Coevolution with toxic prey produces functional trade-offs in sodium channels of predatory snakes

Reviewer #1 (Public Review):

Summary:
This is an analysis of the mutations of Nav1.4 that allow tetrodotoxin resistance in two snake species while reducing the functional capacity of sodium channels in skeletal muscle and thereby reducing muscle function compared to toxin-sensitive snakes.

Strengths:
This is a well-conceived, solid, and well-presented manuscript. Although the subject is not entirely new, the approach is original and the data obtained is solid. The analysis of the structural changes implications in the channel function is certainly an important contribution to the field.

Weaknesses:
A short discussion on nerve sodium channels would be useful.

Reviewer #2 (Public Review):

Summary:
The story of the co-evolution of TTX-bearing newts and their independently evolved TTX-resistant garter snake predators is a classic in evolutionary ecology/physiology. Over the years specific amino acid substitutions in the muscle-expressing (and other) sodium channels have been identified and the behavioral assays of snake crawling performance have indicated that the attainment of TTX-resistance comes at a cost in mobility. One previous study also examined how the amino acid mutations affected the biophysics of Nav channel properties. The present paper starts with this foundation and builds by adding in details and making causal connections across multiple snake populations with different degrees of resistance. The addition of muscle physiology bridges the gap between organismal performance and sodium channel biophysics. Moving in the other direction, examining molecular models of Nav channel structure and energetics allows a deep understanding of how amino acid substitutions affect channel properties. In the end, a clear picture is painted from molecular to organismal levels in two different parallel evolutions of TTX resistance.

Strengths:
This study is a tour de force. It is clearly written, and nicely illustrated, and the methods and procedures are meticulously documented.

Weaknesses:
One caveat is that the Nav channels used to test mutations in expression systems are rat channels engineered with TTX-resisting substitutions observed in snake populations. The ideal experiment would have been to use the snake channels. While the rat channels appear to be a good substitute for the snake channels and the authors have taken pains to show that the important amino acids are conserved between garter snakes and rats, the authors might explain why they did not use snake channels.

The noise analysis seems like a reasonable way to get at the question of single-channel conductance. But why did the authors not just measure single channel conductance in patches as opposed to this much more complex and roundabout method? It is recommended that the authors discuss how noise analysis deals with the problem of having the number of open channels changing rapidly during activation and fast inactivation. Is this a potential problem for deriving the total number of channels?

Reviewer #3 (Public Review):

Summary:
This paper explores the cost of toxin resistance in snakes that prey on newts defended by highly potent TTX. Two species of garter snakes, T. atratus and T. sirtalis, are examined. Both species have resistant and sensitive populations. Resistance is achieved by substitutions in the voltage-gated sodium channels, which block TTX binding. Resistant T. atratus carry the triple substitutions EPN while resistant T. sirtalis carry the quadruple LVNV. These substitutions occur on the third and fourth intracellular domains of the voltage-gated sodium-channel gene Nav1.4, which is the paralog found in skeletal muscle. EPN and LVNV have been previously attributed to conferring resistance to TXX through target-site insensitivity of the channel. Previous work has also shown that snakes from resistant populations have reduced locomotor capabilities compared to their non-resistant counterparts.

The authors systematically test the hypothesis that the resistance-conferring substitutions affect other phenotypes related to the function of the voltage-gated sodium channel, which is, in turn, responsible for the reduced locomotor capabilities. First, they compare the effects of EPN and LVNV on recombinantly expressed rat Nav1.4 with and without EPN and LVNV (in vitro). They find that both EPN and LVNV significantly reduce the channel's conductance. On top of that, LVNV also causes premature deactivation of the channel, thus reducing the current passing through the membrane. Next, they compare muscle tissue function between resistant and non-resistant populations of T. atratus and T. sirtalis (ex vivo). They find that both resistant populations have reduced twitch force (with T. sirtalis, carrying LVNV, having an even stronger reduction), reduced peak rate of force development, and overall reduced force. In addition, T. sirtalis (LVNV) muscle also has reduced peak tetanic force. Finally, they compare the biophysical effects of EPN and LVNV through homology modeling of Nav1.4 to explain the in vitro and in vivo results (in silico). They found that E1248 (of EPN) has a counteracting effect on the destabilizing effect of N1539, shared by both species. T. sirtalis (LVNV) lacks such a counteracting mutation, which could explain the stronger negative effects observed in LVNV channels and muscles.

