Start Me Up: Endocytosis on demand

We have a new paper out. The title is New tools for ‘hot-wiring’ clathrin-mediated endocytosis with temporal and spatial precision. You can read it here.

Cells have a plasma membrane which is the barrier between the cell’s interior and the outside world. In order to import material from outside, cells have a special process called endocytosis. During endocytosis, cells form a tiny bubble of plasma membrane and pull it inside – taking with it a little pocket of the outside world. This process is very important to the cell. For example, it is one way that cells import nutrients to live. It also controls cell movement, growth, and how cells talk to one another. Because it is so important, cell biologists have studied how endocytosis works for decades.

Studying endocytosis is tricky. Like naughty children, cells simply do not do what they are told. There is no way to make a cell in the lab “do endocytosis”. It does it all the time, but we don’t know when or where on the cell surface a vesicle will be made. Not only that, but when a vesicle is made, we don’t really know what cargo it contains. It would be helpful to cell biologists if we could bring cells under control. This paper shows a way to do this. We demonstrate that clathrin-mediated endocytosis can be triggered, so that we can make it happen on-demand.

Endocytosis on-demand

Using a chemical which diffuses into the cell, we can trigger endocytosis to happen all over the cell. The movie on the right shows vesicles (bright white spots) forming after we add the chemical (at 0:00). The way that we designed the system means that the vesicles that form have one type of cargo in there. This is exciting because it means that we can now deliver things into cells using this cargo. So, we can trigger endocytosis on-demand and we can control the cargo, but we still cannot control where on the plasma membrane this happens.

We solved this problem by engineering a light-sensitive version of our system. With this new version we can use blue light to trigger endocytosis. Whereas the chemical diffused everywhere, the light can be focussed in a narrow region on the cell and endocytosis can be trigger only in that region. This means we control where, as well as when, a vesicle will form.

What does hot-wiring mean?

It is possible to start a car without a key by “hot-wiring” it. This happens in the movies, when the bad guy breaks into a car and just twists some wires together to start the car and make a getaway. To trigger endocytosis we used the cell’s own proteins, but we modified them. We chopped out all the unnecessary parts and just left the bare essentials. We call the process of triggering endocytosis “hot-wiring” because it is similar to just twisting the wires together rather than having a key.

It turns out that movies are not like real life, and hot-wiring a car is actually quite difficult and takes a while. So our systems are more like the Hollywood version than real life!

What is this useful for?

As mentioned above, the systems we have made are useful for cell biologists because they allow cells to be “tamed”. This means that we can accurately study the timing of endocytosis and which proteins are required in a very controlled way. It also potentially means that molecules can be delivered to cells that cannot normally enter. So we have a way to “force feed” cells with whatever we want. This would be most useful for drugs or nanoparticles that are not actively taken up by cells.

Who did the work?

Almost all of the work in the paper was by Laura Wood, a PhD student in the lab. She had help from fellow lab members Nick Clarke, who did the correlative light-electron microscopy, and Sourav Sarkar who did the binding experiments. Gabrielle Larocque, another PhD student did some fantastic work to revise the paper after Laura had departed for a post-doc position at another University. We put the paper up on bioRxiv in Summer 2016 and the paper has slowly made its way through peer review to be published in J Cell Biol today.

Wait? I’m a cell biologist! I want to know how this thing really works!

OK. The design is shown to the right. We made a plasma membrane “anchor” and a clathrin “hook” which is a fragment of protein which binds clathrin. The anchor and the hook have an FRB domain and an FKBP domain and these can be brought together by rapamycin. When the clathrin hook is at the membrane this is recognised by clathrin and vesicle formation can begin. The main hook we use is the appendage and hinge from the beta2 subunit of the AP2 complex.

Normally AP2, which has four subunits, needs to bind to PIP2 in the plasma membrane and undergo a conformational change to recognise a cargo molecule with a specific motif, only then can clathrin bind the beta2 appendage and hinge. By hot-wiring, we effectively remove all of those other proteins and all of those steps to just bring the clathrin binding bit to the membrane when we want. Being able to recreate endocytosis using such a minimalist system was a surprise. In vitro work from Dannhauser and Ungewickell had suggested this might be possible, but it really seems that the steps before clathrin engagement are not a precursor for endocytosis.

To make the light inducible version we used TULIPs (tunable light-controlled interacting proteins). So instead of FRB and FKBP we had a LOVpep and PDZ domain on the hook and anchor.

The post title comes from “Start Me Up” by The Rolling Stones. Originally on Tattoo You, but perhaps better known for its use by Microsoft in their Windows 95 advertising campaign. I’ve finally broken a rule that I wouldn’t use mainstream song titles for posts on this blog.

Zero Tolerance

We were asked to write a Preview piece for Developmental Cell. Two interesting papers which deal with the insertion of amphipathic helices in membranes to influence membrane curvature during endocytosis were scheduled for publication and the journal wanted some “front matter” to promote them.

Our Preview is paywalled – sorry about that – but I can briefly tell you why these two papers are worth a read.

