Functional abrogation of p53 is required for T-Ag induced proliferation in cardiomyocytes

Application of Three-Dimensional Culture Method in the Cardiac Conduction System Research

Congenital heart defects (CHD) are the most common type of birth defects. Several human case studies and genetically altered animal models have identified abnormalities in the development of ventricular conduction system (VCS) in the heart. While cell-based therapies hold promise for treating CHDs, translational efforts are limited by the lack of suitable in vitro models for feasibility and safety studies. A better understanding of cell differentiation pathways can lead to development of cell-based therapies for individuals living with CHD/VCS disorders. Here, we describe a new and reproducible 3-D cell culture method for studying cardiac cell lineage differentiation in vitro. We used primary ventricular cells isolated from embryonic day 11.5 (E11.5) mouse embryos, which can differentiate into multiple cardiac cell types including VCS cells. We compared 3-D cultures with three types of basement membrane extracts (BME) for their abilities to support E11.5 ventricular cell differentiation. In addition, the effects of atrial natriuretic peptide (ANP) and an inhibitor for its high affinity receptor were tested on cell differentiation in 3-D cultures. Following the cell culture, protocols for immunofluorescence imaging, cell extraction and protein isolation from the 3-D culture matrix and in-cell western methods are described. Further, these approaches can be used to study the effects of various ligands and genetic interventions on VCS cell development. We propose that these methodologies may also be extended for differentiation studies using other sources of stem cells such as induced pluripotent stem cells.

Expression of a transgene encoding mutant p193/CUL7 preserves cardiac function and limits infarct expansion after myocardial infarction

BACKGROUND

Transgenic mice expressing the dominant interfering p193 protein in cardiomyocytes (MHC-1152stop mice) exhibit an induction of cell cycle activity and altered remodelling after experimental myocardial infarction (MI).

OBJECTIVE

To determine whether the altered remodelling results in improved cardiac function in the MHC-1152stop mice after MI, as compared with non-transgenic mice.

METHODS

MHC-1152stop mice and non-transgenic littermates were subjected to experimental MI via permanent occlusion of the coronary artery. Infarct size was determined at 24 h and at 4 weeks after MI, and left ventricular pressure-volume measurements were performed at 4 weeks after MI in infarcted and sham-operated animals.

RESULTS

Infarct size in MHC-1152stop mice and non-transgenic littermates was not statistically different at 24 h after MI, as measured by tetrazolium staining. Morphometric analysis showed that infarct scar expansion at 4 weeks after MI was reduced by 10% in the MHC-1152stop mice (p<0.05). No differences in cardiac function were detected between sham-operated MHC-1152stop mice and their non-transgenic littermates. However, at 4 weeks after MI, the ventricular isovolumic relaxation time constant (tau) was decreased by 19% (p<0.05), and the slope of the dP/dt(max)-EDV relationship was increased 99% (p<0.05), in infarcted MHC-1152stop mice as compared with infarcted non-transgenic littermates.

CONCLUSION

Expression of the dominant interfering p193 transgene results in a decrease in infarct scar expansion and preservation of myocardial function at 4 weeks after MI. Antagonism of p193 activity may represent an important strategy for the treatment of MI.

Notch activates cell cycle reentry and progression in quiescent cardiomyocytes

The inability of heart muscle to regenerate by replication of existing cardiomyocytes has engendered considerable interest in identifying developmental or other stimuli capable of sustaining the proliferative capacity of immature cardiomyocytes or stimulating division of postmitotic cardiomyocytes. Here, we demonstrate that reactivation of Notch signaling causes embryonic stem cell–derived and neonatal ventricular cardiomyocytes to enter the cell cycle. The proliferative response of neonatal ventricular cardiomyocytes declines as they mature, such that late activation of Notch triggers the DNA damage checkpoint and G2/M interphase arrest. Notch induces recombination signal-binding protein 1 for Jκ (RBP-Jκ)-dependent expression of cyclin D1 but, unlike other inducers, also shifts its subcellular distribution from the cytosol to the nucleus. Nuclear localization of cyclin D1 is independent of RBP-Jκ. Thus, the influence of Notch on nucleocytoplasmic localization of cyclin D1 is an unanticipated property of the Notch intracellular domain that is likely to regulate the cell cycle in multiple contexts, including tumorigenesis as well as cardiogenesis.

