MAP kinase activation in avian cardiovascular development

Aerobic performance in tinamous is limited by their small heart. A novel hypothesis in the evolution of avian flight

Some biomechanical studies from fossil specimens suggest that sustained flapping flight of birds could have appeared in their Mesozoic ancestors. We challenge this idea because a suitable musculoskeletal anatomy is not the only requirement for sustained flapping flight. We propose the “heart to fly” hypothesis that states that sustained flapping flight in modern birds required an enlargement of the heart for the aerobic performance of the flight muscles and test it experimentally by studying tinamous, the living birds with the smallest hearts. The small ventricular size of tinamous reduces cardiac output without limiting perfusion pressures, but when challenged to fly, the heart is unable to support aerobic metabolism (quick exhaustion, larger lactates and post-exercise oxygen consumption and compromised thermoregulation). At the same time, cardiac growth shows a crocodilian-like pattern and is correlated with differential gene expression in MAPK kinases. We integrate this physiological evidence in a new evolutionary scenario in which the ground-up, short and not sustained flapping flight displayed by tinamous represents an intermediate step in the evolution of the aerobic sustained flapping flight of modern birds.

More than Just a Simple Cardiac Envelope; Cellular Contributions of the Epicardium

The adult pumping heart is formed by distinct tissue layers. From inside to outside, the heart is composed by an internal endothelial layer, dubbed the endocardium, a thick myocardial component which supports the pumping capacity of the heart and exteriorly covered by a thin mesothelial layer named the epicardium. Cardiac insults such as coronary artery obstruction lead to ischemia and thus to an irreversible damage of the myocardial layer, provoking in many cases heart failure and death. Thus, searching for new pathways to regenerate the myocardium is an urgent biomedical need. Interestingly, the capacity of heart regeneration is present in other species, ranging from fishes to neonatal mammals. In this context, several lines of evidences demonstrated a key regulatory role for the epicardial layer. In this manuscript, we provide a state-of-the-art review on the developmental process leading to the formation of the epicardium, the distinct pathways controlling epicardial precursor cell specification and determination and current evidences on the regenerative potential of the epicardium to heal the injured heart.

Optical coherence tomography captures rapid hemodynamic responses to acute hypoxia in the cardiovascular system of early embryos

BACKGROUND

The trajectory to heart defects may start in tubular and looping heart stages when detailed analysis of form and function is difficult by currently available methods. We used a novel method, Doppler optical coherence tomography (OCT), to follow changes in cardiovascular function in quail embryos during acute hypoxic stress. Chronic fetal hypoxia is a known risk factor for congenital heart diseases (CHDs). Decreased fetal heart rates during maternal obstructive sleep apnea suggest that studying fetal heart responses under acute hypoxia is warranted.

RESULTS

We captured responses to hypoxia at the critical looping heart stages. Doppler OCT revealed detailed vitelline arterial pulsed Doppler waveforms. Embryos tolerated 1 hr of hypoxia (5%, 10%, or 15% O(2) ), but exhibited changes including decreased systolic and increased diastolic duration in 5 min. After 5 min, slower heart rates, arrhythmic events and an increase in retrograde blood flow were observed. These changes suggested slower filling of the heart, which was confirmed by four-dimensional Doppler imaging of the heart itself.

CONCLUSIONS

Doppler OCT is well suited for rapid noninvasive screening for functional changes in avian embryos under near physiological conditions. Analysis of the accessible vitelline artery sensitively reflected changes in heart function and can be used for rapid screening. Acute hypoxia caused rapid hemodynamic changes in looping hearts and may be a concern for increased CHD risk.

