During male germ cell postmeiotic maturation, dramatic chromatin reorganization occurs, which is driven by completely unknown mechanisms. For the first time, we describe a specific reprogramming of mouse pericentric heterochromatin. Initiated when histones undergo global acetylation in early elongating spermatids, this process leads to the establishment of new DNA packaging structures organizing the pericentric regions in condensing spermatids. Five new histone variants were discovered, which are expressed in late spermiogenic cells. Two of them, which we named H2AL1 and H2AL2, specifically mark the pericentric regions in condensing spermatids and participate in the formation of new nucleoprotein structures. Moreover, our investigations also suggest that TH2B, an already identified testis-specific H2B variant of unknown function, could provide a platform for the structural transitions accompanying the incorporation of these new histone variants.

Spermiogenesis entails a major biochemical and morphological restructuring of the germ cell involving replacement of the somatic histones by protamines packing the DNA into the condensed spermatid nucleus and elimination of the cytoplasm during the elongation phase. We describe H1T2, an histone H1 variant selectively and transiently expressed in male haploid germ cells during spermiogenesis. In round and elongating spermatids, H1T2 specifically localizes to a chromatin domain at the apical pole, revealing a polarity in the spermatid nucleus. Inactivation by homologous recombination shows that H1T2 is critical for spermiogenesis as male H1t2 ؊/؊ mice have greatly reduced fertility. Analysis of spermiogenesis in H1t2 mutant mice shows delayed nuclear condensation and aberrant elongation. As a result, mutant spermatids are characterized by the presence of residual cytoplasm, acrosome detachment, and fragmented DNA. Hence, H1T2 is a protein required for proper cell restructuring and DNA condensation during the elongation phase of spermiogenesis.chromatin ͉ mouse ͉ protamine ͉ acrosome M ammalian spermatogenesis involves the differentiation of diploid spermatogonia into spermatocytes and then, through two successive meiotic divisions, into haploid round spermatids. During spermiogenesis, the haploid round spermatids undergo an elongation phase, transforming them into mature spermatozoa. This process entails a major biochemical and morphological restructuring of the germ cell where the majority of the somatic histones are replaced, first by transition proteins, then protamines packing the DNA into the sperm cell nucleus.Differentiating male germ cells use specialized transcriptional regulatory mechanisms involving testis-specific paralogues of transcription factors such as TLF͞TRF2, ALF, and TAF7L (1-3). 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 (6-9). 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). Thus, whereas H1 histones have been proposed to play a role in the formation of higher-order chromatin structure, the physiological functions of the tissue-specific variants remain obscure. Here, we describe a male germ cellspecific H1-related protein, H1T2, that plays a critical role during the elongation phase of spermiogenesis. Materials and MethodsCloning and Inactivation of H1t2. The H1t2 cDNA was cloned from a mouse testis cDNA library by screening with the short mouse TEST640 EST probe as described (2, 13). 5Ј RACE using mouse testis RNA was performed to confirm the sequence o...

The Sox2 transcription factor is expressed early in the stem cells of the blastocyst inner cell mass and, later, in neural stem cells. We previously identified a Sox2 5-regulatory region directing transgene expression to the inner cell mass and, later, to neural stem cells and precursors of the forebrain. Here, we identify a core enhancer element able to specify transgene expression in forebrain neural precursors of mouse embryos, and we show that the same core element efficiently activates transcription in inner cell mass-derived embryonic stem (

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