Rapid alterations of the axon membrane in antibody-mediated demyelination

Optimizing IgG therapy in chronic autoimmune neuropathies: a hypothesis driven approach

Prolonged intravenous immunoglobulin (IVIG) therapy is used for the chronic autoimmune neuropathies chronic idiopathic demyelinating polyneuropathy and multifocal motor neuropathy, but the doses and treatment intervals are usually chosen empirically due to a paucity of data from dose-response studies. Recent studies of the electrophysiology and immunology of these diseases suggest that antibody-induced reversible dysfunction of nodes of Ranvier may play a role in conduction block and disability which responds to immunotherapy more rapidly than would be expected for demyelination or axonal damage per se. Clinical reports suggest that in some cases, the effects of each dose of IVIG may be transient, wearing-off before the next dose is due. These observations lead us to hypothesize that that therapeutic IgG acts by competing with pathologic autoantibodies and that individual patients may require different IgG levels for optimal therapeutic effects. Frequent IVIG dosing and weekly subcutaneous IgG have been tried as ways of continuously maintaining high serum IgG levels, resulting in stabilization of neuromuscular function in small case series. Frequent grip strength and disability measurements, performed by the patient at home and reported electronically, can be used to assess the extent and duration of responses to IgG doses. Individualization of IgG treatment regimens may optimize efficacy, minimize disability, and identify nonresponders.

Rapid and reversible responses to IVIG in autoimmune neuromuscular diseases suggest mechanisms of action involving competition with functionally important autoantibodies

Intravenous immunoglobulin (IVIG) is widely used in autoimmune neuromuscular diseases whose pathogenesis is undefined. Many different effects of IVIG have been demonstrated in vitro, but few studies actually identify the mechanism(s) most important in vivo. Doses and treatment intervals are generally chosen empirically. Recent studies in Guillain-Barré syndrome and chronic inflammatory demyelinating polyneuropathy show that some effects of IVIG are readily reversible and highly dependent on the serum IgG level. This suggests that in some autoantibody-mediated neuromuscular diseases, IVIG directly competes with autoantibodies that reversibly interfere with nerve conduction. Mechanisms of action of IVIG which most likely involve direct competition with autoantibodies include: neutralization of autoantibodies by anti-idiotypes, inhibition of complement deposition, and increasing catabolism of pathologic antibodies by saturating FcRn. Indirect immunomodulatory effects are not as likely to involve competition and may not have the same reversibility and dose-dependency. Pharmacodynamic analyses should be informative regarding most relevant mechanism(s) of action of IVIG as well as the role of autoantibodies in the immunopathogenesis of each disease. Better understanding of the role of autoantibodies and of the target(s) of IVIG could lead to more efficient use of this therapy and better patient outcomes.

The effects of aerobic exercise training on the age-related lipid peroxidation, Schwann cell apoptosis and ultrastructural changes in the sciatic nerve of rats

The potential role of exercise in preventing the age-related spontaneous peripheral neuropathy has not been studied. We examined the effects of long-term aerobic exercise training on lipid peroxidation, Schwann cell (SC) apoptosis and ultrastructural changes in the sciatic nerve of rats during aging. Three groups of 12-week old Wistar rats ran on a treadmill for 6, 9 and 12 months (exercise trained (ET) group, n=10 each) according to an exercise training program targeted at a speed of 22 m/min (at 7 degrees incline), 60 min/day, 6 days/week. Three corresponding groups of untrained rats were used as the controls (sedentary (SED) group). At the end of each period, sciatic nerve biopsies were performed, and processed for biochemical, immunohistochemical and ultrastructural analyses. The results showed that aging was associated with an increased level of nerve malondialdehyde (MDA, marker of lipid peroxidation) and a higher number of SC apoptosis in SED group. The SED group showed irregular nerve fibers with thin myelin sheaths and areas of myelin-axon detachment. However, the ET group had significantly diminished nerve lipid peroxidation and SC apoptosis. In the ET group, nerve fibers had a thick myelin sheath with frequent folding. These findings suggest that aerobic exercise training protects peripheral nerves by attenuating oxidative reactions, and preserving SCs and myelin sheath from pathologic changes, which occur during normal aging.

