CARDIAC AND EXTRACARDIAC EXPRESSION OF CSX NKX2.5 HOMEODOMAIN PROTEIN/

Csx/Nkx2.5 is an evolutionary conserved homeobox gene related to the D rosophila tinman gene, which is essential for the dorsal mesoderm form ation. Expression of Csx/Nkx2.5 mRNA is the earliest marker for heart precursor cells in all vertebrates so far examined. Previous studies h ave demonstrated that Csx/Nkx2.5 mRNA is highly expressed in the heart and at lower levels in the spleen, tongue, stomach, and thyroid in th e murine embryo, Since some developmental genes are regulated by postt ranscriptional mechanisms, we analyzed the developmental pattern of Cs x protein expression at the single-cell level using Csx-specific antib odies. Immunohistochemical analysis of murine embryos at 7.8 days post coitum revealed that Csx protein is strongly expressed in the nucleus of endodermal and mesodermal cells in the cardiogenic plate. Subseque ntly, in the heart, Car protein was detected only in the nucleus of my ocytes of the atrium and the ventricle through the adult stage. During the fetal period, Csx protein expression in the nucleus was also note d in the spleen, stomach, liver, tongue, and anterior larynx. Unexpect edly, confocal microscopy revealed that Csx immunoreactivity was detec ted only in the cytoplasm of a subset of cranial skeletal muscles. Csx protein was not detected in the thyroid glands. The expression of Csx protein in all organs was markedly downregulated after birth except i n the heart. These results raise the possibility that Csx/Nkx2.5 may p lay a role in the early developmental process of multiple tissues in a ddition to its role in early heart development.