Myocyte proliferation in the developing heart

Mechanical regulation of cardiac development

Mechanical forces are essential contributors to and unavoidable components of cardiac formation, both inducing and orchestrating local and global molecular and cellular changes. Experimental animal studies have contributed substantially to understanding the mechanobiology of heart development. More recent integration of high-resolution imaging modalities with computational modeling has greatly improved our quantitative understanding of hemodynamic flow in heart development. Merging these latest experimental technologies with molecular and genetic signaling analysis will accelerate our understanding of the relationships integrating mechanical and biological signaling for proper cardiac formation. These advances will likely be essential for clinically translatable guidance for targeted interventions to rescue malforming hearts and/or reconfigure malformed circulations for optimal performance. This review summarizes our current understanding on the levels of mechanical signaling in the heart and their roles in orchestrating cardiac development.

Targeted inactivation of Cerberus like-2 leads to left ventricular cardiac hyperplasia and systolic dysfunction in the mouse

Previous analysis of the Cerberus like 2 knockout (Cerl2^−/−) mouse revealed a significant mortality during the first day after birth, mostly due to cardiac defects apparently associated with randomization of the left-right axis. We have however, identified Cerl2-associated cardiac defects, particularly a large increase in the left ventricular myocardial wall in neonates that cannot be explained by laterality abnormalities. Therefore, in order to access the endogenous role of Cerl2 in cardiogenesis, we analyzed the embryonic and neonatal hearts of Cerl2 null mutants that did not display a laterality phenotype. Neonatal mutants obtained from the compound mouse line Cer2^−/−::Mlc1v-nLacZ24⁺, in which the pulmonary ventricle is genetically marked, revealed a massive enlargement of the ventricular myocardium in animals without laterality defects. Echocardiography analysis in Cerl2^−/− neonates showed a left ventricular systolic dysfunction that is incompatible with a long lifespan. We uncovered that the increased ventricular muscle observed in Cerl2^−/− mice is caused by a high cardiomyocyte mitotic index in the compact myocardium which is mainly associated with increased Ccnd1 expression levels in the left ventricle at embryonic day (E) 13. Interestingly, at this stage we found augmented left ventricular expression of Cerl2 levels when compared with the right ventricle, which may elucidate the regionalized contribution of Cerl2 to the left ventricular muscle formation. Importantly, we observed an increase of phosphorylated Smad2 (pSmad2) levels in embryonic (E13) and neonatal hearts indicating a prolonged TGFβs/Nodal-signaling activation. Concomitantly, we detected an increase of Baf60c levels, but only in Cerl2^−/− embryonic hearts. These results indicate that independently of its well-known role in left-right axis establishment Cerl2 plays an important role during heart development in the mouse, mediating Baf60c levels by exerting an important control of the TGFβs/Nodal-signaling pathway.

Studying dynamic events in the developing myocardium

Differentiation and conduction properties of the cardiomyocytes are critically dependent on physical conditioning both in vitro and in vivo. Historically, various techniques were introduced to study dynamic events such as electrical currents and changes in ionic concentrations in live cells, multicellular preparations, or entire hearts. Here we review this technological progress demonstrating how each improvement in spatial or temporal resolution provided answers to old and provoked new questions. We further demonstrate how high-speed optical mapping of voltage and calcium can uncover pacemaking potential within the outflow tract myocardium, providing a developmental explanation of ectopic beats originating from this region in the clinical settings.

