Identification of the spermatogenic zip protein Spz1 as a putative protein phosphatase-1 (PP1) regulatory protein that specifically binds the PP1cgamma2 splice variant in mouse testis

Advances in non-hormonal male contraception targeting sperm motility

BACKGROUND

The high rates of unintended pregnancy and the ever-growing world population impose health, economic, social, and environmental threats to countries. Expanding contraceptive options, including male methods, are urgently needed to tackle these global challenges. Male contraception is limited to condoms and vasectomy, which are unsuitable for many couples. Thus, novel male contraceptive methods may reduce unintended pregnancies, meet the contraceptive needs of couples, and foster gender equality in carrying the contraceptive burden. In this regard, the spermatozoon emerges as a source of druggable targets for on-demand, non-hormonal male contraception based on disrupting sperm motility or fertilization.

OBJECTIVE AND RATIONALE

A better understanding of the molecules governing sperm motility can lead to innovative approaches toward safe and effective male contraceptives. This review discusses cutting-edge knowledge on sperm-specific targets for male contraception, focusing on those with crucial roles in sperm motility. We also highlight challenges and opportunities in male contraceptive drug development targeting spermatozoa.

SEARCH METHODS

We conducted a literature search in the PubMed database using the following keywords: 'spermatozoa', 'sperm motility', 'male contraception', and 'drug targets' in combination with other related terms to the field. Publications until January 2023 written in English were considered.

OUTCOMES

Efforts for developing non-hormonal strategies for male contraception resulted in the identification of candidates specifically expressed or enriched in spermatozoa, including enzymes (PP1γ2, GAPDHS, and sAC), ion channels (CatSper and KSper), transmembrane transporters (sNHE, SLC26A8, and ATP1A4), and surface proteins (EPPIN). These targets are usually located in the sperm flagellum. Their indispensable roles in sperm motility and male fertility were confirmed by genetic or immunological approaches using animal models and gene mutations associated with male infertility due to sperm defects in humans. Their druggability was demonstrated by the identification of drug-like small organic ligands displaying spermiostatic activity in preclinical trials.

WIDER IMPLICATIONS

A wide range of sperm-associated proteins has arisen as key regulators of sperm motility, providing compelling druggable candidates for male contraception. Nevertheless, no pharmacological agent has reached clinical developmental stages. One reason is the slow progress in translating the preclinical and drug discovery findings into a drug-like candidate adequate for clinical development. Thus, intense collaboration among academia, private sectors, governments, and regulatory agencies will be crucial to combine expertise for the development of male contraceptives targeting sperm function by (i) improving target structural characterization and the design of highly selective ligands, (ii) conducting long-term preclinical safety, efficacy, and reversibility evaluation, and (iii) establishing rigorous guidelines and endpoints for clinical trials and regulatory evaluation, thus allowing their testing in humans.

Disruption of protein phosphatase 1 complexes with the use of bioportides as a novel approach to target sperm motility

OBJECTIVE

To design protein phosphatase 1 (PP1)-disrupting peptides covalently coupled to inert cell-penetrating peptides (CPPs) as sychnologically organized bioportide constructs as a strategy to modulate sperm motility.

DESIGN

Experimental study.

SETTING

Academic research laboratory.

PATIENT(S)/ANIMAL(S)

Normozoospermic men providing samples for routine analysis and Holstein Frisian bulls.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Effect of the bioportides on the activity and interactions of PP1γ2-a PP1 isoform expressed exclusively in testicular germ cells and sperm-and on sperm vitality and motility.

RESULT(S)

PP1-disrupting peptides were designed based on the sequences from: 1) a sperm-specific PP1 interactor (A kinase anchor protein 4); and 2) a PP1 inhibitor (protein phosphatase inhibitor 2). Those sequences were covalently coupled to inert CPPs as bioportide constructs, which were successfully delivered to the flagellum of sperm cells to induce a marked impact on PP1γ2 activity and sperm motility. Molecular modeling studies further facilitated the identification of an optimized PP1-binding sequence and enabled the development of a modified stop-sperm bioportide with reduced size and increased potency of action. In addition, a bioportide mimetic of the unique 22-amino acid C-terminus of PP1γ2 accumulated within spermatozoa to significantly reduce sperm motility and further define the PP1γ2-specific interactome.

