Peripheral Nerve Myelin Research Articles

Antibodies to LM1 and LM1-containing ganglioside complexes in Guillain–Barré syndrome and chronic inflammatory demyelinating polyneuropathy

LM1 is localized in human peripheral nerve myelin. Antibodies to ganglioside complexes (GSCs) have been reported in Guillain–Barré syndrome (GBS). We investigated IgG antibodies to LM1 and two GSCs (GM1 and LMI, or GD1b and LM1) in the sera of each 40 patients with chronic inflammatory demyelinating polyneuropathy (CIDP) and GBS, using ELISA. We detected anti-LM1 antibody in five with GBS and seven with CIDP; anti-GM1/LM1 antibody in three with GBS and one with CIDP; and anti-GD1b/LM1 antibody in two with CIDP. Antibodies to LM1 and LM1-containing GSCs may be among the targets for autoimmunity in GBS and CIDP.

Identification of a Negative Regulatory Region for the Exchange Activity and Characterization of T332I Mutant of Rho Guanine Nucleotide Exchange Factor 10 (ARHGEF10)

The T332I mutation in Rho guanine nucleotide exchange factor 10 (ARHGEF10) was previously found in persons with slowed nerve conduction velocities and thin myelination of peripheral nerves. However, the molecular and cellular basis of the T332I mutant is not understood. Here, we show that ARHGEF10 has a negative regulatory region in the N terminus, in which residue 332 is located, and the T332I mutant is constitutively active. An N-terminal truncated ARHGEF10 mutant, ARHGEF10 ΔN (lacking amino acids 1-332), induced cell contraction that was inhibited by a Rho kinase inhibitor Y27632 and had higher GEF activity for RhoA than the wild type. The T332I mutant also showed the phenotype similar to the N-terminal truncated mutant. These data suggest that the ARHGEF10 T332I mutation-associated phenotype observed in the peripheral nerves is due to activated GEF activity of the ARHGEF10 T332I mutant.

Interplay between LXR and Wnt/ -Catenin Signaling in the Negative Regulation of Peripheral Myelin Genes by Oxysterols

Oxysterols are reactive molecules generated from the oxidation of cholesterol. Their implication in cholesterol homeostasis and in the progression of neurodegenerative disorders is well known, but few data are available for their functions in the peripheral nervous system. Our aim was to study the influence of oxysterols on myelin gene expression and myelin sheath formation in peripheral nerves. We show by gas chromatography/mass spectrometry that Schwann cells and sciatic nerves contain 24(S)-hydroxycholesterol, 25-hydroxycholesterol, and 27-hydroxycholesterol and that they express their biosynthetic enzymes and receptors (liver X receptors LXRα and LXRβ). We demonstrate that oxysterols inhibit peripheral myelin gene expression [myelin protein zero (MPZ) and peripheral myelin protein-22 (PMP22)] in a Schwann cell line. This downregulation is mediated by either LXRα or LXRβ, depending on the promoter context, as suggested by siRNA strategy and chromatin immunoprecipitation assays in Schwann cells and in the sciatic nerve of LXR knock-out mice. Importantly, the knock-out of LXR in mice results in thinner myelin sheaths surrounding the axons. Oxysterols repress myelin genes via two mechanisms: by binding of LXRs to myelin gene promoters and by inhibiting the Wnt/β-catenin pathway that is crucial for the expression of myelin genes. The Wnt signaling components (Disheveled, TCF/LEF, β-catenin) are strongly repressed by oxysterols. Furthermore, the recruitment of β-catenin at the levels of the MPZ and PMP22 promoters is decreased. Our data reveal new endogenous mechanisms for the negative regulation of myelin gene expression, highlight the importance of oxysterols and LXR in peripheral nerve myelination, and open new perspectives of treating demyelinating diseases with LXR agonists.

Anti-myelin associated glycoprotein antibodies recognize HNK-1 epitope on CNS

Antibodies to myelin-associated glycoprotein (MAG) are associated with demyelinating polyneuropathy and are specific for the HNK-1 epitope. To test if anti-MAG IgM recognize HNK-1 on CNS, sera from 20 patients and 238 controls were tested on rat slices by indirect immunofluorescence (IIF). IgM from anti-MAG positive patients, but not from control sera, stained rat brain with perineuronal or neuropil pattern, depending on the CNS region. IIF titers significantly correlated with ELISA anti-MAG titers. The staining of patients’ sera were inhibited by mouse anti-HNK-1 monoclonal antibody. Our results demonstrate that anti-MAG IgM recognizes HNK-1 outside the peripheral nerve myelin carriers.

