Interactions of nuclear proteins with DNA, during sperm differentiation in the ram

Biology of sperm chromatin structure and relationship to male fertility and embryonic survival

Embryonic mortality in mammals is typically thought to result from 'female factor' infertility. There is growing evidence, however, that the status of sperm chromatin (DNA) at the time of fertilisation can also influence embryonic survival. During the final stages of spermatogenesis (spermiogenesis) a number of unique biochemical, morphological and physiological processes take place that are associated with marked changes in the structure of sperm chromatin. In early stages of spermatogenesis, sperm DNA is associated with histone nucleoproteins and structured into classical nucleosome core particles similar to other somatic cells. As spermiogenesis proceeds, the histone nucleoproteins are replaced by transition proteins which are subsequently replaced by protamines. At the completion of spermiogenesis the chromatin of mature sperm has a toroidal structure that is tightly compacted and resistant to denaturation. The compaction is necessary to protect sperm chromatin during transit through the epididymis and female reproductive tract. Disruption to chromatin remodelling during spermiogenesis results in chromatin that is susceptible to denaturation. Inappropriate chromatin structure has been shown in a number of mammalian species to be related to male infertility, and specifically the failure of embryonic development. A range of techniques are available to assess chromatin status in sperm but arguably the most informative is the sperm chromatin structure assay (SCSA). The SCSA is a flow cytometric assay that uses the metachromatic properties of acridine orange to measure the susceptibility of sperm chromatin to acid-induced denaturation. A relationship has been demonstrated, primarily in men, between the SCSA outcome and the probability of continued embryonic development and the establishment of pregnancy after fertilisation. The contribution of sperm chromatin instability to reproductive wastage in both natural mating and assisted reproduction warrants further investigation as it may prove valuable as a means of decreasing the incidence of embryonic mortality. In this regard, it is possible that 'male factor' infertility may emerge as an even more important component in embryonic development.

Formation of native-like mammalian sperm cell chromatin with folded bull protamine

The DNA of most vertebrate sperm cells is packaged by protamines. The primary structure of mammalian protamine I can be divided into three domains, a central DNA binding domain that is arginine-rich and amino- and carboxyl-terminal domains that are rich in cysteine residues. In native bull sperm chromatin, intramolecular disulfide bonds hold the terminal domains of bull protamine folded back onto the central DNA binding domain, whereas intermolecular disulfide bonds between DNA-bound protamines help stabilize the chromatin of mature mammalian sperm cells. Folded bull protamine was used to condense DNA in vitro under various solution conditions. Using transmission electron microscopy and light scattering, we show that bull protamine forms particles with DNA that are morphologically similar to the subunits of native bull sperm chromatin. In addition, the stability provided by intermolecular disulfide bonds formed between bull protamine molecules within in vitro DNA condensates is comparable with that observed for native bull sperm chromatin. The importance of the bull protamine terminal domains in controlling the bull sperm chromatin morphology is indicated by our observation that DNA condensates formed under identical conditions with a fish protamine, which lacks cysteine-rich terminal domains, do not produce as uniform structures as bull protamine. A model is also presented for the bull protamine.DNA complex in native sperm cell chromatin that provides an explanation for the positions of the cysteine residues in bull protamine that form intermolecular disulfide bonds.

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.

Expression of a zinc-binding domain of boar spermatidal transition protein 2 in Escherichia coli

