Aquaporin Modulation by Cations, a Review
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe reviewed manuscript by Mom and colleagues (CIMB-3117623) is dedicated to summarize the current knowledge on aquaporin regulation by cations and to describe their molecular modes of action. Aquaporins are integral membrane proteins that mediate transport of water and other small molecules across cellular membranes. They are ubiquitously expressed in living cells where they are key players in maintaining cellular homeostasis.
The review begins by briefly recalling the molecular structure of aquaporins, and then the authors catalog the effects that different cations have on different aquaporins and in different species. If possible, the amino acids involved in the effect are recalled, as well as the molecular mechanism proposed in the various studies to explain these effects.
It is a tedious job that the authors have done, and the result is sometimes confusing for the reader, who finds himself wandering from one aquaporin to another, or from one species to another. However, this is due to the sequence of studies over the last two decades, not the authors of the reviewed manuscript.
Finally, this review contains a great deal of information and numerous references that will allow the reader to explore certain points in greater depth.
I see no objection to the publication of this review in Current Issues in Molecular Biology as the topic of the review, aquaporins and their regulation, should be of interest to a wide audience.
Author Response
Dear reviewer,
First of all, thank you for your work.
For your knowledge, we added a sentence for clarity purposes in the discussion section based on Reviewer 2 comments.
Robin Mom on behalf of all co-authors
Reviewer 2 Report
Comments and Suggestions for AuthorsTitle: Aquaporins modulation by cations, a review.
Overall evaluation
In this article, Robin Mom et al. reviewed the modulation of aquaporins (AQPs) by cations. This review comprehensively summarized the current knowledge about aquaporin regulation by cations, described the associated molecular mechanisms, and highlighted the key residues involved in their interactions. However, without revision, this manuscript is not publishable in Current Issues in Molecular Biology.
Major comments are itemized below.
1. -line 45, “AQPs are found in tetrameric assemblies, with each of the four subunits being a functional pore.”
AQPs are found in many species. Are they structurally similar? If AQPs are structurally distinct between species, it may be better to use a separate section to discuss their structural similarities and differences.
2. The presentation of the Table 1 should be improved.
3. AQPs are regulated by many cations as summarized in this manuscript. Some ions are critical to cell physiology or disease pathology. Consider adding more information about the physiologically relevance of their regulation.
4. -lines 475-479, “On top of the traditional regulation of their genes expression, many other factors have been shown to significantly modulate AQPs function: sub-cellular trafficking[53,163], membrane composition[34], protein-protein interactions[35], allosteric mechanisms between sub-units of the same tetramer[164] or gating[165].”
How does the cation regulation interact with other regulatory mechanisms? Are they working cooperatively or competitively? Give a few examples when discussing the regulation of AQPs by different cations.
5. This review article gives a good summary of the cation regulation of AQPs. Are there significant conflicting results or controversies in their regulation mechanisms? The authors should provide their perspectives on the controversies and future directions in this research field.
Author Response
Dear Reviewer,
First of all, thank you for your work.
Comment 1:
1-line 45, “AQPs are found in tetrameric assemblies, with each of the four subunits being a functional pore.”
AQPs are found in many species. Are they structurally similar? If AQPs are structurally distinct between species, it may be better to use a separate section to discuss their structural similarities and differences.
Answer:
Indeed AQPs are structurally similar between life branches. The archetype hourglass AQP fold is conserved between prokarotes, archae prokaryotes and eukaryotes. The ancestral fold present in bacteria and archae bacteria diversified greatly in multi-cellular organisms and especially in plants. To read further about the conservation and diversification of the AQP fold across species, we draw your attention to reference 1:
- Abascal, F.; Irisarri, I.; Zardoya, R. Diversity and Evolution of Membrane Intrinsic Proteins. Biochimica et Biophysica 536 Acta (BBA) - General Subjects2014, 1840, 1468–1481, doi:10.1016/j.bbagen.2013.12.001.
Regarding the tetrameric assemblies, according to all of the experimental structures of AQPs, the sub-units assemble in tetramers. For further details we referred the reader to reference 27 which is an excellent review about the AQP fold:
- Walz, T.; Fujiyoshi, Y.; Engel, A. The AQP Structure and Functional Implications. Handbook of Experimental 596 Pharmacology2009, 31–56, doi:10.1007/978-3-540-79885-9_2.
