Overexpression of the short isoform of p53 (p44) has unexpectedly uncovered a role for p53 in the regulation of size and life span in the mouse. Hyperactivation of the insulin-like growth factor (IGF) signaling axis by p44 sets in motion a kinase cascade that clamps potentially unimpeded growth through p21Cip1. This suggests that pathways of gene activity known to regulate longevity in lower organisms are linked in mammals via p53 to mechanisms for controlling cell proliferation. Thus, appropriate expression of the short and long p53 isoforms might maintain a balance between tumor suppression and tissue regeneration, a major requisite for long mammalian life span.

Binding between sperm and egg plasma membranes is an essential step in fertilization. Whereas fertilin, a mammalian sperm surface protein, is involved in this crucial interaction, sperm receptors on the egg plasma membrane have not been identified. Because fertilin contains a predicted integrin ligand domain, we investigated the expression and function of integrin subunits in unfertilized mouse eggs. Polymerase chain reactions detected mRNAs for alpha 5, alpha 6, alpha v, beta 1, beta 3, and beta 5. Immunofluorescence revealed alpha 6 beta 1 and alpha v beta 3 on the plasma membrane. GoH3, a function-blocking anti-alpha 6 monoclonal antibody, abolished sperm binding, but a nonfunction-blocking anti-alpha 6 monoclonal antibody, a function-blocking anti-alpha v beta 3 polyclonal antibody, and an RGD peptide had no effect. Somatic cells bound sperm avidly, but only if they expressed alpha 6 beta 1. A peptide analog of the fertilin integrin ligand domain inhibited sperm binding to eggs and alpha 6 beta 1+ cells and diminished GoH3 staining of eggs. Our results indicate a novel role for the integrin alpha 6 beta 1 as a cell-cell adhesion receptor that mediates sperm-egg binding.

Integrin receptors for extracellular matrix receptors are important effectors of cell adhesion, differentiation, and migration in cultured cells and are believed to be critical effectors of these processes during development. To determine when 131 integrins become critical during embryonic development, we generated mutant mice with a targeted disruption of the [31 integrin subunit gene. Heterozygous mutant mice were normal. Homozygous loss of ~1 integrin expression was lethal during early postimplantation development. Homozygous embryos lacking B1 integrins formed normal-looking blastocysts and initiated implantation at E4.5. However, the E4.5 Ill-null embryos in situ had collapsed blastocoeles, and whereas the trophoblast penetrated the uterine epithelium, extensive invasion of the decidua was not observed. Laminin-positive endoderm cells were detected in the inner cell mass area, but endoderm morphogenesis and migration were defective. By E5.5 ~l-null embryos had degenerated extensively. In vitro analysis showed that trophoblast function in Ill-null peri-implantation embryos was largely normal, including expression of tissue-specific markers, and outgrowth on fibronectin-and vitronectin-coated, although not on laminin-coated substrates. In contrast, the inner cell mass region of 131-null blastocyst outgrowths, and inner cell masses isolated from [31-null blastocysts, showed highly retarded growth and defective extraembryonic endoderm morphogenesis and migration. These data suggest that 131 integrins are required for normal morphogenesis of the inner cell mass and are essential mediators of growth and survival of cells of the inner cell mass. Failure of continued trophoblast development in 131-null embryos after inner cell mass failure could be attributable to either an intrinsic requirement for 131 integrins for later stages of trophoblast development, or to the lack of trophic signals from the 131-null inner cell mass.[Key Words: 131 integrins; e~V-integrins; trophoblast; extraembryonic endoderm; survival; migration] Received April 4, 1995; revised version accepted June 15, 1995.Cell--extracellular matrix (ECM) interactions play critical roles in morphogenesis and in the regulation of gene expression (Damsky and Werb 1992;Hynes 1992Hynes , 1994Adams and Watt 1993;Ashkenas et al. 1994;Cross et al. 1994). The integrin family of heterodimeric transmembrane glycoproteins constitutes the major class of receptors mediating cell-ECM interactions. These receptors link the ECM to the internal cytoskeleton and to intracellular signaling pathways. In this role, they mediate cell adhesion and migration, and transduce mechanical and informational signals from the complex extracellular environment, thereby influencing both cytoarchitecture and gene expression. Integrin heterodimeric receptors for ECM can be classified into two major families: {lJ those containing the 131 subunit, and, {2} those containing the oLV subunit. There is extensive apparent redundancy in the ligandbinding preferences of these integrins. For exampl...

Summary The mechanisms of tissue convergence and extension (CE) driving axial elongation in mammalian embryos, and in particular, the cellular behaviors underlying CE in the epithelial neural tissue, have not been identified. Here we show that mouse neural cells undergo mediolaterally biased cell intercalation and exhibit both apical boundary rearrangement and polarized basolateral protrusive activity. Planar polarization and coordination of these two cell behaviors is essential for neural CE, as shown by failure of mediolateral intercalation in embryos mutant for two proteins associated with planar cell polarity signaling: Vangl2 and Ptk7. Embryos with mutations in Ptk7 fail to polarize cell behaviors within the plane of the tissue, while Vangl2 mutant embryos maintain tissue polarity and basal protrusive activity, but are deficient in apical neighbor exchange. Neuroepithelial cells in both mutants fail to apically constrict, leading to craniorachischisis. These results reveal a cooperative mechanism for cell rearrangement during epithelial morphogenesis.

Laminins are components of all basement membranes and have well demonstrated roles in diverse developmental processes, from the peri-implantation period onwards. Laminin 1 (α1β1γ1) is a major laminin found at early stages of embryogenesis in both embryonic and extraembryonic basement membranes. The laminin γ1 chain has been shown by targeted mutation to be required for endodermal differentiation and formation of basement membranes; Lamc1-/- embryos die within a day of implantation. We report the generation of mice lacking lamininα1 and laminin β1, the remaining two laminin 1 chains. Mutagenic insertions in both Lama1 and Lamb1 were obtained in a secretory gene trap screen. Lamb1-/- embryos are similar to Lamc1-/- embryos in that they lack basement membranes and do not survive beyond embryonic day (E) 5.5. However, in Lama1-/- embryos, the embryonic basement membrane forms,the embryonic ectoderm cavitates and the parietal endoderm differentiates,apparently because laminin 10 (α5β1γ1) partially compensates for the absent laminin 1. However, such compensation did not occur for Reichert's membrane, which was absent, and the embryos died by E7. Overexpression of laminin α5 from a transgene improved the phenotype of Lama1-/- embryos to the point that they initiated gastrulation, but this overexpression did not rescue Reichert's membrane, and trophoblast cells did not form blood sinuses. These data suggest that both the molecular composition and the integrity of basement membranes are crucial for early developmental events.