Axon-glia interactions and the domain organization of myelinated axons requires neurexin IV/Caspr/Paranodin

Age-dependent regulation of axoglial interactions and behavior by oligodendrocyte AnkyrinG

The bipolar disorder (BD) risk gene ANK3 encodes the scaffolding protein AnkyrinG (AnkG). In neurons, AnkG regulates polarity and ion channel clustering at axon initial segments and nodes of Ranvier. Disruption of neuronal AnkG causes BD-like phenotypes in mice. During development, AnkG is also expressed at comparable levels in oligodendrocytes and facilitates the efficient assembly of paranodal junctions. However, the physiological roles of glial AnkG in the mature nervous system, and its contributions to BD-like phenotypes, remain unexplored. Here, we generated oligodendroglia-specific AnkG conditional knockout mice and observed the destabilization of axoglial interactions in aged but not young adult mice. In addition, these mice exhibited profound histological, electrophysiological, and behavioral pathophysiologies. Unbiased translatomic profiling revealed potential compensatory machineries. These results highlight the critical functions of glial AnkG in maintaining proper axoglial interactions throughout aging and suggests a previously unrecognized contribution of oligodendroglial AnkG to neuropsychiatric disorders.

Progression of axonal excitability abnormalities with increasing clinical severity of diabetic peripheral neuropathy

OBJECTIVE

Diabetic peripheral neuropathy (DPN) is a frequent complication for persons with type 2 diabetes. Previous studies have failed to demonstrate any significant impact of treatment for DPN. The present study assessed the role of axonal ion channel dysfunction in DPN and explored the hypothesis that there may be a progressive change in ion channel abnormalities that varied with disease stage.

METHODS

Neurophysiological studies were conducted using axonal excitability techniques, a clinical method of assessing ion channel dysfunction. Studies were conducted in 178 persons with type 2 diabetes, with participants allocated into four groups according to clinical severity of neuropathy, assessed using the Total Neuropathy Grade.

RESULTS

Analysis of excitability data demonstrated a progressive and stepwise reduction in two parameters that are related to the activity of Kv1.1 channels, namely superexcitability and depolarizing threshold electrotonus at 10-20 ms (p < 0.001), and mathematical modelling of axonal excitability findings supported progressive upregulation of Kv1.1 conductances with increasing greater disease severity.

CONCLUSION

The findings are consistent with a progressive upregulation of juxtaparanodal Kv1.1 conductances with increasing clinical severity of diabetic peripheral neuropathy.

SIGNIFICANCE

From a translational perspective, the study suggests that blockade of Kv1.1 channels using 4-aminopyridine derivatives such as fampridine may be a potential treatment for DPN.

Protocol for isolating and processing mouse sciatic nerve fibers for confocal immunohistochemistry

Many motor and neurodegenerative diseases affect the peripheral nervous system (PNS). The myelinated axons in the sciatic nerves offer valuable insights into the pathology of these diseases. Here, we present a protocol for isolating and processing mouse sciatic nerves for confocal immunohistochemistry. We describe steps for mouse perfusion, removing and fixing the sciatic nerve, transferring nerves onto slides, staining, and imaging. This protocol can assist in characterizing pathologies of myelinated fibers resulting from diseases affecting the PNS. For complete details on the use and execution of this protocol, please refer to Chang et al. (2023).1.

Grafted human-induced pluripotent stem cells-derived oligodendrocyte progenitor cells combined with human umbilical vein endothelial cells contribute to functional recovery following spinal cord injury

BACKGROUND

Spinal cord injury (SCI) is a devastating disease that causes extensive damage to oligodendrocytes and neurons leading to demyelination and axonal degeneration. In this study, we co-transplanted cell grafts containing oligodendrocyte progenitor cells (OPCs) derived from human-induced pluripotent stem cells (iPSCs) combined with human umbilical vein endothelial cells (HUVECs), which were reported to promote OPCs survival and migration, into rat contusion models to promote functional recovery after SCI.

METHODS

OPCs were derived from iPSCs and identified by immunofluorescence at different time points. Functional assays in vitro were performed to evaluate the effect of HUVECs on the proliferation, migration, and survival of OPCs by co-culture and migration assay, as well as on the neuronal axonal growth. A combination of OPCs and HUVECs was transplanted into the rat contusive model. Upon 8 weeks, immunofluorescence staining was performed to test the safety of transplanted cells and to observe the neuronal repairment, myelination, and neural circuit reconstruction at the injured area; also, the functional recovery was assessed by Basso, Beattie, and Bresnahan open-field scale, Ladder climb, SEP, and MEP. Furthermore, the effect of HUVECs on grafts was also determined in vivo.

RESULTS

Data showed that HUVECs promote the proliferation, migration, and survival of OPCs both in vitro and in vivo. Furthermore, 8 weeks upon engraftment, the rats with OPCs and HUVECs co-transplantation noticeably facilitated remyelination, enhanced functional connection between the grafts and the host and promoted functional recovery. In addition, compared with the OPCs-alone transplantation, the co-transplantation generated more sensory neurons at the lesion border and significantly improved the sensory functional recovery.

CONCLUSIONS

Our study demonstrates that transplantation of OPCs combined with HUVECs significantly enhances both motor and sensory functional recovery after SCI. No significance was observed between OPCs combined with HUVECs group and OPCs-alone group in motor function recovery, while the sensory function recovery was significantly promoted in OPCs combined with HUVECs groups compared with the other two groups. These findings provide novel insights into the field of SCI research.

Mouse models of human CNTNAP1-associated congenital hypomyelinating neuropathy and genetic restoration of murine neurological deficits

The Contactin-associated protein 1 (Cntnap1) mouse mutants fail to establish proper axonal domains in myelinated axons. Human CNTNAP1 mutations are linked to hypomyelinating neuropathy-3, which causes severe neurological deficits. To understand the human neuropathology and to model human CNTNAP1C323R and CNTNAP1R764C mutations, we generated Cntnap1C324R and Cntnap1R765C mouse mutants, respectively. Both Cntnap1 mutants show weight loss, reduced nerve conduction, and progressive motor dysfunction. The paranodal ultrastructure shows everted myelin loops and the absence of axo-glial junctions. Biochemical analysis reveals that these Cntnap1 mutant proteins are nearly undetectable in the paranodes, have reduced surface expression and stability, and are retained in the neuronal soma. Postnatal transgenic expression of Cntnap1 in the mutant backgrounds rescues the phenotypes and restores the organization of axonal domains with improved motor function. This study uncovers the mechanistic impact of two human CNTNAP1 mutations in a mouse model and provides proof of concept for gene therapy for CNTNAP1 patients.

Adult-onset depletion of sulfatide leads to axonal degeneration with relative myelin sparing

3-O-sulfogalactosylceramide (sulfatide) constitutes a class of sphingolipids that comprise about 4% of myelin lipids in the central nervous system. Previously, our group characterized a mouse with sulfatide's synthesizing enzyme, cerebroside sulfotransferase (CST), constitutively disrupted. Using these mice, we demonstrated that sulfatide is required for establishment and maintenance of myelin, axoglial junctions, and axonal domains and that sulfatide depletion results in structural pathologies commonly observed in Multiple Sclerosis (MS). Interestingly, sulfatide is reduced in regions of normal appearing white matter (NAWM) of MS patients. Sulfatide reduction in NAWM suggests depletion occurs early in disease development and consistent with functioning as a driving force of disease progression. To closely model MS, an adult-onset disease, our lab generated a "floxed" CST mouse and mated it against the PLP-creERT mouse, resulting in a double transgenic mouse that provides temporal and cell-type specific ablation of the Cst gene (Gal3st1). Using this mouse, we demonstrate adult-onset sulfatide depletion has limited effects on myelin structure but results in the loss of axonal integrity including deterioration of domain organization accompanied by axonal degeneration. Moreover, structurally preserved myelinated axons progressively lose the ability to function as myelinated axons, indicated by the loss of the N1 peak. Together, our findings indicate that sulfatide depletion, which occurs in the early stages of MS progression, is sufficient to drive the loss of axonal function independent of demyelination and that axonal pathology, which is responsible for the irreversible loss of neuronal function that is prevalent in MS, may occur earlier than previously recognized.

