imaging modalities for in vivo monitoring and characterization of biomaterials and stem cells. This comprehensive review should serve as a guide for those planning preclinical and clinical studies on intrathecal stem cell transplantation.

Imaging of extracellular vesicles derived from human bone marrow mesenchymal stem cells using fluorescent and magnetic labels

Background

Mesenchymal stem cells have been shown therapeutic in various neurological disorders. Recent studies support the notion that the predominant mechanism by which MSCs act is through the release of extracellular vesicles (EVs). EVs seem to have similar therapeutic activity as their cellular counterparts and may represent an interesting alternative standalone therapy for various diseases. The aim of the study was to optimize the method of EV imaging to better understand therapeutic effects mediated by EVs.

Methods

The fluorescent lipophilic stain PKH26 and superparamagnetic iron oxide nanoparticles conjugated with rhodamine (Molday ION Rhodamine B™) were used for the labeling of vesicles in human bone marrow MSCs (hBM-MSCs). The entire cycle from intracellular vesicles to EVs followed by their uptake by hBM-MSCs has been studied. The identity of vesicles has been proven by antibodies against: anti-CD9, -CD63, and -CD81 (tetraspanins). NanoSight particle tracking analysis (NTA), high-resolution flow cytometric analysis, transmission electron microscopy (TEM), ELYRA PS.1 super-resolution microscopy, and magnetic resonance imaging (MRI) were used for the characterization of vesicles.

Results

The PKH26 and Molday ION were exclusively localized in intracellular vesicles positively stained for EV markers: CD9, CD63, and CD81. The isolated EVs represent heterogeneous population of various sizes as confirmed by NTA. The TEM and MRI were capable to show successful labeling of EVs using ION. Co-culture of EVs with hBM-MSCs revealed their uptake by cells in vitro, as visualized by the co-localization of PKH26 or Molday ION with tetraspanins inside hBM-MSCs.

Conclusion

PKH26 and Molday ION seem to be biocompatible with EVs, and the labeling did not interfere with the capability of EVs to re-enter hBM-MSCs during co-culture in vitro. Magnetic properties of IONs provide an additional advantage for the imaging of EV using TEM and MRI.

Stem Cell Tracing Through MR Molecular Imaging

Stem cell therapy opens a new window in medicine to overcome several diseases that remain incurable. It appears such diseases as cardiovascular disorders, brain injury, multiple sclerosis, urinary system diseases, cartilage lesions and diabetes are curable with stem cell transplantation. However, some questions related to stem cell therapy have remained unanswered. Stem cell imaging allows approval of appropriated strategies such as selection of the type and dose of stem cell, and also mode of cell delivery before being tested in clinical trials. MRI as a non-invasive imaging modality provides proper conditions for this aim. So far, different contrast agents such as superparamagnetic or paramagnetic nanoparticles, ultrasmall superparamagnetic nanoparticles, fluorine, gadolinium and some types of reporter genes have been used for imaging of stem cells. The core subject of these studies is to investigate the survival and differentiation of stem cells, contrast agent's toxicity and long term following of transplanted cells. The promising results of in vivo and some clinical trial studies may raise hope for clinical stem cells imaging with MRI.

Stroke promotes survival of nearby transplanted neural stem cells by decreasing their activation of caspase 3 while not affecting their differentiation

