Motor Cortex Activity Organizes the Developing Rubrospinal System

More Than a Small Brain: The Importance of Studying Neural Function during Development

The nervous system contains complex circuits comprising thousands of cell types and trillions of connections. Here, we discuss how the field of "developmental systems neuroscience" combines the molecular and genetic perspectives of developmental neuroscience with the (typically adult-focused) functional perspective of systems neuroscience. This combination of approaches is critical to understanding how a handful of cells eventually produce the wide range of behaviors necessary for survival. Functional circuit development typically lags behind neural connectivity, leading to intermediate stages of neural activity that are either not seen in adults or, if present, are considered pathophysiological. Developmental systems neuroscience examines these intermediate stages of neural activity, mapping out the critical phases and inflection points of neural circuit function to understand how neural activity and behavior emerge across development. Beyond understanding typical development, this approach provides invaluable insight into the pathophysiology of neurodevelopmental disorders by identifying when and how functional development diverges between health and disease. We argue that developmental systems neuroscience will identify important periods of neural development, reveal novel therapeutic windows for treatment, and set the stage to answer fundamental questions about the brain in health and disease.

Early motor repertoire and developmental function outcomes in infants with West syndrome: a case series

BACKGROUND

West syndrome (WS), also known as infantile spasm, is a rare form of severe epilepsy that begins during early infancy. This case series aimed to describe the early motor repertoire and examine the developmental function outcomes of infants with WS.

CASE DESCRIPTIONS

Three infants (one female) with WS were assessed for early motor repertoire using the General Movement Assessment (GMA) which determined General Movement Optimality Scores (GMOS) at 4 post-term weeks of age, and Motor Optimality Scores (MOS) at 12 post-term weeks of age. Cognitive, language, and motor development were evaluated with the Bayley Scales of Infant and Toddler Development - Third Edition (Bayley-III) at 3, 6, 12, and 24 months of age.

OUTCOMES

At 4-weeks post-term, one infant showed poor repertoire movements, while the other two showed cramped-synchronized movements with their GMOS ranging from 6 to 16 (out of 42). All infants showed sporadic/absent fidgety movements at 12 weeks post-term with their MOS ranging from 5 to 9 (out of 28). All sub-domain scores of Bayley-III were <2 SD at all follow-up assessments, that is <70, indicating severe developmental delay.

CONCLUSION

These infants with WS had less than optimal scores of early motor repertoire, and developmental delay at a later age. Early motor repertoire might be an early sign for developmental function outcome at a later age in this population suggesting the need for additional research.

Distinct neural networks derived from galanin-containing nociceptors and neurotensin-expressing pruriceptors

Pain and itch are distinct sensations arousing evasion and compulsive desire for scratching, respectively. It's unclear whether they could invoke different neural networks in the brain. Here, we use the type 1 herpes simplex virus H129 strain to trace the neural networks derived from two types of dorsal root ganglia (DRG) neurons: one kind of polymodal nociceptors containing galanin (Gal) and one type of pruriceptors expressing neurotensin (Nts). The DRG microinjection and immunosuppression were performed in transgenic mice to achieve a successful tracing from specific types of DRG neurons to the primary sensory cortex. About one-third of nuclei in the brain were labeled. More than half of them were differentially labeled in two networks. For the ascending pathways, the spinothalamic tract was absent in the network derived from Nts-expressing pruriceptors, and the two networks shared the spinobulbar projections but occupied different subnuclei. As to the motor systems, more neurons in the primary motor cortex and red nucleus of the somatic motor system participated in the Gal-containing nociceptor-derived network, while more neurons in the nucleus of the solitary tract (NST) and the dorsal motor nucleus of vagus nerve (DMX) of the emotional motor system was found in the Nts-expressing pruriceptor-derived network. Functional validation of differentially labeled nuclei by c-Fos test and chemogenetic inhibition suggested the red nucleus in facilitating the response to noxious heat and the NST/DMX in regulating the histamine-induced scratching. Thus, we reveal the organization of neural networks in a DRG neuron type-dependent manner for processing pain and itch.

