Genetic ablation of phosphatidylinositol transfer protein function in murine embryonic stem cells

Phosphatidylinositol transfer proteins (PITPs) regulate the interface between signal transduction, membrane-trafficking, and lipid metabolic pathways in eukaryotic cells. The best characterized mammalian PITPs are PITPalpha and PITPbeta, two highly homologous proteins that are encoded by distinct genes. Insights into PITPalpha and PITPbeta function in mammalian systems have been gleaned exclusively from cell-free or permeabilized cell reconstitution and resolution studies. Herein, we report for the first time the use of genetic approaches to directly address the physiological functions of PITPalpha and PITPbeta in murine cells. Contrary to expectations, we find that ablation of PITPalpha function in murine cells fails to compromise growth and has no significant consequence for bulk phospholipid metabolism. Moreover, the data show that PITPalpha does not play an obvious role in any of the cellular activities where it has been reconstituted as an essential stimulatory factor. These activities include protein trafficking through the constitutive secretory pathway, endocytic pathway function, biogenesis of mast cell dense core secretory granules, and the agonist-induced fusion of dense core secretory granules to the mast cell plasma membrane. Finally, the data demonstrate that PITPalpha-deficient cells not only retain their responsiveness to bulk growth factor stimulation but also retain their pluripotency. In contrast, we were unable to evict both PITPbeta alleles from murine cells and show that PITPbeta deficiency results in catastrophic failure early in murine embryonic development. We suggest that PITPbeta is an essential housekeeping PITP in murine cells whereas PITPalpha plays a far more specialized function in mammals than that indicated by in vitro systems that show PITP dependence.