CONCLUSION(S)

These investigations demonstrate the utility of CPPs to deliver peptide sequences that target unique protein-protein interactions in spermatozoa to achieve a significant impact upon spermatozoa motility, a key prognostic indicator of male fertility.

The protein phosphatase isoform PP1γ1 substitutes for PP1γ2 to support spermatogenesis but not normal sperm function and fertility†

Four isoforms of serine/threonine phosphatase type I, PP1α, PP1β, PP1γ1, and PP1γ2, are derived from three genes. The PP1γ1 and PP1γ2 isoforms are alternately spliced transcripts of the protein phosphatase 1 catalytic subunit gamma gene (Ppp1cc). While PP1γ1 is ubiquitous in somatic cells, PP1γ2 is expressed exclusively in testicular germ cells and sperm. Ppp1cc knockout male mice (-/-), lacking both PP1γ1 and PP1γ2, are sterile due to impaired sperm morphogenesis. Fertility and normal sperm function can be restored by transgenic expression of PP1γ2 alone in testis of Ppp1cc (-/-) mice. The purpose of this study was to determine whether the PP1γ1 isoform is functionally equivalent to PP1γ2 in supporting spermatogenesis and male fertility. Significant levels of transgenic PP1γ1 expression occurred only when the transgene lacked a 1-kb 3΄UTR region immediately following the stop codon of the PP1γ1 transcript. PP1γ1 was also incorporated into sperm at levels comparable to PP1γ2 in sperm from wild-type mice. Spermatogenesis was restored in mice expressing PP1γ1 in the absence of PP1γ2. However, males from the transgenic rescue lines were subfertile. Sperm from the PP1γ1 rescue mice were unable to fertilize eggs in vitro. Intrasperm localization of PP1γ1 and the association of the protein regulators of the phosphatase were altered in epididymal sperm in transgenic PP1γ1 compared to PP1γ2. Thus, the ubiquitous isoform PP1γ1, not normally expressed in differentiating germ cells, could replace PP1γ2 to support spermatogenesis and spermiation. However, PP1γ2, which is the PP1 isoform in mammalian sperm, has an isoform-specific role in supporting normal sperm function and fertility.

Two novel ligand-independent variants of the VEGFR-1 receptor are expressed in human testis and spermatozoa, one of them with the ability to activate SRC proto-oncogene tyrosine kinases

The vascular endothelial growth factor receptor 1 (VEGFR-1) family of receptors is preferentially expressed in endothelial cells, with the full-length and mostly the soluble (sVEGFR-1) isoforms being the most expressed ones. Surprisingly, cancer cells (MDA-MB-231) express, instead, alternative intracellular VEGFR-1 variants. We wondered if these variants, that are no longer dependent on ligands for activation, were expressed in a physiological context, specifically in spermatogenic cells, and whether their expression was maintained in spermatozoa and required for human fertility. By interrogating a human library of mature testis cDNA, we characterized two new truncated intracellular variants different from the ones previously described in cancer cells. The new isoforms were transcribed from alternative transcription start sites (aTSS) located respectively in intron-19 (i₁₉VEGFR-1) and intron-28 (i₂₈VEGFR-1) of the VEGFR-1 gene (GenBank accession numbers JF509744 and JF509745) and expressed in mature testis and spermatozoa. In this paper, we describe the characterization of these isoforms by RT-PCR, northern blot, and western blot, their preferential expression in human mature testis and spermatozoa, and the elements that punctuate their proximal promoters and suggest cues for their expression in spermatogenic cells. Mechanistically, we show that i₁₉VEGFR-1 has a strong ability to phosphorylate and activate SRC proto-oncogene non-receptor tyrosine kinases and a significant bias toward a decrease in expression in patients considered infertile by WHO criteria.

Functions and therapeutic potential of protein phosphatase 1: Insights from mouse genetics

Protein phosphatase 1 (PP1) catalyzes more than half of all phosphoserine/threonine dephosphorylation reactions in mammalian cells. In vivo PP1 does not exist as a free catalytic subunit but is always associated with at least one regulatory PP1-interacting protein (PIP) to generate a large set of distinct holoenzymes. Each PP1 complex controls the dephosphorylation of only a small subset of PP1 substrates. We screened the literature for genetically engineered mouse models and identified models for all PP1 isoforms and 104 PIPs. PP1 itself and at least 49 PIPs were connected to human disease-associated phenotypes. Additionally, phenotypes related to 17 PIPs were clearly linked to altered PP1 function, while such information was lacking for 32 other PIPs. We propose structural reverse genetics, which combines structural characterization of proteins with mouse genetics, to identify new PP1-related therapeutic targets. The available mouse models confirm the pleiotropic action of PP1 in health and diseases.

