Design and Development of Calcium Sensitive Contrast Agents for fMRI

Abstract

There is a considerable interest in new technologies that allow noninvasive imaging of physiological parameters in living systems, especially in the neuroscience. Magnetic resonance imaging (MRI) is a very powerful tool for neuroimaging, because it can image the tissue noninvasively, in depth, at a good spatial (~100 pm) and temporal (~1s) resolution. In order to study neural activity at a cellular level, it would be of notable significance to combine MRI with calcium-sensitive contrast agents because of the important role of calcium as a second messenger in cellular signaling pathways. Here we describe a family of calcium sensors for MRI based on the conjugation of superparamagnetic iron oxide nanoparticles (SPIOS) to calcium sensing protein calmodulin and its target peptides. In the presence of calcium, interaction between the two protein domains drives the aggregation of SPIOs which results in up to four fold T2 changes. The calcium sensing is reversible and occurs at midpoint of roughly 1 ptM Ca , which makes these contrast agents suitable for imaging of the cytosolic calcium fluctuations in cells. We introduce two generations of these sensors: the first one based on commercial larger SPIOs, and the second one that uses small crosslinked lipid coated nanoparticles (xLCIOs) which have potential to overcome some of the limitations of the prototype sensor, such as inadequate diffusivity and relatively slow kinetic response. When combined with technologies for cellular deliveries of nanoparticles, these sensors and their derivatives may be useful for functional molecular imaging of biological signaling network in live, opaque specimens.