Myelinated peripheral axons are more vulnerable to mechanical trauma in a model of enlarged axonal diameters.

The velocity of axonal impulse propagation is facilitated by myelination and axonal diameters. Both parameters are frequently impaired in peripheral nerve disorders, but it is not known if the diameters of myelinated axons affect the liability to injury or the efficiency of functional recovery. Mice lacking the adaxonal myelin protein chemokine-like factor-like MARVEL-transmembrane domain-containing family member-6 (CMTM6) specifically from Schwann cells (SCs) display appropriate myelination but increased diameters of peripheral axons. Here we subjected Cmtm6-cKo mice as a model of enlarged axonal diameters to a mild sciatic nerve compression injury that causes temporarily reduced axonal diameters but otherwise comparatively moderate pathology of the axon/myelin-unit. Notably, both of these pathological features were worsened in Cmtm6-cKo compared to genotype-control mice early post-injury. The increase of axonal diameters caused by CMTM6-deficiency thus does not override their injury-dependent decrease. Accordingly, we did not detect signs of improved regeneration or functional recovery after nerve compression in Cmtm6-cKo mice; depleting CMTM6 in SCs is thus not a promising strategy toward enhanced recovery after nerve injury. Conversely, the exacerbated axonal damage in Cmtm6-cKo nerves early post-injury coincided with both enhanced immune response including foamy macrophages and SCs and transiently reduced grip strength. Our observations support the concept that larger peripheral axons are particularly susceptible toward mechanical trauma.

Axon-derived PACSIN1 binds to the Schwann cell survival receptor, LRP1, and transactivates TrkC to promote gliatrophic activities.

Schwann cells (SCs) undergo phenotypic transformation and then orchestrate nerve repair following PNS injury. The ligands and receptors that activate and sustain SC transformation remain incompletely understood. Proteins released by injured axons represent important candidates for activating the SC Repair Program. The low-density lipoprotein receptor-related protein-1 (LRP1) is acutely up-regulated in SCs in response to injury, activating c-Jun, and promoting SC survival. To identify novel LRP1 ligands released in PNS injury, we applied a discovery-based approach in which extracellular proteins in the injured nerve were captured using Fc-fusion proteins containing the ligand-binding motifs of LRP1 (CCR2 and CCR4). An intracellular neuron-specific protein, Protein Kinase C and Casein Kinase Substrate in Neurons (PACSIN1) was identified and validated as an LRP1 ligand. Recombinant PACSIN1 activated c-Jun and ERK1/2 in cultured SCs. Silencing Lrp1 or inhibiting the LRP1 cell-signaling co-receptor, the NMDA-R, blocked the effects of PACSIN1 on c-Jun and ERK1/2 phosphorylation. Intraneural injection of PACSIN1 into crush-injured sciatic nerves activated c-Jun in wild-type mice, but not in mice in which Lrp1 is conditionally deleted in SCs. Transcriptome profiling of SCs revealed that PACSIN1 mediates gene expression events consistent with transformation to the repair phenotype. PACSIN1 promoted SC migration and viability following the TNFα challenge. When Src family kinases were pharmacologically inhibited or the receptor tyrosine kinase, TrkC, was genetically silenced or pharmacologically inhibited, PACSIN1 failed to induce cell signaling and prevent SC death. Collectively, these studies demonstrate that PACSIN1 is a novel axon-derived LRP1 ligand that activates SC repair signaling by transactivating TrkC.

Focused ion beam-scanning electron microscopy links pathological myelin outfoldings to axonal changes in mice lacking Plp1 or Mag.

