Axon initial segments: structure, function, and disease

Na+/K+-ATPase: ion pump, signal transducer, or cytoprotective protein, and novel biological functions

Na+/K+-ATPase is a transmembrane protein that has important roles in the maintenance of electrochemical gradients across cell membranes by transporting three Na+ out of and two K+ into cells. Additionally, Na+/K+-ATPase participates in Ca2+-signaling transduction and neurotransmitter release by coordinating the ion concentration gradient across the cell membrane. Na+/K+-ATPase works synergistically with multiple ion channels in the cell membrane to form a dynamic network of ion homeostatic regulation and affects cellular communication by regulating chemical signals and the ion balance among different types of cells. Therefore, it is not surprising that Na+/K+-ATPase dysfunction has emerged as a risk factor for a variety of neurological diseases. However, published studies have so far only elucidated the important roles of Na+/K+-ATPase dysfunction in disease development, and we are lacking detailed mechanisms to clarify how Na+/K+-ATPase affects cell function. Our recent studies revealed that membrane loss of Na+/K+-ATPase is a key mechanism in many neurological disorders, particularly stroke and Parkinson's disease. Stabilization of plasma membrane Na+/K+-ATPase with an antibody is a novel strategy to treat these diseases. For this reason, Na+/K+-ATPase acts not only as a simple ion pump but also as a sensor/regulator or cytoprotective protein, participating in signal transduction such as neuronal autophagy and apoptosis, and glial cell migration. Thus, the present review attempts to summarize the novel biological functions of Na+/K+-ATPase and Na+/K+-ATPase-related pathogenesis. The potential for novel strategies to treat Na+/K+-ATPase-related brain diseases will also be discussed.

Neuronal diversity in the caudate nucleus: A comparative study between camel and human brains

Caudate nucleus (CN) neurons in camels and humans were examined using modified Golgi impregnation methods. Neurons were classified based on soma morphology, dendritic characteristics, and spine distribution. Three primary neuron types were identified in both species: rich-spiny (Type I), sparsely-spiny (Type II), and aspiny (Type III), each comprising subtypes with specific features. Comparative analysis revealed significant differences in soma size, dendritic morphology, and spine distribution between camels and humans. The study contributes to our understanding of structural diversity in CN neurons and provides insights into evolutionary neural adaptations.

How does Nogo-A signalling influence mitochondrial function during multiple sclerosis pathogenesis?

Multiple sclerosis (MS) is a severe neurological disorder that involves inflammation in the brain, spinal cord and optic nerve with key disabling neuropathological outcomes being axonal damage and demyelination. When degeneration of the axo-glial union occurs, a consequence of inflammatory damage to central nervous system (CNS) myelin, dystrophy and death can lead to large membranous structures from dead oligodendrocytes and degenerative myelin deposited in the extracellular milieu. For the first time, this review covers mitochondrial mechanisms that may be operative during MS-related neurodegenerative changes directly activated during accumulating extracellular deposits of myelin associated inhibitory factors (MAIFs), that include the potent inhibitor of neurite outgrowth, Nogo-A. Axonal damage may occur when Nogo-A binds to and signals through its cognate receptor, NgR1, a multimeric complex, to initially stall axonal transport and limit the delivery of important growth-dependent cargo and subcellular organelles such as mitochondria for metabolic efficiency at sites of axo-glial disintegration as a consequence of inflammation. Metabolic efficiency in axons fails during active demyelination and progressive neurodegeneration, preceded by stalled transport of functional mitochondria to fuel axo-glial integrity.

The Axonal Actin Filament Cytoskeleton: Structure, Function, and Relevance to Injury and Degeneration

Early investigations of the neuronal actin filament cytoskeleton gave rise to the notion that, although growth cones exhibit high levels of actin filaments, the axon shaft exhibits low levels of actin filaments. With the development of new tools and imaging techniques, the axonal actin filament cytoskeleton has undergone a renaissance and is now an active field of research. This article reviews the current state of knowledge about the actin cytoskeleton of the axon shaft. The best understood forms of actin filament organization along axons are axonal actin patches and a submembranous system of rings that endow the axon with protrusive competency and structural integrity, respectively. Additional forms of actin filament organization along the axon have also been described and their roles are being elucidated. Extracellular signals regulate the axonal actin filament cytoskeleton and our understanding of the signaling mechanisms involved is being elaborated. Finally, recent years have seen advances in our perspective on how the axonal actin cytoskeleton is impacted by, and contributes to, axon injury and degeneration. The work to date has opened new venues and future research will undoubtedly continue to provide a richer understanding of the axonal actin filament cytoskeleton.

Changes in mitochondrial distribution occur at the axon initial segment in association with neurodegeneration in Drosophila

Changes in mitochondrial distribution are a feature of numerous age-related neurodegenerative diseases. In Drosophila, reducing the activity of Cdk5 causes a neurodegenerative phenotype and is known to affect several mitochondrial properties. Therefore, we investigated whether alterations of mitochondrial distribution are involved in Cdk5-associated neurodegeneration. We find that reducing Cdk5 activity does not alter the balance of mitochondrial localization to the somatodendritic versus axonal neuronal compartments of the mushroom body, the learning and memory center of the Drosophila brain. We do, however, observe changes in mitochondrial distribution at the axon initial segment (AIS), a neuronal compartment located in the proximal axon involved in neuronal polarization and action potential initiation. Specifically, we observe that mitochondria are partially excluded from the AIS in wild-type neurons, but that this exclusion is lost upon reduction of Cdk5 activity, concomitant with the shrinkage of the AIS domain that is known to occur in this condition. This mitochondrial redistribution into the AIS is not likely due to the shortening of the AIS domain itself but rather due to altered Cdk5 activity. Furthermore, mitochondrial redistribution into the AIS is unlikely to be an early driver of neurodegeneration in the context of reduced Cdk5 activity.

Mechanically induced topological transition of spectrin regulates its distribution in the mammalian cell cortex

The cell cortex is a dynamic assembly formed by the plasma membrane and underlying cytoskeleton. As the main determinant of cell shape, the cortex ensures its integrity during passive and active deformations by adapting cytoskeleton topologies through yet poorly understood mechanisms. The spectrin meshwork ensures such adaptation in erythrocytes and neurons by adopting different organizations. Erythrocytes rely on triangular-like lattices of spectrin tetramers, whereas in neurons they are organized in parallel, periodic arrays. Since spectrin is ubiquitously expressed, we exploited Expansion Microscopy to discover that, in fibroblasts, distinct meshwork densities co-exist. Through biophysical measurements and computational modeling, we show that the non-polarized spectrin meshwork, with the intervention of actomyosin, can dynamically transition into polarized clusters fenced by actin stress fibers that resemble periodic arrays as found in neurons. Clusters experience lower mechanical stress and turnover, despite displaying an extension close to the tetramer contour length. Our study sheds light on the adaptive properties of spectrin, which participates in the protection of the cell cortex by varying its densities in response to key mechanical features.

Neuroprotective effects of Si-based hydrogen-producing agent on 6-hydroxydopamine-induced neurotoxicity in juvenile mouse model

Neurotoxins have been extensively investigated, particularly in the field of neuroscience. They induce toxic damage, oxidative stress, and inflammation on neurons, triggering neuronal dysfunction and neurodegenerative diseases. Here we demonstrate the neuroprotective effect of a silicon (Si)-based hydrogen-producing agent (Si-based agent) in a juvenile neurotoxic mouse model induced by 6-hydroxydopamine (6-OHDA). The Si-based agent produces hydrogen in bowels and functions as an antioxidant and anti-inflammatory agent. However, the effects of the Si-based agent on neural degeneration in areas other than the lesion and behavioral alterations caused by it are largely unknown. Moreover, the neuroprotective effects of Si-based agent in the context of lactation and use during infancy have not been explored in prior studies. In this study, we show the neuroprotective effect of the Si-based agent on 6-OHDA during lactation period and infancy using the mouse model. The Si-based agent safeguards against the degradation and neuronal cell death of dopaminergic neurons and loss of dopaminergic fibers in the striatum (STR) and ventral tegmental area (VTA) caused by 6-OHDA. Furthermore, the Si-based agent exhibits a neuroprotective effect on the length of axon initial segment (AIS) in the layer 2/3 (L2/3) neurons of the medial prefrontal cortex (mPFC). As a result, the Si-based agent mitigates hyperactive behavior in a juvenile neurotoxic mouse model induced by 6-OHDA. These results suggest that the Si-based agent serves as an effective neuroprotectant and antioxidant against neurotoxic effects in the brain, offering the possibility of the Si-based agent as a neuroprotectant for nervous system diseases.

