Nuclear myosin is ubiquitously expressed and evolutionary conserved in vertebrates

Mechanochemical properties of human myosin-1C are modulated by isoform-specific differences in the N-terminal extension

Myosin-1C is a single-headed, short-tailed member of the myosin class I subfamily that supports a variety of actin-based functions in the cytosol and nucleus. In vertebrates, alternative splicing of the MYO1C gene leads to the production of three isoforms, myosin-1C⁰, myosin-1C¹⁶, and myosin-1C³⁵, that carry N-terminal extensions of different lengths. However, it is not clear how these extensions affect the chemomechanical coupling of human myosin-1C isoforms. Here, we report on the motor activity of the different myosin-1C isoforms measuring the unloaded velocities of constructs lacking the C-terminal lipid-binding domain on nitrocellulose-coated glass surfaces and full-length constructs on reconstituted, supported lipid bilayers. The higher yields of purified proteins obtained with constructs lacking the lipid-binding domain allowed a detailed characterization of the individual kinetic steps of human myosin-1C isoforms in their productive interaction with nucleotides and filamentous actin. Isoform-specific differences include 18-fold changes in the maximum power output per myosin-1C motor and 4-fold changes in the velocity and the resistive force at which maximum power output occurs. Our results support a model in which the isoform-specific N-terminal extensions affect chemomechanical coupling by combined steric and allosteric effects, thereby reducing both the length of the working stroke and the rate of ADP release in the absence of external loads by a factor of 2 for myosin-1C³⁵. As the large change in maximum power output shows, the functional differences between the isoforms are further amplified by the presence of external loads.

Myosin VI in the nucleus of neurosecretory PC12 cells: Stimulation-dependent nuclear translocation and interaction with nuclear proteins

Myosin VI (MVI) is a unique actin-based motor protein moving towards the minus end of actin filaments, in the opposite direction than other known myosins. Besides well described functions of MVI in endocytosis and maintenance of Golgi apparatus, there are few reports showing its involvement in transcription. We previously demonstrated that in neurosecretory PC12 cells MVI was present in the cytoplasm and nucleus, and its depletion caused substantial inhibition of cell migration and proliferation. Here, we show an increase in nuclear localization of MVI upon cell stimulation, and identification of potential nuclear localization (NLS) and nuclear export (NES) signals within MVI heavy chain. These signals seem to be functional as the MVI nuclear presence was affected by the inhibitors of nuclear import (ivermectin) and export (leptomycin B). In nuclei of stimulated cells, MVI colocalized with active RNA polymerase II, BrUTP-containing transcription sites and transcription factor SP1 as well as SC35 and PML proteins, markers of nuclear speckles and PML bodies, respectively. Mass spectrometry analysis of samples of a GST-pull-down assay with the MVI tail domain as a "bait" identified several new potential MVI binding partners. Among them are proteins involved in transcription and post-transcriptional processes. We confirmed interaction of MVI with heterogeneous nuclear ribonucleoprotein U (hnRNPU) and nucleolin, proteins involved in pre-mRNA binding and transport, and nucleolar function, respectively. Our data provide an insight into mechanisms of involvement of MVI in nuclear processes via interaction with nuclear proteins and support a notion for important role(s) for MVI in gene expression.

Nuclear myosin I regulates cell membrane tension

Plasma membrane tension is an important feature that determines the cell shape and influences processes such as cell motility, spreading, endocytosis and exocytosis. Unconventional class 1 myosins are potent regulators of plasma membrane tension because they physically link the plasma membrane with adjacent cytoskeleton. We identified nuclear myosin 1 (NM1) - a putative nuclear isoform of myosin 1c (Myo1c) - as a new player in the field. Although having specific nuclear functions, NM1 localizes predominantly to the plasma membrane. Deletion of NM1 causes more than a 50% increase in the elasticity of the plasma membrane around the actin cytoskeleton as measured by atomic force microscopy. This higher elasticity of NM1 knock-out cells leads to 25% higher resistance to short-term hypotonic environment and rapid cell swelling. In contrast, overexpression of NM1 in wild type cells leads to an additional 30% reduction of their survival. We have shown that NM1 has a direct functional role in the cytoplasm as a dynamic linker between the cell membrane and the underlying cytoskeleton, regulating the degree of effective plasma membrane tension.

