Polyploidy in Cardiomyocytes: Roadblock to Heart Regeneration?

Novel Insights into Post-Myocardial Infarction Cardiac Remodeling through Algorithmic Detection of Cell-Type Composition Shifts

Background

Recent advances in single cell sequencing have led to an increased focus on the role of cell-type composition in phenotypic presentation and disease progression. Cell-type composition research in the heart is challenging due to large, frequently multinucleated cardiomyocytes that preclude most single cell approaches from obtaining accurate measurements of cell composition. Our in silico studies reveal that ignoring cell type composition when calculating differentially expressed genes (DEGs) can have significant consequences. For example, a relatively small change in cell abundance of only 10% can result in over 25% of DEGs being false positives.

Methods

We have implemented an algorithmic approach that uses snRNAseq datasets as a reference to accurately calculate cell type compositions from bulk RNAseq datasets through robust data cleaning, gene selection, and multi-sample cross-subject and cross-cell-type deconvolution. We applied our approach to cardiomyocyte-specific α1A adrenergic receptor (CM-α1A-AR) knockout mice. 8-12 week-old mice (either WT or CM-α1A-KO) were subjected to permanent left coronary artery (LCA) ligation or sham surgery (n=4 per group). Transcriptomes from the infarct border zones were collected 3 days later and analyzed using our algorithm to determine cell-type abundances, corrected differential expression calculations using DESeq2, and validated these findings using RNAscope.

Results

Uncorrected DEGs for the CM-α1A-KO X LCA interaction term featured many cell-type specific genes such as Timp4 (fibroblasts) and Aplnr (cardiomyocytes) and overall GO enrichment for terms pertaining to cardiomyocyte differentiation (P=3.1E-4). Using our algorithm, we observe a striking loss of cardiomyocytes and gain in fibroblasts in the α1A-KO + LCA mice that was not recapitulated in WT + LCA animals, although we did observe a similar increase in macrophage abundance in both conditions. This recapitulates prior results that showed a much more severe heart failure phenotype in CM-α1A-KO + LCA mice. Following correction for cell-type, our DEGs now highlight a novel set of genes enriched for GO terms such as cardiac contraction (P=3.7E-5) and actin filament organization (P=6.3E-5).

Conclusions

Our algorithm identifies and corrects for cell-type abundance in bulk RNAseq datasets opening new avenues for research on novel genes and pathways as well as an improved understanding of the role of cardiac cell types in cardiovascular disease.

Binucleated human hepatocytes arise through late cytokinetic regression during endomitosis M phase

Binucleated polyploid cells are common in many animal tissues, where they arise by endomitosis, a non-canonical cell cycle in which cells enter M phase but do not undergo cytokinesis. Different steps of cytokinesis have been shown to be inhibited during endomitosis M phase in rodents, but it is currently unknown how human cells undergo endomitosis. In this study, we use fetal-derived human hepatocyte organoids (Hep-Orgs) to investigate how human hepatocytes initiate and execute endomitosis. We find that cells in endomitosis M phase have normal mitotic timings, but lose membrane anchorage to the midbody during cytokinesis, which is associated with the loss of four cortical anchoring proteins, RacGAP1, Anillin, SEPT9, and citron kinase (CIT-K). Moreover, reduction of WNT activity increases the percentage of binucleated cells in Hep-Orgs, an effect that is dependent on the atypical E2F proteins, E2F7 and E2F8. Together, we have elucidated how hepatocytes undergo endomitosis in human Hep-Orgs, providing new insights into the mechanisms of endomitosis in mammals.

Biological Evaluation of Thermosensitive Hydrogels of Chitosan/Hydrolyzed Collagen/β-GP in an In Vitro Model of Induced Cardiac Ischemia

Ischemic events can culminate in acute myocardial infarction, which is generated by irreversible cardiac lesions that cannot be restored due to the limited regenerative capacity of the heart. Cardiac cell therapy aims to replace injured or necrotic cells with healthy and functional cells. Tissue engineering and cardiovascular regenerative medicine propose therapeutic alternatives using biomaterials that mimic the native extracellular environment and improve cellular and tissue functionality. This investigation evaluates the effect of thermosensitive hydrogels, and murine fetal ventricular cardiomyocytes encapsulated in thermosensitive hydrogels, on the contractile function of cardiomyocyte regeneration during an ischemic event. Chitosan and hydrolyzed collagen thermosensitive hydrogels were developed, and they were physically and chemically characterized. Likewise, their biocompatibility was evaluated through cytotoxicity assays by MTT, LDH, and their hemolytic capacity. The hydrogels, and cells inside the hydrogels, were used as an intervention for primary cardiomyocytes under hypoxic conditions to determine the restoration of the contractile capacity by measuring intracellular calcium levels and the expressions of binding proteins, such as a-actinin and connexin 43. These results evidence the potential of natural thermosensitive hydrogels to restore the bioelectrical functionality of ischemic cardiomyocytes.

Efficient and reproducible generation of human iPSC-derived cardiomyocytes and cardiac organoids in stirred suspension systems

Human iPSC-derived cardiomyocytes (hiPSC-CMs) have proven invaluable for cardiac disease modeling and regeneration. Challenges with quality, inter-batch consistency, cryopreservation and scale remain, reducing experimental reproducibility and clinical translation. Here, we report a robust stirred suspension cardiac differentiation protocol, and we perform extensive morphological and functional characterization of the resulting bioreactor-differentiated iPSC-CMs (bCMs). Across multiple different iPSC lines, the protocol produces 1.2E6/mL bCMs with ~94% purity. bCMs have high viability after cryo-recovery (>90%) and predominantly ventricular identity. Compared to standard monolayer-differentiated CMs, bCMs are more reproducible across batches and have more mature functional properties. The protocol also works with magnetically stirred spinner flasks, which are more economical and scalable than bioreactors. Minor protocol modifications generate cardiac organoids fully in suspension culture. These reproducible, scalable, and resource-efficient approaches to generate iPSC-CMs and organoids will expand their applications, and our benchmark data will enable comparison to cells produced by other cardiac differentiation protocols.

Dynamics of collagen oxidation and cross linking in regenerating and irreversibly infarcted myocardium

In mammalian hearts myocardial infarction produces a permanent collagen-rich scar. Conversely, in zebrafish a collagen-rich scar forms but is completely resorbed as the myocardium regenerates. The formation of cross-links in collagen hinders its degradation but cross-linking has not been well characterized in zebrafish hearts. Here, a library of fluorescent probes to quantify collagen oxidation, the first step in collagen cross-link (CCL) formation, was developed. Myocardial injury in mice or zebrafish resulted in similar dynamics of collagen oxidation in the myocardium in the first month after injury. However, during this time, mature CCLs such as pyridinoline and deoxypyridinoline developed in the murine infarcts but not in the zebrafish hearts. High levels of newly oxidized collagen were still seen in murine scars with mature CCLs. These data suggest that fibrogenesis remains dynamic, even in mature scars, and that the absence of mature CCLs in zebrafish hearts may facilitate their ability to regenerate.

Revisiting Cardiac Biology in the Era of Single Cell and Spatial Omics

Throughout our lifetime, each beat of the heart requires the coordinated action of multiple cardiac cell types. Understanding cardiac cell biology, its intricate microenvironments, and the mechanisms that govern their function in health and disease are crucial to designing novel therapeutical and behavioral interventions. Recent advances in single-cell and spatial omics technologies have significantly propelled this understanding, offering novel insights into the cellular diversity and function and the complex interactions of cardiac tissue. This review provides a comprehensive overview of the cellular landscape of the heart, bridging the gap between suspension-based and emerging in situ approaches, focusing on the experimental and computational challenges, comparative analyses of mouse and human cardiac systems, and the rising contextualization of cardiac cells within their niches. As we explore the heart at this unprecedented resolution, integrating insights from both mouse and human studies will pave the way for novel diagnostic tools and therapeutic interventions, ultimately improving outcomes for patients with cardiovascular diseases.

