Pericentric heterochromatin reprogramming by new histone variants during mouse spermiogenesis.

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.

A balanced transcription between telomerase and the telomeric DNA-binding proteins TRF1, TRF2 and Pot1 in resting, activated, HTLV-1-transformed and Tax-expressing human T lymphocytes.

BACKGROUND: The functional state of human telomeres is controlled by telomerase and by a protein complex named shelterin, including the telomeric DNA-binding proteins TRF1, TRF2 and Pot1 involved in telomere capping functions. The expression of hTERT, encoding the catalytic subunit of telomerase, plays a crucial role in the control of lymphocyte proliferation by maintaining telomere homeostasis. It has been previously found that hTERT activity is down-regulated by the human T cell leukaemia virus type 1 (HTLV-1) Tax protein in HTLV-1 transformed T lymphocytes. In this study, we have examined the effects of Tax expression on the transcriptional profile of telomerase and of shelterin in human T lymphocytes. RESULTS: We first provide evidence that the up-regulation of hTERT transcription in activated CD4+ T lymphocytes is associated with a down-regulation of that of TERF1, TERF2 and POT1 genes. Next, the down-regulation of hTERT transcription by Tax in HTLV-1 transformed or in Tax-expressing T lymphocytes is found to correlate with a significant increase of TRF2 and/or Pot1 mRNAs. Finally, ectopic expression of hTERT in one HTLV-1 T cell line induces a marked decrease in the transcription of the POT1 gene. Collectively, these observations predict that the increased transcriptional expression of shelterin genes is minimizing the impact on telomere instability induced by the down-regulation of hTERT by Tax. CONCLUSION: These findings support the notion that Tax, telomerase and shelterin play a critical role in the proliferation of HTLV-1 transformed T lymphocytes.

HMGB4, a novel member of the HMGB family, is preferentially expressed in the mouse testis and localizes to the basal pole of elongating spermatids.

We identified HMGB4, a novel member of the HMGB family lacking the acidic tail typically found in this family. HMGB4 is strongly and preferentially expressed in the adult mouse testis and weakly in the brain, but not in many other tissues. HMGB4 associates with chromatin, and in transfection assays, in contrast to HMGB1, it acts as a potent transcriptional repressor. During spermatogenesis, HMGB4 is present in the euchromatin of late pachytene spermatocytes and haploid round spermatids, whereas stronger expression is observed during the elongation phase, where it localizes to the basal pole of the nucleus in a manner mutually exclusive with H1FNT (H1T2) localized at the apical pole. HMGB4 basal localization is lost in H1FNT-mutant spermatids, showing that H1FNT provides a positional cue for organizing chromatin domains within the nucleus. These results show that HMGB4 and H1FNT specify distinct chromatin domains at the apical and basal poles of the elongating spermatid nucleus.

Post-meiotic shifts in HSPA2/HSP70.2 chaperone activity during mouse spermatogenesis.

HSPA2 (formerly HSP70.2) is a testis-specific member of the HSP70 family known to play a critical role in the completion of meiosis during male germ cell differentiation. Although abundantly present in post-meiotic cells, its function during spermiogenesis remained obscure. Here, using a global proteomic approach to identify genome-organizing proteins in condensing spermatids, we discovered an unexpected role for HSPA2, which acquires new functions and becomes tightly associated with major spermatid DNA-packaging proteins, transition proteins 1 and 2. Hence, HSPA2 is identified here as the first transition protein chaperone, and these data shed a new light on the yet totally unknown process of genome-condensing structure assembly in spermatids.