Biophysical mechanisms underlying tefluthrin-induced modulation of gating changes and resurgent current generation in the human Nav1.4 channel.

Human skeletal muscle contraction is triggered by activation of Nav1.4 channels. Nav1.4 channels can generate resurgent currents by channel reopening at hyperpolarized potentials through a gating transition dependent on the intracellular Navß4 peptide in the physiological conditions. Tefluthrin (TEF) is a pyrethroid insecticide that can disrupt electrical signaling in nerves and skeletal muscle, resulting in seizures, muscle spasms, fasciculations, and mental confusion. TEF can also induce tail currents through other voltage-gated sodium channels in the absence of Navß4 peptide, suggesting that muscle spasms may be caused by resurgent currents. Further, intracellular Navß4 peptide and extracellular TEF may show competitive or synergistic effects; however, their binding sites are still unknown. To address these issues, electrophysiological recordings were performed on CHO-K1 cells expressing Nav1.4 channels with intracellular Navß4 peptide, extracellular TEF, or both. TEF and Navß4 peptide induced a hyperpolarizing shift of activation and inactivation curves in the Nav1.4 channel. TEF also substantially prolonged the inactivation time constants, while simultaneous application of Navß4 peptide partially reversed this effect. Resurgent currents were enhanced by TEF and Navß4 peptide at negative potentials, but TEF more potently enhances resurgent currents and dampens decay of resurgent currents. With longer depolarization, peak resurgent currents decay was fastest with the TEF alone. Molecular docking suggested that TEF and Navß4 peptide binding site(s) are not in the narrowest part of the channel pore, but rather in the bundle-crossing regions and in the domain linkers, respectively. TEF can induce resurgent currents independently and synergistically with Navß4 peptide, which may explain the muscle spasms observed in TEF intoxication.

Deep brain stimulation rectifies the noisy cortex and irresponsive subthalamus to improve parkinsonian locomotor activities.

The success of deep brain stimulation (DBS) therapy indicates that Parkinson's disease is a brain rhythm disorder. However, the manifestations of the erroneous rhythms corrected by DBS remain to be established. We found that augmentation of α rhythms and α coherence between the motor cortex (MC) and the subthalamic nucleus (STN) is characteristically prokinetic and is decreased in parkinsonian rats. In multi-unit recordings, movement is normally associated with increased changes in spatiotemporal activities rather than overall spike rates in MC. In parkinsonian rats, MC shows higher spike rates at rest but less spatiotemporal activity changes upon movement, and STN burst discharges are more prevalent, longer lasting, and less responsive to MC inputs. DBS at STN rectifies the foregoing pathological MC-STN oscillations and consequently locomotor deficits, yet overstimulation may cause behavioral restlessness. These results indicate that delicate electrophysiological considerations at both cortical and subcortical levels should be exercised for optimal DBS therapy.

The genesis and functional consequences of cortico-subthalamic beta augmentation and excessive subthalamic burst discharges after dopaminergic deprivation.

The cardinal electrophysiological signs in Parkinson's disease (PD) include augmented beta oscillations in the motor cortex-subthalamic nucleus (MC-STN) axis and excessive burst discharges in STN. We have shown that excessive STN burst discharges have a direct causal relation with the locomotor deficits in PD. To investigate the correlation between the two cardinal signs, we characterized the courses of development of the electrophysiological abnormalities in the hemiparkinsonian rat model. The loss of dopaminergic neurons develops fast, and is histologically completed within 4-7 days of the lesion. The increase in STN burst discharges is limited to the lesioned side, and follows a very similar course. In contrast, beta augmentation has a bilateral presentation, and requires 14-21 days for full development. Behaviorally, the gross locomotor deficits in open field test and limb akinesia in stepping test match the foregoing fast and slow time courses, respectively. A further look into the spike entrainment shows that the oscillations in local field potential (LFP) of the MC effectively entrain the multi-unit (MU) spikes of MC, STN and entopeduncular nucleus (EPN), a rat homolog of human globus pallidus interna (GPi), whereas the LFP of STN or EPN (GPi) cannot entrain the spikes in MC. We conclude that excessive STN burst discharges are a direct consequence, whereas beta augmentation is probably a secondary or adaptive changes in the cortico-subcortical re-entrant loops, to dopaminergic deprivation. Beta augmentation is therefore not so consistently present as excessive STN burst discharges, but could signal more delicate derangements at the level of cortical programming in PD.

An electrophysiological perspective on Parkinson's disease: symptomatic pathogenesis and therapeutic approaches.

