Processes of DNA Methylation Are Involved in the Mechanisms of Amnesia Induction and Conditioned Food Aversion Memory Reconsolidation

DNA Methylation Machinery in Gastropod Mollusks

The role of DNA methylation in mollusks is just beginning to be understood. This review synthesizes current knowledge on this potent molecular hallmark of epigenetic control in gastropods-the largest class of mollusks and ubiquitous inhabitants of diverse habitats. Their DNA methylation machinery shows a high degree of conservation in CG maintenance methylation mechanisms, driven mainly by DNMT1 homologues, and the presence of MBD2 and MBD2/3 proteins as DNA methylation readers. The mosaic-like DNA methylation landscape occurs mainly in a CG context and is primarily confined to gene bodies and housekeeping genes. DNA methylation emerges as a critical regulator of reproduction, development, and adaptation, with tissue-specific patterns being observed in gonadal structures. Its dynamics also serve as an important regulatory mechanism underlying learning and memory processes. DNA methylation can be affected by various environmental stimuli, including as pathogens and abiotic stresses, potentially impacting phenotypic variation and population diversity. Overall, the features of DNA methylation in gastropods are complex, being an essential part of their epigenome. However, comprehensive studies integrating developmental stages, tissues, and environmental conditions, functional annotation of methylated regions, and integrated genomic-epigenomic analyses are lacking. Addressing these knowledge gaps will advance our understanding of gastropod biology, ecology, and evolution.

Dynamic association of epigenetic H3K4me3 and DNA 5hmC marks in the dorsal hippocampus and anterior cingulate cortex following reactivation of a fear memory

Epigenetic mechanisms such as DNA methylation and histone methylation are critical regulators of gene transcription changes during memory consolidation. However, it is unknown how these epigenetic modifications coordinate control of gene expression following reactivation of a previously consolidated memory. Here, we found that retrieval of a recent contextual fear conditioned memory increased global levels of H3 lysine 4-trimethylation (H3K4me3) and DNA 5-hydroxymethylation (5hmC) in area CA1 of the dorsal hippocampus. Further experiments revealed increased levels of H3K4me3 and DNA 5hmC within a CpG-enriched coding region of the Npas4, but not c-fos, gene. Intriguingly, retrieval of a 30-day old memory increased H3K4me3 and DNA 5hmC levels at a CpG-enriched coding region of c-fos, but not Npas4, in the anterior cingulate cortex, suggesting that while these two epigenetic mechanisms co-occur following the retrieval of a recent or remote memory, their gene targets differ depending on the brain region. Additionally, we found that in vivo siRNA-mediated knockdown of the H3K4me3 methyltransferase Mll1 in CA1 abolished retrieval-induced increases in DNA 5hmC levels at the Npas4 gene, suggesting that H3K4me3 couples to DNA 5hmC mechanisms. Consistent with this, loss of Mll1 prevented retrieval-induced increases in Npas4 mRNA levels in CA1 and impaired fear memory. Collectively, these findings suggest an important link between histone methylation and DNA hydroxymethylation mechanisms in the epigenetic control of de novo gene transcription triggered by memory retrieval.

Reconsolidation of Reminder-Induced Amnesia: Role of NMDA and AMPA Glutamate Receptors

We studied the role of glutamate receptors and reminder in the mechanisms of amnesia maintenance caused by disruption of conditioned food aversion reconsolidation with an antagonist of NMDA glutamate receptor in snails. At the early stage of amnesia (day 3 after induction), injection or NMDA of AMPA glutamate receptor antagonists prior to reminder (presentation of the conditioned food stimulus) led to memory recovery. Reminder alone or injection of antagonists without reminder or after reminder was ineffective. At the late stage of amnesia (day 10), antagonists/reminder had no effect on amnesia maintenance. It was hypothesized that reminder at the early stage of amnesia led to reactivation and reconsolidation of the molecular processes of amnesia including activation NMDA and AMPA glutamate receptors. Injection of antagonists of these receptors prior to reminder led to disruption of reactivation/reconsolidation of amnesia and recovery of the conditioned food aversion memory.