A Pex13 knockout in germ cells induces a spermatogenic arrest

Abstract

Peroxisomes are cell organelles with important functions in metabolic processes, including β-oxidation of very long-chain fatty acids and branched-chain fatty acids, ether lipid synthesis, catabolism of D-amino acids and polyamines, as well as the degradation of reactive oxygen species (ROS), particularly hydrogen peroxide. As lipids have a specific functional significance in the nervous system, peroxisomal disorders lead to disturbed myelination of axons, resulting in severe neurological diseases. In addition, patients show testicular abnormalities, including the reduction of seminiferous tubules or a spermatogenic arrest that leads to azoospermia. Underlying alterations in the testes due to a peroxisomal defect are not well characterized. Peroxisomes do not synthesise proteins on their own, therefore they require an import machinery to translocate proteins from the cytosol into the organelle. The peroxisomal membrane protein PEX13 is part of this translocation machinery. Its inactivation leads to a biogenesis defect of peroxisomes with loss of all metabolic functions. Based on the Cre-lox technique, two mouse models with a conditional knockout for Pex13 were established in either pre- or post-meiotic germ cells mediated by the transgenic Stra8-Cre or Prm-Cre promoter, respectively. The effects of a peroxisomal knockout were analysed with focus on germ cell differentiation and maturation. The Stra8-Cre mediated peroxisomal knockout induced a severe impairment of spermatogenesis. Germ cell differentiation was interrupted at the round spermatid stage, leading to the generation of multinucleated giant cells and thus infertility of male mice. More lipid droplets were accumulated in the cytoplasm of Sertoli cells, compared to control mice. In addition, gas chromatography revealed an alteration in the composition of polyunsaturated fatty acids in the testis. Whereas precursors of the peroxisomal β-oxidation were up-regulated, docosapentaenoic acid (C22:5(ω-6)) and docosahexaenoic acid (C22:6(ω-3)) were significantly down-regulated. Fatty acid elongases and desaturases that are involved in peroxisomal β-oxidation were significantly increased. Peroxisomal genes involved in the metabolite transport, β-oxidation, ether lipid synthesis as well as retinoid and ROS metabolism were strongly down-regulated in the Stra8-Cre mediated Pex13 KO mice. In contrast to that, spermatogenesis was not affected in the Prm-Cre mediated Pex13 knockout mice. Mice were still fertile and showed no obvious impairments of the peroxisomal compartment. This study provides evidence that peroxisomes are inevitable to ensure proper germ cell differentiation. Moreover, peroxisomes seem to be more important for germ cells prior to meiosis than for cells that further differentiate into spermatozoa.