Factors in the cerebrospinal fluid of multiple sclerosis patients interfering with voltage-dependent sodium channels

Temperature-Dependent Auditory Neuropathy: Is it an Acoustic Uhthoff-like Phenomenon?; A Case Report

Objectives:

We describe the case of a young girl in whom transient deafness occurred when her core body temperature rose.

Methods:

The patient was referred for a series of audiological and neurologic evaluations performed over time in both afebrile and febrile states, as well as after a stress test (with a treadmill) in which the body temperature rise simulated the febrile state.

Results:

The patient was found to have a temporary bilateral hearing loss, but had normal distortion product otoacoustic emissions. Moreover, auditory brain stem responses revealed the absence of neural synchrony when her core body temperature increased.

Conclusions:

These results are consistent with a temperature-dependent auditory neuropathy, a rare condition in which patients show normal outer hair cell function and abnormal neural function of the eighth cranial nerve. The symptom is reminiscent of Uhthoff's phenomenon, which is described as transient visual loss and is usually observed in multiple sclerosis. This case of temperature-dependent auditory neuropathy is noteworthy because it sheds light on a disorder of which there have been few reports in the literature. We discuss its similarity to Uhthoff's phenomenon.

The Pentapeptide QYNAD Does Not Inhibit Neuronal Network Activity

Background:

Controversial data was published about the sodium channel-blocking effect of the endogenous pentapeptide QYNAD, which is elevated in patients with multiple sclerosis and Guillain-Barré-syndrome. In some experiments with single cells and nerve preparations QYNAD inhibited sodium currents to the same extent as the known sodium channel blocker lidocaine whereas in other laboratory testing QYNAD failed to show any effect at all.

Methods:

Micro-electrode arrays with cultured neuronal networks are highly suitable to determine neuroactive activity of applied substances. The impact on electrophysiological parameter changes was compared between QYNAD and the established sodium channel blockers lidocaine and tetrodotoxin (TTX).

Results:

QYNAD did not alter network activity whereas the sodium channel blockers lidocaine (IC50 14.9 µM) and tetrodotoxin (IC50 1.1 nM) reversibly decreased network activity in similar concentrations as in patch-clamp experiments. This decrease of spontaneous electrophysiological activity was achieved by prolonging the interburst-interval.

Conclusion:

Although QYNAD might have mild effects on single-cell sodium currents, there is no significant effect on neuronal network function. These results raise concerns about QYNAD exhibiting a relevant impact on functional disability of the central nervous system in patients.

Modulation of neuronal activity by the endogenous pentapeptide QYNAD

Inflammation and demyelination both contribute to the neurological deficits characteristic of multiple sclerosis. Neurological dysfunctions are attributable to inflammatory demyelination and, in addition, to soluble factors such as nitric oxide, cytokines and antibodies. QYNAD, an endogenous pentapeptide identified in the cerebrospinal fluid of patients with demyelinating disorders, has been proposed to promote axonal dysfunction by blocking sodium channels. The present study aimed at characterizing the properties of QYNAD in acutely isolated thalamic neurons in vitro. QYNAD, but not a scrambled peptide (NYDQA), blocked sodium channels in neurons by shifting the steady-state inactivation to more negative potentials. Blocking properties followed a dose-response curve with a maximum effect at 10 microm. A fluorescently labelled QYNAD analogue with retained biological activity specifically stained thalamic neurons, positive for type II sodium channels, thus demonstrating the specificity of QYNAD binding. Our study confirms and extends previous observations describing QYNAD as a potent sodium channel-blocking agent. These data as well as our preliminary observations in in vivo experiments in an animal model of inflammatory CNS demyelination warrant further in vivo studies in order to clarify the exact pathogenetic role of QYNAD in inflammatory neurological diseases.

