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Scientists have identified a unique gene that regulates brain development and regeneration of amputated tails and limbs in frog tadpoles. When this gene was blocked, tadpoles lost their ability to re-grow appendages, leading researchers to believe that appendage regeneration in warm-blooded animals, like humans, might have been caused by the loss of the now-named c-Answer gene at some point during the evolutionary process.
"We suppose that genes can only disappear if removing them has advantages for the animal," said first author Daria Korotkova in a statement. "So, we suggest that when this gene disappeared from warm-blooded species it was by a mutation, acting as a trade-off for the loss of appendage regeneration."
To come to their conclusions, scientists used a computer algorithm called ClusterZSL to search the DNA of African clawed frogs (Xenopus laevis) to find the gene responsible for regeneration by comparing the genetic sequences to those of warm-blooded creatures like chicks, taking note of any place a gene differed between the two. Eight identified genes disappeared in the genomes of warm-blooded vertebrates but were present in cold-blooded animals. One in particular – c-Answer – was found to code for a membrane protein and was unique to cold-blooded species.
Scientists then overexpressed or blocked c-Answer in tadpole embryos using CRISPR/Cas9 and amputated the tails and hindlimb buds. When c-Answer was enhanced, tadpoles were able to regenerate lost tails earlier than those that were born naturally. When blocked, tadpoles could still transition to frogs but lost their ability to regrow amputated limbs.
"C-Answer modulates at least two important molecular pathways that are common to all vertebrates," said senior author Andrey Zaraisky. "Its loss in evolution might alter the functioning of these pathways and, accordingly, lead to major physiological transformations."
An overexpression of c-Answer was also linked to “advanced brain growth and larger eyes” while blockage resulted in smaller brain size – a surprising discovery that indicates the protein may play a role in brain development.
“Obviously, this result confirms that complete gene deletion is a quite rare phenomenon and that indeed the phenotypic and physiological differences between different classes of vertebrates are mostly based on [the] rearrangement of the genomic regulatory networks instead of changes in the gene repertoire,” wrote the authors in Cell Reports.
The ability of cold-blooded animals such as fish, amphibians, and reptiles to regenerate is poorly understood and we are still largely unsure as to why birds and mammals do not have this ability. The results confirm that “significant evolutionary changes” resulted in the loss of the ability to regenerate major body appendages in warm-blooded animals. This “may have been caused by loss of a gene set in the ancestral species that regulates regeneration and brain development in cold-blooded species.”
The study authors are quick to note that there is “still much work to be done” in order to better understand the role of c-Answer in regeneration and brain development.