Time-lapse imaging of cell cycle dynamics during development in living cardiomyocyte

Mammalian cardiomyocytes withdraw from the cell cycle shortly after birth, although it remains unclear how cardiomyocyte cell cycles behave during development. Compared to conventional immunohistochemistry in static observation, time-lapse imaging can reveal comprehensive data in hard-to-understand biological phenomenon. However, there are no reports of an established protocol of successful time-lapse imaging in mammalian heart. Thus, it is valuable to establish a time-lapse imaging system to enable the observation of cell cycle dynamics in living murine cardiomyocytes. This study sought to establish time-lapse imaging of murine heart to study cardiomyocyte cell cycle behavior. The Fucci (fluorescent ubiquitination-based cell cycle indicator) system can effectively label individual G1, S/G2/M, and G1/S-transition phase nuclei red, green and yellow, respectively, in living mammalian cells, and could therefore be useful to visualize the real-time cell cycle transitions in living murine heart. To establish a similar system for time-lapse imaging of murine heart, we first developed an ex vivo culture system, with the culture conditions determined in terms of sample state, serum concentration, and oxygen concentration. The optimal condition (slice culture, oxygen concentration 20%, serum concentration 10%) successfully mimicked physiological cardiomyocyte proliferation in vivo. Time-lapse imaging of cardiac slices from E11.5, E14.5, E18.5, and P1 Fucci-expressing transgenic mice revealed an elongated S/G2/M phase in cardiomyocytes during development. Our time-lapse imaging of murine heart revealed a gradual elongation of the S/G2/M phase during development in living cardiomyocytes.

N-Acetylcysteine prevents congenital heart defects induced by pregestational diabetes

BACKGROUND

Pregestational diabetes is a major risk factor of congenital heart defects (CHDs). Glutathione is depleted and reactive oxygen species (ROS) production is elevated in diabetes. In the present study, we aimed to examine whether treatment with N-acetylcysteine (NAC), which increases glutathione synthesis and inhibits ROS production, prevents CHDs induced by pregestational diabetes.

METHODS

Female mice were treated with streptozotocin (STZ) to induce pregestational diabetes prior to breeding with normal males to produce offspring. Some diabetic mice were treated with N-acetylcysteine (NAC) in drinking water from E0.5 to the end of gestation or harvesting of the embryos. CHDs were identified by histology. ROS levels, cell proliferation and gene expression in the fetal heart were analyzed.

RESULTS

Our data show that pregestational diabetes resulted in CHDs in 58% of the offspring, including ventricular septal defect (VSD), atrial septal defect (ASD), atrioventricular septal defects (AVSD), transposition of great arteries (TGA), double outlet right ventricle (DORV) and tetralogy of Fallot (TOF). Treatment with NAC in drinking water in pregestational diabetic mice completely eliminated the incidence of AVSD, TGA, TOF and significantly diminished the incidence of ASD and VSD. Furthermore, pregestational diabetes increased ROS, impaired cell proliferation, and altered Gata4, Gata5 and Vegf-a expression in the fetal heart of diabetic offspring, which were all prevented by NAC treatment.

CONCLUSIONS

Treatment with NAC increases GSH levels, decreases ROS levels in the fetal heart and prevents the development of CHDs in the offspring of pregestational diabetes. Our study suggests that NAC may have therapeutic potential in the prevention of CHDs induced by pregestational diabetes.

Embryonic cardiac chamber maturation: Trabeculation, conduction, and cardiomyocyte proliferation

Congenital heart diseases are some of the most common human birth defects. Though some congenital heart defects can be surgically corrected, treatment options for other congenital heart diseases are very limited. In many congenital heart diseases, genetic defects lead to impaired embryonic heart development or growth. One of the key development processes in cardiac development is chamber maturation, and alterations in this maturation process can manifest as a variety of congenital defects including non-compaction, systolic dysfunction, diastolic dysfunction, and arrhythmia. During development, to meet the increasing metabolic demands of the developing embryo, the myocardial wall undergoes extensive remodeling characterized by the formation of muscular luminal protrusions called cardiac trabeculae, increased cardiomyocyte mass, and development of the ventricular conduction system. Though the basic morphological and cytological changes involved in early heart development are clear, much remains unknown about the complex biomolecular mechanisms governing chamber maturation. In this review, we highlight evidence suggesting that a wide variety of basic signaling pathways and biomechanical forces are involved in cardiac wall maturation.

