Paranodal Myelin Research Articles

IgG subclass shifts occurring at acute exacerbations in autoimmune nodopathies.

Autoimmune nodopathy associated with anti-contactin1 (CNTN1) IgG4 antibodies frequently manifests as acute axonal degeneration in addition to detachment of the paranodal myelin loops. The acute destruction of myelinated nerve fibers does not match the function of IgG4, which cannot activate the complement pathway. IgG subclass switching from IgG1 or IgG3 to IgG4 has been observed in some patients with autoimmune diseases associated with IgG4 throughout their disease course. Serial changes in IgG subclasses, clinico-neurophysiological features, and nerve and renal pathology were reviewed in three patients with anti-CNTN1-associated autoimmune nodopathy and one patient with anti-contactin-associated protein1 (Caspr1) autoimmune nodopathy. All four patients had predominantly IgG4 autoantibodies, whereas they showed evidence of acute axonal degeneration. The IgG1 subclass was present in all patients at their progressing stage but then disappeared at follow-up. Nerve pathology in the patients with anti-CNTN1 and anti-Caspr1 autoimmune nodopathies showed both structural changes in the paranodes and evidence of acute axonal degeneration. Renal biopsy specimens from two patients with membranous glomerulonephritis and anti-CNTN1 autoimmune nodopathy showed deposition of IgG1 and complement on the glomerular basement membrane, as well as IgG4. In patients with autoimmune nodopathies associated with anti-CNTN1 and anti-Caspr1 IgG4 antibodies, IgG1 subclass autoantibodies were present at their acute exacerbations and might have contributed to the axonal degeneration and glomerular injury. IgG1 disappeared with the cessation of disease progression, which indicates that the IgG1 subclass is a possible biomarker of disease activity.

Targeted inactivation of the Septin2 and Septin9 genes in myelinating Schwann cells of mice.

The formation of axon-enwrapping myelin sheaths by oligodendrocytes in the central nervous system involves the assembly of a scaffolding septin filament comprised of the subunits SEPTIN2, SEPTIN4, SEPTIN7 and SEPTIN8. Conversely, in the peripheral nervous system (PNS), myelin is synthesized by a different cell type termed Schwann cells, and it remained unknown if septins also assemble as a multimer in PNS myelin. According to prior proteome analysis, PNS myelin comprises the subunits SEPTIN2, SEPTIN7, SEPTIN8, SEPTIN9, and SEPTIN11, which localize to the paranodal and abaxonal myelin subcompartments. Here, we use the Cre/loxP-system to delete the Septin9-gene specifically in Schwann cells, causing a markedly reduced abundance of SEPTIN9 in sciatic nerves, implying that Schwann cells are the main cell type expressing SEPTIN9 in the nerve. However, Septin9-deficiency in Schwann cells did not affect the abundance or localization of other septin subunits. In contrast, when deleting the Septin2-gene in Schwann cells the abundance of all relevant septin subunits was markedly reduced, including SEPTIN9. Notably, we did not find evidence that deleting Septin2 or Septin9 in Schwann cells impairs myelin biogenesis, nerve conduction velocity or motor/sensory capabilities, at least at the assessed timepoints. Our data thus show that SEPTIN2 but not SEPTIN9 is required for the formation or stabilization of a septin multimer in PNS myelin in vivo; however, its functional relevance remains to be established.

Anti-pan-neurofascin antibodies induce subclass-related complement activation and nodo-paranodal damage.

