Maria Vartiainen - Academia.edu (original) (raw)

Papers by Maria Vartiainen

Current Topics in Microbiology and Immunology, Feb 1, 2004

The actin cytoskeleton is fundamental for various motile and morphogenetic processes in cells. Th... more The actin cytoskeleton is fundamental for various motile and morphogenetic processes in cells. The structure and dynamics of the actin cytoskeleton are regulated by a wide array of actin-binding proteins, whose activities are controlled by various signal transduction pathways. Recent studies have shown that certain membrane phospholipids, especially PI(4,5)P2 and PI(3,4,5)P3, regulate actin filament assembly in cells and in cell extracts. PI(4,5)P2 appears to be a general regulator of actin polymerization at the plasma membrane or at membrane microdomains, whereas PI(3,4,5)P3 promotes the assembly of specialized actin filament structures in response to some growth factors. Biochemical studies have demonstrated that the activities of many proteins promoting actin assembly are upregulated by PI(4,5)P2, whereas proteins that inhibit actin assembly or promote filament disassembly are down-regulated by PI(4,5)P2. PI(3,4,5)P3 promotes its effects on the actin cytoskeleton mainly through activation of the Rho family of small GTPases. In addition to their effects on actin dynamics, both PI(4,5)P2 and PI(3,4,5)P3 promote the formation of specific actin filament structures through activation/inactivation of actin filament cross-linking proteins and proteins that mediate cytoskeleton-plasma membrane interactions.

Journal of cell science, Jan 28, 2015

Nuclear actin plays an important role in many processes that regulate gene expression. Cytoplasmi... more Nuclear actin plays an important role in many processes that regulate gene expression. Cytoplasmic actin dynamics are tightly controlled by numerous actin-binding proteins, but regulation of nuclear actin has remained unclear. Here we have performed a genome-wide RNAi screen in Drosophila cells to identify proteins that influence either nuclear polymerization or import of actin. We validate 19 factors as specific hits, and show that Chinmo/Bach2, SNF4Aγ/Prkag1 and Rab18 play a role in nuclear localization of actin in both fly and mammalian cells. We identify several novel regulators of cofilin activity, and characterize modulators of both cofilin kinases and phosphatase. For example, Chinmo/Bach2, which regulates nuclear actin levels also in vivo, maintains active cofilin by repressing Cdi/Tes kinase expression. Finally, we show that Nup98 and Lam are candidates for regulating nuclear actin polymerization. Our screen therefore reveals novel aspects of actin regulation and links nucl...

Methods in molecular biology (Clifton, N.J.), 2013

A great number of molecules are constantly being exchanged between the nucleus and the cytoplasm ... more A great number of molecules are constantly being exchanged between the nucleus and the cytoplasm via nuclear pore complexes (NPCs). Importantly, this nucleocytoplasmic trafficking is used to transfer information between the two compartments, thereby permitting the manipulation of critical nuclear processes such as transcription. Constant shuttling of actin is an example of the versatility of this regulatory avenue, as this protein has the capability to drive the transcriptional activity of certain gene sets as well as influence transcription on a global scale. Nuclear import and export are extremely dynamic phenomena and require imaging tools capable of rapid sampling rates for proper quantitative observation. Here we describe live-cell imaging assays based on fluorescence recovery after photobleaching (FRAP) and fluorescence loss in photobleaching (FLIP) for monitoring both import and export of fluorescently labelled molecules. Our assays are performed with GFP-actin, but the same ...

Transcription

Nuclear actin levels have recently been linked to different cellular fates, suggesting that actin... more Nuclear actin levels have recently been linked to different cellular fates, suggesting that actin could act as a switch between altered transcriptional states. Here we discuss our latest results on the mechanisms by which nuclear actin levels are regulated and their implications to the functional significance of nuclear actin.

Current topics in microbiology and immunology, 2004

The Journal of Cell Biology, 2011

The EMBO Journal, 2010

Myocardin-related transcription factors (MRTFs) are actinregulated transcriptional coactivators, ... more Myocardin-related transcription factors (MRTFs) are actinregulated transcriptional coactivators, which bind G-actin through their N-terminal RPEL domains. In response to signal-induced actin polymerisation and concomitant G-actin depletion, MRTFs accumulate in the nucleus and activate target gene transcription through their partner protein SRF. Nuclear accumulation of MRTFs in response to signal is inhibited by increased G-actin level. Here, we study the mechanism by which MRTF-A enters the nucleus. We show that MRTF-A contains an unusually long bipartite nuclear localisation signal (NLS), comprising two basic elements separated by 30 residues, embedded within the RPEL domain. Using siRNA-mediated protein depletion in vivo, and nuclear import assays in vitro, we show that the MRTF-A extended bipartite NLS uses the importin (Imp)a/b-dependent import pathway, and that import is inhibited by G-actin. Interaction of the NLS with the Impa-Impb heterodimer requires both NLS basic elements, and is dependent on the Impa major and minor binding pockets. Binding of the Impa-Impb heterodimer to the intact MRTF-A RPEL domain occurs competitively with G-actin. Thus, MRTF-A contains an actin-sensitive nuclear import signal.

