Shank2 Mutation in a Rat Model Induces Behavioral, Molecular and Electrophysiological Alterations Consistent with an ASD-like Phenotype

Background:  A number of the genes identified as anomalous in genetic screens of the autism spectrum disorders (ASD) population code for proteins that regulate synaptic plasticity, including the Shank family of genes. The link between mutation of these ubiquitously expressed genes and the selective social impairments and repetitive behaviors featured in ASD, though, is still unclear. Transgenic introduction of mutations identified in human ASD subjects into rodent models enables the association of specific genotypes to phenotypes.  Rat models present several advantages over mouse models for characterizing ASD mutations due to their rich social behavioral repertoire, metabolic similarities to humans and amenability to complex electrophysiological manipulations. 

Objectives:  To investigate the relationship between decreased synaptic function and social impairment, we have characterized the behavioral, molecular and electrophysiological phenotype of rats expressing a mutation of the Shank2 gene associated with ASD. The characterization of neural activity associated with aberrant behavior has the potential to identify reliable biomarkers of both disease and drug response.

Methods:   Transgenic Sprague-Dawley rats containing a targeted deletion of exon 31 of the SHANK2 gene were generated using zinc finger nuclease technology. The subsequent animals were characterized through a behavioral battery, molecularly, through western blot protein quantification of regional synaptosomal brain homogenates, and electrophysiologically, through in vivo, chronically implanted, surface and depth electrodes. 

Results:    Mutation of the Shank2 gene results in alterations in social behavior seen throughout development. Homozygous Shank2 mutant rats (HOs) engage in less species typical juvenile play than wild type rats (WTs). This deficit is maintained into adulthood as evidenced by decreased social investigation of conspecifics and transmission of food preference. However, Shank2 mutation in the rat, unlike in the mouse, does not result in decreased social approach.  HOs also exhibited several forms of restricted and repetitive behaviors analogous to those observed in ASD, including increased locomotion and abnormal circling and checking behaviors. Uniquely, HOs also show increased motivation in a progressive ratio task that is independent of hyperactivity. The behavioral deficits associated with Shank2 mutation are accompanied by upregulation of Shank3 and mGluR1 in the striatum and downregulation SHANK1 and Homer1 in the hippocampus. Local field potential recordings in multiple nodes of the social brain circuit, including the amygdala, entorhinal cortex and hippocampus were collected to assess differences in network activity resulting from Shank2 mutation. 

Conclusions:    Shank2 mutation in the rat model recapitulates many of the behavioral features of ASD and has broad effects on the expression of synaptic proteins. Exploration of electrophysiological phenotypes associated with these behaviors in this model may shed light on the neural underpinnings of the symptomatology of the disorder.