Translational control in filial imprinting

Abstract

The brain implements experience-dependent changes in behavior throughout life. Yet certain forms of learning can only occur within restricted windows of time, known as critical periods. The mechanisms engaged during these developmental stages remain largely unknown. Moreover, it is unclear whether young and adult brains use the same mechanisms to support plasticity.;It is well established that persistent experience-dependent changes in behavior require protein synthesis. Thus the molecular hubs controlling experience-dependent protein synthesis may hold the answer to what mechanisms underlie behavioral changes early in life, when protein synthesis peaks. Uncovering the molecular mechanisms controlling protein synthesis during critical periods may help to rejuvenate adult plasticity. My thesis tested this hypothesis, investigating the signaling pathways underlying imprinting in chickens, a protein-synthesis-dependent form of learning constrained to an early critical period. To unravel the molecular mechanism controlling the formation of imprinted memories, I established a behavioral assay to measure both auditory and visual imprinting. Training-dependent behavioral and structural changes were used to track brain plasticity during the critical period and the effect of manipulations. Direct quantification of translation rates in vivo with surface sensing of translation (SUnSET), within imprinting-relevant forebrain areas, confirmed that our training method enhanced protein synthesis in the medial nidopallium/mesopallium (MNM), an area required for auditory imprinting and in the intermediate medial mesopallium (IMM), a nucleus important for visual imprinting. Additionally, combining DiOlistic labeling and confocal imaging I demonstrated that training leads to an experience-dependent increase in the number of mushroom-type spines in these brain regions.;Translational control through dephosphorylation of the Eukaryotic translation initiation factor 2 alpha (eIF2alpha) mediates memory formation and synaptic plasticity in adult rodents. I found that imprinting to virtual objects and arbitrary sounds, triggers the dephosphorylation of elF2alpha in MNM but not in IMM. Consistently with western blot analysis, inhibition elF2alpha upstream phosphatases (GADD34-PP1 and CReP-PP1) selectively impaired auditory imprinting. In contrast, blockade of the elF2alpha kinase PKR and direct inhibition of phosphorylated elF2alpha (pelF2alpha) facilitated auditory imprinting and restored memory formation outside of the critical period (4 days after hatching). Structural plasticity in MNM was abolished in chickens where elF2alpha dephosphorylation was blocked, suggesting that elF2alpha dephosphorylation supports the reorganization of neural circuits involved in auditory imprinting. Together these findings provide the first evidence of a translational control mechanism regulating the formation of imprinted auditory memories within a critical period.;Protein synthesis supports not only auditory but also visual imprinting. However, elF2a dephosphorylation was not required for visual imprinting. Thus, I next focused on the mechanistic target of rapamycin complex 1 (mTORC1) in imprinting, a protein complex that can regulate translation initiation, polypeptide elongation and mRNA transcription in response to a learning experience. mTORC1-mediated protein synthesis regulation is important for adult memory formation but its role early in life is also largely unknown. I found that imprinting training activates mTORC1 in MNM and IMM, as measured by means of increased activation of its downstream target S6K. Inactivation of mTORC1 disrupted imprinting across sensory modality and blocked structural plasticity of dendritic spines in MNM and IMM. Importantly, direct pharmacological activation of the AKT/mTORC1 pathway, or through the thyroid hormone receptor (THr), reopened the critical period, recovering visual and auditory memory formation 4 days after hatching. These results demonstrate for the first time in vertebrates that, early in life, mTORC1 controls the formation of imprinted memories.