Final Report Summary - DMANDYB ASTROMORPH (Studying the dynamic structural interactions between neurons and glial cells at synapses of the central nervous system by using novel technical approaches)
Neuron-glial communication in the central nervous system (CNS) is fundamentally important for many brain processes including synaptic transmission and plasticity. Astrocytic processes contact the majority of excitatory synapses, forming tripartite structures with neurons. In the hippocampus, the morphology of those perisynaptic astrocytic processes (PAP) remodels rapidly and continuously. The present study investigates the physiological mechanisms driving PAP structural plasticity. In order to quantify PAP movement adjacent to dendritic spines, an index of motility was calculated. Viral gene delivery of farnesylated fluorescent proteins was used to achieve specific labelling of neurons and astrocytes membranes with Semliki forest viruses (SFV) having either a glial or a neuronal tropism. SFV were injected in hippocampal slice culture and neuronal spines and astrocytic processes were imaged for 20 min by confocal microscopy. The motility of PAP correlated with the degree of spine coverage. PAP covering less than 50 % of the spine surface were significantly more motile than others. Application of carbachol that elevates neuronal activity induced an increased motility of PAP covering more than 50 % of spine surface. Consistently, silencing neurons with TTX induced a decrease of PAP motility. In accordance with this, Schaffer Collaterals stimulation increases the rate of PAP movements. This evoked-increase of motility was inhibited by TTX and by metabotropic glutamate receptors antagonists LY367385 and MPEP. Intracellular calcium signalling appears to be central in astrocytes during their interactions with neurons. Whether astrocytic calcium excitability could influence PAP motility was then investigated. Genes of the exogenous Gq-coupled receptors MrgA1 or MrgC11 were specifically targeted to astrocytes. Indeed, application of Mrg receptors agonist FMRFa induced calcium increases. Moreover, FMRFa accelerated PAP motility by 20 ± 9 % to 36 ± 14 % in Mrg-expressing astrocytes. Indeed, chelating intracellular calcium with BAPTA significantly reduces PAP motility. We next investigate what could be the physiological role of PAP movements. It has been demonstrated that LTP induce higher spine coverage by astrocytes, suggesting a role of PAP during synaptic plasticity. Interestingly, theta-burst LTP stabilised PAP component as MI decreased after LTP. Taking together, those data strongly suggest that astrocytes participate in the mechanisms taking place during synaptic plasticity by a repositioning of PAP around the synapse. One could speculate that this stabilisation would help synapse efficacy by reducing the volume of the synaptic cleft. Moreover, the stabilisation of astrocytic membranes associated with an increased coverage of the synapse would undoubtedly elevate their membrane docking with neurons through adhesion proteins and then help stabilising spine during LTP. This may have important implication for understanding synapthopathies. It suggests, in turn, that astrocytes do participate in the mechanism of synaptic plasticity through calcium-drive morphological plasticity of their processes.