Neonatal Tbr1 Dosage Controls Cortical Layer 6 Connectivity.

Area of Science:

• Neuroscience
• Genetics
• Developmental Biology

Background:

• Autism spectrum disorder (ASD) mechanisms involving heterozygous loss-of-function mutations in risk genes like TBR1 are not well understood.
• TBR1 plays a critical role in the development of cortical layer 6 neurons, which are implicated in neurodevelopmental disorders.

Purpose of the Study:

• To investigate the function of TBR1 in cortical layer 6 neurons during mouse development.
• To explore the contribution of TBR1 heterozygous loss-of-function mutations to ASD-related phenotypes.

Main Methods:

• Conditional Tbr1 deletion in mouse cortical layer 6 neurons during late gestation.
• Analysis of dendritic patterning, synaptogenesis, and cell-intrinsic physiology in Tbr1 heterozygotes.
• Chromatin immunoprecipitation sequencing (ChIP-seq) and RNA sequencing (RNA-seq) to identify TBR1 targets and regulatory mechanisms.
• Behavioral testing for anxiety-like behaviors in Tbr1 heterozygotes.

Main Results:

• Tbr1 deletion in layer 6 neurons (Tbr1^layer6 mutants) leads to phenotypes in heterozygotes, including altered dendritic patterning and synaptogenesis.
• Restoring Wnt7b expression partially rescues the synaptic deficit in Tbr1^layer6 mutant neurons.
• Tbr1^layer6 heterozygotes exhibit increased anxiety-like behavior, a common ASD phenotype.
• Integration of ChIP-seq and RNA-seq data reveals TBR1's role in regulating layer 6 neuron properties and transcriptional circuits.
• Several identified TBR1 targets are known ASD risk genes, positioning TBR1 centrally in ASD pathophysiology.

Conclusions:

• Heterozygous loss-of-function of TBR1 in cortical layer 6 neurons contributes to ASD-relevant phenotypes, including synaptic deficits and anxiety-like behavior.
• TBR1 regulates crucial developmental processes in layer 6 neurons through transcriptional control, impacting neural circuit assembly.
• Wnt7b signaling is implicated in the synaptic deficits associated with TBR1 mutations, suggesting potential therapeutic targets.