Casein kinase 2 and microtubules control axon initial segment formation

Contribution of Axon Initial Segment Structure and Channels to Brain Pathology

Brain channelopathies are a group of neurological disorders that result from genetic mutations affecting ion channels in the brain. Ion channels are specialized proteins that play a crucial role in the electrical activity of nerve cells by controlling the flow of ions such as sodium, potassium, and calcium. When these channels are not functioning properly, they can cause a wide range of neurological symptoms such as seizures, movement disorders, and cognitive impairment. In this context, the axon initial segment (AIS) is the site of action potential initiation in most neurons. This region is characterized by a high density of voltage-gated sodium channels (VGSCs), which are responsible for the rapid depolarization that occurs when the neuron is stimulated. The AIS is also enriched in other ion channels, such as potassium channels, that play a role in shaping the action potential waveform and determining the firing frequency of the neuron. In addition to ion channels, the AIS contains a complex cytoskeletal structure that helps to anchor the channels in place and regulate their function. Therefore, alterations in this complex structure of ion channels, scaffold proteins, and specialized cytoskeleton may also cause brain channelopathies not necessarily associated with ion channel mutations. This review will focus on how the AISs structure, plasticity, and composition alterations may generate changes in action potentials and neuronal dysfunction leading to brain diseases. AIS function alterations may be the consequence of voltage-gated ion channel mutations, but also may be due to ligand-activated channels and receptors and AIS structural and membrane proteins that support the function of voltage-gated ion channels.

Calcium modeling of spine apparatus-containing human dendritic spines demonstrates an “all-or-nothing” communication switch between the spine head and dendrite

Dendritic spines are highly dynamic neuronal compartments that control the synaptic transmission between neurons. Spines form ultrastructural units, coupling synaptic contact sites to the dendritic shaft and often harbor a spine apparatus organelle, composed of smooth endoplasmic reticulum, which is responsible for calcium sequestration and release into the spine head and neck. The spine apparatus has recently been linked to synaptic plasticity in adult human cortical neurons. While the morphological heterogeneity of spines and their intracellular organization has been extensively demonstrated in animal models, the influence of spine apparatus organelles on critical signaling pathways, such as calcium-mediated dynamics, is less well known in human dendritic spines. In this study we used serial transmission electron microscopy to anatomically reconstruct nine human cortical spines in detail as a basis for modeling and simulation of the calcium dynamics between spine and dendrite. The anatomical study of reconstructed human dendritic spines revealed that the size of the postsynaptic density correlates with spine head volume and that the spine apparatus volume is proportional to the spine volume. Using a newly developed simulation pipeline, we have linked these findings to spine-to-dendrite calcium communication. While the absence of a spine apparatus, or the presence of a purely passive spine apparatus did not enable any of the reconstructed spines to relay a calcium signal to the dendritic shaft, the calcium-induced calcium release from this intracellular organelle allowed for finely tuned “all-or-nothing” spine-to-dendrite calcium coupling; controlled by spine morphology, neck plasticity, and ryanodine receptors. Our results suggest that spine apparatus organelles are strategically positioned in the neck of human dendritic spines and demonstrate their potential relevance to the maintenance and regulation of spine-to-dendrite calcium communication.

During the past decade it has become increasingly clear that abnormal synaptic plasticity is a major hallmark of neurological and cognitive disorders. Developing a better understanding of the synaptic plasticity process, which describes the ability of neurons to adapt their contacts in an activity-dependent manner, will lead to improved treatment of many neurological and cognitive disorders. It is known that calcium-dependent events such as synaptic transmission, intracellular calcium release, and calcium wave propagation, are required for many types of synaptic plasticity expression. However, the biological significance of these processes in neurons of the adult human cortex remains unknown. Due to technical limitations and ethical concerns, experimental data addressing this biologically and clinically relevant topic are not available. Therefore, we have implemented a computational model to study the intracellular calcium dynamics in realistic human dendritic spines based on detailed morphological reconstructions. With our model and simulations, we have established the morphological and biological requirements for the propagation of calcium from spines into the dendrites. Our results suggest a critical role for the calcium-storing spine apparatus organelle in regulating calcium homeostasis and propagation in human dendritic spines.

Formin Activity and mDia1 Contribute to Maintain Axon Initial Segment Composition and Structure

The axon initial segment (AIS) is essential for maintaining neuronal polarity, modulating protein transport into the axon, and action potential generation. These functions are supported by a distinctive actin and microtubule cytoskeleton that controls axonal trafficking and maintains a high density of voltage-gated ion channels linked by scaffold proteins to the AIS cytoskeleton. However, our knowledge of the mechanisms and proteins involved in AIS cytoskeleton regulation to maintain or modulate AIS structure is limited. In this context, formins play a significant role in the modulation of actin and microtubules. We show that pharmacological inhibition of formins modifies AIS actin and microtubule characteristics in cultured hippocampal neurons, reducing F-actin density and decreasing microtubule acetylation. Moreover, formin inhibition diminishes sodium channels, ankyrinG and βIV-spectrin AIS density, and AIS length, in cultured neurons and brain slices, accompanied by decreased neuronal excitability. We show that genetic downregulation of the mDia1 formin by interference RNAs also decreases AIS protein density and shortens AIS length. The ankyrinG decrease and AIS shortening observed in pharmacologically inhibited neurons and neuron-expressing mDia1 shRNAs were impaired by HDAC6 downregulation or EB1-GFP expression, known to increase microtubule acetylation or stability. However, actin stabilization only partially prevented AIS shortening without affecting AIS protein density loss. These results suggest that mDia1 maintain AIS composition and length contributing to the stability of AIS microtubules.

