SEARCH · PubMed Health

Results for “nucleation and spreading”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

7 recordsLinked to original sources

PubMed2025

Heterochromatin de novo formation and maintenance in Plasmodium falciparum.

In the malaria parasite Plasmodium falciparum, the expression of many genes is regulated by heterochromatin (HC) based on the histone mark tri-methylation of histone H3 lysine 9 (H3K9me3). HC assembly involves three distinct steps: de novo nucleation, spreading and maintenance. Nucleation, which consists in formation of HC in a previously euchromatic region, determines the specific genomic locations where HC occurs. This process is not well understood in malaria parasites. Here we investigated the DNA sequence cis determinants of HC nucleation in P. falciparum, using a screening approach based on integration of fragments from different heterochromatic genes into an euchromatic locus, followed by H3K9me3 chromatin immunoprecipitation (ChIP) analysis. We found that fragments of var gene upstream regions nucleated HC efficiently, whereas fragments from the pfap2-g upstream region or from the mspdbl2 locus did not nucleate HC. Fragments from the beginning of the coding sequence (CDS) of pfap2-g nucleated HC with low efficiency, as evidenced by nucleation requiring long fragments of ~2 kb and occurring only in a fraction of the parasites. These results demonstrate that the primary DNA sequence is a main determinant of HC nucleation in P. falciparum. We also studied HC maintenance at the pfap2-g locus, which demonstrated that specific parts of the upstream region, different from the regions competent for HC nucleation, are required for maintenance. Together, our results provide initial insight into how HC is directed to specific loci and maintained in P. falciparum.

Plasmodium falciparum↗

PubMed2025

Discrete Subdomains Establish Epigenetic Diversity in Subtelomeric Heterochromatin.

Subtelomeres are imperfect repeats adjacent to telomeres that are repressed by heterochromatin. Although essential for genome integrity, their repetitive nature has thwarted dissection of local heterochromatin assembly and maintenance mechanisms. Here, we engineered Schizosaccharomyces pombe strains carrying fluorescent reporters at a single subtelomere. We find that subtelomeric heterochromatin is organized into discrete subdomains that nucleate at telomere-proximal and cryptic internal sites. Telomere-proximal regions depend on canonical shelterin or RNA interference nucleation pathways, while telomere-distal regions require nucleosome remodelers, histone chaperones, and boundary-associated factors. Using multi-generational live imaging and targeted perturbations, we show that subtelomeric subdomains display position-specific, clonally variable silencing across a spectrum of robust to fragile epigenetic states. This clonal variegation is also induced by naturally occurring subtelomeric structural variants. These findings demonstrate that subtelomeric heterochromatin maintenance is not uniform but rather governed by local chromatin context and architecture.

H3K9 methylation↗

PubMed2026

Homologous recombination mutants cause differing lethality between h- and h+ Schizosaccharomyces pombe strains due to mat1 heterochromatin.

Homologous recombination (HR) is generally considered dispensable in yeast and vertebrates, yet mounting evidence indicates that its essentiality depends on cellular context. Here, we dissect the basis of this context dependency in Schizosaccharomyces pombe. In the homothallic h90 strain, regarded as wild type, mating-type switching (MTS) occurs every other cell division and requires HR to repair programmed double-strand breaks (DSBs) at the mat1 locus. We show that the widely used heterothallic h-S strain is likewise dependent on HR for viability. HR-deficient h-S mutants (rad51Δ, rad52Δ, or rad54Δ), still frequently employed in the literature, survive only when carrying secondary suppressor mutations that abolish mat1 DSB formation, such as smt-0, swi1Δ, or fml1Δ. In contrast, HR is dispensable in the h+N strain, where duplication of the mat2/3 region into mat1 introduces the cenH and REIII elements. These elements nucleate H3K9 methylation and heterochromatin spreading across the imprint site, blocking imprintosome recruitment and thereby preventing both imprinting and DSB formation. Disruption of this heterochromatin, via deletion of cenH or key chromatin modifiers, restores DSB formation in h+N cells and reinstates HR essentiality in the absence of the Clr4 methyltransferase. Collectively, our findings demonstrate that HR is indispensable for S. pombe survival due to its critical role in repairing mat1 DSBs, except under genetic or epigenetic conditions that suppress their formation.

Homologous Recombination↗

PubMed2026

LINE-1 repeats are a defining feature of the Xce.

During early development, female mammals inactivate one X chromosome to balance their X-linked gene dosage with males. While allelic choice is random in inbred mouse populations, choice can be significantly skewed in interstrain hybrids. The genetic basis of skewing has long been attributed to the mysterious "X chromosome controlling element(s)" (Xce) with different strengths among species, subspecies, and strains. When two X-chromosomes with different Xce strengths are inherited by offspring, the X chromosome with the stronger Xce will have a higher probability of remaining active. Here, we provide evidence that L1Tf repeats-a subfamily of long interspersed nuclear elements 1-plays a role in determining Xce strength. L1Tf elements form a condensed core within the inactive X (Xi) territory. Mouse strains with varying Xce strengths differ in the L1Tf copy number on the X chromosome, with the strength of the Xce allele being inversely related to L1Tf copy number. L1Tf expression mediates the Xce effect. However, in contrast to a prior report, L1Tf RNA does not coat the Xi. Rather, L1Tf promotes condensation of the Xi core. Intriguingly, L1Tfs recruit and sequester YY1 from active genes, accelerating XCI in cis. Thus, L1Tf copy number, expression, and binding of YY1 are key defining features of the Xce. We propose a model in which the Xce influences the choice of Xist alleles by promoting YY1 binding to the nucleation site for the initiation of Xist spreading.

