Calcineurin signaling mediates activity-dependent relocation of the axon initial segment

Actin polymerization and longitudinal actin fibers in axon initial segment plasticity

The location of the axon initial segment (AIS) at the junction between the soma and axon of neurons makes it instrumental in maintaining neural polarity and as the site for action potential generation. The AIS is also capable of large-scale relocation in an activity-dependent manner. This represents a form of homeostatic plasticity in which neurons regulate their own excitability by changing the size and/or position of the AIS. While AIS plasticity is important for proper functionality of AIS-containing neurons, the cellular and molecular mechanisms of AIS plasticity are poorly understood. Here, we analyzed changes in the AIS actin cytoskeleton during AIS plasticity using 3D structured illumination microscopy (3D-SIM). We showed that the number of longitudinal actin fibers increased transiently 3 h after plasticity induction. We further showed that actin polymerization, especially formin mediated actin polymerization, is required for AIS plasticity and formation of longitudinal actin fibers. From the formin family of proteins, Daam1 localized to the ends of longitudinal actin fibers. These results indicate that active re-organization of the actin cytoskeleton is required for proper AIS plasticity.

Static magnetic stimulation induces structural plasticity at the axon initial segment of inhibitory cortical neurons

Static magnetic stimulation (SMS) is a form of non-invasive brain stimulation that alters neural activity and induces neural plasticity that outlasts the period of stimulation. This can modify corticospinal excitability or motor behaviours, suggesting that SMS may alter the intrinsic excitability of neurons. In mammalian neurons, the axon initial segment (AIS) is the site of action potential initiation and undergoes structural plasticity (changes in length and position from the soma) as a homeostatic mechanism to counteract chronic changes in neuronal activity. We investigated whether the chronic application of SMS (6 and 48 h, 0.5 T) induces structural AIS plasticity in postnatally derived primary cortical neurons. Following 6 h of SMS, we observed a shortening in mean AIS length compared to control, that persisted 24 h post stimulation. In contrast, 48 h of SMS induced an immediate distal shift that persisted 24 h post-stimulation. Pharmacological blockade of voltage gated L/T-type calcium channels during stimulation did not prevent SMS-induced AIS structural plasticity. Our findings provide the foundation to expand the use of chronic SMS as a non-invasive method to promote AIS plasticity.

Concussive Head Trauma Deranges Axon Initial Segment Function in Axotomized and Intact Layer 5 Pyramidal Neurons

The axon initial segment (AIS) is a critical locus of control of action potential (AP) generation and neuronal information synthesis. Concussive traumatic brain injury gives rise to diffuse axotomy, and the majority of neocortical axonal injury arises at the AIS. Consequently, concussive traumatic brain injury might profoundly disrupt the functional specialization of this region. To investigate this hypothesis, one and two days after mild central fluid percussion injury in Thy1-YFP-H mice, we recorded high-resolution APs from axotomized and adjacent intact layer 5 pyramidal neurons and applied a second derivative (2o) analysis to measure the AIS- and soma-regional contributions to the AP upstroke. All layer 5 pyramidal neurons recorded from sham animals manifested two stark 2o peaks separated by a negative intervening slope. In contrast, within injured mice, we discovered a subset of axotomized layer 5 pyramidal neurons in which the AIS-regional 2o peak was abolished, a functional perturbation associated with diminished excitability, axonal sprouting and distention of the AIS as assessed by staining for ankyrin-G. Our analysis revealed an additional subpopulation of both axotomized and intact layer 5 pyramidal neurons that manifested a melding together of the AIS- and soma-regional 2o peaks, suggesting a more subtle aberration of sodium channel function and/or translocation of the AIS initiation zone closer to the soma. When these experiments were repeated in animals in which cyclophilin-D was knocked out, these effects were ameliorated, suggesting that trauma-induced AIS functional perturbation is associated with mitochondrial calcium dysregulation.

Sodium channel endocytosis drives axon initial segment plasticity

Activity-dependent plasticity of the axon initial segment (AIS) endows neurons with the ability to adapt action potential output to changes in network activity. Action potential initiation at the AIS highly depends on the clustering of voltage-gated sodium channels, but the molecular mechanisms regulating their plasticity remain largely unknown. Here, we developed genetic tools to label endogenous sodium channels and their scaffolding protein, to reveal their nanoscale organization and longitudinally image AIS plasticity in hippocampal neurons in slices and primary cultures. We find that N-methyl-d-aspartate receptor activation causes both long-term synaptic depression and rapid internalization of AIS sodium channels within minutes. The clathrin-mediated endocytosis of sodium channels at the distal AIS increases the threshold for action potential generation. These data reveal a fundamental mechanism for rapid activity-dependent AIS reorganization and suggests that plasticity of intrinsic excitability shares conserved features with synaptic plasticity.

Axon Initial Segment GABA Inhibits Action Potential Generation throughout Periadolescent Development

Neurons are remarkably polarized structures: dendrites spread and branch to receive synaptic inputs while a single axon extends and transmits action potentials (APs) to downstream targets. Neuronal polarity is maintained by the axon initial segment (AIS), a region between the soma and axon proper that is also the site of action potential (AP) generation. This polarization between dendrites and axons extends to inhibitory neurotransmission. In adulthood, the neurotransmitter GABA hyperpolarizes dendrites but instead depolarizes axons. These differences in function collide at the AIS. Multiple studies have shown that GABAergic signaling in this region can share properties of either the mature axon or mature dendrite, and that these properties evolve over a protracted period encompassing periadolescent development. Here, we explored how developmental changes in GABAergic signaling affect AP initiation. We show that GABA at the axon initial segment inhibits action potential initiation in layer (L)2/3 pyramidal neurons in prefrontal cortex from mice of either sex across GABA reversal potentials observed in periadolescence. These actions occur largely through current shunts generated by GABAA receptors and changes in voltage-gated channel properties that affected the number of channels that could be recruited for AP electrogenesis. These results suggest that GABAergic neurons targeting the axon initial segment provide an inhibitory "veto" across the range of GABA polarity observed in normal adolescent development, regardless of GABAergic synapse reversal potential.Significance Statement GABA receptors are a major class of neurotransmitter receptors in the brain. Typically, GABA receptors inhibit neurons by allowing influx of negatively charged chloride ions into the cell. However, there are cases where local chloride concentrations promote chloride efflux through GABA receptors. Such conditions exist early in development in neocortical pyramidal cell axon initial segments (AISs), where action potentials (APs) initiate. Here, we examined how chloride efflux in early development interacts with mechanisms that support action potential initiation. We find that this efflux, despite moving membrane potential closer to action potential threshold, is nevertheless inhibitory. Thus, GABA at the axon initial segment is likely to be inhibitory for action potential initiation independent of whether chloride flows out or into neurons via these receptors.

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.

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.

