Differentiation-linked changes in the biosynthesis and turnover of sphingomyelins in rat male germ cells: Genes involved and effects of testosterone

In rodents, sphingomyelins (SMs) species with very-long-chain polyunsaturated fatty acid (VLCPUFA) are required for normal spermatogenesis. Data on the expression of enzymes with roles in their biosynthesis and turnover during germ cell differentiation and on possible effects on such expression of testosterone (Tes), known to promote this biological process, were lacking. Here we quantified, in isolated pachytene spermatocytes (PtS), round spermatids (RS), and later spermatids (LS), the mRNA levels from genes encoding ceramide (Cer), glucosylceramide (GlcCer), and SM synthases (Cers3, Gcs, Sms1, and Sms2) and sphingomyelinases (aSmase, nSmase) and assessed products of their activity in cells in culture using nitrobenzoxadiazole (NBD)-labeled substrates and [³H]palmitate as precursor. Transcript levels from Cers3 and Gcs were maximal in PtS. While mRNA levels from Sms1 increased with differentiation in the direction PtS→RS→LS, those from Sms2 increased between PtS and RS but decreased in LS. In turn, the nSmase transcript increased in the PtS→RS→LS order. During incubations with NBD-Cer, spermatocytes produced more GlcCer and SM than did spermatids. In total germ cells cultured for up to 25 h with NBD-SM, not only abundant NBD-Cer but also NBD-GlcCer were formed, demonstrating SM→Cer turnover and Cer recycling. After 20 h with [³H]palmitate, PtS produced [³H]SM and RS formed [³H]SM and [³H]Cer, all containing VLCPUFA, and Tes increased their labeling. In total germ cells, Tes augmented in 5 h the expression of genes with roles in VLCPUFA synthesis, decreased the mRNA from Sms2, and increased that from nSmase. Thus, Tes enhanced or accelerated the metabolic changes occurring to VLCPUFA-SM during germ cell differentiation.

Elovl4 and Fa2h expression during rat spermatogenesis: a link to the very-long-chain PUFAs typical of germ cell sphingolipids

The sphingolipids (SLs) of rodent spermatogenic cells (spermatocytes, spermatids) and spermatozoa contain nonhydroxylated and 2-hydroxylated versions of very-long-chain (C26-C32) PUFAs (n-V and h-V, respectively) not present in Sertoli cells (SCs). Here, we investigated the expression of selected fatty acid elongases [elongation of very-long-chain fatty acid protein (Elovl)], with a focus on Elovl4, and a fatty acid 2-hydroxylase (Fa2h) in rat testes with postnatal development and germ cell differentiation. Along with Elovl5 and Elovl2, Elovl4 was actively transcribed in the adult testis. Elovl4 mRNA levels were high in immature testes and SCs, though the protein was absent. The Elovl4 protein was a germ cell product. All cells under study elongated [³H]arachidonate to tetraenoic and pentaenoic C24 PUFA, but only germ cells produced C26-C32 PUFAs. Spermatocytes displayed the highest Elovl4 protein levels and enzymatic activity. Fa2h mRNA was produced exclusively in germ cells, mostly round spermatids. As a protein, Fa2h was mainly concentrated in late spermatids, in the step of spermiogenesis in which they elongate and their heads change shape. The expression of Elovl4 and Fa2h thus correlate with the abundance of n-Vs and h-Vs in the SLs of rat spermatocytes and spermatids, respectively.

Sphingomyelins and ceramides with VLCPUFAs are excluded from low-density raft-like domains in differentiating spermatogenic cells

Rat spermatogenic cells contain sphingomyelins (SMs) and ceramides (Cers) with very long-chain PUFAs (VLCPUFAs) in nonhydroxylated (n-V) and 2-hydroxylated (h-V) forms. How these atypical species distribute among membrane fractions during differentiation was investigated here using a detergent-free procedure to isolate a small light raft-like low-density fraction and a large heavy fraction, mostly derived from the plasma membrane of spermatocytes, round spermatids, and late spermatids. The light fraction contained cholesterol, glycerophospholipids (GPLs), and SM with the same saturated fatty acids in all three stages. In the heavy fraction, as PUFA increased in the GPL and VLCPUFA in SM from spermatocytes to spermatids, the concentration of cholesterol was also augmented. The heavy fraction had mostly n-V SM in spermatocytes, but accumulated h-V SM and h-V Cer in spermatids. A fraction containing intracellular membranes had less SM and more Cer than the latter, but in both fractions SM and Cer species with h-V increased over species with n-V with differentiation. This accretion of h-V was consistent with the differentiation-dependent expression of fatty acid 2-hydroxylase (Fa2h), as it increased significantly from spermatocytes to spermatids. The non-raft region of the plasma membrane is thus the main target of the dynamic lipid synthesis and remodeling that is involved in germ cell differentiation.

The E2F2 transcription factor sustains hepatic glycerophospholipid homeostasis in mice

Increasing evidence links metabolic signals to cell proliferation, but the molecular wiring that connects the two core machineries remains largely unknown. E2Fs are master regulators of cellular proliferation. We have recently shown that E2F2 activity facilitates the completion of liver regeneration after partial hepatectomy (PH) by regulating the expression of genes required for S-phase entry. Our study also revealed that E2F2 determines the duration of hepatectomy-induced hepatic steatosis. A transcriptomic analysis of normal adult liver identified “lipid metabolism regulation” as a major E2F2 functional target, suggesting that E2F2 has a role in lipid homeostasis. Here we use wild-type (E2F2^+/+) and E2F2 deficient (E2F2^−/−) mice to investigate the in vivo role of E2F2 in the composition of liver lipids and fatty acids in two metabolically different contexts: quiescence and 48-h post-PH, when cellular proliferation and anabolic demands are maximal. We show that liver regeneration is accompanied by large triglyceride and protein increases without changes in total phospholipids both in E2F2^+/+ and E2F2^−/− mice. Remarkably, we found that the phenotype of quiescent liver tissue from E2F2^−/− mice resembles the phenotype of proliferating E2F2^+/+ liver tissue, characterized by a decreased phosphatidylcholine to phosphatidylethanolamine ratio and a reprogramming of genes involved in generation of choline and ethanolamine derivatives. The diversity of fatty acids in total lipid, triglycerides and phospholipids was essentially preserved on E2F2 loss both in proliferating and non-proliferating liver tissue, although notable exceptions in inflammation-related fatty acids of defined phospholipid classes were detected. Overall, our results indicate that E2F2 activity sustains the hepatic homeostasis of major membrane glycerolipid components while it is dispensable for storage glycerolipid balance.

