Abstract P2029: The Role Of Runx1 In Cardiomyocyte Cell Cycle Activity And Its Impact On Cardiac Regeneration

Factors responsible for cardiomyocyte proliferation may serve as a potential therapeutic to stimulate endogenous myocardial regeneration following insult, such as ischemic injury. A previously published forward genetics approach assessing the frequency of a rare and presumed proliferation-competent subpopulation of cardiomyocytes, mononuclear diploid cardiomyocytes (MNDCMs), led us to the transcription factor RUNX1. It is established that RUNX1 induction in cardiomyocytes increases after injury. Here, we examine the effect of RUNX1 on cardiomyocyte cell cycle and establishment of the MNDCM population during postnatal development and cardiac regeneration using both cardiomyocyte-specific gain-and loss of function mouse models. We hypothesize that RUNX1 overexpression (OE) increases cardiomyocyte cell cycle activity with expansion of the MNDCM population, thereby extending the neonatal regenerative window and positively impacting adult cardiac remodeling post injury. During postnatal development, RUNX1 KO decreased postnatal cardiomyocyte cell cycle activity, while RUNX1 OE extended the period of cell cycle activation. This extension observed in RUNX1 OE mice is complete with cytokinesis resulting in an expansion of the MNDCM population and total cardiomyocyte endowment. To determine whether RUNX1 could similarly regulate cardiomyocyte cell cycle in a regenerative and non-regenerative model, we induce P6 and 8-week MIs to measure cell cycle activity and cardiac function post injury. RUNX1 OE neonatal mice with a P6 MI displayed no difference in cardiomyocyte cell cycle activity 7 post injury compared to control littermates. However, RUNX1 OE in adult mice with an 8-week MI showed increased cardiomyocyte cell cycle activity with completion of cytokinesis 2 weeks post injury and limited improvement in cardiac function 28 days post injury. RUNX1 influences cardiomyocyte cell cycle activation in the context of normal postnatal development and adult MI. Conversely, this phenomenon does not appear to translate to the neonatal injury context. We are examining this possible discrepancy with further experiments to fully understand .

Abstract P2049: Myofibroblast Depletion Of Yap And Wwtr1 Improves Cardiac Function After Myocardial Infarction Via Ccn3 Suppression

The Hippo-Yap pathway is an area of interest in cardiac fibroblasts, as inhibition Lats1/2 promotes substantial fibrosis and Yap/Wwtr1 facilitate maturation of fibroblasts to myofibroblasts. While the Hippo-Yap pathway has been characterized in resting and activated fibroblasts, we take the novel approach of investigating its role in myofibroblasts. We assess depletion of Yap alone ( Yap fl/fl ;Postn MCM ) or Yap and Wwtr1 ( Yap fl/fl ;Wwtr1 fl/+ ;Postn MCM ) in myofibroblasts immediately following myocardial infarction and focus on identifying and validating downstream factors mediating pathological remodeling in mice. Following injury, depletion of Yap alone had no effect on heart function, but depletion of Yap and Wwtr1 resulted in improved ejection fraction and fractional shortening at 60 days post injury. Yap fl/fl , Wwtr1 fl/+ ;Postn MCM hearts also displayed smaller scars, reduced interstitial fibrosis, and more denatured collagen. Single cell RNA sequencing of interstitial cells 7 days post injury showed suppression of pro-fibrotic and pro-inflammatory genes and suppression of a matrifibrocyte phenotype in fibroblasts derived from the Yap fl/fl , Wwtr1 fl/+ ;Postn MCM heart. Analysis of in vivo and in vitro transcriptomics revealed depletion of Yap/Wwtr1 resulted in dramatically decreased fibroblast expression of the matricellular protein Ccn3. Administration of recombinant CCN3 to mice following injury aggravated cardiac function and scarring over the course of 28 days. Thus, we show Yap/Wwtr1 depletion in myofibroblasts attenuates fibrosis and significantly improves cardiac outcomes after injury and identify Ccn3 as a novel factor that contributes to adverse cardiac function. Given therapeutic strategies would likely be implemented after myocardial infarction when myofibroblasts are already activated, our work suggests targeting Yap and Wwtr1, or Ccn3 directly as an approach for suppressing pathological remodeling.

