Distinct roles of haspin in stem cell division and male gametogenesis.

The kinase haspin phosphorylates histone H3 at threonine-3 (H3T3ph) during mitosis. H3T3ph provides a docking site for the Chromosomal Passenger Complex at the centromere, enabling correction of erratic microtubule-chromosome contacts. Although this mechanism is operational in all dividing cells, haspin-null mice do not exhibit developmental anomalies, apart from aberrant testis architecture. Investigating this problem, we show here that mouse embryonic stem cells that lack or overexpress haspin, albeit prone to chromosome misalignment during metaphase, can still divide, expand and differentiate. RNA sequencing reveals that haspin dosage affects severely the expression levels of several genes that are involved in male gametogenesis. Consistent with a role in testis-specific expression, H3T3ph is detected not only in mitotic spermatogonia and meiotic spermatocytes, but also in non-dividing cells, such as haploid spermatids. Similarly to somatic cells, the mark is erased in the end of meiotic divisions, but re-installed during spermatid maturation, subsequent to methylation of histone H3 at lysine-4 (H3K4me3) and arginine-8 (H3R8me2). These serial modifications are particularly enriched in chromatin domains containing histone H3 trimethylated at lysine-27 (H3K27me3), but devoid of histone H3 trimethylated at lysine-9 (H3K9me3). The unique spatio-temporal pattern of histone H3 modifications implicates haspin in the epigenetic control of spermiogenesis.

Haspin-dependent and independent effects of the kinase inhibitor 5-Iodotubercidin on self-renewal and differentiation.

The kinase Haspin phosphorylates histone H3 at threonine-3 (H3T3ph), creating a docking site for the Chromosomal Passenger Complex (CPC). CPC plays a pivotal role in preventing chromosome misalignment. Here, we have examined the effects of 5-Iodotubercidin (5-ITu), a commonly used Haspin inhibitor, on self-renewal and differentiation of mouse embryonic stem cells (ESCs). Treatment with low concentrations of 5-ITu eliminates the H3T3ph mark during mitosis, but does not affect the mode or the outcome of self-renewal divisions. Interestingly, 5-ITu causes sustained accumulation of p53, increases markedly the expression of histone genes and results in reversible upregulation of the pluripotency factor Klf4. However, the properties of 5-ITu treated cells are distinct from those observed in Haspin-knockout cells generated by CRISPR/Cas9 genome editing, suggesting "off-target" effects. Continuous exposure to 5-ITu allows modest expansion of the ESC population and growth of embryoid bodies, but release from the drug after an initial treatment aborts embryoid body or teratoma formation. The data reveal an unusual robustness of ESCs against mitotic perturbants and suggest that the lack of H3T3ph and the "off-target" effects of 5-ITu can be partially compensated by changes in expression program or accumulation of suppressor mutations.

Heterochromatin remodeling in embryonic stem cells proceeds through stochastic de-stabilization of regional steady-states.

Cell differentiation is associated with progressive immobilization of chromatin proteins, expansion of heterochromatin, decrease of global transcriptional activity and induction of lineage-specific genes. However, how these processes relate to one another remains unknown. We show here that the heterochromatic domains of mouse embryonic stem cells (ESCs) are dynamically distinct and possesses a mosaic sub-structure. Although random spatio-temporal fluctuations reshuffle continuously the chromatin landscape, each heterochromatic territory maintains its dynamic profile, exhibiting robustness and resembling a quasi-steady state. Transitions towards less dynamic states are detected sporadically as ESCs downregulate Nanog and exit the self-renewal phase. These transitions increase in frequency after lineage-commitment, but evolve differently depending on cellular context and transcriptional status. We propose that chromatin remodeling is a step-wise process, which involves stochastic de-stabilization of regional steady states and formation of new dynamic ensembles in coordination to changes in the gene expression program.

Phosphorylation of histone H3 at Thr3 is part of a combinatorial pattern that marks and configures mitotic chromatin.

We have previously shown that histone H3 is transiently phosphorylated at Thr3 during mitosis. Extending these studies, we now report that phosphorylated Thr3 is always in cis to trimethylated Lys4 and dimethylated Arg8, forming a new type of combinatorial modification, which we have termed PMM. PMM-marked chromatin emerges at multiple, peripheral sites of the prophase nucleus, then forms distinct clusters at the centric regions of metaphase chromosomes, and finally spreads (as it wanes) to the distal areas of segregating chromatids. The characteristic prophase pattern can be reproduced by expressing ectopically the kinase haspin at interphase, suggesting that the formation of the PMM signature does not require a pre-existing mitotic environment. On the other hand, the ;dissolution' and displacement of PMM clusters from a centric to distal position can be induced by partial dephosphorylation or chromosome unravelling, indicating that these changes reflect the regulated grouping and scrambling of PMM subdomains during cell division. Formation of PMM is prevented by haspin knockdown and leads to delayed exit from mitosis. However, PMM-negative cells do not exhibit major chromosomal defects, suggesting that the local structures formed by PMM chromatin may serve as a ;licensing system' that allows quick clearance through the metaphase-anaphase checkpoint.

Chromatin remodeling during mitosis: a structure-based code?

Histone modifications have been associated with particular states of transcriptional activity and are thought to serve as an "information code". However, this principle does not apply to histone phosphorylation, which can be detected in two, seemingly contrasting situations, i.e., in a transcriptionally hyperactive state following growth factor stimulation and in transcriptionally paused mitotic chromosomes. There are several indications that mitotic phosphorylation of histone H3 at serine-10 by the Aurora B kinase and trimethylation at lysine-9 by the methyl transferase Suvar3,9 operate as a "binary switch", which determines recruitment or eviction of heterochromatin-specific proteins from pericentromeric repeats. Moreover, threonine-3 phosphorylation of histone H3 by the newly identified haspin kinase seems to promote chromatid cohesion during mitosis. We discuss here emerging information and new ideas suggesting that these modifications, in combination to upstream and downstream marks, constitute a system of intrinsic folding determinants that facilitate chromatin condensation and confer topological specificity to mitotic chromosomes.

Transcriptional analysis of a gene cluster involved in glucose tolerance in Zymomonas mobilis: evidence for an osmoregulated promoter.

Exponentially growing cells of Zymomonas mobilis normally exhibit a lag period of up to 3 h when they are transferred from a liquid medium containing 2% glucose to a liquid medium containing 10% glucose. A mutant of Z. mobilis (CU1) exhibited a lag period of more than 20 h when it was grown under the same conditions, whereas it failed to grow on a solid medium containing 10% glucose. The glucose-defective phenotype of mutant CU1 was due to a spontaneous insertion in a putative gene (ORF4) identified as part of an operon (glc) which includes three additional putative genes (ORF1, ORF2, and ORF3) with no obvious involvement in the glucose tolerance mechanism. The common promoter controlling glc operon transcription, designated P(glc), was found to be osmoregulated and stimulated by the putative product of ORF4 in an autoregulated fashion, as indicated by expression of the gfp reporter gene. Additionally, reverse transcriptase PCR analysis showed that the gene cluster produces a single mRNA, which verified the operon organization of this transcription unit. Further transcriptional analysis demonstrated that glc operon expression is regulated by the concentration of glucose, which supported the hypothesis that this operon is directly involved in the uncharacterized glucose tolerance mechanism of Z. mobilis.