Celsr3 Inactivation in the Brainstem Impairs Rubrospinal Tract Development and Mouse Behaviors in Motor Coordination and Mechanic-Induced Response

Inactivation of Celsr3 in the forebrain results in defects of longitudinal axonal tracts such as the corticospinal tract. In this study, we inactivated Celsr3 in the brainstem using En1-Cre mice (Celsr3 cKO) and analyzed axonal and behavioral phenotypes. Celsr3 cKO animals showed an 83% reduction of rubrospinal axons and 30% decrease of corticospinal axons in spinal segments, associated with increased branching of dopaminergic fibers in the ventral horn. Decreases of spinal motoneurons, neuromuscular junctions, and electromyographic signal amplitude of the biceps were also found in mutant animals. Mutant mice had impaired motor coordination and defective response to heavy mechanical stimulation, but no disability in walking and food pellet handling. Transsynaptic tracing demonstrated that rubrospinal axons synapse on spinal neurons in the deep layer of the dorsal horn, and mechanical stimulation of hindpaws induced strong calcium signal of red nuclei in control mice, which was less prominent in mutant mice. In conclusion, Celsr3 regulates development of spinal descending axons and the motor network in cell and non-cell autonomous manners, and the maturation of the rubrospinal system is required for motor coordination and response to mechanical stimulation.

3D imaging of supraspinal inputs to the thoracic and lumbar spinal cord mapped by retrograde tracing and light-sheet microscopy

The supraspinal inputs play a major role in tuning the hindlimb locomotion function. While most research on spinal cord injury (SCI) with rodents is based on thoracic segments, the difference in connectivity of the supraspinal centers to the thoracic and lumbar cord is still unknown. Here, we combined retrograde tracing and 3D imaging to map the connectivity of supraspinal neurons projecting to thoracic (T9-vertebral) and lumbar (T13-vertebral) spinal levels in adult female mice. We dissected the difference in connections of corticospinal neurons (CSNs), rubrospinal neurons, and reticulospinal neurons projecting to thoracic and lumbar cords. The ratio of double-labeled neurons is higher in T13-vertebral projection CSNs and parvocellular part of the red nucleus (RPC) than in T9-vertebral projection. Using the Cre-DIO system, we precisely targeted CSNs projecting to T9-vertebral or T13-vertebral. We found that abundant axon branches communicated with the red nucleus and reticular formation and distributed from cervical gray matter to the lumbar cord. Their collateral branches showed a distinct innervation pattern in thoracic and lumbar gray matters and a similar distribution pattern in the cervical spinal cord. These results revealed the difference in connectivity between the thoracic and lumbar projection supraspinal centers and clarified the collateralization of thoracic/lumbar projection CSNs throughout the brain and spinal cord. This study highlights brain-spinal cord neural networks and the complexity of the axon terminals of spinal projection CSNs, which could contribute to the development of targeted therapeutic strategies connecting CST fibers and hindlimb function recovery.

Temporal Course of Transient Direct Corticomotoneuronal Connections during Development in Rodents

