Runx1 is sufficient but not required for cardiomyocyte cell-cycle activation.

Factors responsible for cardiomyocyte proliferation could serve as potential therapeutics to stimulate endogenous myocardial regeneration following insult, such as ischemic injury. A previously published forward genetics approach on cardiomyocyte cell cycle and ploidy led us to the transcription factor, Runx1. Here, we examine the effect of Runx1 on cardiomyocyte cell cycle during postnatal development and cardiac regeneration using cardiomyocyte-specific gain- and loss-of-function mouse models. RUNX1 is expressed in cardiomyocytes during early postnatal life, decreases to negligible levels by 3 wk of age, and increases upon myocardial injury, all consistent with observed rates of cardiomyocyte cell-cycle activity. Loss of Runx1 transiently stymied cardiomyocyte cell-cycle activity during normal postnatal development, a result that corrected itself and did not extend to the context of neonatal heart regeneration. On the other hand, cardiomyocyte-specific Runx1 overexpression resulted in an expansion of diploid cardiomyocytes in uninjured hearts and expansion of 4 N cardiomyocytes in the context of neonatal cardiac injury, suggesting Runx1 overexpression is sufficient to induce cardiomyocyte cell-cycle responses. Persistent overexpression of Runx1 for >1 mo continued to promote cardiomyocyte cell-cycle activity resulting in substantial hyperpolyploidization (≥8 N DNA content). This persistent cell-cycle activation was accompanied by ventricular dilation and adverse remodeling, raising the concern that continued cardiomyocyte cell cycling can have detrimental effects.NEW & NOTEWORTHY Runx1 is sufficient but not required for cardiomyocyte cell cycle.

IL-13 promotes functional recovery after myocardial infarction via direct signaling to macrophages.

There is great interest in identifying signaling pathways that promote cardiac repair after myocardial infarction (MI). Prior studies suggest a beneficial role for IL-13 signaling in neonatal heart regeneration; however, the cell types mediating cardiac regeneration and the extent of IL-13 signaling in the adult heart after injury are unknown. We identified an abundant source of IL-13 and the related cytokine, IL-4, in neonatal cardiac type 2 innate lymphoid cells, but this phenomenon declined precipitously in adult hearts. Moreover, IL-13 receptor deletion in macrophages impaired cardiac function and resulted in larger scars early after neonatal MI. By using a combination of recombinant IL-13 administration and cell-specific IL-13 receptor genetic deletion models, we found that IL-13 signaling specifically to macrophages mediated cardiac functional recovery after MI in adult mice. Single transcriptomics revealed a subpopulation of cardiac macrophages in response to IL-13 administration. These IL-13-induced macrophages were highly efferocytotic and were identified by high IL-1R2 expression. Collectively, we elucidated a strongly proreparative role for IL-13 signaling directly to macrophages following cardiac injury. While this pathway is active in proregenerative neonatal stages, reactivation of macrophage IL-13 signaling is required to promote cardiac functional recovery in adults.

Cardiomyocyte ploidy is dynamic during postnatal development and varies across genetic backgrounds.

Somatic polyploidization, an adaptation by which cells increase their DNA content to support growth, is observed in many cell types, including cardiomyocytes. Although polyploidization is believed to be beneficial, progression to a polyploid state is often accompanied by loss of proliferative capacity. Recent work suggests that genetics heavily influence cardiomyocyte ploidy. However, the developmental course by which cardiomyocytes reach their final ploidy state has only been investigated in select backgrounds. Here, we assessed cardiomyocyte number, cell cycle activity, and ploidy dynamics across two divergent mouse strains: C57BL/6J and A/J. Both strains are born and reach adulthood with comparable numbers of cardiomyocytes; however, the end composition of ploidy classes and developmental progression to reach the final state differ substantially. We expand on previous findings that identified Tnni3k as a mediator of cardiomyocyte ploidy and uncover a role for Runx1 in ploidy dynamics and cardiomyocyte cell division, in both developmental and injury contexts. These data provide novel insights into the developmental path to cardiomyocyte polyploidization and challenge the paradigm that hypertrophy is the sole mechanism for growth in the postnatal heart.