Imbalances in N-CAM, SAM and polysialic acid may underlie the paranodal ion channel barrier defect in diabetic neuropathy

Breakdown of protective tissue barrier systems characterizes the chronic diabetic complications affecting the retina, and peripheral and central nerve tracts. The progressive damages to the blood-retina-, blood-nerve-, and paranodal ion channel barriers have pathophysiological consequences for the relentless progression of these complications. The continuing damage to the paranodal ion channel barrier in the spontaneously diabetic BB/W rat is associated with an increasingly irreversible nerve conduction defect, due to impaired nodal Na+ currents associated with displacement of nodal Na+ channels across the damaged paranodal barrier. The structural substrate for the mechanical barrier of the paranode is provided by electron-dense junctional complexes made up by a moiety of neural cell adhesive-(N-CAM), neural-glial adhesive (Ng-CAM), substrate adhesive molecules (SAMs) and polysialic acid (PSA). To further explore the mechanism underlying the protective barrier defect in diabetic neuropathy we examined the expression and immunolocalization of these molecules in peripheral nerve. In 6-month diabetic BB/W rats, direct and indirect ELISAs revealed significantly up-regulated N-CAM (P < 0.05), tenascin (Ng-CAM), (P < 0.001) and N-cadherin (A-CAM) (P < 0.03). On the other hand, SAMs showed little change, except for PSA which showed a significantly (P < 0.03) decreased concentration in the diabetic nerve. Immunocytochemical identification of these molecules revealed no visually detectable differences between diabetic and control rats. In conclusion, these data suggest that imbalances between highly interactive molecules responsible for the adhesiveness between terminal Schwann cell loops and the paranodal axolemma may underlie the critical paranodal barrier defect in diabetic neuropathy.

An orderly development of paranodal axoglial junctions and bracelets of Nageotte in the rat sural nerve

The present study was designed to assess the normal development of the paranodal apparatus with particular emphasis on axoglial junctions (AGJs) which constitute the paranodal barrier system. The sural nerve was examined in 10- and 31-day-old rats. During the early phase of myelination AGJ attachment of terminal myelin loops to the axolemma proceeded from the node to the internode. The frequency of terminal loops with AGJ attachment increased with fiber growth. As myelination advanced internodal-most loops became almost 100% attached to the axolemma by AGJs, whereas at the same time an increasing number of nodal-most loops were unattached, suggesting a lack of AGJ formation at this site. The formation of bracelets of Nageotte increased with the progressive addition of myelin loops. They formed most frequently at the juxtanodal interface between unattached and attached loops, probably reflecting crowding of terminal loops along the unchanged length of the paranodal axolemma. The findings suggest a complex but orderly age- and fiber size-dependent maturation process of the paranode and its structural barrier system. The present data will serve as a basis for the evaluation of this anatomical region in regenerating and remyelinating fibers in various neuropathies.

Chronic inflammatory demyelinating polyneuropathy: histological and immunopathological studies on biopsied sural nerves

We undertook histological and immunopathological studies on biopsied sural nerves from 9 patients with chronic inflammatory demyelinating polyneuropathy (CIDP). The diagnosis of CIDP was based on the research criteria proposed by the Ad Hoc Subcommittee of the American Academy of Neurology AIDS Task Force. The nerve pathology in these patients comprised macrophage-associated active demyelination and subsequent remyelination of various proportions. The presence of T cells in the endoneurium correlated with activity of demyelination. An analysis of T cell subsets demonstrated that the number of CD8-positive cells predominated over that of CD4-positive ones. Infiltration of B cells, and depositions of immunoglobulin and complement were not seen. These observations suggest that a T cell-mediated process is of pathogenic significance in CIDP. Furthermore, a double immunofluorescence staining revealed that most HLA-DR antigen-positive cells in the nerves in which active demyelination was seen coexpressed a macrophage-specific determinant. Conversely, HLA-DR-positive Schwann cells were found in the nerves in which remyelination was predominant. The expression of HLA-DR antigen on Schwann cells might not play a pathogenic role in the active demyelination in CIDP.

In vivo effects of sera from Guillain-Barré subgroups: an electrophysiological and histological study on rat nerves

Serum from 20 patients with Guillain-Barré syndrome (GBS), 10 healthy controls and 10 patients with recent cytomegalovirus, Epstein-Barr virus, or Campylobacter jejuni/coli infections was injected into rat sciatic nerve. The 20 GBS patients consisted of 2 groups of 10 patients with different electrophysiological and clinical disease patterns. The main aim of the study was to investigate possible differences in humoral (auto)-immunity between these subgroups. We found no statistically significant differences in electrophysiological or histological parameters between nerves injected with sera from the 2 GBS groups. The sera of the GBS groups caused significantly more compound muscle action potential reduction at 3 to 5 days postinjection than the healthy control sera. No significant difference in nerve conduction was found between nerves injected with GBS serum and serum of patients with proven infections without GBS. Histological analysis of the same nerves that were studied electrophysiologically showed no significant differences in demyelination or other histological parameters between patients and controls at 5 days postinjection. Based on the findings in this study that sera of GBS groups with important differences in disease pattern and sera of patients with proven infection but without GBS show similar in vivo effects on rat nerves, we suggest it may be more likely that these effects are caused by aspecific serum factors associated with immune-system activation, especially by precedent infections, than by specific disease-related factors such as anti-myelin antibodies.

