Engineered metalloproteins as contrast sensors for molecular fMRI

Abstract

Functional brain imaging technologies seek to expand our understanding of intact neural systems. Present day functional MRI (fMRI) measures the delayed hemodynamic response that is indirectly associated with neural activity. To study underlying molecular systems noninvasively but precisely, tools must be developed to modulate MRI contrast as a function of discrete molecular events within pathways of interest. In optical imaging modalities, genetically encoded sensors based on fluorescent proteins have provided an engineerable platform on which to optimize desirable device characteristics by exploiting the tools of molecular biology and protein biochemistry. Analogously, we seek to build genetically encodable sensors based on engineered metalloproteins whose effects on MRI contrast are regulated by specific biochemical interactions. In this work, we present two technological advancements toward realizing fMRI contrast sensors for molecular neuroimaging. First, a genetically encodable sensor for free calcium is described, consisting of a novel ferritin-based device that reversibly enhances NMR transverse relaxation times (T2) by Ca - dependent crosslinking. Second, we show that the T1 contrast effect of a recently proposed family of cytochrome P450-based MRI sensors can be significantly enhanced by substitution of the protein's native heme with a high spin manganese porphyrin.