Multiple glycolytic enzymes are tightly bound to the fibrous sheath of mouse spermatozoa

Human sperm tail proteome suggests new endogenous metabolic pathways

Proteomic studies are contributing greatly to our understanding of the sperm cell, and more detailed descriptions are expected to clarify additional cellular and molecular sperm attributes. The aim of this study was to characterize the subcellular proteome of the human sperm tail and, hopefully, identify less concentrated proteins (not found in whole cell proteome studies). Specifically, we were interested in characterizing the sperm metabolic proteome and gaining new insights into the sperm metabolism issue. Sperm were isolated from normozoospermic semen samples and depleted of any contaminating leukocytes. Tail fractions were obtained by means of sonication followed by sucrose-gradient ultracentrifugation, and their purity was confirmed via various techniques. Liquid chromatography and tandem mass spectrometry of isolated sperm tail peptides resulted in the identification of 1049 proteins, more than half of which had not been previously described in human sperm. The categorization of proteins according to their function revealed two main groups: proteins related to metabolism and energy production (26%), and proteins related to sperm tail structure and motility (11%). Interestingly, a great proportion of the metabolic proteome (24%) comprised enzymes involved in lipid metabolism, including enzymes for mitochondrial beta-oxidation. Unexpectedly, we also identified various peroxisomal proteins, some of which are known to be involved in the oxidation of very long chain fatty acids. Analysis of our data using Reactome suggests that both mitochondrial and peroxisomal pathways might indeed be active in sperm, and that the use of fatty acids as fuel might be more preponderant than previously thought. In addition, incubation of sperm with the fatty acid oxidation inhibitor etomoxir resulted in a significant decrease in sperm motility. Contradicting a common concept in the literature, we suggest that the male gamete might have the capacity to obtain energy from endogenous pools, and thus to adapt to putative exogenous fluctuations.

RAB-like 2 has an essential role in male fertility, sperm intra-flagellar transport, and tail assembly

A significant percentage of young men are infertile and, for the majority, the underlying cause remains unknown. Male infertility is, however, frequently associated with defective sperm motility, wherein the sperm tail is a modified flagella/cilia. Conversely, a greater understanding of essential mechanisms involved in tail formation may offer contraceptive opportunities, or more broadly, therapeutic strategies for global cilia defects. Here we have identified Rab-like 2 (RABL2) as an essential requirement for sperm tail assembly and function. RABL2 is a member of a poorly characterized clade of the RAS GTPase superfamily. RABL2 is highly enriched within developing male germ cells, where it localizes to the mid-piece of the sperm tail. Lesser amounts of Rabl2 mRNA were observed in other tissues containing motile cilia. Using a co-immunoprecipitation approach and RABL2 affinity columns followed by immunochemistry, we demonstrated that within developing haploid germ cells RABL2 interacts with intra-flagella transport (IFT) proteins and delivers a specific set of effector (cargo) proteins, including key members of the glycolytic pathway, to the sperm tail. RABL2 binding to effector proteins is regulated by GTP. Perturbed RABL2 function, as exemplified by the Mot mouse line that contains a mutation in a critical protein–protein interaction domain, results in male sterility characterized by reduced sperm output, and sperm with aberrant motility and short tails. Our data demonstrate a novel function for the RABL protein family, an essential role for RABL2 in male fertility and a previously uncharacterised mechanism for protein delivery to the flagellum.

A greater understanding of the mechanism of male fertility is essential in order to address the medical needs of the 1 in 20 men of reproductive age who are infertile. Conversely, there remains a critical need for additional contraceptive options, including those that target male gametes. Towards the aim of filling these knowledge gaps, we have used random mutagenesis to produce the Mot mouse line and to identify RABL2 as an essential regulator of male fertility. Mice carrying a mutant Rabl2 gene are sterile as a consequence of severely compromised sperm motility. Using biochemical approaches we have revealed that RABL2 binds to components of the intraflagellar transport machinery and have identified a number of RABL2 binding (effector) proteins. The presence of the Mot mutation in RABL2 leads to a significantly compromised ability to deliver binding proteins into the sperm tail. RABL2 is predominantly produced in male germ cells; however, lower levels are notably produced in organs that contain motile cilia (hair like structures involved in fluid/cell movement), thus raising the possibility that RABL2 may be involved in a broader set of human diseases collectively known as primary cilia dyskinesia.

Sperm-specific glyceraldehyde-3-phosphate dehydrogenase is expressed in melanoma cells

Sperm-specific glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDS) is normally expressed only in sperms, but not in somatic tissues. Analysis of the expression of GAPDS mRNA in different cancer cell lines shows that the content of GAPDS mRNA is enhanced in some lines of melanoma cells. The purpose of the study was to assay melanoma cells for the expression of protein GAPDS. Three different lines of melanoma cells were investigated. By data of Western blotting, all investigated cells contain a 37-kDa fragment of GAPDS polypeptide chain, which corresponds to the enzyme GAPDS lacking N-terminal amino acid sequence that attaches the enzyme to the cytoskeleton of the sperm flagellum. The results suggest that GAPDS is expressed in melanoma cells without N-terminal domain. The immunoprecipitation of proteins from melanoma cell extracts using rabbit polyclonal antibodies against native GAPDS allowed isolation of complexes containing 37-kDa subunit of GAPDS and full-length subunit of somatic glyceraldehyde-3-phosphate dehydrogenase (GAPD). The results indicate that melanoma cells express both isoenzymes, which results in the formation of heterotetrameric complexes. Immunocytochemical staining of melanoma cells revealed native GAPDS in the cytoplasm. It is assumed that the expression of GAPDS in melanoma cells may facilitate glycolysis and prevent the induction of apoptosis.