Transition protein 2 (TP2; 137 amino acid residues) from boar late spermatid nuclei has three potential zinc finger motifs in the N-terminal 34 region. Gel shift assays revealed that boar TP2 recognized a CpG island sequence in a zinc-dependent manner. However, there was some nonspecific recognition of the oligonucleotide. Then, we constructed the expression system of zinc-binding domain of TP2 (TP2Z) (residues 1-103) in Escherichia coli. Double-stranded DNA fragments encoding TP2Z were synthesized as 18 fragments with 103 residues, annealed, and cloned into the expression plasmid pET11d. TP2Z was expressed upon induction with 1 mM isopropylthiogalactoside and extracted with acid including 0.71 M 2-mercaptoethanol. TP2Z was purified by ion-exchange chromatography on Fractogel EMD SO(-)(3) and HPLC on Nucleosil 300 7C18 and on Diol-120. Atomic absorption and CD spectroscopy showed that TP2Z bound three atoms of zinc per molecule of the protein and underwent a zinc-dependent conformational change in a manner similar to that for intact TP2. Gel shift assays indicated that TP2Z recognized a CpG island sequence more specifically than intact TP2 and that the specificity is dependent on zinc.

Transition protein 4 from boar late spermatid nuclei is a topological factor that stimulates DNA-relaxing activity of topoisomerase I

Transition protein 4 (TP4) from boar late spermatid nuclei, having higher affinity for double-stranded DNA and a local melting activity of DNA, stimulated SV40 DNA-relaxing activity of eukaryotic topoisomerase I at TP4/DNA molar ratios of 6.6-11. A TP4-spermidine mixture stimulated the activity of topoisomerase I much more than spermidine alone, but no more than TP4 alone, and poly-L-arginine did not. These results suggest that TP4 contributes to the chromatin reorganization in the late spermatid nuclei from nucleosomal-type structure with negatively supercoiled DNA to nucleoprotamine structure with no supercoiled DNA.

Casein kinase II activity of buffalo sperm chromatin

Two cyclic AMP-independent protein kinase activities have been found associated with buffalo sperm chromatin: a histone kinase highly specific for arginine-rich histone was reported recently (Mudgal et al., 1997: Arch Andrology 38:191-199) and a casein kinase II is described here. Casein kinase activity was solubilized with 0.35 M NaCl, which extracted 90% of the initial enzyme activity associated with buffalo sperm chromatin. Of the two acidic proteins tested, casein was preferred substrate over phosvitin. Among the casein fractions, the order of preference for casein kinase was beta-casein > alpha-casein > casein. Cyclic AMP at concentrations up to 50 microM had no effect on the phosphorylation of casein. Phosphoamino acid analysis using casein as the substrate showed threonine to be the acceptor amino acid for phosphoester link. Phosphorylation specificity was determined by phosphorylating buffalo beta-casein followed by the preparation of tryptic peptides and identification of amino acid residue phosphorylated. Threonine residue at position 41 having clusters of acidic amino acid residues (Thr. Glu. Asp. Glu) C-terminal to it was phosphorylated, a phosphorylation specificity akin to CKII. It is thought that phosphorylation of histones decreases their association with DNA and probably makes the DNA more available for replication, while phosphorylation of nonhistone proteins modifies their interaction with histones, allowing control of template activity. Two protein kinases found in buffalo sperm chromatin may perform a similar function.

AFM analysis of DNA-protamine complexes bound to mica

A novel method for reconstituting sperm chromatin was used to investigate how protamine 1 condenses DNA. Complexes formed in vitro using linearized plasmid DNA were imaged and measured by atomic force microscopy (AFM). The structures formed were found to be highly dependent on the sample preparation method used for reconstitution. Interstrand, side-by-side fasiculation of DNA and toroidal-like structures only 1-2 DNA diameters thick were observed for complexes formed in solution following direct mixing of the DNA and protamine. Large chromatin aggregates were also observed on the mica. However, if the DNA was first allowed to attach to the mica prior to addition of the protamine, well-defined toroidal complexes were formed without any observed DNA fasiculation or aggregate formation. The diameter of the toroids measured 30.6-50.2 nm (mean 39.4 nm). The dimensions of these structures indicate that the condensed DNA is stacked vertically by four to five turns, with each coil containing as little as 360-370 bp of 'B'-form DNA. This approach for preparing and imaging DNA-protamine complexes permits the analysis of intermediate structures 'trapped' on the mica as partially formed toruses of nucleoprotamine.