Cell Lines as In Vitro Models for Drug Screening and Toxicity Studies

Cell culture is highly desirable, as it provides systems for ready, direct access and evaluation of tissues. The use of tissue culture is a valuable tool to study problems of clinical relevance, especially those related to diseases, screening, and studies of cell toxicity mechanisms. Ready access to the cells provides the possibility for easy studies of cellular mechanisms that may suggest new potential drug targets and, in the case of pathological-derived tissue, it has an interesting application in the evaluation of therapeutic agents that potentially may treat the dysfunction. However, special considerations must be addressed to establish stable in vitro function. In primary culture, these factors are primarily linked to greater demands of tissue to adequately survive and develop differentiated conditions in vitro. Additional requirements include the use of special substrates (collagen, laminin, extracellular matrix preparations, etc.), growth factors and soluble media supplements, some of which can be quite complex in their composition. These demands, along with difficulties in obtaining adequate tissue amounts, have prompted interest in developing immortalized cell lines which can provide unlimited tissue amounts. However, cell lines tend to exhibit problems in stability and/or viability, though they serve as a feasible alternative, especially regarding new potential applications in cell transplant therapy. In this regard, stem cells may also be a source for the generation of various cell types in vitro. This review will address aspects of cell culture system application, with focus on immortalized cell lines, in studying cell function and dysfunction with the primary aim being to identify cell targets for drug screening.

Assessment of embryonic myocardial cell differentiation using a dual fluorescent reporter system

Recent studies have identified the existence of undifferentiated myocardial cells during early embryonic as well as post-natal stages of heart development. While primitive cells present in the precardiac mesoderm can differentiate into multiple cell types of the cardiovascular system, the developmental potential of undifferentiated cells identified in the ventricular myocardium after chamber formation is not well characterized. A deeper understanding of mechanisms regulating myocardial cell differentiation will provide further insights into the normal and pathological aspects of heart development. Here, we showed that Nkx2.5 positive and sarcomeric myosin negative cells were predominantly localized in the right ventricular myocardium of CD1 mice at E11.5 stage. We confirmed that myocardial regions negative for saromeric myosin were also devoid of atrial natriuretic factor (ANF). These observations are consistent with our previous study, which showed that ANF expression is restricted to moderately differentiated and mature myocardial cells in E11.5 myocardium of C3H/FeJ mice. Further, we found that the receptor c-Kit, a marker for early embryonic myocardial progenitor cells, is not expressed in the undifferentiated cells of the E11.5 myocardium. To monitor the differentiation potential of Nkx2.5⁺/ANF⁻ cells in vitro, we developed a novel double fluorescent reporter system. Subsequently, we confirmed that the majority of Nkx2.5⁺/ANF⁻ cells expressed mature myocyte markers such as sarcomeric myosin, MLC2V and alpha-cardiac actin after 48 hrs in culture, albeit at lower levels compared to Nkx2.5⁺/ANF⁺ or Nkx2.5⁻/ANF⁺ cell populations. Our results suggest that fluorescent reporters under the control of lineage-specific promoters can be used to study myocardial cell differentiation in response to various exogenous or pharmacological agents.

Cardiac myocyte cell cycle control in development, disease, and regeneration

Cardiac myocytes rapidly proliferate during fetal life but exit the cell cycle soon after birth in mammals. Although the extent to which adult cardiac myocytes are capable of cell cycle reentry is controversial and species-specific differences may exist, it appears that for the vast majority of adult cardiac myocytes the predominant form of growth postnatally is an increase in cell size (hypertrophy) not number. Unfortunately, this limits the ability of the heart to restore function after any significant injury. Interest in novel regenerative therapies has led to the accumulation of much information on the mechanisms that regulate the rapid proliferation of cardiac myocytes in utero, their cell cycle exit in the perinatal period, and the permanent arrest (terminal differentiation) in adult myocytes. The recent identification of cardiac progenitor cells capable of giving rise to cardiac myocyte-like cells has challenged the dogma that the heart is a terminally differentiated organ and opened new prospects for cardiac regeneration. In this review, we summarize the current understanding of cardiomyocyte cell cycle control in normal development and disease. In addition, we also discuss the potential usefulness of cardiomyocyte self-renewal as well as feasibility of therapeutic manipulation of the cardiac myocyte cell cycle for cardiac regeneration.

