Lenz-Majewski mutations in PTDSS1 affect phosphatidylinositol 4-phosphate metabolism at ER-PM and ER-Golgi junctions

Abstract

Lenz-Majewski syndrome (LMS) is a rare disease characterized by complex craniofacial, dental, cutaneous, and limb abnormalities combined with intellectual disability. Mutations in the PTDSS1 gene 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.

Lipid Tales of Viral Replication and Transmission

“…After de novo synthesis from PC and/or PE at the ER, they can remain in the ER, be transferred to the mitochondria [85,86], or to the inner leaflet of the plasma membrane, the latter in exchange for PI4P lipids [87-89]. From the plasma membrane they can then be trafficked to endosomes, MVBs and lysosomes.…”

Quantifying lipid changes in various membrane compartments using lipid binding protein domains

“…FRET measurements can be done in cell suspensions of transfected cells in a fluorescence spectrophotometer [121] or in a plate reader but it has a poor signal to noise ratio. In order to quantitate the lipid changes in cell suspension, we have recently developed a set of BRET-based biosensors allowing the measurements of various phospholipids [PI4P, PI(4,5)P 2 , PI(3,4,5)P 3 , or PS] in biological membranes, especially in the plasma membrane [127, 146]. The principle of how these sensors work is similar to the FRET approach: these new sensors are based on an energy transfer process with the difference that the donor energy is generated by the enzymatic effect of a luciferase enzyme on its substrate, coelenterazine (energy donor), and the acceptor is a wavelength-matched fluorescent protein, such as Venus.…”

ER-plasma membrane junctions: Why and how do we study them?

“…Similar to Osh6 in yeast, mammalian ORP5 and ORP8 can mediate PI4P/PS countertransport at ER-PM junctions [139, 140]. ORP5 and ORP8 are ER transmembrane proteins that not only localize at ER-PM junctions but also at ER-mitochondria contacts [141].…”