Even the very first bony fish on Earth were susceptible to arthritis, a new study has found a discovery that may fast-track therapeutic research in preventing and treating the painful joint disorder.
The finding contradicts the widely held belief that lubricated joints enabling mobility - called synovial joints - evolved as vertebrates ventured onto land.
For example, human knees and hips have synovial joints, which are highly susceptible to osteoarthritis.
"Developing the first arthritis model in the zebrafish - an emerging regenerative model for medical research - opens up fundamentally new approaches toward finding a cure for arthritis," said Gage Crump, from the University of Southern California.
"While arthritis is the leading cause of disability in the United States, there are no treatments beyond artificial joint replacement. Our research offers new hope for finding a biological cure," said Crump.
Researchers found that certain joints in the zebrafish jaw and fins have features that resemble the synovial joints found in mammals.
The similarity makes sense because water resistance places considerable strain on joints.
Arthritis affects more than 52 million people or about 23 per cent of adults. The number may spike to 78 million - or about 26 per cent of adults - by the year 2040, researchers said.
Four-limbed bony vertebrates such as humans evolved from lobe-finned fish. However, a good laboratory model for this group does not exist, so researchers focused their study on zebrafish, a member of the more evolutionarily distant ray-finned fish.
Using CT scans and genetic tools, the scientists noted that two other ray-finned fish - the three-spined stickleback and the spotted gar - also have synovial joints that produce a protein very similar to what lubricates joints in humans. It is aptly named Lubricin.
Previous research showed that humans and mice lacking Lubricin have poor joint lubrication and develop early onset arthritis.
Researchers found that removing the Lubricin gene from the zebrafish genome causes the same early onset arthritis in their jaws and fins.
Given that fish and humans diverged hundreds of millions of years ago, when bony vertebrates first evolved, this similarity in arthritis susceptibility reveals that synovial joints are at least as ancient as bone itself.
The study appears in the journal eLife.
