The effect of docosahexaenoic acid (DHA) on neuronal survival was studied in cultured cells isolated from newborn rat retina. In vivo, the content of DHA in the retina increased nearly fourfold from days 2 to 12 after birth, whereas in retinal cells in culture it remained constant. Unlike amacrine cells, the photoreceptor cells in control cultures underwent a selective degeneration, starting at day 7, that led to their massive death by day 11. The addition of DHA at day 7 led to its active incorporation by the cultures, increasing from 6 to 21 % of total fatty acids in cell lipids, and completely prevented photoreceptor cell death. When other fatty acids were tested, both neuronal fatty acid composition and photoreceptor death were the same as in control cultures. These results indicate that DHA is specifically required for the survival of retinal photoreceptors. Key Words: Photoreceptor survival-Docosahexaenoic acid-Neuronal survival-Retinal neurons.

About one-fourth the phosphatidylcholines (PCs) from bovine disk photoreceptor membranes contain very long chain (24-36 carbons) polyunsaturated (4, 5, and 6 double bonds) fatty acids of the n-3 and n-6 series (VLCPUFA). Such fatty acids, exclusively occurring in dipolyunsaturated species, are esterified to the sn-1 position of their glycerol backbone, docosahexaenoate being the major fatty acid at sn-2. Chromatographically, such PCs display a weakly polar character relative to other species, ascribable to their exceedingly large number of carbons. After hexane extraction of lyophilized disks, PC is the major component of the fraction of lipids that remains associated with rhodopsin, followed by phosphatidylserine, while a large proportion of the phosphatidylethanolamine is removed. The fatty acid composition of the hexane-removable and protein-bound lipid fractions markedly differs, the latter being enriched in lipid species containing long-chain and very long chain polyenes. This is observed for all lipid classes except free fatty acids. VLCPUFA-containing PCs are the most highly concentrated species in the rhodopsin-associated lipid fraction. The very long chain polyenes these PCs have at sn-1 may account for their resistance to being separated from the protein. It is hypothesized that their unusually long polyenoic fatty acids could be well suited to partially surround alpha-helical segments of rhodopsin.

Abstract:When rat retinal cells are cultured in a serumfree medium, the photoreceptor cells start dying after 7 days. The addition of docosahexaenoic acid (DHA) to the cultures prevents the selective death of photoreceptors. Here it is shown that, unlike other retinal neurons, photoreceptors die through an apoptotic pathway. Hallmarks of apoptosis, such as nuclear fragmentation and condensation and DNA cleavage forming a ladder pattern on an agarose gel, were observed. The timing and high selectivity of the triggering of photoreceptor cell apoptosis suggest the existence of a programmed cell death. Compared with other fatty acids, DHA not only was the most effective in promoting photoreceptor survival, but also the only one to decrease the number of apoptotic nuclei. The results suggest that DHA is important among the factors preventing apoptosis of photoreceptors in the developing retina. A limitation in the availability of this fatty acid might trigger apoptosis as a result of the failure to develop functional photoreceptor outer segments.

Mammalian spermatogenesis is an extraordinary cell transformation process. It includes asymmetric division of spermatogonia, rapid proliferation and differentiation into primary spermatocytes, a meiotic phase in which the genetic material in spermatocytes is recombined and segregated to produce haploid cells, a postmeiotic phase in which secondary spermatocytes develop into round spermatids, and differentiation of the latter into late spermatids and spermatozoa ( 1 ). During this last stage, known as spermiogenesis, spermatids undergo relevant cytological transformations, with changes in nuclear shape, chromatin condensation, formation of acrosome and fl agellum, and disposal of surplus organelles and materials by production and release of membrane-enclosed remnants known as "residual bodies" ( 2, 3 ). These minute, densely packed particles are then engulfed by Sertoli cells ( 4 ).The sphingomyelin (SM) ( 5 ) and ceramide (Cer) ( 6 ) of cells located in mammalian seminiferous tubules are rich in very long chain (C24 to C34) polyunsaturated fatty acids (VLCPUFA) of the n-6 or the n-3 series, the main ones being 28:4n-6 and 30:5n-6, followed by 32:5n-6 in the rat. These lipids belong to spermatogenic cells, as indicated by the facts that i) in the prepubertal rat testis, VLCPUFAcontaining species of SM and Cer are not detected; ii) in