Gpr126 is essential for peripheral nerve development and myelination in mammals

In peripheral nerves, Schwann cells form the myelin sheath that insulates axons and allows rapid propagation of action potentials. Although a number of regulators of Schwann cell development are known, the signaling pathways that control myelination are incompletely understood. In this study, we show that Gpr126 is essential for myelination and other aspects of peripheral nerve development in mammals. A mutation in Gpr126 causes a severe congenital hypomyelinating peripheral neuropathy in mice, and expression of differentiated Schwann cell markers, including Pou3f1, Egr2, myelin protein zero and myelin basic protein, is reduced. Ultrastructural studies of Gpr126-/- mice showed that axonal sorting by Schwann cells is delayed, Remak bundles (non-myelinating Schwann cells associated with small caliber axons) are not observed, and Schwann cells are ultimately arrested at the promyelinating stage. Additionally, ectopic perineurial fibroblasts form aberrant fascicles throughout the endoneurium of the mutant sciatic nerve. This analysis shows that Gpr126 is required for Schwann cell myelination in mammals, and defines new roles for Gpr126 in axonal sorting, formation of mature non-myelinating Schwann cells and organization of the perineurium.

Standard and escalating treatment of chronic inflammatory demyelinating polyradiculoneuropathy

Chronic inflammatory demyelinating polyneuropathy (CIDP) is an acquired, immune-mediated polyradiculoneuritis that is progressive or relapsing over a period of at least 8 weeks. Although the exact pathogenesis is unclear, it is thought to be mediated by both cellular and humoral immune reactions directed against the peripheral nerve myelin or axon. CIDP also involves spinal nerve roots. Early medical treatment of CIDP is important to prevent axonal loss. Only three treatment regimens for CIDP have demonstrated benefit in randomized, controlled studies: corticosteroids, plasma exchange, and intravenous immunoglobulins (IVIg). Approximately 25% of patients respond inadequately to corticosteroids, plasma exchange or IVIg. Large placebo-controlled trials with alternative immunosuppressive compounds, e.g. mycophenolate mofetil, cyclosporine, cyclophosphamide, or monoclonal antibodies, are lacking.

The Early Growth Response Gene Egr2 (Alias Krox20) Is a Novel Transcriptional Target of Transforming Growth Factor-β that Is Up-Regulated in Systemic Sclerosis and Mediates Profibrotic Responses

Although the early growth response-2 (Egr-2, alias Krox20) protein shows structural and functional similarities to Egr-1, these two related early-immediate transcription factors are nonredundant. Egr-2 plays essential roles in peripheral nerve myelination, adipogenesis, and immune tolerance; however, its regulation and role in tissue repair and fibrosis remain poorly understood. We show herein that transforming growth factor (TGF)-β induced a Smad3-dependent sustained stimulation of Egr2 gene expression in normal fibroblasts. Overexpression of Egr-2 was sufficient to stimulate collagen gene expression and myofibroblast differentiation, whereas these profibrotic TGF-β responses were attenuated in Egr-2-depleted fibroblasts. Genomewide transcriptional profiling revealed that multiple genes associated with tissue remodeling and wound healing were up-regulated by Egr-2, but the Egr-2-regulated gene expression profile overlapped only partially with the Egr-1-regulated gene profile. Levels of Egr-2 were elevated in lesional tissue from mice with bleomycin-induced scleroderma. Moreover, elevated Egr-2 was noted in biopsy specimens of skin and lung from patients with systemic sclerosis. These results provide the first evidence that Egr-2 is a functionally distinct transcription factor that is both necessary and sufficient for TGF-β-induced profibrotic responses and is aberrantly expressed in lesional tissue in systemic sclerosis and in a murine model of scleroderma. Together, these findings suggest that Egr-2 plays an important nonredundant role in the pathogenesis of fibrosis. Targeting Egr-2 might represent a novel therapeutic strategy to control fibrosis.

