Human protamines and male infertility

Protamines: lessons learned from mouse models

In brief

Protamines package and shield the paternal DNA in the sperm nucleus and have been studied in many mouse models over decades. This review recapitulates and updates our knowledge about protamines and reveals a surprising complexity in protamine function and their interactions with other sperm nuclear proteins.

Abstract

The packaging and safeguarding of paternal DNA in the sperm cell nucleus is a critical feature of proper sperm function. Histones cannot mediate the necessary hypercondensation and shielding of chromatin required for motility and transit through the reproductive tracts. Paternal chromatin is therefore reorganized and ultimately packaged by protamines. In most mammalian species, one protamine is present in mature sperm (PRM1). In rodents and primates among others, however, mature sperm contain a second protamine (PRM2). Unlike PRM1, PRM2 is cleaved at its N-terminal end. Although protamines have been studied for decades due to their role in chromatin hypercondensation and involvement in male infertility, key aspects of their function are still unclear. This review updates and integrates our knowledge of protamines and their function based on lessons learned from mouse models and starts to answer open questions. The combined insights from recent work reveal that indeed both protamines are crucial for the production of functional sperm and indicate that the two protamines perform distinct functions beyond simple DNA compaction. Loss of one allele of PRM1 leads to subfertility whereas heterozygous loss of PRM2 does not. Unprocessed PRM2 seems to play a distinct role related to the eviction of intermediate DNA-bound proteins and the incorporation of both protamines into chromatin. For PRM1, on the other hand, heterozygous loss leads to strongly reduced sperm motility as the main phenotype, indicating that PRM1 might be important for processes ensuring correct motility, apart from DNA compaction.

Loss of Prm1 leads to defective chromatin protamination, impaired PRM2 processing, reduced sperm motility and subfertility in male mice

One of the key events during spermiogenesis is the hypercondensation of chromatin by substitution of the majority of histones by protamines. In humans and mice, protamine 1 (PRM1/Prm1) and protamine 2 (PRM2/Prm2) are expressed in a species-specific ratio. Using CRISPR-Cas9-mediated gene editing, we generated Prm1-deficient mice and demonstrated that Prm1+/- mice were subfertile, whereas Prm1-/- mice were infertile. Prm1-/- and Prm2-/- sperm showed high levels of reactive oxygen species-mediated DNA damage and increased histone retention. In contrast, Prm1+/- sperm displayed only moderate DNA damage. The majority of Prm1+/- sperm were CMA3 positive, indicating protamine-deficient chromatin, although this was not the result of increased histone retention in Prm1+/- sperm. However, sperm from Prm1+/- and Prm1-/- mice contained high levels of incompletely processed PRM2. Furthermore, the PRM1:PRM2 ratio was skewed from 1:2 in wild type to 1:5 in Prm1+/- animals. Our results reveal that PRM1 is required for proper PRM2 processing to produce mature PRM2, which, together with PRM1, is able to hypercondense DNA. Thus, the species-specific PRM1:PRM2 ratio has to be precisely controlled in order to retain full fertility.

Direct visualization of pre-protamine 2 detects protamine assembly failures and predicts ICSI success

Histone-to-protamine transition is an essential step in the generation of fully functional spermatozoa in various mammalian species. In human and mouse, one of the two protamine-encoding genes produces a precursor pre-protamine 2 (pre-PRM2) protein, which is then processed and assembled. Here we design an original approach based on the generation of pre-PRM2-specific antibodies to visualize the unprocessed pre-PRM2 by microscopy, flow cytometry and immunoblotting. Using mouse models with characterized failures in histone-to-protamine replacement, we show that pre-Prm2 retention is tightly linked to nucleosome disassembly. Additionally, in elongating/condensing spermatids, we observe that pre-Prm2 and transition protein are co-expressed spatiotemporally, and their physical interaction suggests that these proteins act simultaneously rather than successively during histone replacement. By using our anti-human pre-PRM2 antibody we also measured pre-PRM2 retention rates in the spermatozoa from 49 men of a series of infertile couples undergoing ICSI, which shed new light on the debated relation between pre-PRM2 retention and sperm parameters. Finally, by monitoring 2-pronuclei (2PN) embryo formation following ICSI, we evaluated the fertilization ability of the sperm in these 49 patients. Our results suggest that the extent of pre-PRM2 retention in sperm, rather than pre-PRM2 accumulation per se, is associated with fertilization failure. Hence, anti-pre-PRM2/pre-Prm2 antibodies are valuable tools which could be used in routine monitoring of sperm parameters in fertility clinics, as well as in experimental research programmes to better understand the obscure process of histone-to-protamine transition.

