Joint secrets

Whenever you clench a fist, bend your knee, or kick a ball, you're benefiting from the evolutionary processes that endowed us with synovial joints. These are the cartilage line sites of articulation that make our skeletons so mobile. Scientists had thought that this trait evolved exclusively in land dwelling animals, but it turns out to be far older as Gage Crump explained to Chris Smith…

Gage - For a long time, it was thought that when animals came on the land, they had these new mechanical forces walking around in response to gravity and that drove these really special types of lubricated joints which are the type of joints that go wrong in arthritis. And so, we want to understand well – is it really true that these lubricated joints are just in land animals or could they be much more ancient? Surprisingly, there is very little previously known about when lubricated joints first came around. So we used zebrafish which is a model organism for vertebrate biology to really interrogate whether they also have similar lubricated joints to the type that we do.

Chris - And you're telling me no one has actually really studied this in detail despite the enormity of the question.

计-是的。It was actually very surprising. Several years ago, starting this project, I would Google ‘evolution of lubricated joints’ and I could find almost nothing. There was one study in 1942 by a scientist named Hanes but that was in the premolecular era and a lot of these old papers have really been lost to a lot of modern scientists.

Chris - So how can you use genetics to probe this then?

Gage - Zebrafish is a great system especially with gene editing now where we can very simply go and knockout any gene we want very easily. And so, what we did is we focused on this protein called lubricin because it actually functions as a kind of an oil for joints. It’s known that both in people or mice that lack this protein, joints wear very quickly and they get early onset arthritis. So we figured, if we get rid of this in zebrafish and their joints also degenerate very early, that would make a strong argument that the zebrafish joints are lubricated because they need this lubricant protein.

Chris - And that’s what you did is that you went in there and used genome editing tools to get rid of the lubricin gene from the zebrafish.

Gage - Yeah, exactly. So the first thing we need to do is actually show that this lubricin protein – this lubricant – was actually made at fish joints. We did that by looking at not only zebrafish but also a couple of other fish. We then removed the gene from the zebrafish genome and it was really spectacular when we saw the phenotype because it looked almost identical to what you see in mice or people lacking this gene.

Chris - Now obviously, we have a very different set of joints than our fish ancestors do, so what joints were you looking at in the fish?

Gage - Initially, we were looking at the jaw joint. The reason for that is that it’s thought to be one of the most ancient joints in all of animals. And so really, the first fish that evolved were thought to have biting jaws before they had arms, legs, or even fins. And so, we had started with the jaw joint, but then we found later that there's actually these very small joints at the base of the fins in fish which also require this lubricant protein. And it turns out that these really tiny bones at the base of the fin are really what became the arms and legs of land animals with limbs.

Chris - Is the pathology that the zebrafish lacking the gene develop – is that the same as the mice that you knocked this gene out from? Do they get the same sort of joint damage?

计-是的。这是最remarkable to us. There was a set of cellular changes that really characterised loss of this gene which is different from other syndrome. And so, what you get is all these cells dying at the joint surface, this material sloughing off. But then you also have these underlying cartilage cells that divide too rapidly and build up a process called hyperplasia. And really, we saw this whole spectrum of phenotypic changes that you would see in a mouse or human also lacking that gene. So it’s remarkable conservation of the pathology that surprised even us.

Chris - So what you're saying here, is that historically it’s quite a nice compelling story that lifting themselves out of water forced fish and those descendants to develop the sorts of synovial joints that we haven't take for granted today. But in fact, they were already there well before those animals decided to get out of water.

计-是的。We think it makes sense because if you’ve ever gone swimming, you know that it’s very hard to move your arms and legs in the water. there's a lot of pressure from the water. so, it’s a different type of force than the gravity when you're walking but it’s also a significant force. If you imagine these very early fish, they're constantly snapping their jaws on prey. There's going to be a lot of compressive forces on their joints that could cause them to wear down every time. so actually, if you step back and think about it, it makes sense that these early fishes would have these lubricated synovial program in their jaw joint even before that was then used again in limbs when animals came on the land.