A Comparative Study of Upconverting Nanoparticles Versus Lentiviral GFP Transduction for Labeling Mesenchymal Stem Cells

Transcription

1 A Comparative Study of Upconverting Nanoparticles Versus Lentiviral GFP Transduction for Labeling Mesenchymal Stem Cells Artem Kutikov, B.S., Liang Zhao, Gang Han, Ph.D., Jie Song, Ph.D.. University of Massachusetts Medical School, Worcester, MA, USA. Disclosures: A. Kutikov: None. L. Zhao: None. G. Han: None. J. Song: None. Introduction: Bone marrow-derived mesenchymal stem cells (MSCs) are multipotent cells that have shown great promise for bone and cartilage tissue repair and regeneration. Exogenous MSCs, delivered via a biomaterial scaffold/carrier to a tissue defect, may expedite healing through the secretion of soluble factors, proliferation and differentiation into other cell types, or a combination of both. To fully understand how these exogenously delivered stem/progenitor cells participate in tissue repair in a spatially and temporally defined manner, real-time in vivo tracking of these cells using suitable imaging probes that do not interfere with normal cellular functions is desired. Organic dyes, fluorescent proteins, or fluorescent nanoparticles have been traditionally explored for cellular tracking. However, such approaches typically suffer from poor imaging characteristics (e.g. limited tissue penetration depth, tissue autofluorescence). Recently, upconverting nanoparticles (UCNPs) that can absorb and emit near-infrared light, thereby minimizing soft tissue background and enabling deeper tissue penetration than conventional imaging probes, have been developed. 1 Such UCNPs are being investigated for labeling and tracking MSCs, 2 and explored as potential tools for orthopedic tissue engineering applications. To enable these investigations, direct comparison of how UCNP labeling of MSCs vs. the more widely used technique of fluorescent protein expression impacts the cellular viability, proliferation and differentiation is necessary, yet has not been carried out. Here we compare the performance of polyethylenimine (PEI)- conjugated (α-naybf 4 :Tm 3+ )/CaF 2 UCNPs vs. lentiviral transduction of green fluorescent protein (GFP) for labeling rat bone marrow-derived MSCs (rmscs). Specifically, we examined the effect of varying UCNP loading doses on MSC labeling efficiency, cell viability, proliferation, and osteogenic and adipogenic differentiation in vitro. Meanwhile, we compared three constitutive promoters and a range of lentiviral loads for optimal GFP transduction efficiency and minimal cytotoxicity in rmscs. The impact of GFP-transduction on rmsc proliferation and differentiation in vitro is compared with UCNP labeling. Methods: α-(naybf 4 :0.5% Tm 3+ )/CaF 2 UCNPs were synthesized as previously described 1. PEI (MW 25KD) was conjugated to the surface of the UCNPs by ligand exchange and subsequent covalent linkage to poly(acrylic acid). UCNPs were added to passage 1 rmscs at concentrations of 0, 20, 50, or 100 µg/ml in MSC expansion media (αmem + 20% FBS + 1% L-Glutamine + 1% Pen- Strep). Following a 24-h exposure, the particles were removed by 3 washes with PBS. The cells were fixed in 3.7% methanol-free formaldehyde in PBS and imaged with a 2-photon microscope (Zeiss LSM 7 MP, 980nm excitation). To determine the cytotoxicity of the UCNPs, an MTT cell viability assay was performed at 24 h after UCNP exposure at varying doses. The viability of untreated rmscs was used a control. To optimize the GFP lentiviral transduction in rmscs, the ubiquitous promoters Ubiquitin C (UBC), cytomegalovirus (CMV), and human elongation factor-1 alpha (EF1α) were tested at three multiplicities of infection (MOI: 5, 25, 50). Passage 1 rmscs were seeded at a density of ~10,000 cells/cm 2 in transduction media (αmem, 20% heat-inactivated FBS, 1% L-Glutamine) and cultured for 24 h before the respective lentiviral vectors (Cellomics Technology) were added. The plates were spun for 30 min (1200 g, 32 ºC) to increase transduction efficiency. After 24-h transduction, the media were changed to expansion media (αmem, 20% FBS, 1% L-Glutamine, 1% Pen-Strep) and cells were cultured for an additional 24 h before being imaged on an epifluorescence microscope (Zeiss Axiovert 40 CFL). Viability of cells was determined at 24 h following the lentiviral transduction of rmscs by EF1α-GFP. For both UCNP and GFP treated cells, proliferation was determined by MTT assay over 72 h and presented as fold of changes over the initial viable cells. Osteogenic and adipogenic differentiations were performed by incubating the UCNP-rMSCs, GFP-rMSCs, or untreated rmscs in osteogenic or adipogenic media 2 for 14 days. Alizarin red O and oil red O staining was performed to assess the potency of osteogenic and adipogenic differentiation, respectively. Results: Uptake of PEI-conjugated UCNPs by the rmscs positively correlated with the UCNP exposure doses (Fig. 1A). The PEIcoated particles appear to be distributed within the cytoplasm with some degrees of clustering but not inside the cell nucleus. At a concentration of 100 µg/ml, over 90% of the rmscs were labeled with UCNPs. Lentiviral vectors were chosen for the GFP transduction due to their stable integration into the genome. Coupled with a ubiquitously expressed promoter, the GFP should be continuously expressed throughout cell division and differentiation. Of the lentiviral vectors with GFP driven by UBC, CMV, and EF1α promoters, CMV and EF1α led to the higher levels of GFP transduction in rmscs, with EF1α providing the most uniform GFP signal (Fig. 1B). GFP intensity increased with increasing MOI. Both the UCNP and GFP-based methods were able to label a high percentage of cells. Unlike the internalized UCNP particles, the GFP signal appeared to be more evenly distributed throughout the cell, enabling individual cells to be readily identified without requiring a counterstain of the cell nuclei. The MTT cell viability assay revealed that the rmsc viability dropped with increasing UCNP treatment dose. At the dose of 100

