Axonal Segments Research Articles

SARM1-specific motifs in the TIR domain enable NAD+ loss and regulate injury-induced SARM1 activation

Axon injury in response to trauma or disease stimulates a self-destruction program that promotes the localized clearance of damaged axon segments. Sterile alpha and Toll/interleukin receptor (TIR) motif-containing protein 1 (SARM1) is an evolutionarily conserved executioner of this degeneration cascade, also known as Wallerian degeneration; however, the mechanism of SARM1-dependent neuronal destruction is still obscure. SARM1 possesses a TIR domain that is necessary for SARM1 activity. In other proteins, dimerized TIR domains serve as scaffolds for innate immune signaling. In contrast, dimerization of the SARM1 TIR domain promotes consumption of the essential metabolite NAD+ and induces neuronal destruction. This activity is unique to the SARM1 TIR domain, yet the structural elements that enable this activity are unknown. In this study, we identify fundamental properties of the SARM1 TIR domain that promote NAD+ loss and axon degeneration. Dimerization of the TIR domain from the Caenorhabditis elegans SARM1 ortholog TIR-1 leads to NAD+ loss and neuronal death, indicating these activities are an evolutionarily conserved feature of SARM1 function. Detailed analysis of sequence homology identifies canonical TIR motifs as well as a SARM1-specific (SS) loop that are required for NAD+ loss and axon degeneration. Furthermore, we identify a residue in the SARM1 BB loop that is dispensable for TIR activity yet required for injury-induced activation of full-length SARM1, suggesting that SARM1 function requires multidomain interactions. Indeed, we identify a physical interaction between the autoinhibitory N terminus and the TIR domain of SARM1, revealing a previously unrecognized direct connection between these domains that we propose mediates autoinhibition and activation upon injury.

In vivo functional analysis of Drosophila Robo1 immunoglobulin-like domains.

BackgroundIn animals with bilateral symmetry, midline crossing of axons in the developing central nervous system is regulated by Slit ligands and their neuronal Roundabout (Robo) receptors. Multiple structural domains are present in an evolutionarily conserved arrangement in Robo family proteins, but our understanding of the functional importance of individual domains for midline repulsive signaling is limited.MethodsWe have examined the functional importance of each of the five conserved immunoglobulin-like (Ig) domains within the Drosophila Robo1 receptor. We generated a series of Robo1 variants, each lacking one of the five Ig domains (Ig1-5), and tested each for their ability to bind Slit when expressed in cultured Drosophila cells. We used a transgenic approach to express each variant in robo1’s normal expression pattern in wild-type and robo1 mutant embryos, and examined the effects of deleting each domain on receptor expression, axonal localization, regulation, and midline repulsive signaling in vivo.ResultsWe show that individual deletion of Ig domains 2–5 does not interfere with Robo1’s ability to bind Slit, while deletion of Ig1 strongly disrupts Slit binding. None of the five Ig domains (Ig1-5) are individually required for proper expression of Robo1 in embryonic neurons, for exclusion from commissural axon segments in wild-type embryos, or for downregulation by Commissureless (Comm), a negative regulator of Slit-Robo repulsion in Drosophila. Each of the Robo1 Ig deletion variants (with the exception of Robo1∆Ig1) were able to restore midline crossing in robo1 mutant embryos to nearly the same extent as full-length Robo1, indicating that Ig domains 2–5 are individually dispensable for midline repulsive signaling in vivo.ConclusionsOur findings indicate that four of the five Ig domains within Drosophila Robo1 are dispensable for its role in midline repulsion, despite their strong evolutionary conservation, and highlight a unique requirement for the Slit-binding Ig1 domain in the regulation of midline crossing.

CSPGs inhibit axon branching by impairing mitochondria-dependent regulation of actin dynamics and axonal translation.

