H1 Linker Histones Are Essential for Mouse Development and Affect Nucleosome Spacing In Vivo

Abstract

Most eukaryotic cells contain nearly equimolar amounts of nucleosomes and H1 linker histones. Despite their abundance and the potential functional specialization of H1 subtypes in multicellular organisms, gene inactivation studies have failed to reveal essential functions for linker histones in vivo. Moreover, in vitro studies suggest that H1 subtypes may not be absolutely required for assembly of chromosomes or nuclei. By sequentially inactivating the genes for three mouse H1 subtypes (H1c, H1d, and H1e), we showed that linker histones are essential for mammalian development. Embryos lacking the three H1 subtypes die by midgestation with a broad range of defects. Triple-H1-null embryos have about 50% of the normal ratio of H1 to nucleosomes. Mice null for five of these six H1 alleles are viable but are underrepresented in litters and are much smaller than their littermates. Marked reductions in H1 content were found in certain tissues of these mice and in another compound H1 mutant. These results demonstrate that the total amount of H1 is crucial for proper embryonic development. Extensive reduction of H1 in certain tissues did not lead to changes in nuclear size, but it did result in global shortening of the spacing between nucleosomes.DNA in the eukaryotic nucleus is organized into a highly compact nucleoprotein complex referred to as chromatin (48,53). The histones constitute a family of proteins that are intimately involved in organizing chromatin structure. The nucleosome core particle, the highly conserved unit of chromatin organization in all eukaryotes, consists of an octamer of four core histones (H2A, H2B, H3, and H4) around which about 145 bp of DNA is wrapped. The chromatin fiber also contains a fifth histone, the linker histone (usually referred to as H1), which can bind to core particles and protect an additional ϳ20 bp of DNA (linker DNA) from nuclease digestion. The precise location and stoichiometry of H1 within the chromatin fiber are uncertain (45,46,49), but in higher eukaryotes, there is, on average, nearly one H1 molecule for each core particle (48). Most of our knowledge about the role of H1 in chromatin structure is based on in vitro experiments. These studies indicate that two principal functions of linker histones are to organize and stabilize the DNA as it enters and exits the core particle and to facilitate the folding of nucleosome arrays into more compact structures (19,38). Despite the presumed fundamental role of linker histones in chromatin structure, elimination of H1 in Tetrahymena, Saccharomyces cerevisiae, and Aspergillus nidulans and silencing of H1 in Ascobolus immersus showed that H1 is not essential in these unicellular eukaryotes (3,34,39,41,47).In higher organisms, additional levels of control on chromatin organization and function are available because of the existence of multiple nonallelic linker histone variants or subtypes (6, 33). In mice, there are at least eight H1 subtypes, including the widely expressed subtypes H1a through H1e, the testis-specific subtype H1...

H1 linker histone promotes epigenetic silencing by regulating both DNA methylation and histone H3 methylation

“…Expression of the chimeric H1 also led to recruitment of DNMT1 and DNMT3B to the H19 and Gtl2 ICRs, whereas expression of H1c and a truncated H1d lacking this region of the CTD did not lead to recruitment of the two DNMTs. Gene inactivation studies in mice have not revealed essential functions for any of these histone H1 subtypes, including H1c and H1(0) (15)(16)(17). Interestingly however, we reported previously that the genes encoding H1(0) and H1c exhibit differences in their regulation compared with the genes encoding the four other subtypes (40)(41)(42).…”

“…Because the H1d gene was the last gene to be inactivated in the H1 TKO ES cells (16), we sought to restore its expression by stable transfection of the cells with an H1d expression vector. The vector consisted of the H1d gene and 7.1 kb and 1.9 kb of the 5′ and 3′ flanking sequences, respectively.…”

“…Mammals contain at least eight histone H1 subtypes or variants that differ in amino acid sequences and expression during development (12)(13)(14). Functional differences among these subtypes have been difficult to identify because they appear to act redundantly in development (14)(15)(16)(17). Although the precise location of histone H1 within the chromatin fiber is uncertain, it is known that histone H1 resides outside the nucleosome core particle, where it associates with the DNA as it enters and exits the core particle and protects an additional ∼20 bp of DNA (linker DNA).…”

Independent Biological and Biochemical Functions for Individual Structural Domains of Drosophila Linker Histone H1

“…The H1 counterpart in Saccharomyces cerevisiae is not an essential protein (29). On the other hand, the existence of multiple, compensating H1 isoforms in many metazoans has slowed progress in the H1 field (30,31). The current study was designed to overcome these problems by taking advantage of fly genetics and the fact that D. melanogaster expresses a single linker histone protein during much of its development.…”

Isolation and Characterization of a Novel H1.2 Complex That Acts as a Repressor of p53-mediated Transcription

“…Msx1 recruits a linker histone H1 to the MyoD gene, and this selective localization correlates with a repressive chromatin state and gene repression (16). Simultaneous inactivation of three H1 subtype genes (H1.2, H1.3, and H1.4) in mouse embryonic cells significantly affects the expression of a subset of genes, supporting a rather specific action of H1 in gene regulation (17). A recent study demonstrating that specific sets of ribosomal proteins interact with H1 to suppress transcription also provides support for a rather complex mechanism for the effect of H1 in gene regulation (18).…”