Electrophysiology and Beyond: Multiple roles of Na⁺ channel β subunits in development and disease

Abstract

Voltage-gated Na⁺ channel (VGSC) β subunits are not “auxiliary.” These multifunctional molecules not only modulate Na⁺ current (I_Na), but also function as cell adhesion molecules (CAMs) – playing roles in aggregation, migration, invasion, neurite outgrowth, and axonal fasciculation. β subunits are integral members of VGSC signaling complexes at nodes of Ranvier, axon initial segments, and cardiac intercalated disks, regulating action potential propagation through critical intermolecular and cell-cell communication events. At least in vitro, many β subunit cell adhesive functions occur both in the presence and absence of pore-forming VGSC α subunits, and in vivo β subunits are expressed in excitable as well as non-excitable cells, thus β subunits may play important functional roles on their own, in the absence of α subunits. VGSC β1 subunits are essential for life and appear to be especially important during brain development. Mutations in β subunit genes result in a variety of human neurological and cardiovascular diseases. Moreover, some cancer cells exhibit alterations in β subunit expression during metastasis. In short, these proteins, originally thought of as merely accessory to α subunits, are critical players in their own right in human health and disease. Here we discuss the role of VGSC β subunits in the nervous system.

β subunits are multifunctional CAMs

VGSCs in brain are heterotrimers, containing a single α subunit associated with one non-covalently (β1 or β3) and one covalently (β2 or β4) linked β subunit [9, 85]. In mammals, SCN1B - SCN4B encode β1 - β4, respectively. β1, β2, β3, and β4 are type I transmembrane proteins, containing an extracellular N-terminal signal peptide and Ig domain, one transmembrane domain, and an intracellular C-terminal domain (Fig. 1). SCN1B gives rise to two splice variants, β1 and β1B (also called β1A) [32, 61]. β1B is formed through retention of intron 3, containing a stop codon and thus excluding the transmembrane domain in exon 4. The predicted amino acid sequence of the retained intronic region exhibits very low homology between species [61], however, hydrophobicity analysis of these sequences reveals no transmembrane domains in any species, predicting that β1B is a secreted protein that may function as a ligand for cell adhesion [58]. All of the β subunits, including β1B, belong to the Ig superfamily of CAMs [26, 86]. Two SCN1B splice variants, including transmembrane and secreted forms, is consistent with other Ig superfamily CAMs [28, 64]

Fig. 1. Subunit structure of VGSCs.

VGSCs in the CNS are multiprotein complexes composed of a single pore-forming α subunit, one non-covalently-linked β subunit (β1 or β3), and one covalently-linked β subunit (β2 or β4). β1B is a secreted, soluble subunit. α and β subunits interact with multiple cell adhesion, ECM, cytoskeletal, and intracellular signal transduction proteins. Models of the β1 and β2 Ig loops and the C-terminus of β1B were obtained by analyzing amino acid sequences on the I-TASSER server [82, 87, 88]. We thank Mauricio Patino for his help in the design of this figure.

VGSC β subunits are widely expressed in excitable and non-excitable cells in the nervous system. Table 1 provides a detailed description of nervous system expression of β1–β4 in mammals and zebrafish. In some tissues there is evidence that β subunits may be expressed in the absence of α, suggesting that they may play cell adhesive roles in the absence of ion conduction in vivo. β subunit expression in the CNS is developmentally regulated. Of the four genes, β1B and β3 mRNA predominate in fetal brain, with levels decreasing during late gestation and after birth. In contrast, levels of β1 and β2 mRNA increase progressively and become dominant after birth [26, 32, 65, 69].

Table 1. Tissue specific and subcellular domain specific expression of VGSC β subunits in the nervous system.