Protamine-2 Deficiency Initiates a Reactive Oxygen Species (ROS)-Mediated Destruction Cascade during Epididymal Sperm Maturation in Mice

Protamines are the safeguards of the paternal sperm genome. They replace most of the histones during spermiogenesis, resulting in DNA hypercondensation, thereby protecting its genome from environmental noxa. Impaired protamination has been linked to male infertility in mice and humans in many studies. Apart from impaired DNA integrity, protamine-deficient human and murine sperm show multiple secondary effects, including decreased motility and aberrant head morphology. In this study, we use a Protamine-2 (Prm2)-deficient mouse model in combination with label-free quantitative proteomics to decipher the underlying molecular processes of these effects. We show that loss of the sperm's antioxidant capacity, indicated by downregulation of key proteins like Superoxide dismutase type 1 (SOD1) and Peroxiredoxin 5 (PRDX5), ultimately initiates an oxidative stress-mediated destruction cascade during epididymal sperm maturation. This is confirmed by an increased level of 8-OHdG in epididymal sperm, a biomarker for oxidative stress-mediated DNA damage. Prm2-deficient testicular sperm are not affected and initiate the proper development of blastocyst stage preimplantation embryos in vitro upon intracytoplasmic sperm injection (ICSI) into oocytes. Our results provide new insight into the role of Prm2 and its downstream molecular effects on sperm function and present an important contribution to the investigation of new treatment regimens for infertile men with impaired protamination.

The "omics" of human male infertility: integrating big data in a systems biology approach

Spermatogenesis is a complex process in which >2300 genes are temporally and spatially regulated to form a terminally differentiated sperm cell that must maintain the ability to contribute to a totipotent embryo which can successfully differentiate into a healthy individual. This process is dependent on fidelity of the genome, epigenome, transcriptome, and proteome of the spermatogonia, supporting cells, and the resulting sperm cell. Infertility and/or disease risk may increase in the offspring if abnormalities are present. This review highlights the recent advances in our understanding of these processes in light of the "omics revolution". We briefly review each of these areas, as well as highlight areas of future study and needs to advance further.

Protamine 2 precursors (Pre-P2), protamine 1 to protamine 2 ratio (P1/P2), and assisted reproduction outcome

OBJECTIVE

To determine whether the presence of protamine 2 precursors (pre-P2/P2 ratio) and the protamine 1 to protamine 2 ratio (P1/P2) are related to the assisted reproduction outcome.

DESIGN

Prospective study.

SETTING

Assisted Reproduction Unit and University laboratory.

PATIENT(S)

One hundred two infertile patients undergoing treatment at the Assisted Reproduction Unit of the Hospital Clinic of Barcelona.

INTERVENTION(S)

Intracytoplasmic sperm injection (ICSI) and/or IVF treatment of the infertile patients, sperm protamine analysis through electrophoresis and densitometry, and pre-P2 analysis through Western blot.

MAIN OUTCOME MEASURE(S)

The presence of protamine 2 precursors (pre-P2/P2 ratio), sperm P1/P2 ratio, fertilization rates by IVF and/or ICSI, and pregnancy outcome.

RESULT(S)

Pre-P2/P2 and P1/P2 ratios are positively associated with the pregnancy rate. In addition, the P1/P2 ratio is positively associated with the proportion of embryos obtained by IVF, but not by ICSI. The pre-P2/P2 ratio was not related to the fertilization rate.

CONCLUSION(S)

Decreased pre-P2/P2 and P1/P2 ratios are related to a poor pregnancy outcome, but not with the proportion of embryos obtained after ISCI.