N-WASp is required for Schwann cell cytoskeletal dynamics, normal myelin gene expression and peripheral nerve myelination

Schwann cells elaborate myelin sheaths around axons by spirally wrapping and compacting their plasma membranes. Although actin remodeling plays a crucial role in this process, the effectors that modulate the Schwann cell cytoskeleton are poorly defined. Here, we show that the actin cytoskeletal regulator, neural Wiskott-Aldrich syndrome protein (N-WASp), is upregulated in myelinating Schwann cells coincident with myelin elaboration. When N-WASp is conditionally deleted in Schwann cells at the onset of myelination, the cells continue to ensheath axons but fail to extend processes circumferentially to elaborate myelin. Myelin-related gene expression is also severely reduced in the N-WASp-deficient cells and in vitro process and lamellipodia formation are disrupted. Although affected mice demonstrate obvious motor deficits these do not appear to progress, the mutant animals achieving normal body weights and living to advanced age. Our observations demonstrate that N-WASp plays an essential role in Schwann cell maturation and myelin formation.

Sustained increase in the excitability of myelinated peripheral axons to depolarizing current is mediated by Nav1.6

Changes in the excitability of peripheral myelinated axons in response to long-lasting subthreshold depolarizing or hyperpolarizing currents (threshold electrotonus) are used as a complementary electrophysiological parameter in the study of peripheral nerve diseases in people. However, the contribution made by various axonal ion channels to specific components of threshold electrotonus remains incompletely understood. In this study, we have recorded threshold electrotonus responses from isolated nerve segments of sural nerve from control and Scn8amed mice, which lack functional Nav1.6 voltage-gated sodium channel. In med mice, the increase in axonal excitability produced by application of subthreshold depolarizing currents for 100–200ms was not sustained. In contrast, there was no difference in threshold electrotonus responses to subthreshold hyperpolarizing current application between Scn8amed and control mice. These data reveal the specific functional role of an identified subtype of voltage-gated sodium channel (Nav1.6) in mediating the depolarizing threshold electrotonus response of peripheral myelinated nerve fibers.

N-WASP is required for membrane wrapping and myelination by Schwann cells

During peripheral nerve myelination, Schwann cells sort larger axons, ensheath them, and eventually wrap their membrane to form the myelin sheath. These processes involve extensive changes in cell shape, but the exact mechanisms involved are still unknown. Neural Wiskott-Aldrich syndrome protein (N-WASP) integrates various extracellular signals to control actin dynamics and cytoskeletal reorganization through activation of the Arp2/3 complex. By generating mice lacking N-WASP in myelinating Schwann cells, we show that N-WASP is crucial for myelination. In N-WASP-deficient nerves, Schwann cells sort and ensheath axons, but most of them fail to myelinate and arrest at the promyelinating stage. Yet, a limited number of Schwann cells form unusually short internodes, containing thin myelin sheaths, with the occasional appearance of myelin misfoldings. These data suggest that regulation of actin filament nucleation in Schwann cells by N-WASP is crucial for membrane wrapping, longitudinal extension, and myelination.

Two Cases of Scrub Typhus Presenting with Guillain-Barré Syndrome with Respiratory Failure

Two Cases of Scrub Typhus Presenting with Guillain-Barré Syndrome with Respiratory Failure

P0 (Protein Zero) Mutation S34C Underlies Instability of Internodal Myelin in S63C Mice

P0 constitutes 50-60% of protein in peripheral nerve myelin and is essential for its structure and stability. Mutations within the P0 gene (MPZ) underlie a variety of hereditary neuropathies. MpzS63C transgenic mice encode a P0 with a serine to cysteine substitution at position 34 in the extracellular domain of mature P0 (P0S34C), associated with the hypomyelinating Déjérine-Sottas syndrome in human. S63C mice develop a dysmyelinating neuropathy, with packing defects in peripheral myelin. Here, we used x-ray diffraction to examine time-dependent packing defects in unfixed myelin. At ∼7 h post-dissection, WT and S63C(+/+) myelin showed native periods (175 Å) with the latter developing at most a few percent swollen myelin, whereas up to ∼50% of S63C(+/-) (mutant P0 on heterozygous P0 null background) or P0(+/-) myelin swelled to periods of ∼205 Å. In the same time frame, S63C(-/-) myelin was stable, remaining swollen at ∼210 Å. Surprisingly, treatment of whole S63C(-/-) nerves with a reducing agent completely reverted swollen arrays to native spacing and also normalized the swollen arrays that had formed in S63C(+/-) myelin, the genotype most closely related to the human disorder. Western blot revealed P0-positive bands at ∼27 and ∼50 kDa, and MALDI-TOF mass spectrometry showed these bands consisted of Ser(34)-containing peptides or P0 dimers having oxidized Cys(34) residues. We propose that P0S34C forms ectopic disulfide bonds in trans between apposed Cys(34) side chains that retard wrapping during myelin formation causing hypomyelination. Moreover, the new bonds create a packing defect by stabilizing swollen membrane arrays that leads to demyelination.

