A class-specific effect of dysmyelination on the excitability of hippocampal interneurons
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eLife assessment
This important study identifies the functional consequence of myelination of interneuronal axons on circuit function by showing that 4.1B deletion leads to altered myelination in a subset of interneurons and altered intrinsic and synaptic physiological parameters. The authors' conclusions about how myelination of inhibitory axons affects physiological properties are based on solid evidence using a combination of imaging and electrophysiological approaches.
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Abstract
The role of myelination for axonal conduction is well-established in projection neurons but little is known about its significance in GABAergic interneurons. Myelination is discontinuous along interneuron axons and the mechanisms controlling myelin patterning and segregation of ion channels at the nodes of Ranvier have not been elucidated. Protein 4.1B is implicated in the organization of the nodes of Ranvier as a linker between paranodal and juxtaparanodal membrane proteins to the spectrin cytoskeleton. In the present study, 4.1B KO mice are used as a genetic model to analyze the functional role of myelin in Lhx6-positive parvalbumin (PV) and somatostatin (SST) neurons, two major classes of GABAergic neurons in the hippocampus. We show that 4.1B-deficiency induces disruption of juxtaparanodal K + channel clustering and mislocalization of nodal or heminodal Na + channels. Strikingly, 4.1B-deficiency causes loss of myelin in GABAergic axons in the hippocampus. In particular, stratum oriens SST cells display severe axonal dysmyelination and a reduced excitability. This reduced excitability is associated with a decrease in occurrence probability of small amplitude synaptic inhibitory events on pyramidal cells. In contrast, stratum pyramidale fast-spiking PV cells do not appear affected. In conclusion, our results indicate a class-specific effect of dysmyelination on the excitability of hippocampal interneurons associated with a functional alteration of inhibitory drive.
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Author Response
Reviewer #1 (Public Review):
The Authors of this study have investigated the consequence of knocking out protein 4.1B on hippocampal interneurons. They observed that in 4.1B KO mice, the myelinization of axons of PV and SST interneurons was altered. In addition, the molecular organization of the nodal, heminodal, and juxtaparanodal parts of the interneuron axons was disrupted in 4.1B KO mice. Further, the authors found some changes in spiking features of SST, but not PV interneurons as well as synaptic inhibition recorded in CA1 pyramidal cells. Lastly, 4.1B KO mice showed some impairment in spatial memory.
Strengths
One of the strengths of this MS is the multilevel approach to the question of how myelinization of interneuron axons can contribute to hippocampal functions. Further, the cell biological results …
Author Response
Reviewer #1 (Public Review):
The Authors of this study have investigated the consequence of knocking out protein 4.1B on hippocampal interneurons. They observed that in 4.1B KO mice, the myelinization of axons of PV and SST interneurons was altered. In addition, the molecular organization of the nodal, heminodal, and juxtaparanodal parts of the interneuron axons was disrupted in 4.1B KO mice. Further, the authors found some changes in spiking features of SST, but not PV interneurons as well as synaptic inhibition recorded in CA1 pyramidal cells. Lastly, 4.1B KO mice showed some impairment in spatial memory.
Strengths
One of the strengths of this MS is the multilevel approach to the question of how myelinization of interneuron axons can contribute to hippocampal functions. Further, the cell biological results support the claim of the reorganization of channel distributions at axonal nodes.
Weaknesses
- Although the authors acknowledge that SST is expressed in different GABAergic cell types in the hippocampus, they claim that OLM cells, which express SST are subject to changes in 4.1B KO mice. However, this claim is not supported by data. Both OLM cells and GABAergic projection cells expressing SST have many long-running axons in the stratum radiatum, where the investigations have been conducted (e.g. Gulyas et al., 2003; Jinno et al., 2007). Thus, the SST axons can originate from any of these cell types. In addition, both these GABAergic cells have a sag in their voltage responses upon negative current injections (e.g. Zemankovics et al., 2010), making it hard to separate these two SST inhibitory cell types based on the single-cell features. In summary, it would be more appropriate to name the sampled interneurons as SST interneurons. Alternatively, the authors may want to label intracellularly individual interneurons to visualize their dendrites and axons, which would allow them to verify that the de-myelinization occurs along the axons of OLM cells, but not SST GABAergic projection neurons.
