Subplate-dependent mechanisms of cortical circuit assembly - PROJECT SUMMARY/ABSTRACT Cerebral cortex function requires correct assembly of circuit connectivities during development. Subplate neurons, strategically positioned at the gray and white matter interface, play essential roles in assembling cortical circuits, notably in guidance of thalamocortical axons and formation of sensory maps. Subplate neurons have also been implicated in corpus callosum formation. Three non-cell autonomous mechanisms by which subplate neurons support axon pathfinding have been proposed: 1) pioneering, wherein subplate neurons extend the first axons and lay a path for cortical axons to follow; 2) co-fasciculation, wherein subplate descending axons closely interact (“handshake”) with ascending axons to guide reciprocal connectivity; 3) extracellular matrix, wherein subplate neurons provide a substrate for axon growth in nascent white matter. Subplate ablation studies have shown subplate neurons to be indispensable. However, the genetic underpinnings of subplate neuron-mediated axon guidance are largely unknown, in part due to a lack of genetic access to subplate neurons at embryonic stages of circuit development. In a recent study (Doyle et al., PNAS, 2021), we reported a strategy to genetically target subplate neurons. Using this approach to interrogate gene necessity and sufficiency in subplate neuron- mediated circuit assembly, we discovered that the chromatin remodeler Arid1a is essential for the wiring functions of subplate neurons. Human ARID1A mutations are a cause of Coffin-Siris syndrome, a developmental disorder characterized by callosal agenesis. We found that cortical Arid1a deletion led to callosal agenesis and thalamocortical axon misrouting reminiscent of subplate ablation. These miswiring phenotypes coincided with disruptions in the transcriptional identity of subplate neurons, and deficits in subplate neuron wiring functions, including subplate-thalamocortical axon “handshake” and extracellular matrix. Thus, in Arid1a, we identified a multifunctional regulator of subplate neuron-dependent axon guidance functions – a key discovery that opens doors to molecular and mechanistic studies on subplate neurons. In preliminary studies using the same genetic strategy, we further identified an additional regulator of subplate-mediated circuit wiring in the transcription factor Sox5. Mutations in human SOX5 cause Lamb-Shaffer syndrome, a neurodevelopmental disorder characterized by intellectual disability. Here, we will leverage the exceptional opportunities that Arid1a and Sox5 provide to study subplate neurons. We will use our expertise in molecular genetics, circuit neurobiology, genomics, and chromatin biology, to identify the transcriptomic and genomic targets of Arid1a and Sox5 (Aims 1 and 2) and gain a functional understanding of candidate genes in subplate neuron-dependent axon guidance (Aim 3). A key implication of our work is that deficits in subplate neurons may be an underappreciated contributor to neural circuit miswiring in neurodevelopmental disorders, including those associated with chromatin dysregulation. Successful completion of the proposed study on subplate neurons will thus illuminate fundamental mechanisms of circuit development, and the potential consequences of subplate dysfunction in developmental brain disorders.