Parkinson's disease (PD), or paralysis agitans, is a common neurodegenerative disease characterized by dopaminergic deprivation in the basal ganglia because of neuronal loss in the substantia nigra pars compacta. Clinically, PD apparently involves both hypokinetic (e.g. akinetic rigidity) and hyperkinetic (e.g. tremor/propulsion) symptoms. The symptomatic pathogenesis, however, has remained elusive. The recent success of deep brain stimulation (DBS) therapy applied to the subthalamic nucleus (STN) or the globus pallidus pars internus indicates that there are essential electrophysiological abnormalities in PD. Consistently, dopamine-deprived STN shows excessive burst discharges. This proves to be a central pathophysiological element causally linked to the locomotor deficits in PD, as maneuvers (such as DBS of different polarities) decreasing and increasing STN burst discharges would decrease and increase the locomotor deficits, respectively. STN bursts are not so autonomous but show a "relay" feature, requiring glutamatergic synaptic inputs from the motor cortex (MC) to develop. In PD, there is an increase in overall MC activities and the corticosubthalamic input is enhanced and contributory to excessive burst discharges in STN. The increase in MC activities may be relevant to the enhanced beta power in local field potentials (LFP) as well as the deranged motor programming at the cortical level in PD. Moreover, MC could not only drive erroneous STN bursts, but also be driven by STN discharges at specific LFP frequencies (~ 4 to 6 Hz) to produce coherent tremulous muscle contractions. In essence, PD may be viewed as a disorder with deranged rhythms in the cortico-subcortical re-entrant loops, manifestly including STN, the major component of the oscillating core, and MC, the origin of the final common descending motor pathways. The configurations of the deranged rhythms may play a determinant role in the symptomatic pathogenesis of PD, and provide insight into the mechanism underlying normal motor control. Therapeutic brain stimulation for PD and relevant disorders should be adaptively exercised with in-depth pathophysiological considerations for each individual patient, and aim at a final normalization of cortical discharge patterns for the best ameliorating effect on the locomotor and even non-motor symptoms.

Changes in Resurgent Sodium Current Contribute to the Hyperexcitability of Muscles in Patients with Paramyotonia Congenita.

Paramyotonia congenita (PMC) is a rare hereditary skeletal muscle disorder. The major symptom, muscle stiffness, is frequently induced by cold exposure and repetitive exercise. Mutations in human SCN4A gene, which encodes the α-subunit of Nav1.4 channel, are responsible for PMC. Mutation screening of SCN4A gene from two PMC families identified two missense mutations, p.T1313M and p.R1448H. To elucidate the electrophysiological abnormalities caused by the mutations, the p.T1313M, p.R1448H, and wild-type (WT) SCN4A genes were transient expressed on Chinese hamster ovary (CHO-K1) cells. The detailed study on the gating defects of the mutant channels using the whole-cell patch clamping technique was performed. The mutant Nav1.4 channels impaired the basic gating properties with increasing sustained and window currents during membrane depolarization and facilitated the genesis of resurgent currents during repolarization. The mutations caused a hyperpolarization shift in the fast inactivation and slightly enhanced the slow inactivation with an increase in half-maximal inactivation voltage. No differences were found in the decay kinetics of the tail current between mutant and WT channels. In addition to generating the larger resurgent sodium current, the time to peak in the mutant channels was longer than that in the WT channels. In conclusion, our results demonstrated that the mutations p.T1313M and p.R1448H in Nav1.4 channels can enhance fast inactivation, slow inactivation, and resurgent current, revealing that subtle changes in gating processes can influence the clinical phenotype.

Mitochondrial UQCRC1 mutations cause autosomal dominant parkinsonism with polyneuropathy.