The first paper – a collaboration between EMBL scientists led by Marko Kaksonen – deals with the yeast proteins Ent1 and Sla2. Ent1 has an ENTH domain and Sla2 has an ANTH domain. ENTH stands for Epsin N-terminal homology whereas ANTH means AP180 N-terminal homology. These two domains are known to bind membrane and in the case of ENTH to tubulate and vesiculate giant unilamellar vesicles (GUVs). Ent1 does this via an amphipathic helix “Helix 0” that inserts into the outer leaflet to bend the membrane. The new paper shows that Ent1 and Sla2 can bind together (regulated by PIP2) and that ANTH regulates ENTH so that it doesn’t make lots of vesicles, instead the two team up to make regular membrane tubules. The tubules are decorated with a regular “coat” of these adaptor proteins. This coat could prepattern the clathrin lattice. Also, because Sla2 links to actin, then actin can presumably pull on this lattice to help drive the formation of a new vesicle. The regular spacing might distribute the forces evenly over large expanses of membrane.

The second paper – from David Owen’s lab at CIMR in Cambridge – shows that CALM (a protein with an ANTH domain) actually has a secret Helix 0! They show that this forms on contact with lipid. CALM influences the size of clathrin-coated pits and vesicles, by influencing curvature. They propose a model where cargo size needs to be matched to vesicle size, simply due to the energetics of pit formation. The idea is that cells do this by regulating the ratio of AP2 to CALM.

You can read our preview and the papers by Skruzny et al and Miller et al in the latest issue of Dev Cell.

The post title and the title of our Preview is taken from “Zero Tolerance” by Death from their Symbolic LP. I didn’t want to be outdone by these Swedish scientists who have been using Bob Dylan song titles and lyrics in their papers for years.

Insane In The Brain

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

\(4 \pi r^2\)

which is 5026 nm2, or 5.026 x 10-15 m2.

Synaptic vesicles in the presynaptic terminal (asterisk).
Synaptic vesicles in the presynaptic terminal (asterisk)

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).

grestyroad
Gresty Road. Small but perfectly formed

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.

Outer Limits

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

Background
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.

Killer experiment
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.

People
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!
25.coverNow 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.

What else?
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:

  1. 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.
  2. 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.
  3. 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.

Reference:

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

Pubmed | Paper

The post title is taken from “Outer Limits” a 7″ Single by Sleep ∞ Over released in 2010.

Into The Great Wide Open

We have a new paper out! You can read it here.

I thought I would write a post on how this paper came to be and also about our first proper experience with preprinting.

Title of the paper: Non-specificity of Pitstop 2 in clathrin-mediated endocytosis.

In a nutshell: we show that Pitstop 2, a supposedly selective clathrin inhibitor acts in a non-specific way to inhibit endocytosis.

Authors: Anna Willox, who was a postdoc in the lab from 2008-2012, did the flow cytometry measurements and Yasmina Sahraoui who was a summer student in my lab, did the binding experiments. And me.

Background: The description of “pitstops” – small molecules that inhibit clathrin-mediated endocytosis – back in 2011 in Cell was heralded as a major step-forward in cell biology. And it really would be a breakthrough if we had ways to selectively switch off clathrin-mediated endocytosis. Lots of nasty things gain entry into cells by hijacking this pathway, including viruses such as HIV and so if we could stop viral entry this could prevent cellular infection. Plus, these reagents would be really handy in the lab for cell biologists.

The rationale for designing the pitstop inhibitors was that they should block the interaction between clathrin and adaptor proteins. Adaptors are the proteins that recognise the membrane and cargo to be internalised – clathrin itself cannot do this. So if we can stop clathrin from binding adaptors there should be no internalisation – job done! Now, in 2000 or so, we thought that clathrin binds to adaptors via a single site on its N-terminal domain. This information was used in the drug screen that identified pitstops. The problem is that, since 2000, we have found that there are four sites on the N-terminal domain of clathrin that can each mediate endocytosis. So blocking one of these sites with a drug, would do nothing. Despite this, pitstop compounds, which were shown to have a selectivity for one site on the N-terminal domain of clathrin, blocked endocytosis. People in the field scratched their hands at how this is possible.

A damning paper was published in 2012 from Julie Donaldson’s lab showing that pitstops inhibit clathrin-independent endocytosis as well as clathrin-mediated endocytosis. Apparently, the compounds affect the plasma membrane and so all internalisation is inhibited. Many people thought this was the last that we would hear about these compounds. After all, these drugs need to be highly selective to be any use in the lab let alone in the clinic.

Our work: we had our own negative results using these compounds, sitting on our server, unpublished. Back in February 2011, while the Pitstop paper was under revision, the authors of that study sent some of these compounds to us in the hope that we could use these compounds to study clathrin on the mitotic spindle. The drugs did not affect clathrin binding to the spindle (although they probably should have done) and this prompted us to check whether the compounds were working – they had been shipped all the way from Australia so maybe something had gone wrong. We tested for inhibition of clathrin-mediated endocytosis and they worked really well.

