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Traumatic spinal cord injury can cause immediate physical damage to the spinal cord and result in severe neurological deficits. The primary, mechanical tissue damage triggers a variety of secondary damage mechanisms at the injury site which significantly contribute to a larger lesion size and increased functional damage. Inflammatory mechanisms which directly involve both microglia (MG) and monocyte-derived macrophages (MDM) play important roles in the post-injury processes, including inflammation and debris clearing. In the current study, we investigated changes in the structure and function of MG/MDM in the injured spinal cord of adult female mice, 7 days after a thoracic contusion SCI. With the use of chip mapping scanning electron microscopy, which allows to image large samples at the nanoscale, we performed an ultrastructural comparison of MG/MDM located near the lesion vs adjacent regions to provide novel insights into the mechanisms at play post-injury. We found that MG/MDM located near the lesion had more mitochondria overall, including mitochondria with and without morphological alterations, and had a higher proportion of altered mitochondria. MG/MDM near the lesion also showed an increased number of phagosomes, including phagosomes containing myelin and partiallydigested materials. MG/MDM near the injury interacted differently with the spinal cord parenchyma, as shown by their reduced number of direct contacts with synaptic elements, axon terminals and dendritic spines. In this study, we characterized the ultrastructural changes of MG/MDM in response to spinal cord tissue damage in mice, uncovering changes in phagocytic activity, mitochondrial ultrastructure, and inter-cellular interactions within the spinal cord parenchyma. [ABSTRACT FROM AUTHOR]

Abstract Background The human orbitofrontal cortex (OFC) is involved in assessing the emotional significance of events and stimuli, emotion-based learning, allocation of attentional resources, and social cognition. Little is known about the structure, connectivity and excitatory/inhibitory circuit interactions underlying these diverse functions in human OFC, as well as how the circuit is disrupted in individuals with autism spectrum disorder (ASD). Methods We used post-mortem brain tissue from neurotypical adults and individuals with ASD. We examined the morphology and distribution of myelinated axons across cortical layers in OFC, at the single axon level, as a proxy of excitatory pathways. In the same regions, we also examined the laminar distribution of all neurons and neurochemically- and functionally-distinct inhibitory neurons that express the calcium-binding proteins parvalbumin (PV), calbindin (CB), and calretinin (CR). Results We found that the density of myelinated axons increased consistently towards layer 6, while the average axon diameter did not change significantly across layers in both groups. However, both the density and diameter of myelinated axons were significantly lower in the ASD group compared with the Control group. The distribution pattern and density of the three major types of inhibitory neurons was comparable between groups, but there was a significant reduction in the density of excitatory neurons across OFC layers in ASD. Limitations This study is limited by the availability of human post-mortem tissue optimally processed for high-resolution microscopy and immunolabeling, especially from individuals with ASD. Conclusions The balance between excitation and inhibition in OFC is at the core of its function, assessing and integrating emotional and social cues with internal states and external inputs. Our preliminary results provide evidence for laminar-specific changes in the ratio of excitation/inhibition in OFC of adults with ASD, with an overall weakening and likely disorganization of excitatory signals and a relative strengthening of local inhibition. These changes likely underlie pathology of major OFC communications with limbic or other cortices and the amygdala in individuals with ASD, and may provide the anatomic basis for disrupted transmission of signals for social interactions and emotions in autism.

Charcot-Marie-Tooth disease type 1 A (CMT1A) lacks an effective treatment. We provide a therapy for CMT1A, based on siRNA conjugated to squalene nanoparticles (siRNA PMP22-SQ NPs). Their administration resulted in normalization of Pmp22 protein levels, restored locomotor activity and electrophysiological parameters in two transgenic CMT1A mouse models with different severity of the disease. Pathological studies demonstrated the regeneration of myelinated axons and myelin compaction, one major step in restoring function of myelin sheaths. The normalization of sciatic nerve Krox20, Sox10 and neurofilament levels reflected the regeneration of both myelin and axons. Importantly, the positive effects of siRNA PMP22-SQ NPs lasted for three weeks, and their renewed administration resulted in full functional recovery. Beyond CMT1A, our findings can be considered as a potent therapeutic strategy for inherited peripheral neuropathies. They provide the proof of concept for a new precision medicine based on the normalization of disease gene expression by siRNA. Boutary et al. describe siRNA based therapy conjugated with squalene nanoparticles as an efficient approach to normalize PMP22 protein levels, restore locomotor activity, electrophysiological parameters and function of myelin sheath in CMT1A mouse models. These findings could be useful to develop therapeutic strategies for inherited peripheral neuropathies. [ABSTRACT FROM AUTHOR]

The nervous system is one of the most complex expressions of biological evolution. Its high performance mostly relies on the basic principle of the action potential, a sequential activation of local ionic currents along the neural fiber. The implications of this essentially electrical phenomenon subsequently emerged in a more comprehensive electromagnetic perspective of neurotransmission. Several studies focused on the possible role of photons in neural communication and provided evidence of the transfer of photons through myelinated axons. A hypothesis is that myelin sheath would behave as an optical waveguide, although the source of photons is controversial. In a previous work, we proposed a model describing how photons would arise at the node of Ranvier. In this study we experimentally detected photons in the node of Ranvier by Ag+ photoreduction measurement technique, during electrically induced nerve activity. Our results suggest that in association to the action potential a photonic radiation takes place in the node. [ABSTRACT FROM AUTHOR]

Strength-duration time constants, rheobase currents, and recovery cycles allow the nerve adaptive processes to single or pairs of depolarizing stimuli to be assessed. This study investigates the temperature dependence of these excitability indices of human motor nerve fibers with one of three increasingly-severe type of amyotrophic lateral sclerosis pathology, referred to as ALS1, ALS2, and ALS3. The temperature dependence of the excitability indices was investigated during hypothermia (≤25 ℃), hyperthermia (≥40 ℃), and in the physiological temperature range (30-37 ℃). Numerical solutions were computed using a temperature-dependent multi-layered model. Results showed the following trends: (i) while the strength-duration time constants gradually decreased with a temperature increase from 20 ℃ to 42 ℃, they were longer in the three ALS cases than those of the normal case; (ii) the reciprocally dependent strength-duration time constants and rheobase currents were more sensitive to hyperthermia, especially at 42oC, than at temperatures across the physiological range of 30-37 ℃; (iii) the shape of temperature-dependent recovery cycles was similar for both the normal and ALS1 cases; (iv) in the ALS2 case, each test stimulus applied at the end of 100 ms recovery cycle failed to initiate a second action potential during hypothermia at 20 ℃; and (v) in the ALS3 case during hypothermia, hyperthermia and across the physiological temperature range, each test stimulus applied beyond a given conditioning-test interval was blocked. This blockage was a result of the spontaneous action potential generation caused by the conditioning (first) stimulus. The changes obtained for the temperature-dependent strength-duration time constants, rheobase currents, and recovery cycles reflect nodal and internodal ion channel dysfunctions in the three amyotrophic lateral sclerosis cases. It is proposed that these excitability indices can be applied clinically as specific indicators for amyotrophic lateral sclerosis motor neuron disease.