Role of ERK1/2 signaling in congenital valve malformations in Noonan syndrome

Noonan syndrome (NS) is the most common nonchromosomal genetic disorder associated with cardiovascular malformations. The most prominent cardiac defects in NS are pulmonary valve stenosis and hypertrophic cardiomyopathy. Gain-of-function mutations in the protein tyrosine phosphatase Shp2 have been identified in 50% of NS families. We created a NS mouse model with selective overexpression of mutant Shp2 (Q79R-Shp2) in the developing endocardial cushions. In our model, Cre recombinase driven by the Tie2 promoter irreversibly activates transgenic Q79R-Shp2 expression in the endothelial-derived cell lineage. Q79R-Shp2 expression resulted in embryonic lethality by embryonic day 14.5. Importantly, mutant embryos showed significantly enlarged endocardial cushions in the atrioventricular canal and in the outflow tract. In contrast, overexpression of wild-type Shp2 protein at comparable levels did not enhance endocardial cushion growth or alter the morphology of the mature adult valves. Expression of Q79R-Shp2 was accompanied by increased ERK1/2 activation in a subset of cells within the cushion mesenchyme, suggesting that hyperactivation of this signaling pathway may play a pathogenic role. To test this hypothesis in vivo, Q79R-Shp2-expressing mice were crossed with mice carrying either a homozygous ERK1 or a heterozygous ERK2 deletion. Deletion of ERK1 completely rescued the endocardial cushion phenotype, whereas ERK2 protein reduction did not affect endocardial cushion size. Constitutive hyperactivation of ERK1/2 signaling alone with a transgenic approach resulted in a phenocopy of the valvular phenotype. The data demonstrate both necessity and sufficiency of increased ERK activation downstream of Shp2 in mediating abnormal valve development in a NS mouse model.

BMP and FGF regulatory pathways in semilunar valve precursor cells

In the developing atrioventricular (AV) valve, limb bud, and somites, cartilage cell lineage differentiation is regulated by bone morphogenetic protein (BMP), while fibroblast growth factor (FGF) controls tendon cell fate. We observed aggrecan and sox9, characteristic of cartilage cell types, and scleraxis and tenascin, characteristic of tendon cell types, in developing avian semilunar valves. Addition of BMP4 to outflow tract (OFT) precursor cells of young (E4.5) but not older (E6) chick embryos activated Smad1/5/8 and induced sox9 and aggrecan expression, while FGF4 treatment increased phosphorylated MAPK (dpERK) signaling and promoted expression of scleraxis and tenascin. These results identify BMP and FGF pathways that promote expression of cartilage- or tendon-like characteristics in semilunar valve precursor cells. In contrast to AV valve precursor cells, which diversify into leaflets (cartilage-like) or chordae tendineae (tendon-like), semilunar valve cells exhibit both cartilage- and tendon-like characteristics in the developing and mature valve cusp.

BMP and FGF regulatory pathways control cell lineage diversification of heart valve precursor cells

The atrioventricular heart valve leaflets and chordae tendineae are composed of diverse cell lineages and highly organized extracellular matrices that share characteristics with cartilage and tendon cell types in the limb buds and somites. During embryonic chicken valvulogenesis, aggrecan and sox9, characteristic of cartilage cells, are observed in the AV valve leaflets, in contrast to tendon-associated genes scleraxis and tenascin, present in the chordae tendineae. In the limb buds and somites, cartilage cell lineage differentiation is regulated by BMP2, while FGF4 controls tendon cell fate. The ability of BMP2 and FGF4 to induce similar patterns of gene expression in heart valve precursor cells was examined. In multiple assays of cells from prefused endocardial cushions, BMP2 is sufficient to activate Smad1/5/8 phosphorylation and induce sox9 and aggrecan expression, while FGF4 treatment increases phosphorylated MAPK (dpERK) signaling and promotes expression of scleraxis and tenascin. However, these treatments do not alter differentiated lineage gene expression in valve progenitors from fused cushions of older embryos. Together, these studies define regulatory pathways of AV valve progenitor cell diversification into leaflets and chordae tendineae that share inductive interactions and differentiation phenotypes with cartilage and tendon cell lineages.

