The use of anticlusterin monoclonal antibodies for the combined assessment of human sperm morphology and acrosome integrity

Sperm Functional Genome Associated With Bull Fertility

Bull fertility is an important economic trait in sustainable cattle production, as infertile or subfertile bulls give rise to large economic losses. Current methods to assess bull fertility are tedious and not totally accurate. The massive collection of functional data analyses, including genomics, proteomics, metabolomics, transcriptomics, and epigenomics, helps researchers generate extensive knowledge to better understand the unraveling physiological mechanisms underlying subpar male fertility. This review focuses on the sperm phenomes of the functional genome and epigenome that are associated with bull fertility. Findings from multiple sources were integrated to generate new knowledge that is transferable to applied andrology. Diverse methods encompassing analyses of molecular and cellular dynamics in the fertility-associated molecules and conventional sperm parameters can be considered an effective approach to determine bull fertility for efficient and sustainable cattle production. In addition to gene expression information, we also provide methodological information, which is important for the rigor and reliability of the studies. Fertility is a complex trait influenced by several factors and has low heritability, although heritability of scrotal circumference is high and that it is a known fertility maker. There is a need for new knowledge on the expression levels and functions of sperm RNA, proteins, and metabolites. The new knowledge can shed light on additional fertility markers that can be used in combination with scrotal circumference to predict the fertility of breeding bulls. This review provides a comprehensive review of sperm functional characteristics or phenotypes associated with bull fertility.

Identification of novel substrates of a disintegrin and metalloprotease 17 by specific labeling of surface proteins

A disintegrin and metalloprotease 17 (ADAM17) catalyzes the cleavage and release of the ectodomains of its substrates at the cell surface in a process termed ectodomain shedding. However, not all ADAM17 substrates have been identified. Here, we used cell surface protein-specific labeling and proteomic approaches to detect and identify ADAM17 substrates. HeLa cell surface proteins were labeled with a fluorescent dye and cultured with or without TAPI-2, an ADAM17 inhibitor. Labeled proteins released into the culture medium were detected by 2-dimensional gel electrophoresis (2DE). Protein spots showing decreased intensity in response to TAPI-2 were selected as substrates of ADAM17 or their binding proteins, and identified by mass spectrometry. ADAM17 knockdown was preformed to examine the behavior of identified proteins. Of 347 proteins detected by 2DE, 49 showed lower intensity in TAPI-2 (+) than in TAPI-2 (-) samples (p < 0.05), and were considered as candidate substrates of ADAM17. Mass spectrometric analysis of 14 protein spots showing >50% decreased intensity identified clusterin as a novel ADAM17 substrate, in addition to known substrates such as desmoglein-2. Western blot analysis showed that ADAM17 knockdown decreased the levels of clusterin fragments cleaved and released from the cell surface. The results identified clusterin as a novel ADAM17 substrate. The method used to identify clusterin could be used to identify the substrates of other sheddases involved in ectodomain shedding.

Fibrinogen-related protein, FGL2, of hamster cauda epididymal fluid: Purification, kinetic analysis of its prothrombinase activity, and its role in segregation of nonviable spermatozoa

Although the epididymal environment promotes the maturation and survival of spermatozoa, not all spermatozoa remain viable during passage through the epididymis. Does the epididymis has a protective mechanism(s) to segregate the viable sperm from defective spermatozoa? Previously, we identified 260/280 kDa oligomers (termed eFGL-Epididymal Fibrinogen-Like oligomer) are composed of two disulfide-linked subunits: a 64 kDa polypeptide identified as fibrinogen-like protein-2 (FGL2) and a 33 kDa polypeptide identified as fibrinogen-like protein-1 (FGL1). Our morphological studies demonstrated that the eFGL, secreted from the principal cells of the cauda epididymis, is polymerized into a death cocoon-like complex (DCF), masking defective luminal spermatozoa but, not the viable sperm population. In the present study, we purified FGL2 from hamster cauda epididymal fluid toward homogeneity and its prothrombinase catalytic activity was examined. Time-course conversion studies revealed that all prothrombin was converted to thrombin by purified hamster FGL2. Our biochemical studies demonstrate that FGL2 is a lipid-activated serine protease and functions as a lectin by binding specific carbohydrate residues. Co-immunoprecipitation analysis demonstrated that FGL2 of cauda epididymal fluid is ubiquitinated but not the FGL1. We propose that FGL2/FGL1 oligomers represent a novel and unique mechanism to shield the viable sperm population from degenerating spermatozoa contained within the tubule lumen.

