The neuroprotective factor Wlds does not attenuate mutant SOD1-mediated motor neuron disease

Selective degeneration and death of motor neurons in SOD1 mutant-mediated amyotrophic lateral sclerosis (ALS) is accompanied by axonal disorganization and reduced slow axonal transport in the three most frequently used mouse models of mutant SOD1-mediated ALS. To test whether suppression of axonal degeneration (frequently known as Wallerian degeneration) could slow disease development, we took advantage of a spontaneous mouse mutant Wld(s) (Wallerian degeneration slow) in which the programmed axonal degenerative process that is normally activated after axonal injury is significantly delayed. Despite its effectiveness in delaying axonal loss in other neurodegenerative models, the presence of Wld(s) did not slow disease onset, ameliorate mutant motor neuron death, axonal degeneration, or preserve synaptic attachments in mice that develop disease from ALS-linked SOD1 mutants SOD1G37R or SOD1G85R. However, presynaptic endings in both the presence and absence of Wld(s) showed high accumulations of mitochondria and synaptic vesicles, implicating errors of retrograde transport as a consequence of SOD1-mutant damage to axons.     

Axonal degeneration in motor neuron disease

Growing evidence from animal models and patients with amyotrophic lateral sclerosis (ALS) suggests that distal axonal degeneration begins very early in this disease, long before symptom onset and motor neuron death. The cause of axonal degeneration is unknown, and may involve local axonal damage, withdrawal of trophic support from a diseased cell body, or both. It is increasingly clear that axons are not passive extensions of their parent cell bodies, and may die by mechanisms independent of cell death. This is supported by studies in which protection of motor neurons in models of ALS did not significantly improve symptoms or prolong lifespan, likely due to a failure to protect axons. Here, we will review the evidence for early axonal degeneration in ALS, and discuss possible mechanisms by which it might occur, with a focus on oxidative stress. We contend that axonal degeneration may be a primary feature in the pathogenesis of motor neuron disease, and that preventing axonal degeneration represents an important therapeutic target that deserves increased attention.     

Conduction velocity and refractory period of single motor nerve fibres in motor neuron disease.

Electromyographic single motor unit recordings were used to study the axonal conduction velocity and the axonal refractory period of 60 motor units in patients with severe motor neuron disease. Eighteen per cent of the motor units had abnormally low axonal conduction velocity probably due to secondary degenerative changes. Thirty-two per cent of the motor units had abnormally long axonal refractory period but normal conduction velocity. Whether this reflects a primary disease mechanism or secondary changes remains to be established.     

Axonal degeneration and progressive neurologic disability in multiple sclerosis

Accumulating data support axonal degeneration as the major determinant of irreversible neurological disability in patients with multiple sclerosis (MS). The extent of axonal injury correlates with the degree of inflammation in active MS lesions and occurs at early stages of disease, indicating that inflammatory demyelination is an important factor behind axon pathology at the relapsing-remitting stage of MS. Axonal loss from disease onset can remain clinically silent for many years, and permanent neurological disability develops when a threshold of axonal loss is reached and the CNS compensatory resources are exhausted. Lack of myelin-derived trophic support due to long term demyelination may cause continuous axonal degeneration in chronic inactive lesions at the secondary-progressive stage of MS. Axonal pathology is not limited to demyelinated lesions, but also extends into normal appearing white matter. The concept of MS as a neurodegenerative disorder has important clinical implications: First, proactive anti-inflammatory and immunomodulatory treatment should prevent or delay chronic disability since inflammation influences axonal injury. Second, the pathophysiological mechanisms underlying axonal degeneration in MS need to be clarified in order to develop novel neuroprotective therapeutics. Finally, surrogate markers of axonal pathology, such as N-acetyl aspartate, can be used to monitor axonal dysfunction, axonal loss and treatment efficiency in patients with MS.     

Intra‐axonal protein aggregation in the peripheral nervous system

Intracellular protein aggregates are common pathological hallmarks of many neurodegenerative disorders, and a defect in axonal transport is also incriminated. Here, we studied intra‐axonal abnormal protein aggregation and axonopathy by using immunohistochemistry and electron microscopy on peripheral nerve biopsies from 12 patients with chronic axonal peripheral neuropathy (PN) of unknown etiology. Among these patients, three had idiopathic Parkinson's disease (PD). Intra‐axonal ubiquitin aggregates were more numerous in the patients with PD. Intra‐axonal aggregates of tau AT8 were found in five patients without PD. Phosphorylated α‐synuclein aggregation was absent in all cases, while intra‐axonal colocalization of 14‐3‐3 β and ubiquitin was observed in two PD cases. Electron microscopy revealed enlarged axons crowded with organelles in six cases, including the three patients with PD, thus attesting a slowing of the axoplasmic flux. The number of ubiquitin aggregates was correlated with features of reduced axonal flux, while no such correlation was found for tau and 14‐3‐3 β. Age did not correlate with the number of tau, ubiquitin, and 14‐3‐3 aggregates. Thus, both ubiquitin and/or abnormal tau intra‐axonal aggregates may be found in chronic axonal PN. Ubiquitin aggregates might reduce the axonal flux or result from a disease producing slowing of axonal transport.     