Modeling protein-protein interactions in axon initial segment to understand their potential impact on action potential initiation

The axon initial segment (AIS) region is crucial for action potential initiation due to the presence of high-density AIS protein voltage-gated sodium channels (Na_v). Na_v channels comprise several serine residues responsible for the recruitment of Na_v channels into the structure of AIS through interactions with ankyrin-G (AnkG). In this study, a series of computational experiments are performed to understand the role of AIS proteins casein kinase 2 and AnkG on Na_v channel recruitment into the AIS. The computational simulation results using Virtual cell software indicate that Na_v channels with all serine sites available for phosphorylation bind to AnkG with strong affinity. At the low initial concentration of AnkG and casein kinase 2, the concentration of Na_v channels reduces significantly, suggesting the importance of casein kinase 2 and AnkG in the recruitment of Na_v channels.

A Journey through the Cytoskeleton with Protein Kinase CK2

Substrate pleiotropicity, a very acidic phosphorylation consensus sequence, and an apparent uncontrolled activity, are the main features of CK2, a Ser/Thr protein kinase that is required for a plethora of cell functions. Not surprisingly, CK2 appears to affect cytoskeletal structures and correlated functions such as cell shape, mechanical integrity, cell movement and division. This review outlines our current knowledge of how CK2 regulates cytoskeletal structures, and discusses involved pathways and molecular mechanisms.

Identification of compounds inhibiting prion replication and toxicity by removing PrPC from the cell surface

The vast majority of therapeutic approaches tested so far for prion diseases, transmissible neurodegenerative disorders of human and animals, tackled PrP^Sc , the aggregated and infectious isoform of the cellular prion protein (PrP^C ), with largely unsuccessful results. Conversely, targeting PrP^C expression, stability or cell surface localization are poorly explored strategies. We recently characterized the mode of action of chlorpromazine, an anti-psychotic drug known to inhibit prion replication and toxicity by inducing the re-localization of PrP^C from the plasma membrane. Unfortunately, chlorpromazine possesses pharmacokinetic properties unsuitable for chronic use in vivo, namely low specificity and high toxicity. Here, we employed HEK293 cells stably expressing EGFP-PrP to carry out a semi-automated high content screening (HCS) of a chemical library directed at identifying non-cytotoxic molecules capable of specifically relocalizing PrP^C from the plasma membrane as well as inhibiting prion replication in N2a cell cultures. We identified four candidate hits inducing a significant reduction in cell surface PrP^C , one of which also inhibited prion propagation and toxicity in cell cultures in a strain-independent fashion. This study defines a new screening method and novel anti-prion compounds supporting the notion that removing PrP^C from the cell surface could represent a viable therapeutic strategy for prion diseases.

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.

CSNK2B splice site mutations in patients cause intellectual disability with or without myoclonic epilepsy

De novo mutations are a frequent cause of disorders related to brain development. We report the results from the screening of two patients diagnosed with intellectual disability (ID) using exome sequencing to identify new causative de novo mutations. Exome sequencing was conducted in two patient-parent trios to identify de novo variants. In silico and expression studies were also performed to evaluate the functional consequences of these variants. The two patients presented developmental delay with minor facial dysmorphy. One of them presented pharmacoresistant myoclonic epilepsy. We identified two de novo splice variants (c.175+2T>G; c.367+2T>C) in the CSNK2B gene encoding the β subunit of the Caseine kinase 2 (CK2). CK2 is a ubiquitously expressed kinase that is present in high levels in brain and it appears to be constitutively active. The mRNA transcripts were abnormal and significantly reduced in affected fibroblasts and most likely produced truncated proteins. Taking into account that mutations in CSNK2A1, encoding the α subunit of CK2, were previously identified in patients with neurodevelopmental disorders and dysmorphic features, our study confirmed that the protein kinase CK2 plays a major role in brain, and showed that CSNK2, encoding the β subunit, is a novel ID gene. This study adds knowledge to the increasingly growing list of causative and candidate genes in ID and epilepsy, and highlights CSNK2B as a new gene for neurodevelopmental disorders.

MTCL1 plays an essential role in maintaining Purkinje neuron axon initial segment

The axon initial segment (AIS) is a specialized domain essential for neuronal function, the formation of which begins with localization of an ankyrin-G (AnkG) scaffold. However, the mechanism directing and maintaining AnkG localization is largely unknown. In this study, we demonstrate that in vivo knockdown of microtubule cross-linking factor 1 (MTCL1) in cerebellar Purkinje cells causes loss of axonal polarity coupled with AnkG mislocalization. MTCL1 lacking MT-stabilizing activity failed to restore these defects, and stable MT bundles spanning the AIS were disorganized in knockdown cells. Interestingly, during early postnatal development, colocalization of MTCL1 with these stable MT bundles was observed prominently in the axon hillock and proximal axon. These results indicate that MTCL1-mediated formation of stable MT bundles is crucial for maintenance of AnkG localization. We also demonstrate that Mtcl1 gene disruption results in abnormal motor coordination with Purkinje cell degeneration, and provide evidence suggesting possible involvement of MTCL1 dysfunction in the pathogenesis of spinocerebellar ataxia.

