DNA-interacting proteins in the spermiogenesis of the mollusc Murex brandaris

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

Spermiogenic nuclear protein transitions and chromatin condensation. Proposal for an ancestral model of nuclear spermiogenesis

We have chosen three species (Sparus aurata, Dicentrarchus labrax, and Monodonta turbinata) that represent different transition patterns in the composition and structure of spermiogenic nuclei. The transition patterns of these species are representative of spermiogenesis in a large number of animal species. We analyze: (a) nuclear protein exchange; (b) chromatin condensation pattern; and (c) histone acetylation during spermiogenic development. In the simplest spermiogenesis histones and nucleosomes remain in mature sperm. Chromatin of spermatids is organized into 20 nm granules, simultaneous with a nuclear volume reduction. The granules coalesce in the final stage of spermiogenesis. Granular chromatin is correlated with acetylation of histones H3 and H4, whereas final coalescence is associated with histone deacetylation. We also studied two other spermiogenesis where a basic protein substitutes histones. Each species has a very different substituting protein. One has a typical protamine of 34 amino acids; the other has a sperm nuclear basic proteins (SNBP) of 106 amino acids. In both, the structural transitions and histone acetylation pattern are similar: in early spermiogenesis chromatin is organized into 20 nm granules, and histones are significantly acetylated, while the nuclear volume decreases. Subsequently, acetylated histones are displaced by the protamine or SNBP. Histone substitution causes chromatin remodelling and additional reduction in nuclear volume. We analyze these three cases together with earlier works and propose that the formation of 20 nm granules containing acetylated H3 and H4 accomplishes the minimum functional requirement to be considered the most evolutionarily ancestral chromatin conformation preceding condensation in animal spermiogenesis.

Histones and nucleosomes in Cancer sperm (Decapod: Crustacea) previously described as lacking basic DNA-associated proteins: a new model of sperm chromatin

To date several studies have been carried out which indicate that DNA of crustacean sperm is neither bound nor organized by basic proteins and, contrary to the rest of spermatozoa, do not contain highly packaged chromatin. Since this is the only known case of this type among metazoan cells, we have re-examined the composition, and partially the structure, of the mature sperm chromatin of Cancer pagurus, which has previously been described as lacking basic DNA-associated proteins. The results we present here show that: (a) sperm DNA of C. pagurus is bound by histones forming nucleosomes of 170 base pairs, (b) the ratio [histones/DNA] in sperm of two Cancer species is 0.5 and 0.6 (w/w). This ratio is quite lower than the proportion [proteins/DNA] that we found in other sperm nuclei with histones or protamines, whose value is from 1.0 to 1.2 (w/w), (c) histone H4 is highly acetylated in mature sperm chromatin of C. pagurus. Other histones (H3 and H2B) are also acetylated, though the level is much lower than that of histone H4. The low ratio of histones to DNA, along with the high level of acetylation of these proteins, explains the non-compact, decondensed state of the peculiar chromatin in the sperm studied here. In the final section we offer an explanation for the necessity of such decondensed chromatin during gamete fertilization of this species.

All roads lead to arginine: the squid protamine gene

The protamine of squid is one of the most arginine-rich protamines (77%, mol/mol). It possesses a leading sequence that is posttranslationally removed during spermatogenesis in a manner that is analogous to that observed in some of its vertebrate protamine counterparts. In this paper we describe the gene sequence of the protamine of the squid Loligo opalescens. This represents the first complete gene sequence ever reported for an invertebrate protamine. Like those of vertebrate protamines, the messenger RNA is polyadenylated but the gene does not contain an intron. The promoter region contains the major transcriptional regulatory elements (CRE, TATA box, and CAP) that are also characteristic of the vertebrate protamine genes. It is unclear whether the similarities of protamines in species from both the deuterostome and the protostome branches represent the result of phylogenetic conservation or evolutionary convergence.

