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

Effects of activation and assisted reproduction techniques on the composition, structure, and properties of the sauger (Sander Canadensis) spermatozoa plasma membrane

The sperm plasma membrane is a multifunctional organelle essential to fertilization. However, assisted reproduction techniques often negatively affect this structure, resulting in reduced fertility. These reductions have been attributed to plasma membrane damage in a wide array of species, including fish. Considerable research has been conducted on the fish sperm membrane, but few have examined the effect of cryopreservation and other assisted reproduction techniques (ARTs) on not only membrane composition, but also specific characteristics (e.g., fluidity) and organization (e.g., lipid rafts). Herein, we determined the effects of three ARTs (testicular harvest, strip spawning, and cryopreservation) on the sperm plasma membrane, using Sauger (Sander canadensis) sperm as a model. To this end, a combination of fluorescent dyes (e.g., merocyanine 540, filipin III, cholera toxin subunit β), liquid chromatography - mass spectroscopy (LC-MS) analysis of membrane lipids, and membrane ultracentrifugation coupled with plate assays and immunofluorescence were used to describe and compare sperm fluidity, membrane composition, as well as lipid raft composition and distribution among sperm types. Stripped sperm became more fluid following motility activation (40% increase in highly fluid cells characterized by a 2 × increase in fluorescence) and contained lipid rafts restricted to the midpiece. Testicular harvest yielded sperm with characteristics similar to stripped sperm. By contrast, cryopreservation impacted every aspect of membrane physiology. Two cell populations, one highly fluid and the other rigid, resulted from the freeze-thaw process. Cryopreservation reduced lipid raft cholesterol content by 44% and flotilin-2 (a lipid raft marker) was partially displaced owing to a decrease in buoyancy. Unlike stripped and testicular sperm, LC-MS analysis revealed increases in oxidative damage markers, membrane destabilization, and apoptotic signaling in cryopreserved sperm. Ultrastructural analysis also revealed widespread physical damage to the membrane following freeze-thaw. Sperm motility, however, was unrelated to any measure of membrane physiology used in this study. Our results demonstrate that ARTs have the potential to substantially affect the sperm plasma membrane, but not always detrimentally. These results provide multiple potential biomarkers of sperm quality as well as insight into sources of sub-fertility resulting from use of ARTs.

Modification of membrane cholesterol and desmosterol in chicken spermatozoa improves post-thaw survival and prevents impairment of sperm function after cryopreservation

During cryopreservation, spermatozoa are subjected to cryodamage that leads to a decline in fertilisation ability. Due to the complex nature of this process, the initial trigger for cryodamage remains unknown. Recently, we demonstrated that cryopreservation induces early apoptotic changes characterised by phosphatidylserine (PS) translocation via sterol loss from the plasma membrane of chicken spermatozoa. This led us to hypothesise that sterol incorporation into membranes minimises cryodamage, thereby improving the quality of cryopreserved chicken spermatozoa. In the present study, treating spermatozoa with 1.5 mg mL−1 cholesterol- and 3 mg mL−1 desmosterol-loaded cyclodextrin (CLC and DLC respectively) increased post-thaw survival and motility. These effects appeared to be highly dependent the amount of sterol loaded into the spermatozoa. Localisation experiments confirmed the incorporation of exogenous cholesterol into the sperm head region. Detection of PS translocation showed that elevation of these sterols inhibited early apoptotic changes, thereby enhancing post-thaw survival. Furthermore, CLC and DLC treatment suppressed spontaneous acrosome reaction after cryopreservation, preserving the ability of spermatozoa to undergo acrosome reactions in response to physiological stimulation. These results demonstrate that loading sterols into chicken spermatozoa before cryopreservation enhances their quality by inhibiting early apoptotic changes and spontaneous acrosome reactions. The present study provides new mechanistic insight into cryodamage in chicken spermatozoa.