Animals↗

PubMed1978

Nucleation of microtubules in vitro by isolated spindle pole bodies of the yeast Saccharomyces cerevisiae.

Spindle pole bodies (SPBs) were isolated from the yeast Saccharomyces cerevisiae by an adaptation of the Kleinschmidt monolayer technique. Spheroplasts prepared from the cells were lysed on an air-water interface. Spread preparations were picked up on grids, transferred to experimental test solutions, and prepared for whole-mount electron microscopy. Using purified exogenous tubulin from porcine brain tissue, the isolated SPBs were shown to nucleate the assembly of microtubules in vitro. Microtubule growth was directional and primarily onto the intranuclear face of the SPB. Neither the morphology nor the microtubule-initiating capacity of the SPB was affected by treatment with the enzymes DNase, RNase, or phospholipase although both properties were sensitive to trypsin. Analysis of SPBs at various stages of the cell cycle showed that newly replicated SPBs had the capacity to nucleate microtubules. SPBs isolated from exponentially growing cells initiated a subset of the yeast spindle microtubules equivalent to the number of pole-to-pole microtubules seen in vivo. However, SPBs isolated from cells in stationary phase and therefore arrested in G1 nucleated a number of microtubules equal to the total chromosomal and pole-to-pole tubules in the yeast spindle. This may mean that in G1-arrested cells, the SPB is associated with microtubule attachment sites of the yeast chromatin.

Cell Cycle↗

PubMed1977

[In vitro culture of human peritoneal fluid cells].

The author has studied the behaviour of cells from human ascitic fluid in long-term culture (5-6 months). Three cellular types are described with different morphological features, namely the cellular shape, the fashion in which the cell spreads on the glass, the nucleocytoplasmic ratio, the chromatin appearance and the abundance of mitochondria. The three cellular types can phagocytose, but each one in a different way. The first one phagocytoses exclusively erythrocytes 'by contact' without emission of pseudopods; the second one phagocytoses degenerating nucleated cells in the same way as the first; the third type phagocytoses degenerating nucleated cells by emission of long pseudopods. The origin of these three cellular types is discussed; it is felt that they are transformed mesothelial cells. According to this study, it cannot be excluded, especially for the second and the third type, that they are histiocytes coming from serous membranes. The life in vitro of the three cellular types is depending upon the composition of nourishing medium. Cells can divide by mitosis only during the first 10 - 15 days of culture (mitotic index 0.1-3.0(0/00). Nuclear amitosis, nucleolus expulsion into cytoplasm and cytoplasmatic DNA synthesis can be observed in healthy cells.

Aged↗

PubMed1975

Experimental kuru in the spider monkey. Histopathological and ultrastructural studies of the brain during early stages of incubation.

The brains of 10 spider monkeys inoculated intracerebrally with brain suspension from kuru patients have been studied histologically and ultrastructurally. The animals were killed by perfusion of fixative from four to forty-one weeks after inoculation, when healthy and free of neurological signs. Definite histopathological changes had occurred as early as four weeks after inoculation, when moderate numbers of bi-nucleated neurons were found within the limbic cortex, striatum, the hypothalamus and amongst the Purkinje cells of the cerebellum. At later stages of incubation a moderate loss of neurons in the cerebral and cerebellar cortex and a mild to moderate proliferation of fibrous astrocytes here and also in the hypothalamus were the most striking features. None of our cases showed either status spongiosus or the generalized astrocytic proliferation and hypertrophy, characteristic of fully developed experimental kuru, in any region of the brain. The principal ultrastructural abnormalities consisted of the formation of membrane-bound intracytoplasmic vacuoles, predominantly within dendrites, and of concentric laminar arrays derived from the endoplasmic reticulum. The former were seen in all regions of the brain examined and at all stages of incubation. Concentric laminar arrays were confined to the cerebellar nodulus, where they were most numerous in dendrites and neuronal perikarya four weeks after inoculation. Both changes are interpreted as an indication that the kuru agent acts upon the plasma membrane from an early stage onwards and, by stimulating its growth, leads to the formation of complex, membrane-bounded vacuoles and to hyperplasia of the endoplasmic reticulum. The formation of vacuoles is further regarded as the first sign of status spongiosus on an ultrastructural level. Attention is drawn to the great similarities between the changes observed in the present material and those described in the brains of patients dying from kuru and of primates with fully developed experimental kuru. The significance of the relatively rapid spread of the kuru agent throughout the brain is discussed in relation to the concept of "slow virus" diseases.

Animals↗