Glycinergic and GABAergic synaptic activity differentially regulate motoneuron survival and skeletal muscle innervation

NMJ-related diseases beyond the congenital myasthenic syndromes

Neuromuscular junctions (NMJs) are a special type of chemical synapse that transmits electrical stimuli from motor neurons (MNs) to their innervating skeletal muscle to induce a motor response. They are an ideal model for the study of synapses, given their manageable size and easy accessibility. Alterations in their morphology or function lead to neuromuscular disorders, such as the congenital myasthenic syndromes, which are caused by mutations in proteins located in the NMJ. In this review, we highlight novel potential candidate genes that may cause or modify NMJs-related pathologies in humans by exploring the phenotypes of hundreds of mouse models available in the literature. We also underscore the fact that NMJs may differ between species, muscles or even sexes. Hence the importance of choosing a good model organism for the study of NMJ-related diseases: only taking into account the specific features of the mammalian NMJ, experimental results would be efficiently translated to the clinic.

Inhibitory Synaptic Influences on Developmental Motor Disorders

During development, GABA and glycine play major trophic and synaptic roles in the establishment of the neuromotor system. In this review, we summarise the formation, function and maturation of GABAergic and glycinergic synapses within neuromotor circuits during development. We take special care to discuss the differences in limb and respiratory neuromotor control. We then investigate the influences that GABAergic and glycinergic neurotransmission has on two major developmental neuromotor disorders: Rett syndrome and spastic cerebral palsy. We present these two syndromes in order to contrast the approaches to disease mechanism and therapy. While both conditions have motor dysfunctions at their core, one condition Rett syndrome, despite having myriad symptoms, has scientists focused on the breathing abnormalities and their alleviation-to great clinical advances. By contrast, cerebral palsy remains a scientific quagmire or poor definitions, no widely adopted model and a lack of therapeutic focus. We conclude that the sheer abundance of diversity of inhibitory neurotransmitter targets should provide hope for intractable conditions, particularly those that exhibit broad spectra of dysfunction-such as spastic cerebral palsy and Rett syndrome.

Glycine neurotransmission: Its role in development

The accurate function of the central nervous system (CNS) depends of the consonance of multiple genetic programs and external signals during the ontogenesis. A variety of molecules including neurotransmitters, have been implied in the regulation of proliferation, survival, and cell-fate of neurons and glial cells. Among these, neurotransmitters may play a central role since functional ligand-gated ionic channel receptors have been described before the establishment of synapses. This review argues on the function of glycine during development, and show evidence indicating it regulates morphogenetic events by means of their transporters and receptors, emphasizing the role of glycinergic activity in the balance of excitatory and inhibitory signals during development. Understanding the mechanisms involved in these processes would help us to know the etiology of cognitive dysfunctions and lead to improve brain repair strategies.

(-)-Epicatechin Is a Biased Ligand of Apelin Receptor

(-)-Epicatechin (EC) is part of a large family of biomolecules called flavonoids and is widely distributed in the plant kingdom. Several studies have shown the beneficial effects of EC consumption. Many of these reported effects are exerted by activating the signaling pathways associated with the activation of two specific receptors: the G protein-coupled estrogen receptor (GPER), a transmembrane receptor, and the pregnane X receptor (PXR), which is a nuclear receptor. However, the effects of EC are so diverse that these two receptors cannot describe the complete phenomenon. The apelin receptor or APLNR is classified within the G protein-coupled receptor (GPCR) family, and is capable of activating the G protein canonical pathways and the β-arrestin transducer, which participates in the phenomenon of receptor desensitization and internalization. β-arrestin gained interest in selective pharmacology and mediators of the so-called "biased agonism". With molecular dynamics (MD) and in vitro assays, we demonstrate how EC can recruit the β-arrestin in the active conformation of the APLN receptor acting as a biased agonist.

Structure and Function of the Mammalian Neuromuscular Junction

The mammalian neuromuscular junction (NMJ) comprises a presynaptic terminal, a postsynaptic receptor region on the muscle fiber (endplate), and the perisynaptic (terminal) Schwann cell. As with any synapse, the purpose of the NMJ is to transmit signals from the nervous system to muscle fibers. This neural control of muscle fibers is organized as motor units, which display distinct structural and functional phenotypes including differences in pre- and postsynaptic elements of NMJs. Motor units vary considerably in the frequency of their activation (both motor neuron discharge rate and duration/duty cycle), force generation, and susceptibility to fatigue. For earlier and more frequently recruited motor units, the structure and function of the activated NMJs must have high fidelity to ensure consistent activation and continued contractile response to sustain vital motor behaviors (e.g., breathing and postural balance). Similarly, for higher force less frequent behaviors (e.g., coughing and jumping), the structure and function of recruited NMJs must ensure short-term reliable activation but not activation sustained for a prolonged period in which fatigue may occur. The NMJ is highly plastic, changing structurally and functionally throughout the life span from embryonic development to old age. The NMJ also changes under pathological conditions including acute and chronic disease. Such neuroplasticity often varies across motor unit types. © 2022 American Physiological Society. Compr Physiol 12:1-36, 2022.

Impaired signaling for neuromuscular synaptic maintenance is a feature of Motor Neuron Disease

A central event in the pathogenesis of motor neuron disease (MND) is the loss of neuromuscular junctions (NMJs), yet the mechanisms that lead to this event in MND remain to be fully elucidated. Maintenance of the NMJ relies upon neural agrin (n-agrin) which, when released from the nerve terminal, activates the postsynaptic Muscle Specific Kinase (MuSK) signaling complex to stabilize clusters of acetylcholine receptors. Here, we report that muscle from MND patients has an increased proportion of slow fibers and muscle fibers with smaller diameter. Muscle cells cultured from MND biopsies failed to form large clusters of acetylcholine receptors in response to either non-MND human motor axons or n-agrin. Furthermore, levels of expression of MuSK, and MuSK-complex components: LRP4, Caveolin-3, and Dok7 differed between muscle cells cultured from MND patients compared to those from non-MND controls. To our knowledge, this is the first time a fault in the n-agrin-LRP4-MuSK signaling pathway has been identified in muscle from MND patients. Our results highlight the n-agrin-LRP4-MuSK signaling pathway as a potential therapeutic target to prolong muscle function in MND.

Quantifying mitochondrial volume density in phrenic motor neurons

BACKGROUND

Previous assessments of mitochondrial volume density within motor neurons used electron microscopy (EM) to image mitochondria. However, adequate identification and sampling of motor neurons within a particular motor neuron pool is largely precluded using EM. Here, we present an alternative method for determining mitochondrial volume density in identified motor neurons within the phrenic motor neuron (PhMN) pool, with greatly increased sampling.

NEW METHOD

This novel method for assessing mitochondrial volume density in PhMNs uses a combination of intrapleural injection of Alexa 488-conjugated cholera toxin B (CTB) to retrogradely label PhMNs, followed by intrathecal application of MitoTracker Red to label mitochondria. This technique was validated by comparison to 3D EM determination of mitochondrial volume density as a "gold standard".

RESULTS

A mean mitochondrial volume density of ∼11 % was observed across PhMNs using the new MitoTracker Red method. This compared favourably with mitochondrial volume density (∼11 %) measurements using EM.

COMPARISON WITH EXISTING METHOD

The range, mean and variance of mitochondrial volume density estimates in PhMNs were not different between EM and fluorescent imaging techniques.

