Zinc is sufficiently abundant within mammalian sperm nuclei to bind stoichiometrically with protamine 2

Although studies have demonstrated that zinc can bind to sperm nuclear proteins, specifically protamine 2, it has not been shown that the metal is sufficiently abundant inside the sperm nucleus to interact stoichiometrically with these proteins. In this study proton-induced X-ray emission (PIXE) has been used to measure the amount of sulfur and zinc within the nuclei of individual sperm cells to infer the stoichiometry of zinc binding to protamine 2 in six species of mammal: bull, chinchilla, stallion, hamster, human, and mouse (protamine 2 comprises from 0% (bull) to 67% (mouse) of the protamine present in the sperm of these animals). Using the sulfur mass and electrophoretic data on the relative proportion of protamine 1 and protamine 2 in the sperm chromatin of these species, the protamine 1, protamine 2, and total protamine contents within each species sperm nuclei have been determined. The PIXE measurements reveal that the zinc content of the sperm nucleus varies proportionately with the protamine 2 content of sperm chromatin. PIXE analyses of hamster protamines extracted under conditions that appear to at least partially preserve zinc binding also confirm that the majority of the metal is bound to protamine. In five of the species examined, sufficient zinc is present for each protamine 2 molecule to bind one zinc. The results obtained for chinchilla sperm, conversely, indicate the chinchilla protamine 2 molecule may interact differently with zinc. Chinchilla sperm only contain enough zinc for one atom to be bound to two protamine 2 molecules.     

Mapping and Measuring DNA to Protein Ratios in Mammalian Sperm Head by XANES Imaging

We have used XANES imaging, which combines X-ray absorption near edge spectral features (XANES) with 50-nm-resolution X-ray microscopy, to examine the content and distribution of DNA and protein in mature sperm cells. Sperm nuclei from five different species of mammals were examined; these species were chosen for analysis because their sperm contain marked differences in their protamine 1 and protamine 2 contents. The data we've obtained for bull, stallion, hamster, and mouse sperm suggest that the total nuclear protein to DNA ratio is similar in the sperm of many eutherian mammals. Since protamine constitutes the majority of the sperm nuclear protein, these results indicate that the total protamine content of sperm chromatin must be constant among mammalian species, independent of the extent of expression of the protamine 2 gene.     

Sexual selection on protamine and transition nuclear protein expression in mouse species

Post-copulatory sexual selection in the form of sperm competition is known to influence the evolution of male reproductive proteins in mammals. The relationship between sperm competition and regulatory evolution, however, remains to be explored. Protamines and transition nuclear proteins are involved in the condensation of sperm chromatin and are expected to affect the shape of the sperm head. A hydrodynamically efficient head allows for fast swimming velocity and, therefore, more competitive sperm. Previous comparative studies in rodents have documented a significant association between the level of sperm competition (as measured by relative testes mass) and DNA sequence evolution in both the coding and promoter sequences of protamine 2. Here, we investigate the influence of sexual selection on protamine and transition nuclear protein mRNA expression in the testes of eight mouse species that differ widely in levels of sperm competition. We also examined the relationship between relative gene expression levels and sperm head shape, assessed using geometric morphometrics. We found that species with higher levels of sperm competition express less protamine 2 in relation to protamine 1 and transition nuclear proteins. Moreover, there was a significant association between relative protamine 2 expression and sperm head shape. Reduction in the relative abundance of protamine 2 may increase the competitive ability of sperm in mice, possibly by affecting sperm head shape. Changes in gene regulatory sequences thus seem to be the basis of the evolutionary response to sexual selection in these proteins.     