Inflammation, demyelination, neurodegeneration and neuroprotection in the pathogenesis of multiple sclerosis

Although axonal loss has been observed in demyelinated multiple sclerosis (MS) lesions, there has been a major focus on understanding mechanisms of demyelination. However, identification of markers for axonal damage and development of new imaging techniques has enabled detection of subtle changes in axonal pathology and revived interest in the neurodegenerative component of MS. Axonal loss is generally accepted as the main determinant of permanent clinical disability. However, the role of axonal loss early in disease or during relapsing-remitting disease is still under investigation, as are the interactions and interdependency between inflammation, demyelination, neurodegeneration and neuroprotection in the pathogenesis of MS.     

Axonal pathology in multiple sclerosis: relationship to neurologic disability.

In this review, data is summarized supporting the hypothesis that axonal loss is a major pathologic process responsible for irreversible neurologic disability in patients with multiple sclerosis. Pathologic studies implicate inflammatory demyelination as a principal cause of axonal transection and subsequent axonal degeneration. Axonal degeneration caused by chronic demyelination in the absence of active inflammation may also contribute to progressive disability in the later stages of the disease. Studies using magnetic resonance spectroscopy suggest that axonal loss begins at the onset of the disease, and studies using magnetic resonance imaging have documented brain atrophy in the earliest stages of multiple sclerosis. Brain atrophy increases during the relapsing-remitting disease stage without concurrent disability progression. This suggests that compensatory mechanisms maintain neurologic function, despite progressive brain tissue loss during the early stages of the disease. Beyond a threshold, however, further axonal loss leads to continuously progressive neurologic disability. We hypothesize that the rate and extent of axonal loss during relapsing-remitting multiple sclerosis determines when a patient enters the secondary progressive stage of the disease. This view of disease pathogenesis has several important implications. First, surrogate markers of axonal loss are needed to monitor the disease process for patient care and for clinical trials. We propose brain parenchymal fraction, a precise measure of whole-brain atrophy, as an attractive candidate for this purpose. Second, disease-modifying therapy should be used early in multiple sclerosis patients, before extensive axonal loss has occurred. Third, neuroprotective drugs should be tested in combination with anti-inflammatory drugs in multiple sclerosis patients. Finally, studies of the time course of axonal loss, and its mechanisms are critical for effective therapeutic intervention.     

Pattern of axonal injury in murine myelin oligodendrocyte glycoprotein induced experimental autoimmune encephalomyelitis: implications for multiple sclerosis

Axonal damage is a correlate for increasing disability in multiple sclerosis. Animal models such as experimental autoimmune encephalomyelitis (EAE) may help to develop better therapeutical neuroprotective strategies for the human disease. Here we investigate the pattern of axonal injury in murine myelin oligodendrocyte glycoprotein peptide 35-55 (MOG) induced EAE. Inflammatory infiltration, axonal densities and expression of amyloid precursor protein (APP), neurofilaments (SMI31 and 32) as well as expression of sodium channels were quantified in lesions, the perilesional area and normal appearing white matter (NAWM). Quantification of T cells and macrophages revealed a significant reduction of inflammatory infiltration at later disease stages despite an increase of demyelinated areas and persistent clinical disability. In lesions, axonal density was already significantly reduced early and throughout all investigated disease stages. A significant axonal loss was also seen in the grey matter and at later time points in the perilesion as well as NAWM. Numbers of axons characterized by non-phosphorylated neurofilaments and re-distribution of sodium channels 1.2 and 1.6 increased over the course of MOG-EAE whilst APP positive axons peaked at the maximum of disease. Finally, double-labeling experiments revealed a strong colocalization of sodium channels with APP, neurofilaments and the axonal nodal protein Caspr, but not glial and myelin markers in actively demyelinating lesions. In summary, progressive axonal loss distant from lesions is mainly associated with changes in neurofilament phosphorylation, re-distribution of sodium channels and demyelination. This axonal loss is dissociated from acute inflammatory infiltration and markedly correlates with clinical impairment. Consequently, therapeutic intervention may be promising at early stages of EAE focusing on inflammation, or later in disease targeting degenerative mechanisms.     

