Role of dihydroxyacetonephosphate acyltransferase in the biosynthesis of plasmalogens and nonether glycerolipids.

The variant CHO-K1 cell line, NRel-4, is unable to synthesize plasmalogens because of a severe reduction in dihydroxyacetonephosphate acyltransferase (DHAPAT) activity (Nagan, N., A. K. Hajra, L. K. Larkins, P. Lazarow, P. E. Purdue, W. B. Rizzo, and R. A. Zoeller. 1998. Isolation of a Chinese hamster fibroblast variant defective in dihydroxyacetonephosphate acyltransferase activity and plasmalogen biosynthesis: use of a novel two-step selection protocol. Biochem. J. 332: 273-279). Northern analysis demonstrated that the loss of this activity was attributable to a severe reduction in mRNA levels for DHAPAT. Transfection of NRel-4 cells with a plasmid bearing the human DHAPAT cDNA recovered DHAPAT activity and plasmalogen biosynthesis. Examination of clonal isolates from the transfected population showed that recovery of as little as 10% of wild-type DHAPAT activity restored plasmalogen levels to 55% of normal, whereas in one isolate, NRel-4.15, which overexpressed DHAPAT activity by 6-fold over wild-type cells, plasmalogen levels were returned only to wild-type values. Although the rate of plasmenylethanolamine biosynthesis was restored in NRel-4.15, the biosynthesis of nonether glycerolipids was either decreased or unaffected, suggesting that peroxisomal DHAPAT does not normally contribute to nonether glycerolipid biosynthesis. These data demonstrate that a defect in the gene that codes for peroxisomal DHAPAT is the primary lesion in the NRel-4 cell line and that the peroxisomal DHAPAT is essential for the biosynthesis of plasmalogens in animal cells.

Inactivation of ether lipid biosynthesis causes male infertility, defects in eye development and optic nerve hypoplasia in mice.

Although known for almost 80 years, the physiological role of plasmalogens (PLs), the major mammalian ether lipids (ELs), is still enigmatic. Humans that lack ELs suffer from rhizomelic chondrodysplasia punctata (RCDP), a peroxisomal disorder usually resulting in death in early childhood. In order to learn more about the functions of ELs, we generated a mouse model for RCDP by a targeted disruption of the dihydroxyacetonephosphate acyltransferase gene. The mutant mice revealed multiple abnormalities, such as male infertility, defects in eye development, cataract and optic nerve hypoplasia, some of which were also observed in RCDP. Mass spectroscopic analysis demonstrated the presence of highly unsaturated fatty acids including docosahexaenoic acid (DHA) in brain PLs and the occurrence of PLs in lipid raft microdomains (LRMs) isolated from brain myelin. In mutants, PLs were completely absent and the concentration of brain DHA was reduced. The marker proteins flotillin-1 and F3/contactin were found in brain LRMs in reduced concentrations. In addition, the gap junctional protein connexin 43, known to be recruited to LRMs and essential for lens development and spermatogenesis, was down-regulated in embryonic fibroblasts of the EL-deficient mice. Free cholesterol, an important constituent of LRMs, was found in these fibroblasts to be accumulated in a perinuclear compartment. These data suggest that the EL-deficient mice allow the identification of new phenotypes not related so far to EL-deficiency (male sterility, defects in myelination and optic nerve hypoplasia) and indicate that PLs are required for the correct assembly and function of LRMs.