Epigenetic histone H3 phosphorylation marks discriminate between univalent- and bivalent-forming chromosomes during canina asymmetrical meiosis

BACKGROUND AND AIMS

Dogroses (Rosa sect. Caninae) are mostly pentaploid, bearing 2n = 5x = 35 chromosomes in somatic cells. They evolved a unique form of asymmetrical meiosis characterized by two types of chromosomes: (1) chromosomes forming bivalents and distributed in the normal sexual way; and (2) chromosomes occurring as univalents and transferred by a female gamete only. In the mature pollen of pentaploid species, seven bivalent-derived chromosomes are transmitted to offspring, and 21 unpaired univalent chromosomes are eliminated during microsporogenesis. To discriminate between bivalent- and univalent-forming chromosomes, we studied histone H3 phosphorylation patterns regulating meiotic chromosome condensation and segregation.

METHODS

We analysed histone modification patterns during male canina meiosis in two representative dogrose species, 5x Rosa canina and 5x Rosa rubiginosa, by immunohistochemical and molecular cytogenetics approaches. Immunostaining of meiotic cells included α-tubulin, histone H3 phosphorylation (H3S10p, H3S28p and H3T3p) and methylation (H3K4me3 and H3K27me3) marks. In addition, fluorescent in situ hybridization was carried out with an 18S rDNA probe.

KEY RESULTS

In the first meiotic division, univalent chromosomes underwent equational division into chromatids, while homologues in bivalents were segregated as regular dyads. In diakinesis, bivalent chromosomes displayed strong H3 phosphorylation signals in proximal regions, spreading to the rest of the chromosome. In contrast, in univalents, the H3 phosphorylation signals were weaker, occurring mostly outside proximal regions largely overlapping with the H3K4me3 signals. Reduced phosphorylation was associated with relative under-condensation of the univalent chromosomes, particularly at early diakinesis.

CONCLUSIONS

We hypothesize that the absence of pairing and/or recombination in univalent chromosomes negatively affects the histone H3 phosphorylation of their chromatin and perhaps the loading of meiotic-specific cohesins. This apparently destabilizes cohesion of sister chromatids, leading to their premature split in the first meiotic division.

MLK4-mediated phosphorylation of histone H3T3 promotes flowering by transcriptional silencing of FLC/MAF in Arabidopsis thaliana

Casein kinase I (CK1), a ubiquitous Ser/Thr protein kinase in eukaryotes, plays a critical role in higher plant flowering. Arabidopsis CK1 family member MUT9-LIKE KINASEs, such as MLK1 and MLK3, have been shown to phosphorylate histone H3 at threonine 3 (H3T3), an evolutionarily conserved residue, and the modification is associated with the transcriptional repression of euchromatic and heterochromatic loci. This study demonstrates that mlk4-3, a T-DNA insertion mutant of MLK4, flowered late, and that overexpression of MLK4 caused early flowering. The nuclear protein MLK4 phosphorylated histone H3T3 both in vitro and in vivo, and this catalytic activity required the conserved lysine residue K175. mutation of MLK4 at K175 failed to restore the level of phosphorylated H3T3 (H3T3ph) or to complement the phenotypic defects of mlk4-3. The FLC/MAF-clade genes, including FLC, MAF4 and MAF5, were significantly upregulated in mlk4-3. The double mutant mlk4-3 flc-3 flowered earlier than mlk4-3, suggesting that functional FLC is crucial for flowering repression in mlk4-3. Chromatin immunoprecipitation assays showed that MLK4 bound to FLC/MAF chromatin and that H3T3ph occupancy at the promoter of FLC/MAF was negatively associated with its transcriptional level. In accordance, H3T3ph accumulated at FLC/MAF in 35S::MLK4/mlk4-3 but diminished in 35S::MLK4(K175R)/mlk4-3 plants. Moreover, the amount of RNA Pol II deposited at FLC/MAF was clearly enriched in mlk4-3 relative to the wild type. Therefore, MLK4-dependent phosphorylation of H3T3 contributes to accelerating flowering by repressing the transcription of negative flowering regulator FLC/MAF. This study sheds light on the delicate control of flowering by the plant-specific CK1, MLK4, via post-translational modification of histone H3.

40 Years of My Venture with CYTOMETRY

Briefly depicted are the publications in CYTOMETRY that received the highest frequency of citations. Among them are seminal papers describing application of metachromatic fluorochrome acridine orange to differentially stain DNA versus RNA or to analyze susceptibility of DNA in situ to denaturation; both features being markers of different sections of the cell cycle including identification of noncycling quiescent cells. The papers reviewing detection of cyclins D1, E, A or B1, each in relation to cell cycle phase, were also among the highly cited ones. The highest citation rates received publications describing development of the TUNEL methodology to detect apoptotic DNA fragmentation, and more recently expression of ϒH2AX to reveal DNA damage. © 2020 International Society for Advancement of Cytometry.

