Structural requirement of TAG-1 for retinal ganglion cell axons and myelin in the mouse optic nerve

Self-formation of concentric zones of telencephalic and ocular tissues and directional retinal ganglion cell axons

The telencephalon and eye in mammals are originated from adjacent fields at the anterior neural plate. Morphogenesis of these fields generates telencephalon, optic-stalk, optic-disc, and neuroretina along a spatial axis. How these telencephalic and ocular tissues are specified coordinately to ensure directional retinal ganglion cell (RGC) axon growth is unclear. Here, we report self-formation of human telencephalon-eye organoids comprising concentric zones of telencephalic, optic-stalk, optic-disc, and neuroretinal tissues along the center-periphery axis. Initially-differentiated RGCs grew axons towards and then along a path defined by adjacent PAX2+ VSX2+ optic-disc cells. Single-cell RNA sequencing of these organoids not only confirmed telencephalic and ocular identities but also identified expression signatures of early optic-disc, optic-stalk, and RGCs. These signatures were similar to those in human fetal retinas. Optic-disc cells in these organoids differentially expressed FGF8 and FGF9; FGFR inhibitions drastically decreased early RGC differentiation and directional axon growth. Through the RGC-specific cell-surface marker CNTN2 identified here, electrophysiologically excitable RGCs were isolated under a native condition. Our findings provide insight into the coordinated specification of early telencephalic and ocular tissues in humans and establish resources for studying RGC-related diseases such as glaucoma.

Self-formation of concentric zones of telencephalic and ocular tissues and directional retinal ganglion cell axons

The telencephalon and eye in mammals are originated from adjacent fields at the anterior neural plate. Morphogenesis of these fields generates telencephalon, optic-stalk, optic-disc, and neuroretina along a spatial axis. How these telencephalic and ocular tissues are specified coordinately to ensure directional retinal ganglion cell (RGC) axon growth is unclear. Here, we report the self-formation of human telencephalon-eye organoids comprising concentric zones of telencephalic, optic-stalk, optic-disc, and neuroretinal tissues along the center-periphery axis. Initially-differentiated RGCs grew axons towards and then along a path defined by adjacent PAX2+ optic-disc cells. Single-cell RNA sequencing of CONCEPT organoids not only confirmed telencephalic and ocular identities but also identified expression signatures of early optic-disc, optic-stalk, and RGCs. These signatures were similar to those in human fetal retinas. Optic-disc cells in CONCEPT organoids differentially expressed FGF8 and FGF9 ; FGFR inhibitions drastically decreased RGC differentiation and directional axon growth. Through the identified RGC-specific cell-surface marker CNTN2, electrophysiologically-excitable RGCs were isolated under a native condition. Our findings provide insight into the coordinated specification of early telencephalic and ocular tissues in humans and establish resources for studying RGC-related diseases such as glaucoma.

Impact statement

A human telencephalon-eye organoid model that exhibited axon growth and pathfinding from retinal ganglion cell (RGC) axons is reported; via cell surface marker CNTN2 identified using scRNA-seq, early RGCs were isolated under a native condition.

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.

Signaling - transcription interactions in mouse retinal ganglion cells early axon pathfinding -a literature review

Sending an axon out of the eye and into the target brain nuclei is the defining feature of retinal ganglion cells (RGCs). The literature on RGC axon pathfinding is vast, but it focuses mostly on decision making events such as midline crossing at the optic chiasm or retinotopic mapping at the target nuclei. In comparison, the exit of RGC axons out of the eye is much less explored. The first checkpoint on the RGC axons' path is the optic cup - optic stalk junction (OC-OS). OC-OS development and the exit of the RGC pioneer axons out of the eye are coordinated spatially and temporally. By the time the optic nerve head domain is specified, the optic fissure margins are in contact and the fusion process is ongoing, the first RGCs are born in its proximity and send pioneer axons in the optic stalk. RGC differentiation continues in centrifugal waves. Later born RGC axons fasciculate with the more mature axons. Growth cones at the end of the axons respond to guidance cues to adopt a centripetal direction, maintain nerve fiber layer restriction and to leave the optic cup. Although there is extensive information on OC-OS development, we still have important unanswered questions regarding its contribution to the exit of the RGC axons out of the eye. We are still to distinguish the morphogens of the OC-OS from the axon guidance molecules which are expressed in the same place at the same time. The early RGC transcription programs responsible for axon emergence and pathfinding are also unknown. This review summarizes the molecular mechanisms for early RGC axon guidance by contextualizing mouse knock-out studies on OC-OS development with the recent transcriptomic studies on developing RGCs in an attempt to contribute to the understanding of human optic nerve developmental anomalies. The published data summarized here suggests that the developing optic nerve head provides a physical channel (the closing optic fissure) as well as molecular guidance cues for the pioneer RGC axons to exit the eye.

