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Peroxisomes are cell organelles that range in size from 0.1 to 1.5 µm and are present in nearly all eukaryotic cells. They are involved in important functions of the intracellular metabolism of lipids and of reactive oxygen species (ROS). Peroxisomes are highly dynamic organelles that can adapt in shape and protein composition rapidly in response to metabolic demands. Depending on species or tissue type they have distinct functional profiles. However, to date nothing has been described about their function in female germ cells. The aim of this study was to characterize the peroxisomal compartment in mouse oocytes and to analyse possible alterations during oogenesis.
Our results obtained by immunofluorescence microscopic analysis of peroxisomal marker proteins like catalase, Pmp70 and Pex14, show the presence of peroxisomes in oocytes and their significant increase in number during folliculogenesis. We have also used a GFP-PTS1 transgenic mouse strain, whose peroxisomes are labelled by the green fluorescent protein (GFP). This allowed us to analyse the peroxisomes in these oocytes by electron and light microscopy. We could show that they have a typical ultrastructure and that they are sometimes arranged in small groups at the oocyte periphery. To characterize the cell type specific metabolic functions of these peroxisomes we isolated oocytes, analysed the levels of mRNAs encoding peroxisomal proteins by real-time PCR and compared the mRNA expression levels with the levels in total ovary and liver tissue. Relative expression levels of genes representing main functions of peroxisomal metabolism like β-oxidation and ROS-metabolism have been strongly reduced in oocytes compared to the reference. The mRNA levels for glyceronephosphate O-acyltransferase (Gnapat, Dhapat) and alkylglycerone phosphate synthase (Agps, Dhaps-S), however, were significantly increased. These are key enzymes involved in the biosynthesis of ether lipids that are exclusively localized in peroxisomes. We therefore suggest that the synthesis of plasmalogens, the most abundant ether lipids, is an important function of peroxisomes in female germ cells. This increased synthesis of plasmalogens could be explained by an increased need of membranes in the oocyte during folliculogenesis or during cleavage divisions in early embryogenesis. Future studies will focus on lipid analysis of oocytes. This will further clarify the physiological role of peroxisomal plasmalogen biosynthesis in the maturation process of oocytes and early embryogenesis.