Biology subjects

Kulikova, T.

Publications and source records attributed to Kulikova, T..

6 recordsLinked to original sources

bioRxiv2026

Nascent transcripts of LL2R tandem repeat nucleate locus-specific RNP condensates recruiting splicing factors

Spatial organization of the cell nucleus is increasingly understood to be governed by non-coding architectural RNAs that seed the formation of biomolecular condensates. However, the genomic origins and mechanistic principles by which specific architectural RNAs orchestrate ribonucleoprotein (RNP) compartments remain poorly defined. Here, we characterised a novel non-coding architectural RNA that nucleates the formation of a locus-associated membraneless nuclear compartment. Using a combination of T2T genomic, transcriptomic, and high-resolution mapping on giant lampbrush chromosomes, we identified an array of non-coding LL2R tandem repeats on the long arm of chicken chromosome 2. The LL2R tandem repeat array is slowly transcribed by a convoy of RNA polymerases II as a single transcription unit from a retrotransposon promoter. With the use of scanning electron and super-resolution microscopy for detailed analysis, we demonstrate that nascent LL2R repeat transcripts are retained at the site of their transcription and recruit multiple sets of core splicing factors, including TMG-capped snRNAs, core snRNP proteins, and serine/arginine-rich proteins SRRM2 and SC35 that contain intrinsically disordered regions, thus seeding nuclear domain formation. At the same time, the RNP-matrix of the LL2R repeat transcription unit lacks hnRNP L, hnRNP K, and SFPQ, widely distributed along normal transcription loops that bear nascent gene transcripts. In contrast to pre-mRNAs, strand-specific LL2R tandem repeat-containing RNA contains more than threefold excess of predicted donor splice sites compared to acceptor splice sites and is depleted in polyadenylation signals. We suggest that stalling of elongating RNA polymerases at LL2R transcription units and delay in 3 end cleavage may be caused by a deficiency of co-transcriptional splicing of nascent LL2R repeat transcripts. LL2R repeat-containing RNA also forms non-diffusible RNP-aggregates enriched by splicing factors that could serve as a site of post-transcriptional splicing. Our findings reveal how tandem repeat-derived RNAs drive the assembly of oocyte nuclear condensates, providing a functional template for understanding the formation of closely related nuclear speckles and nuclear stress bodies across eukaryotes.

cell biology↗

bioRxiv2025

The 3D genomics of lampbrush chromosomes highlights the role of active transcription in chromatin organization

Lampbrush chromosomes are giant meiotic bivalents in growing oocyte nuclei that have served as a classic model system for studying chromatin organization and RNA synthesis for over a century. Despite their importance, the molecular mechanisms underlying lampbrush chromosomes formation and their distinctive chromomere-loop architecture have remained poorly understood. Moreover, the influence of hypertranscription on chromatin organization during oogenesis remains enigmatic. Here, we provide the first comprehensive genomic, cytological, and biophysical analysis of lampbrush chromosome organization by integrating single-cell Hi-C, RNA-seq, NOMe-seq, FISH mapping, and chromatin simulations. Single-nucleus Hi-C analysis revealed CTCF-independent contact domains with stable boundaries defined by transcription units in a convergent orientation. Contact domains identified through Hi-C analysis correspond to insulated chromomeres in lampbrush chromosomes. Small transcriptionally inactive contact domains surrounded by transcription units in the diverged orientation form "chromatin knots", which are often detached from the chromosome axis. Transcription loops frequently manifest as a "cross" pattern with reduced contacts within chromatin domains. Integrative analysis of the whole-genome data uncovers the mechanisms underlying lampbrush chromosome structure, revealing how hypertranscription modulates chromatin stiffness and repositions SMC complexes to establish the distinctive chromomere-loop organisation. Biophysical modeling through polymer simulation reproduces key features of lampbrush chromosomes, including transcription loop formation, chromomere compaction, and insulation patterns. These findings offer a unifying framework for understanding the remarkable chromatin architecture of lampbrush chromosomes and their transcription-dependent organization. Highlights- First integration of single-cell Hi-C, RNA-seq, NOMe-seq and microscopy methods uncovers molecular mechanisms underlying lampbrush chromosome architecture. - Hi-C reveals contact patterns corresponding to lampbrush chromomeres and transcription loops, validated through BAC-based FISH mapping. - Lampbrush chromosomes are segmented into contact domains formed via a CTCF-independent mechanism, with boundaries coinciding with convergently oriented gene pairs. - Hypertranscription shapes lampbrush chromosome through multiple mechanisms, increasing stiffness and decreasing compaction of transcribed units, generating outward pressure, pushing transcription loops away from the chromosome axis, and repositioning SMC complexes to form transcription-dependent domains with stable boundaries. - Hi-C and RNA-seq data analysis as well as polymer simulations demonstrate that cohesive cohesin functions as a transcription-anchored barrier essential for domain insulation in lampbrush chromosomes. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=144 HEIGHT=200 SRC="FIGDIR/small/635017v4_ufig1.gif" ALT="Figure 1"> View larger version (39K): org.highwire.dtl.DTLVardef@5a425dorg.highwire.dtl.DTLVardef@1bb11e8org.highwire.dtl.DTLVardef@94a8aborg.highwire.dtl.DTLVardef@1cd9adb_HPS_FORMAT_FIGEXP M_FIG C_FIG

