An eye-opening method for imaging probe delivery
Michael J. Lander
After a severe stroke or head injury, patients hardly benefit from surgical procedures involving their brains. Yet to assess the extent of post-trauma neuron repair or blood vessel proliferation, medical practitioners usually have to extract a brain tissue sample. Although important for planning correct treatment, the process may slow recovery and could lead to complications. In response to this problem, Philip K. Liu and colleagues at Massachusetts General Hospital and at Harvard Medical School, both in Charlestown, have created a fully noninvasive diagnostic technique that could be modified to report a wide range of disorders.
A magnetic resonance image of mouse brain tissue taken after the introduction of composite probes reveals regions of extensive gliosis in one hemisphere (left). A corresponding histological view confirms glial cell proliferation (right). Courtesy of Philip K. Liu, Harvard Medical School.
Inspiration for the scientists’ work came from a series of observations, starting with the fact that conditions causing blood-brain barrier leakage allow neuronal magnetic resonance probes to penetrate otherwise sealed-off brain cells. Delivery of the probes, they found, could be achieved invasively through infusion into the cerebral ventricles. Ventricular fluid, however, flows into the lymphatic system — such as the lymph vessels under the conjunctival sac of the eye. Thus, the team reasoned, the probes could be delivered painlessly to patients’ brains via eyedrops.
Before testing their idea using an animal model, the researchers created a pair of composite probes. The first component of each comprised superparamagnetic iron oxide nanoparticles that appeared clearly in magnetic resonance images and that showed good retention in brain cells. To lend the particles binding specificity for intracellular targets of interest, the scientists linked them to short DNA sequences. For detection of gliosis — fibrous outgrowth of cells called glia or astrocytes in a damaged brain — the target for binding was the messenger RNA for glial fibrillary acidic protein. For comparison, the researchers chose to target the messenger RNA of β-actin, a protein found in several nonglial cell types whose production changes little immediately after injury.
Because the brain probe will not reach healthy brains, the investigators had to artificially induce blood-brain barrier rupture in mice prior to delivery to simulate clinical conditions. Reducing blood supply in the cerebral cortex, inflicting puncture wounds or blocking the carotid arteries promoted leakage in experimental mice and mimicked minimal injury, significant trauma and cardiac arrest, respectively. Corresponding control animals underwent mock procedures. The researchers later instilled a solution containing one of the composite probes into the rodents’ eyes.
With a Bruker MRI device, the scientists next generated T2 images and subtraction R2 maps. Using the eyedrops containing gliosis-specific composite on mice that had experienced cortex disruption, the researchers verified barrier leakage and probe effectiveness. Proceeding to brain maps of animals with healing puncture wounds and artery blockage, they found that the probe clustered in regions where subsequent histological examination revealed actual pathology. The β-actin probe also showed expected accumulation patterns in experimental mice, while control animals had normal MRI results in all cases.
The investigators’ findings show not only the possibility of noninvasive delivery but also the potential for probe-linked DNA sequences to reveal the effects of conditions ranging from mental illness to spinal cord injury. They hope to apply their technique to these contexts in the future. Nonetheless, before the method reaches hospitals, researchers must conduct clinical trials on humans to address issues such as dosage, window of detection and toxicity of iron particles. Once perfected, Liu explained, the probe could prove comparatively user-friendly: Patients could use the drops at home and continue their daily routine before the magnetic resonance procedure.
The FASEB Journal, April 2008, pp. 1193-1203.
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