Tag Archives: small molecule inhibitor

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.


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.