Tnni3k alleles influence ventricular mononuclear diploid cardiomyocyte frequency

Revitalizing the heart: strategies and tools for cardiomyocyte regeneration post-myocardial infarction

Myocardial infarction (MI) causes the loss of millions of cardiomyocytes, and current treatments do not address this root issue. New therapies focus on stimulating cardiomyocyte division in the adult heart, inspired by the regenerative capacities of lower vertebrates and neonatal mice. This review explores strategies for heart regeneration, offers insights into cardiomyocyte proliferation, evaluates in vivo models, and discusses integrating in vitro human cardiac models to advance cardiac regeneration research.

A kinase-dead natural polymorphism in the canine Tnni3k gene

Most mammalian cardiomyocytes become polyploid in the neonatal period, concurrent with their loss of proliferative capacity. In mice, natural or engineered mutation of the cardiomyocyte-specific kinase gene Tnni3k causes a higher level of diploid CMs and a higher capacity to support proliferation after adult injury. Here, we identified a polymorphism in the canine Tnni3k gene that is particularly common in the West Highland White Terrier breed, and show that this variant eliminates Tnni3k kinase activity. Thus, in several species, natural Tnni3k polymorphisms exist that are predicted to contribute to variation in diploid CM level and heart regenerative ability.

Two decades of heart regeneration research: Cardiomyocyte proliferation and beyond

Interest in vertebrate cardiac regeneration has exploded over the past two decades since the discovery that adult zebrafish are capable of complete heart regeneration, contrasting the limited regenerative potential typically observed in adult mammalian hearts. Undercovering the mechanisms that both support and limit cardiac regeneration across the animal kingdom may provide unique insights in how we may unlock this capacity in adult humans. In this review, we discuss key discoveries in the heart regeneration field over the last 20 years. Initially, seminal findings revealed that pre-existing cardiomyocytes are the major source of regenerated cardiac muscle, drawing interest into the intrinsic mechanisms regulating cardiomyocyte proliferation. Moreover, recent studies have identified the importance of intercellular interactions and physiological adaptations, which highlight the vast complexity of the cardiac regenerative process. Finally, we compare strategies that have been tested to increase the regenerative capacity of the adult mammalian heart. This article is categorized under: Cardiovascular Diseases > Stem Cells and Development.

Transcription factor NFYa controls cardiomyocyte metabolism and proliferation during mouse fetal heart development

Cardiomyocytes are highly metabolic cells responsible for generating the contractile force in the heart. During fetal development and regeneration, these cells actively divide but lose their proliferative activity in adulthood. The mechanisms that coordinate their metabolism and proliferation are not fully understood. Here, we study the role of the transcription factor NFYa in developing mouse hearts. Loss of NFYa alters cardiomyocyte composition, causing a decrease in immature regenerative cells and an increase in trabecular and mature cardiomyocytes, as identified by spatial and single-cell transcriptome analyses. NFYa-deleted cardiomyocytes exhibited reduced proliferation and impaired mitochondrial metabolism, leading to cardiac growth defects and embryonic death. NFYa, interacting with cofactor SP2, activates genes linking metabolism and proliferation at the transcription level. Our study identifies a nodal role of NFYa in regulating prenatal cardiac growth and a previously unrecognized transcriptional control mechanism of heart metabolism, highlighting the importance of mitochondrial metabolism during heart development and regeneration.

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.

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.

Cardiac Troponin I-Interacting Kinase Affects Cardiomyocyte S-Phase Activity but Not Cardiomyocyte Proliferation

BACKGROUND

Identifying genetic variants that affect the level of cell cycle reentry and establishing the degree of cell cycle progression in those variants could help guide development of therapeutic interventions aimed at effecting cardiac regeneration. We observed that C57Bl6/NCR (B6N) mice have a marked increase in cardiomyocyte S-phase activity after permanent coronary artery ligation compared with infarcted DBA/2J (D2J) mice.

