Spatial Distribution and Burden of Emerging Arboviruses in French Guiana.

Despite the health, social and economic impact of arboviruses in French Guiana, very little is known about the extent to which infection burden is shared between individuals. We conducted a large multiplexed serological survey among 2697 individuals from June to October 2017. All serum samples were tested for IgG antibodies against DENV, CHIKV, ZIKV and MAYV using a recombinant antigen-based microsphere immunoassay with a subset further evaluated through anti-ZIKV microneutralization tests. The overall DENV seroprevalence was estimated at 73.1% (70.6-75.4) in the whole territory with estimations by serotype at 68.9% for DENV-1, 38.8% for DENV-2, 42.3% for DENV-3, and 56.1% for DENV-4. The overall seroprevalence of CHIKV, ZIKV and MAYV antibodies was 20.3% (17.7-23.1), 23.3% (20.9-25.9) and 3.3% (2.7-4.1), respectively. We provide a consistent overview of the burden of emerging arboviruses in French Guiana, with useful findings for risk mapping, future prevention and control programs. The majority of the population remains susceptible to CHIKV and ZIKV, which could potentially facilitate the risk of further re-emergences. Our results underscore the need to strengthen MAYV surveillance in order to rapidly detect any substantial changes in MAYV circulation patterns.

Microglia modulate gliotransmission through the regulation of VAMP2 proteins in astrocytes.

Vesicular release is one of the release mechanisms of various signaling molecules. In neurons, the molecular machinery involved in vesicular release has been designed through evolution to trigger fast and synchronous release of neurotransmitters. Similar machinery with a slower kinetic and a slightly different molecular assembly allows astrocytes to release various transmitters such as adenosine triphosphate (ATP), glutamate, and D-serine. Astrocytes are important modulators of neurotransmission through gliotransmitter release. We recently demonstrated that microglia, another type of glia, release ATP to modulate synaptic transmission using astrocytes as intermediate. We now report that microglia regulate astrocytic gliotransmission through the regulation of SNARE proteins in astrocytes. Indeed, we found that gliotransmission triggered by P2Y1 agonist is impaired in slices from transgenic mice devoid of microglia. Using total internal reflection fluorescence imaging, we found that the vesicular release of gliotransmitter by astrocytes was different in cultures lacking microglia compared to vesicular release in astrocytes cocultured with microglia. Quantification of the kinetic of vesicular release indicates that the overall release appears to be faster in pure astrocyte cultures with more vesicles close to the membrane when compared to astrocytes cocultured with microglia. Finally, biochemical investigation of SNARE protein expression indicates an upregulation of VAMP2 in absence of microglia. Altogether, these results indicate that microglia seems to be involved in the regulation of an astrocytic phenotype compatible with proper gliotransmission. The mechanisms described in this study could be of importance for central nervous system diseases where microglia are activated.

Astrocyte-neuron communication: functional consequences.

Astrocyte-neuron communication has recently been proposed as a potential mechanism participating to synaptic transmission. With the development of this concept and accumulating evidences in favor of a modulation of synaptic transmission by astrocytes, has emerged the term gliotransmission. It refers to the capacity of astrocytes to release various transmitters, such as ATP, glutamate, D-serine, and GABA in the vicinity of synapses. While the cellular mechanisms involved in gliotransmission still need to be better described and, for some, identified, the aim of more and more studies is to determine the role of astrocytes from a functional point of view. This review will summarize the principal studies that have investigated a potential role of astrocytes in the various functions regulated by the brain (sleep, breathing, perception, learning and memory…). This will allow us to highlight the similarities and discrepancies in the signaling pathways involved in the different areas of the brain related to these functions.

Microglia activation triggers astrocyte-mediated modulation of excitatory neurotransmission.

Fine control of neuronal activity is crucial to rapidly adjust to subtle changes of the environment. This fine tuning was thought to be purely neuronal until the discovery that astrocytes are active players of synaptic transmission. In the adult hippocampus, microglia are the other major glial cell type. Microglia are highly dynamic and closely associated with neurons and astrocytes. They react rapidly to modifications of their environment and are able to release molecules known to control neuronal function and synaptic transmission. Therefore, microglia display functional features of synaptic partners, but their involvement in the regulation of synaptic transmission has not yet been addressed. We have used a combination of pharmacological approaches with electrophysiological analysis on acute hippocampal slices and ATP assays in purified cell cultures to show that activation of microglia induces a rapid increase of spontaneous excitatory postsynaptic currents. We found that this modulation is mediated by binding of ATP to P2Y1R located on astrocytes and is independent of TNFα or NOS2. Our data indicate that, on activation, microglia cells rapidly release small amounts of ATP, and astrocytes, in turn, amplified this release. Finally, P2Y1 stimulation of astrocytes increased excitatory postsynaptic current frequency through a metabotropic glutamate receptor 5-dependent mechanism. These results indicate that microglia are genuine regulators of neurotransmission and place microglia as upstream partners of astrocytes. Because pathological activation of microglia and alteration of neurotransmission are two early symptoms of most brain diseases, our work also provides a basis for understanding synaptic dysfunction in neuronal diseases.

Is astrocyte calcium signaling relevant for synaptic plasticity?

Astrocytes constitute a major group of glial cells which were long regarded as passive elements, fulfilling nutritive and structural functions for neurons. Calcium rise in astrocytes propagating to neurons was the first demonstration of direct interaction between the two cell types. Since then, calcium has been widely used, not only as an indicator of astrocytic activity but also as a stimulator switch to control astrocyte physiology. As a result, astrocytes have been elevated from auxiliaries to neurons, to cells involved in processing synaptic information. Curiously, while there is evidence that astrocytes play an important role in synaptic plasticity, the data relating to calcium's pivotal role are inconsistent. In this review, we will detail the various mechanisms of calcium flux in astrocytes, then briefly present the calcium-dependent mechanisms of gliotransmitter release. Finally, we will discuss the role of calcium in plasticity and present alternative explanations that could reconcile the conflicting results published recently.

MafA transcription factor identifies the early ret-expressing sensory neurons.

Dorsal root ganglia proceed from the coalescence of cell bodies of sensory neurons, which have migrated dorsoventrally from the delaminating neural crest. They are composed of different neuronal subtypes with specific sensory functions, including nociception, thermal sensation, proprioception, and mechanosensation. In contrast to proprioceptors and thermonociceptors, little is known about the molecular mechanisms governing the early commitment and later differentiation into mechanosensitive neurons. This is mainly due to the absence of specific molecular markers for this particular cell type. Using knockout mice, we identified the bZIP transcription factor MafA as the first specific marker of a subpopulation of "early c-ret" positive neurons characterized by medium-to-large diameters. This marker will allow further functional characterization of these neurons.