Sarcomere function activates a p53-dependent DNA damage response that promotes polyploidization and limits in vivo cell engraftment

Molecular gatekeepers of endogenous adult mammalian cardiomyocyte proliferation

Irreversible cardiac fibrosis, cardiomyocyte death and chronic cardiac dysfunction after myocardial infarction pose a substantial global health-care challenge, with no curative treatments available. To regenerate the injured heart, cardiomyocytes must proliferate to replace lost myocardial tissue - a capability that adult mammals have largely forfeited to adapt to the demanding conditions of life. Using various preclinical models, our understanding of cardiomyocyte proliferation has progressed remarkably, leading to the successful reactivation of cell cycle induction in adult animals, with functional recovery after cardiac injury. Central to this success is the targeting of key pathways and structures that drive cardiomyocyte maturation after birth - nucleation and ploidy, sarcomere structure, developmental signalling, chromatin and epigenetic regulation, the microenvironment and metabolic maturation - forming a complex regulatory framework that allows efficient cellular contraction but restricts cardiomyocyte proliferation. In this Review, we explore the molecular pathways underlying these core mechanisms and how their manipulation can reactivate the cell cycle in cardiomyocytes, potentially contributing to cardiac repair.

Glycogen synthase kinase-3 inhibition and insulin enhance proliferation and inhibit maturation of human iPSC-derived cardiomyocytes via TCF and FOXO signaling

Embryonic signaling pathways exert stage-specific effects during cardiac development, yet the precise signals for proliferation or maturation remain elusive. To uncover the cues for proliferation, we performed a combinatory cell-cycle screen for insulin and glycogen synthase kinase-3 (GSK3) inhibition in spontaneously beating human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). Our analysis for proliferation, and subsequential downstream sarcomere development, gene expression analysis, and molecular interventions identified a temporal interplay between insulin/Akt/FOXO and CHIR99021/Wnt/GSK3/TCF signaling. Combined pathway activation led to proliferation of immature hiPSC-CMs with low sarcomere and mitochondria content, while, in the absence of pathway activators, cardiomyocytes rapidly exited the cell cycle and fetched higher organization of sarcomeres and mitochondria. Our data demonstrate two important pathways, which enhance proliferation and inhibit maturation, and provide molecular mechanistic understanding of these cell fate decisions in immature hiPSC-CMs.

Engineered Cardiac Tissues as a Platform for CRISPR-Based Mitogen Discovery

Improved understanding of cardiomyocyte (CM) cell cycle regulation may allow researchers to stimulate pro-regenerative effects in injured hearts or promote maturation of human stem cell-derived CMs. Gene therapies, in particular, hold promise to induce controlled proliferation of endogenous or transplanted CMs via transient activation of mitogenic processes. Methods to identify and characterize candidate cardiac mitogens in vitro can accelerate translational efforts and contribute to the understanding of the complex regulatory landscape of CM proliferation and postnatal maturation. In this study, A CRISPR knockout-based screening strategy using in vitro neonatal rat ventricular myocyte (NRVM) monolayers is established, followed by candidate mitogen validation in mature 3-D engineered cardiac tissues (ECTs). This screen identified knockout of the purine metabolism enzyme adenosine deaminase (ADA-KO) as an effective pro-mitogenic stimulus. RNA-sequencing of ECTs further reveals increased pentose phosphate pathway (PPP) activity as the primary driver of ADA-KO-induced CM cycling. Inhibition of the pathway's rate limiting enzyme, glucose-6-phosphate dehydrogenase (G6PD), prevented ADA-KO induced CM cycling, while increasing PPP activity via G6PD overexpression increased CM cycling. Together, this study demonstrates the development and application of a genetic/tissue engineering platform for in vitro discovery and validation of new candidate mitogens affecting regenerative or maturation states of cardiomyocytes.

Transition from fetal to postnatal state in the heart: Crosstalk between metabolism and regeneration

Cardiovascular disease is the leading cause of mortality worldwide. Myocardial injury resulting from ischemia can be fatal because of the limited regenerative capacity of adult myocardium. Mammalian cardiomyocytes rapidly lose their proliferative capacities, with only a small fraction of adult myocardium remaining proliferative, which is insufficient to support post-injury recovery. Recent investigations have revealed that this decline in myocardial proliferative capacity is closely linked to perinatal metabolic shifts. Predominantly glycolytic fetal myocardial metabolism transitions towards mitochondrial fatty acid oxidation postnatally, which not only enables efficient production of ATP but also causes a dramatic reduction in cardiomyocyte proliferative capacity. Extensive research has elucidated the mechanisms behind this metabolic shift, as well as methods to modulate these metabolic pathways. Some of these methods have been successfully applied to enhance metabolic reprogramming and myocardial regeneration. This review discusses recently acquired insights into the interplay between metabolism and myocardial proliferation, emphasizing postnatal metabolic transitions.

