Ancient molecule tells starfish and humans when to stop eating

The feeling of fullness after a nice meal usually prevents us from eating more. This is a familiar experience for humans, but we are not alone. We share this with starfish and many other organisms as well.

New research indicates that the same message instructing us to eat no more has existed for more than 500 million years. The same molecule affecting our feeding behavior in this way also acts on starfish and other sea animals.

Researchers at Queen Mary University of London have followed the evolutionary trail of a neurohormone named bombesin, which regulates appetite in humans.

Collaborating with colleagues at the Institut Pasteur in Paris and the University of Warwick, the researchers discovered that this satiety-inducing peptide is much older than was thought, and is found in animals that lived before the first vertebrates set foot on Earth.

What is bombesin?

Bombesin is a small peptide that helps regulate appetite by sending a message when it’s time to stop eating. While it’s been studied in humans and other mammals, its origins were less understood – until now.

Researchers discovered that a bombesin-like molecule is present in echinoderms like starfish, sea urchins, and sea cucumbers.

If you’re wondering about the unusual name, bombesin isn’t explosive. It’s named after the fire-bellied toad, Bombina bombina, where it was first identified in 1971.

Scientists noticed that injecting this compound into mammals caused them to eat less and take longer between meals. This led to the idea that bombesin-like neurohormones are part of the body’s natural appetite control system.

Today, compounds that mimic bombesin are being explored as potential treatments for obesity, alongside well-known medications like Ozempic.

A needle in a haystack

The research team began by searching for bombesin-like genes in marine invertebrates.

“It was a bit like looking for a needle in a haystack,” said Professor Maurice Elphick. “But eventually we discovered genes encoding a bombesin-like neurohormone in the genomes of starfish and their relatives.”

Once the gene was identified in the common starfish (Asterias rubens), scientists named the molecule ArBN.

Collaborators at the University of Warwick used mass spectrometry to determine the exact structure of ArBN, which allowed the molecule to be synthesized for lab testing.

Watching starfish eat – or not eat

Starfish feed in a bizarre and fascinating way: they push their stomachs out through their mouths to digest prey like mussels. The team wanted to know how ArBN affected this feeding behavior.

“When I tested ArBN, I saw that it caused contraction of the starfish stomach. This suggested that ArBN might be involved in stimulating stomach retraction when starfish stop feeding. And this is exactly what I found,” explained Dr. Weiling Huang, who conducted the behavioral tests.

“When I injected ArBN into starfish with their stomachs everted, it triggered the stomach to retract back into the mouth. What’s more, ArBN also delayed the initiation of feeding, as starfish injected with ArBN took longer to enclose a mussel compared to those injected with water.”

A molecule older than starfish

Understanding that bombesin-like molecules have been regulating feeding behavior since before the rise of vertebrates adds an important piece to the puzzle of how appetite evolved.

“We can infer that this function extends back half-a-billion years to the common ancestor of starfish, humans, and other vertebrates,” said Professor Elphick.

But the implications go beyond biology textbooks. Some species of starfish are moving into new habitats due to climate change. In particular, these species are seeking cooler waters where shellfish farms are located, and may threaten aquaculture operations.

“One of the consequences of climate change is that some starfish species are invading cooler waters where shellfish are cultivated for human consumption,” said Professor Elphick.

“Discovering molecules that inhibit feeding in starfish could be useful in managing these invasions.”

More than just an ancient molecule

This study connects two very different worlds: the sophisticated brain-gut interactions in humans and the slow, alien feeding behavior of starfish. Yet both rely on the same ancient molecule to know when it’s time to stop eating.

As scientific discoveries reveal the biological links between species, the deep interconnectedness of all life becomes ever more apparent.

The full study was published in the journal Proceedings of the National Academy of Sciences.

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