Effects on axonal conduction of anti-ganglioside sera and sera from patients with Guillain-Barré syndrome

The efficacy of plasma exchange as a therapy for Guillain-Barré syndrome (GBS) suggests that humoral factors might contribute to the axonal conduction block responsible for the major symptoms of the disease. To explore this possibility, we have applied sera to rat spinal roots in vitro while monitoring axonal conduction. Neither fresh sera from 12 patients with GBS or Miller-Fisher syndrome (MFS), nor serum from rabbits immunised with Campylobacter jejuni from patients with GBS, MFS or gastroenteritis were effective in causing acute conduction block, despite the presence of antibodies to gangliosides GD3, GM1, GQ1b and GT1a. Potential explanations are advanced.

An endogenous pentapeptide acting as a sodium channel blocker in inflammatory autoimmune disorders of the central nervous system

Reversible blockade of sodium channels by endogenous substances has been claimed to account for the fast exacerbations and relapses commonly seen in demyelinating autoimmune diseases. Evidence has been provided that in the cerebrospinal fluid of patients with multiple sclerosis or Guillain-Barré syndrome, a sodium-channel-blocking factor exists that has properties of local anesthetic agents. This factor could contribute to the nerve conduction block and paresis seen in these disorders. We describe here a previously unknown endogenous substance in human cerebrospinal fluid with distinct channel-blocking properties even at very low (0.00001 M) concentrations. The pentapeptide with the sequence Gln-Tyr-Asn-Ala-Asp exerted its blocking action by shifting the steady-state inactivation curve of the sodium channels to more-negative potentials, as most local anesthetics do. In the cerebrospinal fluid of healthy individuals, its concentration was about 3 microM, whereas in patients with multiple sclerosis and Guillain-Barré syndrome, it increased 300-1,400%. At these concentrations, the peptide's blocking efficacy was higher than that of 50 microM lidocaine. At a concentration of 10 microM, lidocaine is able to 'unmask' subclinical lesions in multiple sclerosis; thus, the endogenous pentapeptide may well contribute to the fast changes of symptoms. Furthermore, it may become valuable as a marker of disease activity.

The small sodium-channel blocking factor in the cerebrospinal fluid of multiple sclerosis patients is probably an oligopeptide

An endogenous factor that is able to reduce the fast transient sodium current of excitable cells has been reported to exist in the cerebrospinal fluid (CSF) of multiple sclerosis (MS) patients. This was confirmed with nine clinically definite MS patients in the acute relapse. In order to purify and chemically identify the factor, microconcentration and gel filtration high-performance liquid chromatography (HPLC) were applied. After each purification step the activity-containing fraction was determined using a biological assay. With all CSFs the activity was contained in the fraction corresponding to 600-800 Da molecular weight, indicating that the factor is chemically homogeneous. The biological activity of the CSF specimens was not correlated to the laboratory CSF data; however, it was correlated to the area under the 210 nm UV light absorption peak in the corresponding chromatogram, i.e. the 600-800 Da MW fraction. As the factor was degradable by acid hydrolysis and a carboxypeptidase, it is suggested that it might be a small peptide.

