Molecular Architecture of the Human Sinus Node
Insights Into the Function of the Cardiac Pacemaker
Background— Although we know much about the molecular makeup of the sinus node (SN) in small mammals, little is known about it in humans. The aims of the present study were to investigate the expression of ion channels in the human SN and to use the data to predict electrical activity.
Methods and Results— Quantitative polymerase chain reaction, in situ hybridization, and immunofluorescence were used to analyze 6 human tissue samples. Messenger RNA (mRNA) for 120 ion channels (and some related proteins) was measured in the SN, a novel paranodal area, and the right atrium (RA). The results showed, for example, that in the SN compared with the RA, there was a lower expression of Nav1.5, Kv4.3, Kv1.5, ERG, Kir2.1, Kir6.2, RyR2, SERCA2a, Cx40, and Cx43 mRNAs but a higher expression of Cav1.3, Cav3.1, HCN1, and HCN4 mRNAs. The expression pattern of many ion channels in the paranodal area was intermediate between that of the SN and RA; however, compared with the SN and RA, the paranodal area showed greater expression of Kv4.2, Kir6.1, TASK1, SK2, and MiRP2. Expression of ion channel proteins was in agreement with expression of the corresponding mRNAs. The levels of mRNA in the SN, as a percentage of those in the RA, were used to estimate conductances of key ionic currents as a percentage of those in a mathematical model of human atrial action potential. The resulting SN model successfully produced pacemaking.
Conclusions— Ion channels show a complex and heterogeneous pattern of expression in the SN, paranodal area, and RA in humans, and the expression pattern is appropriate to explain pacemaking.
Received July 2, 2008; accepted January 23, 2009.
The sinus node (SN), the primary cardiac pacemaker, is complex and heterogeneous.1 The action potential originates in the SN and then propagates into the surrounding muscle of the right atrium (RA).1 The SN is highly adapted to its role as the primary pacemaker; although its mix of ionic currents fits it for pacemaking, its poor electrical coupling protects it from the inhibitory hyperpolarizing influence of the surrounding RA.1,2 Since its discovery by Keith and Flack in 1907,3 a wealth of data has been accrued on this specialized tissue in small mammals, particularly concerning the action potential and the underlying ionic currents. However, only 1 action potential has been published from the adult human SN,4 and this came from a diseased heart subjected to a range of treatments. Consequently, there is little information available on the nature of the action potential in the human SN and the ionic currents that underlie it. The molecular basis of the ionic currents involved in SN action potential has recently been established for both the rabbit5 and mouse6; however, the “molecular architecture” of the human SN has not been elucidated. Of particular interest are ion channels and Ca2+-handling proteins that are key to electrical activity.1,2 Mutations in ion channels in humans (Nav1.5, HCN4) and the knockout of ion channels in mice (Cav1.3, Cav3.1) have been linked with sick sinus syndrome or bradycardia.7
Editorial p 1556
Clinical Perspective p 1575
The primary aim of the present study was to investigate ion channels, along with Ca2+-handling proteins, connexins, receptors, and cell-type markers, at the messenger RNA (mRNA) and protein levels in the SN and the surrounding RA in humans. Our data revealed a complex expression of ion channels and demonstrated a previously undescribed paranodal area (PN) within the terminal crest. The secondary aim of the study was to use a functional genomics approach, which integrated the expression of ion channels by mathematical modeling, to predict electrical activity in the SN. Using a new method of building cell membrane models in which mRNA levels were used to estimate ionic conductances, we were able to demonstrate the potential for spontaneous electrical activity within the SN.
We obtained 6 specimens of the area of RA that contained the SN and PN from nonfailing healthy hearts from the Prince Charles Hospital District, Chermside, Australia (ethics approval, EC2565; work was also ethically approved by the University of Manchester). A clinical profile of the donors is shown in the online Data Supplement (supplemental Table I). We used quantitative polymerase chain reaction (qPCR), in situ hybridization, histology, immunofluorescence, and mathematical modeling to investigate the molecular makeup (and predict function) of the human SN, PN, and RA. Detailed descriptions of the methods are given in the Data Supplement.
Sampling of SN, PN, and Atrial Muscle for qPCR
We sampled SN, PN, and RA (outlined in red, green, and yellow in Figure 1A) from 50 to 100 histological sections from the head, body, and tail of the SN (as defined by Sanchez-Quintana et al8). After extraction of RNA, the amounts of ≈120 mRNAs for cell-type markers, ion channels, Ca2+-handling proteins, connexins, and receptors were measured with qPCR. Here, we describe only the mRNAs for major ion channels or mRNAs that varied among tissues. All data are shown in the Data Supplement (Figures I through VI).
