Effects of combined dorsolateral and dorsal funicular lesions on sensorimotor behaviour in rats

Red nucleus structure and function: from anatomy to clinical neurosciences

The red nucleus (RN) is a large subcortical structure located in the ventral midbrain. Although it originated as a primitive relay between the cerebellum and the spinal cord, during its phylogenesis the RN shows a progressive segregation between a magnocellular part, involved in the rubrospinal system, and a parvocellular part, involved in the olivocerebellar system. Despite exhibiting distinct evolutionary trajectories, these two regions are strictly tied together and play a prominent role in motor and non-motor behavior in different animal species. However, little is known about their function in the human brain. This lack of knowledge may have been conditioned both by the notable differences between human and non-human RN and by inherent difficulties in studying this structure directly in the human brain, leading to a general decrease of interest in the last decades. In the present review, we identify the crucial issues in the current knowledge and summarize the results of several decades of research about the RN, ranging from animal models to human diseases. Connecting the dots between morphology, experimental physiology and neuroimaging, we try to draw a comprehensive overview on RN functional anatomy and bridge the gap between basic and translational research.

ErbB receptor signaling directly controls oligodendrocyte progenitor cell transformation and spontaneous remyelination after spinal cord injury

We recently discovered a novel role for neuregulin‐1 (Nrg1) signaling in mediating spontaneous regenerative processes and functional repair after spinal cord injury (SCI). We revealed that Nrg1 is the molecular signal responsible for spontaneous functional remyelination of dorsal column axons by peripheral nervous system (PNS)‐like Schwann cells after SCI. Here, we investigate whether Nrg1/ErbB signaling controls the unusual transformation of centrally derived progenitor cells into these functional myelinating Schwann cells after SCI using a fate‐mapping/lineage tracing approach. Specific ablation of Nrg1‐ErbB receptors in central platelet‐derived growth factor receptor alpha (PDGFRα)‐derived lineage cells (using PDGFRαCreERT2/Tomato‐red reporter mice crossed with ErbB3fl/fl/ErbB4fl/fl mice) led to a dramatic reduction in P0‐positive remyelination in the dorsal columns following spinal contusion injury. Central myelination, assessed by Olig2 and proteolipid protein expression, was unchanged. Loss of ErbB signaling in PDGFRα lineage cells also significantly impacted the degree of spontaneous locomotor recovery after SCI, particularly in tests dependent on proprioception. These data have important implications, namely (a) cells from the PDGFRα‐expressing progenitor lineage (which are presumably oligodendrocyte progenitor cells, OPCs) can differentiate into remyelinating PNS‐like Schwann cells after traumatic SCI, (b) this process is controlled by ErbB tyrosine kinase signaling, and (c) this endogenous repair mechanism has significant consequences for functional recovery after SCI. Thus, ErbB tyrosine kinase receptor signaling directly controls the transformation of OPCs from the PDGFRα‐expressing lineage into PNS‐like functional remyelinating Schwann cells after SCI.

Motor deficits following dorsal corticospinal tract transection in rats: voluntary versus skilled locomotion readouts

Following spinal cord injury, severe deficits result from damages to ascending and descending tracts, such as the corticospinal tract (CST) which is highly relevant for the motor execution in humans. Unfortunately, no curative treatment is available and intensive efforts are deployed in animal models, such as the CST transection model, to identify interventions providing functional regeneration after spinal cord injury.

The CatWalk XT is a system for multi-parameter gait analysis of voluntary locomotion. In this study, the performance of the CatWalk XT for monitoring of functional deficits associated with dorsal CST lesion in rats was compared to skilled locomotion tests. Motor deficits associated with dorsal CST transection could be reliably monitored over seven weeks based on skilled locomotion testing, i.e. Horizontal Ladder Walk and Grid Walk. The collateral lesion to the overlaying gracile and cuneate funiculi occurring during dorsal CST transection resulted in slight hyposensitivity and proprioceptive deficit, which likely contributed to the lowered performance in skilled locomotion. In contrast, parameters of voluntary locomotion were not significantly affected by dorsal CST transection. Finally, an abnormal adduction reflex was detected immediately after lesion of the CST and could be conveniently used to confirm successful CST lesion in rats of experimental groups.

The functional relevance of the dorsal CST in locomotion of rats is not as prominent as compared to in humans and thus challenging the motor execution is mandatory to reliably investigate CST function. A detailed analysis of voluntary walking using the CatWalk XT is not adequate to detect deficits following dorsal CST lesion in rats.

