Inhibitory synaptic regulation of motoneurons: a new target of disease mechanisms in amyotrophic lateral sclerosis

F-waves responses derived from low-intensity electrical stimulation: A method to explore split-hand pathogenesis

OBJECTIVES

The "split-hand syndrome" is a common clinical sign in amyotrophic lateral sclerosis (ALS), being characterized by more severe atrophy of the hand muscles on the radial side of the hand compared to the ulnar side. We aimed to investigate possible physiological differences between relevant hand muscles using low-intensity F-wave stimulation to assess spinal motoneuron excitability.

METHODS

We recruited 36 healthy volunteers. F-waves were recorded from the abductor pollicis brevis (APB), first dorsal interosseous (FDI) and abductor digiti minimi (ADM), using 20 supramaximal stimuli followed by 20 stimuli at a low-intensity required to obtain M-waves with 10 % amplitude of maximal CMAP. We evaluated the following F-wave parameters: F-M latency, chronodispersion, persistence, amplitude, F/CMAP amplitude ratio and number of F-wave repeaters (with low-intensity). In 10 subjects, low-intensity stimulation F-waves were compared after 20 and 50 stimuli in each muscle.

RESULTS

Low-intensity stimulation resulted in lower F-wave amplitude and persistence and higher F/CMAP amplitude ratios. There were no significant differences in F-wave latencies and chronodispersion. When comparing the three muscles, we found higher F-wave persistence and F/CMAP amplitude ratios when recording over the ADM and APB compared to the FDI. We also found a higher number of F-wave repeaters in the ADM with low-intensity stimulation. Results from 20 to 50 low-intensity stimuli were similar.

DISCUSSION

A small number of low-intensity stimuli is appropriate to study F-wave latencies and chronodispersion. We found differences in some physiological properties of the ADM spinal motoneuron pool compared to other hand muscles.

Potential contribution of spinal interneurons to the etiopathogenesis of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) consists of a group of adult-onset fatal and incurable neurodegenerative disorders characterized by the progressive death of motor neurons (MNs) throughout the central nervous system (CNS). At first, ALS was considered to be an MN disease, caused by cell-autonomous mechanisms acting specifically in MNs. Accordingly, data from ALS patients and ALS animal models revealed alterations in excitability in multiple neuronal populations, including MNs, which were associated with a variety of cellular perturbations such as protein aggregation, ribonucleic acid (RNA) metabolism defects, calcium dyshomeostasis, modified electrophysiological properties, and autophagy malfunctions. However, experimental evidence rapidly demonstrated the involvement of other types of cells, including glial cells, in the etiopathogenesis of ALS through non-cell autonomous mechanisms. Surprisingly, the contribution of pre-motor interneurons (INs), which regulate MN activity and could therefore critically modulate their excitability at the onset or during the progression of the disease, has to date been severely underestimated. In this article, we review in detail how spinal pre-motor INs are affected in ALS and their possible involvement in the etiopathogenesis of the disease.

Preferential potentiation of AMPA-mediated currents in brainstem hypoglossal motoneurons by subchronic exposure of mice expressing the human superoxide dismutase 1 G93A gene mutation to neurotoxicant methylmercury in vivo

The exact causes of Amyotrophic lateral sclerosis (ALS), a progressive and fatal neurological disorder due to loss of upper and/or lower motoneurons, remain elusive. Gene-environment interactions are believed to be an important factor in the development of ALS. We previously showed that in vivo exposure of mice overexpressing the human superoxide dismutase 1 (hSOD1) gene mutation (hSOD1G93A; G93A), a mouse model for ALS, to environmental neurotoxicant methylmercury (MeHg) accelerated the onset of ALS-like phenotype. Here we examined the time-course of effects of MeHg on AMPA receptor (AMPAR)-mediated currents in hypoglossal motoneurons in brainstem slices prepared from G93A, hSOD1wild-type (hWT) and non-carrier WT mice following in vivo exposure to MeHg. Mice were exposed daily to 3 ppm (approximately 0.7 mg/kg/day) MeHg via drinking water beginning at postnatal day 28 (P28) and continued until P47, 64 or 84, then acute brainstem slices were prepared, and spontaneous excitatory postsynaptic currents (sEPSCs) or AMPA-evoked currents were examined using whole cell patch-clamp recording technique. Brainstem slices of untreated littermates were prepared at the same time points to serve as control. MeHg exposure had no significant effect on either sEPSCs or AMPA-evoked currents in slices from hWT or WT mice during any of those exposure time periods under our experimental conditions. MeHg also did not cause any significant effect on sEPSCs or AMPA-currents in G93A hypoglossal motoneurons at P47 and P64. However, at P84, MeHg significantly increased amplitudes of both sEPSCs and AMPA-evoked currents in hypoglossal motineurons from G93A mice (p < 0.05), but not the sEPSC frequency, suggesting a postsynaptic action on AMPARs. MeHg exposure did not cause any significant effect on GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs). Therefore, MeHg exposure in vivo caused differential effects on AMPARs in hypoglossal motoneurons from mice with different genetic backgrounds. MeHg appears to preferentially stimulate the AMPAR-mediated currents in G93A hypoglossal motoneurons in an exposure time-dependent manner, which may contribute to the AMPAR-mediated motoneuron excitotoxicity, thereby facilitating development of ALS-like phenotype.

Potential proteomic alteration in the brain of Tg(SOD1*G93A)1Gur mice: A new pathogenesis insight of amyotrophic lateral sclerosis

The pathogenesis of amyotrophic lateral sclerosis (ALS) remains unclear. The recent studies have suggested that the protein abnormalities could play some important roles in ALS because several protein mutations were found in individuals with this disease. However, proteins that are currently known to be associated with ALS only explain the pathogenesis of this disease in a minority of cases, thus, further screening is needed to identify other ALS-related proteins. In this study, we systematically analyzed and compared the brain proteomic alterations between a mouse model of ALS, the Tg(SOD1*G93A)1Gur model, and wild-type mice using isobaric tags for relative and absolute quantitation (iTRAQ) as well as bioinformatics methods. The results revealed some significant up- and downregulated proteins at the different developmental stages in the ALS-like mice as well as the possibly related cellular components, molecular functions, biological processes, and pathways in the development of ALS. Our results identified some possible proteins that participate in the pathogenesis of ALS as well as the cellular components that are damaged by these proteins, we additionally identified the molecular functions, the biological processes, and the pathways of these proteins as well as the molecules that are associated with these pathways. This study represents an important preliminary investigation of the role of proteomic abnormalities in the pathogenesis of ALS, both in human patients and other animal models. We present some novel findings that may serve as a basis for further investigation of abnormal proteins that are involved in the pathogenesis of ALS.

Transient increase in recurrent inhibition in amyotrophic lateral sclerosis as a putative protection from neurodegeneration

AIM

Adaptive mechanisms in spinal circuits are likely involved in homeostatic responses to maintain motor output in amyotrophic lateral sclerosis. Given the role of Renshaw cells in regulating the motoneuron input/output gain, we investigated the modulation of heteronymous recurrent inhibition.

METHODS

Electrical stimulations were used to activate recurrent collaterals resulting in the Hoffmann reflex depression. Inhibitions from soleus motor axons to quadriceps motoneurons, and vice versa, were tested in 38 patients and matched group of 42 controls.

