Clustered protocadherin family.

The brain is a complex system composed of enormous numbers of differentiated neurons, and brain structure and function differs among vertebrates. To examine the molecular mechanisms underlying brain structure and function, it is important to identify the molecules involved in generating neural diversity and organization. The clustered protocadherin (Pcdh) family is the largest subgroup of the diverse cadherin superfamily. The clustered Pcdh proteins are predominantly expressed in the brain and their gene structures in vertebrates are diversified. In mammals, the clustered Pcdh family consists of three gene clusters: Pcdh-alpha, Pcdh-beta, and Pcdh-gamma. During brain development, this family is upregulated by neuronal differentiation, and Pcdh-alpha is then dramatically downregulated by myelination. Clustered Pcdh expression continues in the olfactory bulb, hippocampus, and cerebellum until adulthood. Structural analysis of the first cadherin domain of the Pcdh-alpha protein revealed it lacks the features that classical cadherins require for homophilic adhesiveness, but it contains Pcdh-specific loop structures. In Pcdh-alpha, an RGD motif on a specific loop structure binds beta1-integrin. For gene expression, the gene clusters are regulated by multiple promoters and alternative cis splicing. At the single-cell level, several dozen Pcdh-alpha and -gamma mRNA are regulated monoallelically, resulting in the combinatorial expression of distinct variable exons. The Pcdh-alpha and Pcdh-gamma proteins also form oligomers, further increasing the molecular diversity at the cell surface. Thus, the unique features of the clustered Pcdh family may provide the molecular basis for generating individual cellular diversity and the complex neural circuitry of the brain.