CONCLUSIONS

Fluorescent imaging may be used to estimate mitochondrial volume density in a large sample of motor neurons, with results similar to EM, although EM did distinguish finer mitochondrion morphology compared to MitoTracker fluorescence. Compared to EM methods, the assessment of a larger sample size and unambiguous identification of motor neurons belonging to a specific motor neuron pool represent major advantages over previous methods.

Maternally derived hormones, neurosteroids and the development of behaviour

In a wide range of taxa, there is evidence that mothers adaptively shape the development of offspring behaviour by exposing them to steroids. These maternal effects have major implications for fitness because, by shaping early development, they can permanently alter how offspring interact with their environment. However, theory on parent-offspring conflict and recent physiological studies showing that embryos rapidly metabolize maternal steroids have placed doubt on the adaptive significance of these hormone-mediated maternal effects. Reconciling these disparate perspectives requires a mechanistic understanding of the pathways by which maternal steroids can influence neural development. Here, we highlight recent advances in developmental neurobiology and psychiatric pharmacology to show that maternal steroid metabolites can have direct neuro-modulatory effects potentially shaping the development of neural circuitry underlying ecologically relevant behavioural traits. The recognition that maternal steroids can act through a neurosteroid pathway has critical implications for our understanding of the ecology and evolution of steroid-based maternal effects. Overall, compared to the classic view, a neurosteroid mechanism may reduce the evolutionary lability of hormone-mediated maternal effects owing to increased pleiotropic constraints and frequently influence long-term behavioural phenotypes in offspring.

Diaphragm neuromuscular transmission failure in a mouse model of an early-onset neuromotor disorder

The spa transgenic mouse displays spasticity and hypertonia that develops during the early postnatal period, with motor impairments that are remarkably similar to symptoms of human cerebral palsy. Previously, we observed that spa mice have fewer phrenic motor neurons innervating the diaphragm muscle (DIAm). We hypothesize that spa mice exhibit increased susceptibility to neuromuscular transmission failure (NMTF) due to an expanded innervation ratio. We retrogradely labeled phrenic motor neurons with rhodamine and imaged them in horizontal sections (70 µm) using confocal microscopy. Phrenic nerve-DIAm strip preparations from wild type and spa mice were stretched to optimal length, and force was evoked by phrenic nerve stimulation at 10, 40, or 75 Hz in 330-ms duration trains repeated each second (33% duty cycle) across a 120-s period. To assess NMTF, force evoked by phrenic nerve stimulation was compared to force evoked by direct DIAm stimulation superimposed every 15 s. Total DIAm fiber number was estimated in hematoxylin and eosin-stained strips. Compared to wild type, spa mice had over twofold greater NMTF during the first stimulus train that persisted throughout the 120 s period of repetitive activation. In both wild type and spa mice, NMTF was stimulation-frequency dependent. There was no difference in neuromuscular junction morphology or the total number of DIAm fibers between wild type and spa mice, however, there was an increase innervation ratio (39%) in spa mice. We conclude that early-onset developmental neuromotor disorders impair the efficacy of DIAm neuromuscular transmission, likely to contribute to respiratory complications.NEW & NOTEWORTHY Individuals with motor control deficits, including cerebral palsy (CP) often have respiratory impairments. Glycine-receptor mutant spa mice have early-onset hypertonia, and limb motor impairments, similar to individuals with CP. We hypothesized that in the diaphragm of spa mice, disruption of glycinergic inputs to MNs would result in increased phrenic-DIAm neuromuscular transmission failure. Pathophysiologic abnormalities in neuromuscular transmission may contribute to respiratory dysfunction in conditions where early developmental MN loss or motor control deficits are apparent.

Impaired neuromuscular transmission of the tibialis anterior in a rodent model of hypertonia

Early-onset hypertonia is characteristic of developmental neuromotor disorders, including cerebral palsy (CP). The spa transgenic mouse displays early-onset spasticity, abnormal gait, and motor impairments that are remarkably similar to symptoms of human CP. Previously, we showed that spa mice have fewer motor neurons innervating the tibialis anterior (TA). An expanded innervation ratio may result in increased susceptibility to neuromuscular transmission failure (NMTF). We assessed NMTF in an ex vivo TA muscle nerve preparation from spa and wild-type (WT) mice by comparing forces elicited by nerve versus muscle stimulation. TA muscle innervation ratio was assessed by counting the number of muscle fibers and dividing by the number of TA motor neurons. Muscle fiber cross-sectional areas were also assessed in the TA muscle. We observed that NMTF was immediately present in spa mice, increased with repetitive stimulation, and associated with increased innervation ratio. These changes were concomitant with reduced TA muscle fiber cross-sectional area in spa mice compared with WT. Early-onset hypertonia is associated with increased innervation ratio and impaired neuromuscular transmission. These disturbances may exacerbate the underlying gait abnormalities present in individuals with hypertonia.NEW & NOTEWORTHY Nerve-muscle interaction is poorly understood in the context of early-onset spasticity and hypertonia. In an animal model of early-onset spasticity, spa mice, we found a marked impairment of tibialis anterior neuromuscular transmission. This impairment is associated with an increased innervation ratio (mean number of muscle fibers innervated by a single motor neuron). These disturbances may underlie weakness and gait disturbances observed in individual with developmental hypertonia and spasticity.

Phrenic motor neuron loss in an animal model of early onset hypertonia

Phrenic motor neuron (PhMN) development in early onset hypertonia is poorly understood. Respiratory disorders are one of the leading causes of morbidity and mortality in individuals with early onset hypertonia, such as cerebral palsy (CP), but they are largely overshadowed by a focus on physical function in this condition. Furthermore, while the brain is the focus of CP research, motor neurons, via the motor unit and neurotransmitter signaling, are the targets in clinical interventions for hypertonia. Furthermore, critical periods of spinal cord and motor unit development also coincide with the timing that the supposed brain injury occurs in CP. Using an animal model of early-onset spasticity (spa mouse [B6.Cg-Glrbspa/J] with a glycine receptor mutation), we hypothesized that removal of effective glycinergic neurotransmitter inputs to PhMNs during development will result in fewer PhMNs and reduced PhMN somal size at maturity. Adult spa (Glrb-/-), and wild-type (Glrb+/+) mice underwent unilateral retrograde labeling of PhMNs via phrenic nerve dip in tetramethylrhodamine. After three days, mice were euthanized, perfused with 4% paraformaldehyde, and the spinal cord excised and processed for confocal imaging. Spa mice had ~30% fewer PhMNs (P = 0.005), disproportionately affecting larger PhMNs. Additionally, a ~22% reduction in PhMN somal surface area (P = 0.019), an 18% increase in primary dendrites (P < 0.0001), and 24% decrease in dendritic surface area (P = 0.014) were observed. Thus, there are fewer larger PhMNs in spa mice. Fewer and smaller PhMNs may contribute to impaired diaphragm neuromotor control and contribute to respiratory morbidity and mortality in conditions of early onset hypertonia.NEW & NOTEWORTHY Phrenic motor neuron (PhMN) development in early-onset hypertonia is poorly understood. Yet, respiratory disorders are a common cause of morbidity and mortality. In spa mice, an animal model of early-onset hypertonia, we found ~30% fewer PhMNs, compared with controls. This PhMN loss disproportionately affected larger PhMNs. Thus, the number and heterogeneity of the PhMN pool are decreased in spa mice, likely contributing to the hypertonia, impaired neuromotor control, and respiratory disorders.

