Preparation of magnetically labeled cells for cell tracking by magnetic resonance imaging

19F MRI-fluorescence imaging dual-modal cell tracking with partially fluorinated nanoemulsions

As a noninvasive “hot-spot” imaging technology, fluorine-19 magnetic resonance imaging (19F MRI) has been extensively used in cell tracking. However, the peculiar physicochemical properties of perfluorocarbons (PFCs), the most commonly used 19F MRI agents, sometimes cause low sensitivity, poor cell uptake, and misleading results. In this study, a partially fluorinated agent, perfluoro-tert-butyl benzyl ether, was used to formulate a 19F MRI-fluorescence imaging (FLI) dual-modal nanoemulsion for cell tracking. Compared with PFCs, the partially fluorinated agent showed considerably improved physicochemical properties, such as lower density, shorter longitudinal relaxation times, and higher solubility to fluorophores, while maintaining high 19F MRI sensitivity. After being formulated into stable, monodisperse, and paramagnetic Fe3+-promoted nanoemulsions, the partially fluorinated agent was used in 19F MRI-FLI dual imaging tracking of lung cancer A549 cells and macrophages in an inflammation mouse model.

Imaging of a high concentration of iron labeled cells with positive contrast in a rat knee

PURPOSE

The sweep imaging with Fourier transformation (SWIFT) imaging technique has been shown to provide positive contrast from diluted cell suspensions labeled with super-paramagnetic iron oxide (SPIO) in a tissue, as an alternative to T2*-weighted imaging. Here we demonstrate a variation of the SWIFT technique that yields a hyperintense signal from a concentrated cell suspension. The proposed technique provides minimal background signal from host tissue and facilitates visualization of injected cells.

METHODS

The proton resonance frequency and linewidth were determined for SPIO solutions of different concentrations. The original SWIFT sequence was modified and a dual saturation Gaussian shape RF pulse with ~200 Hz bandwidth was incorporated into the acquisition protocol to suppress host tissue and fat signals. This modification of the original acquisition protocol permits the detection of a hyperintense signal from grafted cells with minimal background signal from the host tissue.

RESULTS

SPIO particles not only induce broadening of NMR line-width but also an initiate proton resonance frequency shift. This shift is linearly proportional to the concentration of the iron oxide particles and induced by the bulk magnetic susceptibility of SPIOs. The shift of the resonance frequency of iron labeled cells allowed us effectively suppress the host tissues with saturation RF pulse to improve MRI detection of grafted cells.

CONCLUSIONS

Iron oxide particles increase the resonance frequency of water proton signal. This shift permitted us to add the tissue/fat saturation RF pulse into the original SWIFT acquisition protocol and detect distinct hyperintense signals from grafted cells with minimal background signal from the host tissue.

Positive contrast from cells labeled with iron oxide nanoparticles: Quantitation of imaging data

PURPOSE

Conventional T₂ -weighted MRI produces a hypointense signal from iron-labeled cells, which renders quantification unfeasible. We tested a SWeep Imaging with Fourier Transformation (SWIFT) MRI pulse sequence to generate a quantifiable hyperintense signal from iron-labeled cells.

METHODS

Mesenchymal stem cells (MSCs) were labeled with different concentrations of iron oxide particles and examined for cell viability, proliferation, and differentiation. The SWIFT sequence was optimized to detect and quantify the amount of iron in the muscle tissue after injection of iron oxide solution and iron-labeled MSCs.

RESULTS

The incubation of MSCs with iron oxide and low concentration of poly-L-lysine mixture resulted in an internalization of up to 22 pg of iron per cell with no adverse effect on MSCs. Phantom experiments showed a dependence of SWIFT signal intensity on the excitation flip angle. The hyperintense signal from iron-labeled cells or solutions was detected, and an amount of the iron oxide in the tissue was quantified with the variable flip angle method.

CONCLUSIONS

The SWIFT sequence can produce a quantifiable hyperintense MRI signal from iron-labeled cells. The graft of 18 x 10⁶ cells was detectable for 19 days after injection and the amount of iron was quantifiable. The proposed protocol simplifies the detection and provides a means to quantify cell numbers. Magn Reson Med 78:1900-1910, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

High-resolution three-dimensional macromolecular proton fraction mapping for quantitative neuroanatomical imaging of the rodent brain in ultra-high magnetic fields

A well-known problem in ultra-high-field MRI is generation of high-resolution three-dimensional images for detailed characterization of white and gray matter anatomical structures. T₁-weighted imaging traditionally used for this purpose suffers from the loss of contrast between white and gray matter with an increase of magnetic field strength. Macromolecular proton fraction (MPF) mapping is a new method potentially capable to mitigate this problem due to strong myelin-based contrast and independence of this parameter of field strength. MPF is a key parameter determining the magnetization transfer effect in tissues and defined within the two-pool model as a relative amount of macromolecular protons involved into magnetization exchange with water protons. The objectives of this study were to characterize the two-pool model parameters in brain tissues in ultra-high magnetic fields and introduce fast high-field 3D MPF mapping as both anatomical and quantitative neuroimaging modality for small animal applications. In vivo imaging data were obtained from four adult male rats using an 11.7T animal MRI scanner. Comprehensive comparison of brain tissue contrast was performed for standard R₁ and T₂ maps and reconstructed from Z-spectroscopic images two-pool model parameter maps including MPF, cross-relaxation rate constant, and T₂ of pools. Additionally, high-resolution whole-brain 3D MPF maps were obtained with isotropic 170µm voxel size using the single-point synthetic-reference method. MPF maps showed 3-6-fold increase in contrast between white and gray matter compared to other parameters. MPF measurements by the single-point synthetic reference method were in excellent agreement with the Z-spectroscopic method. MPF values in rat brain structures at 11.7T were similar to those at lower field strengths, thus confirming field independence of MPF. 3D MPF mapping provides a useful tool for neuroimaging in ultra-high magnetic fields enabling both quantitative tissue characterization based on the myelin content and high-resolution neuroanatomical visualization with high contrast between white and gray matter.

Tracking and Quantification of Magnetically Labeled Stem Cells using Magnetic Resonance Imaging

Stem cell based therapies have critical impacts on treatments and cures of diseases such as neurodegenerative or cardiovascular disease. In vivo tracking of stem cells labeled with magnetic contrast agents is of particular interest and importance as it allows for monitoring of the cells' bio-distribution, viability, and physiological responses. Herein, recent advances are introduced in tracking and quantification of super-paramagnetic iron oxide (SPIO) nanoparticles-labeled cells with magnetic resonance imaging, a noninvasive approach that can longitudinally monitor transplanted cells. This is followed by recent translational research on human stem cells that are dual-labeled with green fluorescence protein (GFP) and SPIO nanoparticles, then transplanted and tracked in a chicken embryo model. Cell labeling efficiency, viability, and cell differentiation are also presented.

