Proapoptotic Bcl-2 family members have been proposed to play a central role in regulating apoptosis. However, mice lacking bax display limited phenotypic abnormalities. As presented here, bak(-/-) mice were found to be developmentally normal and reproductively fit and failed to develop any age-related disorders. However, when Bak-deficient mice were mated to Bax-deficient mice to create mice lacking both genes, the majority of bax(-/-)bak(-/-) animals died perinatally with fewer than 10% surviving into adulthood. bax(-/-)bak(-/-) mice displayed multiple developmental defects, including persistence of interdigital webs, an imperforate vaginal canal, and accumulation of excess cells within both the central nervous and hematopoietic systems. Thus, Bax and Bak have overlapping roles in the regulation of apoptosis during mammalian development and tissue homeostasis.

In the nematode Caenorhabditis elegans, the zinc finger transcriptional regulator TRA-1A directs XX somatic cells to adopt female fates. The membrane protein TRA-2A indirectly activates TRA-1A by binding and inhibiting a masculinizing protein, FEM-3. Here we report that a part of the intracellular domain of TRA-2A, distinct from the FEM-3 binding region, directly binds TRA-1A. Overproduction of this TRA-1A-binding region has tra-1-dependent feminizing activity in somatic tissues, indicating that the interaction enhances TRA-1A activity. Consistent with this hypothesis, we show that tra-2(mx) mutations, which weakly masculinize somatic tissues, disrupt the TRA-2/TRA-1A interaction. Paradoxically, tra-2(mx) mutations feminize the XX germ line, as do tra-1 mutations mapping to the TRA-2 binding domain. We propose that these mutations render tra-2 insensitive to a negative regulator in the XX germ line, and we speculate that this regulator targets the TRA-2/TRA-1 complex. The intracellular domain of TRA-2A is likely to be produced as a soluble protein in vivo through proteolytic cleavage of TRA-2A or through translation of an XX germ line-specific mRNA. We further show that tagged derivatives of the intracellular domain of TRA-2 localize to the nucleus, supporting the hypothesis that this domain is capable of modulating TRA-1A activity in a manner reminiscent of Notch and Su(H).

Although most cells undergo growth arrest during hypoxia, endothelial cells and placental cytotrophoblasts proliferate in response to low O 2 . We demonstrate that proliferation of embryonic multilineage hematopoietic progenitors is also regulated by a hypoxia-mediated signaling pathway. This pathway requires HIF-1 (HIF-1␣/ ARNT heterodimers) because Arnt −/− embryoid bodies fail to exhibit hypoxia-mediated progenitor proliferation. Furthermore, Arnt −/− embryos exhibit decreased numbers of yolk sac hematopoietic progenitors. This defect is cell extrinsic, is accompanied by a decrease in ARNTdependent VEGF expression, and is rescued by exogenous VEGF. Therefore, "physiologic hypoxia" encountered by embryos is essential for the proliferation or survival of hematopoietic precursors during development. Received July 9, 1999; revised version accepted August 17, 1999. Low O 2 tension (hypoxia) is a condition commonly associated with pathology. Mammals adapt to hypoxia on an organismal level by increasing expression of erythropoietin (EPO) resulting in increased red blood cell production (Semenza et al. 1991), and vascular endothelial growth factor (VEGF) to promote increased vascularization of affected tissues (Shweiki et al. 1992;Forsythe et al. 1996). On a cellular level, mammals adapt to decreased O 2 by increasing expression of glycolytic enzymes (Bunn and Poyton 1996;Semenza et al. 1996;Wenger and Gassmann 1997) and glucose transporters (Bashan et al. 1992) for increased anaerobic respiration.Whereas the growth and division of many cell types is suppressed until O 2 tensions return to normoxic levels (20% O 2 ) (Graeber et al. 1996;Carmeliet et al. 1998), certain cell types must grow and proliferate in response to decreased O 2 . These include placental cytotrophoblasts (Genbacev et al. 1997), which form the maternalfetal interface in the womb (Rodesch et al. 1992;Fischer and Bavister 1993), and vascular endothelial cells, which proliferate to form new capillaries in hypoxic tissues (Phillips et al. 1995). A critical component of the hypoxic response machinery is the bHLH-PAS transcription factor complex hypoxia-inducible factor 1 (HIF-1). HIF-1 activates the expression of genes involved in a broad spectrum of adaptive responses to oxygen deprivation ranging from basic metabolism to angiogenesis and erythropoiesis (Bunn and Poyton 1996;Wenger and Gassmann 1997;Maltepe and Simon 1998), and may impact cell-cycle regulation by interacting with and stabilizing the tumor supressor protein p53 . HIFs are obligate heterodimers comprised of the bHLH-PAS proteins HIF-1␣ (or HIF-2␣) and the arylhydrocarbon receptor nuclear translocator (ARNT; HIF-1␤). All components are constitutively expressed, although HIF-1␣ and HIF-2␣ are quickly degraded under normoxia and stabilized under hypoxia (Salceda and Caro 1997;Wiesener et al. 1998), allowing for HIF activity specifically under hypoxic conditions.Before establishment of a circulatory system capable of delivering oxygenated blood to the embryo, mammalian development occur...