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      Photoacoustic Neuroimaging - Perspectives on a Maturing Imaging Technique and its Applications in Neuroscience

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          Abstract

          A prominent goal of neuroscience is to improve our understanding of how brain structure and activity interact to produce perception, emotion, behavior, and cognition. The brain’s network activity is inherently organized in distinct spatiotemporal patterns that span scales from nanometer-sized synapses to meter-long nerve fibers and millisecond intervals between electrical signals to decades of memory storage. There is currently no single imaging method that alone can provide all the relevant information, but intelligent combinations of complementary techniques can be effective. Here, we thus present the latest advances in biomedical and biological engineering on photoacoustic neuroimaging in the context of complementary imaging techniques. A particular focus is placed on recent advances in whole-brain photoacoustic imaging in rodent models and its influential role in bridging the gap between fluorescence microscopy and more non-invasive techniques such as magnetic resonance imaging (MRI). We consider current strategies to address persistent challenges, particularly in developing molecular contrast agents, and conclude with an overview of potential future directions for photoacoustic neuroimaging to provide deeper insights into healthy and pathological brain processes.

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          Most cited references268

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          Ultra-sensitive fluorescent proteins for imaging neuronal activity

          Summary Fluorescent calcium sensors are widely used to image neural activity. Using structure-based mutagenesis and neuron-based screening, we developed a family of ultra-sensitive protein calcium sensors (GCaMP6) that outperformed other sensors in cultured neurons and in zebrafish, flies, and mice in vivo. In layer 2/3 pyramidal neurons of the mouse visual cortex, GCaMP6 reliably detected single action potentials in neuronal somata and orientation-tuned synaptic calcium transients in individual dendritic spines. The orientation tuning of structurally persistent spines was largely stable over timescales of weeks. Orientation tuning averaged across spine populations predicted the tuning of their parent cell. Although the somata of GABAergic neurons showed little orientation tuning, their dendrites included highly tuned dendritic segments (5 - 40 micrometers long). GCaMP6 sensors thus provide new windows into the organization and dynamics of neural circuits over multiple spatial and temporal scales.
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            Imaging intracellular fluorescent proteins at nanometer resolution.

            We introduce a method for optically imaging intracellular proteins at nanometer spatial resolution. Numerous sparse subsets of photoactivatable fluorescent protein molecules were activated, localized (to approximately 2 to 25 nanometers), and then bleached. The aggregate position information from all subsets was then assembled into a superresolution image. We used this method--termed photoactivated localization microscopy--to image specific target proteins in thin sections of lysosomes and mitochondria; in fixed whole cells, we imaged vinculin at focal adhesions, actin within a lamellipodium, and the distribution of the retroviral protein Gag at the plasma membrane.
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              Fluorescent indicators for Ca2+ based on green fluorescent proteins and calmodulin.

              Important Ca2+ signals in the cytosol and organelles are often extremely localized and hard to measure. To overcome this problem we have constructed new fluorescent indicators for Ca2+ that are genetically encoded without cofactors and are targetable to specific intracellular locations. We have dubbed these fluorescent indicators 'cameleons'. They consist of tandem fusions of a blue- or cyan-emitting mutant of the green fluorescent protein (GFP), calmodulin, the calmodulin-binding peptide M13, and an enhanced green- or yellow-emitting GFP. Binding of Ca2+ makes calmodulin wrap around the M13 domain, increasing the fluorescence resonance energy transfer (FRET) between the flanking GFPs. Calmodulin mutations can tune the Ca2+ affinities to measure free Ca2+ concentrations in the range 10(-8) to 10(-2) M. We have visualized free Ca2+ dynamics in the cytosol, nucleus and endoplasmic reticulum of single HeLa cells transfected with complementary DNAs encoding chimaeras bearing appropriate localization signals. Ca2+ concentration in the endoplasmic reticulum of individual cells ranged from 60 to 400 microM at rest, and 1 to 50 microM after Ca2+ mobilization. FRET is also an indicator of the reversible intermolecular association of cyan-GFP-labelled calmodulin with yellow-GFP-labelled M13. Thus FRET between GFP mutants can monitor localized Ca2+ signals and protein heterodimerization in individual live cells.
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                Author and article information

                Contributors
                Journal
                Front Neurosci
                Front Neurosci
                Front. Neurosci.
                Frontiers in Neuroscience
                Frontiers Media S.A.
                1662-4548
                1662-453X
                10 June 2021
                2021
                : 15
                : 655247
                Affiliations
                [1] 1Department of Chemistry and School of Medicine, Technical University of Munich (TUM) , Munich, Germany
                [2] 2Institute for Synthetic Biomedicine, Helmholtz Center Munich , Munich, Germany
                Author notes

                Edited by: Biqin Dong, Fudan University, China

                Reviewed by: Junhui Shi, Zhejiang Lab, China; Chengbo Liu, Chinese Academy of Sciences (CAS), China

                *Correspondence: Silviu-Vasile Bodea, silviu.bodea@ 123456helmholtz-muenchen.de
                Gil Gregor Westmeyer, gil.westmeyer@ 123456tum.de

                This article was submitted to Brain Imaging Methods, a section of the journal Frontiers in Neuroscience

                Article
                10.3389/fnins.2021.655247
                8253050
                34220420
                7811cf31-5530-4e38-9722-1babc77377c5
                Copyright © 2021 Bodea and Westmeyer.

                This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

                History
                : 18 January 2021
                : 08 March 2021
                Page count
                Figures: 2, Tables: 0, Equations: 0, References: 268, Pages: 20, Words: 0
                Categories
                Neuroscience
                Review

                Neurosciences
                optoacoustic imaging,functional brain imaging,molecular contrast agents,calcium and voltage sensors,stroke,brain tumors,image-guided therapy,translational photoacoustic imaging

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