Selective expression of myosin IC Isoform A in mouse and human cell lines and mouse prostate cancer tissues

Myosin IC is a single headed member of the myosin superfamily. We recently identified a novel isoform and showed that the MYOIC gene in mammalian cells encodes three isoforms (isoforms A, B, and C). Furthermore, we demonstrated that myosin IC isoform A but not isoform B exhibits a tissue specific expression pattern. In this study, we extended our analysis of myosin IC isoform expression patterns by analyzing the protein and mRNA expression in various mammalian cell lines and in various prostate specimens and tumor tissues from the transgenic mouse prostate (TRAMP) model by immunoblotting, qRT-PCR, and by indirect immunohistochemical staining of paraffin embedded prostate specimen. Analysis of a panel of mammalian cell lines showed an increased mRNA and protein expression of specifically myosin IC isoform A in a panel of human and mouse prostate cancer cell lines but not in non-cancer prostate or other (non-prostate-) cancer cell lines. Furthermore, we demonstrate that myosin IC isoform A expression is significantly increased in TRAMP mouse prostate samples with prostatic intraepithelial neoplasia (PIN) lesions and in distant site metastases in lung and liver when compared to matched normal tissues. Our observations demonstrate specific changes in the expression of myosin IC isoform A that are concurrent with the occurrence of prostate cancer in the TRAMP mouse prostate cancer model that closely mimics clinical prostate cancer. These data suggest that elevated levels of myosin IC isoform A may be a potential marker for the detection of prostate cancer.

Tissue specific expression of myosin IC isoforms

Background

Myosin IC is a single headed member of the myosin superfamily that localizes to the cytoplasm and the nucleus and is implicated in a variety of processes in both compartments. We recently identified a novel isoform of myosin IC and showed that the MYOIC gene in mammalian cells encodes three isoforms (isoforms A, B, and C) that differ only in the addition of short isoform-specific N-terminal peptides. The expression pattern of the isoforms and the mechanisms of expression regulation remain unknown.

Results

To determine the expression patterns of myosin IC isoforms, we performed a comprehensive expression analysis of the two myosin IC isoforms (isoform A and B) that contain isoform-specific sequences. By immunoblotting with isoform-specific antibodies and by qRT-PCR with isoform-specific primer we demonstrate that myosin IC isoforms A and B have distinct expression patterns in mouse tissues. Specifically, we show that myosin IC isoform A is expressed in a tissue specific pattern, while myosin IC isoform B is ubiquitously expressed at comparable levels in mouse tissues.

Conclusions

The differences in the expression profile of the myosin IC isoforms indicate a tissue-specific MYOIC gene regulation and further suggest that the myosin IC isoforms, despite their high sequence homology, might have tissue-specific and isoform-specific functions.

Mouse nuclear myosin I knock-out shows interchangeability and redundancy of myosin isoforms in the cell nucleus

BACKGROUND

Nuclear myosin I (NM1) is a nuclear isoform of the well-known "cytoplasmic" Myosin 1c protein (Myo1c). Located on the 11(th) chromosome in mice, NM1 results from an alternative start of transcription of the Myo1c gene adding an extra 16 amino acids at the N-terminus. Previous studies revealed its roles in RNA Polymerase I and RNA Polymerase II transcription, chromatin remodeling, and chromosomal movements. Its nuclear localization signal is localized in the middle of the molecule and therefore directs both Myosin 1c isoforms to the nucleus.

METHODOLOGY/PRINCIPAL FINDINGS

In order to trace specific functions of the NM1 isoform, we generated mice lacking the NM1 start codon without affecting the cytoplasmic Myo1c protein. Mutant mice were analyzed in a comprehensive phenotypic screen in cooperation with the German Mouse Clinic. Strikingly, no obvious phenotype related to previously described functions has been observed. However, we found minor changes in bone mineral density and the number and size of red blood cells in knock-out mice, which are most probably not related to previously described functions of NM1 in the nucleus. In Myo1c/NM1 depleted U2OS cells, the level of Pol I transcription was restored by overexpression of shRNA-resistant mouse Myo1c. Moreover, we found Myo1c interacting with Pol II. The ratio between Myo1c and NM1 proteins were similar in the nucleus and deletion of NM1 did not cause any compensatory overexpression of Myo1c protein.

