Myosin VI regulates the spatial organisation of mammalian transcription initiation

Myosin VI in the nucleolus of neurosecretory PC12 cells: its involvement in the maintenance of nucleolar structure and ribosome organization

We have previously shown that unconventional myosin VI (MVI), a unique actin-based motor protein, shuttles between the cytoplasm and nucleus in neurosecretory PC12 cells in a stimulation-dependent manner and interacts with numerous proteins involved in nuclear processes. Among the identified potential MVI partners was nucleolin, a major nucleolar protein implicated in rRNA processing and ribosome assembly. Several other nucleolar proteins such as fibrillarin, UBF (upstream binding factor), and B23 (also termed nucleophosmin) have been shown to interact with MVI. A bioinformatics tool predicted the presence of the nucleolar localization signal (NoLS) within the MVI globular tail domain, and immunostaining confirmed the presence of MVI within the nucleolus. Depletion of MVI, previously shown to impair PC12 cell proliferation and motility, caused disorganization of the nucleolus and rough endoplasmic reticulum (rER). However, lack of MVI does not affect nucleolar transcription. In light of these data, we propose that MVI is important for nucleolar and ribosome maintenance but not for RNA polymerase 1-related transcription.

Nuclear actin dynamics and functions at a glance

Actin is well known for its cytoskeletal functions, where it helps to control and maintain cell shape and architecture, as well as regulating cell migration and intracellular cargo transport, among others. However, actin is also prevalent in the nucleus, where genome-regulating roles have been described, including it being part of chromatin-remodeling complexes. More recently, with the help of advances in microscopy techniques and specialized imaging probes, direct visualization of nuclear actin filament dynamics has helped elucidate new roles for nuclear actin, such as in cell cycle regulation, DNA replication and repair, chromatin organization and transcriptional condensate formation. In this Cell Science at a Glance article, we summarize the known signaling events driving the dynamic assembly of actin into filaments of various structures within the nuclear compartment for essential genome functions. Additionally, we highlight the physiological role of nuclear F-actin in meiosis and early embryonic development.

Genome maintenance meets mechanobiology

Genome stability is key for healthy cells in healthy organisms, and deregulated maintenance of genome integrity is a hallmark of aging and of age-associated diseases including cancer and neurodegeneration. To maintain a stable genome, genome surveillance and repair pathways are closely intertwined with cell cycle regulation and with DNA transactions that occur during transcription and DNA replication. Coordination of these processes across different time and length scales involves dynamic changes of chromatin topology, clustering of fragile genomic regions and repair factors into nuclear repair centers, mobilization of the nuclear cytoskeleton, and activation of cell cycle checkpoints. Here, we provide a general overview of cell cycle regulation and of the processes involved in genome duplication in human cells, followed by an introduction to replication stress and to the cellular responses elicited by perturbed DNA synthesis. We discuss fragile genomic regions that experience high levels of replication stress, with a particular focus on telomere fragility caused by replication stress at the ends of linear chromosomes. Using alternative lengthening of telomeres (ALT) in cancer cells and ALT-associated PML bodies (APBs) as examples of replication stress-associated clustered DNA damage, we discuss compartmentalization of DNA repair reactions and the role of protein properties implicated in phase separation. Finally, we highlight emerging connections between DNA repair and mechanobiology and discuss how biomolecular condensates, components of the nuclear cytoskeleton, and interfaces between membrane-bound organelles and membraneless macromolecular condensates may cooperate to coordinate genome maintenance in space and time.

Chromatin organization drives the search mechanism of nuclear factors

Nuclear factors rapidly scan the genome for their targets, but the role of nuclear organization in such search is uncharted. Here we analyzed how multiple factors explore chromatin, combining live-cell single-molecule tracking with multifocal structured illumination of DNA density. We find that factors displaying higher bound fractions sample DNA-dense regions more exhaustively. Focusing on the tumor-suppressor p53, we demonstrate that it searches for targets by alternating between rapid diffusion in the interchromatin compartment and compact sampling of chromatin dense regions. Efficient targeting requires balanced interactions with chromatin: fusing p53 with an exogenous intrinsically disordered region potentiates p53-mediated target gene activation at low concentrations, but leads to condensates at higher levels, derailing its search and downregulating transcription. Our findings highlight the role of disordered regions on factors search and showcase a powerful method to generate traffic maps of the eukaryotic nucleus to dissect how its organization guides nuclear factors action.

