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

Abstract

Transverse-axial tubules (TATs) are commonly assumed to be sparse or absent in atrial myocytes from small animals. Atrial myocytes from rats, cats and rabbits lack TATs, which results in a characteristic “V”-shaped Ca release pattern in confocal line-scan recordings due to the delayed rise of Ca in the center of the cell. To examine TAT expression in isolated mouse atrial myocytes, we loaded them with the membrane dye Di-4-ANEPPS to label TATs. We found that >80% of atrial myocytes had identifiable TATs. Atria from male mice had a higher TAT density than female mice, and TAT density correlated with cell width. Using the fluorescent Ca indicator Fluo-4-AM and confocal imaging, we found that wild type (WT) mouse atrial myocytes generate near-synchronous Ca transients, in contrast to the “V”-shaped pattern typically reported in other small animals such as rat. In atrial-specific Na–Ca exchanger (NCX) knockout (KO) mice, which develop sinus node dysfunction and atrial hypertrophy with dilation, we found a substantial loss of atrial TATs in isolated atrial myocytes. There was a greater loss of transverse tubules compared to axial tubules, resulting in a dominance of axial tubules. Consistent with the overall loss of TATs, NCX KO atrial myocytes displayed a “V”-shaped Ca transient with slower and reduced central (CT) Ca re-lease and uptake in comparison to subsarcolemmal (SS) Ca release. We compared chemically detubulated (DT) WT cells to KO, and found similar slowing of CT Ca release and uptake. However, SS Ca transients in the WT DT cells had faster uptake kinetics than KO cells, consistent with the presence of NCX and normal sarcolemmal Ca efflux in the WT DT cells. We conclude that the remodeling of NCX KO atrial myocytes is accompanied by a loss of TATs leading to abnormal Ca release and uptake that could impact atrial contractility and rhythm.

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.…”

Studying signal compartmentation in adult cardiomyocytes

“…Unfortunately, controversy remains regarding the exact nature of the connection between cell size and the configuration of the TATS. The current dogma is that a larger cell demands a more extensive tubular system to function which sits uneasily with data concerning species differences, the loss of T-tubules in compensatory hypertrophy in disease and the lack of correlation between atrial cardiomyocyte size and TAT density (when excluding very small cells) [40]. We demonstrated intra-chamber variability in LV cardiomyocyte TAT organisation in adult rat cells [41].…”

May the Force Not Be With You During Culture: Eliminating Mechano-Associated Feedback During Culture Preserves Cultured Atrial and Pacemaker Cell Functions

“…By synchronizing Ca 2+ release throughout the cell (Hong and Shaw, 2017) and shifting the electro-mechanical feedback to occur in the peripheral areas of the cell, t-tubules allow cardiac cells to contract more forcefully. Atrial cells have a different t-tubule system than ventricular cells; they are termed transverse-axial tubules (TATs) and present as heterogenous sarcolemmal membrane invaginations that appear near the z-lines and assist in atrial cell contraction (Kirk et al, 2003;Hund and Mohlerp, 2016;Yue et al, 2017). When an AP is initiated, the atrial cells activate intracellular Ca 2+ release and sarcomeric contraction through the TAT junctions (Brandenburg et al, 2018).…”

“…The existence of the TATs contributes to nearsynchronous Ca 2+ transients and to synchronous subcellular depolarization and intracellular Ca 2+ release. TATs may lead to region-specific electro-mechanical coupling and heterogeneous atrial contraction (Hund and Mohlerp, 2016;Yue et al, 2017;Brandenburg et al, 2018). There is evidence of dense and welldeveloped TATs in both small and large animals (Kirk et al, 2003;Glukhov et al, 2015;Yue et al, 2017;Brandenburg et al, 2018).…”

“…TATs may lead to region-specific electro-mechanical coupling and heterogeneous atrial contraction (Hund and Mohlerp, 2016;Yue et al, 2017;Brandenburg et al, 2018). There is evidence of dense and welldeveloped TATs in both small and large animals (Kirk et al, 2003;Glukhov et al, 2015;Yue et al, 2017;Brandenburg et al, 2018). In addition, studies have shown the existence of an atrial t-tubule system in larger mammals, such as sheep and humans (Richards et al, 2011;Trafford et al, 2013).…”