Single-Cell Identity Generated by Combinatorial Homophilic Interactions between α, β, and γ Protocadherins

Individual mammalian neurons stochastically express distinct repertoires of α, β, and γ protocadherin (Pcdh) proteins, which function in neural circuit assembly. We report that all three subfamilies of clustered Pcdhs can engage in specific homophilic interactions, that cell surface delivery of Pcdhα isoforms requires cis interactions with other Pcdhs, and that the extracellular cadherin domain EC6 plays a critical role in this process. Examination of homophilic interactions between specific combinations of multiple Pcdh isoforms revealed that Pcdh combinatorial recognition specificities depend on the identity of all of the expressed isoforms. A single mismatched Pcdh isoform can interfere with these combinatorial homophilic interactions. A theoretical analysis reveals that assembly of Pcdh isoforms into multimeric recognition units and the observed tolerance for mismatched isoforms can generate cell surface diversity sufficient for single-cell identity. However, the competing demands of nonself discrimination and self-recognition place limitations on the mechanisms by which homophilic recognition units can function. [Display omitted] •Pcdhα, Pcdhβ, and Pcdhγ isoforms mediate specific homophilic interactions•EC6 domains of Pcdhα and PcdhγC4 isoforms inhibit cell surface delivery•Homophilic specificity of coexpressed Pcdh isoforms provides single-cell identity•Conflicting requirements of self-recognition and nonself discrimination Neural circuit assembly during development requires mechanisms by which individual neurons distinguish self from nonself. A combination of in vitro cell aggregation assays and theoretical analyses reveals that the clustered protocadherins are capable of generating the diversity necessary for neuronal self-recognition in mammalian cells.