The mammalian neocortex is a six-layered structure that creates a representation of the world by integrating sensory information and controls behavior by generating the appropriate motor output. Each layer processes information differently, so understanding how layer-specific neuronal networks are organized will provide a foundation for understanding how different layers function within circuits. The medial prefrontal cortex (mPFC) integrates information from many brain regions and is involved in complex behaviors including social interaction and decision-making. Importantly, mPFC circuits are often disrupted in neurodevelopmental disorders such as autism, but little is known about the fine scale organization of mPFC or how mPFC layers integrate different kinds of information. The Luo lab recently developed a modified rabies-based mono-trans- synaptic tracing technique that allows for detailed analysis of local circuitry in addition to whole brain long- distance mapping. This new rabies technique can be combined with layer-specific Cre driver mouse lines to generate detailed maps of layer-specific circuits in medial prefrontal cortex (mPFC). Cre-dependent rabies tracing will also be combined with cell-type specific gene knockout, so these maps will provide a basis for studying genetic regulation of neocortical connectivity in development and disease with unprecedented precision and scope. To begin to dissect the molecular pathways involved in establishing specific mPFC connectivity, this project will focus on the role of the autism-related gene Tsc1. Recent studies showed that Tsc1 deletion increases excitatory synaptic connectivity and alters the balance of excitation and inhibition, causing hyperexcitability in hippocampal neurons. This phenotype may be related to the role of Tsc1 in Tuberous Sclerosis Complex, a disease in which patients suffer from benign tumors, epilepsy, and autism. Performing Cre-dependent rabies tracing in conditional Tsc1fl/fl mice crossed with layer-specific Cre-drivers will facilitate investigation of the cell-autonomous role o Tsc1 in the development of mPFC connectivity and layer- specific organization. As Tsc1 is a negative regulator of the mammalian target of rapamycin complex, mTORC1, the molecular mechanisms underlying the in vivo function of Tsc1 will also be investigated to elucidate how these signaling pathways regulate brain development. This work will provide a deeper understanding of the molecular and the circuit-level mechanisms that give rise to autism and epilepsy in patients with Tsc1 mutations.
New research suggests that prefrontal cortical circuitry is disrupted in neurodevelopmental disorders such as autism, but little is known about the fine-scale organization of prefrontal cortex or about molecular mechanisms governing its assembly. The tumor-suppressor gene Tsc1 is implicated in autism and has been shown regulate synaptic connectivity, but the role of Tsc1 in prefrontal circuits is unknown. To date, it has been difficul to study the molecular mechanisms of cortical circuit assembly because different cell types and synaptic connections are highly intermingled in the neocortex. Using a new rabies virus-based trans-synaptic tracing approach will allow for investigations of Tsc1 in prefrontal cortex circuit development. By performing rabies tracing from wildtype and Tsc1-knockout neurons in each cortical layer, it will be possible create precise maps of the synaptic inputs to neurons in each layer mPFC, and to see how these patterns change in the absence of Tsc1. In the future, this rabies strategy can be combined with many genes to further investigate the mechanisms of cortical circuit assembly.