Nav1.6
Description: sodium channel, voltage gated, type VIII, alpha subunit Gene: Scn8a Alias: nav1.6, scn8a
Nav1.6, encoded by the gene scn8a, is a sodium, voltage-gated, type 8, alpha subunit channel. Nav1.6 is most abundantly expressed in the CNS during adulthood. It is involved in the control of backpropagation and peaking of the action potential. Mutations to the channel are linked to developmental and epileptic encephalopathy .
Experimental data
Mouse Nav1.6 gene in CHO host cells |
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scn8a, the gene encoding for Nav1.6, is located on chromosome 12 at position 13 (12q13.13) in humans and is composed of 27 exons 26 of which are coding and exon 1 being non-coding.
The amino acid sequence of Nav1.6 is 84% identical to the sequences of Nav1.1 and Nav1.2, but it is more distant from those two than they are to each other [52]
There exists multiple Nav1.6 transcript variants across species as a result of the alternative splicing of scn8a (see Protein Isoform table).
Of the many variants that have been identified, only a few have been properly studied, such the ones resulting from the alternative splicing of exons 10B, 12, 18, and 5.
Notably, like other Navs, the alternative splicing of exon 5 results in a “neonatal” variant of Nav1.6. This splicing event is mirrored by the alternative splicing of exon 18, leading to exon18A and exon18N variants. Exons 5 and 18 encode the corresponding transmembrane segments in domains I and III respectively, indicating possible shared evolutionary origin. Both variants are developmentally regulated, with their expression being highest during the fetal development stage before switching to the adult splice form. [2184] [21845] [2186]
mRNA transcripts, including exon 18N, are expressed at a low level in non-neuronal tissues and are subject to nonsense-mediated decay. 18A variants appear to be restricted to neurons, and are mediated by neuron-specific splice factors. The reasons for this control haven’t been determined yet but a hypothesis is that exon 18 may represent a fail-safe mechanism to prevent damage to non-neuronal cells [2186].
| Species | NCBI accession | Length (nt) | |
|---|---|---|---|
| Human | NM_014191.4 | 11559 | |
| Mouse | NM_001077499.2 | 11340 | |
| Rat | NM_019266.3 | 6548 |
The canonical Nav1.6 isoform is made of around 1980 amino acid residues, the sequence of which is highly conserved across species. The channel has a molecular weight of 229 Kda. [2184]
There exists a number of protein isoforms that arise from the translation of the aforementioned transcript variants:
- The neonatal isoform, resulting from the alternative splicing of exon 5, differs by a single amino acid from the canonical adult form 5A.
- Proteins resulting from the translation of the exon 18N variant contain an in-frame stop codon that results in a truncated channel protein. [2186]
Isoforms
Glycosylation is a particularly important PTM as it is crucial for the biosynthesis, folding, and trafficking of voltage gated sodium channels. Glycosylation plays an important role in the proper subcellular localization of Nav1.6. Mice with aberrant glycosylation sites have reduced channel activity and defective neuronal localization, which manifested itself as chronic movement disorders. [21867][2188]
Nav1.6 appears to be relatively resistant to the modulation by kinases that are classically involved in the phosphorylation of neuronal channels, such as PKA and PKC. However, the channel is still subject to phosphorylation by a number of other enzymes. These include CamKII, p38 mitogen-activated protein kinase (MAPK), and glycogen synthase kinase-3 (GSK3). CaMKII and GCK3 both play a role in the regulation of excitability, as inhibition of the enzymes lead to a decrease in transient and persistent Nav1.6 sodium current and a depolarising shift in the voltage dependence of activation. MAPK is also indirectly involved in neuronal excitability by modulating the surface expression of the channel. Indeed p38 MAPK phosphorylation of Nav1.6 promotes Nedd4-ubiquitination, leading to increased degradation of the protein and thus decreased sodium current. [2189][2188]
Ubiquitination is responsible for the internalization and degradation of proteins, a process mediated by ubiquitin ligases which bind to PY motifs. Nav1.6 contains several PY motifs and undergoes ubiquitin-dependent modulation by Nedd4-a. Abrogation of Nedd4 leads to decreased Nav.6 internalization and an increase in current. However, the activity of Nedd4 is promoted by the previously mentioned p38 MAPK, indicating potential complex interactions between different PTMs, that modulate channel physiology and excitability. [2188]
Palmitoylation is involved in multiple stages of the life cycle of ion channels. Nav1.6 contains multiple identified palmitoylation sites, whose binding leads to the modulation of voltage-dependence of inactivation and increases in current density. Removal of these sites results in a reduction in Nav1.6 excitability [2188][2190].
