Dysregulation of sphingolipid and cholesterol homeostasis imposes oxidative stress in human spermatozoa

Infertility is a significant public health concern, affecting one in six couples globally, with male-factor infertility representing half of all cases. Obesity is an important health problem and is increasingly linked to poor reproductive outcomes as it induces metabolic disturbances and dyslipidemia, both of which impair male fertility. Dyslipidemia, characterized by abnormal lipid profiles, is associated with declining sperm quality and alters cholesterol and sphingolipid abundances in the seminal plasma. Sperm capacitation enables spermatozoa to acquire fertilizing competence by promoting cholesterol efflux from the plasma membrane, modifying lipid composition and membrane fluidity, and enhancing the ability to recognize and fertilize the oocyte. However, the intricate interplay between cholesterol efflux and capacitation-associated modifications in human spermatozoa remains poorly understood. Sphingolipids, including Ceramide (Cer) and sphingosine-1-phosphate (S1P), are critical regulators of cellular functions that play a significant role in male reproductive health. These bioactive lipids and cholesterol promote sperm capacitation, motility, and the acrosome reaction (AR). We hypothesize that dysregulated lipid homeostasis, as seen in cases of dyslipidemia, can impair capacitation by disrupting lipid-signalling pathways and promoting oxidative stress. Our results demonstrate that methyl-β-cyclodextrin (MβCD), which extracts cholesterol from the sperm plasma membrane, enhances a variety of capacitation-associated adaptations, including tyrosine phosphorylation, hyperactive motility, and AR. Moreover, sphingolipid signalling, through the S1PR1/3 receptors, is crucial in mediating these effects. MβCD-cholesterol extraction is also associated with nitric oxide (NO) and superoxide (O₂^•-) production, key signalling molecules involved in capacitation. However, dysregulated lipid metabolism, characterized by elevated cholesterol sulfate (Chol-SO₄), Cer, and Sphingosine (Sph), impairs sperm capacitation through excessive oxidative stress and promotes oxidative damage. The findings suggest that disruptions in lipid metabolism resulting from conditions like obesity can interfere with sperm capacitation and fertilizing potential, providing new insights into male infertility mechanisms.