Polyploidy in Cancer: Causal Mechanisms, Cancer-Specific Consequences, and Emerging Treatments

Drug resistance is the major determinant for metastatic disease and fatalities, across all cancers. Depending on the tissue of origin and the therapeutic course, a variety of biological mechanisms can support and sustain drug resistance. Although genetic mutations and gene silencing through epigenetic mechanisms are major culprits in targeted therapy, drug efflux and polyploidization are more global mechanisms that prevail in a broad range of pathologies, in response to a variety of treatments. There is an unmet need to identify patients at risk for polyploidy, understand the mechanisms underlying polyploidization, and to develop strategies to predict, limit, and reverse polyploidy thus enhancing efficacy of standard-of-care therapy that improve better outcomes. This literature review provides an overview of polyploidy in cancer and offers perspective on patient monitoring and actionable therapy.

CRISPR Activation Reverses Haploinsufficiency and Functional Deficits Caused by TTN Truncation Variants

BACKGROUND

TTN truncation variants (TTNtvs) are the most common genetic lesion identified in individuals with dilated cardiomyopathy, a disease with high morbidity and mortality rates. TTNtvs reduce normal TTN (titin) protein levels, produce truncated proteins, and impair sarcomere content and function. Therapeutics targeting TTNtvs have been elusive because of the immense size of TTN, the rarity of specific TTNtvs, and incomplete knowledge of TTNtv pathogenicity.

METHODS

We adapted CRISPR activation using dCas9-VPR to functionally interrogate TTNtv pathogenicity and develop a therapeutic in human cardiomyocytes and 3-dimensional cardiac microtissues engineered from induced pluripotent stem cell models harboring a dilated cardiomyopathy-associated TTNtv. We performed guide RNA screening with custom TTN reporter assays, agarose gel electrophoresis to quantify TTN protein levels and isoforms, and RNA sequencing to identify molecular consequences of TTN activation. Cardiomyocyte epigenetic assays were also used to nominate DNA regulatory elements to enable cardiomyocyte-specific TTN activation.

RESULTS

CRISPR activation of TTN using single guide RNAs targeting either the TTN promoter or regulatory elements in spatial proximity to the TTN promoter through 3-dimensional chromatin interactions rescued TTN protein deficits disturbed by TTNtvs. Increasing TTN protein levels normalized sarcomere content and contractile function despite increasing truncated TTN protein. In addition to TTN transcripts, CRISPR activation also increased levels of myofibril assembly-related and sarcomere-related transcripts.

CONCLUSIONS

TTN CRISPR activation rescued TTNtv-related functional deficits despite increasing truncated TTN levels, which provides evidence to support haploinsufficiency as a relevant genetic mechanism underlying heterozygous TTNtvs. CRISPR activation could be developed as a therapeutic to treat a large proportion of TTNtvs.

Vascularization of PEGylated fibrin hydrogels increases the proliferation of human iPSC-cardiomyocytes

Studies have long sought to develop engineered heart tissue for the surgical correction of structural heart defects, as well as other applications and vascularization of this tissue has presented a challenge. Recent studies suggest that vascular cells and a vascular network may have regenerative effects on implanted cardiomyocytes (CM) and nearby heart tissue separate from perfusion of oxygen and nutrients. The goal of this study was to test whether vascular cells or a formed vascular network in a fibrin-based hydrogel would alter the proliferation of human iPSC-derived CM. First, vascular network formation in a slowly degrading PEGylated fibrin hydrogel was optimized by altering the cell ratio of human umbilical vein endothelial cells to human dermal fibroblasts, the inclusion of growth factors, and the total cell concentration. An endothelial to fibroblast ratio of 5:1 and a total cell concentration of 1.1 × 106 cells/mL without additional growth factors generated robust vascular networks while minimizing the number of cells required. Using this optimized system, human iPSC-derived CM were cultured on hydrogels without vascular cells, hydrogels with unorganized encapsulated vascular cells, or hydrogels with encapsulated vascular cells organized into networks for 7 days. CM proliferation and gene expression were assayed following 7 days of culture on the hydrogels. The presence of vascular cells in the hydrogel, whether unorganized or in vascular networks, significantly increased CM proliferation compared to an acellular hydrogel. Hydrogels with unorganized vascular cells resulted in lower CM maturity evidenced by decreased expression of cardiac troponin t (TNNT2), myosin light chain 7, and phospholamban compared to hydrogels without vascular cells and hydrogels with vascular networks. Altogether, this study details a robust method of forming rudimentary vascular networks in a fibrin-based hydrogel and shows that a hydrogel containing endothelial cells and fibroblasts can induce proliferation in adjacent CM, and these cells do not hinder CM gene expression when organized into a vascular network.

