MRI technique reveals genes’ roles in learning and memory

Viewing brain activity at the molecule level
March 26, 2014

MIT researchers used a novel MRI contrast agent to track the location of a reporter protein called SEAP, indicated by the bright spot on the left side (inset). The larger image shows the corresponding optical image of the same contrast agent. (Credit: Gil Westmeyer and Alan Jasanoff)

MIT bioengineers have adapted MRI to visualize gene activity inside the brains of living animals.

Tracking these genes with MRI would enable scientists to learn more about how the genes control processes such as forming memories and learning new skills, says Alan Jasanoff, an MIT associate professor of biological engineering and leader of the research team.

“The dream of molecular imaging is to provide information about the biology of intact organisms, at the molecule level,” says Jasanoff, who is also an associate member of MIT’s McGovern Institute for Brain Research. “The goal is to not have to chop up the brain, but instead to actually see things that are happening inside.”

On/off reporter gene

MRI reporter detection. SEAP (red) is produced by genetically modified cells and acts on the MRI contrast agent Mn-TPPP4 (structure shown), cleaving up to four phosphates and inducing precipitation of the products that have low solubility compared with Mn-TPPP4. Accumulation of dephosphorylated contrast agents leads to local increases in MRI contrast in the vicinity of cells expressing SEAP. (Credit: Gil G. Westmeyer et al./Cell: Chemistry & Biology)

To help reach that goal, Jasanoff and colleagues have developed a new way to image a “reporter gene” — an artificial gene that turns on or off to signal events in the body, much like an indicator light on a car’s dashboard.

In the new study, the reporter gene encodes an enzyme that interacts with a magnetic contrast agent injected into the brain, making the agent visible with MRI.

This approach, described in a recent issue of the journal Chemistry & Biology, allows researchers to determine when and where that reporter gene is turned on.

MRI uses magnetic fields and radio waves that interact with protons in the body to produce detailed images of the body’s interior.

In brain studies, neuroscientists commonly use functional MRI to measure blood flow, which reveals which parts of the brain are active during a particular task. When scanning other organs, doctors sometimes use magnetic “contrast agents” to boost the visibility of certain tissues.

The new MIT approach includes a contrast agent called a manganese porphyrin and the new reporter gene, which codes for a genetically engineered enzyme that alters the electric charge on the contrast agent. Jasanoff and colleagues designed the contrast agent so that it is soluble in water and readily eliminated from the body, making it difficult to detect by MRI.

However, when the engineered enzyme, known as SEAP, slices phosphate molecules from the manganese porphyrin, the contrast agent becomes insoluble and starts to accumulate in brain tissues, allowing it to be seen.

The natural version of SEAP is found in the placenta, but not in other tissues. By injecting a virus carrying the SEAP gene into the brain cells of mice, the researchers were able to incorporate the gene into the cells’ own genome. Brain cells then started producing the SEAP protein, which is secreted from the cells and can be anchored to their outer surfaces.

That’s important, Jasanoff says, because it means that the contrast agent doesn’t have to penetrate the cells to interact with the enzyme.

Researchers can then find out where SEAP is active by injecting the MRI contrast agent, which spreads throughout the brain but accumulates only near cells producing the SEAP protein.

Exploring brain function

In this study, which was designed to test this general approach, the detection system revealed only whether the SEAP gene had been successfully incorporated into brain cells. However, in future studies, the researchers intend to engineer the SEAP gene so it is only active when a particular gene of interest is turned on.

Jasanoff first plans to link the SEAP gene with so-called “early immediate genes,” which are necessary for brain plasticity — the weakening and strengthening of connections between neurons, which is essential to learning and memory.

“As people who are interested in brain function, the top questions we want to address are about how brain function changes patterns of gene expression in the brain,” Jasanoff says. “We also imagine a future where we might turn the reporter enzyme on and off when it binds to neurotransmitters, so we can detect changes in neurotransmitter levels as well.”

The research was funded by the Raymond and Beverly Sackler Foundation, the National Institutes of Health, and an MIT-Germany Seed Fund grant.

Abstract of Chemistry & Biology paper

  • A reporter enzyme catalyzes precipitation of paramagnetic phosphoporphyrins
  • Enzymatic product accumulation is visible by light microscopy and MRI
  • The contrast mechanism detects virally driven expression ex vivo and in vivo

The ability to map patterns of gene expression noninvasively in living animals could have impact in many areas of biology. Reporter systems compatible with MRI could be particularly valuable, but existing strategies tend to lack sensitivity or specificity. Here we address the challenge of MRI-based gene mapping using the reporter enzyme secreted alkaline phosphatase (SEAP), in conjunction with a water-soluble metalloporphyrin contrast agent. SEAP cleaves the porphyrin into an insoluble product that accumulates at sites of enzyme expression and can be visualized by MRI and optical absorbance. The contrast mechanism functions in vitro, in brain slices, and in animals. The system also provides the possibility of readout both in the living animal and by postmortem histology, and it notably does not require intracellular delivery of the contrast agent. The solubility switch mechanism used to detect SEAP could be adapted for imaging of additional reporter enzymes or endogenous targets.