This work was financed through the Bristol Centre for Functional Nanomaterials, sponsored by the Engineering and Physical Sciences Research Council (EPSRC grant code EP/G036780/1).

Human mesenchymal stem cells (hMSC) were harvested from the proximal femur bone marrow of osteoarthritic patients undergoing total hip replacement surgery, in full accordance with Bristol Southmead Hospital Research Ethics Committee guidelines (reference #078/01) and after patient consent was obtained.
1. Magnetoferritin Synthesis and Purification
<ol>
	<li>General remarks
	<ol>
		<li>Perform magnetoferritin synthesis in a sealable, double-jacketed reaction vessel at 65 &#176;C under nitrogen atmosphere to restrict oxidation of the metal precursors. Inject metal salt solutions and hydrogen peroxide through access ports in the lid into the reaction vessel using a syringe pump.</li>
		<li>After magnetoferritin synthesis, purify the protein by anion-exchange chromatography (see section 1.4) to remove nanoparticles not enclosed in the protein cavity, followed by size-exclusion chromatography (see section 1.5) to isolate protein monomers. Determine protein concentration using a Bradford assay (see section 1.6).</li>
	</ol>
	</li>
	<li>Preparations prior to synthesis
	<ol>
		<li>Deoxygenate 500 ml of deionized water by placing a tube connected to a nitrogen gas cylinder in the water, sealing the vessel with cling film and bubbling through nitrogen gas for approximately 60 min.</li>
		<li>Heat a water bath connected to the double jacketed reaction vessel to 65 &#176;C. Add 75 ml of 50 mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) buffer (pH 8.6) into the reaction vessel, seal the vessel, and deoxygenate by bubbling nitrogen gas through the buffer solution for approximately 20 min. At the same time, stir the buffer solution using a magnetic stirrer.</li>
		<li>After deoxygenating the HEPES buffer solution, remove the tube supplying the nitrogen gas from the buffer and keep it suspended in the vessel to maintain a nitrogen atmosphere. Add apo-ferritin to achieve a final concentration of 3 mg/ml. Continue the magnetic stirring, but reduce the stirring speed if foaming occurs.</li>
		<li>Prepare a 25 mM stock solution of cobalt sulfate heptahydrate by dissolving 0.19 g in 50 ml of deoxygenated deionized water.</li>
		<li>Prepare a 25 mM solution of ammonium iron sulfate hexahydrate solution by dissolving 0.98 g in 100 ml of deoxygenated deionized water.</li>
		<li>Remove 2.5 ml of the ammonium iron sulfate hexahydrate solution and replace with 2.5 ml of the cobalt sulfate heptahydrate.</li>
		<li>Prepare an 8.33 mM solution of hydrogen peroxide in deoxygenated deionized water by first adding 965 &#956;l of a hydrogen peroxide solution (30% w/w) to 9 ml of deoxygenated deionized water, and then adding 1 ml of this sub-stock to 99 ml of deoxygenated deionized water.</li>
	</ol>
	</li>
	<li>Magnetoferritin synthesis
	<ol>
		<li>Inject 10.1 ml of both the iron-cobalt precursor and the hydrogen peroxide simultaneously into the apo-ferritin solution (magnetically stirred) at a flow rate of 0.15 ml/min using two syringe pump (total volume injected: 20.2 ml).</li>
		<li>During the injection period, deoxygenate another 500 ml of deionized water. 
		NOTE: The contents of the reaction vessel gradually adopt a dark brown color as the reaction proceeds.</li>
		<li>Shortly before completion of the first injection, prepare another 100 ml of iron-cobalt precursor and hydrogen peroxide solution using the freshly deoxygenated deionized water.</li>
		<li>Repeat the injection step described in 1.3.1 using freshly prepared solutions of iron-cobalt and hydrogen peroxide.</li>
		<li>During the injection period, deoxygenate another 500 ml of deionized water, and proceed as described in 1.3.3 and 1.3.4.</li>
		<li>After the third injection, leave the solution to mature for 15 min under stirring.</li>
		<li>Add 1.5 ml of a 1 M sodium citrate solution to the reaction vessel to chelate free metal ions in solution.</li>
		<li>Remove the solution from the reaction vessel into 50 ml centrifuge tubes, centrifuge for 30 min at 4,350 x g and pass the supernatant through a 0.22 &#181;m syringe filter. At this stage, the filtered supernatant can be stored at 4 &#176;C until purification.</li>
	</ol>
	</li>
	<li>Ion exchange chromatography
	<ol>
		<li>Load the sample onto a column (2.5 cm diameter, 20 cm long) containing a cationic matrix using a peristaltic pump at a flow rate of 10 ml/min.</li>
		<li>Wash the column with approximately 100 ml of running buffer (50 mM Tris(hydroxymethyl)aminomethane (Tris) buffer, pH 8.0) using a gradient pump at a flow rate of 10 ml/min.</li>
		<li>To elute the protein, wash the column at 10 ml/min with increasing concentrations of sodium chloride (NaCl) in Tris buffer: 150, 500 and 1,000 mM final NaCl concentration, 150 ml of each concentration.</li>
		<li>As the protein elutes at a NaCl concentration of 500 mM, collect it in 50 ml fractions using an automated fraction collector. 
		NOTE: For a detailed protocol of ion exchange chromatography consult previously published protocols.26</li>
	</ol>
	</li>
	<li>Size exclusion chromatography
	<ol>
		<li>Concentrate the 150 ml of magnetoferritin to a volume of approximately 2 ml using a 15 ml centrifugal filter unit followed by a 4 ml volume unit. Refer to the manufacturer&#39;s instructions of the centrifugal filter units (see material list) for a detailed protocol of this procedure.</li>
		<li>Load the concentrated sample onto a gel filtration column using an injection loop.</li>
		<li>Wash the column with running buffer (50 mM Tris buffer, pH 8.0, containing 150 mM NaCl) at a flow rate of 1.3 ml/min.</li>
		<li>Collect 6 ml fractions using an automated fraction collector. Protein monomers elute last. At this point, purified magnetoferritin can be stored at 4 &#176;C until cationization. 
		NOTE: For a detailed protocol of size exclusion chromatography using a gel filtration column consult previously published protocols.27</li>
	</ol>
	</li>
	<li>Determining protein concentration
	<ol>
		<li>Prepare ferritin standards of 0.06, 0.125, 0.25, 0.5 and 1 mg/ml in 50 mM Tris buffer using commercially available horse spleen ferritin. 
		NOTE: The protein concentration of commercially available ferritin is stated on the bottle. If not, contact the supplier for this information.</li>
		<li>Dilute magnetoferritin samples of unknown concentration in 50 mM Tris buffer until the color of the solution approximately matches the color of the 0.5 mg/ml standard. Make a note of the dilution factor.</li>
		<li>Add 10 &#956;l of each standard and magnetoferritin sample in triplicate into wells of a 96 well plate.</li>
		<li>Add 200 &#956;l of ready-made Bradford reagent to each well (refer to materials list for Bradford assay reagent details).</li>
		<li>Incubate at room temperature for 8 minutes.</li>
		<li>Measure the absorbance at &#955; = 595 nm using a microplate reader.</li>
