Quantitative Proteomics Reveals Distinct Molecular Signatures of Different Cerebellum-Dependent Learning Paradigms

Distinct Proteomic Brain States Underlying Long-Term Memory Formation in Aversive Operant Conditioning

Long-term memory (LTM) formation relies on de novo protein synthesis; however, the full complement of proteins crucial to LTM formation remains unknown in any system. Using an aversive operant conditioning model of aerial respiratory behavior in the pond snail mollusk, Lymnaea stagnalis (L. stagnalis), we conducted a transcriptome-guided proteomic analysis on the central nervous system (CNS) of LTM, no LTM, and control animals. We identified 366 differentially expressed proteins linked to LTM formation, with 88 upregulated and 36 downregulated in LTM compared to both no LTM and controls. Functional annotation highlighted the importance of balancing protein synthesis and degradation for LTM, as indicated by the upregulation of proteins involved in proteasome activity and translation initiation, including EIF2D, mRNA levels of which were confirmed to be upregulated by conditioning and implicated nuclear factor Y as a potential regulator of LTM-related transcription in this model. This study represents the first transcriptome-guided proteomic analysis of LTM formation ability in this model and lays the groundwork for discovering orthologous proteins critical to LTM in mammals.

Kdm3b haploinsufficiency impairs the consolidation of cerebellum-dependent motor memory in mice

Histone modifications are a key mechanism underlying the epigenetic regulation of gene expression, which is critically involved in the consolidation of multiple forms of memory. However, the roles of histone modifications in cerebellum-dependent motor learning and memory are not well understood. To test whether changes in histone methylation are involved in cerebellar learning, we used heterozygous Kdm3b knockout (Kdm3b+/-) mice, which show reduced lysine 9 on histone 3 (H3K9) demethylase activity. H3K9 di-methylation is significantly increased selectively in the granule cell layer of the cerebellum of Kdm3b+/- mice. In the cerebellum-dependent optokinetic response (OKR) learning, Kdm3b+/- mice show deficits in memory consolidation, whereas they are normal in basal oculomotor performance and OKR acquisition. In addition, RNA-seq analyses revealed that the expression levels of several plasticity-related genes were altered in the mutant cerebellum. Our study suggests that active regulation of histone methylation is critical for the consolidation of cerebellar motor memory.

Decreased intrinsic excitability of cerebellar Purkinje cells following optokinetic learning in mice

The optokinetic response (OKR), a reflexive eye movement evoked by a motion of the visual field, is known to adapt its strength to cope with an environmental change throughout life, which is a type of cerebellum-dependent learning. Previous studies suggested that OKR learning induces changes in in-vivo spiking activity and synaptic transmission of the cerebellar Purkinje cell (PC). Despite the recent emphasis on the importance of the intrinsic excitability related to learning and memory, the direct correlation between the intrinsic excitability of PCs and OKR learning has not been tested. In the present study, by utilizing the whole-cell patch-clamp recording, we compared the responses of cerebellar PCs to somatic current injection between the control and learned groups. We found that the neurons from the learned group showed a significant reduction in mean firing rate compared with neurons in the control group. In the analysis of single action potential (AP), we revealed that the rheobase current for the generation of single AP was increased by OKR learning, while AP threshold, AP amplitude, and afterhyperpolarization amplitude were not altered. Taken together, our result suggests that the decrease in the intrinsic excitability was induced in the cerebellar PC of learned group by an increase in the current threshold for generating AP.