Histone-binding domains in a human nuclear autoantigenic sperm protein

The histone chaperone Nrp1 is required for chromatin stability and nuclear division in Tetrahymena thermophila

Histone chaperones facilitate DNA replication and repair by promoting chromatin assembly, disassembly and histone exchange. Following histones synthesis and nucleosome assembly, the histones undergo posttranslational modification by different enzymes and are deposited onto chromatins by various histone chaperones. In Tetrahymena thermophila, histones from macronucleus (MAC) and micronucleus (MIC) have been comprehensively investigated, but the function of histone chaperones remains unclear. Histone chaperone Nrp1 in Tetrahymena contains four conserved tetratricopepeptide repeat (TPR) domains and one C-terminal nuclear localization signal. TPR2 is typically interrupted by a large acidic motif. Immunofluorescence staining showed that Nrp1 is located in the MAC and MICs, but disappeared in the apoptotic parental MAC and the degraded MICs during the conjugation stage. Nrp1 was also colocalized with α-tubulin around the spindle structure. NRP1 knockdown inhibited cellular proliferation and led to the loss of chromosome, abnormal macronuclear amitosis, and disorganized micronuclear mitosis during the vegetative growth stage. During sexual developmental stage, the gametic nuclei failed to be selected and abnormally degraded in NRP1 knockdown mutants. Affinity purification combined with mass spectrometry analysis indicated that Nrp1 is co-purified with core histones, heat shock proteins, histone chaperones, and DNA damage repair proteins. The physical direct interaction of Nrp1 and Asf1 was also confirmed by pull-down analysis in vitro. The results show that histone chaperone Nrp1 is involved in micronuclear mitosis and macronuclear amitosis in the vegetative growth stage and maintains gametic nuclei formation during the sexual developmental stage. Nrp1 is required for chromatin stability and nuclear division in Tetrahymena thermophila.

Intrinsic Disorder in Tetratricopeptide Repeat Proteins

Among the realm of repeat containing proteins that commonly serve as “scaffolds” promoting protein-protein interactions, there is a family of proteins containing between 2 and 20 tetratricopeptide repeats (TPRs), which are functional motifs consisting of 34 amino acids. The most distinguishing feature of TPR domains is their ability to stack continuously one upon the other, with these stacked repeats being able to affect interaction with binding partners either sequentially or in combination. It is known that many repeat-containing proteins are characterized by high levels of intrinsic disorder, and that many protein tandem repeats can be intrinsically disordered. Furthermore, it seems that TPR-containing proteins share many characteristics with hybrid proteins containing ordered domains and intrinsically disordered protein regions. However, there has not been a systematic analysis of the intrinsic disorder status of TPR proteins. To fill this gap, we analyzed 166 human TPR proteins to determine the degree to which proteins containing TPR motifs are affected by intrinsic disorder. Our analysis revealed that these proteins are characterized by different levels of intrinsic disorder and contain functional disordered regions that are utilized for protein-protein interactions and often serve as targets of various posttranslational modifications.

Molecular mimicry: An explanation for autoimmune diseases and infertility

Microorganisms execute an enthralling range of adjustments to survive in the host. Among the various strategies employed by microorganisms to surmount the host immune response, the phenomenon of molecular mimicry empowers the microorganisms to manoeuvre host physiology and cellular functions for their own advantage by mimicking the host proteins and initiating autoimmunity. This phenomena, by and large, has been studied in context of autoimmune diseases; however, its implications have also been reported in infertility. Hence, in this article, we provide a review of the various instances of molecular mimicry initiated by bacteria, parasites and viruses in the world of autoimmune diseases and infertility.

Molecular evolution of NASP and conserved histone H3/H4 transport pathway

BACKGROUND

NASP is an essential protein in mammals that functions in histone transport pathways and maintenance of a soluble reservoir of histones H3/H4. NASP has been studied exclusively in Opisthokonta lineages where some functional diversity has been reported. In humans, growing evidence implicates NASP miss-regulation in the development of a variety of cancers. Although a comprehensive phylogenetic analysis is lacking, NASP-family proteins that possess four TPR motifs are thought to be widely distributed across eukaryotes.

