Biochemical and ultrastructural study of the sperm chromatin from Mytilus galloprovincialis

Alterations in the properties of sperm protamine-like II protein after exposure of Mytilus galloprovincialis (Lamarck 1819) to sub-toxic doses of cadmium

Protamine-like proteins (PL-II, PL-III and PL-IV) represent the major basic nuclear component of Mytilus galloprovincialis L sperm chromatin. The present study investigates the effects induced on the properties of PL-II protein after exposure of Mytilus galloprovincialis L for 24 h to 1.5 and 5 µM CdCl₂. We found cadmium accumulation in protamine-like proteins with a linear grow up with the exposition dose. In particular, after 5 µM CdCl₂ mussels exposure, the mobility of PL-II band changed in SDS-PAGE, suggesting structural rearrangement in presence of cadmium. Structural analysis using fluorescent probes, indicated that at 5 µM CdCl₂ the complete conformational change of PL-II protein was reached. In the same condition of mussels exposure of 5 µM CdCl₂, PL-II protein changed its DNA binding mode, which determined a closer DNA binding, because higher amount of NaCl were required for PL-II protein release by sperm nuclei. These results supported the hypothesis that mussel exposure to this CdCl₂ dose, although lower to toxic ones, affects the properties of this protein and as a consequence chromatin organization of spermatozoa that is essential for the success of fertilization.

The characterization of macroH2A beyond vertebrates supports an ancestral origin and conserved role for histone variants in chromatin

Histone variants play a critical role in chromatin structure and epigenetic regulation. These "deviant" proteins have been historically considered as the evolutionary descendants of ancestral canonical histones, helping specialize the nucleosome structure during eukaryotic evolution. Such view is now challenged by 2 major observations: first, canonical histones present extremely unique features not shared with any other genes; second, histone variants are widespread across many eukaryotic groups. The present work further supports the ancestral nature of histone variants by providing the first in vivo characterization of a functional macroH2A histone (a variant long defined as a specific refinement of vertebrate chromatin) in a non-vertebrate organism (the mussel Mytilus) revealing its recruitment into heterochromatic fractions of actively proliferating tissues. Combined with in silico analyses of genomic data, these results provide evidence for the widespread presence of macroH2A in metazoan animals, as well as in the holozoan Capsaspora, supporting an evolutionary origin for this histone variant lineage before the radiation of Filozoans (including Filasterea, Choanoflagellata and Metazoa). Overall, the results presented in this work help configure a new evolutionary scenario in which histone variants, rather than modern "deviants" of canonical histones, would constitute ancient components of eukaryotic chromatin.

New insights into protamine-like component organization in Mytilus galloprovincialis' sperm chromatin

We have analyzed Mytilus galloprovincialis' sperm chromatin, which consists of three protamine-like proteins, PL-II, PL-III, and PL-IV, in addition to a residual amount of the four core histones. We have probed the structure of this sperm chromatin through digestion with micrococcal nuclease (MNase) in combination with salt fractionation. Furthermore, we used the electrophoretic mobility shift assay to define DNA-binding mode of PL-II and PL-III and turbidimetric assays to determine their self-association ability in the presence of sodium phosphate. Although in literature it is reported that M. galloprovincialis' sperm chromatin lacks nucleosomal organization, our results obtained by MNase digestion suggest the existence of a likely unusual organization, in which there would be a more accessible location of PL-II/PL-IV when compared with PL-III and core histones. So, we hypothesize that in M. galloprovincialis' sperm chromatin organization DNA is wrapped around a PL-III protein core and core histones and PL-II and PL-IV are bound to the flanking DNA regions (similarly to somatic histone H1). Furthermore, we propose that PL's K/R ratio affects their DNA-binding mode and self-association ability as reported previously for somatic and sperm H1 histones.

Analysis of DNA looped domains organization duringTriturus cristatusspermatogenesis

Chromatin from eukaryotes is organized in DNA loops with sequential attachments to a nucleoskeleton named nuclear matrix. This organization plays major roles in replication, transcription, recombination, DNA repair, chromosome condensation and segregation. During spermatogenesis, chromatin undergoes several dynamic transitions, which are often associated with important changes not only in its physical conformation but even in its compositions and structure. To understand the periodical change in the functional organization of chromatin during spermatogenesis, the higher order organization of chromatin in different testicular cell types (pachytene spermatocytes, round spermatids) and the epididymal sperm ofTriturus cristatushave been investigated. The expansion and the contraction of nucleoid DNA were measured with a fluorescence microscope following exposure of nucleoids to increasing concentrations of ethidium bromide (EtBr) (2.5–200 μg/ml) as an intercalating dye to induce DNA-positive supercoils. Nucleoids from all studied cell types exhibited a biphasic change (condensed–relaxed–condensed) in size as a consequence of exposure to increasing concentrations of EtBr, indicating that they contained negatively supercoiled DNA. At higher EtBr concentrations, maximum positive supercoiling occurred in pachytene DNA loops. Our data suggest that pachytene DNA is the most open chromatin conformation in terms of EtBr intercalation.

Macromolecular and ultrastructural organization of the mitotic chromosome scaffold

Using electron microscopy (EM), we have examined three structural domains of the mitotic chromosome scaffold of mouse erythroleukemia (MEL) Friend cells with different morphologic organization: centromeric, intermediate, and telomeric. The intermediate, most extensive, domain exhibited a specific fibrogranular structure representing tightly packed granular bodies with diameters between 20 and 60 nm. The chromosome scaffold contained three main components: proteins (81%), RNA (12%), and DNA (7%). The residual DNA extracted from the scaffold represented short fragments, 300 bp on average, belonging to the class of tandemly arranged repetitive DNA. In situ hybridization experiments confirmed its typical centromeric location. Scaffold RNA represented three fractions: a major RNA fraction with an electrophoretic mobility corresponding to that of 5S RNA and two minor fractions with electrophoretic mobilities somewhat lower than that of 18S RNA. Scaffold RNA was localized mainly in the centromeric region. We show that the newly synthesized protein component of the chromosome scaffolds migrates slowly to the chromosomes, reaching a maximum specific radioactivity 12 h from the onset of the chase period.

Nucleosomal organization of chromatin in sperm nuclei of the bivalve mollusc Aulacomya ater

The sperm nuclei of Aulacomya ater, family Mitylidae, contain three proteins (X, Aa5 and Aa6) which are specific to this cell type coexisting with a set of five somatic-type histones. Information about the chromatin structure resulting from this kind of association is scarce. Therefore, we have probed the structure of this sperm chromatin through digestion with micrococcal nuclease in combination with salt fractionation. The data obtained have allowed us to propose a nucleosomal arrangement for this chromatin. However, two types of nucleosomes would be present in agreement with their protein components.

Nucleosomal organization of a part of chromatin in mollusc sperm nuclei with a mixed basic protein composition

The structural organization of mature sperm chromatin from three representatives of the Mytilidae family has been studied. The acid-soluble proteins in these species nuclei are primarily sperm-specific (approximately 80%) with the remainder being core histones. Previously, we have shown that the mature sperm nuclei of these molluscs are compact, dense structures formed by interaction of the spermspecific proteins with DNA (1). Here we show that: a) although the histones are minor chromatin protein fraction, they still organize a part (20-25%) of the total DNA into nucleosomes; b) one of the sperm-specific proteins, different from somatic H1 or H5 histones participates in the formation of the beaded structures.