The SCN5A gene directs synthesis of cardiac-type, voltage-dependent, Na⁺ channels (Na_v1.5), abundant in the heart. The simultaneous and transient openings of large numbers of voltage-gated Na⁺ channels mediate the rapid inward current responsible for the rising phase of the cardiac action potential (AP). This is fundamental to initiation, propagation, and maintenance of the normal cardiac rhythm that ensures synchronized atrial and ventricular contraction and therefore the normal heartbeat. SCN5A mutations are associated with cardiac arrhythmic syndromes ranging from chronic bradyarrhythmias to acute life-threatening tachyarrhythmias. These include the congenital long QT syndrome subtype 3 (LQT3; Wang et al., 1995), Brugada syndrome (BrS; Chen et al., 1998; Brugada et al., 2000), isolated cardiac conduction disease (CCD; Schott et al., 1999), sinus node dysfunction (SND; Benson et al., 2003), and sudden infant death syndrome (SIDS; Otagiri et al., 2008). Over the past decade, mouse models with genetic modifications in SCN5A have provided invaluable insights into these cardiac disorders. This review summarizes recent studies on Na_v1.5 disorders and their bearing on cardiac conduction properties.

The last two decades have seen significant progress in our understanding of the molecular structure of voltage-gated Na⁺ channels (reviews Goldin, 2001, 2002; Catterall et al., 2003). These channels comprise pore-forming α-subunits, with molecular weights of ~260 kDa, and associated auxiliary β-subunits with molecular weights ~36 kDa (Goldin, 2002; Catterall et al., 2003). Expression of the α-subunit alone is sufficient for functional Na⁺ current expression, but the presence or absence of β-subunits modifies the kinetics and voltage-dependence of channel gating. The α-subunits are organized into four homologous domains (I–IV). Each contains six transmembrane α-helices (S1–S6) and includes an additional pore loop located between the S5 and S6 helices. The various α-subunit isoforms are differentially expressed in different tissues and have distinct pharmacological properties (Goldin, 2002; Catterall et al., 2003).

Recent investigations also demonstrate that in addition to Na_v1.5, different cardiac tissues co-express a range of different α-subunit isoforms. These include several neuronal, Na_v1.1, Na_v1.3, and Na_v1.6, isoforms primarily expressed in brain (Maier et al., 2002; Lei et al., 2004). This pattern is exemplified by the SA node, which shows a complex co-expression of multiple Na_v isoforms. It contains both Na_v1.5 and Na_v1.1 with Na_v1.5 absent in the central nodal cells (Lei et al., 2004). Nav1.3 is also present in mouse (Maier et al., 2002) but not rabbit or rat SA node (Baruscotti et al., 1997; Maier et al., 2002). More recent, competitive RT-PCR and immunohistological, studies have demonstrated Nav1.1, Nav1.2, and Nav1.5 expression in canine SA and atrioventricular (AV) nodes (Haufe et al., 2005).

Recent studies have also clarified the functional roles of these distinct cardiac and neuronal-type Na⁺ channels in the SA node. First, it is likely that the tetrodotoxin (TTX)-sensitive neuronal (Na_v1.1) Na⁺ current is involved in pacemaker function in the SA node. Nanomolar TTX concentrations known to only inhibit neuronal Na⁺ current slowed down pacemaker rates of intact mouse hearts by ~65% (Maier et al., 2002) isolated SA nodes by ~22% and isolated SA nodal pacemaker cells by ~15% (Lei et al., 2004). Furthermore, studies using the AP clamp technique demonstrated that the neuronal Na⁺ current could be activated within the voltage ranges of the pacemaker potential (Lei et al., 2004).

In contrast, the TTX-resistant cardiac (Na_v1.5) Na⁺ current is likely to be important in AP propagation and conduction through the SA node. Thus, measurements of SA node conduction times demonstrated that block of both TTX-sensitive and TTX-resistant Na⁺ current by μM TTX slowed or even blocked AP conduction through the SA node periphery, from the leading pacemaker site in the center of the SA node to the surrounding atrial muscle. However, block of the TTX-sensitive Na⁺ current alone by 10 or 100 nM TTX did not produce such effects (Lei et al., 2004).

Together these findings implicate both neuronal (Na_v1.1) and cardiac (Na_v1.5) Na⁺ channels in pacemaker activities as well as AP conduction through the SA node and from the SA node to surrounding atrial muscle. Their demonstration of physiological roles for Na_v1.5 in SA node function underpins our understanding of the SND attributed to genetic defects in Na_v1.5 channel, as discussed in the sections below.