Healthy myelin sheaths consist of multiple compacted membrane layers closely encasing the underlying axon. The ultrastructure of CNS myelin requires specialized structural myelin proteins, including the transmembrane-tetraspan proteolipid protein (PLP) and the Ig-CAM myelin-associated glycoprotein (MAG). To better understand their functional relevance, we asked to what extent the axon/myelin-units display similar morphological changes if PLP or MAG are lacking. We thus used focused ion beam-scanning electron microscopy (FIB-SEM) to re-investigate axon/myelin-units side-by-side in Plp- and Mag-null mutant mice. By three-dimensional reconstruction and morphometric analyses, pathological myelin outfoldings extend up to 10 µm longitudinally along myelinated axons in both models. More than half of all assessed outfoldings emerge from internodal myelin. Unexpectedly, three-dimensional reconstructions demonstrated that both models displayed complex axonal pathology underneath the myelin outfoldings, including axonal sprouting. Axonal anastomosing was additionally observed in Plp-null mutant mice. Importantly, normal-appearing axon/myelin-units displayed significantly increased axonal diameters in both models according to quantitative assessment of electron micrographs. These results imply that healthy CNS myelin sheaths facilitate normal axonal diameters and shape, a function that is impaired when structural myelin proteins PLP or MAG are lacking.

A role for axon-glial interactions and Netrin-G1 signaling in the formation of low-threshold mechanoreceptor end organs.

Low-threshold mechanoreceptors (LTMRs) and their cutaneous end organs convert light mechanical forces acting on the skin into electrical signals that propagate to the central nervous system. In mouse hairy skin, hair follicle-associated longitudinal lanceolate complexes, which are end organs comprising LTMR axonal endings that intimately associate with terminal Schwann cell (TSC) processes, mediate LTMR responses to hair deflection and skin indentation. Here, we characterized developmental steps leading to the formation of Aß rapidly adapting (RA)-LTMR and Aδ-LTMR lanceolate complexes. During early postnatal development, Aß RA-LTMRs and Aδ-LTMRs extend and prune cutaneous axonal branches in close association with nascent TSC processes. Netrin-G1 is expressed in these developing Aß RA-LTMR and Aδ-LTMR lanceolate endings, and Ntng1 ablation experiments indicate that Netrin-G1 functions in sensory neurons to promote lanceolate ending elaboration around hair follicles. The Netrin-G ligand (NGL-1), encoded by Lrrc4c, is expressed in TSCs, and ablation of Lrrc4c partially phenocopied the lanceolate complex deficits observed in Ntng1 mutants. Moreover, NGL-1-Netrin-G1 signaling is a general mediator of LTMR end organ formation across diverse tissue types demonstrated by the fact that Aß RA-LTMR endings associated with Meissner corpuscles and Pacinian corpuscles are also compromised in the Ntng1 and Lrrc4c mutant mice. Thus, axon-glia interactions, mediated in part by NGL-1-Netrin-G1 signaling, promote LTMR end organ formation.

Conservation and divergence of myelin proteome and oligodendrocyte transcriptome profiles between humans and mice.

Human myelin disorders are commonly studied in mouse models. Since both clades evolutionarily diverged approximately 85 million years ago, it is critical to know to what extent the myelin protein composition has remained similar. Here, we use quantitative proteomics to analyze myelin purified from human white matter and find that the relative abundance of the structural myelin proteins PLP, MBP, CNP, and SEPTIN8 correlates well with that in C57Bl/6N mice. Conversely, multiple other proteins were identified exclusively or predominantly in human or mouse myelin. This is exemplified by peripheral myelin protein 2 (PMP2), which was specific to human central nervous system myelin, while tetraspanin-2 (TSPAN2) and connexin-29 (CX29/GJC3) were confined to mouse myelin. Assessing published scRNA-seq-datasets, human and mouse oligodendrocytes display well-correlating transcriptome profiles but divergent expression of distinct genes, including Pmp2, Tspan2, and Gjc3. A searchable web interface is accessible via www.mpinat.mpg.de/myelin. Species-dependent diversity of oligodendroglial mRNA expression and myelin protein composition can be informative when translating from mouse models to humans.

