Why Search for Alternative GPCR Agonists?

Round 1

Reviewer 1 Report

Boutin and Leprince provide a good overview on the efforts in identifying different natural and synthetic agonists and antagonists at G protein-coupled receptors, with an emphasis on aminergic receptors, such as serotonin receptors, but also touching on peptidergic receptors. The review has a quite entertaining and scholarly style and will certainly be interesting for students starting in the field, but also established scientists.

Concrete examples are given for ligand discovery campaigns in different receptors, and ligand selectivity is discussed in great detail. Overall, the idea is supported that diversity of ligands at GPCRs can indeed be helpful for specific, clinically relevant receptor targeting, and great discoveries can also be made by serendipity along the way.

 

I have a few comments that should be addressed:

Figures 1 and 2 lack legends, and I assume, Figure 2 is incomplete? There are just panels A and B and lots of blank space

End of section 3; p. 5, l. 180 ff. (End of section 3): It is explained that GPCR agonists from other natural sources might be interesting for their higher metabolic stability and increased potencies and better subtype-selectivity. However, as impressively shown for salmon calcitonin as ligand at human amylin receptors  the divergent chemical space might also profoundly affect receptor activation and thereby lead to divergent biological outcomes, which is quite unexpected when focusing only on binding affinities and selectivity (e.g., Cao J, et al. Science. 2022 Mar 25;375(6587):eabm9609. doi: 10.1126/science.abm9609.). Similarly, the signaling pattern arising from exendin at GLP-1R is different from GLP-1 (e.g., Deganutti G,  et al. Nat Commun. 2022 Jan 10;13(1):92. doi: 10.1038/s41467-021-27760-0.; Fletcher MM, et al. Biochem Pharmacol. 2018 Oct;156:406-419. doi: 10.1016/j.bcp.2018.09.003.). The authors might also want to briefly introduce the concept of biased signaling here and refer to the later section in the manuscript that discusses this in more detail.

Section 8, page 12: The intrinsic signaling bias in peptide-activated GPCR families is an intensively studied area, and there are certainly examples beyond urotensin II. For instance, at amylin and GLP1 receptors (see above), but also calcitonin and adrenomedullin receptors, different endogenous ligands are investigated for different signaling patterns. Also in the NPY family, differential engagement of arrestins in response to NPY is studied (e.g., Wanka L, Behr V, Beck-Sickinger AG. Biol Chem. 2021 May 26;403(2):133-149. doi: 10.1515/hsz-2021-0128).

Table 2: the y6 receptor is a pseudogene in humans, this should be indicated in the table. I believe it is correct in the Guide to Pharmacology, which is intended as citation, I guess? However, reference 96 should then be:

Alexander SP, Christopoulos A, Davenport AP, Kelly E, Mathie A, Peters JA, Veale EL et al. (2021) THE CONCISE GUIDE TO PHARMACOLOGY 2021/22: G protein-coupled receptors. Br J Pharmacol. 176 Suppl 1:S27-S156.

The current reference is about angiotensin receptors.

Formal: page 7, line 312: While Google helped me to learn about Jacques Prévert, I don’t think that “Prévert-Style” renders a good linguistic image for any reader outside of France ;)

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

 

The authors discuss the motivation for the search of new agonists of GPCRs and make some interesting points that the field should keep in mind. However, the manuscript contains at least one totally wrong line of reasoning (see item 1) and too many cases of erroneous word usage and/or typos.

 

1.     Lines 111-140. The numbers given are rather misleading, the statement “the available quantity of dopamine never reaches a level at which it could activate its receptors” is wrong. Neurotransmitter receptors, such as dopamine receptors, respond to the concentrations of neurotransmitter that arise upon its release in the synaptic cleft, which are orders of magnitude greater that an average concentration in a particular tissue or brain area. This average is totally meaningless: tissues and cells are highly structured and should not be treated as bags of well-stirred liquid.

2.     Extensive editing, preferably by a native speaker, is needed to eliminate typos and incorrect word usage. E.g., line 138, “rises” should be “raises”; line 147 and below, “Nature” should not be capitalized (with a capital N it’s a well-known journal); line 227, “responsible of a plethora of actions” should be “responsible for a plethora of actions”; lines 272-3, “in certain extend” should be “to a certain extent”; line 273, “encompass” should be “include”; line 289, “comprising” should be “including”; line 312, “a couple of questioning” should be “a couple of questions”; lines 362-3 “accurately cartography the receptor binding site” should be “accurately map the receptor binding site”; line 376, “For examples” should be “For example”; etc.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

The manuscript is interesting. Some changes are required to improve presentation. 

a) Two figures should be edited, the figure caption should be clear and it would include the name of compounds.  

b) The 5 Lipinski rule help to predict bioavailability of a drug by oral administration. It not useful to support a compound which could be a drug or a bioactive compound. Some other theoretical proposals exist in this regard. 

c) The pki should be expressed as higher or lower. Maybe in some context is an improved affinity, but 'better pki' could be high or low in dependence of the aim in an experiment.

d) In the figure 2, both compounds include the dopamine into the depicted structure, the catechol ring and the alpha and beta carbons with an amine are present; albeit if multiple changes and addition of moieties let one to built a molecule with a disimilar structure to endogenous ligand. Then, I suggest to edit the sentence: ...the structures of these two compounds are not even closely related to the dopamine’s one (Figure 2).

e) I suggest you to add some sentences in conclusion section which let to the reader comprehension regarding '...must be dictated by the finality of the need.' Agonist could be useful to study phenomena where the receptors target are clearly involved, or as a target for mechanism which could confer a potential improvement in therapy...It is the meaning?

