Effects of freezing on mesenchymal stem cells labeled with gold nanoparticles.

Abstract

Stem cell therapies are a promising treatment for many patients suffering from diseases with poor prognosis. However, clinical translation is inhibited by a lack of in vivo monitoring techniques to track stem cells throughout the course of treatment. Ultrasound-guided photoacoustic imaging of nanoparticle-labeled stem cells may be a solution. To allow photoacoustic tracking, stem cells must be labeled with an optically absorbing contrast agent. Gold nanoparticles are one option due to their cytocompatibility and strong optical absorption in the near-infrared region. However, stem cell labeling can require up to 24-hour incubation with nanoparticles in culture prior to use. Although stem cell monitoring is critically needed, the additional preparation time may not be feasible - it is cost prohibitive and stem cell treatments should be readily available in emergency situations as well as scheduled procedures. To remedy this, stem cells can be labeled prior to freezing and long-term storage. While it is well known that stem cells retain their cellular function after freezing, storage, and thawing, the impact of gold nanoparticles on this process has yet to be investigated. Therefore, we assessed the viability, multipotency, and photoacoustic activity of gold nanosphere-labeled mesenchymal stem cells after freezing, storage, and thawing for one week, one month, or two months and compared to unlabeled, naïve mesenchymal stem cells which were frozen, stored, and thawed at the same time points. Results indicated no substantial change in viability as assessed by the MTT assay. Differentiation, observed through adipogenesis and osteogenesis, was also comparable to controls. Lastly, strong photoacoustic signals and similar photoacoustic spectral signatures remained. Further studies involving more diverse stem cell types and nanoparticles are required, but our data suggests that function and imaging properties of nanoparticle-labeled stem cells are maintained after freezing and storage, which improves translation of stem cell monitoring techniques by simplifying integration with clinical protocols.