Extent of sperm chromatin hydration determined by atomic force microscopy

Volume measurements were performed on intact bull and mouse sperm heads and amembranous sperm nuclei, both in the fully hydrated (fluid cell) and dehydrated (air-dried on glass coverslips) states by atomic force microscopy (AFM). Data were obtained by analyzing a small population of cells/nuclei, as well as by performing repeated measurements on single cells imaged following the addition of increasing concentrations of propanol. Results show that the volume of fully hydrated, intact sperm heads and amembranous sperm chromatin particles are at least twice the volume of their air-dried counterparts. Dehydration occurs rapidly in air, and the reduction in volume of chromatin induced by water loss appears to be completely reversible. These studies demonstrate that both mouse and bull sperm chromatin are extensively hydrated in the native state, and are not as compact as previous studies have suggested.

The amino acid sequence and interaction with the nucleosome core DNA of transition protein 4 from boar late spermatid nuclei

The primary structure of transition protein 4 (TP4) from boar late spermatid nuclei was determined by automated Edman degradation of S-pyridylethylated protein and of peptides generated by cleavage with Staphylococcus aureus V8 protease, lysyl endopeptidase and CNBr. Boar TP4 is a basic protein consisting of a highly basic amino-terminal half (residues 1-73) and a less basic carboxy-terminal half (residues 74-138). The latter half includes a highly hydrophobic segment, a four-times tandemly repeated sequence, N(G)QNKR(K)X, and a carboxy-terminal segment containing Trp126. Ultraviolet absorption and CD spectra of TP4-rat-liver-nucleosome-core-DNA (double-stranded DNA) complexes suggest a TP4-induced local melting of DNA. Although at 1 mM NaCl TP4 brought about a slight stabilization of the DNA against thermal melting, a destabilization of the DNA was observed at 50 mM NaCl. From the results of quenching of tryptophan (Trp126) fluorescence of TP4 upon its binding to double-stranded and single-stranded boar liver nucleosome-core DNA at 50 mM NaCl, the apparent association constants for the binding of TP4 to double-stranded and single-stranded DNA were calculated to be 7.3 x 10(3) M-1 and 4.1 x 10(3) M-1, respectively. These results suggest that TP4, having different domain structures from TP1-3 and a higher affinity for double-stranded DNA, induces a local destabilization of DNA probably through the stacking of Trp126 with nucleic acid bases.

Genome organisation in the murine sperm nucleus

The organisation of DNA sequences in the murine sperm nucleus was studied using in situ hybridisation of biotinylated DNA probes. The efficiency of this reaction was assessed using a dispersed repetitive DNA probe. Telomeric DNA was distributed around the nucleus. Centromeric and ribosomal DNA sequences occupied restricted domains in the sperm nucleus. DNA sequences for a transgene and a cluster of homeogenes occupied different, and rather less defined, domains. Together these results imply that both repetitive and protein-coding sequences are arranged in the nucleus in an ordered fashion.

The nuclear status of human sperm cells

In the last decade, and in particular since the development of in vitro fertilization techniques, the nuclear status of human sperm cells has shown to be a key parameter in the assessment of male fertility. The shape and condensed state of the mature sperm nucleus are determined by structural and functional events that occur during spermiogenesis. This paper reviews essential findings on re-organization of the nucleus during sperm differentiation and maturation, and reports recent data on the architecture, biochemical composition and stability of the nucleus in human ejaculated spermatozoa. Different methods used to evaluate nuclear maturity in relation to male fertility are critically appraised.