Tissue – subcellular domain	β1	β1B	β2	β3	β4
Central Nervous System
Hippocampal neurons	+ [12]		+ [86]	+ (RNA) [52, 69]	+ [86]
Cortical neurons		+ [32, 61]	+ [86]	+ (RNA) [52, 69]	+ [86]
Basal Ganglia	- (RNA) [52]		+ [86]	+ (RNA) [52, 69]	+ [86]
Retinal ganglion cells	+ [18, 29]		+ [29]
Optic nerve nodes of Ranvier	+ [12, 18]		+ [29]
Optic nerve myelin	+ [19], [29]
Astrocytes	+ (RNA) [2, 55]		+ [54]
Dorsal root ganglia neurons		+ [61]	+ [60, 86]	+ [8]	+ [86]
Ventral horn neurons		+ [32]	+ [86]		+ [86]
Radial glia	+ [19]
Cerebellar Purkinje cells soma		+ [32, 61]	+ [86]	+ (RNA, transitory) [69]	+ [86]
Cerebellar granule neurons soma	+ [4]			- (RNA) [52, 69]
Cerebellar granule neurons AIS and growth cone	+ [4]
Deep cerebellar nuclei		+ [32]	+ [86]		+ [86]
Bergmann glia	+ [14]	- [32]	- [14]	- [14]	+ [14]
Peripheral Nervous System
Peripheral nerves		+ [61]	+ [60]	+ [8]
Sciatic nerve nodes of Ranvier	+ [12]
Schwann cells (zebrafish)	+ [19]

β subunits associate with multiple VGSC α subunits [16, 41, 48, 70]. In addition, β subunits interact both in cis and in trans with multiple CAMs, with components of the extracellular matrix (ECM), and with intracellular cytoskeletal and signaling molecules. A summary of interactions is presented in Table 2, with the majority of these studies focusing on β1. Similar studies have not been carried out for β1B, however, because β1 and β1B share the extracellular Ig domain, it is safe to assume that these two CAMs share many, if not all, extracellular binding partners.

Table 2.

VGSC β subunit protein-protein interactions

	β SUBUNITS			EXTRACELLUL AR MATRIX PROTEINS		CYTOSKELETAL PROTEINS		ION CHANNELS	ENZYMES	CELL ADHESION MOLECULES
	β1	β2	β3	TN-C	TN-R	AnkyrinG	AnkyrinB	Kv4.2 / Kv4.3	RPTPβ	Contactin	NF-155	NF-186	N-Cadherin	Nr CAM
β1	trans [39]	trans [43]	(-) [43]		(+) [83]	Non-phosphorylated β1 [39, 40, 44]	Non-phosphorylated β1 [41]	(+) [15]	(+) [62]	cis [30, 44] trans [43]	trans [43]	cis [63] trans [43]	(+) [41]	cis, trans [43]
β2	trans [43]	trans [39]		(+) [71, 83]	(+) [71, 83]	(+) [39]			(-) [62]	(-) [30]		(-) [63]
β3	(-) [43]		(-) [42]							(-) [42]		cis [63] trans [43]
β4	Cis [1]

Abbreviations: NF: neurofascin, RPTPβ: receptor tyrosine phosphatase β, TN: tenascin.

β1 associates with multiple CAMs, including itself, contactin, neurofascin-186 and -155, NrCAM, N-cadherin, and VGSC β2 [30, 41, 43, 63]. β2 does not associate with contactin, but does associate with β1 and the ECM proteins, tenascin-C and tenascin-R [43, 71]. Fibroblasts expressing β1 or β2 are repelled by tenascin-R substrates, suggesting initial binding of this ECM molecule [83]. Trans homophilic β1 or β2 (but not β3) association results in recruitment of ankyrin to points of cell-cell contact [39, 42]. Phosphorylation of a critical tyrosine residue (β1Y181) abolishes β1 association with ankyrin_B or ankyrin_G and this is postulated to be a mechanism regulating β1 subcellular localization [40, 44]. Indirect evidence suggests that β1 may associate with the lipid raft tyrosine kinase fyn in response to extracellular trans β1-β1 adhesion [5]. Association of the β1 intracellular domain with receptor phosphotyrosine phosphatase β may provide a yin-yang mechanism of phosphorylation and dephosphorylation [62]. β1 and β2 also participate in heterophilic extracellular interactions and some of these interactions require the intracellular domain of at least one of the partners. For example, the intracellular domains of NrCAM and β2, respectively, are necessary for their extracellular association with β1, suggesting inside-out signaling mechanisms [43].