BMP7 promotes cardiomyocyte regeneration in zebrafish and adult mice

Zebrafish have a lifelong cardiac regenerative ability after damage, whereas mammals lose this capacity during early postnatal development. This study investigated whether the declining expression of growth factors during postnatal mammalian development contributes to the decrease of cardiomyocyte regenerative potential. Besides confirming the proliferative ability of neuregulin 1 (NRG1), interleukin (IL)1b, receptor activator of nuclear factor kappa-Β ligand (RANKL), insulin growth factor (IGF)2, and IL6, we identified other potential pro-regenerative factors, with BMP7 exhibiting the most pronounced efficacy. Bmp7 knockdown in neonatal mouse cardiomyocytes and loss-of-function in adult zebrafish during cardiac regeneration reduced cardiomyocyte proliferation, indicating that Bmp7 is crucial in the regenerative stages of mouse and zebrafish hearts. Conversely, bmp7 overexpression in regenerating zebrafish or administration at post-mitotic juvenile and adult mouse stages, in vitro and in vivo following myocardial infarction, enhanced cardiomyocyte cycling. Mechanistically, BMP7 stimulated proliferation through BMPR1A/ACVR1 and ACVR2A/BMPR2 receptors and downstream SMAD5, ERK, and AKT signaling. Overall, BMP7 administration is a promising strategy for heart regeneration.

β-Adrenergic signaling drives structural and functional maturation of mouse cardiomyocytes

Cardiac maturation represents the last phase of heart development and is characterized by morphofunctional alterations that optimize the heart for efficient pumping. Its understanding provides important insights into cardiac regeneration therapies. Recent evidence implies that adrenergic signals are involved in the regulation of cardiac maturation, but the mechanistic underpinnings involved in this process are poorly understood. Herein, we explored the role of β-adrenergic receptor (β-AR) activation in determining structural and functional components of cardiomyocyte maturation. Temporal characterization of tyrosine hydroxylase and norepinephrine levels in the mouse heart revealed that sympathetic innervation develops during the first 3 wk of life, concurrent with the rise in β-AR expression. To assess the impact of adrenergic inhibition on maturation, we treated mice with propranolol, isolated cardiomyocytes, and evaluated morphofunctional parameters. Propranolol treatment reduced heart weight, cardiomyocyte size, and cellular shortening, while it increased the pool of mononucleated myocytes, resulting in impaired maturation. No changes in t-tubules were observed in cells from propranolol mice. To establish a causal link between β-AR signaling and cardiomyocyte maturation, mice were subjected to sympathectomy, followed or not by restoration with isoproterenol treatment. Cardiomyocytes from sympathectomyzed mice recapitulated the salient immaturity features of propranolol-treated mice, with the additional loss of t-tubules. Isoproterenol rescued the maturation deficits induced by sympathectomy, except for the t-tubule alterations. Our study identifies the β-AR stimuli as a maturation promoting signal and implies that this pathway can be modulated to improve cardiac regeneration therapies.NEW & NOTEWORTHY Maturation involves a series of morphofunctional alterations vital to heart development. Its regulatory mechanisms are only now being unveiled. Evidence implies that adrenergic signaling regulates cardiac maturation, but the mechanisms are poorly understood. To address this point, we blocked β-ARs or performed sympathectomy followed by rescue experiments with isoproterenol in neonatal mice. Our study identifies the β-AR stimuli as a maturation signal for cardiomyocytes and highlights the importance of this pathway in cardiac regeneration therapies.

Large animal models of pressure overload-induced cardiac left ventricular hypertrophy to study remodelling of the human heart with aortic stenosis

Pathologic cardiac hypertrophy is a common consequence of many cardiovascular diseases, including aortic stenosis (AS). AS is known to increase the pressure load of the left ventricle, causing a compensative response of the cardiac muscle, which progressively will lead to dilation and heart failure. At a cellular level, this corresponds to a considerable increase in the size of cardiomyocytes, known as cardiomyocyte hypertrophy, while their proliferation capacity is attenuated upon the first developmental stages. Cardiomyocytes, in order to cope with the increased workload (overload), suffer alterations in their morphology, nuclear content, energy metabolism, intracellular homeostatic mechanisms, contractile activity, and cell death mechanisms. Moreover, modifications in the cardiomyocyte niche, involving inflammation, immune infiltration, fibrosis, and angiogenesis, contribute to the subsequent events of a pathologic hypertrophic response. Considering the emerging need for a better understanding of the condition and treatment improvement, as the only available treatment option of AS consists of surgical interventions at a late stage of the disease, when the cardiac muscle state is irreversible, large animal models have been developed to mimic the human condition, to the greatest extend. Smaller animal models lack physiological, cellular and molecular mechanisms that sufficiently resemblance humans and in vitro techniques yet fail to provide adequate complexity. Animals, such as the ferret (Mustello purtorius furo), lapine (rabbit, Oryctolagus cunigulus), feline (cat, Felis catus), canine (dog, Canis lupus familiaris), ovine (sheep, Ovis aries), and porcine (pig, Sus scrofa), have contributed to research by elucidating implicated cellular and molecular mechanisms of the condition. Essential discoveries of each model are reported and discussed briefly in this review. Results of large animal experimentation could further be interpreted aiming at prevention of the disease progress or, alternatively, at regression of the implicated pathologic mechanisms to a physiologic state. This review summarizes the important aspects of the pathophysiology of LV hypertrophy and the applied surgical large animal models that currently better mimic the condition.

Decoding the Impact of the Hippo Pathway on Different Cell Types in Heart Failure

The evolutionarily conserved Hippo pathway plays a pivotal role in governing a variety of biological processes. Heart failure (HF) is a major global health problem with a significant risk of mortality. This review provides a contemporary understanding of the Hippo pathway in regulating different cell types during HF. Through a systematic analysis of each component's regulatory mechanisms within the Hippo pathway, we elucidate their specific effects on cardiomyocytes, fibroblasts, endothelial cells, and macrophages in response to various cardiac injuries. Insights gleaned from both in vitro and in vivo studies highlight the therapeutic promise of targeting the Hippo pathway to address cardiovascular diseases, particularly HF.

HOIL-1L deficiency induces cell cycle alteration which causes immaturity of skeletal muscle and cardiomyocytes

HOIL-1L deficiency was recently reported to be one of the causes of myopathy and dilated cardiomyopathy (DCM). However, the mechanisms by which myopathy and DCM develop have not been clearly elucidated. Here, we sought to elucidate these mechanisms using the murine myoblast cell line C2C12 and disease-specific human induced pluripotent stem cells (hiPSCs). Myotubes differentiated from HOIL-1L-KO C2C12 cells exhibited deteriorated differentiation and mitotic cell accumulation. CMs differentiated from patient-derived hiPSCs had an abnormal morphology with a larger size and were excessively multinucleated compared with CMs differentiated from control hiPSCs. Further analysis of hiPSC-derived CMs showed that HOIL-1L deficiency caused cell cycle alteration and mitotic cell accumulation. These results demonstrate that abnormal cell maturation possibly contribute to the development of myopathy and DCM. In conclusion, HOIL-1L is an important intrinsic regulator of cell cycle-related myotube and CM maturation and cell proliferation.

Polyploid tubular cells: a shortcut to stress adaptation

Tubular epithelial cells (TCs) compose the majority of kidney parenchyma and play fundamental roles in maintaining homeostasis. Like other tissues, mostly immature TC with progenitor capabilities are able to replace TC lost during injury via clonal expansion and differentiation. In contrast, differentiated TC lack this capacity. However, as the kidney is frequently exposed to toxic injuries, evolution positively selected a response program that endows differentiated TC to maintain residual kidney function during kidney injury. Recently, we and others have described polyploidization of differentiated TC, a mechanism to augment the function of remnant TC after injury by rapid hypertrophy. Polyploidy is a condition characterized by >2 complete sets of chromosomes. Polyploid cells often display an increased functional capacity and are generally more resilient to stress as evidenced by being conserved across many plants and eukaryote species from flies to mammals. Here, we discuss the occurrence of TC polyploidy in different contexts and conditions and how this integrates into existing concepts of kidney cell responses to injury. Collectively, we aim at stimulating the acquisition of novel knowledge in the kidney field as well as accelerating the translation of this basic response mechanism to the clinical sphere.