The Node of Ranvier as an Interface for Axo-Glial Interactions: Perturbation of Axo-Glial Interactions in Various Neurological Disorders

The action potential conduction along the axon is highly dependent on the healthy interactions between the axon and myelin-producing glial cells. Myelin, which facilitates action potential, is the protective insulation around the axon formed by Schwann cells and oligodendrocytes in the peripheral (PNS) and central nervous system (CNS), respectively. Myelin is a continuous structure with intermittent gaps called nodes of Ranvier, which are the sites enriched with ion channels, transmembrane, scaffolding, and cytoskeletal proteins. Decades-long extensive research has identified a comprehensive proteome with strictly regularized localization at the node of Ranvier. Concurrently, axon-glia interactions at the node of Ranvier have gathered significant attention as the pathophysiological targets for various neurodegenerative disorders. Numerous studies have shown the alterations in the axon-glia interactions culminating in neurological diseases. In this review, we have provided an update on the molecular composition of the node of Ranvier. Further, we have discussed in detail the consequences of disruption of axon-glia interactions during the pathogenesis of various CNS and PNS disorders.

Super-resolution imaging pinpoints the periodic ultrastructure at the human node of Ranvier and its disruption in patients with polyneuropathy

The node of Ranvier is the key element in saltatory conduction along myelinated axons, but its specific protein organization remains elusive in the human species. To shed light on nanoscale anatomy of the human node of Ranvier in health and disease, we assessed human nerve biopsies of patients with polyneuropathy by super-resolution fluorescence microscopy. We applied direct stochastic optical reconstruction microscopy (dSTORM) and supported our data by high-content confocal imaging combined with deep learning-based analysis. As a result, we revealed a ~ 190 nm periodic protein arrangement of cytoskeletal proteins and axoglial cell adhesion molecules in human peripheral nerves. In patients with polyneuropathy, periodic distances increased at the paranodal region of the node of Ranvier, both at the axonal cytoskeleton and at the axoglial junction. In-depth image analysis revealed a partial loss of proteins of the axoglial complex (Caspr-1, neurofascin-155) in combination with detachment from the cytoskeletal anchor protein ß2-spectrin. High content analysis showed that such paranodal disorganization occurred especially in acute and severe axonal neuropathy with ongoing Wallerian degeneration and related cytoskeletal damage. We provide nanoscale and protein-specific evidence for the prominent, but vulnerable role of the node of Ranvier for axonal integrity. Furthermore, we show that super-resolution imaging can identify, quantify and map elongated periodic protein distances and protein interaction in histopathological tissue samples. We thus introduce a promising tool for further translational applications of super resolution microscopy.

LGI3/2-ADAM23 interactions cluster Kv1 channels in myelinated axons to regulate refractory period

Along myelinated axons, Shaker-type potassium channels (Kv1) accumulate at high density in the juxtaparanodal region, directly adjacent to the paranodal axon-glia junctions that flank the nodes of Ranvier. However, the mechanisms that control the clustering of Kv1 channels, as well as their function at this site, are still poorly understood. Here we demonstrate that axonal ADAM23 is essential for both the accumulation and stability of juxtaparanodal Kv1 complexes. The function of ADAM23 is critically dependent on its interaction with its extracellular ligands LGI2 and LGI3. Furthermore, we demonstrate that juxtaparanodal Kv1 complexes affect the refractory period, thus enabling high-frequency burst firing of action potentials. Our findings not only reveal a previously unknown molecular pathway that regulates Kv1 channel clustering, but they also demonstrate that the juxtaparanodal Kv1 channels that are concealed below the myelin sheath, play a significant role in modifying axonal physiology.

Effect of monovalency on anti-contactin-1 IgG4

Introduction

Autoimmune nodopathies (AN) have been diagnosed in a subset of patients fulfilling criteria for chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) who display no or poor response to intravenous immunoglobulins. Biomarkers of AN are autoantibodies, mainly IgG4, directed against the ternary paranodal complex composed by neurofascin-155, contactin-1 (CNTN1), and Contactin-associated-protein-1 (CASPR1) or against the nodal isoforms of neurofascin. IgG4 can undergo a Fab-arm exchange (FAE) which results in functionally monovalent antibody. This phenomenon differentially affects the pathogenicity of IgG4 depending on the target of autoantibodies. Here, we have evaluated this issue by examining the impact of valency on anti-CNTN1 IgG4 which induces paranodal destruction through a function blocking activity.

Methods

Sera were obtained from 20 patients with AN associated with anti-CNTN1 antibodies. The proportion of monospecific/bispecific anti-CNTN1 antibodies was estimated in each patient by ELISA by examining the ability of serum antibodies to cross-link untagged CNTN1 with biotinylated CNTN1. To determine the impact of monovalency, anti-CNTN1 IgG4 were enzymatically digested into monovalent Fab and tested in vitro on cell aggregation assay. Also, intraneural injections were performed to determine whether monovalent Fab and native IgG4 may penetrate paranode, and antibody infiltration was monitored 1- and 3-days post injection.

Results and discussion

We found that the percentage of monospecific antibodies were lower than 5% in 14 out of 20 patients (70%), suggesting that IgG4 have undergone extensive FAE in situ. The levels of monospecific antibodies correlated with the titers of anti-CNTN1 antibodies. However, no correlation was found with clinical severity, and patients with low or high percentage of monospecific antibodies similarly showed a severe phenotype. Native anti-CNTN1 IgG4 were shown to inhibit the interaction between cells expressing CNTN1/CASPR1 and cells expressing neurofascin-155 using an in vitro aggregation assay. Similarly, monovalent Fab significantly inhibited the interaction between CNTN1/CASPR1 and neurofascin-155. Intraneural injections of Fab and native anti-CNTN1 IgG4 indicated that both mono- and bivalent anti-CNTN1 IgG4 potently penetrated the paranodal regions and completely invaded this region by day 3. Altogether, these data indicate anti-CNTN1 IgG4 are mostly bispecific in patients, and that functionally monovalent anti-CNTN1 antibodies have the pathogenic potency to alter paranode.

DAP12 deletion causes age-related motor function impairment but promotes functional recovery after sciatic nerve crush injury

DNAX activating protein of 12 kDa (DAP12)-deficiency mice showed impaired differentiation of oligodendrocytes and reduced myelin in the central nervous system. Whether DAP12 is expressed by Schwann cells and its roles in the peripheral nervous system (PNS) remains unknown. In this study, expression of DAP12 was detected in Schwann cells in vivo and in vitro. The DAP12-knockout (KO) mice showed age-related motor deficits and thinner myelin in the sciatic nerve than WT mice but significantly faster clinical recovery after sciatic nerve crush injury. In sciatic nerves of DAP12 KO and WT mice, proteomic profiles analysis identified 158 differentially expressed proteins (DEPs) at 8-week-old, 29 DEPs at 54-week-old and 33 DEPs at two weeks after crush injury. Typically, of the DEPs at 54-week-old, up-regulated Lgmn and down-regulated RecK and Yap1 were associated with myelin loss in the sciatic nerve of DAP12 KO mice. Upregulation of nicotinamide nucleotide transhydrogenase and haptoglobin were associated with the accumulation of macrophages in the crushed sciatic nerve of DAP12 KO mice. After crush injury, there were significantly more M1 macrophages at one-week and more M2 macrophages at two-week in sciatic nerve of DAP12 KO mice than WT mice, indicating that DAP12 deletion promotes the phenotype conversion of macrophages from M1 to M2. Collectively, our findings suggest that DAP12 may exert dual roles in the PNS including promoting the physiological myelin formation and maintenance of Schwann cells but delaying nerve repair after injury by modulating the recruitment of macrophages and phenotype conversion.