Although transplantation of stem cells improves recovery of the nervous tissue, little is known about the influence of different brain regions on transplanted cells. After we confirmed that cells with uniform differentiation potential can be generated in independent experiments, one million of neural stem cells isolated from B6.Cg-Tg(Thy1-YFP)16Jrs/J mouse embryos were transplanted into the brain 24 h after induction of stroke. The lateral ventricles, the corpus callosum and the striatum were tested. Two and four weeks after the transplantation, the cells transplanted in all three regions have been attracted to the ischemic core. The largest number of attracted cells has been observed after transplantation into the striatum. Their differentiation pattern and expression of neuroligin 1, SynCAM 1, postsynaptic density protein 95 and synapsin 1 followed the same pattern observed during in vitro cultivation and it did not differ among the tested regions. Differentiation pattern of the cells transplanted in the stroke-affected and healthy animals was the same. On the other hand, neural stem cells transplanted in the striatum of the animals affected by stroke exhibited significantly increased survival rates reaching 260 ± 19%, when compared to cells transplanted in their wild type controls. Surprisingly, improved survival two and four weeks after transplantation was not due to increased proliferation of the grafted cells and it was accompanied by decreased levels of activity of Casp3 (19.56 ± 3.1% in the stroke-affected vs. 30.14 ± 2.4% in healthy animals after four weeks). We assume that the decreased levels of Casp3 in cells transplanted near the ischemic region was linked to increased vasculogenesis, synaptogenesis, astrocytosis and axonogenesis detected in the host tissue affected by ischemia.

Migratory potential of transplanted glial progenitors as critical factor for successful translation of glia replacement therapy: The gap between mice and men

Neurological disorders are a major threat to public health. Stem cell-based regenerative medicine is now a promising experimental paradigm for its treatment, as shown in pre-clinical animal studies. Initial attempts have been on the replacement of neuronal cells only, but glial progenitors (GPs) are now becoming strong alternative cellular therapeutic candidates to replace oligodendrocytes and astrocytes as knowledge accumulates about their important emerging role in various disease processes. There are many examples of successful therapeutic outcomes for transplanted GPs in small animal models, but clinical translation has proved to be challenging due to the 1,000-fold larger volume of the human brain compared to mice. Human GPs transplanted into the mouse brain migrate extensively and can induce global cell replacement, but a similar extent of migration in the human brain would only allow for local rather than global cell replacement. We review here the mechanisms that govern cell migration, which could potentially be exploited to enhance the migratory properties of GPs through cell engineering pre-transplantation. We furthermore discuss the (dis)advantages of the various cell delivery routes that are available, with particular emphasis on intra-arterial injection as the most suitable route for achieving global cell distribution in the larger brain. Now that therapeutic success has proven to be feasible in small animal models, future efforts will need to be directed to enhance global cell delivery and migration to make bench-to-bedside translation a reality.

Peripheral blood detection of systemic graft-specific xeno-antibodies following transplantation of human neural progenitor cells into the porcine spinal cord

Extensive pre-clinical and clinical studies have searched for therapeutic efficacy of cell-based therapeutics in diseases of the Central Nervous System (CNS) with no other viable options. Allogeneic cells represent the primary source of these therapies and immunosuppressive regimens have been empirically employed based on experience with solid organ transplantation, attempting to avoid immune mediated graft rejection. In this study, we aimed to 1) characterize the host immune response to stem cells transplanted into the CNS and 2) develop a non-invasive method for detecting immune response to transplanted cell grafts. Human neural progenitor cells were transplanted into the spinal cord of 10 Göttingen minipigs, of which 5 received no immunosuppression and 5 received Tacrolimus. Peripheral blood samples were collected longitudinally for flow cytometry cross match studies. Necropsy was performed at day 21 and spinal cord tissue analysis. We observed a transient increase in xeno-reactive antibodies was detected on post-operative day 7 and 14 in pigs that did not receive immunosuppression. This response was not detected in pigs that received Tacrolimus immunosuppression. No difference in graft survival was observed between the groups. Infiltration of numerous immune mediators including granulocytes, T lymphocytes, and activated microglia, and complement deposition were detected. In summary, a systemic immunologic response to stem cell grafts was detected for two weeks after transplantation using peripheral blood. This could be used as a non-invasive biomarker by investigators for detection of immunologic rejection. However, the absence of a detectable response in peripheral blood does not rule out a parenchymal immune response.