RESULTS

We characterize the molecular evolution of NASP by systematically identifying putative NASP orthologs across diverse eukaryotic lineages ranging from excavata to those of the crown group. We detect extensive silent divergence at the nucleotide level suggesting the presence of strong purifying selection acting at the protein level. We also observe a selection bias for high frequencies of acidic residues which we hypothesize is a consequence of their critical function(s), further indicating the role of functional constraints operating on NASP evolution. Our data indicate that TPR1 and TPR4 constitute the most rapidly evolving functional units of NASP and may account for the functional diversity observed among well characterized family members. We also show that NASP paralogs in ray-finned fish have different genomic environments with clear differences in their GC content and have undergone significant changes at the protein level suggesting functional diversification.

CONCLUSION

We draw four main conclusions from this study. First, wide distribution of NASP throughout eukaryotes suggests that it was likely present in the last eukaryotic common ancestor (LECA) possibly as an important innovation in the transport of H3/H4. Second, strong purifying selection operating at the protein level has influenced the nucleotide composition of NASP genes. Further, we show that selection has acted to maintain a high frequency of functionally relevant acidic amino acids in the region that interrupts TPR2. Third, functional diversity reported among several well characterized NASP family members can be explained in terms of quickly evolving TPR1 and TPR4 motifs. Fourth, NASP fish specific paralogs have significantly diverged at the protein level with NASP2 acquiring a NNR domain.

2-D DIGE identification of differentially expressed heterogeneous nuclear ribonucleoproteins and transcription factors during neural differentiation of human embryonic stem cells

Neural stem cells (NSC) are progenitors that can give rise to all neural lineages. They are found in specific niches of fetal and adult brains and grow in vitro as non-adherent colonies, the neurospheres. These cells express the intermediate filament nestin, commonly considered an NSC marker. NSC can be derived as neurospheres from human embryonic stem cells (hESC). The mechanisms of cellular programming that hESC undergo during differentiation remain obscure. To investigate the commitment process of hESC during directed neural differentiation, we compared the nuclear proteomes of hESC and hESC-derived neurospheres. We used 2-D DIGE to conduct a quantitative comparison of hESC and NSC nuclear proteins and detected 1521 protein spots matched across three gels. Statistical analysis (ANOVA n = 3 with false discovery correction) revealed that only 2.1% of the densitometric signal was significantly changed. The ranges of average ratios varied from 1.2- to 11-fold at a statistically significant p-value <0.05. MS/MS identified 15 regulated proteins previously shown to be involved in chromatin remodeling, mRNA processing and gene expression regulation. Notably, three members of the heterogeneous nuclear ribonucleoprotein family (AUF-1, and FBP-1 and FBP-2) register a 54, 70 and 99% increased expression, highlighting them as potential markers for NSC in vitro derivation. By contrast, Cpsf-6 virtually disappears with differentiation with an 11-fold drop in NSC, highlighting this protein as a novel marker for undifferentiated ESC.

Vertebrate nucleoplasmin and NASP: egg histone storage proteins with multiple chaperone activities

Recent reviews have focused on the structure and function of histone chaperones involved in different aspects of somatic cell chromatin metabolism. One of the most dramatic chromatin remodeling processes takes place immediately after fertilization and is mediated by egg histone storage chaperones. These include members of the nucleoplasmin (NPM2/NPM3), which are preferentially associated with histones H2A-H2B in the egg and the nuclear autoantigenic sperm protein (NASP) families. Interestingly, in addition to binding and providing storage to H3/H4 in the egg and in somatic cells, NASP has been shown to be a unique genuine chaperone for histone H1. This review revolves around the structural and functional roles of these two families of chaperones whose activity is modulated by their own post-translational modifications (PTMs), particularly phosphorylation. Beyond their important role in the remodeling of paternal chromatin in the early stages of embryogenesis, NPM and NASP members can interact with a plethora of proteins in addition to histones in somatic cells and play a critical role in processes of functional cell alteration, such as in cancer. Despite their common presence in the egg, these two histone chaperones appear to be evolutionarily unrelated. In contrast to members of the NPM family, which share a common monophyletic evolutionary origin, the different types of NASP appear to have evolved recurrently within different taxa.

sNASP, a histone H1-specific eukaryotic chaperone dimer that facilitates chromatin assembly

NASP has been described as a histone H1 chaperone in mammals. However, the molecular mechanisms involved have not yet been characterized. Here, we show that this protein is not only present in mammals but is widely distributed throughout eukaryotes both in its somatic and testicular forms. The secondary structure of the human somatic version consists mainly of clusters of alpha-helices and exists as a homodimer in solution. The protein binds nonspecifically to core histone H2A-H2B dimers and H3-H4 tetramers but only forms specific complexes with histone H1. The formation of the NASP-H1 complexes is mediated by the N- and C-terminal domains of histone H1 and does not involve the winged helix domain that is characteristic of linker histones. In vitro chromatin reconstitution experiments show that this protein facilitates the incorporation of linker histones onto nucleosome arrays and hence is a bona fide linker histone chaperone.