Molecular Influence of Resiniferatoxin on the Urinary Bladder Wall Based on Differential Gene Expression Profiling

Resiniferatoxin (RTX) is a potent capsaicin analog used as a drug for experimental therapy to treat neurogenic disorders associated with enhanced nociceptive transmission, including lower urinary tract symptoms. The present study, for the first time, investigated the transcriptomic profile of control and RTX-treated porcine urinary bladder walls. We applied multistep bioinformatics and discovered 129 differentially expressed genes (DEGs): 54 upregulated and 75 downregulated. Metabolic pathways analysis revealed five significant Kyoto Encyclopedia of Genes and Genomes (KEGG) items ('folate biosynthesis', 'metabolic pathways', 'sulfur relay system', 'sulfur metabolism' and 'serotonergic synapse') that were altered after RTX intravesical administration. A thorough analysis of the detected DEGs indicated that RTX treatment influenced the signaling pathways regulating nerve growth, myelination, axon specification, and elongation. Many of the revealed DEGs are involved in the nerve degeneration process; however, some of them were implicated in the initiation of neuroprotective mechanisms. Interestingly, RTX intravesical installation was followed by changes in the expression of genes involved in synaptic plasticity and neuromodulation, including 5-HT, H2S, glutamate, and GABA transmission. The obtained results suggest that the toxin may exert a therapeutic, antinociceptive effect not only by acting on TRPV1 receptors.

Overall Role of Contactins Expression in Neurodevelopmental Events and Contribution to Neurological Disorders

BACKGROUND

Neurodegenerative disorders may depend upon a misregulation of the pathways which sustain neurodevelopmental control. In this context, this review article focuses on Friedreich ataxia (FA), a neurodegenerative disorder resulting from mutations within the gene encoding the Frataxin protein, which is involved in the control of mitochondrial function and oxidative metabolism.

OBJECTIVE

The specific aim of the present study concerns the FA molecular and cellular substrates, for which available transgenic mice models are proposed, including mutants undergoing misexpression of adhesive/morphoregulatory proteins, in particular belonging to the Contactin subset of the immunoglobulin supergene family.

METHODS

In both mutant and control mice, neurogenesis was explored by morphological/morphometric analysis through the expression of cell type-specific markers, including -tubulin, the Contactin-1 axonal adhesive glycoprotein, as well as the Glial Fibrillary Acidic Protein (GFAP).

RESULTS

Specific consequences were found to arise from the chosen misexpression approach, consisting of a neuronal developmental delay associated with glial upregulation. Protective effects against the arising phenotype resulted from antioxidants (essentially epigallocatechin gallate (EGCG)) administration, which was demonstrated through the profiles of neuronal (-tubulin and Contactin 1) as well as glial (GFAP) markers, in turn indicating the concomitant activation of neurodegeneration and neuro repair processes. The latter also implied activation of the Notch-1 signaling.

CONCLUSION

Overall, this study supports the significance of changes in morphoregulatory proteins expression in the FA pathogenesis and of antioxidant administration in counteracting it, which, in turn, allows to devise potential therapeutic approaches.

Multi-omic profiling reveals the ataxia protein sacsin is required for integrin trafficking and synaptic organization

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is a childhood-onset cerebellar ataxia caused by mutations in SACS, which encodes the protein sacsin. Cellular ARSACS phenotypes include mitochondrial dysfunction, intermediate filament disorganization, and progressive death of cerebellar Purkinje neurons. It is unclear why the loss of sacsin causes these deficits or why they manifest as cerebellar ataxia. Here, we perform multi-omic profiling in sacsin knockout (KO) cells and identify alterations in microtubule dynamics and mislocalization of focal adhesion (FA) proteins, including multiple integrins. Deficits in FA structure, signaling, and function can be rescued by targeting PTEN, a negative regulator of FA signaling. ARSACS mice possess mislocalization of ITGA1 in Purkinje neurons and synaptic disorganization in the deep cerebellar nucleus (DCN). The sacsin interactome reveals that sacsin regulates interactions between cytoskeletal and synaptic adhesion proteins. Our findings suggest that disrupted trafficking of synaptic adhesion proteins is a causal molecular deficit in ARSACS.

The role of gangliosides in the organisation of the node of Ranvier examined in glycosyltransferase transgenic mice

Gangliosides are a family of sialic acid containing glycosphingolipids highly enriched in plasma membranes of the vertebrate nervous system. They are functionally diverse in modulating nervous system integrity, notably at the node of Ranvier, and also act as receptors for many ligands including toxins and autoantibodies. They are synthesised in a stepwise manner by groups of glycosyl- and sialyltransferases in a developmentally and tissue regulated manner. In this review, we summarise and discuss data derived from transgenic mice with different transferase deficiencies that have been used to determine the role of glycolipids in the organisation of the node of Ranvier. Understanding their role at this specialised functional site is crucial to determining differential pathophysiology following directed genetic or autoimmune injury to peripheral nerve nodal or paranodal domains, and revealing the downstream consequences of axo-glial disruption.

Pathogenic autoantibodies in multiple sclerosis - from a simple idea to a complex concept

The role of autoantibodies in multiple sclerosis (MS) has been enigmatic since the first description, many decades ago, of intrathecal immunoglobulin production in people with this condition. Some studies have indicated that MS pathology is heterogeneous, with an antibody-associated subtype - characterized by B cells (in varying quantities), antibodies and complement - existing alongside other subtypes with different pathologies. However, subsequent evidence suggested that some cases originally diagnosed as MS with autoantibody-mediated demyelination were more likely to be neuromyelitis optica spectrum disorder or myelin oligodendrocyte glycoprotein antibody-associated disease. These findings raise the important question of whether an autoantibody-mediated MS subtype exists and whether pathogenic MS-associated autoantibodies remain to be identified. Potential roles of autoantibodies in MS could range from specific antibodies defining the disease to a non-disease-specific amplification of cellular immune responses and other pathophysiological processes. In this Perspective, we review studies that have attempted to identify MS-associated autoantibodies and provide our opinions on their possible roles in the pathophysiology of MS.

Oligodendroglia are emerging players in several forms of learning and memory

Synaptic plasticity is the fundamental cellular mechanism of learning and memory, but recent research reveals that myelin-forming glia, oligodendrocytes (OL), are also involved. They contribute in ways that synaptic plasticity cannot, and the findings have not been integrated into the established conceptual framework used in the field of learning and memory. OLs and their progenitors are involved in long-term memory, memory consolidation, working memory, and recall in associative learning. They also contribute to short-term memory and non-associative learning by affecting synaptic transmission, intrinsic excitability of axons, and neural oscillations. Oligodendroglial involvement expands the field beyond synaptic plasticity to system-wide network function, where precise spike time arrival and neural oscillations are critical in information processing, storage, and retrieval.

A Perspective highlights current evidence that supports oligodendrocytes and their progenitors’ involvement in cognition and proposes that our understanding of learning and memory can be expanded beyond the classic view of synaptic plasticity to a system-wide network function.

Heterogeneity and regulation of oligodendrocyte morphology

Oligodendrocytes form multiple myelin sheaths in the central nervous system (CNS), which increase nerve conduction velocity and are necessary for basic and higher brain functions such as sensory function, motor control, and learning. Structures of the myelin sheath such as myelin internodal length and myelin thickness regulate nerve conduction. Various parts of the central nervous system exhibit different myelin structures and oligodendrocyte morphologies. Recent studies supported that oligodendrocytes are a heterogenous population of cells and myelin sheaths formed by some oligodendrocytes can be biased to particular groups of axons, and myelin structures are dynamically modulated in certain classes of neurons by specific experiences. Structures of oligodendrocyte/myelin are also affected in pathological conditions such as demyelinating and neuropsychiatric disorders. This review summarizes our understanding of heterogeneity and regulation of oligodendrocyte morphology concerning central nervous system regions, neuronal classes, experiences, diseases, and how oligodendrocytes are optimized to execute central nervous system functions.

