Conservation and function of a bovine sperm A-kinase anchor protein homologous to mouse AKAP82

A proteomic approach to identifying spermatozoa proteins in Indonesian native Madura bulls

Proteins assist sperm mature, transit the female reproductive tract, and recognise sperm oocytes. Indigenous Indonesian bulls, Madura bulls, have not been studied for reproductive proteomics. As local Indonesian beef livestock, Madura cattle assist in achieving food security; hence, their number must be improved. Thus, the identification of molecular proteomics-based bull fertility biomarkers is needed. This study aimed to characterise the sperm fertility function of the superior Madura bull (Bos indicus × Bos Javanicus) spermatozoa proteome. Frozen semen from eight Madura superior bulls (Bos indicus × Bos javanicus) aged 4-8 years was obtained from the artificial insemination centre (AIC) in Singosari and Lembang. Madura superior bulls are those that have passed the bull breeding soundness evaluation. Frozen sperm were thawed and centrifuged at 3000 × g for 30 min. Proteins in sperm were characterised through proteomic analysis using liquid chromatography-tandem mass spectrometry (LC-MS/MS). The resulting gene symbols for each protein were then subjected to bioinformatics tools, including UniProt, DAVID, and STRING databases. Regarding sperm fertility, the analysis revealed that 15 proteins were identified in the sperm of Madura bulls. Amongst the identified proteins, the superior Madura bull sperm contained several motilities, energy-related proteins, and chaperone proteins. A substantial portion of characterised proteins are linked to metabolic pathways and the tricarboxylic acid (TCA) cycle, contributing to sperm energy production. In conclusion, the first in-depth proteome identification of sperm related to sperm quality and bull fertility of a unique indigenous Madura breed of Indonesia was performed using the LC-MS/MS proteomic method. These findings may serve as a reference point for further studies related to the functions of bovine sperm and biomarkers of fertility and sperm quality.

Effect of Honey, Coenzyme Q10, and β-Carotene/α-Tocopherol as Novel Additives in Rabbit-Sperm Cryopreservation Extender

The effectiveness of rabbit-sperm cryopreservation is still below average compared to other domestic species. After the sperm cryopreservation process, post-thawing parameters like motility and membrane integrity are significantly compromised. The use of new extender constituents is an approach that can be used to improve the effectiveness of cryopreservation. Accordingly, we used honey (1.25, 2.5, 5, and 10%), coenzyme Q10 (100 and 200 μM), and β-carotene/α-tocopherol (500 μM/620 μM and 250 μM/310 μM) as candidate components for rabbit-sperm extenders during cryopreservation. Ejaculates from commercial adult rabbit bucks (n = 5) were cryopreserved using conventional freezing. Several post-thawing sperm parameters were assessed, including total motility, membrane integrity, viability, nuclear membrane integrity, acrosome reaction, and mitochondrial membrane potential and activation. Additionally, we performed hormonal analyses of the seminal plasma. Moreover, we analyzed the post-thawing levels of a molecular marker of sperm quality, proAKAP4, which was used in rabbits for the first time. Our findings showed that the 2.5% honey supplementation increased the post-thawing sperm motility (13.75 ± 3.75%) compared to the greater concentrations employed. However, the post-thawing motility was negatively affected by the coenzyme Q10 (0%, in both groups) but was not affected by the β-carotene/α-tocopherol supplementation (22 ± 18.15%, and 11.67 ± 10.17%). In conclusion, the cryopreservation protocols of this study did not help to maintain the sperm parameters after thawing. Further studies are required to identify novel protocols to mitigate the damage caused to rabbit sperm during cryopreservation.

Testis-specific serine kinase 3 is required for sperm morphogenesis and male fertility

BACKGROUND

The importance of phosphorylation in sperm during spermatogenesis has not been pursued extensively. Testis-specific serine kinase 3 (Tssk3) is a conserved gene, but TSSK3 kinase functions and phosphorylation substrates of TSSK3 are not known.

OBJECTIVE

The goals of our studies were to understand the mechanism of action of TSSK3.

MATERIALS AND METHODS

We analyzed the localization of TSSK3 in sperm, used CRISPR/Cas9 to generate Tssk3 knockout (KO) mice in which nearly all of the Tssk3 open reading frame was deleted (ensuring it is a null mutation), analyzed the fertility of Tssk3 KO mice by breeding mice for 4 months, and conducted phosphoproteomics analysis of male testicular germ cells.

RESULTS

TSSK3 is expressed in elongating sperm and localizes to the sperm tail. To define the essential roles of TSSK3 in vivo, heterozygous (HET) or homozygous KO male mice were mated with wild-type females, and fertility was assessed over 4 months; Tssk3 KO males are sterile, whereas HET males produced normal litter sizes. The absence of TSSK3 results in disorganization of all stages of testicular seminiferous epithelium and significantly increased vacuolization of germ cells, leading to dramatically reduced sperm counts and abnormal sperm morphology; despite these histologic changes, Tssk3 null mice have normal testis size. To elucidate the mechanisms causing the KO phenotype, we conducted phosphoproteomics using purified germ cells from Tssk3 HET and KO testes. We found that proteins implicated in male infertility, such as GAPDHS, ACTL7A, ACTL9, and REEP6, showed significantly reduced phosphorylation in KO testes compared to HET testes, despite unaltered total protein levels.

