Neutral sterols of rat epididymis: high concentrations of dehydrocholesterols in rat caput epididymidis

The importance of choosing the appropriate cholesterol quantification method: enzymatic assay versus gas chromatography

Cholesterol is a major lipid of the animal realm with many biological roles. It is an important component of cellular membranes and a precursor of steroid hormones and bile acids. It is particularly abundant in nervous tissues, and dysregulation of cholesterol metabolism has been associated with neurodegenerative diseases such as Alzheimer's and Huntington's diseases. Deciphering the pathophysiological mechanisms of these disorders often involves animal models such as mice and Drosophila. Accurate quantification of cholesterol levels in the chosen models is a critical point of these studies. In the present work, we compare two common methods, gas chromatography coupled to flame-ionization detection (GC/FID) and a cholesterol oxidase-based fluorometric assay to measure cholesterol in mouse brains and Drosophila heads. Cholesterol levels measured by the two methods were similar for the mouse brain, which presents a huge majority of cholesterol in its sterol profile. On the contrary, depending on the method, measured cholesterol levels were very different for Drosophila heads, which present a complex sterol profile with a minority of cholesterol. We showed that the enzyme-based assay is not specific for cholesterol and detects other sterols as well. This method is therefore not suited for cholesterol measurement in models such as Drosophila. Alternatively, chromatographic methods, such as GC/FID, offer the required specificity for cholesterol quantification. Understanding the limitations of the quantification techniques is essential for reliable interpretation of the results in cholesterol-related research.

Molecular changes and signaling events occurring in spermatozoa during epididymal maturation

After leaving the testis, spermatozoa have not yet acquired the ability to move progressively and are unable to fertilize oocytes. To become fertilization competent, they must go through an epididymal maturation process in the male, and capacitation in the female tract. Epididymal maturation can be defined as those changes occurring to spermatozoa in the epididymis that render the spermatozoa the ability to capacitate in the female tract. As part of this process, sperm cells undergo a series of biochemical and physiological changes that require incorporation of new molecules derived from the epididymal epithelium, as well as post-translational modifications of endogenous proteins synthesized during spermiogenesis in the testis. This review will focus on epididymal maturation events, with emphasis in recent advances in the understanding of the molecular basis of this process.

Sterols in spermatogenesis and sperm maturation

Mammalian spermatogenesis is a complex developmental program in which a diploid progenitor germ cell transforms into highly specialized spermatozoa. One intriguing aspect of sperm production is the dynamic change in membrane lipid composition that occurs throughout spermatogenesis. Cholesterol content, as well as its intermediates, differs vastly between the male reproductive system and nongonadal tissues. Accumulation of cholesterol precursors such as testis meiosis-activating sterol and desmosterol is observed in testes and spermatozoa from several mammalian species. Moreover, cholesterogenic genes, especially meiosis-activating sterol-producing enzyme cytochrome P450 lanosterol 14α-demethylase, display stage-specific expression patterns during spermatogenesis. Discrepancies in gene expression patterns suggest a complex temporal and cell-type specific regulation of sterol compounds during spermatogenesis, which also involves dynamic interactions between germ and Sertoli cells. The functional importance of sterol compounds in sperm production is further supported by the modulation of sterol composition in spermatozoal membranes during epididymal transit and in the female reproductive tract, which is a prerequisite for successful fertilization. However, the exact role of sterols in male reproduction is unknown. This review discusses sterol dynamics in sperm maturation and describes recent methodological advances that will help to illuminate the complexity of sperm formation and function.

Epididymis cholesterol homeostasis and sperm fertilizing ability

Cholesterol, being the starting point of steroid hormone synthesis, is a long known modulator of both female and male reproductive physiology especially at the level of the gonads and the impact cholesterol has on gametogenesis. Less is known about the effects cholesterol homeostasis may have on postgonadic reproductive functions. Lately, several data have been reported showing how imbalanced cholesterol levels may particularly affect the post-testicular events of sperm maturation that lead to fully fertile male gametes. This review will focus on that aspect and essentially centers on how cholesterol is important for the physiology of the mammalian epididymis and spermatozoa.