Neuronal, but not glial, Contactin 2 negatively regulates axon regeneration in the injured adult optic nerve

Mammalian adult neurons of the central nervous system (CNS) display limited ability to regrow axons after trauma. The developmental decline in their regenerative ability has been attributed to both intrinsic and extrinsic factors, including postnatal suppression of transcription factors and non-neuronal inhibitory components, respectively. The cell adhesion molecule Contactin 2 (CNTN2) is expressed in neurons and oligodendrocytes in the CNS. Neuronal CNTN2 is highly regulated during development and plays critical roles in axon growth and guidance and neuronal migration. On the other hand, CNTN2 expressed by oligodendrocytes interferes with the myelination process, with its ablation resulting in hypomyelination. In the current study, we investigate the role of CNTN2 in neuronal survival and axon regeneration after trauma, in the murine optic nerve crush (ONC) model. We unveil distinct roles for neuronal and glial CNTN2 in regenerative responses. Surprisingly, our data show a conflicting role of neuronal and glial CNTN2 in axon regeneration. Although glial CNTN2 as well as hypomyelination are dispensable for both neuronal survival and axon regeneration following ONC, the neuronal counterpart comprises a negative regulator of regeneration. Specifically, we reveal a novel mechanism of action for neuronal CNTN2, implicating the inhibition of Akt signalling pathway. The in vitro analysis indicates a BDNF-independent mode of action and biochemical data suggest the implication of the truncated form of TrkB neurotrophin receptor. In conclusion, CNTN2 expressed in CNS neurons serves as an inhibitor of axon regeneration after trauma and its mechanism of action involves the neutralization of Akt-mediated neuroprotective effects.

Using the Allen gene expression atlas of the adult mouse brain to gain further insight into the physiological significance of TAG-1/Contactin-2

The anatomic gene expression atlas (AGEA) of the adult mouse brain of the Allen Institute for Brain Science is a transcriptome-based atlas of the adult C57Bl/6 J mouse brain, based on the extensive in situ hybridization dataset of the Institute. This spatial mapping of the gene expression levels of mice under baseline conditions could assist in the formation of new, reasonable transcriptome-derived hypotheses on brain structure and underlying biochemistry, which could also have functional implications. The aim of this work is to use the data of the AGEA (in combination with Tabula Muris, a compendium of single cell transcriptome data collected from mice, enabling direct and controlled comparison of gene expression among cell types) to provide further insights into the physiology of TAG-1/Contactin-2 and its interactions, by presenting the expression of the corresponding gene across the adult mouse brain under baseline conditions and to investigate any spatial genomic correlations between TAG-1/Contactin-2 and its interacting proteins and markers of mature and immature oligodendrocytes, based on the pre-existing experimental or bibliographical evidence. The across-brain correlation analysis on the gene expression intensities showed a positive spatial correlation of TAG-1/Contactin-2 with the gene expression of Plp1, Myrf, Mbp, Mog, Cldn11, Bace1, Kcna1, Kcna2, App and Nfasc and a negative spatial correlation with the gene expression of Cspg4, Pdgfra, L1cam, Ncam1, Ncam2 and Ptprz1. Spatially correlated genes are mainly expressed by mature oligodendrocytes (like Cntn2), while spatially anticorrelated genes are mainly expressed by oligodendrocyte precursor cells. According to the data presented in this work, we propose that even though Contactin-2 expression during development correlates with high plasticity events, such as neuritogenesis, in adulthood it correlates with pathways characterized by low plasticity.

Revisiting the role of Dcc in visual system development with a novel eye clearing method

The Deleted in Colorectal Carcinoma (Dcc) receptor plays a critical role in optic nerve development. Whilst Dcc is expressed postnatally in the eye, its function remains unknown as Dcc knockouts die at birth. To circumvent this drawback, we generated an eye-specific Dcc mutant. To study the organization of the retina and visual projections in these mice, we also established EyeDISCO, a novel tissue clearing protocol that removes melanin allowing 3D imaging of whole eyes and visual pathways. We show that in the absence of Dcc, some ganglion cell axons stalled at the optic disc, whereas others perforated the retina, separating photoreceptors from the retinal pigment epithelium. A subset of visual axons entered the CNS, but these projections are perturbed. Moreover, Dcc-deficient retinas displayed a massive postnatal loss of retinal ganglion cells and a large fraction of photoreceptors. Thus, Dcc is essential for the development and maintenance of the retina.

Diffusion time dependency along the human corpus callosum and exploration of age and sex differences as assessed by oscillating gradient spin-echo diffusion tensor imaging

Conventional diffusion imaging uses pulsed gradient spin echo (PGSE) waveforms with diffusion times of tens of milliseconds (ms) to infer differences of white matter microstructure. The combined use of these long diffusion times with short diffusion times (<10 ​ms) enabled by oscillating gradient spin echo (OGSE) waveforms can enable more sensitivity to changes of restrictive boundaries on the scale of white matter microstructure (e.g. membranes reflecting the axon diameters). Here, PGSE and OGSE images were acquired at 4.7 ​T from 20 healthy volunteers aged 20-73 years (10 males). Mean, radial, and axial diffusivity, as well as fractional anisotropy were calculated in the genu, body and splenium of the corpus callosum (CC). Monte Carlo simulations were also conducted to examine the relationship of intra- and extra-axonal radial diffusivity with diffusion time over a range of axon diameters and distributions. The results showed elevated diffusivities with OGSE relative to PGSE in the genu and splenium (but not the body) in both males and females, but the OGSE-PGSE difference was greater in the genu for males. Females showed positive correlations of OGSE-PGSE diffusivity difference with age across the CC, whereas there were no such age correlations in males. Simulations of radial diffusion demonstrated that for axon sizes in human brain both OGSE and PGSE diffusivities were dominated by extra-axonal water, but the OGSE-PGSE difference nonetheless increased with area-weighted outer-axon diameter. Therefore, the lack of OGSE-PGSE difference in the body is not entirely consistent with literature that suggests it is composed predominantly of axons with large diameter. The greater OGSE-PGSE difference in the genu of males could reflect larger axon diameters than females. The OGSE-PGSE difference correlation with age in females could reflect loss of smaller axons at older ages. The use of OGSE with short diffusion times to sample the microstructural scale of restriction implies regional differences of axon diameters along the corpus callosum with preliminary results suggesting a dependence on age and sex.

