Immunological evidence for a P2 protamine precursor in mature rat sperm

The poly(A)-binding protein partner Paip2a controls translation during late spermiogenesis in mice

Translational control plays a key role in late spermiogenesis. A number of mRNAs encoding proteins required for late spermiogenesis are expressed in early spermatids but are stored as translationally inactive messenger ribonucleoprotein particles (mRNPs). The translation of these mRNAs is associated with shortening of their poly(A) tail in late spermiogenesis. Poly(A)-binding protein (Pabp) plays an important role in mRNA stabilization and translation. Three Pabp-interacting proteins, Paip1, Paip2a, and Paip2b, have been described. Paip2a is expressed in late spermatids. To investigate the role of Paip2 in spermiogenesis, we generated mice with knockout of either Paip2a or Paip2b and double-KO (DKO) mice lacking both Paip2a and Paip2b. Paip2a-KO and Paip2a/Paip2b-DKO mice exhibited male infertility. Translation of several mRNAs encoding proteins essential to male germ cell development was inhibited in late spermiogenesis in Paip2a/Paip2b-DKO mice, resulting in defective elongated spermatids. Inhibition of translation in Paip2a/Paip2b-DKO mice was caused by aberrant increased expression of Pabp, which impaired the interaction between eukaryotic initiation factor 4E (eIF4E) and the cap structure at the 5' end of the mRNA. We therefore propose a model whereby efficient mRNA translation in late spermiogenesis occurs at an optimal concentration of Pabp, a condition not fulfilled in Paip2a/Paip2b-DKO mice.

Protamine 2 precursors, protamine 1/protamine 2 ratio, DNA integrity and other sperm parameters in infertile patients

BACKGROUND

The protamine 1-to-protamine 2 ratio (P1/P2) is altered in the sperm cells of some infertile patients. Also, evidence for increased protamine 2 precursors (pre-P2) in a few patients has been reported. But so far, there have been no studies measuring simultaneously these two variables in a large number of patients.

METHODS

We measured the P1/P2 ratio and the presence of pre-P2 using, for the first time, an antibody specific to the precursor pre-P2, together with other sperm parameters in 224 infertile patients. Additionally, the DNA integrity was assessed by terminal transferase dUTP nick-end labelling (TUNEL) in a subset of the samples.

RESULTS

Pre-P2 levels show a significant positive correlation with the P1/P2 ratio, with the presence of other proteins and, at low pre-P2 levels, with TUNEL-positive sperm. An inverse correlation with sperm count, normal morphology and motility was detected.

CONCLUSIONS

The levels of pre-P2 may provide clues into the pathogenic mechanisms of infertility. The increased proportion of pre-P2 in some patients with increased P1/P2 ratio suggests an involvement of pre-P2 processing. The positive correlation between TUNEL-positive sperm and pre-P2 at low pre-P2/P2 ratios also suggests a link between deficient protamine processing and decreased DNA integrity.

Protamines and male infertility

Protamines are the major nuclear sperm proteins. The human sperm nucleus contains two types of protamine: protamine 1 (P1) encoded by a single-copy gene and the family of protamine 2 (P2) proteins (P2, P3 and P4), all also encoded by a single gene that is transcribed and translated into a precursor protein. The protamines were discovered more than a century ago, but their function is not yet fully understood. In fact, different hypotheses have been proposed: condensation of the sperm nucleus into a compact hydrodynamic shape, protection of the genetic message delivered by the spermatozoa, involvement in the processes maintaining the integrity and repair of DNA during or after the nucleohistone-nucleoprotamine transition and involvement in the epigenetic imprinting of the spermatozoa. Protamines are also one of the most variable proteins found in nature, with data supporting a positive Darwinian selection. Changes in the expression of P1 and P2 protamines have been found to be associated with infertility in man. Mutations in the protamine genes have also been found in some infertile patients. Transgenic mice defective in the expression of protamines also present several structural defects in the sperm nucleus and have variable degrees of infertility. There is also evidence that altered levels of protamines may result in an increased susceptibility to injury in the spermatozoan DNA causing infertility or poor outcomes in assisted reproduction. The present work reviews the articles published to date on the relationship between protamines and infertility.

Nucleoprotein Transitions During Spermiogenesis in Mice with Transition Nuclear Protein Tnp1 and Tnp2 Mutations1

Chromatin remodeling during spermiogenesis is characterized by a series of nuclear protein replacements. Histones are replaced by transition nuclear proteins, which are in turn replaced by protamines. The transition nuclear proteins, TP1 and TP2, and the protamines, PRM1 and PRM2, are the major nuclear proteins involved in this process. Biochemical studies of mice with null mutations in one of the Tnp genes showed that the absence of one TP led to an apparent elevation in the amount of the remaining TP in the testis. To investigate the mechanism of changes of protein levels and effects of one Tnp mutation on other nuclear proteins, we used immunohistochemistry techniques to determine the distribution of these nuclear proteins. In contrast to previous biochemical analyses, which indicated that nuclear protein replacement was sequential with little overlap between the protein types, we found considerable overlap in the nucleoprotein types during spermiogenesis. The TPs, which appear in the nucleus before histone displacement is complete, were shared among genetically inequivalent spermatids. The absence of one TP did not affect the time of appearance of the other TP or of the protamines, but it did affect the displacement of the other TP, leading to its abnormal retention in the nucleus. The elevated levels of the remaining TP in Tnp-mutant mice appeared to be a consequence of the prolonged retention, rather than increased synthesis. Thus the absence of one of the TPs did not significantly affect transcription or translation of the other basic proteins, but it did affect posttranslational events.

