MABE446 Sigma-AldrichAnti-Histone H1° Antibody, clone 34

Oocytes and embryos of many species, including mammals, contain a unique linker (H1) histone, termed H1oo in mammals. It is uncertain, however, whether other H1 histones also contribute to the linker histone complement of these cells. Using immunofluorescence and radiolabeling, we have examined whether histone H10, which frequently accumulates in the chromatin of nondividing cells, and the somatic subtypes of H1 are present in mouse oocytes and early embryos. We report that oocytes and embryos contain mRNA encoding H10. A polymerase chain reaction-based test indicated that the poly(A) tail did not lengthen during meiotic maturation, although it did so beginning at the four-cell stage. Antibodies raised against histone H10 stained the nucleus of wild-type prophase-arrested oocytes but not of mice lacking the H10 gene. Following fertilization, H10 was detected in the nuclei of two-cell embryos and less strongly at the four-cell stage. No signal was detected in H10 -/- embryos. Radiolabeling revealed that species comigrating with the somatic H1 subtypes H1a and H1c were synthesized in maturing oocytes and in one- and two-cell embryos. Beginning at the four-cell stage in both wild-type and H10 -/- embryos, species comigrating with subtypes H1b, H1d, and H1e were additionally synthesized. These results establish that histone H10 constitutes a portion of the linker histone complement in oocytes and early embryos and that changes in the pattern of somatic H1 synthesis occur during early embryonic development. Taken together with previous results, these findings suggest that multiple H1 subtypes are present on oocyte chromatin and that following fertilization changes in the histone H1 complement accompany the establishment of regulated embryonic gene expression.

A striking feature of early embryogenesis in a number of organisms is the use of embryonic linker histones or high mobility group proteins in place of somatic histone H1. The transition in chromatin composition towards somatic H1 appears to be correlated with a major increase in transcription at the activation of the zygotic genome. Previous studies have supported the idea that the mouse embryo essentially follows this pattern, with the significant difference that the substitute linker histone might be the differentiation variant H1 degrees, rather than an embryonic variant. We show that histone H1 degrees is not a major linker histone during early mouse development. Instead, somatic H1 was present throughout this period. Though present in mature oocytes, somatic H1 was not found on maternal metaphase II chromatin. Upon formation of pronuclear envelopes, somatic H1 was rapidly incorporated onto maternal and paternal chromatin, and the amount of somatic H1 steadily increased on embryonic chromatin through to the 8-cell stage. Microinjection of somatic H1 into oocytes, and nuclear transfer experiments, demonstrated that factors in the oocyte cytoplasm and the nuclear envelope, played central roles in regulating the loading of H1 onto chromatin. Exchange of H1 from transferred nuclei onto maternal chromatin required breakdown of the nuclear envelope and the extent of exchange was inversely correlated with the developmental advancement of the donor nucleus.

There exists a close relationship between core histone acetylation and the induced expression of the histone H1(0) gene. We took advantage of this fact to evaluate the influence of chromatin hyperacetylation on the developmentally regulated expression of this specific gene. In this study, the in situ immunodetection approach has been used to analyze both the acetylated histone H4 isoforms and histone H1(0) accumulation during early Xenopus laevis development. We have chosen two stages of development, gastrula stage, when H1(0) is not expressed and not inducible by butyrate treatment, and stage 27 when H1(0) is not expressed but is inducible by butyrate. At stage 27 of development, the early induced accumulation of histone H1(0) under butyrate treatment, occurs mainly in tissues that express the protein normally during later development. These experiments suggest that histone acetylation may be part of a pathway which, in a specific set of cells, keeps H1(0) and probably a series of specific genes, competent for transcription, but cell-specific factors are involved in the induced expression of these genes.

It is known that a transition in the linker-histone variants takes place within chromatin during early development of Xenopus laevis; a cleavage-type H1 is replaced by the somatic type. Based on cytofluorimetric analysis of the distribution of the embryo cells in the cell cycle, we showed that this previously described transition occurs when significant modifications of the proliferative capacities of the cells occur. Moreover, this analysis allowed us to show that cell proliferation decreases gradually after the gastrula stage of development. This period terminates with the arrest of more than 90% of cells in the G0/G1 phase of the cell cycle at stage 45. We showed that the major accumulation of the differentiation-specific H1 subtype, histone H1(0), occurs at this time. H1(0), first detected in a restricted set of tissues, is then widely expressed during the later development at stage 45. Moreover, the double staining of nuclei isolated from embryo cells, for H1(0) and DNA, allowed us to show that this accumulation of H1(0) is not restricted to arrested cells. The example of the Xenopus early development shows that there may be an adaptation of the type of H1 expressed to the proliferative abilities of cells. This observation may provide insight into the significance of the expression of different H1 subtypes during development.