SUMMARY Translational control of mRNAs allows for rapid and selective changes in synaptic protein expression, changes that are required for long-lasting plasticity and memory formation in the brain. Fragile X Related Protein 1 (FXR1P) is an RNA-binding protein that controls mRNA translation in non-neuronal cells and co-localizes with translational machinery in neurons. However, its neuronal mRNA targets and role in the brain are unknown. Here, we demonstrate that removal of FXR1P from the forebrain of postnatal mice selectively enhances long-term storage of spatial memories, hippocampal late-phase LTP (L-LTP) and de novo GluA2 synthesis. Furthermore, FXR1P binds specifically to the 5’UTR of GluA2 mRNA to repress translation and limit the amount of GluA2 incorporated at potentiated synapses. This study uncovers a new mechanism for regulating long-lasting synaptic plasticity and spatial memory formation and reveals an unexpected divergent role of FXR1P among Fragile X proteins in brain plasticity.

Astrocytes secrete factors that promote neuron survival, synapse formation, and plasticity. Understanding how these factors perform these roles requires a robust in vitro system that can effectively assess the impact of individual glial factors on neuronal properties. A classical approach to studying neuron-glial interactions in vitro uses a system where dissociated embryonic rat neurons are suspended over a feeder layer of rat astrocytes. Here, we describe a useful "sandwich" co-culture system where postnatal mouse hippocampal neurons are grown suspended above a feeder layer of mouse hippocampal astrocytes. We demonstrate that neurons in these cultures remain healthy beyond 3 weeks in vitro and develop more synapses compared to neurons grown without astrocytes. An advantage of this method is that astrocytes and neurons can be prepared separately from postnatal transgenic or knock-out mouse lines allowing one to study, for example, how wild-type neurons develop in the presence of astrocytes from a knock-out mouse line that lacks the expression of a specific astrocyte-secreted factor. We find this culture system to be a convenient and powerful approach to study the contribution of astrocyte-secreted molecules to neuron development.

The formation and storage of memories in neuronal networks relies on new protein synthesis, which can occur locally at synapses using translational machinery present in dendrites and at spines. These new proteins support long-lasting changes in synapse strength and size in response to high levels of synaptic activity. To ensure that proteins are made at the appropriate time and location to enable these synaptic changes, messenger RNA (mRNA) translation is tightly controlled by dendritic RNA-binding proteins. Fragile X Related Protein 1 (FXR1P) is an RNA-binding protein with high homology to Fragile X Mental Retardation Protein (FMRP) and is known to repress and activate mRNA translation in non-neuronal cells. However, unlike FMRP, very little is known about the role of FXR1P in the central nervous system. To understand if FXR1P is positioned to regulate local mRNA translation in dendrites and at synapses, we investigated the expression and targeting of FXR1P in developing hippocampal neurons in vivo and in vitro. We found that FXR1P was highly expressed during hippocampal development and co-localized with ribosomes and mRNAs in the dendrite and at a subset of spines in mouse hippocampal neurons. Our data indicate that FXR1P is properly positioned to control local protein synthesis in the dendrite and at synapses in the central nervous system.