Role of Mononuclear Cardiomyocytes in Cardiac Turnover and Regeneration

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.

Distinct protein kinase C isoforms drive the cell cycle re-entry of two separate populations of neonatal rat ventricular cardiomyocytes

The present study tested the hypothesis that protein kinase C-α (PKC-α) recruitment in the presence of the p38α/β MAPK inhibitor SB203580 facilitated the appearance and cell cycle re-entry of nestin(+)-neonatal rat ventricular cardiomyocytes (NNVMs) and induced a transcript profile delineating a proliferative phenotype. Phorbol 12,13-dibutyrate (PDBu) treatment did not induce de novo nestin expression or increase the cell cycle re-entry of 1-day-old NNVMs but significantly increased runt-related transcription factor 1 (Runx1) and p16 cell cycle inhibitor (CDKN2a) mRNA levels and downregulated epithelial cell transforming 2 (ECT2) mRNA expression. SB203580 administration to PDBu-treated NNVMs induced de novo nestin expression, preferentially increased the density (normalized to 500 NNVMs) of nestin(+)-NNVMs that incorporated 5-bromo-2'-deoxyuridine (PDBu, 1.4 ± 3 vs. PDBu/SB203580, 128 ± 34; n = 5 independent litters), significantly inhibited CDKN2a and Runx1 mRNA upregulation and reversed ECT2 mRNA downregulation. PDBu treatment of NNVMs reduced PKC-α, protein kinase-δ (PKC-δ) and protein kinase-ε (PKC-ε) protein levels and GF109203X (conventional PKC isoform inhibitor) selectively attenuated PKC-α protein downregulation. GF109203X administration to PDBu/SB203580-treated NNVMs significantly reduced the density of nestin(+)-NNVMs that incorporated 5-bromo-2'-deoxyuridine (PDBu/SB203580/GF109203X, 40 ± 46; n = 5). Moreover, GF109203X/PDBu/SB203580 treatment unmasked the predominant appearance of a separate NNVM population that incorporated 5-bromo-2'-deoxyuridine (PDBu/SB203580/GF109203X, 192 ± 42; n = 5) delineated by the absence of de novo nestin expression. Sotrastaurin (conventional/novel PKC isoform inhibitor) administration to PDBu/SB203580-treated NNVMs significantly attenuated the density of nestin(+)-NNVMs (PDBu/SB203580/sotrastaurin, 8 ± 10; n = 4) and nestin(-)-NNVMs (PDBu/SB203580/sotrastaurin, 64 ± 30; n = 4) that incorporated 5-bromo-2'-deoxyuridine. These data reveal that the neonatal rat heart contains at least two separate populations of NNVMs that re-enter the cell cycle and the preferential appearance of nestin(+)- or nestin(-)-NNVMs is driven by distinct PKC isoforms in the presence of SB203580.NEW & NOTEWORTHY The appearance of nestin(+)-neonatal rat ventricular cardiomyocytes that re-entered the cell cycle following phorbol ester stimulation in the presence of p38α/β MAPK inhibitor SB203580 was associated with the inhibition of Runx1 and CDKN2a mRNA upregulation. PKC-α selectively induced the cell cycle re-entry of nestin(+)-neonatal rat ventricular cardiomyocytes. Pharmacological inhibition of PKC-α with concomitant p38α/β MAPK suppression unmasked the cell cycle re-entry of a second population of neonatal rat ventricular cardiomyocytes in the absence of nestin expression.

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.

Observation on the effect of intraoperative risk management combined with comfort nursing in gynecological laparoscopic surgery

To explore the effect of intraoperative risk management combined with comfort nursing in gynecological laparoscopic surgery. A retrospective analysis of 384 cases of laparoscopic surgery subjects admitted from February 2019 to June 2022 in the department of gynecology in our hospital were performed and divided into the control group (n = 192) and observation group (n = 192) according to the difference care that they received. Comfort nursing was taken in the control group, and intraoperative risk management combined with comfort nursing was given in the observation group. The coagulation indexes (activated partial thromboplastin time, prothrombin time, and thrombin time), immune inflammation indexes (monocytes and neutrophils), the incidence of lower limb deep vein thrombosis (DVT) and the satisfaction of nursing care were compared between the 2 groups. After care, activated partial thromboplastin time, prothrombin time, and thrombin time in the observation group were significantly longer than those in the control group, and the counts of monocytes and neutrophils in the observation group were significantly lower than those in the control group (P < .05). The incidence of DVT in the observation group was notablely lower than those in the control group (P = .008). The nursing satisfaction in the observation group was notablely higher than those in the control group (P < .001). Intraoperative risk management combined with comfort nursing intervention can improve the level of coagulation indicators and immune inflammation indicators after gynecological laparoscopic surgery, reduce the incidence of DVT, and improve subject nursing satisfaction.

