Measuring cardiomyocyte cell-cycle activity and proliferation in the age of heart regeneration

Intronic RNAscope probes enable precise identification of cardiomyocyte nuclei and cell cycle activity

Cardiac regeneration studies have been plagued by technical challenges in unequivocally identifying cardiomyocyte (CM) nuclei in cardiac sections, crucial for accurate identification of cycling CMs. The use of antibodies to sarcomeric proteins is error-prone, the CM specificity of common nuclear markers is controversial, and utilizing genetically modified mouse models poses risk of inducing unintended cardiac phenotypes. The application of RNAscope intronic probes overcomes the above shortcomings. Intronic probes label intronic RNAs within nuclei and can therefore be utilized as a method for nuclear localization. A Tnnt2 intronic RNAscope probe highly colocalized with Obscurin-H2B-GFP in adult mouse hearts, demonstrating CM specificity. Studies in embryos demonstrated that the Tnnt2 intronic RNAscope probe labeled CM nuclei that had undergone DNA replication, and remained closely associated with CM chromatin at all stages of mitosis, even with nuclear envelope breakdown. The efficiency, accuracy, and perdurance of the Tnnt2 intronic RNAscope probe even with nuclear envelope breakdown facilitated reliable investigation of dynamics of DNA synthesis and potential mitoses in CMs in both border and infarct zones after myocardial infarction (MI). Furthermore, we designed Myl2 and Myl4 intronic RNAscope probes, which labeled ventricular and atrial CM nuclei, respectively, and may help identify CM subtypes generated in vitro.

Squamate ventricular cardiomyocytes: Ploidy, proliferation, and heart muscle cell size in the leopard gecko (Eublepharis macularius)

BACKGROUND

While heart function is broadly conserved across vertebrates, the cellular phenotype of muscle cells (cardiomyocytes) varies across taxa and throughout ontogeny. Emerging evidence suggests that some attributes may correlate with the capacity for spontaneous cardiomyocyte replacement following injury. For example, among non-regenerating taxa like adult mammals and birds, cardiomyocytes are polyploid, rarely proliferate, and are large in size. In contrast, in regeneration-competent zebrafish and amphibians, cardiomyocytes are diploid, spontaneously proliferate, and are comparatively small. For other species, less is known.

RESULTS

Here, we investigate these attributes in the squamate Eublepharis macularius, the leopard gecko. Using the nuclear counterstain DAPI to measure fluorescence intensity as a proxy for DNA content, we found that >90% of adult cardiomyocytes are diploid. Using serial histology and immunostaining for markers of DNA synthesis and mitosis, we determined that adult gecko cardiomyocytes spontaneously proliferate, albeit at significantly lower levels than previously reported in subadults. Furthermore, using wheat germ agglutinin, we found that the cross-sectional area is maintained across ontogeny and that gecko cardiomyocytes are 10× smaller than those of mice.

CONCLUSIONS

Taken together, our data show that gecko cardiomyocytes share several key cellular attributes with regeneration-competent species and that postnatal ventricular growth occurs via cardiomyocyte hyperplasia.

Potential of Proliferative Markers in Pancreatic Cancer Management: A Systematic Review

Background and Aims

Pancreatic cancer is an aggressive malignancy with poor prognosis and limited treatment options. Chemotherapy remains a primary therapeutic approach, but patient responses vary significantly, emphasizing the need for reliable biomarkers. This review explores the potential role of proliferative markers, including Ki-67, PCNA, Cyclin D1, and PHH3, as predictive and prognostic indicators in pancreatic cancer management, aiming to enhance personalized treatment strategies.

Methods

We conducted a narrative review by searching Scopus, PubMed, and Google Scholar for studies focusing on Ki-67, PCNA, Cyclin D1, and PHH3 in relation to pancreatic cancer and chemotherapy. The literature was reviewed to evaluate the role of these markers in predicting chemotherapy response, tumor progression, and overall patient survival.

Results

The review highlights the clinical significance of these markers. Ki-67 and PCNA are associated with cell proliferation, while Cyclin D1 regulates cell cycle progression and PHH3 is linked to mitotic activity. High expression levels of these markers often correlate with increased tumor aggressiveness and poorer patient outcomes. Moreover, they show promise in predicting chemotherapy response, which can inform tailored therapeutic strategies. However, challenges remain, including standardization of detection methods and determination of optimal cutoff values.

Conclusion

Proliferative markers such as Ki-67, PCNA, Cyclin D1, and PHH3 hold potential as predictive and prognostic tools in pancreatic cancer management. Their integration into clinical practice could improve the accuracy of treatment decisions and enhance patient outcomes. Further research and validation are necessary to overcome existing challenges and optimize their application in personalized oncology.