High-resolution single-cell transcriptomic survey of cardiomyocytes from patients with hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a common inherited cardiovascular disease, which can cause heart failure and lead to death. In this study, we performed high-resolution single-cell RNA-sequencing of 2115 individual cardiomyocytes obtained from HCM patients and normal controls. Signature up- and down-regulated genes in HCM were identified by integrative analysis across 37 patients and 41 controls from our data and published human single-cell and single-nucleus RNA-seq datasets, which were further classified into gene modules by single-cell co-expression analysis. Using our high-resolution dataset, we also investigated the heterogeneity among individual cardiomyocytes and revealed five distinct clusters within HCM cardiomyocytes. Interestingly, we showed that some extracellular matrix (ECM) genes were up-regulated in the HCM cardiomyocytes, suggesting that they play a role in cardiac remodelling. Taken together, our study comprehensively profiled the transcriptomic programs of HCM cardiomyocytes and provided insights into molecular mechanisms underlying the pathogenesis of HCM.

Efficient and reproducible generation of human iPSC-derived cardiomyocytes using a stirred bioreactor

In the last decade human iPSC-derived cardiomyocytes (hiPSC-CMs) proved to be valuable for cardiac disease modeling and cardiac regeneration, yet challenges with scale, quality, inter-batch consistency, and cryopreservation remain, reducing experimental reproducibility and limiting clinical translation. Here, we report a robust cardiac differentiation protocol that uses Wnt modulation and a stirred suspension bioreactor to produce on average 124 million hiPSC-CMs with >90% purity using a variety of hiPSC lines (19 differentiations; 10 iPSC lines). After controlled freeze and thaw, bioreactor-derived CMs (bCMs) showed high viability (>90%), interbatch reproducibility in cellular morphology, function, drug response and ventricular identity, which was further supported by single cell transcriptomes. bCMs on microcontact printed substrates revealed a higher degree of sarcomere maturation and viability during long-term culture compared to monolayer-derived CMs (mCMs). Moreover, functional investigation of bCMs in 3D engineered heart tissues showed earlier and stronger force production during long-term culture, and robust pacing capture up to 4 Hz when compared to mCMs. bCMs derived from this differentiation protocol will expand the applications of hiPSC-CMs by providing a reproducible, scalable, and resource efficient method to generate cardiac cells with well-characterized structural and functional properties superior to standard mCMs.

CD151 expression marks atrial- and ventricular- differentiation from human induced pluripotent stem cells

Current differentiation protocols for human induced pluripotent stem cells (hiPSCs) produce heterogeneous cardiomyocytes (CMs). Although chamber-specific CM selection using cell surface antigens enhances biomedical applications, a cell surface marker that accurately distinguishes between hiPSC-derived atrial CMs (ACMs) and ventricular CMs (VCMs) has not yet been identified. We have developed an approach for obtaining functional hiPSC-ACMs and -VCMs based on CD151 expression. For ACM differentiation, we found that ACMs are enriched in the CD151low population and that CD151 expression is correlated with the expression of Notch4 and its ligands. Furthermore, Notch signaling inhibition followed by selecting the CD151low population during atrial differentiation leads to the highly efficient generation of ACMs as evidenced by gene expression and electrophysiology. In contrast, for VCM differentiation, VCMs exhibiting a ventricular-related gene signature and uniform action potentials are enriched in the CD151high population. Our findings enable the production of high-quality ACMs and VCMs appropriate for hiPSC-derived chamber-specific disease models and other applications.

Spatiotemporal modulation of nitric oxide and Notch signaling by hemodynamic-responsive Trpv4 is essential for ventricle regeneration

Zebrafish have a remarkable ability to regenerate injured hearts. Altered hemodynamic forces after larval ventricle ablation activate the endocardial Klf2a-Notch signaling cascade to direct zebrafish cardiac regeneration. However, how the heart perceives blood flow changes and initiates signaling pathways promoting regeneration is not fully understood. The present study demonstrated that the mechanosensitive channel Trpv4 sensed the altered hemodynamic forces in injured hearts and its expression was regulated by blood flow. In addition to mediating the endocardial Klf2a-Notch signal cascade around the atrioventricular canal (AVC), we discovered that Trpv4 regulated nitric oxide (NO) signaling in the bulbus arteriosus (BA). Further experiments indicated that Notch signaling primarily acted at the early stage of regeneration, and the major role of NO signaling was at the late stage and through TGF-β pathway. Overall, our findings revealed that mechanosensitive channels perceived the changes in hemodynamics after ventricle injury, and provide novel insights into the temporal and spatial coordination of multiple signaling pathways regulating heart regeneration.

Perlecan (HSPG2) promotes structural, contractile, and metabolic development of human cardiomyocytes

Perlecan (HSPG2), a heparan sulfate proteoglycan similar to agrin, is key for extracellular matrix (ECM) maturation and stabilization. Although crucial for cardiac development, its role remains elusive. We show that perlecan expression increases as cardiomyocytes mature in vivo and during human pluripotent stem cell differentiation to cardiomyocytes (hPSC-CMs). Perlecan-haploinsuffient hPSCs (HSPG2+/-) differentiate efficiently, but late-stage CMs have structural, contractile, metabolic, and ECM gene dysregulation. In keeping with this, late-stage HSPG2+/- hPSC-CMs have immature features, including reduced ⍺-actinin expression and increased glycolytic metabolism and proliferation. Moreover, perlecan-haploinsuffient engineered heart tissues have reduced tissue thickness and force generation. Conversely, hPSC-CMs grown on a perlecan-peptide substrate are enlarged and display increased nucleation, typical of hypertrophic growth. Together, perlecan appears to play the opposite role of agrin, promoting cellular maturation rather than hyperplasia and proliferation. Perlecan signaling is likely mediated via its binding to the dystroglycan complex. Targeting perlecan-dependent signaling may help reverse the phenotypic switch common to heart failure.

Spatial multi-omics: novel tools to study the complexity of cardiovascular diseases

Spatial multi-omic studies have emerged as a promising approach to comprehensively analyze cells in tissues, enabling the joint analysis of multiple data modalities like transcriptome, epigenome, proteome, and metabolome in parallel or even the same tissue section. This review focuses on the recent advancements in spatial multi-omics technologies, including novel data modalities and computational approaches. We discuss the advancements in low-resolution and high-resolution spatial multi-omics methods which can resolve up to 10,000 of individual molecules at subcellular level. By applying and integrating these techniques, researchers have recently gained valuable insights into the molecular circuits and mechanisms which govern cell biology along the cardiovascular disease spectrum. We provide an overview of current data analysis approaches, with a focus on data integration of multi-omic datasets, highlighting strengths and weaknesses of various computational pipelines. These tools play a crucial role in analyzing and interpreting spatial multi-omics datasets, facilitating the discovery of new findings, and enhancing translational cardiovascular research. Despite nontrivial challenges, such as the need for standardization of experimental setups, data analysis, and improved computational tools, the application of spatial multi-omics holds tremendous potential in revolutionizing our understanding of human disease processes and the identification of novel biomarkers and therapeutic targets. Exciting opportunities lie ahead for the spatial multi-omics field and will likely contribute to the advancement of personalized medicine for cardiovascular diseases.