Proteomics Profiling of Autologous Blood and Semen Exosomes from HIV-infected and Uninfected Individuals Reveals Compositional and Functional Variabilities

Blood and semen are important body-fluids that carry exosomes for bioinformation transmission. Therefore, characterization of their proteomes is necessary for understanding body-fluid-specific physiologic and pathophysiologic functions. Using systematic multifactorial proteomic profiling, we characterized the proteomes of exosomes and exosome-free fractions from autologous blood and semen from three HIV-uninfected and three HIV-infected participants (total of 24 samples). We identified exosome-based protein signatures specific to blood and semen along with HIV-induced tissue-dependent proteomic perturbations. We validated our findings with samples from 16 additional donors and showed that unlike blood exosomes (BE), semen exosomes (SE) are enriched in clusterin. SE but not BE promote Protein·Nucleic acid binding and increase cell adhesion irrespective of HIV infection. This is the first comparative study of the proteome of autologous BE and SE. The proteins identified may be developed as biomarkers applicable to different fields of medicine, including reproduction and infectious diseases.

Relative abundance of heat shock proteins and clusterin transcripts in spermatozoa collected from boar routinely utilised in an artificial insemination centre: preliminary results

It is widely accepted that mature sperm contains RNA. The first hypothesis was that sperm RNAs have no functions of their own but are simply residues of spermatogenesis reflecting the events that occurred during their formation in the testes. More recently new discoveries have essentially expanded these views, showing that sperm mRNAs constitute a population of stable full-length transcripts, many of which are selectively retained during spermatogenesis and delivered to oocytes contributing to early embryo development. It is well known that semen quality can be influenced by occasional physical stress, infection, and variation in temperature and the definition of new markers for evaluation of semen could offer knowledge about the fertility potential of a semen sample. The aim of the present study was to evaluate the presence and the relative quantity of transcripts and protein of heat shock protein 70 (HSP70), 90 (HSP90) and clusterin (CLU) in Percoll-selected spermatozoa collected from seven adult boars of proven fertility routinely employed for artificial insemination. Our results showed the presence of HSP70, HSP90 and CLU transcripts with different level of expression: high for HSPs and low for CLU transcripts. The transcript level of both HSPs are similar among selected spermatozoa derived from high quality sperm with the exception of one boar that showed a reduced content of HSP70 and HSP90 mRNA together with a lower semen quality. At protein level, both HSPs were detected with similar amount among all seven boars whilst no band was evidenced for CLU protein.

Proteomic analysis of mature and immature ejaculated spermatozoa from fertile men

Dysfunctional spermatozoa maturation is the main reason for the decrease in sperm motility and morphology in infertile men. Ejaculated spermatozoa from healthy fertile men were separated into four fractions using three-layer density gradient. Proteins were extracted and bands were digested on a LTQ-Orbitrap Elite hybrid mass spectrometer system. Functional annotations of proteins were obtained using bioinformatics tools and pathway databases. Western blotting was performed to verify the expression levels of the proteins of interest. 1469 proteins were identified in four fractions of spermatozoa. The number of detected proteins decreased according to the maturation level of spermatozoa. During spermatozoa maturation, proteins involved in gamete generation, cell motility, energy metabolism and oxidative phosphorylation processes showed increasing expression levels and those involved in protein biosynthesis, protein transport, protein ubiquitination, and response to oxidative stress processes showed decreasing expression levels. We validated four proteins (HSP 70 1A, clusterin, tektin 2 and tektin 3) by Western blotting. The study shows protein markers that may provide insight into the ejaculated spermatozoa proteins in different stages of sperm maturation that may be altered or modified in infertile men.

