The microtubule cytoskeleton in cardiac mechanics and heart failure

Abstract

The microtubule network of cardiac muscle cells has unique architectural and biophysical features to accommodate the demands of the working heart. Advances in live-cell imaging and in deciphering the 'tubulin code' have shone new light on this cytoskeletal network and its role in heart failure. Microtubule-based transport orchestrates the growth and maintenance of the contractile apparatus through spatiotemporal control of translation, while also organizing the specialized membrane systems required for excitation-contraction coupling. To withstand the high mechanical loads of the working heart, microtubules are post-translationally modified and physically reinforced. In response to stress to the myocardium, the microtubule network remodels, typically through densification, post-translational modification and stabilization. Under these conditions, physically reinforced microtubules resist the motion of the cardiomyocyte and increase myocardial stiffness. Accordingly, modified microtubules have emerged as a therapeutic target for reducing stiffness in heart failure. In this Review, we discuss the latest evidence on the contribution of microtubules to cardiac mechanics, the drivers of microtubule network remodelling in cardiac pathologies and the therapeutic potential of targeting cardiac microtubules in acquired heart diseases.

Fig. 1 |. The tubulin code in the heart.

Diverse isoforms and post-translational modifications of tubulin give rise to discrete microtubule populations. a | The microtubule minus end is stabilized and nucleated at the microtubule-organizing centre (MTOC), whereas the dynamic plus end undergoes cyclic rounds of polymerization and depolymerization. Kinesin and dynein power cargo transport, whereas microtubule-associated proteins, such as MAP4, bind to the microtubule, regulating its stability, transport and interactome. Tubulin plus-end-interacting proteins, such as microtubule-associated protein RP/ED family member 1 (EB1), specifically bind to the GTP–tubulin cap to regulate microtubule dynamic instability. b | The C-terminal tails of the α-tubulin and β-tubulin heterodimer are hot spots for post-translational modifications, such as detyrosination, Δ2 formation (removal of the next glutamine residue after detyrosination), glutamylation and glycylation. Tubulin can also be acetylated, for example, at lysine 40 (K40). In humans, at least nine α-tubulin and nine β-tubulin isoforms exist, further contributing to tubulin diversity. In the lower panel, the circles are scaled to show the relative abundances of different tubulin isoforms in the heart of humans (left) and mice (right)^,. c | The most-studied post-translational modifications of microtubules in the heart are C-terminal detyrosination and luminal acetylation, both of which are induced in heart failure. The tyrosination cycle is regulated by the tyrosinating enzyme tubulin–tyrosine ligase (TTL) and the detyrosinating enzymatic complex formed by vasohibin (VASH) and small vasohibin-binding protein (SVBP). Acetylation is regulated by histone deacetylase 6 (HDAC6) and α-tubulin N-acetyltransferase 1 (αTAT1).

Fig. 2 |. Microtubules organize subcellular domains in cardiomyocytes.

a | Longitudinally oriented microtubules anchor at the nucleus, Z-disc and intercalated disc of cardiomyocytes. b | Detyrosinated microtubules anchor to the nucleus via A-kinase anchor protein 6 (AKAP6) binding to the LINC (linker of nucleoskeleton and cytoskeleton) complex. After export from the nuclear pore, mRNA binds to RNA-binding proteins (RBPs) or the ribosome, where it is transported on the microtubule motors kinesin and dynein to translational hubs flanking the sarcomeric Z-disc. c | The intercalated disc is a specialized region of mechanical and electrical connectivity consisting of two types of anchoring complex — adherens junctions and desmosomes — as well as low-resistance transmembrane channels at gap junctions. Microtubule-associated protein RP/ED family member 1 (EB1) anchors and stabilizes the microtubule plus tips at all three intercalated disc complexes by interfacing with N-cadherin via p150^Glued and β-catenin at the adherens junction, desmoplakin at the desmosome and with the C-terminal tails of connexin 43 channel proteins at gap junctions. d | The dyad is the specialized region of excitation–contraction coupling in cardiomyocytes. Invaginations of the sarcolemma, called transverse tubules, are stabilized in close proximity to the sarcoplasmic reticulum (SR) calcium store. L-type calcium channels (LTCCs) initiate calcium-induced calcium-release from ryanodine receptor 2 channels (RYR2), which are held in close proximity by junctophilin 2 (JP2). Microtubule-dependent redistribution of JP2 is implicated in the loss of dyad organization in heart failure. The microtubule plus-end-interacting protein EB1 interacts with CAP-Gly domain-containing linker protein 1 (CLIP170) to stabilize transverse tubules, whereas cytoskeleton-associated protein 4 (CLIMP63) spans the SR membrane to anchor microtubules to the transmembrane protein triadin. Myc box-dependent-interacting protein 1 (BIN1) binds to the transverse tubule membrane to support its curvature and interacts with the coiled-coil domain of CLIP170 to stabilize the microtubule tip.

Fig. 3 |. The mechanical properties of the microtubule cytoskeleton.

Cytoskeletal filaments are rigid, interconnected polymers that bestow mechanical integrity on cells. a | The three cytoskeletal filaments of mammalian cells are actin, intermediate filaments and microtubules. Microtubules are the largest in diameter and the stiffest (as measured by persistence length, L_p). b | Crosslinked networks of filaments impart non-linear stiffening that is a hallmark of biological materials. In response to being stretched to a length at which the polymers align, crosslinkers engage and the network becomes harder to stretch. Non-crosslinked networks continue to stretch and do not stiffen in response to strain. c | The organization of filaments and their crosslinking dramatically influence their mechanical properties. Dynamic microtubules in mitotic cells have a minor role in cell mechanics, whereas the stabilized microtubules in cardiomyocytes resist contraction and stretch (left panel). Detyrosinated microtubules crosslink to desmin intermediate filaments at the Z-disc, where they buckle to resist contraction and slide to resist stretch (centre panel). Dynamic crosslinking between the microtubules and intermediate filaments imparts viscoelasticity to cardiomyocytes (right panel). Crosslinks break and reform when strained and, as the strain rate becomes faster, the breaking force increases, giving rise to microtubule-based stiffness (viscoelasticity) that increases with the speed of stretch or contraction. MTOC, microtubule-organizing centre.

Fig. 4 |. Microtubule remodelling in heart failure.

a | In the healthy heart, the microtubules are dynamic and undergo balanced detyrosination and binding to microtubule-associated proteins (MAPs), such as MAP4. Microtubule network stabilization and detyrosination are hallmarks of heart disease, but arise from divergent processes in response to mechanical or oxidative stress. b | Mechanical stress such as haemodynamic overload leads to cardiac hypertrophy driven by cardiomyocyte growth. At the microtubule level, mechanical stress induces hypophosphorylation of MAP4, driving MAP4 accumulation on the microtubule and subsequent microtubule stabilization. Long-lived microtubules accumulate post-translational modifications (PTMs), such as detyrosination and acetylation, which further stabilize the microtubules and increase crosslinking between the microtubule network and desmin intermediate filament networks, which can occur through kinesin, plectin and other MAPs (TABLE 1). c | In the ischaemic heart, oxidative stress and inflammation lead to myocardial fibrosis and cardiomyocyte remodelling. At the microtubule level, hyperphosphorylation of MAP4 by MAP–microtubule affinity-regulating kinase 4 (MARK4) dissociates MAP4 from the microtubule, allowing increased vasohibin 2 (VASH2) binding and microtubule detyrosination. Oxidative stress also damages the microtubule and promotes repair by GTP–tubulin, which exerts a stabilizing effect on the microtubule. P_i, inorganic phosphate.