Genetic Analysis of the Organization, Development, and Plasticity of Corneal Innervation in Mice

The cornea has the densest sensory innervation of the body, originating primarily from neurons in the trigeminal ganglion. The basic principles of cornea nerve patterning have been established many years ago using classic neuroanatomical methods, such as immunocytochemistry and electrophysiology. Our understanding of the morphology and distribution of the sensory nerves in the skin has considerably progressed over the past few years through the generation and analysis of a variety of genetically modified mouse lines. Surprisingly, these lines were not used to study corneal axons. Here, we have screened a collection of transgenic and knockin mice (of both sexes) to select lines allowing the visualization and genetic manipulation of corneal nerves. We identified multiple lines, including some in which different types of corneal axons can be simultaneously observed with fluorescent proteins expressed in a combinatorial manner. We also provide the first description of the morphology and arborization of single corneal axons and identify three main types of branching pattern. We applied this genetic strategy to the analysis of corneal nerve development and plasticity. We provide direct evidence for a progressive reduction of the density of corneal innervation during aging. We also show that the semaphorin receptor neuropilin-1 acts cell-autonomously to control the development of corneal axons and that early axon guidance defects have long-term consequences on corneal innervation.SIGNIFICANCE STATEMENT We have screened a collection of transgenic and knockin mice and identify lines allowing the visualization and genetic manipulation of corneal nerves. We provide the first description of the arborization pattern of single corneal axons. We also present applications of this genetic strategy to the analysis of corneal nerve development and remodeling during aging.

Distinct roles for the cell adhesion molecule Contactin2 in the development and function of neural circuits in zebrafish

Contactin2 (Cntn2)/Transient Axonal Glycoprotein 1 (Tag1), a neural cell adhesion molecule, has established roles in neuronal migration and axon fasciculation in chick and mouse. In zebrafish, antisense morpholino-based studies have indicated roles for cntn2 in the migration of facial branchiomotor (FBM) neurons, the guidance of the axons of the nucleus of the medial longitudinal fascicle (nucMLF), and the outgrowth of Rohon-Beard (RB) central axons. To study functions of Cntn2 in later stages of neuronal development, we generated cntn2 mutant zebrafish using CRISPR-Cas9. Using a null mutant allele, we detected genetic interactions between cntn2 and the planar cell polarity gene vangl2, as shown previously with cntn2 morphants, demonstrating a function for cntn2 during FBM neuron migration in a sensitized background of reduced planar cell polarity signaling. In addition, maternal-zygotic (MZ) cntn2 mutant larvae exhibited aberrant touch responses and swimming, suggestive of defects in sensorimotor circuits, consistent with studies in mice. However, the nucMLF axon convergence, FBM neuron migration, and RB outgrowth defects seen in morphants were not seen in the mutants, and we show here that they are likely off-target effects of morpholinos. However, MLF axons exhibited local defasciculation in MZcntn2 mutants, consistent with a role for Cntn2 in axon fasciculation. These data demonstrate distinct roles for zebrafish cntn2 in neuronal migration and axon fasciculation, and in the function of sensorimotor circuits.

The function of contactin-2/TAG-1 in oligodendrocytes in health and demyelinating pathology

The oligodendrocyte maturation process and the transition from the pre-myelinating to the myelinating state are extremely important during development and in pathology. In the present study, we have investigated the role of the cell adhesion molecule CNTN2/TAG-1 on oligodendrocyte proliferation, differentiation, myelination, and function during development and under pathological conditions. With the combination of in vivo, in vitro, ultrastructural, and electrophysiological methods, we have mapped the expression of CNTN2 protein in the oligodendrocyte lineage during the different stages of myelination and its involvement on oligodendrocyte maturation, branching, myelin-gene expression, myelination, and axonal function. The cuprizone model of central nervous system demyelination was further used to assess CNTN2 in pathology. During development, CNTN2 can transiently affect the expression levels of myelin and myelin-regulating genes, while its absence results in reduced oligodendrocyte branching, hypomyelination of fiber tracts and impaired axonal conduction. In pathology, CNTN2 absence does not affect the extent of de- and remyelination. However during remyelination, a novel, CNTN2-independent mechanism is revealed that is able to recluster voltage gated potassium channels (VGKCs) resulting in the improvement of fiber conduction.

