Supplementary MaterialsSupplementary Information 41467_2017_482_MOESM1_ESM. the observation of cell behavior in health and disease1. In addition to its importance for basic research, cell tracking has many potential applications in regenerative and Ryanodine individualized medicine and it facilitates the development of new diagnostic tools and therapeutic strategies2C5. Numerous imaging techniques are used to visualize cells in vivo, including ultrasound, optical imaging, magnetic resonance imaging (MRI) and positron emission tomography (PET). These methods require conceptually different labeling and detection strategies that each have inherent advantages and disadvantages. Direct cell labeling makes use of radioactive, fluorescent or paramagnetic compounds, which are, however, eventually washed out and get diluted. Thus, longitudinal and quantitative monitoring of cells becomes challenging. In contrast, strategies based on stable expression of a chromosomally integrated reporter transgene permit long-term labeling of cells and their progeny1. The Cre/lox recombination system has emerged as a powerful tool to generate time- and tissue-specific mouse mutants6, 7. In addition, this technology can be used to genetically label specific cell populations to map their fate Ryanodine during development8 or in adult mice in the context of physiological or pathophysiological processes9. For genetically inducible fate mapping, cell type-specific expression of the tamoxifen-inducible CreERT2 recombinase is usually combined with Cre-activatable reporter transgenes that are driven by ubiquitous promoters. With this approach, stable, inheritable reporter gene expression can be achieved in a distinct cell population labeled by Cre recombination at a predetermined time. Cre reporter transgenes encoding histochemical, fluorescent or bioluminescent reporter proteins have been integrated into the murine Rosa26 (R26) locus, which is accessible to the transcriptional machinery in most if not all cell types10. With the currently available R26 Cre reporter mouse lines, however, non-invasive quantitative detection of labeled cells in vivo at the whole-body Ryanodine level is not possible, because detection of the aforementioned reporter proteins relies on either ex lover vivo methods requiring tissue fixation, invasive methods with a small field of view such as intravital microscopy, or semi-quantitative non-invasive methods such as bioluminescence imaging. PET is usually a powerful non-invasive imaging modality in both preclinical and clinical settings. It has a high sensitivity and generates quantitative data, and recent improvements in PET-MRI scanner technology enable simultaneous acquisition of functional and morphological information from KGFR living mice11. Reporter genes for detection of cells by PET cause the accumulation of radiolabeled probes on or in reporter gene-expressing cells12, 13. One such PET reporter gene is the computer virus type 1 thymidine kinase (HSV1-tk). It is used in combination with 18F- or 124I-labeled nucleoside analogues, which are phosphorylated by HSV1-tk, but not by mammalian thymidine kinases. In their non-phosphorylated form, PET tracers such as 9-(4[18F]-Fluoro-3-[hydroxymethyl]butyl)guanine are cell-permeable, but after phosphorylation by HSV1-tk they are retained inside the cells. HSV1-tk or an improved variant that enables PET with higher sensitivity, sr39tk14, 15, have Ryanodine been used for PET imaging of rodents, larger animals and humans12, 13. Cre-mediated activation of HSV1-tk expression has been achieved via the delivery of an adenovirus transporting a Cre-activatable HSV1-tk construct to the liver16 or myocardium17 of mice expressing Cre in the respective target tissues. However, transgenic mice with a chromosomally integrated Cre-responsive PET Ryanodine reporter gene have not been explained to date. In such a mouse line, Cre-expressing cell populations will be labeled for PET imaging through Cre-mediated activation of reporter gene expression.