Mean±SEM values are shown. Significant differences in the abundance of mRNAs and proteins in different tissues were identified with 1-way ANOVA and paired t tests. A difference was assumed to be significant at P<0.05. In some Figures, absolute probability values are shown. For cluster analysis, ΔCT values for each mRNA were transformed to log10 and centered. This resulted in a distribution for each mRNA with a mean of 0 and an SD of 1. Because not all the resulting data were normally distributed, statistical analysis of microarrays, a bootstrap method that is tolerant to nonnormality, was used to test differences in mRNA abundance between SN and PN, SN and RA, and PN and RA. A more detailed description of the statistical analysis used in the present study can be found in the Data Supplement.
The authors had full access to and take full responsibility for the integrity of the data. All authors have read and agree to the manuscript as written.
Morphological Characteristics of SN, PN, and Atrial Muscle
Sectioning of the terminal crest revealed the location of the SN on staining with Masson’s trichrome (Figure 1A). The SN was clearly identifiable on the basis of the presence of the SN artery and the large amount of connective tissue (stained blue; myocytes stained purple/pink). Interestingly, sections through the SN also showed a discrete area, composed of loosely packed myocytes, which we have termed the paranodal area (PN). The PN is close to but not continuous with the SN, as shown in Figure 1A. Immunofluorescence showed that this area consisted of a mixture of myocytes expressing and lacking 2 marker proteins: Connexin43 (Cx43), which is responsible for electrical coupling, and atrial natriuretic peptide (ANP), a hormone secreted by atrial muscle (Figure 1B and 1C; white and blue arrows highlight myocytes expressing or lacking markers, respectively). Both Cx43 and ANP are known to be absent from the SN but abundantly expressed in the RA in small mammals.5,6 The present immunofluorescence analysis revealed this was also the case in humans (Figure 1B and 1C).
Housekeeping Genes and Nodal Markers
For qPCR, the level of each transcript is given relative to a housekeeping gene to correct for variations in input RNA. mRNA levels of 2 housekeeper genes were measured: 28S and GAPDH. The level of both was similar in each of the 3 tissues. We chose 28S for the present analysis, although similar results were obtained with GAPDH. ANP and Tbx3 (a transcription factor known to be expressed in SN of small mammals9) were used to characterize the tissues studied by qPCR. qPCR showed ANP to be highly abundant in the PN and RA but poorly expressed in the SN (Figure 2A). This was confirmed by in situ hybridization (Figure 3A and 3B; note the mixture of ANP-expressing [eg, green arrows] and ANP-nonexpressing [eg, red arrows] myocytes in the PN in Figure 3B). qPCR showed Tbx3 to be abundant in the SN but poorly expressed in the RA; there was an intermediate level of expression in the PN (Figure 2B). The poor expression of ANP but high expression of Tbx3 in the SN demonstrated by qPCR confirms that during dissection, the SN tissue was not contaminated with RA tissue.
Voltage-gated Na+ channels are responsible for the inward Na+ current, INa. INa is responsible for the rapid action potential upstroke seen in the working myocardium. Studies in small mammals show that there is little or no measurable INa in the center of the SN.1,2 qPCR showed that mRNA levels for the cardiac Na+ channel, Nav1.5, were SN<PN<RA (Figure 2E). This was confirmed by in situ hybridization (Figure 3C), and immunohistochemistry showed that Nav1.5 protein was expressed in a similar manner (Figure 4).
Recently, in small mammals, other Na+ channels have also been shown to be expressed in cardiac myocytes and to have a role in the SN action potential.10 There was complex expression of other Na+ channels in the 3 regions (Figure 2C and 2D; supplemental Figure II). At the mRNA level, their expression levels were >100-fold lower than Nav1.5, which suggests that these Na+ channels may not play a significant role in humans. Of the Na+ channel β-subunits (Navβ1 through Navβ4), Navβ1 was the most abundant β-subunit at the mRNA level, and its expression was SN<PN<RA (Figure 2F; supplemental Figure II).