Metabolic substrates exhibit differential effects on functional parameters of mouse sperm capacitation

Although substantial evidence exists that sperm ATP production via glycolysis is required for mammalian sperm function and male fertility, conflicting reports involving multiple species have appeared regarding the ability of individual glycolytic or mitochondrial substrates to support the physiological changes that occur during capacitation. Several mouse models with defects in the signaling pathways required for capacitation exhibit reductions in sperm ATP levels, suggesting regulatory interactions between sperm metabolism and signal transduction cascades. To better understand these interactions, we conducted quantitative studies of mouse sperm throughout a 2-h in vitro capacitation period and compared the effects of single substrates assayed under identical conditions. Multiple glycolytic and nonglycolytic substrates maintained sperm ATP levels and comparable percentages of motility, but only glucose and mannose supported hyperactivation. These monosaccharides and fructose supported the full pattern of tyrosine phosphorylation, whereas nonglycolytic substrates supported at least partial tyrosine phosphorylation. Inhibition of glycolysis impaired motility in the presence of glucose, fructose, or pyruvate but not in the presence of hydroxybutyrate. Addition of an uncoupler of oxidative phosphorylation reduced motility with pyruvate or hydroxybutyrate as substrates but unexpectedly stimulated hyperactivation with fructose. Investigating differences between glucose and fructose in more detail, we demonstrated that hyperactivation results from the active metabolism of glucose. Differences between glucose and fructose appeared to be downstream of changes in intracellular pH, which rose to comparable levels during incubation with either substrate. Sperm redox pathways were differentially affected, with higher levels of associated metabolites and reactive oxygen species generated during incubations with fructose than during incubations with glucose.

Structure and function of the human sperm-specific isoform of protein kinase A (PKA) catalytic subunit Cα2

Protein kinase A (PKA) exists as several tissue-specific isoforms that through phosphorylation of serine and threonine residues of substrate proteins act as key regulators of a number of cellular processes. We here demonstrate that the human sperm-specific isoform of PKA named Cα2 is important for sperm motility and thus male fertility. Furthermore, we report on the first three-dimensional crystal structure of human apo Cα2 to 2.1 Å. Apo Cα2 displays an open conformation similar to the well-characterized apo structure of murine Cα1. The asymmetric unit contains two molecules and the core of the small lobe is rotated by almost 13° in the A molecule relative to the B molecule. In addition, a salt bridge between Lys72 and Glu91 was observed for Cα2 in the apo-form, a conformation previously found only in dimeric or ternary complexes of Cα1. Human Cα2 and Cα1 share primary structure with the exception of the amino acids at the N-terminus coded for by an alternative exon 1. The N-terminal glycine of Cα1 is myristoylated and this aliphatic chain anchors the N-terminus to an intramolecular hydrophobic pocket. Cα2 cannot be myristoylated and the crystal structure revealed that the equivalent hydrophobic pocket is unoccupied and exposed. Nuclear magnetic resonance (NMR) spectroscopy further demonstrated that detergents with hydrophobic moieties of different lengths can bind deep into this uncovered pocket. Our findings indicate that Cα2 through the hydrophobic pocket has the ability to bind intracellular targets in the sperm cell, which may modulate protein stability, activity and/or cellular localization.

Tubulin-dynein system in flagellar and ciliary movement

Eukaryotic flagella and cilia have attracted the attention of many researchers over the last century, since they are highly arranged organelles and show sophisticated bending movements. Two important cytoskeletal and motor proteins, tubulin and dynein, were first found and described in flagella and cilia. Half a century has passed since the discovery of these two proteins, and much information has been accumulated on their molecular structures and their roles in the mechanism of microtubule sliding, as well as on the architecture, the mechanism of bending movement and the regulation and signal transduction in flagella and cilia. Historical background and the recent advance in this field are described.

Exogenous pyruvate accelerates glycolysis and promotes capacitation in human spermatozoa

BACKGROUND

There has been an ongoing debate in the reproductive field about whether mammalian spermatozoa rely on glycolysis, oxidative phosphorylation or both for their energy production. Recent studies have proposed that human spermatozoa depend mainly on glucose for motility and fertilization but the mechanism behind an efficient glycolysis in human spermatozoa is not well understood. Here, we demonstrate how human spermatozoa utilize exogenous pyruvate to enhance glycolytic ATP production, motility, hyperactivation and capacitation, events that are crucial for male fertility.

METHODS

Purified human spermatozoa from healthy donors were incubated under capacitating conditions (including albumin, bicarbonate and glucose) and tested for changes in ATP levels, motility, hyperactivation and tyrosine phosphorylation after treatment with pyruvate. The experiments were repeated in the presence of sodium cyanide in order to assess the contribution from mitochondrial respiration. The metabolism of ¹³C labeled glucose and pyruvate was traced by a combination of liquid chromatography and mass spectrometry.

RESULTS

The treatment of human spermatozoa with exogenous pyruvate increased intracellular ATP levels, progressive motility and hyperactivation by 56, 21 and 130%, respectively. In addition, added pyruvate induced a significant increase in tyrosine phosphorylation levels. Blocking of the electron transport chain did not markedly affect the results, indicating that the mechanism is independent of oxidative phosphorylation. However, the observed effects could be counteracted by oxamate, an inhibitor of lactate dehydrogenase (LDH). Metabolic tracing experiments revealed that the observed rise in ATP concentration resulted from an enhanced glycolytic flux, which was increased by more than 50% in the presence of exogenous pyruvate. Moreover, all consumed ¹³C labeled pyruvate added was converted to lactate rather than oxidized in the tricarboxylic acid cycle.

CONCLUSIONS

Human spermatozoa seem to rely mainly, if not entirely, on glycolysis as the source of ATP fueling the energy-demanding processes of motility and capacitation. The efficient glycolysis is dependent on exogenous pyruvate, which indirectly feeds the accelerated glycolysis with NAD⁺ through the LDH-mediated conversion of pyruvate to lactate. Pyruvate is present in the human female reproductive tract at concentrations in accordance with our results. As seen in other mammals, the motility and fertility of human spermatozoa seem to be dictated by the available energy substrates present in the conspecific female.

Identification and validation of mouse sperm proteins correlated with epididymal maturation

Sperm need to mature in the epididymis to become capable of fertilization. To understand the molecular mechanisms of mouse sperm maturation, we conducted a proteomic analysis using saturation dye labeling to identify proteins of caput and cauda epididymal sperm that exhibited differences in amounts or positions on two-dimensional gels. Of eight caput epididymal sperm-differential proteins, three were molecular chaperones and three were structural proteins. Of nine cauda epididymal sperm-differential proteins, six were enzymes of energy metabolism. To validate these proteins as markers of epididymal maturation, immunoblotting and immunofluorescence analyses were performed. During epididymal transit, heat shock protein 2 was eliminated with the cytoplasmic droplet and smooth muscle γ-actin exhibited reduced fluorescence from the anterior acrosome while the signal intensity of aldolase A increased, especially in the principal piece. Besides these changes, we observed protein spots, such as glutathione S-transferase mu 5 and the E2 component of pyruvate dehydrogenase complex, shifting to more basic isoelectric points, suggesting post-translational changes such dephosphorylation occur during epididymal maturation. We conclude that most caput epididymal sperm-differential proteins contribute to the functional modification of sperm structures and that many cauda epididymal sperm-differential proteins are involved in ATP production that promotes sperm functions such as motility.