Human male infertility may be due to a decrease of the protamine P2 content in sperm chromatin

Basic chromosomal proteins were extracted from the sperm of fertile and infertile human males. The relative proportions of protamine 1, 2, and 3 were determined by scanning microdensitometry following electrophoresis of total protamine in polyacrilamide gels. The findings were as follows: (1) The proportion of protamine P(2 + 3) in sperm obtained from infertile males was lower than that in fertile males. (2) Protamine P(2 + 3) in infertile human males showed reduced affinity to DNA. The possibility that some cases of human male infertility may be due to mutation within the protamine P2 gene is discussed. © 1993 Wiley-Liss, Inc.     

DNA and protein content of mouse sperm: Implications regarding sperm chromatin structure

A variety of biochemical and histochemical techniques have been used to compare the composition of chromatin in sperm nuclei isolated from the epididymides of five mouse strains. The DNA content was determined by phosphorus analysis, deoxyribose analysis, absorption spectroscopy at 260 nm, and cytomorphometry following gallocyanine chrome alum staining. All four methods indicate that the mouse sperm nucleus contains approx. 3.3 pg DNA and that the DNA content does not vary significantly among the strains tested. Three different techniques, quantitative amino acid analysis, absorption spectroscopy at 230 nm, and sperm head density analysis in cesium chloride, were used to determine the protein content. Sperm nuclei from each strain of mouse were found to have a protein to DNA ratio of 0.9 and a chromatin protein content of 3 pg/nucleus. Comparisons of the basic proteins by disc gel electrophoresis demonstrate that the sperm nuclei contain only protamine and lack significant levels of somatic histones or transition proteins. The sperm from each strain contained both mouse protamine variants and the relative distribution of the two proteins did not appear to differ among strains. Using this information, we have been able to draw certain conclusions regarding DNA-protamine interactions and the mode of DNA packaging in the sperm nucleus. The most important of these is that the DNA in the mouse sperm nucleus cannot be packaged in nucleosomes. The protamines in sperm chromatin do not function as structural proteins, providing a subunit core around which the DNA is wrapped, but appear to completely neutralize the phosphodiester backbone of the DNA molecule, thereby minimizing the repulsion between neighboring segments of DNA and allowing it to be condensed into a biochemically inactive particle of genetic information.     

Analysis of hamster protamines: primary sequence and species distribution.

Basic nuclear proteins were isolated from the sperm of the Syrian hamster Mesocricetus auratus and characterized by gel electrophoresis, amino acid analysis, and sequencing. Analyses of the proteins by gel electrophoresis show that sperm of this species contain both protamines 1 and 2. The two proteins were purified by HPLC and the complete primary sequence of hamster protamine 1 was determined by automated amino acid sequence analysis. The protein sequence was subsequently confirmed by sequencing the PCR-amplified protamine 1 gene. The first forty-two residues of the hamster protamine 2 sequence were obtained by amino acid sequence analysis of the isolated protein, and this sequence was also confirmed and extended by sequencing the gene. Total basic nuclear protein was also isolated from sperm of six other species of hamsters, the protamines were identified by HPLC and amino acid analysis, and the proportion of protamines 1 and 2 in each species was determined. Marked differences in the protamine 2 content of sperm were observed among the different species of hamster. This variation and the high level of sequence similarity between mouse and hamster protamines provide insight into how the two protamines may be organized in sperm chromatin. Mol. Reprod. Dev. 54:273-282, 1999. Published 1999 Wiley-Liss, Inc.     

Loss of the cleaved-protamine 2 domain leads to incomplete histone-to-protamine exchange and infertility in mice

