Mesencephalic locomotor region stimulation—cuneiform or pedunculopontine?

Roussel et al.1Rousell M.L.-Z.,D. Josset N. Lemieux M. Bretzner F. Functional Contribution of Mesencephalic Locomotor Region Nuclei to Locomotor Recovery after Spinal Cord Injury.Cell Rep. Med. 2023; 4: 100946Abstract Full Text Full Text PDF PubMed Scopus (1) Google Scholar provide new insight into mecencephalic locomotor region (MLR) stimulation to treat spinal cord injury in mice. Previously, it was unclear which part of the MLR to target. Now, evidence converges on cuneiform nucleus activation. Roussel et al.1Rousell M.L.-Z.,D. Josset N. Lemieux M. Bretzner F. Functional Contribution of Mesencephalic Locomotor Region Nuclei to Locomotor Recovery after Spinal Cord Injury.Cell Rep. Med. 2023; 4: 100946Abstract Full Text Full Text PDF PubMed Scopus (1) Google Scholar provide new insight into mecencephalic locomotor region (MLR) stimulation to treat spinal cord injury in mice. Previously, it was unclear which part of the MLR to target. Now, evidence converges on cuneiform nucleus activation. Experiments in the mid-1960s identified the mecencephalic locomotor region (MLR), a midbrain structure that, when stimulated, elicits stepping and even running in the decerebrated cat.2Shik M.L. Severin F.V. Orlovskiĭ G.N. Control of walking and running by means of electric stimulation of the midbrain.Biofizika. 1966; 11: 659-666PubMed Google Scholar The MLR and its function are evolutionarily conserved across species, including lamprey, rodents, and primates.3Orlovsky G. Deliagina T. Grillner S. Neuronal Control of Locomotion: From Mollusc to Man.1999Crossref Google Scholar The effects of MLR stimulation on locomotion have been studied across health and disease. Here, the authors investigate the effects of MLR stimulation in a murine model of chronic spinal cord injury as a therapeutic avenue to promote recovery. In technically demanding experiments, multiple behavioral, kinematic, and electrophysiological readouts are used to refine our understanding of MLR stimulation in the context of optogenetic and genetic manipulations. Spinal cord injury disrupts descending input from supraspinal centers, resulting in paralysis. However, below the lesion, circuitry that generates many aspects of hindlimb locomotion is often intact. Spinal cord motor networks themselves are neuronal assemblies capable of producing different patterns and rhythms of locomotion,4Kiehn O. Decoding the organization of spinal circuits that control locomotion.Nat. Rev. Neurosci. 2016; 17: 224-238https://doi.org/10.1038/nrn.2016.9Crossref PubMed Scopus (402) Google Scholar but even though the majority of spinal cord injuries are incomplete, remaining spared supraspinal input fails to engage these networks. The MLR projects to the medial medullary reticular formation and, via spared reticulospinal axons, could serve to activate intact locomotory circuitry below the level of the lesion. This makes it an attractive supraspinal candidate to target using deep brain stimulation (DBS). While early stimulation studies defined an anatomical region able to initiate movement and modulate speed, the molecular and cellular heterogeneity of the MLR was underappreciated until quite recently. There are differential roles for neuronal populations within the cuneiform (CnF) and pedunculopontine nuclei (PPN) within the MLR. Groundbreaking studies using genetic and viral tools dissected glutamatergic and cholinergic function in the CnF and PPN. These broadly converged on a model where CnF orchestrates fast-escape responses from stationary and change speed, whereas PPN acts mostly downstream of basal ganglia during slower exploratory behavior.5Roseberry T.K. Lee A.M. Lalive A.L. Wilbrecht L. Bonci A. Kreitzer A.C. Cell-type-specific control of brainstem locomotor circuits by basal ganglia.Cell. 2016; 164: 526-537https://doi.org/10.1016/j.cell.2015.12.037Abstract Full Text Full Text PDF PubMed Scopus (219) Google Scholar,6Josset N. Roussel M. Lemieux M. Lafrance-Zoubga D. Rastqar A. Bretzner F. Distinct contributions of Mesencephalic locomotor region nuclei to locomotor control in the Freely behaving mouse.Curr. Biol. 2018; 28: 884-901.e3https://doi.org/10.1016/j.cub.2018.02.007Abstract Full Text Full Text PDF PubMed Scopus (98) Google Scholar,7Caggiano V. Leiras R. Goñi-Erro H. Masini D. Bellardita C. Bouvier J. Caldeira V. Fisone G. Kiehn O. Midbrain circuits that set locomotor speed and gait selection.Nature. 