A Latent Cardiomyocyte Regeneration Potential in Human Heart Disease

BACKGROUND

Cardiomyocytes in the adult human heart show a regenerative capacity, with an annual renewal rate of ≈0.5%. Whether this regenerative capacity of human cardiomyocytes is employed in heart failure has been controversial.

METHODS

We determined cardiomyocyte renewal in 52 patients with advanced heart failure, 28 of whom received left ventricular assist device support. We measured the concentration of nuclear bomb test-derived 14C in cardiomyocyte genomic DNA and performed mathematical modeling to establish cardiomyocyte renewal in heart failure with and without LVAD unloading.

RESULTS

We show that cardiomyocyte generation is minimal in end-stage heart failure patients at rates 18 to 50× lower compared with the healthy heart. However, patients receiving left ventricle support device therapy, who showed significant functional and structural cardiac improvement, had a >6-fold increase in cardiomyocyte renewal relative to the healthy heart.

CONCLUSIONS

Our findings reveal a substantial cardiomyocyte regeneration potential in human heart disease, which could be exploited therapeutically.

Histone H2A.Z Deacetylation and Dedifferentiation in Infarcted/Tip60-depleted Cardiomyocytes

Myocardial infarction (MI) results in the loss of billions of cardiomyocytes (CMs), resulting in cardiac dysfunction. To re-muscularize injured myocardium, new CMs must be generated via renewed proliferation of surviving CMs. Approaches to induce proliferation of CMs after injury have been insufficient. Toward this end we are targeting the acetyltransferase Tip60, encoded by the Kat5 gene, based on the rationale that its pleiotropic functions combine to block CM proliferation at multiple checkpoints. We previously demonstrated that genetic depletion of Tip60 in a mouse model after MI reduces scarring, retains cardiac function, and activates the CM cell-cycle, although it remains unclear whether this culminates in the generation of daughter CMs. In order for pre-existing CMs in the adult heart to undergo proliferation, it has become accepted that they must first dedifferentiate, a process highlighted by loss of maturity, epithelial to mesenchymal transitioning (EMT), and reversion from fatty acid oxidation to glycolytic metabolism, accompanied by softening of the myocardial extracellular matrix (ECM). Based on recently published findings that Tip60 induces and maintains the differentiated state of hematopoietic stem cells and neurons via site-specific acetylation of the histone variant H2A.Z, we assessed levels of acetylated H2A.Z and dedifferentiation markers after depleting Tip60 in CMs post-MI. We report that genetic depletion of Tip60 from CMs after MI results in the near obliteration of acetylated H2A.Z in CM nuclei, accompanied by the altered expression of genes indicative of EMT induction, ECM softening, decreased fatty acid oxidation, and depressed expression of genes that regulate the TCA cycle. In accord with the possibility that site-specific acetylation of H2A.Z maintains adult CMs in a mature state of differentiation, CUT&Tag revealed enrichment of H2A.ZacK4/K7 in genetic motifs and in GO terms respectively associated with CM transcription factor binding and muscle development/differentiation. Along with our previous findings, these results support the notion that Tip60 has multiple targets in CMs that combine to maintain the differentiated state and prevent proliferation.

Polyphenol-Nanoengineered Monocyte Biohybrids for Targeted Cardiac Repair and Immunomodulation

Myocardial infarction is one of the leading cause of cardiovascular death worldwide. Invasive interventional procedures and medications are applied to attenuate the attacks associated with ischemic heart disease by reestablishing blood flow and restoring oxygen supply. However, the overactivation of inflammatory responses and unsatisfactory drug delivery efficiency in the infarcted regions prohibit functional improvement. Here, a nanoengineered monocyte (MO)-based biohybrid system, referred to as CTAs @MOs, for the heart-targeted delivery of combinational therapeutic agents (CTAs) containing anti-inflammatory IL-10 and cardiomyogenic miR-19a to overcome the limitation of malperfusion within the infarcted myocardium through a polyphenol-mediated interfacial assembly, is reported. Systemic administration of CTAs@MOs bypasses extensive thoracotomy and intramyocardial administration risks, leading to infarcted heart-specific accumulation and sustained release of therapeutic agents, enabling immunomodulation of the proinflammatory microenvironment and promoting cardiomyocyte proliferation in sequence. Moreover, CTAs@MOs, which serve as a cellular biohybrid-based therapy, significantly improve cardiac function as evidenced by enhanced ejection fractions, increased fractional shortening, and diminished infarct sizes. This polyphenol nanoengineered biohybrid system represents a general and potent platform for the efficient treatment of cardiovascular disorders.