Like the electrical wires in our homes, the processes of nerve cells ­ the axons, thin extensions that project from the cell bodies ­ need to be insulated to work effectively. This insulation takes the form of layers of a membrane called myelin, which is made of proteins and fats and produced by specialized cells called oligodendrocytes in the brain and the spinal cord. If this layer of insulation becomes damaged, the electrical impulses travelling along the nerves slow down, affecting the ability to walk, speak, see or think. This is the cause of several illnesses, including multiple sclerosis and a group of rare genetic diseases known as leukodystrophies. A lot of the research into myelin, oligodendrocytes and the diseases caused by myelin damage uses mice as an experimental model for humans. Using mice for this type of research is appropriate because of the ethical and technical limitations of experiments on humans. This approach can be highly effective because mice and humans share a large proportion of their genes. However, there are many obvious physical differences between the two species, making it important to determine whether the results of experiments performed in mice are applicable to humans. To do this, it is necessary to understand how myelin differs between these two species at the molecular level. Gargareta, Reuschenbach, Siems, Sun et al. approached this problem by studying the proteins found in myelin isolated from the brains of people who had passed away and donated their organs for scientific research. They used a technique called mass spectrometry, which identifies molecules based on their weight, to produce a list of proteins in human myelin that could then be compared to existing data from mouse myelin. This analysis showed that myelin is very similar in both species, but some proteins only appear in humans or in mice. Gargareta, Reuschenbach, Siems, Sun et al. then compared which genes are turned on in the oligodendrocytes making the myelin. The results of this comparison reflected most of the differences and similarities seen in the myelin proteins. Despite the similarities identified by Gargareta, Reuschenbach, Siems, Sun et al., it became evident that there are unexpected differences between the myelin of humans and mice that will need to be considered when applying results from mice research to humans. To enable this endeavor, Gargareta, Reuschenbach, Siems, Sun et al. have created a searchable web interface of the proteins in myelin and the genes expressed in oligodendrocytes in the two species.

Progressive axonopathy when oligodendrocytes lack the myelin protein CMTM5.

Oligodendrocytes facilitate rapid impulse propagation along the axons they myelinate and support their long-term integrity. However, the functional relevance of many myelin proteins has remained unknown. Here, we find that expression of the tetraspan-transmembrane protein CMTM5 (chemokine-like factor-like MARVEL-transmembrane domain containing protein 5) is highly enriched in oligodendrocytes and central nervous system (CNS) myelin. Genetic disruption of the Cmtm5 gene in oligodendrocytes of mice does not impair the development or ultrastructure of CNS myelin. However, oligodendroglial Cmtm5 deficiency causes an early-onset progressive axonopathy, which we also observe in global and tamoxifen-induced oligodendroglial Cmtm5 mutants. Presence of the WldS mutation ameliorates the axonopathy, implying a Wallerian degeneration-like pathomechanism. These results indicate that CMTM5 is involved in the function of oligodendrocytes to maintain axonal integrity rather than myelin biogenesis.

Differential Contribution of Cadm1-Cadm3 Cell Adhesion Molecules to Peripheral Myelinated Axons.

Cell adhesion proteins of the Cadm (SynCAM/Necl) family regulate myelination and the organization of myelinated axons. In the peripheral nervous system (PNS), intercellular contact between Schwann cells and their underlying axons is believed to be mediated by binding of glial Cadm4 to axonal Cadm3 or Cadm2. Nevertheless, given that distinct neurons express different combinations of the Cadm proteins, the identity of the functional axonal ligand for Cadm4 remains to be determined. Here, we took a genetic approach to compare the phenotype of Cadm4 null mice, which exhibit abnormal distribution of Caspr and Kv1 potassium channels, with mice lacking different combinations of Cadm1-Cadm3 genes. We show that in contrast to mice lacking the single Cadm1, Cadm2, or Cadm3 genes, genetic ablation of all three phenocopies the abnormalities detected in the absence of Cadm4. Similar defects were observed in double mutant mice lacking Cadm3 and Cadm2 (i.e., Cadm3-/-/Cadm2-/-) or Cadm3 and Cadm1 (i.e., Cadm3-/-/Cadm1-/-), but not in mice lacking Cadm1 and Cadm2 (i.e., Cadm1-/-/Cadm2-/-). Furthermore, axonal organization abnormalities were also detected in Cadm3 null mice that were heterozygous for the two other axonal Cadms. Our results identify Cadm3 as the main axonal ligand for glial Cadm4, and reveal that its absence could be compensated by the combined action of Cadm2 and Cadm1.SIGNIFICANCE STATEMENT Myelination by Schwann cells enables fast conduction of action potentials along motor and sensory axons. In these nerves, Schwann cell-axon contact is mediated by cell adhesion molecules of the Cadm family. Cadm4 in Schwann cells regulates axonal ensheathment and myelin wrapping, as well as the organization of the axonal membrane, but the identity of its axonal ligands is not clear. Here, we reveal that Cadm mediated axon-glia interactions depend on a hierarchical adhesion code that involves multiple family members. Our results provide important insights into the molecular mechanisms of axon-glia communication, and the function of Cadm proteins in PNS myelin.