Autoimmune neuropathy associated with antibodies against pan-neurofascin is a new subtype of nodo-paranodopathy. It is relevant because it is associated with high morbidity and mortality. Affected patients often require intensive care unit treatment for several months, and data on the reversibility and long-term prognosis are limited. The pathogenicity including IgG subclass-associated mechanisms has not been unravelled, nor directly compared to anti-neurofascin-155 IgG4-related pathology. Understanding the underlying pathology might have a direct impact on treatment of these severely affected patients. By a multicentre combined prospective and retrospective approach, we provide clinical data of a large cohort of patients with anti-neurofascin-associated neuropathy (n = 18) including longitudinal titre and neurofilament light chain assessment via Ella® and relate clinical data to in vitro pathogenicity studies of anti-neurofascin antibodies. We assessed antibody binding characteristics and the pathogenic effects of anti-pan-neurofascin versus neurofascin-155 antibodies on living myelinating dorsal root ganglia co-cultures. Additionally, we analysed the IgG subclass profile and the complement binding capacity and effector functions considering the effects of intravenous immunoglobulin preparations via enzyme-linked immunosorbent and cell-based assays. In contrast to chronic neurofascin-155 IgG4-associated neuropathy, anti-pan-neurofascin-associated disease presented with a high morbidity and mortality, but as a monophasic and potentially reversible disorder. During follow-up, antibodies were no longer detectable in 8 of 11 patients. Anti-pan-neurofascin had direct access to the nodes of Ranvier in myelinating cultures titre-dependently, most probably inducing this severe phenotype. Antibody preincubation led to impaired paranode formation, destruction of paranodal architecture and alterations on paranodal myelin and sensory neurons in the cultures, with more severe effects than neurofascin-155 antibodies. Besides IgG4, subclass IgG3 was detected and associated with complement binding and cytotoxic effects in vitro. As a possible correlate of axonal damage in vivo, we detected highly increased serum neurofilament light chain levels (sNF-L), correlating to serum C3a. Still, sNF-L was not identified as a marker for poor prognosis, but rather as an intra- and interindividual marker for acuteness, severity and course, with a strong decrease during recovery. Our data provide evidence that anti-pan-neurofascin antibodies directly attack the node and induce severe and acute, but potentially reversible, nodo-paranodal pathology, possibly involving complement-mediated mechanisms. Screening for autoantibodies thus is crucial to identify this subset of patients who benefit from early antibody-depleting therapy. Titre and sNF-L might serve as valuable follow-up parameters. The prospect of a favourable outcome has high relevance for physicians, patients and relatives during months of critical care.

White matter tract conductivity is resistant to wide variations in paranodal structure and myelin thickness accompanying the loss of Tyro3: an experimental and simulated analysis.

Myelination within the central nervous system (CNS) is crucial for the conduction of action potentials by neurons. Variation in compact myelin morphology and the structure of the paranode are hypothesised to have significant impact on the speed of action potentials. There are, however, limited experimental data investigating the impact of changes in myelin structure upon conductivity in the central nervous system. We have used a genetic model in which myelin thickness is reduced to investigate the effect of myelin alterations upon action potential velocity. A detailed examination of the myelin ultrastructure of mice in which the receptor tyrosine kinase Tyro3 has been deleted showed that, in addition to thinner myelin, these mice have significantly disrupted paranodes. Despite these alterations to myelin and paranodal structure, we did not identify a reduction in conductivity in either the corpus callosum or the optic nerve. Exploration of these results using a mathematical model of neuronal conductivity predicts that the absence of Tyro3 would lead to reduced conductivity in single fibres, but would not affect the compound action potential of multiple myelinated neurons as seen in neuronal tracts. Our data highlight the importance of experimental assessment of conductivity and suggests that simple assessment of structural changes to myelin is a poor predictor of neural functional outcomes.

Pathogenesis of acute and chronic inflammatory neuropathies

Guillain-Barre syndrome (GBS) is classified into axonal and demyelinating categories. Acute motor axonal neuropathy (AMAN), an axonal subtype, was widely recognized in the 1990's. Whereas in Europe and North America, classical demyelinating GBS is the major subtype, AMAN is found for 30–80% of GBS cases in Asia. Over the past 25 years, major advances have been made in understanding the immunopathogenesis of AMAN; animal model studies have shown that there is good evidence to support an anti-GM1 antibody-mediated immune process, triggered by molecular mimicry between gangliosides of motor axons and the microorganism. Complement-mediated attack leads to disruption of nodal sodium channel clusters, and paranodal myelin detachment that is responsible for axonal dysfunction/degeneration in AMAN. CIDP is currently classified into “typical CIDP” and other variants such as “multifocal CIDP (mononeuropathy multiplex type)” and distal CIDP. Typical CIDP is a classic form, characterized by symmetric proximal and distal muscle weakness and motor-dominant manifestation. In typical CIDP, demyelination predominantly affects the distal nerve terminals and nerve roots, presumably because of the lack of the blood-nerve barrier in these regions, and this could be responsible for the “non-nerve length-dependent” distribution of muscle weakness. These findings suggest an importance of humoral immunity in typical CIDP. In contrast, multifocal CIDP is characterized by multifocal demyelination in the intermediate nerve trunks, suggesting that cellular immunity may be predominantly involved in breakdown of the blood-nerve barrier at the site of conduction block. The understanding of the pathophysiology of each CIDP subtype is important for appropriate immune-treatments.

