It remains a mystery why pulmonary arterial hypertension (PAH) is irreversible in some congenital heart disease patients while not in others who have very similar underlying cardiac defects. Smadja and coworkers identified a proliferative apoptosis-resistant endothelial phenotype in lung biopsies of patients with irreversible PAH.1 As a less invasive means of identifying this phenotype for irreversible PAH the same group is now measuring circulating endothelial cells as a biomarker to distinguish reversible from irreversible PAH secondary to congenital heart disease.2
Twenty-six congenital heart disease patients with PAH at the Hospital Necker-Enfants Malades, Paris, France were retrospectively labeled with reversible or irreversible PAH based on pulmonary artery pressure 6 months after surgery. Ten patients had irreversible PAH. Lung biopsies taken during surgery showed all patients with irreversible PAH had an apoptosis-resistant phenotype with Bcl2 expression and elevated vascular endothelial growth factor levels. The circulating endothelial cell count in the peripheral blood was 10-fold higher in patients with irreversible PAH. Endothelial cells were quantitated by immunocapture from peripheral blood with magnetic beads coated with an antibody against the endothelial antigen CD146.
Circulating endothelial cells may be a useful biomarker for identifying patients at risk for developing irreversible PAH after surgery for congenital heart disease, a reflection of pulmonary endothelial remodeling. Further studies with longer follow-up and larger numbers of patients will be important to determine whether circulating endothelial cell counts can replace the more invasive lung biopsies to predict irreversible PAH.
Although the transcriptional regulation of the developing heart has been extensively studied, the specific factors that are required to induce the cardiac program for mammalian cells was unknown. Takeuchi and Bruneau3 have identified a group of transcription factors that control cellular differentiation and might allow for regeneration of cardiac myocytes. Studies of ectopic induction of cardiac differentiation in mouse embryos (embryonic day 7.0-8.0) determined that Gata4 and Baf60c can initiate cardiac gene expression but the addition of Tbx5 is necessary for induction of contractile tissue from mesoderm. This minimal combination of transcription factors can induce non-cardiac mammalian mesoderm to differentiate into beating cardiac myocytes. Gata4 is the key factor to initiate the cardiac gene program, with Baf60c allowing binding of Gata4. Tbx5 is required for full differentiation into contractile myocytes. Determination of this novel, tissue-specific differentiation pathway might allow for production of beating cardiac cells for therapeutic uses.
It is realized that diurnal rhythms play a role in cardiovascular physiology and that patients with sleep disturbances have increased risk of heart attack and stroke. However it is less recognized that there are diurnal variations at the molecular level with gene expression variations during the night compared with the day. Martino and Sole summarize the importance of recognizing these diurnal gene alterations in an interesting review, the first of a series on circadian rhythm and cardiovascular function in Circulation Research.4 As cardiovascular tissues have daily variation in physiological processes, along with gene and protein expression, treatment should consider not only the right compartment in the right cell but also the right time. For example, plasminogen activator inhibitor-1mRNA expression, a fibrinolytic regulator, peaks in the morning, a documented time of high risk for myocardial infarcts.5 Under normal 24-hour day and night conditions as much as 13% of the entire mouse cardiac transcriptome exhibited a rhythmic pattern.6 Expression of many of the crucial myocardial renewal genes are highest during the period that the subject normally allocates for sleep. In addition, rat hearts isolated during the subject’s perceived day have greater contractility power than those isolated during the subjective night.7 Disturbances of the diurnal rhythms can also exacerbate heart disease with studies of hamsters of a naturally cardiomyopathic strain showing increased morbidity and mortality with disruption of their light and dark cycle.8
Clinically the application of principles may be highlighted as the risk/benefit and the efficacy of therapeutics may vary with diurnal cycle. In addition, intensive care units are places of often preventable light and sleep disturbances that subject critically ill patients to disturbance of gene expression patterns important for renewal of cardiac cells.
Continuing on the trail of another Holy Grail, investigators from Boston Children’s Hospital demonstrated that indeed, differentiated heart muscle cells, cardiomyocytes, can be induced to proliferate and regenerate. Bersell and colleagues in their study published in the journal Cell,9 discover that the growth factor neuregulin1 (NRG1) and its tyrosine kinase receptor, ErbB4, can induce mononucleated, but not binucleated, cardiomyocytes to divide. They show through in vivo experiments that genetic inactivation of ErbB4 reduces cardiomyocyte proliferation, whereas increasing ErbB4 expression enhances it. Further, they show that by injecting NRG1 in adult mice, they can induce cardiomyocyte cell-cycle activity and promote myocardial regeneration, leading to improved function after induction of myocardial infarction. Importantly, in their studies, undifferentiated progenitor cells did not contribute to NRG1-induced cardiomyocyte proliferation. These findings suggest that increasing the activity of the NRG1/ErbB4 signaling pathway may provide a molecular strategy to promote myocardial regeneration or that may play an important part in the pathogenesis of certain congenital heart diseases.
The Nature paper published with the above title10 focuses just on that: how did the reptilian heart evolve and do they possess a single ventricular chamber or two incompletely septated ventricles? The Bruneau group from the Gladstone Institute of Cardiovascular Diseases in San Francisco examine heart development in the red-eared slider turtle, Trachemys scripta elegans (a chelonian), and the green anole, Anolis carolinensis (a squamate), focusing on gene expression in the developing ventricles. In their studies, the investigators focus on the expression of the T-box transcription factor Tbx5. They find that in contrast to birds and mammals which restrict their expression of Tbx5 to the left ventricle precursors, in both reptiles Tbx5 is homogenously expressed in the ventricular chamber. In later stages, however, Tbx5 expression in the turtle (but not anole) heart is gradually restricted to a distinct left ventricle, forming a left-right gradient. This suggests that Tbx5 expression was refined during evolution to pattern the ventricles. In further support of the preceding hypothesis, the authors show that loss of Tbx5 expression in the mouse ventricle results in a single chamber lacking distinct identity and misexpression of Tbx5 in the developing myocardium, mimicking the reptilian pattern, and results in a single, unseptated ventricle. These findings prove that appropriate Tbx5 expression is requisite for proper septation. These findings are consistent with prior observations related to involvement of Tbx5 in certain single ventricle pathologies seen in humans.
Publication Date: 18-Mar-2010
Last Modified: 18-Mar-2010