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Biological and molecular characterization of lifeguard : an inhibitor of the Fas apoptotic pathway


Apoptosis or programmed cell death participates in several biological processes, including embryonic development, tissue remodeling, tumor surveillance and immune system regulation. The Fas pathway is among the most studied apoptotic pathways. Its misregulation can contribute to many diseases including autoimmunity and cancer. Lifeguard (LFG) is a membrane protein isolated as an inhibitor of the Fas apoptotic pathway. LFG has wide tissue distribution with highest expression in the brain, predominantly in neurons of hippocampus and cerebellum. In order to study the biological role of LFG, we used two different models; mice with reduced LFG expression by siRNA lentiviral transgenesis and LFG null mice. Phenotypic characterization of these mice indicated that loss of LFG affected several organs, such as lungs, kidneys, spleen, thymus and brain. The work presented here reports the investigation of LFG's role in the CNS (1), in the immune system (2) as well as the molecular and biochemical characterization of LFG (3). 1. We focused in the study of LFG's biological role in the brain, especially in cerebellum, where the phenotype was most severe. LFG affected cerebellar size in early development, internal granular layer size, and Purkinje cell differentiation, morphology and susceptibility to apoptosis. In the last decade there has been increasing evidence for an involvement of the Fas apoptotic pathway in cerebral ischemia. Therefore, we have started to test the hypothesis that LFG could have a neuroprotective role against stroke, by inhibiting Fas induced apoptosis. 2. We also investigated the role of LFG in the immune system, especially in T cell development. Based on LFG's expression pattern and on the phenotypical characterization of the T cell subpopulations in the thymus of siLFG mice we hypothesized that LFG is a survival factor whose expression is upregulated after the major selection checkpoints in T cell development. 3. We demonstrated that LFG blocks the Fas apoptotic cascade at the level of caspase 8 activation. We also showed that LFG is ubiquitinated and it interacts with the E3 Ubiquitin ligase AIP4. Finally, deletion mutant studies revealed that the last intracellular loop and transmembrane domain of LFG are essential for its antiapoptotic function

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