Chondroitin sulfate proteoglycans (CSPGs) inhibit the formation of axon collateral branches. The regulation of the axonal cytoskeleton and mitochondria are important components of the mechanism of branching. Actin-dependent axonal plasticity, reflected in the dynamics of axonal actin patches and filopodia, is greatest along segments of the axon populated by mitochondria. It is reported that CSPGs partially depolarize the membrane potential of axonal mitochondria, which impairs the dynamics of the axonal actin cytoskeleton and decreases the formation and duration of axonal filopodia, the first steps in the mechanism of branching. The effects of CSPGs on actin cytoskeletal dynamics are specific to axon segments populated by mitochondria. In contrast, CSPGs do not affect the microtubule content of axons, or the localization of microtubules into axonal filopodia, a required step in the mechanism of branch formation. It is also reported that CSPGs decrease the mitochondria-dependent axonal translation of cortactin, an actin associated protein involved in branching. Finally, the inhibitory effects of CSPGs on axon branching, actin cytoskeletal dynamics and the axonal translation of cortactin are reversed by culturing neurons with acetyl-l-carnitine, which promotes mitochondrial respiration. Collectively these data indicate that CSPGs impair mitochondrial function in axons, an effect which contributes to the inhibition of axon branching. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 419-437, 2017.

(368) Sodium channel NaV1.8 mediates axonal TTX-resistant conduction in peripheral C-fibres in mice

NaV1.8 is a tetrodotoxin-resistant (TTX-r) voltage-gated sodium ion channel known to be expressed in sensory neurons of the dorsal root ganglion (DRG) and involved in the pain pathway. NaV1.8 knockout mice studies have shown that the channel is associated with inflammatory, neuropathic and cold pain. However, the distribution of NaV1.8 in different neuronal compartments has not been determined. The present study investigated how NaV1.8 is distributed in DRG neurons specifically along their unmyelinated peripheral C-fibre axons. A transgenic mouse line lacking NaV1.8 was used. The spatial distribution of NaV1.8 was determined functionally by comparing axonal conduction in the distal and proximal segments of the sural and saphenous nerves. To isolate TTX-r NaV isoforms, 500nM TTX was applied while monitoring the size of the compound axonal C-fibre response to electrical stimulation. In proximal sural and saphenous nerve segments approximately 14% of the C-fibre response was resistant to TTX at 32°C and 10-16% at 23°C, while in distal segments 27% of the C-fibre response was TTX-r at 32°C and 57% was TTX-r at 23°C. The relative increase in TTX-r compound action potential (CAP) amplitude during cooling suggests that NaV isoforms such as NaV1.8, which resists inactivation during cooling, maybe involved. To examine the role of NaV1.8, the effect of TTX on C-fibre conduction in sural and saphenous nerve segments was determined in NaV1.8-/- mice. TTX (500nM) completely blocked the electrically-evoked C-fibre CAP in NaV1.8-/- mice, i.e. there was no TTX-r axonal C-fibre conduction. Our results indicate that TTX-r action potential conduction in peripheral C-fibre axons is mediated exclusively via NaV1.8 channels and NaV1.8 is functionally enriched in distal axonal segments.

Radial glia inhibit peripheral glial infiltration into the spinal cord at motor exit point transition zones.

In the mature vertebrate nervous system, central and peripheral nervous system (CNS and PNS, respectively) GLIA myelinate distinct motor axon domains at the motor exit point transition zone (MEP TZ). How these cells preferentially associate with and myelinate discrete, non-overlapping CNS versus PNS axonal segments, is unknown. Using in vivo imaging and genetic cell ablation in zebrafish, we demonstrate that radial glia restrict migration of PNS glia into the spinal cord during development. Prior to development of radial glial endfeet, peripheral cells freely migrate back and forth across the MEP TZ. However, upon maturation, peripherally located cells never enter the CNS. When we ablate radial glia, peripheral glia ectopically migrate into the spinal cord during developmental stages when they would normally be restricted. These findings demonstrate that radial glia contribute to both CNS and PNS development and control the unidirectional movement of glial cell types across the MEP TZ early in development. GLIA 2016. GLIA 2016;64:1138-1153.

Synaptic Organization of Perisomatic GABAergic Inputs onto the Principal Cells of the Mouse Basolateral Amygdala.