Parkinson's disease is a neurodegenerative disorder with a multifactorial aetiology. Nevertheless, the genetic predisposition in many families with multi-incidence disease remains unknown. This study aimed to identify novel genes that cause familial Parkinson's disease. Whole exome sequencing was performed in three affected members of the index family with a late-onset autosomal-dominant parkinsonism and polyneuropathy. We identified a novel heterozygous substitution c.941A>C (p.Tyr314Ser) in the mitochondrial ubiquinol-cytochrome c reductase core protein 1 (UQCRC1) gene, which co-segregates with disease within the family. Additional analysis of 699 unrelated Parkinson's disease probands with autosomal-dominant Parkinson's disease and 1934 patients with sporadic Parkinson's disease revealed another two variants in UQCRC1 in the probands with familial Parkinson's disease, c.931A>C (p.Ile311Leu) and an allele with concomitant splicing mutation (c.70-1G>A) and a frameshift insertion (c.73_74insG, p.Ala25Glyfs*27). All substitutions were absent in 1077 controls and the Taiwan Biobank exome database from healthy participants (n = 1517 exomes). We then assayed the pathogenicity of the identified rare variants using CRISPR/Cas9-based knock-in human dopaminergic SH-SY5Y cell lines, Drosophila and mouse models. Mutant UQCRC1 expression leads to neurite degeneration and mitochondrial respiratory chain dysfunction in SH-SY5Y cells. UQCRC1 p.Tyr314Ser knock-in Drosophila and mouse models exhibit age-dependent locomotor defects, dopaminergic neuronal loss, peripheral neuropathy, impaired respiratory chain complex III activity and aberrant mitochondrial ultrastructures in nigral neurons. Furthermore, intraperitoneal injection of levodopa could significantly improve the motor dysfunction in UQCRC1 p.Tyr314Ser mutant knock-in mice. Taken together, our in vitro and in vivo studies support the functional pathogenicity of rare UQCRC1 variants in familial parkinsonism. Our findings expand an additional link of mitochondrial complex III dysfunction in Parkinson's disease.

Changes of Resurgent Na+ Currents in the Nav1.4 Channel Resulting from an SCN4A Mutation Contributing to Sodium Channel Myotonia.

Myotonia congenita (MC) is a rare disorder characterized by stiffness and weakness of the limb and trunk muscles. Mutations in the SCN4A gene encoding the alpha-subunit of the voltage-gated sodium channel Nav1.4 have been reported to be responsible for sodium channel myotonia (SCM). The Nav1.4 channel is expressed in skeletal muscles, and its related channelopathies affect skeletal muscle excitability, which can manifest as SCM, paramyotonia and periodic paralysis. In this study, the missense mutation p.V445M was identified in two individual families with MC. To determine the functional consequences of having a mutated Nav1.4 channel, whole-cell patch-clamp recording of transfected Chinese hamster ovary cells was performed. Evaluation of the transient Na+ current found that a hyperpolarizing shift occurs at both the activation and inactivation curves with an increase of the window currents in the mutant channels. The Nav1.4 channel's co-expression with the Navß4 peptide can generate resurgent Na+ currents at repolarization following a depolarization. The magnitude of the resurgent currents is higher in the mutant than in the wild-type (WT) channel. Although the decay kinetics are comparable between the mutant and WT channels, the time to the peak of resurgent Na+ currents in the mutant channel is significantly protracted compared with that in the WT channel. These findings suggest that the p.V445M mutation in the Nav1.4 channel results in an increase of both sustained and resurgent Na+ currents, which may contribute to hyperexcitability with repetitive firing and is likely to facilitate recurrent myotonia in SCM patients.

Electrophysiological parameters that contribute to the pathogenesis of familial amyloid polyneuropathy caused by transthyretin mutations.

Familial amyloid polyneuropathy (FAP) is a rare, hereditary peripheral neuropathy commonly caused by mutations in human transthyretin (TTR) gene. Clinically, FAP caused by TTR mutations (TTR-FAP) involves both large and small nerve fibers. Details of early electrophysiological features in TTR-FAP remain unclear. To address this issue, we evaluated nerve excitability (NET) results in motor axons of control mice and two transgenic mouse models carrying V30M (TTRV30M) or A97S (TTRA97S) mutations that simulate clinical features of TTR-FAP. Transgenic TTRV30M and TTRA97S mice demonstrated significant increases in latency in hindlimb withdraw tests, as well as, poor rotarod test performance, compared to TTRORF mice. NET evaluation showed reduced S2 accommodation, and increased TEdundershoot during threshold electrotonus (TE) in motor axons of both TTRV30M and TTRA97S mice, indicating that axonal membranes were in a depolarized state. Decreased rheobase combined with increased refractoriness in the transgenic mice suggested that there were reduced sodium currents. Further immunohistochemical study of the sciatic nerves revealed the significantly decrease of voltage-gated sodium channel expression in the transgenic mice. Moreover, superexcitability during the recovery cycle is significantly increased in transgenic mice compared with control mice, which is attributed to increased internodal capacitance. Finally, the electron microscopy demonstrated the reduced g-ratio in TTRA97S mice may correlate with an atrophic change of axons and/or increase of myelin thickness. In summary, we evaluated NET results in transgenic mice which modeled the clinical features presented in TTR-FAP patients. Reduced sodium channel expression and increased internodal capacitance are factors contributing to the electrophysiological changes in TTR-FAP.

