Neural representations of head direction have been discovered in many species. A large body of theoretical work has proposed that the dynamics associated with these representations is generated, maintained, and updated by recurrent network structures called ring attractors. We performed electron microscopy-based circuit reconstruction and RNA profiling of identified cell types in the heading direction system of Drosophila melanogaster to directly determine the underlying neural network. We identified network motifs that have been hypothesized to maintain the heading representation in darkness, update it when the animal turns, and tether it to visual cues. Functional studies provided additional support for the proposed roles of individual circuit elements. We also discovered recurrent connections between neuronal arbors with mixed pre-and post-synaptic specializations. Overall, our results confirm that the Drosophila heading direction network contains the core components of a ring attractor while also revealing unpredicted structural features that might enhance the network's computational power..

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-associated protein 9 (CRISPR-Cas9) systems have emerged as the method of choice for genome editing, but large variations in on-target efficiencies continue to limit their applicability. Here, we investigate the effect of chromatin accessibility on Cas9-mediated gene editing efficiency for 20 gRNAs targeting 10 genomic loci in HEK293T cells using both SpCas9 and the eSpCas9(1.1) variant. Our study indicates that gene editing is more efficient in euchromatin than in heterochromatin, and we validate this finding in HeLa cells and in human fibroblasts. Furthermore, we investigate the gRNA sequence determinants of CRISPR-Cas9 activity using a surrogate reporter system and find that the efficiency of Cas9-mediated gene editing is dependent on guide sequence secondary structure formation. This knowledge can aid in the further improvement of tools for gRNA design.