Spike generation is most effectively controlled by inhibitory inputs that target the perisomatic region of neurons. Despite the critical importance of this functional domain, very little is known about the organization of the GABAergic inputs contacting the perisomatic region of principal cells (PCs) in the basolateral amygdala. Using immunocytochemistry combined with in vitro single-cell labeling we determined the number and sources of GABAergic inputs of PCs at light and electron microscopic levels in mice. We found that the soma and proximal dendrites of PCs were innervated primarily by two neurochemically distinct basket cell types expressing parvalbumin (PVBC) or cholecystokinin and CB1 cannabinoid receptors (CCK/CB1BC). The innervation of the initial segment of PC axons was found to be parceled out by PVBCs and axo-axonic cells (AAC), as the majority of GABAergic inputs onto the region nearest to the soma (between 0 and 10 μm) originated from PVBCs, while the largest portion of the axon initial segment was innervated by AACs. Detailed morphological investigations revealed that the three perisomatic region-targeting interneuron types significantly differed in dendritic and axonal arborization properties. We found that, although individual PVBCs targeted PCs via more terminals than CCK/CB1BCs, similar numbers (15–17) of the two BC types converge onto single PCs, whereas fewer (6–7) AACs innervate the axon initial segment of single PCs. Furthermore, we estimated that a PVBC and a CCK/CB1BC may target 800–900 and 700–800 PCs, respectively, while an AAC can innervate 600–650 PCs. Thus, BCs and AACs innervate ~10 and 20% of PC population, respectively, within their axonal cloud. Our results collectively suggest, that these interneuron types may be differently affiliated within the local amygdalar microcircuits in order to fulfill specific functions in network operation during various brain states.

Effects of extracellular calcium and surgical techniques on restoration of axonal continuity by polyethylene glycol fusion following complete cut or crush severance of rat sciatic nerves.

Complete crush or cut severance of sciatic nerve axons in rats and other mammals produces immediate loss of axonal continuity. Loss of locomotor functions subserved by those axons is restored only after months, if ever, by outgrowths regenerating at ∼1 mm/day from the proximal stumps of severed axonal segments. The distal stump of a severed axon typically begins to degenerate in 1-3 days. We recently developed a polyethylene glycol (PEG) fusion technology, consisting of sequential exposure of severed axonal ends to hypotonic Ca(2+) -free saline, methylene blue, PEG in distilled water, and finally Ca(2+) -containing isotonic saline. This study examines factors that affect the PEG fusion restoration of axonal continuity within minutes, as measured by conduction of action potentials and diffusion of an intracellular fluorescent dye across the lesion site of rat sciatic nerves completely cut or crush severed in the midthigh. Also examined are factors that affect the longer-term PEG fusion restoration of lost behavioral functions within days to weeks, as measured by the sciatic functional index. We report that exposure of cut-severed axonal ends to Ca(2+) -containing saline prior to PEG fusion and stretch/tension of proximal or distal axonal segments of cut-severed axons decrease PEG fusion success. Conversely, trimming cut-severed ends in Ca(2+) -free saline just prior to PEG fusion increases PEG fusion success. PEG fusion prevents or retards the Wallerian degeneration of cut-severed axons, as assessed by measures of axon diameter and G ratio. PEG fusion may produce a paradigm shift in the treatment of peripheral nerve injuries. © 2016 Wiley Periodicals, Inc.

Physical interactions between activated microglia and injured axons: do all contacts lead to phagocytosis?

Axonal injury is a pathological hallmark of both head injury and inflammatory-mediated neurological disorders, including multiple sclerosis (Schirmer et al., 2013). Such axonal disruptions and/or disconnections typically result in proximal axonal segments that remain in continuity with the neuronal soma while losing contact with their distal targets. These disconnected axonal segments contribute to loss of signal transduction and overall circuit disruption, via subsequent deafferentation. Due to the disruption of anterograde transport in this cohort of axons, immunolabeling of the normally transported protein, amyloid precursor protein (APP), in post-mortem brain tissue is the most commonly used method for the visualization of the swollen end of proximal axonal segments. While proximal axonal segments remain connected to the neuronal cell body, axonal segments distal to the point of injury progress to anterograde Wallerian degeneration, forming myelin and axonal debris that negatively affect the surrounding tissue (Vargas and Barres, 2007; Mietto et al., 2015).

MOBP levels are regulated by Fyn kinase and affect the morphological differentiation of oligodendrocytes.