Schwann cell development: From neural crest to myelin sheath.

Vertebrate nervous system function requires glial cells, including myelinating glia that insulate axons and provide trophic support that allows for efficient signal propagation by neurons. In vertebrate peripheral nervous systems, neural crest-derived glial cells known as Schwann cells (SCs) generate myelin by encompassing and iteratively wrapping membrane around single axon segments. SC gliogenesis and neurogenesis are intimately linked and governed by a complex molecular environment that shapes their developmental trajectory. Changes in this external milieu drive developing SCs through a series of distinct morphological and transcriptional stages from the neural crest to a variety of glial derivatives, including the myelinating sublineage. Cues originate from the extracellular matrix, adjacent axons, and the developing SC basal lamina to trigger intracellular signaling cascades and gene expression changes that specify stages and transitions in SC development. Here, we integrate the findings from in vitro neuron-glia co-culture experiments with in vivo studies investigating SC development, particularly in zebrafish and mouse, to highlight critical factors that specify SC fate. Ultimately, we connect classic biochemical and mutant studies with modern genetic and visualization tools that have elucidated the dynamics of SC development. This article is categorized under: Signaling Pathways > Cell Fate Signaling Nervous System Development > Vertebrates: Regional Development.

Precise Spatiotemporal Control of Nodal Na+ Channel Clustering by Bone Morphogenetic Protein-1/Tolloid-like Proteinases.

During development of the peripheral nervous system (PNS), Schwann-cell-secreted gliomedin induces the clustering of Na+ channels at the edges of each myelin segment to form nodes of Ranvier. Here we show that bone morphogenetic protein-1 (BMP1)/Tolloid (TLD)-like proteinases confine Na+ channel clustering to these sites by negatively regulating the activity of gliomedin. Eliminating the Bmp1/TLD cleavage site in gliomedin or treating myelinating cultures with a Bmp1/TLD inhibitor results in the formation of numerous ectopic Na+ channel clusters along axons that are devoid of myelin segments. Furthermore, genetic deletion of Bmp1 and Tll1 genes in mice using a Schwann-cell-specific Cre causes ectopic clustering of nodal proteins, premature formation of heminodes around early ensheathing Schwann cells, and altered nerve conduction during development. Our results demonstrate that by inactivating gliomedin, Bmp1/TLD functions as an additional regulatory mechanism to ensure the correct spatial and temporal assembly of PNS nodes of Ranvier.

Glial Derived TGF-ß Instructs Axon Midline Stopping.

