Critical roles of junctophilin-2 in T-tubule and excitation-contraction coupling maturation during postnatal development

Aims: Emerging evidence indicates a critical role for junctophilin-2 (JP2) in T-tubule integrity and assembly of cardiac dyads in adult ventricular myocytes. In the postnatal stage, one of the critical features of myocyte maturation is development of the T-tubule system, though the mechanisms remain poorly understood. In this study, we aim to determine whether JP2 is required for normal cardiac T-tubule maturation. Methods and results: Using in situ confocal imaging of intact murine hearts, we found T-tubules were absent in both left- and right-ventricular myocytes at postnatal Day 8 and did not appear until Day 10. Quantification of T-tubule structural integrity using the T-tubule power (TT_power) index revealed a progressive increase in TT _power between postnatal Days 10 and 19. By postnatal Day 19, TT _powerwas similar to that in adult murine cardiomyocytes, indicative of a nearly matured T-tubule network. JP2 levels increased dramatically during development, reaching levels observed in adult hearts by postnatal Day 14. Deficiency of JP2, using a mouse model in which a JP2-specific shRNA is expressed during embryonic development, severely impaired T-tubule maturation, with equivalent decreases in the left- and right-ventricular TT_power. We also detected a gradual increase in the density of transverse but not longitudinal tubules during development, and JP2 deficiency abolished the increase in the density of transverse elements. Alterations in T-tubules caused significant reduction in Ca²⁺ transient amplitude and marked increase in Ca²⁺ release dyssynchrony, Ca²⁺ alternans, and spontaneous Ca²⁺ waves, leading to contractile failure. Conclusion: Our data identify a critical role for JP2 in T-tubule and excitation-contraction coupling maturation during development.