The production of contrast agents sensitive to neuronal signaling events is a rate-limiting step in the development of molecular-level functional magnetic resonance imaging (molecular fMRI) approaches for studying the brain. High-throughput generation and evaluation of potential probes are possible using techniques for macromolecular engineering of protein-based contrast agents. In an initial exploration of this strategy, we used the method of directed evolution to identify mutants of a bacterial heme protein that allowed detection of the neurotransmitter dopamine in vitro and in living animals. The directed evolution method involves successive cycles of mutagenesis and screening that could be generalized to produce contrast agents sensitive to a variety of molecular targets in the nervous system.
Magnetic resonance imaging Directed evolution Protein engineering Cytochrome P450 Dopamine
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We are grateful to Robert Langer and to collaborators in the Arnold and Jasanoff laboratories for participation in the initial research and discussions that helped establish this protocol. We thank Eric Brustad for preliminary data used in some of the figures. MGS acknowledges the Fannie and John Hertz Foundation and the Paul and Daisy Soros Fellowship for support. This work was funded by NIH grant numbers R01-DA28299 and DP2-OD2441 (New Innovator Award) to A.J., and NIH grant number R01-GM068664 and a grant from the Caltech Jacobs Institute for Molecular Medicine to F.H.A.
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