RESULTS

Compared with controls, the mean depression of quadriceps reflex was larger in patients, while that of soleus was smaller, suggesting that heteronymous recurrent inhibition was enhanced in quadriceps but reduced in soleus. The modulation of recurrent inhibition was linked to the size of maximal direct motor response and lower limb dysfunctions, suggesting a significant relationship with the integrity of the target motoneuron pool and functional abilities. No significant link was found between the integrity of motor axons activating Renshaw cells and the level of inhibition. Enhanced inhibition was particularly observed in patients within the first year after symptom onset and with slow progression of lower limb dysfunctions. Normal or reduced inhibitions were mainly observed in patients with motor weakness first in lower limbs and greater dysfunctions in lower limbs.

CONCLUSION

We provide the first evidence for enhanced recurrent inhibition and speculate that Renshaw cells might have transient protective role on motoneuron by counteracting hyperexcitability at early stages. Several mechanisms likely participate including cortical influence on Renshaw cell and reinnervation by slow motoneurons.

Emerging Mechanisms Underpinning Neurophysiological Impairments in C9ORF72 Repeat Expansion-Mediated Amyotrophic Lateral Sclerosis/Frontotemporal Dementia

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are characterized by degeneration of upper and lower motor neurons and neurons of the prefrontal cortex. The emergence of the C9ORF72 hexanucleotide repeat expansion mutation as the leading genetic cause of ALS and FTD has led to a progressive understanding of the multiple cellular pathways leading to neuronal degeneration. Disturbances in neuronal function represent a major subset of these mechanisms and because such functional perturbations precede degeneration, it is likely that impaired neuronal function in ALS/FTD plays an active role in pathogenesis. This is supported by the fact that ALS/FTD patients consistently present with neurophysiological impairments prior to any apparent degeneration. In this review we summarize how the discovery of the C9ORF72 repeat expansion mutation has contributed to the current understanding of neuronal dysfunction in ALS/FTD. Here, we discuss the impact of the repeat expansion on neuronal function in relation to intrinsic excitability, synaptic, network and ion channel properties, highlighting evidence of conserved and divergent pathophysiological impacts between cortical and motor neurons and the influence of non-neuronal cells. We further highlight the emerging association between these dysfunctional properties with molecular mechanisms of the C9ORF72 mutation that appear to include roles for both, haploinsufficiency of the C9ORF72 protein and aberrantly generated dipeptide repeat protein species. Finally, we suggest that relating key pathological observations in C9ORF72 repeat expansion ALS/FTD patients to the mechanistic impact of the C9ORF72 repeat expansion on neuronal function will lead to an improved understanding of how neurophysiological dysfunction impacts upon pathogenesis.

TDP-43 regulates GAD1 mRNA splicing and GABA signaling in Drosophila CNS

Alterations in the function of the RNA-binding protein TDP-43 are largely associated with the pathogenesis of amyotrophic lateral sclerosis (ALS), a devastating disease of the human motor system that leads to motoneurons degeneration and reduced life expectancy by molecular mechanisms not well known. In our previous work, we found that the expression levels of the glutamic acid decarboxylase enzyme (GAD1), responsible for converting glutamate to γ-aminobutyric acid (GABA), were downregulated in TBPH-null flies and motoneurons derived from ALS patients carrying mutations in TDP-43, suggesting that defects in the regulation of GAD1 may lead to neurodegeneration by affecting neurotransmitter balance. In this study, we observed that TBPH was required for the regulation of GAD1 pre-mRNA splicing and the levels of GABA in the Drosophila central nervous system (CNS). Interestingly, we discovered that pharmacological treatments aimed to potentiate GABA neurotransmission were able to revert locomotion deficiencies in TBPH-minus flies, revealing novel mechanisms and therapeutic strategies in ALS.

Brain Stimulation as a Therapeutic Tool in Amyotrophic Lateral Sclerosis: Current Status and Interaction With Mechanisms of Altered Cortical Excitability

In the last 20 years, several modalities of neuromodulation, mainly based on non-invasive brain stimulation (NIBS) techniques, have been tested as a non-pharmacological therapeutic approach to slow disease progression in amyotrophic lateral sclerosis (ALS). In both sporadic and familial ALS cases, neurophysiological studies point to motor cortical hyperexcitability as a possible priming factor in neurodegeneration, likely related to dysfunction of both excitatory and inhibitory mechanisms. A trans-synaptic anterograde mechanism of excitotoxicity is thus postulated, causing upper and lower motor neuron degeneration. Specifically, motor neuron hyperexcitability and hyperactivity are attributed to intrinsic cell abnormalities related to altered ion homeostasis and to impaired glutamate and gamma aminobutyric acid gamma-aminobutyric acid (GABA) signaling. Several neuropathological mechanisms support excitatory and synaptic dysfunction in ALS; additionally, hyperexcitability seems to drive DNA-binding protein 43-kDA (TDP-43) pathology, through the upregulation of unusual isoforms directly contributing to ASL pathophysiology. Corticospinal excitability can be suppressed or enhanced using NIBS techniques, namely, repetitive transcranial magnetic stimulation (rTMS) and transcranial direct current stimulation (tDCS), as well as invasive brain and spinal stimulation. Experimental evidence supports the hypothesis that the after-effects of NIBS are mediated by long-term potentiation (LTP)-/long-term depression (LTD)-like mechanisms of modulation of synaptic activity, with different biological and physiological mechanisms underlying the effects of tDCS and rTMS and, possibly, of different rTMS protocols. This potential has led to several small trials testing different stimulation interventions to antagonize excitotoxicity in ALS. Overall, these studies suggest a possible efficacy of neuromodulation in determining a slight reduction of disease progression, related to the type, duration, and frequency of treatment, but current evidence remains preliminary. Main limitations are the small number and heterogeneity of recruited patients, the limited "dosage" of brain stimulation that can be delivered in the hospital setting, the lack of a sufficient knowledge on the excitatory and inhibitory mechanisms targeted by specific stimulation interventions, and the persistent uncertainty on the key pathophysiological processes leading to motor neuron loss. The present review article provides an update on the state of the art of neuromodulation in ALS and a critical appraisal of the rationale for the application/optimization of brain stimulation interventions, in the light of their interaction with ALS pathophysiological mechanisms.

BMAA and MCLR Interact to Modulate Behavior and Exacerbate Molecular Changes Related to Neurodegeneration in Larval Zebrafish

Exposure to toxins produced by cyanobacteria (ie, cyanotoxins) is an emerging health concern due to their increasing prevalence and previous associations with neurodegenerative diseases including amyotrophic lateral sclerosis. The objective of this study was to evaluate the neurotoxic effects of a mixture of two co-occurring cyanotoxins, β-methylamino-l-alanine (BMAA) and microcystin leucine and arginine (MCLR), using the larval zebrafish model. We combined high-throughput behavior-based toxicity assays with discovery proteomic techniques to identify behavioral and molecular changes following 6 days of exposure. Although neither toxin caused mortality, morphological defects, nor altered general locomotor behavior in zebrafish larvae, both toxins increased acoustic startle sensitivity in a dose-dependent manner by at least 40% (p < .0001). Furthermore, startle sensitivity was enhanced by an additional 40% in larvae exposed to the BMAA/MCLR mixture relative to those exposed to the individual toxins. Supporting these behavioral results, our proteomic analysis revealed a 4-fold increase in the number of differentially expressed proteins in the mixture-exposed group. Additionally, prediction analysis reveals activation and/or inhibition of 8 enriched canonical pathways (enrichment p-value < .01; z-score≥|2|), including ILK, Rho Family GTPase, RhoGDI, and calcium signaling pathways, which have been implicated in neurodegeneration. We also found that expression of TDP-43, of which cytoplasmic aggregates are a hallmark of amyotrophic lateral sclerosis pathology, was significantly upregulated by 5.7-fold following BMAA/MCLR mixture exposure. Together, our results emphasize the importance of including mixtures of cyanotoxins when investigating the link between environmental cyanotoxins and neurodegeneration as we reveal that BMAA and MCLR interact in vivo to enhance neurotoxicity.

