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In myelinated peripheral axons, Kv1 potassium channels are clustered at the juxtaparanodal region and at an internodal line located along the mesaxon and below the Schmidt-Lanterman incisures. This polarized distribution is controlled by Schwann cells and requires specific cell adhesion molecules (CAMs). The accumulation of Kv1 channels at the juxtaparanodal region depends on the presence of Caspr2 at this site, as well as on the presence of Caspr at the adjacent paranodal junction. However, the localization of these channels along the mesaxonal internodal line still persists in the absence of each one of these CAMs. By generating mice lacking both Caspr and Caspr2 (caspr-/-/caspr2-/-), we now reveal compensatory functions of the two proteins in the organization of the axolemma. Although Kv1 channels are clustered along the inner mesaxon and in a circumferential ring below the incisures in the single mutants, in sciatic nerves of caspr-/-/caspr2-/- mice, these channels formed large aggregates that were dispersed along the axolemma, demonstrating that internodal localization of Kv1 channels requires either Caspr or Caspr2. Furthermore, deletion of both Caspr and Caspr2 also resulted in widening of the nodes of Ranvier, suggesting that Caspr2 (which is present at paranodes in the absence of Caspr) can partially compensate for the barrier function of Caspr at this site even without the formation of a distinct paranodal junction. Our results indicate that Caspr and Caspr2 are required for the organization of the axolemma both radially, manifested as the mesaxonal line, and longitudinally, demarcated by the nodal domains. [ABSTRACT FROM AUTHOR]

Postnatal synapse elimination plays a critical role in sculpting and refining neural connectivity throughout the central and peripheral nervous systems, including the removal of supernumerary axonal inputs from neuromuscular junctions (NMJs). Here, we reveal a novel and important role for myelinating glia in regulating synapse elimination at the mouse NMJ, where loss of a single glial cell protein, the glial isoform of neurofascin (Nfasc155), was sufficient to disrupt postnatal remodeling of synaptic circuitry. Neuromuscular synapses were formed normally in mice lacking Nfasc155, including the establishment of robust neuromuscular synaptic transmission. However, loss of Nfasc155 was sufficient to cause a robust delay in postnatal synapse elimination at the NMJ across all muscle groups examined. Nfasc155 regulated neuronal remodeling independently of its canonical role in forming paranodal axo-glial junctions, as synapse elimination occurred normally in mice lacking the axonal paranodal protein Caspr. Rather, high-resolution proteomic screens revealed that loss of Nfasc155 from glial cells was sufficient to disrupt neuronal cytoskeletal organization and trafficking pathways, resulting in reduced levels of neurofilament light (NF-L) protein in distal axons and motor nerve terminals. Mice lacking NF-L recapitulated the delayed synapse elimination phenotype observed in mice lacking Nfasc155, suggesting that glial cells regulate synapse elimination, at least in part, through modulation of the axonal cytoskeleton. Together, our study reveals a glial cell-dependent pathway regulating the sculpting of neuronal connectivity and synaptic circuitry in the peripheral nervous system. [ABSTRACT FROM AUTHOR]

Clustering of Na+ channels at the nodes of Ranvier is coordinated by myelinating glia. In the peripheral nervous system, axoglial contact at the nodes is mediated by the binding of gliomedin and glial NrCAM to axonal neurofascin 186 (NF186). This interaction is crucial for the initial clustering of Na+ channels at heminodes. As a result, it is not clear whether continued axon-glial contact at nodes of Ranvier is required to maintain these channels at the nodal axolemma. Here, we report that, in contrast to mice that lack either gliomedin or NrCAM, absence of both molecules (and hence the glial clustering signal) resulted in a gradual loss of Na+ channels and other axonal components from the nodes, the formation of binary nodes, and dysregulation of nodal gap length. Therefore, these mice exhibit neurological abnormalities and slower nerve conduction. Disintegration of the nodes occurred in an orderly manner, starting with the disappearance of neurofascin 186, followed by the loss of Na+ channels and ankyrin G, and then IV spectrin, a sequence that reflects the assembly of nodes during development. Finally, the absence of gliomedin and NrCAM led to the invasion of the outermost layer of the Schwann cell membrane beyond the nodal area and the formation of paranodal-like junctions at the nodal gap. Our results reveal that axon-glial contact mediated by gliomedin, NrCAM, and NF186 not only plays a role in Na+ channel clustering during development, but also contributes to the long-term maintenance of Na+ channels at nodes of Ranvier. [ABSTRACT FROM AUTHOR]