Neuregulin-1 controls an endogenous repair mechanism after spinal cord injury

Spontaneous remyelination after spinal cord injury is mediated largely by Schwann cells of unknown origin. Bartus et al. show that neuregulin-1 promotes differentiation of spinal cord-resident precursor cells into PNS-like Schwann cells, which remyelinate central axons and promote functional recovery. Targeting the neuregulin-1 system could enhance endogenous regenerative processes.

Following traumatic spinal cord injury, acute demyelination of spinal axons is followed by a period of spontaneous remyelination. However, this endogenous repair response is suboptimal and may account for the persistently compromised function of surviving axons. Spontaneous remyelination is largely mediated by Schwann cells, where demyelinated central axons, particularly in the dorsal columns, become associated with peripheral myelin. The molecular control, functional role and origin of these central remyelinating Schwann cells is currently unknown. The growth factor neuregulin-1 (Nrg1, encoded by NRG1) is a key signalling factor controlling myelination in the peripheral nervous system, via signalling through ErbB tyrosine kinase receptors. Here we examined whether Nrg1 is required for Schwann cell-mediated remyelination of central dorsal column axons and whether Nrg1 ablation influences the degree of spontaneous remyelination and functional recovery following spinal cord injury. In contused adult mice with conditional ablation of Nrg1, we found an absence of Schwann cells within the spinal cord and profound demyelination of dorsal column axons. There was no compensatory increase in oligodendrocyte remyelination. Removal of peripheral input to the spinal cord and proliferation studies demonstrated that the majority of remyelinating Schwann cells originated within the injured spinal cord. We also examined the role of specific Nrg1 isoforms, using mutant mice in which only the immunoglobulin-containing isoforms of Nrg1 (types I and II) were conditionally ablated, leaving the type III Nrg1 intact. We found that the immunoglobulin Nrg1 isoforms were dispensable for Schwann cell-mediated remyelination of central axons after spinal cord injury. When functional effects were examined, both global Nrg1 and immunoglobulin-specific Nrg1 mutants demonstrated reduced spontaneous locomotor recovery compared to injured controls, although global Nrg1 mutants were more impaired in tests requiring co-ordination, balance and proprioception. Furthermore, electrophysiological assessments revealed severely impaired axonal conduction in the dorsal columns of global Nrg1 mutants (where Schwann cell-mediated remyelination is prevented), but not immunoglobulin-specific mutants (where Schwann cell-mediated remyelination remains intact), providing robust evidence that the profound demyelinating phenotype observed in the dorsal columns of Nrg1 mutant mice is related to conduction failure. Our data provide novel mechanistic insight into endogenous regenerative processes after spinal cord injury, demonstrating that Nrg1 signalling regulates central axon remyelination and functional repair and drives the trans-differentiation of central precursor cells into peripheral nervous system-like Schwann cells that remyelinate spinal axons after injury. Manipulation of the Nrg1 system could therefore be exploited to enhance spontaneous repair after spinal cord injury and other central nervous system disorders with a demyelinating pathology.

Restoration of sensorimotor functions after spinal cord injury

The purpose of this review is to discuss the achievements and perspectives regarding rehabilitation of sensorimotor functions after spinal cord injury. In the first part we discuss clinical approaches based on neuroplasticity, a term referring to all adaptive and maladaptive changes within the sensorimotor systems triggered by a spinal cord injury. Neuroplasticity can be facilitated through the training of movements with assistance as needed, and/or by electrical stimulation techniques. The success of such training in individuals with incomplete spinal cord injury critically depends on the presence of physiological proprioceptive input to the spinal cord leading to meaningful muscle activations during movement performances. The addition of rehabilitation technology, such as robotic devices allows for longer training times and provision of feedback information regarding changes in movement performance. Nevertheless, the improvement of function by such approaches for rehabilitation is limited. In the second part, we discuss preclinical approaches to restore function by compensating for the loss of descending input to spinal networks following complete spinal cord injury. This can be achieved with stimulation of spinal networks or approaches to restore their descending input. Electrical and pharmacological stimulation of spinal neural networks is still in an experimental stage; and despite promising repair studies in animal models, translations to humans up to now have not been convincing. It is likely that combinations of techniques targeting the promotion of axonal regeneration and meaningful plasticity are necessary to advance the restoration of function. In the future, refinement of animal studies may contribute to greater translational success.