Human iPSC-Derived Neurons with Reliable Synapses and Large Presynaptic Action Potentials

Understanding the function of the human brain requires determining basic properties of synaptic transmission in human neurons. One of the most fundamental parameters controlling neurotransmitter release is the presynaptic action potential, but its amplitude and duration remain controversial. Presynaptic action potentials have so far been measured with high temporal resolution only in a limited number of vertebrate but not in human neurons. To uncover properties of human presynaptic action potentials, we exploited recently developed tools to generate human glutamatergic neurons by transient expression of Neurogenin 2 (Ngn2) in pluripotent stem cells. During maturation for 3 to 9 weeks of culturing in different established media, the proportion of cells with multiple axon initial segments decreased, while the amount of axonal tau protein and neuronal excitability increased. Super-resolution microscopy revealed the alignment of the pre- and postsynaptic proteins, Bassoon and Homer. Synaptic transmission was surprisingly reliable at frequencies of 20, 50, and 100 Hz. The synchronicity of synaptic transmission during high-frequency transmission increased during 9 weeks of neuronal maturation. To analyze the mechanisms of synchronous high-frequency glutamate release, we developed direct presynaptic patch-clamp recordings from human neurons. The presynaptic action potentials had large overshoots to ∼25 mV and short durations of ∼0.5 ms. Our findings show that Ngn2-induced neurons represent an elegant model system allowing for functional, structural, and molecular analyses of glutamatergic synaptic transmission with high spatiotemporal resolution in human neurons. Furthermore, our data predict that glutamatergic transmission is mediated by large and rapid presynaptic action potentials in the human brain.

A Multimodal Fitting Approach to Construct Single-Neuron Models With Patch Clamp and High-Density Microelectrode Arrays

In computational neuroscience, multicompartment models are among the most biophysically realistic representations of single neurons. Constructing such models usually involves the use of the patch-clamp technique to record somatic voltage signals under different experimental conditions. The experimental data are then used to fit the many parameters of the model. While patching of the soma is currently the gold-standard approach to build multicompartment models, several studies have also evidenced a richness of dynamics in dendritic and axonal sections. Recording from the soma alone makes it hard to observe and correctly parameterize the activity of nonsomatic compartments. In order to provide a richer set of data as input to multicompartment models, we here investigate the combination of somatic patch-clamp recordings with recordings of high-density microelectrode arrays (HD-MEAs). HD-MEAs enable the observation of extracellular potentials and neural activity of neuronal compartments at subcellular resolution. In this work, we introduce a novel framework to combine patch-clamp and HD-MEA data to construct multicompartment models. We first validate our method on a ground-truth model with known parameters and show that the use of features extracted from extracellular signals, in addition to intracellular ones, yields models enabling better fits than using intracellular features alone. We also demonstrate our procedure using experimental data by constructing cell models from in vitro cell cultures. The proposed multimodal fitting procedure has the potential to augment the modeling efforts of the computational neuroscience community and provide the field with neuronal models that are more realistic and can be better validated.

Kinesin Regulation in the Proximal Axon is Essential for Dendrite-selective Transport

Neurons are polarized and typically extend multiple dendrites and one axon. To maintain polarity, vesicles carrying dendritic proteins are arrested upon entering the axon. To determine whether kinesin regulation is required for terminating anterograde axonal transport, we overexpressed the dendrite-selective kinesin KIF13A. This caused mistargeting of dendrite-selective vesicles to the axon and a loss of dendritic polarity. Polarity was not disrupted if the kinase MARK2/Par1b was coexpressed. MARK2/Par1b is concentrated in the proximal axon, where it maintains dendritic polarity-likely by phosphorylating S1371 of KIF13A, which lies in a canonical 14-3-3 binding motif. We probed for interactions of KIF13A with 14-3-3 isoforms and found that 14-3-3β and 14-3-3ζ bound KIF13A. Disruption of MARK2 or 14-3-3 activity by small molecule inhibitors caused a loss of dendritic polarity. These data show that kinesin regulation is integral for dendrite-selective transport. We propose a new model in which KIF13A that moves dendrite-selective vesicles in the proximal axon is phosphorylated by MARK2. Phosphorylated KIF13A is then recognized by 14-3-3, which causes dissociation of KIF13A from the vesicle and termination of transport. These findings define a new paradigm for the regulation of vesicle transport by localized kinesin tail phosphorylation, to restrict dendrite-selective vesicles from entering the axon.

ANK3 rs10994336 and ZNF804A rs7597593 polymorphisms: genetic interaction for emotional and behavioral symptoms of alcohol withdrawal syndrome

OBJECTIVE

Alcohol withdrawal syndrome (AWS) is a complex condition associated with alcohol use disorder (AUD), characterized by significant variations in symptom severity among patients. The psychological and emotional symptoms accompanying AWS significantly contribute to withdrawal distress and relapse risk. Despite the importance of neural adaptation processes in AWS, limited genetic investigations have been conducted. This study primarily focuses on exploring the single and interaction effects of single-nucleotide polymorphisms in the ANK3 and ZNF804A genes on anxiety and aggression severity manifested in AWS. By examining genetic associations with withdrawal-related psychopathology, we ultimately aim to advance understanding the genetic underpinnings that modulate AWS severity.

METHODS

The study involved 449 male patients diagnosed with alcohol use disorder. The Self-Rating Anxiety Scale (SAS) and Buss-Perry Aggression Questionnaire (BPAQ) were used to assess emotional and behavioral symptoms related to AWS. Genomic DNA was extracted from peripheral blood, and genotyping was performed using PCR.

RESULTS

Single-gene analysis revealed that naturally occurring allelic variants in ANK3 rs10994336 (CC homozygous vs. T allele carriers) were associated with mood and behavioral symptoms related to AWS. Furthermore, the interaction between ANK3 and ZNF804A was significantly associated with the severity of psychiatric symptoms related to AWS, as indicated by MANOVA. Two-way ANOVA further demonstrated a significant interaction effect between ANK3 rs10994336 and ZNF804A rs7597593 on anxiety, physical aggression, verbal aggression, anger, and hostility. Hierarchical regression analyses confirmed these findings. Additionally, simple effects analysis and multiple comparisons revealed that carriers of the ANK3 rs10994336 T allele experienced more severe AWS, while the ZNF804A rs7597593 T allele appeared to provide protection against the risk associated with the ANK3 rs10994336 mutation.

CONCLUSION

This study highlights the gene-gene interaction between ANK3 and ZNF804A, which plays a crucial role in modulating emotional and behavioral symptoms related to AWS. The ANK3 rs10994336 T allele is identified as a risk allele, while the ZNF804A rs7597593 T allele offers protection against the risk associated with the ANK3 rs10994336 mutation. These findings provide initial support for gene-gene interactions as an explanation for psychiatric risk, offering valuable insights into the pathophysiological mechanisms involved in AWS.

