All-trans- retinoic acid stimulates translation and induces spine formation in hippocampal neurons through a membrane-associated RAR alpha

Differentiation and patterning in the developing nervous system require the vitamin A metabolite all-trans-retinoic acid (atRA). Recent data suggest that higher cognitive functions, such as creation of hippocampal memory, also require atRA and its receptors, RAR, through affecting synaptic plasticity. Here we show that within 30 min atRA increased dendritic growth similar to 2- fold, and PSD-95 and synaptophysin puncta intensity similar to 3- fold, in cultured mouse hippocampal neurons, suggesting increased synapse formation. atRA (10 nM) increased ERK1/2 phosphorylation within 10 min. In synaptoneurosomes, atRA rapidly increased phosphorylation of ERK1/2, its target 4E-BP, and p70S6K, and its substrate, ribosome protein S6, indicating activation of MAPK and mammalian target of rapamycin (mTOR). Immunofluoresence revealed intense dendritic expression of RAR alpha in the mouse hippocampus and localization of RAR alpha on the surfaces of primary cultures of hippocampal neurons, with bright puncta along soma and neurites. Surface biotinylation confirmed the locus of RAR alpha expression. Knockdown of RAR alpha by shRNA impaired atRA-induced spine formation and abolished dendritic growth. Prolonged atRA stimulation reduced surface/total RAR alpha by 43%, suggesting internalization, whereas brain-derived nerve growth factor or bicuculline increased the ratio by similar to 1.8fold. atRA increased translation in the somatodendritic compartment, similar to brain-derived nerve growth factor. atRA specifically increased dendritic translation and surface expression of the alpha-amino-3-hydroxyl-5methyl-4-isoxazole propionate receptor (AMPAR) subunit 1 (GluR1), without affecting GluR2. These data provide mechanistic insight into atRA function in the hippocampus and identify a unique membrane-associated RAR alpha that mediates rapid induction of neuronal translation by atRA.