Axoglial junctions: separate the channels or scramble the message

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.

Three-dimensional neuron-astrocyte construction on matrigel enhances establishment of functional voltage-gated sodium channels

This study aimed to investigate and compare cell growth manners and functional differences of primary cortical neurons cultured on either poly-d-lysine (PDL) and or Matrigel, to delineate the role of extracellular matrix on providing resemblance to in vivo cellular interactions in nervous tissue. Primary cortical neurons, obtained from embryonic day 15 mice pups, seeded either on PDL- or Matrigel-coated culture ware were investigated by DIC/bright field and fluorescence/confocal microscopy for their morphology, 2D and 3D structure, and distribution patterns. Patch clamp, western blot, and RT-PCR studies were performed to investigate neuronal firing thresholds and sodium channel subtypes Nav1.2 and Nav1.6 expression. Cortical neurons cultured on PDL coating possessed a 2D structure composed of a few numbers of branched and tortuous neurites that contacted with each other in one to one manner, however, neurons on Matrigel coating showed a more complicated dimensional network that depicted tight, linear axonal bundles forming a 3D interacted neuron-astrocyte construction. This difference in growth patterns also showed a significant alteration in neuronal firing threshold which was recorded between 80 < Iinj > 120 pA on PDL and 2 < Iinj > 160 pA on Matrigel. Neurons grown up on Matrigel showed increased levels of sodium channel protein expression of Nav1.2 and Nav1.6 compared to neurons on PDL. These results have demonstrated that a 3D interacted neuron-astrocyte construction on Matrigel enhances the development of Nav1.2 and Nav1.6 in vitro and decreases neuronal firing threshold by 40 times compared to conventional PDL, resembling in vivo neuronal networks and hence would be a better in vitro model of adult neurons.

The effect of age-related risk factors and comorbidities on white matter injury and repair after ischemic stroke

White matter injury is a crucial component of human stroke, but it has often been neglected in preclinical studies. Most human stroke is associated with one or more comorbidities, including aging, hypertension, diabetes and metabolic syndrome including hyperlipidemia. The purpose of this review is to examine how age and hypertension impact stroke-induced white matter injury as well as white matter repair in both human stroke and preclinical models. It is essential that comorbidities be examined in preclinical trials as they may impact translatability to the clinic. In addition, understanding how comorbidities impact white matter injury and repair may provide new therapeutic opportunities for patients with those conditions.

The oligodendroglial precursor cell line Oli-neu represents a cell culture system to examine functional expression of the mouse gap junction gene connexin29 (Cx29)

The potential gap junction forming mouse connexin29 (Cx29) protein is concomitantly expressed with connexin32 (Cx32) in peripheral myelin forming Schwann cells and together with both Cx32 and connexin47 (Cx47) in oligodendrocytes of the CNS. To study the genomic structure and functional expression of Cx29, either primary cells or cell culture systems might be selected, from which the latter are easier to cultivate. Both structure and expression of Cx29 is still not fully understood. In the mouse sciatic nerve, brain and the oligodendroglial precursor cell line Oli-neu the Cx29 gene is processed in two transcript isoforms both harboring a unique reading frame. In contrast to Cx32 and Cx47, only Cx29 protein is abundantly expressed in undifferentiated as well as differentiated Oli-neu cells but the absence of Etbr dye transfer after microinjection concealed the function of Cx29-mediated gap junction communication between those cells. Although HeLa cells stably transfected with Cx29 or Cx29-eGFP neither demonstrated any permeability for Lucifer yellow nor for neurobiotin, blocking of Etbr uptake from the media by gap junction blockers does suppose a role of Cx29 in hemi-channel function. Thus, we conclude that, due to its high abundance of Cx29 expression and its reproducible culture conditions, the oligodendroglial precursor cell line Oli-neu might constitute an appropriate cell culture system to study molecular mechanisms or putative extracellular stimuli to functionally open Cx29 channels or hemi-channels.

Molecular disruptions of the panglial syncytium block potassium siphoning and axonal saltatory conduction: pertinence to neuromyelitis optica and other demyelinating diseases of the central nervous system