At the time we were testing the function of each of the four interaction sites on clathrin in endocytosis, so we added Pitstop 2 to our experiments to test for specificity. We found  that Pitstop 2 inhibits clathrin-mediated endocytosis even when the site where Pitstops are supposed to bind, has been mutated! The picture shows that the compound (pink) binds where sequences from adaptors can bind. Mutation of this site doesn’t affect endocytosis, because clathrin can use any three of the other four sites. Yet Pitstop blocks endocytosis mediated by this mutant, so it must act elsewhere, non-specifically.

So the compounds were not as specific as claimed, but what could we do with this information? There didn’t seem enough to publish and I didn’t want people in the lab working on this as it would take time and energy away from other projects. Especially when debunking other people’s work is such a thankless task (why this is the case, is for another post). The Dutta & Donaldson paper then came out, which was far more extensive than our results and so we moved on.

What changed?

A few things prompted me to write this work up. Not least, Yasmina had since shown that our mutations were sufficient to prevent AP-2 binding to clathrin. This result filled a hole in our work. These things were:

  1. People continuing to use pitstops in published work, without acknowledging that they may act non-specifically. The turning point was this paper, which was critical of the Dutta & Donaldson work.
  2. People outside of the field using these compounds without realising their drawbacks.
  3. AbCam selling this compound and the thought of other scientists buying it and using it on the basis of the original paper made me feel very guilty that we had not published our findings.
  4. It kept getting easier and easier to publish “negative results”. Journals such as Biology Open from Company of Biologists or PLoS ONE and preprint servers (see below) make this very easy.

Finally, it was a twitter conversation with Jim Woodgett convinced me that, when I had the time, I would write it up.

To which, he replied:

I added an acknowledgement to him in our paper! So that, together with the launch of bioRxiv, convinced me to get the paper online.

The Preprinting Experience

This paper was our first proper preprint. We had put an accepted version of our eLife paper on bioRxiv before it came out in print at eLife, but that doesn’t really count. For full disclosure, I am an affiliate of bioRxiv.

The preprint went up on 13th February and we submitted it straight to Biology Open the next day. I had to check with the Journal that it was OK to submit a deposited paper. At the time they didn’t have a preprint policy (although I knew that David Stephens had submitted his preprinted paper there and he told me their policy was about to change). Biology Open now accept preprinted papers – you can check which journals do and which ones don’t here.

My idea was that I just wanted to get the information into the public domain as fast as possible. The upshot was, I wasn’t so bothered about getting feedback on the manuscript. For those that don’t know: the idea is that you deposit your paper, get feedback, improve your paper then submit it for publication. In the end I did get some feedback via email (not on the bioRxiv comments section), and I was able to incorporate those changes into the revised version. I think next time, I’ll deposit the paper and wait one week while soliciting comments and then submit to a journal.

It was viewed quite a few times in the time while the paper was being considered by Biology Open. I spoke to a PI who told me that they had found the paper and stopped using pitstop as a result. I think this means getting the work out there was worth it after all.

Now it is out “properly” in Biology Open and anyone can read it.

Verdict: I was really impressed by Biology Open. The reviewing and editorial work were handled very fast. I guess it helps that the paper was very short, but it was very uncomplicated. I wanted to publish with Biology Open rather than PLoS ONE as the Company of Biologists support cell biology in the UK. Disclaimer: I am on the committee of the British Society of Cell Biology which receives funding from CoB.

Depositing the preprint at bioRxiv was easy and for this type of paper, it is a no-brainer. I’m still not sure to what extent we will preprint our work in the future. This is unchartered territory that is evolving all the time, we’ll see. I can say that the experience for this paper was 100% positive.

References

Dutta, D., Williamson, C. D., Cole, N. B. and Donaldson, J. G. (2012) Pitstop 2 is a potent inhibitor of clathrin-independent endocytosis. PLoS One 7, e45799.

Lemmon, S. K. and Traub, L. M. (2012) Getting in Touch with the Clathrin Terminal Domain. Traffic, 13, 511-9.

Stahlschmidt, W., Robertson, M. J., Robinson, P. J., McCluskey, A. and Haucke, V. (2014) Clathrin terminal domain-ligand interactions regulate sorting of mannose 6-phosphate receptors mediated by AP-1 and GGA adaptors. J Biol Chem. 289, 4906-18.

von Kleist, L., Stahlschmidt, W., Bulut, H., Gromova, K., Puchkov, D., Robertson, M. J., MacGregor, K. A., Tomilin, N., Pechstein, A., Chau, N. et al. (2011) Role of the clathrin terminal domain in regulating coated pit dynamics revealed by small molecule inhibition. Cell 146, 471-84.

Willox, A.K., Sahraoui, Y.M.E. & Royle, S.J. (2014) Non-specificity of Pitstop 2 in clathrin-mediated endocytosis Biol Open, doi: 10.1242/​bio.20147955.

Willox, A.K., Sahraoui, Y.M.E. & Royle, S.J. (2014) Non-specificity of Pitstop 2 in clathrin-mediated endocytosis bioRxiv, doi: 10.1101/002675.

The post title is taken from ‘Into The Great Wide Open’ by Tom Petty and The Heartbreakers from the LP of the same name.