genomics↗

bioRxiv2024

The first chicken oocyte nucleus whole transcriptomic profile defines the spectrum of maternal mRNA and non-coding RNA genes transcribed by the lampbrush chromosomes

Lampbrush chromosomes, with their unusually high rate of nascent RNA synthesis, provide a valuable model for studying the mechanisms of global transcriptome up-regulation. Here, we aimed to establish a full set of sequences transcribed on the lateral loops of chicken lampbrush chromosomes. For the first time, a whole-genomic profile of transcription along the entire length of all lampbrush chromosomes in the chicken karyotype including sex chromosomes and dot chromosomes was obtained. For that, we performed RNA-seq of oocyte nuclear and cytoplasmic total RNA, poly(A) RNA and small RNA libraries and aligned expressed RNA sequences against the chicken genome. This led to the identification of a full spectrum of maternal RNAs that accumulate in the nucleus and cytoplasm of chicken oocytes at the lampbrush chromosome stage, are then transferred to the zygote and can be used during the early stages of embryogenesis, including transcripts of protein-coding genes, long non-coding RNAs and small housekeeping non-coding RNAs. We also present the first high-throughput transcriptome characterisation of miRNAs and piRNAs in chicken oocytes at the lampbrush chromosome stage. Major targets of predicted piRNAs include CR1 and LTR containing retrotransposable elements. Transcription of tandem repeat arrays including 41bp higher-order repeats as well as unique centromere sequences was demonstrated by alignment against the whole telomere-to-telomere chromosome assemblies. We show that transcription of telomere-derived RNAs, including telomeric repeat-containing RNA (TERRA) and subtelomeric repeat-containing RNA, is initiated at adjacent LTR elements. With nuclear RNA-seq, we obtained information about a wider set of transcripts, including long non-coding RNAs retained in the nucleus and stable intronic sequence RNAs (sisRNAs). We identified 5 UTR sisRNAs that may potentially support host gene transcription both during oogenesis and after activation of the embryonic genome. For a number of protein-coding genes, we visualised nascent transcripts and demonstrated their co-transcriptional splicing on the lateral loops of lampbrush chromosomes by RNA-FISH. The nuclear RNA-seq profile predicted the chromomere-loop organisation of the genomic regions. During oocyte maturation, transcriptional unit boundaries were maintained, while transcriptional output tended to decrease. We conclude that cytoplasmic and nuclear transcripts emerge from nascent transcripts on the lateral loops of lampbrush chromosomes. Most of gene transcripts are initiated at promoters, normally spliced, terminated and polyadenylated. The set of genes transcribed on the lampbrush chromosomes is required for basic cellular processes, is characterised by a broad expression pattern and is similar to sets of genes expressed in other hypertranscriptional systems. We conclude that hypertranscription on the lateral loops of giant lampbrush chromosomes is the main mechanism for synthesising large amounts of transferred to the embryo maternal RNA for thousands of genes.

