YAP Partially Reprograms Chromatin Accessibility to Directly Induce Adult Cardiogenesis In Vivo

Cardiac enhancers: Gateway to the regulatory mechanisms of heart regeneration

The adult mammalian heart has limited regenerative capacity. Cardiac injury, such as a myocardial infarction (MI), leads to permanent scarring and impaired heart function. In contrast, neonatal mice and zebrafish possess the ability to repair injured hearts. Cardiac regeneration is driven by profound transcriptional changes, which are controlled by gene regulatory elements, such as tissue regeneration enhancer elements (TREEs). Here, we review recent studies on cardiac injury/regeneration enhancers across species. We further explore regulatory mechanisms governing TREE activities and their associated binding regulators. We also discuss the potential of TREE engineering and how these enhancers can be utilized for heart repair. Decoding the regulatory logic of cardiac regeneration enhancers presents a promising avenue for understanding heart regeneration and advancing therapeutic strategies for heart failure.

Ploidy in cardiovascular development and regeneration

Somatic polyploidy, a non-inheritable form of genome multiplication, plays cell-type specific and context-dependent roles in organ development and regeneration. In the mammalian heart, embryonic cardiomyocytes are primarily diploid, which lose their ability to complete cell division and become polyploid as they mature. Unlike lower vertebrates like zebrafish, polyploid cardiomyocytes are commonly found across mammals, including humans. Intriguingly, the degree, timing, and modes of cardiomyocyte polyploidization vary greatly between species. In addition to the association with cardiomyocyte development and maturation, recent studies have established polyploidy as a barrier against cardiomyocyte proliferation and heart regeneration following cardiac injury. Hence, a thorough understanding of how and why cardiomyocyte become polyploid will provide insights into heart development and may help develop therapeutic strategies for heart regeneration. Here, we review the dynamics of cardiomyocyte polyploidization across species and how cardiomyocyte-intrinsic, -extrinsic, and environmental factors regulate this process as well as the impact of cardiomyocyte polyploidization on heart development and regeneration.

HBO1 functions as an epigenetic barrier to hepatocyte plasticity and reprogramming during liver injury

Hepatocytes can reprogram into biliary epithelial cells (BECs) during liver injury, but the underlying epigenetic mechanisms remain poorly understood. Here, we define the chromatin dynamics of this process using single-cell ATAC-seq and identify YAP/TEAD activation as a key driver of chromatin remodeling. An in vivo CRISPR screen highlights the histone acetyltransferase HBO1 as a critical barrier to reprogramming. HBO1 is recruited by YAP to target loci, where it promotes histone H3 lysine 14 acetylation (H3K14ac) and engages the chromatin reader zinc-finger MYND-type containing 8 (ZMYND8) to suppress YAP/TEAD-driven transcription. Loss of HBO1 accelerates chromatin remodeling, enhances YAP binding, and enables a more complete hepatocyte-to-BEC transition. Our findings position HBO1 as an epigenetic brake that restrains YAP-mediated reprogramming, suggesting that targeting HBO1 may enhance hepatocyte plasticity for liver regeneration.

Cardiac regeneration: Unraveling the complex network of intercellular crosstalk

The heart is composed of multiple cell types, including cardiomyocytes, endothelial/endocardial cells, fibroblasts, resident immune cells and epicardium and crosstalk between these cell types is crucial for proper cardiac function and homeostasis. In response to cardiac injury or disease, cell-cell interactions and intercellular crosstalk contribute to remodeling to compensate reduced heart function. In some vertebrates, the heart can regenerate following cardiac injury. While cardiomyocytes play a crucial role in this process, additional cell types are necessary to create a pro-regenerative microenvironment in the injured heart. Here, we review recent literature regarding the importance of cellular crosstalk in promoting cardiac regeneration and provide insight into emerging technologies to investigate cell-cell interactions in vivo. Lastly, we explore recent studies highlighting the importance of inter-organ communication in response to injury and promotion of cardiac regeneration. Importantly, understanding how intercellular and inter-organ crosstalk promote cardiac regeneration is essential for the development of therapeutic strategies to stimulate regeneration in the human heart.

A retinoic acid:YAP1 signaling axis controls atrial lineage commitment

In cardiac progenitor cells (CPCs), retinoic acid (RA) signaling induces atrial lineage gene expression and acquisition of an atrial cell fate. To achieve this, RA coordinates a complex regulatory network of downstream effectors that is not fully identified. To address this gap, we applied a functional genomics approach (i.e., scRNA-seq and snATAC-seq) to untreated and RA-treated human embryonic stem cell (hESC)-derived CPCs. Unbiased analysis revealed that the Hippo effectors YAP1 and TEAD4 are integrated with the atrial transcription factor enhancer network and that YAP1 activates RA enhancers in CPCs. Furthermore, Yap1 deletion in mouse embryos compromises the expression of RA-induced genes, such as Nr2f2, in the CPCs of the second heart field. Accordingly, in hESC-derived patterned heart organoids, YAP1 regulates the formation of an atrial chamber but is dispensable for the formation of a ventricle. Overall, our findings revealed that YAP1 cooperates with RA signaling to induce atrial lineages during cardiogenesis.

HDAC7 promotes cardiomyocyte proliferation by suppressing myocyte enhancer factor 2

Postnatal mammalian cardiomyocytes (CMs) rapidly lose proliferative capacity and exit the cell cycle to undergo further differentiation and maturation. Cell cycle activation has been a major strategy to stimulate postnatal CM proliferation, albeit achieving modest effects. One impediment is that postnatal CMs may need to undergo dedifferentiation before proliferation, if not simultaneously. Here, we report that overexpression of Hdac7 in neonatal mouse CMs results in significant CM dedifferentiation and proliferation. Mechanistically, we show that histone deacetylase 7 (HDAC7)-mediated CM proliferation is contingent on dedifferentiation, which is accomplished by suppressing myocyte enhance factor 2 (MEF2). Hdac7 overexpression in CM shifts the chromatin state from binding with MEF2, which favors the transcriptional program toward differentiation, to binding with AP-1, which favors the transcriptional program toward proliferation. Furthermore, we found that HDAC7 interacts with minichromosome maintenance complex components to initiate cell cycle progression. Our findings reveal that HDAC7 promotes CM proliferation by its dual action on CM dedifferentiation and proliferation, uncovering a potential new strategy for heart regeneration/repair.

Border-zone cardiomyocytes and macrophages regulate extracellular matrix remodeling to promote cardiomyocyte protrusion during cardiac regeneration

Despite numerous advances in our understanding of zebrafish cardiac regeneration, an aspect that remains less studied is how regenerating cardiomyocytes invade and replace the collagen-containing injured tissue. Here, we provide an in-depth analysis of the process of cardiomyocyte invasion. We observe close interactions between protruding border-zone cardiomyocytes and macrophages, and show that macrophages are essential for extracellular matrix remodeling at the wound border zone and cardiomyocyte protrusion into the injured area. Single-cell RNA-sequencing reveals the expression of mmp14b, encoding a membrane-anchored matrix metalloproteinase, in several cell types at the border zone. Genetic mmp14b mutation leads to decreased macrophage recruitment, collagen degradation, and subsequent cardiomyocyte protrusion into injured tissue. Furthermore, cardiomyocyte-specific overexpression of mmp14b is sufficient to enhance cardiomyocyte invasion into the injured tissue and along the apical surface of the wound. Altogether, our data provide important insights into the mechanisms underlying cardiomyocyte invasion of the collagen-containing injured tissue during cardiac regeneration.

