Nodes, paranodes and neuropathies

This review summarises recent evidence supporting the involvement of the specialised nodal and perinodal domains (the paranode and juxtaparanode) of myelinated axons in the pathology of acquired, inflammatory, peripheral neuropathies.The identification of new target antigens in the inflammatory neuropathies heralds a revolution in diagnosis, and has already begun to inform increasingly targeted and individualised therapies. Rapid progress in our basic understanding of the highly specialised nodal regions of peripheral nerves serves to strengthen the links between their unique microstructural identities, functions and pathologies. In this context, the detection of autoantibodies directed against nodal and perinodal targets is likely to be of increasing clinical importance. Antiganglioside antibodies have long been used in clinical practice as diagnostic serum biomarkers, and associate with specific clinical variants but not to the common forms of either acute or chronic demyelinating autoimmune neuropathy. It is now apparent that antibodies directed against several region-specific cell adhesion molecules, including neurofascin, contactin and contactin-associated protein, can be linked to phenotypically distinct peripheral neuropathies. Importantly, the immunological characteristics of these antibodies facilitate the prediction of treatment responsiveness

Functional impairment following axonal injury

Following trauma or other neurological disorders, a series of events happen that cause axonal dysfunction or ultimately lead to axonal death. Computational modeling of the nervous system facilitates systematic study of the effects of each injury parameter on the output. The overall goal of this research was to develop a new method of simulating axon damage in a biophysical model and quantify the effects of structural damage on signal conduction. To achieve this, three objectives were addressed 1) quantify the effects of normal morphological variation and demyelination on axonal conduction characteristics, 2) develop a new computationally efficient method for modeling damage in axons, and 3) characterize the structure changes observed in human axons and quantify the relationship between these observed changes and axonal function. Biophysical computational models developed in NEURON were employed to characterize morphological changes in damaged axons and study the effects of some of the most common axonal injuries such as myelin damage and spheroid formation on signal propagation in axons with different calibers. To facilitate efficient computational simulation, a new approach for increasing geometrical resolution in NEURON was developed and assessed. To investigate the effects of axonal swelling on action potential conduction in myelinated axons, the morphological properties of axonal spheroids were characterized by analyzing a series of confocal images captured from post-mortem human brain samples of patients with MS and infarction. Our results indicate that subtle abnormalities in nodal, paranodal and juxtaparanodal regions may have sizable effects on action potential amplitude and velocity and more targeted treatments need to be developed that focus on these regions. In addition, the results of our histopathological and computational studies suggest that axons with different diameters may respond differently to injuries and diseases. Therefore, it is important to perform experimental injury models across a wide range of axons to get a more comprehensive understanding of the relationship between axonal morphological features, injury parameters and functional responses. We expect this research to lay the quantitative foundation for finding new potential functional markers of white matter tissue damage and provide further insights into how myelin damage and axonal spheroids may affect function

Neuroinflammation causes changes to the nodes of Ranvier in Multiple Sclerosis normal-appearing white matter.

Background: In addition to the focal demyelinating lesions in multiple sclerosis (MS), both imaging and neuropathological analyses have demonstrated the presence of a more diffuse pathology in both the white and grey matter, including changes to the structure of nodes of Ranvier in the normal-appearing white matter (NAWM). Objective: We have examined the expression of the paranodal axonal protein Caspr1, the voltage-gated channels Nav and Kv1.2 at nodes and juxtaparanodes respectively, and SMI32+ (dephosphorylated neurofilament) axons in NAWM areas from post-mortem progressive MS brains compared to controls. This axo-geometrical data on nodal changes was then integrated into a computational model of an axon developed with NEURON. To test our hypothesis, rats were injected into the cerebral subarachnoid space with lentiviral vectors for lymphotoxin-α and interferon-γ, and structural changes were examined 3 months later. Furthermore, a cerebellar tissue culture model was used to induce nodal pathology by the activation of microglia with TNF, interferon-γ, conditioned microglial medium and glutamate administration. Results: The paranodal domain in MS NAWM tissue was longer on average than in control and Kv1.2 channels appeared dislocated towards the paranode. These changes were associated with stressed axons and activation of microglia. When these changes were inserted into the computational model, a rapid decrease in velocity was observed as the paranodal peri-axonal space was increased, reaching conduction failure when the axons were less than 1mm of diameter. The same structural changes were observed in the corpus callosum of our rat model and were associated with microglia/astrocyte activation. TNF, interferon-γ, conditioned microglial medium and glutamate also generated paranodal elongation in the cerebellar cultures axons and was reversed/halted by an NMDA blocker. Conclusion: Microglia activated by pro-inflammatory cytokines may release high levels of glutamate, which triggers paranodal pathology in MS NAWM, contributing to axonal damage and subsequent conduction deficits.Open Acces

Neuroinflammation in the normal-appearing white matter (NAWM) of the multiple sclerosis brain causes abnormalities at the nodes of Ranvier

Changes to the structure of nodes of Ranvier in the normal-appearing white matter (NAWM) of multiple sclerosis (MS) brains are associated with chronic inflammation. We show that the paranodal domains in MS NAWM are longer on average than control, with Kv1.2 channels dislocated into the paranode. These pathological features are reproduced in a model of chronic meningeal inflammation generated by the injection of lentiviral vectors for the lymphotoxin-α (LTα) and interferon-γ (IFNγ) genes. We show that tumour necrosis factor (TNF), IFNγ, and glutamate can provoke paranodal elongation in cerebellar slice cultures, which could be reversed by an N-methyl-D-aspartate (NMDA) receptor blocker. When these changes were inserted into a computational model to simulate axonal conduction, a rapid decrease in velocity was observed, reaching conduction failure in small diameter axons. We suggest that glial cells activated by pro-inflammatory cytokines can produce high levels of glutamate, which triggers paranodal pathology, contributing to axonal damage and conduction deficits