Motor learning changes the axon initial segment of the spinal motoneuron

We are studying the mechanisms of H-reflex operant conditioning, a simple form of learning. Modelling studies in the literature and our previous data suggested that changes in the axon initial segment (AIS) might contribute. To explore this, we used blinded quantitative histological and immunohistochemical methods to study in adult rats the impact of H-reflex conditioning on the AIS of the spinal motoneuron that produces the reflex. Successful, but not unsuccessful, H-reflex up-conditioning was associated with greater AIS length and distance from soma; greater length correlated with greater H-reflex increase. Modelling studies in the literature suggest that these increases may increase motoneuron excitability, supporting the hypothesis that they may contribute to H-reflex increase. Up-conditioning did not affect AIS ankyrin G (AnkG) immunoreactivity (IR), p-p38 protein kinase IR, or GABAergic terminals. Successful, but not unsuccessful, H-reflex down-conditioning was associated with more GABAergic terminals on the AIS, weaker AnkG-IR, and stronger p-p38-IR. More GABAergic terminals and weaker AnkG-IR correlated with greater H-reflex decrease. These changes might potentially contribute to the positive shift in motoneuron firing threshold underlying H-reflex decrease; they are consistent with modelling suggesting that sodium channel change may be responsible. H-reflex down-conditioning did not affect AIS dimensions. This evidence that AIS plasticity is associated with and might contribute to H-reflex conditioning adds to evidence that motor learning involves both spinal and brain plasticity, and both neuronal and synaptic plasticity. AIS properties of spinal motoneurons are likely to reflect the combined influence of all the motor skills that share these motoneurons. KEY POINTS: Neuronal action potentials normally begin in the axon initial segment (AIS). AIS plasticity affects neuronal excitability in development and disease. Whether it does so in learning is unknown. Operant conditioning of a spinal reflex, a simple learning model, changes the rat spinal motoneuron AIS. Successful, but not unsuccessful, H-reflex up-conditioning is associated with greater AIS length and distance from soma. Successful, but not unsuccessful, down-conditioning is associated with more AIS GABAergic terminals, less ankyrin G, and more p-p38 protein kinase. The associations between AIS plasticity and successful H-reflex conditioning are consistent with those between AIS plasticity and functional changes in development and disease, and with those predicted by modelling studies in the literature. Motor learning changes neurons and synapses in spinal cord and brain. Because spinal motoneurons are the final common pathway for behaviour, their AIS properties probably reflect the combined impact of all the behaviours that use these motoneurons.

Antisense oligonucleotides restore excitability, GABA signalling and sodium current density in a Dravet syndrome model

Dravet syndrome is an intractable developmental and epileptic encephalopathy caused by de novo variants in SCN1A resulting in haploinsufficiency of the voltage-gated sodium channel Nav1.1. We showed previously that administration of the antisense oligonucleotide STK-001, also called ASO-22, generated using targeted augmentation of nuclear gene output technology to prevent inclusion of the nonsense-mediated decay, or poison, exon 20N in human SCN1A, increased productive Scn1a transcript and Nav1.1 expression and reduced the incidence of electrographic seizures and sudden unexpected death in epilepsy in a mouse model of Dravet syndrome. Here, we investigated the mechanism of action of ASO-84, a surrogate for ASO-22 that also targets splicing of SCN1A exon 20N, in Scn1a+/- Dravet syndrome mouse brain. Scn1a +/- Dravet syndrome and wild-type mice received a single intracerebroventricular injection of antisense oligonucleotide or vehicle at postnatal Day 2. We examined the electrophysiological properties of cortical pyramidal neurons and parvalbumin-positive fast-spiking interneurons in brain slices at postnatal Days 21-25 and measured sodium currents in parvalbumin-positive interneurons acutely dissociated from postnatal Day 21-25 brain slices. We show that, in untreated Dravet syndrome mice, intrinsic cortical pyramidal neuron excitability was unchanged while cortical parvalbumin-positive interneurons showed biphasic excitability with initial hyperexcitability followed by hypoexcitability and depolarization block. Dravet syndrome parvalbumin-positive interneuron sodium current density was decreased compared to wild-type. GABAergic signalling to cortical pyramidal neurons was reduced in Dravet syndrome mice, suggesting decreased GABA release from interneurons. ASO-84 treatment restored action potential firing, sodium current density and GABAergic signalling in Dravet syndrome parvalbumin-positive interneurons. Our work suggests that interneuron excitability is selectively affected by ASO-84. This new work provides critical insights into the mechanism of action of this antisense oligonucleotide and supports the potential of antisense oligonucleotide-mediated upregulation of Nav1.1 as a successful strategy to treat Dravet syndrome.

Physical and functional convergence of the autism risk genes Scn2a and Ank2 in neocortical pyramidal cell dendrites

Dysfunction in sodium channels and their ankyrin scaffolding partners have both been implicated in neurodevelopmental disorders, including autism spectrum disorder (ASD). In particular, the genes SCN2A, which encodes the sodium channel NaV1.2, and ANK2, which encodes ankyrin-B, have strong ASD association. Recent studies indicate that ASD-associated haploinsufficiency in Scn2a impairs dendritic excitability and synaptic function in neocortical pyramidal cells, but how NaV1.2 is anchored within dendritic regions is unknown. Here, we show that ankyrin-B is essential for scaffolding NaV1.2 to the dendritic membrane of mouse neocortical neurons and that haploinsufficiency of Ank2 phenocopies intrinsic dendritic excitability and synaptic deficits observed in Scn2a+/- conditions. These results establish a direct, convergent link between two major ASD risk genes and reinforce an emerging framework suggesting that neocortical pyramidal cell dendritic dysfunction can contribute to neurodevelopmental disorder pathophysiology.

Non-homogenous axonal bouton distribution in whole-brain single-cell neuronal networks

We examined the distribution of pre-synaptic contacts in axons of mouse neurons and constructed whole-brain single-cell neuronal networks using an extensive dataset of 1,891 fully reconstructed neurons. We found that bouton locations were not homogeneous throughout the axon and among brain regions. As our algorithm was able to generate whole-brain single-cell connectivity matrices from full morphology reconstruction datasets, we further found that non-homogeneous bouton locations have a significant impact on network wiring, including degree distribution, triad census, and community structure. By perturbing neuronal morphology, we further explored the link between anatomical details and network topology. In our in silico exploration, we found that dendritic and axonal tree span would have the greatest impact on network wiring, followed by synaptic contact deletion. Our results suggest that neuroanatomical details must be carefully addressed in studies of whole-brain networks at the single-cell level.

Analysis of the effect of the scorpion toxin AaH-II on action potential generation in the axon initial segment

The toxin AaH-II, from the scorpion Androctonus australis Hector venom, is a 64 amino acid peptide that targets voltage-gated Na+ channels (VGNCs) and slows their inactivation. While at macroscopic cellular level AaH-II prolongs the action potential (AP), a functional analysis of the effect of the toxin in the axon initial segment (AIS), where VGNCs are highly expressed, was never performed so far. Here, we report an original analysis of the effect of AaH-II on the AP generation in the AIS of neocortical layer-5 pyramidal neurons from mouse brain slices. After determining that AaH-II does not discriminate between Nav1.2 and Nav1.6, i.e. between the two VGNC isoforms expressed in this neuron, we established that 7 nM was the smallest toxin concentration producing a minimal detectable deformation of the somatic AP after local delivery of the toxin. Using membrane potential imaging, we found that, at this minimal concentration, AaH-II substantially widened the AP in the AIS. Using ultrafast Na+ imaging, we found that local application of 7 nM AaH-II caused a large increase in the slower component of the Na+ influx in the AIS. Finally, using ultrafast Ca2+ imaging, we observed that 7 nM AaH-II produces a spurious slow Ca2+ influx via Ca2+-permeable VGNCs. Molecules targeting VGNCs, including peptides, are proposed as potential therapeutic tools. Thus, the present analysis in the AIS can be considered a general proof-of-principle on how high-resolution imaging techniques can disclose drug effects that cannot be observed when tested at the macroscopic level.

Changes in mitochondrial distribution occur at the axon initial segment in association with neurodegeneration in Drosophila

Changes in mitochondrial distribution are a feature of numerous age-related neurodegenerative diseases. In Drosophila, reducing the activity of Cdk5 causes a neurodegenerative phenotype and is known to affect several mitochondrial properties. Therefore, we investigated whether alterations of mitochondrial distribution are involved in Cdk5-associated neurodegeneration. We find that reducing Cdk5 activity does not alter the balance of mitochondrial localization to the somatodendritic vs. axonal neuronal compartments of the mushroom body, the learning and memory center of the Drosophila brain. We do, however, observe changes in mitochondrial distribution at the axon initial segment (AIS), a neuronal compartment located in the proximal axon involved in neuronal polarization and action potential initiation. Specifically, we observe that mitochondria are partially excluded from the AIS in wild-type neurons, but that this exclusion is lost upon reduction of Cdk5 activity, concomitant with the shrinkage of the AIS domain that is known to occur in this condition. This mitochondrial redistribution into the AIS is not likely due to the shortening of the AIS domain itself but rather due to altered Cdk5 activity. Furthermore, mitochondrial redistribution into the AIS is unlikely to be an early driver of neurodegeneration in the context of reduced Cdk5 activity.

