Comparison of Membrane Characteristics between Freshly Ejaculated and Cryopreserved Sperm in the Chicken

Cryopreserved sperm undergoes serious damage which affects its fertilizing ability. Despite progress in understanding the nature of functional deterioration in mammalian sperm, little is known about the mechanism involved in the induction of functional damage in avian sperm. Cellular membranes are considered the primary site of cryodamage to sperm. Membrane rafts are specific membrane regions enriched in sterols, ganglioside G_M1, and functional proteins and they play important roles in the regulation of diverse functions exerted in mammalian sperm during fertilization. Several reports investigating cryopreservation-induced membrane changes in mammalian sperm have suggested that cryopreservation induces a compositional alteration of membrane rafts via a loss of membrane sterols, leading to impaired fertilizing ability. Recently, we demonstrated that membrane rafts are present in chicken sperm. Therefore, we investigated a possible mechanism for the induction of functional damage in cryopreserved chicken sperm, with particular attention to cryopreservation-induced compositional changes in membrane rafts. Sterol quantification showed that loss of sterols from sperm membranes occurred following cryopreservation. Biochemical analyses of detergent-insoluble membranes showed that the lipid and protein compositions of membrane rafts were altered dramatically by cryopreservation. To determine the physiological role of these changes, we examined external translocation of phosphatidylserine (PS), representing an early apoptotic change, and found that cryopreservation induced apoptotic changes in chicken sperm. Furthermore, methyl-β-cyclodextrin-induced loss of sterols from the plasma membranes stimulated PS translocation that was not accompanied with caspase-3 activation, which plays an important role downstream of the apoptotic cascade. Based on the results obtained in this study, we discuss a new mechanism for reduction of the fertilizing ability in avian sperm after cryopreservation.