Pathogenesis of Guillain-Barré syndrome

Recent neurophysiological and pathological studies have led to a reclassification of the diseases that underlie Guillain-Barré syndrome (GBS) into acute inflammatory demyelinating polyradiculoneuropathy (AIDP), acute motor and sensory axonal neuropathy (AMSAN) and acute motor axonal neuropathy (AMAN). The Fisher syndrome of ophthalmoplegia, ataxia and areflexia is the most striking of several related conditions. Significant antecedent events include Campylobacter jejuni (4-66%), cytomegalovirus (5-15%), Epstein-Barr virus (2-10%), and Mycoplasma pneumoniae (1-5%) infections. These infections are not uniquely associated with any clinical subtype but severe axonal degeneration is more common following C. jejuni and severe sensory impairment following cytomegalovirus. Strong evidence supports an important role for antibodies to gangliosides in pathogenesis. In particular antibodies to ganglioside GM1 are present in 14-50% of patients with GBS, and are more common in cases with severe axonal degeneration associated with any subtype. Antibodies to ganglioside GQ1b are very closely associated with Fisher syndrome, its formes frustes and related syndromes. Ganglioside-like epitopes exist in the bacterial wall of C. jejuni. Infection by this and other organisms triggers an antibody response in patients with GBS but not in those with uncomplicated enteritis. The development of GBS is likely to be a consequence of special properties of the infecting organism, since some strains such as Penner 0:19 and 0:41 are particularly associated with GBS but not with enteritis. It is also likely to be a consequence of the immunogenetic background of the patient since few patients develop GBS after infection even with one of these strains. Attempts to match the subtypes of GBS to the fine specificity of anti-ganglioside antibodies and to functional effects in experimental models continue but have not yet fully explained the pathogenesis. T cells are also involved in the pathogenesis of most or perhaps all forms of GBS. T cell responses to any of three myelin proteins, P2, PO and PMP22, are sufficient to induce experimental autoimmune neuritis. Activated T cells are present in the circulation in the acute stage, up-regulate matrix metalloproteinases, cross the blood-nerve barrier and encounter their cognate antigens. Identification of the specificity of these T cell responses is still at a preliminary stage. The invasion of intact myelin sheaths by activated macrophages is difficult to explain according to a purely T cell mediated mechanism. The different patterns of GBS are probably due to the diverse interplay between antibodies and T cells of differing specificities.

The pathophysiology of multiple sclerosis: the mechanisms underlying the production of symptoms and the natural history of the disease

The pathophysiology of multiple sclerosis is reviewed, with emphasis on the axonal conduction properties underlying the production of symptoms, and the course of the disease. The major cause of the negative symptoms during relapses (e.g. paralysis, blindness and numbness) is conduction block, caused largely by demyelination and inflammation, and possibly by defects in synaptic transmission and putative circulating blocking factors. Recovery from symptoms during remissions is due mainly to the restoration of axonal function, either by remyelination, the resolution of inflammation, or the restoration of conduction to axons which persist in the demyelinated state. Conduction in the latter axons shows a number of deficits, particularly with regard to the conduction of trains of impulses and these contribute to weakness and sensory problems. The mechanisms underlying the sensitivity of symptoms to changes in body temperature (Uhthoff's phenomenon) are discussed. The origin of 'positive' symptoms, such as tingling sensations, are described, including the generation of ectopic trains and bursts of impulses, ephaptic interactions between axons and/or neurons, the triggering of additional, spurious impulses by the transmission of normal impulses, the mechanosensitivity of axons underlying movement-induced sensations (e.g. Lhermitte's phenomenon) and pain. The clinical course of the disease is discussed, together with its relationship to the evolution of lesions as revealed by magnetic resonance imaging and spectroscopy. The earliest detectable event in the development of most new lesions is a breakdown of the blood-brain barrier in association with inflammation. Inflammation resolves after approximately one month, at which time there is an improvement in the symptoms. Demyelination occurs during the inflammatory phase of the lesion. An important mechanism determining persistent neurological deficit is axonal degeneration, although persistent conduction block arising from the failure of repair mechanisms probably also contributes.

Immunologically induced electrophysiological dysfunction: implications for inflammatory diseases of the CNS and PNS

