Abstract 1701: Sodium Channel Beta Subunits Modulate Heart Rate, Drug Sensitivity, and Development in Zebrafish Embryos
Background: Sodium channels are macromolecular complexes that include poreforming α subunits and auxiliary β subunits (SCN1B-4B). While β subunits modulate the gating and expression of α subunits in vitro, the function of each subunit in the intact heart is unclear. In this study, we used morpholino antisense oligos to determine the effects of knockdown of each individual β subunit gene in zebrafish.
Methods and Results: Molecular cloning and phylogenetic analysis identified 5 conserved zebrafish gene orthologs with canonical extracel-lular immunoglobulin-like motifs (zβ1, zβ2, zβ3, zβ4.1, zβ4.2, ~50% amino acid identity to human). All zebrafish β subunit genes are expressed in cardiac tissue and during development. Co-expression of zβ 1 with zebrafish Nav1.5 in CHO cells increased sodium current by 68% at a −30 mV depolarizing pulse (n=4, p<0.005) compared to zNav1.5 alone (n=5), and negatively shifted the voltage-dependence of channel activation without altering inactivation or recovery from inactivation. Morpholinos targeting each β subunit gene achieved >90% reduction in mRNA transcripts (total injected embryos > 1500). Knockdown of β1 or β2 but not β3, β4.1, or β4.2 produced bradycardia that was exaggerated by lidocaine challenge (zβ1 shown in table⇓). Conversely, knockdown of β4.2 perturbed heart development with no change in heart rhythm or drug response.
Conclusions: Zebrafish are a useful model for rapidly characterizing candidate modulators of cardiac rhythm and antiarrhythmic drug action in vivo. These data suggest that individual sodium channel β subunit genes modulate heart rate, response to sodium channel blocking drugs, and heart development.