Decreased motility of human spermatozoa presenting phosphatidylserine membrane translocation-cells selection with the swim-up technique

Phosphatidylserine membrane translocation (PST) is considered to be a marker of apoptosis; however, numerous studies have reported on its role in processes not related to cell death. The purpose of the study was to investigate: (1) what is the impact of PST on the motility of spermatozoa, and (2) does the swim-up isolation involve the percentage of cells presenting PST? Semen of 28 normozoospermic men (WHO criteria) was analyzed. High motility spermatozoa were isolated by the swim-up technique. The percentage of spermatozoa with PST in neat semen and after swim-up isolation was assessed with Annexin-V labeled with fluorescein, using flow cytometry technique. The spermatozoas’ motility was measured with a computer-assisted analysis system. The kinetic subpopulations of spermatozoa were identified with dedicated software and analyzed regarding PST. Vital spermatozoa with PST demonstrated progressive movement. The motion analysis system revealed a very strong positive correlation between the percentage of vital spermatozoa with PST and the percentage of spermatozoa belonging to the slow subpopulation (r = 0.83; p < 0.05), as well as a very strong negative correlation between the percentage of vital spermatozoa with PST and the percentage of spermatozoa belonging to the rapid subpopulation (r = −0.86; p < 0.05). After the swim-up isolation, the percentage of vital spermatozoa presenting PST significantly decreased (2.4 ± 2.1% vs. 5.2 ± 2.4%; p < 0.05). Spermatozoa with PST present progressive movement; however, their motility is decreased. Isolation of spermatozoa with the swim-up technique eliminates the cells with PST.

Testis-specific glyceraldehyde-3-phosphate dehydrogenase: origin and evolution

Background

Glyceraldehyde-3-phosphate dehydrogenase (GAPD) catalyses one of the glycolytic reactions and is also involved in a number of non-glycolytic processes, such as endocytosis, DNA excision repair, and induction of apoptosis. Mammals are known to possess two homologous GAPD isoenzymes: GAPD-1, a well-studied protein found in all somatic cells, and GAPD-2, which is expressed solely in testis. GAPD-2 supplies energy required for the movement of spermatozoa and is tightly bound to the sperm tail cytoskeleton by the additional N-terminal proline-rich domain absent in GAPD-1. In this study we investigate the evolutionary history of GAPD and gain some insights into specialization of GAPD-2 as a testis-specific protein.

Results

A dataset of GAPD sequences was assembled from public databases and used for phylogeny reconstruction by means of the Bayesian method. Since resolution in some clades of the obtained tree was too low, syntenic analysis was carried out to define the evolutionary history of GAPD more precisely. The performed selection tests showed that selective pressure varies across lineages and isoenzymes, as well as across different regions of the same sequences.

Conclusions

The obtained results suggest that GAPD-1 and GAPD-2 emerged after duplication during the early evolution of chordates. GAPD-2 was subsequently lost by most lineages except lizards, mammals, as well as cartilaginous and bony fishes. In reptilians and mammals, GAPD-2 specialized to a testis-specific protein and acquired the novel N-terminal proline-rich domain anchoring the protein in the sperm tail cytoskeleton. This domain is likely to have originated by exonization of a microsatellite genomic region. Recognition of the proline-rich domain by cytoskeletal proteins seems to be unspecific. Besides testis, GAPD-2 of lizards was also found in some regenerating tissues, but it lacks the proline-rich domain due to tissue-specific alternative splicing.

Sperm flagella: comparative and phylogenetic perspectives of protein components

Sperm motility is necessary for the transport of male DNA to eggs in species with both external and internal fertilization. Flagella comprise several proteins for generating and regulating motility. Central cytoskeletal structures called axonemes have been well conserved through evolution. In mammalian sperm flagella, two accessory structures (outer dense fiber and the fibrous sheath) surround the axoneme. The axonemal bend movement is based on the active sliding of axonemal doublet microtubules by the molecular motor dynein, which is divided into outer and inner arm dyneins according to positioning on the doublet microtubule. Outer and inner arm dyneins play different roles in the production and regulation of flagellar motility. Several regulatory mechanisms are known for both dyneins, which are important in motility activation and chemotaxis at fertilization. Although dynein itself has certain properties that contribute to the formation and propagation of flagellar bending, other axonemal structures-specifically, the radial spoke/central pair apparatus-have essential roles in the regulation of flagellar bending. Recent genetic and proteomic studies have explored several new components of axonemes and shed light on the generation and regulation of sperm motility during fertilization.

Lactate dehydrogenase C and energy metabolism in mouse sperm

We demonstrated previously that disruption of the germ cell-specific lactate dehydrogenase C gene (Ldhc) led to male infertility due to defects in sperm function, including a rapid decline in sperm ATP levels, a decrease in progressive motility, and a failure to develop hyperactivated motility. We hypothesized that lack of LDHC disrupts glycolysis by feedback inhibition, either by causing a defect in renewal of the NAD(+) cofactor essential for activity of glyceraldehyde 3-phosphate dehydrogenase, sperm (GAPDHS), or an accumulation of pyruvate. To test these hypotheses, nuclear magnetic resonance analysis was used to follow the utilization of labeled substrates in real time. We found that in sperm lacking LDHC, glucose consumption was disrupted, but the NAD:NADH ratio and pyruvate levels were unchanged, and pyruvate was rapidly metabolized to lactate. Moreover, the metabolic disorder induced by treatment with the lactate dehydrogenase (LDH) inhibitor sodium oxamate was different from that caused by lack of LDHC. This supported our earlier conclusion that LDHA, an LDH isozyme present in the principal piece of the flagellum, is responsible for the residual LDH activity in sperm lacking LDHC, but suggested that LDHC has an additional role in the maintenance of energy metabolism in sperm. By coimmunoprecipitation coupled with mass spectrometry, we identified 27 proteins associated with LDHC. A majority of these proteins are implicated in ATP synthesis, utilization, transport, and/or sequestration. This led us to hypothesize that in addition to its role in glycolysis, LDHC is part of a complex involved in ATP homeostasis that is disrupted in sperm lacking LDHC.