Skeletal muscle transcriptional networks linked to type I myofiber grouping in Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disorder impacting cognition, movement, and quality of life in >10 million individuals worldwide. We recently characterized and quantified a skeletal muscle pathology in PD represented by exaggerated type I myofiber grouping presumed to result from denervation-reinnervation processes. Our previous findings indicated that impaired neuromuscular junction integrity may be involved in type I grouping, which is associated with excessive motor unit activation during weight-bearing tasks. In this study, we performed transcriptional profiling to test the hypothesis that type I grouping severity would link to distinct gene expression networks. We generated transcriptome-wide poly(A) RNA-Seq data from skeletal muscle of individuals with PD [n = 12 (9 men, 3 women); 67 ± 2 yr], age- and sex-matched older adults (n = 12; 68 ± 2 yr), and sex-matched young adults (n = 12; 30 ± 1 yr). Differentially expressed genes were evaluated across cohorts. Weighted gene correlation network analysis (WGCNA) was performed to identify gene networks most correlated with indicators of abnormal type I grouping. Among coexpression networks mapping to phenotypes pathologically increased in PD muscle, one network was highly significantly correlated to type I myofiber group size and another to percentage of type I myofibers found in groups. Annotation of coexpressed networks revealed that type I grouping is associated with altered expression of genes involved in neural development, postsynaptic signaling, cell cycle regulation and cell survival, protein and energy metabolism, inflammation/immunity, and posttranscriptional regulation (microRNAs). These transcriptomic findings suggest that skeletal muscle may play an active role in signaling to promote myofiber survival, reinnervation, and remodeling, perhaps to an extreme in PD.NEW & NOTEWORTHY Despite our awareness of the impact of Parkinson's disease (PD) on motor function for over two centuries, limited attention has focused on skeletal muscle. We previously identified type I myofiber grouping, a novel indicator of muscle dysfunction in PD, presumably a result of heightened rates of denervation/reinnervation. Using transcriptional profiling to identify networks associated with this phenotype, we provide insight into potential mechanistic roles of skeletal muscle in signaling to promote its survival in PD.

Muscles of Breathing: Development, Function, and Patterns of Activation

This review is a comprehensive description of all muscles that assist lung inflation or deflation in any way. The developmental origin, anatomical orientation, mechanical action, innervation, and pattern of activation are described for each respiratory muscle fulfilling this broad definition. In addition, the circumstances in which each muscle is called upon to assist ventilation are discussed. The number of "respiratory" muscles is large, and the coordination of respiratory muscles with "nonrespiratory" muscles and in nonrespiratory activities is complex-commensurate with the diversity of activities that humans pursue, including sleep (8.27). The capacity for speech and adoption of the bipedal posture in human evolution has resulted in patterns of respiratory muscle activation that differ significantly from most other animals. A disproportionate number of respiratory muscles affect the nose, mouth, pharynx, and larynx, reflecting the vital importance of coordinated muscle activity to control upper airway patency during both wakefulness and sleep. The upright posture has freed the hands from locomotor functions, but the evolutionary history and ontogeny of forelimb muscles pervades the patterns of activation and the forces generated by these muscles during breathing. The distinction between respiratory and nonrespiratory muscles is artificial, as many "nonrespiratory" muscles can augment breathing under conditions of high ventilator demand. Understanding the ontogeny, innervation, activation patterns, and functions of respiratory muscles is clinically useful, particularly in sleep medicine. Detailed explorations of how the nervous system controls the multiple muscles required for successful completion of respiratory behaviors will continue to be a fruitful area of investigation. © 2019 American Physiological Society. Compr Physiol 9:1025-1080, 2019.

A Critical Evaluation of Current Concepts in Cerebral Palsy

Spastic cerebral palsy (CP), despite the name, is not consistently identifiable by specific brain lesions. CP animal models focus on risk factors for development of CP, yet few reproduce the diagnostic symptoms. Animal models of CP must advance beyond risk factors to etiologies, including both the brain and spinal cord.

Evolution and Functional Differentiation of the Diaphragm Muscle of Mammals

Symmorphosis is a concept of economy of biological design, whereby structural properties are matched to functional demands. According to symmorphosis, biological structures are never over designed to exceed functional demands. Based on this concept, the evolution of the diaphragm muscle (DIAm) in mammals is a tale of two structures, a membrane that separates and partitions the primitive coelomic cavity into separate abdominal and thoracic cavities and a muscle that serves as a pump to generate intra-abdominal (Pab ) and intrathoracic (Pth ) pressures. The DIAm partition evolved in reptiles from folds of the pleural and peritoneal membranes that was driven by the biological advantage of separating organs in the larger coelomic cavity into separate thoracic and abdominal cavities, especially with the evolution of aspiration breathing. The DIAm pump evolved from the advantage afforded by more effective generation of both a negative Pth for ventilation of the lungs and a positive Pab for venous return of blood to the heart and expulsive behaviors such as airway clearance, defecation, micturition, and child birth. © 2019 American Physiological Society. Compr Physiol 9:715-766, 2019.

Decreased signalling of EphA4 improves functional performance and motor neuron survival in the SOD1G93A ALS mouse model

Amyotrophic lateral sclerosis (ALS) is an untreatable, progressive, neurodegenerative disease specifically affecting motor neurons. Recently, the tyrosine kinase receptor EphA4 was directly implicated in ALS disease progression. We report that a long-lived mutated form of the EphA4 antagonist EphA4-Fc (mutEphA4-Fc), which blocks EphA4 binding to its ligands and inhibits its function, significantly improved functional performance in SOD1G93A ALS model mice, as assessed by rotarod and hind-limb grip strength tests. Further, heterozygous motor neuron-specific EphA4 gene deletion in SOD1G93A mice promoted significant improvement in functional performance during the disease course and a delay in disease onset relative to control mice. Importantly, mice in the heterozygous deletion group showed significantly improved survival of motor neurons and architecture of endplates of neuromuscular junctions compared with control and homozygous EphA4-deletion groups. Our novel results show that EphA4 signalling directly regulates motor neuron survival and that mutEphA4-Fc is a promising therapeutic candidate to slow disease progression in ALS.

Differences in lumbar motor neuron pruning in an animal model of early onset spasticity

Motor neuron (MN) development in early onset spasticity is poorly understood. For example, spastic cerebral palsy (sCP), the most common motor disability of childhood, is poorly predicted by brain imaging, yet research remains focused on the brain. By contrast, MNs, via the motor unit and neurotransmitter signaling, are the target of most therapeutic spasticity treatments and are the final common output of motor control. MN development in sCP is a critical knowledge gap, because the late embryonic and postnatal periods are not only when the supposed brain injury occurs but also are critical times for spinal cord neuromotor development. Using an animal model of early onset spasticity [ spa mouse (B6.Cg- Glrb^spa/J) with a glycine (Gly) receptor mutation], we hypothesized that removal of effective glycinergic neurotransmitter inputs to MNs during development will influence MN pruning (including primary dendrites) and MN size. Spa (Glrb-/-) and wild-type (Glrb+/+) mice, ages 4-9 wk, underwent unilateral retrograde labeling of the tibialis anterior muscle MNs via peroneal nerve dip in tetramethylrhodamine. After 3 days, mice were euthanized and perfused with 4% paraformaldehyde, and the spinal cord was excised and processed for confocal imaging. Spa mice had ~61% fewer lumbar tibialis anterior MNs ( P < 0.01), disproportionately affecting larger MNs. Additionally, a ~23% reduction in tibialis anterior MN somal surface area ( P < 0.01) and a 12% increase in primary dendrites ( P = 0.046) were observed. Thus MN pruning and MN somal surface area are abnormal in early onset spasticity. Fewer and smaller MNs may contribute to the spastic phenotype. NEW & NOTEWORTHY Motor neuron (MN) development in early onset spasticity is poorly understood. In an animal model of early onset spasticity, spa mice, we found ~61% fewer lumbar tibialis anterior MNs compared with controls. This MN loss disproportionately affected larger MNs. Thus number and heterogeneity of the MN pool are decreased in spa mice, likely contributing to the spastic phenotype.

