Molecular fMRI

Comprehensive analysis of brain function depends on understanding the dynamics of diverse neural signaling processes over large tissue volumes in intact animals and humans. Most existing approaches to measuring brain signaling suffer from limited tissue penetration, poor resolution, or lack of specificity for well-defined neural events. Here we discuss a new brain activity mapping method that overcomes some of these problems by combining MRI with contrast agents sensitive to neural signaling. The goal of this “molecular fMRI” approach is to permit noninvasive whole-brain neuroimaging with specificity and resolution approaching current optical neuroimaging methods. In this article, we describe the context and need for molecular fMRI as well as the state of the technology today. We explain how major types of MRI probes work and how they can be sensitized to neurobiological processes, such as neurotransmitter release, calcium signaling, and gene expression changes. We comment both on past work in the field and on challenges and promising avenues for future development.National Institutes of Health (U.S.) (Grants R21-MH102470 and U01-NS09045)Massachusetts Institute of Technology. Simons Center for the Social Brain (Seed Grant

Screen-Based Analysis of Magnetic Nanoparticle Libraries Formed Using Peptidic Iron Oxide Ligands

The identification of effective polypeptide ligands for magnetic iron oxide nanoparticles (IONPs) could considerably accelerate the high-throughput analysis of IONP-based reagents for imaging and cell labeling. We developed a procedure for screening IONP ligands and applied it to compare candidate peptides that incorporated carboxylic acid side chains, catechols, and sequences derived from phage display selection. We found that only l-3,4-dihydroxyphenylalanine (DOPA)-containing peptides were sufficient to maintain particles in solution. We used a DOPA-containing sequence motif as the starting point for generation of a further library of over 30 peptides, each of which was complexed with IONPs and evaluated for colloidal stability and magnetic resonance imaging (MRI) contrast properties. Optimal properties were conferred by sequences within a narrow range of biophysical parameters, suggesting that these sequences could serve as generalizable anchors for formation of polypeptide–IONP complexes. Differences in the amino acid sequence affected T[subscript 1]- and T[subscript 2]-weighted MRI contrast without substantially altering particle size, indicating that the microstructure of peptide-based IONP coatings exerts a substantial influence and could be manipulated to tune properties of targeted or responsive contrast agents. A representative peptide–IONP complex displayed stability in biological buffer and induced persistent MRI contrast in mice, indicating suitability of these species for in vivo molecular imaging applications.National Institutes of Health (U.S.) (Grant R01-DA28299)National Institutes of Health (U.S.) (Grant R01-NS76462)National Institutes of Health (U.S.) (Grant R21-MH102470)Japan Society for the Promotion of Science (Postdoctoral Fellowship for Research Abroad

Bioengineering Novel Reporter Proteins

Visualization of gene expression has led to a revolution in biology over the past two decades. Primarily this visualization has occurred using fluorescent proteins, like GFP, that can be directly visualized with microscopy. Fluorescence imaging is limited by depth of penetration when applied to living mice or humans however. For this, MRI, ultrasound and other modalities are under continual development for in vivo applications. Ideally, every in vivo imaging modality would have their own reporter genes, allowing for unconstrained genetic studies of structure and function. The current wealth of bioinformatics data presents a rich pallet of starting materials for bioengineering this next generation of reporter proteins. This work utilized multiple approaches to creating reporters: cell labeling with, "Biotag" derived from a bacterial biotinylation enzyme and substrate; genetically controlled absorption of the MRI contrast agent Mn via the metal transport protein DMT1; and sequestration of Mn using the metal sensing transcription factor MntR. The reporter proteins were implemented in tissue culture and living mice to give a new view of gene expression in processes such as neural and vascular development. Moreover, the development process yielded new insights into the proteins themselves and the context in which they function. Each method has particular strengths and limitations but are, at present, the vanguard of in vivo molecular imaging

Wireless resonant circuits for the minimally invasive sensing of biophysical processes in magnetic resonance imaging

Biological electromagnetic fields arise throughout all tissue depths and types, and correlate with physiological processes and signalling in organs of the body. Most of the methods for monitoring these fields are either highly invasive or spatially coarse. Here, we show that implantable active coil-based transducers that are detectable via magnetic resonance imaging enable the remote sensing of biological fields. These devices consist of inductively coupled resonant circuits that change their properties in response to electrical or photonic cues, thereby modulating the local magnetic resonance imaging signal without the need for onboard power or wired connectivity. We discuss design parameters relevant to the construction of the transducers on millimetre and submillimetre scales, and demonstrate their in vivo functionality for measuring time-resolved bioluminescence in rodent brains. Biophysical sensing via microcircuits that leverage the capabilities of magnetic resonance imaging may enable a wide range of biological and biomedical applications.NIH grant (R01 NS76462)NIH grant (R01 DA038642)NIH grant (U01 NS904051

Membrane-Permeable Mn(III) Complexes for Molecular Magnetic Resonance Imaging of Intracellular Targets