Background: The human orbitofrontal cortex (OFC) is involved in assessing the emotional significance of events and stimuli, emotion-based learning, allocation of attentional resources, and social cognition. Little is known about the structure, connectivity and excitatory/inhibitory circuit interactions underlying these diverse functions in human OFC, as well as how the circuit is disrupted in individuals with autism spectrum disorder (ASD). Methods: We used post-mortem brain tissue from neurotypical adults and individuals with ASD. We examined the morphology and distribution of myelinated axons across cortical layers in OFC, at the single axon level, as a proxy of excitatory pathways. In the same regions, we also examined the laminar distribution of all neurons and neurochemically- and functionally-distinct inhibitory neurons that express the calcium-binding proteins parvalbumin (PV), calbindin (CB), and calretinin (CR). Results: We found that the density of myelinated axons increased consistently towards layer 6, while the average axon diameter did not change significantly across layers in both groups. However, both the density and diameter of myelinated axons were significantly lower in the ASD group compared with the Control group. The distribution pattern and density of the three major types of inhibitory neurons was comparable between groups, but there was a significant reduction in the density of excitatory neurons across OFC layers in ASD. Limitations: This study is limited by the availability of human post-mortem tissue optimally processed for high-resolution microscopy and immunolabeling, especially from individuals with ASD. Conclusions: The balance between excitation and inhibition in OFC is at the core of its function, assessing and integrating emotional and social cues with internal states and external inputs. Our preliminary results provide evidence for laminar-specific changes in the ratio of excitation/inhibition in OFC of adults with ASD, with an overall weakening and likely disorganization of excitatory signals and a relative strengthening of local inhibition. These changes likely underlie pathology of major OFC communications with limbic or other cortices and the amygdala in individuals with ASD, and may provide the anatomic basis for disrupted transmission of signals for social interactions and emotions in autism. [ABSTRACT FROM AUTHOR]

Sexual dimorphism exists at all levels of the nervous system. These sex differences could underlie genderrelated differences in behavior and neuropsychological function, as well as the gender differences in the prevalence of various mental disorders such as autism, attention deficit disorders, and schizophrenia. Myelination, on the other hand, is a unique cellular process that can have a dramatic impact on the structure and physiology of an axon and its surrounding tissue. The corpus callosum (CC) is the largest of the brain commissures, which connects the cerebral cortices of the two hemispheres, and provides interhemispheric connectivity for information transfer and processing between cortical regions. Variation in the axonal properties of CC will alter the interhemispheric connectivity. The CC consists of myelinated and unmyelinated axons, glial cells and blood vessels. Several functional studies have reported that the function of CC is associated with its axons density and myelination properties. The sexual dimorphism in the axonal content of the CC has always been controversial; hence, the aim of this study was to analyze the differences in axons’ diameter and myelin sheath thickness of the CC between male and female rats. For this purpose, five pairs of adult male and female rats were perfused and the CC were removed and sectioned. Four sections from different subregions of the corpus callosum that represent the genu, anterior body, posterior body, and splenium of the CC were stained and electron microscopic images were captured using stereological guidelines. Later, the axons diameter and myelin sheath thickness for each subregion were calculated and compared between males and females. Our preliminary findings of the present study indicated region specific differences in the myelinated axon thickness and diameter in the CC between male and female rats. [ABSTRACT FROM AUTHOR]

We address the field of soft plasmonics in finite electrolyte liquid systems ranged by insulating membranes by an analogy to the plasmonics of metallic nanostructures. The confined electrolyte systems can be encountered on a bio-cell organizational level, taking into account that the characteristics of ion plasmons fall to the micrometer size scale instead of the nanometer in metals because of at least three orders of magnitude larger masses of ions in comparison to electrons. The lower density of ions in electrolytes in comparison to density of electrons in metal may also reduce the energy of plasmons by several orders. We provide the fully analytical description of surface and volume plasmons in finite ionic micro-systems allowing for further applications. We next apply the theory of ionic plasmons to plasmon–polaritons in ionic periodic systems. The complete theory of ionic plasmon–polariton kinetics in the chain of micrometer-sized electrolyte spheres, confined by a dielectric membrane, is formulated and solved. The latter theory has next been applied to the explanation of a mysterious and unclear (for several dozen of years) problem of so-called saltatory conduction of the action potential in myelinated axons of nerve cells. Contrary to conventional models of nerve signaling, the plasmon–polariton model pretty well fits to the queer properties of the saltatory conduction. Moreover, the presented application of soft plasmonics to signaling in periodically myelinated axons may allow for identification of a different role in information processing of the white and gray matters in brain and spinal cord. We have outlined some perspectives to utilize the difference between the electricity of myelinated and non-myelinated nerve cells in brain to develop the topological concept of the memory functioning. The proposed ionic plasmon–polariton model of the saltatory conduction differently recognizes the role of the insulating myelin than previously was thought which may be helpful in the development of a better understanding of the demyelination diseases. [ABSTRACT FROM AUTHOR]

Abstract Neural diseases at the central nervous system including spinal cord injury (SCI) remain therapeutic challenges. Graphene materials are being delineated as alternative tools for neural repair. Herein, the regenerative ability of reduced graphene oxide (rGO) scaffolds to support pivotal features of neural repair at 4 months after SCI is assessed by an interdisciplinary approach. 3D randomly porous foams have been prepared in mechanical compliance with neural cells and tissues (Young's modulus of 1.3 ± 1.0 kPa) as demonstrated by atomic force microscopy techniques applied ex vivo. After implantation, the significant increase in Young's modulus caused by massive cell/protein infiltration does not alter the mechanical performance of the contralateral spinal cord but provides mechanical stability to the lesion. These aerogels appear fully vascularized and populated with neurites, some of them being myelinated excitatory axons. Clinically-inspired magnetic resonance imaging studies demonstrate that the scaffolds significantly reduce perilesional damage with respect to rats without implants and cause no compressive damage in the contralateral hemicord and rostral/caudal regions. The rGO implants do not either alter the rat spontaneous behaviour or induce toxicity in major organs. Finally, preliminary data suggest hints of rGO sheets dissociation and eventual degradation at the injured spinal cord for the first time. In summary, these 3D porous rGO scaffolds are able to induce, without any further biological functionalization, a compilation of positive effects that have been rarely described before, if ever, for any other material implanted in the injured spinal cord. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Myelinated axons are constricted at nodes of Ranvier. These constrictions are important physiologically because they increase the speed of saltatory nerve conduction, but they also represent potential bottlenecks for the movement of axonally transported cargoes. One type of cargo are neurofilaments, which are abundant space-filling cytoskeletal polymers that function to increase axon caliber. Neurofilaments move bidirectionally along axons, alternating between rapid movements and prolonged pauses. Strikingly, axon constriction at nodes is accompanied by a reduction in neurofilament number that can be as much as 10-fold in the largest axons. To investigate how neurofilaments navigate these constrictions, we developed a transgenic mouse strain that expresses a photoactivatable fluorescent neurofilament protein in neurons. We used the pulse-escape fluorescence photoactivation technique to analyze neurofilament transport in mature myelinated axons of tibial nerves from male and female mice of this strain ex vivo. Fluorescent neurofilaments departed the activated region more rapidly in nodes than in flanking internodes, indicating that neurofilament transport is faster in nodes. By computational modeling, we showed that this nodal acceleration can be explained largely by a local increase in the duty cycle of neurofilament transport (i.e., the proportion of the time that the neurofilaments spend moving). We propose that this transient acceleration functions to maintain a constant neurofilament flux across nodal constrictions, much as the current increases where a river narrows its banks. In this way, neurofilaments are prevented from piling up in the flanking internodes, ensuring a stable neurofilament distribution and uniform axonal morphology across these physiologically important axonal domains. [ABSTRACT FROM AUTHOR]