Oligodendrocytes are the myelinating glial cells of the central nervous system (CNS). Myelin is formed by extensive wrapping of oligodendroglial processes around axonal segments, which ultimately allows a rapid saltatory conduction of action potentials within the CNS and sustains neuronal health. The non-receptor tyrosine kinase Fyn is an important signaling molecule in oligodendrocytes. It controls the morphological differentiation of oligodendrocytes and is an integrator of axon-glial signaling cascades leading to localized synthesis of myelin basic protein (MBP), which is essential for myelin formation. The abundant myelin-associated oligodendrocytic basic protein (MOBP) resembles MBP in several aspects and has also been reported to be localized as mRNA and translated in the peripheral myelin compartment. The signals initiating local MOBP synthesis are so far unknown and the cellular function of MOBP remains elusive. Here, we show, by several approaches in cultured primary oligodendrocytes, that MOBP synthesis is stimulated by Fyn activity. Moreover, we reveal a new function for MOBP in oligodendroglial morphological differentiation.

Cortical neuron activation induced by electromagnetic stimulation: a quantitative analysis via modelling and simulation.

Previous simulation works concerned with the mechanism of non-invasive neuromodulation has isolated many of the factors that can influence stimulation potency, but an inclusive account of the interplay between these factors on realistic neurons is still lacking. To give a comprehensive investigation on the stimulation-evoked neuronal activation, we developed a simulation scheme which incorporates highly detailed physiological and morphological properties of pyramidal cells. The model was implemented on a multitude of neurons; their thresholds and corresponding activation points with respect to various field directions and pulse waveforms were recorded. The results showed that the simulated thresholds had a minor anisotropy and reached minimum when the field direction was parallel to the dendritic-somatic axis; the layer 5 pyramidal cells always had lower thresholds but substantial variances were also observed within layers; reducing pulse length could magnify the threshold values as well as the variance; tortuosity and arborization of axonal segments could obstruct action potential initiation. The dependence of the initiation sites on both the orientation and the duration of the stimulus implies that the cellular excitability might represent the result of the competition between various firing-capable axonal components, each with a unique susceptibility determined by the local geometry. Moreover, the measurements obtained in simulation intimately resemble recordings in physiological and clinical studies, which seems to suggest that, with minimum simplification of the neuron model, the cable theory-based simulation approach can have sufficient verisimilitude to give quantitatively accurate evaluation of cell activities in response to the externally applied field.

Presynaptic protein Synaptotagmin1 regulates the neuronal polarity and axon differentiation in cultured hippocampal neurons.

BackgroundHippocampal neurons in the brain polarize to form multiple dendrites and one long axon. The formation of central synapses remains poorly understood. Although several of the intracellular proteins involved in the clustering of central neurotransmitter receptors and ion channels have been identified, the signals involved in pre- and postsynaptic differentiation remain elusive. Synaptotagmin1 is an abundant and important presynaptic vesicle protein that binds Ca2+ (J Biol Chem 277:7629–7632, 2002) in regulation of synaptic vesicle exocytosis at the synapse. Synapse consists of the formation of synaptic connections and requires precise coordination of Synaptotagmin1. It was reported Synaptotagmin1 plays an important roles in the formation of axonal filopodia and branches in chicken forebrain neurons (Dev Neurobiol 73:27–44, 2013). To determine if Synaptotagmin1 could have a role in formation of axon in hippocampal neurons, we investigated the effects of Synaptotagmin1 overexpression and knockdown using the shRNA on the growth and branching of the axons of primary hippocampal neurons. We showed that overexpression of Synaptotagmin1 leads to abnormal multiple axon formation in cultured rat hippocampal neurons.ResultsWe first examined the effects of Synaptotagmin1 on the numbers of axon and dendrites. We found that the overexpression of Synaptotagmin1 led to the formation of multiple axons and induced an increase in the number of endogenous postsynaptic protein Homer1c clusters in cultured hippocampal neurons. Endogenous initial segment of axon was detected with anti-sodium channel (anti-NaCh) antibody and with anti-Tau1 (J Neurosci 24: 4605–4613, 2004). The endogenous initial segment of axon was stained with anti-NaCh antibodies and with anti-Tau1 antibodies. Then the numbers of prominence dyed positive were counted as axon. We attempted to specifically knockdown the endogenous Synaptotagmin1 with small hairpin RNAs (shRNAs). To further dissect the functions of endogenous Synaptotagmin1 in neuronal polarity, we used the shRNA of Synaptotagmin1 that specifically blocks the existence of endogenous Synaptotagmin1. When the shRNA of Synaptotagmin1 was introduced to the cells, the number of axons and dendrites did not change.ConclusionsThese results indicate that the accumulation of Synaptotagmin1 may play an important role in axon/dendrite differentiation.