Absence of Subcerebral Projection Neurons Is Beneficial in a Mouse Model of Amyotrophic Lateral Sclerosis

OBJECTIVE

Recent studies carried out on amyotrophic lateral sclerosis patients suggest that the disease might initiate in the motor cortex and spread to its targets along the corticofugal tracts. In this study, we aimed to test the corticofugal hypothesis of amyotrophic lateral sclerosis experimentally.

METHODS

Sod1G86R and Fezf2 knockout mouse lines were crossed to generate a model that expresses a mutant of the murine Sod1 gene ubiquitously, a condition sufficient to induce progressive motor symptoms and premature death, but genetically lacks corticospinal neurons and other subcerebral projection neurons, one of the main populations of corticofugal neurons. Disease onset and survival were recorded, and weight and motor behavior were followed longitudinally. Hyper-reflexia and spasticity were monitored using electromyographic recordings. Neurodegeneration and gliosis were assessed by histological techniques.

RESULTS

Absence of subcerebral projection neurons delayed disease onset, reduced weight loss and motor impairment, and increased survival without modifying disease duration. Absence of corticospinal neurons also limited presymptomatic hyper-reflexia, a typical component of the upper motoneuron syndrome.

INTERPRETATION

Major corticofugal tracts are crucial to the onset and progression of amyotrophic lateral sclerosis. In the context of the disease, subcerebral projection neurons might carry detrimental signals to their downstream targets. In its entirety, this study provides the first experimental arguments in favor of the corticofugal hypothesis of amyotrophic lateral sclerosis. ANN NEUROL 2020;88:688-702.

Motoneuronal Spinal Circuits in Degenerative Motoneuron Disease

The most evident phenotype of degenerative motoneuron disease is the loss of motor function which accompanies motoneuron death. In both amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA), it is now clear that dysfunction is not restricted to motoneurons but is manifest in the spinal circuits in which motoneurons are embedded. As mounting evidence shows that motoneurons possess more elaborate and extensive connections within the spinal cord than previously realized, it is necessary to consider the role of this circuitry and its dysfunction in the disease process. In this review article, we ask if the selective vulnerability of the different motoneuron types and the relative disease resistance of distinct motoneuron groups can be understood in terms of their intraspinal connections.

Interrogating interneurone function using threshold tracking of the H reflex in healthy subjects and patients with motor neurone disease

OBJECTIVE

The excitability of the lower motoneurone pool is traditionally tested using the H reflex and a constant-stimulus paradigm, which measures changes in the amplitude of the reflex response. This technique has limitations because reflex responses of different size must involve the recruitment or inhibition of different motoneurones. The threshold-tracking technique ensures that the changes in excitability occur for an identical population of motoneurones. We aimed to assess this technique and then apply it in patients with motor neurone disease (MND).

METHODS

The threshold-tracking approach was assessed in 17 healthy subjects and 11 patients with MND. The soleus H reflex was conditioned by deep peroneal nerve stimulation producing reciprocal Ia and so-called D1 and D2 inhibitions, which are believed to reflect presynaptic inhibition of soleus Ia afferents.

RESULTS

Threshold tracking was quicker than the constant-stimulus technique and reliable, properties that may be advantageous for clinical studies. D1 inhibition was significantly reduced in patients with MND.

CONCLUSIONS

Threshold tracking is useful and may be preferable under some conditions for studying the excitability of the motoneurone pool. The decreased D1 inhibition in the patients suggests that presynaptic inhibition may be reduced in MND.

SIGNIFICANCE

Reduced presynaptic inhibition could be evidence of an interneuronopathy in MND. It is possible that the hyperreflexia is a spinal pre-motoneuronal disorder, and not definitive evidence of corticospinal involvement in MND.

5-HT1A Receptor Agonist Promotes Retinal Ganglion Cell Function by Inhibiting OFF-Type Presynaptic Glutamatergic Activity in a Chronic Glaucoma Model

Serotonin receptors are potential neuroprotective agents in degenerative diseases of the central nervous system. The protective effects of serotonin receptor (5-HT1A) agonists on the survival and function of retinal ganglion cells (RGCs) by regulating the release of the presynaptic neurotransmitter γ-aminobutyric acid (GABA) were confirmed in our previous study of a chronic glaucoma rat model. However, the roles of excitatory amino acids and their interactions with the 5-HT1A receptor in glaucoma remain unknown. Here, we found that ocular hypertension increased glutamine synthetase (GS) and excitatory amino acid transporter 2 (EAAT2) expression in rat retinas. In addition, the high expression of GS and EAAT2 induced by glaucoma was downregulated by the 5-HT1A receptor agonist 8-OH-DPAT and the 5-HT1A receptor antagonist WAY-100635, respectively. Patch-clamp techniques were used to record glutamate receptor-mediated spontaneous and miniature glutamatergic excitatory post-synaptic currents (sEPSCs and mEPSCs) as well as L-glutamate-induced current in OFF-type and ON-type RGCs in rat retinal slices. Although there were no significant differences in the frequency and amplitude of sEPSC and mEPSC release between normal and glaucoma OFF- and ON-type RGCs, exogenous 8-OH-DPAT administration specifically reduced the frequency, but not the amplitude, of sEPSC and mEPSC release in glaucoma OFF-type rather than ON-type RGCs; these effects were completely blocked by WAY-100635. In summary, 8-OH-DPAT decreases and increases GS and EAAT2 expression of glaucomatous retina, respectively, while decreasing sEPSC and mEPSC frequency. In contrast, WAY-100635 increases and decreases GS and EAAT2 expression of glaucomatous retina, respectively, while increasing sEPSC and mEPSC frequency. The reduction of glutamatergic presynaptic transmission by 8-OH-DPAT deactivates RGCs at the neural network level and reduces the excitotoxic damage in the pathological process of chronic glaucoma.

Dynamic interplay between H-current and M-current controls motoneuron hyperexcitability in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a type of motor neuron disease (MND) in which humans lose motor functions due to progressive loss of motoneurons in the cortex, brainstem, and spinal cord. In patients and in animal models of MND it has been observed that there is a change in the properties of motoneurons, termed neuronal hyperexcitability, which is an exaggerated response of the neurons to a stimulus. Previous studies suggested neuronal excitability is one of the leading causes for neuronal loss, however the factors that instigate excitability in neurons over the course of disease onset and progression are not well understood, as these studies have looked mainly at embryonic or early postnatal stages (pre-symptomatic). As hyperexcitability is not a static phenomenon, the aim of this study was to assess the overall excitability of upper motoneurons during disease progression, specifically focusing on their oscillatory behavior and capabilities to fire repetitively. Our results suggest that increases in the intrinsic excitability of motoneurons are a global phenomenon of aging, however the cellular mechanisms that underlie this hyperexcitability are distinct in SOD1G93A ALS mice compared with wild-type controls. The ionic mechanism driving increased excitability involves alterations of the expression levels of HCN and KCNQ channel genes leading to a complex dynamic of H-current and M-current activation. Moreover, we show a negative correlation between the disease onset and disease progression, which correlates with a decrease in the expression level of HCN and KCNQ channels. These findings provide a potential explanation for the increased vulnerability of motoneurons to ALS with aging.

