Voltage-Dependent Sodium Channels in Spinal Cord Motor Neurons Display Rapid Recovery From Fast Inactivation in a Mouse Model of Amyotrophic Lateral Sclerosis

Pathophysiology of ion channels in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) stands as the most prevalent and severe form of motor neuron disease, affecting an estimated 2 in 100,000 individuals worldwide. It is characterized by the progressive loss of cortical, brainstem, and spinal motor neurons, ultimately resulting in muscle weakness and death. Although the etiology of ALS remains poorly understood in most cases, the remodelling of ion channels and alteration in neuronal excitability represent a hallmark of the disease, manifesting not only during the symptomatic period but also in the early pre-symptomatic stages. In this review, we delve into these alterations observed in ALS patients and preclinical disease models, and explore their consequences on neuronal activities. Furthermore, we discuss the potential of ion channels as therapeutic targets in the context of ALS.

Nucleoporin Nup358 Downregulation Tunes the Neuronal Excitability in Mouse Cortical Neurons

Nucleoporins (NUPs) are proteins that comprise the nuclear pore complexes (NPCs). The NPC spans the nuclear envelope of a cell and provides a channel through which RNA and proteins move between the nucleus and the cytoplasm and vice versa. NUP and NPC disruptions have a great impact on the pathophysiology of neurodegenerative diseases (NDDs). Although the downregulation of Nup358 leads to a reduction in the scaffold protein ankyrin-G at the axon initial segment (AIS) of mature neurons, the function of Nup358 in the cytoplasm of neurons remains elusive. To investigate whether Nup358 plays any role in neuronal activity, we downregulated Nup358 in non-pathological mouse cortical neurons and measured their active and passive bioelectrical properties. We identified that Nup358 downregulation is able to produce significant modifications of cell-membrane excitability via voltage-gated sodium channel kinetics. Our findings suggest that Nup358 contributes to neuronal excitability through a functional stabilization of the electrical properties of the neuronal membrane. Hypotheses will be discussed regarding the alteration of this active regulation as putatively occurring in the pathophysiology of NDDs.

NAD+ Precursors and Antioxidants for the Treatment of Amyotrophic Lateral Sclerosis

Charcot first described amyotrophic lateral sclerosis (ALS) between 1865 and 1874 as a sporadic adult disease resulting from the idiopathic progressive degeneration of the motor neuronal system, resulting in rapid, progressive, and generalized muscle weakness and atrophy. There is no cure for ALS and no proven therapy to prevent it or reverse its course. There are two drugs specifically approved for the treatment of ALS, riluzol and edaravone, and many others have already been tested or are following clinical trials. However, at the present moment, we still cannot glimpse a true breakthrough in the treatment of this devastating disease. Nevertheless, our understanding of the pathophysiology of ALS is constantly growing. Based on this background, we know that oxidative stress, alterations in the NAD⁺-dependent metabolism and redox status, and abnormal mitochondrial dynamics and function in the motor neurons are at the core of the problem. Thus, different antioxidant molecules or NAD⁺ generators have been proposed for the therapy of ALS. This review analyzes these options not only in light of their use as individual molecules, but with special emphasis on their potential association, and even as part of broader combined multi-therapies.

Slow recovery from the inactivation of voltage-gated sodium channel Nav1.3 in mouse taste receptor cells

Action potentials play an important role in neurotransmitter release in response to taste. Here, I have investigated voltage-gated Na⁺ channels, a primary component of action potentials, in respective cell types of mouse fungiform taste bud cells (TBCs) with in situ whole-cell clamping and single-cell RT-PCR techniques. The cell types of TBCs electrophysiologically examined were determined immunohistochemically using the type III inositol 1,4,5-triphoshate receptor as a type II cell marker and synaptosomal-associated protein 25 as a type III cell marker. I show that type II cells, type III cells, and TBCs not immunoreactive to these markers (likely type I cells) generate voltage-gated Na⁺ currents. The recovery following inactivation of these currents was well fitted with double exponential curves. The time constants in type III cells (~20 ms and ~ 1 s) were significantly slower than respective time constants in other cell types. RT-PCR analysis indicated the expression of Nav1.3, Nav1.5, Nav1.6, and β1 subunit mRNAs in TBCs. Pharmacological inhibition and single-cell RT-PCR studies demonstrated that type II and type III cells principally express tetrodotoxin (TTX)-sensitive Nav1.3 channels and that ~ 30% of type I cells express TTX-resistant Nav1.5 channels. The auxiliary β1 subunit that modulates gating kinetics was rarely detected in TBCs. As the β1 subunit co-expressed with an α subunit is known to accelerate the recovery from inactivation, it is likely that voltage-gated Na⁺ channels in TBCs may function without β subunits. Slow recovery from inactivation, especially in type III cells, may limit high-frequency firing in response to taste substances.

Mislocalisation of TDP-43 to the cytoplasm causes cortical hyperexcitability and reduced excitatory neurotransmission in the motor cortex

Amyotrophic lateral sclerosis (ALS) is a chronic neurodegenerative disease pathologically characterised by mislocalisation of the RNA-binding protein TAR-DNA-binding protein 43 (TDP-43) from the nucleus to the cytoplasm. Changes to neuronal excitability and synapse dysfunction in the motor cortex are early pathological changes occurring in people with ALS and mouse models of disease. To investigate the effect of mislocalised TDP-43 on the function of motor cortex neurons we utilised mouse models that express either human wild-type (TDP-43^WT ) or nuclear localisation sequence-deficient TDP-43 (TDP-43^ΔNLS ) on an inducible promoter that enriches expression to forebrain neurons. Pathophysiology was investigated through immunohistochemistry and whole-cell patch-clamp electrophysiology. Thirty days expression of TDP-43^ΔNLS in adult mice did not cause any changes in the number of CTIP2-positive neurons in the motor cortex. However, at this time-point, the expression of TDP-43^ΔNLS drives intrinsic hyperexcitability in layer V excitatory neurons of the motor cortex. This hyperexcitability occurs concomitantly with a decrease in excitatory synaptic input to these cells and fluctuations in both directions of ionotropic glutamate receptors. This pathophysiology is not present with TDP-43^WT expression, demonstrating that the localisation of TDP-43 to the cytoplasm is crucial for the altered excitability phenotype. This study has important implications for the mechanisms of toxicity of one of the most notorious proteins linked to ALS, TDP-43. We provide the first evidence that TDP-43 mislocalisation causes aberrant synaptic function and a hyperexcitability phenotype in the motor cortex, linking some of the earliest dysfunctions to arise in people with ALS to mislocalisation of TDP-43.

