Cardiomyocytes - a general description, the intercalated discs, the sarcomere, T-tubules and cardiac mitochondria.

Cardiac muscle consists of interlacing bundles of cardiomyocytes (cardiac muscle cells). Like skeletal muscle, cardiac muscle is striated with narrow dark and light bands, due to the parallel arrangement of actin and myosin filaments that extend from end to end of each cardiomyocyte. However, in comparison with skeletal muscle cells, cardiomyocytes are narrower and much shorter, being about 25µm mm wide and 100µm long. Cardiomyocytes are often branched, and contain one nucleus but many mitochondria, which provide the energy required for contraction.

A prominent and unique feature of cardiac muscle is the presence of irregularly-spaced dark bands between cardiomyocytes. These bands are known as intercalated discs, and they are located to areas where the membranes of adjacent cardiomyocytes come very close together. Intercalated discs are, from a mechanical standpoint, the structural entities that enable contractile force to be transmitted from one cardiomyocyte to another. This allows for the heart to work as a single functional organ. By contrast, skeletal muscle consists of multinucleated muscle fibers and exhibit no intercalated discs. A second feature of cardiomyocytes is the sarcomere, which is also present skeletal muscle. The sarcomeres give cardiac muscle their striated appearance and are the repeating sections that make up myofibrils. Figure 1 is an immunofluorescence image of cardiomyocytes in culture and a representation of cardiomyocyte structure. Several other characteristics are unique to muscle cells and in particular to cardiomyocytes, as they provide cardiomyocytes with their unique properties and constitute the main structural components that are crucial for the function of these cells.

1) The intercalated discs. Different junctional complexes exist within the intercalated disc. These junctions are essential for adhesive integrity, morphogenesis, differentiation, and maintenance of cardiac tissue. In the intercalated disc, intercellular adhesion molecules, gap junctions, and the voltage-gated sodium channel complex form macromolecular complexes that interact specifically to maintain cardiac structure and cardiomyocyte synchrony. The intercalated discs consist of 3 main junctional complexes: desmosomes, adherens junctions (fascia adherens in cardiac muscle), and gap junctions (Figure 2). Gap junctions are essential for chemical and electrical coupling of neighbouring cells, whereas desmosomes and adherens junctions constitute the mechanical intercellular junctions in cardiomyocytes. Thus, adherens junctions link the intercalated disc to the actin cytoskeleton and desmosomes attach to intermediate filaments.

v          Desmosomes: The desmosomes constitute a 3-dimensional intercellular network that lends structural support to cardiac tissue. They consist in a symmetrical protein complex, with each end residing in the cytoplasm of one of a pair of adjacent cells, anchoring intermediate filaments in the cytoskeleton to the cell surface. The middle bridges the intercellular space between cytoplasmic membranes. Desmosomes are not only important as structural and adhesive complexes. On the contrary, they have important roles in tissue morphogenesis during development and wound healing. To this end, proteins that constitute the desmosome often detach from the macromolecular complex and translocate to other subcellular compartments, where they participate in a variety of signaling pathways. Desmosomal distribution and structure are intimately related to the primary role in strengthening tissues exposed to continuous mechanical stress, and therefore are highly conserved throughout vertebrate evolution. As illustrated in the image, transmembrane desmosomal cadherins, desmoglein (DSG) and desmocollin (DSC), bind the armadillo family proteins junctional plakoglobin (JUP) and plakophilin (PKP), which in turn anchor the plakin family member desmoplakin (DSP). The cytoplasmic plaque, which is further stabilized by lateral interactions among these proteins, anchors the intermediate filament cytoskeleton to the desmosome (Figure 3). Mutations in components of the desmosome underlie a variety of cardiomyopathies, including arrhythmia syndromes and heart muscle disorders.

v          Adherens junctions: The adherens junctions provide a strong mechanical connection of cardiomyocytes via linkage to the actin cytoskeleton, which provides a uniform mechanical strength to the heart. They keep the cells tightly together as the heart expands and contracts. Adherens junctions are also the anchor-point where myofibrils are attached, enabling transmission of contractile force from one cell to another. Adherens junctions are constructed from cadherins and catenins. Cadherins (in cardiomyocytes N-Cadherin is the main cadherin) are transmembrane proteins that zip together adjacent cells in a homophilic manner over a distance of 0.2–0.5 μm. At the adherens junctions the apposing membranes become separated by ∼ 20 nm. The transmembrane cadherins form complexes with cytosolic α-, β-, γ- (plakoglobin), and p120 catenin, thereby establishing the connection to the actin cytoskeleton (Figure 4).