Protamine 1: protamine 2 stoichiometry in the sperm of eutherian mammals

We have compared the relative proportion of protamine 1 (P1) and protamine 2 (P2) bound to DNA in the sperm of a variety of eutherian mammals to obtain insight into how these two proteins interact in sperm chromatin. Gel electrophoresis (combined with microdensitometry) and high performance liquid chromatography (HPLC) were used to determine the content of the two protamines, and the identity of each protein was confirmed by amino-terminal sequencing or amino acid analysis. The sperm of all species examined contained P1, but P2 was found to be present only in certain species. Unlike the fixed ratio of core histones that package DNA into nucleosomes in all somatic cells, the proportion of P2 present in mature sperm was found to be continuously variable from 0 to nearly 80%. These results show that P1 and P2 do not interact with each other or DNA to form a discrete complex or subunit structure that is dependent upon particular P1/P2 stoichiometries. Data obtained from a number of closely and distantly related species also indicate that while the P2 content of sperm chromatin is allowed to vary over a wide range during the course of evolution, the relative proportion of P1 and P2 are tightly regulated within a genus.

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.

Detection of P2 precursors in the sperm cells of infertile patients who have reduced protamine P2 levels

OBJECTIVE

To determine whether the reduction in the protamine P2 content (increased P1/P2 ratio) reported in some infertile patients could result from incomplete processing of protamine P2 precursors.

DESIGN

Analysis of samples with a marked reduction in the protamine P2 content using polyacrylamide gel electrophoresis and subsequent detection of protamine P2 precursors through Western blot analysis.

SETTING

University departments and laboratories.

PATIENT(S)

One hundred eighty-four men undergoing an evaluation for infertility.

MAIN OUTCOME MEASURE(S)

Comparative Western blot analysis of nuclear sperm proteins using specific antibodies to protamine P1 and protamine P2.

RESULT(S)

After selection of the samples with a marked reduction of the protamine P2 content and subsequent analysis by Western blot, a small proportion of putative P2 precursors was detected in most samples, whereas a significant increase was detected in two of them.

CONCLUSION(S)

In some infertile men, a reduction in the protamine P2 content relative to protamine P1 (increased P1/P2 ratio) is detected concomitant with an increase in the amount of putative P2 precursors. This could represent the first report of incomplete processing of a nuclear sperm protein in humans.

Persistence of protamine precursors in mature sperm nuclei of the mouse

During mouse spermiogenesis, two protamines, mP1 and mP2, are synthesized in replacement of histones. One of them (protamine mP2, 63 residues) appears at first in elongating spermatid nuclei as a protamine of 106 residues (pmP2) with an amino-terminal extension that is progressively excised. The two protamines were previously described as the only proteins associated with DNA in sperm chromatin. This paper shows that the nuclear proteins of mouse spermatozoa are indeed heterogeneous: at least six minor polypeptides in addition to protamines can be identified. The primary structure of four of them has been established. They are intermediate in the maturation of the precursor of protamine mP2 and correspond to polypeptides pmP2/11, pmP2/16, pmP2/20, and pmP2/32, characterized previously in mouse testis. Therefore, these intermediates of proteolysis generated from pmP2 inside spermatid nuclei persist in mature sperm, whereas the largest precursors, pmP2 and pmP2/5, disappear. These findings clearly indicate that limited proteolysis events still occur outside of the testis.

On the evolution of protamines in bony fish: alternatives to the "retroviral horizontal transmission" hypothesis

Fish protamines are highly specialized molecules which are responsible for chromatin condensation during the last stages of spermatogenesis (spermiogenesis). However, not all fish contain protamines in their sperm nuclei; rather, there seems to be a random distribution of protamines within this group. The origin of this sporadic presence of protamines in the sperm and its significance have not yet been precisely determined. In this paper we have conducted an exhaustive survey of the literature available on the different types of nuclear protein composition of the sperm of teleost fish in order to try to correlate these data with what is presently known about the taxonomy of this group. The results of this analysis have allowed us to make the following observations. The divergence between protamines and histones has occurred several times during the evolution of the bony fish. However, the relative frequency of this divergence is almost negligible during the differentiation of genera and species (intrafamily variation) and is very small during the differentiation of families (interfamily variation). Nevertheless, the divergence is very noticeable among the different orders. It is therefore possible to conclude from all this that the sporadic distribution of protamines in bony fish is not a random event as initially believed. Furthermore, such a heterogeneous distribution of protamines cannot be easily accounted for by a mechanism of horizontal retroviral transmission through repeated and independent acquisition of a protamine gene as has been recently proposed (Jankowski, Stater, Dixon (1986) J Mol Evol 23:1-10). Rather, it could possibly be explained by a repeated and independent loss of the expression of the protamine gene (or loss of the gene itself) which mainly occurred during the diversification of the orders of this group.

Identification of the binding site of two monoclonal antibodies to human protamine

We have previously developed a number of monoclonal antibodies (Mabs) that bind to protamine. One of these antibodies, Hup1N, binds to human protamine 1 but not to protamine 2. In contrast, Mab HupA binds both protamine 1 and protamine 2. The epitopes for these two Mabs were observed to overlap, and were localized to the evolutionarily conservative amino-terminal region of protamine 1. This assignment is based on antibody binding to protamine from different species in which the protamine sequence is known, as well as analysis of antibody binding to synthetic peptides and synthetic peptides with specific amino acid substitutions.