Autism as Synapsopathy: Animal Models Based on Genetic Mutations in Trans-Synaptic Cell Adhesion Molecules

Background: A small percentage of patients with autism spectrum disorders carry missense or nonsense mutations in genes encoding neuroligin-3 and -4, which are postsynaptic cell adhesion molecules, and neurexin-1, their presynaptic ligands.  In addition, the neurexin-1 binding partners neuroligin 1 and 2 are located on chromosomal regions linked to autism.  We have recently characterized mouse models lacking neuroligin 1, 2, or 3 as well as an autism-associated neuroligin 3 point mutation.  We describe the behavioral, electrophysiological, and synaptic phenotypes in these mutant mice.  In addition, we are beginning to use pharmacologic approaches to rescue behavioral deficits in these models.

Objectives: To create and characterize a genetically accurate mouse models of autism.

Methods: We have introduced the R451C-substitution in neuroligin-3 into mice by homologous recombination.  Neuroligin 1, 2, and 3 knockout mice have been created through traditional methods.  All behavioral experiments were performed blind to genotype on 19-22 littermate pairs.  A thorough array of behavioral tests relevant to autism and cognitive function, whole cell and extracellular synaptic electrophysiology, electron microscopy, Western blot for synaptic proteins, and immunohistochemistry for synaptic proteins were performed.

Results: R451C-mutant knockin mice showed relatively selective impairment in social approach behavior and increased inhibitory synaptic transmission in cortex, while neuroligin 3 deletion mutants were normal in this regard. 

Neuroligin 1 knockout mice showed decreased NMDA-receptor mediated synaptic transmission, a likely cause of their decreased long-term potentiation in area CA1, decreased hippocampus-dependent spatial learning, and increased repetitive behavior as measured by a doubling of time spent grooming.  Of relevance to autism, neurexin-1 levels are significantly decreased in these mice.  Consistent with a link between repetitive grooming behavior and decreased NMDA receptor-mediated synaptic transmission, the partial NMDA receptor agonist, D-cycloserine, reversed grooming abnormalities.  Neuroligin 2 knockout mice revealed a decrease in inhibitory synaptic transmission and concomitant increased anxiety-like behavior.

Conclusions: The neuroligin-3 R451C knockin mice represent the first-published, genetically accurate model of autism not associated with a broader neuropsychiatric syndrome.  Furthermore, in agreement with recent findings of decreased cortical excitability in mouse models of Rett syndrome, increased inhibitory synaptic transmission may contribute to human ASDs (Tabuchi et al, Science, 2007).
The finding of increased repetitive behaviors and cognitive dysfunction in neuroligin 1 knockout mice may be of relevance to autism since neuroligin 1 is a postsynaptic binding partner of autism-associated neurexin-1 and these mice also have a significant decrease in neurexin protein levels.  We have now pharmacologically linked the NMDA receptor dysfunction in these mice to increased repetitive behaviors implicating a potential therapeutic target in these mice.