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

The authors tried to highlight the importance of searching for alternative GPCR agonists. There are some that need to be improved.

I have the following recommendations:

1. Numerous studies have documented that Ions, such as Ca2+, Zn2+, Na+, Mg2+, could bind to multiple GPCRs, as endogenous modulators.  Which should be discussed in Section 2.

2. In Section 3, the authors only emphasized the binding affinity of ligands on receptor. The efficacy of signaling also is important. Some ligands have weak affinity on receptors but show high efficacy, for example, a-MSH and b-MSH with low affinities at 0.1-0.8 μM (IC50) on MC3R and MC4R show high efficacies at nanomoles on cAMP signaling. In addition, endogenous modulators might also have an important role in ligand binding affinity for GPCR. Many pharmacological parameters are determined in vitro using transient cells, different cell lines or methods might also affect ligand affinity on receptors. I also disagreed ‘It seems that this concept is almost completely wrong, at least for nonpeptide-liganded GPCRs’ (line 111-112).

 

3. In Section 4: allosteric agonists, especially biased allosteric modulators (e.g., small molecular compounds), are important to develop therapeutics with minimizing drug side effects or some receptor mutations with biased signaling. This is also the reason why synthetic agonists are studied for drug development. Additionally, biased signaling should be discussed in this section. AgRP acts as an antagonist or inverse agonist on cAMP signaling for MC3R and MC4R but shows agonist activity on MAPK signaling of MC3R and MC4R.

 

4. Figure legends are missing

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

 

The authors discuss the motivation for the search of new agonists of GPCRs and make some interesting points that the field should keep in mind. The most glaring problem with stated agonist concentration, which is orders of magnitude lower in blood/tissue than in the synaptic cleft, was corrected.  

 

1.     Line 127, “30 pg/mL” should be translated into molar concentration (e.g., in parentheses).

2.     Google translate is not the most reliable source. Some editing was done, but more is needed. E.g., line 316, given authors’ explanation, “a couple of questioning” should be “a couple of lines of questioning”; capitalized Nature means nature only in the expression “Mother Nature”, otherwise it means the journal of that name. In general, the authors should keep in mind that the number of found errors in word usage and typos depends on how carefully the reviewer reads the manuscript. E.g., when I read my own text after a hiatus of a few weeks, I usually find quite a few.

3.     Typo: Lines 467-473, Greek letters are missing in pdf file.   

Author Response

 

Thank you for your comments. The answers are in the following:

   Line 127, “30 pg/mL” should be translated into molar concentration (e.g., in parentheses).

            Amended 0.2 nM

2. Google translate is not the most reliable source. Some editing was done, but more is needed. E.g., line 316, given authors’ explanation, “a couple of questioning” should be “a couple of lines of questioning”; capitalized Nature means nature only in the expression “Mother Nature”, otherwise it means the journal of that name. In general, the authors should keep in mind that the number of found errors in word usage and typos depends on how carefully the reviewer reads the manuscript. E.g., when I read my own text after a hiatus of a few weeks, I usually find quite a few.

            Both suggestions were taken care of. For the rest, I do not use (or only rarely) Google translate. It does not work, as said. I read novels, I read and listen to English all day long (from Journals, TV, etc..). Therefore, for a pure English native my English sounds a bit unusual. Now, the real question, here, is whether it is understandable or not. For instance, would a reader better understand Nature (no quote after) than Mother Nature?? Due to the context of the sentence, he/she will understand what we are talking about – as we do not cite anything from the Journal. Concerning the last sentence, the more I read my own production, the more I rephrase it. This is not because I find typos, but because I found other ways to say similar things. Jérôme (not St Jérôme, but Jéröme Leprince, my co-author, right?), in the other hand, has a more careful usage of English - as a chemist, maybe a more precise one? - and his corrections of my writing are always in the direction of simplification and comprehension. Of course, if one gives the text to an editing company – as I did before with San Francisco Edit - the text is returned entirely red. Why? Because they are paid to do it. It would be completely counterproductive to return it by saying “the text is fine”.

So, as long as the message is understood and that no one is aiming at Literature Nobel Price, well….

3. Typo: Lines 467-473, Greek letters are missing in pdf file

            amended

 

Reviewer 4 Report

authors have made enough modifications

Author Response

Thank you for your comment.