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.

Systemic Hypoxemia Induces Cardiomyocyte Hypertrophy and Right Ventricular Specific Induction of Proliferation

BACKGROUND

A recent study suggests that systemic hypoxemia in adult male mice can induce cardiac myocytes to proliferate. The goal of the present experiments was to confirm these results, provide new insights on the mechanisms that induce adult cardiomyocyte cell cycle reentry, and to determine if hypoxemia also induces cardiomyocyte proliferation in female mice.

METHODS

EdU-containing mini pumps were implanted in 3-month-old, male and female C57BL/6 mice. Mice were placed in a hypoxia chamber, and the oxygen was lowered by 1% every day for 14 days to reach 7% oxygen. The animals remained in 7% oxygen for 2 weeks before terminal studies. Myocyte proliferation was also studied with a mosaic analysis with double markers mouse model.

RESULTS

Hypoxia induced cardiac hypertrophy in both left ventricular (LV) and right ventricular (RV) myocytes, with LV myocytes lengthening and RV myocytes widening and lengthening. Hypoxia induced an increase (0.01±0.01% in normoxia to 0.11±0.09% in hypoxia) in the number of EdU+ RV cardiomyocytes, with no effect on LV myocytes in male C57BL/6 mice. Similar results were observed in female mice. Furthermore, in mosaic analysis with double markers mice, hypoxia induced a significant increase in RV myocyte proliferation (0.03±0.03% in normoxia to 0.32±0.15% in hypoxia of RFP+ myocytes), with no significant change in LV myocyte proliferation. RNA sequencing showed upregulation of mitotic cell cycle genes and a downregulation of Cullin genes, which promote the G1 to S phase transition in hypoxic mice. There was significant proliferation of nonmyocytes and mild cardiac fibrosis in hypoxic mice that did not disrupt cardiac function. Male and female mice exhibited similar gene expression following hypoxia.

CONCLUSIONS

Systemic hypoxia induces a global hypertrophic stress response that was associated with increased RV proliferation, and while LV myocytes did not show increased proliferation, our results minimally confirm previous reports that hypoxia can induce cardiomyocyte cell cycle activity in vivo.

The multifaceted nature of endogenous cardiac regeneration

Since the first evidence of cardiac regeneration was observed, almost 50 years ago, more studies have highlighted the endogenous regenerative abilities of several models following cardiac injury. In particular, analysis of cardiac regeneration in zebrafish and neonatal mice has uncovered numerous mechanisms involved in the regenerative process. It is now apparent that cardiac regeneration is not simply achieved by inducing cardiomyocytes to proliferate but requires a multifaceted response involving numerous different cell types, signaling pathways and mechanisms which must all work in harmony in order for regeneration to occur. In this review we will endeavor to highlight a variety of processes that have been identifed as being essential for cardiac regeneration.

Mitochondrial disorder and treatment of ischemic cardiomyopathy: Potential and advantages of Chinese herbal medicine

Mitochondrial dysfunction is the main cause of damage to the pathological mechanism of ischemic cardiomyopathy. In addition, mitochondrial dysfunction can also affect the homeostasis of cardiomyocytes or endothelial cell dysfunction, leading to a vicious cycle of mitochondrial oxidative stress. And mitochondrial dysfunction is also an important pathological basis for ischemic cardiomyopathy and reperfusion injury after myocardial infarction or end-stage coronary heart disease. Therefore, mitochondria can be used as therapeutic targets against myocardial ischemia injury, and the regulation of mitochondrial morphology, function and structure is a key and important way of targeting mitochondrial quality control therapeutic mechanisms. Mitochondrial quality control includes mechanisms such as mitophagy, mitochondrial dynamics (mitochondrial fusion/fission), mitochondrial biosynthesis, and mitochondrial unfolded protein responses. Among them, the increase of mitochondrial fragmentation caused by mitochondrial pathological fission is the initial factor. The protective mitochondrial fusion can strengthen the interaction and synthesis of paired mitochondria and promote mitochondrial biosynthesis. In ischemia or hypoxia, pathological mitochondrial fission can promote the formation of mitochondrial fragments, fragmented mitochondria can lead to damaged mitochondrial DNA production, which can lead to mitochondrial biosynthesis dysfunction, insufficient mitochondrial ATP production, and mitochondrial ROS. Burst growth or loss of mitochondrial membrane potential. This eventually leads to the accumulation of damaged mitochondria. Then, under the leadership of mitophagy, damaged mitochondria can complete the mitochondrial degradation process through mitophagy, and transport the morphologically and structurally damaged mitochondria to lysosomes for degradation. But once the pathological mitochondrial fission increases, the damaged mitochondria increases, which may activate the pathway of cardiomyocyte death. Although laboratory studies have found that a variety of mitochondrial-targeted drugs can reduce myocardial ischemia and protect cardiomyocytes, there are still few drugs that have successfully passed clinical trials. In this review, we describe the role of MQS in ischemia/hypoxia-induced cardiomyocyte physiopathology and elucidate the relevant mechanisms of mitochondrial dysfunction in ischemic cardiomyopathy. In addition, we also further explained the advantages of natural products in improving mitochondrial dysfunction and protecting myocardial cells from the perspective of pharmacological mechanism, and explained its related mechanisms. Potential targeted therapies that can be used to improve MQS under ischemia/hypoxia are discussed, aiming to accelerate the development of cardioprotective drugs targeting mitochondrial dysfunction.