A Case Report on Parapharyngeal Nerve Cell Tumor (Schwannoma)

The Parapharyngealspace (PPS) is shaped like an inverted pyramid, extending from the skull base superiorly to the greater cornu of the hyoid bone inferiorly. The superior border of the PPS comprises a small area of the temporal and sphenoid bones, including the carotid canal, jugular foramen, and hypoglossal foramen. The PPS is limited anteriorly by the pterygomandibular raphe and pterygoid fascia and posteriorly by the cervical vertebrae and prevertebral muscles. The medial border of the PPS is the pharynx, and the lateral border is comprised of the ramus of the mandible, the medial pterygoid muscle, and the deep lobe of the parotid gland. Below the level of the mandible, the lateral boundary consists of the fascia overlying the posterior belly of the digastric muscle. The fascia from the styloid process to the tensor veli palatini divides the PPS into an anteromedial compartment (i.e. prestyloid) and a posterolateral (i.e. poststyloid) compartment. The prestyloid compartment contains the retromandibular portion of the deep lobe of the parotid gland, adipose tissue, and lymph nodes associated with the parotid gland. The poststyloid compartment contains the internal carotid artery, the internal jugular vein, CNs IX-XII, the sympathetic chain, and lymph nodes. Tumors of the PPS are uncommon, comprising less than 1% of all head and neck neoplasms. Both benign and malignant tumors may arise from any of the structures contained within the PPS. Of PPS tumors, 70–80% are benign, and 20–30% are malignant. Most PPS tumors are of salivary or neurogenic origin, although metastatic lesions; lymphoreticular lesions; and a variety of uncommon, miscellaneous lesions may arise in this location [1]. Schwannoma are the most common peripheral nerve tumor also referred to as neurilemmoma or neurinoma. This benign tumor arises from myelin in peripheral nerve. The most characteristic presentation is a mass lesion with point tenderness and shooting pain on direct palpation.

Nerve excitability studies characterize KV1.1 fast potassium channel dysfunction in patients with episodic ataxia type 1

Episodic ataxia type 1 is a neuronal channelopathy caused by mutations in the KCNA1 gene encoding the fast K(+) channel subunit K(v)1.1. Episodic ataxia type 1 presents with brief episodes of cerebellar dysfunction and persistent neuromyotonia and is associated with an increased incidence of epilepsy. In myelinated peripheral nerve, K(v)1.1 is highly expressed in the juxtaparanodal axon, where potassium channels limit the depolarizing afterpotential and the effects of depolarizing currents. Axonal excitability studies were performed on patients with genetically confirmed episodic ataxia type 1 to characterize the effects of K(v)1.1 dysfunction on motor axons in vivo. The median nerve was stimulated at the wrist and compound muscle action potentials were recorded from abductor pollicis brevis. Threshold tracking techniques were used to record strength-duration time constant, threshold electrotonus, current/threshold relationship and the recovery cycle. Recordings from 20 patients from eight kindreds with different KCNA1 point mutations were compared with those from 30 normal controls. All 20 patients had a history of episodic ataxia and 19 had neuromyotonia. All patients had similar, distinctive abnormalities: superexcitability was on average 100% higher in the patients than in controls (P < 0.00001) and, in threshold electrotonus, the increase in excitability due to a depolarizing current (20% of threshold) was 31% higher (P < 0.00001). Using these two parameters, the patients with episodic ataxia type 1 and controls could be clearly separated into two non-overlapping groups. Differences between the different KCNA1 mutations were not statistically significant. Studies of nerve excitability can identify K(v)1.1 dysfunction in patients with episodic ataxia type 1. The simple 15 min test may be useful in diagnosis, since it can differentiate patients with episodic ataxia type 1 from normal controls with high sensitivity and specificity.

