Clustered Protocadherins Are Required for Building Functional Neural Circuits.

Neuronal identity is generated by the cell-surface expression of clustered protocadherin (Pcdh) isoforms. In mice, 58 isoforms from three gene clusters, αβ, and γ, are differentially expressed in neurons. Since -heteromeric Pcdh oligomers on the cell surface interact homophilically with that in other neurons , it has been thought that the Pcdh isoform repertoire determines the binding specificity of synapses. We previously described the cooperative functions of isoforms from all three gene clusters in neuronal survival and synapse formation in the spinal cord. However, the neuronal loss and the following neonatal lethality prevented an analysis of the postnatal development and characteristics of the clustered-Pcdh-null (Δ) neural circuits. Here, we used two methods, one to generate the chimeric mice that have transplanted Δ neurons into mouse embryos, and the other to generate double mutant mice harboring null alleles of both the gene and the proapoptotic gene to prevent neuronal loss. First, our results showed that the surviving chimeric mice that had a high contribution of Δ cells exhibited paralysis and died in the postnatal period. An analysis of neuronal survival in postnatally developing brain regions of chimeric mice clarified that many Δ neurons in the forebrain were spared from apoptosis, unlike those in the reticular formation of the brainstem. Second, in Δ null double mutants, the central pattern generator (CPG) for locomotion failed to create a left-right alternating pattern even in the absence of neurodegeneraton. Third, calcium imaging of cultured hippocampal neurons showed that the network activity of Δ neurons tended to be more synchronized and lost the variability in the number of simultaneously active neurons observed in the control network. Lastly, a comparative analysis for -homophilic interactions of the exogenously introduced single Pcdh-γA3 isoforms between the control and the Δ neurons suggested that the isoform-specific -homophilic interactions require a complete match of the expressed isoform repertoire at the contacting sites between interactive neurons. These results suggested that combinations of clustered Pcdh isoforms are required for building appropriate neural circuits.