Kavli Affiliate: Daniele Canzio
| Authors: Lea Kiefer, Gabrielle Isabelle F. Servito, Sandy M Rajkumar, Jennifer Langen, Anna Chiosso, Alexander Buckley, Elizabeth S. Cha, Adan Horta, Michael H Mui and Daniele Canzio
| Summary:
Clustered Protocadherin (Pcdh) proteins act as cell-surface recognition barcodes for neural circuit formation. Neurites expressing the same barcode repel each other, but this mechanism is deployed in two different ways. For instance, convergence of olfactory sensory neuron (OSN) projections requires stochastic expression of distinct Pcdh isoforms in individual cells, while tiling of neural arbors of serotonergic neurons (5-HTs) requires expression of the same isoform, Pcdhαc2. Despite their essential role, however, the molecular mechanisms of cell-type specific Pcdh barcoding remain a mystery. Here, we uncover a new role of cohesin: that of regulating distance-independent enhancer-promoter interactions to enable random Pcdh isoform choice via DNA loop extrusion in OSNs. Remarkably, this step mediates DNA demethylation of Pcdh promoters and their CTCF binding sites, thus directing CTCF to the chosen promoter. In contrast, the uniform pattern of Pcdh expression in 5-HTs is achieved through conventional cohesin-independent, distance-dependent enhancer/promoter interactions, that favor choice of the nearest isoform. Thus, cell-type specific cohesin deployment converts a distance-dependent and deterministic regulatory logic into a distance-independent and stochastic one. We propose that this mechanism provides an elegant strategy to achieve distinct patterns of Pcdh expression that generate wiring instructions to meet the connectivity requirements of different neural classes.