CircRNA-RBAC induces cardiac repair by promoting ribosome biogenesis and cell cycle progression in cardiomyocytes

Ribosome biogenesis (RiBi) is an essential process that controls the protein synthesis rate, but its function in regulating endogenous cardiac regeneration remains unknown. Herein, we investigated the function and underlying mechanism of RiBi-associated circRNAs in cardiomyocyte (CM) proliferation and cardiac regeneration. We used high-throughput sequencing, quantitative PCR and in situ hybridization techniques to identify an adult downregulated circRNA, RiBi-associated circRNA (RBAC), in CMs. A functional study further revealed that RBAC overexpression increased ribosome biogenesis activity and cell cycle progression in CMs, while silencing RBAC decreased ribosome biogenesis activity and cell cycle progression. Moreover, RBAC overexpression induced adult CM proliferation and improved cardiac function after myocardial infarction in adult mice. Mechanistically, RBAC controlled ribosome biogenesis and cell proliferation by regulating the proteasome-dependent degradation of Ddx21, thereby altering the localization of Rps14 and reducing Rb expression. Our findings indicate that RBAC upregulation might be a plausible therapeutic strategy to induce endogenous cardiac regeneration.

A meta-analysis and systematic review of myocardial infarction-induced cardiomyocyte proliferation in adult mouse heart

BACKGROUND

The proliferation capacity of adult cardiomyocytes is very limited in the normal adult mammalian heart. Previous studies implied that cardiomyocyte proliferation increases after injury stimulation, but the result is controversial partly due to different methodologies. We aim to evaluate whether myocardial infarction (MI) stimulates cardiomyocyte proliferation in adult mice.

METHODS

A comprehensive literature search was conducted through PubMed/Medline, Embase, and Web of Science databases from 1 January 2000 to 21 December 2023. The SYRCLE's Risk of Bias tool for animal experiments was used to evaluate the quality of the literature by two independent reviewers. Twenty-six studies with cell cycle indicators (Ki67+, PH3+, BrdU/EdU+, and AurkB+) to evaluate cycling cardiomyocytes were collected for a meta-analysis. Another 10 studies with genetic reporter/tracing systems to evaluate cardiomyocyte proliferation were collected for a systematic review.

RESULTS

Evaluating cardiomyocyte proliferation by immunostaining of the cell cycle indicators on heart tissue, the meta-analysis showed that differences of Ki67+, PH3+, and BrdU/EdU+ cycling cardiomyocytes between MI and Sham groups were not statistically significant. In the post-MI heart, the percentages of PH3+, BrdU/EdU+, and AurkB+ cardiomyocytes were not significantly different between the infarct border zone and remote zone. The percentage of Ki67+ cardiomyocytes in the infarct border zone was statistically higher than that in the remote zone. Most of the studies (6 out of 10) using genetic reporter/tracing mouse systems showed that the difference in cardiomyocyte proliferation between MI and Sham groups was not statistically significant. Among the other 4 studies, at least 3 studies could not demonstrate that MI stimulates bona fide cardiomyocyte proliferation because of methodological shortages.

CONCLUSIONS

MI injury increases Ki67+ cycling adult mouse cardiomyocytes in infarct border zone. Very little overwhelming evidence shows that MI stimulates bona fide proliferation in the adult heart.

Cardiomyocyte proliferation and heart regeneration in adult Xenopus tropicalis evidenced by a transgenic reporter line

Cardiomyocyte proliferation in adult Xenopus tropicalis during heart regeneration has remained largely contentious due to the absence of genetic evidence. Here, we generated a transgenic reporter line Tg(mlc2:H2C) expressing mCherry specifically in cardiomyocyte nuclei driven by the promoter of myosin light chain 2 (mlc2). Using the reporter line, we found that traditional whole-cell staining is not a rigorous way to identify cardiomyocytes in adult Xenopus tropicalis when using a cryosection with common thickness (5 μm) which leading to a high error, but this deviation could be reduced by increasing section thickness. In addition, the reporter line confirmed that apex resection injury greatly increased the proliferation of mlc2+ cardiomyocytes at 3-30 days post-resection (dpr), thereby regenerating the lost cardiac muscle by 30 dpr in adult Xenopus tropicalis. Our findings from the reporter line have rigorously defined cardiomyocyte proliferation in adult heart upon injury, thereby contributing heart regeneration in adult Xenopus tropicalis.

