Detection of complement C1q receptors on human spermatozoa

Complement in stem cells and development

From its discovery in the late nineteenth century, as a 'complement' to the cellular immune response, the complement system has been widely affirmed as a powerful controller of innate and adaptive immune responses. In recent decades however, new roles for complement have been discovered, with multiple complement proteins now known to function in a broad array of non-immune systems. This includes during development, where complement exerts control over stem cell populations from fertilization and implantation throughout embryogenesis and beyond post-natal development. It is involved in processes as diverse as cell localisation, tissue morphogenesis, and the growth and refinement of the brain. Such physiological actions of complement have also been described in adult stem cell populations, with roles in proliferation, differentiation, survival, and regeneration. With such a broad range of complement functions now described, it is likely that current research only describes a fraction of the full reach of complement proteins. Here, we review how complement control of physiological cell processes has been harnessed in stem cell populations throughout both development and in adult physiology.

What the sperm says and the egg hears - a tale of two proteins and more

While considerable information exists regarding the early interactions of spermatozoon and egg that lead to successful fertilization, the molecular biology of events that result in the incorporation of the spermatozoon within the cortical ooplasm is largely undefined. There is circumstantial evidence suggesting that this process involves the interactions of specific oolemmal receptors and their ligands on sperm that bear similarities to mechanisms used in phagocytosis by macrophages. We have postulated that the egg may act as a 'non-professional phagocyte' during its association with the spermatozoon. This review surveys those events, provides an historical context, and creates a paradigm for further investigation.

Surface expression of complement receptor gC1q-R/p33 is increased on the plasma membrane of human spermatozoa after capacitation

Evidence is increasing that complement components might play a role in fertilization. C1q, the first component of the classical complement cascade, has the ability to promote sperm agglutination in a capacitation-dependent manner as well as an effect on sperm-oolemma binding and fusion. We have previously detected gC1qR, the receptor for the globular head portion of C1q, on the surface of capacitated sperm. In this study, we examined the expression of gC1qR in both fresh and capacitated human spermatozoa. We performed immunoprecipitation for gC1qR and analyzed biotinylated sperm membrane by Western blot to illustrate an increase in receptor density after overnight capacitation. These results were confirmed by flow cytometric analysis of spermatozoa using fluorescein isothiocyanate-labeled monoclonal anti-gC1qR antibody. Confocal, indirect immunofluorescence microscopy revealed an increase in receptor expression over the rostral portion of the sperm head after capacitation. In addition, the ability of live spermatozoa to bind to monoclonal anti-gC1qR antibody-coated microtiter wells was also increased after capacitation. These results suggest that gC1qR may play a role in human fertilization.

Reduction in the level of hyaluronan binding protein 1 (HABP1) is associated with loss of sperm motility

Hyaluronan binding protein 1 (HABP1) was reported to be present on human sperm surface and its involvement in fertilization has already been elucidated (Mol. Repro. Dev. 38 (1994) 69). In the present communication, we report a significant reduction in the level of this protein in sperms from asthenozoospermic and oligozoospermic patients as compared to normozoospermic one. Further evidence of the absence of HABP1 in sperms, having motility <20% is documented, which again is a determining factor for fertilization. HABP1 was quantitatively determined using anti-HABP1 antibody from sperm extracts isolated from semen samples of both the fertile and infertile groups demonstrating low sperm motility. Sperm samples with low motility revealed a significant reduction in the level of HABP1 in immunoblot detection as well as immunolocalization experiment. It suggests that decreased HABP1 level may be associated with low motility of sperms, which in turn might cause infertility in the patient. Thus, the sperm surface HABP1 level can be correlated with the degree of sperm motility, an important criteria for fertilization.

C1q and mannose binding lectin engagement of cell surface calreticulin and CD91 initiates macropinocytosis and uptake of apoptotic cells

Removal of apoptotic cells is essential for maintenance of tissue homeostasis, organogenesis, remodeling, development, and maintenance of the immune system, protection against neoplasia, and resolution of inflammation. The mechanisms of this removal involve recognition of the apoptotic cell surface and initiation of phagocytic uptake into a variety of cell types. Here we provide evidence that C1q and mannose binding lectin (MBL), a member of the collectin family of proteins, bind to apoptotic cells and stimulate ingestion of these by ligation on the phagocyte surface of the multifunctional protein, calreticulin (also known as the cC1qR), which in turn is bound to the endocytic receptor protein CD91, also known as the alpha-2-macroglobulin receptor. Use of these proteins provides another example of apoptotic cell clearance mediated by pattern recognition molecules of the innate immune system. Ingestion of the apoptotic cells through calreticulin/CD91 stimulation is further shown to involve the process of macropinocytosis, implicated as a primitive and relatively nonselective uptake mechanism for C1q- and MBL-enhanced engulfment of whole, intact apoptotic cells, as well as cell debris and foreign organisms to which these molecules may bind.

C1q and mannose binding lectin engagement of cell surface calreticulin and CD91 initiates macropinocytosis and uptake of apoptotic cells

Removal of apoptotic cells is essential for maintenance of tissue homeostasis, organogenesis, remodeling, development, and maintenance of the immune system, protection against neoplasia, and resolution of inflammation. The mechanisms of this removal involve recognition of the apoptotic cell surface and initiation of phagocytic uptake into a variety of cell types. Here we provide evidence that C1q and mannose binding lectin (MBL), a member of the collectin family of proteins, bind to apoptotic cells and stimulate ingestion of these by ligation on the phagocyte surface of the multifunctional protein, calreticulin (also known as the cC1qR), which in turn is bound to the endocytic receptor protein CD91, also known as the alpha-2-macroglobulin receptor. Use of these proteins provides another example of apoptotic cell clearance mediated by pattern recognition molecules of the innate immune system. Ingestion of the apoptotic cells through calreticulin/CD91 stimulation is further shown to involve the process of macropinocytosis, implicated as a primitive and relatively nonselective uptake mechanism for C1q- and MBL-enhanced engulfment of whole, intact apoptotic cells, as well as cell debris and foreign organisms to which these molecules may bind.

