Sub-microscopic analysis of t-tubule geometry in living cardiac ventricular myocytes using a shape-based analysis method

Tubulator: an automated approach to analysis of t-tubule and dyadic organization in cardiomyocytes

During cardiac disease, t-tubules and dyads are remodelled and disrupted within cardiomyocytes, thereby reducing cardiac performance. Given the pathological implications of such dyadic remodelling, robust and versatile tools for characterizing these sub-cellular structures are needed. While analysis programs for continuous and regular structures such as rodent ventricular t-tubules are available, at least in two dimensions, these approaches are less appropriate for assessment of more irregular structures, such as dyadic proteins and non-rodent t-tubules. Here, we demonstrate versatile, easy-to-use software that performs such analyses. This software, called Tubulator, enables automated analysis of t-tubules and dyadic proteins alike, in both tissue sections and isolated myocytes. The program measures densities of subcellular structures and proteins in individual cells, quantifies their distribution into transversely and longitudinally oriented elements, and supports detailed co-localization analyses. Importantly, Tubulator provides tools for three-dimensional assessment and rendering of image stacks, extending examinations from the single plane to the whole-myocyte level. To provide insight into the consequences of dyadic organization for synchrony of Ca²⁺ handling, Tubulator also creates 'distance maps', by calculating the distance from all cytosolic positions to the nearest t-tubule and/or dyad. In conclusion, this freely accessible program provides detailed automated analysis of the three-dimensional nature of dyadic and t-tubular structures. This article is part of the theme issue 'The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease'.

Deletion of SM22α disrupts the structure and function of caveolae and T-tubules in cardiomyocytes, contributing to heart failure

Aims

Smooth muscle 22-alpha (SM22α) is an actin-binding protein that plays critical roles in mediating polymerization of actin filaments and stretch sensitivity of cytoskeleton in vascular smooth muscle cells (VSMCs). Multiple lines of evidence indicate the existence of SM22α in cardiomyocytes. Here, we investigated the effect of cardiac SM22α on the membrane architecture and functions of cardiomyocytes to pressure overload.

Methods

SM22α knock-out (KO) mice were utilized to assess the role of SM22α in the heart. Echocardiography was used to evaluate cardiac function, transverse aortic constriction (TAC) was used to induce heart failure, cell shortening properties were measured by IonOptix devices in intact cardiomyocytes, Ca²⁺ sensitivity of myofilaments was measured in permeabilized cardiomyocytes. Confocal microscopy, electron microscopy, western blotting, co-immunoprecipitation (co-IP), Real-Time Quantitative Reverse Transcription PCR (qRT-PCR) techniques were used to perform functional and structural analysis.

Results

SM22α ablation did not alter cardiac function at baseline, but mRNA levels of atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and β-myosin heavy chain (β-MHC) were increased significantly compared with wild type (WT) controls. The membrane architecture was severely disrupted in SM22α KO cardiomyocytes, with disassembly and flattening of caveolae and disrupted T-tubules. Furthermore, SM22α was co-immunoprecipitated with caveolin-3 (Cav3), and the interaction between Cav3 and actin was significantly reduced in SM22α KO cells. SM22α KO cardiomyocytes displayed asynchronized SR Ca²⁺ release, significantly increased Ca²⁺ spark frequency. Additionally, the kinetics of sarcomere shortening was abnormal, accompanied with increased sensitivity and reduced maximum response of myofilaments to Ca²⁺ in SM22α KO cardiomyocytes. SM22α KO mice were more prone to heart failure after TAC.

Conclusions

Our findings identified that SM22α may be required for the architecture and function of caveolae and T-tubules in cardiomyocytes.