Identification of protein changes in human spermatozoa throughout the cryopreservation process

Cryoinjury is a consequence of cryopreservation and may have a negative impact on sperm quality regarding motility, morphology, and viability. This study was designed to identify potential proteomic changes in human sperm cells throughout the cryopreservation process. Comparisons made within this study included the detection of the sperm proteomic changes induced by incubation of the sperm cells with a protein-free cryoprotectant (with and without CryoSperm), and the proteomic changes induced by freezing, thawing, and subsequent after-thawing incubation at two different temperatures (0 °C vs. 23 °C). Tandem Mass Tag (TMT) peptide labeling coupled with LC-MS/MS was used for protein quantification. LC-MS/MS resulted in the identification of 769 quantifiable proteins. The abundance of 105 proteins was altered upon CryoSperm incubation. Freezing and thawing also induced substantial protein changes. However, fewer changes were observed when semen was thawed and then maintained after-thawing at approximately 0 °C than when it was maintained after-thawing at 23 °C, with 60 and 99 differential proteins detected, respectively, as compared to unfrozen semen incubated in CryoSperm. Collectively, these differences indicate that substantial changes occur in the sperm proteome at every stage of the cryopreservation process which may ultimately impair the sperm fertilizing capability. This is the first study to compare protein levels in fresh and cryopreserved semen using the TMT technology coupled to LC-MS/MS.

Two fibrinogen-like proteins, FGL1 and FGL2 are disulfide-linked subunits of oligomers that specifically bind nonviable spermatozoa

Nevertheless, a nonviable sperm population is present in the cauda epididymidis of many species. Degenerating spermatozoa release enzymes that could have detrimental effects on the viability of neighboring cells, and they are source of autoantigens that induce an autoimmune response if they escape the blood-epididymis barrier. Does the epididymis have specialized protective mechanism(s) to segregate the viable sperm population from defective spermatozoa? Previously, we identified a fibrinogen-like protein-2 (fgl2) that specifically binds to and polymerizes into a cocoon-like complex coating defective spermatozoa and sperm fragments. The objective of the present study is to identify the subunit composition of the fgl2-containing oligomers both in the soluble and cocoon-like complex. Our proteomic studies indicate that the 260/280kDa oligomers (termed eFGL) contain two distinct disulfide-linked subunits; 64kDa fgl2 and 33kDa fgl1. Utilizing a PCR-based cloning strategy, the 33kDa polypeptide has been identified as fibrinogen-like protein-1 (fgl1). Immunocytochemical studies revealed that fgl1 selectively binds to defective spermatozoa in the cauda epididymidis. Northern blot analysis and in situ hybridization demonstrated the high expression of fgl1 in the principal cells of the proximal cauda epididymidis. Co-immunoprecipitation analyses of cauda epididymal fluid, using anti-fgl2, demonstrate that both fgl1 and fgl2 are present in the soluble eFGL. Our study is the first to show an association of fgl1 and fgl2 both in the soluble and in the sperm-associated eFGL. We conclude that our results provide new insights into the mechanisms by which the potentially unique epididymal protein functions in the recognition and elimination of defective spermatozoa.

Prostasomes: Their Characterisation: Implications for Human Reproduction: Prostasomes and Human Reproduction

The prostate is a principal accessory genital gland that is vital for normal fertility. Epithelial cells lining the prostate acini release in a defined fashion (exocytosis) organellar nanosized structures named prostasomes. They are involved in the protection of sperm cells against immune response in the female reproductive tract by modulating the complement system and by inhibiting monocyte and neutrophil phagocytosis and lymphocyte proliferation. The immunomodulatory function most probably involves small non-coding RNAs present in prostasomes. Prostasomes have also been proposed to regulate the timing of sperm cell capacitation and induction of the acrosome reaction, since they are rich in various transferable bioactive molecules (e.g. receptors and enzymes) that promote the fertilising ability of sperm cells. Antigenicity of sperm cells has been well documented and implicated in involuntary immunological infertility of human couples, and antisperm antibodies (ASA) occur in several body fluids. The propensity of sperm cells to carry attached prostasomes suggests that they are a new category of sperm antigens. Circulating human ASA recognise prostasomes, and among 12 identified prostasomal antigens, prolactin- inducible protein (95 %) and clusterin (85 %) were immunodominant at the expense of the other 10 that were sporadically occurring.

