Back of the envelope calculations for this post.
An old press release for a paper on endocytosis by Tom Kirchhausen contained this fascinating factoid:
The equivalent of the entire brain, or a football field of membrane, is turned over every hour
If this is true it is absolutely staggering. Let’s check it out.
A synaptic vesicle is ~40 nm in diameter. So the surface area of 1 vesicle is
which is 5026 nm2, or 5.026 x 10-15 m2.
Now, an American football field is 5350 m2 (including both endzones), this is the equivalent of 1.065 x 1018 synaptic vesicles.
It is estimated that the human cortex has 60 trillion synapses. This means that each synapse would need to internalise 17742 vesicles to retrieve the area of membrane equivalent to one football field.
The factoid says this takes one hour. This membrane load equates to each synapse turning over 296 vesicles in one minute, which is 4.93 vesicles per second.
Tonic activity of neurons differs throughout the brain and actually 5 Hz doesn’t sound too high (feel free to correct me on this). We’ve only considered cortical neurons, so the factoid seems pretty plausible!
For an actual football field, i.e. Association Football. The calculation is slightly more complicated. This is because there is no set size for football pitches. In England, the largest is apparently Manchester City (7598 m2) while the smallest actually belongs to the greatest football team in the world, Crewe Alexandra (5518 m2).
A brain would hoover up Man City’s ground in an hour if each synapse turned over 7 vesicles per second, while Gresty Road would only take 5 vesicles per second.
What is less clear from the factoid is whether a football field really equates to an “entire brain”. Bionumbers has no information on this. I think this part of the factoid may come from a different bit of data which is that clathrin-mediated endocytosis in non-neuronal cells can internalise the equivalent of the entire surface area of the cell in about an hour. I wonder whether this has been translated to neurons for the purposes of the quote. Either way, it is an amazing factoid that the brain can turnover this huge amount of membrane in such a short space of time.
So there you have it: quanta quantified on quantixed.
The post title is from “Insane In The Brain” by Cypress Hill from the album Black Sunday.
This post is about a paper that was recently published. It was the result of a nice collaboration between me and Francisco López-Murcia and Artur Llobet in Barcelona.
The paper in a nutshell
The availability of clathrin sets a limit for presynaptic function
Clathrin is a three legged protein that forms a cage around membranes during endoctosis. One site of intense clathrin-mediated endocytosis (CME) is the presynaptic terminal. Here, synaptic vesicles need to be recaptured after fusion and CME is the main route of retrieval. Clathrin is highly abundant in all cells and it is generally thought of as limitless for the formation of multiple clathrin-coated structures. Is this really true? In a neuron where there is a lot of endocytic activity, maybe the limits are tested?
It is known that strong stimulation of neurons causes synaptic depression – a form of reversible synaptic plasticity where the neuron can only evoke a weak postsynaptic response afterwards. Is depression a vesicle supply problem?
What did we find?
We showed that clathrin availability drops during stimulation that evokes depression. The drop in availability is due to clathrin forming vesicles and moving away from the synapse. We mimicked this by RNAi, dropping the clathrin levels and looking at synaptic responses. We found that when the clathrin levels drop, synaptic responses become very small. We noticed that fewer vesicles are able to be formed and those that do form are smaller. Interestingly, the amount of neurotransmitter (acetylcholine) in the vesicles was much less than the volume of the vesicles as measured by electron microscopy. This suggests there is an additional sorting problem in cells with lower clathrin levels.
A third reviewer was called in (due to a split decision between Reviewers 1 and 2). He/she asked a killer question: all of our data could be due to an off-target effect of RNAi, could we do a rescue experiment? We spent many weeks to get the rescue experiment to work, but a second viral infection was too much for the cells and engineering a virus to express clathrin was very difficult. The referee also said: if clathrin levels set a limit for synaptic function, why don’t you just express more clathrin? Well, we would if we could! But this gave us an idea… why don’t we just put clathrin in the pipette and let it diffuse out to the synapses and rescue the RNAi phenotype over time? We did it – and to our surprise – it worked! The neurons went from an inhibited state to wild-type function in about 20 min. We then realised we could use the same method on normal neurons to boost clathrin levels at the synapse and protect against synaptic depression. This also worked! These killer experiments were a great addition to the paper and are a good example of peer review improving the paper.
Fran and Artur did almost all the experimental work. I did a bit of molecular biology and clathrin purification. Artur and I wrote the paper and put the figures together – lots of skype and dropbox activity.
Artur is a physiologist and his lab like to tackle problems that are experimentally very challenging – work that my lab wouldn’t dare to do – he’s the perfect collaborator. I have known Artur for years. We were postdocs in the same lab at the LMB in the early 2000s. We tried a collaborative project to inhibit dynamin function in adrenal chromaffin cells at that time, but it didn’t work out. We have stayed in touch and this is our first paper together. The situation in Spain for scientific research is currently very bad and it deteriorated while the project was ongoing. This has been very sad to hear about, but fortunately we were able to finish this project and we hope to work together more in the future.
We were on the cover!
Now the scientific literature is online, this doesn’t mean so much anymore, but they picked our picture for the cover. It is a single cell microculture expressing GFP that was stained for synaptic markers and clathrin. I changed the channels around for artistic effect.
J Neurosci is slightly different to other journals that I’ve published in recently (my only other J Neurosci paper was published in 2002). For the following reasons:
- No supplementary information. The journal did away with this years ago to re-introduce some sanity in the peer review process. This didn’t affect our paper very much. We had a movie of clathrin movement that would have gone into the SI at another journal, but we simply removed it here.
- ORCIDs for authors are published with the paper. This gives the reader access to all your professional information and distinguishes authors with similar names. I think this is a good idea.
- Submission fee. All manuscripts are subject to a submission fee. I believe this is to defray the costs of editorial work. I think this makes sense, although I’m not sure how I would feel if our paper had been rejected.
López-Murcia, F.J., Royle, S.J. & Llobet, A. (2014) Presynaptic clathrin levels are a limiting factor for synaptic transmission J. Neurosci., 34: 8618-8629. doi: 10.1523/JNEUROSCI.5081-13.2014
The post title is taken from “Outer Limits” a 7″ Single by Sleep ∞ Over released in 2010.