Organization of Membrane Rafts in Chicken Sperm

Membrane rafts regulate sperm acrosome reaction via cAMP-dependent pathway in chickens (Gallus gallus domesticus)

Both transcriptionally and translationally inactive sperm need preassembled pathways into specific cellular compartments to function. Although initiation of the acrosome reaction (AR) involves several signaling pathways including protein kinase A (PKA) activation, how these are regulated remains poorly understood in avian sperm. Membrane rafts are specific membrane regions enriched in sterols and functional proteins and play important roles in diverse cellular processes, including signal transduction. Our recent studies on chicken sperm demonstrated that membrane rafts exist and play a role in multistage fertilization. These, combined with the functional importance of membrane rafts in mammalian sperm AR, prompted us to investigate the roles of membrane rafts in signaling pathways leading to AR in chicken sperm. Using 2-hydroxypropyl-β-cyclodextrin (2-OHCD), we found that the disruption of membrane rafts inhibits PKA activity and AR without affecting protein tyrosine phosphorylation; however, these inhibitions were abolished in the presence of a cyclic 3,5-adenosine monophosphate (cAMP) analog. In addition, biochemical experiments showed a decrease in cAMP content in 2-OHCD-treated sperm, suggesting the involvement of soluble adenylyl cyclase (sAC) and transmembrane adenylyl cyclase (tmAC). Pharmacological experiments, combined with transcriptome analysis, showed that sAC and tmAC are present and involved in AR induction in chicken sperm. Furthermore, stimulation of both isoforms reversed the inhibition of PKA activity and AR in 2-OHCD-treated sperm. In conclusion, our results demonstrated that membrane rafts play an important role in AR induction by regulating the cAMP-dependent pathway and that they provide a mechanistic insight into membrane regulation of AR and sperm function in birds.

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.

Characterization of the functions and proteomes associated with membrane rafts in chicken sperm

Cellular membranes are heterogeneous, and this has a great impact on cellular function. Despite the central role of membrane functions in multiple cellular processes in sperm, their molecular mechanisms are poorly understood. Membrane rafts are specific membrane domains enriched in cholesterol, ganglioside GM1, and functional proteins, and they are involved in the regulation of a variety of cellular functions. Studies of the functional characterization of membrane rafts in mammalian sperm have demonstrated roles in sperm-egg binding and the acrosomal reaction. Recently, our biochemical and cell biological studies showed that membrane rafts are present and might play functional roles in chicken sperm. In this study, we isolated membrane rafts from chicken sperm as a detergent-resistant membranes (DRM) floating on a density gradient in the presence of 1% Triton X-100, and characterized the function and proteomes associated with these domains. Biochemical comparison of the DRM between fresh and cryopreserved sperm demonstrated that cryopreservation induces cholesterol loss specifically from membrane rafts, indicating the functional connection with reduced post-thaw fertility in chicken sperm. Furthermore, using an avidin-biotin system, we found that sperm DRM is highly enriched in a 60 KDa single protein able to bind to the inner perivitelline layer. To identify possible roles of membrane rafts, quantitative proteomics, combined with a stable isotope dimethyl labeling approach, identified 82 proteins exclusively or relatively more associated with membrane rafts. Our results demonstrate the functional distinctions between membrane domains and provide compelling evidence that membrane rafts are involved in various cellular pathways inherent to chicken sperm.

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 GM1, 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.