A Targeting Motif Involved in Sodium Channel Clustering at the Axonal Initial Segment

The axonal initial segment is a unique subdomain of the neuron that maintains cellular polarization and contributes to electrogenesis. To obtain new insights into the mechanisms that determine protein segregation in this subdomain, we analyzed the trafficking of a reporter protein containing the cytoplasmic II–III linker sequence involved in sodium channel targeting and clustering (Garrido, J.J., P. Giraud, E. Carlier, F. Fernandes, A. Moussif, M.P. Fache, D. Debanne, and B. Dargent. 2003. Science. 300:2091–2094). Here, we show that this reporter protein is preferentially inserted in the somatodendritic domain and is trapped at the axonal initial segment by tethering to the cytoskeleton, before its insertion in the axonal tips. The nontethered population in dendrites, soma, and the distal part of axons is subsequently eliminated by endocytosis. We provide evidence for the involvement of two independent determinants in the II–III linker of sodium channels. These findings indicate that endocytotic elimination and domain-selective tethering constitute a potential mechanism of protein segregation at the axonal initial segment of hippocampal neurons.

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Ca2+ entry through NaV channels generates submillisecond axonal Ca2+ signaling

eLife ◽

10.7554/elife.54566 ◽

2020 ◽

Vol 9 ◽

Cited By ~ 1

Author(s):

Naomi AK Hanemaaijer ◽

Marko A Popovic ◽

Xante Wilders ◽

Sara Grasman ◽

Oriol Pavón Arocas ◽

...

Keyword(s):

Calcium Channels ◽

High Speed ◽

Initial Segment ◽

Hek 293 ◽

Voltage Gated ◽

Voltage Gated Calcium Channels ◽

293 Cells ◽

Axonal Initial Segment ◽

Nav Channels ◽

Activity Dependent

Calcium ions (Ca2+) are essential for many cellular signaling mechanisms and enter the cytosol mostly through voltage-gated calcium channels. Here, using high-speed Ca2+ imaging up to 20 kHz in the rat layer five pyramidal neuron axon we found that activity-dependent intracellular calcium concentration ([Ca2+]i) in the axonal initial segment was only partially dependent on voltage-gated calcium channels. Instead, [Ca2+]i changes were sensitive to the specific voltage-gated sodium (NaV) channel blocker tetrodotoxin. Consistent with the conjecture that Ca2+ enters through the NaV channel pore, the optically resolved ICa in the axon initial segment overlapped with the activation kinetics of NaV channels and heterologous expression of NaV1.2 in HEK-293 cells revealed a tetrodotoxin-sensitive [Ca2+]i rise. Finally, computational simulations predicted that axonal [Ca2+]i transients reflect a 0.4% Ca2+ conductivity of NaV channels. The findings indicate that Ca2+ permeation through NaV channels provides a submillisecond rapid entry route in NaV-enriched domains of mammalian axons.

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Altered action potential waveform and shorter axonal initial segment in hiPSC-derived motor neurons with mutations in VRK1

Neurobiology of Disease ◽

10.1016/j.nbd.2021.105609 ◽

2022 ◽

pp. 105609

Author(s):

Rémi Bos ◽

Khalil Rihan ◽

Patrice Quintana ◽

Lara El-Bazzal ◽

Nathalie Bernard-Marissal ◽

...

Keyword(s):

Action Potential ◽

Motor Neurons ◽

Initial Segment ◽

Axonal Initial Segment ◽

Action Potential Waveform ◽

Potential Waveform

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Developmental formation of a diffusion barrier in the plasma membrane of the axonal initial segment : single lipid molecule approach

Seibutsu Butsuri ◽

10.2142/biophys.40.s212_2 ◽

2000 ◽

Vol 40 (supplement) ◽

pp. S212

Author(s):

C. Nakada ◽

M. Nozaki ◽

H. Yamashita ◽

K. Yamaguchi ◽

Ken Ritchie ◽

...