Human genetics and neuropathology suggest a link between miR-218 and amyotrophic lateral sclerosis pathophysiology

Motor neuron-specific microRNA-218 (miR-218) has recently received attention because of its roles in mouse development. However, miR-218 relevance to human motor neuron disease was not yet explored. Here, we demonstrate by neuropathology that miR-218 is abundant in healthy human motor neurons. However, in amyotrophic lateral sclerosis (ALS) motor neurons, miR-218 is down-regulated and its mRNA targets are reciprocally up-regulated (derepressed). We further identify the potassium channel Kv10.1 as a new miR-218 direct target that controls neuronal activity. In addition, we screened thousands of ALS genomes and identified six rare variants in the human miR-218-2 sequence. miR-218 gene variants fail to regulate neuron activity, suggesting the importance of this small endogenous RNA for neuronal robustness. The underlying mechanisms involve inhibition of miR-218 biogenesis and reduced processing by DICER. Therefore, miR-218 activity in motor neurons may be susceptible to failure in human ALS, suggesting that miR-218 may be a potential therapeutic target in motor neuron disease.

Shorter axon initial segments do not cause repetitive firing impairments in the adult presymptomatic G127X SOD-1 Amyotrophic Lateral Sclerosis mouse

Increases in axonal sodium currents in peripheral nerves are some of the earliest excitability changes observed in Amyotrophic Lateral Sclerosis (ALS) patients. Nothing is known, however, about axonal sodium channels more proximally, particularly at the action potential initiating region - the axon initial segment (AIS). Immunohistochemistry for Nav1.6 sodium channels was used to investigate parameters of AISs of spinal motoneurones in the G127X SOD1 mouse model of ALS in adult mice at presymptomatic time points (~190 days old). In vivo intracellular recordings from lumbar spinal motoneurones were used to determine the consequences of any AIS changes. AISs of both alpha and gamma motoneurones were found to be significantly shorter (by 6.6% and 11.8% respectively) in G127X mice as well as being wider by 9.8% (alpha motoneurones). Measurements from 20–23 day old mice confirmed that this represented a change during adulthood. Intracellular recordings from motoneurones in presymptomatic adult mice, however, revealed no differences in individual action potentials or the cells ability to initiate repetitive action potentials. To conclude, despite changes in AIS geometry, no evidence was found for reduced excitability within the functional working range of firing frequencies of motoneurones in this model of ALS.

Increase of hyperpolarization-activated cyclic nucleotide-gated current in the aberrant excitability of spinal muscular atrophy

OBJECTIVE

The pathophysiology of spinal muscular atrophy (SMA) is still unclear.

METHODS

The nerve excitability test in SMA patients and a mouse model of SMA was carried out to explore the pathophysiology of nodal and internodal currents, and quantitative PCR, western blotting, and whole-cell patch-clamp recording were used for the identified hypothesis.

RESULTS

The nerve excitability test in SMA patients showed increased inward rectification in the current-threshold relationship and increased overshoot after hyperpolarizing threshold electrotonus, which indicates increased hyperpolarization-activated cyclic nucleotide-gated (HCN) current; these findings correlated with disease severity. Increased inward rectification in the current-threshold relationship was reproducible in a mouse model of mild SMA, and the abnormality preceded the decline of compound motor action potential amplitudes. Furthermore, quantitative PCR of spinal cord tissues and western blotting of the spinal cord and sciatic nerves showed increased HCN1 and HCN2 expression in SMA mice, and voltage-clamp recording in dissociated spinal motor neurons from SMA mice also showed increased HCN current density. Treatment with ZD7288, an HCN channel blocker, also reduced early mortality, improved motor function, and restored neuromuscular junction architecture in a mouse model of severe SMA.

INTERPRETATION

This study shows that increased HCN current underlies the pathophysiology of SMA and can be a novel non-SMN target for SMA therapy. Ann Neurol 2018;83:494-507.

An ALS-Associated Mutant SOD1 Rapidly Suppresses KCNT1 (Slack) Na+-Activated K+ Channels in Aplysia Neurons

Mutations that alter levels of Slack (KCNT1) Na⁺-activated K⁺ current produce devastating effects on neuronal development and neuronal function. We now find that Slack currents are rapidly suppressed by oligomers of mutant human Cu/Zn superoxide dismutase 1 (SOD1), which are associated with motor neuron toxicity in an inherited form of amyotrophic lateral sclerosis (ALS). We recorded from bag cell neurons of Aplysia californica, a model system to study neuronal excitability. We found that injection of fluorescent wild-type SOD1 (wt SOD1YFP) or monomeric mutant G85R SOD1YFP had no effect on net ionic currents measured under voltage clamp. In contrast, outward potassium currents were significantly reduced by microinjection of mutant G85R SOD1YFP that had been preincubated at 37°C or of cross-linked dimers of G85R SOD1YFP. Reduction of potassium current was also seen with multimeric G85R SOD1YFP of ∼300 kDa or >300 kDa that had been cross-linked. In current clamp recordings, microinjection of cross-linked 300 kDa increased excitability by depolarizing the resting membrane potential, and decreasing the latency of action potentials triggered by depolarization. The effect of cross-linked 300 kDa on potassium current was reduced by removing Na⁺ from the bath solution, or by knocking down levels of Slack using siRNA. It was also prevented by pharmacological inhibition of ASK1 (apoptosis signal-regulating kinase 1) or of c-Jun N-terminal kinase, but not by an inhibitor of p38 mitogen-activated protein kinase. These results suggest that soluble mutant SOD1 oligomers rapidly trigger a kinase pathway that regulates the activity of Na⁺-activated K⁺ channels in neurons.SIGNIFICANCE STATEMENT Slack Na⁺-activated K⁺ channels (KCNT1, K_Na1.1) regulate neuronal excitability but are also linked to cytoplasmic signaling pathways that control neuronal protein translation. Mutations that alter the amplitude of these currents have devastating effects on neuronal development and function. We find that injection of oligomers of mutant superoxide dismutase 1 (SOD1) into the cytoplasm of invertebrate neurons rapidly suppresses these Na⁺-activated K⁺ currents and that this effect is mediated by a MAP kinase cascade, including ASK1 and c-Jun N-terminal kinase. Because amyotrophic lateral sclerosis is a fatal adult-onset neurodegenerative disease produced by mutations in SOD1 that cause the enzyme to form toxic oligomers, our findings suggest that suppression of Slack channels may be an early step in the progression of the disease.

