A spinal synergy of excitatory and inhibitory neurons coordinates ipsilateral body movements

Reviewer #1 (Public Review):

This is a well-written manuscript addressing a fundamental question regarding the functional organization of spinal circuits controlling the execution of locomotor movements. The authors take advantage of the power of mouse genetics to exploit the expression of Hes2 to study the function of the whole population of V2 interneurons. Previous studies could only focus on either the excitatory V2a or inhibitory V2b subpopulations. Here, by combining two different genetic manipulations based on either silencing or acute ablation of V2 interneurons with rigorous functional analysis the authors showed that V2 interneurons can act together to control interlimb coordination and antagonistically to regulate joint movements. The data are convincing and properly analyzed, the conclusions are in line with the results, and the limitations of the study are appropriately addressed. The discussion nicely frames the work in a conceptual framework that takes into account the current literature on the mode of operation of spinal motor circuits. There are a few weaknesses that should be addressed and would further improve what is already a very nice study.

1. While previous work from the authors has consistently shown the validity and reliability of these neuronal silencing and ablation approaches, the study presents no data showing the efficiency and specificity of these genetic manipulations. These are critical parameters for interpreting the results and should be presented, especially considering that the strategies employed are susceptible to the limitations of a lineage-tracing approach. These data would also be important for the discussion section to interpret the differences between the two genetic models and could address some of the options proposed by the authors, as well as the possibility of incomplete and/or unexpected recombination.

2. The authors suggest that the changes in interlimb coordination are "consistent with mice keeping the limbs closer to the body, limiting forward movements in the attempt to preserve body stability". A common reaction to body instability in quadrupeds is a widening of the limbs to lower the center of gravity: limbs are positioned further away from the body. Not quite sure whether I would be so certain of the interpretation that the observed phenotypes are due to body/postural instability. It is possible that the changes in gait are just a direct consequence of the inactivation of V2 interneurons. To clarify this issue, it could be useful to test whether other features of postural control are affected by perturbation of V2 neurons, for example, swimming and rearing analyses would provide interesting insights.

Reviewer #2 (Public Review):

The manuscript by Hayashi et al provides the characterization of a new mouse line that targets V2 neurons and demonstrates the locomotor consequences of manipulating the large V2 population. Prior work has examined the effects of silencing and/or ablation of the excitatory V2a and inhibitory V2b neuronal populations independently. Since the two populations are derived from the same V2 lineage but have opposite transmitter phenotypes, one may expect some common synaptic targets and/or similar or complementary functional roles that require excitatory/inhibitory balance. Overall, the value and importance of the study is that comparison of prior manipulations of the V2a and V2b populations (individually in prior studies) with the more global V2 manipulation (here) provides additional insights into spinal locomotor circuitry.

The authors successfully generate a new Hes2cre mouse line that targets the V2 population with high accuracy. The characterizations as far as the specificity and efficiency of the line are compelling. This line is then used to examine the locomotor effects of, first, synaptically silencing all Hes2 neurons throughout the neuroaxis beginning in early development and, then, ablating spinal Hes2 neurons in the adult. The phenotypes of both groups of mice are quite similar, with some small exceptions. The most obvious disturbance in both is the shortened steps, faster step cycle, and more steps required to travel the same distance. As the authors point out, much of the phenotype may be due to a disruption in balance. Interestingly, the hyperextension that is characteristic of V2b neuronal ablation is lost when the function of V2a neurons is compromised as well, suggesting antagonistic functions of these populations in intralimb coordination.

The experiments are rigorous and the data are clearly presented. The findings are interesting to consider in context with prior work. Some comparisons are difficult since gait is not considered and one of the major roles of spinal V2a neurons has been demonstrated to be speed/gait-dependent. The ipsilateral deficits are a major conclusion but some of the supporting data are not clearly derived (or there was an error in the figure?). The use of spinal restricted manipulation removes many of the potential confounds of the full Hes2 silencing. It is still, however, not possible to disentangle the local spinal circuit effects from altered proprioceptive input pathways or ascending information from the lumbar cord to the cervical regions or the brainstem. Although of value to inform future experiments, this impacts the strength of the conclusions that can be drawn.