Two separate Ca2+ currents have been recorded from SN cells, the long-lasting type (ICa,L) and the transient type (ICaT).1,2,7 ICaL has been shown to have a role in the upstroke of the SN action potential and may have a role in diastolic depolarization.1,2,7 ICaT has been shown to have a role in diastolic depolarization.1,2,7 The ion channels responsible for ICaL are Cav1.2 and Cav1.3; for ICaT, they are Cav3.1 through Cav3.3. Cav1.2 mRNA was ≈10-fold more abundant than Cav1.3 mRNA (Figure 2G and 2H). As measured by qPCR, the Cav1.2 mRNA level tended to be SN<PN=RA, but this difference was not significant (Figure 2G). At the protein level, as determined by immunofluorescence, the amount of Cav1.2 was SN<PN=RA (supplemental Figure VII). Both qPCR (Figure 2H) and in situ hybridization (Figure 3D) showed the level of Cav1.3 mRNA to be SN>PN=RA. Cav3.1 mRNA was at least 10-fold more abundant than Cav3.2 and Cav3.3 mRNAs; Cav3.2 mRNA was below the detection threshold (supplemental Figure II). Like Cav1.3 mRNA, Cav3.1 mRNA (Figure 2I) and protein (supplemental Figure VII) were SN>PN=RA; Cav3.3 mRNA tended to show a similar pattern (supplemental Figure II). Perhaps like Cav1.2 mRNA, mRNAs for the Ca2+ channel subunits, Cavα2δ3 and Cavγ4, were SN<PN=RA (supplemental Figure II). mRNAs for other Ca2+ channel subunits (Cavβ1 through Cavβ3, Cavα2δ1, and Cavα2δ2) did not vary among the 3 regions (supplemental Figure II).
The HCN (hyperpolarization-activated cyclic nucleotide-gated) channels are responsible for the hyperpolarization-activated or “funny” current, If.1,2,7 This has a key role in the pacemaker potential of the SN.2 There are 4 members of the HCN family, HCN1 through HCN4. The levels of both HCN1 and HCN4 mRNAs were SN>PN=RA (Figure 2J and 2L). The level of HCN2 mRNA, in contrast, was SN<PN<RA (Figure 2K). HCN3 mRNA expression was negligible in all tissues (supplemental Figure II). HCN4 mRNA was more abundant than mRNA for the other isoforms, and it was investigated at the protein level with immunofluorescence. Figure 5 shows HCN4 protein to be expressed in the sarcolemma of the SN myocytes, but it was below the detection threshold with the immunohistochemical method in both the PN and RA.
Transient Outward K+ Channels
The transient outward K+ current (Ito) is responsible for the early phase of repolarization (phase 1) of the action potential. The ion channels responsible for Ito are the voltage-dependent K+ channels, Kv1.4, Kv4.2, and Kv4.3. Kv4.3 is regarded as the major ion channel underlying Ito in humans, and Kv4.3 mRNA was at least 10-fold more abundant than Kv1.4 and Kv4.2 mRNAs (Figure 6A through 6C). The pattern of expression of Kv4.3 mRNA (Figure 6C) and protein (supplemental Figure VIII) was SN=PN<RA. Interestingly, Kv1.4 mRNA was SN=RA>PN, whereas Kv4.2 mRNA was SN=PN>RA (Figures 6A and 6B).
Delayed-Rectifier K+ Channels
There are 3 delayed-rectifier K+ currents, ultrarapid (IKur), rapid (IKr), and slow (IKs). The ion channels responsible for IKur, IKr, and IKs are Kv1.5, ERG (ether-a-go-go-related gene), and KvLQT1, respectively. At the mRNA level, the amounts of the 3 ion channels were comparable (Figure 6D through 6F). Both Kv1.5 and ERG mRNA levels were SN<PN=RA, and KvLQT1 mRNA tended to be distributed in a similar manner (Figure 6D through 6F). Immunofluorescence showed Kv1.5 protein to be distributed in the same manner as its mRNA (supplemental Figure VIII).
Inward-Rectifier K+ Channels
The background inward-rectifier K+ current, IK1, is generated by the ion channels Kir2.1 through Kir2.4. Kir2.1 and Kir2.3 mRNAs were ≈10-fold more abundant than Kir2.2 and Kir2.4 mRNAs (supplemental Figure IV). Kir2.1 mRNA level was SN=PN<RA (Figure 6G). Kir2.3 mRNA also tended to be lower in the SN (Figure 6H). Previously, we have also shown that in humans, Kir2.1 protein is expressed in the RA but not in the SN.7 The acetylcholine-activated K+ current, IKACh, is generated by a heteromultimer of Kir3.1 and Kir3.4.11 Kir3.1 mRNA was ≈10-fold more abundant than Kir2.1 and Kir2.3 mRNAs; neither Kir3.1 mRNA and protein nor Kir3.4 mRNA varied among tissues (supplemental Figures IV and VIII). In the heart, the ATP-sensitive K+ current, IKATP, is thought to be generated by the α-subunit, Kir6.2, and the β-subunit, SUR2A. Surprisingly, the abundance of Kir6.1 mRNA was similar to that of Kir6.2 mRNA (supplemental Figure IV). Kir6.2 mRNA level was SN<PN=RA (supplemental Figure IV), but interestingly, Kir6.1 mRNA was SN=RA<PN (Figure 6I). SUR2A mRNA expression did not vary among the tissues (supplemental Figure IV).