We agree and named the sampled interneurons as SST interneurons throughout the text. We acknowledge that SST GABAergic projection cells have long-running axons in the stratum radiatum (Gulyas et al., 2003; Jinno et al., 2007) that may be also dysmyelinated. See Results lanes 200 and 350.
- Although both the cellular part and the behavioral part are interesting, there is no link between them at present. The changes observed in spatial memory tests may not be caused by the changes in the axonal de-myelinization of hippocampal interneurons. Such a claim can be made only using rescue experiments, since changes in 4.1B KO mice leading to behavioral alterations may occur i) in other cell types and ii) in other regions, which have not been investigated.
Alteration of spatial memory has not been previously reported in the 4.1B KO mice. Our results leave open the possibility that dysmyelination of inhibitory interneurons in the hippocampus may induce impaired cognitive ability (see preprint). We agree that future studies investigating a putative rescue of spatial memory by means of virus-mediated expression of 4.1B in hippocampal Lhx6 interneurons would be very informative.
Reviewer #2 (Public Review):
In this study, Pinatel et al. address the role of interneuron myelination in the hippocampus using a 4.1B protein mouse knockout model. They show that deficiency in 4.1B significantly reduces myelin in CA1 stratum radiatum, specifically myelin along axons of parvalbumin and somatostatin hippocampal interneurons. In addition, there are striking defects in the distribution of ion channels along myelinated axons, with misplacement of Na channel clusters along the nodes of Ranvier and the heminodes, and a pronounced decrease in potassium channels (Kv1) at juxtaparanodes. The axon initial segments of SST are also shorter. Because the majority of myelinated axons in the stratum radiatum of the hippocampus belong to PV and SST interneurons such profound changes in myelination are expected to affect interneuronal function. Interestingly, the authors show that PV basket cells' properties appear largely unaffected, while there are substantial changes in stratum oriens O-LM cells. Inhibitory inputs to pyramidal neurons are also changed. Behaviorally, the 4.1B KO mice exhibit deficits in spatial working memory, supporting the role of interneuronal myelination in hippocampal function. This study provides important insights into the role of myelination for the function of inhibitory interneurons, as well as in the mechanisms of axonal node development and ion channel clustering, and thus will be of interest to a broad audience of circuit and cellular neuroscientists. However, the claims of the specificity of the reported changes in myelination need to be better supported by evidence.
Strengths:
The authors combine a wide array of genetic, immunolabeling, optical, electrophysiological, and behavioral tools to address a still unresolved complex problem of the role of myelination of locally projecting inhibitory interneurons in the hippocampus. They convincingly show that changing myelination and ion channel distribution along nodes and heminodes significantly impairs the function of at least some interneuron types in the hippocampus and that this is accompanied by behavioral deficits in spatial memory.
Regarding the organization of myelinated axons, the lack of 4.1B causes striking changes at the nodes of Ranvier that are convincingly and beautifully presented in the Figures. While the reduction in Kv1 in 4.1B KO mice has been previously reported, the mislocalization of sodium channels at the nodes and heminodes had only been observed in developing but not adult spinal cords. This difference in the dependence of the sodium channel distribution on 4.1B in adult hippocampus vs spinal cord may hold important clues for the varying role of myelin along axons of different neuronal types.
The manuscript is very well written, the discussion is comprehensive, and provides detailed background and analysis of the current findings and their implications.