Mitofusin2 mutations disrupt axonal mitochondrial positioning and promote axon degeneration

Alterations in mitochondrial dynamics (fission, fusion, and movement) are implicated in many neurodegenerative diseases, from rare genetic disorders such as Charcot-Marie-Tooth disease, to common conditions including Alzheimer's disease. However, the relationship between altered mitochondrial dynamics and neurodegeneration is incompletely understood. Here we show that disease associated MFN2 proteins suppressed both mitochondrial fusion and transport, and produced classic features of segmental axonal degeneration without cell body death, including neurofilament filled swellings, loss of calcium homeostasis, and accumulation of reactive oxygen species. By contrast, depletion of Opa1 suppressed mitochondrial fusion while sparing transport, and did not induce axonal degeneration. Axon degeneration induced by mutant MFN2 proteins correlated with the disruption of the proper mitochondrial positioning within axons, rather than loss of overall mitochondrial movement, or global mitochondrial dysfunction. We also found that augmenting expression of MFN1 rescued the axonal degeneration caused by MFN2 mutants, suggesting a possible therapeutic strategy for Charcot-Marie-Tooth disease. These experiments provide evidence that the ability of mitochondria to sense energy requirements and localize properly within axons is key to maintaining axonal integrity, and may be a common pathway by which disruptions in axonal transport contribute to neurodegeneration.     

Mechanisms of disease: inherited demyelinating neuropathies--from basic to clinical research

The hereditary motor and sensory neuropathies (also known as Charcot-Marie-Tooth disease or CMT) are characterized by a length-dependent loss of axonal integrity in the PNS, which leads to progressive muscle weakness and sensory deficits. The 'demyelinating' neuropathies (CMT disease types 1 and 4) are genetically heterogeneous, but their common feature is that the primary defect perturbs myelination. As we discuss in this Review, several new genes associated with CMT1 and CMT4 have recently been identified. The emerging view is that a range of different subcellular defects in Schwann cells can cause axonal loss, which represents the final common pathway of all CMT disease and is independent of demyelination. We propose that Schwann cells provide a first line of axonal neuroprotection. A better understanding of axon-glia interactions should open the way to therapeutic interventions for demyelinating neuropathies. Transgenic animal models have become essential for dissecting CMT disease mechanisms and exploring novel therapies.     

Axonal protection using flecainide in experimental autoimmune encephalomyelitis

Axonal degeneration is a major cause of permanent neurological deficit in multiple sclerosis (MS), but no current therapies for the disease are known to be effective at axonal protection. Here, we examine the ability of a sodium channel-blocking agent, flecainide, to reduce axonal degeneration in an experimental model of MS, chronic relapsing experimental autoimmune encephalomyelitis (CR-EAE). Rats with CR-EAE were treated with flecainide or vehicle from either 3 days before or 7 days after inoculation (dpi) until termination of the experiment at 28 to 30 dpi. Morphometric examination of neurofilament-labeled axons in the spinal cord of CR-EAE animals showed that both flecainide treatment regimens resulted in significantly higher numbers of axons surviving the disease (83 and 98% of normal) compared with controls (62% of normal). These findings indicate that flecainide and similar agents may provide a novel therapy aimed at axonal protection in MS and other neuroinflammatory disorders.     

A novel system to accelerate the progression of nerve degeneration in transgenic mouse models of neuropathies

Axon degeneration is a common hallmark of many neurodegenerative diseases. There is now an abundance of spontaneous and genetically engineered mice available to study the mechanisms of axonal degeneration and to screen for axonal protective agents. However, many of these mouse models exhibit slow progressive axonal loss which can span over many months. Consequently, there is a pressing need to accelerate the pace of axonal loss over a short interval for high-throughput screening of pharmacological and genetic therapies. Here, we present a novel technique using acrylamide, an axonal neurotoxin, to provoke rapid axonal degeneration in murine models of neuropathies. The progressive axonal loss which typically occurs over 8months was reproduced within 7 to 10days of the acrylamide intoxication. This approach was successfully applied to Myelin Associated Glycoprotein knockout (MAG-/-) mouse and Trembler-J mouse, a popular murine model of Charcot-Marie-Tooth disease type 1 (CMT-1). Acrylamide intoxication in transgenic mouse models offers a novel experimental approach to accelerate the rate of axonal loss over short intervals for timely in vivo studies of nerve degeneration. This report also provides for the first time an animal model for medication or toxin-induced exacerbation of pre-existing neuropathies, a phenomenon widely reported in patients with neuropathies.     