During inflammation of the central or peripheral nervous system, a high number of immunologically active molecules, including bacterial or viral products as well as host-derived cytokines, are released. Patients suffering from inflammatory CNS or PNS diseases often develop transient symptoms with a rapid recovery, which obviously cannot be accounted for by immunologically induced tissue damage. These observations led to the hypothesis that immunologically active molecules can affect directly the electrophysiological functions of neurons and glial cells. Evidence for this hypothesis came from in vitro studies showing that cytokines, such as interleukins or tumor necrosis factors, arachidonic acid and its metabolites, interfere with electrophysiological properties of neurons or glial cells. These molecules affect ion currents, intracellular Ca2+ homeostasis, membrane potentials, and suppress or enhance the induction and maintenance of long-term potentiation. Similarly, virus proteins from human immunodeficiency virus type I were found to alter intracellular Ca2+ concentrations of neurons and astrocytes by modulating either transmitter receptors and channels or membrane transporters. Cerebrospinal fluid from MS patients contains factors which increase Na+ current inactivation and thereby reduce neuronal excitability. Immunoglobulins in sera of patients suffering from multifocal motor neuropathy and from acquired neuromyotonia interfere with nerve fibers, inducing alterations of conduction. Increased knowledge of these mechanisms will help to explain the pathogenesis of neurological symptoms and may provide a rationale for new therapeutic strategies.

Mechanism of modulation of the voltage-gated skeletal and cardiac muscle sodium channels by fatty acids

Voltage-gated rat skeletal muscle and cardiac Na+ channels are modulated by exogenous unsaturated fatty acids. Application of 1-10 microM arachidonic or oleic acids reversibly depressed Na+ channel conductance and shifted the inactivation curve to hyperpolarizing potentials. These effects were not prevented by inhibitors of lipoxygenase, cyclooxygenase, cytochrome P-450 epoxygenase, or protein kinase C. Neither palmitic acid nor methyl ester oleate had an effect on the inward Na+ current, suggesting that trivial variations in membrane fluidity are not responsible for the Na+ current depression or kinetic changes. Arachidonic acid altered fast Na+ inactivation without changing the slow inactivation kinetics. Moreover, skeletal muscle Na+ channel gating currents were markedly decreased by 2 microM arachidonic acid. Finally, nonstationary noise analysis indicated that both the number of channels and the open probability were slightly decreased without change in the single-channel conductance. These data suggest that unsaturated fatty acids such as arachidonic and oleic acids 1) specifically regulate voltage-gated Na+ channels and 2) interact directly with Na+ channels, perhaps at a fatty acid binding domain, by decreasing the total gating charge and altering fast-inactivation kinetics.

The human endogenous local anesthetic-like factor (ELLF) is functionally neutralized by serum albumin

The cerebrospinal fluid (CSF) of patients with multiple sclerosis or Guillain-Barré syndrome contains a factor that inhibits excitation of nerve and muscle cells like local anesthetics. CSF samples containing the endogenous local anesthetic-like factor (ELLF) were analyzed by gel filtration chromatography and ultraviolet (UV) absorption at 210 nm. The active component was in a single peak corresponding to a molecular weight of 600-800 Da. This peak was decreased and the Na+ channel blocking activity was neutralized by the addition of 40 g/l human serum albumin to the CSF. When the albumin was separated from the CSF/albumin mixture by acetonitrile treatment, the Na+ channel blocking activity reappeared. The ELLF and its neutralization may be of relevance for the clinical fluctuations known with these diseases.

On the nature of endogenous antiexcitatory factors in the cerebrospinal fluid of patients with demyelinating neurological disease

The cerebrospinal fluid (CSF) of patients with demyelinating neurological disease, such as Guillain-Barré syndrome or multiple sclerosis, contains factors that inhibit the excitatory Na+ current. Such antiexcitatory factors are occasionally also detectable in CSF from patients with other neurological diseases but were absent from an artificial CSF containing all major CSF constituents (electrolytes, amino acids, vitamins, metabolites, albumin). In an attempt to characterize these factors, unphysiological pCa or pH values were excluded by the application of the Ca2+ chelator EGTA and the use of buffers. Heating the CSF for 10 min to 95 degrees C or digesting it with proteases did not destroy the antiexcitatory potency. Fractionation of the CSF contents according to molecular weight showed that the factors have a molecular weight < 3 kD. This excludes proteins, such as antibodies or cytokines, as candidates. Small peptides are known to be resistant to some proteases and heating.