CDK5/p35-Dependent Microtubule Reorganization Contributes to Homeostatic Shortening of the Axon Initial Segment

The structural plasticity of the axon initial segment (AIS) contributes to the homeostatic control of activity and optimizes the function of neural circuits; however, the underlying mechanisms are not fully understood. In this study, we prepared a slice culture containing nucleus magnocellularis from chickens of both sexes that reproduces most features of AIS plasticity in vivo, regarding its effects on characteristics of AIS and cell-type specificity, and revealed that microtubule reorganization via activation of CDK5 underlies plasticity. Treating the culture with a high-K⁺ medium shortened the AIS and reduced sodium current and membrane excitability, specifically in neurons tuned to high-frequency sound, creating a tonotopic difference in AIS length in the nucleus. Pharmacological analyses revealed that this AIS shortening was driven by multiple Ca²⁺ pathways and subsequent signaling molecules that converge on CDK5 via the activation of ERK1/2. AIS shortening was suppressed by overexpression of dominant-negative CDK5, whereas it was facilitated by the overexpression of p35, an activator of CDK5. Notably, p35(T138A), a phosphorylation-inactive mutant of p35, did not shorten the AIS. Moreover, microtubule stabilizers occluded AIS shortening during the p35 overexpression, indicating that CDK5/p35 mediated AIS shortening by promoting disassembly of microtubules at distal AIS. This study highlights the importance of microtubule reorganization and regulation of CDK5 activity in structural AIS plasticity and the tuning of AIS characteristics in neurons.SIGNIFICANCE STATEMENT The structural plasticity of AIS has a strong impact on the output of neurons and plays a fundamental role in the physiology and pathology of the brain. However, the mechanisms linking neuronal activity to structural changes in AIS are not well understood. In this study, we prepared an organotypic culture of avian auditory brainstem, reproducing most AIS plasticity features in vivo, and we revealed that activity-dependent AIS shortening occurs through the disassembly of microtubules at distal AIS via activation of CDK5/p35 signals. This study emphasizes the importance of microtubule reorganization and regulation of CDK5 activity in structural AIS plasticity and tonotopic differentiation of AIS structures in the brainstem auditory circuit.

Structural plasticity of the axon initial segment in rat hippocampal granule cells following high frequency stimulation and LTP induction

The axon initial segment (AIS) is the site of action potential initiation and important for the integration of synaptic input. Length and localization of the AIS are dynamic, modulated by afferent activity and contribute to the homeostatic control of neuronal excitability. Synaptopodin is a plasticity-related protein expressed by the majority of telencephalic neurons. It is required for the formation of cisternal organelles within the AIS and an excellent marker to identify these enigmatic organelles at the light microscopic level. Here we applied 2 h of high frequency stimulation of the medial perforant path in rats in vivo to induce a strong long-term potentiation of dentate gyrus granule cells. Immunolabeling for βIV-spectrin and synaptopodin were performed to study structural changes of the AIS and its cisternal organelles. Three-dimensional analysis of the AIS revealed a shortening of the AIS and a corresponding reduction of the number of synaptopodin clusters. These data demonstrate a rapid structural plasticity of the AIS and its cisternal organelles to strong stimulation, indicating a homeostatic response of the entire AIS compartment.

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.

Tracking axon initial segment plasticity using high-density microelectrode arrays: A computational study

Despite being composed of highly plastic neurons with extensive positive feedback, the nervous system maintains stable overall function. To keep activity within bounds, it relies on a set of negative feedback mechanisms that can induce stabilizing adjustments and that are collectively termed “homeostatic plasticity.” Recently, a highly excitable microdomain, located at the proximal end of the axon—the axon initial segment (AIS)—was found to exhibit structural modifications in response to activity perturbations. Though AIS plasticity appears to serve a homeostatic purpose, many aspects governing its expression and its functional role in regulating neuronal excitability remain elusive. A central challenge in studying the phenomenon is the rich heterogeneity of its expression (distal/proximal relocation, shortening, lengthening) and the variability of its functional role. A potential solution is to track AISs of a large number of neurons over time and attempt to induce structural plasticity in them. To this end, a promising approach is to use extracellular electrophysiological readouts to track a large number of neurons at high spatiotemporal resolution by means of high-density microelectrode arrays (HD-MEAs). However, an analysis framework that reliably identifies specific activity signatures that uniquely map on to underlying microstructural changes is missing. In this study, we assessed the feasibility of such a task and used the distal relocation of the AIS as an exemplary problem. We used sophisticated computational models to systematically explore the relationship between incremental changes in AIS positions and the specific consequences observed in simulated extracellular field potentials. An ensemble of feature changes in the extracellular fields that reliably characterize AIS plasticity was identified. We trained models that could detect these signatures with remarkable accuracy. Based on these findings, we propose a hybrid analysis framework that could potentially enable high-throughput experimental studies of activity-dependent AIS plasticity using HD-MEAs.

Mild membrane depolarization in neurons induces immediate early gene transcription and acutely subdues responses to successive stimulus

Immediate early genes (IEGs) are transcribed in response to neuronal activity from sensory stimulation during multiple adaptive processes in the brain. The transcriptional profile of IEGs is indicative of the duration of neuronal activity, but its sensitivity to the strength of depolarization remains unknown. Also unknown is whether activity history of graded potential changes influence future neuronal activity. In this work with dissociated rat cortical neurons, we found that mild depolarization—mediated by elevated extracellular potassium (K⁺)—induces a wide array of rapid IEGs and transiently depresses transcriptional and signaling responses to a successive stimulus. This latter effect was independent of de novo transcription, translation, and signaling via calcineurin or mitogen-activated protein kinase. Furthermore, as measured by multiple electrode arrays and calcium imaging, mild depolarization acutely subdues subsequent spontaneous and bicuculline-evoked activity via calcium- and N-methyl-d-aspartate receptor–dependent mechanisms. Collectively, this work suggests that a recent history of graded potential changes acutely depress neuronal intrinsic properties and subsequent responses. Such effects may have several potential downstream implications, including reducing signal-to-noise ratio during synaptic plasticity processes.

Microtubules as Regulators of Neural Network Shape and Function: Focus on Excitability, Plasticity and Memory

Neuronal microtubules (MTs) are complex cytoskeletal protein arrays that undergo activity-dependent changes in their structure and function as a response to physiological demands throughout the lifespan of neurons. Many factors shape the allostatic dynamics of MTs and tubulin dimers in the cytosolic microenvironment, such as protein–protein interactions and activity-dependent shifts in these interactions that are responsible for their plastic capabilities. Recently, several findings have reinforced the role of MTs in behavioral and cognitive processes in normal and pathological conditions. In this review, we summarize the bidirectional relationships between MTs dynamics, neuronal processes, and brain and behavioral states. The outcomes of manipulating the dynamicity of MTs by genetic or pharmacological approaches on neuronal morphology, intrinsic and synaptic excitability, the state of the network, and behaviors are heterogeneous. We discuss the critical position of MTs as responders and adaptative elements of basic neuronal function whose impact on brain function is not fully understood, and we highlight the dilemma of artificially modulating MT dynamics for therapeutic purposes.

Regulation of Axon Initial Segment Diameter by COUP-TFI Fine-tunes Action Potential Generation

The axon initial segment (AIS) is a specialized structure that controls neuronal excitability via action potential (AP) generation. Currently, AIS plasticity with regard to changes in length and location in response to neural activity has been extensively investigated, but how AIS diameter is regulated remains elusive. Here we report that COUP-TFI (chicken ovalbumin upstream promotor-transcription factor 1) is an essential regulator of AIS diameter in both developing and adult mouse neocortex. Either embryonic or adult ablation of COUP-TFI results in reduced AIS diameter and impaired AP generation. Although COUP-TFI ablations in sparse single neurons and in populations of neurons have similar impacts on AIS diameter and AP generation, they strengthen and weaken, respectively, the receiving spontaneous network in mutant neurons. In contrast, overexpression of COUP-TFI in sparse single neurons increases the AIS diameter and facilitates AP generation, but decreases the receiving spontaneous network. Our findings demonstrate that COUP-TFI is indispensable for both the expansion and maintenance of AIS diameter and that AIS diameter fine-tunes action potential generation and synaptic inputs in mammalian cortical neurons.