Expression of a sperm flagellum component encoded by the Als2cr12 gene

Genes exclusively expressed in male germ cells encode proteins that play important roles in spermatogenesis and fertilization. In this study, we investigated the expression of a novel spermatogenic cell-specific gene known as amyotrophic lateral sclerosis 2 chromosome region candidate 12 (Als2cr12). Our in silico and in vitro analyses revealed that the mouse Als2cr12 gene produces two transcript isoforms by alternative splicing and that one of the isoforms is unique to spermatogenic cells. Using an antibody against the ALS2CR12 protein, we found that a protein from the germ cell-specific Als2cr12 transcript is present in mature sperm from the epididymis as well as germ cells in the testis. Further analysis of the ALS2CR12 protein in sperm disclosed the localization of the protein in the sperm tail. Specifically, our data suggest that the ALS2CR12 protein is associated with the fibrous sheath in the sperm flagellum. Thus, our study provides the first information regarding the expression of the Als2cr12 gene at the transcriptional, protein and cellular levels.

Intralocus sexual conflict resolved through gene duplication

Gene duplication is mainly recognized by its primary role in the origin of new genes and functions. However, the idea that gene duplication can be a central player in resolving sexual genetic conflicts through its potential to generate sex-biased and sex-specifically expressed genes, has been almost entirely overlooked. We review recent data and theory that support gene duplication as a theoretically predicted and experimentally supported means of resolving intralocus sexual antagonism. We believe that this role is probably the consequence of sexual conflict for housekeeping genes that are required in males and females, and which are expressed in sexually dimorphic tissues (i.e. where sexually antagonistic selection is exerted). We think that these genes cannot evolve tissue-specific expression unless they duplicate.

CABYR binds to AKAP3 and Ropporin in the human sperm fibrous sheath

Calcium-binding tyrosine phosphorylation-regulated protein (CABYR) is a highly polymorphic calcium-binding tyrosine- and serine-/threonine-phosphorylated fibrous sheath (FS) protein involved in capacitation. A putative domain (amino acids 12-48) homologous to the regulatory subunit of type II cAMP-dependent protein kinase A (RII) dimerisation and A kinase-anchoring protein (AKAP)-binding domains of protein kinase A at the N-terminus suggests that CABYR may self-assemble and bind to AKAPs. Moreover, there is evidence that CABYR has limited interaction with AKAPs. However, further evidence and new relationships between CABYR and other FS proteins, including AKAPs, will be helpful in understanding the basic physiology of FS. In this study, a new strategy for co-immunoprecipitation of insoluble proteins, as well as the standard co-immunoprecipitation method in combination with mass spectrometry and western blot, was employed to explore the relationship between CABYR, AKAP3 and Ropporin. The results showed that AKAP3 was co-immunoprecipitated with CABYR by the anti-CABYR-A polyclonal antibody, and, conversely, CABYR was also co-immunoprecipitated with AKAP3 by the anti-AKAP3 polyclonal antibody. Another RII-like domain containing protein, Ropporin, was also co-immunoprecipitated with CABYR, indicating that Ropporin is one of CABYR's binding partners. The interactions between CABYR, AKAP3 and Ropporin were confirmed by yeast two-hybrid assays. Further analysis showed that CABYR not only binds to AKAP3 by its RII domain but binds to Ropporin through other regions besides the RII-like domain. This is the first demonstration that CABYR variants form a complex not only with the scaffolding protein AKAP3 but also with another RII-like domain-containing protein in the human sperm FS.

Recombinant human sperm-specific glyceraldehyde-3-phosphate dehydrogenase (GAPDHS) is expressed at high yield as an active homotetramer in baculovirus-infected insect cells

The sperm-specific glyceraldehyde-3-phosphate dehydrogenase (GAPDHS) isoform is a promising contraceptive target because it is specific to male germ cells, essential for sperm motility and male fertility, and well suited to pharmacological inhibition. However, GAPDHS is difficult to isolate from native sources and recombinant expression frequently results in high production of insoluble enzyme. We chose to use the Bac-to-Bac baculovirus-insect cell system to express a His-tagged form of human GAPDHS (Hu his-GAPDHS) lacking the proline-rich N-terminal sequence. This recombinant Hu his-GAPDHS was successfully produced in Spodoptera frugiperda 9 (Sf9) cells by infection with recombinant virus as a soluble, enzymatically active form in high yield, >35 mg/L culture. Biochemical characterization of the purified enzyme by mass spectrometry and size exclusion chromatography confirmed the presence of the tetrameric form. Further characterization by peptide ion matching mass spectrometry and Edman sequencing showed that unlike the mixed tetramer forms produced in bacterial expression systems, human his-GAPDHS expressed in baculovirus-infected insect cells is homotetrameric. The ability to express and purify active human GAPDHS as homotetramers in high amounts will greatly aid in drug discovery efforts targeting this enzyme for discovery of novel contraceptives and three compounds were identified as inhibitors of Hu his-GAPDHS from a pilot screen of 1120 FDA-approved compounds.

Testicular postgenomics: targeting the regulation of spermatogenesis

Sperm are, arguably, the most differentiated cells produced within the body of any given species. This is owing to the fact that spermatogenesis is an intricate and highly specialized process evolved to suit the individual particularities of each sexual species. Despite a vast diversity in method, the aim of spermatogenesis is always the same, the idealized transmission of genetic patrimony. Towards this goal certain requirements must always be met, such as a relative twofold reduction in ploidy, repackaging of the chromatin for transport and specialized enhancements for cell motility, recognition and fusion. In the past 20 years, the study of molecular networks coordinating male germ cell development, particularly in mammals, has become more and more facilitated thanks to large-scale analyses of genome expression. Such postgenomic endeavors have generated landscapes of data for both fundamental and clinical reproductive biology. Continuous, large-scale integration analyses of these datasets are undertaken which provide access to very precise information on a myriad of biomolecules. This review presents commonly used transcriptomic and proteomic workflows applied to various testicular germ cell studies. We will also provide a general overview of the technical possibilities available to reproductive genomic biologists, noting the advantages and drawbacks of each technique.

