The role of microRNAs in learned safety

As fear can be generated and increased by learning processes, likewise also learned inhibition mechanisms of fear do exist. Conditioned inhibition of fear is a fear inhibitory mechanism, which involves learning of safety signals and is therefore referred to as learned safety. Learned safety includes acquisition of safety signals, which, beyond the regulation of fear responses and identification of episodes of security also relate to positive affective conditions, eliciting reward- related approach and a reduction of depression-like behavior in mice. While some selected insights into the neural underpininngs have been obtained and evidence for its translational potential exist, the molecular mechanisms of learned safety remain incompletely understood. The objective of the present project is to shed light on the role of small RNA molecules, microRNAs (miRNAs) which are important regulators of neuronal physiology and pathology, in learned safety. Specifically, we aim to investigate the contribution of selected miRNA species to the structure and function of the amygdala, a brain region critically involved in learned safety, examine their role in the behavioral learned safety response in mice. Additionally we will use an unbiased screening approach to characterize additional miRNA regulatory networks in the amygdala in order to reveal the involvement of hitherto undiscovered molecular pathways with possible relevance for learned safety. Learned safety can be considered as relatively unexplored tool for neuropsychiatric research along the newly established Research Domain Criteria system where it can be applied as paradigm for the examination with in the sub-construct reward prediction within the positive valence system. Learned safety suits well this effort targeting the identification and characterization of core features with relevance spanning more than one of the traditional disorder categories. Data obtained from the proposed approach may elucidate some of the relevant molecular processes of this behavioral feature and present a starting for future research endeavors with the ultimate goal to fill, as tight as possible, the matrix which can be applied to characterize the learned safety-domain from all different angles amenable to currently available to tools of neuroscientific research.

Micro RNAs (miRNAs) are small RNA molecules that modulate the post-transcriptional expression of genes in many body organs, including the brain, where they regulate functions like neuronal outgrowth and synaptic functions. The role of brain miRNAs in the control of synaptic plasticity and fear responses remained however unknown. This project studied the involvement of miRNAs in Learned Safety (LS); the association of certain neutral stimuli (like places and objects) with the absence of threats and security. Two main brain regions crucial for emotional processing were examined; amygdala and hippocampus. It was discovered that the amygdalar miRNAs 132/212 were critical for LS expression, and new target genes regulated by these miRNAs were identified. The research also found that the lack of miRNAs 132/212 altered the expression of hippocampal acetylcholine receptors and kinase proteins, leading to stronger synaptic depression and weakened synaptic strengthening in response to nicotine. These findings unveiled a critical interaction between miRNAs and nicotinergic signaling influencing neuroplasticity at brain structures required for LS, highlighting the significance of amygdalar miRNAs for LS. Fear is a natural response vital for the subsistence of many species, as it allows behavioral adaptation to potential dangers in the environment. Remembering what to fear is hence critical for survival and well-being. Interestingly, brain hypoxia disrupts the formation of fear memories through still unclarified mechanisms. Using an ischemia model, the project revealed that miRNAs 132/212 gene deletion worsened hippocampal synaptic transmission disruption and impaired the expression of proteins important for acetylcholine signaling under hypoxic conditions. These findings therefore proposed miRNAs 132/212 as part of a mechanism protecting fear-processing synaptic circuits during acute cerebral ischemia. This work also unveiled that the miRNAs 132/212 were essential for tuning the differential effects that nicotine exert on synaptic plasticity depending on both, age and hippocampal sub-region, indicating that miRNAs are age-sensitive regulators of the actions of neurotransmitter that modulate fear. In humans, the steroid hormone cortisol plays also crucial roles in emotional processing, modulating fear-related responses and fear-extinction learning. This research also revealed that the miRNAs 132/212 could selectively regulate the effects of steroid hormones on hippocampal functions depending on the region, thus likely aiding in the fine-tuning of emotional memories involved in the expression of LS. This research therefore provided novel insights into the brain mechanisms underlying the expression of fear and emotions, and highlighted that specific miRNAs in different brain regions act in a multi-circuited and age-dependent manner. The project generated five peer-reviewed scientific articles already garnering tens of citations despite being recently published, showcasing the impact of this research across disciplines. Taken together, this work shaded lights on the intricate brain molecular machinery underlying LS processing and emotional behaviors.