Cardiac manifestations and prognostic implications of hereditary transthyretin amyloidosis associated with transthyretin Ala97Ser.

BACKGROUND: The cardiac manifestations of late-onset hereditary transthyretin amyloidosis with p.A97S variant have not been extensively studied, and the prognostic factors remain unclear. METHODS: The clinical profile, echocardiography, and ECG of patients diagnosed with ATTR p.A97S polyneuropathy between 2000 and 2016 were retrospectively collected. 67 patients with ATTR p.A97S were collected. RESULTS: A total of 82% of patients met the criteria for left ventricular (LV) hypertrophy. Reduced global longitudinal strain (GLS) was noted in 42.1% of patients, and 14% of patients had a relative apical sparing pattern. A low voltage pattern in the ECG was observed in 31.3% of patients, while 64.2% presented with a pseudoinfarction pattern. End-systolic LV inner dimension (HR: 2.25 (95% CI: 1.01-5.01), p = 0.048), reduced GLS (HR: 5.26 (1.08-25.0), p = 0.039), relative apical longitudinal strain (RALS>1, HR: 8.57 (1.69-43.3), p = 0.009), increased E/A ratio (HR: 6.51 (1.17-36.4), p = 0.033), and increased QRS duration (HR: 1.02 (1.00-1.04), p = 0.05) were correlated with reduced survival in univariate analysis. Multivariate analysis revealed reduced RALS was significantly correlated with reduced survival (HR: 13.00 (1.81-93.45), p = 0.011). CONCLUSION: Our findings reveal that ATTR p.A97S is a cardiomyopathy as well as a polyneuropathic syndrome. Routine use of more contemporary echocardiographic techniques are recommended to identify cardiac amyloidosis and provide prognostic information.

Anomalous enhancement of resurgent Na+ currents at high temperatures by SCN9A mutations underlies the episodic heat-enhanced pain in inherited erythromelalgia.

Inherited erythromelalgia (IEM), caused by mutations in Nav1.7 channel is characterized by episodic neuropathic pain triggered especially by warm temperature. However, the mechanism underlying the temperature-dependent episodic attacks of IEM remains elusive. We investigated the electrophysiological effect of temperature changes on Nav1.7 channels with three different mutations, p.I136V, p. I848T, and p.V1316A, both in vitro and in vivo. In vitro biophysical studies of the mutant channels show consistent temperature-dependent enhancement of the relative resurgent currents if normalized to the transient currents, as well as temperature-dependent changes in the time to peak and the kinetics of decay of the resurgent currents, but no congruent temperature-dependent changes in steady-state parameters such as shift of activation/inactivation curves and changes of the absolute size of the window or resurgent currents. In vivo nerve excitability tests (NET) in IEM patients reveal the essentially normal indices of NET at a single stimulus. However, there are evident abnormalities if assessed with preconditioning pulses, such as the decrease of threshold elevation in hyperpolarizing threshold electrotonus (50-100 ms), the increase of inward rectification in current-voltage curve, and the increase of refractoriness at the interpulse interval of 2-6 ms in recovery cycle, probably also implicating derangements in temperature dependence of inactivation and of recovery from inactivation in the mutant channels. The pathogenesis of heat-enhanced pain in IEM could be attributed to deranged temperature dependence of Nav1.7 channels responsible for the genesis of resurgent currents.

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.

The Biophysical Basis Underlying Gating Changes in the p.V1316A Mutant Nav1.7 Channel and the Molecular Pathogenesis of Inherited Erythromelalgia.

The Nav1.7 channel critically contributes to the excitability of sensory neurons, and gain-of-function mutations of this channel have been shown to cause inherited erythromelalgia (IEM) with neuropathic pain. In this study, we report a case of a severe phenotype of IEM caused by p.V1316A mutation in the Nav1.7 channel. Mechanistically, we first demonstrate that the Navß4 peptide acts as a gating modifier rather than an open channel blocker competing with the inactivating peptide to give rise to resurgent currents in the Nav1.7 channel. Moreover, there are two distinct open and two corresponding fast inactivated states in the genesis of resurgent Na+ currents. One is responsible for the resurgent route and practically existent only in the presence of Navß4 peptide, whereas the other is responsible for the "silent" route of recovery from inactivation. In this regard, the p.V1316A mutation makes hyperpolarization shift in the activation curve, and depolarization shift in the inactivation curve, vividly uncoupling inactivation from activation. In terms of molecular gating operation, the most important changes caused by the p.V1316A mutation are both acceleration of the transition from the inactivated states to the activated states and deceleration of the reverse transition, resulting in much larger sustained as well as resurgent Na+ currents. In summary, the genesis of the resurgent currents in the Nav1.7 channel is ascribable to the transient existence of a distinct and novel open state promoted by the Navß4 peptide. In addition, S4-5 linker in domain III where V1316 is located seems to play a critical role in activation-inactivation coupling, chiefly via direct modulation of the transitional kinetics between the open and the inactivated states. The sustained and resurgent Na+ currents may therefore be correlatively enhanced by specific mutations involving this linker and relevant regions, and thus marked hyperexcitability in corresponding neural tissues as well as IEM symptomatology.