Nano-scale morphology of cardiomyocyte t-tubule/sarcoplasmic reticulum junctions revealed by ultra-rapid high-pressure freezing and electron tomography

Detailed knowledge of the ultrastructure of intracellular compartments is a prerequisite for our understanding of how cells function. In cardiac muscle cells, close apposition of transverse (t)-tubule (TT) and sarcoplasmic reticulum (SR) membranes supports stable high-gain excitation-contraction coupling. Here, the fine structure of this key intracellular element is examined in rabbit and mouse ventricular cardiomyocytes, using ultra-rapid high-pressure freezing (HPF, omitting aldehyde fixation) and electron microscopy. 3D electron tomograms were used to quantify the dimensions of TT, terminal cisternae of the SR, and the space between SR and TT membranes (dyadic cleft). In comparison to conventional aldehyde-based chemical sample fixation, HPF-preserved samples of both species show considerably more voluminous SR terminal cisternae, both in absolute dimensions and in terms of junctional SR to TT volume ratio. In rabbit cardiomyocytes, the average dyadic cleft surface area of HPF and chemically fixed myocytes did not differ, but cleft volume was significantly smaller in HPF samples than in conventionally fixed tissue; in murine cardiomyocytes, the dyadic cleft surface area was higher in HPF samples with no difference in cleft volume. In both species, the apposition of the TT and SR membranes in the dyad was more likely to be closer than 10 nm in HPF samples compared to CFD, presumably resulting from avoidance of sample shrinkage associated with conventional fixation techniques. Overall, we provide a note of caution regarding quantitative interpretation of chemically-fixed ultrastructures, and offer novel insight into cardiac TT and SR ultrastructure with relevance for our understanding of cardiac physiology.

Beat-by-Beat Cardiomyocyte T-Tubule Deformation Drives Tubular Content Exchange

Supplemental Digital Content is available in the text.

The sarcolemma of cardiomyocytes contains many proteins that are essential for electromechanical function in general, and excitation-contraction coupling in particular. The distribution of these proteins is nonuniform between the bulk sarcolemmal surface and membrane invaginations known as transverse tubules (TT). TT form an intricate network of fluid-filled conduits that support electromechanical synchronicity within cardiomyocytes. Although continuous with the extracellular space, the narrow lumen and the tortuous structure of TT can form domains of restricted diffusion. As a result of unequal ion fluxes across cell surface and TT membranes, limited diffusion may generate ion gradients within TT, especially deep within the TT network and at high pacing rates.

The mechanism of osmotically induced sealing of cardiac t tubules

Cardiac t tubules undergo significant remodeling in various pathological and experimental conditions, which can be associated with mechanical or osmotic stress. In particular, it has been shown that removal of hyposmotic stress can lead to sealing of t tubules. However, the mechanisms underlying the sealing process remain essentially unknown. In this study we used dextran trapping assay to demonstrate that in adult mouse cardiomyocytes, t-tubular sealing can also be induced by hyperosmotic challenge and that both hypo- and hyperosmotic sealing display a clear threshold behavior requiring ≈100 mosmol/L minimal stress. Importantly, during both hypo- and hyperosmotic challenges, the sealing of t tubules occurs only during the shrinking phase. Analysis of the time course of t-tubular remodeling following removal of hyposmotic stress shows that t tubules become sealed essentially instantly, well before any significant reduction in cell size can be observed. Overall, the data support the hypothesis that the critical event in the process of t-tubular sealing during osmotic challenges is detachment (peeling) of the membrane from the underlying cytoskeleton due to suprathreshold stress.NEW & NOTEWORTHY This study provides new insights into how t-tubular membranes respond to osmotic forces. In particular, the data show that osmotically induced sealing of cardiac t tubules is a threshold phenomenon initiated by detachment of t-tubular membrane from the underlying cytoskeleton. The findings are consistent with the hypothesis that final sealing of t tubules is driven by negative hydrostatic intracellular pressure coincident with cell shrinking.

Calcium Signaling in Cardiomyocyte Function

Rhythmic increases in intracellular Ca²⁺ concentration underlie the contractile function of the heart. These heart muscle-wide changes in intracellular Ca²⁺ are induced and coordinated by electrical depolarization of the cardiomyocyte sarcolemma by the action potential. Originating at the sinoatrial node, conduction of this electrical signal throughout the heart ensures synchronization of individual myocytes into an effective cardiac pump. Ca²⁺ signaling pathways also regulate gene expression and cardiomyocyte growth during development and in pathology. These fundamental roles of Ca²⁺ in the heart are illustrated by the prevalence of altered Ca²⁺ homeostasis in cardiovascular diseases. Indeed, heart failure (an inability of the heart to support hemodynamic needs), rhythmic disturbances, and inappropriate cardiac growth all share an involvement of altered Ca²⁺ handling. The prevalence of these pathologies, contributing to a third of all deaths in the developed world as well as to substantial morbidity makes understanding the mechanisms of Ca²⁺ handling and dysregulation in cardiomyocytes of great importance.