The Orphan Adhesion-GPCR GPR126 Is Required for Embryonic Development in the Mouse

Adhesion-GPCRs provide essential cell-cell and cell-matrix interactions in development, and have been implicated in inherited human diseases like Usher Syndrome and bilateral frontoparietal polymicrogyria. They are the second largest subfamily of seven-transmembrane spanning proteins in vertebrates, but the function of most of these receptors is still not understood. The orphan Adhesion-GPCR GPR126 has recently been shown to play an essential role in the myelination of peripheral nerves in zebrafish. In parallel, whole-genome association studies have implicated variation at the GPR126 locus as a determinant of body height in the human population. The physiological function of GPR126 in mammals is still unknown. We describe a targeted mutation of GPR126 in the mouse, and show that GPR126 is required for embryonic viability and cardiovascular development.

Four novel cases of periaxin-related neuropathy and review of the literature

To report 4 cases of autosomal recessive hereditary neuropathy associated with novel mutations in the periaxin gene (PRX) with a review of the literature. Periaxin protein is required for the maintenance of peripheral nerve myelin. Patients with PRX mutations have early-onset autosomal recessive demyelinating Charcot-Marie-Tooth disease (CMT4F) or Déjèrine-Sottas neuropathy (DSN). Only 12 different mutations have been described thus far. Case reports and literature review. Four patients from 3 unrelated families (2 siblings and 2 unrelated patients) were affected by an early-onset, slowly progressive demyelinating neuropathy with relevant sensory involvement. All carried novel frameshift or nonsense mutations in the PRX gene. The 2 siblings were compound heterozygotes for 2 PRX null mutations (p.Q547X and p.K808SfsX2), the third patient harbored a homozygous nonsense mutation (p.E682X), and the last patient had a homozygous 2-nt insertion predicting a premature protein truncation (p.S259PfsX55). Electrophysiologic analysis showed a severe slowing of motor nerve conduction velocities (MNCVs, between 3 and 15.3 m/s) with undetectable sensory nerve action potentials (SNAPs). Sural nerve biopsy, performed in 2 patients, demonstrated a severe demyelinating neuropathy and onion bulb formations. Interestingly, we observed some variability of disease severity within the same family. These cases and review of the literature indicate that PRX-related neuropathies have early onset but overall slow progression. Typical features are prominent sensory involvement, often with sensory ataxia; a moderate-to-dramatic reduction of MNCVs and almost invariable absence of SNAPs; and pathologic demyelination with classic onion bulbs, and less commonly myelin folding and basal lamina onion bulbs.

Krox20/EGR2 deficiency accelerates cell growth and differentiation in the monocytic lineage and decreases bone mass

AbstractKrox20/EGR2, one of the 4 early growth response genes, is a highly conserved transcription factor implicated in hindbrain development, peripheral nerve myelination, tumor suppression, and monocyte/macrophage cell fate determination. Here, we established a novel role for Krox20 in postnatal skeletal metabolism. Microcomputed tomographic analysis of 4- and 8-week-old mice revealed a low bone mass phenotype (LBM) in both the distal femur and the vertebra of Krox20+/− mice. This was attributable to accelerated bone resorption as demonstrated in vivo by increased osteoclast number and serum C-terminal telopeptides, a marker for collagen degradation. Krox20 haploinsufficiency did not reduce bone formation in vivo, nor did it compromise osteoblast differentiation in vitro. In contrast, growth and differentiation were significantly stimulated in preosteoclast cultures derived from Krox20+/− splenocytes, suggesting that the LBM is attributable to Krox20 haploinsufficiency in the monocytic lineage. Furthermore, Krox20 silencing in preosteoclasts increased cFms expression and response to macrophage colony-stimulating factor, leading to a cell-autonomous stimulation of cell-cycle progression. Our data indicate that the antimitogenic role of Krox20 in preosteoclasts is the predominant mechanism underlying the LBM phenotype of Krox20-deficient mice. Stimulation of Krox20 expression in preosteoclasts may present a viable therapeutic strategy for high-turnover osteoporosis.