Hippo Signaling Is Essential for the Maintenance of Zona Glomerulosa Cell Fate in the Murine Adrenal Cortex

Cells of the zona glomerulosa (zG), the outermost zone of the adrenal cortex, secrete aldosterone and transdifferentiate into glucocorticoid-producing cells of the zona fasciculata (zF) during adrenal homeostasis. However, our understanding of the signaling pathways mediating zG cell maintenance or their transdifferentiation into zF cells is incomplete. Hippo is a major pathway that regulates cell proliferation/differentiation during embryogenesis and postnatal tissue homeostasis. Hypothesizing that Hippo signaling could be involved in zG cell maintenance or transdifferentiation, we generated a mouse model in which the two main kinases of the Hippo signaling cascade large tumor suppressor homolog kinases 1/2 (Lats1 and Lats2) are specifically inactivated in zG cells. Here we show that loss of function of Lats1 and Lats2 impairs zG steroidogenesis and leads to zG cell transdifferentiation into cells sharing characteristics with chondroblasts/osteoblasts rather than zF cells. Furthermore, we demonstrate that this phenotype can be rescued by the concomitant inactivation of the transcriptional coactivators Yes-associated protein (Yap) and transcriptional coactivator with PDZ-binding motif (Taz) with Lats1 and Lats2. Finally, we show that expression of a constitutively active form of YAP (YAP5SA) in zG cells does not alter their fate as severely as the loss of Lats1 and Lats2 but leads to adrenal hyperplasia. Together, these findings highlight the critical role of Hippo signaling in maintaining zG cell fate and function and provide key insights into broader mechanisms underlying cellular differentiation.

The effects of YAP/TAZ in cardiomyocytes: a scoping review

The Hippo signaling pathway, through its effectors' yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ), plays a pivotal role in heart development, regeneration, and repair. Despite the well-recognized role of YAP in promoting cardiomyocyte proliferation and differentiation, the underlying mechanisms require further explanation. Therefore, this scoping review was conducted to explore the underlying mechanisms of YAP and TAZ in cardiomyocyte biology. In this scoping review, 138 studies were screened using PRISMA extension for scoping reviews guidelines to examine the upstream regulators, mechanisms, and therapeutic potential of YAP/TAZ in cardiomyocytes. Articles were selected based on relevance to YAP/TAZ signaling in cardiac regeneration and focused on upstream regulators, signaling pathways, and therapeutic applications. Data were extracted using standardized forms, and thematic analysis was performed iteratively. YAP activation regulated several processes, including cardiomyocyte proliferation, differentiation, and protection against oxidative stress. Mechanotransduction factors influence YAP activity, linking the biomechanical environment to cardiac regeneration. Novel upstream regulators, such as prorenin receptors, melatonin, and ERBB2, were identified as YAP/TAZ modulators. Moreover, downstream pathways such as Wnt/β-catenin, PI3K/Akt, and TLR-mediated inflammation confer their effects on cellular proliferation, mitochondrial dynamics, and inflammation. Several therapeutic targets involving YAP that could enhance cardiac regeneration while reducing fibrosis and inflammation were identified. However, significant research gaps remain, including the underexplored role of TAZ, necessity for in vivo studies and transcriptomics to elucidate cell-specific effects, and intricate regulatory networks of YAP/TAZ.

The Hippo pathway and p27Kip1 cooperate to suppress mitotic regeneration in the organ of Corti and the retina

The mature mammalian auditory sensory organ, the organ of Corti (OC), lacks the capacity for regenerating hair cells, leading to permanent hearing impairment. In contrast, the vestibular system has a limited capacity for hair cell regeneration, which we have shown to be further enhanced by inhibiting the Hippo pathway. Here, we demonstrate that, despite similar transcriptional responses, only vestibular and not auditory supporting cells proliferate as a result of Yap activation following Hippo inhibition. Mechanistically, we identify p27Kip1, a cell cycle kinase inhibitor encoded by Cdkn1b, as an additional barrier preventing cell cycle reentry specifically in the OC. We show that while in both systems Yap stimulates p27Kip1 degradation through activation of its direct target gene Skp2, this protein-level control is antagonized by an unusually high level of Cdkn1b transcription in the cochlea. Consequently, p27Kip1 activity is maintained in the OC even in the presence of constitutively active Yap5SA, counteracting its mitogenic effects. Supporting this model, inactivation of the Hippo pathway in the Cdkn1b-deficient background is sufficient to induce adult auditory supporting cell proliferation in vivo. Furthermore, we show that the synergistic interaction between Hippo and p27Kip1 is conserved in the retina where inhibition of both pathways potently induces Müller glia proliferation and initiates neuronal regeneration. Our work uncovers the molecular mechanism preventing quiescent adult sensory progenitor cells, supporting cells in the ear and Müller glia in the eye, from reentering the cell cycle after damage-the key step toward sensory receptor regeneration blocked in mammals.

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.

The Hippo pathway: Organ size control and beyond

The Hippo signaling pathway is a highly conserved signaling network for controlling organ size, tissue homeostasis, and regeneration. It integrates a wide range of intracellular and extracellular signals, such as cellular energy status, cell density, hormonal signals, and mechanical cues, to modulate the activity of YAP/TAZ transcriptional coactivators. A key aspect of Hippo pathway regulation involves its spatial organization at the plasma membrane, where upstream regulators localize to specific membrane subdomains to regulate the assembly and activation of the pathway components. This spatial organization is critical for the precise control of Hippo signaling, as it dictates the dynamic interactions between pathway components and their regulators. Recent studies have also uncovered the role of biomolecular condensation in regulating Hippo signaling, adding complexity to its control mechanisms. Dysregulation of the Hippo pathway is implicated in various pathological conditions, particularly cancer, where alterations in YAP/TAZ activity contribute to tumorigenesis and drug resistance. Therapeutic strategies targeting the Hippo pathway have shown promise in both cancer treatment, by inhibiting YAP/TAZ signaling, and regenerative medicine, by enhancing YAP/TAZ activity to promote tissue repair. The development of small molecule inhibitors targeting the YAP-TEAD interaction and other upstream regulators offers new avenues for therapeutic intervention. SIGNIFICANCE STATEMENT: The Hippo signaling pathway is a key regulator of organ size, tissue homeostasis, and regeneration, with its dysregulation linked to diseases such as cancer. Understanding this pathway opens new possibilities for therapeutic approaches in regenerative medicine and oncology, with the potential to translate basic research into improved clinical outcomes.

The role of mechanosignaling in the control of myocardial mass

Regulation of myocardial mass is key for maintaining cardiovascular health. This review highlights the complex and regulatory relationship between mechanosignaling and myocardial mass, influenced by many internal and external factors including hemodynamic and microgravity, respectively. The heart is a dynamic organ constantly adapting to changes in workload (preload and afterload) and mechanical stress exerted on the myocardium, influencing both physiological adaptations and pathological remodeling. Mechanosignaling pathways, such as the mitogen-activated protein kinases (MAPKs) and the phosphoinositide 3-kinases and serine/threonine kinase (PI3K/Akt) pathways, mediate downstream effects on gene expression and play key roles in transducing mechanical cues into biochemical signals, thereby modulating cellular processes, including control of myocardial mass. Dysregulation of these processes can lead to pathological cardiac remodeling, such as hypertrophic cardiomyopathy. Furthermore, recent studies have highlighted the importance of protein quality control mechanisms, such as the ubiquitin-proteasome system, in settings of extreme physiological conditions that alter the heart workload such as pregnancy and microgravity. Overall, this review provides a thorough insight into how mechanical signals are converted into chemical signals to regulate myocardial mass in both healthy and diseased conditions.