Collagen-Specific HSP47+ Myofibroblasts and CD163+ Macrophages Identify Profibrotic Phenotypes in Deceased Hearts With SARS-CoV-2 Infections.

Background Cardiac fibrosis complicates SARS-CoV-2 infections and has been linked to arrhythmic complications in survivors. Accordingly, we sought evidence of increased HSP47 (heat shock protein 47), a stress-inducible chaperone protein that regulates biosynthesis and secretion of procollagen in heart tissue, with the goal of elucidating molecular mechanisms underlying cardiac fibrosis in subjects with this viral infection. Methods and Results Using human autopsy tissue, immunofluorescence, and immunohistochemistry, we quantified Hsp47+ cells and collagen α 1(l) in hearts from people with SARS-CoV-2 infections. Because macrophages are also linked to inflammation, we measured CD163+ cells in the same tissues. We observed irregular groups of spindle-shaped HSP47+ and CD163+ cells as well as increased collagen α 1(I) deposition, each proximate to one another in "hot spots" of ≈40% of hearts after SARS-CoV-2 infection (HSP47+ P<0.05 versus nonfibrotics and P<0.001 versus controls). Because HSP47+ cells are consistent with myofibroblasts, subjects with hot spots are termed "profibrotic." The remaining 60% of subjects dying with COVID-19 without hot spots are referred to as "nonfibrotic." No control subject exhibited hot spots. Conclusions Colocalization of myofibroblasts, M2(CD163+) macrophages, and collagen α 1(l) may be the first evidence of a COVID-19-related "profibrotic phenotype" in human hearts in situ. The potential public health and diagnostic implications of these observations require follow-up to further define mechanisms of viral-mediated cardiac fibrosis.

Interleukin 4/13 signaling in cardiac regeneration and repair.

Interleukin 4 (IL4) and interleukin 13 (IL13) are closely related cytokines that have been classically attributed to type II immunity, namely, differentiation of T-helper 2 (TH2) cells and alternative activation of macrophages. Although the role of IL4/13 has been well described in various contexts such as defense against helminth parasites, pathogenesis of allergic disease, and several models of wound healing, relatively little is known about the role of IL4/13 in the heart following injury. Emerging literature has identified various roles for IL4/13 in animal models of cardiac regeneration as well as in the adult mammalian heart following myocardial injury. Notably, although IL4 and IL13 signal to hematopoietic cell types following myocardial infarction (MI) to promote wound healing phenotypes, there is substantial evidence that these cytokines can signal directly to non-hematopoietic cell types in the heart during development, homeostasis, and following injury. Comprehensive understanding of the molecular and cellular actions of IL4/13 in the heart is still lacking, but overall evidence to date suggests that activation of these cytokines results in beneficial outcomes with respect to cardiac repair. Here, we aim to comprehensively review the role of IL4 and IL13 and their prospective mechanisms in cardiac regeneration and repair.

The Evolving Roles of Cardiac Macrophages in Homeostasis, Regeneration, and Repair.

Macrophages were first described as phagocytic immune cells responsible for maintaining tissue homeostasis by the removal of pathogens that disturb normal function. Historically, macrophages have been viewed as terminally differentiated monocyte-derived cells that originated through hematopoiesis and infiltrated multiple tissues in the presence of inflammation or during turnover in normal homeostasis. However, improved cell detection and fate-mapping strategies have elucidated the various lineages of tissue-resident macrophages, which can derive from embryonic origins independent of hematopoiesis and monocyte infiltration. The role of resident macrophages in organs such as the skin, liver, and the lungs have been well characterized, revealing functions well beyond a pure phagocytic and immunological role. In the heart, recent research has begun to decipher the functional roles of various tissue-resident macrophage populations through fate mapping and genetic depletion studies. Several of these studies have elucidated the novel and unexpected roles of cardiac-resident macrophages in homeostasis, including maintaining mitochondrial function, facilitating cardiac conduction, coronary development, and lymphangiogenesis, among others. Additionally, following cardiac injury, cardiac-resident macrophages adopt diverse functions such as the clearance of necrotic and apoptotic cells and debris, a reduction in the inflammatory monocyte infiltration, promotion of angiogenesis, amelioration of inflammation, and hypertrophy in the remaining myocardium, overall limiting damage extension. The present review discusses the origin, development, characterization, and function of cardiac macrophages in homeostasis, cardiac regeneration, and after cardiac injury or stress.