CONCLUSIONS

We demonstrated that TSSK3 is essential for male fertility and crucial for phosphorylation of multiple infertility-related proteins. These studies and the pathways in which TSSK3 functions have implications for human male infertility and nonhormonal contraception.

Modulatory effect of MG-132 proteasomal inhibition on boar sperm motility during in vitro capacitation

A series of biochemical and biophysical changes during sperm capacitation initiates various signaling pathways related to protein phosphorylation leading to sperm hyperactivation, simultaneously with the regulation of proteasomal activity responsible for protein degradation and turnover. Our study aimed to unveil the role of the proteasome in the regulation of boar sperm motility, hyperactivated status, tyrosine phosphorylation, and total protein ubiquitination. The proteolytic activity of the 20S proteasomal core was inhibited by MG-132 in concentrations of 10, 25, 50, and 100 μM; and monitored parameters were analyzed every hour during 3 h of in vitro capacitation (IVC). Sperm motility and kinematic parameters were analyzed by Computer Assisted Sperm Analysis (CASA) during IVC, showing a significant, negative, dose-dependent effect of MG-132 on total and progressive sperm motility (TMOT, PMOT, respectively). Furthermore, proteasomal inhibition by 50 and 100 μM MG-132 had a negative impact on velocity-based kinematic sperm parameters (VSL, VAP, and VCL). Parameters related to the progressivity of sperm movement (LIN, STR) and ALH were the most affected by the highest inhibitor concentration (100 μM). Cluster analysis revealed that the strongest proteasome-inhibiting treatment had a significant effect (p ≤ 0.05) on the hyperactivated sperm subpopulation. The flow cytometric viability results proved that reduced TMOT and PMOT were not caused by disruption of the integrity of the plasma membrane. Neither the protein tyrosine phosphorylation profile changes nor the accumulation of protein ubiquitination was observed during the course of capacitation under proteasome inhibition. In conclusion, inhibition of the proteasome reduced the ability of spermatozoa to undergo hyperactivation; however, there was no significant effect on the level of protein tyrosine phosphorylation and accumulation of ubiquitinated proteins. These effects might be due to the presence of compensatory mechanisms or the alteration of various ubiquitin-proteasome system-regulated pathways.

ProAKAP4 Concentration Is Related to Sperm Motility and Motile Sperm Subpopulations in Frozen-Thawed Horse Semen

ProAKAP4 is the precursor of AKAP4 (A-kinase Anchor protein 4), the main structural protein of the fibrous sheath of sperm. The amount of proAKAP4 reflects the ability of spermatozoa to maintain the flagellum activity and functionality up to the site of fertilization and is positively correlated with progressive motility in several mammalian species. The aim of this study was to investigate the relationship between proAKAP4 concentration with horse sperm motility descriptors and spermatic motile subpopulations. For this purpose, a total of 48 ejaculates from 13 different stallions were analyzed. Spermatic motility descriptors were obtained by the CASA system, and four motile subpopulations (SP) with specific motility patterns were statistically identified. ProAKAP4 concentrations were evaluated by ELISA. The relationship between motility descriptors of sperm subpopulations and proAKAP4 concentrations was evaluated. Following a hierarchical cluster statistical analysis, ejaculates were divided into two groups according to their proAKAP4 concentrations, either having low proAKAP4 concentrations (5.06−35.61 ng/10M spz; n = 23) or high (39.92−82.23 ng/10M spz; n = 25) proAKAP4 concentrations (p < 0.001). ProAKAP4 concentrations were positively correlated (p < 0.05) with total and progressive motility, as well as with parameters of velocity. ProAKAP4 amount also showed a negative correlation (p < 0.05) with sperm motile subpopulation number 3, which was the subpopulation with the lowest velocity parameters. In conclusion, proAKAP4 concentration in stallion semen positively reflects sperm progressive motility with the functional velocity kinematic descriptors. Concentrations of proAKAP4 higher than 37.77 ng/10M spz were correlated with a very good quality frozen/thawed stallion semen.