Cholesterol and desmosterol in two sperm populations separated on Sil-Select gradient

Sperm lipids are important for sperm viability, maturity and function. This study aimed to identify cholesterol and desmosterol composition of human spermatozoa of two sperm populations separated on Sil-Select gradient. Forty-eight males were divided into four groups namely healthy men (n = 13), asthenozoospermia (n = 11), asthenoteratozoospermia (n = 10) and oligoasthenoteratozoospermia (n = 14). Sperm cholesterol and desmosterol were estimated in two human sperm population separated by centrifugation in a discontinuous Sil-Select gradient. The results showed that cholesterol and desmosterol were the major sterols in human spermatozoa. Spermatozoa recovered from upper/lower layer interface (fraction I) had low fertilization potential, while those from the base (fraction II) had high fertilization potential. Median values of cholesterol and desmosterol in fraction I were 2.55 micromol and 0.77 micromol/10(9) spermatozoa and in fraction II were 1.16 micromol and 0.27 micromol/10(9) spermatozoa. Cholesterol/desmosterol ratio was significantly higher in fraction II than I (4.8 vs. 3.2, p < 0.01). Cholesterol, desmosterol, total phospholipids and sterols/phospholipids were negatively correlated with sperm concentrations, sperm motility, linear velocity, normal sperm morphology and acrosome reaction percentage whereas cholesterol/desmosterol ratio was positively correlated with these parameters. It is concluded that the difference in sterol composition of sperm subpopulations separated on Sil-Select gradient suggests that composition of sterols is related to sperm functions.

Comparison of cholesterol and its direct precursors along the biosynthetic pathway: effects of cholesterol, desmosterol and 7-dehydrocholesterol on saturated and unsaturated lipid bilayers

Despite extensive studies, the remarkable structure-function relationship of cholesterol in cellular membranes has remained rather elusive. This is exemplified by the fact that the membrane properties of cholesterol are distinctly different from those of many other sterols. Here we elucidate this issue through atomic-scale simulations of desmosterol and 7-dehydrocholesterol (7DHC), which are immediate precursors of cholesterol in its two distinct biosynthetic pathways. While desmosterol and 7DHC differ from cholesterol only by one additional double bond, we find that their influence on saturated lipid bilayers is substantially different from cholesterol. The capability to form ordered regions in a saturated (dipalmitoyl-phosphatidylcholine) membrane is given by cholesterol > 7DHC > desmosterol, indicating the important role of cholesterol in saturated lipid environments. For comparison, in an unsaturated (dioleoyl-phosphatidylcholine) bilayer, the membrane properties of all sterols were found to be essentially identical. Our studies indicate that the different membrane ordering properties of sterols can be characterized by a single experimentally accessible parameter, the sterol tilt. The smaller the tilt, the more ordered are the lipids around a given sterol. The molecular level mechanisms responsible for tilt modulation are found to be related to changes in local packing around the additional double bonds.

Testis specific lactate dehydrogenase as target for immunoliposomes

PROBLEM

Is it possible to deliver therapeutic agents to testis through specific targeting?

METHOD OF STUDY

Immunoliposomes are designed by incorporating antibodies to lactate dehydrogenase-C4 (LDH-C(4)), which is the product of a testis specific gene. Their targeting and delivering ability is investigated in vitro and in vivo.

RESULTS

It is observed that LDHC(4)-immunoliposomes are able to discriminate and recognize antigens on spermatozoa and testes both in vitro and in vivo.

CONCLUSION

Specific targeting through LDH-C(4) appears to be a feasible strategy for delivering therapeutic as well as anti-spermatogenic agents to testis.

Pre-cholesterol precursors in gametogenesis

Meiosis activating sterols (MAS) are biologically active post-lanosterol intermediates of cholesterol biosynthesis that are synthetized primarily in the gonads, including the sperm. MAS reinitiate the meiosis of oocytes in vitro while in vivo they seem to contribute to the oocyte quality and the progression of meiosis. The mRNAs for the MAS-producing enzyme lanosterol 14alpha-demethylase (CYP51) arise by alternative poly (A) signal selection. Only signals with low cleavage activity are used in the testis. Translation of mammalian CYP51s starts at one of the tandem in-frame ATGs. CYP51 protein of the bull is shorter compared to the human due to the usage of a more downstream translation start site. CYP51 proteins are post-translationally modified by glycosylations in the Golgi and on acrosomal membranes of the sperm. Green fluorescence protein-based ex vivo system has been developed to aid studying the intracellular transport of the MAS-producing CYP51. The influence of the post-translational modifications on MAS-synthesizing capacity is under investigation.