Global sperm deoxyribonucleic acid methylation is unaffected in protamine-deficient infertile males

Sperm protamine-1 (P1) and protamine-2 (P2) concentrations were evaluated concomitantly with global DNA methylation patterns in a population of male infertility patients. Protamine quantification and immunofluorescence microscopy in conjunction with quantitative image analysis revealed no significant relationships between the P1/P2 ratio, P1 concentrations, or P2 concentrations and levels of global DNA methylation in mature spermatozoa.

Sperm protamine 1/protamine 2 ratios are related to in vitro fertilization pregnancy rates and predictive of fertilization ability

OBJECTIVE

To evaluate whether aberrant sperm P1/P2 ratios are predictive of abnormal fertilizing ability and are related to in vitro fertilization (IVF) outcome.

DESIGN

Prospective case-control study.

SETTING

University-based infertility and IVF clinic.

PATIENT(S)

Forty-three male infertility patients with an abnormally reduced P1/P2 ratio, 251 patients with a normal P1/P2 ratio, and 121 patients with an abnormally elevated P1/P2 ratio.

INTERVENTION(S)

Human IVF, the sperm penetration assay (SPA), and sperm protamine quantification via nuclear protein extraction, gel electrophoresis, and densitometry analysis.

MAIN OUTCOME MEASURE(S)

Sperm P1/P2 ratios; P1 and P2 quantities; SPA scores; and IVF-fertilization, embryo-quality, pregnancy, delivery, and spontaneous-abortion rates.

RESULT(S)

Standard IVF fertilization rates and SPA scores were significantly reduced in patients with abnormally low and high P1/P2 ratios. In vitro fertilization embryo quality was comparable between these groups, but pregnancy rates were significantly reduced in patients with abnormally reduced P1/P2 ratios.

CONCLUSION(S)

The P1/P2 ratio has a significant relationship to sperm fertilization ability. The relationship between protamines and fertilization ability is not understood but may be either a reflection of generalized abnormalities during spermiogenesis or an indication of protamine deficiency acting as a regulator or checkpoint of spermatogenesis.

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.

A novel mechanism of protamine expression deregulation highlighted by abnormal protamine transcript retention in infertile human males with sperm protamine deficiency

Sperm protamine deficiency has been associated with human male infertility. However, the aetiology of deregulated protamine expression remains elusive. The objective of this study was to evaluate the underlying aetiology of protamine deficiency in male infertility patients with deregulated protamine expression. Protamine-1 (P1) and protamine-2 (P2) protein concentrations were compared against P1 and P2 mRNA levels in the sperm of 166 male infertility patients and 27 men of known fertility. Protamine protein concentrations were quantified by nuclear protein extraction, gel electrophoresis and densitometry analysis. Semi-quantitative real-time RT-PCR was used to quantify P1 and P2 mRNA levels. P1 mRNA concentrations were significantly increased in patients underexpressing P1 protein versus those with normal and increased P1 levels. In patients with an abnormally low ratio of P1 to P2 (P1/P2 <0.8), there was a significant increase in P1 mRNA retention. Patients underexpressing P2 also had significantly increased mean P2 mRNA levels, although the majority of these P2-deficient patients showed an increased frequency of significantly reduced P2 mRNA levels. This is the first study to concomitantly evaluate P1 and P2 protein and mRNA levels in mature human sperm. Abnormally elevated protamine mRNA retention appears to be associated with aberrant protamine expression in infertile human males. These data suggest that defects in protamine translation regulation may contribute to protamine deficiency in infertile males.

Fine resolution of human sperm nucleoproteins by two-dimensional electrophoresis

Human sperm nucleoproteins consist of protamines and histones. Changes in composition of these proteins are thought to correlate with spermatogenesis and may be involved in some instances of male infertility. We sought to separate sperm nucleoproteins including variants of protamine using an improved two-dimensional electrophoretic method, with the aim of comprehensively analysing all sperm nucleoprotein constituents. After extracting nuclear basic proteins from the sperm of normal volunteers, we analysed these proteins on a gel sheet by a radical free, highly reducing method based on Kaltschmidt and Whittmann's two-dimensional electrophoresis. Basic proteins from sperm nuclei were separated clearly into 12 spots. By amino acid sequence analysis, these spots corresponded to protamine 1 (P1)- (five spots), protamine 2 (P2)-related proteins (six spots) and testis-specific histone H2B (one spot). The N-terminal amino acid sequences of the six P2-related proteins were compatible with those of HPI1, HPI2, HPS1, HPS2, HP2 and HP3, and quantitative comparison could be performed. In conclusion, human sperm nucleoproteins including all P2-related variants could be analysed quantitatively with high resolution on a single electrophoretic gel.