SY7.2. Electrophysiological features in GBS and CIDP

GBS is currently classified into axonal and demyelinating categories. Acute motor axonal neuropathy (AMAN), an axonal subtype of GBS, was widely recognized in the 1990s. Whereas in Europe and North America, acute inflammatory demyelinating polyneuropathy (AIDP) is considered the major subtype of GBS, in East Asia, and Central and South America, AMAN is found for 30–80% of GBS cases. Over the past 20 years, major advances have been made in understanding the immunopathogenesis and pathophysiology of AMAN. Clinically, AMAN is characterized by pure motor involvement, frequent antecedent Campylobacter jejuni enteritis, and serum anti-ganglioside antibodies. Electrophysiological studies frequently reveal, as well as axonal degeneration, rapidly reversible nerve conduction block/slowing (resolved within days to a few weeks); the time-course suggests functional or microstructural changes at the nodes of Ranvier. It is now established that disrupted nodal sodium channel clusters, and paranodal myelin detachment is responsible for conduction block in AMAN. CIDP is currently classified into “typical CIDP” and other variants such as “multifocal acquired demyelinating sensory and motor neuropathy (MADSAM)”. Typical CIDP is a classic form, clinically characterized by symmetric proximal and distal muscle weakness and motor- dominant manifestation. In typical CIDP, demyelination predominantly affects the distal nerve terminals and nerve roots, presumably because of the lack of the blood-nerve barrier in these regions, and this could be responsible for the “non-nerve length-dependent” distribution of muscle weakness. These findings suggest an importance of humoral immunity in typical CIDP. In contrast, MADSAM is characterized by multifocal demyelination in the intermediate nerve trunks. In MADSAM neuropathy, cellular immunity may be predominantly involved in breakdown of the blood-nerve barrier at the site of conduction block. Among the CIDP spectrum, typical CIDP and MADSAM are the major subtypes, and their pathophysiology appears to be distinct. The understanding of the pathophysiology of each CIDP subtype is important for appropriate immune treatments.

P-PN029. Clinical and electrophysiological profile of patients with Guillain-Barré Syndrome with conduction blocks

Introduction . Clinical data is limited in patients with Guillain Barre syndrome (GBS) who show conduction block (CB) on nerve conduction study (NCS). We aim to describe the clinical and electrophysiological profiles of GBS with CB. Methods . Between 2017 and 2020, 111 patients fulfilling the National Institute of Neurological Disorders and Stroke (NINDS) criteria for GBS and Brighton level of certainty one or two were seen in a single neurology unit. NCS of three motor (median, ulnar and common peroneal (CP)) and three sensory (median, ulnar and sural) nerves were performed. Definite and probable CBs were defined as proximal/distal latency of 20% of lower limit of normal (LLN). Results . Mean interval from symptom-onset to NCS was 9.4±6.5 days. Twenty patients had CB (one nerve: 15, two nerves: 4, three nerves: 1). This included probable CB in median (n=3), ulnar (n=2) and CP (n=4) nerves, and definite CB in median (n=5), ulnar (n=6) and CP (n=6) nerves. Electrophysiological subtypes included demyelinating (n=11), axonal (n=4) and equivocal (n=5). Demographic and clinical features included M:F of 3:1, mean age of 38.5±19.9 years, facial weakness (n=8), ophthalmoparesis (n=1), respiratory muscle weakness (n=2) and antecedent infection (n=12). At the time of peak neurological deficits, the mean Medical Research Council (MRC) sum score was 35.4±14.9 and Hughes disability score (HDS) was 3.5±0.8. The mean duration of hospital stay was 14.7±15.9 days. At the time of discharge, the mean MRC sum score was 39.8±15.2 and mean HDS was 3.1±0.7. Conclusion . We report CBs across non-entrapment sites in both demyelinating and axonal sub-types of GBS. Interruption of saltatory conduction due to distortion of paranodal myelin leads to reversible conduction failure and rapid recovery as noted in the present study.