Expression of a mutant p193/CUL7 molecule confers resistance to MG132- and etoposide-induced apoptosis independent of p53 or Parc binding

p193/CUL7 is an E3 ubiquitin ligase initially identified as an SV40 Large T Antigen binding protein. Expression of a dominant interfering variant of mouse p193/CUL7 (designated 1152stop) conferred resistance to MG132- and etoposide-induced apoptosis in U2OS cells. Immune precipitation/Western analyses revealed that endogenous p193/CUL7 formed a complex with Parc (a recently identified parkin-like ubiquitin ligase) and p53. Apoptosis resistance did not result from 1152stop-mediated disruption of the endogenous p193/CUL7 binding partners. Moreover, 1152stop molecule did not directly bind to endogenous p193/CUL7, Parc or p53. These data suggested a role for p193/CUL7 in the regulation of apoptosis independently of p53 and Parc activity.

Expression of Mutant p193 and p53 Permits Cardiomyocyte Cell Cycle Reentry After Myocardial Infarction in Transgenic Mice

Previous studies have demonstrated that expression of p193 and p53 mutants with dominant-interfering activities renders embryonic stem cell-derived cardiomyocytes responsive to the growth promoting activities of the E1A viral oncoproteins. In this study, the effects of p53 and p193 antagonization on cardiomyocyte cell cycle activity in normal and infarcted hearts were examined. Transgenic mice expressing the p193 and/or the p53 dominant-interfering mutants in the heart were generated. Transgene expression had no effect on cardiomyocyte cell cycle activity in uninjured adult hearts. In contrast expression of either transgene resulted in a marked induction of cardiomyocyte cell cycle activity at the infarct border zone at 4 weeks after permanent coronary artery occlusion. Expression of the p193 dominant-interfering mutant was also associated with an induction of cardiomyocyte DNA synthesis in the interventricular septa of infarcted hearts. A concomitant and marked reduction in hypertrophic cardiomyocyte growth was observed in the septa of hearts expressing the p193 dominant-interfering transgene, suggesting that cell cycle activation might partially counteract the adverse ventricular remodeling that occurs after infarction. Collectively these data suggest that antagonization of p193 and p53 activity relaxes the otherwise stringent regulation of cardiomyocyte cell cycle reentry in the injured adult heart.

Molecular mechanisms for cardiovascular stem cell apoptosis and growth in the hearts with atherosclerotic coronary disease and ischemic heart failure

In the heart with atherosclerotic coronary disease, chronic ischemia causes progressive loss of cardiovascular cells and ultimately triggers myocardial dysfunctions or heart failure. Various types of stem cells from embryonic and adult tissues have potentials for regenerating functional cardiovascular cells in the heart undergoing ischemic injury. However, native or exogenous stem cells in the ischemic hearts are exposed to various proapoptotic or cytotoxic factors. Furthermore, during repopulation and differentiation, certain numbers of newly produced cells may die by apoptosis during neocardiovascular tissue remodeling and morphogenesis. Embryonic and adult stem cells may have different life spans, as being programmed genetically to apoptosis. The endogenous and environmental factors play important roles in regulation of stem cells, including inflammatory cytokines, growth factors, surface receptors, proteolytic enzymes, mitochondrial respiration, nuclear proteins, telomerase activities, hypoxia-responding proteins, and stem cell-host cell interaction. Clarification of the molecular mechanisms may help us understand and design stem cell therapies.

