Zebrafish models of Major Depressive Disorder as a replacement for rodent models

Why did we fund this project?

This award aims to replace mice in some studies of genetic mutations that contribute to major depressive disorder (MDD) using larval zebrafish.

Treatments for MDD are effective in only around 30% of patients. Analysing patient genomes has identified a number of candidate genes that are associated with MDD and may provide new targets for treatment. Breeding transgenic mice to understand the impact of genetic mutations typically requires stable lines of animals to be bred and maintained. However, genetic mutations can be induced in zebrafish embryos without needing to for breed a stable line. Zebrafish share many genes with humans and based on current scientific thinking, their larvae are not considered capable of suffering before they independently feed at five days post-fertilisation. The student, with Professor Robert Hindges, will develop a larval zebrafish model to assess the impact of candidate genes on behaviours linked with MDD, including sleep and reduced appetite and locomotion.

Major Depressive Disorder (MDD) is a debilitating psychiatric disorder affecting millions of people world-wide. It is characterised by a persistent low mood and/or a loss of pleasure or interest in activities which were once enjoyed (anhedonia) together with other symptoms, including loss of appetite or disruption of sleep. The causes are poorly understood and drug treatments are only effective in about 30% of the patients. Genome Wide Association Studies using genomic data from thousands of patients and controls, have identified candidate loci associated with MDD. Animal models have been widely used to study the genetic basis for MDD. However, to date those models are almost exclusively based on the use of mice. This is problematic in terms of 3Rs, due to the severity levels of procedures, the high number of adult mice that need to be produced and assessed and the low throughput for drug screens for validity. Thus, there is a significant need for better, more 3Rs-relevant alternatives.

This project aims to establish and disseminate the zebrafish model as such an alternative. Zebrafish have a highly similar genome to humans, conserver neurotransmitter pathways and brain areas and show relevant behavioural responses. Importantly, they can be used at unprotected larval stages (at or before 5 days of age), even in behavioural assays or functional brain imaging. Furthermore, using novel genome editing techniques, it is possible to create gene deletions in an embryo and then study the developing larva directly. This eliminates the need for making stable mutant lines as usually done in mice. Finally, zebrafish larvae can be used for high-throughput drug screen, increasing the chance to making a significant advance in the  search for novel therapeutic strategies.

Our project will have a significant 3Rs impact. To generate a novel mouse model and assess it subsequently in different behavioural, cellular and functional assays, about 500 mice are used per gene mutation. With our new zebrafish model, we plan to assess 10 candidate genes. This means we are replacing the use of about 5000 mice locally. Furthermore, through our high-throughput screening ability in zebrafish larvae, we provide a pre-screening method, should there be the necessity to test compounds also in a mammalian system, leading to a significant reduction of used rodents for screens. World-wide, MDD is intensely studied and in the last five years >6500 publications have described the use of mice for this purpose. With our clear dissemination and open promotion plan, we estimate a measurable change away from rodent models towards the use of zebrafish alternatives to create a clear legacy of our work.

In addition to the high significance for 3Rs impact, the project will result in novel high-quality data to advance our general knowledge on the genetic basis for MDD. We will use strict selection criteria to identify appropriate candidate genes, based on the latest available GWAS data from our collaborator. We then will create transient mutants for at least ten candidate genes using our novel genome editing approach and validate them in an established assessment pipeline we used previously in a pilot experiment. The behavioural assessment will focus on locomotion/anxiety, stress, sleep, appetite, and social interactions (as a measure for anhedonia). Furthermore, we will determine some of the biochemical markers relevant in depression. Finally, we will use an automated high throughput screening platform, already existing in our lab, to identify potential new compounds relevant for the therapeutic treatment of MDD.

The PhD student will be integrated in a high quality and interactive environment to provide excellent scientific and 3Rs relevant support throughout the project.