The hNav1.5 channel

1- A new field of cardiac ‘channelopathies’

Mechanisms by which mutation-induced alterations of the hNav1.5 currents predispose long-QT (LQT3) patients to arrhythmias have been explored and reviewed (1). More recent findings demonstrate a new paradigm for human cardiac arrhythmia based not on gene mutations that affect channel biophysical properties, but instead on mutations that affect ion channel/transporter localization at excitable membranes in heart (2). The principal voltage-gated Na channel in heart, Nav1.5, is directly associated with ankyrin-G, an “adaptor” protein, encoded by a distinct gene from ankyrin-B, and which coordinate activity of ion channels and transporters in cardiac muscle. As for an example, the human mutation (E1053K) in the ankyrin-binding motif of Na(v)1.5 causing loss of binding to ankyrin-G results in Brugada syndrome, a potentially fatal arrhythmia (3). The E1053K mutation also prevents accumulation of Na(v)1.5 at cell surface sites in ventricular cardiomyocytes. (4). Such recent data define an exciting new field of cardiac "channelopathies" due to defects in proper channel targeting/localization.

2- Ligands inducing cardiac arrythmias

Inhibition of hNav1.5 can have dramatic affects on the cardiac action potential waveform. It can result in shortening of both the peak amplitude and duration of the action potential, and may be associated with arrhythmias, and sudden death. When pharmaceuticals for non-cardiovascular use block these channels, the effects on the cardiac waveform can be life threatening, presenting an unacceptable risk. Therefore, the ability to better predict cardiac liability on hNav1.5 targets earlier in the drug development process remains crucial.

3- hNav1.5 : a target of interest for safety screening

The new functional high throughput screening technologies have enabled to increase the number of assays on ion channels, now fully exploited as drug targets. In addition to hERG channels, automated patch-clamp platforms are also used to screen new drugs on hNav1.5, to early detect adverse side effects (5), (6). 

4- hNav1.5 channel : structural and functional characteristics

Mammalian voltage-gated Na+ channels (NaChs) are responsible for the generation of action potentials in excitable membranes. To date, 10 Nav alpha subunits from the S4 superfamily of voltage-gated ion channels genes have been identified (Nav1.1-1.9 and Nav2.1), with Nav1.5 being thought to be prominent cardiac subunit. Mammalian NaChs consist of a large pore-forming alpha-subunit (230-270 kDa) and one or two smaller beta-subunits (37-39 kDa) (7), but the alpha-subunit alone can form functional channels when transiently expressed in human embryonic kidney cells which appears comparable with the native Na+ channel (8). The alpha subunit consists of four homologous domains (I to IV), each of which contains six transmembrane-spanning regions (S1-S6) and these four domains come together to form the Na+-selective pore. The cytoplasmic linker between domains III and IV has been shown to play a pivotal role in voltage-dependant Nav channel inactivation (refs in  9 and 10).
hNav1.5 channels display rapid activation gating (opening) as well as slow inactivation gating (closing) during depolarization, undergoing conformational changes (11). They exhibit varying degrees of tetrodotoxin resistance and are more sensitive to lidocaine block than the other isoforms (ref in 12). It is now clear that accessory subunits play an important role in the functionality of Nav channels in cardiac cells. To date, 4 accessory beta subunits, beta1-4, have been identified. Beta1-3 are expressed in cardiac tissue with beta1 and 3 appearing to be the most important to cardiac function. These subunits modify the cell surface expression and the kinetic properties of Nav channels and may also target channels to the plasma membrane.

hNav1.5 Cell Line Performance Data

BACK to CREACELL ION CHANNEL CELL LINES

References :

(1) : Corrado D. et al. (2005). “Is it time to include ion channel diseases among cardio-myopathies? J. Electrocardiology 38 (4), Suppl.1 : 81-87. (2) : Mohler PJ (2006). “Ankyrins and Human Disease: What the Electrophysiologist Should Know”. Journal of Cardiovascular Electrophysiology 17 (10), 1153–1159. (3) : Mohler PJ et al. et al. (2004). “Nav1.5 E1053K mutation causing Brugada syndrome blocks binding to ankyrin-G and expression of Nav1.5 on the surface of cardiomyocytes.” PNAS 101 (50) : 17533-8. (4) : Mohler PJ and Bennett V. (2005). “Ankyrin-based cardiac arrhythmias: a new class of channelopathies due to loss of cellular targeting”. Current opinion cardiol 20 (3) : 189-93. (5) : Chiu P (2006) “Fast, Reliable, Early Screening for Potentially Arrythmia-Causing hERG inhibitors”, MDS Pharma Services Technical Q&A. (6) Comley J (2005/6) “Automated Patch Clamping; Setting a New Standard for Early hERG”, Drug Discovery World, Winter, p. 62. (7) : Catterall, W.A. (2000). From ionic currents to molecular mechanisms: the structure and function of voltage-gated sodium channels. Neuron. 26:13–25. (8) : Ukomadu, C. et al. (1992). Na+ channels expressed transiently in human embryonic kidney cells: biochemical and biophysical properties. Neuron. 8:663–676. (9) : Wang S-Y et al. (2005) : « tryptophan substitution of a putative D4S6 gating hinge alters slow inactivation in cardiac sodium channels. Biophysical J. 88 : 3991-3999. (10) : Xiao Y-F et al. (2006). Potent block of inactivation-deficient na+ channels by n-3 polyinsaturated fatty acuds. Am J Physiol Cell Cell Physiol 290 : C362-C370. (11) : O’Reilly JP et al. (2006) « Slow-inactivation induced conformational change in domain 2-segment 6 of cardiac Na+ channel” Biochem. Biophys. Res. Comm. 345 (1) : 59-66. (12) : McNulty MM et al. (2004). “State-dependant mibefradil block of na+ channels”. Mol. Pharmacol. 66 : 1652-1661.

Last Updated: August of 2007