Co-Registration of Bioluminescence Tomography, Computed Tomography, and Magnetic Resonance Imaging for Multimodal In Vivo Stem Cell Tracking

We present a practical approach for coregistration of bioluminescence tomography (BLT), computed tomography (CT), and magnetic resonance (MR) images. For this, we developed a customized animal shuttle composed of nonfluorescent, MR-compatible Delrin plastic that fits a commercially available MR surface coil. Mouse embryonic stem cells were transfected with the luciferase gene and labeled with superparamagnetic iron oxide nanoparticles. Cells were stereotaxically implanted in the mouse brain and imaged weekly for 4 weeks with bioluminescent imaging (IVIS Spectrum CT scanner) and magnetic resonance imaging (MRI; 11.7 T horizontal bore scanner). Without the use of software coregistration, in vitro phantom studies yielded root-mean-square errors of 7.6 × 10⁻³, 0.93 mm, and 0.78 mm along the medial–lateral (ML), dorsal–ventral (DV), and anterior–posterior (AP) axes, respectively. Rotation errors were negligible. Software coregistration by translation along the DV and AP axes resulted in consistent agreement between the CT and MR images, without the need for rotation or warping. In vivo coregistered BLT/MRI mouse brain data sets showed a single diffuse region of bioluminescent imaging photon signal and MRI hypointensity. Over time, the transplanted cells formed tumors as histopathologically validated. Disagreement between BLT and MRI tumor location was greatest along the DV axis (1.4 ± 0.2 mm) than along the ML (0.5 ± 0.3 mm) and the AP axes (0.6 mm) because of the uncertainty of the depth of origin of the BLT signal. Combining the high spatial anatomical information of MRI with the cell viability/proliferation data from BLT should facilitate preclinical evaluation of novel therapeutic candidate stem cells.

Perspective of Fe3O4 Nanoparticles Role in Biomedical Applications

In recent years, although many review articles have been presented about bioapplications of magnetic nanoparticles by some research groups with different expertise such as chemistry, biology, medicine, pharmacology, and materials science and engineering, the majority of these reviews are insufficiently comprehensive in all related topics like magnetic aspects of process. In the current review, it is attempted to carry out the inclusive surveys on importance of magnetic nanoparticles and especially magnetite ones and their required conditions for appropriate performance in bioapplications. The main attentions of this paper are focused on magnetic features which are less considered. Accordingly, the review contains essential magnetic properties and their measurement methods, synthesis techniques, surface modification processes, and applications of magnetic nanoparticles.

Quantitative "Hot Spot" Imaging of Transplanted Stem Cells using Superparamagnetic Tracers and Magnetic Particle Imaging (MPI)

Magnetic labeling of stem cells enables their noninvasive detection by magnetic resonance imaging (MRI). In practical terms, most MRI studies have been limited to the visualization of local engraftment because other sources of endogenous hypointense contrast complicate the interpretation of systemic (whole-body) cell distribution. In addition, MRI cell tracking is inherently nonquantitative in nature. We report herein on the potential of magnetic particle imaging (MPI) as a novel tomographic technique for noninvasive “hot-spot” imaging and quantification of stem cells using superparamagnetic iron oxide (SPIO) tracers. Neural and mesenchymal stem cells, representing small and larger cell bodies, were labeled with 3 different SPIO tracer formulations, including 2 preparations (Feridex and Resovist) that have previously been used in clinical MRI cell-tracking studies. Magnetic particle spectroscopy measurements demonstrated a linear correlation between MPI signal and iron content for both free particles in homogeneous solution and for internalized and aggregated particles in labeled cells over a wide range of concentrations. The overall MPI signal ranged from 1 × 10⁻³ to 3 × 10⁻⁴ Am²/g Fe, which was equivalent to 2 × 10⁻¹⁴ to 1 × 10⁻¹⁵ Am² per cell, indicating that cell numbers can be quantified with MPI analogous to the use of radiotracers in nuclear medicine or fluorine tracers in ¹⁹F MRI. When SPIO-labeled cells were transplanted in the mouse brain, they could be readily detected by MPI at a detection threshold of about 5 × 10⁴ cells, with MPI/MRI overlays showing an excellent agreement between the hypointense MRI areas and MPI hot spots. The calculated tissue MPI signal ratio for 100 000 vs 50 000 implanted cells was 2.08. Hence, MPI can potentially be further developed for quantitative and easy-to-interpret, tracer-based noninvasive cell imaging, preferably with MRI as an adjunct anatomical imaging modality.

Cellular magnetic resonance imaging contrast generated by the ferritin heavy chain genetic reporter under the control of a Tet-On switch

INTRODUCTION

Despite the strong appeal of ferritin as a magnetic resonance imaging (MRI) reporter for stem cell research, no attempts have been made to apply this genetic imaging reporter in stem cells in an inducible manner, which is important for minimizing the potential risk related to the constitutive expression of an imaging reporter. The aim of the present study was to develop an inducible genetic MRI reporter system that enables the production of intracellular MRI contrast as needed.

METHODS

Ferritin heavy chain (FTH1) was genetically modified by adding a Tet-On switch. A C3H10T1/2 cell line carrying Tet-FTH1 (C3H10T1/2-FTH1) was established via lentiviral transduction. The dose- and time-dependent expression of FTH1 in C3H10T1/2 cells was assessed by western blot and immunofluorescence staining. The induced "ON" and non-induced "OFF" expressions of FTH1 were detected using a 3.0 T MRI scanner. Iron accumulation in cells was analyzed by Prussian blue staining and transmission electron microscopy (TEM).

RESULTS

The expression of FTH1 was both dose- and time-dependently induced, and FTH1 expression peaked in response to induction with doxycycline (Dox) at 0.2 μg/ml for 72 h. The induced expression of FTH1 resulted in a significant increase in the transverse relaxation rate of C3H10T1/2-FTH1 cells following iron supplementation. Prussian blue staining and TEM revealed extensive iron accumulation in C3H10T1/2-FTH1 cells in the presence of Dox.

CONCLUSIONS

Cellular MRI contrast can be produced as needed via the expression of FTH1 under the control of a Tet-On switch. This finding could lay the groundwork for the use of FTH1 to track stem cells in vivo in an inducible manner.

Feasibility Study of Canine Epidermal Neural Crest Stem Cell Transplantation in the Spinal Cords of Dogs

This pilot feasibility study aimed to determine the outcome of canine epidermal neural crest stem cell (cEPI-NCSC) grafts in the normal spinal cords of healthy bred-for-research dogs. This included developing novel protocols for (a) the ex vivo expansion of cEPI-NCSCs, (b) the delivery of cEPI-NCSCs into the spinal cord, and (c) the labeling of the cells and subsequent tracing of the graft in the live animal by magnetic resonance imaging. A total of four million cEPI-NCSCs were injected into the spinal cord divided in two locations. Differences in locomotion at baseline and post-treatment were evaluated by gait analysis and compared with neurological outcome and behavioral exams. Histopathological analyses of the spinal cords and cEPI-NCSC grafts were performed at 3 weeks post-transplantation. Neurological and gait parameters were minimally affected by the stem cell injection. cEPI-NCSCs survived in the canine spinal cord for the entire period of investigation and did not migrate or proliferate. Subsets of cEPI-NCSCs expressed the neural crest stem cell marker Sox10. There was no detectable expression of markers for glial cells or neurons. The tissue reaction to the cell graft was predominantly vascular in addition to a degree of reactive astrogliosis and microglial activation. In the present study, we demonstrated that cEPI-NCSC grafts survive in the spinal cords of healthy dogs without major adverse effects. They persist locally in the normal spinal cord, may promote angiogenesis and tissue remodeling, and elicit a tissue response that may be beneficial in patients with spinal cord injury.