Proteolipid protein modulates preservation of peripheral axons and premature death when myelin protein zero is lacking.

Protein zero (P0) is the major structural component of peripheral myelin. Lack of this adhesion protein from Schwann cells causes a severe dysmyelinating neuropathy with secondary axonal degeneration in humans with the neuropathy Dejerine-Sottas syndrome (DSS) and in the corresponding mouse model (P0(null)-mice). In the mammalian CNS, the tetraspan-membrane protein PLP is the major structural myelin constituent and required for the long-term preservation of myelinated axons, which fails in hereditary spastic paraplegia (SPG type-2) and the relevant mouse model (Plp(null)-mice). The Plp-gene is also expressed in Schwann cells but PLP is of very low abundance in normal peripheral myelin; its function has thus remained enigmatic. Here we show that the abundance of PLP but not of other tetraspan myelin proteins is strongly increased in compact peripheral myelin of P0(null)-mice. To determine the functional relevance of PLP expression in the absence of P0, we generated P0(null)*Plp(null)-double-mutant mice. Compared with either single-mutant, P0(null)*Plp(null)-mice display impaired nerve conduction, reduced motor functions, and premature death. At the morphological level, axonal segments were frequently non-myelinated but in a one-to-one relationship with a hypertrophic Schwann cell. Importantly, axonal numbers were reduced in the vital phrenic nerve of P0(null)*Plp(null)-mice. In the absence of P0, thus, PLP also contributes to myelination by Schwann cells and to the preservation of peripheral axons. These data provide a link between the Schwann cell-dependent support of peripheral axons and the oligodendrocyte-dependent support of central axons.

Nodal versus Total Axonal Strain and the Role of Cholesterol in Traumatic Brain Injury.

Traumatic brain injury (TBI) is a health threat that affects every year millions of people involved in motor vehicle and sporting accidents, and thousands of soldiers in battlefields. Diffuse axonal injury (DAI) is one of the most frequent types of TBI leading to death. In DAI, the initial traumatic event is followed by a cascade of biochemical changes that take time to develop in full, so that symptoms may not become apparent until days or weeks after the original injury. Hence, DAI is a dynamic process, and the opportunity exists to prevent its progression provided the initial trauma can be predicted at the molecular level. Here, we present preliminary evidence from micro-finite element (FE) simulations that the mechanical response of central nervous system myelinated fibers is dependent on the axonal diameter, the ratio between axon diameter and fiber diameter (g-ratio), the microtubules density, and the cholesterol concentration in the axolemma and myelin. A key outcome of the simulations is that there is a significant difference between the overall level of strain in a given axonal segment and the level of local strain in the Ranvier nodes contained in that segment, with the nodal strain being much larger than the total strain. We suggest that the acquisition of this geometric and biochemical information by means of already available high resolution magnetic resonance imaging techniques, and its incorporation in current FE models of the brain will enhance the models capacity to predict the site and magnitude of primary axonal damage upon TBI.

Human Neuroma-in-Continuity Contains Focal Deficits in Myelination.

Functional recovery does not occur in 10% of patients with neonatal brachial plexus palsy. In these patients, resection of a neuroma-in-continuity (NIC) and surgical nerve reconstruction are required. The formation of a NIC seems to prohibit functional recovery, but the underlying biologic mechanisms for this failure are poorly understood. We systematically analyzed a large series of NIC tissue samples from 17 neonatal and 3 adult patients using an array of immunohistochemical techniques. In a large proportion of patients (74%), the NIC contained multiple focal globular areas with markedly diminished myelination. These focal myelin deficits (FMDs) contain Schwann cells that enwrap axons in an apparently normal configuration but do not form myelin. Biomathematical analysis of a 2-cm neuroma predicted a higher-than-95% probability that an axon would encounter 10 FMDs. Axon segments in FMDs also had disturbed nodes of Ranvier (i.e., FMDs contained significantly fewer clustered Na(v)1.6 channels and decreased Caspr and ankyrin G). These observations indicate that axons in NIC course through multiple FMDs and that this may be the pathobiologic basis for conduction blocks in patients with neonatal brachial plexus palsy. These observations indicate the need for novel strategies to promote functional recovery after neonatal brachial plexus palsy by improving myelination in the NIC.