Profiling cardiomyocytes at single cell resolution reveals COX7B could be a potential target for attenuating heart failure in cardiac hypertrophy

Cardiac hypertrophy can develop to end-stage heart failure (HF), which inevitably leading to heart transplantation or death. Preserving cardiac function in cardiomyocytes (CMs) is essential for improving prognosis in hypertrophic cardiomyopathy (HCM) patients. Therefore, understanding transcriptomic heterogeneity of CMs in HCM would be indispensable to aid potential therapeutic targets investigation. We isolated primary CM from HCM patients who had extended septal myectomy, and obtained transcriptomes in 338 human primary CM with single-cell tagged reverse transcription (STRT-seq) approach. Our results revealed that CMs could be categorized into three subsets in nonfailing HCM heart: high energy synthesis cluster, high cellular metabolism cluster and intermediate cluster. The expression of electron transport chain (ETC) was up-regulated in larger-sized CMs from high energy synthesis cluster. Of note, we found the expression of Cytochrome c oxidase subunit 7B (COX7B), a subunit of Complex IV in ETC had trends of positively correlation with CMs size. Further, by assessing COX7B expression in HCM patients, we speculated that COX7B was compensatory up-regulated at early-stage but down-regulated in failing HCM heart. To test the hypothesis that COX7B might participate both in hypertrophy and HF progression, we used adeno associated virus 9 (AAV9) to mediate the expression of Cox7b in pressure overload-induced mice. Mice in vivo data supported that knockdown of Cox7b would accelerate HF and Cox7b overexpression could restore partial cardiac function in hypertrophy. Our result highlights targeting COX7B and preserving energy synthesis in hypertrophic CMs could be a promising translational direction for HF therapeutic strategy.

Single-cell morphological and transcriptome analysis unveil inhibitors of polyploid giant breast cancer cells in vitro

Considerable evidence suggests that breast cancer therapeutic resistance and relapse can be driven by polyploid giant cancer cells (PGCCs). The number of PGCCs increases with the stages of disease and therapeutic stress. Given the importance of PGCCs, it remains challenging to eradicate them. To discover effective anti-PGCC compounds, there is an unmet need to rapidly distinguish compounds that kill non-PGCCs, PGCCs, or both. Here, we establish a single-cell morphological analysis pipeline with a high throughput and great precision to characterize dynamics of individual cells. In this manner, we screen a library to identify promising compounds that inhibit all cancer cells or only PGCCs (e.g., regulators of HDAC, proteasome, and ferroptosis). Additionally, we perform scRNA-Seq to reveal altered cell cycle, metabolism, and ferroptosis sensitivity in breast PGCCs. The combination of single-cell morphological and molecular investigation reveals promising anti-PGCC strategies for breast cancer treatment and other malignancies.

The characteristics of proliferative cardiomyocytes in mammals

Better understanding of the mechanisms regulating the proliferation of pre-existing cardiomyocyte (CM) should lead to better options for regenerating injured myocardium. The absence of a perfect research model to definitively identify newly formed mammalian CMs is lacking. However, methodologies are being developed to identify and enrich proliferative CMs. These methods take advantages of the different proliferative states of CMs during postnatal development, before and after injury in the neonatal heart. New approaches use CMs labeled in lineage tracing animals or single cell technique-based CM clusters. This review aims to provide a timely update on the characteristics of the proliferative CMs, including their structural, functional, genetic, epigenetic and metabolic characteristics versus non-proliferative CMs. A better understanding of the characteristics of proliferative CMs should lead to the mechanisms for inducing endogenous CMs to self-renew, which is a promising therapeutic strategy to treat cardiac diseases that cause CM death in humans.

Whole-genome doubling in tissues and tumors

The overwhelming majority of proliferating somatic human cells are diploid, and this genomic state is typically maintained across successive cell divisions. However, failures in cell division can induce a whole-genome doubling (WGD) event, in which diploid cells transition to a tetraploid state. While some WGDs are developmentally programmed to produce nonproliferative tetraploid cells with specific cellular functions, unscheduled WGDs can be catastrophic: erroneously arising tetraploid cells are ill-equipped to cope with their doubled cellular and chromosomal content and quickly become genomically unstable and tumorigenic. Deciphering the genetics that underlie the genesis, physiology, and evolution of whole-genome doubled (WGD+) cells may therefore reveal therapeutic avenues to selectively eliminate pathological WGD+ cells.

A Fluorescent Reporter Mouse for In Vivo Assessment of Genome Editing with Diverse Cas Nucleases and Prime Editors

CRISPR-based genome-editing technologies, including nuclease editing, base editing, and prime editing, have recently revolutionized the development of therapeutics targeting disease-causing mutations. To advance the assessment and development of genome editing tools, a robust mouse model is valuable, particularly for evaluating in vivo activity and delivery strategies. In this study, we successfully generated a knock-in mouse line carrying the Traffic Light Reporter design known as TLR-multi-Cas variant 1 (TLR-MCV1). We comprehensively validated the functionality of this mouse model for both in vitro and in vivo nuclease and prime editing. The TLR-MCV1 reporter mouse represents a versatile and powerful tool for expediting the development of editing technologies and their therapeutic applications.

Quantitative Three-dimensional Label-free Digital Holographic Imaging of Cardiomyocyte Size, Ploidy, and Cell Division

Cardiac regeneration in newborn rodents depends on the ability of pre-existing cardiomyocytes to proliferate and divide. This capacity is lost within the first week of postnatal development when these cells rapidly switch from hyperplasia to hypertrophy, withdraw from the cell cycle, become binucleated, and increase in size. How these dynamic changes in size and ploidy impact cardiomyocyte proliferative potential is not well understood. In this study, we innovate the application of a commercially available digital holographic imaging microscope, the Holomonitor M4, to evaluate the proliferative responses of mononucleated diploid and binucleated tetraploid cardiomyocytes. This instrument coupled with the powerful Holomonitor App Suite software enables long-term label-free quantitative three-dimensional tracking of primary cardiomyocyte dynamics in real-time with single-cell resolution. Our digital holographic imaging results provide direct evidence that mononucleated cardiomyocytes retain significant proliferative potential as most can successfully divide with high frequency. In contrast, binucleated cardiomyocytes exhibit a blunted response to a proliferative stimulus with the majority not attempting to divide at all. Nevertheless, some binucleated cardiomyocytes were capable of complete division, suggesting that these cells still do retain limited proliferative capacity. By quantitatively tracking cardiomyocyte volume dynamics during these proliferative responses, we reveal that both mononucleated and binucleated cells reach a unique size threshold prior to attempted cell division. The absolute threshold is increased by binucleation, which may limit the ability of binucleated cardiomyocytes to divide. By defining the interrelationship between cardiomyocyte size, ploidy, and cell cycle control, we will better understand the cellular mechanisms that drive the loss of mammalian cardiac regenerative capacity after birth.

The consequences of tetraploidy on Caenorhabditis elegans physiology and sensitivity to chemotherapeutics

Polyploid cells contain more than two copies of each chromosome. Polyploidy has important roles in development, evolution, and tissue regeneration/repair, and can arise as a programmed polyploidization event or be triggered by stress. Cancer cells are often polyploid. C. elegans nematodes are typically diploid, but stressors such as heat shock and starvation can trigger the production of tetraploid offspring. In this study, we utilized a recently published protocol to generate stable tetraploid strains of C. elegans and compared their physiological traits and sensitivity to two DNA-damaging chemotherapeutic drugs, cisplatin and doxorubicin. As prior studies have shown, tetraploid worms are approximately 30% longer, shorter-lived, and have a smaller brood size than diploids. We investigated the reproductive defect further, determining that tetraploid worms have a shorter overall germline length, a higher rate of germ cell apoptosis, more aneuploidy in oocytes and offspring, and larger oocytes and embryos. We also found that tetraploid worms are modestly protected from growth delay from the chemotherapeutics but are similarly or more sensitive to reproductive toxicity. Transcriptomic analysis revealed differentially expressed pathways that may contribute to sensitivity to stress. This study reveals phenotypic consequences of whole-animal tetraploidy that make C. elegans an excellent model for ploidy differences.

