Non-lysosomal accumulation of glucosylceramide alters cytoskeletal dynamics causing globozoospermia

Abstract

Glycosphingolipids are important constituents of cellular membranes. Glucosylceramide (GlcCer) is the simplest glycosphingolipid and serves as a building block for the synthesis of higher-order glycosphingolipids. Defects in the lysosomal beta-glucosidase 1 (GBA1), which cleaves GlcCer to glucose and ceramide, causes accumulation of GlcCer in lysosomes and, thereby, the severe lipid-storage disorder Gaucher disease. Knockout-mice lacking the non lysosomal beta glucosidase 2 (GBA2) accumulate GlcCer outside the lysosomes, resulting in globozoospermia – a severe male fertility defect. The molecular mechanisms underlying this fertility defect are unknown. In my PhD thesis, I (1) investigated the subcellular localization of GBA2 and (2) analyzed how the lack of GBA2 causes globozoospermia in mice. First, I could demonstrate that GBA2 is attached to the cytosolic side of the endoplasmic reticulum (ER) and Golgi membranes. Second, my results revealed that accumulation of non-lysosomal GlcCer disrupts cytoskeletal dynamics, affecting both the microtubule and actin cytoskeleton: microtubule persistence and the rate of actin polymerization are increased in GBA2 knockout-mice. In particular, cytoskeletal structures in the testis that shape the sperm head are disturbed: the microtubule manchette in sperm of GBA2 knockout-mice persist longer and the F actin organization in the apical ectoplasmic specialization (ES) is disrupted. In addition, acrosome formation is impaired due to a defect in vesicle fusion. My results indicate that accumulation of GlcCer outside the lysosomes increases lipid stacking in the plasma membrane, thereby, interfering with protein function, particularly with the function of proteins that control cytoskeletal dynamics. My work provides an insight into how GlcCer accumulation affects cellular signaling and, therefore, how the lack of GBA2 leads to a defect in male infertility.