Cardiomyocyte maturation and its reversal during cardiac regeneration

Cardiovascular disease is a leading cause of death worldwide. Due to the limited proliferative and regenerative capacity of adult cardiomyocytes, the lost myocardium is not replenished efficiently and is replaced by a fibrotic scar, which eventually leads to heart failure. Current therapies to cure or delay the progression of heart failure are limited; hence, there is a pressing need for regenerative approaches to support the failing heart. Cardiomyocytes undergo a series of transcriptional, structural, and metabolic changes after birth (collectively termed maturation), which is critical for their contractile function but limits the regenerative capacity of the heart. In regenerative organisms, cardiomyocytes revert from their terminally differentiated state into a less mature state (ie, dedifferentiation) to allow for proliferation and regeneration to occur. Importantly, stimulating adult cardiomyocyte dedifferentiation has been shown to promote morphological and functional improvement after myocardial infarction, further highlighting the importance of cardiomyocyte dedifferentiation in heart regeneration. Here, we review several hallmarks of cardiomyocyte maturation, and summarize how their reversal facilitates cardiomyocyte proliferation and heart regeneration. A detailed understanding of how cardiomyocyte dedifferentiation is regulated will provide insights into therapeutic options to promote cardiomyocyte de-maturation and proliferation, and ultimately heart regeneration in mammals.

Recent advances in regulating the proliferation or maturation of human-induced pluripotent stem cell-derived cardiomyocytes

In the last decade, human-induced pluripotent stem cell-derived cardiomyocyte (hiPSC-CM)-based cell therapy has drawn broad attention as a potential therapy for treating injured hearts. However, mass production of hiPSC-CMs remains challenging, limiting their translational potential in regenerative medicine. Therefore, multiple strategies including cell cycle regulators, small molecules, co-culture systems, and epigenetic modifiers have been used to improve the proliferation of hiPSC-CMs. On the other hand, the immaturity of these proliferative hiPSC-CMs could lead to lethal arrhythmias due to their limited ability to functionally couple with resident cardiomyocytes. To achieve functional maturity, numerous methods such as prolonged culture, biochemical or biophysical stimulation, in vivo transplantation, and 3D culture approaches have been employed. In this review, we summarize recent approaches used to promote hiPSC-CM proliferation, and thoroughly review recent advances in promoting hiPSC-CM maturation, which will serve as the foundation for large-scale production of mature hiPSC-CMs for future clinical applications.

CRISPR screening in cardiovascular research

The recent advent and widespread application of CRISPR-based genome editing tools have revolutionized biomedical research and beyond. Taking advantage of high perturbation efficiency and scalability, CRISPR screening has been regarded as one of the most powerful technologies in functional genomics which allows investigation of different genetic subjects at a large scale in parallel. Significant progress has been made using various CRISPR screening tools especially in cancer research, however, fewer attempts and less success are reported in other contexts. In this mini-review, we discuss how CRISPR screening has been implemented in studies on cardiovascular research and related metabolic disorders, highlight the scientific progress utilizing CRISPR screening, and further envision how to fully unleash the power of this technique to expedite scientific discoveries in these fields.

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.