		<li>Plot the mean absorbance of the ferritin standards as a function of protein concentration and use the slope and intercept of the linear fit to calculate the concentration of the magnetoferritin sample. 
		NOTE: Take into account the dilution factor that was used to adjust the magnetoferritin sample to the standard curve.</li>
	</ol>
	</li>
</ol>
2. Magnetoferritin Cationization
<ol>
	<li>General remarks 
	NOTE: For magnetoferritin cationization, N,N-dimethyl-1,3-propanediamine (DMPA) was coupled to aspartic and glutamic acid residues on the MF surface using N-(3-dimethylaminopropyl)-N&#39;-ethylcarbodiimide hydrochloride (EDC).
	<ol>
		<li>Carry out the reaction at room temperature under stirring. The protocol given below is for the cationization of 10 mg of magnetoferritin in a total volume of 5 ml (protein concentration 2 mg/ml). However, scale up or down by keeping the protein:DMPA:EDC ratios consistent, as well as the volumes of the buffer and magnetoferritin solution.</li>
		<li>Prior to the cationization reaction, dialyze the magnetoferritin samples into 50 mM phosphate buffer (pH 7) containing 50 mM NaCl. This is important to remove Tris elution buffer and avoid unwanted reactions between the Tris amine and the EDC-activated carboxylic acid residues. Determine the protein concentration after dialysis and adjust it to 4 mg/ml prior to cationization.</li>
	</ol>
	</li>
	<li>Cationization protocol
	<ol>
		<li>Weigh out 374 mg of DMPA and dissolve in 2.5 ml of 200 mM 2-(N-morpholino)ethanesulfonic acid (MES) buffer.</li>
		<li>Adjust the solution pH to approximately 7 using concentrated (6 M) hydrochloric acid (HCl). 
		CAUTION: Perform this step in a fume hood, as addition of the acid to DMPA releases toxic fumes!</li>
		<li>Add 2.5 ml of magnetoferritin solution at 4 mg/ml (final concentration of MES is 100 mM, final concentration of magnetoferritin is 2 mg/ml, total amount of protein is 10 mg).</li>
		<li>Add a magnetic stirrer and stir for 2 hr to equilibrate.</li>
		<li>Carefully adjust the solution to pH 5.0 using 1 M HCl.</li>
		<li>Add 141 mg of EDC powder to the DMPA/magnetoferritin solution.</li>
		<li>Continue stirring for 3.5 hr.</li>
		<li>Filter the solution through a 0.22 &#181;m syringe filter to remove any precipitates.</li>
		<li>Add the solution to dialysis tubing with a 12-14 kDa molecular weight cut-off and dialyze against 4 L of 50 mM phosphate buffer (pH 7) containing 50 mM NaCl for 2 days at 4 &#176;C, replacing the dialysis buffer at least three times during that period. 
		NOTE: At this point, cationized magnetoferritin can be stored at 4 &#176;C until further use.</li>
	</ol>
	</li>
</ol>
3. Human Mesenchymal Stem Cell Labeling with Cationized Magnetoferritin
<ol>
	<li>General remarks
	<ol>
		<li>Perform all cell culture using class II laminar flow cabinets and humidified incubators at 37 &#176;C and 5% carbon dioxide atmosphere.</li>
		<li>Culture cells as monolayers using 175 cm2 flasks and 20 ml of culture medium, which was replenished every 3 - 4 days. Culture medium consisted of Dulbecco&#39;s Modified Eagle&#39;s Medium (DMEM), containing 1,000 mg/L glucose, 10% (v/v) fetal bovine serum, 1% (v/v) penicillin/streptomycin solution, 1% (v/v) L-alanyl-L-glutamine solution and 5 ng/ml freshly supplemented human fibroblast growth factor.</li>
	</ol>
	</li>
	<li>Magnetic labeling of hMSC with cationized magnetoferritin
	<ol>
		<li>Seed 150,000 hMSCs into 25 cm2 flasks and leave to adhere overnight at 37 &#176;C.</li>
		<li>Filter sterilize cationized magnetoferritin through a 0.22 &#181;m syringe filter and determine the protein concentration.</li>
		<li>Keeping sterile conditions, prepare a 0.5 &#956;M solution of cationized magnetoferritin in phosphate buffered saline (PBS). Keep capped in a sterile culture hood until use.</li>
		<li>Wash the plated cells with 2 ml of room temperature PBS.</li>
		<li>Add 1 ml of the sterilized cationized magnetoferritin solution to the plated cells and incubate for the desired time period (1 min to 1 hr).</li>
		<li>Wash cells with PBS, and then harvest cells by adding 0.5 ml of trypsin/ethylenediaminetetraacetic acid (EDTA) and incubating at 37 &#176;C for 5 min.</li>
		<li>Add 1 ml of culture medium to inactivate the trypsin/EDTA, transfer the solution to a 15 ml centrifuge tube and centrifuge for 5 min at 524 x g.</li>
		<li>Discard the supernatant and re-suspend the cell pellet in 0.5 ml magnetic separation buffer, consisting of 0.5% (w/v) fetal bovine serum and 2 mM EDTA in PBS.</li>
	</ol>
	</li>
	<li>Magnetic cell separation
	<ol>
		<li>Attach the magnet to the multi stand, and add a magnetic separation column to the magnet. Place a pre-separation filter on the column.</li>
		<li>Place 0.5 ml of magnetic separation buffer in the pre-separation filter and let it run through both the filter and the column to wash and wet them.</li>
		<li>Place a 15 ml centrifuge tube under the column.</li>
		<li>Add 0.5 ml of the cell suspension in the filter reservoir of the magnetic separation column.</li>
		<li>When the reservoir is empty, add 0.5 ml of magnetic separation buffer.</li>
		<li>When the reservoir is empty, add further 0.5 ml of magnetic separation buffer.</li>
		<li>Repeat one more time (the total volume of magnetic separation buffer used for washing should be 1.5 ml). This wash step elutes all non-magnetized cells from the column (non-magnetized cell fraction).</li>
		<li>Remove the column from the magnet and place it in a fresh 15 ml centrifuge tube. Remove filter from the column reservoir.</li>
		<li>Add 1 ml of magnetic separation buffer to the reservoir and immediately push through the column using the plunger supplied by the manufacturer. This elutes the magnetized cells from the column into the centrifuge tube (magnetized cell fraction).</li>
		<li>Centrifuge both tubes for 5 min at 524 x g.</li>
		<li>Discard the supernatant and re-suspend the cell pellets in 0.3 ml PBS.</li>
		<li>Count the number of cells numbers in each fraction using a hemocytometer.</li>
		<li>Determine the fraction of magnetized cells, M(%), using the following equation: 
		<img alt="Equation" src="/files/ftp_upload/54785/54785eq1.jpg" />&#160; 
		where n(M) is the number of cells in the magnetized fraction and n(M+NM) is the sum of the number of cells in the magnetized and non-magnetized fractions.</li>
	</ol>
	</li>
	<li>Preparing hMSCs labeled with cationized magnetoferritin for iron quantification using inductively coupled plasma optical emission spectroscopy (ICP-OES) 
	NOTE: The protocol may need to be adapted for other ICP-OES instruments. It is advisable to consult with the responsible technician.
	<ol>
		<li>Centrifuge a known number of cells for 5 min at 524 x g.</li>
		<li>Remove the PBS supernatant and add 0.25 ml of 50% (v/v) nitric acid.</li>
		<li>Vortex mix the acidified cell suspension.</li>
		<li>Incubate at room temperature overnight.</li>
		<li>Add 4.75 ml of distilled water to each sample and vortex mix.</li>
		<li>Analyze with ICP-OES.28</li>
		<li>Normalize the measured iron content to the number of cells to determine a value for cellular iron content.</li>
	</ol>
	</li>
</ol>