β subunits are substrates for sequential cleavage by the β-site amyloid precursor protein-cleaving enzyme 1 (BACE1) and γ-secretase [81]. β2 is also cleaved by the α-secretase ADAM10 [34]. Cleavage of β subunits by BACE1 or α-secretase at sites in the extracellular juxtamembrane region results in ectodomain shedding, leaving membrane-bound C-terminal fragments (CTFs) [34, 81]. The shed ectodomain of β1 may function as a soluble ligand for cell adhesion to promote neurite outgrowth [14, 39]. The CTFs are processed by γ-secretase, resulting in freed intracellular domains (ICDs) [34, 81]. Inhibition of β2 cleavage by γ-secretase reduces cell-cell adhesion and migration [34] and β4 processing by BACE1 increases neurite outgrowth [49], predicting that proteolytic processing events are critical to the in vivo functioning of these subunits. The β2 ICD translocates to the nucleus and increases Scn1a expression, suggesting that this fragment may function as a transcriptional regulator of VGSC α subunits [33]. Although all four β subunits are BACE substrates in vitro, in vivo processing has only been confirmed for β2 and β4 [81], suggesting that this role may be specific to subunits that can be covalently linked to α.

What do β subunits do?

1. β subunits modulate I_Na

A large body of literature shows that β subunits modulate I_Na in heterologous systems. While this approach has yielded valuable structure-function information, these data may have little relevance to the in vivo situation. Given α-β subunit combinations can yield different electrophysiological results in different cell backgrounds, e.g. [27]. β1-mediated effects on Na_v1.5 have been particularly difficult to resolve using heterologous expression (see [46] for review). This may be due to the presence of varying levels of endogenous β subunits in cell lines [50, 51] and/or the differential expression of other endogenous VGSC interacting or modifying proteins. Native cells express multiple VGSC α subunits in specific complexes of signaling, cytoskeletal, and adhesion molecules in particular subcellular domains, a situation that cannot be mimicked using a heterologous system. Importantly, while co-expression of β subunits, especially β1, produces significant changes in I_Na in heterologous systems, mouse models tell us otherwise. While Scn1b null mice have a severe neurological phenotype, only subtle, cell type specific changes in I_Na are reported [1, 4, 12, 59, 75]. Thus, heterologous systems are not reliable predictors of the effects of β subunits, especially β1, on I_Na in vivo.

β subunit modulation of I_Na in vivo is cell type specific and subtle, yet may translate into significant changes in electrical excitability. Scn1b null cerebellar granule neurons (CGNs) have normal transient I_Na but reduced resurgent I_Na, likely contributing to the ataxic phenotype of these mice [4]. Minor changes in hippocampal excitability in Scn1b null mice may contribute to severe seizures [59]. A population of Scn2b null hippocampal neurons show negative shifts in the voltage-dependence of inactivation [11, 75], while in small-fast DRG neurons there is slowing of I_Na activation and inactivation with no change in voltage-dependence [37]. Persistent I_Na is increased in hippocampal neurons transfected with β4. β4-expressing neurons from Scn1b and Scn1b/Scn2b null mice show slowed entry into inactivated states [1]. β1 and β4 play antagonistic roles in hippocampus in vivo, with the former favoring inactivation, and the latter favoring activation. Because increased VGSC availability may facilitate action potential firing, this may suggest a mechanism for seizure susceptibility of both mice and humans with SCN1B mutations [1].

The interaction of β subunits with other CAM, cytoskeletal, or signaling molecules may influence I_Na. β1 association with contactin or NF-186 results in increased VGSC cell surface expression [30, 43]. β1 and β2 are ankyrin-binding proteins. Ankyrin_B null mice exhibit reduced I_Na density and abnormal I_Na kinetics [10], suggesting that β subunits play important roles in the VGSC-ankyrin complex. These interactions may be particularly critical at nodes of Ranvier of myelinated axons. Scn1b null mice have reduced numbers of nodes, dysmyelination, and disruption of axo-glial cell-cell contacts [12]. While nodal proteins are localized normally in these mice, association between VGSCs and contactin is disrupted. Loss of critical β1 subunit-dependent protein-protein interactions may lead to instability of the node, resulting in disrupted saltatory conduction [12, 31]. Finally, β1 and Na_v1.6 reciprocally modulate each other in terms of neurite outgrowth, subcellular localization, and regulation of resurgent I_Na in CGNs [4]. Taken together, these data implicate β1 in brain development.