METHODS

Cardiomyocyte cell cycle activity after infarction was monitored in D2J, (D2J×B6N)-F1, and (D2J×B6N)-F1×D2J backcross mice by means of bromodeoxyuridine or 5-ethynyl-2'-deoxyuridine incorporation using a nuclear-localized transgenic reporter to identify cardiomyocyte nuclei. Genome-wide quantitative trait locus analysis, fine scale genetic mapping, whole exome sequencing, and RNA sequencing analyses of the backcross mice were performed to identify the gene responsible for the elevated cardiomyocyte S-phase phenotype.

RESULTS

(D2J×B6N)-F1 mice exhibited a 14-fold increase in cardiomyocyte S-phase activity in ventricular regions remote from infarct scar compared with D2J mice (0.798±0.09% versus 0.056±0.004%; P<0.001). Quantitative trait locus analysis of (D2J×B6N)-F1×D2J backcross mice revealed that the gene responsible for differential S-phase activity was located on the distal arm of chromosome 3 (logarithm of the odds score=6.38; P<0.001). Additional genetic and molecular analyses identified 3 potential candidates. Of these, Tnni3k (troponin I-interacting kinase) is expressed in B6N hearts but not in D2J hearts. Transgenic expression of TNNI3K in a D2J genetic background results in elevated cardiomyocyte S-phase activity after injury. Cardiomyocyte S-phase activity in both Tnni3k-expressing and Tnni3k-nonexpressing mice results in the formation of polyploid nuclei.

CONCLUSIONS

These data indicate that Tnni3k expression increases the level of cardiomyocyte S-phase activity after injury.

Knockout of Cardiac Troponin I-Interacting Kinase leads to cardiac dysfunction and remodeling

Cardiac troponin I-interacting kinase (TNNI3K) is a cardiac-specific kinase that has been identified as a diagnostic marker and a therapeutic target in cardiovascular diseases. However, the biological function of TNNI3K in cardiac dysfunction and remodeling remain elusive. In the present study, a Tnni3k cardiomyocyte-specific knockout (Tnni3k-cKO) mouse model was established. Echocardiography was used to evaluate cardiac function in mice. Heart failure markers were detected using enzyme-linked immunosorbent assay. Hematoxylin and eosin staining, wheat germ agglutinin staining, Masson's trichrome staining, Sirius red staining, and TUNEL staining were used to assess histopathological changes, cardiac hypertrophy, collagen deposition, and myocardial apoptosis, respectively. Expression levels of TNNI3K, apoptosis-related proteins, and p38 mitogen-activated protein kinase were measured using Western blot analysis. Compared to wild-type controls, cardiac dysfunction and cardiac remodeling of Tnni3k-cKO mice increased gradually with age. Tnni3k-cKO mice exhibited cardiac hypertrophy, cardiac fibrosis, and cardiomyocyte apoptosis. Upregulation of cleaved caspase-3 in Tnni3k-cKO mice appeared to be related to phosphorylation and activation of the p38 mitogen-activated protein kinase signaling pathway. In conclusion, this study shows that TNNI3K is essential for cardiac development and function, providing new insights into the development of novel therapeutic strategies for cardiac diseases.

RBPMS is an RNA-binding protein that mediates cardiomyocyte binucleation and cardiovascular development

Noncompaction cardiomyopathy is a common congenital cardiac disorder associated with abnormal ventricular cardiomyocyte trabeculation and impaired pump function. The genetic basis and underlying mechanisms of this disorder remain elusive. We show that the genetic deletion of RNA-binding protein with multiple splicing (Rbpms), an uncharacterized RNA-binding factor, causes perinatal lethality in mice due to congenital cardiovascular defects. The loss of Rbpms causes premature onset of cardiomyocyte binucleation and cell cycle arrest during development. Human iPSC-derived cardiomyocytes with RBPMS gene deletion have a similar blockade to cytokinesis. Sequencing analysis revealed that RBPMS plays a role in RNA splicing and influences RNAs involved in cytoskeletal signaling pathways. We found that RBPMS mediates the isoform switching of the heart-enriched LIM domain protein Pdlim5. The loss of Rbpms leads to an abnormal accumulation of Pdlim5-short isoforms, disrupting cardiomyocyte cytokinesis. Our findings connect premature cardiomyocyte binucleation to noncompaction cardiomyopathy and highlight the role of RBPMS in this process.

