DNA methylation and the regulation of stimulus-specific memory formation

Abstract

Memory formation is a crucial task of the brain. It allows animals to dynamically respond to a changing environment by combining information about current and previous experiences. Thus, it promotes complex behaviours such as living in social groups and elaborate foraging tactics. The ability to form memories is present across the animal kingdom in a variety of forms. The honeybee -an eusocial insect -is capable of both 'simple' associative and complex rule learning. Despite decades of research into the mechanisms of memory formation, however, much remains unknown.One little-understood aspect is the regulation of molecular mechanisms during memory formation.Understanding regulation of transcription is particularly important in this context, as transcription is a requirement for any stable memory. Epigenetic mechanisms regulate transcription by directly interacting with chromatin. Importantly, epigenetic mechanisms are conserved across species as distinct as humans and honeybees. This thesis aims to investigate one particular epigenetic mechanism -DNA methylation -and its role in honey bee memory formation by using behavioural, physiological and molecular assays.First, I studied the role of DNA methyltransferases (Dnmts), which catalyse DNA methylation, after olfactory reward learning. Bees were trained to associate an odour with a sugar reward. Dnmts were then blocked after conditioning using a pharmacological inhibitor. 24 hours later, bees were tested for memory retention and generalisation. Dnmt inhibition increased the generalisation to an odour that was not present during the training, thus impairing stimulus-specific memory. This effect was learning-dependent, as bees' response to odours or sugar water alone did not change after treatment. Furthermore, this effect was robust against alterations in the training paradigm, but the directionality depended on the number of training trials. Next, I used Ca2+ -imaging of the bees' primary olfactory centre (i.e. antennal lobe, AL) to investigate whether Dnmts affect changes in AL processing established during memory formation. The AL is involved in odour discrimination and olfactory learning; both processes are also crucial for stimulus-specific memory formation. If Dnmts were inhibited after olfactory reward learning, the AL response to a new odour changed 48 hours later. This effect potentially serves as a functional explanation for the behavioural phenotype observed after Dnmt inhibition. Furthermore, this study provides the first in vivo evidence for Dnmt-mediated regulation of neuronal networks during memory formation.As DNA methylation regulates transcription, I next investigated the effect of Dnmt inhibition on gene expression. Using qPCR I studied 30 memory-associated genes. In response to Dnmt inhibition, 9 genes were upregulated after conditioning. For some of these genes I then investigated their i methylation patterns using a bisulfite conversion based Mass spectrometry approach. Memory formation changed the methylation pattern compared to both...