Alterations in hypoglossal motor neurons due to GAD67 and VGAT deficiency in mice

There is an emerging body of evidence that glycinergic and GABAergic synaptic inputs onto motor neurons (MNs) help regulate the final number of MNs and axonal muscle innervation patterns. Using mutant glutamate decarboxylase 67 (GAD67) and vesicular inhibitory amino acid transporter (VGAT) deficient mice, we describe the effect that deficiencies of presynaptic GABAergic and/or glycinergic release have on the post-synaptic somato-dendritic structure of motor neurons, and the development of excitatory and inhibitory synaptic inputs to MNs. We use whole-cell patch clamp recording of synaptic currents in E18.5 hypoglossal MNs from brainstem slices, combined with dye-filling of these recorded cells with Neurobiotin™, high-resolution confocal imaging and 3-dimensional reconstructions. Hypoglossal MNs from GAD67- and VGAT-deficient mice display decreased inhibitory neurotransmission and increased excitatory synaptic inputs. These changes are associated with increased dendritic arbor length, increased complexity of dendritic branching, and increased density of spiny processes. Our results show that presynaptic release of inhibitory amino acid neurotransmitters are potent regulators of hypoglossal MN morphology and key regulators of synaptic inputs during this critical developmental time point.

Developmental changes in the morphology of mouse hypoglossal motor neurons

Hypoglossal motor neurons (XII MNs) innervate tongue muscles important in breathing, suckling and vocalization. Morphological properties of 103 XII MNs were studied using Neurobiotin™ filling in transverse brainstem slices from C57/Bl6 mice (n = 34) from embryonic day (E) 17 to postnatal day (P) 28. XII MNs from areas thought to innervate different tongue muscles showed similar morphology in most, but not all, features. Morphological properties of XII MNs were established prior to birth, not differing between E17-18 and P0. MN somatic volume gradually increased for the first 2 weeks post-birth. The complexity of dendritic branching and dendrite length of XII MNs increased throughout development (E17-P28). MNs in the ventromedial XII motor nucleus, likely to innervate the genioglossus, frequently (42 %) had dendrites crossing to the contralateral side at all ages, but their number declined with postnatal development. Unexpectedly, putative dendritic spines were found in all XII MNs at all ages, and were primarily localized to XII MN somata and primary dendrites at E18-P4, increased in distal dendrites by P5-P8, and were later predominantly found in distal dendrites. Dye-coupling between XII MNs was common from E18 to P7, but declined strongly with maturation after P7. Axon collaterals were found in 20 % (6 of 28) of XII MNs with filled axons; collaterals terminated widely outside and, in one case, within the XII motor nucleus. These results reveal new morphological features of mouse XII MNs, and suggest that dendritic projection patterns, spine density and distribution, and dye-coupling patterns show specific developmental changes in mice.

Glycinergic Neurotransmission: A Potent Regulator of Embryonic Motor Neuron Dendritic Morphology and Synaptic Plasticity

Emerging evidence suggests that central synaptic inputs onto motor neurons (MNs) play an important role in developmental regulation of the final number of MNs and their muscle innervation for a particular motor pool. Here, we describe the effect of genetic deletion of glycinergic neurotransmission on single MN structure and on functional excitatory and inhibitory inputs to MNs. We measured synaptic currents in E18.5 hypoglossal MNs from brain slices using whole-cell patch-clamp recording, followed by dye-filling these same cells with Neurobiotin, to define their morphology by high-resolution confocal imaging and 3D reconstruction. We show that hypoglossal MNs of mice lacking gephyrin display increased dendritic arbor length and branching, increased spiny processes, decreased inhibitory neurotransmission, and increased excitatory neurotransmission. These findings suggest that central glycinergic synaptic activity plays a vital role in regulating MN morphology and glutamatergic central synaptic inputs during late embryonic development.

SIGNIFICANCE STATEMENT

MNs within the brainstem and spinal cord are responsible for integrating a diverse array of synaptic inputs into discrete contractions of skeletal muscle to achieve coordinated behaviors, such as breathing, vocalization, and locomotion. The last trimester in utero is critical in neuromotor development, as this is when central and peripheral synaptic connections are made onto and from MNs. At this time-point, using transgenic mice with negligible glycinergic postsynaptic responses, we show that this deficiency leads to abnormally high excitatory neurotransmission and alters the dendritic architecture responsible for coherently integrating these inputs. This study compliments the emerging concept that neurodevelopmental disorders (including autism, epilepsy, and amyotrophic lateral sclerosis) are underpinned by synaptic dysfunction and therefore will be useful to neuroscientists and neurologists alike.

Emerging Roles of Filopodia and Dendritic Spines in Motoneuron Plasticity during Development and Disease

Motoneurons develop extensive dendritic trees for receiving excitatory and inhibitory synaptic inputs to perform a variety of complex motor tasks. At birth, the somatodendritic domains of mouse hypoglossal and lumbar motoneurons have dense filopodia and spines. Consistent with Vaughn's synaptotropic hypothesis, we propose a developmental unified-hybrid model implicating filopodia in motoneuron spinogenesis/synaptogenesis and dendritic growth and branching critical for circuit formation and synaptic plasticity at embryonic/prenatal/neonatal period. Filopodia density decreases and spine density initially increases until postnatal day 15 (P15) and then decreases by P30. Spine distribution shifts towards the distal dendrites, and spines become shorter (stubby), coinciding with decreases in frequency and increases in amplitude of excitatory postsynaptic currents with maturation. In transgenic mice, either overexpressing the mutated human Cu/Zn-superoxide dismutase (hSOD1^G93A) gene or deficient in GABAergic/glycinergic synaptic transmission (gephyrin, GAD-67, or VGAT gene knockout), hypoglossal motoneurons develop excitatory glutamatergic synaptic hyperactivity. Functional synaptic hyperactivity is associated with increased dendritic growth, branching, and increased spine and filopodia density, involving actin-based cytoskeletal and structural remodelling. Energy-dependent ionic pumps that maintain intracellular sodium/calcium homeostasis are chronically challenged by activity and selectively overwhelmed by hyperactivity which eventually causes sustained membrane depolarization leading to excitotoxicity, activating microglia to phagocytose degenerating neurons under neuropathological conditions.

Myofiber branching rather than myofiber hyperplasia contributes to muscle hypertrophy in mdx mice

BACKGROUND

Muscle hypertrophy in the mdx mouse model of Duchenne muscular dystrophy (DMD) can partially compensate for the loss of dystrophin by maintaining peak force production. Histopathology examination of the hypertrophic muscles suggests the hypertrophy primarily results from the addition of myofibers, and is accompanied by motor axon branching. However, it is unclear whether an increased number of innervated myofibers (myofiber hyperplasia) contribute to muscle hypertrophy in the mdx mice.

METHODS

To better understand the cellular mechanisms of muscle hypertrophy in mdx mice, we directly compared the temporal progression of the dystrophic pathology in the extensor digitorum longus (EDL) muscle to myofiber number, myofiber branching, and innervation, from 3 to 20 weeks of age.