Expression of the putative sterol binding protein Stard6 gene is male germ cell specific

Mammalian spermatogenesis is orchestrated by many specific molecular and cellular events. To understand the detailed mechanism by which spermatogenesis is controlled, the specific genes involved in this process must be identified and studied. From the subtracted cDNA library of rat testis prepared using the representational difference analysis (RDA) method, we isolated the cDNA clone of steroidogenic acute regulatory (StAR) protein-related lipid transfer (START) protein 6 (Stard6). Stard6 cDNA consists of 1146 base pairs of nucleotides and has the longest open reading frame, of 227 amino acids. Northern blot analysis revealed Stard6 mRNA to be testis-specific. The mRNA transcript appeared from the third week of postnatal development, and the expression level increased up to adulthood. Moreover, in situ hybridization showed Stard6 mRNA expression to be germ cell-specific and expressed only during the maturation stages of round and elongated spermatids of adult rat testis. Western blot analysis with Stard6 antibody revealed a 28-kDa Stard6 protein only in testis. Immunohistochemistry further confirmed localization of Stard6 protein expressed in mature germ cells, in concert with the in situ hybridization result. Taken together, these results suggest that Stard6, a member of the START protein family, may play a role during germ cell maturation in adult rat testis.

Ternary gradient elution markedly improves silver-ion high performance liquid chromatography of unsaturated sterols

A wide variety of unsaturated sterols can accumulate in eukaryotic cells as a consequence of normal metabolism, genetic disorders, and actions of enzyme inhibitors. Resolving these sterol mixtures into individual components by conventional chromatographic methods is inefficient because unsaturated sterols differ little in polarity, hydrophobicity, and volatility. Although sterol mixtures are well-resolved by silver-ion high performance liquid chromatography (Ag(+)-HPLC), existing methods require derivatization to acetates for best results, and the isocratic mobile phases lead to long analysis times and low sensitivity for late-eluting sterols. We show that these problems can be overcome with ternary gradient elution using acetone, hexanes, and acetonitrile. Separation of a mixture of 20 underivatized sterols gave dramatically shortened analysis times, with good peak shapes for both early- and late-eluting components. In a similar separation of blood sterols from a patient with Smith-Lemli-Opitz syndrome, the band for 7-dehydrocholesterol was much narrower than with isocratic elution. Column re-equilibration was rapid, and the separations could be monitored with ultraviolet spectroscopy at 210 nm, which affords universal, non-destructive detection of unsaturated sterols. Also discussed are retention mechanisms and reproducibility of Ag(+)-HPLC separations. The overall results represent a major advance in chromatographic methods for resolving mixtures of unsaturated sterols differing in the number and position of olefinic bonds.

The cholesterol-regulated StarD4 gene encodes a StAR-related lipid transfer protein with two closely related homologues, StarD5 and StarD6

Using cDNA microarrays, we identified StarD4 as a gene whose expression decreased more than 2-fold in the livers of mice fed a high-cholesterol diet. StarD4 expression in cultured 3T3 cells was also sterol-regulated, and known sterol regulatory element binding protein (SREBP)-target genes showed coordinate regulation. The closest homologues to StarD4 were two other StAR-related lipid transfer (START) proteins named StarD5 and StarD6. StarD4, StarD5, and StarD6 are 205- to 233-aa proteins consisting almost entirely of START domains. These three constitute a subfamily among START proteins, sharing approximately 30% amino acid identity with one another, approximately 20% identity with the cholesterol-binding START domains of StAR and MLN64, and less than 15% identity with phosphatidylcholine transfer protein (PCTP) and other START domains. StarD4 and StarD5 were expressed in most tissues, with highest levels in liver and kidney, whereas StarD6 was expressed exclusively in the testis. In contrast to StarD4, expression of StarD5 and MLN64 was not sterol-regulated. StarD4, StarD5, and StarD6 may be involved in the intracellular transport of sterols or other lipids.