Visual Representation of Nav1.6 Structure
Methodology for visual representation of structure available here
Like all voltage gated sodium channels, Nav1.6 is made up of a single protein comprised of 4 homologous domains (DI-DIV). Each domain is made up of 6 transmembrane subunits (S1-S6). S1-4 form the voltage sensing domain (VSD) whereas the S5-6 form the pore module (PM). The S4 subunit of each domain contains a series of positively charged residues. When membrane depolarization occurs, these charged residues cause the movement of the S4 subunit, inducing a conformational change in S5-S6, opening of the channel and allowing the entry of sodium ions into the cell. Soon after opening, rapid inactivation of Nav1.6 is instigated by the binding of the IFM motif, found in the loop between D3 and D4, to a hydrophobic receptor site next to the S6 in D4. This binding causes the shift of S6, allosterically closing the channel, thus deactivating the channel. Nav1.6 then returns to its resting state following the hyperpolarization of the cell membrane [2115].
The structure of rat Nav1.6 was resolved via cryo-electron microscopy, giving us a detailed insight into the channel’s architecture. Structural resolution of the ion channel highlighted certain features such as the structural variations of the voltage sensing domain IV, which has a wider pocket due to the displacement of its S1–S3. VSDIV represents a major target for most Nav inhibitors of high affinity and selectivity. [2191] However, it is worth noting that the aforementioned study was done on Nav1.6 bound to other molecules and the protein was not in an active conformation state. Further research is needed to confirm the results of the structure of Nav1.6 alone in and active state
Nav1.6 predicted AlphaFold size
Methodology for AlphaFold size prediction and disclaimer are available here
Nav1.6 displays similar kinetics to other voltage-gated sodium channels, namely fast activation and fast inactivation kinetics. However, it does have its own distinct current properties. Nav1.6 has a hyperpolarized shift in voltage-dependent activation compared to other Navs [2188]. Nav1.6 also has a more hyperpolarized voltage-dependence of steady-state inactivation, faster development of closed-state inactivation, slower kinetics of open-channel inactivation and a greater propensity to generate persistent and resurgent currents.[2192]
Single channel unitary conductance
Single channel unitary conductance is determined experimentally.
For Nav1.6, single channel unitary conductance has yet to be determined experimentally.
Model
A single kinetic model for all human voltage-gated sodium channels (Balbi et al, 2017)
https://modeldb.science/230137
Origin : HEK293 cells | Recording temperature: RT
Temperature dependence: Q10 | Formalism: Markov
States: C1, C2, O1, O2, I1, I2 | Implementation: NEURON
Simulation (Nav16_a.mod):
Membrane Systems Group 2022
Origin : Mouse scn8a | Host cell: CHO
Recording temperature: 25 C | Formalism: Hodgkin-Huxley
Gates: m, h | Implementation: NEURON
Simulation (Nav16__mCHO25c__0106.mod):
Model Nav1.6 (ID=33)
| Animal | rat | |
| CellType | L5PC | |
| Age | 21 Days | |
| Temperature | 23.0°C | |
| Reversal | 50.0 mV | |
| Ion | Na + | |
| Ligand ion | ||
| Reference | [288] A L Goldin et. al; J. Neurosci. 1998 Aug 15 | |
| mpower | 1.0 | |
| m Inf | 1.0000/(1+ exp(-0.03937*4.2*(v - -17.000))) | |
| m Tau | 1 | |
Tissue & cellular
Nav1.6 is broadly expressed in the central nervous system (CNS) where it is predominantly expressed in a variety of excitatory and inhibitory neuronal cell types. These include the hippocampal pyramidal and granule cells, retinal ganglion cells, cortical pyramidal neurons, motor neurons, and cerebellar Purkinje and granule cells.
Nav1.6 is also expressed, at lower levels, in the peripheral nervous system (PNS) and is present in a variety of ganglion cells, such as the dorsal root ganglion and trigeminal ganglion neurons, where it is plays a crucial role in peripheral sensory neuron transduction. Nav1.6 is also found at a low level in cardiomyocytes [2188].