Sound-Evoked Activity Influences Myelination of Brainstem Axons in the Trapezoid Body

Plasticity of myelination represents a mechanism to tune the flow of information by balancing functional requirements with metabolic and spatial constraints. The auditory system is heavily myelinated and operates at the upper limits of action potential generation frequency and speed observed in the mammalian CNS. This study aimed to characterize the development of myelin within the trapezoid body, a central auditory fiber tract, and determine the influence sensory experience has on this process in mice of both sexes. We find that in vitro conduction speed doubles following hearing onset and the ability to support high-frequency firing increases concurrently. Also in this time, the diameter of trapezoid body axons and the thickness of myelin double, reaching mature-like thickness between 25 and 35 d of age. Earplugs were used to induce ∼50 dB elevation in auditory thresholds. If introduced at hearing onset, trapezoid body fibers developed thinner axons and myelin than age-matched controls. If plugged during adulthood, the thickest trapezoid body fibers also showed a decrease in myelin. These data demonstrate the need for sensory activity in both development and maintenance of myelin and have important implications in the study of myelin plasticity and how this could relate to sensorineural hearing loss following peripheral impairment.SIGNIFICANCE STATEMENT The auditory system has many mechanisms to maximize the dynamic range of its afferent fibers, which operate at the physiological limit of action potential generation, precision, and speed. In this study we demonstrate for the first time that changes in peripheral activity modifies the thickness of myelin in sensory neurons, not only in development but also in mature animals. The current study suggests that changes in CNS myelination occur as a downstream mechanism following peripheral deficit. Given the required submillisecond temporal precision for binaural auditory processing, reduced myelination might augment sensorineural hearing impairment.

Establishing Wiring Specificity in Visual System Circuits: From the Retina to the Brain

The retina is a tremendously complex image processor, containing numerous cell types that form microcircuits encoding different aspects of the visual scene. Each microcircuit exhibits a distinct pattern of synaptic connectivity. The developmental mechanisms responsible for this patterning are just beginning to be revealed. Furthermore, signals processed by different retinal circuits are relayed to specific, often distinct, brain regions. Thus, much work has focused on understanding the mechanisms that wire retinal axonal projections to their appropriate central targets. Here, we highlight recently discovered cellular and molecular mechanisms that together shape stereotypic wiring patterns along the visual pathway, from within the retina to the brain. Although some mechanisms are common across circuits, others play unconventional and circuit-specific roles. Indeed, the highly organized connectivity of the visual system has greatly facilitated the discovery of novel mechanisms that establish precise synaptic connections within the nervous system.

Autoantibodies to Synaptic Receptors and Neuronal Cell Surface Proteins in Autoimmune Diseases of the Central Nervous System

Investigations in the last 10 years have revealed a new category of neurological diseases mediated by antibodies against cell surface and synaptic proteins. There are currently 16 such diseases all characterized by autoantibodies against neuronal proteins involved in synaptic signaling and plasticity. In clinical practice these findings have changed the diagnostic and treatment approach to potentially lethal, but now treatable, neurological and psychiatric syndromes previously considered idiopathic or not even suspected to be immune-mediated. Studies show that patients' antibodies can impair the surface dynamics of the target receptors eliminating them from synapses (e.g., NMDA receptor), block the function of the antigens without changing their synaptic density (e.g., GABAb receptor), interfere with synaptic protein-protein interactions (LGI1, Caspr2), alter synapse formation (e.g., neurexin-3α), or by unclear mechanisms associate to a new form of tauopathy (IgLON5). Here we first trace the process of discovery of these diseases, describing the triggers and symptoms related to each autoantigen, and then review in detail the structural and functional alterations caused by the autoantibodies with special emphasis in those (NMDA receptor, amphiphysin) that have been modeled in animals.

Contactin-2/TAG-1, active on the front line for three decades

Contactin-2/transiently expressed axonal surface glycoprotein-1 (TAG-1) is a cell adhesion molecule belonging to the immunoglobulin superfamily (IgSF). It has six immunoglobulin-like extracellular domains and four fibronectin III-like ones, with anchoring to the cell membrane through glycosylphosphatidyl inositol. Contactin-2/TAG-1 is expressed in specific neurons transiently on the axonal surface during the fetal period. In postnatal stages, Contactin-2/TAG-1 is expressed in cerebellar granule cells, hippocampal pyramidal cells, and the juxtaparanodal regions of myelinated nerve fibers. In the embryonic nervous system, Contactin-2/TAG-1 plays important roles in axonal elongation, axonal guidance, and cellular migration. In the postnatal nervous system, it also plays an essential role in the formation of myelinated nerve fibers. Moreover, Contactin-2/TAG-1 has been linked to autoimmune diseases of the human nervous system. Taken together, Contactin-2/TAG-1 plays a central role in a variety of functions from development to disease.

The role of Gpi-anchored axonal glycoproteins in neural development and neurological disorders

This review article focuses on the Contactin (CNTN) subset of the Immunoglobulin supergene family (IgC2/FNIII molecules), whose components share structural properties (the association of Immunoglobulin type C2 with Fibronectin type III domains), as well as a general role in cell contact formation and axonal growth control. IgC2/FNIII molecules include 6 highly related components (CNTN 1-6), associated with the cell membrane via a Glycosyl Phosphatidyl Inositol (GPI)-containing lipid tail. Contactin 1 and Contactin 2 share ~50 (49.38)% identity at the aminoacid level. They are components of the cell surface, from which they may be released in soluble forms. They bind heterophilically to multiple partners in cis and in trans, including members of the related L1CAM family and of the Neurexin family Contactin-associated proteins (CNTNAPs or Casprs). Such interactions are important for organising the neuronal membrane, as well as for modulating the growth and pathfinding of axon tracts. In addition, they also mediate the functional maturation of axons by promoting their interactions with myelinating cells at the nodal, paranodal and juxtaparanodal regions. Such interactions also mediate differential ionic channels (both Na⁺ and K⁺) distribution, which is of critical relevance in the generation of the peak-shaped action potential. Indeed, thanks to their interactions with Ankyrin G, Na⁺ channels map within the nodal regions, where they drive axonal depolarization. However, no ionic channels are found in the flanking Contactin1-containing paranodal regions, where CNTN1 interactions with Caspr1 and with the Ig superfamily component Neurofascin 155 in cis and in trans, respectively, build a molecular barrier between the node and the juxtaparanode. In this region K⁺ channels are clustered, depending upon molecular interactions with Contactin 2 and with Caspr2. In addition to these functions, the Contactins appear to have also a role in degenerative and inflammatory disorders: indeed Contactin 2 is involved in neurodegenerative disorders with a special reference to the Alzheimer disease, given its ability to work as a ligand of the Alzheimer Precursor Protein (APP), which results in increased Alzheimer Intracellular Domain (AICD) release in a γ-secretase-dependent manner. On the other hand Contactin 1 drives Notch signalling activation via the Hes pathway, which could be consistent with its ability to modulate neuroinflammation events, and with the possibility that Contactin 1-dependent interactions may participate to the pathogenesis of the Multiple Sclerosis and of other inflammatory disorders.

