Nucleolus-like body of mouse oocytes contains lamin A and B and TRF2 but not actin and topo II

Special Nuclear Structures in the Germinal Vesicle of the Common Frog with Emphasis on the So-Called Karyosphere Capsule

The karyosphere (karyosome) is a structure that forms in the oocyte nucleus-germinal vesicle (GV)-at the diplotene stage of meiotic prophase due to the assembly of all chromosomes in a limited portion of the GV. In some organisms, the karyosphere has an extrachromosomal external capsule, the marker protein of which is nuclear F-actin. Despite many years of theories about the formation of the karyosphere capsule (KC) in the GV of the common frog Rana temporaria, we present data that cast doubt on its existence, at least in this species. Specific extrachromosomal strands, which had been considered the main elements of the frog's KC, do not form a continuous layer around the karyosphere and, according to immunogold labeling, do not contain structural proteins, such as actin and lamin B. At the same time, F-actin is indeed noticeably concentrated around the karyosphere, creating the illusion of a capsule at the light microscopy/fluorescence level. The barrier-to-autointegration factor (BAF) and one of its functional partners-LEMD2, an inner nuclear membrane protein-are not localized in the strands, suggesting that the strands are not functional counterparts of the nuclear envelope. The presence of characteristic strands in the GV of R. temporaria late oocytes may reflect an excess of SMC1 involved in the structural maintenance of diplotene oocyte chromosomes at the karyosphere stage, since SMC1 has been shown to be the most abundant protein in the strands. Other characteristic microstructures-the so-called annuli, very similar in ultrastructure to the nuclear pore complexes-do not contain nucleoporins Nup35 and Nup93, and, therefore, they cannot be considered autonomous pore complexes, as previously thought. Taken together, our data indicate that traditional ideas about the existence of the R. temporaria KC as a special structural compartment of the GV are to be revisited.

Chromatin Configuration in Diplotene Mouse and Human Oocytes during the Period of Transcriptional Activity Extinction

In the oocyte nucleus, called the germinal vesicle (GV) at the prolonged diplotene stage of the meiotic prophase, chromatin undergoes a global rearrangement, which is often accompanied by the cessation of its transcriptional activity. In many mammals, including mice and humans, chromatin condenses around a special nuclear organelle called the atypical nucleolus or formerly nucleolus-like body. Chromatin configuration is an important indicator of the quality of GV oocytes and largely predicts their ability to resume meiosis and successful embryonic development. In mice, GV oocytes are traditionally divided into the NSN (non-surrounded nucleolus) and SN (surrounded nucleolus) based on the specific chromatin configuration. The NSN-SN transition is a key event in mouse oogenesis and the main prerequisite for the normal development of the embryo. As for humans, there is no single nomenclature for the chromatin configuration at the GV stage. This often leads to discrepancies and misunderstandings, the overcoming of which should expand the scope of the application of mouse oocytes as a model for developing new methods for assessing and improving the quality of human oocytes. As a first approximation and with a certain proviso, the mouse NSN/SN classification can be used for the primary characterization of human GV oocytes. The task of this review is to analyze and discuss the existing classifications of chromatin configuration in mouse and human GV oocytes with an emphasis on transcriptional activity extinction at the end of oocyte growth.

Nuclear Lamins: Key Proteins for Embryonic Development

Simple Summary

The biology of a multicellular organism is extremely complex, leaving behind a realm of compound yet systematic mechanisms still to be unraveled. The nucleus is a vital cellular organelle adapted to storing and regulating the hereditary genetic information. Dysregulation of the nucleus can have profound effects on the physiology and viability of cells. This becomes extremely significant in the context of development, where the whole organism arises from a single cell, the zygote. Therefore, even a mild aberration at this stage can have profound effects on the whole organism. However, studying the function of individual nuclear components at this point is exceptionally complicated because this phase is inherently under the control of maternal factors stored in the female germ cell, the egg. Here, we focus on the lamins, as essential nuclear components, and summarize the current knowledge of their role in development. Although scientists encounter challenges working with these miniscule yet key proteins, the demand to know more is increasing gradually due to the mutations caused in lamins leading to irreversible phenotypic conditions in humans.

