Zellweger Syndrome Knockout Mouse Models Challenge Putative Peroxisomal β-Oxidation Involvement in Docosahexaenoic Acid (22:6n-3) Biosynthesis

Abstract

The putative involvement of peroxisomal β-oxidation in the biosynthetic pathway of docosahexaenoic acid (22:6n-3, DHA) synthesis is critically reviewed in light of experiments with two recently developed knockout mouse models for Zellweger syndrome, a peroxisomal disorder affecting brain development. These mice were generated by targeted disruption of the PEX2 and PEX5 peroxisomal assembly genes encoding targeting signal receptor peroxins for the recognition and transport of a set of peroxisomal enzymes, including those of peroxisomal β-oxidation, to the peroxisomal matrix. Analysis of esterified 22:6n-3 concentrations in PEX2−/− and PEX5−/− mice do not support the hypothesized requirement of peroxisomal β-oxidation in 22:6n-3 synthesis, as only brain, but not liver or plasma, 22:6n-3 levels were decreased. Supplementation of PEX5+/− dams with 22:6n-3, although restoring the levels of brain 22:6n-3 in total lipids to that of controls, did not normalize the phenotype. These decreased brain 22:6n-3 concentrations appear to be secondary to impaired plasmalogen (sn-1-alkyl-, alkenyl-2-acyl glycerophospholipids) synthesis, probably at the level of the dihydroxyacetonephosphate acyltransferase (DHAP-AT), a peroxisomal enzyme catalyzing the first step in the synthesis of 22:6n-3-rich plasmalogens. To diminish the confounding effects of impaired plasmalogen synthesis in the brains of these Zellweger syndrome mouse models, kinetic experiments with labeled precursors, such as 18:3n-3 or 20:5n-3, in liver or isolated hepatocytes, which have negligible amounts of plasmalogens, are suggested to establish the rates of 22:6n-3 biosynthesis and precursor–product relationships. Similar experiments using brain of the acyl-CoA oxidase knockout mouse model are proposed to confirm the lack of peroxisomal β-oxidation involvement in 22:6n-3 synthesis, since this mutation would not impair plasmalogen synthesis.