Investigation of a Calcium-Responsive Contrast Agentin Cellular Model Systems: Feasibility for Use as a Smart MolecularProbe in Functional MRI

摘要

Responsive or smart contrast agents (SCAs) represent a promising direction for development of novel functional MRI (fMRI) methods for the eventual noninvasive assessment of brain function. In particular, SCAs that respond to Ca²⁺ may allow tracking neuronal activity independent of brain vasculature, thus avoiding the characteristic limitations of current fMRI techniques. Here we report an in vitro proof-of-principle study with a Ca²⁺-sensitive, Gd³⁺-based SCA in an attempt to validate its potential use as a functional in vivo marker. First, we quantified its relaxometric response in a complex 3D cell culture model. Subsequently, we examined potential changes in the functionality of primary glial cells following administration of this SCA. Monitoring intracellular Ca²⁺ showed that, despite a reduction in the Ca²⁺ level, transport of Ca²⁺ through the plasma membrane remained unaffected, while stimulation with ATP induced Ca²⁺-transients suggested normal cellular signaling in the presence of low millimolar SCA concentrations. SCAs merely lowered the intracellular Ca²⁺ level. Finally, we estimated the longitudinal relaxation times (T1) foran idealized in vivo fMRI experiment with SCA, for extracellular Ca²⁺ concentration level changes expected during intense neuronalactivity which takes place upon repetitive stimulation. The valueswe obtained indicate changes in T1 of around 1–6%,sufficient to be robustly detectable using modern MRI methods in highfield scanners. Our results encourage further attempts to developeven more potent SCAs and appropriate fMRI protocols. This would resultin novel methods that allow monitoring of essential physiologicalprocesses at the cellular and molecular level.