CONCLUSION/SIGNIFICANCE

We observed that Myo1c can replace NM1 in its nuclear functions. Amount of both proteins is nearly equal and NM1 knock-out does not cause any compensatory overexpression of Myo1c. We therefore suggest that both isoforms can substitute each other in nuclear processes.

Specific nuclear localizing sequence directs two myosin isoforms to the cell nucleus in calmodulin-sensitive manner

Background

Nuclear myosin I (NM1) was the first molecular motor identified in the cell nucleus. Together with nuclear actin, they participate in crucial nuclear events such as transcription, chromatin movements, and chromatin remodeling. NM1 is an isoform of myosin 1c (Myo1c) that was identified earlier and is known to act in the cytoplasm. NM1 differs from the “cytoplasmic” myosin 1c only by additional 16 amino acids at the N-terminus of the molecule. This amino acid stretch was therefore suggested to direct NM1 into the nucleus.

Methodology/Principal Findings

We investigated the mechanism of nuclear import of NM1 in detail. Using over-expressed GFP chimeras encoding for truncated NM1 mutants, we identified a specific sequence that is necessary for its import to the nucleus. This novel nuclear localization sequence is placed within calmodulin-binding motif of NM1, thus it is present also in the Myo1c. We confirmed the presence of both isoforms in the nucleus by transfection of tagged NM1 and Myo1c constructs into cultured cells, and also by showing the presence of the endogenous Myo1c in purified nuclei of cells derived from knock-out mice lacking NM1. Using pull-down and co-immunoprecipitation assays we identified importin beta, importin 5 and importin 7 as nuclear transport receptors that bind NM1. Since the NLS sequence of NM1 lies within the region that also binds calmodulin we tested the influence of calmodulin on the localization of NM1. The presence of elevated levels of calmodulin interfered with nuclear localization of tagged NM1.

Conclusions/Significance

We have shown that the novel specific NLS brings to the cell nucleus not only the “nuclear” isoform of myosin I (NM1 protein) but also its “cytoplasmic” isoform (Myo1c protein). This opens a new field for exploring functions of this molecular motor in nuclear processes, and for exploring the signals between cytoplasm and the nucleus.

Extending the knowledge in histochemistry and cell biology

Central to modern Histochemistry and Cell Biology stands the need for visualization of cellular and molecular processes. In the past several years, a variety of techniques has been achieved bridging traditional light microscopy, fluorescence microscopy and electron microscopy with powerful software-based post-processing and computer modeling. Researchers now have various tools available to investigate problems of interest from bird's- up to worm's-eye of view, focusing on tissues, cells, proteins or finally single molecules. Applications of new approaches in combination with well-established traditional techniques of mRNA, DNA or protein analysis have led to enlightening and prudent studies which have paved the way toward a better understanding of not only physiological but also pathological processes in the field of cell biology. This review is intended to summarize articles standing for the progress made in "histo-biochemical" techniques and their manifold applications.

Nuclear myosin 1 is in complex with mature rRNA transcripts and associates with the nuclear pore basket

In rRNA biogenesis, nuclear myosin 1 (NM1) and actin synergize to activate rRNA gene transcription. Evidence that actin is in preribosomal subunits and NM1 may control rRNA biogenesis post-transcriptionally prompted us to investigate whether NM1 associates with and accompanies rRNA to nuclear pores (NPC). Ultracentrifugation on HeLa nucleolar extracts showed RNA-dependent NM1 coelution with preribosomal subunits. In RNA immunoprecipitations (RIPs), NM1 coprecipitated with pre-rRNAs and 18S, 5.8S, and 28S rRNAs, but failed to precipitate 5S rRNA and 7SL RNA. In isolated nuclei and living HeLa cells, NM1 or actin inhibition and selective alterations in actin polymerization impaired 36S pre-rRNA processing. Immunoelectron microscopy (IEM) on sections of manually isolated Xenopus oocyte nuclei showed NM1 localization at the NPC basket. Field emission scanning IEM on isolated nuclear envelopes and intranuclear content confirmed basket localization and showed that NM1 decorates actin-rich pore-linked filaments. Finally, RIP and successive RIPs (reRIPs) on cross-linked HeLa cells demonstrated that NM1, CRM1, and Nup153 precipitate same 18S and 28S rRNAs but not 5S rRNA. We conclude that NM1 facilitates maturation and accompanies export-competent preribosomal subunits to the NPC, thus modulating export.

