Studies on the mechanism of capacitation: albumin-mediated changes in plasma membrane lipids during in vitro incubation of rat sperm cells.

Abstract

Plasma membrane isolated from rat sperm cells after incubation in vitro had a significantly lower cholesterol/phospholipid mole ratio when the medium contained serum albumin. Transfer of albumin-bound phospholipids to the membrane can largely account for this effect. The result is broadly consistent with a previously proposed model for albumin-induced destabilization of sperm membrane (capacitation) and its reversal by seminal plasma membrane vesicles. Albumin also decreased sialic acid and, more specifically, ganglioside levels, presumably by promoting release of sperm neuraminidase. Cholesteryl ester comprised up to 0.5 mol/mol of cholesterol in these plasma mem rane preparations. Mammalian sperm cells can express fertilizing capacity after incubation in vitro for an interval of usually a few hours in a chemically defined medium containing serum albumin (1-6). Although the molecular changes underlying transformation to a capacitated state are unclear, albumin specifically facilitates this process (4). Substantial changes in sperm plasma membrane proteins accompany incubation with albumin (7); however, they seem to involve proteolysis after the acrosome reaction. A clue that lipid binding by albumin might be involved in its ability to promote capacitation was provided by the observation that presaturation of the protein with cholesterolt abolished this capability (4). In view of this finding and the discovery that cholesterol-bearing decapitation factor (DF) vesicles from seminal plasma and synthetic vesicles containing the sterol block capacitation in rabbit and rat spermatozoa (8-12), changes in the lipid bilayer of the sperm plasma membrane seem to be necessary for expression of fertilizing capacity.One interpretation of these observations is that albumin decreases the cholesterol/phospholipid (Chol/PL) ratio in the sperm plasma membrane (13). Spectroscopic data reveal that cholesterol restricts the thermal motion of fatty acid chains in a phospholipid bilayer above the gel-to-liquid transition temperature (14-16), and this causes a "condensation effect" and decreases permeability (17). An increase in bilayer microviscosity evidently accounts for the inhibit'ry effect of cholesterol on fusion between plasma membranes of cultured muscle cells (18) and synthetic phospholipid membranes (19). Decreasing the Chol/PL ratio in sperm plasma mqmbrane, therefore, may be anticipated to promote fusion with the outer acrosomal membrane; this intracellular fusion occurring in the acrosome reaction is a precondition for mammalian fertilization (20). A vesicle-induced reversal of this putative change in the plasma membrane Chol/PL ratio might account for the ability of these vesicles to inhibit spermatozoan fertilizing capacity reversibly (11,12).To test this idea, plasma membrane preparations isolated from rat spermatozoa after incubation in vitro were partially characterized with respect to their lipid composition. It was anticipated bovine serum albumin would decrease the Chol/PL ratio in sperm plasma membrane i...

Effect of Semen Collection in Extender Solution on the Characteristics of Goat Spermatozoa

“…On the other hand, several studies have provided direct evidence that BSA causes influx of Ca 2+ ions into the plasma membrane [29], decreases the cholesterol and phospholipid ratio in the outer acrosomal membrane of the spermatozoa [30][31][32], and stimulates sperm hyperactivation [33]. BSA is most commonly used as a capacitating agent, and capacitation induces hyperactive motility and 106.3 ± 6.7 C TCG 10 ml 85.6 ± 1.2 66.6 ± 2.0 101.3 ± 6.5 TCG 10 ml + BSA 0.1% 86.0 ± 2.5 65.6 ± 2.1 106.0 ± 5.1 TCG 10 ml + BSA 1% 89.0 ± 0.5 60.6 ± 2.3 95.6 ± 4.0 TCG 10 ml + BSA 5%…”

Enhanced Freezability of Goat Spermatozoa Collected into Tubes Containing Extender Supplemented with Bovine Serum Albumin (BSA)

“…To our knowledge, this is the first report of improving sperm freezability by means of a semen collection method. Several researchers have provided direct evidence that BSA, which in extender adheres rapidly to the sperm membrane at the moment of dilution [11], modifies the sperm lipid composition through lipid exchange or hydrolysis [12], promotes plasma membrane protein hydrolysis [13], causes influx of Ca 2+ ions into the cytoplasm [ 1 1 ] , a n d d e c r e a s e s t h e c h o l e s t e r o l a n d phospholipid ratio in the plasma membrane of spermatozoa [14,15]. Moreover, it is well established that cholesterol efflux from the sperm membrane induces enhanced membrane fluidity [16,17], while elevation of membrane cholesterol is associated with decreased membrane fluidity [18][19][20].…”

Cholesterol efflux from sperm: approaches and applications

“…Their results showed increased cholesterol efflux (50%) in the presence of 5% human female serum or follicular fluid, whereas low cholesterol efflux (14%) was observed in spermatozoa incubated with protein-free medium. In contrast, Davis et al (17) found that cholesterol efflux from sperm membranes was not statistically significant using comparable concentrations of serum albumin. Sugkraroek et al (61) found that using 3.5% human serum albumin (HSA) medium only caused a 5% cholesterol removal from fertile donors' sperm.…”

“…In 1971 Toyoda et al (33) reported successful in vitro fertilization in mice by using a chemically defined medium supplemented with BSA. Later, the role of BSA in capacitation media was tested for mouse, rat, and human sperm capacitation (7,(16)(17)(18)(19)22,34). Likewise, BSA has been used in most media to maintain sperm motility and to promote membrane cholesterol efflux (18)(19)34).…”

“…The presence of several cholesterol acceptors such as albumin and highdensity lipoprotein (HDL) in the follicular and oviductal fluid mediates cholesterol efflux from the sperm membrane in vivo (14,15). However, in vitro, cholesterol efflux from the sperm plasma membranes has been facilitated by cholesterol acceptors such as bovine serum albumin (BSA) (6,(16)(17)(18), HDL (14,19), and β-cyclodextrin (20). Likewise, seminal proteins and Percoll gradient centrifugation have been used to induce cholesterol efflux from sperm to promote the acrosome reaction (21)(22)(23).…”