The interaction between myelinating Schwann cells and the axons they ensheath is mediated by cell adhesion molecules of the Cadm/ Necl/SynCAM family. This family consists of four members: Cadm4/Necl4 and Cadml/Necl2 are found in both glia and axons, whereas Cadm2/Necl3 and Cadm3/Necl 1 are expressed by sensory and motor neurons. By generating mice lacking each of the Cadm genes, we now demonstrate that Cadm4 plays a role in the establishment of the myelin unit in the peripheral nervous system. Mice lacking Cadm4 (PGK-Cre/Cadmfl/fl), but not Cadm 1, Cadm2, or Cadm3, develop focal hypermyelination characterized by tomacula and myelin outfold-ings, which are the hallmark of several Charcot-Marie-Tooth neuropathies. The absence of Cadm4 also resulted in abnormal axon- glial contact and redistribution of ion channels along the axon. These neuropathological features were also found in transgenic mice express-ing a dominant-negative mutant of Cadm4 lacking its cytoplasmic domain in myelinating glia Tg(mbp-Cadm4dCT), as well as in mice lacking Cadm4 specifically in Schwann cells (DHH-Cre/Cadm4fl/fl). Consistent with these abnormalities, both PGK-Cre/Cadm4fl/fl and Tg(mbp-Cadm4dCT) mice exhibit impaired motor function and slower nerve conduction velocity. These findings indicate that Cadm4 regulates the growth of the myelin unit and the organization of the underlying axonal membrane. [ABSTRACT FROM AUTHOR]

Extension of axonal and dendritic processes in the CNS is tightly regulated by outgrowth-promoting and -inhibitory cues to assure precision of synaptic connections. We identify a novel role for contactin-associated protein (Caspr) as an inhibitory cue that reduces neurite outgrowth from CNS neurons. We show that proteolysis of Caspr at the cell surface is regulated by the cellular form of prion protein (PrP), which directly binds to Caspr. PrP inhibits Reelin-mediated shedding of Caspr from the cell surface, thereby increasing surface levels of Caspr and potentiating the inhibitory effect of Caspr on neurite outgrowth. PrP deficiency results in reduced levels of Caspr at the cell surface, enhanced neurite outgrowth in vitro, and more efficient regeneration of axons in vivo following spinal cord injury. Thus, we reveal a previously unrecognized role for Caspr and PrP in inhibitory modulation of neurite outgrowth in CNS neurons, which is counterbalanced by the proteolytic activity of Reelin. [ABSTRACT FROM AUTHOR]

Caspr and Caspr2 regulate the formation of distinct axonal domains around the nodes of Ranvier. Caspr is required for the generation of a membrane barrier at the paranodal junction (PNJ), whereas Caspr2 serves as a membrane scaffold that clusters Kv1 channels at the juxtaparanodal region (JXP). Both Caspr and Caspr2 interact with protein 4.1B, which may link the paranodal and juxtaparanodal adhesion complexes to the axonal cytoskeleton. To determine the role of protein 4.1B in the function of Caspr proteins, we examined the ability of transgenic Caspr and Caspr2 mutants lacking their 4.1-binding sequence (d4.1) to restore Kv1 channel clustering in Caspr- and Caspr2-null mice, respectively. We found that Caspr-d4.1 was localized to the PNJ and is able to recruit the paranodal adhesion complex components contactin and NF155 to this site. Nevertheless, in axons expressing Caspr-d4.1, Kv1 channels were often detected at paranodes, suggesting that the interaction of Caspr with protein 4.1B is necessary for the generation of an efficient membrane barrier at the PNJ. We also found that the Caspr2-d4.1 transgene did not accumulate at the JXP, even though it was targeted to the axon, demonstrating that the interaction with protein 4.1B is required for the accumulation of Caspr2 and Kv1 channels at the juxtaparanodal axonal membrane. In accordance, we show that Caspr2 and Kv1 channels are not clustered at the JXP in 4.1B-null mice. Our results thus underscore the functional importance of protein 4.1B in the organization of peripheral myelinated axons. [ABSTRACT FROM AUTHOR]