TAZ interactome analysis using nanotrap-based affinity purification-mass spectrometry

Characterization of protein-protein interactions (PPIs) is a fundamental goal in the post-genomic era. Here, we document a generally applicable approach to identify cellular protein interactomes using a combination of nanobody-based affinity purification (AP) coupled with liquid chromatography and tandem mass spectrometry (LC-MS/MS). The Hippo signaling regulator TAZ (also known as WWTR1) functions as a transcriptional co-repressor or activator depending on its PPI network; we therefore undertook an unbiased proteomic screen to identify TAZ PPIs in striated muscle cells. A GFP nanotrap-based AP approach coupled with protein identification through LC-MS/MS was used to document a comprehensive list of known and novel TAZ interactome components. Informatic analysis of the interactome documented known components of the Hippo signaling pathway and multiple epigenetic regulators such as the NuRD, FACT and SWI/SNF complexes and the pro-myogenic CARM1 methyltransferase. Hippo pathway reporter gene (HOP/HIP) analysis indicated that CARM1 represses TAZ transcriptional co-activator function, promoting TAZ Ser89 phosphorylation and TAZ cytoplasmic sequestration. MS analysis revealed that CARM1 dimethylates TAZ at Arg77 in a PGPR*LAGG consensus peptide, resulting in enhanced TAZ Ser89 phosphorylation. These studies underline the utility of a nanobody-based AP approach for interactome analysis.

Dynamic map illuminates Hippo-cMyc module crosstalk driving cardiomyocyte proliferation

Numerous regulators of cardiomyocyte (CM) proliferation have been identified, yet how they coordinate during cardiac development or regeneration is poorly understood. Here, we developed a computational model of the CM proliferation regulatory network to obtain key regulators and systems-level understanding. The model defines five modules (DNA replication, mitosis, cytokinesis, growth factor, Hippo pathway) and integrates them into a network of 72 nodes and 88 reactions that correctly predicts 74 of 81 (91.35%) independent experiments from the literature. The model predicts that in response to YAP activation, the Hippo module crosstalks to the growth factor module via PI3K and cMyc to drive cell cycle activity. This predicted YAP-cMyc axis is validated experimentally in rat CMs and further supported by YAP-stimulated cMyc open chromatin and mRNA in mouse hearts. This validated computational model predicts how individual regulators and modules coordinate to control CM proliferation.

Spiny mice (Acomys) have evolved cellular features to support regenerative healing

Spiny mice (Acomys spp.) are warm-blooded (homeothermic) vertebrates whose ability to restore missing tissue through regenerative healing has coincided with the evolution of unique cellular and physiological adaptations across different tissue types. This review seeks to explore how these bizarre rodents deploy unique or altered injury response mechanisms to either enhance tissue repair or fully regenerate excised tissue compared to closely related, scar-forming mammals. First, we examine overall trends in healing Acomys tissues, including the cellular stress response, the ability to activate and maintain cell cycle progression, and the expression of certain features in reproductive adults that are normally associated with embryos. Second, we focus on specific cell types that exhibit precisely regulated proliferation to restore missing tissue. While Acomys utilize many of the same cell types involved in scar formation, these cells exhibit divergent activation profiles during regenerative healing. Considered together, current lines of evidence support sustained deployment of proregenerative pathways in conjunction with transient activation of fibrotic pathways to facilitate regeneration and improve tissue repair in Acomys.

Cardiac Regeneration in Adult Zebrafish: A Review of Signaling and Metabolic Coordination

PURPOSE OF REVIEW

This review investigates how post-injury cellular signaling and energy metabolism are two pivotal points in zebrafish's cardiomyocyte cell cycle re-entry and proliferation. It seeks to highlight the probable mechanism of action in proliferative cardiomyocytes compared to mammals and identify gaps in the current understanding of metabolic regulation of cardiac regeneration.

RECENT FINDINGS

Metabolic substrate changes after birth correlate with reduced cardiomyocyte proliferation in mammals. Unlike adult mammalian hearts, zebrafish can regenerate cardiomyocytes by re-entering the cell cycle, characterized by a metabolic switch from oxidative metabolism to increased glycolysis. Zebrafish provide a valuable model for studying metabolic regulation during cell cycle re-entry and cardiac regeneration. Proliferative cardiomyocytes have upregulated Notch, hippo, and Wnt signaling and decreased ROS generation, DNA damage in different zebrafish cardiac regeneration models. Understanding the correlation between metabolic switches during cell cycle re-entry of already differentiated zebrafish cardiomyocytes is being increasingly recognized as a critical factor in heart regeneration. Zebrafish studies provide insights into metabolic adaptations during heart regeneration, emphasizing the importance of a metabolic switch. However, there are mechanistic gaps, and extensive studies are required to aid in formulating therapeutic strategies for cardiac regenerative medicine.

Microtubules Sequester Acetylated YAP in the Cytoplasm and Inhibit Heart Regeneration

BACKGROUND

The Hippo pathway effector YAP (Yes-associated protein) plays an essential role in cardiomyocyte proliferation and heart regeneration. In response to physiological changes, YAP moves in and out of the nucleus. The pathophysiological mechanisms regulating YAP subcellular localization after myocardial infarction remain poorly defined.

METHODS

We identified YAP acetylation at site K265 by in vitro acetylation followed by mass spectrometry analysis. We used adeno-associated virus to express YAP-containing mutations that either abolished acetylation (YAP-K265R) or mimicked acetylation (YAP-K265Q) and studied how acetylation regulates YAP subcellular localization in mouse hearts. We generated a cell line with YAP-K265R mutation and investigated the protein-protein interactors by YAP immunoprecipitation followed by mass spectrometry, then validated the YAP interaction in neonatal rat ventricular myocytes. We examined colocalization of YAP and TUBA4A (tubulin α 4A) by superresolution imaging. Furthermore, we developed YAP-K265R and αMHC-MerCreMer (MCM); Yap-loxP/K265R mutant mice to examine the pathophysiological role of YAP acetylation in cardiomyocytes during cardiac regeneration.

RESULTS

We found that YAP is acetylated at K265 by CBP (CREB-binding protein)/P300 (E1A-binding protein P300) and is deacetylated by nicotinamide phosphoribosyltransferase/nicotinamide adenine dinucleotide/sirtuins axis in cardiomyocytes. After myocardial infarction, YAP acetylation is increased, which promotes YAP cytoplasmic localization. Compared with controls, mice that were genetically engineered to express a K265R mutation that prevents YAP K265 acetylation showed improved cardiac regenerative ability and increased YAP nuclear localization. Mechanistically, YAP acetylation facilitates its interaction with TUBA4A, a component of the microtubule network that sequesters acetylated YAP in the cytoplasm. After myocardial infarction, the microtubule network increased in cardiomyocytes, resulting in the accumulation of YAP in the cytoplasm.

CONCLUSIONS

After myocardial infarction, decreased sirtuin activity enriches YAP acetylation at K265. The growing TUBA4A network sequesters acetylated YAP within the cytoplasm, which is detrimental to cardiac regeneration.

YAP Overcomes Mechanical Barriers to Induce Mitotic Rounding and Adult Cardiomyocyte Division

BACKGROUND

Many specialized cells in adult organs acquire a state of cell cycle arrest and quiescence through unknown mechanisms. Our limited understanding of mammalian cell cycle arrest is derived primarily from cell culture models. Adult mammalian cardiomyocytes, a classic example of cell cycle arrested cells, exit the cell cycle postnatally and remain in an arrested state for the life of the organism. Cardiomyocytes can be induced to re-enter the cell cycle by YAP5SA, an active form of the Hippo signaling pathway effector YAP.