A fundamental question that underlies the proper wiring and function of the nervous system is how axon extension stops during development. However, our mechanistic understanding of axon stopping is currently poor. The stereotypic development of the Drosophila mushroom body (MB) provides a unique system in which three types of anatomically distinct neurons (γ, α'/ß', and α/ß) develop and interact to form a complex neuronal structure. All three neuronal types innervate the ipsi-lateral side and do not cross the midline. Here we find that Plum, an immunoglobulin (Ig) superfamily protein that we have previously shown to function as a TGF-ß accessory receptor, is required within MB α/ß neurons for their midline stopping. Overexpression of Plum within MB neurons is sufficient to induce retraction of α/ß axons. As expected, rescue experiments revealed that Plum likely functions in α/ß neurons and mediates midline stopping via the downstream effector RhoGEF2. Finally, we have identified glial-derived Myoglianin (Myo) as the major TGF-ß ligand that instructs midline stopping of MB neurons. Taken together, our study strongly suggests that TGF-ß signals originating from the midline facilitate midline stopping of α/ß neuron in a Plum dependent manner.

High-affinity heterotetramer formation between the large myelin-associated glycoprotein and the dynein light chain DYNLL1.

The close association of myelinated axons and their myelin sheaths involves numerous intercellular molecular interactions. For example, myelin-associated glycoprotein (MAG) mediates myelin-to-axon adhesion and signalling via molecules on the axonal surface. However, knowledge about intracellular binding partners of myelin proteins, including MAG, has remained limited. The two splice isoforms of MAG, S- and L-MAG, display distinct cytoplasmic domains and spatiotemporal expression profiles. We used yeast two-hybrid screening to identify interaction partners of L-MAG and found the dynein light chain DYNLL1 (also termed dynein light chain 8). DYNLL1 homodimers are known to facilitate dimerization of target proteins. L-MAG and DYNLL1 associate with high affinity, as confirmed with recombinant proteins in vitro. Structural analyses of the purified complex indicate that the DYNLL1-binding segment is localized close to the L-MAG C terminus, next to the Fyn kinase Tyr phosphorylation site. The crystal structure of the complex between DYNLL1 and its binding segment on L-MAG shows 2 : 2 binding in a parallel arrangement, indicating a heterotetrameric complex. The homology between L-MAG and previously characterized DYNLL1-ligands is limited, and some details of binding site interactions are unique for L-MAG. The structure of the complex between the entire L-MAG cytoplasmic domain and DYNLL1, as well as that of the extracellular domain of MAG, were modelled based on small-angle X-ray scattering data, allowing structural insights into L-MAG interactions on both membrane surfaces. Our data imply that DYNLL1 dimerizes L-MAG, but not S-MAG, through the formation of a specific 2 : 2 heterotetramer. This arrangement is likely to affect, in an isoform-specific manner, the functions of MAG in adhesion and myelin-to-axon signalling. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/. Read the Editorial Highlight for this article on page 712.

Axonal Ensheathment in the Nervous System of Lamprey: Implications for the Evolution of Myelinating Glia.

In the nervous system, myelination of axons enables rapid impulse conduction and is a specialized function of glial cells. Myelinating glia are the last cell type to emerge in the evolution of vertebrate nervous systems, presumably in ancient jawed vertebrates (gnathostomata) because jawless vertebrates (agnathans) lack myelin. We have hypothesized that, in these unmyelinated species, evolutionary progenitors of myelinating cells must have existed that should still be present in contemporary agnathan species. Here, we used advanced electron microscopic techniques to reveal axon-glia interactions in the sea lamprey Petromyzon marinus By quantitative assessment of the spinal cord and the peripheral lateral line nerve, we observed a marked maturation-dependent growth of axonal calibers. In peripheral nerves, all axons are ensheathed by glial cells either in bundles or, when larger than the threshold caliber of 3 µm, individually. The ensheathing glia are covered by a basal lamina and express SoxE-transcription factors, features of mammalian Remak-type Schwann cells. In larval lamprey, the ensheathment of peripheral axons leaves gaps that are closed in adults. CNS axons are also covered to a considerable extent by glial processes, which contain a high density of intermediate filaments, glycogen particles, large lipid droplets, and desmosomes, similar to mammalian astrocytes. Indeed, by in situ hybridization, these glial cells express the astrocyte marker Aldh1l1 Specimens were of unknown sex. Our observations imply that radial sorting, ensheathment, and presumably also metabolic support of axons are ancient functions of glial cells that predate the evolutionary emergence of myelin in jawed vertebrates.SIGNIFICANCE STATEMENT We used current electron microscopy techniques to examine axon-glia units in a nonmyelinated vertebrate species, the sea lamprey. In the PNS, lamprey axons are fully ensheathed either individually or in bundles by cells ortholog to Schwann cells. In the CNS, axons associate with astrocyte orthologs, which contain glycogen and lipid droplets. We suggest that ensheathment, radial sorting, and metabolic support of axons by glial cells predate the evolutionary emergence of myelin in ancient jawed vertebrates.