ProAKAP4 Semen Concentrations as a Valuable Marker Protein of Post-Thawed Semen Quality and Bull Fertility: A Retrospective Study

Functional sperm quality markers to predict bull fertility have been actively investigated. Among them, proAKAP4, which is the precursor of AKAP4, the main structural protein in the fibrous sheath of spermatozoa; appears to be promising, especially since spermatozoa lacking AKAP4 expression were shown to be immotile, abnormal, and infertile. In this study, the objective was to evaluate proAKAP4 concentration values with the classic sperm motility descriptors and fertility outcomes (NRR at 90 days) in post-thawed conditions of 10 bulls' semen. ProAKAP4 expression was confirmed by Western blotting and proAKAP4 concentrations were determined by ELISA. Variations in proAKAP4 concentrations were observed independently of the motility sperm descriptors measured using computer-assisted semen analysis (CASA). A ProAKAP4 concentration of 38.67 ± 8.55 ng/10 million spermatozoa was obtained as a statistical mean of all samples. Threshold values of proAKAP4 were then determined between 19.96 to 96.95 ng/10 million spermatozoa. ProAKAP4 concentrations were positively correlated with progressive motility and the linearity coefficient. The sperm showing the lowest progressive motility were the samples exhibiting proAKAP4 concentrations below 20 ng/10 million spermatozoa. Furthermore, proAKAP4 concentrations were significantly higher in bulls with a higher NRR in the field. Our results demonstrate a correlation between the semen concentration of proAKAP4 and NRR-90d (p = 0.05) in post-thawed bull semen, highlighting the potential of proAKAP4 as a predictive marker of bull fertility.

Signaling Enzymes Required for Sperm Maturation and Fertilization in Mammals

In mammals, motility and fertilizing ability of spermatozoa develop during their passage through the epididymis. After ejaculation, sperm undergo capacitation and hyperactivation in the female reproductive tract - a motility transition that is required for sperm penetration of the egg. Both epididymal initiation of sperm motility and hyperactivation are essential for male fertility. Motility initiation in the epididymis and sperm hyperactivation involve changes in metabolism, cAMP (cyclic adenosine mono-phosphate), calcium and pH acting through protein kinases and phosphatases. Despite this knowledge, we still do not understand, in biochemical terms, how sperm acquire motility in the epididymis and how motility is altered in the female reproductive tract. Recent data show that the sperm specific protein phosphatase PP1γ2, glycogen synthase kinase 3 (GSK3), and the calcium regulated phosphatase calcineurin (PP2B), are involved in epididymal sperm maturation. The protein phosphatase PP1γ2 is present only in testis and sperm in mammals. PP1γ2 has a isoform-specific requirement for normal function of mammalian sperm. Sperm PP1γ2 is regulated by three proteins - inhibitor 2, inhibitor 3 and SDS22. Changes in phosphorylation of these three inhibitors and their binding to PP1γ2 are involved in initiation and activation of sperm motility. The inhibitors are phosphorylated by protein kinases, one of which is GSK3. The isoform GSK3α is essential for epididymal sperm maturation and fertility. Calcium levels dramatically decrease during sperm maturation and initiation of motility suggesting that the calcium activated sperm phosphatase (PP2B) activity also decreases. Loss of PP2B results in male infertility due to impaired sperm maturation in the epididymis. Thus the three signaling enzymes PP1γ2, GSK3, and PP2B along with the documented PKA (protein kinase A) have key roles in sperm maturation and hyperactivation. Significantly, all these four signaling enzymes are present as specific isoforms only in placental mammals, a testimony to their essential roles in the unique aspects of sperm function in mammals. These findings should lead to a better biochemical understanding of the basis of male infertility and should lead to novel approaches to a male contraception and managed reproduction.

A Kinase Anchor Protein 4 Is Vulnerable to Oxidative Adduction in Male Germ Cells

Oxidative stress is a leading causative agent in the defective sperm function associated with male infertility. Such stress commonly manifests via the accumulation of pathological levels of the electrophilic aldehyde, 4-hydroxynonenal (4HNE), generated as a result of lipid peroxidation. This highly reactive lipid aldehyde elicits a spectrum of cytotoxic lesions owing to its propensity to form stable adducts with biomolecules. Notably however, not all elements of the sperm proteome appear to display an equivalent vulnerability to 4HNE modification, with only a small number of putative targets having been identified to date. Here, we validate one such target of 4HNE adduction, A-Kinase Anchor Protein 4 (AKAP4); a major component of the sperm fibrous sheath responsible for regulating the signal transduction and metabolic pathways that support sperm motility and capacitation. Our data confirm that both the precursor (proAKAP4), and mature form of AKAP4, are conserved targets of 4HNE adduction in primary cultures of post-meiotic male germ cells (round spermatids) and in mature mouse and human spermatozoa. We further demonstrate that 4HNE treatment of round spermatids and mature spermatozoa results in a substantial reduction in the levels of both proAKAP4 and AKAP4 proteins. This response proved refractory to pharmacological inhibition of proteolysis, but coincided with an apparent increase in the degree of protein aggregation. Further, we demonstrate that 4HNE-mediated protein degradation and/or aggregation culminates in reduced levels of capacitation-associated phosphorylation in mature human spermatozoa, possibly due to dysregulation of the signaling framework assembled around the AKAP4 scaffold. Together, these findings suggest that AKAP4 plays an important role in the pathophysiological responses to 4HNE, thus strengthening the importance of AKAP4 as a biomarker of sperm quality, and providing the impetus for the design of an efficacious antioxidant-based intervention strategy to alleviate sperm dysfunction.

