The Sialic Acid Component of the β1 Subunit Modulates Voltage-gated Sodium Channel Function

Abstract

Voltage-gated sodium channels (Na v ) are responsible for initiation and propagation of nerve, skeletal muscle, and cardiac action potentials. Na v are composed of a pore-forming ␣ subunit and often one to several modulating ␤ subunits. Previous work showed that terminal sialic acid residues attached to ␣ subunits affect channel gating. Here we show that the fully sialylated ␤ 1 subunit induces a uniform, hyperpolarizing shift in steady state and kinetic gating of the cardiac and two neuronal ␣ subunit isoforms. Under conditions of reduced sialylation, the ␤ 1 -induced gating effect was eliminated. Consistent with this, mutation of ␤ 1 N-glycosylation sites abolished all effects of ␤ 1 on channel gating. Data also suggest an interaction between the cis effect of ␣ sialic acids and the trans effect of ␤ 1 sialic acids on channel gating. Thus, ␤ 1 sialic acids had no effect on the gating of the heavily glycosylated skeletal muscle ␣ subunit. However, when glycosylation of the skeletal muscle ␣ subunit was reduced through chimeragenesis such that ␣ sialic acids did not impact gating, ␤ 1 sialic acids caused a significant hyperpolarizing shift in channel gating. Together, the data indicate that ␤ 1 N-linked sialic acids can modulate Na v gating through an apparent saturating electrostatic mechanism. A model is proposed in which a spectrum of differentially sialylated Na v can directly modulate channel gating, thereby impacting cardiac, skeletal muscle, and neuronal excitability.

Role of Extracellular Sialic Acid in Regulation of Neuronal and Network Excitability in the Rat Hippocampus

“…Sialic acids play an important role in many neuronal processes including axonal growth and synaptic activity-induced and neuronal-glial plasticity (Bonfanti, 2006) Charged sialic acid residues have also been proposed to be the moieties responsible for the effects of divalent ions on channel gating behavior (RecioPinto et al, 1990;Bennett et al, 1997;Castillo et al, 1997;Zhang et al, 1999;Johnson et al, 2004). Here we show that sialylation of the external surface of plasma membrane modulates sodium currents and AP threshold in CA3 pyramidal cells and excitability of the hippocampal network.…”

“…The level of sialylation of sodium channels changes during postnatal development, and such alteration in the sialylation level correlates with changes in voltage-dependent channel gating (Castillo et al, 1997;Tyrrell et al, 2001;Stocker and Bennett, 2006). The isoform-specific effects of sialic acid on sodium channel gating have also been well documented (Zhang et al, 1999;Bennett, 2002;Johnson et al, 2004;Johnson and Bennett, 2006). However, the functional role of glycosylation of a central neuronal voltage-gated sodium channel remains unclear.…”

Sodium channel β1 subunit mutations associated with Brugada syndrome and cardiac conduction disease in humans

“…Some groups have reported that β1 increases Na V 1.5 currents with or without affecting voltage dependence or channel kinetics, while others have reported no effect of β1 on Na V 1.5 current (20,(33)(34)(35)(36)(37). The β1B variant has to date only been studied in coexpression studies with the neuronal sodium channel Na V 1.2 (encoded by SCN2A), where it was shown to increase sodium current and cause a small negative shift in voltage dependence of activation (19).…”

Regulated and aberrant glycosylation modulate cardiac electrical signaling

“…We reported previously that sialic acids attached to the ␤ 1 and ␤ 2 Na v auxiliary subunits modulate Na v 1.5 gating (12,29). Our data show that the regulated expression of Na v 1.5 with ␤ 1 and/or ␤ 2 (or neither) can establish a sialic acid-dependent Ϸ15 mV range of channel activation voltages partially dependent on the array of glycosylation signatures created as the Na v complex is remodeled (e.g., Ϯ␤ 1 and/or Ϯ␤ 2 ).…”