Cannabinoid Receptors Modulate Neuronal Morphology and AnkyrinG Density at the Axon Initial Segment

Neuronal polarization underlies the ability of neurons to integrate and transmit information. This process begins early in development with axon outgrowth, followed by dendritic growth and subsequent maturation. In between these two steps, the axon initial segment (AIS), a subcellular domain crucial for generating action potentials (APs) and maintaining the morphological and functional polarization, starts to develop. However, the cellular/molecular mechanisms and receptors involved in AIS initial development and maturation are mostly unknown. In this study, we have focused on the role of the type-1 cannabinoid receptor (CB1R), a highly abundant G-protein coupled receptor (GPCR) in the nervous system largely involved in different phases of neuronal development and differentiation. Although CB1R activity modulation has been related to changes in axons or dendrites, its possible role as a modulator of AIS development has not been yet explored. Here we analyzed the potential role of CB1R on neuronal morphology and AIS development using pharmacological and RNA interference approaches in cultured hippocampal neurons. CB1R inhibition, at a very early developmental stage, has no effect on axonal growth, yet CB1R activation can promote it. By contrast, subsequent dendritic growth is impaired by CB1R inhibition, which also reduces ankyrinG density at the AIS. Moreover, our data show a significant correlation between early dendritic growth and ankyrinG density. However, CB1R inhibition in later developmental stages after dendrites are formed only reduces ankyrinG accumulation at the AIS. In conclusion, our data suggest that neuronal CB1R basal activity plays a role in initial development of dendrites and indirectly in AIS proteins accumulation. Based on the lack of CB1R expression at the AIS, we hypothesize that CB1R mediated modulation of dendritic arbor size during early development indirectly determines the accumulation of ankyrinG and AIS development. Further studies will be necessary to determine which CB1R-dependent mechanisms can coordinate these two domains, and what may be the impact of these early developmental changes once neurons mature and are embedded in a functional brain network.

Cooperative Interactions between 480 kDa Ankyrin-G and EB Proteins Assemble the Axon Initial Segment

The axon initial segment (AIS) is required for generating action potentials and maintaining neuronal polarity. Significant progress has been made in deciphering the basic building blocks composing the AIS, but the underlying mechanisms required for AIS formation remains unclear. The scaffolding protein ankyrin-G is the master-organizer of the AIS. Microtubules and their interactors, particularly end-binding proteins (EBs), have emerged as potential key players in AIS formation. Here, we show that the longest isoform of ankyrin-G (480AnkG) selectively associates with EBs via its specific tail domain and that this interaction is crucial for AIS formation and neuronal polarity in cultured rodent hippocampal neurons. EBs are essential for 480AnkG localization and stabilization at the AIS, whereas 480AnkG is required for the specific accumulation of EBs in the proximal axon. Our findings thus provide a conceptual framework for understanding how the cooperative relationship between 480AnkG and EBs induces the assembly of microtubule-AIS structures in the proximal axon.

SIGNIFICANCE STATEMENT

Neuronal polarity is crucial for the proper function of neurons. The assembly of the axon initial segment (AIS), which is the hallmark of early neuronal polarization, relies on the longest 480 kDa ankyrin-G isoform. The microtubule cytoskeleton and its interacting proteins were suggested to be early key players in the process of AIS formation. In this study, we show that the crosstalk between 480 kDa ankyrin-G and the microtubule plus-end tracking proteins, EBs, at the proximal axon is decisive for AIS assembly and neuronal polarity. Our work thus provides insight into the functional mechanisms used by 480 kDa ankyrin-G to drive the AIS formation and thereby to establish neuronal polarity.

Axon Initial Segment Cytoskeleton: Architecture, Development, and Role in Neuron Polarity

The axon initial segment (AIS) is a specialized structure in neurons that resides in between axonal and somatodendritic domains. The localization of the AIS in neurons is ideal for its two major functions: it serves as the site of action potential firing and helps to maintain neuron polarity. It has become increasingly clear that the AIS cytoskeleton is fundamental to AIS functions. In this review, we discuss current understanding of the AIS cytoskeleton with particular interest in its unique architecture and role in maintenance of neuron polarity. The AIS cytoskeleton is divided into two parts, submembrane and cytoplasmic, based on localization, function, and molecular composition. Recent studies using electron and subdiffraction fluorescence microscopy indicate that submembrane cytoskeletal components (ankyrin G, βIV-spectrin, and actin filaments) form a sophisticated network in the AIS that is conceptually similar to the polygonal/triangular network of erythrocytes, with some important differences. Components of the AIS cytoplasmic cytoskeleton (microtubules, actin filaments, and neurofilaments) reside deeper within the AIS shaft and display structural features distinct from other neuronal domains. We discuss how the AIS submembrane and cytoplasmic cytoskeletons contribute to different aspects of AIS polarity function and highlight recent advances in understanding their AIS cytoskeletal assembly and stability.