The protein tyrosine phosphorylation during in vitro capacitation and cryopreservation of mammalian spermatozoa

Before the process of fertilization, spermatozoa necessitate a period of residence in the female reproductive environment, and undergo a sequence of physiological and biochemical changes collectively referred to as capacitation. Accumulated evidences from several laboratories indicated that the protein tyrosine phosphorylation (PTP) is one of the most important intracellular signaling events regulating sperm function, and is a meaningful indicator of capacitation. Different factors that affect PTP are cholesterol efflux, influx of HCO3(-), increased intracellular Ca(2+), cAMP and reactive oxygen species (ROS). cAMP/PKA and extracellular signal regulated kinases (ERKs) are the known important signaling pathways primarily involved in PTP. Advanced proteomics approaches have revealed several proteins that undergo tyrosine phosphorylation during capacitation. Semen cryopreservation subjects spermatozoa to frequent stressors, which result in capacitation like changes (cryo-capacitation). The cryo-capacitated spermatozoa usually show different patterns of PTP than the normal in vitro capacitated spermatozoa. In the current manuscript, we have summarized some information about the proteins undergoing tyrosine phosphorylation during capacitation and the effect of cryopreservation on PTP as well as the possibilities to reduce the changes associated with cryopreservation process.

PCSK4-null sperm display enhanced protein tyrosine phosphorylation and ADAM2 proteolytic processing during in vitro capacitation

OBJECTIVE

To study the molecular basis for the accelerated capacitation rate in PCSK4-null sperm.

DESIGN

Comparative and controlled experimental research study.

SETTING

Academic medical institute.

ANIMAL(S)

Male mice C57BL/6J wild-type or null congenics for the Pcsk4 allele.

INTERVENTION(S)

Cauda and epididymal sperm were capacitated for varying times.

MAIN OUTCOME MEASURE(S)

Differences in sperm protein tyrosine phosphorylation and proteolytic processing of sperm-egg ligands ADAM2 and ADAM3.

RESULT(S)

The PCSK4-null sperm proteins are hyper-tyrosine phosphorylated during capacitation. This hyperphosphorylation is dependent on protein kinase A (PKA), albumin, and calcium. There is also more ADAM2 proteolytic processing from a 46-kDa form of ADAM2 to a 27-kDa form in PCSK4-null sperm than in wild-type sperm. This processing is dependent on cholesterol efflux.

CONCLUSION(S)

Lack of PCSK4 is associated with quantitative changes in the phosphorylation and proteolysis of sperm proteins during capacitation; therefore, alterations in signal transduction and proteolytic processing during capacitation may underlie the fertilization incompetence of PCSK4-null sperm. More investigation is needed to determine how and to what extent these changes might contribute to the loss of fertilizing ability of PCSK4-null sperm.

The Mouse t Complex Gene Tsga2, Encoding Polypeptides Located in the Sperm Tail and Anterior Acrosome, Maps to a Locus Associated with Sperm Motility and Sperm-Egg Interaction Abnormalities1

Previous studies of sperm from mice heterozygous for a t haplotype (t) and heterospecific combinations of the t complex identified two tightly linked genetic factors responsible for t/t male sterility related to expression of the flagellar waveform aberration, curlicue. Dnahc8, an axonemal dynein heavy chain gene, is a strong candidate for the proximal factor, Ccua, but the identity of the distal factor, Ccub, is unknown. In the present study, we employ motility assays of sperm from males heterozygous for t and novel heterospecific combinations of the t complex to demonstrate that Ccub is a composite of at least two synergic elements, Ccub1, positioned within a genomic interval spanning approximately 0.6 Mb immediately distal to Dnahc8, and Ccub2, situated in a region approximately 4-7 Mb distal to Ccub1. We also show that Tsga2, a testis-restricted gene, fulfills many of the prerequisites required to make it a strong candidate for Ccub1. These include: 1) its location within the aforementioned genomic interval; 2) a highly reduced level of testis expression by its heterospecific allele relative to the level of expression of its t allele; 3) determination that TSGA2(t) carries numerous nonsynonymous mutations in residues otherwise highly conserved in all known orthologous proteins; 4) the detection of major TSGA2 polypeptides in sperm protein extracts; and 5) the apparent distribution of these polypeptides in major sperm tail structures. Surprisingly, these TSGA2 isoforms appear to localize in the vicinity of the anterior acrosome, as well, suggesting that Tsga2 may also play a role in sperm-egg interaction. Finally, our results indicate that a TSGA2 polypeptide with apparent similarities to the smaller of the two sperm isoforms is expressed by epididymal cells.