Mechanisms of compensatory plasticity for respiratory motor neuron death

Respiratory motor neuron death arises from multiple neurodegenerative and traumatic neuromuscular disorders. Despite motor neuron death, compensatory mechanisms minimize its functional impact by harnessing intrinsic mechanisms of compensatory respiratory plasticity. However, the capacity for compensation eventually reaches limits and pathology ensues. Initially, challenges to the system such as increased metabolic demand reveal sub-clinical pathology. With greater motor neuron loss, the eventual result is de-compensation, ventilatory failure, ventilator dependence and then death. In this brief review, we discuss recent advances in our understanding of mechanisms giving rise to compensatory respiratory plasticity in response to respiratory motor neuron death including: 1) increased central respiratory drive, 2) plasticity in synapses on spared phrenic motor neurons, 3) enhanced neuromuscular transmission and 4) shifts in respiratory muscle utilization from more affected to less affected motor pools. Some of these compensatory mechanisms may prolong breathing function, but hasten the demise of surviving motor neurons. Improved understanding of these mechanisms and their impact on survival of spared motor neurons will guide future efforts to develop therapeutic interventions that preserve respiratory function with neuromuscular injury/disease.

Neurointegrity and neurophysiology: astrocyte, glutamate, and carbon monoxide interactions

Astrocyte contributions to brain function and prevention of neuropathologies are as extensive as that of neurons. Astroglial regulation of glutamate, a primary neurotransmitter, is through uptake, release through vesicular and non-vesicular pathways, and catabolism to intermediates. Homeostasis by astrocytes is considered to be of primary importance in determining normal central nervous system health and central nervous system physiology - glutamate is central to dynamic physiologic changes and central nervous system stability. Gasotransmitters may affect diverse glutamate interactions positively or negatively. The effect of carbon monoxide, an intrinsic central nervous system gasotransmitter, in the complex astrocyte homeostasis of glutamate may offer insights to normal brain development, protection, and its use as a neuromodulator and neurotherapeutic. In this article, we will review the effects of carbon monoxide on astrocyte homeostasis of glutamate.

Adipose-derived stem cell conditioned medium impacts asymptomatic peripheral neuromuscular denervation in the mutant superoxide dismutase (G93A) transgenic mouse model of amyotrophic lateral sclerosis

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is devastating, leading to paralysis and death. Disease onset begins pre-symptomatically through spinal motor neuron (MN) axon die-back from musculature at ∼47 days of age in the mutant superoxide dismutase 1 (mSOD1G93A) transgenic ALS mouse model. This period may be optimal to assess potential therapies. We previously demonstrated that post-symptomatic adipose-derived stem cell conditioned medium (ASC-CM) treatment is neuroprotective in mSOD1G93A mice. We hypothesized that early disease onset treatment could ameliorate neuromuscular junction (NMJ) disruption.

OBJECTIVE

To determine whether pre-symptom administration of ASC-CM prevents early NMJ disconnection.

METHODS

We confirmed the NMJ denervation time course in mSOD1G93A mice using co-labeling of neurofilament and post-synaptic acetylcholine receptors (AchR) by α-bungarotoxin. We determined whether ASC-CM ameliorates early NMJ loss in mSOD1G93A mice by systemically administering 200μl ASC-CM or vehicle medium daily from post-natal days 35 to 47 and quantifying intact NMJs through co-labeling of neurofilament and synaptophysin with α-bungarotoxin in gastrocnemius muscle.

RESULTS

Intact NMJs were significantly decreased in 47 day old mSOD1G93A mice (p < 0.05), and daily systemic ASC-CM prevented disease-induced NMJ denervation compared to vehicle treated mice (p < 0.05).

CONCLUSIONS

Our results lay the foundation for testing the long-term neurological benefits of systemic ASC-CM therapy in the mSOD1G93A mouse model of 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.

Abnormal cortical brain integration of somatosensory afferents in ALS

OBJECTIVES

Infraclinical sensory alterations have been reported at early stages of amyotrophic lateral sclerosis (ALS). While previous studies mainly focused on early somatosensory evoked potentials (SEPs), late SEPs, which reflect on cortical pathways involved in cognitive-motor functions, are relatively underinvestigated. Early and late SEPs were compared to assess their alterations in ALS.

METHODS

Median and ulnar nerves were electrically stimulated at the wrist, at 9 times the perceptual threshold, in 21 ALS patients without clinical evidence of sensory deficits, and 21 age- and gender-matched controls. SEPs were recorded at the Erb point using surface electrodes and using a needle inserted in the scalp, in front of the primary somatosensory area (with reference electrode on the ear lobe).

RESULTS

Compared to controls, ALS patients showed comparable peripheral (N9) and early cortical component (N20, P25, N30) reductions, while the late cortical components (N60, P100) were more depressed than the early ones.

CONCLUSIONS

The peripheral sensory alteration likely contributed to late SEP depression to a lesser extent than that of early SEPs.

SIGNIFICANCE

Late SEPs may provide new insights on abnormal cortical excitability affecting brain areas involved in cognitive-motor functions.

Protective effects of the neurosteroid allopregnanolone in a mouse model of spontaneous motoneuron degeneration

Amyotrophic lateral sclerosis (ALS) is a devastating disorder characterized by progressive death of motoneurons. The Wobbler (WR) mouse is a preclinical model sharing neuropathological similarities with human ALS. We have shown that progesterone (PROG) prevents the progression of motoneuron degeneration. We now studied if allopregnanolone (ALLO), a reduced metabolite of PROG endowed with gabaergic activity, also prevents WR neuropathology. Sixty-day old WRs remained untreated or received two steroid treatment regimens in order to evaluate the response of several parameters during early or prolonged steroid administration. ALLO was administered s.c. daily for 5days (4mg/kg) or every other day for 32days (3, 3mg/kg), while another group of WRs received a 20mg PROG pellet s.c. for 18 or 60days. ALLO administration to WRs increased ALLO serum levels without changing PROG and 5 alpha dihydroprogesterone (5α-DHP), whereas PROG treatment increased PROG, 5α-DHP and ALLO. Untreated WRs showed higher basal levels of serum 5α-DHP than controls. In the cervical spinal cord we studied markers of oxidative stress or associated to trophic responses. These included nitric oxide synthase (NOS) activity, motoneuron vacuolation, MnSOD immunoreactivity (IR), brain derived neurotrophic factor (BDNF) and TrkB mRNAs, p75 neurotrophin receptor (p75NTR) and, cell survival or death signals such as pAKT and the stress activated kinase JNK. Untreated WRs showed a reduction of MnSOD-IR and BDNF/TrkB mRNAs, associated to high p75NTR in motoneurons, neuronal and glial NOS hyperactivity and neuronal vacuolation. Also, low pAKT, mainly in young WRs, and a high pJNK in the old stage characterized WŔs spinal cord. Except for MnSOD and BDNF, these alterations were prevented by an acute ALLO treatment, while short-term PROG elevated MnSOD. Moreover, after chronic administration both steroids enhanced MnSOD-IR and BDNF mRNA, while attenuated pJNK and NOS in glial cells. Long-term PROG also increased pAKT and reduced neuronal NOS, parameters not modulated by chronic ALLO. Clinically, both steroids improved muscle performance. Thus, ALLO was able to reduce neuropathology in this model. Since high oxidative stress activates p75NTR and pJNK in neurodegeneration, steroid reduction of these molecules may provide adequate neuroprotection. These data yield the first evidence that ALLO, a gabaergic neuroactive steroid, brings neuroprotection in a model of motoneuron degeneration.