Background: Alzheimer's disease (AD) is a devastating neurodegenerative disease. Despite intensive research, all current therapeutic attempts have failed, highlighting the need for new therapeutic approaches. While most research traditionally attributed cognitive impairment to neuronal damage, recent studies have highlighted the role of non‐neuronal cells, mainly the brain innate‐immune cells, the microglia. However, the contribution of non‐immune glial cells remains elusive.Oligodendrocytes are the central nervous system myelin‐forming cells and are crucial for proper neuronal signaling and brain function. However, little is known about their fate in AD. In our study, we aimed to reveal cellular heterogeneity among non‐immune glia, and particularly oligodendrocytes, in AD. Method: To this end, we used the 5xFAD model of AD and performed single cell RNA‐sequencing, bulk RNA‐sequencing, computational meta‐analysis and immunofluorescence stainings in mouse and human AD brain. Result: We found that oligodendrocytes are the most affected cell type and unveiled a novel oligodendrocyte cell‐state, which we termed 'Disease‐associated OLigodendrocytes' (DOLs). DOLs were found in the AD mouse brains but not in the wild‐type (WT), spatially associated with amyloid‐beta plaques and increased with disease progression. DOL signature points at possible interactions with the immune system and immune‐related signaling, plaque propagation, and blood‐brain‐barrier interference. Meta‐analysis across other neuroinflammatory and neurodegenerative conditions revealed a conserved oligodendrocyte response similar to DOLs. Bulk RNA‐sequencing of oligodendrocytes following different stimulations revealed partial similarity between DOL signature and oligodendrocyte response to specific inflammatory cues. Lastly, we showed that DOLs are present also in human AD brains but not in healthy controls. Conclusion: Overall, this study demonstrates that oligodendrocytes have a common response to neuroinflammation and neurodegeneration, which is recapitulated in humans, and can serve as a potential therapeutic target in multiple pathologies. [ABSTRACT FROM AUTHOR]

Clustering of Kv1 channels at the juxtaparanodal region (JXP) in myelinated axons depends on their association with the Caspr2/TAG-1 adhesion complex. The interaction between these channels and Caspr2 was suggested to depend on PDZ (PSD-95/Discs large/zona occludens-1) scaffolding proteins. Here, we show that at a subset of the JXP, PSD-93 colocalizes with Caspr2, K+ channels and its related protein postsynaptic density protein-95 (PSD-95). The localization of PSD-93 and PSD-95 depends on the presence of Caspr2, as both scaffolding proteins failed to accumulate at the JXP in mice lacking either Caspr2 or TAG-1. In contrast, Caspr2 and K+ channels still colocalized and associated in PSD-93, PSD-95 or double PSD-93/PSD-95 null mice. To directly evaluate the role of PDZ domain proteins in the function of Caspr2, we examined the ability of transgenic Caspr2 molecules lacking either their cytoplasmic domain (Caspr2dCT), or their PDZ-binding sequence (Caspr2dPDZ), to restore Kv1 channel clustering in Caspr2 null mice.Wefound that while Kv1 channels were distributed throughout internodes in nerves expressing Caspr2dCT, they were clustered at the JXP in axons expressing a full-length Caspr2 (Caspr2FL) or the Caspr2dPDZ transgene. Further proteomic analysis revealed that Caspr2 interacts with a distinct set of scaffolding proteins through its PDZ- and protein 4.1-binding sequences. These results demonstrate that while the molecular assembly of the JXP requires the cytoplasmic domain of Caspr2, its carboxy-terminal PDZ-binding motif is dispensable for Kv1 channel clustering. This mechanism is clearly distinct from the one operating at the axon initial segment, which requires PSD-93 for Kv1 channel clustering. [ABSTRACT FROM AUTHOR]