Voltage-gated calcium channels are abnormal in cultured spinal motoneurons in the G93A-SOD1 transgenic mouse model of ALS

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by progressive loss of motoneurons. Hyperexcitability and excitotoxicity have been implicated in the early pathogenesis of ALS. Studies addressing excitotoxic motoneuron death and intracellular Ca(2+) overload have mostly focused on Ca(2+) influx through AMPA glutamate receptors. However, intrinsic excitability of motoneurons through voltage-gated ion channels may also have a role in the neurodegeneration. In this study we examined the function and localization of voltage-gated Ca(2+) channels in cultured spinal cord motoneurons from mice expressing a mutant form of human superoxide dismutase-1 with a Gly93→Ala substitution (G93A-SOD1). Using whole-cell patch-clamp recordings, we showed that high voltage activated (HVA) Ca(2+) currents are increased in G93A-SOD1 motoneurons, but low voltage activated Ca(2+) currents are not affected. G93A-SOD1 motoneurons also have altered persistent Ca(2+) current mediated by L-type Ca(2+) channels. Quantitative single-cell RT-PCR revealed higher levels of Ca1a, Ca1b, Ca1c, and Ca1e subunit mRNA expression in G93A-SOD1 motoneurons, indicating that the increase of HVA Ca(2+) currents may result from upregulation of Ca(2+) channel mRNA expression in motoneurons. The localizations of the Ca1B N-type and Ca1D L-type Ca(2+) channels in motoneurons were examined by immunocytochemistry and confocal microscopy. G93A-SOD1 motoneurons had increased Ca1B channels on the plasma membrane of soma and dendrites. Ca1D channels are similar on the plasma membrane of soma and lower on the plasma membrane of dendrites of G93A-SOD1 motoneurons. Our study demonstrates that voltage-gated Ca(2+) channels have aberrant functions and localizations in ALS mouse motoneurons. The increased HVA Ca(2+) currents and PCCa current could contribute to early pathogenesis of ALS.

Mutant SOD1 protein increases Nav1.3 channel excitability

Amyotrophic lateral sclerosis (ALS) is a lethal paralytic disease caused by the degeneration of motor neurons in the spinal cord, brain stem, and motor cortex. Mutations in the gene encoding copper/zinc superoxide dismutase (SOD1) are present in ~20% of familial ALS and ~2% of all ALS cases. The most common SOD1 gene mutation in North America is a missense mutation substituting valine for alanine (A4V). In this study, we analyze sodium channel currents in oocytes expressing either wild-type or mutant (A4V) SOD1 protein. We demonstrate that the A4V mutation confers a propensity to hyperexcitability on a voltage-dependent sodium channel (Nav1.3) mediated by heightened total Na(+) conductance and a hyperpolarizing shift in the voltage dependence of Nav1.3 activation. To estimate the impact of these channel effects on excitability in an intact neuron, we simulated these changes in the program NEURON; this shows that the changes induced by mutant SOD1 increase the spontaneous firing frequency of the simulated neuron. These findings are consistent with the view that excessive excitability of neurons is one component in the pathogenesis of this disease.

Spinal muscular atrophy patient-derived motor neurons exhibit hyperexcitability

Spinal muscular atrophy (SMA) presents severe muscle weakness with limited motor neuron (MN) loss at an early stage, suggesting potential functional alterations in MNs that contribute to SMA symptom presentation. Using SMA induced pluripotent stem cells (iPSCs), we found that SMA MNs displayed hyperexcitability with increased membrane input resistance, hyperpolarized threshold, and larger action potential amplitude, which was mimicked by knocking down full length survival motor neuron (SMN) in non-SMA MNs. We further discovered that SMA MNs exhibit enhanced sodium channel activities with increased current amplitude and facilitated recovery, which was corrected by restoration of SMN1 in SMA MNs. Together we propose that SMN reduction results in MN hyperexcitability and impaired neurotransmission, the latter of which exacerbate each other via a feedback loop, thus contributing to severe symptoms at an early stage of SMA.

miR-34a regulates cell proliferation, morphology and function of newborn neurons resulting in improved behavioural outcomes

miR-34a is involved in the regulation of the fate of different cell types. However, the mechanism by which it controls the differentiation programme of neural cells remains largely unknown. Here, we investigated the role of miR-34a in neurogenesis and maturation of developing neurons and identified Doublecortin as a new miR-34a target. We found that the overexpression of miR-34a in vitro significantly increases precursor proliferation and influences morphology and function of developing neurons. Indeed, miR-34a overexpressing neurons showed a decreased expression of several synaptic proteins and receptor subunits, a decrement of NMDA-evoked current density and, interestingly, a more efficient response to synaptic stimulus. In vivo, miR-34a overexpression showed stage-specific effects. In neural progenitors, miR-34a overexpression promoted cell proliferation, in migratory neuroblasts reduced the migration and in differentiating newborn neurons modulated process outgrowth and complexity. Importantly, we found that rats overexpressing miR-34a in the brain have better learning abilities and reduced emotionality.

Human iPSC-derived motoneurons harbouring TARDBP or C9ORF72 ALS mutations are dysfunctional despite maintaining viability

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease for which a greater understanding of early disease mechanisms is needed to reveal novel therapeutic targets. We report the use of human induced pluripotent stem cell (iPSC)-derived motoneurons (MNs) to study the pathophysiology of ALS. We demonstrate that MNs derived from iPSCs obtained from healthy individuals or patients harbouring TARDBP or C9ORF72 ALS-causing mutations are able to develop appropriate physiological properties. However, patient iPSC-derived MNs, independent of genotype, display an initial hyperexcitability followed by progressive loss of action potential output and synaptic activity. This loss of functional output reflects a progressive decrease in voltage-activated Na(+) and K(+) currents, which occurs in the absence of overt changes in cell viability. These data implicate early dysfunction or loss of ion channels as a convergent point that may contribute to the initiation of downstream degenerative pathways that ultimately lead to MN loss in ALS.

