Defining the Importance of Phosphatidylserine Synthase-1 (PSS1)

Abstract

Phosphatidylserine (PS) is a quantitatively minor, but physiologically important, phospholipid in mammalian cells. PS is synthesized by two distinct base-exchange enzymes, PS synthase-1 (PSS1) and PS synthase-2 (PSS2), that are encoded by different genes. PSS1 exchanges serine for choline of phosphatidylcholine, whereas PSS2 exchanges ethanolamine of phosphatidylethanolamine for serine. We previously generated mice lacking PSS2 (Bergo, M. O., Gavino, B. J., Steenbergen, R., Sturbois, B., Parlow, A. F., Sanan, D. A., Skarnes, W. C., Vance, J. E., and Young, S. G. (2002) J. Biol. Chem. 277, 47701-47708) and found that PSS2 is not required for mouse viability. We have now generated PSS1-deficient mice. In light of the markedly impaired survival of Chinese hamster ovary cells lacking PSS1 we were surprised that PSS1-deficient mice were viable, fertile, and had a normal life span. Total serine-exchange activity (contributed by PSS1 and PSS2) in tissues of Pss1 ؊/؊ mice was reduced by up to 85%, but except in liver, the PS content was unaltered. Despite the presumed importance of PS in the nervous system, the rate of axonal extension of PSS1-deficient neurons was normal. Intercrosses of Pss1 ؊/؊ mice and Pss2 ؊/؊ mice yielded mice with three disrupted Pss alleles but no double knockout mice. In Pss1 /Pss2؉/؊ and Pss1 ؉/؊ /Pss2؊/؊ mice, serineexchange activity was reduced by 65-91%, and the tissue content of PS and phosphatidylethanolamine was also decreased. We conclude that (i) elimination of either PSS1 or PSS2, but not both, is compatible with mouse viability, (ii) mice can tolerate as little as 10% of normal total serine-exchange activity, and (iii) mice survive with significantly reduced PS and phosphatidylethanolamine content. Phosphatidylserine (PS)2 is a quantitatively minor phospholipid comprising 3-10% of total phospholipids of mammalian cell membranes. PS is synthesized by two distinct base-exchange enzymes, phosphatidylserine synthase-1 (PSS1) and phosphatidylserine synthase-2 (PSS2) (for a review, see Ref. 1). Each PS synthase is encoded by a different gene, but the enzymes share 32% amino acid identity (2). These two synthases are integral membrane proteins located primarily in mitochondria-associated membranes (MAM) (3). MAM comprise a specialized domain of the endoplasmic reticulum and are thought to mediate the import of PS into mitochondria via transient contact between MAM and mitochondrial outer membranes (4, 5). The PS synthases catalyze base-exchange reactions in which serine is exchanged for the polar head group of either phosphatidylcholine (PC) (PSS1 exchanges choline for serine) or phosphatidylethanolamine (PE) (PSS2 exchanges ethanolamine for serine).Mutant Chinese hamster ovary (CHO) cells lacking PSS1 require supplementation with either ethanolamine (20 M) or PS (10 -50 M) for survival (6, 7). In the absence of added ethanolamine, the mutant cells grow for two doublings, exhibit rounded morphology, and subsequently die, whereas the addition of ethanolamine supports growth (6). Normal growt...

Gain-of-function mutations in the phosphatidylserine synthase 1 (PTDSS1) gene cause Lenz-Majewski syndrome

“…The absence of an overtly aberrant phenotype in Ptdss1 knockout mice 21,22 and the positions of the PTDSS1 missense mutations identified in our subjects with LMS suggest that these mutations cause a gain of function, possibly associated with regulatory dysfunction. This reasoning is supported by the description of at least two individuals with large 8q22.1 deletions encompassing PTDSS1 that do not have features of LMS 23 .…”

“…Furthermore, overexpression of PSS1 or PSS2 in McArdle hepatoma cells does not increase cellular phosphatidylserine or phosphatidylethanolamine content 19,28 . In addition, in the majority of PSS1-deficient 21 or PSS2-deficient 22 mouse tissues (including the brain), the levels of phosphatidylserine and phosphatidylethanolamine are not reduced, despite the 65-98% reduction in total serine-exchange activity 21 . Nevertheless, it is possible that, in individuals with LMS, the phospholipid content of specific tissues and subcellular organelles is altered (particularly with regard to phosphatidylserine and the distribution of its subclasses) so that only signaling processes important in the development and function of affected tissues are disturbed.…”

“…Mice lacking either Ptdss1 or Ptdss2 are viable, phenotypically normal and fertile, indicating that, in mice, PSS1 and PSS2 can compensate for each other 21,22 . In contrast, simultaneous disruption of both genes is lethal, implying that phosphatidylserine is absolutely required for viability 21 . In exome data from the four LMS cases analyzed, there was good coverage of PTDSS2, and no previously unreported variants were identified.…”

Phosphatidylserine dictates the assembly and dynamics of caveolae in the plasma membrane

“…PSS1 catalyzes PtdSer biosynthesis from phosphatidylcholine, and PSS2 uses phosphatidylethanolamine as a substrate (PtdEtn) (35). Mice deficient for both PSS1 and PSS2 are not viable (39). However, previous studies identified and isolated a mutant Chinese hamster ovary (CHO) cell line, termed PSB-2, with greatly reduced PtdSer synthase activity, resulting in an ϳ80% reduction in PtdSer (40).…”

Role of Cathepsin D in U18666A-induced Neuronal Cell Death

“…Real-time Polymerase Chain Reaction-Quantitative realtime polymerase chain reaction (PCR) was performed as described elsewhere (40). In brief, cellular RNA was first extracted with the RNeasy minikit and then reverse-transcribed using reverse transcriptase as described earlier (41).…”