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. We have identified the dynein light chain DYNLL1 as a specific high‐affinity binding partner for the cytoplasmic domain of the large myelin‐associated glycoprotein (L‐MAG) isoform. Our structural data identify L‐MAG as a non‐canonical binding partner of DYNLL1. Heterotetramer formation between two disordered L‐MAG cytoplasmic tails and a homodimer of DYNLL1 provides a means for the isoform‐specific dimerization of L‐MAG. This interaction may regulate the binding properties of the MAG extracellular domain during myelination. Read the Editorial Highlight for this article on page 712. Open Science: This manuscript was awarded with the Open Materials Badge. For more information see: https://cos.io/our-services/open-science-badges/ [ABSTRACT FROM AUTHOR]

Myelination of axons by oligodendrocytes is a key feature of the remarkably fast operating CNS. Oligodendrocytes not only tune axonal conduction speed but are also suggested to maintain long-term axonal integrity by providing metabolic support to the axons they ensheath. However, how myelinating oligodendrocytes impact axonal energy homeostasis remains poorly understood and difficult to investigate. Here, we provide a method of how to study electrically active myelinated axons expressing genetically encoded sensors by combining electrophysiology and two-photon imaging of acutely isolated optic nerves. We show that intravitreal adeno-associated viral (AAV) vector delivery is an efficient tool to achieve functional sensor expression in optic nerve axons, which is demonstrated by measuring axonal ATP dynamics following AAV-mediated sensor expression. This novel approach allows for fast expression of any optical sensor of interest to be studied in optic nerve axons without the need to go through the laborious process of producing new transgenic mouse lines. Viral-mediated biosensor expression in myelinated axons and the subsequent combination of nerve recordings and sensor imaging outlines a powerful method to investigate oligodendroglial support functions and to further interrogate cellular mechanisms governing axonal energy homeostasis under physiological and pathological conditions.

Abstract Understanding diffusion-weighted MR signal in brain white matter (WM) has been a long-sought-after goal. Modern research pursues this goal by focusing on the biological compartments that contributes essentially to the signal. In this study, we experimentally address the apparent presence of a compartment in which water motion is restricted in all spatial directions. Using isotropic diffusion encoding, we establish an upper bound on the fraction of such a compartment, which is shown to be about 2% of the unweighted signal for moderate diffusion times. This helps to eliminate such a compartment that have been assumed in literature on biophysical modeling. We also used the diffusion decay curve obtained from the isotropic encoding to establish a lower limit on the mean diffusivities of either of intra- or extra-axonal compartment as a function of their relative water fraction. Highlights • Isotropic diffusion measurement shows an absence of still water compartment in brain white matter. • The lower limit on the trace of intra- and extra-axonal compartment was estimated. • Orientation dispersion of axons and glial processes have to be accounted to fit isotropic measurement. [ABSTRACT FROM AUTHOR]

Abstract Microstructural imaging and connectomics are two research areas that hold great potential for investigating brain structure and function. Combining these two approaches can lead to a better and more complete characterization of the brain as a network. The aim of this work is characterizing the connectome from a novel perspective using the myelination measure given by the g-ratio. The g-ratio is the ratio of the inner to the outer diameters of a myelinated axon, whose aggregated value can now be estimated in vivo using MRI. In two different datasets of healthy subjects, we reconstructed the structural connectome and then used the g-ratio estimated from diffusion and magnetization transfer data to characterize the network structure. Significant characteristics of g-ratio weighted graphs emerged. First, the g-ratio distribution across the edges of the graph did not show the power-law distribution observed using the number of streamlines as a weight. Second, connections involving regions related to motor and sensory functions were the highest in myelin content. We also observed significant differences in terms of the hub structure and the rich-club organization suggesting that connections involving hub regions present higher myelination than peripheral connections. Taken together, these findings offer a characterization of g-ratio distribution across the connectome in healthy subjects and lay the foundations for further investigating plasticity and pathology using a similar approach. Highlights • We integrated myelination into the structural connectome using the g-ratio. • The average g-ratio and the number of streamlines follow different patterns. • A myelin blueprint can be observed looking at the hub structure. • The anatomical organization follows patterns consistent with existing literature. [ABSTRACT FROM AUTHOR]

Abstract The fiber g-ratio is the ratio of the inner to the outer diameter of the myelin sheath of a myelinated axon. It has a limited dynamic range in healthy white matter, as it is optimized for speed of signal conduction, cellular energetics, and spatial constraints. In vivo imaging of the g-ratio in health and disease would greatly increase our knowledge of the nervous system and our ability to diagnose, monitor, and treat disease. MRI based g-ratio imaging was first conceived in 2011, and expanded to be feasible in full brain white matter with preliminary results in 2013. This manuscript reviews the growing g-ratio imaging literature and speculates on future applications. It details the methodology for imaging the g-ratio with MRI, and describes the known pitfalls and challenges in doing so. Highlights • This article reviews the recently developed g-ratio imaging framework. • Confounds in the methodology are detailed. • Recent progress and applications are reviewed. [ABSTRACT FROM AUTHOR]