SIGNIFICANCE

It has been established that mouse and human epidermal neural crest stem cells are somatic multipotent stem cells with proved innovative potential in a mouse model of spinal cord injury (SCI) offering promise of a valid treatment for SCI. Traumatic SCI is a common neurological problem in dogs with marked similarities, clinically and pathologically, to the syndrome in people. For this reason, dogs provide a readily accessible, clinically realistic, spontaneous model for evaluation of epidermal neural crest stem cells therapeutic intervention. The results of this study are expected to give the baseline data for a future clinical trial in dogs with traumatic SCI.

Superparamagnetic iron oxide nanoparticles for in vivo molecular and cellular imaging

In the last decade, the biomedical applications of nanoparticles (NPs) (e.g. cell tracking, biosensing, magnetic resonance imaging (MRI), targeted drug delivery, and tissue engineering) have been increasingly developed. Among the various NP types, superparamagnetic iron oxide NPs (SPIONs) have attracted considerable attention for early detection of diseases due to their specific physicochemical properties and their molecular imaging capabilities. A comprehensive review is presented on the recent advances in the development of in vitro and in vivo SPION applications for molecular imaging, along with opportunities and challenges.

Mesenchymal stem cell tracking in the intervertebral disc

Low back pain is a common clinical problem, which leads to significant social, economic and public health costs. Intervertebral disc (IVD) degeneration is accepted as a common cause of low back pain. Initially, this is characterized by a loss of proteoglycans from the nucleus pulposus resulting in loss of tissue hydration and hydrostatic pressure. Conservative management, including analgesia and physiotherapy often fails and surgical treatment, such as spinal fusion, is required. Stem cells offer an exciting possible regenerative approach to IVD disease. Preclinical research has demonstrated promising biochemical, histological and radiological results in restoring degenerate IVDs. Cell tracking provides an opportunity to develop an in-depth understanding of stem cell survival, differentiation and migration, enabling optimization of stem cell treatment. Magnetic Resonance Imaging (MRI) is a non-invasive, non-ionizing imaging modality with high spatial resolution, ideally suited for stem cell tracking. Furthermore, novel MRI sequences have the potential to quantitatively assess IVD disease, providing an improved method to review response to biological treatment. Superparamagnetic iron oxide nanoparticles have been extensively researched for the purpose of cell tracking. These particles are biocompatible, non-toxic and act as excellent MRI contrast agents. This review will explore recent advances and issues in stem cell tracking and molecular imaging in relation to the IVD.

Patient-specific computational modeling and magnetic nanoconstructs: tools for maximizing the efficacy of stem cell-based therapies

Stem cell transplantation has the potential to restore heart function following myocardial infarction. However, the success of any stem cell-based therapy is critically linked to the effective homing and early engraftment of the injected cells at the infarcted site. Here, a hierarchical multiscale computational model is proposed for predicting the patient-specific vascular transport and intratissue homing and migration of stem cells injected either systemically or locally. Starting with patient-specific data, such as the vascular geometry, blood flow, and location of the infarcted area, the computational model can be used to perform parametric analysis to identify optimal injection conditions in terms of administration route, injection site, catheter type, and infusion velocity. In addition to this, a new generation of magnetic nanoconstructs is introduced for labeling stem cells and monitoring their behavior in vivo via magnetic resonance imaging. These nanoconstructs also can be used for multimodal imaging, merging MRI and nuclear imaging, and the intracellular delivery of active agents to support stem cell differentiation. The convergence of computational modeling and novel nanoconstructs for stem cell labeling could improve our understanding in cell homing and early engraftment at the infarcted site and thus pave the way to more effective stem cell-based therapies.

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

BACKGROUND

The purpose of the study was to evaluate the long-term clinical tracking of magnetically labeled stem cells after intracerebroventricular transplantation as well as to investigate in vitro feasibility for magnetic guidance of cell therapy within large fluid compartments.

METHOD

After approval by our Institutional Review Board, an 18-month-old patient, diagnosed as being in a vegetative state due to global cerebral ischemia, underwent cell transplantation to the frontal horn of the lateral ventricle, with umbilical cord blood-derived stem cells labeled with superparamagnetic iron oxide (SPIO) contrast agent. The patient was followed over 33 months with clinical examinations and MRI. To evaluate the forces governing the distribution of cells within the fluid compartment of the ventricular system in vivo, a gravity-driven sedimentation assay and a magnetic field-driven cell attraction assay were developed in vitro.

RESULTS

Twenty-four hours post-transplantation, MR imaging (MRI) was able to detect hypointense cells in the occipital horn of the lateral ventricle. The signal gradually decreased over 4 months and became undetectable at 33 months. In vitro, no significant difference in cell sedimentation between SPIO-labeled and unlabeled cells was observed (p = NS). An external magnet was effective in attracting cells over distances comparable to the size of human lateral ventricles.

CONCLUSIONS

MR imaging of SPIO-labeled cells allows monitoring of cells within lateral ventricles. While the initial biodistribution is governed by gravity-driven sedimentation, an external magnetic field may possibly be applied to further direct the distribution of labeled cells within large fluid compartments such as the ventricular system.

Effect of transplantation route on stem cell migration to fibrotic liver of rats via cellular magnetic resonance imaging

BACKGROUND AIMS

Assessing mesenchymal stromal cells (MSCs) after grafting is essential for understanding their migration and differentiation processes. The present study sought to evaluate via cellular magnetic resonance imaging (MRI) if transplantation route may have an effect on MSCs engrafting to fibrotic liver of rats.

METHODS

Rat MSCs were prepared, labeled with superparamagnetic iron oxide and scanned with MRI. Labeled MSCs were transplanted via the portal vein or vena caudalis to rats with hepatic fibrosis. MRI was performed in vitro before and after transplantation. Histologic examination was performed. MRI scan and imaging parameter optimization in vitro and migration under in vivo conditions were demonstrated.

RESULTS

Strong MRI susceptibility effects could be found on gradient echo-weighted, or T2∗-weighted, imaging sequences from 24 h after labeling to passage 4 of labeled MSCs in vitro. In vivo, MRI findings of the portal vein group indicated lower signal in liver on single shot fast spin echo-weighted, or T2-weighted, imaging and T2∗-weighted imaging sequences. The low liver MRI signal increased gradually from 0-3 h and decreased gradually from 3 h to 14 days post-transplantation. The distribution pattern of labeled MSCs in liver histologic sections was identical to that of MRI signal. It was difficult to find MSCs in tissues near the portal area on day 14 after transplantation; labeled MSCs appeared in fibrous tuberculum at the edge of the liver. No MRI signal change and a positive histologic examination were observed in the vena caudalis group.