Axonal Charcot-Marie-Tooth disease patient-derived motor neurons demonstrate disease-specific phenotypes including abnormal electrophysiological properties

OBJECTIVE

Charcot-Marie-Tooth (CMT) disease is a group of inherited peripheral neuropathies associated with mutations or copy number variations in over 70 genes encoding proteins with fundamental roles in the development and function of Schwann cells and peripheral axons. Here, we used iPSC-derived cells to identify common pathophysiological mechanisms in axonal CMT.

METHODS

iPSC lines from patients with two distinct forms of axonal CMT (CMT2A and CMT2E) were differentiated into spinal cord motor neurons and used to study axonal structure and function and electrophysiological properties in vitro.

RESULTS

iPSC-derived motor neurons exhibited gene and protein expression, ultrastructural and electrophysiological features of mature primary spinal cord motor neurons. Cytoskeletal abnormalities were found in neurons from a CMT2E (NEFL) patient and corroborated by a mouse model of the same NEFL point mutation. Abnormalities in mitochondrial trafficking were found in neurons derived from this patient, but were only mildly present in neurons from a CMT2A (MFN2) patient. Novel electrophysiological abnormalities, including reduced action potential threshold and abnormal channel current properties were observed in motor neurons derived from both of these patients.

INTERPRETATION

Human iPSC-derived motor neurons from axonal CMT patients replicated key pathophysiological features observed in other models of MFN2 and NEFL mutations, including abnormal cytoskeletal and mitochondrial dynamics. Electrophysiological abnormalities found in axonal CMT iPSC-derived human motor neurons suggest that these cells are hyperexcitable and have altered sodium and calcium channel kinetics. These findings may provide a new therapeutic target for this group of heterogeneous inherited neuropathies.

Long-term measurement of muscle denervation and locomotor behavior in individual wild-type and ALS model mice

The increasing number of mouse models of human degenerative and injury-related diseases that affect motor behavior raises the importance of in vivo methodologies allowing measurement of physiological and behavioral changes over an extended period of time in individual animals. A method that provides long-term measurements of muscle denervation and its behavioral consequences in individual mice for several months is presented in this article. The method is applied to mSod1G93A mice, which model human amyotrophic lateral sclerosis (ALS). The denervation process of gastrocnemius and soleus muscles in mSod1G93A mice is demonstrated for up to 3 mo. The data suggest that as muscle denervation progresses, massive behavioral compensation occurs within the spinal cord that allows animals to walk almost normally until late ages. Only around the age of 84 days is the first sign of abnormal movement during walking behavior detected as an abnormal tibialis anterior activity profile that is manifested in subtle but abnormal swing movement during walking. Additionally, this method can be used with other mouse models of human diseases, such as spinal cord injury, intracerebral hemorrhage, Parkinson's diseases, and spinal muscular atrophy.

Adult spinal motoneurones are not hyperexcitable in a mouse model of inherited amyotrophic lateral sclerosis

In amyotrophic lateral sclerosis (ALS), an adult onset disease in which there is progressive degeneration of motoneurones, it has been suggested that an intrinsic hyperexcitability of motoneurones (i.e. an increase in their firing rates), contributes to excitotoxicity and to disease onset. Here we show that there is no such intrinsic hyperexcitability in spinal motoneurones. Our studies were carried out in an adult mouse model of ALS with a mutated form of superoxide dismutase 1 around the time of the first muscle fibre denervations. We showed that the recruitment current, the voltage threshold for spiking and the frequency-intensity gain in the primary range are all unchanged in most spinal motoneurones, despite an increased input conductance. On its own, increased input conductance would decrease excitability, but the homeostasis for excitability is maintained due to an upregulation of a depolarizing current that is activated just below the spiking threshold. However, this homeostasis failed in a substantial fraction of motoneurones, which became hypoexcitable and unable to produce sustained firing in response to ramps of current. We found similar results both in lumbar motoneurones recorded in anaesthetized mice, and in sacrocaudal motoneurones recorded in vitro, indicating that the lack of hyperexcitability is not caused by anaesthetics. Our results suggest that, if excitotoxicity is indeed a mechanism leading to degeneration in ALS, it is not caused by the intrinsic electrical properties of motoneurones but by extrinsic factors such as excessive synaptic excitation.

Reach task-associated excitatory overdrive of motor cortical neurons following infusion with ALS-CSF

Converging evidence from transgenic animal models of amyotrophic lateral sclerosis (ALS) and human studies suggest alterations in excitability of the motor neurons in ALS. Specifically, in studies on human subjects with ALS the motor cortex was reported to be hyperexcitable. The present study was designed to test the hypothesis that infusion of cerebrospinal fluid from patients with sporadic ALS (ALS-CSF) into the rat brain ventricle can induce hyperexcitability and structural changes in the motor cortex leading to motor dysfunction. A robust model of sporadic ALS was developed experimentally by infusing ALS-CSF into the rat ventricle. The effects of ALS-CSF at the single neuron level were examined by recording extracellular single unit activity from the motor cortex while rats were performing a reach to grasp task. We observed an increase in the firing rate of the neurons of the motor cortex in rats infused with ALS-CSF compared to control groups. This was associated with impairment in a specific component of reach with alterations in the morphological characteristics of the motor cortex. It is likely that the increased cortical excitability observed in the present study could be the result of changes in the intrinsic properties of motor cortical neurons, a dysfunctional inhibitory mechanism and/or an underlying structural change culminating in a behavioral deficit.

