Gene targeting MRI: nucleic acid-based imaging and applications.

Gene action plays a role in neural cell migration, learning processes, stress response, drug addiction, cancer, mental health, psychiatric and neurological disorders, as well as neurodegenerative diseases. Studies also show that upregulation of certain gene activities in neurons may contribute to the development of Alzheimer's disease and other progressive cognitive disorders many decades after the alteration itself occurs. Endogenous, environmental stress-related, or drug-induced chemical imbalances in the brain affect the homeostasis of gene activities in neurons in specific brain regions and contribute to the comorbidity of mental illness and substance dependence. On the other hand, altered gene activities are also a necessary part of repair processes after brain injury. Our general well-being is governed by the highly regulated gene activities in our brains. A better understanding of gene activities and their relationship to the progression of neurological disease can help the research and medical communities develop necessary measures for early intervention, as well as plan more appropriate interventions or new therapeutic approaches that can benefit a broad spectrum of patients who will be or have been affected by brain diseases. We developed a non-invasive imaging technique that allows real-time assessment of gene transcription profiles in live brains. This imaging method has the potential to provide first-hand information about the progression of neurological disorders by gene targeting and cell typing, and it could elucidate a surrogate marker for therapeutic efficacy for future planning of treatments for human diseases. We have established a workable and reproducible MRI technique in live rodent brains.

Evaluation of DNA aptamers directed to thrombin as potential thrombus imaging agents.

Two DNA aptamers directed against two separate exosites on human alpha-thrombin were evaluated for thrombus-imaging potential. Aptamer ODN 1 is directed to the thrombin substrate binding site (exosite 1). Our finding that ODN 1 competes with fibrin for binding to exosite 1 on thrombin suggests that ODN 1 will not be useful for thrombus imaging. Aptamer ODN 2 is directed against the thrombin heparin binding site (exosite 2). ODN 2 bound to model thrombi that were formed either by clotting purified fibrinogen with thrombin, or by recalcifying citrated plasma. As the thrombin content of thrombi was increased the rate of ODN 2 uptake into preformed thrombi increased, whereas the rate of release of ODN 2 out of preformed thrombi decreased. This in vitro data suggested that ODN 2 might be useful for thrombus imaging because it can bind to exosite 2 on fibrin-bound thrombin. However, in a rabbit jugular vein model using thrombus supplemented with human thrombin, ODN 2 uptake was equal to the ovalbumin control, and did not reflect thrombin content. While the in vitro results with ODN 2 were consistent with thrombus imaging, the rapid clearance of ODN 2 from circulation, combined with slow mass transfer in the clot, seem to work against in vivo thrombin-dependent imaging or washout analysis.