Myelinated axons segregate the axonal membrane into four defined regions: the node of Ranvier, paranode, juxtaparanode, and internode. The paranodal junction consists of specific component proteins, such as neurofascin155 (NF155) on the glial side, and Caspr and Contactin on the axonal side. Although paranodal junctions are thought to play crucial roles in rapid saltatory conduction and nodal assembly, the role of their interaction with neurons is not fully understood. In a previous study, conditional NF155 knockout in oligodendrocytes led to disorganization of the paranodal junctions. To examine if disruption of paranodal junctions affects neuronal gene expression, we prepared total RNA from the retina of NF155 conditional knockout, and performed expression analysis. We found that the expression level of 433 genes changed in response to paranodal junction ablation. Interestingly, expression of aquaporin 3 (AQP3) was significantly reduced in NF155 conditional knockout mice, but not in cerebroside sulfotransferase knockout (CST‐KO) mice, whose paranodes are not originally formed during development. Copy number variations have an important role in the etiology of schizophrenia (SCZ). We observed rare duplications of AQP3 in SCZ patients, suggesting a correlation between abnormal AQP3 expression and SCZ. To determine if AQP3 over‐expression in NF155 conditional knockout mice influences neuronal function, we performed adeno‐associated virus (AAV)‐mediated over‐expression of AQP3 in the motor cortex of mice and found a significant increase in caspase 3‐dependent neuronal apoptosis in AQP3‐transduced cells. This study may provide new insights into therapeutic approaches for SCZ by regulating AQP3 expression, which is associated with paranodal disruption. The aquaporin 3 was related to the list of genes identified with copy number variations of schizophrenia, suggesting a correlation between abnormal aquaporin 3 expression and schizophrenia. We found that aquaporin 3 was sensitive to paranodal abnormalities. We further showed that dysregulation of aquaporin 3 expression through disruption of paranode affected neuronal viability. Further understanding of aquaporin 3 function may provide new insights into the etiology of schizophrenia caused by oligodendrocyte abnormalities and potential therapeutic approaches. [ABSTRACT FROM AUTHOR]

Abstract Acute encephalopathy with biphasic seizures and late reduced diffusion (AESD) is the most common pediatric encephalopathy in Japan, however, the exact neuropathology remains uncertain. The postmortem neuropathology in a patient with AESD revealed reduction of myelinated axons with early stage of astrocytosis in the absence of neuronal loss, which suggests the primary pathological damage in AESD involves myelinated axons and astrocytes rather than cortical neurons. An increased number of gemistocytic astrocytes at the corticomedullary junction may cause reduced diffusion, leading to the so-called bright tree appearance on magnetic resonance imaging, characteristic to AESD. [ABSTRACT FROM AUTHOR]

Myelination of axons by oligodendrocytes is a key feature of the remarkably fast operating CNS. Oligodendrocytes not only tune axonal conduction speed but are also suggested to maintain long-term axonal integrity by providing metabolic support to the axons they ensheath. However, how myelinating oligodendrocytes impact axonal energy homeostasis remains poorly understood and difficult to investigate. Here, we provide a method of how to study electrically active myelinated axons expressing genetically encoded sensors by combining electrophysiology and two-photon imaging of acutely isolated optic nerves. We show that intravitreal adeno-associated viral (AAV) vector delivery is an efficient tool to achieve functional sensor expression in optic nerve axons, which is demonstrated by measuring axonal ATP dynamics following AAV-mediated sensor expression. This novel approach allows for fast expression of any optical sensor of interest to be studied in optic nerve axons without the need to go through the laborious process of producing new transgenic mouse lines. Viral-mediated biosensor expression in myelinated axons and the subsequent combination of nerve recordings and sensor imaging outlines a powerful method to investigate oligodendroglial support functions and to further interrogate cellular mechanisms governing axonal energy homeostasis under physiological and pathological conditions. [ABSTRACT FROM AUTHOR]

We have analyzed a mouse model of Charcot-Marie-Tooth disease 2E (CMT2E) harboring a heterozygous p.Asn98Ser (p.N98S) Nefl mutation, whose human counterpart results in a severe, early-onset neuropathy. Behavioral, electrophysiological, and pathological analyses were done on separate cohorts of Nefl N98S/+ mutant mice and their wild type Nefl +/+ littermates between 8 and 48 weeks of age. The motor performance of Nefl N98S/+ mice, as evidenced by altered balance and gait measures, was impaired at every age examined (from 6 to 25 weeks of age). At all times examined, myelinated axons were smaller and contained markedly fewer neurofilaments in Nefl N98S/+ mice, in all examined aspects of the PNS, from the nerve roots to the distal ends of the sciatic and caudal nerves. Similarly, the myelinated axons in the various tracts of the spinal cord and in the optic nerves were smaller and contained fewer neurofilaments in mutant mice. The myelinated axons in both the PNS and the CNS of mutant mice had relatively thicker myelin sheaths. The amplitude and the nerve conduction velocity of the caudal nerves were reduced in proportion with the diminished sizes of myelinated axons. Conspicuous aggregations of neurofilaments were only seen in primary sensory and motor neurons, and were largely confined to the cell bodies and proximal axons. There was evidence of axonal degeneration and regeneration of myelinated axons, mostly in distal nerves. In summary, the p.N98S mutation causes a profound reduction of neurofilaments in the myelinated axons of the PNS and CNS, resulting in substantially reduced axonal diameters, particularly of large myelinated axons, and distal axon loss in the PNS. [ABSTRACT FROM AUTHOR]

Ctactile (CT) afferents respond to gentle tactile stimulation, but only a handful of studies in humans and animals have investigated whether their firing is modified by temperature. We describe the effects of radiant thermal stimuli, and of stationary and very slowly moving mechanothermal stimuli, on CT afferent responses. We find that CT afferents are primarily mechanoreceptors, as they fired little during radiant thermal stimuli, but they exhibited different patterns of firing during combined mechano-cool stimulation compared with warming. CTs fired optimally to gentle, very slowly moving, or stationary mechanothermal stimuli delivered at neutral temperature (~32°C, normal skin temperature), but they responded with fewer spikes (median 67% decrease) and at significantly lower rates (47% decrease) during warm (~42°C) tactile stimuli. During cool tactile stimuli (~18°C), their mean instantaneous firing frequency significantly decreased by 35%, but they often fired a barrage of afterdischarge spikes at a low frequency (~5 Hz) that outlasted the mechanical stimulus. These effects were observed under a variety of stimulus conditions, including during stationary and slowly moving touch (0.1 cm/s), and we complemented these tactile approaches using a combined electrical-thermal stimulation experiment where we found a suppression of spiking during warming. Overall, CT afferents are exquisitely sensitive to tactile events, and we show that their firing is modulated with touch temperatures above and below neutral skin temperature. Warm touch consistently decreased their propensity to fire, whereas cool touch produced lower firing rates but afterdischarge spiking. NEW & NOTEWORTHY C-tactile (CT) afferents are thought to underpin pleasant touch, and previous work has shown that they respond optimally to a slow caress delivered at typical (neutral) skin temperature. Here, we show that, although CTs are primarily mechanoreceptive afferents, they are modified by temperature: warm touch decreases their firing, whereas cool touch produces lower firing rates but long-lasting spiking, frequently seen as afterdischarges. This has implications for the encoding of affective sensory events in human skin. [ABSTRACT FROM AUTHOR]