Over-expression of N-type calcium channels in cortical neurons from a mouse model of Amyotrophic Lateral Sclerosis

Voltage-gated Ca(2+) channels (VGCCs) mediate calcium entry into neuronal cells in response to membrane depolarisation and play an essential role in a variety of physiological processes. In Amyotrophic Lateral Sclerosis (ALS), a fatal neurodegenerative disease caused by motor neuron degeneration in the brain and spinal cord, intracellular calcium dysregulation has been shown, while no studies have been carried out on VGCCs. Here we show that the subtype N-type Ca(2+) channels are over expressed in G93A cultured cortical neurons and in motor cortex of G93A mice compared to Controls. In fact, by western blotting, immunocytochemical and electrophysiological experiments, we observe higher membrane expression of N-type Ca(2+) channels in G93A neurons compared to Controls. G93A cortical neurons filled with calcium-sensitive dye Fura-2, show a net calcium entry during membrane depolarization that is significantly higher compared to Control. Analysis of neuronal vitality following the exposure of neurons to a high K(+) concentration (25 mM, 5h), shows a significant reduction of G93A cellular survival compared to Controls. N-type channels are involved in the G93A higher mortality because ω-conotoxin GVIA (1 μM), which selectively blocks these channels, is able to abolish the higher G93A mortality when added to the external medium. These data provide robust evidence for an excess of N-type Ca(2+) expression in G93A cortical neurons which induces a higher mortality following membrane depolarization. These results may be central to the understanding of pathogenic pathways in ALS and provide novel molecular targets for the design of rational therapies for the ALS disorder.

Neural precursors (NPCs) from adult L967Q mice display early commitment to "in vitro" neuronal differentiation and hyperexcitability

The pathogenic factors leading to selective degeneration of motoneurons in ALS are not yet understood. However, altered functionality of voltage-dependent Na(+) channels may play a role since cortical hyperexcitability was described in ALS patients and riluzole, the only drug approved to treat ALS, seems to decrease glutamate release via blockade or inactivation of voltage-dependent Na(+) channels. The wobbler mouse, a murine model of motoneuron degeneration, shares some of the clinical features of human ALS. At early stages of the wobbler disease, increased cortical hyperexcitability was observed. Moreover, riluzole reduced motoneuron loss and muscular atrophy in treated wobbler mice. Here, we focussed our attention on specific electrophysiological properties, like voltage-activated Na(+) currents and underlying regenerative electrical activity, as read-outs of the neuronal maturation process of neural stem/progenitor cells (NPCs) isolated from the subventricular zone (SVZ) of adult early symptomatic wobbler mice. In self-renewal conditions, the rate of wobbler NPC proliferation "in vitro" was 30% lower than that of healthy mice. Conversely, the number of wobbler NPCs displaying early neuronal commitment and action potentials was significantly higher. Upon switching from proliferative to differentiative conditions, NPCs underwent significant changes in the key properties of voltage gated Na(+) currents. The most notable finding, in cells with neuronal morphology, was an increase in Na(+) current density that strictly correlated with an increased probability to generate action potentials. This feature was remarkably more pronounced in neurons differentiated from wobbler NPCs that upon sustained stimulation, displayed short trains of pathological facilitation. In agreement with this result, an increase in the number of c-Fos positive cells, a surrogate marker of neuronal network activation, was observed in the mesial cortex of the wobbler mice "in situ". Thus these findings, all together, suggest that a state of early neuronal hyperexcitability may be a major contributor of motoneuron vulnerability.

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

Amyotrophic lateral sclerosis (ALS) is the third most common adult-onset neurodegenerative disease. It causes the degeneration of motoneurons and is fatal due to paralysis, particularly of respiratory muscles. ALS can be inherited, and specific disease-causing genes have been identified, but the mechanisms causing motoneuron death in ALS are not understood. No effective treatments exist for ALS. One well-studied theory of ALS pathogenesis involves faulty RNA editing and abnormal activation of specific glutamate receptors as well as failure of glutamate transport resulting in glutamate excitotoxicity; however, the excitotoxicity theory is challenged by the inability of anti-glutamate drugs to have major disease-modifying effects clinically. Nevertheless, hyperexcitability of upper and lower motoneurons is a feature of human ALS and transgenic (tg) mouse models of ALS. Motoneuron excitability is strongly modulated by synaptic inhibition mediated by presynaptic glycinergic and GABAergic innervations and postsynaptic glycine receptors (GlyR) and GABA(A) receptors; yet, the integrity of inhibitory systems regulating motoneurons has been understudied in experimental models, despite findings in human ALS suggesting that they may be affected. We have found in tg mice expressing a mutant form of human superoxide dismutase-1 (hSOD1) with a Gly93 → Ala substitution (G93A-hSOD1), causing familial ALS, that subsets of spinal interneurons degenerate. Inhibitory glycinergic innervation of spinal motoneurons becomes deficient before motoneuron degeneration is evident in G93A-hSOD1 mice. Motoneurons in these ALS mice also have insufficient synaptic inhibition as reflected by smaller GlyR currents, smaller GlyR clusters on their plasma membrane, and lower expression of GlyR1α mRNA compared to wild-type motoneurons. In contrast, GABAergic innervation of ALS mouse motoneurons and GABA(A) receptor function appear normal. Abnormal synaptic inhibition resulting from dysfunction of interneurons and motoneuron GlyRs is a new direction for unveiling mechanisms of ALS pathogenesis that could be relevant to new therapies for ALS.

Mitochondria-derived superoxide and voltage-gated sodium channels in baroreceptor neurons from chronic heart-failure rats

Our previous study has shown that chronic heart failure (CHF) reduces expression and activation of voltage-gated sodium (Na(v)) channels in baroreceptor neurons, which are involved in the blunted baroreceptor neuron excitability and contribute to the impairment of baroreflex in the CHF state. The present study examined the role of mitochondria-derived superoxide in the reduced Na(v) channel function in coronary artery ligation-induced CHF rats. CHF decreased the protein expression and activity of mitochondrial complex enzymes and manganese SOD (MnSOD) and elevated the mitochondria-derived superoxide level in the nodose neurons compared with those in sham nodose neurons. Adenoviral MnSOD (Ad.MnSOD) gene transfection (50 multiplicity of infection) into the nodose neurons normalized the MnSOD expression and reduced the elevation of mitochondrial superoxide in the nodose neurons from CHF rats. Ad.MnSOD also partially reversed the reduced protein expression and current density of the Na(v) channels and the suppressed cell excitability (the number of action potential and the current threshold for inducing action potential) in aortic baroreceptor neurons from CHF rats. Data from the present study indicate that mitochondrial dysfunction, including decreased protein expression and activity of mitochondrial complex enzymes and MnSOD and elevated mitochondria-derived superoxide, contributes to the reduced Na(v) channel activation and cell excitability in the aortic baroreceptor neurons in CHF rats.