Toward the identification of a subset of unexplained infertility: a sperm proteomic approach

OBJECTIVE

To investigate the male gamete proteome and its relation to blastocyst development and reproductive success.

DESIGN

Experimental study.

SETTING

Research laboratory.

PATIENT(S)

Male infertility patients (n=12) with no known male factor infertility, donated motile sperm after intracytoplasmic sperm injection during an oocyte donor in vitro fertilization cycle.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Proteomic profiles of sperm from normozoospermic males.

RESULT(S)

Patients were grouped based on day-5 embryo development: group A=good blastocyst development (>35%≥grade 3 BB) and group B=poor blastocyst development (<15%≥grade 3 BB). No differences between the groups were observed for sperm concentration, motility, or Kruger morphology. The in vitro fertilization outcome was statistically significantly different with higher viable implantation rates observed for group A (A=80% vs. B=48%). Proteomic analysis of the motile sperm samples revealed 49 proteins with statistically significantly differential abundance in relation to blastocyst development (>1.5-fold). Twenty-nine proteins showed decreased abundance for group B, including several proteins involved in spermatogenesis, and 20 proteins showed increased abundance for group B, including several heat shock proteins.

CONCLUSION(S)

An altered sperm proteome was observed with respect to poor blastocyst development and in vitro fertilization outcome in donor oocyte cycles despite normal sperm testing parameters. These data could represent a novel subset of male factor infertility. Ongoing investigation into the male factor contribution to idiopathic infertility may result in improved patient care and enhanced outcomes.

Unraveling the sperm proteome and post-genomic pathways associated with sperm nuclear DNA fragmentation

PURPOSE

Sperm DNA fragmentation has been suggested as a marker for infertility diagnosis and prognosis. Hence, understanding its impact on male physiology and post-genomic pathways would be clinically important. We performed the proteomics and functional enrichment analyses of viable spermatozoa from ejaculates with low and high sperm DNA fragmentation to identify protein expression and pathways altered in association with sperm DNA fragmentation.

METHODS

Sperm DNA fragmentation using the Comet assay and the Komet 6.0.1 software was assessed in raw samples from 89 subjects from a human reproduction service. The Low and High sperm DNA fragmentation groups were formed according to the Olive Tail Moment variable. Spermatozoa proteins from these groups were pooled and analyzed by a shotgun proteomic approach (2D nanoUPLC-ESI-MS(E)). Differentially expressed proteins were used for a functional enrichment study.

RESULTS

Two hundred and fifty-seven proteins were identified or quantified in sperm from the Low and High sperm DNA fragmentation groups. Of these, seventy-one proteins were exclusively or overexpressed in the Low group, whereas twenty-three proteins were exclusively or overexpressed in the High group. One hundred and sixty-three proteins were conserved between these groups. We also functionally related the differentially expressed proteins in viable spermatozoa from the groups. Processes such as triacylglycerol metabolism, energy production, protein folding, response to unfolded proteins, and cellular detoxification were found to be altered in these cells.

CONCLUSIONS

Sperm DNA fragmentation is associated with differential protein expression in viable spermatozoa. These proteins may potentially be used as biomarkers for sperm DNA integrity.

Epididymosomes transfer epididymal sperm binding protein 1 (ELSPBP1) to dead spermatozoa during epididymal transit in bovine

