Gene mutations in the phosphoinositide-metabolizing enzymes are linked to various human diseases. In mammals, PIKfyve synthesizes PtdIns(3,5)P2 and PtdIns5P lipids that regulate endosomal trafficking and responses to extracellular stimuli. The consequence of pikfyve gene ablation in mammals is unknown. To clarify the importance of PIKfyve and PIKfyve lipid products, in this study, we have characterized the first mouse model with global deletion of the pikfyve gene using the Cre-loxP approach. We report that nearly all PIKfyveKO/KO mutant embryos died before the 32–64-cell stage. Cultured fibroblasts derived from PIKfyveflox/flox embryos and rendered pikfyve-null by Cre recombinase expression displayed severely reduced DNA synthesis, consistent with impaired cell division causing early embryo lethality. The heterozygous PIKfyveWT/KO mice were born at the expected Mendelian ratio and developed into adulthood. PIKfyveWT/KO mice were ostensibly normal by several other in vivo, ex vivo, and in vitro criteria despite the fact that their levels of the PIKfyve protein and in vitro enzymatic activity in cells and tissues were 50–55% lower than those of wild-type mice. Consistently, steady-state levels of the PIKfyve products PtdIns(3,5)P2 and PtdIns5P selectively decreased, but this reduction (35–40%) was 10–15% less than that expected based on PIKfyve protein reduction. The nonlinear decrease of the PIKfyve protein versus PIKfyve lipid products, the potential mechanism(s) discussed herein, may explain how one functional allele in PIKfyveWT/KO mice is able to support the demands for PtdIns(3,5)P2/PtdIns5P synthesis during life. Our data also shed light on the known human disorder linked to PIKFYVE mutations
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