Nav1.6 has been identified in a number non-canonical, non-excitable cells where they are thought to play non-negligeable roles. For example, microglia have been shown to express Nav1.6 and removal of the channel lead to a 65% decrease in mice microglia phagocytic capacity of microglia lacking Nav1.6 [2117].
Developmental
Nav1.6 can be found at low levels during prenatal development before its expression levels increase postnatally [365][2193]
Within the neurons of the CNS, Nav1.6 is found mainly in the distal end of the axon initial segment (AIS), around 25–50 μm from the soma [2194]. It is also present at the nodes of Ranvier within myelinated neurons [2195].
Given its fast kinetics, Nav1.6 is involved in the upstroke of the action potential. However, Nav1.6 is also responsible for persistent and resurgent currents.
Navs produce a noncanonical type of non-inactivating sodium current, known as persistent current. This phenomenon is 5-fold higher in Nav1.6 [2196]. Though these currents are generally small, the summation of persistent sodium currents from multiple channels can amplify subthreshold neuronal inputs. This leads to the lowering of AP initiation threshold and enables repetitive AP firing. [2188].
Resurgent current is a voltage and time-dependent property of Nav1.6 which occurs after depolarization at intermediate repolarizing potentials and elicits a small, transient current. These contribute to the spontaneous firing and multi-peaked AP[2188].
Given the location and high channel density at the AIS, Nav1.6’s fast kinetics and persistent and resurgent current properties make it responsible for the initiation of AP as well as repetitive firing of neurons [2194][2197]
Channelopathies
Given its role in action potential initiation, changes in Nav1.6 channel excitability lead to a number of pathologies, the vast majority of which are linked to severe developmental and epileptic encephalopathy (DEE) [2198]
The type of pathology often depends on the type of mutation that occurs [2199]:
- Loss of function: Intellectual disability, movement disorder, ataxia
- Mild gain of function: benign infantile onset seizures, paroxysmal dyskinesia with normal cognition
- Moderate gain of function: Moderate epileptic encephalopathy, developmental arrest and regression
- Severe gain of function: developmental delay. early onset refractory seizures, non ambulatory. lack of speech and language, cortical visual impairement
Nav1.6 also presents high expression in various metastatic tumors, including cancers of the breast, prostate, lymph node, and cervix, and is believed to contribute toward cancer metastasis [2188]
Nav1.6, like other voltage-gated sodium channels, is subject to extensive regulation by various auxiliary proteins and secondary messengers. These regulatory proteins play an important role in the development, localization, and expression of the channel protein.
The binding of Nav β subunits to Nav1.6 regulates neuronal function of the channel. Navβ1 was shown to be necessary for proper localization of Nav1.6 to the AIS during development [2200] Navβ4 is responsible for increased neuronal excitability as a result of increased resurgent current when bound to Nav1.6 [2201] However, expression of Navβ4 and Nav1.6 alone, in a heterologous system, was not sufficient to produce increases in sodium current, suggesting that interactions with other proteins are necessary to mediate this phenomenon [2188]
The activity of fibroblast growth factor homologous factors (FHF) influence current density and gating properties of Nav1.6. FHF4B was shown to suppress Nav1.6 sodium current and regulate localisation to the AIS. FHF2A and FHF2B both impact the gating properties of the channel, resulting in changes in sodium current kinetics and changes in neuron excitability [2188]
Calmodulin (CaM) is a small ubiquitously expressed protein which binds/senses calcium ions. CaM affects several different properties of Nav1.6, including channel inactivation and persistent current. Disruption of Cam activity led to a 62% reduction in current amplitude. The interaction between Nav1.6 and CaM displays calcium dependent inactivation (CDI) as calmodulin was shown to be able to modulate the inactivation kinetics of Nav1.6 currents in a calcium-dependent manner [53]
Table of known and predicted drug interactions with Nav1.6
Table of known and predicted animal toxin interactions with Nav1.6
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Contributors: Katherine Johnston, Rajnish Ranjan, Michael Schartner
To cite this page: [Contributors] Channelpedia https://channelpedia.epfl.ch/wikipages/125/ , accessed on 2024 Apr 19