Quantitative label-free digital holographic imaging of cardiomyocyte optical volume, nucleation, 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 cell size and nucleation 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 and binucleated cardiomyocytes after CHIR99021 treatment, a model proliferative stimulus. This system enables long-term label-free quantitative tracking of primary cardiomyocyte dynamics in real-time with single-cell resolution. Our results confirm that chemical inhibition of glycogen synthase kinase 3 with CHIR99021 promotes complete cell division of both mononucleated and binucleated cardiomyocytes with high frequency. Quantitative tracking of cardiomyocyte volume dynamics during these proliferative events revealed that both mononucleated and binucleated cardiomyocytes reach a similar size-increase threshold prior to attempted cell division. Binucleated cardiomyocytes attempt to divide with lower frequency than mononucleated cardiomyocytes, which may be associated with inadequate increases in cell size. By defining the interrelationship between cardiomyocyte size, nucleation, and cell cycle control, we may better understand the cellular mechanisms that drive the loss of mammalian cardiac regenerative capacity after birth.

Murine neonatal cardiac regeneration depends on Insulin-like growth factor 1 receptor signaling

Unlike adult mammals, the hearts of neonatal mice possess the ability to completely regenerate from myocardial infarction (MI). This observation has sparked vast interest in deciphering the potentially lifesaving and morbidity-reducing mechanisms involved in neonatal cardiac regeneration. In mice, the regenerative potential is lost within the first week of life and coincides with a reduction of Insulin-like growth factor 1 receptor (Igf1r) expression in the heart. Igf1r is a well-known regulator of cardiomyocyte maturation and proliferation in neonatal mice. To test the role of Igf1r as a pivotal factor in cardiac regeneration, we knocked down (KD) Igf1r specifically in cardiomyocytes using recombinant adeno-associated virus (rAAV) delivery and troponin T promotor driven shRNAmirs. Cardiomyocyte specific Igf1r KD versus control mice were subjected to experimental MI by permanent ligation of the left anterior descending artery (LAD). Cardiac functional and morphological data were analyzed over a 21-day period. Neonatal Igf1r KD mice showed reduced systolic cardiac function and increased fibrotic cardiac remodeling 21 days post injury. This cardiac phenotype was associated with reduced cardiomyocyte nuclei mitosis and decreased AKT and ERK phosphorylation in Igf1r KD, compared to control neonatal mouse hearts. Our in vivo murine data show that Igf1r KD shifts neonatal cardiac regeneration to a more adult-like scarring phenotype, identifying cardiomyocyte-specific Igf1r signaling as a crucial component of neonatal cardiac regeneration.

A broadly applicable method for quantifying cardiomyocyte cell division identifies proliferative events following myocardial infarction

Cardiomyocyte proliferation is a challenging metric to assess. Current methodologies have limitations in detecting the generation of new cardiomyocytes and technical challenges that reduce widespread applicability. Here, we describe an improved cell suspension and imaging-based methodology that can be broadly employed to assess cardiomyocyte cell division in standard laboratories across a multitude of model organisms and experimental conditions. We highlight additional metrics that can be gathered from the same cell preparations to enable additional relevant analyses to be performed. We incorporate additional antibody stains to address potential technical concerns of miscounting. Finally, we employ this methodology with a dual-thymidine analog-labeling approach to a post-infarction murine model, which allowed us to robustly identify unique cycling events, such as cardiomyocytes undergoing multiple rounds of cell division.

Runx1 is sufficient but not required for cardiomyocyte cell-cycle activation

Factors responsible for cardiomyocyte proliferation could serve as potential therapeutics to stimulate endogenous myocardial regeneration following insult, such as ischemic injury. A previously published forward genetics approach on cardiomyocyte cell cycle and ploidy led us to the transcription factor, Runx1. Here, we examine the effect of Runx1 on cardiomyocyte cell cycle during postnatal development and cardiac regeneration using cardiomyocyte-specific gain- and loss-of-function mouse models. RUNX1 is expressed in cardiomyocytes during early postnatal life, decreases to negligible levels by 3 wk of age, and increases upon myocardial injury, all consistent with observed rates of cardiomyocyte cell-cycle activity. Loss of Runx1 transiently stymied cardiomyocyte cell-cycle activity during normal postnatal development, a result that corrected itself and did not extend to the context of neonatal heart regeneration. On the other hand, cardiomyocyte-specific Runx1 overexpression resulted in an expansion of diploid cardiomyocytes in uninjured hearts and expansion of 4 N cardiomyocytes in the context of neonatal cardiac injury, suggesting Runx1 overexpression is sufficient to induce cardiomyocyte cell-cycle responses. Persistent overexpression of Runx1 for >1 mo continued to promote cardiomyocyte cell-cycle activity resulting in substantial hyperpolyploidization (≥8 N DNA content). This persistent cell-cycle activation was accompanied by ventricular dilation and adverse remodeling, raising the concern that continued cardiomyocyte cell cycling can have detrimental effects.NEW & NOTEWORTHY Runx1 is sufficient but not required for cardiomyocyte cell cycle.