Tau reduction affects excitatory and inhibitory neurons differently, reduces excitation/inhibition ratios, and counteracts network hypersynchrony

The protein tau has been implicated in many brain disorders. In animal models, tau reduction suppresses epileptogenesis of diverse causes and ameliorates synaptic and behavioral abnormalities in various conditions associated with excessive excitation-inhibition (E/I) ratios. However, the underlying mechanisms are unknown. Global genetic ablation of tau in mice reduces the action potential (AP) firing and E/I ratio of pyramidal cells in acute cortical slices without affecting the excitability of these cells. Tau ablation reduces the excitatory inputs to inhibitory neurons, increases the excitability of these cells, and structurally alters their axon initial segments (AISs). In primary neuronal cultures subjected to prolonged overstimulation, tau ablation diminishes the homeostatic response of AISs in inhibitory neurons, promotes inhibition, and suppresses hypersynchrony. Together, these differential alterations in excitatory and inhibitory neurons help explain how tau reduction prevents network hypersynchrony and counteracts brain disorders causing abnormally increased E/I ratios.

The Type 2 Diabetes Factor Methylglyoxal Mediates Axon Initial Segment Shortening and Alters Neuronal Function at the Cellular and Network Levels

Recent evidence suggests that alteration of axon initial segment (AIS) geometry (i.e., length or location along the axon) contributes to CNS dysfunction in neurological diseases. For example, AIS length is shorter in the prefrontal cortex of type 2 diabetic mice with cognitive impairment. To determine the key type 2 diabetes-related factor that produces AIS shortening we modified levels of insulin, glucose, or the reactive glucose metabolite methylglyoxal in cultures of dissociated cortices from male and female mice and quantified AIS geometry using immunofluorescent imaging of the AIS proteins AnkyrinG and βIV spectrin. Neither insulin nor glucose modification altered AIS length. Exposure to 100 but not 1 or 10 μm methylglyoxal for 24 h resulted in accumulation of the methylglyoxal-derived advanced glycation end-product hydroimidazolone and produced reversible AIS shortening without cell death. Methylglyoxal-evoked AIS shortening occurred in both excitatory and putative inhibitory neuron populations and in the presence of tetrodotoxin (TTX). In single-cell recordings resting membrane potential was depolarized at 0.5-3 h and returned to normal at 24 h. In multielectrode array (MEA) recordings methylglyoxal produced an immediate ∼300% increase in spiking and bursting rates that returned to normal within 2 min, followed by a ∼20% reduction of network activity at 0.5-3 h and restoration of activity to baseline levels at 24 h. AIS length was unchanged at 0.5-3 h despite the presence of depolarization and network activity reduction. Nevertheless, these results suggest that methylglyoxal could be a key mediator of AIS shortening and disruptor of neuronal function during type 2 diabetes.

Conventional measures of intrinsic excitability are poor estimators of neuronal activity under realistic synaptic inputs

Activity-dependent regulation of intrinsic excitability has been shown to greatly contribute to the overall plasticity of neuronal circuits. Such neuroadaptations are commonly investigated in patch clamp experiments using current step stimulation and the resulting input-output functions are analyzed to quantify alterations in intrinsic excitability. However, it is rarely addressed, how such changes translate to the function of neurons when they operate under natural synaptic inputs. Still, it is reasonable to expect that a strong correlation and near proportional relationship exist between static firing responses and those evoked by synaptic drive. We challenge this view by performing a high-yield electrophysiological analysis of cultured mouse hippocampal neurons using both standard protocols and simulated synaptic inputs via dynamic clamp. We find that under these conditions the neurons exhibit vastly different firing responses with surprisingly weak correlation between static and dynamic firing intensities. These contrasting responses are regulated by two intrinsic K-currents mediated by Kv1 and Kir channels, respectively. Pharmacological manipulation of the K-currents produces differential regulation of the firing output of neurons. Static firing responses are greatly increased in stuttering type neurons under blocking their Kv1 channels, while the synaptic responses of the same neurons are less affected. Pharmacological blocking of Kir-channels in delayed firing type neurons, on the other hand, exhibit the opposite effects. Our subsequent computational model simulations confirm the findings in the electrophysiological experiments and also show that adaptive changes in the kinetic properties of such currents can even produce paradoxical regulation of the firing output.

Plasmalogens regulate the AKT-ULK1 signaling pathway to control the position of the axon initial segment

The axon initial segment (AIS) is a specialized region in neurons that encompasses two essential functions, the generation of action potentials and the regulation of the axodendritic polarity. The mechanism controlling the position of the axon initial segment to allow plasticity and regulation of neuron excitability is unclear. Here we demonstrate that plasmalogens, the most abundant ether-phospholipid, are essential for the homeostatic positioning of the AIS. Plasmalogen deficiency is a hallmark of Rhizomelic Chondrodysplasia Punctata (RCDP) and Zellweger spectrum disorders, but Alzheimer's and Parkinson's disease, are also characterized by plasmalogen defects. Neurons lacking plasmalogens displaced the AIS to more distal positions and were characterized by reduced excitability. Treatment with a short-chain alkyl glycerol was able to rescue AIS positioning. Plasmalogen deficiency impaired AKT activation, and we show that inhibition of AKT phosphorylation at Ser473 and Thr308 is sufficient to induce a distal relocation of the AIS. Pathway analysis revealed that downstream of AKT, overtly active ULK1 mediates AIS repositioning. Rescuing the impaired AKT signaling pathway was able to normalize AIS position independently of the biochemical defect. These results unveil a previously unknown mechanism that couples the phospholipid composition of the neuronal membrane to the positional assembly of the AIS.

Mitochondrial Structure and Polarity in Dendrites and the Axon Initial Segment Are Regulated by Homeostatic Plasticity and Dysregulated in Fragile X Syndrome

Mitochondrial dysfunction has long been overlooked in neurodevelopmental disorders, but recent studies have provided new links to genetic forms of autism, including Rett syndrome and fragile X syndrome (FXS). Mitochondria show plasticity in morphology and function in response to neuronal activity, and previous research has reported impairments in mitochondrial morphology and function in disease. We and others have previously reported abnormalities in distinct types of homeostatic plasticity in FXS. It remains unknown if or how activity deprivation triggering homeostatic plasticity affects mitochondria in axons and/or dendrites and whether impairments occur in neurodevelopmental disorders. Here, we test the hypothesis that mitochondria are structurally and functionally modified in a compartment-specific manner during homeostatic plasticity using a model of activity deprivation in cortical neurons from wild-type mice and that this plasticity-induced regulation is altered in Fmr1-knockout (KO) neurons. We uncovered dendrite-specific regulation of the mitochondrial surface area, whereas axon initial segment (AIS) mitochondria show changes in polarity; both responses are lost in the Fmr1 KO. Taken together, our results demonstrate impairments in mitochondrial plasticity in FXS, which has not previously been reported. These results suggest that mitochondrial dysregulation in FXS could contribute to abnormal neuronal plasticity, with broader implications to other neurodevelopmental disorders and therapeutic strategies.

