Evolutionary approach to studying brain chemistry

Researchers have used a technique known as "directed evolution" to devise a novel contrast agent that could enable non-invasive magnetic resonance studies of the neurotransmitter, dopamine, in the brain.

Functional magnetic resonance imaging (fMRI) has enhanced our understanding of how the brain works immensely since it was first introduced to researchers approximately two decades ago. However, fMRI essentially measures nothing more than blood flow as a proxy for neural activity. When a brain region becomes active, blood vessels in that region become dilated, causing increased blood flow, which is observed as an increase in the signal due to iron in the blood's haemoglobin. Scientists would much prefer a direct way of addressing brain functionality.

Now, neuroscientists at the Massachusetts Institute of Technology, MIT, have designed a new MRI sensor that responds to the neurotransmitter dopamine, an achievement that may significantly improve the specificity and resolution of future brain imaging procedures.

"We have designed an artificial molecular probe that changes its magnetic properties in response to the neurotransmitter dopamine," explains biological engineer Alan Jasanoff. Jasanoff is senior author on a Nature Biotechnology paper describing the research. "This new tool connects molecular phenomena in the nervous system with whole-brain imaging techniques, allowing us to probe very precise processes and relate them to the overall function of the brain and of the organism."

By using molecular fMRI, researchers can learn much more specific things about the brain's activity and circuitry than is possible with conventional fMRI, which is insensitive to specific molecules in the brain.

Dopamine is a catecholamine neurotransmitter present in a wide variety of animals, both vertebrates and invertebrates. In the brain, its role is as a neurotransmitter, activating five distinct dopamine receptors and their variants. Dopamine is produced in several areas of the brain, including the substantia nigra and the ventral tegmental area. The compound also acts as a neurohormone released by the hypothalamus. Measuring dopamine in the living brain is obviously of particular interest to neuroscience because this neurotransmitter plays a key role in motivation, reward, addiction, and several neurodegenerative conditions, including Parkinson's disease.

To design a molecular probe that binds to dopamine, Jasanoff and his team worked in collaboration with Robert Langer's group and that of Frances Arnold at California Institute of Technology. Rather than starting from scratch the team took an evolutionary approach to developing the compound. They began with a magnetically active protein similar to haemoglobin, which they already knew would be functional in MRI. They then introduced mutations and selected only those offspring proteins that had a stronger tendency to bind to dopamine. By carrying out several rounds of artificial mutation and selection they could evolve a compound that could target dopamine selectively.

"By harnessing the power of protein engineering we now have the ability to advance neuroscience through more precise non-invasive imaging of the brain," explains co-author Mikhail Shapiro, a former graduate student supervised by Jasanoff and Langer. Shapiro devised this directed evolution approach for making the MRI sensor molecule.

The team carried out in vitro tests to confirm that the protein responds appropriately to the presence of dopamine produced by cells and then moved on to in vivo studies. They were able to absorb a change in the MRI intensity precisely when they artificially triggered dopamine release in the presence of the sensor.

"This [result] means that we can see signal changes in the brain due to the modulation of dopamine," explains Gil Westmeyer, a postdoctoral fellow in Jasanoff's laboratory who directed the in vivo study. "This novel MRI sensor will enable us to study the spatial and temporal patterns of dopamine transmission over the vast and heterogeneous dopamine network in the brain."

While synthetic molecules are typically introduced into the brain with external devices, Jasanoff's new sensor is based on a protein, which means that researchers may also have the ability to genetically encode the sensor to be produced by cells of their own accord.

The next stage in the research will be to use the new MRI sensor to study how the spatial and temporal patterns of dopamine release relate to an animal's experience of reward, learning, and reinforcement. They hope to develop a related suite of new tools to detect different molecular events across the whole brain, and they expect to see additional gains in sensitivity through improved experimental paradigms and further molecular engineering. The new dopamine sensor could become an important tool for biomedical research with animals. The researchers hope, however, that related agents might also be developed that can measure neural activity in the human brain.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.