Tuning the Beat
Differential Expression of Ion Channels in the Sinus Node
See related article, pages 1384–1393
In this issue Tellez et al1 give a detailed analysis of differentially expressed genes regulating excitation and conduction in the sinoatrial node (SAN) in rabbit. Quantitative PCR and in situ hybridization, as well as action potential recordings, enabled the investigators to assign specific patterns to central and peripheral regions of SAN, and to compare these with the corresponding properteis of atrial tissue. Cluster analysis revealed that the SAN transcript profile is significantly different from that of atrial muscle. More importantly, there are apparent isoform switches on moving from atrial muscle to the SAN center: RYR2 to RYR3, Nav1.5 to Nav1.1, Cav1.2 to Cav1.3 and Kv1.4 to Kv4.2. In this context the transcript profile of the SAN periphery represents an intermediate pattern between that of central SAN and atrial muscle.
Differential expression of a variety of genes has been demonstrated in anatomically distinct regions of the heart, for example atrial versus ventricular myocardium,2 or ventricular endocardium versus ventricular epicardium,3 which gives clues to the molecular substrates controlling distinct myocardial electrical properties of specific regions in the heart.
Tellez et al meticulously dissected SAN tissue based on previous work and functional studies, and the authors were able to demonstrate that differential expression corresponds to specific electrical properties of SAN central, SAN periphery and free atrial myocardium. SAN central is characterized by poor electrical coupling to protect against inhibitory hyperpolarizing influence of surrounding atrial muscle. It provides ionic currents appropriate for pacemaking, resulting in spontaneous activity, a pacemaker potential, low take-off potential of the action potential, slow upstroke, small overshoot, small amplitude, long duration, low maximum diastolic potential (MDP). SAN periphery (specifically, the anatomically defined right branch of the sinoatrial ring bundle [RSARB]) has strong electrical coupling, ionic currents composed of higher take-off potential of the action potential, faster upstroke (25-fold), large amplitude and short duration with a high maximum MDP. SAN periphery serves to insulate SAN central from atrial myocardium on the one hand, and to conduct and propagate impulses to atrial tissue on the other hand.
Differential expression of gap junction proteins is a major factor controlling the extended conduction system and has recently been described by this group in detail.4 In SAN central and SAN periphery, gap junction protein expression corresponds to differences in electrical coupling. Whereas messenger RNA for Cx43, a medium conductance gap junction protein, is abundant in atrial myocardium, (present in SAN periphery but absent in SAN central) messenger RNA for Cx45 (and 30.2), a low conductance gap junction protein, is present in SAN central.
The importance of SR Ca2+ release in SAN pacemaker rate, and response to β-adrenergic activation, has recently been substantiated by Bogdanov et al5 and highlighted by an editorial by Bers6 in this journal. Differences in intracellular Ca2+ handling between the center and the periphery of the rabbit SAN may be related to the observed isoform switch from RYR2 to RYR3, from atrial muscle to SAN central. The isoform switch from Cav1.2 to Cav1.3, from atrial muscle to SAN center with Cav1.3 activating at more negative potentials than Cav1.2 is another example for fine tuning the pacemaking and corresponds to the absence of Nav1.5 messenger RNA in SAN central but not in SAN periphery and surrounding atrial muscle.
A century past the discovery of the pacemaker of the heart7 and half a century past the first functional studies of the rabbit sinoatrial node,8 building on the wealth of previous studies of sinus node physiology in rabbit, we are reaching a level of detailed understanding that challenges us to compile a comprehensive model with qualitative and quantitative signaling. This will help our understanding of malfunction or loss of sinus node pacemaker cells because of disease or aging, and will direct us toward more sophisticated cures. Very recently, bioartificial pacemaker have been designed successfully to modulate excitation either by transfer of a key pacemaking channel9 or by a synthetic pacemaker channel10 designed to minimize interference with intrinsic ion channels and maximize flexibility with regard to frequency tuning.
In addition detailed analysis of rare model diseases, such as familial sick sinus syndrome that has been linked to loss of function mutations in SCN5A, the gene encoding Nav1.5 may help to focus our attention on key elements of pacemaking and conduction.11–14 On the other hand, these rare model diseases may help to comprehend the pathology and potential genetic susceptibility to the more common forms of sinus node dysfunction and conduction disease. In this context the complexity of our current understanding of sinus node physiology asks for a more complex polygenic substrate even in many cases that are currently viewed as monogenic.
Sources of Funding
Bundesministerium für Bildung und Forschung (BMBF) grants 01GS0499, and 01GI0204.
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
Tellez JO, Dobrzynski H, Greener ID, Graham GM, Laing E, Honjo H, Hubbard SJ, Boyett MR, Billeter R. Differential expression of ion channel transcripts in atrial muscle and sinoatrial node in rabbit. Circ Res. 2006; 99: 1384–1393.
Nabauer M, Beuckelmann DJ, Uberfuhr P, Steinbeck G. Regional differences in current density and rate-dependent properties of the transient outward current in subepicardial and subendocardial myocytes of human left ventricle. Circulation. 1996; 93: 168–177.
Yamamoto M, Dobrzynski H, Tellez J, Niwa R, Billeter R, Honjo H, Kodama I, Boyett MR. Extended atrial conduction system characterised by the expression of the HCN4 channel and connexin45. Cardiovasc Res. 2006; 72: 271–281.
Bogdanov KY, Maltsev VA, Vinogradova TM, Lyashkov AE, Spurgeon HA, Stern MD, Lakatta EG. Membrane potential fluctuations resulting from submembrane Ca2+ releases in rabbit sinoatrial nodal cells impart an exponential phase to the late diastolic depolarization that controls their chronotropic state. Circ Res. 2006; 99: 979–987.
Bers DM. The beat goes on: diastolic noise that just won’t quit. Circ Res. 2006; 99: 921–923.
Keith A, Flack MW. The form and nature of the muscular connections between the primary divisions of the vertebrate heart. Journal of Anatomy and Physiology. 1907; 41: 172–189.
De Carvalho AP, De Mello WC, Hoffman BF. Electrophysiological evidence for specialized fiber types in rabbit atrium. Am J Physiol. 1959; 196: 483–488.
Tse HF, Xue T, Lau CP, Siu CW, Wang K, Zhang QY, Tomaselli GF, Akar FG, Li RA. Bioartificial sinus node constructed via in vivo gene transfer of an engineered pacemaker HCN Channel reduces the dependence on electronic pacemaker in a sick-sinus syndrome model. Circulation. 2006; 114: 1000–1011.
Kashiwakura Y, Cho HC, Barth AS, Azene E, Marban E. Gene transfer of a synthetic pacemaker channel into the heart: a novel strategy for biological pacing. Circulation. 2006; 114: 1682–1686.
Smits JP, Koopmann TT, Wilders R, Veldkamp MW, Opthof T, Bhuiyan ZA, Mannens MM, Balser JR, Tan HL, Bezzina CR, Wilde AA. A mutation in the human cardiac sodium channel (E161K) contributes to sick sinus syndrome, conduction disease and Brugada syndrome in two families. J Mol Cell Cardiol. 2005; 38: 969–981.