Previously, we showed that epididymal sperm binding protein 1 (ELSPBP1) characterizes spermatozoa already dead before ejaculation in bovine. In this study, we investigated the presence of ELSPBP1 in bull genital tract as well as its acquisition by spermatozoa during epididymal transit. As assessed by real-time RT-PCR, ELSPBP1 was highly expressed in the caput and the corpus epididymis but was present in lower expression levels in the testis and the cauda epididymis. Immunohistochemistry revealed the same expression pattern. However, Western blot on tissue homogenates showed some discrepancies, as ELSPBP1 was found in a comparable concentration all along the epididymis. This difference was due to the presence of ELSPBP1 in the epididymal fluid. In both caput and cauda epididymal fluid, ELSPBP1 was associated with the epididymosomes, small membranous vesicles secreted by epithelial cells of the epididymis and implicated in the transfer of proteins to spermatozoa. As assessed by immunocytometry, ELSPBP1 was found on a subset of dead spermatozoa in caput epididymis but was found on all dead spermatozoa in cauda epididymis. To assess ELSPBP1 acquisition by spermatozoa, caput epididymal spermatozoa were incubated with cauda epididymosomes under various conditions. ELSPBP1 detection by immunocytometry assay revealed that only spermatozoa already dead before incubation were receptive to ELSPBP1 transfer by epididymosomes. This receptivity was enhanced by the presence of zinc in the incubation medium. This specificity for a sperm subpopulation suggests that an underlying mechanism is involved and that ELSPBP1 could be a tag for the recognition of dead spermatozoa during epididymal transit.

Seminal clusterin gene expression associated with seminal variables in fertile and infertile men

PURPOSE

CLU is a disulfide linked, heterodimeric protein associated with the clearance of cellular debris and apoptosis. We assessed the association of seminal CLU gene expression with seminal variables in fertile and infertile men.

MATERIALS AND METHODS

A total of 124 men were divided into healthy, fertile men with normozoospermia, and men with asthenozoospermia, asthenoteratozoospermia and oligoasthenoteratozoospermia. History was obtained, and clinical examination and semen analysis were done. In semen we assessed sperm acrosin activity, sperm DNA fragmentation and seminal CLU gene expression.

RESULTS

CLU RNA and CLU protein gene expression were significantly increased in semen samples of infertile men with oligoasthenoteratozoospermia > asthenoteratozoospermia > asthenozoospermia compared with healthy, fertile controls. CLU gene expression significantly correlated negatively with sperm count, motility, acrosin activity index, linearity index and linear velocity, and significantly correlated positively with the percent of sperm abnormal forms and DNA fragmentation.

CONCLUSION

CLU gene expression was significantly increased in the semen samples of infertile men. It correlated negatively with sperm count, motility, acrosin activity, linearity index and linear velocity, and positively with the percent of sperm abnormal forms and DNA fragmentation.

Proteomic characteristics of spermatozoa in normozoospermic patients with infertility

Male infertility is a vexing yet common problem for men all over the world. However, its etiology remains unknown in most cases. The aim of this study was to screen and investigate the differentially expressed proteins in the sperm of infertile patients, whose sperm clinical parameters met the WHO guidelines. Using MALDI-TOF/TOF analysis, we identified 24 differentially expressed proteins from the 31 most abundant different protein spots in 2D gels of sperm samples, then verified and analyzed localization in sperm of the proteins. Following data mining analysis showed that these 24 proteins were categorized into five functional clustering groups: sexual reproduction, response to wounding, metabolic process, cell growth and/or maintenance, not clear. Additionally, 9 of the 24 differentially expressed proteins are involved in a main pathway network including TGF-β1, MYC, β-estradiol, MYCN, and TP53, which are known to be involved in cell communication, proliferation and differentiation. The observed differences in signaling and metabolic pathways between the infertile sperm and the normal fertile spermatozoa have implications in sperm motility, capacitation, acrosomal reaction and sperm-oocyte communication. These proteins are potential diagnostic markers, and the study of these proteins could help gain further insight into the pathogenic mechanisms in infertility.

Presence, localization, and origin of clusterin in normal human spermatozoa

PURPOSE

Clusterin in mammalian semen is a secretory form of clusterin (sCLU) with the heterodimeric structure. It is secreted by the epididymis and seminal vesicle. It is generally agreed that clusterin mainly exists on the surface of abnormal spermatozoa and is implicated in decreased sperm motility, sperm aggregation and infertility. However, few studies observe clusterin in normal spermatozoa, which is presumed to be a novel form. Up to now, the systematical information about the presence, localization, origin and function of clusterin in normal human spermatozoa has yet not been established. The aim of our current study is to systematically research clusterin in normal human spermatozoa.

METHODS

We detected the presence of clusterin via western blot, explored the localization of clusterin using immunofluorescence, and investigated the origin and distribution of clusterin in human testis by western blot and immunohistochemistry.