Axonal loss in the pathology of MS: consequences for understanding the progressive phase of the disease

Axonal degeneration has been identified as the major determinant of irreversible neurological disability in patients with multiple sclerosis (MS). Axonal injury begins at disease onset and correlates with the degree of inflammation within lesions, indicating that inflammatory demyelination influences axon pathology during relapsing-remitting MS (RR-MS). This axonal loss remains clinically silent for many years, and irreversible neurological disability develops when a threshold of axonal loss is reached and compensatory CNS resources are exhausted. Experimental support for this view-the axonal hypothesis-is provided by data from various animal models with primary myelin or axonal pathology, and from pathological or magnetic resonance studies on MS patients. In mice with experimental autoimmune encephalomyelitis (EAE), 15-30% of spinal cord axons can be lost before permanent ambulatory impairment occurs. During secondary progressive MS (SP-MS), chronically demyelinated axons may degenerate due to lack of myelin-derived trophic support. In addition, we hypothesize that reduced trophic support from damaged targets or degeneration of efferent fibers may trigger preprogrammed neurodegenerative mechanisms. The concept of MS as an inflammatory neurodegenerative disease has important clinical implications regarding therapeutic approaches, monitoring of patients, and the development of neuroprotective treatment strategies.     

Identification of novel GDAP1 mutations causing autosomal recessive Charcot-Marie-Tooth disease

Mutations in the ganglioside-induced differentiation-associated protein 1 gene cause either autosomal recessive demyelinating Charcot-Marie-Tooth disease type 4A or autosomal recessive axonal Charcot-Marie-Tooth disease with vocal cord paresis. We sequenced the ganglioside-induced differentiation-associated protein 1 gene in 138 patients from 119 unrelated families diagnosed with either demyelinating or axonal autosomal recessive Charcot-Marie-Tooth disease. We detected six distinct mutant alleles in four families, four of which are novel. Electrophysiological studies show severely slowed motor nerve conduction velocities with severely reduced compound muscle action potentials. However, one patient had a normal conduction velocity in the ulnar nerve. Based on the electrophysiological tests, patients with ganglioside-induced differentiation-associated protein 1 mutations will therefore be classified as either axonal or demyelinating Charcot-Marie-Tooth disease. The neuropathological aspect shows a divergent pattern; nerve biopsies taken from two siblings at the same age and sharing the same ganglioside-induced differentiation-associated protein 1 gene mutation showed a dissimilar severity stage.     

Axonal swellings in human intramuscular nerves.

The presence, morphology, distribution, and abundance of axonal swellings in intramuscular nerves were evaluated. Axonal swellings were present in intramuscular nerves in 42% of 127 muscle biopsies from patients with a variety of conditions. The incidence was highest in muscle from patients with peripheral neuropathy, but swellings were present in muscle from patients with motor neuron disease, primary muscle diseases, and some individuals without clinical or histological evidence of neuromuscular disease. The greatest number of swellings in intramuscular nerves was in muscle from patients with chronic inflammatory demyelinating neuropathy. Swellings were spherical or elliptical, 4-20 microns in diameter, 5-30 microns in length, and composed of neurofilaments. Swellings were present only in myelinated axons of intramuscular nerves, proximal to nodes of Ranvier or in internodal regions. Swellings were not associated with axonal degeneration. They were probably not transported. The formation or accumulation of swellings may reflect altered axonal dynamics common to a number of disease processes.     

Damage to bulbospinal serotonin-, tyrosine hydroxylase-, and TRH-containing axons occurs early in the development of experimental allergic encephalomyelitis in rats.

Spinal cord monoaminergic and peptidergic axonal damage occurring during the development of experimental allergic encephalomyelitis (EAE) was assessed using immunohistochemistry. Spinal cord axons immunoreactive for serotonin, catecholamines, or a thyrotropin-releasing hormone marker peptide were found to be markedly swollen and distorted by the earliest stage of detectable paralysis during EAE development (the flaccid tail stage). As clinical signs progressed to complete hindlimb paralysis, axonal damage became increasingly extensive. Axonal damage was equally pronounced whether EAE was induced by inoculation with purified myelin basic protein or with whole spinal cord homogenate, suggesting that the damage did not result from an immune attack directed against specific monoaminergic and/or peptidergic antigens present in the inoculant. However, two observations suggested that mechanical or chemical factors associated with the inflammatory foci contribute to the axonal damage: first, distorted axons were nearly always located adjacent to blood vessels or the pial surface, sites at which inflammation occurs during EAE. Second, the severity of axonal damage correlated with the severity of the inflammation. The early onset of axonal damage during development of EAE and the close correlation that was found between the severity of axonal damage and the severity of clinical signs suggested that the axonal damage may contribute to the clinical signs of the disease.     

