Distinct Functions of Acyl/Alkyl Dihydroxyacetonephosphate Reductase in Peroxisomes and Endoplasmic Reticulum.

Masanori Honsho,Megumi Tanaka,Raphael A Zoeller,Yukio Fujiki

doi:10.3389/fcell.2020.00855

Masanori Honsho, Megumi Tanaka + Show 2 more

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https://doi.org/10.3389/fcell.2020.00855

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Abstract

Plasmalogens are a subclass of ether glycerophospholipids characterized by a vinyl-ether bond at the sn-1 position of the glycerol backbone. Plasmalogen biosynthesis is initiated in peroxisomes. At the third step of plasmalogen synthesis, alkyl-dihydroxyacetonephosphate (DHAP) is enzymatically reduced to 1-alkyl-sn-glycero-3-phospate by acyl/alkyl DHAP reductase (ADHAPR), whose activity is found in both peroxisomes and microsomes. We herein show that knockdown of ADHAPR in HeLa cells reduced the synthesis of ethanolamine plasmalogen (PlsEtn), similar to the Chinese hamster ovary cell mutant FAA.K1B deficient in ADHAPR activity. Endogenous ADHAPR and ectopically expressed FLAG-tagged ADHAPR were localized to peroxisomes and endoplasmic reticulum (ER) as a type I integral membrane protein in HeLa cells. ADHAPR targets to peroxisomes via a Pex19p-dependent class I pathway. In addition, it is also inserted into the ER via the SRP-dependent mechanism. The ADHAPR mutant lacking the N-terminal domain preferentially targets to the ER, restoring the reduced level of PlsEtn synthesis in FAA.K1B cell. In contrast, the expression of full-length ADHAPR in the mutant cells elevates the synthesis of phosphatidylethanolamine, but not PlsEtn. Taken together, these results suggest that the third step of plasmalogen synthesis is mediated by ER-localized ADHAPR.

Highlights

Plasmalogen is a major class of glycerophospholipid containing a characteristic vinyl-ether bond at the sn-1 position of the glycerol backbone
Synthesis of ethanolamine plasmalogen (PlsEtn) is initiated in peroxisomes and completed in the endoplasmic reticulum (ER) via totally seven steps of reactions where alkyl-DHAP is reduced by the alkyl DHAP reductase (ADHAPR) at the third step of seven-step reactions in the PlsEtn synthesis
Upon transfecting siRNA against DHRS7b, synthesis of PlsEtn and phosphatidylethanolamine (PtdEtn) was reduced about 40% of those in mock-treated cells (Figures 1C,D). Such reduced synthesis of PlsEtn and PtdEtn is observed in the Chinese hamster ovary (CHO) cell mutant FAA.K1B in a short metabolic-labeling period with [1-3H]ethanolamine (Etn) due to the absence of ADHAPR activity (James and Zoeller, 1997). These results suggest that the enzyme, ADHAPR, encoded by DHRS7b most likely catalyzes the reduction of acyl/alkyl-DHAP in plasmalogen biosynthesis

Summary

Introduction

Plasmalogen is a major class of glycerophospholipid containing a characteristic vinyl-ether bond at the sn-1 position of the glycerol backbone. Plasmalogens account for about 20% of total phospholipids in humans (Nagan and Zoeller, 2001). Ethanolamine plasmalogens (PlsEtns) are major constituents of biological membranes in most human tissues where they constitute approximately 5–20% of the phospholipids, while choline plasmalogens are major constituents primarily of cardiac tissue and skeletal muscle membranes (Braverman and Moser, 2012). PlsEtns are synthesized in seven steps (Nagan and Zoeller, 2001). The initial two steps of plasmalogen biosynthesis in peroxisomes are well characterized. The first-step synthesis of plasmalogens is catalyzed by dihydroxyacetonephosphate acyltransferase/glyceronephosphate

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