Fig. 7.

Dysregulation of sarcomere length and titin isoform transition occur in Tβ4 KO mice during postnatal cardiac growth. Tracking sarcomere maturation through late embryonic and postnatal development (A, phalloidin staining) reveals the earliest evidence of shortened sarcomeres in −/Y hearts at P5 (B; p < 0.001; n = 3–7 per genotype, per stage). +/Y embryos displayed the expected postnatal transition from predominantly N2BA titin to predominantly N2B titin (C, D). This was emulated in −/Y embryos until P1, at which point the shorter N2B isoform became precociously up-regulated and continued to be expressed at an elevated ratio in −/Y hearts throughout later development (C). The titin transition visualized by dual immunofluorescence with exon 49/exon 224 (N2B, red/PEVK segment, green) antibodies (E). Discrete N2B/PEVK banding, around the Z line, is seen in +/Y hearts throughout the P1-P5-P21 transition, whereas, in −/Y hearts, bands become closely apposed from P5 onwards, such that discrete bands could not be discerned. Scale bar in A (all panels): 20 μm. Scale bar in E (all panels): 10 μm *: p < 0.05; **: p < 0.01; ***: p < 0.001. Error bars in B, C: SEM. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)