Research Highlight: Real Time Evaluation of Cell Viability Using Nanoprobes
Highlighting one of the latest articles in regenerative medicine
Evaluation of: Lee SK, Mortensen LJ, Lin CP, Tung CH (2014) An authentic imaging probe to track cell fate from beginning to end. Nature communications 5:5216.
Recent advances in implanted cell therapies could benefit from non-invasive monitoring of cell fate and location in real time. Without introducing transgenic reporters, visualizing cell viability in vivo has been a major challenge for determining the outcomes of implanted cells for tissue engineering applications due to limitations in probe retention and cell toxicity (1).
In this study, Lee et al. describe a nanoparticle-based system designed to serve as an in vivo live-dead probe and cell tracker (2). Nanoprobes were synthesized by layer-by-layer (LBL) assembly of cy5.5-labeled poly-L-lysine (PLL), FITC-labeled poly-D-lysine (PDL), and poly(acrylic acid) (PAA) on a gold nanoparticle (AuNP) core. Their previous work showed the use of a live cell tracker, L3, that consisted of alternating PLL and PAA layers (3). Fluorescence signal was initially quenched and could be visualized upon degradation by living cells. These particles were efficiently taken up by cells with no noticeable toxicity and could track viability in Jurkat cells for up to 21 days. The number of layers was optimized to give a prolonged signal. They then produced the D3 dead cell tracker by alternating PDL and PAA layers on AuNPs. As PDL is not naturally degraded in cells, fluorescence signal could only be seen during cell death. The live-dead cell tracker (DL2) consisted of layers of PDL, PAA, and PLL with a final size of 113 nm. In MDA-MB-231 human breast cancer cells, the DL2 label was taken up by cells for 4 days with stable cy5.5 signal and no discernible FITC signal until apoptosis or necrosis was induced. Human mesenchymal stem cells (hMSCs) were also labeled with the nanoprobes and tracked for 4 days in vitro. Similar to the labeled cancer cells, FITC signal was not detected until induced apoptosis. The labeled hMSCs were injected directly into the mouse ear pinna and monitored over 2 h using confocal microscopy, and the majority of the cells maintained only cy5.5 signal indicating viability. In an effort to determine the route of PDL degradation in the cells, the authors propose a reactive oxygen species (ROS)-mediated mechanism of degradation shown through an ROS quencher that decreases FITC signal upon initiation of apoptosis.
This work utilizes a multi-layered nanoprobe for visualizing cell viability in real time. Although the authors showed that the L3 probes lasted in cells for 21 days in Jurkat cells in a previous publication (3), they did not analyze the DL2 probes for longer than 4 days in this paper. More validation in vivo over longer periods of time for multiple cells types will be necessary for further application of these probes. Nonetheless, this study makes notable progress for regenerative medicine applications of in vivo cell tracking and could be used for the determination of the success of cell therapies.
References
1. Rodriguez-Porcel M (2010) In vivo imaging and monitoring of transplanted stem cells: clinical applications. Current cardiology reports 12(1):51-58.
2. Lee SK, Mortensen LJ, Lin CP, Tung CH (2014) An authentic imaging probe to track cell fate from beginning to end. Nature communications 5:5216.
3. Lee SK, Han MS, Tung CH (2012) Layered nanoprobe for long-lasting fluorescent cell label. Small 8(21):3315-3320.
Scheduled to be published in Volume 10 Issue 02 of Regenerative Medicine.