Abstract

Lamins are essential components of the nuclear envelope and have been studied for decades due to their involvement in several devastating human diseases, the laminopathies. Despite intensive research, the molecular basis behind the disease state remains mostly unclear with a number of conflicting results regarding the different cellular functions of nuclear lamins being published. The field of developmental biology is no exception. Across model organisms, the types of lamins present in early mammalian development have been contradictory over the years. Due to the long half-life of the lamin proteins, which is a maternal factor that gets carried over to the zygote after fertilization, investigators are posed with challenges to dive into the functional aspects and significance of lamins in development. Due to these technical limitations, the role of lamins in early mammalian embryos is virtually unexplored. This review aims in converging results that were obtained so far in addition to the complex functions that ceases if lamins are mutated.

The Long Linker Region of Telomere-Binding Protein TRF2 Is Responsible for Interactions with Lamins

Telomere-binding factor 2 (TRF2) is part of the shelterin protein complex found at chromosome ends. Lamin A/C interacts with TRF2 and influences telomere position. TRF2 has an intrinsically disordered region between the ordered dimerization and DNA-binding domains. This domain is referred to as the long linker region of TRF2, or udTRF2. We suggest that udTRF2 might be involved in the interaction between TRF2 and lamins. The recombinant protein corresponding to the udTRF2 region along with polyclonal antibodies against this region were used in co-immunoprecipitation with purified lamina and nuclear extracts. Co-immunoprecipitation followed by Western blots and mass spectrometry indicated that udTRF2 interacts with lamins, preferably lamins A/C. The interaction did not involve any lamin-associated proteins, was not dependent on the post-translation modification of lamins, nor did it require their higher-order assembly. Besides lamins, a number of other udTRF2-interacting proteins were identified by mass spectrometry, including several heterogeneous nuclear ribonucleoproteins (hnRNP A2/B1, hnRNPA1, hnRNP A3, hnRNP K, hnRNP L, hnRNP M), splicing factors (SFPQ, NONO, SRSF1, and others), helicases (DDX5, DHX9, and Eif4a3l1), topoisomerase I, and heat shock protein 71, amongst others. Some of the identified interactors are known to be involved in telomere biology; the roles of the others remain to be investigated. Thus, the long linker region of TRF2 (udTRF2) is a regulatory domain responsible for the association between TRF2 and lamins and is involved in interactions with other proteins.

Decreased expression of TERT and telomeric proteins as human ovaries age may cause telomere shortening

OBJECTIVE

Telomeres are repetitive sequences localized at the ends of eukaryotic chromosomes comprising noncoding DNA and telomere-binding proteins. TRF1 and TRF2 both bind to the double-stranded telomeric DNA to regulate its length throughout the lifespan of eukaryotic cells. POT1 interacts with single-stranded telomeric DNA and contributes to protecting genomic integrity. Previous studies have shown that telomeres gradually shorten as ovaries age, coinciding with fertility loss. However, the molecular background of telomere shortening with ovarian aging is not fully understood.

METHODS

The present study aimed to determine the spatial and temporal expression levels of the TERT, TRF1, TRF2, and POT1 proteins in different groups of human ovaries: fetal (n = 11), early postnatal (n = 10), premenopausal (n = 12), and postmenopausal (n = 14). Also, the relative telomere signal intensity of each group was measured using the Q-FISH method.

RESULTS

We found that the telomere signal intensities decreased evenly and significantly from fetal to postmenopausal groups (P < 0.05). The TERT, TRF1, TRF2, and POT1 proteins were localized in the cytoplasmic and nuclear regions of the oocytes, granulosa and stromal cells. Furthermore, the expression levels of these proteins reduced significantly from fetal to postmenopausal groups (P < 0.05).

CONCLUSION

These findings suggest that decreased TERT and telomere-binding protein expression may underlie the telomere shortening of ovaries with age, which may be associated with female fertility loss. Further investigations are required to elicit the molecular mechanisms regulating the gradual decrease in the expression of TERT and telomere-binding proteins in human oocytes and granulosa cells during ovarian aging.

