β3: An additional auxiliary subunit of the voltage-sensitive sodium channel that modulates channel gating with distinct kinetics

Abstract

The voltage-sensitive sodium channel confers electrical excitability on neurons, a fundamental property required for higher processes including cognition. The ion-conducting ␣-subunit of the channel is regulated by two known auxiliary subunits, ␤1 and ␤2. We have identified rat and human forms of an additional subunit, ␤3. It is most closely related to ␤1 and is the product of a separate gene localized to human chromosome 11q23.3. When expressed in Xenopus oocytes, ␤3 inactivates sodium channel opening more slowly than ␤1 does. Structural modeling has identified an amino acid residue in the putative ␣-subunit binding site of ␤3 that may play a role in this difference. The expression of ␤3 within the central nervous system differs significantly from ␤1. Our results strongly suggest that ␤3 performs a distinct neurophysiological function.T he voltage-sensitive sodium channel plays a fundamental role in excitable cells, transiently increasing the sodium permeability of the plasma membrane in response to changes in membrane potential and thus propagating the action potential (1, 2). Not surprisingly, mutations in sodium channel genes are implicated in several pathologies, including epilepsy and cardiac arrhythmias (3-5), and therapeutic drugs, including antiepileptics, local anesthetics, and anticonvulsants (6), act on the channel.In the central nervous system, the channel is conventionally described as a heterotrimer composed of a 260-kDa ␣-subunit, a noncovalently associated 36-kDa ␤1-subunit, and a disulfidelinked 33-kDa ␤2-subunit (2). The ␣-subunit forms the ion pore and is responsible for the voltage-sensitive characteristics of the complex. There are multiple isoforms of the ␣-subunit expressed in different regions of the brain and peripheral nervous system that differ in their kinetic properties (1). The ␤-subunits are auxiliary components acting in a regulatory capacity (7). ␤1 increases the fraction of ␣-subunits operating in a fast gating mode, thus accelerating the activation and inactivation kinetics of the channel and modulating the frequency with which neurons fire (8). The ␤2-subunit is required for the efficient assembly of the channel but has minor effects on gating kinetics. These two ␤-subunits are distantly related by sequence (9).We now report the cloning and analysis of the rat and human forms of a previously uncharacterized sequence that we call ␤3. It is homologous to ␤1, but differs from ␤1 both in its distribution within the brain and in some of its kinetic properties. The discovery of this subunit increases the complexity of the sodium channel and raises further questions about the role of these auxiliary subunits. Materials and MethodsCloning Methodology. We isolated a variant of the rat pheochromocytoma cell line PC12 (termed A35C), which lacks typical neuronal properties (10). To discover previously unidentified neuroendocrine-specific genes, subtractive cloning was used to identify transcripts expressed at a level in the variant cells lower than that in normal PC12 cells. Total RNA wa...

Modulatory Proteins Can Rescue a Trafficking Defective Epileptogenic Nav1.1 Na+Channel Mutant

“…␤ subunits can increase Na ϩ current density and membrane insertion of wild-type ␣ subunits in some expression systems (Isom et al, 1992(Isom et al, , 1995 but not in others (Morgan et al, 2000;Qu et al, 2001;Yu et al, 2003). Thus, in some conditions, ␤ subunits can act as chaperones also for wild-type ␣ subunits, but in our experimental conditions, they did not have any significant effects.…”

“…In addition to highlighting the importance of mutations and polymorphisms in their promoters, this possibility opens a complex scenario, because expression patterns in CNS neurons are highly variable. For instance, ␤ subunits show neuron-specific expression in many brain areas (Morgan et al, 2000;Yu et al, 2003). Little is known about interneuron-specific expression levels of Na ϩ channel interacting proteins, and this is particularly important because Na v 1.1 loss-of-function mutations have a prominent effect on the excitable properties of GABAergic neurons (Yu et al, 2006).…”

Sodium channel Scn1b null mice exhibit prolonged QT and RR intervals

“…Mutations in SCN5A encoding Na v 1.5, the major cardiac channel, result in Long QT and Brugada syndromes [1]. Sodium channels are heterotrimers, composed of a single, pore-forming α subunit and two β subunits [3]: a noncovalently linked subunit (β1 or β3) [4,5] and a disulfide-linked subunit (β2 or β4) [6,7]. β subunits are multi-functional molecules that regulate channel cell-surface expression levels and modulate channel function, affecting channel kinetics and voltage-dependence in vitro [8].…”

The genetic basis of Brugada syndrome: A mutation update

“…The SCN3B gene is a member of the gene family SCNB gene family [Meadows and Isom, 2005] described above under SCN1B. In the heart the function of the b3 subunit is to modify the function of Na v 1.5, by increasing the I Na , as for the b1 subunit, albeit with another kinetics [Morgan et al, 2000].…”

“…The gene spans 25.6 kb at chromosome 11q23 and is composed of six exons [Morgan et al, 2000]. The SCN3B gene is translated into a 215 amino acid protein.…”