2. β subunits modulate channel cell surface expression

β subunits, especially β2, increase I_Na density in heterologous systems by enhancing α subunit cell surface expression [26]. α subunit association with β2 and concomitant plasma membrane insertion are the final steps in VGSC biosynthesis, suggesting that β2 is critical for establishment and maintenance of channel cell surface expression and excitability [67, 68]. Experiments in Scn2b null neurons support this conclusion. Acutely dissociated Scn2b null hippocampal neurons have I_Na density that is ∼50% of wildtype. Scn2b null neuronal cultures exhibit a ∼50% reduction in cell surface ³H-saxitoxin binding compared to wildtype [11]. Similar to the effects of β1, however, the ability of β2 to increase I_Na density appears to be cell-type specific. For example, neurons isolated from Scn2b null dentate gyrus have similar I_Na densities as wildtype [75]. Further, the effects of β2 may be specific to tetrodotoxin-sensitive channels. Scn2b null small-fast DRG neurons have significantly decreased protein expression of Na_v1.1 and Na_v1.7 and subsequent reduction in tetrodotoxin-sensitive I_Na density compared to wildtype, but unchanged tetrodotoxin-resistant I_Na [37].

β1 may be required for the expression or subcellular localization of certain VGSC α subunits in specific cell types. Scn1b null hippocampal neurons in the CA3 region express decreased levels of Na_v1.1 and increased levels of Na_v1.3 compared to wildtype [12]. Subpopulations of Scn1b null CGNs exhibit reduced expression of Na_v1.6 channels, but increased levels of Na_v1.1 channels, at axon initial segments compared to wildtype [4].

3. β subunits modulate cellular migration, neurite extension, and axonal fasciculation

As is true for other neuronal and glial CAMs, β subunits are critical for cellular migration, neurite outgrowth, and axonal fasciculation. Interestingly, β subunits are postulated to modulate migration of cancer cells through similar mechanisms [7]. β1 and β2 mediate migration of fibroblasts away from a tenascin-R substrate [83]. β1 and β4 promote neurite extension in a cell type-dependent manner [14, 49]. β1 promotes and β2 inhibits CGN neurite outgrowth, while β4 has no detectable effect [14]. β3 was not tested, but recent results predict that β3 would not affect neurite outgrowth through homophilic interactions [42]. β1-mediated neurite outgrowth in CGNs requires β1-β1 trans interactions: β1 expressed at the CGN cell surface must interact with another β1 that is located either on the cell surface of an adjacent neuron or glial cell [14]. In addition, β1-mediated neurite outgrowth in CGNs requires the presence of Na_v1.6 at the axon initial segment, tetrodotoxin-sensitive I_Na, the CAM contactin, and fyn kinase [4, 5]. Scn1b null mice exhibit significant defasciculation of the corticospinal tract, abnormal migration of CGNs, and defasciculation of cerebellar parallel fibers [5]. Cerebellar defects may contribute to the ataxic phenotype of these mice [12]. Zebrafish scn1bb morphants exhibit defasciculation of the olfactory nerve [19]. Taken together, these data suggest that VGSC β subunits are critical modulators of brain development.

The roles of β subunits in cell migration, adhesion, and neurite extension depend not only on extracellular CAM interactions, but also on intracellular signal transduction events. β1-mediated neurite outgrowth in CGNs requires fyn kinase [5], suggesting that extracellular β1-β1 cell adhesion results in intracellular activation of fyn and initiation of a tyrosine phosphorylation signal transduction cascade. Blockade of processing of β2 by γ-secretase inhibits cell migration [34], suggesting that β2-ankyrin interactions are disrupted by γ-secretase cleavage to allow cytoskeletal remodeling.

What is the role of β subunits in disease?