Increased expression of the beta3 subunit of voltage-gated Na+ channels in the spinal cord of the SOD1G93A mouse

Amyotrophic lateral sclerosis (ALS) is an adult-onset disease characterized by the progressive degeneration of motoneurons (MNs). Altered electrical properties have been described in familial and sporadic ALS patients. Cortical and spinal neurons cultured from the mutant Cu,Zn superoxide dismutase 1 (SOD1G93A) mouse, a murine model of ALS, exhibit a marked increase in the persistent Na+ currents. Here, we investigated the effects of the SOD1G93A mutation on the expression of the voltage-gated Na+ channel alpha subunit SCN8A (Nav1.6) and the beta subunits SCN1B (beta1), SCN2B (beta2), and SCN3B (beta3) in MNs of the spinal cord in presymptomatic (P75) and symptomatic (P120) mice. We observed a significant increase, within lamina IX, of the beta3 transcript and protein expression. On the other hand, the beta1 transcript was significantly decreased, in the same area, at the symptomatic stage, while the beta2 transcript levels were unaltered. The SCN8A transcript was significantly decreased at P120 in the whole spinal cord. These data suggest that the SOD1G93A mutation alters voltage-gated Na+ channel subunit expression. Moreover, the increased expression of the beta3 subunit support the hypothesis that altered persistent Na+ currents contribute to the hyperexcitability observed in the ALS-affected MNs.

Electrophysiological analysis of a murine model of motoneuron disease

OBJECTIVE

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by loss of motoneurons of the primary motor cortex, the brainstem and the spinal cord, for which there are not effective treatments. Several transgenic mice that mimic motoneuron disease have been used to investigate potential treatments. The objective of this work is to characterize electrophysiologically the SOD1(G93A) transgenic mouse model of ALS, and to provide useful markers to improve early detection and monitoring of progression of the disease.

METHODS

We performed nerve conduction tests, motor unit number estimation (MUNE), H reflex tests and motor evoked potentials (MEPs) in a cohort of transgenic and wild type mice from 4 to 16 weeks of age.

RESULTS

The results revealed dysfunction of spinal motoneurons evidenced by deficits in motor nerve conduction tests starting at 8 weeks of age, earlier in proximal than in distal muscles of the hindlimb. MUNE demonstrated that spinal motoneurons loss muscle innervation and have a deficit in their sprouting capacity. Motor evoked potentials revealed that, coexisting with peripheral deficits, there was a dysfunction of central motor tracts that started also at 8 weeks, indicating progressive dysfunction of upper motoneurons.

CONCLUSIONS

These electrophysiological results provide important information about the SOD1(G93A) mouse model, as they demonstrate by the first time alterations of central motor pathways simultaneously to lower motoneuron dysfunction, well before functional abnormalities appear (by 12 weeks of age).

SIGNIFICANCE

The finding of concomitant dysfunction of upper and lower motoneurons contributes to the validation of the SOD1(G93A) mouse as model of ALS, because this parallel involvement is a diagnostic condition for ALS. Electrophysiological tests can be used as early markers of the disease and to evaluate the potential benefits of new treatments on both upper and lower motoneurons.

Intrinsic properties of lumbar motor neurones in the adult G127insTGGG superoxide dismutase-1 mutant mouse in vivo: evidence for increased persistent inward currents

AIM

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterized by a preferential loss of motor neurones. Previous publications using in vitro neonatal preparations suggest an increased excitability of motor neurones in various superoxide dismutase-1 (SOD1) mutant mice models of ALS which may contribute to excitotoxicity of the motor neurones.

METHODS

Using intracellular recording, we tested this hypothesis in vivo in the adult presymptomatic G127insTGGG (G127X) SOD1 mutant mouse model of ALS.

RESULTS

At resting membrane potentials the basic intrinsic properties of lumbar motor neurones in the adult presymptomatic G127X mutant are not significantly different from those of wild type. However, at more depolarized membrane potentials, motor neurones in the G127X SOD1 mutants can sustain higher frequency firing, showing less spike frequency adaption (SFA) and with persistent inward currents (PICs) being activated at lower firing frequencies and being more pronounced.

CONCLUSION

We demonstrated that, in vivo, at resting membrane potential, spinal motor neurones of the adult G127X mice do not show an increased excitability. However, when depolarized they show evidence of an increased PIC and less SFA which may contribute to excitotoxicity of these neurones as the disease progresses.

Persistent inward currents in spinal motoneurons: important for normal function but potentially harmful after spinal cord injury and in amyotrophic lateral sclerosis

Meaningful body movements depend on the interplay between synaptic inputs to motoneurons and their intrinsic properties. Injury and disease often alter either or both of these factors and cause motoneuron and movement dysfunction. The ability of the motoneuronal membrane to generate persistent inward currents (PICs) is especially potent in setting the intrinsic excitability of motoneurons and can drastically change the motoneuron output to a given input. In this article, we review the role of PICs in modulating the excitability of spinal motoneurons during health, and their contribution to motoneuron excitability after spinal cord injury (SCI) and in amyotrophic lateral sclerosis (ALS) leading to exaggerated long-lasting reflexes and muscle spasms, and contributing to neuronal degeneration, respectively.

Intrinsic properties of mouse lumbar motoneurons revealed by intracellular recording in vivo

We have developed an in vivo model for intracellular recording in the adult anesthetized mouse using sharp microelectrode electrodes as a basis for investigations of motoneuron properties in transgenic mouse strains. We demonstrate that it is possible to record postsynaptic potentials underlying identified circuits in the spinal cord. Forty-one motoneurons with antidromic spike potentials (>50 mV) from the sciatic nerve were investigated. We recorded the intrinsic properties of the neurons, including input resistance (mean: 2.4 +/- 1.2 MOmega), rheobase (mean: 7.1 +/- 5.9 nA), and the duration of the afterhyperpolarization (AHP; mean: 55.3 +/- 14 ms). We also measured the minimum firing frequencies (F(min), mean 23.5 +/- 5.7 SD Hz), the maximum firing frequencies (F(max); >300 Hz) and the slope of the current-frequency relationship (f-I slope) with increasing amounts of current injected (mean: 13 +/- 5.7 Hz/nA). Signs of activation of persistent inward currents (PICs) were seen, such as accelerations of firing frequency or jumps in the membrane potential with increasing amounts of injected current. It is likely that the particular anesthetic regime with a mixture of Hypnorm and midazolam is essential for the possibility to evoke PICs. The data demonstrate that mouse spinal motoneurons share many of the same properties that have been demonstrated previously for cat, rat, and human motoneurons. The shorter AHP duration, steeper f-I slopes, and higher F(min) and F(max) than those in rats, cats, and humans are likely to be tailored to the characteristics of the mouse muscle contraction properties.