Background End-to-side (ETS) neurorrhaphy is a promising procedure for peripheral nerve repair, yet controversies regarding the efficacy of this repair in facial nerve anastomosis for facial paralysis still exist. Materials and Methods Thirty rats were divided into three groups: intact control group, direct facial-hypoglossal ETS neurorrhaphy, and end-to-end (ETE) neurorrhaphy. Nerve regeneration was assessed with vibrissae motor performance, electrophysiological tests, retrograde labeling, and histomorphological analysis at 4 and 8 months postoperatively. Results Both ETS and ETE neurorrhaphies resulted in axonal regeneration and functional recovery of the recipient nerve but did not reach the level of intact controls. Significantly higher numbers of myelinated axons and labeled neurons giving regenerating fibers were found in group ETE compared with group ETS at both time points, consistent with the functional and electrophysiological recovery. Group ETS showed significantly smaller fiber diameter and thinner myelin thickness than group ETE at 4 months, but the difference became nonsignificant at 8 months. ETS neurorrhaphy had a very slight effect on the donor nerve, as determined electrophysiologically and histomorphologically. Sparsely distributed double-labeled neurons and relatively large amounts of single-labeled neurons contributing to reinnervation were found through double retrograde neuronal labeling in group ETS. Further quantitative analysis of the percentage of double-labeled neurons showed a pronounced tendency to decline from 19.8% at 4 months to 6.0% at 8 months postoperatively. Conclusion Successful reinnervation after ETS neurorrhaphy could be achieved through both collateral sprouting and terminal sprouting, with the latter seeming to be the principal origin of motor nerve sprouting. [ABSTRACT FROM AUTHOR]

Action potential conduction along myelinated axons depends on high densities of voltage-gated Na+ channels at the nodes of Ranvier. Flanking each node, paranodal junctions (paranodes) are formed between axons and Schwann cells in the peripheral nervous system (PNS) or oligodendrocytes in the CNS. Paranodal junctions contribute to both node assembly and maintenance. Despite their importance, the molecular mechanisms responsible for paranode assembly and maintenance remain poorly understood. βII spectrin is expressed in diverse cells and is an essential part of the submembranous cytoskeleton. Here, we show that Schwann cell βII spectrin is highly enriched at paranodes. To elucidate the roles of glial βII spectrin, we generated mutant mice lacking βII spectrin in myelinating glial cells by crossing mice with a floxed allele of Sptbn1 with Cnp-Cre mice, and analyzed both male and female mice. Juvenile (4 weeks) and middle-aged (60 weeks) mutant mice showed reduced grip strength and sciatic nerve conduction slowing, whereas no phenotype was observed between 8 and 24 weeks of age. Consistent with these findings, immunofluorescence microscopy revealed disorganized paranodes in the PNS and CNS of both postnatal day 13 and middle-aged mutant mice, but not in young adult mutant mice. Electron microscopy confirmed partial loss of transverse bands at the paranodal axoglial junction in the middle-aged mutant mice in both the PNS and CNS. These findings demonstrate that a spectrin-based cytoskeleton in myelinating glia contributes to formation and maintenance of paranodal junctions. [ABSTRACT FROM AUTHOR]

The gray short-tailed opossum (Monodelphis domestica) is a small marsupial gaining recognition as a laboratory animal in biomedical research. Despite numerous studies on opossum neuroanatomy, a consistent and comprehensive neuroanatomical reference for this species is still missing. Here we present the first three-dimensional, multimodal atlas of the Monodelphis opossum brain. It is based on four complementary imaging modalities: high resolution ex vivo magnetic resonance images, micro-computed tomography scans of the cranium, images of the face of the cutting block, and series of sections stained with the Nissl method and for myelinated fibers. Individual imaging modalities were reconstructed into a three-dimensional form and then registered to the MR image by means of affine and deformable registration routines. Based on a superimposition of the 3D images, 113 anatomical structures were demarcated and the volumes of individual regions were measured. The stereotaxic coordinate system was defined using a set of cranial landmarks: interaural line, bregma, and lambda, which allows for easy expression of any location within the brain with respect to the skull. The atlas is released under the Creative Commons license and available through various digital atlasing web services. [ABSTRACT FROM AUTHOR]

Objective To determine the circuit elements required to theoretically describe the stimulus waveforms generated by an implantable pulse generator (IPG) during clinical deep brain stimulation (DBS). Methods We experimentally interrogated the Medtronic Activa PC DBS IPG and defined an equivalent circuit model that accurately captured the output of the IPG. We then compared the detailed circuit model of the clinical stimulus waveforms to simplified representations commonly used in computational models of DBS. We quantified the errors associated with these simplifications using theoretical activation thresholds of myelinated axons in response to DBS. Results We found that the detailed IPG model generated substantial differences in activation thresholds compared to simplified models. These differences were largest for bipolar stimulation with long pulse widths. Average errors were ∼3 to 24% for voltage-controlled stimulation and ∼2 to 11% for current-controlled stimulation. Conclusions Our results demonstrate the importance of including basic circuit elements (e.g. blocking capacitors, lead wire resistance, electrode capacitance) in model analysis of DBS. Significance Computational models of DBS are now commonly used in academic research, industrial technology development, and in the selection of clinical stimulation parameters. Incorporating a realistic representation of the IPG output is necessary to improve the accuracy and utility of these clinical and scientific tools. [ABSTRACT FROM AUTHOR]

White matter structures composed of myelinated axons in the living human brain are primarily studied by diffusion-weighted MRI (dMRI). These long-range projections are typically characterized in a two-step process: dMRI signal is used to estimate the orientation of axon segments within each voxel, then these local orientations are linked together to estimate the spatial extent of putative white matter bundles. Tractography, the process of tracing bundles across voxels, either requires computationally expensive (probabilistic) simulations to model uncertainty in fiber orientation or ignores it completely (deterministic). Furthermore, simulation necessarily generates a finite number of trajectories, introducing “simulation error” to trajectory estimates. Here we introduce a method to analytically (via a closed-form solution) take an orientation distribution function (ODF) from each voxel and calculate the probabilities that a trajectory projects from a voxel into each directly adjacent voxels. We validate our method by demonstrating experimentally that probabilistic simulations converge to our analytically computed transition probabilities at the voxel level as the number of simulated seeds increases. We then show that our method accurately calculates the ground-truth transition probabilities from a publicly available phantom dataset. As a demonstration, we incorporate our analytic method for voxel transition probabilities into the Voxel Graph framework, creating a quantitative framework for assessing white matter structure, which we call “analytic tractography”. The long-range connectivity problem is reduced to finding paths in a graph whose adjacency structure reflects voxel-to-voxel analytic transition probabilities. We demonstrate that this approach performs comparably to the current most widely-used probabilistic and deterministic approaches at a fraction of the computational cost. We also demonstrate that analytic tractography works on multiple diffusion sampling schemes, reconstruction method or parameters used to define paths. Open source software compatible with popular dMRI reconstruction software is provided. [ABSTRACT FROM AUTHOR]

Approximately half of the human brain consists of myelinated axons. Central nervous system (CNS) myelin is made by oligodendrocytes and is essential for nervous system formation, health, and function. Once thought simply as a static insulator that facilitated rapid impulse conduction, myelin is now known to be made and remodeled in to adult life. Oligodendrocytes have a remarkable capacity to differentiate by default, but many aspects of their development can be influenced by axons. However, how axons and oligodendrocytes interact and cooperate to regulate myelination in the CNS remains unclear. Here, we review recent advances in our understanding of how such interactions generate the complexity of myelination known to exist in vivo. We highlight intriguing results that indicate that the cross-sectional size of an axon alone may regulate myelination to a surprising degree. We also review new studies, which have highlighted diversity in the myelination of axons of different neuronal subtypes and circuits, and structure-function relationships, which suggest that myelinated axons can be exquisitely fine-tuned to mediate precise conduction needs. We also discuss recent advances in our understanding of how neuronal activity regulates CNS myelination, and aim to provide an integrated overview of how axon-oligodendrocyte interactions sculpt neuronal circuit structure and function. [ABSTRACT FROM AUTHOR]