Proceedings of the National Academy of Sciences, 2012

Author contributions: M.K.V. designed research; J.D., K.-P.S., E.K.R., and M.K.V. performed resea... more Author contributions: M.K.V. designed research; J.D., K.-P.S., E.K.R., and M.K.V. performed research; M.K.V. contributed new reagents/analytic tools;

Nature Reviews Molecular Cell Biology, 2013

The paradigm states that cytoplasmic actin operates as filaments and nuclear actin is mainly mono... more The paradigm states that cytoplasmic actin operates as filaments and nuclear actin is mainly monomeric, acting as a scaffold in transcription complexes. However, why should a powerful function of actin, namely polymerization, not be used in the nucleus? Recent progress in the field forces us to rethink this issue, as many actin filament assembly proteins have been linked to nuclear functions and new experimental approaches have provided the first direct visualizations of polymerized nuclear actin.

Nature, 2013

Laminopathies, caused by mutations in the LMNA gene encoding the nuclear envelope proteins lamins... more Laminopathies, caused by mutations in the LMNA gene encoding the nuclear envelope proteins lamins A and C, represent a diverse group of diseases that include Emery-Dreifuss Muscular Dystrophy (EDMD), dilated cardiomyopathy (DCM), limb-girdle muscular dystrophy, and Hutchison-Gilford progeria syndrome (HGPS). 1 The majority of LMNA mutations affect skeletal and cardiac muscle by mechanisms that remain incompletely understood. Loss of structural function and disturbed interaction of mutant lamins with (tissue-specific) transcription factors have been proposed to explain the tissue-specific phenotypes. We report here that lamin A/Cdeficient (Lmna −/− ) and Lmna N195K mutant cells have impaired nuclear translocation and downstream signaling of the mechanosensitive transcription factor megakaryoblastic leukaemia 1 (MKL1), a myocardin family member that is pivotal in cardiac development and function. 2 Disturbed nucleo-cytoplasmic shuttling of MKL1 was caused by altered actin dynamics in Lmna −/− and N195K mutant cells. Ectopic expression of the nuclear envelope protein emerin, which is mislocalized in Lmna mutant cells and also linked to EDMD and DCM, restored MKL1 nuclear translocation and rescued actin dynamics in mutant cells. These findings present a novel mechanism that could provide insight into the disease etiology for the cardiac phenotype in many laminopathies, whereby lamins A/C and emerin regulate gene expression through modulation of nuclear and cytoskeletal actin polymerization.

Molecular Biology of the Cell, 2014

The endoplasmic reticulum (ER) comprises a dynamic three-dimensional (3D) network with diverse st... more The endoplasmic reticulum (ER) comprises a dynamic three-dimensional (3D) network with diverse structural and functional domains. Proper ER operation requires an intricate balance within and between dynamics, morphology, and functions, but how these processes are coupled in cells has been unclear. Using live-cell imaging and 3D electron microscopy, we identify a specific subset of actin filaments localizing to polygons defined by ER sheets and tubules and describe a role for these actin arrays in ER sheet persistence and, thereby, in maintenance of the characteristic network architecture by showing that actin depolymerization leads to increased sheet fluctuation and transformations and results in small and less abundant sheet remnants and a defective ER network distribution. Furthermore, we identify myosin 1c localizing to the ER-associated actin filament arrays and reveal a novel role for myosin 1c in regulating these actin structures, as myosin 1c manipulations lead to loss of the actin filaments and to similar ER phenotype as observed after actin depolymerization. We propose that ER-associated actin filaments have a role in ER sheet persistence regulation and thus support the maintenance of sheets as a stationary subdomain of the dynamic ER network.