CABYR isoforms expressed in late steps of spermiogenesis bind with AKAPs and ropporin in mouse sperm fibrous sheath

BACKGROUND

CABYR is a polymorphic calcium-binding protein of the sperm fibrous sheath (FS) which gene contains two coding regions (CR-A and CR-B) and is tyrosine as well as serine/threonine phosphorylated during in vitro sperm capacitation. Thus far, the detailed information on CABYR protein expression in mouse spermatogenesis is lacking. Moreover, because of the complexity of this polymorphic protein, there are no data on how CABYR isoforms associate and assemble into the FS.

METHODS

The capacity of mouse CABYR isoforms to associate into dimers and oligomers, and the relationships between CABYR and other FS proteins were studied by gel electrophoresis, Western blotting, immunofluorescence, immunoprecipitation and yeast two-hybrid analyses.

RESULTS

The predominant form of mouse CABYR in the FS is an 80 kDa variant that contains only CABYR-A encoded by coding region A. CABYR isoforms form dimers by combining the 80 kDa CABYR-A-only variant with the 50 kDa variant that contains both CABYR-A and CABYR-B encoded by full length or truncated coding region A and B. It is proposed that this step is followed by the formation of larger oligomers, which then participate in the formation of the supramolecular structure of the FS in mouse sperm. The initial expression of CABYR occurs in the cytoplasm of spermatids at step 11 of spermiogenesis and increases progressively during steps 12-15. CABYR protein gradually migrates into the sperm flagellum and localizes to the FS of the principal piece during steps 15-16. Deletion of the CABYR RII domain abolished the interaction between CABYR and AKAP3/AKAP4 but did not abolish the interaction between CABYR and ropporin suggesting that CABYR binds to AKAP3/AKAP4 by its RII domain but binds to ropporin through another as yet undefined region.

CONCLUSIONS

CABYR expresses at the late stage of spermiogenesis and its isoforms oligomerize and bind with AKAPs and ropporin. These interactions strongly suggest that CABYR participates in the assembly of complexes in the FS, which may be related to calcium signaling.

Frequent and recent retrotransposition of orthologous genes plays a role in the evolution of sperm glycolytic enzymes

Background

The central metabolic pathway of glycolysis converts glucose to pyruvate, with the net production of 2 ATP and 2 NADH per glucose molecule. Each of the ten reactions in this pathway is typically catalyzed by multiple isozymes encoded by a multigene family. Several isozymes in this pathway are expressed only during spermatogenesis, and gene targeting studies indicate that they are essential for sperm function and male fertility in mouse. At least three of the novel glycolytic isozymes are encoded by retrogenes (Pgk2, Aldoart1, and Aldoart2). Their restricted expression profile suggests that retrotransposition may play a significant role in the evolution of sperm glycolytic enzymes.

Results

We conducted a comprehensive genomic analysis of glycolytic enzymes in the human and mouse genomes and identified several intronless copies for all enzymes in the pathway, except Pfk. Within each gene family, a single orthologous gene was typically retrotransposed frequently and independently in both species. Several retroposed sequences maintained open reading frames (ORFs) and/or provided evidence of alternatively spliced exons. We analyzed expression of sequences with ORFs and <99% sequence identity in the coding region and obtained evidence for the expression of an alternative Gpi1 transcript in mouse spermatogenic cells.

Conclusions

Our analysis detected frequent, recent, and lineage-specific retrotransposition of orthologous glycolytic enzymes in the human and mouse genomes. Retrotransposition events are associated with LINE/LTR and genomic integration is random. We found evidence for the alternative splicing of parent genes. Many retroposed sequences have maintained ORFs, suggesting a functional role for these genes.

Sequential reactions of surface- tethered glycolytic enzymes

The development of complex hybrid organic-inorganic devices faces several challenges, including how they can generate energy. Cells face similar challenges regarding local energy production. Mammalian sperm solve this problem by generating ATP down the flagellar principal piece by means of glycolytic enzymes, several of which are tethered to a cytoskeletal support via germ-cell-specific targeting domains. Inspired by this design, we have produced recombinant hexokinase type 1 and glucose-6-phosphate isomerase capable of oriented immobilization on a nickel-nitrilotriacetic acid modified surface. Specific activities of enzymes tethered via this strategy were substantially higher than when randomly adsorbed. Furthermore, these enzymes showed sequential activities when tethered onto the same surface. This is the first demonstration of surface-tethered pathway components showing sequential enzymatic activities, and it provides a first step toward reconstitution of glycolysis on engineered hybrid devices.

Phosphoglycerate kinase 2 (PGK2) is essential for sperm function and male fertility in mice

Phosphoglycerate kinase 2 (PGK2), an isozyme that catalyzes the first ATP-generating step in the glycolytic pathway, is encoded by an autosomal retrogene that is expressed only during spermatogenesis. It replaces the ubiquitously expressed phosphoglycerate kinase 1 (PGK1) isozyme following repression of Pgk1 transcription by meiotic sex chromosome inactivation during meiotic prophase and by postmeiotic sex chromatin during spermiogenesis. The targeted disruption of Pgk2 by homologous recombination eliminates PGK activity in sperm and severely impairs male fertility, but does not block spermatogenesis. Mating behavior, reproductive organ weights (testis, excurrent ducts, and seminal vesicles), testis histology, sperm counts, and sperm ultrastructure were indistinguishable between Pgk2(-/-) and wild-type mice. However, sperm motility and ATP levels were markedly reduced in males lacking PGK2. These defects in sperm function were slightly less severe than observed in males lacking glyceraldehyde-3-phosphate dehydrogenase, spermatogenic (GAPDHS), the isozyme that catalyzes the step preceding PGK2 in the sperm glycolytic pathway. Unlike Gapdhs(-/-) males, the Pgk2(-/-) males also sired occasional pups. Alternative pathways that bypass the PGK step of glycolysis exist. We determined that one of these bypass enzymes, acylphosphatase, is active in mouse sperm, perhaps contributing to phenotypic differences between mice lacking GAPDHS or PGK2. This study determined that PGK2 is not required for the completion of spermatogenesis, but is essential for sperm motility and male fertility. In addition to confirming the importance of the glycolytic pathway for sperm function, distinctive phenotypic characteristics of Pgk2(-/-) mice may provide further insights into the regulation of sperm metabolism.