Protamines are unique sperm-specific proteins that package and protect paternal chromatin until fertilization. A subset of mammalian species expresses two protamines (PRM1 and PRM2), while in others PRM1 is sufficient for sperm chromatin packaging. Alterations of the species-specific ratio between PRM1 and PRM2 are associated with infertility. Unlike PRM1, PRM2 is generated as a precursor protein consisting of a highly conserved N-terminal domain, termed cleaved PRM2 (cP2), which is consecutively trimmed off during chromatin condensation. The carboxyterminal part, called mature PRM2 (mP2), interacts with DNA and together with PRM1, mediates chromatin-hypercondensation. The removal of the cP2 domain is believed to be imperative for proper chromatin condensation, yet, the role of cP2 is not yet understood. We generated mice lacking the cP2 domain while the mP2 is still expressed. We show that the cP2 domain is indispensable for complete sperm chromatin protamination and male mouse fertility. cP2 deficient sperm show incomplete protamine incorporation and a severely altered protamine ratio, retention of transition proteins and aberrant retention of the testis specific histone variant H2A.L.2. During epididymal transit, cP2 deficient sperm seem to undergo ROS mediated degradation leading to complete DNA fragmentation. The cP2 domain therefore seems to be a key aspect in the complex crosstalk between histones, transition proteins and protamines during sperm chromatin condensation. Overall, we present the first step towards understanding the role of the cP2 domain in paternal chromatin packaging and open up avenues for further research.     

Scrotal heat stress induces altered sperm chromatin structure associated with a decrease in protamine disulfide bonding in the stallion

A variety of testicular insults can induce changes in the structure of spermatozoal chromatin, resulting in spermatozoal DNA that is more susceptible to acid-induced denaturation. The degree of change in the DNA can be measured using the sperm chromatin structure assay (SCSA). The SCSA measures the relative amounts of single- and double-stranded DNA after staining with the metachromatic dye, acridine orange. Here we used a stallion model (n = 4) to study the effects of scrotal heat stress on spermatozoal DNA. This model was created by insulating stallion testes for 48 h and collecting sperm daily thereafter for 60 days. Changes in the SCSA were then correlated with protamine disulfide content and protamine types and levels. Results of the SCSA indicated that the susceptibility of spermatozoal DNA to denaturation was dependent on the spermatogenic cell stage that the ejaculated sperm was in at the time of the heat stress. Spermatozoa with altered DNA had a decrease in the extent of disulfide bonding that was associated with an increase in the susceptibility of DNA to denaturation. However, there were no detectable changes in either the protamine type or level. Thus, in this model, decreased disulfide bonding is associated with an increased susceptibility of spermatozoal DNA to denaturation in the absence of protamine changes.     

Formation of native-like mammalian sperm cell chromatin with folded bull protamine

The DNA of most vertebrate sperm cells is packaged by protamines. The primary structure of mammalian protamine I can be divided into three domains, a central DNA binding domain that is arginine-rich and amino- and carboxyl-terminal domains that are rich in cysteine residues. In native bull sperm chromatin, intramolecular disulfide bonds hold the terminal domains of bull protamine folded back onto the central DNA binding domain, whereas intermolecular disulfide bonds between DNA-bound protamines help stabilize the chromatin of mature mammalian sperm cells. Folded bull protamine was used to condense DNA in vitro under various solution conditions. Using transmission electron microscopy and light scattering, we show that bull protamine forms particles with DNA that are morphologically similar to the subunits of native bull sperm chromatin. In addition, the stability provided by intermolecular disulfide bonds formed between bull protamine molecules within in vitro DNA condensates is comparable with that observed for native bull sperm chromatin. The importance of the bull protamine terminal domains in controlling the bull sperm chromatin morphology is indicated by our observation that DNA condensates formed under identical conditions with a fish protamine, which lacks cysteine-rich terminal domains, do not produce as uniform structures as bull protamine. A model is also presented for the bull protamine.DNA complex in native sperm cell chromatin that provides an explanation for the positions of the cysteine residues in bull protamine that form intermolecular disulfide bonds.     

