Bolton, M. McLean

Autism spectrum disorder (ASD) is a neurodevelopmental disorder that effects about 1 in 44 children and has steadily increased in prevalence over the last decades. ASD is characterized by the diagnostic criteria of a persistent deficit in social communication and interaction and restricted or repetitive behaviors. The amygdala plays an essential role in regulating these behaviors and has continuously been shown to be affected in patients with ASD. As the amygdala is connected throughout the brain with cortical and subcortical areas, it is crucial to understand potential circuit impairments that contribute to the development and progression of behavioral characteristics. In this study, we investigated the role of ASD-associated TBR1 haploinsufficiency on morphological and functional amygdala connectivity. While we don’t see differences in inputs to the basal amygdala (BA), we demonstrated a difference in the BA to prefrontal cortex (PFC) pathway. Interestingly, we show a specific innervation difference of layer 5 neurons in the infralimbic (IL) but not prelimbic (PL) nuclei in the PFC. In accordance with the overall reduced density of BA axons in the IL, we show a decreased density of excitatory synapses. To investigate possible functional consequences of this projection deficit, we characterized pre-and postsynaptic functions of BA-PFC synapses. TBR1 haploinsufficiency impairs the postsynaptic function of BA-PL layer 2/3 and IL layer 5 synapses. BA-PL layer 2/3 synapses show an increased AMPA/NMDA receptor ratio, while this is not observed in BA-IL layer 5 synapses. However, TBR1 haploinsufficiency increases the AMPA and NMDA receptor-mediated currents at these synapses, further highlighting that BA-PL and BA-IL synapses are different and that partial loss of TBR1 affects circuits differently. This novel characterization of the consequences of a TBR1 haploinsufficiency on BA connectivity contributes to the critical understanding of this ASD-associated gene and its detrimental effects that contribute to the underlying behavioral phenotype.

Autism spectrum disorder (ASD) is a complex disorder with large individual variability, where every case has differences in the type and severity of symptoms. Despite the recent increase in diagnoses, scientists have advanced considerably less in their understanding of the mechanisms of ASD because few individual genes that are implicated in ASD are mutated in much more than 1% of patients. One proposed mechanism is that the dysfunction of GABAergic interneurons may play a role in the development and progression of the disorder by interrupting the excitatory and inhibitory balance of neural networks. In our research, we elucidate the role of one class of interneurons in ASD by knocking out a high-risk gene (phosphatase and tensin homologue on chromosome ten, or PTEN) selectively in somatostatinexpressing (SOM+) interneurons. Since many symptoms of autism spectrum disorder present themselves as social anxieties, we test our mouse model in a variety of settings to observe social interaction and social preference, anxiety-like behavior, and repetitive stereotyped behavior. We found that in the SOM+ conditional knockout of PTEN, mice had elevated levels of anxiety and fear recall, suggesting a potential disruption of amygdala function. We then investigated potential dysfunction at the cellular and circuit levels using confocal microscopy, electrophysiology, and 2P local circuit mapping. We found that SOM+ cells lacking PTEN were overgrown morphologically, with larger cell bodies and larger, more complex dendritic arbors. Additionally, SOM+ cells in the central amygdala (CeA) lacking PTEN had elevated levels of excitatory drive from the basolateral amygdala (BLA) as well as a drastic disruption of lateral inhibition within the CeA, seen by decreased connection probability and reduced inhibitory post synaptic currents. Given what is known about central amygdala circuitry, these deficits in CeA SOM+ neuron activity conceivably underlie the fear and anxiety-related phenotype observed in mice with a conditional SOM+ PTEN knockout.