On the diversity of sperm basic proteins in the vertebrates: V. Cytochemical and amino acid analysis in Squamata, Testudines, and Crocodylia

Insights into the sperm chromatin and implications for male infertility from a protein perspective

Male germ cells undergo an extreme but fascinating process of chromatin remodeling that begins in the testis during the last phase of spermatogenesis and continues through epididymal sperm maturation. Most of the histones are replaced by small proteins named protamines, whose high basicity leads to a tight genomic compaction. This process is epigenetically regulated at many levels, not only by posttranslational modifications, but also by readers, writers, and erasers, in a context of a highly coordinated postmeiotic gene expression program. Protamines are key proteins for acquiring this highly specialized chromatin conformation, needed for sperm functionality. Interestingly, and contrary to what could be inferred from its very specific DNA-packaging function across protamine-containing species, human sperm chromatin contains a wide spectrum of protamine proteoforms, including truncated and posttranslationally modified proteoforms. The generation of protamine knock-out models revealed not only chromatin compaction defects, but also collateral sperm alterations contributing to infertile phenotypes, evidencing the importance of sperm chromatin protamination toward the generation of a new individual. The unique features of sperm chromatin have motivated its study, applying from conventional to the most ground-breaking techniques to disentangle its peculiarities and the cellular mechanisms governing its successful conferment, especially relevant from the protein point of view due to the important epigenetic role of sperm nuclear proteins. Gathering and contextualizing the most striking discoveries will provide a global understanding of the importance and complexity of achieving a proper chromatin compaction and exploring its implications on postfertilization events and beyond. This article is categorized under: Reproductive System Diseases > Genetics/Genomics/Epigenetics Reproductive System Diseases > Molecular and Cellular Physiology.

Protamines from liverwort are produced by post-translational cleavage and C-terminal di-aminopropanelation of several male germ-specific H1 histones

Protamines are small, highly-specialized, arginine-rich, and intrinsically-disordered chromosomal proteins that replace histones during spermiogenesis in many organisms. Previous evidence supports the notion that, in the animal kingdom, these proteins have evolved from a primitive replication-independent histone H1 involved in terminal cell differentiation. Nevertheless, a direct connection between the two families of chromatin proteins is missing. Here, we primarily used electron transfer dissociation MS-based analyses, revealing that the protamines in the sperm of the liverwort Marchantia polymorpha result from post-translational cleavage of three precursor H1 histones. Moreover, we show that the mature protamines are further post-translationally modified by di-aminopropanelation, and previous studies have reported that they condense spermatid chromatin through a process consisting of liquid-phase assembly likely involving spinodal decomposition. Taken together, our results reveal that the interesting evolutionary ancestry of protamines begins with histone H1 in both the animal and plant kingdoms.

Recent and rapid amplification of the sperm basic nuclear protein genes in winter flounder

The high molecular weight basic nuclear proteins (HMrBNPs), which are tightly bound to sperm chromatin in winter flounder, are made up of imperfect reiterations of simple peptide sequences that contain phosphorylatable DNA-binding motifs. Genomic Southern blots hybridized with probes to the coding and non-coding regions of HMrBNP mRNA showed that HMrBNP sequences form a complex multi-gene family. Previously, one gene (2B) was used to establish an evolutionary link between histone H1 and the HMrBNPs. Further examination of this complex, multi-gene family has now revealed that the majority of the HMrBNP genes are linked as 4.5 kb direct tandem repeats that each contain a 2.8 kb coding region and a 1.7 kb intergenic region (IR). These findings, combined with the cloning of the IR, established that the tandemly repeated genes lack introns and code for the abundant 3 kb HMrBNP mRNAs that produce the prominent 110 kDa HMrBNP. Southern blotting of DNAs from other righteye flounder species showed that HMrBNP multi-gene families were present in closely related species, though with substantial differences in restriction patterns and band intensities, but were not detected in more distantly related flounders. These observations are consistent with recent and rapid elaboration of the HMrBNP gene family.

The high molecular weight chromatin proteins of winter flounder sperm are related to an extreme histone H1 variant

Unlike mammals, birds, and most other fishes, winter flounder completes spermatogenesis without replacing its germ cell histones with protamines. Instead, during spermiogenesis, these fish produce a family of high molecular weight (80,000-200,000) basic nuclear proteins (HMrBNPs) that bind to sperm chromatin containing the normal complement of histones. These large, basic proteins are built up of tandem iterations of oligopeptide repeats that contain phosphorylatable DNA-binding motifs. Although the HMrBNPs have no obvious homology to histones, protamines, or other sperm-specific chromatin proteins, we report here the isolation of a clone (2B) from a winter flounder genomic DNA library that establishes a link between the HMrBNPs and histone H1. The 2B sequence contains an open reading frame, which, when conceptually translated, encodes a 265-residue protein. At its N terminus the translation product contains numerous simple repeats that match the oligopeptides contained within the HMrBNPs. Unexpectedly, the C terminus of the putative protein shows 66% identity and 76% conservation to the histone H1 globular domain. This connection suggests that the HMrBNPs may have originated from the extended N-terminal tail region of a testis-specific, H1-like linker histone.