Intraoperative characterization of cardiac tissue: the potential of light scattering spectroscopy

Significance

Damage to the cardiac conduction system remains one of the most significant risks associated with surgical interventions to correct congenital heart disease. This work demonstrates how light-scattering spectroscopy (LSS) can be used to non-destructively characterize cardiac tissue regions.

Aim

To present an approach for associating tissue composition information with location-specific LSS data and further evaluate an LSS and machine learning system as a method for non-destructive tissue characterization.

Approach

A custom LSS probe was used to gather spectral data from locations across 14 excised human pediatric nodal tissue samples (8 sinus nodes, 6 atrioventricular nodes). The LSS spectra were used to train linear and neural-network-based regressor models to predict tissue composition characteristics derived from the 3D models.

Results

Nodal tissue region nuclear densities were reported. A linear model trained to regress nuclear density from spectra achieved a prediction r-squared of 0.64 and a concordance correlation coefficient of 0.78.

Conclusions

These methods build on previous studies suggesting that LSS measurements combined with machine learning signal processing can provide clinically relevant cardiac tissue composition.

Targeting cardiomyocyte cell cycle regulation in heart failure

Heart failure continues to be a significant global health concern, causing substantial morbidity and mortality. The limited ability of the adult heart to regenerate has posed challenges in finding effective treatments for cardiac pathologies. While various medications and surgical interventions have been used to improve cardiac function, they are not able to address the extensive loss of functioning cardiomyocytes that occurs during cardiac injury. As a result, there is growing interest in understanding how the cell cycle is regulated and exploring the potential for stimulating cardiomyocyte proliferation as a means of promoting heart regeneration. This review aims to provide an overview of current knowledge on cell cycle regulation and mechanisms underlying cardiomyocyte proliferation in cases of heart failure, while also highlighting established and novel therapeutic strategies targeting this area for treatment purposes.

E3 Ubiquitin Ligase ASB14 Inhibits Cardiomyocyte Proliferation by Regulating MAPRE2 Ubiquitination

The ubiquitin proteasome system is a highly specific and selective protein regulatory system that plays an essential role in the regulation of the cell cycle. Despite its significance, the role of ubiquitination in cardiomyocyte proliferation remains largely unclear. This study aimed to investigate the potential impact of E3 ubiquitin ligase ASB14 (Ankyrin Repeat And SOCS Box Containing 14) on cardiac regeneration. We conducted a microarray analysis of apical resection ventricle tissues, and our findings revealed that ASB14 was down-regulated during the cardiac regenerative response. Subsequently, we examined the effect of ASB14 silencing on cardiomyocyte nuclear proliferation both in vitro and in vivo. Our results indicated that ASB14 silencing promoted cardiomyocyte nuclear proliferation, suggesting that ASB14 may play a role in regulating cardiac regeneration. To further investigate the potential therapeutic implications of ASB14 deficiency, we examined the cardiac function of mice with ASB14 deficiency in response to ischemic injury. Our findings showed that mice with ASB14 deficiency exhibited preserved cardiac function and a therapeutic effect in response to ischemic injury, which was attributed to the enhancement of cardiomyocyte nuclear proliferation. To elucidate the underlying mechanisms, we investigated the effect of ASB14 on microtubule-associated protein RP/EB family member 2 (MAPRE2) protein degradation. Our results indicated that the loss of ASB14 decreased the degradation of MAPRE2 protein, subsequently promoting cardiomyocyte nuclear proliferation and enhancing cardiac repair after myocardial infarction (MI). In conclusion, our study provides evidence that inhibition of ASB14-mediated MAPRE2 ubiquitination promotes cardiomyocyte nuclear proliferation, which may serve as a potential target for treating heart failure induced by MI injury.

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.

