O'Brien, G. K. | bioRxiv Science

bioRxiv2026

Sex-specific plasticity mechanisms mediating fear extinction

Strong evidence suggests synaptic plasticity is the critical cellular mechanism underlying learning and memory. Although the forms of synaptic plasticity used by different circuits vary, a widespread presumption is that the male and female brain has evolved to use the same form of plasticity within the same circuits during learning. We used complimentary approaches to determine how synaptic plasticity within the mouse frontal cortex supports extinction of associative memories. Here, we show that both male and female mice have similar ensemble dynamics in excitatory infralimbic cortical neurons during learning. However, activity in amygdala-projecting neurons was required for extinction memories only in male mice. Likewise, only male mice showed evidence for structural synaptic remodeling and clustering of dendritic spines on infralimbic-amygdala projection neurons. Projection-specific deletion of the glutamate receptor subunit GRIN2B blocked synaptic plasticity and impaired extinction memory in male but not female mice. These distinct mechanisms could be leveraged for precise therapies for mental health conditions relative to the present one-size-fits-all approach.

neuroscience↗

bioRxiv2024

Detailed phenotyping of Tbr1-2A-CreER knock-in mice demonstrates significant impacts on TBR1 protein levels and axon development

Spatiotemporal control of Cre-mediated recombination has been an invaluable tool for understanding key developmental processes. For example, knock-in of Cre into cell type marker gene loci drives Cre expression under endogenous promoter and enhancer sequences, greatly facilitating the study of diverse neuronal subtypes in the cerebral cortex. However, insertion of exogenous DNA into the genome can have unintended effects on local gene regulation or protein function that must be carefully considered. Here, we analyze a recently generated Tbr1-2A-CreER knock-in mouse line, where a 2A-CreER cassette was inserted in-frame just before the stop codon of the transcription factor gene Tbr1. Heterozygous TBR1 mutations in humans and mice are known to cause autism or autism-like behavioral phenotypes accompanied by structural brain malformations, most frequently a reduction of the anterior commissure. Thus, it is critical for modified versions of Tbr1 to exhibit true wild-type-like activity. We evaluated the Tbr1-2A-CreER allele for its potential impact on Tbr1 function and complementation to Tbr1 loss-of-function alleles. In mice with one copy of the Tbr1-2A-CreER allele, we identified reduction of TBR1 protein in early postnatal cortex along with thinning of the anterior commissure, suggesting hypersensitivity of this structure to TBR1 dosage. Comparing Tbr1-2A-CreER and Tbr1-null heterozygous and homozygous mice to Tbr1-null complementation crosses showed reductions of TBR1 dosage ranging from 28.4% to 95.9%. Using these combinatorial genotypes, we found that low levels of TBR1 protein ([~]16%) are sufficient to establish cortical layer positioning, while greater levels (>50%) are required for normal suppression of layer 5 identity. In total, these results strongly support the conclusion that Tbr1-2A-CreER is a hypomorphic allele. We advise caution when interpreting experiments using this allele, such as transcriptomic studies, considering the sensitivity of various corticogenic processes to TBR1 dosage and the association of heterozygous TBR1 mutations with complex neurodevelopmental disorders.

developmental biology↗