The long-term fate of mesenchymal stem cells labeled with magnetic resonance imaging-visible polymersomes in cerebral ischemia

Understanding the long-term fate and potential mechanisms of mesenchymal stem cells (MSCs) after transplantation is essential for improving functional benefits of stem cell-based stroke treatment. Magnetic resonance imaging (MRI) is considered an attractive and clinically translatable tool for longitudinal tracking of stem cells, but certain controversies have arisen in this regard. In this study, we used SPION-loaded cationic polymersomes to label green fluorescent protein (GFP)-expressing MSCs to determine whether MRI can accurately reflect survival, long-term fate, and potential mechanisms of MSCs in ischemic stroke therapy. Our results showed that MSCs could improve the functional outcome and reduce the infarct volume of stroke in the brain. In vivo MRI can verify the biodistribution and migration of grafted cells when pre-labeled with SPION-loaded polymersome. The dynamic change of low signal volume on MRI can reflect the tendency of cell survival and apoptosis, but may overestimate long-term survival owing to the presence of iron-laden macrophages around cell graft. Only a small fraction of grafted cells survived up to 8 weeks after transplantation. A minority of these surviving cells were differentiated into astrocytes, but not into neurons. MSCs might exert their therapeutic effect via secreting paracrine factors rather than directing cell replacement through differentiation into neuronal and/or glial phenotypes.

Transplanted adipose-derived stem cells can be short-lived yet accelerate healing of acid-burn skin wounds: a multimodal imaging study

The incidence of accidental and intentional acid skin burns is rising. Current treatment strategies are mostly inadequate, leaving victims disfigured and without treatment options. Here, we have shown that transplantation of adipose-derived stem cells (ASCs) accelerates the process of acid burn wound-healing. Pre-conditioning of ASCs using ascorbic acid (AA) or hypoxic conditions provided additional benefit. While the wounds were ultimately healed in all mice, histological analysis revealed that, in non-transplanted animals, the number of hair follicles was reduced. Bioluminescent imaging (BLI) of transplanted ASCs revealed a gradual loss of transplanted cells, with a similar rate of cell death for each treatment group. The signal of fluorinated cells detected by a clinically applicable ¹⁹F MRI method correlated with the BLI findings, which points to ¹⁹F MRI as a reliable method with which to track ASCs after transplantation to skin wounds. No difference in therapeutic effect or cell survival was observed between labeled and non-labeled cells. We conclude that, despite being short-lived, transplanted ASCs can accelerate wound-healing and reduce hair loss in acid-burn skin injury. The fluorine nanoemulsion is a clinically applicable cell label capable of reporting on the survival of transplanted cells.

Superparamagnetic Iron Oxide Nanoparticles-Complexed Cationic Amylose for In Vivo Magnetic Resonance Imaging Tracking of Transplanted Stem Cells in Stroke

Cell-based therapy with mesenchymal stem cells (MSCs) is a promising strategy for acute ischemic stroke. In vivo tracking of therapeutic stem cells with magnetic resonance imaging (MRI) is imperative for better understanding cellular survival and migrational dynamics over time. In this study, we develop a novel biocompatible nanocomplex (ASP-SPIONs) based on cationic amylose, by introducing spermine and the image label, ultrasmall superparamagnetic iron oxide nanoparticles (SPIONs), to label MSCs. The capacity, efficiency, and cytotoxicity of the nanocomplex in transferring SPIONs into green fluorescence protein-modified MSCs were tested; and the performance of in vivo MRI tracking of the transplanted cells in acute ischemic stroke was determined. The results demonstrated that the new class of SPIONs-complexed nanoparticles based on biodegradable amylose can serve as a highly effective and safe carrier to transfer magnetic label into stem cells. A reliable tracking of transplanted stem cells in stroke was achieved by MRI up to 6 weeks, with the desirable therapeutic benefit of stem cells on stroke retained. With the advantages of a relatively low SPIONs concentration and a short labeling period, the biocompatible complex of cationic amylose with SPIONs is highly translatable for clinical application. It holds great promise in efficient, rapid, and safe labeling of stem cells for subsequent cellular MRI tracking in regenerative medicine.