Drug Discovery for Adult Cardiomyocyte Regeneration: Opportunities and Challenges

Significance: Cardiovascular disease is a major contributor to human mortality and morbidity. The cardiac tissue undergoes fibrotic healing after injury because of the limited regenerative capacity of adult mammalian cardiomyocyte (CM). Extensive research has been performed to identify therapeutic targets for CM regeneration, as the success of promoting adult human CM regeneration to repair the injured heart is considered the Holy Grail in the field. Recent Advances: To date, more than 30 target genes have been shown to regulate adult mammalian CM proliferation. More than 20 targets have been validated in adult mouse myocardial infarction (MI) model in a therapeutic setting. In this review, the translational efficacy readouts from 17 selected pharmaceutical targets are summarized, among which the Hippo-yes-associated protein (Yap) pathway is the most extensively investigated and fits the criteria for a promising target for pro-CM-regeneration therapy development. Critical Issues and Future Directions: As the pro-CM-regeneration potential of current drug treatment for cardiovascular patients is limited, to help identify and fill the gap between basic research and drug discovery in this specific field, details regarding target identification, validation in mouse MI models, high-throughput screening assay development, and preclinical in vivo efficacy model optimization are discussed. Finally, suggestions and recommendations are also provided to help establish a common guideline for in vivo translational studies for drug discovery focusing on CM regeneration. Antioxid. Redox Signal. 39, 1070-1087.

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.

Tnni3k influences cardiomyocyte S-phase activity and proliferation

Cardiomyocyte proliferation is a difficult phenomenon to capture and prove. Here we employ a retrospective analysis of single cell ventricular suspensions to definitively identify cardiomyocytes that have completed cell division. Through this analysis we determined that the capacity of cardiomyocytes to re-enter the cell cycle and complete cell division after injury are separate and variable traits. Further, we provide evidence that Tnni3k definitively influences both early and final stages of the cell cycle.

A latent cardiomyocyte regeneration potential in human heart disease

Cardiomyocytes in the adult human heart show a regenerative capacity, with an annual renewal rate around 0.5%. Whether this regenerative capacity of human cardiomyocytes is employed in heart failure has been controversial. Using retrospective 14C birth dating we analyzed cardiomyocyte renewal in patients with end-stage heart failure. We show that cardiomyocyte generation is minimal in end-stage heart failure patients at rates 18-50 times lower compared to the healthy heart. However, patients receiving left ventricle support device therapy, who showed significant functional and structural cardiac improvement, had a >6-fold increase in cardiomyocyte renewal relative to the healthy heart. Our findings reveal a substantial cardiomyocyte regeneration potential in human heart disease, which could be exploited therapeutically.

Cardiomyocyte nuclear remodeling after mechanical unloading

Cardiomyocytes increase DNA content in response to stress in humans. DNA content is reported to decrease in association with increased markers of proliferation in cardiomyocytes following left ventricular assist device (LVAD) unloading. However, cardiac recovery resulting in LVAD explant is rare. Thus, we sought to test the hypothesis that changes in DNA content with mechanical unloading occurs independent of cardiomyocyte proliferation by quantifying cardiomyocyte nuclear number, cell size, DNA content, and the frequency of cell-cycling markers using a novel imaging flow cytometry methodology comparing human subjects undergoing LVAD implantation or primary transplantation. We found that cardiomyocyte size was 15% smaller in unloaded versus loaded samples without differences in the percentage of mono-, bi-, or multinuclear cells. DNA content per nucleus was significantly decreased in unloaded hearts versus loaded controls. Cell-cycle markers, Ki67 and phospho-histon3 (H3P), were not increased in unloaded samples. In conclusion, unloading of failing hearts is associated with decreased DNA content of nuclei independent of nucleation state within the cell. As these changes were associated with a trend to decreased cell size but not increased cell-cycle markers, they may represent a regression of hypertrophic nuclear remodeling and not proliferation.NEW & NOTEWORTHY Our data suggest that increases in DNA content that occur with cardiomyocyte hypertrophy in heart failure may reverse with mechanical unloading.

The Microenvironment of the Pathogenesis of Cardiac Hypertrophy

Pathological cardiac hypertrophy is a key risk factor for the development of heart failure and predisposes individuals to cardiac arrhythmia and sudden death. While physiological cardiac hypertrophy is adaptive, hypertrophy resulting from conditions comprising hypertension, aortic stenosis, or genetic mutations, such as hypertrophic cardiomyopathy, is maladaptive. Here, we highlight the essential role and reciprocal interactions involving both cardiomyocytes and non-myocardial cells in response to pathological conditions. Prolonged cardiovascular stress causes cardiomyocytes and non-myocardial cells to enter an activated state releasing numerous pro-hypertrophic, pro-fibrotic, and pro-inflammatory mediators such as vasoactive hormones, growth factors, and cytokines, i.e., commencing signaling events that collectively cause cardiac hypertrophy. Fibrotic remodeling is mediated by cardiac fibroblasts as the central players, but also endothelial cells and resident and infiltrating immune cells enhance these processes. Many of these hypertrophic mediators are now being integrated into computational models that provide system-level insights and will help to translate our knowledge into new pharmacological targets. This perspective article summarizes the last decades' advances in cardiac hypertrophy research and discusses the herein-involved complex myocardial microenvironment and signaling components.