Guillain–Barré syndrome: pathophysiology, etiology, causes, and treatment

Guillain–Barré syndrome (GBS) is a polyradiculoneuropathy autoimmune disease that is characterized by significant inflammation that affects the peripheral nervous system in a rapidly progressive pattern that is mainly clinically presented by muscle weakness. The present literature review aims to broadly discuss GBS: etiology, pathophysiology and management in order to gain an understading of the existing studies that are relevant to this literature review. Among the reported antibodies, anti-GM1 and anti-GQ1B have been reported to be responsible for attacking and damaging either the neuromuscular junctions or peripheral nerves. Moreover, it has been found that the anti-GD1a antibodies in patients bind to the neuromuscular junction and also bind to the nodes of Ranvier of the peripheral nerves and the paranodal myelin of the affected nerves. Reports have shown that this disease is identified as special forms of neuropathies that develop in immune-mediated, post-infection sequelae. Furthermore, in another study it was reported that Molecular mimicry has been previously reported to significantly correlate with the development of the disease as it was investigated in animal models. In addition, Campylobacter jejuni, a pathogen that causes gastrointestinal infections has been previously reported to predispose to the development of GBS in humans. However, scientists have found that plasma exchange and intravenous immunoglobulins (IVIG) remain the most significant and efficacious factors in managing the disease. Nevertheless, recent trials have investigated other approaches that are less efficacious and can lead to serious adverse events and complications.

The role of macrophages in Guillain‐Barré syndrome and chronic inflammatory demyelinating polyneuropathy

AbstractMacrophages play an important role in the pathogenesis of immune‐mediated neuropathies, particularly Guillain‐Barré syndrome (GBS) and chronic inflammatory demyelinating polyneuropathy (CIDP). Early reports of pathological findings in patients with GBS and CIDP suggested demyelination resulting from phagocytosis of myelin by macrophages as the cause of aberrant nerve conduction in these diseases. Later studies established a concept that molecular mimicry of gangliosides in the peripheral nervous system to surface epitopes of pathogens results in the production of autoantibodies in patients with an axonal form of GBS. In some of the patients diagnosed with CIDP, IgG4 autoantibodies against paranodal junction proteins, such as neurofascin 155 and contactin 1, were found to induce aberrant nerve conduction resulting from the detachment of paranodal myelin terminal loops from axolemma. In contrast, the mechanisms of neuropathy in cases with classical macrophage‐induced demyelination are still to be elucidated. Electron microscopic examination suggested that macrophages recognize specific sites of myelinated fibers as the initial target of demyelination in both GBS and CIDP. It seems that the components that distinguish between the nodal regions, such as the nodes of Ranvier, paranodes, and internodes play a pivotal role in the behavior of macrophages that initiate the phagocytosis of myelin. Further studies are required to clarify the mechanisms of macrophage‐induced demyelination to facilitate the development of new therapeutic strategies directed against the pathology of macrophages.

AAV9-mediated Schwann cell-targeted gene therapy rescues a model of demyelinating neuropathy

Mutations in the GJB1 gene, encoding the gap junction (GJ) protein connexin32 (Cx32), cause X-linked Charcot-Marie-Tooth disease (CMT1X), an inherited demyelinating neuropathy. We developed a gene therapy approach for CMT1X using an AAV9 vector to deliver the GJB1/Cx32 gene under the myelin protein zero (Mpz) promoter for targeted expression in Schwann cells. Lumbar intrathecal injection of the AAV9-Mpz.GJB1 resulted in widespread biodistribution in the peripheral nervous system including lumbar roots, sciatic and femoral nerves, as well as in Cx32 expression in the paranodal non-compact myelin areas of myelinated fibers. A pre-, as well as post-onset treatment trial in Gjb1-null mice, demonstrated improved motor performance and sciatic nerve conduction velocities along with improved myelination and reduced inflammation in peripheral nerve tissues. Blood biomarker levels were also significantly ameliorated in treated mice. This study provides evidence that a clinically translatable AAV9-mediated gene therapy approach targeting Schwann cells could potentially treat CMT1X.

Ultrastructural mechanisms of macrophage-induced demyelination in Guillain-Barré syndrome

ObjectiveTo describe the pathological features of Guillain-Barré syndrome focusing on macrophage-associated myelin lesions.MethodsLongitudinal sections of sural nerve biopsy specimens from 11 patients with acute inflammatory demyelinating polyneuropathy (AIDP) exhibiting macrophage-associated...

The formation of paranodal spirals at the ends of CNS myelin sheaths requires the planar polarity protein Vangl2.