Electrical match between initial segment and somatodendritic compartment for action potential backpropagation in retinal ganglion cells

The action potential of most vertebrate neurons initiates in the axon initial segment (AIS) and is then transmitted to the soma where it is regenerated by somatodendritic sodium channels. For successful transmission, the AIS must produce a strong axial current, so as to depolarize the soma to the threshold for somatic regeneration. Theoretically, this axial current depends on AIS geometry and Na⁺ conductance density. We measured the axial current of mouse retinal ganglion cells using whole cell recordings with post hoc AIS labeling. We found that this current is large, implying high Na⁺ conductance density, and carries a charge that covaries with capacitance so as to depolarize the soma by ∼30 mV. Additionally, we observed that the axial current attenuates strongly with depolarization, consistent with sodium channel inactivation, but temporally broadens so as to preserve the transmitted charge. Thus, the AIS appears to be organized so as to reliably backpropagate the axonal action potential.NEW & NOTEWORTHY We measured the axial current produced at spike initiation by the axon initial segment of mouse retinal ganglion cells. We found that it is a large current, requiring high sodium channel conductance density, which covaries with cell capacitance so as to ensure a ∼30 mV depolarization. During sustained depolarization the current attenuated, but it broadened to preserve somatic depolarization. Thus, properties of the initial segment are adjusted to ensure backpropagation of the axonal action potential.

Imaging Voltage in Complete Neuronal Networks Within Patterned Microislands Reveals Preferential Wiring of Excitatory Hippocampal Neurons

Voltage imaging with fluorescent dyes affords the opportunity to map neuronal activity in both time and space. One limitation to imaging is the inability to image complete neuronal networks: some fraction of cells remains outside of the observation window. Here, we combine voltage imaging, post hoc immunocytochemistry, and patterned microisland hippocampal culture to provide imaging of complete neuronal ensembles. The patterned microislands completely fill the field of view of our high-speed (500 Hz) camera, enabling reconstruction of the spiking patterns of every single neuron in the network. Cultures raised on microislands are similar to neurons grown on coverslips, with parallel developmental trajectories and composition of inhibitory and excitatory cell types (CA1, CA3, and dentate granule cells, or DGC). We calculate the likelihood that action potential firing in one neuron triggers action potential firing in a downstream neuron in a spontaneously active network to construct a functional connection map of these neuronal ensembles. Importantly, this functional map indicates preferential connectivity between DGC and CA3 neurons and between CA3 and CA1 neurons, mimicking the neuronal circuitry of the intact hippocampus. We envision that patterned microislands, in combination with voltage imaging and methods to classify cell types, will be a powerful method for exploring neuronal function in both healthy and disease states. Additionally, because the entire neuronal network is sampled simultaneously, this strategy has the power to go further, revealing all functional connections between all cell types.

Brief Sensory Deprivation Triggers Cell Type-Specific Structural and Functional Plasticity in Olfactory Bulb Neurons

Can alterations in experience trigger different plastic modifications in neuronal structure and function, and if so, how do they integrate at the cellular level? To address this question, we interrogated circuitry in the mouse olfactory bulb responsible for the earliest steps in odor processing. We induced experience-dependent plasticity in mice of either sex by blocking one nostril for one day, a minimally invasive manipulation that leaves the sensory organ undamaged and is akin to the natural transient blockage suffered during common mild rhinal infections. We found that such brief sensory deprivation produced structural and functional plasticity in one highly specialized bulbar cell type: axon-bearing dopaminergic neurons in the glomerular layer. After 24 h naris occlusion, the axon initial segment (AIS) in bulbar dopaminergic neurons became significantly shorter, a structural modification that was also associated with a decrease in intrinsic excitability. These effects were specific to the AIS-positive dopaminergic subpopulation because no experience-dependent alterations in intrinsic excitability were observed in AIS-negative dopaminergic cells. Moreover, 24 h naris occlusion produced no structural changes at the AIS of bulbar excitatory neurons, mitral/tufted and external tufted cells, nor did it alter their intrinsic excitability. By targeting excitability in one specialized dopaminergic subpopulation, experience-dependent plasticity in early olfactory networks might act to fine-tune sensory processing in the face of continually fluctuating inputs.SIGNIFICANCE STATEMENT Sensory networks need to be plastic so they can adapt to changes in incoming stimuli. To see how cells in mouse olfactory circuits can change in response to sensory challenges, we blocked a nostril for just one day, a naturally relevant manipulation akin to the deprivation that occurs with a mild cold. We found that this brief deprivation induces forms of axonal and intrinsic functional plasticity in one specific olfactory bulb cell subtype: axon-bearing dopaminergic interneurons. In contrast, intrinsic properties of axon-lacking bulbar dopaminergic neurons and neighboring excitatory neurons remained unchanged. Within the same sensory circuits, specific cell types can therefore make distinct plastic changes in response to an ever-changing external landscape.

Chronic α1-Na/K-ATPase inhibition reverses the elongation of the axon initial segment of the hippocampal CA1 pyramidal neurons in Angelman syndrome model mice

Angelman syndrome (AS) is a neurodevelopmental disorder caused by the loss of function of the maternal UBE3A gene. The hippocampus is one of the most prominently affected brain regions in AS model mice, manifesting in severe hippocampal-dependent memory and plasticity deficits. Previous studies in AS mice reported an elongated axon initial segment (AIS) in pyramidal neurons (PNs) of the hippocampal CA1 region. These were the first reports in mammals to show AIS elongation in vivo. Correspondingly, this AIS elongation was linked to enhanced expression of the α1 subunit of Na⁺/K⁺-ATPase (α1-NaKA). Recently, it was shown that selective pharmacological inhibition of α1-NaKA by marinobufagenin (MBG) in adult AS mice rescued the hippocampal-dependent deficits via normalizing their compromised activity-dependent calcium (Ca⁺²) dynamics. In the herein study, we showed that a chronic selective α1-NaKA inhibition reversed the AIS elongation in hippocampal CA1 PNs of adult AS mice, and differentially altered their excitability and intrinsic properties. Taken together, our study is the first to demonstrate in vivo structural plasticity of the AIS in a mammalian model, and further elaborates on the modulatory effects of elevated α1-NaKA levels in the hippocampus of AS mice.

Sensory input drives rapid homeostatic scaling of the axon initial segment in mouse barrel cortex

The axon initial segment (AIS) is a critical microdomain for action potential initiation and implicated in the regulation of neuronal excitability during activity-dependent plasticity. While structural AIS plasticity has been suggested to fine-tune neuronal activity when network states change, whether it acts in vivo as a homeostatic regulatory mechanism in behaviorally relevant contexts remains poorly understood. Using the mouse whisker-to-barrel pathway as a model system in combination with immunofluorescence, confocal analysis and electrophysiological recordings, we observed bidirectional AIS plasticity in cortical pyramidal neurons. Furthermore, we find that structural and functional AIS remodeling occurs in distinct temporal domains: Long-term sensory deprivation elicits an AIS length increase, accompanied with an increase in neuronal excitability, while sensory enrichment results in a rapid AIS shortening, accompanied by a decrease in action potential generation. Our findings highlight a central role of the AIS in the homeostatic regulation of neuronal input-output relations.