Regulating the regulator: Insights into the cardiac protein phosphatase 1 interactome

Reversible phosphorylation of proteins is a delicate yet dynamic balancing act between kinases and phosphatases, the disturbance of which underlies numerous disease processes. While our understanding of protein kinases has grown tremendously over the past decades, relatively little is known regarding protein phosphatases. This may be because protein kinases are great in number and relatively specific in function, and thereby amenable to be studied in isolation, whereas protein phosphatases are much less abundant and more nonspecific in their function. To achieve subcellular localization and substrate specificity, phosphatases depend on partnering with a large number of regulatory subunits, protein scaffolds and/or other interactors. This added layer of complexity presents a significant barrier to their study, but holds the key to unexplored opportunities for novel pharmacologic intervention. In this review we focus on serine/threonine protein phosphatase type-1 (PP1), which plays an important role in cardiac physiology and pathophysiology. Although much work has been done to investigate the role of PP1 in cardiac diseases including atrial fibrillation and heart failure, most of these studies were limited to examining and manipulating the catalytic subunit(s) of PP1 without adequately considering the PP1 interactors, which give specificity to PP1's functions. To complement these studies, three unbiased methods have been developed and applied to the mapping of the PP1 interactome: bioinformatics approaches, yeast two-hybrid screens, and affinity-purification mass spectrometry. The application of these complementary methods has the potential to generate a detailed cardiac PP1 interactome, which is an important step in identifying novel and targeted pharmacological interventions.

The power of the yeast two-hybrid system in the identification of novel drug targets: building and modulating PPP1 interactomes

Since the description of the yeast two-hybrid (Y2H) method, it has become more and more evident that it is the most commonly used method to identify protein-protein interactions (PPIs). The improvements in the original Y2H methodology in parallel with the idea that PPIs are promising drug targets, offer an excellent opportunity to apply the principles of this molecular biology technique to the pharmaceutical field. Additionally, the theoretical developments in the networks field make PPI networks very useful frameworks that facilitate many discoveries in biomedicine. This review highlights the relevance of Y2H in the determination of PPIs, specifically phosphoprotein phosphatase 1 interactions, and its possible outcomes in pharmaceutical research.

TMEM225: a possible protein phosphatase 1γ2 (PP1γ2) regulator localizes to the equatorial segment in mouse spermatozoa

Tmem225 encodes a putative four-transmembrane domain protein that has an RVxF motif, which is known to be a consensus site for interacting with serine/threonine protein phosphatase 1 (PP1). We previously identified Tmem225 as one of 53 spermatogenesis-associated transmembrane protein genes, with no known physiological function. In this study, we investigated the expression and molecular characteristics of TMEM225 in mice. Tmem225 production was found to be specific to testicular germ cells, with expression increasing during spermatogenesis. In mature spermatozoa, TMEM225 is localized to the equatorial segment of the acrosome but not to the midpiece or tail. TMEM225 appears to be an outer and/or inner acrosomal membrane protein that is lost from the dorsal region of the acrosome after the acrosome reaction. TMEM225 interacts with PP1 in vivo, and a pull-down assay revealed that the carboxy-terminal region of TMEM225 can bind to PP1γ2, the predominant isoform of PP1 in male germ cells. In addition, TMEM225 inhibited PP1γ2 activity in vitro via its RVxF motif. Our results suggest that in mice, TMEM225 is involved in the differentiation and function of spermatozoa through the regulation of PP1γ2 activity, which is necessary for normal spermatogenesis as well as spermatozoa capacitation and motility.

Protein phosphatase 1 catalytic isoforms: specificity toward interacting proteins

The coordinated and reciprocal action of serine-threonine protein kinases and protein phosphatases produces transitory phosphorylation, a fundamental regulatory mechanism for many biological processes. Phosphoprotein phosphatase 1 (PPP1), a major serine-threonine phosphatase, in particular, is ubiquitously distributed and regulates a broad range of cellular functions, including glycogen metabolism, cell cycle progression, and muscle relaxation. PPP1 has evolved effective catalytic machinery but in vitro lacks substrate specificity. In vivo, its specificity is achieved not only by the existence of different PPP1 catalytic isoforms, but also by binding of the catalytic moiety to a large number of regulatory or targeting subunits. Here, we will address exhaustively the existence of diverse PPP1 catalytic isoforms and the relevance of their specific partners and consequent functions.