CK2 activity is required for the interaction of FGF14 with voltage-gated sodium channels and neuronal excitability

Recent data shows that fibroblast growth factor 14 (FGF14) binds to and controls the function of the voltage-gated sodium (Nav) channel with phenotypic outcomes on neuronal excitability. Mutations in the FGF14 gene in humans have been associated with brain disorders that are partially recapitulated in Fgf14(-/-) mice. Thus, signaling pathways that modulate the FGF14:Nav channel interaction may be important therapeutic targets. Bioluminescence-based screening of small molecule modulators of the FGF14:Nav1.6 complex identified 4,5,6,7 -: tetrabromobenzotriazole (TBB), a potent casein kinase 2 (CK2) inhibitor, as a strong suppressor of FGF14:Nav1.6 interaction. Inhibition of CK2 through TBB reduces the interaction of FGF14 with Nav1.6 and Nav1.2 channels. Mass spectrometry confirmed direct phosphorylation of FGF14 by CK2 at S228 and S230, and mutation to alanine at these sites modified FGF14 modulation of Nav1.6-mediated currents. In 1 d in vitro hippocampal neurons, TBB induced a reduction in FGF14 expression, a decrease in transient Na(+) current amplitude, and a hyperpolarizing shift in the voltage dependence of Nav channel steady-state inactivation. In mature neurons, TBB reduces the axodendritic polarity of FGF14. In cornu ammonis area 1 hippocampal slices from wild-type mice, TBB impairs neuronal excitability by increasing action potential threshold and lowering firing frequency. Importantly, these changes in excitability are recapitulated in Fgf14(-/-) mice, and deletion of Fgf14 occludes TBB-dependent phenotypes observed in wild-type mice. These results suggest that a CK2-FGF14 axis may regulate Nav channels and neuronal excitability.-Hsu, W.-C. J., Scala, F., Nenov, M. N., Wildburger, N. C., Elferink, H., Singh, A. K., Chesson, C. B., Buzhdygan, T., Sohail, M., Shavkunov, A. S., Panova, N. I., Nilsson, C. L., Rudra, J. S., Lichti, C. F., Laezza, F. CK2 activity is required for the interaction of FGF14 with voltage-gated sodium channels and neuronal excitability.

Microtubule Dynamics in Neuronal Development, Plasticity, and Neurodegeneration

Neurons are the basic information-processing units of the nervous system. In fulfilling their task, they establish a structural polarity with an axon that can be over a meter long and dendrites with a complex arbor, which can harbor ten-thousands of spines. Microtubules and their associated proteins play important roles during the development of neuronal morphology, the plasticity of neurons, and neurodegenerative processes. They are dynamic structures, which can quickly adapt to changes in the environment and establish a structural scaffold with high local variations in composition and stability. This review presents a comprehensive overview about the role of microtubules and their dynamic behavior during the formation and maturation of processes and spines in the healthy brain, during aging and under neurodegenerative conditions. The review ends with a discussion of microtubule-targeted therapies as a perspective for the supportive treatment of neurodegenerative disorders.

CK2-regulated schwannomin-interacting protein IQCJ-SCHIP-1 association with AnkG contributes to the maintenance of the axon initial segment

The axon initial segment (AIS) plays a central role in electrogenesis and in the maintenance of neuronal polarity. Its molecular organization is dependent on the scaffolding protein ankyrin (Ank) G and is regulated by kinases. For example, the phosphorylation of voltage-gated sodium channels by the protein kinase CK2 regulates their interaction with AnkG and, consequently, their accumulation at the AIS. We previously showed that IQ motif containing J-Schwannomin-Interacting Protein 1 (IQCJ-SCHIP-1), an isoform of the SCHIP-1, accumulated at the AIS in vivo. Here, we analyzed the molecular mechanisms involved in IQCJ-SCHIP-1-specific axonal location. We showed that IQCJ-SCHIP-1 accumulation in the AIS of cultured hippocampal neurons depended on AnkG expression. Pull-down assays and surface plasmon resonance analysis demonstrated that AnkG binds to CK2-phosphorylated IQCJ-SCHIP-1 but not to the non-phosphorylated protein. Surface plasmon resonance approaches using IQCJ-SCHIP-1, SCHIP-1a, another SCHIP-1 isoform, and their C-terminus tail mutants revealed that a segment including multiple CK2-phosphorylatable sites was directly involved in the interaction with AnkG. Pharmacological inhibition of CK2 diminished both IQCJ-SCHIP-1 and AnkG accumulation in the AIS. Silencing SCHIP-1 expression reduced AnkG cluster at the AIS. Finally, over-expression of IQCJ-SCHIP-1 decreased AnkG concentration at the AIS, whereas a mutant deleted of the CK2-regulated AnkG interaction site did not. Our study reveals that CK2-regulated IQJC-SCHIP-1 association with AnkG contributes to AIS maintenance. The axon initial segment (AIS) organization depends on ankyrin (Ank) G and kinases. Here we showed that AnkG binds to CK2-phosphorylated IQCJ-SCHIP-1, in a segment including 12 CK2-phosphorylatable sites. In cultured neurons, either pharmacological inhibition of CK2 or IQCJ-SCHIP-1 silencing reduced AnkG clustering. Overexpressed IQCJ-SCHIP-1 decreased AnkG concentration at the AIS whereas a mutant deleted of the CK2-regulated AnkG interaction site did not. Thus, CK2-regulated IQJC-SCHIP-1 association with AnkG contributes to AIS maintenance.

