Like other kinds of genes, brain-building genes show much less species-to-species variation than you might imagine, given the enormous differences in brain size and complexity between, say, humans and chickens. But even fairly similar gene collections can produce widely divergent brain-development outcomes if, during gene transcription, alternative-splicing programs are activated. As it happens, a fairly subtle alternative-splicing event unique to mammals may trigger a cascade of like events, altering the timing of neuron differentiation, favoring the growth of larger, more complex brains.
Alternative splicing is a gene-expression process that allows a single gene to code for multiple proteins. It usually works by including or excluding different exons, nucleotide sequences, during transcription. In general, the prevalence of alternative splicing increases with vertebrate complexity. Alternative splicing, by enhancing protein diversity, allows a given number of genes to influence more functions that would be possible otherwise.
Aware that alternative splicing is especially widespread in brain tissue, researchers at the University of Toronto decided to investigate whether this gene-function-amplification mechanism could account for the morphological differences in the brains of different vertebrate species.
These researchers, led by Benjamin Blencowe, Ph.D., focused on a protein called PTBP1. This protein, they knew from previous studies, occurs in a form common to all vertebrates. In addition, it occurs in a form specific to mammals. The common form of PTBP1 inhibits alternative splicing in a wide variety of other proteins, preventing cells from becoming neurons. The mammal-specific form of PTBP1, however, is smaller due to exon skipping during transcription. This alternative form, the researchers surmised, might instigate a different chain of events.
The scientists explained their reasoning in a paper (“An alternative splicing event amplifies evolutionary differences between vertebrates”) that appeared August 21 in Science.
[Mammalian-specific] skipping of polypyrimidine tract–binding protein 1 (PTBP1) exon 9 alters the splicing regulatory activities of PTBP1 and affects the inclusion levels of numerous exons,” wrote the authors. “During neurogenesis, skipping of exon 9 reduces PTBP1 repressive activity so as to facilitate activation of a brain-specific [alternative-splicing] program.”
The scientists also described how they engineered chicken cells to make the shorter, mammalian-like PTBP1, the PTBP1 variant known to trigger a cascade of alternative-splicing events.
“Engineered skipping of the orthologous exon in chicken cells induces a large number of mammalian-like [alternative-splicing] changes in PTBP1 target exons,” informed the authors. “These results thus reveal that a single exon-skipping event in an RNA binding regulator directs numerous [alternative-splicing] changes between species.”
Essentially, the researchers demonstrated that a small change in PTBP1 can spur the creation of neurons. This change, they suggested, could account for how mammalian brains evolved to become the largest and most complex among vertebrates.
“One interesting implication of our work is that this particular switch between the two versions of PTBP1 could have affected the timing of when neurons are made in the embryo in a way that creates differences in morphological complexity and brain size,” noted Dr. Blencowe. “This is the tip of an iceberg in terms of the full repertoire of alternative-splicing changes that likely have contributed major roles in driving evolutionary differences.”