PPP1CC2 can form a kinase/phosphatase complex with the testis-specific proteins TSSK1 and TSKS in the mouse testis

The mouse protein phosphatase gene Ppp1cc is essential for male fertility, with mutants displaying a failure in spermatogenesis including a widespread loss of post-meiotic germ cells and abnormalities in the mitochondrial sheath. This phenotype is hypothesized to be responsible for the loss of the testis-specific isoform PPP1CC2. To identify PPP1CC2-interacting proteins with a function in spermatogenesis, we carried out GST pull-down assays in mouse testis lysates. Amongst the identified candidate interactors was the testis-specific protein kinase TSSK1, which is also essential for male fertility. Subsequent interaction experiments confirmed the capability of PPP1CC2 to form a complex with TSSK1 mediated by the direct interaction of each with the kinase substrate protein TSKS. Interaction between PPP1CC2 and TSKS is mediated through an RVxF docking motif on the TSKS surface. Phosphoproteomic analysis of the mouse testis identified a novel serine phosphorylation site within the TSKS RVxF motif that appears to negatively regulate binding to PPP1CC2. Immunohistochemical analysis of TSSK1 and TSKS in the Ppp1cc mutant testis showed reduced accumulation to distinct cytoplasmic foci and other abnormalities in their distribution consistent with the loss of germ cells and seminiferous tubule disorganization observed in the Ppp1cc mutant phenotype. A comparison of Ppp1cc and Tssk1/2 knockout phenotypes via electron microscopy revealed similar abnormalities in the morphology of the mitochondrial sheath. These data demonstrate a novel kinase/phosphatase complex in the testis that could play a critical role in the completion of spermatogenesis.

Tandem affinity purification in transgenic mouse embryonic stem cells identifies DDOST as a novel PPP1CC2 interacting protein

Members of the PP1 family of protein phosphatases achieve functional diversity through numerous and varied protein-protein interactions. In mammals, there are four PP1 isoforms, the ubiquitously expressed PPP1CA, PPP1CB, and PPP1CC1, and the testis specific splice isoform PPP1CC2. When the mouse Ppp1cc gene is deleted, the only phenotypic consequence is a failure of spermatogenesis in homozygous males. To elucidate the function of the Ppp1cc gene, we sought to identify novel protein-protein interactions. To this end, we have created SBP-3XFLAG-PPP1CC1 and SBP-3XFLAG-PPP1CC2 knock-in mouse embryonic stem cell lines using a gene-trap-based system. Tandem affinity purification using our knock-in cell lines identified 11 significant protein-protein interactions, including nine known PP1 interacting proteins and two additional proteins (ATP5C1 and DDOST). Reciprocal in vitro sedimentation assays confirmed the interaction between PPP1CC2 and DDOST that may have physiological implications in spermatogenesis. Immunolocalization studies revealed that DDOST localized to the nuclear envelope in dissociated spermatogenic cells and persists throughout spermatogenesis. The knock-in system described in this paper can be applied in creating tandem affinity-tagged knock-in embryonic stem cell lines with any gene for which a compatible gene-trap line is available.

Protein phosphatase 1α interacting proteins in the human brain

Protein Phosphatase 1 (PP1) is a major serine/threonine-phosphatase whose activity is dependent on its binding to regulatory subunits known as PP1 interacting proteins (PIPs), responsible for targeting PP1 to a specific cellular location, specifying its substrate or regulating its action. Today, more than 200 PIPs have been described involving PP1 in panoply of cellular mechanisms. Moreover, several PIPs have been identified that are tissue and event specific. In addition, the diversity of PP1/PIP complexes can further be achieved by the existence of several PP1 isoforms that can bind preferentially to a certain PIP. Thus, PP1/PIP complexes are highly specific for a particular function in the cell, and as such, they are excellent pharmacological targets. Hence, an in-depth survey was taken to identify specific PP1α PIPs in human brain by a high-throughput Yeast Two-Hybrid approach. Sixty-six proteins were recognized to bind PP1α, 39 being novel PIPs. A large protein interaction databases search was also performed to integrate with the results of the PP1α Human Brain Yeast Two-Hybrid and a total of 246 interactions were retrieved.