To ease the comprehension of the manuscript, reference 27 has been added at the end of the mentioned sentence : “AQPs are found in tetrameric assemblies, with each of the four subunits being a functional pore [27].”
Comment 2:
“2. The presentation of the Table 1 should be improved.”
Answer:
The legend of table 1 has been completed. Regarding the table appearance, we added colors for clarity purposes, however we leave the final decision to the editorial board of CIMB regarding these kind of cosmetic details.
Comment 3:
“3. AQPs are regulated by many cations as summarized in this manuscript. Some ions are critical to cell physiology or disease pathology. Consider adding more information about the physiologically relevance of their regulation.”
Answer:
Information about the physiological relevance of cations are already present in the review. Moreover, many dedicated reviews already exist about the physiological relevance of ions. Finally, the whole point of this review is to go further into the details of such physiological relevance of ions.
Comment 4:
“-lines 475-479, “On top of the traditional regulation of their genes expression, many other factors have been shown to significantly modulate AQPs function: sub-cellular trafficking[53,163], membrane composition[34], protein-protein interactions[35], allosteric mechanisms between sub-units of the same tetramer[164] or gating[165].”
How does the cation regulation interact with other regulatory mechanisms? Are they working cooperatively or competitively? Give a few examples when discussing the regulation of AQPs by different cations. »
Answer:
The whole review is a compilation of examples for cations interaction with different levels of regulation of AQPs. The thing is that there are no general trends that could be extracted from the types of interactions as even for the same ion type, opposite effects can be seen depending on the AQP considered, i.e. zinc fixation increases AQP0 water permeability while it decreases AQP4’s. To give you one example per level of AQP regulation:
_AQP3 gene transcription increase is triggered by magnesium
_sub-cellular trafficking modulation of AQP2 by calcium and calmodulins.
_hypothesis for mercury interaction with membrane phospholipidic heads to explain their action upon plant AQPs
_protein-protein interactions: AQPs binding to calmodulin is calcium dependant
_allosteric mechanisms between sub-units: zinc effect on AQP0 and AQP5
_gating of plant AQPs involves the fixation of calcium on intra-cellular loop D
We believe going back to this level of details in the discussion section would not be relevant especially when it was already done in the previous sections of the review.
Comment 5:
“This review article gives a good summary of the cation regulation of AQPs. Are there significant conflicting results or controversies in their regulation mechanisms? The authors should provide their perspectives on the controversies and future directions in this research field.”
Answer:
Yes the major interrogation is related to the mode of action of mercury and the involvement of cysteines in the transduction of the signal. This point is already mentioned:
“While it appears clearly that cations are relevant physiological regulators of AQPs in all types of organisms, a global understanding of their modes of action is lacking. The initially proposed steric inhibition of water permeability by mercury bound to pore lining cysteine residues is for sure insufficient or too restrictive to explain all types of cation-associated regulations of AQPs. Indeed, there is for instance no clear explanation for allosteric modulations not associated with the targeting of pore-lining residues by cations nor for mercury mode of action in plant AQPs which does not seem to involve cysteines at all.”
Our perspectives about the remaining discrepancies and interrogations are discussed in the last paragraph (to ease the comprehension, the sentence in bold has been added to the main text):
“The arginine of the ar/R constriction is among the most conserved residues of the AQP fold and is also known to be determinant for solute selectivity of the pore. Because of their positively charged sidechains, arginine can act as voltage sensors in voltage-gated channels[166,167]. Considering the perturbations of the protein electrostatic field induced by cation binding, one could hypothesize a similar role for the arginine of the ar/R constriction in AQPs. Several studies have demonstrated the possible modulations of AQPs through the application of external electric fields in silico[168–174]. While other pore lining residues such as histidines were involved, the ar/R constriction arginine was indeed implicated in external electric field sensing[168,171–174]. Other studies highlighted the modulation of AQPs water fluxes by non-physiological membrane potentials through all-atoms molecular dynamics[175,176]. Once again, the ar/R arginine played a central role. Interestingly, high but realistic concentrations of KCl ions could also induce the same type of modulation[176]. Better understanding how charges repartition at different location of the AQP fold could impact ar/R constriction arginine conformation inside the pore lumen could hence be a way to propose a unifying molecular mechanism of AQPs modulation by cations.”
Robin Mom, on behalf of all co-authors
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsThe quality of this manuscript is improved.