Proliferation and Maturation: Janus and the Art of Cardiac Tissue Engineering

During cardiac development and morphogenesis, cardiac progenitor cells differentiate into cardiomyocytes that expand in number and size to generate the fully formed heart. Much is known about the factors that regulate initial differentiation of cardiomyocytes, and there is ongoing research to identify how these fetal and immature cardiomyocytes develop into fully functioning, mature cells. Accumulating evidence indicates that maturation limits proliferation and conversely proliferation occurs rarely in cardiomyocytes of the adult myocardium. We term this oppositional interplay the proliferation-maturation dichotomy. Here we review the factors that are involved in this interplay and discuss how a better understanding of the proliferation-maturation dichotomy could advance the utility of human induced pluripotent stem cell-derived cardiomyocytes for modeling in 3-dimensional engineered cardiac tissues to obtain truly adult-level function.

Exercise, programmed cell death and exhaustion of cardiomyocyte proliferation in aging zebrafish

Exercise may ameliorate the eventual heart failure inherent in human aging. In this study, we use zebrafish to understand how aging and exercise affect cardiomyocyte turnover and myocardial remodelling. We show that cardiomyocyte proliferation remains constant throughout life but that onset of fibrosis is associated with a late increase in apoptosis. These findings correlate with decreases in voluntary swimming activity, critical swimming speed (Ucrit), and increases in biomarkers of cardiac insufficiency. The ability to respond to severe physiological stress is also impaired with age. Although young adult fish respond with robust cardiomyocyte proliferation in response to enforced swimming, this is dramatically impaired in older fish and served by a smaller proliferation-competent cardiomyocyte population. Finally, we show that these aging responses can be improved through increased activity throughout adulthood. However, despite improvement in Ucrit and the proliferative response to stress, the size of the proliferating cardiomyocyte population remained unchanged. The zebrafish heart models human aging and reveals the important trade-off between preserving cardiovascular fitness through exercise at the expense of accelerated fibrotic change.

Proximity to injury, but neither number of nuclei nor ploidy define pathological adaptation and plasticity in cardiomyocytes

The adult mammalian heart consists of mononuclear and binuclear cardiomyocytes (CMs) with various ploidies. However, it remains unclear whether a variation in ploidy or number of nuclei is associated with distinct functions and injury responses in CMs, including regeneration. Therefore, we investigated transcriptomes and cellular as well as nuclear features of mononucleated and binucleated CMs in adult mouse hearts with and without injury. To be able to identify the role of ploidy we analyzed control and failing human ventricular CMs because human CMs show a larger and disease-sensitive degree of polyploidization. Using transgenic Myh6-H2BmCh to identify mononucleated and binucleated mouse CMs, we found that cellular volume and RNA content were similar in both. On average nuclei of mononuclear CMs showed a 2-fold higher ploidy, as compared to binuclear CMs indicating that most mononuclear CMs are tetraploid. After myocardial infarction mononucleated and binucleated CMs in the border zone of the lesion responded with hypertrophy and corresponding changes in gene expression, as well as a low level of induction of cell cycle gene expression. Human CMs allowed us to study a wide range of polyploidy spanning from 2n to 16n. Notably, basal as well as pathological gene expression signatures and programs in failing CMs proved to be independent of ploidy. In summary, gene expression profiles were induced in proximity to injury, but independent of number of nuclei or ploidy levels in CMs.