Screening of an antigen target for immunocontraceptives from cross-reactive antigens between human sperm and Ureaplasma urealyticum

Epidemiologic studies indicated that some infertile men who were infected with Ureaplasma urealyticum displayed positive antisperm antibodies in their serum and/or semen. The purpose of this study was to investigate the possible mechanism of antisperm antibodies production after infection with U. urealyticum and to analyze the relationship between U. urealyticum and infertility. The existence of cross-reactive antigens (61, 50, and 25 kDa) between U. urealyticum and human sperm membrane proteins was confirmed. Among the cross-reactive antigens, the urease complex component UreG of U. urealyticum was determined. By searching the Swiss-Prot protein database, a pentapeptide identity (IERLT) between UreG and human nuclear autoantigenic sperm protein (NASP) was found. Furthermore, using Western blot analysis and enzyme-linked immunosorbent assay, the cross-reaction between the NASP and UreG was verified. Both anti-rUreG antibody and the antiserum against the synthetic peptide NASP393-408 containing the pentapeptide inhibited mouse sperm egg binding and fusion. After immunization by rUreG or the synthetic peptide, 81.2 and 75% female mice became sterile, respectively. The effect on fertility in mice immunized with the synthetic peptide was reversible. These findings proved for the first time that it was feasible to screen antigens for immunocontraceptives from cross-reactive antigens between sperm and microorganisms which induce infertility.

Nuclear autoantigenic sperm protein (NASP), a linker histone chaperone that is required for cell proliferation

A multichaperone nucleosome-remodeling complex that contains the H1 linker histone chaperone nuclear autoantigenic sperm protein (NASP) has recently been described. Linker histones (H1) are required for the proper completion of normal development, and NASP transports H1 histones into nuclei and exchanges H1 histones with DNA. Consequently, we investigated whether NASP is required for normal cell cycle progression and development. We now report that without sufficient NASP, HeLa cells and U2OS cells are unable to replicate their DNA and progress through the cell cycle and that the NASP(-/-) null mutation causes embryonic lethality. Although the null mutation NASP(-/-) caused embryonic lethality, null embryos survive until the blastocyst stage, which may be explained by the presence of stored NASP protein in the cytoplasm of oocytes. We conclude from this study that NASP and therefore the linker histones are key players in the assembly of chromatin after DNA replication.

The Transcriptional Corepressor, PELP1, Recruits HDAC2 and Masks Histones Using Two Separate Domains

PELP1 (proline-, glutamic acid-, and leucine-rich protein 1) has been recognized as a coactivator of estrogen receptor (ER)-recruiting p300/CREB-binding protein histone acetyltransferase to the target chromosome. The present study shows that PELP1 does indeed coactivate ER-mediated transcription but also serves as a corepressor of other nuclear hormone receptors (NR)- and non-NR sequence-specific transcription factors tested, including GR, Nur77, AP1, NF-kappaB, and TCF/SRF. PELP1 expression also retarded the proliferation of mouse fibroblast cell lines in which there was no detectable ER. This was due, at least in part, to the suppressed activation of serum-response genes such as c-fos that in turn resulted from the blocked histone hyperacetylation of nucleosomes containing the c-fos promoter region. The N-terminal leucine-abundant region of PELP1 was observed to interact with HDAC2 and exhibited repressive activity when tethered to the chromatin. In addition, the C-terminal glutamic acid-abundant region bound to the hypoacetylated histones H3 and H4 and prevented them from becoming substrates of histone acetyltransferase. Thus PELP1 promotes and maintains the hypoacetylated state of histones at the target genomic site, and ER binding reverses its role to hyperacetylate histones through an as yet unidentified mechanism.

