Caractérisation d'une nouvelle voie de signalisation PTEN/PLCXD régulant le PtdIns(4,5)P2 endolysosomal
Phosphatidylinositol(4,5)P2 (PtdIns(4,5)P2) is essential for various cellular processes, including cell signaling, intracellular traffic and cytokinesis. Therefore, strict control of its homeostasis is crucial. Indeed, the deregulation of the kinases, phosphatases and phospholipases which controls PtdIns(4,5)P2 leads to multiple pathologies. Among them, the Lowe syndrome is a rare and incurable disease caused by mutations in the OCRL gene which codes for PtdIns(4,5)P2 phosphatase OCRL1. Depletion of dOCRL, the orthologue of OCRL1 in drosophila, alters the homeostasis of PtdIns(4,5)P2 with (i) an abnormal accumulation of PtdIns(4,5)P2 on the endomembranes leading (ii) to cytokinesis defects and multinucleation. The objective of this thesis was to understand how PtdIns(4,5)P2 is regulated on endomembranes. In drosophila cells, we have discovered a new and unexpected function for the tumor suppressor PTEN independent of its phosphatase activity. Indeed, we have found that PTEN reduces the levels of PtdIns(4,5)P2 on endolysosomes thanks to the enzymatic action of dPLCXD, an atypical phospholipase C (PLC). Thus, the PTEN/dPLCXD signaling pathway can compensate for cytokinesis defects due to the loss of dOCRL. Finally, we identified a chemical activator of PLC that restores the functional loss of OCRL in three distinct Lowe syndrome models. Next, we studied the role of this newly identified PTEN/PLCXD pathway during autophagy, a self-digestion mechanism. Indeed, the homeostasis of lysosomal PtdIns(4,5)P2 is essential for the fusion of autophagosomes with lysosomes during autophagy. We have observed that depletion of PLCXD and overexpression of a catalytically inactive mutant of PTEN both alter autophagy in Drosophila and mammalian cells. These data suggest that this newly identified PTEN/PLCXD pathway regulates the autophagic flux. In this thesis, we have highlighted a new PTEN/dPLCXD signaling pathway which controls the levels of PtdIns(4,5)P2 on endolysosomes. This new PTEN function is independent of its phosphatase activity and the first biological function for PLCXD can regulate autophagy and compensate for the loss of dOCRL. This discovery led to the identification of a potential therapeutic strategy for treating patients with Lowe’s syndrome.
Phosphoinositides PtdIns(4,5)P2 PTEN PLC PLCXD OCRL Autophagy Cytokinesis Lowe syndrome