A novel histochemical method of simultaneous detection by a single- or double-immunofluorescence and Bielschowsky's silver staining in teased rat sciatic nerves.

BACKGROUND: The Golgi silver method has been widely used in neuroscience for the study of normal and pathological morphology of neurons. The method has been steadily improved and Bielschowsky's silver staining method (BSSM) is widely used in various pathological conditions, like Alzheimer's disease. NEW METHOD: In this work, teased sciatic nerves were silver impregnated using BSSM. We also developed simultaneous staining by silver impregnation and single- or double-immunofluorescence of the same section in teased nerve preparations. We immunostained against non-myelinating Schwann cells and different myelinating Schwann cell domains. RESULTS: BSSM teased nerves show a strong staining of axons (black) and a gold-brown staining of myelinating and non-myelinating Schwann cells. We were also able to stain by immunofluorescence these BSSM teased nerves with specific molecular markers against non-myelinating Schwann cells, also against non-compact myelin such as the Schmidt-Lanterman incisures or paranodal regions and compact myelin, but not axons. COMPARISON WITH EXISTING METHODS AND CONCLUSIONS: In peripheral nerves, several silver impregnation methods have been used to stain nerves in paraffin sections, but not in teased nerves to enable the assessment of isolated nerve fibers. In conclusion, BSSM gives accurate information of nerve morphology and combining the procedure with immunofluorescence it would be very useful to study the molecular nerve domain organization of the nerve fibers, and to study the molecular pathology of axon degeneration, or myelin disorders, or of any peripheral neuropathy, also to study demyelination diseases in the central nervous system.

Ultrastructural anatomy of nodes of Ranvier in the peripheral nervous system as revealed by STED microscopy.

We used stimulated emission depletion (STED) superresolution microscopy to analyze the nanoscale organization of 12 glial and axonal proteins at the nodes of Ranvier of teased sciatic nerve fibers. Cytoskeletal proteins of the axon (betaIV spectrin, ankyrin G) exhibit a high degree of one-dimensional longitudinal order at nodal gaps. In contrast, axonal and glial nodal adhesion molecules [neurofascin-186, neuron glial-related cell adhesion molecule (NrCAM)] can arrange in a more complex, 2D hexagonal-like lattice but still feature a ∼190-nm periodicity. Such a lattice-like organization is also found for glial actin. Sodium and potassium channels exhibit a one-dimensional periodicity, with the Nav channels appearing to have a lower degree of organization. At paranodes, both axonal proteins (betaII spectrin, Caspr) and glial proteins (neurofascin-155, ankyrin B) form periodic quasi-one-dimensional arrangements, with a high degree of interdependence between the position of the axonal and the glial proteins. The results indicate the presence of mechanisms that finely align the cytoskeleton of the axon with the one of the Schwann cells, both at paranodal junctions (with myelin loops) and at nodal gaps (with microvilli). Taken together, our observations reveal the importance of the lateral organization of proteins at the nodes of Ranvier and pave the way for deeper investigations of the molecular ultrastructural mechanisms involved in action potential propagation, the formation of the nodes, axon-glia interactions, and demyelination diseases.

The ins and outs of polarized axonal domains.