Lgi4 promotes the proliferation and differentiation of glial lineage cells throughout the developing peripheral nervous system.

The mechanisms that regulate peripheral nervous system (PNS) gliogenesis are incompletely understood. For example, gut neural crest stem cells (NCSCs) do not respond to known gliogenic factors, suggesting that yet-unidentified factors regulate gut gliogenesis. To identify new mechanisms, we performed gene expression profiling to identify factors secreted by gut NCSCs during the gliogenic phase of development. These cells highly expressed leucine-rich glioma inactivated 4 (Lgi4) despite the fact that Lgi4 has never been implicated in stem cell function or enteric nervous system development. Lgi4 is known to regulate peripheral nerve myelination (having been identified as the mutated gene in spontaneously arising claw paw mutant mice), but Lgi4 is not known to play any role in PNS development outside of peripheral nerves. To systematically analyze Lgi4 function, we generated gene-targeted mice. Lgi4-deficient mice exhibited a more severe phenotype than claw paw mice and had gliogenic defects in sensory, sympathetic, and enteric ganglia. We found that Lgi4 is required for the proliferation and differentiation of glial-restricted progenitors throughout the PNS. Analysis of compound-mutant mice revealed that the mechanism by which Lgi4 promotes enteric gliogenesis involves binding the ADAM22 receptor. Our results identify a new mechanism regulating enteric gliogenesis as well as novel functions for Lgi4 regulating the proliferation and maturation of glial lineage cells throughout the PNS.

Membrane attack complex of complement is not essential for immune mediated demyelination in experimental autoimmune neuritis

Antibody deposition and complement activation, especially membrane attack complex (MAC) formation are considered central for immune mediated demyelination. To examine the role of MAC in immune mediated demyelination, we studied experimental allergic neuritis (EAN) in Lewis rats deficient in complement component 6 (C6) that cannot form MAC. A C6 deficient Lewis (Lewis/C6−) strain of rats was bred by backcrossing the defective C6 gene, from PVG/C6− rats, onto the Lewis background. Lewis/C6− rats had the same C6 gene deletion as PVG/C6− rats and their sera did not support immune mediated haemolysis unless C6 was added. Active EAN was induced in Lewis and Lewis/C6− rats by immunization with bovine peripheral nerve myelin in complete Freund's adjuvant (CFA), and Lewis/C6− rats had delayed clinical EAN compared to the Lewis rats. Peripheral nerve demyelination in Lewis/C6− was also delayed but was similar in extent at the peak of disease. Compared to Lewis, Lewis/C6− nerves had no MAC deposition, reduced macrophage infiltrate and IL-17A, but similar T cell infiltrate and Th1 cytokine mRNA expression. ICAM-1 and P-selectin mRNA expression and immunostaining on vascular endothelium were delayed in Lewis C6− compared to Lewis rats' nerves. This study found that MAC was not required for immune mediated demyelination; but that MAC enhanced early symptoms and early demyelination in EAN, either by direct lysis or by sub-lytic induction of vascular endothelial expression of ICAM-1 and P-selectin.

Ablation of Dicer from Murine Schwann Cells Increases Their Proliferation while Blocking Myelination

The myelin sheaths that surround the thick axons of the peripheral nervous system are produced by the highly specialized Schwann cells. Differentiation of Schwann cells and myelination occur in discrete steps. Each of these requires coordinated expression of specific proteins in a precise sequence, yet the regulatory mechanisms controlling protein expression during these events are incompletely understood. Here we report that Schwann cell-specific ablation of the enzyme Dicer1, which is required for the production of small non-coding regulatory microRNAs, fully arrests Schwann cell differentiation, resulting in early postnatal lethality. Dicer−/− Schwann cells had lost their ability to myelinate, yet were still capable of sorting axons. Both cell death and, paradoxically, proliferation of immature Schwann cells was markedly enhanced, suggesting that their terminal differentiation is triggered by growth-arresting regulatory microRNAs. Using microRNA microarrays, we identified 16 microRNAs that are upregulated upon myelination and whose expression is controlled by Dicer in Schwann cells. This set of microRNAs appears to drive Schwann cell differentiation and myelination of peripheral nerves, thereby fulfilling a crucial function for survival of the organism.