The long-term cellular response to taxol in peripheral nerve: Schwann cell and endoneurial cell changes

Taxol, an agent known to stabilize and increase the assembly of microtubules, causes long-lasting nerve damage when injected into peripheral nerve. In the present study, the cellular response to taxol in rat sciatic nerve was studied for up to 6 months after a single injection. The initial response of Schwann cells to taxol at the lesion site involved the accumulation of cytoplasmic microtubules which persisted up to 4 months after injection. Some novel microtubule-related cytoplasmic structures were also noted; these included microtubule-lined cytoplasmic crypts and channels. Despite these structural abnormalities, Schwann cells were able to produce myelin sheaths around taxol-induced axonal bulbs. This myelination showed some anomalies up to 4 months consisting of the widening of myelin lamellae, variability in sheath thickness, paranodal myelin infoldings and myelin protrusions. With time the diameter of the axonal bulbs decreased and, concomitant with this, more normal-appearing remyelination occurred. By 5 months, the previously noted myelin abnormalities were rare. By 6 months only a few naked axonal segments occurred at the lesion site. In endoneurial fibroblasts and macrophages cytoplasmic lamellar microtubule formations were frequent at 10 weeks. Needle-like cytoplasmic structures appeared within endoneurial cells at the site of the lesion after 10 weeks. By 3 months these inclusions were numerous and were often surrounded by extended cytoplasmic processes. The needles were up to 50 microns long and 3 microns wide and probably represented cholesterol. By 4 months the number of cytoplasmic needles decreased and at 5 months onwards none was observed. The present findings confirm and extend previous findings that taxol has a long-lasting effect upon both Schwann cells and endoneurial cells and that this is related to abnormal tubulin synthesis.

Altered mechanisms of muscular force generation in lower motor neuron disease

Recruitment and firing rate modulation (FRM) of single motor units (MUs) were evaluated in the first dorsal interosseus muscle in patients with chronic lower motor neuron disorders of primarily neuroaxonal or demyelinating pathology. Residual muscle function was estimated by maximal voluntary force, twitch tension, and compound muscle action potential. The recruitment range of MUs was expanded toward higher relative force levels in all patients. Changes in firing rates per unit force increment were larger in patients with more pronounced muscle atrophy. When this effect was accounted for by calculating FRM for increments of 10% of residual maximal force, patients with subnormal motor nerve conduction velocities showed selective impairment of rate modulation. This was not due to intermittent conduction failure. We conclude that the two force-generating mechanisms, recruitment and FRM, show unspecific compensatory changes related to the loss of MUs and also alterations that are specifically related to the neuroaxonal or demyelinating nature of the neuropathy.

Role of endoneural cells in experimental allergic neuritis and characterisation of a resident phagocytic cell

Electrophysiological, clinical and histological techniques were used to monitor the time course of events related to experimental allergic neuritis (EAN) in 48 Lewis rats. The primary lesion was found to be paranodal demyelination without cellular infiltration. Endoneural phagocytes derive from hematogenous ED1+ED2- monocytes and possibly from resident ED1-ED2+ monocytic cells, not from Schwann cells and fibroblasts. We demonstrate a population of monocytic Ia-bearing, ED1-ED2+ spindle-shaped cells residing in normal peripheral nerve and provide evidence for their transformation into macrophages in the course of EAN.

Pathogenesis and prevention of diabetic neuropathy

Diabetic neuropathy, long-recognized as an important but complex and poorly understood clinical complication of diabetes, is finally yielding to more than a decade of intense clinical and laboratory investigation. At least one basic biochemical mechanism involving sorbitol and MI metabolism, phosphoinositides, protein kinase C, and the (Na,K)-ATPase has been identified that can rationally account for the neurotoxicity of glucose. This biochemical sequence has been examined in some detail in vitro, but some of its elements, such as the link between abnormal sorbitol and MI metabolism, and between protein kinase C and the (Na,K)-ATPase, remain the subject of ongoing investigation. Through its effect on the (Na,K)-ATPase, this metabolic sequence can explain both the rapidly-reversible functional impairment and the early structural lesions of nerve fibers, such as paranodal swelling in acute diabetes. Extrapolation of early paranodal swelling to the more advanced stages of nerve fiber damage remains somewhat speculative, although axo-glial dysjunction is a likely intermediate step. Impaired axonal transport or microvascular dysfunction may be additional contributing factors, possibly also related to abnormal sorbitol and MI metabolism. Blunted phosphoinositide-mediated signal transduction could potentially explain a putative insensitivity to neurotrophic factors and a diminished regenerative response in diabetic neuropathy. Human morphometric studies and ARI trials support the relevance of these pathogenetic processes to human diabetic neuropathy, and suggest that specific metabolic therapy with agents such as ARIs hold promise as important new elements in the treatment and possibly prevention of diabetic neuropathy.