The biomechanics of locomotor compensation after peripheral nerve lesion in the rat

Functional recovery in animal models of nervous system disorders commonly involves behavioural compensation, in which animals alter the use of their limbs after injury, making it difficult to distinguish 'true' recovery from substitution of novel movements. The purpose of this study is to investigate how abnormal movements are produced by using biomechanical assessment of limb joint motion, an approach commonly used to diagnose human pathological gait. Rats were trained to cross a runway whilst kinetic (ground reaction forces) and kinematic (limb segment positions) data were synchronously recorded. Inverse dynamic analysis was used to calculate limb joint moments, or torques, and joint mechanical power throughout the stride for major joints of the forelimbs and hindlimbs, both before and after denervation of a major ankle extensor muscle. Before surgery, rats moved with joint moment and power profiles comparable to other quadrupeds, with differences attributable to species variation in limb posture. After surgery, rats trotted asymmetrically, with a near plantigrade stance of the left hindlimb. Surprisingly, ankle joint moments and power were largely preserved, with dramatic reductions in range of motion and joint moments at the proximal joints of the affected limb. Stiffening of the proximal limb compensated for increased compliance at the ankle but decreased the total mechanical work done by the injured limb. In turn, more work was done by the opposite, i.e. uninjured, hindlimb. This is the first study to quantify the biomechanical adjustments made within and between limbs in laboratory rodents after nervous system injury.

Rehabilitative training and plasticity following spinal cord injury

Rehabilitative training is currently one of the most successful treatments to promote functional recovery following spinal cord injury. Nevertheless, there are many unanswered questions including the most effective and beneficial design, and the mechanisms underlying the training effects on motor recovery. Furthermore, rehabilitative training will certainly be combined with pharmacological treatments developed to promote the "repair" of the injured spinal cord. Thus, insight into training-induced mechanisms will be of great importance to fine tune such combined treatments. In this review we address current challenges of rehabilitative training and mechanisms involved in promoting motor recovery with the focus on animal models. These challenges suggest that although rehabilitative training appears to be a relatively straight forward treatment approach, more research is needed to optimize its effect on functional outcome in order to enhance our chances of success when combining pharmacological treatments promoting axonal growth and rehabilitative training in the clinic.

Bone marrow stromal cells promote skilled motor recovery and enhance contralesional axonal connections after ischemic stroke in adult mice

BACKGROUND AND PURPOSE

We tested the effect of bone marrow stromal cells (BMSCs) on neuronal remodeling of the corticospinal tract originating from the contralesional cortex in mice subjected to unilateral pyramidotomy (PT) followed by middle cerebral artery occlusion (MCAO).

METHODS

Adult mice with transgenic yellow fluorescent protein labeling in the corticospinal tract were subjected to right hemispheric PT and right permanent or sham MCAO. One day later, the mice were treated intravenously with BMSCs or phosphate-buffered saline. A Foot-Fault test and a single pellet-reaching test were performed before surgery, 3 days after MCAO, and weekly thereafter. Pseudorabies virus-614-monomeric red fluorescent protein was injected into the left forelimb flexor muscles 28 days after surgery (4 days before euthanasia). The brain and cervical cord were processed for fluorescent microscopy to detect red fluorescent protein and yellow fluorescent protein labeling, respectively.

RESULTS

Significant functional improvements were evident in PT-MCAO mice treated with BMSCs (n=9) compared with phosphate-buffered saline-treated mice (n=9, P<0.05), but not in mice with PT-sham MCAO treated with either phosphate-buffered saline (n=9) or BMSCs (n=10). Furthermore, in PT-MCAO mice, both corticospinal tract axonal density in the denervated side of the cervical gray matter and red fluorescent protein-labeled pyramidal neurons in the left intact cortex were significantly increased compared with PT-sham MCAO mice (P<0.05). BMSCs significantly enhanced both corticospinal tract density and red fluorescent protein labeling in PT-MCAO mice (P<0.05) only. The behavioral outcome was highly correlated with corticospinal tract density and red fluorescent protein labeling.

CONCLUSIONS

BMSCs amplify stroke-induced contralesional neuronal remodeling, which contributes to motor recovery after stroke.

Alterations of action potentials and the localization of Nav1.6 sodium channels in spared axons after hemisection injury of the spinal cord in adult rats

Previously, we reported a pronounced reduction in transmission through surviving axons contralateral to chronic hemisection (HX) of adult rat spinal cord. To examine the cellular and molecular mechanisms responsible for this diminished transmission, we recorded intracellularly from lumbar lateral white matter axons in deeply anesthetized adult rats in vivo and measured the propagation of action potentials (APs) through rubrospinal/reticulospinal tract (RST/RtST) axons contralateral to chronic HX at T10. We found decreased excitability in these axons, manifested by an increased rheobase to trigger APs and longer latency for AP propagation passing the injury level, without significant differences in axonal resting membrane potential and input resistance. These electrophysiological changes were associated with altered spatial localization of Nav1.6 sodium channels along axons: a subset of axons contralateral to the injury exhibited a diffuse localization (>10 μm spread) of Nav1.6 channels, a pattern characteristic of demyelinated axons (Craner MJ, Newcombe J, Black JA, Hartle C, Cuzner ML, Waxman SG. Proc Natl Acad Sci USA 101: 8168-8173, 2004b). This result was substantiated by ultrastructural changes seen with electron microscopy, in which an increased number of large-caliber, demyelinated RST axons were found contralateral to the chronic HX. Therefore, an increased rheobase, pathological changes in the distribution of Nav1.6 sodium channels, and the demyelination of contralateral RST axons are likely responsible for their decreased conduction chronically after HX and thus may provide novel targets for strategies to improve function following incomplete spinal cord injury.