Overexpression of the Linker histone-binding protein tNASP affects progression through the cell cycle

NASP is an H1 histone-binding protein that is cell cycle-regulated and occurs in two major forms: tNASP, found in gametes, embryonic cells, and transformed cells; and sNASP, found in all rapidly dividing somatic cells (Richardson, R. T., Batova, I. N., Widgren, E. E., Zheng, L. X., Whitfield, M., Marzluff, W. F., and O'Rand, M. G. (2000) J. Biol. Chem. 275, 30378-30386). When full-length tNASP fused to green fluorescent protein (GFP) is transiently transfected into HeLa cells, it is efficiently transported into the nucleus within 2 h after translation in the cytoplasm, whereas the NASP nuclear localization signal (NLS) deletion mutant (NASP-DeltaNLS-GFP) is retained in the cytoplasm. In HeLa cells synchronized by a double thymidine block and transiently transfected to overexpress full-length tNASP or NASP-DeltaNLS, progression through the G(1)/S border is delayed. Cells transiently transfected to overexpress the histone-binding site (HBS) deletion mutant (NASP-DeltaHBS) or sNASP were not delayed in progression through the G(1)/S border. By using a DNA supercoiling assay, in vitro binding data demonstrate that H1 histone-tNASP complexes can transfer H1 histones to DNA, whereas NASP-DeltaHBS cannot. Measurement of NASP mobility in the nucleus by fluorescence recovery after photobleaching indicates that NASP mobility is virtually identical to that reported for H1 histones. These data suggest that NASP-H1 complexes exist in the nucleus and that tNASP can influence cell cycle progression through the G(1)/S border through mediation of DNA-H1 histone binding.

Comparison of mouse and human NASP genes and expression in human transformed and tumor cell lines

We previously cloned and sequenced cDNAs encoding mouse NASP (mNASP), a cell cycle regulated histone H1 binding protein. Here we report the genomic sequence and organization for mNASP along with its 5' regulatory region and compare these with human NASP (hNASP). The mNASP gene contains 16 exons interrupted by 15 introns. The sequence encoding testis mNASP uses all 16 exons while the somatic form uses 13 exons by differential splicing. All the exons conform to the AG/GT splicing rule. Putative TATA box-containing transcription initiation sites are present for somatic NASP in human and mouse and for testis hNASP. Comparison of the promoter regions of mNASP and hNASP approximately 1 kb upstream of the transcription start sites for the two splice variants revealed a number of possible transcription factor binding sites relevant to specific patterns of NASP tissue expression. The presence of single bands on Southern blots of mouse genomic DNA suggests that mNASP is a single copy gene although pseudogenes exist in both the mouse and human genomes. Chromosome fluorescence by in situ hybridization revealed that mNASP is present on chromosome 4, in an area that corresponds to band 4D1, a region syntenic to the locus of hNASP on chromosome 1. Additionally, we report that human somatic and testis NASP mRNAs are expressed at varying levels in all the transformed cell lines and human tumors tested, further supporting NASP's role in the cell cycle of dividing cells.

Characterization of the histone H1-binding protein, NASP, as a cell cycle-regulated somatic protein

Nuclear autoantigenic sperm protein (NASP), initially described as a highly autoimmunogenic testis and sperm-specific protein, is a histone-binding protein that is a homologue of the N1/N2 gene expressed in oocytes of Xenopus laevis. Here, we report a somatic form of NASP (sNASP) present in all mitotic cells examined, including mouse embryonic cells and several mouse and human tissue culture cell lines. Affinity chromatography and histone isolation demonstrate that NASP from myeloma cells is complexed only with H1, linker histones. Somatic NASP is a shorter version of testicular NASP (tNASP) with two deletions in the coding region arising from alternative splicing and differs from tNASP in its 5' untranslated regions. We examined the relationship between NASP mRNA expression and the cell cycle and report that in cultures of synchronized mouse 3T3 cells and HeLa cells sNASP mRNA levels increase during S-phase and decline in G(2), concomitant with histone mRNA levels. NASP protein levels remain stable in these cells but become undetectable in confluent cultures of nondividing CV-1 cells and in nonmitotic cells in various body tissues. Expression of sNASP mRNA is regulated during the cell cycle and, consistent with a role as a histone transport protein, NASP mRNA expression parallels histone mRNA expression.