The panglial syncytium maintains ionic conditions required for normal neuronal electrical activity in the central nervous system (CNS). Vital among these homeostatic functions is "potassium siphoning," a process originally proposed to explain astrocytic sequestration and long-distance disposal of K(+) released from unmyelinated axons during each action potential. Fundamentally different, more efficient processes are required in myelinated axons, where axonal K(+) efflux occurs exclusively beneath and enclosed within the myelin sheath, precluding direct sequestration of K(+) by nearby astrocytes. Molecular mechanisms for entry of excess K(+) and obligatorily-associated osmotic water from axons into innermost myelin are not well characterized, whereas at the output end, axonally-derived K(+) and associated osmotic water are known to be expelled by Kir4.1 and aquaporin-4 channels concentrated in astrocyte endfeet that surround capillaries and that form the glia limitans. Between myelin (input end) and astrocyte endfeet (output end) is a vast network of astrocyte "intermediaries" that are strongly inter-linked, including with myelin, by abundant gap junctions that disperse excess K(+) and water throughout the panglial syncytium, thereby greatly reducing K(+)-induced osmotic swelling of myelin. Here, I review original reports that established the concept of potassium siphoning in unmyelinated CNS axons, summarize recent revolutions in our understanding of K(+) efflux during axonal saltatory conduction, then describe additional components required by myelinated axons for a newly-described process of voltage-augmented "dynamic" potassium siphoning. If any of several molecular components of the panglial syncytium are compromised, K(+) siphoning is blocked, myelin is destroyed, and axonal saltatory conduction ceases. Thus, a common thread linking several CNS demyelinating diseases is the disruption of potassium siphoning/water transport within the panglial syncytium. Continued progress in molecular identification and subcellular mapping of glial ion and water channels will lead to a better understanding of demyelinating diseases of the CNS and to development of improved treatment regimens.

Axonal targeting of Caspr2 in hippocampal neurons via selective somatodendritic endocytosis

Contactin-associated protein 2 (Caspr2) is a neuronal membrane protein that is mutated in autism and related disorders. Although it is highly enriched at juxtaparanodes of Ranvier where it is essential for Shaker-type K(+) channel clustering, little is known about its function and regulation. In the present study, we examined the polarized expression of Caspr2 in hippocampal neurons using extracellular hemagglutinin (HA)-tagged Caspr2 constructs. We found that Caspr2 was targeted to the axonal surface, but colocalized with early endosomes in the somatodendritic compartment. The inhibition of endocytosis using a Dynamin-1 mutant or treatment with Dynasore prevented Caspr2 internalization from the dendrites and cell body. We identified a short sequence included into the 4.1B-binding domain that is required for the endocytosis of Caspr2. This sequence contains a protein kinase C (PKC) substrate motif on Thr1292, and point mutation of this residue or treatment with a PKC inhibitor prevented the somatodendritic internalization of Caspr2. Thus, the PKC-dependent trafficking of Caspr2 underlies its polarized expression in hippocampal neurons.

Structure-function analysis of protein complexes involved in the molecular architecture of juxtaparanodal regions of myelinated fibers

Demyelinating disorders, including multiple sclerosis (MS), are common causes of neurological disability. One critical step towards the management and therapy of demyelinating diseases is to understand the basic functions of myelinating glia and their relationship with axons. Axons and myelinating glia, oligodendrocytes in the central (CNS) and Schwann cells in the peripheral (PNS) nervous systems, reciprocally influence each other's development and trophism. These interactions are critical for the formation of distinct axonal domains in myelinated fibers that ensure the rapid propagation of action potentials. Macromolecular complexes mediating axo-glial interactions in these domains have been identified, consisting of members of the immunoglobulin superfamily (IgSF) of adhesion molecules and the neurexin/NCP superfamily as well as other proteins. We have investigated the molecular details of axo-glial interactions in the juxtaparanodal region of myelinated fibers by utilizing domain-specific GFP constructs and immunoprecipitation assays on transfected cells. We have shown that the immunoglobulin domains of the IgSF member TAG-1/Cnt-2 are necessary and sufficient for the direct, cis interaction of this protein with Caspr2 and potassium channels.

Action Potentials Induce Uniform Calcium Influx in Mammalian Myelinated Optic Nerves

The myelin sheath enables saltatory conduction by demarcating the axon into a narrow nodal region for excitation and an extended, insulated internodal region for efficient spread of passive current. This anatomical demarcation produces a dramatic heterogeneity in ionic fluxes during excitation, a classical example being the restriction of Na influx at the node. Recent studies have revealed that action potentials also induce calcium influx into myelinated axons of mammalian optic nerves. Does calcium influx in myelinated axons show spatial heterogeneity during nerve excitation? To address this, we analyzed spatial profiles of axonal calcium transients during action potentials by selectively staining axons with calcium indicators and subjected the data to theoretical analysis with parameters for axial calcium diffusion empirically determined using photolysis of caged compounds. The results show surprisingly that during action potentials, calcium influx occurs uniformly along an axon of a fully myelinated mouse optic nerve.