Trafficking mechanisms underlying Nav channel subcellular localization in neurons

Voltage gated sodium channels (Nav) play a crucial role in action potential initiation and propagation. Although the discovery of Nav channels dates back more than 65 years, and great advances in understanding their localization, biophysical properties, and links to disease have been made, there are still many questions to be answered regarding the cellular and molecular mechanisms involved in Nav channel trafficking, localization and regulation. This review summarizes the different trafficking mechanisms underlying the polarized Nav channel localization in neurons, with an emphasis on the axon initial segment (AIS), as well as discussing the latest advances regarding how neurons regulate their excitability by modifying AIS length and location. The importance of Nav channel localization is emphasized by the relationship between mutations, impaired trafficking and disease. While this review focuses on Nav1.6, other Nav isoforms are also discussed.

Increased Axon Initial Segment Length Results in Increased Na+ Currents in Spinal Motoneurones at Symptom Onset in the G127X SOD1 Mouse Model of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease preferentially affecting motoneurones. Transgenic mouse models have been used to investigate the role of abnormal motoneurone excitability in this disease. Whilst an increased excitability has repeatedly been demonstrated in vitro in neonatal and embryonic preparations from SOD1 mouse models, the results from the only studies to record in vivo from spinal motoneurones in adult SOD1 models have produced conflicting findings. Deficits in repetitive firing have been reported in G93A SOD1^{(high copy number)} mice but not in presymptomatic G127X SOD1 mice despite shorter motoneurone axon initial segments (AISs) in these mice. These discrepancies may be due to the earlier disease onset and prolonged disease progression in G93A SOD1 mice with recordings potentially performed at a later sub-clinical stage of the disease in this mouse. To test this, and to explore how the evolution of excitability changes with symptom onset we performed in vivo intracellular recording and AIS labelling in G127X SOD1 mice immediately after symptom onset. No reductions in repetitive firing were observed showing that this is not a common feature across all ALS models. Immunohistochemistry for the Na⁺ channel Nav1.6 showed that motoneurone AISs increase in length in G127X SOD1 mice at symptom onset. Consistent with this, the rate of rise of AIS components of antidromic action potentials were significantly faster confirming that this increase in length represents an increase in AIS Na⁺ channels occurring at symptom onset in this model.

Actomyosin Contractility in the Generation and Plasticity of Axons and Dendritic Spines

Actin and non-muscle myosins have long been known to play important roles in growth cone steering and neurite outgrowth. More recently, novel functions for non-muscle myosin have been described in axons and dendritic spines. Consequently, possible roles of actomyosin contraction in organizing and maintaining structural properties of dendritic spines, the size and location of axon initial segment and axonal diameter are emerging research topics. In this review, we aim to summarize recent findings involving myosin localization and function in these compartments and to discuss possible roles for actomyosin in their function and the signaling pathways that control them.

Activity-Dependent Plasticity of Axo-axonic Synapses at the Axon Initial Segment

The activity-dependent rules that govern the wiring of GABAergic interneurons are not well understood. Chandelier cells (ChCs) are a type of GABAergic interneuron that control pyramidal cell output through axo-axonic synapses that target the axon initial segment. In vivo imaging of ChCs during development uncovered a narrow window (P12-P18) over which axons arborized and formed connections. We found that increases in the activity of either pyramidal cells or individual ChCs during this temporal window result in a reversible decrease in axo-axonic connections. Voltage imaging of GABAergic transmission at the axon initial segment (AIS) showed that axo-axonic synapses were depolarizing during this period. Identical manipulations of network activity in older mice (P40-P46), when ChC synapses are inhibitory, resulted instead in an increase in axo-axonic synapses. We propose that the direction of ChC synaptic plasticity follows homeostatic rules that depend on the polarity of axo-axonic synapses.

Microglial process convergence on axonal segments in health and disease

Microglia dynamically interact with neurons influencing the development, structure, and function of neuronal networks. Recent studies suggest microglia may also influence neuronal activity by physically interacting with axonal domains responsible for action potential initiation and propagation. However, the nature of these microglial process interactions is not well understood. Microglial-axonal contacts are present early in development and persist through adulthood, implicating microglial interactions in the regulation of axonal integrity in both the developing and mature central nervous system. Moreover, changes in microglial-axonal contact have been described in disease states such as multiple sclerosis (MS) and traumatic brain injury (TBI). Depending on the disease state, there are increased associations with specific axonal segments. In MS, there is enhanced contact with the axon initial segment and node of Ranvier, while, in TBI, microglia alter interactions with axons at the site of injury, as well as at the axon initial segment. In this article, we review the interactions of microglial processes with axonal segments, analyzing their associations with various axonal domains and how these interactions may differ between MS and TBI. Furthermore, we discuss potential functional consequences and molecular mechanisms of these interactions and how these may differ among various types of microglial-axonal interactions.

Calcineurin Signaling Mediates Disruption of the Axon Initial Segment Cytoskeleton after Injury

The axon initial segment (AIS) cytoskeleton undergoes rapid and irreversible disruption prior to cell death after injury, and loss of AIS integrity can produce profound neurological effects on the nervous system. Here we described a previously unrecognized mechanism for ischemia-induced alterations in AIS integrity. We show that in hippocampal CA1 pyramidal neurons Na_v1.6 mostly preserves at the AIS after disruption of the cytoskeleton in a mouse model of middle cerebral artery occlusion. Genetic removal of neurofascin-186 leads to rapid disruption of Na_v1.6 following injury, indicating that neurofascin is required for Na_v1.6 maintenance at the AIS after cytoskeleton collapse. Importantly, calcineurin inhibition with FK506 fully protects AIS integrity and sufficiently prevents impairments of spatial learning and memory from injury. This study provides evidence that calcineurin activation is primarily involved in initiating disassembly of the AIS cytoskeleton and that maintaining AIS integrity is crucial for therapeutic strategies to facilitate recovery from injury.

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Ion channels are mostly retained at the AIS after ischemic injury

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Neurofascin is required for clustering ion channels at the AIS after ischemia

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Calcineurin inhibition protects AIS structural integrity and function from ischemia

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Calcineurin inhibition protects cognitive function against impairment by ischemia

Shorter axon initial segments do not cause repetitive firing impairments in the adult presymptomatic G127X SOD-1 Amyotrophic Lateral Sclerosis mouse

Increases in axonal sodium currents in peripheral nerves are some of the earliest excitability changes observed in Amyotrophic Lateral Sclerosis (ALS) patients. Nothing is known, however, about axonal sodium channels more proximally, particularly at the action potential initiating region - the axon initial segment (AIS). Immunohistochemistry for Nav1.6 sodium channels was used to investigate parameters of AISs of spinal motoneurones in the G127X SOD1 mouse model of ALS in adult mice at presymptomatic time points (~190 days old). In vivo intracellular recordings from lumbar spinal motoneurones were used to determine the consequences of any AIS changes. AISs of both alpha and gamma motoneurones were found to be significantly shorter (by 6.6% and 11.8% respectively) in G127X mice as well as being wider by 9.8% (alpha motoneurones). Measurements from 20–23 day old mice confirmed that this represented a change during adulthood. Intracellular recordings from motoneurones in presymptomatic adult mice, however, revealed no differences in individual action potentials or the cells ability to initiate repetitive action potentials. To conclude, despite changes in AIS geometry, no evidence was found for reduced excitability within the functional working range of firing frequencies of motoneurones in this model of ALS.