Synaptic scaffolding molecule (S-SCAM) is a synaptic protein, which harbors five or six PSD-95/Discs large/ZO-1 (PDZ), a guanylate kinase and two WW domains. It interacts with NMDA receptor subunits, neuroligin and β-catenin, and is involved in the accumulation of neuroligin at excitatory synapses. In this study, we have demonstrated S-SCAM is localized at inhibitory synapses in rat primary cultured hippocampal neurons. We have identified β-dystroglycan (β-DG) as a binding partner for S-SCAM at inhibitory synapses. WW domains of S-SCAM bind to three sequences of β-DG. We have also revealed that S-SCAM can interact with neuroligin 2, which is known to be exclusively localized at inhibitory synapses. The WW domains and the second PDZ domain of S-SCAM are involved in the interaction with neuroligin 2. β-DG, neuroligin 2 and S-SCAM form a tripartite complex in vitro. Neuroligin 2 is detected in the immunoprecipitates by anti-β-DG antibody from rat brain. S-SCAM, β-DG and neuroligin 2 are partially co-localized in rat hippocampal neurons. These data suggest that S-SCAM is associated with β-DG and neuroligin 2 at inhibitory synapses, and functions as a linker between the dystrophin glycoprotein complex and the neurexin–neuroligin complex. [ABSTRACT FROM AUTHOR]

Paranodal junctions of myelinated nerve fibers are important for saltatory conduction and function as paracellular and membrane protein diffusion barriers flanking nodes of Ranvier. The formation of these specialized axoglial contacts depends on the presence of three cell adhesion molecules: neurofascin 155 on the glial membrane and a complex of Caspr and contactin on the axon. We isolated axonal and glial membranes highly enriched in these paranodal proteins and then used mass spectrometry to identify additional proteins associated with the paranodal axoglial junction. This strategy led to the identification of three novel components of the paranodal cytoskeleton: ankyrinB, αII spectrin, and βII spectrin. Biochemical and immunohistochemical analyses revealed that these proteins associate with protein 4.1B in a macromolecular complex that is concentrated at central and peripheral paranodal junctions in the adult and during early myelination. Furthermore, we show that the paranodal localization of ankyrinB is disrupted in Caspr-null mice with aberrant paranodal junctions, demonstrating that paranodal neuron-glia interactions regulate the organization of the underlying cytoskeleton. In contrast, genetic disruption of the juxtaparanodal protein Caspr2 or the nodal cytoskeletal protein βIV spectrin did not alter the paranodal cytoskeleton. Our results demonstrate that the paranodal junction contains specialized cytoskeletal components that may be important to stabilize axon-glia interactions and contribute to the membrane protein diffusion barrier found at paranodes. [ABSTRACT FROM AUTHOR]

Oligodendrocytes form an insulating multilamellar structure of compact myelin around axons, thereby allowing rapid propagation of action potentials. Despite the considerable clinical importance of myelination, little is known about the molecular mechanisms that enable oligodendrocytes to generate their specialized membrane wrapping. Here, we used microarray expression profiling of oligodendrocyte-ablated mutant mice to identify new glial molecules that are involved in CNS myelination. This effort resulted in the identification of Ermin, a novel cytoskeletal molecule that is exclusively expressed by oligodendrocytes. Ermin appears at a late stage during myelination, and in the mature nerves, it is localized to the outer cytoplasmic lip of the myelin sheath and the paranodal loops. In cultured oligodendrocytes, Ermin becomes visible in well differentiated MBP-positive cells, where it is concentrated at the tip of F-actinrich processes (termed "Ermin spikes"). Ectopic expression of Ermin, but not of a mutant protein lacking its actin-binding domain, induced the formation of numerous cell protrusions and a pronounced change in cell morphology. Our results demonstrate that Ermin is a novel marker of myelinating oligodendroglia and suggest that it plays a role in cytoskeletal rearrangements during the late wrapping and/or compaction phases of myelinogenesis. [ABSTRACT FROM AUTHOR]

Describes the mechanisms and roles of axon-Schwann cell interactions. Origins and adult characteristics of Schwann cells; Discussion on axoglial interactions at the node of Ranvier; Role of perisynaptic Schwann cells in neuromuscular junction development and maintenance.

