Retinoic Acid Synthesis and Signaling during Early Organogenesis

Recent studies indicate that mammals, including humans, maintain some capacity to renew cardiomyocytes throughout postnatal life. Yet, there is little or no significant cardiac muscle regeneration after an injury such as acute myocardial infarction. By contrast, zebrafish efficiently regenerate lost cardiac muscle, providing a model for understanding how natural heart regeneration may be blocked or enhanced. In the absence of lineage-tracing technology applicable to adult zebrafish, the cellular origins of newly regenerated cardiac muscle have remained unclear. Using new genetic fate-mapping approaches, here we identify a population of cardiomyocytes that become activated after resection of the ventricular apex and contribute prominently to cardiac muscle regeneration. Through the use of a transgenic reporter strain, we found that cardiomyocytes throughout the subepicardial ventricular layer trigger expression of the embryonic cardiogenesis gene gata4 within a week of trauma, before expression localizes to proliferating cardiomyocytes surrounding and within the injury site. Cre-recombinase-based lineage-tracing of cells expressing gata4 before evident regeneration, or of cells expressing the contractile gene cmlc2 before injury, each labelled most cardiac muscle in the ensuing regenerate. By optical voltage mapping of surface myocardium in whole ventricles, we found that electrical conduction is re-established between existing and regenerated cardiomyocytes between 2 and 4 weeks post-injury. After injury and prolonged fibroblast growth factor receptor inhibition to arrest cardiac regeneration and enable scar formation, experimental release of the signalling block led to gata4 expression and morphological improvement of the injured ventricular wall without loss of scar tissue. Our results indicate that electrically coupled cardiac muscle regenerates after resection injury, primarily through activation and expansion of cardiomyocyte populations. These findings have implications for promoting regeneration of the injured human heart.

/pdf/primary-contribution-to-zebrafish-heart-regeneration-by-1i5d0mx641.pdf

Primary contribution to zebrafish heart regeneration by gata4+ cardiomyocytes

A dynamic epicardial injury response supports progenitor cell activity during zebrafish heart regeneration.

Appreciable advances into the process of transcript elongation by RNA polymerase II (RNAP II) have identified this stage as a dynamic and highly regulated step of the transcription cycle. Here, we discuss the many factors that regulate the elongation stage of transcription. Our discussion includes the classical elongation factors that modulate the activity of RNAP II, and the more recently identified factors that facilitate elongation on chromatin templates. Additionally, we discuss the factors that associate with RNAP II, but do not modulate its catalytic activity. Elongation is highlighted as a central process that coordinates multiple stages in mRNA biogenesis and maturation.

/pdf/elongation-by-rna-polymerase-ii-the-short-and-long-of-it-t9rldqv37f.pdf

Elongation by RNA polymerase II: the short and long of it.

Congenital heart disease is the leading cause of infant morbidity in the Western world, but only in the past ten years has its aetiology been understood. Recent studies have uncovered the genetic basis for some common forms of the disease and provide new insight into how the heart develops and how dysregulation of heart development leads to disease.

The developmental genetics of congenital heart disease

Organogenesis begins with specification of a progenitor cell population, the size of which provides a foundation for the organ's final dimensions. Here, we present a new mechanism for regulating the number of progenitor cells by limiting their density within a competent region. We demonstrate that retinoic acid signaling restricts cardiac specification in the zebrafish embryo. Reduction of retinoic acid signaling causes formation of an excess of cardiomyocytes, via fate transformations that increase cardiac progenitor density within a multipotential zone. Thus, retinoic acid signaling creates a balance between cardiac and noncardiac identities, thereby refining the dimensions of the cardiac progenitor pool.

https://science.sciencemag.org/content/sci/307/5707/247.full.pdf

Retinoic acid signaling restricts the cardiac progenitor pool.

Vessel and blood specification override cardiac potential in anterior mesoderm

Organogenesis requires the specification of a variety of cell types and the organization of these cells into a particular three-dimensional configuration. The embryonic vertebrate heart is organized into two major chambers, the ventricle and atrium, each consisting of two tissue layers, the myocardium and endocardium. The cellular and molecular mechanisms responsible for the separation of ventricular and atrial lineages are not well understood. To test models of cardiac chamber specification, we generated a high-resolution fate map of cardiac chamber progenitors in the zebrafish embryo at 40% epiboly, a stage prior to the initiation of gastrulation. Our map reveals a distinct spatial organization of myocardial progenitors: ventricular myocardial progenitors are positioned closer to the margin and to the dorsal midline than are atrial myocardial progenitors. By contrast, ventricular and atrial endocardial progenitors are not spatially organized at this stage. The relative orientations of ventricular and atrial myocardial progenitors before and after gastrulation suggest orderly movements of these populations. Furthermore, the initial positions of myocardial progenitors at 40% epiboly indicate that signals residing at the embryonic margin could influence chamber fate assignment. Indeed, via fate mapping, we demonstrate that Nodal signaling promotes ventricular fate specification near the margin, thereby playing an important early role during myocardial patterning.

Organization of cardiac chamber progenitors in the zebrafish blastula

Hoxb5b acts downstream of retinoic acid signaling in the forelimb field to restrict heart field potential in zebrafish

Precise temporal and spatial control of transcription is a fundamental component of embryonic development. Regulation of transcription elongation can act as a rate-limiting step during mRNA synthesis. The mechanisms of stimulation and repression of transcription elongation during development are not yet understood. We have identified a class of zebrafish mutations (pandora, sk8 and s30) that cause multiple developmental defects, including discrete problems with pigmentation, tail outgrowth, ear formation and cardiac differentiation. We demonstrate that the pandora gene encodes a protein similar to Spt6, a proposed transcription elongation factor. Additionally, the sk8 and s30 mutations are null alleles of the foggy/spt5 locus, which encodes another transcription elongation factor. Through real-time RT-PCR analysis, we demonstrate that Spt6 and Spt5 are both required for efficient kinetics of hsp70 transcription in vivo. Altogether, our results suggest that Spt6 and Spt5 play essential roles of comparable importance for promoting transcription during embryogenesis. This study provides the first genetic evidence for parallel functions of Spt6 and Spt5 in metazoans and establishes a system for the future analysis of transcription elongation during development.

/pdf/the-elongation-factors-pandora-spt6-and-foggy-spt5-promote-z5oq2kicvf.pdf

Brian R. Keegan

Papers

Retinoic acid signaling restricts the cardiac progenitor pool.

Vessel and blood specification override cardiac potential in anterior mesoderm

Organization of cardiac chamber progenitors in the zebrafish blastula

Hoxb5b acts downstream of retinoic acid signaling in the forelimb field to restrict heart field potential in zebrafish

The elongation factors Pandora/Spt6 and Foggy/Spt5 promote transcription in the zebrafish embryo