Keyword(s):

Plasma Membrane ◽

Diffusion Barrier ◽

Initial Segment ◽

Lipid Molecule ◽

Axonal Initial Segment

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Paranodal transverse bands are required for maintenance but not initiation of Nav1.6 sodium channel clustering in CNS optic nerve axons

Glia ◽

10.1002/glia.10284 ◽

2003 ◽

Vol 44 (2) ◽

pp. 173-182 ◽

Cited By ~ 18

Author(s):

Matthew N. Rasband ◽

Christopher M. Taylor ◽

Rashmi Bansal

Keyword(s):

Optic Nerve ◽

Sodium Channel ◽

Sodium Channel Clustering

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Nr-CAM and neurofascin interactions regulate ankyrin G and sodium channel clustering at the node of Ranvier

Current Biology ◽

10.1016/s0960-9822(01)00586-3 ◽

2001 ◽

Vol 11 (23) ◽

pp. 1864-1869 ◽

Cited By ~ 48

Author(s):

M. Lustig ◽

G. Zanazzi ◽

T. Sakurai ◽

C. Blanco ◽

S.R. Levinson ◽

...

Keyword(s):

Sodium Channel ◽

Node Of Ranvier ◽

Ankyrin G ◽

Sodium Channel Clustering

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The basis of sharp spike onset in standard biophysical models

10.1101/037549 ◽

2016 ◽

Author(s):

Maria Telenczuk ◽

Bertrand Fontaine ◽

Romain Brette

Keyword(s):

Initial Segment ◽

Compartment Model ◽

Coupling Model ◽

Initiation Site ◽

Current Loop ◽

Electrical Dipole ◽

Biophysical Models ◽

Axonal Initial Segment ◽

Spike Initiation ◽

As If

AbstractIn most vertebrate neurons, spikes initiate in the axonal initial segment (AIS). When recorded in the soma, they have a surprisingly sharp onset, as if sodium (Na) channels opened abruptly. The main view stipulates that spikes initiate in a conventional manner at the distal end of the AIS, then progressively sharpen as they backpropagate to the soma. We examined the biophysical models used to substantiate this view, and we found that orthodromic spikes do no initiate through a local axonal current loop that propagates along the axon, but through a global current loop encompassing the AIS and soma, which forms an electrical dipole. Therefore, the phenomenon is not adequately modeled as the backpropagation of an electrical wave along the axon, since the wavelength would be as large as the entire system. Instead, in these models, we found that spike initiation rather follows the critical resistive coupling model proposed recently, where the Na current entering the AIS is matched by the axial resistive current flowing to the soma. Besides demonstrating it by examining the balance of currents at spike initiation, we show that the observed increase in spike sharpness along the axon is artifactual and disappears when an appropriate measure of rapidness is used; instead, somatic onset rapidness can be predicted from spike shape at initiation site. Finally, we reproduce the phenomenon in a two-compartment model, showing that it does not rely on propagation. In these models, the sharp onset of somatic spikes is therefore not an artifact of observing spikes at the incorrect location, but rather the signature that spikes are initiated through a global soma-AIS current loop forming an electrical dipole.Author summaryIn most vertebrate neurons, spikes are initiated in the axonal initial segment, next to the soma. When recorded at the soma, action potentials appear to suddenly rise as if all sodium channels opened at once. This has been previously attributed to the backpropagation of spikes from the initial segment to the soma. Here we demonstrate with biophysical models that backpropagation does not contribute to the sharpness of spike onset. Instead, we show that the phenomenon is due to the resistive coupling between the large somatodendritic compartment and the small axonal compartment, a geometrical discontinuity that leads to an abrupt variation in voltage.

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Specific Role of the Truncated βIV-Spectrin Σ6 in Sodium Channel Clustering at Axon Initial Segments and Nodes of Ranvier

Journal of Biological Chemistry ◽

10.1074/jbc.m609223200 ◽

2006 ◽

Vol 282 (9) ◽

pp. 6548-6555 ◽

Cited By ~ 30

Author(s):

Yoko Uemoto ◽

So-ichiro Suzuki ◽

Nobuo Terada ◽

Nobuhiko Ohno ◽

Shinichi Ohno ◽

...

Keyword(s):

Sodium Channel ◽

Specific Role ◽

Nodes Of Ranvier ◽

Axon Initial Segments ◽

Sodium Channel Clustering

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