In most mammals, including humans, the postnatal acquisition of normal social and nonsocial behavior critically depends on interactions with peers. Here we explore the possibility that mixed-group housing of mice carrying a deletion of Nlgn3, a gene associated with autism spectrum disorders, and their wild-type littermates induces changes in each other’s behavior. We have found that, when raised together, male Nlgn3 knockout mice and their wild-type littermates displayed deficits in sociability. Moreover, social submission in adult male Nlgn3 knockout mice correlated with an increase in their anxiety. Re-expression of Nlgn3 in parvalbumin-expressing cells in transgenic animals rescued their social behavior and alleviated the phenotype of their wild-type littermates, further indicating that the social behavior of Nlgn3 knockout mice has a direct and measurable impact on wild-type animals’ behavior. Finally, we showed that, unlike male mice, female mice lacking Nlgn3 were insensitive to their peers’ behavior but modified the social behavior of their littermates. Altogether, our findings show that the environment is a critical factor in the development of behavioral phenotypes in transgenic and wild-type mice. In addition, these results reveal that the social environment has a sexually dimorphic effect on the behavior of mice lacking Nlgn3, being more influential in males than females.

Deletions in the 15q11.2 region of the human genome are associated with neurobehavioral deficits, and motor development delay, as well as in some cases, symptoms of autism or schizophrenia. The cytoplasmic FMRP-interacting protein 1 (CYFIP1) is one of the four genes contained within this locus and has been associated with other genetic forms of autism spectrum disorders (ASD). In mice, Cyfip1 haploinsufficiency leads to alteration of dendritic spine morphology and defects in synaptic plasticity, two pathophysiological hallmarks of mouse models of ASD. At the behavioral level, however, Cyfip1 haploinsufficiency leads to minor phenotypes, not directly relevant for 15q11.2 deletion syndrome or ASD. A fundamental question is whether neuronal phenotypes caused by the mutation of Cyfip1 are relevant for the human condition. Here, we describe a synaptic cluster of ASD-associated proteins centered on CYFIP1 and the adhesion protein Neuroligin-3. Cyfip1 haploinsufficiency in mice led to decreased dendritic spine density and stability associated with social behavior and motor learning phenotypes. Behavioral training early in development resulted in alleviating the motor learning deficits caused by Cyfip1 haploinsufficiency. Altogether, these data provide new insight into the neuronal and behavioral phenotypes caused by Cyfip1 mutation and proof-of-concept for the development of a behavioral therapy to treat phenotypes associated with 15q11.2 syndromes and ASD.

The regulation of neuronal soma size is essential for appropriate brain circuit function and its dysregulation is associated with several neurodevelopmental disorders. A defect in the dendritic growth and elaboration of motor neocortical pyramidal neurons in neonates lacking neuregulin-4 (NRG4) has previously been reported. In this study, we investigated if the loss of NRG4 causes further morphological defects that are specific to these neurons. We analysed the soma size of pyramidal neurons of layers 2/3 and 5 of the motor cortex and a subpopulation of multipolar interneurons in this neocortical region in Nrg4 +/+ and Nrg4 -/mice. There were significant decreases in pyramidal neuron soma size in Nrg4 -/mice compared with Nrg4 +/+ littermates at all stages studied (P10, P30 and P60). The reduction was especially marked at P10 and in layer 5 pyramidal neurons. Soma size was not significantly different for multipolar interneurons at any age. This in vivo phenotype was replicated in pyramidal neurons cultured from Nrg4 -/mice and was rescued by neuregulin-4 treatment. Analysis of a public single-cell RNA sequencing repository revealed discrete Nrg4 and Erbb4 expression in subpopulations of layer 5 pyramidal neurons, suggesting that the observed defects were due in part to loss of autocrine Nrg4/ErbB4 signalling. The pyramidal phenotype in the motor cortex of Nrg4 -/mice was associated with a lack of Rotarod test improvement in P60 mice, suggesting that absence of NRG4 causes alterations in motor performance. Significance StatementNeuregulins are growth factors that are abundantly expressed in the nervous system where they regulate a plethora of processes essential for normal nervous system development and function in adulthood. Dysregulation of neuregulin signalling has been implicated in neurodevelopmental disorders, thus characterising the particular functions of members of this family of proteins is highly relevant for understanding how such disorders emerge. This study shows that neuregulin-4 is required to maintain motor cortex pyramidal neuron soma size, and that altered pyramidal neuronal morphology is associated with motor defects in mice.