Very long-chain (C24 to C34) polyunsaturated fatty acids (VLCPUFA) are important constituents of sphingomyelin (SM) and ceramide (Cer) in testicular germ cells. In the present paper we focused on the SM and Cer and their fatty acids in spermatozoa and their main regions, heads and tails. In bull and ram spermatozoa, SM was the third most abundant phospholipid and VLCPUFA were the major acyl groups (ϳ70%) of SM and Cer. In rat epididymal spermatozoa the SM/Cer ratio was low in the absence of and could be maintained high in the presence of the cation chelator EDTA, added to the medium used for sperm isolation. This fact points to the occurrence of an active divalent cation-dependent sphingomyelinase. Bull and rat sperm had an uneven head-tail distribution of phospholipid, with virtually all the VLCPUFA-rich SM located at the head, the lower SM content in the rat being determined by the lower sperm head/tail size ratio. Most of the SM from bull sperm heads was readily solubilized with 1% Triton X-100 at 4°C. The detergent-soluble SM fraction was richer in VLCPUFA than the nonsoluble fraction and richer in saturated fatty acids. Cer was produced at the expense of SM, thus decreasing severalfold the SM/Cer ratio in rat spermatozoa incubated for 2 h in presence of the spermcapacitating agents, calcium, bicarbonate, and albumin. The generation of Cer from SM in the sperm head surface may be an early step among the biochemical and biophysical changes known to take place in the spermatozoon in the physiological events preceding fertilization.In a number of mammals including humans a series of very long-chain polyunsaturated fatty acids (VLCPUFA), 2 i.e. n-6 and n-3 tetraenoic, pentaenoic and hexaenoic fatty acids with up to 32 or 34 carbon atoms, depending on the species, was characterized in the sphingomyelin (SM) from testis and spermatozoa (1, 2). In the testis of various mammals, we focused on the fatty acids of the ceramide (Cer), a lipid molecule with which SM bears a close precursor-product relationship, showing that SM and Cer species containing VLCPUFA are a specific feature of cells of the spermatogenic lineage (3). Because these testicular cells are predecessors of spermatozoa, the question arose as to the quantitative importance of these molecules in spermatozoa, where they could play a role in sperm functions related to fertilization. Transit through the epididymis is a crucial phase in sperm maturation. Spermatozoa exiting the testis are immotile, unable to bind to eggs and to undergo the acrosomal reaction in vitro in response to commonly used stimuli. By the time they reach the region of cauda epididymis, sperm cells have acquired their progressive motility and their ability to bind, penetrate, and fertilize eggs (4). One of the questions we addressed was whether epididymal maturation gives rise to spermatozoa with a larger or a smaller proportion of SM and Cer containing these VLCPUFA as opposed to other fatty acids.Spermatozoa are functionally regionalized cells. Sperm-oocyte interactions are head-relat...

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Very long-chain polyunsaturated fatty acids (VLCPUFA) have previously been shown to be components of sphingomyelin (SM) of mammalian testis and spermatozoa. Here we examined the fatty acids of testicular ceramide (Cer) in comparison with those of SM in some mammals with a special focus on the rat testis. In bull, cat, dog, rabbit, mouse, and rat, VLCPUFA were found in both testicular lipids, Cer having a higher percentage of VLCPUFA than SM. Rat testis had the highest percentage of VLCPUFA in both lipids, the major ones being 28:4n-6 and 30:5n-6. VLCPUFA-containing SM and Cer occurred in cells located in the seminiferous tubules, where germ cells had a higher percentage of these species than Sertoli cells. Seminiferous tubule fractionation showed that SM and Cer of mitochondria and lysosomes had mostly saturates and negligible VLCPUFA, the latter being important in the SM and Cer of microsomes and other membrane fractions. VLCPUFA were absent from the SM and Cer of rat prepuberal testis, increased with the onset of spermatogenesis to account for nearly 15 and 40% of the total fatty acids of testicular SM and Cer, respectively, remained at those levels throughout the adult life of fertile rats and tended to decrease at advanced ages. Four conditions that lead to selective death of germ cells in vivo, namely experimental cryptorchidism, post-ischemic reperfusion, focalized x-ray irradiation and treatments with the antineoplasic drug doxorubicin, caused the VLCPUFA to disappear from the testicular SM and Cer of adult fertile rats, showing that these lipids are specific traits of spermatogenic cells.

In their transit from the caput to the cauda segments of the epididymis, rat spermatozoa undergo significant modifications in lipid content and composition. The amount of lipid phosphorus per cell decreases, and most lipid classes show specific changes in their constituent fatty acids. A depletion of phosphatidylcholine and phosphatidylethanolamine, concomitant with a virtually unchanged amount of the corresponding plasmalogens, are the major alterations, plasmenylcholine thereby becoming the major phospholipid. Diphosphatidylglycerol, sphingomyelin and the phosphoinositides decrease to a lesser extent or do not change at all, also resulting in relative increases with sperm maturation. Concerning the fatty acids, the proportions of oleate (C18:1, n-9) and linoleate (C18:2, n-6) in most lipids decrease on movement of sperm from caput to cauda, augmenting in turn the proportions of longer-chain (C20 to C24) and more unsaturated fatty acids. Docosapentaenoate (C22:5, n-6) is a major acyl chain present in all lipids at both stages, but uncommon long-chain polyenoic fatty acids of the n-9 series are also present, being almost exclusively found in the choline glycerophospholipids. These fatty acids are found to undergo the most significant changes during sperm maturation. They are minor components of plasmenylcholine in immature spermatozoa, but increase severalfold on maturation, representing more than half of the acyl chains of this major lipid in cells from the cauda. The high concentration of n-9 polyenes in mature sperm plasmenylcholine raises intriguing questions on the possible role epididymal cells may play in providing spermatozoa with such an unusual phospholipid. These plasmenylcholines could contribute to the characteristic lipid domain organization of the mature spermatozoa plasma membrane.