Complex chromatin condensation patterns and nuclear protein transitions during spermiogenesis: examples from mollusks

In this paper we review and analyze the chromatin condensation pattern during spermiogenesis in several species of mollusks. Previously, we had described the nuclear protein transitions during spermiogenesis in these species. The results of our study show two types of condensation pattern: simple patterns and complex patterns, with the following general characteristics: (a) When histones (always present in the early spermatid nucleus) are directly replaced by SNBP (sperm nuclear basic proteins) of the protamine type, the spermiogenic chromatin condensation pattern is simple. However, if the replacement is not direct but through intermediate proteins, the condensation pattern is complex. (b) The intermediate proteins found in mollusks are precursor molecules that are processed during spermiogenesis to the final protamine molecules. Some of these final protamines represent proteins with the highest basic amino acid content known to date, which results in the establishment of a very strong electrostatic interaction with DNA. (c) In some instances, the presence of complex patterns of chromatin condensation clearly correlates with the acquisition of specialized forms of the mature sperm nuclei. In contrast, simple condensation patterns always lead to rounded, oval or slightly cylindrical nuclei. (d) All known cases of complex spermiogenic chromatin condensation patterns are restricted to species with specialized sperm cells (introsperm). At the time of writing, we do not know of any report on complex condensation pattern in species with external fertilization and, therefore, with sperm cells of the primitive type (ect-aquasperm). (e) Some of the mollusk an spermiogenic chromatin condensation patterns of the complex type are very similar (almost identical) to those present in other groups of animals. Interestingly, the intermediate proteins involved in these cases can be very different.In this study, we discuss the biological significance of all these features and conclude that the appearance of precursor (intermediate) molecules facilitated the development of complex patterns of condensation and, as a consequence, a great diversity of forms in the sperm cell nuclei     

Both P1 and P2 protamine genes are expressed in mouse, hamster, and rat

To date, in mammals except for the mouse and human, only one protamine variant has been isolated from sperm. These mammalian protamines share amino acid sequence homology with mouse protamine 1 (mP1), the tyrosine-containing variant. Southern blot analysis of restriction enzyme digests of hamster and rat liver DNA reveals the presence of sequences homologous to mP1, and also to mouse protamine 2 (mP2) cDNAs. Northern blots of hamster and rat total testis RNA probed with mP2 cDNA confirm that the protamine 2 gene in these species is transcribed into two size classes of mRNA of approximately 830 and 700 nucleotides. However, the relative abundance of the rat and hamster protamine 2 mRNAs (rP2 and hP2) in total testis is approximately 50-fold lower and 2- to 5-fold lower, respectively, than the mouse protamine 2 mRNA. Northern blot analysis of hamster and rat testis polysome gradients demonstrates that although the amount of rP2 mRNA and hP2 mRNA is reduced, both are present on polysomes. The decreased expression of rat and hamster protamine 2 mRNA relative to their protamine 1 counterparts contrasts protamine expression in the mouse testis, where approximately equal amounts of mP1 and mP2 protamine mRNAs are present. These results suggest differential expression of the P1 and P2 protamine genes in three closely related mammals.     

Protamine 2 deficiency leads to sperm DNA damage and embryo death in mice

Cytokinesis is incomplete in spermatogenic cells, and the descendants of each stem cell form a clonal syncytium. As a result, a heterozygous mutation in a gene expressed postmeiotically affects all of the haploid spermatids within a syncytium. Previously, we have found that disruption of one copy of the gene for either protamine 1 (PRM1) or protamine 2 (PRM2) in the mouse results in a reduction in the amount of the respective protein, abnormal processing of PRM2, and inability of male chimeras to transmit either the mutant or wild-type allele derived from the 129-genotype embryonic stem cells to the next generation. Although it is believed that protamines are essential for compaction of the sperm nucleus and to protect the DNA from damage, this has not been proven experimentally. To test the hypothesis that failure of chimeras to transmit the 129 genotype to offspring was due to alterations in the organization and integrity of sperm DNA, we used the single-cell DNA electrophoresis (comet) assay, ultrastructural analysis, and the intracytoplasmic sperm injection (ICSI) procedure. Comet assay demonstrated a direct correlation between the fraction of sperm with haploinsufficiency of PRM2 and the frequency of sperm with damaged DNA. Ultrastructural analysis revealed reduced compaction of the chromatin. ICSI with PRM2-deficient sperm resulted in activation of most metaphase II-arrested mouse eggs, but few were able to develop to the blastocyst stage. These findings suggest that development fails because of damage to paternal DNA and that PRM2 is crucial for maintaining the integrity of sperm chromatin.     