Weaknesses:
Because of the wide diversity of interneuron types in the hippocampus, and also the presence of myelinated axons from other neuron types as well, including pyramidal neurons, it is very difficult to disentangle the effects of the observed changes in the 4.1 B KO mouse model. While the authors have been careful to explore different possibilities, some of the claims of the specificity of the reported changes in myelination are not completely founded. For example, there is no compelling evidence that the myelination of axons other than the local interneurons is unchanged. The evidence strongly supports the claims of changes in interneuronal myelination, but it leaves open the question of whether 4.1B lack affects the myelination of hippocampal pyramidal neurons or of long-range projections.
This is an important question also raised by Reviewer 1. We have now quantified the density of paranodes in the alveus as shown in Figure 1I. Paranode density was not affected in the alveus nor in the stratum lacunosum-moleculare suggesting that myelinated axons connecting extra-hippocampal areas may be preserved. In particular, this is an indication that the axons of pyramidal neurons that run into the alveus should be properly myelinated.
To be able to better interpret the changes in the 4.1B KO mice, knowledge of the distribution of 4.1B in the hippocampus of control mice will be very helpful. The authors state that 4.1B is observed in PV neurons but not in pyramidal neurons, however, the evidence is not convincing. Thus, the lack of immunolabeling at the pyramidal neuron cell bodies does not indicate that 4.1B is missing at the axonal level. The analysis also leaves out the question of whether 4.1 B is seen in the axons of somatostatin neurons.
We agree and do not exclude that 4.1B may be expressed along the axons of pyramidal neurons. We performed double-staining for SST and 4.1B to show that 4.1B is localized along the internode and enriched at the paranodes of SST axons as observed for PV axons (Figure 4F). The enrichment of 4.1B in GABAergic neurons was previously observed in premyelinated hippocampal cell culture (Bonetto et al. 2019).
Reviewer #3 (Public Review):
Pinatel and colleagues addressed a currently understudied topic in neurobiology, namely, the architecture and function of myelination in subsets of Parvalbumin (PV)- and Somatostatin (SST)-positive GABAergic hippocampal interneurons and its dependence on juxtaparanodal organizer proteins. In order to elucidate the structural and functional implications of interneuron myelination, the authors visualized inhibitory neurons by utilizing a Lhx2-Lhx6 tdTomato reporter line in combination with mutants for crucial membrane and cytoskeletal linker proteins such as Contactin2/TAG-1, Caspr2, and Protein 4.1B. They then applied a comprehensive set of histological, electrophysiological, and behavioral experiments to dissect the role these proteins play in proper myelination and function of PV- and SST-interneurons.
The bulk of the study's data is based on immunofluorescence, which is presented in a number of figures comprised of high-quality images. As much as this is a strength of the study, the underlying image analysis as described in the methods falls short. All structural data rely on the measurements of physical parameters such as length of internodes, the distance between (juxta)paranode and node, the distance between node and myelin sheath, length of the axon initial segment (AIS), etc. In light of this, and considering the small physical dimensions of the nodal region in general, the methods remain unclear about the depth of 3D reconstruction/deconvolution applied to the samples. Measurements presented in the results show significant differences in sub-micrometer dimension, which at least according to the stated methods, are unlikely to be precise given that the confocal imaging parameters do not seem to reach Nyquist conditions. For a study in which a third of all data is aimed at elucidating (sub)micrometer changes, this is crucial and the study would benefit from a more rigorous method description by the authors.
Another methodological weakness is the somewhat small n, and its incoherence across the experiments and therefore, the statistics performed in some of the experiments. Statistics are based on either n for animals, or n for individual data points from several animals. Why is not all data represented as mean/animal? Also, the sampling in general with n = 3 animals is borderline acceptable; in some cases, it seems that only 2 animals were used, and in others, no number is given at all (please refer to author comments for details). This needs to be addressed, either by explaining why so few animals were used, or by adding more data from individual animals.
Assigning structures (AIS, nodes) as n results in overstating effects, since especially for AIS, there is significant heterogeneity in the length across neurons from the same type, and this is masked when 100 AIS are considered as individual n instead 100 AIS per animal, and the animal is (correctly) the n.Since the study seems to switch back and forth between these assignments, it would be helpful to level these data across all experiments unless there are specific reasons not to do so, which then need to be explained. As outlined in the methods, all values are given as means {plus minus} SEM; this needs to be corrected for those cases where the standard deviation is the appropriate choice (e.g. all graphs showing n = individual structure, and not the mean of an animal).