2018; 553: 455-460https://doi.org/10.1038/nature25448Crossref PubMed Scopus (210) Google Scholar But what does this mean for stimulation strategies to promote recovery from spinal cord injury? In the context of a current clinical trial that aims to use MLS-DBS following spinal cord injury,8Stieglitz L.H. Hofer A.S. Bolliger M. Oertel M.F. Filli L. Willi R. Cathomen A. Meyer C. Schubert M. Hubli M. et al.Deep brain stimulation for locomotion in incomplete human spinal cord injury (DBS-SCI): protocol of a prospective one-armed multi-centre study.BMJ Open. 2021; 11e047670https://doi.org/10.1136/bmjopen-2020-047670Crossref PubMed Scopus (7) Google Scholar this is important to define. Indeed, in rats, both CnF stimulation9Hofer A.-S. Scheuber M.I. Sartori A.M. Good N. Stalder S.A. Hammer N. Fricke K. Schalbetter S.M. Engmann A.K. Weber R.Z. et al.Stimulation of the cuneiform nucleus enables training and boosts recovery after spinal cord injury.Brain. 2022; 145: 3681-3697https://doi.org/10.1093/brain/awac184Crossref PubMed Scopus (4) Google Scholar and PPN stimulation10Bonizzato M. James N.D. Pidpruzhnykova G. Pavlova N. Shkorbatova P. Baud L. Martinez-Gonzalez C. Squair J.W. DiGiovanna J. Barraud Q. et al.Multi-pronged neuromodulation intervention engages the residual motor circuitry to facilitate walking in a rat model of spinal cord injury.Nat. Commun. 2021; 12: 1925https://doi.org/10.1038/s41467-021-22137-9Crossref PubMed Scopus (19) Google Scholar have been proposed to promote recovery, the latter when combined with local lumbar epidural stimulation. Now, combining mouse genetics and activation and ablation strategies alongside sensitive outcome measures, Roussel et al.1Rousell M.L.-Z.,D. Josset N. Lemieux M. Bretzner F. Functional Contribution of Mesencephalic Locomotor Region Nuclei to Locomotor Recovery after Spinal Cord Injury.Cell Rep. Med. 2023; 4: 100946Abstract Full Text Full Text PDF PubMed Scopus (1) Google Scholar aim to probe and pinpoint any distinction with greater accuracy. Mice exhibit capacity for some spontaneous recovery following spinal injury. Roussel et al.1Rousell M.L.-Z.,D. Josset N. Lemieux M. Bretzner F. Functional Contribution of Mesencephalic Locomotor Region Nuclei to Locomotor Recovery after Spinal Cord Injury.Cell Rep. Med. 2023; 4: 100946Abstract Full Text Full Text PDF PubMed Scopus (1) Google Scholar first focused on this spontaneous recovery following a thoracic lateral hemisection model of spinal cord injury. This is a one-sided transection lesion of the spinal cord, abolishing the MLR input on that same (left, ipsilesional) side but largely sparing that coming from the contralesional MLR. Consequently, these mice have paralysis in the left hindlimb, which gradually improves over days and weeks: they eventually step but retain deficits. They first found that the anatomical organization of glutamatergic and cholinergic MLR neurons that project to brainstem locomotor circuits, which subsequently project to spinal cord, is maintained following this injury model. Second, they characterized the contribution of the contralesional CnF and PPN to spontaneous recovery. These are axons spared by the injury and still projecting to the lumbar spinal cord, which could mediate spontaneous improvements in limb function. They did this by diptheria toxin (DTX) ablation of each population after spontaneous recovery occurred. In both overground locomotion and swimming performance measured by hindlimb kinematics, deletion of CnF had a more profound impact than deletion of PPN. Next, before injury and at various timepoints following spontaneous