To elucidate the impact of myelinating Schwann cells on the molecular architecture of the node of Ranvier, we investigated the nodal expression of voltage-gated sodium channel (VGSC) isoforms and the localization of paranodal and juxtaparanodal membrane proteins in a severely affected Schwann cell mutant, the mouse deficient in myelin protein zero (P0). The abnormal myelin formation and compaction was associated with immature nodal cluster types of VGSC. Most strikingly, P0-deficient motor nerves displayed an ectopic nodal expression of the Nav1.8 isoform, where it is coexpressed with the ubiquitous Nav1.6 channel. Furthermore, Caspr was distributed asymmetrically or was even absent in the mutant nerve fibers. The potassium channel Kv1.2 and Caspr2 were not confined to juxtaparanodes, but often protruding into the paranodes. Thus, deficiency of P0 leads to dysregulation of nodal VGSC isoforms and to altered localization of paranodal and juxtaparanodal components of the nodal complex. [Copyright &y& Elsevier]

Discusses research on the molecular mechanisms that underlie the generation and maintenance of unique axonal domains which are necessary for normal nerve function. Cells that regulated the local differentiation of myelinated axons; Result of the rapid and efficient conduction of action potentials in vertebrate neurons; Structural feature of myelinated fibres; Function of the nodes of Ranvier in the myelin sheath; Characterization of nodal axolemma.

Examines the junctional protein MAGI-3 with receptor tyrosine phosphatase &beta and tyrosine-phosphorylated proteins. Localization of MAGI-3 with ZO-1 and cingulin at tight junctions; Role for MAGI-3 as a scaffolding molecule; Link of glioblastoma SF763T cells MAGI-3 with a tyrosine-phosphorylated protein.

Provides information on a study that identified a novel cell growth regulator named Falkor using a functional approach to mammalian gene cloning. Results and discussion; Materials and methods used.

We have examined the localization of contactin-associated protein (Caspr), the Shaker-type potassium channels, Kv1.1 and Kv1.2, their associated β subunit, Kvβ2, and Caspr2 in the myelinated fibers of the CNS. Caspr is localized to the paranodal axonal membrane, and Kv1.1, Kv1.2, Kvβ2 and Caspr2 to the juxtaparanodal membrane. In addition to the paranodal staining, an internodal strand of Caspr staining apposes the inner mesaxon of the myelin sheath. Unlike myelinated axons in the peripheral nervous system, there was no internodal strand of Kv1.1, Kv1.2, Kvβ2, or Caspr2. Thus, the organization of the nodal, paranodal, and juxtaparanodal axonal membrane is similar in the central and peripheral nervous systems, but the lack of Kv1.1/Kv1.2/Kvβ2/Caspr2 internodal strands indicates that the oligodendrocyte myelin sheaths lack a trans molecular interaction with axons, an interaction that is present in Schwann cell myelin sheaths. [ABSTRACT FROM AUTHOR]

The extracellular domain of receptor protein tyrosine phosphatase β (RPTPβ) is composed of several domains which mediate its interactions with distinct ligands present on the surface of either neurons or glial cells. Here, we demonstrate that the fibronectin type III domain (FNIII) of RPTPβ binds to glial tumor-derived cell lines and primary astrocytes. We used affinity purification to isolate several proteins that specifically bind to the FNIII domain of RPTPβ. One of these, a 240 kDa protein that was purified from U118MG glioblastoma cell, was identified as tenascin C based on the amino acid sequence of several tryptic peptides. The interaction of RPTPβ with tenascin C was found to mediate cell adhesion. Adhesion and spreading of SF763T astrocytoma cells expressing RPTPβ on tenascin C was specifically abolished by the addition of a soluble fragment containing the FNIII domain of the receptor. RPTPβ-dependent cell adhesion was mediated by binding to the alternatively spliced FNIII repeats A1,2,4 (TnfnA1,2,4) of tenascin C. Furthermore, COS cells expressing RPTPβ adhere to TnfnA1,2,4, while the parental cells did not. These results demonstrate that the FNIII domain of RPTPβ binds to tenascin C and suggest that RPTPβ present on glial tumor cells is a primary adhesion receptor system to the extracellular matrix. Oncogene (2001) 20, 609–618. [ABSTRACT FROM AUTHOR]