The bone transcription factor Osterix controls extracellular matrix- and node of Ranvier-related gene expression in oligodendrocytes

Oligodendrocytes are the primary producers of many extracellular matrix (ECM)-related proteins found in the CNS. Therefore, oligodendrocytes play a critical role in the determination of brain stiffness, node of Ranvier formation, perinodal ECM deposition, and perineuronal net formation, all of which depend on the ECM. Nevertheless, the transcription factors that control ECM-related gene expression in oligodendrocytes remain unknown. Here, we found that the transcription factor Osterix (also known as Sp7) binds in proximity to genes important for CNS ECM and node of Ranvier formation and mediates their expression. Oligodendrocyte-specific ablation of Sp7 changes ECM composition and brain stiffness and results in aberrant node of Ranvier formation. Sp7 is known to control osteoblast maturation and bone formation. Our comparative analyses suggest that Sp7 plays a conserved biological role in oligodendrocytes and in bone-forming cells, where it mediates brain and bone tissue stiffness by controlling expression of ECM components.

Current perspectives on microglia-neuron communication in the central nervous system: Direct and indirect modes of interaction

BACKGROUND

The incessant communication that takes place between microglia and neurons is essential the development, maintenance, and pathogenesis of the central nervous system (CNS). As mobile phagocytic cells, microglia serve a critical role in surveilling and scavenging the neuronal milieu to uphold homeostasis.

AIM OF REVIEW

This review aims to discuss the various mechanisms that govern the interaction between microglia and neurons, from the molecular to the organ system level, and to highlight the importance of these interactions in the development, maintenance, and pathogenesis of the CNS.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Recent research has revealed that microglia-neuron interaction is vital for regulating fundamental neuronal functions, such as synaptic pruning, axonal remodeling, and neurogenesis. The review will elucidate the intricate signaling pathways involved in these interactions, both direct and indirect, to provide a better understanding of the fundamental mechanisms of brain function. Furthermore, gaining insights into these signals could lead to the development of innovative therapies for neural disorders.

Immunoproximity biotinylation reveals the axon initial segment proteome

The axon initial segment (AIS) is a specialized neuronal compartment required for action potential generation and neuronal polarity. However, understanding the mechanisms regulating AIS structure and function has been hindered by an incomplete knowledge of its molecular composition. Here, using immuno-proximity biotinylation we further define the AIS proteome and its dynamic changes during neuronal maturation. Among the many AIS proteins identified, we show that SCRIB is highly enriched in the AIS both in vitro and in vivo, and exhibits a periodic architecture like the axonal spectrin-based cytoskeleton. We find that ankyrinG interacts with and recruits SCRIB to the AIS. However, loss of SCRIB has no effect on ankyrinG. This powerful and flexible approach further defines the AIS proteome and provides a rich resource to elucidate the mechanisms regulating AIS structure and function.

Fibroblast growth factor 13-mediated regulation of medium spiny neuron excitability and cocaine self-administration

Cocaine use disorder (CUD) is a prevalent neuropsychiatric disorder with few existing treatments. Thus, there is an unmet need for the identification of new pharmacological targets for CUD. Previous studies using environmental enrichment versus isolation paradigms have found that the latter induces increased cocaine self-administration with correlative increases in the excitability of medium spiny neurons (MSN) of the nucleus accumbens shell (NAcSh). Expanding upon these findings, we sought in the present investigation to elucidate molecular determinants of these phenomena. To that end, we first employed a secondary transcriptomic analysis and found that cocaine self-administration differentially regulates mRNA for fibroblast growth factor 13 (FGF13), which codes for a prominent auxiliary protein of the voltage-gated Na+ (Nav) channel, in the NAcSh of environmentally enriched rats (i.e., resilient behavioral phenotype) compared to environmentally isolated rats (susceptible phenotype). Based upon this finding, we used in vivo genetic silencing to study the causal functional and behavioral consequences of knocking down FGF13 in the NAcSh. Functional studies revealed that knockdown of FGF13 in the NAcSh augmented excitability of MSNs by increasing the activity of Nav channels. These electrophysiological changes were concomitant with a decrease in cocaine demand elasticity (i.e., susceptible phenotype). Taken together, these data support FGF13 as being protective against cocaine self-administration, which positions it well as a pharmacological target for CUD.

SPTBN1 Mediates the Cytoplasmic Constraint of PTTG1, Impairing Its Oncogenic Activity in Human Seminoma

Seminoma is the most common testicular cancer. Pituitary tumor-transforming gene 1 (PTTG1) is a securin showing oncogenic activity in several tumors. We previously demonstrated that nuclear PTTG1 promotes seminoma tumor invasion through its transcriptional activity on matrix metalloproteinase 2 (MMP-2) and E-cadherin (CDH1). We wondered if specific interactors could affect its subcellular distribution. To this aim, we investigated the PTTG1 interactome in seminoma cell lines showing different PTTG1 nuclear levels correlated with invasive properties. A proteomic approach upon PTTG1 immunoprecipitation uncovered new specific securin interactors. Western blot, confocal microscopy, cytoplasmic/nuclear fractionation, sphere-forming assay, and Atlas database interrogation were performed to validate the proteomic results and to investigate the interplay between PTTG1 and newly uncovered partners. We observed that spectrin beta-chain (SPTBN1) and PTTG1 were cofactors, with SPTBN1 anchoring the securin in the cytoplasm. SPTBN1 downregulation determined PTTG1 nuclear translocation, promoting its invasive capability. Moreover, a PTTG1 deletion mutant lacking SPTBN1 binding was strongly localized in the nucleus. The Atlas database revealed that seminomas that contained higher nuclear PTTG1 levels showed significantly lower SPTBN1 levels in comparison to non-seminomas. In human seminoma specimens, we found a strong PTTG1/SPTBN1 colocalization that decreases in areas with nuclear PTTG1 distribution. Overall, these results suggest that SPTBN1, along with PTTG1, is a potential prognostic factor useful in the clinical management of seminoma.

Thalamic Neuron Resilience during Osmotic Demyelination Syndrome (ODS) Is Revealed by Primary Cilium Outgrowth and ADP-ribosylation factor-like protein 13B Labeling in Axon Initial Segment

A murine osmotic demyelinating syndrome (ODS) model was developed through chronic hyponatremia, induced by desmopressin subcutaneous implants, followed by precipitous sodium restoration. The thalamic ventral posterolateral (VPL) and ventral posteromedial (VPM) relay nuclei were the most demyelinated regions where neuroglial damage could be evidenced without immune response. This report showed that following chronic hyponatremia, 12 h and 48 h time lapses after rebalancing osmolarity, amid the ODS-degraded outskirts, some resilient neuronal cell bodies built up primary cilium and axon hillock regions that extended into axon initial segments (AIS) where ADP-ribosylation factor-like protein 13B (ARL13B)-immunolabeled rod-like shape content was revealed. These AIS-labeled shaft lengths appeared proportional with the distance of neuronal cell bodies away from the ODS damaged epicenter and time lapses after correction of hyponatremia. Fine structure examination verified these neuron abundant transcriptions and translation regions marked by the ARL13B labeling associated with cell neurotubules and their complex cytoskeletal macromolecular architecture. This necessitated energetic transport to organize and restore those AIS away from the damaged ODS core demyelinated zone in the murine model. These labeled structures could substantiate how thalamic neuron resilience occurred as possible steps of a healing course out of ODS.

