Unravelling the Mysteries of the Human AV Node

Login or register to view PDF.
Received date
12 March 2018
Accepted date
12 March 2018
Citation
Arrhythmia & Electrophysiology Review 2018;7(1):63–4.
DOI
https://doi.org/10.15420/aer.2018.7.1.L1

Dear Sir,

I read with great interest the elegant review written by Dr Efimov and colleagues on the structure and properties of the atrioventricular (AV) node in the last issue of this journal (AER 6(4):179–85).1 However, there are several points that need further clarification.

The authors state that total protein and mRNA levels of Cx40, Cx43 and/or Cx45 can be assessed qualitatively and quantitatively. However, representative immunolabelled sections are shown only for Cx43. From this, as well as other publications by the same team,2,3 I am left with the impression that genotyping data have only been presented for Cx43. However, Cx43 is one of four connexins that have been described to date (Cx40, Cx43, Cx45 and Cx30). Why do no data exist for them, and what is the impact of this lack of information on the characterisation of AV nodal properties? Is it just technically not feasible to obtain this information or is it just a matter of conventional priorities? I do believe that such genotyping is important for the full characterisation of the properties of the AV node and, perhaps, the unravelling of the mysteries of the circuit of AV nodal re-entrant tachycardia.4,5

Maria Kokladi, Department of Cardiology, Athens Euroclinic, Greece

Authors’ Reply

 

Citation: Arrhythmia & Electrophysiology Review 2018;7(1):63–4. DOI 10.15420/aer.2018.7.1.L1.R1
Authors: Igor R Efimov & Sharon George, George Washington University, Washington, DC, USA

 

Dear Sir,

Thank you for the opportunity to address the reader’s very important question regarding the roles of various connexin isoforms in the complex function of the human AV node.1 These isoforms can serve as a rate-dependent AV conduction axis during normal sinus rhythm or a lifesaving filter of high-frequency excitation produced by atrial tachyarrhythmias or as a backup junctional pacemaker.

Cx43, Cx40 and Cx45 are the major connexin isoforms expressed in the human AV junction. Cx43 is the most well characterised and anatomically-mapped isoform, especially in humans.2 Several important studies3–5 have reported the mRNA and protein expression of Cx40 and Cx45 in different regions of the AV junction, which appear to play an important role in the complex function of the human AV node. Cx31.9 remains controversial: its mRNA was reported to be present in the human AV node but its protein expression was undetectable in humans, unlike evidence of its ortholog Cx302 in the mouse sinoatrial and AV nodes.6 This is a good example of how mRNA expression data do not necessarily correlate with protein expression and function.

Though the mRNA and protein expression data are robust and clearly address important questions, to investigate this particular issue it would be necessary to immunolable serial sections of the human AV junction, as was done with Cx43 in Hucker et al.2 This method is more advantageous as it allows us to directly compare histological data with protein expression in the region of interest. The availability of histological data confirms the location and boundaries of the various regions of the AV junction (which have morphological differences) that are otherwise not easy to define.

The serial sectioning protocol is technically challenging and strongly dependent on the quality of antibodies. For more than a decade we have tried to anatomically map Cx45 and Cx40 protein expression using immunofluorescence microscopy. Unfortunately only Cx43 mapping was possible due to the low quality of antibodies available on the market and those produced by our collaborators and us. Despite numerous attempts to immunolable serial sections of the human AV and sinoatrial nodes, we have been unable to obtain the consistent high-quality signals from either Cx40 or Cx45 antibodies that are necessary for 3D reconstruction of their distribution in the AV junction.

Over the past few years, we have tested and validated several new antibodies that might work well for this purpose, especially with the advent of the novel CLARITY method, which allows protein mapping in 3D anatomical structures without the need for tissue sectioning.7 This topic is an on-going collaborative project in our laboratory and we hope to have data that directly answer this question in the near future.

Igor R Efimov, George Washington University, Washington, DC, USA
Sharon George, George Washington University, Washington, DC, USA

  1. George SA, Faye NR, Murillo-Berlioz A, et al. At the atrioventricular crossroads: dual pathway electrophysiology in the atrioventricular node and its underlying heterogeneities. Arrhythm Electrophysiol Rev 2017;6(4):179–85.
    Crossref | PubMed
  2. Hucker WJ, McCain ML, Laughner JI, et al. Connexin 43 expression delineates two discrete pathways in the human atrioventricular junction. Anat Rec 2008;291:204–15.
    Crossref | PubMed
  3. Dobrzynski H, Anderson RH, Atkinson A, et al. Structure, function and clinical relevance of the cardiac conduction system, including the atrioventricular ring and outflow tract tissues. Pharmacol Ther 2013;139:260–88.
    Crossref | PubMed
  4. Dobrzynski H, Atkinson A, Borbas Z, et al. Molecular investigation into the human atrioventricular node in heart failure. Anat Physiol 2015;5:164.
    Crossref
  5. Greener ID, Monfredi O, Inada S. Molecular architecture of the human specialised atrioventricular conduction axis. J Mol Cell Cardiol 2011;50:642–51.
    Crossref | PubMed
  6. Kreuzberg MM, Liebermann M, Segschneider S, et al. Human connexin31.9, unlike its orthologous protein connexin30.2 in the mouse, is not detectable in the human cardiac conduction system. J Mol Cell Cardiol 2009;46:553–9.
    Crossref | PubMed
  7. Hsueh B, Burns VM, Pauerstein P, et al. Pathways to clinical CLARITY: volumetric analysis of irregular, soft, and heterogeneous tissues in development and disease. Sci Rep 2017;7:5899.
    Crossref | PubMed
References
  1. George SA, Faye NR, Murillo-Berlioz A, et al. At the atrioventricular crossroads: dual pathway electrophysiology in the atrioventricular node and its underlying heterogeneities. Arrhythm Electrophysiol Rev 2017;6(4):179–85.
    Crossref | PubMed
  2. Hucker WJ, McCain ML, Laughner JI, et al. Connexin 43 expression delineates two discrete pathways in the human atrioventricular junction. Anat Rec (Hoboken) 2008;291:204–15.
    Crossref | PubMed
  3. Hucker WJ, Sharma V, Nikolski VP, Efimov IR. Atrioventricular conduction with and without AV nodal delay: two pathways to the bundle of His in the rabbit heart. Am J Physiol Heart Circ Physiol 2007;293:H1122–30.
    Crossref | PubMed
  4. Katritsis DG, Marine JE, Latchamsetty R, et al. Coexistent types of atrioventricular nodal re-entrant tachycardia: implications for the tachycardia circuit. Circ Arrhythm Electrophysiol 2015;8:1189–93.
    Crossref | PubMed
  5. Katritsis DG, Marine JE, Contreras FM, et al. Catheter ablation of atypical atrioventricular nodal reentrant tachycardia. Circulation 2016;134:1655–63.
    Crossref | PubMed