Fig. 8.

Appropriate sarcomere length can be restored with exogenous Thymosin β4. The N2BA to N2B titin postnatal isoform switch can be recapitulated in cultured cardiomyocytes from E18.5 mouse hearts. A: Wild type (WT) cardiomyocytes displayed an organised sarcomere whereas those from Tβ4 knockout (KO) hearts were shorter in length and partly disordered. Supplementation of culture medium with 100 ng/ml synthetic Tβ4 restored normal morphology and sarcomere organisation in KO cardiomyocytes. Rescue coincided with restoration of normal titin splicing (B; p < 0.01 + Tβ4 vs –Tβ4 in KO cardiomyocytes) and sarcomere length (C), by day 9 in culture. A summary schematic (D) to illustrate the shortened titin and sarcomere/TFL in Tβ4 −/Y cardiomyocytes. The precise mechanism by which Tβ4 impacts titin splicing and/or TFL remains unclear; two possibilities are depicted i) a direct effect on filament extension by regulating the availability of G-actin monomers or ii) a direct effect of titin splicing, via incorporation into splicing complexes, which may also include monomeric nuclear actin. Scale bar in A: 20 μm. *: p < 0.05; ***: p < 0.001, + Tβ4 vs –Tβ4, or as indicated by lines. ††: p < 0.01, KO vs WT. n = 6 per genotype from 3 independent experiments. Error bars in B, C: SEM.