Distinct mononuclear diploid cardiac subpopulation with minimal cell-cell communications persists in embryonic and adult mammalian heart

A small proportion of mononuclear diploid cardiomyocytes (MNDCMs), with regeneration potential, could persist in adult mammalian heart. However, the heterogeneity of MNDCMs and changes during development remains to be illuminated. To this end, 12 645 cardiac cells were generated from embryonic day 17.5 and postnatal days 2 and 8 mice by single-cell RNA sequencing. Three cardiac developmental paths were identified: two switching to cardiomyocytes (CM) maturation with close CM-fibroblast (FB) communications and one maintaining MNDCM status with least CM-FB communications. Proliferative MNDCMs having interactions with macrophages and non-proliferative MNDCMs (non-pMNDCMs) with minimal cell-cell communications were identified in the third path. The non-pMNDCMs possessed distinct properties: the lowest mitochondrial metabolisms, the highest glycolysis, and high expression of Myl4 and Tnni1. Single-nucleus RNA sequencing and immunohistochemical staining further proved that the Myl4+Tnni1+ MNDCMs persisted in embryonic and adult hearts. These MNDCMs were mapped to the heart by integrating the spatial and single-cell transcriptomic data. In conclusion, a novel non-pMNDCM subpopulation with minimal cell-cell communications was unveiled, highlighting the importance of microenvironment contribution to CM fate during maturation. These findings could improve the understanding of MNDCM heterogeneity and cardiac development, thus providing new clues for approaches to effective cardiac regeneration.

Effects of maternal hypothyroidism on postnatal cardiomyocyte proliferation and cardiac disease responses of the progeny

Maternal hypothyroidism (MH) could adversely affect the cardiac disease responses of the progeny. This study tested the hypothesis that MH reduces early postnatal cardiomyocyte (CM) proliferation so that the adult heart of MH progeny has a smaller number of larger cardiac myocytes, which imparts adverse cardiac disease responses following injury. Thyroidectomy (TX) was used to establish MH. The progeny from mice that underwent sham or TX surgery were termed Ctrl (control) or MH (maternal hypothyroidism) progeny, respectively. MH progeny had similar heart weight (HW) to body weight (BW) ratios and larger CM size consistent with fewer CMs at postnatal day 60 (P60) compared with Ctrl (control) progeny. MH progeny had lower numbers of EdU+, Ki67+, and phosphorylated histone H3 (PH3)+ CMs, which suggests they had a decreased CM proliferation in the postnatal timeframe. RNA-seq data showed that genes related to DNA replication were downregulated in P5 MH hearts, including bone morphogenetic protein 10 (Bmp10). Both in vivo and in vitro studies showed Bmp10 treatment increased CM proliferation. After transverse aortic constriction (TAC), the MH progeny had more severe cardiac pathological remodeling compared with the Ctrl progeny. Thyroid hormone (T4) treatment for MH mothers preserved their progeny's postnatal CM proliferation capacity and prevented excessive pathological remodeling after TAC. Our results suggest that CM proliferation during early postnatal development was significantly reduced in MH progeny, resulting in fewer CMs with hypertrophy in adulthood. These changes were associated with more severe cardiac disease responses after pressure overload.NEW & NOTEWORTHY Our study shows that compared with Ctrl (control) progeny, the adult progeny of mothers who have MH (MH progeny) had fewer CMs. This reduction of CM numbers was associated with decreased postnatal CM proliferation. Gene expression studies showed a reduced expression of Bmp10 in MH progeny. Bmp10 has been linked to myocyte proliferation. In vivo and in vitro studies showed that Bmp10 treatment of MH progeny and their myocytes could increase CM proliferation. Differences in CM number and size in adult hearts of MH progeny were linked to more severe cardiac structural and functional remodeling after pressure overload. T4 (synthetic thyroxine) treatment of MH mothers during their pregnancy, prevented the reduction in CM number in their progeny and the adverse response to disease stress.

Polyploid tubular cells initiate a TGF-β1 controlled loop that sustains polyploidization and fibrosis after acute kidney injury

Polyploidization of tubular cells (TC) is triggered by acute kidney injury (AKI) to allow survival in the early phase after AKI, but in the long run promotes fibrosis and AKI-chronic kidney disease (CKD) transition. The molecular mechanism governing the link between polyploid TC and kidney fibrosis remains to be clarified. In this study, we demonstrate that immediately after AKI, expression of cell cycle markers mostly identifies a population of DNA-damaged polyploid TC. Using transgenic mouse models and single-cell RNA sequencing we show that, unlike diploid TC, polyploid TC accumulate DNA damage and survive, eventually resting in the G1 phase of the cell cycle. In vivo and in vitro single-cell RNA sequencing along with sorting of polyploid TC shows that these cells acquire a profibrotic phenotype culminating in transforming growth factor (TGF)-β1 expression and that TGF-β1 directly promotes polyploidization. This demonstrates that TC polyploidization is a self-sustained mechanism. Interactome analysis by single-cell RNA sequencing revealed that TGF-β1 signaling fosters a reciprocal activation loop among polyploid TC, macrophages, and fibroblasts to sustain kidney fibrosis and promote CKD progression. Collectively, this study contributes to the ongoing revision of the paradigm of kidney tubule response to AKI, supporting the existence of a tubulointerstitial cross talk mediated by TGF-β1 signaling produced by polyploid TC following DNA damage.NEW & NOTEWORTHY Polyploidization in tubular epithelial cells has been neglected until recently. Here, we showed that polyploidization is a self-sustained mechanism that plays an important role during chronic kidney disease development, proving the existence of a cross talk between infiltrating cells and polyploid tubular cells. This study contributes to the ongoing revision of kidney adaptation to injury, posing polyploid tubular cells at the center of the process.

Conserved chamber-specific polyploidy maintains heart function in Drosophila

Developmentally programmed polyploidy (whole-genome duplication) of cardiomyocytes is common across evolution. Functions of such polyploidy are essentially unknown. Here, in both Drosophila larvae and human organ donors, we reveal distinct polyploidy levels in cardiac organ chambers. In Drosophila, differential growth and cell cycle signal sensitivity leads the heart chamber to reach a higher ploidy/cell size relative to the aorta chamber. Cardiac ploidy-reduced animals exhibit reduced heart chamber size, stroke volume and cardiac output, and acceleration of circulating hemocytes. These Drosophila phenotypes mimic human cardiomyopathies. Our results identify productive and likely conserved roles for polyploidy in cardiac chambers and suggest that precise ploidy levels sculpt many developing tissues. These findings of productive cardiomyocyte polyploidy impact efforts to block developmental polyploidy to improve heart injury recovery.

The origin, progress, and application of cell-based cardiac regeneration therapy

Cardiovascular disease (CVD) has become a severe threat to human health, with morbidity and mortality increasing yearly and gradually becoming younger. When the disease progresses to the middle and late stages, the loss of a large number of cardiomyocytes is irreparable to the body itself, and clinical drug therapy and mechanical support therapy cannot reverse the development of the disease. To explore the source of regenerated myocardium in model animals with the ability of heart regeneration through lineage tracing and other methods, and develop a new alternative therapy for CVDs, namely cell therapy. It directly compensates for cardiomyocyte proliferation through adult stem cell differentiation or cell reprogramming, which indirectly promotes cardiomyocyte proliferation through non-cardiomyocyte paracrine, to play a role in heart repair and regeneration. This review comprehensively summarizes the origin of newly generated cardiomyocytes, the research progress of cardiac regeneration based on cell therapy, the opportunity and development of cardiac regeneration in the context of bioengineering, and the clinical application of cell therapy in ischemic diseases.