Other K+ Channels
mRNAs for Kv1.2, Kv1.6, Kv1.7, and Kv2.1 were generally weakly expressed in the 3 tissues (supplemental Figure III). Nevertheless, Kv1.2 mRNA expression was significantly less in the PN than in the RA, whereas Kv1.6 and Kv1.7 mRNAs were SN<PN=RA (supplemental Figure III). TASK1 and TWIK1 are twin-pore domain K+ channels that are expressed in cardiac muscle, although their function is unclear. mRNAs for both channels were highly abundant, comparable to Kv1.5, KvLQT1, and Kir2.1 mRNAs, for example (supplemental Figure IV). TASK1 mRNA was SN=RA<PN (Figure 6J), whereas TWIK1 mRNA was SN<PN=RA (supplemental Figure IV). There are 3 small conductance Ca2+-activated K+ channels, SK1 through SK3. SK channels are responsible for the Ca2+-activated K+ current, IKCa, and play a role in repolarization of the human atrial action potential12; SK2 has been shown to be coupled to Cav1.2 and Cav1.3 via α-actinin 2.13 SK2 and SK3 mRNAs were abundant (abundance comparable to that of Kir2.1 mRNA, for example); SK1 mRNA was ≈10-fold less abundant (supplemental Figure IV). SK2 mRNA expression was SN=RA<PN (Figure 6K).
We also studied accessory proteins for K+ channels. Three β-subunits are thought to regulate the delayed-rectifier K+ channels, Kvβ1 through Kvβ3. Kvβ2 mRNA tended to be the most abundant; Kvβ2 mRNA was SN<PN (supplemental Figure IV). MinK-related peptides (MiRPs) are suggested to regulate delayed-rectifier K+, transient outward K+, and HCN channels.14 At the mRNA level, MiRP2 and MiRP3 were more abundant than MiRP1 (supplemental Figure IV); MiRP2 mRNA expression was SN=RA<PN (Figure 6L). Of accessory proteins thought to regulate transient outward K+ channels, at the mRNA level, frequenin (a Ca2+-binding protein known to regulate Kv4 channels)15 was SN>PN=RA and KChIP2 (K+ channel-interacting protein 2) was SN=PN<RA, whereas DPP6 (dipeptidyl aminopeptidase-like protein 6) and KChAP (K+ channel-associated protein) did not vary among tissues (supplemental Figure III).
Intracellular Ca2+ has been suggested to play an important role in pacemaking in small mammals.16 The Na+-Ca2+ exchanger, NCX1, did not vary among tissues at the mRNA (Figure 7A) and protein (supplemental Figure IX) levels. Ryanodine receptor 2 (cardiac; RyR2) mRNA (Figure 7G) and protein (supplemental Figure IX) expression levels were SN<PN=RA, whereas RyR3 mRNA was SN=PN>RA (Figure 7H). However, RyR2 mRNA was ≈1000-fold more abundant than RyR3 mRNA (Figures 7G and 7H). The sarcoplasmic reticulum (SR) Ca2+ pump, SERCA2a, both at the mRNA (Figure 7I) and protein (supplemental Figure IX) levels, was SN<PN=RA. Phospholamban and sarcolipin regulate SERCA2a, and at the mRNA level, they were of comparable abundance; sarcolipin expression was SN<PN=RA, and phospholamban had a similar distribution (supplemental Figure V). PMCA1 and PMCA4 are plasmalemmal Ca2+ ATPases. PMCA4 mRNA was ≈10-fold more abundant than PMCA1 mRNA (supplemental Figure V). PMCA1 mRNA was SN=RA<PN, whereas PMCA4 mRNA was SN>PN=RA (supplemental Figure V). At the mRNA level, all 3 inositol triphosphate (IP3) receptors (isoforms 1 through 3) were detected; isoforms 2 and 3 tended to be most abundantly expressed in the PN (supplemental Figure V). Ca2+/calmodulin-dependent protein kinase IIb and FKBP12 (FK506-binding protein) were detected, but levels did not vary among tissues (supplemental Figure V). TRPC (transient receptor potential canonical) channels have been suggested to be store-operated Ca2+ channels and to be involved in pacemaking in the SN17; 5 isoforms were detected at the mRNA level (supplemental Figure V). TRPC6 mRNA was SN>PN>RA (supplemental Figure V).