Metabolic Regulation of Cardiac Regeneration

The mortality due to heart diseases remains highest in the world every year, with ischemic cardiomyopathy being the prime cause. The irreversible loss of cardiomyocytes following myocardial injury leads to compromised contractility of the remaining myocardium, adverse cardiac remodeling, and ultimately heart failure. The hearts of adult mammals can hardly regenerate after cardiac injury since adult cardiomyocytes exit the cell cycle. Nonetheless, the hearts of early neonatal mammals possess a stronger capacity for regeneration. To improve the prognosis of patients with heart failure and to find the effective therapeutic strategies for it, it is essential to promote endogenous regeneration of adult mammalian cardiomyocytes. Mitochondrial metabolism maintains normal physiological functions of the heart and compensates for heart failure. In recent decades, the focus is on the changes in myocardial energy metabolism, including glucose, fatty acid, and amino acid metabolism, in cardiac physiological and pathological states. In addition to being a source of energy, metabolites are becoming key regulators of gene expression and epigenetic patterns, which may affect heart regeneration. However, the myocardial energy metabolism during heart regeneration is majorly unknown. This review focuses on the role of energy metabolism in cardiac regeneration, intending to shed light on the strategies for manipulating heart regeneration and promoting heart repair after cardiac injury.

Defining the molecular underpinnings controlling cardiomyocyte proliferation

Shortly after birth, mammalian cardiomyocytes (CM) exit the cell cycle and cease to proliferate. The inability of adult CM to replicate renders the heart particularly vulnerable to injury. Restoration of CM proliferation would be an attractive clinical target for regenerative therapies that can preserve contractile function and thus prevent the development of heart failure. Our review focuses on recent progress in understanding the tight regulation of signaling pathways and their downstream molecular mechanisms that underly the inability of CM to proliferate in vivo. In this review, we describe the temporal expression of cell cycle activators e.g., cyclin/Cdk complexes and their inhibitors including p16, p21, p27 and members of the retinoblastoma gene family during gestation and postnatal life. The differential impact of members of the E2f transcription factor family and microRNAs on the regulation of positive and negative cell cycle factors is discussed. This review also highlights seminal studies that identified the coordination of signaling mechanisms that can potently activate CM cell cycle re-entry including the Wnt/Ctnnb1, Hippo, Pi3K-Akt and Nrg1-Erbb2/4 pathways. We also present an up-to-date account of landmark studies analyzing the effect of various genes such as Argin, Dystrophin, Fstl1, Meis1, Pitx2 and Pkm2 that are responsible for either inhibition or activation of CM cell division. All these reports describe bona fide therapeutically targets that could guide future clinical studies toward cardiac repair.

The Cardiac Sarcomere and Cell Cycle

PURPOSE OF REVIEW

The lack of adult human cardiomyocyte proliferative capacity impairs cardiac regeneration such as after myocardial injury. The sarcomere, a specialized actin cytoskeletal structure that is essential for twitch contraction in cardiomyocytes, has been considered a critical factor limiting adult human cardiomyocyte proliferation through incompletely understood mechanisms.

RECENT FINDINGS

This review summarizes known and emerging regulatory mechanisms connecting the human cardiomyocyte sarcomere to cell cycle regulation including structural and signaling mechanisms. Cardiac regeneration could be augmented through targeting the inhibitory effects of the sarcomere on cardiomyocyte proliferation.

AP-1 activation mediates postnatal cardiomyocyte maturation

AIMS

Postnatal maturation of mammalian cardiomyocytes proceeds rapidly after birth, with most of the myocytes exiting cell cycle, becoming binucleated, and adopting oxidative phosphorylation as the primary metabolic route. The triggers and transcriptional programs regulating cardiomyocyte maturation have not been fully understood yet. We performed single cell RNA-Seq in postnatal rat hearts in order to identify the important factors for this process.

METHODS AND RESULTS

Single cell RNA-Seq profiling was performed of postnatal day 1 and day 7 rat hearts, and we found that members of the AP-1 transcription factors showed a transient upregulation in the maturing cardiomyocytes, suggesting their functional involvement in the process. Activating members of the AP-1 family by palmitate or adrenergic stimulation inhibited cardiomyocyte cytokinesis and promoted cardiomyocyte maturation. In contrast, knocking down AP-1 members Atf3 and Jun promoted cardiomyocyte cytokinesis, reduced polyploidy and inhibited maturation. Mechanistically, RNA-Seq results and rescue experiments indicated that AP-1 members activate the expression of fatty acid metabolic genes to promote cardiomyocyte maturation. Finally, intraperitoneal injection of AP-1 inhibitor T-5224 in neonatal mice inhibits cardiomyocyte maturation in vivo.