Moving to the beat: a review of mammalian sperm motility regulation

Because it is generally accepted that a high percentage of poorly motile or immotile sperm will adversely affect male fertility, analysis of sperm motility is a central part of the evaluation of male fertility. In spite of its importance to fertility, poor sperm motility remains only a description of a pathology whose underlying cause is typically poorly understood. The present review is designed to bring the clinician up to date with the most current understanding of the mechanisms that regulate sperm motility and to raise questions about how aberrations in these mechanisms could be the underlying causes of this pathology.

Characterization of an A-kinase anchor protein in equine spermatozoa and examination of the effect of semen cooling and cryopreservation on the binding of that protein to the regulatory subunit of protein kinase-A

OBJECTIVE

To determine whether a homologue of A-kinase anchor protein 4 (AKAP4) is present and functional as an AKAP in equine spermatozoa and examine the effect of semen cooling and cryopreservation on binding of equine AKAP4 to the regulatory (RII) subunit of protein kinase-A (PK-A).

SAMPLE POPULATION

Ejaculated semen collected from 2 fertile stallions, 3 bulls, and 3 humans.

PROCEDURE

Identification of an equine homologue of AKAP4 was investigated via DNA sequencing. Protein was extracted from the spermatozoa of each species for immunoblot analysis to identify AKAP4 and its precursor protein, pro-AKAP4; immunofluorescence microscopy was used to localize those proteins in spermatozoa. Ligand overlay assays were used to determine whether the identified proteins bound to the RII subunit of PK-A and whether cooling or cryopreservation of spermatozoa affected that binding.

RESULTS

The partial genomic sequence of AKAP4 was identified in equine spermatozoa, and immunoblot analysis confirmed that AKAP4 and pro-AKAP4 are present in equine spermatozoa. Via immunofluorescence microscopy, these proteins were localized to the spermatozoal principal piece. Results of ligand overlay assays indicated that equine AKAP4 and pro-AKAP4 bind to the RII subunit of PK-A and are AKAPs; AKAP4-RII binding was not affected by cooling or cryopreservation of spermatozoa.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that equine AKAP4 anchors PK-A to the spermatozoal flagellum (where the kinase is likely to be required for the regulation of spermatozoal motility), but decreases in spermatozoal motility in cooled or cryopreserved semen are not associated with decreased binding of AKAP4 and PK-A.

Role of tyrosine phosphorylation in sperm capacitation / acrosome reaction

Capacitation is an important physiological pre-requisite before the sperm cell can acrosome react and fertilize the oocyte. Recent reports from several laboratories have amply documented that the protein phosphorylation especially at tyrosine residues is one of the most important events that occur during capacitation. In this article, we have reviewed the data from our and other laboratories, and have constructed a heuristic model for the mechanisms and molecules involved in capacitation/acrosome reaction.

Increased phosphorylation of AKAP by inhibition of phosphatidylinositol 3-kinase enhances human sperm motility through tail recruitment of protein kinase A

Sperm motility is regulated by a complex balance between kinases and phosphatases. Among them, phosphatidylinositol 3-kinase (PI 3-kinase) has been recently suggested to negatively regulate sperm motility (Luconi, M., Marra, F., Gandini, L., Lenzi, A., Filimberti, E., Forti, G. and Baldi, E. (2001). Hum. Reprod. 16, 1931-1937). We demonstrate the presence and activity of PI 3-kinase in human spermatozoa and have investigated the molecular mechanism(s) by which the PI 3-kinase inhibitor, LY294002, triggers an increase in sperm motility. PI 3-kinase inhibition results in an increase in intracellular cAMP levels and in tyrosine phosphorylation of the protein kinase A-anchoring protein AKAP3. These effects finally result in a stimulation of protein kinase A (PKA) binding to AKAP3 in sperm tails through the regulatory subunit RIIβ. The increased binding of RIIβ to AKAP3 induced by LY294002 is mainly due to tyrosine phosphorylation of AKAP3, since it is completely blocked by the tyrosine kinase inhibitor erbstatin, which also reverses the effects of LY294002 on motility and suppresses PKA-AKAP3 interaction. The requirement of PKA binding to AKAP3 for sperm motility is confirmed by the reduction of motility induced by an inhibitor of RIIβ-AKAP3 binding, Ht31, whose effects on sperm motility and PKA binding to AKAP3 are reversed by LY294002.

These results demonstrate that PI 3-kinase negatively regulates sperm motility by interfering with AKAP3-PKA binding, providing the first evidence of a molecular mechanism by which PKA can be targeted to sperm tails by interaction with tyrosine phosphorylated form of AKAP3.