Family planning: today and in the future

This review covers the state of contraceptive development noting new entries in the clinic (mainly steroidal and different delivery methods) and novel leads for nonsteroidal female- and male-methods in the pipeline. The time taken to market and the absence of partnerships with industry are stressed as major factors for the slow progress in their development.

Molecular complex of three testis-specific isozymes associated with the mouse sperm fibrous sheath: hexokinase 1, phosphofructokinase M, and glutathione S-transferase mu class 5

Mammalian sperm require ATP for motility, and most of it is generated by glycolysis. The glycolytic enzymes segregate to the principal piece region of the flagellum, where some are bound tightly to a novel cytoskeletal structure defining this region, the fibrous sheath (FS), and others are easily extracted with detergents. One of the latter is the spermatogenic cell-specific variant isozyme of hexokinase type 1 (HK1S), characterized by an N-terminal 24-amino acid spermatogenic cell-specific region (SSR). Yeast two-hybrid screens carried out using the SSR as bait determined that HK1S is tethered to muscle-type phosphofructokinase (PFKM) in the principal piece region. This led to the identification of four testis-specific Pfkm splice variants, one that overlapped a variant reported previously (Pfkm_v1) and three that were novel (Pfkm_v2, Pfkm_v3, and Pfkm_v4). They differ from Pfkm transcripts found in somatic cells by encoding a novel 67-amino acid N-terminal extension, the testis-specific region (TSR), producing a spermatogenic cell-specific PFKM variant isozyme (PFKMS). An antiserum generated to the TSR demonstrated that PFKMS is present in the principal piece and is insoluble in 1% Triton X-100 detergent. In subsequent yeast two-hybrid screens, the TSR was found to interact with glutathione S-transferase mu class 5 (GSTM5), identified previously as a spermatogenic cell-specific component of the FS. These results demonstrated that HK1S is tethered in the principal piece region by PFKMS, which in turn is bound tightly to GSTM5 in the FS.

LDHC: the ultimate testis-specific gene

Lactate dehydrogenase C (LDHC) was, to the best of our knowledge, the first testis-specific isozyme discovered in male germ cells. In fact, this was accomplished shortly before "isozymes or isoenzymes" became a field of study. LDHC was detected initially in human spermatozoa and spermatogenic cells of the testes by gel electrophoresis. Immunohistochemistry was used to localize LDHC first in early-pachytene primary spermatocytes, with an apparent increase in quantity after meiosis, to its final localization in and on the principal piece of the sperm tail. After several decades of biologic, biochemical, and genetic investigations, we now know that the lactate dehydrogenase isozymes are ubiquitous in vertebrates, developmentally regulated, tissue and cell specific, and multifunctional. Here, we will review the history of LDHC and the work that demonstrates clearly that it is required for sperm to accomplish their ultimate goal, fertilization.

Sperm-surface ATP in boar spermatozoa is required for fertilization: relevance to sperm proteasomal function

Extracellular ATP has been implicated in a number of cellular events, including mammalian sperm function. The complement of ATP-dependent sperm proteins includes six subunits of the 26S proteasome, a multi-subunit protease specific to ubiquitinated substrate-proteins. Proteolysis of ubiquitinated proteins by the 26S proteasome is necessary for the success of mammalian fertilization, including but not limited to acrosomal exocytosis (AE) and sperm-zona pellucida (ZP) penetration. The 26S proteasome is uniquely present on the sperm acrosomal surface during mammalian, ascidian, and invertebrate fertilization. The proteasome is a multi-subunit protease complex of approximately 2 MDa composed of the 19S regulatory complex and a 20S proteolytic core. Integrity of the 19S complex is maintained by six 19S ATPase subunits (PSMC1 through PSMC6). Consequently, we hypothesized that fertilization will be blocked by the depletion of sperm-surface associated ATP (ssATP). Depletion of ssATP by the Solanum tuberosum apyrase, a 49 kDa, non-cell permeant enzyme, significantly reduced the ATP content measured by an adapted luminescence-ATP assay from which all permeabilizing agents were excluded. Addition of active apyrase to porcine in vitro fertilization (IVF) medium caused a concentration dependent reduction in the overall fertilization rate. No such outcomes were observed in control groups using heat-inactivated apyrase. Apyrase treatment altered the band pattern of 19S ATPase subunits PSMC1 (Rpt2) and PSMC4 (Rpt3) in Western blotting, suggesting that it had an effect on the integrity of the sperm proteasomal 19S complex. Apyrase only altered the proteasomal core activities slightly, since these activities are not directly dependent on external ATP. In contrast, sperm treatment with MG132, a specific inhibitor of the proteasomal core chymotrypsin-like activity, inhibited the target proteolytic activity, but also induced a compensatory elevation in proteasomal peptidyl-glutamyl peptide hydrolase activity. Altogether, the present data provide an important missing piece of evidence in support of the ssATP-dependent, proteasomal-proteolytic model of sperm-ZP interactions.

In guinea pig sperm, aldolase A forms a complex with actin, WAS, and Arp2/3 that plays a role in actin polymerization

Glycolytic enzymes have, in addition to their role in energy production, other functions in the regulation of cellular processes. Aldolase A has been reported to be present in sperm, playing a key role in glycolysis; however, despite its reported interactions with actin and WAS, little is known about a non-glycolytic role of aldolase A in sperm. Here, we show that in guinea pig spermatozoa, aldolase A is tightly associated to cytoskeletal structures where it interacts with actin, WAS, and Arp2/3. We show that aldolase A spermatozoa treatment increases their polymerized actin levels. In addition, we show that there is a direct correlation between the levels of polymerized actin and the levels of aldolase A-actin interaction. Our results suggest that aldolase A functions as a bridge between filaments of actin and the actin-polymerizing machinery.

The biology of infertility: research advances and clinical challenges

Reproduction is required for the survival of all mammalian species, and thousands of essential 'sex' genes are conserved through evolution. Basic research helps to define these genes and the mechanisms responsible for the development, function and regulation of the male and female reproductive systems. However, many infertile couples continue to be labeled with the diagnosis of idiopathic infertility or given descriptive diagnoses that do not provide a cause for their defect. For other individuals with a known etiology, effective cures are lacking, although their infertility is often bypassed with assisted reproductive technologies (ART), some accompanied by safety or ethical concerns. Certainly, progress in the field of reproduction has been realized in the twenty-first century with advances in the understanding of the regulation of fertility, with the production of over 400 mutant mouse models with a reproductive phenotype and with the promise of regenerative gonadal stem cells. Indeed, the past six years have witnessed a virtual explosion in the identification of gene mutations or polymorphisms that cause or are linked to human infertility. Translation of these findings to the clinic remains slow, however, as do new methods to diagnose and treat infertile couples. Additionally, new approaches to contraception remain elusive. Nevertheless, the basic and clinical advances in the understanding of the molecular controls of reproduction are impressive and will ultimately improve patient care.