Lamin A/C modulates spatial organization and function of the Hsp70 gene locus via nuclear myosin I

The structure-function relationship of the nucleus is tightly regulated, especially during heat shock. Typically, heat shock activates molecular chaperones that prevent protein misfolding and preserve genome integrity. However, the molecular mechanisms that regulate nuclear structure-function relationships during heat shock remain unclear. Here, we show that lamin A and C (hereafter lamin A/C; both lamin A and C are encoded by LMNA) are required for heat-shock-mediated transcriptional induction of the Hsp70 gene locus (HSPA genes). Interestingly, lamin A/C regulates redistribution of nuclear myosin I (NM1) into the nucleus upon heat shock, and depletion of either lamin A/C or NM1 abrogates heat-shock-induced repositioning of Hsp70 gene locus away from the nuclear envelope. Lamins and NM1 also regulate spatial positioning of the SC35 (also known as SRSF2) speckles - important nuclear landmarks that modulates Hsp70 gene locus expression upon heat shock. This suggests an intricate crosstalk between nuclear lamins, NM1 and SC35 organization in modulating transcriptional responses of the Hsp70 gene locus during heat shock. Taken together, this study unravels a novel role for lamin A/C in the regulation of the spatial dynamics and function of the Hsp70 gene locus upon heat shock, via the nuclear motor protein NM1.This article has an associated First Person interview with the first author of the paper.

Lamin A/C and Emerin depletion impacts chromatin organization and dynamics in the interphase nucleus

Background

Nuclear lamins are type V intermediate filament proteins that maintain nuclear structure and function. Furthermore, Emerin - an interactor of Lamin A/C, facilitates crosstalk between the cytoskeleton and the nucleus as it also interacts with actin and Nuclear Myosin 1 (NM1).

Results

Here we show that the depletion of Lamin A/C or Emerin, alters the localization of the nuclear motor protein - Nuclear Myosin 1 (NM1) that manifests as an increase in NM1 foci in the nucleus and are rescued to basal levels upon the combined knockdown of Lamin A/C and Emerin. Furthermore, Lamin A/C-Emerin co-depletion destabilizes cytoskeletal organization as it increases actin stress fibers. This further impinges on nuclear organization, as it enhances chromatin mobility more toward the nuclear interior in Lamin A/C-Emerin co-depleted cells. This enhanced chromatin mobility was restored to basal levels either upon inhibition of Nuclear Myosin 1 (NM1) activity or actin depolymerization. In addition, the combined loss of Lamin A/C and Emerin alters the otherwise highly conserved spatial positions of chromosome territories. Furthermore, knockdown of Lamin A/C or Lamin A/C-Emerin combined, deregulates expression levels of a candidate subset of genes. Amongst these genes, both KLK10 (Chr.19, Lamina Associated Domain (LAD+)) and MADH2 (Chr.18, LAD-) were significantly repressed, while BCL2L12 (Chr.19, LAD-) is de-repressed. These genes differentially reposition with respect to the nuclear envelope.

Conclusions

Taken together, these studies underscore a remarkable interplay between Lamin A/C and Emerin in modulating cytoskeletal organization of actin and NM1 that impinges on chromatin dynamics and function in the interphase nucleus.

Electronic supplementary material

The online version of this article (10.1186/s12860-019-0192-5) contains supplementary material, which is available to authorized users.

Actin depolymerization disrupts karyosphere capsule integrity but not residual transcription in late oocytes of the grass frog Rana temporaria

Late diplotene oocytes are characterized by an essential decrease in transcriptional activity. At this time, chromosomes condense and form a compact structure named a karyosphere. The karyosphere of grass frogs Rana temporaria is surrounded by a fibrillar karyosphere capsule (KC). One of the main protein constituents of R. temporaria KC is actin. In this study, we used antibodies against different actin epitopes to trace different forms of actin in the KC. We also investigated the effect of F-actin depolymerization on the oocyte nuclear structures and transcription of chromatin DNA and rDNA in the amplified nucleoli. It was determined that disruption of actin filaments leads to chromosome shrinkage, nucleoli fusion, and distortion of the KC structure, but does not inhibit residual transcription in both the karyosphere and the nucleoli.