Age-related decreases in the conduction velocity (CV) of action potentials along myelinated axons have been linked to morphological changes in the myelin sheath. In particular, evidence suggests the presence of segmental demyelination and remyelination of axons. In remyelinated segments, the distance between adjacent nodes of Ranvier is typically shorter, and myelin sheaths are thinner. Both experimental and computational evidence indicates that shortened internodes slows CV. In this computational study, we determine the impact of progressive segmental demyelination and remyelination, modeled by shorter internodes with thinner myelin sheaths interspersed with normal ones, upon the CV. We find that CV progressively decreases as the number of remyelinated segments increases, but this decrease is greater than one would expect from an estimate of the CV based merely upon the number of short and long internodes. We trace the additional suppression of the CV to transitions between long and short internodes. Our study presents an important consideration for the precise modeling of neural circuits with remyelinated neurons. [ABSTRACT FROM AUTHOR]

Schwann cells (SCs) are specialized glial cells that myelinate and protect axons in the peripheral nervous system (PNS). Although myelinating SCs are more commonly studied, the PNS also contains a variety of non-myelinating SCs, including but not limited to Remak SCs (RSCs), terminal SCs, enteric glia. Although the field currently lacks many robust tools for interrogating the functions of non-myelinating SCs, recent evidence suggests that, like their myelinating counterparts, non-myelinating SCs are critical for proper PNS function. In this review, we focus specifically on RSCs and highlight recent advances in understanding regulators of RSC development, function, and participation in PNS regeneration. [ABSTRACT FROM AUTHOR]

Grey matter tissue damage is a characteristic pathological hallmark of multiple sclerosis. Although evidence is emerging that cortical atrophy is strongly associated with sustained disease progression, the underlying pathological processes that contribute to neurodegeneration remain poorly understood. One reason for this is that key features of progressive MS, such as cortical demyelination, microglial cell activation and neurodegeneration are absent in most of the commonly used rodent MS models. In this short review we will provide an overview of experimental MS animal models that exhibit MS-like cortical pathology. These models will be instrumental in unravelling the pathogenic mechanisms that contribute to cortical pathology and testing the efficacy of neuroprotective therapies for progressive MS patients. [ABSTRACT FROM AUTHOR]

We address the field of soft plasmonics in finite electrolyte liquid systems ranged by insulating membranes by an analogy to the plasmonics of metallic nanostructures. The confined electrolyte systems can be encountered on a bio-cell organizational level, taking into account that the characteristics of ion plasmons fall to the micrometer size scale instead of the nanometer in metals because of at least three orders of magnitude larger masses of ions in comparison to electrons. The lower density of ions in electrolytes in comparison to density of electrons in metal may also reduce the energy of plasmons by several orders. We provide the fully analytical description of surface and volume plasmons in finite ionic micro-systems allowing for further applications. We next apply the theory of ionic plasmons to plasmon–polaritons in ionic periodic systems. The complete theory of ionic plasmon–polariton kinetics in the chain of micrometer-sized electrolyte spheres, confined by a dielectric membrane, is formulated and solved. The latter theory has next been applied to the explanation of a mysterious and unclear (for several dozen of years) problem of so-called saltatory conduction of the action potential in myelinated axons of nerve cells. Contrary to conventional models of nerve signaling, the plasmon–polariton model pretty well fits to the queer properties of the saltatory conduction. Moreover, the presented application of soft plasmonics to signaling in periodically myelinated axons may allow for identification of a different role in information processing of the white and gray matters in brain and spinal cord. We have outlined some perspectives to utilize the difference between the electricity of myelinated and non-myelinated nerve cells in brain to develop the topological concept of the memory functioning. The proposed ionic plasmon–polariton model of the saltatory conduction differently recognizes the role of the insulating myelin than previously was thought which may be helpful in the development of a better understanding of the demyelination diseases.

Sleep spindles promote the consolidation of motor skill memory in young adults. Older adults, however, exhibit impoverished sleep-dependent motor memory consolidation. The underlying pathophysiological mechanism(s) explaining why motor memory consolidation in older adults fails to benefit from sleep remains unclear. Here, we demonstrate that male and female older adults show impoverished overnight motor skill memory consolidation relative to young adults, with the extent of impairment being associated with the degree of reduced frontal fast sleep spindle density. The magnitude of the loss of frontal fast sleep spindles in older adults was predicted by the degree of reduced white matter integrity throughout multiple white matter tracts known to connect subcortical and cortical brain regions. We further demonstrate that the structural integrity of selective white matter fiber tracts, specifically within right posterior corona radiata, right tapetum, and bilateral corpus callosum, statistically moderates whether sleep spindles promoted overnight consolidation of motor skill memory. Therefore, white matter integrity within tracts known to connect cortical sensorimotor control regions dictates the functional influence of sleep spindles on motor skill memory consolidation in the elderly. The deterioration of white matter fiber tracts associated with human brain aging thus appears to be one pathophysiological mechanism influencing subcortical-cortical propagation of sleep spindles and their related memory benefits. [ABSTRACT FROM AUTHOR]

The pannexin family of channels consists of three members--pannexin-1 (Panx1), pannexin-2 (Panx2), and pannexin-3 (Panx3) that enable the exchange of metabolites and signaling molecules between intracellular and extracellular compartments. Pannexin-mediated release of intracellular ATP into the extracellular space has been tied to a number of cellular activities, primarily through the activity of type P2 purinergic receptors. Previous work indicates that the opening of Panx1 channels and activation of purinergic receptors by extracellular ATP may cause inflammation and apoptosis. In the CNS (central nervous system) and PNS (peripheral nervous system), coupled pannexin, and P2 functions have been linked to peripheral sensitization (pain) pathways. Purinergic pathways are also essential for other critical processes in the PNS, including myelination and neurite outgrowth. However, whether such pathways are pannexin-dependent remains to be determined. In this study, we use a Panx1 knockout mouse model and pharmacological inhibitors of the Panx1 and the ATP-mediated signaling pathway to fill gaps in our understanding of Panx1 localization in peripheral nerves, roles for Panx1 in axonal outgrowth and myelination, and neurite extension. Our data show that Panx1 is localized to axonal, myelin, and vascular compartments of the peripheral nerves. Knockout of Panx1 gene significantly increased axonal caliber in vivo and axonal growth rate in cultured dorsal root ganglia (DRG) neurons. Furthermore, genetic knockout of Panx1 or inhibition of components of purinergic signaling, by treatment with probenecid and apyrase, resulted in denser axonal outgrowth from cultured DRG explants compared to untreated wild-types. Our findings suggest that Panx1 regulates axonal growth in the peripheral nervous system. [ABSTRACT FROM AUTHOR]