Differential extraction and enrichment of human sperm surface proteins in a proteome: identification of immunocontraceptive candidates

The objective of this study was to discover previously unknown human sperm surface proteins that may be candidate contraceptive vaccinogens. To this end, methods of concentrating human sperm proteins for microsequencing by mass spectrometry were used, which increased the likelihood of identifying surface proteins. Vectorial labeling, differential extraction and two-dimensional (2-D) gel electrophoresis were employed to identify and isolate proteins accessible at the cell surface. Percoll harvested or swim-up sperm were either solubilized directly or solubilized after surface labeling with sulfo-succinimidyl-6-(biotinamido)hexanoate (sulfo-NHS-LC-biotin). Comparisons were made of proteins extracted with four lysis buffers: (i) Celis buffer containing 9.8 M urea and 2% Igepal CA-630; (ii) 1% Triton X (TX)-100; (iii) 1.7% TX-114 followed by phase partitioning; or (iv) 1 M NaCl. Blots of proteins separated by high-resolution 2-D electrophoresis were probed with avidin and antibodies to known proteins specific for three domains: the sperm surface (SAGA-1), the acrosome (SP-10), and the cytoskeleton (alpha-tubulin). Celis buffer (45 min) extracted proteins from all three major compartments. However, a 20-s extraction in Celis buffer enriched for several proteins and enabled the identification of several novel peptides by mass spectrometry. Mild extraction with TX-100 or 1 M NaCl solubilized mainly membrane and acrosomal proteins, but not cytoskeletal proteins. Comparison of biotinylated proteins extracted by each method showed that the major vectorially labeled proteins solubilized by Celis buffer were also solubilized by TX-100, TX-114, and 1 M NaCl. Extraction with TX-114 followed by phase-partitioning significantly enriched hydrophobic surface proteins and aided resolution and isolation. Eight protein spots microsequenced following all these extraction methods proved to be novel sperm molecules.

A monoclonal antibody to human SP-10 inhibits in vitro the binding of human sperm to hamster oolemma but not to human Zona pellucida

SP-10 is a sperm intra-acrosomal protein, specific to the testis, that is believed to play an important role in egg-sperm binding. While the molecular characterization of the SP-10 protein has been clarified, little is yet known of its functional role in fertilization. We therefore established a monoclonal antibody (mAb pep-SP10) against a peptide (pep-SP10) that included the most hydrophilic portion of human SP-10 between the 135th and 149th amino acids. Human SP-10 was found to be localized in the equatorial region of acrosome-reacted sperm by immunofluorescent staining using our mAb pep-SP10. Monoclonal Ab pep-SP10 inhibited sperm-oolemma binding in the zona-free hamster egg penetration test, but it did not inhibit sperm-zona binding in the hemizona assay. Furthermore, we demonstrated that the oolemmal ligands of human SP-10 did not include beta(1) integrins, the most promising candidates for oocyte ligands involved in sperm-oolemma binding, based on the findings of a human sperm-cultured cell binding assay using F9 mouse embryonal carcinoma cells and F9-transformed cells lacking beta(1) integrins. In conclusion, our present data suggest that human SP-10, expressed on the equatorial region of acrosome-reacted sperm, indeed mediates sperm-oolemma binding in a beta(1) integrin-independent manner, but not sperm-zona binding.

An investigation of the latency period between sperm oolemmal adhesion and oocyte penetration

In clinical studies of the ability of capacitated human sperm to penetrate zona-free hamster eggs, we have previously observed that the ratio of oolemmal adherent to penetrating sperm varied between men. Sperm incorporation did not occur immediately following gamete adhesion and not all adherent sperm penetrated the egg. To further investigate this phenomenon, comparisons were made of the kinetics of gamete adhesion, membrane fusion, and sperm incorporation of capacitated mouse and human spermatozoa by zona-free hamster eggs and of mouse sperm by zona-free mouse and hamster eggs. Eggs were inseminated with either capacitated human or mouse sperm or combinations of both, washed out of sperm suspension after initial gamete adherence, and further incubated in sperm-free medium. Gamete membrane fusion was judged by dye transfer of Hoechst 33342 and sperm entry of the cortical ooplasm by observation of expanded sperm heads within acridine orange stained eggs. Oolemmal adherent mouse and human sperm fused with and penetrated zona-free hamster eggs at different times whether eggs were inseminated in parallel or with combinations of sperm of both species. Oolemmal adherent mouse sperm penetrated zona-free hamster eggs prior to their penetration of zona-free mouse eggs. Ultrastructural studies of zona-free human eggs inseminated with human sperm confirmed prior observations with hamster eggs that only acrosome-reacted human sperm adhere to the oolemma. These results have lead us to postulate that sperm entry into the egg may occur through a "zipper" mechanism involving the ligation of local gamete receptors similar to the incorporation of target particles by phagocytes and suggest that not all oolemmal adherent human sperm are capable of being incorporated although they have undergone an acrosome reaction.