RESULTS

We found native clusterin in the inner plasma membrane of normal human spermatozoa. It was derived from the testis and showed similar molecular weight and heterodimeric structure compared with sCLU in semen and on the surface of abnormal spermatozoa.

CONCLUSION

Clusterin in normal spermatozoa should be self-synthesized during the later stage of spermatogenesis. The different localization and origin suggested that the clusterin observed by us may be a novel form compared with conventional sCLU on the surface of abnormal spermatozoa.

Evaluation of sperm proteins in infertile men: a proteomic approach

In this study, the sperm protein profile was compared between fertile and infertile men using 2-dimensional gel electrophoresis, liquid chromatography mass spectrometer analysis, and matrix-assisted laser desorption ionization-time-of-flight-mass spectrometry. Four unique proteins, semenogelin II precursor, prolactin-induced protein, clusterin isoform 1, and prostate-specific antigen isoform 1 preproprotein, were predominantly present in the semen of healthy men; however, semenogelin II precursor and clusterin isoform 1 were not seen in the semen of infertile men, suggesting unique differences in the spermatozoa protein profiles of fertile and infertile men.

Identification of gene products present in Triton X-100 soluble and insoluble fractions of human spermatozoa lysates using LC-MS/MS analysis

A comprehensive analysis of the proteins found in human spermatozoa is essential for understanding the events leading up to, and including, fertilization and development. Proteomics offers a platform for investigating this process, provided that the dynamic range is relatively low. In this report, spermatozoa from a number of human sperm ejaculates were isolated in a pure state using discontinuous Percoll gradient centrifugation. Triton X-100 soluble and insoluble proteins were recovered and separated by SDS-PAGE. The separation lanes were dissected into 96 fractions and analyzed individually by LC-MS(n) . A comprehensive protocol, involving LC-MS/MS analysis eventually down to the ninth most intense peak found in the MS-survey scan, was performed. Analysis of purified human sperm populations resulted in the identification of 1056 gene products, of which approximately 8% have not previously been characterized. The data were supported by the large number of proteins represented by expressed sequence tags in the testis. Bioinformatic analysis demonstrated that 437 of the gene products were involved in various metabolic pathways including glycolysis and oxidative phosphorylation. The inventory of proteins present in the human sperm proteome includes a number of notable discoveries including the first description of a nicotinamide adenine dinucleotide phosphate oxidase, dual-oxidase 2, finally laying to rest any doubts about the presence of such enzymes in spermatozoa. Furthermore, a number of different classes of receptor have also been detected in these cells and are potential regulators of sperm function. This list includes at least six seven-pass transmembrane receptors, six tyrosine kinase receptors, a tyrosine phosphatase receptor, glutamate-gated ion channel receptors, transient receptor potential cation channels, and a non-genomic progesterone receptor. This is the first published list of identified proteins in human spermatozoa using LC-MS/MS analysis.

Post-testicular sperm environment and fertility

When mammalian spermatozoa exit the testis, they show a highly specialized morphology; however, they are not yet able to carry out their task: to fertilize an oocyte. This property, that includes the acquisition of motility and the ability to recognize and to fuse with the oocyte investments, is gained only after a transit through the epididymis during which the spermatozoa from the testis travel to the vas deferens. The exact molecular mechanisms that turn these cells into fertile gametes still remain mysterious, but surface-modifying events occurring in response to the external media are key steps in this process. Our laboratory has established cartographies of secreted (secretomes) and present proteins (proteomes) in the epididymal fluid of different mammals and have shown the regionalized variations in these fluid proteins along the epididymis. We have found that the main secreted proteins are common in different species and that enzymatic activities, capable of controlling the sperm surface changes, are present in the fluid. Our studies also indicate that the epididymal fluid is more complex than previously thought; it contains both soluble and particulate compartments such as exosome-like vesicles (epididymosomes) and certainly specific glycolipid-protein micelles. Understanding how these different compartments interplay to modify sperm components during their transit will be a necessary step if one wants to control and to ameliorate sperm quality and to obtain valuable fertility markers helpful to establish a male fertility based genetic selection.