RESULTS

We found that a 28% increase in the number of fibers in transverse sections of muscle correlated with a 31% increase in myofiber branching. Notably, the largest increases in myofiber number and myofiber branching occurred after 12 weeks of age when the proportion of myofibers with central nuclei had stabilized and the mdx mouse had reached maturity. The dystrophic pathology coincided with profound changes to innervation of the muscles that included temporary denervation of necrotic fibers, fragmentation of synapses, and ultra-terminal axon sprouting. However, there was little evidence of synapse formation in the mdx mice from 3 to 20 weeks of age. Only 4.4% of neuromuscular junctions extended ultra-terminal synapses, which failed to mature, and the total number of neuromuscular junctions remained constant.

CONCLUSIONS

Muscle hypertrophy in mdx mice results from myofiber branching rather than myofiber hyperplasia.

Cell biology in neuroscience: Death of developing neurons: new insights and implications for connectivity

The concept that target tissues determine the survival of neurons has inspired much of the thinking on neuronal development in vertebrates, not least because it is supported by decades of research on nerve growth factor (NGF) in the peripheral nervous system (PNS). Recent discoveries now help to understand why only some developing neurons selectively depend on NGF. They also indicate that the survival of most neurons in the central nervous system (CNS) is not simply regulated by single growth factors like in the PNS. Additionally, components of the cell death machinery have begun to be recognized as regulators of selective axonal degeneration and synaptic function, thus playing a critical role in wiring up the nervous system.

Genetic absence of the vesicular inhibitory amino acid transporter differentially regulates respiratory and locomotor motor neuron development

During mid to late embryonic development (E13 to birth in mice), the neuromotor system is refined by reducing motor neuron (MN) numbers and establishing nascent synaptic connections onto and by MNs. Concurrently, the response to GABAergic and glycinergic synaptic activity switches from postsynaptic excitation to inhibition. Our previous studies on mutant mice lacking glycinergic transmission or deficient in GABA suggests that altered MN activity levels during this developmental period differentially regulates MN survival and muscle innervation for respiratory and non-respiratory motor pools. To determine if combined loss of GABAergic and glycinergic transmission plays a similar or exaggerated role, we quantified MN number and muscle innervation in two respiratory (hypoglossal and phrenic) and two locomotor (brachial and lumbar) motor pools, in mice lacking vesicular inhibitory amino acid transporter, which display absent or severely impaired GABAergic and glycinergic neurotransmission. For respiratory MNs, we observed significant decreases in MN number (-20 % hypoglossal and -36 % phrenic) and diaphragm axonal branching (-60 %). By contrast, for non-respiratory brachial and lumbar MNs, we observed increases in MN number (+62 % brachial and +84 % lumbar) and axonal branching for innervated muscles (+123 % latissimus dorsi for brachial and +61 % gluteal for lumbar). These results show that combined absence of GABAergic and glycinergic neurotransmission causes distinct regional changes in MN number and muscle innervation, which are dependent upon the motor function of the specific motor pool.

A method for the three-dimensional reconstruction of Neurobiotin™-filled neurons and the location of their synaptic inputs

Here, we describe a robust method for mapping the number and type of neuro-chemically distinct synaptic inputs that a single reconstructed neuron receives. We have used individual hypoglossal motor neurons filled with Neurobiotin by semi-loose seal electroporation in thick brainstem slices. These filled motor neurons were then processed for excitatory and inhibitory synaptic inputs, using immunohistochemical-labeling procedures. For excitatory synapses, we used anti-VGLUT2 to locate glutamatergic pre-synaptic terminals and anti-PSD-95 to locate post-synaptic specializations on and within the surface of these filled motor neurons. For inhibitory synapses, we used anti-VGAT to locate GABAergic pre-synaptic terminals and anti-GABA-A receptor subunit α1 to locate the post-synaptic domain. The Neurobiotin-filled and immuno-labeled motor neuron was then processed for optical sectioning using confocal microscopy. The morphology of the motor neuron including its dendritic tree and the distribution of excitatory and inhibitory synapses were then determined by three-dimensional reconstruction using IMARIS software (Bitplane). Using surface rendering, fluorescence thresholding, and masking of unwanted immuno-labeling, tools found in IMARIS, we were able to obtain an accurate 3D structure of an individual neuron including the number and location of its glutamatergic and GABAergic synaptic inputs. The power of this method allows for a rapid morphological confirmation of the post-synaptic responses recorded by patch-clamp prior to Neurobiotin filling. Finally, we show that this method can be adapted to super-resolution microscopy techniques, which will enhance its applicability to the study of neural circuits at the level of synapses.

Dysregulation of the complement cascade in the hSOD1G93A transgenic mouse model of amyotrophic lateral sclerosis

Background

Components of the innate immune complement system have been implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS); however, a comprehensive examination of complement expression in this disease has not been performed. This study therefore aimed to determine the expression of complement components (C1qB, C4, factor B, C3/C3b, C5 and CD88) and regulators (CD55 and CD59a) in the lumbar spinal cord of hSOD1^G93A mice during defined disease stages.

Methods

hSOD1^G93A and wild-type mice were examined at four different ages of disease progression. mRNA and protein expression of complement components and regulators were examined using quantitative PCR, western blotting and ELISA. Localisation of complement components within lumbar spinal cord was investigated using immunohistochemistry. Statistical differences between hSOD1^G93A and wild-type mice were analysed using a two-tailed t-test at each stage of disease progression.

Results

We found several early complement factors increased as disease progressed, whilst complement regulators decreased; suggesting overall increased complement activation through the classical or alternative pathways in hSOD1^G93A mice. CD88 was also increased during disease progression, with immunolocalisation demonstrating expression on motor neurons and increasing expression on microglia surrounding the regions of motor neuron death.

Conclusions

These results indicate that local complement activation and increased expression of CD88 may contribute to motor neuron death and ALS pathology in the hSOD1^G93A mouse. Hence, reducing complement-induced inflammation could be an important therapeutic strategy to treat ALS.

Improvement of contusive spinal cord injury in rats by co-transplantation of gamma-aminobutyric acid-ergic cells and bone marrow stromal cells

BACKGROUND AIMS

Cell therapy is considered a promising option for treatment of spinal cord injury (SCI). The purpose of this study is to use combined therapy of bone marrow stromal cells (BMSCs) and BMSC-derived gamma-aminobutyric acid (GABA)ergic inhibitory neurotransmitter cells (BDGCs) for the contusion model of SCI in rats.

METHODS

BDGCs were prepared from BMSCs by pre-inducing them with β-mercaptoethanol followed by retinoic acid and then inducing them by creatine. They were immunostained with BMSC, proneuronal, neural and GABA markers. The BDGCs were intraspinally transplanted into the contused rats, whereas the BMSCs were delivered intravenously. The animals were sacrificed after 12 weeks.

RESULTS

The Basso, Beattie and Bresnahan test showed improvement in the animals with the combined therapy compared with the untreated animals, the animals treated with GABAergic cells only and the animals that received BMSCs. The immunohistochemistry analysis of the tissue sections prepared from the animals receiving the combined therapy showed that the transplanted cells were engrafted and integrated into the injured spinal cord; in addition, a significant reduction was seen in the cavitation.

CONCLUSIONS

The study shows that the combination of GABAergic cells with BMSCs can improve SCI.