The α1, α2, and α3 isoforms of the Na+/K+ pump were detected at the mRNA level. Expression of all 3 isoforms tended to be SN<PN=RA (supplemental Figure V).
Connexins and Cell Adhesion Proteins
Connexins (Cx) are responsible for electrical coupling in the heart. At the mRNA level, the levels of Cx40, Cx43, and Cx45 (responsible for 200-picosiemen, 60 to 100-picosiemen, and 30- to 40-picosiemen gap junction channels, respectively) were comparable and at least 10-fold greater than that of Cx31.9 (responsible for 30- to 40-picosiemen gap junction channels; Figures 7A through 7D). Both Cx40 mRNA (Figure 7B) and protein (supplemental Figure VII) and Cx43 mRNA (Figure 7C) were SN<PN=RA. The Cx31.9 mRNA level was SN<PN (Figure 7A). Two cell adhesion proteins were investigated at the mRNA level: cadherin and desmoplakin (supplemental Figure I). Desmoplakin mRNA was SN<PN=RA (Figure 7E).
Adenosine, adrenergic, and muscarinic receptors were investigated at the mRNA level. A1a, A2a, A2b, and A3 adenosine receptors were detectable; A3 receptor mRNA was SN<PN (supplemental Figure VI). α1a-, α1b-, α1d-, α2a- through α2c-, β1-, and β2-adrenergic receptors were detectable. The α1a-receptor mRNA was SN=PN>RA, the α2a-receptor tended to be SN=RA<PN, and the β1-receptor was SN=PN<RA (supplemental Figure VI). Of the muscarinic receptors, M2 did not vary among tissues, whereas M1 was undetectable (supplemental Figure VI).
Summary and Consequences for Electrical Activity
A 2-way hierarchical cluster analysis (Pearson distance) for all transcripts is shown in the Data Supplement (Figure X). The resultant clusters for the samples show the SN to be distinct from the other 2 tissues. A subset of 66 transcripts that contained fewer than 2.699 estimated false-positive calls in 3 bootstrap comparisons between SN, PN, and RA did cluster the samples into the 3 tissue groups (Figure 8A). Correspondingly, multidimensional scaling (Euclidian distance) with these 66 “indicator” transcripts could also separate the samples according to tissue (Figure 8B). This suggests that the SN is distinct from both the PN and RA, with the PN being intermediate between the other 2. The application of a number of different clustering algorithms (details in the Data Supplement) yielded 4 different groups of transcripts that consistently clustered together. The first were mRNAs that were expressed at higher levels in the SN (specifically, Cav1.3, Cav3.1, HCN1, HCN4, and frequenin). The second consisted of mRNAs that were expressed at higher levels in the PN (specifically, Kir6.1, TASK1, SK2, PMCA1, and the A3 adenosine receptor). The mRNAs of the third group were expressed at higher levels in the RA; they coded for ANP, Nav1.1, Nav1.4, Nav1.5, HCN2, Kv1.5, Kv1.7, Kir6.2, RyR2, sarcolipin, Cx43, and desmoplakin. The mRNAs of the fourth group were not differentially expressed between the tissues, but the consistent clustering indicates that they correlated well in the individual samples. They all code for accessory subunits to voltage-gated Ca2+ channels: Cavα2δ1, Cavα2δ3, Cavβ1, and Cavβ2.