Identification of a hamster epididymal region-specific secretory glycoprotein that binds nonviable spermatozoa

Even though the epididymis produces an environment promoting sperm maturation and viability, some sperm do not survive transit through the epididymal tubule. Mechanisms that segregate the epididymal epithelium and/or the viable sperm population from degenerating spermatozoa are poorly understood. We report here the identification and characterization of HEP64, a 64-kDa glycoprotein secreted by principal cells of the corpus and proximal cauda epididymidis of the hamster that specifically binds to and coats dead/dying spermatozoa. The HEP64 monomer contains approximately 12 kDa carbohydrate and, following chemical deglycosylation, migrates as a approximately 52-kDa polypeptide. Both soluble (luminal fluid) and sperm-associated HEP64 are assembled into disulfide-linked high molecular weight oligomers that migrate as a doublet band of 260/280 kDa by nonreducing SDS-PAGE. In the epididymal lumen, HEP64 is concentrated into focal accumulations containing aggregates of structurally abnormal or degenerating spermatozoa, and examination of sperm suspensions reveals that HEP64 forms a shroudlike coating surrounding abnormal spermatozoa. The HEP64 glycoprotein firmly binds degenerating spermatozoa and is not released by either nonionic detergent or high salt extraction. Electron microscopic immunocytochemistry demonstrates that HEP64 localized to an amorphous coating surrounding the abnormal spermatozoa. The potential mechanisms by which this epididymal secretory protein binds dead spermatozoa as well as its possible functions in the sperm storage function of the cauda epididymidis are discussed.

A reexamination of the role of clusterin as a complement regulator

Clusterin is a highly conserved glycoprotein which has been proposed to protect host cells against complement-mediated cytolysis. We tested the hypothesis that clusterin is a complement regulator using erythrocytes and cells which had been stably transfected with a membrane-anchored form of clusterin as targets for complement-mediated cytolysis. Clusterin gave dose-dependent protection of antibody-coated sheep erythrocytes against complement-mediated lysis by diluted normal human serum. There was a linear relationship between the concentration of clusterin giving 50% protection and the concentration of serum; extrapolation of this to the case of undiluted human serum showed that a clusterin concentration at least two orders of magnitude greater than its physiological plasma concentration would be needed to confer protection against complement-mediated cytolysis under physiological conditions. Physiological concentrations of clusterin did not protect rabbit erythrocytes against alternative complement pathway-mediated lysis using dilute human serum. Exogenous clusterin had no effect on lysis of human erythrocytes triggered by the addition of inulin to autologous human serum. Induction of cell-surface clusterin expression by L929 (murine fibroblast) cells which had been stably transfected with cDNA for human clusterin linked to DNA coding for the 44 C-terminal amino acid residues of CD55 did not protect the cells against complement-mediated lysis by either normal or clusterin-depleted human serum. These data suggest that clusterin may not be a physiologically relevant regulator of complement activation.

Control of the complement system

The complement system has developed a remarkably simple but elegant manner of regulating itself. It has faced and successfully dealt with how to facilitate activation on a microbe while preventing the same on host tissue. It solved this problem primarily by creating a series of secreted and membrane-regulatory proteins that prevent two highly undesirable events: activation in the fluid phase (no target) and on host tissue (inappropriate target). Also, if not checked, even on an appropriate target, the system would go to exhaustion and have nothing left for the next microbe. Therefore, the complement enzymes have an intrinsic instability and the fluid-phase control proteins play a major role in limiting activation in time. The symmetry of the regulatory process between fluid phase and membrane inhibitors at the C4/C3 step of amplification and convertase formation as well as at the MAC steps are particularly striking features of the self/nonself discrimination system. The use of glycolipid anchored proteins on membranes to decay enzymes and block membrane insertion events is unlikely to be by chance. Finally, it is economical for the cofactor regulatory activity to produce derivatives of C3b that now specifically engage additional receptors. Likewise, C1-Inh leads to C1q remaining on the immune complex to interact with the C1q receptor. Thus the complement system is designed to allow rapid, efficient, unimpeded activation on an appropriate foreign target while regulatory proteins intervene to prevent three undesirable consequences of complement activation: excessive activation on a single target, fluid phase activation, and activation on self.