Cytochemical characteristics of the axon membrane at nodes of Ranvier in resting, tetrodotoxin-blocked, and electrically stimulated peripheral nerves of the rat

To test whether intraaxonal ferric ion-ferrocyanide staining at nodes of Ranvier is influenced by functional state of the nodal membrane, the tibial nerves of Sprague-Dawley rats were subjected to one of the following experimental procedures prior to fixation and staining: General anesthesia to induce nerves at rest, tetrodotoxin blocking of nerve activity, and high-frequency (100 Hz) electrical stimulation. In all cases most but not all nodes were stained. No statistically significant differences were found either in the percentage of total stained nodes or in the percentage of stained nodes from large- and small-diameter fibers among the three conditions tested. An explanation is offered to account for the apparent discrepancy between these results and those from other studies involving related cytochemical markers of the nodal apparatus.

Humoral factors in peripheral nerve disease

Humoral factors including soluble substances transported by the blood stream and factors released at a target tissue may play a role in diseases of the peripheral nervous system. Various criteria have to be met in order to accept humoral factors as potential pathogens. In this review these general criteria are discussed, including the evidence provided by plasma exchange therapy, demonstration of circulating or deposited autoantibodies and immune complexes, identification of antigenic molecules, animal model diseases, passive transfer experiments, and the demonstration of circulating factors not directed against specific targets. In acute, chronic, and chronic relapsing inflammatory polyneuropathies, and in the polyneuropathy associated with monoclonal gammopathy, humoral factors have been identified, but their exact pathogenic role is not fully understood. In the Lambert-Eaton myasthenic syndrome, a disorder of the motor nerve terminal, pathogenic IgG-antibodies have been demonstrated by passive transfer experiments. In the experimental animal model disorders, the acute and chronic variants of experimental allergic neuritis, humoral factors including antibodies to myelin basic proteins and galactocerebroside and nonspecific humoral factors may all contribute to the ultimate peripheral nerve damage, but their relative importance in relation to cell-mediated immune reactions is not yet clear.

Axo-glial dysjunction. A novel structural lesion that accounts for poorly reversible slowing of nerve conduction in the spontaneously diabetic bio-breeding rat

Biochemical abnormalities in peripheral nerve are thought to precede and condition the development of diabetic neuropathy, but metabolic intervention in chronic diabetic neuropathy produces only limited acute clinical response. The residual, metabolically unresponsive neurological deficits have never been rigorously defined in terms of either persistent metabolic derangements or irreversible structural defects because human nerve tissue is rarely accessible for anatomical and biochemical study and experimentally diabetic animals do not develop the structural hallmarks of human diabetic neuropathy. Detailed neuroanatomical-functional-biochemical correlation was therefore undertaken in long-term spontaneously diabetic BB-Wistar rats that functionally and structurally model human diabetic neuropathy. Vigorous insulin replacement in chronically diabetic BB rats essentially normalized both the sural nerve fiber caliber spectrum and the decreased sciatic nerve myo-inositol and (Na,K)-ATPase levels generally associated with conduction slowing in diabetic animals; yet, nerve conduction was only partially restored toward normal. Morphometric analysis revealed a striking disappearance of paranodal axo-glial junctional complexes that was not corrected by insulin replacement. Loss of these strategic junctional complexes, which are thought to limit lateral migration of axolemmal Na channels away from nodes of Ranvier, correlates with and can account for the diminished nodal Na permeability and resultant nodal conduction delay characteristic of chronic diabetic neuropathy in this animal model.

