Aug 2026· Cytoskeleton· pp.
e70187
· 0 citations· 67 references
Medicine
TL;DR
This review will discuss the general principles of myofibril assembly in the heart and cover new findings on myofibril maintenance and turnover.
Abstract
The assembly of actin and myosin to the paracrystalline arrangement of myofibrils in striated muscle, that is, heart and skeletal, has puzzled many a cell biologist. Different models, based on cultured cardiomyocytes, were proposed for the last 30 years to explain the sequence of events. These were then evaluated during heart development in embryos in situ and more recently in cardiomyocytes that were derived from human induced pluripotent stem cells (iPSC-CMs). A lot of the initial work was quite descriptive but recent advances in gene editing technology and in super resolution microscopy allow a more targeted functional analysis and will give unprecedented insight. This review will discuss the general principles of myofibril assembly in the heart and cover new findings on myofibril maintenance and turnover.
Skeletal muscle exhibits an unusually complex architecture, in which large multinucleated myofibers accommodate a small population of resident muscle stem cells (MuSCs) along their surface. In addition, myofibers contain molecularly and functionally specialized domains at junctions with motor neurons and tendons. Regeneration of this tissue, therefore, requires a coordinated series of events, spanning MuSC activation, proliferation, fusion to restore myofiber mass, as well as reconstruction of specialized domains. Recent advances have begun to reveal how myogenic nuclei undergo dynamic state transitions in response to interactions with surrounding cell types, and how these states can be remodeled during regeneration. Here, we discuss emerging principles of myonuclear plasticity and spatial specialization.
Minchul Kim, Yu Xin Wang, Nour El Khazen· Current Opinion in Genetics...· 1 citation
Background: Understanding the mechanisms of cardiomyocyte development is critical for fulfilling the potential of induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs). Although myocyte development is known to depend on internal and external mechanical cues, further investigation is required to understand the contributions of different signals and how they are integrated together to generate an adult cardiomyocyte. Here, we address this gap by examining the role of calcium-activated contractility in sarcomere formation and maturation and its influence on the iPSC-CM response to nanopatterns. Methods: We generated iPSCs with homozygous D65A cardiac troponin C (cTnC) substitutions. This engineered cTnC cannot bind to calcium at site II, resulting in tropomyosin blocking strong myosin binding to the thin filament and inhibiting sarcomere contraction. The iPSCs were differentiated into cardiomyocytes and matured in culture over 60 days. Cells were characterized via imaging, metabolic assays, and calcium transient analysis. Proteomes were examined using mass spectrometry throughout differentiation and maturation. We also replated partially matured cardiomyocytes onto nanopatterned surfaces to investigate how external mechanical signals affect maturation in contractile versus non-contractile cells. Results: Surprisingly, we found that sarcomeres formed in the D65A cTnC cardiomyocytes, though these sarcomeres were underdeveloped and disorganized. The D65A cardiomyocytes also exhibited significant proteomic maturation defects and abnormal calcium transients. Replating the non-contractile cardiomyocytes onto nanopatterns improved several structural and proteomic maturation metrics. In contrast, WT maturation did not benefit from the introduction of nanopatterns. Conclusions: Calcium-activated contractility is dispensable for sarcomerogenesis but critical for cardiomyocyte maturation. In non-contractile, D65A cTnC cardiomyocytes, nanopatterns enhanced maturation, suggesting that external mechanical cues may partially compensate for defective contractility. However, nanopatterns did not facilitate WT maturation, suggesting that maturity may reduce the efficacy of nanopatterns. In addition to these novel findings, these mass spectrometry datasets cataloging iPSC-CM maturation represent a useful resource for the cardiovascular research community.
Laura A. Sherer, Abigail Nagle, M. Papadaki et al.· Circulation Research· 0 citations
Fhod3, a member of the formin family of proteins that regulate actin polymerization, is essential for the organization of cardiac sarcomeres during embryonic cardiogenesis. We previously demonstrated that the perinatal deletion of Fhod3 in mice results in lethality with disorganized sarcomeres, whereas the deletion in the adult period is non-lethal and does not affect sarcomere morphology. To clarify the underlying mechanism of this developmental stage-dependent outcome, we examined the effects of Fhod3 depletion in primary cultured cardiomyocytes derived from Fhod3flox neonates. Long-range time-lapse imaging throughout the entire culture period revealed that Fhod3 depletion around birth using muscle creatine kinase-Cre caused sarcomere disruption only in cells that had undergone mitosis and not in cells that had not undergone mitosis, suggesting that sarcomere breakdown by Fhod3 deletion occurs after mitosis. To verify this, we varied the timing of Fhod3 deletion using a tamoxifen-inducible αMHC-MerCreMer line. Cells treated with tamoxifen from day 1 in vitro (DIV1) exhibited a disruption of sarcomeres, similar to that observed in the Fhod3-depleted cells by MCK-Cre around birth. In contrast, cells treated from DIV9, when postnatal mitosis was almost complete, showed organized sarcomere despite Fhod3 depletion. Thus, in the presence of Fhod3, sarcomere reassembly after mitosis was successful, whereas it failed in the absence of Fhod3, indicating the indispensable role of Fhod3 in sarcomere reorganization after mitosis.