Tsga10 encodes a 65-kilodalton protein that is processed to the 27-kilodalton fibrous sheath protein

We had previously reported the isolation of the testis-specific human gene Tsga10, which is not expressed in testes from two infertile patients. To study its role and function, we cloned the mouse homologue Mtsga10. Mtsga10 localizes to mouse chromosome 1, band B. This region is syntenic with human chromosome 2q11.2, where Tsga10 is located. We demonstrate that Mtsga10 mRNA is expressed in testis, but not in other adult tissues, and in several human fetal tissues and primary tumors. We uncovered that different species use different first exons and, consequently, different promoters. Using several antibodies, we discovered that, in mouse testis, Mtsga10 encodes a 65-kDa spermatid protein that appears to be processed to a 27-kDa protein of the fibrous sheath, a major sperm tail structure, in mature spermatozoa. Mtsga10 protein contains a putative myosin/Ezrin/radixin/moesin (ERM) domain. Transfection of fibroblasts with GFP-Mtsga10 fusion protein results in formation of short, thick filaments and deletion of the myosin/ERM domain abolished filament formation. Our results suggest the possibility that Tsga10 plays a role in the sperm tail fibrous sheath.

Phosphorylation of rat spermatidal protein TP2 by sperm-specific protein kinase A and modulation of its transport into the haploid nucleus

Transition protein 2 (TP2), which is expressed during stages 12-15 of mammalian spermiogenesis, has been shown to undergo phosphorylation immediately after its synthesis. We reported earlier that TP2 is phosphorylated in vitro at threonine 101 and serine 109 by the salt extract of sonication-resistant (elongating and elongated) spermatid nuclei and the protein kinase phosphorylating TP2 was identified to be protein kinase A (PKA). We now report that the cytosol from haploid spermatids but not from premeiotic germ cells is able to phosphorylate recombinant TP2 in vitro at threonine 101 and serine 109. The kinase present in the haploid spermatid cytosol that phosphorylates TP2 has been identified to be the sperm-specific isoform of protein kinase A (Cs-PKA). Reverse transcription-PCR analysis indicated that Cs-PKA was present in the haploid spermatids and absent from premeiotic germ cells. The rat Cs-PKA transcript was amplified and sequenced using the isoform-specific primers. The sequence of rat Cs-PKA at the N terminus differs from mouse and human by one amino acid. Western blot analysis using specific anti-Calpha1 antibodies revealed that Calpha1-PKA is absent in haploid spermatid cytosol. We have also established an in vitro nuclear transport assay for the haploid round spermatids. Using this assay, we have found that the cytoplasmic factors and ATP are absolutely essential for translocation of TP2 into the nucleus. Phosphorylation was found to positively modulate the NLS dependent import of TP2 into the nucleus.

Phosphorylation of protein tyrosine residues in fresh and cryopreserved stallion spermatozoa under capacitating conditions

Phosphorylation of tyrosine residues on sperm proteins is one important intracellular mechanism regulating sperm function that may be a meaningful indicator of capacitation. There is substantial evidence that cryopreservation promotes the capacitation of sperm and this cryocapacitation is frequently cited as one factor associated with the reduced longevity of cryopreserved sperm in the female reproductive tract. This study was designed to determine whether stallion sperm express different levels of tyrosine phosphorylation after in vitro capacitation and whether thawed sperm display similar phosphorylation characteristics in comparison with freshly ejaculated sperm. Experiments were performed to facilitate comparisons of tyrosine phosphorylation, motility, and viability of sperm prior to and following in vitro capacitation in fresh and frozen-thawed sperm. We hypothesized that equine spermatozoa undergo tyrosine phosphorylation during capacitation and that this phosphorylation is modified when sperm have been cryopreserved. We also hypothesized that tyrosine phosphorylation could be enhanced by the use of the activators dibutyryl cAMP (db cAMP) and caffeine, as well as methyl beta-cyclodextrin-which causes cholesterol efflux from the spermatozoa-and inhibited by the protein kinase A (PK-A) inhibitor H-89. Our results indicate that equine sperm capacitation is mediated by a signaling pathway that involves cAMP-dependent PK-A and tyrosine kinases and that cryopreserved sperm may be more sensitive to inducers of capacitation, which could explain their limited life span when compared with fresh sperm.

Reactivation of motility of demembranated hamster spermatozoa: role of protein tyrosine kinase and protein phosphatases

Demembranated cauda epididymidal spermatozoa of hamster, following reactivation with 1 mm ATP, exhibited either a loop or planar type of motility. The spermatozoa with planar motility exhibited increased progressive velocity (VSL), straightness (STR), linearity (LIN) and beat cross frequency (BCF) compared to the spermatozoa with loop type motility. cAMP was observed to have differential effects on the motility parameters of the demembranated spermatozoa depending on the type of motility. For instance, in the loop type, average path velocity (VAP), curvilinear velocity (VCL) and VSL were increased in the presence of cAMP unlike in the planar type. Furthermore, in an attempt to understand the role of protein kinases and protein phosphatases in regulation of sperm motility, the effects of various inhibitors of these enzymes on the motility and phosphorylation of proteins of reactivated demembranated spermatozoa were studied. Inhibitors of PTKase (protein tyrosine kinase) and protein phosphatases inhibited the motility of reactivated demembranated hamster spermatozoa. The activity of the respective enzymes associated with demembranated spermatozoa was concurrently inhibited, thus providing evidence that the effect of the inhibitors on motility was mediated through their inhibitory effects on the activities of the enzymes. The results also demonstrated that two phosphotyrosinylated proteins of molecular weight 65 and 80 kDa showed reduced phosphorylation in the presence of PTKase inhibitors, thus indicating their possible role in reactivation of motility of demembranated hamster spermatozoa.