An Ankyrin-G N-terminal Gate and Protein Kinase CK2 Dually Regulate Binding of Voltage-gated Sodium and KCNQ2/3 Potassium Channels

In many mammalian neurons, fidelity and robustness of action potential generation and conduction depends on the co-localization of voltage-gated sodium (Nav) and KCNQ2/3 potassium channel conductance at the distal axon initial segment (AIS) and nodes of Ranvier in a ratio of ∼40 to 1. Analogous "anchor" peptides within intracellular domains of vertebrate KCNQ2, KCNQ3, and Nav channel α-subunits bind Ankyrin-G (AnkG), thereby mediating concentration of those channels at AISs and nodes of Ranvier. Here, we show that the channel anchors bind at overlapping but distinct sites near the AnkG N terminus. In pulldown assays, the rank order of AnkG binding strength is Nav1.2 ≫ KCNQ3 > KCNQ2. Phosphorylation of KCNQ2 and KCNQ3 anchor domains by protein kinase CK2 (CK2) augments binding, as previously shown for Nav1.2. An AnkG fragment comprising ankyrin repeats 1 through 7 (R1-7) binds phosphorylated Nav or KCNQ anchors robustly. However, mutational analysis of R1-7 reveals differences in binding mechanisms. A smaller fragment, R1-6, exhibits much-diminished KCNQ3 binding but binds Nav1.2 well. Two lysine residues at the tip of repeat 2-3 β-hairpin (residues 105-106) are critical for Nav1.2 but not KCNQ3 channel binding. Another dibasic motif (residues Arg-47, Arg-50) in the repeat 1 front α-helix is crucial for KCNQ2/3 but not Nav1.2 binding. AnkG's alternatively spliced N terminus selectively gates access to those sites, blocking KCNQ but not Nav channel binding. These findings suggest that the 40:1 Nav:KCNQ channel conductance ratio at the distal AIS and nodes arises from the relative strength of binding to AnkG.

Quantitative proteomics reveals protein-protein interactions with fibroblast growth factor 12 as a component of the voltage-gated sodium channel 1.2 (nav1.2) macromolecular complex in Mammalian brain

Voltage-gated sodium channels (Nav1.1–Nav1.9) are responsible for the initiation and propagation of action potentials in neurons, controlling firing patterns, synaptic transmission and plasticity of the brain circuit. Yet, it is the protein–protein interactions of the macromolecular complex that exert diverse modulatory actions on the channel, dictating its ultimate functional outcome. Despite the fundamental role of Nav channels in the brain, information on its proteome is still lacking. Here we used affinity purification from crude membrane extracts of whole brain followed by quantitative high-resolution mass spectrometry to resolve the identity of Nav1.2 protein interactors. Of the identified putative protein interactors, fibroblast growth factor 12 (FGF12), a member of the nonsecreted intracellular FGF family, exhibited 30-fold enrichment in Nav1.2 purifications compared with other identified proteins. Using confocal microscopy, we visualized native FGF12 in the brain tissue and confirmed that FGF12 forms a complex with Nav1.2 channels at the axonal initial segment, the subcellular specialized domain of neurons required for action potential initiation. Co-immunoprecipitation studies in a heterologous expression system validate Nav1.2 and FGF12 as interactors, whereas patch-clamp electrophysiology reveals that FGF12 acts synergistically with CaMKII, a known kinase regulator of Nav channels, to modulate Nav1.2-encoded currents. In the presence of CaMKII inhibitors we found that FGF12 produces a bidirectional shift in the voltage-dependence of activation (more depolarized) and the steady-state inactivation (more hyperpolarized) of Nav1.2, increasing the channel availability. Although providing the first characterization of the Nav1.2 CNS proteome, we identify FGF12 as a new functionally relevant interactor. Our studies will provide invaluable information to parse out the molecular determinant underlying neuronal excitability and plasticity, and extending the relevance of iFGFs signaling in the normal and diseased brain.

Role of the axonal initial segment in psychiatric disorders: function, dysfunction, and intervention

The progress of developing effective interventions against psychiatric disorders has been limited due to a lack of understanding of the underlying cellular and functional mechanisms. Recent research findings focused on exploring novel causes of psychiatric disorders have highlighted the importance of the axonal initial segment (AIS), a highly specialized neuronal structure critical for spike initiation of the action potential. In particular, the role of voltage-gated sodium channels, and their interactions with other protein partners in a tightly regulated macromolecular complex has been emphasized as a key component in the regulation of neuronal excitability. Deficits and excesses of excitability have been linked to the pathogenesis of brain disorders. Identification of the factors and regulatory pathways involved in proper AIS function, or its disruption, can lead to the development of novel interventions that target these mechanistic interactions, increasing treatment efficacy while reducing deleterious off-target effects for psychiatric disorders.

