scholarly journals Tropomyosin Tpm3.1 is required to maintain the structure and function of the axon initial segment

2019 ◽  
Author(s):  
Amr Abouelezz ◽  
Holly Stefen ◽  
Mikael Segerstråle ◽  
David Micinski ◽  
Rimante Minkeviciene ◽  
...  
Keyword(s):  
Actin Filaments ◽  
Hippocampal Neurons ◽  
Initial Segment ◽  
Actin Polymerization ◽  
Firing Frequency ◽  
Axon Initial Segment ◽  
Action Potential Initiation ◽  
Sodium Ion Channels ◽  
And Function ◽  
Filament Network

ABSTRACTThe axon initial segment (AIS) is the site of action potential initiation and serves as a vesicular filter and diffusion barrier that help maintain neuronal polarity. Recent studies have revealed details about a specialized structural complex in the AIS. While an intact actin cytoskeleton is required for AIS formation, pharmacological disruption of actin polymerization compromises the AIS vesicle filter but does not affect overall AIS structure. In this study, we found that the tropomyosin isoform Tpm3.1 decorates a population of relatively stable actin filaments in the AIS. Inhibiting Tpm3.1 in cultured hippocampal neurons led to the loss of AIS structure, the AIS vesicle filter, the clustering of sodium ion channels, and reduced firing frequency. We propose that Tpm3.1-decorated actin filaments form a stable actin filament network under the AIS membrane which provides a scaffold for membrane organization and AIS proteins.

scholarly journals Axon initial segment cytoskeleton comprises a multiprotein submembranous coat containing sparse actin filaments

2014 ◽  
Vol 205 (1) ◽  
pp. 67-81 ◽  
Author(s):  
Steven L. Jones ◽  
Farida Korobova ◽  
Tatyana Svitkina
Keyword(s):  
Actin Filaments ◽  
Hippocampal Neurons ◽  
Initial Segment ◽  
Neuronal Cell ◽  
Axon Initial Segment ◽  
Actin Dynamics ◽  
Neuronal Cell Adhesion Molecule ◽  
Voltage Gated Sodium Channels ◽  
Actin Structure ◽  
Action Potential Initiation

The axon initial segment (AIS) of differentiated neurons regulates action potential initiation and axon–dendritic polarity. The latter function depends on actin dynamics, but actin structure and functions at the AIS remain unclear. Using platinum replica electron microscopy (PREM), we have characterized the architecture of the AIS cytoskeleton in mature and developing hippocampal neurons. The AIS cytoskeleton assembly begins with bundling of microtubules and culminates in formation of a dense, fibrillar–globular coat over microtubule bundles. Immunogold PREM revealed that the coat contains a network of known AIS proteins, including ankyrin G, spectrin βIV, neurofascin, neuronal cell adhesion molecule, voltage-gated sodium channels, and actin filaments. Contrary to existing models, we find neither polarized actin arrays, nor dense actin meshworks in the AIS. Instead, the AIS contains two populations of sparse actin filaments: short, stable filaments and slightly longer dynamic filaments. We propose that stable actin filaments play a structural role for formation of the AIS diffusion barrier, whereas dynamic actin may promote AIS coat remodeling.

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

2021 ◽  
Author(s):  
Wei Zhang ◽  
María Ciorraga ◽  
Pablo Mendez ◽  
Diana Retana ◽  
Norah Boumedine-Guignon ◽  
...  
Keyword(s):  
Hippocampal Neurons ◽  
Initial Segment ◽  
Protein Transport ◽  
Neuronal Excitability ◽  
Brain Slices ◽  
Axon Initial Segment ◽  
Composition And Structure ◽  
The Stability ◽  
Protein Density ◽  
Voltage Gated Ion Channels

AbstractThe 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.

scholarly journals Homeostatic plasticity rules control the wiring of axo-axonic synapses at the axon initial segment

2018 ◽  
Author(s):  
Alejandro Pan-Vazquez ◽  
Winnie Wefelmeyer ◽  
Victoria Gonzalez Sabater ◽  
Juan Burrone
Keyword(s):  
Initial Segment ◽  
Pyramidal Cells ◽  
Axon Initial Segment ◽  
Homeostatic Plasticity ◽  
Network Activity ◽  
Gabaergic Interneuron ◽  
Chandelier Cells ◽  
Local Circuits ◽  
And Function