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H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=T1+and+T2+relaxivity+of+intracellular+and+extracellular+USPIO+at+1.5+T+and+3T+clinical+MR+scanning.">T1 and T2 relaxivity of intracellular and extracellular USPIO at 1.5 T and 3T clinical MR scanning.</a> Eur. J. Radiol. 16 (3), 738-745 (2006).</li><li>Jubb, A. M., Allen, H. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Vibrational+Spectroscopic+Characterization+of+Hematite,+Maghemite,+and+Magnetite+Thin+Films+Produced+by+Vapor+Deposition.">Vibrational Spectroscopic Characterization of Hematite, Maghemite, and Magnetite Thin Films Produced by Vapor Deposition.</a> ACS Appl. Mater. Interfaces. 2 (10), 2804-2812 (2010).</li><li>Wang, Z. W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-pressure+x-ray+diffraction+and+Raman+spectroscopic+studies+of+the+tetragonal+spinel+CoFe2O4.">High-pressure x-ray diffraction and Raman spectroscopic studies of the tetragonal spinel CoFe2O4.</a> Phys. Rev. B. 68 (9), (2003).</li><li>Wong, K. K., Douglas, T., Gider, S., Awschalom, D. D., Mann, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Biomimetic+synthesis+and+characterization+of+magnetic+proteins+(magnetoferritin).">Biomimetic synthesis and characterization of magnetic proteins (magnetoferritin).</a> Chem. Mater. 10 (1), 279-285 (1998).</li><li>Okuda, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fe3O4+nanoparticles:+protein-mediated+crystalline+magnetic+superstructures.">Fe3O4 nanoparticles: protein-mediated crystalline magnetic superstructures.</a> Nanotechnology. 23 (41), 415601 (2012).</li><li>Perriman, A. W., C&#246;lfen, H., Hughes, R. W., Barrie, C. L., Mann, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Solvent-Free+Protein+Liquids+and+Liquid+Crystals.">Solvent-Free Protein Liquids and Liquid Crystals.</a> Angew. Chem. Int. Ed. 48 (34), 6242-6246 (2009).</li><li>Billotey, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=T-cell+homing+to+the+pancreas+in+autoimmune+mouse+models+of+diabetes:+in+vivo+MR+imaging.">T-cell homing to the pancreas in autoimmune mouse models of diabetes: in vivo MR imaging.</a> Radiology. 236 (2), 579-587 (2005).</li><li>Smirnov, P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In+vivo+cellular+imaging+of+lymphocyte+trafficking+by+MRI:+a+tumor+model+approach+to+cell-based+anticancer+therapy.">In vivo cellular imaging of lymphocyte trafficking by MRI: a tumor model approach to cell-based anticancer therapy.</a> Magn. Reson. Med. 56 (3), 498-508 (2006).</li><li>Wilhelm, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Magnetic+control+of+vascular+network+formation+with+magnetically+labeled+endothelial+progenitor+cells.">Magnetic control of vascular network formation with magnetically labeled endothelial progenitor cells.</a> Biomaterials. 28 (26), 3797-3806 (2007).</li><li>MacLean, I., Sanders, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cationized+ferritin+and+phosvitin+uptake+by+coated+vesicles+of+the+early+chick+embryo.">Cationized ferritin and phosvitin uptake by coated vesicles of the early chick embryo.</a> Anat. Embryol. 166 (3), 385-397 (1983).</li><li>Van Deurs, B., Nilausen, K., Faergeman, O., Meinertz, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Coated+pits+and+pinocytosis+of+cationized+ferritin+in+human+skin+fibroblasts.">Coated pits and pinocytosis of cationized ferritin in human skin fibroblasts.</a> Eur. J. Cell Biol. 27 (2), 270-278 (1982).</li><li>Xing, R. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Characterization+and+cellular+uptake+of+platinum+anticancer+drugs+encapsulated+in+apoferritin.">Characterization and cellular uptake of platinum anticancer drugs encapsulated in apoferritin.</a> J Inorg Biochem. 103 (7), 1039-1044 (2009).</li><li>Wong, K. K., Mann, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Biomimetic+synthesis+of+cadmium+sulfide-ferritin+nanocomposites.">Biomimetic synthesis of cadmium sulfide-ferritin nanocomposites.</a> Adv. Mater. 8 (11), 928-932 (1996).</li></ol>

