Tbr1 haploinsufficiency impairs amygdalar axonal projections and results in cognitive abnormality.

The neuron-specific transcription factor T-box brain 1 (TBR1) regulates brain development. Disruptive mutations in the TBR1 gene have been repeatedly identified in patients with autism spectrum disorders (ASDs). Here, we show that Tbr1 haploinsufficiency results in defective axonal projections of amygdalar neurons and the impairment of social interaction, ultrasonic vocalization, associative memory and cognitive flexibility in mice. Loss of a copy of the Tbr1 gene altered the expression of Ntng1, Cntn2 and Cdh8 and reduced both inter- and intra-amygdalar connections. These developmental defects likely impair neuronal activation upon behavioral stimulation, which is indicated by fewer c-FOS-positive neurons and lack of GRIN2B induction in Tbr1(+/-) amygdalae. We also show that upregulation of amygdalar neuronal activity by local infusion of a partial NMDA receptor agonist, d-cycloserine, ameliorates the behavioral defects of Tbr1(+/-) mice. Our study suggests that TBR1 is important in the regulation of amygdalar axonal connections and cognition.