The long journey of actin and actin-associated proteins from genes to polysomes

During gene expression, multiple regulatory steps make sure that alterations of chromatin structure are synchronized with RNA synthesis, co-transcriptional assembly of ribonucleoprotein complexes, transport to the cytoplasm and localized translation. These events are controlled by large multiprotein complexes commonly referred to as molecular machines, which are specialized and at the same time display a highly dynamic protein composition. The crosstalk between these molecular machines is essential for efficient RNA biogenesis. Actin has been recently proposed to be an important factor throughout the entire RNA biogenesis pathway as a component of chromatin remodeling complexes, associated with all eukaryotic RNA polymerases as well as precursor and mature ribonucleoprotein complexes. The aim of this review is to present evidence on the involvement of actin and actin-associated proteins in RNA biogenesis and propose integrative models supporting the view that actin facilitates coordination of the different steps in gene expression.

Actin dynamics and functions in the interphase nucleus: moving toward an understanding of nuclear polymeric actin

Actin exists as a dynamic equilibrium of monomers and polymers within the nucleus of living cells. It is utilized by the cell for many aspects of gene regulation, including mRNA processing, chromatin remodelling, and global gene expression. Polymeric actin is now specifically linked to transcription by RNA polymerase I, II, and III. An active process, requiring both actin polymers and myosin, appears to drive RNA polymerase I transcription, and is also implicated in long-range chromatin movement. This type of mechanism brings activated genes from separate chromosomal territories together, and then participates in their compartmentalization near nuclear speckles. Nuclear speckle formation requires polymeric actin, and factors promoting polymerization, such as profilin and PIP2, are concentrated there. A review of the literature shows that a functional population of G-actin cycles between the cytoplasm and the nucleoplasm. Its nuclear concentration is dependent on the cytoplasmic G-actin pool, as well as on the activity of import and export mechanisms and the availability of interactions that sequester it within the nucleus. The N-WASP-Arp2/3 actin polymer-nucleating mechanism functions in the nucleus, and its mediators, including NCK, PIP2, and Rac1, can be found in the nucleoplasm, where they likely influence the kinetics of polymer formation. The actin polymer species produced are tightly regulated, and may take on conformations not easily recognized by phalloidin. Many of the factors that cleave F-actin in the cytoplasm are present at high levels in the nucleoplasm, and are also likely to affect actin dynamics there. The absolute and relative G-actin content in the nucleoplasm and the cytoplasm of a cell contains information about the homeostatic state of that cell. We propose that the cycling of G-actin between the nucleus and cytoplasm represents a signal transduction mechanism that can function through both extremes of global cellular G-actin content. MAL signalling within the serum response factor pathway, when G-actin levels are low, represents a well-studied example of actin functioning in signal transduction. The translocation of NCK into the nucleus, along with G-actin, during dissolution of the cytoskeleton in response to DNA damage represents another instance of a unique signalling mechanism operating when G-actin levels are high.

Ancient animal ancestry for nuclear myosin

The identification of nuclear myosin I (NMI) has raised the possibility that myosin might have had an early functional role in the eukaryotic nucleus. To investigate this possibility, we examined the molecular evolution of the vertebrate myosin-I proteins. We found that myosin I has undergone at least five duplication events in the common ancestor of the vertebrates (vertebrate-specific duplications), leading to nine myosin-I vertebrate gene families, followed by two additional myosin-I duplication events in the lineage leading to modern fish. This expansion suggests a large-scale adaptive radiation in myosin-I function in an early phase of vertebrate evolution. The branching order of the evolutionary tree suggests that the functional role of NMI predates this expansion. More specifically, in the tunicate Ciona intestinalis, we found a myosin-I protein that localizes to the nucleus, but that branches on phylogenetic trees before the duplication that led to vertebrate myosin IC and myosin IH. This relationship suggests that the common ancestor of these three proteins encoded a nuclear isoform and that the localization of myosin I to the nucleus predates the origin of the vertebrates. Thus, a functional role for NMI appears to have been present at an early stage of animal evolution prior to the rise of both myosin IC and the vertebrates, as NMI was present in the last common ancestor of vertebrates and tunicates.