Polyploidy as a Fundamental Phenomenon in Evolution, Development, Adaptation and Diseases

DNA replication during cell proliferation is 'vertical' copying, which reproduces an initial amount of genetic information. Polyploidy, which results from whole-genome duplication, is a fundamental complement to vertical copying. Both organismal and cell polyploidy can emerge via premature cell cycle exit or via cell-cell fusion, the latter giving rise to polyploid hybrid organisms and epigenetic hybrids of somatic cells. Polyploidy-related increase in biological plasticity, adaptation, and stress resistance manifests in evolution, development, regeneration, aging, oncogenesis, and cardiovascular diseases. Despite the prevalence in nature and importance for medicine, agri- and aquaculture, biological processes and epigenetic mechanisms underlying these fundamental features largely remain unknown. The evolutionarily conserved features of polyploidy include activation of transcription, response to stress, DNA damage and hypoxia, and induction of programs of morphogenesis, unicellularity, and longevity, suggesting that these common features confer adaptive plasticity, viability, and stress resistance to polyploid cells and organisms. By increasing cell viability, polyploidization can provide survival under stressful conditions where diploid cells cannot survive. However, in somatic cells it occurs at the expense of specific function, thus promoting developmental programming of adult cardiovascular diseases and increasing the risk of cancer. Notably, genes arising via evolutionary polyploidization are heavily involved in cancer and other diseases. Ploidy-related changes of gene expression presumably originate from chromatin modifications and the derepression of bivalent genes. The provided evidence elucidates the role of polyploidy in evolution, development, aging, and carcinogenesis, and may contribute to the development of new strategies for promoting regeneration and preventing cardiovascular diseases and cancer.

Gene losses may contribute to subterranean adaptations in naked mole-rat and blind mole-rat

Background

Naked mole-rats (Heterocephalus glaber, NMRs) and blind mole-rats (Spalax galili, BMRs) are representative subterranean rodents that have evolved many extraordinary traits, including hypoxia tolerance, longevity, and cancer resistance. Although multiple candidate loci responsible for these traits have been uncovered by genomic studies, many of them are limited to functional changes to amino acid sequence and little is known about the contributions of other genetic events. To address this issue, we focused on gene losses (unitary pseudogenes) and systematically analyzed gene losses in NMRs and BMRs, aiming to elucidate the potential roles of pseudogenes in their adaptation to subterranean lifestyle.

Results

We obtained the pseudogene repertoires in NMRs and BMRs, as well as their respective aboveground relatives, guinea pigs and rats, on a genome-wide scale. As a result, 167, 139, 341, and 112 pseudogenes were identified in NMRs, BMRs, guinea pigs, and rats, respectively. Functional enrichment analysis identified 4 shared and 2 species-specific enriched functional groups (EFGs) in subterranean lineages. Notably, the pseudogenes in these EFGs might be associated with either regressive (e.g., visual system) or adaptive (e.g., altered DNA damage response) traits. In addition, several pseudogenes including TNNI3K and PDE5A might be associated with specific cardiac features observed in subterranean lineages. Interestingly, we observed 20 convergent gene losses in NMRs and BMRs. Given that the functional investigations of these genes are generally scarce, we provided functional evidence that independent loss of TRIM17 in NMRs and BMRs might be beneficial for neuronal survival under hypoxia, supporting the positive role of eliminating TRIM17 function in hypoxia adaptation. Our results also suggested that pseudogenes, together with positively selected genes, reinforced subterranean adaptations cooperatively.

Conclusions

Our study provides new insights into the molecular underpinnings of subterranean adaptations and highlights the importance of gene losses in mammalian evolution.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01243-0.