Magnetic Resonance Imaging of Ferumoxytol-Labeled Human Mesenchymal Stem Cells in the Mouse Brain

The success of stem cell therapy is highly dependent on accurate delivery of stem cells to the target site of interest. Possible ways to track the distribution of MSCs in vivo include the use of reporter genes or nanoparticles. The U.S. Food and Drug Administration (FDA) has approved ferumoxytol (Feraheme® [USA], Rienso® [UK]) as a treatment for iron deficiency anemia. Ferumoxytol is an ultrasmall superparamagnetic iron oxide nanoparticle (USPIO) that has recently been used to track the fate of transplanted cells using magnetic resonance imaging (MRI). The major objectives of this study were to demonstrate the feasibility of labeling hUCB-MSCs with ferumoxytol and to observe, through MRI, the engraftment of ferumoxytol-labeled human umbilical cord blood-derived mesenchymal stem cells (hUCB-MSCs) delivered via stereotactic injection into the hippocampi of a transgenic mouse model of familial Alzheimer's disease (5XFAD). Ferumoxytol had no toxic effects on the viability or stemness of hUCB-MSCs when assessed in vitro. Through MRI, hypointense signals were discernible at the site where ferumoxytol-labeled human MSCs were injected. Iron-positive areas were also observed in the engrafted hippocampi. The results from this study support the use of nanoparticle labeling to monitor transplanted MSCs in real time as a follow-up for AD stem cell therapy in the clinical field.

Cell therapy for spinal cord injury informed by electromagnetic waves

Spinal cord injury devastates the CNS, besetting patients with symptoms including but not limited to: paralysis, autonomic nervous dysfunction, pain disorders and depression. Despite the identification of several molecular and genetic factors, a reliable regenerative therapy has yet to be produced for this terminal disease. Perhaps the missing piece of this puzzle will be discovered within endogenous electrotactic cellular behaviors. Neurons and stem cells both show mediated responses (growth rate, migration, differentiation) to electromagnetic waves, including direct current electric fields. This review analyzes the pathophysiology of spinal cord injury, the rationale for regenerative cell therapy and the evidence for directing cell therapy via electromagnetic waves shown by in vitro experiments.

Cellular Imaging With MRI

Cellular magnetic resonance imaging (MRI) is an evolving field of imaging with strong translational and research potential. The ability to detect, track, and quantify cells in vivo and over time allows for studying cellular events related to disease processes and may be used as a biomarker for decisions about treatments and for monitoring responses to treatments. In this review, we discuss methods for labeling cells, various applications for cellular MRI, the existing limitations, strategies to address these shortcomings, and clinical cellular MRI.

Quantifications of in vivo labeled stem cells based on measurements of magnetic moments

Cells labeled by super paramagnetic iron-oxide (SPIO) nanoparticles are more easily seen in gradient echo MR images, but it has not been shown that the amount of nanoparticles or the number of cells can be directly quantified from MR images. This work utilizes a previously developed and improved Complex Image Summation around a Spherical or Cylindrical Object (CISSCO) method to quantify the magnetic moments of several clusters of SPIO nanoparticle labeled cells from archived rat brain images. With the knowledge of mass magnetization of the cell labeling agent and cell iron uptake, the number of cells in each nanoparticle cluster can be determined. Using a high pass filter with a reasonable size has little effect on each measured magnetic moment from the CISSCO method. These procedures and quantitative results may help improve the efficacy of cell-based treatments in vivo.

Ferumoxytol Labeling of Human Neural Progenitor Cells for Diagnostic Cellular Tracking in the Porcine Spinal Cord with Magnetic Resonance Imaging

We report on the diagnostic capability of magnetic resonance imaging (MRI)‐based tracking of ferumoxytol‐labeled human neural progenitor cells (hNPCs) transplanted into the porcine spinal cord. hNPCs prelabeled with two doses of ferumoxytol nanoparticles (hNPC‐F^Low and hNPC‐F^High) were injected into the ventral horn of the spinal cord in healthy minipigs. Ferumoxytol‐labeled grafts were tracked in vivo up to 105 days after transplantation with MRI. Injection accuracy was assessed in vivo at day 14 and was predictive of “on” or “off” target cell graft location assessed by histology. No difference in long‐term cell survival, assessed by quantitative stereology, was observed among hNPC‐F^Low, hNPC‐F^High, or control grafts. Histological iron colocalized with MRI signal and engrafted human nuclei. Furthermore, the ferumoxytol‐labeled cells retained nanoparticles and function in vivo. This approach represents an important leap forward toward facilitating translation of cell‐tracking technologies to clinical trials by providing a method of assessing transplantation accuracy, delivered dose, and potentially cell survival. Stem Cells Translational Medicine2017;6:139–150

