Abstract Loss-of-function mutations in chromatin remodeler gene ARID1A are a cause of Coffin-Siris syndrome, a developmental disorder characterized by dysgenesis of corpus callosum. Here, we characterize Arid1a function during cortical development and find unexpectedly selective roles for Arid1a in subplate neurons. Subplate neurons (SPNs), strategically positioned at the interface of cortical grey and white matter, orchestrate multiple developmental processes indispensable for neural circuit wiring. We find that pan-cortical deletion of Arid1a leads to extensive mistargeting of intracortical axons and agenesis of corpus callosum. Sparse Arid1a deletion, however, does not autonomously misroute callosal axons, implicating non-cell autonomous Arid1a functions in axon guidance. Supporting this possibility, the ascending axons of thalamocortical neurons, which are not autonomously affected by cortical Arid1a deletion, are also disrupted in their pathfinding into cortex and innervation of whisker barrels. Coincident with these miswiring phenotypes, which are reminiscent of subplate ablation, we unbiasedly find a selective loss of SPN gene expression following Arid1a deletion. In addition, multiple characteristics of SPNs crucial to their wiring functions, including subplate organization, subplate-thalamocortical axon co-fasciculation (“handshake”), and extracellular matrix, are severely disrupted. To empirically test Arid1a sufficiency in subplate, we generate a cortical plate deletion of Arid1a that spares SPNs. In this model, subplate Arid1a expression is sufficient for subplate-thalamocortical axon co-fasciculation and extracellular matrix assembly. Consistent with these wiring functions, subplate Arid1a sufficiently enables normal callosum formation, thalamocortical axon targeting, and whisker barrel development. Thus, Arid1a is a multifunctional regulator of subplate-dependent guidance mechanisms essential to cortical circuit wiring. Significance The cognitive, perceptive, and motor capabilities of the mammalian cerebral cortex depend on assembly of circuit connectivity during development. Subplate neurons, strategically located at the junction of grey and white matter, orchestrate the wiring of cortical circuits. Using a new approach to study gene necessity and sufficiency in subplate neurons, we uncover an essential role for chromatin remodeler Arid1a in subplate neuron gene expression and axon guidance functions. Cortical deletion of Arid1a disrupts subplate-dependent formation of corpus callosum, targeting of thalamocortical axons, and development of sensory maps. Together, our study identifies Arid1a as a central regulator of subplate-dependent axon pathfinding, establishes subplate function as essential to callosum development, and highlights non-cell autonomous mechanisms in neural circuit formation and disorders thereof.

Chromatin remodeler Arid1a regulates subplate neuron identity and wiring of cortical connectivity