Paz Martínez¹ and Antoni Morros²

¹ Instituto de Biología Fundamental. Unidad de Inmunología, Universitat Autònoma de Barcelona, 08193 Bellaterra (Barcelona) Spain.

² Unitat de Biofísica. Departament de Bioquímica i de Biologia Molecular,Universitat Autònoma de Barcelona, 08193 Bellaterra (Barcelona) Spain

Received 05/06/96; Accepted 06/18/96; On-line 07/01/96

The lipid composition of plasma membrane of mammalian spermatozoa is markedly different from those of mammalian somatic cells. The major lipid composition of human spermatozoa presented in Table 1 is based on data reported by Alvarez and Storey (19) and Mack et al.. (20) which are in accordance with the composition given by Poulos and White (21). Human spermatozoa have unusually high levels of ether-linked lipids and a high content of unsaturated fatty acyl groups such as docosahexaenoyl (22:6 chains). Sphingomyelin is also a major lipid constituent of the plasma membranes of sperm. Although these molecules do not have an unusual amount of glycolipids, they contain a structure that is unique to sperm: sulfogalactosylglycerolipid or seminolipid. The ratio cholesterol/phospholipid is around 1; this high cholesterol content seems to play an important role in capacitation.

Ether-linked lipids are shown to be very abundant in several mammalian sperm plasma membranes (22, 23). Ether lipids are glycerophospholipids that contain either one alkenylether group at position sn-1 of glycerol (named plasmalogens), or one (at position sn-1) (Figure 1) or two alkylether groups. Plasmalogens are the most abundant ether lipids in human sperm and in animal cells. Choline and ethanolamine plasmalogens have specific molecular conformation (24), probably responsible for a more densely packed structure as compared with diacylglycerophospholipids. A special behavior of polyunsaturated ethanolamine plasmalogens has been described in artificial and biological membranes and they may facilitate membrane fusion (24, 25) as will be described in detail. Choline plasmalogens may contribute to form non-diffusible membrane regions, that confer stability on the membranes (10, 24).

Figure 1 - Comparison of phosphatidylcholine with ether-linked choline phospholipids

Mammalian spermatozoa undergo important changes during their passage through the epididymis that affect their lipid composition. The lipid pattern of cauda epididymis spermatozoa (mature cells), however, is similar to that in ejaculated spermatozoa. During epididymal maturation, a decrease in the amount of sperm lipid has been reported in the sperm of boar, rat, bull and ram (26-28). However, one of the relevant changes associated with the maturation of rat sperm is that the ratio plasmenyl/phosphatidyl increases from 0.33 in caput epididymis to 0.95 in cauda epididymis. As a consequence of these changes, the plasmalogens become a major phospholipid component in the cauda and plasmenylcholine becomes the most abundant phospholipid (28). Similar data were obtained from the analysis of bovine spermatozoa in which choline plasmalogens were the major phospholipid (35% of total lipid phosphorous) (29). Since ether linkages are not easily cleaved by the action of lipases (24), ether lipids might confer stability on membrane during sperm maturation.

Very high amounts of polyunsaturated fatty acid chains, especially docosahexaenoic (22:6 chains), are found in the plasma membrane of human sperm (Table 1). These chains are concentrated in the ether-containing phospholipids, as proposed for the plasma membrane of ram sperm (22, 30). The other dominant fatty acid chains are saturated, predominantly 16:0. A striking feature of ether lipids, described in the bovine sperm, is the composition of their acyl chains: almost exclusively (97%) a 16:0 moiety at carbon sn-1 and either a 22:5 (20%) or 22:6 (75%) moiety at carbon sn-2 (29). A diagram of this plasmalogen molecule is presented in Figure 2; the configuration of the polyunsaturated acyl chain (22:6) is based on the model presented by Stubbs and Smith (31). An approximately helical conformation is proposed, with a shortening of the chain. The motional freedom of the chain is severely limited by the lack of rotation at the six double bonds, highly restricting changes in conformation (31). Epididymal maturation results in significant alterations in the proportions of major fatty acid chains. In several species an increase of 22:4 and 22:5 fatty acid chains is observed (28, 32). This is probably linked to the increase of plasmalogens already discussed. One of the important components of the plasma membrane of sperm is sphingomyelin, which shows maximal level of saturated fatty acids (80-97%) in goat and boar sperm (26, 27).

Figure 2 - Diagrammatic representation of an ethanolamine plasmalogen (1-O-hexadecyl-2-docosahexa-enoylphosphatidylethanolamine) (16:0/22:6), drawn using the computer program Sybyl 6.2.

This lipid is known to exert a rigidifying effect on biomembranes, that is, it tends to condense the bilayer,perhaps due to hydrogen bonding between the amide bonds or via the free hydroxyl group (33, 34). It has been proposed that sphingomyelin preferentially interacts with cholesterol and determines the sterol content in the plasma membrane (35, 36).

The high cholesterol/phospholipid ratio is originated in the epididymis, which displays a high rate of cholesterol synthesis, and it is transferred into the plasma membrane of maturing spermatozoa (2).

A 2-fold increase in the cholesterol/phospholipid molar ratio is observed in ram spermatozoa during sperm maturation (22). Stabilization of the membrane by cholesterol may bebeneficial to spermatozoa that must travel through the female genital tract.

Cholesterol sulfate is a normal constituent of the plasma membrane of sperm and it is also present in seminal plasma (37). During epididymal transit, an 18-fold increase in the amount of sterol sulfate of the plasma membrane of sperm has been described in hamster (38, 39). This anionic lipid is sequestered into the plasma membrane of the head and of the midpiece of the sperm (40). It has been postulated that cholesterol sulfate could regulate the fluidity of the sperm membrane during epididymal maturation and later during capacitation and acrosome reaction in the female genital tract (26). Interestingly, cholesterol sulfate contributes to the net negative charge of the external surface of sperm (40).

Epididymal maturation induces a significant increase in the negative charge of the sperm membrane due to the uptake of sialoglycoproteins (41) and anionic lipids such as cholesterol sulfate (40), sulfogalactolipids (42) and cardiolipin (28).