Figure 5. Decreased Cx40 and Na_v1.5 expression in the VCS after removal of TBX5.

The proximal (A–J) and distal (K–T) AV bundle was identified by acetycholinesterase activity (A, B, F, G, K, L, P, and Q) and contactin-2 expression (C, D, H, I, M, N, R, and S) on serial sections from Tbx5^fl/fl and Tbx5^minKCreERT2 hearts. Whereas the contactin-2–positive AV bundle expressed high levels of Cx40, Na_v1.5, and TBX5 in Tbx5^fl/fl mice, their expression was drastically reduced in that of Tbx5^minKCreERT2 mice. (C, D, H, I, M, N, R, and S) Dual-color immunofluorescence for Cx40 or Na_v1.5 and contactin-2 is shown. Contactin-2 expression in E, J, O, and T is from sections adjacent to those stained for TBX5; contactin-2 and TBX5 antibodies were both raised in goat, preventing dual-color immunofluorescence on the same section. Nuclei were stained with hematoxylin (A, B, F, G, K, L, P, and Q) or DAPI (blue; C–E, H–J, M–O, and R–T). Boxed regions in A, F, K, and P are shown at higher magnification in B, G, L, and Q. Original magnification, ×10 (A, F, K, and P); ×40 (B–E, G–J, L–O, and P–T).