Merits and Limitations of Studying Neuronal Depolarization-Dependent Processes Using Elevated External Potassium

Elevated extracellular potassium chloride is widely used to achieve membrane depolarization of cultured neurons. This technique has illuminated mechanisms of calcium influx through L-type voltage sensitive calcium channels, activity-regulated signaling, downstream transcriptional events, and many other intracellular responses to depolarization. However, there is enormous variability in these treatments, including durations from seconds to days and concentrations from 3mM to 150 mM KCl. Differential effects of these variable protocols on neuronal activity and transcriptional programs are underexplored. Furthermore, potassium chloride treatments in vitro are criticized for being poor representatives of in vivo phenomena and are questioned for their effects on cell viability. In this review, we discuss the intracellular consequences of elevated extracellular potassium chloride treatment in vitro, the variability of such treatments in the literature, the strengths and limitations of this tool, and relevance of these studies to brain functions and dysfunctions.

Pathogenic Tau Impairs Axon Initial Segment Plasticity and Excitability Homeostasis

Dysregulation of neuronal excitability underlies the pathogenesis of tauopathies, including frontotemporal dementia (FTD) with tau inclusions. A majority of FTD-causing tau mutations are located in the microtubule-binding domain, but how these mutations alter neuronal excitability is largely unknown. Here, using CRISPR/Cas9-based gene editing in human pluripotent stem cell (iPSC)-derived neurons and isogenic controls, we show that the FTD-causing V337M tau mutation impairs activity-dependent plasticity of the cytoskeleton in the axon initial segment (AIS). Extracellular recordings by multi-electrode arrays (MEAs) revealed that the V337M tau mutation in human neurons leads to an abnormal increase in neuronal activity in response to chronic depolarization. Stochastic optical reconstruction microscopy of human neurons with this mutation showed that AIS plasticity is impaired by the abnormal accumulation of end-binding protein 3 (EB3) in the AIS submembrane region. These findings expand our understanding of how FTD-causing tau mutations dysregulate components of the neuronal cytoskeleton, leading to network dysfunction.

The Low-Threshold Calcium Channel Cav3.2 Mediates Burst Firing of Mature Dentate Granule Cells

Mature granule cells are poorly excitable neurons that were recently shown to fire action potentials, preferentially in bursts. It is believed that the particularly pronounced short-term facilitation of mossy fiber synapses makes granule cell bursting a very effective means of properly transferring information to CA3. However, the mechanism underlying the unique bursting behavior of mature granule cells is currently unknown. Here, we show that Cav3.2 T-type channels at the axon initial segment are responsible for burst firing of mature granule cells in rats and mice. Accordingly, Cav3.2 knockout mice fire tonic spikes and exhibit impaired bursting, synaptic plasticity and dentate-to-CA3 communication. The data show that Cav3.2 channels are strong modulators of bursting and can be considered a critical molecular switch that enables effective information transfer from mature granule cells to the CA3 pyramids.

The contribution of ion channels in input-output plasticity

Persistent changes that occur in brain circuits are classically thought to be mediated by long-term modifications in synaptic efficacy. Yet, many studies have shown that voltage-gated ion channels located at the input and output side of the neurons are also the subject to persistent modifications. These channels are thus responsible for intrinsic plasticity that is expressed in many different neuronal types including glutamatergic principal neurons and GABAergic interneurons. As for synaptic plasticity, activation of synaptic glutamate receptors initiate persistent modification in neuronal excitability. We review here how synaptic input can be efficiently altered by activity-dependent modulation of ion channels that control EPSP amplification, spike threshold or resting membrane potential. We discuss the nature of the learning rules shared by intrinsic and synaptic plasticity, the mechanisms of ion channel regulation and the impact of intrinsic plasticity on induction of synaptic modifications.

Feedback-Driven Assembly of the Axon Initial Segment

The axon initial segment (AIS) is a unique neuronal compartment that plays a crucial role in the generation of action potential and neuronal polarity. The assembly of the AIS requires membrane, scaffolding, and cytoskeletal proteins, including Ankyrin-G and TRIM46. How these components cooperate in AIS formation is currently poorly understood. Here, we show that Ankyrin-G acts as a scaffold interacting with End-Binding (EB) proteins and membrane proteins such as Neurofascin-186 to recruit TRIM46-positive microtubules to the plasma membrane. Using in vitro reconstitution and cellular assays, we demonstrate that TRIM46 forms parallel microtubule bundles and stabilizes them by acting as a rescue factor. TRIM46-labeled microtubules drive retrograde transport of Neurofascin-186 to the proximal axon, where Ankyrin-G prevents its endocytosis, resulting in stable accumulation of Neurofascin-186 at the AIS. Neurofascin-186 enrichment in turn reinforces membrane anchoring of Ankyrin-G and subsequent recruitment of TRIM46-decorated microtubules. Our study reveals feedback-based mechanisms driving AIS assembly.

The MAP1B Binding Domain of Nav1.6 Is Required for Stable Expression at the Axon Initial Segment

Na_v1.6 (SCN8A) is a major voltage-gated sodium channel in the mammalian CNS, and is highly concentrated at the axon initial segment (AIS). As previously demonstrated, the microtubule associated protein MAP1B binds the cytoplasmic N terminus of Na_v1.6, and this interaction is disrupted by the mutation p.VAVP(77-80)AAAA. We now demonstrate that this mutation results in WT expression levels on the somatic surface but reduced surface expression at the AIS of cultured rat embryonic hippocampal neurons from both sexes. The mutation of the MAP1B binding domain did not impair vesicular trafficking and preferential delivery of Na_v1.6 to the AIS; nor was the diffusion of AIS inserted channels altered relative to WT. However, the reduced AIS surface expression of the MAP1B mutant was restored to WT levels by inhibiting endocytosis with Dynasore, indicating that compartment-specific endocytosis was responsible for the lack of AIS accumulation. Interestingly, the lack of AIS targeting resulted in an elevated percentage of persistent current, suggesting that this late current originates predominantly in the soma. No differences in the voltage dependence of activation or inactivation were detected in the MAP1B binding mutant relative to WT channel. We hypothesize that MAP1B binding to the WT Na_v1.6 masks an endocytic motif, thus allowing long-term stability on the AIS surface. This work identifies a critical and important new role for MAP1B in the regulation of neuronal excitability and adds to our understanding of AIS maintenance and plasticity, in addition to identifying new target residues for pathogenic mutations of SCN8A SIGNIFICANCE STATEMENT Na_v1.6 is a major voltage-gated sodium channel in human brain, where it regulates neuronal activity due to its localization at the axon initial segment (AIS). Na_v1.6 mutations cause epilepsy, intellectual disability, and movement disorders. In the present work, we show that loss of interaction with MAP1B within the Na_v1.6 N terminus reduces the steady-state abundance of Na_v1.6 at the AIS. The effect is due to increased Na_v1.6 endocytosis at this neuronal compartment rather than a failure of forward trafficking to the AIS. This work confirms a new biological role of MAP1B in the regulation of sodium channel localization and will contribute to future analysis of patient mutations in the cytoplasmic N terminus of Na_v1.6.