As far as the analysis of geometrical AIS changes is concerned, the method section should be extended to address how, if at all, AIS length and position were analyzed in 3D, also considering the somewhat "spotty" immunosignal outlined in Fig. 8D.
We agree with all these comments. We improved Fig.1 I and J by adding more data (n=4 mice). We would like to point out that the phenotype of the 4.1B KO mice is highly penetrant. The selective loss of myelin in the hippocampus was observed in the two different genetic background (4.1B-/- and 4.1B-/-;Lhx6;tdTomato mice) and at all the ages examined (P25P180).
For the quantitative morphological analysis: We considered “n=number of animals” in Figure 1 to describe the massive and selective alteration of myelin in the hippocampus of 4.1B KO mice. In the following Figures, we considered n=ROIs (Figure 2, Figure 3, Figure 6) for the density of SST and PV interneurons or oligodendroglial cells and n=individual structures (Figure 4, Figure 5, Figure 8) for a more precise sampling of the structure heterogeneity (internode, node, AIS). Means ± SEM are indicated in the text corresponding to plot boxes and distribution plots in the Figures.
Concerning AIS measurements, we considered “n” as individual AIS in a coherent manner with the electrophysiological recordings in which “n” is the individual cells. We hope that we have now better illustrated the AIS of SST cells in the stratum oriens in the new Figure 8 with single channel images. In contrast to the AIS of pyramidal neurons that display sinuous feature, the AIS of SST neurons (and especially O-LM cells which axons run straight across the stratum radiatum) show a rather straight organization.
We improved our measurements of the AIS structural parameters (onset, length) of SST neurons of the stratum oriens using confocal imaging with a 20x objective, 0.54 µm steps, Nyquist conditions. Indeed, these new measurements confirmed that the AIS of SST neurons was significantly shorter in the 4.1B KO mice.
The observed AIS length change is then discussed in the context of a study conducted in a pharmacological model of myelin loss, however, that particular study (Hamada & Kole, 2015) found not only a length change but a position change after cuprizone-induced AIS plasticity. The authors should therefore discuss this finding in a bit more detail than simply stating "Adaptation of the AIS has been reported in the cuprizone chemical model of demyelination" (p. 14, ll. 512).
We added these sentences in the Discussion:
Lane 527: Supporting this notion, previous studies have reported an adaptive response of the AIS of cortical pyramidal neurons in the cuprizone chemical model of demyelination. Specifically, it was observed that the length of the AIS is reduced together with a more proximal site of the onset. These changes reduce the AIS excitability suggesting a compensatory mechanism to ectopic action potentials generated in demyelinated axons (Hamada and Kole, 2015).
Lane 556: Interestingly, in cortical pyramidal neurons, demyelination induced by cuprizone causes the restructuring of AIS and reduces excitability at this site. “Acute demyelination leads to a more proximal onset of AIS without a change in the length of ßIV spectrin expression. However, the AIS of these acutely demyelinated axons display a reduced length of Nav1.6 channel expression and extended Kv7.3 channel expression at the distal site (Hamada and Kole, 2015).”
Similarly to the points made about structural data above, the data from electrophysiological recordings should be presented in such a way that e.g. the number of cells and/or animals is readily accessible from the graph or legend. In its current form, this information - while available - needs to be pieced together from in-text information supplemented by figure legends. Sometimes, the authors do not include the number of animals behind individual cell data (for details please see author comments). Please carefully review all figures and edit accordingly.
The behavioral data presented in the study is interesting, but the conclusions drawn are not supported by the data presented, as many unknown factors remain in place that could contribute to the observed phenotype.