Ganglioside mimicry as a cause of Guillain-Barré syndrome

PURPOSE OF REVIEW: Campylobacter jejuni is the most frequent agent of antecedent infection in an axonal variant of Guillain-Barré syndrome, acute motor axonal neuropathy, and anti-GM1 or anti-GD1a IgG antibody is also associated with acute motor axonal neuropathy. Molecular mimicry has been found between human GM1 ganglioside and the lipo-oligosaccharide of C. jejuni isolated from an acute motor axonal neuropathy patient. Progress has been made in Guillain-Barré syndrome research, especially on acute motor axonal neuropathy subsequent to C. jejuni enteritis. RECENT FINDINGS: Sensitization of rabbits with C. jejuni lipo-oligosaccharide, as well as GM1, induced the production of anti-GM1 IgG antibody, and the subsequent development of acute flaccid paralysis. Pathological changes in rabbit peripheral nerves were identical to those seen in human acute motor axonal neuropathy. These findings provide conclusive evidence that molecular mimicry is a cause of human autoimmune disease. Ganglioside-like lipo-oligosaccharide is synthesized by sialyltransferase Cst-II, N-acetylgalactosaminyl-transferase CgtA, and galactosyltransferase CgtB. There is a strong association between the simultaneous presence of these genes and Guillain-Barré syndrome-associated C. jejuni strains. Knockout mutants of C. jejuni genes involved in lipo-oligosaccharide sialylation had reduced reactivity with anti-GM1 sera from Guillain-Barré syndrome patients, and did not induce an anti-GD1a IgG antibody response in mice. Lipo-oligosaccharide biosynthesis genes appear to be essential for the induction of anti-GM1 or anti-GD1a IgG antibody and the subsequent development of acute motor axonal neuropathy. SUMMARY: The concept that carbohydrate mimicry causes autoimmune disease provides a clue to the resolution of the pathogenesis of other immune-mediated diseases.     

Axonal dystrophy in a case of connatal thalamic and brain stem degeneration

Summary A neonate with a rapidly fatal disease characterized by connatal hypertonia and arthrogryposis multiplex is described. Neuropathological investigations revealed bilateral thalamus and brain stem degeneration, axonal degeneration of pyramidal and other tracts in the spinal cord, and axonal spheroids in areas of origin of lower motor neurons and in the brain stem reticular substance. Congenital thalamic and brain stem degeneration is generally assumed to be the result of intrauterine asphyxia. The widespread occurence of axonal spheroids in the present neonate points to the possibility of a genetic or toxic origin for at least some of these cases.     

Increased frequencies of serum antibodies to neurofilament light in patients with primary chronic progressive multiple sclerosis

We investigated whether serum and cerebrospinal fluid (CSF) antibodies to the light subunit of the NF protein (NF-L), a main component of the axonal cytoskeleton, may serve as biological markers for axonal pathology and/or disease progression in multiple sclerosis (MS). IgG to NF-L was measured in sera and CSF of MS patients, patients with inflammatory demyelinating diseases of the PNS, with acute inflammatory neurological diseases (including bacterial and viral meningitis), with neurodegenerative diseases, with acute noninflammatory neurological diseases (including stroke, headache and backache) and healthy controls by enzyme-linked immunosorbent assay. We found that serum anti-NF-L IgG antibodies were significantly elevated in MS patients with primary progressive disease course and we provide evidence for an intrathecal production of these antibodies. Our findings support the use of serum antibodies to NF-L as a marker for axonal destruction.     

帕金森小鼠多巴胺能神经元轴突超微结构研究

目的研究1-甲基4苯基-1，2，3，6-四氢吡啶（MPTP）诱导的小鼠多巴胺能神经元轴突变性的超微结构改变，探讨其在帕金森病发病机制中的作用。方法应用免疫组织化学染色、电镜分析等方法，观察MPTP诱导的小鼠黑质-纹状体多巴胺能神经元轴突变性与神经元损伤的超微结构变化，探讨神经元损伤与细胞轴突损伤的相互关系。结果MPTP可诱导小鼠产生典型的帕金森病症状，并可引起黑质-纹状体多巴胺能神经元的凋亡与轴突变性。但在超微结构变化上存在时间差异性，轴突微管的损伤发生较早，而典型的神经元凋亡改变出现较晚。结论轴突变性在帕金森病发病机制中起着十分重要的作用。可能独立于其胞体的凋亡，甚至可能是神经元凋亡的诱因，因此阻止轴突变性，有可能为帕金森病的治疗提供新的策略。