IgG4 Valency Modulates the Pathogenicity of Anti–Neurofascin-155 IgG4 in Autoimmune Nodopathy

Background and Objectives

IgG4 autoantibodies to neurofascin-155 (Nfasc155) are associated with a subgroup of patients with chronic inflammatory demyelinating polyneuropathy (CIDP), currently named autoimmune nodopathy. We previously demonstrated that those antibodies alter conduction along myelinated axons by inducing Nfasc155 depletion and paranode destruction. In blood, IgG4 have the potency to exchange their moiety with other unrelated IgG4 through a process called Fab-arm exchange (FAE). This process results in functionally monovalent antibodies and may affect the pathogenicity of autoantibodies. Here, we examined this issue and whether FAE is beneficial or detrimental for Nfasc155 autoimmune nodopathy.

Methods

The bivalency and monospecificity of anti-Nfasc155 were examined by sandwich ELISA in 10 reactive patients, 10 unreactive CIDP patients, and 10 healthy controls. FAE was induced in vitro using reduced glutathione and unreactive IgG4, and the ratio of the κ:λ light chain was monitored. To determine the pathogenic potential of bivalent anti-Nfasc155 IgG4, autoantibodies derived from patients were enzymatically cleaved into monovalent Fab and bivalent F(ab’)₂ or swapped with unreactive IgG4 and then were injected in neonatal animals.

Results

Monospecific bivalent IgG4 against Nfasc155 were detected in the serum of all reactive patients, indicating that a fraction of IgG4 have not undergone FAE in situ. These IgG4 were, nonetheless, capable of engaging into FAE with unreactive IgG4 in vitro, and this decreased the levels of monospecific antibodies and modulated the ratio of the κ:λ light chain. When injected in animals, monovalent anti-Nfasc155 Fab did not alter the formation of paranodes; by contrast, both native anti-Nfasc155 IgG4 and F(ab’)₂ fragments strongly impaired paranode formation. The promotion of FAE with unreactive IgG4 also strongly diminished the pathogenic potential of anti-Nfasc155 IgG4 in animals and decreased IgG4 clustering on Schwann cells.

Discussion

Our findings demonstrate that monospecific and bivalent anti-Nfasc155 IgG4 are detected in patients and that those autoantibodies are the pathogenic ones. The transformation of anti-Nfasc155 IgG4 into monovalent Fab or functionally monovalent IgG4 through FAE strongly decreases paranodal alterations. Bivalency thus appears crucial for Nfasc155 clustering and paranode destruction.

Paranodal Axoglial Junctions, an Essential Component in Axonal Homeostasis

In vertebrates, a high density of voltage-gated Na⁺ channel at nodes of Ranvier and of voltage-gated K⁺ channel at juxtaparanodes is necessary for rapid propagation of action potential, that is, for saltatory conduction in myelinated axons. Myelin loops attach to the axonal membrane and form paranodal axoglial junctions (PNJs) at paranodes adjacent to nodes of Ranvier. There is growing evidence that the PNJs contribute to axonal homeostasis in addition to their roles as lateral fences that restrict the location of nodal axolemmal proteins for effective saltatory conduction. Perturbations of PNJs, as in specific PNJ protein knockouts as well as in myelin lipid deficient mice, result in internodal axonal alterations, even if their internodal myelin is preserved. Here we review studies showing that PNJs play crucial roles in the myelinated axonal homeostasis. The present evidence points to two functions in particular: 1) PNJs facilitate axonal transport of membranous organelles as well as cytoskeletal proteins; and 2) they regulate the axonal distribution of type 1 inositol 1,4,5-trisphosphate receptor (IP₃R1) in cerebellar Purkinje axons. Myelinated axonal homeostasis depends among others on the state of PNJs, and consequently, a better understanding of this dependency may contribute to the clarification of CNS disease mechanisms and the development of novel therapies.

Schwann cell nodal membrane disruption triggers bystander axonal degeneration in a Guillain-Barré syndrome mouse model

In Guillain-Barré syndrome (GBS), both axonal and demyelinating variants can be mediated by complement-fixing anti-GM1 ganglioside autoantibodies that target peripheral nerve axonal and Schwann cell (SC) membranes, respectively. Critically, the extent of axonal degeneration in both variants dictates long-term outcome. The differing pathomechanisms underlying direct axonal injury and the secondary bystander axonal degeneration following SC injury are unresolved. To investigate this, we generated glycosyltransferase-disrupted transgenic mice that express GM1 ganglioside either exclusively in neurons [GalNAcT-/--Tg(neuronal)] or glia [GalNAcT-/--Tg(glial)], thereby allowing anti-GM1 antibodies to solely target GM1 in either axonal or SC membranes, respectively. Myelinated-axon integrity in distal motor nerves was studied in transgenic mice exposed to anti-GM1 antibody and complement in ex vivo and in vivo injury paradigms. Axonal targeting induced catastrophic acute axonal disruption, as expected. When mice with GM1 in SC membranes were targeted, acute disruption of perisynaptic glia and SC membranes at nodes of Ranvier (NoRs) occurred. Following glial injury, axonal disruption at NoRs also developed subacutely, progressing to secondary axonal degeneration. These models differentiate the distinctly different axonopathic pathways under axonal and glial membrane targeting conditions, and provide insights into primary and secondary axonal injury, currently a major unsolved area in GBS research.

Beyond Wrapping: Canonical and Noncanonical Functions of Schwann Cells

Schwann cells in the peripheral nervous system (PNS) are essential for the support and myelination of axons, ensuring fast and accurate communication between the central nervous system and the periphery. Schwann cells and related glia accompany innervating axons in virtually all tissues in the body, where they exhibit remarkable plasticity and the ability to modulate pathology in extraordinary, and sometimes surprising, ways. Here, we provide a brief overview of the various glial cell types in the PNS and describe the cornerstone cellular and molecular processes that enable Schwann cells to perform their canonical functions. We then dive into discussing exciting noncanonical functions of Schwann cells and related PNS glia, which include their role in organizing the PNS, in regulating synaptic activity and pain, in modulating immunity, in providing a pool of stem cells for different organs, and, finally, in influencing cancer.

Finding New Ways How to Control BACE1

Recently, all applications of BACE1 inhibitors failed as therapeutical targets for Alzheimer´s disease (AD) due to severe side effects. Therefore, alternative ways for treatment development are a hot research topic. The present analysis investigates BACE1 protein-protein interaction networks and attempts to solve the absence of complete knowledge about pathways involving BACE1. A bioinformatics analysis matched the functions of the non-substrate interaction network with Voltage-gated potassium channels, which also appear as top priority protein nodes. Targeting BACE1 interactions with PS1 and GGA-s, blocking of BACE1 access to APP by BRI3 and RTN-s, activation of Wnt signaling and upregulation of β-catenin, and brain delivery of the extracellular domain of p75^NTR, are the main alternatives to the use of BACE 1 inhibitors highlighted by the analysis. The pathway enrichment analysis also emphasized substrates and substrate candidates with essential biological functions, which cleavage must remain controlled. They include ephrin receptors, ROBO1, ROBO2, CNTN-s, CASPR-s, CD147, CypB, TTR, APLP1/APLP2, NRXN-s, and PTPR-s. The analysis of the interaction subnetwork of BACE1 functionally related to inflammation identified a connection to three cardiomyopathies, which supports the hypothesis of the common molecular mechanisms with AD. A lot of potential shows the regulation of BACE1 activity through post-translational modifications. The interaction network of BACE1 and its phosphorylation enzyme CSNK1D functionally match the Circadian clock, p53, and Hedgehog signaling pathways. The regulation of BACE1 glycosylation could be achieved through N-acetylglucosamine transferases, α-(1→6)-fucosyltransferase, β-galactoside α-(2→6)-sialyltransferases, galactosyltransferases, and mannosidases suggested by the interaction network analysis of BACE1-MGAT3. The present analysis proposes possibilities for the alternative control of AD pathology.