Modulation of the Cardiomyocyte Cell Cycle in Genetically Altered Animals

Many forms of cardiovascular disease are associated with cardiomyocyte loss via apoptosis and/or necrosis. Although there is currently debate regarding the level at which adult cardiomyocytes can reenter the cell cycle and proliferate, it is clear that the intrinsic regenerative growth capacity is insufficient to reverse the progression to failure in badly injured hearts. The ability to reactivate cardiomyocyte proliferation in damaged hearts might permit regenerative growth, provided that the nascent cells are able to participate in a functional syncytium with the surviving myocardium. In this review, techniques commonly used to monitor cardiomyocyte cell cycle activity in normal and injured hearts are discussed. In addition, several genetic models are described wherein the expression of fundamental cell cycle regulatory proteins has been altered in cardiomyocytes.

Assessment of the ultrastructural and proliferative properties of human embryonic stem cell-derived cardiomyocytes

Assessment of early ultrastructural development and cell-cycle regulation in human cardiac tissue is significantly hampered by the lack of a suitable in vitro model. Here we describe the possible utilization of human embryonic stem cell (ES) lines for investigation of these processes. With the use of the embryoid body (EB) differentiation system, human ES cell-derived cardiomyocytes at different developmental stages were isolated and their histomorphometric, ultrastructural, and proliferative properties were characterized. Histomorphometric analysis revealed an increase in cell length, area, and length-to-width ratio in late-stage EBs (>35 days) compared with early (10–21 days) and intermediate (21–35 days) stages. This was coupled with a progressive ultrastructural development from an irregular myofibrillar distribution to an organized sarcomeric pattern. Cardiomyocyte proliferation, assessed by double labeling with cardiac-specific antibodies and either [3H]thymidine incorporation or Ki-67 immunolabeling, demonstrated a gradual withdrawal from cell cycle. Hence, the percentage of positively stained nuclei in early-stage cardiomyocytes ([3H]thymidine: 60 ± 10%, Ki-67: 54 ± 23%) decreased to 36 ± 7% and 9 ± 16% in intermediate-stage EBs and to <1% in late-stage cardiomyocytes. In conclusion, a reproducible temporal pattern of early cardiomyocyte proliferation, cell-cycle withdrawal, and ultrastructural maturation was noted in this model. Establishment of this unique in vitro surrogate system may allow to examine the molecular mechanisms underlying these processes and to assess interventions aiming to modify these properties. Moreover, the detailed characterization of the ES cell-derived cardiomyocyte may be crucial for the development of future cell replacement strategies aiming to regenerate functional myocardium.

Cell cycle regulation to repair the infarcted myocardium

Lower vertebrates such as newt and zebrafish are able to reactivate high levels of cardiomyocyte cell cycle activity in response to experimental injury resulting in apparent regeneration. In contrast, damaged myocardium is replaced by fibrotic scar tissue in higher vertebrates. This process compromises the contractile function of the surviving myocardium, ultimately leading to heart failure. Various strategies are being pursued to augment myocyte number in the diseased hearts. One approach entails the reactivation of cell cycle in surviving cardiomyocytes. Here, we provide a summary of methods to monitor cell cycle activity, and interventions demonstrating positive cell cycle effects in cardiomyocytes as well as discuss the potential utility of cell cycle regulation to augment myocyte number in diseased hearts.

Cardiomyocyte cell cycle regulation

Although rapid progress is being made in many areas of molecular cardiology, issues pertaining to the origins of heart-forming cells, the mechanisms responsible for cardiogenic induction, and the pathways that regulate cardiomyocyte proliferation during embryonic and adult life remain unanswered. In the present study, we review approaches and studies that have shed some light on cardiomyocyte cell cycle regulation. For reference, an initial description of cardiomyogenic induction and morphogenesis is provided, which is followed by a summary of published cell cycle analyses during these stages of cardiac ontology. A review of studies examining cardiomyocyte cell cycle analysis and de novo cardiomyogenic induction in the adult heart is then presented. Finally, studies in which cardiomyocyte cell cycle activity was experimentally manipulated in vitro and in vivo are reviewed. It is hoped that this compilation will serve to stimulate thought and experimentation in this intriguing area of cardiomyocyte cell biology.