Mitochondrial function and reactive oxygen species action in relation to boar motility

Flow cytometric assays of viable boar sperm were developed to measure reactive oxygen species (ROS) formation (oxidization of hydroethidine to ethidium), membrane lipid peroxidation (oxidation of lipophilic probe C(11)-BODIPY(581/591)), and mitochondrial inner transmembrane potential (DeltaPsi(m); aggregation of mitochondrial probe JC-1) during hypothermic liquid storage and freeze-thawing of boar semen and to investigate relationships among ROS, motility, DeltaPsi(m), and ATP production. Basal ROS formation and membrane lipid peroxidation were low in viable sperm of both fresh and frozen-thawed semen, affecting < or =4%. Sperm in fresh, liquid-stored and frozen-thawed semen appeared to be equally susceptible to the activity ROS generators xanthine/xanthine oxidase, FeSO(4)/ascorbate, and hydrogen peroxide (H(2)O(2)). Of the ROS generators tested, FeSO(4)/ascorbate was specific for membrane lipid peroxidation, whereas menadione, xanthine/xanthine oxidase, and H(2)O(2) were specific for oxidization of hydroethidine. Menadione (30microM) and H(2)O(2) (300microM) decreased (P<0.05) motility by 90% during 60min of incubation. Menadione decreased (P<0.05) the incidence of sperm with high DeltaPsi(m) by 95% during 60min of the incubation, although ATP content was not decreased (P>0.05) until 120min. In contrast, H(2)O(2) did not affect DeltaPsi(m) or ATP at any time. The formation of ROS was not associated with any change in viability (90%) for either menadione or H(2)O(2) through 120min. Overall, the inhibitory affects of ROS on motility point to a mitochondrial-independent mechanism. The reduction in motility may have been due to an ROS-induced lesion in ATP utilization or in the contractile apparatus of the flagellum.

Comparison of glycolysis and oxidative phosphorylation as energy sources for mammalian sperm motility, using the combination of fluorescence imaging, laser tweezers, and real-time automated tracking and trapping

The combination of laser tweezers, fluorescent imaging, and real-time automated tracking and trapping (RATTS) can measure sperm swimming speed and swimming force simultaneously with mitochondrial membrane potential (MMP). This approach is used to study the roles of two sources of ATP in sperm motility: oxidative phosphorylation, which occurs in the mitochondria located in the sperm midpiece and glycolysis, which occurs along the length of the sperm tail (flagellum). The relationships between (a) swimming speed and MMP and (b) swimming force and MMP are studied in dog and human sperm. The effects of glucose, oxidative phosphorylation inhibitors and glycolytic inhibitors on human sperm motility are examined. The results indicate that oxidative phosphorylation does contribute some ATP for human sperm motility, but not enough to sustain high motility. The glycolytic pathway is shown to be a primary source of energy for human sperm motility.

Expression and compartmentalisation of the glycolytic enzymes GAPDH and pyruvate kinase in boar spermatogenesis

Boar spermatozoa contain isoforms of both glyceraldehyde 3-phosphate dehydrogenase (GAPDH, EC 1.2.1.12) and pyruvate kinase (PK, EC 2.7.1.40). The sperm-specific forms, GAPDH-S and PK-S, are tightly bound to cell structures. By immunofluorescence microscopy GAPDH-S and PK-S were localised in the principal piece of the boar sperm flagellum as well as in the acrosomal region of the sperm head and at the head-midpiece junction. The midpiece of the flagellum, however, contains isoforms of GAPDH and PK that were only recognised by antibodies against somatic GAPDH and PK, respectively, but not by the antibodies against GAPDH-S and PK-S. In sections of boar testis, GAPDH-S and PK-S were first detected in elongating spermatids when both the developing flagellum and the head were labelled with antibodies against GAPDH-S and PK-S. In contrast, antibodies against rabbit muscle GAPDH and PK labelled all developmental stages of germ cells and also neighbouring contractile cells. Thus, the structure-bound sperm-specific enzymes, GAPDH-S and PK-S, appeared only late in spermatogenesis simultaneously with the development of the structures to which they are bound. Anchoring glycolytic enzymes to structures in these mitochondria-free regions may secure ATP-production for both motility and acrosome function.

X-ray analysis of phosphoglycerate kinase 2, a sperm-specific isoform from Mus musculus

Phosphoglycerate kinase 2 (PGK2) is an isozyme of the glycolytic pathway that provides ATP required for sperm motility. It is encoded by an autosomal retrogene that is expressed only during spermatogenesis, concomitant with the inactivation of the X-linked Pgk1 gene. PGK2 from the mouse, Mus musculus, has been overexpressed from a plasmid in bacteria and purified. It was crystallized in three forms: as the apoenzyme, as a complex with 3-phosphoglycerate (3PG), and as a complex with 3PG and ATP. The crystal structures were solved to 2.7, 2.0, and 2.7 A resolutions, respectively. The overall fold is nearly identical with previously solved mammalian PGK1 molecules. The apoenzyme is in the "open" form; that is the N-terminal domain that can bind 3PG and the C-terminal domain that binds ATP are too far apart for the substrates to interact. Binding 3PG causes a 13 degree rotation that partially closes the structure and causes helix 13, which is disordered in the unliganded structure, to stabilize. Binding ATP leaves the protein in the open configuration but also causes helix 13 to be ordered. Sequence alignment suggests that the active site of PGK2 is essentially identical to that of the cytoplasmic PGK1, but significant differences accumulate on a side of the C-terminal domain away from the active site. These changes may mediate the binding of this isoform to other proteins within the sperm flagellum, while still allowing the hinging action between the domains that is essential to catalytic activity.

Characterization of eight novel proteins with male germ cell-specific expression in mouse

BACKGROUND

Spermatogenesis and fertilization are highly unique processes. Discovery and characterization of germ cell-specific genes are important for the understanding of these reproductive processes. We investigated eight proteins encoded by novel spermatogenic cell-specific genes previously identified from the mouse round spermatid UniGene library.

