Mesenchymal stem cells (MSCs) have proved to be a promising and abundant cell source for tissue and organ repair in regenerative medicine. However, the cell fate, distribution and migration of these transplanted cells are still unclear due to the limited tracking methods. It is desirable to develop a biocompatible and photostable probe to label the MSCs for long-term tracking without affecting the cell proliferation and potency. Herein we apply a recently developed nanoprobe system, in which di(thiophene-2-yl)-diketopyrrolopyrrole (DPP) is covalently linked in the middle of polycaprolactone (PCL) forming the PCL-DPP-PCL polymer complex. Although the PCL-DPP-PCL nanoparticles uptaken by the MSCs did not affect the cell viability, it was interesting that they exhibited different effects on the multilineage potency of the MSCs in the subsequent differentiation in vitro. Specifically, we found that the PCL-DPP-PCL labeling was unfavorable to the MSC osteogenic differentiation, whereas the labeled MSCs exhibited the same adipogenic and chondrogenic differentiations compared to the unlabeled controls as verified by gene expressions and histological staining. Furthermore, the PCL-DPP-PCL nanoparticles remained strong fluorescence intensity even after 4 weeks of differentiation. This study indicated that PCL-DPP-PCL nanoparticles could be used for long-term cell tracing in MSC differentiation into adipogenic and chondrogenic lineages.

Chondrogenic differentiation of human mesenchymal stem cells (MSCs) in three-dimensional hydrogel holds promise as a method for repairing injured articular cartilage. Given MSC plasticity (its potential to mature into alternative lineages), nondestructive monitoring is critical for the optimization of chondrogenic differentiation conditions and the evaluation of the final product. However, conventional validation/assessments of the differentiation process (i.e., quantitative reverse transcription polymerase chain reaction [qRT-PCR] and histology) are end-point assays requiring disruption of the sample. This report introduces molecular beacon (MB)-based nanosensors to achieve noninvasive monitoring of chondrogenic differentiation. These nanosensors consist of biodegradable poly(lactic-co-glycolic acid) nanoparticles (PLGA NPs) encapsulating MBs to detect Type II Collagen (Col2) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNAs that serve as lineage-specific and housekeeping biomarkers, respectively. The sustainable release of MBs from MB-NPs allows longitudinal monitoring of MSCs undergoing chondrogenic differentiation over a period of 28 days. Dual-colored MB loading ensures accurate assessment of Col2 mRNA expression level, where potential heterogeneity in nanosensor uptake and retention by MSCs are taken into account. When normalized nanosensor signal was compared against qRT-PCR result, a tight correlation was observed (R = 0.9301). Finally, nanosensor usage was compatible with MSC potency with minimal influence on chondrogenic, adipogenic, and osteogenic differentiation.

The major goal of bringing nanoparticles and microbubbles together is to provide multifunctionality."

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