Chromatin organization during spermiogenesis inOctopus vulgaris. II: DNA-interacting proteins

In this article we study the proteins responsible for chromatin condensation during spermiogenesis in the cephalopod Octopus vulgaris. The DNA of ripe sperm nuclei in this species is condensed by a set of five different proteins. Four of these proteins are protamines. The main protamine (Po2), a protein of 44 amino acid residues, is extraordinarily simple (composed of only three different amino acid types: arginine (R), serine (S), and glycine (G). It is a basic molecule consisting of 79.5 mol% arginine residues. The rest of the protamines (Po3, Po4, Po5) are smaller molecules (33, 28, and 30 amino acid residues, respectively) that are homologous among themselves and probably with the main Po2 protamine. The ripe sperm nucleus of O. vulgaris also contains a small quantity of a molecule (Po1) that is similar to Po2 protamine. This protein could represent a Po2 protamine-precursor in a very advanced step of its processing. We discuss the characteristics of these proteins, as well as the relation between the complexity of chromatin condensation and the transitions of nuclear proteins during spermiogenesis in O. vulgaris.

Chromatin organization during spermiogenesis inOctopus vulgaris. I: Morphological structures

In the process of the chromatin remodeling that occurs during spermiogenesis in some animal species, it is possible to distinguish between two separate aspects: the chromatin condensation pattern itself (granular, fibrillar, or lamellar), and the architecture of this pattern, that is to say, its arrangement within the nucleus. In the cephalopod Octopus vulgaris these two aspects are clearly differentiated. The condensation pattern develops from 25 nm fibers to fibers with a tubular aspect and with a progressively increasing diameter (40-60 nm and then to 80 nm), to end finally in the form of very thin fibers (3-5 nm) product of the coalescence and dissolution of the major fibers. The main directive force that governs this process lies in the global change that occurs in the proteins that interact with all (or the major part) of the genomic DNA. The condensation pattern by itself in this species does not present a fixed order: most of the fibers appear without any predominant spatial direction in the spermiogenic nuclei. However, as the nuclei elongate, the chromatin fibers arrange in parallel following the elongation axis. This parallel disposition of the chromatin fibers appears to be mediated by two specific areas, each of which we call a "polar nuclear matrix" (PNM). These matrices differentiate in the basal and apical nuclear poles adjacent to the centriolar implantation fosse and the acrosome, respectively. The areas that constitute the PNM have the following characteristics: (a) they are the only areas where DNA is found anchored to the nuclear membrane; (b) they are the zones from which the chromatin condensation pattern (fibers/tubules) begins; and (c) they are most probably the points through which the mechanical forces originating from nuclear elongation are transmitted to chromatin, causing the chromatin fibers/tubules to adopt an almost perfectly parallel disposition. Finally, we discuss the importance of the architecture of the chromatin condensation pattern, as it is one of the determining factors of the spatial organization of the mature sperm genome and chromosome positioning.

Basic nuclear protein pattern and chromatin condensation in the male germ cells of a tropical abalone,Haliotis asinina

The basic nuclear proteins (BNPs) in spermatozoa of a tropical abalone, Haliotis asinina, were composed of a majority of protamine-like (PL) protein and a small amount of histones H1 and H4. Abalone H1 and PL proteins exhibited strong immunological cross reactivities among themselves as well as with chick H5 and calf thymus H1. Thus, all these proteins may belong to the same family. Immunolocalization by indirect immunofluorescence and immunoelectron microscopy indicated that H1 and H4 were present in all steps of the male germ cells, however, with decreasing amount in late stage cells, particularly spermatids and spermatozoa. On the other hand, PL was present in middle step cells (secondary spermatocytes) with increasing amount in spermatids and spermatozoa when the chromatin became tightly packed. Thus, PL may be involved in the condensation of chromatin in the spermatozoa of this species.

Protamines in the internally fertilizing neobatrachian frogEleutherodactylus coqui