Chromosome and sperm size of Holsteins with and without bovine leukocyte adhesion deficiency

The objective was to evaluate bull differences in chromosomal and spermatozoal areas related to the occurrence of the bovine leukocyte adhesion deficiency syndrome. Lymphocyte chromosomes from 30 Holstein bulls and 2 Holstein heifers were measured using image analysis and computer-enhanced video-microscopy. Spermatozoal head areas from 29 of the 30 bulls were measured. Autosomal rank was based on decreasing area. Average total autosomal areas were not the same across bulls. One group of bulls had significantly smaller average chromosomal areas than the others; this group carried bovine leukocyte adhesion deficiency syndrome. Area measures of spermatozoal heads showed that bulls with bovine leukocyte adhesion deficiency syndrome had significantly larger head areas than normal bulls. Lymphocyte chromosomes from 3 cattle that were homozygous for bovine leukocyte adhesion deficiency syndrome were significantly smaller than chromosomes from syndrome heterozygotes. Carrier identification was improved by the use of autosomal and sperm area measurements in addition to pedigree evaluation.

Atomic force microscopy of mammalian sperm chromatin

We have used the atomic force microscope (AFM) to image the surfaces of intact bull, mouse and rat sperm chromatin and partially decondensed mouse sperm chromatin attached to coverglass. High resolution AFM imaging was performed in air and saline using uncoated, unfixed and unstained chromatin. Images of the surfaces of intact chromatin from all three species and of an AFM-dissected bull sperm nucleus have revealed that the DNA is organized into large nodular subunits, which vary in diameter between 50 and 100 nm. Other images of partially decondensed mouse sperm chromatin show that the nodules are arranged along thick fibers that loop out away from the nucleus upon decondensation. These fibers appear to stretch or unravel, generating narrow smooth fibers with thicknesses equivalent to a single DNA-protamine complex. High resolution AFM images of the nodular subunits suggest that they are discrete, ellipsoid-shaped DNA packaging units possibly only one level of packaging above the protamine-DNA complex.

Human male infertility may be due to a decrease of the protamine P2 content in sperm chromatin

Basic chromosomal proteins were extracted from the sperm of fertile and infertile human males. The relative proportions of protamine 1, 2, and 3 were determined by scanning microdensitometry following electrophoresis of total protamine in polyacrylamide gels. The findings were as follows: (1) The proportion of protamine P(2 + 3) in sperm obtained from infertile males was lower than that in fertile males. (2) Protamine P(2 + 3) in infertile human males showed reduced affinity to DNA. The possibility that some cases of human male infertility may be due to mutation within the protamine P2 gene is discussed.

Localization of DNase I-hypersensitive regions during rat spermatogenesis: stage-dependent patterns and unique sensitivity of elongating spermatids

DNase I-hypersensitivity of rat spermatogenic cells was analyzed 1) to establish overall patterns of hypersensitivity in individual cell types, 2) to correlate these patterns with known changes in chromatin organization and function, and 3) to provide a foundation for further analyses examining DNase I-hypersensitivity and the localization of specific genes during spermatogenesis. Parameters for in situ nick translation, using radioactive and fluorescent probes to visualize DNase I-hypersensitive regions (DHR), were established for fixed and sectioned testicular preparations, permeabilized cells, and isolated germ cell nuclei. As anticipated, the pattern of DHR changed in a cell-type specific manner during the course of spermatogenesis, reflective of known stage-dependent alterations in the composition and structure of both the chromatin and the nuclear lamina/matrix as well as changes in gene expression. DHR in preleptotene spermatocytes were primarily peripheral, while in pachytene spermatocytes they were localized along the condensed chromosomes. The pattern of DHR changed from "checkerboard" in steps 7-8 round spermatid nuclei to "lamellar" in steps 10-11 elongating spermatids. In steps 12-13 elongating spermatids. DHR were localized throughout the nuclei or in a graded manner--increasing from anterior to posterior and mirroring the pattern of chromatin condensation. However, unlike the case in other stages, DNA of steps 12-13 elongating spermatids was exquisitely sensitive to nick translation even in the absence of exogenous DNase I. In contrast to the labeling of earlier stages, steps 16-19 spermatids and mature spermatozoa did not demonstrate DNase I-hypersensitivity under any conditions employed. A variety of agents that interact with topoisomerase II and DNA (teniposide, novobiocin, ethidium bromide, and adenosine triphosphate) were tested to determine the basis for the unique sensitivity to nick translation of steps 12-13 elongating spermatids. None of the agents tested, however, affected this unique labeling. The sensitivity of steps 12-13 elongating spermatids to nick translation in the absence of exogenous nuclease indicators the presence of endogenous nicks, which may relieve torsional stress and aid rearrangement as the chromatin is packaged into a form characteristic of the mature spermatozoon.

