βIV-spectrin regulates sodium channel clustering through ankyrin-G at axon initial segments and nodes of Ranvier

β-Spectrin and ankyrin are major components of the membrane cytoskeleton. We have generated mice carrying a null mutation in the βIV-spectrin gene using gene trapping in embryonic stem cells. Mice homozygous for the mutation exhibit tremors and contraction of hindlimbs. βIV-spectrin expression is mostly restricted to neurons, where it colocalizes with and binds to ankyrin-G at axon initial segments (AISs) and nodes of Ranvier (NR). In βIV-spectrin–null neurons, neither ankyrin-G nor voltage-gated sodium channels (VGSC) are correctly clustered at these sites, suggesting that impaired action potential caused by mislocalization of VGSC leads to the phenotype. Conversely, in ankyrin-G–null neurons, βIV-spectrin is not localized to these sites. These results indicate that βIV-spectrin and ankyrin-G mutually stabilize the membrane protein cluster and the linked membrane cytoskeleton at AIS and NR.

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βIVΣ1 spectrin stabilizes the nodes of Ranvier and axon initial segments

The Journal of Cell Biology ◽

10.1083/jcb.200408007 ◽

2004 ◽

Vol 166 (7) ◽

pp. 983-990 ◽

Cited By ~ 89

Author(s):

Sandra Lacas-Gervais ◽

Jun Guo ◽

Nicola Strenzke ◽

Eric Scarfone ◽

Melanie Kolpe ◽

...

Keyword(s):

Sodium Channels ◽

Actin Binding ◽

Ultrastructural Changes ◽

Nodes Of Ranvier ◽

Ankyrin G ◽

Voltage Gated Sodium Channels ◽

Dense Membrane ◽

Afferent Fibers ◽

Actin Binding Domain ◽

Axon Initial Segments

Saltatory electric conduction requires clustered voltage-gated sodium channels (VGSCs) at axon initial segments (AIS) and nodes of Ranvier (NR). A dense membrane undercoat is present at these sites, which is thought to be key for the focal accumulation of channels. Here, we prove that βIVΣ1 spectrin, the only βIV spectrin with an actin-binding domain, is an essential component of this coat. Specifically, βIVΣ1 coexists with βIVΣ6 at both AIS and NR, being the predominant spectrin at AIS. Removal of βIVΣ1 alone causes the disappearance of the nodal coat, an increased diameter of the NR, and the presence of dilations filled with organelles. Moreover, in myelinated cochlear afferent fibers, VGSC and ankyrin G clusters appear fragmented. These ultrastructural changes can explain the motor and auditory neuropathies present in βIVΣ1 −/− mice and point to the βIVΣ1 spectrin isoform as a master-stabilizing factor of AIS/NR membranes.

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Ankyrin-G coordinates assembly of the spectrin-based membrane skeleton, voltage-gated sodium channels, and L1 CAMs at Purkinje neuron initial segments

The Journal of Cell Biology ◽

10.1083/jcb.200109026 ◽

2001 ◽

Vol 155 (5) ◽

pp. 739-746 ◽

Cited By ~ 277

Author(s):

Scott M. Jenkins ◽

Vann Bennett

Keyword(s):

Adhesion Molecules ◽

Sodium Channels ◽

Action Potentials ◽

Membrane Skeleton ◽

Purkinje Neuron ◽

Voltage Gated ◽

Ankyrin G ◽

Voltage Gated Sodium Channels ◽

Cerebellar Purkinje Neurons ◽

Axon Initial Segments

The axon initial segment is an excitable membrane highly enriched in voltage-gated sodium channels that integrates neuronal inputs and initiates action potentials. This study identifies Nav1.6 as the voltage-gated sodium channel isoform at mature Purkinje neuron initial segments and reports an essential role for ankyrin-G in coordinating the physiological assembly of Nav1.6, βIV spectrin, and the L1 cell adhesion molecules (L1 CAMs) neurofascin and NrCAM at initial segments of cerebellar Purkinje neurons. Ankyrin-G and βIV spectrin appear at axon initial segments by postnatal day 2, whereas L1 CAMs and Nav1.6 are not fully assembled at continuous high density along axon initial segments until postnatal day 9. L1 CAMs and Nav1.6 therefore do not initiate protein assembly at initial segments. βIV spectrin, Nav1.6, and L1 CAMs are not clustered in adult Purkinje neuron initial segments of mice lacking cerebellar ankyrin-G. These results support the conclusion that ankyrin-G coordinates the physiological assembly of a protein complex containing transmembrane adhesion molecules, voltage-gated sodium channels, and the spectrin membrane skeleton at axon initial segments.

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Nodes of Ranvier and axon initial segments are ankyrin G–dependent domains that assemble by distinct mechanisms

The Journal of Cell Biology ◽

10.1083/jcb.200612012 ◽

2007 ◽

Vol 177 (5) ◽

pp. 857-870 ◽

Cited By ~ 123

Author(s):

Yulia Dzhashiashvili ◽

Yanqing Zhang ◽

Jolanta Galinska ◽

Isabel Lam ◽

Martin Grumet ◽

...

Keyword(s):

Schwann Cells ◽

Sodium Channels ◽

Cytoskeletal Proteins ◽

Action Potential Generation ◽

Potential Generation ◽

Nodes Of Ranvier ◽

Ankyrin G ◽

Sites Of Action ◽

Axon Initial Segments ◽

Inside Out

Axon initial segments (AISs) and nodes of Ranvier are sites of action potential generation and propagation, respectively. Both domains are enriched in sodium channels complexed with adhesion molecules (neurofascin [NF] 186 and NrCAM) and cytoskeletal proteins (ankyrin G and βIV spectrin). We show that the AIS and peripheral nervous system (PNS) nodes both require ankyrin G but assemble by distinct mechanisms. The AIS is intrinsically specified; it forms independent of NF186, which is targeted to this site via intracellular interactions that require ankyrin G. In contrast, NF186 is targeted to the node, and independently cleared from the internode, by interactions of its ectodomain with myelinating Schwann cells. NF186 is critical for and initiates PNS node assembly by recruiting ankyrin G, which is required for the localization of sodium channels and the entire nodal complex. Thus, initial segments assemble from the inside out driven by the intrinsic accumulation of ankyrin G, whereas PNS nodes assemble from the outside in, specified by Schwann cells, which direct the NF186-dependent recruitment of ankyrin G.

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