Abstract P2036: Investigating Variants Of Hdac7 In Mammalian Heart Regeneration

The longstanding belief in the field is the mammalian heart is incapable of regenerating. However, findings regarding the variability of regenerative capacity of mammalian hearts are opening the door for such a possibility. Mononuclear diploid cardiomyocytes (MNDCMs) are a subpopulation of CMs believed to be capable of mounting a regenerative response and the prevalence of the MNDCM population in mammalian hearts is a variable trait. Thus, if we can understand the genetic mechanisms governing the MNDCM variability, it could indicate strategies to stimulate an endogenous regenerative response. Using a genome-wide association approach across a panel of genetically diverse inbred mice, Dr. Patterson identified genes associated with the frequency of MNDCMs in the uninjured adult heart. From this genetic screen, she validated the gene Tnni3k as one regulator of CM ploidy and regenerative competence after injury. Here, we follow a second novel locus identified from this screen found on chromosome 15, which includes the gene Hdac7. Previously published RNA-seq data on isolated CMs indicate that Hdac7 expression levels increase during the first postnatal week, a time that coincides with the onset of cellular senescence, CM polyploidization, and the loss of innate regenerative capacity in the heart. Using a CM specific Hdac7 conditional knockout mouse, we found that loss of Hdac7 in CMs from birth increases CM DNA synthesis, increases CM ploidy, and increases average CM cell length. Moreover, we identified a protein coding variant of Hdac7 that corresponds to differences in frequency of MNDCM between members of the mouse diversity panel. Finally, we have gathered preliminary findings that suggest genetic epistasis between Hdac7 and Tnni3k. Specifically, Hdac7;Tnni3k double knockout animals display more MNDCMs than Tnni3k knockout did on its own. Our findings suggest Hdac7 negatively regulates DNA synthesis; a key part of the cell cycle that has the potential as a therapeutic target for the stimulation of a proliferation response. In addition, these findings showcase complex genetic interactions regulating CM regeneration, suggesting that combinatorial approaches can enhance regenerative responses.

Abstract 118: Interleukin 4 And 13 Signaling In Macrophages Regulates Neonatal Cardiac Regeneration

Introduction: Heart failure (HF) is a prevalent disease, projected to affect over 8 million Americans by 2030. Current therapy partially decreases progression; however, mortality and disease burden continue to be high. Therefore, there is an unmet need to develop new strategies that target HF progression. Our lab employs the neonatal mouse model of cardiac regeneration to identify pro-reparative pathways that can be applied to treating HF in humans. Previously we demonstrated that the anti-inflammatory cytokine, Interleukin 13 (IL13), promotes cardiac regeneration, however, the cell types mediating this response remain unknown. IL13 and the related cytokine, IL4, share a common receptor (IL4Rα) and both cytokines polarize macrophages into a reparative phenotype. Here, we hypothesize that IL4/13 signaling to macrophages promotes heart regeneration after cardiac injury and we explore the cell source of these cytokines during neonatal development and cardiac regeneration.

