Gwendal Le Masson

Dynamic balance of metabotropic inputs causes dorsal horn neurons to switch functional states.

Sensory relay structures in the spinal cord dorsal horn are now thought to be active processing structures that function before supraspinal sensory integration. Dorsal horn neurons directly receive nociceptive (pain) signals from the periphery, express a high degree of functional plasticity and are involved in long-term sensitization and chronic pain. We show here that deep dorsal horn neurons (DHNs) in Wistar rats can switch their intrinsic firing properties from tonic to plateau or endogenous bursting patterns, depending upon the balance of control by metabotropic glutamate (mGlu) and GABAB receptors. We further show that this modulation acts on at least one common target, the inwardly rectifying potassium channel (Kir3). Finally, we found that these firing modes correspond to specific functional states of information transfer in which dorsal horn neurons can faithfully transmit, greatly enhance or block the transfer of nociceptive information.

Neurons are MHC Class I-Dependent Targets for CD8 T Cells upon Neurotropic Viral Infection

Following infection of the central nervous system (CNS), the immune system is faced with the challenge of eliminating the pathogen without causing significant damage to neurons, which have limited capacities of renewal. In particular, it was thought that neurons were protected from direct attack by cytotoxic T lymphocytes (CTL) because they do not express major histocompatibility class I (MHC I) molecules, at least at steady state. To date, most of our current knowledge on the specifics of neuron-CTL interaction is based on studies artificially inducing MHC I expression on neurons, loading them with exogenous peptide and applying CTL clones or lines often differentiated in culture. Thus, much remains to be uncovered regarding the modalities of the interaction between infected neurons and antiviral CD8 T cells in the course of a natural disease. Here, we used the model of neuroinflammation caused by neurotropic Borna disease virus (BDV), in which virus-specific CTL have been demonstrated as the main immune effectors triggering disease. We tested the pathogenic properties of brain-isolated CD8 T cells against pure neuronal cultures infected with BDV. We observed that BDV infection of cortical neurons triggered a significant up regulation of MHC I molecules, rendering them susceptible to recognition by antiviral CTL, freshly isolated from the brains of acutely infected rats. Using real-time imaging, we analyzed the spatio-temporal relationships between neurons and CTL. Brain-isolated CTL exhibited a reduced mobility and established stable contacts with BDV-infected neurons, in an antigen- and MHC-dependent manner. This interaction induced rapid morphological changes of the neurons, without immediate killing or impairment of electrical activity. Early signs of neuronal apoptosis were detected only hours after this initial contact. Thus, our results show that infected neurons can be recognized efficiently by brain-isolated antiviral CD8 T cells and uncover the unusual modalities of CTL-induced neuronal damage.

A Computational Model of Motor Neuron Degeneration

To explore the link between bioenergetics and motor neuron degeneration, we used a computational model in which detailed morphology and ion conductance are paired with intracellular ATP production and consumption. We found that reduced ATP availability increases the metabolic cost of a single action potential and disrupts K+/Na+ homeostasis, resulting in a chronic depolarization. The magnitude of the ATP shortage at which this ionic instability occurs depends on the morphology and intrinsic conductance characteristic of the neuron. If ATP shortage is confined to the distal part of the axon, the ensuing local ionic instability eventually spreads to the whole neuron and involves fasciculation-like spiking events. A shortage of ATP also causes a rise in intracellular calcium. Our modeling work supports the notion that mitochondrial dysfunction can account for salient features of the paralytic disorder amyotrophic lateral sclerosis, including motor neuron hyperexcitability, fasciculation, and differential vulnerability of motor neuron subpopulations.

L‐type calcium channels and NMDA receptors: a determinant duo for short‐term nociceptive plasticity

In the dorsal horn of the spinal cord, pain‐transmitting neurons exhibit action potential windup, a form of short‐term plasticity, which consists of a progressive increase in neuronal response during repetitive stimulation of nociceptive input fibers. Windup depends on N‐methyl‐d‐aspartate (NMDA) receptor activation, but previous in vitro studies indicated that windup also relies on intrinsic plateau properties of spinal neurons. In the present study, we considered the possible involvement of these properties in windup in vivo. For this purpose, we first studied a nociceptive flexion reflex in the rat. We showed that windup of the reflex is actually suppressed by blockers of L‐type calcium current and Ca2+‐activated non‐specific cationic current (Ican), the two main depolarizing conductances of plateau potentials. We further showed that, during windup, NMDA receptors provide a critical excitatory component in a dynamic balance of excitatory and inhibitory inputs which ultimately activates L‐type calcium channels. The nociceptive reflex involves at least two neuronal groups, which may express intrinsic amplification properties, motor neurons and dorsal horn neurons. By means of extracellular recordings in the dorsal horn, we showed that windup of dorsal horn neuron discharge was sensitive to the modulators of L‐type calcium current. Altogether, our results suggest that, in vivo, windup also depends on the amplification properties of spinal neurons, the triggering of which requires previous activation of NMDA receptors.

Borna Disease Virus Infection Impairs Synaptic Plasticity

ABSTRACT The mechanisms whereby Borna disease virus (BDV) can impair neuronal function and lead to neurobehavioral disease are not well understood. To analyze the electrophysiological properties of neurons infected with BDV, we used cultures of neurons grown on multielectrode arrays, allowing a real-time monitoring of the electrical activity across the network shaped by synaptic transmission. Although infection did not affect spontaneous neuronal activity, it selectively blocked activity-dependent enhancement of neuronal network activity, one form of synaptic plasticity thought to be important for learning and memory. These findings highlight the original mechanism of the neuronal dysfunction caused by noncytolytic infection with BDV.