A novel 4.1 ezrin radixin moesin (FERM)-containing protein, ‘Willin’

The 4.1 superfamily of proteins contain a 4.1 Ezrin Radixin Moesin (FERM) domain and are described as linking the cytoskeleton with the plasma membrane. Here, we describe a new FERM domain-containing protein called Willin. Willin has a recognizable FERM domain within its N-terminus and is capable of binding phospholipids. Its intra-cellular distribution can be cytoplasmic or at the plasma membrane where it can co-localize with actin. However, the plasma membrane location of Willin is not influenced by cytochalasin D induced actin disruption but it is induced by the addition of epidermal growth factor.

Emerging complexities in identity and function of glial connexins

Recent research results indicate that glial gap-junction communication is much more complex and widespread than originally thought, and has diverse roles in brain homeostasis and the response of the brain to injury. The situation is far from clear, however. Pharmacological agents that block gap junctions can abolish neuron-glia long-range signaling and can alleviate neuronal damage whereas, intriguingly, opposite effects are observed in mice lacking connexin43, a major gap-junction subunit protein in astrocytes. How can the apparently contradictory results be explained, and how is specificity achieved within the glial gap-junction system? Another key issue in understanding glial connexin function is that oligodendrocytes and astrocytes, each of which express distinct connexin isotypes, are thought to participate in brain homeostasis by forming a panglial syncytium. Molecular analysis has revealed a surprising diversity of connexin expression and function, and this has led to new hypotheses regarding their roles in the brain, which could be tested using new approaches.

Four classes of intercellular channels between glial cells in the CNS

Astrocytes form extensive gap junctions with other astrocytes and with oligodendrocytes. Junctional communication between CNS glia is likely of critical importance because loss of the gap junction channel-forming proteins, connexins Cx32 and Cx47, result in severe demyelination. However, CNS glia express at least six connexins, and the cellular origins and relationships of these proteins have not been determined. We produced a Cx29 reporter mouse in which the connexin coding sequence was replaced with a histological marker, which was used to demonstrate that Cx29, Cx32, and Cx47 are expressed specifically in oligodendrocytes. To determine the relationships between astrocyte and oligodendrocyte connexins, we used double- and triple-immunofluorescence microscopy using semithin sections (<1 microm) of adult mouse spinal cord. Astrocytes form two distinct classes of gap junctions with each other; those composed of Cx26 and those composed of Cx43 and Cx30. In addition, astrocytes establish two classes of intercellular channels with oligodendrocytes, heterotypic Cx26-Cx32 channels and heterotypic Cx30/Cx43-Cx47 channels that may also be heteromeric. In contrast, Cx29 does not colocalize with any of the other five connexins. The data provide the first in vivo demonstration of heterotypic intercellular channels and reveal an unexpected complexity in the composition of glial gap junctions.

Coupling of calcium homeostasis to axonal sodium in axons of mouse optic nerve

Axonal populations in neonatal and mature optic nerves were selectively stained with calcium dyes for analysis of calcium homeostasis and its possible coupling to axonal Na. Repetitive nerve stimulation causes a rise in axonal [Ca(2+)](i) the posttetanus recovery of which is impeded by increasing the number of action potentials in the tetanus. This effect is augmented in 4-aminopyridine (4-AP; 1 mM), which dramatically increases the calcium and presumably sodium load during the tetanus. Increasing axonal [Na](i) with the Na-ionophore monensin (4-50 microM) and ouabain (30 microM) retards posttetanus calcium decline, suggesting that efficient calcium clearance depends on a low level of axonal [Na](i). Posttetanus calcium clearance is not affected by K-mediated depolarization. To further examine coupling between axonal [Na](i) and [Ca(2+)](i), the resting axonal [Ca(2+)](i) was monitored as axonal [Na(+)](i) was elevated with ouabain, veratridine, and monensin. In all cases, elevation of axonal [Na(+)](i) evokes a calcium influx into axons. This influx is unrelated to activation of calcium channels but is consistent with calcium influx via reversal of the Na/Ca exchanger expected as a consequence of axonal [Na(+)](i) elevation. In conclusion, this study demonstrates that calcium homeostasis in the axons of the optic nerve is strongly coupled to axonal [Na(+)](i) in a manner consistent with the Na/Ca exchanger playing a major role in extruding calcium following nerve activity.