Methods and Results: We generated a genetic model whereby IL4Rα is depleted in macrophages by crossing IL4Rα floxed (IL4Rα fl/fl ) mice with transgenic mice CX3Cr1 driven-Cre recombinase (CX3CR1 Cre ). Flow cytometry analysis confirmed depletion of IL4Rα in cardiac macrophages. We performed myocardial infarction (MI) on postnatal day 1 (P1) mice and assessed cardiac regeneration by ultrasonography 21 days post-injury (dpi). We found that IL4Rα fl/fl CX3CR1 Cre mice had lower ejection fraction compared to IL4Rα fl/fl littermate controls. Preliminary results suggest there is a reduced capillary density in peri-ischemic myocardium. In addition, we used fluorescent reporter mouse lines; IL4-enhanced green fluorescent protein (IL4-GFP) and IL13-yellow fluorescent protein (IL13-YFP), to assess the cellular source of IL4 and IL13 expression in the hearts of neonatal mice. In unoperated mice, we found no detectable expression of IL13 by any cell type in the heart, whereas IL4 was expressed in innate lymphoid cells (ILCs) and T cells. 4 days after MI in P1 mice IL13 was upregulated in ILCs and T cells and IL4 was expressed by ILCs and T cells and upregulated in myeloid cells.

Conclusions and Discussion: We found that IL13 and IL4 are primarily expressed in ILCs and T cells following neonatal injury, suggesting a novel role for ILCs and T cells in the production of IL4 and 13 during the neonatal regeneration process. In addition, we found that lack of IL4/13 signaling in macrophages via depletion IL4Rα impairs cardiac regeneration after MI in neonatal mice, and results in reduced capillary density in peri-ischemic myocardium. We hypothesize that IL4Rα depletion in macrophages impairs reparative macrophage polarization after MI and promotes an inflammatory polarization. Future studies will be aimed to assess macrophage phenotypes in response to IL13/IL4 signaling by transcriptional profiling and flow cytometry.

Abstract 223: Yap and Its Homolog Wwtr1 Are Regulators of Myofibroblast Activation Following Ischemic Injury

Introduction: Following ischemic injury in adult mammals, cardiac fibroblasts differentiate into myofibroblasts and promote secretion of matrix fibers. Myofibroblast activation is critical for initial scar formation and preventing heart rupture, however, extended activity can lead to heart failure progression. Thus, there is a need to identify the mechanisms mediating persistent activation of myofibroblasts to prevent excessive fibrosis and adverse cardiac remodeling. Here we demonstrate that Hippo-Yap pathway offers a target for modulating myofibroblast activation and thus the fibrotic response.

Methods and Results: We tested the hypothesis that Yap and its homolog Wwtr1 (known as ‘Taz’) are regulators of myofibroblast activation following ischemic injury. We implemented a Cre-lox system whereby Yap alone or both Yap and Wwrt1 were depleted using an inducible Cre expressed under a myofibroblast specific promoter ( Postn MCM ). Following permeant ligation of the left anterior descending artery in adult mice, we found that myofibroblast depletion of Yap alone resulted in a significant reduction in left ventricular dilation 28 days post injury (dpi) and decreased proliferation of scar associated cells. Strikingly, myofibroblast specific depletion of Yap and one copy of Wwrt1 resulted in further attenuation of left ventricular dilation as well as improved fractional shortening and ejection fraction at 28 and 60 dpi. Histological assessment revealed that depletion of both Yap and Wwrt1 resulted in greater than 50% reduction in scar size (by midline) at 60 dpi. RNAseq of whole hearts collected at 4 dpi suggested that Hippo-Yap pathway expression specifically in myofibroblasts facilitates immune cell recruitment in the heart. Collectively These data illustrate a role for Hippo-Yap signaling mediating myofibroblast activity and immune cell coordination following injury and therefore cardiac fibrosis and remodeling.

Conclusions: Our data demonstrate that endogenous Yap and Wwrt1 deletion in myofibroblasts suppresses the fibrotic response, mediates inflammation, and improves cardiac function after ischemic injury. These results therefore offer a regulatory pathway that can be targeted therapeutically to prevent progressive heart failure.

Abstract 336: Interleukin 4 and 13 Signaling in Myeloid Cells Regulates Cardiac Regeneration

Introduction: Heart failure is a very important disease in the United States and worldwide, with a projected prevalence of 8 million by 2030. Current therapy partially decreases progression; however, mortality continues to be high, and progression to terminal HF is still significant, requiring heart transplant and other interventions that further increase morbidity and overall disease burden and costs. Therefore, the need to develop new therapies that target HF progression. Our prior work and published studies demonstrate that IL13 promotes cardiac regeneration, however, the mechanism mediating this response is currently unknown. IL13 and IL4 share a common receptor, and both cytokines are known to polarize macrophages into a pro-reparative phenotype. Here, we hypothesize that IL4/13 signaling in myeloid cells promotes heart regeneration after cardiac injury.