Impact of Neurofascin on Chronic Inflammatory Demyelinating Polyneuropathy via Changing the Node of Ranvier Function: A Review

The effective conduction of action potential in the peripheral nervous system depends on the structural and functional integrity of the node of Ranvier and paranode. Neurofascin (NF) plays an important role in the conduction of action potential in a saltatory manner. Two subtypes of NF, NF186, and NF155, are involved in the structure of the node of Ranvier. In patients with chronic inflammatory demyelinating polyneuropathy (CIDP), anti-NF antibodies are produced when immunomodulatory dysfunction occurs, which interferes with the conduction of action potential and is considered the main pathogenic factor of CIDP. In this study, we describe the assembling mechanism and anatomical structure of the node of Ranvier and the necessary cell adhesion molecules for its physiological function. The main points of this study are that we summarized the recent studies on the role of anti-NF antibodies in the changes in the node of Ranvier function and its impact on clinical manifestations and analyzed the possible mechanisms underlying the pathogenesis of CIDP.

The structure of sensory afferent compartments in health and disease

Primary sensory neurons are a heterogeneous population of cells able to respond to both innocuous and noxious stimuli. Like most neurons they are highly compartmentalised, allowing them to detect, convey and transfer sensory information. These compartments include specialised sensory endings in the skin, the nodes of Ranvier in myelinated axons, the cell soma and their central terminals in the spinal cord. In this review, we will highlight the importance of these compartments to primary afferent function, describe how these structures are compromised following nerve damage and how this relates to neuropathic pain.

Dynamic early clusters of nodal proteins contribute to node of Ranvier assembly during myelination of peripheral neurons

Voltage-gated sodium channels cluster in macromolecular complexes at nodes of Ranvier to promote rapid nerve impulse conduction in vertebrate nerves. Node assembly in peripheral nerves is thought to be initiated at heminodes at the extremities of myelinating Schwann cells, and fusion of heminodes results in the establishment of nodes. Here we show that assembly of 'early clusters' of nodal proteins in the murine axonal membrane precedes heminode formation. The neurofascin (Nfasc) proteins are essential for node assembly, and the formation of early clusters also requires neuronal Nfasc. Early clusters are mobile and their proteins are dynamically recruited by lateral diffusion. They can undergo fusion not only with each other but also with heminodes, thus contributing to the development of nodes in peripheral axons. The formation of early clusters constitutes the earliest stage in peripheral node assembly and expands the repertoire of strategies that have evolved to establish these essential structures.

Neuronally expressed a-series gangliosides are sufficient to prevent the lethal age-dependent phenotype in GM3-only expressing mice

Gangliosides are expressed on plasma membranes throughout the body and enriched in the nervous system. A critical role for complex a- and b-series gangliosides in central and peripheral nervous system ageing has been established through transgenic manipulation of enzymes in ganglioside biosynthesis. Disrupting GalNAc-transferase (GalNAc-T), thus eliminating all a- and b-series complex gangliosides (with consequent over-expression of GM3 and GD3) leads to an age-dependent neurodegeneration. Mice that express only GM3 ganglioside (double knockout produced by crossing GalNAc-T-/- and GD3 synthase-/- mice, Dbl KO) display markedly accelerated neurodegeneration with reduced survival. Degenerating axons and disrupted node of Ranvier architecture are key features of complex ganglioside-deficient mice. Previously, we have shown that reintroduction of both a- and b-series gangliosides into neurons on a global GalNAcT-/- background is sufficient to rescue this age-dependent neurodegenerative phenotype. To determine the relative roles of a- and b-series gangliosides in this rescue paradigm, we herein reintroduced GalNAc-T into neurons of Dbl KO mice, thereby reconstituting a-series but not b-series complex gangliosides. We assessed survival, axon degeneration, axo-glial integrity, inflammatory markers and lipid-raft formation in these Rescue mice compared to wild-type and Dbl KO mice. We found that this neuronal reconstitution of a-series complex gangliosides abrogated the adult lethal phenotype in Dbl KO mice, and partially attenuated the neurodegenerative features. This suggests that whilst neuronal expression of a-series gangliosides is critical for survival during ageing, it is not entirely sufficient to restore complete nervous system integrity in the absence of either b-series or glial a-series gangliosides.

Disturbance of prefrontal cortical myelination in olfactory bulbectomized mice is associated with depressive-like behavior

Recent studies have reported that demyelination is associated with the development of depression. Olfactory bulbectomized (OBX) rodents are a useful experimental animal model for depressive disorder. However, little is known about the change in myelination in the brain of OBX mice. To address this question, we observed depressive-like behavior of OBX mice in the tail-suspension test, and determined the quantity of myelin proteins in the prefrontal cortex (PFC), striatum and hippocampus on day 14 or 21 after surgery. The number of nodes of Ranvier paired with the paranodal marker contactin-associated protein (Caspr), as well as the numbers of immature and mature oligodendrocytes in the PFC, were also measured on day 21 after surgery. We examined whether these behavioral and neurochemical changes observed in OBX mice were reversed by chronic administration of imipramine. OBX mice showed depressive-like behavior in the tail-suspension test together with a decrease in the levels of myelin proteins such as myelin basic protein, myelin-associated glycoprotein and cyclicnucleotide phosphodiesterase in the PFC on day 21 after surgery. The number of nodes of Ranvier and mature oligodendrocytes were also decreased in the PFC of OBX mice, while the number of immature oligodendrocytes was increased on day 21 after surgery. However, the number of immature oligodendrocytes in the PFC of OBX mice was decreased on day 35 after surgery. Administration of imipramine (20 mg/kg) for 2 weeks from day 21 after surgery improved OBX-induced depressive-like behavior and abnormal myelination in the PFC. The present findings suggest that the disturbance of myelin function in the PFC may contribute to the pathophysiology of depression, and further support the notion that it plays an important role in the psychological state.

Models of Distal Arthrogryposis and Lethal Congenital Contracture Syndrome

Distal arthrogryposis and lethal congenital contracture syndromes describe a broad group of disorders that share congenital limb contractures in common. While skeletal muscle sarcomeric genes comprise many of the first genes identified for Distal Arthrogyposis, other mechanisms of disease have been demonstrated, including key effects on peripheral nerve function. While Distal Arthrogryposis and Lethal Congenital Contracture Syndromes display superficial similarities in phenotype, the underlying mechanisms for these conditions are diverse but overlapping. In this review, we discuss the important insights gained into these human genetic diseases resulting from in vitro molecular studies and in vivo models in fruit fly, zebrafish, and mice.

A CADM3 variant causes Charcot-Marie-Tooth disease with marked upper limb involvement

The CADM family of proteins consists of four neuronal specific adhesion molecules (CADM1, CADM2, CADM3 and CADM4) that mediate the direct contact and interaction between axons and glia. In the peripheral nerve, axon-Schwann cell interaction is essential for the structural organization of myelinated fibres and is primarily mediated by the binding of CADM3, expressed in axons, to CADM4, expressed by myelinating Schwann cells. We have identified-by whole exome sequencing-three unrelated families, including one de novo patient, with axonal Charcot-Marie-Tooth disease (CMT2) sharing the same private variant in CADM3, Tyr172Cys. This variant is absent in 230 000 control chromosomes from gnomAD and predicted to be pathogenic. Most CADM3 patients share a similar phenotype consisting of autosomal dominant CMT2 with marked upper limb involvement. High resolution mass spectrometry analysis detected a newly created disulphide bond in the mutant CADM3 potentially modifying the native protein conformation. Our data support a retention of the mutant protein in the endoplasmic reticulum and reduced cell surface expression in vitro. Stochastic optical reconstruction microscopy imaging revealed decreased co-localization of the mutant with CADM4 at intercellular contact sites. Mice carrying the corresponding human mutation (Cadm3Y170C) showed reduced expression of the mutant protein in axons. Cadm3Y170C mice showed normal nerve conduction and myelin morphology, but exhibited abnormal axonal organization, including abnormal distribution of Kv1.2 channels and Caspr along myelinated axons. Our findings indicate the involvement of abnormal axon-glia interaction as a disease-causing mechanism in CMT patients with CADM3 mutations.