CONCLUSIONS

The portal vein route seemed to be more beneficial than the vena caudalis on MSC migration to fibrotic liver of rats via MRI.

Magnetic Resonance Imaging Tracking of Graft Survival in the Infarcted Heart: Iron Oxide Particles Versus Ferritin Overexpression Approach

The main objective of cell therapy is the regeneration of damaged tissues. To distinguish graft from host tissue by magnetic resonance imaging (MRI), a paramagnetic label must be introduced to cells prior to transplantation. The paramagnetic label can be either exogenous iron oxide nanoparticles or a genetic overexpression of ferritin, an endogenous iron storage protein. The purpose of this work was to compare the efficacy of these 2 methods for MRI evaluation of engrafted cell survival in the infarcted mouse heart. Mouse skeletal myoblasts were labeled either by cocultivation with iron oxide particles or by engineering them to overexpress ferritin. Along with live cell transplantation, 2 other groups of mice were injected with dead-labeled cells. Both particle-labeled and ferritin-tagged grafts were detected as areas of MRI signal hypointensity in the left ventricle of the mouse heart using T2*-weighted sequences, although the signal attenuation decreased with ferritin tagging. Importantly, live cells could not be distinguished from dead cells when labeled with iron oxide particles, whereas the ferritin tagging was detected only in live grafts, thereby allowing identification of viable grafts using MRI. Thus, iron oxide particles can provide information about initial cell injection success but cannot assess graft viability. On the other hand, genetically based cell tagging, such as ferritin overexpression, despite having lower signal intensity in comparison with iron oxide particles, is able to identify live transplanted cells.

Variability in contrast agent uptake by different but similar stem cell types

The need to track and evaluate the fate of transplanted cells is an important issue in regenerative medicine. In order to accomplish this, pre-labelling cells with magnetic resonance imaging (MRI) contrast agents is a well-established method. Uptake of MRI contrast agents by non-phagocytic stem cells, and factors such as cell homeostasis or the adverse effects of contrast agents on cell biology have been extensively studied, but in the context of nanoparticle (NP)-specific parameters. Here, we have studied three different types of NPs (Endorem®, magnetoliposomes [MLs], and citrate coated C-200) to label relatively larger, mesenchymal stem cells (MSCs) and, much smaller yet faster proliferating, multipotent adult progenitor cells (MAPCs). Both cell types are similar, as they are isolated from bone marrow and have substantial regenerative potential, which make them interesting candidates for comparative experiments. Using NPs with different surface coatings and sizes, we found that differences in the proliferative and morphological characteristics of the cells used in the study are mainly responsible for the fate of endocytosed iron, intracellular iron concentration, and cytotoxic responses. The quantitative analysis, using high-resolution electron microscopy images, demonstrated a strong relationship between cell volume/surface, uptake, and cytotoxicity. Interestingly, uptake and toxicity trends are reversed if intracellular concentrations, and not amounts, are considered. This indicates that more attention should be paid to cellular parameters such as cell size and proliferation rate in comparative cell-labeling studies.

Intracellular gene transcription factor protein-guided MRI by DNA aptamers in vivo

The mechanisms by which transcription factor (TF) protein AP-1 modulates amphetamine's effects on gene transcription in living brains are unclear. We describe here the first part of our studies to investigate these mechanisms, specifically, our efforts to develop and validate aptamers containing the binding sequence of TF AP-1 (5ECdsAP1), in order to elucidate its mechanism of action in living brains. This AP-1-targeting aptamer, as well as a random sequence aptamer with no target (5ECdsRan) as a control, was partially phosphorothioate modified and tagged with superparamagnetic iron oxide nanoparticles (SPIONs), gold, or fluorescein isothiothianate contrast agent for imaging. Optical and transmission electron microscopy studies revealed that 5ECdsAP1 is taken up by endocytosis and is localized in the neuronal endoplasmic reticulum. The results of magnetic resonance imaging (MRI) with SPION-5ECdsAP1 revealed that neuronal AP-1 TF protein levels were elevated in neurons of live male C57black6 mice after amphetamine exposure; however, pretreatment with SCH23390, a dopaminergic receptor antagonist, suppressed this elevation. As studies in transgenic mice with neuronal dominant-negative A-FOS mutant protein, which has no binding affinity for the AP-1 sequence, showed a completely null MRI signal in the striatum, we can conclude that the MR signal reflects specific binding between the 5ECdsAP1 aptamer and endogenous AP-1 protein. Together, these data lend support to the application of 5ECdsAP1 aptamer for intracellular protein-guided imaging and modulation of gene transcription, which will thus allow investigation of the mechanisms of signal transduction in living brains.

Selective delivery of a therapeutic gene for treatment of head and neck squamous cell carcinoma using human neural stem cells

Objectives

Based on studies of the extensive tropism of neural stem cells (NSCs) toward malignant brain tumor, we hypothesized that NSCs could also target head and neck squamous cell carcinoma (HNSCC) and could be used as a cellular therapeutic delivery system.

Methods

To apply this strategy to the treatment of HNSCC, we used a human NSC line expressing cytosine deaminase (HB1.F3-CD), an enzyme that converts 5-fluorocytosine (5-FC) into 5-fluorouracil (5-FU), an anticancer agent. HB1. F3-CD in combination with 5-FC were cocultured with the HNSCC (SNU-1041) to examine the cytotoxicity on target tumor cells in vitro. For in vivo studies, an HNSCC mouse model was created by subcutaneous implantation of human HNSCC cells into athymic nude mice. HB1.F3-CD cells were injected into mice using tumoral, peritumoral, or intravenous injections, followed by systemic 5-FC administration.

Results

In vitro, the HB1.F3-CD cells significantly inhibited the growth of an HNSCC cell line in the presence of the 5-FC. Independent of the method of injection, the HB1.F3-CD cells migrated to the HNSCC tumor, causing a significant reduction in tumor volume. In comparison to 5-FU administration, HB1.F3-CD cell injection followed by 5-FC administration reduced systemic toxicity, but achieved the same level of therapeutic efficacy.

Conclusion

Transplantation of human NSCs that express the suicide enzyme cytosine deaminase combined with systemic administration of the prodrug 5-FC may be an effective regimen for the treatment of HNSCC.

Commercial nanoparticles for stem cell labeling and tracking

Stem cell therapy provides promising solutions for diseases and injuries that conventional medicines and therapies cannot effectively treat. To achieve its full therapeutic potentials, the homing process, survival, differentiation, and engraftment of stem cells post transplantation must be clearly understood. To address this need, non-invasive imaging technologies based on nanoparticles (NPs) have been developed to track transplanted stem cells. Here we summarize existing commercial NPs which can act as contrast agents of three commonly used imaging modalities, including fluorescence imaging, magnetic resonance imaging and photoacoustic imaging, for stem cell labeling and tracking. Specifically, we go through their technologies, industry distributors, applications and existing concerns in stem cell research. Finally, we provide an industry perspective on the potential challenges and future for the development of new NP products.

