Generating genetic diversity in the nervous system

Scientists from Baylor College of Medicine (Texas, USA) and the Wellcome Trust Sanger Institute (Cambridge, UK) have deciphered how neurons can synthesize a diverse range of proteins from a relatively limited number of genes – a discovery with important implications for understanding how complex neural circuitry is formed and maintained throughout our lives.

A long-standing question in neurobiology is how each of the tens of thousands of neurons that populate the mammalian brain are instructed to establish the specific connections that give rise to our complex neural networks. Researchers postulate that the expression of distinct sets of proteins in each individual neuron act as molecular cues to direct the course of each neuron’s fate. The protocadherin (Pcdh) family of proteins are prime candidates for this job, as each individual neuron expresses an overlapping but distinct combination of Pcdh proteins.

In the August 1 issue of Genes & Development, Dr. Allan Bradley and colleagues report on their identification of the mechanism of neuron-specific Pcdh expression. The Pcdh family of proteins is encoded by three gene clusters (Pcdh-a, Pcdh-ß, and Pcdh-g) on human chromosome #5, and mouse chromosome #18. The a and g clusters each contain genes with several variable exons (coding regions of DNA). Each variable exon can be separately joined to a constant region of the gene, thereby creating the genetic blueprint for a Pcdh protein that will have a unique variable region and a common constant region.

Dr. Bradley and colleagues have discovered that that although the Pcdh gene clusters share a similar genomic structure to the immunoglobin genes in the immune system — where antibody protein diversity confers antigen-binding specificity — the neuron-specific expression of Pcdh proteins is accomplished by an entirely different mechanism.

As Dr. Bradley explains, “We tested the various models by creating mice with a variety of modified alleles. The most intriguing theory was recombination (like the immunoglobulin genes), but we found no evidence to support this! Rather it appears that diversity is predominately generated using alternative promoters and cis-alternative splicing with a low level of trans-splicing.”

The researchers found that each variable exon is under the regulatory control of its own promoter (a DNA sequence where RNA polymerase binds to initiate transcription of the gene into pre-mRNA). Once transcribed, the pre-mRNA transcript then predominantly undergoes an intramolecular reaction, known as “cis-splicing,” whereby a variable exon is cut out and joined, or “spliced,” to the constant region of that same pre-mRNA transcript. Ultimately, this process enables a neuron to manipulate the Pcdh gene structure to generate a number of mRNAs, each containing different variable regions, which will each be translated into a unique Pcdh protein.

This work establishes that through the use of multiple promoters and cis-splicing, individual neurons are able to express distinct combinations of Pcdh genes, and, in turn, proteins. Further work will delineate how the differential expression of Pcdh proteins may underlie the specificity of neural connectivity.