Species differences in the morphology of transverse tubule openings in cardiomyocytes

Aims

The ultrastructure of ventricular cardiomyocyte T-tubule connections with the outer cell surface ('mouth' regions) has been reported to differ between mice and rabbits. In mice, T-tubule mouths form convoluted narrow spaces filled with electron-dense matter that impedes diffusion between T-tubular lumen and bulk extracellular space. Here, we explore whether T-tubule mouths are also constricted in rat (another murine model used frequently for cardiac research) and whether pig and human T-tubule mouth configurations are structurally more similar to mice or rabbits.

Methods and results

We used chemically-fixed tissue and high-pressure frozen isolated cardiomyocytes to compare T-tubule mouth architecture using transmission electron microscopy and three-dimensional electron tomography. We find that rat T-tubular mouth architecture is more similar to that of rabbits than mice, lacking the marked tortuosity and electron-dense ground substance that obstructs access to deeper portions of the T-tubular system in mice. Pilot observations in larger mammals (pig, human) suggest that mouse may be the least representative animal model of T-tubule connectivity with the outer cell surface in larger mammals.

Conclusion

Rat T-tubular system architecture appears to be more similar in size and topology to larger mammals than mice. T-tubular mouth topology may contribute to differences in experimental model behaviour, underscoring the challenge of appropriate model selection for research into cell and tissue function.

A fundamental evaluation of the electrical properties and function of cardiac transverse tubules

This work discusses active and passive electrical properties of transverse (T-)tubules in ventricular cardiomyocytes to understand the physiological roles of T-tubules. T-tubules are invaginations of the lateral membrane that provide a large surface for calcium-handling proteins to facilitate sarcomere shortening. Higher heart rates correlate with higher T-tubular densities in mammalian ventricular cardiomyocytes. We assess ion dynamics in T-tubules and the effects of sodium current in T-tubules on the extracellular potential, which leads to a partial reduction of the sodium current in deep segments of a T-tubule. We moreover reflect on the impact of T-tubules on macroscopic conduction velocity, integrating fundamental principles of action potential propagation and conduction. We also theoretically assess how the conduction velocity is affected by different T-tubular sodium current densities. Lastly, we critically assess literature on ion channel expression to determine whether action potentials can be initiated in T-tubules.

Diffusional and Electrical Properties of T-Tubules Are Governed by Their Constrictions and Dilations

Cardiac t-tubules (TTs) form a network of complex surface membrane invaginations that is essential for proper excitation-contraction coupling. Although electron and optical microscopy studies provided a wealth of important information about the structure of TTs, assessing their functional properties remains a challenge. In this study, we investigated the diffusional accessibility of TTs in intact isolated adult mouse ventricular myocytes using, to our knowledge, a novel fluorescence-based assay. In this approach, a small part of TTs is first locally filled with fluorescent dextran and then its diffusion out of TTs is monitored after rapid removal of extracellular dextran. In normal cells, diffusion of 3 kDa dextran is characterized by an average time constant of 3.9 ± 1.2 s with the data ranging from 1.8 to 10.5 s. The data are consistent with essentially free diffusion of dextran in TTs although measurable contribution of binding is also evident. TT fluorescence is abolished in cells treated with high concentration of formamide or after hyposmotic stress. Importantly, the assay we use allows for quantitative, repetitive measurements of subtle dynamic changes in TT structure of the same cell that are not possible to observe with other approaches. In particular, dextran diffusion rate decreases two-to-threefold during cell swelling, suggesting significant structural remodeling of TTs. Computer modeling shows that diffusional accessibility and electrical properties of TTs are primarily determined by the constrictions and dilations of individual TTs and that, from a functional perspective, TTs cannot be considered as a network of cylinders of the same average diameter. Constriction/dilation model of cardiac TTs is in a quantitative agreement with previous high-resolution microscopy studies of TT structure and alternative measurements of diffusional and electrical time constants of TTs. The data also show that the apparent electrical length constant of cardiac TTs is likely several-fold smaller than that estimated in earlier studies.