PP1 forms an active complex with TLRR (lrrc67), a putative PP1 regulatory subunit, during the early stages of spermiogenesis in mice

Mammalian spermatogenesis is a highly regulated developmental pathway that demands dramatic rearrangement of the cytoskeleton of the male germ cell. We have described previously a leucine rich repeat protein, TLRR (also known as lrrc67), which is associated with the spermatid cytoskeleton in mouse testis and is a binding partner of protein phosphatase-1 (PP1), an extremely well conserved signaling molecule. The activity of PP1 is modulated by numerous specific regulators of which TLRR is a candidate. In this study we measured the phosphatase activity of the TLRR-PP1 complex in the adult and the developing mouse testis, which contains varying populations of developing germ cell types, in order to determine whether TLRR acts as an activator or an inhibitor of PP1 and whether the phosphatase activity of this complex is developmentally regulated during spermatogenesis. Additionally, we assayed the ability of bacterially expressed TLRR to affect the enzymatic activity of PP1. Furthermore, we examined phosphorylation of TLRR, and elements of the spermatid cytoskeleton during the first wave of spermatogenesis in the developing testis. We demonstrate here that the TLRR complex is associated with a phosphatase activity in adult mouse testis. The relative phosphatase activity of this complex appears to reach a peak at about 21 days after birth, when pachytene spermatocytes and round spermatids are abundant in the seminiferous epithelium of the mouse testis. TLRR, in addition to tubulin and kinesin-1B, is phosphorylated during the first wave of spermatogenesis. These findings indicate that the TLRR-PP1 complex is active prior to translocation of TLRR toward the sperm flagella and that TLRR, and constituents of the spermatid cytoskeleton, may be subject to regulation by reversible phosphorylation during spermatogenesis in murine testis.

Protein phosphatase 1 complexes modulate sperm motility and present novel targets for male infertility

Infertility is a growing concern in modern society, with 30% of cases being due to male factors, namely reduced sperm concentration, decreased motility and abnormal morphology. Sperm cells are highly compartmentalized, almost devoid of transcription and translation consequently processes such as protein phosphorylation provide a key general mechanism for regulating vital cellular functions, more so than for undifferentiated cells. Reversible protein phosphorylation is the principal mechanism regulating most physiological processes in eukaryotic cells. To date, hundreds of protein kinases have been identified, but significantly fewer phosphatases (PPs) are responsible for counteracting their action. This discrepancy can be explained in part by the mechanism used to control phosphatase activity, which is based on regulatory interacting proteins. This is particularly true for PP1, a major serine/threonine-PP, for which >200 interactors (PP1 interacting proteins-PIPs) have been indentified that control its activity, subcellular location and substrate specificity. For PP1, several isoforms have been described, among them PP1γ2, a testis/sperm-enriched PP1 isoform. Recent findings support our hypothesis that PP1γ2 is involved in the regulation of sperm motility. This review summarizes the known sperm-specific PP1-PIPs, involved in the acquisition of mammalian sperm motility. The complexes that PP1 routinely forms with different proteins are addressed and the role of PP1/A-kinase anchoring protein complexes in sperm motility is considered. Furthermore, the potential relevance of targeting PP1-PIPs complexes to infertility diagnostics and therapeutics as well as to male contraception is also discussed.

New candidate targets of protein phosphatase-1c-gamma-2 in mouse testis revealed by a differential phosphoproteome analysis

Reversible phosphorylation has been implicated in many developmental processes. Dephosphorylation is mediated by several families of phosphatases, including type 1 serine/threonine phosphatases (protein phosphatase-1 or PP1). The loss of the murine Ppp1cc gene causes male infertility as a result of impaired spermatogenesis. Ppp1cc encodes two splice isoforms, PPP1CC1 and PPP1CC2, with the latter being the most abundant isoform in the testis. However, the details of PPP1CC2's involvement in spermatogenesis are still unknown. As a phosphatase has been removed from the mutant mouse, a search for hyperphosphorylated proteins in the mutant testis may reveal the direct downstream targets of PPP1CC2. Using a whole tissue proteomics approach to identify testis-specific dephosphorylation targets of PPP1CC2, we found that two-dimensional electrophoresis identified 10 potential targets in the Ppp1cc null testis several of which are factors known to be important for spermatogenesis, such as HSPA2. Another potential target, tubulin, was found to be misregulated during Ppp1cc(-/-) spermatogenesis, disrupting manchette development. This work represents the first survey of the testicular phosphoproteome under pathological conditions.

Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 4: intercellular bridges, mitochondria, nuclear envelope, apoptosis, ubiquitination, membrane/voltage-gated channels, methylation/acetylation, and transcription factors

As germ cells divide and differentiate from spermatogonia to spermatozoa, they share a number of structural and functional features that are common to all generations of germ cells and these features are discussed herein. Germ cells are linked to one another by large intercellular bridges which serve to move molecules and even large organelles from the cytoplasm of one cell to another. Mitochondria take on different shapes and features and topographical arrangements to accommodate their specific needs during spermatogenesis. The nuclear envelope and pore complex also undergo extensive modifications concomitant with the development of germ cell generations. Apoptosis is an event that is normally triggered by germ cells and involves many proteins. It occurs to limit the germ cell pool and acts as a quality control mechanism. The ubiquitin pathway comprises enzymes that ubiquitinate as well as deubiquitinate target proteins and this pathway is present and functional in germ cells. Germ cells express many proteins involved in water balance and pH control as well as voltage-gated ion channel movement. In the nucleus, proteins undergo epigenetic modifications which include methylation, acetylation, and phosphorylation, with each of these modifications signaling changes in chromatin structure. Germ cells contain specialized transcription complexes that coordinate the differentiation program of spermatogenesis, and there are many male germ cell-specific differences in the components of this machinery. All of the above features of germ cells will be discussed along with the specific proteins/genes and abnormalities to fertility related to each topic.

Identification of interacting transcription factors regulating tissue gene expression in human

BACKGROUND

Tissue gene expression is generally regulated by multiple transcription factors (TFs). A major first step toward understanding how tissues achieve their specificity is to identify, at the genome scale, interacting TFs regulating gene expression in different tissues. Despite previous discoveries, the mechanisms that control tissue gene expression are not fully understood.

RESULTS

We have integrated a function conservation approach, which is based on evolutionary conservation of biological function, and genes with highest expression level in human tissues to predict TF pairs controlling tissue gene expression. To this end, we have identified 2549 TF pairs associated with a certain tissue. To find interacting TFs controlling tissue gene expression in a broad spatial and temporal manner, we looked for TF pairs common to the same type of tissues and identified 379 such TF pairs, based on which TF-TF interaction networks were further built. We also found that tissue-specific TFs may play an important role in recruiting non-tissue-specific TFs to the TF-TF interaction network, offering the potential for coordinating and controlling tissue gene expression across a variety of conditions.

CONCLUSION

The findings from this study indicate that tissue gene expression is regulated by large sets of interacting TFs either on the same promoter of a gene or through TF-TF interaction networks.

PP1gamma2 and PPP1R11 are parts of a multimeric complex in developing testicular germ cells in which their steady state levels are reciprocally related

Mice lacking the protein phosphatase 1 gamma isoforms, PP1gamma1 and PP1gamma2, are male-sterile due to defective germ cell morphogenesis and apoptosis. However, this deficiency causes no obvious abnormality in other tissues. A biochemical approach was employed to learn how expression versus deficiency of PP1gamma2, the predominant PP1 isoform in male germ cells, affects spermatogenesis. Methods used in this study include column chromatography, western blot and northern blot analyses, GST pull-down assays, immunoprecipitation, non-denaturing gel electrophoresis, phosphatase enzyme assays, protein sequencing, and immunohistochemistry. We report for the first time that in wild-type testis, PP1gamma2 forms an inactive complex with actin, protein phosphatase 1 regulatory subunit 7 (PPP1R7), and protein phosphatase 1 regulatory subunit 11 (PPP1R11), the latter, a potent PP1 inhibitor. Interestingly, PPP1R11 protein, but not its mRNA level, falls significantly in PP1gamma-null testis where mature sperm are virtually absent. Conversely, both mature sperm numbers and the PPP1R11 level increase substantially in PP1gamma-null testis expressing transgenic PP1gamma2. PPP1R11 also appears to be ubiquitinated in PP1gamma-null testis. The levels of PP1gamma2 and PPP1R11 were increased in phenotypically normal PP1alpha-null testis. However, in PP1alpha-null spleen, where PP1gamma2 normally is not expressed, PPP1R11 levels remained unchanged. Our data clearly show a direct reciprocal relationship between the levels of the protein phosphatase isoform PP1gamma2 and its regulator PPP1R11, and suggest that complex formation between these polypeptides in testis may prevent proteolysis of PPP1R11 and thus, germ cell apoptosis.

