The Challenges of Translating Knockout Phenotypes into Gene Function

The studies demonstrate several important conclusions. First, the tetraploid rescue experiments demonstrate that the development and differentiation of all cell lineages giving rise to the embryo do not require p38α as had been suggested by other studies. Similarly, tetraploid rescue and hematopoietic reconstitution demonstrate that p38α is not uniquely essential for erythropoiesis as had been proposed. However, the existence of highly related family members evokes the caveat of functional redundancy, an issue that can be addressed by developing mutant strains lacking multiple family members.P38α does play an essential, nonredundant role in extraembryonic cells. Based on expression and aspects of the phenotypes, it is likely that p38α serves this function in labyrinthine trophoblasts. This is interesting when bearing in mind that many of the functions of trophoblasts resemble those of macrophages. Is it possible that a primordial macrophage-like cell was the progenitor of trophoblast lineage cells? What might be the upstream activators of p38α? From the number of growth factors that are critical to the development of the labyrinthine layer (Table 1Table 1) there are ample starting points. Intriguing possibilities include HGF/c-Met based on the expression of c-Met in labyrinthine trophoblasts. VEGF has been shown to activate p38α and that this activation is required for migration of endothelial cells (Rousseau et al. 1997xRousseau, S, Houle, F, Landry, J, and Huot, J. Oncogene. 1997; 15: 2169–2177Crossref | PubMedSee all ReferencesRousseau et al. 1997). Lastly, angiopoietin-1/Tie-2 signaling is critical for vascularization during development (Suri et al. 1996xSuri, C, Jones, P.F, Patan, S, Bartunkova, S, Maisonpierre, P.C, Davis, S, Sato, T.N, and Yancopoulos, G.D. Cell. 1996; 87: 1171–1180Abstract | Full Text | Full Text PDF | PubMed | Scopus (1991)See all ReferencesSuri et al. 1996) including the yolk sac.An equally intriguing possibility is that p38α contributes to the hypoxic response that is critical for vascularization. As noted in Table 1Table 1, a number of genes required for placental development are linked to hypoxic responses with a frequent theme being the production of VEGF. In this regard it is notable that a potential upstream activator of MKKs and thus p38α, Mekk3, is essential for early embryonic vascularization and its deficiency results in a lethal phenotype at E11 (Yang et al. 2000xYang, J, Boerm, M, McCarty, M, Bucana, C, Fidler, I.J, Zhuang, Y, and Su, B. Nat. Genet. 2000; 24: 309–313Crossref | PubMed | Scopus (156)See all ReferencesYang et al. 2000). It will be intriguing to determine whether the embryonic lethality associated with Mekk3 deficiency can also be rescued with tetraploid cells.The immediate activators of p38α in trophoblasts are not known. The lack of a comparable phenotype in MKK3-deficient embryos would suggest that this kinase is not uniquely required for p38α activation. The potential downstream targets of p38α in trophoblasts are numerous (Table 1Table 1). At this point all that can be conclusively deduced is that MAPKAP-2 is not a nonredundant component of the pathway. In summary the properties of the p38α-deficient stains of mice present interesting challenges and opportunities for defining the signaling pathways that require this kinase. With the ability to rescue embryos with tetraploid cells and reconstitute adult mice with fetal liver cells, it will be possible to study the role of p38α in a variety of cell lineages. Indeed, in studies of many signaling pathways, the use of carefully designed and analyzed mutant mice is filtering the noise from critical signals in signal transduction.