v          Gap junctions: The gap junctions mediate direct communication between adjacent cells. These intercellular channels connect the cytoplasm of neighboring cells, enabling passive diffusion of various compounds, like metabolites, water and ions, up to a molecular mass of 1000 Da. Thereby they warrant electrical and metabolic communication between cells. Gap junctions are present in nearly all tissues and cells throughout the entire body. In cardiac muscle gap junctions ensure a proper propagation of the electrical impulse which triggers sequential and coordinated contraction of the cardiomyocytes. A gap junction channel consists of twelve connexin proteins, six of which are contributed by each cell. The six connexin subunits form a hemi-channel in the plasma membrane, which is called a connexon. A connexon docks to another connexon in the intercellular space to create a complete gap junction channel. The intercellular space between adjacent cells at the site of a gap junction is 3.5 nm (Figure 5). Connexins form a large protein family of highly related though functionally distinct connexins. In the ventricular myocardium the most important connexin isoform is Connexin43 (Cx43).

2) The sarcomere. The main function of cardiomyocytes concerns cardiac contraction. To this end, cardiomyocytes are equipped with bundles of myofibrils that contain myofilaments and represent 45 to 60 per cent of the volume of cardiomyocytes (Figure 6). The myofibrils are formed of distinct, repeating units, termed sarcomeres. The sarcomeres represent the basic contractile units of the myocyte, and are defined as the region of myofilament structures between two Z-lines. The distance between Z-lines ranges in human hearts from about 1.6 to 2.2μm. The sarcomere is composed of thick and thin filaments.

The thick filaments are composed of myosin, a protein with a molecular weight of approximately 470 kilodaltons. There are about 300 molecules of myosin per thick filament. Each myosin contains two heads that are the site of the myosin ATPase, that hydrolyzes ATP required for actin and myosin cross bridge formation. These heads interact with a binding site on actin (Figure 7).

The thin filaments are composed of the proteins that form the regulatory protein complex: actin, tropomyosin, and troponin (Figure 7). Actin is a globular protein arranged as a chain of repeating units, forming two strands of an alpha helix. Interdigitated between the actin strands are rod-shaped proteins termed tropomyosin. There are 6-7 actin molecules per tropomyosin. Attached to the tropomyosin at regular intervals is the troponin complex, which is made up of three subunits: troponin-T (TN-T), which attaches to the tropomyosin; troponin-C (TN-C), which serves as a binding site for Ca++ during excitation-contraction coupling (four Ca++ can bind per TN-C); and troponin-I (TN-I), which inhibits the myosin binding site on the actin.

The arrangement of thick and thin filaments makes possible cardiac contraction, which is discussed elsewhere and gives cardiomyocytes a characteristic banded pattern previously shown on Figure 6:

Z-lines. A sarcomere is defined as the segment between two neighbouring Z-lines (or Z-discs, or Z bodies). In electron micrographs of cross-striated muscle, the Z-line (from the German "Zwischenscheibe", the disc in between the I bands) appears as a series of dark lines.

I-band. Surrounding the Z-line is the region of the I-band (for isotropic). I-band is the zone of thin filaments that is not superimposed by thick filaments.

A-band. Following the I-band is the A-band (for anisotropic). Named for their properties under a polarising microscope. An A-band contains the entire length of a single thick filament.

H-zone. Within the A-band is a paler region called the H-zone (from the German "heller", brighter). Named for their lighter appearance under a polarisation microscope. H-band is the zone of the thick filaments that is not superimposed by the thin filaments.

M-line. Within the H-zone is a thin M-line (from the German "Mittelscheibe", the disc in the middle of the sarcomere) formed of cross-connecting elements of the cytoskeleton.

3) T-tubules. In muscle cells including cardiomyocytes, the sarcolemma (i.e. the plasma membrane) forms deep invaginations known as T-tubules (transverse tubules) (Figure 8). These invaginations allow depolarisation of the membrane to penetrate quickly to the interior of the cell. In cells without t-tubules, the wave of calcium ions propagates from the periphery of the cell into the center. However, such a system would first activate the peripheral sarcomeres and then the deeper sarcomeres, resulting in sub-maximal force production. The t-tubules make it possible that current is simultaneously relayed to the core of the cell, and this means that a larger instantaneous force is produced by triggering SR Ca2+release near to all sarcomeres simultaneously. In fact, the t-tubules restrict diffusion of the extracellular fluid, creating a microdomain of ions of a concentration that is relatively stable in comparison with the wider extracellular space. This may also be a mechanism to prevent rapid changes in the extracellular fluid from adversely affecting calcium-induced calcium release.