Improvement of flow cytometry analysis and sorting of bull spermatozoa by optical monitoring of cell orientation as evaluated by DNA specific probing

Flow cytometry is a potential method for the separation of X and Y bearing spermatozoa, on the basis of their relative DNA content evaluated by the fluorescence emission intensity due to specific fluorochrome DNA staining. However, spermatozoa DNA is highly condensed and nuclei exhibit flat non spherical shape, which can produce artefacts impeding accurate analysis. In order to avoid these limitations, decondensation of DNA performed by enzymatic treatment and a modification of the flow cytometer that orients the spermatozoa relative to the laser beam are generally used. In this work, we describe alternative methods and materials for selection of 1) decondensed and thus dead spermatozoa without orientation, sorted on the basis of only the 10% spermatozoa containing the least DNA (expected Y) and the 10% spermatozoa containing the more DNA (expected X), or 2) native spermatozoa homogeneously oriented using a simultaneous measurement of Axial light loss (extinction) and Forward angle light scatter. For testing enrichment of each selected fraction we have worked out a molecular hybridization procedure using X and Y specific DNA probes. We analyse and sort bull spermatozoa on these basis: the purity obtained for these fractions is 80% without orientation after enzymatic treatment, and 70% on live spermatozoa "optically" oriented.

Nuclear transition protein 1 from ram elongating spermatids. Mass spectrometric characterization, primary structure and phosphorylation sites of two variants

The ram transition protein 1 (TP1) is present in spermatid cell nuclei in the nonphosphorylated, monophosphorylated and diphosphorylated forms. Its primary structure was determined by automated Edman degradation of S-carboxamidomethylated protein and of peptides generated by cleavage with thermolysin and endoproteinase Lys-C. The ram TP1 is a small basic protein of 54 residues and structurally very close to other mammalian TP1. The mass spectrometric data obtained from the protein and its fragments reveal that ram TP1 is indeed a mixture (approximately 5:1) of two structural variants (Mr 6346 and 6300). These variants differ only by the nature of the residue at position 27 (Cys in the major variant and Gly in the minor variant). The study of phosphorylation sites has shown that four different serine residues could be phosphorylated in the monophosphorylated TP1, at positions 8, 35, 36 or 39. From previous physical studies, it has been postulated that the Tyr32 surrounded by two highly conserved basic clusters was responsible for the destabilization of chromatin by intercalation of its phenol ring between the bases of double-stranded DNA. The presence of three phosphorylatable serine residues in the very conserved sequence 29-42 is another argument for the involvement of this region in the interaction with DNA.

Boar transition protein 2 and 4 isolated from late spermatid nuclei by high-performance liquid chromatography

The boar late spermatid nuclei retaining transition proteins (TPs) could be obtained from the testis by the use of antipain to inhibit TP-degrading proteinases of the nuclei. The enzymes detected in acid extract including the basic proteins were inactivated by reduction and carboxymethylation of the proteins. The reduced and carboxymethylated basic proteins were fractionated by differential precipitation between 3% trichloroacetic acid (TCA) and 3-20% TCA. From the 3% TCA-precipitate, boar TP2 and TP4 were isolated by high-performance liquid chromatography (HPLC) on Nucleosil 300 7C18. The two TPs were characterized by acid urea- and SDS-polyacrylamide gel electrophoreses and amino acid analysis. Boar TP2 closely resembled rat and mouse TP2s, and ram protein 3 in its high content of serine and basic amino acids, the presence of cysteine and molecular weight. Boar TP4 was similar to ram protein P1 in its high content of basic amino acids, the presence of cysteine and molecular weight. But the TP2 and TP4 differed in electrophoretic mobility on acid urea-gel and solubility in 3% TCA from those of the other species. The HPLC used here also enabled us to efficiently separate boar TP1, TP2, TP3 and TP4, and to estimate that the amount of the TP2, TP3 and TP4 was about 1/8, 1/4 and 1/4 that of the TP1, respectively.

