1 Department of Clinical Medicine - The MR Research Centre, Department of Clinical Medicine, Health, Aarhus University2 Department of Clinical Medicine - The Department of Orthopaedics E, ?AS, Department of Clinical Medicine, Health, Aarhus University3 Department of Clinical Medicine - The Department of Cardiological Medicine B, Department of Clinical Medicine, Health, Aarhus University4 Department of Bioscience - Zoophysiology, Department of Bioscience, Science and Technology, Aarhus University5 Department of Clinical Medicine - Comparative Medicine Lab, Department of Clinical Medicine, Health, Aarhus University6 Department of Clinical Medicine - Comparative Medicine Lab, Department of Clinical Medicine, Health, Aarhus University7 Department of Clinical Medicine - The Department of Orthopaedics E, ?AS, Department of Clinical Medicine, Health, Aarhus University8 Department of Clinical Medicine - The Department of Cardiological Medicine B, Department of Clinical Medicine, Health, Aarhus University9 Department of Bioscience - Zoophysiology, Department of Bioscience, Science and Technology, Aarhus University
Introduction: Regeneration is a widespread phenomenon functioning to maintain and restore normal form and function of cells, tissues, and in some cases organs or appendages. While mammals like mice and rats are typically employed as experimental models in regenerative research, these animals are generally restricted by their limited regenerative potential. Conversely, excellent animal models for regenerative studies exist in lower vertebrates such as the urodele amphibians (salamanders and newts), exemplified in the iconic Mexican axolotl (Ambystoma mexicanum) capable of regenerating whole limbs, tail, jaw, etc. Regeneration in this species is taking place by dedifferentiation of cells to form a collection of stem cells, the blastema, that proliferate and regenerate lost tissue without scar formation. The objective of this study was to non-invasively evaluate regeneration over time in a regenerating axolotl limb model. Superparamagnetic iron oxide particles (SPIOs) sensitive to MRI were used to track cells, and cell viability and regenerative capacity was investigated. Materials and Methods: Limb regeneration was induced by amputation of one hind limb of anaesthetised axolotls. The potential effect on cell viability of two commercially available SPIOs, Resovist (Bayer Schering Pharma, Hydrodynamic diameter 62 nm) and VSOP-C200 (Ferropharm, Hydrodynamic diameter 7 nm) and Resovist in conjugation with the transfection agent poly-L-lysin (PLL) was tested on cultures of axolotl blastema cells from 7 animals in vitro. Cell viability was quantified by PicoGreen-DNA quantification following 3 weeks of culturing. In vivo T1-weighted MRI-tracking of SPIO labelled blastema cells in the regenerating limb of 5 labelled axolotls was tested against 6 sham-operated animals for 84 days, using a clinically available 1.5 T system (Siemens), with a small radiofrequency loop-coil. Image data was processed by ImageJ and comparison of tissue signal intensity and rate of regeneration was conducted using SAS JMP8. Results: SPIO labelling with neither VSOP-C200, Resovist nor Resovist/PLL had any significant effect on blastema cell viability in vitro. MRI revealed that labelled tissue was clearly visibly in vivo 49 days after amputation and a significant decline in signal intensity of labelled limbs versus sham-operated limbs was evident throughout the complete regeneration period of 84 days. SPIO labelling displayed no significant effect on the rate of regeneration. Discussion: SPIO labelling for MRI cell tracking has shown promising results for regenerative therapies using stem cells. This study contributes to broaden the potential of SPIOs to track regenerating tissue in an inherently regenerative model, facilitating the use of SPIOs in future chemically or genetically induced regenerative therapies. In addition, this study concludes that SPIO labelling and MRI tracking of axolotl stem cells allow for non-invasive longitudinal studies in this model, increasing the potential to draw knowledge from the regenerative capacities of this species.