The authors have nothing to disclose.

TEM of magnetoferritin samples stained with aurothioglucose revealed the successful mineralization of nanoparticles inside the protein cage. Electron diffraction and Raman analysis of the nanoparticle core indicated the presence of a cobalt ferrite, indicating successful doping of the nanoparticle core with cobalt. This demonstrates that mixed-oxide nanoparticles can successfully be mineralized within the apo-ferritin cavity. Furthermore, we have shown previously that cobalt doping can be varied by altering the amount of cobalt precursor added to the reaction mixture, which enables tuning of the magnetic properties.18
Magnetoferritin synthesis can be performed in a variety of vessels, as long as they are tightly sealable and have access ports through which reactants can be introduced (e.g., a three-neck round bottom flask). The reaction temperature should be maintained at 65 &#176;C either by placing the vessel in a water/oil bath or using a double-jacketed vessel. Here, we used a double-jacketed electrochemical cell setup to perform the synthesis. To guarantee successful synthesis, maintaining the correct pH and avoiding oxygen contamination of the aqueous solutions is crucial. Metal salt solutions should always be prepared freshly prior to use rather than in advance. Furthermore, commercial apoferritin solutions can vary in quality and affect synthesis outcome (e.g., size of nanoparticle core mineralized). It can help to dialyze the apoferritin solution into 50 mM HEPES buffer (pH 8.6) prior to synthesis to remove any residual reducing agent used by the manufacturer. It is useful to make a note of the batch number of the apo-ferritin solution used for synthesis, so it can be specifically requested from the manufacturer should additional material need to be purchased. Furthermore, the protein concentration of commercially available apo-ferritin should be stated on the bottle, which can be used to calculate the volume of apo-ferritin solution needed for synthesis. If this is not the case, contact the supplier for this information.
The advantage of gradual addition of metal salts and hydrogen peroxide &#8212; as presented here and in previous reports&#160;&#8212; is that mineralization of the nanoparticle core can be controlled such that different loading factors (i.e., nanoparticle sizes) can be achieved.33 Furthermore, it is possible to purify magnetoferritin further using a magnetic separation column, e.g., a column packed with stainless steel powder secured inside an electromagnet.34 Thus, highly monodisperse nanoparticle cores can be isolated from the bulk magnetoferritin sample. However, for magnetic cell labelling as presented here this is not required. A limitation of magnetoferritin synthesis is the relatively low synthesis yield of about 10%, and the relatively high cost of commercial apo-ferritin solutions. However, apo-ferritin may also be prepared from cheaply available horse spleen ferritin by following established de-mineralization protocols.16
Cationization of magnetoferritin was achieved by adding a molar ratio of 250 molecules of DMPA and 50 molecules of EDC per negatively charged residue (calculations based on the amino acid sequence of horse spleen ferritin). This excess of reagent over protein resulted in high cationization efficiencies, comparable also to previously reported results for the cationization of ferritin.35 For MALDI-TOF analysis, apoferritin and cationized apoferritin were used because of the excessive molecular mass of the magnetoferritin core. To yield high cationization efficiencies, optimal pH is also crucial. EDC-mediated crosslinking is most effective under mildly acidic conditions, and we found that pH 5 yielded optimal cationization results for magnetoferritin. However, for other proteins cationization pH may need to be optimized. Cationization at or close to the isoelectric point of the protein should be avoided, because this may lead to severe precipitation.
Stem cell magnetization with cationized magnetoferritin was highly efficient and could be achieved using incubation times well below 30 minutes. Even a one-minute incubation resulted in a cellular iron content of 3.6 pg, which is within the reported range required to influence T2 and T2* contrast for MRI.36,37 It is also remarkable that this efficient labeling is achieved with low extracellular iron concentrations. For example, previous studies using anionic nanoparticles report iron levels of 10 pg per cell after a 30-minute incubation period with 5 mM iron.38 In comparison, incubation with a cationized magnetoferritin solution containing 0.5 &#956;M protein corresponds to incubation with approximately 0.2 mM iron and also yields approximately 10 pg of iron per cell after 30 minutes. We were not able to clearly identify any endocytotic vesicles using TEM. However, previous studies using cationized ferritin found that internalization occurred within the first ten minutes of exposure.39,40 Cationized ferritin could be localized in coated vesicles, indicating clathrin- or caveolin-dependent endocytosis. The same studies also reported that after 30 minutes of incubation, cationized ferritin was still present on the cell surface, as well as in multivesicular bodies, resembling lysosomes.
Further applications could include cationization of apo-ferritin cages loaded with other nanoparticles and/or functional molecules, such as anti-cancer drugs41 or quantum dots42. Cationization of these ferritin constructs could result in faster and more efficient delivery of their cargo to cells.