METHODS

We performed clonal analyses to determine the cell cycle kinetics of YAP5SA cardiomyocytes. We also performed single-cell RNA sequencing, marker gene analysis, and functional studies to examine how YAP5SA cardiomyocytes progress through the cell cycle.

RESULTS

We discovered that YAP5SA-expressing cardiomyocytes divided efficiently, with >20% of YAP5SA cardiomyocyte clones containing ≥2 cardiomyocytes. YAP5SA cardiomyocytes re-entered cell cycle at the G1/S transition and had an S phase lasting ≈48 hours. Sarcomere disassembly is required for cardiomyocyte progression from S to G2 phase and the induction of mitotic rounding. Although oscillatory Cdk expression was induced in YAP5SA cardiomyocytes, these cells inefficiently progressed through G2 phase. This is improved by inhibiting P21 function, implicating checkpoint activity as an additional barrier to YAP5SA-induced cardiomyocyte division.

CONCLUSIONS

Our data reveal that YAP5SA overcomes the mechanically constrained myocardial microenvironment to induce mitotic rounding with cardiomyocyte division, thus providing new insights into the in vivo mechanisms that maintain cell cycle quiescence in adult mammals.

Cross-species comparison reveals that Hmga1 reduces H3K27me3 levels to promote cardiomyocyte proliferation and cardiac regeneration

In contrast to adult mammalian hearts, the adult zebrafish heart efficiently replaces cardiomyocytes lost after injury. Here we reveal shared and species-specific injury response pathways and a correlation between Hmga1, an architectural non-histone protein, and regenerative capacity, as Hmga1 is required and sufficient to induce cardiomyocyte proliferation and required for heart regeneration. In addition, Hmga1 was shown to reactivate developmentally silenced genes, likely through modulation of H3K27me3 levels, poising them for a pro-regenerative gene program. Furthermore, AAV-mediated Hmga1 expression in injured adult mouse hearts led to controlled cardiomyocyte proliferation in the border zone and enhanced heart function, without cardiomegaly and adverse remodeling. Histone modification mapping in mouse border zone cardiomyocytes revealed a similar modulation of H3K27me3 marks, consistent with findings in zebrafish. Our study demonstrates that Hmga1 mediates chromatin remodeling and drives a regenerative program, positioning it as a promising therapeutic target to enhance cardiac regeneration after injury.

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.

Decoding the Impact of the Hippo Pathway on Different Cell Types in Heart Failure

The evolutionarily conserved Hippo pathway plays a pivotal role in governing a variety of biological processes. Heart failure (HF) is a major global health problem with a significant risk of mortality. This review provides a contemporary understanding of the Hippo pathway in regulating different cell types during HF. Through a systematic analysis of each component's regulatory mechanisms within the Hippo pathway, we elucidate their specific effects on cardiomyocytes, fibroblasts, endothelial cells, and macrophages in response to various cardiac injuries. Insights gleaned from both in vitro and in vivo studies highlight the therapeutic promise of targeting the Hippo pathway to address cardiovascular diseases, particularly HF.

A cellular triad for linking cardiac niche to regeneration

Cardiovascular disease is the leading cause of mortality with very limited therapeutic interventions, thus holding great hope for cardiac regenerative medicine. A recent work from Martin's laboratory reports their identification of a fetal-like cardiomyocyte progenitor, adult cardiomyocyte type 2 (aCM2), and its potential interactions with C3+ cardiac fibroblasts and C3ar1+ macrophages to form a regenerative cellular triad, which is only present in the regenerative heart models, YAP5SA-expressing adult hearts and neonatal hearts. The complement signaling is essential for cellular interactions in this regenerative triad. This Highlight summarizes these major findings and provides brief perspectives on the impact of this regenerative niche during cardiac regeneration in the future.

The Multifaceted Roles of Hippo-YAP in Cardiovascular Diseases

The Hippo-yes-associated protein (YAP) signaling pathway plays a crucial role in cell proliferation, differentiation, and death. It is known to have impact on the progression and development of cardiovascular diseases (CVDs) as well as in the regeneration of cardiomyocytes (CMs). However, further research is needed to understand the molecular mechanisms by which the Hippo-YAP pathway affects the pathological processes of CVDs in order to evaluate its potential clinical applications. In this review, we have summarized the recent findings on the role of the Hippo-YAP pathway in CVDs such as myocardial infarction, heart failure, and cardiomyopathy, as well as its in CM development. This review calls attention to the potential roles of the Hippo-YAP pathway as a relevant target for the future treatment of CVDs.

Force-sensing protein expression in response to cardiovascular mechanotransduction

Force-sensing biophysical cues in microenvironment, including extracellular matrix performances, stretch-mediated mechanics, shear stress and flow-induced hemodynamics, have a significant influence in regulating vascular morphogenesis and cardiac remodeling by mechanotransduction. Once cells perceive these extracellular mechanical stimuli, Piezo activation promotes calcium influx by forming integrin-adhesion-coupling receptors. This induces robust contractility of cytoskeleton structures to further transmit biomechanical alternations into nuclei by regulating Hippo-Yes associated protein (YAP) signaling pathway between cytoplasmic and nuclear translocation. Although biomechanical stimuli are widely studied in cardiovascular diseases, the expression of force-sensing proteins in response to cardiovascular mechanotransduction has not been systematically concluded. Therefore, this review will summarize the force-sensing Piezo, cytoskeleton and YAP proteins to mediate extracellular mechanics, and also give the prominent emphasis on intrinsic connection of these mechanical proteins and cardiovascular mechanotransduction. Extensive insights into cardiovascular mechanics may provide some new strategies for cardiovascular clinical therapy.

YAP/TAZ Signalling Controls Epidermal Keratinocyte Fate

The paralogues Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) control cell proliferation and cell fate determination from embryogenesis to ageing. In the skin epidermis, these proteins are involved in both homeostatic cell renewal and injury-induced regeneration and also drive carcinogenesis and other pathologies. YAP and TAZ are usually considered downstream of the Hippo pathway. However, they are the central integrating link for the signalling microenvironment since they are involved in the interplay with signalling cascades induced by growth factors, cytokines, and physical parameters of the extracellular matrix. In this review, we summarise the evidence on how YAP and TAZ are activated in epidermal keratinocytes; how YAP/TAZ-mediated signalling cooperates with other signalling molecules at the plasma membrane, cytoplasmic, and nuclear levels; and how YAP/TAZ ultimately controls transcription programmes, defining epidermal cell fate.

Single-cell multi-modal integrative analyses highlight functional dynamic gene regulatory networks directing human cardiac development

Illuminating the precise stepwise genetic programs directing cardiac development provides insights into the mechanisms of congenital heart disease and strategies for cardiac regenerative therapies. Here, we integrate in vitro and in vivo human single-cell multi-omic studies with high-throughput functional genomic screening to reveal dynamic, cardiac-specific gene regulatory networks (GRNs) and transcriptional regulators during human cardiomyocyte development. Interrogating developmental trajectories reconstructed from single-cell data unexpectedly reveal divergent cardiomyocyte lineages with distinct gene programs based on developmental signaling pathways. High-throughput functional genomic screens identify key transcription factors from inferred GRNs that are functionally relevant for cardiomyocyte lineages derived from each pathway. Notably, we discover a critical heat shock transcription factor 1 (HSF1)-mediated cardiometabolic GRN controlling cardiac mitochondrial/metabolic function and cell survival, also observed in fetal human cardiomyocytes. Overall, these multi-modal genomic studies enable the systematic discovery and validation of coordinated GRNs and transcriptional regulators controlling the development of distinct human cardiomyocyte populations.