recovery, they placed the mice on a treadmill and optogenetically stimulated the contralesional (spared, projecting) CnF or PPN while simultaneously recording flexor-extensor electromyography (EMG) to analyze the degree of muscle movement in response (flexor and extensor muscles work antagonistically during optimal stepping). They found that excitatory CnF, but not PNN, stimulation correlated with spontaneous improvement in locomotor score over time. It looks like CnF could mediate aspects of spontaneous recovery. So could stimulation be used as a therapeutic strategy to promote recovery beyond that which occurs spontaneously? To test this, they took mice with a chronic spinal cord injury who already recovered spontaneous function to the function's limits: these mice step, but not as effectively as an uninjured mouse. They then stimulated the CnF during overground locomotion while measuring EMGs and analyzing walking performance using kinematics. CnF simulation itself initiated walking behavior. EMGs and kinematic analysis of stepping performance improved. Taken together, CnF stimulation modulates spatiotemporal muscle recruitment and improves coordination and walking speed overground. Next, in the chronically injured mice, they tested locomotor performance in a swimming task and directly compared CnF stimulation with PNN stimulation. Here, PNN stimulation decreased swimming speed, whereas CnF stimulation improved performance. Thus, mechanistically, these results suggest that the CnF is important for spontaneous locomotor recovery, and its stimulation can modulate muscle firing to improve locomotion farther than occurs spontaneously in chronic spinal injured mice. Whether the reasonably extensive sparing in this particular lateral hemisection model could influence relative contribution of CnF and PNN could still be questioned. However, this is not the first study to evidence that CnF should be the target for DBS.9Hofer A.-S. Scheuber M.I. Sartori A.M. Good N. Stalder S.A. Hammer N. Fricke K. Schalbetter S.M. Engmann A.K. Weber R.Z. et al.Stimulation of the cuneiform nucleus enables training and boosts recovery after spinal cord injury.Brain. 2022; 145: 3681-3697https://doi.org/10.1093/brain/awac184Crossref PubMed Scopus (4) Google Scholar Indeed, using a chronic severe bilateral contusion injury in rats, which closely models the type of injury most commonly found in the clinic, electrophysiological CnF stimulation was shown to promote recovery.9Hofer A.-S. Scheuber M.I. Sartori A.M. Good N. Stalder S.A. Hammer N. Fricke K. Schalbetter S.M. Engmann A.K. Weber R.Z. et al.Stimulation of the cuneiform nucleus enables training and boosts recovery after spinal cord injury.Brain. 2022; 145: 3681-3697https://doi.org/10.1093/brain/awac184Crossref PubMed Scopus (4) Google Scholar Thus, the convergence of these two studies provides evidence that CnF stimulation could prove an effective therapeutic target for improvement in lower limb function, especially in the population of individuals recruited for the current clinical trial8Stieglitz L.H. Hofer A.S. Bolliger M. Oertel M.F. Filli L. Willi R. Cathomen A. Meyer C. Schubert M. Hubli M. et al.Deep brain stimulation for locomotion in incomplete human spinal cord injury (DBS-SCI): protocol of a prospective one-armed multi-centre study.BMJ Open. 2021; 11e047670https://doi.org/10.1136/bmjopen-2020-047670Crossref PubMed Scopus (7) Google Scholar who have some intact motor function below the level of lesion. Functional contribution of mesencephalic locomotor region nuclei to locomotor recovery after spinal cord injuryRoussel et al.Cell Reports MedicineFebruary 21, 2023In BriefRoussel et al. characterize the distinct contribution of midbrain neurons to locomotor recovery after spinal cord injury (SCI). Although both glutamatergic cuneiform nucleus (CnF) and pedunculopontine nucleus contribute to spontaneous motor recovery, only activation of glutamatergic CnF initiates locomotion and improves stepping ability after chronic SCI. Full-Text PDF Open Access