Surface behavior of sphingomyelins with very long chain polyunsaturated fatty acids and effects of their conversion to ceramides

Molecular species of sphingomyelin (SM) with nonhydroxy (n) and 2-hydroxy (h) very long chain polyunsaturated fatty acids (n- and h-28:4, 30:5, and 32:5) abound in rat spermatogenic cells and spermatozoa. These SMs are located on the sperm head, where they are converted to the corresponding ceramides (Cer) after the completion of the acrosomal reaction, as induced in vitro. The aim of this study was to look into the surface properties of these unique SM species and how these properties change by the SM → Cer conversion. After isolation by HPLC, these SMs were organized in Langmuir films and studied alone, in combination with different proportions of Cer, and during their conversion to Cer by sphingomyelinase. Compression isotherms for all six SMs under study were compatible with a liquid-expanded (LE) state and showed large molecular areas. Only the longest SMs (n-32:5 and h-32:5 SM) underwent a phase transition upon cooling. Interestingly, the abundant h-28:4 Cer exhibited a highly compressible liquid-condensed (LC) phase compatible with a high conformational freedom of Cer molecules but with the characteristic low diffusional properties of the LC phase. In mixed films of h-28:4 SM/h-28:4 Cer, the components showed favorable mixing in the LE phase. The monolayer exhibited h-28:4 Cer-rich domains both in premixed films and when formed by the action of sphingomyelinase on pure h-28:4 SM films. Whereas the SMs from sperm behaved in a way similar to that of shorter acylated SMs, the corresponding Cers showed atypical rheological properties that may be relevant to the membrane structural rearrangements that take place on the sperm head after the completion of the acrosomal reaction.

Altered lipid homeostasis in Sertoli cells stressed by mild hyperthermia

Spermatogenesis is known to be vulnerable to temperature. Exposures of rat testis to moderate hyperthermia result in loss of germ cells with survival of Sertoli cells (SC). Because SC provide structural and metabolic support to germ cells, our aim was to test the hypothesis that these exposures affect SC functions, thus contributing to germ cell damage. In vivo, regularly repeated exposures (one of 15 min per day, once a day during 5 days) of rat testes to 43 °C led to accumulation of neutral lipids. This SC-specific lipid function took 1-2 weeks after the last of these exposures to be maximal. In cultured SC, similar daily exposures for 15 min to 43 °C resulted in significant increase in triacylglycerol levels and accumulation of lipid droplets. After incubations with [3H]arachidonate, the labeling of cardiolipin decreased more than that of other lipid classes. Another specifically mitochondrial lipid metabolic function, fatty acid oxidation, also declined. These lipid changes suggested that temperature affects SC mitochondrial physiology, which was confirmed by significantly increased degrees of membrane depolarization and ROS production. This concurred with reduced expression of two SC-specific proteins, transferrin, and Wilms' Tumor 1 protein, markers of SC secretion and differentiation functions, respectively, and with an intense SC cytoskeletal perturbation, evident by loss of microtubule network (α-tubulin) and microfilament (f-actin) organization. Albeit temporary and potentially reversible, hyperthermia-induced SC structural and metabolic alterations may be long-lasting and/or extensive enough to respond for the decreased survival of the germ cells they normally foster.

Cell-type-specific regulation of genes involved in testicular lipid metabolism: fatty acid-binding proteins, diacylglycerol acyltransferases, and perilipin 2

Male germ cell differentiation entails the synthesis and remodeling of membrane polar lipids and the formation of triacylglycerols (TAGs). This requires fatty acid-binding proteins (FABPs) for intracellular fatty acid traffic, a diacylglycerol acyltransferase (DGAT) to catalyze the final step of TAG biosynthesis, and a TAG storage mode. We examined the expression of genes encoding five members of the FABP family and two DGAT proteins, as well as the lipid droplet protein perilipin 2 (PLIN2), during mouse testis development and in specific cells from seminiferous epithelium.Fabp5expression was distinctive of Sertoli cells and consequently was higher in prepubertal than in adult testis. The expression ofFabp3increased in testis during postnatal development, associated with the functional differentiation of interstitial cells, but was low in germ cells.Fabp9, together withFabp12, was prominently expressed in the latter. Their transcripts increased from spermatocytes to spermatids and, interestingly, were highest in spermatid-derived residual bodies (RB). Both Sertoli and germ cells, which produce neutral lipids and store them in lipid droplets, expressedPlin2. Yet, whileDgat1was detected in Sertoli cells,Dgat2accumulated in germ cells with a similar pattern of expression asFabp9. These results correlated with polyunsaturated fatty acid-rich TAG levels also increasing with mouse germ cell differentiation highest in RB, connecting DGAT2 with the biosynthesis of such TAGs. The age- and germ cell type-associated increases inFabp9,Dgat2, andPlin2levels are thus functionally related in the last stages of germ cell differentiation.

Unique thermal behavior of sphingomyelin species with nonhydroxy and 2-hydroxy very-long-chain (C28-C32) PUFAs

In rat germ cells and spermatozoa, sphingomyelin (SM) contains molecular species with nonhydroxy (n) and 2-hydroxy (h) very-long-chain polyunsaturated fatty acids (V), the most abundant being SMs with (n- and h-) 28:4n-6, 30:5n-6, and 32:5n-6 as acyl chains. The aim of this study was to gain information about their thermotropic behavior and interactions with other lipids. After isolation from rat testis, multilamellar and giant unilamellar vesicles from these SMs were examined using fluorescent probes. Only n-32:5 SM and h-32:5 SM displayed a gel-liquid transition temperature (Tt ∼ 21-22°C), the rest remaining in the liquid state in the 5°C-45°C range. The degree of order was larger in bilayers of any of the h-V SMs than in those of their chain-matched n-V SMs. Both, but n-V SM relatively more than h-V SM, decreased the Tt of dimyristoylphosphatidylcholine as their proportion increased in binary phosphatidylcholine:SM liposomes. In contrast to the established ability of 16:0 SM to form lateral cholesterol/SM-rich ordered domains in ternary dioleoylphosphatidylcholine:cholesterol:SM bilayers, neither n-V SM nor h-V SM showed a tendency to do so. Thus, these SMs are in the fluid state and are not involved in this type of domains in spermatozoa at physiological temperatures. However, this state could be altered at the very low temperatures at which these gametes are usually preserved.

Partition profile of the nicotinic acetylcholine receptor in lipid domains upon reconstitution

The nicotinic acetylcholine receptor (AChR) is in intimate contact with the lipids in its native membrane. Here we analyze the possibility that it is the intrinsic properties of the AChR that determine its partition into a given lipid domain. Torpedo AChR or a synthetic peptide corresponding to the AChR M4 segment (the one in closer contact with lipids) was reconstituted into "raft"-containing model membranes. The distribution of the AChR was assessed by Triton X-100 extraction in combination with fluorescence studies, and lipid analyses were performed on each sample. The influence of rapsyn, a peripheral protein involved in AChR aggregation, was studied. Raft-like domain aggregation was also studied using membranes containing the ganglioside GM1 followed by GM1 crosslinking. The gammaM4 peptide displays a marked preference for raft-like domains. In contrast, AChR alone or in the presence of rapsyn or ganglioside aggregation exhibits no such preference for raft-like domains, but it does cause a significant reduction in the total amount of these domains. The results indicate that the distribution of the AChR in lipid domains cannot be due exclusively to the intrinsic physicochemical properties of the protein and that there must be an external signal in native cell membranes that directs the AChR to a specific membrane domain.