4) Mitochondrial morphology and energy metabolism in cardiomyocytes. Mitochondria have been described as "the powerhouse of the cell" because they generate most of the cell's supply of adenosine triphosphate (ATP). Mitochondria are composed of compartments that carry out specialised functions, and they include the outer membrane, the intermembrane space, the inner membrane, and the cristae and matrix (Figure 9). 

In most cell types, mitochondria adjust their morphology and location depending on energy needs and metabolic conditions of the cell. In cardiomyocytes, the relationship between mitochondrial morphology and location, and function does not seem to be so dependent on cell energy demands: Re-organisation of these organelles depends on the cellular environment and architecture constraints - a large amount of myofilaments, presence of a rigid cytoskeleton and a densely packed mitochondrial network. Moreover, the arrangement of the different organelles between them is so crucial for cardiac cell function that mitochondrial morphology has to be efficiently controlled. Compared to any other cell type, mitochondria from adult cardiomyocytes exhibit the highest density of cristae. However, different types of mitochondria can be distinguished within cardiomyocytes, and their morphological features are usually defined according to their location: intermyofibrillar mitochondria, subsarcolemmal mitochondria and perinuclear mitochondria.

v    Intermyofibrillar mitochondria are strictly ordered between rows of contractile proteins, apparently isolated from each other by repeated arrays of T-tubules, and in close contact with myofibrils and sarcoplasmic reticulum. They are mainly devoted to the energy supply of myosin and SR-ATPases. Intermyofibrillar are elongated in shape with usually one mitochondrion existing per sarcomere. They are 1.5–2.0 μm in length, and their cristae structures also displayed curved configurations.

v    Subsarcolemmal mitochondria present a lower degree of organisation and are probably mainly involved in other roles such as ion homeostasis. They are located beneath the sarcolemma and are more variable in length (0.4–3.0 μm), possessing closely packed cristae.

v    Perinuclear mitochondria are organized in clusters and are most probably involved in transcription and translation processes. They are mostly spherical in shape with lengths ranging from 0.8 to 1.4 μm. These mitochondria contain well-developed curved cristae with relatively little matrix area. 

Given the energy demands derived from cardiomyocyte function, the adult cardiomyocytes contain numerous mitochondria, which can occupy at least 30% of cell volume. Adult cardiomyocytes meet >90% of of the energy requirements by oxidative phosphorylation (OXPHOS) in the mitochondria. Fatty acid oxidation predominates over the oxidation of other nutrients under normal physiological conditions. During periods of stress cardiomyocytes are flexible, and can obtain energy by oxidizing glucose, lactate, amino acids, and ketone bodies. In fact, the ability of to adapt their metabolism to substrate availability results critical for their contraction balance under different physiological and pathophysiological conditions. Proliferation of foetal cardiomyocytes during cardiac development is characterised by high rates of glycolysis and lactate production. Only <15% of the ATP is produced by the fatty acid β-oxidation pathway.

Summary. Cardiomyocytes are the main responsible for heart contraction. The unique structural features discussed here allow them for specialised functions. However, cardiac function needs to be understood in the context of the cardiac tissue, in which other cell types and structures are important in order to get a coordinated cardiac contraction that adapts to the physiological needs of the organ.

A tour of a cardiomyocyte

 For sources and further reading.

This summary has been possible thanks to previous works. The most significant ones are cited here:

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7. Colleen B. Estigoy, Fredrik Pontén, Jacob Odeberg, Benjamin Herbert, Michael Guilhaus & Michael Charleston, Joshua W. K. Ho, Darryl Cameron & Cristobal G. dos Remedios. Intercalated discs: multiple proteins perform multiple functions in non-failing and failing human hearts. Biophys Rev. 2009;1:43–49.

8. Ibrahim M, Gorelik J, Yacoub MH, Terracciano CM. The structure and function of cardiac t-tubules in health and disease. Proc Biol Sci. 2011 Sep 22;278(1719):2714-23.

9. Piquereau J, Caffin F, Novotova M, Lemaire C, Veksler V, Garnier A, Ventura-Clapier R, Joubert F. Mitochondrial dynamics in the adult cardiomyocytes: which roles for a highly specialized cell? Front Physiol. 2013 May 10;4:102.

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11. Gaspar JA, Doss MX, Hengstler JG, Cadenas C, Hescheler J, Sachinidis A. Unique metabolic features of stem cells, cardiomyocytes, and their progenitors. Circ Res. 2014 Apr 11;114(8):1346-60.