The GATA factors are a family of transcriptional regulatory proteins in eukaryotes that share extensive homology in their DNA-binding domains. One enigmatic aspect of GATA factor expression is that several GATA proteins, which ostensibly share the same DNA-binding site specificity, are coexpressed in erythroid cells. To elucidate the roles of individual GATA factors in erythropoiesis, conditional alleles of GATA-I, GATA-2, and GATA-3 were prepared by fusing each of the factors to the hormone-binding domain of the human estrogen receptor (ER). These GATA/ER chimeric factors were shown to be hormone-inducible trans-activating proteins in transient transfection assays. When stably introduced into primary erythroblasts or conditionally transformed erythroid progenitors cells, exogenous GATA-2/ER promoted proliferation and inhibited terminal differentiation in an estrogen-dependent manner. These phenotypic effects are specifically attributable to the action of ectopically expressed GATA-2/ER because erythroblasts expressing exogenous GATA-2 are constitutively arrested in differentiation and because erythroid progenitors expressing either Gal/ER or GATA-3/ER do not display a hormone-responsive block in differentiation. Thus, the GATA-2 transcription factor appears to play a role in regulating the self-renewal capacity of early erythroid progenitor cells.[Key Words: GATA transcription factors; conditional alleles; erythroid differentiation] Received November 22, 1992; revised version accepted March 19, 1993.The GATA factors are a family of (C4) zinc finger transcriptional regulatory proteins in eukaryotes that bind to the consensus DNA sequence WGATAR (for review, see Orkin 1990). In vertebrate organisms, each member of this family has a distinct tissue distribution pattern. Thus, GATA-1 is expressed in the erythroid, megakaryocytic, mast cell, and germ cell lineages (Tsai et al. 1989; Martin et al. 1990;Romeo et al. 1990;Ito et al. 1993), whereas GATA-2 is expressed more widely in both hematopoietic and nonhematopoietic lineages (Yamamoto et al. 1990;Zon et al. 1991). The expression of GATA-3 is confined principally to definitive erythroid cells, specific neuronal cells, and T lymphocytes (Yamamoto et al. 1990;Ko et al. 1991 Engel, in prep.). In addition, during erythroid cell differentiation the relative abundance of the GATA factors with respect to one another undergoes specific developmentally controlled changes that are highly conserved in several species (M.W. Leonard, K.-C. Lim, and J.D. Engel, in prep.). These data therefore suggested that the regulation of the precise quantitative balance of the GATA factors might play a role in influencing the developmental decisions available to erythroid progenitor cells, for example, to undergo either self-renewal or terminal differentiation. Thus, we hypothesize that the ectopic expression of these regulatory proteins in erythroblasts might describe novel phenotypes that would lend insight into the roles that individual GATA family members might play in erythropoiesis....

Limb-bud and heart (LBH) is a novel key transcriptional regulator of vertebrate development. However, the molecular mechanisms upstream of LBH and its role in adult development are unknown. Here we show that in epithelial development, LBH expression is tightly controlled by Wnt signaling.

We report the cloning, protein characterization, and expression of a novel vertebrate gene, termed Lbh (Limb-bud-and-heart), with a spatiotemporal expression pattern that marks embryologically significant domains in the developing limbs and heart. Lbh encodes a highly conserved nuclear protein, which in tissue culture cells possesses a transcriptional activator function. During limb development, expression of Lbh initiates in the ectoderm of the presumptive limb territory in the lateral body wall. As the limb buds appear, Lbh expression is restricted primarily to the distal ventral limb ectoderm and the apical ectodermal ridge, and overlaps in these ectodermal compartments with En1 and Fgf8 expression. During heart formation, Lbh is expressed as early as Nkx2.5 and dHand in the bilateral heart primordia, with the highest levels in the anterior promyocardium. After heart tube fusion and looping, Lbh expression is confined to the ventricular myocardium, with the highest intensity in the right ventricle and atrioventricular canal, as well as in the sinus venosus. Based on the molecular characteristics and the domain-specific expression pattern, it is possible that Lbh functions in synergy with other genes known to be required for heart and limb development.