Studies of activity-driven nervous system plasticity have primarily focused on the gray matter. However, MRI-based imaging studies have shown that white matter, primarily composed of myelinated axons, can also be dynamically regulated by activity of the healthy brain. Myelination in the CNS is an ongoing process that starts around birth and continues throughout life. Myelin in the CNS is generated by oligodendrocytes and recent evidence has shown that many aspects of oligodendrocyte development and myelination can be modulated by extrinsic signals including neuronal activity. Because modulation of myelin can, in turn, affect several aspects of conduction, the concept has emerged that activity-regulated myelination represents an important form of nervous system plasticity. Here we review our increasing understanding of how neuronal activity regulates oligodendrocytes and myelinated axons in vivo, with a focus on the timing of relevant processes. We highlight the observations that neuronal activity can rapidly tune axonal diameter, promote re-entry of oligodendrocyte progenitor cells into the cell cycle, or drive their direct differentiation into oligodendrocytes. We suggest that activityregulated myelin formation and remodeling that significantly change axonal conduction properties are most likely to occur over timescales of days to weeks. Finally, we propose that precise fine-tuning of conduction along already-myelinated axons may also be mediated by alterations to the axon itself. We conclude that future studies need to analyze activity-driven adaptations to both axons and their myelin sheaths tofully understand how myelinated axon plasticity contributes to neuronal circuit formation and function. [ABSTRACT FROM AUTHOR]

The myelin sheath that insulates some axons in the central nervous system allows for faster signal conduction. Previously, axons were thought to be either unmyelinated or fully myelinated. Recent experimental work has discovered a new pattern of myelination (intermittent myelination) along axons in the mouse brain, in which long unmyelinated axon segments are followed by myelinated segments of comparable length. We use a computational model to explore how myelin distribution (in particular intermittent myelination) affects conduction velocity. We find that although fully myelinated axons minimize conduction velocity, varying the spatial distribution of a fixed amount of myelin along a partially myelinated axon leads to considerable variation in the conduction velocity for action potentials. Whether sodium ion channel number or sodium ion channel density is held constant as the area of the unmyelinated segments increases has a strong influence on the optimal pattern of myelin and the conduction velocity. [ABSTRACT FROM AUTHOR]

Neurotransmitter release depends on voltage-gated Na+ channels (Navs) to propagate an action potential (AP) successfully from the axon hillock to a synaptic terminal. Unmyelinated sections of axon are very diverse structures encompassing branch points and numerous presynaptic terminals with undefined molecular partners of Na+ channels. Using optical recordings of Ca2+ and membrane voltage, we demonstrate here that N+ channel β2 subunits (Navβ2s) are required to prevent AP propagation failures across the axonal arborization of cultured rat hippocampal neurons (mixed male and female). When Nav/32 expression was reduced, we identified two specific phenotypes: (1) membrane excitability and AP-evoked Ca2+ entry were impaired at synapses and (2) AP propagation was severely compromised with >40% of axonal branches no longer responding to AP-stimulation. We went on to show that a great deal of electrical signaling heterogeneity exists in AP waveforms across the axonal arborization independent of axon morphology. Therefore, Navβ2 is a critical regulator of axonal excitability and synaptic function in unmyelinated axons. [ABSTRACT FROM AUTHOR]

A set of astrocytic process associated with altered myelinated axons is described in the forebrain of normal adult rodents with confocal, electron microscopy, and 3D reconstructions. Each process consists of a protuberance that contains secretory organelles including numerous lysosomes which polarize and open next to disrupted myelinated axons. Because of the distinctive asymmetric organelle distribution and ubiquity throughout the forebrain neuropil, this enlargement is named paraxial process (PAP). The myelin envelope contiguous to the PAP displays focal disruption or disintegration. In routine electron microscopy clusters of large, confluent, lysosomes proved to be an effective landmark for PAP identification. In 3D assemblies lysosomes organize a series of interconnected saccules that open up to the plasmalemma next to the disruptedmyelin envelope(s). Activity for acid hydrolases was visualized in lysosomes, and extracellularly at the PAP-myelin interface and/or between the glial and neuronal outer aspects. Organelles in astrocytic processes involved in digesting pyknotic cells and debris resemble those encountered in PAPs supporting a likewise lytic function of the later. Conversely, processes entangling tripartite synapses and glomeruli were devoid of lysosomes. Both oligodendrocytic and microglial processes were not associated with altered myelin envelopes. The possible roles of the PAP in myelin remodeling in the context of the oligodendrocyte-astrocyte interactions and in the astrocyte's secretory pathways are discussed. [ABSTRACT FROM AUTHOR]

The present study investigates the temperature dependence of electrotonic potentials in mathematically-simulated myelinated axons with one of three increasingly-severe type of amyotrophic lateral sclerosis (ALS) pathology, termed as ALS1, ALS2 and ALS3, respectively, in the physiological range (30-37°C). These potentials were elicited by long-lasting (100 ms) subthreshold polarizing current stimuli (±40% of the threshold). Numerical solutions were computed using our temperaturedependent multi-layered model. The results showed the following trends: (i) in ALS1, polarizing electrotonic potentials were normal; (ii) in ALS2 and ALS3, action potentials were elicited in the early parts of the depolarizing electrotonic potentials, and (iii) in ALS3, spontaneous discharges were elicited after the termination of applied hyperpolarizing stimuli (i.e., postanodal excitation). The ionic currents underlying electrotonic potentials in the ALS1 case were attributable to the activation of potassium fast (Kf+) and slow (Kf+) channels in the nodal and internodal axolemma beneath the myelin sheath. By contrast, in ALS2 and ALS3, the depolarizing stimuli activated the classical "transient" Na+ channels in the nodal and internodal axolemma beneath the myelin sheath eliciting action potential generation. These results obtained were closer to those observed in hypothermia (≤25°C) than in hyperthermia (≥40°C). [ABSTRACT FROM AUTHOR]

It is well known that acetyl- l -carnitine (ALC) has various neuroprotective effects against neurodegenerative diseases. In addition, it has been reported that ALC facilitates myelination of regenerated axons after peripheral nerve injuries. We previously reported that spontaneous regeneration of the lateral olfactory tract (LOT), the main fiber tract of the central olfactory system, consistently occurred in newborn rats and a majority of these regenerated fibers were unmyelinated in neonatally LOT-transected young adult rats. To investigate the effects of ALC treatment on myelination in LOT, neonatal rats were treated with ALC after LOT transection. Immunohistochemistry for myelin basic protein showed more positive areas in ALC-treated rats than in control rats. Moreover, the number of myelinated axons of regenerated fibers was assessed using electron microscopy and was found to be statistically higher in ALC-treated rats compared to control rats. The study revealed that ALC accelerates myelination of regenerated fibers in neonatally LOT-injured young adult rats. [ABSTRACT FROM AUTHOR]