Accelerated Growth, Differentiation, and Ploidy with Reduced Proliferation of Right Ventricular Cardiomyocytes in Children with Congenital Heart Defect Tetralogy of Fallot

The myocardium of children with tetralogy of Fallot (TF) undergoes hemodynamic overload and hypoxemia immediately after birth. Comparative analysis of changes in the ploidy and morphology of the right ventricular cardiomyocytes in children with TF in the first years of life demonstrated their significant increase compared with the control group. In children with TF, there was a predominantly diffuse distribution of Connexin43-containing gap junctions over the cardiomyocytes sarcolemma, which redistributed into the intercalated discs as cardiomyocytes differentiation increased. The number of Ki67-positive cardiomyocytes varied greatly and amounted to 7.0–1025.5/10⁶ cardiomyocytes and also were decreased with increased myocytes differentiation. Ultrastructural signs of immaturity and proliferative activity of cardiomyocytes in children with TF were demonstrated. The proportion of interstitial tissue did not differ significantly from the control group. The myocardium of children with TF under six months of age was most sensitive to hypoxemia, it was manifested by a delay in the intercalated discs and myofibril assembly and the appearance of ultrastructural signs of dystrophic changes in the cardiomyocytes. Thus, the acceleration of ontogenetic growth and differentiation of the cardiomyocytes, but not the reactivation of their proliferation, was an adaptation of the immature myocardium of children with TF to hemodynamic overload and hypoxemia.

IL4Rα signaling promotes neonatal cardiac regeneration and cardiomyocyte cell cycle activity

Neonatal heart regeneration depends on proliferation of pre-existing cardiomyocytes, yet the mechanisms driving regeneration and cardiomyocyte proliferation are not comprehensively understood. We recently reported that the anti-inflammatory cytokine, interleukin 13 (IL13), promotes neonatal cardiac regeneration; however, the signaling pathway and cell types mediating this regenerative response remain unknown. Here, we hypothesized that expression of the type II heterodimer receptor for IL13, comprised of IL4Rα and IL13Rα1, expressed directly on cardiomyocytes mediates cardiomyocyte cell cycle and heart regeneration in neonatal mice. Our data demonstrate that indeed global deletion of one critical subunit of the type II receptor, IL4Rα (IL4Rα-/-), decreases cardiomyocyte proliferation during early postnatal development and significantly impairs cardiac regeneration following injury in neonatal mice. While multiple myocardial cell types express IL4Rα, we demonstrate that IL4Rα deletion specifically in cardiomyocytes mediates cell cycle activity and neonatal cardiac regeneration. This demonstrates for the first time a functional role for IL4Rα signaling directly on cardiomyocytes in vivo. Reciprocally, we examined the therapeutic benefit of activating the IL4Rα receptor in non-regenerative hearts via IL13 administration. Following myocardial infarction, administration of IL13 reduced scar size and promoted cardiomyocyte DNA synthesis and karyokinesis, but not complete cytokinesis, in 6-day old non-regenerative mice. Our data demonstrate a novel role for IL4Rα signaling directly on cardiomyocytes during heart regeneration and suggest the potential for type II receptor activation as one potential therapeutic target for promoting myocardial repair.

Innate Mechanisms of Heart Regeneration

Heart regeneration is a remarkable process whereby regrowth of damaged cardiac tissue rehabilitates organ anatomy and function. Unfortunately, the human heart is highly resistant to regeneration, which creates a shortage of cardiomyocytes in the wake of ischemic injury, and explains, in part, why coronary artery disease remains a leading cause of death worldwide. Luckily, a detailed blueprint for achieving therapeutic heart regeneration already exists in nature because several lower vertebrate species successfully regenerate amputated or damaged heart muscle through robust cardiomyocyte proliferation. A growing number of species are being interrogated for cardiac regenerative potential, and several commonalities have emerged between those animals showing high or low innate capabilities. In this review, we provide a historical perspective on the field, discuss how regenerative potential is influenced by cardiomyocyte properties, mitogenic signals, and chromatin accessibility, and highlight unanswered questions under active investigation. Ultimately, delineating why heart regeneration occurs preferentially in some organisms, but not in others, will uncover novel therapeutic inroads for achieving cardiac restoration in humans.