We previously observed in rodents that during the 2nd postnatal week corticospinal axons make monosynaptic connections with motoneurons. Prior to that finding, it had been believed that such contacts only occur in higher primates. Although an in vitro electrophysiological study is prerequisite for studying the developmental time course of synaptic connections, the technical difficulty of reliably recording synaptic responses from spinal motoneurons in animals over 2 weeks old hampered the study. Instead, we used retrograde transsynaptic labeling with a genetically modified rabies virus to confirm the presence of direct corticomotoneuronal connections at an early developmental stage and to show that these connections were subsequently eliminated. However, determination of an accurate elimination time course and quantitative evaluation of synaptic connectivity cannot be achieved through viral-tracing experiments. For the present study, we improved the slice preparation procedure and maintenance of slice viability, which enabled us to record postsynaptic responses using the whole cell patch-clamp technique from retrogradely labeled forearm motoneurons up until postnatal week 7. We examined the extent of corticomotoneuronal monosynaptic connections and studied the time course of their accumulation and loss. Positive ratios of monosynaptic corticomotoneuronal EPSCs increased from P6 to P8 and then plateaued (P8-P13: 65%). Thereafter, the monosynaptic connections declined until P21, at which time they were no longer detected. The time course of the falling phase and elimination was confirmed by experiments using optogenetic stimulation. The timing of the elimination fell within the same range (P18-22) estimated in our earlier study using retrograde transsynaptic labeling.

Early Moves: a protocol for a population-based prospective cohort study to establish general movements as an early biomarker of cognitive impairment in infants

INTRODUCTION

The current diagnostic pathways for cognitive impairment rarely identify babies at risk before 2 years of age. Very early detection and timely targeted intervention has potential to improve outcomes for these children and support them to reach their full life potential. Early Moves aims to identify early biomarkers, including general movements (GMs), for babies at risk of cognitive impairment, allowing early intervention within critical developmental windows to enable these children to have the best possible start to life.

METHOD AND ANALYSIS

Early Moves is a double-masked prospective cohort study that will recruit 3000 term and preterm babies from a secondary care setting. Early Moves will determine the diagnostic value of abnormal GMs (at writhing and fidgety age) for mild, moderate and severe cognitive delay at 2 years measured by the Bayley-4. Parents will use the Baby Moves smartphone application to video their babies' GMs. Trained GMs assessors will be masked to any risk factors and assessors of the primary outcome will be masked to the GMs result. Automated scoring of GMs will be developed through applying machine-based learning to the data and the predictive value for an abnormal GM will be investigated. Screening algorithms for identification of children at risk of cognitive impairment, using the GM assessment (GMA), and routinely collected social and environmental profile data will be developed to allow more accurate prediction of cognitive outcome at 2 years. A cost evaluation for GMA implementation in preparation for national implementation will be undertaken including exploring the relationship between cognitive status and healthcare utilisation, medical costs, health-related quality of life and caregiver burden.

ETHICS AND DISSEMINATION

Ethics approval has been granted by the Medical Research Ethics Committee of Joondalup Health Services and the Health Service Human Research Ethics Committee (1902) of Curtin University (HRE2019-0739).

TRIAL REGISTRATION NUMBER

ACTRN12619001422112.

Myelination of Callosal Axons Is Hampered by Early and Late Forelimb Amputation in Rats

Deafferentation is an important determinant of plastic changes in the CNS, which consists of a loss of inputs from the body periphery or from the CNS itself. Although cortical reorganization has been well documented, white matter plasticity was less explored. Our goal was to investigate microstructural interhemispheric connectivity changes in early and late amputated rats. For that purpose, we employed diffusion-weighted magnetic resonance imaging, as well as Western blotting, immunohistochemistry, and electron microscopy of sections of the white matter tracts to analyze the microstructural changes in the corticospinal tract and in the corpus callosum (CC) sector that contains somatosensory fibers integrating cortical areas representing the forelimbs and compare differences in rats undergoing forelimb amputation as neonates, with those amputated as adults. Results showed that early amputation induced decreased fractional anisotropy values and reduction of total myelin amount in the cerebral peduncle contralateral to the amputation. Both early and late forelimb amputations induced decreased myelination of callosal fibers. While early amputation affected myelination of thinner axons, late amputation disrupted axons of all calibers. Since the CC provides a modulation of inhibition and excitation between the hemispheres, we suggest that the demyelination observed among callosal fibers may misbalance this modulation.

