Functional consequences of synapse remodeling following astrocyte-specific regulation of ephrin-B1 in the adult hippocampus

Astrocyte-derived factors can control synapse formation and functions, making astrocytes an attractive target for regulating neuronal circuits and associated behaviors. Abnormal astrocyte-neuronal interactions are also implicated in neurodevelopmental disorders and neurodegenera-tive diseases associated with impaired learning and memory. However, little is known about astrocyte-mediated mechanisms that regulate learning and memory. Here, we propose astrocytic ephrin-B1 as a regulator of synaptogenesis in adult hippocampus and mouse learning behaviors. We found that astrocyte-specific ablation of ephrin-B1 in male mice triggers an increase in the density of immature dendritic spines and excitatory synaptic sites in the adult CA1 hippocampus. However, the prevalence of immature dendritic spines is associated with decreased evoked postsynaptic firing responses in CA1 pyramidal neurons, suggesting impaired maturation of these newly formed and potentially silent synapses or increased excitatory drive on the inhibitory neurons resulting in the overall decreased postsynaptic firing. Nevertheless, astrocyte-specific ephrin-B1 knock-out male mice exhibit normal acquisition of fear memory but enhanced contextual fear memory recall. In contrast, overexpression of astrocytic ephrin-B1 in the adult CA1 hippocampus leads to the loss of dendritic spines, reduced excitatory input, and impaired contextual memory retention. Our results suggest that astrocytic ephrin-B1 may compete with neuronal ephrin-B1 and mediate excitatorysynapseeliminationthroughitsinteractionswithneuronal EphBreceptors. Indeed, adeletionof neuronal EphBreceptorsimpairsthe ability of astrocytes expressing functional ephrin-B1 to engulf synaptosomes in vitro. Our findings demonstrate that astrocytic ephrin-B1 regulates long-term contextual memory by restricting new synapse formation in the adult hippocampus.