We have also extended to study the role of CERT protein in mammalian biology. CERT is an essential gene for mouse development and embryonic survival and, quite strikingly, is critical for mitochondrial integrity. CERT mutant embryos accumulate ceramide in the ER but also mislocalize ceramide to the mitochondria, compromising their function. Cells in mutant embryos show abnormal dilation of the ER and degenerating mitochondria. These subcellular changes manifest as heart defects and cause severely compromised cardiac function and embryonic death around embryonic day 11.5. In spite of ceramide accumulation, CERT mutant mice do not die as a result of enhanced apoptosis. Instead, cell proliferation is impaired, and expression levels of cell cycle-associated proteins are altered. Individual cells survive, perhaps because cell survival mechanisms are activated. Thus, global compromise of ER and mitochondrial integrity caused by ceramide accumulation in CERT mutant mice primarily affects organogenesis rather than causing cell death via apoptotic pathways. Lipid Distribution and Signaling. PL and SL at the plasma membrane play an important role in stimulus-response coupling, cell differentiation, movement, and exo- and endocytosis. They are asymmetrically distributed in biological membranes, and different proteins catalyzing uni- and bi-directional movements of lipids perpetuate asymmetry. Our current efforts focus on scramblase, a protein proposed to be involved in bi-directional transbilayer movement of phospholipids. We have recently completed two genetic screens and obtained Drosophila flies lacking two of the identified scramblase proteins. We have also generated flies lacking both genes (double mutants). Phenotypic analysis of the double mutants indicates that, surprisingly, scramblases do not have a determining role in the scrambling of phospholipids that accompany apoptosis, phagocytosis and fusion. Instead, scramblases play a regulatory role in regulated exocytosis. This has implications for a wide range of cellular processes involving digestive system and endocrine and exocrine secretions with clinical relevance for a wide range of diseases such as diabetes, behavior disorders and even tumor metastasis.
