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In the name of their survival, starfish cut off their own body parts to escape predators. They will eventually regenerate those lost limbs as well, but how this biological process works has remained a mystery. Now a team of scientists has discovered a type of neurohormone responsible for limb severing common starfish (Asteria rubens). The findings are detailed in a study published Aug. 29 in the journal Current biology.
What is autotomy?
Autotomy is an animal’s ability to detach a body part – and still survive – as a way to escape an animal trying to eat it. Lizards can shed their tails to escape salamanders can regrow limbs, eyes, jaws, spinal cord and more. Starfish – or starfish – will cut off one or more of their arms and… let the limbs grow back lost to automation over time.
“The tissues in the region where autotomy occurs are pre-adapted to allow rapid regeneration,” Maurice R. Elphicksays co-author and biologist at Queen Mary University of London (QMUL). Popular science. “It’s as if starfish are built to expect regeneration after autotomy to occur at some point in their lives, i.e. they are attacked by a predator or their arms get stuck under a rock and they have to escape .”
[Related: The sea star’s whole body is a head.]
According to Elphick, when scientists collect starfish along the beach, they often find many organisms that have clearly lost an arm or two at some point in their lives.
“You can tell when autotomy has occurred because the regenerated arm is smaller than the other arms and will probably never quite reach the same size as the original arms,” says Elphick.
While there was automation first scientifically described more than thirty years agothe molecules and hormones behind them have remained elusive. Understanding what is really behind this process at the molecular level could have implications for regenerative medicine in the future.
Stars, they’re just like us (kind of)
Initially the team will discuss this new study was looking for a mammalian neurohormone called cholecystokinin (CCK). They were curious about how a similar neurohormone works in starfish.
“One of its biological actions [in humans] is to stimulate the contraction of the gallbladder and that contraction of the gallbladder forces bile into the intestines and this reflects its role in facilitating the digestion of food,” says Elphick.
CCK too inhibits feeding behavior. When the stomach becomes full and food passes into the intestines, CCK is one of the hormones that sends a signal to the brain that it is time to stop eating.
Considering the role of CCK in mammalian feeding behavior, Elphick and co-authored the study Ana Tinoco from QMUL and the University of Cadiz in Spain investigated how a cholecystokinin-type neurohormone called ArSK/CCK1 works when starfish eat. While examining the neurohormone in the laboratory, they noticed that it actually induced autonomy in some starfish, but not all.
“In some animals, only one arm was lost, but in a few animals, four of the five arms were automated,” says Elphick.
They discovered that at the molecular level it is cholecystokinin-like neurohormones work the same way in both starfish and humans. They bind to a specific receptor protein on the cell surface target cells. The binding then produces a signal that ultimately changes some properties of the target cells.
[Related: Antlion larvae can ‘play dead’ for more than an hour–then, things get random.]
One of these changes is muscle contraction. In humans, it causes the gallbladder to contract. In starfish, the contractions caused by these neurohormones appear to be related to autotomy.
“We have evidence that the process of autotomy is facilitated by the release of this peptide, which we think then causes a muscle at the base of the starfish arm to contract,” says Elphick. “This then facilitates release of the arm and closure of the wound after autotomy has occurred.”
The team is still determining the finer details of how this process works, but has focused on this area for further research.
Future treatments
Although the way cholecystokinin-type neurohormones work in starfish and humans are very similar, both organisms have evolved to use them differently. However, the similarities mean that further research could increase our understanding of human tissue regeneration and better treatments for limb injuries.
“The process of autotomy is complex, like many processes in biology, and is likely regulated by multiple factors. We would like to identify other molecules involved in regulating the autonomy process in starfish,” says Elphick.
[Related: The blueprints for early organs may be hiding in sea stars.]
It also offers a lesson in how scientists can start with one question or project that grows into something completely different.
“I think an important message from this study is the importance of just being able to do experiments where you don’t always know what the outcome will be and you don’t really have a hypothesis,” says Elphick. “You just make observations and then every now and then you discover something that is really intriguing and interesting”