Emerin modulates spatial organization of chromosome territories in cells on softer matrices

Cells perceive and relay external mechanical forces into the nucleus through the nuclear envelope. Here we examined the effect of lowering substrate stiffness as a paradigm to address the impact of altered mechanical forces on nuclear structure-function relationships. RNA sequencing of cells on softer matrices revealed significant transcriptional imbalances, predominantly in chromatin associated processes and transcriptional deregulation of human Chromosome 1. Furthermore, 3-Dimensional fluorescence in situ hybridization (3D-FISH) analyses showed a significant mislocalization of Chromosome 1 and 19 Territories (CT) into the nuclear interior, consistent with their transcriptional deregulation. However, CT18 with relatively lower transcriptional dysregulation, also mislocalized into the nuclear interior. Furthermore, nuclear Lamins that regulate chromosome positioning, were mislocalized into the nuclear interior in response to lowered matrix stiffness. Notably, Lamin B2 overexpression retained CT18 near the nuclear periphery in cells on softer matrices. While, cells on softer matrices also activated emerin phosphorylation at a novel Tyr99 residue, the inhibition of which in a phospho-deficient mutant (emerinY99F), selectively retained chromosome 18 and 19 but not chromosome 1 territories at their conserved nuclear locations. Taken together, emerin functions as a key mechanosensor, that modulates the spatial organization of chromosome territories in the interphase nucleus.

Karyosphere (Karyosome): A Peculiar Structure of the Oocyte Nucleus

The karyosphere, aka the karyosome, is a meiosis-specific structure that represents a "knot" of condensed chromosomes joined together in a limited volume of the oocyte nucleus. The karyosphere is an evolutionarily conserved but morphologically rather "multifaceted" structure. It forms at the diplotene stage of meiotic prophase in many animals, from hydra and Drosophila to human. Karyosphere formation is generally linked with transcriptional silencing of the genome. It is believed that karyosphere/karyosome is a prerequisite for proper completion of meiotic divisions and further development. Here, a brief review on the karyosphere features in some invertebrates and vertebrates is provided. Special emphasis is made on terminology, since current discrepancies in this field may lead to confusions. In particular, it is proposed to distinguish the karyosphere with a capsule and the karyosome (a karyosphere devoid of a capsule). The "inverted" karyospheres are also considered, in which the chromosomes situate externally to an extrachromosomal structure (e.g., in human oocytes).

The karyosphere capsule in Rana temporaria oocytes contains structural and DNA-binding proteins

During the last stages of oogenesis, oocyte chromosomes condense and come close together, forming the so-called karyosphere. Karyosphere formation is accompanied by an essential decrease in transcriptional activity. In the grass frog Rana temporaria, the karyosphere is surrounded by an extrachromosomal capsule that separates the chromosomes from the rest of the nucleoplasm. The karyosphere capsule (KC) of R. temporaria has been investigated in detail at the ultrastructural level, but its protein composition remained largely unknown. We demonstrate here that nuclear actin, especially F-actin, as well as lamins A/C and B are the most abundant proteins of the KC. Key proteins of nuclear pore complexes, such as Nup93 and Nup35, are also detectable in the KC. New antibodies recognizing the telomere-binding protein TRF2 allowed us to localize TRF2 in nuclear speckles. We also found that the R. temporaria KC contains some proteins involved in chromatin remodeling, including topoisomerase II and ATRX. Thus, we believe that KC isolates the chromosomes from the rest of the nucleoplasm during the final period of oocyte growth (late diplotene) and represents a specialized oocyte nuclear compartment to store a variety of factors involved in nuclear metabolism that can be used in future early development. Abbreviations: BrUTP: 5-bromouridine 5'-triphosphate; CytD: cytochalasin D; IGCs: interchromatin granule clasters; IgG: immunoglobulin G; KC: karyosphere capsule; Mw: molecular weight; NE: nuclear envelope; PBS: phosphate buffered saline; SDS-PAGE: sodium dodecyl sulfate polyacrylamide gel electrophoresis; Topo II: topoisomerase II.