Bone morphogenetic protein-4 (BMP-4) is a member of the TGF-I~ superfamily of polypeptide signaling molecules, closely related to BMP-2 and to Drosophila decapentaplegic (DPP). To elucidate the role of BMP-4 in mouse development the gene has been inactivated by homologous recombination in ES cells. Homozygous mutant Bmp-4 t~lblh embryos die between 6.5 and 9.5 days p.c., with a variable phenotype. Most Bmp-4 t~Ibl~ embryos do not proceed beyond the egg cylinder stage, do not express the mesodermal marker 1~Brachyury), and show little or no mesodermal differentiation. Some homozygous mutants develop to the head fold or beating heart/early somite stage or beyond. However, they are developmentally retarded and have truncated or disorganized posterior structures and a reduction in extraembryonic mesoderm, including blood islands. These results provide direct genetic evidence that BMP-4 is essential for several different processes in early mouse development, beginning with gastrulation and mesoderm formation. Moreover, in the presumed absence of zygotic ligand, it appears that homozygous mutants can be rescued partially by related proteins or by maternal BMP-4.[Key Words: BMP-4; mouse embryo; targeted mutation; lethal embryonic phenotype] Received May 3, 1995; revised version accepted July 12, 1995.Bone morphogenetic proteins-2 and -4 (BMP-2 and BMP-4) are two closely related members of the transforming growth factor (TGF)-~ superfamily of secreted polypeptide signaling molecules (for review, see Kingsley 1994; Hogan 1995). The carboxy-terminal mature regions of the two proteins are 92% identical at the amino acid level. Moreover, they have been highly conserved during evolution; in Drosophila a single gene, decapentaplegic (dpp), encodes a protein with -75% sequence identity to BMP-2 and BMP-4 in the carboxy-terminal mature region. At the functional level, human BMP-4 can rescue the dorsal-ventral pattern defects of dpp null mutants, and Drosophila DPP protein can induce ectopic bone in mice (Padgett et al. 1993;Sampath et al. 1993). Genetic analysis has shown that DPP is required at several different stages of Drosophila development. It acts as a dorsalizing morphogen in the dorsal-ventral patterning of the blastoderm embryo, mediates an inductive interaction between mesoderm and endoderm in the larval midgut, and plays a role in the proximal-distal patterning of the leg and wing imaginal discs (for review, see Campbell et al. 1993;Diaz-Benjumea and Cohen 1993;Wall and Hogan 1994).The striking evolutionary conservation of BMP-4 and 1These authors contributed equally to this work. 2Present address:

A fundamental question in stem cell research is whether cultured multipotent adult stem cells represent endogenous multipotent precursor cells. Here we address this question, focusing on SKPs, a cultured adult stem cell from the dermis that generates both neural and mesodermal progeny. We show that SKPs derive from endogenous adult dermal precursors that exhibit properties similar to embryonic neural-crest stem cells. We demonstrate that these endogenous SKPs can first be isolated from skin during embryogenesis and that they persist into adulthood, with a niche in the papillae of hair and whisker follicles. Furthermore, lineage analysis indicates that both hair and whisker follicle dermal papillae contain neural-crest-derived cells, and that SKPs from the whisker pad are of neural-crest origin. We propose that SKPs represent an endogenous embryonic precursor cell that arises in peripheral tissues such as skin during development and maintains multipotency into adulthood.

The division, differentiation, and function of stem cells and multipotent progenitors are influenced by complex signals in the microenvironment, including oxygen availability. Using a genetic "knock-in" strategy, we demonstrate that targeted replacement of the oxygen-regulated transcription factor HIF-1␣ with HIF-2␣ results in expanded expression of HIF-2␣-specific target genes including Oct-4, a transcription factor essential for maintaining stem cell pluripotency. We show that HIF-2␣, but not HIF-1␣, binds to the Oct-4 promoter and induces Oct-4 expression and transcriptional activity, thereby contributing to impaired development in homozygous Hif-2␣ KI/KI embryos, defective hematopoietic stem cell differentiation in embryoid bodies, and large embryonic stem cell (ES)-derived tumors characterized by altered cellular differentiation. Furthermore, loss of HIF-2␣ severely reduces the number of embryonic primordial germ cells, which require Oct-4 expression for survival and/or maintenance. These results identify Oct-4 as a HIF-2␣-specific target gene and indicate that HIF-2␣ can regulate stem cell function and/or differentiation through activation of Oct-4, which in turn contributes to HIF-2␣'s tumor promoting activity.[Keywords: HIF; hypoxia; HIF-2␣; Oct-4; VEGF; TGF-␣; stem cells; cancer] Supplemental material is available at http://www.genesdev.org.

There are two major angiotensin II receptor isoforms, AT1 and AT2. AT1 mediates the well-known pressor and mitogenic effects of angiotensin II, but the signalling mechanism and physiological role of AT2 has not been established. Its abundant expression in fetal tissues and certain brain nuclei suggest possible roles in growth, development and neuronal functions. Here we report the unexpected finding that the targeted disruption of the mouse AT2 gene resulted in a significant increase in blood pressure and increased sensitivity to the pressor action of angiotensin II. Thus AT2 mediates a depressor effect and antagonizes the AT1-mediated pressor action of angiotensin II. In addition, disruption of the AT2 gene attenuated exploratory behaviour and lowered body temperature. Our results show that angiotensin II activates AT1 and AT2, which have mutually counteracting haemodynamic effects, and that AT2 regulates central nervous system functions, including behaviour.