Thursday, January 30, 2020

APEX Proteomics applied to Stress Granules Provides insight into ALS progression!

A great way to see trends in proteomics is to go to ProteomeXchange and see what everyone is uploading.

The word "spatial" is completely blowing up. I think there are a couple versions of the modern "spatial" techniques. APEX, however, might be the best example. There is a great overview of this technique on the Krogan lab website that you can view here (I don't want to steal their nice image!)

Want the 6 18 second description/reminder?
1) You make a version of your protein of interest with a peroxidase on it and then you put it into your biological system and let it interact with all it's friends.
2) You put some biotin-phenols that float around in your system doing (presumably) no harm or alterations to your system
3) !!SURPRISE!! your system by adding hydrogen peroxide!
4) Nothing special happens to any other proteins (except normal H2O2 effects, I guess) but your protein of interest has that peroxidase on it --it reacts with the biotin phenols around it which causes all it's local protein friends to be labeled with biotin!
5) Pull down the biotin labeled proteins and now you know all the proteins in the general area of your protein of interest. Cool, right?

(It is a bit more complex than this. You don't want the APEX fusion being expressed all the time, for example, you can wait and activate it when your cells get ot the right stage -- you also need ot quench the reaction, but now I need to change how many seconds this took again...)

However, if you want to see it in action in a medical context -- check out this new preprint!

This group used multiple APEX Fusions to study stress granules (SG)s which according to the authors "form in response to a variety of cellular stresses by phase-separation of proteins associated with non-translating mRNAs" (yes, stolen from the first sentence of the abstract).

Unlike a lot of spatial labeling techniques -- this one requires nothing special from the mass spectrometer. The biotin labeled proteins are pulled down and digested. This group used a QE Plus for part of the study and it looks like they upgraded to a QE HF at some point. The LC separation was a long (3hour) gradient on 50cm columns. A relatively cycle time is maintained between the two instruments by using 17,500 resolution for MS/MS on the Plus and 30,000 resolution on the HF. The data processing here is MaxQuant and it looks very typical.

With this system set up, the experiment gets more complex, with the addition of ALS linked dipeptides that show alterations in the stress granules and the SUMoylation -- which is where the biology goes beyond me.

What I do get:
1) A fantastic application of this powerful new technique
2) A method that demonstrates that I could definitely do my part of this.
3) Important new understandings into the progression of a protein disease? 

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