AbstractGABAergic interneurons are chiefly responsible for controlling the activity of local circuits in the cortex1,2. However, the rules that govern the wiring of interneurons are not well understood3. Chandelier cells (ChCs) are a type of GABAergic interneuron that control the output of hundreds of neighbouring pyramidal cells through axo-axonic synapses which target the axon initial segment (AIS)4. Despite their importance in modulating circuit activity, our knowledge of the development and function of axo-axonic synapses remains elusive. In this study, we investigated the role of activity in the formation and plasticity of ChC synapses. 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 resulted in a reversible decrease in axo-axonic connections. Voltage imaging of GABAergic transmission at the AIS showed that axo-axonic synapses were depolarising during this period. Identical manipulations of network activity in older mice (P40-P46), when ChC synapses are inhibitory, resulted in an increase in axo-axonic synapses. We propose that the direction of ChC plasticity follows homeostatic rules that depend on the polarity of axo-axonic synapses.

scholarly journals Differential Effects of Axon Initial Segment and Somatodendritic GABAA Receptors on Excitability Measures in Rat Dentate Granule Neurons

2011 ◽  
Vol 105 (1) ◽  
pp. 366-379 ◽  
Author(s):  
Patricio Rojas ◽  
Alejandro Akrouh ◽  
Lawrence N. Eisenman ◽  
Steven Mennerick
Keyword(s):  
Action Potential ◽  
Initial Segment ◽  
Input Resistance ◽  
Receptor Activation ◽  
Axon Initial Segment ◽  
Granule Neurons ◽  
Voltage Threshold ◽  
Granule Neuron ◽  
Action Potential Initiation ◽  
Potential Initiation

GABAA receptors are found on the somatodendritic compartment and on the axon initial segment of many principal neurons. The function of axonal receptors remains obscure, although it is widely assumed that axonal receptors must have a strong effect on excitability. We found that activation of GABAA receptors on the dentate granule neuron axon initial segment altered excitability by depolarizing the voltage threshold for action potential initiation under conditions that minimally affected overall cell input resistance. In contrast, activation of somatic GABAA receptors strongly depressed the input resistance of granule neurons without affecting the voltage threshold of action potential initiation. Although these effects were observed over a range of intracellular chloride concentrations, average voltage threshold was unaffected when ECl rendered GABAA axon initial segment responses explicitly excitatory. A compartment model of a granule neuron confirmed these experimental observations. Low ambient agonist concentrations designed to activate granule neuron tonic currents did not stimulate axonal receptors sufficiently to raise voltage threshold. Using excitatory postsynaptic current (EPSC)-like depolarizations, we show physiological consequences of axonal versus somatic GABAA receptor activation. With axonal inhibition, individual excitatory postsynaptic potentials (EPSPs) largely retained their amplitude and time course, but EPSPs that were suprathreshold under basal conditions failed to reach threshold with GABAA activation. By contrast, somatic inhibition depressed individual EPSPs because of strong shunting. Our results suggest that axonal GABAA receptors have a privileged effect on voltage threshold and that two major measures of neuronal excitability, voltage threshold and rheobase, are differentially affected by axonal and somatic GABAA receptor activation.

scholarly journals Networks of Polarized Actin Filaments in the Axon Initial Segment Provide a Mechanism for Sorting Axonal and Dendritic Proteins

Cell Reports ◽  
2012 ◽  
Vol 2 (6) ◽  
pp. 1546-1553 ◽  
Author(s):  
Kaori Watanabe ◽  
Sarmad Al-Bassam ◽  
Yusuke Miyazaki ◽  
Thomas J. Wandless ◽  
Paul Webster ◽  
...  
Keyword(s):  
Actin Filaments ◽  
Initial Segment ◽  
Axon Initial Segment

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

2016 ◽  
Vol 2016 ◽  
pp. 1-19 ◽  
Author(s):  
Steven L. Jones ◽  
Tatyana M. Svitkina
Keyword(s):  
Actin Filaments ◽  
Initial Segment ◽  
Structural Features ◽  
Axon Initial Segment ◽  
Action Potential Firing ◽  
Potential Firing ◽  
Triangular Network ◽  
Specialized Structure ◽  
Architecture Development ◽  
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.

scholarly journals The Cell Adhesion Molecule Neurofascin Stabilizes Axo-axonic GABAergic Terminals at the Axon Initial Segment

2011 ◽  
Vol 286 (27) ◽  
pp. 24385-24393 ◽  
Author(s):  
Martin Kriebel ◽  
Jennifer Metzger ◽  
Sabine Trinks ◽  
Deepti Chugh ◽  
Robert J. Harvey ◽  
...  
Keyword(s):  
Cell Adhesion ◽  
Adhesion Molecule ◽  
Hippocampal Neurons ◽  
Initial Segment ◽  
Cell Adhesion Molecule ◽  
Synapse Formation ◽  
Growth Factor Receptor ◽  
Axon Initial Segment ◽  
Adult Rat ◽  
Receptor Clusters