Simulations on the influence of myelin water in diffusion-weighted imaging

While myelinated axons present an important barrier to water diffusion, many models used to interpret DWI signal neglect other potential influences of myelin. In this work, Monte Carlo simulations were used to test the sensitivity of DWI results to the diffusive properties of water within myelin. Within these simulations, the apparent diffusion coefficient (D app) varied slowly over several orders of magnitude of the coefficient of myelin water diffusion (D m), but exhibited important differences compared to D app values simulated that neglect D m (=0). Compared to D app, the apparent diffusion kurtosis (K app) was generally more sensitive to D m. Simulations also tested the sensitivity of D app and K app to the amount of myelin present. Unique variations in D app and K app caused by differences in the myelin volume fraction were diminished when myelin water diffusion was included. Also, expected trends in D app and K app with experimental echo time were reduced or inverted when accounting for myelin water diffusion, and these reduced/inverted trends were seen experimentally in ex vivo rat brain DWI experiments. In general, myelin water has the potential to subtly influence DWI results and bias models of DWI that neglect these components of white matter.

Caspr2 autoantibodies target multiple epitopes

Objective:

To better understand the mechanisms of autoantibodies to the axonal protein contactin-associated protein-like 2 (Caspr2) by studying their target epitopes.

Methods:

A plasmid for expressing Caspr2 was modified so that the various extracellular subdomains were deleted individually and in groups. Cultured cells were transfected to express these constructs and assayed by immunofluorescence staining with a commercial Caspr2 antibody and a panel of patient sera known to react with Caspr2. Western blotting was also performed. The role of glycosylation in immunogenicity was tested with tunicamycin and PNGase F treatment.

Results:

Patient antibodies bound to the extracellular domain of Caspr2. Neither native protein structure nor glycosylation was required for immunoreactivity. Caspr2 constructs with single or multidomain deletions were expressed on the plasma membrane. All deletion constructs were recognized by patients' sera, although reactivity was significantly reduced with deletion of the discoidin-like subdomain and strongly reduced or abolished with larger deletions of multiple N-terminal subdomains. Caspr2 with all subdomains deleted except the discoidin-like domain was still recognized by the antibodies.

Conclusion:

Caspr2 autoantibodies recognize multiple target epitopes in the extracellular domain of Caspr2, including one in the discoidin-like domain. Reactivity for some epitopes is not dependent on glycosylation or native protein structure.

Axon diameter and axonal transport: In vivo and in vitro effects of androgens

Testosterone is a sex hormone involved in brain maturation via multiple molecular mechanisms. Previous human studies described age-related changes in the overall volume and structural properties of white matter during male puberty. Based on this work, we have proposed that testosterone may induce a radial growth of the axon and, possibly, modulate axonal transport. In order to determine whether this is the case we have used two different experimental approaches. With electron microscopy, we have evaluated sex differences in the structural properties of axons in the corpus callosum (splenium) of young rats, and tested consequences of castration carried out after weaning. Then we examined in vitro the effect of the non-aromatizable androgen Mibolerone on the structure and bidirectional transport of wheat-germ agglutinin vesicles in the axons of cultured sympathetic neurons. With electron microscopy, we found robust sex differences in axonal diameter (males>females) and g ratio (males>females). Removal of endogenous testosterone by castration was associated with lower axon diameter and lower g ratio in castrated (vs. intact) males. In vitro, Mibolerone influenced the axonal transport in a time- and dose-dependent manner, and increased the axon caliber as compared with vehicle-treated neurons. These findings are consistent with the role of testosterone in shaping the axon by regulating its radial growth, as predicted by the initial human studies.

The microstructural correlates of T1 in white matter

PURPOSE

Several studies have shown strong correlations between myelin content and T1 within the brain, and have even suggested that T1 can be used to estimate myelin content. However, other micro-anatomical features such as compartment size are known to affect longitudinal relaxation rates, similar to compartment size effects in porous media.

METHODS

T1 measurements were compared with measured or otherwise published axon size measurements in white matter tracts of the rat spinal cord, rat brain, and human brain.

RESULTS

In both ex vivo and in vivo studies, correlations were present between the relaxation rate 1/T1 and axon size across regions of rat spinal cord with nearly equal myelin content.

