Towards modelling mental disorders in zebrafish. Neurexins severely modulate anxiety, affiliative social behaviour and aggression

Neurexin ( nrxn ) genes encode synaptic cell-adhesion molecules that have been repetitively associated with neurodevelopmental and mental disorders. While the zebrafish animal model offers tremendous advantages for dissecting neural development/function, no complete loss-of-function (LOF) nrxn zebrafish models are currently available. In this study, we generated the first collection of zebrafish knockout lines for each nrxn gene, with mutations ranging from transmembrane domain- to full genomic locus-deletions. Surprisingly, all homozygous lines developed normally, presenting no gross neurodevelopmental or obvious early behavioural abnormalities. However, this absence of early phenotypes translated into profound, paralog-specific behavioural alterations emerging during later developmental stages. All neurexin knockouts affected mating behaviour, complicating the generation and maintenance of homozygous lines. Except for this shared behavioural alteration, nrxn1 , but not nrxn2 or nrxn3 , led to marked changes in affiliative social behaviour and aggression. In contrast, nrxn2 mutants exhibited severe anxiety-like behaviours, including bottom-dwelling and repetitive freezing/seizure events. Strikingly, nrxn1 full-locus deletion mutants showed opposing behaviour, spending most of their time near the surface. The two also displayed opposite responses to open/closed field transitions; confinement alleviated nrxn2 anxiety but enhanced nrxn1 surface-dwelling. Meanwhile, nrxn3 mutants behaved normally in all our initial tests. In summary, our study introduces a complete set of zebrafish mutants covering the whole nrxn gene family, presenting striking adult behavioural alterations despite the absence of noticeable early defects; echoing the delayed onset of human psychiatric disorders such as schizophrenia. This work confirms the value of zebrafish to study mental disorders and unlock a novel platform to unravel the pathogenic contribution of neurexins and associated subtle neurodevelopmental changes/timing that drive the emergence of mental illnesses.