RNA-Binding Proteins as Critical Post-Transcriptional Regulators of Cardiac Regeneration

Myocardial injury causes death to cardiomyocytes and leads to heart failure. The adult mammalian heart has very limited regenerative capacity. However, the heart from early postnatal mammals and from adult lower vertebrates can fully regenerate after apical resection or myocardial infarction. Thus, it is of particular interest to decipher the mechanism underlying cardiac regeneration that preserves heart structure and function. RNA-binding proteins, as key regulators of post-transcriptional gene expression to coordinate cell differentiation and maintain tissue homeostasis, display dynamic expression in fetal and adult hearts. Accumulating evidence has demonstrated their importance for the survival and proliferation of cardiomyocytes following neonatal and postnatal cardiac injury. Functional studies suggest that RNA-binding proteins relay damage-stimulated cell extrinsic or intrinsic signals to regulate heart regenerative capacity by reprogramming multiple molecular and cellular processes, such as global protein synthesis, metabolic changes, hypertrophic growth, and cellular plasticity. Since manipulating the activity of RNA-binding proteins can improve the formation of new cardiomyocytes and extend the window of the cardiac regenerative capacity in mammals, they are potential targets of therapeutic interventions for cardiovascular disease. This review discusses our evolving understanding of RNA-binding proteins in regulating cardiac repair and regeneration, with the aim to identify important open questions that merit further investigations.

Dynamics of Collagen Oxidation and Cross Linking in Regenerating and Irreversibly Infarcted Myocardium

In mammalian hearts myocardial infarction produces a permanent collagen-rich scar. Conversely, in zebrafish a collagen-rich scar forms but is completely resorbed as the myocardium regenerates. The formation of cross-links in collagen hinders its degradation but cross-linking has not been well characterized in zebrafish hearts. Here, a library of fluorescent probes to quantify collagen oxidation, the first step in collagen cross-link (CCL) formation, was developed. Myocardial injury in mice or zebrafish resulted in similar dynamics of collagen oxidation in the myocardium in the first month after injury. However, during this time, mature CCLs such as pyridinoline and deoxypyridinoline developed in the murine infarcts but not in the zebrafish hearts. High levels of newly oxidized collagen were still seen in murine scars with mature CCLs. These data suggest that fibrogenesis remains dynamic, even in mature scars, and that the absence of mature CCLs in zebrafish hearts may facilitate their ability to regenerate.

Cardiomyocyte Ploidy, Metabolic Reprogramming and Heart Repair

The adult heart is made up of cardiomyocytes (CMs) that maintain pump function but are unable to divide and form new myocytes in response to myocardial injury. In contrast, the developmental cardiac tissue is made up of proliferative CMs that regenerate injured myocardium. In mammals, CMs during development are diploid and mononucleated. In response to cardiac maturation, CMs undergo polyploidization and binucleation associated with CM functional changes. The transition from mononucleation to binucleation coincides with unique metabolic changes and shift in energy generation. Recent studies provide evidence that metabolic reprogramming promotes CM cell cycle reentry and changes in ploidy and nucleation state in the heart that together enhances cardiac structure and function after injury. This review summarizes current literature regarding changes in CM ploidy and nucleation during development, maturation and in response to cardiac injury. Importantly, how metabolism affects CM fate transition between mononucleation and binucleation and its impact on cell cycle progression, proliferation and ability to regenerate the heart will be discussed.

The consequences of tetraploidy on Caenorhabditis elegans physiology and sensitivity to chemotherapeutics

Polyploid cells contain more than two copies of each chromosome. Polyploidy has important roles in development, evolution, and tissue regeneration/repair, and can arise as a programmed polyploidization event or be triggered by stress. Cancer cells are often polyploid. C. elegans nematodes are typically diploid, but stressors such as heat shock and starvation can trigger the production of tetraploid offspring. In this study, we utilized a recently published protocol to generate stable tetraploid strains of C. elegans and compared their physiological traits and sensitivity to two DNA-damaging chemotherapeutic drugs, cisplatin and doxorubicin. As prior studies have shown, tetraploid worms are approximately 30% longer, shorter-lived, and have a smaller brood size than diploids. We investigated the reproductive defect further, determining that tetraploid worms have a shorter overall germline length, a higher rate of germ cell apoptosis, more aneuploidy in oocytes and offspring, and larger oocytes and embryos. We also found that tetraploid worms are modestly protected from growth delay from the chemotherapeutics but are similarly or more sensitive to reproductive toxicity. Transcriptomic analysis revealed differentially expressed pathways that may contribute to sensitivity to stress. Overall, this study reveals the phenotypic consequences of whole-animal tetraploidy in C. elegans.

Perfect duet: Dual recombinases improve genetic resolution

As a powerful genetic tool, site-specific recombinases (SSRs) have been widely used in genomic manipulation to elucidate cell fate plasticity in vivo, advancing research in stem cell and regeneration medicine. However, the low resolution of conventional single-recombinase-mediated lineage tracing strategies, which rely heavily on the specificity of one marker gene, has led to controversial conclusions in many scientific questions. Therefore, different SSRs systems are combined to improve the accuracy of lineage tracing. Here we review the recent advances in dual-recombinase-mediated genetic approaches, including the development of novel genetic recombination technologies and their applications in cell differentiation, proliferation, and genetic manipulation. In comparison with the single-recombinase system, we also discuss the advantages of dual-genetic strategies in solving scientific issues as well as their technical limitations.

Unlocking cardiomyocyte renewal potential for myocardial regeneration therapy

Cardiovascular disease remains the leading cause of mortality worldwide. Cardiomyocytes are irreversibly lost due to cardiac ischemia secondary to disease. This leads to increased cardiac fibrosis, poor contractility, cardiac hypertrophy, and subsequent life-threatening heart failure. Adult mammalian hearts exhibit notoriously low regenerative potential, further compounding the calamities described above. Neonatal mammalian hearts, on the other hand, display robust regenerative capacities. Lower vertebrates such as zebrafish and salamanders retain the ability to replenish lost cardiomyocytes throughout life. It is critical to understand the varying mechanisms that are responsible for these differences in cardiac regeneration across phylogeny and ontogeny. Adult mammalian cardiomyocyte cell cycle arrest and polyploidization have been proposed as major barriers to heart regeneration. Here we review current models about why adult mammalian cardiac regenerative potential is lost including changes in environmental oxygen levels, acquisition of endothermy, complex immune system development, and possible cancer risk tradeoffs. We also discuss recent progress and highlight conflicting reports pertaining to extrinsic and intrinsic signaling pathways that control cardiomyocyte proliferation and polyploidization in growth and regeneration. Uncovering the physiological brakes of cardiac regeneration could illuminate novel molecular targets and offer promising therapeutic strategies to treat heart failure.

A change of heart: understanding the mechanisms regulating cardiac proliferation and metabolism before and after birth

Mammalian cardiomyocytes undergo major maturational changes in preparation for birth and postnatal life. Immature cardiomyocytes contribute to cardiac growth via proliferation and thus the heart has the capacity to regenerate. To prepare for postnatal life, structural and metabolic changes associated with increased cardiac output and function must occur. This includes exit from the cell cycle, hypertrophic growth, mitochondrial maturation and sarcomeric protein isoform switching. However, these changes come at a price: the loss of cardiac regenerative capacity such that damage to the heart in postnatal life is permanent. This is a significant barrier to the development of new treatments for cardiac repair and contributes to heart failure. The transitional period of cardiomyocyte growth is a complex and multifaceted event. In this review, we focus on studies that have investigated this critical transition period as well as novel factors that may regulate and drive this process. We also discuss the potential use of new biomarkers for the detection of myocardial infarction and, in the broader sense, cardiovascular disease.

