Frank Costantini

Manipulating the Mouse Embryo: A Laboratory Manual by B. Hogan, F. Constantini and E. Lacy, Cold Spring Havi~ Laboratmy, 1986. $60.00 (332 pages) ISBN 0 87969 175 1 These are heady days for developmental biologists. Prob- lems that have puzzled scientists for centuries seem to be moving tom the realm of abstract philosophy towards practical solo ution. The power of recombinant DNA technology fuels this opti- mism; at last genes can be engineered, inserted, located, monitored, neutralized and their impact on development asses- sed. The prospect of interfering positively, rather than randomly, with the genetic basis for development is real. The prac- tical basis for this optimism, as recorded in this manual, is clearly well founded. It is impressive that so much pro- gress in the genetic manipulation of the mouse has been made so rapidly. Here are recorded the tech- niques for preparing, inserting and analysing DNA sequences, for retroviral infection of em- bryos, for production and use of EC and EK cells as vehicles for engineered sequences and for nuclear transplantation - all this against a background of the basic procedures required for pro- ducing and handling the em- bryos. If there is one critidsm, it is that the format and content of manual do reveal a fashionable, but perhaps a rather narrow, belief that it is by gene injection (or variants of it) alone that the problem of development will be solved. One might expect a laboratory manual entitled Manilmlating the Mouse Embryo to inform about more general practical aspects of mouse em- bryology than are contained here

Axin2/Conductin/Axil and its ortholog Axin are negative regulators of the Wnt signaling pathway, which promote the phosphorylation and degradation of beta-catenin. While Axin is expressed ubiquitously, Axin2 mRNA was seen in a restricted pattern during mouse embryogenesis and organogenesis. Because many sites of Axin2 expression overlapped with those of several Wnt genes, we tested whether Axin2 was induced by Wnt signaling. Endogenous Axin2 mRNA and protein expression could be rapidly induced by activation of the Wnt pathway, and Axin2 reporter constructs, containing a 5.6-kb DNA fragment including the promoter and first intron, were also induced. This genomic region contains eight Tcf/LEF consensus binding sites, five of which are located within longer, highly conserved noncoding sequences. The mutation or deletion of these Tcf/LEF sites greatly diminished induction by beta-catenin, and mutation of the Tcf/LEF site T2 abolished protein binding in an electrophoretic mobility shift assay. These results strongly suggest that Axin2 is a direct target of the Wnt pathway, mediated through Tcf/LEF factors. The 5.6-kb genomic sequence was sufficient to direct the tissue-specific expression of d2EGFP in transgenic embryos, consistent with a role for the Tcf/LEF sites and surrounding conserved sequences in the in vivo expression pattern of Axin2. Our results suggest that Axin2 participates in a negative feedback loop, which could serve to limit the duration or intensity of a Wnt-initiated signal.