Caspr2 and TAG-1 (also known as CNTNAP2 and CNTN2, respectively) are cell adhesion molecules (CAMs) associated with the voltage-gated potassium channels Kv1.1 and Kv1.2 (also known as KCNA1 and KCNA2, respectively) at regions controlling axonal excitability, namely, the axon initial segment (AIS) and juxtaparanodes of myelinated axons. The distribution of Kv1 at juxtaparanodes requires axo-glial contacts mediated by Caspr2 and TAG-1. In the present study, we found that TAG-1 strongly colocalizes with Kv1.2 at the AIS of cultured hippocampal neurons, whereas Caspr2 is uniformly expressed along the axolemma. Live-cell imaging revealed that Caspr2 and TAG-1 are sorted together in axonal transport vesicles. Therefore, their differential distribution may result from diffusion and trapping mechanisms induced by selective partnerships. By using deletion constructs, we identified two molecular determinants of Caspr2 that regulate its axonal positioning. First, the LNG2-EGF1 modules in the ectodomain of Caspr2, which are involved in its axonal distribution. Deletion of these modules promotes AIS localization and association with TAG-1. Second, the cytoplasmic PDZ-binding site of Caspr2, which could elicit AIS enrichment and recruitment of the membrane-associated guanylate kinase (MAGuK) protein MPP2. Hence, the selective distribution of Caspr2 and TAG-1 may be regulated, allowing them to modulate the strategic function of the Kv1 complex along axons. [ABSTRACT FROM AUTHOR]

ANK3 encodes AnkyrinG (AnkG), a member of the Ankyrin family that is expressed in several different isoforms in many tissues. A unique serine-rich domain and tail domain in the two largest isoforms of AnkG (270 and 480 kDa), restrict AnkG to the axon initial segment and nodes of Ranvier of myelinated neurons. At these sites, AnkG is a master regulator, coordinating the strict clustering of components necessary for proper action potential initiation and propagation along the axon. These components include voltage-gated sodium channels, potassium channels and members of the L1 cell adhesion molecule family. Genetic variation in the ANK3 gene has been linked to a range of neuropsychiatric and neurodevelopmental disorders in human, including schizophrenia, bipolar disorder, intellectual disability and autism spectrum disorders. Here, we study the effect of reduced expression of the large isoforms of Ank3 on cognition and behaviour using a heterozygous knockout mouse model. In three independent behavioural tests, being the open field test, elevated plus maze and social interaction test, we found evidence for increased anxiety in our Ank3 mouse model. Besides, we observed specific neuroanatomical defects in heterozygous knockout mice, including a smaller cingulate cortex, granular retrosplenial cortex, primary motor cortex and fimbria of the hippocampus. [ABSTRACT FROM AUTHOR]

Myelinated axons require the correct localization of key proteins that are essential for nerve conduction and cognitive function. Little is known regarding the altered expression of contactin-associated protein 2 (Caspr2) at the juxtaparanodal regions and Nav1.6 at the node of Ranvier in response to chronic cerebral hypoperfusion (CCH). The aim of the present study was to examine the alterations in the key protein of myelinated axons and the potential mechanisms that may follow CCH. We established a rat model of CCH by controllable partial narrowing of bilateral common carotid arteries. Then, we detected cerebral blood flow (CBF) after surgery. We also evaluated motor-evoked potentials (MEPs), assessed the Morris water maze test, analyzed Caspr2 expression through immunohistochemistry and Nav1.6 protein expression through western blot analysis at 2, 4 and 12 weeks. The results revealed that the mean CBF value was significantly decreased to 33.90±5.48%. The MEP latencies and the escaping latencies were significantly prolonged. There was also an elongation of the first time passing of the hidden platform with a reduction of crossing platform times in spatial probing. Furthermore, the Caspr2 immunoreactivity demonstrated that the Caspr2 level was significantly downregulated with abnormal locations in the corpus callosum. The western blot analysis of Nav1.6 protein revealed that the level was reduced significantly over time. The results demonstrate that CCH leads to central conductive function loss, cognitive function damage and alterations in the key protein of myelinated axons, which may provide a molecular basis and key link for white matter damage. [ABSTRACT FROM AUTHOR]

Summary Nodes of Ranvier in the axons of myelinated neurons are exemplars of the specialized cell surface domains typical of polarized cells. They are rich in voltage-gated sodium channels (Nav) and thus underpin rapid nerve impulse conduction in the vertebrate nervous system [ 1 ]. Although nodal proteins cluster in response to myelination, how myelin-forming glia influence nodal assembly is poorly understood. An axoglial adhesion complex comprising glial Neurofascin155 and axonal Caspr/Contactin flanks mature nodes [ 2 ]. We have shown that assembly of this adhesion complex at the extremities of migrating oligodendroglial processes promotes process convergence along the axon during central nervous system (CNS) node assembly [ 3 ]. Here we show that anchorage of this axoglial complex to the axon cytoskeleton is essential for efficient CNS node formation. When anchorage is disrupted, both the adaptor Protein 4.1B and the cytoskeleton protein βII spectrin are mislocalized in the axon, and assembly of the node of Ranvier is significantly delayed. Nodal proteins and migrating oligodendroglial processes are no longer juxtaposed, and single detached nodal complexes replace the symmetrical heminodes found in both the CNS and peripheral nervous system (PNS) during development. We propose that axoglial adhesion complexes contribute to the formation of an interface between cytoskeletal elements enriched in Protein 4.1B and βII spectrin and those enriched in nodal ankyrinG and βIV spectrin. This clusters nascent nodal complexes at heminodes and promotes their timely coalescence to form the mature node of Ranvier. These data demonstrate a role for the axon cytoskeleton in the assembly of a critical neuronal domain, the node of Ranvier. [ABSTRACT FROM AUTHOR]

K+ channels containing Kv1.1 α subunits, which become prevalent at internodes in demyelinated axons, may underlie their dysfunctional conduction akin to muscle weakness in multiple sclerosis. Small inhibitors were sought with selectivity for the culpable hyper-polarizing K+ currents. Modeling of interactions with the extracellular pore in a Kv1.1-deduced structure identified diaryldi(2-pyrrolyl)methane as a suitable scaffold with optimized alkyl ammonium side chains. The resultant synthesized candidate [2,2'-((5,5'(di-p-topyldiaryldi(2-pyrrolyl)methane)bis(2,2'carbonyl)bis(azanediyl)) diethaneamine·2HCl] (8) selectively blocked Kv1.1 channels (IC50 ≈ 15 μM) recombinantly expressed in mammalian cells, induced a positive shift in the voltage dependency of K+ current activation, and slowed its kinetics. It preferentially inhibited channels containing two or more Kv1.1 subunits regardless of their positioning in concatenated tetramers. In slices of corpus callosum from mice subjected to a demyelination protocol, this novel inhibitor improved neuronal conduction, highlighting its potential for alleviating symptoms in multiple sclerosis. [ABSTRACT FROM AUTHOR]