Differential involvement of rat sperm choline glycerophospholipids and sphingomyelin in capacitation and the acrosomal reaction

Rat spermatozoa main lipid classes and their fatty acids were studied to assess their possible changes in capacitation and the acrosomal reaction (AR), induced in vitro. Capacitation-associated protein tyrosine phosphorylation, and the efflux of 30% of the total cholesterol from gametes to the medium, took place concomitantly with the release of a similar percentage, i.e., a larger amount, of the total phospholipid, mostly after hydrolysis of the major choline glycerophospholipids (CGP). Main medium lipid metabolites after capacitation were lyso-CGP and polyenoic fatty acids typical of CGP (22:4n-9, 22:5n-6), as free fatty acids (FFA). The AR, induced by a calcium ionophore, resulted in further phospholipid loss, but the produced metabolites remained in the gametes. CGP decrease in AR accounted for some additional FFA and lyso-CGP, but mostly for (22:5n-6-rich) diglycerides. Hydrolysis of sphingomyelins (SM) to ceramides also occurred, mostly affecting species with very long chain polyenoic fatty acids. Quantitatively, CGP and SM were the lipid classes decreasing the most after capacitation and AR, respectively. The massive cholesterol and phospholipid loss from the gametes during capacitation is thus associated with protein phosphorylation, a function that has been located to the sperm tail. The lipid metabolites produced during AR, by accumulating in the gamete heads, could be implicated in sperm-oocyte interactions.

Ceramides with 2-hydroxylated, very long-chain polyenoic fatty acids in rodents: From testis to fertilization-competent spermatozoa

Sphingolipids from rodent testis and spermatozoa are known to contain non-hydroxylated (N-) and 2-hydroxylated (2-OH) very-long-chain polyunsaturated fatty acids (VLCPUFA). In this study, the contribution of species with each type of fatty acids to the total ceramides (Cer) and sphingomyelins (SM) was investigated in rat and mouse testis and in rat spermatozoa. The major VLCPUFA in both lipids of testis were N- and 2-OH versions of 28:4n-6, 30:5n-6 and 32:5n-6 in the rat, and predominantly of 30:5n-6 in the mouse. Absent altogether from rat pre-puberal testes, SM and Cer with N-VLCPUFA appeared 10 days earlier than those with 2-OH VLCPUFA in postnatal development, in association with germ cell differentiation. Conversely, in adult fertile rats that were gradually deprived of germ cells in vivo after treatment with doxorubicin, SM and Cer with N-VLCPUFA decreased earlier than their 2-OH counterparts, and neither was present in aspermatogenic testes. In rat epididymal spermatozoa, the content of Cer prevailed over that of SM and 2-OH VLCPUFA prevailed over N-VLCPUFA in both lipids. In mature gametes, the acrosomal reaction resulted in an almost complete hydrolysis of the species of SM that contain both types of VLCPUFA to produce the corresponding Cer. Ceramides are biosynthetic precursors of SM in the testis, but themselves final products in spermatozoa. VLCPUFA-rich SM and Cer are thus produced in germ cells with the teleological objective of fulfilling their ultimate physiological role in spermatozoa that are apt and ready to fertilize an oocyte.

Differentiation-related changes in lipid classes with long-chain and very long-chain polyenoic fatty acids in rat spermatogenic cells

In rat seminiferous tubules (ST), cells that contain polar and neutral lipids with long-chain polyenoic fatty acids (PUFA) and sphingomyelins (SM) and ceramides (Cer) with very long chain (VLC) PUFA of the n-6 series coexist. In this study, pachytene spermatocytes and round spermatids were isolated to determine how these lipids change during spermatogenesis. As the amount per cell of PUFA-rich glycerophospholipids (GPL) decreased with cell size, the 22:5/20:4 ratio increased with cell differentiation. The elovl2 and elovl5 genes, required for 22:5 formation, were expressed (mRNA) in both cell types. Residual bodies- particles with compacted organelles and materials discarded from late spermatids-concentrated cholesterol, 22:5-rich triacylglycerols, and GPL, including plasmalogens and phosphatidylserine. Species of SM and Cer with nonhydroxylated (n-) VLCPUFA (28:4, 30:5, and 32:5) predominated in pachytene spermatocytes, whereas species with the corresponding 2-hydroxy (2-OH) VLCPUFA prevailed in round spermatids. Thus, a dramatic increase in the 2-OH/n-VLCPUFA ratio in SM and Cer was a hallmark of differentiation. A substantial decrease of 2-OH SM occurred between spermatids and mature spermatozoa and 2-OH SM species were collected in residual bodies "en route" to Sertoli cells. Notably, spermatids and spermatozoa gained a significant amount of ceramides devoid of n-VLCPUFA but having 2-OH VLCPUFA as their main fatty acids.

Sequential depletion of rat testicular lipids with long-chain and very long-chain polyenoic fatty acids after X-ray-induced interruption of spermatogenesis

When a single dose of X-rays is applied to the adult rat testis, stem spermatogonia are damaged, and spermatogenesis is interrupted. Supported by Sertoli cells, spermatogenic cells that endure irradiation complete their differentiation and gradually leave the testis as spermatozoa. In this study, the in vivo changes taking place a number of weeks after irradiation revealed cell-specific features of testicular lipid classes. A linear drop, taking about six weeks, in testis weight, nonlipid materials, free cholesterol, and 22:5n-6-rich glycerophospholipids took place with germ cell depletion. Sphingomyelins and ceramides with nonhydroxy very long-chain polyenoic fatty acids (n-VLCPUFA) disappeared in four weeks, together with the last spermatocytes, whereas species with 2-hydroxy VLCPUFA lasted for six weeks, disappearing with the last spermatids and spermatozoa. The amount per testis of 22:5n-6-rich triacylglycerols, unchanged for four weeks, fell between weeks 4 and 6, associating these lipids with spermatids and their residual bodies, detected as small, bright lipid droplets. In contrast, 22:5n-6-rich species of cholesterol esters and large lipid droplets increased in seminiferous tubules up to week 6, revealing they are Sertoli cell products. At week 30, the lipid and fatty acid profiles reflected the resulting permanent testicular involution. Our data highlight the importance of Sertoli cells in maintaining lipid homeostasis during normal spermatogenesis.

Doxorubicin Affects Testicular Lipids with Long-Chain (C18-C22) and Very Long-Chain (C24-C32) Polyunsaturated Fatty Acids

Doxorubicin disrupts spermatogenesis by causing apoptosis of spermatogonia and primary spermatocytes. The aim of this study was to examine the effect of this agent on adult rat testicular lipids and their fatty acids. A single dose (7.5 mg/kg) and a multidose regime (3 mg/kg once a week for 4 weeks) were evaluated. Both treatments resulted in the gradual loss of spermatogenic cells and determined a marked reduction in testicular size and weight 9 weeks after their start. Germ cell loss was accompanied by a decrease in phospholipids, including glycerophospholipids and sphingomyelin. Concomitantly, glycerophospholipids lost selectively their major polyunsaturated fatty acid (PUFA), 22:5n-6, and sphingomyelin lost its major very long-chain PUFA (VLCPUFA), 28:4n-6 and 30:5n-6. The molecular species from which the lost polyenes originated were thus a trait of germ cells. A transient peak of 16:0-ceramide was observed 48 h after the single dose. In both doxorubicin regimes, sphingomyelin and ceramide with reduced amounts of VLCPUFA after about 4 weeks and with no VLCPUFA after 9 weeks resulted. By contrast, triglycerides and especially cholesterol esters (CE) tended to accumulate in the testes undergoing germ cell death, probably in the surviving Sertoli cells, their fatty acid patterns suggesting that initially, these lipids retained part of the PUFA coming from, or no longer used for, the synthesis of germ cell glycerophospholipids. As the latter decreased, CE accumulated massively 9 weeks after starting doxorubicin treatment, 20:4n-6 becoming their major PUFA. Part of these CEs may derive from surviving steroidogenic cells. [Cancer Res 2007;67(14):6973–80]