Magnetization Transfer Ratio and Morphometrics of the Spinal Cord Associates with Surgical Recovery in Patients with Degenerative Cervical Myelopathy

OBJECTIVES

We assessed the prognostic value of the preoperative magnetization transfer ratio (MTR) and morphometrics of the spinal cord in patients with degenerative cervical myelopathy (DCM) in a longitudinal cohort study.

METHODS

Thirteen subjects with DCM underwent 3T magnetization transfer imaging. The MTR was calculated for the spinal cord regions and specific white matter tracts. Morphometric measures were extracted. Clinical (modified Japanese Orthopaedics Association [mJOA] and Nurick scale scores) and health-related quality of life scores were assessed before and after cervical decompression surgery. The association between the magnetic resonance imaging (MRI) metrics and postoperative recovery was assessed (Spearman's correlation). Receiver operating characteristics were used to assess the accuracy of MRI metrics in identifying ≥50% recovery in function.

RESULTS

Preoperative anterior cord MTRs were associated with recovery in mJOA scores (ρ = 0.608; P = 0.036; area under the curve [AUC], 0.66). Preoperative lateral cord MTR correlated with the neck disability index (ρ = 0.699; P = 0.011) and pain interference scale (ρ = 0.732; P = 0.007). Preoperative rubrospinal tract MTR was associated with mJOA score recovery (ρ = 0.573; P = 0.041; AUC, 0.86). Preoperative corticospinal tract and reticulospinal MTRs were related to recovery in pain interference scores (ρ = 0.591; P = 0.033; and ρ = 0.583; P = 0.035, respectively). Eccentricity of the cord was associated with Nurick scores (ρ = 0.606; P = 0.028) and mJOA scores (ρ = 0.651; P = 0.025; AUC, 0.92).

CONCLUSIONS

Preoperative MTR and eccentricity measurements of the spinal cord have prognostic value in assessing the response to surgery and recovery in patients with DCM. Advanced MRI and atlas-based postprocessing techniques can inform interventions and advance the healthcare received by patients with DCM.

Higher primate-like direct corticomotoneuronal connections are transiently formed in a juvenile subprimate mammal

The corticospinal (CS) tract emerged and evolved in mammals, and is essentially involved in voluntary movement. Over its phylogenesis, CS innervation gradually invaded to the ventral spinal cord, eventually making direct connections with spinal motoneurons (MNs) in higher primates. Despite its importance, our knowledge of the origin of the direct CS-MN connections is limited; in fact, there is controversy as to whether these connections occur in subprimate mammals, such as rodents. Here we studied the retrograde transsynaptic connection between cortical neurons and MNs in mice by labeling the cells with recombinant rabies virus. On postnatal day 14 (P14), we found that CS neurons make direct connections with cervical MNs innervating the forearm muscles. Direct connections were also detected electrophysiologically in whole cell recordings from identified MNs retrogradely-labeled from their target muscles and optogenetic CS stimulation. In contrast, few, if any, lumbar MNs innervating hindlimbs showed direct connections on P18. Moreover, the direct CS-MN connections observed on P14 were later eliminated. The transient CS-MN cells were distributed predominantly in the M1 and S1 areas. These findings provide insight into the ontogeny and phylogeny of the CS projection and appear to settle the controversy about direct CS-MN connections in subprimate mammals.

The Relationship Between Hand Function and Overlapping Motor Representations of the Hands in the Contralesional Hemisphere in Unilateral Spastic Cerebral Palsy

BACKGROUND

In many children with unilateral spastic cerebral palsy (USCP), the corticospinal tract to the affected hand atypically originates in the hemisphere ipsilateral to the affected hand. Such ipsilateral connectivity is on average a predictor of poor hand function. However, there is high variability in hand function in these children, which might be explained by the complexity of motor representations of both hands in the contralesional hemisphere.

OBJECTIVE

To measure the link between hand function and the size and excitability of motor representations of both hands, and their overlap, in the contralesional hemisphere of children with USCP.