Mutation of the Protein Kinase C Site in Borna Disease Virus Phosphoprotein Abrogates Viral Interference with Neuronal Signaling and Restores Normal Synaptic Activity

Understanding the pathogenesis of infection by neurotropic viruses represents a major challenge and may improve our knowledge of many human neurological diseases for which viruses are thought to play a role. Borna disease virus (BDV) represents an attractive model system to analyze the molecular mechanisms whereby a virus can persist in the central nervous system (CNS) and lead to altered brain function, in the absence of overt cytolysis or inflammation. Recently, we showed that BDV selectively impairs neuronal plasticity through interfering with protein kinase C (PKC)–dependent signaling in neurons. Here, we tested the hypothesis that BDV phosphoprotein (P) may serve as a PKC decoy substrate when expressed in neurons, resulting in an interference with PKC-dependent signaling and impaired neuronal activity. By using a recombinant BDV with mutated PKC phosphorylation site on P, we demonstrate the central role of this protein in BDV pathogenesis. We first showed that the kinetics of dissemination of this recombinant virus was strongly delayed, suggesting that phosphorylation of P by PKC is required for optimal viral spread in neurons. Moreover, neurons infected with this mutant virus exhibited a normal pattern of phosphorylation of the PKC endogenous substrates MARCKS and SNAP-25. Finally, activity-dependent modulation of synaptic activity was restored, as assessed by measuring calcium dynamics in response to depolarization and the electrical properties of neuronal networks grown on microelectrode arrays. Therefore, preventing P phosphorylation by PKC abolishes viral interference with neuronal activity in response to stimulation. Our findings illustrate a novel example of viral interference with a differentiated neuronal function, mainly through competition with the PKC signaling pathway. In addition, we provide the first evidence that a viral protein can specifically interfere with stimulus-induced synaptic plasticity in neurons.

State-Dependent, Bidirectional Modulation of Neural Network Activity by Endocannabinoids

The endocannabinoid (eCB) system and the cannabinoid CB1 receptor (CB1R) play key roles in the modulation of brain functions. Although actions of eCBs and CB1Rs are well described at the synaptic level, little is known of their modulation of neural activity at the network level. Using microelectrode arrays, we have examined the role of CB1R activation in the modulation of the electrical activity of rat and mice cortical neural networks in vitro. We find that exogenous activation of CB1Rs expressed on glutamatergic neurons decreases the spontaneous activity of cortical neural networks. Moreover, we observe that the net effect of the CB1R antagonist AM251 inversely correlates with the initial level of activity in the network: blocking CB1Rs increases network activity when basal network activity is low, whereas it depresses spontaneous activity when its initial level is high. Our results reveal a complex role of CB1Rs in shaping spontaneous network activity, and suggest that the outcome of endogenous neuromodulation on network function might be state dependent.

Management and therapeutic perspectives in amyotrophic lateral sclerosis

ABSTRACT Introduction: Amyotrophic lateral sclerosis (ALS) is a fatal progressive neurodegenerative disorder affecting both upper and lower motor neurons. Despite much research and effort, no clear insights into a unifying hypothesis for the pathogenesis has so far emerged for this disease. Areas covered: We review the main pathophysiological hypotheses and the potential therapeutic targets in ALS, as well as the management of these patients (in order to improve their survival and quality of life). Expert commentary: ALS is a complex neurodegenerative disease, these days considered as a multisystem disorder with predominant motor symptoms (and various clinical forms). Further comprehension of the pathophysiology of this disease is required, although pathophysiological mechanisms (such as TDP-43) show promise in the search for new therapies. There is still no curative treatment for ALS, but the emergence of multidisciplinary specialized ALS clinics has increased both the quality of life and the survival of these patients.

Current view and perspectives in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS), identified as a distinct clinical entity by Charcot since the end of the nineteenth century, is a devastating and fatal neurodegenerative disorder that affects motor neurons in the brain, brainstem and spinal cord. Survival of patients with ALS is associated with several factors such as clinical phenotype, age at onset, gender, early presence of respiratory failure, weight loss and treatment with Riluzole (the only disease-modifying drug approved for this disease). Nowadays, there is still no curative treatment for ALS: palliative care and symptomatic treatment are therefore essential components in the management of these patients. Nevertheless, the scientific knowledge in the field of ALS motor neuron degeneration is growing, with the prospect of new treatments. Based on this physiopathological knowledge, several new therapeutic targets are being studied, involving various mechanisms such as excitotoxicity, neuroinflammation, mitochondrial dysfunction, oxidative stress, RNA metabolism and other attractive concepts. Moreover, it is also important to identify reliable biomarkers that will be essential components for future therapeutic development and study design in ALS. In this review, we present the main recent advances and promising therapeutics and biomarkers in the field of ALS.

Updating the classification of inherited neuropathies: Results of an international survey

Objective The continual discovery of disease-causing gene mutations has led to difficulties in the complex classification of Charcot-Marie-Tooth diseases (CMT) that needs to be revised. Methods We recently published a proposal to update the classification of inherited neuropathies. The reactions from colleagues prompted us to diffuse the proposal and ask people if they would be ready for such a change. We therefore performed an internet survey (from October 1, 2016, to December 1, 2016) that included more than 300 CMT worldwide specialists (practitioners and scientists) from various countries. A questionnaire (with proposals to update and simplify the way in which CMT is classified) was sent by e-mail to all participants in the last International Charcot-Marie-Tooth and Related Neuropathy Consortium meeting held in Venice, September 8–10, 2016 (as identified through an e-mail list). Results Of the 107 CMT specialists who answered the survey, 65% considered that changes are needed and that our proposals constituted an improvement over the historical classification of CMT. Conclusions Based on recent proposals in the medical literature, these results highlight that most specialists think that changes are needed to the classification of CMT.