DNA packaging in mouse spermatids : Synthesis of protamine variants and four transition proteins

A comparison of the protein compositions of mouse late-step spermatids and cauda epididymal sperm has revealed that the relative distribution of the two amino acid sequence variants of mouse protamine differ markedly in spermatids and sperm. Sonication-resistant spermatids contain the two variants in a ratio of 1:1, while the ratio of these two proteins in cauda epididymal sperm is approx. 2:1. Labeling studies in vivo have shown that this difference is due, in part, to an asynchrony in the time of synthesis of the two protamine variants. Both proteins are synthesized in late-step spermatids, but synthesis of the tyrosine variant in sperm chromatin begins approximately one day before synthesis of the more predominant histidine variant. Analyses of the time of synthesis of protamine and the four transition proteins in late-step spermatids allowed us to estimate the spermatid stage in which these proteins are deposited on DNA and relate these events to the onset of sonication resistance in maturing spermatids. These results indicate that: (1) synthesis and deposition of protamine begins coincident with the onset of sonication resistance in early step 12 spermatids; (2) protamine deposition is complete by mid-step 15; and (3) synthesis of the transition proteins occurs coincident with protamine synthesis.     

Relationships between the dynamics of iatrogenic DNA damage and genomic design in mammalian spermatozoa from eleven species

The dynamic onset of DNA fragmentation in mammalian sperm populations varies widely in different species when the spermatozoa are incubated in vitro at body temperature for several hours, and recent studies have shown that the dynamic rate of DNA fragmentation within a species has considerable predictive value in terms of fertility. The reasons for such variation are unclear, but here we show that differences in protamine sequence and identity could be partially responsible. Sets of 10 normal semen samples from 11 species (ram, goat, boar, white-tailed deer, rabbit, human, domestic and Spanish fighting bull, horse, donkey, rhinoceros, and koala) were cryopreserved, thawed, diluted in an appropriate extender for each species, and then incubated for 4 hr at 37 °C. Semen samples from human infertility patients were also included for comparison with the donors. DNA fragmentation analysis was undertaken immediately after thawing (t(0)) and after 4 hr (t(4)) using the Halomax/Halosperm procedure, and the differences in DNA fragmentation between t(0) and t(4) were examined in the context of the respective protamine genomes. The expression of protamine 2 in a species significantly enhanced the likelihood of sperm DNA fragmentation; greater numbers of cysteine residues in protamine 1 tended to confer increased sperm DNA stability, and there were logical evolutionary relationships between species in terms of their sperm DNA stability. Human spermatozoa from infertility patients exhibited considerably higher DNA instability than the normal semen donors, a difference that could be indirectly attributed to unbalanced protamine 1-to-protamine 2 ratios.     

Histone- and Protamine-DNA Association: Conservation of Different Patterns within the β-Globin Domain in Human Sperm

Most DNA in human sperm is bound to highly basic proteins called protamines, but a small proportion is complexed with histones similar to those found in active chromatin. This raises the intriguing possibility that histones in sperm are marking sets of genes that will be preferentially activated during early development. We have examined the chromatin structure of members of the β-globin gene family, which are expressed at different times in development, and the protamine 2 gene, which is expressed in spermatids prior to the widespread displacement of histones by transition proteins. The genes coding for and γ globin, which are active in the embryonic yolk sac, contain regions which are histone associated in the sperm. No histone-associated regions are present at the sites tested within the β- and δ-globin genes which are silent in the embryonic yolk sac. The trends of histone or protamine association are consistent for samples from the same person, and no significant between-subject variations in these trends are found for 13 of the 15 fragments analyzed in the two donors. The results suggest that sperm chromatin structures are generally similar in different men but that the length of the histone-associated regions can vary. The association of sperm DNA with histones or protamines sometimes changes within as little as 400 bp of DNA, suggesting that there is fine control over the retention of histones.     

