Octopus Brains evolved to share a surprising trait with our brains

EMBARGO Friday 25 November 1900 GMT | Saturday 26 November 0600 AEDT

Our glorious little blue marble of a planet is filled with a surprisingly diverse array of life forms, but some are certainly more peculiar than others.

This is particularly the case of the octopus, an animal so strange that it regularly invites comparisons with extraterrestrials.

Indeed, if there’s one creature on Earth strange enough to have evolved elsewhere, according to British neuroscientist Anil Seth, it’s the octopus. Some fringe theories even propose that octopuses could be extraterrestrials.

However, there is plenty of evidence to firmly link octopus evolution to Earth, and a team of scientists led by systems biologist Nikolaus Rajewsky of the Max-Delbrück-Center for Molecular Medicine has just found a new one.

And it’s really intriguing.

It’s a trait that octopus brains actually share with human brains and the brains of other vertebrates: a huge repertoire of microRNAs in their neural tissue.

“This,” says Rajewsky, “is what connects us to the octopus! »

Octopuses are weird in so many ways. They are also intelligent, just like other cephalopods, like the cuttlefish. And squid brains have been shown to be almost as complex as dog brains. There is even evidence to suggest that octopuses can dream – rarely confirmed in invertebrates.

Unlike other intelligent animals, their nervous system is highly distributed, with a significant proportion of its approximately 500 million neurons distributed in their arms. Each arm is able to make decisions independently and can even continue to react to stimuli after being severed.

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The complex nervous system and intelligence of cephalopods has been something of a puzzle. These traits are relatively common in vertebrates, but they really stand out in invertebrates.

There’s something else really weird about octopuses and other cephalopods. Their bodies can rapidly modify their RNA sequences on the fly to adapt to their environment. This is not how adaptation usually works; normally it starts with DNA, and these changes are passed down to RNA.

This made Rajewsky wonder what other secret RNA octopuses might be hiding.

Analyzing 18 samples obtained from dead octopus – provided by the Stazione Zoologica Anton Dohrn marine research institute in Italy – Rajewsky and his team sequenced RNA mainly from The common octopusthe common octopus. The study also included a whole two-spotted California octopus (bimaculoid octopus) and a Hawaiian bobtail squid (Scolopes of Euprymna).

Sequencing provided a profile of messenger RNAs and the small RNAs they contain. And the results were a surprise.

“There was indeed a lot of RNA editing going on, but not in the areas that we thought were interesting,” says Rajewsky.

What the team discovered is that octopuses have lots of microRNAs, or miRNAs. They found 164 miRNA genes clustered into 138 miRNA families in the common octopus, and 162 miRNA genes clustered into the same 138 families in the California two-spotted octopus. And 42 of the families were new, mostly in brain and nerve tissue.

miRNAs are non-coding RNA molecules that are heavily involved in the regulation of gene expression, binding to larger RNA molecules to help cells fine-tune the proteins they create.

The fact that these families of miRNAs have been preserved in the octopus, along with RNA-binding sites, suggests that they still play a role in octopus biology, although scientists do not yet know what is this role, nor which cells the miRNAs are involved. with.

“This is the third largest expansion of microRNA families in the animal world, and the largest outside of vertebrates,” says biologist Grygoriy Zolotarov, currently at the Center for Genome Regulation in Spain, formerly of Rajewsky’s lab. .

“To give you an idea of ​​the magnitude, oysters, which are also molluscs, have only acquired five new microRNA families since the last ancestors they shared with octopuses – while octopuses have acquired 90!”

The only comparable expansions have occurred in vertebrates, although the scale is a bit different. The human genome encodes, for context, approximately 2,600 mature miRNAs. But the number of miRNAs in the octopus family is comparable to that of animals such as chickens and frogs.

The finding, the researchers say, suggests that complex intelligence, including cephalopod intelligence, may be linked to this expansion of miRNAs.

Interestingly, this is not the only similarity between the brains of octopuses and those of vertebrates. Scientists have previously found that the brains of humans and octopuses both contain high numbers of a type of cell called transposons. There seems to be a lot more going on inside the head (and arms) of an octopus than we realize.

The next step for Rajewsky’s team is to try to figure out exactly what these miRNAs do.

“The notable explosion of the miRNA gene repertoire in coleoid cephalopods may indicate,” the researchers write, “that miRNAs and, perhaps, their specialized neural functions are deeply linked and possibly necessary for the emergence of complex brains in animals”.

The research has been published in Scientists progress.