Tuning of Ranvier node and internode properties in myelinated axons to adjust action potential timing.

Action potential timing is fundamental to information processing; however, its determinants are not fully understood. Here we report unexpected structural specializations in the Ranvier nodes and internodes of auditory brainstem axons involved in sound localization. Myelination properties deviated significantly from the traditionally assumed structure. Axons responding best to low-frequency sounds had a larger diameter than high-frequency axons but, surprisingly, shorter internodes. Simulations predicted that this geometry helps to adjust the conduction velocity and timing of action potentials within the circuit. Electrophysiological recordings in vitro and in vivo confirmed higher conduction velocities in low-frequency axons. Moreover, internode length decreased and Ranvier node diameter increased progressively along the distal axon segments, which simulations show was essential to ensure precisely timed depolarization of the giant calyx of Held presynaptic terminal. Thus, individual anatomical parameters of myelinated axons can be tuned to optimize pathways involved in temporal processing.

Diagnostic tools for evaluating the impact of Focal Axonal Swellings arising in neurodegenerative diseases and/or traumatic brain injury

BackgroundFocal Axonal Swellings arise in several leading neurodegenerative diseases of the central nervous system and are hallmark features of concussions and traumatic brain injuries. Recent theories mapped how the shape of each swelling affects the propagation of spike trains and consequently the information encoded in them. Spikes can be selectively deleted, have their speed affected, or blocked depending upon the severity of the swelling. New methodOur computational toolbox extracts meaningful geometrical parameters from sequential images of injured axon segments. The algorithm provides a principled approach for dealing with imaging distortions caused by experimental artifacts in order to extract the cross-section of an axon by detecting local symmetries, turning points and turning regions. ResultsOur characterization of the Focal Axonal Swelling allows for an assessment of its impact on spike propagation, leading to a color coding of the axon that highlights problematic regions for information propagation. Comparison with existing methodsMany theoretical works reported distortions in spike propagation related to axonal enlargements. Such estimates, however, were not incorporated to a toolbox that could classify axonal swellings directly from experimental images. ConclusionsOur MATLAB toolbox thus highlights potential trouble spots of axonal morphology, and similar to car traffic maps, identify blocked or impaired routes for information flow. This computational framework is a promising starting point for diagnosing and assessing the impact of axonal swellings implicated in concussions, Alzheimer's and Parkinson's disease, Multiple Sclerosis and other neurological pathologies.

Axonal fusion via conduit-based delivery of hydrophilic polymers.

Hydrophilic polymers have been shown to improve physiologic recovery following repair of transected nerves with microsutures. Our study was designed to combine hydrophilic polymer therapy with nerve tubes (NT) to enhance polymer delivery to the site of nerve injury. Using a rat sciatic nerve injury model, a single transection injury was repaired in an end-to-end fashion with NT + polyethylene glycol (PEG) to NT alone. Compound action potentials (CAPs) were recorded before nerve transection and after repair. Behavioral testing was performed for 5weeks. PEG therapy restored CAPS in all, but one, animals, while no CAPS were recorded in animals not receiving PEG. Behavioral nerve function was measured using the standardized functional assessment technique and foot fault asymmetry scores (FF). FF scores were improved for the PEG therapy groups on postoperative days 7, 14, and 21. However, after expected eventual axonal outgrowth, the benefit was less noticeable at days 28 and 35. Immunohistochemistry of the distal axon segments showed an increase number of sensory and motor axons in the NT + PEG group as compared to NT alone. These data suggest that PEG delivery via a conduit may provide a simple, effective way to fuse severed axons and regain early nerve function. For proximal nerve injuries in large animals, recovery of axonal continuity could dramatically improve outcomes, even if fusion only occurs in a small percentage of axons.