What are the consequences of the unique pattern of expression of ion channels in the human SN on electrical activity in this tissue? It was assumed that whole-cell conductance for a particular ionic current was roughly proportional to the abundance of 1 or more mRNAs responsible for the relevant ion channel, ignoring the possible nonlinearity of the relationship between transcript abundance and conductance. For each of the major ionic currents (INa, ICaL, Ito, IKur, IKr, IKs, and IK1), the sum of relevant mRNAs in the SN was calculated and expressed as a percentage of the sum of the same mRNAs in the RA (supplemental Table VII). This was assumed to be equal to the conductance of the relevant ionic current in the SN expressed as a percentage of the conductance in the RA. Na+-Ca2+ exchange, SR Ca2+ release, and SR Ca2+ uptake were scaled in an analogous way (supplemental Table VII). Control values for human RA were taken from the mathematical model of Courtemanche et al18 for the human atrial action potential (based on extensive electrophysiological data from human RA). The human SN expresses the mRNAs responsible for ICaT and If (Figure 2). Because of the absence of these ionic currents in the model (and low expression of corresponding mRNAs in RA; Figure 2), a different approach was adopted for these ionic currents. Conductance for ICaT was assumed to be the same as in rabbit SN cells, and conductance for If was taken from the study of human SN by Verkerk et al.4 To validate this approach, mRNA abundance in human ventricular muscle was used to predict electrical activity in human ventricular muscle; the result was the well-known form of the human ventricular action potential (data not shown). This approach was then used to predict the electrical activity of the human SN by modification of 9 ionic currents and the 3 Ca2+-handling proteins (supplemental Table VII). The calculated human atrial action potential from the unmodified model of Courtemanche et al18 and the calculated human SN action potential as predicted from the model are shown in Figure 8C. The upstroke velocity of the action potential and the accompanying intracellular Ca2+ transients are also shown. The trace produced is a typical pacemaker action potential with a slow upstroke velocity (Figure 8C). There was little change in the action potential when only 1 of the 12 ionic currents and Ca2+-handling proteins was altered, and the pacemaker action potential was only obtained when all 9 ionic currents were altered (supplemental Figures XI and XII). Figure 8C shows that when If was blocked, pacemaking was slowed, as has been observed in human SN myocytes.19 The PN action potential was modeled in the same way as the SN action potential; compared with the RA action potential, the upstroke velocity and amplitude of the action potential were reduced, and the membrane tended to be depolarized during diastole (Figure 8C).
The present study is the first to investigate the complex expression of ion channels in the human SN and its environs. Our study shows 3 distinct patterns of molecular expression, revealing the existence of a PN in addition to the SN and RA. The expression pattern of the SN is appropriate for pacemaking.
Differences Between SN and Atrial Muscle
The cluster analysis in Figure 8A shows that a large number of mRNAs vary between the SN and the RA. These will be discussed in relation to ionic currents.
Compared with the RA, Nav1.5 mRNA and protein and the mRNA for the β-subunit, Navβ1, were less expressed in the SN (Figures 2 through 4⇑⇑). Whereas in the RA in small mammals, there is a large INa, and Nav1.5 is abundantly expressed, in the SN of small mammals, INa is small or absent, and the expression of Nav1.5 mRNA5,6 and protein7,10 is minimal. As a consequence, whereas the upstroke of the RA action potential is fast, the upstroke of the SN action potential is slow (INa is responsible for upstroke of atrial action potential; Figure 8C). Nav1.5 and Navβ1 knockout mice have SN dysfunction, with bradycardia and increased SN conduction times.20,21 In humans, mutations in Nav1.5 have been linked to familial sick sinus syndrome.7 Therefore, despite the lower abundance of Nav1.5 in the SN, it must still have a functional role. Na+ channels other than Nav1.5 are also known to be important in the function of the mouse SN.10 However, Na+ channels other than Nav1.5 were poorly expressed in the human SN (Figure 2), and this suggests that they may not play a large role.
Compared with the RA, Cav1.2 mRNA and protein tended to be less abundant in the SN, whereas Cav1.3 mRNA was more abundant (Figures 2 and 3⇑; supplemental Figure VII). In small mammals, in the absence of INa, ICaL is responsible for the slow upstroke of the action potential in the SN.1 In rabbit, an “isoform switch” from Cav1.2 to Cav1.3 in the SN, similar to that in humans, has been reported.5 Two studies on Cav1.3 knockout mice have shown SN dysfunction with bradycardia and spontaneous arrhythmias.22
ICaT and If
Cav3.1 and HCN4, the major ion channels responsible for ICaT and If at both the mRNA and protein levels, were expressed in greater amounts in the SN than in the RA (Figures 2 and 5⇑; supplemental Figure VII). This is consistent with mRNA data from the mouse6 and rabbit.5 In small mammals, ICaT and If have been shown to be important in pacemaking in the SN, and their pharmacological block slows the pacing rate.7 In addition, the Cav3.1 knockout mouse has bradycardia.23 In the HCN4 knockout mouse, If is reduced by 85% in the SN, the heart rate is reduced by 37%, and the embryo dies between embryonic days 9.5 and 11.5.24 If recently has been recorded from human SN myocytes, and pharmacological blockade of If slows pacemaking4 (see also Figure 8C). In humans, mutations in HCN4 have been linked to familial sick sinus syndrome.7
Kv4.3, the major ion channel responsible for Ito in humans at the mRNA and protein levels, was less abundant in the SN than in the RA (Figure 6; supplemental Figure VIII). Ito is responsible for the early phase of repolarization (phase 1), and it also helps determine action potential duration.1 In the rabbit, the density of Ito is lower in the SN than in the RA,1 and the corresponding ion channel (Kv1.4; there are well-known species differences in ion channels responsible for Ito) is less abundant in the SN than in the RA. It is likely that in humans, as in the rabbit, the density of Ito will be less in the SN than in the RA.