Self-Aggregating Tau Fragments Recapitulate Pathologic Phenotypes and Neurotoxicity of Alzheimer's Disease in Mice

In tauopathy conditions, such as Alzheimer's disease (AD), highly soluble and natively unfolded tau polymerizes into an insoluble filament; however, the mechanistic details of this process remain unclear. In the brains of AD patients, only a minor segment of tau forms β-helix-stacked protofilaments, while its flanking regions form disordered fuzzy coats. Here, it is demonstrated that the tau AD nucleation core (tau-AC) sufficiently induced self-aggregation and recruited full-length tau to filaments. Unexpectedly, phospho-mimetic forms of tau-AC (at Ser324 or Ser356) show markedly reduced oligomerization and seeding propensities. Biophysical analysis reveal that the N-terminus of tau-AC facilitates the fibrillization kinetics as a nucleation motif, which becomes sterically shielded through phosphorylation-induced conformational changes in tau-AC. Tau-AC oligomers are efficiently internalized into cells via endocytosis and induced endogenous tau aggregation. In primary hippocampal neurons, tau-AC impaired axon initial segment plasticity upon chronic depolarization and is mislocalized to the somatodendritic compartments. Furthermore, it is observed significantly impaired memory retrieval in mice intrahippocampally injected with tau-AC fibrils, which corresponds to the neuropathological staining and neuronal loss in the brain. These findings identify tau-AC species as a key neuropathological driver in AD, suggesting novel strategies for therapeutic intervention.

Cdk12 maintains the integrity of adult axons by suppressing actin remodeling

The role of cyclin-dependent kinases (CDKs) that are ubiquitously expressed in the adult nervous system remains unclear. Cdk12 is enriched in terminally differentiated neurons where its conical role in the cell cycle progression is redundant. We find that in adult neurons Cdk12 acts a negative regulator of actin formation, mitochondrial dynamics and neuronal physiology. Cdk12 maintains the size of the axon at sites proximal to the cell body through the transcription of homeostatic enzymes in the 1-carbon by folate pathway which utilize the amino acid homocysteine. Loss of Cdk12 leads to elevated homocysteine and in turn leads to uncontrolled F-actin formation and axonal swelling. Actin remodeling further induces Drp1-dependent fission of mitochondria and the breakdown of axon-soma filtration barrier allowing soma restricted cargos to enter the axon. We demonstrate that Cdk12 is also an essential gene for long-term neuronal survival and loss of this gene causes age-dependent neurodegeneration. Hyperhomocysteinemia, actin changes, and mitochondrial fragmentation are associated with several neurodegenerative conditions such as Alzheimer's disease and we provide a candidate molecular pathway to link together such pathological events.

Projection-Specific Heterogeneity of the Axon Initial Segment of Pyramidal Neurons in the Prelimbic Cortex

The axon initial segment (AIS) is a highly specialized axonal compartment where the action potential is initiated. The heterogeneity of AISs has been suggested to occur between interneurons and pyramidal neurons (PyNs), which likely contributes to their unique spiking properties. However, whether the various characteristics of AISs can be linked to specific PyN subtypes remains unknown. Here, we report that in the prelimbic cortex (PL) of the mouse, two types of PyNs with axon projections either to the contralateral PL or to the ipsilateral basal lateral amygdala, possess distinct AIS properties reflected by morphology, ion channel expression, action potential initiation, and axo-axonic synaptic inputs from chandelier cells. Furthermore, projection-specific AIS diversity is more prominent in the superficial layer than in the deep layer. Thus, our study reveals the cortical layer- and axon projection-specific heterogeneity of PyN AISs, which may endow the spiking of various PyN types with exquisite modulation.

Structural plasticity of axon initial segment in spinal cord neurons underlies inflammatory pain

ABSTRACT

Physiological or pathology-mediated changes in neuronal activity trigger structural plasticity of the action potential generation site-the axon initial segment (AIS). These changes affect intrinsic neuronal excitability, thus tuning neuronal and overall network output. Using behavioral, immunohistochemical, electrophysiological, and computational approaches, we characterized inflammation-related AIS plasticity in rat's superficial (lamina II) spinal cord dorsal horn (SDH) neurons and established how AIS plasticity regulates the activity of SDH neurons, thus contributing to pain hypersensitivity. We show that in naive conditions, AIS in SDH inhibitory neurons is located closer to the soma than in excitatory neurons. Shortly after inducing inflammation, when the inflammatory hyperalgesia is at its peak, AIS in inhibitory neurons is shifted distally away from the soma. The shift in AIS location is accompanied by the decrease in excitability of SDH inhibitory neurons. These AIS location and excitability changes are selective for inhibitory neurons and reversible. We show that AIS shift back close to the soma, and SDH inhibitory neurons' excitability increases to baseline levels following recovery from inflammatory hyperalgesia. The computational model of SDH inhibitory neurons predicts that the distal shift of AIS is sufficient to decrease the intrinsic excitability of these neurons. Our results provide evidence of inflammatory pain-mediated AIS plasticity in the central nervous system, which differentially affects the excitability of inhibitory SDH neurons and contributes to inflammatory hyperalgesia.

Neurofilament accumulations in amyotrophic lateral sclerosis patients' motor neurons impair axonal initial segment integrity

Amyotrophic lateral sclerosis (ALS) is the most common motor neuron (MN) disease in adults with no curative treatment. Neurofilament (NF) level in patient' fluids have recently emerged as the prime biomarker of ALS disease progression, while NF accumulation in MNs of patients is the oldest and one of the best pathological hallmarks. However, the way NF accumulations could lead to MN degeneration remains unknown. To assess NF accumulations and study the impact on MNs, we compared MNs derived from induced pluripotent stem cells (iPSC) of patients carrying mutations in C9orf72, SOD1 and TARDBP genes, the three main ALS genetic causes. We show that in all mutant MNs, light NF (NF-L) chains rapidly accumulate in MN soma, while the phosphorylated heavy/medium NF (pNF-M/H) chains pile up in axonal proximal regions of only C9orf72 and SOD1 MNs. Excitability abnormalities were also only observed in these latter MNs. We demonstrate that the integrity of the MN axonal initial segment (AIS), the region of action potential initiation and responsible for maintaining axonal integrity, is impaired in the presence of pNF-M/H accumulations in C9orf72 and SOD1 MNs. We establish a strong correlation between these pNF-M/H accumulations, an AIS distal shift, increased axonal calibers and modified repartition of sodium channels. The results expand our understanding of how NF accumulation could dysregulate components of the axonal cytoskeleton and disrupt MN homeostasis. With recent cumulative evidence that AIS alterations are implicated in different brain diseases, preserving AIS integrity could have important therapeutic implications for ALS.

Microfluidic Platforms Promote Polarization of Human-Derived Retinal Ganglion Cells That Model Axonopathy

Purpose

Axons depend on long-range transport of proteins and organelles which increases susceptibility to metabolic stress in disease. The axon initial segment (AIS) is particularly vulnerable due to the high bioenergetic demand of action potential generation. Here, we prepared retinal ganglion cells derived from human embryonic stem cells (hRGCs) to probe how axonal stress alters AIS morphology.

Methods

hRGCs were cultured on coverslips or microfluidic platforms. We assayed AIS specification and morphology by immunolabeling against ankyrin G (ankG), an axon-specific protein, and postsynaptic density 95 (PSD-95), a dendrite-specific protein. Using microfluidic platforms that enable fluidic isolation, we added colchicine to the axon compartment to lesion axons. We verified axonopathy by measuring the anterograde axon transport of cholera toxin subunit B and immunolabeling against cleaved caspase 3 (CC3) and phosphorylated neurofilament H (SMI-34). We determined the influence of axon injury on AIS morphology by immunolabeling samples against ankG and measuring AIS distance from soma and length.

Results

Based on measurements of ankG and PSD-95 immunolabeling, microfluidic platforms promote the formation and separation of distinct somatic-dendritic versus axonal compartments in hRGCs compared to coverslip cultures. Chemical lesioning of axons by colchicine reduced hRGC anterograde axon transport, increased varicosity density, and enhanced expression of CC3 and SMI-34. Interestingly, we found that colchicine selectively affected hRGCs with axon-carrying dendrites by reducing AIS distance from somas and increasing length, thus suggesting reduced capacity to maintain excitability.

