Graduation Year

2005

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Physiology and Biophysics

Major Professor

Eric Bennett, Ph.D.

Keywords

Ion channels, Cardiomyocytes, N-glycosylation, Development, Neuraminidase

Abstract

The proper orchestration of voltage-gated ion channel gating is vital to maintaining normal heart rhythms throughout an animal's lifespan. Voltage-gated sodium channels, Nav, are responsible for the initiation of the cardiac action potential, which leads to cardiac systole. Comparison of neonatal ventricular and atrial myocyte Nav gating with adult indicated that the neonatal ventricular Nav gated following a ~10 mV greater depolarization than did atrial or adult ventricular Nav. In this study I questioned whether development- and/or chamber-dependent changes in Nav-associated functional sialic acids could account for these differences. When desialylated with neuraminidase, all gating characteristics for the lower voltage activated atrial and adult ventricular Nav shifted significantly to more depolarized potentials. However, desialylation of the higher voltage activated neonatal ventricular Nav had no effect on channel gating. Furthermore, channels were stripped of

their N-glycosylation via PNGase-F in an attempt to separate the potential effects of the remaining glycosylation structure on Nav gating. Following treatment, neonatal ventricular Nav gating remained unchanged while atrial and adult ventricular Nav gating again shifted to depolarized potentials nearly identical to those of the neonatal ventricular channel. Immunoblot analyses indicated that atrial and adult ventricular Nav a subunits are more heavily sialylated than the neonatal ventricular a subunit, with approximately 15 more sialic acid residues. The data indicate that differential sialylation of myocyte Nav a subunits is responsible for much of the developmental and chamber-specific remodeling of Nav gating observed here. In addition, the Nav1.5 a subunit can associate with b subunits, also believed to be sialylated. The potential for functional b1 trans sialic acids to further modulate Nav1.5 gating was tested via co-transfection of b1 with the Nav1.5 a subunit into the Pro5

/Lec2 mammalian expression system. Co-transfection revealed that the additional b1 trans sialic acids caused a hyperpolarizing shift in all tested gating parameters. When transfected into neonatal ventricular myocytes, b1 expression revealed no effect, implying that b1 expression alone is not responsible. Together, the myocyte and expression system studies describe a novel mechanism by which Nav gating, and subsequently cardiac excitability, are modulated by the regulated change in channel-associated functional sialic acids.

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