Cellular expression of protamine 1 and 2 transcripts in testicular spermatids from azoospermic men submitted to TESE-ICSI

Testicular sperm extraction (TESE) combined with ICSI is used to treat azoospermia. However, the factors that influence the outcome of ICSI in this situation are ill-defined. We sought to investigate the expression of protamine 1 (PRM1) and protamine 2 (PRM2) transcripts in testicular spermatids from obstructive and non-obstructive azoospermic men with impaired spermatogenesis. The relationship between PRM1 and PRM2 transcript levels and the TESE-ICSI outcome was evaluated. The cellular expression of PRM1 and PRM2 mRNAs in single testicular spermatids from 41 azoospermic patients (in whom testicular spermatozoa were subsequently recovered and submitted for TESE-ICSI) was determined by radioactive in situ hybridization. Group I contained seven men with congenital, obstructive azoospermia and whose testicular biopsies indicated quantitatively normal spermatogenesis. Group II consisted of 18 azoospermic men with moderately impaired spermatogenesis. Sixteen men with non-obstructive azoospermia and severely deranged spermatogenesis (i.e. mixed atrophy with small foci of spermatids and spermatozoa) constituted group III. The spermatids of men with severely deranged spermatogenesis exhibited significant lower PRM1 mRNA expression than in the other patient groups. There were no significant inter-group differences in PRM2 mRNA expression. Spermatid PRM1 expression was lower in non-pregnant couples than in pregnant couples. The low number of spermatids in cases of mixed atrophy with small spermatogenic foci is associated with significantly lower PRM1 expression and a lower pregnancy rate. These results emphasize the role of PRM1 as a potentially critical factor in post-ICSI embryonic development.

Identification and evaluation of a novel sperm protamine abnormality in a population of infertile males

BACKGROUND

A significant relationship exists between an abnormally high sperm protamine-1 (P1)/protamine-2 (P2) ratio and male infertility. In this study we investigate whether a decreased P1/P2 ratio is also linked to male infertility and we attempt to describe, at the protein expression level, the underlying cause of sperm P1/P2 deregulation.

METHODS

P1 and P2 protein concentrations were quantified in sperm from 272 infertility patients and 87 fertile donors. P1/P2 ratios and protamine quantity were correlated with fertility status using semen analysis, sperm penetration capacity, and IVF data.

RESULTS

We identified four distinct groups in the study: normal P1/P2 fertile donors, normal P1/P2 patients, low P1/P2 patients, and high P1/P2 patients. P1 and P2 were both under-expressed in patients with a normal P1/P2 ratio, but not in fertile donors. In patients with a low P1/P2 ratio, P1 was under-expressed while P2 was over-expressed; in patients with a high P1/P2 ratio, P1 was normally expressed and P2 was under-expressed. Patients with abnormal P1/P2 ratios displayed significantly reduced semen quality and sperm penetration ability.

CONCLUSIONS

We have identified a novel population of infertile males with a reduced P1/P2 ratio. Aberrant P1/P2 ratios arise from an abnormal concentration of P1 and/or P2, either of which is associated with male infertility.

Semen analysis at the turn of the century: an evaluation of potential uses of new sperm function assays

Semen analysis is a critical assay in the evaluation of infertility and may yield critical information regarding the etiology and prognosis of many types of reduced male fertility. However, basic semen analysis does not directly measure sperm fertilizing capacity, or many of the biochemical events both prior to and subsequent to fertilization. In the last two decades numerous assays of sperm function have been developed. These assays can be classified as: 1) Assays of general biochemistry and ultrastructure, 2) Assays of zona binding and oocyte penetration, and 3) Assays of postpenetration events. Sperm function assays not only allow an accurate diagnosis of many infertilities not diagnosed by the semen analysis, but can also lead to improved treatment modalities. In this review, basic semen analysis and many sperm function assays are briefly reviewed. Novel uses of sperm function are demonstrated in brief case studies.