Electrophysiological Properties of Tetraploid Cardiomyocytes Derived from Murine Pluripotent Stem Cells Generated by Fusion of Adult Somatic Cells with Embryonic Stem Cells

Most cardiomyocytes (CMs) in the adult mammalian heart are either binucleated or contain a single polyploid nucleus. Recent studies have shown that polyploidy in CMs plays an important role as an adaptive response to physiological demands and environmental stress and correlates with poor cardiac regenerative ability after injury. However, knowledge about the functional properties of polyploid CMs is limited. In this study, we generated tetraploid pluripotent stem cells (PSCs) by fusion of murine embryonic stem cells (ESCs) and somatic cells isolated from bone marrow or spleen and performed a comparative analysis of the electrophysiological properties of tetraploid fusion-derived PSCs and diploid ESC-derived CMs. Fusion-derived PSCs exhibited characteristics of genuine ESCs and contained a near-tetraploid genome. Ploidy features and marker expression were also retained during the differentiation of fusion-derived cells. Fusion-derived PSCs gave rise to CMs, which were similar to their diploid ESC counterparts in terms of their expression of typical cardiospecific markers, sarcomeric organization, action potential parameters, response to pharmacologic stimulation with various drugs, and expression of functional ion channels. These results suggest that the state of ploidy does not significantly affect the structural and electrophysiological properties of murine PSC-derived CMs. These results extend our knowledge of the functional properties of polyploid CMs and contribute to a better understanding of their biological role in the adult heart.

Cellular Senescence, a Novel Area of Investigation for Metastatic Diseases

Metastasis is a systemic condition and the major challenge among cancer types, as it can lead to multiorgan vulnerability. Recently, attention has been drawn to cellular senescence, a complex stress response condition, as a factor implicated in metastatic dissemination and outgrowth. Here, we examine the current knowledge of the features required for cells to invade and colonize secondary organs and how senescent cells can contribute to this process. First, we describe the role of senescence in placentation, itself an invasive process which has been linked to higher rates of invasive cancers. Second, we describe how senescent cells can contribute to metastatic dissemination and colonization. Third, we discuss several metabolic adaptations by which senescent cells could promote cancer survival along the metastatic journey. In conclusion, we posit that targeting cellular senescence may have a potential therapeutic efficacy to limit metastasis formation.

Malat1 deficiency prevents neonatal heart regeneration by inducing cardiomyocyte binucleation

The adult mammalian heart has limited regenerative capacity, while the neonatal heart fully regenerates during the first week of life. Postnatal regeneration is mainly driven by proliferation of preexisting cardiomyocytes and supported by proregenerative macrophages and angiogenesis. Although the process of regeneration has been well studied in the neonatal mouse, the molecular mechanisms that define the switch between regenerative and nonregenerative cardiomyocytes are not well understood. Here, using in vivo and in vitro approaches, we identified the lncRNA Malat1 as a key player in postnatal cardiac regeneration. Malat1 deletion prevented heart regeneration in mice after myocardial infarction on postnatal day 3 associated with a decline in cardiomyocyte proliferation and reparative angiogenesis. Interestingly, Malat1 deficiency increased cardiomyocyte binucleation even in the absence of cardiac injury. Cardiomyocyte-specific deletion of Malat1 was sufficient to block regeneration, supporting a critical role of Malat1 in regulating cardiomyocyte proliferation and binucleation, a landmark of mature nonregenerative cardiomyocytes. In vitro, Malat1 deficiency induced binucleation and the expression of a maturation gene program. Finally, the loss of hnRNP U, an interaction partner of Malat1, induced similar features in vitro, suggesting that Malat1 regulates cardiomyocyte proliferation and binucleation by hnRNP U to control the regenerative window in the heart.

Ploidy-stratified single cardiomyocyte transcriptomics map Zinc Finger E-Box Binding Homeobox 1 to underly cardiomyocyte proliferation before birth

Whereas cardiomyocytes (CMs) in the fetal heart divide, postnatal CMs fail to undergo karyokinesis and/or cytokinesis and therefore become polyploid or binucleated, a key process in terminal CM differentiation. This switch from a diploid proliferative CM to a terminally differentiated polyploid CM remains an enigma and seems an obstacle for heart regeneration. Here, we set out to identify the transcriptional landscape of CMs around birth using single cell RNA sequencing (scRNA-seq) to predict transcription factors (TFs) involved in CM proliferation and terminal differentiation. To this end, we established an approach combining fluorescence activated cell sorting (FACS) with scRNA-seq of fixed CMs from developing (E16.5, P1, and P5) mouse hearts, and generated high-resolution single-cell transcriptomic maps of in vivo diploid and tetraploid CMs, increasing the CM resolution. We identified TF-networks regulating the G2/M phases of developing CMs around birth. ZEB1 (Zinc Finger E-Box Binding Homeobox 1), a hereto unknown TF in CM cell cycling, was found to regulate the highest number of cell cycle genes in cycling CMs at E16.5 but was downregulated around birth. CM ZEB1-knockdown reduced proliferation of E16.5 CMs, while ZEB1 overexpression at P0 after birth resulted in CM endoreplication. These data thus provide a ploidy stratified transcriptomic map of developing CMs and bring new insight to CM proliferation and endoreplication identifying ZEB1 as a key player in these processes.

Conserved Chamber-Specific Polyploidy Maintains Heart Function in Drosophila

Developmentally programmed polyploidy (whole-genome-duplication) of cardiomyocytes is common across evolution. Functions of such polyploidy are essentially unknown. Here, we reveal roles for precise polyploidy levels in cardiac tissue. We highlight a conserved asymmetry in polyploidy level between cardiac chambers in Drosophila larvae and humans. In Drosophila , differential Insulin Receptor (InR) sensitivity leads the heart chamber to reach a higher ploidy/cell size relative to the aorta chamber. Cardiac ploidy-reduced animals exhibit reduced heart chamber size, stroke volume, cardiac output, and acceleration of circulating hemocytes. These Drosophila phenotypes mimic systemic human heart failure. Using human donor hearts, we reveal asymmetry in nuclear volume (ploidy) and insulin signaling between the left ventricle and atrium. Our results identify productive and likely conserved roles for polyploidy in cardiac chambers and suggest precise ploidy levels sculpt many developing tissues. These findings of productive cardiomyocyte polyploidy impact efforts to block developmental polyploidy to improve heart injury recovery.

Gender Specificities of Cardiac Troponin Serum Levels: From Formation Mechanisms to the Diagnostic Role in Case of Acute Coronary Syndrome

Cardiac troponins T and I are the main (most sensitive and specific) laboratory indicators of myocardial cell damage. A combination of laboratory signs of myocardial cell damage (elevated levels of cardiac troponins T and I) with clinical (severe chest pain spreading to the left side of the human body) and functional (rise or depression of the ST segment, negative T wave or emergence of the Q wave according to electrocardiography and/or decrease in the contractility of myocardial areas exposed to ischemia according to echocardiography) signs of myocardial ischemia is indicative of the ischemic damage to cardiomyocytes, which is characteristic of the development of acute coronary syndrome (ACS). Today, with early diagnostic algorithms for ACS, doctors rely on the threshold levels of cardiac troponins (99th percentile) and on the dynamic changes in the serum levels over several hours (one, two, or three) from the moment of admission to the emergency department. That said, some recently approved highly sensitive methods for determining troponins T and I show variations in 99th percentile reference levels, depending on gender. To date, there are conflicting data on the role of gender specificities in the serum levels of cardiac troponins T and I in the diagnostics of ACS, and the specific mechanisms for the formation of gender differences in the serum levels of cardiac troponins T and I are unknown. The purpose of this article is to analyze the role of gender specificities in cardiac troponins T and I in the diagnostics of ACS, and to suggest the most likely mechanisms for the formation of differences in the serum levels of cardiac troponins in men and women.