Changes in lipids containing long- and very long-chain polyunsaturated fatty acids in cryptorchid rat testes

The aim of the present study was to examine the effects of experimental cryptorchidism on rat testicular phospholipids and neutral lipids that contain long-chain (C(18)-C(22)) and very long-chain (VLC) (C(24)-C(32)) polyunsaturated fatty acids (PUFA). The weight of the cryptorchid testis was nearly half that of the contralateral control at postsurgical Days 7-10 owing to the depletion of germ cells. Concomitantly, the amounts of major glycerophospholipids (GPL) and sphingomyelin (SM) per testis decreased. Both these lipids lost their characteristic long-chain and very long-chain PUFA, notably 22:5n-6 and 28:4n-6, respectively, which suggests that these species are linked to the membranes of germ cells. In contrast, the amounts and concentrations of triglycerides (TG; triacylglycerols and 1-O-alkyl-2,3-diacylglycerols) and cholesterol esters (CE) increased several fold in the surviving cells (mainly Sertoli cells) in the cryptorchid testis. All these neutral lipids, but especially CE, accumulated large amounts of the major PUFA of the testis, 22:5n-6, as well as pentaenes with longer carbon chains (i.e., 24:5n-6 in TG and 28:5n-6 in CE). This accretion suggests that neutral lipids may store preformed PUFA coming from dying germ cell GPL and also VLCPUFA no longer needed as a source of PUFA destined to assemble new germ cell GPL. The lipid adjustments observed in cryptorchidism suggest a possible role for Sertoli cell CE in the turnover and conservation of PUFA within seminiferous tubules.

Very long-chain polyunsaturated fatty acids are the major acyl groups of sphingomyelins and ceramides in the head of mammalian spermatozoa

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 ( approximately 70%) 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 degrees 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 sperm-capacitating 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.

Ceramides and sphingomyelins with high proportions of very long-chain polyunsaturated fatty acids in mammalian germ cells

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.

Influence of the fatty acid composition of lipids in chylomicron remnants derived from fish or corn oil on the lipid profile of cultured rat hepatocytes

The aim of this work was to characterise the lipid and fatty acid composition of chylomicron remnants enriched in n-3 or n-6 polyunsaturated fatty acids (PUFA) and to investigate their influence on the fatty acid profiles of the lipids of rat hepatocytes cultured in monolayers. Chylomicrons were prepared from the lymph collected from the thoracic duct of rats given an oral dose of fish or corn oil (high in n-3 and n-6 PUFA, respectively), and remnants were prepared in vitro from such chylomicrons using rat plasma containing lipoprotein lipase. The fatty acids predominating in the oils abounded also in their respective chylomicrons and remnants, especially in triacylglycerols. Chylomicrons as well as remnants contained small amounts of phospholipids and long-chain PUFA that were minor in, or absent from, the dietary oils, evidently provided by the intestinal epithelium. The incubation of hepatocytes for 6 h, with either n-3 or n-6 PUFA-rich remnants (0.25-0.75 mM triacylglycerol) resulted in a dose-dependent increase in the amount of triacylglycerols and phospholipids in the cells, which was not affected further by increasing the incubation time to 19 h. Whereas hepatocyte triacylglycerols mostly incorporated the PUFA predominating in each remnant type, the fatty acid profile of cell phospholipids was virtually unchanged. In addition, irrespective of whether they were enriched in n-3 or n-6 PUFA, remnants promoted a relative decrease in the amount of cholesteryl esters, a minor hepatocyte lipid class poor in PUFA. The results demonstrate that the hepatocyte fatty acid profile is modulated in a lipid-class specific way by the amount and type of dietary PUFA delivered to cells in chylomicron remnants.

Very long chain PUFA in murine testicular triglycerides and cholesterol esters

Very long chain (VLC) PUFA of the n-6 and n-3 series are known to occur in mammalian testis. The aim of this work was to characterize further two testicular lipid classes with VLCPUFA, cholesterol esters (CE) and total triglycerides (TG) in rat and mouse testis. The VLCPUFA predominating in these lipids were a series of n-6 pentaenes and tetraenes with 24 to 32 carbons, including small amounts of odd-chain PUFA, 28:5n-6 and 24:5n-6 prevailing in CE and TG, respectively. Most of the VLCPUFA of TG were concentrated in a small fraction of TG, made up by 1-O-alkyl-2,3-DAG. This TG subclass was absent altogether from the TG of sexually immature testis. The TG and the CE with VLCPUFA only occurred in testis of adult fertile animals. The proportion of VLCPUFA in total TG and CE was higher in rodents than in other mammals. In the n-6 PUFA-rich adult mouse testis, the amounts of testicular triacylglycerols decreased significantly after consumption of fish oil for 2 wk. Whereas 18:2n-6 was significantly reduced, the amounts of 22:5n-6 and longer n-6 PUFA were less affected in all major testicular lipids including PC and PE, where they were unchanged. The 1-O-alkyl-2,3-DAG and their n-6 VLCPUFA were virtually unaffected by the diet. The VLCPUFA-containing molecular species of CE and TG may represent a form of storage of cholesterol and polyenoic FA required to sustain spermatogenesis. Via chain-shortening, VLCPUFA stored in the neutral lipids may serve as precursors of the major C22 PUFA typical of cell membrane glycerophospholipids, protecting testicular cells against shifts in FA composition induced by dietary changes.

Thermal behavior of liposomes containing PCs with long and very long chain PUFAs isolated from retinal rod outer segment membranes

About one-fourth the phosphatidylcholines (PC) from retina photoreceptor rod outer segment (ROS) membranes contain docosahexaenoic acid (22:6n-3) at sn-2 and a very long chain polyunsaturated fatty acid (VLCPUFA) (C24 to C36) at the sn-1 position of the glycerol backbone. In order to study the thermotropic behavior of these PCs, subfractions and molecular species of PC (16:0/22:6, 18:0/22:6, 22:6/22:6, 32:5/22:6, 32:6/22:6, 34:5/22:6), were isolated from bovine ROS, and liposomes containing different proportions of these PCs and dimyristoyl-PC (DMPC) or dipalmitoyl PC (DPPC) were compared using the fluorescence probes Laurdan and 1,6-diphenyl-1,3,5-hexatriene (DPH). With both probes, the 22:6n-3 containing PCs from ROS, in all proportions tested, decreased the transition temperature (Tt) of both DMPC and DPPC. Below the transition temperature, coexistence of phases was evidenced in all cases. Liposomes formed with 100% of any of these PCs did not show phase transitions in the temperature range studied (8 degrees C to 50 degrees C). At physiological temperatures, as it is likely to be the case in ROS membranes, all of these PC species were in the liquid-crystalline state. With Laurdan, all dipolyunsaturated PCs seemed to behave similarly: despite the large number of double bonds per molecule, all of them decreased the Tt of DPPC less than did the hexaenoic PCs. With DPH, an ample difference was detected between the dipolyunsaturates, 22:6/22:6-PC and VLCPUFA/22:6-PCs, and between the latter and hexaenoic PCs throughout the temperature range studied. This difference is consistent with the interpretation that the largest "disorder" produced by PCs containing a VLCPUFA like 32:6n-3 at the sn-1 position occurs toward the center of the membrane.