CONCLUSION

While myelination is likely the dominant determinant of T1 in white matter, variations in white matter microstructure, independent of myelin volume fraction, may also be reflected in T1 differences between regions or subjects.

White matter as a transport system

There are two ways to picture white matter: as a grid of electrical wires or a network of roads. The first metaphor captures the classical function of an axon as conductor of action potentials (and information) from one brain region to another. The second one points to the important role of axons in a bi-directional transport of biological molecules and organelles between the cell body and synapse. Given the wide variety of such cargoes, a well-functioning axonal transport is critical for a number of processes, including neurotransmission, metabolism and viability of neurons. This selective review will emphasize the need for considering axonal transport when interpreting functional consequences of inter-individual variations in the structural properties of white matter. We start by describing the space occupied by white matter and techniques used in vivo for its characterization. We then provide examples of key features of maturation and aging of white matter, as well as some of the common abnormalities observed in neurodevelopmental and neurodegenerative disorders. Next, we review work that motivated our focus on axonal diameter, and explain the relationships between transport and cytoskeleton within the axon. We will conclude by describing molecular machinery of axonal transport and genes that may contribute to inter-individual variations in axonal diameter and axonal transport.

Contactin-1 regulates myelination and nodal/paranodal domain organization in the central nervous system

Myelin, a multilayered membrane sheath formed by oligodendrocytes around axons in the CNS, enables rapid nerve impulse conduction and sustains neuronal health. The signals exchanged between axons and oligodendrocytes in myelin remain to be fully elucidated. Here we provide genetic evidence for multiple and critical functions of Contactin-1 in central myelin. We document dynamic Contactin-1 expression on oligodendrocytes in vivo, and progressive accumulation at nodes of Ranvier and paranodes during postnatal mouse development. Nodal and paranodal expression stabilized in mature myelin, but overall membranous expression diminished. Contactin-1-deficiency disrupted paranodal junction formation as evidenced by loss of Caspr, mislocalized potassium Kv1.2 channels, and abnormal myelin terminal loops. Reduced numbers and impaired maturation of sodium channel clusters accompanied this phenotype. Histological, electron microscopic, and biochemical analyses uncovered significant hypomyelination in Contactin-1-deficient central nerves, with up to 60% myelin loss. Oligodendrocytes were present in normal numbers, albeit a minor population of neuronal/glial antigen 2-positive (NG2(+)) progenitors lagged in maturation by postnatal day 18, when the mouse null mutation was lethal. Major contributing factors to hypomyelination were defects in the generation and organization of myelin membranes, as judged by electron microscopy and quantitative analysis of oligodendrocyte processes labeled by GFP transgenically expressed from the proteolipid protein promoter. These data reveal that Contactin-1 regulates both myelin formation and organization of nodal and paranodal domains in the CNS. These multiple roles distinguish central Contactin-1 functions from its specific role at paranodes in the periphery, and emphasize mechanistic differences in central and peripheral myelination.

Effects of duration and timing of prenatal stress on hippocampal myelination and synaptophysin expression

The relationship between prenatal stress (PS) exposure and neurodevelopmental deficits remains inconclusive, especially when assessing the role of PS duration and timing and sex-dependent effects. This study explored a sex-specific association between the duration and timing of exposure and the outcomes of PS-induced neurotoxicity in hippocampal microstructure, synaptophysin expression, and neurobehavioral performance in rats. Pregnant rats were randomly assigned to control, PS-ML (exposed to prenatal restraint stress in the mid-to-late period of pregnancy), or PS-L (exposed in the late period of pregnancy) groups, and offspring in each group were divided into two subgroups by sex. Surface-righting reflex test, cliff avoidance test and Morris water maze test showed that neurodevelopmental levels were reduced in PS-treated pups but without significant sex differences. On postnatal day 22, hippocampal microstructure was examined by electron microscopy, and the expression of hippocampal synaptophysin was assessed by western blot. Abnormal ultrastructural appearance of hippocampal neurons and myelin sheaths, more degenerating neurons and higher G-ratios were found in young PS-ML and PS-L rats as well as reduced expression of hippocampal synaptophysin, although PS-ML pups were more greatly affected than PS-L, with males showing slightly greater impairments than females. These findings suggest that hippocampal hypo-myelination and decreased synaptophysin expression in neurodevelopment may be a duration and time-dependent effect of prenatal stress exposure, modified slightly by sex.

Micro-compartment specific T2* relaxation in the brain

MRI at high field can be sensitized to the magnetic properties of tissues, which introduces a signal dependence on the orientation of white matter (WM) fiber bundles relative to the magnetic field. In addition, study of the NMR relaxation properties of this signal has indicated contributions from compartmentalized water environments inside and outside the myelin sheath that may be separable. Here we further investigated the effects of water compartmentalization on the MRI signal with the goal of extracting compartment-specific information. By comparing MRI measurements of human and marmoset brain at 7T with magnetic field modeling, we show that: (1) water between the myelin lipid bilayers, in the axonal, and in the interstitial space each experience characteristic magnetic field effects that depend on fiber orientation (2) these field effects result in characteristic relaxation properties and frequency shifts for these compartments; and (3) compartmental contributions may be separated by multi-component fitting of the MRI signal relaxation (i.e. decay) curve. We further show the potential application of these findings to the direct mapping of myelin content and assessment of WM fiber integrity with high field MRI.

