Structural evidence for a new elaborate 3D-organization of the cardiomyocyte lateral membrane in adult mammalian cardiac tissues

A novel function of claudin-5 in maintaining the structural integrity of the heart and its implications in cardiac pathology

This study aims to investigate the role of claudin-5 (Cldn5) in cardiac structural integrity. Proteomic analysis was performed to screen the protein profiles in enlarged left atrium from atrial fibrillation (AF) patients. Cldn5 shRNA adeno-associated virus (AAV) or siRNA was injected into the mouse left ventricle or added into HL1 cells respectively to knockdown Cldn5 in cardiomyocytes to observe whether the change of Cldn5 influences cardiac morphology and function, and affects those protein expressions stem from the proteomic analysis. Mitochondrial density and membrane potential were also measured by Mitotracker staining and JC-1 staining under the confocal microscope in HL1 cells. Cldn5 was reduced in cardiomyocytes from the left atrial appendage of AF patients compared to non-AF donors. Proteomic analysis showed 83 proteins were less abundant and 102 proteins were more abundant in AF patients. KEGG pathway analysis showed less abundant CACNA2D2, CACNB2, MYL2 and MAP6 were highly associated with dilated cardiomyopathy. Cldn5 shRNA AAV injection caused severe cardiac atrophy, dilation and myocardial dysfunction in mice. The decreases in mitochondrial numbers and mitochondrial membrane potentials in HL1 cells were observed after Cldn5 knockdown. We demonstrated for the first time the mechanism of Cldn5 downregulation-induced myocyte atrophy and myocardial dysfunction might be associated with the downregulation of CACNA2D2, CACNB2, MYL2 and MAP6, and mitochondrial dysfunction in cardiomyocytes.

Decreasing microtubule detyrosination modulates Nav1.5 subcellular distribution and restores sodium current in mdx cardiomyocytes

AIMS

The microtubule (MT) network plays a major role in the transport of the cardiac sodium channel Nav1.5 to the membrane, where the latter associates with interacting proteins such as dystrophin. Alterations in MT dynamics are known to impact on ion channel trafficking. Duchenne muscular dystrophy (DMD), caused by dystrophin deficiency, is associated with an increase in MT detyrosination, decreased sodium current (INa), and arrhythmias. Parthenolide (PTL), a compound that decreases MT detyrosination, has shown beneficial effects on cardiac function in DMD. We here investigated its impact on INa and Nav1.5 subcellular distribution.

METHODS AND RESULTS

Ventricular cardiomyocytes (CMs) from wild-type (WT) and mdx (DMD) mice were incubated with either 10 µM PTL, 20 µM EpoY, or dimethylsulfoxide (DMSO) for 3-5 h, followed by patch-clamp analysis to assess INa and action potential (AP) characteristics in addition to immunofluorescence and stochastic optical reconstruction microscopy (STORM) to investigate MT detyrosination and Nav1.5 cluster size and density, respectively. In accordance with previous studies, we observed increased MT detyrosination, decreased INa and reduced AP upstroke velocity (Vmax) in mdx CMs compared to WT. PTL decreased MT detyrosination and significantly increased INa magnitude (without affecting INa gating properties) and AP Vmax in mdx CMs, but had no effect in WT CMs. Moreover, STORM analysis showed that in mdx CMs, Nav1.5 clusters were decreased not only in the grooves of the lateral membrane (LM; where dystrophin is localized) but also at the LM crests. PTL restored Nav1.5 clusters at the LM crests (but not at the grooves), indicating a dystrophin-independent trafficking route to this subcellular domain. Interestingly, Nav1.5 cluster density was also reduced at the intercalated disc (ID) region of mdx CMs, which was restored to WT levels by PTL. Treatment of mdx CMs with EpoY, a specific MT detyrosination inhibitor, also increased INa density, while decreasing the amount of detyrosinated MTs, confirming a direct mechanistic link.

CONCLUSION

Attenuating MT detyrosination in mdx CMs restored INa and enhanced Nav1.5 localization at the LM crest and ID. Hence, the reduced whole-cell INa density characteristic of mdx CMs is not only the consequence of the lack of dystrophin within the LM grooves but is also due to reduced Nav1.5 at the LM crest and ID secondary to increased baseline MT detyrosination. Overall, our findings identify MT detyrosination as a potential therapeutic target for modulating INa and subcellular Nav1.5 distribution in pathophysiological conditions.

Differences in Effects of Length-Dependent Regulation of Force and Ca2+ Transient in the Myocardial Trabeculae of the Rat Right Atrium and Ventricle

The comparative differences in the fundamental mechanisms of contractility regulation and calcium handling of atrial and ventricular myocardium remain poorly studied. An isometric force-length protocol was performed for the entire range of preloads in isolated rat right atrial (RA) and ventricular (RV) trabeculae with simultaneous measurements of force (Frank-Starling mechanism) and Ca²⁺ transients (CaT). Differences were found between length-dependent effects in RA and RV muscles: (a) the RA muscles were stiffer, faster, and presented with weaker active force than the RV muscles throughout the preload range; (b) the active/passive force-length relationships were almost linear for the RA and RV muscles; (c) the value of the relative length-dependent growth of passive/active mechanical tension did not differ between the RA and RV muscles; (d) the time-to-peak and amplitude of CaT did not differ between the RA and RV muscles; (e) the CaT decay phase was essentially monotonic and almost independent of preload in the RA muscles, but not in the RV muscles. Higher peak tension, prolonged isometric twitch, and CaT in the RV muscle may be the result of higher Ca²⁺ buffering by myofilaments. The molecular mechanisms that constitute the Frank-Starling mechanism are common in the rat RA and RV myocardium.