Axon initial segments: diverse and dynamic neuronal compartments

The axon initial segment (AIS) is a structurally and molecularly unique neuronal compartment of the proximal axon that functions as both a physiological and physical bridge between the somatodendritic and axonal domains. The AIS has two main functions: to initiate action potentials and to maintain neuronal polarity. The cytoskeletal scaffold ankyrinG is responsible for these functions and clusters ion channels at the AIS. Recent studies reveal how the AIS forms and remarkable diversity in its structure, function, and composition that may be modulated by neuronal activity and posttranslational modifications of AIS proteins. Furthermore, AIS proteins have been implicated in a variety of human diseases. Here, we discuss these findings and what they teach us about the dynamic AIS.

ATP-P2X7 Receptor Modulates Axon Initial Segment Composition and Function in Physiological Conditions and Brain Injury

Axon properties, including action potential initiation and modulation, depend on both AIS integrity and the regulation of ion channel expression in the AIS. Alteration of the axon initial segment (AIS) has been implicated in neurodegenerative, psychiatric, and brain trauma diseases, thus identification of the physiological mechanisms that regulate the AIS is required to understand and circumvent AIS alterations in pathological conditions. Here, we show that the purinergic P2X7 receptor and its agonist, adenosine triphosphate (ATP), modulate both structural proteins and ion channel density at the AIS in cultured neurons and brain slices. In cultured hippocampal neurons, an increment of extracellular ATP concentration or P2X7-green fluorescent protein (GFP) expression reduced the density of ankyrin G and voltage-gated sodium channels at the AIS. This effect is mediated by P2X7-regulated calcium influx and calpain activation, and impaired by P2X7 inhibition with Brilliant Blue G (BBG), or P2X7 suppression. Electrophysiological studies in brain slices showed that P2X7-GFP transfection decreased both sodium current amplitude and intrinsic neuronal excitability, while P2X7 inhibition had the opposite effect. Finally, inhibition of P2X7 with BBG prevented AIS disruption after ischemia/reperfusion in rats. In conclusion, our study demonstrates an involvement of P2X7 receptors in the regulation of AIS mediated neuronal excitability in physiological and pathological conditions.

The making of a node: a co-production of neurons and glia

Nodes of Ranvier are specialized axonal domains formed in response to a glial signal. Recent research advances have revealed that both CNS and PNS nodes form by several overlapping molecular mechanisms. However, the precise nature of these mechanisms and the hierarchy existing between them is considerably different in CNS versus PNS nodes. Namely, the Schwann cells of the PNS, which directly contact the nodal axolemma, secrete proteins that cluster axonodal components at the edges of the growing myelin segment. In contrast, the formation of CNS nodes, which are not contacted by the myelinating glia, is surprisingly similar to the assembly of the axon initial segment and depends largely on axonal diffusion barriers.

In vivo assembly of the axon initial segment in motor neurons

The axon initial segment (AIS) is responsible for both the modulation of action potentials and the maintenance of neuronal polarity. Yet, the molecular mechanisms controlling its assembly are incompletely understood. Our study in single electroporated motor neurons in mouse embryos revealed that AnkyrinG (AnkG), the AIS master organizer, is undetectable in bipolar migrating motor neurons, but is already expressed at the beginning of axonogenesis at E9.5 and initially distributed homogeneously along the entire growing axon. Then, from E11.5, a stage when AnkG is already apposed to the membrane, as observed by electron microscopy, the protein progressively becomes restricted to the proximal axon. Analysis on the global motor neurons population indicated that Neurofascin follows an identical spatio-temporal distribution, whereas sodium channels and β4-spectrin only appear along AnkG⁺ segments at E11.5. Early patch-clamp recordings of individual motor neurons indicated that at E12.5 these nascent AISs are already able to generate spikes. Using knock-out mice, we demonstrated that neither β4-spectrin nor Neurofascin control the distal-to-proximal restriction of AnkG.

GSK3 and β-catenin determines functional expression of sodium channels at the axon initial segment

Neuronal action potentials are generated through voltage-gated sodium channels, which are tethered by ankyrinG at the membrane of the axon initial segment (AIS). Despite the importance of the AIS in the control of neuronal excitability, the cellular and molecular mechanisms regulating sodium channel expression at the AIS remain elusive. Our results show that GSK3α/β and β-catenin phosphorylated by GSK3 (S33/37/T41) are localized at the AIS and are new components of this essential neuronal domain. Pharmacological inhibition of GSK3 or β-catenin knockdown with shRNAs decreased the levels of phosphorylated-β-catenin, ankyrinG, and voltage-gated sodium channels at the AIS, both "in vitro" and "in vivo", therefore diminishing neuronal excitability as evaluated via sodium current amplitude and action potential number. Thus, our results suggest a mechanism for the modulation of neuronal excitability through the control of sodium channel density by GSK3 and β-catenin at the AIS.

Excitable domains of myelinated nerves: axon initial segments and nodes of Ranvier

Neurons are highly polarized cells. They can be subdivided into at least two structurally and functionally distinct domains: somatodendritic and axonal domains. The somatodendritic domain receives and integrates upstream input signals, and the axonal domain generates and relays outputs in the form of action potentials to the downstream target. Demand for quick response to the harsh surroundings prompted evolution to equip vertebrates' neurons with a remarkable glia-derived structure called myelin. Not only Insulating the axon, myelinating glia also rearrange the axonal components and elaborate functional subdomains along the axon. Proper functioning of all theses domains and subdomains is vital for a normal, efficient nervous system.