CONCLUSION

Our results are the first evidence implicating AP-1 transcription factors in postnatal cardiomyocyte maturation both in vitro and in vivo, which expand our understanding of the molecular mechanism of cardiomyocyte maturation, and may lead to novel therapies to treat congenital heart diseases.

TRANSLATIONAL PERSPECTIVE

Postnatal cardiomyocyte maturation is a crucial process of cardiac development that determines fitness of the adult heart, and can be affected by multiple congenital heart diseases which lead to adult heart conditions. Our finding that AP-1 transcription factors transiently activated by multiple cues such as fatty acid and adrenergic signal promote cardiomyocyte maturation provided novel targets for therapeutic intervention, which may be applied during the narrow time window of postnatal cardiomyocyte maturation to treat congenital heart diseases and limit their impact on the adult heart.

Transient Cell Cycle Induction in Cardiomyocytes to Treat Subacute Ischemic Heart Failure

BACKGROUND

The regenerative capacity of the heart after myocardial infarction is limited. Our previous study showed that ectopic introduction of 4 cell cycle factors (4F; CDK1 [cyclin-dependent kinase 1], CDK4 [cyclin-dependent kinase 4], CCNB [cyclin B1], and CCND [cyclin D1]) promotes cardiomyocyte proliferation in 15% to 20% of infected cardiomyocytes in vitro and in vivo and improves cardiac function after myocardial infarction in mice.

METHODS

Using temporal single-cell RNA sequencing, we aimed to identify the necessary reprogramming stages during the forced cardiomyocyte proliferation with 4F on a single cell basis. Using rat and pig models of ischemic heart failure, we aimed to start the first preclinical testing to introduce 4F gene therapy as a candidate for the treatment of ischemia-induced heart failure.

RESULTS

Temporal bulk and single-cell RNA sequencing and further biochemical validations of mature human induced pluripotent stem cell-derived cardiomyocytes treated with either LacZ or 4F adenoviruses revealed full cell cycle reprogramming in 15% of the cardiomyocyte population at 48 hours after infection with 4F, which was associated mainly with sarcomere disassembly and metabolic reprogramming (n=3/time point/group). Transient overexpression of 4F, specifically in cardiomyocytes, was achieved using a polycistronic nonintegrating lentivirus (NIL) encoding 4F; each is driven by a TNNT2 (cardiac troponin T isoform 2) promoter (TNNT2-4Fpolycistronic-NIL). TNNT2-4Fpolycistronic-NIL or control virus was injected intramyocardially 1 week after myocardial infarction in rats (n=10/group) or pigs (n=6-7/group). Four weeks after injection, TNNT2-4Fpolycistronic-NIL-treated animals showed significant improvement in left ventricular ejection fraction and scar size compared with the control virus-treated animals. At 4 months after treatment, rats that received TNNT2-4Fpolycistronic-NIL still showed a sustained improvement in cardiac function and no obvious development of cardiac arrhythmias or systemic tumorigenesis (n=10/group).

CONCLUSIONS

This study provides mechanistic insights into the process of forced cardiomyocyte proliferation and advances the clinical feasibility of this approach by minimizing the oncogenic potential of the cell cycle factors owing to the use of a novel transient and cardiomyocyte-specific viral construct.

The Role of Metabolism in Heart Failure and Regeneration

Heart failure is the leading cause of death worldwide. The inability of the adult mammalian heart to regenerate following injury results in the development of systolic heart failure. Thus, identifying novel approaches toward regenerating the adult heart has enormous therapeutic potential for adult heart failure. Mitochondrial metabolism is an essential homeostatic process for maintaining growth and survival. The emerging role of mitochondrial metabolism in controlling cell fate and function is beginning to be appreciated. Recent evidence suggests that metabolism controls biological processes including cell proliferation and differentiation, which has profound implications during development and regeneration. The regenerative potential of the mammalian heart is lost by the first week of postnatal development when cardiomyocytes exit the cell cycle and become terminally differentiated. This inability to regenerate following injury is correlated with the metabolic shift from glycolysis to fatty acid oxidation that occurs during heart maturation in the postnatal heart. Thus, understanding the mechanisms that regulate cardiac metabolism is key to unlocking metabolic interventions during development, disease, and regeneration. In this review, we will focus on the emerging role of metabolism in cardiac development and regeneration and discuss the potential of targeting metabolism for treatment of heart failure.