RNA editing in Coleoids

A Cephalopod is any member of the molluscan class cephalopoda such as Octopus, Squid and Nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles. Cephalopods, namely octopus, squid and cuttlefish form a subclass referred to as coleoids. The coleoids are invertebrates but may possess some of the most intelligent species among all of invertebrates.

Octopus vulgaris, widely referred to as the common octopus is one such organism. Apart from the fact that octopus has a complex nervous system and sophisticated behavior, arms capable of a wide range of movement, developed eyes, chemoreceptors located in epidermis, suckers, mouth, etc., the recent discovery of high level of RNA editing makes the organism more competent than its companions. This mechanism of RNA editing gives it a way to face environmental changes. It also turns out that this may cause them and their cephalopod brethren to evolve differently from any other organism on the planet. Their great adaptability can be linked to their ability to edit some of their genes, changing RNA that encodes proteins.

In most organisms, the genetic information is transmitted relatively faithfully from DNA to RNA, and on into proteins but studies have shown that octopus along with squids and cuttlefish deviate from this trend and use their own enzymes to edit their RNA, producing proteins that differ from those encoded by DNA.

In 2015, the researchers discovered that the common squid has edited more than 60% of RNA in its nervous system. This brought about a change in its brain physiology and is assumed to confer an ability to adapt to various temperature conditions in the ocean. In 2017, the same team returned with new findings that revealed that at least two species of octopus and one cuttlefish do the same thing. Joshua Rosenthal and his team’s work involved analysis of not only thousands of different squids but also of other molluscan organisms like gastropod slug and nautilus. But unlike the coleoids, the aforementioned didn’t give any hints of straying away from the conventional mechanism. This led to a conclusion that RNA editing couldn’t be generalized for the entire molluscan phylum and was rather limited tot he coleoid subclass. In exchange of this remarkable adaptation, these alien like creatures have given up the ability to evolve relatively quickly. These coleoids don’t evolve like other organisms and this may one day lead them to become useful for humans. During transcription in an animal cell, duplication of the A,T,C and G nucleotides into RNA strands of A,T,C and U takes place. After transcription, introns are spliced out and the remaining sequence is read in a designated fashion. But sometimes, a special sequence in the RNA lets it hook up with an enzyme that edits a single spot in the RNA, turning an A into an I, which the ribosome interprets as a G. It may seem like a minor change but it changes the entire function of a protein. An animal could use RNA editing to change how its proteins work if its environmental changes.

The natural selection seems to have favored RNA editing in coleoids even though it slows down the DNA-based evolution, the reason is wrapped in some double-stranded clover-leaves that form alongside editing sites in the RNA. That information is like a tag for RNA editing. According to Rosenthal studies, RNA editing provides a faster mechanism to try out slightly different protein shapes and sizes and functions. This provides a way for the genome to evolve more slowly than those of other organisms. Eisenberg lists other possible advantages of RNA editing rather than mutations in the DNA. Some of the important contents of the list include- (1)Editing could occur selectively at specific conditions. (2)Editing could be partial, with only a fraction of the reads edited, resulting in coexistence of both edited and non-edited forms of proteins.

In order to maintain flexibility of editing RNA, these coleoids have had to give up on the ability to evolve in their surrounding regions which may have been worth it for them. Rosenthal hypothesizes that development of a complex brain was worth the price of having to evolve slowly. Many of the edited proteins were found in the brain tissue, creating the elaborate dendrites and axons of the neurons turning the shape of the electrical signals that neurons pass. It may also be possible that RNA editing (means of creating a more sophisticated brain) allowed these species to use tools, camouflage themselves, and communicate.

Further objective of Rosenthal’s study is to develop ways of genetically manipulating cephalopods. If he succeeds, he could disable the enzymes responsible for RNA editing and then see what happens.

REFERENCES:

Liscovitch-Brauer N et al. Trade off between transcriptome plasticity and genome evolution in cehalopods. Cell (2017)

Albertin et al. (2012) Cephalopod genomics : A plan of strategy and organization

Doubleday Z A et al(2016) Global proliferation of cephalopods. Curr Biol.

Garrett s, Rosenthal JJC (2012) RNA editing underlies temperature adaptation in K(+) channels from polar octopus. Science

Caroline B Albertin et al. (2015). The octopus genome and the evolution of cephalopod neural and morphological novelties. Nature

Alon S., Eisenberg E. (2015). The majority of transcripts in the squid nervous system are extensively recorded by A- to I- RNA editing.