Developmental expression of Kv potassium channels at the axon initial segment of cultured hippocampal neurons

Axonal outgrowth and the formation of the axon initial segment (AIS) are early events in the acquisition of neuronal polarity. The AIS is characterized by a high concentration of voltage-dependent sodium and potassium channels. However, the specific ion channel subunits present and their precise localization in this axonal subdomain vary both during development and among the types of neurons, probably determining their firing characteristics in response to stimulation. Here, we characterize the developmental expression of different subfamilies of voltage-gated potassium channels in the AISs of cultured mouse hippocampal neurons, including subunits Kv1.2, Kv2.2 and Kv7.2. In contrast to the early appearance of voltage-gated sodium channels and the Kv7.2 subunit at the AIS, Kv1.2 and Kv2.2 subunits were tethered at the AIS only after 10 days in vitro. Interestingly, we observed different patterns of Kv1.2 and Kv2.2 subunit expression, with each confined to distinct neuronal populations. The accumulation of Kv1.2 and Kv2.2 subunits at the AIS was dependent on ankyrin G tethering, it was not affected by disruption of the actin cytoskeleton and it was resistant to detergent extraction, as described previously for other AIS proteins. This distribution of potassium channels in the AIS further emphasizes the heterogeneity of this structure in different neuronal populations, as proposed previously, and suggests corresponding differences in action potential regulation.

NF-κB signaling pathways: role in nervous system physiology and pathology

Intracellular pathways related to cell survival regulate neuronal physiology during development and neurodegenerative disorders. One of the pathways that have recently emerged with an important role in these processes is nuclear factor-κB (NF-κB). The activity of this pathway leads to the nuclear translocation of the NF-κB transcription factors and the regulation of anti-apoptotic gene expression. Different stimuli can activate the pathway through different intracellular cascades (canonical, non-canonical, and atypical), contributing to the translocation of specific dimers of the NF-κB transcription factors, and each of these dimers can regulate the transcription of different genes. Recent studies have shown that the activation of this pathway regulates opposite responses such as cell survival or neuronal degeneration. These apparent contradictory effects depend on conditions such as the pathway stimuli, the origin of the cells, or the cellular context. In the present review, the authors summarize these findings and discuss their significance with respect to survival or death in the nervous system.

The axon initial segment in nervous system disease and injury

The axon initial segment (AIS), with its dense clusters of voltage-gated ion channels decorating the axonal membrane, regulates action potential initiation and modulation. The AIS also functions as a barrier to maintain axodendritic polarity, and its precise axonal location contributes to the fine-tuning of neuronal excitability. Therefore, it is not surprising that mutations in AIS-related genes, disruption of the molecular organization of the AIS and altered AIS ion channel expression, function, location and/or density are emerging as key players in neurological disorders. Here, we consider the role of the AIS in nervous system disease and injury.

IκBα is not required for axon initial segment assembly

The inhibitor of NF-κB alpha (IκBα) protein is an important regulator of the transcription factor NF-κB. In neurons, IκBα has been shown to play a role in neurite outgrowth and cell survival. Recently, a phosphorylated form of IκBα (pIκBα Ser32/36) was reported to be highly enriched at the axon initial segment (AIS) and was proposed to function upstream of ankyrinG in AIS assembly, including ion channel recruitment. However, we report here that the AIS clustering of ankyrinG and Na(+) channels in the brains of IκBα knockout (Nfkbia(-/-)) mice is comparable to that in wild-type littermates. Furthermore, we found that multiple phospho-specific antibodies against pIκBα Ser32/36 non-specifically label AIS in Nfkbia(-/-) cortex and AIS in dissociated Nfkbia(-/-) hippocampal neurons. With the exception of ankyrinG, shRNA-mediated knockdown of known AIS proteins in cultured hippocampal neurons did not eliminate the AIS labeling with pIκBα antibodies. Instead, the pIκBα antibodies cross-react with a phosphorylated epitope of a protein associated with the microtubule-based AIS cytoskeleton that is not integrated into the AIS membrane complex organized by ankyrinG. Our results indicate that pIκBα is neither enriched at the AIS nor required for AIS assembly.

Bioluminescence methodology for the detection of protein-protein interactions within the voltage-gated sodium channel macromolecular complex