Guillain-Barré syndrome: a century of progress

In 1916, Guillain, Barré and Strohl reported on two cases of acute flaccid paralysis with high cerebrospinal fluid protein levels and normal cell counts — novel findings that identified the disease we now know as Guillain–Barré syndrome (GBS). 100 years on, we have made great progress with the clinical and pathological characterization of GBS. Early clinicopathological and animal studies indicated that GBS was an immune-mediated demyelinating disorder, and that severe GBS could result in secondary axonal injury; the current treatments of plasma exchange and intravenous immunoglobulin, which were developed in the 1980s, are based on this premise. Subsequent work has, however, shown that primary axonal injury can be the underlying disease. The association of Campylobacter jejuni strains has led to confirmation that anti-ganglioside antibodies are pathogenic and that axonal GBS involves an antibody and complement-mediated disruption of nodes of Ranvier, neuromuscular junctions and other neuronal and glial membranes. Now, ongoing clinical trials of the complement inhibitor eculizumab are the first targeted immunotherapy in GBS

The axon-myelin unit in development and degenerative disease

Axons are electrically excitable, cable-like neuronal processes that relay information between neurons within the nervous system and between neurons and peripheral target tissues. In the central and peripheral nervous systems, most axons over a critical diameter are enwrapped by myelin, which reduces internodal membrane capacitance and facilitates rapid conduction of electrical impulses. The spirally wrapped myelin sheath, which is an evolutionary specialisation of vertebrates, is produced by oligodendrocytes and Schwann cells; in most mammals myelination occurs during postnatal development and after axons have established connection with their targets. Myelin covers the vast majority of the axonal surface, influencing the axon's physical shape, the localisation of molecules on its membrane and the composition of the extracellular fluid (in the periaxonal space) that immerses it. Moreover, myelinating cells play a fundamental role in axonal support, at least in part by providing metabolic substrates to the underlying axon to fuel its energy requirements. The unique architecture of the myelinated axon, which is crucial to its function as a conduit over long distances, renders it particularly susceptible to injury and confers specific survival and maintenance requirements. In this review we will describe the normal morphology, ultrastructure and function of myelinated axons, and discuss how these change following disease, injury or experimental perturbation, with a particular focus on the role the myelinating cell plays in shaping and supporting the axon

Three dimensional electron microscopy reveals changing axonal and myelin morphology along normal and partially injured optic nerves

Following injury to the central nervous system, axons and myelin distinct from the initial injury site undergo changes associated with compromised function. Quantifying such changes is important to understanding the pathophysiology of neurotrauma; however, most studies to date used 2 dimensional (D) electron microscopy to analyse single sections, thereby failing to capture changes along individual axons. We used serial block face scanning electron microscopy (SBF SEM) to undertake 3D reconstruction of axons and myelin, analysing optic nerves from normal uninjured female rats and following partial optic nerve transection. Measures of axon and myelin dimensions were generated by examining 2D images at 5 µm intervals along the 100 µm segments. In both normal and injured animals, changes in axonal diameter, myelin thickness, fiber diameter, G-ratio and percentage myelin decompaction were apparent along the lengths of axons to varying degrees. The range of values for axon diameter along individual reconstructed axons in 3D was similar to the range from 2D datasets, encompassing reported variation in axonal diameter attributed to retinal ganglion cell diversity. 3D electron microscopy analyses have provided the means to demonstrate substantial variability in ultrastructure along the length of individual axons and to improve understanding of the pathophysiology of neurotrauma

Small Molecule Hsp90 Modulator and Neuregulin-induced Peripheral Demyelination

Abstract Modulating molecular chaperones is emerging as an attractive approach to treat neurodegenerative diseases associated with protein aggregation, diabetic peripheral neuropathy (DPN) and possibly, demyelinating neuropathies. KU-32 is a small molecule inhibitor of heat shock protein 90 (Hsp90) and reverses sensory deficits associated with myelinated fiber dysfunction in DPN. Additionally, KU-32 prevented the loss of myelinated internodes induced by treating myelinated Schwann cell-dorsal root ganglia sensory neuron co-cultures with neuregulin-1 Type 1 (NRG1). Since KU-32 decreased NRG1-induced demyelination in an Hsp70-dependent manner, the goal of the current study was to clarify how Hsp70 may be mechanistically linked to preventing demyelination. The activation of p42/p44 MAPK and induction of the transcription factor c-jun function as negative regulators of myelination. NRG1 activated MAPK, induced c-jun expression and promoted a loss of myelin segments in DRG explants isolated from both wild type and Hsp70 KO mice. Although KU-32 did not block the activation of MAPK, it blocked c-jun induction and protected against a loss of myelinated segments in wildtype (WT) mice. KU-32 did not prevent the NRG1-dependent induction of c-jun and loss of myelin segments in explants from Hsp70 KO mice. Over-expression of Hsp70 in myelinated DRG explants prepared from WT or Hsp70 KO mice was sufficient to block the induction of c-jun and the loss of myelin segments induced by NRG1. Lastly, inhibiting the proteasome prevented KU-32 from decreasing c-jun levels. Collectively, these data support that Hsp70 induction is sufficient to prevent NRG1-induced demyelination by enhancing the proteasomal degradation of c-jun