Conclusions

Thus, microfluidic platforms promote polarized hRGCs that enable modeling of axonopathy.

Translational Relevance

Microfluidic platforms may be used to assay compartmentalized degeneration that occurs during glaucoma.

Retinal ganglion cells adapt to ionic stress in experimental glaucoma

Introduction

Identification of early adaptive and maladaptive neuronal stress responses is an important step in developing targeted neuroprotective therapies for degenerative disease. In glaucoma, retinal ganglion cells (RGCs) and their axons undergo progressive degeneration resulting from stress driven by sensitivity to intraocular pressure (IOP). Despite therapies that can effectively manage IOP many patients progress to vision loss, necessitating development of neuronal-based therapies. Evidence from experimental models of glaucoma indicates that early in the disease RGCs experience altered excitability and are challenged with dysregulated potassium (K+) homeostasis. Previously we demonstrated that certain RGC types have distinct excitability profiles and thresholds for depolarization block, which are associated with sensitivity to extracellular K+.

Methods

Here, we used our inducible mouse model of glaucoma to investigate how RGC sensitivity to K+ changes with exposure to elevated IOP.

Results

In controls, conditions of increased K+ enhanced membrane depolarization, reduced action potential generation, and widened action potentials. Consistent with our previous work, 4 weeks of IOP elevation diminished RGC light-and current-evoked responses. Compared to controls, we found that IOP elevation reduced the effects of increased K+ on depolarization block threshold, with IOP-exposed cells maintaining greater excitability. Finally, IOP elevation did not alter axon initial segment dimensions, suggesting that structural plasticity alone cannot explain decreased K+ sensitivity.

Discussion

Thus, in response to prolonged IOP elevation RGCs undergo an adaptive process that reduces sensitivity to changes in K+ while diminishing excitability. These experiments give insight into the RGC response to IOP stress and lay the groundwork for mechanistic investigation into targets for neuroprotective therapy.

The NG2-glia is a potential target to maintain the integrity of neurovascular unit after acute ischemic stroke

The neurovascular unit (NVU) plays a critical role in health and disease. In the current review, we discuss the critical role of a class of neural/glial antigen 2 (NG2)-expressing glial cells (NG2-glia) in regulating NVU after acute ischemic stroke (AIS). We first introduce the role of NG2-glia in the formation of NVU during development as well as aging-induced damage to NVU and accompanying NG2-glia change. We then discuss the reciprocal interactions between NG2-glia and the other component cells of NVU, emphasizing the factors that could influence NG2-glia. Damage to the NVU integrity is the pathological basis of edema and hemorrhagic transformation, the most dreaded complication after AIS. The role of NG2-glia in AIS-induced NVU damage and the effect of NG2-glia transplantation on AIS-induced NVU damage are summarized. We next discuss the role of NG2-glia and the effect of NG2-glia transplantation in oligodendrogenesis and white matter repair as well as angiogenesis which is associated with the outcome of the patients after AIS. Finally, we review the current strategies to promote NG2-glia proliferation and differentiation and propose to use the dental pulp stem cells (DPSC)-derived exosome as a promising strategy to reduce AIS-induced injury and promote repair through maintaining the integrity of NVU by regulating endogenous NG2-glia proliferation and differentiation.

The Amyloid Precursor Protein Modulates the Position and Length of the Axon Initial Segment

The amyloid precursor protein (APP) is linked to the genetics and pathogenesis of Alzheimer's disease (AD). It is the parent protein of the β-amyloid (Aβ) peptide, the main constituent of the amyloid plaques found in an AD brain. The pathways from APP to Aβ are intensively studied, yet the normal functions of APP itself have generated less interest. We report here that glutamate stimulation of neuronal activity leads to a rapid increase in App gene expression. In mouse and human neurons, elevated APP protein changes the structure of the axon initial segment (AIS) where action potentials are initiated. The AIS is shortened in length and shifts away from the cell body. The GCaMP8f Ca2+ reporter confirms the predicted decrease in neuronal activity. NMDA antagonists or knockdown of App block the glutamate effects. The actions of APP on the AIS are cell-autonomous; exogenous Aβ, either fibrillar or oligomeric, has no effect. In culture, APPSwe (a familial AD mutation) induces larger AIS changes than wild type APP. Ankyrin G and βIV-spectrin, scaffolding proteins of the AIS, both physically associate with APP, more so in AD brains. Finally, in humans with sporadic AD or in the R1.40 AD mouse model, both females and males, neurons have elevated levels of APP protein that invade the AIS. In vivo as in vitro, this increased APP is associated with a significant shortening of the AIS. The findings outline a new role for the APP and encourage a reconsideration of its relationship to AD.SIGNIFICANCE STATEMENT While the amyloid precursor protein (APP) has long been associated with Alzheimer's disease (AD), the normal functions of the full-length Type I membrane protein have been largely unexplored. We report here that the levels of APP protein increase with neuronal activity. In vivo and in vitro, modest amounts of excess APP alter the properties of the axon initial segment. The β-amyloid peptide derived from APP is without effect. Consistent with the observed changes in the axon initial segment which would be expected to decrease action potential firing, we show that APP expression depresses neuronal activity. In mouse AD models and human sporadic AD, APP physically associates with the scaffolding proteins of the axon initial segment, suggesting a relationship with AD dementia.

Early life adversity across different cell- types in the brain

Early life adversity (ELA)- which includes physical, psychological, emotional, and sexual abuse is one of the most common predictors to diverse psychopathologies later in adulthood. As ELA has a lasting impact on the brain at a developmental stage, recent findings from the field highlighted the specific contributions of different cell types to ELA and their association with long lasting consequences. In this review we will gather recent findings describing morphological, transcriptional and epigenetic alterations within neurons, glia and perineuronal nets and their associated cellular subpopulation. The findings reviewed and summarized here highlight important mechanisms underlying ELA and point to therapeutic approaches for ELA and related psychopathologies later in life.

Heparan Sulfates Regulate Axonal Excitability and Context Generalization through Ca2+/Calmodulin-Dependent Protein Kinase II

Our previous studies demonstrated that enzymatic removal of highly sulfated heparan sulfates with heparinase 1 impaired axonal excitability and reduced expression of ankyrin G at the axon initial segments in the CA1 region of the hippocampus ex vivo, impaired context discrimination in vivo, and increased Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) activity in vitro. Here, we show that in vivo delivery of heparinase 1 in the CA1 region of the hippocampus elevated autophosphorylation of CaMKII 24 h after injection in mice. Patch clamp recording in CA1 neurons revealed no significant heparinase effects on the amplitude or frequency of miniature excitatory and inhibitory postsynaptic currents, while the threshold for action potential generation was increased and fewer spikes were generated in response to current injection. Delivery of heparinase on the next day after contextual fear conditioning induced context overgeneralization 24 h after injection. Co-administration of heparinase with the CaMKII inhibitor (autocamtide-2-related inhibitory peptide) rescued neuronal excitability and expression of ankyrin G at the axon initial segment. It also restored context discrimination, suggesting the key role of CaMKII in neuronal signaling downstream of heparan sulfate proteoglycans and highlighting a link between impaired CA1 pyramidal cell excitability and context generalization during recall of contextual memories.