Antibodies to the Caspr1/contactin-1 complex in chronic inflammatory demyelinating polyradiculoneuropathy

Previous studies have described the clinical, serological and pathological features of patients with chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) and antibodies directed against the paranodal proteins neurofascin-155, contactin-1 (CNTN1), contactin-associated protein-1 (Caspr1), or nodal forms of neurofascin. Such antibodies are useful for diagnosis and potentially treatment selection. However, antibodies targeting Caspr1 only or the Caspr1/CNTN1 complex have been reported in few patients with CIDP. Moreover, it is unclear if these patients belong to the same pathophysiological subgroup. Using cell-based assays in routine clinical testing, we identified sera from patients with CIDP showing strong membrane reactivity when both CNTN1 and Caspr1 were co-transfected (but not when CNTN1 was transfected alone). Fifteen patients (10 male; aged between 40 and 75) with antibodies targeting Caspr1/CNTN1 co-transfected cells were enrolled for characterization. The prevalence of anti-Caspr1/CNTN1 antibodies was 1.9% (1/52) in the Sant Pau CIDP cohort, and 4.3% (1/23) in a German cohort of acute-onset CIDP. All patients fulfilled European Federation of Neurological Societies/Peripheral Nerve Society (EFNS/PNS) definite diagnostic criteria for CIDP. Seven (47%) were initially diagnosed with Guillain-Barré syndrome due to an acute-subacute onset. Six (40%) patients had cranial nerve involvement, eight (53%) reported neuropathic pain and 12 (80%) ataxia. Axonal involvement and acute denervation were frequent in electrophysiological studies. Complete response to intravenous immunoglobulin was not observed, while most (90%) responded well to rituximab. Enzyme-linked immunosorbent assay (ELISA) and teased nerve fibre immunohistochemistry confirmed reactivity against the paranodal Caspr1/CNTN1 complex. Weaker reactivity against Caspr1 transfected alone was also detected in 10/15 (67%). Sera from 13 of these patients were available for testing by ELISA. All 13 samples reacted against Caspr1 by ELISA and this reactivity was enhanced when CNTN1 was added to the Caspr1 ELISA. IgG subclasses were also investigated by ELISA. IgG4 was the predominant subclass in 10 patients, while IgG3 was predominant in other three patients. In conclusion, patients with antibodies to the Caspr1/CNTN1 complex display similar serological and clinical features and constitute a single subgroup within the CIDP syndrome. These antibodies likely target Caspr1 primarily and are detected with Caspr1-only ELISA, but reactivity is optimal when CNTN1 is added to Caspr1 in cell-based assays and ELISA.

Mechanisms of Primary Membranous Nephropathy

Membranous nephropathy (MN) is an autoimmune disease of the kidney glomerulus and one of the leading causes of nephrotic syndrome. The disease exhibits heterogenous outcomes with approximately 30% of cases progressing to end-stage renal disease. The clinical management of MN has steadily advanced owing to the identification of autoantibodies to the phospholipase A2 receptor (PLA2R) in 2009 and thrombospondin domain-containing 7A (THSD7A) in 2014 on the podocyte surface. Approximately 50–80% and 3–5% of primary MN (PMN) cases are associated with either anti-PLA2R or anti-THSD7A antibodies, respectively. The presence of these autoantibodies is used for MN diagnosis; antibody levels correlate with disease severity and possess significant biomarker values in monitoring disease progression and treatment response. Importantly, both autoantibodies are causative to MN. Additionally, evidence is emerging that NELL-1 is associated with 5–10% of PMN cases that are PLA2R- and THSD7A-negative, which moves us one step closer to mapping out the full spectrum of PMN antigens. Recent developments suggest exostosin 1 (EXT1), EXT2, NELL-1, and contactin 1 (CNTN1) are associated with MN. Genetic factors and other mechanisms are in place to regulate these factors and may contribute to MN pathogenesis. This review will discuss recent developments over the past 5 years.

Deletion of the Thrombin Proteolytic Site in Neurofascin 155 Causes Disruption of Nodal and Paranodal Organization

In the central nervous system, myelin is attached to the axon in the paranodal region by a trimolecular complex of Neurofascin155 (NF155) in the myelin membrane, interacting with Caspr1 and Contactin1 on the axolemma. Alternative splicing of a single Neurofascin transcript generates several different Neurofascins expressed by several cell types, but NF155, which is expressed by oligodendrocytes, contains a domain in the third fibronectinIII-like region of the molecule that is unique. The immunoglobulin 5–6 domain of NF155 is essential for binding to Contactin1, but less is known about the functions of the NF155-unique third fibronectinIII-like domain. Mutations and autoantibodies to this region are associated with several neurodevelopmental and demyelinating nervous system disorders. Here we used Crispr-Cas9 gene editing to delete a 9 bp sequence of NF155 in this unique domain, which has recently been identified as a thrombin binding site and implicated in plasticity of the myelin sheath. This small deletion results in dysmyelination, eversion of paranodal loops of myelin, substantial enlargement of the nodal gap, a complete loss of paranodal septate junctions, and mislocalization of Caspr1 and nodal sodium channels. The animals exhibit tremor and ataxia, and biochemical and mass spectrometric analysis indicates that while NF155 is transcribed and spliced normally, the NF155 protein is subsequently degraded, resulting in loss of the full length 155 kDa native protein. These findings reveal that this 9 bp region of NF155 in its unique third fibronectinIII-like domain is essential for stability of the protein.

Ensheathment and Myelination of Axons: Evolution of Glial Functions

Myelination of axons provides the structural basis for rapid saltatory impulse propagation along vertebrate fiber tracts, a well-established neurophysiological concept. However, myelinating oligodendrocytes and Schwann cells serve additional functions in neuronal energy metabolism that are remarkably similar to those of axon-ensheathing glial cells in unmyelinated invertebrates. Here we discuss myelin evolution and physiological glial functions, beginning with the role of ensheathing glia in preventing ephaptic coupling, axoglial metabolic support, and eliminating oxidative radicals. In both vertebrates and invertebrates, axoglial interactions are bidirectional, serving to regulate cell fate, nerve conduction, and behavioral performance. One key step in the evolution of compact myelin in the vertebrate lineage was the emergence of the open reading frame for myelin basic protein within another gene. Several other proteins were neofunctionalized as myelin constituents and help maintain a healthy nervous system. Myelination in vertebrates became a major prerequisite of inhabiting new ecological niches.

TDP-43 maximizes nerve conduction velocity by repressing a cryptic exon for paranodal junction assembly in Schwann cells

TDP-43 is extensively studied in neurons in physiological and pathological contexts. However, emerging evidence indicates that glial cells are also reliant on TDP-43 function. We demonstrate that deletion of TDP-43 in Schwann cells results in a dramatic delay in peripheral nerve conduction causing significant motor deficits in mice, which is directly attributed to the absence of paranodal axoglial junctions. By contrast, paranodes in the central nervous system are unaltered in oligodendrocytes lacking TDP-43. Mechanistically, TDP-43 binds directly to Neurofascin mRNA, encoding the cell adhesion molecule essential for paranode assembly and maintenance. Loss of TDP-43 triggers the retention of a previously unidentified cryptic exon, which targets Neurofascin mRNA for nonsense-mediated decay. Thus, TDP-43 is required for neurofascin expression, proper assembly and maintenance of paranodes, and rapid saltatory conduction. Our findings provide a framework and mechanism for how Schwann cell-autonomous dysfunction in nerve conduction is directly caused by TDP-43 loss-of-function.