To divide or not to divide: revisiting liver regeneration

The liver has a remarkable capacity to regenerate. Even with surgical removal (partial hepatectomy) of 70% of liver mass, the remnant tissue grows to recover the original mass and functions. Liver regeneration after partial hepatectomy has been studied extensively since the 19th century, establishing the long-standing model that hepatocytes, which account for most of the liver weight, proliferate to recover the original mass of the liver. The basis of this model is the fact that almost all hepatocytes undergo S phase, as shown by the incorporation of radioactive nucleotides during liver regeneration. However, DNA replication does not necessarily indicate the execution of cell division, and a possible change in hepatocyte size is not considered in the model. In addition, as 15-30% of hepatocytes in adult liver are binuclear, the difference in nuclear number may affect the mode of cell division during regeneration. Thus, the traditional model seems to be oversimplified. Recently, we developed new techniques to investigate the process of liver regeneration, and revealed interesting features of hepatocytes. In this review, we first provide a historical overview of how the widely accepted model of liver regeneration was established and then discuss some overlooked observations together with our recent findings. Finally, we describe the revised model and perspectives on liver regeneration research.

Cardiomyocyte proliferation contributes to heart growth in young humans

The human heart is believed to grow by enlargement but not proliferation of cardiomyocytes (heart muscle cells) during postnatal development. However, recent studies have shown that cardiomyocyte proliferation is a mechanism of cardiac growth and regeneration in animals. Combined with evidence for cardiomyocyte turnover in adult humans, this suggests that cardiomyocyte proliferation may play an unrecognized role during the period of developmental heart growth between birth and adolescence. We tested this hypothesis by examining the cellular growth mechanisms of the left ventricle on a set of healthy hearts from humans aged 0-59 y (n = 36). The percentages of cardiomyocytes in mitosis and cytokinesis were highest in infants, decreasing to low levels by 20 y. Although cardiomyocyte mitosis was detectable throughout life, cardiomyocyte cytokinesis was not evident after 20 y. Between the first year and 20 y of life, the number of cardiomyocytes in the left ventricle increased 3.4-fold, which was consistent with our predictions based on measured cardiomyocyte cell cycle activity. Our findings show that cardiomyocyte proliferation contributes to developmental heart growth in young humans. This suggests that children and adolescents may be able to regenerate myocardium, that abnormal cardiomyocyte proliferation may be involved in myocardial diseases that affect this population, and that these diseases might be treatable through stimulation of cardiomyocyte proliferation.

Biomechanics of the chick embryonic heart outflow tract at HH18 using 4D optical coherence tomography imaging and computational modeling

During developmental stages, biomechanical stimuli on cardiac cells modulate genetic programs, and deviations from normal stimuli can lead to cardiac defects. Therefore, it is important to characterize normal cardiac biomechanical stimuli during early developmental stages. Using the chicken embryo model of cardiac development, we focused on characterizing biomechanical stimuli on the Hamburger–Hamilton (HH) 18 chick cardiac outflow tract (OFT), the distal portion of the heart from which a large portion of defects observed in humans originate. To characterize biomechanical stimuli in the OFT, we used a combination of in vivo optical coherence tomography (OCT) imaging, physiological measurements and computational fluid dynamics (CFD) modeling. We found that, at HH18, the proximal portion of the OFT wall undergoes larger circumferential strains than its distal portion, while the distal portion of the OFT wall undergoes larger wall stresses. Maximal wall shear stresses were generally found on the surface of endocardial cushions, which are protrusions of extracellular matrix onto the OFT lumen that later during development give rise to cardiac septa and valves. The non-uniform spatial and temporal distributions of stresses and strains in the OFT walls provide biomechanical cues to cardiac cells that likely aid in the extensive differential growth and remodeling patterns observed during normal development.