Shuhei Sakaguchi, Yohko Kage, Eka Adip Pradipta et al.· Cellular and Molecular Life...· 0 citations
Multiciliated cells, through the coordinated beating of motile cilia, are vital for moving fluids in the brain and mucus in the airways and in the reproductive organs. Their dysfunction leads to diseases, associated with chronic airway infections, subfertility and ventricular enlargement. A group of transcription factors and a cell cycle variant enable the differentiation of multiciliated cells. However, how the multiciliated differentiation programme is initiated, timed and spatially regulated within developing tissues remains unclear. In this review, we combine findings from airway, brain ventricles and Xenopus epidermal systems to reveal how mechanical cues may interact with transcriptional networks and cell cycle regulators during multiciliogenesis. We suggest a model where mechanical forces serve as contextual regulators that enable, time and control the execution of the multiciliated differentiation programme.
Nathalie Spassky, Nathalie Delgehyr· Open Biology· 0 citations
BACKGROUND: Mature mammalian cardiomyocytes (CMs) develop compact sarcomeric structures that inhibit proliferation. Consequently, CMs must dedifferentiate to a fetal-like state, which is accompanied by sarcomere disassembly, to enable successful cytokinesis. However, the regulation and coordination of CM dedifferentiation, cell cycle progression, and sarcomere reorganization remain unclear. METHODS: We generated adenovirus and adeno-associated virus (MyoAAV) vectors expressing YAP5SA and YAP5SA-S94A under Xon control for LMI070-inducible protein expression. We also developed MyoAAV-cTnT-Tuba1b-shRNA-miR30 for cardiomyocyte-specific knockdown (KD) of Tuba1b. These tools were used to investigate cardiomyocyte dedifferentiation, proliferation, and sarcomere disassembly. We also performed Cleavage Under Targets and Release Using Nuclease (CUT&RUN) to map the genome-wide binding sites of YAP5SA and YAP5SA-S94A, in combination with RNA sequencing to identify YAP target genes. In addition, time-course live-imaging analysis was used to evaluate microtubule and sarcomere dynamics in adult cardiomyocytes. RESULTS: We show that microtubule expression and network density decline with cardiac maturation. Overexpression of YAP5SA, a constitutively active YAP mutant, promotes microtubule growth by stabilizing microtubule dynamics, leading to CM dedifferentiation, cell cycle re-entry and sarcomere disassembly. In contrast, colchicine blocks these processes and significantly attenuates YAP-induced cardiac regeneration. Live imaging reveals a distinct mode of sarcomere disassembly driven by enhanced microtubule polymerization, where microtubule plus-ends directly interact with α-actinin and displace α-actinin fragments, thereby facilitating sarcomere breakdown. Furthermore, the YAP-S94A mutation, which disrupts the YAP and TEAD interaction, significantly reduces YAP5SA-induced microtubule growth, sarcomere disassembly, and cell cycle activity. Mechanistically, CUT&RUN combined with RNA-seq identified direct YAP targets, including Ajuba and Tuba1b, which are critical for microtubule growth. CM-specific KD of Tuba1b attenuates YAP-driven sarcomere disassembly. CONCLUSIONS: These findings identify microtubule networks as an essential regulator modulating CM dedifferentiation and sarcomere reorganization, which is critical for CM cytokinesis and cardiac regenerative repair.
Feng Zhang, Jenna Cusick, Jie Liang et al.· Circulation· 0 citations
Biallelic titin truncation variants (TTNtvs) are linked to severe cardiac and skeletal muscle diseases, due to unclear mechanisms. Using induced pluripotent stem cell-derived cardiomyocytes from a biallelic TTNtv patient with dilated cardiomyopathy, we investigated sarcomere structure/function. Only the longest of the TTNtvs was detected as protein, and this nearly full-length titin was incorporated into the sarcomere. Subtle structural alterations occurred, with shortened A-bands observed in a subset of sarcomeres. Resulting reduction and imbalance of force development was linked to lowered contractility, and many sarcomeres being stretched by their neighbors. Thus, inefficient sarcomere assembly and interaction promotes cardiomyopathy in biallelic TTNtv.
Magnhild Sekse Erdal, Jing Qi, P. Erusappan et al.· Journal of Molecular and Cel...· 0 citations
A new method for surgically removing training examples from a model reveals that as datasets grow, the link between what a model learns and what it produces dissolves.
