Molecular mechanisms regulating early life stress induced alteration of Oxytocin

Early life stress (ELS), such as childhood abuse and/or neglect is associated with a strongly increased risk for developing major psychiatric diseases including depression, bipolar disorder, post-traumatic stress disorder (PTSD). A key feature shared by these disorders is deficient social functioning. Extensive research both in humans and in animal models demonstrate that ELS impacts on social motivation, reduces expression of social behaviours and increases aggressiveness. How does ELS alter social behaviour in adulthood and is it possible to restore normal social behaviour in ELS-exposed individuals? Although we do not have answers to these questions yet, recent progress in neurobiological research identified some key players that can link ELS and altered social behaviour. One of these key players is neuropeptide Oxytocin (Oxt). Oxt is an evolutionarily conserved neuropeptide which regulates various aspects of social behaviour including pair bonding, maternal, affiliative and sexual behaviour. Profound pro-social effects of Oxt have been observed both in laboratory animals and in humans demonstrating the importance of maintaining appropriate levels of Oxt for executing normal social functions. Because of the association between ELS and Oxt reported both in humans and in animal models, synthetic Oxt application for the use in psychiatric diseases has received wide attention. However, questions remain concerning potential side effects of long-term application of synthetic Oxt. An alternative strategy to restore Oxt level would be to use brain's own mechanism of regulating Oxt. However, as of now, we do not know how ELS alters Oxt level.

To identify molecules that regulate Oxt level upon ELS exposure, we need to use an animal model where we can identify and validate their functions through experimental testing in vivo. This work proposes to use zebrafish as a model system because 1) the molecules involved in its stress regulatory system are very similar to those in humans, (ii) the fish is small-sized and easy to house in large numbers making it convenient and cost-efficient for experimental usage, (iii) being a shoaling species, it displays robust social behaviour, (iv) its brain is transparent during development making it easy to study Oxt early in life. Therefore zebrafish is a powerful model to study how ELS affects Oxt. Harnessing the advantages of zebrafish, our lab has recently developed a genetically modified zebrafish to study ELS. In this model, we can change the level of key stress hormone, cortisol, during early life and examine the effects. Using this model, we observed that Oxt level is indeed changed following ELS also in zebrafish.

In this work, we will build upon this pilot data to find regulators that alter Oxt level following ELS. To this end, we will use state-of-art sequencing and genome analyses techniques. We will use a sophisticated gene manipulation techniques to test the function of identified regulators. Upon successful completion, this project will have developed a method to restore Oxt level in ELS-exposed animals and will have identify potential molecular targets that can be manipulated to restore normal social behaviour in ELS-exposed individuals.

This project will identify molecular mechanisms by which early life stress (ELS) exposure alters brain Oxytocin (Oxt) level. ELS profoundly changes adult behaviour and is a primary risk factor for developing psychiatric diseases. Oxt, a key regulator of social behaviour, has been implicated in social dysfunction after ELS as studies have shown alteration of Oxt or Oxt receptor levels following experimental ELS manipulation. Despite this association, currently the underlying molecular mechanism by which ELS regulates Oxt level is poorly understood. Animal studies of ELS using rodents highlight the importance of controlling stressor type, exposure and duration in order to achieve robust results. Recently, we developed a novel ELS model using optogenetics in zebrafish. In this model, using light, we could control precisely the level of cortisol, main stress hormone in zebrafish, during development allowing an unprecedented control of ELS exposure. We have obtained a pilot data demonstrating that ELS alters Oxt level in a subset of Oxt-positive cells. Building upon our pilot data, the goals of this project are to 1) identify exact subset of ELS-sensitve Oxt cells using single-cell RNA sequencing and single-cell projection analyses, 2) identify and validate molecules that regulate Oxt expression following ELS by combining cell-type-specific expression profiling, open chromatin analysis, and candidate molecule testing in zebrafish and 3) manipulate identified Oxt regulators in a cell-type specific manner and assess the effects of the manipulation in restoring normal Oxt level in ELS-exposed animals. Upon successful completion, this project will have identified molecular mechanisms by which ELS regulates Oxt level and developed a method to restore proper Oxt level in ELS-exposed animals. These studies will establish a platform for future approaches to restore normal social behavior in ELS fish and in other vertebrate models of ELS.