METHODS

Polyclonal antibodies were generated against the novel proteins and western blot analysis was performed with various protein samples. Germ cell specificity was investigated using testes from germ cell-less mutant mice. Developmental expression pattern was examined in testicular germ cells, testicular sperm and mature sperm. Subcellular localization was assessed by cell surface biotin labeling and trypsinization. Protein localization and properties in sperm were investigated by separation of head and tail fractions, and extractabilities by a non-ionic detergent and urea.

RESULTS

The authenticity of the eight novel proteins and their specificity to spermatogenic cells were confirmed. In examining the developmental expression patterns, we found the presence of four proteins only in testicular germ cells, a single protein in testicular germ cells and testicular sperm, and three proteins in the testicular stages and mature sperm from the epididymis. Further analysis of the three proteins present in sperm disclosed that one is located at the surface of the acrosomal region and the other two are associated with cytoskeletal structures in the sperm flagellum. We name the genes for these sperm proteins Shsp1 (Sperm head surface protein 1), Sfap1 (Sperm flagellum associated protein 1) and Sfap2 (Sperm flagellum associated protein 2).

CONCLUSION

We analyzed eight novel germ cell-specific proteins, providing new and inclusive information about their developmental and cellular characteristics. Our findings will facilitate future investigation into the biological roles of these novel proteins in spermatogenesis and sperm functions.

The mouse sperm proteome characterized via IPG strip prefractionation and LC-MS/MS identification

Proteomic profiling of the mouse spermatozoon has generated a unique and valuable inventory of candidates that can be mined for potential contraceptive targets and to further our understanding of the PTMs that regulate the functionality of this highly specialized cell. Here we report the identification of 858 proteins derived from mouse spermatozoa, 23 of which demonstrated testis only expression. The list contained many proteins that are known constituents of murine spermatozoa including Izumo, Spaca 1, 3, and 5, Spam 1, Zonadhesin, Spesp1, Smcp, Spata 6, 18, and 19, Zp3r, Zpbp 1 and 2, Spa17, Spag 6, 16, and 17, CatSper4, Acr, Cylc2, Odf1 and 2, Acrbp, and Acrv1. Certain protein families were highly represented in the proteome. For example, of the 42 gene products classified as proteases, 26 belonged to the 26S-proteasome. Of the many chaperones identified in this proteome, eight proteins with a TCP-1 domain were found, as were seven Rab guanosine triphosphatases. Finally, our list yielded three putative seven-transmembrane proteins, two of which have no known tissue distribution, an extragenomic progesterone receptor and three unique testis-specific kinases all of which may have some potential in the future regulation of male fertility.

As the world grows: contraception in the 21st century

Contraceptives that are readily available and acceptable are required in many poorer countries to reduce population growth and in all countries to prevent maternal morbidity and mortality arising from unintended pregnancies. Most available methods use hormonal steroids or are variations of barrier methods. Reports from several fora over the last 12 years have emphasized the number of unwanted pregnancies and resultant abortions, which indicate an unmet need for safe, acceptable, and inexpensive contraceptive methods. This unmet need can be assuaged, in part, by development of new nonhormonal contraceptive methods. This Review addresses the contribution that the "omic" revolution can make to the identification of novel contraceptive targets, as well as the progress that has been made for different target molecules under development.

Expression of the gene for mouse lactate dehydrogenase C (Ldhc) is required for male fertility

The lactate dehydrogenase (LDH) protein family members characteristically are distributed in tissue- and cell type-specific patterns and serve as the terminal enzyme of glycolysis, catalyzing reversible oxidation reduction between pyruvate and lactate. They are present as tetramers, and one family member, LDHC, is abundant in spermatocytes, spermatids, and sperm, but also is found in modest amounts in oocytes. We disrupted the Ldhc gene to determine whether LDHC is required for spermatogenesis, oogenesis, and/or sperm and egg function. The targeted disruption of Ldhc severely impaired fertility in male Ldhc(-/-) mice but not in female Ldhc(-/-) mice. Testis and sperm morphology and sperm production appeared to be normal. However, total LDH enzymatic activity was considerably lower in Ldhc(-/-) sperm than in wild type sperm, indicating that the LDHC homotetramer (LDH-C(4)) is responsible for most of the LDH activity in sperm. Although initially motile when isolated, there was a more rapid reduction in the level of ATP and in motility in Ldhc(-)(/-) sperm than in wild-type sperm. Moreover, Ldhc(-/-) sperm did not acquire hyperactivated motility, were unable to penetrate the zona pellucida in vitro, and failed to undergo the phosphorylation events characteristic of capacitation. These studies showed that LDHC plays an essential role in maintenance of the processes of glycolysis and ATP production in the flagellum that are required for male fertility and sperm function.

A novel pyruvate kinase (PK-S) from boar spermatozoa is localized at the fibrous sheath and the acrosome

Boar spermatozoa contain a novel pyruvate kinase (PK-S) that is tightly bound at the acrosome of the sperm head and at the fibrous sheath in the principal piece of the flagellum, while the midpiece contains a soluble pyruvate kinase (PK). PK-S could not be solubilized by detergents, but by trypsin with no loss of activity. Purified PK-S as well as PK-S still bound to cell structures and soluble sperm PK have all kinetics similar to those of rabbit muscle PK-M1. The PK-S subunit had a relative molecular mass of 64 +/- 1 x 10(3) (n = 3), i.e. slightly higher than that of PK-M1, and carried an N-terminal extension (NH(2)-TSEAM-COOH) that is lacking in native PK-M1. Evidence is provided that PK-S is encoded by the PKM gene. Antibodies produced against the N-terminus of purified PK-S (NH(2)-TSEAMPKAHMDAG-COOH) were specific for PK-S as they did not react with somatic PKs or soluble sperm PK, while anti-PK-M1 recognized both sperm PKs. Immunofluorescence microscopy showed anti-PK-S to label the acrosome and the flagellar principal piece, whereas the midpiece containing the mitochondria was labelled only by anti-PK-M1. Immunogold labelling confirmed the localization of PK-S at the acrosome. In the principal piece, both polyclonal anti-PK-M1 and anti-PK-S were found at the fibrous sheath. Our results suggest that PK-S is a major component in the structural organization of glycolysis in boar spermatozoa.