Genetic deficiency of GABA differentially regulates respiratory and non-respiratory motor neuron development

Central nervous system GABAergic and glycinergic synaptic activity switches from postsynaptic excitation to inhibition during the stage when motor neuron numbers are being reduced, and when synaptic connections are being established onto and by motor neurons. In mice this occurs between embryonic (E) day 13 and birth (postnatal day 0). Our previous work on mice lacking glycinergic transmission suggested that altered motor neuron activity levels correspondingly regulated motor neuron survival and muscle innervation for all respiratory and non respiratory motor neuron pools, during this period of development [1]. To determine if GABAergic transmission plays a similar role, we quantified motor neuron number and the extent of muscle innervation in four distinct regions of the brain stem and spinal cord; hypoglossal, phrenic, brachial and lumbar motor pools, in mice lacking the enzyme GAD67. These mice display a 90% drop in CNS GABA levels ( [2]; this study). For respiratory-based motor neurons (hypoglossal and phrenic motor pools), we have observed significant drops in motor neuron number (17% decline for hypoglossal and 23% decline for phrenic) and muscle innervations (55% decrease). By contrast for non-respiratory motor neurons of the brachial lateral motor column, we have observed an increase in motor neuron number (43% increase) and muscle innervations (99% increase); however for more caudally located motor neurons within the lumbar lateral motor column, we observed no change in either neuron number or muscle innervation. These results show in mice lacking physiological levels of GABA, there are distinct regional changes in motor neuron number and muscle innervation, which appear to be linked to their physiological function and to their rostral-caudal position within the developing spinal cord. Our results also suggest that for more caudal (lumbar) regions of the spinal cord, the effect of GABA is less influential on motor neuron development compared to that of glycine.

Synaptic inhibition and γ-aminobutyric acid in the mammalian central nervous system

Signal transmission through synapses connecting two neurons is mediated by release of neurotransmitter from the presynaptic axon terminals and activation of its receptor at the postsynaptic neurons. γ-Aminobutyric acid (GABA), non-protein amino acid formed by decarboxylation of glutamic acid, is a principal neurotransmitter at inhibitory synapses of vertebrate and invertebrate nervous system. On one hand glutamic acid serves as a principal excitatory neurotransmitter. This article reviews GABA researches on; (1) synaptic inhibition by membrane hyperpolarization, (2) exclusive localization in inhibitory neurons, (3) release from inhibitory neurons, (4) excitatory action at developmental stage, (5) phenotype of GABA-deficient mouse produced by gene-targeting, (6) developmental adjustment of neural network and (7) neurological/psychiatric disorder. In the end, GABA functions in simple nervous system and plants, and non-amino acid neurotransmitters were supplemented.

The relationship between Bayesian motor unit number estimation and histological measurements of motor neurons in wild-type and SOD1(G93A) mice

OBJECTIVE

To assess the relationship between Bayesian MUNE and histological motor neuron counts in wild-type mice and in an animal model of ALS.

METHODS

We performed Bayesian MUNE paired with histological counts of motor neurons in the lumbar spinal cord of wild-type mice and transgenic SOD1(G93A) mice that show progressive weakness over time. We evaluated the number of acetylcholine endplates that were innervated by a presynaptic nerve.

RESULTS

In wild-type mice, the motor unit number in the gastrocnemius muscle estimated by Bayesian MUNE was approximately half the number of motor neurons in the region of the spinal cord that contains the cell bodies of the motor neurons supplying the hindlimb crural flexor muscles. In SOD1(G93A) mice, motor neuron numbers declined over time. This was associated with motor endplate denervation at the end-stage of disease.

CONCLUSION

The number of motor neurons in the spinal cord of wild-type mice is proportional to the number of motor units estimated by Bayesian MUNE. In SOD1(G93A) mice, there is a lower number of estimated motor units compared to the number of spinal cord motor neurons at the end-stage of disease, and this is associated with disruption of the neuromuscular junction.

SIGNIFICANCE

Our finding that the Bayesian MUNE method gives estimates of motor unit numbers that are proportional to the numbers of motor neurons in the spinal cord supports the clinical use of Bayesian MUNE in monitoring motor unit loss in ALS patients.

Electrical activity as a developmental regulator in the formation of spinal cord circuits

Spinal cord development is a complex process involving generation of the appropriate number of cells, acquisition of distinctive phenotypes and establishment of functional connections that enable execution of critical functions such as sensation and locomotion. Here we review the basic cellular events occurring during spinal cord development, highlighting studies that demonstrate the roles of electrical activity in this process. We conclude that the participation of different forms of electrical activity is evident from the beginning of spinal cord development and intermingles with other developmental cues and programs to implement dynamic and integrated control of spinal cord function.

Neuregulin-1 potentiates agrin-induced acetylcholine receptor clustering through muscle-specific kinase phosphorylation

At neuromuscular synapses, neural agrin (n-agrin) stabilizes embryonic postsynaptic acetylcholine receptor (AChR) clusters by signalling through the muscle-specific kinase (MuSK) complex. Live imaging of cultured myotubes showed that the formation and disassembly of primitive AChR clusters is a dynamic and reversible process favoured by n-agrin, and possibly other synaptic signals. Neuregulin-1 is a growth factor that can act through muscle ErbB receptor kinases to enhance synaptic gene transcription. Recent studies suggest that neuregulin-1-ErbB signalling can modulate n-agrin-induced AChR clustering independently of its effects on transcription. Here we report that neuregulin-1 increased the size of developing AChR clusters when injected into muscles of embryonic mice. We investigated this phenomenon using cultured myotubes, and found that in the ongoing presence of n-agrin, neuregulin-1 potentiates AChR clustering by increasing the tyrosine phosphorylation of MuSK. This potentiation could be blocked by inhibiting Shp2, a postsynaptic tyrosine phosphatase known to modulate the activity of MuSK. Our results provide new evidence that neuregulin-1 modulates the signaling activity of MuSK and hence might function as a second-order regulator of postsynaptic AChR clustering at the neuromuscular synapse. Thus two classic synaptic signalling systems (neuregulin-1 and n-agrin) converge upon MuSK to regulate postsynaptic differentiation.

Maturation of the GABAergic transmission in normal and pathologic motoneurons

γ-aminobutyric acid (GABA) acting on Cl⁻-permeable ionotropic type A (GABA_A) receptors (GABA_AR) is the major inhibitory neurotransmitter in the adult central nervous system of vertebrates. In immature brain structures, GABA exerts depolarizing effects mostly contributing to the expression of spontaneous activities that are instructive for the construction of neural networks but GABA also acts as a potent trophic factor. In the present paper, we concentrate on brainstem and spinal motoneurons that are largely targeted by GABAergic interneurons, and we bring together data on the switch from excitatory to inhibitory effects of GABA, on the maturation of the GABAergic system and GABA_AR subunits. We finally discuss the role of GABA and its GABA_AR in immature hypoglossal motoneurons of the spastic (SPA) mouse, a model of human hyperekplexic syndrome.

Structure and development of cochlear afferent innervation in mammals

In mammals, sensorineural deafness results from damage to the auditory receptors of the inner ear, the nerve pathways to the brain or the cortical area that receives sound information. In this review, we first focused on the cellular and molecular events taking part to spiral ganglion axon growth, extension to the organ of Corti, and refinement. In the second half, we considered the functional maturation of synaptic contacts between sensory hair cells and their afferent projections. A better understanding of all these processes could open insights into novel therapeutic strategies aimed to re-establish primary connections from sound transducers to the ascending auditory nerve pathways.

A review of the neural mechanisms of action and clinical efficiency of riluzole in treating amyotrophic lateral sclerosis: what have we learned in the last decade?