ER stress-induced YAP upregulation leads to chondrocyte phenotype loss in age-related osteoarthritis

Background

Osteoarthritis (OA) is a common degenerative joint disease, leading to pain and restricted mobility. Age-related endoplasmic reticulum (ER) stress has been implicated in the pathogenesis of OA, but the underlying mechanisms remain unclear. This study aims to explore the relationship between age-related ER stress, YAP overexpression, and chondrocyte phenotype loss in the development of OA.

Methods

Cartilage samples were collected from patients undergoing amputation, and age-related ER stress markers and YAP expression were assessed using immunohistochemical staining and qPCR. Transgenic mice with cartilage-specific YAP overexpression (YAPOE) were created, and Pamrevlumab was administered to evaluate its therapeutic effects.

Results

Higher expression of ER stress markers and YAP were showed in aged tissues compared to younger tissues. YAP overexpression led to decreased levels of cartilage phenotype markers and increased osteogenesis-related proteins. In vivo, YAPOE mice exhibited OA-like cartilage degeneration, which was mitigated by Pamrevlumab treatment.

Conclusion

Age-related ER stress induces YAP overexpression, contributing to OA pathogenesis. Pamrevlumab effectively prevents this phenotype loss in YAPOE mice, suggesting its potential as a therapeutic agent for OA. These findings provide new insights into the molecular mechanisms of OA and highlight the importance of targeting the ER stress-YAP-CTGF signaling pathway in OA treatment and prevention.

Cardiomyocyte-Specific Overexpression of Activated Yes-Associated Protein Modified-RNA Promotes Cardiomyocyte Proliferation and Myocardial Regeneration

BACKGROUND

The proliferative capacity of cardiomyocytes in adult mammalian hearts is far too low to replace the cells that are lost to myocardial infarction. Both cardiomyocyte proliferation and myocardial regeneration can be improved via the overexpression of a constitutively active variant of YAP5SA (Yes-associated protein, 5SA [active] mutant), but persistent overexpression of proliferation-inducing genes could lead to hypertrophy and arrhythmia, whereas off-target expression in fibroblasts and macrophages could increase fibrosis and inflammation.

METHODS AND RESULTS

Transient overexpression of YAP5SA or GFP (green fluorescent protein; control) was targeted to cardiomyocytes via our cardiomyocyte-specific modified mRNA translation system (YAP5SACM-SMRTs or GFPCM-SMRTs, respectively). YAP5SA-cardiomyocyte specificity was confirmed via in vitro experiments in cardiomyocytes and cardiac fibroblasts that had been differentiated from human induced- pluripotent stem cells and in human umbilical-vein endothelial cells, and the regenerative potency of YAP5SACM-SMRTs was evaluated in a mouse myocardial infarction model. In cultured human induced-pluripotent stem cells-cardiomyocytes, YAP was abundantly expressed for 3 days after YAP5SACM-SMRTs administration and was accompanied by increases in the expression of markers for proliferation, before declining to near-background levels after day 7, whereas GFP fluorescence remained high from days 1 to 3 after GFPCM-SMRTs treatment and then slowly declined. GFP fluorescence was also observed in human induced-pluripotent stem cells-cardiac fibroblasts and human umbilical-vein endothelial cells on day 1 after GFPCM-SMRTs administration but declined substantially by day 3. In the mouse myocardial infarction model, echocardiographic assessments of left-ventricular ejection fraction and fractional shortening were significantly greater, whereas infarct sizes were significantly smaller in YAP5SACM-SMRTs-treated mice than in vehicle-treated control animals, and YAP5SACM-SMRTs appeared to promote cardiomyocyte proliferation.

CONCLUSIONS

The CM-SMRTs can be used to transiently and specifically overexpress YAP5SA in cardiomyocytes, and this treatment strategy significantly promoted cardiomyocyte proliferation and myocardial regeneration in a mouse myocardial infarction model.

Transient proliferation by reversible YAP and mitogen-control of the cyclin D1/p27 ratio

Hippo-YAP signaling orchestrates epithelial tissue repair and is therefore an attractive target in regenerative medicine. Yet it is unresolved how YAP integrates with mitogen signaling and contact inhibition to control the underlying transient proliferative response. Here we show that reduced contact inhibition, increased mitogen signaling, and YAP-TEAD activation converge on increasing the nuclear cyclin D1/p27 protein ratio during G1 phase, towards a threshold ratio that dictates whether individual cells enter or exit the cell cycle. YAP increases this ratio indirectly, in concert with mitogen signaling, by increasing EGFR and other receptors that signal primarily through ERK. After a delay, contact inhibition suppresses YAP activity which gradually downregulates mitogen signaling and the cyclin D1/p27 ratio. Increasing YAP activity by ablating the suppressor Merlin/NF2 reveals a balancing mechanism in which YAP suppression and contact inhibition of proliferation can be recovered but only at higher local cell density. Thus, critical for tissue repair, robust proliferation responses result from the YAP-induced and receptor-mediated prolonged increase in the cyclin D1/p27 ratio, which is only reversed by delayed suppression of receptor signaling after contact inhibition of YAP.

STEMIN and YAP5SA, the future of heart repair?

This review outlines some of the many approaches taken over a decade or more to repair damaged hearts. We showcase the recent breakthroughs in organ regeneration elicited by reprogramming factors OCT3/4, SOX2, KLF4, and C-MYC (OKSM). Transient OKSM transgene expression rejuvenated senescent organs in mice. OKSM transgenes also caused murine heart cell regeneration. A triplet alanine mutation of the N-terminus of Serum Response Factor's MADS box SRF153(A3), termed STEMIN, and the YAP mutant, YAP5SA synergized and activated OKSM and NANOG in adult rat cardiac myocytes; thus, causing rapid nuclear proliferation and blocked myocyte differentiation. In addition, ATAC seq showed induced expression of growth factor genes FGFs, BMPs, Notchs, IGFs, JAK, STATs and non-canonical Wnts. Injected STEMIN and YAP5SA synthetic modifying mRNA (mmRNA) into infarcted adult mouse hearts, brought damaged hearts back to near normal contractility without severe fibrosis. Thus, STEMIN and YAP5SA mmRNA may exert additional regenerative potential than OKSM alone for treating heart diseases.

Discovery of selective LATS inhibitors via scaffold hopping: enhancing drug-likeness and kinase selectivity for potential applications in regenerative medicine

Due to its essential roles in cell proliferation and apoptosis, the precise regulation of the Hippo pathway through LATS presents a viable biological target for developing new drugs for cancer and regenerative diseases. However, currently available probes for selective and highly drug-like inhibition of LATS require further improvement in terms of both activity, selectivity and drug-like properties. Through scaffold hopping aided by docking studies and AI-assisted prediction of metabolic stabilities, we successfully identified an advanced LATS inhibitor demonstrating potent kinase activity, exceptional selectivity against other kinases, and superior oral pharmacokinetic profiles.

Hallmarks of regeneration

Regeneration is a heroic biological process that restores tissue architecture and function in the face of day-to-day cell loss or the aftershock of injury. Capacities and mechanisms for regeneration can vary widely among species, organs, and injury contexts. Here, we describe "hallmarks" of regeneration found in diverse settings of the animal kingdom, including activation of a cell source, initiation of regenerative programs in the source, interplay with supporting cell types, and control of tissue size and function. We discuss these hallmarks with an eye toward major challenges and applications of regenerative biology.