Downregulation of thymosin beta4 in neural progenitor grafts promotes spinal cord regeneration

Thymosin beta4 (Tbeta4) is an actin-binding peptide whose expression in developing brain correlates with migration and neurite extension of neurons. Here, we studied the effects of the downregulation of Tbeta4 expression on growth and differentiation of murine neural progenitor cells (NPCs), using an antisense lentiviral vector. In differentiation-promoting medium, we found twice the number of neurons derived from the Tbeta4-antisense-transduced NPCs, which showed enhanced neurite outgrowth accompanied by increased expression of the adhesion complex N-cadherin-beta-catenin and increased ERK activation. Importantly, when the Tbeta4-antisense-transduced NPCs were transplanted in vivo into a mouse model of spinal cord injury, they promoted a significantly greater functional recovery. Locomotory recovery correlated with increased expression of the regeneration-promoting cell adhesion molecule L1 by the grafted Tbeta4-antisense-transduced NPCs. This resulted in an increased number of regenerating axons and in sprouting of serotonergic fibers surrounding and contacting the Tbeta4-antisense-transduced NPCs grafted into the lesion site. In conclusion, our data identify a new role for Tbeta4 in neuronal differentiation of NPCs by regulating fate determination and process outgrowth. Moreover, NPCs with reduced Tbeta4 levels generate an L1-enriched environment in the lesioned spinal cord that favors growth and sprouting of spared host axons and enhances the endogenous tissue-repair processes.

Early excitability changes in lumbar motoneurons of transgenic SOD1G85R and SOD1G(93A-Low) mice

This work characterizes the properties of wild-type (WT) mouse motoneurons in the second postnatal week and compares these at the same age and in the same conditions to those of two different SOD1 mutant lines used as models of human amyotrophic lateral sclerosis (ALS), the SOD1(G93A) low expressor line and SOD1(G85R) line, to describe any changes in the functional properties of mutant motoneurons (Mns) that may be related to the pathogenesis of human ALS. We show that very early changes in excitability occur in SOD1 mutant Mns that have different properties from those of WT animals. The SOD1(G93A-Low) low expressor line displays specific differences that are not found in other mutant lines including a more depolarized membrane potential, larger spike width, and slower spike rise slope. With current pulses SOD1(G93A-Low) were hyperexcitable, but both mutants had a lower gain with current ramps stimulation. Changes in the threshold and intensities of Na(+) and Ca(2+) persistent inward currents were also observed. Low expressor mutants show reduced total persistant inward currents compared with WT motoneurons in the same recording conditions and give arguments toward modifications of the balance between Na(+) and Ca(2+) persistent inward currents. During the second week postnatal, SOD1(G93A-Low) lumbar motoneurons appear more immature than those of SOD1(G85R) compared with WT and we propose that different time course of the disease, possibly linked with different toxic properties of the mutated protein in each model, may explain the discrepancies between excitability changes described in the different models.

GAB(A) receptors present higher affinity and modified subunit composition in spinal motor neurons from a genetic model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a neurodegenerative disease characterized by the selective degeneration of motor neurons in the spinal cord, brainstem and cerebral cortex. In this study we have analysed the electrophysiological properties of GABA(A) receptors and GABA(A) alpha1 and alpha2 subunits expression in spinal motor neurons in culture obtained from a genetic model of ALS (G93A) and compared with transgenic wild type SOD1 (SOD1) and their corresponding non transgenic litter mates (Control). Although excitotoxic motor neuron death has been extensively studied in relation to Ca(2+)-dependent processes, strong evidence indicates that excitotoxic cell death is also remarkably dependent on Cl(-) ions and on GABA(A) receptor activation. In this study we have analysed the electrophysiological properties of GABA(A) receptors and the expression of GABA(A)alpha(1) and alpha(2) subunits in cultured motor neurons obtained from a genetic model of amyotrophic lateral sclerosis (G93A) and compared them with transgenic wild-type Cu,Zn superoxide dismutase and their corresponding non-transgenic littermates (Control). In all tested motor neurons, the application of gamma-aminobutyric acid (GABA) (0.5-100 mum) evoked an inward current that was reversibly blocked by bicuculline (100 mum), thus indicating that it was mediated by the activation of GABA(A) receptors. Our results indicate that the current density at high GABA concentrations is similar in control, Cu,Zn superoxide dismutase and G93A motor neurons. However, the dose-response curve significantly shifted toward lower concentration values in G93A motor neurons and the extent of desensitization also increased in these neurons. Finally, multiplex single-cell real-time polymerase chain reaction and immunofluorescence revealed that the amount of GABA(A)alpha(1) subunit was significantly increased in G93A motor neurons, whereas the levels of alpha(2) subunit were unchanged. These data show that the functionality and expression of GABA(A) receptors are altered in G93A motor neurons inducing a higher Cl(-) influx into the cell with a possible consequent neuronal excitotoxicity acceleration.

Exposure to cerebrospinal fluid of sporadic amyotrophic lateral sclerosis patients alters Nav1.6 and Kv1.6 channel expression in rat spinal motor neurons

Cerebro Spinal Fluid (CSF) from patients with ALS has been documented to have a toxic effect on motor neurons both in vivo and in vitro. Here we show that the CSF from Amyotrophic Lateral Sclerosis (ALS) patients (ALS-CSF) has the potential to perturb ion channel expression, specifically the Na(v)1.6, and K(v)1.6 channels in newborn rat spinal motor neurons both in vivo and in vitro. ALS-CSF and CSF from nonALS patients (nonALS-CSF) were intrathecally injected into 3-day-old rat pups at the rate of 1 microl/2.5 min using a microinjector. In addition, embryonic rat spinal cord cultures were also exposed to 10% ALS or nonALS-CSF on the 9th day in vitro (9DIV) in serum free DMEM medium. After 48 h of CSF exposure, the cultures and the spinal cord sections were processed for immunostaining of the above mentioned ion channels. We observed a decrease in the expression of Na(v)1.6 and K(v)1.6 channels in motor neurons in ALS-CSF treated group, and the presence of trophic factors like Brain Derived Neurotrophic Factor (BDNF) and Ciliary Neurotrophic Factor CNTF partially reversed the effects produced by ALS-CSF. Altered expression of these voltage-gated channels may interfere with the electrical activity of motor neurons, and thereby lead to the degeneration of neurons.