Chromatin dynamics and subnuclear gene positioning for transcriptional regulation

Plants have been found to exhibit diverse characteristics and functions of chromatin organization, showing both similarities and differences to animals. It is becoming clear how chromatin organization is linked to transcriptional regulation in response to environmental stresses. Regulation of specific chromatin positions in the nuclear space is important for transcription, and the mechanisms that enable such chromatin dynamics are gradually being unveiled. Genes move between subdomains responsible for transcriptional activation or suppression in the subnuclear space in a gene repositioning cycle. We propose a model of localized chromatin interaction in nuclear subdomains, in which the dynamics of local chromatin interactions have a more important impact on the regulation of gene expression than large-scale chromatin organization. In this mini-review, we highlight recent findings on chromatin dynamics, particularly involving transcriptional regulation, and discuss future directions in the study of chromatin organization in plants.

Myosin in chromosome organisation and gene expression

The importance of myosin motor protein is well-characterised within the cytoplasm and cytoskeleton. However, mounting evidence on four nuclear myosins highlights the central role these proteins have in maintaining genomic stability and gene expression. This review focuses on each of their critical roles in chromatin structure, chromosome translocation, transcription regulation, and DNA damage repair in terms of maintaining chromosome and chromatin integrity.

Nuclear myosin VI maintains replication fork stability

The actin cytoskeleton is of fundamental importance for cellular structure and plasticity. However, abundance and function of filamentous actin in the nucleus are still controversial. Here we show that the actin-based molecular motor myosin VI contributes to the stabilization of stalled or reversed replication forks. In response to DNA replication stress, myosin VI associates with stalled replication intermediates and cooperates with the AAA ATPase Werner helicase interacting protein 1 (WRNIP1) in protecting these structures from DNA2-mediated nucleolytic attack. Using functionalized affinity probes to manipulate myosin VI levels in a compartment-specific manner, we provide evidence for the direct involvement of myosin VI in the nucleus and against a contribution of the abundant cytoplasmic pool during the replication stress response.

Autophagy receptor NDP52 alters DNA conformation to modulate RNA polymerase II transcription

NDP52 is an autophagy receptor involved in the recognition and degradation of invading pathogens and damaged organelles. Although NDP52 was first identified in the nucleus and is expressed throughout the cell, to date, there is no clear nuclear functions for NDP52. Here, we use a multidisciplinary approach to characterise the biochemical properties and nuclear roles of NDP52. We find that NDP52 clusters with RNA Polymerase II (RNAPII) at transcription initiation sites and that its overexpression promotes the formation of additional transcriptional clusters. We also show that depletion of NDP52 impacts overall gene expression levels in two model mammalian cells, and that transcription inhibition affects the spatial organisation and molecular dynamics of NDP52 in the nucleus. This directly links NDP52 to a role in RNAPII-dependent transcription. Furthermore, we also show that NDP52 binds specifically and with high affinity to double-stranded DNA (dsDNA) and that this interaction leads to changes in DNA structure in vitro. This, together with our proteomics data indicating enrichment for interactions with nucleosome remodelling proteins and DNA structure regulators, suggests a possible function for NDP52 in chromatin regulation. Overall, here we uncover nuclear roles for NDP52 in gene expression and DNA structure regulation.

Unconventional myosin VI in the heart: Involvement in cardiac dysfunction progressing with age

Hypertrophic cardiomyopathy is the most common cardiovascular disease, which is characterized by structural and functional myocardial abnormalities. It is caused predominantly by autosomal dominant mutations, mainly in genes encoding cardiac sarcomeric proteins, resulting in diverse phenotypical patterns and a heterogenic clinical course. Unconventional myosin VI (MVI) is one of the proteins important for heart function, as it was shown that a point mutation within MYO6 is associated with left ventricular hypertrophy. Previously, we showed that MVI is expressed in the cardiac muscle, where it localizes to the sarcoplasmic reticulum and intercalated discs. Here, we addressed the mechanisms of its involvement in cardiac dysfunction in Snell's waltzer mice (natural MVI knockouts) during heart development. We showed that heart enlargement was already seen in the E14.5 embryos and newborn animals (P0), and was maintained throughout the examined lifespan (up to 12 months). The higher levels of MVI were observed in the hearts of E14.5 embryos and P0 of control heterozygous mice. A search for the mechanisms behind the observed phenotype revealed several changes, accumulation of which resulted in age-progressing heart dysfunction. The main changes that mostly contribute to this functional impairment are the increase in cardiomyocyte proliferation in newborns, disorganization of intercalated discs, and overexpression of SERCA2 in hearts isolated from 12-month-old mice, indicative of functional alterations of sarcoplasmic reticulum. Also, possible aberrations in the heart vascularization, observed in 12-month-old animals could be additional factors responsible for MVI-associated heart dysfunction.