Developmental Aspects of Cardiac Adaptation to Increased Workload

The heart is capable of extensive adaptive growth in response to the demands of the body. When the heart is confronted with an increased workload over a prolonged period, it tends to cope with the situation by increasing its muscle mass. The adaptive growth response of the cardiac muscle changes significantly during phylogenetic and ontogenetic development. Cold-blooded animals maintain the ability for cardiomyocyte proliferation even in adults. On the other hand, the extent of proliferation during ontogenetic development in warm-blooded species shows significant temporal limitations: whereas fetal and neonatal cardiac myocytes express proliferative potential (hyperplasia), after birth proliferation declines and the heart grows almost exclusively by hypertrophy. It is, therefore, understandable that the regulation of the cardiac growth response to the increased workload also differs significantly during development. The pressure overload (aortic constriction) induced in animals before the switch from hyperplastic to hypertrophic growth leads to a specific type of left ventricular hypertrophy which, in contrast with the same stimulus applied in adulthood, is characterized by hyperplasia of cardiomyocytes, capillary angiogenesis and biogenesis of collagenous structures, proportional to the growth of myocytes. These studies suggest that timing may be of crucial importance in neonatal cardiac interventions in humans: early definitive repairs of selected congenital heart disease may be more beneficial for the long-term results of surgical treatment.

Long-Term Transcriptomic Changes and Cardiomyocyte Hyperpolyploidy after Lactose Intolerance in Neonatal Rats

Many cardiovascular diseases originate from growth retardation, inflammation, and malnutrition during early postnatal development. The nature of this phenomenon is not completely understood. Here we aimed to verify the hypothesis that systemic inflammation triggered by neonatal lactose intolerance (NLI) may exert long-term pathologic effects on cardiac developmental programs and cardiomyocyte transcriptome regulation. Using the rat model of NLI triggered by lactase overloading with lactose and the methods of cytophotometry, image analysis, and mRNA-seq, we evaluated cardiomyocyte ploidy, signs of DNA damage, and NLI-associated long-term transcriptomic changes of genes and gene modules that differed qualitatively (i.e., were switched on or switched off) in the experiment vs. the control. Our data indicated that NLI triggers the long-term animal growth retardation, cardiomyocyte hyperpolyploidy, and extensive transcriptomic rearrangements. Many of these rearrangements are known as manifestations of heart pathologies, including DNA and telomere instability, inflammation, fibrosis, and reactivation of fetal gene program. Moreover, bioinformatic analysis identified possible causes of these pathologic traits, including the impaired signaling via thyroid hormone, calcium, and glutathione. We also found transcriptomic manifestations of increased cardiomyocyte polyploidy, such as the induction of gene modules related to open chromatin, e.g., "negative regulation of chromosome organization", "transcription" and "ribosome biogenesis". These findings suggest that ploidy-related epigenetic alterations acquired in the neonatal period permanently rewire gene regulatory networks and alter cardiomyocyte transcriptome. Here we provided first evidence indicating that NLI can be an important trigger of developmental programming of adult cardiovascular disease. The obtained results can help to develop preventive strategies for reducing the NLI-associated adverse effects of inflammation on the developing cardiovascular system.

Purkinje Cardiomyocytes of the Adult Ventricular Conduction System Are Highly Diploid but Not Uniquely Regenerative

Adult hearts are characterized by inefficient regeneration after injury, thus, the features that support or prevent cardiomyocyte (CM) proliferation are important to clarify. Diploid CMs are a candidate cell type that may have unique proliferative and regenerative competence, but no molecular markers are yet known that selectively identify all or subpopulations of diploid CMs. Here, using the conduction system expression marker Cntn2-GFP and the conduction system lineage marker Etv1CreERT2, we demonstrate that Purkinje CMs that comprise the adult ventricular conduction system are disproportionately diploid (33%, vs. 4% of bulk ventricular CMs). These, however, represent only a small proportion (3%) of the total diploid CM population. Using EdU incorporation during the first postnatal week, we demonstrate that bulk diploid CMs found in the later heart enter and complete the cell cycle during the neonatal period. In contrast, a significant fraction of conduction CMs persist as diploid cells from fetal life and avoid neonatal cell cycle activity. Despite their high degree of diploidy, the Purkinje lineage had no enhanced competence to support regeneration after adult heart infarction.