Cryoinjury as a myocardial infarction model for the study of cardiac regeneration in the zebrafish

The zebrafish heart has the capacity to regenerate after ventricular resection. Although this regeneration model has proved useful for the elucidation of certain regeneration mechanisms, it is based on the removal of heart tissue rather than on tissue damage. We recently characterized the cellular response and regenerative capacity of the zebrafish heart after cryoinjury (CI), an alternative procedure that more closely models the pathophysiological process undergone by the human heart after myocardial infarction (MI). After anesthesia, localized CI with a liquid nitrogen-cooled copper probe induced damage in 25% of the ventricle, in a procedure requiring <5 min. Here we present a detailed description of the technique, which provides a valuable system for the study of the mechanisms of heart regeneration and scar removal after MI in a versatile vertebrate model.

Cell lineages, growth and repair of the mouse heart

The formation of the heart involves diversification of lineages which differentiate into distinct cardiac cell types or contribute to different regions such as the four cardiac chambers. The heart is the first organ to form in the embryo. However, in parallel with the growth of the organism, before or after birth, the heart has to adapt its size to maintain pumping efficiency. The adult heart has only a mild regeneration potential; thus, strategies to repair the heart after injury are based on the mobilisation of resident cardiac stem cells or the transplantation of external sources of stem cells. We discuss current knowledge on these aspects and raise questions for future research.

Tbx20 regulation of cardiac cell proliferation and lineage specialization during embryonic and fetal development in vivo

TBX20 gain-of-function mutations in humans are associated with congenital heart malformations and myocardial defects. However the effects of increased Tbx20 function during cardiac chamber development and maturation have not been reported previously. CAG-CAT-Tbx20 transgenic mice were generated for Cre-dependent induction of Tbx20 in myocardial lineages in the developing heart. βMHCCre-mediated overexpression of Tbx20 in fetal ventricular cardiomyocytes results in increased thickness of compact myocardium, induction of cardiomyocyte proliferation, and increased expression of Bmp10 and pSmad1/5/8 at embryonic day (E) 14.5. βMHCCre-mediated Tbx20 overexpression also leads to increased expression of cardiac conduction system (CCS) genes Tbx5, Cx40, and Cx43 throughout the ventricular myocardium. In contrast, Nkx2.5Cre mediated overexpression of Tbx20 in the embryonic heart results in reduced cardiomyocyte proliferation, increased expression of a cell cycle inhibitor, p21(CIP1), and decreased expression of Tbx2, Tbx5, and N-myc1 at E9.5, concomitant with decreased phospho-ERK1/2 expression. Together, these analyses demonstrate that Tbx20 differentially regulates cell proliferation and cardiac lineage specification in embryonic versus fetal cardiomyocytes. Induction of pSmad1/5/8 at E14.5 and inhibition of dpERK expression at E9.5 are consistent with selective Tbx20 regulation of these pathways in association with stage-specific effects on cardiomyocyte proliferation. Together, these in vivo data support distinct functions for Tbx20 in regulation of cardiomyocyte lineage maturation and cell proliferation at embryonic and fetal stages of heart development.

Improving cardiac regeneration after injury: are we a step closer?

During regeneration, lost functional tissue can, in general, be replaced by different mechanisms, including proliferation of terminally differentiated cells or through differentiation of resident stem cells. It is a well-accepted dogma that the mammalian heart cannot efficiently regenerate upon injury as a consequence of insufficient oxygen supply. This is in sharp contrast to the hearts of adult zebrafish or newts that are able to replace lost ventricular tissue. Novel data indicate that the young murine heart also has the ability to regenerate within the first week after birth using mechanisms apparently quite similar to those observed in fish. This now provides us with a good starting point to identify the molecular mechanisms that led to the loss of the regenerative capacity of the adult mammalian heart. These future studies will also indicate whether it will be possible to reawaken the regenerative capability of cardiomyocytes in the human heart by treatment with selected pharmaceuticals.