cell biology↗

bioRxiv2023

Assignment of the somatic A/B compartments to chromatin domains in giant transcriptionally active lampbrush chromosomes

The three-dimensional configuration of the eukaryotic genome is an emerging area of research. Chromosome conformation capture outlined genome segregation into large scale A and B compartments corresponding mainly to transcriptionally active and repressive chromatin. It remains unknown how the compartmentalization of the genome changes in growing oocytes of animals with hypertranscriptional type of oogenesis. In this type of oogenesis, highly elongated chromosomes, called lampbrush chromosomes, acquire a characteristic chromomere-loop appearance, representing one of the classical model systems for studying the structural and functional organization of chromatin domains. Here, we compared the distribution of A/B compartments in chicken somatic cells with chromatin domains in lampbrush chromosomes. We found that in lampbrush chromosomes, the extended chromatin domains, restricted by compartment boundaries in somatic cells, disintegrate into individual chromomeres. Next, we performed FISH-mapping of the genomic loci, which belong to A or B chromatin compartments as well as to A/B compartment transition regions in embryonic fibroblasts on isolated lampbrush chromosomes. We established, that in chicken lampbrush chromosomes, clusters of dense compact chromomeres bearing short lateral loops and enriched with repressive epigenetic modifications generally correspond to constitutive B compartments in somatic cells. These results suggest that gene-poor regions tend to be packed into chromomeres. Clusters of small loose chromomeres with relatively long lateral loops show no obvious correspondence with either A or B compartment identity. Some genes belonging to facultative B (sub-) compartments can be tissue-specifically transcribed during oogenesis, forming distinct lateral loops.

genomics↗

bioRxiv2022

Comparison and critical assessment of single-cell Hi-C protocols

Advances in single-cell sequencing technologies make it possible to study the genome architecture in single cells. The rapid growth of the field has been fueled by the development of innovative single-cell Hi-C protocols. However, the protocols vary considerably in their efficiency, bias, scale and costs, and their relative advantages for different applications are unclear. Here, we compare the two most commonly used single-cell Hi-C protocols. We use long-read sequencing to analyze molecular products of the Hi-C assay and show that whole-genome amplification step results in increased number of artifacts, larger coverage biases, and increased amount of noise compared to PCR-based amplification. Our comparison provides guidance for researchers studying chromatin architecture in individual cells.

genomics↗

bioRxiv2021

Comparison of the somatic TADs and lampbrush chromomere-loop complexes in transcriptionally active prophase I oocytes

In diplotene oocyte nuclei of all vertebrate species, except mammals, chromosomes lack interchromosomal contacts and chromatin is linearly compartmentalized into distinct chromomere-loop complexes forming lampbrush chromosomes. However, the mechanisms underlying the formation of chromomere-loop complexes remain unexplored. Here we aimed to juxtapose somatic topologically associating domains (TADs), recently identified in chicken embryonic fibroblasts, with chromomere-loop complexes in lampbrush meiotic chromosomes. By measuring 3D-distances and colocalization between linear equidistantly located genomic loci, positioned within one TAD or separated by a TAD border, we confirmed the presence of predicted TADs in chicken embryonic fibroblast nuclei. Using three-colored FISH with BAC probes we mapped equidistant genomic regions included in several sequential somatic TADs on isolated chicken lampbrush chromosomes. Eight genomic regions, each comprising two or three somatic TADs, were mapped to non-overlapping neighboring lampbrush chromatin domains - lateral loops, chromomeres or chromomere-loop complexes. Genomic loci from the neighboring somatic TADs could localize in one lampbrush chromomere-loop complex, while genomic loci belonging to the same somatic TAD could be localized in neighboring lampbrush chromomere- loop domains. In addition, FISH-mapping of BAC probes to the nascent transcripts on the lateral loops indicates transcription of at least 17 protein-coding genes and 2 non-coding RNA genes during the lampbrush stage of chicken oogenesis, including genes involved in oocyte maturation and early embryo development.

cell biology↗