Expression of the N-methyl-D-aspartate receptor subunit NR3B regulates dendrite morphogenesis in spinal motor neurons

During postnatal development, the dendrites of spinal motor neurons are refined in an activity-dependent manner that can be influenced by blocking activation of N-methyl-D-aspartate (NMDA) receptors. In late postnatal life, dendritic refinement ceases, and dendrite architecture is unaffected by NMDA antagonists; however the molecular substrate for limiting dendritic plasticity is not understood. During late postnatal development, expression of the NR3B NMDA receptor subunit, a putative dominant-negative subunit that reduces glutamate-induced ionic currents, is upregulated within motor neurons. To investigate whether increasing NR3B expression may contribute to the loss in late development of activity-dependent dendritic reorganization in the spinal cord, we over-expressed NR3B in cultured rat spinal motor neurons, and compared its effects on dendrite morphology with the effects of pharmacological blockade of NMDA receptors. We found that over-expression of the NR3B receptor subunit increased the length and complexity of dendritic arbor, and increased numbers of dendritic filopodia, suggesting that NR3B promotes the addition of branch segments in developing motor neurons. In contrast, blockade of NMDA receptor activity by the NMDA antagonist DL-2-amino-5-phosphonovalerate (AP5) had little effect on the overall length or complexity of dendritic arbor. Instead, treatment with AP5 resulted in significant reorganization of dendritic arbor in a manner that favored addition of dendritic segments of high branch orders, at the expense of those closer to the cell body. These results suggest that expression of the NR3B subunit may participate in activity-dependent reorganization of dendritic architecture, but via a mechanism that may be inconsistent with loss of NMDA receptor activity.

Differential regulation of dendrite complexity by AMPA receptor subunits GluR1 and GluR2 in motor neurons

Activity-dependent developmental mechanisms in many regions of the central nervous system are thought to be responsible for shaping dendritic architecture and connectivity, although the molecular mechanisms underlying these events remain obscure. Since AMPA glutamate receptors are developmentally regulated in spinal motor neurons, we have investigated the role of activation of AMPA receptors in dendritic outgrowth of spinal motor neurons by overexpression of two subunits, GluR1 and GluR2, and find that dendrite outgrowth is differentially controlled by expression of these subunits. Overexpression of GluR1 was associated with greater numbers of filopodia, and an increase in the length and complexity of dendritic arbor. In contrast, GluR2 expression did not alter dendritic complexity, but was associated with a moderate increase in length of arbor, and decreased numbers of filopodia. Neither GluR1 nor GluR2 had any effect on the motility of filopodia. In addition, GluR1 but not GluR2 expression increased the density of dendritic puncta incorporating a GFP-labeled PSD95, suggesting that GluR1 may mediate its effect in part by augmenting the number of excitatory synapses within motor neuron dendrites. Together these results suggest that in spinal motor neurons, AMPA receptors composed of GluR1 subunits may facilitate neurotrophic mechanisms in these neurons, permitting sustained dendrite outgrowth and synaptogenesis, whereas expression of AMPA receptors containing GluR2 acts to preserve existing dendritic arbor. Thus, the observed downregulation of GluR1 in motor neurons during postnatal development may limit the formation of new dendrite segments and synapses, promoting stabilized synaptic connectivity.

Altered calcium homeostasis in motor neurons following AMPA receptor but not voltage-dependent calcium channels' activation in a genetic model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a late-onset progressive neurodegenerative disease characterized by a substantial loss of motor neurons in the spinal cord, brain stem and motor cortex. By combining electrophysiological recordings with imaging techniques, clearance/buffering capacity of cultured spinal cord motor neurons after a calcium accumulation has been analyzed in response to AMPA receptors' (AMPARs') activation and to depolarizing stimuli in a genetic mouse model of ALS (G93A). Our studies demonstrate that the amplitude of the calcium signal in response to AMPARs' or voltage-dependent calcium channels' activation is not significantly different in controls and G93A motor neurons. On the contrary, in G93A motor neurons, the [Ca(2+)](i) recovery to basal level is significantly slower compared to control neurons following AMPARs but not voltage-dependent calcium channels' activation. This difference was not observed in G93A cultured cortical neurons. This observation is the first to indicate a specific alteration of the calcium clearance linked to AMPA receptors' activation in G93A motor neurons and the involvement of AMPA receptor regulatory proteins controlling both AMPA receptor functionality and the sequence of events connected to them.

Astrocytes expressing ALS-linked mutated SOD1 release factors selectively toxic to motor neurons

Mutations in superoxide dismutase-1 (SOD1) cause a form of the fatal paralytic disorder amyotrophic lateral sclerosis (ALS), presumably by a combination of cell-autonomous and non-cell-autonomous processes. Here, we show that expression of mutated human SOD1 in primary mouse spinal motor neurons does not provoke motor neuron degeneration. Conversely, rodent astrocytes expressing mutated SOD1 kill spinal primary and embryonic mouse stem cell-derived motor neurons. This is triggered by soluble toxic factor(s) through a Bax-dependent mechanism. However, mutant astrocytes do not cause the death of spinal GABAergic or dorsal root ganglion neurons or of embryonic stem cell-derived interneurons. In contrast to astrocytes, fibroblasts, microglia, cortical neurons and myocytes expressing mutated SOD1 do not cause overt neurotoxicity. These findings indicate that astrocytes may play a role in the specific degeneration of spinal motor neurons in ALS. Identification of the astrocyte-derived soluble factor(s) may have far-reaching implications for ALS from both a pathogenic and therapeutic standpoint.