Journal of Cell Science, 2013

Cytoskeleton, 2013

A number of studies in the last decade have irrevocably promoted actin into a fully fledged membe... more A number of studies in the last decade have irrevocably promoted actin into a fully fledged member of the nuclear compartment, where it, among other crucial tasks, facilitates transcription and chromatin remodeling. Changes in nuclear actin levels have been linked to different cellular processes: decreased nuclear actin to quiescence and increased nuclear actin to differentiation. Importin 9 and exportin 6 transport factors are responsible for the continuous nucleocytoplasmic shuttling of actin, but the mechanisms, which result in modulated actin levels, have not been characterized. We find that in cells growing under normal growth conditions, the levels of nuclear actin vary considerably from cell to cell. To understand the basis for this, we have extensively quantified several cellular parameters while at the same time recording the import and export rates of green fluorescent protein (GFP)-tagged actin. Surprisingly, our dataset shows that the ratio of nuclear to cytoplasmic fluorescence intensity, but not nuclear shape, size, cytoplasm size, or their ratio, correlates negatively with both import and export rate of actin. This suggests that high-nuclear actin content is maintained by both diminished import and export. The high nuclear actin containing cells still show high mobility of actin, but it is not export competent, suggesting increased binding of actin to nuclear complexes. Creation of such export incompetent actin pool would ensure enough actin is retained in the nucleus and make it available for the various nuclear functions described for actin.

Cover figure: NIH 3T3 cell stained with rhodamine-phalloidin (red) to visualize actin filaments a... more Cover figure: NIH 3T3 cell stained with rhodamine-phalloidin (red) to visualize actin filaments and with FITC-DNAseI (green) to visualize DNA and actin monomers.

FEBS Letters, 2008

Cell biological functions of actin have recently expanded from cytoplasm to nucleus, with actin i... more Cell biological functions of actin have recently expanded from cytoplasm to nucleus, with actin implicated in such diverse processes as gene expression, transcription factor regulation and intranuclear motility. Actin in the nucleus seems to behave differently than in the cytoplasm, raising new questions regarding the molecular mechanisms by which actin functions in cells. In this review, I will discuss dynamic properties of nuclear actin that are related to its polymerization cycle and nucleocytoplasmic shuttling. By comparing the behaviour of nuclear and cytoplasmic actin and their regulators, I try to dissect the underlying differences of these equally important cellular actin pools.

Current Opinion in Cell Biology, 2004

ADF/cofilins are essential regulators of actin filament turnover. Several ADF/cofilin isoforms ar... more ADF/cofilins are essential regulators of actin filament turnover. Several ADF/cofilin isoforms are found in multicellular organisms, but their biological differences have remained unclear. Here we show that three ADF/cofilins exist in mouse and most likely in all other mammalian species. Northern blot, and in situ hybridization analyses demonstrate that cofilin-1 is expressed in most cell types of embryos and adult mice.

Journal of cell science, 2002

Twinfilin is a ubiquitous actin-monomer-binding protein that is composed of two ADF-homology doma... more Twinfilin is a ubiquitous actin-monomer-binding protein that is composed of two ADF-homology domains. It forms a 1:1 complex with ADP-actin-monomers, inhibits nucleotide exchange on actin monomers and prevents assembly of the monomer into filaments. The two ADF-H domains in twinfilin probably have 3D structures similar to those of the ADF/cofilin proteins and overlapping actin-binding sites. Twinfilin also interacts with PtdIns(4,5)P(2), which inhibits its actin-monomer-sequestering activity in vitro. Mutations in the twinfilin gene result in defects in the bipolar budding pattern in S. cerevisiae and in a rough eye phenotype and aberrant bristle morphology in Drosophila melanogaster. These phenotypes are caused by the uncontrolled polymerization of actin filaments in the absence of twinfilin. Studies on budding yeast suggest that twinfilin contributes to actin filament turnover by localizing actin monomers, in their 'inactive' ADP-form, to the sites of rapid filament assemb...

Nature communications, 2015

Controlled transport of macromolecules between the cytoplasm and nucleus is essential for homeost... more Controlled transport of macromolecules between the cytoplasm and nucleus is essential for homeostatic regulation of cellular functions. For instance, gene expression entails coordinated nuclear import of transcriptional regulators to activate transcription and nuclear export of the resulting messenger RNAs for cytoplasmic translation. Here we link these two processes by reporting a novel role for the mRNA export factor Ddx19/Dbp5 in nuclear import of MKL1, the signal-responsive transcriptional activator of SRF. We show that Ddx19 is not a general nuclear import factor, and that its specific effect on MKL1 nuclear import is separate from its role in mRNA export. Both helicase and nuclear pore-binding activities of Ddx19 are dispensable for MKL1 nuclear import, but RNA binding is required. Mechanistically, Ddx19 operates by modulating the conformation of MKL1, which affects its interaction with Importin-β for efficient nuclear import. Thus, Ddx19 participates in mRNA export, translati...