The Temporal Profiles of Changes in Nerve Excitability Indices in Familial Amyloid Polyneuropathy.

Familial amyloid polyneuropathy (FAP) caused by a mutation in transthyretin (TTR) gene is an autosomal dominant inherited disorder. The aim of this study is to explore the pathophysiological mechanism of FAP. We prospectively recruited 12 pauci-symptomatic carriers, 18 patients who harbor a TTR mutation, p.A97S, and two-age matched control groups. Data of nerve excitability test (NET) from ulnar motor and sensory axons were collected.NET study of ulnar motor axons of patients shows increased threshold and rheobase, reduced threshold elevation during hyperpolarizing threshold electrotonus (TE), and increased refractoriness. In sensory nerve studies, there are increased threshold reduction in depolarizing TE, lower slope of recovery and delayed time to overshoot after hyperpolarizing TE, increased refractoriness and superexcitability in recovery cycle. NET profiles obtained from the ulnar nerve of carriers show the increase of threshold and rheobase, whereas no significant threshold changes in hyperpolarizing TE and superexcitability. The regression models demonstrate that the increase of refractoriness and prolonged relative refractory period are correlated to the disease progression from carriers to patients. The marked increase of refractoriness at short-width stimulus suggests a defect in sodium current which may represent an early, pre-symptomatic pathophysiological change in TTR-FAP. Focal disruption of basal lamina and myelin may further increase the internodal capacity, manifested by the lower slope of recovery and delayed time to overshoot after hyperpolarization TE as well as the increase of superexcitability. NET could therefore make a pragmatic tool for monitoring disease progress from the very early stage of TTR-FAP.

Motor and Sensory Axon Excitability Properties From the Median and Ulnar Nerves and the Effects of Age on These Properties.

PURPOSE: Threshold tracking is a new noninvasive approach for detecting axonal excitability changes in vivo. In this study, the authors compared the excitability indices of motor and sensory axons of median and ulnar nerves to determine whether the two nerves behave in a similar or a noninterchangeable way. They also examined whether age affects these indices. METHODS: Seventy normal subjects aged 22-70 years (mean, 36.7 ± 12.5 years) were recruited. Multiple excitability indices were measured in both motor and sensory axons from median and ulnar nerves. RESULTS: The threshold and rheobase were significantly higher for the ulnar motor axons recorded at the first dorsal interosseous muscle than for the median motor axons at the abductor pollicis brevis muscle. In contrast, the strength-duration time constant was decreased, threshold electrotonus reduction (in both depolarizing and hyperpolarizing directions) was significantly smaller, I/V slope was decreased, and subexcitability was reduced for the ulnar motor axons. Excitability indices measured in the sensory axons of both nerves were not overtly different. In R-square analysis, age had a homogeneous influence on sensory axon excitability but heterogeneous influence on motor axon excitability. CONCLUSIONS: The excitability indices may be interchangeable for sensory axons but not motor axons. The authors therefore recommend recording motor axonal excitability in various muscle groups rather than a single muscle group.

Early-onset autosomal-recessive parkinsonian-pyramidal syndrome.

Genetic factors have been known to contribute to familial Parkinson's disease (PD), one of the most common neurodegenerative disorders. During the past decade, six of eleven causative genes linked to familial forms of PD have been identified to associate with autosomal-recessive young-onset Levodopa-responsive parkinsonism. Among these genes, mutations in Parkin, PINKl and DJ-1 are associated with a relatively typical parkinsonian phenotype with sustained treatment response to Levodopa. However, mutations inATP13A2, PLA2G6 and FBX07 are often associated with rapidly progressive parkinsonism and with additional features including pyramidal signs, cognitive decline and loss of sustained Levodopa responsiveness.Clarifying the phenotypes of each of these autosomal-recessive parkinsonian-pyramidal syndromes and understanding the mechanism ot these causative gene products might illuminate the pathogenesis of dopaminergic neuronal degeneration also in the common forms of PD.