L1 endocytosis is controlled by a phosphorylation-dephosphorylation cycle stimulated by outside-in signaling by L1

Dynamic regulation of the cell surface expression of adhesion molecules is an important mechanism for controlling neuronal growth cone motility and guidance. Clathrin-mediated vesicular internalization of L1 via the tyrosine-based endocytosis motif YRSL regulates adhesion and signaling by this Ig superfamily molecule. Here, we present evidence that tyrosine-1176 (Y1176) of the YRSL motif is phosphorylated in vivo. The nonreceptor tyrosine kinase (p60src) is implicated in L1-mediated neurite outgrowth, and we find that p60src phosphorylates Y1176 in vitro. Phosphorylation of Y1176 prevents L1 binding to AP-2, an adaptor required for clathrin-mediated internalization of L1. mAb 74-5H7 recognizes the sequence immediately NH2-terminal to the tyrosine-based motif and binds L1 only when Y1176 is dephosphorylated. 74-5H7 identifies a subset of L1 present at points of cell-cell contact and in vesicle-like structures that colocalize with an endocytosis marker. L1-L1 binding or L1 cross-linking induces a rapid increase in 74-5H7 immunoreactivity. Our data suggest a model in which homophilic binding or L1 cross-linking triggers transient dephosphorylation of the YRSL motif that makes L1 available for endocytosis. Thus, the regulation of L1 endocytosis through dephosphorylation of Y1176 is a critical regulatory point of L1-mediated adhesion and signaling.

Functional binding interaction identified between the axonal CAM L1 and members of the ERM family

A yeast two-hybrid library was screened using the cytoplasmic domain of the axonal cell adhesion molecule L1 to identify binding partners that may be involved in the regulation of L1 function. The intracellular domain of L1 bound to ezrin, a member of the ezrin, radixin, and moesin (ERM) family of membrane-cytoskeleton linking proteins, at a site overlapping that for AP2, a clathrin adaptor. Binding of bacterial fusion proteins confirmed this interaction. To determine whether ERM proteins interact with L1 in vivo, extracellular antibodies to L1 were used to force cluster the protein on cultured hippocampal neurons and PC12 cells, which were then immunolabeled for ERM proteins. Confocal analysis revealed a precise pattern of codistribution between ERMs and L1 clusters in axons and PC12 neurites, whereas ERMs in dendrites and spectrin labeling remained evenly distributed. Transfection of hippocampal neurons grown on an L1 substrate with a dominant negative ERM construct resulted in extensive and abnormal elaboration of membrane protrusions and an increase in axon branching, highlighting the importance of the ERM-actin interaction in axon development. Together, our data indicate that L1 binds directly to members of the ERM family and suggest this association may coordinate aspects of axonal morphogenesis.

The neuronal adhesion protein TAG-1 is expressed by Schwann cells and oligodendrocytes and is localized to the juxtaparanodal region of myelinated fibers

The neural cell adhesion molecule TAG-1, which is a glycosylphosphatidylinositol-linked member of the Ig superfamily, is expressed by various neuronal populations in the developing CNS and PNS. We demonstrate here that Schwann cells and oligodendrocytes also express TAG-1. In the PNS, TAG-1 is detected in ensheathing Schwann cells early postnatally and is maintained throughout adulthood. In mature myelinated fibers of the CNS and PNS, TAG-1 is localized to the juxtaparanodal region. The CNS of the UDP-galactose ceramide galactosyl transferase(-/-) (CGT(-/-)) mouse mutants, which do not synthesize the abundant galactolipids of myelin, display severely disrupted axoglial interactions at the paranodal region. In contrast, axoglial interactions in the PNS of these mutants are less affected. Interestingly, TAG-1 localization is completely undetected in myelinated fibers of the CNS. In the PNS of these mutants, TAG-1 abnormally localizes in the paranodal region. These data raise the intriguing possibility that TAG-1 localization in the juxtaparanodal area mediates, or at least requires, the axoglial contact normally displayed in this region. The abnormal localization of TAG-1 in the CGT mutants might contribute to the disrupted axoglial interactions observed in these animals.

Neurofascin is a glial receptor for the paranodin/Caspr-contactin axonal complex at the axoglial junction

In myelinated fibers of the vertebrate nervous system, glial-ensheathing cells interact with axons at specialized adhesive junctions, the paranodal septate-like junctions. The axonal proteins paranodin/Caspr and contactin form a cis complex in the axolemma at the axoglial adhesion zone, and both are required to stabilize the junction. There has been intense speculation that an oligodendroglial isoform of the cell adhesion molecule neurofascin, NF155, expressed at the paranodal loop might be the glial receptor for the paranodin/Caspr-contactin complex, particularly since paranodin/Caspr and NF155 colocalize to ectopic sites in the CNS of the dysmyelinated mouse Shiverer mutant. We report that the extracellular domain of NF155 binds specifically to transfected cells expressing the paranodin/Caspr-contactin complex at the cell surface. This region of NF155 also binds the paranodin/Caspr-contactin complex from brain lysates in vitro. In support of the functional significance of this interaction, NF155 antibodies and the extracellular domain of NF155 inhibit myelination in myelinating cocultures, presumably by blocking the adhesive relationship between the axon and glial cell. These results demonstrate that the paranodin/Caspr-contactin complex interacts biochemically with NF155 and that this interaction is likely to be biologically relevant at the axoglial junction.