Surface binding or internalization of superparamagnetic iron oxide nanoparticles (SPIONs) has enabled magnetization of a variety of cell types for applications such as imaging and remote manipulation.1 However, achieving sufficient cellular magnetization can be a challenge, particularly when the interaction between the SPION and the cell surface is weak.2 In the past, prolonged exposure or high SPION concentrations have been employed as strategies to overcome this challenge.3,4 Nevertheless, these strategies are problematic because they increase toxicity5,6 and have very limited success in cell types with low internalization rates, such as lymphocytes.7 To enhance cellular uptake of SPIONs, several surface functionalization approaches have been explored. For instance, antibodies have been used to promote receptor-mediated endocytosis,8 while non-specific uptake can be achieved using transfection agents9,10 or cell-penetrating species, such as HIV tat-peptide.11,12 However, antibody and peptide functionalization approaches are limited by expensive reagents and complex synthetic preparation, while transfection agents can induce nanoparticle precipitation and adversely affect cell function.13,14 
We recently reported the synthesis of chemically cationized magnetoferritin, a novel SPION which was highly effective in magnetizing human mesenchymal stem cells (hMSCs) using incubation times as short as one minute.15 Magnetoferritin is synthesized by reconstituting a SPION inside the de-mineralized cavity of the iron storage protein ferritin.16 This protein-based SPION combines many properties that make it well suited for cell magnetization, such as control over the magnetic properties of the magnetic core,17-19 and biocompatibility and aqueous solubility conferred by the protein shell. Furthermore, surface functionalization is easily achieved due to addressable amino acids that can be chemically20-22 or genetically modified.23-25 We have shown that chemical cationization of the acidic amino acid residues of the protein shell generates a stable nanoparticle that readily interacted with anionic domains on the cell surface leading to rapid and persistent cell magnetization. This procedure eliminates the need for laborious functionalization and lengthy incubation protocols, and due the non-specific labelling mechanism this rapid magnetization strategy should find wide-spread application in other cell types. Here, we present an in depth report of the ultra-fast cell labeling method, including detailed protocols of the synthesis, purification and surface functionalization of cationized magnetoferritin.

<table><tbody><tr><td>4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES)</td><td>Fisher Scientific</td><td>BPE310-1</td><td>powder; prepare a 1 M stock solution at pH 8.6 and dilute to 50&amp;nbsp;mM prior to use. Check the pH carefully prior to synthesis!</td></tr><tr><td>apoferritin from equine spleen</td><td>Sigma Aldrich</td><td>A3641</td><td>we used LOT# 081M7011V</td></tr><tr><td>cobalt sulfate heptahydrate&amp;nbsp;</td><td>Sigma Aldrich</td><td>C6768</td><td>prepare fresh solutions from the salt prior to synthesis</td></tr><tr><td>ammonium iron sulphate hexahydrate&amp;nbsp;</td><td>Sigma Aldrich</td><td>F1543</td><td>prepare fresh solutions from the salt prior to synthesis</td></tr><tr><td>hydrogen peroxide solution (30%)</td><td>Sigma Aldrich</td><td>216763</td><td>prepare fresh solutions from the salt prior to synthesis</td></tr><tr><td>sodium citrate</td><td>Sigma Aldrich</td><td>S1804</td><td>powder; a 1 M solution can be prepared and kept at room temperature for several months</td></tr><tr><td>Millex GP filter unit, 0.22&amp;nbsp;micron</td><td>Merck Millipore</td><td>SLGP033RS</td><td>syringe filter</td></tr><tr><td>Trizma base</td><td>Sigma Aldrich</td><td>T1503</td><td>powder; prepare a 1 M stock solution at pH 8.0 and dilute to 50&amp;nbsp;mM prior to use</td></tr><tr><td>sodium chloride</td><td>Sigma Aldrich</td><td>31434</td><td>poweder; add to buffers as required</td></tr><tr><td>Centriprep centrifugal filter units</td><td>Merck Millipore</td><td>4310</td><td>Ultracel YM-50 membrane, 12 ml volume; use for initial concentration until the magnetoferritin solution has been concentrated from about 150 ml to 20&amp;nbsp;ml</td></tr><tr><td>Amicon Ultra-4 centrifugal filter untis</td><td>Merck Millipore</td><td>UFC801024</td><td>Ultracel-10 membrane, 4&amp;nbsp;ml volume; use to concentrate magnetoferritin solution from about 20 ml to&amp;nbsp;2 ml</td></tr><tr><td>ANX Sepharose 4 Fast Flow</td><td>GE Healthcare</td><td>17-1287-04</td><td>we packed this column ourselves</td></tr><tr><td>HiPrep 26/60 Sephacryl S-300 HR column&amp;nbsp;</td><td>GE Healthcare</td><td>17-1196-01</td><td>this column was bought ready packed</td></tr><tr><td>&amp;Auml;KTA purifier system</td><td>GE Healthcare</td><td>28406264</td><td></td></tr><tr><td>sample pump P-960</td><td>GE Healthcare</td><td>18-6727-00</td><td>load sample at a flow rate of 10 ml/min</td></tr><tr><td>automated fraction collector Frac-950</td><td>GE Healthcare</td><td>18-6083-00</td><td></td></tr><tr><td>Bradford assay reagent</td><td>Sigma Aldrich</td><td>B6916</td><td>solution ready to use</td></tr><tr><td>Ferritin, Type I: from horse spleen</td><td>Sigma Aldrich</td><td>F4503</td><td>prepare ferritin standards from this solution to determine magnetoferritin concentration</td></tr><tr><td>&lt;em&gt;N,N&lt;/em&gt;-dimethyl-1,3-propanediamine (DMPA)</td><td>Sigma Aldrich</td><td>308110</td><td>&lt;strong&gt;CAUTION: &lt;/strong&gt;when adjusting the pH of a DMPA solution, perform this step in a fume hood</td></tr><tr><td>&lt;em&gt;N&lt;/em&gt;-(3-dimethylaminopropyl)-&lt;em&gt;N&lt;/em&gt;&amp;rsquo;-ethylcarbodiimide hydrochloride (EDC)</td><td>Sigma Aldrich</td><td>E6383</td><td>keep in freezer but bring to room temperature before opening the bottle</td></tr><tr><td>2-(&lt;em&gt;N&lt;/em&gt;-morpholino)ethanesulfonic acid (MES)</td><td>AppliChem</td><td>A0689,0500</td><td>powder; prepare a 200 M stock solution at pH 5</td></tr><tr><td>dialysis tubing cellulose membrane</td><td>Sigma Aldrich</td><td>D9652</td><td>soak 10 min in deionized water before use</td></tr><tr><td>Dulbecco&amp;rsquo;s Modified Eagle&amp;rsquo;s Medium (DMEM), 1,000 mg/L glucose</td><td>Sigma Aldrich</td><td>D5546</td><td>warm in 37 &amp;deg;C water bath before use</td></tr><tr><td>fetal bovine serum</td><td>Sigma Aldrich</td><td>F7524</td><td>add to stock DMEM bottle, 10% (v/v) final concentration</td></tr><tr><td>penicillin/streptomycin solution</td><td>Sigma Aldrich</td><td>P0781</td><td>add to stock DMEM bottle, 1% (v/v) final concentration</td></tr><tr><td>glutamax solution</td><td>Gibco</td><td>35050-087</td><td>add to stock DMEM bottle, 1% (v/v) final concentration</td></tr><tr><td>human fibroblast growth factor</td><td>PeproTech</td><td>100-18B</td><td>add to DMEM freshly into cell culture flask with each media change; final concentration 5 ng/ml</td></tr><tr><td>phosphate buffered saline</td><td>Sigma Aldrich</td><td>D8537</td><td>sterile solution, for cell cultrue</td></tr><tr><td>trypsin/EDTA solution</td><td>Sigma Aldrich</td><td>E5134</td><td>keep in freezer and defrost in 37&amp;nbsp;&amp;deg;C water bath before use</td></tr><tr><td>ethylenediaminetetraacetic acid (EDTA)</td><td>Sigma Aldrich</td><td>E5134</td><td>powder; make a 2 mM solution in PBS&amp;nbsp;</td></tr><tr><td>bovine serum albumin</td><td>Sigma Aldrich</td><td>A7030</td><td>add 0.5% (w/v) into 2&amp;nbsp;mM EDTA solution in PBS; carefully stir with magnetic stirrer, avoid foaming; filter sterilize through a 0.22 micron syringe filter</td></tr><tr><td>MACS multi stand&amp;nbsp;</td><td>Miltenyi Biotec</td><td>130-042-303</td><td>for attachment of MACS magnet</td></tr><tr><td>MACS MS columns</td><td>Miltenyi Biotec</td><td>130-042-201</td><td>disposable; intended for single use, but if sterility is not required, they can re-used: wash with deionized water and 100% ethanol, and placed in a drying oven; discard if you observe rusty patches</td></tr><tr><td>MiniMACS separator magnet</td><td>Miltenyi Biotec</td><td>130-042-102</td><td>can be bought as a starter kit, together with columns and stands</td></tr><tr><td>MACS column pre-separation filter</td><td>Miltenyi Biotec</td><td>130-041-407</td><td>30 mm filter</td></tr><tr><td>Nitric acid solution, 64-66%</td><td>Sigma Aldrich</td><td>7006</td><td></td></tr><tr><td>Titrando 907, syringe pump</td><td>Metrohm</td><td>2.907.0020</td><td></td></tr><tr><td colspan="4">&lt;strong&gt;Equipment used to characterize magnetoferritin and cationized magnetoferritin&lt;/strong&gt;</td></tr><tr><td>SpectraMax M5</td><td>Molecular Devices</td><td></td><td>Used to measure absorbance in the Bradford assay</td></tr><tr><td>JEM 1200 EX</td><td>JEOL</td><td></td><td>Used for TEM imaging of magnetoferritin</td></tr><tr><td>InVia Raman spectrometer</td><td>Renishaw</td><td></td><td>Used for Raman spectroscopy</td></tr><tr><td>Torus DPSS laser</td><td>Laser Quantum</td><td></td><td>Used for Raman spectroscopy</td></tr><tr><td>Bruker UltrafleXtreme</td><td>Applied Biosystems</td><td></td><td>Used for MALDI-TOF analysis of apoferritin and cationized apoferritin</td></tr><tr><td>ZetaSizer Nano-ZS</td><td>Malvern Instruments</td><td></td><td>Used to measure hydrodynamic diameter and zeta potential of magnetoferritin and cationized magnetoferritin</td></tr><tr><td>Magnetic Property Measurement System</td><td>Quantum Design</td><td></td><td>Used to measure magnetic saturation moment and magnetic susceptibility</td></tr><tr><td>Magnetom Skyra</td><td>Siemens</td><td></td><td>Used to determine longitudinal and transverse relaxivity&amp;nbsp;</td></tr><tr><td>Tecnai 12 BioTwin Spirit</td><td>FEI</td><td></td><td>Used for TEM imaging of hMSC labeled with cationized magnetoferritin</td></tr><tr><td>710 ICP-OES</td><td>Agilent</td><td></td><td>Used to determine iron content in cells</td></tr></tbody></table>