The Vegetative State and Stem Cells: Therapeutic Considerations

The vegetative state (VS), also known as “unresponsive wakefulness syndrome,” is considered one of the most devastating outcomes of acquired brain injury. While diagnosis of this condition is generally well-defined clinically, patients often appear to be awake despite an absence of behavioral signs of awareness, which to the family can be confusing, leading them to believe the loved one is aware of their surroundings. This inequality of agreement can be very distressing. Currently, no cure for the VS is available; as a result, patients may remain in this condition for the rest of their lives, which in some cases amount to decades. Recent advances in stem cell approaches for the treatment of other neurological conditions may now provide an opportunity to intervene in this syndrome. This mini review will address the development of VS, its diagnosis, affected cerebral structures, and the underlying basis of how stem cells can offer therapeutic promise that would take advantage of the often long-term features associated with this maladie to effect a repair of the severely damaged circuitry. In addition, current limitations of this treatment strategy, including a lack of animal models, few long-term clinical studies that might identify benefits of stem cell treatment, and the potential for development of tumors are considered.

Genetic Engineering of Mesenchymal Stem Cells to Induce Their Migration and Survival

Mesenchymal stem cells (MSCs) are very attractive for regenerative medicine due to their relatively easy derivation and broad range of differentiation capabilities, either naturally or induced through cell engineering. However, efficient methods of delivery to diseased tissues and the long-term survival of grafted cells still need improvement. Here, we review genetic engineering approaches designed to enhance the migratory capacities of MSCs, as well as extend their survival after transplantation by the modulation of prosurvival approaches, including prevention of senescence and apoptosis. We highlight some of the latest examples that explore these pivotal points, which have great relevance in cell-based therapies.

Magnetic resonance imaging and cell-based neurorestorative therapy after brain injury

Restorative cell-based therapies for experimental brain injury, such as stroke and traumatic brain injury, substantially improve functional outcome. We discuss and review state of the art magnetic resonance imaging methodologies and their applications related to cell-based treatment after brain injury. We focus on the potential of magnetic resonance imaging technique and its associated challenges to obtain useful new information related to cell migration, distribution, and quantitation, as well as vascular and neuronal remodeling in response to cell-based therapy after brain injury. The noninvasive nature of imaging might more readily help with translation of cell-based therapy from the laboratory to the clinic.

Stem Cell Tracking with Nanoparticles for Regenerative Medicine Purposes: An Overview

Accurate and noninvasive stem cell tracking is one of the most important needs in regenerative medicine to determine both stem cell destinations and final differentiation fates, thus allowing a more detailed picture of the mechanisms involved in these therapies. Given the great importance and advances in the field of nanotechnology for stem cell imaging, currently, several nanoparticles have become standardized products and have been undergoing fast commercialization. This review has been intended to summarize the current use of different engineered nanoparticles in stem cell tracking for regenerative medicine purposes, in particular by detailing their main features and exploring their biosafety aspects, the first step for clinical application. Moreover, this review has summarized the advantages and applications of stem cell tracking with nanoparticles in experimental and preclinical studies and investigated present limitations for their employment in the clinical setting.

Accelerating stem cell trials for Alzheimer's disease

At present, no effective cure or prophylaxis exists for Alzheimer's disease. Symptomatic treatments are modestly effective and offer only temporary benefit. Advances in induced pluripotent stem cell (iPSC) technology have the potential to enable development of so-called disease-in-a-dish personalised models to study disease mechanisms and reveal new therapeutic approaches, and large panels of iPSCs enable rapid screening of potential drug candidates. Different cell types can also be produced for therapeutic use. In 2015, the US Food and Drug Administration granted investigational new drug approval for the first phase 2A clinical trial of ischaemia-tolerant mesenchymal stem cells to treat Alzheimer's disease in the USA. Similar trials are either underway or being planned in Europe and Asia. Although safety and ethical concerns remain, we call for the acceleration of human stem cell-based translational research into the causes and potential treatments of Alzheimer's disease.