2 µg/ml, which resulted in the best UCNP labeling efficiency, only 63% of cells were viable by 24 h compared to the untreated control (Fig. 1C). By contrast, greater than 90% of the GFP-labeled rmscs were viable at the highest multiplicity of infection (MOI=50) (Fig. 1D). The viable UCNP- or GFP-labeled rmscs underwent normal proliferation over a 72-h period compared to unlabeled controls (Fig. 1E). Both UCNP- and GFP-labeled rmscs were able to differentiate into adipocytes and osteoblast upon culture induction, although the osteogenesis of UCNP-labeled cells appear to be noticeably compromised. 2 Quantitative comparison of the responses of these labeled rmscs to osteogenic and adipogenic inductions are undergoing. Discussion: The favorable imaging characteristics of UCNPs (tissue penetration, low background, stable signal) make them promising tools for tracking the fate of transplanted MSCs in tissue engineering. However, whereas high cytoplasmic labeling efficiency (>90%) of rmscs can be achieved with both PEI-coated UCNPs and lentiviral GFP transduction, further optimization of the UCNP-labeling is required to reduce the initial cytotoxicity and negative impact on osteogenic differentiation. Compared to the UCNP labeling, the GFP signal is also more evenly distributed within the cytoplasm and the individual cells can be identified more readily than UCNP labeled cells in vitro. These data suggest that developing UCNP coatings with reduced cytotoxicity and tendency for particle aggregation would be important for their application in MSC tracking. Significance: Successful development of cytocompatible UCNP labels for MSCs can enable the long-term tracking of cell fate to gain mechanistic insight of their role in bone tissue regeneration. Acknowledgments: This work was supported by the University of Massachusetts Medical School and NIH grant R01AR References: 1. Chen G, Shen J, Ohulchanskyy TY, et al. ACS nano. 2012;6(9):

3 2. Zhao L, Kutikov A, Shen J, Duan C, Song J, Han G. Theranostics. 2013;3(4):249-

4

5 ORS 2014 Annual Meeting Poster No: 0307

6