Oxidative Stress Induces Disruption of the Axon Initial Segment

The axon initial segment (AIS), the domain responsible for action potential initiation and maintenance of neuronal polarity, is targeted for disruption in a variety of central nervous system pathological insults. Previous work in our laboratory implicates oxidative stress as a potential mediator of structural AIS alterations in two separate mouse models of central nervous system inflammation, as these effects were attenuated following reactive oxygen species scavenging and NADPH oxidase-2 ablation. While these studies suggest a role for oxidative stress in modulation of the AIS, the direct effects of reactive oxygen and nitrogen species (ROS/RNS) on the stability of this domain remain unclear. Here, we demonstrate that oxidative stress, as induced through treatment with 3-morpholinosydnonimine (SIN-1), a spontaneous ROS/RNS generator, drives a reversible loss of AIS protein clustering in primary cortical neurons in vitro. Pharmacological inhibition of both voltage-dependent and intracellular calcium (Ca²⁺) channels suggests that this mechanism of AIS disruption involves Ca²⁺ entry specifically through L-type voltage-dependent Ca²⁺ channels and its release from IP₃-gated intracellular stores. Furthermore, ROS/RNS-induced AIS disruption is dependent upon activation of calpain, a Ca²⁺-activated protease previously shown to drive AIS modulation. Overall, we demonstrate for the first time that oxidative stress, as induced through exogenously applied ROS/RNS, is capable of driving structural alterations in the AIS complex.

Contribution of the Axon Initial Segment to Action Potentials Recorded Extracellularly

Action potentials (APs) are electric phenomena that are recorded both intracellularly and extracellularly. APs are usually initiated in the short segment of the axon called the axon initial segment (AIS). It was recently proposed that at the onset of an AP the soma and the AIS form a dipole. We study the extracellular signature [the extracellular AP (EAP)] generated by such a dipole. First, we demonstrate the formation of the dipole and its extracellular signature in detailed morphological models of a reconstructed pyramidal neuron. Then, we study the EAP waveform and its spatial dependence in models with axonal AP initiation and contrast it with the EAP obtained in models with somatic AP initiation. We show that in the models with axonal AP initiation the dipole forms between somatodendritic compartments and the AIS, and not between soma and dendrites as in the classical models. The soma-dendrites dipole is present only in models with somatic AP initiation. Our study has consequences for interpreting extracellular recordings of single-neuron activity and determining electrophysiological neuron types, but also for better understanding the origins of the high-frequency macroscopic extracellular potentials recorded in the brain.

Type 2 Diabetes Leads to Axon Initial Segment Shortening in db/db Mice

Cognitive and mood impairments are common central nervous system complications of type 2 diabetes, although the neuronal mechanism(s) remains elusive. Previous studies focused mainly on neuronal inputs such as altered synaptic plasticity. Axon initial segment (AIS) is a specialized functional domain within neurons that regulates neuronal outputs. Structural changes of AIS have been implicated as a key pathophysiological event in various psychiatric and neurological disorders. Here we evaluated the structural integrity of the AIS in brains of db/db mice, an established animal model of type 2 diabetes associated with cognitive and mood impairments. We assessed the AIS before (5 weeks of age) and after (10 weeks) the development of type 2 diabetes, and after daily exercise treatment of diabetic condition. We found that the development of type 2 diabetes is associated with significant AIS shortening in both medial prefrontal cortex and hippocampus, as evident by immunostaining of the AIS structural protein βIV spectrin. AIS shortening occurs in the absence of altered neuronal and AIS protein levels. We found no change in nodes of Ranvier, another neuronal functional domain sharing a molecular organization similar to the AIS. This is the first study to identify AIS alteration in type 2 diabetes condition. Since AIS shortening is known to lower neuronal excitability, our results may provide a new avenue for understanding and treating cognitive and mood impairments in type 2 diabetes.

Axon initial segments: structure, function, and disease

The axon initial segment (AIS) is located at the proximal axon and is the site of action potential initiation. This reflects the high density of ion channels found at the AIS. Adaptive changes to the location and length of the AIS can fine-tune the excitability of neurons and modulate plasticity in response to activity. The AIS plays an important role in maintaining neuronal polarity by regulating the trafficking and distribution of proteins that function in somatodendritic or axonal compartments of the neuron. In this review, we provide an overview of the AIS cytoarchitecture, mechanism of assembly, and recent studies revealing mechanisms of differential transport at the AIS that maintain axon and dendrite identities. We further discuss how genetic mutations in AIS components (i.e., ankyrins, ion channels, and spectrins) and injuries may cause neurological disorders.

Different Neuronal Activity Patterns Induce Different Gene Expression Programs

A vast number of different neuronal activity patterns could each induce a different set of activity-regulated genes. Mapping this coupling between activity pattern and gene induction would allow inference of a neuron's activity-pattern history from its gene expression and improve our understanding of activity-pattern-dependent synaptic plasticity. In genome-scale experiments comparing brief and sustained activity patterns, we reveal that activity-duration history can be inferred from gene expression profiles. Brief activity selectively induces a small subset of the activity-regulated gene program that corresponds to the first of three temporal waves of genes induced by sustained activity. Induction of these first-wave genes is mechanistically distinct from that of the later waves because it requires MAPK/ERK signaling but does not require de novo translation. Thus, the same mechanisms that establish the multi-wave temporal structure of gene induction also enable different gene sets to be induced by different activity durations.

Axon Initial Segment Structural Plasticity is Involved in Seizure Susceptibility in a Rat Model of Cortical Dysplasia

Cortical dysplasia is the most common etiology of intractable epilepsy. Both excitability changes in cortical neurons and neural network reconstitution play a role in cortical dysplasia epileptogenesis. Recent research shows that the axon initial segment, a subcompartment of the neuron important to the shaping of action potentials, adjusts its position in response to changes in input, which contributes to neuronal excitability and local circuit balance. It is unknown whether axon initial segment plasticity occurs in neurons involved in seizure susceptibility in cortical dysplasia. Here, we developed a "Carmustine"- "pilocarpine" rat model of cortical dysplasia and show that it exhibits a lower seizure threshold, as indicated by behavior studies and electroencephalogram monitoring. Using immunofluorescence, we measured the axon initial segment positions of deep L5 somatosensory neurons and show that it is positioned closer to the soma after acute seizure, and that this displacement is sustained in the chronic phase. We then show that Nifedipine has a dose-dependent protective effect against axon initial segment displacement and increased seizure susceptibility. These findings further our understanding of the pathophysiology of seizures in cortical dysplasia and suggests Nifedipine as a potential therapeutic agent.

Localized Myosin II Activity Regulates Assembly and Plasticity of the Axon Initial Segment

The axon initial segment (AIS) is the site of action potential generation and a locus of activity-dependent homeostatic plasticity. A multimeric complex of sodium channels, linked via a cytoskeletal scaffold of ankyrin G and beta IV spectrin to submembranous actin rings, mediates these functions. The mechanisms that specify the AIS complex to the proximal axon and underlie its plasticity remain poorly understood. Here we show phosphorylated myosin light chain (pMLC), an activator of contractile myosin II, is highly enriched in the assembling and mature AIS, where it associates with actin rings. MLC phosphorylation and myosin II contractile activity are required for AIS assembly, and they regulate the distribution of AIS components along the axon. pMLC is rapidly lost during depolarization, destabilizing actin and thereby providing a mechanism for activity-dependent structural plasticity of the AIS. Together, these results identify pMLC/myosin II activity as a common link between AIS assembly and plasticity.