Alpha7 nicotinic acetylcholine receptor agonist promotes retinal ganglion cell function via modulating GABAergic presynaptic activity in a chronic glaucomatous model

Alpha-7 nicotinic acetylcholine receptor (α7-nAChR) agonists can prevent glutamate-induced excitotoxicity in cultured retinal ganglion cells (RGCs). However, the neuroprotective effects and the mechanism of action of PNU-282987, an α7-nAChR agonist, in a chronic in vivo rat glaucoma model are poorly understood. We found that elevated intraocular pressure (IOP) downregulated retinal α7-nAChR expression. Electroretinography revealed that the amplitude of the photopic negative response (PhNR) decreased in parallel with the loss of RGCs caused by elevated IOP. PNU-282987 enhanced RGC viability and function and decreased terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive signals in RGCs. Patch-clamp recordings revealed differences in the baseline frequencies and decay times of the miniature GABAergic inhibitory postsynaptic currents (mIPSCs) of RGCs between control and glaucomatous retinal slices. The results of western blotting and immunostaining showed that glutamic acid decarboxylase 65/67 and GABA deficits persisted in glaucomatous retinas and that these deficits were reversed by PNU-282987. Patch-clamp recordings also showed that PNU-282987 significantly increased the frequency and amplitude of the GABAergic mIPSCs of RGCs. The protective effects of PNU-292987 were blocked by intravitreal administration of selective GABA_A receptor antagonists. The modulation of GABAergic synaptic transmission by PNU-282987 causes de-excitation of ganglion cell circuits and suppresses excitotoxic processes.

Sporadic ALS Astrocytes Induce Neuronal Degeneration In Vivo

Astrocytes from familial amyotrophic lateral sclerosis (ALS) patients or transgenic mice are toxic specifically to motor neurons (MNs). It is not known if astrocytes from sporadic ALS (sALS) patients cause MN degeneration in vivo and whether the effect is specific to MNs. By transplanting spinal neural progenitors, derived from sALS and healthy induced pluripotent stem cells (iPSCs), into the cervical spinal cord of adult SCID mice for 9 months, we found that differentiated human astrocytes were present in large areas of the spinal cord, replaced endogenous astrocytes, and contacted neurons to a similar extent. Mice with sALS but not non-ALS cells showed reduced non-MNs numbers followed by MNs in the host spinal cord. The surviving MNs showed reduced inputs from inhibitory neurons and exhibited disorganized neurofilaments and aggregated ubiquitin. Correspondingly, mice with sALS but not non-ALS cells showed declined movement deficits. Thus, sALS iPSC-derived astrocytes cause ALS-like degeneration in both MNs and non-MNs.

Facilitation of distinct inhibitory synaptic inputs by chemical anoxia in neurons in the oculomotor, facial and hypoglossal motor nuclei of the rat

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by the selective loss of motor neurons in the brainstem and spinal cord. Clinical studies have indicated that there is a distinct region-dependent difference in the vulnerability of motor neurons. For example, the motor neurons in the facial and hypoglossal nuclei are more susceptible to neuronal death than those in the oculomotor nucleus. To understand the mechanism underlying the differential susceptibility to cell death of the neurons in different motor nuclei, we compared the effects of chemical anoxia on the membrane currents and postsynaptic currents in different motor nuclei. The membrane currents were recorded from neurons in the oculomotor, facial and hypoglossal nuclei in brain slices of juvenile Wistar rats by using whole-cell recording in the presence of tetrodotoxin that prevents action potential-dependent synaptic transmission. NaCN consistently induced an inward current and a significant increase in the frequency of spontaneous synaptic inputs in neurons from these three nuclei. However, this increase in the synaptic input frequency was abolished by strychnine, a glycine receptor antagonist, but not by picrotoxin in neurons from the hypoglossal and facial nuclei, whereas that in neurons from the oculomotor nucleus was abolished by picrotoxin, but not by strychnine. Blocking ionotropic glutamate receptors did not significantly affect the NaCN-induced release facilitation in any of the three motor nuclei. These results suggest that anoxia selectively facilitates glycine release in the hypoglossal and facial nuclei and GABA release in the oculomotor nucleus. The region-dependent differences in the neurotransmitters involved in the anoxia-triggered release facilitation might provide a basis for the selective vulnerability of motor neurons in the neurodegeneration associated with ALS.

Syphilis may be a confounding factor, not a causative agent, in syphilitic ALS

Based upon a review of published clinical observations regarding syphilitic amyotrophic lateral sclerosis (ALS), I hypothesize that syphilis is actually a confounding factor, not a causative factor, in syphilitic ALS. Moreover, I propose that the successful treatment of ALS symptoms in patients with syphilitic ALS using penicillin G and hydrocortisone is an indirect consequence of the treatment regimen and is not due to the treatment of syphilis. Specifically, I propose that the observed effect is due to the various pharmacological activities of penicillin G ( e.g., a GABA receptor antagonist) and/or the multifaceted pharmacological activity of hydrocortisone. The notion that syphilis may be a confounding factor in syphilitic ALS is highly relevant, as it suggests that treating ALS patients with penicillin G and hydrocortisone-regardless of whether they present with syphilitic ALS or non-syphilitic ALS-may be effective at treating this rapidly progressive, highly devastating disease.

A Recombinant Human Pluripotent Stem Cell Line Stably Expressing Halide-Sensitive YFP-I152L for GABAAR and GlyR-Targeted High-Throughput Drug Screening and Toxicity Testing

GABA_ARs and GlyRs are considered attractive drug targets for therapeutic intervention and are also increasingly recognized in the context of in vitro neurotoxicity (NT) and developmental neurotoxicity (DNT) testing. However, systematic human-specific GABA_AR and GlyR-targeted drug screening and toxicity testing is hampered due to lack of appropriate in vitro models that express native GABA_ARs and GlyRs. We have established a human pluripotent stem cell line (NT2) stably expressing YFP-I152L, a halide-sensitive variant of yellow fluorescent protein (YFP), allowing for fluorescence-based functional analysis of chloride channels. Upon stimulation with retinoic acid, NT2 cells undergo neuronal differentiation and allow pharmacological and toxicological evaluation of native GABA_ARs and GlyRs at different stages of brain maturation. We applied the cell line in concentration-response experiments with the neurotransmitters GABA and glycine as well as with the drugs strychnine, picrotoxin, fipronil, lindane, bicuculline, and zinc and demonstrate that the established in vitro model is applicable to GABA_AR and GlyR-targeted pharmacological and toxicological profiling. We quantified the proportion of GABA_AR and GlyR-sensitive cells, respectively, and identified percentages of approximately 20% each within the overall populations, rendering the cells a suitable model for systematic in vitro GABA_AR and GlyR-targeted screening in the context of drug development and NT/DNT testing.

Altered development in GABA co-release shapes glycinergic synaptic currents in cultured spinal slices of the SOD1(G93A) mouse model of amyotrophic lateral sclerosis

KEY POINTS

Increased environmental risk factors in conjunction with genetic susceptibility have been proposed with respect to the remarkable variations in mortality in amyotrophic lateral sclerosis (ALS). In vitro models allow the investigation of the genetically modified counter-regulator of motoneuron toxicity and may help in addressing ALS therapy. Spinal organotypic slice cultures from a mutant form of human superoxide dismutase 1 (SOD1G93A) mouse model of ALS allow the detection of altered glycinergic inhibition in spinal microcircuits. This altered inhibition improved spinal cord excitability, affecting motor outputs in early SOD1(G93A) pathogenesis.

