Absence of microglial CX3CR1 impairs the synaptic integration of adult-born hippocampal granule neurons.

Microglia are immune cells that play a crucial role in maintaining brain homeostasis. Among the mechanisms of communication between microglia and neurons, the CX3CL1/CX3CR1 axis exerts a central modulatory role. Animals lacking CX3CR1 microglial receptor (CX3CR1-/- mice) exhibit marked alterations not only in microglia but also in neurons located in various regions of the brain. Here we show that microglial depletion of CX3CR1 leads to the deficient synaptic integration of adult-born granule neurons in the dentate gyrus (DG), both at the afferent and efferent level. Regarding the alterations in the former level, these cells show a reduced number of dendritic spines, which also exhibit morphological changes, namely enlargement and shortening. With respect to changes at the efferent level, these cells show a reduced area of axonal terminals. Both at the afferent and efferent level, synapses show ultrastructural enlargement, but they are depleted of synaptic vesicles, which suggests impaired functionality. We also show that selective increased microglial activation and extracellular matrix deposition in the zones in which the afferent synaptic contacts of these cells occur, namely in the molecular and the granule layer of the DG. In order to evaluate the impact of these structural alterations from a functional point of view, we performed a battery of behavioral tests related to hippocampal-dependent emotional behavior. We observed that female CX3CR1-/- mice exhibit a hyperactive, anxiolytic-like and depressive-like phenotype. These data shed light on novel aspects of the regulation of adult hippocampal neurogenesis by microglia that could be highly relevant for research into mood disorders.

Soluble Tau has devastating effects on the structural plasticity of hippocampal granule neurons.

Tau is a neuronal microtubule-associated protein with countless physiological functions. Although the detrimental effects of insoluble aggregated Tau have been widely studied, recent evidence supports the notion that soluble Tau (composed mostly of monomers and dimers) is also toxic for neurons. Here we evaluated the long-term impact of a single stereotaxic injection of human soluble Tau on hippocampal granule neurons in mice. At the ultrastructural level, soluble Tau reduced the number of afferent synapses and caused a dramatic depletion of synaptic vesicles both in afferent and efferent synapses. Furthermore, the use of an RFP-expressing retrovirus revealed that soluble Tau altered the morphology of newborn granule neurons and reduced their afferent (dendritic spines) and efferent (mossy fiber terminals) connectivity. Finally, soluble Tau caused specific impairment of behavioral pattern separation capacity. Our results thus demonstrate for the first time that soluble Tau causes long-term detrimental effects on the morphology and connectivity of newborn granule neurons and that these effects correlate with impaired behavioral pattern separation skills. These data might be relevant for the field of neurodegenerative disorders, since they contribute to reinforcing the pathological roles played by distinct Tau species in vivo.