Larp1 supports brain growth and spatial memory via post-transcriptional control of the translation machinery.

In the brain, tight regulation of the translation of mRNAs is essential for development and plasticity. The translation machinery itself is largely encoded by mRNAs with terminal oligopyrimidine (TOP) motifs, which can be post-transcriptionally controlled by the mTOR signaling pathway. In neurons, these mRNAs are selectively enriched in axons, dendrites and synapses, suggesting local functions for their regulation. Here, we use a brain-specific knockout of the mTOR effector and TOP mRNA binding protein, Larp1, to uncover its role in brain development and behavior. Loss of Larp1 significantly decreases brain mass and reduces the density of neurons. We find that TOP mRNAs levels are depleted by more than 50% and selectively lost from synapses, reversing the enrichment that occurs when Larp1 is present. In behavior tests, Larp1-deficient mice are severely impaired in spatial learning and memory. These results demonstrate a critical role for Larp1 in maintaining the levels of essential mRNAs necessary for brain growth and highlight the importance of post-transcriptional regulation by mTOR for normal learning and memory.