A mouse model containing a disrupted Na_v1.5 gene, Scn5a, was developed a decade ago (Papadatos et al., 2002). Homozygotes show intrauterine lethality with severe defects in ventricular morphogenesis. Scn5a^+/− heterozygotes show normal survival but a number of electrophysiological defects including impaired atrioventricular conduction, delayed intramyocardial conduction, increased ventricular refractoriness, and ventricular tachycardia with characteristics of re-entrant excitation. Single whole-cell patch clamp studies of isolated adult Scn5a^+/− ventricular myocytes demonstrated a ~50% reduction in Na⁺ conductance. Such mice thus provide a unique and valuable model system for studying physiological effects of cardiac Na⁺ channelopathies.

Recent studies have explored for possible relationships between the pathogenesis of SND, aging processes, and Scn5a-disruption, as well as their possible interactions in the Scn5a^+/− mouse model (Hao et al., 2011). These associated both electrical remodeling and tissue degeneration, detected as a TGF-β₁-mediated fibrosis, with altered pacemaker and conduction function in SND. Such changes occurred with both Scn5a-disruption and aging. A combination of both factors produced the most severe phenotype. Thus, ex vivo SA node preparations isolated from their autonomic inputs then showed increased cycle lengths and sino-atrial conduction times. These changes accompanied alterations in the extent of fibrosis assessed by collagen and fibroblast levels, and ion channel and regulatory gene transcriptional remodeling. Aging and Scn5a-disruption correspondingly resulted in interacting up-regulatory effects on levels of the key modulator of fibrosis, TGF-β₁, and the fibroblast marker, vimentin. The latter changes were also greatest in the old Scn5a^+/− hearts (Figure 2). Altered expression of TGF-β₁ and vimentin transcripts are associated with increased collagen and fibroblast abundance indicating an occurrence of interstitial fibrosis. The fibrosis could potentially slow conduction both within the SA node and from the SA node to the surrounding atrium. Its occurrence parallels previous reports associating the Scn5a^+/− condition and similar, ventricular, fibrotic changes (Royer et al., 2005; van Veen et al., 2005) as well as associating age-dependent SND and fibrosis (Benditt et al., 1990) although such features were not observed in all studies (Alings et al., 1990; Alings and Bouman, 1993). In implicating Na_v1.5 deficiency in such changes these findings additionally suggest novel regulatory roles for Na_v1.5 in cellular biological processes extending beyond its electrical function.

The above findings add to studies that similarly showed that heterozygous Scn5a inactivation in mouse produces ventricular rearrangements and fibrosis with aging (Royer et al., 2005; van Veen et al., 2005; Leoni et al., 2010). They also demonstrated aging resulted in an upregulation of two transcription factors, Atf3, a stress-inducible gene, and Egr1, an early growth response gene, particularly in Scn5a^+/− mice (Royer et al., 2005; van Veen et al., 2005). Furthermore, the variable Na_v1.5 protein expression from the WT allele correlates with the extent of PCCD in the Scn5a^+/− mouse model (Leoni et al., 2010). This study (Leoni et al., 2010) divided 10-week-old Scn5a^+/− mice into two electrocardiographic subgroups showing either severe or mild ventricular conduction defects. These phenotypic differences persisted with aging. At 10 weeks, the Na⁺ channel blocker ajmaline produced similar prolongations of QRS intervals in both groups of Scn5a^+/− mice. However, the effect of ajmaline was greater in the severely affected subgroup in old mice (>53 weeks). Ventricular tachycardia developed in 5- to 10-week-old severely but not mildly affected Scn5a^+/− mice. Such findings therefore matched clinical observations in patients with SCN5A loss-of-function mutations with either severe or mild conduction defects. The severely but not mildly affected old Scn5a^+/− mice also showed extensive cardiac fibrosis. Finally, the severely affected Scn5a^+/− mice had similar Na_v1.5 mRNA but lower Na_v1.5 protein expression, moderately smaller Na⁺ currents, and reduced AP upstroke velocities than the mildly affected Scn5a^+/− mice.

Mice have become powerful tools for clarifying the pathophysiological consequences of SCN5A mutations in SCN5A-related cardiac disorders. Mice harboring SCN5A mutations related to PCCD and SND convincingly recapitulate the clinical phenotypes of PCCD and SND in patients, and provide insight into the mechanisms of SCN5A deficiency-associated cardiac conduction diseases. They should constitute useful tools for future studies addressing as yet unanswered questions, such as the role of genetic and environmental modifiers on SCN5A disease phenotypes.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.