Non-invasive in-vivo imaging of stem cells after transplantation in cardiovascular tissue

Stem cell therapy for degenerative diseases, including ischemic heart disease is now a clinical reality. In the search for the optimal cell type for each patient category, many different stem cell subpopulations have been used. In addition, different cell processing procedures and delivery methods have been utilized. Moreover, choices of endpoints have varied between studies. Diverging results have been reported from clinical experiences, with some studies demonstrating promising results with improved cardiac function and reduced mortality and clinical symptoms, and others have seen no improvements. To better understand the underlying mechanisms of these results, a reverse translation from bedside to bench has been opened. Non-invasive cell tracking after implantation has a pivotal role in this translation. Imaging based methods can help elucidate important issues such as retention, migration and efficacy of the transplanted cells. Great effort is being made in finding new and better imaging techniques for different imaging modalities, and much have already been learned. But there are still many unanswered questions. In this review, we give an overview of the imaging modalities used for cell tracking and summarize the latest advances within the field.

In vivo MRI cell tracking using perfluorocarbon probes and fluorine-19 detection

This article presents a brief review of preclinical in vivo cell-tracking methods and applications using perfluorocarbon (PFC) probes and fluorine-19 ((19) F) MRI detection. Detection of the (19) F signal offers high cell specificity and quantification ability in spin density-weighted MR images. We discuss the compositions of matter, methods and applications of PFC-based cell tracking using ex vivo and in situ PFC labeling in preclinical studies of inflammation and cellular therapeutics. We also address the potential applicability of (19) F cell tracking to clinical trials.

Biocompatible fluorescent nanoparticles for in vivo stem cell tracking

Efficient application of stem cells to the treatment of neurodegenerative diseases requires safe cell tracking to follow stem cell fate over time in the host environment after transplantation. In this work, for the first time, fluorescent and biocompatible methyl methacrylate (MMA)-based nanoparticles (fluoNPs) were synthesized through a free-radical co-polymerization process with a fluorescent macromonomer obtained by linking Rhodamine B and hydroxyethyl methacrylate. We demonstrate that the fluoNPs produced by polymerization of MMA-Rhodamine complexes (1) were efficient for the labeling and tracking of multipotent human amniotic fluid cells (hAFCs); (2) did not alter the main biological features of hAFCs (such as viability, cell growth and metabolic activity); (3) enabled us to determine the longitudinal bio-distribution of hAFCs in different brain areas after graft in the brain ventricles of healthy mice by a direct fluorescence-based technique. The reliability of our approach was furthermore confirmed by magnetic resonance imaging analyses, carried out by incubating hAFCs with both superparamagnetic iron oxide nanoparticles and fluoNPs. Our data suggest that these finely tunable and biocompatible fluoNPs can be exploited for the longitudinal tracking of stem cells.

Rapid spectrophotometric technique for quantifying iron in cells labeled with superparamagnetic iron oxide nanoparticles: potential translation to the clinic

Labeling cells with superparamagnetic iron oxide (SPIO) nanoparticles provides the ability to track cells by magnetic resonance imaging. Quantifying intracellular iron concentration in SPIO labeled cells would allow for the comparison of agents and techniques used to magnetically label cells. Here we describe a rapid spectrophotometric technique (ST) to quantify iron content of SPIO-labeled cells, circumventing the previous requirement of an overnight acid digestion. Following lysis with 10% sodium dodecyl sulfate (SDS) of magnetically labeled cells, quantification of SPIO doped or labeled cells was performed using commonly available spectrophotometric instrument(s) by comparing absorptions at 370 and 750 nm with correction for turbidity of cellular products to determine the iron content of each sample. Standard curves demonstrated high linear correlation (R(2) = 0.998) between absorbance spectra of iron oxide nanoparticles and concentration in known SPIO-doped cells. Comparisons of the ST with inductively coupled plasma-mass spectroscopy (ICP-MS) or nuclear magnetic resonance relaxometric (R(2)) determinations of intracellular iron contents in SPIO containing samples resulted in significant linear correlation between the techniques (R(2) vs ST, R(2) > 0.992, p < 0.0001; ST vs ICP-MS, R(2) > 0.995, p < 0.0001) with the limit of detection of ST for iron = 0.66 µg ml(-1) for 10(6) cells ml(-1). We have developed a rapid straightforward protocol that does not require overnight acid digestion for quantifying iron oxide content in magnetically labeled cells using readily available analytic instrumentation that should greatly expedite advances in comparing SPIO agents and protocols for labeling cells.

Quantification of MRI signal of transgenic grafts overexpressing ferritin in murine myocardial infarcts

The noninvasive detection of transplanted cells in damaged organs and the longitudinal follow-up of cell fate and graft size are important for the evaluation of cell therapy. We have shown previously that the overexpression of the natural iron storage protein, ferritin, permits the detection of engrafted cells in mouse heart by MRI, but further imaging optimization is required. Here, we report a systematic evaluation of ferritin-based stem cell imaging in infarcted mouse hearts in vivo using three cardiac-gated pulse sequences in a 3-T scanner: black-blood proton-density-weighted turbo spin echo (PD TSE BB), bright-blood T(2) -weighted gradient echo (GRE) and black-blood T(2) -weighted GRE with improved motion-sensitized-driven equilibrium (iMSDE) preparation. Transgenic C2C12 myoblast grafts overexpressing ferritin did not change MRI contrast in the PD TSE BB images, but showed a 20% reduction in signal intensity ratio in black-blood T(2) -weighted iMSDE (p < 0.05) and a 30% reduction in bright-blood T(2) -weighted GRE (p < 0.0001). Graft size measurements by T(2) iMSDE and T(2) GRE were highly correlated with histological assessments (r = 0.79 and r = 0.89, respectively). Unlabeled wild-type C2C12 cells transplanted to mouse heart did not change the MRI signal intensity, although endogenous hemosiderin was seen in some infarcts. These data support the use of ferritin to track the survival, growth and migration of stem cells transplanted into the injured heart.

Molecular advances in reporter genes: the need to witness the function of stem cells in failing heart in vivo

Stem cells possess the ability to terminally differentiate in cell phenotypes belonging to several different lineages. Over the last decade, transplant of adult stem cells into the injuried myocardium has been widely studied as a revolutionary approach to promote the non-pharmacological improvement or replacement of the lost function. In spite of the tantalizing perspectives and controversial results, several questions about the viability and biology of transplanted stem cells in the beating heart still remain unanswered, mostly because of the current technological limitations. Recent advances in bio- and nano-technology are allowing the development of molecular probes for imaging thus providing a better understanding of stem cells physiology and fate in vivo. Reporter gene based molecular imaging is a high-throughput and sensitive tool used to unscramble over time the mechanisms underlying cell-induced myocardial repair in vivo. To date, the employed reporter genes have been exogenous (proteins which are expressed after gene engineering), or endogenous (detected by tracer substrates). This review will highlight current and outstanding experimental investigations, which are developing new probes to monitor the fate of stem cells transplanted in failing myocardium in vivo.

