Brain function relies on specificity of synaptic connectivity patterns among different classes of neurons. Yet, the substrates of specificity in complex neuropil remain largely unknown. We search for imprints of specificity in the layout of axonal and dendritic arbors from the rat neocortex. An analysis of 3D reconstructions of pairs consisting of pyramidal cells (PCs) and GABAergic interneurons (GIs) revealed that the layout of GI axons is specific. This specificity is manifested in a relatively high tortuosity, small branch length of these axons, and correlations of their trajectories with the positions of postsynaptic neuron dendrites. Axons of PCs show no such specificity, usually taking a relatively straight course through neuropil. However, wiring patterns among PCs hold a large potential for circuit remodeling and specificity through growth and retraction of dendritic spines. Our results define distinct class-specific rules in establishing synaptic connectivity, which could be crucial in formulating a canonical cortical circuit.

The human CRSP-Med coactivator complex is targeted by a diverse array of sequence-specific regulatory proteins. Using EM and single-particle reconstruction techniques, we recently completed a structural analysis of CRSP-Med bound to VP16 and SREBP-1a. Notably, these activators induced distinct conformational states upon binding the coactivator. Ostensibly, these different conformational states result from VP16 and SREBP-1a targeting distinct subunits in the CRSP-Med complex. To test this, we conducted a structural analysis of CRSP-Med bound to either thyroid hormone receptor (TR) or vitamin D receptor (VDR), both of which interact with the same subunit (Med220) of CRSP-Med. Structural comparison of TR- and VDR-bound complexes (at a resolution of 29 A) indeed reveals a shared conformational feature that is distinct from other known CRSP- Med structures. Importantly, this nuclear receptor-induced structural shift seems largely dependent on the movement of Med220 within the complex.