METHODS

We used single-pulse transcranial magnetic stimulation to measure the size and excitability of motor representations of both hands, and their overlap, in the contralesional hemisphere of 50 children with USCP. We correlated these measures with manual dexterity of the affected hand, bimanual performance, and mirror movement strength.

RESULTS

The main and novel findings were (1) the large overlap in contralesional motor representations of the 2 hands and (2) the moderate positive associations of the size and excitability of such shared-site representations with hand function. Such functional associations were not present for overall size and excitability of representations of the affected hand.

CONCLUSIONS

Greater relative overlap of the affected hand representation with the less-affected hand representation within the contralesional hemisphere was associated with better hand function. This association suggests that overlapping representations might be adaptively "yoked," such that cortical control of the child's less-affected hand supports that of the affected hand.

Motor system plasticity after unilateral injury in the developing brain

In maturity, motor skills depend on the corticospinal tract (CST) and brainstem pathways that together synapse on interneurons and motoneurons in the spinal cord. Descending signals to spinal neurons that mediate voluntary control can be distinguished from peripheral sensory signals, primarily for feedback control. These motor system circuits depend initially on developmental genetic mechanisms to establish their connections and neural activity- and use-dependent synaptic refinement during the early postnatal period to enable motor skills to develop. In this review we consider four key activity-dependent developmental mechanisms that provide insights into how the motor systems establish the proper connections for skilled movement control and how the same mechanisms also inform the mechanisms of motor impairments and developmental plasticity after corticospinal system injury: (1) synaptic competition between the CSTs from each hemisphere; (2) interactions between the CST and spinal cord neurons; (3) synaptic competition between the CST and proprioceptive sensory fibres; and (4) interactions between the developing corticospinal motor system and the rubrospinal tract. Our findings suggest that the corticospinal motor system effectively 'oversees' development of its subcortical targets through synaptic competition and trophic-like interactions and this has important implications for motor impairments after perinatal cortical stroke.

WHAT THIS PAPER ADDS

Neural activity-dependent processes inform the brain and spinal cord response to injury. The corticospinal motor system may 'oversee' development of its downstream subcortical targets through activity, trophic-like interactions, and synaptic competition.

Participatory design in the development of an early therapy intervention for perinatal stroke

BACKGROUND

Perinatal stroke is the leading cause of unilateral (hemiparetic) cerebral palsy, with life-long personal, social and financial consequences. Translational research findings indicate that early therapy intervention has the potential for significant improvements in long-term outcome in terms of motor function. By involving families and health professionals in the development and design stage, we aimed to produce a therapy intervention which they would engage with.

METHODS

Nine parents of children with hemiparesis and fourteen health professionals involved in the care of infants with perinatal stroke took part in peer review and focus groups to discuss evolving therapy materials, with revisions made iteratively. The materials and approach were also discussed at a meeting of the London Child Stroke Research Reference Group. Focus group data were coded using Normalisation Process Theory constructs to explore potential barriers and facilitators to routine uptake of the intervention.

RESULTS

We developed the Early Therapy in Perinatal Stroke (eTIPS) program - a parent-delivered, home-based complex intervention addressing a current gap in practice for infants in the first 6 months of life after unilateral perinatal stroke and with the aim of improving motor outcome. Parents and health professionals saw the intervention as different from usual practice, and valuable (high coherence). They were keen to engage (high cognitive participation). They considered the tasks for parents to be achievable (high collective action). They demonstrated trust in the approach and felt that parents would undertake the recommended activities (high collective action). They saw the approach as flexible and adaptable (high reflexive monitoring). Following suggestions made, we added a section on involving the extended family, and obtained funding for a website and videos to supplement written materials.

CONCLUSIONS

Focus groups with parents and health professionals provided meaningful feedback to iteratively improve the intervention materials prior to embarking on a pilot study. The intervention has a high potential to normalize and become a routine part of parents' interactions with their child following unilateral perinatal stroke.