Spinal cord injury and plasticity: opportunities and challenges

There is still no effective treatment to promote functional recovery following spinal cord injury. However, promoting injury-induced adaptive changes (plasticity) within the central nervous system, associated with repair, promise new treatment strategies. Recent contributions from our group and current challenges of this relatively young field are discussed in this review.

Forelimb locomotor assessment scale (FLAS): novel assessment of forelimb dysfunction after cervical spinal cord injury

We describe here a novel forelimb locomotor assessment scale (FLAS) that assesses forelimb use during locomotion in rats injured at the cervical level. A quantitative scale was developed that measures movements of shoulder, elbow, and wrist joints, forepaw position and digit placement, forelimb-hindlimb coordination, compensatory behaviors adopted while walking, and balance. Female Sprague-Dawley rats received graded cervical contusions ranging from 200 to 230 ("mild," n=11) and 250-290 kdyn ("moderate," n=13) between C5 and C8. Rats were videotaped post-injury as they walked along an alley to determine deficits and recovery of forelimb function. Recovery of shoulder and elbow joint movement occurred rapidly (within 1-7 days post-injury), whereas recovery of wrist joint movement was slower and more variable. Most rats in all groups displayed persistent deficits in forepaw and digit movement, but developed compensatory behaviors to allow functional forward locomotion within 1-2 weeks post-injury. Recovery of forelimb function as measured by the FLAS reached a plateau by 3 weeks post-injury in all groups. Rats with mild contusions displayed greater locomotor recovery than rats with moderate contusions, but exhibited persistent deficits compared to sham controls. Reliability was tested by having seven raters (three internal, four external) from different laboratories, independently and blindly score videos of all rats. The multivariate correlation between all raters, all animals, and all time points ranged from r(2)=0.88-0.96 (p<0.0001), indicating a high inter-rater reliability. Thus, the FLAS is a simple, inexpensive, sensitive, and reliable measure of forelimb function during locomotion following cervical SCI.

Re-expression of Locomotor Function After Partial Spinal Cord Injury

After a complete spinal section, quadruped mammals (cats, rats, and mice) can generally regain hindlimb locomotion on a treadmill because the spinal cord below the lesion can express locomotion through a neural circuitry termed the central pattern generator (CPG). In this review, we propose that the spinal CPG also plays a crucial role in the locomotor recovery after incomplete spinal cord injury.

Task-dependent compensation after pyramidal tract and dorsolateral spinal lesions in rats

The purpose of this research was to investigate whether pathways in the dorsal part of the lateral spinal funiculus (DLF) can compensate for loss of corticospinal input (CST) to the spinal cord. The CST is known to control skilled limb movements in rats. The DLF contains several different pathways, including the rubrospinal tract (RST) which is also thought to influence limb movements. After lesions of either the corticospinal or the rubrospinal system, it is unclear how much of the remaining forelimb function is due to the presence of the alternate pathway. To begin to address this issue, the present study investigates the compensatory role of pathways in the DLF, including the rubrospinal tract, after bilateral lesions of the pyramidal tract (PT). We initially performed bilateral PT lesions in rats, which effectively removed the CST input to the spinal cord. We tested these rats during overground locomotion, skilled locomotion and skilled forelimb usage. After a 6 week recovery period, we then performed bilateral DLF lesions and compared the behavioural abilities of these rats to those of animals which underwent simultaneous PT and DLF lesions. If DLF pathways do compensate for PT lesions, then animals with PT lesions would rely more on DLF pathways than animals without PT lesions. Thus we hypothesized that animals with DLF lesions which were performed 6 weeks after PT lesions would exhibit more deficits on several behavioural tasks compared to animals which received PT and DLF lesions simultaneously. Our hypothesis was supported only for skilled pellet retrieval. Hence some DLF pathways, including the RST, were able to compensate for loss of CST input during skilled reaching but not during overground or skilled locomotion in PT-lesioned rats. These differential responses suggest that behavioural tasks vary in their reliance on specific pathways after injury, and, furthermore, that compensation for loss of specific connections can arise from numerous sources.