Major plasma lipids and fatty acids in four HDL mammals

Lipid classes and their fatty acids were compared in plasma from four mammals: a laboratory rodent, the mouse; two domestic animals, the cat and dog; and a wild animal, the South American armadillo, Chaetophractus villosus. In all, the most abundant lipoprotein was high-density lipoprotein (HDL). In the total lipid of plasma, phospholipids (PL) predominated in all four species, in correlation with the proportion of HDL, both being largest in dogs. The major PL was phosphatidylcholine (PC), followed by sphingomyelin (SM) and lysophosphatidylcholine. The total plasma lipid from the four species contained long-chain n-6 polyunsaturated fatty acids as the predominant acyl groups, followed by comparable proportions of total saturated and monoenoic fatty acids and small percentages of n-3 PUFA. The percentages of these four major groups of fatty acids in PC, SM, triacylglycerols and cholesterol esters were similar among species, but showed significant differences in the ratios between major individual fatty acids composing these groups.

Lipid and fatty acid composition of canine lipoproteins

Lipid classes and their fatty acids were studied in the major lipoprotein fractions from canine, in comparison with human, plasma. In dogs, high-density-lipoprotein (HDL), the main carrier of plasma phospholipid (PL), cholesterol ester (CE) and free cholesterol, was the most abundant lipoprotein, followed by low and very-low density lipoproteins (LDL and VLDL). Notably, LDL and VLDL contributed similarly to the total dog plasma triacylglycerol (TG). The PL composition was similar in all three lipoproteins, dominated by phosphatidylcholine (PC). Even though the content and composition of lipids within and among lipoproteins differed markedly between dog and man, the total amount of circulating lipid was similar. All canine lipoproteins were relatively richer than those from humans in long-chain (C20-C22) n-6 and n-3 polyunsaturated fatty acids (PUFA) but had comparable proportions of total saturated and monoenoic fatty acids, with 18:2n-6 being the main PUFA in both mammals. The fatty acid profile of canine and human lipoproteins differed because they had distinct proportions of their major lipids. There were more n-3 and n-6 long-chain PUFA in canine than in human plasma, because dogs had more HDL, their HDL had more PC and CE, and both these lipids were richer in such PUFA.

Major clofibrate effects on liver and plasma lipids are independent of changes in polyunsaturated fatty acid composition induced by dietary fat

The effects of clofibrate on the content and composition of liver and plasma lipids was studied in mice fed for 4 wk on diets enriched in n-6 or n-3 polyunsaturated fatty acids (PUFA) from sunflower oil (SO) or fish oil (FO), respectively; both oils were fed at 9% of the diet (dry weight basis). Only FO was hypolipidemic. Both oil regimes led to slightly increased concentrations of phospholipids (PL) and triacylglycerols (TG) in liver as compared with a standard chow diet containing 2% fat. Clofibrate promoted hypolipidemia only in animals fed SO. Its main effect was to enlarge the liver, such growth increasing the amounts of major glycerophospholipids while depleting the TG. SO and FO consumption changed the proportion of n-6 or n-3 PUFA in liver and plasma lipids in opposite ways. After clofibrate action, the PUFA of liver PL were preserved better than in the absence of oil supplementation. However, most of the drug-induced changes (e.g., increased 18:1n-9 and 20:3n-6, decreased 22:6/20:5 ratios) occurred irrespective of lipids being rich in n-6 or n-3 PUFA. The concentration of sphingomyelin (SM), a minor liver lipid that virtually lacks PUFA, increased with the dietary oils, decreased with clofibrate, and changed its fatty acid composition in both situations. Thus, oil-increased SM had more 22:0 and 24:0 than clofibrate-decreased SM, which was significantly richer in 22:1 and 24:1.

Docosahexaenoic acid promotes differentiation of developing photoreceptors in culture

PURPOSE

The purpose of this work was to study the effects of diverse fatty acids on the composition, metabolism, differentiation, and characteristics of opsin expression in retina photoreceptors.

METHODS

Cultures of rat retinal neurons were incubated with or without 22:6 n-3, 22:5 n-3, 20:4 n-6, 18:1 n-9, and 16:0, labeled and unlabeled.

RESULTS

In photoreceptor cells incubated with 22:6 n-3 and 22:5 n-3, the proportions of these fatty acids in phospholipids increased four- to sixfold. The remaining fatty acids did not change lipid acyl chain composition. The labeled fatty acids were all actively esterified in neuronal lipids, particularly in phosphatidylcholine. Addition of unlabeled 22:6 n-3 did not affect the distribution among lipids of the other fatty acids but displaced [3H]20:4 n-6 from phosphatidylcholine and phosphatidylethanolamine. These results suggest that retinal neurons have specific mechanisms for processing fatty acids of different lengths and degrees of unsaturation and that 22:6 n-3 incorporation takes priority. Of all fatty acids, 22:6 n-3 was the most effective in promoting photoreceptor differentiation. In 22:6-sufficient photoreceptors, new apical processes formed, the expression of opsin augmented, and its localization improved, concentrating in the apical processes of the cells.

CONCLUSIONS

The advancement in differentiation selectively elicited by 22:6 correlates with the fact that 22:6 n-3, but none of the other fatty acids, delays significantly the onset of apoptosis in photoreceptors in culture. The synthesis of 22:6-containing phospholipid molecules could be required for the proper localization of opsin. This could contribute to furthering the differentiation of photoreceptors, preventing their apoptosis, and extending their survival.

Apoptosis of retinal photoreceptors during development in vitro: protective effect of docosahexaenoic acid

When rat retinal cells are cultured in a serum-free 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.

Active synthesis of C24:5, n-3 fatty acid in retina

The formation of 14C-labelled long-chain and very-long-chain (n-3) pentaenoic and hexaenoic fatty acids was studied in bovine retina by following the metabolism of. [14C]-docosapentaenoate [C22:5, n-3 fatty acid (22:5 n-3)], [14C]-docosahexaenoate (22:6 n-3), and [14C]acetate. With similar amounts of 22:5 n-3 and 22:6 n-3 as substrates, the former was actively transformed into 24:5 n-3, whereas the latter was virtually unmodified. Labelled 24:5, 26:5, 24:6 and 22:6 were formed from [1-14C]22:5 n-3, showing that pentaenoic fatty acids including 24:5 n-3 can be elongated and desaturated within the retina. When retinal microsomes were incubated with [1-14C]22:5 n-3, 24:5 n-3 was the only fatty acid formed. In retinas incubated with [14C]acetate, 24:5 n-3 was the most highly labelled fatty acid among the polyenes synthesized, 24:6 n-3 being a minor product. Such selectivity in the elongation of two fatty acids identical in length, 22:5 n-3 and 22:6 n-3, despite the fact that 22:5 is a minor and 22:6 a major fatty acid constituent of retina, suggests that the active formation of 24:5 n-3 plays a key role in n-3 polyunsaturated fatty acid (PUFA) metabolism. This compound might give rise to even longer pentaenes via elongation, and to the major PUFAs of retina, 22:6 n-3, by 6-desaturation and chain shortening. Of all retinal lipids, a minor component, triacylglycerol (TG), incorporated the largest amounts of [14C]22:5 and 22:6. TG also concentrated most of the [14C]24:5 formed in retina, whether from [14C]22:5 n-3 or from [14C]acetate, suggesting an important role for this lipid in supporting PUFA metabolism and the synthesis of 22:6 n-3.