Primary structures of sperm-specific basic nuclear proteins and gene expression in Japanese newt, Cynops pyrrhogaster

Electrophoretic analyses of acid extracts from mature sperm of newt, Cynops pyrrhogaster, on acid/urea/Triton X-100 polyacrylamide gel showed the exclusive occurrence of sperm-specific nuclear basic proteins (SBPs), which moved faster than somatic histones on the gel. These SBPs were eluted separately by reversed phase-high-performance liquid chromatography as two large peaks and a few small peaks. Of these, only the small peaks disappeared with treatment of the acid extracts with alkaline phosphatase before they were injected into the column, so that there were only two distinct components: NP1 and NP2. Determination of amino acid sequences by the Edman method as well as by sequencing of cDNA for both components indicated that each protein consisted of 43 (NP1) or 48 (NP2) amino acid residues, rich in arginine residues (53.5% in NP1; 47.9% in NP2), forming the clusters. They had molecular masses of 5,386 Da (NP1) and 5,748 Da (NP2), respectively. Northern blot analysis using cDNAs as probes indicated that mRNAs for both NP1 and NP2 occurred not in primary spermatocytes but in round spermatids. In situ hybridization analyses using antisense RNA for NP1 as a probe clearly showed the first appearance of NP1 mRNA at the late stage of round spermatid.

Protamines of reptiles

We have characterized for the first time the complete primary structure of the main protamine components of the sperm from four reptiles: Chrysemys picta (turtle), Elaphe obsoleta (snake), Anolis carolinensis (lizard), and Alligator mississipiensis (crocodilian). These species were chosen to represent one of each of the main phylogenetic branches of this taxonomic group. Comparison of these protamine sequences with those already available from other vertebrate groups allows us to define properly the chemical consensus composition of protamines and provides a unique insight into their molecular evolution and classification.

Ultrastructure of nuclear condensation and localization of DNA and proteins in spermatozoa of a dasyurid marsupial, Sminthopsis crassicaudata

In the dasyurid marsupial, Sminthopsis crassicaudata, the mature spermatozoon has an inner homogeneous (C1) and a peripheral indented (C2) region. Using DNase-gold conjugates, and biotinylated genomic DNA probes, DNA was found to occur in both C1 and C2 regions. The morphogenesis of the spermatozoon nucleus was investigated using ultrastructural and cytochemical studies. Spermiogenesis was divided into 15 steps. By step 10, condensation of the C1 region was complete, and at the caudal extremity of the spermatid nucleus, the nuclear envelope enclosed an electron-lucent space. This space and the surrounding nuclear envelope became very enlarged at step 11. At this stage, a plate of approximately 70 nm in thickness was present along the caudal segment of the C1 region; this "nuclear mantle" did not bind DNase-gold conjugates but stained for lysine-rich proteins using alcoholic phosphotungstic acid. Chromatin condensation resumed at step 12 with the appearance of spherical chromatin structures peripheral to the C1 chromatin. These structures then partially coalesced and the indentations of the C2 region were observed. The expanded nuclear envelope at the caudal extremity persisted in caput epididymal spermatozoa. Spherical inclusions within it did not bind to DNase-gold conjugates but stained for lysine-rich proteins. As the sperm traveled down the epididymis, these inclusions amassed near the nuclear pores and were then removed from the nucleus. In addition, the nuclear mantle was found to have disappeared by the time the spermatozoa reached the corpus epididymidis.

Vertebrate protamine gene evolution I. Sequence alignments and gene structure

The availability of the amino acid sequence for nine different mammalian P1 family protamines and the revised amino acid sequence of the chicken protamine galline (Oliva and Dixon 1989) reveals a much close relationship between mammalian and avian protamines than was previously thought (Nakano et al. 1976). Dot matrix analysis of all protamine genes for which genomic DNA or cDNA sequence is available reveals both marked sequence similarities in the mammalian protamine gene family and internal repeated sequences in the chicken protamine gene. The detailed alignments of the cis-acting regulatory DNA sequences shows several consensus sequence patterns, particularly the conservation of a cAMP response element (CRE) in all the protamine genes and of the regions flanking the TATA box, CAP site, N-terminal coding region, and polyadenylation signal. In addition we have found a high frequency of the CA dinucleotide immediately adjacent to the CRE element of both the protamine genes and the testis transition proteins, a feature not present in other genes, which suggests the existence of an extended CRE motif involved in the coordinate expression of protamine and transition protein genes during spermatogenesis. Overall these findings suggest the existence of an avian-mammalian P1 protamine gene line and are discussed in the context of different hypotheses for protamine gene evolution and regulation.