Effects of Iron Oxide Nanoparticle Labeling on Human Endothelial Cells

Iron oxide nanoparticles (INOPS) are a potential contrast agent for magnetic resonance (MR) tracking of transplanted endothelial cells. The objective of this study was to examine the effect of INOPS labeling on endothelial cells. The mixture of INOPS and poly-l-lysine (PLL) was used to label human endothelial cells. Labeling efficiency was examined by Prussian blue staining, transmission electron microscopy, and atomic absorption spectrometry. The effect of iron oxide concentration on cell viability and proliferation were determined. The correlation of reactive oxygen species (ROS) and apoptosis was also examined. In vitro MRI scanning was carried out using a 1.5T MR system. INOPS-PLL could be readily taken up by endothelial cells and subsequently induce MRI signal intensity changes. However, higher labeling concentration (>50 μg/ml) and longer incubation (48 h) can affect cell viability and proliferation. Mitochondrial damage, apoptosis, and autolysosmes were observed under high INOPS-PLL concentrations, which were correlated to ROS production. INOPS-PLL nanoparticles can be used to label transplanted endothelial cells. However, high concentration of INOPS can impair cell viability, possibly through ROS-mediated apoptosis and autophagy.

Accelerated fluorine-19 MRI cell tracking using compressed sensing

Cell tracking using perfluorocarbon labels and fluorine-19 (19F) MRI is a noninvasive approach to visualize and quantify cell populations in vivo. In this study, we investigated three-dimensional compressed sensing methods to accelerate 19F MRI data acquisition for cell tracking and evaluate the impact of acceleration on 19F signal quantification. We show that a greater than 8-fold reduction in imaging time was feasible without pronounced image degradation and with minimal impact on the image signal-to-noise ratio and 19F quantification accuracy. In 19F phantom studies, we show that apparent feature topology is maintained with compressed sensing reconstruction, and false positive signals do not appear in areas devoid of fluorine. We apply the three-dimensional compressed sensing 19F MRI methods to quantify the macrophage burden in a localized wounding-inflammation mouse model in vivo; at 8-fold image acceleration, the 19F signal distribution was accurately reproduced, with no loss in signal-to-noise ratio. Our results demonstrate that three-dimensional compressed sensing methods have potential for advancing in vivo 19F cell tracking for a wide range of preclinical and translational applications.

Stem cells as a tool for breast imaging

Stem cells are a scientific field of interest due to their therapeutic potential. There are different groups, depending on the differentiation state. We can find lonely stem cells, but generally they distribute in niches. Stem cells don't survive forever. They are affected for senescence. Cancer stem cells are best defined functionally, as a subpopulation of tumor cells that can enrich for tumorigenic property and can regenerate heterogeneity of the original tumor. Circulating tumor cells are cells that have detached from a primary tumor and circulate in the bloodstream. They may constitute seeds for subsequent growth of additional tumors (metastasis) in different tissues. Advances in molecular imaging have allowed a deeper understanding of the in vivo behavior of stem cells and have proven to be indispensable in preclinical and clinical studies. One of the first imaging modalities for monitoring pluripotent stem cells in vivo, magnetic resonance imaging (MRI) offers high spatial and temporal resolution to obtain detailed morphological and functional information. Advantages of radioscintigraphic techniques include their picomolar sensitivity, good tissue penetration, and translation to clinical applications. Radionuclide imaging is the sole direct labeling technique used thus far in human studies, involving both autologous bone marrow derived and peripheral stem cells.

Nanoclusters of iron oxide: effect of core composition on structure, biocompatibility, and cell labeling efficacy

Inorganic nanocrystals have a variety of applications in medicine. They may serve as contrast agents, therapeutics, and for in vitro diagnostics. Frequently, the synthesis route yields hydrophobically capped nanocrystals, which necessitates their subsequent coating to render a water-soluble and biocompatible probe. Biocompatibility is crucial for cellular imaging applications, which require large quantities of diagnostically active nanoparticles to be loaded into cells. We have previously reported the design and synthesis of a fluorescent and magnetic resonance imaging-detectable core-shell nanoparticle that encapsulates hydrophobically coated iron oxide nanocrystals. The core of soybean oil and iron oxide is covered by a shell mixture of phospholipids, some of which contained polyethylene glycol. Despite the biocompatibility of these components, we hypothesize that we can improve this formulation with respect to in vitro toxicity. To this aim, we measured the effect of six different core compositions on nanoparticle structure, cell labeling efficacy, and cell viability, as well as cell tracking potential. We methodically investigated the causes of toxicity and conclude that, even when combining biocompatible materials, the resulting formulation is not guaranteed to be biocompatible.

ICV-transplanted human glial precursor cells are short-lived yet exert immunomodulatory effects in mice with EAE

Human glial precursor cells (hGPs) have potential for remyelinating lesions and are an attractive cell source for cell therapy of multiple sclerosis (MS). To investigate whether transplanted hGPs can affect the pathogenesis of experimental autoimmune encephalomyelitis (EAE), an animal model of MS, we evaluated the therapeutic effects of transplanted hGPs together with the in vivo fate of these cells using magnetic resonance imaging (MRI) and bioluminescence imaging (BLI). At 14 days post-EAE induction, mice (n = 19) were intracerebroventricularly (ICV) injected with 5 × 10(5) hGPs that were magnetically labeled with superparamagnetic iron oxide (SPIO) particles as MR contrast agent and transduced with firefly luciferase for BLI of cell survival. Control mice (n = 18) received phosphate buffered saline (PBS) vehicle only. The severity of EAE clinical disability in the hGP-transplanted group was significantly suppressed (P < 0.05) with concomitant inhibition of ConA and MOG-specific T cell proliferation in the spleen. Astrogliosis was reduced and a lower activity of macrophages and/or microglia was observed in the spinal cord (P < 0.05). On MRI, SPIO signal was detected within the lateral ventricle from 1 day post-transplantation and remained there for up to 34 days. BLI indicated that most cells did not survive beyond 5-10 days, consistent with the lack of detectable migration into the brain parenchyma and the histological presence of an abundance of apoptotic cells. Transplanted hGPs could not be detected in the spleen. We conclude that ICV transplantation of short-lived hGPs can have a remote therapeutic effect through immunomodulation from within the ventricle, without cells directly participating in remyelination.

Assaying macrophage activity in a murine model of inflammatory bowel disease using fluorine-19 MRI

Macrophages have an important role in the pathogenesis of most chronic inflammatory diseases. A means of non-invasively quantifying macrophage migration would contribute significantly towards our understanding of chronic inflammatory processes and aid the evaluation of novel therapeutic strategies. We describe the use of a perfluorocarbon tracer reagent and in vivo (19)F magnetic resonance imaging (MRI) to quantify macrophage burden longitudinally. We apply these methods to evaluate the severity and three-dimensional distribution of macrophages in a murine model of inflammatory bowel disease (IBD). MRI results were validated by histological analysis, immunofluorescence and quantitative real-time polymerase chain reaction. Selective depletion of macrophages in vivo was also performed, further validating that macrophage accumulation of perfluorocarbon tracers was the basis of (19)F MRI signals observed in the bowel. We tested the effects of two common clinical drugs, dexamethasone and cyclosporine A, on IBD progression. Whereas cyclosporine A provided mild therapeutic effect, unexpectedly dexamethasone enhanced colon inflammation, especially in the descending colon. Overall, (19)F MRI can be used to evaluate early-stage inflammation in IBD and is suitable for evaluating putative therapeutics. Due to its high macrophage specificity and quantitative ability, we envisage (19)F MRI having an important role in evaluating a wide range of chronic inflammatory conditions mediated by macrophages.