The anatomy of transcriptionally active chromatin loops in Drosophila primary spermatocytes using super-resolution microscopy

While the biochemistry of gene transcription has been well studied, our understanding of how this process is organised in 3D within the intact nucleus is less well understood. Here we investigate the structure of actively transcribed chromatin and the architecture of its interaction with active RNA polymerase. For this analysis, we have used super-resolution microscopy to image the Drosophila melanogaster Y loops which represent huge, several megabases long, single transcription units. The Y loops provide a particularly amenable model system for transcriptionally active chromatin. We find that, although these transcribed loops are decondensed they are not organised as extended 10nm fibres, but rather they largely consist of chains of nucleosome clusters. The average width of each cluster is around 50nm. We find that foci of active RNA polymerase are generally located off the main fibre axis on the periphery of the nucleosome clusters. Foci of RNA polymerase and nascent transcripts are distributed around the Y loops rather than being clustered in individual transcription factories. However, as the RNA polymerase foci are considerably less prevalent than the nucleosome clusters, the organisation of this active chromatin into chains of nucleosome clusters is unlikely to be determined by the activity of the polymerases transcribing the Y loops. These results provide a foundation for understanding the topological relationship between chromatin and the process of gene transcription.

Binding partners regulate unfolding of myosin VI to activate the molecular motor

Myosin VI is the only minus-end actin motor and it is coupled to various cellular processes ranging from endocytosis to transcription. This multi-potent nature is achieved through alternative isoform splicing and interactions with a network of binding partners. There is a complex interplay between isoforms and binding partners to regulate myosin VI. Here, we have compared the regulation of two myosin VI splice isoforms by two different binding partners. By combining biochemical and single-molecule approaches, we propose that myosin VI regulation follows a generic mechanism, independently of the spliced isoform and the binding partner involved. We describe how myosin VI adopts an autoinhibited backfolded state which is released by binding partners. This unfolding activates the motor, enhances actin binding and can subsequently trigger dimerization. We have further expanded our study by using single-molecule imaging to investigate the impact of binding partners upon myosin VI molecular organization and dynamics.

Genome-Directed Cell Nucleus Assembly

Simple Summary

Speckles and other nuclear bodies, the nucleolus and perinucleolar zone, transcription/replication factories and the lamina-associated compartment, serve as a structural basis for various genomic functions. In turn, genome activity and specific chromatin 3D organization directly impact the integrity of intranuclear assemblies, initiating/facilitating their formation and dictating their composition. Thus, the large-scale nucleus structure and genome activity mutually influence each other. The cell nucleus is frequently considered a compartment in which the genome is placed to protect it from external forces. Here, we discuss the evidence demonstrating that the cell nucleus should be considered, rather, as structure built around the folded genome. Decondensing chromosomes provide a scaffold for the assembly of the nuclear envelope after mitosis, whereas genome activity directs the assembly of various nuclear compartments, including nucleolus, speckles and transcription factories.

Abstract

The cell nucleus is frequently considered a cage in which the genome is placed to protect it from various external factors. Inside the nucleus, many functional compartments have been identified that are directly or indirectly involved in implementing genomic DNA’s genetic functions. For many years, it was assumed that these compartments are assembled on a proteinaceous scaffold (nuclear matrix), which provides a structural milieu for nuclear compartmentalization and genome folding while simultaneously offering some rigidity to the cell nucleus. The results of research in recent years have made it possible to consider the cell nucleus from a different angle. From the “box” in which the genome is placed, the nucleus has become a kind of mobile exoskeleton, which is formed around the packaged genome, under the influence of transcription and other processes directly related to the genome activity. In this review, we summarize the main arguments in favor of this point of view by analyzing the mechanisms that mediate cell nucleus assembly and support its resistance to mechanical stresses.