Intracellular compartments make up roughly two-thirds of the body, but delivery of molecular imaging probes to these spaces can be challenging. This situation is particularly true for probes designed for detection by magnetic resonance imaging (MRI), a high-resolution but relatively insensitive modality. Most MRI contrast agents are polar and membrane impermeant, making it difficult to deliver them in sufficient quantities for measurement of intracellular analytes. Here we address this problem by introducing a new class of planar tetradentate Mn(III) chelates assembled from a 1,2-phenylenediamido (PDA) backbone. Mn(III)-PDA complexes display T₁ relaxivity comparable to that of Gd(III)-based contrast agents and undergo spontaneous cytosolic localization via defined mechanisms. Probe variants incorporating enzyme-cleavable acetomethoxy ester groups are processed by intracellular esterases and accumulate in cells. Probes modified with ethyl esters preferentially label genetically modified cells that express a substrate-selective esterase. In each case, the contrast agents gives rise to robust T₁-weighted MRI enhancements, providing precedents for the detection of intracellular targets by Mn(III)-PDA complexes. These compounds therefore constitute a platform from which to develop reagents for molecular MRI of diverse processes inside cells.United States. National Institutes of Health (R21-MH102470)United States. National Institutes of Health (U01-NS090451)Massachusetts Institute of Technology. Center for Environmental Health Sciences (P30-ES002109

Sensing intracellular calcium ions using a manganese-based MRI contrast agent

There are only few MRI-compatible calcium reporters and they are limited to measuring extracellular calcium levels. Here the authors develop and validate a cell-permeable, manganese-based paramagnetic MRI contrast agent that enables monitoring intracellular calcium signals in vivo in the rat brain

Molecular Magnetic Resonance Imaging of Nitric Oxide in Biological Systems

Copyright © 2020 American Chemical Society. Detection of nitric oxide (NO) in biological systems is challenging due to both physicochemical properties of NO and limitations of current imaging modalities and probes. Magnetic resonance imaging (MRI) could be applied for studying NO in living tissue with high spatiotemporal resolution, but there is still a need for chemical agents that effectively sensitize MRI to biological NO production. To develop a suitable probe, we studied the interactions between NO and a library of manganese complexes with various oxidation states and molecular structures. Among this set, the manganese(III) complex with N,N′-(1,2-phenylene)bis(5-fluoro-2-hydroxybenzamide) showed favorable changes in longitudinal relaxivity upon addition of NO-releasing chemicals in vitro while also maintaining selectivity against other biologically relevant reactive nitrogen and oxygen species, making it a suitable NO-responsive contrast agent for T1-weighted MRI. When loaded with this compound, cells ectopically expressing nitric oxide synthase (NOS) isoforms showed MRI signal decreases of over 20% compared to control cells and were also responsive to NOS inhibition or calcium-dependent activation. The sensor could also detect endogenous NOS activity in antigen-stimulated macrophages and in a rat model of neuroinflammation in vivo. Given the key role of NO and associated reactive nitrogen species in numerous physiological and pathological processes, MRI approaches based on the new probe could be broadly beneficial for studies of NO-related signaling in living subjects

Membrane-Permeable Mn(III) Complexes for Molecular Magnetic Resonance Imaging of Intracellular Targets

Intracellular compartments make up roughly two-thirds of the body, but delivery of molecular imaging probes to these spaces can be challenging. This situation is particularly true for probes designed for detection by magnetic resonance imaging (MRI), a high-resolution but relatively insensitive modality. Most MRI contrast agents are polar and membrane impermeant, making it difficult to deliver them in sufficient quantities for measurement of intracellular analytes. Here we address this problem by introducing a new class of planar tetradentate Mn­(III) chelates assembled from a 1,2-phenylene­diamido (PDA) backbone. Mn­(III)-PDA complexes display <i>T</i>₁ relaxivity comparable to that of Gd­(III)-based contrast agents and undergo spontaneous cytosolic localization via defined mechanisms. Probe variants incorporating enzyme-cleavable aceto­methoxy ester groups are processed by intracellular esterases and accumulate in cells. Probes modified with ethyl esters preferentially label genetically modified cells that express a substrate-selective esterase. In each case, the contrast agents gives rise to robust <i>T</i>₁-weighted MRI enhancements, providing precedents for the detection of intracellular targets by Mn­(III)-PDA complexes. These compounds therefore constitute a platform from which to develop reagents for molecular MRI of diverse processes inside cells

Calcium-dependent molecular fMRI using a magnetic nanosensor

Calcium ions are ubiquitous signalling molecules in all multicellular organisms, where they mediate diverse aspects of intracellular and extracellular communication over widely varying temporal and spatial scales1. Though techniques to map calcium-related activity at a high resolution by optical means are well established, there is currently no reliable method to measure calcium dynamics over large volumes in intact tissue2. Here, we address this need by introducing a family of magnetic calcium-responsive nanoparticles (MaCaReNas) that can be detected by magnetic resonance imaging (MRI). MaCaReNas respond within seconds to [Ca2+] changes in the 0.1-1.0 mM range, suitable for monitoring extracellular calcium signalling processes in the brain. We show that the probes permit the repeated detection of brain activation in response to diverse stimuli in vivo. MaCaReNas thus provide a tool for calcium-activity mapping in deep tissue and offer a precedent for the development of further nanoparticle-based sensors for dynamic molecular imaging with MRI.National Institutes of Health (U.S.) (Grant R01-DA038642)National Institutes of Health (U.S.) (Grant DP2-OD2114)National Institutes of Health (U.S.) (Grant U01-NS090451)National Institutes of Health (U.S.) (Grant R01-EY007023)National Science Foundation (U.S.) (Grant 0070319)National Institutes of Health (U.S.) (Grant S10-OD016326