Effects of induced Na+/Ca2+ exchanger overexpression on the spatial distribution of L-type Ca2+ channels and junctophilin-2 in pressure-overloaded hearts

Abstract

The Na/Ca exchanger 1 (NCX1) is an essential Ca efflux system in cardiomyocytes. Although NCX1 is distributed throughout the sarcolemma, a subpopulation of NCX1 is localized to transverse (T)-tubules. There is growing evidence that T-tubule disorganization is a causal event that shifts the transition from hypertrophy to heart failure (HF). However, the detailed molecular mechanisms have not been clarified. Previously, we showed that induced NCX1 expression in pressure-overloaded hearts attenuates defective excitation-contraction coupling and HF progression. Here, we examined the effects of induced NCX1 overexpression on the spatial distribution of L-type Ca channels (LTCCs) and junctophilin-2 (JP2), a structural protein that connects the T-tubule and sarcoplasmic reticulum membrane, in pressure-overloaded hearts. Quantitative analysis showed that the regularity of NCX1 localization was significantly decreased at 8 weeks after transverse aortic constriction (TAC)-surgery; however, T-tubule organization and the regularities of LTCC and JP2 immunofluorescent signals were maintained at this time point. These observations demonstrated that release of NCX1 from the T-tubule area occurred before the onset of T-tubule disorganization and LTCC and JP2 mislocalization. Moreover, induced NCX1 overexpression at 8 weeks post-TAC not only recovered NCX1 regularity but also prevented the decrease in LTCC and JP2 regularities at 16 weeks post-TAC. These results suggested that NCX1 may play an important role in the proper spatial distribution of LTCC and JP2 in T-tubules in the context of pressure-overloading.

Heterogeneity of transverse-axial tubule system in mouse atria: Remodeling in atrial-specific Na + –Ca 2+ exchanger knockout mice

“…Reynolds et al [54] found junctophilin-2 (JP-2) to be necessary for t-tubule maturation during mouse heart development and the defects of JP-2 can lead to t-tubule remodeling in ventricular myocytes [55]. A recent study suggested that NCX was causally linked to the spatial localization of JP-2 and organization of t-tubules in pressure-overloaded hearts [56], possibly due to abnormal JP-2 proteolysis through Ca-mediated calpain activation as previously described [57,58]. Whether the loss of TATs is a concomitant phenomenon of cellular remodeling or a direct consequence from the intracellular Ca elevation due to the absence of NCX in the KO cells remains to be established.…”

MG53 is dispensable for T-tubule maturation but critical for maintaining T-tubule integrity following cardiac stress

“…NCX1 is a crucial player in Ca 2+ homeostasis in cardiomyocytes [24]. Recent evidence from atrial-specific NCX1 knockout model [40] and cardiac-specific inducible NCX1 overexpression mice [41] suggest that NCX1 may mechanistically contribute to the proper spatial localization of JP2 and maintenance of T-tubule organization. This may represent an additional mechanism for the dysregulated E-C coupling and T-tubule remodeling in MG53-KO mice under stress conditions.…”

Effect of BIN1 on cardiac dysfunction and malignant arrhythmias

“…JP2 is another membrane‐binding protein that physically connects the T‐tubule membrane with the SR membrane . The spatial distributions of LTCC and JP2 are consistent with the invaginations of the T‐tubule membrane . The loss and disorganization of T‐tubules is now understood to be an important cellular‐level contributor to HF, and the plasticity of this system may also be harnessed for therapy .…”

“…Nevertheless, JP2 seems to be more important for controlling T‐tubule orientation and localization of nonjunctional RyRs rather than controlling T‐tubule density in cardiac muscle . It has been reported that T‐tubule disorganization is associated with a significant decrease in JP‐2 expression . However, knowledge of the molecular mechanisms of the downregulation of those proteins in failing hearts and the interaction between them are still unknown.…”

“…19 The spatial distributions of LTCC and JP2 are consistent with the invaginations of the T-tubule membrane. 20 The loss and disorganization of T-tubules is now understood to be an important cellular-level contributor to HF, and the plasticity of this system may also be harnessed for therapy. 3 Downregulation of those proteins, especially BIN1 and JP2, can increase the risk of arrythmias.…”