Ability of hamster spermatozoa to digest their own DNA

Mammalian sperm chromatin is bound by protamines into highly condensed toroids with approximately 50 kilobases (kb) of DNA. It is also organized into loop domains of about the same size that are attached at their bases to the proteinaceous nuclear matrix. In this work, we test our model that each sperm DNA-loop domain is condensed into a single protamine toroid. Our model predicts that the protamine toroids are linked by chromatin that is more sensitive to nucleases than the DNA within the toroids. To test this model, we treated hamster sperm nuclei with DNase I and found that the sperm chromatin was digested into fragments with an average size of about 50 kb, by pulse-field gel electrophoresis (PFGE). Surprisingly, we also found that spermatozoa treated with 0.25% Triton X-100 (TX) and 20 mM MgCl2 overnight resulted in the same type of degradation, suggesting that sperm nuclei have a mechanism for digesting their own DNA at the bases of the loop domains. We extracted the nuclei with 2 M NaCl and 10 mM dithiothreitol (DTT) to make nuclear halos. Nuclear matrices prepared from DNase I-treated spermatozoa had no DNA attached, suggesting that DNase I digested the DNA at the bases of the loop domains. TX-treated spermatozoa still had their entire DNA associated with the nuclear matrix, even though the DNA was digested into 50-kb fragments as revealed by PFGE. The data support our donut-loop model for sperm chromatin structure and suggest a functional role for this type of organization in that sperm can digest its own DNA at the sites of attachment to the nuclear matrix.     

Temporal association of protamine 1 with the inner nuclear membrane protein lamin B receptor during spermiogenesis

During mammalian spermiogenesis, histones are replaced by transition proteins, which are in turn replaced by protamines P1 and P2. P1 protamine contains a short arginine/serine-rich (RS) domain that is highly phosphorylated before being deposited into sperm chromatin and almost completely dephosphorylated during sperm maturation. We now demonstrate that, in elongating spermatids, this phosphorylation is required for the temporal association of P1 protamine with lamin B receptor (LBR), an inner nuclear membrane protein that also possesses a stretch of RS dipeptides at its nucleoplasmic NH(2)-terminal domain. Previous studies have shown that the cellular protein p32 also binds tightly to the unmodified RS domain of LBR. Extending those findings, we now present evidence that p32 prevents phosphorylation of LBR and furthermore that dissociation of this protein precedes P1 protamine association. Our data suggest that docking of protamine 1 to the nuclear envelope is an important intermediate step in spermiogenesis and reveal a novel role for SR protein kinases and p32.     

Origin and evolution of chromosomal sperm proteins

In the eukaryotic cell, DNA compaction is achieved through its interaction with histones, constituting a nucleoprotein complex called chromatin. During metazoan evolution, the different structural and functional constraints imposed on the somatic and germinal cell lines led to a unique process of specialization of the sperm nuclear basic proteins (SNBPs) associated with chromatin in male germ cells. SNBPs encompass a heterogeneous group of proteins which, since their discovery in the nineteenth century, have been studied extensively in different organisms. However, the origin and controversial mechanisms driving the evolution of this group of proteins has only recently started to be understood. Here, we analyze in detail the histone hypothesis for the vertical parallel evolution of SNBPs, involving a “vertical” transition from a histone to a protamine‐like and finally protamine types (H → PL → P), the last one of which is present in the sperm of organisms at the uppermost tips of the phylogenetic tree. In particular, the common ancestry shared by the protamine‐like (PL)‐ and protamine (P)‐types with histone H1 is discussed within the context of the diverse structural and functional constraints acting upon these proteins during bilaterian evolution.