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Abstract

Introduction: How the mammalian brain processes information, stores memories and maintains homeostasis depends on the dynamic communication between neurons, glia cells, blood vessels and many other cell types via signaling molecules such as neurotransmitters, neuropeptides and hormones throughout the entire brain. Currently no technology can measure brain-wide molecular signaling with sufficient spatial and temporal resolution in living mammals, which poses a major roadblock for understanding molecular foundations of brain function and dysfunction in disease states.
Methods: Hemodynamic imaging using functional magnetic resonance imaging (fMRI) or functional ultrasound (fUS) represents the gold standard for imaging whole-brain function in animals and humans and the only method that can measure brain-wide function with a temporal resolution of single seconds and in-plane spatial resolution of 100 μm. Yet, hemodynamic imaging lacks critical information on molecular pathways or signaling molecules underlying the measured imaging signals. We leverage the power of hemodynamic imaging and provide it with molecular information by developing a technology that converts target molecular signal into hemodynamic imaging contrast in fMRI, ultrasound, or optical imaging. We apply this technology for molecular brain imaging in living rats using T2*-weighted MRI.
Results/Discussion: We developed bioluminescence imaging using hemodynamics (BlusH), which utilizes vascular expression of the light-activated cyclase protein PAC to convert light signals from bioluminescent molecular reporters into hemodynamic imaging signals visible in MRI and other modalities (Panel A). We tested the BlusH principle in vitro by quantifying the light output of bioluminescent luciferase proteins (B) and validating light-driven activation of hemodynamic pathways by PAC (C) and direct stimulation of such pathways by the bioluminescent reporter GLuc (D). Targeting expression of PAC to the brain vasculature in rats (F-G) yielded robust light-activated signal changes in T2*-weighted MRI that colocalized with PAC expression and light stimulation patterns (H-I). Finally, wide-field expression of PAC allowed us to accurately map GLuc expression in deep brain regions and three dimensions using MRI (J-L).
Conclusion: Bioluminescence imaging usually suffers from poor tissue penetration of light signals, leading to low-resolution and largely superficial signal detection. BlusH leverages the rich toolbox of bioluminescent reporters to visualize intra- and extracellular molecular processes throughout the brain using noninvasive fMRI providing a bridging technology between brain-wide functional imaging with fMRI and molecular nuclear imaging.
Novelty: Converting molecular signals into hemodynamic imaging contrast offers a unique way to sensitively visualize brain-wide molecular signals in MRI.
Impact: BlusH images molecular brain processes in living mammals and can be combined with optical, electrophysiological or nuclear imaging recordings.
Disclosure: I or one of my co-authors have no financial interest or relationship to disclose regarding the subject matter of this presentation.