Reawakening the Intrinsic Cardiac Regenerative Potential: Molecular Strategies to Boost Dedifferentiation and Proliferation of Endogenous Cardiomyocytes

Despite considerable efforts carried out to develop stem/progenitor cell-based technologies aiming at replacing and restoring the cardiac tissue following severe damages, thus far no strategies based on adult stem cell transplantation have been demonstrated to efficiently generate new cardiac muscle cells. Intriguingly, dedifferentiation, and proliferation of pre-existing cardiomyocytes and not stem cell differentiation represent the preponderant cellular mechanism by which lower vertebrates spontaneously regenerate the injured heart. Mammals can also regenerate their heart up to the early neonatal period, even in this case by activating the proliferation of endogenous cardiomyocytes. However, the mammalian cardiac regenerative potential is dramatically reduced soon after birth, when most cardiomyocytes exit from the cell cycle, undergo further maturation, and continue to grow in size. Although a slow rate of cardiomyocyte turnover has also been documented in adult mammals, both in mice and humans, this is not enough to sustain a robust regenerative process. Nevertheless, these remarkable findings opened the door to a branch of novel regenerative approaches aiming at reactivating the endogenous cardiac regenerative potential by triggering a partial dedifferentiation process and cell cycle re-entry in endogenous cardiomyocytes. Several adaptations from intrauterine to extrauterine life starting at birth and continuing in the immediate neonatal period concur to the loss of the mammalian cardiac regenerative ability. A wide range of systemic and microenvironmental factors or cell-intrinsic molecular players proved to regulate cardiomyocyte proliferation and their manipulation has been explored as a therapeutic strategy to boost cardiac function after injuries. We here review the scientific knowledge gained thus far in this novel and flourishing field of research, elucidating the key biological and molecular mechanisms whose modulation may represent a viable approach for regenerating the human damaged myocardium.

Polyploidy in Tissue Repair and Regeneration

Polyploidy is defined as a cell with three or more whole genome sets and enables cell growth across the kingdoms of life. Studies in model organisms have revealed that polyploid cell growth can be required for optimal tissue repair and regeneration. In mammals, polyploid cell growth contributes to repair of many tissues, including the liver, heart, kidney, bladder, and eye, and similar strategies have been identified in Drosophila and zebrafish tissues. This review discusses the heterogeneity and versatility of polyploidy in tissue repair and regeneration. Polyploidy has been shown to restore tissue mass and maintain organ size as well as protect against oncogenic insults and genotoxic stress. Polyploid cells can also serve as a reservoir for new diploid cells in regeneration. The numerous mechanisms to generate polyploid cells provide an unlimited resource for tissues to exploit to undergo repair or regeneration.

The Diverse Roles of TNNI3K in Cardiac Disease and Potential for Treatment

In the two decades since the discovery of TNNI3K it has been implicated in multiple cardiac phenotypes and physiological processes. TNNI3K is an understudied kinase, which is mainly expressed in the heart. Human genetic variants in TNNI3K are associated with supraventricular arrhythmias, conduction disease, and cardiomyopathy. Furthermore, studies in mice implicate the gene in cardiac hypertrophy, cardiac regeneration, and recovery after ischemia/reperfusion injury. Several new papers on TNNI3K have been published since the last overview, broadening the clinical perspective of TNNI3K variants and our understanding of the underlying molecular biology. We here provide an overview of the role of TNNI3K in cardiomyopathy and arrhythmia covering both a clinical perspective and basic science advancements. In addition, we review the potential of TNNI3K as a target for clinical treatments in different cardiac diseases.