Protamine-histone replacement and DNA replication in the male mouse pronucleus

The protamine to histone replacement in fertilized mouse eggs was studied by using antibodies to these proteins. Its course was followed with respect to DNA replication by autoradiography of 3H-thymidine-labeled fertilized eggs. It was found that protamines were replaced by histones before the onset of DNA replication.

Changes in chromatin structure of boar late spermatids to mature spermatozoa by using modification with dansyl chloride

Changes in the chromatin structure of boar late spermatids maturing to spermatozoa were studied by chemical modification of their nuclei with dansyl (Dns) chloride. Protamine was isolated from the dansylated boar spermatid and sperm nuclei, and its dansylated sites and degrees of dansylation were determined by sequence analysis. The N-terminal Ala-1, Tyr-3 and Tyr-42 of the protamine molecule in cauda epididymal sperm nuclei were dansylated 27%, 22% and 40%, respectively, whereas the respective residues in late spermatid nuclei were about 1.5-times as reactive as those in cauda epididymal sperm nuclei. However, the dansyl ratio of Tyr-3 to Tyr-42 remained unchanged from the late spermatid to mature sperm nuclei. SDS treatment did not affect the reactivity of cauda epididymal protamine and that of Ala-1 of caput epididymal protamine, but raised that of Tyr-3 and Tyr-42 of caput epididymal protamine by a factor of about 1.5. As a result of the SDS treatment, caput epididymal protamine came to have almost the same reactivity as late spermatid protamine. These facts suggest that the fundamental structure, in terms of DNA-protamine interaction, of sperm chromatin was already formed at the stage of the late spermatid, and then during epididymal transit the sperm chromatin was more tightly condensed, with increasing disulfide cross-links, thereby acquiring insensitivity towards the SDS-treatment.

A cytochemical study of the transcriptional and translational regulation of nuclear transition protein 1 (TP1), a major chromosomal protein of mammalian spermatids

Immunocytochemical localization and in situ hybridization techniques were used to investigate the presence of spermatid nuclear transition protein 1 (TP1) and its mRNA during the various stages of spermatogenesis in the rat. A specific antiserum to TP1 was raised in a rabbit and used to show that TP1 is immunologically crossreactive among many mammals including humans. During spermatogenesis the protein appears in spermatids as they progress from step 12 to step 13, a period in which nuclear condensation is underway. The protein is lost during step 15. An asymmetric RNA probe generated from a TP1 cDNA clone identified TP1 mRNA in late round spermatids beginning in step 7. The message could no longer be detected in spermatids of step 15 or beyond. Thus, TP1 mRNA first appears well after meiosis in haploid cells but is not translated effectively for the several days required for these cells to progress to the stage of chromatin condensation. Message and then protein disappear as the spermatids enter step 15. In agreement with a companion biochemical study (Heidaran, M.A., and W.S. Kistler. J. Biol. Chem. 1987. 262:13309-13315), these results establish that translational control is involved in synthesis of this major spermatid nuclear protein. In addition, they suggest that TP1 plays a role in the completion but not the initiation of chromatin condensation in elongated spermatids.

