Molecular mechanisms of chamber-specific myocardial gene expression: transgenic analysis of the ANF promoter

Physiological homology between Drosophila melanogaster and vertebrate cardiovascular systems

The physiology of the Drosophila melanogaster cardiovascular system remains poorly characterized compared with its vertebrate counterparts. Basic measures of physiological performance remain unknown. It also is unclear whether subtle physiological defects observed in the human cardiovascular system can be reproduced in D. melanogaster. Here we characterize the cardiovascular physiology of D. melanogaster in its pre-pupal stage by using high-speed dye angiography and optical coherence tomography. The heart has vigorous pulsatile contractions that drive intracardiac, aortic and extracellular-extravascular hemolymph flow. Several physiological measures, including weight-adjusted cardiac output, body-length-adjusted aortic velocities and intracardiac shear forces, are similar to those in the closed vertebrate cardiovascular systems, including that of humans. Extracellular-extravascular flow in the pre-pupal D. melanogaster circulation drives convection-limited fluid transport. To demonstrate homology in heart dysfunction, we showed that, at the pre-pupal stage, a troponin I mutant, held-up2 (hdp2), has impaired systolic and diastolic heart wall velocities. Impaired heart wall velocities occur in the context of a non-dilated phenotype with a mildly depressed fractional shortening. We additionally derive receiver operating characteristic curves showing that heart wall velocity is a potentially powerful discriminator of systolic heart dysfunction. Our results demonstrate physiological homology and support the use of D. melanogaster as an animal model of complex cardiovascular disease.

Convergent proliferative response and divergent morphogenic pathways induced by epicardial and endocardial signaling in fetal heart development

Heart development requires cardiomyocyte proliferation, coupled with morphogenic differentiation of the inner trabecular myocardium and the outer compact zone myocardium. In mouse embryos lacking the retinoic acid receptor RXRalpha, proliferation and morphogenesis of the compact zone fails. We demonstrated previously that epicardial cells, in response to retinoic acid, secrete an activity that promotes cell proliferation. In this study, we have investigated downstream signaling pathways that are elicited in response to this factor. We find that cells treated in culture activate PI3 kinase and Erk pathways, and that these are required for a proliferative response. In vivo, phosphorylation of Akt and GSK3beta (PI3K pathway) and of Erk1/2 and p90rsk (Erk pathway) is substantially reduced in RXRalpha-deficient heart tissue. Neuregulin, a mitogen secreted from the endocardium which promotes cardiomyocyte proliferation and trabecular differentiation, also activates proliferation via PI3K and Erk pathways. However, the epicardial factor is not neuregulin, and does not function via the neuregulin receptor. Gene markers known to be selectively expressed in trabecular or compact myocardium in vivo are differentially activated in cell culture by treatment with neuregulin or epicardial factor, and are misexpressed in RXRalpha(-/-) heart tissue. We therefore conclude that epicardial and endocardial signals converge on common proliferative components, but diverge in downstream pathways that lead to compact vs. trabecular morphogenic differentiation.