Identification of G2/M phase transition by sequential nuclear and cytoplasmic changes and molecular markers in mice intestinal epithelial cells

Although the regulatory network of G2/M phase transition has been intensively studied in mammalian cell lines, the identification of morphological and molecular markers to identify G2/M phase transition in vivo remains elusive. In this study, we found no obvious morphological changes between the S phase and G2 phase in mice intestinal epithelial cells. The G2 phase could be identified by Brdu incorporation resistance, marginal and scattered foci of histone H3 phosphorylated at Ser10 (pHH3), and relatively intact Golgi ribbon. Prophase starts with nuclear transformation in situ, which was identified by a series of prophase markers including nuclear translocation of cyclinB1, fragmentation of the Golgi complex, and a significant increase in pHH3. The nucleus started to move upwards in the late prophase and finally rounded up at the apical surface. Then, metaphase was initiated as the level of pHH3 peaked. During anaphase and telophase, pHH3 sharply decreased, while Ki67 was obviously bound to chromosomes, and PCNA was distributed throughout the whole cell. Based on the aforementioned markers and Brdu pulse labeling, it was estimated to take about one hour for most crypt cells to go through the G2 phase and about two hours to go through the G2-M phase. It took much longer for crypt base columnar (CBC) stem cells to undergo G2-prophase than rapid transit amplifying cells. In summary, a series of sequentially presenting markers could be used to indicate the progress of G2/M events in intestinal epithelial cells and other epithelial systems in vivo.

Aurora-A mediated histone H3 phosphorylation of threonine 118 controls condensin I and cohesin occupancy in mitosis

Phosphorylation of histone H3 threonine 118 (H3 T118ph) weakens histone DNA-contacts, disrupting the nucleosome structure. We show that Aurora-A mediated H3 T118ph occurs at pericentromeres and chromosome arms during prophase and is lost upon chromosome alignment. Expression of H3 T118E or H3 T118I (a SIN mutation that bypasses the need for the ATP-dependent nucleosome remodeler SWI/SNF) leads to mitotic problems including defects in spindle attachment, delayed cytokinesis, reduced chromatin packaging, cohesion loss, cohesin and condensin I loss in human cells. In agreement, overexpression of Aurora-A leads to increased H3 T118ph levels, causing cohesion loss, and reduced levels of cohesin and condensin I on chromatin. Normal levels of H3 T118ph are important because it is required for development in fruit flies. We propose that H3 T118ph alters the chromatin structure during specific phases of mitosis to promote timely condensin I and cohesin disassociation, which is essential for effective chromosome segregation.

Distinct epigenetic and gene expression changes in rat hippocampal neurons after Morris water maze training

Gene transcription and translation in the hippocampus is of critical importance in hippocampus-dependent memory formation, including during Morris water maze (MWM) learning. Previous work using gene deletion models has shown that the immediate-early genes (IEGs) c-Fos, Egr-1, and Arc are crucial for such learning. Recently, we reported that induction of IEGs in sparse dentate gyrus neurons requires ERK MAPK signaling and downstream formation of a distinct epigenetic histone mark (i.e., phospho-acetylated histone H3). Until now, this signaling, epigenetic and gene transcriptional pathway has not been comprehensively studied in the MWM model. Therefore, we conducted a detailed study of the phosphorylation of ERK1/2 and serine10 in histone H3 (H3S10p) and induction of IEGs in the hippocampus of MWM trained rats and matched controls. MWM training evoked consecutive waves of ERK1/2 phosphorylation and H3S10 phosphorylation, as well as c-Fos, Egr-1, and Arc induction in sparse hippocampal neurons. The observed effects were most pronounced in the dentate gyrus. A positive correlation was found between the average latency to find the platform and the number of H3S10p-positive dentate gyrus neurons. Furthermore, chromatin immuno-precipitation (ChIP) revealed a significantly increased association of phospho-acetylated histone H3 (H3K9ac-S10p) with the gene promoters of c-Fos and Egr-1, but not Arc, after MWM exposure compared with controls. Surprisingly, however, we found very little difference between IEG responses (regarding both protein and mRNA) in MWM-trained rats compared with matched swim controls. We conclude that exposure to the water maze evokes ERK MAPK activation, distinct epigenetic changes and IEG induction predominantly in sparse dentate gyrus neurons. It appears, however, that a specific role for IEGs in the learning aspect of MWM training may become apparent in downstream AP-1- and Egr-1-regulated (second wave) genes and Arc-dependent effector mechanisms.

Structural dynamics of the cell nucleus: basis for morphology modulation of nuclear calcium signaling and gene transcription

Neuronal morphology plays an essential role in signal processing in the brain. Individual neurons can undergo use-dependent changes in their shape and connectivity, which affects how intracellular processes are regulated and how signals are transferred from one cell to another in a neuronal network. Calcium is one of the most important intracellular second messengers regulating cellular morphologies and functions. In neurons, intracellular calcium levels are controlled by ion channels in the plasma membrane such as NMDA receptors (NMDARs), voltage-gated calcium channels (VGCCs) and certain α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) as well as by calcium exchange pathways between the cytosol and internal calcium stores including the endoplasmic reticulum and mitochondria. Synaptic activity and the subsequent opening of ligand and/or voltage-gated calcium channels can initiate cytosolic calcium transients which propagate towards the cell soma and enter the nucleus via its nuclear pore complexes (NPCs) embedded in the nuclear envelope. We recently described the discovery that in hippocampal neurons the morphology of the nucleus affects the calcium dynamics within the nucleus. Here we propose that nuclear infoldings determine whether a nucleus functions as an integrator or detector of oscillating calcium signals. We outline possible ties between nuclear mophology and transcriptional activity and discuss the importance of extending the approach to whole cell calcium signal modeling in order to understand synapse-to-nucleus communication in healthy and dysfunctional neurons.