Structural organization of the sperm chromatin in a fern (Scolopendrium vulgare) studied by spreading methods

To investigate chromatin organization, we applied the spreading techniques to nuclei isolated from Scolopendrium spermatozoids. Well-dispersed chromatin shows three types of fibers: beaded fibers corresponding to a nucleosomal filament with adjacent nucleosomes in close contact, smooth fibers (14 nm in diameter) associated in a complex network, and knobby fibers constituted by local supercoiling of a very thin (4 nm) smooth filament. Along the knobby fibers, beads of variable size are irregularly spaced. The knobby fibers lie parallel and coalesce in thick bundles. The sperms basic proteins identified by electrophoretic analysis probably promote the supercoiling and the side-to-side attachment of the knobby fibers, which are all the more abundant in spread preparations. These results indicate that knobby fibers are probably located in the outer part of the sperm nucleus in which the chromatin is densely packed. As for the nucleosomal and smooth filaments, they may be situated in the inner part.

Isolation of a cDNA clone for transition protein 1 (TP1), a major chromosomal protein of mammalian spermatids

We have isolated a cDNA clone for rat transition protein 1 (TP1), a major chromosomal protein of mammalian spermatids. The clone was identified initially by hybrid selection of TP1 mRNA. The sequence of the 251-nucleotide cDNA includes the entire coding region for the protein, thereby confirming the identity of the clone as well as predicting two changes in the published amino acid sequence.

Changes in DNA topology during spermatogenesis

DNA topology in histone- and protamine-depleted nuclei (nucleoids) from somatic cells, sperm, and spermatogenic cells was studied to determine if the superhelical configuration of DNA looped domains is altered during spermatogenesis. The expansion and contraction of nucleoid DNA was measured with a fluorescence microscope following exposure of nucleoids to different concentrations of ethidium bromide (EB). Nucleoids from Xenopus laevis erythrocytes, primary spermatocytes, and round spermatids, and from Rana catesbeiana sperm all exhibited a biphasic change (condensed-relaxed-condensed) in size as a function of exposure to increasing concentrations (0.5-100 micrograms/ml) of EB, indicating that they contain negatively supercoiled DNA. In contrast, DNA in sperm nucleoids from Xenopus laevis and Bufo fowleri was relaxed and expanded at low (0.5-6 micrograms/ml) EB concentrations, but became gradually condensed as the EB concentration was increased (6-100 micrograms/ml). Nucleoids prepared from all cell types retained the general shape of the nucleus regardless of the superhelical configuration of the nucleoid DNA. Sperm nucleoid DNA condensed by 100 micrograms/ml EB was relaxed by exposure to UV light, DNase I, proteinase K, or 4 M urea, but not by RNase A or 10 mM dithiothreitol. These results demonstrate that the DNA in sperm nucleoids is constrained in domains of supercoiling by nonbasic nuclear proteins. Negatively supercoiled DNA is present in nucleoids from cells with a full complement of histones, including Rana sperm, but not in nucleoids from Xenopus and Bufo sperm in which histones are replaced by "intermediate-type" protamines. Histone replacement in these species, therefore, is accompanied by unfolding of nucleosomal DNA and active removal of the negative supercoils. Results presented also suggest an important role for the nonbasic nuclear proteins of sperm in the morphogenesis of the nucleus and the arrangement of DNA.

High mobility group proteins in ram spermatids

The four major high mobility group proteins HMG 1, 2, 14 and 17, HMG 19B and histone H1(0) were identified in the ram testis by their extraction and solubility characteristics and by their electrophoretic mobilities. HMG 14 and 17 were isolated by chromatography and amino acid analysis revealed that they were similar to their calf thymus analogues. A protein, named 2R and co-extracted with HMG 14, was also purified and analysed. Electrophoretic analyses of the proteins extracted by 0.75 M perchloric acid (PCA) or by 0.35 M NaCl from round and non-round spermatids, separated by centrifugal elutriation, showed that the four major HMG proteins disappear from nuclei in the oldest round spermatids, at the time the nuclear content of protein 2R and histone H1(0) increases in spermatids. Ubiquitin and HMG 19B were present in the round and elongating spermatids, but not in elongated spermatids which contained only protamine. The relation was considered between several protein changes and genetic inactivation and structural reorganization of the spermatid chromatin.