The Axon Initial Segment: An Updated Viewpoint

At the base of axons sits a unique compartment called the axon initial segment (AIS). The AIS generates and shapes the action potential before it is propagated along the axon. Neuronal excitability thus depends crucially on the AIS composition and position, and these adapt to developmental and physiological conditions. The AIS also demarcates the boundary between the somatodendritic and axonal compartments. Recent studies have brought insights into the molecular architecture of the AIS and how it regulates protein trafficking. This Viewpoints article summarizes current knowledge about the AIS and highlights future challenges in understanding this key actor of neuronal physiology.

Enhanced Transmission at the Calyx of Held Synapse in a Mouse Model for Angelman Syndrome

The neurodevelopmental disorder Angelman syndrome (AS) is characterized by intellectual disability, motor dysfunction, distinct behavioral aspects, and epilepsy. AS is caused by a loss of the maternally expressed UBE3A gene, and many of the symptoms are recapitulated in a Ube3a mouse model of this syndrome. At the cellular level, changes in the axon initial segment (AIS) have been reported, and changes in vesicle cycling have indicated the presence of presynaptic deficits. Here we studied the role of UBE3A in the auditory system by recording synaptic transmission at the calyx of Held synapse in the medial nucleus of the trapezoid body (MNTB) through in vivo whole cell and juxtacellular recordings. We show that MNTB principal neurons in Ube3a mice exhibit a hyperpolarized resting membrane potential, an increased action potential (AP) amplitude and a decreased AP half width. Moreover, both the pre- and postsynaptic AP in the calyx of Held synapse of Ube3a mice showed significantly faster recovery from spike depression. An increase in AIS length was observed in the principal MNTB neurons of Ube3a mice, providing a possible substrate for these gain-of-function changes. Apart from the effect on APs, we also observed that EPSPs showed decreased short-term synaptic depression (STD) during long sound stimulations in AS mice, and faster recovery from STD following these tones, which is suggestive of a presynaptic gain-of-function. Our findings thus provide in vivo evidence that UBE3A plays a critical role in controlling synaptic transmission and excitability at excitatory synapses.

Tonotopic Variation of the T-Type Ca2+ Current in Avian Auditory Coincidence Detector Neurons

Neurons in avian nucleus laminaris (NL) are binaural coincidence detectors for sound localization and are characterized by striking structural variations in dendrites and axon initial segment (AIS) according to their acoustic tuning [characteristic frequency (CF)]. T-type Ca²⁺ (CaT) channels regulate synaptic integration and firing behavior at these neuronal structures. However, whether or how CaT channels contribute to the signal processing in NL neurons is not known. In this study, we addressed this issue with whole-cell recording and two-photon Ca²⁺ imaging in brain slices of posthatch chicks of both sexes. We found that the CaT current was prominent in low-CF neurons, whereas it was almost absent in higher-CF neurons. In addition, a large Ca²⁺ transient occurred at the dendrites and the AIS of low-CF neurons, indicating a localization of CaT channels at these structures in the neurons. Because low-CF neurons have long dendrites, dendritic CaT channels may compensate for the attenuation of EPSPs at dendrites. Furthermore, the short distance of AIS from the soma may accelerate activation of axonal CaT current in the neurons and help EPSPs reach spike threshold. Indeed, the CaT current was activated by EPSPs and augmented the synaptic response and spike generation of the neurons. Notably, the CaT current was inactivated during repetitive inputs, and these augmenting effects predominated at the initial phase of synaptic activity. These results suggested that dendritic and axonal CaT channels increase the sensitivity to sound at its onset, which may expand the dynamic range for binaural computation in low-CF NL neurons.SIGNIFICANCE STATEMENT Neurons in nucleus laminaris are binaural coincidence detectors for sound localization. We report that T-type Ca²⁺ (CaT) current was prominent at dendrites and the axonal trigger zone in neurons tuned to low-frequency sound. Because these neurons have long dendrites and a closer trigger zone compared with those tuned to higher-frequency sound, the CaT current augmented EPSPs at dendrites and accelerated spike triggers in the neurons, implying a strategic arrangement of the current within the nucleus. This effect was limited to the onset of repetitive inputs due to progressive inactivation of CaT current. The results suggested that the CaT current increases the sensitivity to sound at its onset, which may expand the dynamic range for binaural computation of low-frequency sound.

M-current inhibition rapidly induces a unique CK2-dependent plasticity of the axon initial segment

Alterations in synaptic input, persisting for hours to days, elicit homeostatic plastic changes in the axon initial segment (AIS), which is pivotal for spike generation. Here, in hippocampal pyramidal neurons of both primary cultures and slices, we triggered a unique form of AIS plasticity by selectively targeting M-type K⁺ channels, which predominantly localize to the AIS and are essential for tuning neuronal excitability. While acute M-current inhibition via cholinergic activation or direct channel block made neurons more excitable, minutes to hours of sustained M-current depression resulted in a gradual reduction in intrinsic excitability. Dual soma-axon patch-clamp recordings combined with axonal Na⁺ imaging and immunocytochemistry revealed that these compensatory alterations were associated with a distal shift of the spike trigger zone and distal relocation of FGF14, Na⁺, and K_v7 channels but not ankyrin G. The concomitant distal redistribution of FGF14 together with Na_v and K_v7 segments along the AIS suggests that these channels relocate as a structural and functional unit. These fast homeostatic changes were independent of l-type Ca²⁺ channel activity but were contingent on the crucial AIS protein, protein kinase CK2. Using compartmental simulations, we examined the effects of varying the AIS position relative to the soma and found that AIS distal relocation of both Na_v and K_v7 channels elicited a decrease in neuronal excitability. Thus, alterations in M-channel activity rapidly trigger unique AIS plasticity to stabilize network excitability.

Activity-dependent axonal plasticity in sensory systems

The rodent whisker-to-barrel cortex pathway is a classic model to study the effects of sensory experience and deprivation on neuronal circuit formation, not only during development but also in the adult. Decades of research have produced a vast body of evidence highlighting the fundamental role of neuronal activity (spontaneous and/or sensory-evoked) for circuit formation and function. In this context, it has become clear that neuronal adaptation and plasticity is not just a function of the neonatal brain, but persists into adulthood, especially after experience-driven modulation of network status. Mechanisms for structural remodeling of the somatodendritic or axonal domain include microscale alterations of neurites or synapses. At the same time, functional alterations at the nanoscale such as expression or activation changes of channels and receptors contribute to the modulation of intrinsic excitability or input-output relationships. However, it remains elusive how these forms of structural and functional plasticity come together to shape neuronal network formation and function. While specifically somatodendritic plasticity has been studied in great detail, the role of axonal plasticity, (e.g. at presynaptic boutons, branches or axonal microdomains), is rather poorly understood. Therefore, this review will only briefly highlight somatodendritic plasticity and instead focus on axonal plasticity. We discuss (i) the role of spontaneous and sensory-evoked plasticity during critical periods, (ii) the assembly of axonal presynaptic sites, (iii) axonal plasticity in the mature brain under baseline and sensory manipulation conditions, and finally (iv) plasticity of electrogenic axonal microdomains, namely the axon initial segment, during development and in the mature CNS.