Single-celled organisms have memories, too
Walking with amoebas
Door Rouèl Gnodde
By Rouèl Gnodde
At first sight, amoebas may look like simple little single-celled blobs that do nothing other than eat bacteria.
However, as retired professor of cell biochemistry has known for years, appearances can be deceiving. Amoebas come in countless shapes and sizes. Some come in the form of fungi, while others are swimming around in our blood. Some amoebas are parasites looking to eat our brains.
Van Haastert became fascinated by these little ‘creatures’ when he was still a student. Back then, not much was known about the amoeboid fungus Dictyostelium. He thought it was a nice challenge. ‘It was perfect for me. I wanted to try and understand these complex processes with all their unknowns.’
He studied how the cells communicated and responded to chemical signals, and he’s taken many an amoeba apart to look at what goes on inside.
Fake legs
What excited him most was the way amoebas move, considering they don’t have eyes, ears, legs, or arms. Instead, they can turn part of their body into leg-like projections called pseudopodia. Actin filaments, akin to the protein fibres our muscles contain, project outward so the creature can pull itself forward.
In order to walk in a straight line, they’d have to be aware of what they’re doing
These fake legs are proof of a fascinating thesis Van Haastert recently published in PlosOne. He discovered that the legs don’t appear randomly, like many scientists originally thought. ‘Amoebas tend to walk in a straight line’, he says. The fact that they do so actually has far-reaching implications. ‘It means they’re aware of what they’re doing, and that they remember what they’ve done previously.’
His research started ten years ago, when a reviewer asked a critical question about something he said about pseudopodia. ‘Somewhere in the paper, I’d written that these little legs always appeared on the amoeba’s back’, says Van Haastert. ‘He asked me how I could be so sure.’
A Post-it note and a projector
The biologist projected a recording of a moving amoeba on a wall, stuck a Post-it on that wall, and drew arrows where the legs started to where they ended. Analysing the movement patterns of these infinitesimally small creatures thusly, he saw a pattern emerging.
His method was simple, unique, and so illuminating that it still works ten years later. These days, he uses clever computer programs and mathematical formulas rather than a Post-it and a projector, but at its core, his research is the same.
‘I initially didn’t think it was that big of a deal’, he says. ‘But once we started doing deeper analyses of angles and the correlations in time, we realised that we were seeing how cells were travelling certain paths.’ This is a fundamentally important fact for many lifeforms.
Memory
Van Haastert spent days at his computer, an Excel sheet on one screen and on another countless hours of microscopic recordings of approximately a hundred different amoeba varieties, mutants where changes had been made to specific proteins. Slowly but surely, he gathered evidence for a particularly bold claim: amoebas have memories, which means they have some kind of consciousness.
The secret was in the footwork. ‘Amoebas sort of skate forward’, Van Haastert explains, ‘alternating between its left and right feet.’ This kind of movement comes natural to organisms that already have legs, but organisms that have to create them need to ‘remember’ where they appeared the last time.
Amoebas sort of skate forward
This requires a precise molecular timepiece. In amoebas, Van Haastert says, this takes the form of a ‘wobbly protein’, which moves back and forth like a little spring. ‘The protein has been phosporylised throughout the cell. That means it contains a negative phosphate group that makes the positive protein close shut like a magnet. In order to form a leg somewhere, the phosphate has to be removed so the protein can open up.’ For amoebas, that’s the signal that tells them the location where they can form a leg.
When the fake foot stops, it takes a while for the phosphate to return. In the meantime, a new foot could appear in the same location. ‘It takes approximately fifteen to twenty seconds’, says Van Haastert. ‘That’s the short-term memory.’
Front and back
It turns out amoebas also have a long-term memory, however. ‘Things are simple for us: we have a top and a bottom, a front and a back. The direction in which we move is logical.’ But amoebas have to decide for themselves which is their front and which is their back, and they have to remember this, too.
To do this, the clever creature has developed another mechanism: a molecular gradient, which runs from high to low concentrations. There is a lot of actin on the side of the amoeba where the legs form. The other side has myosin, which means the creature can’t form any legs there.
It’s not consciousness unless a response isn’t automatic
However, sometimes amoebas have to change direction. ‘Like when it smells a bacterium, or when it has to return to its colony’, says Van Haastert.
When that happens, the amoeba has to overwrite its long-term memory. This process takes approximately two minutes. ‘The smell of bacteria impacts the short-term memory. The signal changes slowly but surely. The legs either cross over or the amoeba moves its legs one by one, like a bad skater’, says Van Haastert.
After two minutes, the molecular gradient in the cell has moved, allowing the amoeba to ‘skate’ in a different direction.
Brain on legs
It’s not a real consciousness just yet, but it’s a start. It’s true that amoebas observe and react to their environment. ‘But it’s not considered a consciousness unless this isn’t automatic’, says Van Haastert. He thinks this precludes self-awareness or ‘the cell having a free will’. For now, Van Haastert considers amoebas a small brain on legs.
That doesn’t make him any less happy with his discovery, and not just because he managed to prove that amoebas have a memory. His method is just as important: it can be applied to other cells, like white blood cells or stem cells.
‘I deliberately made my research as extensive as possible, because I would love it if it helped other people discover things’, says Van Haastert. ‘Take cancer research. People could study an amoeboid cancer call that’s moving through the body and perhaps discover more about metastasising tumours and how to treat them.’ But, he emphasises: ‘That’s a job for future researchers.’