The internally fertilizing primitive frog Ascaphus truei (family Ascaphidae) from the Pacific Northwest is the only frog with an intromittent organ. The more advanced neobatrachian frog Eleutherodactylus coqui (family Leptodactylidae) from Puerto Rico has secondarily acquired internal fertilization but mates by cloacal apposition. Nonetheless, both frogs have introsperm with an elongated head containing highly condensed chromatin. Characterization of sperm nuclear basic proteins (SNBPs) in E. coqui by acid-urea polyacrylamide gel electrophoresis indicates that, as in A. truei, testes from a single animal contain several protamines. Amino acid analysis indicates a composition for the most rapidly moving protamine of each species as follows: in E. coqui, ARG (35.6 mol %) + LYS (3.8 mol %) + HIS (7.6 mol %) = 47 mol % total basic residues and in A. truei, ARG (42.1 mol %) + LYS (11.1 mol %) = 53.2 mol % total basic residues. Transmission electron microscopy shows that E. coqui introsperm, like those in A. truei, are elongate with highly condensed chromatin. However, E. coqui introsperm lacks an axial perforatorium that extends into an endonuclear canal. These morphological features are plesiomorphic (primitive) and shared by A. truei with urodeles and basal amniotes (Jamieson et al. (1993) Herpetologica 49:52-65). In E. coqui introsperm, the nucleoprotein complex has a cross-sectional axis of 420 + 20 angstroms and shows a knobby chromatin structural organization in TEM. The presence of arginine-enriched protamines in both a basal anuran like the ascaphid A. truei and a more advanced neobatrachian like the leptodactylid E. coqui supports the hypothesis that internal fertilization acts as a constraint on the range of SNBP diversity in animals.

Possible mechanisms for early and intermediate stages of sperm chromatin condensation patterning involving phase separation dynamics

During spermiogenesis in some internally fertilizing molluscs and insects, the post-meiotic spermatid nucleus develops via a sequence of complex patterns of the nuclear contents (chromatin and nucleoplasm) on the way to final chromatin condensation. We have examined the TEM data on these sequences for three species: Philaenus spumarius(a homopteran insect), Murex brandaris (a gastropod mollusc), and Eledone cirrhosa(a cephalopod mollusc). For each of these, spatially quantitative study reveals a constant spacing between pattern repeats through changes from granular to fibrillar to lamellar pattern, followed finally by a shrinkage of the spacing. Therefore we distinguish a "patterning" stage followed by a "condensation" stage. The former appears to demand a dynamic explanation, because there is no sign of structural connections to establish the part of the spacing that crosses the nucleoplasm. We consider types of dynamic mechanism, and show that for "nanostructural" dimensions (tens of nanometers as pattern spacing) reaction-diffusion dynamics are quite inappropriate, but that separation of two fluid phases by a mechanism similar to what is known as "spinodal decomposition" is a very attractive possibility.

A walk though vertebrate and invertebrate protamines

An updated comparative analysis of protamines and their corresponding genes is presented, including representative organisms from each of the vertebrate classes and one invertebrate (squid, Loligo opalescens). Special emphasis is placed on the implications for sperm chromatin organization and the evolutionary significance. The review is based on some of the most recent publications in the field and builds upon previously published reviews on this topic.

Condensation of DNA by spermatid basic nuclear proteins

Two transition proteins, TP1 and TP2, participate in the repackaging of the spermatid genome early in mammalian spermiogenesis, coincident with the first detectable changes in chromatin condensation. Using an optical trap and a two-channel flow cell to move single DNA molecules into buffer containing protein, we have measured the rates of DNA condensation and decondensation induced by the binding of Syrian hamster transition proteins TP1 and TP2 and protamines P1 and P2. The results show that both transition proteins condense free DNA, with rates similar to those of protamine 1 and 2. DNA molecules condensed with TP1 were significantly less stable than DNA condensed by protamine or by TP2. Experiments conducted with a peptide corresponding to the C-terminal 25 residues of TP2 showed that this domain is responsible for condensing DNA. Experiments conducted with two fragments of TP1 containing arginine and lysine residues demonstrated that DNA binding by TP1 must involve more than these basic sequences. Zinc facilitated the condensation of DNA by P2 but not by TP2. The dissociation rates of TP2 and P2 from DNA were not affected by the addition of zinc.

