Researchers at MIT’s McGovern Institute for Brain Research have developed a highly sensitive technique for detecting increase in neuronal calcium ion (Ca2+) concentrations. The technique, described online at the Proceedings of the National Academy of Sciences website, could eventually lead to in vivo functional molecular imaging in real time.
Ca2+ signalling is ubiquitous in neurons. Many types of neuronal activity involve an increase in intracellular Ca2+ concentration. The greater the frequency of a neuron’s firing, the more Ca2+ flows into the cell. Calcium enters nerve cells through voltage-gated ion channels, and acts as a second messenger which mediates a wide variety of cellular responses.
Alan Jasonoff and his colleagues modified superparamagnetic iron oxide nanoparticles so that they would aggregate in response to increased levels of intracellular Ca2+. Because the particles are magnetic, they affect the signals produced by functional magnetic resonance imaging (fMRI).
“Using conventional fMRI to study the brain is like trying to understand how a computer works by feeling which parts of it are hot because of energy dissipation in different components,” says Jasonoff. “Chemical sensors for MRI could show what each individual element in each integrated circuit is doing and how it performs the computations and processes information.”
Jasonoff’s technique uses two slightly different types of nanoparticles, each with a different protein attached to their surface. One type has the corkskrew-shaped viral protein M13 attached to it. The other is attached to the calcium-binding protein calmodulin. In the presence of Ca2+ the two proteins bind to each other, causing the nanoparticles to aggregate; this aggregation produces changes in the contrast of the MRI image (left).
The new method enables a big improvement in the spatial and temporal resolution of fMRI. With convenventional fMRI, spatial resolution is limited to groups of about 1,000 neurons because of the organization of capilllaries within the brain, and temporal resolution is limited because the signals produced by changes in blood oxygenation detected by conventional fMRI occur several seconds after the neuronal activity they are associated with. Jasonoff’s technique is sensitive to the influx of Ca2+ that occurs in neurons as soon as they become active. The nanoparticles are sensitive to concentration changes of about 1 microMolar (µM), and because the aggregation of the nanoparticles is reversible, the temporal dynamics of neuronal activity can be imaged in real time.
“These will be tools for making the shift from imaging gross functional properties of the brain through its hemodynamic changes to a fine tuned analysis based on information flow involving cells and circuits,” says Jasonoff.
The McKnight Endowment Fund for Neuroscience has awarded Jasonoff one of four Technological Innovations in Neuroscience Awards; he will receive $200,000 over the next 2 years to develop non-invasive methods for the in vivo delivery of the naoparticles in fruit flies and mice. Other members of Jasonoff’s group are working on producing calcium sensors that are specific for certain types of neurons. One way of doing this is to alter the binding properties of the proteins used to modify the surface of the nanoparticles, by introducing different mutations into their coding sequences.