Docosahexaenoic acid is required for the survival of rat retinal photoreceptors in vitro

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.

Plasma phospholipid fatty acids in X-linked adrenoleukodystrophy

Blood cell and plasma lipid classes and their fatty acids were analyzed in a child with X-linked adrenoleukodystrophy. The increase in saturated fatty acids with very long chains typical of this disease occurred almost exclusively in sphingomyelin. In this lipid, the proportion of lignoceric (24:0) and hexacosanoic (26:0) acids increased while that of 18:0, 20:0, and 24:1 decreased. In the rest of the lipid classes, but especially in cholesteryl esters and triacylglycerols, the proportion of linoleate (18:2) decreased while that of oleate (18:1) increased. In glycerophospholipids, polyunsaturated fatty acids such as 20:4n-6, 22:5n-6, and 22:6n-3 were reduced while their immediate precursors, 20:3n-6, 22:4n-6, and 22:5n-3, respectively, were relatively increased, suggesting a defect in fatty acid desaturation mechanisms. Although less pronounced, a similar trend of changes was seen in the patient's mother; in both, all alterations were more marked in serum than in blood cells.

Plasma phospholipid fatty acids in X-linked adrenoleukodystrophy

Blood cell and plasma lipid classes and their fatty acids were analyzed in a child with X-linked adrenoleukodystrophy. The increase in saturated fatty acids with very long chains typical of this disease occurred almost exclusively in sphingomyelin. In this lipid, the proportion of lignoceric (24:0) and hexacosanoic (26:0) acids increased while that of 18:0, 20:0, and 24:1 decreased. In the rest of the lipid classes, but especially in cholesteryl esters and triacylglycerols, the proportion of linoleate (18:2) decreased while that of oleate (18:1) increased. In glycerophospholipids, polyunsaturated fatty acids such as 20:4n-6, 22:5n-6, and 22:6n-3 were reduced while their immediate precursors, 20:3n-6, 22:4n-6, and 22:5n-3, respectively, were relatively increased, suggesting a defect in fatty acid desaturation mechanisms. Although less pronounced, a similar trend of changes was seen in the patient's mother; in both, all alterations were more marked in serum than in blood cells.

Effects of clofibrate on lipids and fatty acids of mouse liver

Clofibrate administration significantly altered the amount and fatty acid composition of lipids in mouse liver. The net content of phospholipids (PL) increased and that of triacylglycerols (TG) decreased concomitantly with liver enlargement in mice treated for two weeks with this drug (0.5% w/w in the food). The highest increase among PL was in phosphatidylcholine; other components either showed lower increases or, as in the case of sphingomyelin and the plasmalogens, decreased. In all lipid classes the treatment resulted in altered ratios between major saturates, between saturates and monoenes, and between major polyenes. Among these, 20:3n-6 and 22:5n-3 increased several-fold, and the 20:3n-6/20:4n-6 and 22:5n-3/22:6n-3 ratios increased due to a more active formation of the precursors than of the corresponding products. This change affected all glycerolipid classes. Liver sphingomyelin showed a relative enrichment in monoenoic fatty acids like 22:1 and 24:1, caused by a net decrease in the amount of saturates, particularly 22:0 and 24:0. The stimulated membrane proliferation imposed by clofibrate must increase phospholipid synthesis and, hence, the need for fatty acids. The results suggest that these demands are met mostly by TG acyl groups, either directly or after oxidation/desaturation processes. This was apparently the case for the polyenoic fatty acids of the n-6 and n-3 series. The longer chain (C22 and C24) components decreased, suggesting that their oxidation was stimulated to provide part of the required (C20 and C22) polyenes.

Phospholipid solubilization during detergent extraction of rhodopsin from photoreceptor disk membranes

The solubilization of rhodopsin and phospholipids from disks prepared from bovine retinal rods was studied using five different detergents. The relative amounts of rhodopsin and lipid extracted during membrane solubilization differed dramatically with the nature of the surfactant; the two nonpolar detergents, Emulphogene (polyoxyethylene-10 tridecylether) and octylglucoside, removed more protein than lipid; two bile salt-related detergents, 3-[(3-cholamidopropyl)dimethylammonio]-1-propane sulfonate (Chaps) and taurocholate, released relatively more lipid than protein; and digitonin, which shares characteristics with both groups of detergents, extracted more lipid per mole of rhodopsin than the former two but less than the latter two. Solubilization was temperature-dependent with all five detergents, though particularly so with octylglucoside: concentrations adequate for the total micellation of disks at 23 degrees C were ineffectual at 4 degrees C. In total solubilizates of disks, the amount of lipid recovered in rhodopsin-lipid-detergent micelles showed a closer correlation with the critical micellar concentration (CMC) than with the chemical nature of the detergent (octylglucoside > taurocholate > Chaps > digitonin > Emulphogene). The higher the CMC, the larger the amount of lipid associated to the solubilized rhodopsin and the larger the amount of lipid reassociated to rhodopsin upon surfactant dilution. For all five detergents, the lipid progressively extracted from disks during solubilization was relatively richer in phosphatidylcholine (PC) than the lipid in the original membranes. The lipid which tended to be associated with rhodopsin in protein-lipid-detergent mixed micelles was also consistently richer in PC than that present in lipid-detergent micelles. Bleaching of solubilized rhodopsin decreased the amount of lipid in protein-lipid-detergent micelles. Rhodopsin photolytic transitions were faster in nonionic than in bile salt-related detergents.

Long and very long chain polyunsaturated fatty acids of the n-6 series in rat seminiferous tubules. Active desaturation of 24:4n-6 to 24:5n-6 and concomitant formation of odd and even chain tetraenoic and pentaenoic fatty acids up to C32

The formation of long and very long chain (VLC) n-6 polyunsaturated fatty acids (PUFA) in isolated rat seminiferous tubules was investigated by following the metabolism of three 1-14C-labeled n-6 tetraenoic fatty acids (20:4, 24:4, and 32:4) and [U-14C]acetate. In contrast to [14C]32:4, which was poorly incorporated and altered, [14C]20:4 and [14C]24:4 were efficiently taken up by the tubules, esterified into lipids, elongated to VLCPUFA, and desaturated to pentaenoic fatty acids; the rate of [14C]24:4 desaturation to [14C]24:5 was notably high. The main products with [14C]acetate as precursor were labeled saturates and VLCPUFA, most of the label in tetraenoic and pentaenoic acids appearing in 24:4 and 24:5, respectively. These two C24 polyenes, connected by a delta 6 desaturation, may play a central role in n-6 PUFA metabolism, in their capacity as potential precursors of longer polyenes via elongation and of shorter ones, such as 22:5n-6, via retroconversion. Triacylglycerols, rich in C22 and C24 polyenes, incorporated the greatest amounts of both [14C] acetate-derived and exogenous 14C-PUFA, suggesting that this lipid class is involved in the traffic and metabolism of testicular PUFA. The detection of a series of unusual odd-chain tetraenoic and pentaenoic acids, also labeled with [U-14C]acetate, suggests that a PUFA chain shortening mechanism occurs in testis involving alpha- in addition to beta-oxidation. We speculate that alpha-oxidation plays a role in the retroconversion of PUFA.