1. Evidence from mouse models

Scn2b and Scn3b null mice have relatively normal life spans and behaviors [11, 23]. Scn2b null mice have a ∼50% loss of cell surface tetrodotoxin-sensitive VGSCs in neurons, resulting in reduction in compound action potential amplitude and increased action potential threshold. Scn2b null mice show increased susceptibility to seizures and sensitivity to thermal stimuli, and decreased sensitivity in some models of neuropathic pain [11, 37]. The apparent lack of a neurological phenotype in Scn3b mice suggests that Scn1b may compensate for Scn3b in brain [22, 23]. On the other hand, Scn1b null mice have a severe and complex phenotype that includes retarded growth, ataxia, spontaneous seizures, and lethality by postnatal day 21 [12], suggesting that the remaining β subunit genes cannot compensate for the loss of Scn1b. Scn1b null mice may represent a novel model for Dravet Syndrome [59]. In contrast, Scn1b^+/- mice have a normal phenotype [12]. Surprisingly, in spite of an extensive literature describing significant electrophysiological effects of β1 co-expression with α subunits in heterologous systems, only subtle changes in I_Na have been reported in Scn1b null neurons [12, 59, 75]. Thus, the cell adhesive functions of β1 may be more important than I_Na modulation in vivo.

2. Human inherited epilepsy

β subunits are involved in a number of human neurological diseases (Table 3). While β subunit mutations result in cardiac as well as neurological disease, we will focus our discussion on neuropathology.

Table 3. Neurological diseases associated with β subunits.

β Subunit	Disease	Relationship	Model	Reference
β1	Febrile seizures	Causal	Human	[3, 56, 66, 77, 78]
	Dravet Syndrome	Causal	Human, Mouse	[12, 59]
	Temporal lobe epilepsy	Causal	Human	[66]
	Traumatic nerve injury	Downstream target	Human	[13]
β2	Multiple sclerosis	Modifying factor	Mouse	[53]
	Post-traumatic neuropathic pain	Downstream target	Mouse	[60]
	Inflammatory pain	Modifying factor	Mouse	[37]
	Traumatic nerve injury	Downstream target	Human	[13]
β3	Temporal lobe epilepsy without hippocampal sclerosis	Causal vs. Downstream target (?)	Human	[76]
	Traumatic nerve injury	Downstream target	Human	[8]
β4	Huntington's disease	Downstream target	Human, Mouse	[57]

Of the four VGSC β subunits genes, to date only mutations in SCN1B are reported to cause epilepsy (Fig. 2). At least two different studies have purposefully screened SCN2B for mutations in epileptic patients with no positive results, suggesting that this gene may not be a target for epilepsy, even though it is a target for inherited cardiac arrhythmia [25, 77, 80]. All epileptic syndromes associated with SCN1B thus far include febrile seizures, assigning them to the disease spectrum of generalized epilepsy with febrile seizures plus (GEFS+) that includes Dravet Syndrome, a catastrophic pediatric epileptic encephalopathy that includes mental retardation [3, 56, 59, 66, 77, 78]. Functional characterization of these mutants has largely been performed in heterologous systems, but these results may not accurately reflect the in vivo situation (results summarized in [59]). Importantly, the majority of SCN1B epilepsy mutations are loss-of-function [1, 47, 59, 73, 84]. Three of these mutants are trafficking-deficient [59, 84] and it is interesting to speculate that this may be a common underlying mechanism for of SCN1B-linked epilepsies. In all cases but one [59], patients carry a single mutant SCN1B allele, making the data difficult to resolve with the apparently normal phenotype of Scn1b^+/- mice [3, 66, 77, 78], and raising the possibility that SCN1B mutant alleles may have dominant negative functions that have not been detected using heterologous expression systems. It is likely that genetic background as well as epigenetic factors play important roles in the phenotypes of these mutant alleles in human patients. When thinking about the role of β subunits in disease, it is important to consider that β subunits modulate the response of VGSC α subunits to therapeutic agents that are used to treat both epilepsy and cardiac arrhythmia, [24, 35, 38, 75]. Thus, β subunits must be included in analyzing both the cause and treatment of channelopathies.