Pre- and postmortem imaging of transplanted cells

Therapeutic interventions based on the transplantation of stem and progenitor cells have garnered increasing interest. This interest is fueled by successful preclinical studies for indications in many diseases, including the cardiovascular, central nervous, and musculoskeletal system. Further progress in this field is contingent upon access to techniques that facilitate an unambiguous identification and characterization of grafted cells. Such methods are invaluable for optimization of cell delivery, improvement of cell survival, and assessment of the functional integration of grafted cells. Following is a focused overview of the currently available cell detection and tracking methodologies that covers the entire spectrum from pre- to postmortem cell identification.

Nanoparticles and clinically applicable cell tracking

In vivo cell tracking has emerged as a much sought after tool for design and monitoring of cell-based treatment strategies. Various techniques are available for pre-clinical animal studies, from which much has been learned and still can be learned. However, there is also a need for clinically translatable techniques. Central to in vivo cell imaging is labelling of cells with agents that can give rise to signals in vivo, that can be detected and measured non-invasively. The current imaging technology of choice for clinical translation is MRI in combination with labelling of cells with magnetic agents. The main challenge encountered during the cell labelling procedure is to efficiently incorporate the label into the cell, such that the labelled cells can be imaged at high sensitivity for prolonged periods of time, without the labelling process affecting the functionality of the cells. In this respect, nanoparticles offer attractive features since their structure and chemical properties can be modified to facilitate cellular incorporation and because they can carry a high payload of the relevant label into cells. While these technologies have already been applied in clinical trials and have increased the understanding of cell-based therapy mechanism, many challenges are still faced.

Viability and MR detectability of iron labeled mesenchymal stem cells used for endoscopic injection into the porcine urethral sphincter

Direct stem cell therapies for functionally impaired tissue require a sufficient number of cells in the target region and a method for verifying the fate of the cells in the subsequent time course. In vivo MRI of iron labeled mesenchymal stem cells has been suggested to comply with these requirements. The study was conducted to evaluate proliferation, migration, differentiation and adhesion effects as well as the obtained iron load of an iron labeling strategy for mesenchymal stem cells. After injection into the porcine urethral sphincter, the labeled cells were monitored for up to six months using MRI. Mesenchymal stem cells were labeled with ferucarbotran (60/100/200 µg/mL) and ferumoxide (200 µg/mL) for the analysis of migration and viability. Phantom MR measurements were made to evaluate effects of iron labeling. For short and long term studies, the iron labeled cells were injected into the porcine urethral sphincter and monitored by MRI. High resolution anatomical images of the porcine urethral sphincter were applied for detection of the iron particles with a turbo-spin-echo sequence and a gradient-echo sequence with multiple TE values. The MR images were then compared with histological staining. The analysis of cell function after iron labeling showed no effects on proliferation or differentiation of the cells. Although the adherence increases with higher iron dose, the ability to migrate decreases as a presumed effect of iron labeling. The iron labeled mesenchymal stem cells were detectable in vivo in MRI and histological staining even six months after injection. Labeling with iron particles and subsequent evaluation with highly resolved three dimensional data acquisition allows sensitive tracking of cells injected into the porcine urethral sphincter for several months without substantial biological effects on mesenchymal stem cells.

