Proper septation and valvulogenesis during cardiogenesis depend on interactions between the myocardium and the endocardium. By combining use of a hypomorphic Bone morphogenetic protein 4 (Bmp4) allele with conditional gene inactivation, we here identify Bmp4 as a signal from the myocardium directly mediating atrioventricular septation. Defects in this process cause one of the most common human congenital heart abnormalities, atrioventricular canal defect (AVCD). The spectrum of defects obtained through altering Bmp4 expression in the myocardium recapitulates the range of AVCDs diagnosed in patients, thus providing a useful genetic model with AVCD as the primary defect. Congenital heart malformation is the most common human birth defect and the leading cause of death in the first year of life (Hoffman 1995). One of the most frequently diagnosed disorders is atrioventricular canal defect (AVCD), which accounts for 7.3% of all congenital heart abnormalities (Pierpont et al. 2000). During normal development, septation of the AV canal (AVC) is initiated with the formation of the inferior and superior endocardial cushions through epithelial-mesenchymaltransformation (EMT) by some endocardial cells invading into cardiac jelly, the extracellular matrix between the endocardium and myocardium (Nakajima et al. 2000;Markwald and Wessels 2001). Subsequent growth and fusion of the AV cushions produce the central mesenchymal mass, which further develops into the mature AV septum and valves through complex remodeling processes. The central mesenchymal mass also fuses with the atrial septum primum (ASP) and the inlet portion of the ventricular septum to prevent abnormal blood flow between chambers (Marino and Digilio 2000; Markwald and Wessels 2001). AVCD covers a spectrum of abnormalities, from the partial form with defects in the lower part of the ASP to the complete form in which absence of the AV septum results in a single common AVC (Marino and Digilio 2000).The complex cushion morphogenesis during AV septation depends both on signals released from the overlying myocardium and on proper responses of the endocardial and mesenchymal cells (Nakajima et al. 2000;Markwald and Wessels 2001). The molecular pathways for the initiation of cushion formation (EMT) have been studied extensively. In both chicken and mouse explant culture assays, transforming growth factor ␤2 (TGF␤2) is able to replace the overlying myocardium to activate EMT, and inhibition of TGF␤2 activity blocks EMT (Boyer et al. 1999;Nakajima et al. 2000;Camenisch et al. 2002a). Consistent with these in vitro studies, TGF␤2-deficient mice show defects in valvulogenesis (Bartram et al. 2001). In addition to the TGF␤ signaling pathway, the Hyaluronic acid (Ha) synthase 2 (Has2)/ErbB2,ErbB3/Ras pathway has been recently shown to play essential roles in EMT (Camenisch et al. 2000(Camenisch et al. , 2002b. Ha is a prominent component of cardiac jelly in embryonic day 9.5 (E9.5) embryos and its production depends on Has2. In addition to serving as a substrate for migrati...

The transcription factor GATA-1 is expressed in early hematopoietic progenitors and specifically down-regulated in myelomonocytic cells during lineage determination. Our earlier observation that the differentiation of Myb-Ets-transformed chicken hematopoietic progenitors into myeloblasts likewise involves a GATA-I down-regulation, whereas expression is maintained in erythroid, thrombocytic, and eosinophilic derivatives, prompted us to study the effect of forced GATA-I expression in Myb--Ets-transformed myeloblasts. We found that the factor rapidly suppresses myelomonocytic markers and induces a reprogramming of myeloblasts into cells resembling either transformed eosinophils or thromboblasts. In addition, we observed a correlation between the level of GATA-1 expression and the phenotype of the cell, intermediate levels of the factor being expressed by eosinophils and high levels by thromboblasts, suggesting a dosage effect of the factor. GATA-1 can also induce the formation of erythroblasts when expressed in a myelomonocytic cell line transformed with a Myb-Ets mutant containing a lesion in Ets. These cells mature into erythrocytes following temperature-inactivation of the Ets protein. Finally, the factor can reprogram a v-Myc-transformed macrophage cell line into myeloblasts, eosinophils, and erythroblasts, showing that the effects of GATA-1 are not limited to Myb-Ets-transformed myeloblasts. Our results suggest that GATA-1 is a lineage-determining transcription factor in transformed hematopoietic cells, which not only activates lineage-specific genetic programs but also suppresses myelomonocytic differentiation. They also point to a high degree of plasticity of transformed hematopoietic cells.

Growth and differentiation of postnatal hair follicles are controlled by reciprocal interactions between the dermal papilla and the surrounding epidermal hair precursors. The molecular nature of these interactions is largely unknown, but they are likely to involve several families of signaling molecules, including Fgfs, Wnts and Bmps. To analyze the function of Bmp signaling in postnatal hair development, we have generated transgenic mice expressing the Bmp inhibitor, Noggin, under the control of the proximal Msx2 promoter, which drives expression in proliferating hair matrix cells and differentiating hair precursor cells. Differentiation of the hair shaft but not the inner root sheath is severely impaired in Msx2±Noggin transgenic mice. In addition to hair keratins, the expression of several transcription factors implicated in hair development, including Foxn1 and Hoxc13, is severely reduced in the transgenic hair follicles. Proliferating cells, which are normally restricted to the hair matrix surrounding the dermal papilla, are found in the precortex and hair shaft region. These results identify Bmps as key regulators of the genetic program controlling hair shaft differentiation in postnatal hair follicles.

Angular head movements in vertebrates are detected by the three semicircular canals of the inner ear and their associated sensory tissues, the cristae. Bone morphogenetic protein 4 (Bmp4), a member of the Transforming growth factor family (TGF-β), is conservatively expressed in the developing cristae in several species, including zebrafish, frog, chicken, and mouse. Using mouse models in which Bmp4 is conditionally deleted within the inner ear, as well as chicken models in which Bmp signaling is knocked down specifically in the cristae, we show that Bmp4 is essential for the formation of all three cristae and their associated canals. Our results indicate that Bmp4 does not mediate the formation of sensory hair and supporting cells within the cristae by directly regulating genes required for prosensory development in the inner ear such as Serrate1 (Jagged1 in mouse), Fgf10, and Sox2. Instead, Bmp4 most likely mediates crista formation by regulating Lmo4 and Msx1 in the sensory region and Gata3, p75Ngfr, and Lmo4 in the non-sensory region of the crista, the septum cruciatum. In the canals, Bmp2 and Dlx5 are regulated by Bmp4, either directly or indirectly. Mechanisms involved in the formation of sensory organs of the vertebrate inner ear are thought to be analogous to those regulating sensory bristle formation in Drosophila. Our results suggest that, in comparison to sensory bristles, crista formation within the inner ear requires an additional step of sensory and non-sensory fate specification.