Amyotrophic lateral sclerosis (ALS) is a devastating and fatal neurodegenerative disease of adults which preferentially attacks the neuromotor system. Riluzole has been used as the only approved treatment for amyotrophic lateral sclerosis since 1995, but its mechanism(s) of action in slowing the progression of this disease remain obscure. Searching PubMed for "riluzole" found 705 articles published between January 1996 and June 2009. A systematic review of this literature found that riluzole had a wide range of effects on factors influencing neural activity in general, and the neuromotor system in particular. These effects occurred over a large dose range (<1 μM to >1 mM). Reported neural effects of riluzole included (in approximate ascending order of dose range): inhibition of persistent Na(+) current = inhibition of repetitive firing < potentiation of calcium-dependent K(+) current < inhibition of neurotransmitter release < inhibition of fast Na(+) current < inhibition of voltage-gated Ca(2+) current = promotion of neuronal survival or growth factors < inhibition of voltage-gated K(+) current = modulation of two-pore K(+) current = modulation of ligand-gated neurotransmitter receptors = potentiation of glutamate transporters. Only the first four of these effects commonly occurred at clinically relevant concentrations of riluzole (plasma levels of 1-2 μM with three- to four-fold higher concentrations in brain tissue). Treatment of human ALS patients or transgenic rodent models of ALS with riluzole most commonly produced a modest but significant extension of lifespan. Riluzole treatment was well tolerated in humans and animals. In animals, despite in vitro evidence that riluzole may inhibit rhythmic motor behaviors, in vivo administration of riluzole produced relatively minor effects on normal respiration parameters, but inhibited hypoxia-induced gasping. This effect may have implications for the management of hypoventilation and sleep-disordered breathing during end-stage ALS in humans.

Brain derived neurotrophic factor and neurotrophic factor 3 modulate neurotransmitter receptor expressions on developing spiral ganglion neurons

Cochlear spiral ganglion neurons (SGN) provide the only pathway for transmitting sound evoked activity from the hair cells to the central auditory system. Neurotrophic factor 3 (NT-3) and brain derived neurotrophic factor (BDNF) released from hair cells and supporting cells exert a profound effect on SGN survival and neural firing patterns; however, it is unclear what the effects NT-3 and BDNF have on the type of neurotransmitter receptors expressed on SGN. To address this question, the whole-cell patch clamp recording technique was used to determine what effect NT-3 and BDNF had on the function and expression of glutamate, GABA and glycine receptors (GlyR) on SGN of cochlea from postnatal C57 mouse. Receptor currents induced by the agonist of each receptor were recorded from SGN cultured with or without BDNF or NT-3. NT-3 and BDNF exerted different effects. NT-3, and to a lesser extent BDNF, enhanced the expression of GABA receptors and had comparatively little effect on glutamate receptors. Absence of BDNF and NT-3 resulted in the emergence of glycine-induced currents; however, GlyR currents were absent from the short term cultured SGN. In contrast, NT-3 and BDNF suppressed GlyR expression on SGN. These results indicate that NT-3 and BDNF exert a profound effect on the types of neurotransmitter receptors expressed on postnatal SGN, results that may have important implications for neural development and plasticity.

Selective vulnerability of adult cochlear nucleus neurons to de-afferentation by mechanical compression

It is well established that the cochlear nucleus (CN) of developing species is susceptible to loss of synaptic connections from the auditory periphery. Less information is known about how de-afferentation affects the adult auditory system. We investigated the effects of de-afferentation to the adult CN by mechanical compression. This experimental model is quantifiable and highly reproducible. Five weeks after mechanical compression to the axons of the auditory neurons, the total number of neurons in the CN was evaluated using un-biased stereological methods. A region-specific degeneration of neurons in the dorsal cochlear nucleus (DCN) and posteroventral cochlear nucleus (PVCN) by 50% was found. Degeneration of neurons in the anteroventral cochlear nucleus (AVCN) was not found. An imbalance between excitatory and inhibitory synaptic transmission after de-afferentation may have played a crucial role in the development of neuronal cell demise in the CN. The occurrence of a region-specific loss of adult CN neurons illustrates the importance of evaluating all regions of the CN to investigate the effects of de-afferentation. Thus, this experimental model may be promising to obtain not only the basic knowledge on auditory nerve/CN degeneration but also the information relevant to the application of cochlear or auditory brainstem implants.

The complement factor C5a contributes to pathology in a rat model of amyotrophic lateral sclerosis

Complement activation products are elevated in the cerebrospinal fluid and spinal cord of patients with amyotrophic lateral sclerosis (ALS). In this study, we demonstrate complement system involvement in a rodent model of ALS (human SOD1(G93A) transgenic rats). With end-stage disease, SOD1(G93A) rats displayed marked deposition of C3/C3b, and a significant up-regulation of the C5aR in the lumbar spinal cord. This was associated with increased numbers of C5aR-positive astrocytes. However, expression of C5L2, the alternative receptor for C5a, was highest on motor neurons early in the disease process. To determine the contribution of C5a to the pathology displayed by this model of ALS, rats were administered an orally active, selective C5aR antagonist (PMX205; 1 mg/kg/day, oral). Animals treated with PMX205 displayed a significant extension of survival time and a reduction in end-stage motor scores, as compared with vehicle-treated rats. PMX205-treated animals also displayed reduced levels of astroglial proliferation in the lumbar spinal cord. This study provides the first demonstration of an involvement of C5a in an ALS model and suggests that inhibitors of complement activation could be beneficial in the treatment of this neurodegenerative disease.

Neonatal neuronal circuitry shows hyperexcitable disturbance in a mouse model of the adult-onset neurodegenerative disease amyotrophic lateral sclerosis

Distinguishing the primary from secondary effects and compensatory mechanisms is of crucial importance in understanding adult-onset neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). Transgenic mice that overexpress the G93A mutation of the human Cu-Zn superoxide dismutase 1 gene (hSOD1(G93A) mice) are a commonly used animal model of ALS. Whole-cell patch-clamp recordings from neurons in acute slice preparations from neonatal wild-type and hSOD1(G93A) mice were made to characterize functional changes in neuronal activity. Hypoglossal motoneurons (HMs) in postnatal day 4 (P4)-P10 hSOD1(G93A) mice displayed hyperexcitability, increased persistent Na(+) current (PC(Na)), and enhanced frequency of spontaneous excitatory and inhibitory transmission, compared with wild-type mice. These functional changes in neuronal activity are the earliest yet reported for the hSOD1(G93A) mouse, and are present 2-3 months before motoneuron degeneration and clinical symptoms appear in these mice. Changes in neuronal activity were not restricted to motoneurons: superior colliculus interneurons also displayed hyperexcitability and synaptic changes (P10-P12). Furthermore, in vivo viral-mediated GFP (green fluorescent protein) overexpression in hSOD1(G93A) HMs revealed precocious dendritic remodeling, and behavioral assays revealed transient neonatal neuromotor deficits compared with controls. These findings underscore the widespread and early onset of abnormal neural activity in this mouse model of the adult neurodegenerative disease ALS, and suggest that suppression of PC(Na) and hyperexcitability early in life might be one way to mitigate or prevent cell death in the adult CNS.