Role(s) of the serine/threonine protein phosphatase 1 on mammalian sperm motility

Mammalian spermatozoa acquire the capacity for motility and fertilization during the transit through the epididymis under the control of different factors, such as cAMP, intracellular pH, intracellular calcium and phosphorylation of sperm proteins. As the acquisition of functional competence including gaining motility during epididymal transit occurs in the complete absence of contemporaneous gene transcription and translation on the part of the spermatozoa, it is widely accepted that post-translational modifications are the only means by which spermatozoa can acquire functionality. Serine-threonine protein phosphatase 1 (PP1) together with their testis/sperm-specific interacting proteins might be involved in this regulatory mechanism. PP1alpha, PP1beta/delta, PP1gamma1 and PP1gamma2 are all expressed in the testis whereas PP1gamma2 is the only isoform expressed on spermatozoa. I2, I3, sds22, 14-3-3 and hsp90 are associated with PP1gamma2 in spermatozoa located on the sperm head and tail. Activity of PP1gamma2 and the binding pattern to these regulatory proteins changes in spermatozoa recruited from the caput and those from the cauda part of the epididymis. In this review, we summarize the possible roles of PP1 on spermatozoa during spermatogenesis and flagellar motility control. We suggest that PP1 might take part in the inhibition of the sperm motility activation by interacting with AKAPs and CAMKII. A hypothesized signaling pathway of mammalian sperm motility activation and PP1's function has been proposed.

Housekeeping and tissue-specific genes in mouse tissues

BACKGROUND

This study aims to characterize the housekeeping and tissue-specific genes in 15 mouse tissues by using the serial analysis of gene expression (SAGE) strategy which indicates the relative level of expression for each transcript matched to the tag.

RESULTS

Here, we identified constantly expressed housekeeping genes, such as eukaryotic translation elongation factor 2, which is expressed in all tissues without significant difference in expression levels. Moreover, most of these genes were not regulated by experimental conditions such as steroid hormones, adrenalectomy and gonadectomy. In addition, we report previously postulated housekeeping genes such as peptidyl-prolyl cis-trans isomerase A, glyceraldehyde-3-phosphate dehydrogenase and beta-actin, which are expressed in all the tissues, but with significant difference in their expression levels. We have also identified genes uniquely detected in each of the 15 tissues and other tissues from public databases.

CONCLUSION

These identified housekeeping genes could represent appropriate controls for RT-PCR and northern blot when comparing the expression levels of genes in several tissues. The results reveal several tissue-specific genes highly expressed in testis and pituitary gland. Furthermore, the main function of tissue-specific genes expressed in liver, lung and bone is the cell defence, whereas several keratins involved in cell structure function are exclusively detected in skin and vagina. The results from this study can be used for example to target a tissue for agent delivering by using the promoter of tissue-specific genes. Moreover, this study could be used as basis for further researches on physiology and pathology of these tissues.

Zygotically activated genes are suppressed in mouse nuclear transferred embryos

Mammalian oocytes have the ability to confer totipotency to terminally differentiated somatic cell nuclei. Viable cloned animals have been produced by somatic cell nuclear transfer (NT) into oocytes in many mammalian species including mouse. However, the success rates of the production were quite low in all species. Many studies have measured differences in gene expression between NT and fertilized embryos in relatively advanced stages of development such as pre- and post-natal stages or the blastocyst stage. In the present study, we compared gene expression patterns using differential display RT-PCR (DDRT-PCR) between the NT and IVF embryos at the 2-cell stage to detect some abnormalities affecting later development of NT embryos. Aberrant gene expression was detected in NT embryos compared with IVF embryos, and MuERV-L and Dnaja2 genes were down-regulated and Inpp5b and Chst12 genes were up-regulated in the NT embryos. Further analysis showed that the expression of zygotically activated genes such as Interferon-gamma, Dub-1, Spz1, DD2106 (unknown gene), and DD2111 (unknown gene) were suppressed in NT embryos, suggesting that the cellular process involved in the nuclear reprogramming of somatic nucleus is not appropriately regulated.

