Tag Archives: endosome

How can a cytokine still act after it’s gone

Here is yet another new wrinkle in how drugs act.

Type I interferons are  very well studied cytokines, important in the defense against viruses.  There are 16 of them, and in response to infection, they induce the expression of over 300 antiviral genes (ISGs — Interferon Stimulated Genes).

Type I interferon is gone from the plasma after intramuscular injection in a few hours, yet its effects last days to weeks.  Well maybe the proteins it induces hang around a long time.

Not so says Proc. Natl. Acad. Sci. vol. 117 pp. 17510 – 17512 ’20.  It hangs around inside the cell in little vesicles formed from pinching off part of the cell membrane containing type I interferon bound to its receptor.  These are called endosomes.  The authors call this siloing.  The silos persist for days and can actually be seen if you use fluorescently labeled type I interferon.  The endosomes with type I interferon inside persist for days.

I’m sure people are actively studying other cytokines of which there are many, many — e.g. there are more than 50 interleukins that we know about.

Antibodies without antibodies

If you knew exactly how an important class of antibodies interacted with its target, could you design a (relatively) small molecule to act the same way.  These people did, and the work has very exciting implications for infectious disease [ Science vol. 358 pp. 450 – 451, 496 – 502 ’17 ].

The influenza virus is a very slippery target.  Its genome is made of RNA, and copying it is quite error prone, so that mutants are formed all the time.  That’s why the vaccines of yesteryear are useless today.   However there are things called broadly neutralizing antibodies which work against many strains of the virus.  It attacks a vulnerable site on the hemagglutinin protein (HA) of the virus.  It is in the stem of the virus, and binding of the antibody here prevents the conformational change required for the virus to escape the endosome, a fact interesting in itself in that it implies that it only works after the virus enters the cell, although the authors do not explicitly state this.

Study of one broadly neutralizing antibody showed that binding to the site was mediated by a single hypervariable loop.  So the authors worked with a cyclic peptide mimicking the loop.  This has several advantages, in particular the fact that the entropic work of forcing a floppy protein chain into the binding conformation is already done before the peptide meets its target.

The final cyclic peptide contained 11 amino acids, of which 5 weren’t natural. It neutralized pandemic H1 and avaian H5 influenza A strains at nanoMolar concentration.

It’s important that crystal structures of the broadly neutralizing antibody binding to HA were available — this requires atomic level resolution.  I’m not sure cryoEM is there yet.

Time for drug chemists to go to the Multiplex

30 – 40% of all the drugs currently in clinical use are thought to target G Protein Coupled Receptors (GPCRs). Just how many GPCRs inhabit our genome isn’t clear. The latest estimate is 850 which is 4.2% of the 20,077 annotated protein genes we have. That being the case, it behooves drug chemists to know everything about them and how they work.

A recent paper [ Cell vol. 166 pp. 907 – 919 ’16 ] shows that a lot of the old thinking about GPCRs is wrong. Binding of a ligand to a GCPR results in a conformational change in its 7 transmembrane segments, so that the parts inside the cell bind to a heterotrimer of proteins which bind (and hydrolyze) GTP — this is the G protein. So far so good. The trimer splits up into its 3 constituents, unimaginatively called alpha, beta and gamma, each of which can act as a messenger that a ligand from outside the cell has landed on a GPCR, binding to other proteins causing all sorts of effects (e.g. can act as a second messenger)

All good things must end, and termination of GPCR signaling was thought to involve phosphorylation of the intracellular segment of the GPCR, binding of another protein (betaArrestin), removal from the cell membrane (so it can no longer bind its extracellular ligand) and then either destruction or recycling back to the cell membrane. So the old paradigm was betaArrestin binding equals the end of signaling.

It was thought that betaArrestin and the G protein competed for binding to the same intracellular amino acids of the GPCR. Not so says this paper. For some GPCRs both can bind, and signaling can continue, even though the complex of GPCR, G protein and betaArrestin is now inside the cell in an endosome. The complex is called the Multiplex. The examples given are GPCRs for parathyroid hormone (PTH) and Thyroid Stimulating Hormone (TSH). Blurry pictures are given of the complex. GPCRs have been divided into several classes and GPCRs for TSH and PTH are class B GPCRs — which contain a long phosphorylatable tail in the cytoplasm. The G protein binds to these GPCRs by its core region, while betaArrestin binds to the tail. Signaling continues apace.