1: Development. 1999 Mar;126(6):1269-80. 

The cardiac homeobox gene Csx/Nkx2.5 lies genetically upstream of multiple genes
essential for heart development.

Tanaka M, Chen Z, Bartunkova S, Yamasaki N, Izumo S.

Cardiovascular Division, Beth Israel Deaconess Medical Center, and Department of
Medicine, Harvard Medical School, Boston, MA 02215, USA.

Csx/Nkx2.5 is a vertebrate homeobox gene with a sequence homology to the
Drosophila tinman, which is required for the dorsal mesoderm specification.
Recently, heterozygous mutations of this gene were found to cause human
congenital heart disease (Schott, J.-J., Benson, D. W., Basson, C. T., Pease,
W., Silberbach, G. M., Moak, J. P., Maron, B. J., Seidman, C. E. and Seidman, J.
G. (1998) Science 281, 108-111). To investigate the functions of Csx/Nkx2.5 in
cardiac and extracardiac development in the vertebrate, we have generated and
analyzed mutant mice completely null for Csx/Nkx2.5. Homozygous null embryos
showed arrest of cardiac development after looping and poor development of blood
vessels. Moreover, there were severe defects in vascular formation and
hematopoiesis in the mutant yolk sac. Interestingly, TUNEL staining and PCNA
staining showed neither enhanced apoptosis nor reduced cell proliferation in the
mutant myocardium. In situ hybridization studies demonstrated that, among 20
candidate genes examined, expression of ANF, BNP, MLC2V, N-myc, MEF2C, HAND1 and
Msx2 was disturbed in the mutant heart. Moreover, in the heart of adult chimeric
mice generated from Csx/Nkx2.5 null ES cells, there were almost no ES
cell-derived cardiac myocytes, while there were substantial contributions of Csx
/Nkx2.5-deficient cells in other organs. Whole-mount &bgr;-gal staining of
chimeric embryos showed that more than 20% contribution of Csx/Nkx2. 5-deficient
cells in the heart arrested cardiac development. These results indicate that (1)
the complete null mutation of Csx/Nkx2.5 did not abolish initial heart looping,
(2) there was no enhanced apoptosis or defective cell cycle entry in Csx/Nkx2.5
null cardiac myocytes, (3) Csx/Nkx2.5 regulates expression of several essential
transcription factors in the developing heart, (4) Csx/Nkx2.5 is required for
later differentiation of cardiac myocytes, (5) Csx/Nkx2. 5 null cells exert
dominant interfering effects on cardiac development, and (6) there were severe
defects in yolk sac angiogenesis and hematopoiesis in the Csx/Nkx2.5 null
embryos.

PMID: 10021345 [PubMed - indexed for MEDLINE]


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2: Development. 1996 Dec;122(12):3809-16. 

Zebrafish tinman homolog demarcates the heart field and initiates myocardial
differentiation.

Chen JN, Fishman MC.

Cardiovascular Research Center, Massachusetts General Hospital, and Department
of Medicine, Harvard Medical School, Charlestown 02129, USA.

The fashioning of a vertebrate organ requires integration of decisions of cell
fate by individual cells with those that regulate organotypic form. Logical
candidates for this role, in an organ such as the heart, are genes that initiate
the differentiation process leading to heart muscle and those that define the
earliest embryonic heart field, but for neither class are genes defined. We
cloned zebrafish Nkx2.5, a homolog of the tinman homeodomain gene needed for
visceral and cardiac mesoderm formation in Drosophila. In the zebrafish, its
expression is associated with cardiac precursor cells throughout development,
even in the early gastrula, where the level of zebrafish Nkx2.5 is in a gradient
which spatially matches the regional propensity of ventral-marginal cells to
become heart. Overexpression of Nkx2.5 causes formation of disproportionally
larger hearts in otherwise apparently normal embryos. Transplanted cell
expressing high levels of Nkx2.5 express cardiac genes even in ectopic locales.
Fibroblasts transfected with myc-tagged Nkx2.5 express cardiac genes. These
effects require the homeodomain. Thus, Nkx2.5 appears to mark the earliest
embryonic heart field and to be capable of initiating the cardiogenic
differentiation program. Because ectopic cells or transfected fibroblasts do not
beat, Nkx2.5 is likely to be but one step in the determination of cardiac
myocyte cell fate. Its overexpression increases heart size, perhaps by bringing
cells on the edge of the field to a threshold level for initiation of cardiac
differentiation.

PMID: 9012502 [PubMed - indexed for MEDLINE]


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