Modelling the Effects of Electrical Coupling between Unmyelinated Axons of Brainstem Neurons Controlling Rhythmic Activity.

Gap junctions between fine unmyelinated axons can electrically couple groups of brain neurons to synchronise firing and contribute to rhythmic activity. To explore the distribution and significance of electrical coupling, we modelled a well analysed, small population of brainstem neurons which drive swimming in young frog tadpoles. A passive network of 30 multicompartmental neurons with unmyelinated axons was used to infer that: axon-axon gap junctions close to the soma gave the best match to experimentally measured coupling coefficients; axon diameter had a strong influence on coupling; most neurons were coupled indirectly via the axons of other neurons. When active channels were added, gap junctions could make action potential propagation along the thin axons unreliable. Increased sodium and decreased potassium channel densities in the initial axon segment improved action potential propagation. Modelling suggested that the single spike firing to step current injection observed in whole-cell recordings is not a cellular property but a dynamic consequence of shunting resulting from electrical coupling. Without electrical coupling, firing of the population during depolarising current was unsynchronised; with coupling, the population showed synchronous recruitment and rhythmic firing. When activated instead by increasing levels of modelled sensory pathway input, the population without electrical coupling was recruited incrementally to unpatterned activity. However, when coupled, the population was recruited all-or-none at threshold into a rhythmic swimming pattern: the tadpole “decided” to swim. Modelling emphasises uncertainties about fine unmyelinated axon physiology but, when informed by biological data, makes general predictions about gap junctions: locations close to the soma; relatively small numbers; many indirect connections between neurons; cause of action potential propagation failure in fine axons; misleading alteration of intrinsic firing properties. Modelling also indicates that electrical coupling within a population can synchronize recruitment of neurons and their pacemaker firing during rhythmic activity.

Wallerian degeneration in the optic nerve stretch-injury model of traumatic brain injury: a stereological analysis.

Patients with chronic traumatic encephalopathy (CTE) show loss of central white matter, central gray matter, and cortical gray matter with increasing post-traumatic survival. The majority of experimental studies using animals have, however, discussed only the ultrastructural pathophysiology of injured central white matter leading to secondary axotomy and the formation of axonal terminal bulbs. Using the stretch-injured optic nerve model in adult guinea pigs, the present study provides novel quantitative data concerning Wallerian degeneration of disconnected axonal fragments following secondary axotomy out to 12 weeks after injury to an optic nerve. The time course of Wallerian degeneration at the level of an individual nerve fiber is comparable to that reported in earlier studies over 48 h to two weeks after secondary axotomy. But only a relatively small proportion of nerve fibers within the optic tract degenerate via Wallerian degeneration during the first two weeks. Rather, examples of each of the three stages of Wallerian degeneration-acute axonal degeneration, latency of the distal axonal segment, and granular fragmentation-occur within the optic tract across the entire experimental survival of 12 weeks used in the present study. This data suggests that some nerve fibers initiate Wallerian degeneration days and weeks after the initial time of mechanical injury to an optic nerve. The number of intact nerve fibers continues to fall over at least three months after injury in the stretch-injury model of traumatic axonal injury. It is suggested that these novel findings relate to the mechanism(s) whereby central white matter volume decreases over months and years in CTE patients.

The Adhesion GPCR GPR126 Has Distinct, Domain-Dependent Functions in Schwann Cell Development Mediated by Interaction with Laminin-211

Myelin ensheathes axons to allow rapid propagation of action potentials and proper nervous system function. In the peripheral nervous system, Schwann cells(SCs) radially sort axons into a 1:1 relationship before wrapping an axonal segment to form myelin. SC myelination requires the adhesion G protein-coupled receptor GPR126, which undergoes autoproteolytic cleavage into an N-terminal fragment (NTF) and a seven-transmembrane-containing C-terminal fragment (CTF). Here we show that GPR126 has domain-specific functions in SC development whereby the NTF is necessary and sufficient for axon sorting, whereas the CTF promotes wrapping through cAMP elevation. These biphasic roles of GPR126 are governed by interactions with Laminin-211, which we define as a novel ligand for GPR126 that modulates receptor signaling via a tethered agonist. Our work suggests a model in which Laminin-211 mediates GPR126-induced cAMP levels to control early and late stages of SC development.