An in vitro and in vivo evaluation of the capacity of the gene mms6 to be an MRI reporter gene
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Magnetic resonance imaging (MRI) reporter genes produce MRI signal in response to the molecular environment of the cells in which they are expressed. With an MRI scanner, the signal is detected and used to produce an image of the cells. We hypothesized that the magnetotactic bacterial gene mms6 has the potential to function as an MRI reporter gene. Magnetotactic bacteria produce magnetic iron oxide crystals in intracellular organelles called magnetosomes. mms6 encodes an iron-binding, magnetosome membrane protein Mms6, which plays a role in regulating the size and shape of the iron oxide crystals found within the magnetosomes. To test our hypothesis, several mammalian cell lines were transfected with mms6, and mms6-positive clones were genetically engineered. We then used MRI to image these clones in vitro. When the cells were incubated with iron-supplemented culture media, the mms6-positive clones produced more MRI image contrast than mms6-negative cells. Through a systematic process of elimination, the mms6-positive clone that generated the most in vitro MRI image contrast was identified. This clone, named 9L4S, was composed of mms6-positive rat glioma (9L) cells and was used for intracellular iron studies and in vivo imaging. The results of electron microscopy and optical emission spectrometry support the theory that mms6-positive clones enhance MRI image contrast due to an increase in intracellular iron. The main objective of this research was to assess the ability of mms6 to function as an in vivo MRI reporter gene, so a flank tumor animal model was created. Without any exogenous iron supplementation, tumors composed of mms6-positive cells produced greater negative contrast on an MRI image than mms6-negative cells. These results demonstrate that mms6 can be considered for use in studies requiring an MRI reporter gene.