Lamin B2 Levels Regulate Polyploidization of Cardiomyocyte Nuclei and Myocardial Regeneration

Use of stable isotope-tagged thymidine and multi-isotope imaging mass spectrometry (MIMS) for quantification of human cardiomyocyte division

Quantification of cellular proliferation in humans is important for understanding biology and responses to injury and disease. However, existing methods require administration of tracers that cannot be ethically administered in humans. We present a protocol for the direct quantification of cellular proliferation in human hearts. The protocol involves administration of non-radioactive, non-toxic stable isotope 15Nitrogen-enriched thymidine (15N-thymidine), which is incorporated into DNA during S-phase, in infants with tetralogy of Fallot, a common form of congenital heart disease. Infants with tetralogy of Fallot undergo surgical repair, which requires the removal of pieces of myocardium that would otherwise be discarded. This protocol allows for the quantification of cardiomyocyte proliferation in this discarded tissue. We quantitatively analyzed the incorporation of 15N-thymidine with multi-isotope imaging spectrometry (MIMS) at a sub-nuclear resolution, which we combined with correlative confocal microscopy to quantify formation of binucleated cardiomyocytes and cardiomyocytes with polyploid nuclei. The entire protocol spans 3-8 months, which is dependent on the timing of surgical repair, and 3-4.5 researcher days. This protocol could be adapted to study cellular proliferation in a variety of human tissues.

A Roadmap to Heart Regeneration Through Conserved Mechanisms in Zebrafish and Mammals

PURPOSE OF REVIEW

The replenishment of lost or damaged myocardium has the potential to reverse heart failure, making heart regeneration a goal for cardiovascular medicine. Unlike adult mammals, injury to the zebrafish or neonatal mouse heart induces a robust regenerative program with minimal scarring. Recent insights into the cellular and molecular mechanisms of heart regeneration suggest that the machinery for regeneration is conserved from zebrafish to mammals. Here, we will review conserved mechanisms of heart regeneration and their translational implications.

RECENT FINDINGS

Based on studies in zebrafish and neonatal mice, cardiomyocyte proliferation has emerged as a primary strategy for effecting regeneration in the adult mammalian heart. Recent work has revealed pathways for stimulating cardiomyocyte cell cycle reentry; potential developmental barriers for cardiomyocyte proliferation; and the critical role of additional cell types to support heart regeneration. Studies in zebrafish and neonatal mice have established a template for heart regeneration. Continued comparative work has the potential to inform the translation of regenerative biology into therapeutics.

Vertebrate cardiac regeneration: evolutionary and developmental perspectives

Cardiac regeneration is an ancestral trait in vertebrates that is lost both as more recent vertebrate lineages evolved to adapt to new environments and selective pressures, and as members of certain species developmentally progress towards their adult forms. While higher vertebrates like humans and rodents resolve cardiac injury with permanent fibrosis and loss of cardiac output as adults, neonates of these same species can fully regenerate heart structure and function after injury - as can adult lower vertebrates like many teleost fish and urodele amphibians. Recent research has elucidated several broad factors hypothesized to contribute to this loss of cardiac regenerative potential both evolutionarily and developmentally: an oxygen-rich environment, vertebrate thermogenesis, a complex adaptive immune system, and cancer risk trade-offs. In this review, we discuss the evidence for these hypotheses as well as the cellular participators and molecular regulators by which they act to govern heart regeneration in vertebrates.

Changes in Cardiomyocyte Cell Cycle and Hypertrophic Growth During Fetal to Adult in Mammals

The failure of adult cardiomyocytes to reproduce themselves to repair an injury results in the development of severe cardiac disability leading to death in many cases. The quest for an understanding of the inability of cardiac myocytes to repair an injury has been ongoing for decades with the identification of various factors which have a temporary effect on cell‐cycle activity. Fetal cardiac myocytes are continuously replicating until the time that the developing fetus reaches a stage of maturity sufficient for postnatal life around the time of birth. Recent reports of the ability for early neonatal mice and pigs to completely repair after the severe injury has stimulated further study of the regulators of the cardiomyocyte cell cycle to promote replication for the remuscularization of injured heart. In all mammals just before or after birth, single‐nucleated hyperplastically growing cardiomyocytes, 1X2N, undergo ≥1 additional DNA replications not followed by cytokinesis, resulting in cells with ≥2 nuclei or as in primates, multiple DNA replications (polyploidy) of 1 nucleus, 2X2(+)N or 1X4(+)N. All further growth of the heart is attributable to hypertrophy of cardiomyocytes. Animal studies ranging from zebrafish with 100% 1X2N cells in the adult to some strains of mice with up to 98% 2X2N cells in the adult and other species with variable ratios of 1X2N and 2X2N cells are reviewed relative to the time of conversion. Various structural, physiologic, metabolic, genetic, hormonal, oxygenation, and other factors that play a key role in the inability of post‐neonatal and adult myocytes to undergo additional cytokinesis are also reviewed.