Strengths:
A particular strength of this paper is the multi-level approach used to tease apart the negative pleiotropic effects of resistance-conferring substitutions. Each level of experiments informed the next, creating a focused comprehensive analysis of the costs associated with this specialized dietary adaptation in snakes. The results make an important contribution to our understanding of the role of negative pleiotropy in adaptive evolution and would be of broad interest to readers. The paper is well-written, and the data and analyses are clearly presented.

Weaknesses:
The sheer size of the Nav1.4 gene makes it difficult to clone into an expression vector and that's probably why an already cloned rat Nav1.4 was selected for the in vitro experiments. It would be great if the authors could comment on how the level of resistance produced by mutations on the rat Nav1.4 compared to the garter snake Nav1.4s. Are there previous data on tissue-isolated T. sirtalis and T. atratus channels? Is it possible that the snake mutations have slightly different effects on the rat genetic background due to epistatic interactions with sites beyond the 3rd and 4th domains?

Following up on the first comment, sometimes negative pleiotropic effects are mitigated by compensatory mutations in other regions of the protein. This reviewer would recommend that the authors comment on this possibility. Are there substitutions beyond the 3rd and 4th domains that could potentially play a role in this adaptation?

Based on the results, it seems that resistant T. sirtalis got the shorter end of the stick concerning negative pleiotropic effects, despite having similar (the same?) levels of resistance to TTX. Does this difference/disadvantage scale up to locomotor performance as well?

It would be great if the authors could comment on how these resistant populations have persisted despite the locomotor/muscular disadvantages. Are there known differences in predation rates between the populations? The benefit must have outweighed the cost in these cases.

Reviewer #4 (Public Review):

Summary:
The authors set out to address whether TTX resistance in a subset of snakes is due to mutations near the selectivity filters of their Nav1.4 channels. They present an investigation of the properties of two heterologously expressed Nav1.4 channels, bearing the Nav1.4EPN and Nav1.4LVNV mutations found in TTX-resistant snakes. After assessing their sensitivity to TTX, they have studied the biophysical properties of these mutants by electrophysiological methods and discovered that the voltage dependence of their activation and inactivation remains unchanged compared to the TTX-sensitive Nav1.4. These experiments revealed some kinetic differences in Nav1.4LVNV and that both Nav1.4EPN and Nav1.4LVNV show a reduced unitary conductance. The authors also assessed muscle properties (resistance, force development, and contraction timing) of two groups of snakes (in vivo and in dissected muscles) with Nav1.4EPN and Nav1.4LVNV mutations. These experiments showed a reduced performance for the skeletal muscles of snakes bearing Nav1.4EPN and Nav1.4LVNV background. Finally, the authors have built homology models of Nav1.4EPN and Nav1.4LVNV channels to hypothesize a molecular explanation of the altered properties.

Strengths:
• Three levels of analysis are performed in this study: 1) functional characterization of mutated Nav1.4 channels through electrophysiology; 2) molecular level comparisons between human and snake Nav1.4 channels structures through homology modelling; 3) organismal performance/muscle strength experiments on snakes that carry Nav1.4 mutants that render them virtually TTX resistant.

Weaknesses:
• While there is reason to believe that there is a causal link between the observed changes in Nav1.4 and the changes on the organismal level, the evidence presented is not definitive. Specifically, the conclusions from the biophysical/electrophysiological experiments are extrapolated to be causal for the altered muscle performance in TTX-resistant snakes, although there might be alternative explanations. First, the reduction in muscle force could also originate from changes in the calcium release apparatus or other alterations in the electrical properties of the muscle (are there changes in length or duration of muscle action potentials? Is there a change in the fraction of muscle cells that fail action potentials, as would be expected for a significant reduction in conductance?). Second, it remains unclear if, among the different snake Nav channels (e.g. Nav1.6 in motor neurons), Nav1.4 is the only one to display side chain alterations in these TTX-resistant snakes.

• Some of the data presented as part of the NSNA is not sufficiently convincing and should be supplemented with additional evidence or carefully discussed with regard to its limitations.

• The mutations studied are located close to the selectivity filter of Nav1.4. This means that the most likely consequence of the mutations is altered sodium selectivity, possibly along with changes to block extracellular calcium. But these possibilities are not currently addressed.

• The description and accuracy of the homology model remains somewhat unclear, as no validation of the modeled channel has been presented. Therefore, the accuracy of the homology model remains vague, which calls into question to what degree the molecular features of this model can be linked to the electrophysiological findings.