Use of Ferritin Expression, Regulated by Neural Cell-Specific Promoters in Human Adipose Tissue-Derived Mesenchymal Stem Cells, to Monitor Differentiation with Magnetic Resonance Imaging In Vitro

The purpose of this study was to establish a method for monitoring the neural differentiation of stem cells using ferritin transgene expression, under the control of a neural-differentiation-inducible promoter, and magnetic resonance imaging (MRI). Human adipose tissue-derived mesenchymal stem cells (hADMSCs) were transduced with a lentivirus containing the human ferritin heavy chain 1 (FTH1) gene coupled to one of three neural cell-specific promoters: human synapsin 1 promoter (SYN1p, for neurons), human glial fibrillary acidic protein promoter (GFAPp, for astrocytes), and human myelin basic protein promoter (MBPp, for oligodendrocytes). Three groups of neural-differentiation-inducible ferritin-expressing (NDIFE) hADMSCs were established: SYN1p-FTH1, GFAPp-FTH1, and MBPp-FTH1. The proliferation rate of the NDIFE hADMSCs was evaluated using a Cell Counting Kit-8 assay. Ferritin expression was assessed with western blotting and immunofluorescent staining before and after the induction of differentiation in NDIFE hADMSCs. The intracellular iron content was measured with Prussian blue iron staining and inductively coupled plasma mass spectrometry. R2 relaxation rates were measured with MRI in vitro. The proliferation rates of control and NDIFE hADMSCs did not differ significantly (P > 0.05). SYN1p-FTH1, GFAPp-FTH1, and MBPp-FTH1 hADMSCs expressed specific markers of neurons, astrocytes, and oligodendrocytes, respectively, after neural differentiation. Neural differentiation increased ferritin expression twofold, the intracellular iron content threefold, and the R2 relaxation rate two- to threefold in NDIFE hADMSCs, resulting in notable hypointensity in T2-weighted images (P < 0.05). These results were cross-validated. Thus, a link between neural differentiation and MRI signals (R2 relaxation rate) was established in hADMSCs. The use of MRI and neural-differentiation-inducible ferritin expression is a viable method for monitoring the neural differentiation of hADMSCs.

Advances in using MRI probes and sensors for in vivo cell tracking as applied to regenerative medicine

The field of molecular and cellular imaging allows molecules and cells to be visualized in vivo non-invasively. It has uses not only as a research tool but in clinical settings as well, for example in monitoring cell-based regenerative therapies, in which cells are transplanted to replace degenerating or damaged tissues, or to restore a physiological function. The success of such cell-based therapies depends on several critical issues, including the route and accuracy of cell transplantation, the fate of cells after transplantation, and the interaction of engrafted cells with the host microenvironment. To assess these issues, it is necessary to monitor transplanted cells non-invasively in real-time. Magnetic resonance imaging (MRI) is a tool uniquely suited to this task, given its ability to image deep inside tissue with high temporal resolution and sensitivity. Extraordinary efforts have recently been made to improve cellular MRI as applied to regenerative medicine, by developing more advanced contrast agents for use as probes and sensors. These advances enable the non-invasive monitoring of cell fate and, more recently, that of the different cellular functions of living cells, such as their enzymatic activity and gene expression, as well as their time point of cell death. We present here a review of recent advancements in the development of these probes and sensors, and of their functioning, applications and limitations.

Effect of MRI tags: SPIO nanoparticles and 19F nanoemulsion on various populations of mouse mesenchymal stem cells

Transplantation of mesenchymal stem cells (MSCs) has emerged as a promising strategy for the treatment of myriad human disorders, including several neurological diseases. Superparamagnetic iron oxide nanoparticles (SPION) and fluorine nanoemulsion (19F) are characterized by low toxicity and good sensitivity, and, as such, are among the most frequently used cell-labeling agents. However, to date, their impact across the various populations of MSCs has not been comprehensively investigated. Thus, the impact of MRI tags (independent variable) has been set as a primary endpoint. The various populations of mouse MSCs in which the effect of tag was investigated consisted of (1) tissue of cell origin: bone marrow vs. Adipose tissue; (2) age of donor: young vs. old; (3) cell culture conditions: hypoxic vs. normal vs. normal + ascorbic acid (AA); (4) exposure to acidosis: yes vs. no. The impact of those populations has been also analyzed and considered as secondary endpoints. The experimental readouts (dependent variables) included: (1) cell viability; (2) cell size; (3) cell doubling time; (4) colony formation; (5) efficiency of labeling; and (6) cell migration. We did not identify any impact of cell labeling for these investigated populations in any of the readouts. In addition, we found that the harsh microenvironment of injured tissue modeled by a culture of cells in a highly acidic environment has a profound effect on all readouts, and both age of donor and cell origin tissue also have a substantial influence on most of the readouts, while oxygen tension in the cell culture conditions has a smaller impact on MSCs. A detailed characterization of the factors that influence the quality of MSCs is vital to the proper pursuit of preclinical and clinical studies.