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eLife assessment
This important study identifies the functional consequence of myelination of interneuronal axons on circuit function by showing that 4.1B deletion leads to altered myelination in a subset of interneurons and altered intrinsic and synaptic physiological parameters. The authors' conclusions about how myelination of inhibitory axons affects physiological properties are based on solid evidence using a combination of imaging and electrophysiological approaches.
-
Reviewer #1 (Public Review):
The Authors of this study have investigated the consequence of knocking out protein 4.1B on hippocampal interneurons. They observed that in 4.1B KO mice, the myelinization of axons of PV and SST interneurons was altered. In addition, the molecular organization of the nodal, heminodal, and juxtaparanodal parts of the interneuron axons was disrupted in 4.1B KO mice. Further, the authors found some changes in spiking features of SST, but not PV interneurons as well as synaptic inhibition recorded in CA1 pyramidal cells. Lastly, 4.1B KO mice showed some impairment in spatial memory.
Strengths
One of the strengths of this MS is the multilevel approach to the question of how myelinization of interneuron axons can contribute to hippocampal functions. Further, the cell biological results support the claim of the …Reviewer #1 (Public Review):
The Authors of this study have investigated the consequence of knocking out protein 4.1B on hippocampal interneurons. They observed that in 4.1B KO mice, the myelinization of axons of PV and SST interneurons was altered. In addition, the molecular organization of the nodal, heminodal, and juxtaparanodal parts of the interneuron axons was disrupted in 4.1B KO mice. Further, the authors found some changes in spiking features of SST, but not PV interneurons as well as synaptic inhibition recorded in CA1 pyramidal cells. Lastly, 4.1B KO mice showed some impairment in spatial memory.
Strengths
One of the strengths of this MS is the multilevel approach to the question of how myelinization of interneuron axons can contribute to hippocampal functions. Further, the cell biological results support the claim of the reorganization of channel distributions at axonal nodes.Weaknesses
1. Although the authors acknowledge that SST is expressed in different GABAergic cell types in the hippocampus, they claim that OLM cells, which express SST are subject to changes in 4.1B KO mice. However, this claim is not supported by data. Both OLM cells and GABAergic projection cells expressing SST have many long-running axons in the stratum radiatum, where the investigations have been conducted (e.g. Gulyas et al., 2003; Jinno et al., 2007). Thus, the SST axons can originate from any of these cell types. In addition, both these GABAergic cells have a sag in their voltage responses upon negative current injections (e.g. Zemankovics et al., 2010), making it hard to separate these two SST inhibitory cell types based on the single-cell features. In summary, it would be more appropriate to name the sampled interneurons as SST interneurons. Alternatively, the authors may want to label intracellularly individual interneurons to visualize their dendrites and axons, which would allow them to verify that the de-myelinization occurs along the axons of OLM cells, but not SST GABAergic projection neurons.2. Although both the cellular part and the behavioral part are interesting, there is no link between them at present. The changes observed in spatial memory tests may not be caused by the changes in the axonal de-myelinization of hippocampal interneurons. Such a claim can be made only using rescue experiments, since changes in 4.1B KO mice leading to behavioral alterations may occur i) in other cell types and ii) in other regions, which have not been investigated.