Cell and molecular biology of nuclear actin

Actin is a highly conserved protein and one of the major components of the cytoplasm and the nucleus in eukaryotic cells. In the nucleus, actin is involved in a variety of nuclear processes that include transcription and transcription regulation, RNA processing and export, intranuclear movement, and structure maintenance. Recent advances in the field of nuclear actin have established that functions of actin in the nucleus are versatile, complex, and interconnected. It also has become increasingly evident that the cytoplasmic and nuclear pools of actin are functionally linked. However, while the biological significance of nuclear actin has become clear, we are only beginning to understand the mechanisms that lie behind the regulation of nuclear actin. This review provides an overview of our current understanding of the functions of actin in the nucleus.

Enhancing nuclear receptor-induced transcription requires nuclear motor and LSD1-dependent gene networking in interchromatin granules

Although the role of liganded nuclear receptors in mediating coactivator/corepressor exchange is well-established, little is known about the potential regulation of chromosomal organization in the 3-dimensional space of the nucleus in achieving integrated transcriptional responses to diverse signaling events. Here, we report that ligand induces rapid interchromosomal interactions among specific subsets of estrogen receptor alpha-bound transcription units, with a dramatic reorganization of nuclear territories, which depends on the actions of nuclear actin/myosin-I machinery and dynein light chain 1. The histone lysine demethylase, LSD1, is required for these ligand-induced interactive loci to associate with distinct interchromatin granules, long thought to serve as "storage" sites for the splicing machinery, some critical transcription elongation factors, and various chromatin remodeling complexes. We demonstrate that this 2-step nuclear rearrangement is essential for achieving enhanced, coordinated transcription of nuclear receptor target genes.

State-of-the-art technologies, current opinions and developments, and novel findings: news from the field of histochemistry and cell biology

Investigations of cell and tissue structure and function using innovative methods and approaches have again yielded numerous exciting findings in recent months and have added important data to current knowledge, inspiring new ideas and hypotheses in various fields of modern life sciences. Topics and contents of comprehensive expert reviews covering different aspects in methodological advances, cell biology, tissue function and morphology, and novel findings reported in original papers are summarized in the present review.

Subnuclear compartmentalization and function of actin and nuclear myosin I in plants

Actins are highly conserved proteins that serve as the basic building blocks of cytoskeletal microfilaments. In animal cells, specific nuclear actin adopts unconventional conformations that are involved in multiple nuclear functions and that associate with nuclear actin binding proteins. However, there is practically no information available about nuclear actin in plants. Indeed, actin has not been detected in the nuclear proteomes of many plants, and orthologs of the main structural nuclear actin-binding proteins have yet to be identified. Here, we have investigated the characteristics, intranuclear compartmentalization, and function of actin in isolated Allium cepa nuclei as well as that of its motor protein nuclear myosin I (NMI). Using conformation-specific antibodies for nuclear actin isoforms, ss-actin, and NMI, the distribution of these proteins was studied in Western blots and by immunocytochemistry. Moreover, the participation of nuclear actin in transcription was analyzed in run on in situ assays and inhibition of RNA polymerases I and II. We show that actin isoforms with distinct solubilities are present in onion nuclei with a consistent subnuclear compartmentalization. Actin and NMI are highly enriched in foci that are similar to transcription foci, although actin is also distributed diffusely in the nucleus and nucleolus as well as accumulating in a subset of the Cajal bodies. Immunogold labeling identified both proteins in the nuclear transcription subdomains and in other subnuclear compartments. In addition, actin and NMI were diffusely distributed in the nuclear matrix.