Cardiomyocyte ploidy is dynamic during postnatal development and varies across genetic backgrounds

Somatic polyploidization, an adaptation by which cells increase their DNA content to support growth, is observed in many cell types, including cardiomyocytes. Although polyploidization is believed to be beneficial, progression to a polyploid state is often accompanied by loss of proliferative capacity. Recent work suggests that genetics heavily influence cardiomyocyte ploidy. However, the developmental course by which cardiomyocytes reach their final ploidy state has only been investigated in select backgrounds. Here, we assessed cardiomyocyte number, cell cycle activity, and ploidy dynamics across two divergent mouse strains: C57BL/6J and A/J. Both strains are born and reach adulthood with comparable numbers of cardiomyocytes; however, the end composition of ploidy classes and developmental progression to reach the final state differ substantially. We expand on previous findings that identified Tnni3k as a mediator of cardiomyocyte ploidy and uncover a role for Runx1 in ploidy dynamics and cardiomyocyte cell division, in both developmental and injury contexts. These data provide novel insights into the developmental path to cardiomyocyte polyploidization and challenge the paradigm that hypertrophy is the sole mechanism for growth in the postnatal heart.

Gene Therapy for Cardiomyocyte Renewal: Cell Cycle, a Potential Therapeutic Target

Heart disease is the primary cause of death worldwide. Even though extensive research has been done, and many pharmacological and surgical treatments have been introduced to treat heart disease, the mortality rate still remains high. Gene therapy is widely used to understand molecular mechanisms of myocardial infarction and to treat cardiomyocyte loss. It was reported that adult cardiomyocytes proliferate at a very low rate; thus, targeting their proliferation has become a new regenerative therapeutic approach. Currently, re-activating cardiomyocyte proliferation appears to be one of the most promising methods to promote adult cardiomyocyte renewal. In this article, we highlight gene therapeutic targets of cell proliferation presently being pursued to re-activate the cell cycle of cardiomyocytes, including cell cycle regulators, transcription factors, microRNAs, signal transduction, and other contributing factors. We also summarize gene delivery vectors that have been used in cardiac research and major challenges to be overcome in the translation to the clinical approach and future directions.

Myocardial mesostructure and mesofunction

The complex and highly organized structural arrangement of some five billion cardiomyocytes directs the coordinated electrical activity and mechanical contraction of the human heart. The characteristic transmural change in cardiomyocyte orientation underlies base-to-apex shortening, circumferential shortening, and left ventricular torsion during contraction. Individual cardiomyocytes shorten ∼15% and increase in diameter ∼8%. Remarkably, however, the left ventricular wall thickens by up to 30-40%. To accommodate this, the myocardium must undergo significant structural rearrangement during contraction. At the mesoscale, collections of cardiomyocytes are organized into sheetlets, and sheetlet shear is the fundamental mechanism of rearrangement that produces wall thickening. Herein, we review the histological and physiological studies of myocardial mesostructure that have established the sheetlet shear model of wall thickening. Recent developments in tissue clearing techniques allow for imaging of whole hearts at the cellular scale, whereas magnetic resonance imaging (MRI) and computed tomography (CT) can image the myocardium at the mesoscale (100 µm to 1 mm) to resolve cardiomyocyte orientation and organization. Through histology, cardiac diffusion tensor imaging (DTI), and other modalities, mesostructural sheetlets have been confirmed in both animal and human hearts. Recent in vivo cardiac DTI methods have measured reorientation of sheetlets during the cardiac cycle. We also examine the role of pathological cardiac remodeling on sheetlet organization and reorientation, and the impact this has on ventricular function and dysfunction. We also review the unresolved mesostructural questions and challenges that may direct future work in the field.

Porcine Models of Heart Regeneration

Swine are popular large mammals for cardiac preclinical testing due to their similarities with humans in terms of organ size and physiology. Recent studies indicate an early neonatal regenerative capacity for swine hearts similar to small mammal laboratory models such as rodents, inspiring exciting possibilities for studying cardiac regeneration with the goal of improved clinical translation to humans. However, while swine hearts are anatomically similar to humans, fundamental differences exist in growth mechanisms, nucleation, and the maturation of pig cardiomyocytes, which could present difficulties for the translation of preclinical findings in swine to human therapeutics. In this review, we discuss the maturational dynamics of pig cardiomyocytes and their capacity for proliferative cardiac regeneration during early neonatal development to provide a perspective on swine as a preclinical model for developing cardiac gene- and cell-based regenerative therapeutics.