Aberrant Expression of Nodal and Paranodal Molecules in Neuropathy Associated With IgM Monoclonal Gammopathy With Anti-Myelin-Associated Glycoprotein Antibodies

To clarify the pathogenesis of anti-myelin-associated glycoprotein (MAG) antibody neuropathy associated with IgM monoclonal gammopathy (anti-MAG neuropathy), sural nerve biopsy specimens from 15 patients were investigated. Sodium channels, potassium channels, contactin-associated protein 1 (Caspr1), contactin 1, and neurofascin were evaluated by immunofluorescence in teased-fiber preparations. Immunoreactivity to the pan-sodium channel in both anti-MAG neuropathy patients and in normal controls was concentrated at the node of Ranvier unless there was demyelination, which was defined as the widening of the node of Ranvier. However, this immunoreactivity became weak or disappeared as demyelination progressed. In contrast, KCNQ2 immunostaining was nearly absent even in the absence of demyelination. The lengths of Caspr1, contactin 1, and pan-neurofascin immunostaining sites at the paranode were significantly increased compared with those of normal controls despite the absence of demyelination. The length of paranodal neurofascin staining correlated with the anti-MAG antibody titer, nerve conduction indices, the frequency of de/remyelination in teased-fiber preparations, and the frequency of widely spaced myelin (p < 0.05, p < 0.05, p < 0.01, and <0.05, respectively). These findings suggest that nodal and paranodal molecular alterations occur in early stages preceding the morphological changes associated with demyelination in anti-MAG neuropathy.

Morpho-Functional Consequences of Swiss Cheese Knockdown in Glia of Drosophila melanogaster

Glia are crucial for the normal development and functioning of the nervous system in many animals. Insects are widely used for studies of glia genetics and physiology. Drosophila melanogaster surface glia (perineurial and subperineurial) form a blood–brain barrier in the central nervous system and blood–nerve barrier in the peripheral nervous system. Under the subperineurial glia layer, in the cortical region of the central nervous system, cortex glia encapsulate neuronal cell bodies, whilst in the peripheral nervous system, wrapping glia ensheath axons of peripheral nerves. Here, we show that the expression of the evolutionarily conserved swiss cheese gene is important in several types of glia. swiss cheese knockdown in subperineurial glia leads to morphological abnormalities of these cells. We found that the number of subperineurial glia nuclei is reduced under swiss cheese knockdown, possibly due to apoptosis. In addition, the downregulation of swiss cheese in wrapping glia causes a loss of its integrity. We reveal transcriptome changes under swiss cheese knockdown in subperineurial glia and in cortex + wrapping glia and show that the downregulation of swiss cheese in these types of glia provokes reactive oxygen species acceleration. These results are accompanied by a decline in animal mobility measured by the negative geotaxis performance assay.

Implication of Contactins in Demyelinating Pathologies

Demyelinating pathologies comprise of a variety of conditions where either central or peripheral myelin is attacked, resulting in white matter lesions and neurodegeneration. Myelinated axons are organized into molecularly distinct domains, and this segregation is crucial for their proper function. These defined domains are differentially affected at the different stages of demyelination as well as at the lesion and perilesion sites. Among the main players in myelinated axon organization are proteins of the contactin (CNTN) group of the immunoglobulin superfamily (IgSF) of cell adhesion molecules, namely Contactin-1 and Contactin-2 (CNTN1, CNTN2). The two contactins perform their functions through intermolecular interactions, which are crucial for myelinated axon integrity and functionality. In this review, we focus on the implication of these two molecules as well as their interactors in demyelinating pathologies in humans. At first, we describe the organization and function of myelinated axons in the central (CNS) and the peripheral (PNS) nervous system, further analyzing the role of CNTN1 and CNTN2 as well as their interactors in myelination. In the last section, studies showing the correlation of the two contactins with demyelinating pathologies are reviewed, highlighting the importance of these recognition molecules in shaping the function of the nervous system in multiple ways.

Periaxonal and nodal plasticities modulate action potential conduction in the adult mouse brain

Central nervous system myelination increases action potential conduction velocity. However, it is unclear how myelination is coordinated to ensure the temporally precise arrival of action potentials and facilitate information processing within cortical and associative circuits. Here, we show that myelin sheaths, supported by mature oligodendrocytes, remain plastic in the adult mouse brain and undergo subtle structural modifications to influence action potential conduction velocity. Repetitive transcranial magnetic stimulation and spatial learning, two stimuli that modify neuronal activity, alter the length of the nodes of Ranvier and the size of the periaxonal space within active brain regions. This change in the axon-glial configuration is independent of oligodendrogenesis and robustly alters action potential conduction velocity. Because aptitude in the spatial learning task was found to correlate with action potential conduction velocity in the fimbria-fornix pathway, modifying the axon-glial configuration may be a mechanism that facilitates learning in the adult mouse brain.

Assembly and Function of the Juxtaparanodal Kv1 Complex in Health and Disease

The precise axonal distribution of specific potassium channels is known to secure the shape and frequency of action potentials in myelinated fibers. The low-threshold voltage-gated Kv1 channels located at the axon initial segment have a significant influence on spike initiation and waveform. Their role remains partially understood at the juxtaparanodes where they are trapped under the compact myelin bordering the nodes of Ranvier in physiological conditions. However, the exposure of Kv1 channels in de- or dys-myelinating neuropathy results in alteration of saltatory conduction. Moreover, cell adhesion molecules associated with the Kv1 complex, including Caspr2, Contactin2, and LGI1, are target antigens in autoimmune diseases associated with hyperexcitability such as encephalitis, neuromyotonia, or neuropathic pain. The clustering of Kv1.1/Kv1.2 channels at the axon initial segment and juxtaparanodes is based on interactions with cell adhesion molecules and cytoskeletal linkers. This review will focus on the trafficking and assembly of the axonal Kv1 complex in the peripheral and central nervous system (PNS and CNS), during development, and in health and disease.

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.

Current thinking on the mechanisms by which multiple sclerosis gives rise to cumulative neurological disability revolves largely around focal lesions of inflammation and demyelination. However, some of the debilitating symptoms, such as severe fatigue, might be better explained by a more diffuse pathology. This study shows that paranodes in the white matter become abnormal as a result of neuroinflammation, which may be the result of the action of cytokines that cause glia to release glutamate.

Mechanisms of node of Ranvier assembly

The nodes of Ranvier have clustered Na⁺ and K⁺ channels necessary for rapid and efficient axonal action potential conduction. However, detailed mechanisms of channel clustering have only recently been identified: they include two independent axon-glia interactions that converge on distinct axonal cytoskeletons. Here, we discuss how glial cell adhesion molecules and the extracellular matrix molecules that bind them assemble combinations of ankyrins, spectrins and other cytoskeletal scaffolding proteins, which cluster ion channels. We present a detailed molecular model, incorporating these overlapping mechanisms, to explain how the nodes of Ranvier are assembled in both the peripheral and central nervous systems.