Communal living: the role of polyploidy and syncytia in tissue biology

Multicellular organisms are composed of tissues with diverse cell sizes. Whether a tissue primarily consists of numerous, small cells as opposed to fewer, large cells can impact tissue development and function. The addition of nuclear genome copies within a common cytoplasm is a recurring strategy to manipulate cellular size within a tissue. Cells with more than two genomes can exist transiently, such as in developing germlines or embryos, or can be part of mature somatic tissues. Such nuclear collectives span multiple levels of organization, from mononuclear or binuclear polyploid cells to highly multinucleate structures known as syncytia. Here, we review the diversity of polyploid and syncytial tissues found throughout nature. We summarize current literature concerning tissue construction through syncytia and/or polyploidy and speculate why one or both strategies are advantageous.

InteractomeSeq: a web server for the identification and profiling of domains and epitopes from phage display and next generation sequencing data

High-Throughput Sequencing technologies are transforming many research fields, including the analysis of phage display libraries. The phage display technology coupled with deep sequencing was introduced more than a decade ago and holds the potential to circumvent the traditional laborious picking and testing of individual phage rescued clones. However, from a bioinformatics point of view, the analysis of this kind of data was always performed by adapting tools designed for other purposes, thus not considering the noise background typical of the 'interactome sequencing' approach and the heterogeneity of the data. InteractomeSeq is a web server allowing data analysis of protein domains ('domainome') or epitopes ('epitome') from either Eukaryotic or Prokaryotic genomic phage libraries generated and selected by following an Interactome sequencing approach. InteractomeSeq allows users to upload raw sequencing data and to obtain an accurate characterization of domainome/epitome profiles after setting the parameters required to tune the analysis. The release of this tool is relevant for the scientific and clinical community, because InteractomeSeq will fill an existing gap in the field of large-scale biomarkers profiling, reverse vaccinology, and structural/functional studies, thus contributing essential information for gene annotation or antigen identification. InteractomeSeq is freely available at https://InteractomeSeq.ba.itb.cnr.it/.

Adult Cardiomyocyte Proliferation: a New Insight for Myocardial Infarction Therapy

Myocardial infarction leads to cardiomyocyte loss, ensuing ventricular pathological remodeling, dramatic impairment of cardiac function, and ultimately heart failure. Unfortunately, the existing therapeutical treatments cannot directly replenish the lost myocytes in the injured myocardium and the long-term prognosis of heart failure after myocardial infarction remains poor. Growing investigations have demonstrated that the adult mammalian cardiomyocytes possess very limited proliferation capacity, and that was not enough to restore the injured heart. Recently, many studies were targeting to promote cardiomyocyte proliferation via inducing cardiomyocyte cell cycle re-entry for cardiac repair after myocardial infarction. Indeed, these results showed it is a feasible way to stimulate terminally differentiated cardiomyocyte proliferation. Here, we reviewed the major mechanisms and the potential targets for stimulating mammalian adult cardiomyocyte proliferation specifically. This will provide a new therapeutic strategy for the clinical treatment of myocardial infarction by activating the endogenous regeneration. Graphical abstract.

Cardiomyocyte cell cycling, maturation, and growth by multinucleation in postnatal swine

BACKGROUND

Rodent cardiomyocytes (CM) undergo mitotic arrest and decline of mononucleated-diploid population post-birth, which are implicated in neonatal loss of heart regenerative potential. However, the dynamics of postnatal CM maturation are largely unknown in swine, despite a similar neonatal cardiac regenerative capacity as rodents. Here, we provide a comprehensive analysis of postnatal cardiac maturation in swine, including CM cell cycling, multinucleation and hypertrophic growth, as well as non-CM cardiac factors such as extracellular matrix (ECM), immune cells, capillaries, and neurons. Our study reveals discordance in postnatal pig heart maturational events compared to rodents.

METHODS AND RESULTS

Left-ventricular myocardium from White Yorkshire-Landrace pigs at postnatal day (P)0 to 6 months (6mo) was analyzed. Mature cardiac sarcomeric characteristics, such as fetal TNNI1 repression and Cx43 co-localization to cell junctions, were not evident until P30 in pigs. In CMs, appreciable binucleation is observed by P7, with extensive multinucleation (4-16 nuclei per CM) beyond P15. Individual CM nuclei remain predominantly diploid at all ages. CM mononucleation at ~50% incidence is observed at P7-P15, and CM mitotic activity is measurable up to 2mo. CM cross-sectional area does not increase until 2mo-6mo in pigs, though longitudinal CM growth proportional to multinucleation occurs after P15. RNAseq analysis of neonatal pig left ventricles showed increased expression of ECM maturation, immune signaling, neuronal remodeling, and reactive oxygen species response genes, highlighting significance of the non-CM milieu in postnatal mammalian heart maturation.