-
Reviewer #2 (Public Review):
In this study, Pinatel et al. address the role of interneuron myelination in the hippocampus using a 4.1B protein mouse knockout model. They show that deficiency in 4.1B significantly reduces myelin in CA1 stratum radiatum, specifically myelin along axons of parvalbumin and somatostatin hippocampal interneurons. In addition, there are striking defects in the distribution of ion channels along myelinated axons, with misplacement of Na channel clusters along the nodes of Ranvier and the heminodes, and a pronounced decrease in potassium channels (Kv1) at juxtaparanodes. The axon initial segments of SST are also shorter. Because the majority of myelinated axons in the stratum radiatum of the hippocampus belong to PV and SST interneurons such profound changes in myelination are expected to affect interneuronal …
Reviewer #2 (Public Review):
In this study, Pinatel et al. address the role of interneuron myelination in the hippocampus using a 4.1B protein mouse knockout model. They show that deficiency in 4.1B significantly reduces myelin in CA1 stratum radiatum, specifically myelin along axons of parvalbumin and somatostatin hippocampal interneurons. In addition, there are striking defects in the distribution of ion channels along myelinated axons, with misplacement of Na channel clusters along the nodes of Ranvier and the heminodes, and a pronounced decrease in potassium channels (Kv1) at juxtaparanodes. The axon initial segments of SST are also shorter. Because the majority of myelinated axons in the stratum radiatum of the hippocampus belong to PV and SST interneurons such profound changes in myelination are expected to affect interneuronal function. Interestingly, the authors show that PV basket cells' properties appear largely unaffected, while there are substantial changes in stratum oriens O-LM cells. Inhibitory inputs to pyramidal neurons are also changed. Behaviorally, the 4.1B KO mice exhibit deficits in spatial working memory, supporting the role of interneuronal myelination in hippocampal function. This study provides important insights into the role of myelination for the function of inhibitory interneurons, as well as in the mechanisms of axonal node development and ion channel clustering, and thus will be of interest to a broad audience of circuit and cellular neuroscientists. However, the claims of the specificity of the reported changes in myelination need to be better supported by evidence.
Strengths:
The authors combine a wide array of genetic, immunolabeling, optical, electrophysiological, and behavioral tools to address a still unresolved complex problem of the role of myelination of locally projecting inhibitory interneurons in the hippocampus. They convincingly show that changing myelination and ion channel distribution along nodes and heminodes significantly impairs the function of at least some interneuron types in the hippocampus and that this is accompanied by behavioral deficits in spatial memory.Regarding the organization of myelinated axons, the lack of 4.1B causes striking changes at the nodes of Ranvier that are convincingly and beautifully presented in the Figures. While the reduction in Kv1 in 4.1B KO mice has been previously reported, the mislocalization of sodium channels at the nodes and heminodes had only been observed in developing but not adult spinal cords. This difference in the dependence of the sodium channel distribution on 4.1B in adult hippocampus vs spinal cord may hold important clues for the varying role of myelin along axons of different neuronal types.
The manuscript is very well written, the discussion is comprehensive, and provides detailed background and analysis of the current findings and their implications.
Weaknesses:
Because of the wide diversity of interneuron types in the hippocampus, and also the presence of myelinated axons from other neuron types as well, including pyramidal neurons, it is very difficult to disentangle the effects of the observed changes in the 4.1 B KO mouse model. While the authors have been careful to explore different possibilities, some of the claims of the specificity of the reported changes in myelination are not completely founded. For example, there is no compelling evidence that the myelination of axons other than the local interneurons is unchanged. The evidence strongly supports the claims of changes in interneuronal myelination, but it leaves open the question of whether 4.1B lack affects the myelination of hippocampal pyramidal neurons or of long-range projections.To be able to better interpret the changes in the 4.1B KO mice, knowledge of the distribution of 4.1B in the hippocampus of control mice will be very helpful. The authors state that 4.1B is observed in PV neurons but not in pyramidal neurons, however, the evidence is not convincing. Thus, the lack of immunolabeling at the pyramidal neuron cell bodies does not indicate that 4.1B is missing at the axonal level. The analysis also leaves out the question of whether 4.1 B is seen in the axons of somatostatin neurons.
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Reviewer #3 (Public Review):
Pinatel and colleagues addressed a currently understudied topic in neurobiology, namely, the architecture and function of myelination in subsets of Parvalbumin (PV)- and Somatostatin (SST)-positive GABAergic hippocampal interneurons and its dependence on juxtaparanodal organizer proteins. In order to elucidate the structural and functional implications of interneuron myelination, the authors visualized inhibitory neurons by utilizing a Lhx2-tdTomato reporter line in combination with crucial cytoskeletal linker proteins such as Contactin2/TAG-1, Caspr2, and Protein 4.1B. They then applied a comprehensive set of histological, electrophysiological, and behavioral experiments to dissect the role these proteins play in proper myelination and function of PV- and SST-interneurons.