A Testis Specific Isoform of Endophilin B1, Endophilin B1t, Interacts Specifically with Protein Phosphatase-1cγ2 in Mouse Testis and Is Abnormally Expressed in PP1cγ Null Mice

Male mice homozygous for a null mutation in the protein phosphatase-1c gamma (PP1c gamma) gene are infertile, displaying a severe impairment in spermatogenesis that is not compensated by the presence of PP1c alpha and PP1c beta in mutant testes. A lack of the PP1c gamma2 splice variant seems the most likely cause of the mutant phenotype, as it is the most heavily expressed PP1c gamma isoform in wild type testes. Yeast two-hybrid screening using PP1c gamma2 has identified several new binding partners, including endophilin B1t, a testis enriched isoform of endophilin B1a which differs from the somatic form by virtue of a carboxy terminal deletion spanning the last 10 amino acids. The testis isoform did not show an interaction with PP1c alpha, or with a truncated PP1c gamma2 mutant lacking the unique carboxy terminus. In contrast, somatic endophilin B1a did not interact with any of the PP1c isoforms. Sedimentation and co-immunoprecipitation experiments using native testis proteins verified binding of endophilin B1t to PP1c gamma2. Immunohistochemistry on wild type testis sections revealed a stage specific expression pattern for endophilin that appeared concentrated at discrete puncta throughout the seminiferous epithelium. Punctate endophilin expression in cells adjacent to the lumen was absent in PP1c gamma null mice. Phosphatase assays indicate that chimeric endophilin B1t is able to inhibit recombinant PP1c gamma2 activity toward phosphorylase a while having little effect on the activity of PP1c alpha. A potential role for endophilin B1t in mammalian spermatogenesis is discussed within the context of the PP1c gamma knockout testis phenotype.

Expression of a novel T-complex testis expressed 5 (Tctex5) in mouse testis, epididymis, and spermatozoa

Expression of T-complex testis expressed 5 (Tctex5), an orthologue of protein phosphatase-1 inhibitor-3 (PPP1R11), was enhanced in mouse testis and was also expressed in epididymis and spermatozoa. There were three transcripts of Tctex5 including one brain specific and two common transcripts dominant in mouse testis. Tctex5 protein isoforms (75, 52, 32, 25, and 14.3 kDa) were identified. Isoforms of 75 and 52 kDa were spermatogenic-specific and were found in protein fraction containing nuclei, mitochondria, and flagellum accessory, and also in protein fraction containing mainly membranes. Tctex5 was localized in nuclei of pachytene spermatocytes, round spermatocytes, cytoplasm of Sertoli cells in testis; cilia, secretion bodies and nuclei of epithelial cells and interstitium smooth muscle cells in epididymis; and head and principal piece of tail in epididymal spermatozoa. The results suggested that Tctex5 might be a specific protein phosphatase-1 inhibitor in sperm; various Tctex5 transcripts and isoforms and cellular locations imply its different roles in spermatogenesis. Nuclei-type isoforms (75 and 52 kDa) might take part in nucleus remodeling during spermatogenesis whilst membrane-type isoform (52 kDa) might be responsible for dephosphorylation of proteins during capacitation. The other isoforms might play general roles for all kinds of cell types.

bHLH-zip Transcription Factor Spz1 Mediates Mitogen-Activated Protein Kinase Cell Proliferation, Transformation, and Tumorigenesis

BHLH-zip proteins usually play important regulatory roles in cell growth and differentiation. In this study, we show that Spz1, a bHLH-zip transcription factor, acts downstream of mitogen-activated protein kinase (MAPK, extracellular signal-regulated kinase 1/2) to up-regulate cell proliferation and tumorigenesis. In addition, through an interaction with proliferating cell nuclear antigen (PCNA) promoter, Spz1 induced cell proliferation concomitant with an increase in PCNA gene expression. Spz1-transfected cells formed colony foci on soft agar and developed fibrosarcoma tumors in nude mice. MAPK directly interacted and phosphorylated Spz1 protein, which increased PCNA transcription and cell tumorigenic activities. Reduction of endogenous Spz1 expression via RNA interference decreased cell proliferation in p19 embryonic carcinoma cells. High levels of Spz1 expression were detected in murine tumor cell lines and tumor samples of both human and Spz1 transgenic mice. Thus, Spz1 may act as a proto-oncogene, participating in the MAPK signal pathway, and be a potential therapeutic target in the treatment of Ras-induced tumors.