Evidence that the acetyltransferase Tip60 induces the DNA damage response and cell-cycle arrest in neonatal cardiomyocytes

Tip60, a pan-acetyltransferase encoded by the Kat5 gene, is enriched in the myocardium; however, its function in the heart is unknown. In cancer cells, Tip60 acetylates Atm (Ataxia-telangiectasia mutated), enabling its auto-phosphorylation (pAtm), which activates the DNA damage response (DDR). It was recently reported that activation of pAtm at the time of birth induces the DDR in cardiomyocytes (CMs), resulting in proliferative senescence. We therefore hypothesized that Tip60 initiates this process, and that depletion of Tip60 accordingly diminishes the DDR while extending the duration of CM cell-cycle activation. To test this hypothesis, an experimental model was used wherein a Myh6-driven Cre-recombinase transgene was activated on postnatal day 0 (P0) to recombine floxed Kat5 alleles and induce Tip60 depletion in neonatal CMs, without causing pathogenesis. Depletion of Tip60 resulted in reduced numbers of pAtm-positive CMs during the neonatal period, which correlated with reduced numbers of pH2A.X-positive CMs and decreased expression of genes encoding markers of the DDR as well as inflammation. This was accompanied by decreased expression of the cell-cycle inhibitors Meis1 and p27, activation of the cell-cycle in CMs, reduced CM size, and increased numbers of mononuclear/diploid CMs. Increased expression of fetal markers suggested that Tip60 depletion promotes a fetal-like proliferative state. Finally, infarction of Tip60-depleted hearts at P7 revealed improved cardiac function at P39 accompanied by reduced fibrosis, increased CM cell-cycle activation, and reduced apoptosis in the remote zone. These findings indicate that, among its pleiotropic functions, Tip60 induces the DDR in CMs, contributing to proliferative senescence.

The prevalent I686T human variant and loss-of-function mutations in the cardiomyocyte-specific kinase gene TNNI3K cause adverse contractility and concentric remodeling in mice

TNNI3K expression worsens disease progression in several mouse heart pathology models. TNNI3K expression also reduces the number of diploid cardiomyocytes, which may be detrimental to adult heart regeneration. However, the gene is evolutionarily conserved, suggesting a beneficial function that has remained obscure. Here, we show that C57BL/6J-inbred Tnni3k mutant mice develop concentric remodeling, characterized by ventricular wall thickening and substantial reduction of cardiomyocyte aspect ratio. This pathology occurs in mice carrying a Tnni3k null allele, a K489R point mutation rendering the protein kinase-dead, or an allele corresponding to human I686T, the most common human non-synonymous TNNI3K variant, which is hypomorphic for kinase activity. Mutant mice develop these conditions in the absence of fibrosis or hypertension, implying a primary cardiomyocyte etiology. In culture, mutant cardiomyocytes were impaired in contractility and calcium dynamics and in protein kinase A signaling in response to isoproterenol, indicating diminished contractile reserve. These results demonstrate a beneficial function of TNNI3K in the adult heart that might explain its evolutionary conservation and imply that human TNNI3K variants, in particular the widespread I686T allele, may convey elevated risk for altered heart geometry and hypertrophy.

Identification of a nonsense mutation in TNNI3K associated with cardiac conduction disease

BACKGROUND

Cardiac conduction disease (CCD) is a common cardiovascular disease which can lead to life-threatening conditions. The importance of heredity in CCD has been realized in recent years. Several causal genes have been found to be implicated in CCD such as SCN5A, TRPM4, SCN1B, TNNI3K, LMNA, and NKX2.5. To date, only four genetic mutations in TNNI3K have been identified related to CCD.

METHODS

Whole-exome sequencing (WES) was carried out in order to identify the underlying disease-causing mutation in a Chinese family with CCD. The potential mutations were confirmed by Sanger sequencing. Real-time qPCR was used to detect the level of TNNI3K mRNA expression.

RESULTS

A nonsense mutation in TNNI3K (NM_015978.2: g.170891C > T, c.1441C > T) was identified in this family and validated by Sanger sequencing. Real-time qPCR confirmed that the level of TNNI3K mRNA expression was decreased compared with the controls.