CRISPR/Cas9 therapeutics: progress and prospects

Clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) gene-editing technology is the ideal tool of the future for treating diseases by permanently correcting deleterious base mutations or disrupting disease-causing genes with great precision and efficiency. A variety of efficient Cas9 variants and derivatives have been developed to cope with the complex genomic changes that occur during diseases. However, strategies to effectively deliver the CRISPR system to diseased cells in vivo are currently lacking, and nonviral vectors with target recognition functions may be the focus of future research. Pathological and physiological changes resulting from disease onset are expected to serve as identifying factors for targeted delivery or targets for gene editing. Diseases are both varied and complex, and the choice of appropriate gene-editing methods and delivery vectors for different diseases is important. Meanwhile, there are still many potential challenges identified when targeting delivery of CRISPR/Cas9 technology for disease treatment. This paper reviews the current developments in three aspects, namely, gene-editing type, delivery vector, and disease characteristics. Additionally, this paper summarizes successful examples of clinical trials and finally describes possible problems associated with current CRISPR applications.

Cardiac regeneration: Options for repairing the injured heart

Cardiac regeneration is one of the grand challenges in repairing injured human hearts. Numerous studies of signaling pathways and metabolism on cardiac development and disease pave the way for endogenous cardiomyocyte regeneration. New drug delivery approaches, high-throughput screening, as well as novel therapeutic compounds combined with gene editing will facilitate the development of potential cell-free therapeutics. In parallel, progress has been made in the field of cell-based therapies. Transplantation of human pluripotent stem cell (hPSC)-derived cardiomyocytes (hPSC-CMs) can partially rescue the myocardial defects caused by cardiomyocyte loss in large animals. In this review, we summarize current cell-based and cell-free regenerative therapies, discuss the importance of cardiomyocyte maturation in cardiac regenerative medicine, and envision new ways of regeneration for the injured heart.

Immune cells drive new immunomodulatory therapies for myocardial infarction: From basic to clinical translation

The high incidence of heart failure secondary to myocardial infarction (MI) has been difficult to effectively address. MI causes strong aseptic inflammation, and infiltration of different immune cells and changes in the local inflammatory microenvironment play a key regulatory role in ventricular remodeling. Therefore, the possibility of improving the prognosis of MI through targeted immunity has been of interest and importance in MI. However, previously developed immune-targeted therapies have not achieved significant success in clinical trials. Here, we propose that the search for therapeutic targets from different immune cells may be more precise and lead to better clinical translation. Specifically, this review summarizes the role and potential therapeutic targets of various immune cells in ventricular remodeling after MI, especially monocytes/macrophages and neutrophils, as a way to demonstrate the importance and potential of immunomodulatory therapies for MI. In addition, we analyze the reasons for the failure of previous immunomodulatory therapies and the issues that need to be addressed, as well as the prospects and targeting strategies of using immune cells to drive novel immunomodulatory therapies, hoping to advance the development of immunomodulatory therapies by providing evidence and new ideas.

Spermidine Alleviates Intrauterine Hypoxia-Induced Offspring Newborn Myocardial Mitochondrial Damage in Rats by Inhibiting Oxidative Stress and Regulating Mitochondrial Quality Control

Background

Intrauterine hypoxia (IUH) increases the risk of cardiovascular diseases in offspring. As a reactive oxygen species (ROS) scavenger, polyamine spermidine (SPD) is essential for embryonic and fetal survival and growth. However, further studies on the SPD protection and mechanisms for IUH-induced heart damage in offspring are required.

Objectives

This study aimed to investigate the preventive effects of prenatal SPD treatment on IUH-induced heart damage in newborn offspring rats and its underlying mitochondrial-related mechanism.

Methods

The rat model of IUH was established by exposure to 10% O₂ seven days before term. Meanwhile, for seven days, the pregnant rats were given SPD (5 mg.kg^-1.d^-1; ip). The one-day offspring rats were sacrificed to assess several parameters, including growth development, heart damage, cardiomyocytes proliferation, myocardial oxidative stress, cell apoptosis, and mitochondrial function, and have mitochondrial quality control (MQC), including mitophagy, mitochondrial biogenesis, and mitochondrial fusion/fission. In in vitro experiments, primary cardiomyocytes were subjected to hypoxia with or without SPD for 24 hours.

Results

IUH decreased body weight, heart weight, cardiac Ki67 expression, the activity of SOD, and the CAT and adenosine 5'-triphosphate (ATP) levels and increased the BAX/BCL2 expression, and TUNEL-positive nuclei numbers. Furthermore, IUH also caused mitochondrial structure abnormality, dysfunction, and decreased mitophagy (decreased number of mitophagosomes), declined mitochondrial biogenesis (decreased expression of SIRT-1, PGC-1α, NRF-2, and TFAM), and led to fission/fusion imbalance (increased percentage of mitochondrial fragments, increased DRP1 expression, and decreased MFN2 expression) in the myocardium. Surprisingly, SPD treatment normalized the variations in the IUH-induced parameters. Furthermore, SPD also prevented hypoxia-induced ROS accumulation, mitochondrial membrane potential decay, and the mitophagy decrease in cardiomyocytes.

Conclusion

Maternal SPD treatment caused IUH-induced heart damage in newborn offspring rats by improving the myocardial mitochondrial function via anti-oxidation and anti-apoptosis, and regulating MQC.

Skeletal Muscle Nuclei in Mice are not Post-mitotic

The skeletal muscle research field generally accepts that nuclei in skeletal muscle fibers (ie, myonuclei) are post-mitotic and unable to proliferate. Because our deuterium oxide (D2O) labeling studies showed DNA synthesis in skeletal muscle tissue, we hypothesized that resident myonuclei can replicate in vivo. To test this hypothesis, we used a mouse model that temporally labeled myonuclei with GFP followed by D2O labeling during normal cage activity, functional overload, and with satellite cell ablation. During normal cage activity, we observed deuterium enrichment into myonuclear DNA in 7 out of 7 plantaris (PLA), 6 out of 6 tibialis anterior (TA), 5 out of 7 gastrocnemius (GAST), and 7 out of 7 quadriceps (QUAD). The average fractional synthesis rates (FSR) of DNA in myonuclei were: 0.0202 ± 0.0093 in PLA, 0.0239 ± 0.0040 in TA, 0.0076 ± 0. 0058 in GAST, and 0.0138 ± 0.0039 in QUAD, while there was no replication in myonuclei from EDL. These FSR values were largely reproduced in the overload and satellite cell ablation conditions, although there were higher synthesis rates in the overloaded PLA muscle. We further provided evidence that myonuclear replication is through endoreplication, which results in polyploidy. These novel findings contradict the dogma that skeletal muscle nuclei are post-mitotic and open potential avenues to harness the intrinsic replicative ability of myonuclei for muscle maintenance and growth.

Avian cardiomyocyte architecture and what it reveals about the evolution of the vertebrate heart

Bird cardiomyocytes are long, thin and lack transverse (t)-tubules, which is akin to the cardiomyocyte morphology of ectothermic non-avian reptiles, who are typified by low maximum heart rates and low pressure development. However, birds can achieve greater contractile rates and developed pressures than mammals, whose wide cardiomyocytes contain a dense t-tubular network allowing for uniform excitation-contraction coupling and strong contractile force. To address this apparent paradox, this paper functionally links recent electrophysiological studies on bird cardiomyocytes with decades of ultrastructure measurements. It shows that it is the strong transsarcolemmal Ca2+ influx via the L-type Ca2+ current (ICaL) and the high gain of Ca2+-induced Ca2+ release from the sarcoplasmic reticulum (SR), coupled with an internal SR Ca2+ release relay system, that facilitates the strong fast contractions in the long thin bird cardiomyocytes, without the need for t-tubules. The maintenance of an elongated myocyte morphology following the post-hatch transition from ectothermy to endothermy in birds is discussed in relation to cardiac load, myocyte ploidy, and cardiac regeneration potential in adult cardiomyocytes. Overall, the paper shows how little we know about cellular Ca2+ dynamics in the bird heart and suggests how increased research efforts in this area would provide vital information in our quest to understand the role of myocyte architecture in the evolution of the vertebrate heart. This article is part of the theme issue 'The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease'. Please see glossary at the end of the paper for definitions of specialized terms.