Molecular imaging of stem cells: tracking survival, biodistribution, tumorigenicity, and immunogenicity

Being able to self-renew and differentiate into virtually all cell types, both human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) have exciting therapeutic implications for myocardial infarction, neurodegenerative disease, diabetes, and other disorders involving irreversible cell loss. However, stem cell biology remains incompletely understood despite significant advances in the field. Inefficient stem cell differentiation, difficulty in verifying successful delivery to the target organ, and problems with engraftment all hamper the transition from laboratory animal studies to human clinical trials. Although traditional histopathological techniques have been the primary approach for ex vivo analysis of stem cell behavior, these postmortem examinations are unable to further elucidate the underlying mechanisms in real time and in vivo. Fortunately, the advent of molecular imaging has led to unprecedented progress in understanding the fundamental behavior of stem cells, including their survival, biodistribution, immunogenicity, and tumorigenicity in the targeted tissues of interest. This review summarizes various molecular imaging technologies and how they have advanced the current understanding of stem cell survival, biodistribution, immunogenicity, and tumorigenicity.

Tracheal reconstruction by mesenchymal stem cells with small intestine submucosa in rabbits

AIM

The increasing number of newborns requiring intubation and artificial ventilation in the sophisticated premature and intensive care units of recent years has been followed by a concomitant increase in the number of children who develop tracheal stenosis as a sequela of prolonged intubation, with a consequent increasing need for tracheal surgical repair. The aim of this study was to evaluate tracheal reconstruction by monolayered autologous mesenchymal stem cells (MSCs) with small intestine submucosa (SIS) in a rabbit model.

METHODS

Twelve male rabbits were randomly divided into three groups: rabbits with tracheal defects without reconstruction (untreated group, n=4), rabbits with tracheal defects given porcine small intestine submucosa graft (SIS group, n=4), and rabbits with tracheal defects that underwent transplantation of monolayered mesenchymal stem cells on SIS (SIS+MSC group, n=4). Histological and endoscopic analyses were performed by hematoxylin-eosin staining (H&E), Prussian blue staining and endoscopy.

RESULTS

Tracheal stenosis in the SIS+MSC group was minimal, compared to the untreated group and SIS group. Specimens obtained from the untreated and SIS groups showed severe infiltration of inflammatory cells and granulation tissue formation into the trachea. In the SIS+MSC group, however, minimal infiltration of the inflammatory cells and granulation tissue formation were observed. Twelve weeks following the operation, regeneration of pseudostratified columnar epithelium was confirmed by H&E staining with minimal inflammatory cell infiltration in the SIS+MSC group. Moreover, Prussian blue staining clearly demonstrated the presence of labeled MSCs in the regenerated tissue of SIS+MSC group.

CONCLUSIONS

These results demonstrate that tracheal reconstruction by MSCs with SIS is effective in rabbits with tracheal defects with minimal mortality and morbidity, which appears to be a promising strategy in the treatment of tracheal defects.

In vivo Noninvasive Small Animal Molecular Imaging

The remarkable efforts that are made on molecular imaging technologies demonstrate its potential importance and range of applications. The generation of disease-specific animal models, and the developments of target-specific probes and genetically encoded reporters are another important component. Continued improvements in the instrumentation, the identification of novel targets and genes, and the availability of improved imaging probes should be made. Multimodal imaging probes should provide easier transitions between laboratory studies, including small animal studies and clinical applications. Here, we reviewed basic strategies of noninvasive in vivo imaging methods in small animals to introducing the concept of molecular imaging.

Longitudinal tracking of human fetal cells labeled with super paramagnetic iron oxide nanoparticles in the brain of mice with motor neuron disease

Stem Cell (SC) therapy is one of the most promising approaches for the treatment of Amyotrophic Lateral Sclerosis (ALS). Here we employed Super Paramagnetic Iron Oxide nanoparticles (SPIOn) and Hoechst 33258 to track human Amniotic Fluid Cells (hAFCs) after transplantation in the lateral ventricles of wobbler (a murine model of ALS) and healthy mice. By in vitro, in vivo and ex vivo approaches we found that: 1) the main physical parameters of SPIOn were maintained over time; 2) hAFCs efficiently internalized SPIOn into the cytoplasm while Hoechst 33258 labeled nuclei; 3) SPIOn internalization did not alter survival, cell cycle, proliferation, metabolism and phenotype of hAFCs; 4) after transplantation hAFCs rapidly spread to the whole ventricular system, but did not migrate into the brain parenchyma; 5) hAFCs survived for a long time in the ventricles of both wobbler and healthy mice; 6) the transplantation of double-labeled hAFCs did not influence mice survival.

MRI of auto-transplantation of bone marrow-derived stem-progenitor cells for potential repair of injured arteries

Background

This study was to validate the feasibility of using clinical 3.0T MRI to monitor the migration of autotransplanted bone marrow (BM)-derived stem-progenitor cells (SPC) to the injured arteries of near-human sized swine for potential cell-based arterial repair.

Methodology

The study was divided into two phases. For in vitro evaluation, BM cells were extracted from the iliac crests of 13 domestic pigs and then labeled with a T2 contrast agent, Feridex, and/or a fluorescent tissue marker, PKH26. The viability, the proliferation efficiency and the efficacies of Feridex and/or PKH26 labeling were determined. For in vivo validation, the 13 pigs underwent endovascular balloon-mediated intimal damages of the iliofemoral arteries. The labeled or un-labeled BM cells were autotransplanted back to the same pig from which the BM cells were extracted. Approximately three weeks post-cell transplantation, 3.0T T2-weighted MRI was performed to detect Feridex-created signal voids of the transplanted BM cells in the injured iliofemoral arteries, which was confirmed by subsequent histologic correlation.

Principal Findings

Of the in vitro study, the viability of dual-labeled BM cells was 95–98%. The proliferation efficiencies of dual-labeled BM cells were not significantly different compared to those of non-labeled cells. The efficacies of Feridex- and PKH26 labeling were 90% and 100%, respectively. Of the in vivo study, 3.0T MRI detected the auto-transplanted BM cells migrated to the injured arteries, which was confirmed by histologic examinations.

Conclusion

This study demonstrates the capability of using clinical 3.0T MRI to monitor the auto-transplantation of BM cells that migrate to the injured arteries of large animals, which may provide a useful MRI technique to monitor cell-based arterial repair.