Identification of the major tyrosine phosphorylated protein of capacitated hamster spermatozoa as a homologue of mammalian sperm a kinase anchoring protein

The molecular basis of mammalian sperm capacitation is unique in that, it is associated with a protein kinase A (PKA) dependent upregulation of protein tyrosine phosphorylation. Therefore, PKA activity during capacitation would be crucial for the downstream events of protein tyrosine phosphorylation, and mechanisms may exist to ensure that PKA phosphorylates its specific substrate. This could be achieved by bringing PKA close to its substrate, a function normally carried out by an A-kinase anchoring protein (AKAP). We showed previously that cauda epididymidal spermatozoa of hamster undergo a capacitation-dependent increase in protein tyrosine phosphorylation. In the present study, evidence is provided that two major tyrosine phosphorylated proteins of molecular weight 97 and 83 kDa are the hamster homologues of mouse pro-AKAP82 and AKAP82, and have been designated as hamster pro-AKAP83 and AKAP83 respectively. Hamster AKAP83 resembled the mouse AKAP82 with respect to its molecular weight, pI (pH 5-5.5) and cDNA and amino acid sequences. Sequence analysis indicated that the primary structure of pro-AKAP83 was highly conserved and exhibited 91% identity with mouse and rat AKAP82. Further, the functional domains, namely the region involved in binding the regulatory subunit of PKA and the proteolytic cleavage site between pro-AKAP83 and AKAP83, were identical with that observed in rat and mouse pro-AKAP82 and AKAP82. Immunoblot analysis using polyclonal hamster anti-AKAP83 antibodies indicated that AKAP83 was present both in caput and cauda epididymidal spermatozoa. The antibody also identified the pro-AKAP82 and AKAP82 in mouse caput and cauda epididymidal spermatozoa. Immunofluorescence studies indicated that AKAP83 in hamster spermatozoa was localized along the length of principal piece of the tail. It was also demonstrated that hamster pro-AKAP83/AKAP83 gene expression was testis specific and was not expressed in other organs in either sex. This is the first report implicating AKAP in capacitation in rodents.

A-kinase anchor proteins in endocrine systems and reproduction

Over the past few years, significant progress has been made in characterizing the expression and localization of proteins that act as scaffolds for cAMP-dependent protein kinase (PK-A). These A-kinase anchor proteins (AKAPs) tether PK-A to intracellular organelles and structures, sequestering the kinase near its physiological substrates. The compartmentalization of distinct pockets of PK-A activity serves to provide spatial regulation of this signaling pathway. In addition, other signaling proteins bind to AKAPs, as do some newly described proteins of unknown function, suggesting that proteins of various pathways are anchored through AKAPs.

Cloning and chromosomal localization of a gene encoding a novel serine/threonine kinase belonging to the subfamily of testis-specific kinases

Using reverse transcription-polymerase chain reaction (RT-PCR) with degenerate oligonucleotides corresponding to two highly conserved motifs within the protein kinase family of catalytic domains, we isolated a PCR fragment encoding a novel member of the testis-specific serine/threonine kinases (STK) from mouse male mixed germ cell mRNA. This PCR fragment recognized a 1020-bp transcript in male germ cells by northern blot analysis and was used to clone a full-length cDNA from a mouse mixed germ cell cDNA library. This cDNA has an open reading frame of 804 bases encoding a protein of 268 amino acids. This novel gene is almost identical to Stk22c, encoding a recently described testis-specific protein kinase, except for base-pair deletions that result in a shift in the coding region and an alteration of 22 amino acids (residues 109-131). Due to its homology with Stk22c, we have called this protein kinase gene Stk22d. Northern blot analysis revealed that this protein kinase is developmentally expressed in testicular germ cells and is not present in brain, ovary, kidney, liver, or early embryonic cells. We then cloned the human homologue of this protein kinase gene (STK22C) and found it to be expressed exclusively in the testis. Fluorescence in situ hybridization with both the human and mouse cDNA clones revealed syntenic localization on chromosomes 1p34-p35 and 4E1, respectively.