Fig. 2. Localization of human GEFS+ spectrum mutations in SCN1B.

Solid bars, missense mutations; Striped bars, deletions. Numbers correspond to cited references.

3. Is there a role for β subunits in neuroprotection or neurodegeneration?

Rodent models of nerve injury suggest a protective role for β2 reduction in neuropathology. β2 expression is increased in peripheral axons and cell bodies in both the spared nerve injury and spinal nerve ligation models of neuropathic pain [60]. Consistent with this, the behavioral response to these models is significantly attenuated in Scn2b null mice [60] and Scn2b null mice manifest a reduced response to the formalin model of inflammatory pain [37]. Scn2b null mice exhibit significantly attenuated symptoms, mortality, and axonal loss compared to wildtype in the Experimental Allergic Encephalomyelitis (EAE) mouse model of Multiple Sclerosis [53]. Brain levels of Scn4b mRNA are reduced in mouse models Huntington's disease [57]. This change is reflected in β4 protein levels in the basal ganglia, and is confirmed in human patients [57]. β4 reduction in mouse models precedes the appearance of motor symptoms. It has been proposed that, in light of its neurite outgrowth promoting ability, a reduction in β4 may cause neurodegeneration as well as alter I_Na [57].

4. Putative Role of β subunits in Non-epileptic Neurological Disorders

Patients with SCN1B mutations have GEFS+ spectrum epilepsy disorders [59]. Importantly, there is a significant comorbidity of neuropsychiatric disease and seizures [17, 21] and anti-epileptic therapies targeting VGSCs are effective in mood disorders [74], suggesting a shared pathophysiology between these diseases [21]. Bipolar disorder is linked genetically to ANK3, encoding ankyrin_G, a protein that is critical for VGSC targeting and localization in neurons [20]. β1 and β2 are ankyrin_G-binding proteins [40]. SCN8A, encoding Na_v1.6 that associates with β1 and β2 and is also an ankyrin_G-binding protein, is another susceptibility gene for bipolar disorder [79]. Scn1b null mice have neuronal migration, pathfinding, and fasciculation defects in the cerebellum [4, 5]. Cerebellar output targets multiple non-motor areas in the prefrontal cortex and posterior parietal cortex associated with attention, memory, learning, and emotion [72]. Thus, a possible novel direction for β subunit research is the idea that mutations in the genes encoding these subunits may be linked to psychiatric disorders as well as neurological disease. Because SCN1B plays important roles in neuronal pathfinding and VGSC expression, disruptions in its expression during development may lead to a spectrum of childhood and adolescent neurological and neuropsychiatric diseases. In vivo data suggest that the primary role of SCN1B in brain is cell adhesion, further, that disruptions in SCN1B-mediated cell adhesive interactions result in pathology. β subunit function may be modulated in the future by interfering with or enhancing cell adhesive interactions, such that patients with diseases-causing β subunit gene mutations might be treated with small molecules that mimic or alter these interactions.

Conclusions and future directions

Previous work has centered on the functional roles of β subunits in I_Na modulation. However, while β subunits clearly modulate I_Na in heterologous systems, in vivo models indicate that this may not be their most important role. It is critical that we now challenge our previously held concepts and take a fresh look at the biology of these multifunctional subunits. As discussed above, an important direction in ion channel research focuses on the finding that some voltage-gated ion channels are multi-functional. In addition to regulating electrical excitability through ion conduction, some voltage-gated ion channels contribute to processes as diverse as intracellular signaling, transcriptional regulation, scaffolding, and cell adhesion without requiring changes in ion flux [6, 7, 36]. Results reviewed here demonstrate roles for VGSC β subunits in the regulation of VGSC expression, localization, I_Na modulation, action potential conduction, and cell adhesion. While many drugs targeting VGSC α subunits are in common use [45], the potential of VGSC β subunits as therapeutic targets has not been considered. Moreover, therapeutic targeting of the non-conducting functions of VGSCs may be critical. In conclusion, important advances in the therapy of diseases that are associated with VGSC β subunit loss- or modulation-of-function may be achieved by targeting β subunits directly.