During axonal ensheathment, noncompact myelin channels formed at lateral edges of the myelinating process become arranged into tight paranodal spirals that resemble loops when cut in cross section. These adhere to the axon, concentrating voltage-dependent sodium channels at nodes of Ranvier and patterning the surrounding axon into distinct molecular domains. The signals responsible for forming and maintaining the complex structure of paranodal myelin are poorly understood. Here, we test the hypothesis that the planar cell polarity determinant Vangl2 organizes paranodal myelin. We show that Vangl2 is concentrated at paranodes and that, following conditional knockout of Vangl2 in oligodendrocytes, the paranodal spiral loosens, accompanied by disruption to the microtubule cytoskeleton and mislocalization of autotypic adhesion molecules between loops within the spiral. Adhesion of the spiral to the axon is unaffected. This results in disruptions to axonal patterning at nodes of Ranvier, paranodal axon diameter and conduction velocity. When taken together with our previous work showing that loss of the apico-basal polarity protein Scribble has the opposite phenotype-loss of axonal adhesion but no effect on loop-loop autotypic adhesion-our results identify a novel mechanism by which polarity proteins control the shape of nodes of Ranvier and regulate conduction in the CNS.

Gene replacement therapy after neuropathy onset provides therapeutic benefit in a model of CMT1X.

X-linked Charcot-Marie-Tooth disease (CMT1X), one of the commonest forms of inherited demyelinating neuropathy, results from GJB1 gene mutations causing loss of function of the gap junction protein connexin32 (Cx32). The aim of this study was to examine whether delayed gene replacement therapy after the onset of peripheral neuropathy can provide a therapeutic benefit in the Gjb1-null/Cx32 knockout model of CMT1X. After delivery of the LV-Mpz.GJB1 lentiviral vector by a single lumbar intrathecal injection into 6-month-old Gjb1-null mice, we confirmed expression of Cx32 in lumbar roots and sciatic nerves correctly localized at the paranodal myelin areas. Gjb1-null mice treated with LV-Mpz.GJB1 compared with LV-Mpz.Egfp (mock) vector at the age of 6months showed improved motor performance at 8 and 10months. Furthermore, treated mice showed increased sciatic nerve conduction velocities, improvement of myelination and reduced inflammation in lumbar roots and peripheral nerves at 10months of age, along with enhanced quadriceps muscle innervation. Plasma neurofilament light (NEFL) levels, a clinically relevant biomarker, were also ameliorated in fully treated mice. Intrathecal gene delivery after the onset of peripheral neuropathy offers a significant therapeutic benefit in this disease model, providing a proof of principle for treating patients with CMT1X at different ages.

Simulating perinodal changes observed in immune-mediated neuropathies: impact on conduction in a model of myelinated motor and sensory axons.

Immune-mediated neuropathies affect myelinated axons, resulting in conduction slowing or block that may affect motor and sensory axons differently. The underlying mechanisms of these neuropathies are not well understood. Using a myelinated axon model, we studied the impact of perinodal changes on conduction. We extended a longitudinal axon model (41 nodes of Ranvier) with biophysical properties unique to human myelinated motor and sensory axons. We simulated effects of temperature and axonal diameter on conduction and strength-duration properties. We then studied effects of impaired nodal sodium channel conductance and paranodal myelin detachment by reducing periaxonal resistance, as well as their interaction, on conduction in the 9 middle nodes and enclosed paranodes. Finally, we assessed the impact of reducing the affected region (5 nodes) and adding nodal widening. Physiological motor and sensory conduction velocities and changes to axonal diameter and temperature were observed. The sensory axon had a longer strength-duration time constant. Reducing sodium channel conductance and paranodal periaxonal resistance induced progressive conduction slowing. In motor axons, conduction block occurred with a 4-fold drop in sodium channel conductance or a 7.7-fold drop in periaxonal resistance. In sensory axons, block arose with a 4.8-fold drop in sodium channel conductance or a 9-fold drop in periaxonal resistance. This indicated that motor axons are more vulnerable to developing block. A boundary of block emerged when the two mechanisms interacted. This boundary shifted in opposite directions for a smaller affected region and nodal widening. These differences may contribute to the predominance of motor deficits observed in some immune-mediated neuropathies.NEW & NOTEWORTHY Immune-mediated neuropathies may affect myelinated motor and sensory axons differently. By the development of a computational model, we quantitatively studied the impact of perinodal changes on conduction in motor and sensory axons. Simulations of increasing nodal sodium channel dysfunction and paranodal myelin detachment induced progressive conduction slowing. Sensory axons were more resistant to block than motor axons. This could explain the greater predisposition of motor axons to functional deficits observed in some immune-mediated neuropathies.