Proliferation of Schwann cells in demyelinated rat sciatic nerve

Experimental demyelination was induced by intraneural injection of anti-galactocerebroside serum into the sciatic nerves of rats. Schwann cells undergoing mitotic division were observed between days 3 to 9 after the injection and demyelinated segments were still associated with macrophages. Dividing Schwann cells were often present in association with both unmyelinated and myelinated fibers. Whether or not, daughter Schwann cells migrate along the same fiber towards neighboring demyelinated segments remains unclear. When Schwann cells attached to axon membranes of demyelinated segments were studied at later time points, they were present in clusters randomly at various regions of the segments. There was no proximo-distal gradient for the wave of Schwann cell proliferation. Mean Schwann cell internuclear distances were around 40-50 microns at the earliest time of remyelination. Schwann cell redistribution and remyelination progressed regardless of the length of demyelinated segments.

Demyelination-induced plasticity in the axon membrane: an ultrastructural cytochemical study of lead neuropathy in the rat

We examined the distribution of ferric ion-ferrocyanide stain (a marker for excitable regions of myelinated fibers) in the lead-induced demyelinating neuropathy of the rat. By electron microscopy, we found that paranodal degeneration resulted in spreading of the reaction product from nodal to internodal axolemma. During repair, nodal-like stained areas formed at the contact zones between preremyelinating Schwann cells. These data suggest that the location and extent of excitable axonal regions are influenced by axoglial relationships. Additionally, some fibers displayed staining at paranodal axolemma adjacent to demyelinated segments, suggesting it might be an alternative site for impulse generation in demyelinated fibers.

Can the BB-rat help to unravel diabetic neuropathy?

Diabetic neuropathy is probably the most common and one of the most disabling complications of diabetes mellitus. Although several possible pathogenetic mechanisms for this complication have been suggested, they cannot easily be tested in man. Therefore animal models with induced or spontaneous onset of diabetes mellitus have been used. The spontaneously diabetic BB-rat may provide a valuable model, since it displays both metabolic, functional and structural abnormalities of peripheral nerve similar to those present in humans. Based on systematic studies of these abnormalities in peripheral nerve in this model, inter-relationships between diabetic dysmetabolism, dysfunction and structural changes are starting to emerge. These findings are reviewed and a scheme of proven and proposed correlations which now can be tested in this animal model is presented.

Reversible diabetic nerve dysfunction: structural correlates to electrophysiological abnormalities

Structural alterations of the nodal and paranodal areas were examined in the posterior tibial nerve in insulin-depleted and insulin-treated diabetic BB rats. The early metabolic phase of the distal symmetrical polyneuropathy was characterized by paranodal axonal swellings and nodal bulgings of the axon. These alterations correlate with intraaxonal sodium accumulation and decreased sodium equilibrium potentials which account for the early nerve conduction defect. Both the structural and electrophysiological abnormalities were completely normalized after vigorous insulin therapy. In the chronic diabetic polyneuropathy the paranodal area showed loss of paranodal axoglial junctions and paranodal myelin retraction. These changes may be partially responsible for the impaired electrical activity at the node as exemplified by irreversibly impaired sodium permeability and nerve conduction.

Ethylene oxide polyneuropathy: clinical follow-up study with morphometric and electron microscopic findings in a sural nerve biopsy

A case is reported of ethylene oxide polyneuropathy after 5 months of exposure. There was symmetrical distal weakness of both lower extremities and transitory reduced nerve conduction velocities with increased latencies. Sural nerve biopsy revealed nerve fibre degeneration of the Wallerian type, associated with reduction of axonal cross-sectional areas and some degree of nerve fibre regeneration that could be confirmed morphometrically. In addition, there was conspicuous paranodal vesicular disintegration of individual myelin lamellae. Unusual cisternae with introverted hemidesmosomes were noted in endoneurial fibroblasts.

Ligature-induced injury in peripheral nerve: electrophysiological observations on changes in action potential characteristics following blockade of potassium conductance

The effects of the potassium channel blocking agent 4-aminopyridine (4-AP) on action potential properties were studied in chronically injured rat sciatic nerves. In normal, mature myelinated fibers, application of 4-AP does not lead to any significant change in action potential waveform or firing pattern in response to single stimuli. In contrast, application of 4-AP to nerves injured by the placement of loose ligatures results in the appearance of late rippled components in the compound action potential. This alteration in waveform is present at the injury site, but not at nerve segments proximal or distal to this region. Paired stimulation experiments demonstrate that this oscillation of the whole nerve response reflects repetitive firing in response to single stimuli following application of 4-AP. Intra-axonal recording following 4-AP application demonstrates bursts of action potentials, with several spikes of reduced amplitude arising from a depolarizing potential following the initial spike. Refractory period for the late spike is greater than that of the primary action potential. These results demonstrate that potassium channels are present and functional in chronically injured nerves, where blockage of these channels results in repetitive firing in response to single stimuli.