Impaired OTUD7A-dependent Ankyrin regulation mediates neuronal dysfunction in mouse and human models of the 15q13.3 microdeletion syndrome

Copy number variations (CNVs) are associated with psychiatric and neurodevelopmental disorders (NDDs), and most, including the recurrent 15q13.3 microdeletion disorder, have unknown disease mechanisms. We used a heterozygous 15q13.3 microdeletion mouse model and patient iPSC-derived neurons to reveal developmental defects in neuronal maturation and network activity. To identify the underlying molecular dysfunction, we developed a neuron-specific proximity-labeling proteomics (BioID2) pipeline, combined with patient mutations, to target the 15q13.3 CNV genetic driver OTUD7A. OTUD7A is an emerging independent NDD risk gene with no known function in the brain, but has putative deubiquitinase function. The OTUD7A protein-protein interaction network included synaptic, axonal, and cytoskeletal proteins and was enriched for ASD and epilepsy risk genes (Ank3, Ank2, SPTAN1, SPTBN1). The interactions between OTUD7A and Ankyrin-G (Ank3) and Ankyrin-B (Ank2) were disrupted by an epilepsy-associated OTUD7A L233F variant. Further investigation of Ankyrin-G in mouse and human 15q13.3 microdeletion and OTUD7A^L233F/L233F models revealed protein instability, increased polyubiquitination, and decreased levels in the axon initial segment, while structured illumination microscopy identified reduced Ankyrin-G nanodomains in dendritic spines. Functional analysis of human 15q13.3 microdeletion and OTUD7A^L233F/L233F models revealed shared and distinct impairments to axonal growth and intrinsic excitability. Importantly, restoring OTUD7A or Ankyrin-G expression in 15q13.3 microdeletion neurons led to a reversal of abnormalities. These data reveal a critical OTUD7A-Ankyrin pathway in neuronal development, which is impaired in the 15q13.3 microdeletion syndrome, leading to neuronal dysfunction. Furthermore, our study highlights the utility of targeting CNV genes using cell type-specific proteomics to identify shared and unexplored disease mechanisms across NDDs.

Mechanically induced topological transition of spectrin regulates its distribution in the mammalian cortex

The cell cortex is a dynamic assembly that ensures cell integrity during passive deformation or active response by adapting cytoskeleton topologies with poorly understood mechanisms. The spectrin meshwork ensures such adaptation in erythrocytes and neurons. Erythrocytes rely on triangular-like lattices of spectrin tetramers, which in neurons are organized in periodic arrays. We exploited Expansion Microscopy to discover that these two distinct topologies can co-exist in other mammalian cells such as fibroblasts. We show through biophysical measurements and computational modeling that spectrin provides coverage of the cortex and, with the intervention of actomyosin, erythroid-like lattices can dynamically transition into condensates resembling neuron-like periodic arrays fenced by actin stress fibers. Spectrin condensates experience lower mechanical stress and turnover despite displaying an extension close to the contour length of the tetramer. Our study sheds light on the adaptive properties of spectrin, which ensures protection of the cortex by undergoing mechanically induced topological transitions.

Composition, organization and mechanisms of the transition zone, a gate for the cilium

The cilium evolved to provide the ancestral eukaryote with the ability to move and sense its environment. Acquiring these functions required the compartmentalization of a dynein-based motility apparatus and signaling proteins within a discrete subcellular organelle contiguous with the cytosol. Here, we explore the potential molecular mechanisms for how the proximal-most region of the cilium, termed transition zone (TZ), acts as a diffusion barrier for both membrane and soluble proteins and helps to ensure ciliary autonomy and homeostasis. These include a unique complement and spatial organization of proteins that span from the microtubule-based axoneme to the ciliary membrane; a protein picket fence; a specialized lipid microdomain; differential membrane curvature and thickness; and lastly, a size-selective molecular sieve. In addition, the TZ must be permissive for, and functionally integrates with, ciliary trafficking systems (including intraflagellar transport) that cross the barrier and make the ciliary compartment dynamic. The quest to understand the TZ continues and promises to not only illuminate essential aspects of human cell signaling, physiology, and development, but also to unravel how TZ dysfunction contributes to ciliopathies that affect multiple organ systems, including eyes, kidney, and brain.

Pleiotropic Ankyrins: Scaffolds for Ion Channels and Transporters

The ankyrin proteins (Ankyrin-R, Ankyrin-B, and Ankyrin-G) are a family of scaffolding, or membrane adaptor proteins necessary for the regulation and targeting of several types of ion channels and membrane transporters throughout the body. These include voltage-gated sodium, potassium, and calcium channels in the nervous system, heart, lungs, and muscle. At these sites, ankyrins recruit ion channels, and other membrane proteins, to specific subcellular domains, which are then stabilized through ankyrin's interaction with the submembranous spectrin-based cytoskeleton. Several recent studies have expanded our understanding of both ankyrin expression and their ion channel binding partners. This review provides an updated overview of ankyrin proteins and their known channel and transporter interactions. We further discuss several potential avenues of future research that would expand our understanding of these important organizational proteins.

The mammalian nodal action potential: new data bring new perspectives

Since its discovery in the mid-20th century, the Hodgkin-Huxley biophysical model of the squid giant axon's (SGA's) neurophysiology has traditionally served as the basis for the teaching of action potential (AP) dynamics in the physiology classroom. This model teaches that leak conductances set membrane resting potential; that fast, inactivating, voltage-gated sodium channels effect the SGA AP upstroke; and that delayed, rectifying, noninactivating voltage-gated potassium channels carry AP repolarization and the early part of the afterhyperpolarization (AHP). This model serves well to introduce students to the fundamental ideas of resting potential establishment and maintenance, as well as basic principles of AP generation and propagation. Furthermore, the Hodgkin-Huxley SGA model represents an excellent and accessible starting point for discussion of the concept of AP threshold and the role of passive electrical properties of the neuron. Additionally, the introduction of the Hodgkin-Huxley model of the SGA AP permits the integration of physiological principles, as instructors ask students to apply previously studied principles of transporter and channel biophysics to the essential physiological phenomenon of electrical signal conduction. However, both some early observations as well as more recent evidence strongly suggest that this seminal invertebrate model of AP dynamics does not appropriately capture the full story for mammalian axons. We review recent evidence that mammalian axonal nodes of Ranvier repolarize largely (though not exclusively) through the activity of leak potassium-ion (K⁺) conductances carried through two-pore domain (K_2P) channels. We call for changes to physiology textbooks and curricula to highlight this remarkable difference in invertebrate and mammalian AP repolarization mechanisms.NEW & NOTEWORTHY Historically, physiology courses have typically taught that action potential repolarization occurs exclusively due to the activation of delayed-rectifier voltage-gated potassium channels. Here, we review and highlight recent evidence that leak potassium channels of the two-pore domain (K_2P) class may largely serve this repolarization role at mammalian nodes of Ranvier. We call for the inclusion of these ideas in physiology curricula at all levels, from high school to graduate school.

Patient-derived antibodies reveal the subcellular distribution and heterogeneous interactome of LGI1

Autoantibodies against leucine-rich glioma-inactivated 1 (LGI1) occur in patients with encephalitis who present with frequent focal seizures and a pattern of amnesia consistent with focal hippocampal damage. To investigate whether the cellular and subcellular distribution of LGI1 may explain the localization of these features, and hence gain broader insights into LGI1's neurobiology, we analysed the detailed localization of LGI1 and the diversity of its protein interactome, in mouse brains using patient-derived recombinant monoclonal LGI1 antibodies. Combined immunofluorescence and mass spectrometry analyses showed that LGI1 is enriched in excitatory and inhibitory synaptic contact sites, most densely within CA3 regions of the hippocampus. LGI1 is secreted in both neuronal somatodendritic and axonal compartments, and occurs in oligodendrocytic, neuro-oligodendrocytic and astro-microglial protein complexes. Proteomic data support the presence of LGI1-Kv1-MAGUK complexes, but did not reveal LGI1 complexes with postsynaptic glutamate receptors. Our results extend our understanding of regional, cellular and subcellular LGI1 expression profiles and reveal novel LGI1-associated complexes, thus providing insights into the complex biology of LGI1 and its relationship to seizures and memory loss.

Pathology of Initial Axon Segments in Chronic Inflammatory Demyelinating Polyradiculoneuropathy and Related Disorders

The diagnosis of chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) is based on a combination of clinical, electrodiagnostic and laboratory features. The different entities of the disease include chronic immune sensory polyradiculopathy (CISP) and autoimmune nodopathies. It is debatable whether CIDP occurring in the course of other conditions, i.e., monoclonal IgG or IgA gammopathy, should be treated as a separate disease entity from idiopathic CIDP. This study aims to evaluate the molecular differences of the nodes of Ranvier and the initial axon segment (AIS) and juxtaparanode region (JXP) as the potential cause of phenotypic variation of CIDP while also seeking new pathomechanisms since JXP is sequestered behind the paranode and autoantibodies may not access the site easily. The authors initially present the structure of the different parts of the neuron and its functional significance, then discuss the problem of whether damage to the juxtaparanodal region, Schwann cells and axons could cause CIDP or if these damages should be separated as separate disease entities. In particular, AIS's importance for modulating neural excitability and carrying out transport along the axon is highlighted. The disclosure of specific pathomechanisms, including novel target antigens, in the heterogeneous CIDP syndrome is important for diagnosing and treating these patients.