The bulk of the study's data is …
Reviewer #3 (Public Review):
Pinatel and colleagues addressed a currently understudied topic in neurobiology, namely, the architecture and function of myelination in subsets of Parvalbumin (PV)- and Somatostatin (SST)-positive GABAergic hippocampal interneurons and its dependence on juxtaparanodal organizer proteins. In order to elucidate the structural and functional implications of interneuron myelination, the authors visualized inhibitory neurons by utilizing a Lhx2-tdTomato reporter line in combination with crucial cytoskeletal linker proteins such as Contactin2/TAG-1, Caspr2, and Protein 4.1B. They then applied a comprehensive set of histological, electrophysiological, and behavioral experiments to dissect the role these proteins play in proper myelination and function of PV- and SST-interneurons.
The bulk of the study's data is based on immunofluorescence, which is presented in a number of figures comprised of high-quality images. As much as this is a strength of the study, the underlying image analysis as described in the methods falls short. All structural data rely on the measurements of physical parameters such as length of internodes, the distance between (juxta)paranode and node, the distance between node and myelin sheath, length of the axon initial segment (AIS), etc. In light of this, and considering the small physical dimensions of the nodal region in general, the methods remain unclear about the depth of 3D reconstruction/deconvolution applied to the samples. Measurements presented in the results show significant differences in sub-micrometer dimension, which at least according to the stated methods, are unlikely to be precise given that the confocal imaging parameters do not seem to reach Nyquist conditions. For a study in which a third of all data is aimed at elucidating (sub)micrometer changes, this is crucial and the study would benefit from a more rigorous method description by the authors.
Another methodological weakness is the somewhat small n, and its incoherence across the experiments and therefore, the statistics performed in some of the experiments. Statistics are based on either n for animals, or n for individual data points from several animals. Why is not all data represented as mean/animal? Also, the sampling in general with n = 3 animals is borderline acceptable; in some cases, it seems that only 2 animals were used, and in others, no number is given at all (please refer to author comments for details). This needs to be addressed, either by explaining why so few animals were used, or by adding more data from individual animals. Assigning structures (AIS, nodes) as n results in overstating effects, since especially for AIS, there is significant heterogeneity in the length across neurons from the same type, and this is masked when 100 AIS are considered as individual n instead 100 AIS per animal, and the animal is (correctly) the n. Since the study seems to switch back and forth between these assignments, it would be helpful to level these data across all experiments unless there are specific reasons not to do so, which then need to be explained. As outlined in the methods, all values are given as means {plus minus} SEM; this needs to be corrected for those cases where the standard deviation is the appropriate choice (e.g. all graphs showing n = individual structure, and not the mean of an animal).
As far as the analysis of geometrical AIS changes is concerned, the method section should be extended to address how, if at all, AIS length and position were analyzed in 3D, also considering the somewhat "spotty" immunosignal outlined in Fig. 8D. The observed AIS length change is then discussed in the context of a study conducted in a pharmacological model of myelin loss, however, that particular study (Hamada & Kole, 2015) found not only a length change but a position change after cuprizone-induced AIS plasticity. The authors should therefore discuss this finding in a bit more detail than simply stating "Adaptation of the AIS has been reported in the cuprizone chemical model of demyelination" (p. 14, ll. 512).
Similarly to the points made about structural data above, the data from electrophysiological recordings should be presented in such a way that e.g. the number of cells and/or animals is readily accessible from the graph or legend. In its current form, this information - while available - needs to be pieced together from in-text information supplemented by figure legends. Sometimes, the authors do not include the number of animals behind individual cell data (for details please see author comments). Please carefully review all figures and edit accordingly.
The behavioral data presented in the study is interesting, but the conclusions drawn are not supported by the data presented, as many unknown factors remain in place that could contribute to the observed phenotype.
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