A Retinoic Acid:YAP1 signaling axis controls atrial lineage commitment

Vitamin A/Retinoic Acid (Vit A/RA) signaling is essential for heart development. In cardiac progenitor cells (CPCs), RA signaling induces the expression of atrial lineage genes while repressing ventricular genes, thereby promoting the acquisition of an atrial cardiomyocyte cell fate. To achieve this, RA coordinates a complex regulatory network of downstream effectors that is not fully identified. To address this gap, we applied a functional genomics approach (i.e scRNAseq and snATACseq) to untreated and RA-treated human embryonic stem cells (hESCs)-derived CPCs. Unbiased analysis revealed that the Hippo effectors YAP1 and TEAD4 are integrated with the atrial transcription factor enhancer network, and that YAP1 is necessary for activation of RA-enhancers in CPCs. Furthermore, in vivo analysis of control and conditionally YAP1 KO mouse embryos (Sox2-cre) revealed that the expression of atrial lineage genes, such as NR2F2, is compromised by YAP1 deletion in the CPCs of the second heart field. Accordingly, we found that YAP1 is required for the formation of an atrial chamber but is dispensable for the formation of a ventricle, in hESC-derived patterned cardiac organoids. Overall, our findings revealed that YAP1 is a non-canonical effector of RA signaling essential for the acquisition of atrial lineages during cardiogenesis.

Effects and mechanisms of the myocardial microenvironment on cardiomyocyte proliferation and regeneration

The adult mammalian cardiomyocyte has a limited capacity for self-renewal, which leads to the irreversible heart dysfunction and poses a significant threat to myocardial infarction patients. In the past decades, research efforts have been predominantly concentrated on the cardiomyocyte proliferation and heart regeneration. However, the heart is a complex organ that comprises not only cardiomyocytes but also numerous noncardiomyocyte cells, all playing integral roles in maintaining cardiac function. In addition, cardiomyocytes are exposed to a dynamically changing physical environment that includes oxygen saturation and mechanical forces. Recently, a growing number of studies on myocardial microenvironment in cardiomyocyte proliferation and heart regeneration is ongoing. In this review, we provide an overview of recent advances in myocardial microenvironment, which plays an important role in cardiomyocyte proliferation and heart regeneration.

Activation of fetal-like molecular programs during regeneration in the intestine and beyond

Tissue regeneration after damage is generally thought to involve the mobilization of adult stem cells that divide and differentiate into progressively specialized progeny. However, recent studies indicate that tissue regeneration can be accompanied by reversion to a fetal-like state. During this process, cells at the injury site reactivate programs that operate during fetal development but are typically absent in adult homeostasis. Here, we summarize our current understanding of the molecular signals and epigenetic mediators that orchestrate "fetal-like reversion" during intestinal regeneration. We also explore evidence for this phenomenon in other organs and species and highlight open questions that merit future examination.

Abnormal mechanical stress induced chondrocyte senescence by YAP loss-mediated METTL3 upregulation

OBJECTIVES

Abnormal mechanical stress is the pivotal risk factor of temporomandibular joint osteoarthritis (TMJOA). This study investigated the pathogenic mechanism by which abnormal mechanical stress induced chondrocyte senescence.

MATERIALS AND METHODS

Cellular senescence was investigated in the rodent model of unilateral anterior crossbite and in the chondrocytes subjected to mechanical overloading in vitro. The effects of Yes-associated protein (YAP) in chondrocyte senescence and its correlation with methyltransferase-like 3 (METTL3) and N6-methyladenosine (m6A) modification were evaluated. The role of m6A modification in chondrocyte senescence was determined. The therapeutic effects of m6A inhibition in TMJOA were investigated.

RESULTS

Senescent chondrocytes were accumulated in the mechanically induced TMJOA lesions in rats and mechanical overloading could trigger chondrocyte senescence in vitro. This mechanical stress-induced cellular senescence was revealed to be mediated by YAP deficiency that promoted METTL3-dependent m6A modification. Moreover, inhibition of m6A modification rescued chondrocyte senescence in vitro and in vivo, and suppressed TMJOA progression in rats.

CONCLUSIONS

This study uncovered the underlying mechanism of mechanically induced senescence in TMJOA from the perspective of epitranscriptomics and revealed the therapeutic potential of m6A inhibition in TMJOA.

ATP-dependent citrate lyase Drives Left Ventricular Dysfunction by Metabolic Remodeling of the Heart

Background

Metabolic remodeling is a hallmark of the failing heart. Oncometabolic stress during cancer increases the activity and abundance of the ATP-dependent citrate lyase (ACL, Acly ), which promotes histone acetylation and cardiac adaptation. ACL is critical for the de novo synthesis of lipids, but how these metabolic alterations contribute to cardiac structural and functional changes remains unclear.

Methods

We utilized human heart tissue samples from healthy donor hearts and patients with hypertrophic cardiomyopathy. Further, we used CRISPR/Cas9 gene editing to inactivate Acly in cardiomyocytes of MyH6-Cas9 mice. In vivo, positron emission tomography and ex vivo stable isotope tracer labeling were used to quantify metabolic flux changes in response to the loss of ACL. We conducted a multi-omics analysis using RNA-sequencing and mass spectrometry-based metabolomics and proteomics. Experimental data were integrated into computational modeling using the metabolic network CardioNet to identify significantly dysregulated metabolic processes at a systems level.

Results

Here, we show that in mice, ACL drives metabolic adaptation in the heart to sustain contractile function, histone acetylation, and lipid modulation. Notably, we show that loss of ACL increases glucose oxidation while maintaining fatty acid oxidation. Ex vivo isotope tracing experiments revealed a reduced efflux of glucose-derived citrate from the mitochondria into the cytosol, confirming that citrate is required for reductive metabolism in the heart. We demonstrate that YAP inactivation facilitates ACL deficiency. Computational flux analysis and integrative multi-omics analysis indicate that loss of ACL induces alternative isocitrate dehydrogenase 1 flux to compensate.

Conclusions

This study mechanistically delineates how cardiac metabolism compensates for suppressed citrate metabolism in response to ACL loss and uncovers metabolic vulnerabilities in the heart.

YAP dysregulation triggers hypertrophy by CCN2 secretion and TGFβ uptake in human pluripotent stem cell-derived cardiomyocytes

Hypertrophy Cardiomyopathy (HCM) is the most prevalent hereditary cardiovascular disease - affecting >1:500 individuals. Advanced forms of HCM clinically present with hypercontractility, hypertrophy and fibrosis. Several single-point mutations in b-myosin heavy chain (MYH7) have been associated with HCM and increased contractility at the organ level. Different MYH7 mutations have resulted in increased, decreased, or unchanged force production at the molecular level. Yet, how these molecular kinetics link to cell and tissue pathogenesis remains unclear. The Hippo Pathway, specifically its effector molecule YAP, has been demonstrated to be reactivated in pathological hypertrophic growth. We hypothesized that changes in force production (intrinsically or extrinsically) directly alter the homeostatic mechano-signaling of the Hippo pathway through changes in stresses on the nucleus. Using human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), we asked whether homeostatic mechanical signaling through the canonical growth regulator, YAP, is altered 1) by changes in the biomechanics of HCM mutant cardiomyocytes and 2) by alterations in the mechanical environment. We use genetically edited hiPSC-CM with point mutations in MYH7 associated with HCM, and their matched controls, combined with micropatterned traction force microscopy substrates to confirm the hypercontractile phenotype in MYH7 mutants. We next modulate contractility in healthy and disease hiPSC-CMs by treatment with positive and negative inotropic drugs and demonstrate a correlative relationship between contractility and YAP activity. We further demonstrate the activation of YAP in both HCM mutants and healthy hiPSC-CMs treated with contractility modulators is through enhanced nuclear deformation. We conclude that the overactivation of YAP, possibly initiated and driven by hypercontractility, correlates with excessive CCN2 secretion (connective tissue growth factor), enhancing cardiac fibroblast/myofibroblast transition and production of known hypertrophic signaling molecule TGFβ. Our study suggests YAP being an indirect player in the initiation of hypertrophic growth and fibrosis in HCM. Our results provide new insights into HCM progression and bring forth a testbed for therapeutic options in treating HCM.