NMDA receptor blockade maintains correlated motor neuron firing and delays synapse competition at developing neuromuscular junctions

Mammalian neuromuscular synapses undergo an activity-dependent competitive transition from multiple to single innervation during postnatal life. The presence of temporally correlated motor neuron activity, which, in part, is controlled by gap junctional coupling within the spinal cord, appears to modulate synapse elimination. Postnatal injection of dizocilpine maleate (MK801), a specific NMDA antagonist, has been shown to maintain gap junctional coupling among motor neurons. Thus, we tested the hypothesis that MK801 would maintain correlated motor neuron activity and delay postnatal synapse elimination. Temporally correlated motor neuron activity, which is normally lost during the second postnatal week, was maintained and synaptic competition was delayed by several days in 2-week-old mice injected daily with MK801. MK801 appears to modulate motor neuron activity patterns through enhancing mRNA expression of multiple connexins within the spinal cord and delaying motor neuron growth. Our results suggest that MK801 injection preserves correlated neural activity via both synaptic mechanisms and maintenance of gap junctional coupling among neurons within the spinal cord, ultimately delaying synapse elimination.

Pathological apoptosis in the developing brain

More than half of the initially-formed neurons are deleted in certain brain regions during normal development. This process, whereby cells are discretely removed without interfering with the further development of remaining cells, is called programmed cell death (PCD). The term apoptosis is used to describe certain morphological manifestations of PCD. Many of the effectors of this developmental cell death program are highly expressed in the developing brain, making it more susceptible to accidental activation of the death machinery, e.g. following hypoxia-ischemia or irradiation. Recent evidence suggests, however, that activation and regulation of cell death mechanisms under pathological conditions do not exactly mirror physiological, developmentally regulated PCD. It may be argued that the conditions after e.g. ischemia are not even compatible with the execution of PCD as we know it. Under pathological conditions cells are exposed to various stressors, including energy failure, oxidative stress and unbalanced ion fluxes. This results in parallel triggering and potential overshooting of several different cell death pathways, which then interact with one another and result in complex patterns of biochemical manifestations and cellular morphological features. These types of cell death are here called "pathological apoptosis," where classical hallmarks of PCD, like pyknosis, nuclear condensation and caspase-3 activation, are combined with non-PCD features of cell death. Here we review our current knowledge of the mechanisms involved, with special focus on the potential for therapeutic intervention tailored to the needs of the developing brain.

Respiratory activity in brainstem of fetal mice lacking glutamate decarboxylase 65/67 and vesicular GABA transporter

The respiratory neural network in the mammalian medulla oblongata shows rhythmic activity before birth. GABA and glycine are considered to be involved in control of respiratory rhythm. Recently we have demonstrated respiratory failure in glutamic acid decarboxylase (GAD) 67-deficient mice [Tsunekawa N, Arata A, Obata K (2005) Development of spontaneous mouth/tongue movement and related neural activity, and their repression in mouse fetus lacking glutamate decarboxylase 67. Eur J Neurosci 21:173-178]. To further evaluate the involvement of GABA and glycine in fetal respiratory function, we studied neural activities in brainstem-spinal cord blocks prepared from GAD65-/-:67-/- and vesicular GABA transporter (VGAT)-/-mice on embryonic day 14 (E14)-E15 and E18. In these knockout mice, the synthesis of GABA and the vesicular release of GABA and glycine are completely absent, respectively. Spontaneous respiratory discharges were observed in the ventral roots at the cervical cord (C) 4 level from wild-type mice but not from the knockout mice on E18. Administration of substance P induced C4 discharges in GAD65-/-:67-/- preparations but not in VGAT-/- preparations. C4 discharges were observed in the knockout mice on E14-E15, although the frequency was lower than that in the wild-type. Neuronal activities in the respiratory network of the E18 brainstem were recorded using a "blind" patch-clamp technique. Expiratory and inspiratory neurons with their characteristic firing patterns were observed in the wild-type fetuses. Strychnine reversed inspiratory-phase hyperpolarization to large depolarization in expiratory neurons. On the other hand, neurons in the same area of the knockout mice fired spontaneously without any rhythm. Substance P induced hyperpolarizing potentials in medullary neurons of GAD65-/-:67-/- mice. Further administration of strychnine induced large depolarizing potentials. Rhythmic activities were not observed in VGAT-/- mice even in the presence of substance P and strychnine. These results indicate that the lack of GABA and glycine impairs the function of the respiratory network in mouse fetuses and the impairment progresses with fetal age.

Development of spontaneous miniature EPSCs in mouse AVCN neurons during a critical period of afferent-dependent neuron survival

During a critical period prior to hearing onset, cochlea ablation leads to massive neuronal death in the mouse anteroventral cochlear nucleus (AVCN), where cell survival is believed to depend on glutamatergic input. We investigated the development of spontaneous miniature excitatory postsynaptic currents (mEPSCs) in AVCN neurons using whole cell patch-clamp techniques during [postnatal day 7 (P7)] and after (P14, P21) this critical period. We also examined the effects of unilateral cochlea ablation on mEPSC development. The two main AVCN neuron types, bushy and stellate cells, were distinguished electrophysiologically. Bushy cell mEPSCs became more frequent and faster between P7 and P14/P21 but with little change in amplitude. Dendritic filtering of mEPSCs was not detected as indicated by the lack of correlation between 10 and 90% rise times and decay time constants. Seven days after cochlea ablation at P7 or P14, mEPSCs in surviving bushy cells were similar to controls, except that rise and decay times were positively correlated (R = 0.31 and 0.14 for surgery at P7 and P14, respectively). Consistent with this evidence for a shift of synaptic activity from the somata to the dendrites, SV2 staining (a synaptic vesicle marker) forms a ring around somata of control but not experimental bushy cells. In contrast, mEPSCs of stellate cells showed few significant changes over these ages with or without cochlea ablation. Taken together, mEPSCs in mouse AVCN bushy cells show dramatic developmental changes across this critical period, and cochlea ablation may lead to the emergence of excitatory synaptic inputs impinging on bushy cell dendrites.

Glycine receptors regulate interneuron differentiation during spinal network development

Glycinergic and GABAergic excitatory chloride-mediated signaling is often the first form of activity to emerge in the nascent nervous system and has been proposed to be essential for several aspects of nervous system development. However, few studies have examined the effects of disrupting glycinergic transmission. Here we perturbed glycinergic transmission in vivo from the onset of development in zebrafish and examined its impact on the formation of the locomotor circuitry. Targeted knockdown of the embryonic glycine receptor alpha2-subunit disrupted rhythm-generating networks and reduced the frequency of spontaneous glycinergic and glutamatergic events. Immunohistochemistry revealed a reduction in the number of spinal interneurons without affecting sensory and motor neurons. This effect was accompanied by a concomitant increase in the number of mitotic cells, suggesting that glycine receptors regulate interneuron differentiation during early development. Despite the loss of many interneurons, a subthreshold rhythm-generating circuit was still capable of forming. These data provide evidence that glycine receptors, in addition to their role in neurotransmission, regulate interneuron differentiation during development of this central neural network.

Relevance of motoneuron specification and programmed cell death in embryos to therapy of ALS

The molecular cues that generate spinal motoneurons in early embryonic development are well defined. Motoneurons are generated in excess and consequently undergo a natural period of programmed cell death. Although it is not known exactly how motoneurons compete for survival in embryonic development, it is hypothesized that they rely on the ability to access limited amounts of trophic factors from peripheral tissues, a process that is tightly regulated by skeletal muscle activity. Attempts to elucidate the molecular mechanisms that underlie motoneuron generation and programmed cell death in embryos have led to various effective strategies for treating injury and disease in animal models. Such studies provide great hope for the amelioration of human amyotrophic lateral sclerosis (ALS), a devastating progressive motoneuron degenerative disease. Here we review the clinical relevance of studying motoneuron specification and death during embryonic development.