Use of perfluorocarbon nanoparticles for non-invasive multimodal cell tracking of human pancreatic islets

In vivo imaging of engraftment and immunorejection of transplanted islets is critical for further clinical development, with (1)H MR imaging of superparamagnetic iron oxide-labeled cells being the current premier modality. Using perfluorocarbon nanoparticles, we present here a strategy for non-invasive imaging of cells using other modalities. To this end, human cadaveric islets were labeled with rhodamine-perfluorooctylbromide (PFOB) nanoparticles, rhodamine-perfluoropolyether (PFPE) nanoparticles or Feridex as control and tested in vitro for cell viability and c-peptide secretion for 1 week. (19)F MRI, computed tomography (CT) and ultrasound (US) imaging was performed on labeled cell phantoms and on cells following transplantation beneath the kidney capsule of mice and rabbits. PFOB and PFPE-labeling did not reduce human islet viability or glucose responsiveness as compared with unlabeled cells or SPIO-labeled cells. PFOB- and PFPE-labeled islets were effectively fluorinated for visualization by (19)F MRI. PFOB-labeled islets were acoustically reflective for detection by US imaging and became sufficiently brominated to become radiopaque allowing visualization with CT. Thus, perfluorocarbon nanoparticles are multimodal cellular contrast agents that may find applications in real-time targeted delivery and imaging of transplanted human islets or other cells in a clinically applicable manner using MRI, US or CT imaging.

Improving the magnetic resonance imaging contrast and detection methods with engineered magnetic nanoparticles

Engineering and functionalizing magnetic nanoparticles have been an area of the extensive research and development in the biomedical and nanomedicine fields. Because their biocompatibility and toxicity are well investigated and better understood, magnetic nanoparticles, especially iron oxide nanoparticles, are better suited materials as contrast agents for magnetic resonance imaging (MRI) and for image-directed delivery of therapeutics. Given tunable magnetic properties and various surface chemistries from the coating materials, most applications of engineered magnetic nanoparticles take advantages of their superb MRI contrast enhancing capability as well as surface functionalities. It has been found that MRI contrast enhancement by magnetic nanoparticles is highly dependent on the composition, size and surface properties as well as the degree of aggregation of the nanoparticles. Therefore, understanding the relationships between these intrinsic parameters and the relaxivities that contribute to MRI contrast can lead to establishing essential guidance that may direct the design of engineered magnetic nanoparticles for theranostics applications. On the other hand, new contrast mechanism and imaging strategy can be developed based on the novel properties of engineered magnetic nanoparticles. This review will focus on discussing the recent findings on some chemical and physical properties of engineered magnetic nanoparticles affecting the relaxivities as well as the impact on MRI contrast. Furthermore, MRI methods for imaging magnetic nanoparticles including several newly developed MRI approaches aiming at improving the detection and quantification of the engineered magnetic nanoparticles are described.

In vivo and ex vivo MR imaging of slowly cycling melanoma cells

Slowly cycling cells are believed to play a critical role in tumor progression and metastatic dissemination. The goal of this study was to develop a method for in vivo detection of slowly cycling cells. To distinguish these cells from more rapidly proliferating cells that constitute the vast majority of cells in tumors, we used the well-known effect of label dilution due to division of cells with normal cycle and retention of contrast agent in slowly dividing cells. To detect slowly cycling cells, melanoma cells were labeled with iron oxide particles. After labeling, we observed dilution of contrast agent in parallel with cell proliferation in the vast majority of normally cycling cells. A small and distinct subpopulation of iron-retaining cells was detected by flow cytometry after 20 days of in vitro proliferation. These iron-retaining cells exhibited high expression of a biological marker of slowly cycling cells, JARID1B. After implantation of labeled cells as xenografts into immunocompromised mice, iron-retaining cells were detected in vivo and ex vivo by magnetic resonance imaging that was confirmed by Prussian Blue staining. Magnetic resonance imaging detects not only iron retaining melanoma cells but also iron positive macrophages. Proposed method opens up opportunities to image subpopulation of melanoma cells, which is critical for continuous tumor growth.

Emerging MRI methods in translational cardiovascular research

Cardiac magnetic resonance imaging (CMR) has become a reference standard modality for imaging of left ventricular (LV) structure and function and, using late gadolinium enhancement, for imaging myocardial infarction. Emerging CMR techniques enable a more comprehensive examination of the heart, making CMR an excellent tool for use in translational cardiovascular research. Specifically, emerging CMR methods have been developed to measure the extent of myocardial edema, changes in ventricular mechanics, changes in tissue composition as a result of fibrosis, and changes in myocardial perfusion as a function of both disease and infarct healing. New CMR techniques also enable the tracking of labeled cells, molecular imaging of biomarkers of disease, and changes in calcium flux in cardiomyocytes. In addition, MRI can quantify blood flow velocity and wall shear stress in large blood vessels. Almost all of these techniques can be applied in both pre-clinical and clinical settings, enabling both the techniques themselves and the knowledge gained using such techniques in pre-clinical research to be translated from the lab bench to the patient bedside.

In vivo stem cell tracking in neurodegenerative therapies

INTRODUCTION

Cell transplants to replace cells lost due to injury or degenerative diseases, for which there are currently no cures, are being pursued in a wide range of experimental models. Thus, the application of stem cell-based therapies to treat neurodegenerative and traumatic injuries is now a clinical reality. However, the monitoring of cellular grafts, non-invasively, is an important aspect of the ongoing efficacy and safety assessment of cell-based therapies. Hence, there is a need for non-invasive imaging techniques to ensure that transplants are not only administered to the relevant site, but also allow the monitoring of inappropriate cellular migration to improve our understanding of stem cell migration in the context of the whole organism.

AREAS COVERED

This review provides an up to date overview of molecular imaging approaches that have been used for visualizing and tracking transplanted stem cells, in vivo.

EXPERT OPINION

It's important to emphasize that the application of molecular imaging to interrogate transplanted cells may require one or even two imaging modalities to provide a reasonable assessment of transplanted cells in specific organs.

Articular cartilage repair using an intra-articular magnet and synovium-derived cells

The purpose of this study was to investigate the chondrogenic potential of magnetically labeled synovium-derived cells (M-SDCs) and examine whether M-SDCs could repair the articular cartilage using an intra-articular magnet after delivery to the lesion. Synovium-derived cells (SDCs) were cultured from the synovium of a rat knee, and were magnetically labeled with ferumoxides. M-SDCs were examined with a transmission electron microscope. A pellet culture system was used to evaluate the chondrogenic potential of M-SDCs in a magnetic field. In a rat model, allogeneic M-SDCs were injected into the knee after we made an osteochondral defect on the patellar groove and implanted an intra-articular magnet at the bottom of the defect. We histologically examined the defects at 48 h, 4 weeks, 8 weeks, and 12 weeks after treatment. Electron microscopy showed the transfection of ferumoxides into SDCs. The pellet cultures revealed the chondrogenic potential of M-SDCs in a magnetic field. M-SDCs accumulated in the osteochondral defect at 48 h after treatment, and we confirmed the regeneration of the articular cartilage at 4 weeks, 8 weeks, and 12 weeks after treatment using an intra-articular magnet. We demonstrated that articular cartilage defects could be repaired using an intra-articular magnet and M-SDCs. We believe that this system will be useful to repair human articular cartilage defects.