This is a response to a letter by Kelder et al. (1).
We thank Kelder et al. (1) for sharing their valuable insights into the development of atrioventricular (AV) junction. In response to their comments, our replies are as follows. The anatomical identity of the derivatives of dorsal mesenchymal protrusion (DMP) in our studies is in agreement with that described previously (2–5), which can be mapped by Mef2c-AHF-Cre (3, 5), and was also shown to be overlapped with Shox2 expression (2). We also found that Shox2 is co-expressed with Hcn4, Nkx2–5, and cTnT in the DMP derivatives at E11.0 (Fig. 1). The origin of cells that contribute to the compact AV node (AVN) is still controversial. In the letter by Kelder et al. (1), the authors argued that the CTNI+/HCN4+/ISL1+ domain shown in the accompanying figure represents the putative AVN region. However, fate mapping by Isl1-Cre identified only a small subset of AVN cells that are derived from Isl1-expressing cells (6). Thus, whether the CTNI+/HCN4+/ISL1+ domain described by the authors is the primordial AVN warrants further validation.
FIGURE 1.
Shox2 expression overlaps with Nkx2–5, Hcn4, and cTnT in the atrioventricular junction at E11.5. A and B, adjacent sections show co-staining of Shox2 (α-HA), Nkx2–5, Hcn4, and cTnT in the DMP of an E11.5 Shox2HA/+ embryo that carries an HA-tagged Shox2 allele. RA, right atrium; RVV, right venus valve; LVV, left venus valve; SAN, sinoatrial node; DMP, dorsal mesenchymal protrusion.
In addition, in our studies, we showed that Shox2-expressing cells do not contribute to the AVN by fate mapping using Shox2Cre allele (2). Importantly, in our recent studies, we do not observe AV block or a severe change of P-R intervals in the episodes of normal P wave configuration, signs of normal AVN function, in Shox2Nkx2–5Cre mice in which Shox2 is ablated in Nkx2–5-expressing domains including the AVN (Ye, W. and Chen, Y., submitted for publication). Based on these pieces of evidence, we stand on our original conclusion that Shox2 regulates DMP development.
References
- 1. Kelder T. P., Vicente-Steijn R., DeRuiter M. C., Gittenberger-de Groot A. C., Jongbloed M. R. M. (2015) Does the dorsal mesenchymal protrusion act as a temporary pacemaker during heart development? J. Biol. Chem. 290, 8013–8014 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2. Sun C., Yu D., Ye W., Liu C., Gu S., Sinsheimer N. R., Song Z., Li X., Chen C., Song Y., Wang S., Schrader L., Chen Y. (2015) The short stature homeobox 2 (Shox2)-bone morphogenetic protein (BMP) pathway regulates dorsal mesenchymal protrusion development and its temporary function as a pacemaker during cardiogenesis. J. Biol. Chem. 290, 2007–2023 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3. Aanhaanen W. T., Mommersteeg M. T., Norden J., Wakker V., de Gier-de Vries C., Anderson R. H., Kispert A., Moorman A. F., Christoffels V. M. (2010) Developmental origin, growth, and three-dimensional architecture of the atrioventricular conduction axis of the mouse heart. Circ. Res. 107, 728–736 [DOI] [PubMed] [Google Scholar]
- 4. Snarr B. S., O'Neal J. L., Chintalapudi M. R., Wirrig E. E., Phelps A. L., Kubalak S. W., Wessels A. (2007) Isl1 expression at the venous pole identifies a novel role for the second heart field in cardiac development. Circ. Res. 101, 971–974 [DOI] [PubMed] [Google Scholar]
- 5. Briggs L. E., Kakarla J., Wessels A. (2012) The pathogenesis of atrial and atrioventricular septal defects with special emphasis on the role of the dorsal mesenchymal protrusion. Differentiation 84, 117–130 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6. Liang X., Wang G., Lin L., Lowe J., Zhang Q., Bu L., Chen Y., Chen J., Sun Y., Evans S. M. (2013) HCN4 dynamically marks the first heart field and conduction system precursors. Circ. Res. 113, 399–407 [DOI] [PMC free article] [PubMed] [Google Scholar]