Neuronatin is a modifier of estrogen receptor-positive breast cancer incidence and outcome.

PURPOSE: Understanding the molecular mediators of breast cancer survival is critical for accurate disease prognosis and improving therapies. Here, we identified Neuronatin (NNAT) as a novel antiproliferative modifier of estrogen receptor-alpha (ER+) breast cancer. EXPERIMENTAL DESIGN: Genomic regions harboring breast cancer modifiers were identified by congenic mapping in a rat model of carcinogen-induced mammary cancer. Tumors from susceptible and resistant congenics were analyzed by RNAseq to identify candidate genes. Candidates were prioritized by correlation with outcome, using a consensus of three breast cancer patient cohorts. NNAT was transgenically expressed in ER+ breast cancer lines (T47D and ZR75), followed by transcriptomic and phenotypic characterization. RESULTS: We identified a region on rat chromosome 3 (142-178 Mb) that modified mammary tumor incidence. RNAseq of the mammary tumors narrowed the candidate list to three differentially expressed genes: NNAT, SLC35C2, and FAM210B. NNAT mRNA and protein also correlated with survival in human breast cancer patients. Quantitative immunohistochemistry of NNAT protein revealed an inverse correlation with survival in a univariate analysis of patients with invasive ER+ breast cancer (training cohort: n = 444, HR = 0.62, p = 0.031; validation cohort: n = 430, HR = 0.48, p = 0.004). NNAT also held up as an independent predictor of survival after multivariable adjustment (HR = 0.64, p = 0.038). NNAT significantly reduced proliferation and migration of ER+ breast cancer cells, which coincided with altered expression of multiple related pathways. CONCLUSIONS: Collectively, these data implicate NNAT as a novel mediator of cell proliferation and migration, which correlates with decreased tumorigenic potential and prolonged patient survival.

Yap is required for scar formation but not myocyte proliferation during heart regeneration in zebrafish.

AIMS: The Hippo signalling pathway regulates multiple cellular processes during organ development and maintenance by modulating activity of the transcriptional cofactor Yap. Core components of this pathway are required for neonatal mouse heart regeneration, however, investigations to date have typically focused on expression and activity in cardiomyocytes. Due to the regenerative capacity of zebrafish and the fact that global loss of Yap is not fully embryonic lethal in zebrafish, we leveraged a yap null mutant to investigate the impact of constitutive Yap deletion during zebrafish heart regeneration. METHODS AND RESULTS: Following cryoinjury in adult hearts, myocyte proliferation was not decreased in yap mutants, contrary to expectations based on mouse data. Experiments in larval zebrafish (Danio rerio) revealed that deletion of either Yap or Taz had a modest effect on heart growth, reducing gross organ size, while their combined deletion was synergistic; thus, Yap and Taz share some overlapping roles in zebrafish heart development. Surprisingly, adult yap mutants exhibited decreased collagen composition at 7 days post-injury, suggesting a critical role for Yap in scar formation during heart regeneration. siRNA-mediated Yap knockdown in primary rat (Rattus norvegicus) cardiac cells revealed a fibroblast-specific role for Yap in controlling the expression of cytoskeletal and myofibroblast activation genes, as well as pro-inflammatory cyto/chemokines. Corroborating these RNAseq data, we observed increased macrophage infiltration in the scars of yap mutants at 7 days post-injury. CONCLUSION: These results suggest that Yap deletion has minimal effect on myocyte proliferation in adults, but significantly influences scar formation and immune cell infiltration during zebrafish heart regeneration. Collectively, these data suggest an unexpected role for Yap in matrix formation and macrophage recruitment during heart regeneration.

Cardiac myosin binding protein C regulates postnatal myocyte cytokinesis.