A review on decoding the roles of YAP/TAZ signaling pathway in cardiovascular diseases: Bridging molecular mechanisms to therapeutic insights

Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) serve as transcriptional co-activators that dynamically shuttle between the cytoplasm and nucleus, resulting in either the suppression or enhancement of their downstream gene expression. Recent emerging evidence demonstrates that YAP/TAZ is strongly implicated in the pathophysiological processes that contribute to cardiovascular diseases (CVDs). In the cardiovascular system, YAP/TAZ is involved in the orchestration of a range of biological processes such as oxidative stress, inflammation, proliferation, and autophagy. Furthermore, YAP/TAZ has been revealed to be closely associated with the initiation and development of various cardiovascular diseases, including atherosclerosis, pulmonary hypertension, myocardial fibrosis, cardiac hypertrophy, and cardiomyopathy. In this review, we delve into recent studies surrounding YAP and TAZ, along with delineating their roles in contributing to the pathogenesis of CVDs with a link to various physiological processes in the cardiovascular system. Additionally, we highlight the current potential drugs targeting YAP/TAZ for CVDs therapy and discuss their challenges for translational application. Overall, this review may offer novel insights for understanding and treating cardiovascular disorders.

Transient induction of actin cytoskeletal remodeling associated with dedifferentiation, proliferation, and redifferentiation stimulates cardiac regeneration

The formation of new and functional cardiomyocytes requires a 3-step process: dedifferentiation, proliferation, and redifferentiation, but the critical genes required for efficient dedifferentiation, proliferation, and redifferentiation remain unknown. In our study, a circular trajectory using single-nucleus RNA sequencing of the pericentriolar material 1 positive (PCM1+) cardiomyocyte nuclei from hearts 1 and 3 days after surgery-induced myocardial infarction (MI) on postnatal Day 1 was reconstructed and demonstrated that actin remodeling contributed to the dedifferentiation, proliferation, and redifferentiation of cardiomyocytes after injury. We identified four top actin-remodeling regulators, namely Tmsb4x, Tmsb10, Dmd, and Ctnna3, which we collectively referred to as 2D2P. Transiently expressed changes of 2D2P, using a polycistronic non-integrating lentivirus driven by Tnnt2 (cardiac-specific troponin T) promoters (Tnnt2-2D2P-NIL), efficiently induced transiently proliferative activation and actin remodeling in postnatal Day 7 cardiomyocytes and adult hearts. Furthermore, the intramyocardial delivery of Tnnt2-2D2P-NIL resulted in a sustained improvement in cardiac function without ventricular dilatation, thickened septum, or fatal arrhythmia for at least 4 months. In conclusion, this study highlights the importance of actin remodeling in cardiac regeneration and provides a foundation for new gene-cocktail-therapy approaches to improve cardiac repair and treat heart failure using a novel transient and cardiomyocyte-specific viral construct.

Lysine-specific methyltransferase Set7/9 in stemness, differentiation, and development

The enzymes performing protein post-translational modifications (PTMs) form a critical post-translational regulatory circuitry that orchestrates literally all cellular processes in the organism. In particular, the balance between cellular stemness and differentiation is crucial for the development of multicellular organisms. Importantly, the fine-tuning of this balance on the genetic level is largely mediated by specific PTMs of histones including lysine methylation. Lysine methylation is carried out by special enzymes (lysine methyltransferases) that transfer the methyl group from S-adenosyl-L-methionine to the lysine residues of protein substrates. Set7/9 is one of the exemplary protein methyltransferases that however, has not been fully studied yet. It was originally discovered as histone H3 lysine 4-specific methyltransferase, which later was shown to methylate a number of non-histone proteins that are crucial regulators of stemness and differentiation, including p53, pRb, YAP, DNMT1, SOX2, FOXO3, and others. In this review we summarize the information available to date on the role of Set7/9 in cellular differentiation and tissue development during embryogenesis and in adult organisms. Finally, we highlight and discuss the role of Set7/9 in pathological processes associated with aberrant cellular differentiation and self-renewal, including the formation of cancer stem cells.

Inherent Metabolic Adaptations in Adult Spiny Mouse ( Acomys ) Cardiomyocytes Facilitate Enhanced Cardiac Recovery Following Myocardial Infarction

The adult mammalian heart has limited regenerative capacity following injury, leading to progressive heart failure and mortality. Recent studies have identified the spiny mouse ( Acomys ) as a unique model for mammalian cardiac isch3emic resilience, exhibiting enhanced recovery after myocardial infarction (MI) compared to commonly used laboratory mouse strains. However, the underlying cellular and molecular mechanisms behind this unique response remain poorly understood. In this study, we comprehensively characterized the metabolic characteristics of cardiomyocytes in Acomys compared to the non-regenerative Mus musculus . We utilized single-nucleus RNA sequencing (snRNA-seq) in sham-operated animals and 1, 3, and 7 days post-myocardial infarction to investigate cardiomyocytes' transcriptomic and metabolomic profiles in response to myocardial infarction. Complementary targeted metabolomics, stable isotope-resolved metabolomics, and functional mitochondrial assays were performed on heart tissues from both species to validate the transcriptomic findings and elucidate the metabolic adaptations in cardiomyocytes following ischemic injury. Transcriptomic analysis revealed that Acomys cardiomyocytes inherently upregulate genes associated with glycolysis, the pentose phosphate pathway, and glutathione metabolism while downregulating genes involved in oxidative phosphorylation (OXPHOS). These metabolic characteristics are linked to decreased reactive oxygen species (ROS) production and increased antioxidant capacity. Our targeted metabolomic studies in heart tissue corroborated these findings, showing a shift from fatty acid oxidation to glycolysis and ancillary biosynthetic pathways in Acomys at baseline with adaptive changes post-MI. Functional mitochondrial studies indicated a higher reliance on glycolysis in Acomys compared to Mus , underscoring the unique metabolic phenotype of Acomys hearts. Stable isotope tracing experiments confirmed a shift in glucose utilization from oxidative phosphorylation in Acomys . In conclusion, our study identifies unique metabolic characteristics of Acomys cardiomyocytes that contribute to their enhanced ischemic resilience following myocardial infarction. These findings provide novel insights into the role of metabolism in regulating cardiac repair in adult mammals. Our work highlights the importance of inherent and adaptive metabolic flexibility in determining cardiomyocyte ischemic responses and establishes Acomys as a valuable model for studying cardiac ischemic resilience in adult mammals.

Graphical abstract

Metabolic Reprogramming: A Byproduct or a Driver of Cardiomyocyte Proliferation?