Alterations of the axon initial segment in multiple sclerosis grey matter

Grey matter damage has been established as a key contributor to disability progression in multiple sclerosis. Aside from neuronal loss and axonal transections, which predominate in cortical demyelinated lesions, synaptic alterations have been detected in both demyelinated plaques and normal-appearing grey matter, resulting in functional neuronal damage. The axon initial segment is a key element of neuronal function, responsible for action potential initiation and maintenance of neuronal polarity. Despite several reports of profound axon initial segment alterations in different pathological models, among which experimental auto-immune encephalomyelitis, whether the axon initial segment is affected in multiple sclerosis is still unknown. Using immunohistochemistry, we analysed axon initial segments from control and multiple sclerosis tissue, focusing on layer 5/6 pyramidal neurons in the neocortex and Purkinje cells in the cerebellum and performed analysis on the parameters known to control neuronal excitability, i.e. axon initial segment length and position. We found that the axon initial segment length was increased only in pyramidal neurons of inactive demyelinated lesions, compared with normal appearing grey matter tissue. In contrast, in both cell types, the axon initial segment position was altered, with an increased soma-axon initial segment gap, in both active and inactive demyelinated lesions. In addition, using a computational model, we show that this increased gap between soma and axon initial segment might increase neuronal excitability. Taken together, these results show, for the first time, changes of axon initial segments in multiple sclerosis, in active as well as inactive grey matter lesions in both neocortex and cerebellum, which might alter neuronal function.

Axon Initial Segments Are Required for Efficient Motor Neuron Axon Regeneration and Functional Recovery of Synapses

The axon initial segment (AIS) generates action potentials and maintains neuronal polarity by regulating the differential trafficking and distribution of proteins, transport vesicles, and organelles. Injury and disease can disrupt the AIS, and the subsequent loss of clustered ion channels and polarity mechanisms may alter neuronal excitability and function. However, the impact of AIS disruption on axon regeneration after injury is unknown. We generated male and female mice with AIS-deficient multipolar motor neurons by deleting AnkyrinG, the master scaffolding protein required for AIS assembly and maintenance. We found that after nerve crush, neuromuscular junction reinnervation was significantly delayed in AIS-deficient motor neurons compared with control mice. In contrast, loss of AnkyrinG from pseudo-unipolar sensory neurons did not impair axon regeneration into the intraepidermal nerve fiber layer. Even after AIS-deficient motor neurons reinnervated the neuromuscular junction, they failed to functionally recover because of reduced synaptic vesicle protein 2 at presynaptic terminals. In addition, mRNA trafficking was disrupted in AIS-deficient axons. Our results show that, after nerve injury, an intact AIS is essential for efficient regeneration and functional recovery of axons in multipolar motor neurons. Our results also suggest that loss of polarity in AIS-deficient motor neurons impairs the delivery of axonal proteins, mRNAs, and other cargoes necessary for regeneration. Thus, therapeutic strategies for axon regeneration must consider preservation or reassembly of the AIS.SIGNIFICANCE STATEMENT Disruption of the axon initial segment is a common event after nervous system injury. For multipolar motor neurons, we show that axon initial segments are essential for axon regeneration and functional recovery after injury. Our results may help explain injuries where axon regeneration fails, and suggest strategies to promote more efficient axon regeneration.

Neuropathology and neuroanatomy of TDP-43 amyotrophic lateral sclerosis

PURPOSE OF REVIEW

Intracellular inclusions consisting of the abnormal TDP-43 protein and its nucleocytoplasmic mislocalization in selected cell types are hallmark pathological features of sALS. Descriptive (histological, morphological), anatomical, and molecular studies all have improved our understanding of the neuropathology of sporadic amyotrophic lateral sclerosis (sALS). This review highlights some of the latest developments in the field.

RECENT FINDINGS

Increasing evidence exists from experimental models for the prion-like nature of abnormal TDP-43, including a strain-effect, and with the help of neuroimaging-based studies, for spreading of disease along corticofugal connectivities in sALS. Progress has also been made with respect to finding and establishing reliable biomarkers (neurofilament levels, diffusor tensor imaging).

SUMMARY

The latest findings may help to elucidate the preclinical phase of sALS and to define possible mechanisms for delaying or halting disease development and progression.

Genetic interaction between Scn8a and potassium channel genes Kcna1 and Kcnq2

Voltage-gated sodium and potassium channels regulate the initiation and termination of neuronal action potentials. Gain-of-function mutations of sodium channel Scn8a and loss-of-function mutations of potassium channels Kcna1 and Kcnq2 increase neuronal activity and lead to seizure disorders. We tested the hypothesis that reducing the expression of Scn8a would compensate for loss-of-function mutations of Kcna1 or Kcnq2. Scn8a expression was reduced by the administration of an antisense oligonucleotide (ASO). This treatment lengthened the survival of the Kcn1a and Kcnq2 mutants, and reduced the seizure frequency in the Kcnq2 mutant mice. These observations suggest that reduction of SCN8A may be therapeutic for genetic epilepsies resulting from mutations in these potassium channel genes.

Impaired disassembly of the axon initial segment restricts mitochondrial entry into damaged axons

The proteasome is essential for cellular responses to various physiological stressors. However, how proteasome function impacts the stress resilience of regenerative damaged motor neurons remains unclear. Here, we develop a unique mouse model using a regulatory element of the activating transcription factor (Atf3) gene to label mitochondria in a damage‐induced manner while simultaneously genetically disrupting the proteasome. Using this model, we observed that in injury‐induced proteasome‐deficient mouse motor neurons, the increase of mitochondrial influx from soma into axons is inhibited because neurons fail to disassemble ankyrin G, an organizer of the axon initial segment (AIS), in a proteasome‐dependent manner. Further, these motor neurons exhibit amyotrophic lateral sclerosis (ALS)‐like degeneration despite having regenerative potential. Selectively vulnerable motor neurons in SOD1^G93A ALS mice, which induce ATF3 in response to pathological damage, also fail to disrupt the AIS, limiting the number of axonal mitochondria at a pre‐symptomatic stage. Thus, damage‐induced proteasome‐sensitive AIS disassembly could be a critical post‐translational response for damaged motor neurons to temporarily transit to an immature state and meet energy demands for axon regeneration or preservation.

Endocytosis in the axon initial segment maintains neuronal polarity

Neurons are highly polarized cells that face the fundamental challenge of compartmentalizing a vast and diverse repertoire of proteins in order to function properly¹. The axon initial segment (AIS) is a specialized domain that separates a neuron’s morphologically, biochemically and functionally distinct axon and dendrite compartments^2,3. How the AIS maintains polarity between these compartments is not fully understood. Here we find that in Caenorhabditis elegans, mouse, rat and human neurons, dendritically and axonally polarized transmembrane proteins are recognized by endocytic machinery in the AIS, robustly endocytosed and targeted to late endosomes for degradation. Forcing receptor interaction with the AIS master organizer, ankyrinG, antagonizes receptor endocytosis in the AIS, causes receptor accumulation in the AIS, and leads to polarity deficits with subsequent morphological and behavioural defects. Therefore, endocytic removal of polarized receptors that diffuse into the AIS serves as a membrane-clearance mechanism that is likely to work in conjunction with the known AIS diffusion-barrier mechanism to maintain neuronal polarity on the plasma membrane. Our results reveal a conserved endocytic clearance mechanism in the AIS to maintain neuronal polarity by reinforcing axonal and dendritic compartment membrane boundaries.

Endocytosis and degradation of plasma membrane proteins in the axon initial segment, together with the diffusion-barrier mechanism, maintain a polarized distribution of plasma membrane proteins in Caenorhabditis elegans, mouse, rat and human neurons.