Neuronal autoantigens--pathogenesis, associated disorders and antibody testing

The discovery of disorders that are associated with antibodies to neuronal cell-surface proteins has led to a paradigm shift in our understanding of CNS autoimmunity. These disorders can occur in patients with or without cancer-often children or young adults who develop psychosis, catatonic or autistic features, memory problems, abnormal movements, or seizures that were previously considered idiopathic. The autoantigens in such cases have crucial roles in synaptic transmission, plasticity and peripheral nerve excitability. Patients can be comatose or encephalopathic for months and yet fully recover with supportive care and immunotherapy. By contrast, disorders in which the antibodies target intracellular antigens, and in which T-cell-mediated irreversible neuronal degeneration occurs, show a considerably poorer response to treatment. In this article, we review the various targets of neuronal antibodies, focusing predominantly on autoantigens located on the cell surface or synapses-namely, N-methyl-D-aspartate receptors, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors, γ-aminobutyric acid receptors, leucine-rich glioma-inactivated protein 1, contactin-associated protein-like 2, and metabotropic glutamate receptors. We also provide an algorithm to identify and assess antibodies that bind to cell-surface and synaptic antigens.

The cell neural adhesion molecule contactin-2 (TAG-1) is beneficial for functional recovery after spinal cord injury in adult zebrafish

The cell neural adhesion molecule contactin-2 plays a key role in axon extension and guidance, fasciculation, and myelination during development. We thus asked, whether contactin-2 is also important in nervous system regeneration after trauma. In this study, we used an adult zebrafish spinal cord transection model to test the functions of contactin-2 in spinal cord regeneration. The expression patterns of contactin-2 at different time points after spinal cord injury were studied at the mRNA level by qPCR and in situ hybridization, and contactin-2 protein levels and immunohistological localization were detected by Western blot and immunofluorescence analyses, respectively. Contactin-2 mRNA and protein levels were increased along the central canal at 6 days and 11 days after spinal cord injury, suggesting a requirement for contactin-2 in spinal cord regeneration. Co-localization of contactin-2 and islet-1 (a motoneuron marker) was observed in spinal cords before and after injury. To further explore the functions of contactin-2 in regeneration, an anti-sense morpholino was used to knock down the expression of contactin-2 protein by application at the time of injury. Motion analysis showed that inhibition of contactin-2 retarded the recovery of swimming functions when compared to standard control morpholino. Anterograde and retrograde tracing at 6 weeks after injury showed that knock down of contactin-2 inhibited axonal regrowth from NMLF neurons beyond lesion site. The combined observations indicate that contactin-2 contributes to locomotor recovery and successful regrowth of axons after spinal cord injury in adult zebrafish.

Axons and myelinating glia: An intimate contact

The coordination of the vertebrate nervous system requires high velocity signal transmission between different brain areas. High speed nerve conduction is achieved in the myelinated fibers of both the central and the peripheral nervous system where the myelin sheath acts as an insulator of the axon. The interactions between the glial cell and the adjacent axon, namely axo-glial interactions, segregate the fiber in distinct molecular and functional domains that ensure the rapid propagation of action potentials. These domains are the node of Ranvier, the paranode, the juxtaparanode and the internode and are characterized by multiprotein complexes between voltage-gated ion channels, cell adhesion molecules, members of the Neurexin family and cytoskeletal proteins. In the present review, we outline recent evidence on the key players of axo-glial interactions, depicting their importance in myelinated fiber physiology and disease.

The expression of TAG-1 in glial cells is sufficient for the formation of the juxtaparanodal complex and the phenotypic rescue of tag-1 homozygous mutants in the CNS

Myelinated fibers are organized into specialized domains that ensure the rapid propagation of action potentials and are characterized by protein complexes underlying axoglial interactions. TAG-1 (Transient Axonal Glycoprotein-1), a cell adhesion molecule of the Ig superfamily, is expressed by neurons as well as by myelinating glia. It is essential for the molecular organization of myelinated fibers as it maintains the integrity of the juxtaparanodal region through its interactions with Caspr2 and the voltage-gated potassium channels (VGKCs) on the axolemma. Since TAG-1 is the only known component of the juxtaparanodal complex expressed by the glial cell, it is important to clarify its role in the molecular organization of juxtaparanodes. For this purpose, we generated transgenic mice that exclusively express TAG-1 in oligodendrocytes and lack endogenous gene expression (Tag-1(-/-);plp(Tg(rTag-1))). Phenotypic analysis clearly demonstrates that glial TAG-1 is sufficient for the proper organization and maintenance of the juxtaparanodal domain in the CNS. Biochemical analysis shows that glial TAG-1 physically interacts with Caspr2 and VGKCs. Ultrastructural and behavioral analysis of Tag-1(-/-);plp(Tg(rTag-1)) mice shows that the expression of glial TAG-1 is sufficient to restore the axonal and myelin deficits as well as the behavioral defects observed in Tag-1(-/-) animals. Together, these data highlight the pivotal role of myelinating glia on axonal domain differentiation and organization.

Axon-oligodendrocyte interactions during developmental myelination, demyelination and repair

In multiple sclerosis, CNS demyelination is often followed by spontaneous repair, mostly achieved by adult oligodendrocyte precursor cells. Extent of this myelin repair differs, ranging from very low, limited to the plaque border, to extensive, with remyelination throughout the 'shadow plaques.' In addition to restoring neuronal connectivity, new myelin is neuroprotective. It reduces axonal loss and thus disability progression. Reciprocal communication between neurons and oligodendrocytes is essential for both myelin biogenesis and myelin repair. Hence, deciphering neuron-oligodendrocyte communication is not only important for understanding myelination per se, but also the pathophysiology that underlies demyelinating diseases and the development of innovative therapeutic strategies.