ABSTRACT

Amyotrophic lateral sclerosis (ALS) is a fatal, adult-onset neurological disease characterized by a progressive degeneration of motoneurons (MNs). In a previous study, we developed organotypic spinal cultures from an ALS mouse model expressing a mutant form of human superoxide dismutase 1 (SOD1(G93A) ). We reported the presence of a significant synaptic rearrangement expressed by these embryonic cultured networks, which may lead to the altered development of spinal synaptic signalling, which is potentially linked to the adult disease phenotype. Recent studies on the same ALS mouse model reported a selective loss of glycinergic innervation in cultured MNs, suggestive of a contribution of synaptic inhibition to MN dysfunction and degeneration. In the present study, we further exploit organotypic cultures from wild-type and SOD1(G93A) mice to investigate the development of glycine-receptor-mediated synaptic currents recorded from the interneurons of the premotor ventral circuits. We performed single cell electrophysiology, immunocytochemistry and confocal microscopy and suggest that GABA co-release may speed the decay of glycine responses altering both temporal precision and signal integration in SOD1(G93A) developing networks at the postsynaptic site. Our hypothesis is supported by the finding of an increased MN bursting activity in immature SOD1(G93A) spinal cords and by immunofluorescence microscopy detection of a longer persistence of GABA in SOD1(G93A) glycinergic terminals in cultured and ex vivo spinal slices.

Investigating the Mechanism by Which Gain-of-function Mutations to the α1 Glycine Receptor Cause Hyperekplexia

Hyperekplexia is a rare human neuromotor disorder caused by mutations that impair the efficacy of glycinergic inhibitory neurotransmission. Loss-of-function mutations in the GLRA1 or GLRB genes, which encode the α1 and β glycine receptor (GlyR) subunits, are the major cause. Paradoxically, gain-of-function GLRA1 mutations also cause hyperekplexia, although the mechanism is unknown. Here we identify two new gain-of-function mutations (I43F and W170S) and characterize these along with known gain-of-function mutations (Q226E, V280M, and R414H) to identify how they cause hyperekplexia. Using artificial synapses, we show that all mutations prolong the decay of inhibitory postsynaptic currents (IPSCs) and induce spontaneous GlyR activation. As these effects may deplete the chloride electrochemical gradient, hyperekplexia could potentially result from reduced glycinergic inhibitory efficacy. However, we consider this unlikely as the depleted chloride gradient should also lead to pain sensitization and to a hyperekplexia phenotype that correlates with mutation severity, neither of which is observed in patients with GLRA1 hyperekplexia mutations. We also rule out small increases in IPSC decay times (as caused by W170S and R414H) as a possible mechanism given that the clinically important drug, tropisetron, significantly increases glycinergic IPSC decay times without causing motor side effects. A recent study on cultured spinal neurons concluded that an elevated intracellular chloride concentration late during development ablates α1β glycinergic synapses but spares GABAergic synapses. As this mechanism satisfies all our considerations, we propose it is primarily responsible for the hyperekplexia phenotype.

Motor Alterations Induced by Chronic 4-Aminopyridine Infusion in the Spinal Cord In vivo: Role of Glutamate and GABA Receptors

Motor neuron (MN) degeneration is the pathological hallmark of MN diseases, a group of neurodegenerative disorders clinically manifested as muscle fasciculations and hyperreflexia, followed by paralysis, respiratory failure, and death. Ample evidence supports a role of glutamate-mediated excitotoxicity in motor death. In previous work we showed that stimulation of glutamate release from nerve endings by perfusion of the K⁺-channel blocker 4-aminopyridine (4-AP) in the rat hippocampus induces seizures and neurodegeneration, and that AMPA infusion in the spinal cord produces paralysis and MN death. On these bases, in this work we have tested the effect of the chronic infusion of 4-AP in the spinal cord, using implanted osmotic minipumps, on motor activity and on MN survival, and the mechanisms underlying this effect. 4-AP produced muscle fasciculations and motor deficits assessed in two motor tests, which start 2–3 h after the implant, which ameliorated spontaneously within 6–7 days, but no neurodegeneration. These effects were prevented by both AMPA and NMDA receptors blockers. The role of GABA_A receptors was also explored, and we found that chronic infusion of bicuculline induced moderate MN degeneration and enhanced the hyperexcitation produced by 4-AP. Unexpectedly, the GABA_AR agonist muscimol also induced motor deficits and failed to prevent the MN death induced by AMPA. We conclude that motor alterations induced by chronic 4-AP infusion in the spinal cord in vivo is due to ionotropic glutamate receptor overactivation and that blockade of GABAergic neurotransmission induces MN death under chronic conditions. These results shed light on the role of glutamatergic and GABAergic neurotransmission in the regulation of MN excitability in the spinal cord.

Neuregulin 1 confers neuroprotection in SOD1-linked amyotrophic lateral sclerosis mice via restoration of C-boutons of spinal motor neurons

Introduction

Increasing evidence implicates the role of the cell types surrounding motor neurons, such as interneurons and glial cells, in non-cell autonomous neurodegeneration of amyotrophic lateral sclerosis (ALS). C-boutons, the large cholinergic synapses that innervate spinal α-motor neurons to control their excitability, are progressively lost from motor neurons in both human ALS and mutant Cu/Zn superoxide dismutase 1 (SOD1)-ALS mice. Neuregulin-1 (NRG1), a trophic factor implicated in neural development, transmission, and synaptic plasticity, has been reported to localize in the synapse of C-boutons. However, the roles of NRG1 in maintenance of motor neuron health and activity, as well as the functional consequences of its alteration in motor neuron disease, are not fully understood.

Results

NRG1 was localized to the post-synaptic face of C-boutons and its expression was significantly lost in SOD1-ALS mice and human ALS patients. Losses of NRG1 expression and C-boutons occured almost contemporaneously in SOD1-ALS mice. In addition, expressions of ErbB3 and ErbB4, receptors for NRG1, were reduced in the motor neurons of SOD1-ALS mice. Furthermore, viral-mediated delivery of type III-NRG1 to the spinal cord restored the number of C-boutons and extended the survival time of SOD1-ALS mice.

Conclusions

These results suggest that maintenance of NRG1-ErbB4/3 axis by supplementation of NRG1 confers neuroprotection in motor neuron disease, partly through the maintenance of C-boutons of spinal motor neurons.

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-016-0286-7) contains supplementary material, which is available to authorized users.

Intraspinal stem cell transplantation for amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disorder in which the loss of upper and lower motor neurons produces progressive weakness and eventually death. In the decades since the approval of riluzole, the only US Food and Drug Administration-approved medication to moderately slow progression of ALS, no new therapeutics have arisen to alter the course of the disease. This is partly due to our incomplete understanding of the complex pathogenesis of motor neuron degeneration. Stem cells have emerged as an attractive option in treating ALS, because they come armed with equally complex cellular machinery and may modulate the local microenvironment in many ways to rescue diseased motor neurons. Various stem cell types are being evaluated in preclinical and early clinical applications; here, we review the preclinical strategies and advances supporting the recent clinical translation of neural progenitor cell therapy for ALS. Specifically, we focus on the use of spinal cord neural progenitor cells and the pipeline starting from preclinical studies to the designs of phase I and IIa clinical trials involving direct intraspinal transplantation in humans.

