Abstract
All forms of locomotion are repetitive motor activities that require coordinated bilateral activation of muscles. The executive elements of locomotor control are networks of spinal neurons that determine gait pattern through the sequential activation of motor-neuron pools on either side of the body axis1,2,3,4. However, little is known about the constraints that link left–right coordination to locomotor speed. Recent advances have indicated that both excitatory and inhibitory commissural neurons may be involved in left–right coordination5,6,7. But the neural underpinnings of this, and a possible causal link between these different groups of commissural neurons and left–right alternation, are lacking. Here we show, using intersectional mouse genetics, that ablation of a group of transcriptionally defined commissural neurons—the V0 population—leads to a quadrupedal hopping at all frequencies of locomotion. The selective ablation of inhibitory V0 neurons leads to a lack of left–right pattern at low frequencies, mixed coordination at medium frequencies, and alternation at high locomotor frequencies. When ablation is targeted to excitatory V0 neurons, left–right alternation is present at low frequencies, and hopping is restricted to medium and high locomotor frequencies. Therefore, the intrinsic logic of the central control of locomotion incorporates a modular organization, with two subgroups of V0 neurons required for the existence of left–right alternating modes at different speeds of locomotion. The two molecularly distinct sets of commissural neurons may constrain species-related naturally occurring frequency-dependent coordination and be involved in the evolution of different gaits.
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Acknowledgements
This work was supported by the Söderberg Foundation, Swedish Research Council and European Research Council. J.B. is an EMBO fellow. We thank H. U. Zeilhofer for donating the Hoxb8::Cre mouse strain, S. Karaz, A. C. Westerdahl and P. Löw for extensive genotyping, L. Lundfald for participating in early experiments, N. Sleiers for participating in the in vivo experiments, and P. L. Ruffault for providing mouse tissue. We thank colleagues for discussing different aspects of our study.
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O.K., A.E.T., L.B. and J.B. contributed to the conception and design of the study. A.E.T. performed electrophysiological experiments, J.B. performed anatomical experiments and both analysed the data. L.B. carried out and analysed the in vivo experiments. A.P. engineered the Dbx1DTA mice and detected the hopping phenotype in the E1Ngn2::Cre;Dbx1DTA mice. G.F. provided mice and fixed tissue. O.K. supervised all aspects of the work. All authors discussed the results and participated in writing the manuscript.
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This file contains Supplementary Figures 1-7. (PDF 15651 kb)
Quadrupedal hopping in E1Ngn2::Cre;Dbx1DTA mouse
Three weeks old E1Ngn2::Cre;Dbx1DTA mouse running spontaneously on runway. The run was captured at 100 frames/s and is shown at half‐speed. At all frequencies of locomotion the mouse shows a pronounced quadrupedal hopping with sequential synchronized lifting of the forelimbs followed by synchronized lifting of the hindlimbs. (MP4 9969 kb)
Alternating gait in wild-type mouse
Three weeks old wild‐type mouse running spontaneously on a runway. The run was captured at 100 frames/s and is shown at half‐speed. At frequencies of locomotion below 10 Hz, the mouse produces alternating gaits in forelimbs and hindlimbs. (MP4 7525 kb)
Hindlimb hopping in Hoxb8::Cre; DbxDTA mouse
Three weeks old Hoxb8::Cre; DbxDTA mouse running spontaneously on a runway. The run was captured at 100 frames/s and is shown at half‐speed. At all frequencies of locomotion the mouse displays hindlimb hopping while the forelimb are mostly alternating reflecting the rostro‐caudal gradient of Hoxb8 expression in the spinal cord. (MP4 9328 kb)
Speed dependent hopping in Vglut2::Cre; Dbx1DTA mouse
Three weeks old Vglut2::Cre; Dbx1DTA mouse running spontaneously on a runway. The run was captured at 100 frames/s and is shown at half‐speed. The mouse displays left‐right alternating or hopping hindlimb gait in a frequency‐dependent manner: an alternating pattern is present at low locomotor frequencies (2‐4 Hz) and a hopping gait at high frequencies (4‐10 Hz). Forelimbs are alternating at all frequencies of locomotion. (MP4 8464 kb)
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Talpalar, A., Bouvier, J., Borgius, L. et al. Dual-mode operation of neuronal networks involved in left–right alternation. Nature 500, 85–88 (2013). https://doi.org/10.1038/nature12286
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DOI: https://doi.org/10.1038/nature12286
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