synthesis of cationized magnetoferritin for ultra-fast magnetization of cells

Many important biomedical applications, such as cell imaging and remote manipulation, can be achieved by labeling cells with superparamagnetic iron oxide nanoparticles (SPIONs). Achieving sufficient cellular uptake of SPIONs is a challenge that has traditionally been met by exposing cells to elevated concentrations of SPIONs or by prolonging exposure times (up to 72 hr). However, these strategies are likely to mediate toxicity. Here, we present the synthesis of the protein-based SPION magnetoferritin as well as a facile surface functionalization protocol that enables rapid cell magnetization using low exposure concentrations. The SPION core of magnetoferritin consists of cobalt-doped iron oxide with an average particle diameter of 8.2 nm mineralized inside the cavity of horse spleen apo-ferritin. Chemical cationization of magnetoferritin produced a novel, highly membrane-active SPION that magnetized human mesenchymal stem cells (hMSCs) using incubation times as short as one minute and iron concentrations as lows as 0.2 mM.

TEM was used to confirm nanoparticle mineralization inside the apoferritin cavity and determine the average core size (Figures 1A and 1B). Image analysis of unstained magnetoferritin samples gave an average core diameter of 8.2 &#177; 0.7 nm, and aurothioglucose stain confirmed the presence of nanoparticles within the protein cage. Note that the images show a magnetoferritin sample that was further purified using magnetic separation to isolate uniform nanoparticle cores. Magnetoferritin samples that were not magnetically purified have a slightly broader core size distribution.29 Analysis of the structure of the magnetoferritin core using selected-area electron diffraction indicated the possible presence of the inverse spinel structure based on magnetite (Fe3O4) and/or maghemite (&#947;-Fe2O3), as well as the spinel structure due to Co3O4. Furthermore, Raman spectra revealed peaks attributed to Fe3O4, small amounts of &#947;-Fe2O3, and a cobalt ferrite (Figure 1 C). ICP-OES analysis of magnetoferritin showed an average of 102 &#956;g of iron and 0.9 &#956;g of cobalt per milligram of magnetoferritin.
A schematic is included, illustrating the subsequent cationization step (Figure 2 A). The hydrodynamic diameter of magnetoferritin and cationized magnetoferritin was 11.8 &#177; 1.1 nm and 12.5 &#177; 1.4 nm, respectively, as determined by dynamic light scattering. The cationization efficiency of covalent DMPA-coupling to magnetoferritin was assessed using zeta potentiometry and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry. The zeta potential changed from -10.5 mV for MF to + 8.3 mV for cationized magnetoferritin, confirming a change in surface potential from negative to positive (Table 1). Mass spectrometry experiments found a subunit molecular weight of 20.1 kDa for native apo-ferritin and 21.1 kDa for cationized apo-ferritin (Figure 2 B). This mass increase corresponds to approximately 12 coupled DMPA molecules per protein subunit, and the cationization of 288 residues on the entire 24-subunit protein.
Magnetic saturation and susceptibility were measured using SQUID magnetometry, and transverse and longitudinal relaxivity were measured using magnetic resonance imaging. Magnetic properties were similar for magnetoferritin and cationized magnetoferritin, indicating that cationization had negligible impact on the magnetic properties of the enclosed SPION (Table 1). Furthermore, these properties are similar to other iron oxide based nanoparticles,19,30 demonstrating that cationized magnetoferritin would be as suitable as conventional SPION-based MRI contrast agents in enhancing imaging contrast.
After a 30-minute exposure, the cell surface was densely covered with cationized magnetoferritin (Figure 3 A). However, after one week, no nanoparticles were found on the cell surface (Figure 3 B). Cationized magnetoferritin was remarkably effective at magnetically labelling hMSCs. Notably, exposing the cells to cationized magnetoferritin for one minute resulted in the magnetization of 92% of the cell population and the delivery of 3.6 pg of iron per cell. Increasing the incubation time to 15 minutes resulted in the magnetization of the entire cell population (Figure 3 C).
<img alt="Figure 1" src="/files/ftp_upload/54785/54785fig1.jpg" /> 
Figure 1: Characterization of magnetoferritin cores doped with 5% cobalt. TEM images of magnetoferritin stained with aurothioglucose (A) and unstained (B). Inset shows corresponding electron diffraction with magnetite indices. Scale bar: 20 nm. (C) Raman spectrum for magnetoferritin. The arrows indicate the main Raman vibration modes for cobalt ferrite (T2g), magnetite and maghemite (both A1g).31,32 The laser wavelength used was 532 nm. (Image adapted from Okuda et al.18). Note that this magnetoferritin sample had been further purified using magnetic separation, which isolated uniformly loaded magnetoferritin particles. <a href="http://cloudflare.jove.com/files/ftp_upload/54785/54785fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/54785/54785fig2.jpg" /> 
Figure 2: Cationization of magnetoferritin. A) Solvent accessible surface area representations showing the distribution of acidic (red) and basic (yellow) amino acid residues on the protein surface. Magnetoferritin (1) is modified to cationized magnetoferritin (2) by carbodiimide-mediated crosslinking of DMPA to acidic amino acid residues on the protein surface (3). B) Mass spectrometry analysis of apo-ferritin and cationized apo-ferritin subunits. Mass-to-charge (m/z) spectrum of apoferritin (ApoF) and cationized apoferritin (cat-ApoF) generated by MALDI-TOF. A mass increase from 20.1 kDa to 21.1 kDa is observed after cationization. (Image adapted from Correia Carreira et al.15) <a href="http://cloudflare.jove.com/files/ftp_upload/54785/54785fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/54785/54785fig3.jpg" /> 
Figure 3: Magnetic labeling and cell separation of hMSCs incubated with cationized magnetoferritin. A) TEM image of hMSCs after a 30-minute incubation with cationized magnetoferritin. The arrow indicates the presence of magnetoferritin cores densely packed on the cell surface. Scale bars: 200 nm. B) TEM image of hMSC one week after labeling. The cell surface is clear of cationized magnetoferritin. C) Investigating the rapidity of magnetic labeling: 92% of the cell population were magnetized after a one-minute exposure with 0.5 &#956;M cationized magnetoferritin, and the entire cell population was magnetized within 15 minutes. Iron content per cell was determined using ICP-OES. Average and standard deviation from three biological replicates are shown. (Image adapted from Correia Carreira et al.15) <a href="http://cloudflare.jove.com/files/ftp_upload/54785/54785fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td></td>
			<td>MF</td>
			<td>cat-MF</td>
		</tr>
		<tr>
			<td>Hydrodynamic diameter [nm]</td>
			<td>11.8 &#177; 1.1</td>
			<td>12.5 &#177; 1.4</td>
		</tr>
		<tr>
			<td>Zeta potential [mv]</td>
			<td>(-)10.4 &#177; 0.2</td>
			<td>8.3 &#177; 0.7</td>
		</tr>
		<tr>
			<td>Magnetic saturation moment [Am2 kg-1]</td>
			<td>54.9 &#177; 1.6</td>
			<td>55.3 &#177; 1.4</td>
		</tr>
		<tr>
			<td>Mass susceptibility [x 104 m3 kg-1]</td>
			<td>1.75 &#177; 0.08</td>
			<td>1.75&#177; 0.07</td>
		</tr>
		<tr>
			<td>Longitudinal relaxivity [mM-1 sec-1]</td>
			<td>2.6 &#177; 0.1</td>
			<td>2.3 &#177; 0.1</td>
		</tr>
		<tr>
			<td>Transverse relaxivity [mM-1 sec-1]</td>
			<td>44.6 &#177; 1.0</td>
			<td>52.8 &#177; 0.8</td>
		</tr>
	</tbody>
</table>
Table 1: Physicochemical characterization of magnetoferritin (MF) and cationized magnetoferritin (cat-MF). (Table adapted from Correia Carreira et al.15)

Watch this Scientific Journal Video about Synthesis of Cationized Magnetoferritin for Ultra-fast Magnetization of Cells at JoVE.com

Synthesis of Cationized Magnetoferritin for Ultra-fast Magnetization of Cells

A protocol for the synthesis and cationization of cobalt-doped magnetoferritin is presented, as well as a method to rapidly magnetize stem cells with cationized magnetoferritin.

Many important biomedical applications, such as cell imaging and remote manipulation, can be achieved by labeling cells with superparamagnetic iron oxide ...

synthesis-cationized-magnetoferritin-for-ultra-fast-magnetization

Universidad de Cartagena UDEC

Research

JoVE Journal

Bioengineering

9.9K Views.  University of Bristol. A protocol for the synthesis and cationization of cobalt-doped magnetoferritin is presented, as well as a method to rapidly magnetize stem cells with cationized magnetoferritin.

Video: Synthesis of Cationized Magnetoferritin for Ultra-fast Magnetization of Cells

Labeling Stem Cells with Ferumoxytol, an FDA-Approved Iron Oxide Nanoparticle

Stem cell based therapies offer significant potential for the field of regenerative medicine. However, much remains to be understood regarding the in vivo kinetics of transplanted cells. A non-invasive method to repetitively monitor transplanted stem cells in vivo would allow investigators to directly monitor stem cell transplants and identify successful or unsuccessful engraftment outcomes.
A wide range of stem cells continues to be investigated for countless applications. This protocol focuses on 3 different stem cell populations: human embryonic kidney 293 (HEK293) cells, human mesenchymal stem cells (hMSC) and induced pluripotent stem (iPS) cells. HEK 293 cells are derived from human embryonic kidney cells grown in culture with sheared adenovirus 5 DNA. These cells are widely used in research because they are easily cultured, grow quickly and are easily transfected. hMSCs are found in adult marrow. These cells can be replicated as undifferentiated cells while maintaining multipotency or the potential to differentiate into a limited number of cell fates. hMSCs can differentiate to lineages of mesenchymal tissues, including osteoblasts, adipocytes, chondrocytes, tendon, muscle, and marrow stroma. iPS cells are genetically reprogrammed adult cells that have been modified to express genes and factors similar to defining properties of embryonic stem cells. These cells are pluripotent meaning they have the capacity to differentiate into all cell lineages 1. Both hMSCs and iPS cells have demonstrated tissue regenerative capacity in-vivo.
Magnetic resonance (MR) imaging together with the use of superparamagnetic iron oxide (SPIO) nanoparticle cell labels have proven effective for in vivo tracking of stem cells due to the near microscopic anatomical resolution, a longer blood half-life that permits longitudinal imaging and the high sensitivity for cell detection provided by MR imaging of SPIO nanoparticles 2-4. In addition, MR imaging with the use of SPIOs is clinically translatable. SPIOs are composed of an iron oxide core with a dextran, carboxydextran or starch surface coat that serves to contain the bioreactive iron core from plasma components. These agents create local magnetic field inhomogeneities that lead to a decreased signal on T2-weighted MR images 5. Unfortunately, SPIOs are no longer being manufactured. Second generation, ultrasmall SPIOs (USPIO), however, offer a viable alternative. Ferumoxytol (FerahemeTM) is one USPIO composed of a non-stoichiometric magnetite core surrounded by a polyglucose sorbitol carboxymethylether coat. The colloidal, particle size of ferumoxytol is 17-30 nm as determined by light scattering. The molecular weight is 750 kDa, and the relaxivity constant at 2T MRI field is 58.609 mM-1 sec-1 strength4. Ferumoxytol was recently FDA-approved as an iron supplement for treatment of iron deficiency in patients with renal failure 6. Our group has applied this agent in an &ldquo;off label&rdquo; use for cell labeling applications. Our technique demonstrates efficient labeling of stem cells with ferumoxytol that leads to significant MR signal effects of labeled cells on MR images. This technique may be applied for non-invasive monitoring of stem cell therapies in pre-clinical and clinical settings.

We describe a technique for labeling and tracking stem cells with FDA-approved, superparamagnetic iron oxide (SPIO), ferumoxytol (Feraheme). This cellular imaging technique that utilizes magnetic resonance (MR) imaging for visualization, is readily accessible for long-term monitoring and diagnosis of successful or unsuccessful stem cell engraftments in patients.