Presynaptic facilitation of glycinergic mIPSC is reduced in mice lacking α3 glycine receptor subunits

Glycinergic neurons provide an important mechanism to control excitation of motoneurons in the brainstem and a reduction or loss of glycinergic inhibition can be deleterious by leading to hyperexcitation such as in hyperekplexia or neurodegeneration and neuronal death as in amyotrophic lateral sclerosis (ALS). Second messenger systems that change cyclic AMP and lead to phosphorylation of the α3 subunit of the glycine receptor (GlyR α3) have been shown to be potent modulators of synaptic inhibition in the spinal cord and brain stem. In this study we analyzed the role of GlyR α3 in synaptic inhibition to the hypoglossal nucleus using Glra3 (the gene encoding the glycine receptor α3 subunit) knockout mice. We observed that baseline glycinergic synaptic transmission to nucleus of hypoglossal motoneurons is rather normal in Glra3 knockout mice. Interestingly, we found that the modulation of synaptic transmission by cAMP-mediated pathways appeared to be reduced in Glra3 knockout mice. In the second postnatal week the forskolin-induced increase of miniature inhibitory postsynaptic potential (mIPSC) frequency was significantly larger in control as compared to Glra3 knockout mice suggesting that presynaptic glycine release in the hypoglossal nucleus is partially depending on GlyR α3.

ALS Patient Stem Cells for Unveiling Disease Signatures of Motoneuron Susceptibility: Perspectives on the Deadly Mitochondria, ER Stress and Calcium Triad

Amyotrophic lateral sclerosis (ALS) is a largely sporadic progressive neurodegenerative disease affecting upper and lower motoneurons (MNs) whose specific etiology is incompletely understood. Mutations in superoxide dismutase-1 (SOD1), TAR DNA-binding protein 43 (TARDBP/TDP-43) and C9orf72, have been identified in subsets of familial and sporadic patients. Key associated molecular and neuropathological features include ubiquitinated TDP-43 inclusions, stress granules, aggregated dipeptide proteins from mutant C9orf72 transcripts, altered mitochondrial ultrastructure, dysregulated calcium homeostasis, oxidative and endoplasmic reticulum (ER) stress, and an unfolded protein response (UPR). Such impairments have been documented in ALS animal models; however, whether these mechanisms are initiating factors or later consequential events leading to MN vulnerability in ALS patients is debatable. Human induced pluripotent stem cells (iPSCs) are a valuable tool that could resolve this “chicken or egg” causality dilemma. Relevant systems for probing pathophysiologically affected cells from large numbers of ALS patients and discovering phenotypic disease signatures of early MN susceptibility are described. Performing unbiased ‘OMICS and high-throughput screening in relevant neural cells from a cohort of ALS patient iPSCs, and rescuing mitochondrial and ER stress impairments, can identify targeted therapeutics for increasing MN longevity in ALS.

Morphometric and neurochemical alterations found in l-BMAA treated rats

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by progressive muscle paralysis that reflects the motoneurons' degeneration. Several studies support the relationship between β-N-methylamino-l-alanine (l-BMAA), a neurotoxic amino acid produced by cyanobacteria and diatoms, and the sporadic occurrence of ALS and other neurodegenerative diseases. Therefore, the study of its neurotoxicity mechanisms has assumed great relevance in recent years. Recently, our research team has proposed a sporadic ALS animal model by l-BMAA administration in rats, which displays many pathophysiological features of human ALS. In this paper, we deepen the characterization of this model corroborating the occurrence of alterations present in ALS patients such as decreased muscle volume, thinning of the motor cortex, enlarged brain's lateral ventricles, and alteration of both bulbar nuclei and neurotransmitters' levels. Therefore, we conclude that l-BMAA treated rats could be a good model which mimics degenerative features that ALS causes in humans.

Not GABA but glycine mediates segmental, propriospinal, and bulbospinal postsynaptic inhibition in adult mouse spinal forelimb motor neurons

The general view is that both glycine (Eccles, 1964) and GABA (Curtis and Felix, 1971) evoke postsynaptic inhibition in spinal motor neurons. In newborn or juvenile animals, there are conflicting results showing postsynaptic inhibition in motor neurons by corelease of GABA and glycine (Jonas et al., 1998) or by glycine alone (Bhumbra et al., 2012). To resolve the relative contributions of GABA and glycine to postsynaptic inhibition, we performed in vivo intracellular recordings from forelimb motor neurons in adult mice. Postsynaptic potentials evoked from segmental, propriospinal, and bulbospinal systems in motor neurons were compared across four different conditions: control, after gabazine, gabazine followed by strychnine, and strychnine alone. No significant differences were observed in the proportion of IPSPs and EPSPs between control and gabazine conditions. In contrast, EPSPs but not IPSPs were recorded after adding strychnine with gabazine or administering strychnine alone, suggesting an exclusive role for glycine in postsynaptic inhibition. To test whether the injected (intraperitoneal) dose of gabazine blocked GABAergic inhibitory transmission, we evoked GABAA receptor-mediated monosynaptic IPSPs in deep cerebellar nuclei neurons by stimulation of Purkinje cell fibers. No monosynaptic IPSPs could be recorded in the presence of gabazine, showing the efficacy of gabazine treatment. Our results demonstrate that, in the intact adult mouse, the postsynaptic inhibitory effects in spinal motor neurons exerted by three different systems, intrasegmental and intersegmental as well as supraspinal, are exclusively glycinergic. These findings emphasize the importance of glycinergic postsynaptic inhibition in motor neurons and challenge the view that GABA also contributes.

Human immunodeficiency virus-1 Tat protein increases the number of inhibitory synapses between hippocampal neurons in culture

Synaptodendritic damage correlates with cognitive decline in many neurodegenerative diseases, including human immunodeficiency virus-1 (HIV-1)-associated neurocognitive disorders (HAND). Because HIV-1 does not infect neurons, viral-mediated toxicity is indirect, resulting from released neurotoxins such as the HIV-1 protein transactivator of transcription (Tat). We compared the effects of Tat on inhibitory and excitatory synaptic connections between rat hippocampal neurons using an imaging-based assay that quantified clusters of the scaffolding proteins gephyrin or PSD95 fused to GFP. Tat (24 h) increased the number of GFP-gephyrin puncta and decreased the number of PSD95-GFP puncta. The effects of Tat on inhibitory and excitatory synapse number were mediated via the low-density lipoprotein receptor-related protein and subsequent Ca(2+) influx through GluN2A-containing NMDA receptors (NMDARs). The effects of Tat on synapse number required cell-autonomous activation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). Ca(2+) buffering experiments suggested that loss of excitatory synapses required activation of CaMKII in close apposition to the NMDAR, whereas the increase in inhibitory synapses required Ca(2+) diffusion to a more distal site. The increase in inhibitory synapses was prevented by inhibiting the insertion of GABAA receptors into the membrane. Synaptic changes induced by Tat (16 h) were reversed by blocking either GluN2B-containing NMDARs or neuronal nitric oxide synthase, indicating changing roles for pathways activated by NMDAR subtypes during the neurotoxic process. Compensatory changes in the number of inhibitory and excitatory synapses may serve as a novel mechanism to reduce network excitability in the presence of HIV-1 neurotoxins; these changes may inform the development of treatments for HAND.

