How Cephalopods Edit Their Own RNA: A Strange Form of Cognitive Plasticity

One of the strangest discoveries in molecular biology of the last decade is that cephalopods — octopuses, squid, and cuttlefish — recode their own RNA transcripts at vastly higher rates than any other animal lineage. In the cephalopod nervous system, roughly 60% of expressed RNA is edited at one or more sites by the cellular machinery, compared to less than 1% in humans and other mammals. The phenomenon was discovered piecemeal through the 1990s and 2000s, characterized at the genome scale by the Eli Eisenberg and Joshua Rosenthal labs in 2015 and 2017, and is now well enough understood that we can describe what cephalopods are doing and start to ask what it might mean for how they think.

What RNA editing is

Most cells use a simple pipeline: DNA is transcribed to RNA, RNA is translated to protein. The protein sequence is determined by the RNA sequence, which is determined by the DNA sequence. Mutations in DNA propagate to RNA and protein; otherwise the chain is one-to-one.

RNA editing breaks this one-to-one relationship by chemically modifying RNA bases after transcription but before translation. The most common form, A-to-I editing, converts adenosine bases to inosine through the action of an enzyme called ADAR (adenosine deaminase acting on RNA). Inosine is read by the ribosome as guanosine, so the effect is that an A in the DNA becomes a G in the protein-coding readout for that particular RNA molecule. The DNA is unchanged, but the proteins produced from that DNA can have a different sequence than the DNA encodes.

RNA editing is found in essentially every animal that has been examined, but in most lineages it occurs at a small number of sites. In humans, only a handful of edits are known to be functionally important, mostly in genes related to neuronal signaling and immune response. The total fraction of RNA molecules that get edited at all is low, and the fraction of edits that change protein sequence (rather than occurring in non-coding regions) is much lower.

The cephalopod difference

In cephalopods, RNA editing is not a marginal phenomenon. Liscovitch-Brauer et al. published a 2017 Cell paper that surveyed RNA editing across the cephalopod lineage, comparing it to the closely related but uneditable nautilus. The result was startling: octopuses, squid, and cuttlefish edit at hundreds of thousands of sites across their transcriptome, with editing concentrated heavily in the nervous system. Many of the edits change protein sequences in ways that affect neuronal signaling proteins — ion channels, neurotransmitter receptors, cytoskeletal components.

The closely related nautilus, which branched from the cephalopod lineage roughly 500 million years ago and which has a much simpler nervous system, has editing levels comparable to other invertebrates. So the high editing rate in modern cephalopods is a derived feature, evolved in the cephalopod lineage after the split from nautilus and presumably associated with the evolution of the cephalopod nervous system, which is by far the most sophisticated nervous system among invertebrates.

The genome cost of preserving editing

The mechanical detail that ties RNA editing to evolution is striking. ADAR doesn't recognize specific edit sites by sequence alone; it recognizes them by the local secondary structure of the RNA, particularly double-stranded regions formed by the edited region pairing with another part of the same RNA molecule. For an editing site to be preserved through evolution, both the editing site and its pairing partner have to be conserved together — which means the surrounding DNA can't accumulate mutations as freely as it could otherwise.

Liscovitch-Brauer et al. showed that the regions of cephalopod DNA flanking edited sites have unusually low rates of synonymous mutation (mutations that don't change the protein sequence). This is the molecular signature of a lineage that's actively trading genome evolution rate for the ability to maintain RNA editing. The lineage has chosen, over evolutionary timescales, to keep its RNA editing machinery functional even at the cost of slower genome adaptation. That's an unusual evolutionary trade-off and implies that the editing is doing something the lineage can't easily replace.

What the editing does

The edited proteins that have been characterized in cephalopods are concentrated in neuronal signaling. Potassium channels, sodium channels, neurotransmitter receptors, and synaptic-vesicle proteins all show extensive editing in cephalopod nervous systems. In some cases, the edited and unedited versions of the same protein have measurably different functional properties — different gating kinetics, different ligand affinities, different subcellular localization.

One of the cleanest experimental cases involves a potassium channel called Kv1.1. The unedited version of Kv1.1 has different inactivation kinetics than the edited version. The relative ratios of edited and unedited versions vary across cephalopod tissues, across temperature conditions, and even across individual neurons within the same tissue. The interpretation is that the cephalopod is using RNA editing as a mechanism for tuning protein function in a way that isn't fixed by the genome but can be adjusted in response to local conditions.

The 2012 Garrett and Rosenthal paper in Science showed that octopus Kv1 channels are more heavily edited at cold temperatures than at warm temperatures, and that the edited versions have kinetics that compensate for the slower channel gating that low temperatures would otherwise produce. The interpretation is that octopuses use temperature-sensitive RNA editing to maintain consistent neuronal function across the temperature range they experience in different ocean depths.

What it might mean for cognition

The functional implications of pervasive RNA editing in a complex nervous system are not fully understood, but the implications are intriguing. In a system where the same gene produces multiple functionally distinct protein variants in proportions that vary by tissue, by temperature, and possibly by activity history, the relationship between genome and phenotype is much looser than in mammals. A cephalopod neuron's properties depend not just on what genes it expresses but on how those genes are edited at the moment of translation.

This has been speculated to be relevant to the cephalopod's unusual cognitive flexibility — the capacity for tool use, problem-solving, and rapid behavioral learning that distinguishes them from other invertebrates and rivals or exceeds many vertebrates. The argument, made carefully because the evidence is still emerging, is that RNA editing might provide a substrate for plastic neuronal tuning that operates on timescales between transcription (slow) and synaptic plasticity (fast). Whether this is actually a substantive contribution to cephalopod cognition or whether it's a coincidence of two separately evolved features is still being investigated.

The phylogenetic puzzle

The thing that makes the cephalopod RNA editing story most interesting is the phylogenetic context. Cephalopods diverged from the vertebrate lineage roughly 600 million years ago. Their nervous system evolved independently of the vertebrate nervous system, with different organizational principles — distributed processing in the arms, two-thirds of neurons outside the central brain, completely different developmental program. And yet they arrived at a level of cognitive flexibility comparable to vertebrates, using a fundamentally different molecular toolkit including this enormous reliance on RNA editing.

The implication is that complex cognition can be assembled from very different molecular foundations. The vertebrate solution involves a particular combination of synaptic plasticity mechanisms, neurotransmitter systems, and developmental programs. The cephalopod solution involves a different combination, with RNA editing apparently playing a role that doesn't have an obvious analog in vertebrates. If we wanted to design an artificial cognitive system, the cephalopod existence proves that there are at least two viable molecular routes to it, and the cephalopod route uses ingredients we don't currently understand well.

The deeper observation

The schoolroom version of molecular biology is that DNA encodes RNA which encodes protein, and the chain is one-to-one. The actual situation is messier, with a substantial gap between genome and phenotype filled by RNA editing, alternative splicing, post-translational modification, and other mechanisms that change what proteins actually get made. In most lineages this gap is small. In cephalopods, the gap is enormous, and the lineage has been actively maintaining the gap-generating machinery for hundreds of millions of years at evolutionary cost. The cephalopod is, in a literal molecular sense, not what its genome says it is. The fact that this lineage produces some of the most cognitively flexible animals in the ocean is either coincidence or evidence that the relationship between genome, protein, and cognition has more degrees of freedom than the schoolroom version suggests. Either answer is interesting.