Ephrin-B1 regulates the adult diastolic function through a late postnatal maturation of cardiomyocyte surface crests

The rod-shaped adult cardiomyocyte (CM) harbors a unique architecture of its lateral surface with periodic crests, relying on the presence of subsarcolemmal mitochondria (SSM) with unknown role. Here, we investigated the development and functional role of CM crests during the postnatal period. We found in rodents that CM crest maturation occurs late between postnatal day 20 (P20) and P60 through both SSM biogenesis, swelling and crest-crest lateral interactions between adjacent CM, promoting tissue compaction. At the functional level, we showed that the P20-P60 period is dedicated to the improvement of relaxation. Interestingly, crest maturation specifically contributes to an atypical CM hypertrophy of its short axis, without myofibril addition, but relying on CM lateral stretching. Mechanistically, using constitutive and conditional CM-specific knock-out mice, we identified ephrin-B1, a lateral membrane stabilizer, as a molecular determinant of P20-P60 crest maturation, governing both the CM lateral stretch and the diastolic function, thus highly suggesting a link between crest maturity and diastole. Remarkably, while young adult CM-specific Efnb1 KO mice essentially exhibit an impairment of the ventricular diastole with preserved ejection fraction and exercise intolerance, they progressively switch toward systolic heart failure with 100% KO mice dying after 13 months, indicative of a critical role of CM-ephrin-B1 in the adult heart function. This study highlights the molecular determinants and the biological implication of a new late P20-P60 postnatal developmental stage of the heart in rodents during which, in part, ephrin-B1 specifically regulates the maturation of the CM surface crests and of the diastolic function.

A novel role of claudin-5 in prevention of mitochondrial fission against ischemic/hypoxic stress in cardiomyocytes

BACKGROUND

Downregulation of claudin-5 in the heart is associated with the end-stage heart failure. However, the underlying mechanism of claudin-5 is unclear. Here we investigated the molecular actions of claudin-5 in perspective of mitochondria in cardiomyocytes to better understand the role of claudin-5 in cardioprotection during ischemia.

METHODS AND RESULTS

Claudin-5 was detected in the murine heart tissue and the neonatal rat cardiomyocytes (NRCM). Its protein level was severely decreased after myocardial ischemia/reperfusion (I/R; 30 min/24 h) or hypoxia/reoxygenation (H/R; 24 h/4 h). Claudin-5 was present in the mitochondria of NRCM as determined by confocal microscopy. H/R-induced downregulation of claudin-5 was accompanied by mitochondrial fragmentation. The protein level of mitofusin 2 (Mfn2) was dramatically decreased while the expression of dynamin-related protein (Drp) 1 was significantly increased after H/R. H/R-induced mitochondrial swelling and fission were observed by transmission electron microscope (TEM). Overexpression of claudin-5 by adenoviral infection reversed these structural disintegration of mitochondria. The mitochondria-centered intrinsic pathway of apoptosis triggered by H/R and indicated by the expression of cytochrome c and cleaved caspase 3 in the cytoplasm of NRCMs was also reduced by overexpressing claudin-5. Overexpression of claudin-5 in mouse heart also significantly decreased cleaved caspase 3 expression and the infarct size in ischemic heart with improved systolic function.

CONCLUSION

We demonstrated for the first time the presence of claudin-5 in the mitochondria in cardiomyocytes and provided the firm evidence for the cardioprotective role of claudin-5 in the preservation of mitochondrial dynamics and cell fate against hypoxia- or ischemia-induced stress.

Single-Molecule Localization of the Cardiac Voltage-Gated Sodium Channel Reveals Different Modes of Reorganization at Cardiomyocyte Membrane Domains

BACKGROUND

Mutations in the gene encoding the cardiac voltage-gated sodium channel Na_v1.5 cause various cardiac arrhythmias. This variety may arise from different determinants of Na_v1.5 expression between cardiomyocyte domains. At the lateral membrane and T-tubules, Na_v1.5 localization and function remain insufficiently characterized.

METHODS

We used novel single-molecule localization microscopy and computational modeling to define nanoscale features of Na_v1.5 localization and distribution at the lateral membrane, the lateral membrane groove, and T-tubules in cardiomyocytes from wild-type (N=3), dystrophin-deficient (mdx; N=3) mice, and mice expressing C-terminally truncated Na_v1.5 (ΔSIV; N=3). We moreover assessed T-tubules sodium current by recording whole-cell sodium currents in control (N=5) and detubulated (N=5) wild-type cardiomyocytes.

RESULTS

We show that Na_v1.5 organizes as distinct clusters in the groove and T-tubules which density, distribution, and organization partially depend on SIV and dystrophin. We found that overall reduction in Na_v1.5 expression in mdx and ΔSIV cells results in a nonuniform redistribution with Na_v1.5 being specifically reduced at the groove of ΔSIV and increased in T-tubules of mdx cardiomyocytes. A T-tubules sodium current could, however, not be demonstrated.

CONCLUSIONS

Na_v1.5 mutations may site-specifically affect Na_v1.5 localization and distribution at the lateral membrane and T-tubules, depending on site-specific interacting proteins. Future research efforts should elucidate the functional consequences of this redistribution.