Focal loss of the paranodal domain protein Neurofascin155 in the internal capsule impairs cortically induced muscle activity in vivo

Paranodal axoglial junctions are essential for rapid nerve conduction and the organization of axonal domains in myelinated axons. Neurofascin155 (Nfasc155) is a glial cell adhesion molecule that is also required for the assembly of these domains. Previous studies have demonstrated that general ablation of Nfasc155 disorganizes these domains, reduces conduction velocity, and disrupts motor behaviors. Multiple sclerosis (MS), a typical disorder of demyelination in the central nervous system, is reported to have autoantibody to Nfasc. However, the impact of focal loss of Nfasc155, which may occur in MS patients, remains unclear. Here, we examined whether restricted focal loss of Nfasc155 affects the electrophysiological properties of the motor system in vivo. Adeno-associated virus type5 (AAV5) harboring EGFP-2A-Cre was injected into the glial-enriched internal capsule of floxed-Neurofascin (NfascFlox/Flox) mice to focally disrupt paranodal junctions in the cortico-fugal fibers from the motor cortex to the spinal cord. Electromyograms (EMGs) of the triceps brachii muscles in response to electrical stimulation of the motor cortex were successively examined in these awake mice. EMG analysis showed significant delay in the onset and peak latencies after AAV injection compared to control (Nfasc+/+) mice. Moreover, EMG half-widths were increased, and EMG amplitudes were gradually decreased by 13 weeks. Similar EMG changes have been reported in MS patients. These findings provide physiological evidence that motor outputs are obstructed by focal ablation of paranodal junctions in myelinated axons. Our findings may open a new path toward development of a novel biomarker for an early phase of human MS, as Nfasc155 detects microstructural changes in the paranodal junction.

Mutations of CNTNAP1 led to defects in neuronal development

Mutations of CNTNAP1 were associated with myelination disorders, suggesting the role of CNTNAP1 in myelination processes. Whether CNTNAP1 may have a role in early cortical neuronal development is largely unknown. In this study, we identified 4 compound heterozygous mutations of CNTNAP1 in 2 Chinese families. Using mouse models, we found that CNTNAP1 is highly expressed in neurons and is located predominantly in MAP2⁺ neurons during the early developmental stage. Importantly, Cntnap1 deficiency results in aberrant dendritic growth and spine development in vitro and in vivo, and it delayed migration of cortical neurons during early development. Finally, we found that the number of parvalbumin⁺ neurons in the cortex and hippocampus of Cntnap1^–/– mice is strikingly increased by P15, suggesting that excitation/inhibition balance is impaired. Together, this evidence elucidates a critical function of CNTNAP1 in cortical development, providing insights underlying molecular and circuit mechanisms of CNTNAP1-related disease.

Deficiency of CNTNAP1 causes severe cortical developmental deficits, leading to human lethal perinatal symptoms.

Mitochondrial dynamics and bioenergetics regulated by netrin-1 in oligodendrocytes

Mitochondria are dynamic organelles that produce energy and molecular precursors that are essential for myelin synthesis. Unlike in neurons, mitochondria in oligodendrocytes increase intracellular movement in response to glutamatergic activation and are more susceptible to oxidative stress than in astrocytes or microglia. The signaling pathways that regulate these cell type-specific mitochondrial responses in oligodendrocytes are not understood. Here, we visualized mitochondria migrating through thin cytoplasmic channels crossing myelin basic protein-positive compacted membranes and localized within paranodal loop cytoplasm. We hypothesized that local extracellular enrichment of netrin-1 might regulate the recruitment and function of paranodal proteins and organelles, including mitochondria. We identified rapid recruitment of mitochondria and paranodal proteins, including neurofascin 155 (NF155) and the netrin receptor deleted in colorectal carcinoma (DCC), to sites of contact between oligodendrocytes and netrin-1-coated microbeads in vitro. We provide evidence that Src-family kinase activation and Rho-associated protein kinase (ROCK) inhibition downstream of netrin-1 induces mitochondrial elongation, hyperpolarization of the mitochondrial inner membrane, and increases glycolysis. Our findings identify a signaling mechanism in oligodendrocytes that is sufficient to locally recruit paranodal proteins and regulate the subcellular localization, morphology, and function of mitochondria.

Update on IgG4-mediated autoimmune diseases: New insights and new family members

Antibodies of IgG4 subclass are exceptional players of the immune system, as they are considered to be immunologically inert and functionally monovalent, and as such may be part of classical tolerance mechanisms. IgG4 antibodies are found in a range of different diseases, including IgG4-related diseases, allergy, cancer, rheumatoid arthritis, helminth infection and IgG4 autoimmune diseases, where they may be pathogenic or protective. IgG4 autoimmune diseases are an emerging new group of diseases that are characterized by pathogenic, antigen-specific autoantibodies of IgG4 subclass, such as MuSK myasthenia gravis, pemphigus vulgaris and thrombotic thrombocytopenic purpura. The list of IgG4 autoantigens is rapidly growing and to date contains 29 candidate antigens. Interestingly, IgG4 autoimmune diseases are restricted to four distinct organs: 1) the central and peripheral nervous system, 2) the kidney, 3) the skin and mucous membranes and 4) the vascular system and soluble antigens in the blood circulation. The pathogenicity of IgG4 can be validated using our classification system, and is usually excerted by functional blocking of protein-protein interaction.

That's a Wrap! Molecular Drivers Governing Neuronal Nogo Receptor-Dependent Myelin Plasticity and Integrity

Myelin is a dynamic membrane that is important for coordinating the fast propagation of action potentials along small or large caliber axons (0.1-10 μm) some of which extend the entire length of the spinal cord. Due to the heterogeneity of electrical and energy demands of the variable neuronal populations, the axo-myelinic and axo-glial interactions that regulate the biophysical properties of myelinated axons also vary in terms of molecular interactions at the membrane interfaces. An important topic of debate in neuroscience is how myelin is maintained and modified under neuronal control and how disruption of this control (due to disease or injury) can initiate and/or propagate neurodegeneration. One of the key molecular signaling cascades that have been investigated in the context of neural injury over the past two decades involves the myelin-associated inhibitory factors (MAIFs) that interact with Nogo receptor 1 (NgR1). Chief among the MAIF superfamily of molecules is a reticulon family protein, Nogo-A, that is established as a potent inhibitor of neurite sprouting and axon regeneration. However, an understated role for NgR1 is its ability to control axo-myelin interactions and Nogo-A specific ligand binding. These interactions may occur at axo-dendritic and axo-glial synapses regulating their functional and dynamic membrane domains. The current review provides a comprehensive analysis of how neuronal NgR1 can regulate myelin thickness and plasticity under normal and disease conditions. Specifically, we discuss how NgR1 plays an important role in regulating paranodal and juxtaparanodal domains through specific signal transduction cascades that are important for microdomain molecular architecture and action potential propagation. Potential therapeutics designed to target NgR1-dependent signaling during disease are being developed in animal models since interference with the involvement of the receptor may facilitate neurological recovery. Hence, the regulatory role played by NgR1 in the axo-myelinic interface is an important research field of clinical significance that requires comprehensive investigation.

Ultrastructural Lesions of Nodo-Paranodopathies in Peripheral Neuropathies

Whatever the cause of myelin damage of the peripheral nervous system, the initial attack on myelin by a dysimmune process may begin either at the internodal area or in the paranodal and nodal regions. The term "nodo-paranodopathy" was first applied to some "axonal Guillain-Barré syndrome" subtypes, then extended to cases classified as chronic inflammatory demyelinating polyradiculoneuropathy bearing IgG4 antibodies against paranodal axoglial proteins. In these cases, paranodal dissection develops in the absence of macrophage-induced demyelination. In contrast, the mechanisms of demyelination of other dysimmune neuropathies induced by macrophages are unexplained, as no antibodies have been identified in such cases. Electron microscopy of longitudinal sections of nerve biopsies is useful to visualize and authenticate the characteristic lesions of paranodes/nodes. However, it should be borne in mind that identical ultrastructural aspects are seen in other types of polyneuropathies: Genetic, experimental, and in a few polyneuropathies for which there is no obvious etiology. Ultrastructural nerve studies confirm the initial involvement of nodes/paranodes in various types of acquired and genetic neuropathies. For some of them, the antibodies or the proteins involved by mutations are clearly identified such as Caspr-1, Contactin-1, NFasc155, and NFasc186; other unidentified proteins are likely to be involved as well.