Stem cell based therapies offer significant potential for the field of regenerative medicine. However, much remains to be understood regarding the in vivo ...

Medicine

In vitro Labeling of Human Embryonic Stem Cells for Magnetic Resonance Imaging

Human embryonic stem cells (hESC) have demonstrated the ability to restore the injured myocardium. Magnetic resonance imaging (MRI) has emerged as one of the predominant imaging modalities to assess the restoration of the injured myocardium. Furthermore, ex-vivo labeling agents, such as iron-oxide nanoparticles, have been employed to track and localize the transplanted stem cells. However, this method does not monitor a fundamental cellular biology property regarding the viability of transplanted cells. It has been known that manganese chloride (MnCl2) enters the cells via voltage-gated calcium (Ca2+) channels when the cells are biologically active, and accumulates intracellularly to generate T1 shortening effect. Therefore, we suggest that manganese-guided MRI can be useful to monitor cell viability after the transplantation of hESC into the myocardium.
In this video, we will show how to label hESC with MnCl2 and how those cells can be clearly seen by using MRI in vitro. At the same time, biological activity of Ca2+-channels will be modulated utilizing both Ca2+-channel agonist and antagonist to evaluate concomitant signal changes.

In this video, we are showing how to label human embryonic stem cells (hESC) with manganese chloride (MnCl2) which can enter cells via voltage-gated calcium channels when the cells are biologically active. Additionally, we show the use of MnCl2 as cellular MRI contrast agent to determine the in vitro viability of hESC.

Human embryonic stem cells (hESC) have demonstrated the ability to restore the injured myocardium. Magnetic resonance imaging (MRI) has emerged as one of ...

Biology

DNA-magnetic Particle Binding Analysis by Dynamic and Electrophoretic Light Scattering

Isolation of DNA using magnetic particles is a field of high importance in biotechnology and molecular biology research. This protocol describes the evaluation of DNA-magnetic particles binding via dynamic light scattering (DLS) and electrophoretic light scattering (ELS). Analysis by DLS provides valuable information on the physicochemical properties of particles including particle size, polydispersity, and zeta potential. The latter describes the surface charge of the particle which plays major role in electrostatic binding of materials such as DNA. Here, a comparative analysis exploits three chemical modifications of nanoparticles and microparticles and their effects on DNA binding and elution. Chemical modifications by branched polyethylenimine, tetraethyl orthosilicate and (3-aminopropyl)triethoxysilane are investigated. Since DNA exhibits a negative charge, it is expected that zeta potential of particle surface will decrease upon binding of DNA. Forming of clusters should also affect particle size. In order to investigate the efficiency of these particles in isolation and elution of DNA, the particles are mixed with DNA in low pH (~6), high ionic strength and dehydration environment. Particles are washed on magnet and then DNA is eluted by Tris-HCl buffer (pH = 8). DNA copy number is estimated using quantitative polymerase chain reaction (PCR). Zeta potential, particle size, polydispersity and quantitative PCR data are evaluated and compared. DLS is an insightful and supporting method of analysis that adds a new perspective to the process of screening of particles for DNA isolation.

This protocol describes the synthesis of magnetic particles and evaluation of their DNA-binding properties via dynamic and electrophoretic light scattering. This method focuses on monitoring changes in particle size, their polydispersity and zeta potential of particle surface which play major role in binding of materials such as DNA.

Isolation of DNA using magnetic particles is a field of high importance in biotechnology and molecular biology research. This protocol describes the ...

Chemistry

Enrich and Expand Rare Antigen-specific T Cells with Magnetic Nanoparticles

We have developed a tool to both enrich and expand antigen-specific T cells. This can be helpful in cases such as to A) detect the existence of antigen-specific T cells, B) probe the dynamics of antigen-specific responses, C) understand how antigen-specific responses affect disease state such as autoimmunity, D) demystify heterogeneous responses for antigen-specific T cells, or E) utilize antigen-specific cells for therapy. The tool is based on a magnetic particle that we conjugate antigen-specific and T cell co-stimulatory signals, and that we term as artificial antigen presenting cells (aAPCs). Consequently, since the technology is simple to produce, it can easily be adopted by other laboratories; thus, our purpose here is to describe in detail the fabrication and subsequent use of the aAPCs. We explain how to attach antigen-specific and co-stimulatory signals to the aAPCs, how to utilize them to enrich for antigen-specific T cells, and how to expand antigen-specific T cells. Furthermore, we will highlight engineering design considerations based on experimental and biological information of our experience with characterizing antigen-specific T cells.

Antigen-specific T cells are difficult to characterize or utilize in therapies due to their extremely low frequency. Herein, we provide a protocol to develop a magnetic particle which can bind to antigen-specific T cells to enrich these cells and then to expand them several hundred-fold for both characterization and therapy.

We have developed a tool to both enrich and expand antigen-specific T cells. This can be helpful in cases such as to A) detect the existence of ...

Engineering

Cell Labeling and Targeting with Superparamagnetic Iron Oxide Nanoparticles

Targeted delivery of cells and therapeutic agents would benefit a wide range of biomedical applications by concentrating the therapeutic effect at the target site while minimizing deleterious effects to off-target sites. Magnetic cell targeting is an efficient, safe, and straightforward delivery technique. Superparamagnetic iron oxide nanoparticles (SPION) are biodegradable, biocompatible, and can be endocytosed into cells to render them responsive to magnetic fields. The synthesis process involves creating magnetite (Fe3O4) nanoparticles followed by high-speed emulsification to form a poly(lactic-co-glycolic acid) (PLGA) coating. The PLGA-magnetite SPIONs are approximately 120 nm in diameter including the approximately 10 nm diameter magnetite core. When placed in culture medium, SPIONs are naturally endocytosed by cells and stored as small clusters within cytoplasmic endosomes. These particles impart sufficient magnetic mass to the cells to allow for targeting within magnetic fields. Numerous cell sorting and targeting applications are enabled by rendering various cell types responsive to magnetic fields. SPIONs have a variety of other biomedical applications as well including use as a medical imaging contrast agent, targeted drug or gene delivery, diagnostic assays, and generation of local hyperthermia for tumor therapy or tissue soldering.

Targeted cell delivery is useful in a variety of biomedical applications. The goal of this protocol is to use superparamagnetic iron oxide nanoparticles (SPION) to label cells and thereby enable magnetic cell targeting approaches for a high degree of control over cell delivery and localization.

Targeted delivery of cells and therapeutic agents would benefit a wide range of biomedical applications by concentrating the therapeutic effect at the ...

Synthesis of Functionalized Magnetic Nanoparticles, Their Conjugation with the Siderophore Feroxamine and its Evaluation for Bacteria Detection

In the present work, the synthesis of magnetic nanoparticles, its coating with SiO2, followed by its amine functionalization with (3-aminopropyl)triethoxysilane (APTES) and its conjugation with deferoxamine, a siderophore recognized by Yersinia enterocolitica, using a succinyl moiety as a linker are described.
Magnetic nanoparticles (MNP) of magnetite (Fe3O4) were prepared by solvothermal method and coated with SiO2 (MNP@SiO2) using the St&#246;ber process followed by functionalization with APTES (MNP@SiO2@NH2). Then, feroxamine was conjugated with the MNP@SiO2@NH2 by carbodiimide coupling to give MNP@SiO2@NH2@Fa. The morphology and properties of the conjugate and intermediates were examined by eight different methods including powder X-Ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and energy dispersive X-Ray (EDX) mapping. This exhaustive characterization confirmed the formation of the conjugate. Finally, in order to evaluate the capacity and specificity of the nanoparticles, they were tested in a capture bacteria assay using Yersinia enterocolitica.

This work describes protocols for the preparation of magnetic nanoparticles, its coating with SiO2, followed by its amine functionalization with (3-aminopropyl)triethoxysilane (APTES) and its conjugation with deferoxamine using a succinyl moiety as a linker. A deep structural characterization description and a capture bacteria assay using Y. enterocolitica for all the intermediate nanoparticles and the final conjugate are also described in detail.