Occurrence of long and very long polyenoic fatty acids of the n-9 series in rat spermatozoa

Dietary deficiency of essential fatty acids of the n-3 and n-6 series is known to promote a compensatory increase in polyenoic fatty acids of the n-9 series in the lipids of mammalian tissues. In the present study long-chain n-9 polyenes were found to be normal components of the epididymis and especially of sperm isolated from that tissue, in healthy, well-fed, fertile rats maintained on essential fatty acid-sufficient diets. The n-9 polyenes occurred in large concentrations in the choline glycerophospholipids (CGP), the major phospholipid class of spermatozoa in epididymal cauda, and were highly concentrated in plasmenylcholine, the major subclass of CGP. The uncommon polyene 22:4n-9 was found in the highest proportion, followed in order of relative abundance by 22:3n-9, 20:3n-9 and 24:4n-9. These polyenes were probably derived from oleate (18:1n-9) in much the same way as long-chain polyenes of the n-6 and n-3 series are derived from linoleate (18:2n-6) and linolenate (18:3n-3), respectively.

Lipid remodelling during epididymal maturation of rat spermatozoa. Enrichment in plasmenylcholines containing long-chain polyenoic fatty acids of the n-9 series

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.

Solubilization of myelin membranes by detergents

The major proteins of myelin have classically been extracted in organic solvents. Here we investigated some of the characteristics of brain myelin solubilization in aqueous detergent solutions. At comparable molar concentrations, two nonionic detergents, i.e., octyl glucoside and Lubrol PX, proved relatively better myelin solubilizers than the detergents related to the bile salts, i.e., cholate and CHAPS. The two former detergents solubilized more protein than lipid and the two latter ones more lipid than protein from myelin membranes. All four detergents solubilized the phospholipid more efficiently than the cholesterol component of myelin. The detergent concentrations required for myelin solubilization were reduced substantially if the temperature and the salt concentration of the media were increased. As much as 3 mg of lyophilized myelin (about 1 mg of protein) were solubilized readily per milliliter of a solution containing 30 mM octyl glucoside and 0.1 M sodium sulfate in 0.1 M sodium phosphate buffer, pH 6.7. Each of the detergents studied, including the above four, sodium dodecyl sulfate (SDS). Triton X-100, and Zwittergent 3-14, had its own advantages and drawbacks as myelin protein extractors. The nonionic amphiphiles and CHAPS left a small residue mainly composed of proteins of the Wolfgram fraction, as revealed by SDS-polyacrylamide gel electrophoresis. Octyl glucoside was preferred, given its versatility as solubilizer, ultraviolet transparency, and high critical micellar concentration. Observations on possible difficulties that may be encountered are also included.

Asymmetric distribution of phospholipids in acetylcholine receptor-rich membranes from T. marmorata electric organ

1. The distribution of phospholipids between the two leaflets of the lipid bilayer in acetylcholine receptor (AChR)-rich membranes from T. marmorata has been examined with two complementary techniques: chemical derivatization with the membrane-impermeable reagent trinitrobenzenesulphonate (TNBS) and B.cereus phospholipase C hydrolysis. 2. AChR-membranes were reacted with TNBS at 0-4 and 37 degrees C and the accessibility of their aminophospholipids was compared to that of rod outer segment and erythrocyte membranes. The results indicate that more of the total ethanolamine glycerophospholipid (EGP) than of the total phosphatidylserine (PS) is located in the outer monolayer. 3. Nearly half the phospholipid content of AChR membranes is hydrolyzed by phospholipase C with a half-time of ca. 1.6 min at 25 degrees C. Consistent with the TNBS results, more of the total EGP than of the total PS is degraded. Beyond 3 min the reaction slows down, relatively smaller additional amounts of lipids are hydrolyzed, and all phospholipid classes are attacked to a similar extent, indicating that after half the lipid is removed all phospholipids become accessible to the enzyme. 4. The results indicate that the outer leaflet of the bilayer is richer in ethanolamine and choline glycerophospholipids, whereas phosphatidylinositol, most of the sphingomyelin, and ca 65% of the PS are located on the inner leaflet.

Phospholipid species containing long and very long polyenoic fatty acids remain with rhodopsin after hexane extraction of photoreceptor membranes

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.

Synthesis of very long chain (up to 36 carbon) tetra, penta and hexaenoic fatty acids in retina

The synthesis of very long chain (C24 to C36) polyunsaturated (four, five and six double bonds) fatty acids (VLCPUFA) is investigated in bovine retina using [14C]acetate. Saturates on the one hand (mainly palmitate), and polyenes on the other (mainly VLCPUFA), incorporate most of the label found in lipids. Phosphatidylcholine (PC) is the most highly labelled lipid class, since both types of 14C-labelled fatty acids, but especially this novel series of VLCPUFA, are concentrated in this phospholipid. Radioactivity from [14C]acetate is found in very long chain tetra, penta and hexaenoic fatty acids of PC. The labelling of 20:4(n - 6), 20:5(n - 3), 22:5(n - 6) and 22:6(n - 3) is much lower than that of longer polyenes of each of these series, indicating that VLCPUFA are synthesized in situ by successive elongations of the above polyenes, pre-existing in retina lipids. In various subcellular fractions isolated from retinas after incubations with [14C]acetate (including cytosol, microsomes, mitochondria and photoreceptor membranes), the labelling of the VLCPUFA of PC is very high, even at relatively short intervals of incubation. The results suggest that not only the synthesis but also the intracellular traffic among membranes of VLCPUFA-containing species of PC are very active processes in the retina.

Lipid metabolism in electroplax

The in vivo labeling of electrocyte lipids is followed after injection of radioactive glycerol and two fatty acids, oleate and arachidonate, into the electric organ of an elasmobranch (Discopyge tschudii). De novo synthesis of lipids and acyl-exchange reactions are operative in the electrocyte. The three precursors are preferentially incorporated into phosphatidylcholine, phosphatidylinositol, and triacylglycerols. The highest specific activities are attained by triacylglycerols and polyphosphoinositides. Electrocyte stacks from electric organ show an efficient and continuous esterification of oleate and arachidonate into lipids after several hours of incubation. Except for an apparently more active labeling of triacylglycerols, which is attributed to the larger availability of free fatty acid precursors under the in vitro experimental conditions, the pattern of lipid labeling is similar to that attained in vivo. 32P-labeled lipids are also steadily produced in electrocyte stacks (24 h of incubation with [32P]phosphate) using glucose as the sole exogenous source of energy. Polyphosphoinositides are the lipids preferentially labeled. The ability to sustain the labeling of lipids under in vitro conditions renders isolated electrocyte stacks an interesting model for future research on lipid involvement in cholinergic function.

Composition of lipids in elasmobranch electric organ and acetylcholine receptor membranes

The composition of phospholipids from electric organ and from membranes enriched in acetylcholine receptors (AChRs) is analyzed in three elasmobranch fish (Torpedo marmorata, Torpedo californica, and Discopyge tschudii). Irrespective of their purity, AChR-containing membranes are similar to electric organ in lipid and fatty acid composition. The following characteristics are common to the three species: (a) Choline, ethanolamine, and serine glycerophospholipids account for 80-90% of the phospholipids. (b) Their major fatty acid constituents are monoenes, saturates, and long-chain (n-3) polyenes (especially docosahexaenoate). (c) A large proportion of the ethanolamine glycerophospholipids (30-50%) is made up by plasmenylethanolamine, which contains fewer polyenes than phosphatidylethanolamine per mole of lipid. (d) Polyphosphoinositides represent 20-30% of the inositides of electric organ. (e) Phosphatidylinositol and phosphatidate have large proportions of 20- and 22-carbon polyenes. (f) Diphosphatidylglycerol and triacylglycerols are rich in oleate but also contain long-chain polyenes. (g) Sphingomyelin has monoenes and saturates ranging from 14 to 26 carbons. Species-related variations are observed (a) in the ratios between some phospholipid classes and subclasses and (b) in the relative abundance of the major polyunsaturated acyl chains of phospholipids. Despite these differences, the average unsaturation and length of fatty acids in major phospholipid classes are similar for the three species.