O-28 Simulated differential sensitivity of motor and sensory nerves for conduction block in immune-mediated neuropathies

Background Immune-mediated polyneuropathies affect myelinated nerve fibers, resulting in conduction slowing or block, with selective involvement of sensory and motor axons. The underlying mechanisms are not well understood. We studied the impact of putative mechanisms by focusing on nodal and paranodal dysfunction on saltatory conduction in a longitudinal myelinated axon model. Materials and methods We extended our longitudinal axon model, which consisted of 41 nodes of Ranvier, with biophysical characteristics unique for human myelinated motor or sensory axons. We studied the effects of impaired nodal sodium channel conductance, paranodal myelin loop detachment by reducing paranodal seal resistance, and their interaction on saltatory conduction in the nine middle nodes and surrounding paranodes. Results Physiological motor and sensory conduction velocities were approximately 48 m/s and 50 m/s, respectively. Progressive conduction slowing was observed when reducing maximum sodium channel conductance and reducing paranodal seal resistance. Conduction blocks eventually occurred at a 75% (motor) and 78% (sensory) reduction in maximum sodium channel conductance and at a 87% (motor) and 89% (sensory) reduction in paranodal seal resistance. A boundary of block emerged representing the severity levels where both interacting mechanisms induced a conduction block. Conclusion Using our longitudinal myelinated motor and sensory axon model, simulations support the view that biophysical differences between motor and sensory axons potentially contribute to a differential susceptibility for development of a conduction block in immune-mediated polyneuropathies.

P68-S The effect of compression on peripheral nerves in ALS patients

Background Compression over a peripheral nerve leads to several changes in structural and physiological properties of that nerve. Paranodal myelin abnormalities and impaired axonal transport are some pathological incidents defined after acute compression. Materials and methods In sixteen ALS patients and 16 controls, CMAP Scan curves were recorded from first dorsal interosseus (FDI) muscle by stimulating ulnar nerve at elbow before compression, compression in 5 consecutive CMAP scanning cycles, and after compression. Stimulus intensities needed to elicit 10% and 90% of CMAP and step number and step% were calculated for each scan. As a further comparison, areas under CMAP scan curves were examined using natural values, percentages, and differences between compression (comp) cycles. Results For both patients and controls, conventional CMAP scan parameters were not significantly different in consecutive cycles. During compression, a tendency of reduction in area under CMAP scan curve of ALS patients (164.8 mVms, 156.3 mVms, 146.5 mVms, 142.1 mVms, 142.7 mVms, 137 mVms, and 174.9 mVms for pre-comp, 1st comp, 2nd comp, 3rd comp, 4th comp, 5th comp, and post- comp cycles, respectively) and a tendency of increase in CMAP scan curve area of healthy subjects (283.3 mVms, 315.6 mVms, 298.5 mVms, 319.7 mVms, 336.1 mVms, 361.4 and 308.1 mVms for pre-comp, 1st comp, 2nd comp, 3rd comp, 4th comp,5th comp, and post- comp cycles, respectively) were observed. Conclusions Probably due to incidental changes in nerve excitability, ALS patients mostly reacted differently to nerve compression when compared with healthy subjects.

Teasing of Ventral Spinal Cord White Matter Fibers for the Analysis of Central Nervous System Nodes of Ranvier.

In the central nervous system, the formation of nodes of Ranvier, the short, unmyelinated regions of the axon where voltage-gated sodium channels that mediate saltatory conduction in myelinated nerves are concentrated, is orchestrated by oligodendrocytes, the myelinating cells of the CNS. While transmission electron microscopy remains the gold standard for the study of how the nodal region is organized, this approach is both technically demanding and time-consuming. The availability of antibodies that can be used to label paranodal myelin and the underlying axonal domains that are formed as a result of myelination allowsfor the precise analysis of the nodal region. In this chapter, we describe the method used to prepare teased fiber preparations of CNS white matter. Teased fiber preparations facilitate the rapid, quantitative analysis of a large number of nodes of Ranvier per animal compared to conventional histological approaches.