In the present work, the synthesis of magnetic nanoparticles, its coating with SiO2, followed by its amine functionalization with ...

Stable Aqueous Suspensions of Manganese Ferrite Clusters with Tunable Nanoscale Dimension and Composition

Manganese ferrite clusters (MFCs) are spherical assemblies of tens to hundreds of primary nanocrystals whose magnetic properties are valuable in diverse applications. Here we describe how to form these materials in a hydrothermal process that permits the independent control of product cluster size (from 30 to 120 nm) and manganese content of the resulting material. Parameters such as the total amount of water added to the alcoholic reaction media and the ratio of manganese to iron precursor are important factors in achieving multiple types of MFC nanoscale products. A fast purification method uses magnetic separation to recover the materials making production of grams of magnetic nanomaterials quite efficient. We overcome the challenge of magnetic nanomaterial aggregation by applying highly charged sulfonate polymers to the surface of these nanomaterials yielding colloidally stable MFCs that remain non-aggregating even in highly saline environments. These non-aggregating, uniform, and tunable materials are excellent prospective materials for biomedical and environmental applications.

We report a one-pot hydrothermal synthesis of manganese ferrite clusters (MFCs) that offers independent control over material dimension and composition. Magnetic separation allows rapid purification while surface functionalization using sulfonated polymers ensures the materials are non-aggregating in biologically relevant medium. The resulting products are well positioned for biomedical applications.

Manganese ferrite clusters (MFCs) are spherical assemblies of tens to hundreds of primary nanocrystals whose magnetic properties are valuable in diverse ...

Imaging the Motion of Fully Reconstituted Cdc45/Mcm2-7/GINS (CMG)

Hybrid Ensemble and Single-molecule Assay to Image the Motion of Fully Reconstituted CMG

Eukaryotes have one replicative helicase known as CMG, which centrally organizes and drives the replisome, and leads the way at the front of replication forks. Obtaining a deep mechanistic understanding of the dynamics of CMG is critical to elucidating how cells achieve the enormous task of efficiently and accurately replicating their entire genome once per cell cycle. Single-molecule techniques are uniquely suited to quantify the dynamics of CMG due to their unparalleled temporal and spatial resolution. Nevertheless, single-molecule studies of CMG motion have thus far relied on pre-formed CMG purified from cells as a complex, which precludes the study of the steps leading up to its activation. Here, we describe a hybrid ensemble and single-molecule assay that allowed imaging at the single-molecule level of the motion of fluorescently labeled CMG after fully reconstituting its assembly and activation from 36 different purified S. cerevisiae polypeptides. This assay relies on the double functionalization of the ends of a linear DNA substrate with two orthogonal attachment moieties, and can be adapted to study similarly complex DNA-processing mechanisms at the single-molecule level.

Author Spotlight: Investigating the Motion Dynamics of the Eukaryotic Replisome Components at the Single-Molecule Level

We report a hybrid ensemble and single-molecule assay to directly image and quantify the motion of fluorescently labeled, fully reconstituted Cdc45/Mcm2-7/GINS (CMG) helicase on linear DNA molecules held in place in an optical trap.

Eukaryotes have one replicative helicase known as CMG, which centrally organizes and drives the replisome, and leads the way at the front of replication ...

Biochemistry

Using Magnetometry to Monitor Cellular Incorporation and Subsequent Biodegradation of Chemically Synthetized Iron Oxide Nanoparticles

Magnetic nanoparticles, made of iron oxide, present a peculiar interest for a wide range of biomedical applications for which they are often internalized in cells and then left within. One challenge is to assess their fate in the intracellular environment with reliable and precise methodologies. Herein, we introduce the use of the vibrating sample magnetometer (VSM) to precisely quantify the integrity of magnetic nanoparticles within cells by measuring their magnetic moment. Stem cells are first labeled with two types of magnetic nanoparticles; the nanoparticles have the same core produced via a fast and efficient microwave-based nonaqueous sol gel synthesis and differ in their coating: the commonly used citric acid molecule is compared to polyacrylic acid. The formation of 3D cell-spheroids is then achieved via centrifugation and the magnetic moment of these spheroids is measured at different times with the VSM. The obtained moment is a direct fingerprint of the nanoparticles&#8217; integrity, with decreasing values indicative of a nanoparticle degradation. For both nanoparticles, the magnetic moment decreases over culture time revealing their biodegradation. A protective effect of the polyacrylic acid coating is also shown, when compared to citric acid.

Iron oxide nanoparticles are synthesized via a nonaqueous sol gel procedure and coated with anionic short molecules or polymer. The use of magnetometry for monitoring the incorporation and biotransformations of magnetic nanoparticles inside human stem cells is demonstrated using a vibrating sample magnetometer (VSM).

Magnetic nanoparticles, made of iron oxide, present a peculiar interest for a wide range of biomedical applications for which they are often internalized ...

Preparation and In Vitro Characterization of Magnetized miR-modified Endothelial Cells

To date, the available surgical and pharmacological treatments for cardiovascular diseases (CVD) are limited and often palliative. At the same time, gene and cell therapies are highly promising alternative approaches for CVD treatment. However, the broad clinical application of gene therapy is greatly limited by the lack of suitable gene delivery systems. The development of appropriate gene delivery vectors can provide a solution to current challenges in cell therapy. In particular, existing drawbacks, such as limited efficiency and low cell retention in the injured organ, could be overcome by appropriate cell engineering (i.e., genetic) prior to transplantation. The presented protocol describes the efficient and safe transient modification of endothelial cells using a polyethyleneimine superparamagnetic magnetic nanoparticle (PEI/MNP)-based delivery vector. Also, the algorithm and methods for cell characterization are defined. The successful intracellular delivery of microRNA (miR) into human umbilical vein endothelial cells (HUVECs) has been achieved without affecting cell viability, functionality, or intercellular communication. Moreover, this approach was proven to cause a strong functional effect in introduced exogenous miR. Importantly, the application of this MNP-based vector ensures cell magnetization, with accompanying possibilities of magnetic targeting and non-invasive MRI tracing. This may provide a basis for magnetically guided, genetically engineered cell therapeutics that can be monitored non-invasively with MRI.

Preparation and In Vitro Characterization of Magnetized miR-modified Endothelial Cells

This manuscript describes the efficient, non-viral delivery of miR to endothelial cells by a PEI/MNP vector and their magnetization. Thus, in addition to genetic modification, this approach allows for magnetic cell guidance and MRI detectability. The technique can be used to improve the characteristics of therapeutic cell products.

To date, the available surgical and pharmacological treatments for cardiovascular diseases (CVD) are limited and often palliative. At the same time, gene ...

Synthesis of Cationized Magnetoferritin for Ultra-fast Magnetization of Cells

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Chapters in this video

Labeling Stem Cells with Ferumoxytol, an FDA-Approved Iron Oxide Nanoparticle

In vitro Labeling of Human Embryonic Stem Cells for Magnetic Resonance Imaging

DNA-magnetic Particle Binding Analysis by Dynamic and Electrophoretic Light Scattering

Enrich and Expand Rare Antigen-specific T Cells with Magnetic Nanoparticles

Cell Labeling and Targeting with Superparamagnetic Iron Oxide Nanoparticles

Synthesis of Functionalized Magnetic Nanoparticles, Their Conjugation with the Siderophore Feroxamine and its Evaluation for Bacteria Detection

Stable Aqueous Suspensions of Manganese Ferrite Clusters with Tunable Nanoscale Dimension and Composition

Hybrid Ensemble and Single-molecule Assay to Image the Motion of Fully Reconstituted CMG

Using Magnetometry to Monitor Cellular Incorporation and Subsequent Biodegradation of Chemically Synthetized Iron Oxide Nanoparticles

Preparation and In Vitro Characterization of Magnetized miR-modified Endothelial Cells