Growth of white matter in the adolescent brain: myelin or axon?

White matter occupies almost half of the human brain. It contains axons connecting spatially segregated modules and, as such, it is essential for the smooth flow of information in functional networks. Structural maturation of white matter continues during adolescence, as reflected in age-related changes in its volume, as well as in its microstructure. Here I review recent observations obtained with magnetic resonance imaging in typically developing adolescents and point out some of the known variations in structural properties of white matter vis-à-vis brain function in health and disease. I conclude by re-focusing the interpretations of MR-based studies of white matter from myelin to axon.

Could Sex Differences in White Matter be Explained by g ratio?

Recent studies with magnetic resonance imaging suggest that age-related changes in white matter during male adolescence may indicate an increase in g ratio wherein the radial growth of an axon outpaces a corresponding increase in myelin thickness. We review the original Rushton (1951) model where a g ratio of approximately 0.6 represents an optimal relationship between the axon and fibre diameters vis-à-vis conduction velocity, and point out evidence indicating slightly higher g ratio in large-diameter fibres. We estimate that fibres with a diameter larger than 9.6 mum will have a relatively thinner myelin sheath, and brains with increasingly larger proportions of such large-diameter fibres will have progressively lower concentration of myelin. We conclude by pointing out possible implications of "suboptimal" g ratio for the emergence of "disconnection" disorders, such as schizophrenia, in late adolescence.

GPI-anchored proteins at the node of Ranvier

Contactin and TAG-1 are glycan phosphatidyl inositol (GPI)-anchored cell adhesion molecules that play a crucial role in the organization of axonal subdomains at the node of Ranvier of myelinating fibers. Contactin and TAG-1 mediate axo-glial selective interactions in association with Caspr-family molecules at paranodes and juxtaparanodes, respectively. How membrane proteins can be confined in these neighbouring domains along the axon has been the subject of intense investigations. This review will specifically examine the properties conferred by the lipid microenvironment to regulate trafficking and selective association of these axo-glial complexes. Increasing evidences from genetic and neuropathological models point to a role of lipid rafts in the formation or stabilization of the paranodal junctions.

Contactins: emerging key roles in the development and function of the nervous system

Contactins are a subgroup of molecules belonging to the immunoglobulin superfamily that are expressed exclusively in the nervous system. The subgroup consists of six members: contactin, TAG-1, BIG-1, BIG-2, NB-2 and NB-3. Since their identification in the late 1980s, contactin and TAG-1 have been studied extensively. Axonal expression and the neurite extension activity of contactin and TAG-1 attracted researchers to study the function of these molecules in axon guidance during development. After the exciting discovery of the molecular function of contactin and TAG-1 in myelination earlier this decade, these two molecules have come to be known as the principal molecules in the function and maintenance of myelinated neurons. In contrast, the function of the other four members of this subgroup remained unknown until recently. Here, we will give an overview of contactin function, including recent progress on BIG-2, NB-2 and NB-3.

The mouse F3/contactin glycoprotein: structural features, functional properties and developmental significance of its regulated expression

F3/Contactin is an immunoglobulin superfamily component expressed in the nervous tissue of several species. Here we focus on the structural and functional properties of its mouse relative, on the mechanisms driving its regulated expression and on its developmental role. F3/Contactin is differentially expressed in distinct populations of central and peripheral neurons and in some non-neuronal cells. Accordingly, the regulatory region of the underlying gene includes promoter elements undergoing differential activation, associated with an intricate splicing profile, indicating that transcriptional and posttranscriptional mechanisms contribute to its expression. Transgenic models allowed to follow F3/Contactin promoter activation in vivo and to modify F3/Contactin gene expression under a heterologous promoter, which resulted in morphological and functional phenotypes. Besides axonal growth and pathfinding, these concerned earlier events, including precursor proliferation and commitment. This wide role in neural ontogenesis is consistent with the recognized interaction of F3/Contactin with developmental control genes belonging to the Notch pathway.

Impairment of learning and memory in TAG-1 deficient mice associated with shorter CNS internodes and disrupted juxtaparanodes

The cell adhesion molecule TAG-1 is expressed by neurons and glial cells and plays a role in axon outgrowth, migration and fasciculation during development. TAG-1 is also required for the clustering of Kv1.1/1.2 potassium channels and Caspr2 at the juxtaparanodes of myelinated fibers. Behavioral examination of TAG-1 deficient mice (Tag-1(-/-)) showed cognitive impairments in the Morris water maze and novel object recognition tests, reduced spontaneous motor activity, abnormal gait coordination and increased response latency to noxious stimulation. Investigation at the molecular level revealed impaired juxtaparanodal clustering of Caspr2 and Kv1.1/1.2 in the hippocampus, entorhinal cortex, cerebellum and olfactory bulb, with diffusion into the internode. Caspr2 and Kv1.1 levels were reduced in the cerebellum and olfactory bulb. Moreover, Tag-1(-/-) mice had shorter internodes in the cerebral and cerebellar white matter. The detected molecular alterations may account for the behavioural deficits and hyperexcitability in these animals.