Mitochondrial permeability transition pore regulates Parkinson's disease development in mutant α-synuclein transgenic mice

Parkinson's disease (PD) is a movement disorder caused by neurodegeneration in neocortex, substantia nigra and brainstem, and synucleinopathy. Some inherited PD is caused by mutations in α-synuclein (αSyn), and inherited and idiopathic PD is associated with mitochondrial perturbations. However, the mechanisms of pathogenesis are unresolved. We characterized a human αSyn transgenic mouse model and tested the hypothesis that the mitochondrial permeability transition pore (mPTP) is involved in the disease mechanisms. C57BL/6 mice expressing human A53T-mutant αSyn driven by a thymic antigen-1 promoter develop a severe, age-related, fatal movement disorder involving ataxia, rigidity, and postural instability. These mice develop synucleinopathy and neocortical, substantia nigra, and cerebello-rubro-thalamic degeneration involving mitochondriopathy and apoptotic and non-apoptotic neurodegeneration. Interneurons undergo apoptotic degeneration in young mice. Mutant αSyn associated with dysmorphic neuronal mitochondria and bound voltage-dependent anion channels. Genetic ablation of cyclophilin D, an mPTP modulator, delayed disease onset, and extended lifespans of mutant αSyn mice. Thus, mutant αSyn transgenic mice on a C57BL/6 background develop PD-like phenotypes, and the mPTP is involved in their disease mechanisms.

Aberrant regulation of DNA methylation in amyotrophic lateral sclerosis: a new target of disease mechanisms

Amyotrophic lateral sclerosis (ALS) is the third most common adult-onset neurodegenerative disease. A diagnosis is fatal owing to degeneration of motor neurons in brain and spinal cord that control swallowing, breathing, and movement. ALS can be inherited, but most cases are not associated with a family history of the disease. The mechanisms causing motor neuron death in ALS are still unknown. Given the suspected complex interplay between multiple genes, the environment, metabolism, and lifestyle in the pathogenesis of ALS, we have hypothesized that the mechanisms of disease in ALS involve epigenetic contributions that can drive motor neuron degeneration. DNA methylation is an epigenetic mechanism for gene regulation engaged by DNA methyltransferase (Dnmt)-catalyzed methyl group transfer to carbon-5 in cytosine residues in gene regulatory promoter and nonpromoter regions. Recent genome-wide analyses have found differential gene methylation in human ALS. Neuropathologic assessments have revealed that motor neurons in human ALS show significant abnormalities in Dnmt1, Dnmt3a, and 5-methylcytosine. Similar changes are seen in mice with motor neuron degeneration, and Dnmt3a was found abundantly at synapses and in mitochondria. During apoptosis of cultured motor neuron-like cells, Dnmt1 and Dnmt3a protein levels increase, and 5-methylcytosine accumulates. Enforced expression of Dnmt3a, but not Dnmt1, induces degeneration of cultured neurons. Truncation mutation of the Dnmt3a catalytic domain and Dnmt3a RNAi blocks apoptosis of cultured neurons. Inhibition of Dnmt catalytic activity with small molecules RG108 and procainamide protects motor neurons from excessive DNA methylation and apoptosis in cell culture and in a mouse model of ALS. Thus, motor neurons can engage epigenetic mechanisms to cause their degeneration, involving Dnmts and increased DNA methylation. Aberrant DNA methylation in vulnerable cells is a new direction for discovering mechanisms of ALS pathogenesis that could be relevant to new disease target identification and therapies for ALS.

Characterization of early pathogenesis in the SOD1(G93A) mouse model of ALS: part II, results and discussion

Pathological events are well characterized in amyotrophic lateral sclerosis (ALS) mouse models, but review of the literature fails to identify a specific initiating event that precipitates disease pathology. There is now growing consensus in the field that axon and synapses are first cellular sites of degeneration, but controversy exists over whether axon and synapse loss is initiated autonomously at those sites or by pathology in the cell body, in nonneuronal cells or even in nonmotoneurons (MNs). Previous studies have identified pathological events in the mutant superoxide dismutase 1 (SOD1) models involving spinal cord, peripheral axons, neuromuscular junctions (NMJs), or muscle; however, few studies have systematically examined pathogenesis at multiple sites in the same study. We have performed ultrastructural examination of both central and peripheral components of the neuromuscular system in the SOD1(G93A) mouse model of ALS. Twenty percent of MNs undergo degeneration by P60, but NMJ innervation in fast fatigable muscles is reduced by 40% by P30. Gait alterations and muscle weakness were also found at P30. There was no change in axonal transport prior to initial NMJ denervation. Mitochondrial morphological changes are observed at P7 and become more prominent with disease progression. At P30 there was a significant decrease in excitatory axo-dendritic and axo-somatic synapses with an increase in C-type axo-somatic synapses. Our study examined early pathology in both peripheral and central neuromuscular system. The muscle denervation is associated with functional motor deficits and begins during the first postnatal month in SOD1(G93A) mice. Physiological dysfunction and pathology in the mitochondria of synapses and MN soma and dendrites occur, and disease onset in these animals begins more than 2 months earlier than originally thought. This information may be valuable for designing preclinical trials that are more likely to impact disease onset and progression.

Early interneuron dysfunction in ALS: insights from a mutant sod1 zebrafish model

OBJECTIVE

To determine, when, how, and which neurons initiate the onset of pathophysiology in amyotrophic lateral sclerosis (ALS) using a transgenic mutant sod1 zebrafish model and identify neuroprotective drugs.

METHODS

Proteinopathies such as ALS involve mutant proteins that misfold and activate the heat shock stress response (HSR). The HSR is indicative of neuronal stress, and we used a fluorescent hsp70-DsRed reporter in our transgenic zebrafish to track neuronal stress and to measure functional changes in neurons and muscle over the course of the disease.

RESULTS

We show that mutant sod1 fish first exhibited the HSR in glycinergic interneurons at 24 hours postfertilization (hpf). By 96 hpf, we observed a significant reduction in spontaneous glycinergic currents induced in spinal motor neurons. The loss of inhibition was followed by increased stress in the motor neurons of symptomatic adults and concurrent morphological changes at the neuromuscular junction (NMJ) indicative of denervation. Riluzole, the only approved ALS drug and apomorphine, an NRF2 activator, reduced the observed early neuronal stress response.

INTERPRETATION

The earliest event in the pathophysiology of ALS in the mutant sod1 zebrafish model involves neuronal stress in inhibitory interneurons, resulting from mutant Sod1 expression. This is followed by a reduction in inhibitory input to motor neurons. The loss of inhibitory input may contribute to the later development of neuronal stress in motor neurons and concurrent inability to maintain the NMJ. Riluzole, the approved drug for use in ALS, modulates neuronal stress in interneurons, indicating a novel mechanism of riluzole action.

Increased neuronal activity fragments the Golgi complex

The Golgi complex is essential for many aspects of cellular function, including trafficking and sorting of membrane and secretory proteins and posttranslational modification by glycosylation. We observed reversible fragmentation of the Golgi complex in cultured hippocampal neurons cultured in hyperexcitable conditions. In addition, Golgi fragmentation was found in cultured neurons with hyperactivity due to prolonged blockade of GABA(A)-mediated inhibition or withdrawal of NMDA receptor antagonism. The interplay between neuronal hyperactivity and Golgi structure established in this study thus reveals a previously uncharacterized impact of neuronal activity on organelle structure. This finding may have important roles in protein processing and trafficking in the Golgi as well as effects on neuronal signaling.