CONCLUSIONS

This study found the first nonsense TNNI3K mutation associated with CCD in a Chinese family. TNNI3K harboring the mutation (c.1441C > T) implicated a loss-of-function pathogenic mechanism with an autosomal dominant inheritance pattern. This research enriches the phenotypic spectrum of TNNI3K mutations, casting a new light upon the genotype-phenotype correlations between TNNI3K mutations and CCD and indicating the importance of TNNI3K screening in CCD patients.

Allelic variants between mouse substrains BALB/cJ and BALB/cByJ influence mononuclear cardiomyocyte composition and cardiomyocyte nuclear ploidy

Most mouse cardiomyocytes (CMs) become multinucleated shortly after birth via endoreplication and interrupted mitosis, which persists through adulthood. The very closely related inbred mouse strains BALB/cJ and BALB/cByJ differ substantially (6.6% vs. 14.3%) in adult mononuclear CM level. This difference is the likely outcome of a single X-linked polymorphic gene that functions in a CM-nonautonomous manner, and for which the BALB/cByJ allele is recessive to that of BALB/cJ. From whole exome sequence we identified two new X-linked protein coding variants that arose de novo in BALB/cByJ, in the genes Gdi1 (R276C) and Irs4 (L683F), but show that neither affects mononuclear CM level individually. No BALB/cJ-specific X-linked protein coding variants were found, implicating instead a variant that influences gene expression rather than encoded protein function. A substantially higher percentage of mononuclear CMs in BALB/cByJ are tetraploid (66.7% vs. 37.6% in BALB/cJ), such that the overall level of mononuclear diploid CMs between the two strains is similar. The difference in nuclear ploidy is the likely result of an autosomal polymorphism, for which the BALB/cByJ allele is recessive to that of BALB/cJ. The X-linked and autosomal genes independently influence mitosis such that their phenotypic consequences can be combined or segregated by appropriate breeding, implying distinct functions in karyokinesis and cytokinesis.

Mononuclear diploid cardiomyocytes support neonatal mouse heart regeneration in response to paracrine IGF2 signaling

Injury to the newborn mouse heart is efficiently regenerated, but this capacity is lost by one week after birth. We found that IGF2, an important mitogen in heart development, is required for neonatal heart regeneration. IGF2 originates from the endocardium/endothelium and is transduced in cardiomyocytes by the insulin receptor. Following injury on postnatal day 1, absence of IGF2 abolished injury-induced cell cycle entry during the early part of the first postnatal week. Consequently, regeneration failed despite the later presence of additional cell cycle-inducing activities 7 days following injury. Most cardiomyocytes transition from mononuclear diploid to polyploid during the first postnatal week. Regeneration was rescued in Igf2-deficient neonates in three different contexts that elevate the percentage of mononuclear diploid cardiomyocytes beyond postnatal day 7. Thus, IGF2 is a paracrine-acting mitogen for heart regeneration during the early postnatal period, and IGF2-deficiency unmasks the dependence of this process on proliferation-competent mononuclear diploid cardiomyocytes.

Residual Diploidy in Polyploid Tissues: A Cellular State with Enhanced Proliferative Capacity for Tissue Regeneration?

A major objective of modern biomedical research aims to promote tissue self-regeneration after injury, obviating the need for whole organ transplantation and avoiding mortality due to organ failure. Identifying the population of cells capable of regeneration, alongside understanding the molecular mechanisms that activate that population to re-enter the cell cycle, are two important steps to advancing the field of endogenous tissue regeneration toward the clinic. In recent years, an emerging trend has been observed, whereby polyploidy of relevant parenchymal cells, arising from alternative cell cycles as part of a normal developmental process, is linked to restricted proliferative capacity of those cells. An accompanying hypothesis, therefore, is that a residual subpopulation of diploid parenchymal cells retains proliferative competence and is the major driver for any detected postnatal cell turnover. In this perspective review, we examine the emerging literature on residual diploid parenchymal cells and the possible link of this population to endogenous tissue regeneration, in the context of both heart and liver. We speculate on additional cell types that may play a similar role in their respective tissues and discuss outstanding questions for the field.