Recently it was shown that a specific form of male infertility in Holstein cattle was caused by a nonsense variant in the α/β-hydrolase domain-containing 16B (ABHD16B) gene resulting in a protein truncation at amino acid position 218 (p.218Q*) and loss of function. Lipidomics showed that the absence of ABHD16B influenced the content of phosphatidylcholine (PC), ceramide (Cer), diacylglycerol (DAG), and sphingomyelin (SM) in variant carrier sperm membranes. However, the exact cause of infertility in affected sires has remained unclear until now. To elucidate the cause of infertility, we analyzed (i) standard sperm parameters (i.e., total sperm number, morphological intact sperm, total sperm motility), (ii) in vitro fertilizability and effects on early embryonic development, and (iii) sperm survival rates (i.e., capacitation time). The affected spermatozoa showed no changes in the usual sperm parameters and were also capable of fertilization in vitro. Furthermore, the absence of ABHD16B did not affect early embryonic development. Based on these results, it was concluded that the affected spermatozoa appeared to be fertilizable per se. Consequently, the actual cause of the inability to fertilize could only be due to a time- and/or place-dependent process after artificial insemination and before fertilization. A process fundamental to the ability to fertilize after insemination is capacitation. Capacitation is a biochemical maturation process that spermatozoa undergo in the female genital tract and is inevitable for the successful fertilization of the oocyte. It is known that the presence and concentration of certain sperm membrane lipids are essential for the correct course of capacitation. However, precisely these lipids are absent in the membrane of spermatozoa affected by the ABHD16B truncation. Since all other causes of fertilization inability were excluded in the previous experiments, consequently, the only remaining hypothesis was that the loss of function of ABHD16B leads to a capacitation disruption. We were able to show that heterozygous and homozygous affected spermatozoa exhibit premature capacitation and therefore decay before fertilization. This effect of the loss of function of ABHD16B has not been described before and our studies now revealed why sires harboring the variant in the ABHD16B gene are infertile.
Keywords: ABHD16B; Holstein cattle; lipid composition; loss of function; male infertility; premature capacitation; sperm membrane.