IKur, IKr, and IKs
As with Kv4.3, the ion channels responsible for IKur, IKr, and IKs (Kv1.5, ERG, and KvLQT1) at the mRNA level (and protein level in the case of Kv1.5) tended to be less expressed in the SN than in the RA (Figure 6; supplemental Figure VIII). It is well known that in the rabbit, the action potential duration decreases down the conduction pathway from the SN to the RA,1 and it is possible that this is also true in humans as a result of the decrease in expression of Kv4.3, Kv1.5, and ERG, for example, in the SN.
Kir2.1 and Kir2.3, which are likely to be the major ion channels responsible for IK1, tended to be less abundant in the SN than in the RA (Figure 6). This is expected, because in small mammals, the presence of IK1 in the working myocardium is known to be responsible for its stable resting potential, whereas in the SN, IK1 is small or absent, and as a result, there is no stable resting potential; instead, there is pacemaking.7 The knockdown of Kir2.1 by ≈80% in the ventricles has been used as a potential strategy for the creation of a biopacemaker.25
Acetylcholine-Activated K+ Current
The 2 ion channels responsible for IKACh, Kir3.1 and Kir3.4, at the mRNA and protein (Kir3.1 only) levels were abundant and did not vary among tissues (supplemental Figures IV and VIII). This is consistent with mRNA and protein data from small mammals.5,6,26 IKACh is activated on binding of acetylcholine released from vagal nerves to M2 receptors; M2 receptors were abundantly expressed in the human SN (supplemental Figure VI).
At the mRNA level, and in some cases at the protein level, various Ca2+-handling proteins (RyR2, SERCA2a, sarcolipin) were less abundant in the SN than in the RA (Figure 7; supplemental Figures V and IX). Intracellular Ca2+ has been suggested to play an important role in pacemaker activity.16 It has been suggested that some SR Ca2+ release occurs during the pacemaker potential. This occurs either in response to ICaT or spontaneously.16 The diastolic Ca2+ release activates the Na+-Ca2+ exchanger. Because the exchanger is electrogenic, an inward current is produced (INaCa), which contributes to the pacemaker potential. In the rabbit, as in humans, RyR2 mRNA is less abundant in the SN than in the RA.5 Previously, RyR3 mRNA has been shown to be expressed primarily in the cardiac conduction system in mouse6 and rabbit.5 Although a similar tendency was observed in humans, RyR3 mRNA was poorly expressed compared with RyR2 mRNA (Figure 7).
mRNA and protein for the 2 most abundant connexins (Cx40 and Cx43) were less abundant in the SN than in the RA (Figures 1 and 7⇑; supplemental Figure VII). In small mammals, electrical coupling is strong in the RA to allow rapid propagation of the action potential, but it is weak in the SN to protect the SN from the hyperpolarizing influence of the neighboring RA.1 Consistent with this and with observations in humans, the expression of Cx40 and Cx43 in small mammals at the mRNA and protein levels is also reduced in the SN compared with the RA.1 In the SN, electrical coupling is presumably provided by Cx45; in humans, as in small mammals, Cx45 mRNA was abundant in the SN (Figure 7). A mouse deficient in Cx40 shows an increase in SN recovery time and a bradycardia,27 which suggests that at least in the mouse, Cx40 is important in SN function.
Gaborit et al28 have measured the abundance of mRNA for ion channels in various regions of the nondiseased human heart, including the RA; expression levels in the RA are qualitatively similar to those reported here for the RA.