Protein-protein interactions are critical molecular determinants of ion channel function and emerging targets for pharmacological interventions. Yet, current methodologies for the rapid detection of ion channel macromolecular complexes are still lacking. In this study we have adapted a split-luciferase complementation assay (LCA) for detecting the assembly of the voltage-gated Na+ (Nav) channel C-tail and the intracellular fibroblast growth factor 14 (FGF14), a functionally relevant component of the Nav channelosome that controls gating and targeting of Nav channels through direct interaction with the channel C-tail. In the LCA, two complementary N-terminus and C-terminus fragments of the firefly luciferase were fused, respectively, to a chimera of the CD4 transmembrane segment and the C-tail of Nav1.6 channel (CD4-Nav1.6-NLuc) or FGF14 (CLuc-FGF14). Co-expression of CLuc-FGF14 and CD4-Nav1.6-NLuc in live cells led to a robust assembly of the FGF14:Nav1.6 C-tail complex, which was attenuated by introducing single-point mutations at the predicted FGF14:Nav channel interface. To evaluate the dynamic regulation of the FGF14:Nav1.6 C-tail complex by signaling pathways, we investigated the effect of kinase inhibitors on the complex formation. Through a platform of counter screenings, we show that the p38/MAPK inhibitor, PD169316, and the IκB kinase inhibitor, BAY 11-7082, reduce the FGF14:Nav1.6 C-tail complementation, highlighting a potential role of the p38MAPK and the IκB/NFκB pathways in controlling neuronal excitability through protein-protein interactions. We envision the methodology presented here as a new valuable tool to allow functional evaluations of protein-channel complexes toward probe development and drug discovery targeting ion channels implicated in human disorders.

Novel diffusion barrier for axonal retention of Tau in neurons and its failure in neurodegeneration

Missorting of Tau from axons to the somatodendritic compartment of neurons is a hallmark of Alzheimer's disease, but the mechanisms underlying normal sorting and pathological failure are poorly understood. Here, we used several Tau constructs labelled with photoconvertible Dendra2 to analyse its mobility in polarized neurons. This revealed a novel mechanism of sorting-a retrograde barrier in the axon initial segment (AIS) operating as cellular rectifier. It allows anterograde flow of axonal Tau but prevents retrograde flow back into soma and dendrites. The barrier requires binding of Tau to microtubules but does not require F-actin and thus is distinct from the sorting of membrane-associated proteins at the AIS. The barrier breaks down when Tau is phosphorylated in its repeat domain and detached from microtubules, for example, by the kinase MARK/Par1. These observations link the pathological hallmarks of Tau missorting and hyperphosphorylation in neurodegenerative diseases.

In vitro maturation of the cisternal organelle in the hippocampal neuron's axon initial segment

Regulation of Ca(2+) concentrations is essential to maintain the structure and function of the axon initial segment (AIS). The so-called cisternal organelle of the AIS is a structure involved in this regulation, although little is known as to how this organelle matures and is stabilized. Here we describe how the cisternal organelle develops in cultured hippocampal neurons and the interactions that facilitate its stabilization in the AIS. We also characterize the developmental expression of molecules involved in Ca(2+) regulation in the AIS. Our results indicate that synaptopodin (synpo) positive elements considered to be associated to the cisternal organelle are present in the AIS after six days in vitro. There are largely overlapping microdomains containing the inositol 1,4,5-triphosphate receptor 1 (IP(3)R1) and the Ca(2+) binding protein annexin 6, suggesting that the regulation of Ca(2+) concentrations in the AIS is sensitive to IP(3) and subject to regulation by annexin 6. The expression of synpo, IP(3)R1 and annexin 6 in the AIS is independent of the neuron activity, as it was unaffected by tetrodotoxin blockage of action potentials and it was resistant to detergent extraction, indicating that these proteins interact with scaffolding and/or cytoskeleton proteins. The presence of ankyrin G seems to be required for the acquisition and maintenance of the cisternal organelle, while the integrity of the actin cytoskeleton must be maintained for the expression IP(3)R1 and annexin 6 to persist in the AIS.

Treatment of cultured human astrocytes and vascular endothelial cells with protein kinase CK2 inhibitors induces early changes in cell shape and cytoskeleton

Ubiquitous protein kinase CK2 is a key regulator of cell migration, proliferation and tumor growth. CK2 is abundant in retinal astrocytes, and its inhibition suppresses retinal neovascularization in a mouse retinopathy model. In human astrocytes, CK2 co-distributes with GFAP-containing intermediate filaments, which implies its association with cytoskeleton. Contrary to astrocytes, CK2 is co-localized in microvascular endothelial cells (HBMVEC) with microtubules and actin stress fibers, but not with vimentin-containing intermediate filaments. Specific CK2 inhibitors (TBB, TBI, TBCA and DMAT) and nine novel CK2 inhibiting compounds (TID43, TID46, Quinolone-7, Quinolone-39, FNH28, FNH62, FNH64, FNH68 and FNH74) were tested at 10-200 μM for their ability to induce morphological alterations in cultured human astrocytes (HAST-40), and HBMVEC (For explanation of the inhibitor names, see "Methods" section). CK2 inhibitors caused dramatic changes in shape of cultured cells with effective inhibitor concentrations between 50 and 100 μM. Attached cells retracted, acquired shortened processes, and eventually rounded up and detached. CK2 inhibitor-induced morphological alterations were completely reversible and were not blocked by caspase inhibition. However, longer treatment or higher inhibitor concentration did cause apoptosis. The speed and potency of the CK2 inhibitors effects on cell shape and adhesion were inversely correlated with serum concentration. Western analyses showed that TBB and TBCA elicited a significant (about twofold) increase in the activation of p38 and ERK1/2 MAP kinases that may be involved in cytoskeleton regulation. This novel early biological cell response to CK2 inhibition may underlie the anti-angiogenic effect of CK2 suppression in the retina.