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      Cortical Circuit Dysfunction as a Potential Driver of Amyotrophic Lateral Sclerosis

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          Abstract

          Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease that affects selected cortical and spinal neuronal populations, leading to progressive paralysis and death. A growing body of evidences suggests that the disease may originate in the cerebral cortex and propagate in a corticofugal manner. In particular, transcranial magnetic stimulation studies revealed that ALS patients present with early cortical hyperexcitability arising from a combination of increased excitability and decreased inhibition. Here, we discuss the possibility that initial cortical circuit dysfunction might act as the main driver of ALS onset and progression, and review recent functional, imaging and transcriptomic studies conducted on ALS patients, along with electrophysiological, pathological and transcriptomic studies on animal and cellular models of the disease, in order to evaluate the potential cellular and molecular origins of cortical hyperexcitability in ALS.

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          Most cited references134

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          Interneurons of the neocortical inhibitory system.

          Mammals adapt to a rapidly changing world because of the sophisticated cognitive functions that are supported by the neocortex. The neocortex, which forms almost 80% of the human brain, seems to have arisen from repeated duplication of a stereotypical microcircuit template with subtle specializations for different brain regions and species. The quest to unravel the blueprint of this template started more than a century ago and has revealed an immensely intricate design. The largest obstacle is the daunting variety of inhibitory interneurons that are found in the circuit. This review focuses on the organizing principles that govern the diversity of inhibitory interneurons and their circuits.
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            TDP-43 mutant transgenic mice develop features of ALS and frontotemporal lobar degeneration.

            Frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS) are neurodegenerative diseases that show considerable clinical and pathologic overlap, with no effective treatments available. Mutations in the RNA binding protein TDP-43 were recently identified in patients with familial amyotrophic lateral sclerosis (ALS), and TDP-43 aggregates are found in both ALS and FTLD-U (FTLD with ubiquitin aggregates), suggesting a common underlying mechanism. We report that mice expressing a mutant form of human TDP-43 develop a progressive and fatal neurodegenerative disease reminiscent of both ALS and FTLD-U. Despite universal transgene expression throughout the nervous system, pathologic aggregates of ubiquitinated proteins accumulate only in specific neuronal populations, including layer 5 pyramidal neurons in frontal cortex, as well as spinal motor neurons, recapitulating the phenomenon of selective vulnerability seen in patients with FTLD-U and ALS. Surprisingly, cytoplasmic TDP-43 aggregates are not present, and hence are not required for TDP-43-induced neurodegeneration. These results indicate that the cellular and molecular substrates for selective vulnerability in FTLD-U and ALS are shared between mice and humans, and suggest that altered DNA/RNA-binding protein function, rather than toxic aggregation, is central to TDP-43-related neurodegeneration.
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              Corticocortical inhibition in human motor cortex.

              1. In ten normal volunteers, a transcranial magnetic or electric stimulus that was subthreshold for evoking an EMG response in relaxed muscles was used to condition responses evoked by a later, suprathreshold magnetic or electric test shock. In most experiments the test stimulus was given to the lateral part of the motor strip in order to evoke EMG responses in the first dorsal interosseous muscle (FDI). 2. A magnetic conditioning stimulus over the hand area of cortex could suppress responses produced in the relaxed FDI by a suprathreshold magnetic test stimulus at interstimulus intervals of 1-6 ms. At interstimulus intervals of 10 and 15 ms, the test response was facilitated. 3. Using a focal magnetic stimulus we explored the effects of moving the conditioning stimulus to different scalp locations while maintaining the magnetic test coil at one site. If the conditioning coil was moved anterior or posterior to the motor strip there was less suppression of test responses in the FDI. In contrast, stimulation at the vertex could suppress FDI responses by an amount comparable to that seen with stimulation over the hand area. With the positions of the two coils reversed, conditioning stimuli over the hand area suppressed responses evoked in leg muscles by vertex test shocks. 4. The intensity of both conditioning and test shocks influenced the amount of suppression. Small test responses were more readily suppressed than large responses. The best suppression was seen with small conditioning stimuli (0.7-0.9 times motor threshold in relaxed muscle); increasing the intensity to motor threshold or above resulted in less suppression or even facilitation. 5. Two experiments suggested that the suppression was produced by an action on cortical, rather than spinal excitability. First, a magnetic conditioning stimulus over the hand area failed to produce any suppression of responses evoked in active hand muscles by a small (approximately 200 V, 50 microsecond time constant) anodal electric test shock. Second, a vertex conditioning shock had no effect on forearm flexor H reflexes even though responses in the same muscles produced by magnetic cortical test shocks were readily suppressed at appropriate interstimulus intervals. 6. Small anodal electric conditioning stimuli were much less effective in suppressing magnetic test responses than either magnetic or cathodal electric conditioning shocks.(ABSTRACT TRUNCATED AT 400 WORDS)
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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
                29 April 2020
                2020
                : 14
                : 363
                Affiliations
                INSERM UMR_S 1118, Mécanismes Centraux et Périphériques de la Neurodégénérescence, Faculté de Médecine, Université de Strasbourg , Strasbourg, France
                Author notes

                Edited by: Sabine Liebscher, Ludwig-Maximilians-Universität München, Germany

                Reviewed by: Valentina Bonetto, Mario Negri Institute for Pharmacological Research, Italy; Foteini Christidi, National and Kapodistrian University of Athens, Greece

                *Correspondence: Caroline Rouaux, caroline.rouaux@ 123456inserm.fr

                These authors have contributed equally to this work

                This article was submitted to Neurodegeneration, a section of the journal Frontiers in Neuroscience

                Article
                10.3389/fnins.2020.00363
                7201269
                32410944
                971cfcbc-cce2-4ef0-83fe-e880dddb3d13
                Copyright © 2020 Brunet, Stuart-Lopez, Burg, Scekic-Zahirovic and Rouaux.

                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
                : 31 December 2019
                : 25 March 2020
                Page count
                Figures: 3, Tables: 0, Equations: 0, References: 170, Pages: 18, Words: 0
                Funding
                Funded by: H2020 European Research Council 10.13039/100010663
                Award ID: 639737
                Funded by: Association Française contre les Myopathies 10.13039/100007393
                Award ID: 16923
                Categories
                Neuroscience
                Review

                Neurosciences
                amyotrophic lateral sclerosis,cerebral cortex,hyperexcitability,network dysfunction,intrinsic,extrinsic

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