The present study suggests that this area is unique, being similar in some respects to the RA but in other respects to the SN. As shown in most of the Figures, the PN showed an intermediate pattern of expression of many ion channels similar to that seen in the periphery of the rabbit SN.5 Results from in situ hybridization and immunofluorescence suggest that this is due to a mixture of nodal and atrial cells within the PN (eg, Figures 1 and 3⇑), again consistent with results from the periphery of the rabbit SN.5 The PN, however, shows a unique pattern of expression of K+ channels and accessory subunits. For example, it shows greater expression compared with both the SN and RA of Kv4.2, Kir6.1, TASK1, SK2, and MiRP2 (Figure 6). Expression of Kir2.1 mRNA was low in the PN compared with the RA (Figure 6). It is possible, therefore, that the PN is depolarized compared with the RA. This may be important; it may electrotonically depolarize the thin layer of RA that lies between the PN and SN (Figure 1A) and in this way facilitate the conduction of the action potential from the SN into the RA. In addition, as a result of a low abundance of Kir2.1, the PN may be unstable and prone to ectopic activity, and it could be responsible for the cristal tachycardias known to originate from this region.29 The dynamics of atrial fibrillation are thought to be affected by the expression levels of Kir2 channels,30 and therefore, the PN may play a special role in atrial fibrillation. The leading pacemaker site in the SN is not static and is altered in response to external factors such as sympathetic and parasympathetic stimulation.1 This is known as “pacemaker shift” perhaps the PN has a role in this shift. Intracellular recordings from the PN are required to determine the function of this novel region.
Conclusions and Implications
We have characterized the expression of ion channels in the human SN and its environs, including an extensive novel area, the PN. Our results provide insights into the electrical properties of these cardiac tissues, as well as sick sinus syndrome, be it familial, age related, or heart failure related. For example, mutations in Nav1.5 and HCN4 have been shown to cause familial sick sinus syndrome, and the present study shows how these ion channels are normally distributed in the human SN.7 Perhaps the Nav1.5-related dysfunction is the result of loss of INa from the PN (because it is absent in SN). For treatment of sick sinus syndrome, there has been much interest in the creation of a biopacemaker. Recent strategies have included knocking out Kir2.125 or overexpressing HCN31 in ventricular myocardium. The present results show that there are many changes in gene expression between the human SN and the RA, and this suggests that for a successful biopacemaker to be developed, the manipulation of more than 1 gene may be required. In addition, if we understand gene expression, why create a de novo pacemaker in an alien environment? Why not repair the sick sinus using gene therapy of the ion channels?
Why is the heart rate in humans ≈70 bpm, whereas in the mouse, it is ≈400 to 800 bpm? Marionneau et al6 studied ion channel expression at the mRNA level in the mouse SN. Comparison of the 2 data sets (using the mRNA level of Nav1.5 in the RA as a reference) suggests that although the expression of some key ion channels in the SN in the 2 species is comparable (Cav1.2, Cav1.3, and ERG), many key ion channels (and associated accessory subunits) responsible for inward (Nav1.5, Navβ1, Nav1.4, Cav3.1, Cav3.2, and HCN4) and outward (Kv1.5, Kv2.1, Kv4.2, KvLQT1, Kir2.1, Kir2.2, and KChIP2) currents are substantially less abundant in humans than in the mouse. The lower expression of the ion channels responsible for outward K+ currents in the human SN explains the expected longer action potential in the SN in humans than in the mouse; a longer action potential in the SN in humans will by itself slow the heart rate. The lower expression of the ion channels responsible for inward currents in the human SN is expected to further slow the heart rate. Finally, the comparison suggests that the expression of SERCA2 in the SN is substantially less abundant in humans than in the mouse, and this too may have implications for the heart rate.
Sources of Funding
This study was supported by Medtronic, Inc.
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There is a wealth of data surrounding the unique functional and molecular makeup of the sinus node in smaller mammals; however, in humans, there are few data available, with almost no information on the ion channels responsible for pacemaking. This is the first study to investigate the expression of ion channels, Ca2+-handling proteins, connexins, and receptors in the human sinus node, its surrounding atrial muscle, and a novel area (the paranodal area) that runs alongside the sinus node. This study greatly aids our understanding of the molecular and functional makeup of the human sinus node. For example, in patients, naturally occurring mutations in Nav1.5 and HCN4 have been linked with familial sick sinus syndrome, and the present study explains why this is so. For treatment of sick sinus syndrome, there has been much interest in the creation of a biological pacemaker, or biopacemaker. Recent strategies have included knocking down Kir2.1 or overexpressing an HCN (hyperpolarization-activated cyclic nucleotide-gated) channel in the ventricular myocardium; however, the present study results show that there are many differences in gene expression between the human sinus node and working myocardium, which suggests that for a successful and robust biopacemaker to be developed, it is likely that the manipulation of more than 1 gene will be required. In addition, if we understand gene expression, why create a de novo pacemaker in an alien environment? Why not repair the sick sinus by use of gene therapy of the ion channels?
The online-only Data Supplement is available with this article at http://circ.ahajournals.org/cgi/content/full/CIRCULATIONAHA.108.804369/DC1.