Serine-1321-independent regulation of the mu 1 adult skeletal muscle Na+ channel by protein kinase C

The adult skeletal muscle Na+ channel mu1 possesses a highly conserved segment between subunit domains III and IV containing a consensus protein kinase C (PKC) phosphorylation site that, in the neuronal isoform, acts as a master control for "convergent" regulation by PKC and cAMP-dependent protein kinase. It lacks an approximately 200-aa segment between domains I and II though to modulate channel gating. We here demonstrate that mu1 is regulated by PKC (but not cAMP-dependent protein kinase) in a manner distinct from that observed for the neuronal isoforms, suggesting that under the same conditions muscle excitation could be uncoupled from motor neuron input. Maximal phosphorylation by PKC, in the presence of phosphatase inhibitors, reduced peak Na+ currents by approximately 90% by decreasing the maximal conductance, caused a -15 mV shift in the midpoint of steady-state inactivation, and caused a slight speeding of inactivation. Surprisingly, these effects were not affected by mutation of the conserved serine (serine-1321) in the interdomain III-IV loop. the pattern of current suppression and gating modification by PKC resembles the response of muscle Na+ channels to inhibitory factors present in the serum and cerebrospinal fluid of patients with Guillain-Barré syndrome, multiple sclerosis, and idiopathic demyelinating polyradiculoneuritis.

Cerebrospinal fluid and serum from patients with inflammatory polyradiculoneuropathy have opposite effects on sodium channels

The effects of cerebrospinal fluid (CSF) and serum from patients having Guillain-Barré syndrome (GBS) or chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) on voltage-dependent Na+ channels were compared. Bathing human myoballs in CSF substantially reduced their Na+ currents (by > 40% with 8 of 10 patients) elicited at 1 Hz under whole-cell recording conditions. This was because, at the resting potential, more Na+ channels were inactivated (left-shift of the h infinity curve). CSF from patients with other neurological diseases (OND) produces a similar, but smaller, effect. In contrast, serum samples from the same GBS and OND patients caused an increase of the Na+ currents by reducing the number of Na+ channels inactivated at the resting potential. This right-shift of the h infinity curve is in part explained by the effect of serum albumin. We confirm that the CSF of most GBS and CIDP patients contains factors inhibiting voltage-dependent Na+ currents. There is no indication that such factors are effective in the serum of these patients.

Local anaesthetic-like effect of interleukin-2 on muscular Na+ channels: no evidence for involvement of the IL-2 receptor

The effects of interleukin-2 (IL-2) on muscular Na+ currents were studied in human myoballs. The transient Na+ inward currents, elicited by repetitive depolarizations at 1 Hz and recorded in the whole-cell mode, were inhibited by the cytokine, the half-maximum effect occurring at about 500 U/ml. The effects resembled those of local anaesthetics without use dependence, as the inactivation (h infinity) curve was shifted in a negative direction while the current/voltage curve was not affected. As with these local anaesthetics, depolarization at 1, 4 and 8 Hz in the presence of IL-2 did not produce any cumulative block. The interaction of IL-2 with the Na+ channels is very fast (within ms) and it is suggested that it occurs when the Na+ channels are in the state of fast inactivation. The recovery from inactivation was only slightly slowed by IL-2, in agreement with the absence of any use dependence. All effects were readily reversible on washout of the cytokine. The effects were seen both in tetrodotoxin-(TTX)-sensitive adult Na+ channels and in TTX-insensitive juvenile channels. In contrast to the whole-cell configuration, no inhibition was visible in the attached-patch configuration. Further, the preincubation with an anti-IL-2-receptor antibody did not prevent the inhibitory effect of IL-2 on the Na+ currents. It is concluded that the cytokine blocks the voltage-dependent muscular Na+ channels by keeping the channels in the state of fast inactivation. An IL-2 receptor and a second messenger system are not likely to be involved in this reaction.