Nuclear myosin I acts in concert with polymeric actin to drive RNA polymerase I transcription

Actin is associated with all three nuclear RNA polymerases and acts in concert with nuclear myosin I (NM1) to drive transcription. Practically nothing is known regarding the state of actin and the functional interplay of actin and NM1 in transcription. Here we show that actin and NM1 act in concert to promote RNA polymerase I (Pol I) transcription. Drugs that prevent actin polymerization or inhibit myosin function inhibit Pol I transcription in vivo and in vitro. Mutants that stabilize the polymeric state actin are tightly associated with Pol I and activate transcription, whereas a polymerization-deficient mutant does not bind to Pol I and does not promote rDNA transcription. Consistent with nuclear actin and myosin synergizing in transcription activation, NM1 mutants that lack specific functions, such as binding to ATP, actin, or calmodulin, are incapable of associating with Pol I and rDNA. The results show that actin polymerization and the motor function of NM1 are required for association with the Pol I transcription machinery and transcription activation. These observations provide insights into the cooperative action of actin and myosin in the nucleus and reveal an actomyosin-based mechanism in transcription.

Transcriptional control of gene expression by actin and myosin

Recent years have witnessed a new turn in the field of gene expression regulation. Actin and an ever-growing family of actin-associated proteins have been accepted as members of the nuclear crew, regulating eukaryotic gene transcription. In complex with heterogeneous nuclear ribonucleoproteins and certain myosin species, actin has been shown to be an important regulator in RNA polymerase II transcription. Furthermore, actin-based molecular motors are believed to facilitate RNA polymerase I transcription and possibly downstream events during rRNA biogenesis. Probably these findings represent the tip of the iceberg of a rapidly expanding area within the functional architecture of the cell nucleus. Further studies will contribute to clarify how actin mediates nuclear functions with a glance to cytoplasmic signalling. These discoveries have the potential to define novel regulatory networks required to control gene expression at multiple levels.

Recent progress in histochemistry

The progress in discerning the structure and function of cells and tissues in health and disease has been achieved to a large extent by the continued development of new reagents for histochemistry, the improvement of existing techniques and new imaging techniques. This review will highlight some advancements made in these fields.

Mechanotransduction from the ECM to the genome: Are the pieces now in place?

A multitude of biochemical signaling processes have been characterized that affect gene expression and cellular activity. However, living cells often need to integrate biochemical signals with mechanical information from their microenvironment as they respond. In fact, the signals received by shape alone can dictate cell fate. This mechanotrasduction of information is powerful, eliciting proliferation, differentiation, or apoptosis in a manner dependent upon the extent of physical deformation. The cells internal "prestressed" structure and its "hardwired" interaction with the extra-cellular matrix (ECM) appear to confer this ability to filter biochemical signals and decide between divergent cell functions influenced by the nature of signals from the mechanical environment. In some instances mechanical signaling through the tissue microenvironment has been shown to be dominant over genomic defects, imparting a normal phenotype on cells that otherwise have transforming genetic lesions. This mechanical control of phenotype is postulated to have a central role in embryogenesis, tissue physiology as well as the pathology of a wide variety of diseases, including cancer. We will briefly review studies showing physical continuity between the external cellular microenvironment and the interior of the cell nucleus. Newly characterized structures, termed nuclear envelope lamina spanning complexes (NELSC), and their interactions will be described as part of a model for mechanical transduction of extracellular cues from the ECM to the genome.

The histochemistry and cell biology vade mecum: a review of 2005–2006

The procurement of new knowledge and understanding in the ever expanding discipline of cell biology continues to advance at a breakneck pace. The progress in discerning the physiology of cells and tissues in health and disease has been driven to a large extent by the continued development of new probes and imaging techniques. The recent introduction of semi-conductor quantum dots as stable, specific markers for both fluorescence light microscopy and electron microscopy, as well as a virtual treasure-trove of new fluorescent proteins, has in conjunction with newly introduced spectral imaging systems, opened vistas into the seemingly unlimited possibilities for experimental design. Although it oftentimes proves difficult to predict what the future will hold with respect to advances in disciplines such as cell biology and histochemistry, it is facile to look back on what has already occurred. In this spirit, this review will highlight some advancements made in these areas in the past 2 years.