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.

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Protamines and male infertility.

Haploid male germ cells package their DNA into a volume that is typically 10% or less that of a somatic cell nucleus. To achieve this remarkable level of compaction, spermatozoa replace most of their histones with smaller, highly basic arginine and (in eutherians) cysteine rich protamines. One reason for such a high level of compaction is that it may help optimise nuclear shape and hence support the gametes’ swimming ability for the long journey across the female reproductive tract to the oocyte. Super-compaction of the genome may confer additional protection from the effects of genotoxic factors. However, many species including the human retain a fraction of their chromatin in the more relaxed nucleosomal configuration that appears to run counter to the ergonomic, toroidal and repackaging of sperm DNA. Recent research suggests that the composition of this ‘residual’ nucleosomal compartment, a generally overlooked feature of the male gamete, is far more significant and important than previously thought. In this respect, the transport and incorporation of modified paternal histones by the spermatozoon to the zygote has been demonstrated and indicates another potential paternal effect in the epigenetic reprogramming of the zygote following fertilisation that is independent of imprinting status. In this review, the most recent research into mammalian spermatozoal chromatin composition is discussed alongside evidence for conserved, non-randomly located nucleosomal domains in spermatozoal nuclei, all supporting the hypothesis that the spermatozoon delivers a novel epigenetic signature to the egg that may be crucial for normal development. We also provide some thoughts on why this signature may be required in early embryogenesis. Reproduction (2010) 139 287–301

/pdf/paternal-dna-packaging-in-spermatozoa-more-than-the-sum-of-4p53dj8mbq.pdf

Paternal DNA packaging in spermatozoa: more than the sum of its parts? DNA, histones, protamines and epigenetics.

During the elongating spermatid stage of spermiogenesis, human sperm chromatin undergoes a complex transition in which histones are extensively replaced by protamines in a carefully regulated transition including histone modifications and intermediate and temporary replacement of the histones by sperm-specific transition proteins. The replacement of most histones by protamines 1 and 2 facilitates a high order of chromatin packaging necessary for normal sperm function and may also be necessary for DNA silencing and imprinting changes within the sperm cell. Protamines 1 and 2 are usually expressed in nearly equal quantities, but elevated or diminished protamine 1/protamine 2 ratios are observed in some infertile men and is often associated with severe spermatogenesis defects. Human and animal studies demonstrate that expression of the protamine proteins is uniquely regulated by transcription/translation factors, including storage of the mRNA in ribonucleoprotein (RNP) particles composed of the mRNA, transcription factors and a kinesin molecule necessary for transport of the RNP to the cytoplasm and removal of transcriptional activators from the nucleus. Recent studies indicate that most patients with abnormal protamine protein levels have elevated levels of protamine transcript in the mature sperm cell, indicating a possible defect in transcription or translation. The regulation of protamine expression is unique and includes several possible mechanisms which may be responsible for dysregulation of protamine expression and concurrent broad spectrum defects in spermatogenesis. We suggest two hypotheses: (i) that abnormal protamine expression is indicative of a generalized defect in mRNA storage and/or translation which affects other mRNA transcripts or (ii) that protamines may act as a checkpoint of spermatogenesis.

/pdf/altered-protamine-expression-and-diminished-spermatogenesis-1pbfl1usk6.pdf

Altered protamine expression and diminished spermatogenesis: what is the link?

Multiplex mRNA profiling for the identification of body fluids

A dramatic remodeling of sperm chromatin occurs during mammalian spermiogenesis. Nuclear elongation and chromatin condensation are concomitant with modifications in the basic protein complement associated with DNA. A number of biochemical events accompany the displacement of histones and the appearance of protamines in elongating spermatids. The mRNAs of transition proteins and protamines are transcribed and stored in the cytoplasm of spermatids until days later when they are translated. The intrinsic regulation of the expression of the transition protein and protamine genes occurs at three levels: transcription, translation, and posttranslation. The aim of this review is to cover most of the morphological, biochemical, and functional events which concern nuclear protein transitions during spermiogenesis and which are thereby involved in the nuclear status of ejaculated sperm cells.

Expression of mammalian spermatozoal nucleoproteins.

The nuclei of spermatozoa in all mammals examined so far contain P1 protamine. A second protamine variant, protamine P2, has to date been isolated only from human and murine spermatozoa where it represents the major fraction of basic nuclear protein. In order to elucidate the reason for this unusual distribution of the protamine variants among mammals we have investigated the expression of protamine P2 in boar and bull. It can be shown that also in these species protamine 2 is transcribed and translated on low levels. Various mutational events though have altered the primary structure of the protein: In boar, a deletion of 8 aminoacids has removed a sequence motif from the amino-terminus of the molecule, which highly probable is of functional relevance. The bovine sequence, as a consequence of numerous point mutations has accumulated neutral and hydrophobic aminoacids which reduce the affinity of the protamine 2 to DNA.

/pdf/the-lack-of-protamine-2-p2-in-boar-and-bull-spermatozoa-is-56hqav57cg.pdf

The lack of protamine 2 (P2) in boar and bull spermatozoa is due to mutations within the P2 gene

Genomic sequences of human protamines whose genes, PRM1 and PRM2, are clustered.

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The lack of protamine 2 (P2) in boar and bull spermatozoa is due to mutations within the P2 gene

Genomic sequences of human protamines whose genes, PRM1 and PRM2, are clustered.