Cell adhesion molecules regulate synapse formation and maintenance via transsynaptic contact stabilization involving both extracellular interactions and intracellular postsynaptic scaffold assembly. The cell adhesion molecule neurofascin is localized at the axon initial segment of granular cells in rat dentate gyrus, which is mainly targeted by chandelier cells. Lentiviral shRNA-mediated knockdown of neurofascin in adult rat brain indicates that neurofascin regulates the number and size of postsynaptic gephyrin scaffolds, the number of GABAA receptor clusters as well as presynaptic glutamate decarboxylase-positive terminals at the axon initial segment. By contrast, overexpression of neurofascin in hippocampal neurons increases gephyrin cluster size presumably via stimulation of fibroblast growth factor receptor 1 signaling pathways.

scholarly journals The Palmitoyl Acyltransferase ZDHHC14 Controls Kv1-Family Potassium Channel Clustering at the Axon Initial Segment

2020 ◽  
Author(s):  
Shaun S. Sanders ◽  
Luiselys M. Hernandez ◽  
Heun Soh ◽  
Santi Karnam ◽  
Randall S. Walikonis ◽  
...  
Keyword(s):  
Potassium Channels ◽  
Hippocampal Neurons ◽  
Initial Segment ◽  
Neuronal Excitability ◽  
Pdz Domain ◽  
Axon Initial Segment ◽  
Type I ◽  
Action Potential Firing ◽  
Potential Firing ◽  
Palmitoyl Acyltransferase

AbstractThe palmitoyl acyltransferase (PAT) ZDHHC14 is highly expressed in the hippocampus and is the only PAT predicted to bind Type I PDZ domain-containing proteins. However, ZDHHC14’s neuronal roles are unknown. Here, we identify the PDZ domain-containing Membrane-associated Guanylate Kinase (MaGUK) PSD93 as a direct ZDHHC14 interactor and substrate. PSD93, but not other MaGUKs, localizes to the Axon Initial Segment (AIS). Using lentiviral-mediated shRNA knockdown in rat hippocampal neurons, we find that ZDHHC14 controls palmitoylation and AIS clustering of PSD93 and also of Kv1 potassium channels, which directly bind PSD93. Neurodevelopmental expression of ZDHHC14 mirrors that of PSD93 and Kv1 channels and, consistent with ZDHHC14’s importance for Kv1 channel clustering, loss of ZDHHC14 decreases outward currents and increases action potential firing in hippocampal neurons. To our knowledge, these findings identify the first neuronal roles and substrates for ZDHHC14 and reveal a previously unappreciated role for palmitoylation in control of neuronal excitability.Impact StatementZDHHC14 controls palmitoylation and axon initial segment targeting of PSD93 and Kv1-family potassium channels, events that are essential for normal neuronal excitability.

scholarly journals The palmitoyl acyltransferase ZDHHC14 controls Kv1-family potassium channel clustering at the axon initial segment

eLife ◽  
2020 ◽  
Vol 9 ◽  
Author(s):  
Shaun S Sanders ◽  
Luiselys M Hernandez ◽  
Heun Soh ◽  
Santi Karnam ◽  
Randall S Walikonis ◽  
...  
Keyword(s):  
Hippocampal Neurons ◽  
Initial Segment ◽  
Neuronal Excitability ◽  
Pdz Domain ◽  
Axon Initial Segment ◽  
Type I ◽  
Action Potential Firing ◽  
Outward Currents ◽  
Potential Firing ◽  
Palmitoyl Acyltransferase

The palmitoyl acyltransferase (PAT) ZDHHC14 is highly expressed in the hippocampus and is the only PAT predicted to bind Type-I PDZ domain-containing proteins. However, ZDHHC14’s neuronal roles are unknown. Here, we identify the PDZ domain-containing Membrane-associated Guanylate Kinase (MaGUK) PSD93 as a direct ZDHHC14 interactor and substrate. PSD93, but not other MaGUKs, localizes to the axon initial segment (AIS). Using lentiviral-mediated shRNA knockdown in rat hippocampal neurons, we find that ZDHHC14 controls palmitoylation and AIS clustering of PSD93 and also of Kv1 potassium channels, which directly bind PSD93. Neurodevelopmental expression of ZDHHC14 mirrors that of PSD93 and Kv1 channels and, consistent with ZDHHC14’s importance for Kv1 channel clustering, loss of ZDHHC14 decreases outward currents and increases action potential firing in hippocampal neurons. To our knowledge, these findings identify the first neuronal roles and substrates for ZDHHC14 and reveal a previously unappreciated role for palmitoylation in control of neuronal excitability.

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