Myelinated axons are divided into polarized subdomains including axon initial segments and nodes of Ranvier. These domains initiate and propagate action potentials and regulate the trafficking and localization of somatodendritic and axonal proteins. Formation of axon initial segments and nodes of Ranvier depends on intrinsic (neuronal) and extrinsic (glial) interactions. Several levels of redundancy in both mechanisms and molecules also exist to ensure efficient node formation. Furthermore, the establishment of polarized domains at and near nodes of Ranvier reflects the intrinsic polarity of the myelinating glia responsible for node assembly. Here, we discuss the various polarized domains of myelinated axons, how they are established by both intrinsic and extrinsic interactions, and the polarity of myelinating glia.

Localization of aquaporin 1 water channel in the Schmidt-Lanterman incisures and the paranodal regions of the rat sciatic nerve.

Aquaporin 1 (AQP1) is a member of a family of small, integral membrane water-transporting proteins, which facilitate water movement across cell membranes in response to osmotic gradients. Several papers have studied the expression and function of the AQPs in the central nervous system. However, little is known about the AQPs in the peripheral nervous system (PNS). In the PNS, AQP1, AQP2 and AQP4 have been reported in both peripheral neurons and glial cells. In this work we studied the expression and localization of AQP1 in the rat sciatic nerve. We found that from the four AQPs we studied (AQP1, AQP2, AQP4 and AQP9) only AQP1 is expressed in the nerve by reverse transcription polymerase chain reaction (RT-PCR). AQP1 is also observed at the protein level by Western blot analysis. We also studied the localization of AQP1 in the sciatic nerve by immunohistochemistry. The results show that AQP1 is present in both myelinating and non-myelinating Schwann cells. In myelin internodes AQP1 is enriched in the Schmidt-Lanterman incisures and in some internodes it is also present in the abaxonal membrane. At the nodes of Ranvier, AQP1 co-localizes with actin in the paranodal regions of the nerve. Therefore, AQP1 might play an important role in myelin homeostasis maintaining the thermodynamic equilibrium across the plasma membrane in myelinated axons during electrical activity. Also the expression of AQP1 in non-myelinating Schwann cells supports the involvement of AQP1 in pain perception.

Caspr and caspr2 are required for both radial and longitudinal organization of myelinated axons.

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.

Neuronal activity in the hub of extrasynaptic Schwann cell-axon interactions.

The integrity and function of neurons depend on their continuous interactions with glial cells. In the peripheral nervous system glial functions are exerted by Schwann cells (SCs). SCs sense synaptic and extrasynaptic manifestations of action potential propagation and adapt their physiology to support neuronal activity. We review here existing literature data on extrasynaptic bidirectional axon-SC communication, focusing particularly on neuronal activity implications. To shed light on underlying mechanisms, we conduct a thorough analysis of microarray data from SC-rich mouse sciatic nerve at different developmental stages and in neuropathic models. We identify molecules that are potentially involved in SC detection of neuronal activity signals inducing subsequent glial responses. We further suggest that alterations in the activity-dependent axon-SC crosstalk impact on peripheral neuropathies. Together with previously reported data, these observations open new perspectives for deciphering glial mechanisms of neuronal function support.

Emerging roles of exosomes in neuron-glia communication.

Brain function depends on coordinated interactions between neurons and glial cells. Recent evidence indicates that these cells release endosome-derived microvesicles termed exosomes, which are 50-100 nm in size and carry specific protein and RNA cargo. Exosomes can interact with neighboring cells raising the concept that exosomes may mediate signaling between brain cells and facilitate the delivery of bioactive molecules. Oligodendrocytes myelinate axons and furthermore maintain axonal integrity by an yet uncharacterized pathway of trophic support. Here, we highlight the role of exosomes in nervous system cell communication with particular focus on exosomes released by oligodendrocytes and their potential implications in axon-glia interaction and myelin disease, such as multiple sclerosis. These secreted vesicles may contribute to eliminate overproduced myelin membrane or to transfer antigens facilitating immune surveillance of the brain. Furthermore, there is emerging evidence that exosomes participate in axon-glia communication.