Evolution of octopod sperm II: comparison of acrosomal morphogenesis in Eledone and Octopus

The first stages of acrosome development during Eledone cirrhosa spermiogenesis are similar to that in Octopus vulgaris, and comprise the initial elongation of both organelles. However, the acrosome in E. cirrhosa does not continue its elongation as it does in O. vulgaris. Instead, its length remains fixed and it undergoes a process of helicoidization that includes the entire organelle. In each spermatid, helicoidization of the E. cirrhosa acrosome occurs simultaneously with helicoidization of the nucleus. The acrosome is associated with special structures that probably are involved in the helical torsion of the organelle. We propose a hypothesis to explain the evolutionary relationship between the acrosomes of O. vulgaris and E. cirrhosa, particularly as it is influenced by nucleomorphogenesis and microtubular contraction.

Chromatin condensation, cysteine-rich protamine, and establishment of disulphide interprotamine bonds during spermiogenesis of Eledone cirrhosa (Cephalopoda)

During spermiogenesis in Eledone cirrhosa a single protamine substitutes for histones in nuclei of developing spermatids. This protein displays a peculiar primary structure. It contains 22.6 mol% cysteine residues (19 cysteines in 84 residues). This makes it the most cysteine-rich protamine known. The proportion of basic residues is relatively low (arginine 36.9 mol%, lysine 19.0 mol%). The protamine of E. cirrhosa condenses spermiogenic chromatin in a pattern which comprises fibres with a progressively larger diameter and lamellae that finally undergo definitive coalescence. We have also performed a study that estimates the number of interprotamine disulphide bonds formed during the process of spermiogenic chromatin condensation by means of sequential disappearance of MMNA (monomaleimido-nanogold) labelling. During the first step of spermiogenesis, protamines are found spread over very slightly condensed chromatin with their cysteines in a reactive state (protamine-cys-SH). From this stage the interprotamine disulphide bonds are established in a progressive way. First they are formed inside the chromatin fibres. Subsequently, they participate in the mechanism of fibre coalescence and finally, in the last step of spermiogenesis, the remaining free reactive -SH groups of cysteine form disulphide bonds, thus promoting a definitive stabilization of the nucleoprotein complex in the ripe sperm nucleus.

Chromatin reorganization during spermiogenesis of the mollusc Thais hemostoma (Muricidae): implications for sperm nuclear morphogenesis in cenogastropods

Thais is a cenogastropod mollusc belonging to the Muricidae family. The sperm nuclear morphogenesis of Thais develops in two well-defined and peculiar steps. In the first one, the round early spermatidyl nucleus is penetrated by an endonuclear channel, which arranges as a helix at the inner nuclear surface and organizes the condensing chromatin all around. In the second step, the spiral channel stretches, dragging along the associated chromatin and leading to a definitive cylinder-shaped sperm nucleus. Simultaneously with these changes in nuclear shape, the chromatin is sequentially organized in granules, fibres, lamellae, and, finally, in a very condensed structure, whereas the spermiogenic DNA-associated proteins become more basic and simple. The sperm nucleus contains a small group of protamines consisting of only four types of amino acid (lysine, arginine, glycine, and serine). The most remarkable fact on nuclear spermiogenesis in Thais is that, whereas the chromatin condensation process, the nuclear proteins, and the final shape of sperm nucleus are very similar to those in other muricidae studied, the pathway of nuclear morphogenesis is completely different. We propose an independent genetic control for those two spermiogenic events (chromatin condensation and nucleomorphogenesis). Finally we discuss briefly the main traits of nucleomorphogenesis of muricid molluscs.

Selection of cancer chemopreventive agents based on inhibition of topoisomerase II activity

The present study was undertaken to determine if in vitro inhibition of one or both of the two most dominant mammalian DNA topoisomerases (topos) is common among chemopreventive agents. To determine if an agent was a topo I inhibitor, we employed the DNA relaxation and nicking assays. For potential topo II inhibitors, we used the DNA unknotting and linearisation assays. 14 of 30 agents (47%) were ineffective in all four assays (IC(50) >100 microgram/ml), and 11 (37%) inhibited topo II catalytic activity. The sensitivity of the topo II assay was 63%, selectivity 93%, positive predictive value 91%, and total accuracy 77%. For chemopreventive efficacy, the positive predictive value of the unknotting assay was 92%, and the total accuracy was 60%. These data suggest that reduced topo II activity is a desirable property of many known chemopreventive agents. We conclude that the unknotting assay could be a valuable addition to the in vitro tests presently used to select chemopreventive agents.