Transcription MRI: a new view of the living brain.

Altered gene activities are underlying causes of many neurological disorders. The ability to detect, image, and report endogenous gene transcription using magnetic resonance (MR) holds great potential for providing significant clinical benefits. In this review, we present the development of conjugates consisting of gene-targeting short nucleic acids (oligodeoxynucleotides, or sODN) and superparamagnetic iron oxide nanoparticles (SPION, an MR susceptibility T(2) agent) for reporting gene activity using transcription MRI (tMRI). We will discuss 1) the target specificity of sODN, 2) selection of contrast agents for tMRI, 3) the distribution and uptake, 4) sequence specificity, 5) histology of SPION and sODN, 6) data acquisition and quantitative analysis for tMRI, and 7) application of gene transcript-targeting nanoparticles in biology and medicine. We will also discuss methods of validating the correlation between results from conventional assays (in situ hybridization, PCR, histology Prussian blue stain and immunohistochemistry) in postmortem samples and retention of SPION-sODN using tMRI. The application of our novel contrast probe to report and target gene transcripts in the mesolimbic pathways of living mouse brains after amphetamine exposure will be discussed. Because of the targeting ability in the nucleic acid sequence, the concept of tMRI probes with complementary nucleic acid (antisense DNA or short interfering RNA) allows not only tracking, targeting, binding to intracellular mRNA, and manipulating gene action but also tracing cells with specific gene action in living brains. Transcription MRI will lend itself to myriad applications in living organs.