Defining mechanisms of cardiomyocyte proliferation should guide the understanding of endogenous cardiac regeneration and could lead to novel treatments for diseases such as myocardial infarction. In the neonatal heart, energy metabolic reprogramming (phenotypic alteration of glucose, fatty acid, and amino acid metabolism) parallels cell cycle arrest of cardiomyocytes. The metabolic reprogramming occurring shortly after birth is associated with alterations in blood oxygen levels, metabolic substrate availability, hemodynamic stress, and hormone release. In the adult heart, myocardial infarction causes metabolic reprogramming but these changes cannot stimulate sufficient cardiomyocyte proliferation to replace those lost by the ischemic injury. Some putative pro-proliferative interventions can induce the metabolic reprogramming. Recent data show that altering the metabolic enzymes PKM2 [pyruvate kinase 2], LDHA [lactate dehydrogenase A], PDK4 [pyruvate dehydrogenase kinase 4], SDH [succinate dehydrogenase], CPT1b [carnitine palmitoyl transferase 1b], or HMGCS2 [3-hydroxy-3-methylglutaryl-CoA synthase 2] is sufficient to partially reverse metabolic reprogramming and promotes adult cardiomyocyte proliferation. How metabolic reprogramming regulates cardiomyocyte proliferation is not clearly defined. The possible mechanisms involve biosynthetic pathways from the glycolysis shunts and the epigenetic regulation induced by metabolic intermediates. Metabolic manipulation could represent a new approach to stimulate cardiac regeneration; however, the efficacy of these manipulations requires optimization, and novel molecular targets need to be defined. In this review, we summarize the features, triggers, and molecular regulatory networks responsible for metabolic reprogramming and discuss the current understanding of metabolic reprogramming as a critical determinant of cardiomyocyte proliferation.

Hippo pathway in cell-cell communication: emerging roles in development and regeneration

The Hippo pathway is a central regulator of tissue growth that has been widely studied in mammalian organ development, regeneration, and cancer biology. Although previous studies have convincingly revealed its cell-autonomous functions in controlling cell fate, such as cell proliferation, survival, and differentiation, accumulating evidence in recent years has revealed its non-cell-autonomous functions. This pathway regulates cell-cell communication through direct interactions, soluble factors, extracellular vesicles, and the extracellular matrix, providing a range of options for controlling diverse biological processes. Consequently, the Hippo pathway not only dictates the fate of individual cells but also triggers multicellular responses involving both tissue-resident cells and infiltrating immune cells. Here, we have highlighted the recent understanding of the molecular mechanisms by which the Hippo pathway controls cell-cell communication and discuss its importance in tissue homeostasis, especially in development and regeneration.

Gene therapy encoding cell cycle factors to treat chronic ischemic heart failure in rats

AIMS

Gene therapies to induce cardiomyocyte (CM) cell cycle re-entry have shown a potential to treat subacute ischaemic heart failure (IHF) but have not been tested in the more relevant setting of chronic IHF. Our group recently showed that polycistronic non-integrating lentivirus encoding Cdk1/CyclinB1 and Cdk4/CyclinD1 (TNNT2-4Fpolycistronic-NIL) is effective in inducing CM cell cycle re-entry and ameliorating subacute IHF models and preventing the subsequent IHF-induced congestions in the liver, kidneys, and lungs in rats and pigs. Here, we aim to test the long-term efficacy of TNNT2-4Fpolycistronic-NIL in a rat model of chronic IHF, a setting that differs pathophysiologically from subacute IHF and has greater clinical relevance.

METHODS AND RESULTS

Rats were subjected to a 2-h coronary occlusion followed by reperfusion; 4 weeks later, rats were injected intramyocardially with either TNNT2-4Fpolycistronic-NIL or LacZ-NIL. Four months post-viral injection, TNNT2-4Fpolycistronic-NIL-treated rats showed a significant reduction in scar size and a significant improvement in left ventricular (LV) systolic cardiac function but not in the LV dilatation associated with chronic IHF. A mitosis reporter system developed in our lab showed significant induction of CM mitotic activity in TNNT2-4Fpolycistronic-NIL-treated rats.

CONCLUSION

This study demonstrates, for the first time, that TNNT2-4Fpolycistronic-NIL gene therapy induces CM cell cycle re-entry in chronic IHF and improves LV function, and that this salubrious effect is sustained for at least 4 months. Given the high prevalence of chronic IHF, these results have significant clinical implications for developing a novel treatment for this deadly disease.

Border-zone cardiomyocytes and macrophages contribute to remodeling of the extracellular matrix to promote cardiomyocyte invasion during zebrafish cardiac regeneration

Despite numerous advances in our understanding of zebrafish cardiac regeneration, an aspect that remains less studied is how regenerating cardiomyocytes invade, and eventually replace, the collagen-containing fibrotic tissue following injury. Here, we provide an in-depth analysis of the process of cardiomyocyte invasion using live-imaging and histological approaches. We observed close interactions between protruding cardiomyocytes and macrophages at the wound border zone, and macrophage-deficient irf8 mutant zebrafish exhibited defects in extracellular matrix (ECM) remodeling and cardiomyocyte protrusion into the injured area. Using a resident macrophage ablation model, we show that defects in ECM remodeling at the border zone and subsequent cardiomyocyte protrusion can be partly attributed to a population of resident macrophages. Single-cell RNA-sequencing analysis of cells at the wound border revealed a population of cardiomyocytes and macrophages with fibroblast-like gene expression signatures, including the expression of genes encoding ECM structural proteins and ECM-remodeling proteins. The expression of mmp14b , which encodes a membrane-anchored matrix metalloproteinase, was restricted to cells in the border zone, including cardiomyocytes, macrophages, fibroblasts, and endocardial/endothelial cells. Genetic deletion of mmp14b led to a decrease in 1) macrophage recruitment to the border zone, 2) collagen degradation at the border zone, and 3) subsequent cardiomyocyte invasion. Furthermore, cardiomyocyte-specific overexpression of mmp14b was sufficient to enhance cardiomyocyte invasion into the injured tissue and along the apical surface of the wound. Altogether, our data shed important insights into the process of cardiomyocyte invasion of the collagen-containing injured tissue during cardiac regeneration. They further suggest that cardiomyocytes and resident macrophages contribute to ECM remodeling at the border zone to promote cardiomyocyte replenishment of the fibrotic injured tissue.

Cardiac regeneration: Pre-existing cardiomyocyte as the hub of novel signaling pathway

In the mammalian heart, cardiomyocytes are forced to withdraw from the cell cycle shortly after birth, limiting the ability of the heart to regenerate and repair. The development of multimodal regulation of cardiac proliferation has verified that pre-existing cardiomyocyte proliferation is an essential driver of cardiac renewal. With the continuous development of genetic lineage tracking technology, it has been revealed that cell cycle activity produces polyploid cardiomyocytes during the embryonic, juvenile, and adult stages of cardiogenesis, but newly formed mononucleated diploid cardiomyocytes also elevated sporadically during myocardial infarction. It implied that adult cardiomyocytes have a weak regenerative capacity under the condition of ischemia injury, which offers hope for the clinical treatment of myocardial infarction. However, the regeneration frequency and source of cardiomyocytes are still low, and the mechanism of regulating cardiomyocyte proliferation remains further explained. It is noteworthy to explore what force triggers endogenous cardiomyocyte proliferation and heart regeneration. Here, we focused on summarizing the recent research progress of emerging endogenous key modulators and crosstalk with other signaling pathways and furnished valuable insights into the internal mechanism of heart regeneration. In addition, myocardial transcription factors, non-coding RNAs, cyclins, and cell cycle-dependent kinases are involved in the multimodal regulation of pre-existing cardiomyocyte proliferation. Ultimately, awakening the myocardial proliferation endogenous modulator and regeneration pathways may be the final battlefield for the regenerative therapy of cardiovascular diseases.