Differential expression of the C(s) and Calpha1 isoforms of the catalytic subunit of cyclic 3',5'-adenosine monophosphate-dependent protein kinase testicular cells

The amino terminus of the sperm cAMP-dependent protein kinase catalytic subunit (termed C(s)) differs from that of the Calpha1 isoform expressed in most tissues due to the use of alternative transcripts of the Calpha gene. Both Calpha1 and C(s) transcripts are present in testis; C(s) is expressed specifically in spermatogenic cells and is the only C isoform detected in mature sperm. Immunohistochemistry of mouse testis using antibodies specific for C(s) and Calpha1 now shows that Calpha1 is present in somatic testicular cells, spermatogonia, and preleptotene spermatocytes but not in cells that are in later stages of spermatogenesis. In contrast, C(s) is expressed only in midpachytene and later stage spermatocytes and in spermatids. Therefore, C(s) and Calpha1 expression do not overlap. Immunofluorescence microscopic localization of C(s) in murine and ovine sperm reveals that C(s) is located primarily in sperm tail components, including the midpiece mitochondria and the axoneme. Quantitative analysis of Western blots indicates that individual ovine sperm contain approximately 4 x 10(5) molecules of C(s), a seemingly large number for a protein that acts catalytically.

Flagellar radial spoke protein 3 is an A-kinase anchoring protein (AKAP)

Previous physiological and pharmacological experiments have demonstrated that the Chlamydomonas flagellar axoneme contains a cAMP-dependent protein kinase (PKA) that regulates axonemal motility and dynein activity. However, the mechanism for anchoring PKA in the axoneme is unknown. Here we test the hypothesis that the axoneme contains an A-kinase anchoring protein (AKAP). By performing RII blot overlays on motility mutants defective for specific axonemal structures, two axonemal AKAPs have been identified: a 240-kD AKAP associated with the central pair apparatus, and a 97-kD AKAP located in the radial spoke stalk. Based on a detailed analysis, we have shown that AKAP97 is radial spoke protein 3 (RSP3). By expressing truncated forms of RSP3, we have localized the RII-binding domain to a region between amino acids 144-180. Amino acids 161-180 are homologous with the RII-binding domains of other AKAPs and are predicted to form an amphipathic helix. Amino acid substitution of the central residues of this region (L to P or VL to AA) results in the complete loss of RII binding. RSP3 is located near the inner arm dyneins, where an anchored PKA would be in direct position to modify dynein activity and regulate flagellar motility.

Possible roles of protein kinase A in cell motility and excystation of the early diverging eukaryote Giardia lamblia

Since little is known of how the primitive protozoan parasite, Giardia lamblia, senses and responds to its changing environment, we characterized a giardial protein kinase A (gPKA) catalytic subunit with unusual subcellular localization. Sequence analysis of the 1080-base pair open reading frame shows 48% amino acid identity with the cyclic AMP-dependent kinase from Euglena gracilis. Northern analysis indicated a 1.28- kilobase pair transcript at relatively constant concentrations during growth and encystation. gPKA is autophosphorylated, although amino acid residues corresponding to Thr-197 and Ser-338 of human protein kinase A (PKA) that are important for autophosphorylation are absent. Kinetic analysis of the recombinant PKA showed that ATP and magnesium are preferred over GTP and manganese. Kinase activity of the native PKA has also been detected in crude extracts using kemptide as a substrate. A myristoylated PKA inhibitor, amide 14-22, inhibited excystation with an IC(50) of 3 microm, suggesting an important role of gPKA during differentiation from the dormant cyst form into the active trophozoite. gPKA localizes independently of cell density to the eight flagellar basal bodies between the two nuclei together with centrin, a basal body/centrosome-specific protein. However, localization of gPKA to marginal plates along the intracellular portions of the anterior and caudal pairs of flagella was evident only at low cell density and higher endogenous cAMP concentrations or after refeeding with fresh medium. These data suggest an important role of PKA in trophozoite motility during vegetative growth and the cellular activation of excystation.

Relationship between sperm motility and the processing and tyrosine phosphorylation of two human sperm fibrous sheath proteins, pro-hAKAP82 and hAKAP82

Sperm motility is regulated by the cAMP-dependent protein kinase (protein kinase-A)-mediated phosphorylation of a group of largely unidentified flagellar proteins. Human AKAP82 (hAKAP82) and its precursor protein, pro-hAKAP82, are members of the A-kinase anchor protein (AKAP) family. These proteins tether protein kinase-A to the fibrous sheath of human spermatozoa and presumably localize the activity of the kinase near specific targets in the sperm flagellum. In this way, pro-hAKAP82 and hAKAP82 may be involved in regulating sperm motility. Similar to its homologues in other species, pro-hAKAP82 is proteolytically processed to hAKAP82. However, the amount of processing of pro-hAKAP82 in human spermatozoa is less than the amount of processing of the precursor in other species. We postulated that this lower extent of processing may be related to lower percentages of human sperm motility. In addition, both pro-hAKAP82 and hAKAP82 are tyrosine phosphorylated in a capacitation-dependent manner. Since capacitation is associated with hyperactivated motility, we postulated that tyrosine phosphorylation of pro-hAKAP82/hAKAP82 is associated with changes in motility. However, using a combination of immunofluorescence and immunoblotting approaches, we found no evidence for an association between either processing or tyrosine phosphorylation of pro-hAKAP82/hAKAP82 and significant differences in motility in spermatozoa from normal men.