Axial Tubule Junctions Activate Atrial Ca2+ Release Across Species

Rationale: Recently, abundant axial tubule (AT) membrane structures were identified deep inside atrial myocytes (AMs). Upon excitation, ATs rapidly activate intracellular Ca²⁺ release and sarcomeric contraction through extensive AT junctions, a cell-specific atrial mechanism. While AT junctions with the sarcoplasmic reticulum contain unusually large clusters of ryanodine receptor 2 (RyR2) Ca²⁺ release channels in mouse AMs, it remains unclear if similar protein networks and membrane structures exist across species, particularly those relevant for atrial disease modeling.

Objective: To examine and quantitatively analyze the architecture of AT membrane structures and associated Ca²⁺ signaling proteins across species from mouse to human.

Methods and Results: We developed superresolution microscopy (nanoscopy) strategies for intact live AMs based on a new custom-made photostable cholesterol dye and immunofluorescence imaging of membraneous structures and membrane proteins in fixed tissue sections from human, porcine, and rodent atria. Consistently, in mouse, rat, and rabbit AMs, intact cell-wide tubule networks continuous with the surface membrane were observed, mainly composed of ATs. Moreover, co-immunofluorescence nanoscopy showed L-type Ca²⁺ channel clusters adjacent to extensive junctional RyR2 clusters at ATs. However, only junctional RyR2 clusters were highly phosphorylated and may thus prime Ca²⁺ release at ATs, locally for rapid signal amplification. While the density of the integrated L-type Ca²⁺ current was similar in human and mouse AMs, the intracellular Ca²⁺ transient showed quantitative differences. Importantly, local intracellular Ca²⁺ release from AT junctions occurred through instantaneous action potential propagation via transverse tubules (TTs) from the surface membrane. Hence, sparse TTs were sufficient as electrical conduits for rapid activation of Ca²⁺ release through ATs. Nanoscopy of atrial tissue sections confirmed abundant ATs as the major network component of AMs, particularly in human atrial tissue sections.

Conclusion: AT junctions represent a conserved, cell-specific membrane structure for rapid excitation-contraction coupling throughout a representative spectrum of species including human. Since ATs provide the major excitable membrane network component in AMs, a new model of atrial “super-hub” Ca²⁺ signaling may apply across biomedically relevant species, opening avenues for future investigations about atrial disease mechanisms and therapeutic targeting.

Solute movement in the t-tubule system of rabbit and mouse cardiomyocytes

Cardiac transverse (t-) tubules carry both electrical excitation and solutes toward the cell center but their ability to transport small molecules is unclear. While fluorescence recovery after photobleaching (FRAP) can provide an approach to measure local solute movement, extraction of diffusion coefficients is confounded by cell and illumination beam geometries. In this study, we use measured cellular geometry and detailed computer modeling to derive the apparent diffusion coefficient of a 1-kDa solute inside the t-tubular system of rabbit and mouse ventricular cardiomyocytes. This approach shows that diffusion within individual t-tubules is more rapid than previously reported. T-tubule tortuosity, varicosities, and the presence of longitudinal elements combine to substantially reduce the apparent rate of solute movement. In steady state, large (>4 kDa) solutes did not freely fill the t-tubule lumen of both species and <50% of the t-tubule volume was available to solutes >70 kDa. Detailed model fitting of FRAP data suggests that solute diffusion is additionally restricted at the t-tubular entrance and this effect was larger in mouse than in rabbit. The possible structural basis of this effect was investigated using electron microscopy and tomography. Near the cell surface, mouse t-tubules are more tortuous and filled with an electron-dense ground substance, previously identified as glycocalyx and a polyanionic mesh. Solute movement in the t-tubule network of rabbit and mouse appears to be explained by their different geometric properties, which impacts the use of these species for understanding t-tubule function and the consequences of changes associated with t-tubule disease.

