phosphatidylinositol-4-phosphate and Intellectual-Disability

Lenz-Majewski syndrome (LMS) is a rare disease characterized by complex craniofacial, dental, cutaneous, and limb abnormalities combined with intellectual disability. Mutations in thePTDSS1gene coding one of the phosphatidylserine (PS) synthase enzymes, PSS1, were described as causative in LMS patients. Such mutations render PSS1 insensitive to feedback inhibition by PS levels. Here we show that expression of mutant PSS1 enzymes decreased phosphatidylinositol 4-phosphate (PI4P) levels both in the Golgi and the plasma membrane (PM) by activating the Sac1 phosphatase and altered PI4P cycling at the PM. Conversely, inhibitors of PI4KA, the enzyme that makes PI4P in the PM, blocked PS synthesis and reduced PS levels by 50% in normal cells. However, mutant PSS1 enzymes alleviated the PI4P dependence of PS synthesis. Oxysterol-binding protein-related protein 8, which was recently identified as a PI4P-PS exchanger between the ER and PM, showed PI4P-dependent membrane association that was significantly decreased by expression of PSS1 mutant enzymes. Our studies reveal that PS synthesis is tightly coupled to PI4P-dependent PS transport from the ER. Consequently, PSS1 mutations not only affect cellular PS levels and distribution but also lead to a more complex imbalance in lipid homeostasis by disturbing PI4P metabolism.

Topics: Abnormalities, Multiple; Bone Diseases, Developmental; Cell Membrane; Endoplasmic Reticulum; Golgi Apparatus; HEK293 Cells; Humans; Intellectual Disability; Minor Histocompatibility Antigens; Mutation; Nitrogenous Group Transferases; Phosphatidylinositol Phosphates; Phosphotransferases (Alcohol Group Acceptor)

Ciliary transport is required for ciliogenesis, signal transduction, and trafficking of receptors to the primary cilium. Mutations in inositol polyphosphate 5-phosphatase E (INPP5E) have been associated with ciliary dysfunction; however, its role in regulating ciliary phosphoinositides is unknown. Here we report that in neural stem cells, phosphatidylinositol 4-phosphate (PI4P) is found in high levels in cilia whereas phosphatidylinositol (4,5)-bisphosphate (PI(4,5)P2) is not detectable. Upon INPP5E inactivation, PI(4,5)P2 accumulates at the ciliary tip whereas PI4P is depleted. This is accompanied by recruitment of the PI(4,5)P2-interacting protein TULP3 to the ciliary membrane, along with Gpr161. This results in an increased production of cAMP and a repression of the Shh transcription gene Gli1. Our results reveal the link between ciliary regulation of phosphoinositides by INPP5E and Shh regulation via ciliary trafficking of TULP3/Gpr161 and also provide mechanistic insight into ciliary alterations found in Joubert and MORM syndromes resulting from INPP5E mutations.

Topics: Abnormalities, Multiple; Animals; Cell Movement; Cells, Cultured; Cerebellar Diseases; Cerebellum; Cilia; Cyclic AMP; Embryo, Mammalian; Eye Abnormalities; Eye Diseases; Hedgehog Proteins; Hippocampus; Intellectual Disability; Intercellular Signaling Peptides and Proteins; Intracellular Signaling Peptides and Proteins; Kidney Diseases, Cystic; Kruppel-Like Transcription Factors; Mice; Mice, Inbred C57BL; Mice, Transgenic; Neural Stem Cells; Obesity; Penile Diseases; Phosphatidylinositol 4,5-Diphosphate; Phosphatidylinositol Phosphates; Phosphoric Monoester Hydrolases; Protein Transport; Proteins; Receptors, G-Protein-Coupled; Retina; Signal Transduction; Zinc Finger Protein GLI1