Glutamine synthetase is essential in early mouse embryogenesis

Glutamine synthetase (GS) is expressed in a tissue‐specific and developmentally controlled manner, and functions to remove ammonia or glutamate. Furthermore, it is the only enzyme that can synthesize glutamine de novo. Since congenital deficiency of GS has not been reported, we investigated its role in early development. Because GS is expressed in embryonic stem (ES) cells, we generated a null mutant by replacing one GS allele in‐frame with a β‐galactosidase‐neomycine fusion gene. GS+/LacZ mice have no phenotype, but GSLacZ/LacZ mice die at ED3.5, demonstrating GS is essential in early embryogenesis. Although cells from ED2.5 GSLacZ/LacZ embryos and GSGFP/LacZ ES cells survive in vitro in glutamine‐containing medium, these GS‐deficient cells show a reduced fitness in chimera analysis and fail to survive in tetraploid‐complementation assays. The survival of heavily (>90%) chimeric mice up to at least ED16.5 indicates that GS deficiency does not entail cell‐autonomous effects and that, after implantation, GS activity is not essential until at least the fetal period. We hypothesize that GS‐deficient embryos die when they move from the uterine tube to the harsher uterine environment, where the embryo has to catabolize amino acids to generate energy and, hence, has to detoxify ammonia, which requires GS activity. Developmental Dynamics 236:1865–1875, 2007. © 2007 Wiley‐Liss, Inc.