Anti-MAG neuropathy: Role of IgM antibodies, the paranodal junction and juxtaparanodal potassium channels

ObjectiveTo improve understanding of disease pathophysiology in anti-myelin-associated glycoprotein (anti-MAG) neuropathy to guide further treatment approaches. MethodsAnti-MAG neuropathy patients underwent clinical assessments, nerve conduction and excitability studies, and ultrasound assessment. ResultsPatients demonstrated a distinctive axonal excitability profile characterised by a reduction in superexcitability [MAG: −14.2 ± 1.6% vs healthy controls (HC): −21.8 ± 1.2%; p < 0.01] without alterations in most other excitability parameters. Mathematical modelling of nerve excitability recordings suggested that changes in axonal function could be explained by a 72.5% increase in juxtaparanodal fast potassium channel activation and an accompanying hyperpolarization of resting membrane potential (by 0.3 mV) resulting in a 94.2% reduction in discrepancy between anti-MAG data and the healthy control model. Superexcitability changes correlated strongly with clinical and neurophysiological parameters. Furthermore, structural assessments demonstrated a proximal pattern of nerve enlargement (C6 nerve root cross-sectional area: 15.9 ± 8.1 mm2 vs HC: 9.1 ± 2.3 mm2; p < 0.05). ConclusionsThe imaging and neurophysiological results support the pathogenicity of anti-MAG IgM. Widening between adjacent loops of paranodal myelin due to antibodies would expand the pathway from the node to the juxtaparanode, increasing activation of juxtaparanodal fast potassium channels, thereby impairing saltatory conduction. SignificancePotassium channel blockers may prove beneficial in restoring conduction closer to its normal state and improving nerve function in anti-MAG neuropathy.

P345 Another brick in the wall: Is hexane neuropathy a ‘nodo-paranodopathy’?

Objectives To discuss electrophysiological follow-up of patients with n-hexane neuropathy and to remark overlaps with new concept nodo-paranodopathy. Methods 3 males, aged between 18–39 years, working in shoe manufactory were evaluated. Occupational history, toxic substance exposures, neurologic examinations were noted and NCV-EMG were performed upon hospital admission and during clinical follow-up (1., 3., 4., 6. months; 1., 3., 9. months; 3., 4., 5. months, respectively). Results Motor nerve conduction studies, performed in 1–4 months of clinical paresia initiation, revealed focal conduction blocks. Electrophysiological follow-up showed decreased distal CMAP amplitudes and denervation potentials supporting axonal degeneration in some; whereas increment in some other distal CMAP amplitudes, supporting early reversed conduction failures. This different patterns of conduction block resolution were documented both in different nerves of different patients and also in different nerves of the same patient. SNAPs were either decreased or preserved. Urine 2,5-hexandion concentrations were between 1.45–4.52 mg. Discussion n-Hexane gives cause to one of the most common toxic polyneuropathies seen in poorly ventilated factories. It is a sensory-motor polyneuropathy ending up with axonal degeneration. Nerve biopsy reveals paranodal axonal swelling and secondary myelin retraction in early stages. Myelin retraction imitates demyelination causing focal conduction block and failure before axonal degeneration emerges. This brings to mind the new category of nodo-paranodopathy described firstly for anti-ganglioside antibody mediated neuropathies, which can be proved by electrophysiological reevaluations. Significance n-Hexane neuropathy may have a pathogenic mechanism of dysfunction/disruption at the node of Ranvier as another example of nodo-paranodopathy.

The effects of paranodal myelin damage on action potential depend on axonal structure.

Biophysical computational models of axons provide an important tool for quantifying the effects of injury and disease on signal conduction characteristics. Several studies have used generic models to study the average behavior of healthy and injured axons; however, few studies have included the effects of normal structural variation on the simulated axon's response to injury. The effects of variations in physiological characteristics on axonal function were mapped by altering the structure of the nodal, paranodal, and juxtaparanodal regions across reported values in three different caliber axons (1, 2, and 5.7μm). Myelin detachment and retraction were simulated to quantify the effects of each injury mechanism on signal conduction. Conduction velocity was most affected by axonal fiber diameter (89%), while membrane potential amplitude was most affected by nodal length (86%) in healthy axons. Postinjury axonal functionality was most affected by myelin detachment in the paranodal and juxtaparanodal regions when retraction and detachment were modeled simultaneously. The efficacy of simulated potassium channel blockers on restoring membrane potential and velocity varied with axonal caliber and injury type. The structural characteristics of axons affect their functional response to myelin retraction and detachment and their subsequent response to potassium channel blocker treatment.