Analysis of the autoimmune epitopes on human testicular NASP using recombinant and synthetic peptides

The human nuclear autoantigenic sperm protein, NASP, is a testicular histone-binding protein of 787 amino acids to which most vasectomized men develop autoantibodies. In this study to define the boundaries of antigenic regions and epitope recognition pattern, recombinant deletion mutants spanning the entire protein coding sequence and a human NASP cDNA sublibrary were screened with vasectomy patients' sera. Employing panel sera from 21 vasectomy patients with anti-sperm antibodies, a heterogeneous pattern of autoantibody binding to the recombinant polypeptides was detected in ELISA and immunoblotting. The majority of sera (20/21) had antibodies to one or more of the NASP fusion proteins. Antigenic sites preferentially recognized by the individual patients' sera were located within aa 32-352 and aa 572-787. Using a patient's serum selected for its reactivity to the whole recombinant protein in Western blots, cDNA clones positive for the C-terminal domain of the molecule were identified. The number and location of linear epitopes in this region were determined by synthetic peptide mapping and inhibition studies. The epitope-containing segment was delimited to the sequence aa 619-692 and analysis of a series of 74 concurrent overlapping 9mer synthetic peptides encompassing this region revealed four linear epitopes: amino acid residues IREKIEDAK (aa 648-656), KESQRSGNV (aa 656-664), AELALKATL (aa 665-673) and GFTPGGGGS (aa 680-688). All individual patients' sera reacted with epitopes within the sequence IREellipsis.GGS (aa 648-688). The strongest reactivity was displayed by peptides corresponding to the sequence AELALKATL (aa 665-673). Thus, multiple continuous autoimmune epitopes in NASP involving sequences in the conserved C-terminal domain as well as in the less conserved testis-specific N-terminal region comprising the histone-binding sites, as predicted for an antigen-driven immune response, may be a target of autoantibodies in vasectomized men and may provide a relevant laboratory variable to describe more accurately the spectrum of autoantibody specificities associated with the clinical manifestation of vasectomy.

A chimeric sperm peptide induces antibodies and strain-specific reversible infertility in mice

The development of a contraceptive vaccine based on a gamete-specific antigen requires knowledge of the ability of the antigen to elicit an immune response that inhibits fertilization. A well-defined immune response, as elicited by a synthetic peptide comprising a dominant B-cell epitope coupled to a common promiscuous T-cell epitope, might be preferable. In this study, the immunodominant B-cell epitope of sperm antigen Sp17 has been identified and synthesized as a chimeric peptide with the promiscuous T-cell epitope bovine RNase[94-104] at the N terminal. Immunization of female BALB/c mice with this peptide induced a dose-dependent reduction in fertility. Although antibodies to recombinant and native Sp17 were elicited in these mice, there was no strict correlation between the level of these antibodies and the reduction in fertility. Moreover, the induction of infertility was strain-specific since no effect on fertility could be induced in B6AF1 mice. To understand the mechanism behind this apparent strain-specific infertility induction, a more extended study on both the humoral and the cellular immune response to the chimeric peptide was performed. The antigen-specific T-cell response and the levels of antigen-specific cytokines are the major factors that affect fertility outcome.

Autoimmunogenicity of the human sperm protein Sp17 in vasectomized men and identification of linear B cell epitopes

OBJECTIVE

To assess whether human sera positive for antisperm antibodies have detectable levels of Sp17 autoantibodies and to determine the linear B cell epitopes to which these are raised for both native and recombinant Sp17.

DESIGN

Enzyme-linked immunoaborbent assays were performed against recombinant HSp17 on 15 serum samples from prevasovasostomy and postvasovasostomy patients. Positive sera then were used in mimotope analyses to determine HSp17 immunodominant linear B cell epitopes. These were compared with the linear B cell epitopes of recombinant HSp17.

SETTING

University research laboratory.

PATIENT(S)

Fifteen vasectomized or vasovasostomized men.

MAIN OUTCOME MEASURE(S)

Serum antibody reactivity to human Sp17.

RESULT(S)

Sera from vasectomized and vasovasostomized men exhibit Sp17 antibodies raised predominantly to two immunodominant linear B cell epitopes (amino acids 4 to 19 and amino acids 118 to 127), which differed from those of recombinant HSp17 (amino acids 52 to 79 and amino acids 124 to 136).

CONCLUSION(S)

The results show that Sp17 is an antigen to which vasectomized men raise autoantibodies. Two linear B cell epitopes predominate in native Sp17 and these differ from (but overlap with) those of the bacterially expressed recombinant protein.