Methods and Results: We compared cardiac regeneration in IL4Ra null/FLOXed (IL4Rα-/fl) and IL4Rα-/fl lysosome M Cre littermate mice, knocking out IL4Rα in myeloid cells. We performed apical resection on postnatal day 1 (P1) mice and assessed cardiac regeneration by histological analysis. We found that mice lacking IL4Ra in myeloid cells (IL4rafl/-, LysMCre) had significantly impaired cardiac regenerative capacity compared to Cre negative littermate controls. We also performed gene expression analysis by qPCR in whole hearts collected at 2dpi to quantify macrophages markers. Cre positive mice trended higher for expression of arginase 1 and mannose receptor 1 (p=0.055 and 0.057, respectively; student t-test) compared to Cre negative littermates.

Conclusions and Discussion: Lack of IL4/13 signaling in myeloid cells impairs cardiac regeneration after injury in neonatal mice and modulated expression of immune markers. To confirm these results, flow cytometry to quantify immune cell infiltration can be performed. Other components that limit damage extension can also be assessed, like angiogenesis and cardiomyocyte proliferation. Our data suggest that IL4/IL13 signaling polarizes myeloid cells to a pro-reparative state in the neonatal heart. Additional studies are required to delineate the specific cell population mediating this response.

Abstract 302: IL4Ra is Required for Cardiomyocyte Cell Cycle Activity and Cardiac Regeneration

Introduction: During the first week of life, neonatal mice are able to regenerate their hearts after injury with minimal scarring. Work from our lab demonstrates that IL13 signaling is required for neonatal heart regeneration, however multiple IL13 receptors exist. Here, we aim to identify the specific receptor ligand interaction that promotes regenerative healing in the heart. In vitro data suggests the IL4Ra/IL3Ra1 receptor heterodimer may mediate cardiomyocyte (CM) proliferation and heart regeneration. Thus, we aim to test the functional role of this receptor in cardiac regeneration in vivo . We hypothesize that IL13 signals through IL4Ra/IL13Ra1 directly on CMs to promote CM cell cycle activity and cardiac regeneration.

Methods: To delineate IL13 signaling mechanisms in murine hearts, we utilized two knockouts of IL4Ra—global IL4Ra knockout (KO) and CM-specific IL4Ra knockout (IL4Ra fl/fl Myh6 CRE ) mice. To assess regeneration, mice received cardiac apical resection surgery at postnatal day 1 (P1). Regeneration was assessed by echocardiography and histological analysis of residual scars and CM proliferation indices. We next tested if IL13 administration could extend the regenerative window. We performed myocardial infarction (MI) on P7 mice and administered IL13 for two weeks. We assessed scar size through trichrome staining and CM cell cycle activity through immunostaining.

Results: We observed impaired cardiac regeneration, determined by scar formation and decreased cardiac function in IL4Ra KO mice compared to littermate controls. Similar to global KOs, we observed decreased function in IL4Ra fl/fl Myh6 CRE mice. IL13 administration to wildtype mice after P7 MI decreased MI severity and increased CM cell cycle activity, suggesting improved reparative capacity. Interestingly, IL13 administration in IL4Ra fl/fl Myh6 CRE mice did not improve cardiac recovery phenotypes indicating that IL13 functions through IL4Ra directly on CMs to promote cardiac healing.

Conclusion: These results demonstrate that the IL4Ra receptor subunit is required for cardiac regeneration, and activation of this receptor can extend the regenerative window. These findings lay the groundwork for potential therapeutic targets for promoting cardiac healing.