Normal Spermatogenesis in Mice Lacking the Testis-Specific Linker Histone H1t

Abstract

H1 histones bind to linker DNA and nucleosome core particles and facilitate the folding of chromatin into a more compact structure. Mammals contain seven nonallelic subtypes of H1, including testis-specific subtype H1t, which varies considerably in primary sequence from the other H1 subtypes. H1t is found only in pachytene spermatocytes and early, haploid spermatids, constituting as much as 55% of the linker histone associated with chromatin in these cell types. To investigate the role of H1t in spermatogenesis, we disrupted the H1t gene by homologous recombination in mouse embryonic stem cells. Mice homozygous for the mutation and completely lacking H1t protein in their germ cells were fertile and showed no detectable defect in spermatogenesis. Chromatin from H1t-deficient germ cells had a normal ratio of H1 to nucleosomes, indicating that other H1 subtypes are deposited in chromatin in place of H1t and presumably compensate for most or all H1t functions. The results indicate that despite the unique primary structure and regulated synthesis of H1t, it is not essential for proper development of mature, functional sperm.The histones are a family of basic proteins that are involved in organizing the DNA in the nuclei of eukaryotic cells into a compact structure called chromatin. There are five major classes of histones, the core histones H2A, H2B, H3, and H4 and the linker histone H1. Two molecules of each of the core histones constitute the protein octamer of the nucleosome core particle. H1 histones bind to DNA in the nucleosome core particle and to the linker DNA between nucleosomes. These interactions are thought to facilitate the folding of nucleosomes into the 30-nm chromatin fiber and higher-order chromatin structures (39,42). Interactions between histones and DNA would be expected to modulate gene activity, and recent evidence clearly shows that both the core histones and H1 can have a profound effect on transcription (reviewed in references 18, 43, and 44).Among the five classes of histones, the H1 histones exhibit the most diversity. For mice seven H1 subtypes have been described (24, 25), including the "somatic" subtypes H1a through H1e, the replacement subtype H1 o , and the testisspecific linker histone H1t. These seven H1 subtypes are also present in humans, and the genomic organization of the genes encoding the H1 subtypes in humans appears to be very similar to that in mice (12,40).Within the H1 family of proteins, the testis-specific H1t subtype is unique in that it is the only member exhibiting a truly tissue-specific pattern of expression. Although the other six H1 subtypes display distinct patterns of expression during differentiation and development, they are all expressed in numerous tissues (24-26). On the other hand, the H1t gene is transcribed exclusively in mid-and late-pachytene spermatocytes (10, 13, 16), and H1t protein is found only in pachytene spermatocytes and early, haploid spermatids (11,17,29), in which it constitutes up to 55% of the total H1 linker histone in chromatin (29). M...

Polar nuclear localization of H1T2, a histone H1 variant, required for spermatid elongation and DNA condensation during spermiogenesis

“…In addition, male germ cells contain specialized chromatin components such as H1T, a histone H1 variant specifically expressed in pachytene spermatocytes and early haploid cells (4), and HILS1, a histone H1-related protein specifically expressed in elongating and elongated spermatids (5). However, no spermatogenesis abnormalities are observed in mice lacking the H1t gene, perhaps because of redundancy with the somatic H1.1, which is also expressed at this stage (6)(7)(8)(9). This finding contrasts with the reduced fertility and defective spermiogenesis in mice lacking transition proteins 1 or 2 or haploinsufficient for protamines 1 or 2 (10-12).…”

HILS1 is a spermatid-specific linker histone H1-like protein implicated in chromatin remodeling during mammalian spermiogenesis

“…Each toroid contains 60 kb of DNA and is linked to other toroids by uncoiled DNA stretches. Whereas the roles of PRMs in sperm chromatin condensation and nuclear shaping are evident, the roles of H1t (16)(17)(18), TNP1 (20), and TNP2 (21) in the intermediate steps before the appearance of PRMs remain uncertain. Because HILS1 has diverged significantly from other linker histone family members and is expressed in elongating and elongated spermatids where core histones are absent, HILS1 is unlikely to function as a typical linker histone.…”

“…H1t is first detected in mid-pachytene spermatocytes, where it rapidly integrates into the chromatin, replaces most of the other somatic H1 subtypes, and persists until the elongating spermatid stage (15). Surprisingly, knockout of H1t does not affect chromatin remodeling, indicating that other known or unknown linker histones may compensate for its absence (16)(17)(18).…”

“…H1t is first detected in mid-pachytene spermatocytes, where it rapidly integrates into the chromatin, replaces most of the other somatic H1 subtypes, and persists until the elongating spermatid stage (15). Surprisingly, knockout of H1t does not affect chromatin remodeling, indicating that other known or unknown linker histones may compensate for its absence (16)(17)(18).The development of spermatids into spermatozoa, termed spermiogenesis, is characterized by striking morphological and molecular transformations. In elongating and elongated spermatids, major restructuring of the somatic chromatin occurs; this process involves the replacement of somatic histones and H1t by transition nuclear proteins (TNP1 and TNP2), which are subsequently replaced by protamines (PRM1 and PRM2).…”

Transcription and post-transcriptional regulation of spermatogenesis

“…They found that Brdt-mutant testes have elevated levels of H1t mRNA and that the H1t promoter is bound by BRDT, which together suggests that the H1t gene is a direct target of BRDT. While H1t is a good candidate to have a role in mediating BRDT's functions in male germ cells, it cannot be sufficient, as H1t-mutant mice have normal spermatogenesis and fertility (Drabent et al 2000;Lin et al 2000). Thus, future studies will be required to identify other BRDT target genes and to determine which of these mediates BRDT's ability to promote chromatin remodelling and germ-cell development.…”