The Development of Compartmentation of cAMP Signaling in Cardiomyocytes: The Role of T-Tubules and Caveolae Microdomains

3′-5′-cyclic adenosine monophosphate (cAMP) is a signaling messenger produced in response to the stimulation of cellular receptors, and has a myriad of functional applications depending on the cell type. In the heart, cAMP is responsible for regulating the contraction rate and force; however, cAMP is also involved in multiple other functions. Compartmentation of cAMP production may explain the specificity of signaling following a stimulus. In particular, transverse tubules (T-tubules) and caveolae have been found to be critical structural components for the spatial confinement of cAMP in cardiomyocytes, as exemplified by beta-adrenergic receptor (β-ARs) signaling. Pathological alterations in cardiomyocyte microdomain architecture led to a disruption in compartmentation of the cAMP signal. In this review, we discuss the difference between atrial and ventricular cardiomyocytes in respect to microdomain organization, and the pathological changes of atrial and ventricular cAMP signaling in response to myocyte dedifferentiation. In addition, we review the role of localized phosphodiesterase (PDE) activity in constraining the cAMP signal. Finally, we discuss microdomain biogenesis and maturation of cAMP signaling with the help of induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs). Understanding these mechanisms may help to overcome the detrimental effects of pathological structural remodeling.

Caveolae in Rabbit Ventricular Myocytes: Distribution and Dynamic Diminution after Cell Isolation

Caveolae are signal transduction centers, yet their subcellular distribution and preservation in cardiac myocytes after cell isolation are not well documented. Here, we quantify caveolae located within 100 nm of the outer cell surface membrane in rabbit single-ventricular cardiomyocytes over 8 h post-isolation and relate this to the presence of caveolae in intact tissue. Hearts from New Zealand white rabbits were either chemically fixed by coronary perfusion or enzymatically digested to isolate ventricular myocytes, which were subsequently fixed at 0, 3, and 8 h post-isolation. In live cells, the patch-clamp technique was used to measure whole-cell plasma membrane capacitance, and in fixed cells, caveolae were quantified by transmission electron microscopy. Changes in cell-surface topology were assessed using scanning electron microscopy. In fixed ventricular myocardium, dual-axis electron tomography was used for three-dimensional reconstruction and analysis of caveolae in situ. The presence and distribution of surface-sarcolemmal caveolae in freshly isolated cells matches that of intact myocardium. With time, the number of surface-sarcolemmal caveolae decreases in isolated cardiomyocytes. This is associated with a gradual increase in whole-cell membrane capacitance. Concurrently, there is a significant increase in area, diameter, and circularity of sub-sarcolemmal mitochondria, indicative of swelling. In addition, electron tomography data from intact heart illustrate the regular presence of caveolae not only at the surface sarcolemma, but also on transverse-tubular membranes in ventricular myocardium. Thus, caveolae are dynamic structures, present both at surface-sarcolemmal and transverse-tubular membranes. After cell isolation, the number of surface-sarcolemmal caveolae decreases significantly within a time frame relevant for single-cell research. The concurrent increase in cell capacitance suggests that membrane incorporation of surface-sarcolemmal caveolae underlies this, but internalization and/or micro-vesicle loss to the extracellular space may also contribute. Given that much of the research into cardiac caveolae-dependent signaling utilizes isolated cells, and since caveolae-dependent pathways matter for a wide range of other study targets, analysis of isolated cell data should take the time post-isolation into account.

Transverse tubule remodelling: a cellular pathology driven by both sides of the plasmalemma?

Transverse (t)-tubules are invaginations of the plasma membrane that form a complex network of ducts, 200-400 nm in diameter depending on the animal species, that penetrates deep within the cardiac myocyte, where they facilitate a fast and synchronous contraction across the entire cell volume. There is now a large body of evidence in animal models and humans demonstrating that pathological distortion of the t-tubule structure has a causative role in the loss of myocyte contractility that underpins many forms of heart failure. Investigations into the molecular mechanisms of pathological t-tubule remodelling to date have focused on proteins residing in the intracellular aspect of t-tubule membrane that form linkages between the membrane and myocyte cytoskeleton. In this review, we shed light on the mechanisms of t-tubule remodelling which are not limited to the intracellular side. Our recent data have demonstrated that collagen is an integral part of the t-tubule network and that it increases within the tubules in heart failure, suggesting that a fibrotic mechanism could drive cardiac junctional remodelling. We examine the evidence that the linkages between the extracellular matrix, t-tubule membrane and cellular cytoskeleton should be considered as a whole when investigating the mechanisms of t-tubule pathology in the failing heart.