Cardioblast-intrinsic Tinman activity controls proper diversification and differentiation of myocardial cells inDrosophila

Abstract

The NK homeobox gene tinman (tin) is required for the specification of the cardiac, visceral muscle and somatic muscle progenitors in the early dorsal mesoderm of Drosophila. Like its vertebrate counterpart Nkx2.5, the expression of tin is maintained in cardiac cells during cardiac maturation and differentiation; however, owing to the complete lack of a dorsal vessel in tin mutant embryos, the function of tin in these cells has not been defined. Here we show that myocardial cells and dorsal vessels can form even though they lack Tin, and that viable adults can develop, as long as Tin is provided in the embryonic precardiac mesoderm. However, embryos in which tin expression is specifically missing from cardial cells show severe disruptions in the normal diversification of the myocardial cells, and adults exhibit severe defects in cardiac remodeling and function. Our study reveals that the normal expression and activity of Tin in four of the six bilateral cardioblasts within each hemisegment of the heart allows these cells to adopt a cell fate as 'working' myocardium, as opposed to a fate as inflow tract (ostial) cells. This function of tin involves the repression of Dorsocross (Doc) T-box genes and, hence, the restriction of Doc to the Tin-negative cells that will form ostia. We conclude that tin has a crucial role within myocardial cells that is required for the proper diversification, differentiation, and post-embryonic maturation of cardiomyocytes, and we present a pathway involving regulatory interactions among seven-up, midline, tinman and Dorsocross that establishes these developmental events upon myocardial cell specification.

Tinman is a direct activator of midline in the drosophila dorsal vessel

“…Thus, sequences in the proximal 1.7-kb fragment may be required to initiate the expression in both cardioblast lineages but are only able to maintain Tin-CB expression, whereas sequences in the distal 2.6-kb fragment are required to maintain mid expression in Svp-CBs following activation. This is also consistent with the observation that mid expression is maintained in tin null embryos that have early tin expression rescued by a transgene (Zaffran et al, 2006). In these embryos, we presume that mid expression is activated by tin but maintained by tin-independent elements, like those that we propose are found in the mid2.6 fragment.…”

“…The mid-expressing cells are selected from within clusters of cells expressing the transcription factor genes tin, pannier (pnr), which encodes a GATA factor, and Dorsocross 1, 2, and 3 (collectively referred to as Doc), which encode the Drosophila Tbx6 homologs (MiskolcziMcCallum et al, 2005;Qian et al, 2005;Zaffran et al, 2006). Lineage studies have shown that two cells in each hemisegment divide to give 6 cardioblasts (Ward and Skeath, 2000;Han and Bodmer, 2003).…”

“…In tin gain-of-function experiments, the small increase in midexpressing cells could result from increased cardioblast specification (Qian et al, 2005;. Second, in tin mutant embryos where early tin expression (up to stage 12) is restored with a transgene that allows the specification of cardiac tissues, mid expression is maintained in all six cardioblasts (Zaffran et al, 2006). Thus, tin may be required for the specification of cardioblasts and may not be directly involved in mid expression.…”

Tinman/Nkx2-5 acts via miR-1 and upstream of Cdc42 to regulate heart function across species

“…Whereas ablation of tinman in the Drosophila embryo abolishes heart formation, targeted deletion of tinman at later stages reveals additional requirements for tinman in maintaining normal adult heart function and physiology (Zaffran et al, 2006). This is reminiscent of Nkx2-5 in vertebrates, which is required for establishing heart function in the developing mouse embryo, for example, and also later for its maintenance in the adult (Pashmforoush et al, 2004).…”

Cellular components and signals required for the cardiac outflow tract assembly in Drosophila

“…Both cell types can be subdivided into subpopulations expressing a cell subset-specific combinatorial code of transcription factors (6)(7)(8)(9)(10)(11)(12)(13). Such a subdivision suggests that different subsets of cardiac cells differentiate into functionally distinct heart components-a possibility that is supported by the finding that the Svp/Doc-positive pair of cardioblasts has the capacity to develop into the inflow tract (ostial) cells (9,14), whereas the four Tin-expressing cardioblasts adopt a cell fate of ''working myocardium'' (15).…”