Homozygous cardiac myosin binding protein C-deficient (Mybpc(t/t)) mice develop dramatic cardiac dilation shortly after birth; heart size increases almost twofold. We have investigated the mechanism of cardiac enlargement in these hearts. Throughout embryogenesis myocytes undergo cell division while maintaining the capacity to pump blood by rapidly disassembling and reforming myofibrillar components of the sarcomere throughout cell cycle progression. Shortly after birth, myocyte cell division ceases. Cardiac MYBPC is a thick filament protein that regulates sarcomere organization and rigidity. We demonstrate that many Mybpc(t/t) myocytes undergo an additional round of cell division within 10 d postbirth compared with their wild-type counterparts, leading to increased numbers of mononuclear myocytes. Short-hairpin RNA knockdown of Mybpc3 mRNA in wild-type mice similarly extended the postnatal window of myocyte proliferation. However, adult Mybpc(t/t) myocytes are unable to fully regenerate the myocardium after injury. MYBPC has unexpected inhibitory functions during postnatal myocyte cytokinesis and cell cycle progression. We suggest that human patients with homozygous MYBPC3-null mutations develop dilated cardiomyopathy, coupled with myocyte hyperplasia (increased cell number), as observed in Mybpc(t/t) mice. Human patients, with heterozygous truncating MYBPC3 mutations, like mice with similar mutations, have hypertrophic cardiomyopathy. However, the mechanism leading to hypertrophic cardiomyopathy in heterozygous MYBPC3(+/-) individuals is myocyte hypertrophy (increased cell size), whereas the mechanism leading to cardiac dilation in homozygous Mybpc3(-/-) mice is primarily myocyte hyperplasia.

A systematic analysis of neonatal mouse heart regeneration after apical resection.

The finding that neonatal mice are able to regenerate myocardium after apical resection has recently been questioned. We determined if heart regeneration is influenced by the size of cardiac resection and whether surgical retraction of the ventricular apex results in an increase in cardiomyocyte cell cycle activity. We performed moderate or large apical ventricular resections on neonatal mice and quantified scar infiltration into the left ventricular wall at 21 days post-surgery. Moderately resected hearts had 15±2% of the wall infiltrated by a collagen scar; significantly greater scar infiltration (23±4%) was observed in hearts with large resections. Resected hearts had higher levels of cardiomyocyte cell cycle activity relative to sham hearts. Surgically retracting the ventricle often resulted in fibrosis and induced cardiomyocyte cell cycle activity that were comparable to that of resected hearts. We conclude that apical resection in neonatal mice induces cardiomyocyte cell cycle activity and neomyogenesis, although scarring can occur. Surgical technique and definition of approach to assessing the extent of regeneration are both critical when using the neonatal mouse apical resection model.

Role of genetic modifiers in an orthologous rat model of ARPKD.

Human data and animal models of autosomal recessive polycystic kidney disease (ARPKD) suggest that genetic factors modulate the onset and severity of the disease. We report here for the first time that ARPKD susceptibility is attenuated by introgressing the mutated Pkhd1 disease allele from the polycystic kidney (PCK) rat onto the FHH (Fawn-Hooded Hypertensive) genetic background. Compared with PCK, the FHH.Pkhd1 strain had significantly decreased renal cyst formation that coincided with a threefold reduction in mean kidney weights. Further analysis revealed that the FHH. Pkhd1 is protected from increased blood pressure as well as elevated plasma creatinine and blood urea nitrogen levels. On the other hand, liver weight and biliary cystogenesis revealed no differences between PCK and FHH.Pkdh1, indicating that genes within the FHH genetic background prevent the development of renal, but not hepatic, manifestations of ARPKD. Microarray expression analysis of kidneys from 30-day-old PCK rats revealed increased expression of genes previously identified in PKD renal expression profiles, such as inflammatory response, extracellular matrix synthesis, and cell proliferation genes among others, whereas the FHH.Pkhd1 did not show activation of these common markers of disease. This newly developed strain can serve as a tool to map modifier genes for renal disease in ARPKD and provides further insight into disease variability and pathophysiology.