CONCLUSIONS

CM maturational events such as decline of mononucleation and cell cycle arrest occur over a 2-month postnatal period in pigs, despite reported loss of heart regenerative potential by P3. Moreover, CMs grow primarily by multinucleation and longitudinal hypertrophy in older pig CMs, distinct from mice and humans. These differences are important to consider for preclinical testing of cardiovascular therapies using swine, and may offer opportunities to study aspects of heart regeneration unavailable in other models.

Mydgf promotes Cardiomyocyte proliferation and Neonatal Heart regeneration

Myeloid-derived growth factor (Mydgf), a paracrine protein secreted by bone marrow-derived monocytes and macrophages, was found to protect against cardiac injury following myocardial infarction (MI) in adult mice. We speculated that Mydgf might improve heart function via myocardial regeneration, which is essential for discovering the target to reverse heart failure. Methods: Two genetic mouse lines were used: global Mydgf knockout (Mydgf-KO) and Mydgf-EGFP mice. Two models of cardiac injury, apical resection was performed in neonatal and MI was performed in adult mice. Quantitative reverse transcription-polymerase chain reaction, western blot and flow cytometry were performed to study the protein expression. Immunofluorescence was performed to detect the proliferation of cardiomyocytes. Heart regeneration and cardiac function were evaluated by Masson's staining and echocardiography, respectively. RNA sequencing was employed to identify the key involved in Mydgf-induced cardiomyocyte proliferation. Mydgf recombinant protein injection was performed as a therapy for cardiac repair post MI in adult mice. Results: Mydgf expression could be significantly induced in neonatal mouse hearts after cardiac injury. Unexpectedly, we found that Mydgf was predominantly expressed by endothelial cells rather than macrophages in injured neonatal hearts. Mydgf deficiency impeded neonatal heart regeneration and injury-induced cardiomyocyte proliferation. Mydgf recombinant protein promoted primary mouse cardiomyocyte proliferation. Employing RNA sequencing and functional verification, we demonstrated that c-Myc/FoxM1 pathway mediated Mydgf-induced cardiomyocyte expansion. Mydgf recombinant protein improved cardiac function in adult mice after MI injury with inducing cardiomyocyte proliferation. Conclusion: Mydgf promotes cardiomyocyte proliferation by activating c-Myc/FoxM1 pathway and improves heart regeneration both in neonatal and adult mice after cardiac injury, providing a potential target to reverse cardiac remodeling and heart failure.

The nuclear envelope: LINCing tissue mechanics to genome regulation in cardiac and skeletal muscle

Regulation of the genome is viewed through the prism of gene expression, DNA replication and DNA repair as controlled through transcription, chromatin compartmentalisation and recruitment of repair factors by enzymes such as DNA polymerases, ligases, acetylases, methylases and cyclin-dependent kinases. However, recent advances in the field of muscle cell physiology have also shown a compelling role for 'outside-in' biophysical control of genomic material through mechanotransduction. The crucial hub that transduces these biophysical signals is called the Linker of Nucleoskeleton and Cytoskeleton (LINC). This complex is embedded across the nuclear envelope, which separates the nucleus from the cytoplasm. How the LINC complex operates to mechanically regulate the many functions of DNA is becoming increasingly clear, and recent advances have provided exciting insight into how this occurs in cells from mechanically activated tissues such as skeletal and cardiac muscle. Nevertheless, there are still some notable shortcomings in our understanding of these processes and resolving these will likely help us understand how muscle diseases manifest at the level of the genome.

Polyploidy: A Biological Force From Cells to Ecosystems

Polyploidy, resulting from the duplication of the entire genome of an organism or cell, greatly affects genes and genomes, cells and tissues, organisms, and even entire ecosystems. Despite the wide-reaching importance of polyploidy, communication across disciplinary boundaries to identify common themes at different scales has been almost nonexistent. However, a critical need remains to understand commonalities that derive from shared polyploid cellular processes across organismal diversity, levels of biological organization, and fields of inquiry - from biodiversity and biocomplexity to medicine and agriculture. Here, we review the current understanding of polyploidy at the organismal and suborganismal levels, identify shared research themes and elements, and propose new directions to integrate research on polyploidy toward confronting interdisciplinary grand challenges of the 21^st century.

Lamin B2, Guardian of Cardiomyocyte Nuclear Division

Regenerative capacity is robust in the neonatal mouse heart but is lost during postnatal development when cardiomyocytes undergo cell-cycle arrest and polyploidization. In this issue of Developmental Cell, Han et al. (2020) show that Lamin B2, a nuclear lamina filament supporting cardiomyocyte karyokinesis, also facilitates cell division and cardiac regeneration.