Labeling of phosphatidylcholines of retina subcellular fractions by [1-14C]eicosatetraenoate (20:4(n-6)), docosapentaenoate (22:5(n-3)) and docosahexaenoate (22:6(n-3))

The labeling of molecular species of phosphatidylcholine (PC) has been studied in bovine retinas incubated for 2 h with (1-14C)-labeled (n-6) eicosatetraenoate (n-3) docosapentaenoate and (n-3) docosahexaenoate (20:4, 22:5 and 22:6, respectively) and in four subcellular fractions isolated after such incubations. Of the total radioactivity incorporated in PC, the following percentages of the above fatty acids, respectively, are found in its dipolyunsaturated species: 58, 56 and 53% in rod outer segments; 29, 41 and 49% in mitochondria; 24, 28 and 39% in microsomes; 12, 14 and 16% in postmicrosomal supernatants; 28, 36 and 58% in entire retinas. The remainder percentages are in tetra-, penta- and hexaenoic species of PC, respectively. The levels of pentaenoic species in the PCs of all fractions are similar, while tetraenes are lowest and hexaenes highest in photoreceptor membranes. Dipolyunsaturated species are highly concentrated in photoreceptor membranes, but are minor components of mitochondrial, microsomal and cytosolic PC. The specific radioactivities of tetraenoic, pentaenoic and hexaenoic PCs are decreasingly lower in the following order: postmicrosomal supernatants, microsomes, mitochondria, photoreceptor membranes. In contrast, the specific radioactivities of dipolyunsaturated PCs are higher in mitochondria and microsomes than in the other fractions, especially with 22:5 and 22:6. It is suggested that mitochondria as well as the endoplasmic reticulum could play a role in the synthesis and further modifications of dipolyunsaturated PCs before being supplied to photoreceptor membranes.

Labeling of lipids of retina subcellular fractions by [1-14C]eicosatetraenoate (20:4(n-6)) docosapentaenoate (22:5(n-3)) and docosahexaenoate (22:6(n-3))

(1-14C)-labeled (n-6) eicosatetraenoate, (n-3) docosapentaenoate and (n-3) docosahexaenoate (20:4, 22:5 and 22:6, respectively) are efficiently taken up and actively esterified into the lipids of bovine retina after 2 h incubation. Photoreceptor membranes, mitochondria, microsomes and postmicrosomal supernatants, which display significant differences in phospholipid and fatty acid compositions, are isolated after such incubations to study the labeling of lipids. The lipid classes preferentially labeled with the acids (1) largely differ among and within subcellular fractions, while (2) some common features in the treatment of the three polyenes are observed in each fraction. In all of them, the three acids are actively incorporated in phosphatidylcholine; ethanolamine glycerophospholipid, phosphatidylserine (PS) and phosphatidylinositol (PI) are highly labeled with 22:6, 22:5 and 20:4 respectively; within ethanolamine glycerophospholipid, the three label phosphatidylethanolamine in preference to plasmenylethanolamine. Most of the 14C esterified in mitochondria is in phospholipids. The endoplasmic reticulum produces in addition highly labeled triacylglycerols, also found in cytosol. High levels of 14C-labeled diacylglycerols are observed exclusively in photoreceptor membranes, where the specific radioactivity of PI is very high. The total amounts of 14C incorporated (1) are in general similar within a given fraction for the three polyenes, but (2) largely differ among fractions. The labeling of the highly unsaturated phospholipids of photoreceptor membranes is the lowest, while the postmicrosomal supernatant (whose lipids are relatively the poorest in polyenoic fatty acids) contains most of the labeled lipids isolated from retinas under these conditions. The results indicate that polyunsaturated species of retina phospholipids undergo an active synthesis and turnover, as well as an intense intracellular traffic among membranes.

Effects of aging on the composition and metabolism of docosahexaenoate-containing lipids of retina

The amount of docosahexaenoate (22:6n-3)-containing phospholipid species decreases with aging in the rat retina. Most lipids, but especially choline and serine glycerophospholipids, show a significant fall in 22:6n-3, which is not compensated by increases in other polyenoic fatty acids. The decrease not only affects 22:6 but also various very long chain n-3 hexaenoic fatty acids which, in phosphatidylcholine, have up to 36 carbon atoms, and which are probably synthesized by successive elongations of 22:6n-3. The in vitro incorporation of [2-3H]glycerol into retinal lipids indicates that the de novo biosynthetic pathways are not impaired by aging. The incorporation of [1-14C]docosahexaenoate is significantly stimulated into all lipids of aged retinas, but to the largest extent in those showing the largest decreases in 22:6, especially in choline glycerophospholipids. The results indicate that the decreased levels of 22:6 with aging are due not to an impaired activity of the enzymes involved in the synthesis and turnover of phospholipids but to a decreased availability of this polyene in the retina. It is suggested that this may stem from a defect in some of the enzymatic steps that lead to the synthesis of 22:6n-3, probably that catalyzed by delta 4 desaturase, the effect on longer hexaenes being secondary to the decreased synthesis of 22:6.

A novel group of very long chain polyenoic fatty acids in dipolyunsaturated phosphatidylcholines from vertebrate retina

Dipolyunsaturated phosphatidylcholines from bovine retina contain a whole series of unusual fatty acids. Methyl esters from these acids are very strongly retained on polar and nonpolar gas-liquid chromatography stationary phases. On thin layers of silica-AgNO3, they separate as tetra-, penta-, and hexaenoic fatty acid methyl esters. After hydrogenation, the three polyunsaturated fractions give the same series of saturated methyl esters, having 20 (or 22)-36 carbon atoms. High pressure liquid chromatography, as well as gas-liquid chromatography, indicates that the new components of the three fractions are even-carbon homologs of well known polyenoic fatty acids of the n-6 and n-3 families, since they behave as series of 20-36-carbon tetraenoic (n-6), pentaenoic (n-3 and n-6), and hexaenoic (n-3) fatty acids. Their occurrence in phospholipid molecules also having docosahexaenoate (22:6) explains the separation of major dipolyunsaturated phosphatidylcholines from retina into dodecaenoic, undecaenoic, and decaenoic fractions after argentation thin layer chromatography. Using high pressure liquid chromatography, the latter are resolved into individual species having 10-12 double bonds and 42-58 carbon atoms. The unusual PCs are thus endowed not only with the highest degree of unsaturation, but with the longest hydrocarbon chains yet reported for vertebrate glycerophospholipids. It is shown that phosphatidylcholines containing the novel fatty acids are highly concentrated in photoreceptor membranes and that they occur in the retina of vertebrates so distant in evolution as fish, birds, and various mammals.