Reviewer #3 (Public Review):

Hayashi et al., investigate the role of spinal neurons derived from the V2 progenitor domain. They identify a molecular marker, Hes2, specific to the V2 lineage in the spinal cord. The authors use this result to generate a new mouse line allowing specific access to the Hes2 lineage and show that this lineage is composed of excitatory V2a and inhibitory V2b spinal interneurons plus some populations of supraspinal neurons. Taking advantage of this new tool, they demonstrate that the developmental silencing of the Hes2 lineage leads to a disruption of mouse locomotor gait characterized by shorter strides and an increased cadence with no alteration of the alternation between flexion and extension. In addition, the authors show that the silencing of the Hes2 lineage also leads to an alteration of the interlimb coordination and a decreased capacity of the mice to achieve complex motor tasks. Using an intersectional genetic approach, the authors further demonstrate that the selective ablation of spinal V2 neurons in adult mice recapitulates the festination phenotype as well as the altered execution of complex motor tasks.

By identifying a novel marker of the V2 lineage in the spinal cord and using this finding to generate a new mouse line Hayashi and colleagues suggest an intriguing interplay between excitatory and inhibitory V2 spinal neurons modulating differentially, multiple facets of motor behavior.

The conclusions of this study are for the vast majority well supported by data. However, a few additional validations of the mouse model that is used and clarification about the methods of statistical analysis would improve the quality of this manuscript.

1. Additional validations of the Hes2iCre mouse line generated and used in this study would improve the quality of the manuscript as well as shed light on the potential value of the use of the Hes2iCre mouse line for future investigations.

- When reporting the cell population labeled by GFP in Hes2iCre; R26LSL-Sun1-GFP the authors need to report the number of animals on which these quantifications were performed to strengthen their conclusions (Figure 3C-E). Similarly, when showing the number of Hes2+, Chx10+ (V2a) and GATA3+ (V2b) neurons in Hes2iCre heterozygous vs homozygous the number of animals should be reported (Figure 3G; Figure S2E-F).

- The numbers of Hes2+, Chx10+ (V2a) and GATA3+ (V2b) neurons in Hes2iCre heterozygous vs homozygous is reported. However, it would improve the validation of the mouse line, if the authors could provide a quantification of the numbers of Chx10+ and GATA3+ cells in heterogygous Hes2WT/iCre animals versus littermates lacking the Cre.

- Although the study focus on spinal V2 neurons and the intersectional approach used in the last part of the paper is compelling, a better description of the supraspinal neurons that are part of the Hes2 lineage would give a better insight into the potential contribution of supraspinal Hes2 lineage to the motor phenotype described in Hes2-silenced mouse. In particular, an experiment showing if V2 (especially Chx10+ V2a) neurons from the medullary reticular nucleus are part of the Hes2 lineage would allow us to get a better grasp on the potential supraspinal effect of Hes2 neurons silencing.

1. Adding a part in the methods explaining the statistical analysis applied is needed. In this part, the choices of the statistical analysis performed should be clearly explained and the assumptions stated. Although the intersectional genetic approach is challenging and does not allow for obtaining numerous animals, the use of parametric Student's t-tests on groups with only 4 animals is discussable and at least needs to be justified in the methods (results presented in Figure 6 and Figure S5). When the number of statistical units allows it, the normality of the distributions and the homoscedasticity should be tested prior to the use a parametric test. In some instances, tests taking into account the hierarchical structure of the data could be used. Furthermore, running statistical analysis on what seems to be a group of n=2 statistical units (Figure S3L) is not appropriate.

2. Although this decision belongs to the authors, the use of the term "synergy" in the title and abstract might be misleading and might lead to confusion regarding the important outcome of this study. The authors show compelling evidence that the spinal ablation of the V2 lineage leads to a disruption of the ipsilateral coordination of body movements. However, as well explained by the authors, prior studies ablating individual V2a and V2b populations did not show any abnormal ipsilateral body coordination. This rather suggests a redundant or complementary function of inhibitory and excitatory V2 spinal neurons in spinal circuits, with the possibility for one individual population to compensate for the effect on the ipsilateral coordination following the ablation of the other population. Alternatively, "synergy" may suggest a simultaneous activity of V2a and V2b neurons that is not in the scope of this work.