⁵²Mn production for PET/MRI tracking of human stem cells expressing divalent metal transporter 1 (DMT1)

There is a growing demand for long-term in vivo stem cell imaging for assessing cell therapy techniques and guiding therapeutic decisions. This work develops the production of ⁵²Mn and establishes proof of concept for the use of divalent metal transporter 1 (DMT1) as a positron emission tomography (PET) and magnetic resonance imaging (MRI) reporter gene for stem cell tracking in the rat brain. ⁵²Mn was produced via proton irradiation of a natural chromium target. In a comparison of two ⁵²Mn separation methods, solvent-solvent extraction was preferred over ion exchange chromatography because of reduced chromium impurities and higher ⁵²Mn recovery. In vitro uptake of Mn-based PET and MRI contrast agents (⁵²Mn²⁺ and Mn²⁺, respectively) was enhanced in DMT1 over-expressing human neural progenitor cells (hNPC-DMT1) compared to wild-type control cells (hNPC-WT). After cell transplantation in the rat striatum, increased uptake of Mn-based contrast agents in grafted hNPC-DMT1 was detected in in vivo manganese-enhanced MRI (MEMRI) and ex vivo PET and autoradiography. These initial studies indicate that this approach holds promise for dual-modality PET/MR tracking of transplanted stem cells in the central nervous system and prompt further investigation into the clinical applicability of this technique.

Theranostic Magnetic Nanostructures (MNS) for Cancer

Despite the complexities of cancer, remarkable diagnostic and therapeutic advances have been made during the past decade, which include improved genetic, molecular, and nanoscale understanding of the disease. Physical science and engineering, and nanotechnology in particular, have contributed to these developments through out-of-the-box ideas and initiatives from perspectives that are far removed from classical biological and medicinal aspects of cancer. Nanostructures, in particular, are being effectively utilized in sensing/diagnostics of cancer while nanoscale carriers are able to deliver therapeutic cargo for timed and controlled release at localized tumor sites. Magnetic nanostructures (MNS) have especially attracted considerable attention of researchers to address cancer diagnostics and therapy. A significant part of the promise of MNS lies in their potential for "theranostic" applications, wherein diagnostics makes use of the enhanced localized contrast in magnetic resonance imaging (MRI) while therapy leverages the ability of MNS to heat under external radio frequency (RF) field for thermal therapy or use of thermal activation for release of therapy cargo. In this chapter, we report some of the key developments in recent years in regard to MNS as potential theranostic carriers. We describe that the r₂relaxivity of MNS can be maximized by allowing water (proton) diffusion in the vicinity of MNS by polyethylene glycol (PEG) anchoring, which also facilitates excellent fluidic stability in various media and extended in vivo circulation while maintaining high r₂values needed for T₂-weighted MRI contrast. Further, the specific absorption rate (SAR) required for thermal activation of MNS can be tailored by controlling composition and size of MNS. Together, emerging MNS show considerable promise to realize theranostic potential. We discuss that properly functionalized MNS can be designed to provide remarkable in vivo stability and accompanying pharmacokinetics exhibit organ localization that can be tailored for specific applications. In this context, even iron-based MNS show extended circulation as well as diverse organ accumulation beyond liver, which otherwise renders MNS potentially toxic to liver function. We believe that MNS, including those based on iron oxides, have entered a renaissance era where intelligent synthesis, functionalization, stabilization, and targeting provide ample evidence for applications in localized cancer theranostics.