Hearts and bones: shared regulatory mechanisms in heart valve, cartilage, tendon, and bone development

The mature heart valves are dynamic structures composed of highly organized cell lineages and extracellular matrices. The discrete architecture of connective tissue within valve leaflets and supporting structures allows the valve to withstand life-long functional demands and changes in hemodynamic forces and load. The dysregulation of ECM organization is a common feature of heart valve disease and can often be linked to genetic defects in matrix protein structure or developmental regulation. Recent studies have identified specific regulatory pathways that are active in the developing valve structures and also control cartilage, tendon, and bone development. This review will focus on the regulatory hierarchies that control normal and abnormal heart valve development in parallel with other connective tissue cell types.

Physiopathology of the embryonic heart (with special emphasis on hypoxia and reoxygenation)

The adaptative response of the developing heart to adverse intrauterine environment such as reduced O2 delivery can result in alteration of gene expression with short- and long-term consequences including adult cardiovascular diseases. The tolerance of the developing heart of acute or chronic oxygen deprivation, its capacity to recover during reperfusion and the mechanisms involved in reoxygenation injury are still under debate. Indeed, the pattern of response of the immature myocardium to hypoxia-reoxygenation differs from that of the adult. This review deals with the structural and metabolic characteristics of the embryonic heart and the functional consequences of hypoxia and reoxygenation. The relative contribution of calcium and sodium overload, pH disturbances and oxidant stress to the hypoxia-induced cardiac dysfunction is examined, as well as various cellular signaling pathways (e.g. MAP kinases) involved in cell survival or death. In the context of the recent advances in developmental cardiology and fetal cardiac surgery, a better understanding of the physiopathology of the stressed developing heart is required.

Ras-related signaling pathways in valve development: ebb and flow

Congenital heart defects affect approximately 1 in every 100 live births, and deficits in the formation of the mitral, tricuspid, and outflow tract valves account for 20-25% of all cardiac malformations. Mutations in genes that affect Ras signaling have been identified in individuals with congenital valve disease associated with Noonan syndrome and neurofibromatosis type 1. Dissection of Ras-related signaling pathways during valvulogenesis provides seminal insight into cellular and molecular mechanisms that contribute to congenital heart disease.

Noonan syndrome mutation Q79R in Shp2 increases proliferation of valve primordia mesenchymal cells via extracellular signal-regulated kinase 1/2 signaling

The molecular pathways regulating valve development are only partially understood. Recent studies indicate that dysregulation of mitogen-activated protein kinase (MAPK) signaling might play a major role in the pathogenesis of congenital valvular malformations, and, in this study, we explored the role of extracellular signal-regulated kinase (ERK) 1/2 activation in valve primordia expressing the Noonan syndrome mutation Q79R-Shp2. Noonan syndrome is an autosomal dominant disease characterized by dysmorphic features and cardiac abnormalities, with frequent pulmonic stenosis. The Q79R mutation of PTPN11 previously identified in Noonan syndrome families results in a gain-of-function of the encoded protein tyrosine phosphatase Shp2. We compared the effects of wild-type Shp2 and Q79R-Shp2 on endocardial cushion development. Atrioventricular and outflow tract endocardial cushions were excised from chick embryos, infected with wild-type Shp2 or Q79R-Shp2 adenovirus and embedded in a gel matrix. Q79R-Shp2, but not wild-type-Shp2, expression resulted in increased outgrowth of cells into the gel. The dependence of the Q79R-Shp2 effect on ERK1/2 and p38 MAPK signaling was then determined. The MAPK/ERK kinase (MEK)-1 inhibitor U0126, but not the p38-MAPK pathway inhibitor SB203580, abolished the effect of Q79R-Shp2 on cushion outgrowth. Coinfection with Q79R-Shp2 and dominant negative MEK-1 prevented enhanced endocardial cushion outgrowth, whereas expression of constitutively active MEK-1 mimicked the effect of Q79R-Shp2. Furthermore, dissociated cushion cells displayed increased 5-bromodeoxyuridine incorporation when infected with Q79R-Shp2 but not with wild-type Shp2. This promitotic effect was eliminated by U0126. Our results demonstrate that ERK1/2 activation is both necessary and sufficient to mediate the hyperproliferative effect of a gain-of-function mutation of Shp2 on mesenchymal cells in valve primordia.